Metal–Ligand Exchange Coupling Alters the Open-Shell Ligand Electronic Structure in a Bis(semiquinone) Complex

Abstract

The electronic structure of the bis(dioxolene) bridging ligand -SQ 2 Th 2 -is responsive to metal−ligand magnetic exchange coupling. Comparison of the crystal structure of (NiSQ) 2 Th 2 to that of (ZnSQ) 2 Th 2 indicates an open-shell biradical ground state for the dinuclear Ni(II) complex compared to the closed-shell quinoidal character found in the dinuclear Zn(II) complex. Consistent with a comparison of bond lengths obtained by X-ray diffraction, the analysis of the variable-temperature magnetic susceptibility data for crystalline (NiSQ) 2 Th 2 yields reduced SQ−SQ radical−radical magnetic exchange coupling (J SQ−SQ = −203 cm −1 ) compared to that of (ZnSQ) 2 Th 2 (J SQ−SQ = −321 cm −1 ). The reduced SQ−SQ exchange coupling in (NiSQ) 2 Th 2 derives from an attenuation of the SQ spin densities, which in turn is derived from the Ni−SQ antiferromagnetic exchange interactions. This reduction in SQ−-SQ exchange that we observe for (NiSQ) 2 Th 2 correlates with an effective lengthening of the bridge unit by ∼2.1 Å relative to that of (ZnSQ) 2 Th 2 . This magnitude of the effective increase in the bridge distance is consistent with the (NiSQ) 2 Th 2 J SQ−SQ value lying between those of (ZnSQ) 2 Th 2 and (ZnSQ) 2 Th 3 . The ability to modulate spin populations on an organic radical via pairwise Ni−SQ magnetic exchange interactions is a general way to affect electronic coupling in the Th−Th bridge. Our results suggest that metal−radical exchange coupling represents a powerful mechanism for tuning organic molecular electronic structure, with important implications for molecular electronics and molecular electron transport.

Innocent or Noninnocent Interactions of the Cations on Spin-Frustrated Lindqvist Hexavanadate: Structural Analysis and Magnetic Properties

“…Spin-free wave functions show a doublet ground state followed by a quartet close in energy (∼4.2 cm –1 1 , ∼ 4.4 cm –1 for 2 , and ∼6.3 cm –1 for 3 ). The contributions of excitations (single, double) from the doubly occupied orbitals (bonding orbitals with a 2p contribution of oxido bridges, see Figure ) to the wave function of these states are almost negligible and allow one to explain the spin density, also negligible, on the bridging ligands, thus justifying the previous conclusion of a coupling mechanism dominated by interaction through space . As a consequence of spin–orbital coupling (introduced in a second step of calculations from the RASSCF wave functions), the electronic states are distributed in a series of Kramers doublets, which, in the framework of the interpretation of the magnetic properties, behaves as a S = 1/2 system which does not represent the actual state of the electron spin but also a pseudospin acting in the model space of | M S ⟩ wave functions of the pseudospin projection onto a quantization axis.…”