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-tolylimi no-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 et a(2)-bound TTP ligand. When 1 is heated to 45 degrees C in hydrocarbon solv ents, toluene is eliminated and the orthometalated product 2 is formed. The molecular structure of 2 indicates eta(1) and eta(2) benzyl groups. The va riable-temperature H-1 NMR (-78 to 50 degrees C) spectra exhibit a single b enzyl resonance. The magnitude of (1)J(CH) for the benzyl methylene resonan ce is consistent with a rapid exchange between eta(1) and eta(2) bonding mo des in solution. Isotopic labeling experiments employing (PPP-d(10))Zr(CH2P h)(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 exh ibits a large kinetic isotope effect: k(H)/k(D) = 5.2(5) at 65 degrees C. T he thermal stability of alkyl complexes is improved by replacing the ortho protons with isopropyl groups. (DDP)ZrMe3 (5) can be prepared from (DDP)ZrC l3 via halide metathesis using MeLi (DDP = 2-(2,6-diisopropyl)phenylamino-4 -(2,6-diisopropyl)phenylimino-2-pentenato). The thermal stability of 5 is g reatly enhanced compared to those of 1 and 3.