EPR study of the photolysis of methyl- and adenosylcobinamides in the presence of phosphine and pyridine bases. Evidence for the need of a judicious choice of irradiation temperature and solvent to assess ligand binding

• Type: Article in International Scientific Journal
• Date: 2008

Authors

Joao Gomes (Author) Other; Baltazar de Castro (Author) FCUP; Maria Rangel (Author) ICBAS

Journal

Title: Organometallics

Vol. 27

Pages: 2536-2543

ISSN: 0276-7333

Publisher: American Chemical Society

Indexing

ISI Web of Knowledge - 4 Citations

Scopus - 4 Citations

Scientific classification

FOS: Natural sciences > Chemical sciences

Other information

Authenticus ID: P-003-YJJ

DOI: 10.1021/om700837d

Abstract (EN): The products of anaerobic photolysis of methylcobinamide (MeCbi(+)) and adenosylcobinamide (AdoCbi(+)) in the presence of phosphine and pyridine bases have been studied in methanol, at two irradiation temperatures (77 and 300 K) by EPR spectroscopy. The presence of cobalt(II) species and organic radicals in solution is indicative that homolytic cleavage of the Co-C bond is induced upon irradiation with visible light. The EPR results obtained upon photolysis of cobinamide solutions in the presence of several cobinamide:phosphorus base molar ratios show that only for a large excess of base is the 1:1 adduct quantitatively formed, and is, in those experimental conditions, the single Co(II) species in solution. For molar ratios below 1:20 the EPR spectra allow identification of a mixture of the 1:1 adduct and a species with no phosphorus ligand bound to the cobalt(II) center. For pyridine derivatives, (a) in equimolar solutions of cobinamide:nitrogen base, EPR spectra exhibit one signal characteristic of the five-coordinate species, and (b) in the presence of a large excess of base a less intense EPR signal characteristic of a 1:2 adduct is observed together with that of the 1:1 adduct. Photolysis in frozen matrix allowed us to confirm that phosphines do not show a propensity to bind to the organocobalt(III)(corrin)(+) compounds but do bind to the [Co(II)(corrin)](+) species formed upon photolysis, thus evidencing the reactivity of the latter. Another aspect concerning ligand binding is the fact that the choice of the solvent may be determinant in the assignment of coordination numbers. EPR parameters and spin densities calculated for the Co(II) species are in agreement with those described for a (2)A(1) ground state and vary according to basicity of pyridine bases and with Tolman cone angle for the phosphines, confirming that both electronic and steric effects are important to determine the type of adducts formed and the usefulness of EPR parameters to get insight into structural changes regarding the coordination of axial ligands. The possibility of establishing such correlations is recently becoming more important, as in many of the processes catalyzed by B(12) cofactors displacement of axial ligands is observed and proved to be determinant to the enzymatic mechanism.

Language: English

Type (Professor's evaluation): Scientific

No. of pages: 8