"Hydridic Hydrogen-Bond Donors" Are Not Hydrogen-Bond Donors.

Herein, we dismiss a recent proposal by Civis, Hobza, and co-workers to modify the IUPAC definition of hydrogen bonds in order to expand the scope from protonic Y-Hδ+ to hydridic Y-Hδ- hydrogen-bond donor fragments [J. Am. Chem. Soc. 2023, 145, 8550]. Based on accurate Kohn-Sham molecular orbital (KS-MO) analyses, we falsify the conclusion that interactions involving protonic and hydridic hydrogens are both hydrogen bonds; they are not. Instead, our quantitative KS-MO, energy decomposition, and Voronoi deformation density analyses reveal two fundamentally different bonding mechanisms for protonic Y-Hδ+ and hydridic Y-Hδ- fragments which go with charge transfer in opposite directions. On one hand, we confirm the IUPAC definition for regular hydrogen bonds in the case of protonic Y-Hδ+ fragments. On the other hand, complexes involving Y-Hδ- fragments are, in fact, acceptors in other well-known families of Lewis-acid/base interactions, such as halogen bonds, chalcogen bonds, and pnictogen bonds. These mechanisms lead to the same spectroscopic phenomenon in both the Y-Hδ+ and Y-Hδ- fragments, that is, the redshift in the Y-H stretching frequency, which is, thus, not an exclusive indicator for hydrogen bonding.

The nature of metallophilic interactions in closed-shell d8-d8 metal complexes.

We have quantum chemically analyzed the closed-shell d8-d8 metallophilic interaction in dimers of square planar [M(CO)2X2] complexes (M = Ni, Pd, Pt; X = Cl, Br, I) using dispersion-corrected density functional theory at ZORA-BLYP-D3(BJ)/TZ2P level of theory. Our purpose is to reveal the nature of the [X2(CO)2M]⋯[M(CO)2X2] bonding mechanism by analyzing trends upon variations in M and X. Our analyses reveal that the formation of the [M(CO)2X2]2 dimers is favored by an increasingly stabilizing electrostatic interaction when the M increases in size and by more stabilizing dispersion interactions promoted by the larger X. In addition, there is an overlooked covalent component stemming from metal-metal and ligand-ligand donor-acceptor interactions. Thus, at variance with the currently accepted picture, the d8-d8 metallophilicity is attractive, and the formation of [M(CO)2X2]2 dimers is not a purely dispersion-driven phenomenon.

Blueshift in Trifurcated Hydrogen Bonds: A Tradeoff between Tetrel Bonding and Steric Repulsion.

The front cover artwork is provided by the TheoCheM group at the Vrije Universiteit Amsterdam. The image shows how, in X- •••H3 C-Y complexes, the Lewis base X- tetrel-binds to the central C while sterically pushing the H atoms towards C; hence, the compression and blueshift of the H-C bonds. Read the full text of the Research Article at 10.1002/cphc.202300480.

Blueshift in Trifurcated Hydrogen Bonds: A Tradeoff between Tetrel Bonding and Steric Repulsion.

We have quantum chemically investigated the origin of the atypical blueshift of the H-C bond stretching frequency in the hydrogen-bonded complex X- •••H3 C-Y (X, Y=F, Cl, Br, I), as compared to the corresponding redshift occurring in Cl- •••H3 N and Cl- •••H3 C-H, using relativistic density functional theory (DFT) at ZORA-BLYP-D3(BJ)/QZ4P. Previously, this blueshift was attributed, among others, to the contraction of the H-C bonds as the H3 C moiety becomes less pyramidal. Herein, we provide quantitative evidence that, instead, the blueshift arises from a direct and strong X- •••C interaction of the HOMO of A- with the backside lobe on carbon of the low-lying C-Y antibonding σ* LUMO of the H3 C-Y fragment. This X- •••C bond, in essence a tetrel bond, pushes the H atoms towards a shorter H-C distance and makes the H3 C moiety more planar. The blueshift may, therefore, serve as a diagnostic for tetrel bonding.

Stabilization of Diborynes versus Destabilization of Diborenes by Coordination of Lewis Bases: Unravelling the Dichotomy.

We have quantum chemically investigated the boron-boron bonds in B2 , diborynes B2 L2 , and diborenes B2 H2 L2 (L=none, OH2 , NH3 ) using dispersion-corrected relativistic density functional theory at ZORA-BLYP-D3(BJ)/TZ2P. B2 has effectively a single B-B bond provided by two half π bonds, whereas B2 H2 has effectively a double B=B bond provided by two half π bonds and one σ 2p-2p bond. This different electronic structure causes B2 and B2 H2 to react differently to the addition of ligands. Thus, in B2 L2 , electron-donating ligands shorten and strengthen the boron-boron bond whereas, in B2 H2 L2 , they lengthen and weaken the boron-boron bond. The aforementioned variations in boron-boron bond length and strength become more pronounced as the Lewis basicity of the ligands L increases.

Multiple hydrogen-bonded dimers: are only the frontier atoms relevant?

Non-frontier atom exchanges in hydrogen-bonded aromatic dimers can induce significant interaction energy changes (up to 6.5 kcal mol-1). Our quantum-chemical analyses reveal that the relative hydrogen-bond strengths of N-edited guanine-cytosine base pair isosteres, which cannot be explained from the frontier atoms, follow from the charge accumulation in the monomers.

Intermolecular Covalent Interactions: Nature and Directionality.

Quantum chemical methods were employed to analyze the nature and the origin of the directionality of pnictogen (PnB), chalcogen (ChB), and halogen bonds (XB) in archetypal Fm Z⋅⋅⋅F- complexes (Z=Pn, Ch, X), using relativistic density functional theory (DFT) at ZORA-M06/QZ4P. Quantitative Kohn-Sham MO and energy decomposition analyses (EDA) show that all these intermolecular interactions have in common that covalence, that is, HOMO-LUMO interactions, provide a crucial contribution to the bond energy, besides electrostatic attraction. Strikingly, all these bonds are directional (i.e., F-Z⋅⋅⋅F- is approximately linear) despite, and not because of, the electrostatic interactions which, in fact, favor bending. This constitutes a breakdown of the σ-hole model. It was shown how the σ-hole model fails by neglecting both, the essential physics behind the electrostatic interaction and that behind the directionality of electron-rich intermolecular interactions. Our findings are general and extend to the neutral, weaker ClI⋅⋅⋅NH3 , HClTe⋅⋅⋅NH3 , and H2 ClSb⋅⋅⋅NH3 complexes.

Iodine Gauche Effect Induced by an Intramolecular Hydrogen Bond.

The gauche conformer in 1-X,2-Y-disubstituted ethanes, that is, the staggered orientation in which X and Y are in closer contact, is only favored for relatively small substituents that do not give rise to large X···Y steric repulsion. For more diffuse substituents, weakly attractive orbital interactions between antiperiplanar bonds (i.e., hyperconjugation) cannot overrule the repulsive forces between X and Y. Our quantum chemical analyses of the rotational isomerism of XCH2CH2Y (X = F, OH; Y = I) at ZORA-BP86-D3(BJ)/QZ4P reveal that indeed the anti conformer is generally favored due to a less destabilizing I···F and I···O-H steric repulsion. The only case when the gauche conformer is preferred is when the hydroxyl hydrogen is oriented toward the iodine atom in the 2-iodoethanol. This is because of the significantly stabilizing covalent component of the I···H-O intramolecular hydrogen bond. Therefore, we show that strong intramolecular interactions can overcome the steric repulsion between bulky substituents in 1,2-disubstituted ethanes and cause the gauche effect. Our quantum chemical computations have guided nuclear magnetic resonance experiments that confirm the increase in the gauche population as X goes from F to OH.

Cleaner and stronger: how 8-quinolinolate facilitates formation of Co(III)-thiolate from Co(II)-disulfide complexes.

The formation of Co(III)-thiolate complexes from Co(II)-disulfide complexes using the anionic ligand 8-quinolinolate (quin-) has been studied experimentally and quantum chemically. Two Co(II)-disulfide complexes [Co2(LxSSLx)(Cl)4] (x = 1 or 2; L1SSL1 = 2,2'-disulfanediylbis(N,N-bis(pyridin-2-ylmethyl)ethan-1-amine; L2SSL2 = 2,2'-disulfanedylbis (N-((6-methylpyridin-2-yl)methyl)-N-(pyridin-2-ylmethyl) ethan-1-amine) have been successfully converted with high yield to their corresponding Co(III)-thiolate complexes upon addition of the ligand 8-quinolinolate. Using density functional theory (DFT) computations the d-orbital splitting energies of the cobalt-thiolate compounds [Co(L1S)(quin)]+ and [Co(L2S)(quin)]+ were estimated to be 3.10 eV and 3.07 eV, indicating a slightly smaller ligand-field strength of ligand L2SSL2 than of L1SSL1. Furthermore, the orientation of the quin- ligand in the thiolate compounds determines the stability of the thiolate complex. DFT computations show that the thiolate structure benefits from more electrostatic attraction when the oxygen atom of the quin- ligand is positioned trans to the sulfur atom of the [Co(L1S)]2+ fragment. Quin- is the first auxiliary ligand with which it appeared possible to induce the redox-conversion reaction in cobalt(II) compounds of the relatively weak-field ligand L2SSL2.

Probing the redox-conversion of Co(II)-disulfide to Co(III)-thiolate complexes: the effect of ligand-field strength.

The redox-conversion reaction of cobalt(II)-disulfide to cobalt(III)-thiolate complexes triggered by addition of the bidentate ligand 2,2'-bipyridine has been investigated. Reaction of the cobalt(II)-disulfide complex [Co2(L1SSL1)(X)4] (L1SSL1 = di-2-(bis(2-pyridylmethyl)amino)-ethyldisulfide; X = Cl or Br) [1X] with 2,2'-bipyridine (bpy) resulted in the formation of two different products, namely the cobalt(III)-thiolate complex [Co(L1S)(bpy)]X2 and the unexpected side product [Co2(L1SSL1)(bpy)2(X)2]X2. Crystals of [Co2(L1SSL1)(bpy)2(Cl)2](BPh4)2 [2Cl](BPh4)2 obtained after anion exchange showed the cobalt(II) ions to be in octahedral geometries with the nitrogen donors of the disulfide ligand arranged in a facial conformation and the chloride ion trans to the tertiary amine nitrogen. Remarkably, this side product cannot be converted to the cobalt(III)-thiolate compound [Co(L1S)(bpy)](SbF6)2 [3](SbF6)2 by removal of the chloride ion with use of a silver salt, as this causes scrambling of the ligands, resulting in the formation of [Co(bpy)3]n+. [Co(L1S)(bpy)](SbF6)2 was obtained in a pure form by addition of bpy to a solution in acetonitrile of the compound [Co(L1S)(MeCN)2]2+ [4]2+. Addition of NEt4Cl to [3](SbF6)2 regenerates the cobalt(II)-disulfide complex [1Cl] as confirmed spectroscopically. DFT studies revealed that the conversion from [1Cl] to [3]2+ most likely occurs via the hypothetical intermediate species [2Cl]2+mer, in which the nitrogen atoms of the disulfide ligand are arranged in a meridional conformation. Interestingly, the estimated d-orbital splitting energy of [3]2+ is lower than that of [4]2+, indicating that the ligand-field strength of bpy is lower than anticipated, which hampers clean conversion in the redox-conversion reaction. This study shows that the redox-conversion reaction between cobalt(II)-disulfide and cobalt(III)-thiolate complexes is intricate rather than straightforward.

σ-Electrons Responsible for Cooperativity and Ring Equalization in Hydrogen-Bonded Supramolecular Polymers.

Invited for this month's cover are collaborators from the TheoCheM group of the Vrije Universiteit Amsterdam and the University of Perugia. The cover picture shows a σ-electron traveling through a hydrogen-bonded squaramide linear chain. The charge transfer within the σ-electronic system is the cause for the cooperativity in the investigated urea, deltamide, and squaramide polymers. More information can be found in the Full Paper by Célia Fonseca Guerra, and co-workers.

σ-Electrons Responsible for Cooperativity and Ring Equalization in Hydrogen-Bonded Supramolecular Polymers.

We have quantum chemically analyzed the cooperative effects and structural deformations of hydrogen-bonded urea, deltamide, and squaramide linear chains using dispersion-corrected density functional theory at BLYP-D3(BJ)/TZ2P level of theory. Our purpose is twofold: (i) reveal the bonding mechanism of the studied systems that lead to their self-assembly in linear chains; and (ii) rationalize the C-C bond equalization in the ring moieties of deltamide and squaramide upon polymerization. Our energy decomposition and Kohn-Sham molecular orbital analyses reveal cooperativity in all studied systems, stemming from the charge separation within the σ-electronic system by charge transfer from the carbonyl oxygen lone pair donor orbital of one monomer towards the σ* N-H antibonding acceptor orbital of the neighboring monomer. This key orbital interaction causes the C=O bonds to elongate, which, in turn, results in the contraction of the adjacent C-C single bonds that, ultimately, makes the ring moieties of deltamide and squaramide to become more regular. Notably, the π-electron delocalization plays a much smaller role in the total interaction between the monomers in the chain.

Dipolar repulsion in α-halocarbonyl compounds revisited.

The concept of dipolar repulsion has been widely used to explain several phenomena in organic chemistry, including the conformational preferences of carbonyl compounds. This model, in which atoms and bonds are viewed as point charges and dipole moment vectors, respectively, is however oversimplified. To provide a causal model rooted in quantitative molecular orbital theory, we have analyzed the rotational isomerism of haloacetaldehydes OHC-CH2X (X = F, Cl, Br, I), using relativistic density functional theory. We have found that the overall trend in the rotational energy profiles is set by the combined effects of Pauli repulsion (introducing a barrier around gauche that separates minima at syn and anti), orbital interactions (which can pull the anti minimum towards anticlinal to maximize hyperconjugation), and electrostatic interactions. Only for X = F, not for X = Cl-I, electrostatic interactions push the preference from syn to anti. Our bonding analyses show how this trend is related to the compact nature of F versus the more diffuse nature of the heavier halogens.

The pnictogen bond: a quantitative molecular orbital picture.

We have analyzed the structure and stability of archetypal pnictogen-bonded model complexes D3PnA- (Pn = N, P, As, Sb; D, A = F, Cl, Br) using state-of-the-art relativistic density functional calculations at the ZORA-M06/QZ4P level. We have accomplished two tasks: (i) to compute accurate trends in pnictogen-bond strength based on a set of consistent data; and (ii) to rationalize these trends in terms of detailed analyses of the bonding mechanism based on quantitative Kohn-Sham molecular orbital (KS-MO) theory in combination with a canonical energy decomposition analysis (EDA) and Voronoi deformation density (VDD) analyses of the charge distribution. We have found that pnictogen bonds have a significant covalent character stemming from strong HOMO-LUMO interactions between the lone pair of A- and σ* of D3Pn. As such, the underlying mechanism of the pnictogen bond is similar to that of hydrogen, halogen, and chalcogen bonds.

A Quantitative Molecular Orbital Perspective of the Chalcogen Bond.

Invited for this month's cover are the groups of Prof. Dr. Teodorico C. Ramalho (Federal University of Lavras and University Hradec Kralove) and Prof. Dr. F. Matthias Bickelhaupt (Vrije Universiteit Amsterdam and Radboud University). The cover picture shows the key message of their work, that is, the covalency of the chalcogen bonds, in an elegantly simple and attractive manner. To that end, the chalcogen bonds are represented by schematic 3D structures of the bond donor D2 Ch and the bond acceptor A- , and their attractive interaction in green. Then, a colorful molecular orbital (MO) diagram where the HOMO-LUMO mixing is represented by the mixing of red (HOMO) and blue (LUMO) into purple (MO) is presented. Read the full text of their Full Paper at 10.1002/open.202000323.

Chalcogen bonds: Hierarchical ab initio benchmark and density functional theory performance study.

We have performed a hierarchical ab initio benchmark and DFT performance study of D2 Ch•••A- chalcogen bonds (Ch = S, Se; D, A = F, Cl). The ab initio benchmark study is based on a series of ZORA-relativistic quantum chemical methods [HF, MP2, CCSD, CCSD(T)], and all-electron relativistically contracted variants of Karlsruhe basis sets (ZORA-def2-SVP, ZORA-def2-TZVPP, ZORA-def2-QZVPP) with and without diffuse functions. The highest-level ZORA-CCSD(T)/ma-ZORA-def2-QZVPP counterpoise-corrected complexation energies (ΔECPC ) are converged within 1.1-3.4 kcal mol-1 and 1.5-3.1 kcal mol-1 with respect to the method and basis set, respectively. Next, we used the ZORA-CCSD(T)/ma-ZORA-def2-QZVPP (ΔECPC ) as reference data for analyzing the performance of 13 different ZORA-relativistic DFT approaches in combination with the Slater-type QZ4P basis set. We find that the three-best performing functionals are M06-2X, B3LYP, and M06, with mean absolute errors (MAE) of 4.1, 4.2, and 4.3 kcal mol-1 , respectively. The MAE for BLYP-D3(BJ) and PBE amount to 8.5 and 9.3 kcal mol-1 , respectively.

The Gauche Effect in XCH2 CH2 X Revisited.

We have quantum chemically investigated the rotational isomerism of 1,2-dihaloethanes XCH2 CH2 X (X = F, Cl, Br, I) at ZORA-BP86-D3(BJ)/QZ4P. Our Kohn-Sham molecular orbital (KS-MO) analyses reveal that hyperconjugative orbital interactions favor the gauche conformation in all cases (X = F-I), not only for X = F as in the current model of this so-called gauche effect. We show that, instead, it is the interplay of hyperconjugation with Pauli repulsion between lone-pair-type orbitals on the halogen substituents that constitutes the causal mechanism for the gauche effect. Thus, only in the case of the relatively small fluorine atoms, steric Pauli repulsion is too weak to overrule the gauche preference of the hyperconjugative orbital interactions. For the larger halogens, X⋅⋅⋅X steric Pauli repulsion becomes sufficiently destabilizing to shift the energetic preference from gauche to anti, despite the opposite preference of hyperconjugation.

A Quantitative Molecular Orbital Perspective of the Chalcogen Bond.

We have quantum chemically analyzed the structure and stability of archetypal chalcogen-bonded model complexes D2 Ch⋅⋅⋅A- (Ch = O, S, Se, Te; D, A = F, Cl, Br) using relativistic density functional theory at ZORA-M06/QZ4P. Our purpose is twofold: (i) to compute accurate trends in chalcogen-bond strength based on a set of consistent data; and (ii) to rationalize these trends in terms of detailed analyses of the bonding mechanism based on quantitative Kohn-Sham molecular orbital (KS-MO) theory in combination with a canonical energy decomposition analysis (EDA). At odds with the commonly accepted view of chalcogen bonding as a predominantly electrostatic phenomenon, we find that chalcogen bonds, just as hydrogen and halogen bonds, have a significant covalent character stemming from strong HOMO-LUMO interactions. Besides providing significantly to the bond strength, these orbital interactions are also manifested by the structural distortions they induce as well as the associated charge transfer from A- to D2 Ch.

Nature and Strength of Lewis Acid/Base Interaction in Boron and Nitrogen Trihalides.

We have quantum chemically investigated the bonding between archetypical Lewis acids and bases. Our state-of-the-art computations on the X3 B-NY3 Lewis pairs have revealed the origin behind the systematic increase in B-N bond strength as X and Y are varied from F to Cl, Br, I, H. For H3 B-NY3 , the bonding trend is driven by the commonly accepted mechanism of donor-acceptor [HOMO(base)-LUMO(acid)] interaction. Interestingly, for X3 B-NH3 , the bonding mechanism is determined by the energy required to deform the BX3 to the pyramidal geometry it adopts in the adduct. Thus, Lewis acids that can more easily pyramidalize form stronger bonds with Lewis bases. The decrease in the strain energy of pyramidalization on going from BF3 to BI3 is directly caused by the weakening of the B-X bond strength, which stems primarily from the bonding in the plane of the molecule (σ-like) and not in the π system, at variance with the currently accepted mechanism.

Halogen Bonds in Ligand-Protein Systems: Molecular Orbital Theory for Drug Design.

Halogen bonds are highly important in medicinal chemistry as halogenation of drugs, generally, improves both selectivity and efficacy toward protein active sites. However, accurate modeling of halogen bond interactions remains a challenge, since a thorough theoretical investigation of the bonding mechanism, focusing on the realistic complexity of drug-receptor systems, is lacking. Our systematic quantum-chemical study on ligand/peptide-like systems reveals that halogen bonding is driven by the same bonding interactions as hydrogen bonding. Besides the electrostatic and the dispersion interactions, our bonding analyses, based on quantitative Kohn-Sham molecular orbital theory together with energy decomposition analysis, reveal that donor-acceptor interactions and steric repulsion between the occupied orbitals of the halogenated ligand and the protein need to be considered more carefully within the drug design process.