Probing bistability in Fe(II) and Co(II) complexes with an unsymmetrically substituted quinonoid ligand.

Margarethe Van Der Meer,Biprajit Sarkar,Petr Neugebauer,Joris Van Slageren,Yvonne Rechkemmer,Frauke D Breitgoff,Raphael Marx,Sebastian Dechert,María Dörfel

doi:10.1039/c6dt00757k

Abstract

The generation of molecular platforms, the properties of which can be influenced by a variety of external perturbations, is an important goal in the field of functional molecular materials. We present here the synthesis of a new quinonoid ligand platform containing an [O,O,O,N] donor set. The ligand is derived from a chloranilic acid core by using the [NR] (nitrogen atom with a substituent R) for [O] isoelectronic substitution. Mononuclear Fe(II) and Co(II) complexes have been synthesized with this new ligand. Results obtained from single crystal X-ray crystallography, NMR spectroscopy, (spectro)electrochemistry, SQUID magnetometry, multi-frequency EPR spectroscopy and FIR spectroscopy are used to elucidate the electronic and geometric structures of the complexes. Furthermore, we show here that the spin state of the Fe(II) complex can be influenced by temperature, pressure and light and the Co(II) complex displays redox-induced spin-state switching. Bistability is observed in the solid-state as well as in solution for the Fe(II) complex. The new ligand presented here, owing to the [NR] group present in it, will likely have more adaptability while investigating switching phenomena compared to its [O,O,O,O] analogues. Thus, such classes of ligands as well as the results obtained on the reversible changes in physical properties of the metal complexes are likely to contribute to the generation of multifunctional molecular materials.

Highlights

The ligand 2,5-dihydroxy-1,4-benzoquinone containing a symmetrically substituted [O,O,O,O] donor set has been extensively used in coordination chemistry.1 In recent years, the combination of dhbq with CoII showed valence tautomerism (VT).2 A related ligand, chloranilic acid (CA, Fig. 1) and its derivatives, were used with FeII,3 to generate spin crossover (SCO) properties4 and with CoII,5 to generate VT
We had reported earlier on a similar method to prepare ligands with an [O,N,O,N] donor set (Fig. 1) starting from dhbq.6a In those ligands, usually two of the [O] in dhbq are substituted with two [NR] groups leading to the formation of a symmetrically substituted quinonoid ligand as the major product
The net result is the formation of an unsymmetrically substituted H2L ligand, examples of which are rare in the literature.6a,h The purity and structure of H2L was established using a combination of 1H and 13C NMR data and elemental analyses

Summary

Introduction

The ligand 2,5-dihydroxy-1,4-benzoquinone (dhbq, Fig. 1) containing a symmetrically substituted [O,O,O,O] donor set has been extensively used in coordination chemistry.1 In recent years, the combination of dhbq with CoII showed valence tautomerism (VT).2 A related ligand, chloranilic acid (CA, Fig. 1) and its derivatives, were used with FeII,3 to generate spin crossover (SCO) properties4 and with CoII,5 to generate VT. Functional metal complexes with such [O,N,O,N] donor sets have been useful for investigating electron transfer processes, spin state switching, spin– spin coupling and other properties.6 In these ligands with an

Results

Conclusion