Synthesis and preliminary DNA-binding studies of diimineplatinum(ii) complexes containing 3- or 4-pyridineboronic acid

A series of platinum(II) complexes of the type [Pt(NN)(pyB)2](NO3)2 (NN = bipy, phen; pyB = 3- or 4-pyridineboronic acid) were successfully prepared and fully characterised by 1D- and 2D-multinuclear NMR spectroscopy and ESI-MS. Using VT 1H NMR spectroscopy, rotational isomers for [Pt(NN)(3-pyB)2](NO3)2 were identified and the free energies of activation for rotation of 3-pyB about the Pt-N bond were determined to be DeltaG++310) = 69.2 +/- 0.1 kJ mol(-1) and DeltaG++(305) = 66.0 +/- 0.1 kJ mol(-1) for [Pt(bipy)(3-pyB)2](NO3)2 and [Pt(phen)(3-pyB)2](NO3)2, respectively. The 3- and 4-pyB ligands readily deboronate in boiling H2O to afford [Pt(NN)(py)2](NO3)2; the structure of [Pt(phen)(py)2](2+) (as its PF6- salt) was confirmed by X-ray crystallography. Preliminary thermal denaturation studies revealed only minimal interactions between [Pt(NN)(pyB)2](NO3)2 and calf-thymus DNA and is attributed to hydroxylation of the boronic acid groups at pH 7.4 to afford the corresponding zwitterionic boronate species. This was confirmed by means of variable pH 1H and 11B{1H} NMR spectroscopy.