Solution Structure and Dynamics of Hf-Al and Hf-Zn Heterobimetallic Adducts Mimicking Relevant Intermediates in Chain Transfer Reactions.

Cationic cyclometalated hafnocenes [CpCpHf][B(CF)] () and [CpCpHf][B(CF)] ( and ) were synthesized from the corresponding [(Cp)HfMe][B(CF)] () and [(Cp)HfMe][B(CF)] () complexes via C-H activation. , , and , mimicking a propagating M-polymeryl species (M = transition metal) with or without a β-methyl branch on the metalated chains, serve to investigate whether and how the nature of the last inserted olefin molecules changes the structure, stability, and reactivity of the corresponding heterobimetallic complexes, formed in the presence of aluminum- or zinc-alkyl chain transfer agents (CTAs), which are considered relevant intermediates in coordinative chain transfer polymerization (CCTP) and chain shuttling polymerization (CSP) technologies. NMR and DFT data indicate no major structural difference between the resulting heterobridged complexes, all characterized by the presence of multiple α-agostic interactions.

Interception of Elusive Cationic Hf-Al and Hf-Zn Heterobimetallic Adducts with Mixed Alkyl Bridges Featuring Multiple Agostic Interactions.

Invited for the cover of this issue is the group of Cristiano Zuccaccia at the Università degli Studi di Perugia. The image depicts a relation of the nuances of chemical bonding to the diverse ways that animals "bind" to their natural surroundings. Read the full text of the article at 10.

Interception of Elusive Cationic Hf-Al and Hf-Zn Heterobimetallic Adducts with Mixed Alkyl Bridges Featuring Multiple Agostic Interactions.

Heterobimetallic complexes with inequivalent bridging alkyl chains are very often invoked as key intermediates in many catalytic processes, yet their interception and structural characterization are lacking. Such complexes have been prepared from reactions of the cationic cyclometalated hafnocene [Cp Cp Hf][B(C F ) ] (1) with main group metal alkyls to afford the corresponding hetero-bridged cationic products, [Cp Cp Hf(μ-R)E(R) ][B(C F ) ] (E=Al or Zn; R=Me, Et, or iBu). NMR and DFT studies demonstrate that both bridging alkyls establish agostic interactions with Hf, which are appreciably stronger for ethyl rather than methyl groups.

Mechanism of activation of a hafnium pyridyl-amide olefin polymerization catalyst: ligand modification by monomer.

We have investigated the olefin polymerization mechanism of hafnium catalysts supported by a pyridyl-amide ligand with an ortho-metalated naphthyl group. Ethylene-alpha-olefin copolymers from these catalysts have broad molecular weight distributions that can be fit to a bimodal distribution. We propose a unique mechanism to explain this behavior involving monomer modification of the catalyst, which generates multiple catalyst species when multiple monomers are present.

Catalytic production of olefin block copolymers via chain shuttling polymerization.

We report a catalytic system that produces olefin block copolymers with alternating semicrystalline and amorphous segments, achieved by varying the ratio of alpha-olefin to ethylene in the two types of blocks. The system uses a chain shuttling agent to transfer growing chains between two distinct catalysts with different monomer selectivities in a single polymerization reactor. The block copolymers simultaneously have high melting temperatures and low glass transition temperatures, and therefore they maintain excellent elastomeric properties at high temperatures.

Structure and H(2)-Loss Energies of OsHX(H(2))(CO)L(2) Complexes (L = P(t-Bu)(2)Me, P(i-Pr)(3); X = Cl, I, H): Attempted Correlation of (1)J(H-D), T(1min), and DeltaG().

1J(H-D), T(1min) and k(1) for H(2) dissociation from OsHX(H(2))(CO)L(2) have been measured for X = Cl, I, H (L = P(t-Bu)(2)Me or P(i-Pr)(3)), as well as for OsCl(2)(H(2))(CO)(P(i-Pr)(3))(2). For comparison, new data (including previously unobserved coupling constants) have been reported for W(HD)(CO)(3)(P(i-Pr)(3))(2). A comprehensive consideration of T(1min) data for over 20 dihydrogen complexes containing only 1-2 phosphines cis to H(2), together with a consideration of the shortest "conceivable" H-H distance for H(2) bound to a d(4) or d(6) metal, is used to argue that the "fast spinning" model is not appropriate for determining r(H-H) in such complexes.