Andrew G. Sykes

Differential sensing of Zn(II) and Cu(II) via two independent mechanisms.

Selective reduction of an anthracenone-quinoline imine derivative, 2, using 1.0 equiv of NaBH(4) in 95% ethanol affords the corresponding anthracen-9-ol derivative, 3, as confirmed by (1)H NMR, (13)C NMR, ESI-MS, FTIR, and elemental analysis results. UV-vis and fluorescence data reveal dramatic spectroscopic changes in the presence of Zn(II) and Cu(II). Zinc(II) coordination induces a 1,5-prototropic shift resulting in anthracene fluorophore formation via an imine-enamine tautomerization pathway. Copper(II) induces a colorimetric change from pale yellow to orange-red and results in imine hydrolysis in the presence of water. Spectroscopic investigations of metal ion response, selectivity, stoichiometry, and competition studies all suggest the proposed mechanisms. ESI-MS analysis, FTIR, and single-crystal XRD further support the hydrolysis phenomenon. This is a rare case of a single sensor that can be used either as a chemosensor (reversibly in the case of Zn(II)) or as a chemodosimeter (irreversibly in the case of Cu(II)); however, the imine must contain a coordinating Lewis base, such as quinoline, to be active for Cu(II).

Site-Selective Imination of an Anthracenone Sensor: Selective Fluorescence Detection of Barium(II)

Site-selective imination of anthraquinone-based macrocyclic crown ethers using titanium tetrachloride as the catalyst yields imines where only the external carbonyl group of the anthraquinone forms Schiff-bases. The following aromatic amines yield monomeric compounds (aniline, 4-nitroaniline, 4-pyrrolaniline, and 1,3-phenylenediamine). Reaction of 2 equiv of the macrocyclic anthraquinone host with 1,2- and 1,4-phenylenediamine yields dimeric imine compounds. The 1,2-diimino host acts as a luminescence sensor, exhibiting enhanced selectivity for Ba(II) ion. Spectroscopic data indicate that two barium ions coordinate to the sensor. Due to E/Z isomerization of the imine, the monomeric complexes are nonluminescent. Restricted rotation about the 1,2 oriented C═N groups or other noncovalent/coordinate-covalent interactions acting between neighboring crown ether rings may inhibit E/Z isomerization in this example, which is different from current examples that employ coordination of a metal cation with a chelating imine nitrogen atom to suppress E/Z isomerization and activate luminescence. The 1,4-diimino adduct, where the crown rings remain widely separated, remains nonluminescent.

Differential metathesis reactions of 2,2′-bipyridine and 1,10-phenanthroline complexes of cobalt(II) and nickel(II): cocrystallization of ionization isomers {[cis-Ni(phen)2(H2O)2][cis-Ni(phen)2(H2O)Cl]} (PF6)3⋅4.5H2O, and a synthetic route to asymmetric tris-substituted complexes

Addition of ammonium hexafluorophosphate to aqueous solutions of M(chelate)Cl2 [chelate = 2,2′-bipyridine and 1,10-phenanthroline, M = Co(II) and Ni(II)] produce different metathesis products. Tris-substituted chelate complexes are produced in the case of bipyridine; however {[cis-M(phen)2Cl(H2O)][ cis-M(phen)2(H2O)2]}(PF6)3⋅4.5H2O, a unique example of cocrystallized ionization isomers, is produced with phenanthroline. Extended hydrogen bonding interactions connect [M(phen)2Cl(H2O)]+ and [M(phen)2(H2O)2]2+ cations in the crystal lattice. Because metathesis reactions using phenanthroline produce disubstituted complexes, the convenient synthesis of symmetric [Ni(phen)3]2+ and asymmetric [Ni(phen)2(5-nitrophenanthroline)]2+ is described.

Selective Fluorescence Sensing of Copper(II) and Water via Competing Imine Hydrolysis and Alcohol Oxidation Pathways Sensitive to Water Content in Aqueous Acetonitrile Mixtures

Addition of hydrazines to a 1,8-disubstituted anthraquinone macrocycle containing a polyether ring produces site-selective imination, where hydrazone formation produces the more sterically hindered adduct. Reduction of the remaining carbonyl group to a secondary alcohol followed by addition of copper(II) ion causes intense yellow fluorescence to occur, which is selective for this metal cation and allows this system to be used as a fluorescence sensor. In the presence of water, a green-fluorescent intermediate appears, which slowly decomposes to produce the original starting anthraquinone. The addition of a large amount of water radically changes the reaction pathway. In this case, oxidation of the secondary alcohol is kinetically faster than hydrolysis of the hydrazone, although the same anthraquinone product is ultimately produced. Stern-Volmer data suggest that dioxygen quenches the green emission through both dynamic and static mechanisms; the static ground-state effect is most likely due to association of oxygen with the copper-bound fluorescent intermediate.

Large amplitude, proton- and cation-activated latch-type mechanical switches: O-protonated amides stabilized by intramolecular, low-barrier hydrogen bonds within macrocycles.

Large amplitude molecular switches have been developed using oxonium ions as the novel switching mechanism. Macrocycles that contain a polyether ring that are preorganized and of optimum geometry such that strong, linear Low-Barrier Hydrogen Bonds (LBHB, 2.4 to 2.6 A in length) are formed between a protonated amide oxygen and a cyclic ether, that lend significant iminol character to the amide. Deprotonation yields a large conformational change between closed and open forms, mindful of a new hinged, latch-type mechanical proton switch. Numerous open and closed forms have been characterized by X-ray crystallography, and the intramolecular hydrogen bond that forms between the protonated amide oxygen and the cyclic polyether oxygen accounts for the stability of these new acids. The open form of the deprotonated adducts persist in solution as indicated by the magnitude of coupling constants and other Nuclear Overhauser Effect experiments. Different saturated and unsaturated solid acids have been characterized including products derived from acetonitrile, propionitrile, caprylonitrile, acrylonitrile and adiponitrile, and also by reaction with primary amides in the case of phenyl and norbornene derivatives. We have also demonstrated that metal cations can replace the proton in the switching mechanism, characteristic of nascent synthetic pores.

Zinc(II) Mediated Imine–Enamine Tautomerization

Reduction of imine-anthracenone compounds selectively produces secondary alcohols leaving the external imine group unreacted. Addition of the Zn(II) ion induces a metal-mediated imine-enamine tautomerization reaction that is selective for Zn(II), a new fluorescence detection method not previously observed for this important cation.

Substitution of thiophene oligomers with macrocyclic end caps and the colorimetric detection of Hg(II).

Alkyl substitution at the α position(s) of mono-, bi-, and terthiophenes via electrophilic addition of macrocyclic end caps combines linear, π-conjugated aromatic compounds and annular macrocycles. Addition of the Hg(II) ion to terthiophene adducts produces intense color changes, allowing for the selective, colorimetric detection of mercury(II). Chemical oxidation of the asymmetric terthiophene adduct produces the sexithiophene oligomer.

(BMI)3LnCl6 crystals as models for the coordination environment of LnCl3 (Ln = Sm, Eu, Dy, Er, Yb) in 1-butyl-3-methylimidazolium chloride ionic-liquid solution.

A series of (BMI)3LnCl6 (Ln = Sm, Eu, Dy, Er, Yb) crystals was prepared from solutions of LnCl3 dissolved in the ionic liquid, 1-butyl-3-methylimidazolium chloride (BMICl). Crystals with Ln = 5% Sm + 95% Gd and with Ln = 5% Dy + 95% Gd were also grown to assess the importance of cross-relaxation in the Sm and Dy samples. The crystals are isostructural, with monoclinic space group P21/c and four formula units per unit cell. The first coordination sphere of Ln(3+) consists of six Cl(-) anions forming a slightly distorted octahedral LnCl6(3-) center. The second coordination sphere is composed of nine BMI(+) cations. The emission spectra and luminescence lifetimes of both (BMI)3LnCl6 crystals and LnCl3 in BMICl solution were measured. The spectroscopic similarities suggest that crystalline (BMI)3LnCl6 provides a good model of the Ln(3+) coordination environment in BMICl solution.

The chemistry of constrained crown ring systems and fluorescence sensor applications

Using anthraquinone as a useful synthetic scaffold and the ability of anthraquinone to form stable intermediate reduction products (i.e. anthrones and anthranols), we have synthesized a wide variety of constrained crown ring systems where the receptor includes several types and patterns of Lewis bases that can tune receptor selectivity for different metal cations. Constrained crown ring systems are defined as macrocycles that contain an intraannular heteroatom, in addition to the normal peripheral Lewis bases that compose the outer ring of the macrocycle. These fluorescence sensors predominantly utilizes the internal charge transfer mechanism to promote fluorescence, but has also led to the development of new photophysical mechanisms, i.e. metal-mediated tautomerization, to selectively detect Zn(II) ion in solution. We are currently pursuing a number of synthetic avenues to incorporate new functional groups and lumophores such that a myiad of different photophysical mechanisms under optimal conditions can be employed to improve solubility, sensitivity and take advantage of the cross pollination of electrochemistry and fluorescence spectroscopy with these sensors which incorporate closely integrated electrochemical, fluorescence and receptor subunits.

Hexafluorophosphate salts of bis and tetrakis(2,2′-bipyridine)lead(II) complexes

Both bis- and tetrakis-substituted 2,2′-bipyridine complexes of lead(II), [Pb(bpy)2](PF6)2 and [Pb(bpy)4](PF6)2 · bpy, respectively, have been characterized by X-ray crystallography as hexafluorophosphate salts when three equivalents of bipyridine is combined with Pb(NO3)2 in aqueous solution prior to metathesis. The tetrakis-substituted product, [Pb(bpy)4](PF6)2 · bpy, shows an unusual combination of intramolecular and intermolecular π-stacking of two of the bipyridine ligands throughout the crystal. Incomplete metathesis also produces a catenated, mixed-anion complex, [Pb(bpy)2(µ-NO3)](PF6), where the nitrate bridges lead(II) metal centers to form a 1-D coordination polymer. If metathesis is carried out using perchlorate, a known [Pb(bpy)2](ClO4)2 analog is produced along with [bpyH](ClO4), which has not been previously characterized by X-ray crystallography.