The Platinum Blues

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.  

DFT calculations on platinum blue compounds provide insight on their electronic structures. The four d/(o) orbitals of metals form four nondegenerate a MOs. The HOMO is singly occupied. The blue color of the compound is the consequence of the excitation from one of the three filled molecular orbitals into the half-filled a orbital. Further, it illustrates that a one-electron oxidation from the HOMO should lead to a strengthening of the Pt-Pt bonds. 

Matsumoto s group has carried out complete characterization of [ cw-Pt2(pyrr)2 m] (pyrr = 2-pyrrolidonate) compounds. The a-pyrrolidone green and a-pyrrolidone violet have been characterized as [Pt4(pyrr)4(NH3)8](N03)5.48] a 3H2O and [Pt4(pyrr)4(NH3)8](PF6)2(N03)2.56] 5H2O, respectively. The nonstoichiometric composition of these compounds is attributed to the mixed-valency. 

The X-ray structure of 2-pyrrolidone tan reveals a tetranuelear structure similar to that of 2-pyridone blue, except that the axial nitrates are missing in the former. The Pt-Pt distances are 2.702(6) A, 2.710(5) A, and 

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.  

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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-... 

Since tetranuclear platinum-blues are oxidized by 02 to Pt(III) dinuclear complexes and are reversively reduced to the platinum-blues and further to the Pt(II) dinuclear complexes, an attempt was made to use these complexes as catalysts for olefin oxidation to ketones and epoxides. The catalysts used were a-pyrrolidonato-bridged Pt-tan [Pt4(NH3)8(C4H6N0)4](N03)6 -2H20 (19), pivalamidato-bridged Pt-blue [Pt4(NH3)8(C5H10NO)4](NO3)5 (57), a-pyrrolidonato-tan [Pt4(NH3)8... 

PtCl2(py)DMF] is in equilibrium with [Pt2Cl4py2] and free DMF.51 The platinum blue product from the reaction of K2[PtCl4] with acetamide now appears to contain some Ptm and to have MeCONH- groups and not neutral acetamide.52... 

Matsumoto etal. [103] [104] used a-pyrrolidone as bridging ligand and obtained a tan colored compound having analogous structure to the platinum blue of Lippard, except that the average oxidation state was +2.50... 

Radiochem. 26, 235 (1983). Related to these molecules are the platinum blues , in which one finds oxidized Pt(II) oligomers. For a leading reference, see O Halloran, T. 

Of interest in view of the antitumor properties of cw-PtCl2(NH3)2 (see below) are the cations such as ciy-[Pt(NH3)2(H20)2]2+. Exposure of phosphate containing solutions to air gives blue species that may be related to the platinum blues discussed later. There are also well-established amine complexes such as [M(am)4]2+ and [M(en)2]2+. 

Although the isolation of the first blue platinum compound was reported as early as 1908, the nature of the bonding in the platinum blues re-... 

The platinum blues are mixed-valence complexes containing discrete Pt chains (Figure 22.23). They are formed by hydrolysis of CM-[PtCl2(NH3)2] or cis-[PtCl2(en)] in aqueous AgN03 (i.e. replacing Cl by H2O... 

Figure 8. G r) for the platinum blue [Pt2(NH3)4(//-HP04)2] . Different types of atom are labeled with numbers in the diagram of the model - 1, 2, 3, 4 indicate the Pt atoms of the polymetallic chain, 5 is the oxygen atom, 6 the ammonia ligand, and 7 the phosphorus atom.

The solution electronic spectrum exhibits live absorptions whose positions and intensities are a function of concentration and pH, suggesting interaction in solution as well as in the solid state (157). Conductivity measurements have not been reported for the platinum blue substance, but since there is no evidence for partial oxidation the material is not expected to show a high conductivity. 

Soc., vol. 106, p. 6427]. (b) Examples of lV,0-donor ligands present in platinum blues, (c) Schematic representation of the platinum blue... 

The understanding of the chemical nature of the platinum blues received a considerable impulse with the resolution of the first crystal structure of a... 

Extensive chemical and structural studies have now elucidated the essential features of the platinum blues and these studies have utilized almost exclusively precursor complexes based on the ligands noted above. Before the details of their formation are examined a brief description of the blue structure is given. 

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