Mechanisms of Reactions of Open-Shell, Heavier Group 14 Derivatives with Small Molecules: n-π* Back-Bonding in Isocyanide Complexes, C-H Activation under Ambient Conditions, CO Coupling, and Ancillary Molecular Interactions

The main themes of this review are the mechanisms of the reactions of germanium and tin analogues of carbenes with isocyanides, CO, ammonia, and related molecules. The treatment of Ge(Ar-Me6)(2) (Ar-Me6 = C6H3-2,6(C6H2-2,4,6-Me-3)(2)) with MeNC or (BuNC)-N-t afforded 1 :1 complexes, but the increase in the electron density at germanium leads to C H activation at the isocyanide methyl or tert-butyl substituents. For MeNC, the initial adduct formation is followed by a migratory insertion of the MeNC carbon into a Ge-C(ipso) bond of an aryl substituent. The addition of excess MeNC led to sequential insertions of two further MeNC molecules. The third insertion led to methylisocyanide methyl group C-H activation, to afford an azagermacyclopentadienyl species The (BuNC)-N-t complex (Ar-Me6)(2)GeCNBut spontanously transforms into (Ar-Me6)(2)Ge(H)CN and isobutene with C-H activation of the Bu-t substituent. The germylene Ge(Ar-Me6)(Ar-Pr4i) [Ar-Pr4i = C6H3-2,6(C6H3-2,6-Pr-2(i))(2)] reacted with CO to afford alpha-germyloiryketones. The initial step is the formation of a 11 complex, followed by migratory insertion into the Ge-C bond of the Ar-Pr4i ligand to give (ArGeC)-Ge-Me6(O)Ar-Pr4i. Insertion of a second CO gave (ArGeC)-Ge-Me6(O)C(O)Ar-Pr4i, which rearranges to afford alpha-germyloxyketone. No reaction was observed for Sn(Ar-Me6)(2) with RNC (R = Me, Bu-t) or CO. Spectroscopic (IR) results and density functional theory (DFT) calculations showed that the reactivity can be rationalized on the basis of Ge-C (isocyanide or CO) Ge(n) -> pi* (ligand) back bonding The reaction of Ge(Ar-Me6)(2) and Sn(Ar-Me6)(2) with ammonia or hydrazines initially gave 1:1 adducts. However, DFT calculations show that there are ancillary N-H center dot center dot center dot N interactions with a second ammonia or hydrazine, which stabilizes the transition state to form germanium(IV) hydride (amido or hydrazido) products. For tin, arene elimination is favored by a buildup of electron density at the tin, as well as the greater polarity of the Sn-C(ipso) bond. Germanium(IV) products were observed upon reaction of Ge(Ar-Me6)(2) with acids, whereas reactions of Sn(Ar-Me6)(2) with acids did not give tin (II) products In contrast to reactions with NH3, there is no buildup of negative charge at tin upon protonation, and its subsequent reaction with conjugate bases readily affords the tin(IV) products.