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The hydrogen-bond donor strength of ureas, widely used in hydrogen-bond donor catalysis, molecular recognition, and self-assembly, can be enhanced by increasing the size of the chalcogen X in the C＝X bond from O to S to Se and by introducing more electron-withdrawing substituents because both modifications increase the positive charge on the NH groups which become better hydrogen-bond donors. However, in 1,3-diaryl X-ureas, a steric mechanism disrupts the positive additivity of these two tuning factors, as revealed by our quantum-chemical analyses. This leads to an enhanced hydrogen-bond donor strength, despite a lower NH acidity, for 1,3-diaryl substituted O-ureas compared to the S- and Se-urea analogs. In addition, we provide a strategy to overcome this steric limitation using a predistorted urea-type hydrogen-bond donor featuring group 14 elements in the C＝X bond so that the hydrogen-bond donor strength of X-urea derivatives bearing two aryl substituents can be enhanced upon varying X down group 14. [ABSTRACT FROM AUTHOR]

Nickel carbenes are key reactive intermediates in the catalytic cyclopropanation of olefins and other reactions, but isolated examples are scarce and generally rely on low coordination numbers (≤3) to stabilize the metal−ligand multiple bond. Here we report the isolation and characterization of a stable tetracoordinated nickel carbene bearing a triphosphine pincer ligand. Its nucleophilic character is evidenced by reaction with acids, and it can transfer the carbene fragment to CO to form a ketene. A computational study of the Ni=C chemical bond sheds light on the role of the third phosphine in the pincer framework to the stabilization of the nickel carbene fragment. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Introducing a rigid linear π‐conjugated acetylene linker into a supramolecular building block consisting of a hydrogen‐bond donor side and a hydrogen‐bond acceptor side yields a decrease in cooperativity in the resulting formed quartet. This follows from our Kohn‐Sham molecular orbital and Voronoi deformation density analyses of hydrogen‐bonded macrocycles based on guanine and cytosine nucleobases. The acetylene linker abstracts electron density from the hydrogen‐bond acceptor and donor, making the hydrogen‐bond acceptor more negatively charged and the hydrogen‐bond donor more positively charged and hence suppressing the donor–acceptor charge transfer interaction between the interacting fragments. This, ultimately, hampers the cooperativity in the hydrogen‐bonded macrocycle. We envision that these findings could open the door to new design principles for the development of novel hydrogen‐bond supramolecular macrocycles. [ABSTRACT FROM AUTHOR]

Exchanging oxygen in the functional group C=O (i. e. carbonyl) for the less electronegative Group 16 elements, sulfur or selenium, unexpectedly enhances the electronegativity of the C=X group in π‐conjugated molecules and reduces the molecular π HOMO–LUMO energy gap. Quantum‐chemical analyses revealed that the steric size of the chalcogen atom X is at the origin of this seemingly counterintuitive behavior. This tuning of the chemical properties of carbonyl compounds by varying the chalcogen atom size in the C=X bond can be applied in many fields of chemistry. This concept article delineates several useful applications in the fields of organocatalysis, supramolecular chemistry, and photo(electro)chemistry. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Opposite to what one might expect, we find that the C=X group can become effectively more, not less, electronegative when the Pauling electronegativity of atom X decreases down Groups 16, 15, and 14 of the Periodic Table. Our quantum‐chemical analyses, show that, and why, this phenomenon is a direct consequence of the increasing size of atom X down a group. These findings can be applied to tuning and improving the hydrogen‐bond donor strength of amides H2NC(=X)R by increasingly withdrawing density from the NH2 group. A striking example is that H2NC(=SiR2)R is a stronger hydrogen‐bond donor than H2NC(=CR2)R. [ABSTRACT FROM AUTHOR]

The symmetry‐decomposed Voronoi deformation density (VDD) charge analysis is an insightful and robust computational tool to aid the understanding of chemical bonding throughout all fields of chemistry. This method quantifies the atomic charge flow associated with chemical‐bond formation and enables decomposition of this charge flow into contributions of (1) orbital interaction types, that is, Pauli repulsive or bonding orbital interactions; (2) per irreducible representation (irrep) of any point‐group symmetry of interacting closed‐shell molecular fragments; and now also (3) interacting open‐shell (i.e., radical) molecular fragments. The symmetry‐decomposed VDD charge analysis augments the symmetry‐decomposed energy decomposition analysis (EDA) so that the charge flow associated with Pauli repulsion and orbital interactions can be quantified both per atom and per irrep, for example, for σ, π, and δ electrons. This provides detailed insights into fundamental aspects of chemical bonding that are not accessible from EDA. [ABSTRACT FROM AUTHOR]

We have investigated the stereoselectivity and reactivity of the sodium borohydride reduction of 2‐X‐cyclohexanones (X=H, Cl, Br) using a combined approach of competitive experiments and density functional theory calculations. Our results show that the hydride addition proceeds via a late transition state in which the C–H bond is nearly formed, consistent with the mild reducing power of NaBH4. The reaction barrier decreases from the 2‐halocyclohexanones to the unsubstituted cyclohexanone, in line with relative reactivities observed in the competitive experiments. Furthermore, we provide a protocol to solve the longstanding issue of properly modelling the axial–equatorial facial selectivity of hydride addition to the carbonyl group substituted with a vicinal polar group. The inclusion of implicit solvation in combination with an explicit solvent molecule is crucial to reproduce the stereoselective formation of the cis product observed experimentally. [ABSTRACT FROM AUTHOR]

Our quantum chemical analyses elucidated how the replacement of O in the amide bonds of benzene‐1,3‐5‐tricarboxamides (OBTAs) with the larger chalcogens S and Se enhances the intermolecular interactions and thereby the stability of the obtained hydrogen‐bonded supramolecular polymers due to two unexpected reasons: i) the SBTA and SeBTA monomers have a better geometry for self‐assembly and ii) induce stronger covalent (hydrogen‐bond) interactions besides enhanced dispersion interactions. In addition, it is shown that the cooperativity in benzene‐1,3,5‐triamide (BTA) self‐assembly is caused by charge separation in the σ‐electronic system following the covalency in the hydrogen bonds. [ABSTRACT FROM AUTHOR]

Hydrogen‐bonded supramolecular systems are known to obtain extra stabilization from the complexation with ions, like guanine quadruplex (GQ). They experience strong hydrogen bonds due to cooperative effects. To gain deeper understanding of the interplay between ions and hydrogen‐bonding cooperativity, relativistic dispersion‐corrected density functional theory (DFT‐D) computations were performed on triple‐layer hydrogen‐bonded rosettes of ammeline interacting with alkali metal cations and halides. Our results show that when ions are placed between the stacks, the hydrogen bonds are weakened but, at the same time, the cooperativity is strengthened. This phenomenon can be traced back to the shrinkage of the cavity as the ions pull the monomers closer together and therefore the distance between the monomers becomes smaller. On one hand this results in a larger steric repulsion, but on the other hand, the donor‐acceptor interactions are enhanced due to the larger overlap between the donating and accepting orbitals leading to more charge donation and therefore an enhanced electrostatic attraction. [ABSTRACT FROM AUTHOR]

The synthesis of the selenium‐based ligand L1SeSeL1 (2,2'‐diselanediylbis(N,N‐bis(pyridin‐2‐ylmethyl)ethan‐1‐amine) is described along with its reactivity with cobalt(II) salts. The cobalt(II)‐diselenide complex [Co2(L1SeSeL1)Cl4] was obtained in good yield, and its spectroscopic properties closely resemble that of its sulfur analog. Reaction of L1SeSeL1 with Co(II) thiocyanate results in the formation of the cobalt(III) compound [Co(L1Se)(NCS)2], similar to reaction of L1SSL1. The redox‐conversion reactions from the Co(II)‐diselenide compound [Co(L1SeSeL1)Cl4] using external triggers such as removal of the halide ions or the addition of the strong‐field ligand 8‐quinolinolate resulted in good yields of the Co(III)‐selenolate complexes [Co(L1Se)(MeCN)2](SbF6)2 and [Co(L1Se)(quin)]Cl. Our computational studies show that the ligand‐field splitting energy of the selenium compounds is smaller than their sulfur analogs, indicating that redox‐conversion of cobalt(II)‐diselenide to cobalt(III)‐selenolate complexes may be more arduous than that for the related sulfur compounds. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

We discuss the fundamental aspects of the EDA-NOCV method and address some critical comments that have been made recently. The EDA-NOCV method unlike most other methods focuses on the process of bond formation between the interacting species and not just only on the analysis of the finally formed bond. This is demonstrated using LiF as an example. There is a difference between the interactions between the initial species which form the bond and are also the final product of bond cleavage, and the interactions between the fragments in the eventually formed molecule. The flexibility of the method allows the choice of the interacting fragments which helps to identify the charge and electron configuration of the fragments which describe the bond. This is very helpful in cases where the bond may be described with several Lewis structures. We reject the idea that it would be a disadvantage to have "bond path functions" as the energy components in the EDA, which actually indicate the variability of the method. The bonding analysis in a different sequence of the bond formation gives important results for the various questions that can be asked. This is demonstrated by using CH2, CO2 and the formation of a guanine quartet as examples. The fact that a bond is always defined by the bound molecule, the fragments, and their states is universal and deeply physical, as we show here again for various examples. The results of the EDA-NOCV method are in full accordance with the physical mechanism of the chemical bond as revealed by Ruedenberg. [ABSTRACT FROM AUTHOR]

The amino groups of thio‐ and selenoamides can act as stronger hydrogen‐bond donors than of carboxamides, despite the lower electronegativity of S and Se. This phenomenon has been experimentally explored, particularly in organocatalysis, but a sound electronic explanation is lacking. Our quantum chemical investigations show that the NH2 groups in thio‐ and selenoamides are more positively charged than in carboxamides. This originates from the larger electronic density flow from the nitrogen lone pair of the NH2 group towards the lower‐lying π*C=S and π*C=Se orbitals than to the high‐lying π*C=O orbital. The relative energies of the π* orbitals result from the overlap between the chalcogen np and carbon 2p atomic orbitals, which is set by the carbon‐chalcogen equilibrium distance, a consequence of the Pauli repulsion between the two bonded atoms. Thus, neither the electronegativity nor the often‐suggested polarizability but the steric size of the chalcogen atom determines the amide's hydrogen‐bond donor capability. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

We have quantum chemically analyzed the influence of nucleotide composition and sequence (that is, order) on the stability of double‐stranded B‐DNA triplets in aqueous solution. To this end, we have investigated the structure and bonding of all 32 possible DNA duplexes with Watson–Crick base pairing, using dispersion‐corrected DFT at the BLYP‐D3(BJ)/TZ2P level and COSMO for simulating aqueous solvation. We find enhanced stabilities for duplexes possessing a higher GC base pair content. Our activation strain analyses unexpectedly identify the loss of stacking interactions within individual strands as a destabilizing factor in the duplex formation, in addition to the better‐known effects of partial desolvation. Furthermore, we show that the sequence‐dependent differences in the interaction energy for duplexes of the same overall base pair composition result from the so‐called "diagonal interactions" or "cross terms". Whether cross terms are stabilizing or destabilizing depends on the nature of the electrostatic interaction between polar functional groups in the pertinent nucleobases. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

A fundamental and ubiquitous phenomenon in chemistry is the contraction of both C−H and C−C bonds as the carbon atoms involved vary, in s–p hybridization, along sp3 to sp2 to sp. Our quantum chemical bonding analyses based on Kohn–Sham molecular orbital theory show that the generally accepted rationale behind this trend is incorrect. Inspection of the molecular orbitals and their corresponding orbital overlaps reveals that the above‐mentioned shortening in C−H and C−C bonds is not determined by an increasing amount of s‐character at the carbon atom in these bonds. Instead, we establish that this structural trend is caused by a diminishing steric (Pauli) repulsion between substituents around the pertinent carbon atom, as the coordination number decreases along sp3 to sp2 to sp. [ABSTRACT FROM AUTHOR]

We have quantum chemically studied the structure and nature of alkali‐ and coinage‐metal bonds (M‐bonds) versus that of hydrogen bonds between A−M and B− in archetypal [A−M⋅⋅⋅B]− model systems (A, B=F, Cl and M=H, Li, Na, Cu, Ag, Au), using relativistic density functional theory at ZORA‐BP86‐D3/TZ2P. We find that coinage‐metal bonds are stronger than alkali‐metal bonds which are stronger than the corresponding hydrogen bonds. Our main purpose is to understand how and why the structure, stability and nature of such bonds are affected if the monovalent central atom H of hydrogen bonds is replaced by an isoelectronic alkali‐ or coinage‐metal atom. To this end, we have analyzed the bonds between A−M and B− using the activation strain model, quantitative Kohn‐Sham molecular orbital (MO) theory, energy decomposition analysis (EDA), and Voronoi deformation density (VDD) analysis of the charge distribution. [ABSTRACT FROM AUTHOR]

When carbonyl ligands coordinate to transition metals, their bond distance either increases (classical) or decreases (nonclassical) with respect to the bond length in the isolated CO molecule. C−O expansion can easily be understood by π‐back‐donation, which results in a population of the CO's π*‐antibonding orbital and hence a weakening of its bond. Nonclassical carbonyl ligands are less straightforward to explain, and their nature is still subject of an ongoing debate. In this work, we studied five isoelectronic octahedral complexes, namely Fe(CO)62+, Mn(CO)6+, Cr(CO)6, V(CO)6− and Ti(CO)62−, at the ZORA‐BLYP/TZ2P level of theory to explain this nonclassical behavior in the framework of Kohn–Sham molecular orbital theory. We show that there are two competing forces that affect the C−O bond length, namely electrostatic interactions (favoring C−O contraction) and π‐back‐donation (favoring C−O expansion). It is a balance between those two terms that determines whether the carbonyl is classical or nonclassical. By further decomposing the electrostatic interaction ΔVelstat into four fundamental terms, we are able to rationalize why ΔVelstat gives rise to the nonclassical behavior, leading to new insights into the driving forces behind C−O contraction. [ABSTRACT FROM AUTHOR]

We have quantum chemically investigated the bonding between archetypical Lewis acids and bases. Our state‐of‐the‐art computations on the X3B−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 H3B−NY3, the bonding trend is driven by the commonly accepted mechanism of donor−acceptor [HOMO(base)−LUMO(acid)] interaction. Interestingly, for X3B−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. [ABSTRACT FROM AUTHOR]

The hydroaminomethylation (HAM) reaction converts alkenes into N‐alkylated amines and has been well studied for rhodium‐ and ruthenium‐based catalytic systems. Cobalt‐based catalytic systems are able to perform the essential hydroformylation reaction, but are also known to form very active hydrogenation catalysts, therefore we examined such a system for its potential use in the HAM reaction. Thus, we have quantum‐chemically explored the hydrogenation activity of [HCo(CO)3] in model reactions with ethene, methyleneamine, formaldehyde, and vinylamine using dispersion‐corrected relativistic density functional theory at ZORA‐BLYP‐D3(BJ)/TZ2P. Our computations reveal essentially identical overall barriers for the catalytic hydrogenation of ethene, formaldehyde, and vinylamine. This strongly suggests that a cobalt‐based catalytic system will lack hydrogenation selectivity in experimental HAM reactions. Our HAM experiments with a cobalt‐based catalytic system (consisting of Co2(CO)8 as cobalt source and P(n‐Bu)3 as ligand) resulted in the formation of the desired N‐alkylated amine. However, significant amounts of hydrogenated starting material as well as alcohol (hydrogenated aldehyde) were always formed. The use of cobalt‐based catalysts in the HAM reaction to selectively form N‐alkylated amines seems therefore not feasible. This confirms our computational prediction and highlights the usefulness of state‐of‐the‐art DFT computations for guiding future experiments. [ABSTRACT FROM AUTHOR]