Carbene–Calcium Silylamides and Amidoboranes

Abstract

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.

Illuminating Advances: Photochemistry and Photophysics of N-Heterocyclic Carbenes (NHCs) and Its Structural Correlation

“…The utilization of sterically unencumbered NHCs enables the extraction of Me 2 N= BH 2 from Ca(NMe 2 BH 3 ) or Ca-(NMe 2 BH 2 NMe 2 BH 3 ) units by reacting with calcium amido borane complexes. This procedure yields a thermally-stable calcium complex that is highly enriched with amido borane [67]. Some of the examples of typical calcium NHCs complexes is presented in Figure 17.…”