London Dispersion Enables the Shortest Intermolecular Hydrocarbon H···H Contact

Article abstract of DOI:10.1021/jacs.7b01879

Neutron diffraction of tri(3,5-tert-butylphenyl)methane at 20 K reveals an intermolecular C-H···H-C distance of only 1.566(5) ?, which is the shortest reported to date. The compound crystallizes as a C₃-symmetric dimer in an unusual head-to-hea

Full text of DOI:10.1021/jacs.7b01879

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Communication
London Dispersion Enables the Shortest
Intermolecular Hydrocarbon H•••H Contact
Sören Rösel, Henrik Quanz, Christian Logemann, Jonathan Becker, Estelle Mossou,
Laura Cañadillas-Delgado, Eike Caldeweyher, Stefan Grimme, and Peter R. Schreiner
J. Am. Chem. Soc., Just Accepted Manuscript • Publication Date (Web): 13 May 2017
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London Dispersion Enables the Shortest Intermolecular Hy-
drocarbon H•••H Contact
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Sören Rösel,# Henrik Quanz,# Christian Logemann,‡ Jonathan Becker,‡ Estelle Mossou,§, ¶ Laura
Cañadillas-Delgado,§,¤ Eike Caldeweyher,+ Stefan Grimme,+ and Peter R. Schreiner#,*
# Institut für Organische Chemie, Justus-Liebig Universität, Heinrich-Buff-Ring 17, 35392 Giessen (Germany)
‡ Institut für Anorganische und Analytische Chemie, Justus-Liebig Universität, Heinrich-Buff-Ring 17, 35392 Giessen
(Germany)
§ Institute Laue-Langevin, Rue Jules Horowitz 6, BP 156, 38042 Grenoble Cedex 9 (France)
¤ Centro Universitario de la Defensa de Zaragoza. Ctra Huesca s/n, 50090 Zaragoza (Spain)
¶ Faculty of Natural Sciences, Keele University, Staffordshire, ST5 5BG, United Kingdom
+ Mulliken Center for Theoretical Chemistry, Institute of Physical and Theoretical Chemistry, University Bonn, Ber-
ingstraße 4, 53115 Bonn (Germany)
Supporting Information Placeholder
ro-pentacyclododecane 4 with a short H···H contact of
ABSTRACT: Neutron diffraction of tri(3,5-tert-butylphenyl)-
1.617(3) Å (NRD), thereby significantly undercutting the sum
methane at 20 K reveals an intermolecular C–H···H–C dis-
tance of only 1.566(5) Å, which is the shortest reported to
date. The compound crystallizes as a C3-symmetric dimer in
of the van-der-Waals (vdW) radii of 2.40 Å15 by ∆RvdW = –
0.78 Å (Figure 1).16 Such compressions are not uncommon in
heteroatomic polycycles, with Pascal’s record holder 5 that
displays an intramolecular Si–H···H–Si contact of roughly
1.56 Å, as judged from quantum chemical computations and
the Si···Si distance of 4.433(2) Å derived from X-ray single
crystal diffraction (XRD) data; unfortunately, no NRD study
of 5 has been published.17-18
an unusual head-to-head fashion. Quantum chemical com-
putations of the solid state at the HSE-3c level of theory
reproduce the structure and the close contact very well
(1.555 Å at 0 K) and emphasize the significance of packing
effects; the gas-phase dimer structure at the same level
shows a 1.634 Å
C–H···H–C distance. Intermolecu-
lar London dispersion interactions between contacting tert-
butyl substituents surrounding the central contact deliver
the decisive energetic contributions to enable this remarka-
ble bonding situation.
Continuously probing the limits of chemical bonding helps
improve our current understanding and sophistication of
molecular structure theories. Many of us are enthralled by
the notion of going beyond carbon-carbon triple bonds,1
inorganic quadruple and higher bond orders,2-3 twisted dou-
ble bonds,4-6 compressed7-8 and stretched covalent bonds9-11
and non-covalent interactions such as extremely short C···C
contacts,12-13 and many more. To the best of our knowledge,
we report here the shortest intermolecular H···H contact in a
hydrocarbon as evident from the exceptional congruence of
both neutron and X-ray diffraction experimental data with
sophisticated quantum chemical crystal structure computa-
tions.
Figure 1. The very short intermolecular H···H contact in all-
t
meta Bu-triphenylmethane dimer 12 and the very long RC–C
t
in LD stabilized all-meta Bu-hexaphenylethane 2. Polycyclic
structures with very short intramolecular H···H contacts:
exo,exo-tetracyclododecane 3 (NRD), “half-cage” 4 (NRD)
and in,in-bis(hydrosilane) 5 (XRD; RH···H ~ 1.56 Å by computa-
tions and 1.531(8), NRD18).
Impressive examples for very short intramolecular C–
H···H–C contacts are some bowl-like structures such as
exo,exo-tetracyclo[6.2.1.13,6.02,7]dodecane 3 derivatives with
RH•••H down to 1.713(3) Å14 (neutron diffraction data, NRD)
and the famous current record holder “half-cage” pentachlo-
Structures 3–5 share very tight transannular H···H contacts
within a sterically confined environment, and at first glance
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such short contacts would not be expected intermolecularly
cantly shorter because of the well known underestimation of
XRD C–H bond distances.
because of the energetic penalty associated with bringing
atoms much within their comfortable vdW-radii.19-22 Here
we demonstrate not only that this energetic drawback can be
overcome in an intermolecular bonding situation via highly
attractive London dispersion23-25 (LD) interactions, but also
that such short nonbonding distances can be compressed
even more than in the shortest published intramolecular
case.
Using NRD at the lowest achievable temperature of 20 K
utilizing a large crystal with 2 mm edges, we determined the
cubic, yet less symmetric space group P213.35 The new
asymmetric unit consists of two distinguishable fragments,
lowering the symmetry in the dimer to C3. While the Cα-Hα
bond lengths are ordinary36 (Table 1) with 1.088(5) Å and
1.098(5) Å the extremely short intermolecular Hα···Hα’ con-
tact of only 1.566(5) Å is the shortest reported to date.
To allow such short contacts well below their vdW radii,15,
37 the overall stabilization must derive from other parts of the
molecule, which, based on the measured NRD data, do not
suffer from significant deformations: The sp3 angles around
the central carbon Cα amounts to 106.0° (Hα-Cα-Ci) and 112.7°
(Ci-Cα-Ci’); the C–C bonds are ordinary.36 More importantly,
there are 33 contacts within the attractive vdW-range below
the sum of the atom radii (3.08 Å)38 down to 2.39 Å. NRD
measurements at higher temperatures (Table 1) reveal the
expected structural temperature dependence resulting in an
H···H distance change of ΔRH···H(20 Kꢀ200 K) = 0.03 Å; linear
extrapolation leads to a minimum RH···H of 1.563 Å at 0 K.
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In molecular design large alkyl groups are used to intro-
duce bulk and sterically shield reactive moieties. The fact
that bulky groups are highly polarizable –thereby increasing
their ability to engage in non-negligible, stabilizing LD inter-
actions– is often disregarded. Such groups are appropriately
termed “dispersion energy donors”, DEDs.26 This thermody-
namic stabilization can be utilized to isolate otherwise highly
reactive molecular entities within an LD shell,27 as demon-
strated recently for bulky NHC coordinated main group
compounds28 and the exceedingly crowded hexa-
phenylethane
derivatives
such
as
all-meta
tBu-
hexaphenylethane (2).29-30
While hexaphenylethane is experimentally unknown, steri-
cally much more crowded 2 is isolable and was characterized
via XRD31 and NMR-spectroscopy.32 The origin of the stabili-
Table 1. Structural data of 12 determined by NRD and
DFT computations.
t
zation was traced back to LD interactions between the Bu-
groups, which are deemed excellent DEDs.29-30 The other-
wise delicate organic peroxide functionality33 is also highly
NRD
100
HSE-3cb
t
stabilized when embedded in the all-meta Bu-trityl motive
T [K]
20
200
0
0
leading to a m.p. (decomp.) of 253 °C in bis(tri(3,5-di-tert-bu-
(state)
(s)
(g)
tylphenyl)methyl) peroxide.34 These peripheral LD interac-
t
tions also give rise to an all-meta Bu-triphenylmethane di-
R-value
0.031
0.054
0.096
mer 12 [Bis(tri(3,5-di-tert-butylphenyl)methane)], featuring
the shortest C–H···H–C contact reported to date.
Volume [ų] 7833.1(4) 7901.4(4) 8040.4(4 7676.
)
9
Hα···Hα'
1.566(5) 1.577(6) 1.594(9) 1.555 1.634
1.093(5)c,
d
Cα–Hα
1.093(3)
1.091(5) 1.088 1.092
1.091
1.085(6) 1.085(2) 1.076(4) 1.082 1.082
Ave. Calk–H 1.090(7) 1.078(29) 1.068(45) 1.091
Ave. Carom
–
H
Cα–Ci
1.515(1)c
106.0(1)c 106.0(1)
1.515(1)
1.515(2)
1.509
1.513
Hα–Cα–Ci
Ci–Cα–Ci
105.9(1) 105.9 104.8
112.7(1)c
112.7(1)
112.8(1)
40.1
112.8
39.8
113.7
45.3
Hα–Cα–Ci–
Co
39.8
39.8
Ho···Pha
3.632
2.927(1)
5.192
3.639
3.654
3.613 3.637
Figure 2. ORTEP representation of 12 derived from neutron
diffraction data with ellipsoids drawn at 50% probability.
Ho···Co'
2.958(1) 2.978(1) 2.917 3.062
tBu···Ph'a
tBu··· tBu'a
5.136
6.115
b
5.140
6.113
5.025 4.982
ꢂ
Hydrocarbon 1 crystallizes in the cubic space group ꢀꢁ3 as
6.166
6.037 5.889
determined by XRD at 100 K. The asymmetric unit consists
of one 3,5-di-tBu-phenyl group attached to the central Cα–Hα
moiety. Application of the S6 symmetry operation present in
aGroup centroid distances; Basis set: def2-mSVP; Ave. of
both molecules in 12 (cf. SI); dCα–Hα = 1.088(5) Å; C’α–
H’α = 1.098(5) Å.
c
ꢂ
ꢀꢁ3 along the Cα–Hα axis result in dimer 12 with a RCα···Cα of
3.780(7) Å. As the XRD C–H bond length of RC–H = 1.00(4) Å
already revealed a remarkably short C–H···H–C distance of
1.77(7) Å, we reckoned that the true distance must be signifi-
To rationalize the structural peculiarities of 12 we employed
DFT computations within the CRYSTAL14 software suite39
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utilizing the recently developed screened Fock exchange
therefore is unfavored by ∆Gd298 = 7.7 kcal mol–1.46 By omit-
ting the Bu moieties the head-to-head arrangement of 62
density functional composite scheme termed HSE-3c.40 This
efficient method includes the D3 correction41 and a geomet-
rical counterpoise correction scheme42 accounting for the
basis set superposition error (BSSE) and improving the de-
scription of LD interactions. The primitive cell starting from
the 20 K NRD data (ꢀ2ꢃ3) was fully optimized featuring
relaxation of all atom positions as well as cell parameters.
Due to the high symmetry of the molecular crystal we were
able to reduce the atom count within the primitive cell from
856 reducible to 74 irreducible atoms.
t
becomes unstable and about 77% of LD interaction is lost
resulting in ∆Gd298 = –8.5 kcal mol–1. Solvation free energy
contributions computed with BP86-COSMO-RS47-49 for
CHCl3 destabilize the dimerization of 12 by ΔΔGd298 = –
8.1 kcal mol–1. In addition we note a relatively large contribu-
tion of the (often repulsive) three-body Axilrod-Teller-Muto
(ATM) dispersion terms46, 50-51 that enlarge the H···H distance
by about 0.01 to 0.02 Å while decreasing the De from 30.9 to
27.7 kcal mol–1.
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The HSE-3c computed solid state structure is in good
agreement with the NRD structure of 12 (Table 1). The H···H
distance of 1.555 Å at 0 K reproduces the NRD extrapolated
value within the error bounds. The volume of the primitive
cell is 2% smaller than observed experimentally. This equals
a thermal density gradient of ≈1 mg cm–3 K–1 (cf. SI), which
deviates from the experimental value (0.1 mg cm–3 K–1) but is
in the typical range for organic crystals.43 To estimate pack-
ing effects, 12 was computed in the gas phase at the same
level of theory. This elongates the central H···H contact to
1.634 Å. Crystal packing therefore provides some non-
negligible stabilization to the tight H···H contact. Note that
the often employed approximation of using molecularly
(non-periodic) optimized structures as substitute for period-
ic crystal data is insufficient in our case and would lead to
inconsistencies between theory and experiment of about 0.07
Å.
Dimer 12 is formally the hydrogenation product of 2. Upon
dissociation of 2 a second, local minimum was predicted to
occur along the intrinsic reaction coordinate.29 Such radical
“van-der-Waals-complexes” –better termed LD-complexes–
are related to the “cage-effect”.52 The bis(all-meta tBu-
triphenylmethyl radical) LD-complex 82 with a C···C distance
of 5.28 Å can be taken as a hydrocarbon analogue of struc-
tures with frustrated Lewis pairs (FLPs),53 where the split
central C–C bond corresponds to the unsaturated dative
DꢀA bond. This analogy was recognized in the very first
appearance of FLPs, where the authors depicted HPE deriva-
tives as structures analogous to FLPs.54 As FLPs are able to
split H2,55 it seemed plausible to assume 82 may be able to
split H2 with 12 being the formal product. Unfortunately, all
experimental attempts to split dihydrogen with the corre-
sponding radical 8· were unsuccessful, as solutions of 8·
eventually hydrolyze over a period of one month to all-meta
t
tBu-triphenylmethanol and all-meta Bu-triphenylmethane 1
Closely related unsubstituted triphenylmethane 644 and the
all-meta methyl derivative 7 (cf. SI) crystallize in space
when pressurized with H2 or D2 (for ease of identification by
NMR, for details see SI). We attributed this finding to the
fact that the barrier for H2 cleavage is the highest when there
is no polarization as is the case for 8·.
ꢂ
groups different from 12, namely in ꢀꢄꢁ2ꢃ and ꢀ1, respective-
ly, and no linear head-to-head dimers can be discerned.
An energy decomposition analysis (EDA, Figure 3 and cf.
SI)45 of 12 at the B3LYP-D3ATM(BJ)/def2-TZVPP level is in-
structive in understanding the roles of the various (arguably
arbitrary) energy contributions to the overall stability of 12;
the trends are mirrored by an SAPT(0) analysis (Figure S23).
From a different viewpoint, 12 may perhaps also be viewed
as a “frozen early transition state” because of the very close
contact of the Hα’s. Indeed, our computational analysis of
the contact between the close hydrogens using the quantum
theory of atoms in molecules (QTAIM)56 reveals a bond
critical point (Fig. S25) and the non-covalent interaction
(NCI) plot57 displays a strongly attractive region (Fig. S28).
Such computed attractions in tight H···H contacts were also
found in a study of 5.58 As such arrangements are far off
equilibrium, an interpretation of the analysis of the density
gradient is delicate and might lead to contradictory results.
A bonding interaction between the Hα’s should weaken the
respective Cα–Hα bond, measurable by a red-shifted C–H
bond stretching frequency. However, experimentally the
stretching vibration frequency of, e.g., 5 increases.16 Unfortu-
nately, the central C–H IR absorptions are buried under
other vibrational bands for 12. Deuteration at the central
methine carbon red-shifts the corresponding C–2H bond
stretching frequency into the uncongested region around
2100 to 2300 cm–1 but we did not observe additional absorp-
tions (cf. Figure S15). The PBEh-3c computed central asym-
metric (IR active) ν(Cα-H···H-Cα) is about 56 cm–1 blue-
shifted as compared to ν(Cα-H) of the monomer, similar to
what was found for 5.
t
Of course, the steric bulk of the Bu-groups increase the
overall Pauli exchange repulsion between the monomers
significantly. However, all other contributions including
electrostatics stabilize the dimer interactions, with the dis-
persion component being by far the most important.
Figure 3. The energy decomposition analysis of 12 at B3LYP-
D3ATM(BJ)/def2-TZVPP.
To estimate the overall stabilizing energy contributions of
t
the Bu groups, the dissociation energy of 12 was computed
by optimization at B3LYP-D3ATM(BJ)/def2-TZVPP including
thermostatistical corrections at PBEh-3c and compared to
the unsubstituted symmetric triphenylmethane dimer 62.
Structure 12 is bound by 48.9 kcal mol–1 of LD, dissociation
Table 2. B3LYP-D3ATM(BJ)/def2-TZVPP//PBEh-3c com-
putations of the dimeric structures of unsubstituted
t
triphenylmethane 6 and all-meta Bu-substituted 1.
Distances in Å, energies in kcal mol–1.
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12
62
8.2
De
27.8
298
∆Hd
26.6
18.9
8.7
17.2
–8.5b
–8.3c
11.3
298
T∆Sd
298
∆Gd
7.7
298
∆Gd;solv
–0.4c
48.9
–0.007
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Edisp
a
∆RC–H
–0.005
1.717
RCH···HC
1.601
b
aDifference to monomer; ∆G < 0 means dissociation;
cafter solvent correction [BP86-COSMO-RS:CHCl3].
In summary, we have experimentally identified the shortest
intermolecular H···H contact (1.566(5) Å) reported to date in
the crystal structure of the all-meta Bu-triphenylmethane
dimer 12, as analyzed by NRD at temperatures as low as 20 K.
Solid state DFT computations reveal that crystal packing
does affect this distance but is not chiefly responsible for this
short H···H contact. Rather, large LD interactions exerted via
the Bu-groups surrounding the compressed H···H contact
stabilize 12. The Bu-groups act as DEDs and counter the
otherwise energetically unfavorable head-to-head arrange-
ment of Hα’s.
t
t
t
ASSOCIATED CONTENT
Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website. Experimental details, NMR, IR,
XRD and NRD data as well as computational details (coordi-
nates, NCI Plot, QTAIM) (PDF)
AUTHOR INFORMATION
Corresponding Author
*prs@org.chemie.uni-giessen.de
Notes
The authors declare no competing financial interests.
ACKNOWLEDGMENT
This work was supported by the Priority Program “Disper-
sion” (www.spp1807.de) of the Deutsche Forschungsgemein-
schaft.
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