Carbene–Calcium Silylamides and Amidoboranes

The stereoelectronic effects of N-heterocyclic carbene (NHC) and cyclic(alkyl)(amino) carbene (CAAC) coordination to calcium silylamides and amidoboranes have been investigated. The straightforward complexation of a sterically unencumbered NHC (i.e., N,N'-diisopropyl-2,3-dimethylimidazol-2-ylidine) and {Ca[N(SiMe3)(2)](2)}(2) or (THF)(2)Ca[N(SiMe3)(2)](2) in equimolar amounts afforded (NHC)Ca[N(SiMe3)(2)](2) (2) or (NHC)(THF)Ca[N(SiMe3)2]2 (4), respectively. Spectroscopic analyses reveal negligible electronic differences in 2 and 4, and the latter can be desolvated under prolonged vacuum. Similarly, CAAC complexation to {Ca[N(SiMe3)(2)](2)}(2) afforded (CAAC)Ca[N(SiMe3)(2)](2) (5) as the first crystallographically characterized CAAC-Ca coordination complex, but this compound is thermally unstable and rapidly decomposes to intractable mixtures. The reaction of {Ca[N(SiMe3)(2)](2)}(2) and HNMe2BH3 afforded the bis(amidoborane) [Ca(NMe2BH3)(2)](n) (6), which does not react with bulky carbenes but readily complexes with unencumbered NHCs with subsequent non-innocent participations in calcium-mediated amine borane dehydrocoupling. Significantly, (NHC)2Ca(NMe2BH3)(2) (7) decomposes under mild heating (50 degrees C, 16 h) to form [CaH2](n) and the hydride-rich complex [(NHC-BH2NMe2)Ca(NMe2BH3)(2)](2) (8). Compound 8 was independently prepared from the reaction of 6 and NHC-BH2NMe2 and is remarkably thermally stable in refluxing benzene (85 degrees C, 24 h). In the absence of carbenes, the dehydrocoupling of 6 and HNMe2BH3 afforded (THF)(2)Ca(NMe2BH3)(NMe2BH2NMe2BH3) (9), but subsequent reactions with NHC resulted in the immediate abstraction and migration of Me2N=BH2 toward the formation of 8.