Platinum-pyridone blues

Magnetic susceptibility data show that the platinum—pyridone blue complex behaves as a simple Curie paramagnet with an effective magnetic moment of 1.81 [//B(spin-only) = 1.73], indicating the presence of one unpaired electron [24]. Comparative values for the ethylenediamine and 1-MeUracil complexes are 1.934 and 1.89 respectively. 

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. 

Followed by the X-ray studies on cc-pyridonate-blue, various platinum-blues and the related complexes of exocyclic amidate and imidate ligands (see Fig. 3) have been prepared and structurally ana-... 

As mentioned above, the Pt(2.25+)4 species (B1 and B2) possesses one unpaired electron of Pt(III) (S = 1/2) and is the only species which exhibits paramagnetism among the four oxidation states. The ESR spectrum of the ce-pyridonate-blue species shown in Fig. 7 exhibits an axial signal characteristic of the platinum-blues (g = —2.4 and g = —2.0) (48, 49, 70, 96). Similar signal patterns are also observed for several other blue compounds (34, 47, 57, 86, 88, 89), and these common features show that the unpaired electron resides on the dz2 orbital located along the Pt chain. The g values observed for the para-... 

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]. 

Platinum-acetamide-blue and platinum-uracil-blue compounds have been reported. The tetranuclear uracil blue [Pt4(l-MeU)4(NH3)g](N03)5-H20, where 1-MeU is the mono-anion of 1-methyluracil, has structure and properties resembling 2-pyridone-blues. Platinum blue compounds with uridine, isonicotinamide, malonamide, and biuret have been described. ... 

In general, such derivatives are constituted by tetraplatinum complexes having as the main ligand a wide range of bio-compatible molecules. The first, structurally characterized platinum blue complex was the oc-pyridonate [Pt4(NH3)8(C5H4N0)4](N03)5, Figure l.15... 

The work by Lippard and coworkers [2][24][25][88][95][98-100] derives its chief motivation from the understanding of the interaction between the anticancer drug cA-[PtCl2(NH3)2] and pyrimidine nucleobases. Unfortunately, the reaction of c7v-[PtCl2(NH3)2] with molecules such as uracil or thymine leads to non-crystalline dark blue materials ( platinum blues ) which are difficult to characterize. The use of a ligand with similar but more restricted number of donor sites, such as a-pyridone (hp), allowed isolation and full characterization of relevant platinum complexes. Related work has used 1-methyluracil (1-Me-urac) and 1-methylthymine (1-Me-thym) in which one of the pyrimidine nitrogens has been blocked [101]. 

The 2-pyridone platinum blue and other related tetranuelear compounds bear a close structural relationship with [ Rh2(n-pz)(CN Bu)2 4], [ Ir2 (p-pz)2(l)(CN Bu) 2], (pz = pyrazolates) and [ Ir2(n-bztzt)2(I)(CO)4 2] (bztzt = benzothiazol-2-thiolate) [Rh4Cl(bridge)8] (bridge = 1,3-diisocyanopro-pane), termed rhodium and iridium blues. ... 

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