Analysis of the Putative Cr–Cr Quintuple Bond in Ar′CrCrAr′ (Ar′ = C6H3-2,6(C6H3-2,6-Pri2)2 Based on the Combined Natural Orbitals for Chemical Valence and Extended Transition State Method

Advances and Prospects in Understanding London Dispersion Interactions in Molecular Chemistry

London dispersion (LD) interactions are the main contribution of the attractive part of the van der Waals potential. Even though LD effects are the driving force for molecular aggregation and recognition, the role of these omnipresent interactions in structure and reactivity had been largely underappreciated over decades. However, in the recent years considerable efforts have been made to thoroughly study LD interactions and their potential as a chemical design element for structures and catalysis. This was made possible through a fruitful interplay of theory and experiment. This review highlights recent results and advances in utilizing LD interactions as a structural motif to understand and utilize intra- and intermolecularly LD-stabilized systems. Additionally, we focus on the quantification of LD interactions and their fundamental role in chemical reactions.

Silyl Groups Are Strong Dispersion Energy Donors

We present an experimental and computational study to investigate noncovalent interactions between silyl groups that are often employed as "innocent" protecting groups. We chose an extended cyclooctatetraene (COT)-based molecular balance comprising unfolded (1,4-disubstituted) and folded (1,6-disubstituted) valance bond isomers that typically display remote and close silyl group contacts, respectively. The thermodynamic equilibria were determined using nuclear magnetic resonance measurements. Additionally, we utilized Boltzmann weighted symmetry-adapted perturbation theory (SAPT) at the sSAPT0/aug-cc-pVDZ level of theory to dissect and quantify noncovalent interactions. Apart from the extremely bulky tris(trimethylsilyl)silyl "supersilyl" group, there is a preference for the folded 1,6-COT valence isomer, with London dispersion interactions being the main stabilizing factor. This makes silyl groups excellent dispersion energy donors, a finding that needs to be taken into account in synthesis planning.

Antiferromagnetic Coupling Supported by Metallophilic Interactions: Theoretical View

The antiferromagnetic coupling supported by metallophilic interactions has been studied in the framework of the broken symmetry approach (BS) and multiconfigurational calculations (CASSCF). A series of heterobimetallic complexes of the form [PtCo(X)₄(Y)]₂ (X = tba thiobenzoate, SAc thioacetate, and Y = H₂O, NO₂py, py), previously reported, have been used as model systems. Magnetic coupling constants were found in good agreement with the experimental reports, and it could be concluded that axial ligands with a pure σ-donor character have a marked effect on the J value strengthening the antiferromagnetic coupling, as shown for [PtCo(SAc)₄(H₂O)]₂ and [PtNi(SAc)₄(H₂O)]₂. The latter complex, included for comparative purposes, also made it possible to evidence that the interaction between magnetic orbitals and low-level excitation in the Pt···Pt region is also relevant favoring the stronger antiferromagnetic coupling found in this case. A careful analysis of the energetic components involved in Pt···Pt interaction suggests that the stabilization arises from a combination of favorable orbital contributions, which allows a weak covalent Pt···Pt σ(dz²...dz²) bond. Theoretical tools evidence that the weak σ-bond found between monomeric units is responsible for a spin polarization mechanism resulting in the observed antiferromagnetic interaction. Multiconfigurational calculations finally allowed us to establish that the spin polarization mechanism involves not only the d_z² orbitals in the M-Pt···Pt-M bond direction but also the empty 6p_z orbitals of Pt atoms. The inclusion of these orbitals favors a correlation-induced delocalization of magnetic orbitals and therefore a better balance among direct and kinetic exchange. The results shown in this work are relevant in the molecular design of systems supported by metallophilic interactions not only between platinum atoms but also could be extended to other cases with similar interactions.

Polarizability and isotope effects on dispersion interactions in water

True understanding of dispersion interaction in solution remains elusive because of difficulty in the precise evaluation of its interaction energy. Here, the effect of substituents with different polarizability on dispersion interactions in water is discussed based on the thermodynamic parameters determined by isothermal titration calorimetry for the formation of discrete aggregates from gear-shaped amphiphiles (GSAs). The substituents with higher polarizability enthalpically more stabilize the nanocube, which is due to stronger dispersion interactions and to the hydrophobic effect. The differences in the thermodynamic parameters for the nanocubes from the GSAs with CH3 and CD3 groups are also discussed to lead to the conclusion that the H/D isotope effect on dispersion interactions is negligibly small, which is due to almost perfect entropy-enthalpy compensation between the two isotopomers.

Metal-Metal (MM) Bond Distances and Bond Orders in Binuclear Metal Complexes of the First Row Transition Metals Titanium Through Zinc

This survey of metal-metal (MM) bond distances in binuclear complexes of the first row 3d-block elements reviews experimental and computational research on a wide range of such systems. The metals surveyed are titanium, vanadium, chromium, manganese, iron, cobalt, nickel, copper, and zinc, representing the only comprehensive presentation of such results to date. Factors impacting MM bond lengths that are discussed here include (a) the formal MM bond order, (b) size of the metal ion present in the bimetallic core (M₂) ⁿ⁺, (c) the metal oxidation state, (d) effects of ligand basicity, coordination mode and number, and (e) steric effects of bulky ligands. Correlations between experimental and computational findings are examined wherever possible, often yielding good agreement for MM bond lengths. The formal bond order provides a key basis for assessing experimental and computationally derived MM bond lengths. The effects of change in the metal upon MM bond length ranges in binuclear complexes suggest trends for single, double, triple, and quadruple MM bonds which are related to the available information on metal atomic radii. It emerges that while specific factors for a limited range of complexes are found to have their expected impact in many cases, the assessment of the net effect of these factors is challenging. The combination of experimental and computational results leads us to propose for the first time the ranges and "best" estimates for MM bond distances of all types (Ti-Ti through Zn-Zn, single through quintuple).

Metal-interacted histidine dimer: an ETS-NOCV and XANES study

We have analyzed the metal coordination in a histidine dimer, hydrated with a water molecule, based on the extended transition state scheme with the theory of natural orbitals for chemical valence (ETS-NOCV).

The multiple bonding in heavier group 14 element alkene analogues is stabilized mainly by dispersion force effects

Computations on the heavier group 14 dimetallenes [E{CH(SiMe₃)₂}₂]₂ and [E{N(SiMe₃)₂}₂]₂ (E = Ge, Sn, or Pb) and their respective monomers indicated that empirically observed dimerization is principally driven by attractive dispersion forces.

The structures and bonding in the heavier group 14 element olefin analogues [E{CH(SiMe₃)₂}₂]₂ and [E{N(SiMe₃)₂}₂]₂ (E = Ge, Sn, or Pb) and their dissociation into :E{CH(SiMe₃)₂}₂ and :E{N(SiMe₃)₂}₂ monomers were studied computationally using hybrid density functional theory (DFT) at the B3PW91 with basis set superposition error and zero point energy corrections. The structures were reoptimized with the dispersion-corrected B3PW91-D3 method to yield dispersion force effects. The calculations generally reproduced the experimental structural data for the tetraalkyls with a few angular exceptions. For the alkyls, without the dispersion corrections, dissociation energies of –2.3 (Ge), +2.1 (Sn), and –0.6 (Pb) kcal mol^–1 were calculated, indicating that the dimeric E–E bonded structure is favored only for tin. However, when dispersion force effects are included, much higher dissociation energies of 28.7 (Ge), 26.3 (Sn), and 15.2 (Pb) kcal mol^–1 were calculated, indicating that all three E–E bonded dimers are favored. Calculated thermodynamic data at 25 °C and 1 atm for the dissociation of the alkyls yield ΔG values of 9.4 (Ge), 7.1 (Sn), and –1.7 (Pb) kcal mol^–1, indicating that the dimers of Ge and Sn, but not Pb, are favored. These results are in harmony with experimental data. The dissociation energies for the putative isoelectronic tetraamido-substituted dimers [E{N(SiMe₃)₂}₂]₂ without dispersion correction are –7.0 (Ge), –7.4 (Sn), and –4.8 (Pb) kcal mol^–1, showing that the monomers are favored in all cases. Inclusion of the dispersion correction yields the values 3.6 (Ge), 11.7 (Sn), and 11.8 (Pb) kcal mol^–1, showing that dimerization is favored but less strongly so than in the alkyls. The calculated thermodynamic data for the amido germanium, tin, and lead dissociation yield ΔG values of –12.2, –3.7, and –3.6 kcal mol^–1 at 25 °C and 1 atm, consistent with the observation of monomeric structures. Overall, these data indicate that, in these sterically-encumbered molecules, dispersion force attraction between the ligands is of greater importance than group 14 element–element bonding, and is mainly responsible for the dimerization of the metallanediyls species to give the dimetallenes. In addition, calculations on the non-dissociating distannene [Sn{SiMe^tBu₂}₂]₂ show that the attractive dispersion forces are key to its stability.

Energy decomposition analysis approaches and their evaluation on prototypical protein–drug interaction patterns

The partitioning of the interaction energy into chemical components such as electrostatics, polarization, and charge transfer is possible with energy decomposition analysis approaches. We review and evaluate these for biomolecular applications.

Theory, synthesis and reactivity of quintuple bonded complexes

Recent achievements in the area of metal–metal quintuple bonding are highlighted, including synthesis of quintuple bonded complexes, metal-to-metal bonding schemes, and their reactivity.

The important role of the Mo–Mo quintuple bond in catalytic synthesis of benzene from alkynes. A theoretical study

The reaction mechanism of catalytic synthesis of benzene from alkynes by the Mo–Mo quintuple bond and the electronic structure and bonding nature of dimetallacyclobutadiene and dimetallabenzyne were studied theoretically.

Discovering complexes containing a metal–metal quintuple bond: from theory to practice

Recent advances in quintuple bonding are highlighted based on theoretical predictions and experimental achievements.

Density functional theory studies on the structure and electron distribution in the peroxide intermediate of the catalytic cycle of multicopper oxidases

The peroxide intermediate (PI) is obtained in the first step of the reduction of O2 by multicopper oxidases. Earlier density functional theory (DFT) studies as well as spectral and structural comparison to a fully oxidized structural analogue of the PI known as the peroxide adduct (PA) reveal that O2 bridges all three copper atoms of the trinuclear cluster in the PI. This orientation of the oxygen moiety has been discussed as a result of the influence from the second coordination sphere. In the present study, we investigate by DFT and quantum mechanics/molecular mechanics (QM/MM) the potential energy surface (PES) of the PI as a function of the orientation of O2 within the copper cluster to examine the influence of the second coordination sphere on the structure of the PI. We use the second order spin-flip constricted variational DFT method to probe a possible multideterminantal nature of the PI and to devise a computational strategy for its treatment. Our results suggest that the PI can be approximated to a closed shell singlet. Additionally, for the determination of the oxidation states of the three copper atoms in the PI, the electron redistribution upon the formation of the PI has been investigated with the extended transition state–natural orbitals for chemical valence method. We observe a flat PES on which oxygen can easily rotate between the copper atoms. The fully bridged PI structure emerges in the absence of atoms from the second coordination sphere and has been attributed to the coordination unsaturation of the copper atoms in the cluster. The good Cu–O overlap leads to the participation of all copper atoms in the reduction of O2.