STRUCTURES AND PROPERTIES OF DIBRIDGED (MU-OXO)DIIRON(III) COMPLEXES - EFFECTS OF THE FE-O-FE ANGLE

A series of (μ-oxo)diiron(Ill) complexes of tris(2-pyridylmethyl)amine (TPA), [Fe2(TPA)20(L)](C104)nwere 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-O-Fe angle (125--180°) on the electronic spectral and magnetic properties of the (μ-oxo)diiron(III) core. [Fe2(TPA)2O(CO3)](ClO4)2·2CH3OH (1) crystallizes in the monoclinic space group P2i/c with a = 11.282 (7) &, b = 18.253 (9) &, c = 20.390 (7) &, 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) Å, 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)l(C1O4)2•CH3OH•H2O (9) crystallizes in the monoclinic space group Pl with a = 12.170 (5) Å, b = 12.982 (9) Å, c = 17.070 (7) Å, α = 77.26 (7)°, β = 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-l,3-carboxylates while 9 exhibits a symmetric diiron core bridged by a μ-l,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-O-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. lH NMR spectra indicate that the iron(IIl) 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-O-Fe angle increases. The latter trend suggests that the visible bands are dominated by oxo-to-Fe(III) charge-transfer transitions. © 1990, American Chemical Society. All rights reserved.