Exploiting the Strong Hydrogen Bond Donor Properties of a Borinic Acid Functionality for Fluoride Anion Recognition

Pyrrole βC-B-N Fused Porphyrins: Molecular Structures and Opto-Electrochemical Studies

Herein, we report the design, synthesis, structure, and electrochemical study of doubly βC-B-N fused Ni(II) porphyrins (1-trans, 1-cis, 2-trans, and 2-cis). These compounds have been synthesized from A2B2 type dipyridyl Ni(II) porphyrins (Ar=Ph for 1 a; Ar=C6F5 for 2 a) via Lewis base-directed electrophilic aromatic borylation reactions. The solution state structures of these compounds have been established using 1H NMR, 11B NMR, 1H-1H COSY, 1H-13C HSQC, and 19F-13C HSQC NMR techniques. Single crystal X-ray analysis have revealed that 1-trans, 1-cis, and 2-trans adopt ruffled conformations, with alternate meso-carbons on the opposite sides of the mean porphyrin plane. The Soret bands in the absorption spectra of the B-N fused molecules are ~40 nm redshifted compared to unfused Ni(II) porphyrin precursors. The B-N fusion have diminished the redox potential of fused porphyrins. Although 1-trans and 1-cis, show four oxidation processes, 2-trans and 2-cis show only three oxidation processes. DFT studies have revealed that the tetrahedral geometry of the boron has induced a twist in the π-conjugation, which destabilizes the HOMO and stabilizes the LUMO in 1-trans, 1-cis, 2-trans, and 2-cis.

A Bidentate Ligand Featuring Ditopic Lewis Acids in the Second Sphere for Selective Substrate Capture and Activation

We present a ligand platform featuring appended ditopic Lewis acids to facilitate capture/activation of diatomic substrates. We show that incorporation of two 9-borabicyclo[3.3.1]nonane (9-BBN) units on a single carbon tethered to a pyridine pyrazole scaffold maintains a set of unquenched nitrogen donors available to coordinate FeII , ZnII , and NiII . Using hydride ion affinity and competition experiments, we establish an additive effect for ditopic secondary sphere boranes, compared to the monotopic analogue. These effects are exploited to achieve high selectivity for binding NO2 - in the presence of competitive anions such as F- and NO3 - . Finally, we demonstrate hydrazine capture within the second-sphere of metal complexes, followed by unique activation pathways to generate hydrazido and diazene ligands on Zn and Fe, respectively.

Fluorescent small molecules with alternating triarylamine-substituted selenophenothiophene and triarylborane: synthesis, photophysical properties and anion sensing studies

Two novel D-π-A fluorophores based on selenopheno[3,2-b]thiophene, possessing triphenylamine and 4,4'-dimethoxytriphenylamine units as donors and dimesitylborane as an acceptor, linked through a π-conjugated thiophene spacer (BTPAST and BOMeTPAST, respectively) were synthesized. Their photophysical properties were investigated in both solution and the state of aggregation and compared to those of their corresponding donor parts, having no dimesitylborane units (TPAST and OMeTPAST). All the compounds displayed large Stokes shifts between 100 and 140 nm with positive solvatochromism in solvents having different polarities. While BTPAST displayed both aggregation induced emission (AIE) and twisted intramolecular charge transfer (TICT) characteristics, the others preponderated with TICT effects. The sensing abilities of BTPAST and BOMeTPAST towards different anions were studied. Both exhibited chromogenic and fluorogenic responses to small anions such as fluoride and cyanide, for which the detection limits were found to be 0.12 and 2.43 ppm with BTPAST and 0.59 and 0.92 ppm with BOMeTPAST, respectively. These results provide guidance for the development of novel fused selenophenothiophene sensors in the field of anion sensing.

Easily-prepared Hydroxy-containing Receptors Recognize Anions in Aqueous Media

Despite their ready availability, O-H groups have received relatively little attention as anion recognition motifs. Here, we report two simple hydroxy-containing anion receptors that are prepared in two facile steps followed by anion exchange, without the need for chromatographic purification at any stage. These receptors contain a pyridinium bis(amide) motif as well as hydroxyphenyl groups, and bind mono- and divalent anions in 9:1 CD₃ CN:D₂ O, showing a selectivity preference for sulfate. Notably, a "model" receptor that does not contain hydroxy groups shows only very weak sulfate binding in this competitive solvent mixture. In the solid state, X-ray crystallographic studies show that the receptors tend to form extended assemblies with anions; however, ¹ H and DOSY NMR studies as well as molecular dynamics simulations show that only 1:1 complexes are present in solution. Molecular dynamics simulations suggest that one of the receptors suffers from competing intramolecular hydrogen bonding, while another binds partially-hydrated anions, with the receptor's O-H groups forming hydrogen bonds to water molecules within the anion's coordination sphere.

Bidentate Boron Lewis Acids: Selectivity in Host-Guest Complex Formation

Bidentate boron Lewis acids based on 1,8-diethynylanthracene were synthesised in two steps by initial stannylation of the terminal alkynes and subsequent tin-boron exchange with different chloroboranes. The reactions were very selective, and the target compounds were obtained in high purity and good to excellent yields. Complexation experiments of 1,8-bis[(diphenylboranyl)ethynyl]anthracene with nitrogen bases (pyridine, pyrimidine, TMEDA) afforded three stable adducts, which were structurally characterised by X-ray diffraction. Competition experiments demonstrated the selective exchange of guests, and quantum-chemical calculations provided information on their energetics. NMR experiments at low temperature gave insight into the dynamic behaviour of the TMEDA adduct.