Electron Localization and Delocalization in Mixed-Valence Transition Metal Clusters:  Structural and Spectroscopic Studies of Oxo-Centered Trinuclear Complexes [Fe3O(OOCCMe3)6(py)3]+/0and [Mn3O(OOCCMe3)6(py)3]+/0

Abstract

Crystal structures are reported for the following complexes [Fe3O(OOCCMe3)6(py)3]ClO4·0.5py, 1; [Mn3O(OOCCMe3)6(py)3]ClO4·MeCN, 2; [Mn3O(OOCCMe3)6(py)3], 4; [Fe3O(OOCCMe3)6(py)3], 5. Crystal data are as follows: 1, system monoclinic, space group P21/n, a = 11.658(5) Å, b = 35.450(5) Å, c = 14.084(5) Å, β = 100.10(1)°, V = 5730(3) Å3, Z = 4; 2, orthorhombic, P212121, a = 18.592(11) Å, b = 25.924(11) Å, c = 13.290(5) Å, V = 6405(3) Å3, Z = 4; 4, monoclinic, P21/c, a = 17.941(4) Å, b = 14.945(3) Å, c = 21.655(4) Å, β = 96.50(3)°, V = 5769(2) Å3, Z = 4; 5, monoclinic, P21, a = 11.800(3) Å, b = 20.238(7) Å, c = 12.003(2) Å, β = 106.97(2)°, V = 2741.6(13) Å3, Z = 2. In 1 and 2, the triangular metal ion clusters are close to equilateral geometry, but the coordination polyhedra of the metal ions are significantly different. Those of the fully oxidized triiron(III) cluster are close to the ideal tetragonal symmetry. Those of the trimanganese(III) cluster show strong Jahn−Teller distortions, the four Mn−carboxylate bonds defining a long and a short O−Mn−O axis and these axes around the three Mn centers being correlated so as to define an approximate 3-fold screw axis for the whole cluster. In the mixed-valence trimanganese complex 4 the coordination implies localized valence states close to (2Mn3+ + Mn2+) but in the triiron complex 5 the bond distances are close to those expected for a partially averaged state (2Fe2.5++ Fe3+). Mössbauer spectra of the mixed-valence iron complex indicate a phase transition to a localized (2Fe3+ + Fe2+) state below T = 96 K. In the high-temperature phase, IR spectra support a model in which the electron transfer takes place between two adjacent iron centers at a rate which remains lower than that of the infrared time scale up to room temperature.