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First-Row Transition Metal Complexes Designed for Small Molecule Activation: Synthesis, Characterization, Reactivity, and Mechanisms

  • Author(s): Ziegler, Micah Samuel
  • Advisor(s): Tilley, T. Don
  • et al.

Chapter 1. The synthesis of discrete, cationic binuclear μ-aryl dicopper complexes [Cu2(µ-η1:η1-Ar)DPFN]X (Ar = C6H5, 3,5-(CF3)2C6H3, and C6F5; DPFN = 2,7-bis(fluoro-di(2-pyridyl)methyl)-1,8-naphthyridine; X = BAr4– and NTf2–; Tf = SO2CF3) was achieved by treatment of a dicopper complex [Cu2(µ-η1:η1-NCCH3)DPFN]X2 (X = PF6– and NTf2–) with tetraarylborates. Structural characterization revealed symmetrically bridging aryl groups, and 1H NMR spectroscopy evidenced the same structure in solution at 24 °C. Electrochemical investigation of the resulting arylcopper complexes uncovered reversible redox events that led to the synthesis and isolation of a rare mixed-valence organocopper complex [Cu2(µ-η1:η1-Ph)DPFN](NTf2)2, in high yield. The solid-state structure of the mixed-valence μ-phenyl complex exhibits inequivalent copper centers, despite a short Cu···Cu distance. Electronic and variable-temperature electron paramagnetic resonance spectroscopy of the mixed-valence μ-phenyl complex suggest that the degree of spin localization is temperature-dependent, with a high degree of spin localization observed at lower temperatures. Electronic structure calculations agree with the experimental results and suggest that the spin is localized almost entirely on one metal center.

Chapter 2. A discrete, dicopper μ-alkynyl complex, [Cu2(μ-η1:η1-C≡C(C6H4)CH3)DPFN]NTf2 (DPFN = 2,7-bis(fluoro-di(2-pyridyl)methyl)-1,8-naphthyridine; NTf2– = N(SO2CF3)2–), reacts with p-tolylazide to yield a dicopper complex with a symmetrically bridging 1,2,3-triazolide, [Cu2(µ-η1:η1-(1,4-bis(4-tolyl)-1,2,3-triazolide))DPFN]NTf2. This transformation exhibits bimolecular reaction kinetics and represents a key step in a proposed, bimetallic mechanism for copper-catalyzed azide–alkyne cycloaddition (CuAAC). The μ-alkynyl and μ-triazolide complexes undergo reversible redox events (by cyclic voltammetry), suggesting that a cycloaddition pathway involving mixed-valence dicopper species might also be possible. Synthesis and characterization of the mixed-valence μ-alkynyl dicopper complex, [Cu2(μ-η1:η1-C≡C(C6H4)CH3)DPFN](NTf2)2, revealed an electronic structure with an unexpected partially delocalized spin, as evidenced by electron paramagnetic resonance spectroscopy. Studies of the mixed-valence μ-alkynyl complex’s reactivity suggest that a mixed-valence pathway is less likely than one involving intermediates with only copper(I).

Chapter 3. Discrete, cationic µ-alkyl dicopper complexes [Cu2(µ-η1:η1-R)DPFN]NTf2 (R = CH3, CH2C(CH3)3, DPFN = 2,7-bis(fluoro-di(2-pyridyl)-methyl)-1,8-naphthyridine, NTf2– = N(SO2CF3)2–) were synthesized by treatment of an acetonitrile bridged dicopper complex [Cu2(µ-η1:η1-NCCH3)DPFN](NTf2)2 with LiR or MgMe2. Structural characterization by X-ray crystallography and NMR spectroscopy revealed that the alkyl ligands symmetrically bridge the two copper centers, and the complexes persist in room temperature solution. Notably, the µ-methyl complex showed less than 20% decomposition after 34 days in room temperature THF solution. Treatment of the µ-methyl complex with acids demonstrates the ability to install a range of monoanionic bridging ligands, and electrochemical characterization revealed oxidation-reduction events that evidence putative mixed-valence dicopper alkyl complexes. Computational studies suggest that the dicopper–carbon bonds are highly covalent, possibly explaining their relative persistence.

Chapter 4. Recent interest in the development of more environmentally sustainable chemical catalysis has encouraged the study of inorganic complexes that employ relatively earth abundant, first-row transition metals. To aid these efforts, a ligand was designed and employed to support low-coordinate, low-valent cobalt complexes. A neutral Co(II) complex was synthesized using the dianionic ligand TIPSDAX (2,7-di-tert-butyl-9,9-dimethyl-N4,N5-bis(triisopropylsilyl)-4,5-diamido-9H-xanthene) and reduced chemically to afford an anionic Co(I) complex. Both complexes were characterized by single-crystal X-ray diffraction. Measurements of their magnetic moments reveal an unexpectedly high spin for the Co(I) complex.

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