Hieber anion

The low-valent ferrate [Fe(CO)3(NO)] 76 or Hieber anion was discovered some 50 years ago by Hieber and Beutner [43, 44] in order to extend the Hieber base reaction [45,46], in which iron pentacarbonyl 78 reacts with alkaline bases to form the [Fe(CO)4] anion [47, 48]. Compared to its homoleptic analogue, the Hieber anion is more stable because the electron-withdrawing character of the nitrosyl ligand stabilizes the negative charge at the iron atom. 

With Roustan et al. using the sodium salt of the Hieber anion 76-Na, the procedure was improved by Xu and Zhou in 1987 when they introduced the corresponding shelf-stable tetrabutylammonium salt 76-[Bu4N] which is available from Fe(CO)5 78, NaN02 and Bu4NBr (Scheme 17) [61,62]. As well as discovered by Roustan they obtained the substitution products with an ipso-preference (Scheme 16) albeit in a significantly lower yield. In order to maintain the catalytic activity of the product, the reactions were performed under CO-gas atmosphere. 

After these encouraging results, it is surprising that no further investigations have been performed on this reaction, which might be attributed to problems regarding catalyst stability, reproducibility, or the use of a problematic CO gas atmosphere. The comeback of the Hieber anion dates back to 2006 when our group... 

Scheme 4-196. Allylic alkylation of C-nucleophiles with allyl carbonates catalyzed by the Hieber anion [Fe(CO)3NO] .

Apart from this mechanistic hypothesis, another scenario, with a ferrate complex as intermediate, may be possible. In 1928, Hieber discovered that Fe(CO)5 78 underwent a disproportionation in the presence of ethylenediamine 122 [97-101]. Depending on the reaction temperature, different ferrate complexes were formed that incorporated a [Fe(en)3] cation (en = ethylenediamine) and mono-, di- or trinuclear ferrate anions (Scheme 32) [102-107]. As the reaction discussed above is also performed with amines at high temperatures, these ferrates may well be involved in the catalytic cycle of the carbonylation discussed above. 

Hieber and coworkers first prepared the pentacarbonyl hydride complex of rhenium1 by protonation of the corresponding pentacarbonyl anion, [Re(CO)5] . Modified versions of the original syntheses have been reported,2 but these procedures still require extensive vacuum-line manipulations. Furthermore, the yield of ReH(CO)s is only 30% based on pure Na[Re(CO)s],1,3 which means that the overall yield from Re2(CO)i0 is much lower. 

Anionic clusters are good nucleophiles (see Section III,A) and are often easy to make. On the other hand, the electrophilic nature of most monometallic complexes is obvious from ligand substitutions. The combination of these properties makes a strategy for cluster expansion. This strategy was used for the first time by Hieber (130) in making Fe4(CO)fc from Fe3(CO),7 and Fe(CO)s. It is probably active in many syntheses of large metal carbonyl clusters because the Re, Os, Rh, Ir, Ni, and Pt clusters involved are almost always anionic. However, simple stoichiometries can rarely be written for such reactions (122). This route makes mixed metal clusters accessible, e.g.,... 

The r(NO) frequencies for these three anions decrease with increasing CN- substitution so that the [Co(NO)(CN)3]3 anion shows the very low value of 1485 cm-1. This anion is isoelectronic with [Ni(NO)(CN)3]2 prepared by Hieber and Fiihrling (137). 

Fe2(CO)9, and Fe3(CO)12, respectively. Similar disproportionations occurred with Ni(CO)4 and Co2(CO)8 which gave anionic species such as [Ni2(CO)6]2, [Ni3(CO)8]2, [Co(CO)4] , etc., upon treatment with ammonia or other amines. In contrast to the carbonyls of iron, nickel and cobalt, those of chromium, molybdenum and tungsten reacted with pyridine and 1,2-ethylenediamine to afford substitution products of the general composition M(CO)6 (py) (n = 1, 2, and 3) and M(CO)4(en) with the metal remaining in the oxidation state zero [25], Mainly as the result of this work, Hieber became convinced that the metal carbonyls should be regarded as true coordination compounds, and the coordinated CO should not be considered a radical but a monodentate ligand like NH3, pyridine, etc. He held this view despite the criticism by several of his contemporaries [3, 19] and was very pleased to see that in most textbooks published after 1940 this view had been accepted. 

A more favorable synthesis of salts of the iron, cobalt and nickel carbonyl anions, which were initially prepared by disproportionation reactions of Fe(CO)5, Co2(CO)8, and Ni(CO)4 with pyridine and other amines, was found by treatment of the neutral carbonyls with alkali in aqueous or alcoholic solutions. Careful studies by Hieber revealed that Fe(CO)5 as well as Fe3(CO)12 reacted with exactly four equivalents of hydroxide ions to give the corresponding dianionic iron carbonylates (Scheme 4.4). These dianions are relatively strong bases and readily accept a proton from a water molecule to give the monoanionic hydrido carbonylates [I IFe(CO)4] and [HFe3(CO)n], respectively [36]. The related carbonylates of cobalt and manganese, [Co(CO)4] and [Mn(CO)5], were obtained by a similar way as [Fe(CO)4]2 [25]. With regard to the mechanism of Hieber s Basenreaktion , the most plausible explanation is based on an initial nucleophilic attack by the hydroxide ion at the carbon atom of a CO... 

The chemistry of the metal carbonyl hydrides and metal carbonylates remained the principal research topic for Hieber until the 1960s. He mentioned in his account [25], that it was a particular pleasure for him that in his laboratory the first hydrido carbonyl complexes of the manganese group, HMn(CO)5 and HRe(CO)5, were prepared by careful addition of concentrated phosphoric acid to solid samples of the sodium salts of the [M(CO)5] anions, giving the highly volatile hydrido derivatives in nearly quantitative yield [45, 46]. In contrast to HCo(CO)4 and its rhodium and iridium analogues, the pentacarbonyl hydrido compounds of manganese and rhenium are thermally remarkably stable, and in... 

In the last three decades of the twentieth century, following Walter Hieber s retirement, four aspects of the research on mono- and polynuclear metal carbonyl complexes found particular attention. These were the preparation of highly reduced carbonyl metallate anions, the generation of stable metal carbonyl cations, the matrix isolation of uncharged metal carbonyls obeying or not the 18-electron rule and, last but not least, the giant metal carbonyl clusters. 

The anion [Fe3H(C0)u]" was first isolated by Hieber and Brendle1 in 1957, who employed a base disproportionation of Fe3(CO)i2 followed by protonation. 

Heteropoly anions. 760-764 Hexagonal crystal system, 75. 78 Hexamiciear clusters. 816 HexgcnuA cSos st packed system, 120 Hieber, Walter. 643 Highest occupied molecular orbital (HOMO), 351,428, 721, 722... 

The first step is likely to be a so-called Hieber base reaction (0 in Scheme 1), describing a nucleophilic attack at metal-coordinated carbon monoxide by hydroxide. While eq. (3) is the prototypical example as first reported by Walter Hieber in 1932 [5], several other variants have since become known. For example, azides and alkyl/aryl anions attack metal carbonyls according to eqs. (4) and (5), yielding metal isocyanates (via a Curtius-type azide degradation step) and metal carbenes, respectively [6, 7]. 

Hieber and his school have carried out a systematic investigation of the transition metal carhonyl anions and derivative hydrides 169), and have synthesized many carbonyl anion and hydride clusters, particularly of the first row transition metals. The preparations of these clusters were all based on the reduction of a carbonyl compound. More recently a number of heteronuclear carbonyl anion and hydride clusters have been produced by condensation reactions. 

Another area of major interest in the field is that of mixed metal cluster systems. Hieber very early in the study of metal carbonyl chemistry was able to prepare mercury complexes of the type Fe(CO)4(HgCl)2. The related gold ° compounds were prepared in the 1960s by reaction of phosphine gold halides with carbonyl anions. 

Some very large carbonyl cluster anions of the late transition elements have been prepared by condensation of carbonyl anions with polynuclear carbonyls. The first example of such a reaction was discovered by Hieber in 1965. 

Because the most synthetically useful anionic metal carbonyl derivatives are those of the alkali metals, the chemistry of this class of substances will be emphasized. These materials are oftenpreparedfromthefreeoramalgamated alkali metal and an appropriate metal carbonyl derivative in an ether or other inert aprotic solvent. The metal ammine and related salts of metal carbonyl anions, thoroughly studied by Hieber and co-workers in Munich over the last 15 years, will be treated in less detail since they are less useful synthetically. For a more detailed description of these compounds the reader is referred to the review article by Hieber et al. (7). 

The related reactions between Lewis bases, especially amines, and various metal carbonyl derivatives have been utilized largely by Hieber and coworkers to prepare a variety of metal carbonyl anions. Examples of reactions of this type are illustrated by the following equations ... 

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