Evidence for Charge Delocalization in Diazafluorene Ligands Supporting Low-Valent [Cp*Rh] Complexes

Ligands based upon the 4,5-diazafluorene core are an important class of emerging ligands in organometallic chemistry, but the structure and electronic properties of these ligands have received less attention than they deserve. Here, we show that 9,9'-dimethyl-4,5-diazafluorene (Me₂ daf) can stabilize low-valent complexes through charge delocalization into its conjugated π-system. Using a new platform of [Cp*Rh] complexes with three accessible formal oxidation states (+III, +II, and +I), we show that the methylation in Me₂ daf is protective, blocking Brønsted acid-base chemistry commonly encountered with other daf-based ligands. Electronic absorption spectroscopy and single-crystal X-ray diffraction analysis of a family of eleven new compounds, including the unusual Cp*Rh(Me₂ daf), reveal features consistent with charge delocalization driven by π-backbonding into the LUMO of Me₂ daf, reminiscent of behavior displayed by the workhorse 2,2'-bipyridyl ligand. Taken together with spectrochemical data demonstrating clean conversion between oxidation states, our findings show that 9,9'-dialkylated daf-type ligands are promising building blocks for applications in reductive chemistry and catalysis.

A novel off-on fluorescent chemosensor for Al3+derived from a 4,5-diazafluorene Schiff base derivative

The performance of a chemosensor is closely related to its structure. A new Schiff bass (DFSB) based on 4,5-diazafluorene units has been synthesized in this work. The interaction of DFSB with different metal ions has been studied using UV-vis absorption spectra and fluorescent spectra. The results show that DFSB is a highly selective and sensitive probe for Al³⁺ ions over other commonly coexisting metal ions in ethanol. A very obvious fluorescence enhancement effect was observed, and a turn-on ratio over 1312-fold was triggered with the addition of 10 equiv. of Al³⁺ ions. What is more, such fluorescent responses could be detected by the naked eye under a UV-lamp. The lowest detection limit for Al³⁺ was determined as 3.7 × 10⁻⁸ M. The complex solution (DFSB–Al³⁺) exhibited reversibility with EDTA. These results may be caused by the unique molecular structure.

A highly selective and sensitive“turn-on” fluorescent sensor for detecting Al³⁺ derivated from 4,5-diazafluorene.

Coordination chemistry and applications of versatile 4,5-diazafluorene derivatives

This review article highlights the versatile nature of 4,5-diazafluorene derivatives as ligands, and details some recent advances made using this ligand family.

5,5-Bis(4-methoxy-phen-yl)-2,8-bis-[3-(trifluoro-meth-yl)phen-yl]-5H-cyclo-penta-[2,1-b:3,4-b']dipyridine

The title compound, C₃₉H₂₆F₆N₂O₂, showed two melting transitions 477.4 and 506.5 K in a differential scanning calorimetry (DSC) study. The first of these can be attributed to a melting phase transition arising from the rotation of two trifluoro­methyl groups. In the crystal structure, both trifluoro­methyl groups are disordered over two sites with occupancy factors of 0.660 (17) and 0.340 (17) for the major and minor orientations, respectively. The introduction of trifluoro­methyl groups inhibits π-stacking between the diaza­fluorene (cyclo­penta­[2,1-b:3,4-b’]dipyridine) units. Three short F⋯O contacts between 2.80 (3) and 2.95 (1) Å are observed in the crystal structure.