Micromodulation: induction of intramolecular electron transfer by solvate molecule dynamics in the iron acetato oxo methylpyridine mixed-valence complex [Fe3O(O2CCH3)6(3-Me-py)3](solvate)

Abstract

The oxo-centered, trinuclear, mixed-valence iron acetate complexes [Fe30(02CCH3)6(3-Me-py)w3h]e.reS S, = CH,CN (I), toluene (Z), 3-methylpyridine (3), and benzene (4), have been prepared, and the intramolecular electron transfer properties in the solid state have been studied. It is found that the rate of electron transfer is dramatically influenced by changing the solvate molecule. The complex 1 crystallizes in the monoclinic space group P2,/c with 4 molecules in a unit cell which has dimensions a = 12.760 ( 5 ) A, b = 26.233 (1 1) A, c = 14.819 ( 5 ) A, and /3 = 116.20 (3)'. Compound 2 crystallizes in the triclinic space group Pi with 2 = 2, a = 13.056 (4) A, b = 15.902 (3) A, c = 12.896 (2) A, a = 102.82 (2)O, /3 = 118.96 (Z)', and y = 70.02 (2)'. The complex 3, isostructural with 2, was characterized in the triclinic space group Al with Z = 4: at 128 K, a = 12.964 ( 5 ) A, b = 12.976 (4) A, c = 29.293 (9) A, a = 84.01 ( 1 ) O , /3 = 94.41 (l)', and y = 121.76 (1)'; at 298 K, a = 13.114 (4) A, 6 = 13.052 (4) A, c = 30.054 ( 5 ) A, a = 85.81 (4)', /3 = 95.23 (4)', and y = 120.99 ( 5 ) ' . The complex 1 has valence-trapped electronic states from 120 to 298 K on the Mossbauer time scale, and non-equivalent metal sites are seen in its single-crystal X-ray structure. The other three complexes are isostructural as indicated by the single-crystal X-ray structures of 2 and 3 and by a comparison of room-temperature powder X-ray diffraction patterns determined for all three complexes. The three complexes 2, 3, and 4 also exhibit similar Mossbauer spectra. At temperatures below - 100 K there are two doublets in the area ratio of two (high-spin Fell') to one (high-spin Fe"). Increasing the sample temperature of 2, 3, and 4 above - 100 K leads to the appearance of a third average-valence doublet with a small spectral area. Eventually the spectrum changes upon increasing the temperature to become a single-average doublet. The X-ray structural work for 3 carried out at 298 and 128 K shows that the dimensions of the Fe30 triangular complex change appreciably with temperature. The Fe, triangle of 3 at 128 K is asymmetric with three inequivalent Fe ions; the triangle becomes more equilateral at the higher temperature. The solid-state packing arrangement of 2, 3, and presumably 4 consists of Fe30 units arranged twodimensionally in layers with the solvate molecule located in an open space made by three neighboring Fe30 molecules. In the case of 1 CH3CN molecules are sandwiched between pairs of Fe30 molecules. The Fe30 complexes in 2 and 3 adopt a symmetric conformation with all three 3-methylpyridine ligands approximately perpendicular to the Fe30 plane, while in 1 two of the 3-methylpyridine ligands are parallel to the Fe30 plane. The effects of conformation on the charge distribution and intramolecular electron-transfer rate are discussed in light of these and previously reported results. Intramolecular electron transfer is slow in 1 because the environment about the Fe30 complexes in 1 leads to an asymmetric complex with inequivalent iron ions. This inequivalence introduces appreciable potential-energy barriers for intramolecular electron transfer. In the case of 2,3, and 4 it is suggested that an order-disorder phase transition involving motion of the solvate molecules influences the rate of intramolecular electron transfer. A single-crystal 2H NMR study of 2 with perdeuterated toluene shows that at 293 K the toluene solvate molecules are involved in two motions. There is a twofold ring flip about the para axis, and the toluene solvate molecule is jumping between two lattice positions. Variable-temperature (290 to 140 K) 2H NMR data are presented for a microcrystalline sample of 2 with perdeuterated toluene to show that, while the solvate molecule motion has not completely stopped by 140 K, the motion has slowed down considerably.