Isomorphism and polymorphism in zinc(II) and cobalt(II) phosphinate polymers

Abstract

A study has been made of the iso- and poly-morphism phenomena in a series of mixed-ligand species, {M[(Buⁿ₂PO₂)_{2 – 2x}(Ph₂PO₂)_2x]}_n(M = Zn^II or Co^II), by means of differential scanning calorimetry and X-ray analysis. These compounds are linear copolymers and the di-n-butylphosphinate and diphenylphosphinate groups are randomly distributed along the molecular chains. The homopolymer, [Zn(BuⁿPhPO₂)₂]_n, has been studied as a model system with a disordered positioning of the n-butylphenylphosphinate groups along the molecular chains. This compound exhibits polymorphism and its γ-form is isomorphous with that of [Zn(Ph₂PO₂)₂]_n.

The zinc(II) mixed-ligand species are crystalline and isomorphous with the blue cobalt(II) compounds. Their solubility in non-co-ordinating solvents decreases with increasing the content of Ph₂PO₂^– groups. Isomorphous substitution of some Ph₂PO₂^– groups with Buⁿ₂PO₂^– groups in the crystal lattice of the [Zn(Ph₂PO₂)₂]_n homopolymer or of some Buⁿ₂PO₂^– groups with Ph₂PO₂^– groups in the crystal lattice of the [Zn(Buⁿ₂PO₂)₂]_n homopolymer takes place. On the basis of the structures of the crystalline form of [Zn(Buⁿ₂PO₂)₂]_n, stable at room temperature, and of the γ-form of [Zn(BuⁿPhPO₂)₂]_n, the solid-state transitions and the isomorphism and polymorphism phenomena of the various co-ordination polymers are discussed.