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

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 BNHx 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.