Cobalt sandwich complexes

By cobalt-lithium exchange, the group of Sekiguchi and coworkers generated several dilithium salts of variously substituted cyclobutadiene dianions . By the reaction of the functionalized acetylenes (e.g. compound 137) with CpCo(CO)2 (136), the corresponding cobalt sandwich complexes, related to compound 138, were obtained (Scheme 50). These can be interconverted into the dilithium salts of the accordant cyclobutadiene dianions (e.g. dilithium compound 139) by reaction with metallic lithium in THF. Bicyclic as well as tricyclic (e.g. dilithium compound 141, starting from cobalt complex 140) silyl substituted systems were generated (Scheme 51) . ... 

The molecular structures obtained by X-ray crystallography for the cobalt sandwich complex with two B-Me- or B-OMe-containing rings show that both molecules have planar and parallel rings arranged centro-symmetrically (74CB3786). Similar structures were found for metallic complexes with M — Fe or Mn, and R = Me or Ph (72CB3484). 

Figure 14 Boracychc ligands used in cobalt sandwich complexes...

One of the most intensely studied boracychc compound is the borabenzene ligand synthesized by Her-berich and coworkers. Bis- and monoborabenzene cobalt sandwich complexes were first obtained by treating cobaltocene with boron trihalides BX3 (R = Cl, Br) or alkyl/arylboron dihalides (R = alkyl, aryl) as outlined in... 

Studies of Ln(in) nitrilotriacetate (539) and ethylenediamine (540) complexes have been reported. In the latter case a contact mechanism is responsible for the shifts in the lighter lanthanides (up to Ho) whereas a pseudocontact shift is dominant for the remainder of the series. Some novel lanthanide-cobalt sandwich complexes have also been reported. (587) Uranium(iv) complexes continue to be the most extensively studied of the actinide ion complexes. Appreciable dipolar interactions are expected to arise from the orbitally degenerate ground term arising from the 5f configuration of U(iv). However, in practice isotropic shifts of U(iv) complexes are not dominated by dipolar interactions. 

A mixture of the ten-membered cyclic cw-diol 648 and its trans- om v is obtained from the enediyne dialdehyde 647. A new family of hydrocarbon rings, the three skipped cyclic C12H12 dienediynes 649 - 651, has been described. These compounds were prepared from dilithium derivatives of enediynes and diiodo- or dibromoalkenes as shown in Scheme 6. Treatment of 651 with CpCo(CO)2 gave the cobalt sandwich complex 652 ". ... 

Aranzaes JR, Daniel M-C, Astruc D (2006) Metallocenes as references for the determination of redox potentials by cyclic voltammetry - permethylated iron and cobalt sandwich complexes, inhibition by polyamine dendrimers, and the role of hydroxy - containing ferrocenes. Can J Chem 84(2) 288-299. doi 10.1139/v05-262... 

Takanashi K, Lee VY, Ichinohe M, Sekiguchi A (2007) (q -Cycloptaitadienyl)(q -tetrasila-and ti -tiisilagCTmacyclobutadiene)cobalt sandwich complexes featuring heavy cyclobutadiene ligands. Eur J Inorg Chem 5471... 

Cobalt complex 83 was obtained by the reaction of [CpCo(SMe2)3l2(BF4)2 with pentamethylpyrrole [88AG(E)579]. Full cobalt sandwich of 2,5-di-terr-butyl-pyrrole is also known [91JCS(CC)1368]. Meanwhile, attempts to synthesize other pyrrolyl complexes of cobalt, (Ti -2,5-dimethylpyrrole)cobalttricarbonyl and the 3,4-dimethyl analog, have been unsuccessful [87JOM(330)231]. 

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... 

Carboranes also offer the capacity for Co111—C bond formation. Perhaps the best-known example of a mixed cyclopentadienyl-metallocarborane sandwich complex is the cobalt(III) compound (Cp)Co(l,2-C2B9Hn), which serves as a precursor for other species.544... 

These sandwich complexes have been prepared [(ir-B9C2Hn)2M]-, j(jr-B9C2-H,oC6H6)2M]-, [(7r-B9C2H9(CH3)2)2M] (M = Fe, Co substituents are on the carbon atoms). The unsubstituted complexes of iron and cobalt and the phenyl-substituted complex of cobalt are stable toward base and may be prepared by either... 

Reduction of a dicohaltacarhorane sandwich complex (5) is a two-electron process, and the metal centers are independent, noncommunicating sites [52]. In related work [53], it was observed that a cyclic voltammogram for the reduction of (6) at a platinum cathode in either dimethoxyethane or dimethylformamide containing tetra- -butylammonium hex-afluorophosphate consists of three well-resolved stages of reduction, each corresponding to a one-electron process at each cobalt center. 

There is a certain analogy between the aromatic anions of cyclopentadienide (C5H5 ) and boratabenzene (C5H6B ). l-Methylbora-2,5-cyclohexadienehas a more acidic proton connected to the, sp3-hybridized ring carbon atom than cyclopentadi-ene, due to the same tendency of aromatic anion formation [252, 253]. The related 1-phenyl-1,4-dihydroborabenzene affords the lithium salt of 1-phenylborataben-zene on treatment with tert-butyllithium. Like metallic complexes such as ferrocene formed by cyclopentadiene, boratabenzene also forms such sandwich -complexes with iron and cobalt. The iron complex can be acetylated under Friedel-Crafts conditions. 

The ligands triphos or/ and np in presence of compounds of iron, cobalt, nickel, rhodium, iridium and palladium, by reaction with THF solutions of white phosphorus, P, or yellow arsenic, As, form mononuclear or dinuclear sandwich complexes containing the cyclo-triphosphorus or cyclo-triarsenic units which behave as 3n-electrons rings. 

Curiously, only chromium (33), cobalt, and nickel complexes of this 11-atom ligand have been prepared thus far. The complex (l,7-B9C2Hu)2Co has been prepared by both the anhydrous and aqueous methods (18). An X-ray diffraction study of the cesium salt of the cobalt(III) complex confirmed the symmetrical sandwich bonding of the carborane cages but failed to locate the carbon atoms because of disorder in the crystal (48). 

Scheme 21. Syntheses of cationic nonaamido-sandwich complexes of iron and cobalt (18-electron forms) using the same core as that used in Scheme 20.

Similar reaction conditions for manganese, iron and cobalt complexes lead to more stable arene systems, presumably due in part to a higher valence electron count for these metal centres. Reduction of [3,5- Pr2-2,6-Trip2C6HMX]ra (M = Mn, X = I, n= 1 M = Fe, X = C1, n= 1 M = Co, X = C1, n — 2) gives rise to the Mn(I) inverted sandwich complex [3,5- Pr2-... 

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