Ferromagnetic Nanoscale Electron Correlation Promoted by Organic Spin-Dependent Delocalization

Abstract

We describe the electronic structure and the origin of ferromagnetic exchange coupling in two new metal complexes, NN-SQ-CoIII(py)2Cat-NN (1) and NN-Ph-SQ-CoIII(py)2Cat-Ph-NN (2) (NN = nitronylnitroxide radical, Ph = 1,4-phenylene, SQ = S = 1/2 semiquinone radical, Cat = S = 0 catecholate, and py = pyridine). Near-IR electronic absorption spectroscopy for 1 and 2 reveals a low energy optical band that has been assigned as a Ψu → Ψg transition involving bonding and antibonding linear combinations of delocalized dioxolene (SQ/Cat) valence frontier molecular orbitals. The ferromagnetic exchange interaction in 1 is so strong that only the high-spin quartet state (ST = 3/2) is thermally populated at temperatures up to 300 K. The temperature-dependent magnetic susceptibility data for 2 reveals that an excited state spin doublet (ST = 1/2) is populated at higher temperatures, indicating that the phenylene spacer modulates the magnitude of the magnetic exchange. The valence delocalization within the dioxolene dyad of 2 results in ferromagnetic alignment of two localized NN radicals separated by over 22 Å. The ferromagnetic exchange in 1 and 2 results from a spin-dependent delocalization (double exchange type) process and the origin of this strong electron correlation has been understood in terms of a valence bond configuration interaction (VBCI) model. We show that ferromagnetic coupling promoted by organic mixed-valency provides keen insight into the ability of single molecules to communicate spin information over nanoscale distances. Furthermore, the strong interaction between the itinerant dioxolene electron and localized NN electron spins impacts our ability to understand the exchange interaction between delocalized electrons and pinned magnetic impurities in technologically important dilute magnetic semiconductor materials. The long correlation length (22 Å) of the itinerant electron that mediates this coupling indicates that high-spin π-delocalized organic molecules could find applications as nanoscale spin-polarized electron injectors and molecular wires.

An electron transfer driven magnetic switch: ferromagnetic exchange and spin delocalization in iron verdazyl complexes

“…This interpretation is supported by the results of DFT calculations which place the bulk of the spin density on the metal ion, but leave part of it delocalized across both ligands (Figure 5). We note that this is similar to the mechanism invoked to explain the long range coupling of two nitronyl-nitroxides coordinated to a metal center 32 and the exchange between metal ions in quinonoid bridged systems. 14, 15, 33…”

Valence‐Tautomerism‐Driven Aromatic Nucleophilic Substitution in Cobalt‐Bound Tetrabromocatecholate Ligands – Influence of Positive Charge at the Ligand Backbone on Phenoxazinone Synthase Activity

“…The two aromatic rings in the 3,5,6‐tribromo‐4‐pyridiniumcatecholate ligand are mutually perpendicular, as suggested by the dihedral angle between the pyridyl and phenyl rings of 86.98°, which is usual for such organic molecules , . The Co–N(pyridine) and average Co–O(catecholate) bond lengths in 1 of 1.948(8) and 1.885(6) Å, respectively, are comparable with those in previously reported cobalt(III) catecholate complexes with similar coordination environments . The C–O and ring C–C (bearing catecholate –OH groups) bond lengths of the coordinated o ‐dioxolene ligand provide useful information regarding the oxidation state of the coordinated redox‐active dioxolene ligand.…”

“…1.34 Å). Similarly, the C–C bond lengths for the semiquinonate and catecholate states usually vary in the ranges 1.45–1.48 and 1.36–1.44 Å, respectively . In the present complex, the C–O and ring C–C bond lengths of 1.353(11) Å (average value) and 1.395(15) Å, respectively, indicate that the dioxolene ligand is in the catecholate state.…”

Quantum Information and Computation for Chemistry

“…mixed-valence species with organic radicals to make new "double-exchange" molecules (Kirk et al, 2009), and radical-substituted conjugated polymers and oligomers (Oyaizu and Nishide, 2009) are promising architectures.…”