Synthesis, Photophysical and Electrochemical Investigation of Dinuclear Tetrazolato-Bridged Rhenium Complexes

Starting from anionic tetrazole-based ligands, namely,
5-(4′-cyanophenyl)­tetrazolate
and 5-(4′-pyridyl)­tetrazolate, mononuclear and dinuclear complexes
of fac-[Re­(CO)3(phen)]+ (phen
= 1,10-phenanthroline) were prepared and characterized. For the mononuclear
complexes, regioselective coordination of the metal fragments on the
negatively charged tetrazolato ring is exclusively obtained. Coordination
to the benzonitrile and pyridine groups was achieved by previous alkylation
of the tetrazole ring. Dinuclear complexes were obtained by treatment
of the corresponding mononuclear tetrazole-bound complexes with fac-[Re­(CO)3(phen)­(THF)]+. The second
rhenium fragment coordinated either to the pyridine ring or, in the
case of the benzonitrile ligand, to the tetrazole ring. The electrochemical
properties were probed in an imidazolium ionic liquid, highlighting
reduction processes centered on the phen ligand and oxidation processes
localized on the metal. The photophysical properties of the complexes
are characterized by phosphorescent emission from triplet metal-to-ligand
charge transfer excited states, with trends in the lifetime and quantum
yield in qualitative agreement with the energy gap law. The two dinuclear
complexes show almost superimposable emission profiles: in the 5-(4′-cyanophenyl)­tetrazolate-bridged
complex, the two metal fragments coordinated to the tetrazole are
equivalent and share a positive charge of +1. On the other hand, the
photophysical properties of the 5-(4′-pyridyl)­tetrazolate-bridged
dinuclear complex suggest energy transfer between the two metal centers.