Preparation, spectroscopic, and structural characterization of the first Co(III) cyanoxime complex: two polymorphs of fac-, tris(benzoylcyanoximato)cobalt(III), Co(BCO)₃

Abstract

Coordination chemistry of cyanoximes–low molecular weight organic molecules having general formula NC–C(=NOH)–R–received attention in recent years due to their pronounced biological activity and interesting properties. The preparation, spectroscopic properties, electrochemical properties, and crystal structure of the first representative of a family of cobalt(III) tris-cyanoximates–Co(BCO)₃ (where BCO is the anion of the benzoylcyanoxime, C₆H₅–C(O)–C(NO)–CN^–)–are now reported. The complex crystallizes as red prisms from CHCl₃ solution as the fac-isomer in centrosymmetric R3 space group with three pairs of enantiomers in the unit cell (Z = 6). Slow evaporation of CH₃CN solution of Co(BCO)₃ leads to crystallization of the same fac-complex without a solvent molecule in the lattice, only this time in the centrosymmetric monoclinic C2/c space group (Z = 8). Both structures have converged with final values R ₁ = 0.0462 (wR ₂ = 0.1273) and R ₁ = 0.0323 (wR ₂ = 0.0809), respectively. Therefore, two polymorphs of Co(BCO)₃ were observed, which is a rare case for this transition metal in Werner-type complexes. The cyanoxime anion forms a five-membered chelate ring by the nitrogen of the nitroso and the oxygen of the carbonyl, and the ligand adopts cis-anti configuration. The anion is bound to the metal center via short, covalent Co–N and Co–O bonds, and represents the second known case of very tight binding of an oxime ligand to Co(III). There is a significant degree of ligand-to-metal charge transfer reflected in an unusually low value of the redox potential Co(III) + e = Co(II) (E _1/2 = −0.0975 V) as opposed to that tabulated for aqueous solutions +1.82 V, and low-field shifts of the positions of the signals in ¹³C NMR spectrum of fac-Co(BCO)₃ compared to the free ligand.

Keywords:

• Benzoylcyanoxime
• Cobalt(III) complex
• UV-Vis spectroscopy
• Polymorphs
• X-ray analysis
• Cyclic voltammetry

Acknowledgments

N. Gerasimchuk is very thankful to the MSU Graduate College for the initial support of this investigation, to Prof. Mark Richter for his help with electrochemical measurements, to Dr Svitlana Silchenko for her help with pK measurements for the HBCO ligand, to Dr Konstantin V. Domasevitch for his help with the partial resolution of the solvent disorder in the crystal structure of the cobalt(III) complex, and to Ms. Alex Corbett for technical help.