Abstract
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.
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