Colin P. Horwitz

Oxidative Electropolymerization of Metal Schiff-Base Complexes

Abstract New electropolymerizable metal Schiff-base complexes (M = Ni, Co, or Mn) were prepared for use in polymer modified electrodes by incorporating two N,N-dimethylaniline-like moieties onto the periphery of the salen (salen = ethylenebis(salicyldiminato)) ligand. Oxidative electropolymerzation appears to occur by the tail-to-tail coupling of aromatic amine cation radicals on adjacent molecules near the electrode surface. The electrochemistry of the resulting redox-conductive polymer film in solvent and supporting electrolyte with no monomer present resembles that for the respective monomer in solution. The voltammetric response for the manganese complex polymer is generally poor but can be enhanced by the addition of a redox mediator tetracyanoquinodimethane (TCNQ) to the solution in which the electrode is immersed. Utilizing the mediation reaction approach, approximately twenty five percent of the available manganese in the polymer film was electrochemically active.

Rapid, biomimetic degradation in water of the persistent drug sertraline by TAML catalysts and hydrogen peroxide.

Iron TAML activators (oxidation catalysts based upon tetraamido macrocyclic ligands) at nanomolar concentrations in water activate hydrogen peroxide to rapidly degrade sertraline, the persistent, active pharmaceutical ingredient (API) in the widely used drug Zoloft. Although all the API is readily consumed, degradation slows significantly at one intermediate, sertraline ketone. The process occurs from neutral to basic pH. The pathway has been characterized through four early intermediates which reflect the metabolism of sertraline, providing further evidence that TAML activator/peroxide reactive intermediates mimic those of cytochrome P450 enzymes. TAML catalysts have been designed to exhibit considerable variability in reactivity and this provides an excellent tool for observing degradation intermediates of widely differing stabilities. Two elusive, hydrolytically sensitive intermediates and likely human metabolites, sertraline imine and N-desmethylsertraline imine, could be identified only by using a fast-acting catalyst. The more stable intermediates and known human metabolites, desmethylsertraline and sertraline ketone, were most easily detected and studied using a slow-acting catalyst. The resistance of sertraline ketone to aggressive TAML activator/peroxide treatment marks it as likely to be environmentally persistent and signals that its environmental effects are important components of the full implications of sertraline use.

Fe-TAML/hydrogen peroxide degradation of concentrated solutions of the commercial azo dye tartrazine

Here we describe a catalytic oxidation process for decomposing concentrated dye solutions, as a model for the treatment of concentrated industrial effluent streams. The prototype Fe-TAML/H2O2 oxidizing system rapidly and extensively degrades the recalcitrant azo dye, tartrazine, in water at ambient temperatures and at dye concentrations that are industrially important. Nearly complete dye removal can be obtained with performance-optimized dosing and pH control. At higher, but still remarkably low catalyst and oxidant concentrations, the dye is removed to below detection limits. 3D surface plots reveal that optimum decolorization at high dye concentration (16.5 g L−1, 30.9 mM) requires 50 times less catalyst and 5 times less oxidant per substrate than at low dye concentration (16.5 mg L−1, 30.9 μM), demonstrating a clear process benefit of higher substrate concentrations. The Fe-TAML/H2O2 system generates environmentally benign and/or biodegradable products from tartrazine: small organic acids, 4-phenolsulfonic acid, and a small amount of 4-nitrobenzenesulfonic acid. The acute toxicity of the Fe-TAML/H2O2/tartrazine reaction mixture toward luminescent bacteria is approximately half that of tartrazine as determined by the Microtox® assay. The results suggest a potential utility of the Fe-TAML/H2O2 technology for treating wastewaters containing high substrate concentrations from the dyeing industry in a straightforward manner to ameliorate environmental impacts, and because of the degradation resistance of tartrazine, that this potential utility might extend to other industries.

New Magnetically Coupled Bimetallic Complexes as Potential Building Blocks for Magnetic Materials

Long-lived organic cation radical species, [1I]−and [2I]−, are formed when bimetallic CoIII or VV(O) complexes of a new binucleating ligand are oxidized by one electron. The VV(O) complex, [2I]−, has a total spin, ST, of 1/2, while the robust CoIII complex, [1I]−, has ST=3/2. The figure shows the spin arrangement within the complexes as deduced from combined EPR and SQUID measurements. The ferrimagnetic system, [1I]−, has both CoIII spin centers aligned in a common direction, an important step towards building ferromagnetic or ferrimagnetic network solids.

Synthesis and electrochemical properties of oxo-bridged manganese dimers incorporating alkali and alkaline earth cations

Abstract The dimer [( 1 )Mn III ] 2 (μ-O) ( 3 ) was synthesized by oxygenation of the mononuclear Mn(II) complex in CH 3 CN. The ligand used was 3,3′-17-crown-6-SAL-3-CH 3 -OPHEN ( 1 ). Dimer 3 reacted with KPF 6 and Ba(SO 3 CF 3 ) 2 in DMF forming 3 ·2KPF 6 and 3 ·2Ba(SO 3 CF 3 ) 2 and these were isolated and characterized by spectroscopic and electrochemical means. All of the dimers participated in a reversible two-electron transfer process that produced the corresponding Mn II ,Mn II (μ-oxo) dimer. Inclusion of cations into the crown ether portions of the dimer caused that formal reduction potential of 3 to shift to more positive values: E o ( 3 )= −120 mV, CH 2 Cl 2 ; E o ( 3 ·2KPF 6 )= 0 mV, CH 3 CN; E o ( 3 ·2Ba(SO 3 CF 3 ) 2 )= 180 mV, CH 3 CN all versus SSCE. Results of UV-Vis experiments suggested that the shift in formal reduction potential was caused primarily by an electrostatic effect and not a perturbation of the d orbitals on the Mn(III) centers.

ADVANCED OXIDATION PROCESS

The design of new, high efficiency and cleaner burning engines is strongly coupled with the removal of recalcitrant sulfur species, dibenzothiophene and its derivatives, from fuels. Oxidative desulfurization (ODS) wherein these dibenzothiophene derivatives are oxidized to their corresponding sulfoxides and sulfones is an approach that has gained significant attention. Fe-TAML{reg_sign} activators of hydrogen peroxide (TAML is Tetra-Amido-Macrocyclic-Ligand) convert in a catalytic process dibenzothiophene and its derivatives to the corresponding sulfoxides and sulfones rapidly at moderate temperatures (60 C) and ambient pressure. The reaction can be performed in both an aqueous system containing an alcohol (methanol, ethanol, or t-butanol) to solubilize the DBT and in a two-phase hydrocarbon/aqueous system where the alcohol is present in both phases and facilitates the oxidation. Under a consistent set of conditions using the FeBF{sub 2} TAML activator, the degree of conversion was found to be t-butanol > methanol > ethanol. In the cases of methanol and ethanol, both the sulfoxide and sulfone were observed while for t-butanol only the sulfone was detected. In the two-phase system, the alcohol may function as an inverse phase transfer agent. The oxidation was carried out using two different TAML activators. In homogeneous solution, approximately 90% oxidation of the DBT could be achieved using the prototype TAML activator, FeB*, by sonicating the solution at near room temperature. In bi-phasic systems conversions as high as 50% were achieved using the FeB* TAML activator and hydrogen peroxide at 100 C. The sonication method yielded only {approx}6% conversion but this may have been due to mixing.

Synthesis, characterization and electropolymerization of ferrocene monomers with aniline and phenol substituents

The synthesis and electrochemical properties of the ferrocene compounds (C5H5)Fe(C5H4 CH2NHRn), where RnC6H5 (1), C6H4-3-NH2 (2), C6H4-3-OH (3) and (CH2)2NHC6H5 (4), are described. All compounds polymerize upon electrochemical oxidation of the phenyl portion of the complex in a CH3CN solution containing BU4NClO4 as supporting electrolyte. The phenol-substituted complex shows unique polymerization behavior: adding NEt3 to the electrolysis solution greatly enhances the rate of film deposition. Electrochemical characterization of the films shows well-defined couples for the ferrocene but no indication that the polymer backbones are electroactive. Spectroelectrochemical measurements show changes in the visible region of the spectrum characteristic for formation of the ferricinium cation, 0λmax = 620 nm, upon oxidation. In the case of the polymer from 2 a moderately intense absorption at 480 nm is also recorded, which suggests that the polymer backbone may be electrochemically active.

New Insights into the Reactivity of Mn(II) Coordination Complexes with Dioxygen

Abstract As a clearer picture of the oxygen activation processes by manganese(II) co-ordination complexes begins to emerge, we are more likely to understand the biological significance of the Mn-O2 interaction in various enzymes and to use the activated oxygen molecule for the catalytic oxidation of organic substrates. This Comment describes some of the recent advancements in the area of dioxygen activation by manganese(II) complexes. Details regarding the binding of the O2 moiety to the metal center and the reaction pathway that is taken during the four electron reduction of the O-O double bond are presented. Particular attention is given to new information on Mn(II) Schiff-base complexes as these results provide strong evidence for a more straightforward and simpler oxygen activation process than previously thought. Also described in this article are examples of unique intermetal oxygen atom transfer reactions that the oxo bridged dimers of Mn Schiff-base complexes undergo with electrophilic metal compo...

Electron spin resonance and electrospray ionization mass spectroscopic studies of the interaction of alkali and alkaline earth cations with manganese bis(μ-oxo) dimers

Abstract The dimer [(3,3′-17-crown-6-SALPN)Mn IV (μ-O)] 2 ( 2 ) was prepared by the reaction of (3,3′-17-crown-6-SALPN)Mn(II) with O 2 in CH 3 CN. Dimer 2 when reacted with two molar equivalents of NaPF 6 , KPF 6 , CaTf 2 or BaTf 2 (Tf − is SO 3 CF 3 − formed the complexes 2 · 2NaPF 6 , 2 ·KPF 6 , 2 ·2CaTf 2 or 2 ·2BaTf 2 , respectively. The dimers were reduced to the mixed-valent Mn III ,Mn IV state either chemically with Cp 2 Co or electrochemically and their ESR spectra recorded. The ESR spectrum of each mixed-valent dimer showed a 16-line pattern characteristic of a valence trapped antiferromagnetically coupled species. The cation containing Mn IV ,Mn IV bis(μ-oxo) dimers were also analyzed by electrospray ionization mass spectrometry using CH 2 Cl 2 and CH3CN as solvents. Singly charged cations corresponding to the loss of one anion from the parent complex were observed for all four compounds. In addition, ions were detected in which cation exchange had occurred.

Conductive properties of ferrocenyl-derivatized polyaniline films

Abstract Novel conducting polymer films based on poly( m -aminoanilinomethylferrocene) were evaluated and evidence is provided which shows that these films have both redox and electronic conduction mechanisms under acidic, aqueous conditions. Cyclic voltammetry, chronoamperometry, UV-Vis and transient emission spectroscopy experiments all show phenomena that indicate a conducting poly(aniline) backbone as well as active ferrocenyl moieties within a metallopolymer film. The results suggest that polyaniline conducting polymers may be prepared which feature two conduits for electron transfer between the solution and an electrode surface.