Clusters of Iron

The first carbidocarbonyl transition metal cluster to be recognized was Fe5C(CO),5 (1), which was isolated in very low yield from the reaction of triiron dodecacarbonyl with methylphenylacetylene and characterized by X-ray diffraction by Dahl and co-workers (2). The molecule (Fig. 1) comprises a square pyramidal Fe5 core with the carbide situated. 08 A below the center of a square face. Each iron atom bears three terminal carbonyls. Improved syntheses of 1 by protonation (5) or oxidation of [Fe6C(CO)l6]2- 

The reactions of 1 with phosphines and phosphites proceed smoothly at room temperature yielding products with varying degrees of substitution, Fe5C(CO)l5 (L) , n= 1-3, depending on the phosphorus donor ligand L 

All attempts to protonate the partially exposed carbon atom in 1 have been unsuccessful (1,5). Reaction with base yields [Fe5C(CO)14]2- (2), which was isolated as its tetraethylammonium salt (5). The dianion may also be synthesized by the reaction of Fe5C(CO)15 and [Fe(CO)4]2-, and although its structure is as yet undetermined, it presumably retains a square pyramidal core (4). 

Muetterties has reported the reaction of 2 with a number of mono- or di- 

As will be the case for all perspective drawings in this survey, metal atoms are represented by larger filled circles, the carbide carbon atom as a small filled circle, and carbonyls as small open circles, with metal-metal bonds as heavy lines to emphasize the core geometry. Mean Fe-Fe distances apical-basal - 2.63 A, intrabasal = 2.66 A. Fewicrf-CMrt4de= 1.96 A, mean Fe -i-C.. = 1.89 A. The carbide carbon lies 0.08 A below the basal plane. 

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The reactions of some transition metal cluster ions have been described in a review by Parent and Anderson (201). The review covered reactions reported up to 1992 and so the reactions reported here are generally later than 1992. A recent review by Knickelbein (202) discusses the reactions of cation clusters of iron, cobalt, nickel, copper, silver, niobium, and tungsten with small molecules such as H2 and D2. Some of the reactions in Knickelbein s review are included in the following tables of reactions (Tables IV and V). Table IV gives examples of the reactions of transition metal cluster ions and includes the vaporization source, experimental apparatus, the reactants, and the observed product ions. A few examples from these tables will be selected for further discussion. 

Iron-sulfur proteins are a group of enzymes and other electron carriers that contain clusters of iron and sulfide linked directly to amino-acyl side chains, usually cysteines. They are widely distributed in nature. Soon after their discovery. Hall et al. (1971) proposed that they could be used to follow the course of evolution. Studies of genome sequences have revealed that iron-sulfur cluster binding motifs are among the most commonly recognized sequences. 

Rearrangements of clusters, i.e. changes of cluster shape and increase and decrease of the number of cluster metal atoms, have already been mentioned with pyrolysis reactions and heterometallic cluster synthesis in chapter 2.4. Furthermore, cluster rearrangements can occur under conditions which are similar to those used to form simple clusters, e.g. simple redox reactions interconvert four to fifteen atom rhodium clusters (12,14, 280). Hard-base-induced disproportionation reactions lead to many atom clusters of rhenium (17), ruthenium and osmium (233), iron (108), rhodium (22, 88, 277), and iridium (28). And the interaction of metal carbonyl anions and clusters produces bigger clusters of iron (102, 367), ruthenium, and osmium (249). 

The synthetic methods used involve reaction of a cluster anion with [AuCIL], elimination of methane between a cluster hydride and [AuMeL] or addition of LAu+ units to metal-metal bonds. The emphasis here will be on structure and reactions of the complexes. Some examples of mixed gold clusters are given in Table 15, where it can be seen that most work has been on derivatives of clusters of iron, ruthenium and osmium. 

The recent upsurge of interest in iron-sulfur-nitrosyl complexes has been stimulated in part by the reported isolation of [Fe2(SMe)2(NO)4] from natural sources (12), by the obvious resemblances between these complexes and the naturally occurring [2Fe-2S] and [4Fe-4S] clusters of iron sulfur proteins (23, 14), and by the connections between tetrairon-sulfur-nitrosyls and cubane-type clusters (15). Most of the work in this area has been published in the past 5 years or so, and no review has previously been made. However, a number of excellent reviews of the wider aspects of metal-nitrosyl chemistry have appeared (16 19). 

Only two kinds of reagent have been extensively used for this type of reaction so far. These are the anionic carbido carbonyl clusters of iron and... 

Because of the broad variety of iron-containing proteins, they are usually divided into three distinct classes, based on their structural composition. Iron-sulfur proteins, which contain clusters of iron and inorganic sulfur and in which the metal... 

Much of the chemistry of peripheral C2 clusters of iron and ruthenium has been reviewed.35 Evolution of permetallated ethynes via cluster build-up to permetallated ethenes and ethynes is discussed, sequential additions of metal fragments occurring in addition to thermal redox and thermal condensation reactions. 

Closer to industrial application however, is the gas phase hydroxylation with nitrous oxide as the oxidant (Equation 39). The reaction is carried out at 350°C with a selectivity to phenol of 98%, at 27% benzene conversion. The catalyst is Fe-ZSM-5 a zeolite containing A1 and Fe in the silicalite-1 framework. Active sites are thought to be binuclear clusters of iron oxide, formed in the channels by the migration of Fe, during thermal treatments of the zeolite. Selectivity is of... 

In summary then, at least some of the Fe + appears to be oxidized in the protein coat, though it seems unlikely that significant amounts of dimeric, or higher polymeric, clusters of iron will be produced there. The displacement of one Fe + from a site in the coat by incoming Fe + is difficult enough to envisage, let alone the displacement of an iron cluster. 

The correlation implicit in Eq. (21) is quantitatively best for clusters of iron and vanadium consisting of more than eight atoms. For the case of niobium clusters, the correlation is more qualitative, but a distinct one-to-one correspondence of local minima in reactivity with local maxima in IP is indeed observed for clusters containing eight or more atoms. 

The main idea one has to keep in mind from this study is that inorganic clusters of iron octahedra with iron at two oxidation states are active for the reaction studied and that one has to consider the active sites as these clusters, preferentially as ensemble of two trimers although clusters of other sizes (dimers, tetramers, pentamers...) are also active and selective but to a lesser extent. The iron oxidation state is changing during the reaction in a similar way as the... 

What may seem to be an intrinsic decrease in TOF with particle size may sometimes be caused by the decreasing extent of reduction of smaller particles. Thus, for nickel, small particles are not completely reduced, and the degree of reduction must be considered in evaluating the effect of particle size on TOF (59). Attempts to produce small clusters of iron... 

One of the most important enzymes in the world— nitrogenase, the plant protein that catalyzes nitrogen fixation— contains active clusters of iron, sulfur, and molybdenum atoms. Crystalline molybdenum (Mo) has a body-centered cubic unit cell d of Mo = 10.28 g/cm ). (a) Determine the edge length of the unit cell, (b) Calculate the atomic radius of Mo. 

According to Niu and Millot (1999), at low/ 02, where the majority defects are iron interstitials, the prevailing oxygen defects are free oxygen vacancies. At high P02, where the majority defects are cation vacancies, the observed slope of 1/6 supports the presence of clusters of iron vacancies with oxygen vacancies formed as a result of Coulombic attractive forces. The main two points here are (a) oxygen diffusivities in select oxides (but not all) may follow a complex pattern as a function of Po2 md (b) information of this type is necessary in order to assess the relation between rates and defect structures. 

More recently, however, a monosubstituted carbonyl cluster of iron, Pe3(CO)ii(PPh3), has been isolated from the mixture of products of the reaction of Feg(C0)i2 with PPhg, which consisted of 98% mononuclear compounds (18). The clusters Co4(CO)nL (L = PPhg, AsPhg, SbPhg) (75) were the products of reaction of L with Co4(CO)i2 under mild conditions. By reaction with phosphite esters the clusters Fe3(CO)42- [P(OMe)3] (n = 1-3) have also been obtained from Fe3(CO)i2 (331). Apparently substitution without breakdown of the cluster is favored by the higher... 

Particularly conspicuous exposure age clusters in bold face, uncertain ones marked by a Howardites-Eucrites-Diogenites. b Ages of clusters of iron meteorites refer to C1 age-scale (see text). In the " K-scale, the double peak at 255 207 Ma in group IVA corresponds to a single peak at 375 Ma, whereas the IILAB peak at 460 Ma is shifted to 650 Ma (Voshage 1978, 1984). Oldest stated age of 2300 Myr (Deep Springs) refers to °K-scale. 

The unification of the various interpretations with respect to the active sites is extremely complicated due to the intrinsic heterogeneous nature of iron species in the catalyst. Particularly challenging in practice is suppressing clustering of iron species into large inactive iron oxide particles. A further complicating aspect for a rational unification is the application of Fe-zeolites in a wide range of catalytic reactions with a different mechanism. 

The preparation method of iron-zeolites has been recomized as critical in order to obtain reproducible catalysts with a desired performance." A distribution of iron species is normally obtained upon activation of catalysts by available methods. Suppressing clustering of iron species into iron oxide is convenient, since these species are proven inactive at low temperatures in the various reactions catalyzed by Fe-zeolites. " Steam activation of isomorphously substituted FeMFI zeolites enables a certain control of the degree of iron clustering, and thus on the relative amount of certain species in the final catalyst, as compared toother methods. A rather unique achievement has been attained here with Fe-silicalite (873 K), in view of the remarkable uniform nature of extraframework species in isolated positions. A minor association of iron species is present... 

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