From paramagnetic to single-molecule magnet behaviour in heterobimetallic compounds containing the tetrakis(thiocyanato-κN)cobaltate(II) anion

Abstract

Four heterobimetallic compounds of formula [Ni(Me6trans[14]dieneN4)][Co(NCS)4] (1 and 2), [Ni(Me6trans[14]dieneN4)]2[Co(NCS)4](ClO4)2·H2O (3) and [Ni(Me6trans[14]dieneN4)]2[Co(NCS)4](PF6)2 (4) (Me6trans[14]dieneN4 = 5,7,7,12,14,14-hexamethyl-1,4,8,11-tetraazacyclotetradeca-4,11-diene) have been synthesised and structurally characterised. Their structures are formed of square-planar [Ni(Me6trans[14]dieneN4)]2+ cations and tetrahedral [Co(NCS)4]2– anions. The variable-temperature magnetic properties of 2–4 mainly obey the zero-field splitting effects (D) of the 4A2 ground-state term of the pseudo-tetrahedral [Co(NCS)4]2– anionic complex (2D is the energy gap between the |±1/2> and |±3/2> levels of the spin quadruplet when E = 0), the square-planar nickel(II) unit being diamagnetic. Analysis of their magnetic data through the spin Hamiltonian H = D[Sz2 – S(S + 1)/3] + E(Sx2 – Sy2) + gβHS led to the following best-fit parameters: g = 2.216(17), D = +3.74(12) cm–1 and E/D = 0.0137(6) for 2, g = 2.335(20), D = +11.6(4) cm–1 and E/D = 0.002(4) for 3 and g = 2.298(15), D = +7.29(17) cm–1 and E/D = 0.068(3) for 4. The Q-band EPR spectra of powdered samples of 2–4 in the temperature range 4.0–20 K support the positive signs of D. With an applied dc field, the ac susceptibility data for 3 and 4 below 4.5 K reveal a slow magnetic relaxation, which is a characteristic of single-ion magnet (SIM) behaviour.