Anionic zirconium and hafnium borohydride complexes

Zirconium and hafnium tetrachlorides react with NaBH₄, in dimethoxyethane (DME) to give [Na(DME)₃][M(BH₄)₅]. These compounds react with Bu₄NBH₄ and Ph₄PBH₄ to give (R₄E)[M(BH₄)₅]. Bidentate and tridentate BH ₄ ^– occur in [M(BH₄)₅]^– according to IR spectroscopy. Data from¹H and¹H-{¹¹B} NMR spectra are consistent with intermolecular exchange of BH₄ ligands in solutions of complexes (I)–(VI). The BH₄groups and the bridging and terminal protons in each BH₄ group equilibrate rapidly. Heating the complexes (I)–(VI) reduces the central atom, releases diborane, and decomposes the outer-sphere cation. The neutral borohydrides M(BH₄)₄, can be prepared by thermolysis of the sodium salts (I) and (II).Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 6, pp. 1207–1214, June, 1990.     

Bis(alkylamido)phenylborane complexes of zirconium,hafnium, and vanadium

The coordination chemistry of the bis(tert-butylamido)phenylborane ligand, [(t)BuN-B(Ph)-N(t)Bu](2)(-), is developed. The ligand can be delivered to metals of groups 4 and 5 from its dilithio salt. The reactions of PhB((t)BuNLi)(2), 1, with metal halides of zirconium, hafnium, and vanadium generate complexes of the general formulas ((t)BuN-B(Ph)-N(t)Bu)(2)M(THF) (M = Zr (2), Hf (3)), Li(2)[M((t)BuN-B(Ph)-N(t)Bu)(3)] (M = Zr (4), Hf (5)), and M((t)BuN-B(Ph)-N(t)Bu)(2) (M = V (6)). (1)H and (11)B[(1)H] NMR and single-crystal X-ray analysis show that these amido metal complexes are structurally analogous to amidinates.     

Catalytic dehydrogenation of cyclooctane with titanium, zirconium and hafnium metallocene complexes

Metallocene complexes in combination with cocatalysts like methylalumoxane (MAO) are not only excellent catalysts for olefin polymerization but also appropriate catalysts for the activation of alkanes in homogeneous (autoclave) and heterogeneous (fixed bed reactor) reactions. The activities of the catalysts depend on the temperature, the cocatalysts, additives, the central metal and the ligand structure. Generally, complexes with low steric demands and MAO as cocatalyst gave the highest activities. The comparison of different π-ligands resulted in the following activity order: cyclopentadienyl > indenyl > fluorenyl. The influence of σ-ligands and n-donor ligands gave the following activity order: -Cl > -PMe₃ > -CH₂Ph > -(CH₂)₄CH₃ > -NPh₃. The activities depended on the nature of the cocatalyst and decreased in the following order: MAO ? AlMe₃ > AlEt₃. The addition of aluminum powder and the Lewis base NPh₃ increased the activity of the Cp₂ZrCl₂/MAO catalyst. The Cp₂ZrCl₂/MAO/NPh₃ catalyst showed the highest activity in homogeneous reactions with 458 turnovers in 16 h at 300 °C. The Cp₂ZrCl₂/MAO/NPh₃/SI1102 catalyst gave the highest activity in heterogeneous catalysis with 206 turnovers in 5 h at 350 °C. None of the catalysts required a hydrogen acceptor like an external olefin.     

Addition and metathesis reactions of zirconium and hafnium imido complexes

The zirconium and hafnium imido metalloporphyrin complexes (TTP)M = NArtPr (TTP = meso-5,10,15,20-tetra-p-tolylporphyrinato dianion; M = Zr (1), Hf; AriPr = 2,6-diisopropylphenyl) were used to mediate addition reactions of carbonyl species and metathesis of nitroso compounds. The imido complexes react in a stepwise manner in the presence of 2 equiv of pinacolone to form the enediolate products (TTP)M[OC(tBu)CHC(tBu)(Me)O] (M = Zr (2), Hf (3)), with elimination of H2NAriPr. The bis(mu-oxo) complex [(TTP)ZrO]2 (4) is formed upon reaction of (TTP)Zr = NAriPr with PhNO. Treatment of compound 4 with water or treatment of compound 2 with acetone produced the (mu-oxo)bis(mu-hydroxo)-bridged dimer [(TTP)Zr]2(mu-O)(mu-OH)2 (5). Compounds 2, 4, and 5 were structurally characterized by single-crystal X-ray diffraction.     

Dynamic transformations of cyclopentadienyl chelate complexes of zirconium and hafnium

Russian Chemical Bulletin -     

Synthesis of zirconium, hafnium, and tantalum complexes with sterically demanding hydrazide ligands

The bulky hydrazine t-BuN(H)NMe2 was synthesized via hydrazone and t-BuN(H)N(H)Me intermediates as the major component in a 90:5:5 mixture consisting of t-BuN(H)NMe2, t-BuN(Me)N(H)Me, and t-BuN(Me)NMe2. Reacting the mixture with n-BuLi followed by distillation and fractional crystallization led to the isolation of the ligand precursor LiN(t-Bu)NMe2. Lithium hydrazides, LiN(R)NMe2, were reacted with metal chlorides to afford the hydrazide complexes M(N(Et)NMe2)4 (M = Zr or Hf), MCl(N(R)NMe2)3 (M = Zr, R = i-Pr or t-Bu; M = Hf, R = t-Bu), and TaCl3(N(i-Pr)NMe2)2. The X-ray crystal structures of [LiN(i-Pr)NMe2]4, [LiN(t-Bu)NMe2.THF]2, ZrCl(N(R)NMe2)3 (R = i-Pr or t-Bu), and TaCl3(N(i-Pr)NMe2)2 were determined. The structural analyses revealed that the hydrazide ligands in ZrCl(N(R)NMe2)3 (R = i-Pr or t-Bu) and TaCl3(N(i-Pr)NMe2)2 are eta2 coordinated.     

Synthesis and reactivity of zirconium and hafnium complexes incorporating chelating diamido-N-heterocyclic-carbene ligands

Early transition metal complexes employing a diamido N-heterocyclic carbene (NHC) ligand set (denoted [NCN]) render the centrally disposed NHC moiety stable to dissociation. Aminolysis reactions with the mesityl-substituted ligand precursor (^Mes[NCN]H₂) and M(NMe₂)₄ (M = Zr, Hf) provide bis(amido)-NHC-metal complexes that can be further converted to chloro and alkyl derivatives. Activation of ^Mes[NCN]M(CH₃)₂ with [Ph₃C][B(C₆F₅)₄] yields {^Mes[NCN]MCH₃}{B(C₆F₅)₄}, which is surprisingly inactive for the polymerization of 1-hexene. The zirconium cation did, however, show moderate ability to catalytically polymerize ethylene. The hafnium dialkyls are thermally stable with the exception of the diethyl complex, ^Mes[NCN]Hf(CH₂CH₃)₂, which undergoes β-hydrogen transfer and subsequent C–H bond activation with an ortho-methyl substituent on the mesityl group. The hafnium dialkyl complexes also insert carbon monoxide and substituted isocyanides to yield η²-acyls and η²-iminoacyls, respectively. In some circumstances, further C–C bond coupling occurs to yield enediolates and eneamidolate metallocycles. The molecular structures of ^Mes[NCN]Hf(CH₂CHMe₂)₂, ^Mes[NCN]Hf(η²-(2,6-Me₂C₆H₃NCCH₃)(CH₃), ^Mes[NCN]Hf(η²-(2,6-Me₂C₆H₃NCCH₃)₂, ^Mes[NCN]Hf(OC(CH₃)C(CH₃)NXy), and [^Mes[NCN]Hf(OC(ⁱBu)C(ⁱBu)O)]₂ are included.     

Spectrophotometric studies of the complexes of quadrivalent titanium,zirconium, and hafnium with dibromopyrogallol sulfonphthalein

The characteristics of thee colored chelates of titanium(IV), zirconium(IV), and hafnium(IV) with dibromopyrogallol red have been described. The studies include the determination of molar ratio by two different methods, the range of pH for the stability of the chelates and the evaluation of the conditional stability constants by two methods, i.e., method of Dey and mole ratio method. The λ_max of the ligand was found to be at 430 mμ and that of the chelates of Ti(IV), Zn(IV), Hf(IV) were 560, 550, and 550 mμ, at the pH of study, i.e., 1.5, 0.6, and 1.0, respectively.     

Dinitrogen functionalization with bis(cyclopentadienyl) complexes of zirconium and hafnium

The rich chemistry of substituted bis(cyclopentadienyl)zirconium and hafnium complexes bearing side-on coordinated dinitrogen ligands is highlighted in this Perspective. Our studies in this area were initially motivated by the desire to understand side-on vs. end-on dinitrogen coordination in bimetallic zirconocene and hafnocene N2 compounds. In the cases where eta2,eta2-dinitrogen compounds were isolated, both structural and computational data have established significant imido character in the metal-nitrogen bonds. This additional bonding interaction, which is diminished in end-on complexes bearing both terminal and bridging N2 ligands, facilitates dinitrogen functionalization by non-polar reagents including dihydrogen, carbon-hydrogen bonds and weak Br?nsted acids such as water and ethanol. In hafnocene chemistry, where unwanted side-on, end-on isomerization is suppressed, cycloaddition of phenylisocyanate to coordinated N2 has also been accomplished. For N-H bond forming reactions involving H2, kinetic measurements, in addition to isotopic labelling and computational studies, are consistent with dinitrogen functionalization by 1,2-addition involving a highly ordered, four-centred transition structure.     

Complexometric determination of aluminium, zirconium, and hafnium

Summary Methods for the complexometric titration of aluminium, zirconium and hafnium are described using pyrocatechol violet as indicator. The EDTA titration of zirconium and hafnium is performed in acid medium and that of aluminium in a solution buffered with acetate.

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Zusammenfassung Methoden zur komplexometrischen Titration von Aluminium, Zirkonium und Hafnium unter Verwendung von Brenzcatechinviolett als Indicator werden beschrieben. Die ÄDTA-Titration von Zirkonium und Hafnium erfolgt in relativ stark saurer Lösung, während die des Aluminiums in acetatgepufferter Lösung ausgeführt wird.

     

Diamido/diamine cyclam-based zirconium and hafnium complexes: Synthesis and characterization

The synthesis and characterization of amido-amine cyclam based metal complexes is described. A novel tetraazamacrocycle ligand precursor (Li₂[1,8-Bn₂-1,4,8,11-tetraazacyclotetradecane], Li₂Bn₂cyclam, 2) is reported. Reactions of 2 with MCl₄(THF)₂ afforded M(Bn₂cyclam)Cl₂ (M = Zr 3, Hf 4). The two complexes show trigonal prismatic metal coordination geometries in the solid-state molecular structures. The cross-bridged cyclam 1,4,8,11-tetraazabicyclo[6.6.2]hexadecane (CB-H₂cyclam 5) was used to prepare the lithiated ligand precursor (CB-Li₂cyclam 6) and (CB-(Me₃Si)₂cyclam 7). M(CB-cyclam)Cl₂ (M = Zr 8, Hf 9) were synthesized from reactions of MCl₄(THF)₂ with 6. The structures of 3 and 4 are compared with those of zirconium and hafnium complexes derived from cyclam and unsaturated tetraazamacrocyclic ligands.     

Crystal structures of crown and aza-crown ether complexes with zirconium,hafnium, niobium,and tantalum fluorometallates

A systematic X-ray diffraction study of the interaction products of Zr(IV), Hf(IV), Nb(V), and Ta(V) oxides (fluorides) with crown-ethers (CEs) in aqueous solutions of hydrofluoric acid is performed. It is shown that oxygen-containing CEs form oxonium complexes with [NbF6]^s- and [TaF6]^s- hexafluorometallate anions. In two systems, [cis-syn-cis-DCH18C6-H₃O][TaF₆] and [B18C6·H₃O][TaF₆], the phenomenon of supramolecular isomerism is found, which is caused by a change in the conformation of the macrocycle or by a partial redistribution of intermolecular hydrogen bonds. The use of aza-crown ethers as extractants made it possible to extract unique hydrolytically unstable anions, the products of incomplete fluorine substitution for oxygen atoms in the starting oxides in the form of crystalline complexes with a composition of [(HA15C5)₂][Ta₂F₁₀O] and [(HA18C6·H₂O)(A18C6·H₂O)] [(H₂O)Nb₂F₉O]. In [(18C6)(H₇O₃)₂×(Hf₂F₁₀·2H₂O)], [(HA18C6)(M₂F₁₀·2H₂O)·(H₃O)·H₂O], and [(H₂DA18C6) (M₂F₁₀·2H₂O)·2H₂O] (M=Zr, Hf) complexes, the metals are extracted in the form of identical (M₂F₁₀·2H₂O)^2s- anions with a similar topology. The performed study demonstrates that macrocyclic complexones are undoubtedly promising to extract Zr(IV), Hf(IV), Nb(V), and Ta(V) from fluorine-containing aqueous solutions.     

Pyrrolide-imine benzyl complexes of zirconium and hafnium: synthesis, structures, and efficient ethylene polymerization catalysis

A series of pyrrolyl-imines HL^1-6 was prepared by the condensation of pyrrole-2-carboxyaldehyde with different amines. The reaction of 2 equiv of pyrrolyl-imine with tetrabenzyl complexes of hafnium and zirconium M(CH₂Ph)₄ (M=Hf or Zr) gave dibenzyl complexes (L^3-6)₂M(CH₂Ph)₂, which were characterized by NMR spectroscopy and crystal structure analysis. NMR spectra of the complexes with secondary alkyl substituents at the imine nitrogen (isopropyl: 3a, 4-tert-butylcyclohexyl: 4a and 4b) suggest that rapid racemization between Δ and Λ configurations occurs in solution on the NMR time scale. The complexes with pyrrolide-imine ligands with a tertiary alkyl group such as tert-butyl (5a and 5b) or 1-adamantyl (6a and 6b) at the imine nitrogen possess cis-configured benzyl groups. Hafnium complexes 5a and 6a react with B(C₆F₅)₃ in bromobenzene-d₅ to give the corresponding cationic benzyl complexes, which exhibit high activity for ethylene polymerization (5a: 2242 kg-polymer/ mol-Hf h bar, 6a: 2096 kg-polymer/ mol-Hf h bar). Zirconium complexes 5b and 6b display a remarkably high ethylene polymerization activity when activated with methylaluminoxane (5b: 17,952 kg-polymer/mol-Zr h bar, 6b: 22,944 kg-polymer/mol-Zr h bar).     

(Amidomethyl)pyridine zirconium and hafnium complexes: Synthesis and structural characterization

A series of 2-formyl and 2-acetylpyridines was condensed with 2,6-diisopropylaniline to yield the corresponding imines. Their reaction with sodium borohydride gave the respective N-arylaminomethylpyridines. Treatment of the N-arylformimino- or -acetiminopyridines with trimethylaluminum followed by hydrolysis furnished a series of the respective substituted N-arylaminoethylpyridine derivatives. Their reaction with tetrabenzylzirconium or tetrakis(dimethylamido)zirconium or -hafnium gave the corresponding (chelate ligand)MX₃ systems in a variety of cases. Some of these gave very active ethene polymerization catalysts upon activation with methylalumoxane. Six of the neutral aminoalkylpyridines were characterized by X-ray diffraction, as were eight of the zirconium or hafnium complexes and two aluminum chelate complex systems.     

Chemical detection of zirconium and hafnium

Summary 3-Hydroxyflavone, morin and quercetin are suitable for the fluorescent detection of hafnium and zirconium under proper acidity and reagent concentration conditions. 3-Hydroxyflavone is the most sensitive reagent for both metals. Substantially acid solutions are preferred for the reactions of hafnium and zirconium to increase selectivity despite decreasing reaction sensitivity. The hydroxyflavone hafnium chelates are more stable to acid than the zirconium chelates especially with morin and quercetin. The very low background fluorescence of quercetin permits the detection of hafnium in strong perchloric acid where the fluorescence of the other two reagents hinders detection.

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Zusammenfassung 3-Hydroxyflavon(I), Morin(II) und Quercetin(III) eignen sich bei bestimmter Acidität und Reagenskonzentration zum Fluoreszenznachweis von Hafnium und Zirkonium. I ist das empfindlichste Reagens für beide Metalle. Deutlich saure Lösungen steigern die Selektivität der Reaktionen, setzen aber die Empfindlichkeit herab. Das Hafnium-I-Chelat ist gegenüber Säure stabiler als die Zirkoniumchelate, besonders von II und III. Die sehr geringe Untergrundfluoreszenz von III ermöglicht den Hafniumnachweis in starker Perchlorsäure, worin die Fluoreszenz der beiden anderen Reagenzien den Nachweis verhindert.