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The dominating catalytic approach to aromatic hydrocarbons from renewables, deoxygenation of phenol-rich depolymerized lignin bio-oils, is hard to achieve: hydrodeoxygenation (HDO) of phenols typically leads to the loss of aromaticity and to non-negligible fractions of cyclohexanones and cyclohexanols. Here, we report a catalyst, niobia-supported iridium nanoparticles (Ir@Nb

Here we report the first heterogeneous iridium-based catalyst (Ir@Nb2O5) able to combine full conversion in hydrodeoxygenation (HDO) of lignin-derived phenols to hydrocarbons with appreciable selectivity for aromatics. With temperature and pressure optimization, and coking inhibition by selective removal of acid sites, the aromatic yields reach 69% under mild conditions.

Critical to advancing the uptake of olefin metathesis in leading contexts, including pharmaceutical manufacturing, is identification of highly active catalysts that resist decomposition. Amines constitute an aggressive challenge to ruthenium metathesis catalysts. Examined here is the impact of 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU), morpholine, n-butylamine, and triethylamine on Ru metathesis catalysts that represent the current state of the art, including cyclic alkyl amino carbene (CAAC) and N-heterocyclic carbene (NHC) complexes. Accordingly, the amine-tolerance of the nitro-Grela catalyst RuCl2(H2IMes)(=CHAr) (nG; Ar = C6H4-2-OiPr-5-NO2) is compared with that of its CAAC analogues nGC1 and nGC2, and the Hoveyda-class catalyst RuCl2(C2)(=CHAr′) HC2 (Ar′ = C6H4-2-OiPr). In C1, the carbene carbon is flanked by an N-2,6-Et2C6H3 group and a CMePh quaternary carbon; in C2, by an N-2-iPr-6-MeC6H3 group and a CMe2 quaternary carbon. The impact of 1 equiv amine per Ru on turnover numbers (TONs) in ring-closing metathesis of diethyl diallylmalonate was assessed at 9 ppm Ru, at RT and 70 °C. The deleterious impact of amines followed the trend NEt3 ∼ NH2nBu ≪ DBU ∼ morpholine. Morpholine is shown to decompose nGC1 by nucleophilic abstraction of the methylidene ligand; DBU, by proton abstraction from the metallacyclobutane. Decomposition was minimized at 70 °C, at which nGC1 enabled TONs of ca. 60 000 even in the presence of morpholine or DBU, vs ca. 80 000 in the absence of base. Unexpectedly, H2IMes catalyst nG delivered 70–90% of the performance of nGC1 at high temperatures, and underwent decomposition by Brønsted base at a similar rate. Density functional theory (DFT) analysis shows that this similarity is due to comparable net electron donation by the H2IMes and C1 ligands. Catalysts bearing the smaller C2 ligand were comparatively insensitive to amines, owing to rapid, preferential bimolecular decomposition.

Ru-alkylidenes bearing sterically demanding arylthiolate ligands (SAr) constitute one of only two classes of catalyst that are Z-selective in metathesis of 1-alkenes. Of particular interest are complexes bearing pyridine as a stabilizing donor ligand, [RuCl(SAr)(═CHR)(NHC)(py)] (R = phenyl or 2-thienyl, NHC = N-heterocyclic carbene, py = pyridine), which initiate catalysis rapidly and give appreciable yields combined with moderate to high Z-selectivity within minutes at room temperature. Here, we extend this chemistry by synthesizing and testing the first two such complexes (5a and 5b) bearing 3-phenylindenylidene, a ligand known to promote stability in other ruthenium-based olefin metathesis catalysts. The steric pressure resulting from the three bulky ligands (the NHC, the arylthiolate, and the indenylidene) forces the thiolate ligand to position itself trans to the NHC ligand, a configuration different from that of the corresponding alkylidenes. Surprisingly, although this configuration is incompatible with Z-selectivity and slows down pyridine dissociation, the two new complexes initiate readily at room temperature. Although their thermal stability is lower than that of typical indenylidene-bearing catalysts, 5a and 5b are fairly stable in catalysis (TONs up to 2200) and offer up to ca. 80% of the Z-isomer in prototypical metathesis homocoupling reactions. Density functional theory (DFT) calculations confirm the energetic cost of dissociating pyridine from 5a (= M1-Py) to generate 14-electron complex M1. Whereas the latter isomer does not give a metathesis-potent allylbenzene π-complex, it may isomerize to M1-trans and M2, which both form π-complexes in which the olefin is correctly oriented for cycloaddition. The olefin orientation in these complexes is also indicative of Z-selectivity. publishedVersion

Ruthenium–cyclic(alkyl)(amino)carbene (CAAC) catalysts, used at ppm levels, can enable dramatically higher productivities in olefin metathesis than their N-heterocyclic carbene (NHC) predecessors. A key reason is the reduced susceptibility of the metallacyclobutane (MCB) intermediate to decomposition via β-H elimination. The factors responsible for promoting or inhibiting β-H elimination are explored via density functional theory (DFT) calculations, in metathesis of ethylene or styrene (a representative 1-olefin) by Ru–CAAC and Ru–NHC catalysts. Natural bond orbital analysis of the frontier orbitals confirms the greater strength of the orbital interactions for the CAAC species, and the consequent increase in the carbene trans influence and trans effect. The higher trans effect of the CAAC ligands inhibits β-H elimination by destabilizing the transition state (TS) for decomposition, in which an agostic MCB Cβ–H bond is positioned trans to the carbene. Unproductive cycling with ethylene is also curbed, because ethylene is trans to the carbene ligand in the square pyramidal TS for ethylene metathesis. In contrast, metathesis of styrene proceeds via a ‘late’ TS with approximately trigonal bipyramidal geometry, in which carbene trans effects are reduced. Importantly, however, the positive impact of a strong trans-effect ligand in limiting β-H elimination is offset by its potent accelerating effect on bimolecular coupling, a major competing means of catalyst decomposition. These two decomposition pathways, known for decades to limit productivity in olefin metathesis, are revealed as distinct, antinomic, responses to a single underlying phenomenon. Reconciling these opposing effects emerges as a clear priority for design of robust, high-performing catalysts. publishedVersion

Whereas transition-metal-catalyzed decarbonylative dehydration of fatty acids shows promise as a more sustainable route to α-olefins, the solvents used for this process have so far been toxic compo...

ispartof: FARADAY DISCUSSIONS vol:220 issue:0 pages:144-178 ispartof: location:England status: published

Bimolecular catalyst decomposition is a fundamental, long-standing challenge in olefin metathesis. Emerging ruthenium–cyclic(alkyl)(amino)carbene (CAAC) catalysts, which enable breakthrough advances in productivity and general robustness, are now known to be extraordinarily susceptible to this pathway. The details of the process, however, have hitherto been obscure. The present study provides the first detailed mechanistic insights into the steric and electronic factors that govern bimolecular decomposition. Described is a combined experimental and theoretical study that probes decomposition of the key active species, RuCl2(L)(py)(═CH2) 1 (in which L is the N-heterocyclic carbene (NHC) H2IMes, or a CAAC ligand: the latter vary in the NAr group (NMes, N-2,6-Et2C6H3, or N-2-Me,6-iPrC6H3) and the substituents on the quaternary site flanking the carbene carbon (i.e., CMe2 or CMePh)). The transiently stabilized pyridine adducts 1 were isolated by cryogenic synthesis of the metallacyclobutanes, addition of pyridine, and precipitation. All are shown to decompose via second-order kinetics at −10 °C. The most vulnerable CAAC species, however, decompose more than 1000-fold faster than the H2IMes analogue. Computational studies reveal that the key factor underlying accelerated decomposition of the CAAC derivatives is their stronger trans influence, which weakens the Ru−py bond and increases the transient concentration of the 14-electron methylidene species, RuCl2(L)(═CH2) 2. Fast catalyst initiation, a major design goal in olefin metathesis, thus has the negative consequence of accelerating decomposition. Inhibiting bimolecular decomposition offers major opportunities to transform catalyst productivity and utility, and to realize the outstanding promise of olefin metathesis. publishedVersion

The selective transformation of 1-alkenes into E-olefins is a long-standing challenge in olefin metathesis. Density functional theory (DFT) calculations predict high E-selectivity for catalysts incorporating a bidentate, dianionic thio-indolate ligand within a RuXX’(NHC)(py)(= CHR) platform (NHC = N-heterocyclic carbene; py = pyridine). Such complexes are predicted to yield E-olefins by favoring anti-disposed substituents in the transition state expected to be rate-determining: specifically, that for cycloreversion of the metallacyclobutane intermediate. Three pyridine-stabilized catalysts Ru21a-c were synthesized, in which the thio-indolate ligand bears a H, Me, or Ph substituent at the C2 position, and the NHC ligand is the unsaturated imidazoline-2-ylidene Me2IMes (which bears N-mesityl groups and methyl groups on the C4,5 backbone). Single-crystal X-ray diffraction analysis of Ru21c confirms the ligand orientation required for E-selective metathesis, with the thio-indolate sulfur atom binding cis to the NHC, and the indolate nitrogen atom trans to the NHC. However, whereas the new complexes mediated metathetic exchange of their 2-thienylmethylidene ligand in the presence of the common metathesis substrates styrene and allylbenzene, no corresponding self-metathesis products were obtained. Only small amounts of 2-butene (73% (Z)-2-butene) were obtained in self-metathesis of propene using Ru21a. Detailed DFT analysis of this process revealed that product release is surprisingly slow, limiting the reaction rate and explaining the low metathesis activity. With the barrier to dissociation of (Z)-2-butene being lower than that of (E)-2-butene, the calculations also account for the observed Z-selectivity of Ru21a. These findings provide guidelines for catalyst redesign in pursuit of the ambitious goal of E-selective 1-alkene metathesis. Graphic abstract

Tridentate, bis-phenolate N-heterocyclic carbenes (NHCs) are among the ligands giving the most selective and active group 4-based catalysts for the copolymerization of cyclohexene oxide (CHO) with CO2. In particular, ligands based on imidazolidin-2-ylidene (saturated NHC) moieties have given catalysts which exclusively form polycarbonate in moderate-to-high yields even under low CO2 pressure and at low copolymerization temperatures. Here, to evaluate the influence of the NHC moiety on the molecular structure of the catalyst and its performance in copolymerization, we extend this chemistry by synthesizing and characterizing titanium complexes bearing tridentate bis-phenolate imidazol-2-ylidene (unsaturated NHC) and benzimidazol-2-ylidene (benzannulated NHC) ligands. The electronic properties of the ligands and the nature of their bonds to titanium are studied using density functional theory (DFT) and natural bond orbital (NBO) analysis. The metal&ndash, NHC bond distances and bond strengths are governed by ligand-to-metal &sigma, and &pi, donation, whereas back-donation directly from the metal to the NHC ligand seems to be less important. The NHC &pi, acceptor orbitals are still involved in bonding, as they interact with THF and isopropoxide oxygen lone-pair donor orbitals. The new complexes are, when combined with [PPN]Cl co-catalyst, selective in polycarbonate formation. The highest activity, albeit lower than that of the previously reported Ti catalysts based on saturated NHC, was obtained with the benzannulated NHC-Ti catalyst. Attempts to synthesize unsaturated and benzannulated NHC analogues based on Hf invariably led, as in earlier work with Zr, to a mixture of products that include zwitterionic and homoleptic complexes. However, the benzannulated NHC-Hf complexes were obtained as the major products, allowing for isolation. Although these complexes selectively form polycarbonate, their catalytic performance is inferior to that of analogues based on saturated NHC.

Catalyst discovery is increasingly relying on computational chemistry, and many of the computational tools are currently being automated. The state of this automation and the degree to which it may contribute to speeding up development of catalysts are the subject of this Perspective. We also consider the main challenges associated with automated catalyst design, in particular the generation of promising and chemically realistic candidates, the tradeoff between accuracy and cost in estimating the catalytic performance, the opportunities associated with automated generation and use of large amounts of data, and even how to define the objectives of catalyst design. Throughout the Perspective, we take a cross-disciplinary approach and evaluate the potential of methods and experiences from fields other than homogeneous catalysis. Finally, we provide an overview of software packages available for automated in silico design of homogeneous catalysts. publishedVersion

Recently reported thiolate-coordinated ruthenium alkylidene complexes show promise in Z-selective and stereoretentive olefin metathesis reactions. Herein we describe the immobilization of three Ru complexes containing a bulky aryl thiolate on mesostructured silica via surface organometallic chemistry. The applied methodology gives isolated catalytic sites homogeneously distributed on the silica surface. The catalytic results with two model substrates show comparable Z-selectivities to those of the homogeneous counterparts. acceptedVersion

The Grubbs methylidene RuCl2(PCy3)2(═CH2) GIm is immediately decomposed by the small N-heterocyclic carbene (NHC) tetramethylimidazol-2-ylidene (IMe4), even at −80 °C. Labeling experiments with RuCl2(PCy3)2(═13CH2) reveal capture of the methylidene ligand as ethylimidazolium chloride [13CH313CH2–IMe4]Cl, A*. Density functional calculations indicate that IMe4 first abstracts methylidene from five-coordinate GIm (a pathway unavailable to bulkier NHCs), generating the N-heterocyclic olefin (NHO) H2C═IMe4. The latter attacks a second GIm; protonolysis of highly basic [Ru]–CH2CH2IMe4 then liberates A. Identical, slightly slower, reactivity is seen for RuCl2(H2IMes)(PCy3)(═CH2), GIIm. The high efficiency of this process opens up opportunities for tandem metathesis enabled by sterically accessible NHCs and NHOs.

Scientists are exploring renewables as sources for a more sustainable production of chemicals. Linear α-olefins (LAOs) are key commodity chemicals and petrochemical intermediates, which are currently almost exclusively obtained from fossil resources. The most important renewable alternative is plant oil, from which fatty acids and their derivatives can be converted into LAOs by ethenolysis of monounsaturated fatty acids or deoxygenation of saturated ones. The variety, in terms of length, is greater for the saturated fatty acids found in plant oils, and this review covers deoxygenation processes mediated by homogeneous, heterogeneous and enzymatic catalysts and discusses the strengths and weaknesses of different approaches for the selective production of LAOs. Although progress has been made in recent years, the best catalysts in each category are still far from fulfilling the industrial requirements for efficiency, atom economy, and turnover numbers. We hope that our focus on the main remaining challenges will stimulate future research to develop catalysed deoxygenation of fatty acid derivatives as a sustainable and industrially viable route to a range of α-olefins.

Pyridine as a stabilizing donor ligand drastically improves the performance of ruthenium monothiolate catalysts for olefin metathesis in comparison with previous versions based on a stabilizing benzylidene ether ligand. The new pyridine-stabilized ruthenium alkylidenes undergo fast initiation and reach appreciable yields combined with moderate to high Z selectivity in self-metathesis of terminal olefins after only a few minutes at room temperature. Moreover, they can be used with a variety of substrates, including acids, and promote self-metathesis of ω-alkenoic acids. The pyridine-stabilized ruthenium monothiolate catalysts are also efficient at the high substrate dilutions of macrocylic ring-closing metathesis and resist temperatures above 100 °C during catalysis. publishedVersion

Heterogeneous catalysts have so far not been very efficient [turnover frequency (TOF) 95%) for one such deoxygenation, decarbonylative dehydration, by using a heterogeneous Pd/C catalyst in the presence of phosphine ligands. The process is solvent free, allows for catalyst recycling, and does not require in situ distillation of the product to maintain high selectivity.

Ruthenium-based olefin metathesis catalysts are used in laboratory-scale organic synthesis across chemistry, largely thanks to their ease of handling and functional group tolerance. In spite of this robustness, these catalysts readily decompose, via little-understood pathways, to species that promote double-bond migration (isomerization) in both the 1-alkene reagents and the internal-alkene products. We have studied, using density functional theory (DFT), the reactivity of the Hoveyda–Grubbs second-generation catalyst 2 with allylbenzene, and discovered a facile new decomposition pathway. In this pathway, the alkylidene ligand is lost, via ring expansion of the metallacyclobutane intermediate, leading to the spin-triplet 12-electron complex (SIMes)RuCl2 (3R21, SIMes = 1,3-bis(2,4,6-trimethylphenyl)-4,5-dihydroimidazol-2-ylidene). DFT calculations predict 3R21 to be a very active alkene isomerization initiator, either operating as a catalyst itself, via a η3-allyl mechanism, or, after spin inversion to give R21 and formation of a cyclometalated Ru-hydride complex, via a hydride mechanism. The calculations also suggest that the alkylidene-free ruthenium complexes may regenerate alkylidene via dinuclear ruthenium activation of alkene. The predicted capacity to initiate isomerization is confirmed in catalytic tests using p-cymene-stabilized R21 (5), which promotes isomerization in particular under conditions favoring dissociation of p-cymene and disfavoring formation of aggregates of 5. The same qualitative trends in the relative metathesis and isomerization selectivities are observed in identical tests of 2, indicating that 5 and 2 share the same catalytic cycles for both metathesis and isomerization, consistent with the calculated reaction network covering metathesis, alkylidene loss, isomerization, and alkylidene regeneration. publishedVersion

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ispartof: FARADAY DISCUSSIONS vol:220 issue:0 pages:464-488 ispartof: location:England status: published

One of the most important tasks for chemistry in our time is to contribute to sustainable chemical production. A green industrial process for linear α-olefins, the arguably most important class of petrochemical intermediates, from renewable resources would be a major contribution to this end. Plant oils are attractive renewable feedstocks for this purpose because their triglycerides can be hydrolyzed to fatty acids that contain valuable long-chain hydrocarbons (C16-C22). These hydrocarbons may, in turn, be converted to α-olefins by the deoxygenation of the fatty acids. For the most selective of these deoxygenation reactions, transition-metal catalyzed decarbonylative dehydration, the density functional theory (DFT) calculations have just started to offer valuable mechanistic insight, and the use of this insight in rational catalyst design has been facilitated by the arrival of the first well-defined precatalyst for this reaction, Pd(cinnamyl)Cl(DPEphos) (1). Here, we present DFT calculations showing how, in 1, the hemilability of DPEphos, a classical P-O-P diphosphine, contributes to a low overall barrier and high α-selectivity. DPEphos facilitates decarbonylation by first switching from bidentate to monodentate binding to create a coordination site for CO. The recoordination of the dangling phosphine displaces the Pd-bound CO, a co-product that must leave the reactor for the reaction to proceed, and the escaping CO is here modelled using a low pressure in the calculation of its thermochemical corrections. Finally, the role of the hemilabile ligand suggests that further improvements in the decarbonylative dehydration of fatty acids to α-olefins might be achieved by exploring new, potentially asymmetric, hemilabile ligands.

A general-purpose software package, termed DE Novo OPTimization of In/organic Molecules (DENOPTIM), for

International audience; Thiolate-coordinated ruthenium alkylidene complexes can give high Z-selectivity and stereoretentivity in olefin metathesis. To investigate their applicability as heterogeneous catalysts, we have successfully developed a methodology to easily immobilize prototype ruthenium alkylidenes onto hybrid mesostructured silica via a thiolate tether. In contrast, the preparation of the corresponding molecular complexes appeared very challenging in solution. These prototype supported complexes contain small thiolates but still, they are slightly more Z-selective than their molecular analogues. These results open the door to more active and selective heterogeneous catalysts by supporting more advanced thiolate Ru-complexes.

A general-purpose software package, termed DE Novo OPTimization of In/organic Molecules (DENOPTIM), for de novo design and virtual screening of functional molecules is described. Molecules of any element and kind, including metastable species and transition states, are handled as chemical objects that go beyond valence-rules representations. Synthetic accessibility of the generated molecules is ensured via detailed control of the kinds of bonds that are allowed to form in the automated molecular building process. DENOPTIM contains a combinatorial explorer for screening and a genetic algorithm for global optimization of user-defined properties. Estimates of these properties may be obtained to form the fitness function (figure of merit or scoring function) from external molecular modeling programs via shell scripts. Examples of a range of different fitness functions and DENOPTIM applications, including an easy-to-do test case, are described. DENOPTIM is available as Open Source from https://github.com/denoptim-project/DENOPTIM. acceptedVersion

Transition-metal-catalyzed decarbonylative dehydration of even-numbered fatty acids to give valuable odd-numbered alpha olefins has so far been performed using approaches in which the catalyst is formed in situ. We show that well-defined Pd(bis-phosphine) precatalysts eliminate the need for excess phosphine and selectively produce linear alpha olefins under mild conditions and with substantially increased turnover frequency (TOF) and substrate scope. In particular, the new precatalyst Pd(cinnamyl)Cl(DPEPhos) (5) selectively converts substrates containing unprotected aliphatic-alcohol and amine functional groups to their corresponding linear alpha olefins.

Reactive absorption of CO2 by aqueous amine solutions is widely used in the oil, gas and petrochemical industries. However, the energy cost associated with CO2 capture is still a major hurdle to global implementation. Finding new absorbent systems with improved properties is therefore critical to the safety and efficiency of the carbon capture process. Owing to a wide range of tunable properties, imidazoles or imidazoles/amine mixtures have recently been identified as promising to address some of these challenges. In this work, evolutionary de novo design is used to propose new imidazole-based compounds suitable for carbon capture. At the core of this scheme is a genetic algorithm that optimizes the acid dissociation constant (pKa), an important factor governing the performance of solvents. Calculation of the pKa using quantum chemical methods may produce accurate results but is often too computationally demanding to be suitable for a de novo design setup, where thousands of structures are evaluated over many iterations. To improve the efficacy of the evolutionary process while maintaining high accuracy, we apply a quantitative structure–property relationship (QSPR) model that relates molecular structure descriptors calculated at the semi-empirical level to experimentally determined pKa values. Several promising compounds with high pKa (>10) were identified by the evolutionary design approach and further validated using density functional theory calculations. QSPR models for equally relevant physical properties (such as density, viscosity, vapour pressure) and biodegradability were used as additional filters to ensure that the high pKa structures also have favourable values for these properties.

Titanium(IV) complexes bearing an unsubstituted tridentate bis(phenolate) N-heterocyclic carbene (NHC) were synthesized and structurally identified. While sterically unencumbered NHC–Ti(IV) complexes bearing chloro and alkoxy co-ligands tend to dimerize in solution and in solid-state, the use of a bulky aryloxy as co-ligand favors the monomeric species. Upon activation by onium salts, all these complexes were found to be highly selective towards the copolymerization of cyclohexene oxide (CHO) with CO2 under mild conditions (PCO2 < 1 bar), albeit the sterically unencumbered NHC–Ti(IV) complexes are less stable and active than their structural analogues bearing bulkier substituents.

The correlation between rapid initiation and rapid decomposition in olefin metathesis is probed for a series of fast-initiating, phosphine-free Ru catalysts: the Hoveyda catalyst HII, RuCl2(L)(═CHC6H4-o-OiPr); the Grela catalyst nG (a derivative of HII with a nitro group para to OiPr); the Piers catalyst PII, [RuCl2(L)(═CHPCy3)]OTf; the third-generation Grubbs catalyst GIII, RuCl2(L)(py)2(═CHPh); and dianiline catalyst DA, RuCl2(L)(o-dianiline)(═CHPh), in all of which L = H2IMes = N,N′-bis(mesityl)imidazolin-2-ylidene. Prior studies of ethylene metathesis have established that various Ru metathesis catalysts can decompose by β-elimination of propene from the metallacyclobutane intermediate RuCl2(H2IMes)(κ2-C3H6), Ru-2. The present work demonstrates that in metathesis of terminal olefins, β-elimination yields only ca. 25–40% propenes for HII, nG, PII, or DA, and none for GIII. The discrepancy is attributed to competing decomposition via bimolecular coupling of methylidene intermediate RuCl2(H2IMes)(═CH2), ...

International audience; Single crystal structural analysis of [Fe-II(tame)(2)]Cl-2 center dot MeOH (tame = 1,1,1-tris(aminomethyl)-ethane) as a function of temperature reveals a smooth crossover between a high temperature high-spin octahedral d(6) state and a low temperature low-spin ground state without change of the symmetry of the crystal structure. The temperature at which the high and low spin states are present in equal proportions is T-1/2 = 140K. Single crystal, variable-temperature optical spectroscopy of [Fe-II(tame)(2)]Cl-2 center dot MeOH is consistent with this change in electronic ground state. These experimental results confirm the spin activity predicted for [Fe-II(tame)(2)](2+) during its de novo artificial evolution design as a spin-crossover complex [Chem. Inf. Model. 2015, 55, 1844], offering the first experimental validation of a functional transition-metal complex predicted by such in silico molecular design methods. Additional quantum chemical calculations offer, together with the crystal structure analysis, insight into the role of spin-passive structural components. A thermodynamic analysis based on an Ising-like mean field model (Slichter-Drickammer approximation) provides estimates of the enthalpy, entropy and cooperativity of the crossover between the high and low spin states.

We describe a method for the design of multicyclic compounds from three-dimensional (3D) molecular fragments. The 3D building blocks are assembled in a controlled fashion, and closable chains of such fragments are identified. Next, the ring-closing conformations of such formally closable chains are identified, and the 3D model of a cyclic or multicyclic molecule is built. Embedding this method in an evolutionary algorithm results in a de novo design tool capable of altering the number and nature of cycles in species such as transition metal compounds with multidentate ligands in terms of, for example, ligand denticity, type and length of bridges, identity of bridgehead terms, and substitution pattern. An application of the method to the design of multidentate nitrogen-based ligands for Fe(II) spin-crossover (SCO) compounds is presented. The best candidates display multidentate skeletons new to the field of Fe(II) SCO yet resembling ligands deployed in other fields of chemistry, demonstrating the capability of the approach to explore structural variation and to suggest unexpected and realistic molecules, including structures with cycles not found in the building blocks.

Traditional approaches for improving the photovoltaic performance of dye-sensitized solar cells (DSSCs) have mainly relied on judicious molecular design and device level modifications. Such schemes, however, are bound by costly and time-consuming synthesis procedures. In this paper, we demonstrate the efficacy of an alternative approach based on in silico evolutionary de novo design of novel dye structures with improved DSSC power conversion efficiency (PCE) values. Because the PCE, cannot as yet be directly computed from first principles, the evolutionary fitness function utilizes predictive structure–property relationship (QSPR) models calibrated from empirical data. Our design approach is applied to phenothiazine-based dye sensitizers. The chemical structure space is explored using a genetic algorithm that systematically assembles molecules from fragments in a synthetically tractable manner. Five novel phenothiazine dyes are proposed using our approach where all have predicted PCE values above 9%.

Linear alpha olefins (LAOs) are key commodity chemicals and petrochemical intermediates that are currently produced from fossil resources. Fatty acids are the obvious renewable starting material for LAOs, which can be obtained via transition-metal-catalyzed decarbonylative dehydration. However, even the best catalysts that have been obtained to date, which are based on palladium, are not active and stable enough for industrial use. To provide insight for design of better catalysts, we here present the first computationally derived mechanism for another attractive transition-metal for this reaction, rhodium. By comparing the calculated mechanisms and free energy profiles for the two metals, Pd and Rh, we single out important factors for a facile, low-barrier reaction and for a stable catalyst. While the olefin formation is rate limiting for both of the metals, the rate-determining intermediate for Rh is, in contrast to Pd, the starting complex, (PPh3)2Rh(CO)Cl. This complex largely draws its stability from the strength of the Rh(I)–CO bond. CO is a much less suitable ligand for the high-oxidation state Rh(III). However, for steric reasons, rhodium dissociates a bulkier triphenylphosphine and keeps the carbonyl during the oxidative addition, which is less favorable than for Pd. When compared to Pd, which dissociates two phosphine ligands at the start of the reaction, the catalytic activity of Rh also appears to be hampered by its preference for high coordination numbers. The remaining ancillary ligands leave less space for the metal to mediate the reaction.

Bronsted bases of widely varying strength are shown to decompose the metathesis-active Ru intermediates formed by the second-generation Hoveyda and Grubbs catalysts. Major products, in addition to propenes, are base·HCl and olefin-bound, cyclometalated dimers [RuCl(κ2-H2IMes−H)(H2C═CHR)]2 Ru-3. These are generated in ca. 90% yield on metathesis of methyl acrylate, styrene, or ethylene in the presence of either DBU, or enolates formed by nucleophilic attack of PCy3 on methyl acrylate. They also form, in lower proportions, on metathesis in the presence of the weaker base NEt3. Labeling studies reveal that the initial site of catalyst deprotonation is not the H2IMes ligand, as the cyclometalated structure of Ru-3 might suggest, but the metallacyclobutane (MCB) ring. Computational analysis supports the unexpected acidity of the MCB protons, even for the unsubstituted ring, and by implication, its overlooked role in decomposition of Ru metathesis catalysts.

DFT calculations have been used to elucidate the chain termination mechanisms for neutral nickel ethylene oligo- and polymerization catalysts and to rationalize the kind of oligomers and polymers produced by each catalyst. The catalysts studied are the (κ(2)-O,O)-coordinated (1,1,1,5,5,5-hexafluoro-2,4-acetylacetonato)nickel catalyst I, the (κ(2)-P,O)-coordinated SHOP-type nickel catalyst II, the (κ(2)-N,O)-coordinated anilinotropone and salicylaldiminato nickel catalysts III and IV, respectively, and the (κ(2)-P,N)-coordinated phosphinosulfonamide nickel catalyst V. Numerous termination pathways involving β-H elimination and β-H transfer steps have been investigated, and the most probable routes identified. Despite the complexity and multitude of the possible termination pathways, the information most critical to chain termination is contained in only few transition states. In addition, by consideration of the propagation pathway, we have been able to estimate chain lengths and discriminate between oligo- and polymerization catalysts. In agreement with experiment, we found the Gibbs free energy difference between the overall barrier for the most facile propagation and termination pathways to be close to 0 kcal mol(-1) for the ethylene oligomerization catalysts I and V, whereas values of at least 7 kcal mol(-1) in favor of propagation were determined for the polymerization catalysts III and IV. Because of the shared intermediates between the termination and branching pathways, we have been able to identify the preferred cis/trans regiochemistry of β-H elimination and show that a pronounced difference in σ donation of the two bridgehead atoms of the bidentate ligand can suppress hydride formation and thus branching. The degree of rationalization obtained here from a handful of key intermediates and transition states is promising for the use of computational methods in the screening and prediction of new catalysts of the title class.

We have investigated the metal–ligand bond energies in C2H4–Pt(PH3)2 and C60–Pt(PH3)2 by means of ab initio correlated methods (MP2, MP4(SDQ), MP4, QCISD, CCSD and CCSD(T)) in conjunction with large basis sets. For D e (C2H4–Pt(PH3)2), an accurate value of 17.2 kcal/mol is established at the CCSD(T) level of theory. Due to the size of the system, the bond energy for the Buckminsterfullerene system was explored in terms of a sequence of model systems of increasing size, providing a D e estimate of 28.2 kcal/mol at the ONIOM(CCSD(T)/C14H8:MP2/C60) level of theory. The performance of a range of high-end density functionals (with and without dispersion) is evaluated for these systems by comparison to the best ab initio results. Among these, we find density functionals BLYP and B3LYP, augmented by Grimme’s D3 dispersion correction (to give the corresponding DFT-D methods), to provide good and consistent agreement with our best estimates. Next, DFT-optimised structures for C2H4–Pt(PPh3)2 and C60–Pt(PPh3)2 are p...

Density functional theory calculations accounting for dispersion, thermochemical, and continuum solvent effects have been applied to study the metathesis of ethyl vinyl ether (EVE) as mediated by ruthenium catalysts L(PCy3)(X)2Ru═CHPh (L = PCy3, IMes (1,3-dimesitylimidazol-2-ylidene), H2IMes (1,3-dimesityl-4,5-dihydroimidazol-2-ylidene); X = Cl, Br, I) in toluene. The computational approach has been validated against experimental data for ruthenium-based olefin metathesis catalysts and is found to give acceptable accuracy even for weakly bound transition states of phosphine and olefin dissociation and association. All relevant stationary points of the EVE metathesis reaction have been included in the study, allowing comparison with experimental kinetic data (Sanford, M. S.; Love, J. A; Grubbs, R. H.J. Am. Chem. Soc. 2001, 123, 6543). From the active 14-electron complex, the barriers to both phosphine association (at a rate proportional to k–1) and olefin binding (k2) involve contributions from entropy and...

Whereas a number of highly Z-selective ruthenium-based olefin metathesis catalysts bearing N-heterocyclic carbene ligands have been reported in recent years, Zselectivity has so far been difficult to achieve for phosphinebased catalysts. Guided by predictive density functional theory (DFT) calculations, we have developed phosphine-based ruthenium olefin metathesis catalysts giving 70−95% of the Zisomer product in homocoupling of terminal alkenes such as allylbenzene, 1-octene, allyl acetate, and 2-allyloxyethanol. Starting from a moderately selective catalyst, [P(Cy)3](-S-2,4,6-Ph-C6H2)ClRu(==CH-o-OiPrC6H4) (4, Cy = cyclohexyl, iPr = isopropyl), obtained by substituting a chloride of the Hoveyda−Grubbs first-generation catalyst with 2,4,6- triphenylbenzenethiolate, we moved on to replace Cl and PCy3 by chelating, anionic phosphine ligands. Such ligands increase selectivity by limiting rotation around the P−Ru bond and by specifically directing the steric bulk of the phosphine substituents toward the selectivity-inducing thiolate ligand. In particular, DFT calculations predicted that o-(dialkylphosphino)phenolate ligands should improve selectivity and activity compared to 4. The most promising of these compounds (8b), based on the o-(ditert- butylphosphino)phenolate ligand, directs the two P-bonded tert-butyl substituents toward the 2,4,6-triphenylbenzenethiolate and has little steric hindrance trans to the thiolate. This compound metathesizes terminal olefins such as allylbenzene and 1- octene with Z-selectivities above 80% and allylacetate above 90%. Although these phosphine-based ruthenium monothiolate catalysts in general achieve somewhat lower activities and Z-selectivities than their second-generation counterparts, they also offer examples giving less substrate and product isomerization and thus higher yields. publishedVersion