Diffusion NMR spectroscopy was applied to investigate all individual components and combinations thereof for the Cp2ZrMe2/MAO (DMAO)/TBP (MAO = methylaluminoxane, DMAO = AlMe3 depleted MAO, TBP = 2,6-di-tert-butylphenol) ternary system, selected as a prototypical catalytic pool for homogeneous olefin polymerisation. Both MAO and DMAO were found to self-aggregate in C6D6 with the latter having a higher propensity. TBP reacts with DMAO affording MeAl(2,6-di-tert-butylphenoxide)2 and causing a structural modification of DMAO, whose aggregates become much larger. The actual dimensions and self-aggregation tendency of (D)MAO, which depend on Al concentration and the possible presence of TBP, turned out to carry over to [Cp2Zr(μ-Me)2AlMe2]MeMAO (1) OSIP (outer sphere ion pair) and [Cp2Zr(+)Me···MeMAO(-)] (2) ISIP (inner sphere ion pair) that form upon activation of Cp2ZrMe2. Once the intrinsic self-aggregation tendency of MAO has been subtracted, OSIP 1 and ISIP 2 behave exactly as analogous ion pairs with borate ions: ISIP 2 does not self-aggregate, whereas OSIP 1 exhibits the same self-aggregation trends of zirconocene OSIPs with borate counterions.
NMR spectroscopy and DFT studies indicate that the Symyx/Dow Hf(IV)-pyridylamido catalytic system for olefin polymerization, [{N(-),N,CNph(-)}HfMe][B(C6F5)4] (1, Nph = naphthyl), interacts with ER(n) (E = Al or Zn, R = alkyl group) to afford unusual heterobimetallic adducts [{N(-),N}HfMe(μ-CNph)(μ-R)ER(n-1)][B(C6F5)4 in which the cyclometalated Nph acts as a bridge between Hf and E. (1)H VT (variable-temperature) EXSY NMR spectroscopy provides direct evidence of reversible alkyl exchanges in heterobimetallic adducts, with ZnR2 showing a higher tendency to participate in this exchange than AlR3. 1-Hexene/ERn competitive reactions with 1 at 240 K reveal that the formation of adducts is strongly favored over 1-hexene polymerization. Nevertheless, a slight increase in the temperature (to >265 K) initiates 1-hexene polymerization.
NMR spectroscopy has been exploited to investigate the reactions of Hf(IV) organometallic complexes with trialkylaluminum and dialkylzinc, with the aim of obtaining insights into the elementary steps of coordinative chain transfer polymerization (CCTP). Bis(cydopentadienyl)hafnium dimethyl (Cp2HfMe2, 1Me(2)) and [N-(2,6-diisopropylphenyl)-alpha-(2-isopropylphenyl)-6-(1-naphthalenyl)-2-pyridinemethanaminato]hafnium dimethyl (2Me(2)) complexes have been chosen as case studies for understanding the differences between poorly performing and highly active CCTP catalysts, in an attempt to assess the effect of the ancillary ligand on the transalkylation rate. 2Me(2) was found to react much more quickly with both AlEt3 and ZnEt2 in comparison to 1Me(2), mainly due to a remarkably lower activation enthalpy. In addition, while the ethylation rate was found to depend on the nature of the alkylating agent for 1Me(2), it does not for 2Me(2). This difference in reactivity was observed also in the case of the ion pairs obtained by reacting 1Me(2) and 2Me(2) with [CPh3][B(C6F5)(4)]. For the latter species, NMR indicated that two main deactivation pathways, namely anion decomposition and a -bond methatesis of Hf-alkyl groups, occur.\ud
Keyword
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.