Atom Transfer Processes in Oxidative Quenching of Triplet State Tetrakis(μ-pyrophosphite-P,P‘)diplatinate(II) by Acidopentacyanocobaltate(III) Anions

Abstract

In the presence of high electrolyte concentration to screen the Coulomb repulsion, the emission of 3Pt2(pop)4 4- (where pop = μ-pyrophosphite-P,P‘) can be quenched by the complexes Co(CN)5X3- (where X = I, Br, Cl, N3). In acidic aqueous solutions the quenching involves the eventual reduction of the cobalt complexes to Co2+ aq and gives the oxidative quenching products Pt2 III(pop)4X2 4- (X = I, Br, Cl). For the reaction of 3Pt2(pop)4 4- with Co(CN)5I3- and Co(CN)5Br3- in 0.5 M KCl/pH 2 solution, the product yields per quenching event are 0.34 and 0.21, respectively. Steady-state photolysis using the Co(CN)5I3- quencher in 0.5 M KCl medium gave predominantly Pt2(pop)4I2 4- as product with only small amounts of Pt2(pop)4ICl4-, pointing to a direct iodine atom-transfer process. In contrast, Co(CN)5Br3- quencher gave Pt2(pop)4Cl2 4- under the same conditions, possibly indicative of electron transfer. However, study of the mixed-valence platinum dimer intermediates formed by laser flash transient spectroscopy provides conclusive evidence for atom transfer as the dominant primary reactive quenching step for all three halopentacyanocobaltates. The steady-state results for Co(CN)5Br3- quencher arose via the interconversion of the monobromo mixed-valence platinum dimer intermediate to its chloro analogue in chloride solutions. Co(CN)5N3 3- gave a mixture of oxidation products and no intermediate corresponding to electron transfer, indicating a complex interaction with 3Pt2(pop)4 4-.

Electronic structures and photophysics of d8-d8 complexes

“…Several weak UV absorption bands were attributed to LMMCT (ligand to metal-metal CT) transitions of strongly mixed singlet-triplet character [50]. Pt(pop) undergoes interesting photoreactivity from the 3 dσ*pσ excited state, including: H-atom abstraction from main-group metal hydrides whose kinetics point to radical-like behavior [51][52][53]; catalytic dehydrogenation of secondary alcohols [26,28,53]; and inner-sphere electron transfer with [Co(CN) 5 X] 3complexes accompanied by transfer of the halide ligand X [54,55]. The 3 dσ*pσ state is quenched by energy transfer as well as by oxidative (e.g.…”

Structural Control of 1A2u-to-3A2u Intersystem Crossing in Diplatinum(II,II) Complexes

“…Owing to its long lifetime (∼10 μs) 12 and biradical character, 13,14 the 3 dσ*pσ state reacts with a variety of substrates, including alcohols, hydrocarbons, alkylhalides, 2−5 DNA, 15 and metal complexes. 13,14,16 These reactions, which involve transfer of hydrogen or halogen atoms, can be viewed as photochemical oxidative-addition or innersphere electron-transfer processes.…”

“…The eight terminal P(O)(OH) groups, which are exposed to solvent, likely play a role in excited-state deactivation. , Pt(pop) shows very weak fluorescence from a 1 dσ*pσ state that decays on a tens-of-picoseconds time scale with temperature- and solvent-dependent lifetimes, , and the corresponding triplet state exhibits intense green phosphorescence. Owing to its long lifetime (∼10 μs) and biradical character, , the 3 dσ*pσ state reacts with a variety of substrates, including alcohols, hydrocarbons, alkylhalides, − DNA, and metal complexes. ,, These reactions, which involve transfer of hydrogen or halogen atoms, can be viewed as photochemical oxidative-addition or inner-sphere electron-transfer processes.…”