IRON(II) COMPLEXES OF 2,2'-BIIMIDAZOLE AND 2,2'-BIBENZIMIDAZOLE AS MODELS OF THE PHOTOSYNTHETIC MONONUCLEAR NONHEME FERROUS SITES - SYNTHESIS, MOLECULAR AND CRYSTAL-STRUCTURE, AND MOSSBAUER AND MAGNETIC STUDIES

A series of seven high-spin ferrous complexes of 2,2′-biimidazole and 2,2′-bibenzimidazole ligands have been prepared and studied with IR and Mossbauer spectroscopy and variable-temperature magnetic susceptibility. The crystal and molecular structures of [Fe(bbzimH2)3](CIO4)2-H2O (1) and [Fe(bbzimH2)3]Cl2·-MeOH-4.33H2O (2) have been determined. 1 crystallizes in the monoclinic system, space group P21/c, with Z = 4 and a = 13.564 (3) ⇋, b = 19.122 (3) ⇋, c = 18.328 (5) ⇋, and β = 109.82 (2)°. 2 crystallizes in the hexagonal system, space group R3, with Z = 6 and a = 15.509 (3) ⇋ and c = 37.989 (7) ⇋. The structure of 1 consists of the mononuclear cations [Fe(bbzimH2)3]2+separated by C1O4−anions and molecules of water of crystallization. The structure of 2 includes [Fe(bbzimH2)3]2+cations with two chloride anions and one water molecule statistically disordered over three sites. Steric hindrance considerations indicate that the bbzimH2ligands of Fe(bbzimH2)2]Cl2(4) and Fe(bbzimH2)2](HCO2)2(7) are cis to each other as confirmed by the Mossbauer spectroscopy results. A trans arrangement of the bimH2ligands of Fe(bimH2)2Cl2(3) and Fe(bimH2)2(HCO2)2(6), although implying some steric hindrance, is favored by the Mossbauer results. The Mossbauer spectroscopy and variable-temperature magnetic susceptibility results obtained provide evidence that the seven complexes described herein are mononuclear high-spin iron(II) species experiencing zero-field splitting of the iron(II) ground state. The distortion of the coordination sphere is predominantly axial for 2–4, 6 and 7 and rhombic for 1 and 5 (Fe(bimH2)2(CH3CO2)2). Complex 5 shows two iron(II) sites possibly corresponding to a mixture of cis and trans arrangements. This complex shows the closest similarity to the bacterial reaction centers and photosystem 2 ferrous sites. The large variability in the Mossbauer properties in this rather similar set of complexes indicates that in addition to the nature of the ligands the distortion of the Fe(II) coordination sphere is a factor that has to be taken into account in designing proper spectral analogues of the photosynthetic iron(II) complex. © 1990, American Chemical Society. All rights reserved.