Effect of pi-aromatic spacers on the magnetic properties and slow relaxation of double stranded metallacyclophanes with a Ln(III)-M-II-M-II-Ln(III) (Ln(III) = Gd-III, Dy-III, Y-III; M-II = Ni-II, Co-II) linear topology

The coordination-driven self-assembly of two polydentate linear Schiff base ligands (either N,N-bis(2-hydroxy-3-methoxy-benzyliden)-1,4-diaminobenzene, L2, or N,N-bis(2-hydroxy-3-methoxy-benzyliden)-1,5-diaminonaphthalene, L3) with two transition metal ions (M-II = Ni-II or Co-II) and two lanthanide ions (Ln(III) = Gd-III or Dy-III) afforded seven linear M(2)Ln(2) complexes of formula [Ni(2)Ln(2)(L2)(2)(CH3CN)(3)(H2O)(NO3)(6)](CH3CN)(2)(H2O) (Ln(III) = Gd 1 and Dy 2) and [M(2)Ln(2)(L3)(2)(CH3CN)(4)(NO3)(6)](CH3CN)(x) (M = Ni-II, Co-II; Ln = Dy-III, Gd-III,Y-III; x = 0-4) (3-7). Within the tetranuclear units of these complexes, two ligands coordinate through the N,O-phenoxide donor sets to two M(II) ions, giving rise to M-2 metallacycles. In the case of complexes 1-2, the Ni-2-metallacycle contains 14-members, where the Ni-II ions are bridged by para-phenylenediimine groups. In complexes 3-7, the M-2-metallacycle consists of 18-members, where the transition metal ions are linked by naphthalenediimine bridging groups. At both sides of these metallacycles, the M-II ions are connected to Ln(III) ions through phenoxido bridging groups. The analysis of the dc and ac magnetic properties of these complexes reveals that: (i) all the compounds exhibit weak ferromagnetic exchange interactions between the M-II and Ln(III) ions through the bis(phenoxido) bridging groups and weak antiferromagnetic M-II-M-II interactions transmitted by the acenediimine bridging groups; (ii) DFT calculations not only support the nature and magnitude of the magnetic exchange interactions, but also the polarization mechanism for the M-II-M-II interactions through the acene bridging legends; (iii) the antiferromagnetic interaction for 1 is stronger than for 3, which can be justified by the longer intermetallic Ni center dot center dot center dot Ni distance and alpha,alpha'-substitution for the latter; (iv) complexes 2 and 4 show slow relaxation of the magnetization below 5 K at zero static magnetic field with U-eff/k(B) values of 19 K and 15.9 K respectively, the higher U-eff/k(B) value corresponding to the stronger J(DyGd) coupling constant; (v) the change of -Delta S-m for the M2Gd2 complexes 1, 3 and 6 has been analyzed by taking into account the values of their J and J(1) magnetic exchange interactions and single-ion anisotropies. (C) 2019 Elsevier Ltd. All rights reserved.