Long, Multicenter Bonding Present in Radical-Radical InteractionsA Computational Study

Resumen

It is well established that organic radical-anions, such as reduced tetracyanoethylene (i.e., [TCNE]•(-)), may form dianionic dimers exhibiting long, multicenter bonding. This bond arises from the overlap of the b(2g) SOMOs of [TCNE]•- to form a doubly-occupied bonding and an empty antibonding combination of orbitals of b2u and b1g symmetry, respectively, leading to a diamagnetic pi-[TCNE](2(2-)) dimer. Long, multicenter bonds were first experimentally and theoretically characterized in crystals of [TCNE]•- salts, whose dimers exhibit an intermonomer distance of ~2.9 Å, and they were found to present the same electronic properties as conventional covalent bonds. Further studies in dichloromethane solution at low temperature determined its equilibrium constant KD and enthalpy and entropy of dimerization (delta-HD and delta-SD) by UV-vis and EPR measurements. Since their first characterization several other organic radicals have been shown to exhibit long, multicenter bonding; among them, other radicalanions such as tetracyanobenzene (TCNB) or tetracyanopyrazine (TCNP), radical-cations such as tetrathiafulvalene (TTF), or neutral radicals such as phenalenyl derivatives. Salts of the aforementioned organic radical-ions are well known to exhibit technologically important properties such as magnetic ordering, metal-like electrical conductivity, and even superconductivity. TCNE, TCNQ, and TCNP, to list a few examples, are building blocks in molecule-based bulk ferromagnets having magnetic ordering temperatures above room temperature. However, the electron pairing resulting from the formation of long, multicenter dimers would impede these physical properties. Moreover, recent experimental and theoretical work showed the presence of long, multicenter bonded pi-[TTF](2(2+)) dimers in supramolecular aggregates at room temperature. These supramolecular aggregates were recently proposed as an emerging class of materials with potential ability on molecular switching applications. It is thus of high importance to achieve a complete understanding on the formation and nature of such interactions. The present thesis focuses in the computational study of long, multicenter bonding. The work is divided into two parts: a first part where the fundamentals of long, multicenter bonding are evaluated, and a second part where room-temperature long, multicenter bonding is studied. In the first part, an extensive characterization with a wide variety of density functionals and active spaces is performed in the gas phase, for the pi-[TCNE](2(2-)), pi-[TTF](2(2+)), pi-[TCNB](2(2-)), and pi-[TCNP](2(2-)) dimers. Moreover, a comprehensive study of the pi-[TCNE](2(2-)) dimer in explicit dichloromethane solution is performed by means of molecular dynamics. Finally, the last two articles focus in solid state long, multicenter bonded dimers. The relative orientation of the pi-[TCNB] (2(2-)) and pi-[TCNP]2(2(2-)) dimers is challenged in the former, while a thorough characterization of the four polymorphs of the [TTF][TCNE] charge-transfer crystal is done in the latter. In the latter, the two established kinds of pi-[TCNE](2(2-)) dimers, actually only found in the pi and pi- polymorphs of [TTF][TCNE], are revised. The second part of the thesis focuses in room-temperature long, multicenter bonding. There are two categories where long, multicenter bonding is still preserved at room temperature: oligomeric donors or acceptors and supramolecular aggregations. In this thesis, some examples of both categories of room-temperature long, multicenter bonding are analyzed. The dimerization of oxidized alpha, beta-substituted heptathienoacene, [D4T7]•+ is investigated, as well as the effect of adding bulky substituents. Furthermore, the room-temperature formation of pi- [TTF] (2(2+)) involving its inclusion in the cavity of a CB[8], and as a building block of a molecular clip and a [3]catenane is also carefully studied. It is quantitatively shown in the thesis that the extra stability, which permits the room-temperature observation of long, multicenter bonding, is due to an increase of dispersion interactions while a decrease of the internal repulsion between the radical-ions.