Slow Magnetic Relaxation in Co(III)-Co(II) Mixed-Valence Dinuclear Complexes with a CoIIO5X (X = Cl, Br, NO3) Distorted-Octahedral Coordination Spheres

The reaction of the multisite coordination ligand (LH4) with CoX2 center dot nH(2)O in the presence of tetrabutylammonium hydroxide affords a series of homometallic dinuclear mixed-valence complexes, [(CoCoII)-Co-III(LH2)(2)(X)(H2O)](H2O)(m) (1, X = Cl and m = 4; 2, X = Br and m = 4; 3, X = NO3 and m = 3). All of the complexes have been structurally characterized by X-ray crystallography. Both cobalt ions in these dinuclear complexes are present in a distorted-octahedral geometry. Detailed magnetic studies on 1-3 have been carried out. M vs H data at different temperatures can be fitted with S = 3/2, the best fit leading to D-3/2 = 7.4 cm(-1), vertical bar E/D vertical bar < 1 x 10(-3), and g = 2.32 for 1 and D-3/2 = -9.7 cm(-1) vertical bar E/D vertical bar < 1 x 10(-4), and g = 2.52 for 2. In contrast to 1 and 2, M vs H data at different temperatures suggest that compound 3 has comparatively little magnetic anisotropy. In accordance with the large negative D values observed for compounds 1 and 2, they are single-molecule magnets (SMMs) and exhibit slow relaxation of magnetization at low temperatures under an applied magnetic field of 1000 Oe with the following energy barriers: 7.9 cm(-1) (tau(o) = 6.1 x 10(-6) s) for 1 and 14.5 cm(-1) (tau(o) = 1.0 x 10(-6) s) for 2. Complex 3 does not show any SMM behavior, as expected from its small magnetic anisotropy. The tau(o) values observed for 1 and 2 are much larger than expected for a SMM, strongly suggesting that the quantum pathway of relaxation at very low temperatures is not fully suppressed by the effects of the applied field.