Effect of Heme-Heme Interactions and Modulation of Metal Spins by Counter Anions in a Series of Diiron(III)-μ-hydroxo Bisporphyrins: Unusual Stabilization of Two Different Spins in a Single Molecular Framework

Abstract

A new family of five ethene-bridged diiron(III)-μ-hydroxo bisporphyrins with the same core structure but different counter anions, represented by the general formula [Fe2(bisporphyrin)]OH⋅X (X=counter anion), is reported herein. In these complexes, two different spin states of Fe are stabilized in a single molecular framework. Protonation of the oxo-bridged dimer 1 by strong Brønsted acids such as HI, HBF4, HPF6, HSbF6, and HClO4 produces the μ-hydroxo complexes with I5− (2), BF4− (3), PF6− (4), SbF6− (5), and ClO4− (6) as counter anions, respectively. The X-ray structures of 2 and 6 have been determined, which provide a rare opportunity to investigate structural changes upon protonation. Spectroscopic characterization has revealed that the two iron(III) centers in 2 are nonequivalent with nearly high and admixed-intermediate spins in both the solid state and solution. Moreover, the two different FeIII centers of 3–5 are best described as having admixed-high and admixed-intermediate spins with variable contributions of S=5/2 and 3/2 for each state in the solid, but two different admixed-intermediate spins in solution. In contrast, the two FeIII centers in 6 are equivalent and are assigned as having high and intermediate spin states in the solid and solution, respectively. The X-ray structures reveal that the FeO bond length increases on going from the μ-oxo to the μ-hydroxo complexes, and the Fe-O(H)-Fe unit becomes more bent, with the dihedral angle decreasing from 150.9(2)° in 1 to 142.3(3)° and 143.85(2)° in 2 and 6, respectively. Variable-temperature magnetic data have been subjected to a least-squares fitting using the expressions derived from the spin Hamiltonians H=−2JS1⋅S2−μ⋅B+D[equation image−1/3S(S+1)] (for 2, 3, 4, and 5) and H=−2JS1⋅S2 (for 6). The results show that strong antiferromagnetic coupling between the two FeIII centers in 1 is attenuated to nearly zero (−2.4 cm−1) in 2, whereas the values are −46, −32.6, −33.5, and −34 cm−1 for 3, 4, 5, and 6, respectively.