Synthesis and small molecule chemistry of the niobaziridine-hydride functional group

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Chemistry, 2005.Vita.Includes bibliographical references.Chapter 1. Synthesis and Divergent Reactivity of the Niobaziridine-Hydride Functional Group The synthesis, characterization and reactivity of the niobaziridine-hydride complex Nb(H)([eta]²-t- ]Bu(H)C=NAr)(N[Np]Ar)₂ (la-H; Np = neopentyl, Ar = 3,5-Me₂C₆H₃) is discussed. The niobaziridine-hydride functional group in complex la-H serves as a protecting group for the reactive three-coordinate d² species, Nb(N[Np]Ar)₃ (2a), via reversible C-H activation of the 3- ]H-containing N[Np]Ar ligand. At elevated temperatures, complex la-H rapidly converts to the neopentylimido complex, Nb(NNp)(Ar)(N[Np]Ar)₃ (3a), which is the product of N[Np]Ar ligand C'-N oxidative addition by putative three-coordinate 2a. Although not observed directly, the evidence for the intermediacy of 2a in N[Np]Ar ligand C-H and C-N activation processes has been obtained through isotopic labeling (H/D) studies. To further ascertain the propensity of la- H to serve as a masked form of 2a, its reactivity with small-molecule substrates was surveyed. Treatment of la-H with nitrous oxide (N₂0) or triphenylphosphine oxide (OPPh₃), readily generated the oxo Nb(V) complex, ONb(N[Np]Ar)₃, thus establishing la-H as a source of the potent two-electron reductant 2a. Complex la-H was also found to effect the two-electron reduction of a host of other inorganic substrates. However, when treated with certain unsaturated organic molecules, insertion into the Nb-H moiety of complex la-H is observed in which the niobaziridine ring is left intact. Based on synthetic studies, a coordinatively induced, C-H bond reductive elimination mechanism is proposed for reactions between la-H and small molecules.(cont.) This mechanistic proposal accounts for both the observed insertion and two-electron reduction behavior exhibited by niobaziridine-hydride la-H. To extend the generality of niobaziridine- hydride functional group as a protecting group for three-coordinate Nb(NR₂)₃ species, the complexes Nb(H)([eta]²-Me₂C=NAr)(N[i-Pr]Ar)₂ (lb-H) and Nb(H)([eta]²-Ad(H)C=NAr)(N[CH₂- Ad]Ar)₂ (1c-H) were synthesized. The thermal behavior of niobaziridine-hydrides lb-H and lc- 1-I is compared and contrasted to that of the N-neopentyl variant la-H. Chapter 2. Activation of Elemental Phosphorus: Synthesis of an Anionic Terminal Phosphide of Niobium The niobazinidine-hydride complex Nb(H)( [eta]²-t-Bu(H)C=NAr)(N[Np]Ar)₂ (la-H; Np neopentyl, Ar = 3,5-Me₂C₆H₃) was found to react quantitatively with elemental phosphorus (P₄) to provide the bridging diphosphide complex ([mu]₂:[eta]²,[eta]²-P₂)[Nb(N[Np]Ar)₃]₂ ([mu]-P₂)[2a]₂. Reductive cleavage of ([mu]-P₂)[2a]₂ with sodium amalgam afforded the sodium salt of the terminal niobium phosphide anion, [PNb(N[Np]Ar)₃)]⁻ ([2a-P]⁻), which is best formulated as containing a Nb-P triple bond. The phosphorus atom of [2a-P]⁻ has proven to be nucleophilic and is readily functionalized upon addition of an electrophile. Treatment of [2a-P]⁻ with trimethylstannyl chloride provided the terminal phosphinidene complex Me₃SnP=Nb(N[Np]Ar)₃ which contains a P-Sn single bond. However, treatment of [2a-P]⁻ with ClP(t-Bu)₂ or ClP(Ph)₂ provided niobium- complexed [eta]²-phosphinophosphinidene complexes, which contain considerable P-P multiple bonding character. Thus, substantial electronic reorganization of the Nb [equal] P moiety in [2a-P]⁻ is induced upon functionalization.(cont.) The tendency for the Nb[equal]P unit in [2a-P]- to undergo electronic reorganization has been exploited, resulting in the synthesis of a complexed [eta]²-P,P- diphosphaorganoazide (PPNR) species, which eliminates a 'P2' unit when heated. Furthermore, treatment of the phosphido anion [2a-P]⁻ with divalent group 14 salts affords complexes of the formulation ([mu]₂:[eta]₃,[eta]₃ -cyclo-EP₂)[Nb(N[Np]Ar)₃]₂ (E = Ge, Sn, Pb). The bridging cyclo-EP₂ units in these complexes can be considered as neutral 2[pi]-electron, three-membered rings isolobal to the cyclopropenium ion. The molecular and electronic structure of anion [2a-P]⁻ and several of its derivatives are discussed. Chapter 3. Isovalent Pnictogen for O(Cl) Exchange Mediated by Terminal Pnictide Anions of Niobium Reported herein is a new, metathetical P for O(Cl) exchange mediated by an anionic niobium phosphide complex which furnished phosphaalkynes (RC [equal] P) from acyl chlorides (RC(O)Cl) under mild conditions. The niobaziridine hydride complex, Nb(H)(t-Bu(H)C=NAr)(N[Np]Ar)₂ (la-H, Np = neopentyl, Ar = 3,5-Me₂C₆H₃), has been shown in chapter 2 to react with elemental phosphorus (P₄) affording the [mu]-diphosphide complex, ([mu]²:[eta]²,[eta]²-P₂)[Nb(N[Np]Ar)₃]₂ (([mu]- P)[2a]₂), which can be subsequently reduced by sodium amalgam to the anionic, terminal phosphide complex, [Na][PNb(N[Np]Ar)₃] (Na[2a-P]).(cont.) Treatment of Na[2a-P] with either ]pivaloyl (t-BuC(O)Cl) or 1-adamantoyl (1-AdC(O)Cl) chloride provides the thermally unstable, niobacycles, (t-BuC(O)P)Nb(N[Np]Ar)₃ (2-t-Bu) and (1-AdC(O)P)Nb(N[Np]Ar)₂ (2-1-Ad) which are intermediates along the pathway to ejection of the known phosphaalkynes t-BuC-P (3- t-Bu) and 1-AdC [equal] P (3-1-Ad). Phosphaalkyne ejection from 2-t-Bu and 2-1-Ad proceeds with formation of the niobium(V) oxo complex ONb(N[Np]Ar)₃ (2a-0) as a stable byproduct. Preliminary kinetic measurements for fragmentation of 2-t-Bu to 3-t-Bu and 2a-0 in C₆D₆ solution are consistent with a first order process. Separation of volatile 3-t-Bu from 2a-0 after thermolysis has been readily achieved by vacuum transfer in yields of 90%. Pure 2a-0 is recovered after vacuum transfer and can be treated with 1.0 equivalent of triflic anhydride (Tf₂O, Tf = SO₂CF₃) to afford the bistriflate complex, Nb(OTf)₂(N[Np]Ar)₃ (2a-(OTf)₂), in high yield. Complex 2a-(OT'f)₂ provides direct access to la-H upon reduction with magnesium anthracene, thus completing a cycle of element activation, small-molecule generation via metathetical P-atom transfer and deoxygenative recycling of the final niobium(V) oxo product. Extension of this metathetical P for O(C1) exchange to the synthesis of novel phosphaalkynes is discussed. In addition, the analogous N for O(C1) exchange reaction for the synthesis of organonitriles from the niobium nitrido anion, [NNb(N[Np]Ar)₃]⁻ ([2a-N]⁻) has been developed. Nitrido anion [2a- N]⁻ is obtained in a heterodinuclear N₂ scission reaction which employs the molybdenum trisamide system as a reaction partner.(cont.) Treatment of [2a-N]⁻ with acyl chloride substrates rapidly furnishes organonitriles concomitant with the formation of niobium oxo 2a-0. Deoxgenative recycling of 2a-0 to a niobium complex appropriate for heterodinuclear N2 scission has been developed as well. Utilization of ¹⁵N-labeled ¹⁵N₂ gas in this chemistry has afforded a series of ¹⁵N-labeled organonitriles which have been characterized by solution ¹⁵N NMR. While, no intermediate complexes are observed during the organonitrile formation process, synthetic and computational studies on model systems provide strong evidence for the intermediacy of niobacyclic species.by Joshua S. Figueroa.Ph.D

Na[Tc(CO)(CNp-F-ArDArF2)4]: an isocyanide analogue of the elusive Na[Tc(CO)5]

The first crystalline technetium complex in a negative oxidation state, [Tc−I(CO)(CNp-F-ArDArF2)4]−, was isolated and structurally characterized as its [Na(Crypt-2.2.2)]+ salt. It mirrors the properties of the textbook organometallic compound Na[Tc(CO)5], which has eluded isolation and structural characterization until today. [Na(Crypt-2.2.2)][Tc−I(CO)(CNp-F-ArDArF2)4] reacts expectedly as a nucleophile, which is demonstrated by reactions with HCl and ClSnMe3. They give the unprecedented monohydrido and trimethylstannyl complexes of technetium

Rhenium Complexes with p-Fluorophenylisocyanide

p-Fluorophenylisocyanide (CNPhpF) reacts with [Re(CO)5Br] under stepwise exchange of the carbonyl ligands depending on the conditions applied. The reaction stops with the formation of fac-[Re(CO)3Br(CNPhpF)2] in boiling THF. An ongoing carbonyl exchange is observed at higher temperatures, e. g. in refluxing toluene, with the final formation of the [Re(CNPhpF)6]+ cation. The progress of the reactions has been studied by 19F NMR spectroscopy and the structures of [Re(CO)Br(CNPhpF)4] and [Re(CNPhpF)6](BPh4) have been elucidated by X-ray diffraction

Superlattices, polymorphs, and solid state NMR spin-lattice relaxation measurements of 2,6-di-t-butylnaphthalene

Two polymorphs of 2,6-di-tert-butylnaphthalene, which differ by a factor of twelve in the number of crystallographically independent tert-butyl group environments, have been characterized by a synergistic combination of low-frequency 1H NMR spin–lattice relaxation rate measurements and conventional crystallographic structure determinations

The Chemistry of Phenylimidotechnetium(V) Complexes with Isocyanides: Steric and Electronic Factors

Organometallic approaches are of ongoing interest for the development of novel functional 99mTc radiopharmaceuticals, while the basic organotechnetium chemistry seems frequently to be little explored. Thus, structural and reactivity studies with the long-lived isotope 99Tc are of permanent interest as the foundation for further progress in the related radiopharmaceutical research with this artificial element. Particularly the knowledge about the organometallic chemistry of high-valent technetium compounds is scarcely developed. Here, phenylimido complexes of technetium(V) with different isocyanides are introduced. They have been synthesized by ligand-exchange procedures starting from [Tc(NPh)Cl3(PPh3)2]. Different reactivity patterns and products have been obtained depending on the steric and electronic properties of the individual ligands. This involves the formation of 1:1 and 1:2 exchange products of Tc(V) with the general formulae [Tc(NPh)Cl3(PPh3)(isocyanide)], cis- or trans-[Tc(NPh)Cl3(isocyanide)2], but also the reduction in the metal and the formation of cationic technetium(I) complex of the formula [Tc(isocyanide)6]+ when p-fluorophenyl isocyanide is used. The products have been studied by single-crystal X-ray diffraction and spectroscopic methods, including IR and multinuclear NMR spectroscopy. DFT calculations on the different isocyanides allow the prediction of their reactivity towards electron-rich and electron-deficient metal centers by means of the empirical SADAP parameter, which has been derived from the potential energy surface of the electron density on their potentially coordinating carbon atoms

Phenylimido complexes of rhenium: fluorine substituents provide protection, reactivity, and solubility

Reactions of [Re(NPhF)Cl3(PPh3)2] ({NPhF}2− = p-fluorophenylimide) with a variety of alkyl and aryl isocyanides have been studied. Different reactivity patterns and products have been obtained depending on the steric and electronic properties of the individual ligands. This involves the formation of 1 : 1 and 1 : 2 exchange products of Re(V) with the general formulae mer-[Re(NPhF)Cl3(PPh3)(isocyanide)] and cis- or trans-[Re(NPhF)Cl3(isocyanide)2]. The stability of the obtained products is correlated with the substitution pattern of the isocyanide ligands. The products have been studied by single-crystal X-ray diffraction and spectroscopic methods, including IR and multinuclear NMR spectroscopy as well as mass spectrometry. The use of partially fluorinated starting materials and ligands allows the modulation of the solubilities of the starting materials and the products as well as the monitoring of the reactions by means of 19F NMR. The attachment of the CF3 or F substituent on the isocyanides gives control over the steric bulk and the electronic properties of the ligands and, thus, their reactivity

A Complete Triad of Zero-Valent 17-Electron Monoradicals of Group 7 Elements Stabilized by m-Terphenyl Isocyanides

The first consistent series of mononuclear 17-electron complexes of three Group 7 elements has been isolated in crystalline form and studied by X-ray diffraction and spectroscopic methods. The paramagnetic compounds have a composition of [M0(CO)(CNp-F-ArDarF2)4] (M = Mn (1), Tc (3), Re (6); ArDArF2 = 2,6-(3,5-(CF3)2C6H3)2­C6H3)) and are stabilized by four sterically encumbering isocyanides, which prevent the metalloradicals from dimerization. They have a square pyramidal structure with the carbonyl ligands as apexes. The frozen-solution EPR spectra of the rhenium and technetium compounds are clearly anisotropic with large 99Tc and 185,187Re hyperfine interactions for one component. High-field EPR (Q band and W band) has been applied for the elucidation of the EPR parameters of the manganese(0) complex

H-1 Nuclear Magnetic Resonance Spin-Lattice Relaxation, C-13 Magic-Angle-Spinning Nuclear Magnetic Resonance Spectroscopy, Differential Scanning Calorimetry, and X-Ray Diffraction of Two Polymorphs of 2,6-Di-Tert-Butylnaphthalene

Polymorphism, the presence of structurally distinct solid phases of the same chemical species, affords a unique opportunity to evaluate the structural consequences of intermolecular forces. The study of two polymorphs of 2,6-di-tert-butylnaphthalene by single-crystal x-ray diffraction, differential scanning calorimetry (DSC), C-13 magic-angle-spinning (MAS) nuclear magnetic resonance (NMR) spectroscopy, and H-1 NMR spin-lattice relaxation provides a picture of the differences in structure and dynamics in these materials. The subtle differences in structure, observed with x-ray diffraction and chemical shifts, strikingly affect the dynamics, as reflected in the relaxation measurements. We analyze the dynamics in terms of both discrete sums and continuous distributions of Poisson processes

Sterically Induced Binding Selectivity of Single m-Terphenyl Isocyanide Ligands

Sterically encumbering m-terphenyl isocyanides are a class of metal-binding group that foster low-coordinate metal-center environments in coordination chemistry by exerting considerable intermolecular steric pressures between neighboring ligands. In the context of metal surfaces, the encumbering steric properties of the m-terphenyl isocyanides are shown to weaken the interaction between the metal-binding group and a planar substrate, leading to a preference for molecular adsorption at sites with convex curvature, such as the step edges and herringbone elbow sites on Au(111). Here, we investigate the site-selective binding of individual m-terphenyl isocyanide ligands on a Au(111) surface through scanning tunneling microscopy (STM) and inelastic electron tunneling spectroscopy (IETS). The site-dependent steric pressure alters the vibrational fingerprint of the m-terphenyl isocyanides, which is characterized with single-molecule precision through joint experimental and theoretical approaches. This study for the first time provides molecular-level insights into the steric-pressure-enabled surface binding selectivity as well as its effect on the chemical properties of individual m-terphenyl isocyanide ligands, thereby highlighting the potential to control the physical and chemical properties of metal surfaces through tailored ligand design

Quantized Nambu-Poisson Manifolds in a 3-Lie Algebra Reduced Model

We consider dimensional reduction of the Bagger-Lambert-Gustavsson theory to a zero-dimensional 3-Lie algebra model and construct various stable solutions corresponding to quantized Nambu-Poisson manifolds. A recently proposed Higgs mechanism reduces this model to the IKKT matrix model. We find that in the strong coupling limit, our solutions correspond to ordinary noncommutative spaces arising as stable solutions in the IKKT model with D-brane backgrounds. In particular, this happens for S^3, R^3 and five-dimensional Neveu-Schwarz Hpp-waves. We expand our model around these backgrounds and find effective noncommutative field theories with complicated interactions involving higher-derivative terms. We also describe the relation of our reduced model to a cubic supermatrix model based on an osp(1|32) supersymmetry algebra.Comment: 22 page