Insights into crystallization mechanism: a synchrotron study of polymorphism in a cobalt acetate cluster compound.

The structure of the title compound, di-mu3-acetato-1kappa2O,O':2kappaO';2kappaO:3kappa2O,O'-di-mu2-acetato-1kappaO:2kappaO';2kappaO:3kappaO'-octapyridyl-1kappa3N,2kappa2N,3kappa3N-tricobalt(II) bis(hexafluorophosphate), [Co3(C2H3O2)4(C5H5N)8](PF6)2, consists of divalent multinuclear cations in which three CoII ions are bridged by four mu2-acetate groups. The CoII ions are arranged in an approximately linear manner. The bridging acetates adopt two distinct coordination geometries: one pair bridges via a single O atom and the other pair employs both O atoms. The coordination octahedron around each CoII ion is completed by three pyridine molecules for the two outer CoII ions and by two for the inner ion. Charge is balanced by two PF6- anions. Single-crystal synchrotron X-ray studies indicate the existence of two polymorphs, both triclinic, which are distinguished primarily by differences in the relative orientations of the multinuclear cations, which in form 1 are tilted with respect to each other, but in form 2 are co-parallel as a result of the central Co atom lying on an inversion centre. The results of the structural studies allow an insight into the crystallization mechanism and resultant polymorphism. They suggest that a (bidentate carboxyl)C-O...H-C(pyridine) interaction exists in solution. For form 1, crystallized from pyridine, the interaction is not structure determining, as it is satisfied by interactions between solvent and solute. For form 2, crystallized from CH2Cl2, the interaction is between a bound acetate carboxyl group on one cation and a bound pyridine on another and is thus structure directing.