Synthesis, Structures, and Reactivity of Ruthenium Complexes with PNP-pincer Type Phosphaalkene Ligands

2,6-Bis­[1-phenyl-2-(2,4,6-tri-tert-butylphenyl)-2-phosphaethenyl]­pyridine
(BPEP-Ph) reacts with [Ru3(CO)12] or [RuCl­(η3-allyl)­(CO)3] in toluene at elevated temperatures
to afford the three types of PNP-pincer complexes 1–3. The reaction of BPEP-Ph with [Ru3(CO)12] under vacuum forms a mixture of [Ru­(CO)2(BPEP-Ph)] (1) and a dicarbonylruthenium(0) complex (2);
the latter complex has an unsymmetrical PNP-pincer ligand with 1-phenyl-2-phosphaethenyl
and phenyl­(benzo­[b]­phospholan-1-yl)­methyl groups
at the 2,6-positions of the pyridine core, which is formed by intramolecular
C–H addition of a t-Bu group to the PC
bond. On the other hand, the reaction of BPEP-Ph with [RuCl­(η3-allyl)­(CO)3] affords a dicarbonylruthenium­(II)
chloride (3) with an unsymmetrical PNP-pincer ligand
bearing a dearomatized pyridyl group with 1-phenyl-2-phosphaethenyl
and α-(benzo­[b]­phospholan-1-yl)­benzylidene
substituents at the 2,6-positions. Treatment of 3 with i-PrOK in THF results in selective formation of 2. Complexes 1–3 have been isolated
as crystalline compounds and examined in detail by IR and NMR spectroscopy,
X-ray diffraction studies, and DFT calculations. Complex 1 exhibits catalytic activity toward N-alkylation
of amines (RNH2) with alcohols (R′CH2OH) to give imines (RNCHR′) as major products.