Mixing and matching N,N- and N,O-chelates in anionic Mg(I) compounds: synthesis and reactivity with RNCNR and CO.

Reduction of [Mg(NON)]2 ([NON]2- = [O(SiMe2NDipp)2]2-, Dipp = 2,6-iPr2C6H3) affords Mg(I) species containing NON- and NNO-ligands ([NNO]2- = [N(Dipp)SiMe2N(Dipp)SiMe2O]2-). The products of reactions with iPrNCNiPr and CO are consistent with the presence of reducing Mg(I) centres. Extraction with THF affords [K(THF)2]2[(NNO)Mg-Mg(NNO)] with a structurally characterised Mg-Mg bond that was examined using density functional theory.

Trends in Structure and Ethylene Polymerization Reactivity of Transition-Metal Permethylindenyl-phenoxy (PHENI*) Complexes.

A family of ansa-permethylindenyl-phenoxy (PHENI*) transition-metal chloride complexes has been synthesized and characterized (1-7; {(η5-C9Me6)Me(R″)Si(2-R-4-R'-C6H2O)}MCl2; R,R' = Me, tBu, Cumyl (CMe2Ph); R″ = Me, nPr, Ph; M = Ti, Zr, Hf). The ancillary chloride ligands could readily be exchanged with halides, alkyls, alkoxides, aryloxides, or amides to form PHENI* complexes [L]TiX2 (8-17; X = Br, I, Me, CH2SiMe3, CH2Ph, NMe2, OEt, ODipp). The solid-state crystal structures of these PHENI* complexes indicate that one of two conformations may be preferred, parametrized by a characteristic torsion angle (TA'), in which the η5 system is either disposed away from the metal center or toward it. Compared to indenyl PHENICS complexes, the permethylindenyl (I*) ligand appears to favor a conformation in which the metal center is more accessible. When heterogenized on solid polymethylaluminoxane (sMAO), titanium PHENI* complexes exhibit exceptional catalytic activity toward the polymerization of ethylene. Substantially greater activities are reported than for comparable PHENICS catalysts, along with the formation of ultrahigh-molecular-weight polyethylenes (UHMWPE). Catalyst-cocatalyst ion pairing effects are observed in cationization experiments and found to be significant in homogeneous catalytic regimes; these effects are also related to the influence of the ancillary ligand leaving groups in slurry-phase polymerizations. Catalytic efficiency and polyethylene molecular weight are found to increase with pressure, and PHENI* catalysts can be categorized as being among the most active for the controlled synthesis of UHMWPE.

Towards designer polyolefins: highly tuneable olefin copolymerisation using a single permethylindenyl post-metallocene catalyst.

Using a highly active permethylindenyl-phenoxy (PHENI*) titanium catalyst, high to ultra-high molecular weight ethylene-linear-α-olefin (E/LAO) copolymers are prepared in high yields under mild conditions (2 bar, 30-90 °C). Controllable, efficient, and predictable comonomer enchainment provides access to a continuum of copolymer compositions and a vast range of material properties using a single monomer-agnostic catalyst. Multivariate statistical tools are employed that combine the tuneability of this system with the analytical and predictive power of data-derived models, this enables the targeting of polyolefins with designer properties directly through predictive alteration of reaction conditions.

Fully tuneable ethylene-propylene elastomers using a supported permethylindenyl-phenoxy (PHENI*) catalyst.

Using a highly active supported permethylindenyl-phenoxy (PHENI*) titanium catalyst, high molecular weight ethylene-propylene (EPM) and ethylene-propylene-diene (EPDM) elastomers are prepared using slurry-phase catalysis. Final copolymer composition was found to reflect the monomer feed ratio in a linear fashion, to access a continuum of material properties with a single catalyst. Post-polymerisation crosslinking of EPDM was also demonstrated in a model sulfur vulcanisation system.

Hydrogen-bonding and resonance stabilisation effects in cationic bis(iminium) phenoxide diacids.

The synthesis and characterisation of a bis(iminium)phenoxide diacid cation [4-tBu-C6H2-2,6-(HCN(H)Dipp)-1-O]+ ([H2tBu,DippL]+), is discussed. [H2tBu,DippL][BF4] (1) and [H2tBu,DippL][H2N{B(C6F5)3}2] (2) were synthesised in high yields via protonation of the bis(imino)phenol conjugate base with ethereal HBF4 or Bochmann's acid ([H(OEt2)2][H2N{B(C6F5)3}2]). Both species were fully characterised using NMR and IR spectroscopy as well as X-ray crystallography. The cationic fragment adopts an unusual tautomeric form in which both acidic protons are located on the nitrogen atoms: [HN〈O〉NH]+. This bis(iminium) phenoxide tautomer is stabilised by delocalisation of electron density from oxygen, into the extended π-system of the planar cation, and was found to be 22.6 and 263.1 kJ mol-1 lower in energy (ΔG) than the alternative [N〈OH〉NH]+ and [N〈OH2〉N]+ tautomers respectively. Topological analysis confirmed the presence of two electrostatic N+H⋯O- hydrogen bonds which contribute -111.2 kJ mol-1 towards the stabilisation of the diacid. The pKa values of the cations were estimated, from NMR experiments, to be 4.2 in THF (1) and 11.4 in acetonitrile (2).

Bimolecular Reductive Elimination of Ethane from Pyridine(diimine) Iron Methyl Complexes: Mechanism, Electronic Structure, and Entry into [2+2] Cycloaddition Catalysis.

The application of bimolecular reductive elimination to the activation of iron catalysts for alkene-diene cycloaddition is described. Key to this approach was the synthesis, characterization, electronic structure determination, and ultimately solution stability of a family of pyridine(diimine) iron methyl complexes with diverse steric properties and electronic ground states. Both the aryl-substituted, (MePDI)FeCH3 and (EtPDI)FeCH3 (RPDI = 2,6-(2,6-R2-C6H3N═CMe)2C5H3N), and the alkyl-substituted examples, (CyAPDI)FeCH3 (CyAPDI = 2,6-(C6H11N═CMe)2C5H3N), have molecular structures significantly distorted from planarity and S = 3/2 ground states. The related N-arylated derivative bearing 2,6-di-isopropyl aryl substituents, (iPrPDI)FeCH3, has an idealized planar geometry and exhibits spin crossover behavior from S = 1/2 to S = 3/2 states. At 23 °C under an N2 atmosphere, both (MePDI)FeCH3 and (EtPDI)FeCH3 underwent reductive elimination of ethane to form the iron dinitrogen precatalysts, [(MePDI)Fe(N2)]2(µ-N2) and [(EtPDI)Fe(N2)]2(µ-N2), respectively, while (iPrPDI)FeCH3 proved inert to C-C bond formation. By contrast, addition of butadiene to all three iron methyl complexes induced ethane formation and generated the corresponding iron butadiene complexes, (RPDI)Fe(η4-C4H6) (R = Me, Et, iPr), known precatalysts for the [2+2] cycloaddition of olefins and dienes. Kinetic, crossover experiments, and structural studies were combined with magnetic measurements and Mössbauer spectroscopy to elucidate the electronic and steric features of the iron complexes that enable this unusual reductive elimination and precatalyst activation pathway. Transmetalation of methyl groups between iron centers was fast at ambient temperature and independent of steric environment or spin state, while the intermediate dimer underwent the sterically controlled rate-determining reaction with either N2 or butadiene to access a catalytically active iron compound.

Multimetallic Permethylpentalene Hydride Complexes.

The synthesis and characterization of group 4 permethylpentalene (Pn* = C8Me6) hydride complexes are explored; in all cases, multimetallic hydride clusters were obtained. Group 4 lithium metal hydride clusters were obtained when reacting the metal dihalides with hydride transfer reagents such as LiAlH4, and these species featured an unusual hexagonal bipyramidal structural motif. Only the zirconium analogue was found to undergo hydride exchange in the presence of deuterium. In contrast, a trimetallic titanium hydride cluster was isolated on reaction of the titanium dialkyl with hydrogen. This diamagnetic, mixed valence species was characterized in the solid state, as well as by solution electron paramagnetic resonance and nuclear magnetic resonance spectroscopy. The structure was further probed and corroborated by density functional theory calculations, which illustrated the formation of a metal-cluster bonding orbital responsible for the diamagnetism of the complex. These permethylpentalene hydride complexes have divergent structural motifs and reactivity in comparison with related classical cyclopentadienyl analogues.

Sterically rigid bismuth pincer complexes; observation of the growing polymer chain in polar monomer polymerisation.

A family of pyridine dipyrrolide bismuth complexes (Mes,PhL)MX (1-6) (M = Bi, X = O-2,6-Me-C6H3 = OXyl (1); M = Sb, X = OXyl (2); M = Bi, X = O-2,6-iPr-C6H3 = ODipp (3), O-2,6-tBu-C6H3 = OArtBu (4), OtBu (5) and OCMe2Et = OAm (6), N(SiMe3)2 = N'' (7) and CH2Ph (8)) have been prepared and investigated as initiators for the ring-opening polymerisation of lactide monomers. Bismuth lactate complexes (Mes,PhL)Bi{OC(H)(Me)C(O)OR} were prepared as models for the propagating species (R = tBu (9), Me (10), iPr (11)). The first insertion of the lactide monomer is rate limiting and the second and subsequent insertions are more rapid (kinit ≪ kLA2 < kprop), leading to a significant induction period. The sterically demanding, rigid pincer ligand affords a well-defined coordination environment at the metal centre and allows for the enchainment of two lactide monomers to be differentiated spectroscopically ((Mes,PhL)Bi{OC(H)(Me)C(O)}4OX (12-X)), with this species also implied to be the true initiator for the regime of propagation with first order kinetics. Well-controlled first order kinetic data for the polymerisation of L-, D-, rac- and meso-lactide are observed.

Supported permethylindenyl titanium catalysts for the synthesis of disentangled ultra-high molecular weight polyethylene (disUHMWPE).

Novel permethylindenyl-phenoxide (PHENI*) ansa-metallocene titanium complexes have been synthesised and immobilised on inorganic solid supports to afford highly effective catalysts for slurry-phase ethylene polymerisation. When supported on solid polymethylaluminoxane these complexes were both extremely active (up to 3.7 × 106 gPE molTi-1 h-1 bar-1) and produced substantially disentangled polyethylene with a weight-average molecular weight (Mw) of 3.4 MDa (disUHMWPE).

CO2 activation by permethylpentalene amido zirconium complexes.

We report the synthesis and characterisation of new permethylpentalene zirconium bis(amido) and permethylpentalene zirconium cyclopentadienyl mono(amido) complexes, and their reactivity with carbon dioxide.

Ring-opening polymerisation of l- and rac-lactide using group 4 permethylpentalene aryloxides and alkoxides.

A new family of group 4 permethylpentalene (C8Me62-; Pn*) aryloxide and alkoxide complexes have been synthesised and fully characterised by multinuclear NMR spectroscopy and single-crystal X-ray diffraction; (η8-C8Me6)Zr(OR)2 (R = tBu (1), 2,6-Me-C6H3 (2), 2,6-iPr-C6H3 (3) and 4-OMe-C6H4 (4)), (η8-C8Me6)Zr (OR) (R = 2,6-tBu-C6H3 (5) and 2,6-tBu-4-Me-C6H2 (6)), (η8-C8Me6)ZrCp(OR) (R = tBu (7), 2,6-Me-C6H3 (8) and 2,6-iPr-C6H3 (9)), (η8-C8Me6)TiCp(O-2,6-Me-C6H3) (10) and (η8-C8Me6)ZrCpMe(OR) (R = 2,6-Me-C6H3 (11), 2,6-iPr-C6H3 (12) and 2,4-tBu-C6H3 (13)). 2, 3, 6, 7, 9, 10 and 12 were studied as initiators for the ring-opening polymerisation (ROP) of l-lactide, and 2, 3, 6, 7 and 10 were studied as initiators for the ROP of rac-lactide. 3 was found to be the most active initiator for the ROP of l-lactide (kobs = 0.35 h-1) and 2 for the ROP of rac-lactide (kobs = 0.21 h-1). These initiators produced isotactic PLA for the ROP of l-lactide and moderately heterotactic enriched (maximum Pr of 0.69) or atactic PLA for the ROP of rac-lactide with polymer chains consisting of polylactic acid repeat units with -OR and -OH end groups.

Synthesis of zirconocene complexes and their use in slurry-phase polymerisation of ethylene.

A new family of zirconocene complexes of the type (3-RInd#)2ZrX2 (where Ind# = C6Me5H and R = Me, Et and Ph) have been synthesised and fully characterised. Six new crystal structures have been reported (meso-(3-EtInd#)2ZrBr2, rac-(3-EtInd#)2ZrCl2, rac-(3-EtInd#)2Zr(CH2Ph)2, meso-(3-EtInd#)2Zr(CH2Ph)2, meso-(3-MeInd#)2ZrBr2 and meso-(3-MeInd#)2Zr(CH2Ph)2). The complexes were studied for slurry-phase ethylene polymerisation when immobilised on solid polymethylaluminoxane (sMAO). Variation in the initiation group was found to have greater influence over polymerisation activity for meso-catalysts than rac-catalysts, with meso-alkyl catalysts showing higher polymerisation activities than meso-halide. Below 70 °C, polymerisation activity follows the order sMAO-meso-(3-EtInd#)2Zr(CH2Ph)2, sMAO-meso-(3-EtInd#)2ZrCl2 and sMAO-meso-(3-EtInd#)2ZrBr2 (activities of 657, 561, and 452 kgPE molM -1 h-1 bar-1, respectively). sMAO-meso-(3-EtInd#)2ZrBr2 produces HDPE with the highest molecular weight, followed by sMAO-meso-(3-EtInd#)2ZrCl2 and sMAO-meso-(3-EtInd#)2Zr(CH2Ph)2 (M w of 503, 406, and 345 kg mol-1, respectively, at 50 °C). sMAO-meso-(3-MeInd#)2ZrBr2 produced HDPE with almost identical molecular weights to sMAO-meso-(3-EtInd#)2ZrCl2 (395 kg mol-1 at 50 °C).

Bismuth Pyridine Dipyrrolide Complexes: a Transient Bi(II) Species Which Ring Opens Cyclic Ethers.

A family of group 15 MIII pyridine dipyrrolide complexes has been prepared and fully characterized; the reduction of these complexes was investigated with traditional strong metal reductants, which led either to over-reduction in the case of Mg and Zn or to ligand redistribution and "ate" complex formation when KC8 was used. However, by utilizing organosilanes as soluble reductants, the ring opening and two electron reduction of thf solvent was observed with concomitant formation of Bi-C and Si-O bonds; this is an example of a main group complex that is capable of ring opening a cyclic ether in the absence of additional metal reducing agents. The proposed BiII intermediate in this mechanism could be trapped using the stable organic radical TEMPO.

Constrained geometry scandium permethylindenyl complexes for the ring-opening polymerisation of l- and rac-lactide.

The synthesis and characterisation of constrained geometry scandium permethylindenyl chloride, aryloxide and borohydride complexes Me2SB(tBuN,I*)Sc(Cl)(THF) (1), Me2SB(tBuN,I*)Sc(O-2,6-iPr-C6H3)(THF) (2), Me2SB(tBuN,I*)Sc(O-2,4-tBu-C6H3)(THF) (3) and Me2SB(tBuN,I*)Sc(BH4)(THF) (4) are reported. The activity of complexes 1-4 as initiators for the ring-opening polymerisation (ROP) of l- and rac-lactide is presented. The ROP of l- and rac-lactide using complexes 2 and 3 show first-order dependence on monomer concentration, and produced isotactic polylactide (PLA) and moderately heterotactic PLA (Pr = 0.68-0.72), respectively. Good agreement between experimental and theoretical molecular weights of PLA (Mn) and relatively narrow dispersities (Mw/Mn < 1.20) were obtained. Complex 4 showed higher activity for ROP of l- and rac-lactide than 2 or 3, with second-order dependence on monomer concentrations. However, poorly controlled molecular weights and lower heteroselectivity (Pr = 0.61-0.67) were observed. The effect of temperature and catalyst concentration for the ROP of l-lactide using 2 and 3 was also studied.

Synthesis, Structure, and Hydrogenolysis of Pyridine Dicarbene Iron Dialkyl Complexes.

Two methods for the synthesis of bis(imidazol-2-ylidene)pyridine iron dialkyl complexes, (CNC)Fe(CH2SiMe3)2, have been developed. The first route consists of addition of two equivalents of LiCH2SiMe3 to the iron dihalide complex, (CNC)FeBr2, while the second relies on addition of the free CNC ligand to readily-prepared (py)2Fe(CH2SiMe3)2 (py = pyridine). With aryl-substituted CNC ligands, octahedral complexes of the type ( Ar CNC)Fe(CH2SiMe3)2(N2) ( Ar CNC = bis(arylimidazol-2-ylidene)pyridine) were isolated, where the dinitrogen ligand occupies the site trans to the pyridine of the CNC-chelate. In contrast, the alkyl-substituted variant, (tBuACNC)Fe(CH2SiMe3)2 (tBuACNC = 2,6-(tBu-imidazol-2-ylidene)2pyridine) was isolated as the five-coordinate compound lacking dinitrogen. Exposure of the ( Ar CNC)Fe(CH2SiMe3)2(N2) derivatives to an H2 atmosphere resulted in formation of the corresponding iron hydride complexes ( Ar CNC)FeH4. These compounds catalyzed hydrogen isotope exchange between the deuterated benzene solvent and H2, generating isotopologues and isotopomers of ( Ar CNC)Fe(H n )(D4-n ) (n = 0-4). When (3,5-Me2 MesCNC)Fe(CH2SiMe3)2(N2) (3,5-Me2 MesCNC = 2,6-(2,4,6-Me3-C6H2-imidazol-2-ylidene)2-3,5-Me2-pyridine) was treated successively with H2 and then N2, the corresponding reduced dinitrogen complex (3,5-Me2 MesCNC)Fe(N2)2 was isolated. The same product was also obtained following addition of pinacolborane to (3,5-Me2 MesCNC)Fe(CH2SiMe3)2(N2).

Synthesis, characterisation and redox properties of anti-bimetallic permethylpentalene complexes.

We report a series of anti-bimetallic transition metal complexes based on the permethylpentanyl (Pn*, C8Me6) ligand: anti-(MCpR5)2Pn* (where M = Fe, Co and Ni and R = H or Me). Compounds (FeCp*)2Pn* (1), (CoCp*)2Pn* (2), (NiCp*)2Pn* (3), (CoCp)2Pn* (4) and (NiCp)2Pn* (5) were fully characterised by NMR spectroscopy, X-ray crystallography, DFT calculations and cyclic voltammetry. All anti-(MCpR5)Pn* structures have diamagnetic ground states. 1, 2 and 4 have isolated non-degenerate ground states with closed shell configurations. A study of the magnetic properties of the dinickel complexes 3 and 5 reveal Boltzmann population of a Curie triplet state in the solution. Cyclic voltammetry reveals multiple reversible or quasi-reversible redox couples for all the dinuclear complexes.

Group 4 permethylindenyl complexes for the polymerisation of l-, d- and rac-lactide monomers.

A series of well-defined group 4 permethylindenyl complexes have been prepared and fully characterised by NMR spectroscopy and X-ray crystallography. Me2SB(Cp,I*)ZrCl2 ({(η5-C9Me6)Me2Si(η5-C5H4)}ZrCl2; 1), Me2SB(CpMe,I*)ZrCl2 ({(η5-C9Me6)Me2Si(η5-C5H3Me)}ZrCl2; 2), Me2SB(Cp,I*)HfCl2 (3) and Z-Me2SB(Cp,I*)ZrCl(O-2,6-Me-C6H3) (4) were investigated as initiators for the ring-opening polymerisation (ROP) of l-, d- and rac-lactide monomers in the presence of benzyl alcohol. 1-4 displayed second order dependence on monomer concentration and produced isotactic and heterotactic (Pr = 0.81) polylactides for the polymerisation of l-, d- and rac-lactide respectively. The effects of temperature, catalyst concentration, co-initiator concentration, solvent and scale were studied. At 80 °C, with two equivalents of benzyl alcohol, 4 was the most active initiator for the ROP of l-, d- and rac-lactide (kobs = 6.39, 6.38 and 5.89 M-1 h-1 respectively). The polylactides were characterised by NMR spectroscopy, GPC and MALDI-TOF mass spectrometry.

Magnesium, calcium and zinc [N2N'] heteroscorpionate complexes.

A new family of sterically demanding N2N' heteroscorpionate pro-ligands (HC(tBu2pz)2SiMe2N(H)R (R = iPr, tBu, Ph, Xyl)) has been prepared via a straightforward modular synthetic route. An extensive study into the synthesis and characterisation of lithium, magnesium, calcium and zinc complexes supported by both 3,5-tBu and 3,5-Me substituted N2N' ligand families has been conducted. Attempted deprotonation of the pro-ligands with nBuLi afforded the corresponding lithium salts Li{HC(tBu2pz)2SiMe2NR} (R = iPr (1), tBu (2), Ph (3) and Xyl (4)) but air- and thermal-sensitivity limited the yields of these potentially useful precursors; only the sterically encumbered ligand system allowed clean reactivity. Magnesium methyl complexes Mg{HC(tBu2pz)2SiMe2NR}Me (R = iPr (5) and R = Ph (6)) were prepared using an excess of the Grignard reagent MeMgCl. Magnesium butyl complexes were synthesised in good yields using the dialkyl precursor MgnBu2 to afford Mg{HC(R'2pz)2SiMe2NR}nBu (R' = Me; R = iPr (7), tBu (8), Ad (9), Ph (10). R' = tBu; R = iPr (11), Ph (12)). Protonolylsis reactions were used to synthesise magnesium and calcium amide complexes Mg{HC(R'2pz)2SiMe2NR}{N(SiHMe2)2} (R' = Me; R = iPr (13), tBu (14), Ph (15). R' = tBu; R = Ph (16)) or Mg{HC(R'2pz)2SiMe2NR}{N(SiMe3)2} (R' = Me; R = iPr (17), tBu (18), Ph (19). R' = tBu; R = Ph (20)), and Ca{HC(R'2pz)2SiMe2NR}{N(SiMe2)2} (L) (R' = Me; L = thf; R = iPr (21), tBu (22), Ph (23). R' = tBu; L = none; R = Ph (24). Zinc methyl complexes Zn{HC(R'2pz)2SiMe2NR}Me (R' = Me; R = iPr (25), tBu (26), Ph (27). R' = tBu; R = Ph (28)) were prepared by reaction of the N2N' heteroscorpionate pro-ligands with ZnMe2. In preliminary studies, magnesium amide complexes 16 and 20 were evaluated as initiators for the ring-opening polymerisation (ROP) of ε-caprolactone (ε-CL) and rac-lactide (rac-LA). Although the overall polymerisation control was poor, 16 and 20 were found to be active initiators.

Popcorn-shaped polyethylene synthesised using highly active supported permethylindenyl metallocene catalyst systems.

Unsymmetrical permethylindenyl bent metallocene complexes have been synthesised and reacted with inorganic solid supports to afford catalysts for the slurry phase polymerisation of ethylene. Those supported on solid polymethylaluminoxane were both highly active catalysts and afforded polymers with a desirable, low aggregation, "popcorn" morphology.

Ammonia Activation, H2 Evolution and Nitride Formation from a Molybdenum Complex with a Chemically and Redox Noninnocent Ligand.

Treatment of the bis(imino)pyridine molybdenum η6-benzene complex (iPrPDI)Mo(η6-C6H6) (iPrPDI, 2,6-(2,6-iPr2C6H3N═CMe)2C5H3N) with NH3 resulted in coordination induced haptotropic rearrangement of the arene to form (iPrPDI)Mo(NH3)2(η2-C6H6). Analogous η2-ethylene and η2-cyclohexene complexes were also synthesized, and the latter was crystallographically characterized. All three compounds undergo loss of the η2-coordinated ligand followed by N-H bond activation, bis(imino)pyridine modification, and H2 loss. A dual ammonia activation approach has been discovered whereby reversible M-L cooperativity and coordination induced bond weakening likely contribute to dihydrogen formation. Significantly, the weakened N-H bonds in (iPrPDI)Mo(NH3)2(η2-C2H4) enabled hydrogen atom abstraction and synthesis of a terminal nitride from coordinated ammonia, a key step in NH3 oxidation.