Structural and spectroscopic studies on rhenium(III) diphenyl(2-pyridyl)phosphine oxide complexes

The binuclear dual emitter [Br(CO)3Re(PN)(NP)Re(CO)3Br] (PN): 3-chloro-6-(4-diphenylphosphinyl)butoxypyridazine, a new bridging P,N-bidentate ligand resulting from the ring opening of tetrahydrofuran

Lithium diphenylphosphide unexpectedly provokes the ring-opening of tetrahydrofuran (THF) and by reaction with 3,6-dichloropyridazine leads to the formation of the ligand 3-chloro-6-(4-diphenylphosphinyl)butoxypyridazine (P⋯N), which was isolated. The reaction of this ligand with the (Re(CO)₃(THF)Br)₂ dimer yields the novel complex [Br(CO)₃Re(μ-3-chloro-6-(4-diphenylphosphinyl)butoxypyridazine)₂Re(CO)₃Br] (BrRe(P⋯N)(N⋯P)ReBr), which was crystallized in the form of a chloroform solvate, (C₄₆H₄₀Br₂Cl₂N₄O₈P₂Re₂)·(CHCl₃). The monoclinic crystal (P2₁/n) displays a bimetallic cage structure with a symmetry inversion centre in the middle of the rhenium to rhenium line. The molecule shows two oxidation signals occurring at +1.50 V and +1.76 V which were assigned to the Re^I/Re^II and Re^II/Re^III metal-centered couples, respectively, while signals observed at -1.38 V and -1.68 V were assigned to ligand centered reductions. Experimental and DFT/TDDFT results indicate that the UV-Vis absorption maximum of BrRe(P⋯N)(N⋯P)ReBr occurring near 380 nm displays a metal to ligand charge transfer (MLCT) character, which is consistent with CV results. Upon excitation at this wavelength, a weak emission (Φ_em < 1 × 10^-3) is observed around 580 nm (in dichloromethane) which decays with two distinct lifetimes τ₁ and τ₂ of 24 and 4.7 ns, respectively. The prevalence of non-radiative deactivation pathways is consistent with efficient internal conversion induced by the high conformational flexibility of the P⋯N ligand's long carbon chain. Measurements in a frozen solvent at 77 K, where vibrational deactivation is hindered, show intense emission associated with the ³MLCT state. These results demonstrate that BrRe(P⋯N)(N⋯P)ReBr preserves the dual emitting nature previously reported for the mononuclear complex RePNBr, with emission associated with and states.