Design of a Binuclear Ni(II) Complex with Large Ising-type Anisotropy and Weak Anti-Ferromagnetic Coupling

Abstract

The preparation of a binuclear Ni(II) complex with a pentacoordinate environment using a cryptand organic ligand and the imidazolate bridge is reported. The coordination sphere is close to trigonal bipyramidal (tbp) for one Ni(II) and to square pyramidal (spy) for the other. The use of the imidazolate bridge that undergoes π-π stacking with two benzene rings of the chelating ligand induces steric hindrance that stabilizes the pentacoordinate environment. Magnetic measurements together with theoretical studies of the spin states energy levels allow fitting the data and reveal a large Ising-type anisotropy and a weak anti-ferromagnetic exchange coupling between the metal ions. The magnitude and the nature of the magnetic anisotropy and the difference in anisotropy between the two metal ions are rationalized using wave-function-based calculations. We show that a slight distortion of the coordination sphere of Ni(II) from spy to tbp leads to an Ising-type anisotropy. Broken-symmetry density functional calculations rationalize the weak anti-ferromagnetic exchange coupling through the imidazolate bridge.

Semiquinone radical-bridged M2 (M = Fe, Co, Ni) complexes with strong magnetic exchange giving rise to slow magnetic relaxation

“…For example, the trigonal 22 Note that the only previously reported example of a complex with multiple trigonal bipyramidal nickel ions is an imidazole-bridged Ni 2 complex, which features a non-magnetic ground state. 23 Based on these precedents, we postulate that the strong magnetic exchange through the semiquinone bridge in 6 serves to limit fast relaxation via Raman and direct processes and thus enables observation of Orbach relaxation. Our results support a growing number of studies that demonstrate the utility of judiciously selected multinuclear single-molecule magnets featuring radical-bridged, high-anisotropy metal ions for suppressing through-barrier relaxation processes.…”

“…Moreover, the only example of any complex incorporating multiple trigonal bipyramidal Ni 2+ centers is a dinuclear imidazole-bridged [Ni(m-Im)NiL](ClO 4 ) complex, which features weak antiferromagnetic coupling between Ni 2+ centers. 23 This conspicuous dearth of compounds likely stems from synthetic difficulties associated with the tendency of Ni 2+ to adopt octahedral coordination.…”

A Bis‐Binuclear NiII Complex with Easy and Hard Axes of Magnetization: Complementary Experimental and Theoretical Insights

“…All of the magnetic α and β spin orbitals have non‐negligible tails on the oxygen atoms of the nitrate bridge, which ensures that both exchange pathways between MO1 and MO3 on the one hand and between MO2 and MO4 on the other contribute to the coupling . It may be noted that in the case of the imidazolate bridge that we recently reported, only one of the two exchange pathways was active because only one of the Ni II magnetic orbitals has delocalization tails on the imidazolate bridge. This rationalizes the significantly larger value of the exchange coupling in the present complex.…”

“…A possible strategy for studying nonsymmetrical binuclear Ni II complexes in which local magnetic anisotropy is of the same order of magnitude as the exchange coupling is to perform theoretical calculations on the whole system to determine the exchange coupling and only on mononuclear fragments to determine the local magnetic anisotropy terms , . Such knowledge is necessary for the design of binuclear species that can behave as double qubits , .…”

“…These ligands have two tetradentate coordination sites and may form symmetrical complexes, particularly when Co 2+ ions, which are stable in a trigonal‐bipyramidal geometry, are used with an azido bridge, for instance . However, for Ni 2+ ions, which prefer square‐pyramidal or octahedral geometry, nonsymmetrical binuclear species are obtained if bridging ligands such as CO 3 – or CH 3 OCO 2 – are used, and, more recently, an imidazolate bridge …”