Influence of the co-ligand on the magnetic and relaxation properties of layered cobalt(II) thiocyanato coordination polymers.

Reaction of Co(NCS)2 with 1,2-bis(4-pyridyl)-ethane (bpa) leads to the formation of [Co(NCS)2(bpa)2]n, which, on heating, transforms into the new layered coordination polymer [Co(NCS)2(bpa)]n. This compound can also be prepared in solution, but because no reasonable single crystals are available, its crystal structure was determined from X-ray powder data from scratch. In the crystal structure of [Co(NCS)2(bpa)]n, the cobalt(II) cations are coordinated by two S-bonded and two N-bonded thiocyanato anions and two N atoms of the bpa co-ligands in a distorted octahedral geometry. The cobalt(II) cations are linked into chains by pairs of μ-1,3 bridging thiocyanato anions. These chains are further connected into layers by the 1,2-bis(4-pyridyl)-ethane ligand. The compound was magnetically characterized, and, for comparative purposes, the complementary magnetic study of a known and very similar compound, [Co(NCS)2(bpe)]n (bpe = 1,2-bis(4-pyridyl)-ethylene), was also undertaken. The compounds differ in their interchain interactions, which are antiferromagnetic but significantly greater for [Co(NCS)2(bpe)]n. Magnetic measurements indicate that [Co(NCS)2(bpa)]n is a canted antiferromagnet with Néel temperature TN = 3.1 K and that Co(NCS)2(bpe) is an antiferromagnet with TN = 4.0 K. Both compounds show a metamagnetic transition with a critical field HC ∼ 40 Oe and ∼ 400 Oe, respectively. Magnetic relaxations were studied by means of dc and ac methods and analyzed using the Argand diagrams. Except for the thermally activated single chain and domain wall relaxations observed for both compounds, temperature-independent slow relaxations were observed for [Co(NCS)2(bpa)]n.