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

A novel multitopic, two-pocket ligand HLnit, containing the nitronyl nitroxide radical, has been designed for a self-assembled [2 × 2] grid system. HLnit 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 HLnit with Cu(BF 4)2 in methanol depending on the reaction time affords self-assembled [2 × 2] grids (Lnit)2(L) 2Cu(ii)4(BF4)4·CH 3OH (1) or (Lnit)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 JCu-Cu = +3.1 cm-1 and JCu-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 JCu-Cu = +5 cm-1 and JCu-Nit = -282 cm-1, which are in very good agreement with the experimentally obtained values. © 2013 The Royal Society of Chemistry.