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

The structure of the title compound, di‐μ 3 ‐acetato‐1κ 2 O , O ′:2κ O ′;2κ O :3κ 2 O , O ′‐di‐μ 2 ‐acetato‐1κ O :2κ O ′;2κ O :3κ O ′‐octapy­ridyl‐1κ 3 N ,2κ 2 N ,3κ 3 N ‐tri­cobalt(II) bis­(hexa­fluoro­phosphate), [Co 3 (C 2 H 3 O 2 ) 4 (C 5 H 5 N) 8 ](PF 6 ) 2 , consists of divalent multinuclear cations in which three Co II ions are bridged by four μ 2 ‐acetate groups. The Co II 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 Co II ion is completed by three pyridine mol­ecules for the two outer Co II ions and by two for the inner ion. Charge is balanced by two PF 6 − 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 CH 2 Cl 2 , the inter­action is between a bound acetate carboxyl group on one cation and a bound pyridine on another and is thus structure directing.