Reactivity of boraguanidinato germylenes toward carbonyl compounds and isocyanides: C-O, C-F and C-N bond activation

The reactions of two equivalents of germylene [(i-Pr)₂NB(N-2,6-Me₂C₆H₃)₂]Ge (1) with carbonyl compounds RC(O)R' resulted in carbonyl functionality activation and the formation of 4-(R,R')-1,2-digerma-3-oxa-cyclobutanes (R/R' = Ph/CF₃ (2) or C₆F₅/H (3)). Surprisingly, the analogous reaction of 1 with C₆F₅C(O)Me led to the insertion of the germanium atom into the C-F bond of the perfluorophenyl group, thus producing a spiro compound (4) with a germanium atom sharing 1,2-digerma-3,5-diaza-4-bora-cyclopentane and 1-germa-2,4-diaza-3-boracyclobutane rings. Furthermore, the reaction of 1 with 2e^- donors was investigated. In the case of 4-dimethylaminopyridine (DMAP), an expected complex [(i-Pr)₂NB(N-2,6-Me₂C₆H₃)₂]Ge(DMAP) (5) was isolated, but using t-BuNC resulted in the formation of germanium(iv) cyanide [(i-Pr)₂NB(N-2,6-Me₂C₆H₃)₂]Ge(CN)(t-Bu) (6) as a result of C-N bond activation in the starting isocyanide. In contrast, mixing other isocyanides RNC (R = Cy or Ad) with 1 in solution led only to an equilibrium between the starting compounds and most probably the corresponding complexes [(i-Pr)₂NB(N-2,6-Me₂C₆H₃)₂]Ge(CNR) (R = Cy (7a) or Ad (8a)) based on NMR studies. From these equilibrium mixtures, fortuitously, single crystals of digerma-spiro-complexes (7 and 8) containing two germanium atoms (one of them coordinated to a particular isocyanide) were obtained and structurally authenticated by the X-ray diffraction technique.