Structures and Properties of Dibridged (μ-Oxo)diiron(III) Complexes. Effects of the Fe-O-Fe Angle
A series of (μ-oxo)diiron(lll) complexes of tris(2-pyridylmethy1)amine (TPA), [Fe2(TPA)2O(L)] (CIO4)n, were synthesized and characterized where L represents the bridging ligands carbonate, hydrogen maleate, diphenyl phosphate, diphenylphosphinate, maleate, and phthalate. Together with the linear dichloride complex, this series of compounds provides a unique opportunity to systematically study the effects of the Fe-0-Fe angle (125-180°) on the electronic spectral and magnetic properties of the (μ-oxo)diiron(lll) core. [Fe2(TPA)2O(CO3)](CIO4)2.2CH3OH (1) crystallizes in the monoclinic space group P21/c with a = 11.282 (7) Å, b = 18.253 (9) Å, c = 20.390 (7) Å, and β= 95.02 (4)°. The structure was determined at -50°C from 4544 out of a total of 8154 reflections with R = 0.068 and Rw = 0.080. [Fe2(TPA)2O(maleateH)](C104)3.2CH3COCH3(4) crystallizes in the monoclinic space group P21/n with a = 21.604 (6)°, b = 11.76 (1) Å, c = 22.150 (7) Å, and β = 115.62 (3)°. The structure was determined at -50°C from 4832 out of a total of 7043 reflections with R = 0.072 and Rw = 0.089. [Fe2(TPA)2O(phthalate)](C104)2CH3OH.H2O(9) crystallizes in the monoclinic space group PI with a = 12.170 (5) Å, b = 12.982 (9) Å, c = 17.070 (7) Å, ∞=115.62 (3)°, and γ=62.76 (6)°. The structure was determined at -32°C from 3592 out of a total of 8329 reflections with R = 0.059 and Rw = 0.069. X-ray crystallographic studies of 1,4, and 9 establish the presence of a doubly bridged diiron core in which complexes 1 and 4 contain distinct iron centers bridged by μ-1,3-carboxylates while 9 exhibits a symmetric diiron core bridged by a μp-1,6-phthalate. These studies also reveal that the (μ-oxo)diiron(III) core expands on going from 1 to 4 to 9 due to the increasing bites of the bridging ligands. The Fe-0-Fe bond angles of 1, 4, and 9 are 125.4, 131.0, and 143.4°, respectively, while the Fe-Fe distances are 3.196, 3.261, and 3.402 Å, respectively. 1H NMR spectra indicate that the iron(ll1) centers of 1 and 4 remain distinct in solution while 9 retains its symmetric structure. While the magnetic properties of the complexes appear to be independent of the Fe-O-Fe angle, the visible absorption features of the complexes systematically blue shift as the Fe-0-Fe angle increases. The latter trend suggests that the visible bands are dominated by oxo-to-Fe(1II) charge-transfer transitions.