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Targeting pour point depressants of low-molecular weight and branched polyethylenes, a series of 9-[2,4-bis(benzhydryl)-6- R -phenylimino]-5,6,7,8-tetrahydro-cycloheptapyridine-nickel complexes (Ni1-Ni10) were developed as efficient precatalysts. Upon activation with either EASC or MAO, all nickel complex precatalysts exhibited high activity [up to 8.12 × 10 6 g PE (mol of Ni) -1 h -1 ] with single-site behavior toward ethylene polymerization, producing low-molecular weight and unimodal polyethylenes. The resultant polyethylenes possessed high branching with predominant methyl groups and longer chains, along with either internal vinylene or vinyl end groups. The activities of these complex precatalysts were heavily rationalized on the basis of the electronic and steric influences of their 6- R -substituents, with bromides following the order of Ni5 (F) > Ni4 (Cl) > Ni1 (Me) > Ni2 (Et) > Ni3 (iPr) and chlorides following the order of Ni10 (F) > Ni9 (Cl) > Ni6 (Me) > Ni7 (Et) > Ni8 (iPr). DFT calculations revealed the crucial role of agostic interactions (-Ni⋯H-C(Ph 2 )) between the nickel metal and the hydrogen atom of the ortho bulky group in achieving high catalytic activity and intramolecular hydrogen bonding with the fluoride atom in producing low M w PE wax. Moreover, the organic compounds and nickel complexes were well characterized, including representative complexes Ni3 and Ni4, via single-crystal X-ray diffraction.

In this study, a series of structurally rigid cyclooctyl-fused iminopyridine iron complexes, [L 2 FeCl][FeCl 4 ] and [2L 3 Fe][Cl][3FeCl 4 ], was synthesized via a one-pot method and investigated as precatalysts in conjunction with methylaluminoxane for isoprene (Ip) polymerization. Combined characterization through FTIR analysis, elemental analysis and single crystal XRD analysis fully verified the structure of these complexes. The most active iron complex, FeH, exhibited a trisligated nature, with its cation adopting an octahedral geometry around the metal center. In contrast, all the other iron complexes (Fe2Me, Fe2Et, Fe2iPr, Fe3Me, Fe2Et,Me) displayed bisligated configurations, with distorted trigonal bipyramidal geometry of cations. During isoprene polymerization, the extent of steric hindrance of the ligand framework exerted a significant impact on catalytic performance. The FeH precatalyst with less steric hindrance demonstrated excellent performance, producing high molecular weight polyisoprenes with conversions exceeding 99% for 4000 equiv. of monomer. Even at very low catalyst loadings, as low as 0.0025 mol% (Fe/Ip), the polymerization of isoprene could proceed smoothly with an exceptionally high activity of 4.0 × 10 6 g PI (mol Fe , h) -1 . Moreover, this precatalyst exhibited good thermal stability, maintaining high activity levels (typically 10 5 g PI (mol Fe , h) -1 ) across a broad temperature range from -20 °C to 100 °C. Additionally, by adjusting steric substituents and the reaction temperature, the 1,4/3,4 regioselectivity could be modulated from 9/91 to 69/31 while maintaining a high stereoselectivity of cis -1,4 structures ( cis / trans : >99/1).

A series of manganese(I) carbonyl complexes bearing structurally related NN - and NNN -chelating ligands have been synthesized and assessed as catalysts for transfer hydrogenation (TH). Notably, the NN -systems based on N -R functionalized 5,6,7,8-tetrahydroquinoline-8-amines, proved the most effective in the manganese-promoted conversion of acetophenone to 1-phenylethanol. In particular, the N -isopropyl derivative, Mn1, when conducted in combination with t -BuONa, was the standout performer mediating not only the reduction of acetophenone but also a range of carbonyl substrates including (hetero)aromatic-, aliphatic- and cycloalkyl-containing ketones and aldehydes with especially high values of TON (up to 17 200; TOF of 3550 h -1 ). These findings, obtained through a systematic variation of the N -R group of the NN ligand, are consistent with an outer-sphere mechanism for the hydrogen transfer. As a more general point, this Mn-based catalytic TH protocol offers an attractive and sustainable alternative for producing alcoholic products from carbonyl substrates.

The interest in the late transition metal catalyst based design of new architectures of polyethylene (PE) has continuously been increasing over the last few years. The structure of these catalysts is predominantly important in controlling the morphological and architectural properties of the resulting polyethylene. Particularly, iminopyridine is a versatile bidentate support for Ni and Pd catalysts in ethylene (oligo)polymerization providing a wide variety of products ranging from volatile oligomers to ultra-high molecular weight polyethylene. Extensive structural modifications have been induced in the iminopyridine ligand through steric and electronic substitution, tuning the catalyst behavior in terms of activity and properties of the resulting polymer. Carbocyclic-fused iminopyridine and N -oxide iminopyridine are the new state of the art iminopyridine ligand designs. In this review, we aim to summarize all the developments in mononuclear iminopyridine-nickel and -palladium catalysts for ethylene (oligo)polymerization since the first report published in 1999 to present, focusing on the correlation among the pre-catalyst, co-catalyst type, thermal stability and polymer/oligomer structure. For comparison, the structural variations in the binuclear iminopyridine-nickel catalysts are also described. The detailed comparison of the structural variations in these catalysts with respect to their polymerization performance will give deep understanding in the development of new efficient catalyst designs.

Molybdenum complexes ligated with N 1 , N 1 -dialkyl- N 2 -(5,6,7,8-tetrahydroquinolin-8-yl)ethane-1,2-diamines and auxiliary ligands, providing various structural features, were developed: [NN H /NN H N]Mo(CO) 4/3 (Mo1-Mo3), [NN H N]Mo(CO) 2 Br (Mo4-Mo5), [NN H ]Mo(CO)(η 3 -C 3 H 5 )Br (Mo6) and [NN H N/S]Mo(CO)(PPh 3 ) 2 (Mo7-Mo8). All the complexes were highly active in the transfer hydrogenation (TH) of a model substrate (acetophenone), providing excellent yields of 1-phenylethanol. The structural variation in the ligand framework had a modest effect on the catalyst performance as compared to the changes in the auxiliary ligands Br, PPh 3 and CO. This structural evolution provided the complex [Mo(NN H )(η 3 -C 3 H 5 )(CO) 2 Br] (Mo6) as the most effective catalyst not only for the transfer hydrogenation of acetophenone but also for a wide range of diverse ketones (up to 43 examples). Moreover, easy purification of the products by only removing the acetone byproduct is another noteworthy feature of this environmentally friendly route.

In this work, fluorinated 2,6-bis(arylimino)pyridyl iron(II) complexes, [2-[CMeN{2,4-{(4-FC 6 H 4 ) 2 CH} 2 -6-F}]-6-(CMeNAr)C 5 H 3 N]FeCl 2 (Ar = 2,6-Me 2 C 6 H 3 Fe1, 2,6-Et 2 C 6 H 3 Fe2, 2,6- i Pr 2 C 6 H 3 Fe3, 2,4,6-Me 3 C 6 H 2 Fe4, and 2,6-Et 2 -4-MeC 6 H 2 Fe5) and [2-[CMeN{2-{(4-FC 6 H 4 ) 2 CH}-4-{(C 6 H 5 )CHAr'}-6-F}]-6-(CMeN(2,6- i Pr 2 C 6 H 3 ))C 5 H 3 N]FeCl 2 (Ar' = 3-{(4-FC 6 H 4 ) 2 CH} 2 -4-NH 2 -5-FC 6 H 2 Fe6), verified with different steric substituents, were synthesized and characterized. The molecular structures of Fe2 and Fe3 were determined by X-ray diffraction, revealing a pseudo-square-pyramidal geometry. High activities were achieved toward ethylene polymerization in each iron complex case. The sterically least demanding ligand enhanced the activity of its complex Fe1 with the highest activity up to 16.8 × 10 6 g of PE (mol of Fe) -1 h -1 at 70 °C, while the bulkiest ligand led to the formation of the highest molecular weight of the resulting polyethylene using Fe6. Generally, the resulting polyethylenes are highly linear and most of them have a tendency to display bimodal distributions by virtue of the presence of multiple sites or competing chain transfer reactions. End-group analysis of polyethylenes confirms that the end groups include both unsaturated vinyl-end groups and saturated n -propyl or i -butyl, revealing the co-existence of two chain termination pathways including primary chain transfer to aluminium and secondary β-H transfer. The chain termination processes were interpreted with the 1D sequence inverse-gated decoupled 13 C NMR measurement of the resulting polyethylenes and DFT calculations along with the relevant polymerization mechanism.

A well-defined PNN-Ru catalyst was revisited to self-condense 2-aminobenzyl alcohol in forming a series of novel aza-crown compounds [aza-12-crown-3 (1), aza-16-crown-4 (2) and aza-20-crown-5 (3)]. All aza-crown compounds are separated and determined by NMR, IR, and ESI-MS spectroscopy as well as X-ray crystallography, indicating the saddle structure of 1 and the twisted 1,3-alternate conformation structure of 3. These aza-crown compounds have been explored to study ferric initiation of transfer hydrogenation (TH) of ketones into their corresponding secondary alcohols in the presence of 2-propanol with a basic t-BuOK solution, achieving a high conversion (up to 95%) by a ferric complex with 2 in a low loading (0.05 mol%).

The α,α'-bis(imino)-2,3:5,6-bis(pentamethylene)pyridyl-cobalt(ii) chlorides, [2,3:5,6-{C4H8C(N(2-R1-4-R3-6-R2C6H2))}2C5HN] CoCl2 (R1 = Me, R2 = R3 = CH(p-FPh)2Co1; R1 = Et, R2 = R3 = CH(p-FPh)2Co2; R1 = i-Pr, R2 = R3 = CH(p-FPh)2Co3; R1 = Cl, R2 = R3 = CH(p-FPh)2Co4; R1 = F, R2 = R3 = CH(p-FPh)2Co5; R1 = F, R2 = R3 = CHPh2Co5'', R1 = R2 = Me, R3 = CH(p-FPh)2Co6; R1 = R3 = Me, R2 = CH(p-FPh)2Co7), have been synthesized by a one-pot template reaction of α,α'-dioxo-2,3:5,6-bis(pentamethylene)pyridine, cobalt(ii) chloride and the respective aniline in n-butanol. By contrast, the mixed cobalt(ii) chloride/acetate complex, [2,3:5,6-{C4H8C(N(2-F-4,6-(CH(p-FPh)2)2C6H2))}2C5HN]CoCl(OAc) (Co5'), was isolated when the corresponding template reaction was carried out in acetic acid. Structural characterization of Co4, Co5 and Co5'' revealed distorted square pyramidal geometries while six-coordinate Co5', incorporating a chelating acetate ligand, exhibited a distorted octahedral geometry. On activation with either MAO or MMAO, 2-fluoride-4,6-bis{di(p-fluorophenyl)methyl}-substituted Co5 showed maximum catalytic activity for ethylene polymerization at a high operating temperature of 60 °C (up to 2.1 × 107 g (PE) mol-1 (Co) h-1), producing highly linear (Tms > 121 °C), low molecular weight polyethylene waxes (Mw range: 1.5-5.0 kg mol-1) with narrow dispersity (Mw/Mn range: 1.7-2.9). End-group analysis of the waxes reveals β-H elimination as the dominant chain transfer process.

The 4,4'-difluorobenzhydryl-modified bis(imino)pyridylferrous chlorides, [2-{CMeN(2,6-(4-FC6H4)2CH2-4-t-BuC6H2)}-6-(CMeNAr)C5H3N] FeCl2 (Ar = 2,6-Me2C6H3Fe1, 2,6-Et2C6H3Fe2, 2,6-i-Pr2C6H3Fe3, 2,4,6-Me3C6H2Fe4, 2,6-Et-4-MeC6H2Fe5 and 2,6-(4-FC6H4)2CH2-4-t-BuC6H2Fe6), were synthesized in good yields. All iron complexes were characterized by 1H/19F NMR and elemental analysis, and the molecular structures of representative complexes Fe1 and Fe6 were determined by single crystal X-ray diffraction, which revealed a slightly distorted square pyramid around the iron center. Activated with either MAO or MMAO, Fe1-Fe5 exhibited very high activities (up to 17.2 × 106 g (PE) mol-1 (Fe) h-1 for Fe1/MMAO) toward ethylene polymerization, producing highly linear polyethylenes with narrow dispersities as required by industry and value added PEs. Importantly, the Fe1/MAO maintained an activity of 9.5 × 106 g (PE) mol-1 (Fe) h-1 at 100 °C of operating temperature, making the catalytic system suitable for practical application. Simulation quantitatively revealed the mechanism of the enhanced catalytic performance from the electronic and steric point of view.

A one-pot template strategy has been utilized to synthesize sterically enhanced bis(imino)cyclohepta[b]pyridine-cobalt(ii) chlorides, [2-{(Ar)N[double bond, length as m-dash]CMe}-9-{N(Ar)}C10H10N]CoCl2 (Ar = 2-(C5H9)-4,6-(CHPh2)2C6H2Co1, 2-(C6H11)-4,6-(CHPh2)2C6H2Co2, 2-(C8H15)-4,6-(CHPh2)2C6H2Co3, 2-(C12H23)-4,6-(CHPh2)2C6H2Co4, 2,6-(C5H9)2-4-(CHPh2)C6H2Co5). All five complexes have been characterized by a combination of FT-IR spectroscopy, elemental analysis and single crystal X-ray diffraction. The molecular structures of Co1, Co3 and Co5 highlight the substantial steric hindrance imparted by the 2-cycloalkyl-6-benzhydryl or 2,6-dicyclopentyl ortho-substitution pattern; distorted square pyramidal geometries are exhibited in each case. On activation with methylaluminoxane (MAO) or modified methylaluminoxane (MMAO), all the complexes (apart from Co4/MAO) were active ethylene polymerization catalysts (up to 3.70 × 106 g PE per mol (Co) per h for Co5/MMAO), operating effectively at temperatures between 50 °C and 60 °C, producing polyethylenes with high molecular weights (up to 589.5 kg mol-1 for Co3/MAO). Furthermore, all polymers were highly linear (Tm > 130 °C) with narrow dispersities (Mw/Mn range: 2.0-3.0). The coexistence of two chain termination pathways, β-H elimination and transfer to aluminum, has been demonstrated using 13C/1H NMR spectroscopy.

The syntheses of six bis(imino)-5,6,7,8-tetrahydrocycloheptapyridine-iron(ii) chloride complexes, [2-{(Ar)NCMe}-9-{N(Ar)}C 10 H 10 N]FeCl 2 (Ar = 2-(C 5 H 9 )-6-MeC 6 H 3 Fe1, 2-(C 6 H 11 )-6-MeC 6 H 3 Fe2, 2-(C 8 H 15 )-6-MeC 6 H 3 Fe3, 2-(C 5 H 9 )-4,6-Me 2 C 6 H 2 Fe4, 2-(C 6 H 11 )-4,6-Me 2 C 6 H 2 Fe5, 2-(C 8 H 15 )-4,6-Me 2 C 6 H 3 Fe6), are reported in which the ring size of the ortho-cycloalkyl group has been varied as has the type of para-substituent. The molecular structures of Fe3 and Fe6 reveal square pyramidal geometries at iron while the ortho-cyclooctyl rings adopt boat-chair conformations. On treatment with either methylaluminoxane (MAO) or modified methylaluminoxane (MMAO), all six complexes showed optimal activities at 80 °C [up to 1.9 × 10 7 g of PE per mol Fe per h for Fe5/MMAO] for ethylene polymerization forming linear polyethylene (T m 's > 126 °C). Notably, the catalytic activities showed a marked correlation with the ring size of the ortho-cycloalkyl substituent: cyclohexyl (Fe2 and Fe5) > cyclooctyl (Fe3 and Fe6) > cyclopentyl (Fe1 and Fe4) for either para-substituent, H or Me. Furthermore, this family of iron catalysts exhibited remarkable thermostability by remaining highly active even at temperatures as high as 100 °C (1.1 × 10 7 g of PE per mol Fe per h); the wide variation in polymer molecular weights (M w : 2.4-166 kg mol -1 ), influenced through choice of precatalyst and co-catalyst as well as by temperature and pressure, further highlights the versatility of these catalysts.

Five structurally related bis(arylimino)pyridine-iron(ii) chloride complexes, [2-[CMeN{2,6-{(4-FC6H4)2CH}2-4-NO2}]-6-(CMeNAr)C5H3N]FeCl2 (Ar = 2,6-Me2C6H3Fe1, 2,6-Et2C6H3Fe2, 2,6-i-Pr2C6H3Fe3, 2,4,6-Me3C6H2Fe4, and 2,6-Et2-4-MeC6H2Fe5), incorporating one N-2,6-bis{di(4-fluorophenyl)methyl}-4-nitrophenyl group and one distinct N-aryl group, have been prepared in good yield through the interaction of the corresponding free ligands (L1-L5) with FeCl2·4H2O. All ferrous complexes were paramagnetic which was manifested by broad and highly shifted peaks in their 1H NMR spectra. The marked steric imbalance imposed by the two inequivalent N-aryl groups was a key feature highlighted in the molecular structures of representative complexes Fe1 and Fe2. Upon activation with either MAO or MMAO, Fe1-Fe5 all exhibited high activities for ethylene polymerization with good thermal stability [activities as high as 1.58 × 107 g (PE) mol-1 (Fe) h-1 at 60 °C], affording especially high molecular weight linear polyethylenes (3.92 × 105 g mol-1 at 70 °C; Tm > 130 °C). To the best of our knowledge, the molecular weights of the polyethylenes produced by the current class of iron catalysts exceed the highest values reported for related bis(imino)pyridine-iron catalysts to date; changes in the ortho-R1 substitution pattern offered some additional fine control of the molecular weight. Moreover, the nature of the aluminoxane co-catalyst employed had a noticeable effect on the polymer end group composition. When using MAO, unsaturated polymers containing both vinyl and n-propyl end groups were evident, whereas with MMAO, fully saturated polymers were generated containing both isobutyl and n-propyl end groups.

Six examples of dinuclear bis(imino)pyridine-cobalt(ii) complexes, [1,5-{2-(CMe[double bond, length as m-dash]N)-6-(CMe[double bond, length as m-dash]N(2,6-R 1 2 -4-R 2 -C 6 H 2 ))C 5 H 3 N} 2 (C 10 H 6 )]Co 2 Cl 4 (R 1 = Me, R 2 = H Co1; R 1 = Et, R 2 = H Co2; R 1 = i Pr, R 2 = H Co3; R 1 = Me, R 2 = Me Co4; R 1 = Et, R 2 = Me Co5; R 1 = CHPh 2 , R 2 = Me Co6), have been prepared from the corresponding bis(tridentate) compartmental ligands (L1-L6) in reasonable yields. The molecular structures of Co3 and Co5 revealed two N,N,N-cobalt dichloride units to adopt anti-positions about the 1,5-naphthyl linking unit, with each cobalt center exhibiting a distorted trigonal bipyramidal geometry. On activation with either MAO or MMAO, Co1-Co6 were shown to promote both polymerization and oligomerization of ethylene with high overall activities (up to 1.03 × 10 7 gPE per·mol(Co) per·h for Co1/MAO at 70 °C). Curiously, on increasing the reaction temperature a larger proportion of polymer was noted, while at lower temperature an enhanced selectivity for oligomer was seen. In general, the oligomeric products displayed Schulz-Flory distributions with high selectivities for α-olefins (>99%). On the other hand, the highly linear polymers displayed narrow dispersities and comprised both fully saturated and unsaturated chain ends with the vinyl content (-CH[double bond, length as m-dash]CH 2 ) found to rise with the reaction temperature. By modulating the steric hindrance exerted by the ortho-R 1 substituents in the precatalyst, polyethylenes displaying a remarkably broad range of molecular weights could be obtained [from 4.52 kg mol -1 (R 1 = Me) to 246.7 kg mol -1 (R 1 = CHPh 2 )].

Six types of 2,8-bis(imino)-7,7-dimethyl-5,6-dihydroquinoline, 2-(ArN[double bond, length as m-dash]CMe)-8-(ArN)-7,7-Me 2 C 9 H 6 N (Ar = 2,6-Me 2 C 6 H 3 L1, 2,6-Et 2 C 6 H 3 L2, 2,6- i Pr 2 C 6 H 3 L3, 2,4,6-Me 3 C 6 H 2 L4, 2,6-Et 2 -4-MeC 6 H 2 L5, 2,4,6- t Bu 3 C 6 H 3 L6), distinguishable by their steric and electronic profile, are described that can readily undergo complexation with cobaltous chloride to form their corresponding LCoCl 2 chelates, Co1-Co6. The molecular structures of Co2 and Co3 reveal square pyramidal geometries with ring puckering a feature of the gem-dimethyl section of their unsymmetrical N,N,N'-ligands. On activation with either methylaluminoxane (MAO) or modified methylaluminoxane (MMAO), all the cobalt complexes exhibited exceptionally high activities for ethylene polymerization with levels reaching up to 1.19 × 10 7 g PE per mol (Co) per h for mesityl-containing Co4. Significantly, these catalysts exhibited good thermal stability by displaying their optimal performance at temperatures up to 70 °C whilst also maintaining appreciable catalytic lifetimes. With the exception of that obtained using the most sterically hindered Co6 (2,4,6-t-butyl), the polyethylenes are of low molecular weight (M w ≤16.0 kg mol -1 ) and of narrow dispersity (M w /M n ≤3.4). Moreover, end-group analysis of these highly linear polymer waxes reveals evidence for unsaturated as well as various levels of fully saturated materials highlighting the role of both β-H elimination and chain transfer to aluminum as termination pathways.

α-Diimine and related complexes of late transition metals such as palladium and nickel have been attracting continuing interest as single-site catalysts of ethylene homopolymerization to branched polyolefins, having challenging mechanical properties. The state-of-the art catalysts demonstrate promising catalytic activities, and enhanced thermal stabilities, affording polyethylenes with a variable degree of branching and, in addition, are able to incorporate polar co-monomers into polyethylene structures. At the same time, fundamental understanding of the structure-reactivity relationships of such catalysts mostly remains at the phenomenological level, due to the lack of experimental data on the solution structures of intermediates that drive the polymerization process. In this perspective, we discuss recent advances of α-diimine nickel based catalysts of ethylene polymerization, focusing on the relationships between the catalyst structures on the one hand, and their thermal stabilities and properties of the resulting polyethylene, on the other hand. In addition, some intriguing novel mechanistic findings of these catalyst systems are presented.

A novel family of 2,6-bis[1-(p-dibenzocycloheptylarylimino)ethyl]pyridylcobalt dichlorides Co1-Co4 were synthesized and fully characterized by FT-IR, 1H NMR, and elemental analysis as well as X-ray diffraction analysis. The 1H NMR spectra of these paramagnetic cobalt complexes Co1-Co3 showed similar spectra with one set of signals, while Co4 possessed two sets of signals due to the different conformations caused by different substituents on the ortho-position of the N-aryl group. Crystal structures of Co1, Co2 and Co4 revealed similar square-pyrimidal geometry around the cobalt ion, in which two dibenzocycloheptyl groups adapted as cis-conformers, whereas Co3 possessed bistrigonal geometry and the two dibenzocycloheptyl groups were adapted as trans-conformers. On activation with methylaluminoxane (MAO) or modified MAO (MMAO), all these cobalt complexes displayed high catalytic activities towards ethylene polymerization (up to 1.21 × 107 g (PE) mol-1 (Co) h-1) and produced polyethylenes with narrow molecular distributions (PDI ≈ 2.0). Co3 bearing bulkier isopropyl substituents on the ortho-position gave much higher molecular weight of resultant polyethylene than the other cobalt complexes reported herein. The microstructure analysis of the PEs demonstrated that they are highly linear and contain a vinyl end group as the major group and saturated methyl as the minor group, in which the content of the unsaturated polymer chain relied on the ortho-substituent of the N-aryl group.

Herein, a series of dialkylaluminum 2-substituted 6,6-dimethylcyclopentyl pyridin-7-oxylates Al1-Al7 were synthesized and characterized by 1H- and 13C-NMR spectroscopy and elemental analysis. The molecular structure of Al3 was proven to be a dimer of an aluminum complex. These aluminum complexes could efficiently initiate the ring-opening polymerization (ROP) of ε-caprolactone (CL), and the structural differentiations of the resultant PCL were strongly dependent on the amount of BnOH (PhCH2OH) used. In the absence of BnOH, the resultant PCL showed a cyclic structure, whereas BnO-capped linear PCL was obtained in the presence of >2.0 equivalents of BnOH; the resultant PCL was a mixture of linear and cyclic PCLs in the presence of 1.0 equivalent of BnOH. Moreover, these aluminum complexes exhibited high efficiency towards the ROP of l-lactide (LLA); however, the activities were lower than those for the ROP of ε-CL. Without BnOH, the resultant PLLA showed a highly linear structure with the alkyl-end group from aluminum complexes; on the other hand, PLLA displayed a major cyclic structure and minor BnO-capped linear PLLA if 1.0 equivalent of BnOH was employed, and the percentage of BnO-capped linear PLLA was increased by increasing the amount of BnOH.

The unsymmetrical diketone, 1,2,3,7,8,9,10-heptahydrocyclohepta[b]quinoline-4,6-dione, based on a central pyridine unit fused by both 6- and 7-membered rings, has been synthesized via a sequence of reactions including ruthenium-catalyzed coupling cyclization. Templating this diketone with a mixture of cobalt(ii) chloride hexahydrate and the corresponding aniline in acetic acid at reflux afforded five examples of carbocyclic-fused bis(arylimino)pyridine-cobalt(ii) chlorides (aryl = 2,6-Me2Ph Co1, 2,6-Et2Ph Co2, 2,6-i-Pr2Ph Co3, 2,4,6-Me3Ph Co4, 4-Me-2,6-Et2Ph Co5) in good yield. All cobalt complexes have been fully characterized including by 1H NMR spectroscopy which reveals broad but assignable paramagnetically shifted peaks. The molecular structures of Co1, Co3 and Co4 highlight the inequivalency of the two fused rings with the cobalt center adopting a distorted trigonal bipyramidal geometry. Treatment of Co1-Co5 with MAO gave highly active catalysts (up to 5.03 × 106 g PE mol-1 (Co) h-1 at 40 °C, with Co4 > Co5 > Co1 > Co2 > Co3) for ethylene polymerization generating strictly linear vinyl-terminated polymers with low molecular weights (Mw range: 1.53-22.77 kg mol-1). By comparison, polymerizations conducted using Co1-Co5/MMAO were less active and displayed a lower selectivity for unsaturated polymers. Common to both MAO and MMAO, the most sterically hindered precatalyst Co3 gave the highest molecular weight polymer of the series (up to 22.77 kg mol-1) but exhibited the lowest activity.

A new set of five unsymmetrical N,N'-diiminoacenaphthenes, 1-[2,6-{(4-FC 6 H 4 ) 2 CH} 2 -4-NO 2 C 6 H 4 N]-2-(ArN)C 2 C 10 H 6 (Ar = 2,6-Me 2 C 6 H 3 L1, 2,6-Et 2 C 6 H 3 L2, 2,6- i Pr 2 C 6 H 3 L3, 2,4,6-Me 3 C 6 H 2 L4, 2,6-Et 2 -4-MeC 6 H 2 L5), have been synthesized and used to prepare their corresponding nickel(ii) halide complexes, LNiBr 2 (Ni1-Ni5) and LNiCl 2 (Ni6-Ni10). The molecular structures of Ni3(OH 2 ) and Ni4 reveal distorted square pyramidal and tetrahedral geometries, respectively, while the 1 H NMR spectra of all the nickel(ii) (S = 1) complexes show broad paramagnetically shifted peaks. Upon activation with either methylaluminoxane (MAO) or ethylaluminum sesquichloride (Et 3 Al 2 Cl 2 , EASC), Ni1-Ni10 displayed very high activities for ethylene polymerization with the optimal performance being observed using 2,6-dimethyl-containing Ni1 in combination with EASC (1.66 × 10 7 g PE mol -1 (Ni) h -1 at 50 °C) which produced high molecular weight plastomeric polyethylene (M w = 3.93 × 10 5 g mol -1 , T m = 70.6 °C) with narrow dispersity (M w /M n = 2.97). Moreover, Ni1/EASC showed good thermal stability by operating effectively at an industrially relevant 80 °C with a level of activity (6.01 × 10 6 g of PE mol -1 (Ni) h -1 ) that exceeds previously disclosed N,N'-nickel catalysts under comparable reaction conditions. This improved thermal stability and activity has been ascribed to the combined effects imparted by the para-nitro and fluoride-substituted benzhydryl ortho-substituents.

Five chromium(iii) chloride complexes, [2-{(Ar)N[double bond, length as m-dash]CMe}-9-{N(Ar)}C10H10N]CrCl3 (Ar = 2,6-Me2C6H3Cr1, 2,6-Et2C6H3Cr2, 2,6-i-Pr2C6H3Cr3, 2,4,6-Me3C6H2Cr4, 2,6-Et2-4-MeC6H2Cr5), each chelated by a sterically and electronically different cycloheptyl-fused N,N,N'-bis(imino)pyridine, have been synthesized by the reactions of CrCl3(THF)3 with the corresponding ligand (L1/L1'-L5/L5'). The molecular structure of Cr2 highlights both the steric properties exerted by the inequivalent N-2,6-ethylphenyl groups and the puckering of the fused cycloheptyl ring; a distorted octahedral geometry is conferred about the metal center. On activation with methylaluminoxane (MAO) or modified MAO (MMAO), Cr1-Cr5 displayed their optimal activity for ethylene polymerization at temperatures between 70 and 80 °C with the least sterically demanding Cr1 proving the most productive (1.44 × 107 g (PE) per mol (Cr) per h). The polyethylenes formed are of low molecular weight (Mw range: 0.66-3.56 kg mol-1) with narrow molecular weight distributions and display high levels of end-group unsaturation. Furthermore, the amenability of these vinyl-terminated polyethylenes to undergo functionalization via epoxidation has been demonstrated.

Reaction of 8-amino-5,6,7,8-tetrahydroquinoline with RuCl2(PPh3)3 at room temperature affords the ruthenium(ii) chelate (8-NH2-C9H10N)RuCl2(PPh3)2 (E), in which the two triphenylphosphine ligands are disposed mutually cis. By contrast, when the reaction is performed at reflux ligand oxidation/dehydrogenation occurs along with cis-trans reorganization of the triphenylphosphines to form the 8-imino-5,6,7-trihydroquinoline-ruthenium(ii) complex, (8-NH-C9H9N)RuCl2(PPh3)2 (F). Complex F can also be obtained in higher yield by heating a solution of E alone to reflux. Comparison of their molecular structures highlights the superior binding properties of the bidentate imine ligand in F over its amine-containing counterpart in E. Both complexes are highly effective in the transfer hydrogenation of a wide range of alkyl-, aryl- and cycloalkyl-containing ketones affording their corresponding secondary alcohols with loadings of as low as 0.1 mol%. Significantly, F can deliver excellent conversions even in bench quality 2-propanol in reaction vessels open to the air, whereas the catalytic efficiency of E is diminished by the presence of air but only operates efficiently under inert conditions.

Four examples of phenol-substituted methylene-bridged bis(imino)pyridines, CH(C6H4-4-OH){2'-(4-C6H2-2,6-R22N[double bond, length as m-dash]CMe)-6'-(2'',6''-R12C6H3N[double bond, length as m-dash]CMe)C5H3N}2 [R1 = R2 = Me L1, R1 = R2 = Et L2, R1 = Et, R2 = Me L3, R1 = iPr, R2 = Me L4], have been synthesized and fully characterized. Treatment of L1-L4 with two equivalents of cobaltous chloride affords the bimetallic complexes, [(L)Co2Cl4] (L = L1Co1, L2Co2, L3Co3, L4Co4), in good yield. The molecular structure of Co1 shows the two metal centers to be separated by a distance of 13.339 Å with each cobalt displaying a distorted trigonal bipyramidal geometry. On activation with either MAO or MMAO, Co1-Co4 exhibited high activities for ethylene polymerization (up to 1.46 × 107 g(PE) mol-1(Co) h-1 at 50 °C) with their relative values influenced by the steric properties of the N-aryl groups: Co1 > Co3 > Co4 > Co2. Highly linear polyethylenes incorporating high degrees of vinyl end-groups are a feature of all the materials produced with the molecular weights of the MAO-promoted systems (Mw range = 2-8 kg mol-1) generally higher than seen with MMAO (Mw range = 1-3 kg mol-1), while the distributions using MMAO are narrower (PDI < 2.0).

A family of six unsymmetrical N,N'-diiminoacenaphthene-nickel(ii) bromide complexes, [1-{2,6-(Ph 2 CH) 2 -4-MeC 6 H 2 N}-2-(ArN)C 2 C 10 H 6 ]NiBr 2 (Ar = 2-(C 6 H 11 )-6-MeC 6 H 2 Ni1, 2-(C 5 H 9 )-6-MeC 6 H 2 Ni2, 2-(C 8 H 15 )-6-MeC 6 H 2 Ni3, 2-(C 6 H 11 )-4,6-Me 2 C 6 H 2 Ni4, 2-(C 5 H 9 )-4,6-Me 2 C 6 H 2 Ni5, 2-(C 8 H 15 )-4,6-Me 2 C 6 H 2 Ni6), each bearing one ring-size variable 4-R-2-methyl-6-cycloalkyl-substituted N-aryl group and one N'-4-methyl-2,6-dibenzhydrylphenyl group, have been prepared and fully characterized. The molecular structures of Ni1, Ni2, Ni3 and Ni5 reveal distorted tetrahedral geometries with different degrees of steric protection imparted by the two inequivalent N-aryl groups. On activation with either EASC or MMAO, all the precatalysts are highly active (up to 17.45 × 10 6 g PE mol -1 (Ni) h -1 ) for ethylene polymerization at 20-50 °C with their activities correlating with the type of cycloalkyl ortho-substituent: cyclooctyl (Ni6, Ni3) > the cyclopentyl (Ni5, Ni2) > cyclohexyl (Ni4, Ni1) for either R = H or Me. Moderately branched to hyperbranched polyethylenes (T m 's as low as 44.2 °C) can be obtained with molecular weights in the range 2.14-6.68 × 10 5 g mol -1 with the branching content enhanced by the temperature of the polymerization. Dynamic mechanical analysis (DMA) and monotonic tensile stress-strain tests have been employed on the polyethylene samples and reveal the more branched materials to show good elastic recovery properties (up to 75.5%).

A series of highly sensitive aluminum hydroquinolin-8-olates (C1-C8) was synthesized and characterized by 1 H/ 13 C NMR spectroscopy. The molecular structures of compounds C1, C3, C4, and C5 were confirmed by single crystal X-ray crystallography and demonstrated the binuclear form. In the presence of BnOH, all the aluminum complexes exhibited moderate to high activities towards the ring-opening polymerization of ε-CL at high temperatures, but quite low activities at ambient temperature. Microstructure analysis of the resultant polycaprolactones showed that the polymers were linear in nature with a BnO- end group. In addition, the mechanism was investigated by monitoring the 1 H NMR and 27 Al NMR of C1 and these results suggested that the complexes existed as dimeric species at low temperature and partly converted into active mononuclear species at higher temperatures, which was easily coordinated by BnOH to afford the active species for the ring-opening polymerization of ε-caprolactone.

A new family of nickel halides (bromides Ni1-Ni5 and chlorides Ni6-Ni10) ligated by 1-(2,6-dibenzhydryl-4-nitrophenylimino)-2-(arylimino)acenaphthylene (Aryl = 2,6-Me 2 C 6 H 3 L1, 2,6-Et 2 C 6 H 3 L2, 2,6- i Pr 2 C 6 H 3 L3, 2,4,6-Me 3 C 6 H 2 L4, and 2,6-Et 2 -4-MeC 6 H 2 L5) have been prepared and well characterized, and the scope of their catalytic properties toward the polymerization of ethylene has been investigated. Upon activation with either Et 2 AlCl or EASC, the nickel bromide complexes displayed better activities than their nickel chloride counterparts and produced higher-molecular-weight polyethylene in the range of 10 6 g mol -1 with a very narrow range of molecular weight distributions. In comparison with reference precatalysts with non-nitro substituents (CH 3 , F or Cl), the title complexes experienced a modest negative effect on catalytic activity upon replacement with a NO 2 moiety (activity up to 4.61 × 10 6 g PE (mol Ni) -1 h -1 at 20 °C). Conversely, the NO 2 moiety exerted a positive effect to increase the molecular weight of the resulting polyethylene, and Ni4/Et 2 AlCl gave polyethylene with a maximum molecular weight of as high as 32.8 × 10 5 g mol -1 , which is not only the highest among the title complexes but also higher than any literature values reported with 1,2-diiminoacenaphthylnickel halides.

Five examples of α,α'-bis(arylimino)-2,3:5,6-bis(pentamethylene)pyridyl-chromium(iii) chlorides (aryl = 2,6-Me 2 Ph Cr1, 2,6-Et 2 Ph Cr2, 2,6-i-Pr 2 Ph Cr3, 2,4,6-Me 3 Ph Cr4, 2,6-Et 2 -4-MePh Cr5) have been synthesized by the one-pot template reaction of α,α'-dioxo-2,3:5,6-bis(pentamethylene)pyridine, CrCl 3 ·6H 2 O and the corresponding aniline. The molecular structures of Cr1 and Cr4 reveal distorted octahedral geometries with the N,N,N-ligand adopting a mer-configuration. On activation with an aluminium alkyl co-catalyst, Cr1-Cr5 exhibited high catalytic activities in ethylene polymerization and showed outstanding thermal stability operating effectively at 80 °C with activities up to 1.49 × 10 7 g of PE (mol of Cr) -1 h -1 . Significantly, the nature of the co-catalyst employed had a dramatic effect on the molecular weight of the polymeric material obtained. For example, using diethylaluminium chloride (Et 2 AlCl) in combination with Cr4 gave high density/high molecular weight polyethylene with broad molecular weight distributions (30.9-39.3). By contrast, using modified methylaluminoxane (MMAO), strictly linear polyethylene waxes of lower molecular weight and narrow molecular weight distribution (1.6-2.0) were obtained with vinyl end-groups.

Five examples of selectively fluorinated unsymmetrical diiminoacenaphthylenes, 1-[2,6-{(4-FC 6 H 4 ) 2 CH} 2 -4-FC 6 H 4 N]-2-(ArN) C 2 C 10 H 6 (Ar = 2,6-Me 2 C 6 H 3 L1, 2,6-Et 2 C 6 H 3 L2, 2,6-iPr 2 C 6 H 3 L3, 2,4,6-Me 3 C 6 H 2 L4, 2,6-Et 2 -4-MeC 6 H 2 L5), have been synthesized and used to prepare their corresponding nickel(ii) halide complexes, LNiBr 2 (Ni1-Ni5) and LNiCl 2 (Ni6-Ni10). Both 1 H and 19 F NMR spectroscopy techniques have been employed to characterize paramagnetic Ni1-Ni10; an inequivalent fluorine environment is a feature of the tetrahedral complexes in solution. Upon activation with relatively low ratios (ca. 600 equiv.) of ethylaluminum sesquichloride (Et 3 Al 2 Cl 2 , EASC), all the nickel complexes displayed high activities toward ethylene polymerization at 30 °C with precatalyst Ni4 the standout performer at 2.20 × 10 7 g of PE per mol of Ni per h, producing highly branched polyethylenes. In comparison with related diiminoacenaphthylene-nickel catalysts, these current systems, incorporating a high fluorine content on one N-aryl group, display superior productivity. In addition, the molecular structures of Ni2 and Ni4 are reported and the active catalyst is probed using 19 F NMR spectroscopy.

A series of 2-(1-aryliminoethyl)-9-arylimino-5,6,7,8-tetrahydrocycloheptapyridylcobalt chlorides were synthesized and characterized using FT-IR and elemental analysis, and the molecular structures of complexes , and were confirmed to present a pseudo-square-pyramidal or trigonal-bipyramidal geometry around the cobalt center using single-crystal X-ray diffraction. Upon activation with either methylaluminoxane (MAO) or modified methylaluminoxane (MMAO), all cobalt precatalysts gave high activities up to the level of 10(7) gPE mol(-1) (Co) h(-1) toward ethylene polymerization, being one of most active cobalt-based precatalysts. In comparison with cobalt analogues, the title precatalysts generally possessed longer lifetime along with good thermo-stability; moreover, the resultant polyethylenes were highly linear and unimodal in most cases.

The stoichiometric reactions of 5,6,7,8-tetrahydrocycloheptapyridin-9-one (cycloheptapyridin-9-one) with various anilines lead to corresponding mixtures of 9-aryliminocycloheptapyridine and the isomeric 9-arylamino-5,6,7-trihydrocycloheptapyridine derivatives; these compounds further reacted with nickel dichloride to form 9-aryliminocycloheptapyridyl nickel chlorides, respectively. The new organic compounds were analyzed by the NMR measurements, and all the organic and complex compounds were characterized by the FT-IR spectroscopy and elemental analysis. In addition, the molecular structures of representative nickel complexes and , determined by means of single-crystal X-ray diffraction, were found to be binuclear dimers with distorted square-pyramidal geometry around the nickel centers. On activation with either ethylaluminium sesquichloride (Et3Al2Cl3) or methylaluminoxane (MAO), all nickel complex pre-catalysts exhibited high activities of up to 7.80 × 10(6) g PE mol(-1) (Ni) h(-1) toward ethylene polymerization and produced highly branched polyethylenes in narrow polydispersity. The title nickel complexes showed comparable activities with 8-arylimino-5,6,7-trihydroquinolyl nickel analogues; whilst both exhibited higher activities than did the 2-iminopyridyl nickel analogues due to the enhancement of the ring-tension of cyclic-fused pyridine derivatives.

A series of 1-[2-(bis(4-fluorophenyl)methyl)-4,6-dimethylphenylimino]-2-aryliminoacenaphthylene derivatives together with the corresponding nickel bromide complexes was synthesized and characterized. Representative complexes C2 and C5 were characterized by the single-crystal X-ray diffraction, revealing a distorted tetrahedral geometry. Upon activation with either methylaluminoxane (MAO) or ethylaluminum sesquichloride (EASC), all nickel complexes exhibited high activities towards ethylene polymerization, producing polyethylene with a relatively low degree of branching and narrow polydispersity. Complex C1 maintained good activity at elevated reaction temperatures, which indicates significant thermal stability of the active species.

A series of 2-(1-arylimino)ethyl-9-arylimino-5,6,7,8-tetrahydrocycloheptapyridine derivatives was synthesized and fully characterized, and thereafter reacted with iron dichloride to form their corresponding iron(II) complexes. The single crystals of representative organic and iron complex compounds were obtained and analyzed by the X-ray diffraction analysis, indicating the distorted bipyramidal geometry around the iron core. Moreover, DFT calculations were performed on selected species to determine their structural features. On treatment with either MAO or MMAO, all iron complex pre-catalysts showed high activities (up to 1.56 × 10(7) gPE mol(-1)(Fe) h(-1)) toward ethylene polymerization. Regarding the nature of the ligands and reaction parameters, their catalytic activities and the characters of the obtained polyethylenes have been carefully investigated. The ring strain of the fused-cycloheptane of the ligands within iron complexes was considered to affect their catalytic performance in ethylene polymerization. The active species were activated and controlled by using a co-catalyst of MMAO preferred over MAO, and the obtained polyethylenes with MMAO showed narrower molecular polydispersity than the corresponding polyethylenes with MAO.

Reaction of [VO(OnPr)3] with the Schiff bases 3,5-(tBu)2-2-OH-C6H2CH(N(x-OR-C6H4)) (R = Me; x = 2, L(1)H; x = 3, L(2)H; x = 4, L(3)H; R = Et (L(4)H), CF3 (L(5)H), Ph (L(6)H)) or 4-methyl-3-(R)-2-(OH)-C6H4C[double bond, length as m-dash]N(2'-(2''-(OR1)C6H4)C6H4) (R = adamantyl, R(1) = Ph (L(7)H) or R = C(Me)2Ph, R(1) = Ph (L(8)H)) afforded the bis(chelate) vanadium(iv) complexes [VO(L(n))2] (n = 1 (1·2MeCN); n = 2 (2); n = 3 (3·2MeCN); n = 4 (4); n = 5 (5); n = 6 (6); n = 6, (7·1.5MeCN); n = 7, (8); n = 8, (9)); in the case of L(6)H, the oxo-bridged vanadium(v) complexes [VO(μ-O)(L(6))]2 (10) was also isolated. By contrast, interaction of 4-methyl-3-(R)-2-(OH)-C6H4C[double bond, length as m-dash]N(2'-(2''-(OR1)C6H4)C6H4) (R = adamantyl, R(1) = Me (L(9)H); R = tBu, R(1) = Me (L(10)H); R = C(Me)2Ph, R(1) = Me (L(11)H)) with [VO(OnPr)3] led to the isolation of the dinuclear complexes [VO(μ-OH)(μ-OnPr)(L(n))]2 (n = 9, (·4MeCN); 10, (12); 11, (13)), respectively. The molecular structures of 1 to 13 are reported. All complexes have been screened as pre-catalysts for the polymerization of ethylene in the presence of the co-catalyst diethylaluminium chloride (DEAC) with or without ethyltrichloroacetate (ETA) present at 1 or 10 bar of ethylene. Under high pressure, all pre-catalysts exhibited high activity and afforded high molecular weight (Mw ≈ 200 000 to 675 000 g mol(-1)), linear polyethylene with activities (in the presence of ETA) in the range 4960-16 400 g mmol(-1) h(-1); at one bar, the products were generally of lower molecular weight. The use of methylaluminoxane (MAO) or modified MAO (MMAO) as co-catalyst led to trace or poor (≤110 g mmol(-1) h(-1)) activity, respectively.

Pyrene-4,5,9,10-tetraone was prepared via the oxidation of pyrene, and reacted with various anilines to afford a series of 4,5,9,10-tetra(arylimino)pyrenylidene derivatives (L1-L4). The tetraimino-pyrene compounds L1 and L2 were reacted with two equivalents of (DME)NiBr2 in CH2Cl2 to afford the corresponding dinickel bromide complexes (Ni1 and Ni2). The organic compounds were fully characterized, whilst the bi-metallic complexes were characterized by FT-IR spectra and elemental analysis. The molecular structures of representative organic and nickel compounds were confirmed by single-crystal X-ray diffraction studies. These nickel complexes exhibited high activities towards ethylene polymerization in the presence of either MAO or Me2AlCl, maintaining a high activity over a prolonged period (longer than previously reported dinickel complex pre-catalysts). The polyethylene obtained was characterized by GPC, DSC and FT-IR spectroscopy and was found to possess branched features.

A series of 2-(2-benzhydrylnaphthyliminomethyl)pyridine derivatives (L1-L3) was prepared and used to synthesize the corresponding bis-ligated nickel(II) halide complexes (Ni1-Ni6) in good yield. The molecular structures of representative complexes, namely the bromide Ni3 and the chloride complex Ni6, were confirmed by single crystal X-ray diffraction, and revealed a distorted octahedral geometry at nickel. Upon activation with either methylaluminoxane (MAO) or modified methylaluminoxane (MMAO), all nickel complex pre-catalysts exhibited high activities (up to 2.02 × 10(7) g(PE) mol(-1)(Ni) h(-1)) towards ethylene polymerization, producing branched polyethylene of low molecular weight and narrow polydispersity. The influence of the reaction parameters and the nature of the ligands on the catalytic behavior of the title nickel complexes were investigated.

A series of 2-(1-(2-benzhydrylnaphthylimino)ethyl)pyridine derivatives (L1-L3) was synthesized and fully characterized. The organic compounds acted as bi-dentate ligands on reacting with nickel halides to afford two kinds of nickel complexes, either mononuclear bis-ligated L2NiBr2 (Ni1-Ni3) or chloro-bridged dinuclear L2Ni2Cl4 (Ni4-Ni6) complexes. The nickel complexes were fully characterized, and the single crystal X-ray diffraction revealed for Ni2, a distorted square pyramidal geometry at nickel comprising four nitrogens of two ligands and one bromide; whereas for Ni4, a centrosymmetric dimer possessing a distorted octahedral geometry at nickel was formed by two nitrogens of one ligand, two bridging chlorides and one terminal chloride along with oxygen from methanol (solvent). When activated with diethylaluminium chloride (Et2AlCl), all nickel complexes performed with high activities (up to 1.22 × 10(7) g (PE) mol(-1) (Ni) h(-1)) towards ethylene polymerization; the obtained polyethylene possessed high branching, low molecular weight and narrow polydispersity, suggestive of a single-site active species. The effect of the polymerization parameters, including the nature of the ligands/halides on the catalytic performance is discussed.

A series of nickel(II) dihalides complexes bearing 4,5-bis(arylimino)pyrenylidenes, NiX2(2,6-R(1)-4-R(2)C6H2N)2C16H8, was synthesized and characterized by FT-IR spectroscopy, elemental analysis, and single crystal X-ray diffraction for the organic compounds (L2 and L3) and the nickel complexes (C1 and C2). The molecular structures of C1 (R(1) = Me, R(2) = H, X = Br) and C2 (R(1) = R(2) = Me, X = Br) revealed a distorted tetrahedral geometry around the nickel centre. Upon treatment with the co-catalysts MAO, EASC and MMAO, all the nickel pre-catalysts exhibited high activities (of up to 4.42 × 10(6) g(PE) mol(Ni)(-1) h(-1)) for ethylene polymerization, and produced polyethylene products with a high degree of branching (up to 130 branched per 1000 carbons) and narrow molecular weight distribution. The influence of the reaction parameters and the nature of the ligands on the catalytic behavior of the title nickel complexes have been investigated.

A series of 1,2-bis(arylimino)acenaphthylidenes (L1-L5) and their corresponding 4,4'-methylenebis(1-(2,6-diisopropylphenylimino)-2-(arylimino)acenaphthylene) derivatives (L6-L10) were synthesized and used to form mono-nuclear nickel bromides LnNiBr2 (n = 1-5, Ni1-Ni5) and bi-nuclear nickel halides LnNi2X4 (n = 6-10: X = Br, Ni2-1-Ni2-5; n = 4, X = Cl, Ni2-6). All the organic compounds were fully characterized by FT-IR spectra, NMR measurements and elemental analysis. The nickel complexes were characterized by FT-IR spectra and elemental analysis and the molecular structures of the representative complexes Ni1, Ni2-1 and Ni2-3 were confirmed by single-crystal X-ray diffraction. Upon activation with either Et2AlCl or MAO, all the nickel complex pre-catalysts exhibited high activity toward ethylene polymerization over the temperature range from ambient to 50 °C. In general, the bi-nuclear complexes showed a positive synergetic effect with higher activity than their mono nuclear analogs. The resultant polyethylene possessed higher molecular weight and a high degree of branching.

A series of 2-[1-(4,6-dimethyl-2-benzhydrylphenylimino)ethyl]-6-[1-(arylimino)ethyl]pyridines was synthesized and used to prepare the iron(II) and cobalt(II) chloride complexes thereof. All organic compounds were fully characterized by elemental analysis, IR and NMR ((1)H/(13)C) spectroscopy, whilst the metal complexes were characterized by elemental analysis and IR spectroscopy as well as by single-crystal X-ray diffraction studies (for two representative cobalt complexes), which revealed that the geometry at the metal was either pseudo-square-pyramidal or trigonal bipyramidal. Upon activation with either methylaluminoxane (MAO) or modified methylaluminoxane (MMAO), all metal complex pre-catalysts exhibited high activities for ethylene polymerization. The iron pre-catalysts show much higher activity than did their cobalt analogues; however, the iron catalytic systems generally produced polyethylene of wide molecular weight polydispersity. At elevated reaction temperature, the polyethylene was of lower molecular weight, but revealed narrow polydispersity.

Recent progress in the use of iron-based complex pre-catalysts for ethylene reactivity is reviewed, illustrating the current state-of-the-art and the potential usefulness of such systems for delivering solely ethylene oligomerization or polymerization products. The problems associated with the industrial use of late transition metal complex pre-catalysts are generally regarded as catalyst deactivation and the formation of more products of lower molecular weight at elevated temperature. These problems have been addressed for iron-based complex pre-catalysts via the fine tuning of substituents of existing ligands and/or the design of new ligand sets. Results revealed that modified bis(imino)pyridyliron dichlorides were capable of operating at elevated temperatures, and were capable of delivering highly linear polyethylene. Other new models of iron complexes have achieved high activity for ethylene oligomerization and/or polymerization. Particularly successful has been the use of the 2-iminophenanthrolyliron pre-catalyst, which have now been utilized in a 500 tonne pilot plant.

A series of 2-(2-benzhydrylbenzenamino)pyridine ligands (L1–L13) was synthesized and used as bidentate N^N ligands with nickel halides to afford the corresponding nickel dihalide complexes L(2)Ni(2)Cl(4) C1–C13 and L(2)NiBr(2) D1–D13. All ligands and complexes were characterized by IR and NMR spectroscopy, and by elemental analysis. The molecular structures of the representative complexes C1·2CH(3)OH, C5·2H(2)O, D4, D7 and D9 were confirmed by single-crystal X-ray diffraction studies. Upon activation with either methylaluminoxane (MAO) or ethylaluminium sesquichloride (Et(3)Al(2)Cl(3), EASC), these nickel pre-catalysts exhibited high activities (up to the range of 10(7) g mol(-1) (Ni) h(-1)) towards ethylene polymerization, producing branched polyethylenes with narrow polydispersity.

The stoichiometric reaction of the salicylaldimine derivatives (L1-L12) with trimethylaluminium afforded the corresponding dimethylaluminium aldiminophenolates (C1-C12), which were fully characterized by NMR spectroscopy and elemental analysis. The molecular structures of the representative complexes C1, C6, and C8 were determined by the single-crystal X-ray diffraction, which revealed distorted tetrahedral geometry at aluminium. Activation of the dimethylaluminium aldiminophenolates for the ring-opening polymerization required one equivalent of BnOH. On the basis of the polymerization results for L-lactide, D-lactide or rac-lactide, higher efficiency was observed for the ROP of D-lactide, and the nature of the ligands present significantly affected the observed catalytic activities and the properties of the resultant polylactides.

A series of N-(5,6,7-trihydroquinolinylidene)-2-benzhydrylbenzenamine ligands was synthesized and characterized by (1)H/(13)C NMR and IR spectroscopy, and by elemental analysis. These ligands reacted with NiCl(2) or NiBr(2)(DME) to form the title halide complexes, which were also characterized by IR spectroscopy and elemental analysis. Single crystal X-ray diffraction revealed that the representative nickel complexes crystallized as centro-symmetric dimers with chloro-bridges linking distorted octahedral nickel centers. On activation with either methylaluminoxane (MAO) or diethylaluminium chloride (Et(2)AlCl), all nickel pre-catalysts showed high activities for ethylene polymerization, producing polyethylene with narrow molecular weight distribution, consistent with single-site catalysis. The nature of the ligands and reaction parameters were investigated and discussed in terms of their influence on the catalytic behavior of these nickel pre-catalysts.

The 2-(N-alkylcarboxamide)-6-iminopyridine ligands (L1-L7) can bind as either mono-anionic tridentate N^N^N ligands on reaction with PdCl(2)(CH(3)CN)(2), to form complexes LPdCl (C1-C7), or as neutral tridentate N^N^O ligands with NiCl(2)·6H(2)O, to produce complexes LNiCl(2) (C8-C14). All metal complexes were characterized by IR spectroscopy and elemental analysis, and in the case of the palladium complexes, by (1)H and (13)C NMR spectroscopy. The crystal structures of C3, C4, C6, C10, and C12 were determined by X-ray crystallography, and revealed a distorted square geometry around the palladium centre, whereas for nickel, a distorted square-pyramidal geometry was adopted. The representative palladium complex (C3) was further reacted with AgBF(4) in acetonitrile affording the salt [L3Pd(CH(3)CN)][BF(4)] (C15) and the structure of this was confirmed by single-crystal X-ray diffraction. By contrast, carrying out the reaction in dichloromethane rather than acetonitrile, in the presence of malononitrile (CNCH(2)CN), resulted in the formation of the bimetallic palladium complex [L3Pd(CNCH(2)CN)PdL3]·2[BF(4)] (C16). Upon activation with diethylaluminium chloride, all the nickel complexes showed high activity for ethylene dimerization. Furthermore, the palladium complexes exhibited good activities in the vinyl-polymerization of norbornene upon activation with MAO., (This journal is © The Royal Society of Chemistry 2011)

A series of unsymmetrical 2,6-bis(imino)pyridylcobalt(II) complexes, {2-[2,6-(CH(C(6)H(5))(2))(2)-4-Me-C(6)H(2)N==C(CH(3))]-6-(2,6-R(1)(2)-4-R(2)-C(6)H(2)N==CCH(3))-C(5)H(3)NCoCl(2)} where R(1) = Me, Et or (i)Pr, R(2) = H or Me, together with the new symmetrical complex 2,6-[2,6-(CH(C(6)H(5))(2))(2)-4-Me-C(6)H(2)N==C(CH(3))](2)-C(5)H(3)NCoCl(2), were synthesized. All of the compounds were fully characterized by (1)H NMR and IR spectroscopy, as well as by elemental analysis. The molecular structures of Co1 (R(1) = Me, R(2) = H) and Co5 (R(1) = Et, R(2) = Me) were further confirmed by single crystal X-ray diffraction, which indicated that the cobalt centres were penta-coordinate with a pseudo square-pyramidal geometry. Upon treatment with MAO or MMAO, these cobalt pre-catalysts exhibited higher activities than any previously reported cobalt pre-catalysts, with values as high as 4.64 × 10(6) g PE mol(-1)(Co) h(-1) for ethylene polymerization at atmospheric pressure. The polyethylenes obtained were of high molecular weight and narrow molecular weight distribution., (This journal is © The Royal Society of Chemistry 2011)

A series of N-(5,6,7-trihydroquinolin-8-ylidene)arylamine ligands was synthesized and fully characterized by NMR, IR spectroscopy and elemental analysis. Dimeric N-(5,6,7-trihydroquinolin-8-ylidene)arylaminonickel dichlorides were prepared and examined by IR spectroscopy and elemental analysis, and the molecular structures of the representative nickel complexes were determined by the single crystal X-ray diffraction. On treatment with various alkylaluminiums, all the title complexes exhibited highly active, single-site active behavior for ethylene polymerization producing polyethylene (PE) waxes. The catalytic systems using the co-catalysts diethylaluminium chloride (Et(2)AlCl) or methylaluminoxane (MAO) were investigated in detail, and the molecular weights and distributions of the PEs obtained were found to significantly rely on the nature of the different ligands present and reaction parameters such as the molar ratios of Al/Ni, reaction temperature and reaction time., (This journal is © The Royal Society of Chemistry 2011)

A series of 2-(1-(arylimino)propyl)quinolin-8-olate half-titanocene dichlorides, Cp'TiCl(2)L (Cp' = η(5)-C(5)H(5) or η(5)-C(5)Me(5), L = 2-(1-(2,6-R(1)-4-R(2)-phenylimino)propyl)quinolin-8-olate), was synthesized via the stoichiometric reaction of Cp'TiCl(3) with the corresponding potassium 2-(1-(2,6-R(1)-4-R(2)-phenylimino)propyl)quinolin-8-olate salt. All titanium compounds were characterized by elemental analysis, (1)H NMR and (13)C NMR spectroscopy; the molecular structures of two representative compounds were determined by single crystal X-ray diffraction. On activation with methylaluminoxane (MAO), all half-titanocene compounds showed high activity in ethylene polymerization, and furthermore, performed with good to high activities in the co-polymerization of ethylene with either 1-hexene or 1-octene affording polyethylenes with high co-monomer incorporation. Less bulky ortho-substituents (R(1)) on the phenylimino groups were found to enhance the catalytic activities of their titanium compounds. In general, the titanium pro-catalysts containing η(5)-C(5)Me(5) (C7-C12) exhibited higher activities than did their analogues bearing η(5)-C(5)H(5) (C1-C6). Some of the resultant polyolefins were ultrahigh molecular weight polyethylene., (This journal is © The Royal Society of Chemistry 2011)

The stoichiometric reactions of 2-(2,6-R-phenylimino)quinolin-8-ol (L1-L5, L1: R = Me, L2: R = Et, L3: R = (i)Pr, L4: R = Cl, L5: R = F) with Me(3)Al afforded the dimeric aluminium complexes [Me(2)AlL](2) (1-5) in good yields. By contrast, stoichiometric reactions of 2-(1-(2,6-R-phenylimino)propyl) quinolin-8-ol (L6-L10, L6: R = Me, L7: R = Et, L8: R = (i)Pr, L9: R = Cl, L10: R = F)) with Me(3)Al gave the mononuclear aluminium complexes Me(2)AlL (6-10) accompanied with by-products of the form Me(2)AlL·Me(3)Al (11-15). All methylaluminium complexes were characterized by NMR spectroscopy, elemental analysis, and the molecular structures of complexes 3, 6 and 8 were determined by single-crystal X-ray diffraction. Aluminium compounds 1-5 possessed negligible activity towards the ring-opening polymerization of ε-caprolactone either in the presence or absence of BnOH. In contrast, in the presence of BnOH, the mononuclear aluminium compounds 6-10 could efficiently initiate the ring-opening polymerization of ε-caprolactone; the polymerization proceeded in a living manner.

A series of 2-benzoimidazol-8-ethoxyquinolines (L1-L6) and the nickel dihalides thereof (C1-C12) were synthesized and characterized by elemental analysis and infrared spectroscopy. The molecular structures of representative complexes were determined by single crystal X-ray diffraction which revealed a distorted pyramidal geometry at nickel. All complexes, when activated with Et(2)AlCl, showed high activities towards ethylene oligomerization and enhanced thermodynamic stability.

The stoichiometric reactions of 2-(benzimidazol-2-yl)-6-methylpyridine (L1) or 8-(benzimidazol-2-yl)quinaldine (L2) with trialkylaluminium reagents R(3)Al (R = Me, Et and iBu) afforded the corresponding dialkylaluminium benzimidazolate complexes R(2)AlL (L1, R = Me (1), Et (2), iBu (3); L2 R = Me (4), Et (5), iBu (6)). Treatment of L1 with one or two equivalents of Et(2)AlCl led to the adducts EtAl(L1)(2)·AlEtCl(2) (7) or Et(2)AlL1·AlEtCl(2) (8), respectively. Complex 7 was also available via treatment of 8 with one equivalent of L1. Reaction of L1 with two equivalents of AlR(3) (R = Me or Et) afforded R(2)AlL1·AlR(3) (R = Me, 9; R = Et, 10), which were also formed when 1 or 2 were reacted with AlR(3). Reaction of L2 with two equivalents of AlR(3) (R = Me or Et) gave the complexes R(2)AlL2·AlR(3) (R = Me, 11; R = Et, 12), which were also formed in the stoichiometric reaction of 4 or 5 with AlR(3) (R = Me or Et). Screening of these complexes in the presence of BnOH, for the ring-open polymerisation of ε-caprolactone, revealed appreciable activities. Only the aluminium compounds ligated by 2-(benzimidazol-2-yl)-6-methylpyridine maintained high activity in the absence of BnOH. In all cases, polymers with bi- or multi-modal characteristics were produced.

Treatment of N-aryl-2-methylquinolin-8-amines (L1-L3) with one equivalent AlMe3 or AlEt3 afforded dialkyl aluminum compounds (C1-C5), whereas the stoichiometric reaction of 2-((2-(1H-benzo[d]imidazol-2-yl)quinolin-8-ylimino)methyl)phenols (L4-L6) with either AlMe3 or AlEt3 produced monoalkyl aluminum compounds (C6-C10). All the organoaluminum compounds were characterized by 1H, 13C NMR and elemental analysis, and the molecular structures of representative compounds were confirmed by X-ray crystallography. With bidentate ligands, compounds C1 and C3 showed tetrahedron geometry around Al center, while compound C7 has a distorted square pyramidal geometry around Al center with the framework comprising the tetradentate ligand. The dialkyl aluminum compounds (C1-C5) performed high catalytic activities towards the ring opening polymerization (ROP) of epsilon-caprolactone (epsilon-CL), in parallel, the monoalkyl aluminum compounds (C6-C10) showed negative results for the polymerization of epsilon-CL. In the presence or absence of benzyl alcohol (BnOH), the four-coordination aluminum compounds (C1-C5) are both highly active towards the ring opening polymerization (ROP) of epsilon-caprolactone with resulting high conversation of epsilon-caprolactone and polymers with high molecular weight. In the presence of one equivalent of BnOH, polymerization of epsilon-caprolactone proceeded in a living manner and molecular weights of the obtained poly(epsilon-caprolactone)s could be precisely controlled by adapting the reaction conditions.