Metal complex compounds metallocenes

This review summarizes some earlier qualitative work as well as recent quantitative studies of redistribution equilibria and describes the principles underlying the mathematical treatment of such equilibria as well as the general implications of these equilibria with respect to general chemistry. In line with the general objective of the Advances in Organometallic Chemistry series, this article is limited to carbon-metal-bonded systems, metal hydrides, metal carbonyl compounds, metallocenes, and similar complexes. Excluded therefore are halogen-, sulfur-, nitrogen, and phosphorus-based systems.Various aspects of redistribution reactions were reviewed previously (42, 74, 87, 88,150,186, 285, 286, 288). 

Planar chiral compounds usually (and for the purpose of this review, always) contain unsymmetrically substituted aromatic systems. Chirality arises because the otherwise enantiotopic faces of the aromatic ring are differentiated by the coordination to a metal atom - commonly iron (in the ferrocenes) or chromium (in the arenechromium tricarbonyl complexes). Withdrawal of electrons by the metal centre means that arene-metal complexes and metallocenes are more readily lithiated than their parent aromatic systems, and the stereochemical features associated with the planar chirality allow lithiation to be diastereoselective (if the starting material is chiral) or enantioselective (if only the product is chiral). 

Metalcarbonyls, organo- s. Acyl-cobaltcarbonyl complexes Metal complex compounds cobalt porphyrin complexes complex salts manganese complex compounds, ar. metalcarbonyl, organo-metallocenes... 

Very little is known as yet of the chemistry of cyclopentadienylthallium(I) and the related compounds listed in Table I. The parent compound gives tribromocyclopentane on treatment with bromine and the hexabromo derivative with potassium hypobromite 112). By far the most important use discovered so far for these organothallium(I) compounds is the preparation of metallocenes and cyclopentadiene-transition metal complexes. These preparations are, in general, characterized by manipulative simplicity and high yields, and details of the reactions reported thus far are summarized in Tables II-IV. 

Related to these catalysts are the systems based on lanthanide metal systems or rare earth metal complexes [46, 47]. The main problem with these catalyst systems is their instability. When the catalyst solution is prepared by reachng a metallocene with an organolithium compound in a polar solvent, the prepared catalyst soluhon is unstable and decomposes quickly, even under a nitrogen atmosphere. The activity of these catalysts can be high only if the catalyst is added to the polymer soluhon immediately after preparation. Attempts have been made to overcome the stability problem by using an additive in the system to improve the stability and the activity of the catalyst [33-35, 41, 57, 58, 61]. Re-... 

Substituted ethanes, e.g., benzpinacol Carbonium ions Organometallic compounds ti-Complexes with transition metals, e.g., metallocenes... 

One of the most important carbon 7i-donors are the cyclopentadienes and their heteroanalogues, for instance 15 and 16. In general, cyclopentadiene itself forms three general types of mononuclear Cp transition metal complexes Cp2M 3 (symmetric molecules with mutually parallel Cp rings examples M = Fe, Cr, Ni), Cp2ML 124 [bent metallocenes, L = H, R, CO, etc. (n=l-3)], and half-sandwich compounds CpML 125 (n — 1 4) [21b], In the bent sandwich complexes... 

On the basis of Cp and its heteroanalogues, a wide series of metallocene structures of types 3, 5, 126, 127 [2-5,7,8,13,132,133,192-205] have been obtained. These complexes are typical for Fe, Co, Ni, Mn, V, Cr, and Ru, and are the standard examples of complex compounds with multicenter delocalized coordination bonds formed as a result of combined grouping orbitals of the cyclopentadienyl rings with rf-metal orbitals. 

The electronic spectrum is yet another property which illustrates the similarities between the metallocenes and (7r-ollyl) metal compounds. In Table VI are listed some data for a series of Coin(absorption bands with the small extinction coefficients are probably two of the spin-allowed d-d transitions. Scott (34) has developed an approximate axial ligand field model for the carborane-transition metal complexes and has discussed the optical spectra in relation to this bonding theory. The actual assessment of bonding in the (7r-ollyl) metal compound as well as the metallocenes would be greatly aided by accurate assignments of the electronic spectra. 

Caruso F, Rossi M (2004) Antitumor titanium compounds and related metallocenes. In Sigel H (ed) Metal Complexes in Tumor Diagnosis and as Anticancer Agents. CRC Press, USA, pp 353-384... 

In a slightly different approach, the introduction of a metal complex is also possible as a metallo amino acid. For example, ferrocenyl-alanine (Fer) was prepared as early as 1957 [67]. Also, stereoselective preparations [68-71] and its biological activity were reported [72], and the compound has been inserted into peptide sequences just as a normal amino acid would (see Fig. 7 for an example) [35, 71]. In this case, the metallocene constitutes the side chain. Alternatively, the amino and carboxylic acid function can be on two different Cp rings, yielding, in the simplest possible case, l -aminoferrocene-l-carboxylic acid (Fca, see Fig. 7 for an example). This compound has also been reported in the literature, although it was difficult to obtain in pure form [73,74], It has since been incorporated into oligomers... 

Unlike the compounds obtained with the heavier halogens, polyfluori-nated metallocene derivatives cannot be prepared from the permercurated metallocenes. Thermally stable pentafluorinated ruthenocenes have been made from the (oxocyclohexadienyl)(cyclopentadienyl)metal complexes by flash vacuum pyrolysis (Scheme 6).53 54 Decafluorometallocenes, [M(C5F5)2], are still unknown potential routes to these compounds starting from fluorinated cyclopentadiene (C5F5H)55 or from the decachlorometal-locenes48 have been unsuccessful. However, there seems to be no fundamental steric or electronic barrier to their eventual preparation. 

Some years ago we observed that Group IV metallocenes, i.e., the elusive monomeric units bis(i7-cyclopentadienyl)zirconium and -hafnium, readily form 1 1 complexes with conjugated dienes in their 5-trans conformation (22). To our knowledge, these complexes (3) are the first and, at the time, only known examples of mononuclear (i-/ranj-i7 -diene)metal complexes, i.e., compounds in which both double bonds of the i-irans conformer of a conjugated diene are coordinated to the same transition metal center 21). Some examples of this class of transition metal complexes are stable, isolable compounds even at room temperature 23). 

Dyotropic Rearrangements and Related cr-o- Exchange Processes, 16, 33 Electronic Effects in Metallocenes and Certain Related Systems, 10, 79 Electronic Structure of Alkali Metal Adducts of Aromatic Hydrocarbons, 2, 115 Electron-Transfer Reactions of Mononuclear Organotransition Metal Complexes, 23, I Electron-Transfer Reactions of Polynuclear Organotransition Metal Complexes, 24, 87 Fast Exchange Reactions of Group 1, 11, and 111 Organometallic Compounds, 8, 167 Fischer-Tropsch Reaction, 17, 61 Fluorocarbon Derivatives of Metals, 1, 143... 

The use of the boratabenzene heterocycle as a ligand for transition metal complexes dates back to 1970 with the synthesis of (C H5B-Ph)CpCo+ (1) (Cp = cyclopentadienyl).1 Since boratabenzene and Cp are 6 it electron donors, 1 can be considered isoelectronic to cobaltocenium. Many other transition metal compounds have been prepared that take advantage of the relationship between Cp and boratabenzene.2 In 1996, the synthesis of bis(diisopropylaminoboratabenzene)zirconium dichloride (CsHsB-NPr ZrCh (2) was reported Of particular interest is that 2 can be activated with methylaluminoxane (MAO) to produce ethylene polymerization catalysts with activities similar to those characteristic of group 4 metallocenes.4 Subsequent efforts showed that, under similar reaction conditions, (CsHjB-Ph ZrCh/MAO (3/MAO) gave predominantly 2-alkyl-1-alkenes5 while (CsHsB-OEt ZrCh/MAO (4/MAO) produced exclusively 1-alkenes.6 Therefore, as shown in Scheme 1, it is possible to modulate the specificity of the catalytic species by choice of the exocyclic group on boron. 

Unlike the compounds obtained with the heavier halogens, polyfluori-nated metallocene derivatives cannot be prepared from the permercurated metallocenes. Thermally stable pentafluorinated ruthenocenes have been made from the (oxocyclohexadienyl)(cyclopentadienyl)metal complexes by flash vacuum pyrolysis (Scheme Decafluorometallocenes,... 

As far as organometallics are concerned, and important exception is represented by the (per)alkylated metallocenes and the related bis(arene) complexes of various transition metals. These compounds have been intensively studied in connection with the magnetic properties of their CT complexes, mainly by Miller and co-workers [11]. These studies led to the discovery of the first organometallic compound displaying bulk ferromagnetic properties (vide infra). 

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