Diversity of New Structural Types in Polynuclear Iron Chemistry with a Tridentate N,N,O Ligand

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posted on 2007-05-28, 00:00 authored by Rashmi Bagai, Saiti Datta, Amalia Betancur-Rodriguez, Khalil A. Abboud, Stephen Hill, George Christou

The syntheses, crystal structures, and magnetochemical characterization of four new iron clusters [Fe7O4(O2CPh)11(dmem)2] (1), [Fe7O4(O2CMe)11(dmem)2] (2), [Fe6O2(OH)4(O2CBut)8(dmem)2] (3), and [Fe3O(O2CBut)2(N3)3(dmem)2] (4) (dmemH = Me2NCH2CH2N(Me)CH2CH2OH) = 2-{[2-(dimethylamino)ethyl]methylamino}ethanol) are reported. The reaction of dmemH with [Fe3O(O2CR)6(H2O)3](NO3) (R = Ph (1), Me (2), and But (3)) gave 1, 2, and 3, respectively, whereas 4 was obtained from the reaction of 3 with sodium azide. The complexes all possess rare or novel core topologies. The core of 1 comprises two [Fe4(μ3-O)2]8+ butterfly units sharing a common body Fe atom. The core of 2 consists of a [Fe3O3] ring with each doubly bridging O2- ion becoming μ3 by also bridging to a third, external Fe atom; a seventh Fe atom is attached on the outside of this core via an additional μ3-O2- ion. The core of 3 consists of a [Fe4(μ3-O)2]8+ butterfly unit with an Fe atom attached above and below this by bridging O atoms. Finally, the core of 4 is an isosceles triangle bridged by a μ3-O2- ion with a rare T-shaped geometry and with the azide groups all bound terminally. Variable-temperature, solid-state dc, and ac magnetization studies were carried out on complexes 1−4 in the 5.0−300 K range. Fitting of the obtained magnetization (M) vs field (H) and temperature (T) data by matrix diagonalization and including only axial anisotropy (zero-field splitting) established that 1, 2, and 4 each possess an S = 5/2 ground state spin, whereas 3 has an S = 5 ground state. As is usually the case, good fits of the magnetization data could be obtained with both positive and negative D values. To obtain more accurate values and to determine the sign of D, high-frequency EPR studies were carried out on single crystals of representative complexes 1·4MeCN and 3·2MeCN, and these gave D = +0.62 cm-1 and |E| ≥ 0.067 cm-1 for 1·4MeCN and D = −0.25 cm-1 for 3·2MeCN. The magnetic susceptibility data for 4 were fit to the theoretical χM vs T expression derived by the use of an isotropic Heisenberg spin Hamiltonian and the Van Vleck equation, and this revealed the pairwise exchange parameters to be antiferromagnetic with values of Ja = −3.6 cm-1 and Jb = −45.9 cm-1. The combined results demonstrate the ligating flexibility of dmem and its usefulness in the synthesis of a variety of Fex molecular species.