Water-Mediated Charge Transfer and Electron Localization in a Co3Fe2 Cyanide-Bridged Trigonal Bipyramidal Complex

The effects of temperature and chemical environment on a pentanuclear cyanide-bridged, trigonal bipyramidal molecular magnet has been investigated. Using element and oxidation state specific near-ambient pressure XPS (NAP-XPS) to probe charge transfer and second order, non- linear vibrational spectroscopy, which is sensitive to symmetry changes based on charge (de)localization coupled with DFT a detailed picture of environmental effects on charge-transfer induced spin transitions is presented. The molecular cluster, Co3Fe2(tmphen)6(μ-CN)6(t-CN)6, abbrev. Co3Fe2, shows changes in electronic behavior depending on chemical environment. NAP-XPS shows that temperature changes induce a metal-to-metal charge transfer (MMCT) in Co3Fe2 between a Co and Fe center, while cycling between ultrahigh vacuum and 2 mbar water at constant temperature, causes oxidation state changes not fully captured by the MMCT picture. Sum frequency generation vibrational spectroscopy (SFG-VS) probes the role of the cyanide ligand, which controls the electron (de)localization via the superexchange coupling. Spectral shifts and intensity changes indicate a change from a charge delocalized, Robin-Day Class II/III high spin state to a charge localized, Class I low spin state consistent with DFT. In the presence of a H-bonding solvent the complex adopts a localized electronic structure, while removal of the solvent delocalizes the charges and drives a MMCT. This change in Robin-Day classification of the complex as a function of chemical environment results in reversible switching of the dipole moment, analogous to molecular multiferroics. These results illustrate the important role of chemical environment and solvation on underlying charge and spin transitions in this and related complexes.