Agostic Bond Research Articles

Electron Density and Molecular Orbital Analyses of the Nature of Bonding in the η3-CCH Agostic Rhodium Complexes Preceding the C-C and C-H Bond Cleavages.

In our recent work, we revisited C-H and C-C bond activation in rhodium (I) complexes of pincer ligands PCP, PCN, PCO, POCOP, and SCS. Our findings indicated that an η3-Csp2Csp3H agostic intermediate acts as a common precursor to both C-C and C-H bond activation in these systems. We explore the electronic structure and bonding nature of these precleavage complexes using electron density and molecular orbital analyses. Using NBO, IBO, and ESI-3D methods, the bonding in the η3-CCH agostic moiety is depicted by two three-center agostic bonds: Rh-Csp2-Csp3 and Rh-Csp3-H, with all three atoms datively bound to Rh(I). IBO analysis specifically highlights the involvement of three orbitals (CC→Rh and CH→Rh σ donation, plus Rh→CCH π backdonation) in both C-C and C-H bond cleavages. NCIPLOT and QTAIM analyses highlight anagostic (Rh-H) or β-agostic (Rh-Csp2-H) interactions and the absence of Rh-Csp3 interactions. QTAIM molecular graphs suggest bond path instability under dynamic conditions due to the nearness of line and ring critical points. Several low-frequency and low-force vibrational modes interconvert various bonding patterns, reinforcing the dynamic η3-CCH agostic nature. The kinetic preference for C-H bond breaking is attributed to the smaller reduced mass of C-H vibrations compared to C-C vibrations.

α,β‐C—C—C Agostic Bonding Interactions in Ruthenacyclobutane and π‐Complex Assisted Olefin Metathesis Catalyzed by Ruthenium‐Alkylidene Complexes

ABSTRACTComputational aspects of concerted [2+2] oxidative‐retrocycloaddition‐cycloreversion reaction through ruthenium alkylidene π‐complexes and ruthenacyclobutane with α,β‐(C—C—C) agostic bonding interactions in olefin metathesis are presented. d6‐Ruthenium carbene complexes, with ruthenium in the oxidation state +2, undergo successive [2+2] cycloaddition and cycloreversion steps, through associative, dissociative, or interchange mechanisms. This process involves coordination of the olefin to 16‐electron Ru complex followed by phosphine dissociation, or first phosphine dissociation then coordination of the olefin to the 14‐electron Ru complex with rearrangement to a ruthenacyclobutane intermediate, followed by symmetrical reverse steps. Donation of σ‐electron density from the two C—C σ‐bonds to the metal center leads to α,β‐(C—C—C) agostic bonds, which stabilized metallacyclobutane as a formally 16‐electron complex, with lower energy than the corresponding π‐complex. In the transformation from π‐complex to ruthenacyclobutane the ruthenium atom is formally oxidized to Ru(IV). The most efficient ligands are those that stabilize the high‐oxidation state metallacyclobutane (IV) intermediate relative to the ruthenium carbene.

C-H functionalization of quinoline N-oxides catalyzed by Pd(II) complexes: a computational study.

Pd(II) catalysts, particularly the acetate salt in acetic acid, tended to favor regioselective C-H activation of quinoline N-oxides (QOs) at the C2 position. However, Pd(II)Cl2 was shown to catalyze their C-H activation at C8 and, in the presence of water, C8-H activation was accompanied by the formation of 2-quinolinones. The aim of the DFT study described in this work was to shed light on the complete mechanism of these competing catalytic reactions, when PdCl2 reacts with QO and benzaldehyde in dichloroethane. C-H activation of QO was the first step of the reaction and involved either a metallacycle, with a CQO-Pd(II) σ-bond and a C(8)-H-Pd(II) agostic bond, or an η3-QO complex, with three carbon atoms of the heteroring of QO binding PdCl2. The first situation led to the unusual C8 activation and the second to C2 activation. The σ-metallacycle undergoes C8-H activation and the energy of the TOF determining the transition state to form the product is ∼17 kcal mol-1, while for the reaction through the π-metallacycle (C2-H activation) the corresponding energy is higher (∼29 kcal mol-1) and thus is not competitive under the same conditions. The reaction proceeding through the σ-complex, activating the C8 position, is preferred, in agreement with experimental results. Both reactions involve oxidation of Pd(II) to Pd(IV) and the catalyst is regenerated. When small amounts of water are added to the reaction mixture, C8-H activation (acylation) results from the same σ-metallacycle with the same barrier, but the simultaneous formation of 2-quinolinones is more complicated. It starts with OH- attack at the C2 position, and is followed by the migration of two hydrogen atoms, and the final reductive elimination step ends with Pd(0). The higher barriers for the migration and reoxidation of Pd(0) are associated with the more demanding reaction conditions. The different reactivity of Pd(II)(OAc)2 under analogous conditions is clarified, as it is only capable of forming the above mentioned π-complex and thus of activating the C2 position of QO. This catalyst can preferentially activate the C8-H bond under rather different conditions, including in particular acetic acid medium, as shown by other authors.

Adsorption of a water molecule on the surface of neutral and charged titanium clusters: Tin-H2O, Tin+1-H2O, Tin-1-H2O, n ≤ 9

Adsorption of a water molecule on the surface of neutral and charged titanium, Tin0, ±1 H2O, n ≤ 9, clusters was studied in this work using density functional theory, BPW91-D2, all-electron calculations. Water adsorption is crucial for understanding the nascent solvation of Tin and H-O bond activation. Computed Tin0,±1 ground states, GS, allow a characterization of the structural, electronic, and energetic properties. Compact forms appear for the Tin0,±1, n ≤ 9 GS, and the estimated ionization energies (IE), electron affinities (EA) and binding energies (D0) follow the trends of available experimental results. The most stable clusters are Ti4+1, Ti50, -1, Ti70, ±1, and Ti90, ± 1, being in accord with the measured D0 for the cations. Overall, the cations show larger D0 that neutral and anions: D0(Tin+) > D0 (Tin) > D0(Tin-). Further, the IE are reduced by water adsorption on the Tin clusters, this being due to delocalization effects, through the network of the Ti–Ti and Ti–O bonds, of the most external valence electrons of Tin-H2O. For Tin+H2O, the lowest D0 occur at n = 4, 7, and 9, with maximums at n = 5 and 8, the highest D0 is for n = 1; being different from those of bare Tin+. Metal-oxygen Ti-O bond and Ti-H bond interactions account for the adsorption of water on Tin0,±1, which occurs on a metal atom of a triangular Ti3 face. In the Ti-O and Ti-H bond pattern, the metal atom is roughly placed in between the O and H atoms, originating agostic bonds, which produce H-O bond activation, being larger in the Tin––H2O anions. Thus, the titanium clusters may show catalytic behavior in their interaction with water molecules.

Contrasting behaviour under pressure reveals the reasons for pyramidalization in tris(amido)uranium(III) and tris(arylthiolate) uranium(III) molecules

A range of reasons has been suggested for why many low-coordinate complexes across the periodic table exhibit a geometry that is bent, rather a higher symmetry that would best separate the ligands. The dominating reason or reasons are still debated. Here we show that two pyramidal UX3 molecules, in which X is a bulky anionic ligand, show opposite behaviour upon pressurisation in the solid state. UN″3 (UN3, N″ = N(SiMe3)2) increases in pyramidalization between ambient pressure and 4.08 GPa, while U(SAr)3 (US3, SAr = S-C6H2-tBu3−2,4,6) undergoes pressure-induced planarization. This capacity for planarization enables the use of X-ray structural and computational analyses to explore the four hypotheses normally put forward for this pyramidalization. The pyramidality of UN3, which increases with pressure, is favoured by increased dipole and reduction in molecular volume, the two factors outweighing the slight increase in metal-ligand agostic interactions that would be formed if it was planar. The ambient pressure pyramidal geometry of US3 is favoured by the induced dipole moment and agostic bond formation but these are weaker drivers than in UN3; the pressure-induced planarization of US3 is promoted by the lower molecular volume of US3 when it is planar compared to when it is pyramidal.

Three-centre electron sharing indices (3c-ESIs) as a tool to differentiate among (an)agostic interactions and hydrogen bonds in transition metal complexes.

The agostic bond plays an important role in chemistry, not only in transition metal chemistry but also in main group chemistry. In some complexes with M⋯H-X (X = C, N) interactions, differentiation among agostic, anagostic, and hydrogen bonds is challenging. Here we propose the use of three-centre electron sharing indices to classify M⋯H-X (X = C, N) interactions.

Theoretical Study of N-Heterocyclic-Carbene-ZnX2 (X = H, Me, Et) Complexes.

This article discusses the properties of as many as 30 carbene–ZnX (X = H, Me, Et) complexes featuring a zinc bond C⋯Zn. The group of carbenes is represented by imidazol-2-ylidene and its nine derivatives (labeled as IR), in which both hydrogen atoms of N-H bonds have been substituted by R groups with various spatial hindrances, from the smallest Me, Pr, Bu through Ph, Tol, and Xyl to the bulkiest Mes, Dipp, and Ad. The main goal is to study the relationship between type and size of R and X and both the strength of C⋯Zn and the torsional angle of the ZnX plane with respect to the plane of the imidazol-2-ylidene ring. Despite the considerable diversity of R and X, the range of is quite narrow: 2.12–2.20 Å. On the contrary, is characterized by a fairly wide range of 18.5–27.4 kcal/mol. For the smallest carbenes, the ZnX molecule is either in the plane of the carbene or is only slightly twisted with respect to it. The twist angle becomes larger and more varied with the bulkier R. However, the value of this angle is not easy to predict because it results not only from the presence of steric effects but also from the possible presence of various interatomic interactions, such as dihydrogen bonds, tetrel bonds, agostic bonds, and hydrogen bonds. It has been shown that at least some of these interactions may have a non-negligible influence on the structure of the IR–ZnX2 complex. This fact should be taken into account in addition to the commonly discussed R⋯X steric repulsion.

A One-Dimensional Coordination Polymer Based on 1H-Benzimidazole-2-methanol

A coordination polymer, [CuL]n (HL = 1H-benzimidazole-2-methanol), has been hydrothermally synthesized and structurally characterized by single-crystal X-ray diffraction. The complex (C8H7CuN2O) crystallizes in orthorhombic sp. gr. Pbca with a = 11.3962(13) A, b = 9.3779(10) A, c = 13.7003(15) A, Z = 8. In the complex, the Cu+ ions and the coordination anion L– are connected with each other to form a series of 1D CuL chains, which are further linked through interchain weak interactions (anagostic interaction and O···π interaction) to generate a wave-shaped 3D supramolecular coordination network. The agostic bond O–H⇀Cu+ between the Cu+ ion and the hydroxyl group in coordination anion L– with the H–Cu+ bond length of 1.952 A and the O1–H1–Cu1 bond angle of 140.05(1)° is observed.

Iridium complexes featuring a tridentate SiPSi ligand: from dimeric to monomeric 14, 16 or 18-electron species.

In the solid state, the dinuclear iridium complex [μ-Cl-Ir(SiMe2CH2-o-C6H4)2PPh]2, 1, is shown by X-ray diffraction to bear dibenzylsilylphosphine ligands in SiPSi tridentate coordination modes as well as chloride bridges. In C6D6 solution, 1 dissociates into the 14-electron species [IrCl(SiMe2CH2-o-C6H4)2PPh] prone to coordinate one or two L-type ligands such as PR3 (R = Cy, Ph, OEt), CO and CH3CN giving rise to the corresponding mononuclear 16- or 18-electron complexes [IrCl(SiMe2CH2-o-C6H4)2PPh(L)x] (x = 1, 2) as evidenced by X-ray and NMR studies. The dinuclear structure is retained upon reaction with Et3SiH which results in the formation of [μ-Cl,μ-H-Ir2{(SiMe2CH2-o-C6H4)2PPh}2] with a bridging hydride. On the basis of NMR studies, the reaction of the triphenylphosphine complex [IrCl(SiMe2CH2-o-C6H4)2PPh(PPh3)] with LiBHEt3 leads to the hydride complex [IrH(SiMe2CH2-o-C6H4)(η2-H-SiMe2CH-o-C6H4)PPh(PPh3)] in which one SiPSi ligand has been transformed and is now bonded to iridium in a tetradentate mode via P, Si, an agostic Si-H bond, and C of a methine as a result of the activation of one methylene group.

A Quasi-Atomic Analysis of Three-Center Two-Electron Zr-H-Si Interactions.

A comprehensive analysis of the bonding structure of the disilyl zirconocene amide cation {Cp2Zr[N(SiHMe2)2]}+ is conducted by application of an intrinsic orbital localization method that yields quasi-atomic orbitals (QUAOs). An emphasis is placed on describing a previously characterized three-center two-electron interaction between zirconium, hydrogen, and silicon that presents structural and spectroscopic features similar to that of agostic bonds. Expressions of the first-order density matrix in terms of the QUAOs yields bond orders (BOs), kinetic bond orders (KBOs), and the extent of transfer of charge that are useful to determine the electronic nature of the Zr-H-Si bond. The interactions between the QUAOs demonstrate the importance of vicinal interactions in the stabilization of the molecule. In addition, the evolution of the QUAOs during reactions with Lewis bases reveals the role of the Zr-H-Si interaction in facilitating the reaction.

Polly L. Arnold

Polly L. Arnold

Synthesis and Reactivity of Group Six Metal PCP Pincer Complexes: Reversible CO Addition Across the Metal–Caryl Bond

In the present study, Cr(III) complexes of the type trans-[Cr(PCPNEt-iPr)(solvent)Cl2] (solvent = CH3CN, THF), the Cr(0) complex [Cr(κ3P,CH,P-P(CH)PNEt-iPr)(CO)3] which features an η2-Caryl–H agostic bond as well as seven coordinate cationic chloro carbonyl Mo(II) and W(II) complexes of the type [M(PCPNEt-iPr)(CO)3Cl] featuring PCP pincer ligands based on a 1,3-diaminobenzene scaffold were prepared and characterized. The seven coordinate chloro tricarbonyl complexes exhibit fluxional behavior in solution due to rapid CO ligand interconversions. Another interesting aspect is a rapid and reversible addition of one CO ligand across the metal–Cipso bond. The mechanism of the dynamic process of the chloro carbonyl complexes was investigated by means of DFT calculations. The Mo(II) and W(II) tricarbonyl complexes could be reduced to the respective anionic Mo(0) and W(0) complexes [Mo(PCPNEt-iPr)(CO)3]− and [Mo(PCPNEt-iPr)(CO)3]−. These air sensitive compounds are readily protonated by MeOH to form agostic and h...

Iron PCP Pincer Complexes in Three Oxidation States: Reversible Ligand Protonation To Afford an Fe(0) Complex with an Agostic C-H Arene Bond.

In the current investigation, the reaction of Fe2(CO)9 with the ligand precursor 2-chloro-N1,N3-bis(diisopropylphosphanyl)-N1,N3-diethylbenzene-1,3-diamine (P(C-Cl)PNEt- iPr) (1) was investigated. When a suspension of Fe2(CO)9 and 1 in CH3CN was transferred in a sealed microwave glass vial and stirred for 18 h at 110 °C the complex [Fe(PCPNEt- iPr)(CO)2Cl] (2) was obtained. In an attempt to prepare the hydride Fe(II) complex [Fe(PCPNEt- iPr)(CO)2H] (3), 2 was reacted with 1 equiv of Li[HBEt3] in THF. Instead of ligand substitution, this complex underwent a one electron reduction which led to the formation of the low-spin d7 Fe(I) complex [Fe(PCPNEt- iPr)(CO)2] (4). Exposure of a benzene solution of 4 to NO gas (1 bar) at room temperature affords the diamagnetic complex [Fe(PCPNEt- iPr)(CO)(NO)] (5). This is the first iron PCP nitrosyl complex. Protonation of 5 with HBF4·Et2O affords the cationic Fe(0) complex [Fe(κ3 P,CH,P-P(CH)PNEt- iPr)(CO)(NO)]BF4 (6) which features an η2-Caryl-H agostic bond. Even with relatively weak bases such as NEt3 the agostic C-H bond can be deprotonated with reformation of the starting material 5. Therefore, protonation of 5 is completely reversible.

Reversible Ligand Protonation of a Mn(I) PCP Pincer Complex To Afford a Complex with an η2-Caryl–H Agostic Bond

The first manganese PCP pincer complex, [Mn(PCPNEt-iPr)(CO)3], was obtained by treating [Mn2(CO)10] and 2-chloro-N1,N3-bis(diisopropylphosphanyl)-N1,N3-diethylbenzene-1,3-diamine (P(C-Cl)PNEt-iPr) in CH3CN under solvothermal conditions in a sealed microwave glass tube for 8 h at 140 °C. Protonation of this complex with HBF4·Et2O leads to the formation of the cationic Mn(I) complex [Mn(κ3P,CH,P-P(CH)PNEt-iPr)(CO)3]BF4, which features an η2-Caryl–H agostic bond. There was no evidence for the formation of a Mn(III) hydride species. The agostic C–H bond is comparatively acidic, and even relatively weak bases, such as NEt3, deprotonate this bond to re-form the starting material [Mn(PCPNEt-iPr)(CO)3]; protonation is fully reversible. Substitution of one CO ligand was achieved upon treatment with NO+, affording the cationic complex [Mn(PCPNEt-iPr)(CO)2(NO)]+, where the two CO ligands are trans to one another.

Ligand-Enforced Switch of the Coordination Mode in Low-Valent Group 6 Carbonyl Complexes Containing Pyrimidine-Based Bisphosphines

The reaction of M(CO)6 (M = Cr, Mo, W) with N,N′-bis(diisopropylphosphine)-N,N′-dimethylpyrimidine-4,6-diamines (PymR-iPr) bearing R = Me, Ph, tBu substituents in the 2-position was investigated. The pyrimidine-based bisphosphine ligands with R = Me, Ph reacted with M(CO)6 to yield mononuclear and homobimetallic complexes of the types [M(κ2P,N-PymMe-iPr)(CO)4] and [M(CO)4-μ2-(κ2P,N-PymPh-iPr)M(CO)4], respectively. Heterobimetallic complexes of the type [M1(CO)4-μ2-(κ2P,N-PymMe-iPr)M2(CO)4] were obtained by reacting mononuclear complexes [M1(κ2P,N-PymMe-iPr)(CO)4] with 1 equiv of the respective M2(CO)6. Replacing these substituents by a bulky tBu group led to a switch in the coordination mode of the pyrimidine ligand. In the case of chromium, the complex [Cr(κ3P,CH,P-PymtBu-iPr)(CO)3] containing an η2-Caryl–H agostic bond was selectively formed and no C–H bond cleavage took place. However, in the case of molybdenum, the reaction led to the formation of an inseparable mixture of the agostic complex [Mo(κ3P,...

Structural Characterization of β-Agostic Bonds in Pd-Catalyzed Polymerization

β-agostic Pd complexes are critical intermediates in catalytic reactions, such as olefin polymerization and Heck reactions. Pd β-agostic complexes, however, have eluded structural characterization, due to the fact that these highly unstable molecules are difficult to isolate. Herein, we report the single-crystal X-ray and neutron diffraction characterization of β-agostic (α-diimine)Pd–ethyl intermediates in polymerization. Short Cα–Cβ distances and acute Pd–Cα–Cβ bond angles combined serve as unambiguous evidence for the β-agostic interaction. Characterization of the agostic structure and the kinetic barrier for β-H elimination offer important insight into the fundamental understanding of agostic bonds and the mechanism of polymerization.

Intramolecular hydrogenation of triethylsilylethylene catalyzed by Ru(II) complex: Agostic bond formation and trizonal transition states with ten acting atoms

Hydrogenation of olefins catalyzed by a ruthenium II complex, the RuH2(η2-H2)2(P(C6H11)3)2 have been studied using DFT/B3LYP method. The establishment of agostic Ru-C bonds followed by the migration of hydrogen atoms from one cyclohexyl of phosphine ligand is the main particularity of this reaction. Transition states associated with the latter migrations were determined. Each one is trizonal and involves three different areas and ten active atoms. Intrinsic Reaction Coordinate (IRC) calculations allowed following their movements during this reaction. Energy profiles confirm the simultaneous movement of all these active atoms. Molecular orbitals of all involved species were analyzed and related to ruthenium atomic orbitals. Obtained results highlight the catalytic effect of Ru atom able, thanks to its versatility, to redistribute its AOs and to control the exchange process of concerned ligands. Those transition states are currently among the most complicated ones to be calculated up today.

Mechanistic Insights into the Directed Hydrogenation of Hydroxylated Alkene Catalyzed by Bis(phosphine)cobalt Dialkyl Complexes.

The mechanism of directed hydrogenation of hydroxylated alkene catalyzed by bis(phosphine)cobalt dialkyl complexes has been studied by DFT calculations. The possible reaction channels of alkene hydrogenation catalyzed by catalytic species (0T, 0P, and 0) were investigated. The calculated results indicate that the preferred catalytic activation processes undergo a 1,2 alkene insertion. 0P and 0 prefer the β hydrogen elimination mechanism with an energy barrier of 9.5 kcal/mol, and 0T prefers the reductive elimination mechanism with an energy barrier of 11.0 kcal/mol. The second H2 coordination in the σ bond metathesis mechanism needs to break the agostic H2-βC bond of metal-alkyl intermediates (21P and 21T), which owns the larger energetic span compared to the reductive elimination. This theoretical study shows that the most favorable reaction pathway of alkene hydrogenation is the β hydrogen elimination pathway catalyzed by the planar (dppe)CoH2. The hydrogenation activity of Co(II) compounds with redox-innocent phosphine donors involves the Co(0)-Co(II) catalytic mechanism.

Transition‐Metal Complexes with (C–C)→M Agostic Interactions

The coordination of sigma bonding electron density to a Lewis acid was for some time largely the domain of boron hydride chemistry. Subsequently it was found that C–H bonds could undergo intramolecular coordination to metal centers, forming what are now known as “agostic bonds” and “agostic complexes”. Thereafter, even the H–H bond in molecular hydrogen was found to be capable of coordination, leading to what are commonly referred to as “sigma complexes”. Eventually, coordination by much more shielded C–C bonds was established, and is of particular significance with regard to to the interest in C–C bond activation. This review focuses on the key developments in this field, including the nature of the bonding interactions in these and some related species, as revealed by spectroscopic, structural, and theoretical studies, their involvement in important chemical reactions such as alkene polymerizations and metathesis, and comparisons with the other types of electron‐deficient bridge bonding.

Thorium and Uranium Hydride Phosphorus and Arsenic Bearing Molecules with Single and Double Actinide-Pnictogen and Bridged Agostic Hydrogen Bonds.

Thorium atoms from laser ablation react with phosphine during condensation in excess argon to produce two new infrared absorptions at 1467.2 and 1436.6 cm-1 near weak bands for ThH and ThH2, which increase on annealing to 25 and 30 K, indicating spontaneous reactions. Analogous experiments with uranium produced two similar bands at 1473.4 and 1456.7 cm-1 above UH at 1423.8 cm-1 and another absorption at 1388.2 cm-1. Electronic structure calculations at the coupled cluster CCSD(T) for Th and density functional theory calculations for U as well as their proximity to other actinide hydride absorptions support assignments of these bands to the simplest molecules HP═ThH2, HP═UH2, and PH2-UH. Arsine gave the analogous products HAs═ThH2, HAs═UH2, and AsH2-UH. The HE═AnH2 molecules (E = P, As; An = Th, U) have strong agostic An-H(E) interactions with H-E-An angles in the range of 60-64°. The calculated agostic bond distances are 9% to 12% longer than terminal single An-H bonds, which suggests that these strong agostic bonds can be considered as bridge bonds since similar relationships are found for the dibridged M2H6 molecules (M = Al, Ga, In). The NBO analysis and the molecular orbitals show the presence of a σ and a π bond for HE═AnH2 molecules that are heavily polarized with most of the density on the P or As.