The Mechanism of Alkene Addition to a Nickel Bis(dithiolene) Complex: The Role of the Reduced Metal Complex

Abstract

The binding of an alkene by Ni(tfd)(2) [tfd = S(2)C(2)(CF(3))(2)] is one of the most intriguing ligand-based reactions. In the presence of the anionic, reduced metal complex, the primary product is an interligand adduct, while in the absence of the anion, dihydrodithiins and metal complex decomposition products are preferred. New kinetic (global analysis) and computational (DFT) data explain the crucial role of the anion in suppressing decomposition and catalyzing the formation of the interligand product through a dimetallic complex that appears to catalyze alkene addition across the Ni-S bond, leading to a lower barrier for the interligand adduct.

Computational investigation of cobalt and copper bis (oxothiolene) complexes as an alternative for olefin purification

“…In summary, all cobalt bis (oxothiolene) complexes (in both spin states) favor the formation of desired interligand adducts over the formation of the respective intraligand adducts, both kinetically and thermodynamically. This complex exceeds the performances of parent 1 _Ni_SS complex [21,22] for olefin purification and display better properties than 1 _Co_SS [23] . This can be viewed in terms of enhanced kinetic and thermodynamic selectivity toward the formation of interligand adduct, although the rate-determining barriers are somewhat lower in the case of 1 _Co_SS (Electronic Supplementary Material; Fig.…”

“…We start with the thermodynamic prediction of the formation of relevant adducts (2, 2y, and 3), which is briefly presented in the next section. Selected complexes from the next section with favorable thermodynamics, in accordance with above requirements, have been tested in reaction with ethylene, by locating Scheme 2 The anion-catalyzed reaction competes with the intrinsic reactivity of 1 [21] the transition states (reaction barriers) to explore reaction profiles.…”

“…The cis -interligand adduct can be formed via a complicated four-step process that includes different bimetallic species ( 1/1 − ) and the formation of an intermediate with a Ni-S coordination of the ethylene (Scheme 2 ). The revised mechanism was proposed by Hall and co-workers as a result of combined experimental and theoretical studies, which highlighted the crucial role of the reduced anionic complex acting as a co-catalyst [ 21 , 22 ].

Scheme 2 The anion-catalyzed reaction competes with the intrinsic reactivity of 1 [ 21 ]

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What Happens Without Nickel? Cyclization Reactions of Ethylene with Ethanedithial and Related Molecules

“…Since 2001, it has been established that nickel bis(dithiolene) is a promising candidate for purification of ethylene from petroleum feedstocks . Hence, the reaction of ethylene with nickel bis‐dithiolene has intensively studied both experimentally and theoretically, as well as theoretical work on several derivatives including nickel bis(oxothiolene) and nickel bis(dioxolene) …”

“…Theoretical work has shown that the direct reaction of ethylene with nickel bis‐dithiolene to form the interligand adduct is symmetrically forbidden, and so Fan and Hall suggested a mechanism where the interligand adduct is formed via a twisted intermediate; note that this was proposed prior to the decomposition route being known. This was later refined due to experimental evidence; specifically, another mechanism was proposed following Fekl's experimental results wherein the presence of nickel bis‐dithiolene anion is crucial to avoiding the decomposition products. In the current mechanism proposal, an indirect addition of ethylene on the nickel atom occurs while the nickel complex has formed an anionic dimer, which splits, and the resultant neutral complex then rearranges to the interligand adduct.…”