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| Author | : Jennifer L. Scott |
| Publisher | : |
| Total Pages | : 574 |
| Release | : 2007 |
| Genre | : Catalysis |
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| Author | : Jennifer L. Scott |
| Publisher | : |
| Total Pages | : 0 |
| Release | : 2007 |
| Genre | : Catalysis |
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| Author | : |
| Publisher | : |
| Total Pages | : 946 |
| Release | : 2008 |
| Genre | : Dissertations, Academic |
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| Author | : Aaron M. Hollas |
| Publisher | : |
| Total Pages | : 188 |
| Release | : 2016 |
| Genre | : |
| ISBN | : 9781369227055 |
The work described herein focuses on the ability of redox-active ligands to enable multi-electron reactivity at transition metal centers. A parallel theme is the effect of ancillary ligands on controlling and modulating the electronic structure of the redox-active ligand and metal center in addition to ancillary ligand effects as they relate to controlling the primary coordination sphere of the metal. (Abstract shortened by ProQuest.).
| Author | : Preeyanuch Sangtrirutnugul |
| Publisher | : |
| Total Pages | : 684 |
| Release | : 2007 |
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| Author | : Amanda Catherine Bowman |
| Publisher | : |
| Total Pages | : 0 |
| Release | : 2010 |
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The synthesis, reactivity and electronic structures of a series of bis(imino)pyridine iron and cobalt complexes was investigated. A series of monomeric bis(imino)pyridine cobalt dinitrogen complexes was investigated using a combination of 1H NMR and infrared spectroscopies, X-ray crystallography, EPR spectroscopy, solution and solid state magnetic measurements and density functional theory. The neutral bis(imino)pyridine cobalt dinitrogen complexes have doublet ground states and are best described as low-spin cobalt(I) centers with an unpaired electron on the singly reduced chelate, while the anionic bis(imino)pyridine cobalt dinitrogen complexes are also best described as low-spin cobalt(I) centers with dianionic chelates. These investigations established that reduction of monochloride precursors occurs at the metal center, in contrast to the related bis(imino)pyridine iron bis(dinitrogen) complex, (iPrPDI)Fe(N2)2, where reduction of the chelate is observed. A series of bis(imino)pyridine iron imide complexes was also investigated. A combination of Xray crystallography, variable temperature SQUID magnetization data and Mössbauer spectroscopy was used to elucidate the electronic structures of these complexes. In contrast to the previously reported N-aryl substituted bis(imino)pyridine iron imide complexes, where an iron(III) metal center and a singly reduced chelate was observed, an iron(IV) metal center and a triplet diradical chelate was observed for N-alkyl substituted bis(imino)pyridine iron imide complexes. For (iPrPDI)FeN(2Ad) (iPrPDI = 2,6-(2,6-iPr2-C6H3-N=CMe)2C5H3N), thermal spin crossover from S = 0 to S = 1 was observed when warming from 15 K to 200 K. (ArPDI)FeNR compounds with an S = 0 ground state promoted C-H bond activation of both imine methyl groups of the bis(imino)pyridine ligand. The C-H bond activation with (iPrPDI)FeN(CyOct) was firstorder in iron with a rate constant of k = 3.4(2)x10-5 s-1 at 25 °C and a primary kinetic isotope effect of 3.3(2), consistent with a rate-determining step of intramolecular C-H bond activation. In contrast, no C-H bond activation of the ligand was observed for the iron imide complexes that are S = 1 at 23 °C. The reactivity of bis(imino)pyridine iron imide compounds with hydrogen, silanes, terminal alkynes and organic azides, and the electronic structures of the resulting iron complexes, was also investigated.
| Author | : Amela Arnold (Drljevic) |
| Publisher | : |
| Total Pages | : 0 |
| Release | : 2020 |
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This dissertation discusses the synthesis, characterization and electronic delocalization of bis(imino)pyridine (I2P) complexes of the Group 13 metals Al, Ga and In. This work aims to understand the electronic communication between redox-active ligands bridged by a main group metal and apply this research for storing multiple electrons in nonaqueous redox-flow battery applications. The introduction to this dissertation details the background on redox-active ligands, mixed valency and redox flow batteries. A future outlook on this field is presented. The work presented here has contributed to our understanding of delocalization of charge in organic molecules, and how delocalization is affected by increasing charge, appending electron-donating or withdrawing functional groups or varying the metal bridge. Evidence for a symmetric nonaqueous redox flow battery is presented. In Chapter 2, ligand-based mixed valency is introduced. Water stable organic mixed valence (MV) compounds were prepared by reaction of reduced bis(imino)pyridine ligands (I2P) with the trichloride salts of Al, Ga, and In. Coordination of two tridentate ligands to each ion affords octahedral complexes that are accessible with five ligand charge states: [(I2P0)(I2P−)M]2+, [(I2P−)2M]+, (I2P−)(I2P2−)M, [(I2P2−)2M]−, [(I2P2−)(I2P3−)M]2−, and for M = Al only, [(I2P3−)2M]3−. In solid-state structures the anionic members of the redox series are stabilized by intercalation of Na+ cations within the ligands. The MV members of the redox series, (I2P−)(I2P2−)M and [(I2P2−)(I2P3−)M]2−, show characteristic intervalence transitions, in the near-infrared region between 6800 - 7400 and 7800 - 9000 cm-1, respectively. Cyclic voltammetry (CV), NIR spectroscopic, and X-ray structural studies support the assignment of Class II for compounds [(I2P2−)(I2P3−)M]2− and Class III for M = Al and Ga in (I2P−)(I2P2−)M. All compounds containing a singly reduced I2P− ligand exhibit a sharp, low energy transition in the region 5100 - 5600 cm−1 that corresponds to a [pi]* - [pi]* transition. CV studies demonstrate that the electron transfer events in each of the redox series, Al, Ga, and In span 2.2, 1.4 and 1.2 V, respectively. In Chapter 3, ligand-based mixed valent (MV) complexes of Al(III) incorporating electron donating (ED) and electron withdrawing (EW) substituents on bis(imino)pyridine ligands (I2P) are presented. The MV states (I2P−)(I2P2−)Al and [(I2P2−)(I2P3−)Al]2− prepared containing EW groups are both assigned as Class II/III. The MV states prepared with incorporation of ED functional groups are Class III and Class II/III in the (I2P−)(I2P2−)Al and [(I2P2−)(I2P3−)Al]2− charge states, respectively. The assignments of the delocalized electronic structures were made using cyclic voltammetry (CV), and near infra-red (NIR) spectroscopy. The MV ligand charge states (I2P−)(I2P2−)Al and [(I2P2−)(I2P3−)Al]2− show intervalence charge transfer (IVCT) transitions at 6850-7740 and 7410-9780 cm−1, respectively. Alkali metal cations in solution had no effect on the IVCT bands of [(I2P2−)(I2P3−)Al]2− complexes containing ED -PhNMe2 substituents or EW -PhF5 substituents on the I2P ligands. Localization of charge in [(I2P2−)(I2P3−)Al]2− was observed when -PhOMe substituents are included on the I2P ligands, so that those complexes are Class II/III with K+ and Class III with K:(18-crown-6)+. In Chapter 4, the application of these redox-active octahedral Al(III) complexes as analytes for symmetric nonaqueous redox flow batteries is presented. Redox flow batteries (RFBs) operate by storing electrons on soluble molecular anolytes and catholytes, however large increases in the energy density of RFBs could be achieved if multiple electrons could be stored in each molecular analyte. Others have suggested and employed various transition element - redox active ligands combinations to realize multi-electron storage in anolytes, and a challenge with those efforts has been the analyte's stability over extended charging and discharging of multi-electron cycles. We reported an organoaluminum analyte in which four electrons can be stored on organic ligands, and for which charging and discharging cycles performed in a symmetric nonaqueous RFB configuration remain stable for over 100 cycles at 70% state of charge and 97% Coulombic efficiency. Stability is promoted by the kinetic inertness of the anolyte to trace water in solvents and by the redox stability of the Al(III) ion to the applied current. Proof-of principle experiments performed with an asymmetric NRFB configuration further demonstrate that up to four electrons can be stored in the cell with no degradation of the analyte over multiple cycles that show 96% Coulombic efficiency.
| Author | : Ryan Michael Clarke |
| Publisher | : |
| Total Pages | : 194 |
| Release | : 2018 |
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Transition metal complexes with pro-radical ligands have received considerable research attention due to their interesting electronic structures, photophysical properties, and applications in catalysis. The relative ordering of metal and ligand frontier orbitals in a complex incorporating pro-radical ligands dictates whether oxidation/reduction occurs at the metal centre or at the ligand. Many metalloenzymes couple redox events at multiple metal centres or between metals and pro-radical ligands to facilitate multielectron chemistry. Owing to the simplicity of the active sites, many structural and functional models have been studied. One class of pro-radical ligand that has been investigated extensively are bis-imine bis-phenoxide ligands (i.e. salen) due to their highly modular syntheses. In this thesis, projects related to the synthesis, electronic structure, and reactivity of mono and bimetallic complexes incorporating the salen framework are explored. Chapter 2 presents a systematic investigation of the effects of geometry on the electronic structure of four bis-oxidized bimetallic Ni salen species. The tunability of their intense intervalence charge transfer (IVCT) transitions in the near infrared (NIR) by nearly 400 nm due to exciton coupling in the excited states is described. For the first time, this study demonstrates the applicability of exciton coupling to ligand radical systems absorbing in the NIR region. Chapter 3 investigates the ground-state electronic structure of a bis-oxidized Co dimer. Enhanced metal participation to the singly occupied molecular orbitals results in both high spin Co(III) and Co(II)-L• character in the ground state, and no observable band splitting in the NIR due to exciton coupling. Finally, Chapter 4 describes a series of oxidized nitridomanganese(V) salen complexes with different para ring substituents (R = CF3, tBu, and NMe2), demonstrating that nitride activation is dictated by remote ligand electronics. Upon one-electron oxidation, electron deficient ligands afford a Mn(VI) species and nitride activation, whereas an electron-rich ligand results in ligand based oxidation and resistance to N coupling of the nitrides. This study highlights the alternative reactivity pathways that pro-radical ligands impose on metal complexes and represents a key step in the use of NH3 as a hydrogen storage medium. The results presented herein provide a starting point for further efforts in reactivity with the salen platform.
| Author | : Thomas Winfield Myers |
| Publisher | : |
| Total Pages | : |
| Release | : 2014 |
| Genre | : |
| ISBN | : 9781321212495 |
This dissertation discusses the synthesis, reactivity and characterization of iminopyridine and bis(imino)pyridine complexes of aluminum and other electrophilic main group metal ions. It is shown that aluminum complexes of iminopyridine ligands can undergo stoichiometric redox transformations while aluminum complexes of bis(imino)pyridine ligands can facilitate heterolytic substrate activation and catalytic dehydrogenation reactions. In chapter 2, the reaction of disulfides, nitrogen group transfer reagents, and zinc(II) salts with [Na(THF)6][(IP2−)2Al] (2.1c) (IP = 2,6-bis(isopropyl)-N-(2-pyridinyl-methylene)phenylamine) affords aluminum complexes of the form (IP−)2AlX (X = Cl 2.2, [mu]2-SSCN(CH3)2 2.4, SCH3 2.5, NaNTs 2.6, CCPh 2.7, N3 2.8, SPh 2.9, and NHPh 2.10). Additionally, the oxidation of (IP−)2Al(Me) (2.3) by single electron oxidants leads to formation of [(IP−)(IP)Al(Me)]+. When TrBPh4 (Tr = triphenylmethyl) is employed as the oxidant, carbon-carbon coupling between one of the IP ligands and the Tr group is observed to form [(TrIP)(IP)Al(Me)]+ (2.11). This bond formation can be reversed when bulkier anions are added, or can be avoided by using TrB(C6F5)4 and TrBAr[superscript F] as the one electron oxidants. In chapter 3, the formation of Al(III) and Ga(III) oxo intermediates are proposed resulting from the oxidation of [(IP2−)2M]− (M = Al, Ga) with pyO (pyO = pyridine-N-oxide). These reactive intermediates homolytically and heterolytically cleave C-H bonds to form [Na(DME)(THF)][(IP2−)(IP−)Al(OH)] (3.3) and (IP−)2M(OH) (M = Al 3.4, Ga 3.7). The identity of the counter cation directs the reactivity of [(IP2−)2Al]−. When [Na(DME)3][(IP2−)2Al] is employed, C-H activation of solvent is observed, while when [Bu4N][(IP2−)2Al] is employed proton abstraction from Bu4N+ is observed. The oxidation of [(IP2−)2Ga]− by pyO leads to acid base chemistry when either Na+ or Bu4N+ is employed as the counter cation. The reaction of 3.4 and 3.7 with CO2 leads to formation of [(IP−)2M]2([mu][eta]1:[kappa]2-OCO2) (M = Al 3.10, Ga 3.11). Reduction of 3.10 and 3.11 with alkali or alkali earth metals and subsequent oxidation allows for the reformation of 3.4 and 3.7. In chapter 4, the reduction of [superscript Me]IP[subscript Mes] ([superscript Me]IP[subscript Mes] = 2,6-bis(isopropyl)-N-(2-(5-mesityl-pyridinyl)-methylene)phenylamine) with sodium metal followed by metathesis with MCl[subscript n]X[subscript 3-n] (M = Al, Ga, X = Cl, CH3) leads to the formation of ([superscript Me]IP[subscript Mes−)MX2 (M = Al, X = Cl, 4.1a, 4.2a; M = Ga, X = Cl 4.5), ([superscript CH2]IP[subscript Mes]−)AlX2 (X = Cl, 4.1b, 4.2b), ([superscript Me]IP[subscript Mes]2−)MX(OEt2) (M = Al, X = Cl, 4.3, 4.4; M = Ga, X = Cl 4.6) . Unlike the IP ligand system, only one [superscript Me]IP[subscript Mes] ligand coordinates to the metal center in these complexes. Selective deprotonation of the [superscript Me]IP[subscript Mes] ligand is observed in ether solvents, while selective reduction is observed in alkane and aromatic solvents. In chapter 5, complexes of bis(imino)pyridine ligands with aluminum are presented. Reduction of [superscript Ph]I2P ([superscript Ph]I2P = 2,6-(2,6-[superscript i]Pr2-C6H3N=CPh)2C5H3N) by 2 equivalents of sodium metal followed by salt metathesis with AlCl2X (X = Cl, H) affords ([superscript Ph]I2P2−)AlX(THF) (X = Cl 5.1, H 5.2a) and ([superscript Ph]I2P2−)AlH (5.2b). The [superscript Ph]I2P2− ligands in these complexes are shown to be chemically non-innocent. The addition of polar N-H and O-H bonds across the aluminum-amido bonds leads to the formation of ([superscript Ph]HI2P2−)AlH(X) (X = NHDipp 5.3a, NHPh 5.3b, [mu]-O 5.5, OPh 5.8) (Dipp = 2,6-diisopropylphenyl). 5.2b also catalyzes the dehydrogenative coupling of benzylamine with 3.5 turnovers over 24 hours. In chapter 6, complexes of the form ([superscript Ph]I2P2−)AlX(THF) (X = H, Me) are shown to be active catalysts for the selective dehydrogenation of formic acid with an initial TOF of up to 5200 hr−1 and up to 2200 total turnovers observed. The mechanism of the transformation is examined through a series of stoichiometric reactions. In the presence of formic or acetic acid, the [superscrpt Ph]I2P2− ligand is protonated at both the amido nitrogen and at the ipso carbon position effectively hydrogenating one of the imine arms of the ligand. The Al(III) complexes of the [superscript Ph]HI2P− and [superscript Ph]H2I2P forms of the ligand favor [beta]-hydride abstraction from formate, while the Al(III) complexes of the [superscript Ph]I2P2− form of the ligand favors the reverse reaction: insertion of CO2 into the Al-H bond. The liberation of CO2 from formate is investigated through a series of deuterium labeling studies which show [beta]-hydride transfer from formate to the aluminum center. Finally, in chapter 7, the variety of electronic states adopted by complexes of methyl-substituted bis(imino)pyridine ligands is discussed. Reduction of [superscript Me]I2P ([superscript Me]I2P = 2,6-bis(1-methylethyl)-N-(2-pyridinylmethylene)phenylamine) with sodium metal leads to the formation of ([superscript Me]I2P−)Na(OEt2) (7.1). Reduction of [superscript Me]I2P by sodium metal followed by salt metathesis with MgCl2, Mg(OTf)2, AlCl3, and AlCl2H affords [([superscript Me]I2P2−)Mg(THF)](MgCl2) (7.2), ([superscript Me]I2P2−)Mg(THF)2 (7.3), ([superscript Me]I2P−)AlCl2 (7.4), ([superscript Me]I2P2−)AlCl(THF) (7.5) and ([superscript Me]I2P2−)AlH(THF) (7.6), respectively. The electronic states of 7.1 to 7.6 are shown to be dependent on the reaction conditions used to synthesize the complexes with certain conditions leading to dimer formation. Initial reactivity studies with 7.5 and 7.6 are discussed.
| Author | : Adam Ruddy |
| Publisher | : |
| Total Pages | : |
| Release | : 2014 |
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