Chemical and spectroscopic characterization of cis-diammineplatinum α-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 complexes. The extinction coefficient of the blue chromophore is a sensitive function of the anions present and of the temperature. The approximately axial electron spin resonance spectrum is characterized by g values of 2.38 and 1.976, very similar to those reported from single-crystal ESR studies. Extensive '95Pt hyperfine interactions indicate that PPB maintains its oligomeric structure upon initial dissolution in water. The decomposition of PPB in solution has been monitored by intensity losses with time in both the blue absorption band and the ESR signal. The concomitant decrease observed indicates the correspondence of the blue chromophore with the unpaired spin. Oxidative titrations of PPB show a linear decrease in ,4680 and ESR signal intensity with the addition of 3 equiv of ceric ion per tetramer. The chemical characteristics of PPB in solution are compared to those of platinum acetamide blue (PAB), cis-diammineplatinum uracil blue (PUB), and the newly characterized cisdiammineplatinum hypoxanthine green (PHG), the first reported platinum purine "blue". The results establish the similarity in 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 to that of PPB, although of lower absolute signal intensity. This result may be due to spin-spin coupling since PUB is shown to be a longer chain oligomer. Both PHG and PAB exhibit a weak ESR transition. Oxidative titrations using ceric ion of PAB and PHG monitored spectrophotometrically show that 1.75 e-/Pt is required for the complete loss of blue color; this result is consistent with oxidation to platinum(lV) from the initial platinum formal oxidation state of 2.25. Classical reductive titrations of the cis-diammineplatinum blues to platinum mttal further support the oxidation-state assignment of 2.25. Gel electrophoresis of the cis-diammineplatinum blues, assuming the identical charge per monomeric unit, indicates the length of the oligomers to increase in the series PPB < PHG < PUB. These studies demonstrate that blue platinum complexes share the properties of mixed valency and oligomeric structure.
Ⓒ 1979 American Chemical Society. This work was supported by a grant from the National Cancer Institute, DHEW (Grant CA-15826 to S.J.L.). J.K.B. is the recipient of a National Science Foundation Predoctoral Fellowship. We thank J. Lehmann and D. Hutt for excellent technical assistance, Dr. P. Aisen for performing the ESR integrations, and Dr. H. Patterson for a preprint of ref 24. A generous loan of K_2PtC_(14) from Engelhard Industries is gratefully acknowledged.