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

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(-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(-C₇H₈)GeL^† (4). Similarly, L∗GeGeL∗ doubly reduces cyclooctatetraene (COT) to give the planar cyclooctateraenediyl inverse sandwich complex L∗Ge(μ-^η2η2-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(-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{-HC=C(Me)}GeL^† (8), L∗Ge{-(Me₃Si)C=C(CCSiMe₃)}GeL∗ (9), and ∗∗Ge{-(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.