Ligand Field Strength Mediates Electron Delocalization in Octahedral [(^HL)₂Fe₆(L′)_m]ⁿ⁺ Clusters

Abstract

To assess the impact of terminal ligand binding on a variety of cluster properties (redox delocalization, ground-state stabilization, and breadth of redox state accessibility), we prepared three electron-transfer series based on the hexanuclear iron cluster [(HL)2Fe6(L′)m]n+ in which the terminal ligand field strength was modulated from weak to strong (L′ = DMF, MeCN, CN). The extent of intracore M–M interactions is gauged by M–M distances, spin ground state persistence, and preference for mixed-valence states as determined by electrochemical comproportionation constants. Coordination of DMF to the [(HL)2Fe6] core leads to weaker Fe–Fe interactions, as manifested by the observation of ground states populated only at lower temperatures (<100 K) and by the greater evidence of valence trapping within the mixed-valence states. Comproportionation constants determined electrochemically (Kc = 104–108) indicate that the redox series exhibits electronic delocalization (class II–III), yet no intervalence charge transfer (IVCT) bands are observable in the near-IR spectra. Ligation of the stronger σ donor acetonitrile results in stabilization of spin ground states to higher temperatures (~300 K) and a high degree of valence delocalization (Kc = 102– 108) with observable IVCT bands. Finally, the anionic cyanide-bound series reveals the highest degree of valence delocalization with the most intense IVCT bands (Kc = 1012–1020) and spin ground state population beyond room temperature. Across the series, at a given formal oxidation level, the capping ligand on the hexairon cluster dictates the overall properties of the aggregate, modulating the redox delocalization and the persistence of the intracore coupling of the metal sites.

Meta-Atom Behavior in Clusters Revealing Large Spin Ground States

“…55 Via the strong electron delocalization apparent in these weak field clusters, the electronic structure begins to approximate that of a single transition-metal ion, facilitating the isolation of the observed ground states from lowest-lying electronic excited states. 54 Consistent with this description, the formally mixed valent 2 does not exhibit intervalence charge-transfer bands in the near-infrared (12000 > ν > 5000 cm −1 , Figure S23). …”

“…While this is yet to be verified, we can account for the observed spin state of the pyridine bound 2 by considering how pyridine ligation alters the frontier orbital picture in Figure 5. Akin to the redox directed binding of acetonitrile or dimethylformamide to [( H L) 2 Fe 6 ] + , 34,54 two nominally ${\mathrm{d}}_{z^2}$-based SALCs (originating from 1e g and 2t 1u ) are destabilized upon binding of solvent to the [( H L) 2 Fe 6 ] − core, leaving two orbitals out of energetic range to be populated. Thus, populating the 37 valence electrons in the remaining 28 orbitals affords a maximum spin ground state of S = 19/2 (Figure 5d) consistent with that determined experimentally, and suggesting Δ E (2t 2u –2t 2g ) ≤ 19150 cm −1 (2.37 eV) the mean spin-pairing energy for a ferrous ion.…”

A Thermodynamic Model for Redox-Dependent Binding of Carbon Monoxide at Site-Differentiated, High Spin Iron Clusters

“…Changes in the identity of an ancillary ligand (DMF, MeCN, or - CN) have been shown to modulate the extent of valence delocalization in a series of hexairon clusters. 62 However, the site-differentiated nature of the clusters examined here allows us to distinguish the effects of CO binding on the electronic properties of the binding site from those on remote metal centers.…”

Nitric oxide activation facilitated by cooperative multimetallic electron transfer within an iron-functionalized polyoxovanadate–alkoxide cluster

“…Linearity of each fully-reversible redox event was confirmed by plotting the current density ( j p reported in A cm –2 ) versus the square root of the scan rate ( v 1/2 reported in V s –1 ). 25 , 47 Comparison of the CV of complex 2-V 5 FeNO to that of the starting material, 1-V 5 Fe , illustrates the drastic changes in the redox properties of the cluster upon coordination of NO, suggesting substrate coordination has a direct effect on the electronic structure of the multimetallic assembly.…”