Tuning the excited-state properties of luminescent rhenium(V) benzylidyne complexes containing phosphorus and nitrogen donor ligands

Abstract

Efficient methods are developed for the synthesis of rhenium(V)-substituted benzylidyne complexes with various auxiliary ligands to facilitate the tuning of their excited-state properties. The electronic structures and spectroscopic and photophysical properties of [Re(≡CAr′)(pdpp)2Cl] + (Ar′ = C 6H 2Me 3-2,4,6, pdpp = o-phenylenebis(diphenylphosphine), 2), [Re(≡CAr′)L 2(CO)(H 2O)Cl] + (L = PPh 3, 3; P(C 6H 4OMe-p) 3, 4; PPh 2Me, 5), [Re(≡CAr′)(dppe)-(CO) 2Cl] + (dppe = 1,2-bis(diphenylphosphino)ethane, 6), [Re(≡CAr′)(L-L)(CO) 2Cl] + (L-L = 2,2′-bipyridine, 7; 4,4′-dichloro-2,2′-bipyridine, 8; 4,4′-dimethoxycarbonyl-2,2′-bipyridine, 9), [Re(≡CAr′)(TpO(CO) 2] + (Tp′ = tris(3,5-dimethyl-1-pyrazolyl)borohydride, 10), and [Re(≡CC 6H 4-R)(pdPp)(CO) 2(O 3SCF 3)] + (13, R = OMe, a; Me, b; H, c; Cl, d; Br, e; CN, f) are studied and compared. The molecular structures of 7·CHCl 3, 10, 12f, 13a·CH 3OH·H 2O, and 13d·2CH 2Cl 2 are determined by X-ray crystallography and reveal Re≡C distances in the 1.766(8)-1.786(7) Å range. HF-SCF calculations on the model compounds [Re(≡CC 6H 5)(H 2-PCH=CHPH 2) 2C1] + (2m), [Re(≡CC 6H 5)(PH 3) 2(H 2O)(CO)Cl] + (3m), and [Re(≡CC 6H 5)(H 2-PCH=CHPH 2)(CO) 2(OH)] + (13m) suggest that the HOMO is π(Re≡C-Ph) and the LUMO is ¶*(Re=C-Ph). CI-singles calculations on the excited state of optimized 2m indicate that the lowest energy UV-vis absorption of 2-6, 10, and 13 originates from a HOMO to LUMO spin-forbidden transition. This is identified as 3[π(Re≡CAr) → π* (Re≡CAr)], where d(Re) → p(≡C) MLCT character is apparent and the p(≡C) orbital and phenyl π system are conjugated. The UV-vis absorption spectra of 7-9 are significantly different, and their lowest energy absorption is assigned d(Re) → π*(X 2-bpy). The rhenium(V) benzylidyne complexes are highly emissive at room temperature and 77 K. The combination of spectroscopic studies and theoretical calculations suggest that the emitting state of 2-6, 10, and 13 is 3[π(Re≡CAr)→π*(Re=CAr)] but that of 7-9 is d(Re) →*(X 2-bpy). The emission energies in dichloromethane can be adjusted from 520 to 610 nm by variation of the benzylidyne and ancillary ligands. Their electrochemical behaviors are examined and provide further evidence to support the excited-state assignment.