Titanium, zirconium and hafnium

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HfCl4 and TiCl3 in SOCl2 yielding salts of [TiHfCl10]2А (edge-sharing bioctahedral structure). In solution this exists in equilibrium with [Ti2Cl10]2А and ...
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www.rsc.org/annrepa | Annual Reports A

Titanium, zirconium and hafnium S. A. Cotton DOI: 10.1039/b716569m

This chapter reviews the literature reported during 2007 on titanium, zirconium and hafnium. Because of the limitations of space, this review is very selective, and much fine work has not been cited, especially in the area of organometallic chemistry.

1. Highlights Highlights include significant developments in dinitrogen complexes,79–82 whilst burgeoning interest in TiO2 is sustained.11,13,15,18

2. Introduction Continued progress in functionalisation of N2 using complexes of these metals has been reviewed,1,2 as have developments in TiCp2Cl2-type antitumour metallocenes,3 and modified Group 4 metallocenes in bioorganometallic chemistry.4 Five-membered metallacycles of Ti and Zr have been surveyed.5 Many aspects of TiO2 nanomaterials have been examined,6 and the role of TiO2 in UV filters reviewed.7 Hitherto obscure HfO2, which has a higher dielectric constant than SiO2, is attracting attention as the transistor gate material in the 45 nm generation chips. In turn, this focusses attention on volatile Hf precursors.8,9 Zircon (ZrSiO4) has been suggested as a material for containing nuclear waste, but a report that it suffers more damage than expected from the a-decay of Pu casts this into doubt.10

3. TiO2 and other binary compounds Multi-walled carbon nanotubes have been used as structural templates in the synthesis of oriented arrays of rutile and anatase nanorods.11 TiCl4 combines with water formed in aldol condensation of ketones to form single-phase titania nanocrystallites and nanofibres, in autoclave reactions of TiCl4 solutions in propanone.12 Tetraalkylammonium salts of [TiF6]2 gave largely water-free solutions that proved suitable precursors for the synthesis of nanocrystalline TiO2.13 Ab initio calculations of the mechanism for the reaction between TiI4 and H2O for the deposition of TiO2 upon SiO2 surfaces have been reported; extension to the reaction with TiCl4 predicts purer films.14 A simple synthesis of mesocrystals of NH4TiOF3 and their facile topotaxial conversion, by washing or annealing, to anatase TiO2 is reported,15 as is a convenient microwave synthesis of uniform anatase nanocrystals using ionic liquids.16 A one step synthesis is reported for TiO2/WO3 nanocomposites, photocatalysts for gas phase trichloroethylene degradation.17 The key parameters controlling crystallization of anatase-TiO2 nanomaterials have been analysed.18 Particle size has a considerable effect upon the Li capacity of Uppingham School, Uppingham, Rutland, UK LE15 9QE

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lithiated anatase TiO2.19 On heating at 500 1C, sonochemically-coated LiOH-TiO2 nanoparticles form Li4Ti5O12 nanoparticles.20 Photoelectron spectra of (TiO2)n clusters (n = 1–10) have been used to study the band gap, thereby modelling clusters for TiO2 itself, a wide-band-gap semiconductor.21 Surface structures of rutile TiO2 (011) have been examined.22 Cd doping stabilizes mesoporous TiO2.23 The interfacial phosphate groups in phosphate-based mesoporous TiO2 are good sites for cation-exchange, e.g. Cd2+.24 Use of TiO2 nanoparticles in sunscreens risks the formation of photoelectrons and hence damaging free radicals; grafting antioxidants with a hydrophobic polymer coating directly onto the TiO2 particles eliminates any photocatalytic degradation.25 On irradiation with visible light, platinized TiO2 systems form hydrogen from aqueous protons and the sacrificial electron donor, triethanolamine.26 The photophysics of bis(terpy)RuII complexes, containing nanostructured TiO2 attached to one of the terpyridines and a potential electron donor attached to the other, have been examined.27 Grafting Ce3+ onto nanocrystalline TiO2 particles causes intense metal-to-metal charge transfer (MMCT) bands in the visible spectrum, the system having very high quantum efficiency for the oxidative decomposition of 2-propanol.28 An asymmetric squaraine sensitizer anchored to a nanocrystalline TiO2 film displays remarkable far red sensitization.29 Using an amphipathic viral peptide destabilizes the vesicles leading to lipid bilayer formation on TiO2 (and Au) substrates, potentially permitting a wider choice of substrates to support bilayers.30 Micrometer-sized Ag(core)–AgCl(shell) composites on TiO2 thin films can probe photoinduced interfacial electron transfer.31 Apatitecoated Ag/AgBr/TiO2 acts as a photocatalyst for the inactivation of bacteria such as E. coli by visible light.32 Highly fluorescent charged hyperbranched conjugated polyelectrolytes have been employed as bilayered sensitisers in TiO2 hybrid solar cells.33 In situ polymerisation of an amphiphilic diacetylene within a TiO2 nanostructure, leading to a polydiacetylene/TiO2 nanocomposite using visible light irradiation, is reported.34 Similarly, polymerisation of a functional monomer in the presence of TiO2 nanoparticles and target molecules leads to molecular imprinted polymer coated photocatalysts.35 Solar cells have been designed based upon co-sensitization of organic dyes, on nanocrystalline TiO2 films, with complementary spectral absorptions, leading to a panchromatic response and very high photon-current conversions.36 Ru-phthalocyanine sensitized TiO2 permits both slow and efficient electron injection, owing to the long-lived Ru-pc excited state.37 Sol-gel synthesized TiO2/SiO2 composite nanofibres show photocatalytic activity selectively for small organic substrates.38 It is reported that careful attention to particle morphology and chemistry of the TiO2 surface can greatly enhance the photocatalytic properties of hydrothermal TiO2.39 Mesoporous LnIII–TiO2 (Ln = Tb, Eu, Sm) nanocomposites may find application in LEDs and photocatalysis.40 Mesoporous TiO2 spheres with a tunable chamber structure permit multiple reflections of UV light, leading to much enhanced photocatalytic activity.41 Photoluminescence measurements have been used to follow water photooxidation at flat rutile surfaces, the rate decreasing with increasing pH.42 Changing the size of TiO2 nanoparticles containing CdSe quantum dots permits modulation of the electron injection.43 As a change from TiO2, an interesting report on TiSi2 concerns its ability to act as a photocatalyst for the decomposition of water, due to surface layers of impurities TiO2 and SiO2 which create catalytically active sites.44 The synthesis of titanium carbide from its elements has been studied by in situ neutron-diffraction.45

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4. Complexes Interest in the binding of transferrin to TiIV continues; binding occurs in both hydrolysed and unhydrolysed forms, and this is modelled by Ti(HBED) complexes at different pH values. The complex obtained at low pH has distorted octahedral coordination.46 More titanium(IV) citrate complexes, Na3[Ti(H2cit)2(Hcit)]  9H2O, K4[Ti(H2cit)(Hcit)2]  4H2O, K5[Ti(Hcit)3]  4H2O and Na7[TiH(cit)3]  18H2O, have been synthesised, all having a monomeric, octahedrally coordinated, tricitrato Ti unit.47 If Ti wire is dissolved in HOTf and HF, the solution contains equimolar amounts of TiII and TiIV. Excess Fe(III) or Ru(III) oxidise these solutions to Ti(IV), but if titanium is in excess, Ti(III) is formed.48 Reductions of 1,4-benzoquinones and the Mn(IV) complex of biguanide are catalyzed by added Ti(IV); kinetics are consistent with formation of a TiII–TiIV complex.49 Hydrated ZrO2 has an unexpectedly high solubility in CaCl2 solution; the [Zr(OH)6]2 ions are stabilised by associating with Ca2+ ions as the ternary complex [Ca3{Zr(OH)6}]4+ (in contrast ThO2 forms [Ca4{Th(OH)8}]4+).50 Both NEt3BzCl or Ph3PNPPh3Cl react with a mixture of HfCl4 and TiCl3 in SOCl2 yielding salts of [TiHfCl10]2 (edge-sharing bioctahedral structure). In solution this exists in equilibrium with [Ti2Cl10]2 and [Hf2Cl10]2. Hydrolysis results in [Ph3PNPPh3]2[Cl3Ti(m-O)HfCl5].51 Reaction of ZrCl4, [Ph4P]Cl and a NaOH/Na mixture in py gives [Ph4P]2[(ZrCl4Py)2O].52 New bis(b-ketoesterato)zirconium-(IV) and -hafnium-(IV) phthalocyaninates have been characterised.53 The cluster chlorides Na4[(Zr6Be)Cl16] and K[(Zr6Fe)Cl15] exchange both inner- and outer–sphere chlorides with bromide in solutions in Lewis-base ionic liquids EMIm-Br and AlBr3, evidently in a reversible process.54

5. Alkoxides, amides and related species [Ti(OiPr)4] reacts with naphthalene-2,3-diol forming a variety of products. Reaction in CDCl3 affords the symmetric dimer [{TiNp(OiPr)2}2(HOiPr)2], and a partly hydrolyzed 6:6 assembly [Ti3(m3-O)(m-Np)2(Np)(m-OiPr)(OiPr)(HOiPr)2(m-O)]2 has also been obtained, likely from hydrolysis of the 3:3 product [{TiNp(OiPr)2}3(HOiPr)]. Redissolution of [{TiNp(OiPr)2}2(HOiPr)2] is believed to form a 4:4 condensation product. The behaviour of [Ti(OtBu)4] appears much simpler.55 [Zr(OtBu)4] forms (Z2-amidate) complexes on reaction with surface amide groups in polyamide nylon 6/6; in turn these complexes can link the cell-adhesive peptide arginine–glycine–aspartic acid to the polymer, affording exceptionally high surface loadings.56 [Ti(OiPr)4] and [Zr(OtBu)4] react with an S analogue of a Schiff base ‘‘salenH2’’ ligand ((OSSO)H2) forming octahedral [Ti(OiPr)2(OSSO)] and [Zr(OtBu)2(OSSO)]. ((OSSO)H2) reacts with [Zr(CH2Ph)4] forming [Zr(CH2Ph)2(OSSO)], an active 1-hexene polymerization catalyst upon activation with B(C6F5)3.57 A mixture of [La5O(OiPr)13] and [Zr(OiPr)4(iPrOH)] crystallises forming [La2Zr3O(OiPr)16].58 [Ni(acac)2] reacts with [Zr(OiPr)4(iPrOH)] in toluene forming [{Zr(OiPr)3(acac)}2] and [NiZr2(acac)(OiPr)9].59 [Zr(OR)4] (R = iPr, C(CH3)2Et) and R 0 COOH (R 0 = tBu, C(CH3)2Et) react forming [Zr6O4(OH)4(OOCR 0 )12], which have a [Zr6(m3-O)4(m3-OH)4] core.60 [Ti6O6(OiPr)6(O2CR)6] (R = tBu, CH2tBu, C(CH3)2Et) have been studied as potential TiO2 CVD precursors.61 The alkoxide [Ti(OiPr)(OC6Me2H2CH2)3N] reacts with BH3  thf forming [Ti(BH4)(OC6-

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Me2H2CH2)3N], reducible to the dimeric Ti(III) species [{Ti(OC6Me2H2CH2)3N}2]. In contrast, [Ti(OiPr)(OC6Me2H2CH2)3N] reacts with B(C6F5)3 affording the Lewis acid adduct [Ti(OC6Me2H2CH2)3N][HO  B(C6F5)3], whilst [Ti(OiPr)4] reacts with B(C6F5)3 in an exchange reaction forming [{Ti(OiPr)3(C6F5)}2].62 Methacrylic acid reacts with barium–titanium and barium–zirconium double alkoxides forming the oxo-clusters [Ba2Ti10(m3-O)8(m2-OH)5(m2-OMc)20(OiPrOMe)2] and [Ba(OMc)2 (McOH)3]n, potential nanosized building blocks for inorganic–organic hybrid materials.63 A variety of oxime complexes of titanium(IV) isopropoxide have been synthesised, including dinuclear complexes with bridging isopropoxides, and trinuclear complexes containing triply bridging oximes.64 Mono and polyatomic titanium alkoxy complexes with N,O and O,O chelating ligands have been synthesised and the structure of [Ti(OiPr)2{2-(–)-menthoxopyridine}2] determined.65 Grafting the aryloxide/alkyls [(ArO)Zr(CH2tBu)3] and [(ArO)2Zr(CH2tBu)2] onto silica gives monosiloxy surface complexes [(RSiO)Zr(CH2tBu)2(OAr)] and [(RSiO)Zr(CH2tBu)(OAr)2], the former being a catalyst for the homologation of propane.66 Octahedral [Hf(NEt2)2Cl2(DME)] is an intermediate in the synthesis of [Hf(NEt2)2(OiPr)2].67 p-TsNH2 reacts with [Ti(NMe2)4] in toluene forming [{M(m-NTs)(NMe2)2}2], whilst in CH2Cl2 the products are [{Ti{m-N,O-NTs}Cl(NMe2)(NHMe2)2}2] and [Ti{m-N,O-NTs}Cl2(NHMe2)2]n.68 Catalytic amounts of [Ti(NMe2)4] promote highly selective transamidation reactions between primary amines and secondary carboxamides, whilst stoichiometric amounts lead to amidine and oxotitanium products.69 The mixed amide/guanidinate [{(NiPr)2C(NEtMe)}2Zr(NEtMe)2] is thermally stable and a potential material in the thin-film synthesis of ZrO2.70 Pyrazolato complexes including [Zr(Z2-3,5-Me2Pz)2Cl2(Z1-3,5-Me2PzH)2]  (3,5-Me2PzH) and [Zr(Z2-3,5Me2Pz)2(CH2Ph)2] have been studied as alkene polymerization catalysts on activation by MAO.71 A Zr(IV) amide reacts with dioxygen forming a most unusual bis(peroxide) complex (1; Ar = 2,6-diisopropylphenyl), with very long O–O distances ca. 1.50–1.51 A˚.72

A dinuclear zirconium guanidinato complex, [{Zr{ArNC(NMe2)N(SiMe3)} (m2-Cl)Cl2}2] (Ar = 2,6-iPr2C6H3), is an active catalyst for ethene polymerization.73 Titanatranes containing a tetradentate (O3N) trianionic donor, ({(O-2,4-R2C6H2-6CH2)2(OCH2CH2)}N3–) (R = Me, tBu)) have been investigated as ethene polymerisation catalysts. {(HO-2,4-Me2C6H2-6-CH2)2(HOCH2CH2)}N reacts with Ti(OR 0 )4 (R 0 = iPr , tBu) forming dimeric titanatranes, [Ti2(OR 0 )2{((O-2,4Me2C6H2-6-CH2)2(m2-OCH2CH2))N}2], whereas the corresponding reaction with {((O-2,4-tBu2C6H2-6-CH2)2(OCH2CH2))N}3 results in monomeric TBPY [Ti(OR 0 ){((O-2,4-tBu2C6H2-6-CH2)2(OCH2CH2))N}] (2).74

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Titanium imides [Ti(NR)(N2ArNpy)(L)] (N2ArNpy = MeC(2-C5H4N)(CH2NAr)2, Ar = 4-C6H4Me or 3,5-C6H3Me2: L = e.g. py) react rapidly with arylacetylenes HCRCAr forming the {2+2} cycloaddition products, azatitanacyclobutene [Ti(N2ArNpy){k2-N(tBu)CHQCAr 0 }] (Ar 0 = Ph and 4-C6H4Me). These are catalytic intermediates in the anti-Markovnikov hydroamination of terminal alkynes, and also react slowly with another mole of alkyne forming azatitanacyclohexadienes [Ti(N2ArNpy){k2-N(tBu)CHQC(Ar 0 )C(Ar 0 )QCH}].75 At room temperature, [Ti{MeN(CH2CH2NSiMe3)2}(NNPh2)(py)] inserts alkynes such as PhCR CMe selectively, forming [Ti{MeN(CH2CH2NSiMe3)2}(NC(Ph)QC(Me)NPh2) (py)].76 The tris(ketimide) [Ti(NQCtBu2)3Cl] reacts with TlPF6 in MeCN forming [Ti(NQCtBu2)3(NCMe)]PF6. Amongst its many reactions, alkyls [Ti(NQCtBu2)3CH2Ph] and [Ti(NQCtBu2)3Me] have also been made.77 TiCl4 reacts with 3 moles of 2-mercaptopyridine (HSC5H4N) and tert-butylpyridine in toluene to afford seven coordinate [TiCl(SC5H4N)3], a similar reaction occurring with 2-mercaptopyrimidine forming [TiCl(SC4H3N2)3]. Low pressure CVD of these compounds has been studied, [TiCl(SC5H4N)3] giving thin films of TiS2 at 600 1C and TiS2/TiO2 films at 500 1C; [TiCl(SC4H3N2)3] yields TiS2 films contaminated with Cl, C and N.78

6. Dinitrogen complexes Reduction of [(NPN)*ZrCl2] with KC8 in thf under N2 yields the side-on dinitrogen complex [(NPN)*Zr(thf)(m-Z2:Z2-N2)Zr(NPN)*(thf)] (N–N = 1.503 A˚). Reaction with py or PMe2R (R = Me, Ph) gives [(NPN)*Zr(py)(m-Z2:Z2-N2)Zr(NPN)*(py)] and [(NPN)*Zr(PMe2R)(m-Z2:Z2-N2)Zr(NPN)*]; only one phosphine coordinates to the dimer unit. [(NPN)*Zr(PMe2R)(m-Z2:Z2-N2)Zr(NPN)*] reacts with H2 forming [(NPN)*Zr(PMe2R)(m-H)(m-Z2:Z2-N2H)Zr(NPN)*].79 Reduction, using KC8 in thf, of [(Z5-C5Me4R)M{N(R 0 )C(X)N(R 0 )}Cl2] (M = Zr or Hf, X = NMe2 or Me, R = H or Me, R 0 = iPr, Et), gives the side-on bridged [(Z5-C5Me4R)M{N(R 0 )C(X)N(R 0 )}(m-Z2:Z2-N2)(Z5-C5Me4R)M{N(R 0 )C(X)N(R 0 )}]. N–N distances correlate with increasing fold angle of the M2N2 core. Certain of these undergo N-alkylation; all undergo hydrosilylation and hydrogenation with PhSiH3 and H2.80 Na/Hg reduction of the ansa-zirconocene compound [Me2Si(Z5-C5Me4)(Z5-C5H3-3-tBu)ZrCl2] under N2 affords side-on [{Me2Si(Z5-C5Me4)(Z5-C5H3-3-tBu)Zr}2(m2,Z2,Z2-N2)]. This is readily hydrogenated to [{Me2Si(Z5-C5Me4)(Z5-C5H3-3-tBu)Zr}2(m2,Z2,Z2-N2H2)]; in the absence of H2, this is unstable, and tends to dehydrogenate to [{Me2Si(Z5-C5Me4)(Z5-C5H33-tBu)HZr}2(m2,Z2,Z2-N2)].81 [{(Z5-C5H2-1,2,4-Me3)2Ti}2(m2,Z2,Z2-N2)] has a side-on bound N2 ligand, but attempts to make similar complexes using iPr or tBu ring substituents result in products with end-on (and less activated) dinitrogen ligands. The ansa-titanocene complexes rac-[{Me2Si(Z5-C5H2-2-SiMe3-4-tBu)2Ti}2(m2,Z1,Z1-N2)] and [{Me2Si(Z5-

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C5Me4)(Z5-C5H3-3-tBu)Ti}2(m2,Z1,Z1-N2)] both have feebly activated end-on N2 ligands.82 The hafnocene dinitrogen complex [{(Z5-C5H-1,2,3,42 2 Me4)2Hf}2(m2,Z ,Z -N2)] reacts with CO2 forming N–C bonds, subsequent reaction with Me3SiI forming [(Me3C  O2C)2N-N(SiMe3)2], a hydrazine derivative.83 Computational studies of dinitrogen complexes suggest electronic reasons for reactivity differences, thus examination of potential energy surfaces for the reaction + H2 [(Z5-C5MenH5n)2M][(Z5[(Z5-C5MenH5n)2M]2(m2,Z2,Z2-N2) 2 2 C5MenH5n)2MH](m2,Z ,Z -NNH) (M = Ti, Zr, Hf; n = 0,4) indicates triplet ground states for M = Ti and singlet ground states for Zr and Hf, the latter fulfilling required criteria for the addition.84

7. Organometallics Laser-ablated Ti, Zr and Hf atoms react with CH2Cl2 and CHCl3 forming CH2QMCl2 and triplet HCCMCl3.85 Likewise, triplet state species FCCMF3 are formed from Ti, Zr and Hf atoms and CF4, and ClCCMCl3 from Ti, Zr and Hf atoms and CCl4. Similar reactions with CF2Cl2 produce a mixture of FCCMFCl2 and ClCCMF2Cl.86 CH2QMF2 result from reaction between the Group 4 metal atoms and CH2F2, but the corresponding reactions with CHF3 afford CHFQTiF2, triplet HCCZrF3 and HCCHfF3.87 Unlike ferrocene and its brethren, [Cp2Ti] has defied synthesis by the Wilkinson route. Now it has been trapped (as the Ph4C4 adduct) by adding PhCRCPh to the reaction mixture of TiCl2, thf and CpNa; it is believed that chlorides present in the mixture form a stabilizing chloro complex with the [Cp2Ti].88 Gas-phase [Cp2Ti] has been generated by decomposition of a Cp2TiIV compound, and identified by its mass spectrum.89 Replacing one methyl on each ring in [Cp*2Ti] by a SiMe3 group greatly improves its stability, [TiII(Z5-C5Me4(SiMe3))2] being stable in toluene solution up to 90 1C. At 140 1C, it eliminates H2 forming [TiII{Z3:Z4-C5Me2(SiMe3)(CH2)2}{Z5C5Me4(SiMe3)}]. In contrast, the methyl derivatives [TiIIIMe{Z5-C5Me4(SiMe3)}2] and [TiIVMe2{Z5-C5Me4(SiMe3)}2] decompose at lower temperatures than [Cp*2TiMe] and [Cp*2TiMe2], eliminating hydrogen from the SiMe3 group forming [TiIII{Z5:Z1-C5Me4(SiMe2CH2)}{Z5-C5Me4(SiMe3)}] and [TiII{Z6:Z1-C5Me3(CH2) (SiMe2CH2)}{Z5-C5Me4(SiMe3)}] and CH4.90 Mg reduction of [Cp*2TiCl2] in the presence of RCCR affords [Cp*2Ti(Z2-RCCR)] (R = Ph, Et).91 [Cp*TiMe3] reacts with O(SiPh2OH)2 forming the siloxide [Cp*TiMe{(OSiPh2)2O}] hydrolysed to [(Cp*Ti{(OSiPh2)2O})2(m-O)]. Corresponding reactions of [Cp*TiMe3] with HOSiPh3 afford [Cp*TiMe2(OSiPh3)], hydrolysed to [{Cp*TiMe(OSiPh3)}2(m-O)].92 [Cp*2ZrCl2] reacts with Mg/Hg in py by ortho C–H bond activation, forming [Cp*2ZrH(Z2-kC,N-C5H4N)] in which the ring N is positioned laterally; isomerisation on warming yields the isomer with the medial nitrogen.93 Polymer-supported [Cp00 CpMCl2] (M = Ti or Zr) catalyse the polymerisation of ethene under very mild conditions.94 [(CGC)TiMe2] and [(EBICGC)Ti2Me4] (CGC = Me2Si(Me4C5)(NtBu); EBICGC = (m-CH2CH2-3,3 0 ){(Z5-indenyl)[1-Me2Si(tBuN)]}2) are effective precatalysts for alkene polymerization in the presence of alkenylsilanes.95 Extracting 3TiCl3  AlCl3 with thf is an excellent route to the useful material [TiCl3(thf)3]; this reacts with KL [L = {OCMe2CH2(1-C[NCHCHNPri])}] forming the octahedral Ti(III) tris(carbene) complex mer-[TiL3].96 [TiMe2(dmpe)2] reacts with excess PhSiH3 forming the oligosilane complex [Ti(Si3H5Ph3)(dmpe)2], which slowly reacts with further PhSiH3 forming [Ti(Si4H6Ph4)(dmpe)2].97 (Indenyl)titanium(IV)chloride, [(Z5-C9H7)TiCl3], and similar compounds of substituted indenyls have been

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synthesised and their ability as ethene polymerisation catalysts investigated.98 [Cp*TiMe(OtBu)2] and [B(C6F5)3] react to form zwitterionic [Cp*Ti(OtBu)2(m-Me)B(C6F5)3], where all the methyl hydrogens interact agostically with Ti.99 Thiuram disulfides, (R2NCS2)2 (R2 = Me2, Et2, (CH2)5) oxidise [Ti(CO)6]2– forming trigonal prismatic [Ti(CO)4(S2CNR2)].100 The short-lived alkylidyne complex [(PNP)TiRCtBu] (PNP = N[2-P(CHMe2)24-methylphenyl]2) activates the aromatic C–H bonds of anisole and substituted anisoles. Similarly [(PNP)TiRCtBu] activates C6F6 and CF3C6F5 forming [(PNP)TiQ(tBu(ArF))F] (ArF = C6F5, C6F4CF3).101 [(PNP)TiRCtBu] removes the ring nitrogen from pyridine and substituted pyridines (the important hydrodenitrogenation (HDN) process for removing N-heterocycles in crude oil) forming arenes.102 [Cp2Zr(4-F-o-tolyl)(CD2Cl2)]+ reacts with but-2-yne in solution via displacement of CD2Cl2 affording [Cp2Zr(4-F-o-tolyl)(2-butyne)]+, followed by a rate-limiting insertion to yield the product [Cp2Zr(4-F-o-tolyl)(2-butyne)]+.103 On warming the Z6,Z5-bis(indenyl)zirconium complex [(Z6-C9H5-1,3-(SiMe3)2)(Z5C9H5-1,3-(SiMe3)2)Zr(DME)] to 45 1C, an equimolar mixture of the ethylene complex [(Z5-C9H5-1,3-(SiMe3)2)2Zr(Z2-CH2QCH2)] and the bis(methoxide) [(Z5C9H5-1,3-(SiMe3)2)2Zr(OMe)2] forms.104 The titanium–carbide complex [CpTi(m2-Me)(m2-NPiPr3)(m4-C)(AlMe2)3] reacts with ClSnMe3 forming [CpTi(m2-Cl)(m2-NPiPr3)(m4-C)(m2-Cl)(AlMe)(AlMe2)2] and with MeO3SCF3 forming [CpTi(m2-Me)(m2-NPiPr3)(m4-C)(m2-O3SCF3)(AlMe)(AlMe2)2], in both the m4-C moiety is retained.105 New bent-sandwich zirconocene dialkyls and methylalkyls [CpCp*ZrR2], [Cp2Zr(CH3)R] and [CpCp*Zr(CH3)R] (R = CH2CMe3, CH2SiMe3, CH2CEt3, CH2CMe2CH2Ph) have ben synthesised as precatalysts for alkene polymerization. The bulkiness of the alkyl groups is reflected in widened C–Zr–C angles and long zirconium-alkyl bond distances.106 Na reduction of [Cp2ZrCl2] with RZnI (R = C6H3-2,6-(2,4,6-iPr3C6H2)2) resulted in [Cp2Zr(ZnR)2], the first structurally characterized compound with a Zn–Zr bond.107 Alumina-supported [Zr(CH2tBu)4], which can be represented as [(AlsO)2Zr(CH2tBu)]+ [(tBuCH2)Als], reacts with H2 at 150 1C eliminating alkanes and forming various hydride species, including [(AlsO)2Zr(H)(m-H)Al] and [(AlsO)2Zr(H)(m-R)Al], along with cationic [(AlsO)2Zr(H)]+ (Als represents a surface aluminium atom).108 Poly(azolyl)borate complexes [(cot)Zr(k2-L)Cl] (L = H2B(pz)2, HB(pzMe2)3, H2B(mt)2) result from reaction of [(cot)ZrCl2(thf)] with K[H2B(pz)2], K[HB(pzMe2)3], and Na[H2B(mt)2]. Attempts to prepare [(cot)Zr(k2-H2B(pzMe2)2)Cl] resulted in a cluster [(cot)4Zr4(m-O)4(m-Cl)2Cl2].109 Hafnium complexes of a pyridyl–amide ligand with an ortho-metalated naphthyl group are catalysts for the polymerisation of ethene and a alkenes, the mechanism involving monomer 1,2-insertion into the Hf-naphthyl bond, forming more active catalysts.110 Interest in the possibilities of titanocene anti-cancer drugs is sustained,111–113 with one compound studied being 500 times more cytotoxic than titanocene dichloride.111

Abbreviations Cp00 EMIm HBED H2Np H4cit

C5Me4H 1-Ethyl-3-methylimidazolium N,N 0 -di(o-hydroxybenzyl)ethylenediamine-N,N 0 -diacetic acid naphthalene-2,3-diol citric acid

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MAO McO 3,5-Me2Pz mt [NPN]* pz p-TsNH2

methyl aluminoxane methacrylate 3,5-dimethylpyrazol-1-ato methimazolyl {[N-(2,4,6-Me3C6H2)(2-N-5-MeC6H3)]2PPh} pyrazolyl p-toluenesulfonylamide

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