Molecular Pnictogen Activation by Rare Earth and Actinide Complexes

Abstract

This review covers the activation of molecular pnictogens (group 15 elements) by homogeneous rare earth and actinide complexes. All examples of molecular pnictogen activation (dinitrogen, white phosphorus, yellow arsenic) by both rare earths and actinides, to date (2015), are discussed, focusing on synthetic methodology and the structure and bonding of the resulting complexes.

1. Introduction

Rare earth (scandium, yttrium and the lanthanides) and actinide complexes remain underexplored with respect to the transition metals and main group elements but often demonstrate both unique reactivity and molecular properties. Understanding of the bonding and electronic structure of these complexes has particular significance for separation of metals in nuclear waste streams [1].

Activation of molecular pnictogens (group 15 elements) is an area of growing importance; atmospheric dinitrogen (N₂) and white phosphorus (P₄) are principal sources of N- and P-containing compounds (e.g., polymers, pharmaceuticals, agrochemicals, explosives, and specialty chemicals) but are both very challenging to selectively activate. Metal-arsenic, -antimony and -bismuth complexes remain rare [2,3,4], while the study of metal-pnictogen complexes, including these heavier pnictogen homologues, is also of fundamental importance with respect to Ln and An-pnictogen bonding and electronic structure.

Fixation of N₂, the six electron reduction to two molecules of more reactive ammonia, is necessary for further formation of N-element bonds. In nature, nitrogenase enzymes containing metalloproteins (Fe, Mo or V) fix N₂ through proton-coupled electron transfer under ambient conditions [5,6]. In industry, the Haber–Bosch process combines N₂ and high purity H₂ at high temperatures and pressures over heterogeneous iron- or ruthenium-based catalysts [7,8,9,10]. This highly efficient process produces 100 million tons of ammonia per year but is the largest energy-consuming process in the modern world today; the need for direct activation and functionalisation of N₂ under mild conditions is a clear goal. Accessing appropriately reactive phosphorus building blocks presents a different set of challenges based on the sustainability and efficiency of chemical transformations required; from phosphate rock minerals which are mined globally on a 225 million ton scale per year (2013) [11], phosphate fertilisers derived from phosphoric acid are the major products with the remainder used for elemental phosphorus production. Organophosphorus compounds are generally derived from PCl₃, obtained by the chlorination of P₄, and subsequent multi-step procedures [12,13,14]. Attention has turned to direct and selective activation of elemental phosphorus under mild conditions; this approach is more atom-efficient (which is important given the limited accessible deposits of phosphate rock), avoids the need for large scale production of PCl₃ (which is toxic, corrosive and highly reactive), and is both more economically and environmentally sustainable [15].

The area of dinitrogen activation has been reviewed extensively with particular focus on the Haber–Bosch process [16,17,18], and biological nitrogen fixation [5,6,19,20,21,22,23,24,25,26]. There are reviews on transition metal N₂ activation which cover N₂ binding modes [27,28,29], multimetallic N₂ activation [30,31], the relevance of metal hydride complexes to N₂ activation [32,33], N₂ cleavage and functionalisation [34,35] (including electrochemical [36] and photolytic N₂ cleavage [37]), and N₂ activation at bare metal atoms [38] and using surface organometallic chemistry [39]. Specific reviews have also focused on activation by group 4 metals [40,41,42], iron [31,43,44], molybdenum [24,45,46,47], and the mid-to-late transition metal centres [48]. In terms of rare earth N₂ activation; an account of work from Evans and co-workers to 2004 has been reported [49], and Gardiner more recently reviewed the chemistry of the lanthanides with dinitrogen and reduced derivatives [50]. The area of actinide N₂ activation has been discussed in the context of small molecule activation by trivalent uranium complexes [51,52].

Transition metal-mediated white phosphorus activation has been previously reviewed [53,54,55,56], with specific reviews on both early transition metal complexes [57,58] and late transition metal complexes [59]. More broadly, reviews of P₄ activation by p-block compounds have also been reported [60,61,62,63,64].

This review seeks to cover all examples of molecular pnictogen activation (dinitrogen, white phosphorus, yellow arsenic) by both rare earth and actinide complexes to date, focusing on synthetic methodology and the structure and bonding of the resulting complexes. Only well-defined homogeneous complexes will be discussed; heterogeneous and surface chemistry lie beyond the scope of this review.

2. Dinitrogen Activation by Rare Earth Complexes

2.1. Complexes Containing a Formal N₂²⁻ Ligand

The majority of rare earth complexes that activate dinitrogen (N₂) result in its formal reduction to the N₂²⁻ anion and the formation of bimetallic complexes of the general form [A₂(thf)_xLn]₂(μ-η²:η²-N₂) where there is side-on binding of N₂ (“A is defined as a group that exists as an anion in LnA₃ and provides reductive reactivity in combination with an alkali metal or the equivalent”) [65]. The work of Evans and co-workers has led to the development of three key methodologies to access these species: (i) salt metathesis reactions of divalent lanthanide halides with alkali metal salts; (ii) combination of trivalent Ln complexes with alkali metals (LnA₃/M or LnA₂A′/M method); (iii) photochemical activation of LnA₂A′ systems (Figure 1).

Figure 1. Routes to N₂²⁻ complexes using rare earth metals.

Figure 1. Routes to N₂²⁻ complexes using rare earth metals.

For reference, N–N and M–N(N₂) bond lengths obtained from single crystal X-ray diffraction experiments, N–N stretching frequencies (obtained by IR or Raman spectroscopy) and ^14/15N-NMR spectroscopic data are summarised in Table 1.

Table 1. Summary of rare earth N₂²⁻ complexes.

Complex (#) [Reference]	N–N Bond Length (Å)	Ln–N(N2) Bond Lengths (Å)	N–N Frequency (cm−1)	14/15N-NMR Spectroscopy (ppm) a
N2	1.0975 [66]	-	2331 [67]	−75 [68]
[(η5-C5Me4H)2Sc]2(μ-η2:η2-N2) (1) [69]	1.239(3)	2.216(1)	-	-
		2.220(1)
[(η5-C5Me4H)2Sc]2(μ-η2:η2-N2) (1′) [70]	1.229(3)	2.197(2)	-	385
		2.179(2)
[(η5-C5Me5)2Y]2(μ-η2:η2-N2) (2) [71]	1.172(6)	2.279(3)	-	496
		2.292(3)
[(η5-1,2,4-tBu–C5H2)2Nd]2(μ-η2:η2-N2) (3) [67]	1.226(12)	2.495(2)	1622 (14N2)	-
		2.497(2)	1569 (15N2)
[(η5-C5Me5)2Sm]2(μ-η2:η2-N2) b (4) [72]	1.088(12)	2.348(6)	-	−117 (263 K)
		2.367(6)		−161 (203 K)
[(η5-C5Me5)2Dy]2(μ-η2:η2-N2) (5) [73]	-	-	-	-
[(η5-SiMe3–C5H4)2Dy]2(μ-η2:η2-N2) (6) [74]	Connectivity only			-
[(η5-C5Me5)2Tm]2(μ-η2:η2-N2) (7) [75]	Connectivity only			-
[(η5-1,3-SiMe3–C5H3)2Tm]2(μ-η2:η2-N2) (8) [75]	1.259(4)	2.273(2)	-	-
		2.272(2)
[(η5-C5Me5)2Lu]2(μ-η2:η2-N2) (9) [71]	Connectivity only			527
[(η5-C5Me5)(η5-C5Me4H)Lu]2(μ-η2:η2-N2) (10) [76]	1.275(3)	2.291(3)	1736 (14N2)	-
		2.295(3)	1678 (15N2)
[(η5-C5Me4H)2Y(thf)]2(μ-η2:η2-N2) c (11) [77]	1.252(5)	2.338(3)	-	468
		2.370(3)
[(η5-SiMe3–C5H4)2Y(thf)]2(μ-η2:η2-N2) c (12) [78]	1.244(2)	2.3214(14)	-	-
		2.3070(14)
[(η5-C5Me5)2La(thf)]2(μ-η2:η2-N2) d (13) [79]	1.233(5)	2.537(4)	-	569
		2.478(4)
[(η5-C5Me4H)2La(thf)]2(μ-η2:η2-N2) c (14) [79]	1.243(4)	2.457(2)	-	495
		2.503(2)
[(η5-C5Me5)2Ce(thf)]2(μ-η2:η2-N2) c (15) [68]	1.258(9)	2.4548(15)	-	871
		2.542(2)
[(η5-C5Me4H)2Ce(thf)]2(μ-η2:η2-N2) d (16) [68]	1.235(6)	2.428(3)	-	1001
		2.475(3)
[(η5-C5Me5)2Pr(thf)]2(μ-η2:η2-N2) d (17) [68]	1.242(9)	2.4459(14)	-	2231
		2.512(2)
[(η5-C5Me4H)2Pr(thf)]2(μ-η2:η2-N2) c (18) [68]	1.235(7) e	2.418(4) e	-	2383
		2.455(3) e
[(η5-C5Me4H)2Nd(thf)]2(μ-η2:η2-N2) c (19) [79]	1.241(5) e	2.404(3) e	-	-
		2.451(2) e
[(η5-SiMe3–C5H4)2Tm(thf)]2(μ-η2:η2-N2) c (20) [75]	1.236(8)	2.274(4)	-	-
		2.302(4)
[(η5-C5Me4H)2Lu(thf)]2(μ-η2:η2-N2) c (21) [80]	1.243(12)	2.290(6)	-	521
		2.311(6)
[{(Me3Si)2N}2Y(thf)]2(μ-η2:η2-N2) c (22) [81]	1.274(3)	2.297(2)	1425 (14N2)	+513 (t)
		2.308(2)	1377 (15N2)
[{(Me3Si)2N}2La(thf)]2(μ-η2:η2-N2) (23) [81]	-	-	-	516
[{(Me3Si)2N}2Nd(thf)]2(μ-η2:η2-N2) c (24) [81]	1.258(3)	2.3758(16)	-	-
		2.3938(16)
[{(Me3Si)2N}2Gd(thf)]2(μ-η2:η2-N2) c (25) [81]	1.278(4)	2.326(2)	-	-
		2.353(2)
[{(Me3Si)2N}2Tb(thf)]2(μ-η2:η2-N2) c (26) [81]	1.271(4)	2.301(2)	-	-
		2.328(2)
[{(Me3Si)2N}2Dy(thf)]2(μ-η2:η2-N2) c (27) [82]	1.305(6)	2.287(3)	-	-
		2.312(3)
[{(Me3Si)2N}2Ho(thf)]2(μ-η2:η2-N2) c (28) [83]	1.264(4)	2.296(2)	-	-
		2.315(2)
[{(Me3Si)2N}2Er(thf)]2(μ-η2:η2-N2) c (29) [81]	1.276(5)	2.271(3)	-	-
		2.302(3)
[{(Me3Si)2N}2Tm(thf)]2(μ-η2:η2-N2) c (30) [82]	1.261(4)	2.271(2)	-	-
		2.296(2)
[{(Me3Si)2N}2Lu(thf)]2(μ-η2:η2-N2) c (31) [83]	1.285(4)	2.241(2)	1451 (14N2)	557
		2.272(2)
[{(Me3Si)2N}2Y(PhCN)]2(μ-η2:η2-N2) c (32) [84]	1.258(2)	2.2848(13)	-	-
		2.3092(13)
[{(Me3Si)2N}2Y(C5H5N)]2(μ-η2:η2-N2) c (33) [84]	1.255(3)	2.2917(16)	-	-
		2.3107(17)
[{(Me3Si)2N}2Y(4-NMe2–C5H4N)]2(μ-η2:η2-N2) c (34) [84]	1.259(2)	2.2979(12)	-	-
		2.3132(12)
[{(Me3Si)2N}2Y(Ph3PO)]2(μ-η2:η2-N2) c (35) [84]	1.262(2)	2.3000(14)	-	-
		2.3022(14)
[{(Me3Si)2N}2Y(Me3NO)]2(μ-η2:η2-N2) c (36) [84]	1.198(3)	2.2925(17)	-	-
		2.2941(18)
[(2,6-tBu–C6H3O)2Nd(thf)2]2(μ-η2:η2-N2) (37) [82]	1.242(7)	2.397(4)	-	-
		2.401(3)
[(2,6-tBu–C6H3O)2Dy(thf)2]2(μ-η2:η2-N2) (38) [82]	1.257(7) f	2.328(4) f	1526 (14N2)	-
		2.340(4) f
	1.256(9) g	2.336(5) g
		2.336(5) g
[Na4(thf)8][(η5:η1:η5:η1-Et2calix[4]pyrrole)Pr]2(μ-η2:η2-N2) (39) [85]	-	-	-	-
[Na4(dme)5][(η5:η1:η5:η1-Et2calix[4]pyrrole)Pr]2(μ-η2:η2-N2) (40) [85]	1.254(7)	2.414(5)	-	-
		2.457(5)
[Na4(thf)8][(η5:η1:η5:η1-Et2calix[4]pyrrole)Nd]2(μ-η2:η2-N2) (41) [85]	-	-	-	-
[Na4(dioxane)6][(η5:η1:η5:η1-Et2calix[4]pyrrole)Nd]2(μ-η2:η2-N2) (42) [85]	1.234(8)	2.511(4)	-	-
		2.508(4)
[{HB(3-tBu-5-Me–pz)}Tm{NH(2,5-tBu–C6H3)}]2(μ-η2:η2-N2) (43) [86]	1.215(10)	2.274(8)	-	-
		2.286(9)

^a Referenced to CH₃¹⁵NO; ^b In equilibrium with [(η⁵-C₅Me₅)₂Sm]₂; ^c trans arrangement of donor solvent; ^d cis arrangement of donor solvent; ^e The authors indicate poor data quality or significant disorder in these structures; ^f thf solvent of crystallisation in unit cell; ^g toluene solvent of crystallisation in unit cell.

Table 1. Summary of rare earth N₂²⁻ complexes.

Complex (#) [Reference]	N–N Bond Length (Å)	Ln–N(N2) Bond Lengths (Å)	N–N Frequency (cm−1)	14/15N-NMR Spectroscopy (ppm) a
N2	1.0975 [66]	-	2331 [67]	−75 [68]
[(η5-C5Me4H)2Sc]2(μ-η2:η2-N2) (1) [69]	1.239(3)	2.216(1)	-	-
		2.220(1)
[(η5-C5Me4H)2Sc]2(μ-η2:η2-N2) (1′) [70]	1.229(3)	2.197(2)	-	385
		2.179(2)
[(η5-C5Me5)2Y]2(μ-η2:η2-N2) (2) [71]	1.172(6)	2.279(3)	-	496
		2.292(3)
[(η5-1,2,4-tBu–C5H2)2Nd]2(μ-η2:η2-N2) (3) [67]	1.226(12)	2.495(2)	1622 (14N2)	-
		2.497(2)	1569 (15N2)
[(η5-C5Me5)2Sm]2(μ-η2:η2-N2) b (4) [72]	1.088(12)	2.348(6)	-	−117 (263 K)
		2.367(6)		−161 (203 K)
[(η5-C5Me5)2Dy]2(μ-η2:η2-N2) (5) [73]	-	-	-	-
[(η5-SiMe3–C5H4)2Dy]2(μ-η2:η2-N2) (6) [74]	Connectivity only			-
[(η5-C5Me5)2Tm]2(μ-η2:η2-N2) (7) [75]	Connectivity only			-
[(η5-1,3-SiMe3–C5H3)2Tm]2(μ-η2:η2-N2) (8) [75]	1.259(4)	2.273(2)	-	-
		2.272(2)
[(η5-C5Me5)2Lu]2(μ-η2:η2-N2) (9) [71]	Connectivity only			527
[(η5-C5Me5)(η5-C5Me4H)Lu]2(μ-η2:η2-N2) (10) [76]	1.275(3)	2.291(3)	1736 (14N2)	-
		2.295(3)	1678 (15N2)
[(η5-C5Me4H)2Y(thf)]2(μ-η2:η2-N2) c (11) [77]	1.252(5)	2.338(3)	-	468
		2.370(3)
[(η5-SiMe3–C5H4)2Y(thf)]2(μ-η2:η2-N2) c (12) [78]	1.244(2)	2.3214(14)	-	-
		2.3070(14)
[(η5-C5Me5)2La(thf)]2(μ-η2:η2-N2) d (13) [79]	1.233(5)	2.537(4)	-	569
		2.478(4)
[(η5-C5Me4H)2La(thf)]2(μ-η2:η2-N2) c (14) [79]	1.243(4)	2.457(2)	-	495
		2.503(2)
[(η5-C5Me5)2Ce(thf)]2(μ-η2:η2-N2) c (15) [68]	1.258(9)	2.4548(15)	-	871
		2.542(2)
[(η5-C5Me4H)2Ce(thf)]2(μ-η2:η2-N2) d (16) [68]	1.235(6)	2.428(3)	-	1001
		2.475(3)
[(η5-C5Me5)2Pr(thf)]2(μ-η2:η2-N2) d (17) [68]	1.242(9)	2.4459(14)	-	2231
		2.512(2)
[(η5-C5Me4H)2Pr(thf)]2(μ-η2:η2-N2) c (18) [68]	1.235(7) e	2.418(4) e	-	2383
		2.455(3) e
[(η5-C5Me4H)2Nd(thf)]2(μ-η2:η2-N2) c (19) [79]	1.241(5) e	2.404(3) e	-	-
		2.451(2) e
[(η5-SiMe3–C5H4)2Tm(thf)]2(μ-η2:η2-N2) c (20) [75]	1.236(8)	2.274(4)	-	-
		2.302(4)
[(η5-C5Me4H)2Lu(thf)]2(μ-η2:η2-N2) c (21) [80]	1.243(12)	2.290(6)	-	521
		2.311(6)
[{(Me3Si)2N}2Y(thf)]2(μ-η2:η2-N2) c (22) [81]	1.274(3)	2.297(2)	1425 (14N2)	+513 (t)
		2.308(2)	1377 (15N2)
[{(Me3Si)2N}2La(thf)]2(μ-η2:η2-N2) (23) [81]	-	-	-	516
[{(Me3Si)2N}2Nd(thf)]2(μ-η2:η2-N2) c (24) [81]	1.258(3)	2.3758(16)	-	-
		2.3938(16)
[{(Me3Si)2N}2Gd(thf)]2(μ-η2:η2-N2) c (25) [81]	1.278(4)	2.326(2)	-	-
		2.353(2)
[{(Me3Si)2N}2Tb(thf)]2(μ-η2:η2-N2) c (26) [81]	1.271(4)	2.301(2)	-	-
		2.328(2)
[{(Me3Si)2N}2Dy(thf)]2(μ-η2:η2-N2) c (27) [82]	1.305(6)	2.287(3)	-	-
		2.312(3)
[{(Me3Si)2N}2Ho(thf)]2(μ-η2:η2-N2) c (28) [83]	1.264(4)	2.296(2)	-	-
		2.315(2)
[{(Me3Si)2N}2Er(thf)]2(μ-η2:η2-N2) c (29) [81]	1.276(5)	2.271(3)	-	-
		2.302(3)
[{(Me3Si)2N}2Tm(thf)]2(μ-η2:η2-N2) c (30) [82]	1.261(4)	2.271(2)	-	-
		2.296(2)
[{(Me3Si)2N}2Lu(thf)]2(μ-η2:η2-N2) c (31) [83]	1.285(4)	2.241(2)	1451 (14N2)	557
		2.272(2)
[{(Me3Si)2N}2Y(PhCN)]2(μ-η2:η2-N2) c (32) [84]	1.258(2)	2.2848(13)	-	-
		2.3092(13)
[{(Me3Si)2N}2Y(C5H5N)]2(μ-η2:η2-N2) c (33) [84]	1.255(3)	2.2917(16)	-	-
		2.3107(17)
[{(Me3Si)2N}2Y(4-NMe2–C5H4N)]2(μ-η2:η2-N2) c (34) [84]	1.259(2)	2.2979(12)	-	-
		2.3132(12)
[{(Me3Si)2N}2Y(Ph3PO)]2(μ-η2:η2-N2) c (35) [84]	1.262(2)	2.3000(14)	-	-
		2.3022(14)
[{(Me3Si)2N}2Y(Me3NO)]2(μ-η2:η2-N2) c (36) [84]	1.198(3)	2.2925(17)	-	-
		2.2941(18)
[(2,6-tBu–C6H3O)2Nd(thf)2]2(μ-η2:η2-N2) (37) [82]	1.242(7)	2.397(4)	-	-
		2.401(3)
[(2,6-tBu–C6H3O)2Dy(thf)2]2(μ-η2:η2-N2) (38) [82]	1.257(7) f	2.328(4) f	1526 (14N2)	-
		2.340(4) f
	1.256(9) g	2.336(5) g
		2.336(5) g
[Na4(thf)8][(η5:η1:η5:η1-Et2calix[4]pyrrole)Pr]2(μ-η2:η2-N2) (39) [85]	-	-	-	-
[Na4(dme)5][(η5:η1:η5:η1-Et2calix[4]pyrrole)Pr]2(μ-η2:η2-N2) (40) [85]	1.254(7)	2.414(5)	-	-
		2.457(5)
[Na4(thf)8][(η5:η1:η5:η1-Et2calix[4]pyrrole)Nd]2(μ-η2:η2-N2) (41) [85]	-	-	-	-
[Na4(dioxane)6][(η5:η1:η5:η1-Et2calix[4]pyrrole)Nd]2(μ-η2:η2-N2) (42) [85]	1.234(8)	2.511(4)	-	-
		2.508(4)
[{HB(3-tBu-5-Me–pz)}Tm{NH(2,5-tBu–C6H3)}]2(μ-η2:η2-N2) (43) [86]	1.215(10)	2.274(8)	-	-
		2.286(9)

^a Referenced to CH₃¹⁵NO; ^b In equilibrium with [(η⁵-C₅Me₅)₂Sm]₂; ^c trans arrangement of donor solvent; ^d cis arrangement of donor solvent; ^e The authors indicate poor data quality or significant disorder in these structures; ^f thf solvent of crystallisation in unit cell; ^g toluene solvent of crystallisation in unit cell.

2.1.1. Cyclopentadienyl Ancillary Ligands

The first isolated, structurally characterised dinitrogen complex of an f-element metal was reported by Evans and co-workers [72]. [(η⁵-C₅Me₅)₂Sm]₂(μ-η²:η²-N₂) (4) was isolated by slow crystallisation of a toluene solution of the bent metallocene [(η⁵-C₅Me₅)₂Sm]₂ under an N₂ atmosphere (Figure 2). 4 exists in dynamic equilibrium with the metallocene starting material involving reversible Sm^II/Sm^III interconversion. In the solid state, 4 displays tetrahedral coordination around each Sm centre with gearing of the [Sm(C₅Me₅)₂] units and the first example of a co-planar M₂N₂ diamond core for any metal. The bridging, side-on bound N₂ has a short N–N distance of 1.088(12) Å (free N₂: 1.0975 Å [66]) and does not imply reduction to N₂²⁻; however, recent studies by Arnold and co-workers have shown that N–N bond lengths determined using X-ray diffraction experiments can be underestimated and so may not provide the best way of assessing the level of dinitrogen reduction [87,88]. Both the Sm–N/C bond lengths and the ¹³C-NMR spectral data support formulation of the complex as [Sm^III]₂(N₂²⁻). Maron and co-workers have reported calculations on the interaction of N₂ with [(η⁵-C₅Me₅)₂Ln] (Ln = Sm, Eu, Yb) [89].

Figure 2. Rare earth complexes with cyclopentadienyl ligands resulting from N₂ activation to N₂²⁻.

Figure 2. Rare earth complexes with cyclopentadienyl ligands resulting from N₂ activation to N₂²⁻.

Since this landmark discovery, the methodology of using reducing divalent rare earth metal complexes to activate N₂ has resulted in analogous cyclopentadienyl complexes of Dy (5, 6) [74] and Tm (7, 20) [75]. Structurally, these complexes all demonstrate a common planar Ln₂N₂ core (Ln–N–N–Ln dihedral angle = 0°), with the arrangement of the cyclopentadienyl ligands being dependent on the metal centre and the nature of the ligand itself (Figure 3).

The number of dinitrogen complexes has been expanded significantly by the report that combination of trivalent lanthanide complexes LnCp^R₃ or LnCp^R₂A′ with an alkali metal can also reduce dinitrogen affording side-on bound N₂ complexes [(η⁵-Cp^R)₂Ln]₂(μ-η²:η²-N₂) (Ln = Sc (1), Y (2), Nd (3), Dy (5, 6), Lu (9)) [67,69,70,71,76] or [(η⁵-Cp^R)₂Ln(thf)]₂(μ-η²:η²-N₂) (Ln = Y (11, 12), La (13, 14), Ce (15, 16), Pr (17, 18), Nd (19), Lu (21)) [68,78,79,80,83]. The generality of this method has been demonstrated by the wide range of metals utilised as well as the use of both homo- and heteroleptic trivalent lanthanide starting materials with a variety of cyclopentadienyl, amide, aryloxide, hydride, halide and borohydride ligands.

Figure 3. Structural variations in the solid state structures of [(η⁵-Cp^R)₂Ln]₂(μ-η²:η²-N₂) (Ln = Sc (1) and (1′); Nd (3); Sm (4)).

Figure 3. Structural variations in the solid state structures of [(η⁵-Cp^R)₂Ln]₂(μ-η²:η²-N₂) (Ln = Sc (1) and (1′); Nd (3); Sm (4)).

Complexes 11–21 (with donor solvent bound) also have planar Ln₂N₂ cores. However, only 13 (La), 15 (Ce) and 17 (Pr) have a cis arrangement of thf molecules and an asymmetrically-bound N₂²⁻ ligand (as a result of crystallographically non-equivalent N atoms). This is most clearly seen though the difference in Ln–N–Ln′ angles (11: 145.77(16) and 157.33(18)°; 13: 156.9(3) and 144.7(3)°; 15: 157.0(3) and 145.1(3)°) (Figure 4).

Figure 4. Structural variations in the solid state structures of [(η⁵-C₅Me₄H)₂Ln(thf)]₂(μ-η²:η²-N₂) (11, 14, 16, 18, 19, 21) (11 depicted, left) and [(η⁵-C₅Me₅)₂Ln(thf)]₂(μ-η²:η²-N₂) (13, 15, 17) (13 depicted, right). One C₅Me₄ ring in 11 is disordered.

Figure 4. Structural variations in the solid state structures of [(η⁵-C₅Me₄H)₂Ln(thf)]₂(μ-η²:η²-N₂) (11, 14, 16, 18, 19, 21) (11 depicted, left) and [(η⁵-C₅Me₅)₂Ln(thf)]₂(μ-η²:η²-N₂) (13, 15, 17) (13 depicted, right). One C₅Me₄ ring in 11 is disordered.

In terms of bonding, calculations were carried out on 1 and the Sc–N (N₂) bonding interaction was found to be a polar covalent two-electron four-centre bond resulting from donation from a filled Sc 3d orbital into an empty N₂ π_g antibonding orbital in the Sc₂N₂ plane. The lowest unoccupied molecular orbital (LUMO) of 1 is an unperturbed antibonding π_g orbital based on N₂. This bonding scheme can likely be extended for all compounds 1–21, with the donor nd orbital varying based on the nature of the metal ion [69].

Complexes 4 and 15–18 were characterised by ¹⁵N-NMR spectroscopy; the first reported examples of such spectra for paramagnetic N₂ complexes [68]. Trivalent Sm, Ce and Pr were chosen due to the low magnetic susceptibility of these ions (4f⁵ Sm^III, μ = 0.84 μ_B; 4f¹ Ce^III, μ = 2.54 μ_B; 4f² Pr^III, μ = 3.58 μ_B). Broad singlets at high frequency were observed for 15 (871 ppm), 16 (1001 ppm), 17 (2231 ppm) and 18 (2383 ppm). Consistent with the reversible N₂ coordination to 4, only a singlet at −75 ppm corresponding to free N₂ is observed at 298 K [68]. Cooling resulted in a new resonance at −117 ppm at 263 K which shifted linearly to −161 ppm at 203 K and accounts for bound N₂. In the context of pioneering NMR spectroscopic characterisation of organometallic complexes, the solid state ¹⁵N- and ¹³⁹La-NMR spectra of 14–¹⁵N₂ have also been reported [90].

Most recently, it has been demonstrated by Evans and co-workers that photochemical activation of the closed shell LnA₂A′ complexes Ln(η⁵-C₅Me₅)₂(η³-C₅Me₄H) or Ln(η⁵-C₅Me₅)(η⁵-C₅Me₄H)(η³-C₅Me₄H), which feature a novel η³ binding mode of a cyclopentadienyl ligand, yields side-on dinitrogen complexes [(η⁵-C₅Me₅)₂M]₂(μ-η²:η²-N₂) (M = Y (2), Dy (5), Lu (9)) and [(η⁵-C₅Me₅)(η⁵-C₅Me₄H)Lu]₂(μ-η²:η²-N₂) (10), with concomitant formation of (C₅Me₄H)₂ [73]. These reactions typically take place in under 5 h but, in the absence of photochemical activation, normally require a number of weeks. Full conversion to 2 and 9 is achieved via this methodology; this is notable given that other synthetic methods afforded yields of 26%–51% and 49%–59% respectively. Sterically induced reduction, typified by bulky M(η⁵-C₅Me₅)₃ complexes, does not account for this process since the less sterically hindered [C₅Me₄H]⁻ ligand acts as the reductant [91]. Calculations support a mechanism involving electron transfer from the [η³-C₅Me₄H]⁻ ligand into an empty 4d_z2 orbital on the metal centre. This affords a [C₅Me₄H]· radical which dimerises, and excited [Cp₂M]* nd¹ fragment which reduces N₂. Similarly, the allyl complexes Ln(C₅Me₅)₂(η³-C₃RH₄) (R = H or Me) can be photochemically activated to yield 2 and 9; in this case, propene and isobutene are observed as by-products due to H-atom abstraction from solvent rather than radical dimerisation [92].

2.1.2. Amide Ancillary Ligands

The simple silylamide ligand [N(SiMe₃)₂]⁻ has also proved suitable to prepare related complexes of the form [{(Me₃Si)₂N}₂Ln(thf)]₂(μ-η²:η²-N₂) (Ln = Y (22), La (23), Nd (24), Gd (25), Tb (26), Dy (27), Ho (28), Er (29), Tm (30), Lu (31)) (Figure 5, left) [65,81,82,83,93,94,95]. All complexes can be prepared using the LnA₃/M or LnA₂A′/M method and in addition, 24, 27 and 30 can be prepared directly from the divalent starting materials NdI₂, DyI₂ and TmI₂(thf)₃ respectively (though 24 is notable in that it is only isolated in 4% yield using this synthetic approach) [82].

Lewis base coordination of yttrium complex 22 was investigated through a series of substitution reactions to afford [{(Me₃Si)₂N}₂Ln(L)]₂(μ-η²:η²-N₂) (L = PhCN (32), C₅H₅N (33), 4-NMe₂–C₅H₄N (34), Ph₃PO (35), Me₃NO (36)) (Figure 5, right) [84]. Varying the donor ligand had little effect on the planar Ln₂N₂ structural core of 22 and 32–35, though the N–N distance in 36 was unexpectedly short at 1.198(3) Å. Similarly for complexes 1–21, calculations on [{(Me₃Si)₂N}₂Y(L)]₂(μ-η²:η²-N₂) (22) indicate that the Y–N (N₂) bonding interaction involves donation from a filled Y 4d orbital into an antibonding N₂ π_g orbital in the Y₂N₂ plane. The LUMO is an unperturbed antibonding N₂ π_g orbital [84,93]. UV-Vis spectra of 22 and 31–35 all contained a low energy, low intensity absorption around 700 nm which corresponds to the formally electric-dipole forbidden HOMO–LUMO (a_g–a_g) transition and act as a fingerprint of the electronic structure of the Y₂N₂ core.

Figure 5. Rare earth complexes with amide ligands resulting from N₂ activation to N₂²⁻.

Figure 5. Rare earth complexes with amide ligands resulting from N₂ activation to N₂²⁻.

In terms of bonding, the closed shell 4f¹⁴ Lu^III ion in 31 provides an interesting contrast to yttrium complex 22 with 4f° Y^III ions. Calculations support that the bonding is described in analogy with 22, but using higher energy, radially diffuse 5d orbitals which have a good energy match with the N₂ antibonding π_g orbital and are the correct symmetry for overlap [65]. Hughbanks and co-workers have also reported calculations on the 4f⁷ Gd^III complex 33 to analyse magnetic coupling [96].

2.1.3. Aryloxide Ancillary Ligands

Aryloxide ancillary ligands have been used to prepare side-on N₂ complexes of the form [(2,6-^tBu–C₆H₃O)₂Ln(thf)₂]₂(μ-η²:η²-N₂) (Ln = Nd (37) and Dy (38)) which now contain two molecules of coordinating solvent per metal (Figure 6) [65,82,93]. Calculations on 38, which contains open shell 4f⁹ Dy^III ions, indicate that Dy–N₂ bonding is derived from a 5d–π_g interaction in the Dy₂N₂ plane [65].

Figure 6. Rare earth complexes with aryloxide ligands resulting from N₂ activation to N₂²⁻.

Figure 6. Rare earth complexes with aryloxide ligands resulting from N₂ activation to N₂²⁻.

2.1.4. Multidentate Ancillary Ligands

Floriani and co-workers reported N₂ complexes of Pr^III (39, 40) and Nd^III (41, 42) using a calix[4]pyrrole ligand; these complexes were obtained as single crystals suitable for X-ray diffraction studies and no isolated yields were reported (Figure 7) [85]. [Na₄(thf)₈][(η⁵:η¹:η⁵:η¹-^Et2calix[4]pyrrole)Ln]₂(μ-η²:η²-N₂) (Ln = Pr (39), Nd (41)) were prepared by reduction of [Na(thf)₂][(η⁵:η¹:η⁵:η¹-^Et2calix[4]pyrrole)Ln(thf)] (Ln = Pr or Nd) using sodium metal with catalytic napthalene. Addition of dimethoxyethane (dme) to 39 led to solvent exchange to afford [Na₄(dme)₅][(η⁵:η¹:η⁵:η¹-^Et2calix[4]pyrrole)Ln]₂(μ-η²:η²-N₂) (40). Similarly, addition of dioxane to 41 affords [Na₄(dioxane)₆][(η⁵:η¹:η⁵:η¹-^Et2calix[4]pyrrole)Ln]₂(μ-η²:η²-N₂) (42). The solid state structures of 40 and 42 revealed different coordination modes of the sodium counterions and feature N–N bond distances of 1.254(7) and 1.234(8) Å which are consistent with reduction to N₂²⁻. Measured magnetic moments are also consistent with Ln^III oxidation state for both metal centres.

Figure 7. Rare earth complexes with multidentate ligands resulting from N₂ activation to N₂²⁻.

Figure 7. Rare earth complexes with multidentate ligands resulting from N₂ activation to N₂²⁻.

Takats and co-workers reported the scorpionate complex [{HB(3-^tBu-5-Me–pz)}Tm{NH(2,5-^tBu–C₆H₃)}]₂(μ-η²:η²-N₂) (43) (pz = C₃HN₂ = pyrazolyl) which was prepared by the protonolysis reaction of 2,5-^tBu–C₆H₃NH₂ with the heteroleptic Tm^II hydrocarbyl compound {HB(3-^tBu-5-Me–pz)}Tm{CH(SiMe₃)₂} [86]. The N–N bond distance of 1.215(10) Å is consistent with reduction to N₂²⁻ and the Ln₂N₂ core is slightly bent with an Ln–N–N–Ln torsion angle of 0.37°.

2.2. Complexes Containing a Formal N₂³⁻ Ligand

2.2.1. Amide Ancillary Ligands

For reference, N–N and M–N(N₂) bond lengths obtained from single crystal X-ray diffraction experiments and N–N stretching frequencies (obtained by IR or Raman spectroscopy) are summarised in Table 2.

Table 2. Summary of rare earth N₂³⁻ complexes.

Complex (#) [Reference]	N–N Bond Length (Å)	Ln–N (N2) Bond Lengths (Å)	Ln–N–N–Ln Torsion Angle (°)	N–N Frequency (cm−1)
N2	1.0975 [66]	-	-	2331 [67]
[K(thf)6][{(Me3Si)2N}2Y(thf)]2(μ-η2:η2-N2) b (44) [93]	1.401(6)	2.194(3)	0	1002 (14N2) (calculated)
		2.218(3)
	1.401(6) a	2.190(3) a
		2.213(3) a
[K(thf)6][{(Me3Si)2N}2La(thf)]2(μ-η2:η2-N2) (45) [65]	-	-	0	-
[K(thf)6][{(Me3Si)2N}2Lu(thf)]2(μ-η2:η2-N2) b (46) [65]	1.414(8)	2.163(4)	0	979 (14N2)
		2.180(4)
[K][{(Me3Si)2N}2Y(thf)]2(μ3-η2:η2:η2-N2) b (47) [93]	1.405(3)	2.225(2)	14.22	989 (14N2)
		2.242(2)		956 (15N2)
[K][{(Me3Si)2N}2Gd(thf)]2(μ3-η2:η2:η2-N2) b (48) [97]	1.395(3)	2.248(2)	13.64	-
		2.274(2)
[K][{(Me3Si)2N}2Tb(thf)]2(μ3-η2:η2:η2-N2) b (49) [97]	1.401(3)	2.235(2)	16.12	-
		2.260(2)
[K][{(Me3Si)2N}2Dy(thf)]2(μ3-η2:η2:η2-N2) b (50) [97]	1.404(5)	2.229(4)	15.27	-
		2.242(4)
[K(18c6)(thf)2][{(Me3Si)2N}2Y(thf)]2(μ-η2:η2-N2) b (51) [97]	1.396(3)	2.1909(17)	0	-
		2.2136(16)
[K(18c6)(thf)2][{(Me3Si)2N}2Gd(thf)]2(μ-η2:η2-N2) b (52) [94]	1.401(4)	2.224(2)	0	-
		2.249(2)
[K(18c6)(thf)2][{(Me3Si)2N}2Tb(thf)]2(μ-η2:η2-N2) b (53) [94]	1.394(3)	2.2056(15)	0	-
		2.2345(15)
[K(18c6)(thf)2][{(Me3Si)2N}2Dy(thf)]2(μ-η2:η2-N2) b (54) [94]	1.393(7)	2.199(4)	0	-
		2.213(4)
[K(18c6)(thf)2][{(Me3Si)2N}2Ho(thf)]2(μ-η2:η2-N2) b (55) [95]	1.404(4)	2.188(2)	0	-
		2.210(2)
[K(18c6)(thf)2][{(Me3Si)2N}2Er(thf)]2(μ-η2:η2-N2) b (56) [65]	1.409(4)	2.178(2)	0	-
		2.204(2)
[Na(thf)6][{(Me3Si)2N}2Y(thf)]2(μ-η2:η2-N2) b (57) [65]	1.393(7)	2.199(4)	0	-
		2.213(4)
[Na(thf)6][{(Me3Si)2N}2Er(thf)]2(μ-η2:η2-N2) b (58) [65]	1.403(4)	2.1817(19) 2.2019(19)	0	-
[K(thf)6][(2,6-tBu–C6H3O)2Dy(thf)]2(μ-η2:η2-N2) (59) [93]	1.396(7)	2.197(3)	0	962 (14N2)
		2.203(4)
[K(thf)][(2,6-tBu–C6H3O)2Dy(thf)]2(μ3-η2:η2:η2-N2) (60) [93]	1.402(7)	2.235(5)	6.59	-
		2.209(5)

^a Second independent molecule in unit cell; ^b trans arrangement of thf.

Table 2. Summary of rare earth N₂³⁻ complexes.

Complex (#) [Reference]	N–N Bond Length (Å)	Ln–N (N2) Bond Lengths (Å)	Ln–N–N–Ln Torsion Angle (°)	N–N Frequency (cm−1)
N2	1.0975 [66]	-	-	2331 [67]
[K(thf)6][{(Me3Si)2N}2Y(thf)]2(μ-η2:η2-N2) b (44) [93]	1.401(6)	2.194(3)	0	1002 (14N2) (calculated)
		2.218(3)
	1.401(6) a	2.190(3) a
		2.213(3) a
[K(thf)6][{(Me3Si)2N}2La(thf)]2(μ-η2:η2-N2) (45) [65]	-	-	0	-
[K(thf)6][{(Me3Si)2N}2Lu(thf)]2(μ-η2:η2-N2) b (46) [65]	1.414(8)	2.163(4)	0	979 (14N2)
		2.180(4)
[K][{(Me3Si)2N}2Y(thf)]2(μ3-η2:η2:η2-N2) b (47) [93]	1.405(3)	2.225(2)	14.22	989 (14N2)
		2.242(2)		956 (15N2)
[K][{(Me3Si)2N}2Gd(thf)]2(μ3-η2:η2:η2-N2) b (48) [97]	1.395(3)	2.248(2)	13.64	-
		2.274(2)
[K][{(Me3Si)2N}2Tb(thf)]2(μ3-η2:η2:η2-N2) b (49) [97]	1.401(3)	2.235(2)	16.12	-
		2.260(2)
[K][{(Me3Si)2N}2Dy(thf)]2(μ3-η2:η2:η2-N2) b (50) [97]	1.404(5)	2.229(4)	15.27	-
		2.242(4)
[K(18c6)(thf)2][{(Me3Si)2N}2Y(thf)]2(μ-η2:η2-N2) b (51) [97]	1.396(3)	2.1909(17)	0	-
		2.2136(16)
[K(18c6)(thf)2][{(Me3Si)2N}2Gd(thf)]2(μ-η2:η2-N2) b (52) [94]	1.401(4)	2.224(2)	0	-
		2.249(2)
[K(18c6)(thf)2][{(Me3Si)2N}2Tb(thf)]2(μ-η2:η2-N2) b (53) [94]	1.394(3)	2.2056(15)	0	-
		2.2345(15)
[K(18c6)(thf)2][{(Me3Si)2N}2Dy(thf)]2(μ-η2:η2-N2) b (54) [94]	1.393(7)	2.199(4)	0	-
		2.213(4)
[K(18c6)(thf)2][{(Me3Si)2N}2Ho(thf)]2(μ-η2:η2-N2) b (55) [95]	1.404(4)	2.188(2)	0	-
		2.210(2)
[K(18c6)(thf)2][{(Me3Si)2N}2Er(thf)]2(μ-η2:η2-N2) b (56) [65]	1.409(4)	2.178(2)	0	-
		2.204(2)
[Na(thf)6][{(Me3Si)2N}2Y(thf)]2(μ-η2:η2-N2) b (57) [65]	1.393(7)	2.199(4)	0	-
		2.213(4)
[Na(thf)6][{(Me3Si)2N}2Er(thf)]2(μ-η2:η2-N2) b (58) [65]	1.403(4)	2.1817(19) 2.2019(19)	0	-
[K(thf)6][(2,6-tBu–C6H3O)2Dy(thf)]2(μ-η2:η2-N2) (59) [93]	1.396(7)	2.197(3)	0	962 (14N2)
		2.203(4)
[K(thf)][(2,6-tBu–C6H3O)2Dy(thf)]2(μ3-η2:η2:η2-N2) (60) [93]	1.402(7)	2.235(5)	6.59	-
		2.209(5)

^a Second independent molecule in unit cell; ^b trans arrangement of thf.

The first definitive evidence for an N₂³⁻ reduction product of dinitrogen was demonstrated by Evans and co-workers [93]. The LnA₃/M system of Y{N(SiMe₃)₂}₃ with KC₈ in thf afforded a mixture of [{(Me₃Si)₂N}₂Y(thf)]₂(μ-η²:η²-N₂) (22), [K(thf)₆][{(Me₃Si)₂N}₂Y(thf)]₂(μ-η²:η²-N₂) (44) and [K][{(Me₃Si)₂N}₂Y(thf)]₂(μ₃-η²:η²:η²-N₂) (47) from which 44 and 47 could be isolated (Figure 8). The EPR spectrum of 44–¹⁵N₂ has a 9-line pattern consistent with a triplet of triplets due to two ¹⁵N and two ⁸⁹Y nuclei and has a hyperfine coupling constant of 8.2 G implying a N-centred radical, while 47–¹⁵N₂ shows extra coupling to potassium; all spectra indicate the presence of the N₂³⁻ ion. The N–N bond distances are 1.401(6) and 1.405(3) Å respectively and are intermediate between N–N single bonds (1.47 Å in N₂H₄) and N=N double bonds (1.25 Å in PhN=NPh) [98]. The N–N vibrational stretching frequency in 47 is 989 cm⁻¹, significantly reduced from 1425 cm⁻¹ for 22. Similarly to complexes 22–36, the Y–N₂ bonding interaction in these complexes can be described by the donation from a filled Y 4d orbital into an antibonding N₂ π_g orbital (HOMO). However, the orthogonal antibonding N₂ π_g orbital is now also occupied by a single electron.

Figure 8. Rare earth complexes with amide ligands resulting from N₂ activation to N₂³⁻.

Figure 8. Rare earth complexes with amide ligands resulting from N₂ activation to N₂³⁻.

Following this remarkable report, these types of complexes have been extended to more rare earth metal centres; [K(thf)₆][{(Me₃Si)₂N}₂Ln(thf)]₂(μ-η²:η²-N₂) (Ln = La (45), Lu (46)) and [K][{(Me₃Si)₂N}₂Ln(thf)]₂(μ₃-η²:η²:η²-N₂) (Ln = Gd (48), Tb (49), Dy (50)). Additionally, the solvated counterion can also be varied to include 18-crown-6 in [K(18c6)(thf)₂][{(Me₃Si)₂N}₂Ln(thf)]₂(μ-η²:η²-N₂) (Ln = Y (51), Gd (52), Tb (53), Dy (54), Ho (55), Er (56)) or can be exchanged for sodium in [Na(thf)₆][{(Me₃Si)₂N}₂Ln(thf)]₂(μ-η²:η²-N₂) (Ln = Y (57), Er (58)) [65,94,95,97].

The solid state structures of 44–46 and 51–58 which contain outer-sphere counterions have the common Ln₂N₂ planar core like those of rare earth N₂²⁻ complexes 1–42 which have been structurally characterised. However, complexes 47–50, which have inner-sphere K⁺ ions, display bent Ln₂N₂ cores with torsion angles of 14.22° (47), 13.64° (48), 16.12° (49) and 15.27° (50).

Complexes 48–50 (inner-sphere K⁺) and 52–56 (outer-sphere K⁺) all display interesting magnetic properties; 48 and 52 have the strongest magnetic exchange couplings in a Gd^III complex with exchange constants of −27 cm⁻¹, and 49, 50 and 53–56 demonstrate single-molecule-magnet behaviour [94,95,99,100]. Combination of rare earth complexes which demonstrate both high anisotropy and strong exchange coupling potentially provides a route to single-molecule magnets with high blocking temperatures. The diffuse nature of the N₂³⁻ radical facilitates strong coupling in these systems by overlap of the Ln 4f orbitals with the bridging dinitrogen ligand. 53 and 54 exhibit magnetic hysteresis up to record blocking temperatures of 13.9 K (0.9 mTs⁻¹ sweep rate) and 8.3 K (0.08 Ts⁻¹ sweep rate) respectively. Competing Ln^III–Ln^III antiferromagnetic coupling is observed in complexes at low temperatures in 48–50, which have a non-zero Ln–N–N–Ln dihedral angle, demonstrating the importance of geometry of the Ln₂N₂ unit to magnetic behaviour.

2.2.2. Aryloxide Ancillary Ligands

As for yttrium complexes 44 and 47, the dysprosium aryloxide complexes [K(thf)₆][(2,6-^tBu–C₆H₃O)₂Dy(thf)]₂(μ-η²:η²-N₂) (59) and [K(thf)][(2,6-^tBu–C₆H₃O)₂Dy(thf)]₂(μ₃-η²:η²:η²-N₂) (60) were first isolated from reaction of DyI₂ with KO-2,6-^tBu–C₆H₃ (Figure 9) [65,93]. Reoxidation of 59 with AgBPh₄ affords the N₂²⁻ complex [(2,6-^tBu–C₆H₃O)₂Dy(thf)₂]₂(μ-η²:η²-N₂) (38). From the solid state structures the N–N bond lengths are 1.396(7) (59) and 1.402(7) Å (60), which fall in the range of reported N₂³⁻ complexes (Table 2). As anticipated, 59 has a planar Ln₂N₂ core in the solid state whereas it is bent in complex 60 with a Ln–N–N–Ln torsion angle of 6.59°.

Figure 9. Rare earth complexes with aryloxide ligands resulting from N₂ activation to N₂³⁻.

Figure 9. Rare earth complexes with aryloxide ligands resulting from N₂ activation to N₂³⁻.

2.3. Complexes Containing a Formally N₂⁴⁻ Ligand

For reference, N–N and M–N(N₂) bond lengths obtained from single crystal X-ray diffraction experiments are summarised in Table 3.

Table 3. Summary of rare earth N₂⁴⁻ complexes.

Complex (#) [Reference]	N–N Bond Length (Å)	Ln–N (N2) Bond Lengths (Side-on) (Å)	Ln–N (N2) Bond Lengths (End-on) (Å)
N2	1.0975 [66]	-	-
[Li(thf)2]2[(Et2calix[4]pyrrole)Sm]2(N2Li4) (61) [101]	1.525(4)	2.357(2)	-
		2.342(2)
[{Ph2C(C4H3N)2}Sm(thf)]4(μ4-η1:η1:η2:η2-N2) (62) [102]	1.412(17)	2.327(3)	2.177(8)
		2.327(3)	2.177(8)
[{CyC(C4H3N)2}Sm(thf)]4(μ4-η1:η1:η2:η2-N2) (63) [103]	1.392(16)	2.339(3)	2.160(8)
		2.339(3)	2.160(8)
[{Et2C(C4H3N)2}Sm(thf)]4(μ4-η1:η1:η2:η2-N2) (64) [104]	1.415(4)	2.328(3)	2.145(3)
		2.342(3)
[{Ph(Me)C(C4H3N)2}Sm(dme)]4(μ4-η1:η1:η2:η2-N2) (65) [104]	1.42(2)	2.316(13)	2.149(11)
		2.316(12)
[Na(thf)]2[{CyC(C4H3N)2}Sm(thf)]4(μ6-η1:η1:η1:η1:η2:η2-N2) (66) [103]	1.371(16)	2.332(11)	2.178(10)
		2.324(11)
[{Li(thf)}3(μ3-Cl)][(Cycalix[4]pyrrole)Sm]2(μ-η2:η2-N2) (67) [105]	1.08(3)	2.880(18)	-
		2.974(18)
[(Li(thf)2][(Cycalix[4]pyrrole)2Sm3Li2](μ5-η1:η1:η2:η2:η2-N2) (68) [105]	1.502(5)	2.249(4) (Sm(1)–N(1))	-
		2.253(4) (Sm(1)–N(1))
		2.355(4) (Sm(2)–N(1))
		2.370(4) (Sm(2)–N(1))
		2.398(4) (Sm(3)–N(1))
		2.376(4) (Sm(3)–N(1))

Table 3. Summary of rare earth N₂⁴⁻ complexes.

Complex (#) [Reference]	N–N Bond Length (Å)	Ln–N (N2) Bond Lengths (Side-on) (Å)	Ln–N (N2) Bond Lengths (End-on) (Å)
N2	1.0975 [66]	-	-
[Li(thf)2]2[(Et2calix[4]pyrrole)Sm]2(N2Li4) (61) [101]	1.525(4)	2.357(2)	-
		2.342(2)
[{Ph2C(C4H3N)2}Sm(thf)]4(μ4-η1:η1:η2:η2-N2) (62) [102]	1.412(17)	2.327(3)	2.177(8)
		2.327(3)	2.177(8)
[{CyC(C4H3N)2}Sm(thf)]4(μ4-η1:η1:η2:η2-N2) (63) [103]	1.392(16)	2.339(3)	2.160(8)
		2.339(3)	2.160(8)
[{Et2C(C4H3N)2}Sm(thf)]4(μ4-η1:η1:η2:η2-N2) (64) [104]	1.415(4)	2.328(3)	2.145(3)
		2.342(3)
[{Ph(Me)C(C4H3N)2}Sm(dme)]4(μ4-η1:η1:η2:η2-N2) (65) [104]	1.42(2)	2.316(13)	2.149(11)
		2.316(12)
[Na(thf)]2[{CyC(C4H3N)2}Sm(thf)]4(μ6-η1:η1:η1:η1:η2:η2-N2) (66) [103]	1.371(16)	2.332(11)	2.178(10)
		2.324(11)
[{Li(thf)}3(μ3-Cl)][(Cycalix[4]pyrrole)Sm]2(μ-η2:η2-N2) (67) [105]	1.08(3)	2.880(18)	-
		2.974(18)
[(Li(thf)2][(Cycalix[4]pyrrole)2Sm3Li2](μ5-η1:η1:η2:η2:η2-N2) (68) [105]	1.502(5)	2.249(4) (Sm(1)–N(1))	-
		2.253(4) (Sm(1)–N(1))
		2.355(4) (Sm(2)–N(1))
		2.370(4) (Sm(2)–N(1))
		2.398(4) (Sm(3)–N(1))
		2.376(4) (Sm(3)–N(1))

To date, all examples of complexes containing an N₂⁴⁻ ligand, derived from dinitrogen activation by a rare earth metal centre involve multidentate ligands (Figure 10) [101,102,103,104,105,106]. The first example of a structurally characterised rare earth complex containing an N₂⁴⁻ ligand was reported by Gambarotta and co-workers [101]. The octametallic complex [Li(thf)₂]₂[(^Et2calix[4]pyrrole)Sm]₂(N₂Li₄) (61) was prepared by reaction of SmCl₃(thf)₃ with [Li(thf)]₄[^Et2calix[4]pyrrole] and subsequent reduction with Li metal under an argon atmosphere, followed by exposure to N₂. The reaction by-products were not determined. In the solid state structure, there is an octahedron of metal ions coordinating to the N₂⁴⁻ ligand which is η²-bound to the two Sm^III in the apical sites, and both η²- and η¹-bound to the opposite pairs of the four Li⁺ ions in the equatorial plane. The N–N distance of 1.525(4) Å, combined with a total magnetic moment of 2.72 μ_B, is consistent with formulation of the complex as [Sm^III]₂(N₂⁴⁻).

Figure 10. Lanthanide complexes with multidentate donors resulting from N₂ activation to N₂⁴⁻.

Figure 10. Lanthanide complexes with multidentate donors resulting from N₂ activation to N₂⁴⁻.

A family of tetrametallic Sm dipyrrolide complexes [{R₂C(C₄H₃N)₂}Sm(L)]₄(μ₄-η¹:η¹:η²:η²-N₂) (L = thf, CR₂ = CPh₂ (62), CCy (63), CEt₂ (64); L = dme, CR₂ = C(Me)Ph (65)) was prepared by reaction of SmI₂(thf)₂ with the corresponding alkali metal dipyrrolide salt, and display both side-on and end-on N₂ coordination [102,103,104]. 62–65 were stable both thermally and in vacuo. All compounds were structurally characterised using X-ray diffraction experiments; the N–N bond lengths range from 1.392(16) to 1.42(2) Å, and variable coordination modes of the pyrrolide ligands are observed across all complexes. Reduction of 63 with Na afforded [Na(thf)]₂[{R₂C(C₄H₃N)₂}Sm(thf)]₄(μ₆-η¹:η¹:η¹:η¹:η²:η²-N₂) (66). The N₂⁴⁻ ligand is bound end-on to two Sm ions and side-on to the other two, as well as being end-on bound to two Na⁺ ions. Assignment of the reduction level of dinitrogen to N₂⁴⁻ leads to a formal oxidation state of +2.5 for each samarium centre. This is proposed on the basis of the N–N bond distance (1.371(16) Å; slightly shorter than that expected for an N–N single bond), magnetic moment (4.05 μ_B; lower than the analogous divalent complex 63) and short Sm–Sm contacts in the solid state which may promote magnetic couplings.

Tetra-calix-pyrrole complexes [{Li(thf)}₃(μ₃-Cl)][(^Cycalix[4]pyrrole)Sm]₂(μ-η²:η²-N₂) (67) and the unusual trimetallic [Li(thf)₂][(^Cycalix[4]pyrrole)₂Sm₃Li₂](μ₅-η¹:η¹:η²:η²:η²-N₂) (68) contain formal N₂⁴⁻ ligands, on the basis of charge neutrality implied from the presence of Sm^III centres but have disparate N–N bond lengths of 1.08(3) Å and 1.502(5) Å respectively [105]. It should be noted that the crystallographic data for 67 was only of a good enough quality to obtain structural connectivity.

3. Dinitrogen Activation by Actinide Complexes

3.1. Complexes Containing an Activated N₂ Ligand

Perhaps surprisingly, given the number of examples of dinitrogen activation by rare earth complexes, there are very few examples of N₂ activation with actinide complexes despite the presence of uranium in early catalysts for the Haber–Bosch process (Figure 11) [107]. Actinide-element bonding in the model system [X₃An]₂(μ-η²:η²-N₂) (An = Th–Pu, X = F, Cl, Br, Me, H, OPh) has recently been studied using relativistic DFT calculations [108].

Figure 11. Actinide complexes resulting from N₂ activation (no N–N bond cleavage).

Figure 11. Actinide complexes resulting from N₂ activation (no N–N bond cleavage).

For reference, N–N bond lengths obtained from single crystal X-ray diffraction experiments, N–N stretching frequencies (obtained by IR or Raman spectroscopy) and ^14/15N-NMR spectroscopic data of actinide N₂ complexes are summarised in Table 4.

Table 4. Summary of actinide N₂ complexes.

Complex (#) [Reference]	Stability	N–N Bond Length (Å)	N–N Frequency (cm−1)	14/15N-NMR Spectroscopy (ppm) a
N2	-	1.0975 [66]	2331 [67]	−75 [68]
[{N(CH2CH2NSiMe2tBu)3}U]2(μ-η2:η2-N2) (69) [109]	Stable under N2 (1 atm)	1.109(7)	-	-
	N2 dissociation in vacuo
{Ph(tBu)N}3Mo(μ2-η1:η1-N2)U{N(tBu)(3,5-Me–C6H3)}3 (70) [110]	Stable in vacuo at 25 °C	1.232(11)	- (14N2)	-
	“Thermally stable”		1547 (15N2)
{(3,5-Me-C6H3)(Ad)N}3Mo(μ2-η1:η1-N2)U{N(tBu)(3,5-Me–C6H3)}3 (71) [110]	Stable in vacuo at 25 °C	1.23(2)	1568 (14N2)	-
	“Thermally stable”		1527 (15N2)
[(η5-C5Me5)(η8-1,4-SiiPr3–C8H4)U]2(μ-η2:η2-N2) (72) [111]	75% conversion to 72 at 50 psi N2	1.232(10)	-	-
	N2 dissociation in vacuo, in solution and solid state
(η5-C5Me5)3U(η1-N2) (73) [112]	Crystallisation at 80 psi N2	1.120(14)	2207 (14N2)	-
	N2 dissociation in vacuo or in solution under N2 (1 atm)		2134 (15N2)
[(2,6-tBu–C6H3O)3U]2(μ-η2:η2-N2) (74) [88]	N2 dissociation in vacuo and in solution at 25 °C	1.163(19)	-	-
		1.204 (17)
		1.201(19)
[(2,4,6-tBu–C6H2O)3U]2(μ-η2:η2-N2) (75) [88]	Stable in vacuo at 25 °C	1.236(5)	1451 (14N2)	-
	N2 dissociation at 80 °C in solution		1404 (15N2)
[{(Mes)3SiO}3U]2(μ-η2:η2-N2) (76) [87]	Stable in vacuo at 25 °C	1.124(12) (eclipsed)	1437 (14N2)	4213.5
	Slowly forms U{OSi(Mes)3}4 at 100 °C in solution	1.080(11) (staggered)	1372 (15N2)

^a Referenced to CH₃¹⁵NO.

Table 4. Summary of actinide N₂ complexes.

Complex (#) [Reference]	Stability	N–N Bond Length (Å)	N–N Frequency (cm−1)	14/15N-NMR Spectroscopy (ppm) a
N2	-	1.0975 [66]	2331 [67]	−75 [68]
[{N(CH2CH2NSiMe2tBu)3}U]2(μ-η2:η2-N2) (69) [109]	Stable under N2 (1 atm)	1.109(7)	-	-
	N2 dissociation in vacuo
{Ph(tBu)N}3Mo(μ2-η1:η1-N2)U{N(tBu)(3,5-Me–C6H3)}3 (70) [110]	Stable in vacuo at 25 °C	1.232(11)	- (14N2)	-
	“Thermally stable”		1547 (15N2)
{(3,5-Me-C6H3)(Ad)N}3Mo(μ2-η1:η1-N2)U{N(tBu)(3,5-Me–C6H3)}3 (71) [110]	Stable in vacuo at 25 °C	1.23(2)	1568 (14N2)	-
	“Thermally stable”		1527 (15N2)
[(η5-C5Me5)(η8-1,4-SiiPr3–C8H4)U]2(μ-η2:η2-N2) (72) [111]	75% conversion to 72 at 50 psi N2	1.232(10)	-	-
	N2 dissociation in vacuo, in solution and solid state
(η5-C5Me5)3U(η1-N2) (73) [112]	Crystallisation at 80 psi N2	1.120(14)	2207 (14N2)	-
	N2 dissociation in vacuo or in solution under N2 (1 atm)		2134 (15N2)
[(2,6-tBu–C6H3O)3U]2(μ-η2:η2-N2) (74) [88]	N2 dissociation in vacuo and in solution at 25 °C	1.163(19)	-	-
		1.204 (17)
		1.201(19)
[(2,4,6-tBu–C6H2O)3U]2(μ-η2:η2-N2) (75) [88]	Stable in vacuo at 25 °C	1.236(5)	1451 (14N2)	-
	N2 dissociation at 80 °C in solution		1404 (15N2)
[{(Mes)3SiO}3U]2(μ-η2:η2-N2) (76) [87]	Stable in vacuo at 25 °C	1.124(12) (eclipsed)	1437 (14N2)	4213.5
	Slowly forms U{OSi(Mes)3}4 at 100 °C in solution	1.080(11) (staggered)	1372 (15N2)

^a Referenced to CH₃¹⁵NO.

The first example of dinitrogen activation by an actinide complex was reported by Scott and co-workers; the trivalent uranium complex {N(CH₂CH₂NSiMe₂^tBu)₃}U reacts with N₂ (1 atm) to yield [{N(CH₂CH₂NSiMe₂^tBu)₃}U]₂(μ-η²:η²-N₂) (69) [109]. In solution, the reaction is reversible and 69 converts back to the trivalent uranium starting material when freeze-pump-thaw degassed (Scheme 1). The solid state structure of 69 illustrates the side-on binding mode of N₂ and features an N–N bond length of 1.109(7) Å. Alongside solution magnetic susceptibility measurements of 3.22 μ_B per uranium centre, these data agree with the dimer being formulated as [U^III]₂(N₂⁰).

Scheme 1. Reversible N₂ binding in 69.

Scheme 1. Reversible N₂ binding in 69.

Cummins and co-workers reported thermally stable heterobimetallic U–Mo N₂ complexes featuring the end-on binding mode of N₂; the U^III tris(amide) U{N(^tBu)Ar}₃(thf) (Ar = 3,5-Me–C₆H₃) reacts with Mo{N(R)Ar′}₃ under an N₂ atmosphere (1 atm) to yield {Ar′(R)N}₃Mo(μ₂-η¹:η¹-N₂)U{N(^tBu)Ar}₃ (R = ^tBu, Ar′ = Ph (70); R = Ad, Ar′ = 3,5-Me–C₆H₃ (71)) [110]. The solid state molecular structure of 70 shows an N–N bond distance of 1.232(11) Å, consistent with N₂²⁻ and a formal oxidation state of U^IV. In principal, U^III and Mo^III metal centres can provide the 6 electrons required for N₂ bond cleavage and the stability of 70 and 71 should be noted with reference to Mo{N(^tBu)Ar′}₃ which cleaves N₂ and forms a terminal nitride (Mo≡N) under mild conditions [113,114,115].

Reversible side-on binding and N₂ activation was demonstrated by Cloke et al., using a mixed sandwich U^III pentalene complex. (η⁵-C₅Me₅)(η⁸-1,4-SiⁱPr₃–C₈H₄)U reacts with N₂ to yield [(η⁵-C₅Me₅)(η⁸-1,4-SiⁱPr₃–C₈H₄)U]₂(μ-η²:η²-N₂) (72) [111]. The solid state structure features an N–N bond length of 1.232(10) Å which is consistent with reduction to N₂²⁻; regardless of the formal reduction level, the relief of steric crowding in 72 likely drives the facile loss of N₂ when there is no overpressure.

To date, the only example of a monometallic f-element complex of N₂ was reported by Evans et al.; sterically crowded U(η⁵-C₅Me₅)₃ reacts with N₂ (80 psi) to afford (η⁵-C₅Me₅)₃U(η¹-N₂) (73) [112]. N₂ binding is reversible and lowering the pressure results in N₂ dissociation. The solid state structure of 73 shows the N₂ ligand is bound end-on and linearly (U–N–N = 180°) and the N–N distance is 1.120(14) Å which is statistically equivalent with free N₂.

Arnold and co-workers reported that the trivalent uranium aryloxides U(OAr)₃ (Ar = 2,6-^tBu–C₆H₃ or 2,4,6-^tBu–C₆H₂) bind N₂ (1 atm) to form the side-on bound N₂ adducts [(ArO)₃U]₂(μ-η²:η²-N₂) (74 and 75 respectively) [88]. Though 74 was obtained as a minor product, the more sterically hindered 75 was formed in quantitative yield and was stable under dynamic vacuum and in the presence of coordinating solvents and polar small molecules (CO and CO₂) under ambient conditions. N₂ loss was observed when 75 was heated to 80 °C in a toluene solution. The solid state N–N bond lengths are 1.163(19), 1.204(17), 1.201(19) Å (74) and 1.236(5) Å (75) which indicate significant N₂ reduction by the electron rich U^III metal centres. Consistent with this, Raman spectroscopy performed on 75 showed a strong band at 1451 cm⁻¹ for the N–N stretch (1404 cm⁻¹ in the 75–¹⁵N₂) which is significantly lower than in free N₂ (2331 cm⁻¹) [67]. DFT calculations indicate a ⁵A_g ground state which agrees with the [U^IV]₂(N₂²⁻) description of 75. Significantly, the U–N (N₂) interaction derives from two occupied MOs showing π backbonding from uranium f orbitals into an N₂ antibonding π_g orbitals and that the interaction is strongly polarised. The bonding description is very similar to that in previously calculated models for 69 (formally N₂⁰) [116,117] and 72 (N₂²⁻) [118] which display very different N–N bond lengths. While experimental N–N bond distances determined by X-ray diffraction experiments are undeniably useful for quick comparisons, the bond length is likely underestimated since the data is based on electron density rather than atomic positions and thus may not reflect the level of dinitrogen reduction. In these studies, N–N stretching wavenumbers were more accurately reproduced by calculation than bond length and it is proposed that this would be a more suitable measurement for probing N₂ reduction.

The most robust actinide N₂ complex prepared to date is [{(Mes)₃SiO}₃U]₂(μ-η²:η²-N₂) (76) (Mes = 2,4,6-Me–C₆H₂) and is stable both in vacuo and in toluene solution up to 100 °C, at which point U{OSi(Mes)₃}₄ is slowly formed as the major product (52% conversion after 18 h) [87]. 76 is isolated from the reaction of U{N(SiMe₃)₂}₃ with 3 equivalents of HOSi(Mes)₃ under an N₂ atmosphere (1 atm). Raman spectroscopy shows a peak at 1437 cm⁻¹ assigned to the N–N stretching mode, indicating a significant level of reduction with respect to free dinitrogen (2331 cm⁻¹) and comparing well with 1451 cm⁻¹ recorded for 75 where reduction to N₂²⁻ was assigned. The N–N distances in the solid state are 1.124(12) Å (76-Eclipsed) and 1.080(11) Å (76-Staggered), which are statistically equivalent to that of free N₂; the disparity in implied reduction of N₂ from Raman spectroscopy and X-ray diffraction experiments again highlighting that the latter may not be best suited for assigning reduction in these systems.

3.2. Complexes Resulting from N₂ Cleavage

Tetra-calix-pyrrole ligands bound to Sm^II centres have been demonstrated to activate N₂ by Gambarotta and co-workers [101]. With U^III, an unprecedented example of N–N bond cleavage using an molecular f-element complex was observed; when [K(dme)][(^Et2calix[4]pyrrole)U(dme)] is treated with potassium naphthalenide under an atmosphere of N₂, N–N bond cleavage occurs to afford [K(dme)₄][{K(dme)(^Et2calix[4]pyrrole)U}₂(μ-NK)₂] (77) (Scheme 2) [119]. 77 contains two bridging nitrides (U–N: 2.076(6) and 2.099(5) Å) which have contacts with potassium ions (N–K: 2.554(6) Å) that bridge two pyrrolide units on separate ligands. It was postulated that 77 is a Class 1 U^IV–U^V mixed valence complex on the basis of an absorption at 1247 nm in the near-IR spectrum which is characteristic of U^V. The paramagnetism of 77 resulted in NMR silence in both ¹⁵N- and ¹⁴N-NMR spectra.

Scheme 2. N–N bond cleavage using a U^III complex to form 71.

Scheme 2. N–N bond cleavage using a U^III complex to form 71.

Reduction of the thorium bisphenolate complex [K(dme)₂][{(2-^tBu-4-Me–C₆H₂O)₂-6-CH₂}₂ThCl(dme)] (A) with potassium naphthalenide under an atmosphere of dinitrogen unexpectedly resulted in the amide complex [K(dme)₄][{(2-^tBu-4-Me–C₆H₂O)₂-6-CH₂}₂Th(NH₂)(dme)] 78 which is the first example of N₂ functionalisation using an f-element complex (Scheme 3). The parent [NH₂]⁻ amide ligand is confirmed through ¹⁵N-NMR spectroscopy which shows a triplet at 155.01 ppm (¹J_NH = 57.2 Hz) [120]. The proposed mechanism of this transformation involves formation of a formally zero-valent thorium intermediate which contains two bound [C₁₀H₈K(18c6)] fragments (identified through a single crystal X-ray diffraction experiment). This intermediate can then react with the starting material A leading to N₂ activation, cleavage and hydrogenation as a result of H atom abstraction from solvent molecules.

Scheme 3. N₂ cleavage and hydrogenation by a thorium complex.

Scheme 3. N₂ cleavage and hydrogenation by a thorium complex.

4. White Phosphorus Activation by Rare Earth Complexes

Rare earth complexes resulting from P₄ activation are illustrated in Figure 12. For reference, average P–P and Ln–P bond lengths obtained from single crystal X-ray diffraction experiments, and ³¹P-NMR spectroscopic resonances are summarised in Table 5. The structural cores of complexes 79–84 are shown in Figure 13 for clarity.

Figure 12. Rare earth complexes resulting from P₄ activation.

Figure 12. Rare earth complexes resulting from P₄ activation.

Figure 13. Overview of the Ln_xP_n structural cores resulting from P₄ activation by rare earth complexes.

Figure 13. Overview of the Ln_xP_n structural cores resulting from P₄ activation by rare earth complexes.

Table 5. Summary of rare earth P₄ activation complexes.

Complex (#) [Reference]	Average P–P Bond Lengths (Å)	Average M–P Bond Length (Å)	31P-NMR Spectroscopy (298 K) (ppm) a
P4	2.21 [121]	-	−488 to −527 [122]
[(η5-C5Me5)2Sm]4(μ4-η2:η2:η2:η2-P8) (79) [123]	2.195 (Pcorner–Pinner)	3.047	-
	2.291 (Pinner–Pinner)
[{Fe(1-NSitBuMe2–C5H4)2}Sc]4(μ4-η2:η2:η2:η2-P8) (80) [124]	2.204 (Pcorner–Pinner)	2.768	+45.7
	2.308 (Pinner–Pinner)		+96.2
[{Fe(1-NSitBuMe2–C5H4)2}Sc]3(μ3-η2:η2:η2-P7) (81) [124]	2.229 (Pbottom–Pbottom)	2.750	+23.1
	2.197 (Pedge–Pbottom)		−118.9
	2.201 (Papex–Pedge)		−131.4
[{Fe(1-NSitBuMe2–C5H4)2}Y(thf)]3(μ3-η2:η2:η2-P7) (82) [124]	2.238 (Pbottom–Pbottom)	2.950	−21.1
	2.176 (Pedge–Pbottom)		−82.4
	2.188 (Papex–Pedge)		−130.3
[{Fe(1-NSitBuMe2–C5H4)2}La(thf)]3(μ3-η2:η2:η2-P7) (83) [125]	2.258 (Pbottom–Pbottom)	3.120	−75
	2.161 (Pedge–Pbottom)
	2.191 (Papex–Pedge)
[{Fe(1-NSitBuMe2–C5H4)2}Lu(thf)]3(μ3-η2:η2:η2-P7) (84) [125]	2.233 (Pbottom–Pbottom)	2.893	+0.8
	2.181 (Pedge–Pbottom)		−96.8
	2.183 (Papex–Pedge)		−133.3

^a Referenced to 85% H₃PO₄.

Table 5. Summary of rare earth P₄ activation complexes.

Complex (#) [Reference]	Average P–P Bond Lengths (Å)	Average M–P Bond Length (Å)	31P-NMR Spectroscopy (298 K) (ppm) a
P4	2.21 [121]	-	−488 to −527 [122]
[(η5-C5Me5)2Sm]4(μ4-η2:η2:η2:η2-P8) (79) [123]	2.195 (Pcorner–Pinner)	3.047	-
	2.291 (Pinner–Pinner)
[{Fe(1-NSitBuMe2–C5H4)2}Sc]4(μ4-η2:η2:η2:η2-P8) (80) [124]	2.204 (Pcorner–Pinner)	2.768	+45.7
	2.308 (Pinner–Pinner)		+96.2
[{Fe(1-NSitBuMe2–C5H4)2}Sc]3(μ3-η2:η2:η2-P7) (81) [124]	2.229 (Pbottom–Pbottom)	2.750	+23.1
	2.197 (Pedge–Pbottom)		−118.9
	2.201 (Papex–Pedge)		−131.4
[{Fe(1-NSitBuMe2–C5H4)2}Y(thf)]3(μ3-η2:η2:η2-P7) (82) [124]	2.238 (Pbottom–Pbottom)	2.950	−21.1
	2.176 (Pedge–Pbottom)		−82.4
	2.188 (Papex–Pedge)		−130.3
[{Fe(1-NSitBuMe2–C5H4)2}La(thf)]3(μ3-η2:η2:η2-P7) (83) [125]	2.258 (Pbottom–Pbottom)	3.120	−75
	2.161 (Pedge–Pbottom)
	2.191 (Papex–Pedge)
[{Fe(1-NSitBuMe2–C5H4)2}Lu(thf)]3(μ3-η2:η2:η2-P7) (84) [125]	2.233 (Pbottom–Pbottom)	2.893	+0.8
	2.181 (Pedge–Pbottom)		−96.8
	2.183 (Papex–Pedge)		−133.3

^a Referenced to 85% H₃PO₄.

Roesky and co-workers reported the first example of a molecular polyphosphide of the rare earth elements, [(η⁵-C₅Me₅)₂Sm]₄(μ₄-η²:η²:η²:η²-P₈) (79) [123]. The samarocene (η⁵-C₅Me₅)₂Sm activates P₄ to yield a P₈⁴⁻ fragment with a realgar-type structure, a process proposed to be driven by the one-electron oxidation of the divalent samarium metal centre. 79 has molecular D_2d symmetry and the [Cp*₂Sm] units bridge the P₈⁴⁻ cage with Sm–P distances in the range of 2.997(2) to 3.100(2) Å. DFT calculations support the strongly ionic character of the Sm–P bonds.

Activation of P₄ by group 3 metal centres was first reported by Diaconescu and co-workers [124]. Reaction of the scandium arene inverse-sandwich complexes [{Fe(1-NSi^tBuMe₂–C₅H₄)₂}Sc]₂(μ-arene) (arene = C₁₀H₈ or C₁₄H₁₀) with P₄ resulted in displacement of the neutral arene and formation of a mixture of the tetrametallic [{Fe(1-NSi^tBuMe₂–C₅H₄)₂}Sc]₄(μ₄-η²:η²:η²:η²-P₈) (80) and trimetallic [{Fe(1-NSi^tBuMe₂–C₅H₄)₂}Sc]₃(μ₃-η²:η²:η²-P₇) (81). The mixtures were readily separated and the product distribution could be controlled by the stoichiometry of P₄ and the nature of the arene starting material. 80 possesses a realgar-type P₈⁴⁻ unit whereas 81 contains a Zintl-type P₇³⁻ unit [126], the first example of its formation in the absence of strong alkali metal reducing agents. Solution phase ³¹P-NMR spectroscopy demonstrates a diagnostic AA′A′′MM′M′′X spin system. The analogous yttrium arene inverse-sandwich complex [{Fe(1-NSi^tBuMe₂–C₅H₄)₂}Y(thf)]₂(μ-C₁₀H₈) activates P₄ to yield [{Fe(1-NSi^tBuMe₂–C₅H₄)₂}Y(thf)]₃(μ₃-η²:η²:η²-P₇) (82) as the sole product where the larger coordination sphere of yttrium is saturated with an additional thf molecule [127]. Importantly, in the context of functionalisation of white phosphorus to organophosphorus compounds, both 81 and 82 were shown to react with 3 equivalents of Me₃SiI to yield P₇(SiMe₃)₃ and {Fe(1-NSi^tBuMe₂–C₅H₄)₂}MI (Scheme 4).

Scheme 4. P₇³⁻ functionalisation with Me₃SiI.

Scheme 4. P₇³⁻ functionalisation with Me₃SiI.

This chemistry was later extended to lanthanum and lutetium using the same methodology, forming [{Fe(1-NSi^tBuMe₂–C₅H₄)₂}Ln(thf)]₃(μ₃-η²:η²:η²-P₇) (Ln = La (83), Lu (84)) [125]. The valence tautomerisation of P₇³⁻ in 83, which occurs at a similar temperature to Li₃P₇ but does not require donor solvents, was proposed to take place by a lanthanum-assisted mechanism involving simultaneous formation and breaking of 4 La–P bonds.

5. White Phosphorus Activation by Actinide Complexes

Actinide complexes resulting from P₄ activation are illustrated in Figure 14. For reference, average P–P and An–P bond lengths obtained from single crystal X-ray diffraction experiments, and ³¹P-NMR spectroscopic resonances are summarised in Table 6. The structural cores of complexes 85–91 are shown in Figure 15 for clarity.

Table 6. Summary of actinide P₄ activation complexes.

Complex (#) [Reference]	Average P–P Bond Lengths (Å)	Average An–P Bond Length (Å)	31P-NMR Spectroscopy (ppm) a
P4	2.21 [121]	-	−488 to −527 [122]
[(η5-1,3-tBu–C5H3)2Th](μ-η3:η3-cyclo-P3)[(η5-1,3-tBu–C5H3)2ThCl] (85) [128]	2.185	2.913	−75.7 (293 K)
[(η5-1,3-tBu–C5H3)2Th]2(μ-η3:η3-P6) (86) [128]	2.234	2.904 (Th-η2-P)	+125.4 (293 K)
		2.844 (Th-η1-P)	−41.9 (293 K)
[{(3,5-Me–C6H3)(tBu)N}3U](μ-η4:η4-cyclo-P4) (87) [129]	2.160	3.127	+794
[{(3,5-Me–C6H3)(Ad)N}3U](μ-η4:η4-cyclo-P4) (88) [129]	2.159	3.124	+803
[(η5-C5Me5)(η8-1,4-SiiPr3–C8H6)U]2(μ-η2:η2-cyclo-P4) (89) [130]	2.150	2.977	+718
[HC(SiMe2N-4-Me–C6H4)3U]3(μ3-η2:η2:η2-P7) (90) [130]	2.249 (Pbottom–Pbottom)	2.990	-
	2.187 (Pedge–Pbottom)
	2.209 (Papex–Pedge)
[{N(CH2CH2NSiiPr3)3}U]2(μ-η5:η5-cyclo-P5) (91) [131]	2.006	3.280	-

^a Referenced to 85% H₃PO₄.

Table 6. Summary of actinide P₄ activation complexes.

Complex (#) [Reference]	Average P–P Bond Lengths (Å)	Average An–P Bond Length (Å)	31P-NMR Spectroscopy (ppm) a
P4	2.21 [121]	-	−488 to −527 [122]
[(η5-1,3-tBu–C5H3)2Th](μ-η3:η3-cyclo-P3)[(η5-1,3-tBu–C5H3)2ThCl] (85) [128]	2.185	2.913	−75.7 (293 K)
[(η5-1,3-tBu–C5H3)2Th]2(μ-η3:η3-P6) (86) [128]	2.234	2.904 (Th-η2-P)	+125.4 (293 K)
		2.844 (Th-η1-P)	−41.9 (293 K)
[{(3,5-Me–C6H3)(tBu)N}3U](μ-η4:η4-cyclo-P4) (87) [129]	2.160	3.127	+794
[{(3,5-Me–C6H3)(Ad)N}3U](μ-η4:η4-cyclo-P4) (88) [129]	2.159	3.124	+803
[(η5-C5Me5)(η8-1,4-SiiPr3–C8H6)U]2(μ-η2:η2-cyclo-P4) (89) [130]	2.150	2.977	+718
[HC(SiMe2N-4-Me–C6H4)3U]3(μ3-η2:η2:η2-P7) (90) [130]	2.249 (Pbottom–Pbottom)	2.990	-
	2.187 (Pedge–Pbottom)
	2.209 (Papex–Pedge)
[{N(CH2CH2NSiiPr3)3}U]2(μ-η5:η5-cyclo-P5) (91) [131]	2.006	3.280	-

^a Referenced to 85% H₃PO₄.

Figure 14. Actinide complexes resulting from P₄ activation.

Figure 14. Actinide complexes resulting from P₄ activation.

Figure 15. Overview of the An_xP_n structural cores resulting from P₄ activation by actinide complexes.

Figure 15. Overview of the An_xP_n structural cores resulting from P₄ activation by actinide complexes.

The first report of P₄ activation by an actinide complex came from Scherer et al., using thorium [128]. The butadiene complex (η⁵-1,3-^tBu–C₅H₃)₂Th(η⁴-C₄H₆) reacts with P₄ at 100 °C in the presence of MgCl₂(OEt₂) to yield [(η⁵-1,3-^tBu–C₅H₃)₂Th](μ-η³:η³-cyclo-P₃)[(η⁵-1,3-^tBu–C₅H₃)₂ThCl] (85). In the absence of MgCl₂(OEt₂), [(η⁵-1,3-^tBu–C₅H₃)₂Th]₂(μ-η³:η³-P₆) (86) was afforded as a consequence of P₄ fragmentation and subsequent catenation. 85 features a cyclo-P₃³⁻ unit and formal Th^IV centres. In the solid state structure, the coordination environment about the thorium centres is trigonal planar and tetrahedral (Cl bound) with Th–P distances ranging from 2.809(6) to 2.974(8) Å. The P–P distances are 2.171(9), 2.192(9) and 2.192(8) Å. Only a single broad resonance is observed in the ³¹P-NMR spectrum at 293 K but cooling to 193 K leads to splitting and the observation of an A₂B system; the barrier to rotation of the cyclo-P₃ unit was estimated to be ca. 44 kJ·mol⁻¹. 86 contains a P₆⁴⁻ bicycle with thorium metal centres capping the five-membered rings. Th–P bond lengths in the solid state structure range from 2.840(7) to 2.919(7) Å.

The following report of P₄ activation came over a decade later and was the first using a uranium complex [129]. The U^III tris(amide) U{N(R)Ar}₃(thf) (R = ^tBu or Ad, Ar = 3,5-Me–C₆H₃) reacts with 0.5 equivalents of P₄ to yield [{Ar(R)N}₃U](μ-η⁴:η⁴-cyclo-P₄) (R = ^tBu (87), Ad (88)) which contains a cyclo-P₄²⁻ unit and where the metal centres have been formally oxidised to U^IV. In the solid state structures, the average P–P bond distance is 2.159 Å and the P–P–P angle is 90°; both statistically equivalent across the two structures. Resonances at 794 and 803 ppm were observed for 87 and 88 respectively in the ³¹P-NMR spectrum. Computational studies implied that the U–P bonding character is largely ionic with the presence of a weak δ-bonding interaction between filled U df hybrid orbitals and the P₄²⁻ LUMO.

Cloke and co-workers described the related cyclo-P₄ example [(η⁵-C₅Me₅)(η⁸-1,4-SiⁱPr₃–C₈H₆)U]₂(μ-η²:η²-cyclo-P₄) (89) which was prepared from (η⁵-C₅Me₅)(η⁸-1,4-SiⁱPr₃–C₈H₆)U(thf) and 0.5 equivalents of P₄ [130]. This was the first example of the μ-η²:η²-P₄ coordination mode [132,133] and DFT studies on a model system [(η⁵-C₅H₅)(η⁸-C₈H₈)U](μ-η²:η²-cyclo-P₄) support the formulation of the dimer with P₄²⁻ and U^IV oxidation states. The tilted cyclo-P₄²⁻ unit leads to U–P bonding interactions involving both σ and π orbitals. The wedge shaped nature of the sterically demanding (η⁵-C₅Me₅)(η⁸-1,4-SiⁱPr₃–C₈H₆)U fragment likely results in the slipped μ-η²:η² coordination mode.

Following on from the rare earth inverse sandwich complexes that resulted in P₈⁴⁻ and P₇³⁻ clusters, and P₇³⁻ functionalisation [124,125], Liddle and co-workers reported the reaction of [HC(SiMe₂NAr)₃U]₂(μ-η⁶-η⁶-C₆H₅CH₃) (Ar = 4-Me–C₆H₄) with 1.1 equivalents of P₄ which afforded the first actinide Zintl complex [HC(SiMe₂NAr)₃U]₃(μ₃-η²:η²:η²-P₇) (90) [134]. U–P bonding was determined to be essentially ionic. Interestingly, reaction of 90 with a number of electrophiles under ambient conditions led to functionalisation of the P₇³⁻ unit and liberation of P₇R₃ (R = SiMe₃, Me, Ph, Li(tmeda)) after P–Si, P–C or P–Li bond formation. Though not catalytic, 90 could be regenerated from this reaction mixture and two turnovers achieved demonstrating a significant step towards controlled P₄ activation under mild conditions.

Very recently, Liddle and co-workers described the first example of a cyclo-P₅ complex resulting from activation of P₄ by an f-block complex [131]. [{N(CH₂CH₂NSiⁱPr₃)₃}U]₂(μ-η⁵:η⁵-cyclo-P₅) (91) was prepared by reaction of {N(CH₂CH₂NSiⁱPr₃)₃}U with 0.25 equivalents of P₄. Spectroscopic and magnetic measurements support oxidation to afford U^IV centres and charge transfer resulting in a formal P₅²⁻ ligand in this inverse sandwich complex. Despite the isolobal analogy of cyclo-P₅ with the cyclopentadienyl anion, which bonds to metal centres using primarily σ- and π-bonding, calculations on 91 suggest that the principal U–P interactions involve polarised δ-bonding and this can be attributed to the energetically available uranium 5f orbitals of correct δ-symmetry.

Compared to both rare earth and actinide complexes, the activation of P₄ by transition metal complexes have proven to result in a wide variety of activation products [58,59]; with notable examples including fragmentation resulting in terminal and bridging P₁ ligands [135,136,137], P₂ ligands [138], cyclo-P₃ ligands [56,139], fragmentation to other P₄ ligands [140], coordination of P₄ tetrahedra [141,142], and expansion to P_n (n = 5–14) ligands [143,144,145,146]. More significantly, functionalisation of these phosphorus units has also been observed.

6. Arsenic, Antimony and Bismuth Activation by Rare Earth and Actinide Complexes

There is only a single example of molecular arsenic activation by a rare earth or actinide metal complex. Reaction of the thorium butadiene complex (η⁵-1,3-^tBu–C₅H₃)₂Th(η⁴-C₄H₆) with As₄ (yellow arsenic) in boiling xylene affords [(η⁵-1,3-^tBu–C₅H₃)₂Th]₂(μ-η³:η³-As₆) in analogy to the previously reported P₄ chemistry of Scherer et al. (Figure 16) [147]. In the solid state molecular structure; the average Th–(η²-As₆) bonds are 3.027 Å, average Th–(η¹-As) bonds are 2.922 Å and average As–As bonds are 2.459 Å.

Figure 16. Actinide complexes resulting from As₄ activation.

Figure 16. Actinide complexes resulting from As₄ activation.

In contrast, activation of molecular arsenic by homogeneous transition metal complexes is considerably more diverse; formation of cyclo-As_n ligands (n = 3–6, 8) [54,148], metal arsenic clusters [149], coordination of intact As₄ tetrahedra to metal ions [133,150,151], fragmentation into As₂ and other As₄ ligands [152,153,154], catenation to As₁₀ and As₁₂ ligands [155], reactions to form P_nAs_m ligands [156], and full As₄ fragmentation resulting in terminal M≡As arsenide bonds [157] have all been reported.

Beyond arsenic in group 15 are antimony and bismuth. While activation of molecular forms of these elements is unlikely, it is worth noting that Scheer and co-workers reported a tungsten terminal stibido complex {N(CH₂CH₂NSiMe₃)₃}₃W≡Sb prepared from reaction of {N(CH₂CH₂NSiMe₃)₃}₃WCl with LiSb(H){CH(SiMe₃)₂} [158], while Breunig et al., have reported [(η⁵-C₅Me₅)₂Mo(CO)₂](cyclo-Sb₃) and [(η⁵-C₅H₅)₂Mo(CO)₂](cyclo-Sb₃), which are a result of reaction of [(η⁵-Cp^R)₂Mo(CO)₃]₂ with (^tBuSb)₄ [2]. In terms of rare earth complexes; [(η⁵-C₅Me₅)₂Sm]₂(μ₃-η¹:η²:η²-Sb₃){(η⁵-C₅Me₅)₂(thf)Sm} and [(η⁵-C₅Me₅)₂Sm]₂(η²:η²-Bi₂) were prepared by reaction of [(η⁵-C₅Me₅)₂Sm]₂ with SbPh₃ and BiPh₃ respectively [3,4].

7. Conclusions and Perspectives

To date, a wide range of rare earth dinitrogen complexes have been prepared (1–68), including group 3 metal ions and 4f elements at both ends of the periodic table, despite the limited radial extension of the 4f orbitals and the trivalent oxidation state being the most prevalent. In fact, apart from the very first f-element dinitrogen complex (4), all of the other complexes are air-sensitive but stable to N₂ dissociation in vacuo. Reduction of N₂ to N₂²⁻ has most commonly been achieved with the [A₂(thf)_xLn]₂(μ-η²:η²-N₂) structural motif (1–38) whereas examples of reduction to N₂⁴⁻ have all involved more complex multidentate ligands (61–68). The nature of the bonding in complexes of the form [A₂(thf)_xLn]₂(μ-η²:η²-N₂) has been extensively studied for both group 3, and closed and open shell 4fⁿ metal ions demonstrating that the Ln–N (N₂) bonding is based on a Ln nd–N₂ π* interaction. These systems have allowed for the first definitive characterisation of the N₂³⁻ radical reduction product of dinitrogen (44, 47) and this has now been extended to many of the rare earth elements (45, 46, 48–60). Isolation of the N₂³⁻ radical in homogeneous complexes is significant since it is likely to be a transient species in other transition metal systems and may also have a role in biological N₂ fixation.

In terms of reactivity, the rare earth N₂ complexes prepared thus far react with N₂ dissociation rather than N₂ cleavage and functionalisation [77,159,160,161]. Related to this, the understanding of the nature of bonding of Ln–N multiple bonds is of fundamental interest and the isolation of terminal imido (Ln=NR) complexes has only recently been reported [162,163,164]. It is also significant to consider the reactivity of Ln–N₂ complexes in the context of other low electron count early transition metal complexes; here, N₂ cleavage or functionalisation can be achieved through ligand induced reductive cleavage [165,166,167,168,169,170,171,172,173], and more generally, N–N bond scission of reduced N₂ derivatives can be attained through metal-ligand cooperativity [174,175].

Actinide dinitrogen complexes are much rarer, with just 8 examples of well-defined molecular uranium complexes (69–76). Of these, only the heterobimetallic U–Mo end-on N₂ complexes (70, 71) and recently prepared U^IV aryloxide and siloxide side-on N₂ complexes (75, 76) are thermally robust and stable when exposed to vacuum. Key steps forward have been made in the understanding of U–N (N₂) bonding in the side-on N₂ complexes as a polar covalent U 5f-N₂ π* interaction, and in the rationalisation of the differences between solid state N–N bond lengths and the overall electronic structure of these complexes. This understanding, in combination with well-designed ligand sets may lead the way in the preparation of other isolable actinide dinitrogen complexes for further study.

Though the isolated actinide N₂ complexes, like the rare earth N₂ complexes, tend to react with N₂ loss rather than N₂ functionalisation or cleavage, there are two reports of such reactivity. Importantly, in both cases, the putative An(N₂) complex was not observed. Cleavage of N₂ by a uranium tetra-calix-pyrrole complex results in a bimetallic complex with bridging nitrides (78) whereas a thorium bisphenolate complex activates and functionalises N₂ to a parent amide ligand [NH₂]⁻ (79) through an unknown mechanism. Both reactions occur in the presence of an external reductant. Examples of isolable terminal uranium nitrides (U≡N), derived from NaN₃, have only recently been reported [176,177,178], but it has already been demonstrated that these systems are capable of nitride functionalisation. 78 and 79 remain standout examples in demonstrating that actinide complexes can both cleave and functionalise N₂, but also highlight how much more remains to be understood in this field.

P₄ activation by rare earth complexes has led to both P₈⁴⁻ ions with realgar-type structures (79, 80), and P₇³⁻ ions (81–84) using cyclopentadienyl and amido ancillary ligands. Promisingly, functionalisation of the P₇³⁻ unit in 81 and 82 was found possible using Me₃SiI to afford P₇(SiMe₃)₃. This is an interesting prospect for the synthesis of organophosphorus compounds from a P₄ building block. Actinide P₄ activation results in a more diverse array of phosphorus ligands; P₃³⁻ (85), P₆⁴⁻, (86), cyclo-P₄²⁻ (87–89), P₇³⁻ (90), and cyclo-P₅²⁻ (91). Similar to rare earth chemistry, the P₇³⁻ ions in 90 could be functionalised by P–Si, P–C or P–Li bond formation to afford P₇R₃ units and this reaction cycle could be completed with two turnovers. 91 is the first example of an f-element being able to fragment and catenate P₄ to cyclo-P₅²⁻. Despite the parallels with the cyclopentadienyl ligand, calculations suggest the U–P (cyclo-P₅) interaction to be based on polarised δ-bonding and electronic structure in these systems can be described as [U^IV]₂(P₅²⁻). Putting this area into perspective, transition metals have already been shown to activate molecular phosphorus and, in limited examples, to result in further functionalisation.

There remains only a lone example of arsenic activation by a thorium butadiene complex leading to a P₆ cage (92) and no examples using rare earth metals. The first examples of crystallographically characterised uranium arsenide (U–AsH₂), arsenidene (U=AsH) and arsenido (U≡AsK) complexes have only recently been reported, using KAsH₂ as a source of arsenic [179]. These compounds raise the question of the diverse reactivity that actinide complexes could be expected to show with molecular arsenic and whether the formation of an unsupported, terminal actinide arsenide bond (M≡As) is accessible.

It is clear that molecular pnictogen activation by rare earth and actinide metal complexes is an exciting field of study which remains underdeveloped with respect to transition metals and main group elements. These unique metals offer the potential of new reactivity and functionalisation chemistry with the pnictogen elements, while the fundamental study of M–pnictogen bonds remains important.