Investigations of the Magnetic and Spectroscopic Properties of V(III) and V(IV) Complexes

Herein, we utilize a variety of physical
methods including magnetometry
(SQUID), electron paramagnetic resonance (EPR), and magnetic circular
dichroism (MCD), in conjunction with high-level ab initio theory to
probe both the ground and ligand-field excited electronic states of
a series of V­(IV) (S = 1/2)
and V­(III) (S = 1) molecular complexes. The ligand
fields of the central metal ions are analyzed with the aid of ab initio
ligand-field theory (AILFT), which allows for a chemically meaningful
interpretation of multireference electronic structure calculations
at the level of the complete-active-space self-consistent field with
second-order N-electron valence perturbation theory. Our calculations
are in good agreement with all experimentally investigated observables
(magnetic properties, EPR, and MCD), making our extracted ligand-field
theory parameters realistic. The ligand fields predicted by AILFT
are further analyzed with conventional angular overlap parametrization,
allowing the ligand field to be decomposed into individual σ-
and π-donor contributions from individual ligands. The results
demonstrate in VO2+ complexes that while the axial vanadium–oxo
interaction dominates both the ground- and excited-state properties
of vanadyl complexes, proximal coordination can significantly modulate
the vanadyl bond covalency. Similarly, the electronic properties of
V­(III) complexes are particularly sensitive to the available σ
and π interactions with the surrounding ligands. The results
of this study demonstrate the power of AILFT-based analysis and provide
the groundwork for the future analysis of vanadium centers in homogeneous
and heterogeneous catalysts.