Abstract:
Heterodinuclear mixed valence [Zn(II) - Fe(III)] and the homodinuclear [Zn(II) - Zn(II)] and [Ni(II) - Ni(II)] complexes of a bicompartmental ligand containing a bridging phenoxy as a O - donor and four pyridyl moieties and two amine moieties as the N-donors exhibit phosphoester hydrolysis activity similar to the hydrolase family of enzymes. While the heterodinuclear [Zn(II)-Fe(III)] (2) complex was obtained by the sequential addition of Fe(NO3)(3).9H(2)O and Zn(OAc)(2).2H(2)O to the ligand 2,6-bis{[bis(2-pyridylmethyl)amino]methyl}-4-t-butylphenol (HL) (1) in moderate yield of 37%, the homodinuclear [Zn(II) - Zn(II)] (3) and [Ni(II) - Ni (II)] (4) complexes were obtained by the direct reaction of the ligand (1) with Zn(OAc)(2).2H(2)O and Ni (OAc)(2).2H(2)O respectively, in good to moderate yields (43-63%). Based on the spectrophotometric titration and the mass spectrometry studies, a monoaquated and dihydroxo species 2C, 3C and 4C has been identified as the catalytically active species responsible for the phosphodiester hydrolysis of the bis(2,4 - dinitrophenyl)phosphate (2,4 - BDNPP) substrate in the pH range 5.5-10.5. The kinetic studies further revealed that the homodinuclear [Ni(II) - Ni(II)] complexes (4) (k(cat) = 1.26 x 10(-2)s(-1)) is more active by 39 times than the homodinuclear [Zn(II) - Zn(II)] complexes (3) (k(cat) = 3.20 x 10(-4)s(-1)) and 27 times more active than the heterodinuclear [Zn(II) - Fe(III)] complex (2) (k(cat) = 4.62 x 10(-4)s(-1)) in the phosphodiester hydrolysis activity. Significantly enough, the catalyst - substrate adduct species (2E, 2F and 3F) containing a metal bound bis (2,4-dinitrophenyl)phosphate has been detected by mass spectrometry for the first time.