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

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

Cyclometalated N‑Heterocyclic Carbene Complexes of Ruthenium for Access to Electron-Rich Silylene Complexes That Bind the Lewis Acids CuOTf and AgOTf

The synthesis of the cyclometalated complexes Cp*Ru­(IXy-H) (<b>2</b>) [IXy = 1,3-bis­(2,6-dimethylphenyl)­imidazol-2-ylidene; IXy-H = 1-(2-CH₂C₆H₃-6-methyl)-3-(2,6-dimethylphenyl)­imidazol-2-ylidene-1-yl (the deprotonated form of IXy); Cp* = η⁵-C₅Me₅] and Cp*Ru­(IXy-H)­(N₂) (<b>3</b>) was achieved by dehydrochlorination of Cp*Ru­(IXy)Cl (<b>1</b>) with KCH₂Ph. Complexes <b>2</b> and <b>3</b> activate primary silanes (RSiH₃) to afford the silyl complexes Cp*­(IXy-H)­(H)­RuSiH₂R [R = <i>p</i>-Tol (<b>4</b>), Mes (<b>5</b>), Trip (<b>6</b>)]. Density functional theory studies indicated that these complexes are close in energy to the corresponding isomeric silylene species Cp*­(IXy)­(H)­RuSiHR. Indeed, reactivity studies indicated that various reagents trap the silylene isomer of <b>6</b>, Cp*­(IXy)­(H)­RuSiHTrip (<b>6a</b>). Thus, benzaldehyde reacts with <b>6</b> to give the [2 + 2] cycloaddition product <b>7</b>, while 4-bromoacetophenone reacts via C–H bond cleavage and formation of the enolate Cp*­(IXy)­(H)₂RuSiH­[OC­(CH₂)­C₆H₄Br]­Trip (<b>8</b>). Addition of the O–H bond of 2,6-dimethylphenol across the RuSi bond of <b>6a</b> gives Cp*­(IXy)­(H)₂RuSiH­(2,6-Me₂C₆H₃O)­Trip (<b>9</b>). Interestingly, CuOTf and AgOTf also react with <b>6</b> to provide unusual Lewis acid-stabilized silylene complexes in which MOTf bridges the Ru–Si bond. The AgOTf complex, which was crystallographically characterized, exhibits a structure similar to that of [Cp*­(^<i>i</i>Pr₃P)­Ru­(μ-H)₂SiHMes]⁺, with a three-center, two-electron Ru–Ag–Si interaction. Natural bond orbital analysis of the MOTf complexes supported this type of bonding and characterized the donor interaction with Ag (or Cu) as involving a delocalized interaction with contributions from the carbene, silylene, and hydride ligands of Ru

Donor-Promoted 1,2-Hydrogen Migration from Silicon to a Saturated Ruthenium Center and Access to Silaoxiranyl and Silaiminyl Complexes

Masked silylene complexes Cp*­(IXy-H)­(H)­RuSiH₂R (R = Mes (<b>3</b>) and Trip (<b>4</b>); IXy = 1,3-bis­(2,6-dimethylphenyl)­imidazol-2-ylidene; “IXy-H” is the deprotonated form of IXy) exhibit metallosilylene-like (L_<i>n</i>M–Si–R) reactivity, as observed in reactions of nonenolizable ketones, enones, and tosyl azides, to give unprecedented silaoxiranyl, oxasilacyclopentenyl, and silaiminyl complexes, respectively. Notably, these silicon-containing complexes are derived from the primary silanes MesSiH₃ and TripSiH₃ via activation of all three Si–H bonds. DFT calculations suggest that the mechanism of formation for the silaoxiranyl complex Cp*­(IXy)­(H)₂Ru–Si­(OCPh₂)­Trip (<b>6</b>) involves coordination of benzophenone to a silylene silicon atom, followed by a single-electron transfer in which Si-bonded, non-innocent benzophenone accepts an electron from the reactive, electron-rich ruthenium center. Importantly, this electron transfer promotes an unusual 1,2-hydrogen migration to the resulting, more electron-deficient ruthenium center via a diradicaloid transition state

Donor-Promoted 1,2-Hydrogen Migration from Silicon to a Saturated Ruthenium Center and Access to Silaoxiranyl and Silaiminyl Complexes

Masked silylene complexes Cp*­(IXy-H)­(H)­RuSiH₂R (R = Mes (<b>3</b>) and Trip (<b>4</b>); IXy = 1,3-bis­(2,6-dimethylphenyl)­imidazol-2-ylidene; “IXy-H” is the deprotonated form of IXy) exhibit metallosilylene-like (L_<i>n</i>M–Si–R) reactivity, as observed in reactions of nonenolizable ketones, enones, and tosyl azides, to give unprecedented silaoxiranyl, oxasilacyclopentenyl, and silaiminyl complexes, respectively. Notably, these silicon-containing complexes are derived from the primary silanes MesSiH₃ and TripSiH₃ via activation of all three Si–H bonds. DFT calculations suggest that the mechanism of formation for the silaoxiranyl complex Cp*­(IXy)­(H)₂Ru–Si­(OCPh₂)­Trip (<b>6</b>) involves coordination of benzophenone to a silylene silicon atom, followed by a single-electron transfer in which Si-bonded, non-innocent benzophenone accepts an electron from the reactive, electron-rich ruthenium center. Importantly, this electron transfer promotes an unusual 1,2-hydrogen migration to the resulting, more electron-deficient ruthenium center via a diradicaloid transition state

Symmetrical Hydrogen Bonds in Iridium(III) Alkoxides with Relevance to Outer Sphere Hydrogen Transfer

A chelating ligand formed by deprotonation of 2-(2′-pyridyl)-2-propanol stabilizes a distorted trigonal bipyramidal geometry in a 16e^– d⁶ 5-coordinate iridium complex with the alkoxide acting as a π donor. Ambiphilic species such as AcOH bearing both nucleophilic and electrophilic functionality form adducts with the unsaturated iridium complex which contain strong intramolecular O···H···O hydrogen bonds that involve the basic alkoxide oxygen. Density functional theory (DFT) calculations on the isolated cations reproduce with high accuracy the geometrical features obtained via X-ray diffraction and corroborate the presence of very short hydrogen bonds with O···O distances of about 2.4 Å. Calculations further confirm the known trend that the hydrogen position in these bonds is sensitive to the O···O distance, with the shortest distances giving rise to symmetrical O···H···O interactions. Dihydrogen is shown to add across the Ir–O π bond in a presumed proton transfer reaction, demonstrating bifunctional behavior by the iridium alkoxide

Symmetrical Hydrogen Bonds in Iridium(III) Alkoxides with Relevance to Outer Sphere Hydrogen Transfer

A chelating ligand formed by deprotonation of 2-(2′-pyridyl)-2-propanol stabilizes a distorted trigonal bipyramidal geometry in a 16e^– d⁶ 5-coordinate iridium complex with the alkoxide acting as a π donor. Ambiphilic species such as AcOH bearing both nucleophilic and electrophilic functionality form adducts with the unsaturated iridium complex which contain strong intramolecular O···H···O hydrogen bonds that involve the basic alkoxide oxygen. Density functional theory (DFT) calculations on the isolated cations reproduce with high accuracy the geometrical features obtained via X-ray diffraction and corroborate the presence of very short hydrogen bonds with O···O distances of about 2.4 Å. Calculations further confirm the known trend that the hydrogen position in these bonds is sensitive to the O···O distance, with the shortest distances giving rise to symmetrical O···H···O interactions. Dihydrogen is shown to add across the Ir–O π bond in a presumed proton transfer reaction, demonstrating bifunctional behavior by the iridium alkoxide

1,2-Hydrogen Migration to a Saturated Ruthenium Complex via Reversal of Electronic Properties for Tin in a Stannylene-to-Metallostannylene Conversion

An intramolecular 1,2­(α)-H migration in a saturated ruthenium stannylene complex, to form a ruthenostannylene complex, involves a reversal of the role for a coordinated stannylene ligand, from that of an electron donor to an acceptor in the transition state. This change in the bonding properties for a stannylene group, with a simple molecular motion, lifts the usual requirement for generation of an unsaturated metal center in migration chemistry

Metathesis Activity Encoded in the Metallacyclobutane Carbon-13 NMR Chemical Shift Tensors

Metallacyclobutanes are an important class of organometallic intermediates, due to their role in olefin metathesis. They can have either planar or puckered rings associated with characteristic chemical and physical properties. Metathesis active metallacyclobutanes have short M–C_α/α′ and M···C_β distances, long C_α/α′–C_β bond length, and isotropic ¹³C chemical shifts for both early d⁰ and late d⁴ transition metal compounds for the α- and β-carbons appearing at ca. 100 and 0 ppm, respectively. Metallacyclobutanes that do not show metathesis activity have ¹³C chemical shifts of the α- and β-carbons at typically 40 and 30 ppm, respectively, for d⁰ systems, with upfield shifts to ca. −30 ppm for the α-carbon of metallacycles with higher d^<i>n</i> electron counts (<i>n</i> = 2 and 6). Measurements of the chemical shift tensor by solid-state NMR combined with an orbital (natural chemical shift, NCS) analysis of its principal components (δ₁₁ ≥ δ₂₂ ≥ δ₃₃) with two-component calculations show that the specific chemical shift of metathesis active metallacyclobutanes originates from a low-lying empty orbital lying in the plane of the metallacyclobutane with local π*­(M–C_α/α′) character. Thus, in the metathesis active metallacyclobutanes, the α-carbons retain some residual alkylidene character, while their β-carbon is shielded, especially in the direction perpendicular to the ring. Overall, the chemical shift tensors directly provide information on the predictive value about the ability of metallacyclobutanes to be olefin metathesis intermediates

Adult preference - Laying site

The file ‘Adult preference - Laying site’ contains the data of the laying site in the ‘oviposition experiment’. - The first column, entitled ‘ID_eggmasses’, gives the unique identifer of each egg-mass sampled in the experimental cages. - The second column, entitled ‘ID_cage’, gives the unique numerical identifier of each experimental cage. - The third column, entitled ‘Ostrinia_species’, refer to O. nubilalis (ECB) and O. scapulalis (ABB) species used in the experiment. - The fourth column, called ‘Setup’, corresponds to the three experimental set-ups tested in the oviposition experiment: choice (with maize and mugwort plants), maize_only (with maize plants only) and mugwort_only (with mugwort plants only). - the fifth column, entitled ‘Laying_site’, corresponds to the three laying sites where adult females laid eggs: mugwort, maize or the netting cage. - the sixth column, entitled ‘Sampling_Date’, corresponds to the sampling date of egg-masses. - the seventh column, called ‘Hatching’ is a binomial variable, with 0 corresponding to not hatched egg-masses and 1 to hatched egg-masses. - the eighth column, called ‘Hatching_Date’, corresponds to the date of egg-masses hatching (‘NA’ for the egg-masses that did not hatch)