Interplay between structure and properties in dicyanoaurate-based coordination polymers

This research project has focused on the design and synthesis of capping ligand-free dicyanoaurate-based coordination polymers. Dicyanoaurate is a building block that readily bridges two transition metals and increases the connectivity of the system through gold-gold interactions. A series of isostructural M(μ-OH2)2[Au(CN)2]2 coordination polymers (Cu, Ni, Co, Fe, Mn) was prepared. Their structure contains an unprecedented motif in which metal centers are doubly bridged by water molecules, generating chains. The magnetic interactions present in these water-bridged polymers were investigated; both ferromagnetic (Cu, Ni, Co) and antiferromagnetic interactions (Fe, Mn) were observed. Muon spin relaxation studies showed different magnetic ground states for these polymers, including long-range ordered (Cu, TC = 0.2 K) and spin-glass states (Ni, Tf = 3.5 K). Several Cu[Au(CN)2]2(analyte)x polymers (analyte = dimethylsulfoxide, N,N-dimethylformamide, water, pyridine, ammonia) were prepared and found to be vapochromic, i.e. change colour upon exposure to volatile analytes. The analyte incorporated can easily be identified as each colour is specific to one analyte, with maximum reflection ranging from 433 to 560 nm. The infrared cyanide vibration signature of each polymer is also unique and allows another way of identifying the analyte present. The analyte can be exchanged at room temperature, without any thermal treatment. A survey of analyte sensing capabilities was performed by replacing the Cu ions with Ni and Co ions in M[Au(CN)2]2(analyte)x. The preparation of [cation]{M[Au(CN)2]3} polymers (cation = K+, [PPN]+, [nBu4N]+; M = Ni, Co) was also investigated. The incorporation of different cations affected the structural arrangement of the anionic framework, but did not prevent the 3-D connectivity between the metal centers. Despite the high connectivity, very weak antiferromagnetic interactions were observed which suggests that dicyanoaurate is a poor mediator of magnetic exchange. Studies by transmission electron microscopy showed the presence of trace amounts of NiO and Au nanoparticles in hydrothermally recrystallized samples of dicyanoaurate-containing coordination polymers. These products were hardly detectable by standard analysis techniques, but the NiO nanoparticles dominated the magnetic response at low temperatures. This finding illustrates the care that must be taken when investigating samples prepared by hydrothermal methods.