Tuning ligand electronics and peripheral substitution on cobalt salen complexes: structure and polymerisation activity.

A series of cobalt salen complexes, where salen represents an N2O2 bis-Schiff-base bis-phenolate framework, are prepared, characterised and investigated for reversible-termination organometallic mediated radical polymerisation (RT-OMRP). The salen ligands contain a cyclohexane diimine bridge and systematically altered para-substituted phenoxide moieties as a method to examine the electronic impact of the ligand on complex structure and reactivity. The complexes are characterised by single crystal X-ray diffraction, cyclic voltammetry, X-ray photoelectron spectroscopy, electron paramagnetic resonance spectroscopy and computational methods. Structural studies all support a tailorable metal centre reactivity altered by the electron-donating ability of the salen ligand. RT-OMRP of styrene, methyl methacrylate and vinyl acetate is reported and suggests that cobalt-carbon bond strength varies with the ligand substitution. Competing ß-hydrogen abstraction affords long-chain olefin-terminated polymer chains and well controlled vinyl acetate polymerisations, contrasting with the lower temperature associative exchange mechanism of degenerative transfer OMRP.

Organometallic mediated radical polymerization of vinyl acetate using bis(imino)pyridine vanadium trichloride complexes.

The synthesis and characterization of one novel proligand and six novel vanadium(III) trichloride complexes is described. The controlled radical polymerization activity towards vinyl acetate of these, and eight other bis(imino)pyridine vanadium trichloride complexes previously reported, is investigated. Those complexes possessing variation at the N-aryl para-position with no steric protection offered by ortho-substituents (4 examples) result in poor control over poly(vinyl acetate) polymerization. Control is improved with increasing steric bulk at the ortho-position of the N-aryl substituent (4 examples) although attempts to increase steric bulk past isopropyl were unsuccessful. Synthesizing bis(imino)pyridine vanadium trichloride complexes with substituted imine backbones restores polymerization control when aliphatic substituents are used (4 examples) but ceases to make any drastic improvements on catalyst lifetime. Modification of the polymerization conditions is also investigated, in an attempt to improve the catalyst lifetime. Expansion of the monomer scope to include other vinyl esters, particularly those derived from renewable resources, shows promising results.

Controlled radical polymerization mediated by amine-bis(phenolate) iron(III) complexes.

Tetradentate amine-bis(phenolate) iron(III) halide complexes containing chloro substituents on the aromatic ring are extremely efficient catalysts for controlled radical polymerization. Molecular weights are in good agreement with theoretical values and polydispersity indexes (PDIs) are as low as 1.11 for styrene and methyl methacrylate polymerizations. Complexes containing alkyl substituents on the aromatic ring are less efficient. Kinetic data reveal activity for styrene polymerization among the fastest reported to date and initial studies implicate a multimechanism system. Despite the highly colored polymerization media, simple work-up procedures yield pure white polymers.

Origins of stereoselectivity in optically pure phenylethaniminopyridine tris-chelates M(NN')3(n+) (M = Mn, Fe, Co, Ni and Zn).

One-pot reactions of 2-pyridinecarboxaldehyde, chiral phenylethanamines and Fe(II) give single diastereomer fac diimine complexes at thermodynamic equilibrium so that no chiral separations are required (d.r. > 200 : 1). The origins of this stereoselectivity are partly steric and partly a result of the presence of three sets of inter-ligand parallel-offset π-stacking interactions. Mn(II), Co(II), Co(III), Ni(II) and Zn(II) give similar fac structures, alongside the imidazole analogues for Fe(II). While most of the complexes are paramagnetic, the series of molecular structures allows us to assess the influence of the π-stacking present, and there is a strong correlation between this and the M-N bond length. Fe(II) is close to optimal. For the larger Zn(II) ion, very weak π-stacking leads to poorer measured stereoselectivity (NMR) but this is improved with increased solvent polarity. The mechanism of stereoselection is further investigated via DFT calculations, chiroptical spectroscopy and the use of synthetic probes.

Structural and electronic modulation of magnetic properties in a family of chiral iron coordination polymers.

The complexes FeL(2) [L = bidentate Schiff base ligands obtained from (R)-(+)-α-phenylethanamine and 4-substituted salicylaldehydes, substituent R = H, (t)Bu, NO(2), OMe, CN, OH] react with ditopic proligands 1,4-pyrazine (pz) or 4,4'-bipyridine (bpy), to give a family of optically pure Fe(II) polymeric chain complexes of formula {FeL(2)(µ-pz)}(∞) and {FeL(2)(µ-bpy)}(∞). Crystallographic studies show that a range of structures are formed including unidirectional and bidirectional linear polymers and canted zigzag chains. Interchain interactions via π-contacts and hydrogen bonding are also observed. SQuID magnetometry studies on all of the complexes reveal antiferromagnetic interactions, the magnitudes of which are rationalized on the basis of substituent electronic properties and bridging ligand identity. For complexes with bridging pz, the antiferromangnetic interaction is enhanced by electron-releasing substituents on the Fe units, and this is accompanied by a contraction in the intrachain distance. For complexes bridged with the longer bpy the intrachain antiferromagnetic couplings are much weaker as a result of the longer intrachain distance. The magnetic data for this series of chain complexes follow a Bonner-Fisher 1D chain model, alongside a zero field splitting (ZFS) model for Fe(II) (S = 2) as appropriate. The intrachain antiferromagnetic coupling J values, g-factors, and the axial ZFS parameter D were obtained.

TTF salts of optically pure cobalt pyridine amidates; detection of soluble assemblies with stoichiometry corresponding to the solid state.

Optically pure anionic complexes of pyridinecarboxamide ligands, N(2),N(6)-bis((R)-α-methylbenzyl)pyridine-2, 6-dicarboxamide H(2)(R,R-L(1)) and N(2),N(6)-bis((S)-1-methoxypropan-2-yl)pyridine-2, 6-dicarboxamide H(2)(S,S-L(2)) have been synthesised and fully characterised. The complexes: (18-crown-6)K[Co(III)(R,R-L(1))(2)], (18-crown-6)K[Fe(III)(R,R-L(1))(2)] and K[Co(III)(S,S-L(2))(2)]·3H(2)O show interesting extended structures from 0D discrete units through 1D zigzag chains to 2D honeycomb layers. The complex anions were used in the synthesis of radical cation salts with tetrathiafulvalene (TTF). The salts (TTF)[Co(III)(R,R-L(1))(2)] and (TTF)[Co(III)(S,S-L(2))(2)]·EtOAc were characterised by single crystal X-ray diffraction and conductivity measurements. Both compounds comprise mono-oxidised TTF molecules and exhibit similar layered structures with no direct TTF stacking but in which phenyl substituents from the complex anion or co-crystallised ethyl acetate alternate with TTF(+) units. Solution spectroscopic and cyclic voltammetric evidence points to the formation of soluble assemblies between TTF(+) and the counterion which correspond to the stoichiometry observed by crystallography and other methods in the solid state.

Mechanism of catalytic cyclohydroamination by zirconium salicyloxazoline complexes.

The mechanism of hydroamination/cyclization of primary aminoalkenes by catalysts based on Cp*LZr(NMe(2))(2) (L = κ(2)-salicyloxazoline) is investigated in a range of kinetic, stoichiometric, and structural studies. The rate law is found to be d[substrate]/dt = k[catalyst](1)[substrate](0) for all catalysts and aminoalkenes studied. The overall rate is similar for formation of five- and six-membered rings, and a substantial KIE (k(H)/k(D)) is observed, indicating the involvement of N-H bond-breaking in a rate-determining step (RDS) which is not ring-closure. Remarkably, the reaction proceeds at the same rate in THF as it does in toluene, but added non-cyclizable amine slows the reaction, indicating that while the metal is not acting as a Lewis acid in the RDS, the activated substrate is involved. Also in contrast to other catalysts, increasing steric bulk improves the rate, and the origins of this are investigated by X-ray crystallography. Thermodynamic parameters extracted from eight independent kinetic studies indicate moderate ordering (ΔS(double dagger) = -13 to -23 cal/K·mol) and substantial overall bond disruption (ΔH(double dagger) = 17 to 21 kcal/mol) in the rate-determining transition state. Secondary amines are unreactive, as is a catalyst with a single aminolyzable site, thus excluding an amido mechanism. A catalytic cycle involving rate-determining formation of a reactive imido species is proposed. Stoichiometric steps in the process are shown to be feasible and have appropriate rates by synthetic and in situ NMR spectroscopic studies. The fate of the catalyst in the absence of excess amine (at the end of the catalytic reaction) is conversion to a metallacyclic species arising from CH activation of a peripheral substituent.

Organic-soluble optically pure anionic metal complexes PPh4[M(III)(S,S-EDDS)].2H2O (M = Fe, Co, Cr).

The first organic-soluble, optically and diastereomerically pure EDDS metal complexes have been synthesised. A number of synthetic approaches were attempted, but finally the tetraphenylphosphonium series emerged as providing readily accessible compounds of trivalent Cr, Fe and Co in reasonable yields via the silver salts without the need to perform ion-exchange chromatography. The species PPh(4)[M(III)(S,S-EDDS)] are very soluble in methanol, acetonitrile and even THF but isolation was facilitated by addition of stoichiometric water giving the highly crystalline but still conveniently soluble title compounds. The structures of the three isomorphous crystals comprise H(2)O-bridged extended hydrogen bonded structures with large channels occupied by the counterion molecules. The magnetic properties and circular dichroism spectra are reported along with comparative data for water-soluble NH(4)[Fe(III)(S,S-EDDS)]. Phase purity (and hence diastereomeric purity) in the paramagnetic systems is assessed through powder XRD. The practical utility of this type of compound was confirmed by optical resolution of (+/-)-[Ru(II)(bpy)(3)]Cl(2).

Self-assembling optically pure Fe(A-B)3 chelates.

Optically pure, single diastereomer fac-tris(diimine) complexes of Fe(II) are available from a remarkably facile one-pot procedure using a range of readily available (R)-2-phenylglycinol derivatives.

Correlation of metal spin-state in alpha-diimine iron catalysts with polymerization mechanism.

The alpha-diimine iron complexes, (R',R'')[N,N]FeCl(2) ((R',R'')[N,N] = R'-N=CR' '-CR' '=N-R', where R' = tert-butyl (tBu), cyclohexyl (Cy) and R' ' = phenyl (Ph), para-fluorophenyl (F-Ph), para-bromophenyl (Br-Ph), para-methylphenyl (Me-Ph), or para-methoxyphenyl (MeO-Ph)), are found to polymerize styrene through a catalytic chain transfer (CCT) mechanism. Magnetic moment measurements indicate that Fe(III) complexes containing these ligands possess intermediate (S = 3/2) spin-state iron centers. In contrast, Fe(III) complexes bearing proton (R' ' = H) and para-dimethylaminophenyl (R' ' = NMe(2)-Ph) substituents are high-spin and are efficient atom transfer radical polymerization (ATRP) catalysts. Hammett plots show a linear correlation of the substituent constant, sigma, with polymerization rate and polymer molecular weight, respectively.