Ethylene oligomerization in metal–organic frameworks bearing nickel(ii) 2,2′-bipyridine complexes

Abstract

The metal–organic frameworks Zr₆O₄(OH)₄(bpydc)₆ (1; bpydc²⁻ = 2,2′-bipyridine-5,5′-dicarboxylate) and Zr₆O₄(OH)₄(bpydc)_0.84(bpdc)_5.16 (2; bpdc²⁻ = biphenyl-4,4′-dicarboxylate) were readily metalated with Ni(DME)Br₂ (DME = dimethoxyethane) to produce the corresponding metalated frameworks 1(NiBr₂)₆ and 2(NiBr₂)_0.84. Both nickel(II)-containing frameworks catalyze the oligomerization of ethylene in the presence of Et₂AlCl. In these systems, the pore environment around the active nickel sites significantly influences their selectivity for formation of oligomers over polymer. Specifically, the single-crystal structure of 1(NiBr₂)_5.64 reveals that surrounding metal–linker complexes enforce a steric environment on each nickel site that causes polymer formation to become favorable. Minimizing this steric congestion by isolating the nickel(II) bipyridine complexes in the mixed-linker framework 2(NiBr₂)_0.84 markedly improves both the catalytic activity and selectivity for oligomers. Furthermore, both frameworks give product mixtures that are enriched in shorter olefins (C_4–10), leading to deviations from the expected Schulz–Flory distribution of oligomers. Although these deviations indicate possible pore confinement effects on selectivity, control experiments using the nickel-treated biphenyl framework Zr₆O₄(OH)₄(bpdc)₆(NiBr₂)_0.14 (3(NiBr₂)_0.14) reveal that they likely arise at least in part from the presence of nickel species that are not ligated by bipyridine within 1(NiBr₂)_5.64 and 2(NiBr₂)_0.84.