Pentacoordinated cloro-bis-o-iminosemiquinonato Mn and Fe complexes

The new pentacoordinated chloro-bis-o-iminosemiquinonato complexes (imSQOMe)2MnIIICl (1) and (imSQOMe)2FeIIICl (2) based on 4,6-di-tert-butyl-N-(2-methoxyphenyl)-o-aminophenol were synthesized. As established by X-ray diffraction analysis the compounds 1 and 2 are structural analogues. No additional coordination of methoxy groups to metal center was found in the case of both investigated complexes. Electrochemical oxidation leads to the bonding of functional –OMe group with iron(III) ion in compound 2. Triplet ground spin state (S = 1) for 1 is realized due to a prevailing strong antiferromagnetic interaction between unpaired electrons of high-spin manganese(III) (d4, SMn = 2) and two o-iminosemiquinonato radicals (SR = 1/2). The temperature dependence of effective magnetic moment (μeff) indicates the doublet ground spin state (S = 1/2) and thermally occupied quartet spin state (S = 3/2) for complex 2. The performed DFT calculations demonstrate that the magnetic properties of 2 are conditioned by the strong antiferromagnetic exchange of Fe-imSQ type as well as the transformation of iron(III) ion from high-spin configuration to intermediate-spin state (SFe = 5/2 → S = 3/2) with a decrease in temperature as a result of spin-crossover. The latter phenomenon was proved by Mossbauer spectroscopy. At the same time the intermediate-spin manganese(III) isomer (i.s.MnIII, SMn = 1) is destabilized by more than 20 kcal/mol, and therefore spin-crossover in compound 1 is impossible.