Thermodynamically metastable chain and stable layered Co(NCS)2 coordination polymers: thermodynamic relations and magnetic properties

Abstract

Reaction of Co(NCS)₂ with 4-bromopyridine leads to the formation of discrete complexes with the composition Co(NCS)₂(4-bromopyridine)₄·(CH₃CN)_0.67 (1), Co(NCS)₂(4-bromopyridine)₂(H₂O)₂ (2), Co(NCS)₂(4-bromopyridine)₂(CH₃OH)₂ (3) and Co(NCS)₂(4-bromopyridine)₂(CH₃CN)₂ (4). Upon heating compounds 2 and 4 transform into a crystalline product with the composition Co(NCS)₂(4-bromopyridine)₂ (5-I) that also can easily be obtained from solution. In this compound, the Co cations are linked by single μ-1,3-bridging thiocyanate anions into layers. Thermal decomposition of 3 leads to a second isomer (5-II), which is thermodynamically metastable and can also be synthesized from solution under kinetic control. In contrast to 5-I, the Co cations are linked by pairs of anionic ligands into linear chains. The magnetic exchange is very weak in 5-I, but much stronger and ferromagnetic along the linear chains in 5-II. AF ordering in 5-II is reached at 3.05 K, and magnetic relaxation is observed at the metamagnetic transition with an Arrhenius barrier of 17.1(3) cm⁻¹. Ab initio computational studies reveal a different type of magnetic anisotropy to be present in the two crystallographically – independent Co centers in 5-II.