Diplatinum complex, excited

Yasuda N, Uekusa H, Ohashi Y (2004) X-Ray analysis of excited-state structures of the diplatinum complex anions in five crystals with different cations. Bull Chem Soc Jpn 77 933-944... 

The excited diplatinum complex is now predisposed to react further. The incipient Pt—Pt bond is partially formed and addition of hydrogen is enhanced 202... 

As a diplatinum complex [Pt2(pop)4]" (pop = diphosphite) has an excited state with a long lifetime, ca. 7 ts, we selected it as the first sample [37]. For the cations, five alkylammonium cations were selected, as shown in Figure 5-23. For the tetrabutylammonium (Bu), tetrapentylammonium (Pn) and benzyltriethylammo-nium (Bzte) cations, there are two crystal forms, named Bui and Bu2, Pnl and Pn2, and Bztel and Bzte2, respectively. On analysis of these eight crystal structures, the Pt anions in all crystals except Bzdmp were found to have two extra protons the Pt anions should be represented as [Pt2(pop)2(popFI)2] hence only two cations are included for a Pt anion in those crystals. The Bu2 and Bzte2 crystals have a... 

It is noticeable that the shortenings of the Pt-Pt and Pt-P distances, A(Pt-Pt) and A(Pt-P), of the Pn and Bzte crystals are 0.0081(3) and 0.005(1)A, and 0.0127(5) and 0.008(1) A, respectively, which are the largest two among the five crystals. The ratios of A(Pt-Pt) / A(Pt-P) are 1.62 and 1.59 for the Pn and Bzte crystals, respectively, which are very close to each other. This suggests that the excited diplatinum complex anion may have the same structure in the two crystals, and the concentrations of the excited molecules may be different between the two crystals. On the other hand, the excited structures in the Bu, Bztbu and Bzdmp crystals may be different due to the different crystal environment. The change at the excited state is schematically drawn in Fig. 7.26. Not only the Pt-Pt distance but also Pt-P distance is shortened at the excited state of the diplatinum complex anion, which is in good agreement with the experimental results observed from the combination of the EXAFS method with rapid-flow laser spectroscopy [54]. 

Photocleavage activity was also reported for tetra-anionic diplatinum(II) complexes with pyrophosphito ligands. The photocleavage proceeds via hydrogen atom abstraction, which occurs on collision between an excited state of complex and DNA, and is strongly enhanced by magnesium ion, probably due to screening of the electrostatic repulsion between the two reactants [88],... 

The most widely documented diplatinum(II) complex is [Pt2(P205H2)4]" which contains bridging (P,P-bonded) diphosphito ligands (Figs. 3.11, 3.12). Photophysical studies confirm that the properties of the photoactive excited state are a manifestation of tf-tf metal-metal interactions [57]. 

A mechanism is proposed that involves the phototochemical activation of a diplatinum //-alkynylidene precursor complex to give a long-lived excited state that is quenched by diphenylacetylene through [2+2] cycloaddition or insertion reactions. The failure to observe emission from this complex, however, makes it difficult to verify these proposals. ... 

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