Five- and Six-Coordinate Group 4 Compounds Stabilized by β-Ketiminate and Diketiminate Ligands:  Syntheses and Comparisons between Solid-State and Solution Structures

The preparation and reaction chemistry of β-diketiminate titanium and zirconium complexes is described. Amine elimination reactions work well for introducing Tolnacnac or Tolnacac to the metal centers (TolnacnacH = 2-(p-tolylamino)-4-(p-tolylimino)-2-pentene; TolnacacH = 4-p-toluidinopent-3-en-2-one). In certain cases, the iminium salt of the diketimine can be used to circumvent the unfavorable reaction kinetics. Salt elimination reactions starting from group 4 metal halides and β-diketiminate lithium reagents are the most versatile method for introducing β-diketiminate ligands to the metal. For (β-diketiminate)MCl3 compounds (M = Ti, Zr) η 5- and η 2-coordination modes can be controlled by modifying the diketiminate ligands. Several structures of five- and six-coordinate metal complexes were solved by X-ray diffraction methods. Five-coordinate metal complexes adopt both trigonal bipyramidal and square pyramidal geometries, and the six-coordinate metal complexes possess pseudooctahedral metal centers. For (Tolnacnac)2ZrX2 (X = Cl, OR, NMe2) the activation parameters for Λ/Δ conversion have been probed by dynamic NMR and are consistent with a Bailar-twist mechanism. At a common temperature, the isomerization rates follow the order Cl > OR > NMe2.