Preparation, Structure Characterization, and Oxidation Activity of Ruthenium Complexes with Tripodal Ligands Bearing Noncovalent Interaction Sites

Ruthenium(II/III) complexes with tripodal tris(pyridylmethyl)amine ligands bearing one, two, or three pivalamide
groups (MPPA, BPPA, TPPA:  amide-series ligands) or neopentylamine ones (MNPA, BNPA, TNPA:  amine-series
ligands) at the 6-position of the pyridine ring have been synthesized and structurally characterized. The X-ray
structure analyses of the single crystals of these complexes reveal that they complete an octahedral geometry
with the tripodal ligand and some monodentate ligands. The amide-series ligands prefer to form a Ru(II) complex,
while the amine-series ones give a Ru(III) complex. In the presence of PhIO oxidant, the catalytic activities for
epoxidation of olefins, hydroxylation of alkane, and dehydrogenation of alcohol have been investigated using the
six ruthenium complexes [RuII(tppa)Cl2] (1), [RuIII(tnpa)Cl2]PF6 (2), [RuII(bppa)Cl]PF6 (3), [RuIII(bnpa)Cl2]PF6 (4),
[RuII(mppa)Cl]PF6 (5), and [RuIII(mnpa)Cl2]PF6 (6). Among them, the amide-series complexes, 1, 3, and 5, showed
a higher epoxidation activity in comparison with the amine-series ones, 2, 4, and 6. On the other hand, the latter
showed a higher reactivity for hydroxylation, allylic oxidation, and CC bond cleavage reactions compared with
the former. Such a complementary reactivity is interpreted by the character of the ruthenium−oxo species involving
electronically equivalent formulas, Ru(V)O and Ru(IV)−O•.