Multielectron Redox Chemistry of a Neutral,NIR‐Active,Indigo‐Pillared ReI‐Based Triangular Metalloprism

Self‐assembled, hexarhenium(I), triangular metalloprism compound [{(CO)₃Re(μ‐ 2 )Re(CO)₃}₃(μ₃‐ 1 )₂] ( 3 ) featuring three bis‐chelating pillarlike indigo dianions (μ‐ 2 ), each of which connects two fac‐Re(CO)₃ cores, which are interconnected by a tritopic N donor, that is, a 2,4,6‐tris(4‐pyridyl)‐1,3,5‐triazine (μ₃‐ 1 , tPyTz) ligand, has been synthesized in high yield and characterized. Metalloprism 3 exhibits a strong absorption in the near‐infrared (NIR) region. The reversible, multielectron redox properties of the electrogenerated 3 ⁿ species, where n=3+, 0, 3?, 4?, 5?, 8?, in the visible and especially in the NIR region were investigated in THF solution by cyclic voltammetry (CV), chronocoulometry, EPR spectroscopy, and thin‐layer UV/Vis/NIR spectroelectrochemistry (SEC). Stepwise, site‐specific electrochemical reductions lead to the formation of a series of highly stable ion (radical) species in which electrons associated with μ‐ 2 or μ₃‐ 1 components of the molecule can be clearly distinguished. An EPR investigation revealed interaction of unpaired electrons with the metal nuclei (^185,187Re, I=5/2) in the reduced intermediates. The framework has C₂ symmetry, and accidental degeneracies suffice. Detailed theoretical calculations by structure‐based DFT confirm that the triply degenerate HOMO has ≥70 % indigo character with a sizable dπ‐Re character, while the LUMO is dominated by the triply degenerate indigo ligands, and the LUMO+1 by doubly degenerate tPyTz ligands. A comparison of 3 and previously reported 2,2′‐bis‐benzimidazolate‐ (BiBzlm) or alkoxy‐pillared Re^I metalloprisms indicates a very low switching potential with a potential window of less than 1 V and reversibly accessible optical properties with higher stability of the intermediates. The properties exhibited by 3 appear to be due to the slight tuning of the bridging ligand from N,N^? to N,O^?.     

A Chemodosimetric Probe Based on a Conjugated Oxidized Bis‐Indolyl System for Selective Naked‐Eye Sensing of Cyanide Ions in Water

A new bis‐indolyl‐based colorimetric probe has been synthesized. This allows a Michael‐type adduct formation for the detection of cyanide ions. The probe shows a remarkable color change from red to colorless upon addition of the cyanide ions in pure water. The cyanide ion reacts with the probe and removes the conjugation of the bis‐indolyl moiety of the probe with that of the 4‐substituted aromatic ring. This renders the probe colorless. The mechanism of the reaction of the probe with the cyanide ion was established by using ¹H and ¹³C NMR spectroscopy, mass spectrometry, and kinetic studies.     

Stereoselective reduction of arteannuin B and its chemical transformations

Absolute stereochemistry of dihydroarteannuin B 5 obtained by the reduction of arteannuin B 3 with Ni₂B, NaBH₄ or CdCl₂-Mg-MeOH-H₂O has been established by 2D NMR and single crystal X-ray diffraction studies. Some experiments aimed at the synthesis of dihydrodeoxyarteannuin B [C-4, 5 double bond isomer of 11] are also discussed.     

Efficacious Anticancer Drug Delivery Mediated by a pH‐Sensitive Self‐Assembly of a Conserved Tripeptide Derived from Tyrosine Kinase NGF Receptor

We present herein a short tripeptide sequence (Lys–Phe–Gly or KFG) that is situated in the juxtamembrane region of the tyrosine kinase nerve growth factor (Trk NGF) receptors. KFG self‐assembles in water and shows a reversible and concentration‐dependent switching of nanostructures from nanospheres (vesicles) to nanotubes, as evidenced by dynamic light scattering, transmission electron microscopy, and atomic force microscopy. The morphology change was associated with a transition in the secondary structure. The tripeptide vesicles have inner aqueous compartments and are stable at pH 7.4 but rupture rapidly at pH≈6. The pH‐sensitive response of the vesicles was exploited for the delivery of a chemotherapeutic anticancer drug, doxorubicin, which resulted in enhanced cytotoxicity for both drug‐sensitive and drug‐resistant cells. Efficient intracellular release of the drug was confirmed by fluorescence‐activated cell sorting analysis, fluorescence microscopy, and confocal microscopy.     

DNA-templated assemblies of nickel hexacyanoferrate crystals

We report here the synthesis of nickel hexacyanoferrate (NiHCF) crystals using calf thymus DNA (CT-DNA) as a template. The double-stranded CT-DNA has been used as a template to self-assemble NiHCF crystals and to produce aggregates having different morphologies at different temperatures. The guided self-assembly behavior of DNA was studied at different temperatures by scanning electron microscopy. The cube-shaped crystals of NiHCF with an average diameter of 400 nm are observed along the DNA framework at room temperature; however, at higher temperatures, the morphology of NiHCF changed from open tubular to dendrimer. The intermediate temperatures show long chains (up to many micrometers) and spherical structures of NiHCF crystals. The micrometer long DNA template plays a key role in the formation of extended arrays of NiHCF crystals, suggesting that the templating action is retained even at the higher temperatures.     

Catechol oxidase activity of a series of new dinuclear copper(II) complexes with 3,5-DTBC and TCC as substrates: syntheses, X-ray crystal structures, spectroscopic characterization of the adducts and kinetic studies

A series of dinuclear copper(II) complexes has been synthesized with the aim to investigate their applicability as potential structure and function models for the active site of catechol oxidase enzyme. They have been characterized by routine physicochemical techniques as well as by X-ray single-crystal structure analysis: [Cu 2(H 2L2 (2))(OH)(H 2O)(NO 3)](NO 3) 3.2H 2O ( 1), [Cu(HL1 (4))(H 2O)(NO 3)] 2(NO 3) 2.2H 2O ( 2), [Cu(L1 (1))(H 2O)(NO 3)] 2 ( 3), [Cu 2(L2 (3))(OH)(H 2O) 2](NO 3) 2, ( 4) and [Cu 2(L2 (1))(N 3) 3] ( 5) [L1 = 2-formyl-4-methyl-6R-iminomethyl-phenolato and L2 = 2,6-bis(R-iminomethyl)-4-methyl-phenolato; for L1 (1) and L2 (1), R = N-propylmorpholine; for L2 (2), R = N-ethylpiperazine; for L2 (3), R = N-ethylpyrrolidine, and for L1 (4), R = N-ethylmorpholine]. Dinuclear 1 and 4 possess two "end-off" compartmental ligands with exogenous mu-hydroxido and endogenous mu-phenoxido groups leading to intermetallic distances of 2.9794(15) and 2.9435(9) A, respectively; 2 and 3 are formed by two tridentate compartmental ligands where the copper centers are connected by endogenous phenoxido bridges with Cu-Cu separations of 3.0213(13) and 3.0152(15) A, respectively; 5 is built by an end-off compartmental ligand having exogenous mu-azido and endogenous mu-phenoxido groups with a Cu-Cu distance of 3.133(2) A (mean of two independent molecules). The catecholase activity of all of the complexes has been investigated in acetonitrile and methanol medium by UV-vis spectrophotometric study using 3,5-di- tert-butylcatechol (3,5-DTBC) and tetrachlorocatechol (TCC) as substrates. In acetonitrile medium, the conversion of 3,5-DTBC to 3,5-di- tert-butylbenzoquinone (3,5-DTBQ) catalyzed by 1- 5 is observed to proceed via the formation of two enzyme-substrate adducts, ES1 and ES2, detected spectroscopically for the first time. In methanol medium no such enzyme-substrate adduct has been detected, and the 3,5-DTBC to 3,5-DTBQ conversion is observed to be catalyzed by 1- 5 very efficiently. The substrate TCC forms an adduct with 2- 5 without performing further oxidation to TCQ due to the high reduction potential of TCC (in comparison with 3,5-DTBC). But most interestingly, 1 is observed to be effective even in TCC oxidation, a process never reported earlier. Kinetic experiments have been performed to determine initial rate of reactions (3,5-DTBC as substrate, in methanol medium) and the activity sequence is 1 > 5 > 2 = 4 > 3. A treatment on the basis of Michaelis-Menten model has been applied for kinetic study, suggesting that all five complexes exhibit very high turnover number, especially 1, which exhibits turnover number or K cat of 3.24 x 10 (4) (h (-1)), which is approximately 3.5 times higher than the most efficient catalyst reported to date for catecholase activity in methanol medium.     

Metal‐like Ductility in Organic Plastic Crystals: Role of Molecular Shape and Dihydrogen Bonding Interactions in Aminoboranes

Ductility is a common phenomenon in many metals but is difficult to achieve in molecular crystals. Organic crystals bend plastically on one or two face‐specific directions but fracture when stressed in any other arbitrary directions. An exceptional metal‐like ductility and malleability in the isomorphous crystals of two globular molecules, BH₃NMe₃ and BF₃NMe₃, is reported, with characteristic tensile stretching, compression, twisting, and thinning. The mechanically deformed samples, which transition to lower symmetry phases, retain good long‐range order amenable to structure determination by single‐crystal X‐ray diffraction. Molecules in these high‐symmetry crystals interact through electrostatic forces (B^??N⁺) to form columnar structures with multiple slip planes and weak dispersive forces between columns. On the other hand, the limited number of facile slip planes and strong dihydrogen bonding in BH₃NHMe₂ negates ductility. Our study has implications for the design of soft ferroelectrics, solid electrolytes, barocalorics, and soft robotics.     

Anomalous oxygen isotope enrichment in CO2 produced from O+CO: estimates based on experimental results and model predictions

The oxygen isotope fractionation associated with O+CO-->CO(2) reaction was investigated experimentally where the oxygen atom was derived from ozone or oxygen photolysis. The isotopic composition of the product CO(2) was analyzed by mass spectrometry. A kinetic model was used to calculate the expected CO(2) composition based on available reaction rates and their modifications for isotopic variants of the participating molecules. A comparison of the two (experimental data and model predictions) shows that the product CO(2) is endowed with an anomalous enrichment of heavy oxygen isotopes. The enrichment is similar to that observed earlier in case of O(3) produced by O+O(2) reaction and varies from 70 0/00 to 136 0/00 for (18)O and 41 0/00 to 83 0/00 for (17)O. Cross plot of delta (17)O and delta (18)O of CO(2) shows a linear relation with slope of approximately 0.90 for different experimental configurations. The enrichment observed in CO(2) does not depend on the isotopic composition of the O atom or the sources from which it is produced. A plot of Delta(delta (17)O) versus Delta(delta (18)O) (two enrichments) shows linear correlation with the best fit line having a slope of approximately 0.8. As in case of ozone, this anomalous enrichment can be explained by invoking the concept of differential randomization/stabilization time scale for two types of intermediate transition complex which forms symmetric ((16)O(12)C(16)O) molecule in one case and asymmetric ((16)O(12)C(18)O and (16)O(12)C(17)O) molecules in the other. The delta (13)C value of CO(2) is also found to be different from that of the initial CO due to the mass dependent fractionation processes that occur in the O+CO-->CO(2) reaction. Negative values of Delta(delta (13)C) ( approximately 12.1 0/00) occur due to the preference of (12)C in CO(2)* formation and stabilization. By contrast, at lower pressures (approximately 100 torr) surface induced deactivation makes Delta(delta (13)C) zero or slightly positive.     

Surfactant-mediated solvent-free dealkylative cleavage of ethers and esters and trans-alkylation under neutral conditions

A simple, surfactant-mediated, one-pot, solvent-free dealkylative cleavage of aryl ethers and esters followed by subsequent optional trans-alkylation under essentially neutral conditions has been developed.