Diversity of new structural types in polynuclear iron chemistry with a tridentate N,N,O ligand

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)(N-3)(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 Bu-t (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 [Fe-4(mu(3)-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 mu(3) by also bridging to a third, external Fe atom; a seventh Fe atom is attached on the outside of this core via an additional mu(3)-O2- ion. The core of 3 consists of a [Fe-4(mu(3)-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 mu(3)-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 center dot 4MeCN and 3 center dot 2MeCN, and these gave D = +0.62 cm(-1) and parallel to E parallel to >= 0.067 cm(-1) for 1 center dot 4MeCN and D = -0.25 cm(-1) for 3 center dot 2MeCN. The magnetic susceptibility data for 4 were fit to the theoretical chi(M) 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 J(a) = -3.6 cm(-1) and J(b) = -45.9 cm(-1). The combined results demonstrate the ligating flexibility of dmem and its usefulness in the synthesis of a variety of Fe-x molecular species.