Cobalt, complexes cyclobutadiene

Trimethylsilyl groups in these compounds are cleaved with hydrochloric acid giving the first cobalt-cyclobutadiene complexes with unsubstituted positions in the four-membered ring. The protecting role of SiMe3 groups (see also Section III, A) is important here, since phenylacetylene does not give cyclobutadiene complexes under similar conditions, but trimerizes instead. 

In a metallation reminiscent of those described in Section 9.18.4.2.3 (Equation 8), treatment of the biscyclopenta-dienyl compound 124 with (ClMe2Si)2 gives the trisilaheterocycle 125 in 69% yield <1997JOM(541)9>. The cobalt cyclobutadiene complexes 126 are formed via intramolecular cyclizations in 56-70% yield. This demonstrates that silaheterocycles can be formed at a metal center to form complexes as well as being preformed and then added to a suitable metal center to give metal derivatives (Scheme 35) <19970M646>. 

Certain cobalt cyclobutadiene complexes with acetylenic substituents rearrange on vacuum distillation through a hot tube (525 °C) and two mechanisms have been considered. The one in Scheme 15 involves a [2+2] cycloaddition so that... 

Flash vapor pyrolysis of the rf -thiophene l,l-dioxide)cobalt complexes results in extrusion of SO2 to generate (cyclobutadiene)cobalt complexes (Scheme 63)229. The absence of ligand crossover products indicates that this reaction occurs in a unimolecular fashion. Pyrolysis of the diastereomerically pure complex 240 gave the cyclobutadiene complex as an equimolar mixture of diastereomers 241a and 241b. In addition, the recovered starting material (37%) was shown to have ca 40% scramble of the diastereomeric... 

Co-free PAE). In PAE-CoCpl, the fluorescence quantum yield is only 18% of that observed for Co-free PAE, even though the quencher substitutes less than 0.1% of the aryleneethynylene units. The fluorescence in solution disappeared in PAE-CoCp4, where every fifth unit is a cyclobutadiene complex. The mechanism by which this quenching occurs is via the cobalt-centered MLCT states [82,83], conferred onto the polymer by the presence of cyclobutadiene complexes. Even in the solid state the polymers PAE-CoCpl-2 are nonemissive. It was therefore shown that incorporation of CpCo-stabilized cyclobutadiene complexes into PPEs even in small amounts leads to an efficient quenching of fluorescence in solution and in the solid state. Quenching occurs by inter- and intramolecular energy transfer [84]. 

Cobalt carbonyl complexes react with 1,3-diynes to give a variety of complexes in which two molecules of diyne have coupled to form y-cyclobutadiene ligands slightly different conditions result in formation of cluster complexes (see Section VILE.2). In the mixture of complexes obtained from the reaction... 

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) . ... 

These compounds have been obtained indirectly by reactions of silylated acetylenes with metal carbonyls or olefin complexes. Thus, trimethylsilylphenylacetylene reacts with rj5-cyclopentadienylcobalt dicarbonyl, cobaltocene, or rjs-cyclopentadienyl-(l,3-cyclooctadiene) cobalt, in refluxing xylene, to give a mixture of cis- and trans-bis-(trimethylsilyl)cyclobutadiene complexes (R = Me, R = Ph) 68, 127, 137) ... 

Acetylenes are well known to undergo facile trimerizations to derivatives of benzene in the presence of various transition metal catalysts 23). A number of mechanisms for this process have been considered including the intervention of metal-cyclobutadiene complexes 24). This chemistry, however, was subjected to close examination by Whitesides and Ehmann, who found no evidence for species with cyclobutadiene symmetry 25). Cyclotrimeri-zation of 2-butyne-l,l,l-d3 was studied using chromium(III), cobalt(II), cobalt(O), nickel(O), and titanium complexes. The absence of 1,2,3-trimethyl-4,5,6-tri(methyl-d3) benzene in the benzene products ruled out the intermediacy of cyclobutadiene-metal complexes in the formation of the benzene derivatives. The unusual stability of cyclobutadiene-metal complexes, however, makes them dubious candidates for intermediates in this chemistry. Once formed, it is doubtful that they would undergo sufficiently facile cycloaddition with acetylenes to constitute intermediates along a catalytic route to trimers. 

Thiophen Dioxides and their Dihydro and Tetrahvdro Derivatives. - Flash vacuum pyrolysis of cobalt complexes (194) of thiophen 1 1-dioxides led to extrusion of sulphur dioxide and the formation of cyclobutadiene complexes (195). Stereochemical labelling experiments were employed in order to determine the mechanism of this transformation.114 A detailed publication on the reaction of... 

Compounds 33 and 34 are readily formed from 31 by direct reaction with CpCo(CO)2. A possible reaction sequence is formation of the triene, 31, from alkyne dimerization followed by reaction with the cobalt species to give the three complexes. Reaction of 34 with alkynes yielded only cyclobutadiene complexes alkyne metathesis was not observed, probably since the carbon-to-metal bonds are too strong. 

As iron forms cyclobutadiene complexes preferentially in the (0) oxidation state, the best starting materials for these complexes are Fe(0) compounds such as the carbonyls. Cobalt, however, tends to form Co(I) (also cyclobutadiene complexes. The best known one (cyclopentadienyl)(tetra-phenylcyclobutadiene)cobalt (XXVIII), has been prepared from cyclo-pentadienylcobalt(I) derivatives and also from cobaltocene, a cobalt(II)... 

Sekiguchi s work concerning the use of silylated cyclotetrasUadienes and related ligands covered also several cobalt (17) and rhodium complexes [237,422,425]. Related work on a monosila cyclobutadiene complex was reported by Kira and coworkers [426]. 

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... 

Heteroatom-containing (cyclobutadiene)Co complexes (e.g. 237, 238 and 239) have been prepared by the reaction of heteroatom containing cobalt precursors with dipheny-lacetylene or by the reaction of cobalt precursors with phospha-alkynes228. 

Gibson and co-workers have introduced a well-designed latent cyclobutadiene moiety. Compound 49 reacts with a phenylethyne-cobalt complex to give PKR product 50 as a single diastereomer that resulted from the reaction at the less sterically hindered site, and 50 was subjected to the retro-Diels-Alder reaction at 205 °G under a vacuum of 6 torr to give 51 (Equation (23)). ... 

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