Synthesis and Structural Analysis of (Arylimido)vanadium(V) Complexes Containing Phenoxyimine Ligands: New, Efficient Catalyst Precursors for Ethylene Polymerization

Synthesis and Characterization of Two "Tied-Back" Lithium Ketimides and Isolation of a Ketimide-Bridged [Cr2]6+ Dimer with Strong Antiferromagnetic Coupling

Reaction of 1 equiv of KN(SiMe₃)₂ with 9-fluorenone results in the formation of (Me₃Si)N═C₁₃H₈ (1) in high yield after work-up. Addition of 1 equiv of phenol to 1 results in rapid desilylation and formation of 9-fluorenone imine, HN═C₁₃H₈ (2). Subsequent reaction of 2 with 1 equiv of LiNⁱPr₂ results in deprotonation and formation of [Li(Et₂O)]₄[N═C₁₃H₈]₄ (3) in good yield. Reaction of 1 equiv of KN(SiMe₃)₂ with 2-adamantanone for 7 days at room temperature results in the formation of (Me₃Si)N═C₁₀H₁₄ (4) in good yield. Dissolution of 4 in neat MeOH results in rapid desilylation concomitant with formation of 2-adamantanone imine, HN═C₁₀H₁₄ (5). Subsequent reaction of 5 with 1 equiv of LiNⁱPr₂ results in formation of [Li(THF)]₄[N═C₁₀H₁₄]₄ (6). Both 3 and 6 were characterized by X-ray crystallography. Finally, reaction of CrCl₃ with 3.5 equiv of 6 results in formation of the [Cr₂]⁶⁺ dimer, [Li][Cr₂(N═C₁₀H₁₄)₇] (7), which can be isolated in modest yield after work-up. Complex 7 features a Cr-Cr bond length of 2.653(2) Å. Additionally, solid-state magnetic susceptibility measurements reveal strong antiferromagnetic coupling between the two Cr centers, with J = -200 cm^-1.

Vanadium complexes derived from oxacalix[6]arenes: structural studies and use in the ring opening homo-/co-polymerization of ε-caprolactone/δ-valerolactone and ethylene polymerization

Oxacalix[6]arene vanadium complexes have been employed for the ROP of cyclic esters and ethylene polymerization.

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.

Synthesis and Structural Analysis of (Imido)vanadium Dichloride Complexes Containing 2-(2'-Benz-imidazolyl)pyridine Ligands: Effect of Al Cocatalyst for Efficient Ethylene (Co)polymerization

(Imido)vanadium(V) dichloride complexes containing 2-(2'-benzimidazolyl)-6-methylpyridine ligand (L) of type V(NR)Cl2(L) [R = 1-adamantyl (Ad, 1), C6H5 (2), and 2,6-Me2C6H3 (3)] have been prepared, and their structures were determined by X-ray crystallography as distorted trigonal bipyramidal structures around vanadium. Reactions with ethylene using 1-3 in the presence of methylaluminoxane (MAO) afforded a mixture of oligomer and polymers, and the compositions were affected by the imido ligand employed. By contrast, 1-3 exhibited remarkable catalytic activities for ethylene polymerization in the presence of Me2AlCl; the phenylimido complex (2) exhibited the highest activity [80 100 kg-PE/mol-V·h turn over frequency (TOF, 2 850 000 h-1, 792 s-1)]. The ethylene copolymerizations with norbornene afforded ultrahigh-molecular-weight copolymers with uniform molecular weight distributions and compositions [e.g., M n = 1.71-2.66 × 106, M w/M n = 2.27-2.53]. On the basis of V nuclear magnetic resonance (51V NMR), electron spin resonance, and V K-edge X-ray absorption near-edge structure (XANES) spectra of the catalyst solution, the observed difference in the catalyst performance in the presence of (between) MAO and Me2AlCl cocatalyst should be due to the formation of different catalytically active species with different oxidation states. Apparent changes in the oxidation state were observed in the (especially in the NMR and XANES) spectra upon addition of Me2AlCl, whereas no significant changes in the spectra were observed in presence of MAO.

Use of Vanadium Complexes Bearing Naphthalene-Bridged Nitrogen-Sulfonate Ligands as Catalysts for Copolymerization of Ethylene and Propylene

Vanadium complexes bearing naphthalene-bridged nitrogen-sulfonate ligand ([ê²(N,O)-8-(PhN)-1-naphthalenesulfonato]VOCl (1a) and [ê²(N,O)-8-(PhN)-1-naphthalenesulfonato]VCl₂ (1b)) were synthesized. Activated by ethylaluminium sesquichloride (EASC) and in the presence of ethyl trichloroacetate (ETCA) as reactivator, complexes 1a and 1b showed activities of up to 39.1 kg polymer (mol V)⁻¹ h⁻¹, affording the copolymers with high molecular weights (M_w up to 28 × 10⁴) and narrow molecular weight distributions (M_w/M_n ~ 3.0) as well as high propylene incorporation of up to 49.4%. Compared to the traditional VOCl₃ system, these complexes exhibited higher propylene incorporation ability and higher catalytic activities especially at high polymerization temperatures of 50 °C and above. Determined by DSC and ¹³C NMR, the copolymers obtained with 1a and 1b had more random structures than that with the VOCl₃ system.

Theoretical Study of Terminal Vanadium(V) Chalcogenido Complexes Bearing Chlorido and Methoxido Ligands

Solvent (methanol) coordinated vanadium(V) chalcogenido complexes bearing chlorido and methoxido ligands have been studied computationally by means of density functional (DFT) methods. The gas phase complexes were fully optimized using B3LYP/GEN functionals with 6-31+G⁎⁎ and LANL2DZ basis sets. The optimized complexes show distorted octahedral geometries around the central vanadium atom. The ligand pπ-vanadium dπ interactions were analyzed by natural bond order (NBO) and natural population analyses (NPA). These results show strong stabilization of the V=O bond as was further confirmed by the analyses of the frontier molecular orbitals (FMOs). Second-order perturbation analyses also revealed substantial delocalization of lone pair electrons from the oxido ligand into vacant non-Lewis (Rydberg) orbitals as compared to the sulfido and seleno analogues. These results show significant ligand-to-metal charge transfer (LMCT) interactions. Full interaction map (FIM) of the reference complex confirms hydrogen bond interactions involving the methanol (O-H) and the chlorido ligand.

Arylimido zirconium and titanium complexes: characteristic structures and application in ethylene polymerization

Dimeric anilidolithium (ArHNLi·Et2O)2 (Ar=2,6- i Pr2C6H3) reacted with zirconium tetrachloride in THF to give the heterometallic zirconium–lithium complex [(Et2O)2Li(μ-Cl)2(ArHN)(ArN=)Zr(μ-Cl)]2 (C1) and with titanium tetrachloride in toluene to give the titanium complex [(ArN=)TiCl2·(Et2O)2] (C2) each in good isolated yields. Their molecular structures in the solid state were confirmed by X-ray diffraction analysis. Upon activation with methylaluminoxane, both arylimido zirconium and titanium complexes exhibited good catalytic activities toward ethylene polymerization.

Methyl Complexes of the Transition Metals

Organometallic chemistry can be considered as a wide area of knowledge that combines concepts of classic organic chemistry, that is, based essentially on carbon, with molecular inorganic chemistry, especially with coordination compounds. Transition-metal methyl complexes probably represent the simplest and most fundamental way to view how these two major areas of chemistry combine and merge into novel species with intriguing features in terms of reactivity, structure, and bonding. Citing more than 500 bibliographic references, this review aims to offer a concise view of recent advances in the field of transition-metal complexes containing M-CH3 fragments. Taking into account the impressive amount of data that are continuously provided by organometallic chemists in this area, this review is mainly focused on results of the last five years. After a panoramic overview on M-CH3 compounds of Groups 3 to 11, which includes the most recent landmark findings in this area, two further sections are dedicated to methyl-bridged complexes and reactivity.

Isoprene polymerization mediated by vanadium-[ONNO] complexes

Vanadium complexes with amine-bis(phenolate) [ONNO] ligands catalyze the polymerization of isoprene when activated with AlⁱBu₃.

Quantifying the Electron Donor and Acceptor Abilities of the Ketimide Ligands in M(N═C(t)Bu2)4 (M = V, Nb, Ta)

Addition of 4 equiv of Li(N═C(t)Bu2) to VCl3 in THF, followed by addition of 0.5 equiv of I2, generates the homoleptic V(IV) ketimide complex, V(N═C(t)Bu2)4 (1), in 42% yield. Similarly, reaction of 4 equiv of Li(N═C(t)Bu2) with NbCl4(THF)2 in THF affords the homoleptic Nb(IV) ketimide complex, Nb(N═C(t)Bu2)4 (2), in 55% yield. Seeking to extend the series to the tantalum congener, a new Ta(IV) starting material, TaCl4(TMEDA) (3), was prepared via reduction of TaCl5 with Et3SiH, followed by addition of TMEDA. Reaction of 3 with 4 equiv of Li(N═C(t)Bu2) in THF results in the isolation of a Ta(V) ketimide complex, Ta(Cl)(N═C(t)Bu2)4 (5), which can be isolated in 32% yield. Reaction of 5 with Tl(OTf) yields Ta(OTf)(N═C(t)Bu2)4 (6) in 44% yield. Subsequent reduction of 6 with Cp*2Co in toluene generates the homoleptic Ta(IV) congener Ta(N═C(t)Bu2)4 (7), although the yields are poor. All three homoleptic group 5 ketimide complexes exhibit squashed tetrahedral geometries in the solid state, as determined by X-ray crystallography. This geometry leads to a d(x(2)-y(2))(1) ((2)B1 in D(2d)) ground state, as supported by DFT calculations. EPR spectroscopic analysis of 1 and 2, performed at X- and Q-band frequencies (∼9 and 35 GHz, respectively), further supports the (2)B1 ground-state assignment, whereas comparison of 1, 2, and 7 with related group 5 tetra(aryl), tetra(amido), and tetra(alkoxo) complexes shows a higher M-L covalency in the ketimide-metal interaction. In addition, a ligand field analysis of 1 and 2 demonstrates that the ketimide ligand is both a strong π-donor and strong π-acceptor, an unusual combination found in very few organometallic ligands.

Vanadyl calix[6]arene complexes: synthesis, structural studies and ethylene homo-(co-)polymerization capability

Treatment of p-tert-butylcalix[6]areneH6 (L(6)H6) with in situ [LiVO(Ot-Bu)4] afforded, after work-up, the dark green complex [Li(MeCN)4][V2(O)2Li(MeCN)(L(6)H2)2]·8MeCN (1·8MeCN). On one occasion, the reaction led to the formation of a mixture of products, the bulk of which differing from 1 only in the amount of solvate, viz.2·9.67MeCN. The second minor, yellow product has the formula {[(VO2)2(L(6)H2)(Li(MeCN)2)2]·2MeCN}n (3·2MeCN), and comprises a 1D polymeric structure with links through the L(6)H2 ligand and Li2O2 units. When the reverse order of addition was employed such that lithium tert-butoxide (7.5 equivalents) was added to L(6)H6, and subsequently treated with VOCl3 (2 equiv.), the complex {[VO(THF)][VO(μ-O)]2Li(THF)(Et2O)][L(6)]}·2Et2O·0.5THF (4·2Et2O·0.5THF), which contains a trinuclear motif possessing a central, octahedral vanadyl centre linked via oxo bridges to two tetrahedral (C3v) vanadyl centres, was isolated. The calix[6]arene in 4 is severely twisted and adopts a ‘down, down, down, down, out, out’ conformation. Use of excess lithium tert-butoxide led to a complex very similar to 4, differing only in the solvent of crystallization, namely 5·Et2O·2THF. The ability of 1 and 5 to act as pre-catalysts for ethylene polymerization in the presence of a variety of co-catalysts and under various conditions has been investigated. Co-polymerization of ethylene with propylene and with 1-hexene have also been conducted; results are compared versus VO(OEt)Cl2.

Ethylene polymerisation and ethylene/norbornene copolymerisation by using aryloxo-modified vanadium(V) complexes containing 2,6-difluoro-, dichloro-phenylimido complexes

Aryloxo-modified vanadium(V) dichloride complexes containing a dichloro- or difluoro-phenylimido ligand of the type, V(N-2,6-X2C6H3)Cl2(O-2,6-R2C6H3) [X = Cl (1), F (2); R = Me (a), F (b)], showed remarkable catalytic activity in ethylene polymerisation and the copolymerisation with norbornene (NBE) in the presence of Et2AlCl; the activities were higher than that of V(N-2,6-Me2C6H3)Cl2(O-2,6-Me2C6H3) reported previously. Complex 1a is a suitable catalyst precursor in terms of not only the activity, but also synthesis of high molecular weight copolymers with both unimodal molecular weight distributions and uniform compositions.

Vanadium(v) tetra-phenolate complexes: synthesis, structural studies and ethylene homo-(co-)polymerization capability

Bi-metallic vanadium(v) tetra-phenolate complexes, on combination with organoaluminium co-catalysts and a reactivator, are capable of ethylene homo- or ethylene/propylene co-polymerization of ethylene with high activity.

Vanadyl complexes bearing bi-dentate phenoxyimine ligands: synthesis, structural studies and ethylene polymerization capability

Mono- and bi-vanadium bidentate phenoxyimine complexes have been structurally characterized. They are highly active ethylene polymerization catalysts in the presence of diethylaluminum chloride and ethyltrichloroacetate.