2D and 3D coordination polymers based on 2,2′-bipyrimidine and cyanide bridging ligands incorporating coordinated and guest ammonia molecules. Synthesis, crystal structures, magnetic properties and thermal analysis of {[Ni(CN)4]2[(Ni(NH3)2)2(bpym)]·2H2O}n and {[Cu2(CN)2 (bpym)]·NH3}n

Abstract

The coordination polymers {[Ni(CN)₄]₂[(Ni(NH₃)₂)₂(bpym)]·2H₂O}_n1 and {[Cu₂(CN)₂(bpym)]·NH₃}_n2 have been prepared from the reactions of M(NO₃)₂·nH₂O [Cu(II), n = 3 and M = Ni(II), n = 6] with NaCN and bpym (2,2′-bipyrimidine) in concentrated aqueous ammonia, respectively. The structure of 1 is made of bipyrimidine bridged centrosymmetric dinuclear [Ni(NH₃)₂(μ-bpym)Ni(NH₃)₂] fragments connected by four [Ni(CN)₄]²⁻ anions giving rise to a 2D framework. Layers, which adopt a stair-like conformation, are stacked with an ABAB… repeat pattern. Water molecules are located in the interlayer space and are involved in hydrogen bond interactions with the non-bridging cyanide groups of the [Ni(CN)₄]²⁻ anions and one of the ammonia molecules. Magnetic measurements clearly show the existence of antiferromagnetic exchange interactions between the local spin triplet states of the octahedral Ni(II) ions mainly mediated by the bpym exchange pathway with a J value of −16.82 cm⁻¹. The structure of 2 consists of [Cu₂(bpym)] centrosymmetric fragments connected by cyanide groups to four neighbouring Cu(I) ions giving rise to a 3D network with channels perpendicular to the bc plane. Guest ammonia molecules are located into channels. TG-XRD combined studies show that the structure of 1 is retained after dehydration whereas that of 2, however, collapses after ammonia elimination.