[Show abstract][Hide abstract] ABSTRACT: The mechanism of ethylene trimerization and tetramerization with a chromium-diphosphinoamine (Cr-PNP) catalyst system has been studied with combined experimental and theoretical methods. Of the total product output, 1-octene, cyclopentanes, n-alkanes and higher (C10+) olefins are formed with a fractional (ca. 1.4 overall) order response to ethylene concentration, while 1-hexene formation is approximately first order in ethylene. Theoretical studies suggest a mechanism involving a cationic monometallic catalyst in Cr(I) and Cr(III) formal oxidation states. A key feature of the developed model is the occurrence of a double coordination mechanism, where a bis(ethylene) chromacyclopen-tane intermediate is responsible for 1-octene formation as well as the other co-products which have a greater than first order response to ethylene. In contrast, 1-hexene is formed primarily from a mono(ethylene) chromacyclopentane in-termediate. The selectivity of catalysis is governed by the competition between single and double coordination path-ways. The mechanistic model developed displays excellent correlation with experimental observations, and is able to fully explain the formation of all products generated with this catalyst.
[Show abstract][Hide abstract] ABSTRACT: Several benzofurobenzofuran and methanodibenzodioxocine iminopyridine derivatives have been used as ligands to form mononuclear and dinuclear iron complexes. Complexes 6, 7, 8, 9 and 10 were able to promote the catalytic oxidation of benzylic C-H bonds to ketones in moderate to high yields. The effects of backbone scaffold, nuclearity (mononuclear versus dinuclear) and nitrogen hybridization (iminopyridine versus aminopyridine) were studied. A strong effect on the yields of the nature and position of the substituents in the substrates was observed.
[Show abstract][Hide abstract] ABSTRACT: A series of potentially tetradentate and pentadentate ligands modelled on BPMEN has been prepared and their iron(ii) bis(triflate) complexes have been isolated and characterised by spectroscopic and crystallographic techniques (BPMEN = N,N'-bis(pyridylmethyl)ethylenediamine). Changes to the BPMEN ligand have invariably led to complexes with different coordination modes or geometries and with inferior catalytic efficiencies for the oxidation of cyclohexane with H2O2. The reaction of an iron(ii) complex containing a pentadentate BPMEN-type ligand with O2 has resulted in ligand degradation via oxidative N-dealkylation and the isolation of a bis(hydroxo)-bridged dinuclear iron(iii) complex with a picolinate-type ligand.
[Show abstract][Hide abstract] ABSTRACT: Platinum(II) and palladium(II) complexes [M(CH3)(L)]SbF6 with substituted terpyridine ligands L undergo light-driven oxygen insertion reactions into metal methyl bonds resulting in methylperoxo complexes [M(OOCH3)(L)]SbF6. The oxygen insertion reactions occur readily for complexes with methyl ligands that are activated due to steric interaction with substituents (NH2, NHMe or CH3) at the 6,6"-positions on the terpyridine ligand. All complexes exhibit attractive intramolecular π···π or M···M interactions in the solid state and in solution, which lead to excited triplet dinuclear M-M complexes upon irradiation. A mechanism is proposed whereby a dinuclear intermediate is generated upon irradiation that has a weakened M-C bond in the excited state, resulting in the observed oxygen insertion reactions.
Journal of the American Chemical Society 09/2014; 136(40). DOI:10.1021/ja5055143 · 12.11 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: A bio-inspired manganese(ii) complex with a linear pentadentate ligand framework containing soft sulfur donors and an alternating NSNSN binding motif displays excellent dual CAT/SOD-like antioxidant activity with high turnover efficiency and good operation stability in an aqueous environment.
Chemical Communications 03/2014; 50(35). DOI:10.1039/c4cc00758a · 6.83 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Square-planar palladium(II) and platinum(II) methyl complexes with terpyridine and 6,6″-diamino terpyridine ligands undergo methyl exchange reactions upon exposure to light. The half-life of the methyl exchange reactions correlates with the relative Pd–C and Pt–C bond strengths. No exchange between methyl and phenyl groups is observed, probably due to the stronger Pt–C bond in the platinum phenyl complex. A mechanism is proposed whereby a dinuclear intermediate is generated upon irradiation that has a weakened M–C bond in the excited state, resulting in the observed methyl exchange reactions.
[Show abstract][Hide abstract] ABSTRACT: A model for the homogeneous Cr-PNP (PNP≤diphosphinoamine) ethylene trimerisation and tetramerisation catalyst system has been studied theoretically, with the aim of identifying suitable density functional theory methods for treatment of this catalyst, and evaluating the likely oxidation and spin states of the active species. Benchmarking studies involving high-level treatment reveal the difficulty of accurately calculating the thermochemistry of this system, and suggest that local density functionals, such as M06L, probably provide the best option. Density functional theory modelling of catalyst activation and the first steps of oligomerisation up until 1-hexene formation appears to favour a CrI-CrIII mechanism, involving spin surface crossing from sextet to quartet states.
Australian Journal of Chemistry 01/2014; 67(10). DOI:10.1071/CH14436 · 1.56 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Octahedral, tetrahedral, and square planar geometries are the most often encountered coordination geometries for transition metal complexes. In certain cases, coordination equilibria can exist between different geometries, such as between six- and four-coordinate geometries in nickel(II) complexes, which were discovered half a century ago. Here, we present the first examples of a seven-five coordination equilibrium. Extensive spectroscopic studies in solution have provided evidence for a dynamic equilibrium between two iron(II) complexes, one with a seven-coordinate pentagonal bipyramidal geometry and one with a five-coordinate trigonal bipyramidal geometry.
[Show abstract][Hide abstract] ABSTRACT: Tri(pyridylmethyl)phosphine (TPPh), the remarkably elusive congener of tri(pyridylmethyl)amine (TPA), has been prepared, as well as the relative tri(N-methyl-pyridylamino)phosphine (TPAMP). The coordination properties of these new ligands have been evaluated for chromium(III), iron(II), and ruthenium(II) complexes and compared with the related TPA complexes. In all cases, a different coordination behavior has been observed whereby TPPh and TPAMP always act as tridentate ligands. A chromium(III) complex [Cr(TPPh)Cl3] has been prepared, which has shown low ethylene oligomerization activity. Octahedral low spin iron(II) complexes [Fe(TPPh)2](2+) and [Fe(TPAMP)2](2+) were obtained with two ligands bound to the metal center. Ruthenium(II) chloro complexes of TPA and TPPh undergo ligand exchange reactions in acetonitrile, and the ruthenium(II) complex [Ru(MeCN)2(TPA)](2+) can be oxidized by m-CPBA in acetonitrile to give a transient ruthenium(IV) oxo complex [Ru(O)(MeCN)(TPA)](2+). Attempts to generate high valent ruthenium(IV) oxo TPPh or TPAMP complexes could not be achieved, probably due to insufficient stabilization by these strong field ligands.
[Show abstract][Hide abstract] ABSTRACT: Glycerol is converted to a mixture of butyric and isobutyric acid by rhodium- or iridium-catalysed carbonylation using HI as the co-catalyst. The initial reaction of glycerol with HI results in several intermediates that lead to isopropyl iodide, which upon carbonylation forms butyric and isobutyric acid. At low HI concentration, the intermediate allyl iodide undergoes carbonylation to give vinyl acetic acid and crotonic acid. Higher polyols Cn Hn+2 (OH)n are carbonylated to the corresponding Cn+1 mono-carboxylic acids.
Chemistry - A European Journal 05/2013; 19(21). DOI:10.1002/chem.201203069 · 5.73 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The pentadentate ligand bis(pyridylmethyl)(bipyridylmethyl)amine BPAbipy 1 and the iron(II) complex [Fe(1)(CH3CN)](ClO4)2 have been prepared. The strong field ligand 1 stabilises the low spin iron(II) complex, resulting in a high redox potential of 1.01 V for the Fe(II)/Fe(III) redox couple (vs. SCE). High valent iron complexes are destabilised by strong field ligands and the reaction of [Fe(1)(CH3CN)](ClO4)2 with PhIO is believed to result in a short-lived iron(IV) oxo complex, which is rapidly reduced to the oxo-bridged dinuclear iron(III) complex [(Fe(1))2(μ-O)](ClO4)4. The catalytic properties of the iron(II) complex [Fe(1)(CH3CN)](ClO4)2 for the oxidation of cyclohexane with various amounts of H2O2 has been evaluated. Moderate catalytic activities, comparable to other iron(II) complexes featuring pentadentate ligands but significantly lower than the tetradentate BPMEN ligand, have been observed.
[Show abstract][Hide abstract] ABSTRACT: A series of bis(thiazolinyl)- and bis(thiazolyl)pyridine Thio-Pybox ligands and their metal complexes of chromium(III), iron(II), cobalt(II) and nickel(II) has been prepared, as well as a nickel(II) complex containing a monoanionic bis(thiazolinyl)phenyl Thio-Phebox ligand. These new metal complexes have been characterised and used as catalysts, in combination with the co-catalyst MAO, for the polymerisation of ethylene and for the polymerisation of butadiene. In the case of ethylene polymerisation, the Thio-Pybox and Thio-Phebox metal complexes have shown relatively low polymerisation activities, much lower compared to the related bis(imino)pyridine complexes of the same metals. In the polymerisation of butadiene, several Thio-Pybox cobalt(II) complexes show very high activities, significantly higher than the other metal complexes with the same ligand. It is the metal, rather than the ligand, that appears to have the most profound effect on the catalytic activity in butadiene polymerisation, unlike in the polymerisation of ethylene, where bis(imino)pyridine ligands provide highly active catalysts for a range of 1st row transition metals.
[Show abstract][Hide abstract] ABSTRACT: Acetylene oligomerization with an activated iron bis(imino)pyridine complex with low steric hindrance leads to benzene as the major product, in contrast to this reaction with a more sterically encumbered analogue, which leads to polyacetylene. The mechanism of benzene formation was studied and likely occurs via metallacycle intermediates; this process can be interrupted by addition of ZnEt2 as a chain transfer agent to generate hexadiene instead.
[Show abstract][Hide abstract] ABSTRACT: The synthesis, characterization and ethylene polymerisation behaviour of a series of first row transition metal complexes of the general formula LMXn (M = Fe, Co, Mn, n = 2, X = Cl; M = V, Cr, Ti, n = 3, X = Cl; M = Ni, n = 2, X = Br) with bis(imino)pyridine ligands L are reported, whereby the ligands contain heteroatom substituents DRm at the imine carbon (D = O, S, m = 1, R = Me, Ph, 2,6-Me2C6H3 or D = N, m = 2, R = Me, Ph). Only the O- and S-substituted complexes show catalytic activity for the polymerisation of ethylene, upon activation with methylaluminoxane (MAO). The Mn- and Ni-based catalysts were found to be inactive under these conditions. The S-substituted complexes are generally more active than the O-substituted complexes and the catalytic activities increase with the size of the substituents. Iron-, vanadium- and chromium-based catalysts give highly active catalyst systems, which in some cases are more active than the well-known ketimine catalysts. All catalysts produce highly linear polyethylene with molecular weights affected by M, D and R. O-substituted catalyst systems are generally less active and produce lower molecular weight polyethylene compared to S-substituted systems.
[Show abstract][Hide abstract] ABSTRACT: Bis(imino)pyridine complexes of Co and Fe, and diimine complexes of Ni and Pd, have been tested as catalysts for acetylene oligomerisation and polymerisation. While Co, Ni and Pd complexes displayed low activity, an Fe complex led to an extremely active system for acetylene polymerisation after activation with methylaluminoxane (MAO). The product output could be shifted towards short-chain oligomers by addition of the chain transfer reagent ZnEt2, leading to polyene oligomeric chains bound to Zn. Complex branched and cyclic higher oligomers were partially identified, and some possible mechanisms for their formation proposed. The Fe system did not successfully catalyse copolymerisation of acetylene and ethylene, although there was evidence of limited acetylene incorporation into ethylene oligomers.
[Show abstract][Hide abstract] ABSTRACT: A series of cationic cis-dicarbonylrhodium(I) complexes [Rh(L)(CO)2]SbF6 has been prepared containing 2,2′-bipyridine ligands with proximate H-bonding substituents R in the 6- and 6′-position (R = OH, NH2, COOEt and PO(OEt)2). The solid-state structures have been determined by X-ray crystallography for the complexes where R = OH, NH2 and COOEt. The molecular structures have revealed metal–metal and π–π interactions between the square-planar complexes resulting in the formation of molecular chains in the solid state. IR studies on the reaction of [Rh(bipy)(CO)2]SbF6 with MeI have shown the formation of a neutral complex [RhI(CO)(bipy)], which undergoes oxidative addition of MeI much faster than the cationic complex [Rh(bipy)(CO)2]+. Although all complexes show good activities for the carbonylation of methyl acetate, this is believed to be due to their instability and the formation of [RhI2(CO)2]– under the reaction conditions.
Berichte der deutschen chemischen Gesellschaft 08/2011; 2011(23-23):3511-3522. DOI:10.1002/ejic.201100423 · 2.94 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The application of Lewis acids and Lewis acid-functionalized ligands as activators for methyl acetate in the rhodium-catalyzed carbonylation of methyl acetate to acetic anhydride has been investigated. The reaction of methyl acetate with B(C6F5)3 results in the formation of the adduct [MeOAc·B(C6F5)3]. The combination of this adduct with [Rh(OAc)(CO)2]2 results in a transfer of the Lewis acid to the rhodium complex, rather than an anticipated oxidative addition reaction. In a second approach, novel Lewis acid-functionalized rhodium(I) complexes [Rh(CO)Cl(BPP)], [Rh(CO)2(BPP)]SbF6, and [Rh(MeCN)2(BPP)]SbF6 (BPP = PhB(C6H4PPh2)2) have been prepared. The lack of reactivity of [Rh(CO)2(BPP)]SbF6 toward MeOAc has shown that the rhodium–boron interaction is too strong for activation of methyl acetate, and no carbonylation activity was observed under the conditions used.
[Show abstract][Hide abstract] ABSTRACT: A catalyst system composed of a 2,6-bis(arylimino)pyridineiron(II) dichloride complex and methylaluminoxane is found to be extremely active for acetylene polymerisation. The formation of poly(acetylene) gels and surface films occurs at very low catalyst concentrations, around three orders of magnitude lower than traditional catalyst systems.
Chemical Communications 06/2011; 47(24):6945-7. DOI:10.1039/c1cc12169c · 6.83 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The first square planar rhodium(I) complexes containing the 6,6'-dihydroxy-2,2'-bipyridine ligand have been prepared. The complexes form molecular wires in the solid state and are active catalysts for the carbonylation of methyl acetate.