Extended Data Fig. 2: Schematic of open metal-site generation, hydrogen adsorption and dissociation for subsequent Ar–NO2 reduction with M-BBTA-X catalysts.
In the 1D BBTA system, MeOH initially coordinates to the square pyramidal metal centres, converting species A to B. Subsequent M–X bond cleavage (M = Co, Ni; X = Cl, OH, SH) can proceed either homolytically (C1) or heterolytically (C2), generating an open metal site for H2 adsorption (D1, D2). The H–H bond then dissociates, facilitated by a N-atom on the ligand, forming species E1 and E2. Although our computed results show relatively high energy requirements for the formation of active species E, our goal was to qualitatively understand trends related to MOF structure, metal node, and X ligand. The cluster models employed frozen C atoms in the linkers to maintain structural rigidity of the MOF, potentially overestimating energetic barriers where remarkable ligand relaxation would occur, particularly during the H–H dissociation on the ligand (Conversion of species D to E). Future studies using fully relaxed periodic models of all proposed reaction intermediates should yield lower energy requirements for E formation. Additionally, while intrinsic barriers appear high due to solvent stabilization of species B, the apparent activation energy (Eapp) observed experimentally can be lower when measured relative to separated reactants (Species A).
