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)2Ca[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 °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 °C, 24 h). In the absence of
carbenes, the dehydrocoupling of 6 and HNMe2BH3 afforded (THF)2Ca(NMe2BH3)(NMe2BH2NMe2BH3) (9), but subsequent reactions with NHC resulted in
the immediate abstraction and migration of Me2NBH2 toward the formation of 8.