EPR and XANES studies of anaerobic photolysis of iso-propilpyridinecobaloxime: Elucidation of the reactivity of the Co(II) primary product

• Elucidation of the reaction undergone by the photolysis primary product, [Co(DH)2py].
• Detection of a species [Co(DH)2] with no axially bound pyridine molecules.
• Identification of pyridine source to form photolysis secondary product, [Co(DH)2py2].
• XANES results provide independent structural evidence and corroborate EPR results.
• Structure of [Co(DH)2] and [Co(DH)2py] is notably different as powder or in solution.

Anaerobic photolysis of iso-propilpyridinecobaloxime was monitored by EPR and X-ray Absorption Near Edge spectroscopy (XANES). The spectra of the cobalt(II) photolysis products were analyzed and compared with those obtained for parent cobalt(II) dimethylglyoximato complexes in order to rationalize the involvement of photolysis primary products in further reactions in solution.The results obtained upon photolysis in frozen matrix, confirm that the photolysis primary product is the five coordinate cobalt(II) complex, [Co(DH)2py], and that this species is only observable in frozen solution. As the matrix is relaxed to room temperature the five coordinate species evolves to give rise to a six coordinate cobalt(II) complex, [Co(DH)2py2], that persists at room temperature and that is observable in EPR upon re-cooling to frozen solution. When monitoring the warming up of the irradiated frozen solution we observe an EPR signal which is assigned to a square planar species, [Co(DH)2], providing evidence for the source of pyridine molecules to originate the compound with two bound pyridine molecules. Since the latter signal is lost upon warming we hypothesize that the four coordinate species polymerizes or packs into diamagnetic forms at room temperature. The supposition is corroborated by comparison of the values of the magnetic moments of compounds [Co(DH)2py2], [Co(DH)2py] and [Co(DH)2] as powders and in solution. EPR titration of compound [Co(DH)2] with pyridine substantiates the latter results and shows that the complex with two bound pyridine molecules is the predominant species from a 1:1 M ratio.The XANES profiles of powder and solution samples of compound [Co(DH)2] are meaningfully different and indicative that substantial axial interactions are present in solution while the powder's profile is closely related with those described for square planar complexes. The presence of two solvent molecules bound to [Co(DH)2] in solution is confirmed by the values of the spin-Hamiltonian parameters obtained for the same sample.The use of XANES spectroscopy was particularly valuable to provide independent evidence that the structure of [Co(DH)2] and [Co(DH)2py] is different in powder and in solution and to monitor the change in oxidation state and geometry during the progression of the photolysis process.

Study of anaerobic photolysis of i-C3H7Co(DH)2py allowed elucidation of the reaction undergone by the photolysis primary product, [Co(DH)2py] and characterization of a species whose spin-Hamiltonian parameters are consistent with a square planar [Co(DH)2] complex with no bound pyridine molecules revealing the source of pyridine to form the secondary product, [Co(DH)2py2].Figure optionsDownload as PowerPoint slide