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

Unsymmetrical bis-cyclometalated dicarboxylato complexes (OC-6-32)-[Pt(tpy)2(O2CR)2] [tpy = cyclometalated 2-(p-tolyl)pyridine, R = t-Bu (1), Me (2), Ph (3), CF3 (4)], are obtained from the reaction of cis-[Pt(tpy)2] with the appropriate PhI(O2CR)2 reagent. Treatment of complexes of this type with different carboxylates (R'CO2 -) results in the formation of mixed-carboxylato derivatives, namely (OC-6-43)-[Pt(tpy)2(O2CMe)(O2CR ')] [R ' = t-Bu (5), CF3 (6), Ph (7)], (OC-6-34)-[Pt(tpy)2(O2CCF3)(O2CR ')] [R ' = t-Bu (8), Me (9), Ph (10)], and (OC-6-34)-[Pt(tpy)2(O2C-t-Bu)(O2CMe)] (11). Irradiation of 1-3 and 5-11 with UV light (365 nm) in MeCN gives 5-methyl-2-(2-pyridyl)phenyl pivalate (12), 5-methyl-2-(2-pyridyl)phenyl acetate (13) or 5-methyl-2-(2-pyridyl)phenyl benzoate (14) as the major photoproduct from most complexes, resulting from a reductive C-O coupling between a tpy ligand and a carboxylato ligand. Cyclometalation of 12-14 at the ensuing Pt(II) species to produce cis-[Pt(tpy)(tpyO2CR/R ')], reduction to cis-[Pt(tpy)2] and isomerization to (OC-6-33)-[Pt(tpy)2(O2CR/R ')2] are identified as secondary processes in most cases. In contrast, complex 4 exclusively photoisomerizes to (OC-6-33)-[Pt(tpy)2(O2CCF3)2] (4 '). 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 d sigma* orbital distributed along the N-Pt-O axis, which would trigger the observed processes.