Acetylene complexes with metals

The reactions of electrophilic alkynes, such as DMAD (dimethyl acetylene-dicarboxylate), with metal per- and poly-chalcogenido complexes have been exploited for the synthesis of homoleptic and heteroleptic 1,2-dithiolene,... 

Reaction of acetylenic complexes with triosmium dodecacarbonyl leads to a variety of products involving one, two, or three acetylenic units. As with ruthenium, for the monosubstituted alkynes, hydrogen transfer can occur to the metal cluster. Thus, Os3(CO)12 and phenyl-acetylene (L) yield, in refluxing benzene, the derivatives Os3(CO)10L, Os3(CO)10L2, Os3(CO)9L, and HOs3(CO)9(L-H). The general chemistry is summarized in Scheme 2 (131). 

In support of the concept that acetylenes form tt complexes with a single surface atom of the catalyst, McQuillin el al. (39) have cited the parellelism between the effect of substances such as amines and phosphines as inhibitors for the hydrogenation of butynediols to butenediols on a palladium catalyst with the ability of these same substances to form complexes with metals of the class to which palladium belongs (40). 

The ease with which olefins form complexes with metals naturally led to investigation of acetylenes as ligands but until recent years only a few ill-defined, unstable acetylene complexes of copper and silver were known. Now complexes of acetylenes with metals of the chromium, manganese, iron, cobalt, nickel, and copper subgroups are known. These complexes fall naturally into two classes—those in which the structure of the acetylene is essentially retained and those in which the acetylene is changed into another ligand during complex formation. Complexes of the first class are discussed here and the second class is discussed in Section VI. 

Acetyiene and its derivatives also turned out to be suitable probe-molecules for basic centers in zeolites. Figure 2 depicts IR spectra of acetylene adsorbed on the NaX, Cs/NaX, and Na/Y zeolites. The most intense bands at 3300-2900 cm belong to the stretching vibrations of the C-H bond. Obviously, in the case of alkaline forms of zeolites, two types of complexes with acetylenes may be formed (1) 7t-complexes with metal cations (complex 3) and (2) o-complexes with basic oxygen atoms of the framework (complex4) ... 

The conditions for the synthesis must differ, as the electronic configuration of each metal changes, but the intermediate in each case probably is a complex in which acetylene and carbon monoxide are each linked to two metal atoms. Cobalt and iron compounds having both acetylene and carbonyl bridges have already been synthesized 27). The report of the preparation of a dimeric nickel hydrocarbonyl, [NiH(CO)a]2 by Behrens 28) may well lead to the isolation of a siipilar acetylene complex with nickel. 

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... 

Catalysis by alkali metal ions has recently been reported as an alternative route. In an argon matrix, acetylene forms a n complex with the metal. On irradiation, it isomerizes to the vinylidene form, M C=CH2. When complexed with metals, vinylidene is much more stable, in the same way that metal carbenoids are generally much more stable than carbenes, and rearrangement of a tungsten alkyne complex to a tungsten vinylidene complex has been reported. ... 

Ferrocene was the first organometallic guest incorporated and numerous spectroscopic and electrochemical studies have been performed on ferrocene, substituted ferrocene, and related metallocene (e.g. cobaltocene) inclusion complexes (444-469]. Half-sandwich cyclopentadienyl- and benzene-metal carbonyl complexes have also been studied quite extensively [470-479] as have // -allyl metal (palladium) complexes [480], diene metal (rhodium) complexes [481-484], acetylene cobalt carbonyl cluster complexes [485], and complexes with metal carbonyls, e.g. Fe(CO)5, Mn2(CO)io, and CoNO(CO)3 [485a]. 

Reyiews.—Recent reviews on areas of acetylenic chemistry include synthetic routes to average-ring-size cycloalkynes, a study of the bonding in metal-acetylene complexes, transition-metal complexes of acetylene, intramolecular cyclization reactions with acetylenic bond participation, oligomerization of acetylenes induced by metals of the nickel triad, an article on the handling of acetylenic compounds, and a book on preparative acetylenic chemistry. ... 

Instances in which cyclobutadiene complexes are the major products from the reactions of acetylene complexes with additional alkyne are uncommon. These generally have been found to be significant products with sterically hindered alkynes and with palladium and platinum metals. For example, phenyl tert-butyl acetylene was converted to the corresponding cyclobutadiene complex (one isomer) upon treatment with (PhCN)2 PdCl2 (Hos-okawa and Moritani, 1969) [Eq. (71)]. With sterically less demanding tolane,... 

Primary dialkylboranes react readily with most alkenes at ambient temperatures and dihydroborate terminal acetylenes. However, these unhindered dialkylboranes exist in equiUbtium with mono- and ttialkylboranes and cannot be prepared in a state of high purity by the reaction of two equivalents of an alkene with borane (35—38). Nevertheless, such mixtures can be used for hydroboration if the products are acceptable for further transformations or can be separated (90). When pure primary dialkylboranes are required they are best prepared by the reduction of dialkylhalogenoboranes with metal hydrides (91—93). To avoid redistribution they must be used immediately or be stabilized as amine complexes or converted into dialkylborohydtides. 

All mechanisms proposed in Scheme 7 start from the common hypotheses that the coordinatively unsaturated Cr(II) site initially adsorbs one, two, or three ethylene molecules via a coordinative d-7r bond (left column in Scheme 7). Supporting considerations about the possibility of coordinating up to three ethylene molecules come from Zecchina et al. [118], who recently showed that Cr(II) is able to adsorb and trimerize acetylene, giving benzene. Concerning the oxidation state of the active chromium sites, it is important to notice that, although the Cr(II) form of the catalyst can be considered as active , in all the proposed reactions the metal formally becomes Cr(IV) as it is converted into the active site. These hypotheses are supported by studies of the interaction of molecular transition metal complexes with ethylene [119,120]. Groppo et al. [66] have recently reported that the XANES feature at 5996 eV typical of Cr(II) species is progressively eroded upon in situ ethylene polymerization. 

The reductive cyclization of non-conjugated diynes is readily accomplished by treatment of the acetylenic substrate with stoichiometric amounts of low-valent titanium52 523 and zirconium complexes.53 533 Hence, it is interesting to note that while early transition metal complexes figure prominently as mediators of diyne reductive cyclization, to date, all catalyzed variants of this transformation employ late transition metal complexes based on nickel, palladium, platinum, and rhodium. Nevertheless, catalytic diyne reductive cyclization has received considerable attention and is a topic featured in several review articles. ... 

Several bare lanthanide ions Ln+ react with butadiene to give the acetylene complexes [LnC2H2]+ but the [LnO]+ ions generally react by addition of an organic group, with the exceptions of Eu+ and Yb+, which form Ln+ and [LnOH]+ one of the few cases of the reduction of [LnO]+ ions to the bare metal ion (162). 

This observation may well explain the considerable difference between metal-olefin and metal-acetylene chemistry observed for the trinuclear metal carbonyl compounds of this group. As with iron, ruthenium and osmium have an extensive and rich chemistry, with acetylenic complexes involving in many instances polymerization reactions, and, as noted above for both ruthenium and osmium trinuclear carbonyl derivatives, olefin addition normally occurs with interaction at one olefin center. The main metal-ligand framework is often the same for both acetylene and olefin adducts, and differs in that, for the olefin complexes, two metal-hydrogen bonds are formed by transfer of hydrogen from the olefin. The steric requirements of these two edgebridging hydrogen atoms appear to be considerable and may reduce the tendency for the addition of the second olefin molecule to the metal cluster unit and hence restrict the equivalent chemistry to that observed for the acetylene derivatives. 

Catalytic forms of copper, mercury and silver acetylides, supported on alumina, carbon or silica and used for polymerisation of alkanes, are relatively stable [3], In contact with acetylene, silver and mercury salts will also give explosive acetylides, the mercury derivatives being complex [4], Many of the metal acetylides react violently with oxidants. Impact sensitivities of the dry copper derivatives of acetylene, buten-3-yne and l,3-hexadien-5-yne were determined as 2.4, 2.4 and 4.0 kg m, respectively. The copper derivative of a polyacetylene mixture generated by low-temperature polymerisation of acetylene detonated under 1.2 kg m impact. Sensitivities were much lower for the moist compounds [5], Explosive copper and silver derivatives give non-explosive complexes with trimethyl-, tributyl- or triphenyl-phosphine [6], Formation of silver acetylide on silver-containing solders needs higher acetylene and ammonia concentrations than for formation of copper acetylide. Acetylides are always formed on brass and copper or on silver-containing solders in an atmosphere of acetylene derived from calcium carbide (and which contains traces of phosphine). Silver acetylide is a more efficient explosion initiator than copper acetylide [7],... 

The most efficient catalysts for the homo Diels-Alder reactions of norbornadiene were found to be cobalt327 and nickel328 complexes. The general mechanistic pathway that has been proposed for these reactions has been depicted in equation 161329. According to this mechanism, co-ordination of norbornadiene and the olefin or acetylene to the metal center gives 557, which is in equilibrium with metallocyclopentane complex 558. Then, insertion of the olefin or acetylene in the metal-carbon bond takes place to form 559. Reductive elimination finally liberates the deltacyclane species. 

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