Hafnium Complexes Research Articles

Synthesis and Scale-up of Imino–Enamido Hafnium and Zirconium Olefin Polymerization Catalysts

New imino–enamido hafnium and zirconium trimethyl complexes were prepared in high yield by reacting a mixture of an imino–enamine ligand and hafnium or zirconium tetrachloride with four equivalents of methylmagnesium bromide in toluene. A significantly improved imine formation reaction in the final step of the ligand synthesis was developed that involves the reaction between the keto–enamine and a reagent formed in situ by mixing 0.55 equivalents of titanium tetrachloride and 5.5 equivalents of n-butylamine. Ethylene/1-octene polymerization evaluations at 120 °C revealed that polymerization characteristics of the imino–enamido hafnium and zirconium trimethyl derivatives were very similar to those of the tribenzyl analogues. A scale-up of the hafnium trimethyl derivative, performed on a 255 g scale, was accomplished in an overall yield of 57% in four steps starting with commercially available cyclohexa-1,2-dione. Hafnium and zirconium complexes derived from two isomeric imino–enamine ligands each resulted in very high molecular weight ethylene/1-octene copolymers, which differ significantly, however, in the level of incorporated octene. These features make these new catalysts good candidates for the preparation of olefin block copolymers via chain-shuttling polymerization.

Tetranuclear Hafnium(IV) and Zirconium(IV) Cationic Complexes Sandwiched between Two Di-Lacunary Species of α-Keggin Polyoxometalates: Lewis Acid Catalysis of the Mukaiyama–Aldol Reaction

Abstract The syntheses, X-ray structures, and homogeneous Lewis acid catalytic activities for the Mukaiyama–aldol reaction of four tetranuclear HfIV and ZrIV cluster cations, which are sandwiched between two 1,2-di-lacunary α-Keggin polyoxometalates (POMs) and between two 1,4-di-lacunary POMs, are described, i.e., [[{M(H2O)}2{M(H2O)2}2(µ-OH)3(µ3-OH)2](α-1,2-PW10O37)2]7− (M = Hf 1 and M = Zr 2) and [{M4(H2O)4(µ-OH)2(µ3-O)2}(α-1,4-PW10O37)2]8− (M = Hf 3 and M = Zr 4), respectively. Evaluated was homogeneous Lewis acid catalysis of the Mukaiyama–aldol reaction in aqueous/CH3CN mixed media at room temperature under air by water-soluble sodium or lithium salts of sandwich-structured Hf/Zr-containing Keggin and Dawson POMs. In particular, the sodium salts of tetranuclear Hf/Zr cluster cations sandwiched between two di-lacunary α-Keggin POMs, i.e., Na-1–Na-4, showed the highest activities, compared with other cluster cations. The present POM-based sandwich-structured compounds gave products with high stereoselectivity, i.e., high anti-selectivity, regardless of high or low activities. The excellent stabilities of Na-1–Na-4 as catalysts, i.e., with no reduced activities, were confirmed even after reusing several times.

Structural diversity in tris(cyclopentadienyl) complexes of the Group 4 metals: Synthesis of Cp and MeCp complexes of Zr and Hf, and crystallographic characterization of (MeCp)3HfCl and (MeCp)4Zr (MeCp = C5H4CH3)

Previous theoretical studies have predicted structural differences between the tris(cyclopentadienyl) zirconium and hafnium complexes Cp3MX (Cp=C5H5; M=Zr, Hf). Cp3ZrCl can be isolated via the stoichiometric reaction of Cp2ZrCl2 and NaCp, but forming the analogous Hf complex is complicated by ligand redistribution. Partial crystallographic analysis of these two complexes indicates likely structural differences wherein the Zr complex has three η5 Cp ligands (“3-5” structure) while the Hf complex has two η5 Cp ligands and one η1 Cp ligand (“2-5,1-1” structure). The use of C5H4CH3 (MeCp) simplifies both the syntheses and the procurement of suitable crystals. The preparation of a number of MeCp complexes of Zr and Hf is reported along with crystallographic characterization of (MeCp)3HfCl, which has a 2-5,1-1 structure and (MeCp)4Zr, which has a 3-5,1-1 structure.

Formation of polyphosphorus ligands mediated by zirconium and hafnium complexes

The reactions of R2P–P(SiMe3)Li (R=tBu, iPr, iPr2N) with [Cp2MCl2] (M=Zr, Hf), [Cp∗CpZrCl2] and [CpZrCl3] yielded polyphosphorus (mainly hexaphosphorus) compounds which can be viewed as intermediates between R2P–P(SiMe3)Li and derivatives of 1,2,3,4-tetraphosphabicyclo[1.1.0] butane. Thus R2P–P(SiMe3)Li can act as a building block for the formation of the P2 ligand and the R2P–P(P2) and R2P–P(P2)P–PR2 moieties. Solid state structures of zirconium(IV) and hafnium(IV) complexes with R2P–P(P2) and R2P–P(P2)P–PR2 ligands were established by X-ray studies.

Zirconium and hafnium complexes containing N-alkyl substituted amine biphenolate ligands: coordination chemistry and living ring-opening polymerization catalysis

The coordination chemistry of zirconium and hafnium complexes containing the tridentate amine biphenolate ligands [RN(CH2-2-O-3,5-C6H2(tBu)2)2](2-) ([R-ONO](2-); R = tBu (1a), iPr (1b), nPr (1c)) featuring distinct N-alkyl substituents is described. Alcoholysis of Zr(OiPr)4(HOiPr) or Hf(OiPr)4(HOiPr) with H2[1a] in diethyl ether solutions at -35 °C generates the corresponding five-coordinate [1a]M(OiPr)2 (M = Zr (2a), Hf (3a)) in high isolated yield. Similar reactions employing H2[1b] produce six-coordinate [1b]M(OiPr)2(HOiPr) (M = Zr (2b·HOiPr), Hf (3b·HOiPr)) as an isopropanol adduct. Repetitive trituration of 2b·HOiPr and 3b·HOiPr with diethyl ether gives five-coordinate 2b and 3b, respectively. Treatment of M(OiPr)4(HOiPr) with H2[1c] under similar conditions affords six-coordinate [1c]M(OiPr)2(HOiPr) (M = Zr (2c·HOiPr), Hf (3c·HOiPr)), subsequent recrystallization of which from acetonitrile-diethyl ether solutions leads to acetonitrile adducts 2c·MeCN and 3c·MeCN. Reactivity studies of these zirconium and hafnium complexes revealed that they are all active catalysts for ring-opening polymerization of ε-caprolactone. Among them, the N-isopropyl derived complexes are most reactive. Polymerizations catalyzed by 2b, 3b and 3c·MeCN were proved to be living. The X-ray structures of 2a·HOiPr, 2a·MeCN, 2c·HOiPr, 2c·MeCN, and 3c·MeCN are presented.

Ring opening polymerization of rac-lactide by group 4 tetracarbamato complexes: activation, propagation and role of the metal

A series of group 4 metal tetracarbamates M(O(2)CNR(2))(4) (M = Ti, R = Et, 1a; M = Zr, R = Et, 1b; (i)Pr, 1c; M = Hf, R = Et, 1d; R = (i)Pr, 1e) were studied as catalytic precursors in the solution polymerization of rac-lactide. The titanium complex but not the zirconium and hafnium ones increase the activity by addition of (i)PrOH. The structure of the carbamato ligand markedly influences the molar mass of polymer; the complexes with isopropyl carbamato ligands produced PLA with molar masses up to 94,000 g mol(-1). The main mechanistic aspects of the initial stages of the polymerization reactions were outlined by spectroscopic and computational analyses. In the case of zirconium and hafnium complexes, an interaction between a carbamato ligand and the CH unit of one lactide molecule is established at room temperature. This interaction is followed by the high temperature proton transfer from the lactide to the carbamato O-atom, affording a catalytic active alkoxy complex with release of CO(2) and NHR(2). The polymerization mediated by Ti(O(2)CNEt(2))(4) involves the release of a radical fragment [O(2)CNEt(2)]˙, with consequent generation of a Ti(III) center. The propagating chain is an alcoholate ligand coordinated to a Ti(IV) centre and containing a radical mainly localized at the tail of the chain (DFT, EPR).

Zirconium and hafnium Salalen complexes in isospecific polymerisation of propylene

The activity of dibenzylzirconium and dibenzylhafnium Salalen complexes in polymerisation of propylene with MAO as a cocatalyst is described. Three Salalen ligand precursors combining a bulky alkyl group (1-adamantyl) on the imine-side phenol and electron withdrawing halo groups of different sizes on the amine-side phenol were explored. All metal complexes were obtained as single diastereomers. An X-ray crystallographic structure of a hafnium complex of an additional ligand carrying the combination of tert-butyl and chloro substituted phenolates, 4-Hf, revealed a fac-mer wrapping of the Salalen ligand around the metal centre. All complexes led to active catalysts in propylene polymerisation and to isotactic polypropylene of high regioregularity. The zirconium complexes led to polypropylene having molecular weights of Mw = 132,000-200,000 and isotacticities of [mmmm] = 65.7-75.0%. The hafnium complexes led to polypropylene of higher molecular weights of Mw = 375,000-520,000 and higher stereoregularities of [mmmm] = 80.6-89.3%, the highest isotacticity obtained with 3-Hf.

Synthesis, structure, and reactivity of tris(amidate) mono(amido) and tetrakis(amidate) complexes of group 4 transition metals

The syntheses of a series of tris(amidate) mono(amido) titanium and zirconium complexes are reported. The binding motif of the amidate ligand has been determined to depend on the size of the metal centre for these sterically demanding N,O-chelating ligands; the larger zirconium metal centre supports three κ(2)-(N,O) bound amidate ligands while the titanium analogue has one ligand bound in a κ(1)-(O) fashion to alleviate steric strain. Reactivity studies indicate that, despite high steric crowding about the tris(amidate) mono(amido) zirconium metal centre, transamination of the reactive dimethylamido ligand can be achieved using aniline. This complex is also an active precatalyst for intramolecular alkene hydroamination, in which protonolysis of one amidate ligand in the presence of excess amine is observed as an initiation step prior to catalytic turnover. Eight-coordinate homoleptic κ(2)-amidate complexes of zirconium and hafnium have also been prepared.

Synthesis, Spectral Characterization, Cyclic Voltammetry, Molecular Modeling and Catalytic Activity of Sulfa-Drug Divalent Metal Complexes

stract Complexes of cobalt(II), nickel(II), copper(II), zinc(II) and Hafnium(II) of general composition [M(L)2(Cl)2] [L = 4-(phenylphosphinylideneamino-N-thiazolyl benzene-sulfonamide] have been synthesized. The elemental analysis, molar conductance measurements, magnetic susceptibility measurements, mass, IR, UV, NMR, SEM, EDX, thermal and EPR spectral studies of the compounds led to the conclusion that the ligand acts as a bidentate manner. The molar conductance of the complexes in fresh solution of DMSO lies in the range of 7.46–9.13 Ω-1 cm2 mol-1 indicating their non-electrolytic behavior. On the basis of analytical and spectroscopic techniques, octahedral geometry of the complexes was proposed. The ligand binds in a bidentate manner, coordinated through sulfonamide oxygen and thiazole nitrogen atoms. The ligand field parameters were calculated for Co(II), Ni(II) and Cu(II) complexes and their values were found in the range reported for a octahedral structure. The catalytic activities of the divalent metal complexes have been studied in the oxidation of cyclohexane by hydrogen peroxide, an environmental friendly oxidant. The molecular modeling parameters of the ligand and its Co(II) and Hf(II) complexes have been calculated.

Zirconium and hafnium complexes based on 2-aryl-8-arylaminoquinoline ligands: synthesis, molecular structure, and catalytic performance in ethylene copolymerization

A general and efficient approach toward new zirconium and hafnium complexes based on 2-aryl-8-arylaminoquinoline ligands was developed. These precursors, when activated with MAO/borate cocatalyst and supported on silica, result in active olefin polymerization catalysts. The ethylene copolymers produced under industrially relevant conditions show very high molecular weights and unique microstructures defined by the multisite nature of the catalyst. A site-diversification mechanism is proposed to explain the presence of at least five individual sites, as deduced from 3D-TREF analysis of ethylene-butene copolymers.

CO assisted N2functionalization activated by a dinuclear hafnium complex: a DFT mechanistic exploration

In this paper, the reaction mechanisms of CO assisted N(2) cleavage and functionalization activated by a dinuclear hafnium complex are studied using a density function theory (DFT) method. Several key intermediates (Ia, Ib, Ic and Id) with axial/equatorial N=C=O coordination structures are found to be of importance along reaction pathways of CO assisted N(2) functionalization, which could provide a profound theoretical insight into the C-N bond formation and N-N bond cleavage. There are two different attack directions to insert the first CO molecule into the Hf-N bonds of the dinuclear hafnium complex, which lead to C-N bond formation. The calculated results imply that CO insertion into the Hf(1)-N(3) bond (Path A1) reacts more easily than that into the Hf(2)-N(3) bond (Path A3). But for the insertion of the second CO insertion to give 2A, there are two possibilities (Path A1 and Path A2) according to this insertion being after/before N-N bond cleavage. Two pathways (Path A1 and Path A2) are proved to be possible to form final dinitrogen functionalized products (oxamidide 2A, 2B and 2C) in this study, which explain the formation of different oxamidide isomers in CO assisted N(2) functionalization activated by a dinuclear hafnium complex.

Activation of CO2 by phosphinoamide hafnium complexes

Hf-phosphinoamide cation complexes behave as metal-based frustrated Lewis pairs and bind one or two equivalent of CO2 and in as well can activate CO2 in a bimetallic fashion to give a pseudo-tetrahedral P2CO2 fragment linking two Hf centres.

Isospecific polymerization of 1-hexene by C1-symmetric half-metallocene dimethyl complexes of group 4 metals with bidentate N-substituted iminomethylpyrrolyl ligands

Non-bridged half-metallocene dimethyl complexes of group 4 metals 2a-4a with an N-4-methoxyphenyl(iminomethyl)pyrrolyl ligand 1a were synthesized and characterized by NMR spectroscopy and X-ray analysis. Upon activation with [Ph3C][B(C6F5)4], these complexes became active catalysts for the polymerization of 1-hexene. A series of hafnium complexes with various N-substituents on the imine group of ligands 1b-1g were also prepared and applied as catalysts for 1-hexene polymerization. The activation parameters for the exchange process between the two methyl groups bound to the metal for Cp*MMe2(R-pyr) complexes were estimated by NMR shape analysis at various temperatures. The findings indicated that the transition state of the ligand flipping process might be associated with the isoselectivity of the polymerization reaction.

Synthesis of hafnium(IV) β-ketoiminates as potential precursors for the MOCVD of HfO2

An amine-elimination pathway has been used to produce two new hafnium compounds starting from the tetrakis(dimethylamido)hafnium(IV) complex and an amine – bis[(Z)-4-(2,6-dimethylphenylamino)pent-3-en-2-one]bis(dimethylamido)hafnium(IV) and [(Z)-4-(4-methylphenylamino)pent-3-en-2-one]bis(dimethylamido)hafnium(IV). The resulting complexes were characterized by single crystal X-ray diffraction and NMR. In addition, their volatilities were studied by thermogravimetric analysis (TGA), where [(Z)-4-(4-methylphenylamino)pent-3-en-2-one]bis(dimethylamido)hafnium(IV) showed better properties for MOCVD applications with a ΔHsub of 27kJ/mol and a Tsub of 104°C.

Hafnium Amidoquinoline Complexes: Highly Active Olefin Polymerization Catalysts with Ultrahigh Molecular Weight Capacity

The preparation and characterization of a new class of polyolefin procatalysts is described. Hafnium tribenzyl procatalysts, supported by amidoquinoline ligands, were prepared in two steps from commercially available materials. Solid-state structures, determined by single-crystal X-ray analyses, revealed that all the hafnium complexes display approximate trigonal-bipyramidal geometry around the metal center. The complexes were evaluated in an ethylene/1-octene copolymerization study and were found to be highly active at elevated temperatures (120 °C). The best catalyst, derived from ((2,6-dimethylphenyl)(2,4-dimethylquinolin-8-yl)amino)tribenzylhafnium (6d), compares favorably to previously reported systems supported by bidentate nitrogen-based ligands. In particular, this catalyst exhibits very high molecular weight capacity and high catalytic activity, with a moderate 1-octene response. Alkyl substitution at the carbon ortho to the quinolino nitrogen was found to be an important factor for improving pol...

Crystal structures of hafnium(IV) and zirconium(IV) complexes with β-diketones

Preparation methods are developed for a number of 7- and 8-coordinated derivatives of Hf(IV) and Zr(IV) with β-diketonate ligands (R1-CO-CH-CO-R2). The complexes obtained are examined by IR spectroscopy, mass spectrometry, and X-ray crystallography. All structures are molecular. The M-O distances fall within 2.09–2.28 A. In the crystals, the molecules are joined only by van der Waals interactions. It is demonstrated that the series of hafnium(IV) and zirconium(IV) chelates with identical ligands are isostructural, and the introduction of CF3- or tert-butyl groups in the terminal positions of the ligand as well as chlorine substitution for one of the ligands do not essentially affect the basic geometric characteristics of the ligand in the complexes.

Synthesis, Characterization, and Lactide Polymerization Activity of Group 4 Metal Complexes Containing Two Bis(phenolate) Ligands

A series of group 4 metal complexes Zr-(1)(2), Zr-(2)(2), Zr-(3)(2), Zr-(4)(2), Zr-(5)(2), Hf-(1)(2), and Hf-(4)(2) containing two bridged bis(phenolate) ligands of the (OSSO)-type were prepared by the reaction of the corresponding bis(phenol) and group 4 metal precursor MX(4) (X = O(i)Pr, CH(2)Ph) and isolated as robust, colorless crystals. NMR spectra indicate D(2) symmetry, in agreement with the solid state structure determined by single crystal X-ray diffraction study of the complexes Zr-(1)(2), Hf-(1)(2), Zr-(3)(2), Zr-(4)(2), and Zr-(5)(2). The complexes with the 1,4-dithiabutanediyl bridged ligands exhibit a highly symmetric coordination around the metal center. The introduction of the rigid trans-1,2-cyclohexanediyl bridged ligands led to a distorted coordination around the metal center in Zr-(4)(2) and Zr-(5)(2) when the ortho substituent is tert-butyl and the para substituent is larger than methyl. The complexes Zr-(1)(2), Zr-(2)(2), Zr-(3)(2), Zr-(4)(2) as well as Hf-(1)(2) and Hf-(4)(2) initiated the ring-opening polymerization of meso-lactide at 100 °C to give heterotactic polylactide with pronounced heterotacticity (>70%) and varying polydispersity (1.05 < M(w)/M(n) < 1.61). As shown by kinetic studies, zirconium complex Zr-(1)(2) polymerized meso-lactide faster than the homologous hafnium complex Hf-(1)(2).

Hafnium Trialkyls Stabilized by Bulky, Electron‐Rich Aminopyridinates

AbstractA series of hafnium aminopyridinates were synthesized and spectroscopically analyzed. In addition, selected examples of these hafnium complexes were characterized by X‐ray single crystal structure analysis. The aminopyridinato ligands used here carry an additional dialkylamine substituent to enhance the electron donating ability of the ligands. Structural data and low temperature NMR spectroscopic investigations are indicative of the increased donating ability of the ligands. Ethylene polymerization studies revealed a rather low catalytic activity most likely due to catalyst instability. Details of the deactivation reaction were investigated by NMR spectroscopy.

Novel zirconium (IV) and hafnium (IV) phthalocyanines with dibenzoylmethane as out-of-plane ligand: Synthesis, X-ray structure and fluorescent properties

Novel phthalocyanine complexes of zirconium (IV) and hafnium (IV) containing dibenzoylmethane groups as out-of-plane ligands were synthesized and characterized by elemental analyses, GALDI mass spectrometry, IR, 1H NMR, UV–vis and fluorescence spectroscopy. The structures of the complexes were determined by single-crystal X-Ray diffraction. The synthesized compounds are isomorphous, with their central atoms exhibiting (ZrN4O4 and HfN4O4) square-antiprism coordination geometry, which consists of four nitrogen atoms from the phthalocyanine macrocycle and four oxygen atoms from the two bidentate dibenzoylmethane ligands. GALDI mass spectrometry studies have shown that hafnium phthalocyanine forms adducts with masses higher than the mass of the molecular ion, indicating a high ability of this complex to self-associate in the gas phase. The spectral-luminescent properties of the compounds in different solvents were studied. The splitting of the Q-band observed in the absorption spectra is related to the lowering of complexes symmetry due to the incorporation of two rigid ligands to the central metal atom of the macrocycle. Study of the absorption spectra of phthalocyanines over a wide concentration range does not reveal any indications for the formation of aggregates. Fluorescence quantum yield values of the complexes were determined by a comparative method in toluene and DMSO.

Synthesis, Structure and Ethylene Polymerization Behavior of Group 4 Metal Complexes Bearing Salicylaldaminato Ligands with Appended Donor Functionality

A series of group 4 metal complexes were synthesized by an equimolar reaction of M(CH2Ph)(4) with (E)-2,4-di-tert-butyl-6-[(phenylimino)methyl]phenol bearing donor functionality. This process involved toluene elimination followed by an intramolecular migration reaction of one benzyl group from metal to the imino carbon, which was supported by the isolation and structural characterization of the intermediate. These new complexes were fully characterized by multinuclear NMR spectroscopy, elemental analyses and X-ray analyses. The titanium complexes showed a very high activity in both ethylene polymerization and ethylene/1-hexene copolymerization upon activation with modified methylaluminoxane (MMAO), whereas the corresponding zirconium and hafnium complexes were almost inactive under the same reaction conditions.