Binuclear metal-alkyne complexes

The species responsible for alkyne polymerization, which is kinetically more facile than eyelotrimerization since only a small fraction of the added alkyne is converted to benzenes, is not yet known. Carbene-metal complexes, both mononuclear (54) and binuclear (y2-CR2) complexes (55,56), have been shown to act as alkyne polymerization initiators and several years ago it was shown that terminal alkynes and alcohols can react to give alkoxycarbene ligands (57) As yet, we have no evidence... 

In Table 2 appear complexes of transition metals with acetylenic compounds. A study was made of the structure and interconversion of acetylene complexes with binuclear transition metals, where acetylene lies parallel or perpendicular to the metal-metal bond" . Carbonyl alkyne complexes with binuclear iron give good separations in reverse-phase HPLC 4... 

Half-sandwich rare earth metal alkyl complexes can act as catalyst precursors for the cross-coupling of various terminal alkynes with isocyanides selectively affording the (Z)-l-aza-l,3-enyne products (Scheme 20). The unprecedented Z selectivity could arise from the formation of an alkynide-bridged binuclear catalyst species, in which the crosscoupling reaction took place at the two metal centers in an intermolecular fashion. 

The first complexes of a-keto ylides and group 5 early transition metals have only recently been obtained by reaction of Nb(III) derivatives [[NbCl3(dme) (R C=CR")] with 25 (R = thiazolyl) (Scheme 16). The chelation of the ylide occurs through an N,0-coordination to the metal center and in presence of MeLi a deprotonation of a phenyl ring takes place with the loss of alkyne, leading to the formation of a new orfho-metallated binuclear compound 32. The two ylides involved in the complexation behave as tridentate anionic ligands and are mutually in a trans disposition in order to minimize the steric hindrance [71,72]. Another binuclear niobium complex 33 has been obtained from 25 (R = Me, Ph) with this time an 0-coordinated a-keto ylide [68]. 

Because compounds 4 and 33 are efficient catalyst precursors for the hydrogenation of diphenylacetylene and both contain an alkyne bound parallel to a metal-metal bond, E. Sappa and coworkers have made the hypothesis that clusters with an alkyne bound parallel to one edge of a triangular metal array could act as catalyst precursors or as intermediates in the hydrogenation of alkynes. - i In fact, the unsaturated binuclear complex [Ru2(Cp)2(//-Ph2C2)(CO)] (35), which has the alkyne bonded in a perpendicular mode to the metal-metal bond (Fig. 5), is a poor hydrogenation catalyst. ... 

The binuclear compounds as well as the mononuclear seven-coordinate bis(nitrile) compounds are catalysts for the polymerisation of terminal alkynes such as phenylacetylene (PA) or ferf-butylacetylene (f-BA) and for the ROMP of norbornene (NBE) and norbornadiene (NBD) [20, 38-42]. The first step in all catalytic reactions is the coordination of the organic substrate to metal and the formation of a complex. Sometimes it was possible to isolate the products in crystalline form but frequently such adducts were observed only in situ by NMR investigations. 

When more than one metal is present in an organometallic complex rather different electrochemical behaviour is possible. For example, metal-metal bonds can function as electron sinks so that extended redox series, linked by sequential one-electron transfer steps, may become available. In addition, the paramagnetic members of such series may show unusual properties associated with mix valency. Here we describe not only the reactions of binuclear species which undergo two or more sequential electron-transfer reactions but also a study of the bonding in alkyne-bridged binuclear species. 

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