Rhodium(III)-cyclometalated-imine complexes: Solution behavior and reactivity with molecular hydrogen

Acetone solutions of cis, trans, cis-[Rh(H)(2)(PPh3)(2)(acetone)(2)] PF6 and cis-[Rh(PPh3)(2)(acetone)(2)]PF6 react with the imines RN=CR ’(R ’’), where R = PhCH2, Ph, or C6H11; R ’= H or Me; and R ’’ = Ph or P-OMe-C6H4, to form orthometalated complexes exemplified by [Rh(H){RN= CR ’(o-C6H4)}(PPh3)(2)(acetone)]PF6 that are generally characterized spectroscopically, and in one case (where R = PhCH2, R ’ = Me, and R ’’ = p-OMe-C6H4) by X-ray crystallography. These complexes maintain their integrity in acetone solution and show no reaction toward 1 atm H-2 at ambient conditions. In MeOH, the complexes in which R ’ = Me simply change to the corresponding MeOH derivative, but when R ’ = H they are partially converted to the non-orthometalated, eta(1)-N-imine species [Rh(PPh3)(2)(RN=CHR ’’)(MeOH)]PF6 (I), and in MeOH, stoichiometric hydrogenation of the imine to the corresponding amine occurs via I. We have shown recently (Inorg. Chem. 2004, 43, 4820) that the catalyzed hydrogenation of PhCH2N=C(H)Ph to dibenzylamine using cis, trans, cis- [Rh(H)(2)(PPh3)(2)(MeOH)(2)] PF6 as catalyst precursor in MeOH proceeds via a cationic Rh-(eta(1)-N-imine)(amine) intermediate (II), analogous to I but with the MeOH replaced by the amine that is formed via hydrolysis of the imine by the presence of adventitious water. Intermediate I (R = PhCH2, R ’’ = Ph) is shown to be somewhat less active than II toward H-2, but both intermediates provide viable pathways for hydrogenation of the imine. The orthometalated imine-amine complex [Rh(H){PhCH2N=C(Me)(o-C6H4)}(PPh3)(2)(NH2CH2Ph)]PF6, where the benzylamine comes from imine hydrolysis, is also isolated and characterized crystallographically.