Hafnium Complexes Research Articles

Mono(imidazolin-2-iminato) Hafnium Complexes: Synthesis and Application in the Ring-Opening Polymerization of ε-Caprolactone and rac-Lactide

Mono-substituted imidazolinX-2-iminato hafnium(IV) complexes (X = iPr, tBu, Mesityl, Dipp) were synthesized and fully characterized, including solid-state X-ray diffraction analysis. When the X group is small (iPr), a dimeric structure is obtained. In all the monomeric complexes, the Hf-N bond can be regarded as a double bond with similar electronic properties. The main difference among the monomeric complexes is the cone angle of the ligand, which induces varying steric hindrances around the metal center. When the monomeric complex of mono(bis(diisopropylphenyl)imidazolin-2-iminato) hafnium tribenzyl was reacted with three equivalents (equiv) of iPrOH, the benzyl groups were easily replaced, forming the corresponding tri-isopropoxide complex. However, when BnOH was used, dimeric complexes were obtained. When five equivalents of the corresponding alcohols (BnOH, iPrOH) were reacted with the monomeric complex, different dimeric complexes were obtained. Regardless of the high oxophilicity of the hafnium complexes, all complexes were active catalysts for the ring-opening polymerization (ROP) of ε-caprolactone. Dimeric complexes 5 and 6 were found to be the most active catalysts, enabling polymerization to occur in a living, immortal fashion, as well as the copolymerization of ε-caprolactone with rac-lactide, producing block copolymer PCL-b-LAC. The introduction of imidazolin-2-iminato ligands enables the tailoring of the oxophilicity of the complexes, allowing them to be active in catalytic processes with oxygen-containing substrates.

Terminal and Super‐Basic Parent Imides of Hafnium

AbstractA dinuclear hafnium complex containing the parent imido ligand [(PN)(PNC)Hf=NH{μ2‐K}]2 (2) (PN−=(N‐(2‐PiPr2‐4‐methylphenyl)‐2,4,6‐Me3C6H2; PNC2−=(N‐(2‐PiPr2‐4‐methylphenyl)‐2,4,6‐CH2Me2C6H2), was prepared by reduction of the bisazide trans‐[(PN)2Hf(N3)2] (1) with two equiv of KC8. Encapsulation of K+ in 2 with crown‐ether or cryptand affords the first discrete salt [K(encap)][(PN)(PNC)Hf≡NH] (encap=18‐crown‐6(THF)2, 3; 2,2,2‐Kryptofix, 4), featuring a terminal parent imide and possessing some of the shortest Hf−N bond lengths known to date. DFT calculations revealed formation of 2 to proceed via an extremely basic monomeric nitrido, [(PN)2Hf≡N]−(A), having a computed pKBH+ of ∼57 followed by heterolytic splitting of an inert 1,2‐CH bond of a benzylic methyl group across the Hf≡N triple bond in A. An electronic structure analysis reveals A to possess a covalent Hf≡N triple bond and of super‐basic character. We also showcase reactivity of the Hf≡NH bond with various electrophiles.

Terminal and Super-Basic Parent Imides of Hafnium.

A dinuclear hafnium complex containing the parent imido ligand [(PN)(PNC)Hf=NH{μ2 -K}]2 (2) (PN- =(N-(2-Pi Pr2 -4-methylphenyl)-2,4,6-Me3 C6 H2 ; PNC2- =(N-(2-Pi Pr2 -4-methylphenyl)-2,4,6-CH2 Me2 C6 H2 ), was prepared by reduction of the bisazide trans-[(PN)2 Hf(N3 )2 ] (1) with two equiv of KC8 . Encapsulation of K+ in 2 with crown-ether or cryptand affords the first discrete salt [K(encap)][(PN)(PNC)Hf≡NH] (encap=18-crown-6(THF)2 , 3; 2,2,2-Kryptofix, 4), featuring a terminal parent imide and possessing some of the shortest Hf-N bond lengths known to date. DFT calculations revealed formation of 2 to proceed via an extremely basic monomeric nitrido, [(PN)2 Hf≡N]- (A), having a computed pKBH+ of ∼57 followed by heterolytic splitting of an inert 1,2-CH bond of a benzylic methyl group across the Hf≡N triple bond in A. An electronic structure analysis reveals A to possess a covalent Hf≡N triple bond and of super-basic character. We also showcase reactivity of the Hf≡NH bond with various electrophiles.

Catalytic reductive ring opening of epoxides enabled by zirconocene and photoredox catalysis

Catalytic reductive ring opening of epoxides enabled by zirconocene and photoredox catalysis

Fast-Tracking the l-Lactide Polymerization Activity of Group 4 Metal Complexes of Amine Tris(phenolate) Ligands

Zirconium complexes of amine tris(phenolate) ligands wherein the substituents ortho to the phenolate oxygens are aryl groups are introduced, and their application in the catalysis of ring-opening polymerization of l-lactide at 180 °C is described. Spectroscopic and crystallographic characterization revealed that for ortho-phenyl substituents, bridging dinuclear complexes were obtained, whereas for ortho-mesityl-substituents, fluxional mononuclear complexes were obtained. Melt polymerizations employing catalyst ratios reaching as low as 5 ppm for unpurified l-lactide and 1 ppm for purified l-lactide revealed ultra-high activities for the zirconium complex featuring the mesityl-phenolate substituents, yielding high-molecular-weight stereoerror-free poly(l-lactic acid) that exhibited enhanced stability toward thermal degradation. Slightly lower activities were found for the corresponding hafnium complex and for the zirconium complex featuring the phenyl-phenolate substituents. These catalysts could thus serve as drop-in replacement for the industrially employed stannous octanoate.

Olefin polymerization cocatalysts: Development, applications, and prospects

<p indent="0mm">The annual global production of polyolefin products exceeds 100 million tons, and they are widely used in various fields. In contrast to heterogeneous polymerization that produces polyolefins with broad dispersity, single-site transition-metal-catalyzed homogeneous olefin polymerization enables the synthesis of polyolefins with narrow molecular weight distribution, controllable chain segment length, and uniform insertion ratio. It can flexibly tune the molecular structure of polyolefins to provide polyolefin materials with the required properties, which can be applied to high-end applications such as medical packaging and medical equipment. It is important to choose a suitable combination of a catalyst and cocatalyst to obtain tailor-made polyolefins through high-efficiency homogeneous polymerization. The main function of the cocatalyst is to react with the catalyst precursor to generate and stabilize active metal cations, the latter being the center of catalyzing olefin polymerizations. This review summarizes the most important work regarding cocatalysts in the past two decades, including novel structures: Mono/dinuclear boron-, aluminum-, and polymer-based promoters. This review illustrates the importance of the weakly coordinative nature of the counter anion, providing the design principle for the development of a highly efficient cocatalyst. Activation mechanism: The use of an industrial catalysis-related pyridylamido hafnium complex is an example that showcases the complicated reactions of this catalyst with different cocatalysts, shedding light on the importance of choosing the pair of catalyst and cocatalyst. Applications in polymerization: Binuclear cocatalysts and the cocatalysts combined with coordination chain transfer polymerizations are used to synthesize novel structural polyolefins, indicating that the cocatalyst plays a crucial role in alternating polyolefin structures. Cocatalyst-involved catalyst deactivation reactions: This part emphasizes the reaction pathways of cocatalysts that lead to catalyst deactivation, including the replacement of metal and ligand. Further, this review proposes the possible future development of cocatalysts by investigating the reported compounds with high Lewis acidity and the development of novel functionalized cocatalysts. For instance, the development of external stimuli-responsive cocatalyst structures may be one of the future development directions. Combining cocatalysts with existing catalytic methods, such as the coordinative chain transfer polymerization for synthesizing polyolefins with a novel structure, may be the second development direction in the future. With the advancement of computer hardware and algorithms together with experimental data, computational chemistry helps understand the effect of cocatalysts on polymerization. Predicting the structure of new and efficient cocatalysts through big data and artificial intelligence is the third possible future development direction.

Synthesis and X-Ray Structure Analysis of Cytotoxic Heptacoordinated Salan Hafnium(Iv) Complexes Stabilized with 2,6-Dipicolinic Acid

Synthesis and X-Ray Structure Analysis of Cytotoxic Heptacoordinated Salan Hafnium(Iv) Complexes Stabilized with 2,6-Dipicolinic Acid

A phenyl-substituted germole dianion and its reaction with hafnocene dichloride

The reaction of phenyl-substituted dipotassium germacyclo-pentadienediide with one equivalent of hafnocene dichloride at low temperature provides a bicyclic germylene. With two equivalents of hafnocene dichloride, a dinuclear hafnium complex with a mmm-coordinating germolyl dianion ligand is formed.

Group IV Transition Metal (M = Zr, Hf) Precursors for High-κ Metal Oxide Thin Films.

This paper describes the synthesis of eight novel zirconium and hafnium complexes containing N-alkoxy carboxamidate-type ligands, as potential precursors for metal oxides and atomic layer deposition (ALD) for HfO2. A series of ligands, viz., N-ethoxy-2,2-dimethylpropanamide (edpaH), N-ethoxy-2-methylpropanamide (empaH), and N-methoxy-2,2-dimethylpropanamide (mdpaH), were used to afford complexes Zr(edpa)4 (1), Hf(edpa)4 (2), Zr(empa)4 (3), Hf(empa)4 (4), Zr(mdpa)4 (5), Hf(mdpa)4 (6), ZrCp(edpa)3 (7), and HfCp(edpa)3 (8). Thermogravimetric analysis curves assessed for the evaporation characteristics of complexes 1-8 revealed single-step weight losses with low residues, except for the mdpa-containing complexes. Single-crystal X-ray diffraction studies of 1, 2, 5, and 6 revealed that all the complexes have monomeric molecular structures, with the central metal ion surrounded by eight oxygen atoms from the four bidentate alkoxyalkoxide ligands. Among the complexes prepared, 8 exhibited a low melting point (64 °C), good volatility (1 Torr at 112 °C), high thermal stability, and excellent endurance over 6 weeks at 120 °C. Therefore, an ALD process for the growth of HfO2 was developed using HfCp(edpa)3 (8) as a novel precursor. Furthermore, the HfO2 film exhibited a low capacitance equivalent oxide thickness of ∼1.5 nm, with Jg as low as ∼3 × 10-4 A/cm2 at Vg -1 V in a metal-insulator-semiconductor capacitor (Au/HfO2/p-Si).

Computational mechanistic insights into hafnium catalyzed CO2 activation and reduction

In this study, we present the reaction mechanisms for CO2 activation and reduction reactions using hafnium complex as a frustrated Lewis pair catalyst investigated using density functional theory (DFT) calculations. CO2 activation reactions with different oxidation states of the catalyst were explored. The energy barriers for CO2 activation with oxidised, single-electron reduced and two-electron reduced catalyst were calculated to be 17.18, 14.46 and 15.73 kcal/mol, respectively. The mechanisms of CO2 reduction to CO and HCOO- upon addition of HCl as a proton source were also studied. The activation energies to release CO and HCOO- from the hafnium complex were calculated to be 9.05 and 75.34 kcal/mol, respectively. The results indicated that formate formation is unlikely. The overall analysis of the reaction mechanisms shows that the studied Hf complex effectively activates CO2 and can also potentially reduce the activated CO2 to CO.

Reduced Arene Complexes of Hafnium Supported by a Triamidoamine Ligand.

A series of hafnium complexes with a reduced arene of the general formula [K(L)][Hf(Xy‐N3N)(arene)] (Xy‐N3N={(3,5‐Me2C6H3)NCH2CH2}3N3−, L=THF, 18‐crown‐6; arene=C10H8 2−, C14H10 2−) mimic the chemistry of hafnium in its formal oxidation state +II. All compounds were obtained upon reduction of the chlorido complex [HfCl(Xy‐N3N)(thf)] with two equivalents of potassium naphthalenide or anthracenide. The reducing nature and the basicity of the reduced anthracene ligand were explored in the reaction of benzonitrile and azobenzene, and by deprotonation of tert‐butylacetylene, respectively. The reduction of benzonitrile provides an initial double nitrile insertion product under kinetic control that rearranges after extrusion of one of the inserted nitriles to a hafnium imido complex as the thermodynamic product. The reduction of azobenzene resulted in a diphenylhydrazido(2−) complex. Treatment of terminal alkynes with the anthracene or diphenylhydrazido(2−) complex led to the selective protonation of the corresponding dianionic ligand.

NSSN-Type Group 4 Metal Complexes in the Ring-Opening Polymerization of l-Lactide.

A new class of zirconium and hafnium complexes coordinated by linear dianonic tetradentate NSSN ligands is reported. The ligands feature two amide functions coupled with two thioether groups linked by a central flexible ethane bridge and two lateral rigid phenylene bridges and differ for the substituents on the aniline nitrogen atoms, i.e., isopropyl, cyclohexyl, or mesityl substituents: NSSN-iPr, NSSN-Cy, or NSSN-Mes. They were prepared by reacting 2-aminothiophenol with dibromoethane to afford the NSSN ligands without substituents on the aniline nitrogen atoms, which were subsequently alkylated through a reductive amination of acetone or cyclohexanone or palladium-catalyzed cross-coupling reaction with mesityl bromide. The corresponding zirconium and hafnium complexes 1-5 were obtained through a transamination reaction between the neutral ligands and Zr(NMe2)4 or Hf(NMe2)4 [(NSSN-iPr)Zr(NMe2)2 (1), (NSSN-Cy)Zr(NMe2)2 (2), (NSSN-Mes)Zr(NMe2)2 (3), (NSSN-iPr)Hf(NMe2)2 (4), and (NSSN-Cy)Hf(NMe2)2 (5)]. They were characterized in solution by NMR spectroscopy and in solid state by X-ray diffraction analysis (except for 3). All complexes present an octahedral coordination geometry with a fac-fac ligand wrapping and a cis relationship between the other two monodentate ligands. The catalytic performances of 1-5 in the ring-opening polymerization of cyclic esters were investigated. Complex 1 was the most active: its polymerization activity was superior to those generally displayed by zirconium complexes featuring OSSO ligands and compared well with those of the most active group 4 complexes operating in a toluene solution.

Techniques in the synthesis of organometallic compounds of Hafnium

Abstract Hafnium is a transition metal and it is the 45th most abundant transition element present on the earth. Hafnium has been successfully alloyed with several metals including titanium, iron, and niobium. Hafnium complexes are less active olefin polymerization catalysts. In the current review synthesis of hafnium complexes involving bonding through different linkages like “carbon, nitrogen, oxygen, carbon and oxygen, nitrogen and cobalt nitrogen and oxygen, nitrogen and phosphorus, nitrogen and sulfur, phosphorus and carbon, phosphorus and oxygen, sulfur carbon and oxygen, carbon–nitrogen and oxygen, carbon–nitrogen and phosphorus, carbon–nitrogen oxygen, sulfur and phosphorus, carbon–oxygen phosphorus and nitrogen”. The commonly used solvents for the synthesis of Hafnium complexes are tetrahydrofuran, n-hexane, and toluene, etc. These complexes were mostly reported at different temperatures ranges from −35 to 110 °C with continuous stirring, according to the nature of ligands. An overview of techniques in the synthesis of Hafnium complexes through various routes has been compiled.

Synthesis of Ethylene/1-Octene Copolymers with Ultrahigh Molecular Weights by Zr and Hf Complexes Bearing Bidentate NN Ligands with the Camphyl Linker

Ultrahigh molecular weight polyethylene (UHMWPE) is a class of high-performance engineering plastics, exhibiting a unique set of properties and applications. Although many advances have been achieved in recent years, the synthesis of UHMWPE is still a great challenge. In this contribution, a series of zirconium and hafnium complexes, [2,6-(R1)2-4-R2-C6H2-N-C(camphyl)=C(camphyl)-N-2,6-(R1)2-4-R2-C6H2]MMe2(THF) (1-Zr: R1 = Me, R2 = H, M = Zr; 2-Zr: R1 = Me, R2 = Me, M = Zr; 1-Hf: R1 = Me, R2 = H, M = Hf; 2-Hf: R1 = Me, R2 = Me, M = Hf), bearing bidentate NN ligands with the bulky camphyl backbone were synthesized by the stoichiometric reactions of α-diimine ligands with MMe4 (M = Hf or Zr). All Zr and Hf metal complexes were analyzed using 1H and 13C NMR spectroscopy, and the molecular structures of complexes 1-Zr and 1-Hf were determined by single-crystal X-ray diffraction, revealing that the original α-diimine ligand was selectively reduced into the ene-diamido form and generated an 1,3-diaza-2-metallocyclopentene ring in the metal complexes. Zr complexes 1-Zr and 2-Zr showed moderate activity (up to 388 kg(PE)·mol−1(M)·h−1), poor copolymerization ability, but unprecedented molecular weight capability toward ethylene/1-octene copolymerization. Therefore, copolymers with ultrahigh molecular weights (&gt;600 or 337 × 104 g∙mol−1) were successfully synthesized by 1-Zr or 2-Zr, respectively, with the borate cocatalyst [Ph3C][B(C6F5)4]. Surprisingly, Hf complexes 1-Hf and 2-Hf showed negligible activity under otherwise identical conditions, revealing the great influence of metal centers on catalytic performances.

Study of Novel Hafnium-Based Hybrid Thin Films By Molecular Layer Deposition

Molecular layer deposition (MLD) is a gas-phase reaction technique which relies on self-limiting surface reactions for growth of surface-bound polymers. Hybrid organic-inorganic materials usually possess properties distinct from both inorganic and organic components. MLD techniques can be used to deposit hybrid films with precisely defined composition and tunable properties that can be conformally deposited on high aspect ratio substrate. In this study, novel hybrid organic-inorganic hafnium based polymer films were successfully deposited using sequential exposures of a homoleptic tetrakis(dimethylamido)hafnium (TDMAH) complex and ethanolamine (EA) as the reactants by MLD. Hafnium nitrides and oxides have received much attention in the fields of microelectronics and lithium ion batteries, and thus are of interest as hybrid materials that may impart beneficial mechanical and chemical properties. To grow the Hf hybrid films, TDMAH and EA pulses were alternated with 120s of N2 purge. The processes were investigated as a function of temperature, precursor pulsing time and number of cycles. The results show that the self-limiting surface reactions possess a constant growth rate of ∼1.9 Å per cycle at 120 °C. Temperature-dependent growth characterization was performed for these hybrid materials. By increasing the temperature from 65 °C to 145 °C the growth rate decreases from ∼2.5 Å per cycle to ∼1.1 Å per cycle. The presence of expected C-N, C-O and -CH2 moieties in the MLD films were confirmed by Fourier transform infrared (FTIR) spectroscopy. Hafnium nitride and oxide bonds within hybrid thin films were identified by applying X-ray photoelectron spectroscopy (XPS). The results show that the deposited hafnium based organic-inorganic hybrid films contain both Hf – O and Hf – N bonds which enables flexibility, and thus have potential applications for both battery and microelectronics technologies.

Zirconium and Hafnium Complexes Bearing Salen Ligands for the Polymerization of Ethylene

A series of previously synthesized homoleptic and heteroleptic complexes of zirconium and hafnium were used for the polymerization of ethylene. Upon activation with methylaluminoxane, all the complexes serve as pre-catalysts for the polymerization of ethylene. The effects of polymerization time, reaction temperature, solvents, and methylaluminoxane to catalyst ratio on the polyethylene yield were investigated. The catalyst activity depends on the coordination environment of the metals. The highest activity obtained was 5.91 × 104 g of polyethylene/[(mol of catalyst) h] under the optimized conditions of hexane as a solvent, reaction time and temperature of 30 min and 50°C respectively, and MAO to catalyst ratio of 1000 : 1.

Unsaturated and Benzannulated N-Heterocyclic Carbene Complexes of Titanium and Hafnium: Impact on Catalysts Structure and Performance in Copolymerization of Cyclohexene Oxide with CO2.

Tridentate, bis-phenolate N-heterocyclic carbenes (NHCs) are among the ligands giving the most selective and active group 4-based catalysts for the copolymerization of cyclohexene oxide (CHO) with CO2. In particular, ligands based on imidazolidin-2-ylidene (saturated NHC) moieties have given catalysts which exclusively form polycarbonate in moderate-to-high yields even under low CO2 pressure and at low copolymerization temperatures. Here, to evaluate the influence of the NHC moiety on the molecular structure of the catalyst and its performance in copolymerization, we extend this chemistry by synthesizing and characterizing titanium complexes bearing tridentate bis-phenolate imidazol-2-ylidene (unsaturated NHC) and benzimidazol-2-ylidene (benzannulated NHC) ligands. The electronic properties of the ligands and the nature of their bonds to titanium are studied using density functional theory (DFT) and natural bond orbital (NBO) analysis. The metal–NHC bond distances and bond strengths are governed by ligand-to-metal σ- and π-donation, whereas back-donation directly from the metal to the NHC ligand seems to be less important. The NHC π-acceptor orbitals are still involved in bonding, as they interact with THF and isopropoxide oxygen lone-pair donor orbitals. The new complexes are, when combined with [PPN]Cl co-catalyst, selective in polycarbonate formation. The highest activity, albeit lower than that of the previously reported Ti catalysts based on saturated NHC, was obtained with the benzannulated NHC-Ti catalyst. Attempts to synthesize unsaturated and benzannulated NHC analogues based on Hf invariably led, as in earlier work with Zr, to a mixture of products that include zwitterionic and homoleptic complexes. However, the benzannulated NHC-Hf complexes were obtained as the major products, allowing for isolation. Although these complexes selectively form polycarbonate, their catalytic performance is inferior to that of analogues based on saturated NHC.

Benzimidazolin-2-iminato Hafnium Complexes: Synthesis, Characterization, and Catalytic Addition of Alcohols to Carbodiimides

A series of asymmetric imidazolin-2-iminato and benzimidazolin-2-iminato hafnium(IV) complexes were synthesized and fully characterized including single-crystal X-ray diffraction. The asymmetric im...

Synthesis of New Boron-Containing Ligands and Their Hafnium(IV) Complexes

A two-stage method for the synthesis of a boron-containing complexone on the basis of aminomalonic acid was proposed. The products were studied by multinuclear NMR spectroscopy, IR absorption spectroscopy, and ESI mass spectrometry at all the stages of synthesis. The product (NBu4)[2-B10H9NHC(CH3)NHCH(COOEt)2] was structurally characterized by singe-crystal X-ray diffraction. The complexation reaction between the borylated derivative of aminomalonic acid and Hf(NEt2)4 and Hf(OBu)4 was studied.

Preparation of Pyridylamido Hafnium Complexes for Coordinative Chain Transfer Polymerization.

The pyridylamido hafnium complex (I) discovered at Dow is a flagship catalyst among postmetallocenes, which are used in the polyolefin industry for PO-chain growth from a chain transfer agent, dialkylzinc. In the present work, with the aim to block a possible deactivation process in prototype compound I, the corresponding derivatives were prepared. A series of pyridylamido Hf complexes were prepared by replacing the 2,6-diisopropylphenylamido part in I with various 2,6-R2C6H3N-moieties (R = cycloheptyl, cyclohexyl, cyclopentyl, 3-pentyl, ethyl, or Ph) or by replacing 2-iPrC6H4C(H)- in I with the simple PhC(H)-moiety. The isopropyl substituent in the 2-iPrC6H4C(H)-moiety influences not only the geometry of the structures (revealed by X-ray crystallography), but also catalytic performance. In the complexes bearing the 2-iPrC6H4C(H)-moiety, the chelation framework forms a plane; however, this framework is distorted in the complexes containing the PhC(H)-moiety. The ability to incorporate α-olefin decreased upon replacing 2-iPrC6H4C(H)-with the PhC(H)-moiety. The complexes carrying the 2,6-di(cycloheptyl)phenylamido or 2,6-di(cyclohexyl)phenylamido moiety (replacing the 2,6-diisopropylphenylamido part in I) showed somewhat higher activity with greater longevity than did prototype catalyst I.