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

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%).

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 1–3 (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).

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).

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.

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).

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.