Octa(μ₃-selenido)hexarhenium(III) complexes containing axial monodentate diphosphine or diphosphine-monoxide ligands

A series of the octahedral hexarhenium(III) complexes containing a variable number of diphosphine (diphos) or diphosphine-monoxide (diphosO) ligands have been prepared by the substitution of the diphosphine Ph₂P(CH₂)_nPPh₂ (n=1 to 5) for the iodide ions in the parent octahedral hexarhenium cluster compound [Re₆Se₈I₆]^3-. The diphosphine Ph₂P(CH₂)_nPPh₂ ligands adopt an η¹-bonding mode with the Re₆(μ₃-Se)₈ core, and the P donor atom in the pendant arm is noncoordinated and oxygenated in most cases. The series of new hexarhenium(III) complexes have been well-defined by ¹H, ¹³C, and ³¹P NMR spectroscopic and FAB-MS data. Four compounds among the series were characterized by X-ray structural determination. Geometrical isomers were identified by NMR spectroscopy as well as by the structural determinations. The apical ligand substitution induces significant change in the redox potentials and the photophysical properties of the Re₆(μ₃-Se)₈ core. The E_1/2 value of the reversible process Re^III₆/Re^III₅Re^IV becomes more positive with the increasing number of the coordinated P donors. The phosphine-substituted hexarhenium(III) derivatives are highly luminescent, with microsecond scale emissive lifetime at ambient temperature, and the fully substituted derivatives with the formula [Re₆Se₈(η¹ -diphosO)₆]²⁺ display the strongest luminescence with the longest emission lifetimes.