Self-assembled polymetallic square grids ([2 × 2] M4, [3 × 3] M9) and trigonal bipyramidal clusters (M5)—structural and magnetic properties

Abstract

New self-assembled grids and clusters are reported, with square [2 × 2] M₄ (M = Mn(II)₄, Cu(II)₄), trigonal-bipyramidal Mn(II)₅, and square [3 × 3] M₉ (M = Mn(II), Cu(II)) examples. These are based on a series of ditopic and tritopic hydrazone ligands involving pyridine, pyrimidine and imidazole end groups. In all cases the metal centres are bridged by hydrazone oxygen atoms with large (>125°) bridge angles, leading to antiferromagnetic exchange for all the Mn systems (J = −2 to −5 cm⁻¹), which results in S = 0 (Mn₄), and S = 5/2 (Mn₅, Mn₉) ground states. The copper systems have a 90° alternation of the Jahn–Teller axes within the Cu₄ and Cu₈ grid rings (Cu₉), which leads to magnetic orbital orthogonality, and dominant ferromagnetic coupling. For the Cu₉ grid antiferromagnetic exchange between the ring and the central copper leads to a S = 7/2 ground state, while for the Cu₄ grids S = 4/2 ground states are observed. The magnetic data have been treated using isotropic exchange models in the cases of the Cu₄ and Cu₉ grids, and the Mn₅ clusters. However due to the enormity of a fully isotropic calculation a simplified model is used for the Mn₉ grid, in which the outer Mn₈ ring is treated as the equivalent of an isolated magnetic chain, with no coupling to the central metal ion.