Thermally Persistent High-Spin Ground States in Octahedral Iron Clusters

Abstract

Chemical oxidation and reduction of the all-ferrous (HL)2Fe6 in THF affords isostructural, coordinatively unsaturated clusters of the type [(HL)2Fe6] n : [(HL)2Fe6]­[BArF24] (1, n = +1; where [BArF24]− = tetrakis­[(3,5-trifluoromethyl)­phenyl]­borate), [Bu4N]­[(HL)2Fe6] (2a, n = −1), [P]­[(HL)2Fe6] (2b, n = −1; where [P]+ = tributyl­(1,3-dioxolan-2-ylmethyl)­phosphonium), and [Bu4N]2[(HL)2Fe6] (3, n = −2). Each member of the redox-transfer series was characterized by zero-field 57Fe Mössbauer spectroscopy, near-infrared spectroscopy, single-crystal X-ray crystallography, and magnetometry. Redox-directed trends are observed when comparing the structural metrics within the [Fe6] core. The metal octahedron [Fe6] decreases marginally in volume as the molecular reduction state increases as gauged by the Fe–Feavg distance varying from 2.608(11) Å (n = +1) to 2.573(3) (n = −2). In contrast, the mean Fe–N distances and ∠Fe–N–Fe angles correlate linearly with the [Fe6] oxidation level, or alternatively, the changes observed within the local Fe–N4 coordination planes vary linearly with the aggregate spin ground state. In general, as the spin ground state (S) increases, the Fe–N­(H)avg distances also increase. The structural metric perturbations within the [Fe6] core and measured spin ground states were rationalized extending the previously proposed molecular orbital diagram derived for (HL)2Fe6. Chemical reduction of the (HL)2Fe6 cluster results in an abrupt increase in spin ground state from S = 6 for the all-ferrous cluster, to S = 19/2 in the monoanionic 2b and S = 11 for the dianionic 3. The observation of asymmetric intervalence charge transfer bands in 3 provides further evidence of the fully delocalized ground state observed by 57Fe Mössbauer spectroscopy for all species examined (1–3). For each of the clusters examined within the electron-transfer series, the observed spin ground states thermally persist to 300 K. In particular, the S = 11 in dianionic 3 and S = 19/2 in the monoanionic 2b represent the highest spin ground states isolated up to room temperature known to date. The increase in spin ground state results from population of the antibonding orbital band comprised of the Fe–N σ* interactions. As such, the thermally persistent ground states arise from population of the resultant single spin manifolds in accordance with Hund’s rules. The large spin ground states, indicative of strong ferromagnetic electronic alignment of the valence electrons, result from strong direct exchange electronic coupling mediated by Fe–Fe orbital overlap within the [Fe6] cores, equivalent to a strong double exchange magnetic coupling B for 3 that was calculated to be 309 cm–1.

CaMn3IVO4 Cubane Models of the Oxygen‐Evolving Complex: Spin Ground States S<9/2 and the Effect of Oxo Protonation

“…Thermally well-isolated spin ground states have been observed in multinuclear complexes, with values of |J| in the range of 160~900 cm −1 . [74][75][76] Such systems behave as Curie paramagnets, with no temperature dependence of χT. In the absence of double exchange or close metal-metal contacts, the physical nature of the large increase of J in 2-Ca remains unclear, but we provide the following hypothesis.…”

“…[64] The j 5/2, 1i ground state is separated from the j 3/2, 0i first excited state by 210 AE 50 cm À1 and from the j 7/2, 2i second excited state by 350 AE 50 cm À1 .T hermally well-isolated spin ground states have been observed in multinuclear complexes,w ith values of j J j in the range of 160-900 cm À1 . [74][75][76] Such systems behave as Curie paramagnets,with no temperature dependence of cT. In the absence of double exchange or close metal-metal contacts,t he physical nature of the large increase of J in 2-Ca remains unclear,but we provide the following hypothesis.In1-Ca and 4-Ca where each Mn IV ion is coordinated by mostly s-only ligands,t he principal symmetry axis for each Mn center is ill-defined, and thus several competing exchange pathways can be envisioned between d xy ,d xz ,and d yz ,resulting in the overall weak coupling 1 that weaken the overall magnitude of the antiferromagnetic coupling may be reduced.…”

“… [64] The |5/2, 1⟩ ground state is separated from the |3/2, 0⟩ first excited state by 210±50 cm −1 and from the |7/2, 2⟩ second excited state by 350±50 cm −1 . Thermally well‐isolated spin ground states have been observed in multinuclear complexes, with values of | J | in the range of 160–900 cm −1 [74–76] . Such systems behave as Curie paramagnets, with no temperature dependence of χT .…”

CaMn3IVO4 Cubane Models of the Oxygen‐Evolving Complex: Spin Ground States S<9/2 and the Effect of Oxo Protonation

“…[64] The j 5/2, 1i ground state is separated from the j 3/2, 0i first excited state by 210 AE 50 cm À1 and from the j 7/2, 2i second excited state by 350 AE 50 cm À1 .T hermally well-isolated spin ground states have been observed in multinuclear complexes,w ith values of j J j in the range of 160-900 cm À1 . [74][75][76] Such systems behave as Curie paramagnets,with no temperature dependence of cT. In the absence of double exchange or close metal-metal contacts,t he physical nature of the large increase of J in 2-Ca remains unclear,but we provide the following hypothesis.In1-Ca and 4-Ca where each Mn IV ion is coordinated by mostly s-only ligands,t he principal symmetry axis for each Mn center is ill-defined, and thus several competing exchange pathways can be envisioned between d xy ,d xz ,and d yz ,resulting in the overall weak coupling Angewandte Chemie observed.…”

A high-nuclearity complex containing a decanuclear iron(iii)/oxo cage in a football-like structure and rare (R-/S)-hemiacetalate ligands in a butterfly-like format

“…[1][2][3][4][5][6] There are numerous reasons for the interest in the synthesis and study of high nuclearity 3d molecular metal clusters at moderate-to-high oxidation states. 7,8 High nuclearity 3d metal clusters, 9,10 especially the ones that include Ni II , Co II , Mn III and Fe III , oen display fascinating structures, occasionally attractive properties, [11][12][13][14] including single molecular magnet (SMM) and coupled SMM behavior, [15][16][17] and promising applications for data storage, quantum computing and spintronics. 18,19 The high nuclearity iron(III)-oxo chemistry has being developed over many years by virtue of their important role in bioinorganic chemistry, molecular magnetism and so on.…”