Arylazoindazole Photoswitches: Facile Synthesis and Functionalization via SNAr Substitution.

A straightforward synthetic route to arylazoindazoles via nucleophilic aromatic substitution is presented. Upon deprotonation of the NH group, a C6F5-substituted formazan undergoes facile cyclization as a result of intermolecular nucleophilic substitution (SNAr). This new class of azo photoswitches containing an indazole five-membered heterocycle shows photochemical isomerization with high fatigue resistance. In addition, the Z-isomers have long thermal half-lives in the dark of up to several days at room temperature. The fluorinated indazole group offers a handle for further functionalization and tuning of its properties, as it is shown to be susceptible to a subsequent, highly selective nucleophilic displacement reaction.

Spin-Crossover in a Pseudo-tetrahedral Bis(formazanate) Iron Complex.

Spin-crossover in a pseudo-tetrahedral bis(formazanate) iron(II) complex (1) is described. Structural, magnetic, and spectroscopic analyses indicate that this compound undergoes thermal switching between an S=0 and an S=2 state, which is very rare in four-coordinate complexes. The transition to the high-spin state is accompanied by an increase in Fe-N bond lengths and a concomitant contraction of intraligand N-N bonds. The latter suggests that stabilization of the low-spin state is due to the π-acceptor properties of the ligand. One-electron reduction of 1 leads to the formation of the corresponding anion, which contains a low-spin (S=1/2) Fe(I) center. The findings are rationalized by electronic structure calculations using density functional theory.

Formazanate ligands as structurally versatile, redox-active analogues of ß-diketiminates in zinc chemistry.

A range of tetrahedral bis(formazanate)zinc complexes with different steric and electronic properties of the formazanate ligands were synthesized. The solid-state structures for several of these were determined by X-ray crystallography, which showed that complexes with symmetrical, unhindered ligands prefer coordination to the zinc center via the terminal N atoms of the NNCNN ligand backbone. Steric or electronic modifications can override this preference and give rise to solid-state structures in which the formazanate ligand forms a 5-membered chelate by binding to the metal center via an internal N atom. In solution, these compounds show dynamic equilibria that involve both 5- and 6-membered chelates. All compounds are intensely colored, and the effect of the ligand substitution pattern on the UV-vis absorption spectra was evaluated. In addition, their cyclic voltammetry is reported, which shows that all compounds may be electrochemically reduced to radical anionic (L2Zn(-)) and dianionic (L2Zn(2-)) forms. While unhindered NAr substituents lie in the plane of the ligand backbone (Ar = Ph), the introduction of sterically demanding substituents (Ar = Mes) favors a perpendicular orientation in which the NMes group is no longer in conjugation with the backbone, resulting in hypsochromic shifts in the absorption spectra. The redox potentials in the series of L2Zn compounds may be altered in a straightforward manner over a relatively wide range (∼700 mV) via the introduction of electron-donating or -withdrawing substituents on the formazanate framework.

Alkali metal salts of formazanate ligands: diverse coordination modes as a result of the nitrogen-rich [NNCNN] ligand backbone.

Alkali metal salts of redox-active formazanate ligands were prepared, and their structures in the solid-state and in solution are determined. The nitrogen-rich [NNCNN] backbone of formazanates results in a varied coordination chemistry, with both the internal and terminal nitrogen atoms available for bonding with the alkali metal. The potassium salt K[PhNNC(p-tol)NNPh]·2THF (1-K) is dimeric in the solid state and even in THF solution, as a result of the K atom bridging via interaction with a terminal N atom and the aromatic ring of a second unit. Conversely, for the compounds Na[MesNNC(CN)NNMes]·2THF (2-Na) and Na[PhNNC((t)Bu)NNPh] (3-Na) polymeric and hexameric structures are found in the solid state respectively. The preference for binding the alkali metal through internal N atoms (1-K and 2-Na) to give a 4-membered chelate, or via internal/external N atoms (5-membered chelate in 3-Na), contrasts with the 6-membered chelate mode observed in our recently reported formazanate zinc complexes.