Intermolecular CarbonylÂ·Â·Â·Carbonyl Interactions in Transition-Metal

Article Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

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Intermolecular Carbonyl···Carbonyl Interactions in Transition-Metal Complexes Jorge Echeverría* Departament de Química Inorgànica i Orgànica and Institut de Química Teòrica i Computacional IQTC-UB, Universitat de Barcelona, Martí i Franquès 1-11, 08028 Barcelona, Spain S Supporting Information *

ABSTRACT: We performed a comprehensive analysis of intermolecular carbonyl−carbonyl interactions in transition-metal complexes. Those interactions can be classified in two main types depending on the organometallic or organic nature of the donor carbonyl: M−CO···CO and R−CO···CO, respectively. By means of a combined structural and computational study we unraveled their geometrical features and strength. Moreover, electronic structure, natural bond orbitals, energy decomposition analysis, and quantum theory of atoms in molecules calculations were performed to try to understand their nature. Remarkably, we discovered that these carbonyl−carbonyl contacts have several features of the n → π* interaction. The charge transfer from an oxygen lone pair to an empty antibonding π orbital of the acceptor carbonyl is also accompanied by an electrostatic Oδ−···Cδ+ interaction. To the best of our knowledge this is the first report of an intermolecular n → π* interaction in metal complexes. These results might be significant, for instance, for the catalytic activation of carbonylcontaining small molecules with metal compounds or in the design of hybrid organic− inorganic materials, metal−organic frameworks, and other extended structures.

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despite a small associate energy of ∼0.27 kcal/mol, dictate the conformation of many proteins.13 In a recent report, we showed the existence of short intramolecular carbonyl···carbonyl contacts in transition-metal complexes, which are stabilized by electron delocalization from a donor organic carbonyl into an acceptor carbonyl ligand, giving the first piece of evidence of the n → π* interaction involving metal compounds.14 In the light of those results we wondered if such interactions were limited to occur within a molecule or they can be established between two (or more) different metal complexes. That is interesting, because it would allow their use as a tool in crystal engineering and materials design, evidencing that carbonyl−carbonyl interactions are not limited to intramolecular contacts. We present here the report of intermolecular carbonyl···carbonyl interactions in transitionmetal complexes. A combined structural and theoretical analysis of these interactions was performed to unveil their abundance, strength, and physical nature. Electronic structure calculations and natural bond order (NBO), energy decomposition analysis (EDA), and quantum theory of atoms in molecules (QTAIM) analysis were done to estimate whether the origin of the interaction is based on orbital interactions or electrostatics.

INTRODUCTION Noncovalent interactions, despite being generally weak, are of fundamental importance in chemistry, biology, and materials science.1 The study of dihydrogen interactions,2 π- and σ-hole bonding,3 π/π stacking,4 London forces,5 metallophilicity,6 and many others has attracted increasing interest in the last decades, mainly due to the importance of such interactions in determining the solid-state packing and the properties of many molecules and supramolecular aggregates. The attractive nature of noncovalent interactions encompasses a wide range of forces, the most important being electrostatics, dispersion, and orbital interactions.5a,7 In many cases, all those attractive forces coexist, and it is difficult to discern the contribution of each of them to the overall interaction stability. For example, discussion on the charge transfer or electrostatic nature (or both) of halogen bonding is still ongoing.8 There are some other cases, however, in which the physical origin of the noncovalent interaction seems to be clearer, as in the so-called n → π* interaction.9 The n → π* interaction is associated with the charge transfer from a lone pair into an antibonding empty π orbital. It is mainly present in organic molecules and has been observed to contribute to biomolecular function.10 Perhaps the most representative case of the n → π* interaction is the intimate contact between two carbonyl groups (CO···CO).11 In such a contact, the donor carbonyl approaches the acceptor one along the Burgi−Dunitz trajectory for a nucleophilic attack (O···CO angle of 107°),12 resulting in an O···C distance shorter than the sum of the van der Waals radii (3.27 Å) that facilitates the n/π* orbital overlap. Remarkably, intramolecular carbonyl−carbonyl interactions, © XXXX American Chemical Society

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RESULTS AND DISCUSSION 1. Structural Analysis. A close inspection of experimental crystal structures containing short intermolecular carbonyl− carbonyl contacts in transition-metal complexes reveals the Received: February 12, 2018

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DOI: 10.1021/acs.inorgchem.8b00392 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

could present some steric hindrance toward the establishment of a short contact between its C atom and the O atom of another carbonyl group, which could explain why they are more commonly found as donors than as acceptors. However, this should not be a problem for μ2-CO ligands, with a planar M2CO framework that makes the C atom accessible. A more detailed analysis of the geometrical preferences of μ1-CO → μ1CO, μ2-CO → μ1-CO, and μ3-CO → μ1-CO interactions discloses some interesting features (Figure 3). Whereas all three

existence of two main types of interactions. Whereas in both types the acceptor carbonyl is a ligand directly attached to a metal center, the donor group can be either another carbonyl ligand or an organic carbonyl group (Scheme 1). We termed these two possibilities as M−CO···CO and R−CO···CO interactions, respectively. Next, we analyze in detail those two typologies. Scheme 1. Carbonyl−Carbonyl Interactions in TransitionMetal Complexes

1.1. M−CO···CO Interactions. The first interaction type involves a close contact between two carbonyl ligands (M− CO···CO). This is by far the most abundant case with more than 11 000 structures with O···C distance shorter than the sum of the van der Waals (vdW) radii (a representative example is depicted in Figure 1), which represents 30% of all carbonyl-

Figure 1. Short M−CO···CO (M = Fe) contact in the crystal structure of a metal complex (FOBFUX).16 Colors code: black = carbon, red = oxygen, white = hydrogen, yellow = sulfur, orange = iron.

containing metal complexes existing in the Cambridge Structural Database (CSD).15 Most of these contacts occur between two terminal carbonyl ligands (10 737 structures). Interestingly, contacts in which μ2 and μ3 bridging carbonyls act as donors are much more abundant than those in which they do as acceptors (Figure 2). We also observed that terminal carbonyls seem to be better acceptors than the bridging ones. In the case of μ3-CO ligands, the presence of the three metals

Figure 3. 2D histogram of the distribution of short M−CO···CO contacts as a function of the O···CO angle in transition-metal complexes when the donor carbonyl ligand is coordinated to (a) one, (b) two, and (c) three metal atoms. The acceptor is in all cases a terminal carbonyl ligand. The gray line represents the sum of the vdW radii of C and O (∑rvdW = 3.27 Å). Interval widths: x = 0.03 Å, y = 3°.

cases present a marked preference for O···CO angles at ∼80−85°, the O···C interaction distances are shorter for μ3-CO → μ1-CO interactions, with a clear peak of abundance at 3.1 Å (0.17 Å shorter than the sum of the vdW radii). Note that, for μ1-CO → μ1-CO interactions, the maximum is located at 3.3− 3.4 Å. Remarkably, the interaction distance becomes shorter as more metals are coordinated to the donor carbonyl ligand, following the trend μ3 < μ2 < μ1. The shape of the angledistance maps of Figure 3 is very similar to previously reported maps including proteins and other organic molecules, although in those cases the angle approached 100° as the distance becomes shorter.11a In the present case, however, the angle at short distances tends to 80−90° because the approach region at

Figure 2. Combinations of ligands with different coordination, terminal (μ1) and bridging two (μ2) and three (μ3) metal atoms, acting as donors and acceptors in carbonyl−carbonyl interactions and the number of structures found in the CSD for each case. B


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Inorganic Chemistry ∼100° is not usually available in metal complexes due to the presence of ligands (or the metal electron density). Of course, one could argue that Figure 3 contains data from all transition metals, which makes difficult the comparison between differently coordinated carbonyl ligands. To confirm possible trends, we narrow our focus on rhodium complexes, particularly, in those cases in which μ3-CO···CO and μ1-CO··· CO contacts coexist in the same crystal structure. By doing so, we can compare different ligands belonging to the same complexes, trying to isolate the nature of the ligand, the μ number, as the only variable (or at least the most relevant one). A search in the CSD for structures with those characteristics returns 22 hits, all of them Rh clusters containing both types of carbonyls (μ3- and μ1-CO). In Figure 4, we represent, in the

Figure 5. Examples of the two main R−CO···CO types of short carbonyl···carbonyl intermolecular contacts found in transition-metal complexes: involving (a) an independent molecule (BAJNAD)27 or (b) a complex ligand (CFMCFE).28 Colors code: black = carbon, red = oxygen, green = fluorine, white = hydrogen, light blue = nitrogen, orange = iron, purple = platinum.

1 tBu−COOH). We also found a case in which the donor carbonyl is in an anion (F 3 C−COO − ) (CAKQIR). 26 Remarkably, and despite the structural diversity of the donor groups, in all cases there is a marked directionality for the interaction with a clear preference for O···CO angles close to 80−90° (see Figure 6). This value is very similar to those found

Figure 4. Superimposed histograms for the abundance of short μ3-C O···CO (red bars) and μ1-CO···CO (green bars) contacts in carbonyl rhodium complexes. Interval width = 0.0667 Å.

shape of two histograms, the abundance of O···C distances associated with μ3-CO···CO (in red) and μ1-CO···CO (in green) contacts. As it can be seen, contacts involving μ3-CO ligands are shorter than those involving μ1-CO···CO ones, in good agreement with our previous results. Even though there is not necessarily the same number of μ3-CO···CO and μ1-CO··· CO contacts in a given crystal structure, this should affect the bar height but not the shape of the histogram and the position of the peak that represents the maximum of abundance. Therefore, these results further suggest that the donor ability increases with the number of metals to which the CO group is coordinated. 1.2. R−CO···CO Interactions. We found 386 crystal structures in the CSD15 that contain at least one R−CO··· CO contact shorter than the sum of the van der Waals radii of O and C (3.27 Å).17 Here, the interacting R−CO moiety, that is, the donor, can be on an independent organic molecule or be part of a ligand (not a simple carbonyl ligand). An experimental example of each type is shown in Figure 5a,b, respectively. The latter is in fact the most common case with 363 structures, including, on the one hand, more or less complex ligands in which the interacting donor group can be an ester (137 hits), a ketone (87 hits), or an amide (34 hits), although other examples are found for ligands with less abundant carbonyl-containing motives such as aldehydes (BAMXAO, EGOJAM),18 carboxylic acids (IFUCIV, KACBEX),19 thioesters (FIHYUQ, FISMOK),20 isocyanates (BITKOE),21 azodicarboxylates (CARLUC),22 ureates (MURFEC),23 maleic anhydrides24 (POVZAB), and nucleic acids derivatives (ELIWEC).25 On the other hand, there are only 22 structures in which the donor carbonyl is found in an independent organic molecule (19 (CH3)2CO, 2 (C6H5)2CO,

Figure 6. Intermolecular O···C distance as a function of the O···CO angle, represented as a 2D histogram, for R−CO···CO short contacts in transition-metal complexes. Interval widths: x = 0.03 Å, y = 3°.

for intramolecular carbonyl−carbonyl contacts in transitionmetal complexes.14 On the other hand, we did not find any geometrical preference for CO···C angles or CO···CO dihedrals (Figures S1 and S2 in the Supporting Information). Note that this geometrical scenario is consistent with the existence of a n → π* interaction. 2. Computational Study. Next, we perform a comprehensive computational investigation to understand the nature and strength of intermolecular organic carbonyl−carbonyl ligand (R−CO···CO) short contacts in transition-metal complexes. We focus on this interaction topology, rather than on M−CO···CO contacts, because it represents a lower computational effort, and, moreover, the derived results can C


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Inorganic Chemistry

has been recently unravelled,9c,10d,11,29 to the best of our knowledge, this is the first report of an intermolecular n → π* interaction involving an organic carbonyl (ketone) and a carbonyl ligand acting as the donor and acceptor, respectively. In fact, there are two lone pair orbitals in a carbonyl that can act as donors, one with more s and the other with more p character. The overlap between those two orbitals and the empty π* acceptor is shown in Figure 7a,b for the case of M =

be in principle extrapolated to other interaction types. We built a model system consisting of a dimethyl ketone as the donor and a [trans-MBr2(CO)2] (M = Ni, Pd, Pt) complex as the acceptor. This model is based on the experimental structure shown in Figure 5a (BAJNAD) but with less bulky ligands to allow free mobility of the donor with respect to the acceptor. We fully optimized the donor−acceptor adducts for the three different metals (Ni, Pd, and Pt), confirming by vibrational analysis that they are minima of the corresponding potential energy surfaces. The main geometrical parameters are summarized in Table 1. The first observation is that the Table 1. Geometrical Parameters of Optimized Me2CO··· [trans-MBr2(CO)2] (M = Ni, Pd, Pt) Systemsa M

dO···C (Å)

dO···M (Å)

ΔdCO (Å)

ΔdM−C (Å)

Ni Pd Pt

2.847 2.836 2.849

3.391 3.569 3.646

−0.002 −0.002 −0.003

0.020 0.026 0.028

Δ∠M‑

∠O···C

CO

O

∠CO ···C

(deg)

(deg)

(deg)

−2.4 −3.5 −3.6

92.6 89.6 87.2

154.7 140.7 153.7

d is the distance between the two specified atoms; ∠ is the measured angle between the three specified atoms; Δ for a given parameter is the difference between the value of such parameter in the interacting and the non-interacting carbonyls. a

donor−acceptor O···C distances are below the sum of the van der Waals radii ( Ni. That could be because the CO bond length is shorter for Pd, being the CO ligand more similar to a free CO molecule (more M−CO than MCO character). It is also worth mentioning that the electron density distribution and the resulting electrostatic potentials in CO-containing molecules and adducts are still nowadays intriguing research subjects.31 This scenario is consistent with the existence of an E


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Inorganic Chemistry Table 3. Energy Decomposition Analysis of [MBr2(CO)2] (M = Ni, Pd and Pt) Complexesa metal

ΔEPAULI

ΔEELEC

ΔEFRZ

ΔEPOL

ΔECT

ΔEINT

Ni Pd Pt

4.168 4.241 4.106

−4.865 −5.194 −5.109

−0.697 (25.9) −0.954 (32.2) −1.003 (34.8)

−0.896 (33.3) −0.909 (30.7) −0.856 (29.7)

−1.097 (40.8) −1.095 (37.0) −1.021 (35.4)

−2.690 −2.958 −2.880

All energies are given in kilocalories per mole. The values in parentheses are the contribution in percentage (%) of each component (ΔEFRZ, ΔEPOL, and ΔECT) to the total interaction energy (ΔEINT).

a

trends: ΔEFRZ increases when descending the group, whereas ΔEPOL and ΔECT decrease. Moreover, while charge transfer is the most important component for Ni, it becomes less dominant for Pt, for which all contributions are between 29.7 and 35.4%. Next, we applied Bader’s QTAIM36 to the three systems under study. This methodology has proven to be very useful to analyze noncovalent interactions in the past.2b,f,37 Remarkably, the three metal complexes show bond paths (BPs) between the interacting Odonor and Cacceptor atoms. The main characteristics of the bond critical points (BCPs) associated with those BPs are summarized in Table 4. The values of the electron density

nated carbonyl ligands can be explained, at least in part, in terms of orbital mixing and electrostatics. Future research should be directed to understand the influence of the nature of the metal center and the charge of the complex, two factors that also affect the stretching frequencies of carbonyls, on the establishment of carbonyl···carbonyl interactions.

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CONCLUSIONS In summary, we have presented here a comprehensive combined structural and computational study of short intermolecular carbonyl−carbonyl (CO···CO) contacts in transition-metal complexes. To the best of our knowledge, this is the first time that these interactions are reported and described in detail in the literature. From a structural point of view, we have characterized two main types of interactions: M− CO···CO and R−CO···CO. In the first case the donor is a carbonyl ligand directly attached to a metal and in the second an organic carbonyl group (ketone, amide, ester, etc.). In both cases, the acceptor is a carbonyl ligand. We have also observed that in M−CO···CO interactions, the ability as donor of the carbonyl ligand improves as more metal atoms are attached to it; that is, μ3-CO is a better donor than μ1-CO. For the two interaction typologies, there is a clear directionality, with O··· CO angles approaching 80−90° for O···C distances shorter than the sum of the van der Waals radii (3.27 Å). We have also performed a computational analysis to understand the physical nature and the origin of the interaction. Remarkably, the CO···CO interaction in transition metals has many of the geometrical characteristics of the n → π* interaction. Furthermore, an NBO analysis has shown that the interaction is stabilized by the overlap of a lone pair of the donor and an empty π* orbital of the acceptor carbonyl. For the R−CO···CO case, electrostatics also plays a role in the interaction, with a nice linear correlation between the interaction energy and the value of the electrostatic potential at the acceptor carbon atom. An energy decomposition analysis has shown that both orbital mixing (n → π* interaction) and electrostatics (π-hole interaction) are the two main components of the CO···CO interaction in metal complexes. Finally, its attractive nature has been further confirmed by a QTAIM analysis of the electron density. The results presented here open the door to the use of the CO···CO interaction as a tool for the design of solids composed of transition-metal complexes and organic−inorganic hybrid materials and can contribute to understanding the involvement of metal complexes in molecular magnetism, molecular electronics, or catalysis.

Table 4. Calculated (B3LYP/Def2-TZVP) Values for Several QTAIM Properties at the BCPsa

a

metal

ρ

∇2 ρ

DI(Caccep,Odonor)

H

ε

Ni Pd Pt

0.0094 0.0097 0.0094

0.0374 0.0382 0.0377

0.0422 0.0421 0.0381

0.0020 0.0020 0.0020

0.7088 0.7710 0.9354

All quantities in atomic units.

(ρ) at the BCPs are small and positive (0.0094−0.0097 e/ bohr3) but slightly larger than for previously calculated homopolar38 and heteropolar37a,39 dihydrogen interactions and some π-hole bonds.37b The values for the Laplacian of the electron density (∇2ρ) and for the total electron energy density (H), very close to zero, indicates that the interaction is weak and of noncovalent nature. The delocalization index (DI) between the Cacceptor and Odonor atoms is small but higher than for other closed-shell interactions,37a which indicates some degree of electron delocalization of one molecule into the neighboring. The electron density plots and BPs and BCPs for the three complexes can be found in the Supporting Information (Figure S3). We saw in our structural analysis of M−CO···CO contacts that bridging carbonyl ligands seem to be better donors and worse acceptors than terminal ones. In the light of our computational results, one can try now to understand such experimental behavior. It is known that the stretching frequencies of carbonyl ligands decrease as more metal atoms are attached to the carbonyl, enhancing the π-back-donation, which involves an increase in the population of the π* orbital and a longer C−O bond length.40 Accordingly, the carbonyl π* orbital will increase its energy becoming a worse acceptor in an n → π* interaction, since the latter becomes weaker as the energy difference between donor and acceptor orbital gets larger. Moreover, a carbonyl bonded to three metals will have more double-bond character, the oxygen atom having the two lone pairs more available to engage in a donor−acceptor interaction. Also for the establishment of an electrostatic interaction, the oxygen atom is more negatively charged when the carbonyl group is closer to a CO than to a free CO. Therefore, the interaction preferences of differently coordi-

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COMPUTATIONAL METHODS

All geometry optimizations were performed with Gaussian0941 at the B3LYP level of theory. The triple-ξ quality Def2-TZVPD basis set was employed for all atoms, including the associated pseudopotential for Pd and Pt. The combination of B3LYP and a triple-ξ basis set has been widely used as the reference method for the study of n → π* interactions.11a,13,29a,c All optimized adducts were characterized as true F


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Inorganic Chemistry

in biphenyl revisited. Struct. Chem. 2007, 18, 849−857. (g) Matta, C. F.; Hernández-Trujillo, J.; Tang, T.-H.; Bader, R. F. W. Hydrogen− Hydrogen Bonding: A Stabilizing Interaction in Molecules and Crystals. Chem. - Eur. J. 2003, 9, 1940−1951. (3) (a) Grabowski, S. Triel Bonds, π-Hole-π-Electrons Interactions in Complexes of Boron and Aluminium Trihalides and Trihydrides with Acetylene and Ethylene. Molecules 2015, 20, 11297. (b) Grabowski, S. J. Boron and other Triel Lewis Acid Centers: From Hypovalency to Hypervalency. ChemPhysChem 2014, 15, 2985−2993. (c) Grabowski, S. J. π-Hole Bonds: Boron and Aluminum Lewis Acid Centers. ChemPhysChem 2015, 16, 1470−1479. (d) Bauza, A.; Mooibroek, T. J.; Frontera, A. Directionality of π-holes in nitro compounds. Chem. Commun. 2015, 51, 1491−1493. (e) Bauzá, A.; Mooibroek, T. J.; Frontera, A. Tetrel-Bonding Interaction: Rediscovered Supramolecular Force? Angew. Chem., Int. Ed. 2013, 52, 12317−12321. (f) Bauzá, A.; Mooibroek, T. J.; Frontera, A. The Bright Future of Unconventional σ/π-Hole Interactions. ChemPhysChem 2015, 16, 2496−2517. (g) García-Llinás, X.; Bauzá, A.; Seth, S. K.; Frontera, A. Importance of R−CF3···O Tetrel Bonding Interactions in Biological Systems. J. Phys. Chem. A 2017, 121, 5371−5376. (h) Esrafili, M. D.; Mohammadian-Sabet, F.; Vessally, E. An ab initio study on the nature of sigma-hole interactions in pnicogen-bonded complexes with carbene as an electron donor. Mol. Phys. 2016, 114, 2115−2122. (i) Esrafili, M. D.; Nurazar, R. Chalcogen bonds formed through πholes: SO3 complexes with nitrogen and phosphorus bases. Mol. Phys. 2016, 114, 276−282. (j) Li, W.; Zeng, Y.; Zhang, X.; Zheng, S.; Meng, L. The enhancing effects of group V σ-hole interactions on the F···O halogen bond. Phys. Chem. Chem. Phys. 2014, 16, 19282−19289. (k) Liu, M. X.; Li, Q. Z.; Cheng, J. B.; Li, W. Z.; Li, H. B. Tetrel bond of pseudohalide anions with XH3F (X = C, Si, Ge, and Sn) and its role in S(N)2 reaction. J. Chem. Phys. 2016, 145, 224310. (l) Murray, J. S.; Lane, P.; Clark, T.; Riley, K. E.; Politzer, P. σ-Holes, π-holes and electrostatically-driven interactions. J. Mol. Model. 2012, 18, 541−548. (m) Murray, J. S.; Lane, P.; Politzer, P. Expansion of the σ-hole concept. J. Mol. Model. 2009, 15, 723−729. (4) (a) Grimme, S. Do special noncovalent π-π stacking interactions really exist? Angew. Chem., Int. Ed. 2008, 47, 3430−3434. (b) Janiak, C. A critical account on pi-pi stacking in metal complexes with aromatic nitrogen-containing ligands. J. Chem. Soc., Dalton Trans. 2000, 3885− 3896. (c) Miller, L. L.; Mann, K. R. pi-dimers and pi-stacks in solution and in conducting polymers. Acc. Chem. Res. 1996, 29, 417−423. (5) (a) Wagner, J. P.; Schreiner, P. R. London Dispersion in Molecular ChemistryReconsidering Steric Effects. Angew. Chem., Int. Ed. 2015, 54, 12274−12296. (b) Wang, C.; Mo, Y.; Wagner, J. P.; Schreiner, P. R.; Jemmis, E. D.; Danovich, D.; Shaik, S. The SelfAssociation of Graphane Is Driven by London Dispersion and Enhanced Orbital Interactions. J. Chem. Theory Comput. 2015, 11, 1621−1630. (6) (a) Echeverría, J.; Cirera, J.; Alvarez, S. Mercurophilic interactions: a theoretical study on the importance of ligands. Phys. Chem. Chem. Phys. 2017, 19, 11645−11654. (b) Schmidbaur, H.; Schier, A. Argentophilic Interactions. Angew. Chem., Int. Ed. 2015, 54, 746−784. (c) Schmidbaur, H.; Schier, A. Aurophilic interactions as a subject of current research: an up-date. Chem. Soc. Rev. 2012, 41, 370− 412. (7) (a) Wang, H.; Wang, W.; Jin, W. J. σ-Hole Bond vs π-Hole Bond: A Comparison Based on Halogen Bond. Chem. Rev. 2016, 116, 5072− 5104. (b) Weinhold, F.; Landis, C. R., Intermolecular Interactions. In Discovering Chemistry with Natural Bond Orbitals; John Wiley & Sons, Inc.: Hoboken, NJ, 2012; pp 209−230. (8) (a) Duarte, D. J. R.; Sosa, G. L.; Peruchena, N. M.; Alkorta, I. Halogen bonding. The role of the polarizability of the electron-pair donor. Phys. Chem. Chem. Phys. 2016, 18, 7300−7309. (b) Thirman, J.; Engelage, E.; Huber, S. M.; Head-Gordon, M. Characterizing the interplay of Pauli repulsion, electrostatics, dispersion and charge transfer in halogen bonding with energy decomposition analysis. Phys. Chem. Chem. Phys. 2018, 20, 905−915. (9) (a) Alkorta, I.; Blanco, F.; Elguero, J.; Dobado, J. A.; Melchor Ferrer, S.; Vidal, I. Carbon···Carbon Weak Interactions. J. Phys. Chem.

minima of the corresponding potential energy surfaces by computing their vibrational frequencies. Natural bond orbitals (NBO) analysis was performed with the NBO3.1 program42 included in Gaussian09 at the B3LYP/Def2-TZVPD level. Molecular electrostatic potential maps were built with GaussView43 on the electron density 0.001 au isosurfaces. Energy decomposition analysis was done by means of the ALMO-EDA method implemented in the Q-Chem 5.0 software.34 QTAIM studies were done on the B3LYP/Def2-TZVPD wave function by means of the AIMAll program.44 Structural searches were performed in the CSD15 (version 5.38 (Nov 2016) + two updates). Only structures with their three-dimensional coordinates well-defined, not disordered, not polymeric, and with R < 10% were allowed in searches. CSD refcodes of selected examples are given throughout the text as six-letter codes (e.g., ABCDEF). We used the van der Waals radii proposed by Alvarez.17

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b00392. Plot showing dihedral angle as a function of intermolecular O···C distance in crystal structures, illustrated O···C bond paths and associated BCPs and 2D electron density contours, Cartesian coordinates of optimized structures (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Jorge Echeverría: 0000-0002-8571-0372 Notes

The author declares no competing financial interest.

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ACKNOWLEDGMENTS The author thanks the Spanish Ministerio de Economiá y Competitividad (MINECO) for funding (IJC-2014-20097 and CTQ2015-64579-C3-1-P) and CSUC and IQTC for computational facililties.

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REFERENCES

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Intermolecular CarbonylÂ·Â·Â·Carbonyl Interactions in Transition-Metal

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