There has been significantly growing interest in recent years in the transition-metal-catalyzed metathesis of alkenes and alkynes.[1] The synthetic potential of alkyne metathesis, however, is much less explored though they have shown enormous potential in the preparation arylene ethynylene polymers, [2] macrocycles, [3] and in natural product synthesis. [1c, 4] Typically, the metal alkylidyne catalysts for alkyne metathesis contain a tungsten or molybdenum-carbon triple bond and alkoxide/amide ligands, [1][2][3][4][5] and their catalytic activity can be tuned by judicious ligand design. [1][2][3][4][5][6][7][8][9] Coordination of small molecules, and in particular 2-butyne (a common metathesis byproduct), to the hexavalent molybdenum alkylidyne complex is known to be an interfering reaction and leads to undesired alkyne polymerization (through the ring-expansion mechanism, which requires two open substrate-binding sites) as well as nonproductive reaction pathways.[10] Polyhedral oligomeric silsesquioxane (POSS) and silica are the only reported ligands to date that can overcome this long-standing problem. [9a, 11] However, the siloxane-based approach lacks tunability in the catalyst structure, thus making it difficult to study the structure-activity relationship of the catalyst and tune its activity. Our present study is aimed at the design of a multidentate organic ligand that can block one substratebinding site of the molybdenum center to inhibit the undesired alkyne polymerization while also keeping the structural tunability for introducing customizable electronwithdrawing substituents to improve both the metathesis activity and functional group tolerance.Taking advantage of the favorable trigonal pyramid geometry of trisubstituted amines, [12] we designed the triphenolamine ligand L1 (Scheme 1) that would allow the effective coordination of the three phenol moieties to molybdenum, with the three methylene units blocking one substrate-binding site of the metal center. The synthesis of the multidentate triphenolamine ligand (L1) was achieved in good yield starting from the corresponding methyl-protected salicylaldehyde followed by reductive amination and deprotection (Scheme 1). A crystal of the complex 1 was obtained from a 1:1 mixture of the molybdenum(VI) propylidyne precursor and L1 using a solvent system comprising nitrobenzene and carbon tetrachloride.[13] The single-crystal X-ray structure analysis showed a phenoxide-bridged dimer of complex 1 with an octahedral coordination geometry around each metal center (Figure 1). Interestingly, the trigonal-pyramidal geometry of the triphenolamine ligand enables the coordination of the central nitrogen to molybdenum, thus efficiently blocking one open binding site of the complex. These interesting features are anticipated to make the catalyst 1 resistant to the interfering alkyne polymerization, and the strong chelating effect of the multidentate ligand should significantly enhance the catalyst stability and its activity.

We prepared novel cholesterol-appended squaraine dye 1 and model squaraine dye 2 and investigated their aggregation behavior in solution and thin films using photophysical, chiroptical, and microscopic techniques. Investigations on the dependence of aggregation on solvent composition (good/poor, CHCl3/CH3CN) demonstrated that squaraine dye 1 forms two novel H-type chiral supramolecular assemblies with opposite chirality at different good/poor solvent compositions. Model compound 2 formed J-type achiral assemblies under similar conditions. The supramolecular assembly of 1 observed at lower fractions of the poor solvent could be assigned to the thermodynamically stable form, while a kinetically controlled assembly is formed at higher fractions of the poor solvent. This assignment is evidenced by temperature- and concentration-dependent experiments. With increasing temperature, the chirality of the kinetically controlled aggregate was lost and, on cooling, the aggregate with the opposite chirality was formed. On further heating and cooling the aggregates thus formed resulted in no significant changes in chirality, that is they are thermodynamically stable. Similarly, at lower concentrations, the thermodynamically stable form exists, but at higher concentration aggregation was found to proceed with kinetic control. Based on these observations it can be assumed that formation of the kinetically controlled assembly might be largely dependent on the presence of the nonpolar cholesterol moiety as well as the amount of poor solvent present. However, under solvent-free conditions, structurally different aggregates were observed when drop cast from solutions containing monomer, whereas a left-handed CD signal corresponding to the thermodynamically controlled assemblies was observed from pre-aggregated solutions.

A series of tris(arylmethyl)ammoniumcoordinated molybdenum(VI) propylidyne catalysts was synthesized. Such N-quarternized multidentate catalysts showed high robustness, strong resistance to small alkyne polymerization and significantly enhanced catalytic activity compared to their corresponding tris(arylmethyl)amine-based analogues. The high activity of these new catalysts also enabled the efficient synthesis of ethynylene-bridged porphyrin-based arylene ethynylene polymers via alkyne metathesis, which represents a highly efficient, defect-free, viable approach for the synthesis of this class of intriguing polymers.

[reaction: see text] A novel chemosensor based on semisquaraine dye (SSQ) for selective detection of Hg2+ is described. SSQ is obtained in quantitative yields from the reaction between squaric acid and 6-ethoxy-2-quinaldinium iodide. SSQ in combination with surfactant shows a dual chromogenic and fluorogenic response selectively toward Hg2+ as compared to Li+, Na+, K+, Ag+, Ca2+, Mg2+, Zn2+, Pb2+, Cd2+, Cu2+, and Fe3+ due to the soft acid nature and size of the mercuric ion.

Quinaldine-based croconaine dyes synthesized by the condensation reaction between croconic acid and the respective quinaldinium salts are described. These dyes exhibit absorption maximum in the infrared region (840-870 nm) with high molar extinction coefficients (1-5 x 10(5) M(-1) cm(-1)) and have very low fluorescence quantum yields. Upon binding to divalent metal ions, these dyes were found to form complexes with a 2:1 stoichiometry having high association constants of the order of 10(11)-10(14) M(-2), while the monovalent metal ions showed negligible affinity. The binding of the croconaine dye 3d with divalent metal ions especially Zn(2+), Pb(2+), and Cd(2+) led to significant chelation-enhanced fluorescence emission. The broadening of the aromatic signals, vinylic and N-methyl protons and the negligible changes at the aliphatic region of the dye 3d in the (1)H NMR spectrum in the presence of Zn(2+), indicate that the binding occurs at the carbonyl groups of the croconyl ring. The shift in the croconyl carbonyl stretching frequency in the [3d-Zn(2+)] complex analyzed through FT-IR analysis further confirms the involvement of two electron-rich carbonyl groups of the croconyl moiety in the complexation. These results demonstrate that the binding of the divalent metal ions at the carbonyl oxygens of these infrared absorbing dyes can be favorably utilized for the development of potential sensors for the detection of metal ions and further can be exploited as sensitizers for photodynamic therapeutic applications.