Experimental and theoretical investigations on magneto-structural correlation in trinuclear copper(II) hydroxido propellers

The trinuclear copper(II) compounds [Cu3(μ3-OH)(GL1)3](ClO4)2 (1–4) and [Cu3(μ3-OH)(GL2)3](ClO4)2 (5–8) with tridentate NNO Schiff base ligands GL1− and GL2− derived from 5-G-substituted salicylaldehydes (G = NO2, Br, H, Me) and the diamines 1,2-ethanediamine and 1,3-propanediamine, respectively, were investigated aiming at shedding light on possible magneto-structural correlation in this class of complexes. All derivatives contain [Cu3(μ3-OH)(L)3]2+ cations with partial cubane Cu3O4 cores, and the metal ions are linked together in a pyramidal fashion by a triple-bridging hydroxido group, giving rise to propellers with three [Cu(L)]+ blades. In these spin-frustrated magnetic systems, the three copper(II) ions within a cluster communicate anti-ferromagnetically (−2J Ŝi·Ŝj convention) through the bridging OH group with coupling constants J ranging from −4.5(1) for 4 (G = Me) to −10.1(1) cm−1 for 1 (G = NO2), and stabilization of the doublet S = 1/2 ground state. The structural features of the complexes reveal very minimal deviations upon variation of G or the diamine flexibility along the whole series of compounds, preserving almost constant magnetic cores. Similar conclusions are also drawn by DFT gas-phase geometry optimizations of the [Cu3(μ3-OH)(L)3]2+ cations. Therefore, confident of excluding structural influences on the magnetic super-exchange path, the modulating factor of J in our derivatives can be sought after the different electronic demand of G. Atomic NBO charges support this point, revealing small but systematic variations in the electron density flow along the blades and the positive charge on copper(II) ions with the electronic nature of G, with the most remarkable effect given by the nitro group. Topological analysis of electron density according to the Quantum Theory of Atoms In Molecules further support the distinguishing role of this group with respect to the other substituents taken into consideration, besides providing indirect information about the super-exchange path.