Exploring the Use of Transition Metals in the Synthesis of Novel Metal-Ligand Multiple Bonds, Azide Complexes, and Unprecedented Reactivity with the Phosphaethynolato Reagent

Lauren N Grant, University of Pennsylvania

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 [μ2-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, is...

Subject Area

Inorganic chemistry|Chemistry

Recommended Citation

Grant, Lauren N, "Exploring the Use of Transition Metals in the Synthesis of Novel Metal-Ligand Multiple Bonds, Azide Complexes, and Unprecedented Reactivity with the Phosphaethynolato Reagent" (2019). Dissertations available from ProQuest. AAI13898885.
https://repository.upenn.edu/dissertations/AAI13898885

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