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

Unsymmetrical bis-cyclometalated dicarboxylato complexes (<i>OC</i>-6-32)-[Pt(tpy)₂(O₂CR)₂] [tpy = cyclometalated 2-(<i>p</i>-tolyl)pyridine, R = <i>t</i>-Bu (<b>1</b>), Me (<b>2</b>), Ph (<b>3</b>), CF₃ (<b>4</b>)], are obtained from the reaction of <i>cis</i>-[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 (<i>OC</i>-6-43)-[Pt(tpy)₂(O₂CMe)(O₂CR')] [R' = <i>t</i>-Bu (<b>5</b>), CF₃ (<b>6</b>), Ph (<b>7</b>)], (<i>OC</i>-6-34)-[Pt(tpy)₂(O₂CCF₃)(O₂CR')] [R' = <i>t</i>-Bu (<b>8</b>), Me (<b>9</b>), Ph (<b>10</b>)], and (<i>OC</i>-6-34)-[Pt(tpy)₂(O₂C-<i>t</i>-Bu)(O₂CMe)] (<b>11</b>). Irradiation of <b>1</b>-<b>3</b> and <b>5</b>-<b>11</b> with UV light (365 nm) in MeCN gives 5-methyl-2-(2-pyridyl)phenyl pivalate (<b>12</b>), 5-methyl-2-(2-pyridyl)phenyl acetate (<b>13</b>) or 5-methyl-2-(2-pyridyl)phenyl benzoate (<b>14</b>) as the major photoproduct from most complexes, resulting from a reductive C-O coupling between a tpy ligand and a carboxylato ligand. Cyclometalation of <b>12</b>-<b>14</b> at the ensuing Pt(II) species to produce <i>cis</i>-[Pt(tpy)(tpyO₂CR/R')], reduction to <i>cis</i>-[Pt(tpy)₂] and isomerization to (<i>OC</i>-6-33)-[Pt(tpy)₂(O₂CR/R')₂] are identified as secondary processes in most cases. In contrast, complex <b>4</b> exclusively photoisomerizes to (<i>OC</i>-6-33)-[Pt(tpy)₂(O₂CCF₃)₂] (<b>4'</b>). 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.