9 Titanium, zirconium and hafnium S. A. Cotton Uppingham School, Uppingham, Rutland, UK LE15 9QE 1 Introduction This report follows the pattern of recent years in being selective, particularly in the area of organometallic chemistry. It covers the available 1998 literature, together with some late 1997 papers. An important account1 of bent metallocenes covers aspects of their structure and bonding as well as certain reactions including Ziegler–Natta polymerisation catalysed by [ZrRCp 2 ]` systems.A review of zirconium and hafnium fluoro complexes systematises their structural chemistry on the basis of the F: Zr(Hf) ratio.2 Alkene and alkyne complexes of zirconium formed by thermolysis of [ZrR 2 Cp 2 ] (R\alkyl) have been reviewed3 as have titanocene and zirconocene olefin polymerisation catalysts.4 An account of dicatechol ligands includes several examples of their application to produce helicate complexes of titanium.5 A review of organically pillared micro- and meso-porous materials is relevant.6 A review of fluorenyl complexes of zirconium and hafnium as catalysts for olefin polymerisation covers their preparations and structures as well as their catalytic activity.7 Another review concerned with olefin polymerisation catalysts treats Group 4 ansa-cyclopentadienyl –amide complexes.8 The McMurry reaction for the coupling of ketones and aldehydes to a§ord alkenes, most usually with titanium catalysts, has been reviewed.9 The co-ordination chemistry of titanium for the year 1994 has been reviewed.10 2 Metallacarbohedranes Areview of metal–carbon cage compounds includes metcars11 whilst an overall review of metcars has been published.12 Ab initio SCF calculations of [Ti 8 C 12 (H 2 O) 8 ] and [Ti 8 C 12 (C 2 H 4 ) 4 ] indicate that in the former electron transfer occurs from water to the cluster whilst in the ethene adduct the transfer is in the opposite direction.13 New prominent clusters including [Ti 3 C 8 ]~, [Ti 4 C 8 ]~, [Ti 6 C 13 ]~, [Ti 7 C 13 ]~, [Ti 9 C 15 ]~ and [Ti 13 C 22 ]~ have been observed in negative ion mass spectra.14 3 Halides and their complexes Reaction of TiCl 4 with hexamethyldisilane a§ords an active form of TiCl 3 in high Annu.Rep. Prog. Chem., Sect. A, 1999, 95, 105–115 105yield.15 Raman studies of liquid and gaseous ZrCl 4 establish the presence of dimeric species in equilibrium with monomeric ZrCl 4 in the vapour; and monomeric ZrCl 4 in equilibrium with polymeric [ZrCl 4 ]n in the liquid.16 Study of melts of ZrCl 4 with CsCl establishes the existence of Cs 2 ZrCl 6 and CsZr 2 Cl 9 .HfCl 4 catalyses the intramolecular allylsilylation of alkynes.17 LiCs 4 [Zr 3 F 17 ]·HF has individual [Zr 3 F 17 ]5~ ions, involving seven- and eight-co-ordinate zirconium.18 An NMR spectroscopic study of the reaction of TiCl 4 with HF in MeCN establishes the formation of [TiCl 4~xFx(NCMe) 2 ] (x\1–4).19 In the presence of [PPh 4 ]Cl, partial hydrolysis of TiCl 3 and TiCl 4 gives a variety of complexes.20 Hydrolysis of TiCl 4 in CH 2 Cl 2 gives the heptanuclear mixed-valence compound [PPh 4 ] 3 [Ti 7 Cl 10 O 3 ].In acetone, hydrolysis of TiCl 3 leads to [PPh 4 ] 2 - [Ti 2 Cl 7 (H 2 O)(OH)(OCMe 2 )] and [TiCl 3 (H 2 O) 3 ] as the products whereas hydrolysis of TiCl 4 gives polymeric M[PPh 4 ][Ti 2 Cl 6 X(O)]N (X\OH, Cl) and [PPh 4 ]- [TiCl 5 (H 2 O)] as well as the simple complex [PPh 4 ] 2 [TiCl 6 ]. Reduction of TiCl 4 by HSnBu 3 a§ords a brown solid which reacts with Cl~ forming a precursor for facesharing Ti(III) complexes, including [Ti 2 Cl 7 (PEt 3 ) 2 ]~, [Ti 2 Cl 9 ]3~ and [Ti 3 Cl 12 ]3~, though pyridine cleaves bridges to form trans-[TiCl 4 (py) 2 ]~.A number of complexes, [TiCl 4 (SHR) 2 ] and [TiCl 4 (SR 2 ) 2 ], have been synthesised as possible precursors for titanium sulfide films and the structure of cis-[TiCl 4 (tht) 2 ] determined, along with that of [(TiCl 4 ) 2 (MeSSMe)] which has a Ti 2 Cl 8 core together with a bridging disulfide ligand.21 TiCl 4 reacts with Me 2 Se 2 and Et 2 Se 2 forming [(TiCl 4 ) 2 (Se 2 Me 2 )] and [(TiCl 4 ) 2 (Se 2 Et 2 )] respectively;22 the former is unstable, decomposing to [TiCl 4 (SeMe 2 ) 2 ] and Se, which, like [TiCl 4 (SeEt 2 ) 2 ], can be prepared directly from TiCl 4 and R 2 Se (R\Me, Et).[TiCl 4 (SeEt 2 ) 2 ] gives (moisture-sensitive) TiSe 2 films at 500–600 °C. [MZrF 4 (dmso) 2N2 ] has seven-co-ordinate zirconium with two bridging fluorides;23 other complexes [ZrF 4 L] (L\dmf, dmso, dimethylacetamide, tetramethylurea) have been synthesised. A range of PMe 3 complexes have been reported and structurally characterised,24 [ZrI 4 (PMe 3 ) 3 ] (capped trigonal antiprism) and [HfI 4 (PMe 3 ) 2.5 ] Ma mixture of [HfI 4 (PMe 3 ) 3 ] and [HfI 4 (PMe 3 ) 2 ]N; the bioctahedral M(III) compounds [Ti 2 I 6 (PMe 3 ) 4 ], [Hf 2 I 6 (PMe 3 ) 4 ], [Zr 2 Br 6 (PMe 3 ) 4 ]; and the M(II) species [Hf 2 I 4 (PMe 3 ) 4 (l-g6: g6-C 6 H 6 ].Serendipitously prepared [TiCl 4 (dmpe) 2 ] is the first eight-co-ordinate titanium phosphine complex.25 Monomeric [TiCl 4MMe 2 Si(NPEt 3 ) 2N] has distorted octahedral co-ordination of titanium.The dimer [MTiCl 3 (NPEt 3 )N2 ] has asymmetric Ti 2 N 2 bridges; in the thf adduct [TiCl 3 (NPEt 3 )(thf) 2 ] the thf trans to the phosphoraneiminato ligand has a much longer Ti–O bond (238.0 pm) than that trans to Cl (213.7 pm).26 A trimeric species [Ti 3 Cl 9 (NPEt 3 ) 3 ] is a cluster [Ti 3 Cl 8 (NPEt 3 ) 3 ]Cl in which two nitrogens act as l3 -bridges, the remaining nitrogen and two chlorides acting as l-bridging atoms.27 ABr (A\Na–Cs), Zr, ZrBr 4 and an interstitial element Z (Z\B, Be, H, Mn) react in Ta containers at 850 °C to form two series of quaternary phases that contain A 4 Br3` ions.28 Various cubic [A 4 Br] 2 [Zr 6 XBr 18 ] (A\Na–Cs; X\Be, B, H, Mn) exist.[K 4 Br] 2 [Zr 6 BBr 18 ] has the inverse fluorite structure. [Cs 4 Br] 2 [Zr 6 BBr 18 ] has puckered Zr 6 Br 16 layers with Cs 4 Br3` in tunnels normal to the layers. Neutron and X-ray di§raction studies29 on [PPh 4 ] 3 [Zr 6 Cl 18 H 5 ] show that the hydrogens (which undergo rapid movement at room temperature) occupy the triangular faces of the Zr 6 octahedron with an average Zr–H distance of 1.92Å.Deprotonation by amines yields the new tetrahydride clusters [PPh 4 ] 4 [Zr 6 Cl 18 H 4 ] and [NH 3 Et] 4 [Zr 6 Cl 18 H 4 ]. The Annu. Rep. Prog. Chem., Sect. A, 1999, 95, 105–115 106mixed-halide cluster Na[Zr 6 Cl 10.94 I 3.06 B] has a di§erent cluster connectivity to other zirconium cluster halides.30 Ligand-exchange and solvolysis reactions of the [Zr 6 BCl 12 ]` cluster have been studied by NMR spectroscopy.31 Spectra were observed for [Zr 6 BCl 12 Cl 6~xLx]x~5` (L\MeCN, MeOH, py) as well as for others species such as [Zr 6 BCl 12 (PR 3 ) 6 ]` and [Zr 6 BCl 12 (OPR 3 ) 6~xLx]`.Structures were reported for [Zr 6 BCl 12 Cl(py) 5 ] and [Hpy]cis-[Zr 6 BCl 12 Cl 2 (py) 4 ]. Mo� ssbauer spectra of [Zr 6 Cl 14 Fe], Li[Zr 6 Cl 15 Fe],and Rb[Zr 6 Cl 15 Fe] show extremely negative isomer shifts.32 4 Oxides and other binary and related compounds Among its many useful properties, TiO 2 absorbs UV light and is potentially a very useful photocatalyst for decomposing organic compounds and bacteria; TiO 2 -coated surfaces become hydrophilic and thus resist fogging.33 Sunlight-irradiation of a TiO 2 suspension in MeCN containing maleic anhydride and 4- methoxybenzyl(trimethyl)silane causes a C–C bond-forming reaction leading to benzylated succinic acid in good yield.34 A rapid solid-state route for the synthesis of TiC and ZrC from reaction of the metal halides with CaC 2 or Al 4 C 3 has been reported.35 TiN reacts with Sr 2 N forming SrTiN 2 , a rare ternary nitride of Ti (SP titanium).36 Zr/Ti reacts with BaSe/SrSe and Se in a flux forming columnar compounds Ba 15 Zr 14 Se 42 and Sr 21 Zr 19 Se 57 with mean oxidation states of ]3.86 and ]3.79 respectively.37 The solubility product of Zr(OH) 4 has been determined; logK4 \[55.26.38 Cs 2 ZrSe 3 contains complex anionic chains 1 = [ZrSe 3 ]2~ built up of quadratic ZrSe 5 pyramids sharing oppositeasal edges.39 5 Alkoxides, thiolates, amides and imides FT-IR and polarised FT-Raman spectra of [Ti(OPr*) 4 ], both neat and in solution, have been investigated and assigned.40 EXAFS studies of [Zr(OPr/) 4 ] and [Zr(OBu/) 4 ] indicate the presence of oligomers with six-co-ordinate zirconium; studies of mixed zirconium–titanium species are interpreted in terms of hetero species.41 [Ti(OPr*) 4 ] and PPh 3 mediate a reductive olefination reaction which has been applied to the one-pot synthesis of perfluorinated trans-allylic alcohols.42 The mixed alkyl –aryloxide [Ti(OPr*) 3 (OC 6 H 2 Me 3 -2,4,6)] is dimeric [(2,4,6-Me 3 C 6 H 2 O)(Pr*O) 2 - Ti(l-OPr*) 2 Ti(OC 6 H 2 Me 3 -2,4,6)(OPr*) 2 ].43 Hydrolysis of [Ti(OPr*) 4 ] in the presence of acetylacetone gives [Ti 4 (l3 -O) 2 (l-OPr*) 2 (OPri) 8 (acac) 2 ].44 Controlled addition of water to [MMOSi(OBu5) 3N4 ] results in the isolation of aqua complexes [MMOSi(OBu5) 3N4 (H 2 O)x] (M\Zr, Hf; x\1,2) which are stable indefinitely in the solid state and for weeks in solution.Hydrolysis occurs by an associative mechanism involving a seven-co-ordinate intermediate, postulated to be [MMOSi(OBu5) 3N4 (H 2 O) 3 ].45 [Ti(OPr*) 3 (OCH 2 CH 2 NMe 2 ] and [Ti(OPr*) 2 (OCH 2 - CH 2 NMe 2 ) 2 ] have been synthesized as MOCVD precursors for TiO 2 thin films.46 [Ti(OR) 4 ] (R\Et, Pr*) react with R@CO 2 H forming [Ti 6 (l3 -O) 6 (l-O 2 CR@) 6 (OR) 6 ], confirmed by X-ray di§raction on [Ti 6 (l3 -O) 6 (l-O 2 CC 6 H 4 OPh) 6 (OEt) 6 ].47 With one mole of formic acid, [Ti(OPr*) 4 ] gives [Ti 4 (l4 -O)(l-O)(O 2 CH) 2 (l-OPr*) 4 (OPr*) 6 ], Annu.Rep. Prog. Chem., Sect. A, 1999, 95, 105–115 107which decomposes in solution by transesterification; with two moles of formic acid, the stable [Ti 6 (l3 -O) 6 (l-O 2 CH) 6 (OPr*) 6 ] is formed.48 [Ti(OPr*) 4 ] substitutes protons in the pinacolate complex [Zr 2 (OCMe 2 CMe 2 O) 2 (OCMe 2 CMe 2 OH) 4 ] forming first [Zr 2 (OCMe 2 CMe 2 O) 4 (OCMe 2 CMe 2 OH) 2 Ti(OPr*) 2 ] then [Zr 2 (OCMe 2 CMe 2 O) 6 - MTi(OPr*) 2N2 ].49 In the solid state, [MTi(OR) 3 (acac)N2 ] (R\Me, Et, Pr*) and [MTi(OR) 3 (tmhd)N2 ] (R\Me, Pr/, Pr*) are centrosymmetric dimers with alkoxide bridges; in solution they undergo ligand redistribution reactions to a§ord mixtures of [MTi(OR) 3 (acac)N2 ], [Ti(OR) 2 (acac) 2 ] and [Ti(OR) 4 ].50 Thin films of ZrO 2 have been obtained by liquid injection MOCVD from the mixed ligand precursor [Zr 2 (OPr*) 6 (tmhd) 2 ].51 Several Group 2 heterometallic alkoxides have been reported, including [SrTi 4 (OEt) 18 ], which has a bow-tie Ti 2 SrTi 2 core with strontium bound to two face sharing Ti 2 (OEt) 9 bioctahedra, and [Sr 2 Ti(OPr*) 8 (Pr*OH) 3 ]·2Pr*OH, with a triangular Sr 2 Ti core bridged by three l-Pr*O ligands and capped by two l3 -Pr*O groups.52 Ba(OMe) 2 reacts with Ti(OMe) 4 forming a number of mixed alkoxides53 including BaTi 4 (OMe) 18 (possibly with a similar structure to that of the strontium compound described above), Ba 2 Ti(OMe) 8 and BaTiO(OMe) 4 .[Ti(OPr*) 4 ] reacts with [Pb(O 2 CR) 2 ] to produce heterometallic compounds54 such as [Pb 2 Ti 2 (O)(O 2 CR) 2 (OPr*) 8 ] (R\C 3 F 7 , Bu5) and [Pb 2 Ti 4 (O) 2 (O 2 CR) 2 (OPr*) 14 ] (R\Pr*). Three zirconium alkoxide species share a common Zr 3 (l-OR) 3 (l3 -OR) 2 core: [Zr 3 O(OCH 2 CMe 3 ) 9 Cl], [Zr 3 O(OCMe 3 ) 9 (OH)] and [Na 4 Zr 6 O 2 (OEt) 24 ], the last formed from Zr(OEt) 4 and NaOSiMe 3 .55 [TiCl 3 (thf) 3 ] reacts with LiNPh 2 forming [Ti(NPh 2 ) 4 ], which has distorted tetrahedral co-ordination; partial hydrolysis a§ords the symmetric dimer [(Ph 2 N) 3 Ti(l-O)Ti(NPh 2 ) 3 ].56 [MMN(SiMe 3 ) 2N3 Me] (M\Zr, Hf) react with B(C 6 F 5 ) 3 forming the salt [MMN(SiMe 3 ) 2N3 ][BMe(C 6 F 5 ) 3 )] ; unlike the starting material, the product displays M–Si–C multicentre interactions.57 [MMN(Ph)SiMe 3N3 Cl] and [MMN(Ph)SiMe 3N3 Me] were also reported.The bulky lithium amide LiL·OEt 2 ML\3,5–Me 2 C 6 H 3 N(Ad)N reacts with [ZrCl 4 (thf) 2 ] forming [ZrClL 3 ]; the remaining chloride can be substituted to a§ord [ZrMeL 3 ] and [Zr(BH 4 )L 3 ].58 TBPY complexes of a tridentate diamido amine [MX 2MRC(C 5 H 4 N)(CH 2 NSiMe 3 ) 2N] (M\Ti, Zr; X\Cl, Br; R\H, Me) have been synthesised59 and used as a synthon for mono- and di-alkyls such as [TiXR@MMeC(C 5 H 4 N)(CH 2 NSiMe 3 ) 2N] and [TiR 2MMeC(C 5 H 4 N)(CH 2 NSiMe 3 ) 2N] (R@\PhCH 2 , Me3 SiCH 2 , Me3 SiC 2 ).Compounds of a tridentate diamide ligand (H 2 L\Bu5NHSiMe 2 NHCH 2 CH 2 NMe 2 ) have been made, including [ZrL(NMe 2 ) 2 ] and [ZrLCl 2 ].60 A wide range of Ti-(II), -(III) and -(IV) benzamidinates includes61 a binuclear dinitrogen complex 1.Annu. Rep. Prog. Chem., Sect. A, 1999, 95, 105–115 108Chiral imine(alkoxy) complexes such as 2 have been synthesised.62 Titanium imide complexes of macrocycles include [Ti(NBu5)(ttp)] and [Ti(NBu5)(acen)].63 Heteroallylic imide complexes synthesised64 include [Ti(NCCMe 3 )Cl(py) 2MPhC(NSiMe 3 ) 2N], [Ti(NCCMe 3 )Cl(py) 2MMeOC 6 H 4 C(NSi-Me 3 ) 2N], [Ti(NCCMe 3 )Cl(py)MPh 2 P(NSiMe 3 ) 2N], [Ti(NCCMe 3 )MPh 2 P(NSiMe 3 ) 2N2 ] and [Ti(NCCMe 3 )(py)MPhC(NPr*) 2N]. 6 Complexes of macrocycles ZrCl 4 reacts with 1,2–dicyanobenzene under a stream of I 2 forming [Zr(pc)I 2 (NCC 6 H 4 CN)].65 This contains seven-co-ordinate zirconium; the dicyanobenzene molecule is bound through one nitrogen. The alkyne in [Ti(ttp)(g2- RC–– – CR)] (R\Et, Ph) is displaced by PhN––NPh forming the azobenzene adduct [Ti(ttp)(g2-PhN––NPh)]; at high temperatures this reacts with [Ti(ttp)(g2-RC–– – CR)] forming the phenylimide [Ti(ttp)(–– NPh)].66 Reduction of [Ti(ttp)Cl] a§ords trans- [Ti(ttp)(thf) 2 ], the thf of which can be displaced by Bu5NC giving [Ti(ttp)(NCBu5) 2 ].A ligand binding order of py[methylpyridine[Bu5NC[PhC–– – CPh[ EtC–– – CEt[thf was deduced.67 The structure of [MTi 2 (l3 -O)Cl 2 LN2 ] (L\p-tertbutylcalix[ 4]arene) is based68 on a four-runged Ti 4 O 4 planar ladder; each calixarene is bound to three titanium atoms. 7 Phosphates and phosphonates Dehydration of c-titanium phosphate, c-[Ti(H 2 PO 4 )(PO 4 )]·2H 2 O proceeds via a number of steps, through the anhydrous compound then the layered pyrophosphate [Ti(PO 4 )(H 2 P 2 O 7 ) 0.5 ] and finally the anhydrous pyrophosphate [TiP 2 O 7 ].69 a,/- Alkyldiamines have been intercalated as monolayers into g-titanium phosphate affording compounds with the maximum composition g-[Ti(H 2 PO 4 )(PO 4 )]· 0.67NH 2 CnH 2nNH 2 ·H 2 O (n\2–9).70 a-Zr(HPO 4 ) 2 ·H 2 O (a-ZrP) can be made by decomposing zirconium fluoride complexes in the presence of phosphoric acid.71 a- Zirconium phosphonate can in turn be made from a-ZrP by reaction with molten phenylphosphonic acid.72 The interlayer distance in pillared derivatives of c-zirconium phosphate, ZrPO 4 [O 2 P(OH) 2 ] 1~x[O 2 P(OH)(CH 2 )nHOPO 2 ]x@2 ·mH 2 O (n\4, 6, 8, 10, 12, 16; for example x\0.098,m\2.60; x\0.22,m\2.50; x\0.47,m\2.05), can be modulated by varying the length of the alkyl group.73 A new method of preparing Annu.Rep. Prog. Chem., Sect. A, 1999, 95, 105–115 109colloidal suspensions of a-ZrP, involving the use of a phase swelled with npropylamine vapours, was applied to the synthesis of chromia-pillared materials.74 Intercalation and exfoliation of a-ZrP have been studied microscopically; the ratedetermining step in intercalation is the opening of the interlamellar galleries.75 Hydrothermal reaction76 of [Zr(OPr/) 4 ] with phosphoric acid in the presence of en and HF yields the one-dimensional [H 2 en][Zr(HPO 4 ) 3 ], the two-dimensional [H 2 en] 0.5 [Zr(PO 4 )(HPO 4 )] and the three-dimensional open-framework compound [H 2 en] 0.5 [Zr 2 (PO 4 ) 2 (HPO 4 F)]·H 2 O.The synthesis and structure of a new phase of zirconium phosphate, s-Zr(HPO 4 ) 2 , has been reported.77 The vapour-phase intercalation of amines by a-HfP has been studied.78 Compounds [Hf(HPO 4 ) 2 ]·yamine·H 2 O have been obtained (y\1, amine\piperidine, pyridine; y\2, amine\aniline, benzylamine, cyclohexylamine); when y\1, the amines form a monomolecular layer and for y\2 the arrangement is bimolecular.a-HfP also intercalates the isomeric methylpiperidines. 79 [NH 4 ]Zr[F 2 ][H 3MO 3 PCH 2 NH(CH 2 CO 2 ) 2N2 ]·3H 2 O has a linear chain of zirconiums linked by phosphonate bridges.80 A new layered phosphonate, [Zr(O 3 PCH 2 C 6 H 4 NH 2 )x(O 3 PCH 3 ) 2~x], has been prepared in an intercalated hydrochloride form, [Zr(O 3 PCH 2 C 6 H 4 NH 3 Cl)x(O 3 PCH 3 ) 2~x].81 8 Other complexes Ab initio calculations indicate that the [Ti(H 2 O) 7 ]2` ion could be implicated in water-exchange reactions of the titanium(II) aqua ion.82 EPR studies of g-irradiated titanium in mesoporous molecular sieves have been interpreted in terms of Ti3` at both tetrahedral and octahedral sites.83 EXAFS has been used to detect two di§erent framework Ti sites in a Ti silicalite.84 [ZrCl(PhCOCHCOPh) 3 ] has capped octahedral co-ordination of zirconium.85 In the dimeric anion [MZr(OH)(CO 3 ) 3N2 ]6~ each zirconium is bound to three chelating carbonates and two bridging OH groups in a dodecahedral environment,86 a similar arrangement being found in M[Zr(OH)(C 2 O 4 ) 3 ] 2N6~.A new cluster, [Zr 6 (OH) 8 (OMe) 4 (O 2 CCHPh 2 ) 12 ]·2MeCN, contains an octahedron of zirconiums with bridging carboxylates and l3 -OH bridges.87 [Zr(O 2 CNEt 2 ) 4 ] has a dodecahedral co-ordination at the zirconium atom.88 [TiCl 2 Cp 2 ] reacts with Na 2 H 2 edta forming [Ti(edta)(H 2 O)] which has sevenco- ordinate titanium.89 H 5 O 2 [Hf(dtpa)]·H 2 O is isomorphous with the Sn analogue, having square antiprismatic co-ordination of hafnium.90 TiCl 4 reacts with [ZnS 6 (tmen)] forming a material analysing as TiSx (x[1.9) which is soluble in donor solvents a§ording isolable complexes [TiS 4 L 2 ] (L\py, 4–butylpyridine) and [TiS 4 (Meim) 3 ], the latter containing two g2-S 2 ligands 3.On oxidation, it forms [Ti 2 S 6 (Meim) 4 ], which is [(g2-S 2 )(Meim) 2 Ti(l-g2:g2-S 2 )Ti(g2-S 2 )(Meim) 2 ] with a bridging disulfide ligand.91 Annu. Rep. Prog. Chem., Sect.A, 1999, 95, 105–115 110A range of halide titanium(IV) Schi§ base complexes such as [Ti(H 2 salen)F 4 ], [Ti(salen)Cl 2 ], [Ti(salen)Br 2 ] and [MTiI(salen)N2 (l-O)], in addition to the seven-coordinate [Ti(salen)Cl 2 (thf)], are reported.92 The range of titanium(IV) Schi§ base complexes has been broadened with the synthesis93 of phenolates 4; R\H; R@\Ph, C 6 H 4 Bu5-4; R\Bu5; R@\C 6 H 4 Bu5-4) and triflates (R\CF 3 SO 2 ) as well as unsymmetrically substituted compounds of the type [Ti(salen)Cl(OPr*)].Some titanium imide Schi§ base complexes of the type [Ti(salen)NR] (R\C 6 H 3 Me-2,6) have been made.94 9 Organometallics A highlight is the synthesis and structure of the first ‘titanocene’, [TiMg- C 5 H 4 (SiMe 2 Bu5)N2 ], which has parallel Cp rings in a staggered arrangement.95 A postscript to the recent synthesis of [TiL 2 ] (L\pentalene) is provided by DFT calculations which indicate that only nine of the ten ligand p-orbitals interact with the metal, so that it really is an 18–electron compound.96 Gas-phase electron-di§raction of [TiCl 3 Et] gives no indication of any agostic Ti .. .H–Cb interaction or unusual ethyl geometry, unlike the situation in solid [Ti(dmpe)Cl 3 Et], where b-agostic interactions exist.97 The solid-state structure of [ZrCl(CH 2 CMe 3 ) 3 ] contains linear polymeric chains 5.98 Improved syntheses of [Ti(CH 2 EMe 3 ) 4 ] (E\C, Si) have also been reported.When [Zr(CH 2 CMe 3 ) 4 ] is treated with a partly dehydroxylated silica–alumina, binding to surface silanol groups gives a surface species Zr(–– – SiO)(CH 2 CMe 3 ) 3 which forms a hydride formulated as Zr(–– – SiO)H on hydrogenation at 150 °C.This catalyses the hydrogenation of polythene and polypropene to ‘diesel’ and lighter fraction alkanes under mild conditions.99 [Ti(g3-C 3 H 5 )(C 5 Me 4 CH 2 )Cp*] isomerises to the propenyl [Ti(C 5 Me 4 CH 2 )(g1-CH––CHCH 3 )Cp*] in a series of reversible firstorder steps.100 Zirconium attached to a calixarene binds butadiene ligands in a p2, g4 fashion.101 High-yield syntheses of [MCp 4 ] (M\Ti, Zr, Hf) use the reaction between [MCl 2 Cp 2 ] and NaCp in toluene.102 [ZrCp 4 ] reacts with CF 3 SO 3 Hforming [Zr(CF 3 SO 3 ) 2 Cp 3 ]; with water this gives [MZr(CF 3 SO 3 )Cp 3N2 (l-O)]. Cyclopentadienyl arylkoxides such as [Ti(OC 6 H 3 Pr* 2 -2,6) 3 Cp] have been characterised.103 Annu.Rep. Prog. Chem., Sect. A, 1999, 95, 105–115 111[M(O 2 CNEt 2 ) 4 ] (M\Zr, Hf) reacts with MgCp 2 forming [M(O 2 CNEt 2 ) 3 Cp] which can be grafted onto silica as a [M(O 2 CNEt 2 ) 2 Cp] fragment.104 A series of luminescent zirconium thiolates [Zr(SR) 2 Cp* 2 ] (R\Bu/, C 6 H 4 Bu5-p, C 6 H 4 Cl-p or Ph) and one selenium analogue [Zr(SeR) 2 Cp* 2 ] (R\Ph) have been prepared; the structure of [Zr(SBu/) 2 Cp* 2 ] was determined.105 Fenske–Hall MO calculations imply a thiolate-based HOMO and a LUMO with largely Zr d character.The structure of [ZrPh 2 Cp 2 ] is reported.106 [Zr(thf)(CF 3 SO 3 ) 2 Cp 2 ] reacts with 2,2@-biquinoline (L) forming bright yellow [Zr(L)Cp 2 ]2`, whose lowest energy electronic transition is a Cp~]biquinoline ligand-to-ligand charge-transfer transition.107 [TiMe 2 Cp 2 ] reacts with SiH 4 in the presence of PMe 3 forming paramagnetic [Ti(PMe 3 )(SiH 3 )Cp 2 ].108 In a most detailed study of the e§ect of cation–anion structure upon metallocenecatalysed polymerisation, various cationic species have been characterised.[MMe 3 Cp*] (M\Zr, Hf) reacts with ppb to form the base-free [MMe 2 Cp*]- [pbbMe]. Similar reaction with [CPh 3 ][pba] a§ords [MMe 2 Cp*][pba].Some basefree dimeric species have also been synthesised, such as [(ZrMeCp 2 ) 2 (l-Me)][pbbMe] an e¶cient catalyst for the polymerisation of methylmethacrylate to form syndiotactic poly(methylmethacrylate).109 Tris(b-perfluoronaphthyl)borane (pnb) has been evaluated as a strong organo–Lewis cocatalyst by using it to activate a range of Group 4 metallocenes producing Ziegler–Natta alkene polymerisation catalysts.110 Thus it reacts with [ZrMe 2 Cp* 2 ] forming [ZrMeCp* 2 ][pnbMe], with rac- [ZrMe 2MMe 2 Si(ind) 2N] and [MMe 2 (cgc)] (M\Ti, Zr) to form rac- [ZrMeMMe 2 Si(ind) 2N][pnbMe], and [MMe(cgc)][pnbMe].Binuclear complexes such as [MTiMe(cgc)N2 (l-Me)][pnbMe] and [Ti 2 (l-Me)Cp 2 )][pnbMe] have also been synthesised.These compounds have catalytic activities at least as high as the corresponding B(C 6 F 5 ) derivatives. Reduction of MX 4 (X\halogen) with Al in the presence of AlX 3 and an arene leads to [Ti(AlI 4 ) 2 (g6-C 6 H 6 )], [Zr 3 Br 6 (g6-C 6 Me 6 ) 3 ][Al 3 OBr 8 ] [Al 2 Br 7 ], [Zr 3 Br 6 (g6-C 6 H 3 Me 3 ) 3 ][Al 3 OBr 8 ] 3 and [Zr(AlBr 4 )(g6-C 6 H 6 ) 2 ] [Al 2 Br 7 ].111 A bis(amidate) ligand L supports a Ti–g6-C 6 H 5 CH 3 linkage.112 [HfCl 2 - Me(ind)] is dimeric [HfClMe(ind)(l-Cl) 2 HfClMe(ind)].113 [Ti(py) 3 Cl 2 (NBu5)] reacts with K 2 C 8 H 8 to form [Ti(NBu5)(g8-C 8 H 8 )] whose structure has been determined and PE spectrum assigned.114 References 1 J.C.Green, Chem. Soc.Rev., 1998, 27, 263. 2 R.L. Davidovich, Koord. Khim., 1998, 24, 803; Russ. J. Coord. Chem., 1998, 24, 751. 3 E. Negishi and T. Takahashi, Bull. Chem. Soc. Jpn., 1998, 71, 755. 4 W. Kaminsky, J. Chem. Soc., Dalton Trans., 1998, 1413. 5 M. Albrecht, Chem. Soc. Rev., 1998, 27, 281. 6 A. Clearfield, Chem. Mater., 1998, 10, 2801. 7 H.G. Alt and E. Samuel, Chem. Soc. Rev., 1998, 27, 323. 8 A.L.McKnight and R. M. 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