Alkyne clusters complexes

Hydrogenation of alkynes is catalyzed by alkyne cluster complexes Fe3(GO)9(RG2R ). In addition to hydrogenation, formation of metallacyclic byproducts such as Fe3(GO)8 (RG2R )(RG2R ) was observed, resulting in decreased catalytic activity. 

Surprisingly, in several experiments carried out under a variety of conditions, formation of the / cyclohexyne derivative 19b from cyclohexene and la was never observed. This complex was prepared from cyclohexene with f(C5H5)2Co]/potassium. Reactions with this reagent usually do not proceed cleanly and generate a large amount of insoluble by-products. Nevertheless, separation of the products is usually less laborious, due to the absence of 3 and 4. As a second type of product, the tetranuclear /r4-alkyne cluster complexes 21 were formed in very low yield from the corresponding cycloalkenes and [(C5H5)2Co]/K. ° ... 

Fig. 2.8. Basic structures of some alkyne cluster complexes, (a) Bonded to a butterfly array of metal atoms bearing to an octahedral M4C2 framework (b) and (c) isomeric forms of interaction with a M 3-cluster triangular face...

In organometallic systems in particular, clusters with bridging alkylthiolates are well known, as exemplified by Co2(Cp)2(/i-SCH3)2, which undergoes reactions with alkyne-cobalt complexes to yield trinuclear clusters.170... 

Reaction of the phosphaalkyne complex Pt(dppe)(f/2-P=CtBu) with either Fe2(CO)9 or Fe3(CO)12 affords high yields of the red cluster Fe2Pt-(A<3-( /2--L)-P=OBu)(CO)6(dppe) (55), which contains a novel /i3-(f/2-1) phosphaalkyne ligand (140,140a). This bonding mode is similar to the alkyne coordination observed in some alkyne clusters, such as Fe3(/i3-(f/2- )-PhC=CPh)(CO)9 (141), and the low value of J(195Pt-31P) = 128 Hz indicates that the P atom of the phosphaalkyne is not coordinated to the Pt... 

Metal cluster complexes containing vinylidene ligands have been considered as models of species present when olefins or alkynes are chemisorbed on metal surfaces (114). Vinylidene has been detected in reactions of ethylene or acetylene with Fe(100), Ni( 111), and Pt(l 11) surfaces (115), and was shown to be an intermediate by theoretical studies on a manganese surface (116). The facile cleavage of C-H bonds which occurs in these systems, together with hydrogen addition or abstraction, also occurs on metal clusters. Typical of the reactions considered is the hydrogen transfer reaction... 

H, Me, r-Bu, or Ph or R = H and R = Me, r-Bu, or Ph), was performed. Two possible reactions were investigated (a) the reactions suitable for the gas-phase interactions, which start from a 1 1 Br2-alkyne r-complex and do not enter into a 2 1 Br2-alkyne jt-complex and (b) the processes passing through a 2 1 Br2-alkyne 7r-complex, which appear more realistic for the reactions in solutions. The structures of the reactants and (g) the final products and also the possible stable intermediates have been optimized and the transition states for the predicted process have been found. Both trans- and cw-dibromoalkenes may ensue without the formation of ionic intermediates from a n-complex of two bromine molecules with the alkyne (solution reactions). The geometry around the double bond formed in dibromoalkenes strongly depends on the nature of the substituents at the triple bond. The cluster model was used for the prediction of the solvent influence on the value of the activation barrier for the bromination of the but-2-yne.35... 

The cluster valence electron (c.v.e.) count usually corresponds to 12 + 22 electrons. Bonding of the C2 unit involves stabilization of a, a, and ji orbitals by interaction with radial metal MOs of the same symmetry, together with overlap of ji orbitals with filled metal MOs, i.e., a similar synergic interaction to the familiar bonding mode found in alkyne-metal complexes. For the model [Co8(C2)(n-L)(L)8]4 based on two trigonal... 

Alkyne hydrido cluster complexes of the type [Co3(p.-H)2(/i-alkyne)Cp3] can be prepared by 1,2 double C—H activation of alkenes.132 133 For example, the p.-cycloalkyne complexes 36 can be prepared either from the respective cycloalkenes and [Co(C2H4)2Cp] or by using the reductive cleavage of cobaltocene with potassium as a source of CpCo fragments. (See Formula 36.)... 

Several factors affect the nature of the products in a reaction between a transition metal cluster and an alkyne or alkene. In this section, the various synthetic routes to alkyne or alkene-substituted clusters will be presented, and these will be used to analyze the changes in reactivity of the cluster systems when one or more of the important reaction parameters is altered. In order to simplify the discussion, tri-, tetra-, and higher nuclearity clusters will be treated separately. Finally, in this section, there is a brief description of the chemistry of alkylidyne-substituted clusters since synthetic routes to alkyne-containing complexes may involve these species. 

The best example of chemical activation in cluster chemistry is the use of Me3NO which results in CO replacement under mild conditions (129). In an interesting example taken from alkyne-cluster chemistry, when Me3NO is used dry a monosubstituted cluster derivative is obtained, but with damp Me3NO a tetranuclear vinylidene complex is isolated (130, 131), as illustrated in Eqs. (4) and (5), respectively. 

Although the reactions between alkynes or alkenes and metal clusters are the main source of alkyne-substituted complexes, there are other reagents which can produce similar products. Two such reagents are tetraphenylcyclopentadienone, which in the reaction with Ru3(CO)i2 produces Ru3(CO)10(PhCCPh) (167), and dimethyl-vinylarsine, which has been made to react with several carbonyl clusters [Eq. (8)] (168, 169). In the reaction of M3(CO)12 (M = Ru, Os) with a number of tertiary phosphines and aromatic alcohols, an oxidative addition takes place and benzyne-triosmium compounds are obtained (170-176). The fact that Os3(CO)uPEt3 can be converted into an alkyne compound (177) suggests that the conversion goes through substituted intermediates. Carbene derivatives of clusters have also... 

C NMR is generally applicable to organometallic and organo cluster compounds, but has the advantage of giving a direct probe on the acetylenic carbon atoms in alkyne-substituted clusters. With the development of more powerful NMR instruments, this technique has been used extensively to characterize organo-substituted cluster complexes in solution. For smaller clusters, the combination of 13C NMR with 31P NMR and resonance studies from metallic nuclei, where appropriate, frequently leads to complete structure elucidation. 

The observation of 13C-13C coupling constants in fi2- and pA-t]2-alkyne cobalt clusters has also shed some light on the nature of bonding in these complexes (382). There is a drastic decrease in the (CC) coupling constant on going from the free alkyne to complexes where the ligand is bonded to two or four cobalt atoms. In Co4(CO)10(/i4->/2-HCCH), the 1 J(CC) constant is 21 Hz, compared to that of 171.5 Hz in HCCH, and theoretical calculations indicate that this is consistent with a rehybridization of the formally sp hybridized acetylenic C atoms toward sp3 hybridization. 

Regrettably, alkyne-substituted cluster complexes seem particularly prone to fragmentation and very few accurate mass spectroscopic studies have been reported. A recent exception has been the field-desorption and electron-impact mass spectral investigation of mono-and oligo-nuclear ferracyclic ring systems of the from Fei(CO)J,(C2R2)2 (x = 1, 2, 3 y = 6, 8) (384). These species show intense molecular ion peaks, which enable ready recognition of the molecular composition. 

This technique is not a direct structural probe, but has been used as an experimental method to augment theoretical calculations on the bonding in cluster systems, including a number of alkyne-substituted complexes (389-391). The basis of the technique is that photons in the vacuum ultraviolet region of the spectrum, whose energy is about 10 eV, interact with molecules in the gas phase to cause either promotion of electrons from one bound state to another or their ejection as free electrons. Photoelectron spectroscopy is only concerned with processes that liberate electrons, either by direct ionization [Eq. (15)],... 

IV. Types of Bonding of Alkynes in Cluster Complexes A. Bonding Modes of Alkynes... 

Before considering the bonding of alkynes in cluster complexes it is worth discussing the rather simpler cases of mononuclear alkyne complexes and the free ligand itself. 

Fig. 15. Observed bonding modes of alkyne and alkyne-derived ligands in cluster complexes.

In contrast to alkyne ligands, alkenes are observed to bond side-on to one metal center in a cluster complex. This may reflect the poorer n-acceptor properties of an alkene compared to an alkyne. In all cases reported for single alkene units the ligand has been coordinated to a metal atom in a trinuclear cluster unit, and in the majority of cases the... 

Several complexes of Pd° and Pd containing PR3, DMSO, etc. catalyze hydrogenation of monoal-kenes. " Salen complexes of Pd and Ni are employed as catalysts for alkene hydrogenation. The cluster complex of palladium [Pd5(PPh)2], is a catalyst for the hydrogenation of alkenes, dienes, alkynes and many other kinds of unsaturated compound. ... 

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