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An educational escape activity was developed, embedding elements of both augmented and immersive virtual reality technologies, in an active learning environment. The mental visualization of abstract concepts can be difficult for students, inhibiting their ability to reason cogently regarding important topics of stereochemistry. Consequently, an educational escape room coupled with immersive technologies can be utilized to foster teamwork and critical thinking while encouraging student interaction with three-dimensional virtual molecular phenomena. Nine participants were recruited from the University of East Anglia to complete the activity. From this first experience, student feedback and subsequent thematic analysis show not only the significant positive impacts on student engagement and learning but also the potential of this paradigm to be adopted as an educational tool.

This paper offers a reflective discussion of recent developments in active learning pedagogical approaches, with a focus on class-flipping and peer instruction. We present two case studies based on the experience of the authors in promoting active learning in two large-class undergraduate modules in Introductory Macroeconomics and Descriptive Inorganic Chemistry at the University of East Anglia. Both case-studies are based on a flipping model with in-class active learning delivered through peer instruction. However, the experience of each author differs in the way the teaching approach was introduced and integrated with additional pedagogies to meet specific needs of the student population, as well as the desired learning outcomes for both modules. In the Introductory Macroeconomics case-study, we discuss how combining peer instruction with a self-assessment component can support the development of students’ metacognitive skills. In the Descriptive Inorganic Chemistry case-study, we demonstrate that peer instruction questions can be co-designed in partnership with students to foster engagement and challenge misconceptions. We argue that peer instruction can be used as an effective, scalable, and easily adaptable active learning pedagogy in many different learning environments. Reflecting on our experience, as well as the student voice, our concluding discussion considers: (i) the importance of careful question design, (ii) the role of audience response technologies, as well as (iii) present and future challenges to the promotion of active learning approaches in Higher Education.

The drive to flip teaching is a reaction to the limitations of the traditional lecture, which may be efficient at addressing many students but is ineffective at engaging and teaching those students. The term ‘ flipped lecture ’ describes the process of requiring students to prepare for a timetabled contact hour through directed but independent study in advance. Consequently the academic is freed from the constraints of having to work through the content and can use a variety of technology-facilitated strategies to maximise engagement, probe understanding and ensure students are able to apply the knowledge effectively.

The lithiation of 1,3-dipropoxy-p-tert-butylcalix[4]arene (LH2) followed by reaction with nBuMgBr in THF resulted in the formation of the hetero-bimetallic complex [Li(THF)Mg(nBu)L] (1). By contrast, the treatment of tripropoxy-p-tert-butylcalix[4]arene (L′H) with nBu2Mg afforded a mononuclear complex [L′Mg(nBu)] (2). Single-crystal XRD studies revealed that in both structures the calix[4]arene adopts a cone conformation, and a Li cation resides in the cavity for 1. Both 1 and 2 were active for the ring-opening polymerisation of rac-lactide. Compound 2 displayed not only exceptional activity (100 equivalents, 3 min, 92 % conversion, BnOH, room temperature) but also high selectivity (probability to form an r dyad, Pr=0.85) and exhibited an immortal character in THF. Surprisingly, compound 2 also showed isotactic bias (Pr=0.30–0.36) and an immortal character if toluene was employed as the solvent; 2 D J-resolved 1H NMR spectroscopy was employed for the assignment of the stereoselectivity.

A full grasp of chemistry requires students to be able to connect microscopic reality with symbolic representations. Immersive virtual reality provides a solution for those who need a tangible link between these representations.

Reaction of Zn(C6F5)2·toluene (two equivalents) with 1,3-dipropoxy-p-tert-butyl-calix[4]arene (L(1)H2) led to the isolation of the complex [{Zn(C6F5)}2L(1)] (1), whilst similar use of Zn(Me)2 resulted in the known complex [{Zn(Me)}2L(1)] (2). Treatment of L(1)H2 with in situ prepared Zn{N(SiMe3)2}2 in refluxing toluene led to the isolation of the compound [(Na)ZnN(SiMe3)2L(1)] (3). The stepwise reaction of L(1)H2 and sodium hydride, followed by ZnCl2 and finally NaN(SiMe3)2 yielded the compound [Zn{N(SiMe3)2}2L(1)] (4). The reaction between three equivalents of Zn(C6F5)2·toluene and oxacalix[3]arene (L(2)H3) at room temperature formed the compound {[Zn(C6F5)]3L(2)} (5); heating of 5 in acetonitrile caused the ring opening of the parent oxacalix[3]arene and rearrangement to afford the complex [(L(2))Zn6(C6F5)(R)(RH)OH]·5MeCN R = C6F5CH2-(p-(t)BuPhenolate-CH2OCH2-)2-p-(t)BuPhenolate-CH2O(-))(3-) (6). The molecular structures of the new complexes 1, 3 and 6, together with that of the known complex 2, whose solid state structure has not previously been reported, have been determined. Compounds 1, 3-5 have been screened for the ring opening polymerization (ROP) of ε-caprolactone (ε-CL) and rac-lactide. Compounds featuring a Zn-C6F5 fragment were found to be poor ROP pre-catalysts as they did not react with benzyl alcohol to form an alkoxide. By contrast, compound 4, which contains a zinc silylamide linkage, was the most active of the zinc-based calix[4]arene compounds screened and was capable of ROP at ambient temperature with 65% conversion over 4 h.

The dimethylsulfide adduct of bis(pentafluorophenyl)borane HB(C 6 F 5 ) 2 ·SMe 2 reacts on mixing with alkenes such as 1-hexene and styrene, with selectivities closely following those of the Me 2 S-free reagent. The mono(pentafluorophenyl)borane H 2 B(C 6 F 5 )·SMe 2 also undergoes rapid hydroboration reactions to give the dialkylboranes R 2 B(C 6 F 5 ). Treatment of the boranes with trimethylsilylacetylene results in rapid hydroboration to give the trans-vinylborane. The reaction of H 2 B(C 6 F 5 )·SMe 2 with 1,5-cyclooctadiene at room temperature yields a kinetic mixture of the 1,4- and 1,5-dihydroboration products which quantitatively isomerizes on heating to the 1,5-isomer, pentafluorophenyl-9-borobicyclo[3.3.1]nonane. This bicyclic borane crystallizes as the SMe 2 adduct, which has been characterized by X-ray crystallography.

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A new family of cyclometallated gold(III) thiolato complexes based on pyrazine‐centred pincer ligands has been prepared, (C^Npz^C)AuSR, where C^Npz^C=2,6‐bis(4‐ButC6H4)pyrazine dianion and R=Ph (1), C6H4 tBu‐4 (2), 2‐pyridyl (3), 1‐naphthyl (1‐Np, 4), 2‐Np (5), quinolinyl (Quin, 6), 4‐methylcoumarinyl (Coum, 7) and 1‐adamantyl (8). The complexes were isolated as yellow to red solids in high yields using mild synthetic conditions. The single‐crystal X‐ray structures revealed that the colour of the deep‐red solids is associated with the formation of a particular type of short (3.2–3.3 Å) intermolecular pyrazine⋅⋅⋅pyrazine π‐interactions. In some cases, yellow and red crystal polymorphs were formed; only the latter were emissive at room temperature. Combined NMR and UV/Vis techniques showed that the supramolecular π‐stacking interactions persist in solution and give rise to intense deep‐red photoluminescence. Monomeric molecules show vibronically structured green emissions at low temperature, assigned to ligand‐based 3IL(C^N^C) triplet emissions. By contrast, the unstructured red emissions correlate mainly with a 3LLCT(SR→{(C^Npz^C)2}) charge transfer transition from the thiolate ligand to the π⋅⋅⋅π dimerized pyrazine. Unusually, the π‐interactions can be influenced by sample treatment in solution, such that the emissions can switch reversibly from red to green. To our knowledge this is the first report of aggregation‐enhanced emission in gold(III) chemistry.

[(Me2S)Au]+ cations, generated from (Me2S)AuCl and AgSbF6 in dichloromethane at 0–20 °C, serve as sources of solvated Au+ (alongside unreactive [Au(SMe2)2]+), which reacts with the methyl-substituted arenes C6Me6–nHn (n = 0–2) with C–H bond cleavage to give the sulfonium salts [C6Me5–nHnCH2SMe2]+. There was no evidence for arene π coordination to Au+ or for the formation of σ-bonded Au–benzyl species. Surprisingly, the reaction of Au+ with CH2Ar2 leads to C–C bond cleavage (Ar = 2,4,6-C6H2Me3). The reactions are highly selective for benzylic C–H and C–C bonds, whereas metalation of the arene ring is not observed.

The borane dimethyl sulfide adduct H3B·SMe2 and the diethyl ether adduct of tris(pentafluorophenyl)borane, (C6F5)3B·OEt2, undergo facile exchange of hydride and pentafluorophenyl ligands, yielding (C6F5)2HB·SMe2 (1) and (C6F5)H2B·SMe2 (2) depending upon the ratio of reagents used. In the presence of excess dimethyl sulfide, both compounds can be isolated as colorless crystals, which have been structurally characterized.

Treatment of LiC6F5 with B(C6F5)3 in equal volumes of light petroleum and diethyl ether at low temperature followed by slow warming to room temperature precipitates a microcrystalline solid which dries under vacuum to a material with the composition [Li(OEt2)3][B(C6F5)4]. Crystallization from diethyl ether yields solvent dependent [Li(OEt2)4][B(C6F5)4], which has been crystallographically characterised.

Treatment of (Ar(F)')(2)Zn(OEt(2))(2) (Ar(F)' = 4-C(6)F(5)C(6)F(4)) with 2 equiv. of benzonitrile, 4-(phenyl)benzonitrile, 4(pyrrolyl)benzonitrile, pyridine, 4-(phenyl)pyridine or 4-(pyrrolyl)pyridine in dichloromethane afforded the corresponding adducts (Ar(F)')(2)ZnL(2) in near quantitative yield. The 2,2'-bipyridine adduct was prepared similarly. Multinuclear NMR spectroscopy indicated that zinc's four-coordinate character was maintained in solution. The pyridine complex crystallized from dichloromethane with a solid-state structure free of face-to-face aryl-aryl interactions. In contrast, the 4-(pyrrolyl)pyridine adduct crystallized from both dichloromethane and 1,2-difluorobenzene, with solvent of crystallization, but otherwise essentially identical supramolecular architectures assembled through aryl-aryl synthons, including a face-to-face pentafluorophenyl-pyrrole interaction

The phosphinoborane adduct H(3)P x B(C(6)F(5))(3) can be deprotonated using LiN(SiMe(3))(2) to give the phosphidoborate salt Li[H(2)PB(C(6)F(5))(3)], which was converted to the phosphidodiborates Li[H(2)P{B(C(6)F(5))(3)}(2)] and Li[H(2)P{B(C(6)F(5))(3)}{BH(3)}] by treatment with an equivalent of B(C(6)F(5))(3) or Me(2)S.BH(3), respectively. A series of anions of the form [RR'P{M(C(6)F(5))(3)}{BH(3)}](-), where R = R' = Ph or R= (t)Bu, R' = H, and M = B or Al, were prepared (through treatment of salts Li[RR'P(BH(3))] with the corresponding Lewis acid) and characterized using multinuclear NMR, elemental analysis and X-ray crystallography. The solid state structures of [Li(Et(2)O)(x)][Ph(2)P{M(C(6)F(5))(3)}{BH(3)}] exhibit eta(2)-bonding of the BH(3) group to the cationic lithium center. The attempted preparation of an analogous series with amide cores of the form [R(2)N{B(C(6)F(5))(3)}{BH(3)}](-) proved unsuccessful; among the competing reaction pathways hydride abstraction occurred preferentially to yield Li[HB(C(6)F(5))(3)] and dimers or higher oligomers with the composition (R(2)NBH(2))(n).

The formation of cationic species relevant to olefin polymerization based on (SBI)HfCl2, Me2C(C5H4)(Flu)HfCl2, Ph2C(C5H4)(Flu)HfCl2, and L′HfCl2 activated by MAO, AlMe3/CPh3[B(C6F5)4], and AlBui3/CPh3[B(C6F5)4] (SBI = rac-Me2Si(Ind)2; L′ = C2H4(Flu)(5,6-C3H6-2-MeInd)) was studied by 1H, 13C, and 19F NMR spectroscopy. Thermally stable heterobinuclear intermediates of the type [LHf(μ-Me)2AlMe2]+[MeMAO]− and [LHf(μ-Me)2AlMe2]+[B(C6F5)4]− were identified when using MAO and AlMe3/CPh3[B(C6F5)4] as activators, respectively. The stability of these species explains the low productivity of hafnocene catalysts in the presence of AlMe3-containing activators, compared to zirconocenes. By contrast, in the ternary systems LHfCl2/AlBui3/CPh3[B(C6F5)4] hydride species were detected that must be responsible for the formation of the highly active sites in olefin polymerization. The ionic hydrido species differ significantly in stability. The formation of the mixed-alkyl complex L′Hf(Me)CH2SiMe3 proceeds with surprisingly h...

We report the first crystallographically characterized dimethylaniline complex of a group 4 metallocene. [(IPCF)ZrMe(NMe2Ph)][B(C6F5)4] shows strongly coordinated NMe2Ph which does not interchange in solution. DFT calculations suggest that the Zr-N bond is 23 kJ mol-1 stronger than the Zr-Me-Zr bond in [{(IPCF)ZrMe}2(µ-Me)][MeB(C6F5)3], the structure of which is also reported (IPCF = Me2C(C5H4)(fluorenyl)).

The mixed-alkyl metallocene complexes (IPCF)M(Me)(CH2SiMe3) (M = Zr, Hf; IPCF = Me2C(C5H4)(C13H8)) were synthesized by the reaction of (IPCF)M(Me)Cl (M = Zr, Hf) with Me3SiCH2MgCl. The crystal structures of (IPCF)Zr(CH2SiMe3)2, (IPCF)HfMe2, and (IPCF)Zr(Me)Cl were determined by X-ray diffraction. The kinetics of site epimerization of the ion pairs (IPCF)M(CH2SiMe3)(μ-Me)B(C6F5)3 and [(IPCF)MCH2SiMe3+···B(C6F5)4−] (M = Zr, Hf) were studied by variable-temperature NMR spectroscopy, while the solution ground-state structures of the ion pairs [LZrCH2SiMe3+···B(C6F5)4−] (L = SBI, IPCF; SBI = rac-Me2Si(Ind)2) were probed experimentally by 19F,1H HOESY NMR spectroscopy and theoretically by DFT and molecular dynamics calculations. They reveal differences in the strength of anion interactions between the SBI and IPCF systems which may be significant for their catalytic activity. The tetraarylborate salts are stabilized by agostic interactions to ligand Si−Me moieties, with Hf > Zr. The exchange rates of both the M...

Treatment of (C6F5)2Zn(toluene) with 2 equiv of a series of benzonitrile or pyridine derivatives yielded the complexes (C6F5)2Zn(L)2 (where L = benzonitrile, 4-(phenyl)benzonitrile, 4-(N-pyrrolyl)benzonitrile, pyridine, 4-(phenyl)pyridine, and 4-(N-pyrrolyl)pyridine). The four-coordinate solution-phase nature of these complexes was confirmed by a series of variable-temperature 19F NMR experiments and comparison to (C6F5)2Zn(2,2′-bipy). The solvent-free solid-state structures of each of the four-coordinate adducts and the toluene solvate of (C6F5)2Zn(NCC6H4C6H5)2 were determined by single-crystal X-ray diffraction and have distorted tetrahedral geometries. Analysis of the crystal packing revealed a preponderance of offset face-to-face homo−aryl and embrace-like interactions over the hetero−aryl, pentafluorophenyl−phenyl, interaction. These aryl−aryl synthons serve to assemble paired, one- and three-dimensional supramolecular architectures.

The title ionic solid, [Ti(C(2)H(6)N)3(C(2)H(7)N)2][Ti(C(18)BF(15)N)(C(18)H(2)BF(15)N)Cl(C(2)H(7)N)(2)].C(7)H(8), (I), comprises a cation with three dimethylamide ligands in the equatorial plane and two dimethylamine ligands positioned axially in a trigonal-bipyramidal geometry about the central Ti(IV) atom. The anion has a highly distorted octahedral structure. The two dimethylamine ligands are coordinated mutually trans. The chloride is trans to the tris(pentafluorophenyl)boron-amide, while the sixth coordination site is occupied by an ortho-F atom of the tris(pentafluorophenyl)boron-amide group in a trans disposition with respect to the tris(pentafluorophenyl)boron-nitride ligand. The most significant feature of the anion is the presence of an unprecedented terminal Ti[triple-bond]N moiety [1.665 (2) A], stabilized by coordination to B(C(6)F(5))(3), with a Ti[triple-bond]N-B angle of 169.50 (19) degrees.

Treatment of a toluene solution of Zn(C6F5)2 with p-C4H4N–C6H4CN yields (p-C4H4N–C6H4CN)2Zn(C6F5)2, which has been structurally characterised following recrystallization from 1,2-difluorobenzene by single-crystal X-ray diffraction and possesses the expected distorted tetrahedral molecular geometry. However, crystallization from dichloromethane yields a trigonal planar complex with only one p-C4H4N–C6H4CN ligand. The three-coordinate complex forms an intermolecular dimer in the solid state through a cooperative combination of zinc–arene, offset homoaromatic and dipole–dipole interactions.(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006)

A series of eight adducts of the form (RR‘R‘ ‘N)2·Zn(C6F5)2 have been prepared through treatment of the Lewis acid Zn(C6F5)2 with 2 equiv of the corresponding amine (R = tBu or CH2Ph, R‘ = R‘ ‘ = H; R = R‘ = Me or CH2Ph, R‘ ‘ = H; R = Me, R‘ = CH2Ph, R‘ ‘ = H; RR‘ = cyclo-C4H8 or cyclo-C5H10, R‘ ‘ = H; R = R‘ = Me, R‘ ‘ = CH2Ph). The solid-state structures of all eight compounds have been elucidated by single-crystal X-ray diffraction. In each case the geometry about the zinc atom is essentially tetrahedral. However, there is considerable variation in the supramolecular architectures in the solid state. A number of types of noncovalent interactions are observed including phenyl−pentafluorophenyl stacking, X−H···F−C contacts, and offset face-to-face contacts between pentafluorophenyl rings, giving rise to one-, two-, and three-dimensional supramolecular structures. In our examples we find that no one intermolecular interaction predominates.

Treatment of the mono(salicylaldiminato)titanium complexes {3-But-2-(O)C6H3CHN(Ar)}TiCl3(THF) (Ar = C6H5, 2,4,6-Me3C6H2 or C6F5) with the potassium s-enaminoketonates (C6H5)NC(CH3)C(H)C(R)OK (R = CH3, CF3) yielded the first examples of heteroligated (salicylaldiminato) (s-enaminoketonato)titanium dichloride complexes. The complex {3-But-2-(O)C6H3CHN(C6H5)}{(C6H5)NC(CH3)C(H)C(CH3)O}TiCl2 was structurally characterized by X-ray diffraction and has an orientation with trans-O,O,cis-Cl,Cl, cis-N,N distorted octahedral geometry. These complexes polymerize ethene when activated with MAO; the highest productivity, 5650 kg PE (mol metal)-1 h-1 atm-1, was afforded by {3-But-2-(O)C6H3CHN(C6F5)}{(C6H5)NC(CH3)C(H)C(CF3)O}TiCl2 at 60 °C.

Treatment of Zr{3-Bu-t-2-(O)C6H3CH=N(C6C6F5))Cl-3(THF) with K[2-(C6H5NCH)C4H3N] yields Zr{3Bu(t)-2-(O)C6H3CH=N(C6F5)}{(2-(C6H5NCH)C4H3N)Cl-2, the first example of a (salicylaldiminato)(pyrrolylaldiminato)zirconium complex. The catalytic behavior of both the new zirconium pre-catalyst and its titanium analogue have been determined. The titanium system is the more effective catalyst for both ethene homopolymerization and copolymerizations with hex-1-ene, norbornene, and cyclopentene. The titanium catalyst combines the high productivities of the bis(salicylaldiminato) parent complex with the more favorable comonomer incorporation of the bis(pyrrolylaldiiminato) series.

N -ferrocenyl salicylaldimine (FcPI)H, readily accessible from aminoferrocene and salicylaldehyde, reacts with dibutylmagnesium in THF to afford a red crystalline (FcPI) 2 Mg(THF) 2 ( 2 ). The reactions of the Li(FcPI) with CpTiCl 3 gave CpTiCl 2 (FcPI) · CH 2 Cl 2 ( 4 ), whereas with CpZrCl 3 · DME a mixture of three products is formed: CpZrCl(FcPI) 2 ( 5 ), CpZrCl 2 (FcPI) ( 6 ), and (FcPI) 2 ZrCl 2 · CH 2 Cl 2 ( 3 · CH 2 Cl 2 ). The crystal structures of compounds 2 – 6 are reported. Typically, the N–O chelates are oriented such that O donors are mutually trans , while N donors prefer to be trans to Cl ligands. The redox potentials of these complexes are very similar to ferrocene. On activation with methylaluminoxane, the Ti complex 4 shows moderate activity for ethylene polymerisation, while (FcPI) 2 ZrCl 2 is highly active for the oligomerisation of ethylene, to give products with average degrees of polymerisation of 12–16.

Reactive intermediates in living olefin polymerization over homogeneous catalysts based on bis[N-(3-tert-butylsalicylidene)-2,3,4,5,6-pentafluoroanilinato]titanium(IV) dichloride (L2TiCl2) have been studied. Outer sphere ion pairs [L2TiMe(S)]+[Me-MAO]- and [L2TiMe(S)]+[B(C6F5)4]- (S = solvent molecule), formed by activation of L2TiCl2 with MAO or with AlMe3/[CPh3]+[B(C6F5)4]-, and reactions of these ion pairs with ethene to afford chain-propagating species [L2TiP]+[Me-MAO]- and [L2TiP]+[B(C6F5)4]- (P = growing polymeryl chain) have been characterized by 1H, 13C, and 19F NMR spectroscopy.

The new mixed-alkyl metallocene complexes (SBI)M(Me)CH2SiMe3 (M = Zr, Hf) are accessible by the successive treatment of (SBI)MCl2 with Me3SiCH2MgCl and MeMgCl in toluene (SBI = rac-Me2Si(1-Ind)2). Reaction with B(C6F5)3 or CPh3+[B(C6F5)4]- in toluene or toluene/difluorobenzene affords (SBI)Mδ+(CH2SiMe3)(μ-Me)Bδ-(C6F5)3 and the ion pairs [(SBI)MCH2SiMe3+···B(C6F5)4-], respectively. Both types of compounds are thermally stable in aromatic solvents at ambient temperature. Whereas in the MeB(C6F5)3- complexes the alkyl ligand points away from the metal and tight anion coordination forms the familiar inner-sphere ion pair, in the B(C6F5)4- salts the alkyl ligand adopts a conformation that enables agostic bonding to a γ-CH3 group. Here, and by implication in M-polymeryl species of similar steric requirements, agostic interactions are preferred over anion coordination, leading to an outer-sphere ion pair structure. This alkyl bonding mode retards the −SiMe3 rotation, which for M = Hf is slow on the NMR time scal...

Reaction of TiCl4 with the reagents RN(SiMe3)2 in dichloromethane precipitates red to black powders (R = 1-adamantyl, phenyl, pentafluorophenyl, 3,5-bis(trifluoromethyl)phenyl), which on treatment with tetrahydrofuran gives the new THF adducts [Ti(NR)Cl(μ-Cl)(THF)2]2. The complexes are dimeric in the solid state with bridging chloride and terminal imido ligands. Activation with MAO (methylaluminoxane) gives ethene polymerisation catalysts with productivities of up to 31 kg PE [(mol complex) h bar]−1.

The Schiff base N,N‘-o-phenylenebis(3,5-di-tert-butyl-salicylideneimine) (1; tBu4salophenH2) reacts with 1 equiv of Zr(η5-C5H5)Cl3·DME in the presence of 2.2 equiv of NEt3 in pentane to give the monocyclopentadienyl bis(phenoxo-imino) monochloro zirconium derivative Zr(η5-C5H5)[C6H4-1,2-{NCH(3,5-tBu2C6H2-2-O)}2]Cl (2). Moreover, 2 equiv of the monolithiated reagent derived from 3-tert-butyl-N-ethylsalicylaldimine by reaction with LinBu in hexane reacts at −78 °C with 1 equiv of Zr(η5-C5H5)Cl3·DME in THF to yield the corresponding monocyclopentadienyl bis(phenoxo-imino) monochloro zirconium complex Zr(η5-C5H5)[CH3CH2NCH{(3-tBuC6H3-2-O)}]2Cl (5) after elimination of LiCl. Methylation of 2 with MgClMe in pentane or toluene at −78 °C affords Zr(η5-C5H5)[1,2-C6H4{NCH(3,5-tBu2C6H2-2-O)}{NCH(Me)(3,5-tBu2C6H2-2-O)}] (3) as a result of the reduction of one of the CN bonds of the coligand. The hydrolysis of 3 gives the new organic compound [C6H4{NCH(3,5-tBu2C6H2-2-OH)}{NCH(Me)(3,5-tBu2C6H2-2-OH)}] (4), which was al...

The influence of the concentration of CPh 3 + B(C 6 F 5 ) 4 − as the activator for metallocene polymerisation catalysts has been quantified. Whereas typically a 1:1 molar ratio of activator and metallocene dialkyl is employed, in line with the expected stoichiometry of alkyl abstraction, significant increases in the productivity of some—but not all—propene polymerisation catalysts have been found. The effect is particularly marked for catalysts with more open structures, such as the constrained-geometry complex (C 5 Me 4 SiMe 2 NBu t )TiCl 2 , where the activity could be increased by about an order of magnitude. Most catalysts show an optimum CPh 3 + /Zr ratio, beyond which further increases of [CPh 3 + ] has no effect. While some zirconocenes reach activities of up to 1.2×10 9 g PP (mol Zr) −1 [C 3 H 6 ] −1 h −1 (25 °C, 1 bar), other, typically less active zirconocenes do not show a trityl response at all. Possible reasons for this effect are discussed.

The influence of perfluoroborate anions on catalyst activity and stereospecificity has been determined for propene polymerisations at 20-60°C. For C 2 -symmetric ansa-zirconocenes, activities were strongly ligand-dependent, with systems activated by TIBA/Ph 3 C + X ― , where TIBA=A1 i BU 3 , X={Z[B(C 6 F 5 ) 3 ] 2 }, and Z=CN or NH 2 , giving significantly higher activities than methylaluminoxane under comparable conditions. For high-activity catalysts at 60°C, {H 2 N[B(C 6 F 5 ) 3 ] 2 } - proved superior to {CN[B-(C 6 F 5 ) 3 ] 2 } - , presumably due to the higher thermal stability of the amidoborate. The influence of couteranions on PP stereospecificity for these catalysts is negligible. On the other hand, the conformationally flexible catalyst, (2-PhInd) 2 Zr-C1 2 /TIBA/Ph 3 C + X - , shows not only a strong anion dependence of its activity, but also significant variation in poly(propylene) isotacticity, mainly due to the formation of small fractions of highly isotactic (mmm>70%)PP. This effect was particularly pronounced for {CN[B(C 6 F 5 ) 3 ] 2 } - and the dianion, {Ni[CNB(C 6 F 5 ) 3 ] 4 } 2

The reaction of Cp'(CpB)ZrCl2 [CpB=?5-C5H4B(C6F5)2] with LiNHCMe3 gave Cp'(CpB)(µ-NHCMe3)ZrCl, with a constrained-geometry type Cp---B---N chelate ligand. The 19F-NMR spectrum of the zirconium complexes, as well as that of the titanium analogue, reveals C---F...H---N hydrogen bonding to one of the ortho-F atoms of a C6F5 ring, strong enough to persist in solution at room temperature. The reaction of Cp'(CpB)TiCl2 with LiPPh2 affords the Cp---B---P chelate complex Cp'(CpB)(µ-PPh2)TiCl, the first example of a crystallographically characterised Ti(IV) phosphido compound. A 19F-NMR study of a number of adducts of B(C6F5)3 with prim- and sec-amines demonstrates the importance of intramolecular hydrogen bonding to C6F5 in this class of compounds, while there are no such interactions in B(C6F5)3(PHR2) (R=Cy, Ph). The crystal structures of Cp'(CpB)(µ-PPh2)TiCl, B(C6F5)3(NHMe2) and B(C6F5)3(PHCy2) are reported.

The versatile reactivity of B(C6F5)3 in alkene polymerization reactions is summarized. Adduct formation with basic anions such as CN– and NH2– gives extremely weakly co- ordinating diborates, which are the basis of some of the most active polymerization catalysts known to date. By contrast, the reaction of B(C6F5)3 with zirconium half-sandwich complexes leads to extensive C6F5 transfer, including the surprising formation of borole-bridged triple decker complexes. Main group alkyls undergo such C6F5 exchange reactions very readily unless donor ligands are present. Borate salts of new three-coordinate zinc alkyl cations proved to be highly effective catalysts for the ring-opening polymerization of epoxides and lactones.

Deprotonation of the phenol-imines 2-But-6-(RNCH)C6H3OH (R=2,4,6-Me3C6H2 (1a), C6F5 (1b), C6H11 (1c) and phenol-amines 2,4-But2-6-(R′NCH2)C6H2OH (R′=C4H8 (1d), C5H10 (1e)) with n-BuLi gives the corresponding lithium phenoxides. The reaction with MCl4 in THF solution leads to the bis(ligand) complexes {2-But-6-(RNCH)C6H3O}2MCl2 and {2,4-But2-6-(R′NCH2)C6H2O}2MCl2 (M=Ti: 2a, 2d, 2e, Zr: 3a, 3d and 3e). The cyclopentadienyl phenoxy-imine and -amine complexes Cp{2-But-6-(RNCH)C6H3O}MCl2 and Cp{2,4-But2-6-(R′NCH2)C6H2O}MCl2 (M=Ti: 4a–4e, Zr: 5a–5e) were prepared similarly through reaction with CpMCl3. The crystal and molecular structures of 2a, 3a, 4a and 4e have been determined. 2a and 3a are isostructural and exhibit a distorted octahedral geometry. 4a has a distorted square-pyramidal structure whereas 4e is essentially tetrahedral and the nitrogen does not coordinate. All new complexes are active for the polymerisation of ethene when activated with methyaluminoxane. 4b, 5a, 5d and 5e are active for the copolymerisation of ethene and 1-hexene and the oligomerisation of 1-hexene.

The ansa-borane complex (Me2S)(C6H5)B(C5H4)2ZrCl2 ( 1 ) reacts selectively with 2 equiv of LiC6F5 to give (Me2S)(C6H5)B(C5H4)2Zr(C6F5)2 ( 2 ) and with 3 equiv LiC6F5 to form the borato-bridged complex [Li(Et2O)3][(C6F5)(C6H5)B(C5H4)2Zr(C6F5)2] ( 3a ). The C6F5 groups are exchanged for Me by reaction with AlMe3 to form [Li(Et2O)x][(C6F5)(C6H5)B(C5H4)2ZrMe2] ( 4 ). Treatment of BCl3 with (SiMe3)(SnMe3)C5H4 affords BCl(C5H4SiMe3)2 ( 5 ), which undergoes a dehalosilylation reaction with ZrCl4(Me2S)2 to give Cl(Me2S)B(C5H4)ZrCl2 ( 6 ). The reaction of 6 with LiC6F5 proceeds in a fashion similar to that of 2 , leading first to (Et2O)ClB(C5H4)2Zr(C6F5)2 ( 7 ) and with 5 equiv of LiC6F5 to [Li(Et2O)4][(C6F5)2B(C5H4)2Zr(C6F5)2] ( 9a ) in 28% yield. The yield is improved through the use of C6F5MgBr at 60 °C. The C6F5 groups are readily exchanged for Me through reaction with LiMe to give [Li(Et2O)4][(C6F5)2B(C5H4)2ZrMe2] ( 10a ). Complexes 3 , 9 , and 10 are catalysts for ethene polymerization when activated with MAO or AlBui3/[CPh3][B(C6F5)4].

The formation of adducts of tris(pentafluorophenyl)borane with strongly coordinating anions such as CN(-) and [M(CN)(4)](2)(-) (M = Ni, Pd) is a synthetically facile route to the bulky, very weakly coordinating anions [CN[B(C(6)F(5))(3)](2)](-) and [M[CNB(C(6)F(5))(3)](4)](2-) which are isolated as stable NHMe(2)Ph(+) and CPh(3)(+) salts. The crystal structures of [CPh(3)][CN[B(C(6)F(5))(3)](2)] (1), [CPh(3)][ClB(C(6)F(5))(3)] (2), [NHMe(2)Ph](2)[Ni[CNB(C(6)F(5))(3)](4)].2Me(2)CO (4b.2Me(2)CO), [CPh(3)](2)[Ni[CNB(C(6)F(5))(3)](4)].2CH(2)Cl(2) (4c.2CH(2)Cl(2)), and [CPh(3)](2)[Pd[CNB(C(6)F(5))(3)](4)].2CH(2)Cl(2) (5c.2CH(2)Cl(2)) are reported. The CN stretching frequencies in 4 and 5 are shifted by approximately 110 cm(-1) to higher wavenumbers compared to the parent tetracyano complexes in aqueous solution, although the M-C and C-N distances show no significant change on B(C(6)F(5))(3) coordination. Zirconocene dimethyl complexes L(2)ZrMe(2) [L(2) = Cp(2), SBI = rac-Me(2)Si(Ind)(2)] react with 1, 4c or 5c in benzene solution at 20 degrees C to give the salts of binuclear methyl-bridged cations, [(L(2)ZrMe)(2)(mu-Me)][CN[B(C(6)F(5))(3)](2)] and [(L(2)ZrMe)(2)(mu-Me)](2)[M[CNB(C(6)F(5))(3)](4)]. The reactivity of these species in solution was studied in comparison with the known [[(SBI)ZrMe](2)(mu-Me)][B(C(6)F(5))(4)]. While the latter reacts with excess [CPh(3)][B(C(6)F(5))(4)] in benzene to give the mononuclear ion pair [(SBI)ZrMe(+).B(C(6)F(5))(4)(-)] in a pseudo-first-order reaction, k = 3 x 10(-4) s(-1), [(L(2)ZrMe)(2)(mu-Me)][CN[B(C(6)F(5))(3)](2)] reacts to give a mixture of L(2)ZrMe(mu-Me)B(C(6)F(5))(3) and L(2)ZrMe(mu-NC)B(C(6)F(5))(3). Recrystallization of [Cp' '(2)Zr(mu-Me)(2)AlMe(2)][CN[B(C(6)F(5))(3)](2)] affords Cp' '(2)ZrMe(mu-NC)B(C(6)F(5))(3) 6, the X-ray structure of which is reported. The stability of [(L(2)ZrMe)(2)(mu-Me)](+)X(-) decreases in the order X = [B(C(6)F(5))(4)][M[CNB(C(6)F(5))(3)](4)][CN[B(C(6)F(5))(3)](2)] and increases strongly with the steric bulk of L(2) = Cp(2)SBI. Activation of (SBI)ZrMe(2) by 1 in the presence of AlBu(i)(3) gives extremely active ethene polymerization catalysts. Polymerization studies at 1-7 bar monomer pressure suggest that these, and by implication most other highly active ethene polymerization catalysts, are strongly mass-transport limited. By contrast, monitoring propene polymerization activities with the systems (SBI)ZrMe(2)/1/AlBu(i)(3) and CGCTiMe(2)/1/AlBu(i)(3) at 20 degrees C as a function of catalyst concentration demonstrates that in these cases mass-transport limitation is absent up to [metal] approximately 2 x 10(-5) mol L(-1). Propene polymerization activities decrease in the order [CN[B(C(6)F(5))(3)](2)](-)[B(C(6)F(5))(4)](-)[M[CNB(C(6)F(5))(3)](4)](2-)[MeB(C(6)F(5))(3)](-), with differences in activation barriers relative to [CN[B(C(6)F(5))(3)](2)](-) of DeltaDeltaG = 1.1 (B(C(6)F(5))(4)(-)), 4.1 (Ni[CNB(C(6)F(5))(3)](4)(2-)) and 10.7-12.8 kJ mol(-)(1) (MeB(C(6)F(5))(3)(-)). The data suggest that even in the case of very bulky anions with delocalized negative charge the displacement of the anion by the monomer must be involved in the rate-limiting step.

The (borylcyclopentadienyl)titanium complex (Cp-B)TiCl3 (1; Cp-B = eta(5)-C5H4B(C6F5)(2)) reacts with LiC5H5 (LiCp), LiC5H4SiMe3 (LiCP'), and LiC9H7 (LiInd) to give the titanocene complexes (Cp-B)CpTiCl2 (2), (Cp-B)Cp'TiCl2 (3), and (Cp-B)(Ind)TiCl2 (4), respectively. In contrast to 1, which possesses piano stool geometry with an uncoordinated, trigonal-planar borg moiety, the -B(C6F5)(2) substituents in 2-4 act as intramolecular Lewis acids by coordinating to chloride ligands, with formation of B-Cl-Ti bridges that have relatively short B-Cl and elongated Ti-Cl bonds. The compounds are fluxional, with the -B(C6F5)(2) moiety switching rapidly from one chloride ligand to the other (2: Delta G(double dagger) = 37 kJ mol(-1) (200 K)). Recrystallization of 2 in the presence of traces of moisture afforded (Cp-B)CpTi(mu-OH)Cl (5), with a rigid B-O-Ti chelate arrangement. Treatment of 1 with 1 or 2 equiv of LiHNCMe3 gives the binuclear titanium imido complexes [(Cp-B)TiCl(mu-NCMe3)](2) (7) and [(Cp-B)TiCl(mu-NCMe3). H2NCMe3](2) (8), respectively. These complexes are based on Ti2N2 rings but show no boron-imide interactions. In contrast, the reaction of 2 with LiNHCMe3 affords (Cp-B)CpTi(mu-NHCMe3)Cl (9), which exhibits a constrained-geometry type Cp-B-N arrangement. The crystal structures of 4, 5, 8, and 9 have been determined.

Metallocene complexes of Group 4 metals are playing an increasingly important role in olefin polymerisation catalysis. This contribution surveys several aspects of the chemistry of metallocene catalysts that are being pursued in our laboratory at present, namely the application of half-sandwich complexes as catalysts for high-molecular weight elastomeric polypropene, unusual C–H activation reactions of zirconium half-sandwich complexes, the development of 14-electron borole complexes as neutral analogues of cationic [Cp 2 ZrR] + catalysts, and the heterogenisation of MAO-free zirconocene catalysts on borate-functionalised silica.

Bis(pentafluorophenyl)boron fluoride (C6F5)2BF·OEt2 (1), readily accessible from BF3·OEt2 and 2 equiv of C6F5MgBr, reacts with fluorenyllithium to give (Flu)B(C6F5)2 (4), while the reaction with indenyllithium leads to the regioisomers 1- and 2-IndB(C6F5)2 5 and 6, which are separated by fractional crystallization. 4 and 5 form crystalline adducts with tert-butylamine. The trimethylsilyl derivatives Flu(SiMe3)B(C6F5)2 (9) and Ind(SiMe3)B(C6F5)2 (10) are similarly prepared. Heating (C6F5)2BF·OEt2 leads to ether cleavage and formation of (C6F5)2BOEt. Treatment of 5 and 6 with Zr(NMe2)4 at room temperature gives indenylzirconium amido half-sandwich complexes; however, the reaction is accompanied by the unexpected exchange of one boron-C6F5 substituent by NMe2, to form 1- and 2-{C9H6B(C6F5)(NMe2)}Zr(NMe2)3. Reaction with SiClMe3 affords the trichlorides 1- and 2-{C9H6B(C6F5)(NMe2)}ZrCl3. The NMe2 substituent reduces the Lewis acidity of boron, so that donor ligands such as THF or DME coordinate exclusively to...

Bis(cyclopentadienyl)hafnium diethyl (1) reacts with [CPh3][B(C6F5)4] in dichloromethane at −60 °C with hydride rather than alkyl transfer to give triphenylmethane and the ethyl-bridged dinuclear complex [(Cp2HfEt)2(μ-Et)][B(C6F5)4] (2). The complex is less stable than analogous methyl complexes but is stabilized by the presence of excess Cp2HfEt2. The reaction between Cp2HfEt2, [CPh3][B(C6F5)4], and AlEt3 under analogous conditions leads to [Cp2Hf(μ-Et)2AlEt2][B(C6F5)4] (3). The reaction between Cp2 HfMe2 and AlEt3 leads to alkyl ligand exchange to give, successively, Cp2Hf(Me)(Et) and Cp2 HfEt2. Similar fast ligand exchange reactions, take place between Cp′2ZrMe2 and AlEt3 and can be used for generating the thermally labile complex Cp′2ZrEt2 as a precursor for cationic polymerization catalysts [Cp′2 = Cp2, rac-Me2Si(Ind)2]. Polymerization activities of rac-[Me2Si(Ind)2Zr(μ-R)2AlR2][B(C6F5)4] increase in the order R = Me

Treatment of TiCl(NMe(2))(3) with H(3)N·B(C(6)F(5))(3) results in N-H activation and ligand exchange to yield the structurally characterised salt [TiCl(NMe(2))(2)(NMe(2)H)(2)](+)[Ti[triple bond]NB(C(6)F(5))(3)(Cl)(2)(NMe(2)H)(2)](-). Cation exchange with [Me(4)N]Cl, [Ph(4)P]Cl and [(PhCH(2))Ph(3)P]Cl yields the respective ammonium and phosphonium salts of the [Ti[triple bond]NB(C(6)F(5))(3)(Cl)(2)(NMe(2)H)(2)](-) anion. X-ray crystallography reveals that the essential trigonal bipyramidal geometry and composition of the anion is retained in each of these salts despite some minor variations in the Ti-N-B angle and the nature of the interionic interactions. Electronic investigation by DFT calculations confirmed the Ti-N triple bond character implied by the experimentally determined bond length, with the HOMO and HOMO-1 having Ti-N π-bonding character. The dimethylamine ligands of the anion resist substitution by moderate bases but can be displaced by pyridine to give a pentacoordinate anion. In contrast, addition of 2,2'-bipyridyl gives a neutral octahedral complex. Treatment of the pyridine complex with TlCp results in the formation of a four coordinate anionic cyclopentadienyl complex.

Treatment of Me(2)S·B(C(6)F(5))(n) H(3-n) (n=1 or 2) with ammonia yields the corresponding adducts. H(3)N·B(C(6)F(5))H(2) dimerises in the solid state through N-H···H-B dihydrogen interactions. The adducts can be deprotonated to give lithium amidoboranes Li[NH(2)B(C(6)F(5))(n)H(3-n)]. Reaction of the n=2 reagent with [Cp(2)ZrCl(2)] leads to disubstitution, but [Cp(2)Zr{NH(2)B(C(6)F(5))(2)H}(2)] is in equilibrium with the product of β-hydride elimination [Cp(2)Zr(H){NH(2)B(C(6)F(5))(2)H}], which proves to be the major isolated solid. The analogous reaction with [Cp(2)HfCl(2)] gives a mixture of [Cp(2)Hf{NH(2)B(C(6)F(5))(2)H}(2)] and the N-H activation product [Cp(2)Hf{NHB(C(6)F(5 )(2)H}]. [Cp(2)Zr{NH(2)B(C(6)F(5))(2)H}(2)]·PhMe and [Cp(2)Hf{NH(2)B(C(6)F(5))(2)H}(2)]·4(thf) exhibit β-B-agostic chelate bonding of one of the two amidoborane ligands in the solid state. The agostic hydride is invariably coordinated to the outside of the metallocene wedge. Exceptionally, [Cp(2)Hf{NH(2)B(C(6)F(5))(2)H}(2)]⋅PhMe has a structure in which the two amidoborane ligands adopt an intermediate coordination mode, in which neither is definitively agostic. [Cp(2)Hf{NHB(C(6)F(5))(2)H}] has a formally dianionic imidoborane ligand chelating through an agostic interaction, but the bond-length distribution suggests a contribution from a zwitterionic amidoborane resonance structure. Treatment of the zwitterions [Cp(2)MMe(μ-Me)B(C(6)F(5))(3)] (M=Zr, Hf) with Li[NH(2)B(C(6)F(5))(n)H(3-n)] (n=2) results in [Cp(2) MMe{NH(2)B(C(6)F(5))(2)H}] complexes, for which the spectroscopic data, particularly (1)J(B,H), again suggest β-B-agostic interactions. The reactions proceed similarly for the structurally encumbered [Cp''(2)ZrMe(μ-Me)B(C(6)F(5))(3)] precursor (Cp''=1,3-C(5)H(3)(SiMe(3))(2) , n=1 or 2) to give [Cp''(2)ZrMe{NH(2)B(C(6)F(5))(n)H(3-n)}], both of which have been structurally characterised and show chelating, agostic amidoborane coordination. In contrast, the analogous hafnium chemistry leads to the recovery of [Cp''(2)HfMe(2)] and the formation of Li[HB(C(6)F(5))(3)] through hydride abstraction.