(Arylimido)niobium(V) Complexes Containing 2-Pyridylmethylanilido Ligand as Catalyst Precursors for Ethylene Dimerization That Proceeds via Cationic Nb(V) Species

2-Anilidomethylpyridine-Derived Three-Coordinate Zinc Hydride: The Journey Unveils Anilide Backbone's Reactive Nature

Low-coordinate heteroleptic zinc hydrides are catalytically important but rare and synthetically challenging. We herein report three-coordinate monomeric zinc hydride on a 2-anilidomethylpyridine framework (NNL). The synthetic success comes through systematically screening a few different routes from different precursors. During the process, the ligand's anilide backbone interestingly appears to be more reactive than Zn's terminal site to electrophilic Lewis and Brønsted acids. The proligand NNLH reacts with [Zn{N(SiMe3)2}2] and ZnEt2 to give [(NNL)ZnA] (A = N(SiMe3)2 (1), Et(2)). Both are inert to PhSiH3 and H2 but react with HBpin only through the internal Zn-Nanilide bond to give the borylated ligand NNLBpin (3). The reactions of 1 and 2 with Ph3EOH (E = C, Si) afford a series of divergent compounds like [(NNLH)Zn(OSiPh3)2] (4), [Zn3(OSiPh3)4Et2] (5), and [EtZn(OCPh3)] (6). But in all cases, it is invariably the Zn-Nanilide bond protonated by the -OH with equal or higher preference than the terminal Zn-N or Zn-C bonds. A DFT analysis rationalizes the origin of such a reactivity pattern. Realizing that an acid-free route might be the key, reacting [(NNL)Li] with ZnBr2 gives [(NNL)Zn(μ-Br)]2 (7), which on successively treating with KOSiPh3 and PhSiH3 gives the desired [(NNL)ZnH] (8) as a three-coordinate monomer with a terminal Zn-H bond. Estimating the ligand steric in 8 shows the openness in Zn's coordination sphere, a desired criterion for efficient catalysis. This and a positive influence of the pyridyl sidearm is reflected in 8's superior activity in hydroborating PhC(O)Me by HBpin in comparison to Jones' two-coordinate anilido zinc hydride.

Solution XANES and EXAFS analysis of active species of titanium, vanadium complex catalysts in ethylene polymerisation/dimerisation and syndiospecific styrene polymerisation

Mechanistic studies in homogeneous catalysis through the solution transition metal K Edge XANES (X-ray absorption near-edge structure) and EXAFS (Extended X-ray absorption fine structure) analysis for vanadium and titanium complex catalysts for ethylene polymerisation/dimerization, and syndiospecific styrene polymerisation, including interpretation of the XANES spectra, have been introduced. The core excitation spectra of the complexes based on the time-dependent density functional theory (TD-DFT) can be used to interpret the Ti and V K-edge features and to extract information on the electronic structure from the XANES spectra. Theoretical calculations and experimental XAS analysis should have great potential for analysing the active species.

Solution X-Ray Absorption Spectroscopy (XAS) for Analysis of Catalytically Active Species in Reactions with Ethylene by Homogeneous (Imido)vanadium(V) Complexes—Al Cocatalyst Systems

Solution V K-edge XANES (X-ray absorption near edge structure) and EXAFS (extended X-ray absorption fine structure) analysis of vanadium(V) complexes containing both imido ligands and anionic ancillary donor ligands (L) of type, V(NR)(L)X2 (R = Ar, Ad (1-adamantyl); Ar = 2,6-Me2C6H3; X = Cl, Me, L = 2-(ArNCH2)C5H4N, OAr, WCA-NHC, and 2-(2’-benzimidazolyl)pyridine; WCA-NHC = anionic NHCs containing weak coordinating B(C6F5)3), which catalyze ethylene dimerization and/or polymerization in the presence of Al cocatalysts, has been explored. Different catalytically actives species with different oxidation states were formed depending upon the Al cocatalyst (MAO, Me2AlCl, AliBu3, etc.) and the anionic ancillary donor ligand employed. The method is useful for obtainment of the direct information of the active species (oxidation state, basic framework around the centered metal) in solution, and for better understanding in catalysis mechanism and organometallic as well as coordination chemistry.

XAS Analysis of Reactions of (Arylimido)vanadium(V) Dichloride Complexes Containing Anionic NHC That Contains a Weakly Coordinating B(C6F5)3 Moiety (WCA-NHC) or Phenoxide Ligands with Al Alkyls: A Potential Ethylene Polymerization Catalyst with WCA-NHC Ligands

(Arylimido)vanadium(V) dichloride complexes containing anionic N-heterocyclic carbene (NHC) ligands that contain weakly coordinating B(C₆F₅)₃ moieties (WCA-NHC) of the type [V(NAr)Cl₂(WCA-NHC-Ar')] (5, Ar = 2,6-Me₂C₆H₃, Ar' = 2,6- ⁱ Pr₂C₆H₃) showed significant catalytic activity for ethylene polymerization in the presence of Al cocatalysts (MAO and Al ⁱ Bu₃); the activity by the 5-MAO catalyst (19 500 kg-PE/mol-V·h; TOF 11 600 min^-1) is the highest among those reported using the other (imido)vanadium(V) complexes in the presence of MAO, and the 5-Al ⁱ Bu₃ catalyst showed higher activity (66 000 kg-PE/mol-V·h; TOF 39 200 min^-1). The V K-edge X-ray absorption near-edge structure (XANES) analyses (in toluene) strongly suggest the formation of certain vanadium(III) species by reduction with Al ⁱ Bu₃ accompanying structural changes; the EXAFS analysis suggests the presence of the arylimido ligand and one V-Cl bond (2.34 ± 0.04 Å), which is longer than those [2.1901(8)-2.2462(8) Å] in the reported (imido)vanadium(V) complexes. The XANES analysis of [V(NAr)Cl₂(OAr)] strongly suggests the formation of the other vanadium(III) species by reduction with Me₂AlCl or Et₂AlCl, and the EXAFS analysis suggests the presence of the arylimido ligand and two V-Cl bonds (2.45 ± 0.03 Å). The XANES spectra showed no significant changes in both the pre-edge peak(s) and the edge peak when these complexes were treated with MAO, suggesting that the basic geometry and the high oxidation state were preserved under these conditions.