Syntheses and Structures of Five-Coordinate Zirconium Alkyl Complexes Supported by Diketiminate Ligands

An alkane elimination reaction generates the diketiminate compound (TTP)Zr(CH2Ph)3 (1) from Zr(CH2Ph)4 and TTPH (TTPH = 2-p-tolylamino-4-p-tolylimino-2-pentene). The molecular structure of 1 was solved, and it shows a five-coordinate zirconium with three η 1-coordinated benzyl groups and an η 2-bound TTP ligand. When 1 is heated to 45 °C in hydrocarbon solvents, toluene is eliminated and the orthometalated product 2 is formed. The molecular structure of 2 indicates η 1 and η 2 benzyl groups. The variable-temperature 1H NMR (−78 to 50 °C) spectra exhibit a single benzyl resonance. The magnitude of 1 J CH for the benzyl methylene resonance is consistent with a rapid exchange between η 1 and η 2 bonding modes in solution. Isotopic labeling experiments employing (PPP-d 10)Zr(CH2Ph)3(3-d 10, PPP = 2-phenylamino-4-phenylimino-2-pentenato) support direct C−H activation through a four-centered transition state. Based on kinetic experiments, C−H activation is unimolecular, and the rate-limiting step exhibits a large kinetic isotope effect: k H/k D = 5.2(5) at 65 °C. The thermal stability of alkyl complexes is improved by replacing the ortho protons with isopropyl groups. (DDP)ZrMe3 (5) can be prepared from (DDP)ZrCl3 via halide metathesis using MeLi (DDP = 2-(2,6-diisopropyl)phenylamino-4-(2,6-diisopropyl)phenylimino-2-pentenato). The thermal stability of 5 is greatly enhanced compared to those of 1 and 3.