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.