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

The eta (2)-alkyne complex [Fe(CO)(2){P(OPh)(3)}(2)(eta (2)-PhCCPh)] 1 reac ts readily with PR3 to give [Fe(CO)(2){P(OPh)(3)}(PR3)(eta (2)-PhCCPh)], an d then [Fe(CO)(2)(PR3)(2)(eta (2)-PhCCPh)] (R = OMe, 2a; OEt, 2b; (OBu)-Bu- n, 2c; Me, 2d; Bu-n, 2e; Ph, 2f). The ability of the alkyne ligand to act a s a four-electron donor to a ligand-dissociated 16-electron reaction interm ediate promotes site-specific replacement of the axial phosphite ligands; t here is no evidence for equatorial CO replacement. Reaction of 1 with CO af fords six-coordinate ferracyclopent-3-ene-2,5-dione (maleoyl) complexes [Fe (CO)(m){P(OPh)(3)}(n){eta (1) : eta (1)-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(a lkyne) bonds. Analogues of 4a, [Fe(CO)(2){P(OR)(3)}(2){eta (1) : eta (1)-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 mechani sm proposed for this reaction depends on replacement of an axial phosphite ligand by CO which is then susceptible to migratory attack by C(alkyne), wh ereas 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 an d 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 an d Ph) also proceeds via alkyne-CNR coupling to give [Fe(CO)(2){P(OPh)(3)}{e ta (1) : eta (1) : eta (1) : eta (1)-C(=NR)=C(Ph)C(Ph)=C(=NR)}] which is pr oposed to have a square pyramidal geometry on the basis of spectroscopic da ta.