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A novel axially chiral binaphthyl fluorene based salen ligand, AFX-155 [2,2'-(1E,1'E)-(R)-1,1'-binaphthyl-2,2'-diylbis(azan-1-yl-1-ylidene)bis(methan-1-yl-1-ylidene)bis(4-((7-(diphenylamino)-9,9-dihexyl-9H-fluoren-2-l)ethynyl)phenol)], with potential applications in homogeneous catalysis, biophotonics, and sensing was synthesized. A full comparative theoretical-experimental analysis of the UV-vis and electronic circular dichroism (ECD) spectra of the 10 primary isomers, comprising stereoisomers and optical isomers, revealed the presence of the unique structure in tetrahydrofuran (THF) solution, the trans-R-intra//trans-R-extra. A proposed route of attack of the (R)-(+)-2,2'-diamino-1,1'-binapthalene onto a salicaldehyde 5-(2-(2-(diphenylamino)-9,9-dihexyl-9H-fluoren-7-yl)ethynyl)-2-hydroxybenzaldehyde followed by a consecutive attack of the resulting species onto another salicaldehyde, both via Burgi:Dunitz trajectory, validates the unambiguous formation of the established isomer. Steric hindrances seem to be the determinant factor that defines the 3D structural conformation of this particular stereoisomer of AFX-155 with triple axial chirality. The determination of every optimal structure and the dominant conformers of AFX-155 were calculated evaluating, in CONFLEX, their steric energies using force fields at MMFF94S (2006-11-24HGTEMP) level in gas phase. The geometry of the conformers was optimized in THF (using PCM) using Gaussian 09 at the DFT/B3LYP level of theory and 6-31G* basis set. The first 100 electronic excited states were calculated using the same level of theory and basis set.

Polymeric {TiO(salen)}n prepared from [Ti(OCHMe2)2(acac)2] and salen-H2 in MeOH and with an apparent chain structure, is mostly insoluble but dissolves when strongly heated in aromatic aldehydes or ketones, depositing the insoluble form again on cooling. p-Tolualdehyde performed differently, giving a red solution from which insoluble red crystals were deposited. X-ray crystallography showed the complex was [{Ti(salen)(O2CC6H4Me-4)-O-Ti(salen)(O2CC6H4Me-4)]. Solvent (1) (solvent = p-tolualdehyde or p-toluic acid) containing a Ti–O–Ti bridge. Reaction of {TiO(salen)}n with p-tert-butylbenzoic acid in the presence of excess p-tolualdehyde gave [{Ti(salen)(O2CC6H4CMe3-4)-O-Ti(salen)(O2CC6H4CMe3-4) ]. 4-CH3C6H4CHO (2) for which an X-ray crystal structure determination indicated similar connectivity around the metal as for (1). {TiO(salen)}n heated strongly in isobutyrophenone (PhCOCHMe2) also gave red crystals which X-ray analysis showed structural similarity to (1) and (2) but incorporating a PhCO2-ligand. {TiO(salen)}n heated in acetophenone gave a product which X-ray analysis showed was a dioxo-bridged dimer (4) in which C–C bond formation had occurred on one side of each salen ligand.

Ionic ligands derived from a salen ligand containing two proximal 1,3-disubstituted imidazolium ionic liquid cores form cobalt(III) complexes capable of selectively oxidizing veratryl alcohol, a lignin model compound, to veratraldehyde using air as the source of oxygen. These complexes are easy to prepare, inexpensive, water stable, and soluble in ionic liquids, making them viable candidates for use as oxidation catalysts.

The organic imine and their metal complexes were synthesized and characterized by IR, UV and NMR. The third order non linear optical properties of the compounds were investigated. The measurements of second hyperpolarizabilites were performed using single beam Z-scan technique with 8 ns laser pulses. Ligand and its Copper, Zinc and Nickel complexes show good third order non linearity whereas Manganese complex did not show any activity.

Two new one-component aluminium-based catalysts for the reaction between epoxides and carbon dioxide have been prepared. The catalysts are composed of aluminium–salen chloride complexes with trialkylammonium groups directly attached to the aromatic rings of the salen ligand. With terminal epoxides, the catalysts induced the formation of cyclic carbonates under mild reaction conditions (25–35 °C; 1–10 bar carbon dioxide pressure). However, with cyclohexene oxide under the same reaction conditions, the same catalysts induced the formation of polycarbonate. The catalysts could be recovered from the reaction mixture and reused.

Eleven bis(dimethylphenolato) Ti(IV) complexes of salen ligands with different steric and electronic properties due to different aromatic substituents at the ortho and para positions are reported, and their cytotoxicity toward HT-29 and OVCAR-1 cells and its dependence on hydrolytic behavior are discussed. Eight complexes of this series were analyzed by X-ray crystallography, confirming the trans geometry of the labile ligands with otherwise relatively similar coordination features to those of cis-salan analogues. Relatively high and similar hydrolytic stability is observed for all complexes, with t(1/2) values for labile ligand hydrolysis of 2-11 h in 10% D(2)O solutions. In contrast, varying cytotoxicities were achieved, identifying selected members as the first trans-Ti(IV) complexes reported as anticancer agents. Steric bulk all around the complex diminished the activity, where a complex with no aromatic substitutions is especially active and complexes substituted particularly at the ortho positions are mostly inactive, including ortho-halogenated and ortho-tert-butylated, with one exception of the ortho-methoxylated complex demonstrating appreciable activity. In contrast, para-halogenation provided the complexes of highest cytotoxic activity in this series (IC(50) as low as 1.0 ± 0.3 μM), with activity exceeding that of cisplatin by up to 15-fold. Reaction of a representative complex with ortho-catechol yielded a "cis"-Ti(IV) complex following rearrangement of the salen ligand on the metal center, with highly similar coordination features and geometry to those of the catecholato salan analogues, suggesting that the complexes operate by similar mechanisms and rearrangement of the salen ligand may occur upon introduction of a suitable chelating target. In additional cytotoxicity measurements, a salen complex was preincubated in the biological medium for varying periods prior to cell addition, revealing that marked cytotoxicity of the salen complex is retained for longer preincubation periods relative to known Ti(IV) complexes, suggesting that the hydrolysis products may also induce cytotoxic effects, thus reducing stability concerns.

The authors are grateful to Dr A. S. Azevedo for collecting the powder diffraction data. IKB thanks FCT for the contract under 'Programa Ciencia 2007'. This work was supported by the Centro de Quimica (University of Minho, Portugal) and by Fundacao para a Ciencia e Tecnologia (FCT-Portugal), under programme POCTI-SFA-3-686.

The spectroscopy, electrochemistry and electrogenerated chemiluminescence (ECL) of eight bisalicylideneethylenediamino (salen) metal complexes are reported. Two of the complexes contain an unsubstituted salen ligand and either cobalt(II) or nickel(II). The others have 1,2-cyclohexanediamonio- N , N ′-bis(3,5-di- t -butylsalicylidene) as the ligand, and chromium(III), aluminum(III), cobalt(II), cobalt(III) or manganese(II) as the metal center. The complexes have lowest energy absorption maxima between 350 and 430 nm. When excited at these wavelengths, the complexes emit between 417 and 594 nm in acetonitrile. Photoluminescence efficiencies ( ϕ em ) were between 0.0310 and 23.8 compared to Ru(bpy) 3 2+ (bpy = 2,2′-bipyridine; ϕ em = 1), with the aluminum complexes displaying the most intense photoluminescence. Both reversible and irreversible oxidative electrochemistry is displayed by the metal–salen complexes with oxidation potentials ranging between +0.152 and +1.661 V versus Ag/AgCl. The ECL intensity peaks at a potential corresponding to oxidation of both TPrA and the salen systems, indicating that both are involved in the ECL reaction sequence. ECL efficiencies ( ϕ ecl ) were between 0.0018 and 0.0086 when compared to Ru(bpy) 3 2+ ( ϕ ecl = 1) in acetonitrile (0.05 M tri- n -propylamine (TPrA) as an oxidative–reductive ECL coreactant). Also, qualitative studies using transmission filters suggest that the complexes emit ECL in approximately the same region as their photoluminescence, indicating that the same excited state is formed in both experiments.

A novel enantiopure salen ligand bearing a diphenylphosphine oxide on the 3-position of one aromatic ring was synthesized and combined with Ti(O i -Pr) 4 as a monometallic bifunctional catalyst for asymmetric cyanosilylation reaction of aldehydes with trimethylsilyl cyanide (TMSCN). The catalyst system exhibited excellent activity and moderate enantioselectivity. The addition of TMSCN to 4-nitrobenzaldehyde in the presence of 1 mol% catalyst loading could complete within 10 min at ambient temperature. An intramolecularly cooperative catalysis was observed in this system wherein the central metal Ti(IV) is suggested to play a role of Lewis acid to activate aldehydes while the appended diphenylphosphine oxide worked as Lewis base to activate TMSCN.

Metal complexes based on the salen ligand are highly versatile compounds that have many interesting properties, including magnetism, catalysis, and reversible oxygen binding. The marriage of complexes of salen with porous materials is a promising way to combine the attractive properties of the salen complex with those of the porous structure. In this Microreview, we discuss several recent examples of published research that illustrate the power of combining these two disparate areas of research. Approaches based on supramolecular chemistry are emphasized.

This brief review deals with the development of a general protocol for the synthesis of μ-oxido divanadium(V) compounds [LOV V -(μ-O)-V V O(Salen)] (L = L 1 –L 5 ) ( 1 – 5 ) incorporating coordination asymmetry. One of the vanadium centers in these compounds has an octahedral environment, completed by tetradentate Salen ligand, while the other center has a square pyramidal geometry, made up of tridentate biprotic Schiff-base ligands (H 2 L 1–5 ) with ONO ( 1 – 3 ) and ONS ( 4 , 5 ) type donor combinations. Single crystal X-ray diffraction, ESI-MS, and multi-nuclear NMR ( 1 H and 51 V) spectroscopy have been used extensively for the characterization of these compounds. The V 2 O 3 core in these compounds, save 3 , has a rare type of twist-angular structure. The V(1)⋯V(2) separations (3.7921(7)–3.3084(6) A) are by far the largest in these compounds compared to their peers containing a V 2 O 3 core. The molecules retain their unsymmetrical binuclear structures also in solution as established by NMR spectroscopy. The mixed-oxidation compound (ImH)[L 4 OV IV -(μ-O)-V V OL 5 ] 7 containing two dissimilar ligands has a V 2 O 3 core with a syn -angular structure and exhibits crystallographically imposed mirror symmetry due to static disorder. In solution of donor solvents, this angular core structure changes into a linear one ( anti -linear) by accepting solvents in to the vacant coordination site of the metal centers. Finally, the protocol for the synthesis of heterobimetallic compounds with vanadium(V) and Re(VII) combination flanked by a single μ-oxido bridge has been developed in which the precursor complexes [V IV OL 6,7 ] (H 2 L 6,7 are Salen type of ligands) are allowed to oxidize aerially in the presence of added perrhenate anion. The oxidized [V V OL 6,7 ] + species hold the ReO 4 − anion in the vacant coordination site of the metal ion, trans to the terminal oxido group, thus generating the V V –O–Re VII moiety in the heterobimetallic compounds ( 9 and 10 ). Both X-ray crystallography and 1 H NMR spectroscopy have been used to establish the identities of these compounds. In compound 9 , the Re(1)–O(11)–V(1) bridge angle is barely linear (170.2(3)°) with a Re⋯V separation of 3.9647(9) A. The redox behavior of 9 and 10 are quite interesting, each undergoing two reductions both in the positive potential range at E 1/2 = 0.59 and 0.16 V vs. Ag/AgCl reference and have single-electron stoichiometry, confirmed by constant potential coulometry.

Gas-phase equilibrium measurements have been used to determine the relative binding affinities of 18 ligands to Jacobsen's manganese salen catalyst. The group of ligands spans 5.7 kcal/mol and includes seven functional groups (alcohol, ketone, ester, acyclic ether, cyclic ether, epoxide, and amine). The data follow general trends seen in other gas-phase metal cation affinities, but are influenced to a much greater extent by steric effects. For example, a 2° amine is a stronger binder than a 1° amine (as typical for gas-phase cation binding), but a 3° amine is a much weaker binder due to excessive crowding with the bulky salen ligand. The impact of steric effects was also explored with computational modeling at the B3LYP/6-311+G(d,p)/B3LYP/6-31G(d) level. The study demonstrates the utility of using mass spectrometry to probe the ligand binding characteristics of sterically demanding, metal-centered catalysts.

A variety of transition metal tetrahydro-salen complexes M-[H 4 ]Salen (M = Cu, Co and Fe) and the corresponding metal salen complexes M-Salen were firstly tethered onto amino-functionalized mesoporous SBA-15 and screened as heterogeneous catalysts for aerobic epoxidation of styrene. The mesoporous structural integrity throughout the tethering procedure, the successful tethering of the organometallic complexes, the loadings of metal ions and organic ligands as well as the catalyst surface constitution and location of active organometallic species on the SBA-15 support were determined by comprehensive characterization techniques such as XRD, N 2 adsorption/desorption, TEM, FT-IR, UV–vis spectroscopy, ICP-AES, XPS and TG/DSC. The tethered catalyst M-[H 4 ]Salen-SBA (M = Cu or Co) exhibits improved reactivity than the corresponding M-Salen-SBA catalyst due to the incorporation of a modified coordination environment of the central metal cations by C N bond hydrogenation. However, Fe-[H 4 ]Salen-SBA shows lower reactivity as compared to Fe-Salen-SBA possibly due to the easy formation of relatively inactive dinuclear iron(III) tetrahydro-salen complexes, verified by XPS and ESR. The recycling results of these heterogeneous catalysts exemplified by Co-Salen-SBA, Co-[H 4 ]Salen-SBA and Fe-Salen-SBA, show good recoverability without significant loss of activity and selectivity within successive runs. Heterogeneity tests of three sample catalysts confirm the high stability of cobalt catalysts against leaching of active species into solution.

A dimeric salen ligand derived from L-tartaric acid was synthesized through linking two salen units with 1,6-dibromohexane. The corresponding Ti complex was proved to be an efficient catalyst for asymmetric addition of trimethylsilyl cyanide to aldehydes, providing the corresponding optically active trimethylsilyl ether of cyanohydrins with moderate to good enantiomeric excesses and excellent yields under relatively low catalyst loading condition.

A new chiral salen ligand based on two ferrocenyl groups is designed. Unlike known salen ligands, of which chirality originates from central and axial chiral centers, the chirality of this ligand comes from the planar chiral ferrocenyl groups. The ligand is synthesized stereoselectively using a novel aromatization strategy starting from a ferrocene derivative, which was readily prepared using a known chiral auxiliary approach.

A new metallocapped cyclodextrin (CD) was synthesized by the regioselective debenzylation, induced by diisobutylaluminium hydride (DIBAL-H), of perbenzylated cyclodextrins. This reaction allowed for the efficient preparation of an unprecedented CD-salen type copper(II) complex. The electrochemical behavior of both the bound and unbound CD-salen compounds was investigated by cyclic voltammetry. Notably, it was shown that the presence of tert-butyl groups at the ortho- and para-positions of the salen aromatic rings stabilized the copper(II) phenoxyl radical species that was generated upon the one-electron oxidation of the starting compound. Importantly, this stabilization remained effective when the salen-type ligand was covalently attached to the CD. This allowed for investigations of the reactivity of the copper(II) phenoxyl radical complex towards a primary alcohol to be performed by cyclic voltammetry. This reaction can be considered as mimicking the behavior of galactose oxidase. However, under these conditions, no reactivity was observed in the presence of benzyl alcohol. This may be due to distortion, either of the initially square planar salen ligand after its grafting to the CD primary face, and/or of the CD itself. On the other hand, the electrochemical reduction of the un-grafted copper(II) salen-type ligand led to a transient anionic species that exhibited significant stability on the time-scale of the slow cyclic voltammetry measurement in the absence of the CD, but was unstable in the presence of the CD. In the latter case, it was demonstrated that the anionic species was protonated by the CD. Importantly, this protonation was not fast enough to prevent catalytic activation of iodomethane by the electro-generated copper(I)-capped salen CD complex.

A oxo-bridged binuclear iron complex with chiral salen ligand [Fe2L2]O (H2L = R, R'-N, N'-bis (3,5-di-tert-butylsalicylaldene)-1, 2-cyclohexanediamine) has been synthesized and the structure has been determined by X-ray diffraction analysis. The title complex crystallizes in triclinic system with space group P-1. Crystal data: a = 1.42555(14) nm, b = 1.54889 (15) nm, c = 1.83662 (17) run, = 103.873(2)°, β = 100.506(2)°, 7=101.840(2)°, V = 3.7371 nm3, Dc = 1.082 g/cm3 and Z = 2. In the complex, each iron center is penta-coordination and in distorted square-pyramidal environment. Two Fem atoms are intramolecularity bridged by an oxygen atom with Fe-Fe nonbond distance of 0.3517(3) nm.

The experiments described here clarify the mechanism and origin of the enantioselectivity of the oxidation of racemic secondary alcohols catalyzed by chiral Mn(III)-salen complexes using HOBr, Br(2)/H(2)O/KOAc or PhI(OAc)(2)/H(2)O/KBr as a stoichiometric oxidant. Key points of the proposed pathway include (1) the formation of a Mn(V)-salen dibromide, (2) its subsequent reaction with the alcohol to give an alkoxy-Mn(V) species, and (3) carbonyl-forming elimination to produce the ketone via a highly organized transition state with intramolecular transfer of hydrogen from carbon to an oxygen of the salen ligand.

The chiral triplesalen ligand H(6)chand provides three chiral salen ligand compartments in a meta-phenylene arrangement by a phloroglucinol backbone. The two diastereomeric versions H(6)chand(RR) and H(6)chand(rac) have been used to synthesize the enantiomerically pure chiral complex [(FeCl)(3)-(chand(RR))] (3(RR)) and the racemic complex [(FeCl)(3)(chand(rac))] (3(rac)). The molecular structure of the free ligand H(6)chand(rac) exhibits at the terminal donor sides the O-protonated phenolimine tautomer and at the central donor sides the N-protonated keto-enamine tautomer. The trinuclear complexes are comprised of five-coordinate square-pyramidal Fe-III ions with a chloride at the axial positions. The crystal structure of 3(rac) exhibits collinear chiral channels of similar to 11 angstrom in diameter making up 33.6% of the volume of the crystals, whereas the crystal structure of 3(RR) exhibits voids of 560 angstrom(3). Mossbauer spectroscopy demonstrates the presence of Fe-III high-spin ions. UV/Vis spectroscopy is in accordance with a large delocalized system in the central backbone evidenced by strong low-energy shifts of the imine pi-pi* transitions relative to that of the terminal units. Magnetic measurements reveal weak intramolecular exchange interactions but strong magnetic anisotropies of the FeIII ions. Complexes 3(rac) and 3(RR) are good catalysts for the sulfoxidation of sulfides providing very good yields and high selectivities with 3(RR) being enantioselective. A comparison of 3(RR) and [FeCl-(salen')] provides higher yields and selectivities but lower enantiomeric excess values (ee values) for 3(RR) relative to [FeCl(salen')]. The low ee values of 3(RR) appeared to be connected to a strong ligand folding in 3(RR), opening access to the catalytically active high-valent Fe-O species. The higher selectivity is assigned to a cooperative stabilization of the catalytically active high-valent Fe-O species through the phloroglucinol backbone in the trinuclear complexes.

Two dinuclear cobalt/copper compounds have been isolated from the reaction between N,N′-ethylenebis(salicylideniminato)cobalt(II), [Co(salen)], and copper(II) chloride in different conditions. The first one is a dinuclear cobalt(III)/copper(II) derivative, [Co(salen)Cl2Cu(EtOH)2Cl], 1, that have the cobalt atom six-coordinated to the four donor atoms of the salen ligand and to two chlorine atoms in a slightly distorted octahedral environment and the copper atom five-coordinated to the two bridging oxygen atoms of the salen ligand, two ethanol molecules and one extra chlorine atom. This compound is the only reported example of a cobalt/copper derivative with the cobalt maintaining the salen coordinative site, since the usual reaction takes place by a transmetalation process. This reaction is observed in the second derivative, [Cu(salen)CoCl2], 2, where the copper atom displaces the cobalt from the salen cavity. The copper atom adopts a square-planar coordinative environment, while the cobalt is tetrahedrically coordinated to the two bridging oxygen and two chlorine atoms. Both compounds present several intermolecular contacts that increase the dimensionality in the crystal and some of which can transmit magnetic interactions. The magnetic properties confirm the structural picture, with isolated copper(II) centres in 1, where the cobalt(III) is in the low spin form, and with antiferromagnetically coupled S = 1/2 and S = 3/2 centres in 2.

Condensation of the O-protected planar chiral hydroxyferrocene carbaldehyde (S p )-1 with ethylenediamine, followed by deprotection gave rise to the "ferro-salen" ligand (S p ,S p )-3 in good yield. This scaffold was used for the preparation of a series of metal complexes [(S p ,S p )-4/5/6], which were subsequently applied for the Lewis acid catalyzed asymmetric trimethylsilylcyanation of benzaldehyde.

The cobalt(III) complex of a salen ligand bearing one quaternary ammonium salt on the three-position of one aromatic ring is a highly active catalyst for the alternating copolymerization of cyclohexene oxide (CHO) and CO2 to afford the corresponding poly(cyclohexene carbonate) (PCHC) at various temperatures. The cobalt-based catalyst exhibited excellent activity and selectivity for polymer formation at high temperatures up to 120 °C and even under low CO2 pressures of 0.1 MPa. Also, the cobalt(III)-salen complex could operate very efficiently for the terpolymerization of CHO and aliphatic epoxides with CO2 to provide selectively polycarbonates with a narrow polydispersity at high temperatures. The polycarbonates resulting from the terpolymerization of equimolar CHO and propylene oxide (PO) with CO2 have a close content for both cyclohexene carbonate and propylene carbonate units. This is ascribed to the presence of CHO significantly inhibiting the reactivity of PO and thereby causing a matched reactivity ...

Inspired by pioneering contributions on chiral Bronsted acid mediated reactions and our own studies in aminocatalysis, we are currently exploring asymmetric counteranion-directed catalysis (ACDC) as a general strategy for asymmetric synthesis. According to this concept, catalytic reactions that proceed via cationic intermediates can be performed highly enantioselectively by the incorporation of a chiral counteranion into the catalyst. After our initial proof of concept with organocatalytic transfer hydrogenations and epoxidations, ACDC has recently been extended to transition-metal catalysis with Toste s gold-catalyzed allene cyclizations, and our palladium-catalyzed Tsuji–Trost-type a-allylation of aldehydes. The further exploration of the potential of ACDC in transition-metal catalysis, especially applied to redox reactions, is of great interest and significance. Here we report a highly enantioselective epoxidation of olefins that is catalyzed by a chiral ion pair consisting of an achiral Mn–salen cation and a chiral phosphate counteranion. Stimulated by an important contribution from Kochi et al. , Jacobsen and Katsuki have significantly advanced the catalytic asymmetric epoxidation of unfunctionalized alkenes by introducing chiral Mn–salen catalysts. These complexes display a broad substrate scope although certain olefin classes still fail to be converted with high enantioselectivity. Interestingly, cationic Mn–salen complexes are C2-symmetrical and inherently chiral—even when the salen ligand itself is achiral. In case of the Jacobsen–Kastuki epoxidation, the chiral backbone of the salen ligand fixes the complex in one of the two enantiomorphic confirmations. The neutral donor ligands typically added increase reactivity and enantioselectivity by displacing the apically coordinated anion of the Mn complex. Chiral neutral donor ligands such as sparteine and chiral N-oxides have been used in combination with an achiral Mn–salen complex and shown to shift the equilibrium of enantiomeric conformational isomers of the cationic metal complex to one side. Reasonably good enantioselectivities have been achieved in the corresponding epoxidations. We hypothesized that a chiral counteranion should also be able to induce a preference for one of the two enantiomorphic conformations. Specifically, chiral binol-derived phosphate anions are ideally suited for our purposes, because in addition to possibly inducing one enantiomorphic conformation of the cationic complex, these ions may also amplify the chiral microenvironment around the metal center with suitable substituents at the 3,3’-positions (Figure 1). Overall, this may lead to a new type of chiral Mn–salen catalyst with unique properties.

Alternating copolymerization of propylene oxide (PO) and carbon dioxide (CO2) was realized under mild conditions with a moderate turnover frequency (TOF), employing sole bifunctional cobalt salen complexes containing Lewis acid metal center and covalent bonded Lewis base on the ligand. Variation of the covalent bonded Lewis base substituents on the salen ligands could tailor the catalytic activity with TOF changing from 19.3 to 34.9 h(-1), polymeric/cyclic carbonate selectivity from 95.3 to 72.8%, and the head-to-tail structure in the polymer from 72.2 to 86.0%. The IR analysis confirmed that the Lewis base moiety on one molecule could coordinate with cobalt center of adjacent molecule, playing similar role to the Salen metal complex/Lewis base binary catalytic system. (C) 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48:359-365,2010

Chromium(III) derivatives of the tetradentate phenoxyamine ligand H2salan, where salan = the N,N′-dimethylated bis(aminophenoxide) ligand or the saturated version of the corresponding salen ligand, in the presence of 1 equiv of [PPN]N3 (PPN=Ph3P═N+═PPh3) are shown to effectively catalyze the copolymerization of cyclohexene oxide and carbon dioxide. X-ray crystallographic analysis reveals the structure of the complex to be different from that of its salen analogue, with an all cis arrangement of the nitrogen and oxygen atoms. Although these catalysts are selective for copolymerizing propylene oxide and CO2 at ambient temperature with a high degree of regioselectivity, the copolymerization of cyclohexene oxide and CO2 requires higher temperatures (e.g., 60 °C). Nevertheless, the random polymerization of cyclohexene oxide and propylene oxide with CO2 at ambient temperature provides a terpolymer with a nearly statistical distribution of monomer units. In addition, these catalysts have been shown to be efficie...

[Iron(III)–salen] complexes (salen = N,N′bis(salicylidene)ethylenediaminato) efficiently catalyze the H2O2 oxidation of organic sulfides and sulfoxides. The spectrophotometric kinetic studies show that these reactions follow Michaelis–Menten kinetics. The rate of the reaction is highly sensitive to the nature of the substituent present in the aryl moiety of ArSMe or ArS(O)Me and phenolic moiety of salen ligand. The plot of log k values of p-XC6H4SMe and p-XC6H4S(O)Me with Hammett σ constant give the reaction constant (ρ) values in the range of −0.7 to −1.5 and −0.7 to −1.0, respectively, for different iron(III)–salen complexes But the plot of log k values with σ gives positive ρ value when we introduce substituents in the phenolic moiety of iron(III)–salen complexes. The binding of the substrates with iron(III)–salen complexes is more pronounced with the sulfoxides. The product analyses show the selective oxidation of sulfides to sulfoxides and sulfoxides to sulfones. Based on the spectral and kinetic studies the possible mechanisms have been proposed.

Mononuclear and heterodinuclear complexes of the salen-type ligand H2LH2 [H2LH2 = 2,2′-[1,2-dihydroxybenzene-4,5-diylbis(nitrilomethylidine)]bis(3,5-di-tert-butylphenol)] were prepared, whereof [{Mn(CH3OH)2}LH2]Cl, [ML( ZrCp 2 ∗ )] (M = Ni, Cu, CrCl(py); py = pyridine, Cp∗ = pentamethylcyclopentadienyl) were characterized by X-ray analysis. Additionally, cyclic voltammetric investigations were performed to ascertain the influence of the second transition metal complex fragment [ ZrCp 2 ∗ ]2+ on the metallo salen ligand. Moreover, the complexes were tested in the catalytic epoxidation of styrene. Although the complexes are quite sensitive towards oxidants, monometallic complex [{Mn(CH3OH)2}LH2]Cl exhibited a conversion of 70.6% with styrene oxide selectivity of 88% over 1 h at room temperature.

Methyltrioxorhenium(VII) (MTO) forms 1:1 (mono-) or/and 2:1 (bis-) complexes with salen ligands, undergoing a hydrogen transfer from a ligand-bound OH-group to a ligand nitrogen atom. Some complexes show good stability both in the solid state and in solution, while others must be kept at low temperatures under an argon atmosphere. X-ray crystallography shows distorted trigonal bipyramidal structures of all examined complexes in the solid state, this structure being due to the steric demands of the ligands, with the methyl group of MTO residing in the apical sites in the cis position. Temperature-dependent proton NMR data indicate that the coordination between salen ligands and MTO at low temperatures is considerably stronger than at room temperature. Density functional theory calculations have been performed to find approximate structures for all described complexes and to try to find a rationale for the preferred formation of mono- versus bis-MTO complexes. The formation of mono- or bis-MTO adducts is dependent on both the steric and the electronic influence of the respective salen ligands. The catalytic performance is strongly influenced by the ring substitution. Two MTO molecules coordinated to one salen ligand lead to an additional boost of catalytic activity because there is not only double the amount of catalytic centers present but also a "ligand enhanced" activity increase.

Ethylenebis(5-chlorosalicylideneiminato)vanadium dichloride supported on MgCl2(THF)2 or on the same carrier modified by EtnAlCl3−n, where n = 1–3, was used in ethylene polymerization in the presence of MAO or a common alkylaluminium compounds as a cocatalyst. The support type alter vanadium loading and also change the characteristic of the catalytic active sites. Et2AlCl is the best activator for a catalyst which has been immobilized on a nonmodified support, whereas the systems which contain a carrier which has been modified by an organoaluminium compound reveal the highest activity in conjunction with MAO. That difference, together with different temperature effects on polymerization efficiency (i.e., decrease and increase of catalytic activity for increasing temperatures, respectively) suggest the formation of different types of active sites in the catalytic systems supported on modified and nonmodified magnesium carrier. However, all supported precatalysts possess a long lifetime, still being active towards ethylene polymerization after 2 h. All the systems yield wide MWD polyethylene, while bimodal MWD is found for some part of analyzed samples. Polyethylene with bimodal particle size distribution is formed with the system which contain modified carriers at higher temperatures. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 3480–3489, 2009

A polymeric ionic liquid (IL)-functionalized chiral salen ligand (PICL) was synthesized by covalent polymerization between amino ( NH2) group of 1,3-dipropylamineimidazolium bromide with chloromethyl ( CH2Cl) group at two sides of 5,5′ positions in the typical chiral salen ligand. Treatment of the synthesized PICL with Mn(OAc)2⋅4H2O and LiCl under aerobic oxidation yielded the corresponding polymeric IL-functionalized chiral salen Mn(III) complex (PICC). The typical IR bands at 1613, 1540, 570, and 412 cm−1, as well as the maximum UV–vis absorbed peaks around 433 nm of the PICC were proposed as characteristics of the monomeric salen Mn(III) complex. The PICC was used as a catalyst in the enantioselective epoxidation of styrene. Comparable catalytic activity and enantioselectivity relative to the monomeric chiral salen Mn(III) complex were observed. Furthermore, recovery of the polymeric catalyst was readily accomplished by simple precipitation in n-hexane, and subsequently reused (10 times) without significant loss of reactivity and enantioselectivity.

The reaction of uranyl acetate with (2,2′-(1E,1′E)-(2-benzyl-3-hydroxyquinoxaline-6,7-diyl)bis(azan-1-yl-1-ylidene)bis(methan-1-yl-1-ylidene) diphenol) (H2L1) at room temperature in methanol and chloroform yields the UO2L1 complex. Crystals were grown through solvent diffusion of the ligand–metal complex in dimethyl formamide with diethyl ether to prepare: UO2L1 · DMF (1). Complexes with 2,2′-(1E,1′E)-(2-benzyl-3-hydroxyquinoxaline-6,7-diyl)bis(azan-1-yl-1-ylidene)bis(methan-1-yl-1-ylidene)dibenzene-1,4-diol (H2L2) and 2,2′-(1E,1′E)-(2-hydroxy-3-isopropylquinoxaline-6,7-diyl)bis(azan-1-yl-1-ylidene)bis(methan-1-yl-1-ylidene)diphenol (H2L3) were also prepared, and crystals of the uranyl complexes (UO2L2 · DMF (2) and (3)) grown from DMF/ether. A fourth complex UO2L4 · H2O (4) was prepared through layering a solution of the tetra-tert-butyl substituted 2-quinoxalinol salen ligand H2L4 in acetone with an aqueous solution containing uranyl acetate. The complexes exhibit a symmetric core featuring a slightly distorted bicapped pentagonal geometry around the uranium center with two oxo-groups and two imine groups from the ligand chelating the ligand and the fifth site in the coordination plane of the ligand occupied by a solvent molecule. These compounds have been characterized using solution (NMR and UV–Vis) and solid-state (IR, X-ray crystallography) techniques. Complexes of H2L4 with early transition metals; Mn2+, Co2+, Ni2+, and Cu2+ are also prepared and characterized for comparison of solution and spectroscopic characteristics.

The coordination chemistry of 4-(N,N-dimethylamino)pyridine (DMAP) with Salen- or Salan- (where the tetradentate N,N'-disubstituted bisaminophenoxide is designated as Salan, a saturated version of the Schiff-base Salen ligand) chromium complexes was studied by electrospray ionization mass spectrometry (ESI-MS). The relative stabilities of mono DMAP adducts of these chromium complexes were characterized by collision-induced dissociation (CID) and further discussed with regard to the activity in catalyzing CO(2)/epoxide copolymerization. [SalenCr](+) cations preferably bind two DMAP molecules to form six-coordinated complex ions, while [SalanCr](+) cations usually bind one molecule of DMAP to form five-coordinated complex ions, which were found to be relatively unstable. The remarkable difference in the coordination of DMAP to these two chromium complexes resulted in a significant difference in catalytic activity for the alternating copolymerization of CO(2) and propylene oxide. In the presence of 1 equiv of DMAP, the activity of the chromium-Salan complex 2a was up to 86 h(-1) of TOF at ambient temperature, which was about 30 times that of the corresponding chromium-Salen complex 1a. In sharp contrast to a long induction period up to 2 h with the use of 1a in conjunction with DMAP as catalyst, no initiation time or a very short one was observed in the binary 2a/DMAP catalyst systems. The initiator role of DMAP was confirmed by continuous determination of the propagating polymer species at various intervals using ESI-MS, which in combination with a kinetic study by means of infrared spectroscopy resulted in a mechanistic understanding on the difference in activity of the two catalyst systems for CO(2)/epoxide copolymerization.

Reaction of silver nitrite with Ru(salen)(PPh(3))(Cl) (salen(2-)=N,N'-ethylenebis(salicylideneiminato dianion) using [NEt(4)]OH as a phase transfer agent led to the formation of a stable, N-coordinated nitrite ion complex (Ru(III)-NO(2)) that was characterized by single crystal X-ray diffraction. This reaction also resulted in nitration of the two phenolic rings of the salen ligand, which was unexpected given the basic conditions. With sodium nitrite, the Ru(III)-nitro complex was formed, but no ring nitration was observed. Thus, in a non-acidic medium, the combination of nitrite with redox active metal centers may provide a route to the nitration of phenolic derivatives. Possible mechanisms of this unusual reaction having potential relevance to aromatic nitrations in biological systems are discussed. This is also the first report of the crystal structure of a Ru(III)-nitro complex, a species that has been suggested to be unstable and prone to rapid disproportionation.

The asymmetric alkynylation of aldehydes catalyzed by bifunctional Zn(salen) catalyst was realized. With a Kozlowski salen ligand (7e) bearing secondary basic 1-piperidinylmethyl groups at C-3 and C-3′, the enhanced catalytic reactivity and stereoselectivity in comparison with that of normal salen ligands in the alkynylzinc addition to aldehydes were observed.

A copper(II) complex of the 2-quinoxalinol salen ligand (salquCu) 1 has been tested for use in catalysis. Here, an optimized method for oxidation of aryl methylenes and its potential applications are described. In organic solvents, the yields obtained are higher than with other commonly used catalytic methods. Because this methods does not require high heat or increased pressure, this presents an opportunity for more environmentally friendly or “green” chemistry in a single-phase system. Using this method, a key fragment of natural products Vitamin K1 and K2, 1,4-naphthoquinone, can be easily synthesized from 1,2,3,4-tetrahydronaphthalene in increased yield (65 %) as compared to established methods (30–40 % yield) that require higher temperatures and increased pressure. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)

In situ X-ray absorption spectroscopy at the Mn K- and Br K-edge was employed to study the cycloaddition of carbon dioxide to propylene oxide and styrene oxide, catalyzed by Mn(III) salen bromide complexes. Three homogeneous complexes with varying salen ligand structure and one complex immobilized on silica were used and compared. Ex situ X-ray absorption spectroscopy showed that in all cases octahedral complexes in a distorted symmetry were present and that the bromide ligand was coordinated to the Mn central atom. Under reaction conditions the structure of the catalysts strongly changed. In order to investigate this change, a special spectroscopic batch reactor cell was designed that allowed continuous monitoring of catalyst structure under cycloaddition conditions directly at the synchrotron radiation source, i.e. while maintaining the demanding conditions of elevated pressure and temperature. Complementary XAS investigations on the two key elements of the catalyst, the manganese central atom and the bromide ligand, showed that the coordination sphere of the hexacoordinated manganese central atom changed due to the replacement of the bromo-ligand probably by an epoxide molecule under reaction conditions. This was the case for all three homogeneous catalysts with varying salen ligand structure as well as for the heterogeneous catalyst. In case of the heterogeneous catalyst bromide also went into the solution whereas most of the manganese remained on the solid support. The loss of the direct coordination of the bromine neighbors to the Mn central atom was also evidenced by EXAFS spectra, e.g. loss of Br backscattering in the Mn K-EXAFS spectra and the Mn-backscattering in the Br K-edge spectra. In the catalytic studies it was observed that propylene oxide usually reacted much faster than styrene oxide. This seems to be related to a faster coordination of propylene oxide to the Mn atom, which could be followed in a time-resolved manner at the Br K-edge. On the basis of the analysis of the XANES and EXAFS data together with observations from literature a reaction mechanism is proposed.

Ethylenebis(5‐chlorosalicylideneiminato)vanadium dichloride supported on MgCl2(THF)2 or on the same carrier modified by EtnAlCl3−n, where n = 1–3, was used in ethylene polymerization in the presence of MAO or a common alkylaluminium compounds as a cocatalyst. The support type alter vanadium loading and also change the characteristic of the catalytic active sites. Et2AlCl is the best activator for a catalyst which has been immobilized on a nonmodified support, whereas the systems which contain a carrier which has been modified by an organoaluminium compound reveal the highest activity in conjunction with MAO. That difference, together with different temperature effects on polymerization efficiency (i.e., decrease and increase of catalytic activity for increasing temperatures, respectively) suggest the formation of different types of active sites in the catalytic systems supported on modified and nonmodified magnesium carrier. However, all supported precatalysts possess a long lifetime, still being active towards ethylene polymerization after 2 h. All the systems yield wide MWD polyethylene, while bimodal MWD is found for some part of analyzed samples. Polyethylene with bimodal particle size distribution is formed with the system which contain modified carriers at higher temperatures.

A symmetrical salen ligand is formed by the condensation of (1R,2R)-diaminocyclohexane with 4-hydroxy-3-phenanthrenecarboxaldehyde. Crystallographic characterization of the metal complexes (ZnII, FeII, and FeIII) show 1:1 mixtures of diastereomeric M and P helical molecules. ECD spectra of the ZnII complex combined with DFT simulations strongly suggest that the M conformation is dominant in solution. This is supported by 1H and 13C NMR data, which are consistent with a single species in solution.Key words: zinc, iron, ECD, DFT, salen complexes.

The new tetra dentate dianionic H2PS (N,N'-dipyridoxyl (1,3-propylenediamine)) Schiff-base ligand and its octahedral Co(III) salen complex [Co(PS)(H2O)(CH3OH)]+CH3COO(-) were synthesized, where coordinating atoms of H2PS (N,N,O(-),O(-)) occupied equatorial positions with H2O and CH3OH as axial ligands. The nature of the H2PS and its complex were determined by elemental and spectrochemical (IR, UV-vis, 1H NMR and Mass) analysis. Also, the fully optimized geometries and vibrational frequencies of them together with the 1H NMR chemical shifts of H2PS have been calculated using density functional theory (B3LYP) method. Obtained structural parameters are in good agreement with the experimental data reported for similar compounds. The calculated and experimental results confirmed the suggested structures for the ligand and complex.

Salens, derived from 1,2-ethylenediamine and salicylaldehydes, have been widely used as ligands for metal complexes which have been showing enormous potential in chemical properties of asymmetric catalysts as well as biological properties such as anticancer agents. Almost all of the salen–metal complexes with their corresponding metal (II)-complexes show the evidences of chelation of two oxygens in salens. However, several metal (II) complexes, especially cobalt (II) complexes, could not show NMR spectra due to their paramagnetism. Recently, it has been reported that one of the cobalt (III) complexes was used for NMR spectroscopy to evaluate its stereoselectivity as a catalyst. Even though many salen ligands are known, their NMR data are not assigned completely. It was possible that modification in northern part of salen with 2-hydroxyphenyl group afforded another oxygen chelation site in salen ligand. Here we report that synthesis and full NMR assignment of new salen ligands, which form meso 1,2-bis(2-hydroxyphenyl)ethylenediamine) and their cobalt (III) complexes. The assignments of 1H and 13C NMR data obtained in this experiment can help us to predict the NMR data of other salen ligands. Copyright © 2008 John Wiley & Sons, Ltd.

Vanadium complexes with tetradentate salen-type ligands were first time explored in ethylene polymerizations. The effects of the vanadium complex structure, the alkyl aluminum cocatalysts type (EtAlCl2, Et2AlCl, Et3Al, and MAO), and the polymerization conditions (Al/V molar ratio, temperature) on polyethylene yield were explored. It was found that EtAlCl2 in conjunction with investigated vanadium complexes produced the most efficient catalytic systems. It was shown, moreover, that the structural changes of the tetradentate salen ligand (type of bridge which bond donor nitrogen atoms and type of substituent on aryl rings) affected activity of the catalytic system. The complexes containing ligands with cyclohexylene bridges were more active than those with ethylene bridges. Furthermore, the presence of electron-withdrawing groups at the para position and electron-donating substituents at the ortho position on the aryl rings of the ligands resulted in improved activity in relation to the systems with no substituents (with the exception of bulky t-Bu group). The results presented also revealed that all vanadium complexes activated by common organoaluminum compounds gave linear polyethylenes with high melting points (134.8–137.6 °C), high molecular weights, and broad molecular weight distribution. The polymer produced in the presence of MAO possesses clearly lower melting point (131.4 °C) and some side groups (around 9/1000 C). © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 6940–6949, 2008

Chiral chromium complexes of tetradentate N,N'-disubstituted bis(aminophenoxide) (designated as Salan, a saturated version of Schiff-base Salen ligand) in conjunction with an ionic quaternary ammonium salt can efficiently catalyze the copolymerization of CO 2 with racemic propylene oxide (rac-PO) at mild conditions to selectively afford completely alternating poly(propylene carbonate) (PPC) with ∼ 95% head-to-tail linkages and moderate enantioselectivity. These new catalyst systems predominantly exceed the previously much-studied SalenCr(III) systems in catalytic activity, polymer enantioselectivity, and stereochemistry control. The chiral diamine backbone, sterically hindered substitute groups on the aromatic rings, and the presence of sp 3 -hydridized amino donors and its N,N'-disubstituted groups in chiral SalanCr(III) complexes all play significant roles in controlling polymer stereochemistry and enantioselectivity. Furthermore, a relationship between polycarbonate enantioselectivity and its head-to-tail linkages in relation to regioselective ring-opening of the epoxide was also discussed on the basis of stereochemical studies of PPCs derived from the copolymerization of CO 2 with chiral PO at various conditions.

The catalytic activity of Mn(salen)Cl containing tert-pentyl groups at the 3,5-positions of the salen ligand in the epoxidation of alkenes and hydroxylation of alkanes was studied at room temperature, using sodium periodate as an oxygen source. The effects of various axial ligands were investigated in the epoxidation of cyclooctene. Imidazole, as a strong π-donor ligand, was the best axial ligand. The effect of different solvents was studied in the epoxidation of cyclooctene in CH3CN/H2O solvent mixture. The epoxidation reactions of cyclooctene by different oxygen donors including NaIO4, Bu4NIO4, KHSO5, H2O2, H2O2/urea, NaOCl and tert-BuOOH were also studied and NaIO4 was selected as oxygen source. The presence of bulky substituents in the 3,5-positions of the salen ligand was found to increase the catalytic activity of this complex.

The Co(III)--salen-catalyzed (salen=N,N'-bis(salicylidene)ethylenediamine dianion) hydrolytic kinetic resolution (HKR) of racemic epoxides has emerged as a highly attractive and efficient method of synthesizing chiral C(3) building blocks for intermediates in larger, more complex molecules. HKR reaction rates have displayed a second order dependency on the concentration of active sites, and thus researchers have proposed a bimetallic transition state for the HKR mechanism. Here we report the utilization of pendant Co(III)--salen catalysts on silica supported polymer brushes as a catalyst for the HKR of epichlorohydrin. The novel polymer brush architecture provided a unique framework for promoting site-site interactions as required in the proposed bimetallic transition state of the HKR mechanism. Furthermore, the polymer brushes mimic the environment of soluble polymer-based catalysts, whereas the silica support permitted facile recovery and reuse of the catalyst. The polymer brush catalyst displayed increased activities over the soluble Jacobsen Co--salen catalyst and was observed to retain its high enantioselectivities (>99 %) after each of five reactions despite decreasing activities. Analysis indicated decomposition of the salen ligand as an underlying cause of catalyst deactivation.

Excellent enantioselectivities and isolated yields have been achieved for the hydrolytic kinetic resolution of epoxides using a resin-supported dendronized R,R-(salen)Co catalyst with catalyst loadings as low as 0.04 mol%, the lowest metal loadings of any heterogeneous resin-supported (salen)Co catalyst reported to date. In addition, the supported catalysts can be recycled and reused with comparable enantioselectivities. It is hypothesized that the high catalytic activity can be attributed to the flexible linker and the dendronized framework supporting the (salen)Co moieties on the resin thereby promoting cooperativity between two metal centers. This work opens up new opportunities for the design of highly active resin-supported catalysts that catalyze transformations through a bimetallic pathway.

Salen ligand synthesized from ( 1R , 2R )-(-)-1,2-diaminocyclohexane and 2-hydroxy-3(3-triethoxysilyl-propyl)-benzaldehyde has been used as manganese chelate for the immobilization of the catalytic site. Sol–gel hydrolysis and polycondensation of this chelate with ten equivalents of tetraethyl orthosilicate produced the heterogeneous catalyst. This catalyst is active in enantioselective epoxidations of alkenes in 61–76% yield and 54–86% e.e., using sodium hypochlorite as the primary oxidant. Additionally the immobilized catalyst shows an improvement in the enantioselectivity when compared to the homogeneous Mn(III)–salen analog.

Reaction of [Ru (VI)(N)(L (1))(MeOH)] (+) (L (1) = N, N'-bis(salicylidene)- o-cyclohexylenediamine dianion) with excess pyridine in CH 3CN produces [Ru (III)(L (1))(py) 2] (+) and N 2. The proposed mechanism involves initial equilibrium formation of [Ru (VI)(N)(L (1))(py)] (+), which undergoes rapid N...N coupling to produce [(py)(L (1))Ru (III) N N-Ru (III)(L (1))(py)] (2+); this is followed by pyridine substituion to give the final product. This ligand-induced N...N coupling of Ru (VI)N is utilized in the preparation of a series of new ruthenium(III) salen complexes, [Ru (III)(L)(X) 2] (+/-) (L = salen ligand; X = H 2O, 1-MeIm, py, Me 2SO, PhNH 2, ( t )BuNH 2, Cl (-) or CN (-)). The structures of [Ru (III)(L (1))(NH 2Ph) 2](PF 6) ( 6), K[Ru (III)(L (1))(CN) 2] ( 9), [Ru (III)(L (2))(NCCH 3) 2][Au (I)(CN) 2] ( 11) (L (2) = N, N'-bis(salicylidene)- o-phenylenediamine dianion) and [N ( n )Bu 4][Ru (III)(L (3))Cl 2] ( 12) (L (3) = N, N'-bis(salicylidene)ethylenediamine dianion) have been determined by X-ray crystallography.

The series of salen-type complexes differing in the metal center (M = Ti, Zr, V) and in the coordination spheres (different substituent at salen ligands) after activation with methylalumoxane (MAO) or Et 2 AlCl were studied as catalytic systems in ethylene polymerization. The vanadium and titanium complexes were found to be more active in combination with alkylaluminum compound (Et 2 AlCl) then with MAO, while zirconium complex was active only in combination with MAO. The activity of complexes activated by Et2AlCl decreases in the order V > Ti » Zr ≡ 0, and when MAO is used as the cocatalyst the activity changes the order: Ti > Zr > V. The substituent type in the salen ligand also affects the catalyst activity; the best results were obtained for the catalyst having Cl or Br as substituents. The type of substituent has no significant effect on polyethylene properties, while the type of cocatalyst and the type of metallic centre play here the essential role. The salen titanium complexes in combination with Et 2 AlCl usually produce low molecular weight polyethylene with narrow polydispersity, also accompanied by oligomers. Other catalytic systems give linear high molecular weight polyethylene (M w = 200 000-700 000).

Chromium salen derivatives in the presence of anionic initiators have been shown to be very effective catalytic systems for the selective coupling of oxetane and carbon dioxide to provide the corresponding polycarbonate with a minimal amount of ether linkages. Optimization of the chromium(III) system was achieved utilizing a salen ligand with tert-butyl groups in the 3,5-positions of the phenolate rings and a cyclohexylene backbone for the diimine along with an azide ion initiator. The mechanism for the coupling reaction of oxetane and carbon dioxide has been studied. Based on binding studies done by infrared spectroscopy, X-ray crystallography, kinetic data, end group analysis done by (1)H NMR, and infrared spectroscopy, a mechanism of the copolymerization reaction is proposed. The formation of the copolymer is shown to proceed in part by way of the intermediacy of trimethylene carbonate, which was observed as a minor product of the coupling reaction, and by the direct enchainment of oxetane and CO 2. The parity of the determined free energies of activation for these two processes, namely 101.9 kJ x mol (-1) for ring-opening polymerization of trimethylene carbonate and 107.6 kJ x mol (-1) for copolymerization of oxetane and carbon dioxide supports this conclusion.

A chiral salen ligand containing a carboxylic acid group was attached to macroporous monosized polystyrene-divinylbenzene beads grafted with (4-hydroxybutyl) vinyl ether groups. A Ti(IV) complex ...