Conformational analysis of homodinuclear mixed ligand complexes of iron(II), iron(III),and cobalt(II) with a pyrazolone derivative and pyridine

Conformational analysis with MM+ force field was performed for mixed-ligand homodinuclear complexes of the type: M-2(BAMP)py(2)Cl(n) where for n = 4, M = Co+2, or Fe+2, and for n = 6, M = Fe+3; BAMP = N,N'-bis(antipyrylmethyl)-piperazine.. According to the experimental data, in Co2+ complexes, the cobalt ions are chiral and they have a distorted T-d geometry. Therefore the Co2+ complexes can have at least 8 pairs of diastereoisomers. Nine of them have the MM+ relative energies in the range of 3 kcal/mol. Experimental data indicate a distorted O-h geometry of the iron centers in iron(II) and iron(III) complexes. Accordingly, the iron(III) complexes can have at least 64 position isomers, while the iron(II) complexes can have at least 576 position isomers. Many of them have very close MM+ energies populating the low energy minima on the potential energy surface, PES. The standard MM+ force field gives M-O bond lengths by about 0.1-0.15 A shorter than the known experimental values for M-O bond lengths in the Co2+, Fe2+ or Fe3+ complexes with ligands having similar donor atoms. The ring bond angles and those around metal ions have values in the range of the known experimental ones for the complexes with similar donor atoms and coordination geometry. The great number of the coexisting isomers in the thermally populated minima on the PES could be a reason for the failure to obtain single crystals of the studied mixed-ligand complexes.