Sorption in the β-phases of transition metal(II) tetra-(4-methylpyridine) thiocyanates and related compounds

Abstract

In addition to the previously known clathrate complexes with aromatic hydrocarbons and certain of their derivatives, the β-phases of M^II(4-methylpyridine)₄(SCN)₂(where M^II is a transition metal such as Co, Ni, or Mn) have now been found to sorb numerous other guest species. The β-phases behaved, like the zeolites, as crystals permanently porous in the absence of guest molecules, and the sorption isotherms were reversible.

Sorption was investigated, often over a range of temperatures, for the following guest molecules: O₂, N₂, Ar, Kr, Xe; CH₃Cl, CH₂Cl₂, CHCl₃, CCl₄; C₁ to C₆ n-paraffins; C₆H₆, toluene, ethylbenzene, and o-, m-, and p-xylene; cyclohexane; isopentane and 2,2,4-trimethylpentane; C₁ to C₆ primary alcohols derived from n-paraffins; CO₂, CS₂, and C₂H₅·O·C₂H₅. Sorption is related to size, shape, and polarity of the guest molecules.

Isotherms were of Types I, IV, and V in Brunauer's classification. Isosteric heats of sorption were found to correlate with heats of vapourisation of the liquid sorbates, or their b.p.s, thus emphasising the physical nature of the sorption. For small molecules stepped isotherms or isotherms showing upward inflections were often observed. It is suggested that this behaviour arises from the reversible generation of extra sorption sites above certain threshold pressures.

Of ligands other than 4-methylpyridine, 4-ethylpyridine gave a complex which sorbed benzene, toluene, ethylbenzene, and o- and p-xylene, but not m-xylene, CO₂, Xe, or aliphatic compounds. Other pyridine-based ligands gave complexes which did not form clathrate phases.