Influence of water on the formation of O2-reactive divalent metal enolate complexes of relevance to acireductone dioxygenases

Abstract

Reaction conditions were evaluated for the preparation of [(6-PhTPA)Ni(PhC(O)C(OH)C(O)Ph)]ClO₄ (3) and [(6-Ph₂TPA)Co(PhC(O)C(OH)C(O)Ph)]ClO₄ (7), two complexes of structural relevance to the enzyme/substrate (ES) adduct in Ni(II)- and Co(II)-containing forms of acireductone dioxygenase. The presence of water in reactions directed at the preparation of 3 and 7 was found to result in isomerization of the enolate precursor 2-hydroxy-1,3-diphenylpropane-1,3-dione to give the ester 2-oxo-2-phenylethylbenzoate. Performing synthetic procedures under dryer conditions reduced the amount of ester production and enabled the isolation of 3 in high yield. This complex was comprehensively characterized, including by X-ray crystallography. Using similar conditions for the 6-Ph₂TPACo-containing system, the amount of ester generated was only modestly affected, but the formation of a benzoate complex ([(6-Ph₂TPA)Co(O₂CPh)]ClO₄, 10) resulting from ester hydrolysis was prevented. The best preparation of 7 was found to involve dry conditions and short reaction times. The approach outlined herein toward determining appropriate reaction conditions for the preparation of 3 and 7 involved the preparation and characterization of several air-stable (6-PhTPA)Ni- and (6-Ph₂TPA)Co-containing analog complexes having enolate, solvent, and benzoate ligands. These complexes were used as paramagnetic ¹H NMR standards for evaluation of reaction mixtures containing 3 and 7.