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 = 2p-tolylamino-4-p-tolylimino-2-pentene). The molecular structure of 1 was solved, and it shows a five-coordinate zirconium with three eta(1)-coordinated benzyl groups and an eta(2)-bound TTP ligand. When 1 is heated to 45 degrees C in hydrocarbon solvents, toluene is eliminated and the orthometalated product 2 is formed. The molecular structure of 2 indicates eta(1) and eta(2) benzyl groups. The variable-temperature H-1 NMR (-78 to 50 degrees 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 eta(1) and eta(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 degrees 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.