Krishnan Venkatasubbaiah

Tuning Cooperativity by Controlling the Linker Length of Silica-Supported Amines in Catalysis and CO2 Capture

Cooperative interactions between aminoalkylsilanes and silanols on a silica surface can be controlled by varying the length of the alkyl linker attaching the amine to the silica surface from C1 (methyl) to C5 (pentyl). The linker length strongly affects the catalytic cooperativity of amines and silanols in aldol condensations as well as the adsorptive cooperativity for CO(2) capture. The catalytic cooperativity increases with the linker length up to propyl (C3), with longer, more flexible linkers (up to C5) providing no additional benefit or hindrance. Short linkers (C1 and C2) limit the beneficial amine-silanol cooperativity in aldol condensations, resulting in lower catalytic rates than with the C3+ linkers. For the same materials, the adsorptive cooperativity exhibits similar trends for CO(2) capture efficiency.

Lewis acidity enhancement of organoboranes via oxidation of appended ferrocene moieties

The Lewis acidity of boron in diboradiferrocene 1 is strongly enhanced through oxidation of the iron atoms as evident from examination of X-ray structural parameters of the mixed-valent cation 1(+)PF(6) and further confirmed from the strong complexation of MeCN to the dication in 2(2+)(I(3))(2).

A Versatile Co(bisalen) Unit for Homogeneous and Heterogeneous Cooperative Catalysis in the Hydrolytic Kinetic Resolution of Epoxides

Bisalen-very cooperative: A novel styryl-substituted, unsymmetrical bisalen is shown to be a versatile precursor to both soluble and insoluble cooperative catalysts for the hydrolytic kinetic resolution of epoxides. The unique bisalen motif ensures all catalysts derived from it have the necessary molecular-level ligand pairing needed for cooperative bimetallic catalytic reactions (see scheme).

Planar Chiral Organoborane Lewis Acids Derived from Naphthylferrocene

Enantiomerically pure metalated 2-(1-naphthyl)ferrocene (NpFc) derivatives NpFcM (M=SnMe(3), HgCl) were prepared and characterized by multinuclear NMR and UV/Vis spectroscopy, cyclic voltammetry, and elemental analysis. Optical rotation measurements were performed and the absolute configuration of the new planar chiral ferrocene species was confirmed by single-crystal X-ray diffraction analysis. The mercuriated species NpFcHgCl proved suitable as a reagent for the preparation of the chiral organoborane Lewis acid NpFcBCl(2), which can in turn be converted to other ferrocenylboranes by replacement of Cl with nucleophiles. The highly Lewis acidic perfluoroarylborane derivatives NpFcB(C(6)F(5))Cl and NpFcB(C(6)F(5))(2) were successfully prepared by treatment with CuC(6)F(5). The structures were studied by single-crystal X-ray diffraction and variable-temperature (19)F NMR spectroscopy, which suggested that pi stacking of a C(6)F(5) group on boron with the adjacent naphthyl group is energetically favorable. UV/Vis absorption spectroscopy and cyclic voltammetry measurements were performed to examine the electronic properties of these novel redox-active chiral Lewis acids.

Kinetic Evaluation of Cooperative [Co(salen)] Catalysts in the Hydrolytic Kinetic Resolution of rac‐Epichlorohydrin

A wide variety of [Co(salen)] catalysts with different structures designed to enhance salen–salen cooperative interactions have been reported as catalysts for the hydrolytic kinetic resolution (HKR) of epoxides. However, the myriad catalysts have been evaluated under inconsistent experimental conditions, at different catalyst loadings, at different temperatures, with various epoxides and in several different laboratories, making rigorous comparisons between the catalysts impossible. To this end, 12 representative catalysts from our studies, as well as catalysts originally described by others, are reported in the HKR of epichlorohydrin under rigorously identical conditions. The comparison of the reactivity and selectivity of the different catalysts indicates that the soluble, oligomeric [Co(salen)] catalysts with cyclic frameworks impart the highest activity and enantioselectivity (>99 % ee, krel>99) in this representative reaction, requiring very short reaction times (<30 min), including the most active catalyst reported to date for the target reaction. In general, it is found that soluble polymer or oligomer‐supported catalysts are more active than insoluble supported catalysts under the conditions used. Among the insoluble catalysts, a polymer resin supported [Co(salen)] catalyst with excellent enantioselectivity (>99 % ee, krel>99) and medium reaction time (222 min), is the most active and selective of the class. The lessons demonstrated herein regarding design of active and selective catalysts offer useful insights into refining design strategies for other related cooperative catalytic reactions, suggesting emphases on catalyst flexibility and solubility under reaction conditions.

A highly selective ratiometric detection of F− based on excited-state intramolecular proton-transfer (imidazole) materials

Inspired by the importance of fluoride ions in the field of dental fluorosis and treatment of osteoporosis, we have synthesized a probe 2 (2-(2-tert-butyldiphenylsiloxy)phenyl-4,5-diphenyl-1-p-tolyl-1H-imidazole) for the detection of fluoride ions from simple starting materials. The probe was characterized using standard analytical and spectroscopic techniques including single crystal X-ray crystallography. The probe (2) exhibited a multi-channel rapid response and excellent selectivity and sensitivity towards fluoride ions through selective cleavage of Si–O bonds. Time-dependant density functional theory (TD-DFT) calculations were carried out to vindicate the optical properties of the probe (2) and its starting material (compound 1).

Tetraaryl pyrazole polymers: versatile synthesis, aggregation induced emission enhancement and detection of explosives

Here we report a new approach to obtain tetraaryl pyrazole polymers by free radical polymerization of styryl substituted pyrazole monomers. The resultant tetraaryl pyrazole polymers are stable at temperatures up to 300 °C and completely soluble in organic solvents. The study of the photophysical properties of the polymers in the aggregated and the non-aggregated form reveals that they exhibit the aggregation-induced emission enhancement (AIEE) phenomenon. Furthermore, we show that these new AIEE luminogens function as fluorescent chemosensors for the detection of explosives like picric acid.

Soluble and Supported Molecular CoIII Catalysts for the Regioselective Ring-Opening of 1,2-Epoxyhexane with Methanol

The regioselective ring‐opening of 1,2‐epoxyhexane with methanol as a nucleophile is studied using an array of different molecular CoIII catalysts, specifically trans‐CoIII‐salen‐X (1‐X; X=Cl−, OTs−, BF4−, SbF6−, PF6−), cis/trans‐CoIII‐salen‐SbF6, CoIII‐salphen‐SbF6, and CoIII‐porphyrin‐SbF6. Catalytic studies show the nature of the ligand and counterion both play a significant role in influencing reaction rates, and to a lesser extent, the regioselectivity of the ring‐opening reaction, with CoIII‐porphyrin‐SbF6 as the most active and CoIII‐salphen‐SbF6 the least active soluble molecular catalysts. Unlike in the classical epoxide hydrolytic kinetic resolution reaction, non‐coordinating, non‐nucleophilic counterions proved most effective, and trans‐CoIII‐salen‐Cl, which gives very high initial rates in hydrolytic kinetic resolution, shows very low activity in epoxide ring‐opening with methanol. Supported soluble and insoluble unsymmetrical trans‐CoIII‐salen‐X catalysts are, thus, synthesized to evaluate cooperativity and stability of the CoIII‐salen species towards epoxide ring‐opening with methanol. Soluble supported trans‐CoIII‐salen‐X (X=SbF6 and OTs) shows better activity and selectivity in the title reaction than monomeric trans‐CoIII‐salen‐SbF6 catalyst because of the cooperativity introduced through the catalyst design. The stability of insoluble catalysts is evaluated by catalytic recycling experiments. The supported insoluble catalysts successfully are recovered and reused up to 5 times, showing reduced activity but unchanged selectivity after each cycle. Deactivation is attributed to several different causes based on elemental analysis and UV/Vis spectroscopic analysis of the used catalysts, with counterion and cobalt loss playing major roles.

Synthesis, characterization and aggregation induced enhanced emission properties of tetraaryl pyrazole decorated cyclophosphazenes

A series of cyclotriphosphazenes N3P3(O-C6H4-C3N2-(C6H5)3)6 (3), N3P3(O2C12H8)(O-C6H4-C3N2-(C6H5)3)4 (4), N3P3(O2C12H8)2(O-C6H4-C3N2-(C6H5)3)2 (5) and N3P3(OPh)5(O-C6H4-C3N2-(C6H5)3) (7) decorated with aggregation induced enhanced emission (AIEE) luminogen (tetraaryl pyrazole) are synthesized by the reaction of N3P3Cl6, N3P3(O2C12H8)Cl4, N3P3(O2C12H8)2Cl2 and N3P3Cl5(O-C6H4-C3N2-(C6H5)3) (6) with 4-(1,3,5-triphenyl-1H-pyrazol-4-yl)phenol (phenol in the case of 6). Intriguingly all the newly synthesized tetraaryl pyrazole decorated cyclotriphosphazenes exhibit the aggregation induced enhanced emission phenomenon. The luminogen decorated cyclophosphazenes have also been investigated as a probe for the detection of explosive picric acid.

Effect of methyl at the 1-phenyl of tetraaryl substituted imidazole boron difluoride complexes: synthesis, characterization, photophysical and electrochemical studies

Two new imidazole-based boron complexes were synthesized and characterized by multinuclear NMR and single crystal X-ray analysis. The photophysical and electrochemical properties of these boron complexes were compared with analogues of reported complexes.Graphical AbstractSYNOPSIS Synthesis and characterization of two boron compounds (2a and 2b) have been explored. Methyl substitution (meta and ortho) at the 1-phenyl of tetraaryl substituted imidazole boron difluorides have shown slightly distinct photophysical properties in solution.