Date of Award

2019

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Graduate Group

Chemistry

First Advisor

Daniel J. Mindiola

Abstract

Chapter 1: Titanium Nitrides: Synthesis and Reactivity as Powerful Nucleophiles. In this chapter we explore the synthesis and reactivity of a titanium nitride anion complex [2-K(OEt2)]2[(PN)2Ti≡N]2, supported by two phosphino anilido ligands (PN− = (N-(2-(diisopropylphosphino)-4-methylphenyl)-2,4,6-trimethylanilide). Reactivity studies discussed include the synthesis of a series of imide moieties including rare examples such as methylimido, borylimido, phosphonylimido, and a parent imido. For the parent imide, using various weak acids allowed us to narrow the pKa range of the NH group to be between 26–36. The synthesis of the nitride was accomplished by reductively promoted elimination of N2 from the azide precursor (PN)2TiN3, whereas reductive splitting of N2 could not be achieved using the dinitrogen complex (PN)2Ti=N=N=Ti(PN)2 and a strong reductant. Complete N-atom transfer reactions could also be observed when the nitride complex was treated with ClC(O)tBu and OCCPh2 to form NCtBu and KNCCPh2, respectively, along with the terminal oxo complex, (PN)2Ti≡O, which was also characterized. A combination of solid state 15N NMR (MAS) in collaboration with Prof. Gang Wu and theoretical studies in collaboration with Prof. Balazs Pinter describe the shielding effect of the counter cation in the nitride anion as well as the discrete salt [K(18-crown-6)][(PN)2Ti≡N] and the putative anion [(PN)2Ti≡N]−, and also to probe the origin of the highly downfield 15N NMR resonance when shifting from dimer to monomer or to a terminal nitride (discrete salt). The upfield shift of the 15N nitride resonance in the 15N NMR spectrum was found to be linked to the K+ induced electronic structural change of the titanium-nitride functionality by using a combination of MO analysis and quantum chemical analysis of the corresponding shielding tensors.

Chapter 2: Titanium Nitrides: Reactivity Spanning from the Generation of Nitridyl Radicals to Electrophilic Behavior. The titanium nitride complex discussed in Chapter 1 is instead showcased as a potent source of a nitridyl radical upon oxidation of the nitride with trityl chloride or iodine. This chapter presents a thorough mechanistic study that shows this nitridyl radical is capable of abstracting H-atoms from the PN ligand scaffold to make the rare parent imido discussed in Chapter 1. Alternatively, the nitridyl moiety is competent at oxidation of the phosphorous arm of the PN− ligand to form an asymmetric NPN’ scaffold. A thorough reactivity study to detail all aspects of this mechanism is presented. Eventually, all intermediates result in the formation of halide complexes, (NPN’)(PN)TiX (X = I, Cl), based on two mechanistic pathways. In addition to the oxidation of the nitride, we show that this nitridyl radical can also be formed from photolysis of the azido complex (PN)2TiN3, originally presented in Chapter 1. In addition to reactivity, we also explore matrix EPR studies of (PN)2TiN3 in collaboration with the de Bruin group. From a different reactivity viewpoint, we showcase that the titanium nitride can behave electrophilically in reactivity with isocyanides to form Ti(II) complexes [(PN)2Ti(NCNR)][K(solv)], where the R = Ad or tBu and the interaction of the countercation varies depending on the use of DME (not charge separated) or kryptofix (completely charge separated species). A discussion of the characterization of these complexes, complete with a discussion of other known Ti(II) chemistry is presented.

Chapter 3: Extending Reactivity: Synthesis of A Molecular Zirconium Nitrido Superbase and A Transient Uranium Nitrido. In this chapter, the preparation and characterization of a zirconium complex having a terminally bound parent imide motif, (PN)2Zr≡NH is discussed, along with the zirconium nitride complex {(PN)2Zr≡N[μ2-Li(THF)]}2. This latter complex represents the first structurally characterized terminally bound Zr nitride complex. (PN)2Zr≡NH was prepared by reduction of trans-(PN)2Zr(N3)2 with KC8. Isotopic labeling and spectroscopic studies are described, which were prepared using the respective 15N enriched isotopologues, whereas solid-state structural studies confirmed some of the shortest Zr≡N distances known to date (Zr≡NH, 1.830(3) Å; Zr≡N‒, 1.822(2) Å). It was found that the nitride in {(PN)2Zr≡N[μ2-Li(THF)]}2 is super basic and in the range of −36 to −43 pKb units. Computational studies in collaboration with Prof. Balazs Pinter have been applied to probe the bonding and structure for this new class of zirconium-nitrogen multiple bonds. In addition to this study, the synthesis of U(III) and U(IV) complexes supported by the PN ligand is also discussed, which was conducted in collaboration with the Schelter and Baik labs. New complexes include the halide starting materials, (PN)2UI and (PN)2UCl2, which both yield (PN)2U(N3)2 when treated with NaN3. When reduced with potassium graphite, the azido complex produces a putative, transient uranium−nitrido moiety that undergoes an intramolecular C−H activation to form a rare example of a parent imido complex, [K(THF)3][(PN)UI(NH)[iPr2P(C6H3Me)N(C6H2Me2CH2)]]. Select calculations performed in collaboration with the Baik group are also presented.

Chapter 4: The (First) Structural Characterization of a Terminal Titanium Methylidene. The first example of a structurally characterized mononuclear, terminal titanium methylidene, (PN)2Ti=CH2, is presented in this chapter via one-electron oxidation of (PN)2Ti(CH3), followed by deprotonation using an ylide reagent or by H-atom abstraction using an aryloxyl radical. The use of the PN− ligand to stabilize this rare scaffold supports the molecular orbital discussion in Chapter 1, which shows this ligand scaffold to enforce an ideal orbital overlap for the formation of metal-ligand multiple bonds. The Ti=C distance was found to be 1.939(3) Å by a single crystal X-ray diffraction study. Multinuclear and multidimensional NMR spectroscopic experiments revealed the methylidene to engage in long-range interactions with protons on the ligand framework. Decomposition of the methylidene moiety in solution is also discussed. Computational studies in collaboration with the Baik lab are also presented, which showed that the Ti=C bond displays all the hallmarks of a prototypical Schrock-carbene.

Chapter 5: Stabilizing Unusual Oxidation States with the PN Ligand. This section details the final story of reactivity of our early transition metal complexes with the PN− ligand, in which we use this scaffold to stabilize highly reactive oxidation states. Reduction of the group 4 transition metal precursors (PN)2MCl2 (M = Zr or Hf), both readily prepared by transmetallation of 2 equiv. of LiPN with MCl4(THF)2 with a slight excess of KC8, resulted in the isolation of the trivalent complexes (PN)2MCl (M = Zr or Hf). Structural characterization of Zr and Hf (III) complexes is extremely rare, especially in non-metallocene scaffolds. All complexes were identified by solid-state X-ray diffraction analysis. For the trivalent complexes, access to these rare oxidation states called for use of EPR to determine the location of the radical electron. Low temperature X-band EPR spectroscopy conducted in collaboration with the Meyer group allowed for the identification of these metal-centered d1 radicals. A comparison with the isostructural and isoelectronic but more stable (PN)2TiCl is also presented.

Chapter 6: Phosphaethynolate Chemistry of Scandium to Stabilize Diisophosphaethynolate. This chapter takes a marked turn towards the experiments conducted with the phosphaethynolate reagent, Na(OCP)(dioxane)2.5. At the outset, a brief summary of the reactivity of this reagent with electropositive metals is described. Next, the reactivity with a do Sc precursor supported by nacnac ligands (nacnac− = [ArNC(CH3)]2CH) is described. The unprecedented OCPPCO ligand, diisophosphaethynolate, is formed via reductive coupling of a Sc−OCP precursor. Upon reduction with KC8, isolation of the dinuclear complex, namely [K(OEt2)]2[(nacnac)Sc(OAr)]2(OCPPCO), is observed, leading to a unique motif [OCPPCO]4−, stabilized by two scandium centers. Detailed NMR spectra of all complexes as well as IR and single-crystal X-ray studies were obtained to fully elucidate the nature of these complexes in solution as well as in the solid state. Theoretical calculations in collaboration with Prof. Balazs Pinter were used to probe the electronic structure and orbitals responsible for the bonding interactions in the Sc−OCPPCO−Sc skeleton, with comparison to to the linear mode observed in the precursor.

Chapter 7: Phosphaethynolate Chemistry of Titanium to Form a Ti2P2 Complex. Looking to the right, we describe the reactivity of the phosphaethynolate reagent with the d1 precursor (nacnac)TiCl(OAr), which when treated with –OCP forms the complex [(nacnac)Ti(OAr)]2(2:2,2-P2). The exact mechanism of this reductive decarbonylation of the phosphaethynolate ion, which serves as a P atom source, is discussed. This complex is the first structurally characterized Group 4 transition metal P2 complex. Additionally, its structure reveals the rhombic Ti2P2 core is essentially planar (as opposed to an expected butterfly shaped core), which when paired with alternating short and long Ti−P bonds, implies some degree of multiple bonding character between the Ti–P and P–P sites. In order to study the planar nature of the core, as well as the origin of the highly downfield 31P NMR spectroscopic signal, computational studies in collaboration with Prof. Balazs Pinter are reported.

Chapter 8: Phosphaethynolato Chemistry of Vanadium: Challenging the Binding Mode of –OCP in Early Transition Metals. The last chapter to discuss the reactivity of –OCP, this chapter will focus on the use of the d2 precursor (nacnac)VCl(OAr). From many studies with –OCP, transmetallation has largely resulted in coordination according to classical Lewis acid–base theory. That is, for harder early transition metal ions, O-bound coordination has been observed, whereas in the case of softer late transition metal ions, P-bound coordination predominates. Using our V(III) complex, (nacnac)V(OAr)(PCO), the first example is provided of a 3d early transition metal that binds –OCP via the P-atom. Full characterization studies of this molecule including HFEPR spectroscopy and SQUID measurements in collaboration with Prof. Josh Telser and Dr. Jurek Kryzstek, and theoretical studies are presented in collaboration with Prof. Balazs Pinter.

Chapter 9: A Cobalt Azido Complex in a Cis-Divacent Octahedral Geometry. A family of Co(II) complexes supported by the bulky, dianionic bis(pyrrolyl)pyridine pincer ligand pyrr2py (pyrr2py2− = 3,5-tBu2-bis(pyrrolyl)-pyridine) are discussed in this chapter. These compounds include adducts of ether, toluene, and adamantyl azide, where the core of this complex = (pyrr2py)Co. The cobalt azido complex is notable in that the organic azide binds to the metal through the γ-N in a κ1 fashion. Photolysis of the azido complex results in N2 extrusion and formation of C−H insertion product (pyrrpypyrrNHAd)Co, which we propose to form via insertion of the nitrene (NAd) into one tBu C−H bond, thus resulting in a pincer ligand having a pendant secondary amine. The structural characterization of all complexes is discussed as well as electrochemical data of the ether complex.

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