Novel Rectangular [Fe<sub>4</sub>(μ<sub>4</sub>-OHO)(μ-OH)<sub>2</sub>]<sup>7+</sup> versus “Butterfly” [Fe<sub>4</sub>(μ<sub>3</sub>-O)<sub>2</sub>]<sup>8+</sup> Core Topology in the Fe<sup>III</sup>/RCO<sub>2</sub><sup>-</sup>/phen Reaction Systems (R = Me, Ph; phen = 1,10-Phenanthroline): Preparation and Properties of [Fe<sub>4</sub>(OHO)(OH)<sub>2</sub>(O<sub>2</sub>CMe)<sub>4</sub>(phen)<sub>4</sub>](ClO<sub>4</sub>)<sub>3</sub>, [Fe<sub>4</sub>O<sub>2</sub>(O<sub>2</sub>CPh)<sub>7</sub>(phen)<sub>2</sub>](ClO<sub>4</sub>), and [Fe<sub>4</sub>O<sub>2</sub>(O<sub>2</sub>CPh)<sub>8</sub>(phen)<sub>2</sub>]
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The preparations, X-ray structures, and detailed physical characterizations are presented for three new tetranuclear FeIII/RCO2-/phen complexes, where phen = 1,10-phenanthroline: [Fe4(OHO)(OH)2(O2CMe)4(phen)4](ClO4)3·4.4MeCN·H2O (1·4.4MeCN·H2O); [Fe4O2(O2CPh)7(phen)2](ClO4)·2MeCN (2·2MeCN); [Fe4O2(O2CPh)8(phen)2]·2H2O (3·2H2O). Complex 1·4.4MeCN·H2O crystallizes in space group P21/n, with a = 18.162(9) Å, b = 39.016(19) Å, c = 13.054(7) Å, β = 104.29(2)°, Z = 4, and V = 8963.7 Å3. Complex 2·2MeCN crystallizes in space group P21/n, with a = 18.532(2) Å, b = 35.908(3) Å, c = 11.591(1) Å, β = 96.42(1)°, Z = 4, and V = 7665(1) Å3. Complex 3·2H2O crystallizes in space group I2/a, with a = 18.79(1) Å, b = 22.80(1) Å, c = 20.74(1) Å, β = 113.21(2)°, Z = 4, and V = 8166(1) Å3. The cation of 1 contains the novel [Fe4(μ4-OHO)(μ-OH)2]7+ core. The core structure of 2 and 3 consists of a tetranuclear bis(μ3-O) cluster disposed in a “butterfly” arrangement. Magnetic susceptibility data were collected on 1−3 in the 2−300 K range. For the rectangular complex 1, fitting the data to the appropriate theoretical χM vs T expression gave J1 = −75.4 cm-1, J2 = −21.4 cm-1, and g = 2.0(1), where J1 and J2 refer to the FeIIIO(O2CMe)2FeIII and FeIII(OH)FeIII pairwise exchange interactions, respectively. The S = 0 ground state of 1 was confirmed by 2 K magnetization data. The data for 2 and 3 reveal a diamagnetic ground state with antiferromagnetic exchange interactions among the four high-spin FeIII ions. The exchange coupling constant Jbb (“body−body” interaction) is indeterminate due to prevailing spin frustration, but the “wing−body” antiferromagnetic interaction (Jwb) was evaluated to be −77.6 and −65.7 cm-1 for 2 and 3, respectively, using the appropriate spin Hamiltonian approach. Mössbauer spectra of 1−3 are consistent with high-spin FeIII ions. The data indicated asymmetry of the Fe4 core of 1 at 80 K, which is not detected at room temperature due to thermal motion of the core. The spectra of 2 and 3 analyze as two quadrupole-split doublets which were assigned to the body and wing-tip pairs of metal ions. 1H NMR spectra are reported for 1−3 with assignment of the main resonances.
