Carbene–Calcium Silylamides and Amidoboranes

• Published in: Organometallics Vol. 41; no. 21; pp. 3064 - 3072
• Main Authors: Obi, Akachukwu D., Freeman, Lucas A., Coates, Samuel J., Alexis, Andrew J. H., Frey, Nathan C., Dickie, Diane A., Webster, Charles Edwin, Gilliard, Robert J.
• Format: Journal Article
• Language: English
• Published: American Chemical Society 14.11.2022
• ISSN: 0276-7333; 1520-6041
• DOI: 10.1021/acs.organomet.2c00464

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.