Preparation, Magnetic and Structural Study on Oxido-Bridged Diiron(III) Complexes with Open-Chain Tetrapyrrolic 2,2′-Bidipyrrin Ligands

Four (mu-oxido)diiron(III) complexes of differently alkyl-substituted 2,2'-bidipyrrin ligands [(L)Fe](2)O have been prepared by a one-pot approach from the respectively substituted 2,2'-bipyrroledialdehydes and 3,4-dialkylpyrroles, and from a suitable source of trivalent iron, under alkaline conditions. Four single-crystal X-ray diffraction studies were under-taken on three of the new compounds as different solvates. The iron atoms are five-coordinate in all cases, and the observed Fe-N and Fe-O distances are in the range of 2.024-2.082 angstrom and 1.761-1.818 angstrom, respectively. These bonds are (porphyrinato) iron-like, typical for high spin iron(III) complexes, and clearly. distinct from those observed for the respective mononuclear (chlorido)-, (bromido)- and (iodido)iron(III)-2,2'-bidipyrrins, which contain a central intermediate spin iron(III) ion in the solid state. All complexes contain two helically distorted tetrapyrrolic ligands with the same sense of helicity, but pack quite differently from one case to the other. A cleft is present on one side of the connecting oxygen atom, which is occupied either by a self-complementary second molecule of the complex itself, or by a small molecule (dichloromethane, or pyrrole with N-H center dot center dot center dot O bond). The Fe-O-Fe subunits are bent and can be grouped with respect to the Fe-O-Fe bond angle range of either 153.91-156.89 degrees or 131.89-132.97 degrees. All new compounds were investigated by analytical, spectroscopic and magnetic means and found weakly paramagnetic and dynamic with respect to helix inversion at ambient temperature in solution. SQUID magnetometry revealed antiferromagnetically coupled high spin iron(III) species with exchange coupling constants of J = -111 to -122 cm(-1). The exchange coupling appears independent from the Fe-O-Fe angle as described earlier for related porphyrinoid and salen-type (mu-oxido)diiron(III) species. ((C) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)