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(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.