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Main Group Metal Chemistry

Editor-in-Chief: Jurkschat, Klaus

Editorial Board: Atwood, David / Basu Baul, Tushar S. / Beckmann, Jens / Chandrasekhar, Vadepalli / Izod, Keith / Jones, Cameron / Karlov, Sergey S. / Mehring, Michael / Molloy, Kieran / Naseer, Muhammad Moazzam / Ramasami, Ponnadurai / Ruhlandt-Senge, Karin / Ruzicka, Ales / Saito, Masaichi / Sarazin, Yann / Tokitoh, Norihiro / Wagler, Jörg


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Volume 36, Issue 3-4

Issues

Synthesis and crystal structures of two bulky bis(amido)germylenes

Edwin W.Y. Wong / Terrance J. Hadlington / Cameron Jones
Published Online: 2013-05-01 | DOI: https://doi.org/10.1515/mgmc-2013-0015

Abstract

A new bulky secondary amine, HN(Dip){C(H)Ph2} (Dip=C6H3Pri2-2,6), has been prepared. This and a related amine, HN(Dip)(Mes) (Mes=mesityl), have been utilized in the formation of bulky lithium amides, which when added to a source of GeCl2 have yielded two new, bulky bis(amido)germylenes, [Ge{N(Dip)(Mes)}2] and [Ge{N(Dip)[C(H)Ph2]}2], both of which have been crystallographically characterized and shown to be two-coordinate monomers.

Keywords: bulky amide; germanium; germylene; low coordinate; X-ray crystallography

In recent decades, the chemistry of main group metal compounds has undergone a rapid renaissance. This has stemmed from the realization that such compounds containing the metal center in a low oxidation state and/or a low coordination number can be accessed and that their intrinsically high reactivity can afford them useful applications in synthesis, catalysis, etc. (Fischer and Power, 2010; Asay et al., 2011). The accessibility of almost all low oxidation state/low coordination number main group compounds has relied on the utilization of very bulky ligand systems, which provide kinetic stabilization to those compounds. One of the most widely used ligand classes in this field are anionic N-donor systems of varying denticity. The important examples here include monodentate amides, -NR2 (Lappert et al., 2009), and bidentate b-diketiminates, [(R1NCR2)2CR3]- (Tsai, 2012), and guanidinates/amidinates, [(R1N)2CNR22]-/[(R1N)2CR2]- (Jones, 2010). We have become interested in developing extremely bulky examples of the former, for example, -N(Ar*)(SiR3), (Ar*=C6H2{C(H)Ph2}2Me-2,6,4), which we have utilized to stabilize a variety of low oxidation state p-block metal systems (Li et al., 2011a,b, 2012; Dange et al., 2012). Of most relevance to this study is our prior preparation of the first monomeric amido-germanium(II) halide, [(SiMe3)(Ar*)NGeCl] (Li et al., 2011a,b), which has been reduced to give the only example of an amido-substituted digermyne, [{(SiMe3)(Ar*)NGe}2] (Li et al., 2011a,b). In attempts to prepare less bulky amido-germanium chlorides, LGeCl, to access more reactive examples of amido-digermynes, we instead obtained two new examples of two-coordinate bis(amido)germylenes, the synthesis and X-ray crystal structures of which are reported herein.

At the outset of this study, compounds of the type LGeCl incorporating amide ligands substituted with the Dip group (C6H3Pri2-2,6) were targeted. It was believed that, although such ligands are less sterically encumbered than related Ar*-substituted amides, they should possess sufficient bulk to kinetically stabilize the target chloro-germylenes. The two amides that were employed for this purpose were the known system, -N(Dip)(Mes) (Mes=mesityl) (Zhu et al., 2012), and the new amide, -N(Dip){C(H)Ph2}. The conjugate acid of the latter, HN(Dip){C(H)Ph2} 1, was readily prepared in good yield via treatment of the known imine, DipN=CPh2 (Love et al., 1999) with Bui2AlH, followed by aqueous workup. The lithium salts of both amides were generated in situ and added to a solution of one equivalent of GeCl2·dioxane. In both cases, there was no evidence for the formation of chloro-germylenes, LGeCl; instead, low isolated yields of the bis(amido)germylenes, [Ge{N(Dip)(Mes)}2] 2 and [Ge{N(Dip)[C(H)Ph2]}2] 3 resulted. It can be postulated that these compounds arose from the generation of the expected LGeCl products as intermediates, although these react more readily with excess lithium amide in the reaction mixture than does unreacted GeCl2·dioxane. Subsequently, the intentional preparations of 2 and 3 were attempted, whereby GeCl2·dioxane was treated with two equivalents of the appropriate lithium amide (Scheme 1). These reactions led to a higher yield of 2 (29%), although again compound 3 could only be formed in a very low yield (<5%).

Preparation of compounds 2 and 3 (Dip=C6H3Pri2-2,6; Mes=mesityl).
Scheme 1

Preparation of compounds 2 and 3 (Dip=C6H3Pri2-2,6; Mes=mesityl).

The spectroscopic data for 1 and 2 are fully consistent with their proposed formulations, whereas the very low yield of 3 precluded its full spectroscopic characterization. That said, all three compounds were crystallographically characterized and their molecular structures are depicted in Figures 13 (see also Table 1). The structure of 1 is unexceptional, although it does highlight the steric bulk of the -N(Dip){C(H)Ph2} fragment. The structures of 2 and 3 are similar and reveal them to be monomeric, with two-coordinate Ge centers. In both, the Ge(NC2)2 backbone of the compounds are close to planar, and their Dip substituents take up ‘exo-’ positions on that framework. The Ge-N distances in the compounds are very similar to one another and to those in the seven other structurally characterized examples of bis(amido)germylenes (range, 1.869–1.920 Å; Cambridge Crystallographic Database search, March 2013). Interestingly, the N-Ge-N angles in 2 and 3 are at the upper end of the rather wide known range for similar compounds (88.59–111.51°). This likely reflects the significant steric bulk of their amide ligands. Indeed, the similarly bulky bis(amido)germylene, [Ge{N(Dip)(SiMe3)}2], has the widest reported N-Ge-N angle in the series (Meller et al., 1992).

Thermal ellipsoid plot (30% probability surface) of the molecular structure of 1 (hydrogen atoms, except H(1), omitted). Selected bond lengths (Å) and angles (°): N(1)-C(1) 1.428(3), N(1)-C(13) 1.487(3), C(1)-N(1)-C(13) 117.1(2).
Figure 1

Thermal ellipsoid plot (30% probability surface) of the molecular structure of 1 (hydrogen atoms, except H(1), omitted).

Selected bond lengths (Å) and angles (°): N(1)-C(1) 1.428(3), N(1)-C(13) 1.487(3), C(1)-N(1)-C(13) 117.1(2).

Thermal ellipsoid plot (30% probability surface) of the molecular structure of 2 (hydrogen atoms omitted). Selected bond lengths (Å) and angles (°): Ge(1)-N(1) 1.8878(13), N(1)-C(1) 1.442(2), N(1)-C(10) 1.458(2), N(1)-Ge(1)-N(1)′ 110.83(8), C(1)-N(1)-C(10) 118.10(12), symmetry operation (′)=-x+1, y, -z+3/2.
Figure 2

Thermal ellipsoid plot (30% probability surface) of the molecular structure of 2 (hydrogen atoms omitted).

Selected bond lengths (Å) and angles (°): Ge(1)-N(1) 1.8878(13), N(1)-C(1) 1.442(2), N(1)-C(10) 1.458(2), N(1)-Ge(1)-N(1)′ 110.83(8), C(1)-N(1)-C(10) 118.10(12), symmetry operation (′)=-x+1, y, -z+3/2.

Thermal ellipsoid plot (30% probability surface) of the molecular structure of 3 (hydrogen atoms omitted). Selected bond lengths (Å) and angles (°): Ge(1)-N(1) 1.8643(15), Ge(1)-N(2) 1.8695(15), N(1)-C(1) 1.452(2), N(1)-C(38) 1.491(2), N(2)-C(13) 1.451(2), N(2)-C(25) 1.496(2), N(1)-Ge(1)-N(2) 109.69(7), C(1)-N(1)-C(38) 118.01(14), C(13)-N(2)-C(25) 117.55(14).
Figure 3

Thermal ellipsoid plot (30% probability surface) of the molecular structure of 3 (hydrogen atoms omitted).

Selected bond lengths (Å) and angles (°): Ge(1)-N(1) 1.8643(15), Ge(1)-N(2) 1.8695(15), N(1)-C(1) 1.452(2), N(1)-C(38) 1.491(2), N(2)-C(13) 1.451(2), N(2)-C(25) 1.496(2), N(1)-Ge(1)-N(2) 109.69(7), C(1)-N(1)-C(38) 118.01(14), C(13)-N(2)-C(25) 117.55(14).

Table 1

Summary of crystallographic data for 13.

In summary, a new bulky secondary amine has been prepared. This and a related known amine have been utilized in the in situ formation of bulky lithium amides, which when added to a source of GeCl2 have yielded two new, bulky bis(amido)germylenes, both of which have been crystallographically characterized. We continue to develop the application of bulky amides to the stabilization of low oxidation state/low coordination number main group compounds and will report on our efforts in this direction in due course.

Experimental

General

All manipulations were carried out using standard Schlenk and glove box techniques under an atmosphere of high-purity dinitrogen. Tetrahydrofuran (THF), hexane, and toluene were distilled over potassium. 1H and 13C{1H} nuclear magnetic resonance (NMR) spectra were recorded on Bruker Avance III 400 or Varian Inova 500 spectrometers and were referenced to the resonances of the solvent used. Fourier transform infrared spectra were recorded using an Agilent Cary 630 attenuated total reflectance (ATR) spectrometer. The melting points were determined in sealed glass capillaries under dinitrogen and are uncorrected. The compounds HN(Dip)(Mes) (Zhu et al., 2012) and DipN=CPh2 (Love et al., 1999) were prepared by literature procedures. All other reagents were used as received from the Aldrich Chemical Company, St Louis, MO, USA.

Preparation of HN(Dip){C(H)Ph2} 1

Bui2AlH (13.06 mL of a 1.7 m solution in toluene, 22.2 mmol) was added to a solution of DipN=CPh2 (3.79 g, 11.1 mmol) in toluene (100 mL) and the mixture was heated at reflux for 4.5 h. After cooling to room temperature, the reaction mixture was quenched with water (100 mL) and extracted with ethyl acetate. The extract was washed with water and brine, dried over Na2SO4, and concentrated under reduced pressure to give a pale yellow oil. The product was rigorously dried by stirring in anhydrous hexane over CaH2 for 1 h. This mixture was filtered and the filtrate was concentrated under reduced pressure to give a pale yellow oil, which crystallized on standing (2.40 g, 62%). M.p. 77–80°C; 1H NMR (400 MHz, 298 K, C6D6): δ 1.03 (d, J=6.8 Hz, 12H, CH(CH3)2), 2.94 (sept, J=6.8 Hz, 2H, CH(CH3)2), 3.62 (br d, J=8.9 Hz, 1H, NH), 5.21 (br d, J=8.7 Hz, 1H, CH(C6H5)2), 7.02–7.11 (m, 9H, ArH), 7.22–7.25 (m, 4H, ArH); 13C{1H} NMR (100 MHz, 298 K, C6D6): δ 24.3 (CH(CH3)2), 28.0 (CH(CH3)2), 69.7 (CH(C6H5)2), 124.0, 124.1, 127.2, 127.9, 128.7, 142.3, 142.6, 144.5 (ArC); IR ν cm-1 (ATR): 3374(m), 3063(m), 1491(m), 1444(m), 1244(m), 1177(m), 1110(m), 1079(m), 1048(m), 763(m), 696(m); ESI acc. mass. calc for M+–H: 342.2227; found, 342.2220.

Preparation of [Ge{N(Dip)(Mes)}2] 2

To a solution of HN(Dip)(Mes) (500 mg, 1.7 mmol) in THF (40 mL) at -80°C was added BunLi (1.6 m in THF, 1.16 mL, 1.9 mmol). The mixture was warmed to room temperature, stirred for 2 h, and then added to a solution of GeCl2·dioxane (196 mg, 0.85 mmol) at -80°C. The resultant solution was warmed to room temperature and stirred for 12 h, whereupon volatiles were removed in vacuo. The yellow residue was extracted into a hexane/toluene mix (30/10 mL), the extract filtered, and stored at -20°C yielding pale yellow crystals of 2 (0.16 g, 29%). M.p.: 232–238°C (decomp.); 1H NMR (400 MHz, C6D6, 298 K): δ 0.68 (d, J=6.8 Hz, 6H, CH(CH3)2), 1.16 (d, J=6.8 Hz, 6H, CH(CH3)2), 1.40 (d, J=6.8 Hz, 6H, CH(CH3)2), 1.44 (d, J=6.8 Hz, 6H, CH(CH3)2), 1.87 (s, 6H, Mes-o-CH3), 2.03 (s, 6H, Mes-o-CH3), 2.48 (s, 6H, Mes-p-CH3), 3.18 (sept, J=6.8 Hz, 2H, CH(CH3)2), 4.15 (sept, J=6.8 Hz, 2H, CH(CH3)2), 6.25 (s, 2H, Mes-p-CH), 6.49 (s, 2H, Mes-p-CH), 7.09–7.19 (m, 6H, Dip-Ar-CH); 13C{1H} NMR (75.5 MHz, C6D6) d 20.2 (CH(CH3)2), 21.9 (CH(CH3)2), 22.2 (CH(CH3)2), 22.4 (CH(CH3)2), 24.9 (Mes-p-CH3), 26.2 (Mes-o-CH3), 27.6 (Mes-o-CH3), 29.4 (CH(CH3)2), 30.1 (CH(CH3)2), 125.4, 125.5, 125.8, 129.4, 130.3, 130.5, 131.8, 132.0, 144.0, 145.2, 145.8, 147.9 (Ar-C); IR ν cm-1 (ATR): 3056(w), 1421(s), 1226(s), 1137(m), 1094(m), 1031(w), 873(s), 850(m), 742 (s).

Preparation of [Ge{N(Dip)[C(H)Ph2]}2] 3

BunLi (1.0 mL of 1.6 m in hexanes, 1.6 mmol) was added to a solution of 1 (0.500 g, 1.46 mmol) in THF (30 mL) at -80°C, the mixture warmed to room temperature, and stirred for 2.75 h. The resultant solution was then added to a solution of GeCl2‧dioxane (0.337 g, 1.46 mmol) in THF (50 mL) at -80°C. The reaction mixture was warmed to room temperature and stirred overnight, and the volatiles were removed in vacuo. The residue was extracted with hexane and the extract was filtered. An orange oil was obtained by evaporating the solvent from the filtrate in vacuo. From this oil crystallized a significant quantity of HN(Dip){C(H)Ph2}. This was isolated and the remaining oil was dissolved in 1 mL diethyl ether. The placement of this solution at -25°C gave a low yield (<5%) of 3 as yellow crystals. 1H NMR (500 MHz, 298 K, C6D6): δ 0.90 (d, J=6.8 Hz, 12H, CH(CH3)2), 1.28 (d, J=6.8 Hz, 12H, CH(CH3)2), 3.19 (sept, J=6.8 Hz, 4H, CH(CH3)2), 5.61 (s, 2H, CH(C6H5)2), 6.96–7.08 (m, 20H, ArH), 7.33–7.34 (m, 6H, ArH).

X-ray crystallography

Crystals of 1–3 suitable for X-ray structural determination were mounted in silicone oil. Crystallographic measurements were carried out at 123(2) K and were made using either an Oxford Gemini Ultra diffractometer using a graphite monochromator with Mo Kα radiation (λ=0.71073 Å) or the MX1 beamline of the Australian Synchrotron (λ=0.71080 Å). All structures were solved by direct methods and refined on F2 by full matrix least squares (SHELX97) (Sheldrick, 1997) using all unique data. All nonhydrogen atoms are anisotropic with hydrogen atoms included in calculated positions (riding model). The crystallographic data, details of data collections, and refinement are given in Table 1. The crystallographic data (excluding structure factors) for all structures have been deposited with the Cambridge Crystallographic Data Centre (CCDC no. 928224-928226). Copies of this information may be obtained free of charge from The Director, CCDC, 12 Union Road, Cambridge CB2 1EZ, UK (fax: +44-1223-336033; e-mail: deposit@ccdc.cam.ac.uk or Web site: http://www.ccdc.cam.ac.uk).

This research was supported by the Australian Research Council (grant to C.J.) and the U.S. Air Force Asian Office of Aerospace Research and Development (grant FA2386-11-1-4110 to C.J.). Part of this research was undertaken on the MX1 beamline at the Australian Synchrotron, Victoria, Australia.

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About the article

Corresponding author: Cameron Jones, School of Chemistry, Monash University, P.O. Box 23, Melbourne 3800, Victoria, Australia


Received: 2013-03-07

Accepted: 2013-04-06

Published Online: 2013-05-01

Published in Print: 2013-07-01


Citation Information: Main Group Metal Chemistry, Volume 36, Issue 3-4, Pages 133–136, ISSN (Online) 2191-0219, ISSN (Print) 0792-1241, DOI: https://doi.org/10.1515/mgmc-2013-0015.

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