Half sandwich compounds

Syndiotactic polystyrene was first obtained only recently by Ishihara et al. [5] in polymerisation with a homogeneous catalyst derived from a transition metal compound such as monocyclopentadienyltitanium trichloride and methylalu-minoxane in toluene. Since then, several authors have reported on the synthesis of syndiotactic polystyrene promoted by different catalysts based on metal hydrocarbyls such as benzyl compounds, half-sandwich metallocenes (e.g. monocyclopentadienyl, monopentamethylcyclopentadienyl and monoindenyl metal derivatives), metal alkoxides, metallocenes and some other compounds. These catalysts are commonly derived from titanium or zirconium compounds, either activated with methylaluminoxane or aluminium-free, such as those activated with tris(pentafluorophenyl)boron, and promote the syndiospecific polymerisation of styrene and substituted styrenes [5-10,21,48-70], Representative examples of the syndiospecific polymerisation of styrene using catalysts based on various titanium compounds and methylaluminoxane are shown in Table 4.2 [6,52,53,56,58],... 

Other sandwich compounds Half-sandwich compounds with rf- or y -rings Half-sandwich compounds with y -rings Miscellaneous compounds with rf ligands... 

Although the chemistry of zirconium in its lower oxidation states is still relatively unexplored, it is developing. Examples which offer the possibility of further exploitation include the blue, paramagnetic zirconium(III) compound 32) [L2Zr(/r-Cl)2ZrL2] L = C5H3(SiMe3)2-l,3, and the sandwich and half-sandwich compounds derived from cycloheptatriene red... 

Numerous half-sandwich compounds of the type [M()7 -C5R5)L2], M = Rh, Ir R = H, Me L = CO, phosphine etc.) are known and are useful reagents. [Ir()7 -C5Me5)(CO)2] for instance is an excellent nucleophile and is also used in the photochemical activation of C-H in alkanes. It is particularly effective in the latter role when supercritical CO2 is the solvent. ... 

A half-sandwich titanacarborane guanidinate complex is formed on treatment of the corresponding amide precursor with 1,3-dicyclohexylcarbodiimide (Scheme 106). The compounds have been employed as catalysts for the guanylation of amines (cf. Section V.C). ... 

Goti, A. et al., J. Organomet. Chem., 1994, 470(1-2), C4 This half-sandwich compound is extremely pyrophoric. 

The unsymmetrical complexes LnPcPc can also be prepared in a stepwise manner (Scheme 8.1, D, upper part) [87, 112-114]. Treatment of phthalonitrile with excess of Lu(OAc)3- H2Oand DBU gives the half-sandwich compound LuPc(0Ac)(H20)2. The latter complex reacts with Na2Nc in 1-chloronaphthalene leading to the formation of LuPcNc [87]. Similarly, the template reaction of unsubstituted and crown-substituted phthalonitriles with lutetium acetate was carried out in boding w-hexanol in the presence of DBU [115-117]. Bi- and tri-nuclear [118] lutetium complexes could also be obtained following this approach [111, 119]. 

The experiments described here and many others performed in our group show clearly, that it will be very difficult to find non-nucleophilic anions which allow the isolation of compounds containing the highly electrophilic half-sandwich cation Me5C5Si+. This species so far has been observed only in the gas-phase stabilization in the condensed phase is still an attractive target. 

Attempts to substitute the cyclooctadiene ligand from [(r 4-cod)Rh(r 2-R2P-CH2-SbR2)][BPh4] (R = z -Pr, R = z -Pr or Z-Bu) by H2 led to neutral half-sandwich compounds where one of the phenyl rings of the BPh4 anion is coordinated to rhodium (21) (Scheme 9).100... 

Salts of [ iplatinum compounds. Thus salts of the dianion [2,2,2-(CO)3-c/oxo-2,l-ReCBioHnf have been prepared by... 

Silane a-complexes of the Group 6 metals are among the best studied. Three families of compounds are particularly noteworthy. These are the half-sandwich arene complexes [Cr(ri -HSiMe2H)(CO)2(C6Me6)] (27), the above-mentioned Kubas s complexes and the pentacarbonyl derivatives [M(t -HSiR3)(CO)5]. ... 

Fig.5 Examples of simple metallocene structures a parallel sandwich, b multi-decker sandwich, c half-sandwich, d bent/tilted sandwich and e compounds with differently bonded cyclopentadienyl ligands...

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