Field-induced single molecule magnet behavior of a dinuclear cobalt(ii) complex: a combined experimental and theoretical study

Abstract

Two dinuclear cobalt(II) complexes, [(dmso)Co^IIL¹(μ-(m-NO₂)C₆H₄COO)Co^II(NCS)] (1) and [(dmso)Co^IIL²(μ-(m-NO₂)C₆H₄COO)Co^II(NCS)] (2) [dmso = dimethylsulfoxide, H₂L¹ = (2,2-dimethyl-1,3-propanediyl)bis(iminomethylene)bis(6-methoxyphenol) and H₂L² = (2,2-dimethyl-1,3-propanediyl)bis(iminomethylene)bis(6-ethoxyphenol)] have been synthesized and structurally characterized by single-crystal X-ray diffraction, magnetic-susceptibility measurements and various spectroscopic techniques. Each complex contains a cobalt(II) center with a slightly distorted octahedral geometry and a second cobalt(II) center with a distorted trigonal prismatic one. To obtain insight into the physical nature of weak non-covalent interactions, we have extensively used the Bader's quantum theory of atoms-in-molecules (QTAIM). In addition, the non-covalent interaction reduced density gradient (NCI-RDG) methods established the presence of such non-covalent intermolecular interactions. Variable temperature magnetic susceptibility measurements show that both cobalt centers in each complex are in the high spin state (S = 3/2) and both complexes show weak ferromagnetic couplings through the double phenoxido bridges (J = 3.36(3) cm⁻¹ in 1 and 4.56(2) cm⁻¹ in 2). The magnetic properties of both complexes can be fitted to a Co(II) dimer model including similar orbital reduction factors (α = −0.94(1) for 1 and −0.85(1) for 2) although different zero field splitting parameters D(1) = 11.0(4) cm⁻¹ and D(2) = 19.5(4) cm⁻¹ in 1 and D(1) = 8.2(4) cm⁻¹ and D(2) = −1.3(4) cm⁻¹ in 2. AC magnetic measurements reveal that the Co^II₂ unit in complex 2 exhibits field-induced slow relaxation of the magnetization at low temperatures and high frequencies.