Iron complexes carbonyl exchange

Carbonyl exchange at (FeH(CO)4] in tetrahydrofuran solution is catalysed by sodium ions. Formation of ion pairs (1) is thought to be responsible for the catalysis. A kinetic study of diene exchange at five-co-ordinate iron complexes of the type [( 7 -enone)Fe(CO)3] shows that the reaction proceeds via competing associative and dissociative pathways. ... 

Tris(amido)phosphines 40-43 react with Fc2(CO)9 to give mononuclear iron carbonyl complexes Fe(CO)4(PR3). IR data collected from these complexes, combined with data from similar complexes, revealed the TT-acidity of the phosphines to be 41 w 40 > 43 > P(OPh>3 >42 > PPh3 > P(NMe2)3- Variable-temperature NMR on the Fe(GO)4 complexes of 41-43 showed rapid exchange of axial and equatorial carbonyls from —80 to 20 °C, while complex 40 showed slow axial-equatorial carbonyl exchange even at room temperature, attributed to the steric bulk and rigidity of the ligand. 

The use of iron carbonyl derivatives in organic synthesis is often limited by the difficulty of separation of the iron complexes from the desired product at the end of the reaction. In the synthesis of aldehydes from alkyl halides using tetra-carbonylhydridoferrate, [HFe(CO)4] , this problem has been overcome by attaching the iron complex to an ion-exchange resin [equation (9)]. The resin converts alkyl halides into the homologous aldehyde in over 90% yield, and at the end of the reaction the iron complex is retained on the polymer. Allyl chlorides fail to react, and secondary alkyl halides undergo elimination. 

We do not know exactly where the hydrogen binds at the active site. We would not expect it to be detectable by X-ray diffraction, even at 0.1 nm resolution. EPR (Van der Zwaan et al. 1985), ENDOR (Fan et al. 1991b) and electron spin-echo envelope modulation (ESEEM) (Chapman et al. 1988) spectroscopy have detected hyperfine interactions with exchangeable hydrous in the NiC state of the [NiFe] hydrogenase, but have not so far located the hydron. It could bind to one or both metal ions, either as a hydride or H2 complex. Transition-metal chemistry provides many examples of hydrides and H2 complexes (see, for example. Bender et al. 1997). These are mostly with higher-mass elements such as osmium or ruthenium, but iron can form them too. In order to stabilize the compounds, carbonyl and phosphine ligands are commonly used (Section 6). 

Any interconversion in a sample may give rise to temperature-dependent NMR spectra. For example, the 13C NMR spectrum of dimeric cyclopentadienyl iron dicarbonyl is temperature dependent [114]. This was attributed to intermolecular exchanges of carbonyls and interconversion between cis and trans complexes. 

Thiocarbonyl (see Thiocarbonyl) compounds react rapidly with iron carbonyls, but only in a few cases are mononuclear )] -C=S complexes reahzed. Most of the work in this area has resulted from the discovery that carbon disulfide reacts with iron carbonyl transfer agents, in the presence of phosphines or phosphites, to give complexes (127). In several of these cases, it is possible to exchange one or both of the phosphine/phosphite ligands in high yield. 

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