Luminescent Rhenium(I)-Dipyrrinato Complexes

The first Re­(I)-dipyrrinato complexes are reported. Complexes
with
the general formulas fac-[ReL(CO)3Cl]−, fac-[ReL(CO)3PR3], 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)5Cl] with the dipyrrin (LH) to produce fac-[ReL(CO)3Cl]−. A subsequent reaction with PR3 (R = phenyl, butyl) leads
to displacement of the chloride ligand to generate fac-[ReL(CO)3PR3], 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 1H 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 π–π*
transition of the dipyrrin chromophore. Weak emission (λex = 485 nm, quantum yields <0.01) was observed for [ReL(CO)3Cl]− and [ReL(CO)3PR3] complexes, but no emission was generally
evident from the [ReL(CO)2(PR3)­(PR′3)] complexes. On the basis of the large Stokes shift (∼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)3PR3] 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.