Redox Chemistry of Bis(phosphaethenyl)pyridine Iron Complexes

Redox reactions of iron complexes bearing a PNP-pincer-type phosphaalkene ligand, 2,6-bis[1-phenyl-2-(2,4,6-tri-tert-butylphenyl)-2-phosphaethenyl]pyridine (BPEP), are reported. The Fe(II) dibromide [FeBr2(BPEP)] ( 1) is readily reduced by [Cp2Co] to afford the four-coordinate Fe(I) monobromide [FeBr(BPEP)] (2), while 2 reacts with PhCH2Br to reproduce 1. Treatment of 1 with MesMgBr or Mes(2)Mg(THF)(2) (Mes = 2,4,6-Me3C6H2) results in one-electron reduction of 1, followed by transmetalation of the resulting 2 with mesitylmagnesium compounds to give the Fe(I) mesityl complex [FeMes(BPEP)] (3). The single-crystal diffraction study of 3 has revealed a distorted trigonal monopyramidal arrangement around the iron center. SQUID magnetometry has established a low-spin ground state (S = 1/2) of 3. Complex 2 reacts with Me2Mg(THF)(2) to afford Fe(0) and Fe(II) complexes (4 and 5, respectively) coordinated with novel multidentate ligand systems containing a phosphonium ylide structure. The formation processes of 4 and 5 via an [FeMe(BPEP)] intermediate are discussed on the basis of their X-ray structures.