Molecular Electrostatic Effects from Anionic Borate Ligands in Oxidative Reactivity

Metal−oxo intermediates mediate challenging C−H functionalization and O−O coupling reactions in enzymatic systems. The preparation of metal complexes with comparable reactivity is appealing for the synthesis of fine chemicals and alternative energy schemes. However, model complexes typically display relatively muted reactivity, possibly due to the use of strongly donating ligands rather than the weakly donating imidazole and carboxylate ligands used in enzymes. In lieu of the wide variety of ligands available to synthetic systems, enzyme active sites are known to exert large electric fields on substrates. The involvement of electric fields in enzymatic reactivity has motivated the study of electrostatic effects in molecular complexes featuring non-interacting charged functional groups. Inspired by these examples, this thesis details the use of weakly donating and charged ligands to target metal-oxo model complexes with unusual electronic structures and fundamental studies on the effects of charged groups on ligand properties. In Chapter 2, an unusual series of discrete iodosyl- and iodoxyarene adducts of Co are isolated from the reaction between cobalt metalated tris-pyrazolyl borate (Tp) complexes and the respective O-atom transfer reagents. The reactivity of these adducts with O-atom acceptors and an H-atom donor was investigated. Reactivity data are consistent with the involvement of a transient oxo complex in one case, while the two other systems appear to react with substrates directly as iodosyl- or iodoxyarene adducts. The observation of adduct reactivity suggests that the Tp is generally not sufficiently donating to support high valent intermediates. These results motivated a deeper investigation of the effects of charged moieties on ligand donor strength. In Chapter 3, the synthesis of a novel anionic phosphine, PPh2CH2BF3K, the corresponding selenides [PPh4][SePPh2CH2BF3] and [TEA][SePPh2CH2BF3], and the Rh carbonyl complex [PPh4][Rh(acac)(CO)(PPh2(CH2BF3))] are reported. Solvent-dependent changes in the phosphorus selenium coupling constants (JP–Se) of the selenides were fit using Coulomb's law. These data support that up to 80% of the increase in donor strength of [PPh4][SePPh2CH2BF3] relative to SePPh2Et is a result of electrostatic contributions. This JP–Se method was extended to [PPh4][SePPh2(2-BF3Ph)] and likewise estimates up to a 70% electrostatic contribution to the increase in donor strength. The use of PPh2CH2BF3K also accelerated C–F oxidative addition reactivity with Ni(COD)2 in comparison to the neutral phosphines PEt3 and PCy3. This enhanced reactivity prompted the investigation of catalytic defluoroborylation of fluoroarenes. These results demonstrated that covalently bound charged functionalities can exert a significant electrostatic influence under common solution phase reaction conditions. In Chapter 4, the synthetic approaches to tri-anionic weakly donating tris-pyridyl ligands are outlined. The preparation of a novel BF2CF3− anion to increase the solubility is reported. Future studies with these new ligands will initially be directed at characterizing the donor strength through metalation with Ni-NO and measurement of the nitrosyl stretching frequency. The impact of charge location on the nitrosyl stretching frequency is expected to provide insight on the electric field environment at the metal center. Appendix 1 details the synthesis of pentadentate anionic ligands based on the tetra-pyrazolyl lutidine ligand. Appendices 2-4 contain supporting data for Chapter 2-4