Bi- and tridentate ligand designs incorporating N-heterocyclic carbenes

点击翻译按钮获取中文摘要

This application proposes to investigate the development of novel ligands incorporating N-heterocyclic carbene (NHC) donors and to assess their ability to generate new chemistries with transition metals. The target of the proposal is twofold: to generate new ligands with general applicability in coordination chemistry, and to provide training opportunities for HQP. The proposal is designed for the training of three students, and the three individual subprojects are clearly delineated.***The first subproject targets the synthesis of chelating, sterically encumbered bis(NHC)s that can coordinate to metals in both cis and trans fashion, and the development of their low oxidation state nickel and cobalt chemistries. These are conceptually remarkably simple synthons that have received so far surprisingly little attention. Our investigations so far have shown that in nickel complexes, these bis(NHC)s behave substantially differently than their much investigated counterparts incorporating monodentate ligands, (NHC)2Ni, havig the ability to generate exciting new chemistries. The targeted bis(NHC)s will be further refined to obtain more robust complexes, and the nickel chemistry will be expanded to cobalt.***The second subproject targets the synthesis of tridentate, CSiC and CBC pincer ligands incorporating two NHC donors connected to the central main group donor by flexible alkyl pendant arms. Most common pincer ligands rely on rigid skeletons incorporating aromatic rings, yet our research showed that flexible akyl skeletons are better at accommodating the preferred coordination geometry at the metal and can provide additional stabilization via agostic and anagostic interactions. The ligands pursued within this line of investigation are derived from the chelating ligands described above through incorporation of BH, SiH and SiH2 moieties into their alkyl skeletons. According to preliminary investigations, these pincer ligands should be easy synthetic targets and their strong boron and silicon central donors is expected to render them excellent for the investigation of nickel and cobalt systems in oxidation state I.***The third subproject targets tridentate, NCN and CCC ligands with central NHC moieties and imine or phenyl pendant arms, respectively. The CCC ligands have been obtained via double rollover cyclometalation in ruthenium complexes and showed the ability to activate small molecules in solution and the solid state. Our investigations have shown that the proposed NCN ligands promote a behavior previously not observed for NHCs, which are quintessential ancillary ligands: ligand-assisted reactivity involving the carbene carbon. It was observed that ligands with imine pendant arms support N(CH)N structures, while the analogs with pyridyl pendant arms support NHC-containing NCN structures. Asymmetric NCN analogs with one pyridine and one imine pendant arm will be synthesized, striving to promote facile interconversion between the N(CH)N and NCN species.