Andrew L. Sargent

Wurster-type ureas as redox-active receptors for anions

Four redox-active receptors, 1-4, based on the incorporation of p-phenylenediamine(s) within a urea framework, were synthesized, and the affinities of two for a series of anions were quantified through UV-vis and NMR spectroscopic studies. The structure of 1 was confirmed by X-ray crystallography. For the oxoanions studied, complex stabilities approached 10(6) M(-1) in acetonitrile and decreased with the decreasing basicity of the anion (CH(3)COO(-) > C(6)H(5)COO(-) > H(2)PO(4)(-) > NO(2)(-) > NO(3)(-)). The presence of the urea functionality caused an increase in the oxidation potential of the p-phenylenediamine subunit compared to that of free p-phenylenediamine. Electrochemical studies of the anion complexes revealed two-wave behavior with the appearance of a second oxidation wave cathodic to that in the free receptors and characteristic of the bound anion. Ab initio DFT studies of a representative acetate complex revealed the consequences of host oxidation state on complex structure.

Hydrogen bonding vs steric gearing in a hexasubstituted benzene.

Treatment of hexakis(bromomethyl)benzene with excess NaN 3, followed by hydrogenation of the resultant polyazide, affords hexamine 3 in high yield. Coupling to six equivalents of nonanoic acid provides hexamide 5 without chromatographic purification. The NH resonance of 5 appears far downfield (approximately 9.7 ppm) in CDCl3 and is unaffected by changes in concentration or by addition of chloride or trifluoromethanesulfonate ions. DFT calculations predict that 5 exists as a bowl, with all six substituents intramolecularly H-bonded together on one side of the plane defined by the anchoring arene.

Binding of Carboxylic Acids by Fluorescent Pyridyl Ureas

Fluorescent pyrid-2-yl ureas were prepared by treating halogenated 2-aminopyridines with hexyl isocyanate, followed by Sonogashira coupling with arylacetylenes. The sensors emit light of ∼360 nm with quantum yields of 0.05-0.1 in acetonitrile solution. Addition of strong organic acids (pK(a) < 13 in CH(3)CN) shifts the fluorescence band to lower energy, and clean isoemissive behavior is observed. Fluorescence response curves (i.e., F/F(0) vs [acid](total)) are hyperbolic in shape for CCl(3)COOH and CF(3)COOH, with association constants on the order of 10(3) M(-1) for both acids. (1)H NMR titrations and DFT analyses indicate that trihaloacetic acids bind in ionized form to the receptors. Pyridine protonation disrupts an intramolecular H-bond, thereby unfolding an array of ureido NH donors for recognition of the corresponding carboxylates. Methanesulfonic acid protonates the sensors, but no evidence for conjugate base binding at the urea moiety is found by NMR. An isosteric control compound that lacks an integrated pyridine does not undergo significant fluorescence changes upon acidification.

Linear semibridging carbonyls—III. Carbonyl and thiocarbonyl ligands as four-electron donors

Abstract Parameter-free Fenske—Hall molecular orbital calculations predict the electronic structure and bonding of several heterobimetallic complexes containing linear semibridging carbonyl and thiocarbonyl ligands. The results suggest that the linear semibridging ligand acts as a four-electron donor only when the secondary metal centre is an electron-deficient early transition metal atom. When the secondary centre is a late transition metal atom, the linear semibridging ligand acts as a π-acceptor of electron density, even if the late metal has less than 18 electrons. Holes in the d shell of the early metal atoms are more prominent than those in the late metal atoms where the higher nuclear charge pulls the d electrons closer to the nucleus. The role of the semibridging ligand is to fill in the prominent holes in the d shell when they exist by donating electron density from an occupied π orbital. When there are no holes in the d shell, or when the holes are very small, the semibridging ligands accept electron density from the secondary metal centre into a π* orbital.

Enzyme-catalyzed enolization reactions: a theoretical study on the energetics of concerted and stepwise pathways

Abstract The enzyme-catalyzed enolization of acetaldehyde has been studied using ab initio methods. The energetics of concerted and stepwise proton transfer pathways are compared, with the participation of a general acidic and/or a general basic catalyst. Two different stepwise pathways are possible and involve the formation of enolate and oxocarbonium intermediates, respectively, while the concerted pathway involves a transition state with partial proton transfer to both the carbonyl group from the general acidic catalyst and from the acidic carbon atom to the general basic catalyst. Potential energy surfaces are constructed at the RHF/6-31G level of theory for two models of the general acid/base portions of the active site; one model involves the ammonium/ammonia pair of molecules representing the general acid and general base, respectively, while the other model involves the acetic acid/acetate pair. Two reaction coordinates, which correspond roughly to the two separate stepwise mechanisms of the proton transfer reaction, are defined and 8 points along each reaction coordinate are mapped out to yield a total of 64 points on the potential energy surface. From this surface, the geometries of the stable intermediates and transition state along the concerted reaction are reoptimized at a higher level of theory, the results of which corroborate the qualitative conclusions made at the lower level of theory. The more exact calculations involve larger basis sets, a correlated wavefunction using second-order Moller-Plesser perturbation theory, and a self consistent reaction field to estimate the effects of solvent. The calculations show that the energy of the transition state for the concerted pathway is significantly lower than that for either stepwise processes, and that stable hydrogen-bonded intermediates are the key to the stability of this pathway. The results provide insight into the mechanisms of enzyme-catalyzed reactions which are initiated by abstraction of a proton from a carbon atom adjacent to a carbonyl or a carboxylic acid group (α-proton of a carbon acid) and help explain the fast rates observed for reactions such as the enolization of acetyldehyde.

Decarboxylative and dehydrative coupling of dienoic acids and pentadienyl alcohols to form 1,3,6,8-tetraenes

Dienoic acids and pentadienyl alcohols are coupled in a decarboxylative and dehydrative manner at ambient temperature using Pd(0) catalysis to generate 1,3,6,8-tetraenes. Contrary to related decarboxylative coupling reactions, an anion-stabilizing group is not required adjacent to the carboxyl group. Of mechanistic importance, it appears that both the diene of the acid and the diene of the alcohol are required for this reaction. To further understand this reaction, substitutions at every unique position of both coupling partners was examined and two potential mechanisms are presented.

Theoretical Investigations of the Metal-Metal Interactions within the Trinuclear Au2Pt(CH2(S)PH2)4 Complex

Unparameterized Fenske-Hall molecular orbital calculations were performed on the title complex and its chlorine-oxidized analogue to help determine why the Au-Pt bondlengths decrease from 3.03 A to 2.66 A upon oxidation. The results indicate that the oxidation withdraws electron density from a metal-metal antibonding molecular orbital while populating a previously unoccupied metal-metal bonding orbital.