Diimines, platinum -diimine oxidation

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). 

The long lifetimes of CT excited states of the Pt(diimine)(dithiolate) complexes allow for bimolecular photochemistry, often involving oxidation of the complex. The earliest report of photoreactivity of these complexes dealt with the photooxidation of Pt(bpy)(tdt) (20) following excitation at 577 nm in chloroform (118). The reaction proceeds with a quantum yield of < ) = 0.03 and was attributed to ET to the halocarbon solvent (Eq. 8) similar to the CTTS photooxidation chemistry of the platinum bis(dithiolate) dianions described above. 

Chemical and electrochemical oxidation of these [Pt Me2(a-diimine)] species in MeCN was later studied in more detail in the Tdset group (38). Irreversible electrochemical waves were again observed, and bulk electrolysis revealed consumption of 1.1-1.6 FmoP indicative of a le oxidation process. (Electro)chemical bulk oxidation leads to only marginal formation of methane or ethane, and almost quantitative formation of the species [Pt Me3(a-diimine)(NCMe)] and [Pt Me(a-diimine)(NCMe)] in a 1 1 ratio. It was proposed that the short-lived [Pt Me2(a-diimine)] " " decomposes via a bimolecular methyl transfer from one platinum to another. This explains the product distribution and the lack of products derived from free alkyl radicals. This could either involve a reaction between two [Pt °Me2(a-diimine)] + intermediates or a reaction of [Pt° Me2(a-diimine)] + with the starting material [Pt Me2 (a-diimine)]. 

A study of a platinum /3-diiminate (nacnac) complex provides evidence for oxidative addition to C-H bonds. Starting with a stable Pt(rv) trimethyl adduct, loss of ethane produces a reactive Pt(ll) intermediate that metallates the ligand (Pt(ll) — Pt(iv)) and reductively eliminates methane(Pt(lv) —> Pt(ll)). / -Elimination then generates an observable pendant olefin hydride complex. This species serves as a reversible source of the unsaturated Pt(ll) intermediate, which can reversibly add to alkane solvent C-H or C-D bonds, permitting deuteration of the isopropyl groups of the ligand (Equation (14)). ... 

The use of fluorescent organometallic complexes to label biological substrates is beginning to provide some exciting alternatives to the more traditional organic dyes [126], with suitable iridium [127-129], rhodium [130], platinum [131], rhenium [132-135] and osmium [136] examples having recently been reported. The Re diimine wires (13g and 13h, Fig. 13), have been shown to form complexes with the nitric oxide synthase mutant 5114 [137]. Steady-state luminescence measurements with 13h establish a dissociation constant of 100 nM, while 13h binds with a... 

---