Enforced Proximity Interactions as Vehicle to Drive Quantum Crystallography

• Applicant: Professor Dr. Jens Beckmann, since 8/2019
• Subject Area: Inorganic Molecular Chemistry - Synthesis and Characterisation ; Analytical Chemistry
• Term: from 2017 to 2021
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 383534171

Controlling and tuning intermolecular interactions and interactions between functional groups is of utmost importance in all fields of chemistry, e.g. materials science and drug design, catalysis and synthesis. Often, hydrogen atoms and lone pairs are governing such interactions. Although single-crystal X-ray structure determination is one of the most used and routine techniques to evaluate geometry and interactions, information on hydrogens and lone pairs is often vague or inaccurate; the two most important structural features are the two most difficult to characterize with X-ray crystallography. We have developed a quantum crystallographic methods called Hirshfeld Atom Refinement (HAR) that is so accurate and precise that both hydrogen atoms and lone pairs can easily be located from the electron density maps. However, HAR is also computationally demanding, so that the refinement of experimental data of large compound and of those including heavy elements is impossible. In this project we will extend HAR to complexes that bear late heavy transition metals inside a large ligand system and accurately characterize hydrogen- and lone-pair-to-transition metal interactions.We will synthesize systematic arrays of compounds where M...H and M...LP interactions are enforced through chelation with N or P donor atoms; proximity enforcing ligands (PELs) lead to enforced proximity interactions (EPIs). The nature of these EPIs (precisely, Si-H...M, P-H...M, P-LP...M, S-LP...M) can be of repulsive or attractive, agostic or anagostic nature. Moreover, oxidative addition reactions to the transition metal hydrides Si/P-M-H are possible, which would lead to a systematic series of stable molecule species with terminal M-H bonds. Model compounds with terminal Si-H, P-H, and M-H groups will be measured with neutron diffraction to obtain benchmarking distances involving hydrogen to reference against.The crystallographic analysis of the described systems will become feasible because we suggest a way to approximate the wavefunction used in HAR and not recompute it at every refinement stage. We will use extremely localized molecular orbitals (ELMOs) deposited in databases for each bonding situation that occurs in the ligands like LEGO building blocks to rapidly construct the wavefunction (HAR-ELMO). The results will be nearly as accurate as HAR and as quick as standard shelx refinement. Once the ligand systems are described with ELMO building blocks the E-X...M interactions can be computed very accurately at a high relativistic level of theory, which would not be applicable otherwise (HAR-QM/ELMO). Subsequently, the interactions can be analyzed with modern orbital, energy and real-space bonding descriptors. The new software and the new methods will be made generally accessible through implementation into the broadly used software package Olex2.

• DFG Programme: Research Grants
• Ehemaliger Antragsteller: Privatdozent Dr. Simon Grabowsky, until 7/2019