The Bond between CO and Cp′₃U in Cp′₃U(CO) Involves Back-bonding from the Cp′₃U Ligand-Based Orbitals of π-Symmetry, where Cp′ Represents a Substituted Cyclopentadienyl Ligand

The experimental CO stretching frequencies in the 1:1 adducts between (C5H5−nRn)3U and CO range from 1976 cm−1 in (C5H4SiMe3)3U(CO) to 1900 cm−1 in (C5HMe4)3U(CO). The origin of the large difference between the stretching frequencies in free (2143 cm−1) and coordinated CO and the large effect the substituents on the cyclopentadienyl ligands have on the difference is explored by DFT calculations with a small core effective core potential in which 32 electrons on uranium are explicitly treated. The results of these calculations, along with a NBO analysis, show that a σ-bond is formed between CO and an empty σ orbital on the Cp′3U fragment composed of fσ and dσ parentage orbitals. The back-bonding interaction, which results in lowering the CO stretching frequency, does not originate from nonbonding metal-based orbitals but from the filled ligand-based orbitals of π-symmetry that are used for bonding in the Cp′3U fragment. This model, which is different from the back-bonding model used in the d-transition metal complexes, rationalizes the large substituent effect in the 5f-metal complexes

Shutting Down Secondary Reaction Pathways: The Essential Role of the Pyrrolyl Ligand in Improving Silica Supported d⁰-ML₄ Alkene Metathesis Catalysts from DFT Calculations

The efficiency of silica supported d0 ML4 alkene metathesis catalysts [(≡SiO)M(NR1)(CHR2)(X)] (M = Mo, W; R1 = aryl and alkyl) is influenced by the nature of the X ancillary ligand. Replacing the alkyl ligand by a pyrrolyl ligand dramatically increases the performance of the catalyst. DFT calculations on the metathesis, the deactivation, and the byproduct formation pathways for the imido Mo and W and the alkylidyne Re complexes give a rational for the role of pyrrolyl ligand. Dissymmetry at the metal center leads to more efficient catalyst even when the difference in σ-donating ability between X and OSi is not large. β-H transfer at the square based pyramid metallacyclobutane is the key step for catalyst deactivation and byproduct formation. Overall, the greatest benefit of substituting the ancillary alkyl by a pyrrolyl ligand, [(≡SiO)M(ER1)(CHR2)(pyrrolyl)], is in fact not to improve the efficiency of the catalytic cycle of alkene metathesis, but to shut down deactivation and byproduct formation pathways. Pyrrolyl ligand, and more generally ligands having metal-bound-atoms more electronegative than carbon, disfavor mostly the two first steps (β-H transfer at the metallacyclobutane and subsequent insertion of an ethene in the M−H bond) of the deactivation channel. The [(≡SiO)M(ER1)(CHR2)(pyrrolyl)] catalyst is thus highly efficient because pyrrolyl ligand is optimal: (i) it is still a better electron donor than the siloxy group, thus, favoring the metathesis pathway (dissymmetry at the metal center); and (ii) the nitrogen of the pyrrolyl ligand is more electronegative than the carbon of the alkyl group, thus, specifically disfavoring the decomposition of the metallacyclobutane intermediate via β-H transfer

New Access to Vinylidenes from Ruthenium Polyhydrides

Reaction of the terminal acetylenes RC⋮CH (R = Ph and SiMe3) with RuH3XL2 (X = Cl, I; L = PtBu2Me) occurs (in the time of mixing) in a 2:1 stoichiometry to release RHCCH2 and form the vinylidene complexes RuHX(CCHR)L2. Ab initio DFT (B3LYP) calculations show that the vinylidene complex has a Y structure with a Cl at the foot of the Y. No intermediate is seen for this reaction, even at low temperature, or for the analogous reaction of OsH3Cl(PiPr3)2. Since PhC⋮CD forms only the isotopomer RuDI(CCHPh)L2 and PhHCCHD, a mechanism is proposed where an early event is addition of Ru−H across the C⋮C bond. Preliminary computational studies of the reaction path for the formation of the vinylidene complex support this step of insertion of the acetylene into the Ru−H bond

Oxo vs Imido Alkylidene d⁰‑Metal Species: How and Why Do They Differ in Structure, Activity, and Efficiency in Alkene Metathesis?

Density functional calculations have been carried out to analyze the origin of the differences in reactivity, selectivity, and stability toward deactivation in metathesis of d⁰ oxo alkylidene complexes vs their isoelectronic imido counterparts. DFT calculations show that the elementary steps and geometries of the extrema are similar for the oxo and imido complexes, but that the energy profiles are different, the greatest difference occurring for the deactivation pathway. For the alkene metathesis pathway, replacing the imido by an oxo ligand slightly lowers the energy barrier for alkene coordination but raises that for the [2+2]-cycloaddition and cycloreversion; it also destabilizes the trigonal bipyramidal (<b>TBP</b>) metallacyclobutane intermediate with respect to the separated reactants. The isomeric square-based pyramid (<b>SP</b>) metallacyclobutane is in general more stable, and its stability relative to the separated reactants is similar for oxo and imido systems. Consequently, the oxo complex is associated with a slightly larger energy difference between the lowest energy intermediate (<b>SP</b> or separated reactants) and the highest energy transition state (cycloreversion) than the imido complex, which accounts for a slightly lower activity. Changing the imido into an oxo ligand disfavors strongly the deactivation pathway by raising considerably the energy barrier of the β-H transfer at the <b>SP</b> metallacycle that begins the entry into the channel for deactivation and byproduct formation as well as that of the subsequent ethene insertion. This makes the oxo catalysts more selective and stable toward deactivation than the corresponding imido catalysts, when dimerization can be avoided

Silyl, Hydrido Silylene or Alternative Bonding Modes: The Many Possible Structures of [(C₅H₅)(PH₃)IrX]⁺ (X = SiHR₂ and SiR₃; R = H, CH₃, SiH₃, and Cl)

DFT calculations with the B3LYP functional are carried out on the systems [Cp(PH3)Ir(SiHR2)]+ and [Cp(PH3)Ir(SiR3)]+ (Cp = C5H5, R = H, CH3, SiH3, Cl), which are representative examples of experimental complexes where the silylene ligand can exist. Geometry optimization for the different systems gives a large variety of structures, including the conventional silyl and hydrido silylene isomers, but also other less usual bridged structures, with a variety of groups taking a bridging position. Analysis of the large amount of data, together with those previously reported for [(dhpe)Pt(SiHR2)]+ and [(dhpe)Pt(SiR3)]+ (dhpe = H2P−CH2−CH2−PH2), leads to a better understanding of the general factors governing the relative stabilities of the possible isomeric forms

DFT Investigation of the Catalytic Hydromethylation of α-Olefins by Metallocenes. 1. Differences between Scandium and Lutetium in Propene Hydromethylation

A DFT study of the catalytic properties of Cp2ScCH3 and Cp2LuCH3 in the hydromethylation of propene has been performed. The catalytic behavior of Cp2ScCH3 is confirmed, and the formation of secondary products is rationalized. It is shown that Cp2LuCH3 cannot exhibit catalytic behavior and that only stoichiometric conversion of propene to isobutane could be observed. The difference in reactivities between the two metallocenes has been investigated, and an electronic explanation is given based on differences in the coordination of propene. However, the intrinsic reactivities of the two metallocenes is proposed to be driven by both electronic and steric effects

A Rational Basis for the Axial Ligand Effect in C−H Oxidation by [MnO(porphyrin)(X)]⁺ (X = H₂O, OH⁻, O²⁻) from a DFT Study

Oxyl radical character in the MnO group of the title system is shown from a density functional theory study to be essential for efficient C−H cleavage, which is a key step in C−H oxidation. Since oxyl species have elongated Mn−O bonds relative to the more usual oxo species of type MnO, the normal expectation would be that high trans-influence ligands X should facilitate oxyl character by elongating the Mn−O bond and thus enhance both oxyl character and reactivity. Contrary to this expectation, but in line with the experimental data (Jin, N.; Ibrahim, M.; Spiro, T. G.; Groves, J. T. J. Am. Chem. Soc. 2007, 129, 12416), we find that reactivity increases along the series X = O2− − 2O for the following reasons. The ground-state singlet (S) is unreactive for all X, and only the higher-energy triplet (T) and quintet (Q) states have the oxyl character needed for reactivity, but the higher trans-influence X ligands are also shown to increase the S/T and S/Q gaps, thus making attainment of the needed T and Q states harder. The latter effect is dominant, and high trans-influence X ligands thus disfavor reaction. The higher reactivity in the presence of acid noted by Groves and co-workers is thus rationalized by the preference for having X = H2O over OH− or O2−

d⁰ Re-Based Olefin Metathesis Catalysts, Re(⋮CR)(CHR)(X)(Y): The Key Role of X and Y Ligands for Efficient Active Sites

DFT(B3PW91) calculations show that the reaction pathways for ethylene metathesis with Re(⋮CMe)(CHMe)(X)(Y) (X/Y = CH2CH3/CH2CH3; CH2CH3/OSiH3; OSiH3/CH2CH3; OCH3/OCH3, CH2CH3/OCH3, and OCF3/OCF3) occur in two steps:  first, the pseudo-tetrahedral d0 Re complexes distort to a trigonal pyramid to open a coordination site for ethylene, which remains far from Re (early transition state for C−C bond formation). The energy barrier, determined by the energy required to distort the catalyst, is the lowest for unsymmetrical ligands (X ≠ Y) when the apical site of the TBP is occupied by a good σ-donor ligand (X) and the basal site by a poor σ-donor (Y). Second, the formation of metallacyclobutanes (late transition state for C−C bond formation) has a low energy barrier for any type of ligands, decreasing for poor σ-donor X and Y ligands, because they polarize the Re−C alkylidene bond as Re+δC-δ, which favors the reaction with ethylene, itself polarized by the metal center in the reverse way. The metallacyclobutane is also a TBP, with apical alkylidyne and Y ligands, and it is stabilized by poor σ-donor X and Y. The best catalyst will have the more shallow potential energy surface, and will thus be obtained for the unsymmetrical set of ligands with X = a good σ-donor (alkyl) and Y = a poor σ-donor (O-based ligand). This rationalizes the high efficiency of well-defined Re alkylidene supported on silica, compared to its homogeneous equivalent, Re(⋮CMe)(CHMe)(OR)2

Structures of d⁴ MH₃X: a Computational Study of the Influence of the Metal and the Ligands

Density functional theory (DFT, PBE0, and range separated DFT, RSH + MP2) and coupled-cluster with single and double and perturbative triple excitations (CCSD­(T)) calculations have been used to probe the structural preference of d⁴ MH₃X^<i>q</i> (M = Ru, Os, Rh⁺, Ir⁺, and Re^–; X = H, F, CH₃, CF₃, SiH₃, and SiF₃) and of MX₄ (M = Ru; X = H, F, CH₃, CF₃, SiH₃, and SiF₃). Landis et al. have shown that complexes in which the metal is sd³ hybridized have tetrahedral and non-tetrahedral structures with shapes of an umbrella or a 4-legged piano stool. In this article, the influence of the metal and ligands on the energies of the three isomeric structures of d⁴ MH₃X and MX₄ is established and rationalized. Fluoride and alkyl ligands stabilize the tetrahedral relative to non-tetrahedral structures while hydride and silyl ligands stabilize the non-tetrahedral structures. For given ligands and charge, 4d metal favors more the non-tetrahedral structures than 5d metals. A positive charge increases the preference for the non-tetrahedral structures while a negative charge increases the preference for the tetrahedral structure. The factors that determine these energy patterns are discussed by means of a molecular orbital analysis, based on Extended Hückel (EHT) calculations, and by means of Natural Bond Orbital (NBO) analyses of charges and resonance structures (NRT analysis). These analyses show the presence of through-space interactions in the non-tetrahedral structures that can be sufficiently stabilizing, for specific metals and ligands, to stabilize the non-tetrahedral structures relative to the tetrahedral isomer

Elucidating the Link between NMR Chemical Shifts and Electronic Structure in d⁰ Olefin Metathesis Catalysts

The nucleophilic carbon of d⁰ Schrock alkylidene metathesis catalysts, [M] = CHR, display surprisingly low downfield chemical shift (δ_iso) and large chemical shift anisotropy. State-of-the-art four-component relativistic calculations of the chemical shift tensors combined with a two-component analysis in terms of localized orbitals allow a molecular-level understanding of their orientations, the magnitude of their principal components (δ₁₁ > δ₂₂ > δ₃₃) and associated δ_iso. This analysis reveals the dominating influence of the paramagnetic contribution yielding a highly deshielded alkylidene carbon. The largest paramagnetic contribution, which originates from the coupling of alkylidene σ_MC and π*_MC orbitals under the action of the magnetic field, is analogous to that resulting from coupling σ_CC and π*_CC in ethylene; thus, δ₁₁ is in the MCH plane and is perpendicular to the MC internuclear direction. The higher value of carbon-13 δ_iso in alkylidene complexes relative to ethylene is thus due to the smaller energy gap between σ_MC and π*_MC vs this between σ_CC and π*_CC in ethylene. This effect also explains why the highest value of δ_iso is observed for Mo and the lowest for Ta, the values for W and Re being in between. In the presence of agostic interaction, the chemical shift tensor principal components orientation (δ₂₂ or δ₃₃ parallel or perpendicular to π_MX) is influenced by the MCH angle because it determines the orientation of the alkylidene CHR fragment relative to the MC internuclear axis. The orbital analysis shows how the paramagnetic terms, understood with a localized bond model, determine the chemical shift tensor and thereby δ_iso