Vanadyl as a Spectroscopic Probe of Tunable Ligand Donor Strength in Bimetallic Complexes

Incorporation of secondary metal ions into heterobimetallic
complexes
has emerged as an attractive strategy for rational tuning of compounds’
properties and reactivity, but direct solution-phase spectroscopic
interrogation of tuning effects has received less attention than it
deserves. Here, we report the assembly and study of a series of heterobimetallic
complexes containing the vanadyl ion, [VO]2+, paired with
monovalent cations (Cs+, Rb+, K+,
Na+, and Li+) and a divalent cation (Ca2+). These complexes, which can be isolated in pure form or
generated in situ from a common monometallic vanadyl-containing
precursor, enable experimental spectroscopic and electrochemical quantification
of the influence of the incorporated cations on the properties of
the vanadyl moiety. The data reveal systematic shifts in the V–O
stretching frequency, isotropic hyperfine coupling constant for the
vanadium center, and V(V)/V(IV) reduction potential in the complexes.
These shifts can be interpreted as charge density effects parametrized
through the Lewis acidities of the cations, suggesting broad potential
for the vanadyl ion to serve as a spectroscopic probe in multimetallic
species.