Do the bridging angle affect the luminescent properties of [(CO)₃(phen)Re(µ-OH)Re(phen)(CO)₃]⁺?. An experimental and computational study on three polymorphs

The reaction of [Re^IBr(CO)₃(THF)]₂ with phenanthroline in solution leads to the diimine rhenium(I)tricarbonyl [(phen)Re(CO)₃Br] and the dimer [(CO)₃(phen)Re(µ-OH)(phen)Re(CO)₃]⁺Br⁻ (1⁺Br⁻) as a by-product (~5% yield). This latter bimetallic compound crystallizes as a mixture of three polymorphs, with variable amount of solvent: 1⁺Br⁻·CHCl₃ (triclinic and orthorhombic) and 1⁺Br⁻·2(CHCl₃) (triclinic). Rietveld analysis showed 38, 54 and 8% of 1⁺Br⁻·CHCl₃ (triclinic), 1⁺Br⁻·CHCl₃ (orthorhombic) and 1⁺Br⁻·2(CHCl₃) (triclinic) respectively. The 1⁺ cation shows two [Re^I(phen)(CO)₃]⁺ units connected by means of a central hydroxyl group. Noteworthy, the bridging Re–O–Re angle varies between 130.9(7)° to 140.7(2)° along the polymorphic series. UV–Vis spectrum for the 1⁺ shows a broad absorption band around 390 nm in CH₂Cl₂ which has been attributed to has MLCT character (Re(d_π) → π*(phen)) as confirmed by DFT. Excitation at 405 nm in solution leads to emission at 595 nm (DCM), 605 nm (DMF) and 625 nm (CH₃CN). The emission follows a mono-exponential law of decay and has lifetime of 405.8 ns (DCM). The solid-state polymorphic mixture absorbs around 460 nm. Excitation of the solid at 320 nm leads to emission with a maximum at 575 nm, which follow a bi-exponential law of decay with two components, a short one τ₁ of 27.0 ns and longer one τ₂ of 178.5 ns. These results suggest that the emitting state does not vary as function of the molecular geometry.