Catalytic Synthesis of an Unsymmetrical PNP-Pincer-Type Phosphaalkene Ligand

An
unsymmetrical PNP-pincer-type phosphaalkene ligand, 2-(phospholanylmethyl)-6-(2-phosphaethenyl)­pyridine
(PPEP), has been prepared from 2,6-bis­(2-phosphaethenyl)­pyridine (BPEP)
by intramolecular C–H addition/cyclization of the 2-phosphaethenyl
group with a 2,4,6-tri-tert-butylphenyl substituent
(CHPMes*). The reaction proceeds in hexane in the presence
of a catalytic amount of [Pt­(PCy3)2] (20 mol
%) at 80 °C in a sealed tube, giving PPEP in 32% isolated yield,
along with byproduction of 2,6-bis­(phospholanylmethyl)­pyridine (BPMP)
and a Pt­(II) phosphanido complex (5). The PPEP ligand
reacts with [Rh­(μ-Cl)­(C2H4)2]2 and [RuCl2(PPh3)3]
to afford [RhCl­(PPEP)] (6) and [RuCl2(PPh3)­(PPEP)] (8), respectively. Complex 6 easily undergoes C–H addition/cyclization at the other CHPMes*
group to afford the 2,6-bis­(phospholanylmethyl)­pyridine complex [RhCl­(BPMP)]
(7), whereas 8 is stable against C–H
addition/cyclization. Treatment of 8 with tBuOK forms [RuCl­(PPh3)­(PPEP*)] (9), coordinated with an unsymmetrical PNP-pincer-type phosphaalkene
ligand containing a dearomatized pyridine unit (PPEP*). The X-ray
structures of 5 and 9 are reported. The
reaction processes from BPEP to PPEP and to 5 are discussed
based on NMR observations.