Magnetic properties of isostructural M(H2O)4[Au(CN)4]2-based coordination polymers (M = Mn, Co, Ni, Cu, Zn) by SQUID and μSR studies

Abstract

A series of isomorphous M(H₂O)₄[Au(CN)₄]₂·4H₂O (M = Mn, Co, Ni, Zn; Cu is similar) coordination polymers was synthesized from the reaction of M(II) with KAu(CN)₄; they consist of octahedrally coordinated metal centres with four equatorial water molecules and trans-axial N-cyano ligands from [Au(CN)₄]⁻ moieties, generating a linear 1-D chain of M(H₂O)₄[Au(CN)₄]-units. An additional interstitial [Au(CN)₄]⁻ unit forms Au⋯N and hydrogen bonds with adjacent chains. The Cu(II) system readily loses water to yield Cu[Au(CN)₄]₂(H₂O)₄, which was not structurally characterized. The magnetic properties of these polymers were investigated by a combination of SQUID magnetometry and zero-field muon spin relaxation (ZF-μSR). Only weak antiferromagnetic interactions along the chains are mediated by the [Au(CN)₄]-units, but the ZF-μSR data indicates that interchain interactions yield a phase transition to a magnetically ordered state for Cu[Au(CN)₄]₂(H₂O)₄ below 0.6 K, while for M(H₂O)₄[Au(CN)₄]₂·4H₂O (M = Co), depopulation of zero-field split Kramer's doublets to an effective “S = 1/2” ground state yields a transition to a spin-frozen magnetic state below 0.26 K. On the other hand, only a simple slowing-down of spins above 0.02 K is observed for the more weakly zero-field split M(H₂O)₄[Au(CN)₄]₂·4H₂O (M = Mn, Ni) complexes.