Ligand Field Inversion in Sterically Confined Copper Architectures

Abstract

Ligand field inversion is general for high-valent cupryl (CuIII) complexes in which a physical 3d8 electronic configuration is inaccessible and metal-ligand multiple bonds (MLMBs) are unfeasible. These findings afford refined considerations of how the presence or absence of MLMBs in late transition metal complexes dictate electronic structure and multi-electron catalysis, holding implications in chemical bonding, copper-mediated catalysis, and metalloenzyme activity. Transition metal complexes featuring metal-ligand multiple bonds are powerful intermediates through which heteroatom functionality is transferred to unreactive substrates through C–H functionalization (Chapter 1). We present the synthesis of a copper nitrenoid situated within a sterically confined dipyrrin scaffold flanked with peralkylated hydrindacenes, allowing us to probe the capacity for Cu to engage in MLMB formation (Chapter 3). Inspection of bond metrics by single-crystal X-ray diffraction reveals evidence of nitrenoid dearomatization, in accord with a redox non-innocent moiety. Multiconfigurational SORCI calculations and multinuclear X-ray absorption spectroscopy measurements (Cu K-edge, Cu L2,3-edge, N K-edge) reveal a dominate CuI (3NAr) electronic structure in which Cu is bound by the nitrene fragment through dative bonding with unquenched triplet nitrene character. Nitrene transfer with electron-deficient aryl azides proceeds with retention of stereochemistry for amination and aziridination with density functional theory calculations supporting a stepwise hydrogen-atom abstraction followed by barrierless radical recombination to yield the corresponding C–N bond (Chapter 4). Hammett analysis reveals an electrophilic nitrene transfer process with rate-limiting nitrenoid formation as ascertained through detailed kinetic isotope effect measurements and computational analysis. he hydrophobic cleft produced by the hydrindacene-flanked dipyrrin allows for atypical ligation of denitrification-pertinent species, among additional unprecedented Cu–adducts (Chapter 5). Bond metrics and vibrational spectra reveals minimal activation upon ligation, attributed to the heightened electrophilicity of Cu and consequent of the dipyrrin scaffold. Formation of a cupric superoxide species is observed upon air exposure, with identification and characterization by single-crystal X-ray diffraction, resonance Raman spectroscopy, multinuclear X-ray absorption spectroscopy measurements and multiconfigurational calculations (Chapter 5, 6). Despite the unparalleled thermal and aqueous stability of this species, desorption of the O2 is facilitated under controlled conditions thermally or chemically, providing new pertinent principles for directing sorbents for air separation. Catalytic diarylhydrazine oxidation using air is observed, with intermittency of an observable Cu(H2O2) adduct. The electron structure and reactivity of the Cu(O2) species is juxtaposed against isostructural Ni(O2) and Co(O2) species, all exhibiting substantial O2 character in acceptor orbitals. Thermolysis of the Cu(N2O) adduct in solution affords the corresponding three-coordinate, cupric hydroxide, accessed similarly through controlled hydrolysis of a cupric alkoxide precursor (Chapter 7). Computations reveal unpaired spin on the hydroxide ligand engendered from a covalent Cu−OH bond, supported by H–atom abstraction from exogeneous substrates and radical recombination with persistent alkyl radicals. Multiconfigurational calculations on the copper oxenoid intermediate, potentially accessed through a single-crystal-to-single-crystal transformation, reveal a near-equal CuI (3O)/CuII(2O) contributions in lieu of a CuIII(O) assignment, in accord with ligand field inversion. These results demonstrate the feasibility of cupric hydroxides in biological hydroxylation pathways to engage in C–H bond activation and C–O bond formation