Density Functional Study of Ethylene Dimerization by (Acetylacetonato)nickel Hydride

Abstract

Density functional calculations have been carried out on the thermochemical aspects of catalytic ethylene dimerization by the d8 hydride (propanedialato(l-))Ni-(I), where the propanedialate(l-) anion served as a model for the chelate acac ligand, acetylacetonate(l-). The hydride (I) was found to have a low-spin d8 configuration with a square planar structure where one site is vacated cis to hydrogen. It was shown that ethylene inserts readily into the Ni-H bond of (I) with an exothermicity of 44.6 kcal/mol. The resulting ethyl complex (III) has a strong agostic interaction between nickel and a /3-hydrogen. It is suggested that the ethyl complex is the actual catalyst in the dimerization cycle. The agostic interaction in is estimated to have a strength of 10 kcal/mol. The next insertion of ethylene into the Ni-ethyl bond of III leads to the butyl complex (IV). The insertion is exothermic by 25 kcal/mol. The reaction between (IV) and ethylene leads finally to the release of 1-butene and the regeneration of the ethyl complex (III). This step is nearly thermoneutral with a reaction heat of 0.1 kcal/mol. It is suggested that the elimination of 1-butene takes place via a transition state in which both ethylene and 1-butene are -complexed to I. The internal barrier of activation for the final step is calculated to be 17 kcal/mol. The substitution of hydrogen by CH3 or CF3 groups on the propanedialate(l-) ligand were also considered. It was found that such substitutions only had a minor effect on the thermochemistry of the insertion processes in the dimerization cycle.

Nickel Catalyzed Olefin Oligomerization and Dimerization

“…Ziegler et al calculated the ethylene dimerization path for [Ni­(acac)­H] using DFT. − Insertion of ethylene into the Ni–H or the Ni-ethyl bonds are facile and exothermic steps. While chain termination by β-H elimination is an endothermic step, calculations for alternative paths suggest a preferred β-H transfer to ethylene.…”

Synthesis, Structure, and Reactivity of (tBu2PC2H4PtBu2)Ni(CH3)2 and {(tBu2PC2H4PtBu2)Ni}2(μ-H)2

“…As far as the mechanism of the formation of 6 and propene is concerned, we suggest that complex 5 reacts with ethene by insertion into the Ni−CH 3 bond (rate-determining step) to form the intermediate [(d t bpe)Ni(C 3 H 7 )(OSO 2 CF 3 )] ( G ), which is structurally related to 6 and 6a . Intermediate G , as a result of β-H activation, eliminates propene to give the Ni II −hydride intermediate [(d t bpe)Ni(H)(OSO 2 CF 3 )] ( H ), which rapidly inserts ethene into the Ni−H bond to yield 6 (Scheme ) …”

Nickel Hydride Complexes

“…273−275 For the cationic nickel system bearing a 2-(2-pyridyl)benzimidazole ligand, the hydride intermediate has been identified by ESI-MS. 275 It is generally believed that a 3-coordinate nickel hydride is needed to initiate the oligomerization process (Scheme 101). 240,241 The vacant coordination site is first occupied by ethylene and then left open following ethylene insertion. β-Hydride elimination from the propagating alkyl chain gives α-olefins and regenerates the nickel hydride.…”

“…Reaction of alkenes with nickel hydrides, specifically, has major implications for ethylene oligomerization. DFT calculations on the acac ligand system suggest that a 3-coordinate nickel hydride (acac)­NiH is the key intermediate, which adopts a planar geometry and allows ethane to coordinate cis to the hydride. , Consistent with this mechanistic proposal, the related nickel hydride [κ P , κ O -Ph 2 PCH 2 C­(CF 3 ) 2 O]­Ni­(H)­(PCy 3 ) is inactive for catalytic ethylene oligomerization, but the mixture of Ni­(COD) 2 and Ph 2 PCH 2 C­(CF 3 ) 2 OH is catalytically active. The inertness of the isolated nickel hydride is likely due to PCy 3 blocking the vacant coordination for ethylene.…”