Iron compounds ligand substitution

Today several methods are known, which can be classified into two types, depending on whether they involve the reduction of iron(II) compounds or ligand substitutions on iron(0) complexes. 

Iron complexes with hydrated furane ligands are available from appropriate alkynyl- and alkenyl-substituted iron compounds as precursors 174). 

A different class of iron compounds, viz. model haem complexes (Por)Fe(PMc3)(L ) (Por = substituted porphyrins), were studied by Walker et using a selective double-resonance technique in connection with isotopic enrichment of Fe. The complexes display a linear correlation between S Fe and 5 P of the phosphine ligand, and the variation of iron chemical shifts could be explained in terms of electronic d-d transition energies. 

Inspired by the observation of efficient hydrogenation catalysis with iron compounds bearing redox-active tridentate ligands, our laboratory also explored related chemistry with reduced iron compounds with bidentate chelates [92]. Aryl-substituted a-diimines were attractive supporting ligands due to their ease of synthesis, estabUshed one- and two-electron redox-activity [92-95] and precedent in iron-catalyzed C-C bond forming reactions [96-98]. 

Although additives to induce radical chemistry have allowed ligand substitutions of 18-electron complexes to be conducted under mild conditions, photochemical reactions provide a common and practical alternative. Photochemically induced dissociation of carbonyl ligands is most common, but photochemical dissociations of other dative ligands are known. Several examples are shown in Equations 5.36-5.40. These examples illustrate the dissociation of CO from homoleptic carbonyl compounds of iron - and chromium, the dissociation of CO from piano-stool carbonyl compounds, " ttie dissociation of N, and the dissociation of a carbodiimide to generate an intermediate that coordinates and cleaves the C-H bonds of alkanes. In some cases, like the formation of the two THE complexes, the products of the photochemical process are not isolated instead, they are treated in situ with a ligand, such as a phosphine, to form monosubstitution products selectively. 

Figure 5.15 [Fe(pyrimidine) H20)(M(CN)2)2] H,0, M=Ag, Au is an example of the cds (CdS04) net. The Ag compound is show with iron-centred nodes and Ag ions are shown as nicks on the links. This structure is triply interpenetrated and the compound exhibits spin crossover with hysteresis and dramatic colour change as well as a solid-state ligand substitution reaction with loss of water, making the pyrimidine ligand bridging irons from two different nets [33].

Ligand substitution reactions of the iron-group trimetal dodecacarbonyl clusters and derivatives from them have been extensively studied. M3(CO)i2 cluster, as it frequently occurs with stable small tri and tetranuclear clusters, is constituted by saturated 18-electron metal atoms. Accordingly dissociative pathways for substitutions in these compounds are expected. 

Iron carbonyl halides have not been extensively examined in the past decade for their own sake, but rather most studies where both CO and halide ions are bound to iron have focused on preparing derivatives with other ligand types bound to Fe. Examples of these have already been described in the appropriate sections above. The reader is referred to Equations 54, 75, 76, 78, 79, 105, Schemes 21, 46, 51, 55, 74, 75, 82, 83, and structure 36. General routes to these compounds include substitution reactions of Fe(CO)4X2 complexes and carbonylation of iron halide coordination compounds. 

The simultaneous observation of the two EPR spectra has been reported in particular for several tris(dithiocarbamato)iron(III) complexes [Fe(R2NC(S)S)3] where R = cyclohexyl [143], hydroxyethyl [144], and n-butyl [145, 146]. In addition, a considerable number of iron(III) complexes of the type [Fe" -N402] has been found which show EPR spectra of both the HS and LS isomers. These comprise [Fe(X-SalEen)2] Y2 where X-SalEen is the Schiff-base ligand obtained by condensation of X-substituted salicylaldehyde and IV-ethylethylenediamine [147] and similar compounds [100, 148, 149, 150, 151]. For the cobalt(II) complex [Co(terpy)2] (004)2, it is not completely clear whether the two observed EPR spectra are due to HS and LS states related by a spin-state transformation [152]. 

Ferrocene compounds containing heteroelements are versatile building blocks for polynuclear complexes. They possess electrochemically active iron centers as well as unique cylindrical shapes. An example of ferrocene-substituted thiolato ligands is l,l -ferrocenedithiolate (S2fc). Its complex with Pd(II) lacks a Pd—Fe bond. 

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