The development of heterogeneous catalysts capable of selectively converting lignin model compounds into products of added value offers an exciting avenue to explore in the production of renewable chemical feedstocks. The use of metal-organic frameworks (MOFs) in such chemical transformations relies largely on the presence of accessible open metal sites found within highly porous networks that simultaneously allow for fast transport and strong interactions with desired substrates. Here, we present the first systematic study on the modulation of catalytic performance of a cationic framework, [Cu2(L)(H2O)2](NO3)2·5.5H2O (L = 1,1'-bis(3,5-dicarboxylatophenyl)-4,4'-bipyridinium), achieved through the exchange of anionic guests. Remarkably, the catalytic activity proves to be highly anion-dependent, with a nearly 10-fold increase toward the aerobic C-C bond cleavage of a lignin model compound when different anionic species are incorporated within the MOF. Moreover, we demonstrate that the cationic nature of the MOF, imparted by the incorporation of viologen moieties within the linker, tunes the electrophilicity of the active copper(II) sites, resulting in stronger interactions with the substrate. As such, the copper-based framework exhibits enhanced catalytic performance when compared to its neutral counterpart, emphasizing the appeal of charged frameworks for use as green heterogeneous catalysts.
Keywords: C−C bond cleavage; anion-exchange; copper(II); heterogeneous catalysis; lignin model; metal−organic framework.