The interaction of Lewis acidic boron derivatives with Re(CO)5−nH(PMe3)n complexes

Lewis acid adducts of the hydrides cis- and trans-Re(CO)(PMe3)4H (1) and (2), mer-Re(CO)2(PMe3)3H (3), fac-Re(CO)2(PMe3)3H (4) and trans-Re(CO)3(PMe3)2H (5) were studied with BH3 and 9-borabicyclo[3,3,1] norbonane (BBNH). Using BH3·THF and (BBNH)21 and 2 afforded Re(CO)(PMe3)3(η2-BH4) (6) and Re(CO)(PMe3)3(η2-BBNH2) (7) as stable and isolable products. VT IR studies established for the reaction to 7 that BBNH first attaches in a pre-equilibrium to the OCO atom of 1 or 2. At higher temperatures ReH adduct formation occurs with instantaneous transformation to 7 and elimination of PMe3·BBNH. In a similar way, the hydrides 3 and 4 were converted with BH3·THF and (BBNH)2 to yield the stable complexes Re(CO)2(PMe3)2(η2-BH4) (8) and Re(CO)2(PMe3)2(η2-BBNH2) (9). The intermediacy of the η1-BH4 adducts mer-/fac-Re(CO)2(PMe3)3(η1-BH4) was confirmed by VT 1H, 31P NMR and VT IR experiments. The conversion of 5 with BH3·THF led to equilibria with adducts at the OCO terminus in trans position to H and with HRe as revealed by VT IR studies. Temperature dependent 31P equilibrium studies allowed to calculate ΔH=−4.9 kcal mol−1 and ΔS=+0.034 e.u. for this reaction. These adducts could not be isolated. Compound 5 does not react with (BBNH)2 even at elevated temperatures. DFT calculations were carried out to support the structures of the BH3 adducts of 5. In addition a vibrational analysis helped to unravel the IR band assignments of the involved compounds. DFT calculations on 8 confirmed its C2v structure. X-ray diffraction studies were carried out on single crystals of 6 and 7.