Cyclopentadienyl transition metal compounds

D. E. Bublitz and K. L. Rinehart, Jr., The Synthesis of Substituted Ferrocenes and Other 7r-Cyclopentadienyl-Transition Metal Compounds, Organic Reactions 17, 1 (1969). 

The method involving reductive C5HS removal using alkali metals can also be applied to other i)5-cyclopentadienyl transition metal compounds. In addition, other ligands such as phosphanes, carbon monoxide or dinitrogen can be used instead of olefins. 

Synthesis of substituted ferrocenes and other w-cyclopentadienyl transition metal compounds Some carbonium ions of unusual... 

The discovery of the first cyclopentadienyl transition metal compound - ferrocene [ 1 ] - and the confirmation of its structure [2] opened a new era in chemistry. It showed the direct relationship between organic chemistry and inorganic chemistry of transition metals, and provided the necessary impetus for development of a new area organometallic chemistry. Although, the cy-clopentadienylmetal compounds seemed to be rather a chemical curiosity, further research soon revealed their useful potential in chemistry. Namely, it has been shown that many of them can be used as catalysts to facilitate a plethora of reactions and transformation imder mild reaction conditions that would be otherwise difficult or even impossible to achieve by using classical organic processes. 

Organosilyl substituted 7)5-cyclopentadienyl complexes may be prepared by two main procedures (a) reactions of silylated cyclopenta-dienes or their alkali metal derivatives, with an appropriate transition metal compound (carbonyl or halide) with formation of ir-bonds (b) metalation of a preformed 7j5-cyclopentadienyl complex, followed by treatment with an organohalosilane. 

Chiral Metal Atoms in Optically Active Organo-Transition-Metal Compounds, 18, 151 13C NMR Chemical Shifts and Coupling Constants of Organometallic Compounds, 12, 135 Compounds Derived from Alkynes and Carbonyl Complexes of Cobalt, 12, 323 Conjugate Addition of Grignard Reagents to Aromatic Systems, I, 221 Coordination of Unsaturated Molecules to Transition Metals, 14, 33 Cyclobutadiene Metal Complexes, 4, 95 Cyclopentadienyl Metal Compounds, 2, 365... 

Breslow (139) discovered a homogeneous system well suited for kinetic analysis. He realized that bis(cyclopentadienyl)titanium(IV) compounds, which are very soluble in aromatic hydrocarbons, could be used instead of titanium tetrachloride as the transition-metal compound together with aluminum alkyls to give Ziegler catalysts. Subsequent research on this and other systems with various alkyl groups has been conducted by Natta et al. (140, 141), Belov el at. (142-144), Patat (145), Patat and Sinn (146) Sinn et al. (119, 147), Shilov and co-workers (148-150), Chien and Hsieh (20), Adema (151), Clauss and Bestian (152), Henrici-Olive and Olive (153), and Reichert and Schoetter (154) and Fink (155). 

The following tables summarize the silicon-transition-metal compounds known up to 1973. The abbreviations used for organic substituents are me = CH3, et = C2Hs, pr = C3H7, bu = C4H10, ph = C6H5,bz = CH2C6Hs, cp = w-cyclopentadienyl, diphos = l,2-bis(diphenylphosphino)-ethane. 

Another route to bis( -arene)vanadium(0) compounds is the cocondensation of arenes with vaporized vanadium metal (see Metal Vapor Synthesis of Transition Metal Compounds) On treatment with 1,3-cyclohexadiene and butyllithium, 15-electron vanadocene (5) is converted to 16-electron ( -benzene)( -cyclopentadienyl)vanadium(l) (6) (Scheme 3). Use of potassium naphthalenide affords the corresponding naphthalene complex. 

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. 

On the other hand, transition metal compounds with chelating l,l -ferrocenylene, i.e., l-metalla-[l]ferrocenophanes, are rare, the best-known representatives being the group 4 metal complexes (Fig. 5-11) derived from the di(cyclopentadienyl) metal dichlorides, fc[MCp2] and fc[M(C5H4tBu)2] (M = Ti, Zr, Hf) [188, 212]. 

The compound Cr2(C5Mej)2S5 is the first chromium representative of sulfur rich dinuclear cyclopentadienyl transition metal complexes of general formula M2CP2S, (x 4). It is obtained from the reaction of Cr2(CsMes)2(CO)4 with sulfur in toluene solution. Alternatively, it can be prepared from Cr(C5Me5)2 and sulfur in about the same yield. 

The presence of the cyclopentadienyl ligand at the chromium center provides a catalyst with a unique high response to hydrogen as a chain transfer agent (97). A number of ir- and O-bonded transition metal compounds in solution or supported (34) have been described as polymerization catalysts. Unsupported and supported transition metal-ally 1 compounds have been proposed to initiate polymerization by reaction with monomer in a manner illustrated by Reaction 22 for (allyl)3ZrBr/Si02 and ethylene. 

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