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

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./nChapter 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)./nChapter 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./nChapter 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.

Advisor: Tilley, T. Don