The interaction of Lewis acidic boron derivatives with Re(CO) 5− nH(PMe 3) n complexes

Abstract

Lewis acid adducts of the hydrides cis- and trans-Re(CO)(PMe 3) 4H ( 1) and ( 2), mer-Re(CO) 2(PMe 3) 3H ( 3), fac-Re(CO) 2(PMe 3) 3H ( 4) and trans-Re(CO) 3(PMe 3) 2H ( 5) were studied with BH 3 and 9-borabicyclo[3,3,1] norbonane (BBNH). Using BH 3·THF and (BBNH) 2 1 and 2 afforded Re(CO)(PMe 3) 3(η 2-BH 4) ( 6) and Re(CO)(PMe 3) 3(η 2-BBNH 2) ( 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 O CO atom of 1 or 2. At higher temperatures ReH adduct formation occurs with instantaneous transformation to 7 and elimination of PMe 3·BBNH. In a similar way, the hydrides 3 and 4 were converted with BH 3·THF and (BBNH) 2 to yield the stable complexes Re(CO) 2(PMe 3) 2(η 2-BH 4) ( 8) and Re(CO) 2(PMe 3) 2(η 2-BBNH 2) ( 9). The intermediacy of the η 1-BH 4 adducts mer-/fac-Re(CO) 2(PMe 3) 3(η 1-BH 4) was confirmed by VT 1H, 31P NMR and VT IR experiments. The conversion of 5 with BH 3·THF led to equilibria with adducts at the O CO terminus in trans position to H and with H Re as revealed by VT IR studies. Temperature dependent 31P equilibrium studies allowed to calculate Δ H=−4.9 kcal mol −1 and Δ S=+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 BH 3 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 C 2 v structure. X-ray diffraction studies were carried out on single crystals of 6 and 7.