Author: Kensha Marie ClarkPublisher:
ISBN: 9781124209203
Category :
Languages : en
Pages : 195
Book Description
Redox-active ligands have been previously shown to enhance reactivity at metal centers by serving as electron reservoirs. This class of ligand facilitates intramolecular electron transfer, enabling reactivity pathways such as oxidative addition and reductive elimination at d0 metal centers. Towards the goal of coupling ligand promoted reactivity with the known reactivity of low-coordinate Group IV imido complexes, the sterics, electronics, and reactivity of the N, N'-bis(arylimino)acenaphthylene (BIAN) ligand set were explored. Initially, the coordination geometry enforced by three BIAN ligand derivatives, dpp-BIAN, tmp-BIAN, and dmp-BIAN (dpp = 2,6-diisopropylphenyl; tmp = 2,4,6-trimethylphenyl; dmp = 3,5-dimethylphenyl), in titanium imido complexes was evaluated. These three ligands present different steric profiles, with dpp-BIAN providing the most steric bulk, while dmp-BIAN ligand offers the least steric protection. In these studies, it was observed that the bulkier ligands, dpp-BIAN and tmp-BIAN, stabilize coordinatively unsaturated imido complexes, (dpp-BIAN)TiCl2(=NR) and (tmp-BIAN)TiCl2(=NR) (R = tBu or 2,6-dimethylphenyl). The dmp-BIAN ligand exclusively forms the six-coordinate amino-imido complexes, (dmp-BIAN)TiCl2(=NR)(NH2tBu). Neutral tris-semiquinonate complexes, M(dmp-BIAN isq)3, of titanium, vanadium, and chromium were structurally, spectroscopically, and electrochemically characterized. From these data, the ability of the BIAN ligand to facilitate intramolecular charge redistribution in early metals was determined. In these complexes, the ligand not only enables reversible redox processes, but antiferromagnetic exchange coupling is observed between the radical ligands and metal ions. The bulky dpp-BAAN ligand, which is the two electron reduced form of dpp-BIAN, was used to make unprecedented mixed-valence cluster complexes of zirconium and hafnium, [(dpp-BAAN)3M3(u2-Cl)3(u3-Cl)2] (M = Zr or Hf). In these complexes the metal centers act as non-discrete redox sites, and an unpaired electron is considered delocalized over the entire molecule. The synthesis and subsequent isolation of these complexes demonstrates the stability that the electronically flexible BIAN ligand set provides. After considering the ligand sterics and electronics, Group IV synthons (dpp-BAAN)MCl2 and (dpp-BAAN)MR2 (M = Ti or Zr; R = Np, CH2Si(CH3)3, or OtBu) were synthesized and characterized. The reactivity of (dpp-BAAN)TiCl2 towards oxidants was evaluated. It was found that this complex successfully undergoes oxidative chloride addition and nitrene addition. Nitrene addition to the (dpp-BAAN)TiNp2 complex results in the unexpected extrusion of bineopentyl from the metal center to generate a putative imido species. Protonolysis of the same synthon similarly forms a putative imido species. The addition of chloride to this species yields a five-coordinate imido complex, (dpp-BIAN)TiCl2(=NR) (R = p-MeCH6), and establishes the basis for further reactivity studies.