Investigations into the multielectron reactivity of cobalt complexes with redox-active o-phenylenediamide ligands
Creators: name: Zou, Minzhu
Publisher: name: No Publisher Supplied
Year: 2024

description: Chapter 1.The development of new organometallic complexes and (electro)catalysts based on earth-abundant, non-previous metals that are capable of multielectron reactivity is a pressing research field for improving the sustainability of many organic transformations. The introduction of redox-active ligands into transition metal complexes can lead to novel redox behavior due to the propensity of these ligands to serve as electron reservoirs. This prospect is especially attractive at first-row transition metal systems that traditionally favor one-electron redox processes and radical reactivity over the two-electron redox cycles typically observed at the noble metals. The redox flexibility afforded by redox-active ligands may enable substrate activation and/or bond forming and breaking processes that would otherwise not be accessible at first-row metal centers with traditional redox innocent ligands. Herein, key examples of substrate activation and organic transformations that are facilitated by ligand-promoted multielectron reactivity will be discussed. We highlight the electrochemical properties of these metal complexes in relation to their substrate reactivity, where, in several cases, these systems are limited by well-separated, sequential redox events at two redox-active ligands. This discussion sets the stage for the subsequent chapters of this thesis, in which we describe a family of novel cobalt complexes bearing a single-redox active ligand that are capable of multielectron redox behavior and multielectron substrate activation. Chapter 2. Achieving multielectron activity at first-row transition metal complexes has important implications for homogeneous catalysis using earth-abundant metals. In this chapter, we report a family of cobalt-phenylenediamide complexes that undergo reversible 2e− oxidation regardless of the ligand substituents, enabling unprecedented multielectron redox tuning over 0.5 V and, in each case, affording the dicationic CoIII-benzoquinonediimine species. The neutral complexes are best described as delocalized systems with π-bonding in the metallocycle, consistent with a closed-shell singlet ground-state predicted by density functional theory (DFT) calculations. Our DFT results also predict an ECE pathway for 2e− oxidation (ECE = electrochemical step, chemical step, electrochemical step), where the first 1e− step involves redox-induced electron transfer to yield a CoII intermediate. Disruption of the metallocycle bonding in this state enables a change in the coordination geometry through association of an addition ligand, which is critical for accessing the potential inversion. The electronic properties of the phenylenediamide ligand govern whether the second electron is lost from the ligand or metal, providing a remarkable example of tunable 2e− behavior at first-row systems. Chapter 3. The reactivity of an electron-rich cobalt complex bearing an o-phenylenediamide ligand with electrophilic trifluoromethyl and electrophilic fluorine sources is reported. These reactions lead to rapid generation of the CoIII−CF3 or CoIII−F complex, respectively, promoted by redox-active ligand-to-substrate two-electron transfer. The rate of trifluoromethyl addition at the cobalt center is found to correlate with the potential difference between the cobalt complex and the trifluoromethyl reagent. In this chapter, we also present initial demonstrations of radical trifluoromethylation and nucleophilic fluorination of organic substrates, setting the stage for the development of electrocatalytic pathways for these bond-forming reactions. Chapter 4. In this chapter, the synthesis and characterization of [CoIII–CF3] complexes in three oxidation states are presented. Each species is isolable and has been fully characterized by various methods. The Co–CF3 bond formation occurs between [CpCp(tBuUreaopda)] and an electrophilic trifluoromethyl source, which proceeds without a change in the formal oxidation state of the cobalt center; instead, the redox-active phenylenediamide ligand serves as a 2e– reservoir, generating [CpCp(tBuUreabqdi)(CF3)]+. Electrochemical studies of the monocationic species [Co–CF3]+ reveal two reversible 1e– reductions that are assigned as ligand-based processes. Chemical reduction with 1 or 2 equiv. reducing agent enables isolation of the neutral and anionic complexes, respectively, where the [CoIII–CF3] bond remains intact. Analysis of the crystal structures of these three species show systematic changes to the N–Cphenlyene and C–C bond lengths, consistent with reduction of the ligand from [opda]2– to [s-bqdi]•– to [bqdi]0. In principle, these [Co–CF3]n+ complexes may enable formal umpolung of the CF3 group if the CF3– moiety can be released from [Co–CF3]– and used for organic trifluoromethylation reactions. In contrast, the reaction of [CpCo(tBuUreaopda)] and alkyl triflates results in ligand-centered alkylation of the ureayl substituents instead of the desired Co–alkyl bond formation, suggesting less favorable bond formation at cobalt and greater nucleophilic accessibility of the ureayl groups compared to the metal center. Chapter 5. The o-phenylenediamine (opdaH2) ligand in half-sandwich cobalt complexes [CpCo(opdaH2)(MeCN)]2+ or [Cp*Co(opdaH2)(MeCN)]2+ can react with ligand-centered hydrogen atom transfer. In this chapter, we show that these complexes transfer two hydrogen atoms to molecular oxygen (O2) or the tBu3PhO• radical, with concomitant formation of [Co-bqdi]2+ in which the ligand exists in the diimine form. The reactivity of [Co-opdaH2]2+ and O2 (or air) was monitored by UV-vis and NMR spectroscopy, which revealed the apparent direct conversion of [Co-opdaH2]2+ to [Co-bqdi]2+ without the presence of other intermediates. DFT calculations suggest consecutive hydrogen atom transfer from [Co-opdaH2]2+ in which the BDFE value for the first N–H bond is greater than that of the second N–H bond. The calculated average BDFE value of the N–H bonds (70.8 kcal/mol) is very close to our experimentally determined value (74.1 kcal/mol). Combined with our measurements of the N–H bond acidity, we have fully characterized the thermodynamic parameters of this system, which enables construction of a double square scheme to depict the overall 2e−, 2H+ or 2H• transfer from [Co-opdaH2]2+.