Shedding Light on the Protonation States and Location\nof Protonated N Atoms of Adenine in Metal–Organic Frameworks

Abstract

We report the syntheses\nand structures of five metal–organic frameworks (MOFs) based\non transition metals (Ni^II, Cu^II, and Zn^II), adenine, and di-, tri-, and tetra-carboxylate ligands.\nAdenine, with multiple N donor sites, was found to coordinate to the\nmetal centers in different binding modes including bidentate (through\nN7 and N9, or N3 and N9) and tridentate (through N3, N7, and N9).\nSystematic investigations of the protonation states of adenine in\neach MOF structure via X-ray photoelectron spectroscopy revealed that\nadenine can be selectively protonated through N1, N3, or N7. The positions\nof H atoms connected to the N atoms were found from the electron density\nmaps, and further supported by the study of C–N–C bond\nangles compared to the literature reports. DFT calculations were performed\nto geometrically optimize and energetically assess the structures\nsimulated with different protonation modes. The present study highlights\nthe rich coordination chemistry of adenine and provides a method for\nthe determination of its protonation states and the location of protonated\nN atoms of adenine within MOFs, a task that would be challenging in\ncomplicated adenine-based MOF structures.