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Multiple Bistability in Quinonoid-Bridged Diiron(II) Complexes: Influence of Bridge Symmetry on Bistable Properties

Research paper by Margarethe van der Meer, Yvonne Rechkemmer, Frauke D. Breitgoff, Raphael Marx, Petr Neugebauer, Uta Frank, Joris van Slageren, Biprajit Sarkar

Indexed on: 09 Nov '16Published on: 08 Nov '16Published in: Inorganic Chemistry



Abstract

The influence of the nature of quinone bridges on the electrochemical, spectroscopic, and magnetic properties of diiron(II) complexes is probed. It is shown that the unsymmetrically substituted bridge allows for site-specific electron transfer, site-specific spin state changes, greater thermodynamic stabilization of the mixed-valent form, broad and symmetrical IVCT bands in the mixed-valent state, SCO with large hysteresis, and LIESST effects. None of these properties are observed for the corresponding symmetrically substituted diiron(II) complex.Quinonoid bridges are well-suited for generating dinuclear assemblies that might display various bistable properties. In this contribution we present two diiron(II) complexes where the iron(II) centers are either bridged by the doubly deprotonated form of a symmetrically substituted quinonoid bridge, 2,5-bis[4-(isopropyl)anilino]-1,4-benzoquinone (H2L2′) with a [O,N,O,N] donor set, or with the doubly deprotonated form of an unsymmetrically substituted quinonoid bridge, 2-[4-(isopropyl)anilino]-5-hydroxy-1,4-benzoquinone (H2L5′) with a [O,O,O,N] donor set. Both complexes display temperature-induced spin crossover (SCO). The nature of the SCO is strongly dependent on the bridging ligand, with only the complex with the [O,O,O,N] donor set displaying a prominent hysteresis loop of about 55 K. Importantly, only the latter complex also shows a pronounced light-induced spin state change. Furthermore, both complexes can be oxidized to the mixed-valent iron(II)–iron(III) form, and the nature of the bridge determines the Robin and Day classification of these forms. Both complexes have been probed by a battery of electrochemical, spectroscopic, and magnetic methods, and this combined approach is used to shed light on the electronic structures of the complexes and on bistability. The results presented here thus show the potential of using the relatively new class of unsymmetrically substituted bridging quinonoid ligands for generating intriguing bistable properties and for performing site-specific magnetic switching.

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