Ethylene Oligomerization in Metal–Organic Frameworks Bearing Nickel(II) 2,2′-Bipyridine Complexes
The metal–organic frameworks Zr6O4(OH)4(bpydc)6 (1; bpydc2– = 2,2′-bipyridne-5,5′-dicarboxylate) and Zr6O4(OH)4(bpydc)0.84(bpdc)5.16 (2; bpdc2– = 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 catalytic activity and selectivity for oligomers. Furthermore, both frameworks give product mixtures that are enriched in shorter olefins (C4–10), 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 a non-complexed form of nickel within 1(NiBr2)5.64 and 2(NiBr2)0.84.