Binucleating Jäger-type {(N2O2)2}4− ligands: magnetic and electronic interactions of Fe(ii), Ni(ii) and Cu(ii) across an in-plane TTF-bridge

Abstract

The simultaneous presence of different electrophores provides an interesting playground for responsive materials. Herein, we present the incorporation of a twice-reversibly oxidizable tetrathiafulvalene (TTF) unit into a binucleating ligand, bridging two metal centers in a fully conjugated plane. A two-step synthesis scheme gave the D_2h symmetric Schiff base-like ligand H₄L in moderate yields from which the corresponding copper(II) [Cu₂L], nickel(II) [Ni₂L], [Ni₂L(py)₄] and iron(II) complexes [Fe₂L(py)₄], [Fe₂L(dmap)₄] and [Fe₂L(bpee)₂]·1 Tol could be obtained. Characterization was performed through ¹H-NMR, IR, UV-vis and ⁵⁷Fe-Mössbauer spectroscopy, SQUID magnetometry and cyclic voltammetry, supported by density functional theory (DFT) calculations. Single crystal X-ray analysis of [Ni₂L(py)₄] revealed six-coordinate paramagnetic centers, whereas [Ni₂L] underwent gradual coordination induced spin state switching (CISSS) in solution. The magnetic independence of both metal centers is echoed by close-to-ideal Curie-plots of the [Cu₂L] system and the gradual spin crossover of all iron(II) compounds. By contrast, cyclic voltammetry measurements in solution indicated oxidation-dependent TTF–metal interactions, as well as metal–metal interactions. The reversible TTF-borne events in H₄L and [Ni₂L] are overlaid with metal-borne events in the case of [Fe₂L(py)₄], as is corroborated by an analysis of the frontier orbital landscapes and through diagnostic spectral features upon chemical oxidation.