NMR evidence for formation of new alcohol rhenium complexes as intermediates in ionic hydrogenations of carbonyl groups with systems composed of ReH2(NO)(CO)(PR3)2 (R = Pri, CH3, OPri) and CF3COOH

Low-temperature hydrogenations of benzaldehyde and acetone by systems ReH2(CO)(NO)(PR3)(2)/CF3COOH in CD2Cl2 with R = Pr-i (1a), CH3 (1b) and OPri (1c) result in formation of the new unstable alcohol complexes [ReH(CO) (NO)(PR3)(2)(R*OH) ](+)[CF3COO](-) (R* = C6H5CH2, (CH3)(2)CH) characterized by the low-temperature NMR spectra. Heating the reaction solutions above 210-240 K leads to alcohol elimination to form monohydrides ReH(CO)(NO)(PR3)(2)(CF3COO). Hydrogenation rates decrease in the order 1b > 1c > 1a and C6H5CH=O > (CH3)(2)C=O. Hydrogenation processes remain effective under H-2 (or D-2) atmosphere and in the presence of an excess of CF3COOH when the dihydrides exist as dihydrogen compounds [ReH(H-2)(CO)(NO)(PR3)(2)](+)[CF3COO](-). Relative acidity of the dihydrogen complexes decreases in the order OPri > Pr-i > CH3. The hydrogenation with dihydrogen complexes is discussed in terms of an ionic mechanism. (C) 1998 Elsevier Science S.A. All rights reserved.