A self-assembled Cu(ii)4 [2 × 2] grid with organic radicals

Abstract

A novel multitopic, two-pocket ligand HL_nitnitnitnit, containing the nitronyl nitroxide radical, has been designed for a self-assembled [2 × 2] grid system. HL_nitnitnitnit is not stable in methanol and slowly undergoes a disproportionation, during which the nitronyl nitroxide radical converts to the diamagnetic amidino oxide. In situ reaction of HL_nitnitnitnit with Cu(BF₄)₂ in methanol depending on the reaction time affords self-assembled [2 × 2] grids (L_nitnitnitnit)₂(L)₂Cu(II)₄(BF₄)₄·CH₃OH (1) or (L_nitnitnitnit)_0.5(L)_3.5Cu(II)₄(BF₄)₄·CH₃OH (2), in which L contains an amidino oxide arising from a 3-electron reduction of the nitronyl nitroxide radical. The percentages of stable radical in grids 1 and 2 as determined by X-ray are 50 and 12.5%, respectively. Structures 1 and 2 contain a six-coordinated distorted Cu(II) that is oxo-bridged at 140°. The nitronyl nitroxide radical coordinates via the oxo-atom of the N⁺–O⁻ fragment in a chelating fashion and lies in the equatorial plane of the metal ion. The magnetic properties of 1 could be fitted to a 5-spin Hamiltonian with J_Cu–Cu = +3.1 cm⁻¹ and J_Cu–Nit = −278 cm⁻¹, while those of 2 to a 4-spin Hamiltonian with J_Cu–Cu = +2.9 cm⁻¹. The Q-band EPR spectra of 1 and 2 recorded in solution at 20 K showed intricate anisotropic features of the Cu(II) ion and a much weaker signal of the –NO˙ fragment, associated with a strong Cu(II)–Nit antiferromagentic coupling. The DFT calculated (B3LYP/TZVP/6-31G*) magnetic coupling constants for the grid of 1 were J_Cu–Cu = +5 cm⁻¹ and J_Cu–Nit = −282 cm⁻¹, which are in very good agreement with the experimentally obtained values.