Onset of three-centre, four-electron bonding in peri-substituted acenaphthenes: A structural and computational investigation

Article abstract of DOI:10.1039/c1dt11697e

Two series of sterically crowded peri-substituted acenaphthenes have been prepared, containing mixed halogen-chalcogen functionalities at the 5,6-positions in A1-A6 (Acenap[X][EPh] (Acenap = acenaphthene-5,6-diyl; X = Br, I; E = S, Se, Te) and chalcogen-chalcogen moieties in A7-A12 (Acenap[EPh][E′Ph] (Acenap = acenaphthene-5,6-diyl; E/E′ = S, Se, Te). The related dihalide compounds A13-A16 Acenap[XX′] (XX′ = BrBr, II, IBr, ClCl) have also been prepared. Distortion of the acenaphthene framework away from the ideal was studied as a function of the steric bulk of the interacting halogen and chalcogen atoms occupying the peri-positions. The acenaphthene series experiences a general increase in peri-separation for molecules accommodating heavier congeners and maps the trends observed previously for the analogous naphthalene compounds N1-N12 (Nap[X][EPh], Nap[EPh][E′Ph] (X = Br, I; E/E′ = S, Se, Te). The conformation of the aromatic ring systems and subsequent location of p-type lone-pairs dominates the geometry of the peri-region. The differences in peri-separations observed for compounds adopting differing conformations of the peri-substituted phenyl group can be correlated to the ability of the frontier orbitals of the halogen or chalcogen atoms to take part in attractive or repulsive interactions. Density-functional studies have confirmed these interactions and suggested the onset of formation of three-centre, four-electron bonding under appropriate geometric conditions. The Royal Society of Chemistry 2012.

Full text of DOI:10.1039/c1dt11697e

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An international journal of inorganic chemistry
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Volume 41 | Number 11 | 21 March 2012 | Pages 3097–3340
ISSN 1477-9226
COVER ARTICLE
Woollins et al.
Onset of three-centre, four-electron bonding in peri-substituted
acenaphthenes: A structural and computational investigation
1477-9226(2012)41:11;1-W

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PAPER
Onset of three-centre, four-electron bonding in peri-substituted
acenaphthenes: A structural and computational investigation†
Lara K. Aschenbach, Fergus R. Knight, Rebecca A. M. Randall, David B. Cordes, Alex Baggott,
Michael Bühl, Alexandra M. Z. Slawin and J. Derek Woollins*
Received 7th September 2011, Accepted 14th October 2011
DOI: 10.1039/c1dt11697e
Two series of sterically crowded peri-substituted acenaphthenes have been prepared, containing mixed
halogen-chalcogen functionalities at the 5,6-positions in A1–A6 (Acenap[X][EPh] (Acenap =
acenaphthene-5,6-diyl; X = Br, I; E = S, Se, Te) and chalcogen-chalcogen moieties in A7–A12 (Acenap
[EPh][E′Ph] (Acenap = acenaphthene-5,6-diyl; E/E′ = S, Se, Te). The related dihalide compounds A13–
A16 Acenap[XX′] (XX′ = BrBr, II, IBr, ClCl) have also been prepared. Distortion of the acenaphthene
framework away from the ideal was studied as a function of the steric bulk of the interacting halogen and
chalcogen atoms occupying the peri-positions. The acenaphthene series experiences a general increase in
peri-separation for molecules accommodating heavier congeners and maps the trends observed previously
for the analogous naphthalene compounds N1–N12 (Nap[X][EPh], Nap[EPh][E′Ph] (X = Br, I; E/E′ = S,
Se, Te). The conformation of the aromatic ring systems and subsequent location of p-type lone-pairs
dominates the geometry of the peri-region. The differences in peri-separations observed for compounds
adopting differing conformations of the peri-substituted phenyl group can be correlated to the ability of
the frontier orbitals of the halogen or chalcogen atoms to take part in attractive or repulsive interactions.
Density-functional studies have confirmed these interactions and suggested the onset of formation of
three-centre, four-electron bonding under appropriate geometric conditions.
in many areas of research.2,5,11 They affect thermodynamic
stability, molecular geometry, self-assembly, crystal packing,
reactivity and biological activity and as such have implications
in drug design, supramolecular chemistry, materials and
nanoscience.1,2,5,11,12
Weak interactions acting between bulky heteroatoms con-
strained in unavoidably congested environments have attracted
great attention over the last decade.13,14 Sterically restricted
systems incorporating heteroatoms at distances shorter than the
sum of van der Waals radii, suffer steric compression arising
from non-bonded interactions as a result of direct overlap of
orbitals.14 Such bonding situations can be accomplished by posi-
tioning large heteroatoms of Groups 16 and 17 at suitable
locations in a rigid organic framework.13,14
Introduction
Understanding how atoms interact is a fundamental aspect of
chemistry. Whilst there have been great advances in the knowl-
edge of covalent and ionic bonding, one of the major challenges
is to develop a full understanding of non-covalent interactions.1
These are weak non-bonded forces, acting between adjacent but
unbound atoms within (intramolecular) or between molecules
(intermolecular) and can be attractive or repulsive in nature.1–4
A wide range of interactions have emerged of which hydrogen
bonding, ion-ion interactions, van der Waals forces, ion-dipole
interactions, π–π stacking and dipole–dipole interactions are the
most noteworthy.2,5 Hydrogen bonding (NH⋯O, OH⋯O) is the
principal noncovalent force,5,6 though less common interactions
have emerged such as CH⋯O5,7 and CH⋯π5,8 (non-convention-
al hydrogen bonding) and short van der Waals forces between
halogen and chalcogen congeners.2,5,9,10
Naphthalene15 and related 1,2-dihydroacenaphthylene (ace-
naphthene)16 are polycyclic aromatic hydrocarbons which offer
scaffolds with which to study non-bonded intramolecular inter-
actions. Geometric constraints unique to these frameworks are
imposed by a double substitution of atoms or groups at the peri-
positions (positions 1 and 8 of the naphthalene ring, 5 and 6 of
acenaphthene).15,16 The characteristic naphthalene carbon skel-
eton is perfectly planar with all internal angles ca. 120° inducing
a parallel alignment of the exocyclic bonds which are separated
by a peri-distance of ca. 2.5 Å.15 The introduction of the ethyl-
ene linker at the 1,2-positions of acenaphthene results in a minor
reduction in the planarity of the carbon ring system.16 As a con-
sequence, the internal angles of acenaphthene, unlike the
Focused investigations of X-ray crystal-structure data have
highlighted the importance of weak non-covalent interactions
School of Chemistry, University of St Andrews, St Andrews, Fife, U.K..
E-mail: jdw3@st-andrews.ac.uk; Fax: (+44) 1334 463384; Tel: (+44)
1334 463861
†Electronic supplementary information (ESI) available: Full experimen-
tal details, computaional methods and tables, X-ray crystal structure
data. CCDC reference numbers 816080–816094. For ESI and crystallo-
graphic data in CIF or other electronic format see DOI: 10.1039/
c1dt11697e
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Dalton Trans., 2012, 41, 3141–3153 | 3141

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naphthalene geometry, are unequal in magnitude and range from
111–127°. The most acute angle lies closest to the ethylene
bridge (C1–C1a–C2, Fig. 1) with a greater splay observed nearer
to the peri-region (C5–C5a–C6, Fig. 1). The peri-bonds are sub-
sequently divergent with atoms tilted in opposite directions, thus
increasing the “ideal” peri-distance in acenaphthene to ca. 2.7 Å
compared with naphthalene.15,16
naphthalene backbone. Our early work focused on dichalcogen-
ide ligands27 and unusual phosphorus compounds.28 More
recently we have reported the synthesis and structural study of
chalcogen-chalcogen,29,30 halogen-chalcogen31,32 and mixed-
donor phosphorus–chalcogen33 naphthalene systems.
Herein we report the synthesis and a comprehensive structural
study of two acenaphthene series containing mixed halogen-
chalcogen functionalities at the 5,6-positions in A1–A6 (Acenap
[X][EPh] (Acenap = acenaphthene-5,6-diyl; X = Br, I; E = S,
Se, Te) and chalcogen-chalcogen moieties in A7–A12 (Acenap
[EPh][E′Ph] (Acenap = acenaphthene-5,6-diyl; E/E′ = S, Se, Te).
The preparation and structural study of five halide compounds
A13–A16 Acenap[XX′] (XX′ = BrBr, II, IBr, ClCl) is also dis-
cussed. The molecular structures of acenaphthenes A1–A16 are
compared with the structures for the analogous naphthalene
compounds N1–N12,29,31 previously reported by us and N1334
and N1435 (see Fig. 2).
Fig. 1 The geometry of ortho-substitution in benzenes, peri-substi-
tution in acenaphthenes and naphthalenes and bay-region disubstitution
in phenanthrene. The preferred method of achieving a relaxed geometry
for each motif is shown at the bottom.15,16,20,21
Whilst naphthalene and acenaphthene peri-distances are suffi-
cient for accommodating two hydrogen atoms (sum of the van
der Waals radii (ΣrvdW) of two hydrogen atoms = 2.18 Å),17 it is
conceivable that larger substituents constrained by the organic
backbones would experience considerable steric hindrance.18
Repulsive interactions between the two peri-functionalities,
resulting from sub-van der Waals contacts, leads to deformation
of the carbon frameworks away from their natural geometry via
in-plane and out-of-plane distortions and buckling of the aro-
matic ring system (angular strain).14,18,19 Strain relief can also be
achieved through attractive intramolecular interactions between
peri-substituents due to the presence of weak or strong bonding,
thus relaxing the geometry of the backbone.14,18 It is this
relationship between attraction and repulsion, bonding and non-
bonding, occurring within sub-van der Waals distances, which
gives peri-substituted compounds their unusual reactivity, struc-
ture and bonding.14–16,18,19
The distinguishing features of peri-substitution are highlighted
when comparing the proximity effects of ortho-substitution in
benzenes20 and bay-substitution in phenanthrenes.21 Whilst
ortho- and peri-substituted species are adept at forming com-
pounds with a bridging geometry,22,23 the closer proximity of
the interacting moieties in peri-substituted frameworks allows a
relaxed geometry to form via the formation of a direct bond
between the two peri-atoms (Fig. 1).24 Although the bay-substi-
tuted conformation of phenanthrenes positions the interacting
atoms at even closer distances (ca. 2.0 Å),17,21 the difficult
nature of inserting two bulky substituents on the bay atoms,
limits their chemistry with single-atom bridged species affording
the preferred relaxed geometry.25
Fig. 2 Sterically crowded peri-substituted acenaphthenes A1–A17 and
naphthalenes N1–N14.29,31
Results and discussion
Whilst the chemistry of sterically crowded 1,8-disubstituted
naphthalenes has been well investigated,14,18,26 peri-substituted
acenaphthenes have received much less attention. During our
initial investigations of sterically crowded systems we utilised
the special peri-substituted geometry and stability of the rigid
Compounds A1–A16 were synthesised and crystal structures
were determined for A1–A12, A14–A16. The series of 5-bromo-
6-(phenylchalogeno)acenaphthenes A1–A3 was prepared fol-
lowing a similar procedure to that used previously in the
3142 | Dalton Trans., 2012, 41, 3141–3153
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preparation of analogous naphthalene derivatives N1–N331 and
utilising stepwise halogen-lithium exchange reactions of 5,6-
dibromoacenaphthene A13, as shown in Scheme 1.
Scheme 2 The preparation of 5-iodo-6-bromoacenaphthene A15: (i)
nBuLi (1 equiv), Et2O, −78 °C, 1 h; (ii) I2 (1 equiv), Et2O, −10–0 °C,
2 h.
The 5,6-dilithioacenaphthene·2TMEDA complex A18 was
independently treated with two equivalents of the respective
diphenyl dichalcogenide (diphenyl disulfide, diphenyl disele-
nide, diphenyl ditelluride) to afford the series of 5,6-bis(phenyl-
chalcogeno)acenaphthenes A7–A9 [yield: 54 (A7), 52 (A8),
55% (A9); Scheme 3].
Scheme 1 Synthesis of 5-halo-6-(phenylchalcogeno)acenaphthenes
A1–A3 from A13 and A4–A6 from A14: (i) nBuLi (1 equiv), Et2O,
−78 °C, 1 h; (ii) PhEEPh (1 equiv; E = S, Se, Te), Et2O, −78 °C, 1 h.
For their synthesis, A13 was initially treated with a single
equivalent of n-butyllithium in diethyl ether to afford the precur-
sor 5-bromo-6-lithioacenaphthene, which when treated with the
respective dichalcogenide (diphenyl disulfide, diphenyl disele-
nide, diphenyl ditelluride) consequently afforded A1–A3 [yield:
87 (A1), 66 (A2), 53% (A3); Scheme 1].31 The analogous 5-
iodo-6-(phenylchalogeno)acenaphthene derivatives A4–A6 were
synthesised from corresponding halogen-lithium exchange reac-
tions of 5,6-diiodoacenaphthene A14, similar to the procedure
used in the preparation of naphthalene derivatives N4–N6,31 but
in moderately low yields compared to the synthesis of A1–A3.
A14 reacts independently with a single equivalent of n-butyl-
lithium followed by the respective dichalcogenide to afford A4–
A6 [yield: 55 (A4), 30 (A5), 22% (A6); Scheme 1].
The two iodo-substituted compounds, A4 and A5, could also
be prepared in higher yields via the sequential halogen-lithium
exchange reactions of 5-bromo-6-(phenylsulfanyl)acenaphthene
A1 and 5-bromo-6-(phenylselanyl)acenaphthene A2; treatment
with a single equivalent of n-butyllithium followed by a single
equivalent of diiodine afforded the respective 5-iodo-6-(phenyl-
chalogeno)acenaphthene derivative [yield: 76 (A4), 60% (A5)].
This route avoids the low yielding preparation of 5,6-iodoace-
naphthene A14 (∼33%)36 and affords syntheses in three to four
times higher yield starting from A13.
The preparation of A6 was also attempted using a similar pro-
cedure, starting from 5-bromo-6-(phenyltelluro)acenaphthene
A3 and reacting sequentially with single equivalents of n-butyl-
lithium and diiodine. This was unsuccessful, since A3 is more
susceptible to attack by n-butyllithium at tellurium rather than
bromine, and facile cleavage of the Te–CAcenap bond occurs;29
chalcogen-lithium exchange transpires resulting in the formation
of the precursor 5-lithio-6-bromoacenaphthene. Following the
addition of diiodine, a lithium-halogen exchange reaction takes
place to afford 5-iodo-6-bromoacenaphthene A15 [yield: 17%;
Scheme 2].
Scheme 3 The
preparation
of
5,6-bis(phenylchalcogeno)ace-
naphthenes A7–A9 via 5,6-dilithioacenaphthene-TMEDA complex A18:
(i) N-bromosuccinimide (NBS), N,N-dimethylformamide (DMF),
0–15 °C, 2 h; (ii) EtOH, reflux, 12 h; (iii) Et2O, TMEDA, nBuLi (drop-
wise), −10-0 °C, 1 h; (iv) PhEEPh (2 equiv; E = S, Se, Te), Et2O,
−78 °C, 1 h.36
A7 and A8 were also prepared by further reaction of 5-bromo-
6-(phenylsulfanyl)acenaphthene A1 and 5-bromo-6-(phenylsela-
nyl)acenaphthene A2 with single equivalents of n-butyllithium
and dichalcogenide [yield: 69 (A7), 60% (A8); Scheme 4]. This
was a higher yielding route for the preparation of A7, whereas
the higher yielding route to A8 proceeds via the TMEDA
complex intermediate. Due to the susceptibility of the tellurium
moiety towards n-butyllithium, the corresponding reaction of A3
to form A9 was not attempted. The analogous reaction of iodo-
derivatives A4 and A5 afforded A7 and A8 in lower yield.
The chemistry of 5,6-dilithioacenaphthene, like that of 1,8-
dilithionaphthalene, merits special interest due to the presence of
two reaction sites located at the peri-positions of the organic fra-
mework. Synthetic conditions for the preparation of 5,6-
dilithioacenaphthene developed, utilising significantly milder
conditions and generating a N,N,N′,N′-tetramethyl-1,2-ethanedia-
mine (TMEDA) complex A18,36 comparable to that of the
naphthalene analogue.37
Scheme 4 The
preparation
of
5,6-bis(phenylchalcogeno)ace-
naphthenes A7 and A8 via A1 and A2: (i) nBuLi (1 equiv), Et2O,
−78 °C, 1 h; (ii) PhEEPh (1 equiv; E = S, Se), Et2O, −78 °C, 1 h.
The mixed chalcogen compounds were prepared by further
reaction of the singly substituted 5-halo-6-(phenylchalcogeno)
acenaphthene precursors A1, A2, A4 and A5; employing the
characteristic halogen-lithium exchange procedure afforded three
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Table 1 77Se and 125Te NMR spectroscopic data29,31
peri-atoms
Br, Se
Br, Te
I, Se
I, Te
Se, Se
Te, Te
Se, S
Te, S
Te, Se
A2
423.7
—
N2
447.8
—
A3
—
696.0
N3
—
A5
400.9
—
N5
430.8
—
A6
—
662.0
N6
—
A8
408.3
—
N8
428.6
—
A9
—
585.9
N9
—
A10
433.7
—
N10
455.3
—
A11
—
689.4
N11
—
A12
340.7
663.4 (J = −715.6)
N12
362.8
687.6 (J = −834.0)
Acenaphthene
Naphthalene
77Se NMR
125Te NMR
77Se NMR
125Te NMR
731.2
698.3
619.7
715.2
aAll spectra run in CDCl3; δ (ppm), J (Hz).
mixed chalcogen compounds A10–A12 (Scheme 5), analogous
with the formation of naphthalene derivatives N10–N12.29
interatomic distances, angles and torsion angles are listed in
Table 2. Further crystallographic information can be found in
Figures S6–S8 and Tables S5–S11 in the Electronic Supporting
Information (ESI)†.
The molecular structures of acenaphthenes A1–A12 exhibit
both similarities and differences to the previously reported
naphthalene analogues N1–N12.29,31 The introduction of the
ethylene linker on the acenaphthene organic framework reduces
the planarity of the carbon skeleton and naturally increases the
splay of the exocyclic bonds,16 although the resulting peri-separ-
ation of ca. 2.7 Å16 is still insufficient to fully accommodate the
bulk of the halogen and chalcogen functional groups in A1–
A12. Nevertheless, the minor reduction in repulsive interactions
operating between the acenaphthene peri-functionalities, as a
result of the diverging exocyclic bonds,16 means deviation from
the ideal acenaphthene geometry16 is less than the deformation
Scheme 5 The preparation of mixed (phenylchalcogeno) derivatives
A10–A12: (i) nBuLi (1 equiv), Et2O, −78 °C, 1 h; (ii) PhEEPh (1
equiv; E = S, Se, Te), Et2O, −78 °C, 1 h.
Following lithiation, A1 and A4 reacted with diphenyl disele-
nide to afford 1-(phenylselanyl)-6-(phenylsulfanyl)ace-
naphthene A10 [yields of 37 and 21%, respectively]. A10 could
also be prepared by the corresponding reactions of A2 and A5
with diphenyl disulfide in yields of 36 and 52%, respectively. In
a complementary set of reactions, A1 and A4 reacted with diphe-
nyl ditelluride to afford 1-(phenyltelluro)-6-(phenylsulfanyl)ace-
naphthene A11 and equally A2 and A5 formed 1-
(phenyltelluro)-6-(phenylselanyl)acenaphthene A12 in yields of
30, 35, 62 and 32%, respectively (Scheme 5).
from
ideality
occurring
in
analogous
naphthalene
derivatives.15,29,31
In substituted naphthalenes partial relief of peri-space crowd-
ing is achieved by the elongation of the bonds around C10,
closest to the repulsive interactions, and supplemented by an
increase in the C1–C10–C9 angular splay. In the acenaphthene
systems A1–A12 distortion of the acenaphthene carbon skeleton
also involves further widening of the C1–C10–C9 bay-angle
(mean 132° compared with the ideal 127°),16 however, the
bonds around C5 and C10 (average lengths 1.43 Å and 1.42 Å)
77Se and 125Te NMR spectroscopic data for the series of ace-
naphthene derivatives and their naphthalene analogues is dis-
played in Table 1. The respective acenaphthene NMR signals
display an upfield shift, lying at lower chemical shifts, indicating
the nuclei are more shielded than in equivalent naphthalene
compounds.
retain their ideal length (average lengths 1.418
Å and
1.423 Å).16
In contrast to the naphthalene systems, the central C5–C10
bond in the acenaphthene compounds stretches from 1.39 Å to
an average of 1.43 Å whilst the presence of the ethylene bridge
ensures the C4–C5–C6 bond angle remains close to ideal
(average 111.6° compared with the ideal 111.3°).16
The aforementioned bond stretching and angle widening dis-
tortions of the acenaphthene framework are insufficient to com-
pletely alleviate the steric strain imposed by the bulky chalcogen
and halogen substituents. Like their naphthalene analogues, A1–
A12 experience supplementary displacement of the peri-atoms
away from the acenaphthene mean plane and in-plane tilting of
the exocyclic bonds. Distances from the plane range from
0.03–0.44 Å, with a general increase in out-of-plane distortion
observed as the bulk of the interacting atoms increases.
The large J values for Te–Se coupling observed in the 125Te
NMR spectra for A12 (J = −715.6 Hz) and N12 (J = −834.0
Hz, sign determined from DFT/ZORA calculations, see ESI†)
indicate a potential weakly-attractive through-space interaction.
X-ray investigations
Suitable single crystals of A1–A12, A14, A15 and A16 were
obtained by diffusion of hexane into saturated solutions of the
individual compound in dichloromethane. The molecular struc-
tures of A1–A12 are analysed together here and compared with
the structures of naphthalene derivatives N1–N12 previously
reported.29,31 The molecular study of compounds A14, A15 and
A16 is also presented and contrasted with the known structures
of A13,38 A17,39 N1334 and N14.35 Fig. 3, 4 and 8 show the
structures of A1 and A2, A7–A9 and A16 respectively. Selected
Additional separation is provided by buckling of the ace-
naphthene ring as shown by torsion angles associated with the
central C5–C10 bond (see Table 2). The natural geometry of the
bay-region imposed by the ethane linker ensures that the central
C1–C10–C9 angle is more obtuse in acenaphthene [126.9°]16
3144 | Dalton Trans., 2012, 41, 3141–3153
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Table 2 Selected interatomic distances [Å] and angles [°] for compounds A1–A17
Compound
A1
A2
A3
A4
A5
A6
X, EPh
Br,SPh
Br,SePh
Br,TePh
I,SPh
I,SePh
I,TePh
Peri-region-distances
X(1)⋯E(1)
3.297(3)
90
3.1588(16)
84
3.2503(14)
83
3.428(4)
91
3.3291(11)
86
3.3721(17) [3.3774(17)]
83 [84]
a
% ΣrvdW
Peri-region bond angles
X(1)–C(1)–C(10)
C(1)–C(10)–C(9)
E(1)–C(9)–C(10)
Σ of bay angles
122.3(5)
132.5(6)
125.5()
380.3(11)
20.3
121.8(4)
131.4(5)
121.4(4)
374.6(9)
14.6
121.6(7)
131.7(9)
123.5(7)
376.8(15)
16.8
125.3(9)
132.6(12)
124.4(10)
382.3(20)
22.3
123.8(5)
131.4(6)
122.3(5)
377.5(11)
17.5
124.6(12) [123.7(11)]
130.5(15) [130.3(14)]
123.2(12) [124.4(10)]
378.3(32) [378.4(28)]
18.3 [18.4]
Splay angleb
Out-of-plane displacement
X(1)
E(1)
0.149(1)
−0.121(1)
0.287(1)
−0.359(1)
0.307(1)
−0.298(1)
−0.201(1)
0.092(1)
−0.439(1)
0.364(1)
−0.425(1) [0.333(1)]
0.334(1) [−0.395(1)]
Central naphthalene ring torsion angles
C:(6)–(5)–(10)–(1)
C:(4)–(5)–(10)–(9)
Compound
178.76(1)
176.84(1)
A7
175.27(1)
177.24(1)
A8
178.17(1)
176.39(1)
A9
179.40(1)
177.51(1)
A10
174.34(1)
177.91(1)
A11
175.29(1) [175.81(1)]
−177.33(1) [178.08(1)]
A12
EPh, E′Ph
SPh,SPh
SePh,SePh
TePh,TePh
SePh,SPh
TePh,SPh
TePh,SePh
Peri-region-distances
E(1)⋯E(2)
3.274(4) [3.289(4)]
91 [91]
3.1834(10)
84
3.3674(19)
82
3.113(4)
84
3.1576(15)
82
3.2479(19)
82
a
% ΣrvdW
Peri-region bond angles
E(1)–C(1)–C(10)
C(1)–C(10)–C(9)
E(2)–C(9)–C(10)
Σ of bay angles
124.3(9) [126.3(8)]
131.8(11) [129.2(10)]
125.1(9) [124.3(9)]
381.2(24) [379.8(22)]
21.2 (19.8)
123.4(6)
131.1(8)
122.0(6)
376.5(13)
16.5
123.9(10)
131 7(16)
122.8(12)
378.4(24)
18.4
120.7(7)
131.3(9)
120.7(8)
372.7(16)
12.7
121.5(4)
131.2(5)
122.3(5)
375.0(10)
15
123.1(7)
130.6(9)
123.4(8)
377.1(16)
17.1
Splay angleb
Out-of-plane displacement
E(1)
E(2)
−0.132(1) [0.194(1)]
0.161(1) [−0.128(1)]
0.176(1)
−0.193(1)
0.310(1)
−0.404(1)
0.274(1)
−0.416(1)
0.176(1)
−0.063(1)
−0.403(1)
0.032(1)
Central naphthalene ring torsion angles
C:(6)–(5)–(10)–(1)
C:(4)–(5)–(10)–(9)
Compound
−176.50(1) [177.94(1)]
178.40(1)
179.23(1)
A15
176.19(1)
179.03(1)
A16
175.14(1)
174.65(1)
A1337
179.22(1)
176.81(1)
A1738
179.41(1)
176.12(1)
178.82(1) [−176.17(1)]
A14
I I
X X′
I Br
Cl Cl
Br Br
Br Cl
Peri-region-distances
X(1)⋯X(2)
3.5505(12)
90
3.4145(18)
89
3.1336(11)
90
3.308(1)
89
3.287(1)
91
a
% ΣrvdW
Peri-region bond angles
X(1)–C(1)–C(10)
C(1)–C(10)–C(9)
X(2)–C(9)–C(10)
Σ of bay angles
125.6(5)
132.7(6)
126.9(5)
385.2(13)
25.2
124.4(7)
131.2(8)
125.9(7)
381.5(18)
21.5
122.5(2)
132.1(2)
122.51(17)
377.1(5)
17.1
123.94(1)
132.65(1)
124.20(1)
380.79(1)
20.8
119.77(1)
138.25(1)
120.10(1)
378.12(3)
18.1
Splay angleb
Out-of-plane displacement
X(1)
X(2)
0.098(1)
−0.146(1)
−0.067(1)
0.098(1)
−0.067(1)
0.114(1)
−0.027(1)
0.074(1)
−0.117(1)
0.053(1)
Central naphthalene ring torsion angles
C:(6)–(5)–(10)–(1)
C:(4)–(5)–(10)–(9)
179.13(1)
179.25(1)
179.12(1)
−179.66(1)
−178.19(1)
−179.12(1)
−179.29(1)
−178.71(1)
179.35(1)
−172.53(1)
a van der Waals radii used for calculations: rvdW(S) 1.80 Å, rvdW(Se) 1.90 Å, rvdW(Te) 2.06 Å, rvdW(Cl) 1.75 Å, rvdW(Br) 1.85 Å, rvdW(I) 1.98 Å;17
b Splay angle: Σ of the three bay region angles − 360.
than naphthalene [121.6°].15 Following further in-plane displace-
ment of the exocyclic bonds upon halogen/chalcogen substi-
tution, the observed splay angles for acenaphthenes A1–A12 are
inevitably greater than in comparable naphthalene derivatives.
Consequently, larger intramolecular peri-distances are achieved
by acenaphthenes A1–A12 [3.11–3.42 Å] compared to analo-
gous naphthalene counterparts [3.00–3.34 Å],29,31 though separ-
ations are still shorter than the sum of van der Waals radii for the
two interacting atoms [3.60–4.12 Å].17
the C(ar)–E–C(ar) planes.40 The absolute conformation of the
aromatic rings is calculated from torsion angles θ and γ (Tables
3 and 4; Fig. 5). Angle θ defines the rotation around the E–
CAcenap bond and angle γ the rotation around the E–CPh bond.
When θ and γ approach 90°, the orientation is denoted axial and
when the angles indicate a quasi-planar arrangement (close to
180°), they are termed equatorial.40 Axial conformations of
angle θ align the respective E–CPh bond perpendicular to the
mean least-squares plane and correspond to a type A struc-
ture.14,40 When the E–CPh bond is located on or close to the ace-
naphthene plane, in the case of equatorial angles of θ (∼180°),
the conformation is denoted as type B.14,40 For values of θ
The configurations adopted by acenaphthenes A1–A12, like
naphthalenes N1–N12, can be classified by the relative align-
ment of the E(acenaphthene) and E(phenyl) rings with respect to
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Fig. 3 The molecular structures of A1 and A2 (H atoms have been left off for clarity). The structures of A4 (which adopts a similar structure to A1),
A3, A5 and A6 (adopting conformations similar to A2) are omitted here but can be found in the ESI (Fig S6)†.
Fig. 4 The molecular structures of A7, A8 and A9 (H atoms omitted for clarity). The structures of A10, A11 and A12 (adopting conformations
similar to A8) are omitted here but can be found in the ESI (Fig. S7)†.
Table 3 Torsion angles [°] categorising the acenaphthene and phenyl ring conformations in A1–A6 [values in parentheses are for independent
molecules]
Compound
Acenaphthene ring conformations
Phenyl ring conformations
Torsion angle
C(10)–C(9)–E(1)–C(13)
Conformation
C(9)–E(1)–C(13)–C(14)
Conformation
A1
A2
A3
A4
A5
A6
θ1 −89.44(1)
Acenap1: axial
γ1 170.06(1)
γ1 95.94(1)
γ1 100.03(1)
γ1 151.86(1)
γ1 −92.60(1)
γ1 −101.61(1) [100.49(1)]
Ph1: equatorial
Ph1: axial
Ph1: axial
Ph1: equatorial
Ph1: axial
Ph1: axial
θ1 159.92(1)
Acenap1: equatorial
Acenap1: equatorial
Acenap1: axial
Acenap1: equatorial
Acenap1: equatorial
θ1 162.39(1)
θ1 90.15(1)
θ1 −165.87(1)
θ1 −161.4 (1) [162.48(1)]
aAcenap1: acenaphthene ring E(1); bAcenap2: acenaphthene ring E(2); cPh1: E(1) phenyl ring; dPh2: E(2) phenyl ring; eaxial: perpendicular to C(ar)–
E–C(ar) plane; fequatorial: coplanar with C(ar)–E–C(ar) plane.
which lie out with those of an axial or equatorial structure
(∼135°) the classification is a twist and corresponds to
type C.14,40
The series of 5-halo-6-(phenylchalcogeno)acenphthenes A1–
A6 mimic the geometric configurations adopted by their
naphthalene analogues N1–N6 (Fig. 6).31 Sulfur compounds A1
and A4 adopt an axial arrangement, with the E–CPh bond
perpendicular to the mean plane, and corresponding to type
A.14,40 Considerable distortion occurs within the mean ace-
naphthene plane in both compounds, though tilting of the X–
CAcenap and E–CAcenap bonds is more pronounced in A4 to
accommodate the larger iodine substituent (splay angles: A1
20.3°, A4 22.3°). Subsequently, the non-bonded intramolecular
I⋯S(phenyl) peri-distance in A4 [3.428(4) Å] is notably longer
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Table 4 Torsion angles [°] categorising the acenaphthene and phenyl ring conformations in A7–A12 [values in parentheses are for independent
molecules]
Compound
Acenaphthene ring conformations
Phenyl ring conformations
Torsion angle
C(10)–C(1)–E(1)–C(13)
C(10)–C(9)–E(2)–C(19)
C(1)–E(1)–C(13)–C(14)
C(9)–E(2)–C(19)–C(20)
A7
θ1 75.64(1) [−80.48(1)]
Acenap1: axial
θ1 −97.70(1) Acenap1:
axial
θ2 92.07(1) [−86.23(1)]
Acenap2: axial
θ2 163.05(1) Acenap2:
equatorial
γ1 −4.51(1) [−1.77(1)] Ph1:
equatorial
γ1 −148.68(1) Ph1: equatorial
γ2 30.56(1) [146.62(1)] Ph2: twist
γ2 −84.31(1) Ph2: axial
A8
A9
θ1 166.60(1) Acenap1:
equatorial
θ1 156.26(1) Acenap1:
equatorial
θ1 166.03(1) Acenap1:
equatorial
θ2 79.56(1) Acenap2:
axial
θ2 −100.46(1) Acenap2:
axial
θ2 −95.67(1) Acenap2:
axial
θ2 101.19(1) Acenap2:
axial
γ1 86.37(1) Ph1: axial
γ1 −82.12(1) Ph1: axial
γ1 −83.57(1) Ph1: axial
γ1 83.63(1) Ph1: axial
γ2 −167.34(1) Ph2: equatorial
γ2 5.36(1) Ph2: equatorial
γ2 −150.20(1) Ph2: equatorial
γ2 160.85(1) Ph2: equatorial
A10
A11
A12
θ1 −168.14(1) Acenap1:
equatorial
aAcenap1: acenaphthene ring E(1); bAcenap2: acenaphthene ring E(2); cPh1: E(1) phenyl ring; dPh2: E(2) phenyl ring; eaxial: perpendicular to C(ar)–
E–C(ar) plane; fequatorial: coplanar with C(ar)–E–C(ar) plane.
of the peri-atoms to distances of 0.3–0.4 Å from the plane
throughout. The acenaphthene organic framework of each com-
pound is also twisted with C–C–C–C central torsion angles
deviating from planarity by 1.8–5.7°.
Counterintuitively, the Br⋯E peri-distances in A2 and A3,
involving heavier chalcogens, are shorter than the Br⋯S distance
in A1 and similarly I⋯E separations in A5 and A6 are shorter
than in A4. This appears to be linked to the type A conformation
found for A1 and A4, as opposed to the type B conformation
prevalent for the other compounds (vide infra).
Fig. 5 Conformation of the acenaphthene aromatic rings is calculated
from torsion angles: θ defines rotation around the E–CAcenap bond; γ
The bis(phenylsulfanyl) derivative A7 adopts an axial-axial
conformation of acenaphthene rings, positioning both S–CPh
bonds perpendicular to the mean acenaphthene plane and displa-
cing the phenyl rings to opposite sides of the molecule; charac-
terised as type AA-t (-t corresponds to a trans orientation of
phenyl groups with respect to the acenaphthene plane; see
Fig. 6, Table 4).14,40 Whilst one of the phenyl rings aligns paral-
lel to the C(13)–S(1)–C(1) plane (equatorial alignment), the
second phenyl ring is rotated around the S(2)–C(19) bond by
∼30°, thus adopting a twist conformation with respect to the cor-
responding C(19)–S(2)–C(9) plane.14,40 This structure contrasts
with the configuration of the reciprocal naphthalene compound
N7 which adopts a type AB arrangement.29
The remaining members of the 5,6-(phenylchalcogeno)ace-
naphthene group A8–A12, influenced by the substitution of the
larger chalcogen atoms, adopt similar AB type structures with
one E–CPh bond aligning close to the acenaphthene plane and
one aligning roughly perpendicular to the plane (Fig. 6).14,40
Phenyl moieties are displaced to the same side of the mean ace-
naphthene plane for all compounds except for the bis-telluride
derivative A9 whose aromatic rings adopt a trans-orientation.
The geometrical structures adopted by the heavier members of
the series parallel those of the equivalent naphthalene species,
except for A9; N9 adopts a CC-c orientation.29
defines rotation around the E–CPh bond.
than the Br⋯S(phenyl) distance in A1 [3.297(3) Å], but both
separations are within the sum of the respective van der Waals
radii by ∼0.35 Å. The displacement of the peri-atoms to opposite
sides of the mean acenaphthene plane is comparable in both
derivatives, with only minor distortions observed (A1 Br 0.15 Å,
S −0.12 Å; A4 I −0.20 Å, S 0.09 Å). Furthermore, the ace-
naphthene frameworks of A1 and A4 show similar deviations
from planarity, with central acenaphthene ring torsion angles
around 1–3°.
The heavier chalcogens impose a contrasting geometry on A2,
A3, A5 and A6, aligning the E–CPh bond along the plane with
an equatorial type B conformation (Table 3, Fig. 6).14,40
A
quasi-linear three-body fragment exists in each of the four
derivatives with X⋯E–CPh angles (ψ) in the range 169–176°
(Fig. 6). The substitution of increasingly large heteroatoms at the
close contact peri-positions cramps the bay-region leading to an
increase in steric pressure which is relieved via molecular distor-
tions. Naturally, the non-bonded peri-distances display a general
increase with increasing peri-atom size, with separations
lengthening from 3.1588(16) Å in A2 to 3.3721(17) Å in A6,
14–17% shorter than the sum of van der Waals radii in each
case. The increase in steric congestion is accompanied by a
larger positive splay of the exocyclic X–CAcenap and E–CAcenap
bonds within the acenaphthene plane with splay angles increas-
ing from 14.6° in A2 to 18.3° in A6. Out-of-plane distortion is,
however, comparable in the four derivatives with displacement
The geometrical constraints of the type AB conformation
adopted by A8–A12 affords a quasi-linear three-body fragment
of the type E⋯E′–CPh. In all cases ψ approaches 180°, and non-
bonded peri-distances are within the sum of van der Waals radii
for the two interacting chalcogen atoms. No similar alignment is
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Fig. 6 The orientation of the E(phenyl) groups, the quasi-linear arrangements and structural conformations of A1–A12.
found in A7 which has angles of 75.1° [S(2)⋯S(1)–C(13)] and
85.8° [S(1)⋯S(2)–C(19)].
compounds. The resulting noncovalent S⋯S peri-distance
(3.274(4) Å [3.289(4) Å]) is only 9% shorter than double the
van der Waals radius of sulfur (3.6 Å).17
The double substitution of sulfur moieties in bis(phenylsulfa-
nyl) derivative A7, in accord with the singly substituted 5-halo-
6-(phenylsulfanyl)acenaphthenes A1 and A4, extorts greater ace-
naphthene distortion than might be expected from interactions
between lighter Group 15 and 16 congeners. Considerable repul-
sion between the exocyclic peri-bonds distorts the bay-region
geometry within the acenaphthene plane and results in a large
and positive splay angle of 21.1°. Out-of-plane distortion is com-
parable to A1 and A4 with a similar displacement of the sulfur
atoms to opposite sides of the mean acenaphthene plane [S(1)
−0.132(1) Å, S(2) 0.161(1) Å]. Central acenaphthene ring C–C–
C–C torsion angles (1 and 3°) illustrate the extent of twisting
occurring in the usually rigid aromatic ring systems of these
The larger chalcogen atoms of A8–A12 are accommodated by
a greater deformation of the acenaphthene geometry through an
increase in in-plane and out-of-plane distortion and a significant
buckling of the carbon skeleton. The degree of distortion is gen-
erally related to the size of the atoms residing in the bay-region
and there is a marked lengthening of the peri-gap [3.113(4) Å in
A8 to 3.3674(19) Å in A12] as the heavier congeners are located
at the 5,6-positions. The increasing congestion in the peri-space
causes a greater divergence of the E–CAcenap bonds within the
acenaphthene plane and larger positive splay angles are observed
(12.7° in A2 to 18.4° in A12). The peri-atoms are displaced to
opposite sides of the mean acenaphthene plane in all five
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Fig. 8 Molecular structure of A15 (H atoms omitted for clarity). The
structures of A14 and A16 (adopting similar conformations) are omitted
here but can be found in the ESI (Fig. S8)†.
the phenyl or acenaphthene ring systems. S–C (1.81–1.75 Å),
Se–C (1.95–1.92 Å) and Te–C (2.18–2.11 Å) bond lengths in
A7–A12 are within usual ranges ((1.77 0.05), (1.93 0.05),
(2.12 0.05), Å respectively).41
Fig. 7 Short intermolecular contacts between neighbouring molecules
of A9 form planar Te4 rhombus units.
The series of dihalo-derivatives A13–A17 exhibit a natural
increase in molecular distortion as the heavier members of
Group 17 are fixed in unavoidably close proximity at the 5,6-
positions (see Table 2, Fig. 8). The increasing congestion of the
peri-space is accompanied by significant in-plane distortion
associated with extreme tilting of the exocyclic X–C bonds.
Large positive splay angles feature in all five compounds,
increasing from 18.6° in A16 to a substantial 25.2° in A14 to
accommodate the two large iodine atoms. Subsequently, non-
covalent X⋯X′ peri-distances elongate from 3.1336(11) Å in
dichloride A16 to 3.5505(12) Å in the heaviest dihalide, A14.
Out-of-plane distortion is negligible in all five derivatives with
the halogen atoms located essentially on the mean acenaphthene
plane (displacements < 0.2 Å). The acenaphthene framework of
A17,39 with a maximum C–C–C–C torsion angle of 7.4°, is sig-
nificantly more distorted than the backbones of the remaining
derivatives with distances from the plane in the range 0.1–0.4 Å.
Central acenaphthene ring C–C–C–C torsion angles show a
range of distortions (0.6–5.4°), with A8 displaying the greatest
deviation from planarity [C(6)–C(5)–C(10)–C(1) 175.14(1)°, C
(4)–C(5)–C(10)–C(9) 174.65(1)°].
Short intermolecular contacts operating between neighbouring
molecules of bis-telluride A9 construct Te4 parallelograms. As a
consequence of the symmetry, the Te4 rhombus core is strictly
planar containing two unequal Te⋯Te non-bonded distances and
with interior angles combining to 360°. The larger intermolecu-
lar separation (3.853(1) Å) is longer than the non-bonded intra-
molecular peri-distance (3.3674(19) Å), though both are shorter
than twice the van der Waals radius of tellurium (4.12 Å)17
(Fig. 7). No similar intermolecular arrangement is observed in
the remaining members of the series and there is no overlap of
Fig. 9 The relationship between the size of the heteroatoms positioned at the 5,6-positions in acenaphthene (sum of van der Waals radii) versus the
peri-distance in A1–A16 and their naphthalene analogues.
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members of the halogen series, which barely deviate from pla-
narity (<1.3°).
Similarly, repulsive interactions between lone-pairs also domi-
nate the geometry of the peri-region in the bis-halides A13–A17,
N13 and N14, imposing larger than expected peri-separations
relative to the size of the atoms, in line with distances seen for
the type A and AA structural motifs (Fig. S9, ESI†)
In addition, it appears that an attractive interaction may also
exist in these structure types.14,29,31,40 The presence of a quasi-
linear G⋯E–CPh (G = Br, I, S, Se, Te) three-body fragment
might promote the delocalisation of a chalcogen/halogen lone-
pair (G) to the antibonding σ* (E–C) orbital, thus forming an
attractive three-centre four electron type interaction.14,29,31,40,42
For both the groups of compounds, the type A/AA with the bis
halides (see Fig. S9, ESI†), and the type B/AB compounds (see
Fig. S10, ESI†), quasi-linear relationships may be observed
between peri-distance and peri-atom size.
In order to probe how well these findings could be reproduced
and rationalised computationally, and specifically to determine
the extent of three-centre, four-electron type interactions occur-
ring in the series, density functional theory (DFT) calculations
were performed for the whole set of compounds of this study.
We will concentrate on the acenaphthene series A1–A17; results
for the corresponding naphthalene series are deposited in the
ESI†.
First, the atomic coordinates obtained from X-ray crystallogra-
phy were reoptimised at the B3LYP level to ensure that the
chosen basis sets and effective core potentials (ECPs) are ade-
quate for the problem at hand. Indeed, while the optimised peri-
distances between the chalcogens and/or halogens are noticeably
longer than those observed in the solid, by up to 0.01 Å, this
overestimation (which is a common DFT problem) is highly sys-
tematic: an excellent linear correlation between DFT and X-ray
distances is found, with a slope of 1.15 and a correlation coeffi-
cient of 0.97 (see Fig. S1 ESI†). The scatter about this
Adjacent plots independently displaying the relationship
between peri-distance versus collective peri-atom size (sum of
van der Waals radii of the two peri-atoms) reveals that molecular
distortion occurring in the series of acenaphthene derivatives
maps that observed in the naphthalene compounds (Fig. 9).29,31
The rigidity of the acenaphthene and naphthalene organic fra-
meworks places steric restrictions on systems incorporating large
heteroatoms at peri-distances within sub-van der Waals radii.
Naturally, when larger substituents occupy these positions,
increasing non-bonding interactions result as a direct conse-
quence of the overlap of orbitals and a general increase in peri-
separation is observed in Fig. 9.29,31 Further constraints,
however, can be imposed on the system by the number, size and
nature of functional groups attached to the peri-atoms including
the presence of highly repulsive p-type lone-pairs.40
The conformation of the aromatic ring systems and by infer-
ence the location of the chalcogen and halogen lone-pairs thus
has an important impact on the degree of distortion occurring in
N1–N12 and A1–A12 and dictates the overall molecular confor-
mation. The non-bonded X⋯E and E⋯E′ peri-distances in A1–
A12 and N1–N12 are dominated by the repulsion between p-
type lone pairs.40 Greater repulsion, leading to larger than
expected peri-distances relative to the size of the interacting
atoms (Fig. 9), is observed when the conformation of the
respective derivative conforms to a type A or type AA struc-
ture.14,29,31,40 In this instance, torsion angles θ are roughly 90°
(see Tables 3 and 4), the naphthalene/acenaphthene ring adopts
an axial conformation and the chalcogen p-orbitals lie in the
plane of the aromatic system.14,40 The collinear orientation of the
p orbital axis along the mean plane ensures repulsion is relatively
high.40
Fig. 10 Plot of B3LYP optimised peri-distances vs. sum of van der Waals radii in the acenaphthene series. However, lone-pair lone-pair repulsion is
less effective when angles θ approach 180° and the naphthalene/acenaphthene ring system adopts an equatorial configuration [types B and AB].14,40
Rotation around the E–CNap/Acenap bond now aligns the axis of the chalcogen p-orbitals parallel to the mean plane of the aromatic ring system and
reduces repulsive interactions (Fig. S10, ESI†).40
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correlation is much smaller than the trends discussed in the
following.
Next, a conformational search was undertaken, where all mol-
ecules A1–A17 were optimised in the respective other confor-
mation, i.e. A/B or AA/AB. AA variants with the phenyl groups
in cis rather than in trans orientation were also trialled, but these
invariably optimised to AB forms (or intermediate structures
denoted AC or CC, where one or both of the dihedral angles θ
are close to 140°. In addition, for the unsymmetrical bis(chalco-
gen) species A10–A12, the two possible AB conformations,
labelled AB and BA, were considered. The energy span between
the least and most stable conformer in each case is remarkably
variable, ranging from 0.3 kJ mol−1 for A1 to 24 kJ mol−1 for
A12 (Table S1†).
In all cases, except for A7, the most stable conformer corre-
sponded to the structure observed in the solid. For A7, the opti-
mised AB form is only 3 kJ mol−1 lower in energy than the
observed AAvariant. That the latter is found in the solid is likely
to be due to intermolecular interactions or packing forces. All
optimised peri-distances are plotted versus the sum of the van
der Waals radii in Fig. 10.
It can be seen that the bis(halogen) compounds, the A confor-
mers and the AA species (Fig. 10, empty squares, blue squares
and yellow triangles, respectively) behaved as a related series,
and display greater peri-distances than the other compounds of
the same van der Waals’ radii. This series nicely reproduces the
empirical correlation shown in Fig. 9, extending it to the whole
acenaphthene set.
Fig. 11 Localised natural bond orbitals in A9–AB (B3LYP level): The
lone pair on one Te (top) can act as a weak donor into the σ*C-Te orbital
on the other side (bottom). Hydrogen atoms omitted for clarity.
When one of the Ph substituents is moved from the “perpen-
dicular” A to an “equatorial” B (or C) orientation, a significant
decrease in the peri-distance is observed. The extent of this
decrease is rather variable, ranging from. 0.08 Å for A4 to as
much as 0.3 Å for A9 (Table S1†). In general, the effect is some-
what more pronounced for the bis(chalcogen) species A7–A12
than for the mixed halogen/chalcogen derivatives A1–A6. As a
result, the correlation involving the B and AB conformers
(Fig. 10, orange diamonds, and green and red triangles) shows
some scatter (as does the corresponding empirical relation in
Fig. S10, ESI†), but the overall trend is clearly visible.
The key question thus is, whether the change of the peri-dis-
tance with conformation is due to specific bonding interactions
or not? The extent of covalent bonding is conveniently probed
by the Wiberg bond index (WBI),43 which usually approaches a
value of one for a true single bond. As expected, very small
WBIs are computed for the A and AA conformers with their
longer peri-distances (between ca. 0 and 0.02; see Table S1†).
Significantly higher values are obtained for the B and AB
species, up to 0.14 for A9. In general, there is a trend to higher
WBIs with larger peri-distance contraction on going from A to B
or AA to AB (see Fig. S2†), which is usually paralleled by a
more pronounced energy gain during the conformational change.
In contrast, those members of the series of naphthalene com-
pounds which adopt type B orientations show low WBI values
of 0.05–0.08, indicating only a very minor interaction between
non-bonded chalcogen atoms in these species.29,31
NBOs are plotted for A9 in Fig. 11, where this donor-acceptor
interaction is estimated to be roughly 50 kJ mol−1. While this
number should not be over interpreted, it is clear that this type of
interaction can contribute significantly to the weak interaction
between the two peri substituents and to the stabilisation of the
AB vs. the AAt conformer (ca. 23 kJ mol−1, Table S1†).
A slightly larger donor-acceptor interaction is indicated for the
AB conformer of A8, despite a smaller WBI and energy gain
compared to A9. In a related compound where the “equatorial”
Ph group in A8 is replaced with Br,45 a much better electron
acceptor, the observed Se–Se distance shortens from 3.18 Å
(Table 2) to 2.52 Å (with a concomitant WBI increase from 0.07
to 0.5529). The weak interactions between the peri-atoms of this
study can thus be taken as an onset of 3-center, 4-electron
bonding, which becomes prominent in this known Br,SePh
system.
In addition to these weak donor-acceptor interactions, electro-
static contributions may be non-negligible as well. The p-type
lone pairs on the chalcogens can be discerned in plots of the
electrostatic potential (ESP), where the two opposing lobes are
prominent through a clearly negative potential (see Fig. S3†).
The alignment of the two lone pairs in the AA-t conformers
should thus not only lead to Pauli-, but also to electrostatic repul-
sion. This repulsion is much reduced in the AB conformers,
where there may even be some weak electrostatic attraction due
to an area of a very slightly positive ESP, reminiscent of the “σ-
hole” mechanism proposed to be at the origin of the so-called
halogen bonding observed within R–X…B moieties (where X is
In the second-order perturbation analysis of the natural bond
orbitals (NBOs),44 weak donor–acceptor interactions are appar-
ent in the AB conformers, involving the p-type lone pair on E
and the σ*(E′–C) antibonding orbital on the other side. Key
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a halogen and B a Lewis base).46 A similar mechanism has
recently been predicted for group 16 molecules.45b
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A series of mixed halogen-chalcogen (A1–A6), bis-chalcogen
(A7–A9) and mixed chalcogen-chalcogen (A10–A12) peri-sub-
stituted acenaphthenes have been prepared from standard
halogen-lithium exchange reactions of 5,6-dibromoacenaphthene
A13, 5,6-diiodoacenaphthene A14 and 5,6-dilithioacenaphthe-
ne·2TMEDA complex A18. The molecular structures of A1–
A12 were analysed and compared with analogous naphthalene
compounds N1–N12 previously reported by us.29,31 Molecular
distortion occurring in the series of the acenaphthene derivatives
was found to map the degree of distortion observed in the
naphthalene compounds.
A general increase in the peri-distance is observed when
larger atoms occupy the proximal peri-positions, however, one
particular conformational group of compounds, those adopting
the B or AB conformation, exhibit shorter than expected peri-
separations compared with type A/AA derivatives. The degree of
molecular distortion and hence the magnitude of the non-bonded
peri-distance is dominated by the repulsion between p-type lone
pairs.40 The conformation adopted in type A/AA positions the
chalcogen p-orbitals in line with the aromatic plane. This colli-
near orientation of the p-orbital axis along the mean plane
ensures repulsion is relatively high.40 Conversely, type B/AB
configurations align the axis of the chalcogen p-orbitals parallel
and to the mean plane and repulsive interactions are reduced.40
This configuration in type B/AB structures results in a quasi-
linear three-body fragment across the peri gap, which appears
to promote the formation of an attractive three-centre, four elec-
tron type interaction.14,29,31,40,41 Support for this interpretation
comes from natural-bond-orbital analysis of DFT-derived
wavefunctions.
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Acknowledgements
Elemental analyses were performed by Sylvia Williamson and
Mass Spectrometry was performed by Caroline Horsburgh. Cal-
culations were performed using the EaStCHEM Research Com-
puting Facility maintained by Dr H. Früchtl. The work in this
project was supported by the Engineering and Physical Sciences
Research Council (EPSRC). Michael Bühl wishes to thank EaSt-
CHEM and the University of St Andrews for support.
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