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 assembly and study of a series of heterobimetallic complexes containing the vanadyl ion, [VO]2+, paired with mono-valent cations (Cs+, Rb+, K+, Na+, and Li+) or a di-valent 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 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 and isotropic hyperfine coupling constant for the vanadium center 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.