Determination of Rhodium–Alkoxide Bond Strengths in Tp′Rh(PMe3)­(OR)H

The active fragment [Tp′Rh­(PMe3)], generated from a thermal precursor Tp′Rh­(PMe3)­(CH3)­H, underwent oxidative addition of water and alcohols to give O–H adducts of the type Tp′Rh­(PMe3)­(OR)­H (R = H, Me, Et, nPr, nBu, CH2Ph, iPr, c-pentyl, CH2CF3, CH2CH2OH) at ambient temperature. These activation products eliminate water or alcohols in benzene, which allows determination of the relative metal–oxygen bond energies by using previously established kinetics techniques. Analysis of the relationship between the relative M–O bond strengths and O–H bond strengths showed a linear correlation with RM–O/O–H of 0.97 (3) for aliphatic alcohols. The two extraordinary substrates (R = CH2CF3, CH2CH2OH) both have stronger M–O bonds than would be predicted from this trend, suggesting the stabilization of the M–O bond when an electron-withdrawing substituent is present as previously seen in M–C bond strengths. In addition, the O–H activation products of aliphatic alcohols are thermally unstable at 80 °C, as rearrangement to form Tp′Rh­(PMe3)­H2 from β-elimination is observed after 1 or 2 d. Benzyl alcohol and 2,2,2-trifluoroethanol activation products were stable. For benzyl alcohol, although the O–H activation product was kinetically favored, the C–H activation products of the benzene ring were thermodynamically preferred.