Novel diaminocarbene ligands and their applications in ruthenium-based metathesis catalysts
Rosen, Evelyn Louise
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With the ever expanding utility of transition metal catalysis, there has been a thrust both to develop catalysts with unique selectivites or activites, and to understand the factors which govern these characteristics at both a fundamental and practical level. Olefin metathesis has become an essential reaction for the synthesis of small molecules in addition to polymeric materials. We have pursued two distinct ligand classes based on diaminocarbenes with novel architectures to address specific limitations within this useful class of reactions: 1) limited access to polymeric materials with controlled microstructures and 2) poor stereoselectivity in Ru-catalyzed cross-metathesis (CM) reactions. Numerous phosphines and N-heterocyclic carbenes (NHCs) have been used as ligands for Ru metathesis catalysts, and the resulting activity is very sensitive to their steric and electronic nature. We envisioned that we could take advantage of this dependence by developing a catalyst with tunable ligand donicity. Redox-switchable ligands can lead to catalysts whose selectivity and/or activity are dependent upon the ligand oxidation state. Towards this purpose, we have developed a ligand which incorporates a 1,1’-disubstituted ferrocene moiety into the backbone of a diaminocarbene (FcDAC). Upon ligation of FcDAC to various transition metals, we were able to use cyclic voltammetry and a spectroelectrochemical FT-IR experiment to show electronic communication between FcDAC and the coordinated metal. We then pursued Ru metathesis catalysts incorporating these ligands. The ring-opening metathesis polymerization of 1,5-cyclooctadiene was studied using [(FcDAC)(PPh₃)Cl₂Ru=(3-phenylindenylid-2-ene)] as the catalyst. Chemical redox reactions were used to establish the ability of FcDAC to impart redox-tunable properties to Ru metathesis catalysts. A new ligand class pioneered in our group, N-aryl,N-alkyl acyclic diaminocarbenes (ADCs), was also studied in various Ru metathesis catalysts. To our delight, these catalysts showed lower E : Z ratios than analogous NHC ligands in two representative CM reactions. We also investigated the conformational diversity of these differentially substituted ADCs given their ability to rotate about their C–N bonds, in particular, to determine how this might influence their donicity. Complexes of the type [(ADC)Ir(COD)Cl] and [(ADC)Ir(CO)₂Cl] were studied, given the wealth of structural and spectral data available for analogous compounds incorporating related ligand classes. Different conformations resulted depending on the N-substituents and the nature of the metal complex. Interestingly, the electron donating ability of ADC ligands was found to depend on their conformation, as evidenced by FT-IR and cyclic voltammetry. This established a new avenue for tuning the donor properties of differentially substituted ADC ligands. The unique properties of these novel ligand classes were demonstrated in Ru metathesis catalysts. However, on a broader level, these ligands are expected to have utility in diverse catalytic applications.