Metallocenes analogues

A number of organometallic polymers containing metallocenes and metallocene analogues has been well-known for some time2. Because of the features of high-temperature stability and radiation resistance of the ferrocene nucleus, ferrocene-containing polymers are of special interest. Basically, these polymers may be divided into two classes the metallocene moiety is either located in a pendant group or in a backbone of the polymer chain. The former polymers have been synthesized by vinyl polymerization of vinyl metallocene monomers such as vinylferrocene. The latter polymers have been prepared by polycondensation of l,l -disubstituted metallocenes or metallocene dihalides with a.w-disubstituted monomers, and fell into two main types, (A) and (B). 

A more complex structure was reported for [28bZr-(02CMe)2]2, obtained in 70% yield from the constrained-geometry metallocene analogue [(Me4C5)-... 

The aminolysis of the homoleptic group 4 metal dimethylamides with amines, alkohols, and acidic hydrocarbons such as cyclo-pentadiene was first described by Chandra, G. Lappert, M. F. J. Chew. Soc. A 1968, 1940. This method was first applied to the synthesis of half-sandwich metallocene analogues by Hughes, A. K. Meetsma, A. Teuben, J. H. Organowetallics 1993, 12, 1936. 

Exclusively ir-aromatic metal clusterslike X X=Bq, Mg, Ca, NaX, Na2 3, Y Y= Zn, Cd, Hg, Be, Mg, Ca have also been studied. The NICS(O) value for Ca3 is positive whereas its NICS(l) value is negative. Some of them exhibit bond stretch isomerism . Trigonal dianion aromatic metal clusters " " " like Y F=Be, Mg, Ca containing two TT-electrons can replace the cyclopentadienyl rings to form various metallocene analogues (Fig. 17 and 18) and multidecker sandwich complexes They can stabilize a direct Zn-Zn bond and can trap hydrogen molecules. ... 

However, the best-researched group of catalysts for these substrates is the metallocenes (Fig. 30.3 Table 30.1, entries 7-13) [7-14]. The highest ee-value was obtained with the chiral samarium metallocene 10a. Hydrogenation of 1 at -78°C gave 2-phenylbutane in 96% ee (Table 30.1, entry 12) [12]. Turnover frequencies (TOFs) as high as 1210 IT1 have been recorded for these catalysts [14]. Catalyst 10 b, the titanocene analogue of 10 a, has been synthesized and used to hydrogenate 1 in 60% ee (Table 30.1, entry 13) [13]. 

The ferrocene analogues, like ferrocene, are oxidizable with the loss of one electron. In cases For which structures have been determined, they have been found to correspond to that expected on the basis of metallocene chemistry (Fig. 16.53). 

The principle aim of the reported studies was to model structures, conformational equilibria, and fluxionality. Parameters for the model involving interactionless dummy atoms were fitted to infrared spectra and allowed for the structures of metallocenes (M = Fe(H), Ru(II), Os(II), V(U), Cr(II), Cofll), Co(ni), Fe(III), Ni(II)) and analogues with substituted cyclopentadienyl rings (Fig. 13.3) to be accurately reproduced 981. The preferred conformation and the calculated barrier for cyclopentadienyl ring rotation in ferrocene were also found to agree well with the experimentally determined data (Table 13.1). This is not surprising since the relevant experimental data were used in the parameterization procedure. However, the parameters were shown to be self-consistent and transferable (except for the torsional parameters which are dependent on the metal center). An important conclusion was that the preference for an eclipsed conformation of metallocenes is the result of electronic effects. Van der Waals and electrostatic terms were similar for the eclipsed and staggered conformation and the van der Waals interactions were attractive 981. It is important to note, however, that these conclusions are to some extent dependent on the parameterization scheme, and particularly on the parameters used for the nonbonded interactions. 

A half-sandwich imido complex of niobium (132), which is both isoelec-tronic and isolobal with the bent metallocene Cp2ZrCl2, reacts with two equiv PhMgBr in the presence of PMe3 to give the 18-e complex 6 [Eq. (21)].29 In contrast to the zirconocene analogue, trimethylphosphine is required to stabilize this tr-complex. 

As members of the first family of organometallics, metallocenes and their substituted analogues will undoubtedly continue to influence all areas of inorganic and organometallic chemistry in the foreseeable future. 

The complex SmCpj was synthesized by metal vapour reaction of samarium with Cs(CH3)5H as a green product or by desolvation of the red SmCpj 2THF by sublimation. The crystal structure of the complex SmCpj shows it to be decamethyl samarocene with bent metallocene disposition [138], The Cg-Sm-Cg angle of 140.1° is greater than 136.7 in the desolvated analogue. The Sm-C bond distance is reduced, 2.79(1) A compared to 2.86 (3) A and the structure of SmCpj is shown in Fig. 6.12. 

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