Germanium(II)-Inserted η⁵-closo-Germacarboranes and Their Donor-Acceptor Complexes. Crystal Structure of 1-Ge(C₁₀H₈N₂)-2,3-(Me₃Si)₂-2,3-C₂B₄H₄

The reaction of GeCl₄ with the nido-carborane dianionic salts NaLi[(Me₃Si)(R)C₂B₄H₄] (R = SiMe₃, Me, or H) in a molar ratio of 1:2 in THF produced closo-1-Ge^II-2,3-(SiMe₃)₂-2,3-C₂B₄H₄(I), closo-1-Ge^II-2-(SiMe₃)-3-(Me)-2,3-C₂B₄H₄(II), and closo-1-Ge^II-2-(SiMe₃)-2,3-C₂B₄H₅(III) as air-sensitive, pale yellow liquids, in 36, 49, and 23% yields, respectively. In addition to the corresponding neutral nido-carborane precursors, the Ge(IV)-inserted derivatives of commo-bis(germacarborane), [(Me₃Si)(R)C₂B₄H₄]₂Ge^IV, were also produced in 9.0, 7.4, and 9.5% yields when R = SiMe₃, Me, and H, respectively. The closo-germacarboranes I, II, and III react slowly with 2,2^'-bipyridine in benzene to form the electron donor-acceptor complexes l-Ge^II-(C₁₀H₈N₂)-2,3-(SiMe₃)₂-2,3-C₂B₄H₄ (IV), 1-Ge^II(C₁₀H₈N₂)-2-(SiMe₃)-3-(Me)-2,3-C₂B₄H₄(V), and 1-Ge^II-(C₁₀H₈N₂)-2-(SiMe₃)-2,3-C₂B₄H₅(VI) in 73, 42, and 48% yields, respectively. Both the visible/UV and ¹¹B NMR spectroscopic results on IV-VI indicate that at least two different kinetic processes are taking place: a slow formation of the bipyridine-germacarborane adduct, followed by a rapid intraor intermolecular process which causes all the basal and apical boron atoms to become equivalent on an NMR time scale. The X-ray crystal structure of IV shows an extreme displacement of the germanium from the centroidal position above the C₂B₃ face in its distorted pentagonal-bipyramidal geometry with the Lewis base tilted significantly away from the cage carbons. The slippage of the apical germanium is such that it could be considered as η²-bonded to the C₂B₃ face of the carborane cage. Although some similarities between the closo-germacarborane and the closo-stannacarborane systems exist, the donoracceptor complexes of the two metallacarboranes seem to be quite different. Compound IV crystallizes in the monoclinic space group P2₁/n with a = 7.102 (5) Å, b = 34.61 (2) Å, c = 9.774 (4) Å, β = 102.55 (4)°, U = 2345 (2) ų, and Z = 4. Full-matrix least-squares refinement converged at R = 0.040 and R_w= 0.038.