Metal carbonyls, preparation

Anions of transition-metal carbonyls are prepared by reducing the carbonyl and react with derivatives of group-IIB metals to yield mono-, di- and even trisubstituted derivatives ... 

Our efforts in this area of catalysis began in 1980. Our initial emphasis was on the preparation of supported phase transfer catalysts. We later became interested in the chemistry of anioni-cally activated alumina(25) and the reactivity of metal carbonyl anions prepared under these conditions. A brief description of our work in the preparation of these materials and their synthetic applications follows. 

The first example of the use of metal carbonyls to prepare supported metal particles was reported by Parkyns, who prepared an alumina-supported nickel catalyst from Ni(CO)4 [3]. Since then, many studies of the groups of Professors Yermakov, Ugo, Basset, Ichikawa, Guczi and Gates, among others, have constituted the bases for knowledge in this field [4-10]. 

The following sections detail and update the more relevant aspects concerning the use of binary carbonyl metal complexes in the preparation of tailored metal supported catalysts. In several cases, we discuss the use of other pertinent carbonyl... 

F. Mixed Cyanocarbonyl Metalates Prepared from Metal Carbonyl Derivatives and NaN(SiMes)2. 38... 

The following facts concerning the use of liquid NH3 are of particular importance (1) its behavior as a solvent similar to water, but with a much lower proton activity (2) the access to the reaction temperature range of -78 to +120°C which made possible the preparation and isolation of complexes at low temperatures and (3) the strong reducing character of the solutions of the alkali metals in liquid NH3, which especially allowed for access to the carbonyl metalates having very low oxidation states. 

The reduction of metal carbonyls or their derivatives with sodium in liquid NHS, begun by us 35 years ago, has become a method often used successfully by numerous other research groups throughout the world. A few examples are the preparation of Na2[7j5-C5H5V(CO)3] from 175-C5H5V(CO)4 (43) and [V-m(CO)5]s- and [M m(CO)3]3- (M = Co, Rh, Ir). According to Ellis et al. (44), these carbonyl metalates are obtained in the extremely low oxidation state of -III from the anions [V-1(CO) ]-and Co (CO)4] and also from the neutral complexes M3(CO)i2 (M = Rh, Ir). 

The mono- and dinuclear carbonyl metalates of Cr, Mo, and W, first prepared by us using liquid NHS, have also been of great importance for the synthesis of numerous neutral complexes not accessible by other routes. Special attention is drawn to the deeply colored and very unstable mono- and dinuclear paramagnetic complexes Cr(CO)sI, Cr (CO),oI, Cr(CO)5CN, Cr(CO)sNCS, Cr COl.oNCS, and Cr(CO)sSH. 

The preparation of HNCC in the form of well-shaped crystals suitable for diffractometric analysis is extremely important because of the basic need for this type of characterization. In the last few years, we have used extensively crystallization methods involving slow diffusion of solvents in which the complex is insoluble. Generally, it is sufficient to stratify slowly a lower-density solvent over the solution of the compound (e.g., i-PrOH on acetone, t-Pr20 on MeOH, toluene on THF) crystallization occurs by simple diffusion for a couple of days. Unfortunately, with salts of carbonyl-metalates the ease of formation of crystals is generally unpredictable and dependent on the particular cation. Often it is necessary to try a whole series of cations, which may be very time-consuming, in order to obtain proper crystals. 

Chemical vapor transport is used to synthesize thin films of materials on a substrate. The film can be the same composition as the substrate or different. In order to proceed with chemical vapor transport, the constituent elements of the compound to be deposited as a thin film must be brought into the vapor phase. Given that many of the thin films of commercial importance involve elements with little or no practical vapor pressure, a lot of attention has been focused on preparing volatile compounds that contain the elements needed in thin-film preparations. Most chemical supply companies carry these compounds as stock items. The major classes of compounds include metal alkyl, metal carbonyl, metal alkoxide, metal 3-diketonates, and organometallics. Examples of each are given in Table 3.1. 

Many reviews have been devoted to the use of carbonyl metal complexes for the preparation of supported catalysts [2, 4, 17, 18, 61-67]. 

The main reasons for using carbonyl metal complexes as precursors for catalyst preparation are the following ... 

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 direct reaction between a metal carbonyl and an aromatic hydrocarbon often leads to the replacement of 2 or 3 carbonyl groups yielding arene complexes. Complexes of the type [(arene)V(CO)4][V(CO)g] 50), (arene)Cr(CO)g 327), and (arene)Fe(CO)3 281) have been prepared in this manner. Carbonyl metal halide complexes have also been observed to react with aromatic hydrocarbons in the presence of AlClg yielding cationic arene derivatives, e.g. 71, 104, 448),... 

The carbonyls are in general volatile compounds with an extensive chemistry which presents many problems as regards valence and stereochemistry. Some are reactive and form a variety of derivatives, as shown in Chart 22.1 for the iron compounds, while others are relatively inert, as for example, Cr(CO)6 etc. and Re2(CO)iQ. This rhenium compound, although converted to the carbonyl halides by gaseous halogens, is stable to alkalis and to concentrated mineral acids. A few carbonyls may be prepared by the direct action of CO on the metal, either at atmospheric pressure (Ni(C0)4) or under pressure at elevated temperatures (Fe(CO)s, Co4(CO)i2)- Others are prepared from halides or, in the case of Os and Re, from the highest oxide. The polynuclear carbonyls are prepared photo-synthetically, by heating the simple carbonyls, or by other indirect methods. 

These results are particularly interesting. Indeed, it appears from the literature that there are only a few methods allowing the preparation of carbonyl metals in aprotic solvent from metallic salts, under CO at atmospheric pressure and at low temperature27. ... 

Most catalysts that have been mentioned so far are mononuclear. The few binuclear compounds utilized Co2(CO)8 or phosphinesubstituted derivatives) did not give evidence of any unusual type of binuclear catalysis. However, new products could result with catalysts producing two active centers in close vicinity which would not dissociate in the course of the reaction. The expected difference between mononuclear and binuclear catalysis is shown in the accompanying diagram (52). A series of metal salts of cobalt carbonyl hydride of composition M[Co(C0)4]n (M=Zn, Cd, Hg, n = 2 M = In, = 3) were tested as potential binuclear catalysts. The complex salts are relatively easily accessible Zn[Co(CO)4]2, for instance, may be prepared from cobalt carbonyl, metallic zinc, and CO (at 3000 psi initial pressure) using toluene as the solvent and a temperature of 200°. The compound may also be synthesized directly from metallic cobalt, zinc, and CO... 

The dinuclear [(Tp Pr2)MCo(CO)4] (M = Ni, Co, Fe, Mn) have been prepared by the reaction of cationic complexes [M(Tp Pr2)-(NCMe)3]PF with the corresponding metalates. The tetrahedral, high-spin M(Tp Pr2) moiety and the coordinatively saturated carbonyl-metal fragments are connected by a M-M interaction.11... 

An alternative strategy for the preparation of mononuclear ylide complexes is to start from gold(I) precursors which already contain an ylide ligand. Displacement in such complexes of tetrahydrothiophene, SC4H8, by neutral or anionic ligands (including polyfunctional phosphines, acetylides, carbonyl metalates and methanide complexes) leads to a variety of mono-, di- and tetranuclear compounds of remarkable stability (Scheme 9)55,114,211... 

Ever since Parkyns (60) prepared nickel particles on oxide (A1203, Si02) surfaces by decomposition of nickel carbonyl, metal cluster compounds (alkoxides, carbonyls, organometallics, etc.) have been used increasingly for the production of laboratory metal catalysts (Table I), and several reviews have appeared on this subject (6l-65a,b). 

The carbonyls Ni(CO)4 and Fe(CO)5 (both highly toxic) are the only ones normally obtained by action of CO on the finely divided metal. Formation of Ni(CO)4 (equation 21.4) occurs at 298 K and 1 bar pressure, but Fe(CO)5 is made under 200 bar CO at 420-520 K. Most other simple metal carbonyls are prepared by reductive carbonylation, i.e. action of CO and a reducing agent (which may be excess CO) on a metal oxide, halide or other compound (e.g. reactions 23.3-23.10). Yields are often poor and we have not attempted to write stoichiometric reactions for the preparation of [Tc(H20)3(C0)3]+, see Box 22.7. 

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