Reactivity of Amido-Digermynes, LGeGeL (L = Bulky Amide), toward Olefins and Related Molecules: Facile Reduction, C–H Activation, and Reversible Cycloaddition of Unsaturated Substrates

Abstract

Reactions of two sterically hindered amido-digermynes, L*GeGeL* (1; L* = −N(Ar*)(SiMe3); Ar* = C6H2Me{C(H)Ph2}2-4,2,6) and L†GeGeL† (2; L† = −N(Ar†)(SiPri3); Ar† = C6H2Pri{C(H)Ph2}2-4,2,6), with a variety of olefins and related molecules are investigated. These lead to the facile reduction, C–H activation, dehydrogenation, and/or cycloaddition of the unsaturated substrate. Specifically, reaction of L†GeGeL† with ethylene proceeds via a formal [2 + 2 + 2] cycloaddition to give the digermabicyclo[2.2.0]hexane L†Ge(μ-C2H4)2GeL† (3). In contrast, treating L†GeGeL† with norbornadiene proceeds via reductive insertion of one olefin moiety of the organic substrate into the Ge–Ge bond of 1, yielding the norbornenediyl-bridged bis(germylene) L†Ge(μ-C7H8)GeL† (4). Similarly, L*GeGeL* doubly reduces cyclooctatetraene (COT) to give the planar cyclooctateraenediyl inverse sandwich complex L*Ge(μ-η2,η2-COT)GeL* (5). An indication that this reaction occurs via an initial formal [2 + 2] cycloaddition intermediate comes from the reaction of L†GeGeL† with 1,5-cyclooctadiene (COD). This affords the [2 + 2] cycloaddition product L†Ge(COD)GeL† (6), which exists in solution in equilibrium with 2 and free COD. A computational study indicates that 6 readily dissociates, as the reaction that gave it is close to thermoneutral. Reaction of 1,3-cyclohexadiene (1,3-CHD) with L†GeGeL† yields the 1,4-bis(germylene) substituted cyclohex-2-enediyl L†Ge(μ-C6H8)GeL† (7), which is an isolated intermediate in the transfer hydrogenation, or C–H activation, reaction between L†GeGeL† and 1,3-CHD. Heating 7 gives benzene and the known digermene L†(H)GeGe(H)L†. Reactions of 1 or 2 with propyne, bis(trimethylsilyl)butadiyne, and azobenzene all lead to reductive insertion of the unsaturated substrate into the Ge–Ge bond of the digermyne and formation of L†Ge{μ-HC═C(Me)}GeL† (8), L*Ge{μ-(Me3Si)C═C(CCSiMe3)}GeL* (9), and L*Ge{μ-(Ph)NN(Ph)}GeL* (10), respectively. The reaction of 4-dimethylaminopyridine (DMAP) with L*GeGeL* gives the adduct complex L*(DMAP)GeGe(DMAP)L* (11). Taken as a whole, this study highlights both similarities and significant differences between the reactivities of the amido-digermynes 1 and 2 and those of their previously described terphenyl-substituted counterparts.