Search

The effects of temperature and chemical environment on a pentanuclear cyanide-bridged, trigonal bipyramidal molecular paramagnet have been investigated. Using element- and oxidation state-specific near-ambient pressure X-ray photoemission spectroscopy (NAP-XPS) to probe charge transfer and second order, nonlinear 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, Co 3 Fe 2 (tmphen) 6 (μ-CN) 6 ( t -CN) 6 , abbrev. Co 3 Fe 2 , shows changes in electronic behavior depending on the chemical environment. NAP-XPS shows that temperature changes induce a metal-to-metal charge transfer (MMCT) in Co 3 Fe 2 between a Co and Fe center, while cycling between ultrahigh vacuum and 2 mbar of 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 an 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 the chemical environment and solvation on underlying charge and spin transitions in this and related complexes.

A new series of Rh 2 (II,II) complexes with the formula cis -[Rh 2 (DTolF) 2 (bpnp)(L)] 2+ , where bpnp = 2,7-bis(2-pyridyl)-1,8-naphthyridine, DTolF = N , N' -di( p -tolyl) formamidinate, and L = pdz (pyridazine; 2 ), cinn (cinnoline; 3 ), and bncn (benzo[ c ]cinnoline; 4 ), were synthesized from the precursor cis -[Rh 2 (DTolF) 2 (bpnp)(CH 3 CN) 2 ] 2+ ( 1 ). The first reduction couple in 2-4 is localized on the bpnp ligand at approximately -0.52 V vs Ag/AgCl in CH 3 CN (0.1 M TBAPF 6 ), followed by reduction of the corresponding diazine ligand. Complex 1 exhibits a Rh 2 (δ*)/DTolF → bpnp(π*) metal/ligand-to-ligand charge-transfer ( 1 ML-LCT) absorption with a maximum at 767 nm (ε = 1800 M -1 cm -1 ). This transition is also present in the spectra of 2-4 , overlaid with the Rh 2 (δ*)/DTolF → L(π*) 1 ML-LCT bands at 516 nm in 2 (L = pdz), 640 nm in 3 (L = cinn), and 721 nm in 4 (L = bncn). Complexes 2 and 3 exhibit Rh 2 (δ*)/DTolF → bpnp 3 ML-LCT excited states with lifetimes, τ, of 3 and 5 ns, respectively, in CH 3 CN, whereas the lowest energy 3 ML-LCT state in 4 is Rh 2 (δ*)/DTolF → bncn in nature with τ = 1 ns. Irradiation of 4 with 670 nm light in DMF in the presence of 0.1 M TsOH ( p -toluene sulfonic acid) and 30 mM BNAH (1-benzyl-1,4-dihydronicotinamide) results in the production of H 2 with a turnover number (TON) of 16 over 24 h. The axial capping of the Rh 2 (II,II) bimetallic core with the bpnp ligand prevents the formation of an Rh-H hydride intermediate. These results show that the observed photocatalytic reactivity is localized on the bncn ligand, representing the first example of ligand-centered H 2 production.