Reactivity Studies of N-Heterocyclic Carbene Complexes of Germanium(II)

The chemistry of three N-heterocyclic carbene (NHC) complexes of
GeR2, where R = Cl (1), OtBu (2), Mes (3) (Mes = 2,4,6-trimethylphenyl), toward
2,3-dimethylbutadiene (DMB), 3,5-di-tert-butylorthoquinone,
methyl iodide, pivalic acid, and benzophenone is reported. Upon heating, 2 and 3 cyclize with DMB to yield a germacyclopentene
and free NHC. In contrast, 1 does not react with DMB.
PB1PBE/6-311+G(d,p) model chemistry shows that the cycloaddition reactions
of NHC-GeX2 (X = F, Cl) with butadiene are not thermodynamically
favorable. 3,5-Di-tert-butylorthoquinone reacts rapidly
with 1−3 to form cyclic products;
in the case of 1 and 2, the NHC remains
coordinated to the germanium, resulting in a hypervalent species.
Compounds 1−3 react with methyl iodide
by displacement of I−; in each case [NHC-GeR2Me]+ is produced. Only compound 3 reacts
in a controlled fashion with pivalic acid; both 1:1 and 1:2 adducts
were characterized. Benzophenone failed to react with 1 or 2 but did undergo cycloaddition with 3. In comparison with uncomplexed GeR2 species, the NHC-GeR2 complexes are less reactive. The prospect of using NHC-GeR2 as a synthon for GeR2 appears to be reaction specific.