Chemical and spectroscopic characterization of cis-diammineplatinum .alpha.-pyridone blue in aqueous solution. Comparison with other platinum blues

The spectral, redox, and chemical properties of cis-diammineplatinum a-pyridone blue (PPB), [Pt2(NH3)4(CsH40N)2]2(N03)5-H20, in aqueous solution have been studied and compared with those of other blue platinum . The extinction coefficient of the blue chromophore is a sensitive function of the anions present and of the temperature. approximately axial electron spin resonance spectrum is characterized by g values of 2.38 and 1.976, very similar to reported from single-crystal ESR studies. Extensive ’95Pt hyperfine interactions indicate that PPB maintains its oligomeric upon initial dissolution in water. The decomposition of PPB in solution has been monitored by intensity losses time in both the blue absorption band and the ESR signal. The concomitant decrease observed indicates the correspondence the blue chromophore with the unpaired spin. Oxidative titrations of PPB show a linear decrease in ,4680 and ESR signal with the addition of 3 equiv of ceric ion per tetramer. The chemical characteristics of PPB in solution are compared those of platinum acetamide blue (PAB), cis-diammineplatinum uracil blue (PUB), and the newly characterized cisdiammineplatinum green (PHG), the first reported platinum purine “blue”. The results establish the similarity their chemical composition and electronic structure. The electronic spectra exhibit broad absorption bands in the region 590-720 nm. These blue chromophores also depend upon the anions present. The ESR spectrum of PUB is strikingly similar that of PPB, although of lower absolute signal intensity. This result may be due to spin-spin coupling since PUB is shown be a longer chain oligomer. Both PHG and PAB exhibit a weak ESR transition. Oxidative titrations using ceric ion of PAB PHG monitored spectrophotometrically show that 1.75 e-/Pt is required for the complete loss of blue color; this result is with oxidation to platinum(lV) from the initial platinum formal oxidation state of 2.25. Classical reductive titrations the cis-diammineplatinum blues to platinum mttal further support the oxidation-state assignment of 2.25. Gel electrophoresis the cis-diammineplatinum blues, assuming the identical charge per monomeric unit, indicates the length of the oligomers increase in the series PPB < PHG < PUB. These studies demonstrate that blue platinum complexes share the properties mixed valency and oligomeric structure.