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 ](ClO 4 ) 2 · H 2 O ( 3 ) and [Ni(Me 6 trans [14]dieneN 4 )] 2 [Co(NCS) 4 ](PF 6 ) 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 (2 D 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 ) + gβH S 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.