Theoretical Assessment of the Mechanism and Active Sites in Alkene Dimerization on Ni Monomers Grafted onto Aluminosilicates: (Ni–OH)+ Centers and C–C Coupling Mediated by Lewis Acid–Base Pairs

Ni-based solids are effective catalysts for alkene dimerization, but the nature of active centers and identity and kinetic relevance of bound species and elementary reactions remain speculative and based on organometallic chemistry. Ni centers grafted onto ordered MCM-41 mesopores lead to well-defined monomers that are rendered stable by the presence of an intrapore nonpolar liquid, thus enabling accurate experimental inquiries and indirect evidence for grafted (Ni-OH)+ monomers. Density functional theory (DFT) treatments presented here confirm the plausible involvement of pathways and active centers not previously considered as mediators of high turnover rates for C2-C4 alkenes at cryogenic temperatures. (Ni-OH)+ species act as Lewis acid-base pairs that stabilize C-C coupling transition states by polarizing two alkenes in opposite directions via concerted interactions with the O and H atoms in these pairs. DFT-derived activation barriers for ethene dimerization (59 kJ mol-1) are similar to measured values (46 ± 5 kJ mol-1) and the weak binding of ethene on (Ni-OH)+ is consistent with kinetic trends that require sites to remain essentially bare at subambient temperatures and high alkene pressures (1-15 bar). DFT treatments of classical metallacycle and Cossee-Arlman dimerization routes (Ni+ and Ni2+-H grafted onto Al-MCM-41, respectively) show that such sites bind ethene strongly and lead to saturation coverages, in contradiction with observed kinetic trends. These C-C coupling routes at acid-base pairs in (Ni-OH)+ differ from molecular catalysts in (i) the type of elementary steps; (ii) the nature of active centers; and (iii) their catalytic competence at subambient temperatures without requiring co-catalysts or activators.