Browsing by Subject "inorganic chemistry"
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Item Computational Benchmarking in Biomimetic Nickel, Copper, and Iron Complexes(2012-02-14) Brothers, Scott MichaelSophisticated catalytically active sites of metalloenzymes provide inspiration to synthetic chemists, as the metal coordination environments are often atypical to those found on the chemist's benchtop. Furthermore, metal-ligand cooperativity using earthabundant metals is anticipated to eventually supplant noble metals, currently used in industrial catalysis. Despite progress in synthesis of small molecule active site models, reproduction of the enzymatic function is rarely observed. However, differences that might define catalytic efficiency of enzymes can be addressed by theory. Density functional theory, or DFT, has been developed as an in silico tool to complement and interpret crystallographic and spectroscopic results or to make predictions in the absence of experimental data. In this dissertation, such techniques serve to elucidate the observed reactivity or electronic character of both nickel and copper bound in square planar N?S? ligand fields, and of {Fe(NO)?} units, respectively. Nickel and copper complexes in tetraanionic N?S??? ligand environments were investigated with respect to change of metal, to modification of ligand environment, and to response in reactivity of thiolate sulfur atoms. From the DFT calculations and consistent with experimental observations, it was discovered that binding of a nucleophile at one thiolate sulfur effectively decreases reactivity of the second sulfur, and nucleophilic binding at both sulfurs serves to deactivate the complex toward further thiolate reactivity. Additionally, despite both Cu and Ni binding comfortably in the N?S??? coordination sphere, the former displays increased ionicity versus the latter, demonstrated by electrostatic potential mapping. A methodology for accurate modeling of geometry and vibrational frequencies of complexes containing a {Fe(NO)?} unit was determined from the results of a test set of complexes using a matrix of functionals and basis sets. Utilizing the optimum performer, the BP86 functional and a mixed SDD ECP basis set on iron and 6-311++G(d,p) on other atoms, a series of iron dinitrosyl complexes containing diverse ancillary ligands spanning the spectrochemical series was subsequently investigated. The electrochemical potentials of the pairs of "oxidized" and "reduced" DNIC complexes were evaluated for values occurring in the biological regime. Furthermore, as the {Fe(NO)?} unit is capable of coordination in interesting yet dissimilar geometric motifs, bimetallic, tetrameric, and adamantane-like DNIC complexes have been investigated with our DFT methodology.Item Electronic structure of dimetal bonded systems: ditungsten, dimolybdenum and diruthenium systems(Texas A&M University, 2007-04-25) Villagran Martinez, DinoThis dissertation investigates three topics in the field of multiple-bonded metal chemistry. The first topic concerns the synthetic and theoretical considerations of ditungsten formamidinates and guanidinates compounds. This work presents an enhanced synthetic path to the W2(hpp)4 molecule (Hhpp = 1,3,4,6,7,8-hexahydro-2H-pyrimido[1,2-a]pyrimidine). The reflux of W(CO)6 with Hhpp in o-dichlorobenzene at 200 oC produces W2(hpp)4Cl2 in a one-pot reaction in 92% yield. This compound is stable and easily stored for further use, and it can be efficiently reduced in a one-step reaction to the most easily ionized compound W2(hpp)4. This work also examines the electronic structure and geometry of the intermediates W2(????-CO)2(????- hpp)2(????2-hpp)2 and W2(hpp)4Cl2. The second topic concerns the theoretical investigation by DFT of the electronic structure of [Mo2] units bridged by oxamidate ligands or bridging hydride ions ([Mo2] = (Mo2(DArF)3, where DArF is the anion of a diarylformamidine). It is shown that the effect of the gauche conformation of the ???? oxamidate isomers is due to steric interactions, and that the planar ???? oxamidate isomers have an electronic structure similar to that of naphthalene when it is doubly oxidized. The [Mo2](????-H)2[Mo2] compound shows interdimetal unit interactions between the ???? orbitals of the two [Mo2] units. These interactions are theoretically predicted and experimentally observed by a decrease in the [Mo2]---[Mo2] distance with a one-electron oxidation of [Mo2](????- H)2[Mo2]. The final topic concerns the magnetic and structural properties of two Ru2(DArF)4Cl compounds. The compounds with Ar = p-anisyl (para) and m-anisyl (meta) both show different temperature dependence of their molar magnetic susceptibility, ????. For the para compound, there is a Boltzmann distribution between a ????*3 ground state and a ????*2????* upper state, and this is confirmed by a temperature dependence of the Ru-Ru bond length: 2.4471(5) ???? at 23 K and 2.3968(5) ???? at 300 K. For the meta compound, a ????*2????*configuration persists over the range of 23-300 K as shown by an invariant Ru-Ru bond length and its molar magnetic susceptibility.Item Experimental and Theoretical Investigations of Anion-pi Interactions Metallacyclic Architectures of First-Row Transition Metals and N-Heteroaromatic Ligands(2012-07-16) Giles, IanResearch into anion-pi interactions has shifted from attempts to establish the legitimacy of the interaction to the incorporation of anion-pi interactions into supramolecular architectures. The research discussed in this dissertation explores the subtle effects of ligand, anion, and metal ion on supramolecular architectures of tetrazine-based ligands in the context of anion-pi interactions and their importance in the design and synthesis of supramolecular architectures. Computational studies highlight the importance of the arene quadrupole moment, molecular polarizability, and substituent effects on the strength of anion-pi interactions. More importantly, however, this work establishes that there is a distinct directionality inherent to the anion-pi interaction between polyatomic anions and N-heterocycles, which can be used to direct ligands in supramolecular architectures as demonstrated through the work of the Dunbar group in recent years, particularly that of the square and pentagonal metallacycles. The extension of metallacycles of bptz to CoII and FeII demonstrates the ability to tune the size of the metallacyclic cavity by simply changing the metal ion and results in the surprising encapsulation of two [SbF6]- anions in [Fe5(bptz)5(NCCH3)10][SbF6]10. 1H NMR spectroscopy and electrochemical studies reveal slight but significant differences characteristic of the square and pentagonal metallacycles and support the presence of anion-pi interactions in solution and highlight the importance of the encapsulated anion in the templation and stability of the metallacycles. A study of the interconversion between the square and pentagonal metallacycles via 1H NMR is presented for the first time. Increasing the pi-acidity of the chelating ligand from bptz to bmtz results in the encapsulation of only one [SbF6]- anion in [Fe5(bmtz)5(NCCH3)10][SbF6]10, maximizing anion-pi interactions with the ligand despite the tighter fit. A significant hurdle in the incorporation of different anions into the metallacyclic structures was overcome with the development of a new synthetic protocol for [Fe(NCCH3)6]2+ salts of a wide range of anions from sodium salts and Fe4Cl8(THF)6. Also, the nuclearity of the less stable [Fe5(bptz)5(NCCH3)10][PF6]10 metallacycle was established via a combination of MS, electrochemistry and 1H NMR experiments through comparisons with known FeII metallacycle solution behavior.Item Impact of the ligands on linear trimetal chains(Texas A&M University, 2004-09-30) Lei, PengIncreasing attention has been given to the preparation and study of compounds with linear chains of metal atoms surrounded by four ligands. The majority of linear trimetal complexes are supported by dpa, the anion of dipyridylamine, having the general formula M3(dpa)4X2, where X is typically a monoanion. It has been shown that the behavior of the trinuclear system is far more complicated than might have been expected. Specifically, both symmetrical and unsymmetrical chains can occur and the interpretation of the magnetic properties of certain compounds has been a challenging task. Present in this dissertation is the bulk of work completed on an exploration of syntheses and characterizations of linear trichromium and trinickel compounds with different types of tridentate ligands. These ligands include 2,6-bis(phenylamino)pyridine,H2BPAP, (the corresponding dianion of this is denoted by BPAP) and a set of five unsymmetrical formamidines with different organic substituents ranging from strong electron-donating groups, such as -OCH3, to electron-withdrawing groups, e.g., F. Ligands impact on the trimetal chain in various ways. In the case of the M3(BPAP)42- ions, there are no axial interactions because these anionic species do not attract electron donating ligands. Thus they have properties which are different from those of M3(dpa)4X2 molecules. Most notably, the Ni3(BPAP)42- ion is diamagnetic and all three nickel ions can be described as square-planar, low-spin NiII centers. ivWhen unsymmetrical formamidines are used to support linear trichromium chains with a Cl anion at each end, the separation between terminal chromium atoms is significantly longer (ca. 0.15 - 0.25) than those in trichromium compounds reported earlier. Moreover, the unsymmetrical formamidinates tend to support symmetrical trichromium chains, while the rest of the known tridentate ligands typically yield unsymmetrical Cr36+ chains. The synthesis and structural studies of trinickel compounds with unsymmetrical formamidines are also presented.