Topological control of the spin coupling in dinuclear copper(II) complexes with meta- and para-phenylenediamine bridging ligands

Abstract

A novel series of copper(II) complexes of formula [Cu(tren)(mpda)](ClO 4) 2 · 1/2H 2O ( 1), [Cu 2(tren) 2(mpda)](ClO 4) 4 · 2H 2O ( 2), and [Cu 2(tren) 2(ppda)](ClO 4) 4 · 2H 2O ( 3) containing the tetradentate tris(2-aminoethyl)amine (tren) terminal ligand and the potentially bridging 1, n-phenylenediamine [ n = 3 (mpda) and 4 (ppda)] ligand have been prepared and spectroscopically characterized. X-ray diffraction on single crystals of 1 and 3 show the presence of mono- ( 1) and dinuclear ( 3) copper(II) units where the mpda ( 1) and ppda ( 3) ligands adopt terminal monodentate ( 1) and bridging bis(monodentate) ( 3) coordination modes toward [Cu(tren)] 2+ cations with an overall non-planar, orthogonal disposition of the phenylene group and the N–Cu–N threefold axis of the trigonal bipyramid of each copper(II) ion [values of the Cu–N–C–C torsion angle ( ϕ ) in the range of 50.8(3)–79.2(2) ( 1) and 80.9(2)–86.5(2)° ( 3)]. Variable-temperature magnetic susceptibility measurements on the dinuclear complexes 2 and 3 show the occurrence of moderate ferromagnetic ( J = +8.3 cm −1, 2) and strong antiferromagnetic ( J = −51.4 cm −1, 3) couplings between the two copper(II) ions across the meta- and para-phenylenediamine bridges, leading to S = 1 ( 2) and S = 0 ( 3) ground spin states [ H = − J S 1 · S 2 with S 1 = S 2 = S Cu = 1/2]. Density functional theory (DFT) calculations on the triplet ( 2) and broken-symmetry (BS) singlet ( 3) ground spin states, support the occurrence of a spin polarization mechanism for the propagation of the exchange interaction through the predominantly π-type orbital pathway of the 1, n-phenylenediamine bridge. Finally, a new magneto-structural correlation between the magnitude of the magnetic coupling ( J) and the Cu–N–C–C torsion angle ( ϕ ) has been found which reveals the role of σ- versus π-type orbital pathways in the modulation of the magnetic coupling for m- and p-phenylenediamine-bridged dicopper(II) complexes.