Iron hydrides

Vibrational spectra are often so complicated that assignment of a particular absorption to a given bond is difficult. One way to confirm that an assignment is correct is to carry out selective isotopic substitution. For example, we can replace a hydrogen atom with a deuterium atom. If an iron-hydride (Fe—H) stretch occurs at 1950 cm-1, at what energy will this stretch occur, approximately, for a compound that has deuterium in place of the hydrogen Refer to Major Technique 1, which follows these exercises. 

Abstract Organic syntheses catalyzed by iron complexes have attracted considerable attention because iron is an abundant, inexpensive, and environmentally benign metal. It has been documented that various iron hydride complexes play important roles in catalytic cycles such as hydrogenation, hydrosilylation, hydro-boration, hydrogen generation, and element-element bond formation. This chapter summarizes the recent developments, mainly from 2000 to 2009, of iron catalysts involving hydride ligand(s) and the role of Fe-H species in catalytic cycles. 

Keywords Catalysis Electrochemical reduction Hydroboration Hydrogenation Hydrosilylation Iron hydride complex Photochemical reduction... 

In 1931, Hieber and Leutert reported Fe(CO)4(H)2 not only as the first iron hydride complex but also as the first transition-metal hydride complex (FeH2 was reported in 1929 from FeCl2 and PhMgBr under a hydrogen atmosphere. However, it exists only in a gas phase) [2, 3]. The complex synthesized from Fe(CO)5 and OH (Scheme 1) is isolable only at low temperature and decomposes at room temperature into Fe(CO)5, Fe(CO)3, and H2. 

Fig. 1 The first X-ray crystal structures of three types of the iron hydride complexes...

Iron hydride complexes can be synthesized by many routes. Some typical methods are listed in Scheme 2. Protonation of an anionic iron complex or substitution of hydride for one electron donor ligands, such as halides, affords hydride complexes. NaBH4 and L1A1H4 are generally used as the hydride source for the latter transformation. Oxidative addition of H2 and E-H to a low valent and unsaturated iron complex gives a hydride complex. Furthermore, p-hydride abstraction from an alkyl iron complex affords a hydride complex with olefin coordination. The last two reactions are frequently involved in catalytic cycles. 

A Fe-H bond is generally polarized as Fe -H because H is more electronegative than Fe. However, iron hydride complexes impart much less negative charge to the hydride than early transition-metal hydride complexes. 

The proposed mechanism for Fe-catalyzed 1,4-hydroboration is shown in Scheme 28. The FeCl2 is initially reduced by magnesium and then the 1,3-diene coordinates to the iron center (I II). The oxidative addition of the B-D bond of pinacolborane-tfi to II yields the iron hydride complex III. This species III undergoes a migratory insertion of the coordinated 1,3-diene into either the Fe-B bond to produce 7i-allyl hydride complex IV or the Fe-D bond to produce 7i-allyl boryl complex V. The ti-c rearrangement takes place (IV VI, V VII). Subsequently, reductive elimination to give the C-D bond from VI or to give the C-B bond from VII yields the deuterated hydroboration product and reinstalls an intermediate II to complete the catalytic cycle. However, up to date it has not been possible to confirm which pathway is correct. 

The isolated iron hydride complexes introduced in this chapter are listed in Table 12, where the hydride chemical shifts in the HNMR spectra and the Fe-H bond distances are summarized. 

Use of less sterically hindered examples of 5 in combination with MAO allows for active catalysts for the linear (head-to-head) dimerisation of a-olefins such as 1-butene, 1-hexene, 1-decene and Chevron Phillips C20-24 a-olefin mixture (Scheme 4) [47], The mechanism for dimerisation is thought to involve an initial 1,2-insertion into an iron-hydride bond followed by a 2,1-insertion of the second alkene and then chain transfer to give the dimers. Structurally related cobalt systems have also been shown to promote dimerisation albeit with lower activities [62], Oligomerisation of the a-olefms propene, 1-butene and 1-hexene has additionally been achieved with the CF3-containing iron and cobalt systems 5j and 6j yielding highly linear dimers [23],... 

ESI mass spectrometry has been used to probe the activation of 5a with MAO in THF and has shown the presence of both a four coordinate [2,6- (2,6-t-Pr2CsH3) N=CMe 2C5H3N]FeMe+ cation and an iron hydride species [2,6- (2,6-t-Pr2C6H3) N=CMe 2C5H3N]FeH+ [106], Using low Fe/MAO ratios, the activation reaction of... 

Knowledge of the active site allows for speculation on the mechanism of H2-D20 exchange which these Fe4 systems catalyze 473,483). Ruthe-nium(III) systems catalyze such an exchange via a ruthenium(III) hydride intermediate (7, p. 73 Section II,A), as exemplified in reactions (82) and (83), and iron hydrides must be involved in the hydrogenase systems. Ruthenium(III) also catalyzes the H2 reduction of ruthenium(IV) via reaction (82), followed by reaction (84) (3), and using these ruthenium systems as models, a very tentative scheme has been proposed 473) for... 

PBu3) to eliminate ethylene (in agreement with the enhanced ethylene production in the presence of CO and PBu3) or be further reduced to ethane by an iron hydride or by LAH.— The ethylidene could insert a new CO forming a ketene with three carbons and extend the chain. The importance of coordination of A1H3 to... 

According to this model the iron-hydride bond is cleaved upon illumination and the hydride, together with the proton, leaves the complex. This would lead to re-establishment of the nickel thiolate bond which was weakened in the former state, explaining the changes in the EPR spectrum observed after illumination (Pig. 7.16-III). After a flip of the electronic z-axis the selenium could in this state interact with the unpaired electron in an orbital with d -y2 character. To get EPR-active, CO-treated... 

It is unlikely that the hydride addition to the cation proceeds via an isolable iron hydride, 77-CpFe(CO)2H, plus CH3CH=CH2, followed by readdition of the hydride to the olefin, because it was not possible to transfer the iron to either 1-hexene or butadiene during the reduction. 

Titanium iron hydrides are among the materials which, at the present time, appear to have potential for practical applications as an energy-storage medium (7). The formation and properties of titanium iron hydride have been studied by Reilly and Wiswall (3), who found that the reaction proceeds in two steps as indicated by Reactions 5 and 6. Both hydrides have dissociation pressures above 1 atm at room temperature in contrast to TiH2 which is very stable. Titanium iron is representative of intermetallic compounds that consist of an element (titanium) capable of forming a stable hydride and another element (iron) that is not a hydride former or at best, forms a hydride with great difficulty. Iron presumably plays a role in destabilizing the hydrides. Titanium also forms a 1 1 compound with copper (there are other intermetallic compounds in the titanium-copper system) and this fact, coupled with the observation that copper... 

This method has essentially been applied only to iron hydrides. A correlation between Mossbauer and H NMR chemical shifts has been noted.112 The method has also been applied in cluster hydrides, and the non-equivalence of the metals in [Fe4H(CO)13] was shown in this way, for example. 

The mechanism of the C—H and C—C bond activation of bare Fe+ with n-heptyltrimethylsilane has been elucidated with the help of extensive labeling studies71. The system was found to display a rather rich chemistry. Loss of neutral tetramethylsilane from the ion-molecule complex (equation 11) was explained by an initial insertion of the metal ion into the Cl— C2 bond to form 21, and a subsequent fi-H shift giving rise to the iron-hydride complex 22. This ion can then lose a tetramethylsilane molecule via reductive elimination. 

The product in braces decomposes, giving an intermediate iron hydride species at the expense of (3-hydrogen elimination ... 

It is likely that such iron hydride species is capable of reducing acetone ... 

Disproportionation about the transition metal is much less common one example involves a silyl iron hydride (259). 

The first experiments which were carried out in the author s laboratory on organometallic phase-transfer catalysis were concerned with the reduction of nitrobenzenes (4) to anilines (5) by triiron dodecacarbonyl. Such a conversion was reported to occur in benzene containing methanol at reflux for 10-17 h, with the hydridoundecacarbonyltriferrate anion as the likely key intermediate (16). It was our expectation that the trinuclear iron hydride should be generated by phase-transfer catalysis and if so, effect reduction of nitro compounds (4) under exceedingly mild conditions. Indeed this was the case, as illustrated by the results shown in Table I (17). Not only is the reaction complete in 2 h or less using sodium hydroxide as the aqueous phase, benzene as the organic phase, and benzyltrieth-ylammonium chloride as the phase-transfer catalyst, but it occurs at room temperature and requires less metal carbonyl than when the reaction was... 

It is also conceivable that, instead of the formation of Fe3(CO)u2- by reaction of Fe3(CO)12 with the quaternary ammonium hydroxide, attack by the latter at a carbonyl carbon of Fe3(CO)i2 could occur to give the hydroxycarbonyl species 7 (Scheme 4). Subsequent loss of carbon dioxide would afford the iron hydride. The intermediacy of organometallics... 

Since silene is an unstable species, various transition metal-silene complexes coordinated by the silicon-carbon double bond have been reported. In 1970, Pannel reported the formation of silene by irradiation of an iron complex (Eq. 6) [8]. He obtained an iron-TMS complex that was apparently formed from silene and an iron-hydride complex generated from the starting iron complex by /3-hydrogen elimination [8]. Wrighton confirmed the existence of tungsten-and iron-silene complexes by examination of NMR spectra obtained at low temperature (Eqs. 7 and 8) [9]. 

The ort o-metalated iron hydride complexes [HFe(CO)2 P(OPh)3 (PhO)2POC6H4 ] with R C=CR (R =Ph, R2 = Mc R = R2 = Mc R =Me, R2 = CH(OEt)2 R =Me, R = CH20H R = R = CH20H) in the presence of hydrated zinc chloride give a series of ferracyclopentendione complexes 203 <2000JOM(612)61>. With phenylace-tylene under similar conditions, the ferrole-type species 204 is formed with structural parameters similar to those... 

Iron Hydride.—It is a moot point as to whether or not a hydride of iron is capable of existence. It has been suggested that ferrous hydride results on treating ferrous iodide with zinc ethyl. Thus... 

Iron Hydride—Iron and Fluorine—Ferrofluondes—Ferrifluorides. 

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