Improving a Methane C-H Activation Complex by Metal and Ligand Alterations from Computational Results.

We present results for a series of complexes derived from a titanium complex capable of activating C-H bonds under mild conditions (PNP)Ti═CHtBu(CH2tBu), where PNP = N[2-PiPr2-4-methylphenyl]2-. In addition to the initial activation of methane, a tautomerization reaction to a terminal methylidene is also explored due to methylidene's potential use as a synthetic starting point. Analogous complexes with other low-cost 3d transition metals were studied, such as scandium, titanium, vanadium, and chromium as both isoelectronic and isocharged complexes. Our results predict that V(IV) and V(V) complexes are promising for methane C-H bond activation. The V(V) complex has a low rate-determining barrier for methane activation, specifically 16.6 kcal/mol, which is approximately 12 kcal/mol less than that for the Ti complex, as well as having a moderate tautomerization barrier of 29.8 kcal/mol, while the V(IV) complex has a methane activation barrier of 19.0 kcal/mol and a tautomerization barrier of 31.1 kcal/mol. Scandium and chromium complexes are much poorer for C-H bond activation; scandium has very high barriers, while chromium strongly overstabilizes the alkylidene intermediate, potentially stopping the further reaction. In addition to the original PNP ligand, some of the most promising ligands from a previous work were tested, although (as shown previously) modification of the ligand does not typically have large effects on the activity of the system. Our best ligand modification improves the performance of the V(V) complex via the substitution of the nitrogen in PNP by phosphorus, which reduces the tautomerization barrier by 5 to 24.4 kcal/mol.

New Type of Aromatic π-Systems for Anion Recognition: Strong Anion-π and C-H⋠⋠⋠Anion Interactions Between Halides and Aromatic Ligands in Half-Sandwich Compounds.

Half-sandwich compounds of benzene, cyclopentadienyl, pentamethylcyclopentadienyl, and indenyl were studied as a new type of aromatic π-systems for interactions with halide anions. Although uncoordinated benzene forms only C-H⋅⋅⋅anion interactions, and hexafluorobenzene forms only anion-π interactions, aromatic ligands in half-sandwich compounds can form both types of interactions, because their entire electrostatic potential surface is positive. These aromatic ligands can form stronger anion-π interactions than organic aromatic molecules, as a consequence of more pronounced dispersion and induction energy components. Moreover, C-H⋅⋅⋅anion interactions of aromatic ligands are stronger than anion-π interactions, and significantly stronger than C-H⋅⋅⋅anion interactions of benzene. Our study shows that transition-metal coordination can make aromatic moieties suitable for strong interactions with anions, and gives insight into the design of new anion receptors.

The Affinity of Some Lewis Bases for Hexafluoroisopropanol as a Reference Lewis Acid: An ITC/DFT Study.

To figure out the possible role of 1,1,1,3,3,3-hexafluoropropan-2-ol (HFIP) as well as to provide reference thermochemical data in solution, the formation of Lewis acid-base complexes between HFIP (Lewis acid) and a series of 8 different Lewis bases (3 sulfoxides, 3 Nsp2 pyridine derivatives, 1 aromatic amine, 1 cyclic aliphatic ether) was examined by isothermal titration calorimetry (ITC) experiments and static density functional theory augmented with Dispersion (DFT-D) calculations. Measured ITC association enthalpy values (ΔHa ) lie between -9.3 and -14 kcal mol-1 . Computations including a PCM implicit solvation model produced similar exothermicity of association of all studied systems compared to the ITC data with ΔHa values ranging from -8.5 to -12.7 kcal mol-1 . An additional set of calculations combining implicit and explicit solvation by chlorobenzene of the reactants, pointed out the relatively low interference of the solvent with the HFIP-base complexation: its main effect is to slightly enhance the Gibbs energy of the HFIP-Lewis base association. It is speculated that the interactions of bulk HFIP with Lewis bases therefore may significantly intervene in catalytic processes not only via the dynamic microstructuring of the medium but also more explicitly by affecting bonds' polarization at the Lewis bases.

Stacking interactions of resonance-assisted hydrogen-bridged rings and C6-aromatic rings.

Stacking interactions between six-membered resonance-assisted hydrogen-bridged (RAHB) rings and C6-aromatic rings were systematically studied by analyzing crystal structures in the Cambridge Structural Database (CSD). The interaction energies were calculated by quantum-chemical methods. Although the interactions are stronger than benzene/benzene stacking interactions (-2.7 kcal mol-1), the strongest calculated RAHB/benzene stacking interaction (-3.7 kcal mol-1) is significantly weaker than the strongest calculated RAHB/RAHB stacking interaction (-4.7 kcal mol-1), but for a particular composition of RAHB rings, RAHB/benzene stacking interactions can be weaker or stronger than the corresponding RAHB/RAHB stacking interactions. They are also weaker than the strongest calculated stacking interaction between five-membered saturated hydrogen-bridged rings and benzene (-4.4 kcal mol-1) and between two five-membered saturated hydrogen-bridged rings (-4.9 kcal mol-1). SAPT energy decomposition analyses show that the strongest attractive term in RAHB/benzene stacking interactions is dispersion, however, it is mostly canceled by a repulsive exchange term; hence the geometries of the most stable structures are determined by an electrostatic term.

How flexible is the water molecule structure? Analysis of crystal structures and the potential energy surface.

Water molecules from crystal structures archived in the CSD show a relatively large range both in the bond angle and bond lengths. High level ab initio calculations at the CCSD(T)/CBS level predicted a possibility for energetically low-cost (±1 kcal mol-1) changes of the bond angle and bond lengths in a wide range, from 96.4° to 112.8° and from 0.930 Å to 0.989 Å, respectively.

Stacking interactions of borazine: important stacking at large horizontal displacements and dihydrogen bonding governed by electrostatic potentials of borazine.

Potential energy surfaces of borazine-benzene and borazine-borazine stacking interactions were studied by performing DFT, CCSD(T)/CBS and SAPT calculations. The strongest borazine-benzene stacking was found in a parallel-displaced geometry, with a CCSD(T)/CBS interaction energy of -3.46 kcal mol-1. The strongest borazine-borazine stacking has a sandwich geometry, with a CCSD(T)/CBS interaction energy of -3.57 kcal mol-1. The study showed that borazine forms significant stacking interactions at large horizontal displacements (over 4.5 Å), with energies of -2.20 kcal mol-1 for the borazine-benzene and -1.96 kcal mol-1 for the borazine-borazine system. The strength of interactions and their geometrical preferences can be rationalized by observing the electrostatic potentials of borazine and benzene, which is in agreement with SAPT analysis showing that electrostatics is the most important energy component for borazine stacking. All the interactions found in crystal structures of borazine and related compounds were identified either as potential curve minima or the geometries obtained from their optimizations. We also report a new dihydrogen bonding dimer with a CCSD(T)/CBS interaction energy of -2.37 kcal mol-1, which is encountered in the borazine crystal structures and enables the formation of additional simultaneous interactions that contribute to the overall stability of the crystals.

Influence of chelate ring type on chelate-chelate and chelate-aryl stacking: the case of nickel bis(dithiolene).

Chelate-aryl and chelate-chelate stacking interactions of nickel bis(dithiolene) were studied at the CCSD(T)/CBS and DFT levels. The strongest chelate-aryl stacking interaction between nickel bis(dithiolene) and benzene has a CCSD(T)/CBS stacking energy of -5.60 kcal mol-1. The strongest chelate-chelate stacking interactions between two nickel bis(dithiolenes) has a CCSD(T)/CBS stacking energy of -10.34 kcal mol-1. The most stable chelate-aryl stacking has the benzene center above the nickel atom, while the most stable chelate-chelate dithiolene stacking has the chelate center above the nickel atom. Comparison of chelate-aryl stacking interactions of dithiolene and acac-type nickel chelate shows similar strength. However, chelate-chelate stacking is stronger for dithiolene nickel chelate than for acac-type nickel chelate, which has a CCSD(T)/CBS interaction energy of -9.50 kcal mol-1.

Unraveling the Role of a Flexible Tetradentate Ligand in the Aerobic Oxidative Carbon-Carbon Bond Formation with Palladium Complexes: A Computational Mechanistic Study.

Mechanistic details of the aerobic oxidative coupling of methyl groups by a novel (MeL)PdII(Me)2 complex with the tetradentate ligand, MeL = N, N-dimethyl-2,11-diaza[3.3](2,6)pyridinophane, has been explored by density functional theory calculations. The calculated mechanism sheds light on the role of this ligand's flexibility in several stages of the reaction, especially as the oxidation state of the Pd changes. Ligand flexibility leads to diverse axial coordination modes, and it controls the availability of electrons by modulating the energies of high-lying molecular orbitals, particularly those with major d z2 character. Solvent molecules, particularly water, appear essential in the aerobic oxidation of PdII by lowering the energy of the oxygen molecule's unoccupied molecular orbital and stabilizing the PdX-O2 complex. Ligand flexibility and solvent coordination to oxygen are essential to the required spin-crossover for the transformation of high-valent PdX-O2 complexes. A methyl cation pathway has been predicted by our calculations in transmetalation between PdII and PdIV intermediates to be preferred over methyl radical or methyl anion pathways. Combining an axial and equatorial methyl group is preferred in the reductive elimination pathway where roles are played by the ligand's flexibility and the fluxionality of trimethyl groups.

Probing the Carbon-Hydrogen Activation of Alkanes Following Photolysis of Tp'Rh(CNR)(carbodiimide): A Computational and Time-Resolved Infrared Spectroscopic Study.

Carbon-hydrogen bond activation of alkanes by Tp'Rh(CNR) (Tp' = Tp = trispyrazolylborate or Tp* = tris(3,5-dimethylpyrazolyl)borate) were followed by time-resolved infrared spectroscopy (TRIR) in the υ(CNR) and υ(B-H) spectral regions on Tp*Rh(CNCH2CMe3), and their reaction mechanisms were modeled by density functional theory (DFT) on TpRh(CNMe). The major intermediate species were: κ3-η1-alkane complex (1); κ2-η2-alkane complex (2); and κ3-alkyl hydride (3). Calculations predict that the barrier between 1 and 2 arises from a triplet-singlet crossing and intermediate 2 proceeds over the rate-determining C-H activation barrier to give the final product 3. The activation lifetimes measured for the Tp*Rh(CNR) and Tp*Rh(CO) fragments with n-heptane and four cycloalkanes (C5H10, C6H12, C7H14, and C8H16) increase with alkanes size and show a dramatic increase between C6H12 and C7H14. A similar step-like behavior was observed previously with CpRh(CO) and Cp*Rh(CO) fragments and is attributed to the wider difference in C-H bonds that appear at C7H14. However, Tp'Rh(CNR) and Tp'Rh(CO) fragments have much longer absolute lifetimes compared to those of CpRh(CO) and Cp*Rh(CO) fragments, because the reduced electron density in dechelated κ2-η2-alkane Tp' complexes stabilizes the d8 Rh(I) in a square-planar geometry and weakens the metal's ability for oxidative addition of the C-H bond. Further, the Tp'Rh(CNR) fragment has significantly slower rates of C-H activation in comparison to the Tp'Rh(CO) fragment for the larger cycloalkanes, because the steric bulk of the neopentyl isocyanide ligand hinders the rechelation in κ2-Tp'Rh(CNR)(cycloalkane) species and results in the C-H activation without the assistance of the rechelation.

Insight into the Interactions of Amyloid ß-Sheets with Graphene Flakes: Scrutinizing the Role of Aromatic Residues in Amyloids that Interact with Graphene.

The interaction of amyloid ß-sheet segments with graphene-flake models is investigated by using DFT calculations. The structure of ß-sheets of selected amyloid segments is based on crystal structures obtained from the Protein Data Bank. Our study, based on DFT calculations for model systems, indicates that the interaction in amyloid-graphene aggregates can be stronger than the interaction in the respective amyloid-amyloid aggregates. The results also indicate an important specific role of aromatic sidechains in amyloid-graphene interactions. This work confirms recent experimental evidence that graphene and its modifications inhibit the aggregation of ß-amyloid peptides.

Chelated metal ions modulate the strength and geometry of stacking interactions: energies and potential energy surfaces for chelate-chelate stacking.

Quantum chemical calculations were performed on model systems of stacking interactions between the acac type chelate rings of nickel, palladium, and platinum. CCSD(T)/CBS calculations showed that chelate-chelate stacking interactions are significantly stronger than chelate-aryl and aryl-aryl stacking interactions. Interaction energy surfaces were calculated at the LC-ωPBE-D3BJ/aug-cc-pVDZ level, which gives energies in good agreement with CCSD(T)/CBS. The stacking of chelates in an antiparallel orientation is stronger than the stacking in a parallel orientation, which is in agreement with the larger number of antiparallel stacked chelates in crystal structures from the Cambridge Structural Database. The strongest antiparallel chelate-chelate stacking interaction is formed between two platinum chelates, with a CCSD(T)/CBS interaction energy of -9.70 kcal mol-1, while the strongest stacking between two palladium chelates and two nickel chelates has CCSD(T)/CBS energies of -9.21 kcal mol-1 and -9.50 kcal mol-1, respectively. The strongest parallel chelate-chelate stacking was found for palladium chelates, with a LC-ωPBE-D3BJ/aug-cc-pVDZ energy of -6.51 kcal mol-1. The geometries of the potential surface minima are not the same for the three metals. The geometries of the minima are governed by electrostatic interactions, which are the ones determining the positions of the energy minima. Electrostatic interactions are governed by different electrostatic potentials above the metals, which are very positive for nickel, slightly positive for palladium, and slightly negative for platinum.

Is the R3 Si Moiety in Metal-Silyl Complexes a Z ligand? An Answer from the Interaction Energy.

The computation of metal-silyl interaction energies indicates the existence of situations in which the silyl group behaves as a Z-type ligand according to the Green method of covalent-bond classification. There is a scale of relative intrinsic silylicity Π, defined as the ratio of the intrinsic silyl-to-triflate interaction energy of a silyltriflate as a reference compound relative to the silyl-to-metal interaction of given complex, that can reveal in a straightforward manner the propensity of SiR3 groups to behave chemically as metal-bound "silylium" ions, namely, [SiR3 ]+ . Emblematic cases, either taken from the Cambridge Structural Database (CSD) or constructed for the purpose of this study, were also investigated from the viewpoints of extended transition-state natural orbitals for chemical valence (ETS-NOCV) and quantum theory of atoms in molecules (QTAIM) analyses. It is shown in the case of POBMUP-which is the iridium 1,3-bis[(di-tert-butylphosphino)oxy]benzene (POCOP) complex isolated by Brookhart et al.-how slight variations of molecular charge and structure can drastically affect the relative intrinsic silylicity of the SiEt3 group that is weakly bonded to the hydrido-iridium motif.

Unexpected Importance of Aromatic-Aliphatic and Aliphatic Side Chain-Backbone Interactions in the Stability of Amyloids.

The role of aromatic and nonaromatic amino acids in amyloid formation has been elucidated by calculating interaction energies between ß-sheets in amyloid model systems using density functional theory (B3LYP-D3/6-31G*). The model systems were based on experimental crystal structures of two types of amyloids: (1) with aromatic amino acids, and (2) without aromatic amino acids. Data show that these two types of amyloids have similar interaction energies, supporting experimental findings that aromatic amino acids are not essential for amyloid formation. However, different factors contribute to the stability of these two types of amyloids. In the former, the presence of aromatic amino acids significantly contributes to the strength of interactions between side chains; interactions between aromatic and aliphatic side chains are the strongest, followed by aromatic-aromatic interactions, while aliphatic-aliphatic interactions are the weakest. In the latter, that is, the amyloids without aromatic residues, stability is provided by interactions of aliphatic side chains with the backbone and, in some cases, by hydrogen bonds.

Methane Activations by Titanium Neopentylidene Complexes: Electronic Resilience and Steric Control.

The titanium neopentylidene complex (PNP)Ti═CHtBu(CH2tBu) (PNP = N[2-PiPr2-4-methylphenyl]2-) is capable of activating both sp2 and sp3 C-H bonds under mild conditions. In addition to methane C-H activation, competition between the initial hydrogen abstraction reaction to form the methane activation product and the tautomerization reaction of this product to form a terminal methylidene was also explored. Several modifications of the PNP and CHtBu ligands were explored to determine the effect of these changes on C-H bond activation. In general, on the one hand, the modifications involving electronic effects have small and inconsistent influence on the stability of the intermediates and products and on the reaction barriers. On the other hand, the use of bulky groups in the ligands favors the methane activation process. By replacing the iPr groups in the PNP ligand with tBu groups, both methane activation and tautomerization reactions become more energetically favorable than in the unmodified complex. On the one hand, the largest acceleration of the methane C-H activation occurs when tBu groups in the phosphine are combined with an extra CH2 linker between the aromatic ring and the phosphine. On the other hand, replacing the nitrogen in the PNP ligand by phosphorus results in lower barriers for the tautomerization reaction and the stabilization of the product of the tautomerization although it remains slightly less stable than product of methane C-H activation. While several ligand modifications related to the electronic effects were examined, it is interesting that most of them did not make a significant change on the barriers for either reaction, indicating a significant resilience of this titanium complex, which could be used to enhance the practical aspects of the complex without a significant loss of its activity.

Hydrogen Bonding between Metal-Ion Complexes and Noncoordinated Water: Electrostatic Potentials and Interaction Energies.

The hydrogen bonding of noncoordinated water molecules to each other and to water molecules that are coordinated to metal-ion complexes has been investigated by means of a search of the Cambridge Structural Database (CSD) and through quantum chemical calculations. Tetrahedral and octahedral complexes that were both charged and neutral were studied. A general conclusion is that hydrogen bonds between noncoordinated water and coordinated water are much stronger than those between noncoordinated waters, whereas hydrogen bonds of water molecule in tetrahedral complexes are stronger than in octahedral complexes. We examined the possibility of correlating the computed interaction energies with the most positive electrostatic potentials on the interacting hydrogen atoms prior to interaction and obtained very good correlation. This study illustrates the fact that electrostatic potentials computed for ground-state molecules, prior to interaction, can provide considerable insight into the interactions.

Nature of the water/aromatic parallel alignment interactions.

The water/aromatic parallel alignment interactions are interactions where the water molecule or one of its O-H bonds is parallel to the aromatic ring plane. The calculated energies of the interactions are significant, up to ΔE(CCSD)(T)(limit) = -2.45 kcal mol(-1) at large horizontal displacement, out of benzene ring and CH bond region. These interactions are stronger than CH···O water/benzene interactions, but weaker than OH···π interactions. To investigate the nature of water/aromatic parallel alignment interactions, energy decomposition methods, symmetry-adapted perturbation theory, and extended transition state-natural orbitals for chemical valence (NOCV), were used. The calculations have shown that, for the complexes at large horizontal displacements, major contribution to interaction energy comes from electrostatic interactions between monomers, and for the complexes at small horizontal displacements, dispersion interactions are dominant binding force. The NOCV-based analysis has shown that in structures with strong interaction energies charge transfer of the type π → σ*(O-H) between the monomers also exists.

Stacking of metal chelates with benzene: can dispersion-corrected DFT be used to calculate organic-inorganic stacking?

CCSD(T)/CBS energies for stacking of nickel and copper chelates are calculated and used as benchmark data for evaluating the performance of dispersion-corrected density functionals for calculating the interaction energies. The best functionals for modeling the stacking of benzene with the nickel chelate are M06HF-D3 with the def2-TZVP basis set, and B3LYP-D3 with either def2-TZVP or aug-cc-pVDZ basis set, whereas for copper chelate the PBE0-D3 with def2-TZVP basis set yielded the best results. M06L-D3 with aug-cc-pVDZ gives satisfying results for both chelates. Most of the tested dispersion-corrected density functionals do not reproduce the benchmark data for stacking of benzene with both nickel (no unpaired electrons) and copper chelate (one unpaired electron), whereas a number of these functionals perform well for interactions of organic molecules.

Carbon-hydrogen activation of cycloalkanes by cyclopentadienylcarbonylrhodium--a lifetime enigma.

Carbon-hydrogen bond activation reactions of four cycloalkanes (C5H10, C6H12, C7H14, and C8H16) by the Cp'Rh(CO) fragments (Cp' = η(5)-C5H5 (Cp) or η(5)-C5Me5 (Cp*)) were modeled theoretically by combining density functional and coupled cluster theories, and their reaction rates were measured by fast time-resolved infrared spectroscopy. The reaction has two steps, starting with the formation of a σ-complex intermediate, followed by oxidative addition of the C-H bond by the rhodium. A range of σ-complex stabilities among the electronically unique C-H bonds in a cycloalkane were calculated and are related to the individual strengths of the C-H bond's interactions with the Rh fragment and the steric repulsion that is incurred upon forming the specific σ-complex. The unexpectedly large increase in the lifetimes of the σ-complexes from cyclohexane to cycloheptane was predicted to be due to the large range of stabilities of the different σ-complexes found for cycloheptane. The reaction lifetimes were simulated with two mechanisms, with and without migrations among the different σ-complexes, to determine if ring migrations prior to C-H activation were influencing the rate. Both mechanisms predicted similar lifetimes for cyclopentane, cyclohexane, and, to a lesser extent, cycloheptane, suggesting ring migrations do not have a large impact on the rate of C-H activation for these cycloalkanes. For cyclooctane, the inclusion of ring migrations in the reaction mechanism led to a more accurate prediction of the lifetime, indicating that ring migrations did have an effect on the rate of C-H activation for this alkane, and that migration among the σ-complexes is faster than the C-H activation for this larger cycloalkane.

Stacking of benzene with metal chelates: calculated CCSD(T)/CBS interaction energies and potential-energy curves.

Accurate values for the energies of stacking interactions of nickel- and copper-based six-membered chelate rings with benzene are calculated at the CCSD(T)/CBS level. The results show that calculations made at the ωB97xD/def2-TZVP level are in excellent agreement with CCSD(T)/CBS values. The energies of [Cu(C3H3O2)(HCO2)] and [Ni(C3H3O2)(HCO2)] chelates stacking with benzene are -6.39 and -4.77 kcal mol(-1), respectively. Understanding these interactions might be important for materials with properties that are dependent on stacking interactions.

Parallel water/aromatic interactions of non-coordinated and coordinated water.

The parallel interactions of non-coordinated and coordinated water molecules with an aromatic ring were studied by analyzing data in the Cambridge structural database (CSD) and by using quantum chemical calculations. The CSD data show that water/aromatic contacts prefer parallel to OH/π interactions, which indicates the importance of parallel interactions. The results reveal the influence of water coordination to a metal ion; the interactions of aqua complexes are stronger. Coordinated water molecules prefer a parallel-down orientation in which one OH bond is parallel to the aromatic ring, whereas the other OH bond points to the plane of the ring. The interactions of aqua complexes with parallel-down water/benzene orientation are as strong as the much better known OH/π orientations. The strongest calculated interaction energy is -14.89 kcal mol(-1) . The large number of parallel contacts in crystal structures and the quite strong interactions indicate the importance of parallel orientation in water/benzene interactions.