Probing the Delicate Balance between Pauli Repulsion and London Dispersion with Triphenylmethyl Derivatives

Article abstract of DOI:10.1021/jacs.8b09145

The long-known, ubiquitously present, and always attractive London dispersion (LD) interaction was probed with hexaphenylethane (HPE) derivatives. A series of all-meta hydrocarbyl [Me, ⁱPr, ^tBu, Cy, Ph, 1-adamantyl (Ad)]-substituted triphenylmethyl (TPM) derivatives [TPM-H, TPM-OH, (TPM-O)₂, TPM^?] was synthesized en route, and several derivatives were characterized by single-crystal X-ray diffraction (SC-XRD). Multiple dimeric head-to-head SC-XRD structures feature an excellent geometric fit between the meta-substituents; this is particularly true for the sterically most demanding ^tBu and Ad substituents. NMR spectra of the ⁱPr-, ^tBu-, and Cy-derived trityl radicals were obtained and reveal, together with EPR and UV-Vis spectroscopic data, that the effects of all-meta alkyl substitution on the electronic properties of the trityl scaffold are marginal. Therefore, we concluded that the most important factor for HPE stability arises from LD interactions. Beyond all-meta ^tBu-HPE we also identified the hitherto unreported all-meta Ad-HPE. An intricate mathematical analysis of the temperature-dependent dissociation constants allowed us to extract δG_d²⁹⁸(exptl) = 0.3(5) kcal mol^-1 from NMR experiments for all-meta ^tBu-HPE, in good agreement with previous experimental values and B3LYP-D3(BJ)/def2-TZVPP(C-PCM) computations. These computations show a stabilizing trend with substituent size in line with all-meta Ad-HPE (δG_d²⁹⁸(exptl) = 2.1(6) kcal mol^-1) being more stable than its ^tBu congener. That is, large, rigid, and symmetric hydrocarbon moieties act as excellent dispersion energy donors. Provided a good geometric fit, they are able to stabilize labile molecules such as HPE via strong intramolecular LD interactions, even in solution.

Full text of DOI:10.1021/jacs.8b09145

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Article
Probing the Delicate Balance Between Pauli Repulsion
and London Dispersion with Triphenylmethyl Derivatives
Sören Rösel, Jonathan Becker, Wesley D. Allen, and Peter R. Schreiner
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b09145 • Publication Date (Web): 05 Oct 2018
Downloaded from http://pubs.acs.org on October 5, 2018
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Page 1 of 15
Journal of the American Chemical Society
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Probing the Delicate Balance Between Pauli Repulsion and
London Dispersion with Triphenylmethyl Derivatives
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Sören Rösel,§ Jonathan Becker,# Wesley D. Allen,¶,+ and Peter R. Schreiner§,*
§Institute of Organic Chemistry, Justus Liebig University, Heinrich-Buff-Ring 17, 35392 Giessen (Germany), E-mail:
prs@uni-giessen.de
#Institute of Inorganic and Analytical Chemistry, Justus Liebig University, Heinrich-Buff-Ring 17, 35392 Giessen
(Germany)
¶Department of Chemistry and +Center for Computational Quantum Chemistry, University of Georgia, Athens,
Georgia 30602, USA
Supporting Information Placeholder
London Dispersion (LD) formally describes the attractive part
of the van-der-Waals (vdW) potential and was first
established by Fritz London to rationalize the condensation
of noble gases.1 LD was discovered as an important design
element in chemistry quite a while ago,2 but has only recently
been appreciated in full.3 While LD is most apparent for
nonpolar substances (e.g., hydrocarbons) it is ubiquitous and
not related to a particular functional group.4 It also does not
disappear in solution,5 not even in highly polar ionic liquids6
or in dimers of equally charged species.7 The important role
LD plays for large biomolecules has been well established
over the last decades.8
ABSTRACT: The long known, ubiquitously present and
always attractive London Dispersion (LD) interaction was
probed with hexaphenylethane (HPE) derivatives. A series of
all-meta hydrocarbyl [Me, iPr, tBu, Cy, Ph, 1-Adamantyl (Ad)]
substituted triphenylmethyl (TPM) derivatives [TPM–H, TPM–
OH, (TPM–O)2, TPM•] was synthesized en route and several
derivatives were characterized by single crystal X-ray
diffraction (SC-XRD). Multiple dimeric head-to-head SC-XRD
structures feature the excellent geometric fit between the
meta-substituents; this is particularly true for the sterically
t
most demanding Bu and Ad substituents. NMR spectra of
the iPr-, tBu-, and Cy-derived trityl radicals were obtained and
reveal, together with EPR and UV-Vis spectroscopic data, that
the effects of all-meta alkyl substitution on the electronic
properties of the trityl scaffold are marginal. Therefore, we
concluded that the most important factor for HPE stability
arises from LD interactions. Beyond all-meta tBu-HPE we also
identified the hitherto unreported all-meta Ad-HPE. Intricate
mathematical analysis of the temperature dependent
dissociation constants allowed us to extract ΔGd298(exptl.) =
0.3(5) kcal mol–1 from NMR experiments for all-meta tBu-HPE,
in good agreement with previous experimental values and
The unfortunate but widespread neglect of LD in chemical
reasoning, in particular, in connection with steric effects,3b, 3l
arises from the fact that LD interactions are considered small
and often negligible. While it is true that a single individual
LD interaction indeed is small, the strong gain in count of
these pairwise additive interactions make LD interactions
grow rapidly for increasingly larger molecules. Hence, the
notion of LD overall being “weak” is not very meaningful in
realistic chemical systems. Still, this notion supported the
long standing oversight (despite better knowledge) that
many practical implementations of density functional theory
(DFT) did not include LD. Interpretable results were still
acquired but they are likely due to favorable error
compensation through the use of small basis sets and non-
inclusion of basis set superposition errors (BSSE) as well as
solvation.9 This error compensation surfaced when DFT
results were systematically compared with experiment and
explicitly correlated methods that now have became available
for increasingly larger molecular systems.10
B3LYP-D3(BJ)/def2-TZVPP(C-PCM) computations.
These
computations show a stabilizing trend with substituent size
in line with all-meta Ad-HPE (ΔGd298(exptl.) = 2.1(6) kcal mol–
t
1) being more stable than its Bu congener. That is, large,
rigid, and symmetric hydrocarbon moieties act as excellent
dispersion energy donors (DEDs).
Provided a good
geometrical fit, they are able to stabilize labile molecules such
as HPE via strong intramolecular LD interactions – even in
solution.
Introduction
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tBu
tBu
Precursor Synthesis
tBu
1
2
3
4
5
6
7
8
tBu
tBu
The general approach (Figure 2 top) for the preparation of
triphenylmethyl radicals (R-1•) starts from 3,5-dialkyl-1-
tBu
tBu
tBu
tBu
tBu
halobenzenes (R-2), for which there are multiple synthetic
routes. The corresponding organometallic species, typically
Grignard or lithium reagents, add to a suitable C1-synthon,
e.g., diethyl carbonate to give trityl alcohols (R-3). Their
chlorination with acetyl chloride yields the corresponding
halides (R-4). The radicals can then be generated via a single
electron reduction using metals such as silver or zinc.
2a
= 1.71 Å
tBu
tBu
RC–C
m.p. = 246 °C
t
1
2
Bu-
2b
RC–C = 1.65 Å
disp = 28 kcal mol–1
RC–C = 1.67(3) Å
E
disp = 62 kcal mol–1
E
9
Figure 1.
Representative
hydrocarbons
2-(1-
10
11
12
13
14
15
16
17
18
19
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diamantyl)[121]-tetramantane 2a, bis(1-diamantyl) 2b and
3,5-Dimethyl-1-bromobenzene (Me-2) is commercially
available and 3,5-di-iso-propyl-1-bromobenzene (iPr-2) is
readily accessible starting from commercially available 2,6-di-
iso-proplyaniline via bromination, diazotation, and
reduction.22 Triple Friedel-Crafts alkylation of tert-butyl
chloride with AlCl3 of benzene and subsequent mono-ipso-
bromination leads to 3,5-di-tert-butyl-1-bromobenzene (tBu-
2).23 Similar approaches unfortunately do not work for the
phenyl (Ph-2), cyclohexyl (Cy-2), and 1-adamantyl (Ad-2)
t
all-meta tert-butyl hexaphenylethane Bu-12 with very long
central C–C bonds prepared owing to large stabilizing LD
interaction that outweigh Pauli exchange “steric” repulsion.3d,
11
Some striking examples from our own work are represented
through the remarkable structures of diamondoid12 dimers,
of which the [121]tetramantane-diamantane adduct 2a
displays the longest C–C bond (1.71 Å) reported for an alkane
to date (Figure 1); still, 2a is remarkably stable with an m.p. of
246 °C.3d, 11 A computational analysis [B3LYP-D/6-31G(d,p)]
of the bond dissociation energy of the smaller congener
bis(1-diamantyl) 2b showed that more than one third of its
ΔGd298 = 71 kcal mol–1 arises from LD. Similarly, all-meta tert-
derivatives.
Ph-2
was
synthesized
from
1,3,5-
tribromobenzene Br-5 with phenylboronic acid in a Suzuki C–
C cross coupling reaction.24 Cy-2 and Ad-2 were ultimately
synthesized through Negishi C–C cross coupling (Figure 2
bottom).
t
butyl hexaphenylethane Bu-12 –one of the two isolable
Cyclohexyl zinc chloride 10 was generated via a Grignard
reaction from cyclohexyl bromide
unbridged hexaphenylethanes prepared to date– displays a
9 and subsequent
very long C–C bond (1.67(3) Å).13
TPSS-D3/TZVPP
transmetallation with zinc(II)chloride. The coupling to 1,3-
dibromo-5-chlorobenzene (Cl-5) with 2:1 mol-% ratio of 2-
dicyclohexylphosphino-2',6'-dimethoxybiphenyl
computations demonstrate that 62 kcal mol–1 of the bond
dissociation energy (De = 34 kcal mol–1) are associated with
LD interactions (the difference being other effects including
Pauli repulsion).13 LD therefore is the decisive contribution to
rationalize the unexpected stabilities of these hydrocarbons.
(SPhos25)/Pd(OAc)2
at
r.t.
gave
1-chloro-3,5-
dicyclohexylbenzene 11 in 96% yield. While cyclohexyl
lithium is readily accessible26 and cyclohexyl magnesium
species are even commercially available, 1-adamantyl
lithium27 or 1-adamantyl magnesium species are very difficult
to generate.28 Fortunately, 1-adamantyl zinc chloride 13 is
accessible in one step through an in situ Grignard reaction
scavenged by zinc chloride stabilized with lithium chloride.29
However, the coupling of 13 to Cl-5 with 10:5 mol-% ratio of
SPhos/Pd(OAc)2 at 50 °C gave 3,5-di(1-adamantyl)-1-
chlorobenzene 15 only in up to 15% yield.
Quantifying LD is rather challenging as it is often associated
with, e.g., the hydrophobic effect14 so that it is difficult to
separate LD from other attractive forces.15 This is evident in
test systems originating from protein structures,8b,
16
supramolecular entities17 or molecular balances including
heteroatoms that lead to significant polar effects.5e,
18
Differential thermodynamic methods like the double mutant
cycle19 allow the observation of the interactions between
certain parts of the test systems.
Recently we proposed that all-meta alkyl substituted
i
t
hexaphenylethane [HPE; R-12; R = Me, Pr, Bu, Cy, Ph, 1-
Adamantyl (Ad)] derivatives are suitable testing systems for
determining the relative effects of dispersion energy donors
(DEDs20).21 These molecules are pure hydrocarbons that may
form a weak covalent central C–C-bond in equilibrium with
their triphenylmethyl-type (trityl) radical monomers. Hence,
the equilibrium position reveals some information about the
DED ability of the interacting peripheral groups. Here, we
report the synthesis of the corresponding all-meta
substituted triphenylmethyl chloride precursors, the
properties of the corresponding radicals, and the challenges
associated with capturing the hexaphenylethane derivatives
in their equilibria with their radical monomers.
Results and discussion
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a)
R
R
25 °C was used instead, yielding 95% of the product within
1
5 min.32
R
R
R
R
R
R
2
3
4
5
6
7
8
9
a)
b)
The difficulties in the coupling of 1-adamantyl to the meta
positions of the 1,3,5-substituted tri- or dihalobenzenes can
be rationalized by the increased steric bulk and electron-
richness leading to a slow first transmetallation (TM).33 As the
oxidative addition (OA) in the case of the coupling of 10 or
13 to Cl-5 consists of the same catalyst and substrate there
should be no difference. The reductive elimination (RE) is
enhanced by bulky and electron donating ligands.34 As the
steric demand and electron donating ability of adamantyl is
larger than that of cyclohexyl, the coupling of the former
should be faster. This contradicts the observed drop in yield
and RE can thus be excluded as the rate limiting step.
OH
X
X
R
R
R
R
R
R
2
3
4
R–
R–
R–
R = Me, iPr, tBu, Cy, Ph, Ad
R
R
R
R
R
R
R
10
11
12
13
14
15
16
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19
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K–1
R
R
c)
•
R
R
R
R
R
R
R
R
R
1
1
R–
a)
R–
2
While there are numerous studies on the individual OA and
RE steps, there are only few mechanistic studies for the TM in
b)
Br
ZnCl
Cy
Cy
35
Negishi couplings.33c,
Most likely a square-plane Pd2+
b)
complex of the composition [(η2-SPhos)Pd(Ar)Br] forms after
the OA, where the SPhos ligand can switch to an η1 binding
mode to enable TM.25, 36 Steric congestion only would slow
TM and merely explains the high amount of reduced 13.
99%
96%
X
9
10
11
(X = Cl)
Cy- (X = I)
c)
72%
2
Br
ZnCl
Ad
Ad
d)
85%
e)
Ultimately, the electronic propensities of the benzene
substituents have been included to understand why only fully
substituted products are obtained and why TMS substitution
leads to much higher yield. Our initial attempts to improve
the yield employed an amide to mask the halide functionality.
Subsequent hydrolysis and Sandmeyer reaction should lead
57%
X
(X = TMS)
12
13
14
g)
f)
95%
2
Ad- (X = I)
1%
Figure 2. Top: General approach for the preparation of all-
meta alkyl substituted trityl radicals: a) Li0 or tBuLi or Mg, then
(EtO)2CO or Cl2CO; b) SOCl2 or AcCl; c) Ag0 or Zn0. Bottom:
The synthesis of 3,5-dicyclohexyl-1-iodobenzene Cy-2 and
3,5-di(1-adamantyl)-1-iodobenzene Ad-2. Conditions: a) Mg,
THF, r.t., 60 min; then ZnCl2, r.t., 10 min; b) cat. 2:1
SPhos/Pd(OAc)2, Cl-5, THF, r.t., 3 h; c) Li0, cat. naphthalene,
Et2O, r.t.; then I2, d) Mg, ZnCl2, LiCl, THF, 2 h; e) cat. 2:1
SPhos/Pd(OAc)2, TMS-5, THF, 50 °C, 3 h; f) I2, AgCOOCF3,
MeOH, hexane, r.t., 1 h; g) 1. cat. 2:1 SPhos/Pd(OAc)2, Cl-5,
THF, 50 °C, 3 h, 15%; 2. Li0, cat. naphthalene, Et2O, r.t.; then I2,
7%.
to Ad-2.
Indeed, the synthesis of N-boc-3,5-
di(1-adamantyl)aniline 1737 showed an increased yield of
30%. An analysis of Hammett’s meta values38 indicates a
correlation between the electron donating nature of the
substituents on the aryl and the yields [c.f. supporting
information (SI)].
Successive exchange of electron
withdrawing bromine against electron donating 1-adamantyl
during the reaction therefore eases the subsequent couplings
cycles. Likewise, an initially installed electron donating group
such as TMS leads to a faster overall reaction and improved
yields.
Lithiation of Me-, iPr-, tBu-, Cy-, and Ph-2 with tBuLi at –78 °C
proceeded smoothly. After addition of diethyl carbonate at
25 °C the corresponding trityl alcohols R-3 were isolated in
good to excellent yields [93% (Me); 85% (iPr); 78% (tBu); 94%
(Cy); 71% (Ph)]. The synthesis of Ad-3 was more delicate and
initially failed under the common conditions. The major
product was identified as 1,1’-di(3,5-di(1-adamantyl)phenyl)-
1’’-tert-butylmethanol and laborious to seperate. Performing
Both 11 and 14 were rather difficult to metallate and only
lithiation of 11 using Li0 with catalytic amounts of
naphthalene in a broken glass/Et2O slurry and subsequent
quenching of the 3,5-dicyclohexylphenyl lithium with iodine
gave 3,5-dicyclohexyl-1-iodobenzene Cy-2 in 72% yield.
Treatment of 15 the same way to lithiate the strong C–Cl
bond facilitate solvolysis of the corresponding lithiated
species and mainly led to reduction. Only 7% of 3,5-di(1-
adamantyl)-1-iodobenzene Ad-2 were obtained. A more
suitable precursor to 3,5-di(1-adamantyl)-1-iodobenzene
was found with 3,5-dibromo-1-trimethylsilylbenzene (TMS-
5). The Negishi coupling of 13 to TMS-5 with a 10:5 mol-%
ratio of SPhos/Pd(OAc)2 at 50 °C gave 57% yield of isolated
t
the addition of BuLi in a titration like manner and avoiding
any super- or substoichiometric addition of tBuLi gave up to
64% yield.
Attempts to improve the synthesis of Ad-3 by application of
a Grignard species by metal-halogen exchange would require
i
at least 2.2 equiv. PrMgCl·LiCl at 0 °C for full conversion of
3,5-di(1-adamantyl)-5-TMS-benzene 14.
The iodine
i
Ad-2. However, excess of PrMgCl·LiCl solely led to the iso-
commonly is demasked with ICl within a few minutes.30
Unfortunately, ICl reacts with adamantane under these
conditions,31 and I2 together with AgCOOCF3 as Lewis acid at
t
propylated compound. Grignard reaction of Bu- or Ad-2
with Mg-powder were initiated by substoichiometric
amounts of iPr-MgCl·LiCl or DiBAL–H and gave good
3
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R
R
R
R
conversion to the Grignard species. Unfortunately, treatment
1
2
3
4
5
6
7
8
of these with diethyl carbonate or phosgene did not yield Ad-
R
R
R
R
R
R
R
R
3 and tBu-3 in only 20%.
Zn(Cu)
H2O
Solv-H

OH
Cl
H
C
R
R
R
R
R
R
R
R
R
R
R
R
R
R
R
3
4
1
19
R–
R–
R–
R-
Br
HBr, AcOH,
PhMe, 60 °C
O2
ZnCl2
R
R
R
R = iPr, Ph
R
R
R
R
R
R
R
18
R-
R
R
R = Me, iPr, tBu,
Cy, 1-Ad, Ph
9
R
R
R
OH
O
R
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
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31
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50
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O
R
R
R
R
R
R
R
R
AcCl, PhH, 
R
R
R
R
3
R
R–
R
Cl
R = Me, iPr, tBu,
Cy, Ph, Ad
R-21
R–20
R
R
R
Figure 4. Generation of R-1• and decomposition channels
leading to hydrolysis (R-3), hydrogen abstraction (R-19), and
oxidation (R-20) products. Chloride abstraction from R-4 by
zinc(II)chloride leads to carbocations R-21.
4
R-
Figure 3. Halogenation procedures for R-3.
Fundamental for the synthesis of the HPE derivatives R-12 or
the radical monomers R-1• is the synthesis of the
corresponding all-meta alkyl triphenylmethyl halides.
Chlorination of R-3 to all-meta alkyl triphenylmethyl
chlorides (R-4) with acetyl chloride worked similarly well for
the alkyl derivatives [61% (Me); 59% (iPr); 84% (tBu); 69% (Cy);
55% (Ad)]. Chlorination of Ph-3 was cumbersome and Ph-4
was resistant to single electron transfer (SET) from Zn(Cu) or
Ag. The treatment of Ph-3 with HBr/AcOH gave the
corresponding bromide Ph-18 in 71% yield (Figure 3). The
Hydrocarbons. Hydrocarbons R-19 were synthesized by
treatment of R-3 with NaBH4/AlCl3.39 The single crystal X-ray
t
diffraction (SC-XRD) structure of Bu-19 reveals a head-to-
head arrangement of two independent molecules.3w The
neutron diffraction (NRD) structure displayed the shortest
intermolecular C–H···H–C contact (RH···H
=
1.566(5) Å)
reported to date; for a very short intramolecular contact see
ref.40 The underlying interaction enabling this short RH···H was
t
found to be largely LD. The C–H···H–C contact in Bu-19 is
arranged in a straight line [γ(C–H···C’) = 180°] with an angle
between their CipsoC’ipsoC’’ipso planes (PC) θ(PC;P’C) of 0°. The
phenyl rings around the central carbons are arranged in a
propeller-like fashion (Π-rotamer in Figure 5a).
i
i
same reaction with Pr-3 gave 64% yield of Pr-18 but the
obtained colorless crystals rapidly cracked and turned brown
even under exclusion of moisture and light at –20 °C. This
illustrates the sensitivity of the alkyl derivatives R-18 and
consequently only R-4 was used further.
Similar head-to-head arrangements were found in the SC-
XRD structures of Me-19
modifications found for Cy-19 (
arrangements the C–H···H–C contacts are shifted.
푃1
(
)
and the two crystal
and
Precursors and Related Species
푃1
푃2 /푐
). In these
1
The generation of R-1• sometimes also causes significant
decomposition and side product formation (Figure 4). It was
therefore necessary to prepare these to provide reference
spectral data for the detailed interpretation of the spectra of
As
t
observed for Bu-192 the RH···H of Me-19 and Cy-19 are
distinctly below the sum of their vdW radii. In line with the
DED concept the RH···H distance is 2.08 Å for the smaller and
less effective methyl group DED in Me-193w while it is
reduced to 1.83 Å for the larger Cy-19. The latter is 25%
shorter than expected from the sum of the vdW radii even
when the RC–H distance is fixed to 1.00 Å (as customary for X-
ray structure determination). Not unexpectedly, this makes
cyclohexyl a much better DED than a methyl group on the
basis of their sizes and polarizabilities.
the mixtures containing the radicals.
Hence, the
triphenylmethanol (R-3), triphenylmethane (R-19), and
bis(triphenylmethyl)peroxide derivatives (R-20) as well as the
triphenylmethyl carbocations R-21 were synthesized for
proper spectral analyses.
The phenyl rings in the first independent molecule in the
푃1
asymmetric unit of Cy-19 in
form a propeller like (Π)
arrangement akin to tBu-19 (Figure 5b). The flatter structure
t
of the Cy substituents in comparison to Bu allows for a
parallel shift of the C–H bonds by 0.70 Å to avoid in part the
repulsive interaction through the short H···H contact. The
phenyl rings in the second independent molecule are
arranged like rabbit-ears (Β) (Figure 5c) and the C–H bonds
are significantly shifted (3.54 Å). The second modification
푃2 /푐
found for Cy-19 (
) consists of a mixed Β/Π dimer. Here,
1
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repulsion is reduced through an angled arrangement
(θ(PC;P’C) = 35.9°, RH···H = 2.37 Å; Figure 5d).
tBu-3 in the asymmetric unit. Here the hydrogen of one OH
1
2
3
4
5
6
7
8
function is located between the two oxygen atoms [γ(OD–
H···OA) = 178.1°], the other hydrogen is uncoordinated. Both
molecules have common geometrical features and form an
Both arrangements found in Me-19 consist of a parallel
shifted Β·Β dimer. The tight dimer still contains a 13% shorter
RH···H than the sum of the vdW radii; the C–H bonds are shifted
by 0.68 Å. The second dimer is stronger shifted and therefore
has a RH···H of 2.55 Å. Parent H-1941 and iPr-19 show no such
arrangement. Both crystallize as Π-rotamers. The latter
prefers an in-line head-to-tail arrangement (cf. SI).
angled arrangement [θ(PC;P’C)
arrangement is found for Cy-3 [Figure 6b;
=
28.5°].
A
similar
푃2 /푐
; R = 0.104;
1
θ(PC;P’C) ≈ 29°], but the severe whole molecule disorder of
this SC-XRD structure prohibit an exact analysis.
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55
56
57
58
59
60
Figure 6. The asymmetric units of the SC-XRD structures of
a) Bu-3 and b) Cy-3 consist of two independent molecules.
t
Some tert-butyl groups in (tBu-3)2 show disorder. (Cy-3)2 also
is disordered and the depicted structure has an occupancy of
>48%. Disorders, non-hydroxyl hydrogens and alky groups
were omitted for clarity. Ellipsoids are drawn at 50%
probability.
Figure 5. a) The two observed rotamers for R-19. Depicted
푃1
are the dimers for Cy-19 formed by b) the pure Π-form in
c) the pure Β-form in , and c) the mixed Π·Β dimer in
,
.
푃1 푃2 /푐
1
Disorders, cyclohexyl groups and non-methyl hydrogens are
omitted for clarity. Ellipsoids are drawn at 50% probability.
Alcohols and Chlorides. Alcohols R-3 and chlorides R-4 were
synthesized as described above. Common alcohols form
hydrogen bond networks in the solid state.42
Triphenylmethanol forms a dynamic tetrameric hydrogen
bond bonding arrangement43 resulting in a broad infrared
(IR) signal at 3472 cm–1.44 The all-meta substituted trityl
alcohol derivatives R-3 instead form no hydrogen bond
networks. The attenuated total reflection (ATR)-IR spectra of
the R-3 derivatives feature a sharp O–H stretching vibration
in the range of ν(O–H) = 3550–3610 cm–1 (Figure 7). These
arise from stretching vibrations of isolated hydroxyl
functions. Monomeric MeOH absorbs at 3664 cm–1, dimeric
MeOH show a band splitting that is about 150 cm–1 red-
shifted for the hydrogen bond donor.45 tBu- and Cy-3 also
show red shifts with broad bands at 3428 and 3413 cm–1,
respectively, indicating hydrogen bridged dimers. The IR
spectra are in line with the corresponding SC-XRD structures.
Figure 7. Details of the ATR-IR spectra of the R-3 derivatives.
The marked absorption bands correspond to the ν(O–H), free
(sharp) or involved in hydrogen bonding (lower
wavenumbers, broad).
푃2 /푐
Me-3 crystallizes in space group
(c.f. SI) and shows only
1
one type of isolated OH functions according to one sharp
–1
t
푃2 /푛
ν(O–H) of 3601 cm . The SC-XRD structure of Bu-3 (
;
1
Figure 6a) instead reveals two independent molecules of
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SC-XRD structures of the halogenated derivatives were
Still, the substituents most aptly gear into each other in Ad-
1
2
3
4
5
6
7
8
obtained for iPr-18, iPr-4, and Ad-4 (c.f. SI). These crystallize
as non-dimeric Π-rotamers with common geometrical
20 allowing multiple attractive interactions (Figure 8). Some
of these interactions are lost in Cy-20 due to the flatter
structure of the Cy substituents. Lastly, the numerous
geometrical constrains in Ph-20 allow only for one angled, T-
shaped or one parallel-shifted intramolecular contact. These
features agree well with the computational trends found for
R-12.21
i
features. iPr-18 and Pr-4 show the same disorder at some
iPr groups. The Ad-4 structure has C3 symmetry axis along
the C–Cl bond and displays a similar disorder in all Ad groups.
The disorder arises by twisting the adamantyl groups about
60° around the Cmeta–Cα axis. All R-4 derivatives show a
characteristic downfield chemical shift of the halogenated
versus the hydroxylated methyl carbon (Table 1). The narrow
chemical shift range of the central carbon in the series of
alcohols and chlorides reveals a negligible electronic effect,
in accordance with the very small changes in linear free-
energy relationship parameters for induction or resonance.38b
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Table 1. 13C-NMR shifts of the substituted methyl
carbon in different all-meta substituted trityl
derivatives. Chemical shifts δ measured in C6D6, given
in ppm relative to the solvent peak.
(3,5-R2Ph)3C-
OH
Cl
OO
H
R c)
R2
X
Me
iPr
Cy
82.3
83.1
83.2
83.0
83.6
84.1
57.5
58.2
58.3
94.2
94.3
210.
1
71.5
b)
tBu
83.7
84.0
82.9
84.4
85.2
82.4
95.3
94.5
58.7
59.5
58.1
1-Ad
Ph
..79.3
a)
a)Chemical shift of the bromide. b)Measured in C6D12.
Literature value 70.7 ppm.46 c)δ(Trityl cation · TfO in
CD2Cl2) = 211.4 ppm.47
Peroxides. The peroxides R-20 were obtained through the
reaction of oxygen with R-1• typically giving poorly soluble
materials. Filtration and recrystallization yielded pure R-20.
However, attempts to synthesize Me-20 via this route gave a
complex, inseparable mixture. The synthesis of Me-20 was
not pursued further as it did not form under the reaction
conditions of Me-12 synthesis. SC-XRD structures were
Figure 8. Comparison of the molecular structures (SC-XRD)
of Ad-, Cy-, and Ph-20. Right column: close-ups of the
equivalent representations of the noncovalent interaction
(NCI) plots50 of one substituent on top of the opposing
phenyl ring buried within surrounding substituents (s = 0.5
au/–0.04 < ρ < +0.04 au). Solvent molecules, disorders, and
hydrogens were omitted for clarity.
i
obtained for Pr-20, Cy-20·Et2O, Ad-20·4(C6H6), and Ph-20.
t
The SC-XRD structures of H-2048 and Bu-2049 are literature
The very different NMR and UV-Vis properties of the trityl
known.
cations (R-21) versus the neutrals allowed detailed NMR and
t
UV-Vis spectral investigations of the Bu-21 cation that is
The peroxides do not appear particularly strained, neither
between the trityl moieties nor their substituents. The torsion
angles τ(Cmethyl–O–O’–C’methyl) for R-20 were 180° throughout
and only unsymmetric Π-rotamers were found. The shapes
of the peroxide substructures were very similar within this
representative for the R-21 cation series. The UV-Vis
t
spectrum was recorded after treatment of Bu-3 with MsOH
in benzene and showed an absorption maximum at λmax
431 nm, in good agreement with time dependent (TD)-DFT
at B3LYP-D3(BJ)/cc-pVDZ(conductor-like polarizable
=
series: RC–O = 1.456(+5/–6) Å, RO–O = 1.476(+6/–7) Å, γ(Cmethyl
–
continuum model (C-PCM):Benzene) computations (414 nm,
O–O’) = 107.7(+1.1/–0.4)°; the trityl moieties are very similar.
The averaged γ(Ci–Cmethyl–Ci’) is 112.4(±4)° and deviations
from this value with increasing substituent size are randomly
scattered.
t
f = 0.47). The NMR spectrum of Bu-21 was obtained after
t
reacting Bu-3 with HBF4, TfOH or MsOH in C6D6. The
aromatic hydrogens experience a severe downfield shift from
t
t
7.28/7.04 ppm for Bu-3 to 8.27/7.56 ppm for Bu-21. The
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t
chemical shift of the tBu group was nearly unaffected. Similar
well resolved. The EPR spectrum of Bu-1 revealed mainly
hyperfine splitting due to the ortho and para hydrogens
(Figure 10c) in accordance with the literature.51 In tBu-1• no
spin coupling with the tert-butyl hydrogens was observed but
a 1H NMR signal from the latter was visible. The EPR spectra
of iPr-1•, tBu-1•, Cy-1•, and Ph-1• featured the same
superordinate splitting pattern into ten peaks. Only the EPR
spectrum of Me-1• in benzene is clearly different (Figure 10f).
All α-methyl hydrogens in Me-1• couple strongly with the
unpaired electron to give a complex multiplet. This is in line
with the assumed significant spin density at the benzylic
positions (Table 2) and explains the lack of NMR signals for
Me-1•. iPr-1• represents an intermediate species as it
features additional subtle couplings to the six methine
hydrogens. No coupling to the β-methyl hydrogens was
observed and the NMR spectrum is indicative of a radical
species. The large line width in the EPR spectra of Cy-1• and
Ph-1• prevent a detailed analysis of their spin distributions.
1
2
3
4
5
6
7
8
shift effects were observed in the 13C NMR, thereby also
indicating that meta substitution on the trityl moiety exerts
t
little electronic influence. Consequently, treatment of Bu-4
with ZnCl2 in C6D6 resulted in very similar UV-Vis spectra as
t
1
found for Bu-21 with λmax around 433 nm. The H-NMR of
tBu-4 with ZnCl2 gave two broadened peaks at 8.22 and
7.78 ppm in a 1:2 ratio. These likely arise from an equilibrium
of tBu-4 with ZnCl2 that acts as a weak Lewis acid.
9
Trityl Radical Derivatives
10
11
12
13
14
15
16
17
18
19
20
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22
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Radical generation was performed in benzene via SET from
Zn(Cu) to R-4 for the alkyl derivatives or to Ph-18.
Concentrations where kept at 22–27 mmol L–1 for NMR and
~5 mmol L–1 for EPR experiments. The poor solubility of the
1-adamantyl derivatives limited the concentration to about
1
10 mmol L–1. NMR spectroscopy of radicals is limited to H
as the signal intensity of the other nuclei is already very low
and vanishes completely due to electron spin–core spin
coupling. Very broad signals were observed in case of iPr-1•
t
(2.33 ppm), Bu-1• (1.83 ppm),21 and Cy-1• ( 0.75, 1.04 and
2.09 ppm) as shown in Figure 9. The three 1H-NMR peaks in
Cy-1• have an approx. 5:2:3 ratio. The large amounts of
byproducts upon radical generation of Ad-1• and the
1
generally broadened adamantyl H-NMR signals prevented
the unambiguous identification of Ad-1• via NMR. While an
enhancement of the broadened signals for tBu-1• and Cy-1•
was observed in CD2Cl2 (Figure 9), the reaction mixture
containing Ad-1• in CD2Cl2 showed no such effect. Neither
the samples of the methyl nor the phenyl derivative showed
broadened signals.
Even paramagnetic 1H-NMR
spectroscopy (–80 to 140 ppm; 0.1 s relaxation delay; 0.27 s
acquisition time) did not reveal signals that could be related
to these radicals (for the full spectra see SI). In all cases the
remaining major signals were identified as either R-3, R-19,
R-20, or R-21. We found no evidence for the formation of
van-der-Waals (R-1•)2 or covalent R-12 dimers in benzene.
Figure 9. 1H-NMR spectra of iPr-1•, tBu-1•, and Cy-1• in C6D6
(black lines/blue dots) and CD2Cl2 (grey lines and dots).
An explanation for the invisibility of Ph-1• and in particular
Me-1• was delivered by EPR spectroscopy and spin density
(ρspin) computations. EPR spectra of all R-1• derivatives in
benzene were recorded. tBu-1• yielded a well resolved EPR
spectrum (Figure 10c). Despite equal concentrations and
sample preparation none of the other spectra was equally
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via NMR; the α-hydrogens in Me-1• carry a seven times
1
2
3
4
5
6
7
8
i
higher spin density than Pr-1• or Cy-1•. The aromatic π-
system in Ph-1• spreads the spin density over all hydrogens
(>0.1 mμB/Å–³) and no 1H-NMR signal is observed. Thus, spin
1
densities lower than this value are necessary to observe H-
NMR signals. Similar to the small substitutent effect on the
13C NMR (Table 1) neither in the EPR (Δαmax = 0.06 G [±1%])
spectra nor the computed spin densities (Δρspin = 0.015 μB Å–3
[±2%]) show significant effects.
9
Table 2. Averaged, absolute magnitude of the
computed natural bonding orbital analysis spin
densities of the atoms at the corresponding positions
at B3LYP-D3(BJ)/cc-pVDZ. Values in 1000 · μB Å–3.
Values in italics are likely to be observed by 1H-NMR.
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12
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Positio Ato
Substituent
n
m
Me
iPr
Trityl core
Cy
tBu
Ad
Ph
central
C
H
H
556 561 547 551 531 555
3.53 3.49 3.68 3.48 3.52 3.57
3.63 3.48 3.52 3.50 3.79 3.58
Substituent
p
o
α
β
γ
δ
H
H
H
H
1.34 0.18 0.21
0.01 0.05 0.01 0.02 0.18
0.02
0.00
0.04 0.10
0.01 0.20
Figure 11. Representation of the computed all-meta
cyclohexyl trityl radical [B3LYP-D3(BJ)/cc-pVDZ] carbon atom
spin densities colored in blue (the more intense color
indicates higher spin density). Hydrogens are omitted for
clarity.
Figure 10. EPR spectra of a) Me-1•, b) iPr-1•, c) tBu-1•, d) Cy-
1•, and e) Ph-1• in benzene with c(R-1•) ~ 5 mmol L–1.
Computations of the radicals at the B3LYP-D3(BJ)/cc-pVDZ
level of theory are instructive (Table 2). The more remote the
position of the hydrogens in the substituent regarding the
trityl scaffold, the more likely it is to observe a 1H-NMR signal
The benzene solutions containing Me-, iPr-, tBu-, Cy-, and Ad-
1• appear orange, while the solution containing Ph-1• is red.
UV-Vis spectra of the reaction mixtures containing alkyl R-1•
show an absorption around 433 nm (Figure 12). The UV-Vis
spectrum of the reaction mixture of tBu-1• matches with the
1
(Figure 11). As a reminder, we observed H NMR peaks for
iPr-1•, tBu-1•, and Cy-1• but not for Me-1• and Ph-1•. α-Alkyl
hydrogens carry a ρspin > 0.2 mμB/Å–³ and are not observable
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spectrum of the corresponding cation. Nonetheless, cationic
PCM:CyH)//B3LYP-D3(BJ)/cc-pVDZ(C-PCM:CyH) to reduce
1
2
3
4
5
6
7
8
R-21 was observed via NMR only in case of the reaction
mixture containing Ad-1•. The color of the R-1• reaction
mixtures must therefore emerge from very low
concentrations of R-21. The small change in λmax over the
series of alkyl derivatives in comparison to the parent H-21
(λmax = 432 nm) emphasize the negligible influence of alkyl
substitution on the electronic properties of the trityl
scaffold.52
the BSSE and to allow a more precise assessment of the
relative stabilities of R-12, (R-1•)2, and R-22 (Table 3, Figure
13b). These results agree with experiment for the free
dissociation energy of tBu-12 within 0.2 kcal mol–1. Previously
LD was estimated by comparison of the dispersion corrected
with the uncorrected DFT computations [Edisp = De(B3LYP-
D3(BJ))– De(B3LYP)]. The alternative way employed here is
the reduction of LD upon dissociation, obtained as Edisp
=
9
2 · D3(BJ)[R-1•] – D3(BJ)[R-12]. The magnitude of the D3(BJ)
dispersion correction depends on the functional. The D3(BJ)
values utilizing the B3LYP functional combination compare
reasonably well to more rigorously defined Edisp, e.g., from
symmetry adapted perturbation theory (SAPT).
Time
dependent
(TD)-B3LYP-D3(BJ)/cc-pVDZ
(C-
10
11
12
13
14
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PCM:benzene) computations of tBu-1• give the main
absorption at 361 nm (f = 0.29). This is 52 nm blue-shifted as
t
compared to the most intense absorption of Bu-21 that is
about twice as strong. Longer reaction times in low
t
concentration reaction mixtures of Bu-4 with Zn(Cu) in
These computations show that R-22 contributes only in the
case of the parent triphenylmethyl H-22. Substitution with
Me leads to an overall destabilization in favor of free Me-1•.
Further increase in bulkyness renders iPr-12 and tBu-12 almost
isoenergetic with their monomers R-1•. ΔGd298[(tBu-1•)2
tBu-12] is –1.8 kcal mol–1, in perfect agreement with previous
experiments. R-12 is stabilized by substitution with the larger
Cy group, for which Cy-12 is the most stable form:
ΔGd298[(1•)212] = 1.5 kcal mol–1. Ultimately, Ad-12 is 11.8
kcal mol–1 [ΔGd298[(1•)212] more stable than any other
arrangement. As (Ad-1•)2 displays no symmetry, we were
unable to compute vibrational frequencies at a meaningful
benzene showed a UV-Vis spectrum with a λmax = 352 nm in
good agreement with the computed TD-spectrum of tBu-1•.
This is also in line with experimental and computational UV-
Vis spectrum of the unsubstituted trityl radical (λmax = 338–
340 nm).53 Other uncharged closed-shell trityl derivatives
absorb close to or below the absorption edge of benzene
(e.g., the Jacobsen-Nauta structure: λmax = 315 nm54) and do
not contribute to the UV-Vis spectra in benzene.
298
level of theory and its ΔGd was therefore estimated from
the single point energy of (Ad-1•)2 and the ΔG correction
from Ad-12. (Ph-1•)2 should solely exist as a single species in
the reaction mixture. R-1• and (R-1•)2 are indistinguishable
via NMR but it is plausible to assume based on the
computational analysis solely an equilibrium between R-12
and the energetically next lowest species, which is R-1• for
Me and iPr and (R-1•)2 for all other derivatives.
298
Table 3. Computed ΔGd
and Edisp values for the
dissociation of all-meta substituted trityl radical
complexes (R-1•)2, hexaphenylethanes R-12 and
Jacobsen-Nauta structures R-22 at the B3LYP-
D3(BJ)/def2-TZVPP(C-PCM:CyH)//B3LYP-D3(BJ)/ cc-
pVDZ(C-PCM:CyH) level of theory. All energies in kcal
mol–1.
Figure 12. UV-Vis spectra of the reaction mixtures containing
R-1• in benzene. λmax = 435 (Me); 434 (iPr); 438 (Cy); 432 (tBu);
440 (Ad); 504, 371 (Ph) nm.
Hexaphenylethane Derivatives
t
The SET to Bu-4 in cyclohexane-d12 also produced a bright
αa)
(R-1•)2
R-12
R-22
orange solution; the reaction is slower than in benzene.21
A
1H NMR spectrum of this solution featured the radical signals
as well as a set of five new peaks. These could be assigned
to all-meta tert-butyl hexaphenylethane Bu-12. A variable
temperature (VT)-NMR/van ‘t Hoff analysis revealed a
dissociation energy of ΔGd298 (exp.)= –1.6 kcal mol–1, in good
agreement with theoretical predictions.13
298
29
298
R = [ų] ΔGd
Edis ΔGd
p
Edisp ΔGd
Edisp
8
25.
4
–
0.9
–1.1
–5.2
4.3
–12.6 32.2
16.8
H
0.8
2.6
6.2
8.0
10.0b)
37.
5
–14.1 42.7
0.5 64.4
2.5 70.9
16.4 87.0
–20.1 29.4
–27.7 52.1
–25.5 60.9
–4.3 65.4
9
Me
iPr
Subsequent computations predicted increasing stability in
the series of R-12 due to increasing LD,21 but these
computations quantitatively differ from experiment as they
neither take the equilibrium with a vdW dimer13 (R-1•)2 nor
54.
7
42.
7
tBu
Cy
the Jacobsen-Nauta-structure (R-22; Figure 13a) into
55
account.54a,
performed
An improved computational analysis was
here at B3LYP-D3(BJ)/def2-TZVPP(C-
67.
2
10.
8
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share one spin system according to correlation spectroscopy
(COSY) NMR experiments, and their intensities show a strong,
reversible temperature dependency (Figure 14). The
solubility of all all-meta 1-adamantyl trityl derivatives in C6D12
is very low. It was therefore only possible to record 1H NMR
spectra of the reaction mixture of Ad-12 in C6D12. Other
solvents like CD2Cl2 or toluene-d8 did not show similar peaks
even at temperatures as low as –65 °C.
96.
0
108.
10.
0
25.6
6.9
–4.0 72.3
Ph
Ad
1
2
3
4
5
6
7
8
6
16.6c
96.
3
114.
8
105.
15.
9
28.4
5.0
5
)
a)Isotropic polarizability of the corresponding
hydrogen capped substituent R–H.56 b)The exptl.
298
ΔGd
= 4.7 kcal mol–1 determined via EPR is
presumably too positive due to uncertainties through
invisibility of closed-shell decomposition products.57
c)ΔGd298 was calculated from the single point energy of
(Ad-1•)2 and the ΔG correction from Ad-12.
9
10
11
12
13
14
15
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17
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22
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41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Figure 13. a) Relations between R-12, (R-1•)2, R-1•,and R-22.
b) Computed free dissociation energies ΔGd
298
at B3LYP-
D3(BJ)/def2-TZVPP(C-PCM:CyH)//B3LYP-D3(BJ)/cc-pVDZ(C-
PCM:CyH) of (R-1•)2, R-12, and R-22 relative to 2 R-1•. The
circle diameter correlate with Edisp
.
Due to the unpredictability of solvent effects and the many
challenges associated with solubility of our substrates and
their dissociation products, we decided to investigate the
already available radicals in cyclohexane. The reaction
1
t
Figure 14. H VT-NMR of a) Bu-12 and b) Ad-12 featuring
three unique signals for the three nonequivalent, aromatic
hydrogens as described for tBu-12.21
i
t
mixtures containing Pr-, Bu-, Cy-, and Ad-1• were orange
and became increasingly cloudy with increasing molecular
weight; after filtration clear solutions were obtained. The
intensity of the orange color of these solutions decreased
with molecular weight until an almost colorless solution was
obtained for Ad-1•. This color trend is indicative (although
not necessarily connected) of the decrease of signal intensity
of the radicals R-1• in NMR – from a strong signal for iPr-1•
to no signal for Ad-1•.
Table 4. 1H-NMR chemical shifts in C6D12 from
experimental and computed spectra for tBu- and Ad-
12. Chemical shifts were computed with GIAO
B3LYP/6-31G(d,p)(C-PCM:CyH) and were averaged
per position. Values of tBu-12 are taken from21.
H-position:
oin
p
oout
Rin
Rout
For Ad-1• we were able to assign three aromatic signals at
7.67, 7.09, and 5.95 ppm in the H NMR spectrum to Ad-12.
The chemical shifts agree well with gauge-independent
atomic orbital (GIAO) B3LYP/6-31G(d,p)(C-PCM:CyH)
computations (Table 4). The offset between computational
and experimental chemical shifts is very similar to the offset
for tBu-12.21 The three triplets (4J = 1.7 Hz) have a 1:1:1 ratio,
7.7
5
7.1
9
6.0
2
exp.
0.98
0.95
1
tBu-12
com
p.
8.1
7
7.5
3
6.2
6
1.07
n. a.
1.01
n. a.
7.6
6
7.0
9
5.9
5
Ad-12
exp.
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Journal of the American Chemical Society
R-12 was not observable in any solvent other than
com
p.
7.9
9
7.5
5
6.0
1
0.98–
2.24
0.97–
2.23
1
2
3
4
5
6
7
8
9
cyclohexane. This demonstrates the large influence of the
solvent on the stability of these compounds.5g The solvent
influence is acceptably described by C-PCM for the effect of
non polar solvents on the electronic energy but poorly for the
entropy. On top, there are multiple conformers found within
1 kcal mol–1 by force field conformer searches for R-12. These
could not be treated adequately due to the immense
computational cost. The computations therefore do not
account thoroughly for conformational flexibility and solvent
dynamics. This manifests in a counterintuitive order of
The obscured NMR signals of Ad-1• prevent the direct use of
a van ‘t Hoff plot. Based on the law of mass action, the law
of mass conservation, and the strict proportionality between
concentration and NMR signal intensity we established a
mathematical solution (for the deviation see SI) solely based
on the temperature dependency of the NMR signal intensity
퐼
ퟏ of R-12:
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
2
푒 ―풂
298
i
푇
computed entropies ΔSd with lower values for Pr and Cy
[68 and 74 cal mol–1 K–1] than for Me, tBu, Ad, and Ph [79, 82,
89, and 93 cal mol–1 K–1], respectively.
1 ― 1 + 16퐵0푒풂
푇
퐼ퟏ (푇) = 퐼0 1 +
(
)
(1)
2
[
]
8퐵0
퐼
0
with being the signal intensity at T = 0 K, B0 = K0/C0 the
equilibrium constant K0 at T = 0 K scaled by the initial
concentration C0 of R-12, and
dissociation and the ideal gas constant . A multi-parameter
Solubility proves as a decisive factor that is not taken into
account properly by the computations. The most promising
candidate Ad-12 is almost too poorly soluble to be examined
by NMR while all smaller alkyl derivatives R-1 were readily
soluble. The all-meta phenyl trityl scaffold (Ph-1• etc.) was
not sufficiently soluble to be examined in cyclohexane.
∆퐻298
풂 =
푅
푑
, the enthalpy of
푅
퐼0
fit to the recorded VT-NMR data directly provide
and
ΔHd298. ΔSd298 is then indirectly accessible via the van ‘t Hoff
plot and gives access to the free dissociation energy via
ΔGd298 = ΔHd298 – TΔSd
.
298
Conclusions
As a validation, fitting of the tBu-12 VT-NMR intensities of the
We present the synthesis and spectroscopic as well as
computational analysis of a series of all-meta substituted
triphenylmethyl radicals R-1• with R = Me, iPr, tBu, Cy, and Ph,
including the very challenging 1-adamantyl derivative.
Beyond the two known, unbridged hexaphenylethane
equilibrium between tBu-12 and Bu-1• to equation (1) yields
298
ΔHd
= 9.2(4) kcal mol–1, only 1.3 kcal mol–1 higher than
determined via the classic van ‘t Hoff approach.21 The a
posteriori application of the van ‘t Hoff plot with the observed
298
298
= 30.0(3) cal mol–1 K–1. In sum a ΔGd
of
퐼
0 reveal ΔSd
derivatives
tBu-12
and
hexa(2,6-di-tert-butyl-4-
0.3(5) kcal mol–1 is obtained that is well in line with previous
findings.
biphenylyl)ethane46 we were able to observe one new HPE
derivative, namely Ad-12.
Processing of the Ad-12 VT-NMR intensities in the same
En route to the all-meta substituted triphenylmethyl radicals
we also synthesized the hydrocarbon- (R-19), alcohol- (R-3),
halide- (R-4/R-18), peroxo- (R-20) and carbocation (R-21)
derivatives; several of their structural features are also
affected by LD interactions. Two rotamers were found for R-
19 in the SC-XRD, while only the Β-type was identified for all
other triphenylmethyl (TPM) species. The Β-type rotamers of
Me-, tBu-, and Cy-19 form head-to-head dimers that engage
in short H···H contacts between the central C–H groups with
the shortest being for tBu.3w Similarly, for tBu- and Cy-3 we
found head-to-head dimers in the SC-XRD, whose formation
is supported by red shifted O–H stretching band in the ATR-
IR spectra. Peroxides R-20 show no obvious structural
evidence for particular strain, not even for Ad-20. In contrast,
a better structural fit between bulky (spherical) than between
small (or “flat”) substituents indicates stronger interactions
298
manner gives ΔHd
= 10.5(5) kcal mol–1. Together with
ΔSd = 28.3(3) cal mol–1 K–1 from the van’t Hoff treatment.
298
298
That is, this HPE derivative is slightly more stable with ΔGd
= 2.1(6) kcal mol–1, in qualitative agreement with our earlier
theoretical predictions.21 The strong, reversible temperature
dependence of the NMR intensities is unexpected as Ad-12
should not take part in an observable equilibrium with Ad-1•
according to the computed ΔGd298, which is about a
magnitude higher than the experimental value.
For iPr- and Cy-1• variable temperature NMR was unrevealing
as the concentrations of these species apparently are too low
to be observable via NMR. This threshold is estimated as a
R-1•/R-12 ratio higher than 99.9:0.01 which corresponds to
c(R-12) < 0.025 mmol L–1. These derivatives therefore have a
298
ΔGd
below –4.5 kcal mol–1. The 12-fold interactions
t
i
between the substituents amplify the pairwise effect and
within the Ad and Bu than within the Pr, Cy, and Ph
derivatives. This provides hints as to select and utilize
dispersion energy donors in the design of structures and
catalysts.
298
cause
a
steep
increase
in
ΔGd
with
polarizability/substituent size that readily transcend the
narrow energy regime where an equilibrium can be observed.
Only Bu- and Ad-12 are stabilized enough through their
t
NMR spectroscopy of the series of R-3, R-4, R-19, and R-20
show only a narrow range of chemical shifts with a downfield
trend with increasing substituent size – a hint for increasing
radical stabilization. Similarly, UV-Vis spectroscopy of R-21
show a small red shift in λmax indicating a slight increase of
the HOMO-LUMO gap and stabilization of the carbocations.
substituents to be observable. It is therefore evident that
rigid and spherical tBu and 1-Ad involve significantly stronger
dispersion interactions in solution than flexible and flat iPr or
Cy. This contradicts our computations where both iPr-, tBu-,
and Cy -12 similarly have a ΔGd of about 0 kcal mol–1 but
298
Ad-12 is more stable by one order of magnitude.
11
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Journal of the American Chemical Society
Page 12 of 15
EPR spectroscopy and ρspin computations for R-1• also show
only little changes in the series from Me to Ad but a slight
trend for radical stabilization with increasing substituent size.
These small spectroscopic changes reveal, in accord with
Liebig University Giessen) for support with NMR
1
2
3
4
5
6
7
8
spectroscopy, Astrid Spielmeyer (Justus Liebig University
Giessen) with mass spectrometry, Olaf Burghaus (Philipps-
University Marburg) with EPR and Christian Logemann (Justus
Liebig University Giessen), Victor G. Young Jr., Ph.D.
(University of Minnesota) as well as Frank Hampel (Friedrich-
Alexander University Erlangen-Nürnberg) with SC-XRD
structure elucidation. This work was supported by the Priority
Program “Dispersion” (www.spp1807.de) of the Deutsche
Forschungsgemeinschaft.
linear
free
energy
relationship
parameters
for
a
induction/resonance38b and radical stabilization,58
negligible electronic effect on the trityl scaffold, thereby
slightly favoring (R-1•)2 or R-1• over R-12. Hence, changes in
the stability of R-12 must predominantly be governed by
changes in London dispersion.
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Our conceptually improved computational analysis takes a
possible equilibrium between R-12, (R-1•)2, R-1•, and R-22
into account and agrees well with experiment: Only parent H-
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ASSOCIATED CONTENT
Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website. The SI contains computational and
experimental procedures and original spectra as well as XYZ
coordinates for all computed species, Mathematica outputs
as well as SC-XRD data as PDF file. Crystallographic data were
deposited at the Cambridge Crystallographic Data Centre as
CCDC 1851902-1851919 and 1852233.
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AUTHOR INFORMATION
Corresponding Author
* prs@uni-giessen.de
Notes
The authors declare no competing financial interests.
ACKNOWLEDGMENT
We thank Sebastian Ahles (Justus Liebig University Giessen)
for very valuable discussions and Andrey A. Fokin (Igor
Sikorsky Kyiv Polytechnic Institute and Justus Liebig
University Giessen) for instructive questions. We thank
Vladyslav Bakhonskyi and Finn Wilming (both Justus Liebig
University Giessen) for their preliminary synthetic
contributions. We acknowledge Heike Hausmann (Justus
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