Hydroboration and Deoxygenation of CO 2 Mediated by a Gallium(I) Cation.

Hydroboration of CO 2 to formoxy borane occurs under ambient conditions in acetonitrile using pinacolborane HBpin in the presence of gallium(I) cation [(Me 4 TACD)Ga][BAr 4 ] ( 1 ; Me 4 TACD = N , N ', N ″, N '''-tetramethyl-1,4,7,10-tetraazacyclododecane; Ar = C 6 H 3 -3,5-Me 2 ). Slow turnover was accompanied by side reactions including ligand scrambling of HBpin to give BH 3 (CH 3 CN) and crystalline B 2 pin 3 . When 1 was reacted with CO 2 alone, the formation of the gallium(III) carbonato complex [(Me 4 TACD)Ga(κ 2 -O 2 CO)][BAr 4 ] ( 3 ) along with CO was observed. This complex was assumed to form via the unstable oxido cation [(Me 4 TACD)Ga=O] + ( 4 ). Reaction of 1 with N 2 O in the presence of BPh 3 confirmed the formation of the oxido cation, which was spectroscopically characterized as a triphenylborane adduct [(Me 4 TACD)Ga=O(BPh 3 )][BAr 4 ] ( 4·BPh 3 ). CO was also detected when CO 2 was reacted with 1 in the presence of HBpin, suggesting that compound 3 may also be formed in initial stages of catalysis. Compound 3 reacts with HBpin to give formoxy borane, borane redistribution products, and an unidentified Me 4 TACD-containing species 5 , which was also observed in "catalytic" runs starting from 1 , HBpin, and CO 2 . Hydroboration of CO 2 using HBpin with slow turnover and competitive ligand scrambling was also observed in the presence of gallium(III) hydride dication [(Me 4 TACD)GaH][BAr 4 ] 2 ( 2 ), which is unreactive toward CO 2 in the absence of HBpin.