A-Pyridonate-blue

The first direct evidence for the structure of platinum-blues was provided by the single-crystal X-ray studies of cis-diammineplatinum a-pyridonate-blue, [Pt(2.25+)4(NH3)8(/x-a-pyridonato-N,0)4] (N03)5 H20 (48, 49). In the study, Barton and Lippard selected a-pyridone as a simplified model of pyrimidine bases (see Fig. 3), which must be the primary reason of their success in obtaining the first crystalline-blue material. 

Crystals of PPB are stable in air and dissolve in water to give a deep blue solution, which slowly bleaches with time. The blue color is enhanced and stabilized in the presence of nitric or perchloric acid (pH 1) and/or 0.1 Af sodium nitrate or perchlorate. In 0.1 M nitric acid PPB exhibits three optical spectroscopic bands at 680, 480, and 405 nm. The blue color is rapidly discharged in the presence of chloride ion and can be restored by removing chloride with silver nitrate. cu-Diammineplatinum a-pyridone blue is paramagnetic and exhibits strong ESR signals at room temperature, with g values of 2.37 and 1.99. 

Fig. 5.1. Schematic representation of the structure of platinum—a-pyridone blue, [Pt4(NH3)g(a-pyr)4]. From Reference 28.

Many of the blues mentioned in Section 5.2 have been characterized by UV/visible and EPR spectroscopy, and clearly the oligomeric nature of these species warns against any comparisons with the discrete blues discussed above. The isolation of the crystalline blue materials (Table 5.II) has permitted various spectroscopic and theoretical studies to elucidate the origin of the blue color. Magnetic susceptibility, EPR, optical and X-ray photoelectron spectroscopic parameters have all been obtained [65, 66]. The studies on the original platinum—a-pyridone blue have been supplemented by the more recent structures. 

The optical spectrum of platinum—a-pyridone blue, as with the pyrimidine analogues, varies with pH, counteranions, temperature and time. Polarized single-crystal spectra in conjunction with a SCF-X Sw calculation have elucidated the major features of the electronic structure [67]. The blue color has been attributed to transitions from the inner Pt—Pt bonding o orbital to an antibonding one (a ) and from the outer Pt—Pt 7t bonding orbital to outer Pt—Pt a. The effect of alteration of the Pt—Pt distances in the platinum—ethylenediamine—pyridone blue was correlated with the optical spectrum. The delocalization of the unpaired spin density classes the pyridone blue as a Robin—Day Class III-A compound [68]. 

C20H30CUN4O4, Bis(1,3-propanediamine)copper(II) benzoate, 38B, 996 C2oH3oN4Ni04, Bis(camphorquinone dioximato)nickel(II), 44B, 1000 C20H34Ce2Ni,Oi 8 6 H2O, Hydroxyethylethylenediaminetriacetatodiaqua-cerium(III) trihydrate dimer, 45B, 1121 C20H38Cl6N204Ta2, M (1,2-Dimethyl-1,2-diimidoethenato(2-))-bis(tri-chlorobis(tetrahydrofuran)tantalum), 44B, 1000 C20H40N17O19Pt4 H2O, cis-Diammineplatinum a-pyridone blue nitrate hydrate, 45B, 1268... 

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