Dual activity systems for the synthesis and modulation of macromolecules and transition metal complexes

Chapter 1. Previous work on various dual-activity systems, including chemical systems that change after the application of an external stimulus and systems that contain components that work in a cooperative manner, is discussed. Chapter 2. Controlled and reversible ligand exchange reactions are useful tools for modulating reactivity of transition metal complexes. Here, we present a reversible electropolymerization of nickel azopyridine (azpy) complexes based on the redox-controlled diimine ligand exchange reaction. Upon a single electron reduction, (azpy)NiBr2 disproportionates to afford (azpy)2Ni and a [NiBr4]2-, while in situ 1e- oxidation of this mixture leads to complete restoration of the original complex. Reversible intermetallic ligand exchange is also demonstrated between nickel and cobalt or iron azopyridine complexes. This facile and quantitative ligand exchange reaction was utilized to develop a reversible electropolymerization strategy to generate nickel metallopolymers containing redox-active azobispyridine ligands. Chapter 3. Reduction of the insulating one-dimensional coordination polymer [Cu(abpy)PF6]n, 1a(PF6), (abpy = 2,2-azobispyridine) yields the conductive, porous polymer [Cu(abpy)]n, 2a. Pressed pellets of neutral 2a exhibit a conductivity of 0.093 S·cm−1 at room temperature and a Brunauer-Emmett-Teller (BET) surface area of 56 m2g1. Fine powders of 2a have a BET surface area of 90 m2g1. Cyclic voltammetry shows that the reduction of 1a(PF6) to 2a is quasi-reversible, indicative of facile charge transfer through the bulk material. The BET surface area of the reduced polymer 2 can be controlled by changing the size of the counter anion X in the cationic [Cu(abpy)X]n. Reduction of [Cu(abpy)X]n with X = Br (2b) or BArF (2c; BArF = tetrakis(3,5-bis(trifluoromethyl)phenyl)), affords [Cu(abpy)]n polymers with surface areas of 60 and 200 m2g−1, respectively. Chapter 4. We report the reactivity of porous and electrically conductive copper azobispyridine (abpy) metallopolymers with nitrogen dioxide (NO2). Mixed-valence [Cu(abpy)]n metallopolymers undergo a redox reaction with NO2, resulting in the disproportionation of NO2 gas and the formation of a CuII-nitrate species and nitric oxide gas. Exposure to NO2 results in a decrease in the room temperature conductivity value by four orders of magnitude as well as a complete loss of porosity. We also observe a loss of the intervalence charge transfer band and emergence of nitrate stretches in the infrared spectrum upon NO2 dosing. We obtain crystallographic evidence for the formation of nitrate upon NO2 exposure. We also can reform the porous and conductive [Cu(abpy)]n metallopolymers by reducing the CuII-nitrate species. Chapter 5. Reductive elimination (RE) is a critical step in many catalytic processes. The reductive elimination of unsaturated groups (aryl, vinyl and ethynyl) from Pd(II) species is considerably faster than RE of saturated alkyl groups. Pd(II) dimethyl complexes ligated by chelating diimine ligands are stable towards RE unless subjected to a thermal or redox stimulus. Herein, we report the spontaneous RE of ethane from (azpy)PdMe2 complexes and the unique role of the redox-active azopyridine (azpy) ligands in facilitating this reaction. The (azpy)PdMe2 complexes are air- and moisture-stable in the solid form, but they readily produce ethane upon dissolution in polar solvents at temperatures from 10oC to room temperature without the need for an external oxidant or elevated temperatures. Experimental and computational studies indicate that a bimolecular methyl transfer precedes the reductive elimination step, where both steps are facilitated by the redox-active azopyridine ligand. Chapter 6. A dual catalyst system was developed to synthesize hydrolysable polyether-polyester copolymers from propylene oxide and cyclic esters such as δ-valerolactone, γ-butyrolactone, and ε-caprolactone. A bimetallic chromium catalyst active for the stereoselective polymerization of propylene oxide and an organoctalyst active for the ring opening polymerization of lactones were used in conjunction with alcohol chain shuttling agents to synthesize new copolymers. The monomer and alcohol ratios were varied to yield a wide range of copolymers with varying monomer ratios, molecular weights, and crystallinities. Chapter 7. A recently reported dimeric salen-chromium complex enantioselectively polymerizes propylene oxide. Polymerization is proceeds efficiently with the optimized catalyst system, but only after a period where the catalyst sits dormant and only short oligomers are formed. We undertook a detailed mechanistic study with the use of high resolution electrospray ionization mass spectrometry (ESI-MS) to determine the chromium species responsible for this lag phase. MS reveals that chromium hydroxide and chromium-bound 1,2-diols are present during this dormant period, and kinetic studies show that these species are responsible for an arrest state. In-situ MS shows that the chromium hydroxide reacts with propylene oxide to generate chromium-bound 1,2-diols. MS analysis of early-formed oligomers shows that they are telechelic, supporting the hypothesized chain transfer mechanism even at early time points.

Keywords: Chemistry.
Advisor: Waymouth Robert M. 
Karunadasa Hemamala
Stack T. (T. Daniel P.)