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

Abstract

Several rhenium(V)–benzylidyne complexes [Re(CR)(pdpp)₂Cl]⁺[R = C₆H₂Me₃-2,4,6, pdpp =o-phenylenebis(diphenylphosphine)], [Re(CR)L₂(CO)(H₂O)Cl]⁺[L = PPh₃, P(C₆H₄OMe-p)₃ or PMe₂Ph] and trans-[Re(CR)(dppe)(CO)₂Cl]⁺[dppe = 1,2-bis(diphenylphosphino)ethane] have been prepared. The structures of trans-[Re(CR)(pdpp)₂Cl]ClO₄·CHCl₃·0.25MeOH and [Re(CR)(PPh₃)₂(CO)(H₂O)Cl]ClO₄·1.5MeOH have been determined by X-ray analyses. The ReC distances are 1.802(5) and 1.784(8)Å 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_π*(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_π*)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°(Re^V*–Re^IV) and E°(Re^VI–Re^V*) in acetonitrile have been determined using spectroscopic and electrochemical data as well as by Stern–Volmer quenching experiments.