London Dispersion Decisively Contributes to the Thermodynamic

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London Dispersion Decisively Contributes to the Thermodynamic Stability of Bulky NHC-Coordinated Main Group Compounds J. Philipp Wagner, and Peter R. Schreiner J. Chem. Theory Comput., Just Accepted Manuscript • DOI: 10.1021/acs.jctc.5b01100 • Publication Date (Web): 24 Nov 2015 Downloaded from http://pubs.acs.org on November 30, 2015

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Journal of Chemical Theory and Computation

London Dispersion Decisively Contributes to the Thermodynamic Stability of Bulky NHCCoordinated Main Group Compounds J. Philipp Wagner and Peter R. Schreiner* Justus-Liebig University, Heinrich-Buff-Ring 17, 35392 Giessen, Germany. E-mail: [email protected]

ABSTRACT: We evaluated the dispersion stabilization of a series of seemingly reactive main group compounds coordinated to bulky N-heterocyclic carbene ligands.

We computed the

thermochemistry of hypothetical isodesmic exchange reactions of these ligands with their unsubstituted parent systems employing the B3LYP/6-311G(d,p) level of theory with and without dispersion corrections.

The energy difference of these two approaches gave dispersion

corrections of 30 kcal mol–1 and more.

We therefore conclude that London dispersion

contributes critically to the thermodynamic stabilities of these compounds. As such, these “coreshell” structures undergo reactions of the “reactive core” as long as the dispersion stabilization is conserved.

ACS Paragon Plus Environment

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INTRODUCTION Multiple bonding between elements beyond the first full period is increasingly unfavorable1 and requires stabilization through appropriate ligand systems, which typically are considered innocent bystanders that shield the sensitive “core” against the attack of reactive species. Yet, synthetic chemists have been able to prepare compounds exhibiting multiple bonding2,3 between heavier main group elements and compounds deemed too reactive to be isolable.4 A general strategy to meet the synthetic challenges to overcome the intrinsic lability of a particular bonding arrangement is the use of bulky ligands such as (formally anionic) terphenyls3 or the more recent family of neutral N-heterocyclic carbenes (NHCs).5,6 One of the most remarkable examples of the use of such ligands perhaps is the NHC-stabilized disilicon structure 1 from Robinson’s lab (Figure 1).7 It features a Si=Si double bond and simultaneously a lone electron pair on each of the silicon atoms – a very uncomfortable bonding situation for a heavier carbon congener. Here, silicon is formally in the oxidation state of zero; therefore, it has been termed a soluble main group element allotrope.8

Considering the number of atoms, similar ratios of organic shell

ligands around inorganic cores are often encountered and the ligands are usually described as being necessary “to provide kinetic stabilization to the highly reactive molecule”.3 While this is certainly true in part, one wonders whether there is also an intrinsic thermodynamic stabilization through the ligands. In a pictorial sense, one may consider the large organic shell around a reactive entity as a tight first solvation shell or as the skin of a sporting ball where the filling itself is less important for the stability of the entire structure. Inspired by recent findings we surmised that a key thermodynamic stabilizing factor may derive from attractive intramolecular London dispersion forces between the bulky ligands.9 If that is so, the design of such types of core-shell molecules would take a different direction as one would aim at maximizing the outer-shell stabilization and many new hitherto “impossible” molecules may be realized.


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Figure 1. X-ray structure of NHC-stabilized disilicon compound 1 that may derive a significant part of its stability from London dispersion interactions as do molecules 2 and 3. The thermodynamic stabilization proposed here is found in diamondoid dimers like 2 that exhibit extremely long C–C single bonds.10,11 Despite such long bonds (of up to 1.71 Å, the longest C– C bond in an alkane known to date), 2 shows an unexpectedly high thermal stability that is at variance with well-established bond length-bond strength relationships.12 This behavior was attributed to multiple attractive HH contacts of the juxtaposed internal surfaces that overall result in a large thermodynamic stabilization and a dispersion interaction larger than 30 kcal mol– 1 10,11

.

Another example is the all-meta tert-butyl substituted hexaphenylethane derivative 3 with

a 1.67 Å long central bond. While the parent compound without the tert-butyl substituents is unknown, sterically much more encumbered 3 could even be crystallized!13

The question

therefore is why the obviously more crowded structure can be isolated while the sterically less demanding parent compound has not been experimentally documented.

As we reported

recently, the answer lies in the role the tert-butyl groups play in providing the necessary thermodynamic dispersion stabilization that prevents the compound from dissociating.14 Even more striking, dispersion forces can also be used to overcome the strong Coulombic repulsions exerted by two interacting charges of the same sign.

For instance,

tetrakis(isonitrile)rhodium dimers form in solution because of attractive dispersion interactions that apparently are able to outweigh the electrostatic repulsion.15 It has also been shown that dispersion forces are important for the binding of bulky (phosphine) ligands to transition metal


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centers.16,17 Owing to their ability to stabilize molecules thermodynamically, Grimme termed bulky nonpolar groups dispersion energy donors (DEDs),18 but their utility in the design of chemical structures or even in catalysis19 has not yet been appreciated much. Here we outline that important stabilizing contributions of London dispersion effects to thermodynamic molecular stability has been even less appreciated for main group chemistry, but is likely to be a key factor for understanding the intrinsic stabilities of otherwise highly reactive molecular structures.

For instance, considering the dissociation of group 14 double bond

species E2H4 (E being a group 14 element) into the divalent EH2 (i.e., a carbene or heavier homologue) the energy required to break the double bonds lowers considerably as one goes down in group IV. For lead, the dissociation energy decreases to a value that is on the order of (what is thought to be) a typical van der Waals interaction (5.7 vs. 4.6 kcal mol–1 for the dissociation of two perfectly aligned n-hexane molecules).20-22 Here, dispersion forces must be important not only for a quantitative description of the system.23 Putting more bulk on the plumbylene as shown in a recently synthesized cyclic disilylated structure, stabilizes the dimer through London dispersion forces (although the Pb–Pb bond is closer to a single bond in this case).24 There is growing evidence that dispersion is also important for lighter elements.

The ring

opening of a cyclotrisilene to a disilenyl silylene by an NHC would be endergonic without inclusion

of

dispersion

experimentally.25

into

the

computations;

the

reaction

occurs

spontaneously

Further experimental proofs are provided by dichalcogenolate carbene

analogues (Si to Sn): the central angles become smaller the bulkier the ligands are, indicative of their attractive interactions.26 Encouraged by these documented examples we present here a detailed analysis of the role of dispersion in the stabilization of a selected prototypical series of low-coordination main group compounds.

We employ density functional theory (DFT) computations in combination with


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isodesmic reaction schemes that provide differential interaction energies while largely reducing systematic computational errors. We decided to focus on NHC-stabilized systems as they are ubiquitously used. Their singlet carbene electronic structures can appropriately be treated with single-determinant DFT. Our general conclusions bear implications for other types of bulky ligands as well.

COMPUTATIONAL METHODS To understand the role of London dispersion we compared the results of the widely popular hybrid density functional B3LYP27,28 that is known to lack a proper description of dispersion29,30 with its D3-corrected version (with the original zero-damping function).31 We used the functionals in conjunction with the 6-311G(d,p) Pople basis set of triple-ζ quality that should be sufficient to converge their performance,32 especially in combination with isodesmic (error-balancing) equations.33 To cross-check our results, we additionally employed several other levels of theory, basis sets, and also the D3-correction in combination with the Becke-Johnson damping function34 to reaction (1) (Scheme 1); the results are given in the Supporting Information. Depending on the “repulsiveness” of the functional, the reported values for the stabilization due to London dispersion are likely to represent a lower bound.

The exact magnitude of the

dispersion energy is likely to be even higher as the B3LYP functional does include a small amount of dispersion. It is difficult to compute the absolute bond energy of the NHC-ligands to the inorganic species in the core directly as it will be potentially affected by large errors. The ligand-free species are highly reactive molecules and are often not even electronic ground state singlet structures (as in the case of Robinson’s disilicon compound 1).7 Thus, these core structures are unlikely to be free intermediates in the formation of the NHC-stabilized molecules. Equally, the dissociation of all ligands is not a particularly relevant reaction in understanding the intrinsic stability of these


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unusual compounds. Therefore, we aimed at unveiling the approximate magnitude of London dispersion interactions provided by the DEDs employing isodesmic reaction schemes. Here, we ask the question what happens if one exchanges the typically employed bulky NHC-ligands with hypothetical minimal size ligands. The comparison of the DFT and DFT-D3 energies provides a lower bound estimate of the stabilization due to dispersion by means of computing ∆∆Edisp = ∆H0(B3LYP-D3) – ∆H0(B3LYP). The ∆∆EDisp correction is not exactly equal to the dispersion stabilization as the correction depends on the repulsiveness of the functional employed, but it provides an excellent estimate of the magnitude of this interaction. All structures were optimized at the B3LYP/6-311G(d,p) and the corresponding D3-corrected levels of theory (B3LYP-D3) by taking the X-ray single crystal structures as starting points. Frequency computations were performed to assure that the optimized structures correspond to true minima on the potential energy hypersurface. The energies (∆H0) given are corrected for zero point vibrational energies (ZPVEs).

For all computations we used the Gaussian09

electronic structure code,35 unless noted otherwise.

RESULTS AND DISCUSSION To visualize the presence of attractive contacts in a selected series of molecules we began with a noncovalent interaction (NCI) analysis36,37 of 1. This method is based on the relationship of the electron density to its reduced density gradient that vanishes in regions of noncovalent interactions at low densities. The latter can be visualized in 3D space with attractive (green areas for dispersion, blue for stronger interactions) and repulsive (red areas) contacts being distinguishable by the sign of the second eigenvalue of the Hessian of the density. The NCI plot of 1 displayed in Figure 2 shows large green areas within the individual ligands but also between the ligands and the Si=Si unit. This is in line with our general hypothesis that the bulk of the carbene ligands stabilizes the molecule. To quantify this attraction, we computed the interaction energy of two carbenes frozen in the geometry of B3LYP-D3 optimized 1 with reference to the ACS Paragon Plus Environment

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frozen monomeric ligands (i.e., the super-molecular approach). While the electronic interaction energy is repulsive employing uncorrected DFT (+6.2 kcal mol–1), DFT-D3 identifies a strongly bound complex (–13.7 kcal mol–1).

Thus, the overall dispersion correction to this complex

amounts to a total of 19.9 kcal mol–1. To further strengthen our estimation of the dispersion stabilization we additionally performed a SAPT038,39 computation (i. e., the perturbative approach) with the PSI4 program package.40 We utilized the truncated jun-cc-pVDZ basis set41 showing the best performance in a benchmark study.42 We obtained a comparable interaction energy of –10.3 kcal mol–1 and found that the molecular association is mainly dispersion driven, amounting to a total of 21.3 kcal mol–1. The perturbative treatment supports our estimation of the dispersion stabilization based on the magnitude of the D3 correction and is in line with the general statement that our values rather represent a lower bound and the exact dispersion stabilizations are higher.

Figure 2. NCI plot if 1 optimized at B3LYP-D3/6-311G(d,p). The large green areas suggest considerable attraction between the carbene ligands and between the ligands and the core. To further quantify the dispersion interactions in 1 and other systems, we computed the energetics of the hypothetical ligand exchanges as depicted in Schemes 1 – 4. Note that we made use of the Lewis structures suggested by the authors although there has been some debate on these notations recently.43-45 We grouped the compounds according to the ligands that were used to stabilize them (Figure 3).

Carbene A is substituted by two bulky


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diisopropylphenyl groups on the nitrogens and has been used highly successfully for the preparation of a diversity of main group compounds.7,46-49 Carbene B is also a classical NHC bearing less sterically demanding mesityl groups. A different but also very popular ligand is the cyclic alkyl amino carbene (CAAC) C invented in Bertrand’s group.50,51

This non-aromatic

carbene exhibits a quaternary carbon atom instead of a second nitrogen making it a strong πacceptor. The cyclohexyl moiety is not conformationally locked and thus the concept of “flexible steric bulk” is employed in the molecule.52 We also considered a dihydrogenated version of A that we called h2-A; this carbene is formally not aromatic. Carbene h2-A was also used in combination with C. Carbenes A and B are substituted for the smaller version a, that bears just hydrogen on the nitrogens. Ligand C was substituted with c that is still quite bulky but the quaternary center had to be kept in order not to change the electronic nature of the compound. We view the reactions starting from the hypothetical compounds that are ligated by the minimal size carbenes and offer these the usually employed bulky NHCs to see if the use of the latter is desirable from a thermodynamic point of view. We hope this increases the readability of the equations.

Figure 3. Carbene ligands: A, B and C are typically employed experimentally. We substituted these with the minimal (parent) ligands a and c to quantify dispersion effects. Starting with 1, we evaluated the thermochemistry of reaction (1). The uncorrected DFT results indicate that it would be highly unfavorable to increase the size of the ligand by exchanging a for A! The small a-NHC ligands turn into the plane of the π-bond in 1a; fixing the structure of 1a in the geometry of 1 corresponds to a second order saddle point. When dispersion is explicitly


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included in the computations, the sign of ∆H0 changes and the ligand exchange becomes exothermic, with the overall dispersion stabilization amounts to almost 30 kcal mol–1. This is a remarkably large value that is on the order of, for instance, the aromatic stabilization energy of benzene, and can thus not be ignored in the design of molecular structures. Next we considered a series of boron compounds that exhibit homonuclear single (4), double (5), and triple (6) bonds.46,47,53

The overall dispersion stabilization is in the range of 27–

31 kcal mol–1 and thus is almost constant despite the largely differing geometric arrangements of ligands around the core compound. Hence, a key conclusion is that an important part of the overall structure stabilization derives from the ligand-ligand and ligand-core interactions, nearly independent of the molecular spacer between them. This is supported by the finding that 4–6 can formally be interconverted through (de)hydrogenation reactions, keeping ∆∆EDisp nearly constant. The computed ∆H0 for reaction (4) without explicit inclusion of dispersion is much smaller than in the rest of the serious suggesting that almost no “steric clashes” occur upon substituting a for A in 6a. However, the computed ∆H0 with dispersion interactions included is large and negative yielding an almost constant ∆∆EDisp for the series of boron compounds. Ligand A was also used to prepare diphosphorus compound 7.48

Here, the dispersion

stabilization was computed to be a sizeable 27.1 kcal mol–1. However, one has to note that 7a with a fixed C–P–P–C dihedral of 180° as in 7 corresponds to a rotational transition state with a barrier of about 7 kcal mol–1.48 Adding this barrier to the dispersion stabilization again yields a value of over 30 kcal mol–1. Structure 7 can be oxidized to 8 with molecular oxygen in dry toluene; the latter can be considered an NHC-stabilized phosphorus analogue of N2O4. This is quite remarkable as the phosphorous oxides prefer adamantane-like cage structures.49 Again, we computed a large dispersion stabilization of 33.3 kcal mol–1.


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Scheme 1. Isodesmic reaction schemes for compounds with ligand A. The energies (∆H0) given are in kcal mol–1. dispersion a Si

Si

2A

Si

Si

2 a (1)

1a

−29.9

1

a

B3LYP-D3

17.6

−12.3

A a

A

H B H H B H 4a a a H

B3LYP

A

B

B

a

2A

2A

H 5a

H B H H B H 4 A A H

B

B

A

B a

−31.0

21.4

−9.6

2 a (3)

H 5

−27.0

17.3

−9.7

A

a B

2 a (2)

2A

6a

B A

a

B

2 a (4)

6

−26.8

4.8

−21.9

A

P P

2A

2 a (5)

P P

7a

−27.1

7

a

−3.9

A

a O O P P O O a 8a

2A

A O O P P O O A 8

2 a (6)

−33.3 −7.1

Next we considered molecules with the less bulky ligand B (Scheme 2).

23.2

26.2

Fortunately,

compounds analogous to those with ligand A have been prepared, allowing a direct comparison of the differential stabilization energies. In this regard, compounds 9 and 10 are analogous to 4 and 5, respectively.54 Qualitatively, we find the same trends as for compounds coordinated with ligand A:

uncorrected DFT favors coordination of minimal size ligands while dispersion

corrected DFT predicts the opposite. The overall dispersion stabilization is somewhat smaller than in molecules with ligand A ranging from 20 to 23 kcal mol–1. Diphosphorus structure 11 differs from 7 such that the smaller ligands allow the C–P–P–C dihedral angle to adopt a value of 134° in the crystal structure.48 This is well reproduced by our DFT-D3 computations that yield


10

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an angle of 129° (despite potentially interfering packing effects) while uncorrected B3LYP opens the structure upon optimization and gives a corresponding angle of 167°. Scheme 2. Isodesmic reaction schemes for compounds with ligand B. The energies (∆H0) given are in kcal mol–1. a

B

H B H H B H 4a a

2B

a H

B

B

a

2a

(7)

−22.9 −14.0

B3LYP

B3LYP-D3 8.9

B H 5a

2B

H

B

B

B

a

H 10

2 a (8)

−20.2 −9.9

10.4

B

P P a

dispersion

H B H H B H 9 B

2B 7a

P P B

2 a (9)

−22.2

11

−4.5

17.7

There is also a CAAC stabilized version of diphosphorus 12 (Scheme 3),55 which was prepared by breaking down white phosphorous through direct reaction with the carbene, in contrast to Robinson’s approach of reducing an NHC→PCl3 complex. Here, the C–P–P–C dihedral angle is 149°, which also indicates the somewhat reduced steric demand of C as compared to A. Both DFT and DFT-D3 fail to give good agreement with experiment while DFT-D3 is still closer (172° and 162°, respectively) probably due to a very flat torsional potential. The overall dispersion stabilization is the smallest in this series as it amounts to “only” 13.2 kcal mol–1. We conclude that the electronic stabilization provided by the carbene is much more pronounced here. This conclusion is corroborated with the Lewis structure of 12 with two double bonds instead of the dative bond arrow.43-45 However, in the CAAC coordinated organoboron species 13, that is isoelectronic to an amine, we still compute a large ∆∆EDisp of 24.2 kcal mol–1.56 In this case, the nicely aligned intramolecular contact surfaces of the ligands allow the maximization of dispersion interactions. Hence, at least for the somewhat smaller ligand C we observe a pronounced


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dependence of the dispersion stabilization on the geometric arrangement of the ligands, which is not the case for the larger ligands. An advantage of smaller carbenes in contrast to larger ones is that they allow for a greater flexibility of the inorganic core that can adopt a more favorable geometry. Scheme 3. Isodesmic reactions for evaluating the dispersion stabilization originating from compounds coordinated to ligand C. The energies (∆H0) given are in kcal mol–1. c P P c 12c H B c

13c

C P P C 12

2C

H B

2C c

C

13

dispersion −13.2 −5.0

2 c (10)

2 c (11)

B3LYP

B3LYP-D3 8.2

−24.2

20.6

−3.7

C

Finally, we considered species with ligands h2-A and mixtures of h2-A and C (Scheme 4). Structure 14 is a phosphinonitrene that is enclosed by two h2-A ligands.57 It can be employed as a nitrogen atom transfer reagent and is a stable and thus highly versatile reagent. Due to the similarity of A and h2-A, we compute a similar ∆∆EDisp in excess of 30 kcal mol–1. Species 15 is a carbene-stabilized phosphorous mononitride.58 Here, two different carbenes are applied, i. e., h2-A and C. The dispersion stabilization is somewhat lower (7 kcal mol–1) than that computed for two h2-A carbenes. Scheme 4. Isodesmic reactions for h2-A and mixed ligand types. The energies (∆H0) given are in kcal mol–1.

h2-a

N N P 14a N a-h2 h2-a N P c 15ac

C

2 h2-A

h2-A

h 2-A

dispersion

N 2 h2-a (12) N P 14 N A-h2

h2-A N P C 15

c

h2-a (13)

B3LYP

B3LYP-D3

−32.0

23.3

−8.7

−25.0


−11.8

13.3

12

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Depending on the type of ligand we determined large ∆∆EDisp values that contribute to the thermodynamic stability of the remarkable compounds under consideration. Our findings are perfectly in line with Grimme’s dispersion energy donor concept.18 Despite the fact that the steric bulk is usually interpreted as to provide kinetic stabilization to these molecules, many reactions of main group coordination compounds are known. A number of these compounds can be oxidized to their monoradical cations without decomposition;56,58,59 for instance, 7 can be oxidized to 8,49 13 can be protonated,56 and 14 is a nitrogen atom transfer reagent.57 The most remarkable and striking feature is that in all these reactions, the organic shell of the compounds stays essentially intact while the inorganic core undergoes a chemical reaction owing to the fact that for all cases ∆∆EDisp is (roughly) conserved during the chemical transformation. Again, this is in quantity and predictability comparable to the concept of aromaticity, for which the πaromatic carbon core structure stays intact after an aromatic substitution reaction.

As a

consequence, we propose that the systems under consideration prefer to undergo reactions where the dispersion stabilization energy through the ligand system is conserved or at least retained to a large degree. This would be the case for the hypothetical hydrogenation of 6 to 5 and 5 to 4. Structure 6 is already known to comproportionate with tetrabromo-6 to dibromo-6 (Scheme 5(a)); the dispersion stabilization should be essentially constant.47

Also, when a

molecule of CuCl coordinates to disilicon 1, ∆∆EDisp should be almost constant (or even enhanced, Scheme 5(b)).60

The situation is even more pronounced when 1 reacts with

BH3·THF: Under these conditions the Si=Si double bond is cleaved and two different products form. However, the organic shell provided by the two carbenes is still intact although it is covalently bound to silicon in one case rather than coordinatively (Scheme 5(c)).61 Moreover, 1 can be oxidized with N2O and molecular oxygen to Si2O3 and Si2O4 species, respectively, which are still di-coordinated, (Scheme 5(d)). As over-reaction to silicon oxide powder and the free carbene occurs easily, the utilization of even more powerful DEDs might provide additional stabilization.62


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Scheme 5. Known reactions of main group coordination compounds where the organic shell is preserved. A A B

(a) A

A

Br B Br Br B Br

B

Br

Si

A

Si

N N

(d)

Si A

A

O Si

Si CuCl

A

4 eq. BH 3 THF

O

Si

CuCl

1

A

1

Br

A

A

(c)

B

6 A

(b)

B

H H H H B H B H B H H H Si Si B H H H

+ 3 N 2O Toluene, r.t.

O

1

BH 3 N N

H

N

Si

N

H

+ 2 O2

O

Toluene, r.t.

A

Si O

N

BH 3

A

O Si

N

O

CONCLUSIONS AND OUTLOOK Employing a series of isodesmic reactions we demonstrate that several main group coordination compounds substantially profit from intramolecular dispersion stabilization provided by the organic substituents on the carbenes. The dispersion corrections are as large as ca. 30 kcal mol–1, that is, they are comparable in magnitude to, for instance, aromatic stabilization energies. An immediate conclusion is that computational studies on such types of compounds must include dispersion interactions and that experimental design of novel compounds should systematically utilize novel DEDs. One has to exercise caution with the investigation of smaller model compounds as important interactions might be neglected. In contrast to the general notion


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of kinetic stabilization through shielding of the ligands against reactive species (water, oxygen, etc.), we find that many highly unusual main group ligand complexes are thermodynamically stabilized.

Additional steric bulk even in remote positions might just provide the necessary

quantum of stabilization that makes a particular molecule experimentally viable. Hence, this can be used as a design principle, as captured with the notion of dispersion energy donors. The stabilizing effect of organic DEDs is very likely to be important in other areas of (inorganic) chemistry. One such example is the Pd(0) catalyst 16 (Figure 4), which is a highly versatile catalyst in the Suzuki coupling of hindered substrates: it is firmly embedded in a DED pocket.52 Similarly, “rugby ball” 17 is encapsulated –and possibly kept together– by a dispersive organic shell.

Figure 4. A Pd(0) complex with bulky NHC ligands and inorganic “rugby ball” with an organic shell that is likely to profit from London dispersion stabilization.

ASSOCIATED CONTENT Supporting Information


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Optimized geometries, electronic and zero-point energies and comparisons to other levels of theory are provided in a separate document. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author [email protected] Additional Information The authors declare no competing financial interests.

ACKNOWLEDGMENTS This work was supported by the priority program (SPP 1807) “Dispersion” of the Deutsche Forschungsgemeinschaft (Schr 597/27-1). JPW thanks the Fonds der Chemischen Industrie for a scholarship. Computations were conducted on the LOEWE-CSC high-performance computer of the State of Hesse. We would like to thank HPC-Hessen, funded by the State Ministry of Higher Education, Research and the Arts, for programming advice. We thank Henrik Quanz for technical support.

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London Dispersion Decisively Contributes to the Thermodynamic

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