Apparent Anti-Woodward–Hoffmann Addition to a Nickel Bis(dithiolene) Complex: The Reaction Mechanism Involves Reduced, Dimetallic Intermediates

Nickel
dithiolene complexes have been proposed as electrocatalysts for alkene
purification. Recent studies of the ligand-based reactions of Ni­(tfd)2 (tfd = S2C2(CF3)2) and its anion [Ni­(tfd)2]− with alkenes
(ethylene and 1-hexene) showed that in the absence of the anion, the
reaction proceeds most rapidly to form the intraligand adduct, which
decomposes by releasing a substituted dihydrodithiin. However, the
presence of the anion increases the rate of formation of the stable cis-interligand adduct, and decreases the rate of dihydrodithiin
formation and decomposition. In spite of both computational and experimental
studies, the mechanism, especially the role of the anion, remained
somewhat elusive. We are now providing a combined experimental and
computational study that addresses the mechanism and explains the
role of the anion. A kinetic study (global analysis) for the reaction
of 1-hexene is reported, which supports the following mechanism: (1)
reversible intraligand addition, (2) oxidation of the intraligand
addition product prior to decomposition, and (3) interligand adduct
formation catalyzed by Ni­(tfd)2–. Density
functional theory (DFT) calculations were performed on the Ni­(tfd)2/Ni­(tfd)2–/ethylene system to
shed light on the selectivity of adduct formation in the absence of
anion and on the mechanism in which Ni­(tfd)2– shifts the reaction from intraligand addition to interligand addition.
Computational results show that in the neutral system the free energy
of activation for intraligand addition is lower than that for interligand
addition, in agreement with the experimental results. The computations
predict that the anion enhances the rate of the cis-interligand adduct
formation by forming a dimetallic complex with the neutral complex.
The [(Ni­(tfd)2)2]− dimetallic
complex then coordinates ethylene and isomerizes to form a Ni,S-bound
ethylene complex, which then rapidly isomerizes to the stable interligand
adduct but not to the intraligand adduct. Thus, the anion catalyzes
the formation of the interligand adduct. Significant experimental
evidence for dimetallic species derived from nickel bis­(dithiolene)
complexes has been found. ESI-MS data indicate the presence of a [(Ni­(tfd)2)2]− dimetallic complex as the
acetonitrile adduct. A charge-neutral association complex of Ni­(tfd)2 with the ethylene adduct of Ni­(tfd)2 has been
crystallographically characterized. Despite the small driving force
for the reversible association, very major structural reorganization
(square-planar → octahedral) occurs.