Toward an Iron(II) Spin-Crossover Grafted Phosphazene Polymer

Abstract

Two new cyclotriphosphazene ligands with pendant 2,2':6',2″-terpyridine (Terpy) moieties, namely, (pentaphenoxy){4-[2,6-bis(2-pyridyl)]pyridoxy}cyclotriphosphazene (L(1)), (pentaphenoxy){4-[2,6-terpyridin-4-yl]phenoxy}cyclotriphosphazene (L(2)), and their respective polymeric analogues, L(1P) and L(2P), were synthesized. These ligands were used to form iron(II) complexes with an Fe(II)Terpy(2) core. Variable-temperature resonance Raman, UV-visible, and Mössbauer spectroscopies with magnetic measurements aided by density functional theory calculations were used to understand the physical characteristics of the complexes. By a comparison of measurements, the polymers were shown to behave in the same way as the cyclotriphosphazene analogues. The results showed that spin crossover (SCO) can be induced to start at high temperatures by extending the spacer length of the ligand to that in L(2) and L(2P); this combination provides a route to forming a malleable SCO material.

Avoiding cross-linking in iron-polyphosphazene metallo-polymers

“…This is supported by the colour of the solution and solubility of the complexes. Unlike the metallo-polymers previously reported by us [5,13] both L 1 -Fe and L 2 -Fe, could be dissolved in THF and chloroform and dried repeatedly, suggesting little or no cross-linking.…”

“…This could be avoided by reacting FeCl 2 ·4H 2 O with the free ligand followed by anion metatheses [15,16]. However, we have previously described attempts at anion exchange with non-coordinated anions resulting in incomplete reactions [5] which will be exacerbated Scheme 1. The substituted tricyclophosphazene, N 3 P 3 (bph) 2 (Obpy) 2 .…”

“…The elemental analysis of both L 1 and L 2 suggests that the polymers contain residual NaCl, which is a common feature for post-polymerisation synthesis of polyphosphazenes [5,11].…”

“…To date, the only reported SCO-grafted polymers consisted of tridentate ligands attached to the polymer backbone. Upon coordination to either iron(II) or iron(III), the polymers cross-link, forming insoluble solids [3][4][5]. As a means of avoiding crosslinking, a tetradentate ligand or higher could be employed; however, these are more difficult to synthesise and attach to a polymer.…”

Chemical and physical behaviour of heteroleptic 2,6-bis(1 H -benzimidazol-2-yl)pyridine and 2,2′:6′,2″-terpyridine substituted tricyclophosphazene ruthenium(II) complexes

“…We have shown that varying the spacer between the phosphazene and the metal complex alters the physical behaviour of the complexes [2,3]. Since this behaviour is generally similar for the cyclotrimer and the polymer, the former can be used as a model for the polymer [2,3].…”