Excited-State Contributions to Ground-State Properties of Mixed-Valence Dimers: Spectral and Electronic-Structural Studies of [Fe2(OH)3(tmtacn)2]2+ Related to the [Fe2S2]+ Active Sites of Plant-Type Ferredoxins

The electronic transitions of the S = 9/2 class III mixed-valence dimer [Fe 2 (OH) 3 (tmtacn) 2 ] 2+ are assigned using absorption, magnetic circular dichroism (MCD), and resonance Raman (RR) spectroscopies. This set of excited-state assignments allows the relative contributions of metal−metal and metal−ligand interactions to the D 3 h molecular-orbital energy splittings to be estimated. From this analysis the pathway for valence delocalization in this dimer is determined to involve a significant direct Fe···Fe σ-bonding interaction. The spectroscopic analysis is supported by electronic-structure calculations, which predict a spectrum similar to that observed and provide descriptions of the dimer's molecular orbitals. These results are further supported by the observation of a significant increase in the Fe···Fe internuclear separation with σ−σ* excitation, determined by vibronic analysis of the σ−σ* absorption band shape and associated RR excitation profiles. Combined, these results provide a measure of the geometry dependence of the double-exchange electron-transfer parameter, B, that dominates the ground-state magnetic properties of this dimer. The excited-state assignments also identify the superexchange pathways active in this dimer and provide a spectroscopic measure of the ground-state Heisenberg exchange-coupling constant, yielding −23 < J < +2 cm - 1 . The fact that J is small is confirmed by variable-temperature magnetic-susceptibility experiments, which provide an upper limit of ∼70 cm - 1 for the magnitude of antiferromagnetic coupling in this dimer. The contributions of B, J, and vibronic coupling to the valence-delocalized S = 9/2 ground state of [Fe 2 (OH) 3 (tmtacn) 2 ] 2+ are related to the analogous properties of mixed-valence iron−sulfur and other non-heme iron dimers. From this comparison it is concluded that significant direct Fe−Fe electronic coupling also occurs in [Fe 2 S 2 ] + dimers, but the combination of an order-of-magnitude increase in Heisenberg exchange coupling relative to that of [Fe 2 (OH) 3 (tmtacn) 2 ] 2+ and the greater influence of vibronic-coupling terms in S = 1/2 vs S = 9/2 states outweighs this electronic coupling and leads to the valence-trapped antiferromagnetically-coupled ground state observed in [Fe 2 S 2 ] + dimers. The difference in Heisenberg exchange between the [Fe 2 (OH) 3 (tmtacn) 2 ] 2+ and [Fe 2 S 2 ] + dimers is related to specific differences in the superexchange pathways and covalencies of metal−bridge interactions.