Open Metal Sites in the Metal–Organic Framework CPO-27-Cu: Detection of Regular and Defect Copper Species by CO and NO Probe Molecules

The open copper metal sites in CPO-27-Cu were studied by means of IR spectroscopy of adsorbed CO and NO, and density functional theory calculations. Very low Lewis acidity of the Cu²⁺ sites was established by CO (IR band at 2153–2149 cm^–1). Variable-temperature IR experiments indicate adsorption enthalpy of ca. −20 kJ mol^–1. It was also found that CO is a sensitive probe of the occupation of the neighboring copper sites. In contrast to the general expectations, NO is very weakly adsorbed on the Cu²⁺ sites (−14.5 kJ mol^–1, IR band at 1888 cm^–1). The effect is attributed to the particular Cu²⁺ ion coordination and electronic state, leading to a large Jan–Teller deformation and low effective charge, preventing significant charge transfer effects between the metal center and the guest molecules as well as any significant electrostatic interactions. Thus, dominating are van der Waals interactions which position the adsorbed molecule relatively far away at about 2.7–3.0 Å. Adsorption of CO also revealed that a small fraction of the copper ions are found in the Cu⁺ state (IR band at 2120 cm^–1), and these sites were associated with and modeled as defect undercoordinated sites most probably located at the terminal crystallite surfaces. A small fraction of adsorbed NO was relatively strongly adsorbed (−35 kJ mol^–1) and associated with the same set of defect copper sites.