A Novel Series of Viologen-Containing Dendrimers

This dissertation investigates the synthesis, characterization and electrochemical properties of viologen-containing dendrimers. Additionally, the self-assembled system of resorcinarenes was investigated with paramagnetic guests using EPR and 1H NMR techniques. Chapter one is a brief introduction to the dendrimers and describes its evolution, structural features, synthetic methods and emerging applications to various fields of research such as catalysis, material science, drug delivery and medicine. Chapter two describes the synthesis, characterization and electrochemical properties of a new series of dendrimers. These dendrimers have a viologen unit at the core surrounded by Newkome and Fr¨¦chet dendrons. The potentials of two consecutive one-electron reductions of the viologen core were determined by cyclic voltammetry. The electrochemistry of viologen unit showed a distinct and obvious trend. Newkome and Frechet dendrons having different functional groups as repeating units has opposite effect on the half-wave potentials. The overall effect of these two dendrons is reflected by the corresponding reduction potentials. The redox site encapsulation by the Frechet and Newkome dendrons is indicated by the attenuation in heterogeneous electron transfer rate constants. Chapter three describes the probing of self-assembled capsule of resorcinarenes with 4-amino tempo and 4-trimethyl-ammonium tempo derivative. EPR spectroscopy and 1H NMR spectroscopy were used to investigate the nature of complexations involved in these systems. We observed a subsequent change in their spectroscopic parameters. Careful investigation of rotational correlation times and NMR line-widths at half height revealed that 4-trimethyl-ammonium tempo has stronger binding affinity with these capsules compared to 4-amino tempo due to favorable cation -pi interactions

Preparation, characterization, and electrochemical properties of a new series of hybrid dendrimers containing a viologen core and Frechet and Newkome dendrons

Five new 4,4'-bipyridinium (viologen) core dendrimers containing a Frechet (Fn, n = 1-3, first to third generation) and a Newkome (Nn, n = 1-3) dendron linked to each of the termini of the viologen residue were prepared and characterized. These macromolecules (FnNn) were prepared according to synthetic methodology already developed by our group. The electrochemical behavior of these dendrimers is characterized by the stepwise reduction of the viologen nucleus (V(2+)/V(+) and V(+)/V). The recorded half-wave potentials are affected by dendron growth in the three surveyed solvent media (dichloromethane, tetrahydrofuran, and acetonitrile). The size of the Newkome dendron has a more pronounced effect on the half-wave potentials than the size of the Frechet dendron. However, increasing the size of the Frechet dendron diminishes the magnitude of the cathodic potential shifts resulting from Newkome dendron growth. The largest dendrimers investigated (F1N3 and F2N3) exhibit quasi-reversible voltammetric behavior. The diffusion coefficients of these molecules were also determined using pulse gradient stimulated echo NMR techniques

Mechanistic Studies of Ammonia Borane Dehydrogenation Catalyzed by Iron Pincer Complexes

A series of iron bis­(phosphinite) pincer complexes with the formula of [2,6-(ⁱPr₂PO)₂C₆H₃]­Fe­(PMe₂R)₂H (R = Me, <b>1</b>; R = Ph, <b>2</b>) or [2,6-(ⁱPr₂PO)₂-4-(MeO)­C₆H₂]­Fe­(PMe₂Ph)₂H (<b>3</b>) have been tested for catalytic dehydrogenation of ammonia borane (AB). At 60 °C, complexes <b>1</b>–<b>3</b> release 2.3–2.5 equiv of H₂ per AB in 24 h. Among the three iron catalysts, <b>3</b> exhibits the highest activity in terms of both the rate and the extent of H₂ release. The initial rate for the dehydrogenation of AB catalyzed by <b>3</b> is first order in <b>3</b> and zero order in AB. The kinetic isotope effect (KIE) observed for doubly labeled AB (<i>k</i>_NH3BH3/<i>k</i>_ND3BD3 = 3.7) is the product of individual KIEs (<i>k</i>_NH3BH3/<i>k</i>_ND3BH3 = 2.0 and <i>k</i>_NH3BH3/<i>k</i>_NH3BD3 = 1.7), suggesting that B–H and N–H bonds are simultaneously broken during the rate-determining step. NMR studies support that the catalytically active species is an AB-bound iron complex formed by displacing <i>trans</i> PMe₃ or PMe₂Ph (relative to the hydride) by AB. Loss of NH₃ from the AB-bound iron species as well as catalyst degradation contributes to the decreased rate of H₂ release at the late stage of the dehydrogenation reaction

Towards catalytic ammonia oxidation with Mo and Ru- ammonia complexes

Ammonia is an appealing nitrogen- based fuel due to its high energy d. and ease of storage and distribution. Catalysts for NH_3 oxidn. will be essential to utilize the energy stored in N- H bonds. Understanding the N- H bond breaking steps during ammonia oxidn. is crucial for developing homogeneous catalytic systems for NH oxidn. and also for developing catalysts for the synthesis of ammonia from dinitrogen. Although transition metal ammonia complexes have been known for centuries, information on their N- H bond energies is not well studied. Our current efforts involve modulating the N- H bond strength through coordination to transition metals and studying the N- H bond cleavage reactions by hydrogen atom abstraction or oxidn. and deprotonation steps. The development of mol. M- NH_3 (M = Mo, Ru) complexes supported by chelating phosphine and /or amine based ligands to catalyze NH oxidn. reactions and the initial electrochem. studies of these systems will be described

Iron-Based Catalysts for the Hydrogenation of Esters to Alcohols

Hydrogenation of esters is vital to the chemical industry for the production of alcohols, especially fatty alcohols that find broad applications in consumer products. Current technologies for ester hydrogenation rely on either heterogeneous catalysts operating under extreme temperatures and pressures or homogeneous catalysts containing precious metals such as ruthenium and osmium. Here, we report the hydrogenation of esters under relatively mild conditions by employing an iron-based catalyst bearing a PNP-pincer ligand. This catalytic system is also effective for the conversion of coconut oil derived fatty acid methyl esters to detergent alcohols without adding any solvent

Molecular Insight into Fluorocarbon Adsorption in Pore Expanded Metal–Organic Framework Analogs

International audienc

Ammonia Oxidation by Abstraction of Three Hydrogen Atoms from a Mo–NH₃ Complex

We report ammonia oxidation by homolytic cleavage of all three H atoms from a [MoNH₃]⁺ complex using the 2,4,6-tri-<i>tert</i>-butylphenoxyl radical to yield a Mo-alkylimido ([MoNR]⁺) complex (R = 2,4,6-tri-<i>tert</i>-butylcyclohexa-2,5-dien-1-one). Chemical reduction of [MoNR]⁺ generates a terminal MoN nitride complex upon NC bond cleavage, and a [MoNH]⁺ complex is formed by protonation of the nitride. Computational analysis describes the energetic profile for the stepwise removal of three H atoms from [MoNH₃]⁺ and formation of [MoNR]⁺

Ammonia Oxidation by Abstraction of Three Hydrogen Atoms from a Mo–NH₃ Complex

We report ammonia oxidation by homolytic cleavage of all three H atoms from a [MoNH₃]⁺ complex using the 2,4,6-tri-<i>tert</i>-butylphenoxyl radical to yield a Mo-alkylimido ([MoNR]⁺) complex (R = 2,4,6-tri-<i>tert</i>-butylcyclohexa-2,5-dien-1-one). Chemical reduction of [MoNR]⁺ generates a terminal MoN nitride complex upon NC bond cleavage, and a [MoNH]⁺ complex is formed by protonation of the nitride. Computational analysis describes the energetic profile for the stepwise removal of three H atoms from [MoNH₃]⁺ and formation of [MoNR]⁺