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Communication

Holmium(III) Supermesityl-Imide Complexes Bearing Methylaluminato/Gallato Ligands

by
Dorothea Schädle
1,
Cäcilia Maichle-Mössmer
1,
Karl W. Törnroos
2 and
Reiner Anwander
1,*
1
Institut für Anorganische Chemie, Universität Tübingen, Auf der Morgenstelle 18, 72076 Tübingen, Germany
2
Department of Chemistry, University of Bergen, Allégaten 41, 5007 Bergen, Norway
*
Author to whom correspondence should be addressed.
Inorganics 2015, 3(4), 500-510; https://doi.org/10.3390/inorganics3040500
Submission received: 22 September 2015 / Revised: 28 October 2015 / Accepted: 30 October 2015 / Published: 10 November 2015
(This article belongs to the Special Issue Rare Earth and Actinide Complexes)
Browse Figures
Versions Notes

Abstract

:
Heterobimetallic µ2-imide complexes [Ho(µ2-Nmes*){Al(CH3)4}]2 (1, supermesityl = mes* = C6H2tBu3-2,4,6) and [Ho(µ2-Nmes*){Ga(CH3)4}]2 (2) have been synthesized from homoleptic complexes Ho[M(CH3)4]3 (M = Al, Ga) via deprotonation of H2Nmes* or with K[NH(mes*)] according to a salt metathesis-protonolysis tandem reaction. Single-crystal X-ray diffraction of isostructural complexes [Ho(µ2-Nmes*){M(CH3)4}]2 (M = Al, Ga) revealed asymmetric Ho2N2 metallacycles with very short Ho–N bond lengths and secondary Ho ⋯ arene interactions.

Graphical Abstract

1. Introduction

The emerging field of rare-earth metal imide chemistry has revealed interesting structural motifs [1,2], but studies regarding their fundamental properties and reactivity are lagging behind. Bochkarev and Schumann were the first to report on a tetranuclear ytterbium(III) phenylimide complex obtainable via reduction of azobenzene by ytterbium naphthalenide [3]. Another synthesis strategy was developed by Evans et al., adventitiously identifying [Nd(NPh)(AlMe2)(AlMe4)2]2 (Figure 1, I) via attempted alkylation of [Nd(NHPh)3(KCl)3] with excess AlMe3 [4]. We and others succeeded in the isolation of a series of rare-earth metal complexes (Figure 1, II–III, secondary interactions are not shown), by adopting the strategy of organoaluminum-assisted imide formation [5,6,7,8,9,10]. Similarly, deprotonation of lanthanide anilide complexes was achieved via treatment with butyllithium, affording complexes of type IV [11]. A different synthesis approach has been employed in the reactions of alkyl complexes with amine-boranes to yield Lewis acid (LA)-stabilized imido entities [Ln(NR)(BH3)] (R = H, teraryl) [12,13].
Inorganics 03 00500 g001 1024
Figure 1. Structural motifs of heterobimetallic rare-earth metal imide complexes: I (L = AlMe4−, Ar = Ph), II (L = AlMe3(NHAr)−, Ar = C6H3iPr2-2,6; L = AlMe4−, Ar = C6H2tBu3-2,4,6), III (L* = monoanionic ancillary ligand), IV (M = Na, Li, Ar = C6H3iPr2-2,6).
Figure 1. Structural motifs of heterobimetallic rare-earth metal imide complexes: I (L = AlMe4−, Ar = Ph), II (L = AlMe3(NHAr)−, Ar = C6H3iPr2-2,6; L = AlMe4−, Ar = C6H2tBu3-2,4,6), III (L* = monoanionic ancillary ligand), IV (M = Na, Li, Ar = C6H3iPr2-2,6).
Inorganics 03 00500 g001
In general, complexes of type [(L)Ln(NR)] or [(L)Ln(NR)(AlMe3)] have shown promising usability in a range of areas, including catalytic cyclotrimerization of benzonitrile [14], synthesis of substituted pyridines [15], preparation of Ln/M heterobimetallics [16], hydroelementation reactions [17], and polymerization of dienes [18]. Further, such complexes give access to many different types of new Ln(III) imide complexes comprising alkyl-imide, amide-imide, alkoxide-imide, and cyclopentadienyl-imide compounds [18]. Moreover, derivatization of the Ln=N(R) functionality with small molecules and organic substrates revealed interesting reaction patterns and bonding features [19,20].
To fully investigate the implications of the choice of Ln(III) alkyl precursor for any envisioned synthesis, we also employed rare-earth metal tetramethylgallate complexes. Although the structural parameters of the Ln/M heterobimetalic complexes Ln[M(CH3)4]3 (M = Al, Ga) [21,22,23] and the derived half-sandwich [24], metallocene [25], and scorpionate complexes [10,26,27] are similar, the reactivity, in some cases, is dramatically different [10,26,27]. For example, previous studies from our laboratory revealed that [(TptBu,Me)Ln(CH3)2] (TptBu,Me = hydrotris(3-tert-butyl-5-methylpyrazolyl)borate) and [(TptBu,Me)Ln(CH3){Ga(CH3)4}] gave the Ln(III) anilide complexes [(TptBu,Me)Ln(CH3)(NHR)] (R = alkyl, aryl), as opposed to the aluminum congener [(TptBu,Me)Ln(CH3){Al(CH3)4}], which yields complexes of type III [9,10].
While a series of dimeric LA-stabilized rare-earth metal imides (type II) has been reported previously [7], we now report on additional aniline-derived rare-earth metal imide complexes employing methylaluminate and methylgallate complexes. The intention was to investigate fundamental differences of organogallium versus organoaluminum moieties since GaMe3 should behave as a weaker LA towards nitrogen than AlMe3.

2. Results and Discussion

The µ2-imide complexes [Ln{Al(CH3)4}(µ2-Nmes*)]x (Ln = Y, La, Nd, Lu; mes* = C6H2tBu3-2,4,6) were previously obtained from homoleptic heterobimetallic complexes Ln[Al(CH3)4]3 utilizing two distinct protocols: reaction with 2,4,6-tri-tert-butylaniline in n-hexane via methane elimination or with potassium (2,4,6-tri-tert-butylphenyl)amide in toluene according to a salt metathesis-protonolysis tandem reaction [7]. Similarly, the reaction of Ho[M(CH3)4]3 (M = Al, Ga) with H2Nmes* or K[NH(mes*)], respectively, led to dimeric complexes [Ho{M(CH3)4}(µ2-Nmes*)]2 (M = Al (1), Ga (2)) (Scheme 1). The identities of the holmium imide complexes 1 and 2 were confirmed by elemental analysis and single-crystal X-ray diffraction studies revealing isomorphous dimeric arrangements, featuring a Ho2N2 core on a crystallographic inversion center. The solid-state structures of 1 and 2 are depicted in Figure 2 with selected bond lengths and angles shown in Table 1. The geometry about the four-coordinate metal centers can best be described as distorted tetrahedral with two methyl groups and two imido nitrogen atoms at the four vertices. The rare-earth metal atoms are asymmetrically bridged by two µ2-imido nitrogen atoms, displaying one short (Ho–N, 1: 2.107(1) Å; 2: 2.102(3) Å) and one long contact (Ho–N, 1: 2.283(1) Å; 2: 2.288(3) Å). The Ho–N bond lengths in 1 and 2 are comparable to the distances in yttrium imide complexes, considering the similar ionic radii (Table 2).
Inorganics 03 00500 g003 1024
Scheme 1. Synthesis of holmium(III) imide complexes.
Scheme 1. Synthesis of holmium(III) imide complexes.
Inorganics 03 00500 g003
Inorganics 03 00500 g002 1024
Figure 2. Solid-state structures of [Ho{Al(CH3)4}(µ2-Nmes*)]2 (1) and [Ho{Ga(CH3)4}(µ2-Nmes*)]2 (2) with 30% probability ellipsoids. Hydrogen atoms are omitted for clarity.
Figure 2. Solid-state structures of [Ho{Al(CH3)4}(µ2-Nmes*)]2 (1) and [Ho{Ga(CH3)4}(µ2-Nmes*)]2 (2) with 30% probability ellipsoids. Hydrogen atoms are omitted for clarity.
Inorganics 03 00500 g002
Table
Table 1. Selected bond lengths [Å] and angles [°] of complexes 1 and 2.
Table 1. Selected bond lengths [Å] and angles [°] of complexes 1 and 2.
12
Ho1–N1′2.107(1)Ho1–N1′2.102(3)
Ho1–N12.283(1)Ho1–N12.288(3)
Ho1′–N12.107(1)Ho1′–N12.102(3)
Ho1–C12.512(2)Ho1–C192.512(4)
Ho1–C22.598(2)Ho1–C202.594(5)
Ho1–C52.626(1)Ho1–C12.629(3)
Ho1–C62.715(1)Ho1–C62.750(3)
Ho1–C132.833(1)Ho1–C162.864(3)
Ho1 ⋯ Al13.0838(5)Ho1 ⋯ Ga13.0573(4)
Al1–C12.0836(16)Ga1–C192.115(4)
Al1–C22.0718(16)Ga1–C202.092(4)
Al1–C31.9681(19)Ga1–C211.978(4)
Al1–C41.9727(17)Ga1–C221.974(4)
N1–C51.3811(18)N1–C11.386(4)
N1′–Ho1–N184.63(4)N1′–Ho1–N184.68(11)
Ho1′–N1–Ho195.37(4)Ho1′–N1–Ho195.32(11)
Ho1′–N1–C5175.67(10)Ho1′–N1–C1176.8(2)
Ho1–N1–C587.95(8)Ho1–N1–C187.80(18)
C1–Ho1–C282.76(5)C19–Ho1–C2083.98(14)
C1–Al1–C2108.80(6)C19–Ga1–C20108.63(15)
C3–Al1–C4118.82(8)C21–Ga1–C22117.5(2)
C1–Ho1–C2–Al1−11.34(5)C19–Ho1–C20–Ga114.20(14)
Table
Table 2. Selected Y–N(imido) and Ho–N(imido) bond lengths [Å].
Table 2. Selected Y–N(imido) and Ho–N(imido) bond lengths [Å].
CompoundsLn–NCN aReference
[(TptBu,Me)Y{NC6H3(CH3)2-2,6}(AlMe3)] c2.123(2)–2.128(3)5[10]
[(TptBu,Me)Y{NC6H3(CH3)2-2,6}(HAlMe2)] c2.133(2)5[8]
[(TptBu,Me)Y(NtBu)(AlMe3)] c2.081(3)–2.088(3)5[9]
[(TptBu,Me)Ho(NtBu)(AlMe3)] c2.083(2)–2.084(2)5[9]
[(TptBu,Me)Y(NAd)(AlMe3)] c,d2.092(2)–2.099(2)5[9]
[(TptBu,Me)Ho(NAd)(AlMe3)] c,d2.087(2)–2.090(2)5[9]
[(TptBu,Me)Y{NC6H3(CH3)2-2,6}(DMAP) c,e2.024(4)5[10]
[(C5Me4SiMe3)4Y4(µ3-NCH2CH3)2(µ2-NCHPh)4]2.116(6)–2.418(6)6/7[14]
[L3Y3(µ2-CH3)3(µ3-CH3)(µ3-NR)] f2.308(3)–2.435(7)6[28]
[Y{Al(CH3)4}(µ2-Nmes*)]22.1089(9)–2.2909(9)6[7]
[Ho{Al(CH3)4}(µ2-Nmes*)]22.107(1)–2.283(1)6b
[Ho{Ga(CH3)4}(µ2-Nmes*)]22.102(3)–2.288(3)6b
a CN = coordination number; b this work; c TptBu,Me = hydrotris(3-tert-butyl-5-methylpyrazolyl)borate; d Ad = 1-adamantyl; e DMAP = 4-(dimethylamino)pyridine; f L = [PhC(NC6H3iPr2-2,6)2]−; R = alkyl, aryl.
The pronounced asymmetry of the Ho2N2 core most likely originates from secondary interactions between the holmium centers and the ipso and ortho carbons as well as one CH3 tert-butyl group of the bridging µ2-Nmes* ligands. One distinct difference between [Ho{Al(CH3)4}(µ2-Nmes*)]2 (1) and [Ho{Ga(CH3)4}(µ2-Nmes*)]2 (2) is the bending of the M(CH3)4 moiety being slightly more pronounced for the gallium derivative, since the softer Ga(III) center can achieve shorter Ln(III) ⋯ Ga(III) contacts.
A preliminary reactivity study of yttrium congener [Y{Al(CH3)4}(µ2-Nmes*)]2 [7] was performed in order to assess the feasibility of exchanging the AlMe4− ligand by other ancillary ligands. The reaction of [Y{Al(CH3)4}(µ2-Nmes*)]2 with KCp* (Cp* = C5Me5) in toluene at 80 °C led to the formation of an off-white solid, which is insoluble in n-hexane and toluene. The 1H NMR spectrum of the product indicated the formation of [Cp*Y(Nmes*)]n. However, as shown previously, toluene-soluble imide complexes [(AlMe4)Ln(NC6H3iPr-2,6)(AlMe3)x]2 readily undergo salt-metathesis reactions with a variety of alkaline metal salts [MI(L)] (L = silylamide, cyclopentadienyl, aryloxide) to generate heteroleptic Ln(III) imide complexes [18].

3. Experimental Section

3.1. General Procedures

All operations were performed with rigorous exclusion of air and water, using standard Schlenk, high-vacuum, and glovebox techniques (MBraun MBLab; <1 ppm O2, <1 ppm H2O). Toluene and n-hexane were purified by using Grubbs columns (MBraun SPS, solvent purification system, MBraun, Garching, Germany) and stored in a glovebox; [D6]benzene was obtained from Aldrich (St. Louis, MO, USA), degassed, dried over Na for 24 h, and filtered. Then 2,4,6-tri-tert-butylaniline was obtained from Aldrich and used as received. Potassium (2,4,6-tri-tert-butylphenyl)amide was synthesized according to literature procedures [7]. Homoleptic complexes [Ho{Al(CH3)4}3] [22,29] and [Ho{Ga(CH3)4}3] [30,31] (2) were prepared according to literature methods. DRIFT spectra were recorded on a NICOLET 6700 FTIR spectrometer (Thermo Scientific, Dreieich, Germany) using dried KBr and KBr window. Elemental analyses were performed on an Elementar Vario Micro Cube (Elementar, Hanau, Germany).

3.2. [Ho{M(CH3)4}(µ2-Nmes*)]2 (1 and 2)

3.2.1. Procedure A

A solution of Ho[M(CH3)4]3 in toluene (3 mL) was added to a vigorously stirred suspension of potassium (2,4,6-tri-tert-butylphenyl)amide in toluene (2 mL). The reaction mixture was stirred for 2 h at ambient temperature and the toluene solution then separated by centrifugation, decanted, and filtrated. The solid residue (product and K[M(CH3)4] was extracted with additional toluene (5 × 2 mL). The extract was dried under vacuum and triturated with n-hexane (2 × 2 mL). After that the solid was washed with n-hexane (2 × 2 mL), followed by drying under reduced pressure. Compounds 1 and 2 were obtained as powder or by crystallization from the mother liquor at ambient temperature.

3.2.2. Procedure B

A solution of 2,4,6-tri-tert-butylaniline in n-hexane (3 mL) was added to a solution of Ho[M(CH3)4]3 in n-hexane (2 mL). The reaction mixture was stirred for 8 h at 80 °C. The solution turned orange and a precipitate was formed. The mixture was chilled to ambient temperature, the solid product was separated by centrifugation and washed with n-hexane (2 × 2 mL). The procedure was repeated twice with the combined extracts. Compounds 1 and 2 were dried in vacuo and obtained as orange powder.

3.3. [Ho{Al(CH3)4}(µ2-Nmes*)]2 (1)

3.3.1. Procedure A

Following the procedure described above, Ho[Al(CH3)4]3 (85.3 mg, 0.20 mmol) and potassium(2,4,6-tri-tert-butylphenyl)amide (59.9 mg, 0.20 mmol) yielded [Ho{Al(CH3)4}(µ2-Nmes*)]2 as orange crystals (102.3 mg, 0.10 mmol, ≥99%).

3.3.2. Procedure B

Following the procedure described above, Ho[Al(CH3)4]3 (85.3 mg, 0.20 mmol) and 2,4,6-tri-tert-butylaniline (52.3 mg, 0.20 mmol) yielded [Ho{Al(CH3)4}(µ2-Nmes*)]2 as orange powder (38.4 mg, 0.04 mmol, 40%).

3.3.3. Physical Data of 1

DRIFT IR (KBr): 3014 w, 2962 s, 2869 m, 2773 w, 1588 w, 1477 w, 1462 w, 1394 m, 1377 m, 1361 m, 1340 w, 1274 m, 1244 s, 1227 m, 1200 s, 1108 w, 916 w, 894 w, 874 w, 856 m, 783 w, 755 w, 716 s, 694 vs, 663 w, 638 w, 609 w, 578 m, 547 w, 504 m, 476 w, 458 w, 437 m·cm−1; elemental analysis calcd (%) for C44H82Al2N2Ho2 (1022.96 g·mol−1): C 51.66, H 8.08, N 2.74; found: C 51.76, H 7.84, N 2.74.

3.4. [Ho(GaMe4)(µ2-Nmes*)]2 (2)

3.4.1. Procedure A

Following the procedure described above, Ho[Ga(CH3)4]3 (269.9 mg, 0.47 mmol) and potassium(2,4,6-tri-tert-butylphenyl)amide (139.9 mg, 0.47 mmol) yielded [Ho{Ga(CH3)4}(µ2-Nmes*)]2 as orange crystals (520.1 mg, 0.47 mmol, ≥99%).

3.4.2. Procedure B

Following the procedure described above, Ho[Ga(CH3)4]3 (236.7 mg, 0.43 mmol) and 2,4,6-tri-tert-butylaniline (127.9 mg, 0.43 mmol) yielded [Ho{Ga(CH3)4}(µ2-Nmes*)]2 as orange powder (100.8 mg, 0.09 mmol, 42%).

3.4.3. Physical Data of 2

DRIFT IR (KBr): 2962 vs, 2904 m, 2869 m, 2774 w, 1588 vw, 1477 w, 1460 w, 1394 s, 1378 m, 1360 m, 1274 m, 1246 vs, 1227 m, 1193 m, 1109 w, 915 vw, 875 w, 856 s, 783 w, 755 w, 716 w, 662 vw, 594 w, 556 w, 530 w, 505 m, 476 vw, 428 w·cm−1; elemental analysis calcd for C44H82Ga2N2Ho2 (1108.44 g/mol): C 47.68, H 7.46, N 2.53; found: C 47.91, H 7.42, N 2.52.

3.5. X-Ray Crystallography

Crystal data for compounds 1 and 2 are given in Table 3. Bond lengths and angles are listed in Table 2. Crystals of 1 and 2 were grown using standard techniques from saturated toluene solutions. Suitable single crystals for X-ray structure analyses were selected in a glovebox and coated with Parabar 10312 and fixed on a nylon loop/glass fiber.
Table
Table 3. Crystallographic data for compounds 1 and 2.
Table 3. Crystallographic data for compounds 1 and 2.
12
FormulaC44H82Al2Ho2N2C44H82Ga2Ho2N2
ColorYellowYellow
Mr (g·mol−1)1022.941108.41
Cryst systemMonoclinicTriclinic
Space groupC2/cP 1 ¯
a [Å]24.0888(13)10.1896(4)
b [Å]11.7687(6)11.5565(5)
c [Å]20.4327(11)11.5966(5)
α [°]9065.090(3)
β [°]111.5590(10)83.184(3)
γ [°]9084.962(4)
V [Å3]5387.3(5)1228.69(9)
Z41
F(000)2080556
T [K]103(2)173(2)
ρcalcd (g cm3)1.2611.498
µ(mm−1)2.9744.297
R1 (obsd.) a0.01720.0296
wR2 (all) b0.04660.0661
S c1.0421.082
a R1 = Σ(||F0| − |Fc||)/Σ|F0|, F0 > 4σ(F0); b wR2 = {Σ[w(F02 − Fc2)2/Σ[w(F02)2]}1/2; c S = [Σw(F02 − Fc2)2/(n0 − np)]1/2.
Data for compound 2 were collected on a Stoe IPDS 2T instrument equipped with a fine focus sealed tube and graphite monochromator using MoKα radiation (λ = 0.71073 Å) performing ω scans. Raw data were collected and integrated using Stoe’s X-Area software package [32]. A numerical absorption correction based on crystal shape optimization was applied using Stoe’s X-Red [33] and X-Shape [34]. X-ray data for compound 1 were collected on a Bruker AXS, TXS rotating anode instrument using a Pt135 CCD detector, and graphite monochromated using MoKα radiation (λ = 0.71073 Å), employing ω-scans. Raw data were processed using APEX [35] and SAINT [36], corrections for absorption effects were applied using SADABS [37]. The structure was solved by direct methods and refined against all data by full-matrix least-squares methods on F2 using SHELXTL [38] and ShelXle [39]. All graphics were produced employing ORTEP-3 [40] and POV-Ray [41]. Further details of the refinement and crystallographic data are listed in the CIF files. CCDC 1426090 (1) and 1426091 (2) contain all 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/data_request/cif.

4. Conclusions

Like the homoleptic tetramethylaluminate Ho[Al(CH3)4]3, the respective gallate complex can be converted into imido-bridged complexes [Ho{M(CH3)4}(µ2-Nmes*)]2 (M = Al (1), Ga (2)). Depending on the synthesis protocol, meaning either protonolysis or the salt metathesis-protonolysis tandem reaction, the imide complexes are formed in moderate to excellent yields, respectively. Preliminary experiments on the capability of [Ln{Al(CH3)4}(µ2-Nmes*)]2 to engage in ligand exchange reactions were hampered by solubility issues. Ongoing studies in our group address the feasibility of imide-tetramethylgallate complexes derived from other primary amines/anilines. We have recently shown that switching to the H2Ndipp (dipp = C6H3iPr2-2,6) proligand significantly enhanced the solubility of the isolated bimetallic rare-earth metal imide complexes Ln2(µ2-Ndipp)(µ3-Ndipp)[(µ2-CH3)2Al(CH3)][Al(CH3)4]2, thus allowing for facile derivatization [18].

Supplementary Materials

Supplementary materials can be found at https://www.mdpi.com/2304-6740/3/4/0500/s1.

Acknowledgments

We are grateful to the German Science Foundation for support (Grant: AN 238/15-1).

Author Contributions

All reactions and analyses described were planned and conducted by Dorothea Schädle. Analyses include DRIFT spectroscopy and elemental analysis. Publication writing was done by Dorothea Schädle and Reiner Anwander. The structural analyses by single crystal X-ray diffraction were performed by Cäcilia Maichle-Mössmer and Karl W. Törnroos.

Conflicts of Interest

The authors declare no conflict of interest.

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MDPI and ACS Style

Schädle, D.; Maichle-Mössmer, C.; Törnroos, K.W.; Anwander, R. Holmium(III) Supermesityl-Imide Complexes Bearing Methylaluminato/Gallato Ligands. Inorganics 2015, 3, 500-510. https://doi.org/10.3390/inorganics3040500

AMA Style

Schädle D, Maichle-Mössmer C, Törnroos KW, Anwander R. Holmium(III) Supermesityl-Imide Complexes Bearing Methylaluminato/Gallato Ligands. Inorganics. 2015; 3(4):500-510. https://doi.org/10.3390/inorganics3040500

Chicago/Turabian Style

Schädle, Dorothea, Cäcilia Maichle-Mössmer, Karl W. Törnroos, and Reiner Anwander. 2015. "Holmium(III) Supermesityl-Imide Complexes Bearing Methylaluminato/Gallato Ligands" Inorganics 3, no. 4: 500-510. https://doi.org/10.3390/inorganics3040500

APA Style

Schädle, D., Maichle-Mössmer, C., Törnroos, K. W., & Anwander, R. (2015). Holmium(III) Supermesityl-Imide Complexes Bearing Methylaluminato/Gallato Ligands. Inorganics, 3(4), 500-510. https://doi.org/10.3390/inorganics3040500

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