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

Abstract

The reaction of [Re I Br(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 + Bras 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(dπ)→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.