Energy-minimized structures and MO levels of catalysts related to [RuO(hpsd)(bpy)]+ that competently hydroxylate benzene (hpsd(2-)=(2-hydroxyphenyl)salicyldiminato)

Two series of complexes with formal oxidation state assignments of {RuV(O2−)} have been examined by molecular mechanics and molecular orbital methods at the level of PM3 calculations in order to assess the origin of differences in the activity of these complexes in the conversion of benzene to phenol by oxygen transfer. The first series includes complexes of general formula [RuO(hpsd)(XY)]n+ with hpsd2− (also known in the literature as amp2−)=(2-hydroxyphenyl)salicyldiminato; XY=bpy(2,2′-bipyridine) and other py-X, wherein the second pyridyl group of bpy is changed to X=–CH2N(CH3)2 (stronger σ-donor X), –CH2P(CH3)2 (better π-acceptor X), –CO2 − (weak π-donor X), –CH2S− (strong π-donor X), and –CH2C(CH3)2 − (very strong σ-donor X). A second series of complexes, [RuO(TDL)(bpy)]n+ was also studied with TDL=(tridentate ligand) of the parent hpsd2− (or amp2−); cpsd2−=(N-(2-carboxyphenyl)salicylaldiminato); cppc−=(N-2-carboxyphenylpyridine-2-carboxaldiminato); and hppc−=(2-hydroxyphenyl)2-pyridylcarboxaldiminato (or app−). Experimentally, the activity order based upon the percentage yields of oxygenated products for [RuO(TDL)(bpy)]n+ is as follows for TDL’s=hpsd2− (91%) > cppc− (87%) > cpsd2− (84%) > hppc− (80%). The rates approach toward saturation in reactivity as a function of the fractional positive charge on the apical O center: cppc− (0.233) > hpsd2− (0.166) > cpsd2− (0.105) > hppc− (0.041). The reactivity order follows chelate ring strain influences of the TDL, with 5,6-membered chelate rings; hpsd2− and cppc− > 6,6; cpsd2− > 5,5; hppc−. It was determined that the general structures of these complexes are best described as pentagonal pyramidal (rather than pseudo-octahedral) with the RuO unit apical, the three donors of hpsd2−, cpsd2−, cppc− or hppc−, and the two donors of XY ligands adopting a waffled arrangement around the Ru center as the remaining donors of the pentagonal set. The donor most trans to the apical RuO is approximately at 140°, rather than 180°. Ligands such as hpsd2− (amp2−) are not retained in a single planar array, but rather with one of the aromatic donors turned upward to shield the approach of the RuO unit from one side. The ligand series [RuO(hpsd)(XY)]+ averages angles between adjacent atoms of the pentagonal set of 75.4° instead of a theoretical 72.0°; angles between the apical RuO and adjacent donors average 111° but with wide deviations (±30°) depending upon the donors of the TDL. Small changes in the donor atom positions, and in the capability of the “trans” donor’s σ-donor strength, and whether it is a π-acceptor or a π-donor, modulate the degree of mixing of ligand orbitals and the LUMO/SOMO energy gap which influences reactivity. The presence of a π-acceptor ligand provides the most destabilization of Ru–O π bonding, and this appears to be the best way to increase the activity of these catalysts toward oxidation of C6H6 to C6H5OH. Also, implicated in the activity of the catalysts is the need for two non-innocent phenolate donors that raise the energy of orbitals on the apical O atom. This increases the oxenoid character of the terminal O, and makes the insertion into a C–H bond more favored. The complex [RuO(hpsd)(bpy)]+, hpsd2−=(2-hydroxyphenyl)salicyldiminato (also known as amp2−), is the parent complex of the series [RuO(TDL)(py-X)]n+ that hydroxylate benzene (TDL=tridentate ligand). Although formally octahedral {RuVO2−} species, calculations at the PM3 level show them to be better described as pentagonal pyramidal complexes with resonance character of {RuIII, O-atom} and even {RuII, O+}. The more π-accepting the trans py-X ligand, the higher the catalytic activity. Eleven complexes are discussed.