Mixed‐Valence, Mixed‐Spin‐State, and Heterometallic [2×2] Grid‐type Arrays Based on Heteroditopic Hydrazone Ligands: Synthesis and Electrochemical Features

Abstract

An extended family of heterometallic [(M1)2(M2)2(L-)4](n+) [2x2] grid-type arrays 1-9 has been prepared. The three-tiered synthetic route encompasses regioselective, redox and enantioselective features and is based on the stepwise construction of heteroditopic hydrazone ligands A-C. These ligands contain ionisable NH and nonionisable NMe hydrazone units, which allows the metal redox properties to be controlled according to the charge on the ligand binding pocket. The 2-pyrimidine (R) and 6-pyridine (R') substituents have a significant effect on complex geometry and influence both the electrochemical and magnetic behaviour of the system. 1H NMR spectroscopic studies show that the Fe(II) ions in the grid can be low spin, high spin or spin crossover depending on the steric effect of substituents R and R'. This steric effect has been manipulated to construct an unusual array possessing two low-spin and two spin-crossover Fe(II) centres (grid 8). Electrochemical studies were performed for the grid-type arrays 1-9 and their respective mononuclear precursor complexes 10-13. The grids function as electron reservoirs and display up to eight monoelectronic, reversible reduction steps. These processes generally occur in pairs and are assigned to ligand-based reductions and to the Co(III)/Co(II) redox couple. Individual metal ions in the heterometallic grid motif can be selectively addressed electrochemically (e.g., either the Co(III) or Fe(II) ions can be targeted in grids 2 and 5). The Fe(II) oxidation potential is governed by the charge on the ligand binding unit, rather than the spin state, thus permitting facile electrochemical discrimination between the two types of Fe(II) centre in 7 or in 8. Such multistable heterometallic [2x2] gridlike arrays are of great interest for future supramolecular devices incorporating multilevel redox activity.

Self‐Assembly of Alkoxy‐Substituted Bis(hydrazone)‐Based Organic Ligands and of a Metallosupramolecular Grid on Graphite

“…[37,38] These fully reversible deprotonation steps are reflected in dramatic changes in optical, electrochemical and magnetic properties. [37][38][39] In an extension of the previous work, with the objective of exploiting these effects of deprotonation, we synthesized new pyridylhydrazone ligands (L1 and L2) and their [2 2] gridlike complexes (Figure 1), which have been characterized at the solid/liquid interface by scanning tunnelling microscopy (STM). [40,41] Long octadecyl chains were attached to the ligand to promote its physisorption on HOPG.…”

“…[35,36] Recently, comprehensive optical, electrochemical and magnetic studies have been performed on grid complexes of much more readily synthesized analogues of fused bis(terpyridine) ligands containing pyridylhydrazone units. [22,[37][38][39] These ligands have the additional feature of containing an ionizable centre, the hydrazone C=NNHR unit, the acidity of which is enhanced by metal cation coordination. This means that grids containing both the neutral ligand (so that the grids are cationic) and its dinegative conjugate base (giving neutral grids with 2 + metal Self-assembly of a bis(hydrazone) chelating ligand functionalized with octadecyloxy substituents is described, as well as the reproducible thermally activated self-assembly of its metallosupramolecular cationic Co II A C H T U N G T R E N N U N G [22] grid-type complex, on a highly oriented pyrolitic graphite (HOPG) substrate.…”

Self‐Assembly of Supramolecular Nanostructures: Ordered Arrays of Metal Ions and Carbon Nanotubes

“…(ii) The step-wise synthesis of a [2 Â 2]Co II 2 Co III 2 metal ion array yields molecular species, which show exactly the diagonally localized electron density distribution required for the internal cell structure of such automata devices (Fig. 12.5c) [17].…”

Versatile coordinating behaviour of bis(acylhydrazone) ligands derived from imino- and methyl-iminodiacetic acid diethyl ester. Antimicrobial properties of their trinuclear copper(II) complexes

“…Di-and poly-nuclear metal complexes of hydrazonic ligands play a significant role in the study of multi-electron redox reactions, magnetic exchange interactions and intramolecular binding [21,24]. Acylhydrazone and semicarbazone derivative compounds occupy an important position as ligands in metal coordination chemistry, because this general class of chelating agents shows different stereochemistries and nuclearity in complexation with transition metal ions [4,16].…”