http://hdl.handle.net/1721.1/7595 2013-06-18T06:37:44Z http://hdl.handle.net/1721.1/79328 Small molecule binding to electrophilic trigonal pyramidal platinum, palladium, and nickel Tsay, Charlene Chapter 1 A general introduction to the concepts and background of several types of transition metal complexes that motivate and inform the research described herein. These include a-complexes and molecular adducts of dinitrogen, dihydrogen, and carbon dioxide. Chapter 2 Trigonal bipyramidal platinum(II) complexes of the monoanionic, tetradentate, triphosphine [SiPR₃ ([SiP₃R]- = [(2-R₂PC₆H₄)₃Si]-; R = Ph, iPr) ligand are prepared and shown to provide access to cationic species with divergent behavior. The less electron-rich phenyl-substituted ligand renders the platinum center extremely electrophilic, leading to structurally characterized examples of weakly-donating ligands bound in the fifth, apical coordination site. Of particular interest is the structure of the toluene adduct, which suggests a possible interaction between the platinum center and an aryl C-H bond. When the ligand phosphines are instead substituted by the more electron-rich isopropyl groups, the electrophilicity of the cationic platinum is shown to be mitigated, allowing access to a four-coordinate, trigonal pyramidal platinum center. The crystallographically characterized geometry for this divalent platinum is in contrast to the canonical square planar configuration for d⁸, 16-electron transition metal complexes. The palladium analogue is also synthesized and shown to possess the same coordination. Chapter 3 Cationic nickel complexes of the [SiPR₃] ligand are synthesized and, in contrast to their platinum and palladium congeners, facilitate the surprising binding of molecular dinitrogen to electrophilic nickel(II) centers. The extremely high stretching frequencies of these bound N₂ moieties attest to their minimal activation, and the stability of these complexes is shown to arise from increased adonation from the N₂ to the cationic nickel center, which compensates for the relative lack of it back-bonding that stabilizes N₂ adducts in less electrophilic systems. These cationic nickel species are additionally shown to form thermally stable adducts of molecular dihydrogen. The relative binding strengths of N₂ and H₂ to these nickel centers are explored and shown to be modulated by the ligand phosphine substituents. Furthermore, evidence of linear binding of carbon dioxide is presented, representing an electrophilic approach to carbon dioxide activation that is in contrast to the low-valent, nucleophilic metal paradigm. Chapter 4 The four-coordinate neutral nickel boratrane (TPiPrB = (2-iPr₂PC₆H₄)₃B) reported in the literature represents an isostructural counterpart to the cationic {[SiiPr₃]Ni}+ species presented in Chapter 3. Though these two compounds are formally separated by two oxidation states of nickel, the Lewis-acidic nature of the Z-type borane ligand in (TP'PrB)Ni renders it valence-isoelectronic with {[SiiPr3]Ni}+. The reactivity toward N₂ and H₂ of (TPiPr'B)Ni, as well as that of the new compound (TPPhB)Ni, is explored and discussed in context of what is observed for the {[SiPR3]Ni}+ system. The neutral (TPiPr'B)Ni, while presumably a better [pi] back-bonder than cationic {I[SiPip' 3]Ni}T, is demonstrated not to bind N2, though a very weak, fluxional interaction with H₂ at low temperature is hypothesized. The more electrophilic (TP PhB)Ni exhibits room temperature interactions with both N₂ and H₂, though the nature of these interactions has yet to be confirmed. These results thus underline the importance of [sigma]-donation in stabilizing N₂ and H₂ adducts of poorly 7r back-bonding metal centers. Chapter 5 Cobalt(I) complexes of [SiPR3] provide an additional isostructural, isoelectronic point of comparison to the cationic nickel species presented in Chapter 3. The dinitrogen adducts [SiP'i' 3]Co(N2) and [SiPPh3]Co(N₂), previously reported from our laboratory, feature strongly bound N₂ ligands that are not labile to vacuum. The corresponding dihydrogen adducts are generated slowly under an H₂ atmosphere. The intact nature of both dihydrogen ligands, which also are not labile to vacuum, is reflected in their NMR spectroscopic parameters. The thermal stability of these compounds enabled crystallization of [SiPi'' 3]Co(H₂) which, along with the related (TP'i'B)Co(H₂) complex also developed in our laboratory, represent the first structurally characterized dihydrogen adducts of cobalt. Additional comparisons are made between the relative N₂ and H₂ binding strengths of this system and those of the structurally and electronically related family of [SiPR3] and (TpRB) metal complexes. Appendix A The asymmetric dinucleating ligand [NOPPh], designed to contain both a hard, N-donor binding site and a soft-P-donor binding site, is synthesized and shown to form a diiron complex that features asymmetric bonding to the bridging acetates. The corresponding symmetric, allphosphine dinucleating ligand [POPPh], proves to be more conducive to further study, and provides access to the symmetric diiron, di-([mu]-bromide) starting material {[POPPh ]Fe 2Br2} {BArF4 }. Addition of hydrazine generates the asymmetric, unbridged N₂H₄ adduct, which features localized diamagnetic and paramagnetic iron centers. The conformation of this species additionally demonstrates the flexibility of this ligand framework. Reduction of the diiron(II) starting material in the presence of PMe₃ results in formation of a putative asymmetric iron(O)/iron(I) dimetallic complex, in which an N₂ molecule is bound to the diamagnetic iron center, while the PMe₃ is ligated to the high-spin iron center and rendered NMR silent. The N₂ ligand is shown to be reversibly displaced by H₂ , suggesting the formation of a dihydrogen adduct, as well as by CO₂, which is postulated to bind as a bent, [eta]²(C,O) ligand. Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Chemistry, 2013.; Vita. Cataloged from PDF version of thesis.; Includes bibliographical references. 2013-01-01T00:00:00Z http://hdl.handle.net/1721.1/79274 The use of chirped pulse millimeter-wave spectroscopy in chemical dynamics and kinetics Shaver, Rachel Glyn .Chirped-pulse millimeter wave (CPmmW) spectroscopy is a revolutionary technique that has taken advantage of advances in electronics to give high signal to noise broadband rotational spectra in a very short period of time that provides meaningful line intensities. We have implemented this technique in the 58 - 102 GHz range to study the rotational spectra of molecules with two heavy atoms. Photolysis (at 193 nm) and pyrolysis of vinyl cyanide have produced differing HCN and HNC vibrational population distributions. The photolysis experiment does not sample a collisional regime and the resulting spectra show excited states of HCN and HNC, whereas the pyrolysis experiment, which does sample a collisional regime, results in spectra that are devoid of vibrational satellites. This indicates that the intensities of vibrational satellite transitions sample the photolysis reaction only and not post-photolysis collisional effects. Mono-deuterated vinyl cyanide was photolyzed at 193 nm, in which all HCN/HNC are produced via a four-center mechanism and all DCN/DNC are produced via a three-center mechanism. The HCN and HNC products dominate, demonstrating the greater importance of the three-center mechanism. CPmmW spectroscopy is also a valuable tool in studying unimolecular and bimolecular reactions. We have studied the unimolecular decomposition of deuterated methyl nitrite which produces DNO products and bimolecular hydrogen abstraction reaction of NO with acetaldehyde resulting in HNO products. These reactions demonstrate the potential use of nitric oxide radical as a gas-phase catalyst to perform cracking of hydrocarbons and sugars. Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Chemistry, 2013.; Cataloged from PDF version of thesis.; Includes bibliographical references (p. 51-53). 2013-01-01T00:00:00Z http://hdl.handle.net/1721.1/79273 Optical and spin properties of nitrogen vacancy centers in bulk and nanocrystalline diamond Ofori-Okai, Benjamin Kwasi The NV center is becoming a very hot topic in many areas of science, including, Physics, Chemistry, Biology, and Quantum Information. The Degen group has focused on a specific application of the NV center, namely scanning magnetometry. In my time in the group, I focused on building a microscope and studying NV centers in bulk and nanocrystalline diamond. I began by building a confocal microscope which was capable of observing and determine single NV centers. I made measurements on the photon statistics of different defects to determine if they were single emitters or multiple emitters. I also made microwave frequency magnetic measurements to determine the spin properties of single NV centers by measuring their couplings to neighboring paramagnetic nuclei as well as to a spin bath. Through these efforts, I was able to successfully confirm that the microscope was capable of identifying and measuring single NV centers and their properties. Lastly, I worked on the first steps of improving our understanding of NV centers in bulk diamond crystals. The goal of magnetometry involves putting the NV center as close to the diamond surface as possible. I made measurements that were aimed at studying the spin and coherence properties of the NV when it was within 10 nm of the diamond surface. These studies provided insight into the interactions of the NV center with the diamond surface. Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Chemistry, 2013.; Cataloged from PDF version of thesis.; Includes bibliographical references. 2013-01-01T00:00:00Z http://hdl.handle.net/1721.1/79272 The synthesis and characterization of porous, conductive, and ordered materials Narayan, Tarun Chandru Two different classes of polymers were pursued as candidates for materials possessing porosity, conductivity, and crystalline order. Attempts were made with hexaazatrinaphthylene- and dibenzotetrathiafulvalene-based precursors with boronic acids to prepare covalent-organic frameworks (COFs) possessing boroxole linkages. After preparing the precursors, several different reaction conditions were attempted, but the desired COFs proved elusive. The second class of materials was tetrathiafulvalene-based metal-organic frameworks (MOFs). These materials were constructed with tetrathiafulvalene tetrabenzoic acid and zinc, cobalt, and manganese nitrate to give helical structures with approximately cylindrical pores. This structure type has one close S-S contact of 3.8 Å which acts as a potential avenue for charge transfer. Gas sorption measurements suggest that the materials retain porosity upon evacuation. The material possesses high charge mobility as determined by flash photolysis time-resolved microwave conductivity measurements. Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Chemistry, February 2013.; "February 2013." Vita. Cataloged from PDF version of thesis.; Includes bibliographical references (p. 87-92). 2013-01-01T00:00:00Z