Toward the redox-based allosteric control of the activity of a trinuclear metal complex catalyst

The complexes [(L-H)Co-II(ClO4) (1) and (L-H)Co-III (NO2)(2)] (2a) were prepared by reaction of L with Co(ClO4)(2)-6H(2)O and Na3CoIII (NO2)(6), respectively, in the presence of base. L is a polypyridyl ligand with a tetradentate and two bidentate binding sites. On crystallisation of 2a from CHCl3 the complex ((L-H)Co-III (Cl, NO2)(2)) (2b) was obtained and characterised by X-ray crystallography. As expected, Co(III) is octahedrally coordinated by four N-donors of L and two axial, disordered NO2/CI donors. Trinuclear complexes (L-H)Con(II/III) Cu-2(II) in which the Cu(II) ions coordinate to the bidentate sites of L were prepared in solution. Kinetics of the cleavage of the phosphodiester 2-hydroxypropyl-4-nitrophenyl phosphate (HPNP) by the trinuclear complexes was investigated in detail at pH 7 and 25 degreesC. The two 'functional' copper(II) ions cooperate in the cleavage of HPNP while the 'structural' (or allosteric) cobalt ion determines the conformation of the catalyst. The catalytic rate constants k(cat), of HPNP cleavage are 0.015 s(-1) for the Co(III) complex and 0.0031 s(-1) for the cobalt(II) complex, indicating that the catalytic activity of the complex is strongly dependent on the oxidation state of the Co centre. As a consequence of the significantly shorter Co(III)-N distances, a smaller distance between the functional Cu ions is expected in (L-H)(CoCu2(II))-Cu-(III). This work is an important step toward the allosteric control of the activity of synthetic catalysts using the redox chemistry of metal ions. (C) 2002 Elsevier Science B.V. All rights reserved.