Synthesis and Mechanistic Investigation of Bipyrazolo[1,5-a]pyridines via Palladium-Catalyzed Cross-Dehydrogenative Coupling of Pyrazolo[1,5-a]pyridines.

The synthesis of a range of 3,3'-bipyrazolo[1,5-a]pyridine derivatives via direct cross-dehydrogenative coupling of pyrazolo[1,5-a]pyridine precursors is herein presented. This simple and efficient methodology involving palladium(II)-catalyzed C-H bond activation showed good functional group tolerance and product yield (up to 94%). Through the mechanistic insights gained from both kinetic isotope effect experimental studies and density functional theory calculations, a plausible reaction mechanism was outlined. Furthermore, subsequent derivatizations of the resulting 7,7'-diaryl-3,3'-bipyrazolo[1,5-a]pyridines, executed by performing palladium-mediated ortho C-H bond activation followed by hypervalent iodine-induced chlorination, rendered this series of compounds more extended π-conjugation and twisted conformations. Our study on these bipyrazolo[1,5-a]pyridine-based luminogens provides new opportunities for tailor-made organic luminescent materials.

Novel bipyrazolo[1,5-a]pyridine luminogens with aggregation-induced emission enhancement properties.

A novel 3,3'-bipyrazolo[1,5-a]pyridine molecular scaffold was obtained as a product of serendipity. Both photophysical characterisations and HOMO-LUMO theoretical calculations indicate its potential as a promising fluorophore with notable intramolecular charge transfer. Nonetheless, the emission properties of this compound suffer from the typical aggregation-caused quenching effect. To overcome this situation, we introduced additional diaryl groups onto the skeleton and synthesised a series of 7,7'-diaryl-3,3'-bipyrazolo[1,5-a]pyridines via palladium-catalysed intermolecular C-H/C-H bond cross-coupling reaction in 35-62% yields. This series of tailor-made luminogens with twisted π-structures display aggregation-induced emission enhancement properties.

An Unprecedented Interpenetrating Structure Built from Two Differently Bonded Frameworks: Synthesis, Characteristics, and Efficient Removal of Anionic Dyes from Aqueous Solutions.

The first example of one single crystal (NTOU-5) containing two different organic-inorganic hybrid open-framework structures was obtained using a hydro(solvo)thermal method and structurally characterized by single-crystal X-ray diffraction. Remarkably, under the same synthetic conditions, the zinc ions are respectively coordinated by oxalic acid (OX) and 1,2,4,5-tetrakis(imidazol-1-ylmethyl)benzene (TIMB) linkers to form two significantly different frameworks: anionic [Zn2 (OX)3 ]2- and cationic [Zn(TIMB)]2+ networks that interweave with each other to give an unprecedented interpenetrating structure with two differently-bonded open-frameworks. From the inorganic chemistry perspective, it is extremely difficult to control to which metal center the oxygen-donor linkers or/and nitrogen-donor ligands bind. A mixed Co/Zn analogue was also obtained by a similar method. The single-crystal XRD and EDS analyses indicate that the octahedral Zn ions of the anionic framework are replaced by cobalt cations, whereas the Zn ions in the tetrahedral positions of the cationic networks remain intact. This leads to the formation of the interpenetrating analogue with a mixed metal composition. Furthermore, NTOU-5 shows structural stability and efficiently removes organic dyes from aqueous solutions at concentrations of 10 ppm.