Carboxylic Acid Functionalized Spin-Crossover Iron(II) Grids for Tunable Switching and Hybrid Electrode Fabrication.

Two carboxyl-substituted iron(II) grids, one protonated, [Fe4(HL)4](BF4)4·4MeCN·AcOEt (1), and the other deprotonated, [Fe4(L)4]·DMSO·EtOH (2), where H2L = 4-{4,5-bis[6-(3,5-dimethylpyrazol-1-yl)pyrid-2-yl]-1 H-imidazol-2-yl}benzoic acid, were synthesized. Single-crystal X-ray structure analyses reveal that both complexes have a tetranuclear [2 × 2] grid structure. 1 formed one-dimensional chains through intermolecular hydrogen bonds between the carboxylic acid units of neighboring grids, while 2 formed two-dimensional layers stabilized by π-π-stacking interactions. 1 showed spin transition between the 3HS-1LS and 1.5HS-2.5LS states around 200 K, while 2 showed spin-crossover between the 4LS and 2LS-2HS states above 300 K. A modified indium-tin oxide (ITO) electrode was fabricated by soaking the ITO in a solution of 1. The resultant electrode showed reversible redox waves attributed to the original redox processes of iron(II)/iron(III).

Studies on the Magnetic Ground State of a Spin Möbius Strip.

Here we report the synthesis, structure and detailed characterisation of three n-membered oxovanadium rings, Nan [(V=O)n Nan (H2 O)n (α, ß, or γ-CD)2 ]⋅m H2 O (n=6, 7, or 8), prepared by the reactions of (V=O)SO4 ⋅x H2 O with α, ß, or γ-cyclodextrins (CDs) and NaOH in water. Their alternating heterometallic vanadium/sodium cyclic core structures were sandwiched between two CD moieties such that O-Na-O groups separated the neighbouring vanadyl ions. Antiferromagnetic interactions between the S=1/2 vanadyl ions led to S=0 ground states for the even-membered rings, but to two quasi-degenerate S=1/2 states for the spin-frustrated heptanuclear cluster.

Multiredox active [3 × 3] copper grids.

A nonanuclear copper grid complex, [Cu(II)9(L)6](BF4)6·1-PrOH·5H2O (1·1-PrOH·5H2O; L = 2,6-bis[5-(2-pyridinyl)-1H-pyrazol-3-yl]pyridine), was synthesized with a [3 × 3] grid structure consisting of nine Cu(II) ions and six deprotonated ligands and displayed four-step quasi-reversible redox behavior from [Cu(II)9] to [Cu(I)4Cu(II)5]. The corresponding heterovalent complex [Cu(I)2Cu(II)7(L)6](PF6)4·3H2O (2·3H2O) was successfully isolated and had a distorted core structure that radically changed the intramolecular magnetic coupling pathways.

Contrasting magnetism of [Mn(III)(4)] and [Mn(II)(2)Mn(III)(2)] squares.

Two tetranuclear manganese distorted square-shaped clusters, [Mn(III)(4)(L1)(4)(mu(2)-OMe)(4)].2.5H(2)O (1) and [Mn(II)(2)Mn(III)(2)(L2)(4)(H(2)O)(2)](PF(6))(2).CHCl(3).CH(3)OH.1.5H(2)O (2) (H(2)L1 = 2-[3-(2-hydroxyphenyl)-1H-pyrazol-5-yl]-6-pyridinecarboxylic acid methyl ester; H(2)L2 = 2-[3-(2-hydroxyphenyl)-1H-pyrazol-5-yl]-6-pyridinecarboxylic acid ethyl ester), exhibit antiferromagnetic and ferromagnetic interactions between neighboring manganese ions, respectively.

Undecanuclear mixed-valence 3d-4f bimetallic clusters.

Two undecanuclear 3d-4f clusters with the general formula {Mn(III)(4)Mn(IV)Ln(III)(6)}, where Ln = Gd or Tb, were synthesized, with both showing large spin ground states, and the Tb species acting as a single molecule magnet.

Programmable spin-state switching in a mixed-valence spin-crossover iron grid.

Photo-switchable systems, such as discrete spin-crossover complexes and bulk iron-cobalt Prussian blue analogues, exhibit, at a given temperature, a bistability between low- and high-spin states, allowing the storage of binary data. Grouping different bistable chromophores in a molecular framework was postulated to generate a complex that could be site-selectively excited to access multiple electronic states under identical conditions. Here we report the synthesis and the thermal and light-induced phase transitions of a tetranuclear iron(II) grid-like complex and its two-electron oxidized equivalent. The heterovalent grid is thermally inactive but the spin states of its constituent metal ions are selectively switched using different laser stimuli, allowing the molecule to exist in three discrete phases. Site-selective photo-excitation, herein enabling one molecule to process ternary data, may have major ramifications in the development of future molecular memory storage technologies.

Triple-stranded ferric helices: a π-π interaction-driven structural hierarchy of Fe5, Fe7, and Fe17 clusters.

Three polypyridine ligand-supported multinuclear iron complexes, [Fe5], [Fe7] and [Fe17], were synthesized and their physical properties were investigated. The complexes had triple-stranded helical structures with pseudo threefold symmetry, and were stabilized by varying degrees of intramolecular π-π stacking. The pentanuclear iron complex consisted of two Fe(II) and three Fe(III) ions, supported by three ligands, while the heptanuclear complex comprised four Fe(II) centres, three Fe(III) ions, and six ligands, and the heptadecanuclear complex contained seventeen Fe(III) ions and nine ligands. Electrochemical studies revealed that the pentanuclear and heptanuclear iron complexes showed pseudo-reversible three- and five-step redox behaviours, respectively. Magnetic measurements conducted on the pentanuclear and heptanuclear complexes revealed that antiferromagnetic interactions were operative between neighbouring iron ions through the oxo- and pyrazole-bridges.

A rectangular Ni-Fe cluster with unusual cyanide bridges.

An asymmetric polycyanide iron complex, K(2)[Fe(III)(L1)(CN)(4)](MeOH) (HL1 = 2,2'-(1H-pyrazole-3,5-diyl)bis-pyridine), was synthesized and its complexation compatibility with nickel ions was examined. Two kinds of enantiomeric nickel-iron squares were obtained in the presence of a chiral bidentate capping ligand. The compounds display unusual cyanide bridge geometry and have ferromagnetic interactions between nickel and iron ions.