A new basic motif in cyanometallate coordination polymers: structure and magnetic behavior of M(mu-OH2)2[Au(CN)2]2 (M=Cu, Ni)

The structures of two cyanoaurate-based coordination polymers, M(mu-OH(2))(2)Au(CN)(2)](2) (M=Cu, Ni), were determined by using a combination of powder and single-crystal X-ray diffraction techniques. The basic structural motif for both polymers contains rarely observed M(mu-OH(2))(2)M double aqua-bridges, which generate an infinite chain; two trans Au(CN)(2)](-) units also dangle from each metal center. The chains form ribbons that interact three dimensionally through CNH hydrogen bonding. The magnetic properties of both compounds and of the dehydrated analogue CuAu(CN)(2)](2) were investigated by direct current (dc) and alternating current (ac) magnetometry; muon spin-relaxation data was also obtained to probe their magnetic properties in zero-field. In M(mu-OH(2))(2)Au(CN)(2)](2), ferromagnetic chains of M(mu-OH(2))(2)M are present below 20 K. Interchain magnetic interactions mediated through hydrogen bonding, involving water and cyanoaurate units, yield a long-range magnetically ordered system in Cu(mu-OH(2))(2)Au(CN)(2)](2) below 0.20 K, as indicated by precession in the muon spin polarization decay. Ni(mu-OH(2))(2)Au(CN)(2)](2) undergoes a transition to a spin-glass state in zero-field at 3.6 K, as indicated by a combination of muon spin-relaxation and ac-susceptibility data. This transition is probably due to competing interactions that lead to spin frustration. A phase transition to a paramagnetic state is possible for Ni(mu-OH(2))(2)Au(CN)(2)](2) upon application of an external field; the critical field was determined to be 700 Oe at 1.8 K. The dehydrated compound CuAu(CN)(2)](2) shows weak antiferromagnetic interactions at low temperatures.