Synthesis of new stable and metastable thiocyanato coordination polymers by thermal decomposition and by solution reactions

Abstract

Investigations on the thermal behavior of the 1 : 4 (ratio between metal and neutral co-ligand) compound [Cd(NCS)₂(pyridine)₄] (1-Cd) show that the pyridine ligands can be removed in three separate steps. In the first TG step a pyridine-deficient 1 : 2 compound of composition [Cd(NCS)₂(pyridine)₂]_n (2-Cd/III) is obtained, which transforms into an 1 : 1 compound of composition [Cd(NCS)₂(pyridine)]_n (3-Cd) on further heating. Rietveld refinement of 2-Cd/III proves that this compound represents a new form, which is different from that of the 1 : 2 compounds 2-Cd/I and 2-Cd/II reported recently. In further experiments a fourth 1 : 2 compound of composition [Cd(NCS)₂(pyridine)₂]_n (2-Cd/IV) was crystallized and characterized by single crystal X-ray diffraction. In the crystal structures of 2-Cd/II, 2-Cd/III and 2-Cd/IV the metal cations are linked by the anionic ligands into chains, in which the Cd cations are coordinated by two trans-oriented pyridine ligands and four thiocyanato anions within slightly distorted octahedra. For the literature known compound 2-Cd/I no atomic coordinates are given, but for 2-Cd/II an all-cis, for 2-Cd/III an all-trans and for 2-Cd/IV an alternating cis-cis-trans arrangement of the thiocyanato anions is observed. Solvent mediated conversion experiments prove that 2-Cd/III transforms into 2-Cd/IVvia2-Cd/II as intermediate and therefore, the cis-cis-trans coordinated compound 2-Cd/IV represents the thermodynamic stable form at room-temperature. The 1 : 1 compound 3-Cd can also be prepared from solution. In its crystal structure the Cd cations are coordinated by one pyridine ligand as well as two N-bonded and three S-bonded thiocyanato anions. Two neighboring octahedra are connected by two μ-1,1,3(S,S,N)-coordinating anions into double octahedra, which are further linked into chains by μ-1,1,3(S,S,N)- and μ-1,3-coordinating thiocyanato anions. IR investigations of the 1 : 4, 1 : 2 and 1 : 1 compounds reveal that the different coordination modes of the anionic ligands can be rationalized by analyzing the changes in the values of their asymmetric ν_as(CN) stretching vibration. Additional investigations on 2-Cd/II, 2-Cd/III and 2-Cd/IV using differential scanning calorimetry and X-ray powder diffraction reveal that on heating additional crystalline 1 : 2 compounds are formed in reversible and irreversible phase transitions.