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 (η⁶-arene)Cr(CO)₃ (arene=fluorobenzene, 1a benzene, 1b and mesitylene, 1c) has been studied. Photolysis of 1b in presence of ammonia–borane (H₃N·BH₃, AB) and tert-butylamine–borane (^tBuH₂N·BH₃, TBAB) resulted in H₂ evolution and precipitation of a BNH_x polymer. On the other hand, photolysis in the presence of trimethylamine–borane (Me₃N·BH₃, TMAB) resulted in the formation of a σ-borane complex (2) along with Cr(CO)₅(η¹-HBH₂·NMe₃) (3). The σ-borane complexes (η⁶-arene)Cr(CO)₂(η¹-HBH₂·NMe₃) (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.