A High-Fidelity Molecular Model of the Cu(111) Repeating Unit

Dynamic processes at surfaces are central to heterogeneous catalysis, but their atomistic mechanism(s) can prove difficult to elucidate due to variations in material structure and the corresponding impact on reactivity. Moreover, disparities between reaction conditions and those employed for spectroscopic characterization at surfaces can inhibit detailed understanding of catalysis-relevant chemistries. Herein, we substantiate the so-called "cluster-surface" analogy by leveraging a low-valent tricopper architecture (1) as a model system for small molecule activation at Cu(111). Two reaction classes are explored: the adsorption of carbon monoxide (CO) and the dissociative adsorption of dihydrogen (H₂). These processes serve as an ideal testbed to compare the reactivity of a molecular cluster (1) to that of a heterogeneous surface, as both reactions have empirical data from measurements performed on crystalline Cu(111). Cluster 1 reversibly binds CO. Variable temperature NMR analysis with ¹³CO reveals a favorable enthalpy but large negative entropy (-5.1 kcal × mol^-1 and -22.9 cal × mol^-1 × K^-1, respectively) for CO binding, affording a process that is marginally endergonic at room temperature (ΔG_ads(298.15 K) = 1.7 ± 0.5 kcal × mol^-1). Similarly, analogous to a Cu(111) surface, 1 is shown to oxidatively add (chemisorb) H₂. Kinetic parameters were determined for this process and the activation enthalpy (8.4 ± 0.5 kcal × mol^-1) closely mirrors that established for H₂ binding at the Cu(111) facet (6.0 to 12.4 kcal × mol^-1). Together, these results showcase that a trinuclear cluster can reproduce the small molecule binding and activation energetics of a bulk crystalline surface, setting the stage for studying less-defined surface processes in an atomically precise molecular setting.