Photophysical and Photochemical Trends in Tricarbonyl Rhenium(I) N‑Heterocyclic Carbene Complexes

A family of tricarbonyl Re­(I) complexes
of the formulation fac-[Re­(CO)3(NHC)­L]
has been synthesized and characterized, both spectroscopically and
structurally. The NHC ligand represents a bidentate N-heterocyclic
carbene species where the central imidazole ring is substituted at
the N3 atom by a butyl, a phenyl, or a mesityl group and substituted
at the N1 atom by a pyridyl, a pyrimidyl, or a quinoxyl group. On
the other hand, the ancillary L ligand alternates between chloro and
bromo. For the majority of the complexes, the photophysical properties
suggest emission from the lowest triplet metal-to-ligand charge transfer
states, which are found partially mixed with triplet ligand-to-ligand
charge transfer character. The nature and relative energy of the emitting
states appear to be mainly influenced by the identity of the substituent
on the N3 atom of the imidazole ring; thus, the pyridyl complexes
have blue-shifted emission in comparison to the more electron deficient
pyrimidyl complexes. The quinoxyl complexes show an unexpected blue-shifted
emission, possibly occurring from ligand-centered excited states.
No significant variations were found upon changing the substituent
on the imidazole N3 atom and/or the ancillary ligand. The photochemical
properties of the complexes have also been investigated, with only
the complexes bound to the pyridyl-substituted NHC ligands showing
photoinduced CO dissociation upon excitation at 370 nm, as demonstrated
by the change in the IR and NMR spectra as well as a red shift in
the emission profile after photolysis. Temperature-dependent photochemical
experiments show that CO dissociation occurs at temperatures as low
as 233 K, suggesting that the Re–C bond cleaves from excited
states of metal-to-ligand charge transfer nature rather than thermally
activated ligand field excited states. A photochemical mechanism that
takes into account the reactivity of the complexes bound to the pyridyl-NHC
ligand as well as the stability of those bound to the pyrimidyl- and
quinoxyl-NHC ligands is proposed.