Complex Speciation of bipy- and phen-NNP Pincer Complexes of Ruthenium under Catalytic Ester Reduction Conditions

The N-heterocyclic backbone of NNP pincer
ligands
derived from 2,2′-bipyridine and 1,10-phenanthroline is prone
to hydrogenation under H2 or in alcohols. The new complex
of this work, RuHCl(CO)[phen-NNP] (4), where phen-NNP
= 2-((di-t-butylphosphaneyl)methyl)-1,10-phenanthroline,
undergoes dearomatization when treated with potassium tert-butoxide.
The dearomatized product 5 reacts with ethanol and dihydrogen
to give 16-electron hydrides 7–9 containing
partially hydrogenated phen-NNP ligands. The structure and reactivity
of complexes 5–9 are detailed in
the present report. Similar chemical behavior has been documented
for the Milstein catalyst, RuHCl(CO)[bipy-NNP] where bipy = 6-((di-t-butylphosphaneyl)methyl)-2,2′-bipyridine. The dearomatized
Milstein catalyst (1) is readily hydrogenated in solution
at room temperature to give the main product 2 containing
an NNP ligand with the central piperidine fragment, accompanied by
the dimer 3 possessing one NNP ligand with an outer piperidine
fragment. The dearomatized bipy- and phen-NNP complexes 1 and 5 and their hydrogenated derivatives 2, 3, and 9 were tested in this work in
solventless room-temperature reduction of methyl hexanoate to hexanol
under 50 bar H2 while using a 5000:1 substrate-to-catalyst
ratio. The phen-NNP derived complexes 5 and 9 proved inactive in this reaction. The density functional theory
(DFT) calculations indicated that H2 addition to the 16-electron 9 should be endoergic by at least 8.5 kcal/mol, thus, making
the complex unable to transfer H2 to the substrate. With
the Milstein catalyst 1 and its hydrogenated derivatives 2 and 3 the order of activity in room-temperature
reduction of methyl hexanoate was 3 > 1 > 2 with the conversions reaching 99, 70, and 34%,
respectively.
A distinct induction period observed in this reaction with 1 suggested that the catalytic activity should be attributed to a
highly active complex catalyst formed from 1 in the reaction
solution. A possible structure for this “super-catalyst”
is proposed.