N2 hydrogenation from activated end-on bis(indenyl) zirconium dinitrogen complexes

Sodium amalgam reduction of the bis(indenyl)zirconium dihalide complexes, (eta5-C9H5-1-iPr-3-Me)2ZrX2 (X = Cl, Br, I), yielded the corresponding end-on dinitrogen complexes, [(eta5-C9H5-1-iPr-3-Me)2Zr(NaX)]2(mu2, eta1, eta1-N2), with inclusion of 1 equiv of salt per zirconocene. The solid state structures of the chloro and iodo congeners establish short Zr N and elongated N N bonds, consistent with modest to strong activation of the coordinated dinitrogen molecule. Exposure of the N2 compounds to 1 atm of dihydrogen resulted in rapid N H bond formation to yield a hydrido zirconocene hydrazido compound concomitant with salt elimination. These studies establish a new structural type of zirconocene dinitrogen complex and demonstrate that side-on coordination of the N2 ligand in the ground state is not a prerequisite for dinitrogen hydrogenation.     

Carbon-oxygen bond cleavage with eta9,eta5-bis(indenyl)zirconium sandwich complexes

Treatment of the eta9,eta5-bis(indenyl)zirconium sandwich complex, (eta9-C9H5-1,3-(SiMe3)2)(eta5-C9H5-1,3-(SiMe3)2)Zr, with dialkyl ethers such as diethyl ether, CH3OR (R=Et, nBu, tBu), nBu2O, or iPr2O resulted in facile C-O bond scission furnishing an eta5,eta5-bis(indenyl)zirconium alkoxy hydride complex and free olefin. In cases where ethylene is formed, trapping by the zirconocene sandwich yields a rare example of a crystallographically characterized, base-free eta5,eta5-bis(indenyl)zirconium ethylene complex. Observation of normal, primary kinetic isotope effects in combination with rate studies and the stability of various model compounds support a mechanism involving rate-determining C-H activation to yield an eta5,eta5-bis(indenyl)zirconium alkyl hydride intermediate followed by rapid beta-alkoxide elimination. For isolable eta6,eta5-bis(indenyl)zirconium THF compounds, thermolysis at 85 degrees C also resulted in C-O bond cleavage to yield the corresponding zirconacycle. Both mechanistic and computational studies again support a pathway involving haptotropic rearrangement to eta5,eta5-bis(indenyl)zirconium intermediates that promote rate-determining C-H activation and ultimately C-O bond scission.     

Mono- and bisphenoxy derivatives of bis(indenyl)zirconium(IV)

A series of (C₉H₇)₂Zr(OAr)Cl and (C₉H₇)₂Zr(OAr)₂ complexes, where Ar = C₆H₅, p-ClC₆H₄, α-C₁₀H₇, or β-C₁₀H₇, have been synthesised by the reaction of bis(indenyl)zirconium(IV)-dichloride with an appropriate phenol in a 1:1 and 1:2 molar ratio in refluxing benzene in the presence of triethylamine. These complexes have been characterised by elemental analyses, conductance measurements and spectral (IR, ¹H NMR and electronic) studies.     

Bis(indenyl)zirconium(IV) complexes of monofunctional bidentate salicylidimines

A series of (C₉H₇)₂Zr(SB)Cl complexes whereSB ^– is the anion of bidentateSchiff base derived from salicylaldehyde and 4-substituted anilines, viz. salicylidene-4-ansidine, salicylidene-4-phenetidine, salicylidene-4-chloroaniline, salicylidene-4-bromoaniline, salicylidene-4-iodoaniline and salicylidene-4-nitroaniline, have been synthesized by the reaction of bis(indenyl)zirconium(IV) dichloride andSchiff base (SBH) in 1:1 molar ratio in refluxingTHF in the presence of triethylamine. The new derivatives have been characterized on the basis of their elemental analyses, conductance measurements and spectral (IR,¹H-NMR, UV-VIS) studies.

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Bis(indenyl)zirkonium(IV)-Komplexe monofunktioneller zweizähniger Salicylidimine

Zusammenfassung Es wurde eine Reihe von (C₉H₇)₂Zr(SB)Cl-Komplexen synthetisiert, wobeiSB ^– für das Anion einer zweizähnigenSchiff-Base steht. DieSchiff-Basen sind von Salicylaldehyd und 4-substituierten Anilinen hergeleitet: Salicyliden-4-anisidin,-4-phenetidin, -4-Cl-, -4-Br-, -4-I-anilin und -4-Nitroanilin. Die Synthese erfolgte über die Reaktion von Bis(indenyl)zirkonium(IV)-dichlorid mit derSchiff-Base (SBH) in einem molaren Verhältnis von 1:1 am Rückfluß in Gegenwart von Triethylamin undTHF als Lösungsmittel. Zur Charakterisierung der neuen Derivate wurden Elementaranalysen, Leitfähigkeitsmessungen und spektroskopische Daten (IR,¹H-NMR, UV-VIS) herangezogen.

     

A counterintuitive, yet efficient synthesis of bis(indenyl)zirconium dihalides

The use of bisindenyl zirconium dimethyl in the synthesis of the corresponding dihalides is described. The synthesis of Ind₂ZrCl₂ has been carried out, cleanly and quantitatively, by chlorination of Ind₂ZrMe₂ (2) with a variety of inorganic chlorides, including BCl₃, SCl₂ in either toluene or methylene chloride, and HCl in Et₂O. The latter method is the preferred one, since it produces methane as the only byproduct, and the formed Ind₂ZrCl₂ precipitates quantitatively from the Et₂O solvent: a simple filtration then gives the analytically pure product in quantitative yield. Ind₂ZrBr₂ and Ind₂ZrI₂ are obtained by reacting 2 with Br₂ and I₂, respectively. The ‘inverse’ synthesis of metallocene dichlorides by chlorination of their dimethyl ‘derivatives’ has advantages compared to the classic method: first, Ind₂ZrCl₂ is obtained in higher yield (90-96%); second, it is easier to purify (at the dimethyl stage) and is obtained free from LiCl. Five bisindenyl zirconium complexes with different sigma ligands have been investigated in solution propylene polymerizations: it is found that the σ-ligands have no relevant influence on the catalyst activity, nor on the polymer properties.     

Zirconium bis(indenyl) sandwich complexes with an unprecedented indenyl coordination mode and their role in the reactivity of the parent bent-metallocenes: a detailed DFT mechanistic study

The mechanisms of three closely related reactions were studied in detail by means of DFT/B3 LYP calculations with a VDZP basis set. Those reactions correspond to 1) the reductive elimination of methane from [Zr(eta5-Ind)2(CH3)(H)] (Ind=C9H7-, indenyl), 2) the formation of the THF adduct, [Zr(eta5-Ind)(eta6-Ind)(thf)] and 3) the interconversion between the two indenyl ligands in the Zr sandwich complex, [Zr(eta5-Ind)(eta9-Ind)], which forms the link between the two former reactions. An analysis of the electronic structure of this species indicates a saturated 18-electron complex. A full understanding of the indenyl interchange process required the characterisation of several isomers of the Zr-bis(indenyl) species, corresponding to different spin states (S=0 and S=1), different coordination modes of the two indenyl ligands (eta5/eta9, eta5/eta5 and eta6/eta9), and three conformations for each isomer (syn, anti, and gauche). The fluxionality observed was found to occur in a mechanism involving bis(eta5-Ind) intermediates, and the calculated activation energy (11-14 kcal mol(-1)) compares very well with the experimental values. Two alternative mechanisms were explored for the reductive elimination of methane from the methyl/hydride complex. In the more favourable one, the initial complex, [Zr(eta5-Ind)2(CH3)(H)], yields [Zr(eta5-Ind)2] and methane in one crucial step, followed by a smooth transition of the Zr intermediate to the more stable eta5/eta9-species. The overall activation energy calculated (Ea=29 kcal mol(-1)) compares well with experimental values for related species. The formation of the THF adduct follows a one step mechanism from the appropriate conformer of the [Zr(eta5-Ind)(eta9-Ind)] complex, producing easily (Ea=6.5 kcal mol(-1)) the known product, [Zr(eta5-Ind)(eta6-Ind)(thf)], a species previously characterised by X-ray crystallography. This complex was found to be trapped in a potential well that prevents it from evolving to the 3.4 kcal mol(-1) more stable isomer, [Zr(eta5-Ind)2(thf)], with both indenyl ligands in a eta5-coordination mode and a spin-triplet state (S=1).     

Intra- and inter-ring haptotropic rearrangements in (naphthalene and anthracene)nickel complexes: a DFT study

Density functional quantum chemical calculations of the mechanisms of metallotropic η²,η²-intra- and η²,η²-inter-ring haptotropic rearrangements (HRs) in 16e zero-valent η²-(naphthalene and anthracene)nickel complexes involving migration of the organometallic group within the same ring or from one aromatic ring to the other were carried out. The structures of the initial complexes, transition states, and intermediates were determined. The intra-ring HRs in these systems proceed via low-energy η⁴-cis-butadiene transition states. The inter-ring HRs proceed along the periphery of the naphthalene and anthracene ligands via high-energy η³-allylic transition states. In contrast to well-investigated η⁶,η⁶-inter-ring HRs in (naphthalene and anthracene)tricarbonylchromium complexes, the activation barriers to the η²,η²-inter-ring HRs in the corresponding nickel complexes are much lower. Transition states of these processes are characterized by higher hapticity compared to the initial complexes. This also distinguishes the nickel complexes from the corresponding Cr(CO)₃ complexes for which the hapticities of transition states of the η⁶,η⁶-inter-ring HRs are lower than those of the initial complexes. The calculated activation barriers to the η²,η²-intra-ring HRs in the (naphthalene and anthracene)nickel complexes as well as the barrier to rotation of the organonickel group in the naphthalene complex are in good agreement with the experimental data. The calculated barriers to the η²,η²-inter-ring HRs in the naphthalene and anthracene complexes are 3–5 kcal mol⁻¹ lower than the experimental values. This is probably due to the competition between two mechanisms of this process, a low-energy intramolecular mechanism and a high-energy intermolecular dissociative mechanism.     

Bis(indenyl)titanium(IV) and zirconium(IV) complexes of monofunctional bidentate salicylidimines

Dichlorobis(indenyl)-titanium(IV) and -zirconium(IV), (C₉H₇)₂TiCl₂ and (C₉H₇)₂ZrCl₂, react with bidentate Schiff bases such as salicylidene aniline, salicylidene-o-toluidine, salicylidene-m-toluidine and salicylidene-p-toluidine in a 1:1 molar ratio in refluxing tetrahydrofuran in the presence of triethylamine to yield complexes of the type (C₉H₇)₂Ti(SB)Cl and (C₉H₇)₂Zr(SB)Cl, respectively where SB is the anion of the corresponding Schiff base, SBH. The new derivatives have been characterised on the basis of their elemental analyses, conductance measurements and spectral (IR, ¹H NMR and electronic) studies.     

Spin-state alteration from sterically enforced ligand rotation in bis(indenyl)chromium(II) complexes

The rotational orientation of cyclopentadienyl rings usually has no effect on d-orbital energy levels and splitting in transition metal complexes. With related but less symmetrical carbocyclic ligands, however, the magnetic properties of the associated complexes can be altered by the alignment of the ligands. Examples of this effect are found in substituted organochromium(II) bis(indenyl) complexes. The monosubstituted compounds (1-RC(9)H(6))(2)Cr (R = t-Bu, SiMe(3)) are prepared from the substituted lithium indenides and CrCl(2) in THF; they are high-spin species with four unpaired electrons. Their spin state likely reflects that in the unknown monomeric (C(9)H(7))(2)Cr, which is calculated to have a high-spin (S = 2) ground state in the staggered configuration (180 degrees rotation angle). However, the analogous bis(indenyl) complexes containing t-Bu or SiMe(3) groups in both the 1 and 3 positions on the indenyl ligands ((1,3-R(2)C(9)H(5))(2)Cr) are low-spin compounds with two unpaired electrons. X-ray diffraction results indicate that [1-(t-Bu)C(9)H(6)](2)Cr exists in a staggered conformation, with Cr-C (av) = 2.32(4) A. In contrast, the average Cr-C distances in [1,3-(t-Bu or SiMe(3))(2)C(9)H(5)](2)Cr are 2.22(2) and 2.20(2) A, respectively, and the rings are in a gauche configuration, with rotation angles of 87 degrees. The indenyl conformations are sterically imposed by the bulk of the t-Bu and SiMe(3) substituents. The change from a staggered to a gauche indenyl orientation lowers the symmetry of a (C(9)H(7))(2)M complex and allows greater mixing of metal and ligand orbitals. Calculations indicate that previously nonbonding pi orbitals of the indenyl anion are able to interact with the chromium d orbitals, producing bonding and antibonding combinations. The latter remain unpopulated, and the resulting increase in the HOMO-LUMO gap forces the complexes to adopt a low-spin configuration. The possibility of using sterically imposed ligand rotation as a means of spin-state manipulation makes indenyl compounds a potentially rich source of magnetically adjustable molecules.     

Preparation and characterization of bis(cyclopentadienyl)zirconium (IV) oximate complexes

Summary Oximate complexes of bis(cyclopentadienyl)zirconium(IV) chloride (Cp₂ZrCl₂ ) having the general formulae Cp₂Zr(Ox), Cp₂Zr(OxH)Cl and Cp₂Zr(OxH)₂ [where OxH₂ = RC₆H₄C(OH)R: NOH, R = H, Me, R= H, Me, Et, n-Pr; PhC: N(OH)CH(OH)Ph and RC : N(OH)N(OH): CR, R = H, Me and Ph] have been synthesized by reacting bis(cyclopentadienyl)zirconium(IV) chloride with the appropriate oxime in tetrahydrofuran in the presence of triethylamine at room temperature. The complexes have been characterized by their elemental analysis, i.r. and electronic spectra, molecular weight determination, electrical conductance and magnetic measurements.Reprints of this paper are not available.     

Amineborane dehydrogenation promoted by isolable zirconium sandwich, titanium sandwich and N(2) complexes

Catalytic dehydrogenation of R(2)NHBH(3) (R = Me, H) promoted by a family of bis(cyclopentadienyl)titanium and bis(indenyl)zirconium compounds is reported; structure-reactivity relationships as a function of cyclopentadienyl and indenyl substituents have been examined.     

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.     

Zirconium sandwich complexes with eta(9) indenyl ligands: well-defined precursors for zirconocene-mediated coupling reactions

A family of isolable, well-defined bis-indenyl zirconium sandwich complexes, (eta(5)-C(9)H(5)-1,3-R(2))(eta(9)-C(9)H(5)-1,3-R(2))Zr (R = silyl, alkyl), have been prepared by either alkane reductive elimination or alkali metal reduction of a suitable zirconium(IV) dihalide precursor. Crystallographic characterization of two of these derivatives, R = SiMe(2)CMe(3) and CHMe(2), reveals unprecedented eta(9) coordination of one of the indenyl ligands. Variable-temperature and EXSY NMR studies establish that the eta(5) and eta(9) rings are rapidly interconverting in solution. The sandwich complexes serve as effective sources of low-valent zirconium reacting rapidly with both olefins and alkynes at ambient temperature. In contrast to bis-cyclopentadienyl chemistry, the olefin adducts of the bis-indenyl zirconium sandwiches undergo preferential C-H activation to yield the corresponding allyl hydride compounds, although reaction with excess olefin proceeds through the eta(2)-olefin adduct, forming the corresponding zirconacyclopentane.     

Ethylene and propylene polymerization with bis(indenyl)zirconium/Mao catalytic systems modified by sterically demanding alcohols

Ethylene and propylene polymerization using Ind₂ZrCl₂ and Ind₂Zr(CH₃)₂/MAO catalytic systems modified by the sterically demanding bridged alicyclic alcohols, adamantan‐1‐ol, adamantan‐2‐ol, 2‐methyladamantan‐2‐ol, and fenchyl alcohol, was investigated. Lower alcohols like isopropanol completely deactivate the system, whereas in the case of catalysts modified by these voluminous alcohols only a slight decrease in the catalytic activity proportional to alcohol/metallocene molar ratio was observed. The addition of the modifiers gives rise to polymers with higher molecular weights than the nonmodified systems, but no structural changes in the polyethylenes were observed. The addition of the sterically demanding alcohols to the reaction medium changes the regioregularity of polypropylenes, but does not significantly influence their stereoregularity, at 30 °C. Propylene–ethylene copolymers containing up to 8.6% of ethylene units derived from 1,3‐insertion and significant amount of rr‐centered pentads were obtained by single‐monomer polymerization of propylene with Ind₂ZrCl₂/MMAO/adamantan‐1‐ol, at 70 °C. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 4248–4259, 2005     

二茚基稀土胺化物催化丙烯腈聚合

用二茚基稀土胺化物Ind2LnN(i-Pr)2(Ln=Y,Yb)作为单组分催化剂催化丙烯腈聚合,研究了催化剂用量、单体浓度及聚合温度对标题化合物的催化活性和所得聚丙烯腈的分子量的影响。提高聚合发应温度可明显提高催化活性,当聚合温度达50℃,单体浓度为5.1mol