From Paramagnetic to Single-Molecule Magnet Behaviour in Heterobimetallic Compounds Containing the Tetrakis(thiocyanato-κN)cobaltate(II) Anion: From Paramagnetic to Single-Molecule Magnet Behaviour in Heterobimetallic Compounds Containing the Tetrakis(thiocyanato-κN)cobaltate(II) Anion

Four heterobimetallic compounds of formula [Ni(Me(6)trans[14]dieneN(4))][Co(NCS)(4)] (1 and 2), [Ni(Me(6)trans[14]dieneN(4))](2)[Co(NCS)(4)](ClO4)(2)H2O (3) and [Ni(Me(6)trans[14]dieneN(4))](2)[Co(NCS)(4)](PF6)(2) (4) (Me(6)trans[14]dieneN(4) = 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(Me(6)trans[14]dieneN(4))](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 (4)A(2) 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[S-z(2) - S(S + 1)/3] + E(S-x(2) - S-y(2)) + gHS 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.