Photoinduced Dimerization Reaction Coupled with Oxygenation of a Platinum(II)–Hydrazone Complex

Photoreactivities of Ni­(II)–
and Pt­(II)–hydrazone complexes, [NiCl­(L)] (Ni1) and [PtCl­(L)] (Pt1), respectively [HL = 2-(diphenylphosphino)­benzaldehyde-2-pyridylhydrazone], were investigated
in detail via UV–vis absorption, 1H nuclear magnetic
resonance (NMR) spectroscopy, and electrospray ionization time-of-flight
(ESI-TOF) mass spectrometry; the two photoproducts obtained from the
photoreaction of Pt1 were also successfully identified
via X-ray analysis. The absorption bands of the Ni1 and Pt1 complexes were very similar, centered around 530 nm, and
were assigned as an intraligand charge transfer transition of the
hydrazone moiety. The absorption spectrum of Pt1 in a
CH3CN solution changed drastically upon photoirradiation
(λ = 530 nm), whereas no change was observed for Ni1. 1H NMR and ESI-TOF mass spectra under various conditions
suggested that the photoexcited Pt1* reacts with dissolved
dioxygen to form a reactive intermediate, and the ensuing dark reactions
afforded two different products without any decomposition. In contrast
to the simple photo-oxidation of HL to form a phosphine
oxide HL­(PO), the X-ray crystallographic
analyses of the photoproducts clearly indicate the formation of a
mononuclear Pt complex with the oxygenated hydrazone ligand (Pt1O) and a dinuclear Pt complex with the oxygenated and dimerized
hydrazone ligand (Pt2). The photosensitized reaction
in the presence of an 1O2-generating photosensitizer,
methylene blue (MB), also produced Pt1O and Pt2, indicating that the reaction between 1O2 and
ground-state Pt1 is the important step. In a highly viscous
dimethyl sulfoxide solution, Pt1 was slowly, but quantitatively,
converted to the mononuclear form, Pt1O, without the
formation of the dinuclear product, Pt2, upon photoirradiation
(and in the reaction photosensitized by MB), suggesting that this
photoreaction of Pt1 involves at least one diffusion-controlled
reaction. On the other hand, the same complexes Pt1O and Pt2 were also produced in the degassed solution, probably
because of the reaction of the photoexcited Pt1* with
the biradical character and H2O.