Weak metal-metal interactions for selectivity in organometallic and coordination chemistry – Dispersion Directed Associations – WEAKINTERMET-2DA

The methodology was based on 1) the exploratory synthesis of new unsaturated complexes using ambiphilic ligands possessing several coordination sites, some typically covalent, the other non-covalent; 2) isotherm calorimetric titration, a technique that gives access with high accuracy to association enthalpies of molecular aggregates in solution; 3) the recourse to quantum calculation mainly by means of new methods of the DFT-D, particularly appropriate to analyse the nature of stabilizing interactions in valence electron-depleted complexes, the validity of which was probed in transition metal complex systems by extensive benchmarking against experimentally-determined enthalpies, using conventional solvation models.

From this project has emerged a new concept in coordination chemistry, the first evidences of which were long considered as curiosities. The concept of hemichelation arose from the postulate that an unsaturated metal centre could be stabilized by non-covalent interactions and isolated under a persistent form. This groundbreaking advance questions the Langmuir-Sidgwick rule and opens new directions for applications in catalysis and in molecular sensing.

At midway this project still requires time to evolve in the two main directions defined here, which both contribute to a better understanding of the role of non covalent force to molecular cohesion.

The nature of the publications that were published translates the efforts invested in two distinct directions. The first deals with the investigation of the role of Dispersion in the stabilization of organometallic molecular aggregates and in the control of their molecular conformation. The second deals with the rise of the concept of hemichelation by a recourse to combined theoretical and experimental methods. About 10 publication in high ranking journals were produced in the period 2011-2014, a number of communications in seminars and conferences as well as two promotional highlights in the academic press (GDCh and the CNRS).

Non-covalent interactions are ubiquitous in Nature and their accurate description remains a difficult task. Nonetheless, van der Waals interactions, comprising electrostatic and dispersion based pi-pi, CH-pi, CX-pi, XH-pi interactions and H-bonding, which constitute rather conventional classes of interactions, have been shown to contribute greatly to the stereo-specificity of chemical processes in natural systems. For obvious reasons, i.e because of their directionality in most cases and in spite of the relative unpredictability of their occurrence, these interactions are increasingly being called upon and considered for the rational synthetic design of supramolecular assemblies and coordination polymers. Non-covalent interactions are acutely important in nanomolecular construction too. Understanding them and being able to analyze the origins of attractive interactions that stem generally from various cooperative effects remains a challenge to both the theoreticians and the experimentalists. Non-covalent interactions are so essential in processes whereby molecular recognition, chiral recognition and chiral amplification intervene that efforts are still actively sustained to gain fundamental knowledge. Our proposal consists of devising the methodological threads leading to the rational use of weak metal-metal interactions in closed-shell metal complexes of the 3d, 4d and 5d series for stereo-selective processes. This project crystallizes a number of conclusions drawn recently in a series of publications that we have issued wherein we stress, for instance, the crucial role of dispersion (London force) in the determination of the stereo-chemical courses of very simple coordination chemical processes and in re-enforcing molecular stability in bimetallic adducts. The rational use of dispersion-supported metal-metal interactions for the purpose of promoting stereo-differentiation and operate chiral recognition of matched binuclear adducts has never been attempted to our knowledge. To reach this aim, our proposal seeks a methodology based on solid experimental grounds with support of new theoretical tools and methods. Our experimental approach aims at drawing a reliable picture of the thermodynamic and kinetic aspects of the formation of dispersion-supported weak donor-acceptor interactions between transition metals of the 3d, 4d and 5 d series. It aims also at demonstrating the potential of such weak inter-metallic interaction in chiral recognition.