Photoluminescence, photoredox properties and crystal structures of rhenium(V)-benzylidyne complexes with phosphine ligands

Several rhenium(v)-benzylidyne complexes [Re(CR)(pdpp)(2)Cl](+) [R = C(6)H(2)Me(3)-2,4,6, pdpp = o-phenylenebis(diphenylphosphine)], [Re(CR)L(2)(CO)(H2O)Cl](+) [L = PPh(3), P(C(6)H(4)OMe-p)(3) or PMe(2)Ph] and trans-[Re(CR)(dppe)(CO)(2)Cl](+) [dppe = 1,2-bis(diphenylphosphino)ethane] have been prepared. The structures of trans-[Re(CR)(pdpp)(2)Cl]ClO4 . CHCl3 . 0.25MeOH and [Re(CR)(PPh(3))(2)(CO)(H2O)Cl]ClO4 . 1.5MeOH have been determined by X-ray analyses. The Re=C distances are 1.802(5) and 1.784(8) Angstrom respectively. In acetonitrile and dichloromethane the complexes show intense absorption bands at 318-330 nm and weak ones at 405-450 nm, the latter being tentatively assigned to d(xy) --> d(x*) (d(xz),d(yz)) transitions. Photoexcitation in the solution, solid or glassy state gives intense orange to red emissions, and the-emitting states are tentatively assigned to (3)[(d(xy))(1)(d(pi*))(1)]. The variation in non-radiative decay rate constants for the emissions of the rhenium(v)-benzylidyne complexes are consistent with a prediction from the energy-gap law. The excited states are better oxidants and reductants than the ground states. The values of E(o)(Re-V*-Re-IV) and E(o)(Re-VI-Re-V*) in acetonitrile have been determined using spectroscopic and electrochemical data as well as by Stern-Volmer quenching experiments.