Luminescent Rhenium(I)-Dipyrrinato Complexes

The first Re(I)-dipyrrinato complexes are reported. Complexes with the general formulas fac-[ReL(CO)(3)Cl](-), fac-[ReL(CO)(3)PR3], and [ReL(CO)(2)(PR3)(PR'(3))] have been prepared, where L is one of a series of meso-aryl dipyrrinato ligands. Access to these complexes proceeds via the reaction of [Re(CO)(5)Cl] with the dipyrrin (LH) to produce fac-[ReL(CO)(3)Cl](-). A subsequent reaction with PR3 (R = phenyl, butyl) leads to displacement of the chloride ligand to generate fac-[ReL(CO)(3)PR3], and further reaction with PR'(3) leads to the displacement of the CO ligand trans to the first PR3 ligand to give trans(P), cis(C)-[ReL(CO)(2)(PR3)(PR'(3))]. The structures of the complexes were determined in the solid state by X-ray crystallography and in solution by H-1 NMR spectroscopy. Electronic absorption spectroscopy reveals a prominent band in the visible region at relatively low energy (472-491 nm) for all complexes, which is assigned as a pi-pi* transition of the dipyrrin chromophore. Weak emission (lambda(ex) = 485 nm, quantum yields <0.01) was observed for [ReL(CO)(3)Cl](-) and [ReL(CO)(3)PR3] complexes, but no emission was generally evident from the [ReL(CO)(2)(PR3)(PR'(3))] complexes. On the basis of the large Stokes shift (similar to 6000 cm(-1)), the emission is ascribed to phosphorescence from a triplet excited state. The emission intensity is sensitive to dissolved oxygen and methyl viologen; a Stern-Volmer plot in the latter case gave a straight line. Photochemical ligand substitution reactions of [ReL(CO)(3)PR3] were induced by excitation with a 355 nm laser in acetonitrile. [ReL(CO)(2)(PR3)(CH3CN)] is formed as a putative intermediate, which reacts thermally with added PR'(3) to produce [ReL(CO)(2)(PR3)(PR'(3))] complexes.