Photolysis of arene chromium tricarbonyl complexes in presence of amine–boranes: Observation of σ-borane complexes in solution

Abstract

Activation of the B–H σ-bond of amine–boranes on the chromium(0) center of arene chromium tricarbonyl complexes ( η 6-arene)Cr(CO) 3 (arene = fluorobenzene, 1a; benzene, 1b and mesitylene, 1c) has been studied. Photolysis of 1b in presence of ammonia–borane (H 3N·BH 3, AB) and tert-butylamine–borane ( t BuH 2N·BH 3, TBAB) resulted in H 2 evolution and precipitation of a BNH x polymer. On the other hand, photolysis in the presence of trimethylamine–borane (Me 3N·BH 3, TMAB) resulted in the formation of a σ-borane complex ( 2) along with Cr(CO) 5( η 1-HBH 2·NMe 3) ( 3). The σ-borane complexes ( η 6-arene)Cr(CO) 2( η 1-HBH 2·NMe 3) (arene = fluorobenzene, 2a; benzene, 2b and mesitylene, 2c) were characterized in solution by 1H, 11B, and 13C NMR spectroscopy. Electron withdrawing substituents on the arene ring provide the more stable σ-borane moiety in this series of complexes.