Gauging Donor/Acceptor Properties and Redox Stability of Chelating Click-Derived Triazoles and Triazolylidenes: A Case Study with Rhenium(I) Complexes

Bidentate ligands
containing at least one triazole or triazolylidene (mesoionic carbene,
MIC) unit are extremely popular in contemporary chemistry. One reason
for their popularity is the similarities as well as differences in
the donor/acceptor properties that these ligands display in comparison
to their pyridine or other N-heterocyclic carbene counterparts. We
present here seven rhenium­(I) carbonyl complexes where the bidentate
ligands contain combinations of pyridine/triazole/triazolylidene.
These are the first examples of rhenium­(I) complexes with bidentate
1,2,3-triazol-5-ylidene-containing ligands. All complexes were structurally
characterized through 1H and 13C NMR spectroscopy
as well as through single-crystal X-ray diffraction. A combination
of structural data, redox potentials from cyclic voltammetry, and
IR data related to the CO coligands are used to gauge the donor/acceptor
properties of these chelating ligands. Additionally, a combination
of UV–vis–near-IR/IR/electron paramagnetic resonance
spectroelectrochemistry and density functional theory calculations
are used to address questions related to the electronic structures
of the complexes in various redox states, their redox stability, and
the understanding of chemical reactivity following electron transfer
in these systems. The results show that donor/acceptor properties
in these bidentate ligands are sometimes, but not always, additive
with respect to the individual components. Additionally, these results
point to the fact that MIC-containing ligands confer remarkable redox
stability to their fac-Re­(CO)3-containing
metal complexes. These findings will probably be useful for fields
such as homogeneous- and electro-catalysis, photochemistry, and electrochemistry,
where fac-Re­(CO)3 complexes of triazoles/triazolylidenes
are likely to find use.