Copper Metal–Organic Framework Surface Catalysis: Catalyst Poisoning, IR Spectroscopic, and Kinetic Evidence Addressing the Nature and Number of the Catalytically Active Sites En Route to Improved Applications

The metal–organic framework (MOF) H3[(Cu4Cl)3–(BTTri)8, H3BTTri = 1,3,5-tris­(1H-1,2,3-triazol-5-yl)­benzene] (CuBTTri) is a precatalyst for biomedically relevant nitric oxide (NO) release from S-nitrosoglutathione (GSNO). The questions of the number and nature of the catalytically most active, kinetically dominant sites are addressed. Also addressed is whether or not the well-defined structural geometry of MOFs (as solid-state analogues of molecular compounds) can be used to generate specific, testable hypotheses about, for example, if intrapore vs exterior surface metal sites are more catalytically active. Studies of the initial catalytic rate vs CuBTTri particle external surface area to interior volume ratio show that intrapore copper sites are inactive within the experimental error (≤1.7 × 10–5% of the observed catalytic activity)restated, the traditional MOF intrapore metal site catalysis hypothesis is disproven for the current system. All observed catalysis occurs at exterior surface Cu sites, within the experimental error. Fourier transform infrared (FT-IR) analysis of CN–-poisoned CuBTTri reveals just two detectable Cu sites at a ca. ≥0.5% detection limit, those that bind three or one CN– (“Cu­(CN)3” and “CuCN”), corresponding to the CN– binding expected for exterior surface, 3-coordinate (Cusurface) and intrapore, 5-coordinate (Cupore) sites predicted by the idealized, metal-terminated crystal structure. Two-coordinate Cu defect sites are ruled out at the ≥0.5% FT-IR detection limit as such defect sites would have been detectable by the FT-IR studies of the CN–-poisoned catalyst. Size-selective poisoning studies of CuBTTri exterior surface sites reveal that 1.3 (±0.4)% of total copper in 0.6 ± 0.4 μm particles is active. That counting of active sites yields a normalized turnover frequency (TOF), TOFnorm = (4.9 ± 1.2) × 10–2 mol NO (mol Cusurface)−1 s–1 (in water, at 20 min, 25 °C, 1 mM GSNO, 30% loss of GSNO, and 1.3 ± 0.4 mol % Cusurface)a value ∼100× higher than the TOF calculated without active site counting. Overall, Ockham’s razor interpretation of the data is that exterior surface, Cusurface sites are the catalytically most active sites present at a 1.3 (±0.4)% level of total Cu.