Metal ion complexation in organometallic crown ethers: Structural and rate effects on the migratory insertion of carbon monoxide in indenyl and cyclopentadienyl iron (II) complexes

Abstract The iron complexes [Fe( η 5 -ligand)(CO) 2 R] [ligand=C 5 H 5 (Cp), R=Me ( 1 ); ligand=C 5 H 5 , R=CHMe 2 ( 2 ); ligand=C 9 H 7 (Ind), R=Me ( 3 )] react with the crown ether functionalized phosphine [Ph 2 P–CH 2 -(aza-15-crown-5)] (L) to give the products of alkyl migratory insertion [Fe( η 5 –ligand)(COR)(CO)(L)] 4 , 5 and 6 , respectively. The cyclopentadienyl methyl complex [Fe( η 5 -C 5 H 5 )(CO) 2 Me] reacts with L only in refluxing hexane. The reactions of 2 and 3 , in acetonitrile or chloroform, have been followed by FT-IR spectroscopy, or by UV–Vis. The presence of sodium or calcium salts, NaI, NaPF 6 , or CaI 2 , does not affect the reactivity of the cyclopentadienyl complex 2 , whereas it increases two to three times the rate of reaction of the indenyl complex 3 . The acetyl complexes [Fe( η 5 -ligand)(COMe)(CO)(L)], (ligand=Cp, 4 ; ligand=Ind, 6 ) exhibit a shift of the stretching band ν (COMe) to lower frequencies, in the presence of metal salts, up to 40 cm −1 in the case of 6 ·NaPF 6 in chloroform, and up to 46 cm −1 in the case 6 ·KPF 6 in dichloromethane, as a result of coordination of the carbonyl oxygen to the crown ether held cation. This shift is absent or small in the more polar solvents tetrahydrofuran, acetonitrile, or dioxane, and it is not observed for the isopropyl complex 5 , suggesting that steric hindrance of the alkyl group prevents acyl coordination to the metal ion. The terminal carbonyl group of complexes 4 , 5 and 6 displays a shift of the ν (CO) band to higher frequencies in most solvents, up to 27 cm −1 for 6 ·NaPF 6 in acetonitrile and to 26 cm −1 for 6 ·LiClO 4 in dichloromethane, arising from electronic polarization by the metal ion and indicating electronic communication between the two metal centres.