Platinum diimine complexes

Burmeister and colleagues have described the related pseudohalogen derivatives MfterpyjXj (X = SCN or SeCN) (90-92). The platinum compound exhibits the two thiocyanate stretching frequencies expected for a square-planar complex, and is formulated [Pt(terpy)(NCS)][NCS], However, the palladium complexes are less easily formulated, exhibiting absorptions due to coordinated ECN (E = S or Se) only. These observations were interpreted in terms of a square-planar structure, with a bidentate terpy ligand in view of the known ability for palladium and platinum diimine complexes to form five-coordinate species, this formulation must also be considered. In the absence of definitive structural evidence, the formulation as five-coordinate species must be regarded as speculative. 

Luminescence has been observed from a large number of later transition element complexes and a rich array of excited states have been observed. Related sections from CCC (1987) include Chapter 16.5 on Pt, Rh, and Ir complexes, 36.3 on Mo halide clusters, 43 on Re complexes, 45.4 on Ru polypyridyls, 46.4 on Os polypyridyls, 48.6 on Rh complexes, and sections of Chapter 52 on Pt complexes. Several recent reviews have been published on polynuclear complexes, the photophysics of gold complexes,and platinum diimine complexes. Many other more narrowly focused review articles have been published on transition metal complex luminescence a significant number are published in the journal Coordination Chemistry Reviews and some of these reviews are cited in this chapter. 

The question of which pathway is preferred was very recently addressed for several diimine-chelated platinum complexes (93). It was convincingly shown for dimethyl complexes chelated by a variety of diimines that the metal is the kinetic site of protonation. In the system under investigation, acetonitrile was used as the trapping ligand L (see Fig. 1) which reacted with the methane complex B to form the elimination product C and also reacted with the five-coordinate alkyl hydride species D to form the stable six-coordinate complex E (93). An increase in the concentration of acetonitrile led to increased yields of the methyl (hydrido)platinum(IV) complex E relative to the platinum(II) product C. It was concluded that the equilibration between the species D and B and the irreversible and associative1 reactions of these species with acetonitrile occur at comparable rates such that the kinetic product of the protonation is more efficiently trapped at higher acetonitrile concentrations. Thus, in these systems protonation occurs preferentially at platinum and, by the principle of microscopic reversibility, this indicates that C-H activation with these systems occurs preferentially via oxidative addition (93). 

Ammine ligands complexed to platinum(IV) will undergo condensation reactions with acetylacetone. Thus Pt(NH3) + and acetylacetone react rapidly to give the diimine complex (equation 332).1032... 

A number of platinum diimine-dithiolene complexes display long-lived solution and solid-state luminescence in ambient conditions and in frozen solvent glasses. A linear correlation has been found between emission energies and the difference in the ground-state redox potentials. These results provide the tools for tuning and systematically controlling... 

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