N-Heterocyclic Carbenes in Lewis Acid/Base Stabilised Phosphanylboranes

Abstract

The reactions of 2-borane-1,3,4,5-tetramethylimidazoline (BH3·NHCMe) with selected phosphane adducts of the Lewis acids B(C6F5)3 and Ga(C6F5)3 are studied. Among others, adducts (C6F5)3Ga·PH2Cp* (1a) and (C6F5)3B·PH2Cp* (2) are used as starting materials. When the (C6F5)3Ga-phosphane adducts 1a and (C6F5)3Ga·PPhH2 are treated with BH3·NHCMe, the Lewis acid/base stabilised phosphanylboranes (C6F5)3Ga·P(Cp*)HBH2·NHCMe (3a) and (C6F5)3Ga·P(Ph)HBH2·NHCMe (3b) are formed, respectively, by a hydrogen elimination reaction. In contrast, the reaction of BH3·NHCMe with the (C6F5)3B-phosphane adducts (C6F5)3B·PH2R [R = H, R = Cp* (2) and R = Ph] in CH2Cl2 at room temperature leads to the formation of a salt with the general formula [(C6F5)3BH][RPH2·BH2·NHCMe] (4a: R = H, 4b: R = Cp*, 4c: R = Ph). To synthesise the Lewis acid/base stabilised phosphanylborane with (C6F5)3B as a Lewis acid and 1,3,4,5-tetramethylimidazolylidene (NHCMe) as a Lewis base, a different synthetic pathway was applied: the replacement reaction of the Lewis base. At room temperature, NHCMe displaced the amine in (C6F5)3B·P(Ph)HBH2·NMe3 to yield (C6F5)3B·P(Ph)HBH2·NHCMe (5). All compounds were comprehensively characterised by spectroscopic methods. Compounds 1a, 1b, 2, 3a, 3b and 5 were additionally characterised by X-ray crystallographic analysis.