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 H2 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, η1-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-(η1-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 H2, 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.