Enhanced substitutional lability of [Fe(CO)2{P(OPh)3}2(η2-PhCCPh)]: facile insertion of CO and organoisocyanides into iron–alkyne bonds

Abstract

The η²-alkyne complex [Fe(CO)₂{P(OPh)₃}₂(η²-PhCCPh)] 1 reacts readily with PR₃ to give [Fe(CO)₂{P(OPh)₃}(PR₃)(η²-PhCCPh)], and then [Fe(CO)₂(PR₃)₂(η²-PhCCPh)] (R = OMe, 2a; OEt, 2b; OⁿBu, 2c; Me, 2d; ⁿBu, 2e; Ph, 2f). The ability of the alkyne ligand to act as a four-electron donor to a ligand-dissociated 16-electron reaction intermediate promotes site-specific replacement of the axial phosphite ligands; there is no evidence for equatorial CO replacement. Reaction of 1 with CO affords six-coordinate ferracyclopent-3-ene-2,5-dione (maleoyl) complexes [Fe(CO)_m{P(OPh)₃}_n{η¹ ∶ η¹-C(O)C(Ph)C(Ph)C(O)}] (m = 3, n = 1, 3; m = n = 2, 4a) in which CO groups have inserted into each of the Fe–C(alkyne) bonds. Analogues of 4a, [Fe(CO)₂{P(OR)₃}₂{η¹ ∶ η¹-C(O)C(Ph)C(Ph)C(O)}] (R = Me, 4b; Et, 4c), are similarly obtained as the sole products from the reaction of CO with 2a and 2b, respectively. The mechanism proposed for this reaction depends on replacement of an axial phosphite ligand by CO which is then susceptible to migratory attack by C(alkyne), whereas the existing equatorial CO ligands are not. The crystal structure of 3 has been determined: the complex exhibits a distorted octahedral geometry about the iron centre with a facial arrangement of the three CO ligands and the remaining coordination sites occupied by the P(OPh)₃ ligand and by the two C(O) groups of the maleoyl moiety. Reaction of 1 and CNR (R = Me and Ph) also proceeds via alkyne–CNR coupling to give [Fe(CO)₂{P(OPh)₃}{η¹ ∶ η¹ ∶ η¹ ∶ η¹-C(NR)C(Ph)C(Ph)C(NR)}] which is proposed to have a square pyramidal geometry on the basis of spectroscopic data.