The conversion of seed oil based feedstocks such as methyl oleate into useful commercial raw materials via olefin metathesis has been a research focus for decades, due to their low cost and renewable supply, but technical success has been limited due to poor catalyst activities and turnovers. We report here recent studies on the cross-metathesis of methyl oleate with ethylene (ethenolysis) catalyzed by bis(tricyclohexylphosphine)benzylideneruthenium dichloride (1). At 25 °C/60 psig of ethylene, catalysis by 1 results in the highly selective formation of 1-decene and methyl 9-decenoate. However, reactivity losses limit the catalyst turnovers well below commercial viability in batch reactor operation. In an attempt to address the limitations of this chemistry, a combination of an experimental evaluation of the impact of process parameters, a detailed analysis of the fundamental reaction steps, kinetic modeling, and molecular modeling has been applied to develop a more detailed understanding of this complex catalytic pathway. These fundamental studies have led to a more complete understanding of the factors impacting catalyst performance and the identification of approaches necessary to achieve an economically viable process.
We have applied a nonlocal density functional method to the study of ethylene polymerization with a Ni(II) catalytic center coordinated to diimine (HNdCH-CHdNH). We have investigated chain initialization, chain propagation, as well as chain isomerization and chain termination. Chain initialization proceeds in a stepwise fashion, with an overall activation barrier of 11.1 kcal/mol. Chain propagation can proceed via two different pathways, which have similar activation energies (16.8 and 17.5 kcal/mol, respectively). In contrast to behavior observed for metallocene catalysts, none of the insertion transition states show agostic stabilization. The activation energy for chain isomerization is 12.8 kcal/mol, which proceeds via a concerted mechanism, rotating the chain and simultaneously abstracting the -agostic hydrogen. Chain termination occurs via a stable hydride intermediate, which is formed with a barrier of 9.7 kcal/mol and decays into the termination product with a small activation energy of 1.7 kcal/mol. Production of experimentally observed high molecular weight polymers can only be explained by suppression of the chain termination transition state due to sterically demanding substituents on the diimine ligand.
A number of proposed models of the active sites in TiCl4/MgCl2 heterogeneous Ziegler-Natta catalysts are examined using density functional methods. Using a number of different models for the surface, the sites formed when unreduced TiCl4 is adsorbed onto MgCl2 are predicted to be unstable. In contrast, TiCl3 and TiCl2 are found to bind strongly to MgCl2. Alternative models of the surface where one or all of the Mg atoms is replaced with Ti(II) give larger binding energies for TiCl4. Each step of the ethylene polymerization reaction is considered for each of the site models that are expected to be active. Two possible termination mechanisms, chain transfer to the monomer and β-hydrogen elimination, are also examined. The sites formed from TiCl4 bind ethylene more weakly than those formed from TiCl3. No particular trends with respect to the type of site are found for the barrier to insertion, but the direction of approach of the ethylene molecule is important. The β-hydrogen elimination termination mechanism is predicted to be much less important than chain transfer to the monomer. From a comparison of ethylene insertion and termination barriers, the TiCl 3-based sites on the TiCl2 surface and the TiCl3-based Corradini site on MgCl2 are shown to be very poor catalysts while the others, in particular the TiCl3-based edge site on MgCl2, appear to be more promising models of the actual active sites.
We have studied the solvation of divalent copper by water and ammonia through the optimization of the structures of [Cu(H2O) n ]2+ and [Cu(NH3) n ]2+, n = 3−8, by static density functional theory and ab initio molecular dynamics simulations. We found that as the number of solvent molecules increases to more than four, the additional ligands prefer to be hydrogen-bonded to the planar tetragonal primary hydration shell of [Cu(solvent)4]2+ instead of filling the vacant axial position. The energetic preference of water is about 20−35 kJ/mol for the hydrogen bond compared to the axial position, whereas ammonia shows preference of only a few kJ/mol. Dynamical simulations were successful in reaching the lowest energy conformations. Especially remarkable is the dynamics of [Cu(H2O)8]2+, which has evolved from an eight-coordinate structure to a planar structure with four primary and four secondary solvent molecules in a short 10 ps simulation. Both [Cu(H2O)8]2+ and [Cu(NH3)8]2+ prefer a quasi-planar structure with a total of eight hydrogen bonds between the solvent molecules in the first and second solvation shells. Each secondary water and ammonia is hydrogen-bonded to two adjacent molecules in the primary solvation shell. It is remarkable that ammonia can form two hydrogen bonds with only one lone electron pair. The strong network of hydrogen bonds stabilizes the tetragonal planar primary hydration shell. These calculations indicate that the high kinetic stability of the eight-coordinate clusters in previous mass spectrometry experiments is related to the stabilization of the planar primary solvation shell by the network of hydrogen bonds. We found a correlation between experimental ion signals in the gas phase and the planarity of the first solvation shells.
The resting state structure of the metallocene−alkyl cation, the coordination of the olefin to the preferred resting state structure, and the insertion process of the Ti-constrained geometry catalyst (CpSiH2NH)TiR+ have been studied with density functional theory. A combined static and dynamic approach has been utilized whereby “static” calculations of the stationary points on the potential surface are meshed with first principles Car−Parrinello molecular dynamics simulations. The first molecular dynamics simulation specifically addressing the structure of a metallocene−alkyl cation is presented showing rapid interconversion between γ- and β-agostic conformations. Complementary static calculations show a small energetic preference for a γ-agostic resting state. Coordination of the olefin to the Ti−alkyl resting state is likely to result in the formation of a β-agostic π-complex which is highly favored energetically over other π-complexes that may initially form. The whole propagation cycle was studied from π-complex to subsequent π-complex. The propagation barrier corresponds to the insertion process which was calculated to have a free energy barrier of ΔG ⧧ = 24.3 kJ/mol at 300 K. The initial β-agostic interactions which stabilize the π-complex are replaced by α-agostic bonds which stabilize the insertion transition state. A study of the back-side insertion process reveals that it may be competitive with the front-side insertion process.
We study the homogeneous catalytic copolymerization of olefin and carbon monoxide. − The catalytic center is modeled by Pd(II) coordinated to PH2CHCHPH2. C2H4 is used as a model for the olefin. We investigate the chain propagation mechanism for alternating copolymerization as well as the side reactions resulting from multiple insertion of olefin and CO, respectively. We find that strictly alternating copolymerization is kinetically favored over homopolymerization of olefin and thermodynamically as well as kinetically favored over successive multiple insertions of CO. Insertion of one C2H4−CO unit into the Pd−ethyl bond yields −219 kJ/mol, whereas insertion of a C2H4−C2H4 segment yields exactly −200 kJ/mol. Insertion of a CO−CO segment yields only −88 kJ/mol. Therefore, multiple successive CO insertions are by comparison so unfavorable as to be ruled out completely. We explain the experimentally observed preference of strictly alternating copolymerization over multiple olefin insertions by the higher adduct formation energy of CO (−78 kJ/mol) as opposed to only −51 kJ/mol for C2H4. Furthermore, the activation barriers for the insertion of a CO/C2H4 unit into the chain are only 48 and 58 kJ/mol, respectively, whereas the barrier for C2H4 insertion is 65 kJ/mol. All acyl species encountered are only weakly stabilized by agostic interactions, whereas Pd-alkyl species are strongly stabilized by agostic interactions. The acyl species can stabilize itself by −31 kJ/mol over the most favorable agostic conformation by adopting a η2-carbonyl conformation. The growing polyketone chain is strongly stabilized by forming chelate bonds between the carbonyl oxygens and Pd. The strictly alternating copolymerization pattern originates from a combination of effects: On the one hand, the number of CO units incorporated into the chain is maximized, because CO (as the better π acceptor) stabilizes the reactive center more than ethylene during the adduct formation proceeding insertion and CO also faces a lower barrier during insertion into the chain. On the other hand, subsequent multiple insertions of CO are avoided since they are kinetically as well as thermodynamically highly unfavorable.
We present an extensive theoretical study of the iron(II)-bisimine pyridine based ethylene-) CH 3 , 1A) recently developed by the groups of Brookhart and Gibson. The study was based on density functional theory (DFT) for the "generic" model system 1a and a combined DFT and molecular mechanics approach for the "real" system 1A. It is shown that the rate-determining step for both termination and propagation in the "real" system is the capture of ethylene by 1A. The steric bulk introduced by R ) 2,6-C 6 H 4 (i-Pr) 2 was found to suppress ethylene capture for the termination step and increase the rate of insertion. Termination takes place on the singlet potential energy surface (PES). For propagation the singlet and triplet PES's are close in energy and spin-state change is possible. The quintet states are too high in energy to play any role in polymerization. The model system 1a was found to form an ethylene complex that is too stable for any further chemical transformation to take place.
A systematic study has been carried out on the complexation of ethylene to a number of d0 [L]MC2H5 0,+,2+ fragments [M = Sc(III), Y(III), La(III), Lu(III), Ti(IV), Zr(IV), Hf(IV), Ce(IV), Th(IV), and V(V); L = NH(CH)2NH2- (1), N(BH2)(CH)2(BH2)N2- (2), O(CH)3O- (3), Cp2 2- (4), NHSi(H2)C5H4 2- (5), [(oxo)(O(CH)3O)]3- (6), (NH2)2 2- (7), (OH)2 2- (8), (CH3)2 2- (9), and NH(CH2)3NH2- (10)], where a hydrogen on the β-carbon of the ethyl unit is bound to the metal in an agostic interaction (β-agostic bond). It is shown that the complexation energy of an ethylene molecule to a [L]MC2H5 n + precursor can be predicted to within ±20 kJ/mol by simple empirical rules, based on the accessible metal surface of the [L]MC2H5 n + fragment and its gross charge. Discussions are also given of the relative preference for frontside (ethylene syn to β-agostic bond) versus backside (ethylene anti to β-agostic bond) coordination by the olefin as a function of the central atom, the auxiliary ligand set L and the strength of the β-agostic bond. It is finally shown that the β-agostic bond strength in the [L]MC2H5 n + precursor follows the order Ti ≈ Zr > Th > Hf for [L]MC2H5 + and Sc ≈ Y ≥ La > Lu for [L]MC2H5 for L = 7−9, with agostic interactions for uncharged precursor complexes [L]MC2H5 generally being weaker than for charged precursor complexes.
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