Hydrometallation of amino-dialkynylgermanes – a gallium hydride oligomer and intramolecular Lewis acid-base interactions

1 Introduction

Hydroalumination or hydrogallation of donor-functionalized oligo(alkynyl)silanes and olico(alkynyl)germanes opened facile access to a large variety of unprecedented compounds that show an intramolecular coordination of the coordinatively unsaturated metal atoms by chlorine atoms [1], alkynyl [2–5], or amino groups [6, 7]. These interactions resulted in an activation of Si–X or Ge–X bonds (X = donor) and facilitated fascinating secondary reactions such as the insertion of heterocumulenes [7] or rearrangement processes with the intermediate formation of silyl or germyl cations under mild conditions [1, 8]. We report here on the molecular structures of two compounds that were obtained by the treatment of amino-di(alkynyl)germanes with two equivalents of the hydrides H–M(CMe₃)₂ (M = Al, Ga).

2 Results and discussion

Treatment of the di(alkynyl)germane H₅C₆(Et₂N)Ge(C≡C–CMe₃)₂ with equimolar quantities of H–Ga(CMe₃)₂ afforded the alkenyl-alkynylgermane 1 (Scheme 1) in high yield [8]. 1 shows a strong intramolecular Ga–N bonding interaction that results in the formation of a GaNGeC heterocycle with a significantly lengthened Ge–N bond. One alkynyl group is unaffected and occupies a terminal position at the central Ge atom. It should facilitate a second hydrogallation step to obtain a di(alkenyl)germane with two Lewis acidic metal atoms in a single molecule, which would have interesting coordination properties (chelating Lewis acid). H–Al(CMe₃)₂ gave the corresponding di(alkenyl)germane by dual hydroalumination of the di(alkynyl)germane in high yield [8]. By contrast, the monoaddition product 1 was detected by NMR spectroscopy as the only component when the dialkynyl starting compound was treated with excess H–Ga(CMe₃)₂ (3.3 eq.) in toluene at room temperature. From the reaction mixture, we isolated few single crystals of a new and complex gallium hydride (2; Scheme 1), which was identified by crystal structure determination (Fig. 1). It has a unique and highly interesting molecular structure in which three Ga atoms are bridged by three H atoms to form a six-membered nonplanar Ga₃H₃ heterocycle (H4 being 70 pm above the plane of the five remaining ring atoms). This motif resembles that of trimeric H–Ga(CMe₃)₂ [9], which has a planar Ga₃H₃ ring. One Ga–H–Ga 3c2e bond in 2 is bridged by a di(alkenyl)germanium moiety to give a bicyclic molecule. Both C≡C triple bonds are reduced, and the amino group originally attached to the Ge atom is replaced by an H atom to form a Ge–H bond. Ge–H and Ga–H bond lengths correspond to standard values [9, 10]. A balanced equation for the synthesis of 2 starting from 1 requires the formation of Ga(CMe₃)₃ and Et₂N–Ga(CMe₃)₂ as by-products. However, due to their very low concentration in the reaction mixture, they could not be detected by NMR spectroscopy. We tried to reproduce the synthesis of this fascinating compound by treatment of the dialkyne with the sesquihydride [H₂Ga–CMe₃]₂[H–Ga(CMe₃)₂]₂ [9], which has been applied only recently for the synthesis of a related silicon compound [6]. Decomposition of the germane was observed with the formation of a subhydride [11] containing two Ga–Ga single bonds.

Scheme 1

Schematic drawing of the molecular structures of 1 and 2.

Fig. 1

Molecular structure and numbering scheme of 2. Displacement ellipsoids are drawn at the 40 % level. Hydrogen atoms (except the olefinic and hydridic H, arbitrary radius) and methyl substituents of the Ga–CMe₃ groups have been omitted for clarity. Selected bond lengths (pm) and angles (°): Ge1–H1 153(2), Ge1–C11 194.8(2), Ge1–C21 194.6(2), Ge1–C31 195.9(2), C11–Ga1 197.5(2), C21–Ga3 197.6(2), Ga1–H2 171(2), Ga1–H4 178(2), Ga2–H2 174(2), Ga2–H3 174(2), Ga3–H3 170(2), Ga3–H4 168(2); Ga1–H4–Ga3 124.2, Ga1–H2–Ga2 138.3, Ga2–H3–Ga3 140.6, C11–Ga1–H2 109.7(7), C21–Ga3–H3 106.0(8).

Compound 3 was obtained by dual hydroalumination of Et₂N–Ge(C≡C–CMe₃)₃ [7] (Scheme 2). The structure of the analogous Ga compound has been determined previously, but we failed for more than 2 years to generate single crystals of 3. Its structure is important for a comparison of structural and chemical properties that strongly depend on the different acceptor capabilities of Al and Ga. The strength of intramolecular donor–acceptor interactions influences the activation of Ge–N and Ge–C bonds and the course of secondary reactions. The Al compound, for instance, reacted with chloride anions to form an adduct that eliminated an imine via an intermediate germyl cation at elevated temperature. A similar reaction was not observed for the Ga compound [8]. We obtained single crystals of 3 by slow crystallization from pentafluorobenzene at –20 °C. Its molecular structure (Fig. 2) comprises two structural motifs that result from different intramolecular interactions. The atom Al1 is coordinated by the amino N atom N1 to generate a four-membered, almost planar AlNGeC heterocycle. The Al1–N1 distance of 209.4(2) pm is shorter than the Ga–N distance in the Ga compound (222.6(2) pm) [7], reflecting the different acceptor strengths of the metal atoms. Both values correspond to typical M–N distances in M–N–E bridges (M = Al, Ga) [7, 12–15]. These M–N interactions cause elongated Ge–N bonds (199.4(1) (M = Al) and 197.2(2) pm (M = Ga); 181.5(1) pm in Et₂N–Ge(C≡C–CMe₃)₃ [8]), with the stronger effect observed for the Al compound 3. Quantum-chemical calculations of related systems have shown that these structural details reflect a weakening of the Ge–N bond compared with the starting aminogermane. The WBI bond indices for the Ge–N and Al–N bonds are similar, indicating a chelating coordination of the amino group by Ge and Al, but significant differences were detected for the Ge–N–Ga system with relatively strong Ge–N and weak Ga–N bonds [7]. The second Al atom of 3 (Al2) shows an interaction with the α-C atom of the alkynyl group, which bears a relatively high partial negative charge. The Al2–C11 distance of 253.9 pm is relatively long compared with that of related compounds [2–5], which may be caused by steric crowding in this molecule with two bulky Al(CMe₃)₂ units, but it is shorter than the Ga–C distance in the corresponding Ga compound (281.2 pm) [7]. The different strengths of these interactions result in different Ge–C11 bond lengths (194.6(2) vs. 192.2(2) pm, 189 pm on average in the starting trisalkyne [8]). Compound 3 may be described as a spiro-compound with two four-membered rings joined by a central Ge atom. The Al atoms deviate from the plane of the directly bonded C atoms (alkyl and vinyl groups) by 52.0 (Al1) and 39.4 pm.

Scheme 2

Schematic drawing of the molecular structure of 3 (R = CMe₃).

Fig. 2

Molecular structure and numbering scheme of 3. Displacement ellipsoids are drawn at the 40 % level. Hydrogen atoms (except the olefinic H, arbitrary radius) have been omitted for clarity. Selected bond lengths (pm) and angles (°): Ge1–N1 199.4(1), Al1–N1 209.4(2), Ge1–C11 194.6(2), Al1–C11 253.9; Al1–N1–Ge1 91.34(6), N1–Al1–C21 83.98(7), N1–Ge1–C21 88.19(7), Ge1–C11–C12 173.9(2).

3 Experimental section

All procedures were carried out under an atmosphere of purified argon in dried solvents (toluene with Na/benzophenone; pentafluorobenzene with molecular sieves). Compound 3 [7], H₅C₆(Et₂N)Ge(C≡C–CMe₃)₂ [8] and HGa(CMe₃)₂ [9] were obtained according to literature procedures.