Revisiting the Aufbau Reaction with Acetylene: Further Insights from Experiment and Theory

Abstract

The first steps of acetylene chain growth at AlEt 3 , via migratory insertion, have been investigated both experimentally and theoretically. The first insertion into the Al-Et bond occurs readily, leading to the stable alkenyl-bridged dimer [{Et 2 Al(μ-CHdCHEt)} 2 ] (1). The alkenyl bridging mode has been observed through isolation and structural analysis of Al 2 Et 2 (OC 6 H 3 Ph 2 ) 2 (μ-CHdCHEt)(μ-OC 6 H 3 Ph 2 ) (2), synthesized by way of controlled reaction of 1 with 2,6-Ph 2 C 6 H 3 OH. This stable binding mode increases the barrier to a second insertion of acetylene, as insertion proceeds through monomeric Al-acetylene adducts. The energetics of further chain growth, dimer formation, and chain termination via hydrogenolysis were investigated theoretically. The results provide further insight and explanation for previous experimental findings in relation to the Aufbau reaction with acetylene.

Catalytic Reactions of Acetylene: A Feedstock for the Chemical Industry Revisited

“…It was found that further insertions of acetylene can take place, but only very slowly, and their products can be observed only by sensitive analytical techniques. The reaction itself proceeds too slowly for practical applications and it is also stoichiometric with respect to AlEt 3 , but these studies have shown how differently acetylene behaves compared with ethylene, the former producing branched oligomers whereas the latter leads to linear species. − It was shown, however, that by using an efficient acetylene polymerization catalyst, namely, an iron bisiminopyridine complex with methylaluminoxane as a cocatalyst, the polymerization reaction can be forced to produce oligomers by adding a chain-transfer agent such as ZnEt 2 . The reaction produces mostly C4–C8 oligomers, with traces of higher oligomers present, but it has the disadvantage of being stoichiometric with respect to ZnEt 2 .…”

Boron, aluminium, gallium, indium and thallium

“…The results provide further insight and explanation for experimental findings in relation to the Aufbau reaction with acetylene. 233 The aluminium amides R 2 AlN(Ar)SiMe 3 [R = Et or i Bu; Ar = Dipp or Mes (Dipp = 2,6-i Pr 2 C 6 H 3 , Mes = 2,4,6-Me 3 C 6 H 2 ] were prepared by ethane or hydrogen elimination reaction between Et 3 Al or i Bu 2 AlH and ArN(H)SiMe 3 and characterised by single crystal X-ray diffraction. 234 The chemistry of aluminium hydrazine complexes has been the focus of reports that describe the thermolysis of trialky hydrazine adducts 235 and dialkyl hydrazide complexes 236 to form Al-N cage structures and their potential utility as hydrogen storage compounds as determined by computational studies.…”

Acetylene Cyclotrimerization with an Iron(II) Bis(imino)pyridine Catalyst

“…While a route involving selective acetylene hydrogenation to ethylene, followed by oligomerization, has been developed by Hall and co-workers, , we and others were interested in direct conversion of acetylene to fuel-range products. Initial studies from our group showed that a variety of early-transition-metal metallocene catalysts displayed very low activity, and in fact acetylene oligomerization occurs mainly at the cocatalyst, triethylaluminum (“Aufbau” with acetylene). , More recently, a survey of several middle- to late-transition-metal imine complexes revealed that the iron(II) bis(imino)pyridine complex 1 , previously developed for ethylene polymerization, − is an extremely active catalyst for acetylene polymerization after activation with methylaluminoxane (MAO). , This system produces polyacetylene gels and films with unprecedented activity, although rapid deactivation occurs due to encapsulation by the insoluble polymer. Extension of this catalyst class to oligomer production was therefore of interest, and without polymer formation the catalyst was expected to have a significantly greater lifetime.…”

“…Initial studies from our group showed that a variety of early-transition-metal metallocene catalysts displayed very low activity, 4 and in fact acetylene oligomerization occurs mainly at the cocatalyst, triethylaluminum ("Aufbau" with acetylene). 5,6 More recently, a survey of several middle-to latetransition-metal imine complexes revealed that the iron(II) bis(imino)pyridine complex 1, previously developed for ethylene polymerization, 7−9 is an extremely active catalyst for acetylene polymerization after activation with methylaluminoxane (MAO). 10,11 This system produces polyacetylene gels and films with unprecedented activity, although rapid deactivation occurs due to encapsulation by the insoluble polymer.…”