Thermally induced structural rearrangement of the Fe(ii) coordination geometry in metallo-supramolecular polyelectrolytes

Abstract

Rigid rod-type metallo-supramolecular coordination polyelectrolytes with Fe(II) centres (Fe-MEPEs) are produced via the self-assembly of the ditopic ligand 1,4-bis(2,2':6',2''-terpyridine-4'-yl)benzene (tpy-ph-tpy) and Fe(II) acetate. Fe-MEPEs exhibit remarkable electrochromic properties; they change colour from blue to transparent when an electric potential is applied. This electrochemical process is generally reversible. The blue colour in the ground state is a result of a metal-to-ligand charge transfer at the Fe(II) centre ion in a quasi-octahedral geometry. When annealed at temperatures above 100 °C, the blue colour turns into green and the formerly reversible electrochromic properties are lost, even after cooling down to room temperature. The thermally induced changes in the Fe(II) coordination sphere are investigated in situ during annealing of a solid Fe-MEPE using X-ray absorption fine structure (XAFS) spectroscopy. The study reveals that the thermally induced transition is not accompanied by a redox process at the Fe(II) centre. From the detailed analysis of the XAFS spectra, the changes are attributed to structural changes in the coordination sphere of the Fe(II) site. In the low temperature state, the Fe(II) ion rests in a quasi-octahedral coordination environment surrounded by six nitrogen atoms of the pyridine rings. The axial Fe-N bond length is 1.94 Å, while the equatorial bond length amounts to 1.98 Å. In the high temperature state, the FeN6-site exhibits a distortion with the axial Fe-N bonds being shortened to 1.88 Å and the equatorial Fe-N bonds being elongated to 2.01 Å.

In operando XAFS experiments on flexible electrochromic devices based on Fe(II)-metallo-supramolecular polyelectrolytes and vanadium oxide

“…This means that in fact the Fe complex in Fe-MEPE does not undergo a noticeable structural change during charging and discharging. From a previous XAFS study of these materials [27] we know that the iron ion is coordinated in a pseudo-octahedral environment with the nitrogen atoms of the pyridine rings in the vertices. The axial Fe-N distance is 1.95 Å, and the equatorial Fe-N distance amounts to 1.98 Å.…”

The crystal structure of the triclinic polymorph of 1,4-bis([2,2′:6′,2′′-terpyridin]-4′-yl)benzene

“…1,4-Di([2,2 0 :6 0 ,2 00 -terpyridin]-4 0 -yl)benzene has been used as a ligand in the formation of mononuclear complexes (Santoni et al, 2013;Laramé e-Milette & Hanan, 2017), binuclear complexes (Santoni et al, 2013;Schmittel et al,2006;Maekawa et al, 2004), tetranuclear complexes (Schmittel et al, 2005), one-dimensional coordination polymers (Koo et al, 2003), two-dimensional coordination polymers (Bulut et al, 2015;Jones et al (2010), and numerous metallo-supramolecular polymers (without reported crystal structures), see for example: Vaduvescu & Potvin, 2004;Nishimori et al, 2007;Han et al, 2008;Schwarz et al, 2010;Ding et al, 2012;Muronoi et al, 2013;Szczerba et al, 2014;Munzert et al, 2016;Meded et al, 2017;Bera et al, 2018. A search of the Cambridge Structural Database (CSD, Version 5.40, update August 2019; Groom et al, 2016) for the ISSN 2056-9890 title compound yielded only nine hits (see supporting information), which included the report on the structure of the orthorhombic polymorph, Form II, by Fernandes et al (2010).…”

Crystal Engineering of 6-Carboxy-4-aryl-2,2′-bipyridine Complexes: Potent Chelators with Intrinsic Intermolecular Affinity

“…In the ongoing drive toward new functional supramolecular systems, a constant need exists for the discovery and development of new ligand systems, binding motifs, and molecular scaffolds, in order to diversify the existing chemical toolbox and rationally pursue specific applications. − To this end, significant progress has been made recently in establishing modular ligand sets for further development into supramolecular systems. , Doing so requires a thorough basis in the fundamental coordination chemistry and structural trends of archetypal systems, to establish the necessary understanding of the subtle geometric and electronic effects which are well-known to strongly influence the bulk properties of such systems. − The polypyridine family of ligands are well-known metallosupramolecular ligand scaffolds, widely exploited in transition metal coordination chemistry, due to the predictable coordination behavior and favorable electronic and magnetic properties of bipyridine and terpyridine d-metal ion complexes. , These structures have found myriad applications, including as dye-sensitized solar cells, redox photocatalysts, molecular electronics, and spin crossover devices. − As well-established starting points, functional 2,2′-bipyridine derivatives are ideally suited for the installation of new substitution patterns and functional groups . Although direct derivatization of the deactivated pyridine ring itself can be synthetically cumbersome compared to other nitrogen heterocycles, new synthetic methods are continually emerging for the synthesis of functional bipyridine ligands. , …”