Photooxidation Reactions of Cyclometalated Palladium(II) and Platinum(II) Complexes

Abstract

New C,N,S-cyclometalated palladium­(II) and platinum­(II) complexes have been synthesized and their structural, electrochemical, and photochemical properties examined. The blue color of these complexes in solution changed to yellow under visible-light irradiation. By measurement of the absorption spectra for quantifying changes in color, isosbestic points for each complex clearly indicated the presence of only two species responsible for the change of color. X-ray analysis revealed that the visible-light-induced yellow species were S-oxygenated sulfinato complexes. Photosensitized generation of singlet oxygen (1O2) was confirmed by the direct detection of singlet oxygen luminescence at 1275 nm. The present cyclometalated palladium­(II) and platinum­(II) complexes are efficient photosensitizers of singlet oxygen, which rapidly reacts with coordinating sulfur atoms.

Singlet Oxygen Generation, Quenching and Reactivity with Metal Thiolates†

“…Production of singlet oxygen was directly confirmed by detection of its NIR signal. Upon irradiation under aerobic conditions, complex 16 forms the corresponding sulfinato complex, just as is the case for the Pt(II) analogue of 16 (64). As mentioned during the discussion of the Pt complexes, Bartlett et al .…”

“…The nature of the intermediate(s) for this process is also not fully understood. Recent ab initio calculations by Yasuda et al for their Pt (10) and Pd thiolate ( 16) complexes (64) indicate an asymmetric thiadioxirane-type structure which may collapse directly to the sulfinate product. This mechanism, which appears to resemble a concerted pathway, would be different from any of the reaction channels of organic sulfides.…”

“…The behavior of the alkyl bisthiolate complex (4,4 0 -di-tertbutyl-2,2 0 -bipyridine)Pt(II)(meso-1,2-diphenyl-1,2-ethanedithiolate (7, Scheme 10c) is somewhat unique. The reaction results in formation of an elimination product bearing a 1,2-diphenyl-1, 2-ethenedithiolate ligand (64). Schanze and coworkers hypothesized that a transient persulfoxide intermediate removes the proton at the a-position relative to the sulfur atom leading to formation of an ylide and loss of HO 2 À .…”

“…Yasuda and coworkers explored a new cyclometalated Pt(II) complex (10) which contains a triphenyl phosphine ligand in addition to a 2-imino-benzenthiolate moiety (64). The authors reported that complex 10 is an excellent photosensitizer for the generation of 1 O 2 .…”

“…However, the exact nature of the intermediate (open persulfoxide or closed thiadioxirane) remains uncertain. Yasuda et al have invoked-on the bases of DFT calculations-a triangular species with substantial charge transfer from the sulfur to the dioxygen molecule during the photooxidation of 10 to sulfinate 58 (64). While this species has been referred to as an intermediate, it features very long S-O bond distances of 2.761 and 3.431 A, respectively, which seems to suggest that it is an early transition state rather than a discrete intermediate.…”