Half-sandwich

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... 

Mono(cyclopentadienyl), or half-sandwich poly-oxo complexes are of interest as hydrocarbon-soluble models for oxide catalysts. The action of water on [Ta( j -C5Me5)(PMe3)2] yields the colourless [Ta4( j -C5Me5)407(0H)2] which has a tetranuclear butterfly core (Fig. 22.12c).( >... 

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. ... 

The half-sandwich and sandwich complexes of phospholides and phosphole tetramer are known even for the nontransition metals. The half-sandwich arrangement was studied for lithium tetramethylphospholide [89AG(E)1367] and sodium derivative 127 generated from the phospholide tetramer and sodium in the presence of 1,2-dimethoxyethane [94JA3306 96AG(E)1125]. Potassium and 1,2-dimethoxyethane in THE in these conditions give a full dianionic sandwich 128. 

In the context of pyridin synthesis from alkynes and nitriles homogeneously catalyzed by (cyclopentadienyl)cobalt complexes (Eq. 13), it was found that electron-withdrawing groups on the cyclopentadienyl ring significantly increase the activity [63]. During the screening of various cobalt half-sandwich complexes... 

The remarkable 19-electron molybdenum half-sandwich complex Mo(r 5-C5HPh4)(CO)2L2 (L2 = 2,3-bis(diphenylphosphino)maleic anhydride) (44) was prepared from [Mo(r)5-C5HPh4)(CO)3]2 and L2 and its structure in the solid state determined [65]. The average distance from molybdenum to the ring ligand is... 

Electron rich half-sandwich complexes — metal bases par excellence. H. Werner, Angew. Chem., Int. Ed. Engl., 1983, 22, 927-949 (170). 

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). ... 

The facile insertion of carbodiimides into a Ti-CMe bond of (CsRslTiMes (R = FI, Me) (cf. Section 1II.B.2) has been extended to the insertion of optically pure (R,R)- and meso-(R,S)-1.3-bis(l-phenylethyl)carbodiimide. Treatment of Cp M(NMe2)3 (M = Zr, FFf) with both achiral and optically pure aminooxazoline proligands ITL yielded metastable aminooxazoline half-sandwich diamide... 

In a related study the synthesis of half-sandwich cyclopentadienyl titanium f-butylimido complexes containing pendant-arm-functionalized amidinate ligands... 

Dihydro-lH-l,2-azaboroles derive from cyclopentadiene by the isoelectronic replacement of a C=C group by a BN moiety ". The neutral rings react in xs boiling Fe(CO)j without any other solvent to form red-brown iron-dihydro-1,2-azaborolyldi-carbonyl dimers as their cis and transisomers in 56% yield, or with CojfCOg in petroleum ether at 60-80°C to the half-sandwich complex (dihydro-1,2-azaborolyl)Co(CO)2 (40%). The CO group can easily be substituted by olefins ... 

Some half-sandwich complexes with dihydro-l,2-azaborolyl ring ligands have been described, too ... 

However an unexpected new cyclic ruthenium phosphorus ylide half-sandwich complex 42 has been obtained by reaction of 41 with dichloromethane as solvent [79]. The cyclisation involves a C-Cl activation and corresponds to the incorporation of the methylene moiety in the P-C bond and to the ortho-metal-lation of one phenyl of the phosphine. An other novel unusual phosphonium ylide ruthenium complex 43 has also recently been described [80]. 

Interestingly, the cyclopropanation of styrenes with EDA catalyzed by the half sandwich iron complex [CpFe(CO)2(THF)] BF4 afforded cyclopropanes in good yields and with ds-selectivity cisitrans = 80 20) [62]. With phenyldiazomethane as a carbene source, excellent cA-selectivity (92-100%) was achieved (Scheme 15) [63]. 

Ferrocene and Half Sandwich Complexes as Catalysts with Iron Participation... 

Abstract The unique and readily tunable electronic and spatial characteristics of ferrocenes have been widely exploited in the field of asymmetric catalysis. The ferrocene moiety is not just an innocent steric element to create a three-dimensional chiral catalyst enviromnent. Instead, the Fe center can influence the catalytic process by electronic interaction with the catalytic site, if the latter is directly coimected to the sandwich core. Of increasing importance are also half sandwich complexes in which Fe is acting as a mild Lewis acid. Like ferrocene, half sandwich complexes are often relatively robust and readily accessible. This chapter highlights recent applications of ferrocene and half sandwich complexes in which the Fe center is essential for catalytic applications. 

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