Effect of Inter-Porphyrin Distance on Spin-State in Diiron(III) μ-Hydroxo Bisporphyrins

Abstract

The synthesis, structure, and properties of bischloro, μ-oxo, and a family of μ-hydroxo complexes (with BF4−, SbF6−, and PF6− counteranions) of diethylpyrrole-bridged diiron(III) bisporphyrins are reported. Spectroscopic characterization has revealed that the iron centers of the bischloro and μ-oxo complexes are in the high-spin state (S=5/2). However, the two iron centers in the diiron(III) μ-hydroxo complexes are equivalent with high spin (S=5/2) in the solid state and an intermediate-spin state (S=3/2) in solution. The molecules have been compared with previously known diiron(III) μ-hydroxo complexes of ethane-bridged bisporphyrin, in which two different spin states of iron were stabilized under the influence of counteranions. The dimanganese(III) analogues were also synthesized and spectroscopically characterized. A comparison of the X-ray structural parameters between diethylpyrrole and ethane-bridged μ-hydroxo bisporphyrins suggest an increased separation, and hence, less interactions between the two heme units of the former. As a result, unlike the ethane-bridged μ-hydroxo complex, both iron centers become equivalent in the diethylpyrrole-bridged complex and their spin state remains unresponsive to the change in counteranion. The iron(III) centers of the diethylpyrrole-bridged diiron(III) μ-oxo bisporphyrin undergo very strong antiferromagnetic interactions (J=−137.7 cm−1), although the coupling constant is reduced to only a weak value in the μ-hydroxo complexes (J=−42.2, −44.1, and −42.4 cm−1 for the BF4, SbF6, and PF6 complexes, respectively).