Magnetic properties of 1:2 mixed cobalt(II) salicylaldehyde Schiff-base complexes with pyridine ligands carrying high-spin carbenes (Scar = 2/2, 4/2, 6/2, and 8/2) in dilute frozen solutions: role of organic spin in heterospin single-molecule magnets.

The 1:2 mixtures of Co(p-tolsal)2, p-tolsal = N-p-tolylsalicylideniminato, and diazo-pyridine ligands, DXpy; X = 1, 2, 3l, 3b, and 4, in MTHF solutions were irradiated at cryogenic temperature to form the corresponding 1:2 cobalt-carbene complexes Co(p-tolsal)2(CXpy)2, with Stotal = 5/2, 9/2, 13/2, 13/2, and 17/2, respectively. The resulting Co(p-tolsal)2(CXpy)2, X = 1, 2, 3l, 3b, and 4, showed magnetic behaviors characteristic of heterospin single-molecule magnets with effective activation barriers, Ueff/kB, of 40, 65, 73, 72, and 74 K, for reorientation of the magnetic moment and temperature-dependent hysteresis loops with a coercive force, Hc, of ∼0, 6.2, 10, 6.5, and 9.0 kOe at 1.9 K, respectively. The relaxation times, τQ, due to a quantum tunneling of magnetization (QTM) were estimated to be 1.6 s for Co(p-tolsal)2(C1py)2, ∼2.0 × 10(3) s for Co(p-tolsal)2(C2py)2, and >10(5) s for Co(p-tolsal)2(CXpy)2; X = 3b, 3l, and 4. In heterospin complexes, organic spins, carbenes interacted with the cobalt ion to suppress the QTM pathway, and the τQ value increased with increasing the Stotal values.

Magnetic properties of 1 : 4 complexes of CoCl2 and pyridines carrying carbenes (S(0) = 4/2, 6/2, and 8/2) in diluted frozen solution; influence of carbene multiplicity on heterospin single-molecule magnets.

The microcrystalline sample of a parent complex, [CoCl(2)(py)(4)], showed a single-molecule magnet (SMM) behavior with an effective activation barrier, U(eff)/k(B), of 16 K for reversal of the magnetism in the presence of a dc field of 3 kOe. Pyridine ligands having 2-4 diazo moieties, DYpy; Y = 2, 3l, 3b, and 4, were prepared and confirmed to be quintet, septet, septet, and nonet in the ground state, respectively, after irradiation. The 1 : 4 complexes, CoCl(2)(DYpy)(4); Y = 2, 3l, 3b, and 4 in frozen solutions after irradiation showed the magnetic behaviors of SMMs with total spin multiplicity, S(total) = 17/2, 25/2, 25/2, and 33/2, respectively. Hysteresis loops depending on the temperature were observed and the values of coercive force, H(c), at 1.9 K were 12, 8.4, 11, and 8.1 kOe for CoCl(2)(CYpy)(4); Y = 2, 3l, 3b, and 4, respectively. In dynamic magnetic susceptibility experiments, ac magnetic susceptibility data obeyed the Arrhenius law to give U(eff)/k(B) values of 94, 92, 93, and 87 K for CoCl(2)(CYpy)(4); Y = 2, 3l, 3b, and 4, respectively, while the relaxation times for CoCl(2)(CYpy)(4); Y = 2 and 3l, obtained by dc magnetization decay in the range of 3.5-1.9 K slightly deviated downward from Arrhenius plots on cooling. The dynamic magnetic behaviors for CoCl(2)(CYpy)(4) including [CoCl(2)(py)(4)] and CoCl(2)(C1py)(4) suggested that the generated carbenes interacted with the cobalt ion to increase the relaxation time, τ(q), due to the spin quantum tunneling magnetization, which became larger with increasing S(total) of the complex.