A New Paradigm for Design of High-Spin Organic Molecules: The Mechanism of Spin-dependent Delocalization in Exchange-Coupled Mixed-Valent Organic Species

A theory is developed to describe the electronic structure of mixed-valent, exchange-coupled organic biradicals. The phenomena described are analogous to those of spin-dependent delocalization observed in binuclear inorganic complexes in the sense that coupling of delocalized hole states favors a triplet state over a singlet state. However, the mathematical description of the delocalization is quite similar to Kramers’ treatment of antiferromagnetic and ferromagnetic terms in metal oxides.1 There is a similarity in that a delocalized orbital in the mixed-valent biradical anion can play the role of oxide in conventional superexchange. As in Kramers’ theory, there are two terms that result from the analysis, a second-order term and a third-order term. However, the significance of the terms is different for delocalized magnetic orbitals than for the metalbridge-metal system considered by Kramers. The computational chemistry shows that the third-order ferromagnetic term dominates for large dihedral angles, in agreement with previous experimental results. The mechanism presented suggests a new paradigm for the design of high-spin organic molecules. Introduction High-spin organic molecules (S ≥ 1) have been the focus of intense study from both a theoretical perspective, and as ligands to prepare molecule-based magnetic materials 2-8 Most high-spin molecule design involves attaching paramagnetic functional groups to a π-system (a Coupler) so as to achieve a cross-conjugated π-topology.9,10 Such conncectivity gives nondisjoint NBMOs11 that give rise to substantial exchange integrals that stabilize high-spin ground-states. For example, the structural features of trimethylenemethane and meta-xylylene form the basis of a plethora of high-spin molecules.7,10,12 Up to the present time this remains the only design paradigm for preparing high-spin molecules. Certain mixed-valent metal complexes exhibit a spin-spin coupling mechanism for which there is no organic counterpart: double exchange, also called spin-dependent delocalization (SDD).3,13-15 The salient feature of SDD is that the highest spinmultiplicity state is preferentially stabilized by electron delocalization, according to the transfer (resonance) integral, |β|. Despite the large number of mixed-valent organic compounds, only a few examples have molecular structures that enforce significant exchange coupling. Of these few, none exhibit either new coupling pathways or enhanced ferromagnetic coupling.16-18 We feel that mixed-valent organic ligands hold a great deal of promise for new properties/behaviors,19-23 particularly when the mixedvalent organic ligands have more than one unpaired electron. Recently, we reported the first example of enhanced ferromagnetic coupling in a mixed-valent molecule that lacks an effective π-type ferromagnetic Coupler 9/26/02 Page 2 of 22 ([Na][(LZn)3(SQ2Cat)]), formed from one-electron reduction of an antiferromagnetically-coupled, isovalent triradical, (LZn)3(SQ3).22 (LZn)3(SQ3) CH3 t-Bu O