Density functional theory (DFT) at the M06 level was utilized to compare the reactivity of Pd(PtBu3)2 with that of Pd2(dba)3 in catalyzing carbostannylation of alkynes in the presence of [AuL]+, where L is a phosphine ligand. In both cases, a common active catalyst is found to be responsible for conducting the reaction. The underlying reason for this is that [AuL]+ is capable of acting as a phosphine scavenger and removing both phosphines from Pd(PtBu3)2. The phosphine scavenger property of the cationic gold complexes may find applications in other catalytic coupling reactions. We also found that other Lewis acids such as AuCl, CuCl, and ZnCl2 might have potential for use as phosphine scavengers from palladium(0) bis(phosphine) complexes.

The gold-catalyzed direct functionalization of aromatic C–H bonds has attracted interest for constructing organic compounds which have application in pharmaceuticals, agrochemicals, and other important fields. In the literature, two major mechanisms have been proposed for these catalytic reactions: inner-sphere syn-addition and outer-sphere anti-addition (Friedel–Crafts-type mechanism). In this article, the AuCl3-catalyzed hydrofurylation of allenyl ketone, vinyl ketone, ketone, and alcohol substrates is investigated with the aid of density functional theory calculations, and it is found that the corresponding functionalizations are best rationalized in terms of a novel mechanism called “concerted electrophilic ipso-substitution” (CEIS) in which the gold(III)-furyl σ-bond produced by furan auration acts as a nucleophile and attacks the protonated substrate via an outer-sphere mechanism. This unprecedented mechanism needs to be considered as an alternative plausible pathway for gold(III)-catalyzed arene functionalization reactions in future studies.

The rhenium dioxide anion [ReO2]– reacts with carbon dioxide in a linear ion trap mass spectrometer to produce [ReO3]– corresponding to activation and cleavage of a C–O bond. Isotope labeling experiments using [Re18O2]– reveal that 18O/16O scrambling does not occur prior to cleavage of the C–O bond. Density functional theory calculations were performed to examine the mechanism for this oxygen atom abstraction reaction. Because the spins of the ground states are different for the reactant and product ions (3[ReO2]− versus 1[ReO3]−), both reaction surfaces were examined in detail and multiple [O2Re-CO2]− intermediates and transition structures were located and minimum energy crossing points were calculated. The computational results show that the intermediate [O2Re(η2–C,O–CO2)]– species most likely initiates C–O bond activation and cleavage. The stronger binding affinity of CO2 within this species and the greater instabilities of other [O2Re–CO2)]– intermediates are significant enough that oxygen atom exchange is avoided.

Cummins et al. have observed that 3 equiv of Mo(N[R]Ar)3 (R = C(CD3)2CH3, Ar = 3,5-C6H3Me2) are required for dual S═O bond cleavage within a SO2 molecule. Using density functional theory calculations, this theoretical study investigates a mechanism for this SO2 cleavage reaction that is mediated by MoL3, where L = NH2 or N[tBu]Ph. Our results indicate that an electron transfers into the SO2 ligand, which leads to Mo oxidation and initiates SO2 coordination along the quartet surface. The antiferromagnetic (AF) nature of the (NH2)3Mo–SO2 adduct accelerates intersystem crossing onto the doublet surface. The first S═O bond cleavage occurs from the resulting doublet adduct and leads to formation of L3Mo═O and SO. Afterward, the released SO molecule is cleaved by the two remaining MoL3, resulting in formation of L3Mo═S and an additional L3Mo═O. This mononuclear mechanism is calculated to be strongly exothermic and proceeds via a small activation barrier, which is in accordance with experimental results. An additional investigation into a binuclear process for this SO2 cleavage reaction was also evaluated. Our results show that the binuclear mechanism is less favorable than that of the mononuclear mechanism.

Density functional theory was used to investigate the protodeauration of organogold compounds, a process which is thought to be the final step in the gold-catalyzed nucleophilic addition to activated π bonds wherein a proton is added and the gold catalyst is regenerated. In this context, we have studied two important factors which control the effectiveness of this transformation. We find that the nature of the alkenyl group in PMe3Au(alkenyl) affects the reaction barrier through the strength of the Au–C bond; the stronger the Au–C bond, the higher the activation energy. This, in turn, is determined by the π-accepting/donating ability of the substituents on the alkenyl group. We theoretically confirm that, for protodeauration, the reaction should be rapid when π-donating groups are present. In contrast, when π-accepting substituents are present, the intermediate gold complexes may be stable enough to be isolated experimentally. The second important factor controlling the reaction is the nature of the phosphine ligands. We theoretically confirm that electron-rich ligands such as PMe3 or PPh3 accelerate the reaction. We find that this is due to the strong electron-donating nature of these ligands, which strengthens the Au–P bond in the final product and thus provides a thermodynamic driving force for the reaction. Also, it is shown how the protodeauration is affected by the number of molecules solvating the proton. The protodeauration mechanism of some other organogold compounds such as gold–alkyl, gold–alkynyl, and gold–allyl species was investigated as well. The findings of this study can be used to design more effective systems for transformations of organogold compounds.

We have investigated computationally the gold and palladium cocatalyzed reaction of alkynes with vinylstannane. Our work has involved a careful and thorough exploration of different mechanistic possibilities. We find that palladium acting alone as a catalyst leads to a very high reaction barrier, consistent with the experimental observation that there is no reaction in the presence of just palladium. However, the involvement of a gold(I) complex lowers the reaction barrier considerably, and the vinylstannylation reaction can proceed with a modest activation energy of about 10 kcal/mol. Our key finding is that the introduction of the gold complex avoids the formation of high-energy structures involving vinyl species in a trans arrangement on palladium. Our work confirms the role of intermediates containing both palladium and gold as suggested by Blum. For the gold–palladium cocatalyzed reaction, we also investigated an alternative mechanism suggested by Blum. With some modifications, this mechanism has a slightly higher reaction barrier, but if it does occur, then we predict a strong dependency on the counterion, in agreement with related experimental findings.Keywords: bimetallic catalysis; density functional theory (DFT); gold; palladium; reaction mechanism; vinylstannylation

The cleavage of one N–O bond in NO2 by two equivalents of Mo(NRAr)3 has been shown to occur to form molybdenum oxide and nitrosyl complexes. The mechanism and electronic rearrangement of this reaction was investigated using density functional theory, using both a model Mo(NH2)3 system and the full [N(tBu)(3,5-dimethylphenyl)] experimental ligand. For the model ligand, several possible modes of coordination for the resulting complex were observed, along with isomerisation and bond breaking pathways. The lowest barrier for direct bond cleavage was found to be via the singlet η2-N,O complex (7 kJ mol−1). Formation of a bimetallic species was also possible, giving an overall decrease in energy and a lower barrier for reaction (3 kJ mol−1). Results for the full ligand showed similar trends in energies for both isomerisation between the different isomers, and for the mononuclear bond cleavage. The lowest calculated barrier for cleavage was only 21 kJ mol−1via the triplet η1-O isomer, with a strong thermodynamic driving force to the final products of the doublet metal oxide and a molecule of NO. Formation of the full ligand dinuclear complex was not accompanied by an equivalent decrease in energy seen with the model ligand. Direct bond cleavage via an η1-O complex is thus the likely mechanism for the experimental reaction that occurs at ambient temperature and pressure. Unlike the other known reactions between MoL3 complexes and small molecules, the second equivalent of the metal does not appear to be necessary, but instead irreversibly binds to the released nitric oxide.

The mechanism for the oxidation of 3′-dGMP by [PtCl4(dach)] (dach = diaminocyclohexane) in the presence of [PtCl2(dach)] has been investigated using density functional theory. We find that the initial complexation, i.e., the formation of [PtCl3(dach)(3′-dGMP)], is greatly assisted by the reaction of the encounter pair [PtCl2(dach)···3′-dGMP] with [PtCl4(dach)], leading to migration of an axial chlorine ligand from platinum(IV) to platinum(II). A dinuclear platinum(II)/platinum(IV) intermediate could not be found, but the reaction is predicted to pass through a platinum(III)/platinum(III) transition structure. A cyclization process, i.e., C8–O bond formation, from [PtCl3(dach)(3′-dGMP)] occurs through an intriguing phosphate–water-assisted deprotonation reaction, analogous to the opposite of a proton shuttle mechanism. Followed by this, the guanine moiety is oxidized via dissociation of the PtIV–Clax bond, and the cyclic ether product is finally formed after deprotonation. We have provided rationalizations, including molecular orbital explanations, for the key steps in the process. Our results help to explain the effect of [PtCl4(dach)] on the complexation step and the effect of a strong hydroxide base on the cyclization reaction. The overall reaction cycle is intricate and involves autocatalysis by a platinum(II) species.

A computational analysis of the Pd-catalyzed coupling of 3-methyl-2-phenylpyridine (mppH) with [Ph2I]BF4 to form mppPh is supportive of a prior synthetic and kinetic study implicating binuclear palladium species in a rate-limiting oxidation step. The Pd(OAc)2 precatalyst forms the “clamshell” orthopalladated complex [Pd(mpp)(μ-OAc)]2 (8) as the active catalyst, which is oxidized by [Ph2I]+ in a reaction having the highest energy requirement of all steps in the catalytic cycle. In the oxidation reaction, involving formal transfer of Ph+, the electrophilic iodine center interacts initially with a bridging acetate oxygen atom of [Pd(mpp)(μ-OAc)]2 (8), “Pd–O···IPh2”, which transforms to a transition structure with retention of the O···I interaction and formation of a “Pd(μ-Ph-η1)I” bridge in a four-membered ring, “Pd···Ph···I(Ph)···O–Pd”, followed by elimination of PhI with formation of a binuclear Pd(III) cation containing a Pd–Pd bond, [Ph(mpp)Pd(μ-OAc)2Pd(mpp)]+ (14). Cation 14 undergoes mpp···Ph coupling at one Pd center to form the binuclear Pd(II) cation [(mppPh-N)Pd(μ-OAc)2Pd(mpp)]+ (Da). Cation Da may fragment to release mppPh and mononuclear palladium species, followed by orthopalladation at a mononuclear center. However, in an environment of very low acetate concentration and high nitrogen-donor concentration, it is considered far more likely that Da undergoes ligand exchange with release of mppPh and formation of [(mppH-N)Pd(μ-OAc)2Pd(mpp)]+ (I). Computation shows a low-energy pathway for orthopalladation at cation I that involves nitrogen-donor reagents mppH and mppPh acting as bases to remove a proton as [HN-donor]+. This orthopalladation would complete the cycle and regenerate the catalyst, [Pd(mpp)(μ-OAc)]2 (8). A Hammett plot obtained from a computational analysis of the reaction of [(p-X-C6H4)(Mes)I]BF4 (X = H, Me, OMe, F, Cl, COMe, CF3) has a reaction constant (ρ) of 1.8, which compares well with the experimental result (ρ = 1.7 ± 0.2). Consistent with this, the analysis reveals the dominant role of the interaction energy for palladium- and iodine-containing fragments in the transition structure.

The mechanism for the polymerization of ethylene via a mononuclear aluminum catalyst has been shown previously to lead to inconsistent results. We propose here for the first time a plausible mechanism involving a dinuclear aluminum species which overcomes the problems of the mononuclear catalyst. With the aid of computational quantum chemistry, we have shown that the dinuclear pathway has a much lower activation energy than the mononuclear pathway, a result which can be explained in terms of a greater orbital overlap being maintained in the dinuclear transition structure. We show that the generation of one growing polymer chain is more likely than that of two or three growing polymer chains. Importantly we find that the propagation step is more favorable than termination, which is in contrast to the findings with the mononuclear catalyst.

Oxidation of binuclear Pd(II) complexes with PhICl2 or PhI(OAc)2 has previously been shown to afford binuclear Pd(III) complexes featuring a Pd–Pd bond. In contrast, oxidation of binuclear Pd(II) complexes with electrophilic trifluoromethylating (“CF3+”) reagents has been reported to afford mononuclear Pd(IV) complexes. Herein, we report experimental and computational studies of the oxidation of a binuclear Pd(II) complex with “CF3+” reagents. These studies suggest that a mononuclear Pd(IV) complex is generated by an oxidation–fragmentation sequence proceeding via fragmentation of an initially formed, formally binuclear Pd(III), intermediate. The observation that binuclear Pd(III) and mononuclear Pd(IV) complexes are accessible in the same reactions offers an opportunity for understanding the role of nuclearity in both oxidation and subsequent C–X bond-forming reactions.

Density functional theory has been used to investigate the reactions of 1,5 enynes with alcohols in the presence of a gold catalyst. We have compared the mechanism of the alcohol addition reaction for the enyne with that of the enyne where the carbon at position 3 is replaced with silicon. We find that different intermediates are present in both cases, and in the case of the silicon analogue, the intermediate that we find from the calculations is different from any that have previously been proposed in the literature. For the silicon analogue we have been able to rationalize the observed effects of alcohol concentration and nucleophilicity on the product distribution. For the carbon-based enyne we have shown why different products are observed depending on the substitution at position 3 of the enyne. Overall, we have provided for the first time a consistent explanation and rationalization of several different experiments that have been previously published in the literature. Our mechanism will assist in predicting the outcome of experimental reactions involving different alcohols, reagent concentrations, and substitution patterns of the 1,5 enynes.

Synthetic routes to methyl(aryl)alkynylpalladium(IV) motifs are presented, together with studies of selectivity in carbon–carbon coupling by reductive elimination from PdIV centres. The iodonium reagents IPh(CCR)(OTf) (R = SiMe3, But, OTf = O3SCF3) oxidise PdIIMe(p-Tol)(L2) (1–3) [L2 = 1,2-bis(dimethylphosphino)ethane (dmpe) (1), 2,2′-bipyridine (bpy) (2), 1,10-phenanthroline (phen) (3)] in acetone-d6 or toluene-d9 at −80 °C to form complexes PdIV(OTf)Me(p-Tol)(CCR)(L2) [R = SiMe3, L2 = dmpe (4), bpy (5), phen (6); R = But, L2 = dmpe (7), bpy (8), phen (9)] which reductively eliminate predominantly (>90%) p-Tol-CCR above ∼−50 °C. NMR spectra show that isomeric mixtures are present for the PdIV complexes: three for dmpe complexes (4, 7), and two for bpy and phen complexes (5, 6, 8, 9), with reversible reduction in the number of isomers to two occurring between −80 °C and −60 °C observed for the dmpe complex 4 in toluene-d8. Kinetic data for reductive elimination from PdIV(OTf)Me(p-Tol)(CCSiMe3)(dmpe) (4) yield similar activation parameters in acetone-d6 (66 ± 2 kJ mol−1, ΔH‡ 64 ± 2 kJ mol−1, ΔS‡ −67 ± 2 J K−1 mol−1) and toluene-d8 (Ea 68 ± 3 kJ mol−1, ΔH‡ 66 ± 3 kJ mol−1, ΔS‡ −74 ± 3 J K−1 mol−1). The reaction rate in acetone-d6 is unaffected by addition of sodium triflate, indicative of reductive elimination without prior dissociation of triflate. DFT computational studies at the B97-D level show that the energy difference between the three isomers of 4 is small (12.6 kJ mol−1), and is similar to the energy difference encompassing the six potential transition state structures from these isomers leading to three feasible C–C coupling products (13.0 kJ mol−1). The calculations are supportive of reductive elimination occurring directly from two of the three NMR observed isomers of 4, involving lower activation energies to form p-TolCCSiMe3 and earlier transition states than for other products, and involving coupling of carbon atoms with higher s character of σ-bonds (sp2 for p-Tol, sp for CC–SiMe3) to form the product with the strongest C–C bond energy of the potential coupling products. Reductive elimination occurs predominantly from the isomer with Me3SiCC trans to OTf. Crystal structure analyses are presented for PdIIMe(p-Tol)(dmpe) (1), PdIIMe(p-Tol)(bpy) (2), and the acetonyl complex PdIIMe(CH2COMe)(bpy) (11).

The palladium-catalyzed silaboration of pyridines has been investigated with the use of density functional theory. The results predict a very interesting dearomatization step in the reaction mechanism which is surprisingly facile due to the formation of a very strong covalent bond between nitrogen and boron in the product. Our calculations show that the regioselectivity of the final product is governed by a mixture of electronic and steric effects, and our predicted outcomes are in agreement with the experimental results.

Although organocopper and organosilver compounds are known to decompose by homolytic pathways among others, surprisingly little is known about their bond dissociation energies (BDEs). In order to address this deficiency, the performance of the DFT functionals BLYP, B3LYP, BP86, TPSSTPSS, BHandHLYP, M06L, M06, M06-2X, B97D, and PBEPBE, along with the double hybrids, mPW2-PLYP, B2-PLYP, and the ab initio methods, MP2 and CCSD(T), have been benchmarked against the thermochemistry for the M–C homolytic BDEs (D0) of Cu–CH3 and Ag–CH3, derived from guided ion beam experiments and CBS limit calculations (D0(Cu–CH3) = 223 kJ·mol–1; D0(Ag–CH3) = 169 kJ·mol–1). Of the tested methods, in terms of chemical accuracy, error margin, and computational expense, M06 and BLYP were found to perform best for homolytic dissociation of methylcopper and methylsilver, compared with the CBS limit gold standard. Thus the M06 functional was used to evaluate the M–C homolytic bond dissociation energies of Cu–R and Ag–R, R = Et, Pr, iPr, tBu, allyl, CH2Ph, and Ph. It was found that D0(Ag–R) was always lower (∼50 kJ·mol–1) than that of D0(Cu–R). The trends in BDE when changing the R ligand reflected the H–R bond energy trends for the alkyl ligands, while for R = allyl, CH2Ph, and Ph, some differences in bond energy trends arose. These trends in homolytic bond dissociation energy help rationalize the previously reported (Rijs, N. J.; O’Hair, R. A. J. Organometallics2010, 29, 2282–2291) fragmentation pathways of the organometallate anions, [CH3MR]−.

Reductive elimination of C–Cl and C–C bonds from binuclear organopalladium complexes containing Pd–Pd bonds with overall formal oxidation state +III are explored by density functional theory for dichloromethane and acetonitrile solvent environments. An X-ray crystallographically authenticated neutral complex, [(L-C,N)ClPd(μ-O2CMe)]2 (L = benzo[h]quinolinyl) (I), is examined for C–Cl coupling, and the proposed cation, [(L-C,N)PhPd1(μ-O2CMe)2Pd2(L-C,N)]+ (II), examined for C–C coupling together with (L-C,N)PhPd1(μ-O2CMe)2Pd2Cl(L-C,N) (III) as a neutral analogue of II. In both polar and nonpolar solvents, reaction from III via chloride dissociation from Pd2 to form II is predicted to be favored. Cation II undergoes Ph–C coupling at Pd1 with concomitant Pd1–Pd2 lengthening and shortening of the Pd1–O bond trans to the carbon atom of L; natural bond orbital analysis indicates that reductive coupling from II involves depopulation of the dx2–y2 orbital of Pd1 and population of the dz2 orbitals of Pd1 and Pd2 as the Pd–Pd bond lengthens. Calculations for the symmetrical dichloro complex I indicate that a similar dissociative pathway for C–Cl coupling is competitive with a direct (nondissociative) pathway in acetonitrile, but the direct pathway is favored in dichloromethane. In contrast to the dissociative mechanism, direct coupling for I involves population of the dx2–y2 orbital of Pd1 with Pd1–O1 lengthening, significantly less population occurs for the dz2 orbital of Pd1 than for the dissociative pathway, and dz2 at Pd2 is only marginally populated resulting in an intermediate that is formally a Pd1(I)-Pd2(III) species, (L-Cl-N,Cl)Pd1(μ-O2CMe)Pd2Cl(O2CMe)(L-C,N) that releases chloride from Pd2 with loss of Pd(I)–Pd(III) bonding to form a Pd(II) species. A similar process is formulated for the less competitive direct pathway for C–C coupling from III, in this case involving decreased population of the dz2 orbital of Pd2 and strengthening of the Pd(I)–Pd(III) interaction in the analogous intermediate with η2-coordination at Pd1 by L-Ph-N, C1-C2.

The Laplaza-Cummins catalyst L3Mo (L = N(R)Ar), is experimentally inactive towards carbon dioxide. Previous theoretical analysis identified the cause for this inactivity and suggested that a switch to a d2 transition metal may induce activity towards the inert CO2 molecule. In this manuscript we have tested this hypothesis by altering the central metal to Ta, Nb or V. Our calculations suggest that the tantalum analogue, TaL3, will successfully bind to CO2 in a mononuclear η2 arrangement and, importantly, will strongly activate one C–O bond to a point where spontaneous C–O cleavage occurs. This prediction of a strongly exothermic reaction takes into consideration the initial barrier to formation, spin crossings, ligand bulk and even the choice of density functional in the calculations. The Nb analogue will likely coordinate CO2 but reaction may not proceed further. In contrast, the V analogue faces an initial coordination barrier and is not expected to be sufficiently active to coordinate CO2 to the triamide catalyst. A similar scenario exists for mixed metal interactions involving a d2 and d4 combination in a bridging dinuclear arrangement.

We have used density functional theory to investigate the reductive elimination from platinum(IV) structures of the form L2PtR4 where L = PMe3, PH3, PF3, CO, NH3 and R = vinyl, Me. We conclude that reductive elimination occurs via the L-predissociation pathway for R = Me, irrespective of ligand L. But when R = vinyl, direct elimination is the preferred pathway if the L ligand is PMe3; otherwise both pathways are competitive for R = vinyl. We also note that if L is more π-electron accepting and less σ-electron donating, the reductive elimination from the six-coordinate complexes L2PtR4 will be more rapid. Reductive elimination from the five-coordinate complexes LPtR4 proceeds more easily if the ligand trans to the two R groups being coupled is more σ-electron donating and the ligands cis to the two R groups are more π-electron accepting.

Density functional theory has been used to analyze the detailed reaction mechanism for the reductive cleavage of CO2 by a dinitrogen bridged bis-β-diketoiminatediiron complex, LtBuFe−N2−FeLtBu (I), recently reported by Holland and co-workers. A number of pathways have been investigated and the most likely mechanism correlates well with experimental evidence. A rationale has been provided for the binding of CO2, the release of CO, and the ready formation of CO32−. Our results show that the insertion of CO2 into the diiron complex is the rate determining step of the reductive cleavage reaction. An intramolecular reduction step from the reduced dinitrogen bridge is proposed which serves to increase the activation of CO2. This is followed by an intersystem crossing from the septet to the nonet state which acts as a driving force for the subsequent release of CO. The overall reductive cleavage reaction is exergonic by 120 kJ/mol, and further reaction of the released CO with the starting diiron complex is also predicted to be strongly exergonic.

Ligand effects in bimetallic high oxidation state systems containing a X−Pd−Pd−Y framework have been explored with density functional theory (DFT). The ligand X has a strong effect on the dissociation reaction of Y to form [X−Pd−Pd]+ + Y−. In the model system examined where Y is a weak σ-donor ligand and a good leaving group, we find that dissociation of Y is facilitated by greater σ-donor character of X relative to Y. We find that there is a linear correlation of the Pd−Y and Pd−Pd bond lengths with Pd−Y bond dissociation energy, and with the σ-donating ability of X. These results can be explained by the observation that the Pd dz2 population in the PdY fragment increases as the donor ability of X increases. In these systems, the PdIII−PdIII arrangement is favored when X is a weak σ-donor ligand, while the PdIV−PdII arrangement is favored when X is a strong σ-donor ligand. Finally, we demonstrate that ligand exchange to form a bimetallic cationic species in which each Pd is six-coordinate should be feasible in a high polarity solvent.

The mechanistic subtleties involved with the interaction of an amido/bis(phosphine)-supported (PNP)Ir fragment with a series of linear and cyclic ethers have been investigated using density functional theory. Our analysis has revealed the factors dictating reaction direction toward either an iridium-supported carbene or a vinyl ether adduct. The (PNP)Ir structure will allow carbene formation only from accessible carbons α to the ethereal oxygen, such that d electron back-donation from the metal to the carbene ligand is possible. Should these conditions be unavailable, the main competing pathway to form vinyl ether can occur, but only if the (PNP)Ir framework does not sterically interfere with the reacting ether. In situations where steric hindrance prevents unimpeded access to both pathways, the reaction may progress to the initial C−H activation but no further. Our mechanistic analysis is density functional independent and whenever possible confirmed experimentally by trapping intermediate species experimentally. We have also highlighted an interesting systematic error present in the DFT analysis of reactions where steric environment alters considerably within a reaction.

Experimental results have previously suggested that the transmetalation step in the Stille reaction is hindered at one extreme by very bulky ligands L on the PdL2 catalyst, yet at the other extreme, transmetalation is also found to be slow for small ligands. Our aim in this paper is to resolve this dilemma using computational chemistry and to show which ligand is best and why. With the use of density functional theory we show that the reason why L = PtBu3 retards transmetalation is because the bulky ligand hinders the coordination of the organostannane. On the other hand a small ligand such as L = PMe3 leads to the formation of a very stable intermediate in the catalytic cycle which then requires a large activation energy for the transmetalation to proceed. The L = PPh3 ligand appears to provide just the right balance in that it can readily coordinate the organostannane but avoids forming the very stable intermediate, and is thus the ligand of choice. L = PPh2Me is predicted to be the next best option, but L = PPhMe2 is too small and forms an intermediate whose stability prevents further reaction in the transmetalation step. Our calculations are also able to account for the accelerating role of CsF in the transmetalation step of the Stille reaction. Finally, this work demonstrates the importance of taking into account the steric properties of the full ligand in theoretical studies of such reactions, rather than using small model phosphines.

The Laplaza/Cummins L3Mo (L = N(R)Ar) system is a very important complex for activating small molecules such as N2. Previous experimental work has shown that CS2 binds to the L3Mo system and forms an Mo–CS–Mo intermediate, while the environmentally important CO2 molecule is unreactive. The aim of this paper is to explain why there is this contrast in reactivity. We have used density functional methods to show that at first glance the reaction of 3L3Mo + CO2 should proceed smoothly to give L3Mo–O + L3Mo–CO–MoL3. However initial coordination of the CO2 molecule to L3Mo does not take place because of the bending of CO2, the energy required to cross from the doublet to the quartet state, and the lower metal–CO2 binding energy compared to metal–CS2. The subsequent formation of the L3Mo–CO–MoL3 intermediate is similarly unfeasible due to steric and entropic effects. We have provided a molecular orbital rationalization for these effects and have also shown that it is important to take account of steric factors in order to get an accurate understanding of the energetic picture.

An alternative mechanism for intramolecular C−C coupling between heterocycles and alkenes with rhodium phosphine catalysts is presented involving oxidative addition, alkene insertion, and reductive elimination (route 2), as described previously for similar group 10 reactions by Cavell and McGuinness. Computational studies indicate that the rate-determining step is associated with reductive elimination of the product and overall barriers indicate this mechanism would be competitive with an alternative involving formation of a carbene complex derived from mechanistic work by Bergman, Ellman, and associates (route 1). Activation of the reacting azole through inclusion of an acid catalyst appears to support the route 2 mechanism. A much lower activation barrier is observed under acidic conditions, a result consistent with that found under experimental conditions.

Generation of N-heterocyclic carbene (NHC) complexes [(dmpe)M(azol-2-ylidene)R] via the oxidative addition of a series of 2-substituted azolium salts to Group-10 zerovalent metal complexes has been investigated using density functional theory (2-R = H, Me, Ph; Azole = imidazole, thiazole, oxazole; M = Ni, Pd, Pt). Overall, platinum-based pathways result in the greatest enthalpies of reaction, but due to the reactive nature of Group-10 metals bearing the 1,2-bis(dimethylphosphino)ethane (dmpe) chelate, nickel and palladium species also have little trouble proceeding to stable products in the absence of a significant barrier. Imidazolium salts were found to be the most vulnerable to oxidative addition due to their low stabilisation energies when compared to the oxazolium and thiazolium species. Activation barriers show the general trend of phenyl > methyl > hydrido with regard to the azole 2-substituent, with no observed barrier for all but one of the 2-hydrido cases. Minimal barriers were found to exist in a number of cases for activation of a C(2)–CH3 bond suggesting that synthesis of alkyl–carbene complexes may be possible via this route under certain conditions, and therefore ionic liquids based on these substituted azolium salts may be active participants in catalytic reactions.

The reductive elimination of 2-hydrocarbyl-imidazolium salts from hydrocarbyl–palladium complexes bearing N-heterocyclic carbene (NHC) ligands represents an important deactivation route for catalysts of this type. We have explored the influence that carbene N-substituents have on both the activation energy and the overall thermodynamics of the reductive elimination reaction using density functional theory (DFT). Given the proximity of the N-substituent to the three-centred transition structure, steric bulk has little influence on the activation barrier and it is electronic factors that dominate the barriers' magnitude. Increased electron donation from the departing NHC ligand acts to stabilise the associated complex against reductive elimination, with stability following the trend: Cl < H < Ph < Me < Cy < iPr < neopentyl < tBu. The intimate involvement of the carbene pπ-orbital in determining the barrier to reductive elimination means N-substituents that are capable of removing π-density (e.g. phenyl) act to promote a more facile reductive elimination.

The influence of spectator ligand bite angle and the twist angle of the carbene on the reductive elimination of N-heterocyclic carbenes (NHCs) from palladium bis-phosphine complexes has been investigated using density functional theory. The spectator bite angle was found to have a significant influence on both the activation energy (Eact) and the enthalpy of reaction. Widening of the bite angle was found to lower Eact and increase the enthalpy of reaction. In contrast, rotation of the carbene with respect to the PdL2 plane was found to have little influence on Eact. At carbene twist angles approaching 0° however, relief of the increased steric strain provides a considerable driving force for the decomposition reaction.

The oxidative addition of 1,3-dimethylimidazolium to a model Wilkinson's catalyst (RhCl(PH3)3) has been studied with density functional calculations (B3LYP). According to our free energy calculations, the octahedral rhodium carbene hydride product forms from initial predissociation of a phosphine molecule to subsequently form a 5 ligand intermediate; however, results indicate that a six ligand, associative route with a concerted three-centred transition structure may also be competitive. Exchange of the phosphine molecule on the metal centre with trimethylphosphine had a significant effect in lowering the barrier to oxidative addition and decreasing the endothermicity of the reaction. Solvation was found to have a moderate effect on the overall reaction. Bulk solvent calculations reflected a relative stabilisation of reactants for both pathways, resulting in an endothermic overall reaction. A study of alternative azolium salts revealed the saturated 1,3-dimethyl-4,5-dihydroimidazolium resulted in little change to the reaction geometries or energies, while the use of 3-methylthiazolium salt significantly reduced the barrier to addition and increased the exothermicity of the reaction considerably.

Generation of N-heterocyclic carbene (NHC) complexes [(dmpe)M(azol-2-ylidene)R] via the oxidative addition of a series of 2-substituted azolium salts to Group-10 zerovalent metal complexes has been investigated using density functional theory (2-R = H, Me, Ph; Azole = imidazole, thiazole, oxazole; M = Ni, Pd, Pt). Overall, platinum-based pathways result in the greatest enthalpies of reaction, but due to the reactive nature of Group-10 metals bearing the 1,2-bis(dimethylphosphino)ethane (dmpe) chelate, nickel and palladium species also have little trouble proceeding to stable products in the absence of a significant barrier. Imidazolium salts were found to be the most vulnerable to oxidative addition due to their low stabilisation energies when compared to the oxazolium and thiazolium species. Activation barriers show the general trend of phenyl > methyl > hydrido with regard to the azole 2-substituent, with no observed barrier for all but one of the 2-hydrido cases. Minimal barriers were found to exist in a number of cases for activation of a C(2)–CH3 bond suggesting that synthesis of alkyl–carbene complexes may be possible via this route under certain conditions, and therefore ionic liquids based on these substituted azolium salts may be active participants in catalytic reactions.

A theoretical study into the reactions of the N2O adducts of N-heterocyclic carbenes (NHCs) and a V(III) complex was carried out using DFT calculations. Unlike most transition metal reactions with N2O that simply release N2 following O-atom transfer onto the metal centre, this NHC-based system traps the entire N2O molecule and then cleaves both the N–O and N–N bond in two consecutive reactions. The NHC presence increases the reactivity of N2O by altering the distribution of electron density away from the O-atom towards the two N-atoms. This electronic redistribution enables V–N binding interactions to form a reactive N,O-donor intermediate species. Our results show that bond breaking with concomitant ligand migration occurs via a concerted process for both the N–O and N–N cleavage reactions.

The Laplaza-Cummins catalyst L3Mo (L = N(R)Ar), is experimentally inactive towards carbon dioxide. Previous theoretical analysis identified the cause for this inactivity and suggested that a switch to a d2 transition metal may induce activity towards the inert CO2 molecule. In this manuscript we have tested this hypothesis by altering the central metal to Ta, Nb or V. Our calculations suggest that the tantalum analogue, TaL3, will successfully bind to CO2 in a mononuclear η2 arrangement and, importantly, will strongly activate one C–O bond to a point where spontaneous C–O cleavage occurs. This prediction of a strongly exothermic reaction takes into consideration the initial barrier to formation, spin crossings, ligand bulk and even the choice of density functional in the calculations. The Nb analogue will likely coordinate CO2 but reaction may not proceed further. In contrast, the V analogue faces an initial coordination barrier and is not expected to be sufficiently active to coordinate CO2 to the triamide catalyst. A similar scenario exists for mixed metal interactions involving a d2 and d4 combination in a bridging dinuclear arrangement.

The cleavage of one N–O bond in NO2 by two equivalents of Mo(NRAr)3 has been shown to occur to form molybdenum oxide and nitrosyl complexes. The mechanism and electronic rearrangement of this reaction was investigated using density functional theory, using both a model Mo(NH2)3 system and the full [N(tBu)(3,5-dimethylphenyl)] experimental ligand. For the model ligand, several possible modes of coordination for the resulting complex were observed, along with isomerisation and bond breaking pathways. The lowest barrier for direct bond cleavage was found to be via the singlet η2-N,O complex (7 kJ mol−1). Formation of a bimetallic species was also possible, giving an overall decrease in energy and a lower barrier for reaction (3 kJ mol−1). Results for the full ligand showed similar trends in energies for both isomerisation between the different isomers, and for the mononuclear bond cleavage. The lowest calculated barrier for cleavage was only 21 kJ mol−1via the triplet η1-O isomer, with a strong thermodynamic driving force to the final products of the doublet metal oxide and a molecule of NO. Formation of the full ligand dinuclear complex was not accompanied by an equivalent decrease in energy seen with the model ligand. Direct bond cleavage via an η1-O complex is thus the likely mechanism for the experimental reaction that occurs at ambient temperature and pressure. Unlike the other known reactions between MoL3 complexes and small molecules, the second equivalent of the metal does not appear to be necessary, but instead irreversibly binds to the released nitric oxide.

The Laplaza/Cummins L3Mo (L = N(R)Ar) system is a very important complex for activating small molecules such as N2. Previous experimental work has shown that CS2 binds to the L3Mo system and forms an Mo–CS–Mo intermediate, while the environmentally important CO2 molecule is unreactive. The aim of this paper is to explain why there is this contrast in reactivity. We have used density functional methods to show that at first glance the reaction of 3L3Mo + CO2 should proceed smoothly to give L3Mo–O + L3Mo–CO–MoL3. However initial coordination of the CO2 molecule to L3Mo does not take place because of the bending of CO2, the energy required to cross from the doublet to the quartet state, and the lower metal–CO2 binding energy compared to metal–CS2. The subsequent formation of the L3Mo–CO–MoL3 intermediate is similarly unfeasible due to steric and entropic effects. We have provided a molecular orbital rationalization for these effects and have also shown that it is important to take account of steric factors in order to get an accurate understanding of the energetic picture.

Synthetic routes to methyl(aryl)alkynylpalladium(IV) motifs are presented, together with studies of selectivity in carbon–carbon coupling by reductive elimination from PdIV centres. The iodonium reagents IPh(CCR)(OTf) (R = SiMe3, But, OTf = O3SCF3) oxidise PdIIMe(p-Tol)(L2) (1–3) [L2 = 1,2-bis(dimethylphosphino)ethane (dmpe) (1), 2,2′-bipyridine (bpy) (2), 1,10-phenanthroline (phen) (3)] in acetone-d6 or toluene-d9 at −80 °C to form complexes PdIV(OTf)Me(p-Tol)(CCR)(L2) [R = SiMe3, L2 = dmpe (4), bpy (5), phen (6); R = But, L2 = dmpe (7), bpy (8), phen (9)] which reductively eliminate predominantly (>90%) p-Tol-CCR above ∼−50 °C. NMR spectra show that isomeric mixtures are present for the PdIV complexes: three for dmpe complexes (4, 7), and two for bpy and phen complexes (5, 6, 8, 9), with reversible reduction in the number of isomers to two occurring between −80 °C and −60 °C observed for the dmpe complex 4 in toluene-d8. Kinetic data for reductive elimination from PdIV(OTf)Me(p-Tol)(CCSiMe3)(dmpe) (4) yield similar activation parameters in acetone-d6 (66 ± 2 kJ mol−1, ΔH‡ 64 ± 2 kJ mol−1, ΔS‡ −67 ± 2 J K−1 mol−1) and toluene-d8 (Ea 68 ± 3 kJ mol−1, ΔH‡ 66 ± 3 kJ mol−1, ΔS‡ −74 ± 3 J K−1 mol−1). The reaction rate in acetone-d6 is unaffected by addition of sodium triflate, indicative of reductive elimination without prior dissociation of triflate. DFT computational studies at the B97-D level show that the energy difference between the three isomers of 4 is small (12.6 kJ mol−1), and is similar to the energy difference encompassing the six potential transition state structures from these isomers leading to three feasible C–C coupling products (13.0 kJ mol−1). The calculations are supportive of reductive elimination occurring directly from two of the three NMR observed isomers of 4, involving lower activation energies to form p-TolCCSiMe3 and earlier transition states than for other products, and involving coupling of carbon atoms with higher s character of σ-bonds (sp2 for p-Tol, sp for CC–SiMe3) to form the product with the strongest C–C bond energy of the potential coupling products. Reductive elimination occurs predominantly from the isomer with Me3SiCC trans to OTf. Crystal structure analyses are presented for PdIIMe(p-Tol)(dmpe) (1), PdIIMe(p-Tol)(bpy) (2), and the acetonyl complex PdIIMe(CH2COMe)(bpy) (11).