Photoinduced Reductive C-O Couplings from Unsymmetrical Bis-Cyclometalated Pt(IV) Dicarboxylato Complexes.

Unsymmetrical bis-cyclometalated dicarboxylato complexes (-6-32)-[Pt(tpy)(OCR)] [tpy = cyclometalated 2-(-tolyl)pyridine, R = -Bu (), Me (), Ph (), CF ()], are obtained from the reaction of -[Pt(tpy)] with the appropriate PhI(OCR) reagent. Treatment of complexes of this type with different carboxylates (R'CO) results in the formation of mixed-carboxylato derivatives, namely (-6-43)-[Pt(tpy)(OCMe)(OCR')] [R' = -Bu (), CF (), Ph ()], (-6-34)-[Pt(tpy)(OCCF)(OCR')] [R' = -Bu (), Me (), Ph ()], and (-6-34)-[Pt(tpy)(OC--Bu)(OCMe)] (). Irradiation of - and - with UV light (365 nm) in MeCN gives 5-methyl-2-(2-pyridyl)phenyl pivalate (), 5-methyl-2-(2-pyridyl)phenyl acetate () or 5-methyl-2-(2-pyridyl)phenyl benzoate () as the major photoproduct from most complexes, resulting from a reductive C-O coupling between a tpy ligand and a carboxylato ligand. Cyclometalation of - at the ensuing Pt(II) species to produce -[Pt(tpy)(tpyOCR/R')], reduction to -[Pt(tpy)] and isomerization to (-6-33)-[Pt(tpy)(OCR/R')] are identified as secondary processes in most cases. In contrast, complex exclusively photoisomerizes to (-6-33)-[Pt(tpy)(OCCF)] (). The C-O couplings are favored for the most electron-rich carboxylato ligands and occur predominantly from the carboxylato trans to N. Consistent with this, a computational study reveals that the lowest singlet and triplet LMCT excited states result from electronic transitions to a dσ* orbital distributed along the N-Pt-O axis, which would trigger the observed processes.