Metal Carbon Bond Strength Research Articles

Adaptive Aromaticity in 18e Metallapentalenes.

Complexes with aromaticity in both the lowest singlet state (S0) and the lowest triplet state (T1) (denoted as adaptive aromaticity) are rare because according to Hückel's and Baird's rules, a species could be aromatic in either the S0 or T1 state in most cases. Thus, it is particularly challenging to design species with adaptive aromaticity. Previous reports on adaptive aromaticity were mainly focused on 16e metallapentalenes. Here, we demonstrate that 18e metallapentalenes could possess adaptive aromaticity supported by a set of aromaticity indices when the nitrido and imido ligands are introduced via density functional theory calculations. Further investigation suggests that the metal-carbon bond strength plays an important role in the S0 state aromaticity and the T1 state aromaticity could be attributed to spin electron localization. All these findings could be useful for the development of metallaaromatic chemistry.

Metal–carbon bond strengths under polymerization conditions: 2,1-insertion as a catalyst stress test

Quantitative agreement between experimentally determined M–C bond dissociation energies (BDE) and DFT predictions (M06-2X/TZ//TPSSTPSS/DZ) can be reached by choosing the correct anchor for experimentally derived BDE. For the example of the archetypical metallocene catalyst Cp2TiCl2, it is shown that titanium–carbon bonds are very weak under polymerization conditions and fluctuate; steric strain is introduced after 2,1 insertion and via olefin capture. Thus, homolysis can become competitive with chain propagation. Depending on the catalyst and temperature, 2,1 insertion can be only a temporary inconvenience (dormancy) or a definitive decay event. It is then shown for a set of nine common Ti and Zr polymerization catalysts how ligand variation affects the metal–carbon BDE. Predicted stabilities of the M(IV) oxidation state with respect to homolysis are in nice agreement with the experimentally observed temperature tolerance of the various catalysts: homolysis is easier for Ti than for Zr, and cyclopentadienyl groups in particular facilitate homolysis, especially in bis-cyclopentadienyl systems.

Establishing the most favorable metal–carbon bond strength for carbon nanotube catalysts

We have studied a wide range of transition metals to find potential carbon nanotube (CNT) catalysts for chemical vapor deposition (CVD) production.

Understanding the Mechanisms of Unusually Fast HH, CH, and CC Bond Reductive Eliminations from Gold(III) Complexes

Carbon-carbon bond reductive elimination from gold(III) complexes are known to be very slow and require high temperatures. Recently, Toste and co-workers have demonstrated extremely rapid CC reductive elimination from cis-[AuPPh3 (4-F-C6 H4 )2 Cl] even at low temperatures. We have performed DFT calculations to understand the mechanistic pathway for these novel reductive elimination reactions. Direct dynamics calculations inclusive of quantum mechanical tunneling showed significant contribution of heavy-atom tunneling (>25 %) at the experimental reaction temperatures. In the absence of any competing side reactions, such as phosphine exchange/dissociation, the complex cis-[Au(PPh3 )2 (4-F-C6 H4 )2 ](+) was shown to undergo ultrafast reductive elimination. Calculations also revealed very facile, concerted mechanisms for HH, CH, and CC bond reductive elimination from a range of neutral and cationic gold(III) centers, except for the coupling of sp(3) carbon atoms. Metal-carbon bond strengths in the transition states that originate from attractive orbital interactions control the feasibility of a concerted reductive elimination mechanism. Calculations for the formation of methane from complex cis-[AuPPh3 (H)CH3 ](+) predict that at -52 °C, about 82 % of the reaction occurs by hydrogen-atom tunneling. Tunneling leads to subtle effects on the reaction rates, such as large primary kinetic isotope effects (KIE) and a strong violation of the rule of the geometric mean of the primary and secondary KIEs.

Synthesis and energetics of Tp′Rh[P(OMe)3](R)H: a systematic investigation of ligand effects on C–H activation at rhodium

The thermal precursor Tp′Rh[P(OMe)3](Me)H was used to generate the active [Tp′Rh[P(OMe)3]] fragment, which activates C–H bonds of various hydrocarbons to form products of the type Tp′Rh[P(OMe)3](R)H (Tp′ = tris-(3,5-dimethylpyrazolyl)borate). Only one single activation product was observed in each case. The structures of Tp′Rh[P(OMe)3](R)X (X = H, Br, Cl) have been characterized by NMR spectroscopy, elemental analysis, and X-ray crystallography. The kinetics of reductive elimination of RH from Tp′Rh[P(OMe)3](R)H as well as competition experiments between substrates allow measurement of the Rh–C bond strengths relative to the Rh–Ph bond strength. Two separate linear correlations of the Rh–C bond energies versus H–C bond energies were found based on whether the alkyl group is α-substituted or not. While the correlation for α-substituted substrates gives a slope of 1.45, smaller than the slope (1.55) for unsubstituted hydrocarbons, the Rh–C bond energies of the former group are higher by ∼7 kcal mol−1. In comparison with the analogous [Tp′Rh(PMe3)] and [Tp′Rh(CNneopentyl)] systems, replacing the spectator ligand with a more electronic donating group slightly increases metal–carbon bond strengths as the trend in slopes of the correlations follows an order of CNneopentyl < P(OMe)3 ≤ PMe3.

Kinetic and Thermodynamic Selectivity of Intermolecular C–H Activation at [Tp′Rh(PMe3)]. How Does the Ancillary Ligand Affect the Metal–Carbon Bond Strength?

Tp'Rh(PMe3)(CH3)H was synthesized as a precursor to produce the coordinatively unsaturated fragment [Tp'Rh(PMe3)], which reacts with benzene, mesitylene, 3,3-dimethyl-1-butene, 2-methoxy-2-methylpropane, 2-butyne, acetone, pentane, cyclopentane, trifluoroethane, fluoromethane, dimethyl ether, and difluoromethane at ambient temperature to give only one product in almost quantitative yield in each case. All of the complexes Tp'Rh(PMe3)(R)H were characterized by NMR spectroscopy, and their halogenated derivatives were fully characterized by NMR spectroscopy, elemental analysis, and X-ray crystallography. The active species [Tp'Rh(PMe3)] was also able to activate the alkynyl C-H bond of terminal alkynes to give activation products of the type Tp'Rh(PMe3)(C≡CR)H (R = t-Bu, SiMe3, hexyl, CF3, Ph, p-MeOC6H4, and p-CF3C6H4). The measured relative rhodium-carbon bond strengths display two linear correlations with the corresponding carbon-hydrogen bond strengths, giving a slope of 1.54 for α-unsubstituted hydrocarbons and a slope of 1.71 for substrates with α-substitution. Similar trends of energy correlations were established by DFT calculated metal-carbon bond strengths for the same groups of substrates.

Rhodium–Carbon Bond Energies in Tp′Rh(CNneopentyl)(CH2X)H: Quantifying Stabilization Effects in M–C Bonds

A series of substituted methyl derivatives of the type Tp'Rh(CNneopentyl)(CH2X)H (CH2X = CH2C(═O)CH3, CH2C≡CCH3, CH2O-t-Bu, CH2CF3, CH2F, CHF2) was synthesized either by photolysis of Tp'Rh(CNneopentyl)(PhNCNneopentyl) in neat CH3X or by exchange with the labile hydrocarbon in Tp'Rh(CNneopentyl)(n-pentyl)H or Tp'Rh(CNneopentyl)(CH3)H. Only a single product was observed in each case. Clean reductive elimination was observed for all compounds in C6D6. Structures of these complexes and their corresponding chlorinated derivatives have been characterized by NMR spectroscopy, elemental analysis, and X-ray crystallography. Relative Rh-C bond energies are calculated using previously established kinetic techniques, and two separate linear correlations are observed versus known C-H bond strengths, one for the parent hydrocarbons, and one for the substituted hydrocarbons. Both correlations have slopes of 1.4, and are separated vertically by 7.5 kcal mol(-1) (-CH2X above -CxHy). In addition, it is now clear that preferences for linear vs branched olefin insertion products in substituted derivatives can be predicted on the basis of the strengths of the β-C-H bonds. The DFT calculations of the metal-carbon bond strengths in these Rh-CH2X derivatives with α-substitution show a trend that is in good agreement with the experimental results.

Benchmark calculations of metal carbonyl cations: relativistic vs. electron correlation effects

In this paper we present benchmark results for isoelectronic metal carbonyl complexes of the groups 11 and 12 of the periodic table. The focus is on the geometry, vibrational frequencies, bond dissociation energy and chemical bonding. The description of these complexes requires a good balance between electron correlation and relativistic effects. Our results demonstrate that the combination of the effective core potential and the MP2 method gives quantitative results for the first- and the second-row transition metal complexes and only qualitative agreement for the third-row complexes. In order to obtain quantitative results for the whole series the use of four-component or X2C methods is mandatory. The fourth-row transition metal carbonyl complexes from groups 11 and 12 have been studied for the first time. The metal-carbon bond strength pattern along the group is shown to be highly dependent on the correct description of the relativistic effects. Finally, the relativistic effects on the bonding are studied by means of electron density difference maps, the analysis of the bond critical points of the electron density and the mechanism for σ-donation and π-backdonation. Our analysis indicates that the fourth-row complexes exhibit a strong covalent character induced by relativistic effects.

C–H Activation of Terminal Alkynes by Tris-(3,5-dimethylpyrazolyl)boraterhodiumneopentylisocyanide: New Metal–Carbon Bond Strengths

C-H bond activation of terminal alkynes by [Tp'Rh(CNneopentyl)] (Tp' = hydridotris-(3,5-dimethylpyrazolyl)borate) resulted in the formation of terminal C-H bond activation products Tp'Rh(CNneopentyl)(C≡CR)H (R = t-Bu, SiMe(3), hexyl, CF(3), p-MeOC(6)H(4), Ph, and p-CF(3)C(6)H(4)). A combination of kinetic selectivity determined in competition reactions and activation energy for reductive elimination has allowed for the calculation of relative Rh-C(alkynyl) bond strengths. The bond strengths of Rh-C(alkynyl) products are noticeably higher than those of Rh-C(aryl) and Rh-C(alkyl) analogues. The relationship between M-C and C-H bond strengths showed a linear correlation (slope R(M-C/H-C) = 1.32), and follows energy correlations previously established for unsubstituted sp(2) and sp(3) C-H bonds in aliphatic and aromatic hydrocarbons.

Diversity and design of metal-based carbon monoxide-releasing molecules (CO-RMs) in aqueous systems: revealing the essential trends

The CO-releasing ability of a diverse library of primary metal carbonyl complexes has been assessed using a deoxymyoglobin-carbonmonoxymyglobin assay. A wide spectrum of rates for the CO-release process was observed in aqueous systems. For octahedral d(6) complexes, the rate was found to decrease in the sequence FeI(2)(CO)(4) > [NEt(4)][V(CO)(6)] > MnBr(CO)(5) > Cr(CO)(6) implying that CO-release is not controlled by the metal-carbon bond strengths. Within the series, [NEt(4)][MX(CO)(5)] (M = Cr, Mo, W; X =Cl, Br, I), the rate of CO-release was found to decrease down the group (Cr > Mo > W), whilst within the chromium series a similar trend was observed for the halide (Cl > Br > I). The d(4) complexes [NEt(4)][MI(3)(CO)(4)] (M = Mo, W) exhibit faster release than their d(6) congeners. A mechanistic investigation into the [NEt(4)][MX(CO)(5)] series revealed the intermediacy of [[M(CO)(5)](2)(mu-X)](-) in the CO-release process and that the hydrolysis of the M-X bond, rather than the intrinsic strength of M-CO bonds, controls the rate of CO-release in aqueous systems.

The metal–carbon stretching frequencies in methyl complexes of Rh, Ir, Ga and In with porphyrins and a tetradentate pyridine–amide ligand

Interactions of trans-[Rh(bpb)(CH 3)(H 2O)] ( 1) [bpb=1,2-bis(2-pyridinecarboxamido)benzene] with Lewis bases L afford the respective adducts trans-[Rh(bpb)(CH 3)(L)], where L=4-substituted pyridine 4-Xpy (X=H ( 2), t Bu ( 3), NMe 2 ( 4) CN ( 5)), PMe 2Ph ( 6) or benzimidazole ( 7). The structures of complexes 3 and 6 have been established by X-ray crystallography. The Rh–C distance in complex 6 (2.095(6) Å) is longer than that in complex 3 (2.02(1) Å), indicating that PMe 2Ph has a stronger trans influence than 4- t Bupy. The metal–carbon stretching frequencies for trans-[Rh(bpb)(CH 3)(L)] and [M(TTP)(CH 3)] [TTP=5,10,15,20-tetrakis(4-methylphenyl)porphyrin dianion; M=Rh, Ir, Ga, In] have been determined by near IR FT-Raman spectroscopy. Complex 1 exhibits ν(Rh–C) at 562 cm −1, which downshifts to 532 cm −1 upon deuteriation of the axial methyl group. Replacement of the aquo ligand in complex 1 with nitrogen ligands or phosphine resulted in downshift in ν(Rh–C). The ν(Rh–C) for trans-[Rh(bpb)(CH 3)(L)] was found to decrease in the order L: PMe 2Ph≫4-Xpy∼BzIm>H 2O, consistent the order of trans influence of L. For [M(TTP)(CH 3)] (M=Co, Rh, Ir, Ga, or In) the M–C force constant was found to decrease in the orders Ir>Rh>Co and Ga>In, consistent with the trends of metal–carbon bond strength for these metals. For trans-[Rh(bpb)(CH 3)(4-Xpy)] and trans-[Rh(TTP)(CH 3)(4-Xpy)], the ν(Rh–C) were found to be not very sensitive to the nature of X, suggesting that the electronic factors of the axial pyridine ligand do not have a significant effect on the Rh–C bonds for these rhodium alkyl complexes.

Metallabenzenes.

Many similarities exist between metallabenzenes and conventional arenes. Among these similarities are structural features such as ring planarity and the absence of bond length alternation, spectroscopic features such as downfield chemical shifts for ring protons, and chemical reactions such as electrophilic aromatic substitution and arene displacement from (arene)Mo(CO)3. All of these features, taken together, strongly support the thesis that metallabenzenes represent a new class of aromatic compounds, one in which metal d orbitals participate fully with carbon p orbitals in the formation of ring pi-bonds. However, it is also apparent that metallabenzenes are much more prone to isomerization reactions than are conventional arenes. This appears to be particularly true of first-row and second-row metallabenzenes, where the metal-carbon bond strengths are weaker. In these systems, carbene migratory insertion often leads to cyclopentadienyl-metal products. pi-Coordination of metallabenzenes to other metal centers generally stabilizes the metallabenzene moieties while maintaining their aromatic character. Among metallabenzenes coordinated in this way, there are representatives from all three transition-metal rows (Fe, Ni, Mo, Ru, and Ir). The metal atom in pi-coordinated metallabenzenes is displaced out of the ring and away from the complexing metal center. The reason for this displacement in most cases appears to be steric repulsion between ligands on the two metal centers. However, other subtle effects may contribute. For example, metal displacement leads to more favorable internal angles at the alpha-carbons and a better orientation of C alpha p orbitals toward the complexing metal center. While no dominant synthetic strategy for constructing metallabenzenes has emerged, cyclization reactions involving metal-thiocarbonyl, metal-alkylidyne, and metal-alkylidene precursors have proved useful. In addition, approaches involving pentadienyl reagents as the source of ring carbons have yielded notable successes. Vinylcyclopropene reagents have recently led to isolation of the first example of a metallabenzene valence isomer--a metallabenzvalene--and its subsequent conversion to a planar metallabenzene. Finally, interligand attacks of butadienyls on carbonyls have produced a variety of transient oxy- or alkoxy-substituted metallabenzene species. The development of new synthetic approaches, particularly systematic approaches that can be used with a variety of transition metals, is the key issue facing metallabenzene chemists. One hundred and thirty-five years after Kekulé's celebrated dream, aromatic chemistry continues to be a fascinating and provocative research topic. Metallabenzenes represent one of the "new frontiers" that promise to keep aromatic chemistry vibrant well into the 21st century.

Trapping-mediated dissociative chemisorption of cycloalkanes on Ru(001) and Ir(111): influence of ring strain and molecular geometry on the activation of C-C and C-H bonds.

We have measured the initial probabilities of dissociative chemisorption of perhydrido and perdeutero cycloalkane isotopomers on the hexagonally close-packed Ru(001) and Ir(111) single-crystalline surfaces for surface temperatures between 250 and 1100 K. Kinetic parameters (activation barrier and preexponential factor) describing the initial, rate-limiting C-H or C-C bond cleavage reactions were quantified for each cycloalkane isotopomer on each surface. Determination of the dominant initial reaction mechanism as either initial C-C or C-H bond cleavage was judged by the presence or absence of a kinetic isotope effect between the activation barriers for each cycloalkane isotopomer pair, and also by comparison with other relevant alkane activation barriers. On the Ir(111) surface, the dissociative chemisorption of cyclobutane, cyclopentane, and cyclohexane occurs via two different reaction pathways: initial C-C bond cleavage dominates on Ir(111) at high temperature (T > approximately 600 K), while at low temperature (T < approximately 400 K), initial C-H bond cleavage dominates. On the Ru(001) surface, dissociative chemisorption of cyclopentane occurs via initial C-C bond cleavage over the entire temperature range studied, whereas dissociative chemisorption of both cyclohexane and cyclooctane occurs via initial C-H bond cleavage. Comparison of the cycloalkane C-C bond activation barriers measured here with those reported previously in the literature qualitatively suggests that the difference in ring-strain energies between the initial state and the transition state for ring-opening C-C bond cleavage effectively lowers or raises the activation barrier for dissociative chemisorption via C-C bond cleavage, depending on whether the transition state is less or more strained than the initial state. Moreover, steric arguments and metal-carbon bond strength arguments have been evoked to explain the observed trend of decreasing C-H bond activation barrier with decreasing cycloalkane ring size.

Two-Step Methane Conversion to Higher Hydrocarbons: Comment on the Relevance of Metal–Carbon Bond Strength

Herein, data available in the literature for nonoxidative, two-step CH4 homologation over SiO2-supported transition metals are analyzed according to the principle of Sabatier. Volcano plots are constructed which provide empirical evidence to support the hypothesis that the metal–carbon bond strength is pivotal to determine catalyst efficiency, chain-growth probability, and selectivity to C+2 hydrocarbons.

Calculation of the metal-carbon bond strength of surface carbon deposited on solid oxide fuel cell nickel/zirconia fuel reforming anodes

The activation energy for the removal of surface carbon formed by methane decomposition following high-temperature reforming, from a nickel/zirconia solid oxide fuel cell (SOFC) anode has been calculated using two methods based on temperature-programmed oxidation. It is found that there is a fairly good agreement between the two methods. In addition, it was observed that the addition of small quantities of lithium to the anode resulted in a significant lowering of the activation energy for surface carbon removal by about 50 kJ mol-1.

Theory‐enforced Re‐investigation of the Origin of the Large Metal–Carbon Bond Strength in PdCH2I+ and its reactions with unsaturated hydrocarbons

AbstractState‐of‐the‐art ab initio studies demonstrate that the reaction Pd+ + CH3I → PdCH2I+ + H. is endothermic by ca. 20 kcal/mol, which translates into a bond dissociation energy (BDE) of ca. 83 kcal/mol for the Pd+CH2I bond. This figure is in agreement with an experimental bracket of 68 kcal/mol &lt; BDE(Pd+CH2I) &lt; 92 kcal/mol. Based on these findings, the previously studied Pd+/CH3I system was re‐investigated, and double‐resonance experiments demonstrate that the formation of PdCH2I+ occurs stepwise via PdCH as a reactive intermediate. Further, ion/molecule reactions of PdCH2I+ with unsaturated hydrocarbons are studied, which reveal the formation of carbon–carbon bonds in the gas phase.

Unparalleled, Enormous Metal–Carbon Bond Strength in PdCH2I+

AbstractThermalized Pd+ cations activate methyl iodide by selective cleavage of a CH bond under formation of PdCH2I+ and an H‐atom. This finding implies that the interaction energy between the metal cation and the CH2I fragment and thus the metal–carbon bond strength exceeds 103 kcal/mol. Theory predicts that the energetically most favorable isomer of this ion exhibits the Pd+CH2I structure, which is stabilized by an unprecedented bridging interaction between the two heavy atoms Pd and I.

Trends of Metal-Carbon Bond Strengths in Transition Metal Complexes

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTTrends of Metal-Carbon Bond Strengths in Transition Metal ComplexesPer E. M. SiegbahnCite this: J. Phys. Chem. 1995, 99, 34, 12723–12729Publication Date (Print):August 1, 1995Publication History Published online1 May 2002Published inissue 1 August 1995https://doi.org/10.1021/j100034a007RIGHTS & PERMISSIONSArticle Views3073Altmetric-Citations66LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (798 KB) Get e-Alerts

Characterization and Thermolysis Reactions of CO2-Bridged Iron-Tin and Rhenium-Tin Complexes. Structure-Reactivity Correlations

We report here our studies of the thermolysis behavior of iron-tin complexes and related rhenium-tin complexes and new structural data which allow further correlations to be made between thermolysis activity and bonding parameters involving the CO{sub 2} ligand. All compounds have been characterized by solid-state IR spectral methods which allow a determination of the {mu}{sub 2}-{eta}{sup 2} or {mu}{sub 2}-{eta}{sup 3} bonding mode in each compound when compared with structural data on representative compounds of each type. Factors which control the ease of thermolysis and the mode of decomposition are the bonding type, the presence of electron-donating or electron-withdrawing groups on the tin atom as well as those on iron or rhenium, metal-carbon bond strength, the orientation of the bridging CO{sub 2} ligand between the two metal centers, and, most importantly, the stability of the corresponding metal (iron or rhenium) anion. Reaction mechanisms are proposed which are consistent with structural and electronic differences that lead to separate thermolysis paths. 17 refs., 7 figs., 11 tabs.

Mechanism of carbon—carbon bond formation by transition metals

Both CO dissociation and dissociative methane adsorption on transition-metal catalysts can result in different carbonaceous intermediates. A very reactive carbidic intermediate produces C 2 + hydrocarbons upon hydrogenation. The selectivity for chain growth versus hydrogenation depends on the metal—carbon bond strength and is optimum for ruthenium and cobalt. The recombining CH χ fragments have an average of one hydrogen atom per surface carbon (χ = 1).