^EtIDip (^EtIPr)—Synthesis, Characterisation and Reactivity of a Robust, Backbone-Modified N-Heterocyclic Carbene and Group 13 Element Complexes

Abstract

We report the synthesis, characterisation and reactivity of the stable imidazol-2-ylidene ^EtIDip (^EtIPr), {EtCN(Dip)}₂C:, Dip = 2,6-iPr₂C₆H₃, as a chemically robust alternative to IDip (IPr), {HCN(Dip)}₂C:. The N-heterocyclic carbene ^EtIDip could be further converted to the oxidised species [^EtIDipCl]Cl, ^EtIDipF₂, ^EtIDipO, and ^EtIDipSe, and the group 13 element complexes ^EtIDipEX₃, with E = B, X = Br; E = Al, X = I; E = Ga, X = I; E = Al, X = H. The properties of the ^EtIDip and IDip ligands are compared and the molecular structures of (DipNCEt)₂, [^EtIDipH]Cl, [^EtIDipH]I, ^EtIDip, [^EtIDipCl]Cl, ^EtIDipF₂, ^EtIDipO, ^EtIDipBBr₃, ^EtIDipAlI₃, ^EtIDipGaI₃, and ^EtIDipAlH₃ have been determined.

1. Introduction

The introduction of a stable imidazol-2-ylidene by Arduengo and co-workers in 1991 [1], and many other N-heterocyclic carbenes (NHCs) and related species [2,3,4], was a breakthrough for access to a wide range of both stable and fleeting carbon-based donor molecules. In the following decades, these discoveries transformed fields such as coordination chemistry and catalysis, organocatalysis, material science and other applications in the chemical sciences [2,3,4,5,6]. The term “carbene” implies an electron sextet at carbon, but for NHCs which have ground state singlet character, these display ylide character and are stabilised by adjacent donor atoms, for example, two N atoms for imidazol-2-ylidenes, see Figure 1. Typically, the HOMO of an imidazol-2-ylidene (Figure 1a) shows a carbon-based lone pair character that is responsible for good σ-donating properties. Three occupied π-type orbitals, see the HOMO-1 (Figure 1a) as one example, show that the carbon p-orbital perpendicular to the heterocycle plane is not entirely empty.

In main group chemistry, NHCs, especially sterically demanding examples, have allowed the synthesis and characterisation of a wide range of often quite stable NHC-element complexes that include novel types of donor-stabilised element(0) compounds and donor adducts of highly reactive small molecular entities [7,8,9,10,11,12,13]. Many of these novel compounds advanced our understanding of donor-acceptor interactions and this led to a wider discussion on element-element bonding interpretations [14,15,16,17,18]. The large variety of stable NHCs and related persistent carbenes [2,3,4] allows researchers to choose from a wide variety of donor ligands with various σ-donating and π-accepting properties, steric bulk and other characteristics [19,20]. Especially the sterically demanding NHC IDip, or 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene, often abbreviated IPr in the literature (see Figure 1b), has been a widely applied and successfully employed stable carbene for many applications [21,22,23]. A search of the Cambridge Structural Database (CSD) showed that the IDip motif alone (binding to a non-hydrogen atom via the central carbon centre, including all hydrogen atoms, but excluding examples with additional substituents and derivatives) provided >3300 entries [24]. Although NHCs such as IDip often act as spectator-type donor ligands, they can also produce a wide variety of initially unexpected or unintended reaction outcomes. For example, the 4- and/or 5-positions in imidazolylidenes are not inert and can be deprotonated to so-called abnormal NHCs, aNHCs (e.g., Figure 1c), or mesoionic NHCs, and coordinate to metal centres or take part in derivatisation reactions [25,26,27]. The 4,5-dimethyl substituted analogue of IDip, ^MeIDip (Figure 1b) [28], has these positions blocked, but appears to show a lower solubility compared with IDip for many derivatives. Furthermore, the methyl groups in imidazolium cations and related systems can be deprotonated to nucleophilic N-heterocyclic olefins (NHOs) [29,30] and to mesoionic N-heterocyclic olefins (mNHOs) [31,32,33], see Figure 1d for a general example. Activating these substituents can lead to coordination complexes of NHOs and mNHOs and their further reactivity. A CSD search showed that the ^MeIDip motif alone (binding to a non-hydrogen atom via the central carbon centre, including all hydrogen atoms, but excluding derivatives) provided approximately 100 entries, and the majority are NHO complexes or organic derivatives [24]. In addition to activating various ring positions in NHCs, many reactions are now known where the parent NHC-heterocycle undergoes unexpected ring expansion or ring-opening reactions, and many of these reactions were induced by reactive main group components [34,35,36].

To access a potentially more robust NHC compared with IDip and ^MeIDip, and to retain high solubility and good crystallisation properties of derivatives, we were interested in preparing the 4,5-diethyl derivative of IDip, ^EtIDip (^EtIPr) (Figure 1b) and report on our efforts herein. We envisaged that the ethyl substituents would not distort the central stable imidazolylidene ring and that ^EtIDip would be less prone to being deprotonated or CH-activated at the backbone (alkyl) positions compared with IDip and ^MeIDip, and thus present a more chemically robust alternative. We further envisaged that the introduction of ethyl groups could be achieved by modifying common synthetic methods and would afford economical access to ^EtIDip. Other approaches to NHC ligands with long alkyl chains in 4- and 5-positions have been reported but use a different synthetic approach for forming the central NHC ring fragment than those that are preferred to yield IDip [37,38].

Figure 1. (a) Typical HOMO and HOMO-1 orbitals (isovalue 0.07) of an imidazolylidene NHC, (MeCNMe)₂C: [39], (b) NHCs IDip, with numbering scheme, ^MeIDip and ^EtIDip, (c) a related abnormal, or mesoionic (c.f. the formal charges) NHC, aNHC, (d) two mesomeric forms of a related (abnormal) mesoionic NHO, mHNO.

Figure 1. (a) Typical HOMO and HOMO-1 orbitals (isovalue 0.07) of an imidazolylidene NHC, (MeCNMe)₂C: [39], (b) NHCs IDip, with numbering scheme, ^MeIDip and ^EtIDip, (c) a related abnormal, or mesoionic (c.f. the formal charges) NHC, aNHC, (d) two mesomeric forms of a related (abnormal) mesoionic NHO, mHNO.

2. Results and Discussion

2.1. Synthesis of ^EtIDip 3

To target the backbone-modified ^EtIDip, we employed a synthetic route that mimics previously reported and proven pathways to access IDip and ^MeIDip [2,3,4,20,21,22,23,28,40,41]. Diimine 1 has been mentioned in the literature before [42] but is surprisingly unexploited. To access 1, we started with hexane-3,4-dione and studied the condensation reaction with two equivalents of 2,6-diisopropyl aniline (DipNH₂) in refluxing toluene using a Dean-Stark trap to remove water from condensation with para-toluenesulfonic acid hydrate as a catalyst. By taking reaction mixture aliquots, we found that the reaction was only slowly progressing via a monosubstituted intermediate but required around one week to convert to 1 in a good in-situ yield. The choice of stoichiometric or catalytic quantities of acid in the mixture did not lead to large differences in reaction outcomes but did impact the practicalities of the basic aqueous work-up of reaction mixtures using potassium carbonate solution and dichloromethane. Due to the relatively harsh conditions, we performed this reaction under an atmosphere of nitrogen to help suppress side reactions. In order to accelerate the synthesis, we tested the higher-boiling solvent xylene (mixture of isomers, b.p. ca. 137 °C) instead of toluene (b.p. ca. 111 °C) under the same conditions. We found that some condensation reaction occurred, but that at the higher temperature, the product appeared to decompose again and reformed DipNH₂. We thus switched again to toluene using the conditions provided in Scheme 1 and heated the mixture for overall around eight days. The first overnight heating period was kept at ca. 100 °C with the aim of preventing much of the hexane-3,4-dione from entering the Dean-Stark trap and largely forming the less volatile monosubstituted intermediate (EtC(=O)C(=NDip)Et) before full reflux to remove condensation water from the mixture. The experiments showed that much harsher conditions are required to form 1 compared with syntheses of other diimines such as (DipN=CR)₂, R = H, Me [22,23]. For work-up, most of the solvent was distilled off under atmospheric pressure via the Dean-Stark trap and the brown residue was treated with aqueous K₂CO₃ solution and dichloromethane to afford a brown oil of crude 1 that could be recrystallised from methanol at −40 °C to yield trans-3,4-bis(2,6-diisopropylphenylimino)hexane 1 in good yield (76%). Diimine 1 shows expected NMR spectroscopic features, and a single crystal X-ray diffraction study (Figure 2) shows the overall trans-arrangement of the conjugated diimine unit similar to that in (DipN=CMe)₂ [43].

For economical access to the imidazolium salt [^EtIDipH]Cl 2, we started from the conditions reported by Hintermann [40]. A mixture of diimine 1 and paraformaldehyde (1.1–1.5 equivalents) in ethyl acetate was slowly treated with chlorotrimethylsilane (1.1–1.5 equivalents) as the acid component and chloride source. Variables such as reaction time (e.g., 4 h, 16 h, 3 days), temperature (e.g., room temperature versus reflux), and a solvent change to THF were investigated, and typically afforded the precipitation of crude [^EtIDipH]Cl 2, that could be washed with solvents such as diethyl ether and n-hexane, in moderate yields of around 28–38%. Variations of the conditions did not change the isolated product yields much, and so far, we have not improved on the moderate, but not atypical yield for this compound class, using this synthetic route. The combination of steric bulk and the trans-arrangement of the diimine unit in 1 may impede necessary bond forming and ring-closing reactions for a high yield of the stable imidazolium ring product. [^EtIDipH]Cl 2 shows the expected resonances from NMR spectroscopic studies and both a solvent-free form and chloroform solvate of salt 2 could be structurally characterised by X-ray diffraction (Figure 3 and Figure S39,

Table 1. Selected bond lengths and angles for compounds 2–6 and related species.

Compound	N–C/Å	N–C–N/°	C–E/Å	other/Å or/°
[EtIDipH]Cl 2	N2–C1 1.3332(18)	N2–C1–N2′ 107.95(19)	C1–H1 0.9500	H1∙∙∙∙Cl1 ca. 2.28 a
[EtIDipH]I∙C6H6 b	N2–C1 1.332(2), N5–C1 1.334(2), N42–C41 1.335(2), N45–C41 1.332(2)	N2–C1–N5 108.37(14), N45–C41–N42 108.15(14)	C1–H1 0.9500, C41–H41 0.9500	H1∙∙∙∙I1 ca. 2.761, H41∙∙∙∙I41 ca. 2.833
EtIDip 3	N2–C1 1.356(10), N5–C1 1.365(10)	N2–C1–N5 101.9(6)	-	-
[EtIDipCl]Cl 4	N2–C1 1.334(3), N5–C1 1.334(2)	N5–C1–N2 108.37(17)	Cl1–C1 1.681(2)	Cl1∙∙∙∙Cl2 ca. 3.044,
EtIDipF2 5	N2–C1 1.3928(14), N5–C1 1.3914(13)	N5–C1–N2 105.91(9)	F1–C1 1.4018(13), F2–C1 1.4005(13)	F2–C1–F1 98.80(8)
EtIDipO 6	N2–C1 1.383(3), N5–C1 1.381(3)	N5–C1–N2 104.55(17)	O1–C1 1.227(2)	O1–C1–N2 127.71(19), O1–C1–N5 127.73(19)

^a The chloride position is disordered; ^b two independent molecules.

). In addition, the related [^EtIDipH]I (Figure 3) was also crystallised as part of the wider study, and selected metrical data for ^EtIDip-compounds are collected in Table 1.

The solid-state structure of [^EtIDipH]Cl 2 revealed the expected geometry and showed relatively short cation-CH∙∙∙∙Cl⁻ contacts. To access free NHC ^EtIDip 3 from [^EtIDipH]Cl 2, commercially available KOtBu in THF can be used to conveniently deprotonate 2 to NHC 3. ^EtIDip 3 can be recrystallised from n-hexane to afford pure 3. NMR spectroscopy provided the expected sharp resonances for 3, including an NHC carbene resonance at δ 217.4 ppm in deuterated benzene, close to that reported for IDip (δ 220.6 ppm in C₆D₆) [22,23]. A molecular structure from single crystal X-ray diffraction of 3 (Figure 4, Table 1) shows the expected monomeric carbene-type structure and no close contact of the carbene centre to other atoms in the solid state. The N2–C1–N5 angle is significantly more acute compared with that of the imidazolium salts and other derivatives (Table 1), vide infra, possibly due to the high p-character of the C–N bonds and the high s-character of the NHC lone pair.

2.2. Oxidation Reactions of ^EtIDip 3

As a reactive formally divalent carbon(II) compound, we studied the conversion of ^EtIDip 3 to higher oxidation state carbon reagents. The reaction of 3 with hexachloroethane as a Cl₂ source in a variety of solvents afforded [^EtIDipCl]Cl 4 in analogy to previous examples, Scheme 2 [44,45,46,47]. Compound 4 crystallised as an ion-separated salt (Figure 5, Table 1) with a close Cl1∙∙∙∙Cl2 contact that is typical for a halogen bond distance [48]. The near-linear C–Cl1∙∙∙∙Cl2 geometry (172.52°) supports a donor contact from the chloride (Cl2) to the cationic chloroimidazolium ion (Cl1) having a σ-hole. In analogy to the synthesis of IDipF₂ [47,49], which is commercially available as PhenoFluor and is a widely used deoxyfluorination reagent, we have treated [^EtIDipCl]Cl 4 with four equivalents of CsF in acetonitrile or toluene at 60 °C overnight, Scheme 2.

This afforded ^EtIDipF₂ 5 (see Figure 6 and Table 1). Unfortunately, we always obtained significant amounts of the formal hydrolysis product, ^EtIDipO 6 (Figure 7, Table 1), alongside 5 in our hands, which we ascribe to possible impurities and/or moisture in the heat and vacuum-dried CsF sample. Compounds 5 and 6 show broadly similar solubilities and can co-crystallise, but we were able to separate these sufficiently by fractional crystallisation. An isolated yield of 41% was achieved for 5 and most of the remainder was found to be 6. The ¹⁹F NMR spectrum of 5 shows a sharp singlet at δ = −34.0 ppm in deuterated benzene or a broadened singlet (δ = −34.1 ppm) in deuterated chloroform which compares well with the resonance reported for IDipF₂ (δ = −36.5 ppm in deuterated chloroform [47]).

Starting with a sample of ^EtIDip 3 in deuterated benzene, treatment with elemental selenium afforded ^EtIDipSe 7 in-situ, Scheme 2, and its NMR spectroscopic data was collected and includes a ⁷⁷Se NMR resonance [50,51,52,53] at δ 134.0 ppm. We repeated the experiment under the same conditions with IDip and obtained spectral data of IDipSe for comparison, which shows a ⁷⁷Se NMR resonance at δ 109.8 ppm (c.f. δ 87 ppm in deuterated acetone and δ 90 ppm in deuterated chloroform) [50,51,52,53]. Spectroscopic data in relation to the properties of 3 will be discussed in a later section, vide infra.

2.3. Synthesis and Characterisation of Group 13 Element Complexes of ^EtIDip 3

Since the early days of research with stable NHCs, group 13 element complexes have been targeted, and led to the early synthesis and characterisation of a stable NHC-alane (AlH₃) adduct [54]. These studies showed that NHCs such as IDip are capable of forming chemically robust and thermally stable complexes of reactive fragments such as AlH₃ [55] and Al₂H₄ [56]. IDipAlH₃, for example, only decomposes in the solid state at around 230 °C [55]. NHC-group 13 chemistry has expanded ever since and still reveals surprises and new reactivity nowadays [11,12,13].

We treated ^EtIDip 3 with BBr₃, or BBr₃·SMe₂, affording the adduct ^EtIDipBBr₃ 8 in good yield, Scheme 3. In a similar manner, ^EtIDip 3 could be reacted with the Lewis acids AlI₃ and GaI₃, respectively, to afford the complexes ^EtIDipAlI₃ 9 and ^EtIDipGaI₃ 10 in moderate isolated and higher in-situ yields, Scheme 3.

Reactions of ^EtIDip 3 with “GaI” [57,58] or “GaI₂” also predominantly furnished ^EtIDipGaI₃ 10. The molecular structures of complexes 8 (Figure 8), 9 (Figure 9) and 10 (Figure 10) were determined and selected bond distances and angles are collected in

Table 2. Selected bond lengths in ^EtIDipEX₃, E = B, Al, Ga; X = H, Br, I; complexes 8–11.

Compound (Structure)	N–C/Å	C–E/Å	E–Xapical/Å	E–Xbasal/Å
EtIDipBBr3∙2 C6H6, 8∙2 C6H6	N2–C1 1.372(2), N5–C1 1.372(2)	C1–B1 1.645(3)	Br3–B1 2.053(2)	Br1–B1 2.028(2), Br2–B1 2.020(2)
EtIDipAlI3∙2 C6H6, 9∙2 C6H6	N2–C1 1.362(2), N5–C1 1.360(2)	Al1–C1 2.0404(18)	I2–Al1 2.5410(6)	I1–Al1 2.5141(6), I3–Al1 2.5227(5)
EtIDipGaI3 10 (10′)	N2–C1 1.399(16), N5–C1 1.390(15)	Ga1–C1 2.050(13)	I2–Ga1 2.6015(19)	I1–Ga1 2.5759(17), I3–Ga1 2.5603(17)
EtIDipGaI3∙2 C6H6, 10∙2 C6H6 (10″)	N2–C1 1.358(4), N5–C1 1.355(4)	Ga1–C1 2.043(3)	I3–Ga1 2.5620(5)	I1–Ga1 2.5374(5), I2–Ga1 2.5295(5)
EtIDipAlH3 11 (11′) a,b	N2–C1 1.3551(15)	Al1–C1 2.0588(19)	Al1–H2 1.538(16)	Al1–H1 1.501(15), Al1–H3 1.474(16)
EtIDipAlH3 11 (11″) c	N2–C1 1.3578(18), N5–C1 1.3587(18)	Al1–C1 2.0606(16)	Al1–H1A 1.51(3)	Al1–H1B 1.45(2), Al1–H1C 1.49(2)

^a AlH positions are disordered by symmetry and thus six H positions are found; the H2 orientation is the H most “apical”, the H1 orientation is the most “basal”; ^b AlH distances were refined using distance restraints; ^c The H1A orientation is the most “apical”, and similar to that of H1B, whereas the H1C position is the most “basal”.

and

Table 3. Selected bond angles in ^EtIDipEX₃, E = B, Al, Ga; X = H, Br, I; complexes 8–11.

Compound (Structure)	N–C–N/°	C–E–Xapical/°	C–E–Xbasal/°	X–E–X/°
EtIDipBBr3∙2 C6H6, 8∙2 C6H6	N5–C1–N2 104.53(13)	C1–B1–Br3 107.11(12)	C1–B1–Br1 114.41(12), C1–B1–Br2 113.47(12)	Br2–B1–Br1 106.39(9), Br2–B1–Br3 107.97(9), Br1–B1–Br3 107.18(9)
EtIDipAlI3∙2 C6H6, 9∙2 C6H6	N5–C1–N2 104.26(14)	C1–Al1–I2 108.47(5)	C1–Al1–I1 112.71(5), C1–Al1–I3 114.08(5)	I1–Al1–I2 108.17(2), I1–Al1–I3 107.83(2), I3–Al1–I2 105.18(2)
EtIDipGaI3 10 (10′)	N5–C1–N2 102.8(10)	C1–Ga1–I2 107.7(4)	C1–Ga1–I1 113.6(3), C1–Ga1–I3 114.5(3)	I1–Ga1–I2 107.15(6), I3–Ga1–I1 107.18(6), I3–Ga1–I2 106.18(6)
EtIDipGaI3∙2 C6H6, 10∙2 C6H6 (10″)	N5–C1–N2 105.2(3)	C1–Ga1–I3 108.89(9)	C1–Ga1–I1 114.72(9), C1–Ga1–I2 113.27(9)	I2–Ga1–I1 107.548(17), I1–Ga1–I3 104.472(17), I2–Ga1–I3 107.354(17)
EtIDipAlH3 11 (11′) a,b	N2–C1–N2′ 104.09(14)	C1–Al1–H2 103.7(15)	C1–Al1–H1 105.8(13), C1 Al1 H3 118.6(16)	H1–Al1–H2 105.0(12), H1–Al1–H3 112.9(14), H2–Al1–H3 109.6(13)
EtIDipAlH3 11 (11″) c	N2–C1–N5 103.92(12)	C1–Al1–H1A 107.0(10)	C1–Al1–H1B 105.6(9), C1–Al1–H1C 107.5(8)	H1A–Al1–H1B 113.4(13), H1A–Al1–H1C 109.3(12), H1B–Al1–H1C 113.6(12)

^a AlH positions are disordered by symmetry and thus six H positions are found; the H2 orientation is the H most “apical”, the H1 orientation is the most “basal”; ^b AlH distances were refined using distance restraints; ^c The H1A orientation is the most “apical”, and similar to that of H1B, whereas the H1C position is the most basal.

, respectively.

^EtIDip 3 could also be converted to ^EtIDipAlH₃ 11 in good yields using either AlH₃·NMe₃ or LiAlH₄. The synthetic procedure for the latter route is, however, more problematic, because the relatively low solubility of ^EtIDipAlH₃ 11 in aromatic solvents, which is lower than that of IDipAlH₃ for comparison, makes extraction into hydrocarbon solvents to remove lithium-containing insoluble by-products difficult during work-up. Thus, synthesis of 11 via AlH₃·NMe₃ is preferable even though the reaction of 3 with LiAlH₄ provides a high yield of 11. Two molecular structures of ^EtIDipAlH₃ 11 were obtained (Figure 11, Table 2 and Table 3) and the molecular structures of 8–11 will be discussed below.

NMR spectra of complexes 8–10 show the expected features, for example, two doublets and one septet for the protons of the isopropyl substituents (¹H NMR). The ¹³C{¹H} NMR resonances of the NHC-carbene atom were not observed, which is not uncommon [10,11,12,13]. Compound 8 shows a ¹¹B NMR resonance at δ −15.3 ppm, which is similar to that of IDipBBr₃ (δ −16.5 ppm [59]). ^EtIDipAlH₃ 11 shows comparable NMR spectroscopic characteristics with IDipAlH₃, including a very broad resonance around δ 3.57 ppm for the AlH₃ unit. The IR stretch of the Al–H bonds in 11 (1732 cm⁻¹) is virtually unchanged from that of IDipAlH₃ (1729 cm⁻¹ [55]). In the solid state, complex 11 melts at around 218–220 °C and decomposes above ca. 240 °C to a brown oil. We have attempted reduction reactions of complexes 9–11 with very strong s-block metal-based reducing agents but, so far, these largely resulted in the formation of uncoordinated NHC 3 or unreacted group 13 metal complex (9–11), and are described in the experimental section.

For the system IDipAlH₃ plus IDip in deuterated benzene, we previously found only one set of ¹H NMR resonances for the IDip ligand which suggests that some rapid ligand exchange is occurring under the conditions [60]. The lower solubility of a similar mixture of ^EtIDipAlH₃ 11 and ^EtIDip 3 in deuterated benzene makes spectra more difficult to judge, because the sample contained significant quantities of insoluble crystalline 11 and shows some broad resonances but suggested that separate sets of ligand resonances for 11 and 3 were afforded, which may be a sign for different solution behaviour compared with that for IDipAlH₃ plus IDip. Replacing some hydrides with iodides in the IDipAlH₃ plus IDip system showed that cationic normal-abnormal NHC complexes [(IDip)Al(H)X(aIDip)]I (X = H, I), where IDip represents normal (2-position) NHC coordination and aIDip represents the tautomeric abnormal (4-position) NHC coordination. If we consider that the steric profiles around the carbene centres in ^EtIDip 3 and IDip, and their σ-donor properties, are very similar, vide infra, then the different ligand exchange properties may be an indication of different solution state behaviour. It could be possible that abnormal NHC complexes are already accessible in this room temperature equilibrium and partially responsible for the different ligand exchange behaviour, but it should be noted that the normal-abnormal complex formation of the iodinated salts [(IDip)Al(H)X(aIDip)]I (X = H, I) required elevated temperatures (85 °C). Furthermore, a mixture of IDipAlI₃ plus IDip did not show significant solution state ligand exchange (105 °C) nor facile formation of the normal-abnormal complex [(IDip)AlI₂(aIDip)]I at high temperatures [60], which could mean that the AlH units may be involved in the normal/abnormal NHC interconversion mechanism.

We have heated a solution of ^EtIDipAlH₃ 11 in deuterated benzene at 100 °C followed by NMR spectroscopy. Upon cooling for NMR spectroscopic analysis, some 11 rapidly crystallised, which appeared to broaden spectra and concentrated by-products in solution. We could find, however, that under these conditions, 11 was essentially stable for days and only slowly, resonances for small quantities of uncoordinated ^EtIDip 3 appeared. This is in contrast to alane complexes of sterically demanding NHCs with saturated backbone units of various ring sizes (e.g., imidazolinylidenes) that show ring expansion, ring opening and “decarbonisation” reactions of NHC ligands at 90–95 °C [11,34,37]. Bulky IPr*AlH₃ (IPr* = (HCNAr*)₂C:, Ar* = 4-Me-2,6-(Ph₂CH)-C₆H₂) is thermally stable in the solid state (m.p. ca. 230 °C), but shows some lability to vacuum and the onset of decomposition in solution was determined to be 60 °C, with full decomposition occurring after 16 h producing significant quantities of IPr*H₂ [61]. Furthermore, other products, intermediates, and mechanistic pathways for the decomposition of reactive NHC-group 13 complexes have been observed or considered [10,11,12,13,62,63]. Heating 11 for many days, however, eventually led to its decomposition. After nine days, a noticeable proportion of 11 had decomposed, largely to ^EtIDip 3, aluminium metal, and the resonance of dihydrogen was observed in NMR spectra, and after 14 days, almost all 11 had decomposed. However, this does demonstrate the relatively robust nature of alane complex 11, and NHC ligand 3, which did not significantly degrade upon the decomposition of its complex (11).

2.4. Molecular Structures of ^EtIDipEX₃ and ^EtIDipEH₃ Complexes

The molecular structures of ^EtIDipBBr₃ 8, ^EtIDipAlI₃ 9, ^EtIDipGaI₃ 10 (two structures; 10′ and 10″), and ^EtIDipAlH₃ 11 (two structures; 11′ and 11″), see Figure 8, Figure 9, Figure 10 and Figure 11, Table 2 and Table 3, are similar to those reported for the IDip analogues IDipBBr₃ [59], IDipAlI₃ [64], IDipGaI₃ [65], and IDipAlH₃ [55], and provide similar bond distances. All compounds show the group 13 element in a distorted tetrahedral coordination geometry. E–C and E–I bond lengths between 9 and 10 are highly similar.

Considering the orientation of the EX₃ fragments (E = group 13 element, X = halide, hydride, other anionic substituent) in ^EtIDipEX₃ complexes and inspecting similar complexes in the CSD [24], suggests that the positions of the X-substituents relative to the NHC-plane can be broadly categorised as shown in Figure 12; i.e., either showing one X-ligand perpendicular to the NHC-plane (orientation I), one X-ligand in the NHC plane (orientation II) and an in-between case (orientation III), as conformational isomers. Looking across the set of structures for 8–10 featuring heavy halides, they all have the halides in EX₃ fragments oriented in a way that one X substituent is in a “quasi-apical” position perpendicular to the NHC plane, whereas the other two X substituents are in a “quasi-basal” position, broadly near the NHC plane, see Figure 12, orientation I. Other NHCEX₃ complexes reported in the CSD [24] show a variety of orientations and a clear trend is difficult to ascertain.

In the molecular structures of 8–10, the “quasi-apical” X group shows a longer E–X bond and a more acute C–E–X angle compared with those found for the “quasi-basal” E–X bonds (Table 2 and Table 3). A fine balance of several effects will likely contribute as reasons for favouring some orientations over others and the observed orientation is likely a result of only small energetic differences between various causes. The orientation will be influenced by steric effects between ligand and X-substituents, dispersion [66,67], crystal packing effects, nature of the E–X bond (including bond length and strength, orbital overlap, E–X electronegativity difference), and other orbital effects, for example possible C–EX₃ π-orbital overlap. To judge the steric effects, a space-filling model (using van der Waals spheres [68]) has been generated for solid-state ^EtIDipAlI₃ 9 (and 10 shows virtually the same dimensions), Figure 13, which suggests that ligand bulk may influence both Al–I bond lengths, C–Al–I angles, and AlI₃ orientation via distortion.

We furthermore suggest that other effects play a role in the structure and EX₃ orientation of these complexes that can help explain the trends in the E–X bond lengths and C–E–X angles. In relation to well-known hyperconjugation [69], a σ-orbital forming the E–X_apical bond can form π-overlap with the not fully saturated π-interaction around the orthogonal carbene p-orbital, see Figure 14 for simplified drawings. In relation to this, Frenking, Buchner and co-workers recently described π-backbonding from the BeBr₂ σ-electron density to the NHC carbene ligand in NHC-Be complexes, i.e., from the Be–Br bonds that are arranged perpendicular to the NHC plane [70]. Whether orbital interactions are the main driver of the distortions found in orientation I, or the consequence of dominating steric and packing effects, is difficult to judge without quantifying all influences, and will likely involve only small differences in energy from the various effects. Complexes that show orientation II, and indeed orientation III, in the solid state, may show the same type of orbital interactions as part of a different balance of effects that governs EX₃ orientations. In orientation II, orbital overlap with the carbene π-system by the out-of-plane substituent bonds is expected to be less favourable and weaker compared with that of the quasi-apical substituent bond in orientation I, but the former allows two of those interactions. Orientation II is likely disfavoured for a sterically unsuitable in-plane substituent.

The hydride positions in ^EtIDipAlH₃ 11 located by X-ray diffraction are naturally much less accurately determined compared to the halide position in 8–10, and thus should not be overinterpreted. The two structures (11′, 11″) appear to show hydride orientations between orientations I and III and the same general trends on bond lengths and angles seem to be present but are not significant enough within appropriate standard deviations.

2.5. Comparison of the Electronic Properties of ^EtIDip 3 and IDip

The various electronic effects of the σ-donating and π-accepting properties, plus influences from sterics (repulsion, but also attraction via dispersion) in carbene ligands as part of complexes are difficult to determine separately, and typically, an overall effect is measured through spectroscopic study of carbenes and their derivatives [19]. Considering that IDip has been widely studied and that IDip and ^EtIDip 3 have a similar steric environment around the carbene centre, we wanted to draw direct comparisons between the electronic properties of both imidazolylidenes. Where spectroscopic values from the literature are taken, it should be remembered that some variations in environments, including solvent choice, concentration, temperature, counter ions, spectrometer setup etc can have an influence on the values [19]. We have determined the ¹J(¹³C-¹H) NMR coupling constant of [^EtIDipH]Cl 2 in deuterated DMSO (223.89 Hz) and have found that this value is very close to that reported for [IDipH]Cl in the same solvent (223.68 Hz) [71,72]. Similarly, the ¹³C{¹H} NMR resonances of the free carbene species in deuterated benzene (δ 217.4 ppm for 3, δ 220.6 ppm for IDip [23]) are highly similar. Comparing the Al–H IR stretch of ^EtIDipAlH₃ 11 (1732 cm⁻¹) with that of IDipAlH₃ (1729 cm⁻¹) [55] shows them to be identical within the error margins of such experiments on different spectrometers. The ¹⁹F NMR resonances for ^EtIDipF₂ 5 (δ −34.1 ppm) and IDipF₂ (δ −36.5 ppm) in deuterated chloroform are close but not identical. These data suggest that the σ-donating abilities of 3 and IDip are highly similar. Comparing the ⁷⁷Se NMR chemical shifts [50,51,52,53] of in-situ prepared ^EtIDipSe 7 (δ 134.0 ppm) and IDipSe (109.8 ppm) under the same conditions in deuterated benzene shows that the resonance for 7 is ca. 24 ppm downfield-shifted compared with that of IDipSe. This may suggest a moderately more π-accepting character for ^EtIDip 3 compared with IDip. The reasons for these small electronic trend directions of σ-donating and π-accepting properties between 3 and IDip are not entirely clear, but it is worth reiterating that overall trends are measured. A higher π-acidity of 3 compared with IDip would help favour an orbital interaction as described in Figure 14. Aside from electronic properties, our experience with ^EtIDip 3 and its complexes so far suggests on average a slightly lower solubility in hydrocarbon solvents compared with those of IDip. The introduction of the two ethyl groups in 3 does provide a robust and sterically demanding NHC alternative to IDip that should suppress many side reactions, especially those that involve abnormal NHC derivatives.

3. Materials and Methods

3.1. Experimental Details

All manipulations were carried out using standard Schlenk-line and glove box techniques under a dry argon or dinitrogen atmosphere unless described below (for organic condensation reactions and aqueous organic workup steps). Benzene, toluene, diethyl ether, THF, n-hexane and n-pentane were either dried and distilled under inert gas over LiAlH₄, sodium or potassium or taken from an MBraun solvent purification system and degassed prior to use. ¹H, ¹³C{¹H}, ¹⁹F{¹H}, and ⁷⁷Se{¹H}-NMR spectra were recorded on a Bruker AV 300, Bruker AVII 400 or Bruker AV III 500 spectrometer (Bruker UK, Coventry, United Kingdom, manufactured in Germany) in deuterated chloroform or deuterated benzene and were referenced to the residual ¹H or ¹³C{¹H} resonances of the solvent used, respectively. The ¹⁹F{¹H} NMR spectra were referenced to external CCl₃F in CDCl₃ (δ_F = 0.00 ppm) and the ⁷⁷Se{¹H} NMR spectra were referenced to external Ph₂Se₂ in CDCl₃ (δ_Se = 463 ppm). Chemical shifts are given in ppm. Abbreviations: s = singlet, d = doublet, t = triplet, q = quartet, sept = septet, br = broad, vbr = very broad resonance, m = multiplet. IR spectra were obtained on a Shimadzu IR Affinity spectrometer with ATR attachment. Melting points were determined using a Gallenkamp melting point apparatus in sealed glass capillaries under argon and were uncorrected. Yields or conversions in solution were determined by integration of ¹H NMR spectra against an internal standard (such as hexamethylbenzene). “GaI” and “GaI₂” [57], AlH₃∙NMe₃ [73], Na/NaCl (5% w/w) and K/KI (5% w/w) [74], KC₈ [75], and [{(^Mesnacnac)Mg}₂] [76], were prepared as described in the literature. All other reagents were used as received.

3.2. Syntheses and Characterisation of ^EtIDip 3

3.2.1. Synthesis of trans-3,4-Bis(2,6-diisopropylphenylimino)hexane, (DipNCEt)₂, 1

In air, pTsOH∙H₂O (4.77 g, 25.1 mmol, 0.2 equiv.), toluene (ca. 400 mL), 2,6-diisopropylaniline (49.8 g, 53.0 mL, 281 mmol, 2.2 equiv.), and hexane-3,4-dione (14.3 g, 15.0 mL, 125 mmol, 1.0 equiv.) were combined in a round bottom flask. The flask was fitted with a Dean-Stark trap, condenser, topped with a nitrogen gas inlet, and the apparatus flushed with nitrogen gas for one minute. Under nitrogen, the reaction mixture was first heated to 100 °C and stirred at this temperature for one night. It was then heated to reflux (ca. 125 °C) and stirred at the temperature for 7 to 9 days. Subsequently, most of the solvent was distilled off via the Dean-Stark trap, and the remaining solvent was removed in vacuo leaving a brown residue. The residue was then dissolved in dichloromethane (150 mL) and vigorously stirred with a saturated solution of K₂CO₃ (150 mL) for one hour. The organic layer was separated, and the aqueous phase was further extracted with dichloromethane (1 × 30 mL). The organic phases were combined, and all volatiles were removed in vacuo leaving a brown oil. The residue was crystallised from ice-cold methanol (200 mL) at −40 °C over three days and afforded a bright yellow crystalline solid of 1 which was dried in vacuo. Additional crops of 1 were obtained from the concentration of the resultant supernatant solution and subsequent storage at −40 °C. The crystalline solid was dried under a vacuum. Yield = 41.1 g (76%). ¹H NMR (400.3 MHz, CDCl₃, 295 K) δ = 1.08 (t, J_HH = 7.6 Hz, 6H, CH₂CH₃), 1.16 (d, J_HH = 6.8 Hz, 12H, CH(CH₃)₂), 1.23 (d, 12H, J_HH = 6.9 Hz, CH(CH₃)₂), 2.56 (q, J_HH = 7.6 Hz, 4H, CH₂CH₃), 2.78 (sept, J_HH = 6.8 Hz, 4H, CH(CH₃)₂), 7.09–7.17 (m, 6H, ArH); ¹³C{¹H} NMR (100.7 MHz, CDCl₃, 296 K): δ = 10.4 (CH₂CH₃), 22.5 (CH(CH₃)₂), 23.0 (CH₂CH₃), 23.5 (CH(CH₃)₂), 28.5 (CH(CH₃)₂), 122.9 (ipso-ArC), 123.7 (m-ArC), 135.3 (o-ArC), 145.9 (p-ArC), 171.8 (C=N); M.p.: 66–67 °C (melts to a yellow oil). IR (ATR), ν~/cm⁻¹: 2961 m, 2958 w, 2820 w, 1703 m, 1626 m, 1456 m, 1398 w, 905 m, 750 s.

3.2.2. Synthesis of [^EtIDipH]Cl 2

To a Schlenk flask containing a stirring yellow solution of 1 (10.0 g, 23.1 mmol, 1 equiv.) in ethyl acetate (ca. 90 mL), dry paraformaldehyde (867 mg, 28.9 mmol, 1.25 equiv.) was added using a solid addition flask. To the stirring suspension, chlorotrimethylsilane (3.60 mL, 28.4 mmol, 1.2 equiv.) was added dropwise over 30 min. The reaction mixture was then stirred at room temperature for two days, and a colour change from yellow to red was observed within two hours. The orange suspension was allowed to settle, leaving a dark brownish precipitate, which was filtered, washed with diethyl ether (3 × 20 mL) and dried in vacuo affording the off-white crude product of 2 (3.91 g, 35%). A slightly improved yield (4.23 g, 38%) was obtained in one case when heating the reaction mixture under reflux for two days. ¹H NMR (400.1 MHz, CDCl₃, 294 K) δ = 1.10 (t, J_HH = 7.6 Hz, 6H, CH₂CH₃), 1.30 (d, J_HH = 3.7 Hz, 12H, CH(CH₃)₂), 1.32 (d, J_HH = 3.7 Hz, 12H, CH(CH₃)₂), 2.28 (sept, J_HH = 6.7 Hz, 2H, CH(CH₃)₂), 2.50 (q, J_HH = 7.6 Hz, 4H, CH₂CH₃), 7.35 (d, J_HH = 7.8 Hz, 4H, m-ArH), 7.56 (t, J_HH = 7.8 Hz, 2H, p-ArH), 11.51 (s, 1H, N₂CH); ¹³C{¹H} NMR (100.6 MHz, CDCl₃, 295 K): δ = 13.5 (CH₂CH₃), 16.8 (CH(CH₃)₂), 22.9 (CH(CH₃)₂, 26.2 (CH(CH₃)₂), 29.5 (CH₂CH₃), 124.9 (m-ArC), 128.0 (ipso-ArC), 132.2 (p-ArC), 133.4 (C=C), 139.3 (N₂CH), 145.5 (o-ArC); M.p.: >300 °C (decomp. to a dark-brown oil). IR (ATR), ν~/cm⁻¹: 2963 m, 2938 w, 2872 w, 2405 m, 1535 s, 1464 s, 1142 s, 1063 s, 806 m.

3.2.3. Synthesis of ^EtIDip (^EtIPr) 3

To a Schlenk flask containing [^EtIDipH]Cl 2 (3.37 g, 7.00 mmol, 1 equiv.) and KOtBu (943 mg, 8.40 mmol, 1.2 equiv.) was added THF at 0 °C. The resulting brownish solution was warmed to room temperature and stirred overnight. The reaction mixture was subsequently allowed to settle and filtered. All volatiles from the THF filtrate were removed in vacuo and the orange-brownish residue was extracted with warm (ca. 35 °C) n-hexane (ca. 30 mL). Large colourless crystals were isolated from the above extracts and analysed by NMR spectroscopy to be the pure product 3. Yield: 2.36 g (76%); ¹H NMR (400.1 MHz, C₆D₆, 295 K): δ = 0.84 (t, J_HH = 7.5 Hz, 6H, CH₂CH₃), 1.25 (d, J_HH = 7.0 Hz, 12H, CH(CH₃)₂), 1.35 (d, J_HH = 6.9 Hz, 12H, CH(CH₃)₂), 2.23 (q, J_HH = 7.5 Hz, 4H, CH₂CH₃), 2.90 (sept, J_HH = 6.9 Hz, 4H, CH(CH₃)₂), 7.22 (d, J_HH = 7.4 Hz, 4H, m-ArH), 7.32 (t, J_HH = 7.4 Hz, 2H, p-ArH); ¹³C{¹H} NMR (100.6 MHz, C₆D₆, 295 K): δ = 14.5 (CH₂CH₃), 17.6 (CH₂CH₃), 22.7 (CH(CH₃)₂), 26.1 (CH(CH₃)₂), 29.0 (CH(CH₃)₂), 123.7 (m-ArC), 129.0 (p-ArC), 129.9 (C=C), 137.4 (ipso-ArC), 146.9 (o-ArC), 217.4 (N₂C); M.p.: 173–174 °C (melts). IR (ATR), ν~/cm⁻¹: 2961 m, 2915 w, 2870 w, 2392 m, 1695 s, 1486 s, 805 s.

3.3. Oxidation Reactions of ^EtIDip 3

3.3.1. Synthesis of [^EtIDipCl]Cl 4

To a Schlenk flask containing a mixture of ^EtIDip 3 (200 mg, 451 μmol, 1 equiv.), 1,1,1,2,2,2-hexachloroethane (107 mg, 451 μmol, 1 equiv.) was added to THF (20 mL) slowly at 0 °C. The reaction mixture was warmed to room temperature and stirred vigorously overnight (ca. 16 h). The formation of a brownish solution and an off-white precipitate was observed. The precipitate was filtered and dried under vacuum yielding a crude product of 4. Colourless crystals suitable for single crystal X-ray diffraction analysis were obtained from the THF filtrate at −40 °C overnight and dried under vacuum. Yield: 0.190 g (82%). ¹H NMR (400.3 MHz, C₆D₆, 298 K): δ 0.91 (t, J_HH = 7.7 Hz, 6H, CH₂CH₃). 1.12 (d, J_HH = 6.8 Hz, 12H, CH(CH₃)₂), 1.34 (d, J_HH = 6.6 Hz, 12H, CH(CH₃)₂), 3.07 (q, J_HH = 7.7 Hz, 4H, CH₂CH₃), 3.15 (sept, J_HH = 6.6 Hz, 4H, CH(CH₃)₂), 7.02 (d, J_HH = 7.6 Hz, 4H, m-CH), 7.13 (t, J_HH = 8.3 Hz, 2H, p-CH); ¹H NMR (CDCl₃, 400.1 MHz, 294 K): δ = 1.14 (d, J_HH = 7.0, CH(CH₃)₂), 1.17 (t, J_HH = 7.6, CH₂CH₃), 1.32 (d, J_HH = 6.7 Hz, 12H, CH(CH₃)₂), 2.20 (sept, J_HH = 6.7 Hz, 4H, CH(CH₃)₂), 2.69 (q, J_HH = 7.6 Hz, 4H, CH₂CH₃), 7.44 (d, J_HH = 7.9 Hz, 4H, m-CH), 7.69 (t, J_HH = 7.9 Hz, 2H, p-CH); ¹³C{¹H} NMR (100.6 MHz, CDCl₃, 295 K) δ = 13.4 (CH₂CH₃), 17.8 (CH₂CH₃), 23.6 (CH(CH₃)₂), 24.6 (CH(CH₃)₂), 29.4 (CH(CH₃)₂), 125.89 (NC=CN), 126.0 (m-ArC), 126.2 (p-ArC), 133.4 (ipso-ArC), 136.1 (o-ArC), 145.4 (C-Cl). M.p.: 247–249 °C (melts). IR (ATR), ν~/cm⁻¹: 2963 w, 2932 w, 2868 w, 1732 m (Al-H), 1450 m, 1366m, 787 s, 750 s.

3.3.2. Synthesis of ^EtIDipF₂ 5 and ^EtIDipO 6

To a J. Young flask containing a mixture of [^EtIDipCl]Cl 4 (150 mg, 290 μmol, 1 equiv.) and dried CsF (176 mg, 1.16 mmol, 4 equiv.) was added acetonitrile (20 mL) at room temperature. The reaction mixture was heated at 60 °C and vigorously stirred overnight (ca. 16 h). Subsequently, the reaction mixture was cooled to room temperature and concentrated to ca. 10 mL in vacuo, and toluene (5 mL) was added. The resulting solution was filtered, and the filtrate was concentrated to ca. 7 mL under reduced pressure and stored at −4 °C yielding a crop of colourless crystals of 5. Yield: 57.4 mg (41%). The second crop of colourless crystals obtained from the concentrated mother liquor at −4 °C was analysed to be the ^EtIDipO 6. Yield: 72.0 mg (54%). Toluene can be used as an alternative solvent in the above reaction.

Data for ^EtIDipF₂ 5: ¹H NMR (400.3 MHz, C₆D₆, 298 K): δ 0.90 (t, J_HH = 7.4 Hz, 6H, CH₂CH₃), 1.23 (d, J_HH = 6.9 Hz, 12H, CH(CH₃)₂), 1.39 (d, J_HH = 6.7 Hz, 12H, CH(CH₃)₂), 1.95 (q, J_HH = 7.4 Hz, 4H, CH₂CH₃), 3.50 (sept, J_HH = 6.8 Hz, 4H, CH(CH₃)₂), 7.10–7.15 (m, 4H, m-CH), 7.19–7.32 (t, J_HH = 8.3, 7.1 Hz, 2H, p-CH); ¹H NMR (CDCl₃, 400.3 MHz, 298 K): δ 1.03 (br, 6H, CH₂CH₃), 1.24 (d, J_HH = 6.8 Hz, 12H, CH(CH₃)₂), 1.27 (d, br, 12H, CH(CH₃)₂), 2.09 (vbr, 4H, CH₂CH₃), 3.24 (vbr, 4H, CH(CH₃)₂), 7.12–7.48 (m, br, 6H, ArH); ¹³C{¹H} NMR (100.7 MHz, C₆D₆, 295 K): δ = 14.1 (CH₂CH₃), 16.8 (CH₂CH₃), 24.2 (CH(CH₃)₂), 25.4 (CH(CH₃)₂), 28.4 (CH(CH₃)₂), 120.5 (NC=CN), 124.6 (m-ArC), 126.6 and/or 131.5 (part of the CF₂ triplet?), 129.6 (p-ArC), 130.3 (o-ArC), 151.4 (ipso-ArC); ¹⁹F NMR (376.1 MHz, C₆D₆, 295 K): δ = −34.0 ppm; ¹⁹F NMR (470.4 MHz, CDCl₃, 298 K): δ = −34.1 ppm. Note: NMR spectra of 5 show sharp resonances in deuterated benzene but some relatively broad resonances in deuterated chloroform. We are not confident in assigning the triplet for the CF₂ group in the ¹³C{¹H} NMR spectrum and part of this resonance may be overlapping with the very strong solvent signal of benzene. Thus, some resonances are only tentatively assigned. The spectra for 5 do look broadly similar to those of IDipF₂ [47]. M.p.: 176–178 °C (melts to a brown oil). IR (ATR), ν~/cm⁻¹: 2965 w, 2910 w, 2870 w, 2358 m, 1458 s, 1375 s, 1250 s, 821 s.

Data for ^EtIDipO 6: ¹H NMR (400.1 MHz, C₆D₆, 295 K): δ 0.81 (t, J_HH = 7.4 Hz, 6H, CH₂CH₃), 1.26 (d, J_HH = 6.9 Hz, 12H, CH(CH₃)₂), 1.40 (d, J_HH = 6.8 Hz, 12H, CH(CH₃)₂), 2.08 (q, J_HH = 7.4 Hz, 4H, CH₂CH₃), 3.10 (sept, J_HH = 6.8 Hz, 4H, CH(CH₃)₂), 7.18 (d, 4H, m-CH), 7.29 (t, J_HH = 8.3, 7.1 Hz, 2H, p-CH), ¹³C{¹H} NMR (100.6 MHz, C₆D₆, 295 K): δ = 14.5 (CH₂CH₃), 17.0 (CH₂CH₃), 22.6 (CH(CH₃)₂), 25.4 (CH(CH₃)₂), 29.0 (CH(CH₃)₂), 120.4 (NC=CN), 123.7 (m-ArC), 129.4 (p-ArC), 131.1 (ipso-ArC), 148.2 (o-ArC), 152.5 (C=O). M.p.: 226–227 °C (melts). IR (ATR), ν~/cm⁻¹: 2962 w, 2927 w, 2868 w, 1684 s, 1464 s, 1389 s, 800 s, 752 s.

3.3.3. Synthesis of ^EtIDipSe 7

To a J. Young NMR tube containing ^EtIDip 3 (15.5 mg, 34.9 μmol, 1 equiv.) in C₆D₆ (0.6 mL) was added a slight excess of elemental selenium (ca. 4 mg, 51 μmol). The reaction mixture was shaken to form a solution. Full conversion of 3 to 7 was observed at room temperature within 30 min according to ¹H NMR spectroscopy. ¹H NMR (400.1 MHz, C₆D₆, 294 K): δ = 0.81 (t, J_HH = 7.6 Hz, 3H, CH₂CH₃), 1.18 (d, J_HH = 6.9 Hz, 6H, CH(CH₃)₂), 1.62 (d, J_HH = 6.8 Hz, 6H, CH(CH₃)₂), 2.10 (q, J_HH = 7.6 Hz, 2H, CH₂CH₃), 2.79 (sept, J_HH = 6.8 Hz, 2H, CH(CH₃)₂), 7.16 (d, J_HH = 7.3 Hz, 4H, m-ArH), 7.27 (t, J_HH = 7.3 Hz, 2H, p-ArH); ¹³C{¹H} NMR (100.6 MHz, C₆D₆, 295 K): δ = 13.8 (CH₂CH₃), 18.27 (CH₂CH₃), 23.9 (CH(CH₃)₂), 25.2 (CH(CH₃)₂), 29.5 (CH(CH₃)₂), 124.6 (m-ArC), 130.2 (p-ArC), 130.2 (C=C), 133.4 (ipso-ArC), 147.2 (o-ArC), 161.3 (C=Se); ⁷⁷Se{¹H} NMR (57.3 MHz, C₆D₆, 298 K): δ_Se = 134.0 ppm.

IDipSe was prepared in situ from IDip in an analogous manner for comparison. ¹H NMR (400.1 MHz, C₆D₆, 294 K): δ = 1.17 (d, J_HH = 6.9 Hz, 12H, CH(CH₃)₂), 1.51 (d, J_HH = 6.8 Hz, 6H, CH(CH₃)₂), 2.90 (sept, J_HH = 6.9 Hz, 4H, CH(CH₃)₂), 6.34 (s, 2H, CH=CH), 7.19 (m, 4H, m-ArH), 7.28 (m, 2H, p-ArH); ⁷⁷Se{¹H} NMR (57.3 MHz, C₆D₆, 298 K): δ_Se = 109.8 ppm.

3.4. Group 13 Element Complexes of ^EtIDip 3

3.4.1. Synthesis of ^EtIDipBBr₃ 8

Method 1: To a J. Young NMR tube containing a brownish solution of ^EtIDip 3 (28.3 mg, 63.8 μmol, 1 equiv.) was added BBr₃∙SMe₂ (20.0 mg, 63.8 μmol,1 equiv.) in C₆D₆ (0.6 mL). No colour change was observed. Full conversion of 3 was observed at the first point of analysis (ca. 30 min after the starting materials were mixed in the NMR tube). Colourless crystals formed rapidly when the reaction mixture was stored at ambient temperature for 20 min, which were analysed by single crystal X-ray diffraction to be compound 8.

Method 2: To a Schlenk flask containing a solution of ^EtIDip 3 (300 mg, 676 μmol, 1 equiv.) in toluene (20 mL) was added BBr₃ (64.1 μL, 676 μmol, 1 equiv.) slowly at ca. 0 °C. The resulting brownish solution was warmed to room temperature and stirred for two hours. The reaction mixture was concentrated to ca. 10 mL and n-hexane (ca. 5 mL) was added, forming a white precipitate. This was filtered off and dried in vacuo to yield product 8. Further crops were obtained from the toluene/hexane filtrate at −40 °C. Yield: 0.352 g (75%); ¹H NMR (400.3 MHz, C₆D₆, 296 K): δ = 0.87 (t, J_HH = 7.6 Hz, 6H, CH₂CH₃), 1.06 (d, J_HH = 6.8 Hz, 12H, CH(CH₃)₂), 1.63 (d, J_HH = 6.6 Hz, 12H, CH(CH₃)₂), 2.12 (q, J_HH = 7.5 Hz, 4H, CH₂CH₃), 2.83 (sept, J_HH = 6.7 Hz, 4H, CH(CH₃)₂), 7.18 (d, J_HH = 7.8 Hz, 4H, m-ArH), 7.34 (t, J_HH = 7.7 Hz, 2H, p-ArH). ¹¹B NMR (96.3 MHz, C₆D₆, 298 K): δ_B = −15.3 ppm. ¹³C{¹H} NMR (100.7 MHz, C₆D₆, 297 K): δ = 11.4 (CH₂CH₃), 17.6 (CH₂CH₃), 25.1 (CH(CH₃)₂), 25.2 (CH(CH₃)₂), 29.2 (CH(CH₃)₂), 125.1 (m-Ar), 131.1 (p-Ar), 133.3 (NC=CN), 134.5 (ipso-Ar), 146.7 (o-Ar), (C-B) not observed; M.p.: >278 °C (decomp. from white to grey). IR (ATR), ν~/cm⁻¹: 2988 w, 2872 w, 1454 s, 1196 s, 837 s, 600 s.

3.4.2. Synthesis of ^EtIDipAlI₃ 9

To a Schlenk flask containing a mixture of ^EtIDip 3 (250 mg, 563 μmol, 1 equiv.) and AlI₃ (250 mg, 563 μmol, 1 equiv.) was added toluene (20 mL) at 0 °C. The reaction mixture was slowly warmed to room temperature and stirred overnight (ca. 16 h) resulting in a light-yellow solution. The reaction mixture was concentrated to ca. 10 mL in vacuo and n-hexane (5 mL) was added. The yellowish precipitate was filtered and dried under a vacuum to give the product 9 as an off-white powder. Colourless crystals suitable for single crystal X-ray diffraction were obtained after storing the toluene filtrate at −40 °C overnight. Yield: 0.256 g (53%). ¹H-NMR (400.1 MHz, C₆D₆, 300 K) δ = 0.73 (t, J_HH = 7.6 Hz, 6H, CH₂CH₃), 0.92 (d, J_HH = 6.8 Hz, 12H, CH(CH₃)₂), 1.56 (d, J_HH = 6.7 Hz, 12H, CH(CH₃)₂), 1.97 (q, J_HH = 7.5 Hz, 4H, CH₂CH₃), 2.67 (sept, J_HH = 6.7 Hz, 4H, CH(CH₃)₂), 7.12 (d, J_HH = 7.8 Hz, 4H, m-ArH), 7.25–7.31 (m, 2H, p-ArH), ¹³C{¹H} NMR (100.6 MHz, C₆D₆, 295 K) δ = 10.7 (CH₂CH₃), 17.3 (CH₂CH₃), 24.9 (CH(CH₃)₂), 25.5 (CH(CH₃)₂), 28.4 (CH(CH₃)₂), 125.3 (m-Ar), 131.6 (p-Ar), 131.8 (NC=CN), 135.5 (ipso-Ar), 146.6 (o-Ar), (C-Al) not observed; M.p.: >243 °C (decomposed, colour changed from white to brown). IR (ATR), ν~/cm⁻¹: 2989 w, 2945 w, 2865 w, 2369 m, 1623 s, 1545 s, 1469 s, 803 s.

3.4.3. Attempted Reductions of ^EtIDipAlI₃ 9

Small-scale reduction reactions are described below, although some larger-scale attempts have been made. To a J. Young NMR tube charged with a solution of ^EtIDipAlI₃ 9 (10.0 mg, 11.7 μmol, 1 equiv.) in C₆D₆ (0.6 mL) was added an appropriate amount of reducing agent as described below (a–c). The reaction mixture was shaken and sonicated. The reaction mixtures were stored at room temperature or heated at 60 to 80 °C for up to 5 days. The reaction process was monitored by ¹H NMR spectroscopy.

(a)

^EtIDipAlI₃ 9 (10.0 mg, 11.7 μmol, 1 equiv.) and Na/NaCl (5% w/w, 5.38 mg, 11.7 μmol, 1 equiv.). No reaction occurred at room temperature over two days. The reaction mixture was then heated at 60 °C for 1 h, and the formation of ^EtIDip 3 was observed by ¹H NMR spectroscopy.

(b)

^EtIDipAlI₃ 9 (10.0 mg, 11.7 μmol, 1 equiv.) and KC₈ (4.76 mg, 35.1 μmol, 3 equiv.). No reaction occurred at room temperature over two days. The reaction mixture was then heated at 60 °C for 1 h and the full consumption of ^EtIDipAlI₃ 9 was observed. Colourless crystals of ^EtIDip 3 were obtained after allowing the reaction mixture to stand at room temperature overnight.

(c)

^EtIDipAlI₃ 9 (10.0 mg, 11.7 μmol, 1 equiv.) and [{(^Mesnacnac)Mg}₂] (4.20 mg, 5.90 μmol, 0.5 equiv.). The formation of a white precipitate was observed at room temperature within an hour, likely [{(^Mesnacnac)Mg(µ-I)}₂], plus a mixture of compounds. Colourless crystals suitable for single crystal X-ray diffraction analysis were obtained from the supernatant solution and were shown to be [^EtIDipH]I.

3.4.4. Synthesis of ^EtIDipGaI₃ 10

Method 1. To a Schlenk flask containing a solution of ^EtIDip 3 (250 mg, 563 μmol, 1 equiv.) in toluene (20 mL) was added GaI₃ (250 mg, 555 μmol, 1 equiv.) at 0 °C. The reaction mixture was warmed to room temperature and stirred overnight (ca. 16 h). The resulting orange solution was concentrated to ca. 5 mL under reduced pressure and n-hexane (5 mL) was added. The precipitate was filtered and dried in vacuo to give a fine yellow powder. The solution filtrate was stored at −40 °C affording pure ^EtIDipGaI₃ 10 as colourless crystals. Crystals suitable for single-crystal X-ray diffraction analysis were obtained from benzene. Yield: 0.281 g (56%).

Method 2: To a Schlenk flask containing a solution of ^EtIDip 3 (250 mg, 563 μmol, 1 equiv.) in toluene (20 mL) was added either freshly made “GaI” (112 mg, 563 μmol, 1 equiv.), or “GaI₂” (182 mg, 564 μmol, 1 equiv.) at ca. 0 °C. The reaction mixture was warmed to room temperature and stirred overnight (ca. 16 h). The resulting precipitate of Ga metal was filtered off and the yellowish filtrate was concentrated to ca. 10 mL and stored at −40 °C. This afforded colourless crystals of ^EtIDipGaI₃ 10 in both cases (35% when using “GaI” and 28% when using “GaI₂”) that were analysed by ¹H NMR spectroscopy.

¹H-NMR (400.1 MHz, C₆D₆, 294 K): δ = 0.72 (t, J_HH = 7.6 Hz, 6H, CH₂CH₃), 0.92 (d, J_HH = 6.8 Hz, 12H, CH(CH₃)₂), 1.57 (d, J_HH = 6.7 Hz, 12H, CH(CH₃)₂), 1.96 (q, J_HH = 7.6 Hz, 4H, CH₂CH₃), 2.67 (sept, J_HH = 6.8 Hz, 4H, CH(CH₃)₂), 7.13 (d, J_HH = 7.8 Hz, 4H, m-ArH), 7.22–7.31 (m, 2H, p-ArH). ¹³C{¹H} NMR (101.0 MHz, C₆D₆, 295 K) δ = 11.3 (CH₂CH₃), 17.7 (CH₂CH₃), 25.2 (CH(CH₃)₂), 26.0 (CH(CH₃)₂), 28.9 (CH(CH₃)₂), 125.8 (m-Ar), 132.0 (NC=CN), 135.8 (ipso-Ar), 147.2 (o-Ar), not observed (C-Ga). M.p.: >248 °C (decomp. to a brown oil). IR (ATR), ν~/cm⁻¹: 2965 w, 2930 w, 2868 w, 2361 m, 2330 m, 1514 s, 1454 s, 1383 s, 804 s, 764 s.

3.4.5. Attempted Reductions of ^EtIDipGaI₃ 10

Small-scale reduction reactions are described below, although some larger-scale attempts have been made. To a J. Young NMR tube containing a solution of ^EtIDipGaI₃ 11 (10.0 mg, 11.2 μmol, 1 equiv.) in C₆D₆ (0.6 mL) was added an appropriate reducing agent (a–c). The reaction mixture was shaken and sonicated. The resulting mixture was stored at room temperature or heated at 60 to 80 °C for up to 5 days and monitored at irregular intervals by ¹H NMR spectroscopy. No resonances for significant quantities of a new gallium-containing complex were detected.

A few examples of reaction conditions are given:

(a)

^EtIDipGaI₃ 10 (10.0 mg, 11.2 μmol, 1 equiv.), Na/NaCl (5% w/w, 5.14 mg, 11.2 μmol, 1 equiv.). The formation of a colourless solution and a grey precipitate was observed after storing the mixture at room temperature for two days. Needle-like colourless crystals were analysed by single crystal X-ray diffraction to be compound 10. The formation of ^EtIDip 3 was observed from ¹H NMR spectra when the mixture was heated at 60 °C for three hours.

(b)

^EtIDipGaI₃ 10 (10.0 mg, 11.2 μmol, 1 equiv.), KC₈ (4.54 mg, 33.6 μmol, 3 equiv.). The formation of ^EtIDip 3 was observed from ¹H NMR spectra after leaving the reaction mixture to stand at room temperature for two hours.

(c)

^EtIDipGaI₃ 10 (10.0 mg, 11.2 μmol, 1 equiv.), [{(^Mesnacnac)Mg}₂] (4.02 mg, 5.60 μmol, 0.5 equiv.). The formation of ^EtIDip 3 was observed from ¹H NMR spectra after leaving the reaction mixture to stand at room temperature for one hour.

3.4.6. Synthesis of ^EtIDipAlH₃ 11

Method 1: To a Schlenk flask containing a mixture of ^EtIDip 3 (500 mg, 1.13 mmol, 1 equiv.) and LiAlH₄ (85.5 mg, 2.25 mmol, 2 equiv.) was added diethyl ether (30 mL) in a cold bath (−78 °C). The reaction mixture was warmed to room temperature and stirred vigorously overnight (ca. 16 h) which resulted in the precipitation of a white solid. All volatiles were removed in vacuo and the residue was extracted with warm toluene (2 × 30 mL) and then THF (15 mL). Colourless crystals of 11 suitable for single crystal X-ray diffraction were obtained from the toluene solution at −40 °C overnight. Further crops were obtained from a concentrated THF solution at −40 °C for two days. Yield: 0.41 g (77%).

Method 2: To a Schlenk flask containing a solution of ^EtIDip 3 (200 mg, 451 μmol, 1 equiv.) in toluene (20 mL) was slowly added a solution of AlH₃∙NMe₃ in toluene (0.95 mL of a 0.5 m solution in toluene, 475 μmol, 1.05 equiv.) at ca. −78 °C. The resulting reaction mixture was warmed to room temperature and stirred for two hours. Subsequently, all the volatiles were removed in vacuo, the residue was washed with n-hexane (10 mL) and dried under vacuum affording crude ^EtIDipAlH₃ 11. Another crop of colourless crystals was obtained from the n-hexane wash. Yield: 0.178 g (83%). ¹H NMR (400.1 MHz, C₆D₆, 300 K): δ = 0.81 (t, J_HH = 7.6 Hz, 6H, CH₂CH₃), 1.07 (d, J_HH = 6.8 Hz, 12H, CH(CH₃)₂), 1.56 (d, J_HH = 6.8 Hz, 12H, CH(CH₃)₂), 2.09 (q, J_HH = 7.6 Hz, 4H, CH₂CH₃), 2.61 (sept, J_HH = 6.9 Hz, 4H, CH(CH₃)₂), 3.57 (br s, 3H, AlH₃), 7.13 (d, J_HH = 7.7 Hz, 4H, m-CH), 7.26 (t, J_HH = 7.8 Hz, 2H, p-CH); ¹³C{¹H} NMR (125.7 MHz, C₆D₆, 298 K): δ = 12.5 (CH₂CH₃), 17.3 (CH₂CH₃), 24.4 (CH(CH₃)₂), 24.4 (CH(CH₃)₂), 29.1 (CH(CH₃)₂), 124.5 (m-ArC), 128.4 (p-ArC), 130.7 (NC=CN), 133.1 (ipso-ArC), 146.3 (o-ArC). M.p.: 218–220 °C (melts), >240 °C (decomp. to a brown oil). IR (ATR), ν~/cm⁻¹: 2963 w, 2932 w, 2868 w, 1732 m (Al-H), 1450 m, 1366 m, 787 s, 750 s.

3.4.7. Attempted Reductions of ^EtIDipAlH₃ 11

Small-scale reduction reactions are described below, although some larger-scale attempts have been made. To a J. Young NMR tube charged with a solution of ^EtIDipAlH₃ 11 (10.0 mg, 21.1 μmol, 1 equiv.) in C₆D₆ (0.6 mL) was added an appropriate amount of reducing agent (a–c). The reaction mixture was shaken and sonicated. The reaction mixture was left sitting at room temperature or heated at 60 to 80 °C for up to 5 days. The reaction process was monitored by ¹H NMR spectroscopy.

(a)

^EtIDipAlH₃ 11 (10.0 mg, 21.1 μmol, 1 equiv.) and Na/NaCl (5% w/w, 19.4 mg, 42.2 μmol, 2 equiv.). Resonances of ^EtIDip 3 were observed via ¹H NMR spectroscopy after the reaction mixture was sonicated at room temperature for 5 min.

(b)

^EtIDipAlH₃ 11 (10.0 mg, 21.1 μmol, 1 equiv.) and KC₈ (8.55 mg, 63.3 μmol, 3 equiv.). No reaction occurred at room temperature. The reaction mixture was then heated at 60 °C for 1 h, and, upon cooling, colourless crystals of ^EtIDipAlH₃ 11 formed.

(c)

^EtIDipAlH₃ 11 (10.0 mg, 21.1 μmol, 1 equiv.) and [{(^Mesnacnac)Mg}₂] (7.57 mg, 10.6 μmol, 0.5 equiv.). No reaction occurred at room temperature. The reaction mixture was then heated to 60 °C for 1 h, and the formation of magnesium(II) hydride [{(^MesNacnac)Mg(μ-H)}₂] was observed via ¹H NMR spectroscopy [56]. Two broad peaks with chemical shifts at 1.23 and 1.36 ppm were speculated to originate from the isopropyl hydrogen atoms of the desired dialane [{(^EtIDip)AlH₂}₂], c.f. [{(IDip)AlH₂}₂] [56], but so far, this compound could not be isolated from the mixture in pure form.

3.5. X-Ray Crystallographic Details

Experimental and refinement details of the molecular structure determinations by single crystal X-ray diffraction can be found in the supporting information. CCDC 2407796–2407809 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/structures.

4. Conclusions

In this work, we have presented the synthesis and characterisation of the new stable imidazolylidene ^EtIDip 3 and we suggest that this can serve as a chemically robust alternative to IDip (IPr) with similar steric demand. We have furthermore reported conversion of 3 to the oxidised species [^EtIDipCl]Cl 4, ^EtIDipF₂ 5, ^EtIDipO 6, and ^EtIDipSe 7, and the synthesis and structural characterisation of the NHC-group 13 element adducts ^EtIDipBBr₃, 8 ^EtIDipAlI₃ 9, ^EtIDipGaI₃ 10, and ^EtIDipAlH₃ 11. The alane adduct 11 is thermally stable in solution and in the solid state and indicates that ^EtIDip 3 can serve as a valuable new NHC ligand.