Binding and activation of dihydrogen and other small molecules in a dirhodium paddle wheel unit based three-dimensional porous coordination polymer: FTIR and computational studies

We have studied the electronic structure and reactivity of coordinatively unsaturated rhodium centers, being part of the dirhodium paddle wheel type Secondary Building Unit (SBU), in a microporous Metal-Organic Framework (MOF). This dirhodium structure manifests a Rh(I)-like local electronic structure at the coordinatively unsaturated accessible adsorption sites, prone to relatively strong interactions with small molecule guests like dihydrogen, dinitrogen, nitrous oxide, iodine and iodine containing compounds, as a few examples. Thе local adsorption site Rh(I)-like chemistry is confirmed by in situ FTIR spectroscopy of CO, detecting the corresponding monocarbonyl species at about 2100 cm-1. The corresponding metal-dihydrogen complex is manifested via the stretching dihydrogen(deuterium) vibrations observed at 3808 cm-1 (2737 cm-1). Our computational work predicts optimal binding energy in this complex for hydrogen storage applications, in the order of 24 kJ/mol at ambient temperatures, owing to a substantial dihydrogen -bond electron donation into the frontier Rh 4s state. Interestingly, and in contrast to all other known -bond dihydrogen-metal complexes, here a back-donation from rhodium d-states into the dihydrogen * antibonding states is practically absent, and hence the resultant metal-dihydrogen complex appears a pure -bond complex. Furthermore, the particular electronic structure of the dirhodium unit appears very suitable for applications related to the capture and utilization of iodine (its radionuclides) containing volatile species as well as separation of isoelectronic species like nitrous oxide and carbon dioxide. Computed strong adsorption energies for nitric oxide, in excess of 80 kJ/mol, as compared to about 50 kJ/mol for water, imply possible controlled delivery and continuous release in aqueous solutions, for instance in medicinal and generally biological applications.