Theoretical Study of the Oxidative Addition of Ammonia to Various Unsaturated Low-Valent Transition Metal Species

Reaction profiles for the oxidative addition of NH3 to a number of unsaturated low-valent transition metal complexes have been computed using gradient-corrected density functional theory. The metal complexes studied are d8 CpM(CO) (M = Rh, Ir) and trans-M(PH3)2X (M = Rh, Ir; X = H, Cl) and d10 ML2 (M = Pd, Pt; L = PH3, L2 = H2PCH2CH2PH2, dpe). Reactions with the d8 species are characterized by the formation of strongly bound ammine complexes from which computed activation energies for oxidative addition are in excess of 16 kcal mol-1. Computed reaction enthalpies are all exothermic with these complexes. With d10 M(PH3)2 species computed ammine adducts are weak, activation barriers are in excess of 23 kcal mol-1, and the overall reaction is endothermic for both M = Pd and Pt. The introduction of the chelating dpe ligand results in stronger ammine adducts but only slightly reduced computed activation barriers. Of the d10 species only the reaction with Pt(dpe) is computed to be exothermic. Comparison of the computed reaction profiles for analogous second- and third-row complexes shows the NH3 oxidative addition reaction to be more favorable with the third-row species, which exhibit more strongly bound ammine adducts, lower activation barriers, and more exothermic reactions. Of the species studied the most promising unsaturated fragments for effecting NH3 oxidative addition are CpIr(CO), trans-Ir(PH3)2X (X = H, Cl), and Pt(dpe). The more favorable thermodynamics computed with these third-row species arise from higher M−NH2 and M−H homolytic bond strengths in the hydrido-amido products. M-NH2 bonds are computed to be between 6 and 13 kcal mol-1 and M−H bonds between 5 and 14 kcal mol-1 stronger in the third-row complexes compared to their second-row congeners. For complexes exhibiting no N→M π-donation M−NH2 bonds are computed to be up to 26 kcal mol-1 weaker than M−H bonds. N→M π-donation reduces this differential, and in Ir(PH3)2(H)2(NH2) the Ir−NH2 and Ir−H bonds are calculated to have equal homolytic bond strengths. Computed activation energies for NH3 oxidative addition do not appear to be related to the strength of the ammine adduct, and for metal complexes of the same row the computed activation energy is relatively insensitive to the nature of the unsaturated fragment. These findings are discussed in terms of an NH3 reorientation/N−H bond activation model for the oxidative addition reaction. Although strongly Lewis acidic metal fragments usually promote oxidative addition, with NH3 these form strong ammine adducts from which NH3 reorientation is energetically costly. For metal fragments with lower Lewis acidity NH3 reorientation is more facile, but the subsequent oxidative addition remains difficult. These ideas are supported by the accessibility of η1-H and η3-H,H,H NH3 adducts formed with Pt(dpe), while with Ir(PH3)2Cl only a high-energy η1-H species was located

Computational Study of the Reaction of C₆F₆ with [IrMe(PEt₃)₃]: Identification of a Phosphine-Assisted C−F Activation Pathway via a Metallophosphorane Intermediate

Density functional theory calculations have been used to model the reaction of C6F6 with [IrMe(PEt3)3], which proceeds with both C−F and P−C bond activation to yield trans-[Ir(C6F5)(PEt3)2(PEt2F)], C2H4, and CH4 (Blum, O.; Frolow, F.; Milstein, D. J. Chem. Soc., Chem. Commun. 1991, 258). Using a model species, trans-[IrMe(PH3)2(PH2Et)], a low-energy mechanism involving nucleophilic attack of the electron-rich Ir metal center at C6F6 with displacement of fluoride has been identified. A novel feature of this process is the capture of fluoride by a phosphine ligand to generate a metallophosphorane intermediate [Ir(C6F5)(Me)(PH3)2(PH2EtF)]. These events occur in a single step via a 4-centered transition state, in a process that we have termed “phosphine-assisted C−F activation”. Alternative mechanisms based on C−F activation via concerted oxidative addition or electron-transfer processes proved less favorable. From the metallophosphorane intermediate the formation of the final products can be accounted for by facile ethyl group transfer from phosphorus to iridium followed by β-H elimination of ethene and reductive elimination of methane. The interpretation of phosphine-assisted C−F activation in terms of nucleophilic attack is supported by the reduced activation barriers computed with the more electron-rich model reactant trans-[IrMe(PMe3)2(PMe2Et)] and the higher barriers found with lesser fluorinated arenes. Reactivity patterns for a range of fluoroarenes indicate the dominance of the presence of ortho-F substituents in promoting phosphine-assisted C−F activation, and an analysis of the charge distribution and transition state geometries indicates that this process is controlled by the strength of the Ir-aryl bond that is being formed

Computational Studies on the Stabilities of <i>trans</i>-[Ir(OMe)(CO)(PPh₃)₂] and <i>trans</i>-[Ir(CH₂Me)(CO)(PPh₃)₂] toward β-H Elimination

The relative stabilities of trans-[Ir(XMe)(CO)(PR3)2] species (X = O, CH2) toward β-H elimination have been studied via combination of density functional and hybrid density functional/Hartree−Fock calculations. For both small (R = H) and full (R = Ph) model systems β-H elimination from the methoxide species is found to be disfavored both kinetically and thermodynamically compared to that from the analogous ethyl complexes. This is consistent with the greater stability of alkoxide species seen experimentally (R = Ph). In all cases the major contribution to the activation barrier is phosphine dissociation, and for the alkyl systems this leads directly to an agostically stabilized intermediate from which β-H transfer readily occurs. In contrast, with the trans-[Ir(OMe)(CO)(PR3)2] species a π-stabilized intermediate is formed and a further isomerization barrier must be overcome before β-H transfer can be accessed. Further calculations were performed on the acetophenone complex [Ir(H)(η2-OC(Me)Ph)(CO)(PPh3)], and a low-energy pathway for face exchange of the metal-bound ketone has been characterized. This involves an η1-intermediate and provides a mechanism for facile racemization of the precursor alkoxide. Selected calculations using alternative hybrid calculations showed the sensitivity of PPh3 binding energies to the methodology employed. This is especially the case for the final step in the β-H elimination reaction, the formation of [Ir(H)(CO)(PPh3)3] from [Ir(H)(CO)(PPh3)2] and free PPh3, where the use of the UFF approach appears to be particularly unreliable

Theoretical Study of the Electronic Structure of Group 6 [M(CO)₅X]^- Species (X = NH₂, OH, Halide, H, CH₃) and a Reinvestigation of the Role of π-Donation in CO Lability

Density functional calculations have been employed to investigate the electronic structure of [M(CO)5X]- species (M = Cr, Mo, W; X = NH2, OH, halide, H, CH3) and to compute CO ligand dissociation energies. The calculations indicate that CO loss is most facile from the cis position, and CO dissociation energies are computed to increase along the series X = NH2 3 < H. These results are in agreement with available experimental data. Trends in CO dissociation are related to the ability of X to stabilize the unsaturated 16e [M(CO)4X]- species formed. In addition, π-destabilization of the ground-state [M(CO)5X]- species is equally significant. Analysis of the electronic structure of the 18e species shows that Xπ−dπ 4e destabilization results in hybridization at the metal center which enhances trans M−CO but reduces cis M−CO π-back-donation. Strong π-donation from X also induces σ-antibonding interactions between the metal and the cis CO ligands. A fragment analysis reveals that these effects are strongest for the “hard” fluoride, hydroxide, and amide ligands

Theoretical Studies on the Insertions of Unsymmetrical Alkynes into the Metal−Carbon Bond of Phosphanickelacycles: Electronic Factors

Density functional calculations have been employed to study the insertion reactions of alkynes (RC⋮CR‘:  R = H, R‘ = H, Me, CF3, Ph; R = Me, R‘ = Ph; R = CO2H, R‘ = H, Me, CF3, Ph) with the model phosphanickelacycle [NiBr(CHCHCH2PH2-κC,P)(PH3)]. Calculations with HC⋮CH indicate that associative processes with insertion via 5-coordinate intermediates are preferred kinetically over an alternative mechanism involving initial displacement of a PH3 ligand. Two possible trigonal-bipyramidal 5-coordinate intermediates were located with either Br or PH3 occupying an axial position trans to the Ni−vinyl bond. The preference for an associative process was confirmed with HO2CC⋮CH and HC⋮CMe. Computed 5-coordinate transition state energies for unsymmetrical alkynes are generally consistent with the regioselectivities observed with experimental analogues. One exception is HC⋮CCF3, for which the wrong regioisomer is marginally favored, although the computed energy difference between the transition states leading to opposite regioisomers is negligible. Both the observed and calculated results are discussed in terms of a simple model for predicting the insertion regioselectivities based on the polarization of the alkyne π⊥ orbital. In all cases, this model accounts well for the experimental regioselectivities but analysis of the computational results shows the success of this approach depends on both the alkyne and the 5-coordinate intermediate from which the insertion occurs. In particular, when electron-withdrawing substituents are present, a swap in regioselectivity is often predicted, depending upon whether insertion proceeds from the isomer with Br axial or from that with PH3 axial

Theoretical Study of the CO Migratory Insertion Reactions of Pt(Me)(OMe)(dppe) and Ni(Me)(OR)(bpy) (R = Me, O-<i>p</i>-C₆H₄CN): Comparison of Group 10 Metal−Alkyl, −Alkoxide, and −Aryloxide Bonds

We report the results of theoretical mechanistic studies on the alternative migratory insertion reactions of CO with the metal−oxygen and metal−carbon bonds of Pt(Me)(OMe)(dhpe) (dhpe = H2PCH2CH2PH2) and Ni(Me)(OR)(α-diimine) (R = Me, Ph, α-diimine = NHCHCHNH) as models for Pt(Me)(OMe)(dppe) (dppe = Ph2PCH2CH2PPh2) and Ni(Me)(O-p-C6H4CN)(bpy) (bpy = 2,2‘-bipyridyl), respectively. With Pt(Me)(OMe)(dhpe) the methoxycarbonyl product, Pt(Me)(CO2Me)(dhpe), is favored over the acyl alternative, Pt{C(O)Me}(OMe)(dhpe), by 13 kcal/mol. Two alternative pathways for methoxycarbonyl formation were located, both of which are initiated via displacement of a chelate arm to form two isomers of Pt(Me)(OMe)(CO)(η1-H2PCH2CH2PH2) (2a, CO trans to OMe; 2b, CO trans to Me). Subsequent CO migratory insertion into the Pt−OMe bond of 2b yields the methoxycarbonyl product directly. Alternatively, isomerization of 2a to a third isomer, 2c (CO trans to phosphine), can occur, from which CO migratory insertion again produces the methoxycarbonyl species. This latter isomerization/migratory insertion process represents the lowest energy pathway. Alternative CO migratory insertion reactions involving the Pt−Me bonds of 2a,c suffer from very high activation barriers. The 2a to 2c isomerization is unusual, as it involves transfer of OMe to phosphine to give a metallophosphorane intermediate, followed by OMe transfer back to the metal. The net result is a swapping of the positions of the OMe and phosphine ligands. The computed kinetic and thermodynamic preference for reaction with the Pt−OMe bond is consistent with the observed reactivity of Pt(Me)(OMe)(dppe). With the Ni(Me)(OR)(α-diimine) systems CO migratory insertion proceeds via five-coordinate CO adducts. When R = Me, insertion into the Ni−OMe bond is more accessible kinetically but the acyl product is slightly more stable by 3.5 kcal/mol. Introduction of the Ph substituent dramatically lowers the reactivity of the Ni−OR bond, with the acyl becoming the kinetically more accessible species and being 18.4 kcal/mol more stable than the phenoxycarbonyl alternative. The lower reactivity of the Ni−OPh bond arises primarily from the weak C−O bond in the phenoxycarbonyl product and accounts for the experimental preference for acyl formation in the reaction of Ni(Me)(O-p-C6H4CN)(bpy) with CO

Theoretical Study of CO Migratory Insertion Reactions with Group 10 Metal−Alkyl and −Alkoxide Bonds

The results of density functional calculations on the alternative migratory insertion reactions of CO with the M−OMe and M−Me bonds of group 10 M(Me)(OMe)(PH3)2 model systems are reported. For all three metals insertion into the M−OMe bond to form methoxycarbonyl products is thermodynamically favored over insertion into the M−Me bond to give acyls. This preference is small when M = Ni (ΔΔER = 3 kcal/mol) but increases down the triad and becomes significant for M = Pt (ΔΔER = 12 kcal/mol). Both associative five-coordinate and phosphine displacement four-coordinate mechanisms for migratory insertion were considered. For Ni associative mechanisms are more accessible and the lowest energy pathway is for reaction with the Ni−Me bond. With Pd and Pt the five- and four-coordinate pathways are close in energy, and for Pd there is a small kinetic preference for insertion into the Pd−OMe bond. For Pt however there is a clear kinetic preference for reaction with the Pt−OMe bond. During migratory insertion into M−OMe bonds the methoxide ligand rotates in the transition state to allow the participation of an oxygen lone pair in C−O bond formation while maintaining some residual M···OMe interaction. This M···OMe interaction is retained to some extent in the three-coordinate methoxycarbonyl species formed along the four-coordinate pathways. For an isostructural series of reactive species the trend in activation energy is always Ni Pd < Pt (with Pt > Ni) for reaction with the M−OMe bond. Trends in the computed thermodynamic and kinetic data of the alternative migratory insertions can be understood in terms of metal−ligand homolytic bond strengths. All M−C bonds studied show a marked increase down the group 10 triad, whereas much less variation is seen in the M−OMe bonds, which results in reaction with the M−OMe bonds being generally favored. A key additional driving force, however, is the stronger C−O bond formed in the methoxycarbonyl product compared to the C−C bond of the alternative acyl species

Computational Study of C−C Activation of 1,3-Dimesitylimidazol-2-ylidene (IMes) at Ruthenium: The Role of Ligand Bulk in Accessing Reactive Intermediates

Density functional theory calculations have been employed to model phosphine substitution in Ru(PPh3)3(CO)(H)2 to form Ru(IMes)(PPh3)2(CO)(H)2 (1mono) and Ru(IMes)2(PPh3)(CO)(H)2 (1bis), as well as the novel C(aryl)−C(sp3) intramolecular bond activation of the IMes ligand in 1bis. The computed ligand exchange energies show that 1bis is unstable with respect to displacement of IMes by PPh3 and will thus re-form 1mono over time. PPh3/IMes substitution also leads to a significant labilization of the PPh3 ligand trans to hydride, a result of increasing steric encumbrance upon the introduction of the bulky IMes ligands. The energetics of intramolecular C−C and C−H activation have been computed for both 16e Ru(IMes)n(PPh3)3-n(CO) and 14e Ru(IMes)n(PPh3)2-n(CO) species (n = 1 or 2) and indicate that the introduction of a second IMes ligand does not significantly promote the actual C−C activation step. Instead the need to have two IMes ligands present in the metal coordination sphere before C−C activation can occur is linked to the promotion of PPh3 loss in 1bis, which makes the formation of unsaturated species such as Ru(IMes)2(CO) particularly accessible

Computational Studies of Carboxylate-Assisted C-H Activation and Functionalization at Group 8-10 Transition Metal Centers

Computational studies on carboxylate-assisted C-H activation and functionalization at group 8-10 transition metal centers are reviewed. This Review is organized by metal and will cover work published from late 2009 until mid-2016. A brief overview of computational work prior to 2010 is also provided, and this outlines the understanding of carboxylate-assisted C-H activation in terms of the "ambiphilic metal-ligand assistance" (AMLA) and "concerted metalation deprotonation" (CMD) concepts. Computational studies are then surveyed in terms of the nature of the C-H bond being activated (C(sp(2))-H or C(sp(3))-H), the nature of the process involved (intramolecular with a directing group or intermolecular), and the context (stoichiometric C-H activation or within a variety of catalytic processes). This Review aims to emphasize the connection between computation and experiment and to highlight the contribution of computational chemistry to our understanding of catalytic C-H functionalization based on carboxylate-assisted C-H activation. Some opportunities where the interplay between computation and experiment may contribute further to the areas of catalytic C-H functionalization and applied computational chemistry are identified

Ability of N-Heterocyclic Carbene Ligands to Promote Intermolecular Oxidative Addition Reactions at Unsaturated Ruthenium Centers

We report the results of density functional calculations on the reactivity of a series of coordinatively unsaturated mixed phosphine/N-heterocyclic carbene complexes of ruthenium of the type Ru(CO)(IR)3-n(PH3)n, where n = 1−3 and R = H (1,3-imidazol-2-ylidene) and R = Me (1,3-dimethylimidazol-2-ylidene). The oxidative addition reactions of H2 and CH4 and the C−C bond activation of C2H6 have been studied. For all three processes, substitution of PH3 by IH results in minimal changes in the reaction energetics. In all cases H2 oxidative addition is barrierless and is downhill by around 120 kJ/mol. With CH4 activation barriers of around 75 kJ/mol are computed and the reaction is approximately thermoneutral. With C2H6 activation barriers increase to around 260 kJ/mol and the reaction is disfavored by about + 35 kJ/mol. Introduction of the IMe ligand disfavors oxidative addition, especially for the C2H6 reaction, and this trend is linked to increased steric bulk of the IMe ligand compared to IH. Computed Ru−PH3 and Ru−IR bond strengths and CO stretching frequencies indicate that PH3/IR substitution does create a more electron-rich metal center, and yet this does not facilitate oxidative addition with these Ru species. A fragment analysis reveals that, as expected, PH3/IH substitution enhances the Lewis basicity of the metal reactant. However, a more important effect is a reduction in Lewis acidity, and this factor lies behind the similar reaction energetics computed for analogous PH3- and IH-containing species