Pt II complexes

Pt(II) Complexes. The cyclometallated Pt(II) complexes often exhibit very interesting luminescence properties. Furthermore, several of these complexes undergo photochemical reactions. For these reasons, a relatively large number of cyclometallated Pt(II) complexes have been the object of accurate investigations in the last few years. 

A temperature dependence study of the luminescence lifetime over the temperature range 77-310 K [89] has indicated that the luminescent 3MLCT level can undergo deactivation via a thermally accessible 3MC level. The activation energy of this process, however, is considerably higher (3700 cm J), so that luminescence can compete with the radiationless decay even at room temperature. 

MLCT excited state involving the tpy- ligand. Confirmation of the involvement of the tpy ligand in the MLCT luminescence of Pt(ppzXtpy) is the fact that the complex exhibits a strong luminescence in fluid solution at room temperature, a property that seems to be peculiar to cyclometallated tpy-complexes [78, 80, 85]. The luminescence quantum yield of Pt(ppzXtpy) is even larger (and the emission lifetime longer) than that of the parent Pt(tpy)2 complex. The radiative lifetime, calculated from the luminescence quantum yield and the excited state lifetime, is 3.8 x 10s s [82], quite similar to those of MLCT emitters like Pt(tpy)2 [85] and Ru(bpy)3 + [15]. 

The absorption and luminescence properties of Pt(ppy)2 have been compared with those of the isomeric Pt(bphXbpy) complex (14) [76]. 

For the Pt(bphXCH3CN)2 complex [111], the high energy feature of the structured luminescence spectrum is at 493 nm and the luminescence lifetime in acetonitrile at room temperature is 14 ns. The emitting level is thought to be localized on bph. 

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MeS(0)CHMe2, Et2S=0] support the S-bonded structures and configurations of the complexes shown in Scheme 15. Far-IR studies indicate that the Pd(II) complexes have uniformly the trans structure while the Pt(II) complexes have the cis one196 197. 

The fact that the coordination of a hard N donor atom is preferred in a trans position to the soft C donor atom and reciprocally, constitutes a good illustration of the antisymblotic effect which seems here to occur in presence of the soft Pd(II) metal. Similar observations have also been made in Pt(II) complexes. 

Nucleophilic reactivity toward Pt(II) complexes may be conveniently systematized via linear free energy relationships established between reactions of trans Ptpy2Cl2 (py = pyridine) with various nucleophiles and reactions of other Pt(II) complexes with the same nucleophiles. First, each nucleophile is characterized by a nucleophilicity parameter, derived from its reactivity toward the common substrate, trans Ptpy2Cl2. Reactivity toward other Pt(II) substrates is then quite satisfactorily represented by an equation of the form (21), wherein ky is the value of in the reaction with nucleophile Y... 

The constant s, characteristic of the substrate complex, reflects its sensitivity to variation in nucleophilicity as assessed by the Ptpy2Cl2 reaction. It is called the nucleophilic discrimination factor (ndf). The intercept log k turns out to be related to the value of the k term in the rate law for the solvent in question. Some representative ligands involved in attack on Pt(II) complexes may be listed in order of decreasing as follows ... 

Hydration of unactivated alkynes is an important method for functionalizing this plentiful hydrocarbon source. Therefore, a variety of metal ions have been proposed as catalysts for this reaction, and almost all of the reported additions of water to terminal alkynes follow the Markonikov rule. The hydration of l-aUcynes with Hg(II) salts in sulfuric acid [85], RuCh/aq.HCl [86, 87], K[Ru (edta-H)Cl] 2H20 [88], RhCl,.3H20/aq. HCl [89], RhCl3/NR4 [90], Zeise-type Pt(II) complexes [91-93], and NaAuCl4 [94] produced exclusively methyl ketones (Eq. 6.46). 

Several Pt(II) complexes are also effective for the hydration of internal alkynes (Eq. 6.48). Internal alkynes were converted to the corresponding internal ketones in... 

Table 6. Trans-effect on the rates of reaction of some Pt( II) complexes with Pyridine (12)...

Fig. 8. Electronic absorption spectra of some planar square Pt(II) complexes of chloride and ammonia in aqueous solution (from Chatt, J., Gamlen, G. A., Orgel, L. E. J. Chem. Soc. 486 (1958))...

The range of complexes which can be formed by just one amino-acid, glycine, and one Pt(II) complex, PtCll-, will now be illustrated (45, 46). Direct reaction with glycine gives the bis-(cis)-glycinato Pt(II) complex... 

Scheme 18 Conversion of the Mathey-phosphole to the corresponding Pt(II) complex...

As intracellular reduction mechanisms are important for Pt(II) and Pt(IV) drugs alike, the chemical reduction of cis,trans-[Pt(en)I2(OH)2] (3) by GSH was further explored under biologically relevant conditions (24), which led to the unexpected detection of a long-lived chelate-ring-opened Pt(II) complex capable of forming DNA-Pt adducts. 

We have recently extended our interest to the analogous halfsandwich osmium-arene complexes and are exploring the chemical and biological properties of [Os(r 6-arene)(XY)Z]ra 1 complexes (Fig. 25) (105). Both the aqueous chemistry and the biological activity of osmium complexes have been little studied. Third-row transition metals are usually considered to be more inert than those of the first and second rows. Similar to the five orders of magnitude decrease in substitution rates of Pt(II) complexes compared to Pd(II), the [Os(ri6-arene)(L)X]"+ complexes were expected to display rather different kinetics than their Ru(II)-arene analogs. A few other reports on the anticancer activity of osmium-arene complexes have also appeared recently (106-108). 

Electronic Tuning of the Lability of Inert Co(HI) and Pt(II) Complexes Rudi Van Eldik... 

Chalk and Elarrod (11a) compared the above ethylene Pt(II) complex with chloroplatinic acid for hydrosilation, and found that each gave essentially the same results in terms of rate, yields, and products. Plati-num(II) complexes and rhodium(I) complexes were very much alike in their behavior. No system was found in which a palladium olefin complex brought about hydrosilation. In most systems the palladium complex was very rapidly reduced to the metal. 

Although no exact structures of the hypothetical catalytic species have been isolated, Eaborn (17) has synthesized some compounds with many of the structural features unfortunately, he did not examine them as catalysts for hydrosilation. A stable Pt(II) complex was isolated for study as follows ... 

At 20°C, an induction period was observed in hexane. At 5°C an induction period of about 2 hours was seen in hexane, toluene, and styrene. This suggests that the Pt(II) complex [C6H5CH=CH2PtCl2]2 became catalytic after conversion to C6H5CH=CH2PtH(—Si=). This conversion was not instantaneous thus an induction period was noted, influenced by temperature and by solvents. The kinetic rate constant for formation of the phenylethylsilane was temperature dependent but nearly free of solvent effects. 

The oxidative addition of alkane C-H bonds to Pt(II) has also been observed in these TpRa -based platinum systems. As shown in Scheme 19, methide abstraction from the anionic Pt(II) complex (K2-TpMe2)PtMe2 by the Lewis acid B(C6F5)3 resulted in C-H oxidative addition of the hydrocarbon solvent (88). When this was done in pentane solution, the pentyl(hydrido)platinum(IV) complex E (R = pentyl) was observed as a... 

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