Exploring hypervalency and three-centre, four-electron bonding interactions: Reactions of acenaphthene chalcogen donors and dihalogen acceptors

Article abstract of DOI:10.1039/c2dt12031c

Sterically crowded peri-substituted selenium and tellurium acenaphthene donors D1-D7 [Acenap(EPh)(Br) E = Se, Te; Acenap(SePh)(EPh) E = Se, S; Acenap(TePh)(EPh) E = S, Se, Te] react with dibromine and diiodine acceptors to afford a group of structurally d

Full text of DOI:10.1039/c2dt12031c

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PAPER
Exploring hypervalency and three-centre, four-electron bonding interactions:
Reactions of acenaphthene chalcogen donors and dihalogen acceptors†
Fergus R. Knight, Kasun S. Athukorala Arachchige, Rebecca A. M. Randall, Michael Bühl,
Alexandra M. Z. Slawin and J. Derek Woollins*
Received 25th October 2011, Accepted 15th December 2011
DOI: 10.1039/c2dt12031c
Sterically crowded peri-substituted selenium and tellurium acenaphthene donors D1–D7 [Acenap(EPh)
(Br) E = Se, Te; Acenap(SePh)(EPh) E = Se, S; Acenap(TePh)(EPh) E = S, Se, Te] react with dibromine
and diiodine acceptors to afford a group of structurally diverse addition products 1–12, comparable in
some cases to previously reported naphthalene analogues. Tellurium donors D4–D6 react conventionally
with the dihalogens to afford insertion adducts 6–11 (X-R2Te-X) exhibiting molecular see-saw
geometries, characterised by hypervalent X-Te-X quasi-linear fragments. The reactions of selenium
donors D1–D3 with diiodine afford expected neutral charge-transfer (CT) spoke adducts 1, 4 and 5
(R2Se-I-I) containing quasi-linear Se-I-I alignments. Conversely, treatment of D2 and D3 with dibromine
results in the formation of two tribromide salts 2 and 3 containing bromoselanyl cations [R2Se–Br]+⋯
[Br–Br2]−, each exhibiting a quasi-linear three-body Br–Se⋯E (E = Se, S) fragment. The peri-bonding in
these species can be thought of as a weak hypervalent G⋯Se-X three-centre, four-electron (3c-4e) type
interaction, closely related to the T-shaped 3c-4e interaction. Density-functional calculations performed on
2 and 3 and their bare cations (2a and 3a) reveal Wiberg bond indices of 0.25–0.37, suggesting
substantial 3c-4e character in these systems. The presence of such an interaction operating in 2 and 3
alleviates steric strain within the peri-region and minimises the degree of molecular distortion required to
achieve a relaxed geometry. Ditellurium donor D7 reacts with dibromine to afford an unorthodox
insertion adduct 12 containing a Te–O–Te bridge and two quasi-linear Br–Te–O fragments, with the
central tellurium atoms assuming a molecular see-saw geometry. Whilst DFT calculations indicate 12 is
thermodynamically unfavourable, its formation is viable under experimental conditions.
complexes containing unconventional bonding situations.7–9
wide variety of structural archetypes are known, of which the
neutral charge-transfer (CT) spoke adduct (R2E-X-Y, 10-X-2)
and the see-saw insertion adduct (X-ER2-Y, 10-E-4) are the most
common.7–10 Experimental factors such as the type of chalcogen
donor atom, the form of the dihalogen or inter-halogen, the
stoichiometry of the reactants and the nature of the donor atoms’
R group(s) all play a significant role and control the reaction
pathway.7–10
Spoke and see-saw conformations of RR′E·X2 type adducts
are predicted to be in equilibrium in solution, with reactions
between chalcogen donors [R2E] and di- and interhalogens pro-
gressing via nucleophilic attack at either the halogen or chalco-
gen site of a common [R2E-X]+ cation intermediate.7–9,11–14 The
conformation of the final structure is thus dictated by the degree
of charge-transfer from non-bonding chalcogen donor orbitals
n(E) to the LUMO of the di- or inter-halogen acceptor σ*(X-X′).
Typically, see-saw adducts are formed when halide X is more
electronegative than chalcogen E [R2Eδ+-Xδ−], with nucleophilic
halide attack occurring at the chalcogen atom.7–10,12,15
A
Introduction
The interaction of atoms is an integral aspect of chemistry,
biology and materials science. The pioneering work on the elec-
tronic theory of the covalent bond by Lewis and Langmuir in the
early 1900s,1 led to great advances in the area of strong bonding
(covalent/ionic), but the ambiguity over “hypervalent” species1–4
and the nature of weak, non-covalent interactions,5,6 continues
to intrigue chemists. The search for structures which invoke
new and unusual types of interactions is therefore indispensible
for developing our understanding of non-bonded forces and
the theory of bonding.
Organo-Group 16 donor compounds react with dihalogen
(I2, Br2) and interhalogen (IBr, ICl) acceptors to afford addition
EaSTCHEM 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 850816–850827. For ESI and crystallo-
graphic data in CIF or other electronic format see DOI: 10.1039/
c2dt12031c
The defining quasi-linear E-X-Y and X-E-Y fragments of
spoke and see-saw adducts, appear to violate the octet rule and
3154 | Dalton Trans., 2012, 41, 3154–3165
This journal is © The Royal Society of Chemistry 2012

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are thus classified as hypervalent.1 Hypervalency of the heavier
main group elements has been debated upon since the innovative
work by Langmuir and Lewis1 and remains an area of contention
to chemists. The advancement of molecular orbital theory, led
Rundle and Pimentel to develop the work by Sugden2 and intro-
duce the notion of a three centre four-electron bond (3c-4e).2
This concept has evolved to include related four- and five-centre,
six electron bonds (4c, 6e; 5c, 6e)9 and continues to be a
favoured method for explaining hypervalency without violating
the octet rule or invoking ionic bonding.4 Numerous compounds
containing linear fragments have been well documented7–9,16
with the nature of the bonding described by the Rundle–Pimen-
tel 3c-4e model and a charge-transfer model.2,17
We have previously utilised the rigidity of polycyclic aromatic
hydrocarbons, naphthalene18 and related 1,2-dihydroacenaphthy-
lene (acenaphthene),19 to constrain bulky heteroatoms in un-
avoidably congested environments, at distances within the sum
of the van der Waals radii. The geometric constraints unique to
these frameworks, imposed by a double substitution of atoms or
groups at the peri-positions, offer good scaffolds with which to
study non-bonded intramolecular interactions.5,6,18–22
Initially we focused on naphthalene, investigating dichalco-
genide23 ligands and unusual phosphorus compounds24 before
expanding our research to mixed phosphorus-chalcogen,25
chalcogen-chalcogen26–28 and halogen-chalcogen29 systems.
More recently we have exploited the acenaphthene backbone
to prepare corresponding halogen-chalcogen and chalcogen-
chalcogen derivatives.30
During our investigations of peri-substituted naphthalenes we
prepared a number of chalcogen-donors26,29 and reported their
reactions with dibromine and diiodine.27 The structurally diverse
array of addition adducts exhibited a number of linear fragments
which were classified as hypervalent and represented by the
Rundle–Pimentel 3c-4e model.2,17,27 Hypervalency is a topic of
continued interest, with the quest for linear arrangements a
common priority for tracing hypercoordinate interactions. The
work presented here complements our previous study, reporting
an investigation of related acenaphthene chalcogen donor com-
pounds D1–D7 and their reactions with dibromine and diiodine.
homogeneity of the new compounds was, where possible,
confirmed by microanalysis. 77Se and 125Te NMR spectroscopy
data is displayed in Table 1. Suitable single crystals were
obtained for 1–12 by diffusion of hexane into saturated solutions
of the individual compound in dichloromethane. The molecular
structures of 1–12 are analysed together here and compared with
the structures of acenaphthene donors D1–D730 and previously
reported analogous naphthalene addition adducts.27 Compounds
1–10 crystallise with one molecule in the asymmetric unit, com-
pounds 11 and 12 contain two nearly identical molecules in the
asymmetric unit. Selected interatomic distances, angles and
torsion angles are listed in Tables 2 and 3. Further crystallo-
graphic information can be found in Tables 4–6 and in the
Electronic Supporting Information (ESI)†.
Neutral charge-transfer (CT) spoke adducts (R2E-X-X) are
formed whenever there is a weak coordination involving a trans-
fer of electron density from non-bonding chalcogen orbitals into
the LUMO of the halogen acceptor molecule.7–9,12,15 The degree
of charge transfer can be predicted from electronegativity (χ)
values, with CT adducts generally formed when χ(X) is less than
χ(E).7–9,12,15 Upon CT formation there is a natural lowering of
the dihalogen bond order and a lengthening of the X-X bond.7,8
Strong adducts are characterised by X⋯X bond orders in the
range 0.4–0.6 and contain E-X-X three body fragments.7 Even
stronger interactions between the chalcogen donor and the diha-
logen acceptor, results in the cleavage of the X-X bond and
leads to the formation of an [R2E-X]+ cation which interacts
with a X− anion.7 In this instance, the X-X CT bond is con-
sidered an X+⋯X− ionic interaction, with bond orders less than
0.4 and X⋯X separations larger than the sum of van der Waals
radii.7
Whilst the majority of CT adducts structurally characterised in
the literature are prepared from organosulfur donors7,8,10,31 [χ(S)
2.58]32 and diiodine [χ(I) 2.36],32 the “softer” character of orga-
noselenides [χ(Se) 2.35],32 which implies greater donor proper-
ties compared with sulfur, results in significantly stronger
adducts exhibiting elongated iodine–iodine bond lengths.7,8,33
Selenium donor compounds D1, D2 and D3 react convention-
ally7,8,27,30 with a single equivalent of diiodine to afford similar
CT spoke adducts (1, 4 and 5 respectively; Scheme 1, Fig. 1),
exhibiting quasi-linear Se-I-I fragments [Se-I-I angles 171–173°;
Table 3]. The 77Se NMR spectra for 1 [δ = 437.6 ppm], 4 [δ =
421.0 ppm] and 5 [δ = 460.4 ppm] display the expected
downfield shifts, compared with donor compounds D1 [δ =
423.7 ppm], D2 [δ = 408.3 ppm] and D3 [δ = 433.7 ppm],
Results and discussion
Compounds 1–12 were synthesised and characterised by multi-
nuclear NMR and IR spectroscopy and mass spectrometry. The
Table 1 77Se and 125Te NMR spectroscopic dataa
1
2
3
4
5
Peri-atoms
Se, Br
Se–I–I
437.6
6
Se, Se
Br–Se–Se
405.2
7
Se, S
Br–Se–S
431.2
8
Se, Se
Se–I–I
421
Se, S
Se–I–I
460.4
10
Linear arrangement
77Se NMR
9
11
12
Peri-atoms
Te, Br
Br–Te–Br
—
Te, Br
I–Te–I
—
Te, S
Br–Te–Br
—
Te, Se
Br–Te–Br
332.9
921.9
Te, S
I–Te–I
—
Te, Se
I–Te–I
343.2
871
Te, Te
Br–Te–O
—
Linear arrangement
77Se NMR
125Te NMR
918.8
860.6
930.6
878.9
961.3
a Spectrum of 1 run in CD3CN, spectra of 2 and 3 run in (CD3)2CO, spectra of 4–11 run in CDCl3, spectrum of 12 run in (CD3)2NCOD; δ (ppm).
This journal is © The Royal Society of Chemistry 2012
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Table 2 Selected interatomic distances [Å] and angles [°] for compounds 1–12
Compound
1
2
3
4
5
6
Peri-moieties
Br SePhI2
BrSePh SePh
BrSePh SPh
I2SePh SePh
I2SePh SPh
Br TePhBr2
Peri-region distances and sub-van der Waals contacts
E(1)⋯X/E
3.1753(19)
85
2.801(3)
74
2.740(3)
74
3.326(3)
88
3.252(2)
88
3.2581(19)
83
a
% ΣrvdW
Peri-region bond angles
E(1)–C(1)–C(10)
C(1)–C(10)–C(9)
X/E–C(9)–C(10)
Σ of bay angles
122.3(9)
133.6(11)
120.8(9)
376.7(24)
16.7
117.2(14)
132(2)
118.1(15)
367.3(39)
7.3
118.0(6)
129.5(8)
117.9(6)
365.4(16)
5.4
122.6(11)
132.6(13)
125.3(11)
380.5(29)
20.5
123.4(5)
132.5(6)
123.1(6)
379.0(14)
19
122.8(10)
132.0(12)
120.0(9)
374.8(25)
14.8
Splay angleb
Out-of-plane displacement
E(1)
0.125(1)
−0.101(1)
−0.073(1)
0.121(1)
0.069(1)
−0.163(1)
−0.091(1)
0.179(1)
0.130(1)
−0.133(1)
0.531(1)
−0.205(1)
X/E(1)
Central acenaphthene ring torsion angles
C:(6)–(5)–(10)–(1)
C:(4)–(5)–(10)–(9)
Compound
178.85(1)
179.14(1)
7
179.29(1)
−176.49(1)
8
−178.60(1)
177.79(1)
9
−179.55(1)
−178.75(1)
10
178.01(1)
178.30(1)
11
174.62(1)
−179.67(1)
12
Peri-moieties
Br TePhI2
Br2TePh SPh
Br2TePh SePh
I2TePh SPh
I2TePh SePh
(BrTePh)2O
Peri-region distances and sub-van der Waals contacts
E(1)⋯X/E
3.2050(11)
82
3.218(3)
83
3.2729(8)
83
3.141(4)
81
3.2677(18) [3.2862(18)]
83 [83]
3.335(1) [3.385(1)]
81 [82]
a
% ΣrvdW
Peri-region bond angles
E(1)–C(1)–C(10)
C(1)–C(10)–C(9)
X/E–C(9)–C(10)
Σ of bay angles
123.2(4)
131.8(5)
121.8(4)
376.8(11)
16.8
121.8(10)
132.2(14)
121.7(10)
375.7(27)
15.7
123.6(5)
131.1(7)
122.6(5)
377.3(14)
17.3
122.8(8)
129.0(11)
122.7(10)
374.5(23)
14.5
124.4(8) [122.3(8)]
128.7(9) [131.7(10)]
122.2(7) [121.6(9)]
375.3(19) [375.6(22)]
15.3 [15.6]
124.0(3) [123.2(4)]
132.1(4) [132.3(4)]
122.9(3) [124.6(3)]
379.0(8) [380.1(9)]
19.0 [20.1]
Splay angleb
Out-of-plane displacement
E(1)
X/E(1)
−0.055(1)
0.154(1)
0.493(1)
−0.257(1)
−0.447(1)
0.217(1)
0.122(1)
0.162(1)
−0.463(1) [−0.582(1)]
0.487(1) [0.402(1)]
0.045(1) [0.099(1)]
−0.139(1) [0.115(1)]
Central acenaphthene ring torsion angles
C:(6)–(5)–(10)–(1)
C:(4)–(5)–(10)–(9)
179.52(1)
−177.77(1)
176.92(1)
175.99(1)
−175.53(1)
−177.49(1)
−179.00(1)
178.10(1)
−175.72(1) [−176.60(1)]
175.82(1) [−172.96(1)]
179.78(1) [179.01(1)]
176.60(1) [178.03(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(Br) 1.85 Å;34 b Splay angle: Σ of the three bay
region angles −360.
Table 3 Selected interatomic Se–I–I, Br–Se–E′, Br–Br–Br, X–Te–X and Br–Te–O distances [Å] and angles [°] for 1–12
Compound
1
4
5
2
3
Se(1)–I(1)
I(1)–I(2)
2.9565(17)
2.8161(14)
2.901(2)
2.8267(19)
2.9102(12)
2.8168(10)
Se(1)–Br(1)
Se(1)—E(2)
Br(2)–Br(3)
Br(3)–Br(4)
Br(1)–Se(1)–E(2)
Br(2)–Br(3)–Br(4)
9
2.6808(9)
2.6859(9)
175.54(3)
2.458(3)
2.801(3)
2.518(4)
2.582(4)
171.42(11)
178.19(9)
10
2.8860(19)
2.9648(19)
177.73(4)
2.4409(17)
2.740(3)
2.614(2)
2.502(2)
174.95(1)
179.24(5)
11
2.9630(14) [2.9327(13)]
2.8976(14) [2.9173(14)]
177.18(4) [177.67(4)]
Se(1)–I(1)–I(2)
171.32(4)
173.22(6)
173.16(3)
Compound
Te(1)–X(1)
Te(1)–X(2)
X(1)–Te(1)–X(2)
Compound
Br(1)–Te(1)
Te(1)–O(1)
6
7
8
2.7025(17)
2.6583(19)
174.23(6)
12
2.8261(7) [3.024(3)]
2.005(3) [1.942(3)]
176.46(8) [174.73(8)]
2.9613(8)
2.9199(8)
173.04(2)
2.6766(17)
2.6850(17)
175.49(5)
Te(2)–O(1)
Te(2)–Br(2)
Br(2)–Te(2)–O(1)
1.992(3) [2.045(3)]
2.8862(7) [2.7654(7)]
172.52(9) [175.41(9)]
Br(1)–Te(1)–O(1)
respectively (Table 1).30 In all three derivatives, coordination
of selenium to the diiodine molecule results in a reduction of
bond order, of the I–I bond, which extends from 2.66 Å (free
iodine)35 to an average 2.82 Å (Table 3). CT spoke adducts
are classified by the directional parameter θ, which calculates the
position of the E-X vector, with respect to the plane containing
the chalcogen two electron-pairs in a tetrahedral sp3 geometry
(Fig. 2).7 In 1, 4 and 5, the diiodine molecule lies quasi-perpen-
dicular to the R2Se plane, with dihedral angles 69–79°,
indicative of donor molecules having sterically crowded chalco-
gen lone-pairs.7
Upon the formation of CT spoke 1, there is no significant
alteration in the acenaphthene geometry compared with the
structure of donor D1.30 The phenyl ring orientates with a
similar equatorial conformation, aligning the Se–CPh bond along
the mean plane, corresponding to a type B configuration
(Fig. 3).36 Molecular distortion and by inference the peri-dis-
tance, is comparable in both donor and adduct, with non-bonded
3156 | Dalton Trans., 2012, 41, 3154–3165
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Table 4 Crystallographic data for compounds 1–4
1
2
3
4
Empirical formula
Formula weight
T/°C
Crystal colour, habit
Crystal dimensions (mm3)
Crystal system
C18H13BrI2Se
641.97
−148(1)
red, platelet
0.210 × 0.090 × 0.020
Monoclinic
a = 14.174(7) Å
b = 14.165(7) Å
c = 9.265(5) Å
—
β = 106.194(10)°
—
V = 1787(2)
P21/c
C24H18Br6Se2
943.75
−148(1)
orange, chip
0.150 × 0.060 × 0.030
Triclinic
a = 10.870(5) Å
b = 11.404(6) Å
c = 11.938(6) Å
α = 106.223(7)°
β = 91.894(8)°
γ = 108.294(9)°
V = 1337.3(11)
ˉ
P1
2
2.344
C24.25H18.5Br4Cl0.5SSe
758.28
−148(1)
red, platelet
0.240 × 0.170 × 0.030
Triclinic
a = 10.822(8) Å
b = 11.568(8) Å
c = 11.940(10) Å
α = 81.42(3)°
β = 73.07(3)°
γ = 71.07(3)°
V = 1350.2(17)
ˉ
P1
2
1.865
C24H18Se2I2
718.14
−148(1)
colourless, prism
0.20 × 0.20 × 0.20
Monoclinic
a = 9.555(5) Å
b = 8.242(4) Å
c = 28.147(14) Å
—
β = 97.390(13)°
—
V = 2198.4(20)
P21/n
4
2.17
Lattice parameters
Volume/Å3
Space group
Z value
4
D
calc (g cm−3
)
2.387
F000
1184
77.97
13238
0.0869
0.341–0.856
3140(199)
15.78
0.0493
0.0605
880
117.691
11213
0.0849
0.348–0.703
5355(289)
18.53
0.1066
0.147
725
74.64
11677
0.0707
0.528–0.799
4740(293)
16.18
1344
1.822
18239
0.1087
0.152–0.290
5113(253)
20.21
0.0952
0.1343
μ(Mo-Kα)/cm−1
No. of reflections measured
Rint
Min. and max. transmissions
Observed reflection (no. variables)
Reflection/parameter ratio
Residuals: R1 (I > 2.00σ(I))
Residuals: R (all reflections)
Residuals: wR2 (all reflections)
Goodness of fit indicator
Maximum peak in final diff. map
Minimum peak in final diff. map
0.0866
0.1292
0.2791
1.059
0.1936
0.3337
1.203
0.3043
1.022
1.274
1.81 e−/Å3
−2.05 e−/Å3
2.46 e−/Å3
−2.40 e−/Å3
1.64 e−/Å3
−1.27 e−/Å3
2.29 e−/Å3
−2.48 e−/Å3
Table 5 Crystallographic data for compounds 5–8
5
6
7
8
Empirical formula
Formula weight
T/°C
Crystal colour, habit
Crystal dimensions (mm3)
Crystal system
C24H18I2SSe
671.24
−148(1)
red, platelet
0.240 × 0.040 × 0.020
Monoclinic
a = 9.578(4) Å
b = 8.184(3) Å
c = 28.084(10) Å
—
C18H13Br3Te
596.61
C18H13BrI2Te
690.61
−148(1)
orange/red, platelet
0.24 × 0.06 × 0.03
Monoclinic
a = 26.071(7) Å
b = 9.378(2) Å
c = 15.682(4) Å
—
C24H18Br2STe
625.87
−148(1)
yellow, chunk
0.150 × 0.120 × 0.030
Monoclinic
a = 12.516(5) Å
b = 12.616(5) Å
c = 14.436(6) Å
—
−148(1)
yellow, chip
0.24 × 0.15 × 0.06
Orthorhombic
a = 15.745(7) Å
b = 10.066(4) Å
c = 22.162(9) Å
—
Lattice parameters
β = 96.485(9)°
—
V = 2187.4(13)
P21/n
—
—
β = 105.281(5)°
—
V = 3698.4(16)
C2/c
8
2.48
β = 112.682(8)°
—
V = 2103(2)
P21/n
Volume/Å3
Space group
Z value
V = 3512(3)
Pbcn
8
4
4
Dcalc (g cm−3
)
2.038
2.256
1.977
F000
1272
46.443
2224
85.335
2512
71.107
1200
53.334
μ(Mo-Kα)/cm−1
No. of reflections measured
Rint
15951
0.0473
0.448–0.911
3848(253)
15.21
0.0411
0.0529
0.1625
26513
0.1228
0.308–0.599
3563(199)
17.9
0.0801
0.0988
0.2488
14642
0.0415
0.418–0.808
3729(199)
18.74
0.0379
0.0468
0.149
15436
0.1181
0.425–0.852
3692(253)
14.59
0.0956
0.1257
0.2865
Min. and max. transmissions
Observed reflection (no. variables)
Reflection/parameter ratio
Residuals: R1 (I > 2.00σ(I))
Residuals: R (all reflections)
Residuals: wR2 (all reflections)
Goodness of fit indicator
Maximum peak in final diff. map
Minimum peak in final diff. map
1.215
1.405
1.267
1.347
1.66 e−/Å3
−1.76 e−/Å3
1.39 e−/Å3
−1.62 e−/Å3
2.67 e−/Å3
−2.88 e−/Å3
1.99 e−/Å3
−2.23 e−/Å3
Se⋯Br separations for 1 [3.1753(19) Å] and D1 [3.1588(16)
Å]30 ∼15% shorter than the sum of van der Waals radii.
Maximum C–C–C–C central acenaphthene ring torsion angles
indicate greater planarity in the organic backbone upon diiodine
coordination, with deviation by ∼1° compared with 3–5°
in D1.30
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Table 6 Crystallographic data for compounds 9–12
9
10
11
12
Empirical formula
Formula weight
T/°C
Crystal colour, habit
Crystal dimensions (mm3)
Crystal system
C24H18SeTeBr2
672.77
−148(1)
yellow, prism
0.15 × 0.15 × 0.06
Monoclinic
a = 12.680(4) Å
b = 12.776(3) Å
c = 14.386(4) Å
—
β = 113.068(6)°
—
V = 2144.2(10)
P21/n
C24H18I2STe
719.88
−148(1)
brown, chunk
0.150 × 0.150 × 0.020
Monoclinic
a = 11.129(6) Å
b = 16.603(9) Å
c = 12.582(7) Å
—
β = 102.416(10)°
—
V = 2270(2)
P21/n
C24.05H18.10 Cl0.1I2SeTe
770.99
−148(1)
orange, platelet
0.09 × 0.03 × 0.03
Triclinic
a = 11.079(3) Å
b = 14.105(4) Å
c = 16.134(5) Å
α = 82.21(2)°
β = 78.41(2)°
γ = 77.42(2)°
V = 2399.4(12)
ˉ
P1
4
2.134
C25.5H21.5Br2N0.5O1.5Te2
773.96
−148(1)
colourless, prism
0.150 × 0.150 × 0.150 mm
Monoclinic
a = 11.599(3) Å
b = 23.952(6) Å
c = 17.983(4) Å
—
β = 95.095(7)°
—
V = 4976(2)
P21/n
Lattice parameters
Volume/Å3
Space group
Z value
4
4
8
D
calc (g cm−3
)
2.084
2.106
2.066
F000
1272
68.314
1344
41.306
1424
53.49
2912
55.861
μ(Mo-Kα)/cm−1
No. of reflections measured
Rint
16726
0.0473
0.419–0.664
4324(253)
17.09
0.0453
0.0535
0.1624
18531
0.102
0.208–0.921
4609(253)
18.22
0.0758
0.1013
0.2485
19966
31149
0.0472
0.347–0.433
9013(568)
15.87
0.0329
0.0403
0.0692
0.0619
0.310–0.852
9555(516)
17.96
Min and max transmissions
Observed reflection (no. variables)
Reflection/parameter ratio
Residuals: R1 (I > 2.00σ(I))
Residuals: R (all reflections)
Residuals: wR2 (all reflections)
Goodness of fit indicator
Maximum peak in final diff. map
Minimum peak in final diff. map
0.0664
0.0941
0.2394
1.205
1.129
1.287
1.081
1.58 e−/Å3
−1.79 e−/Å3
1.94 e−/Å3
−2.58 e−/Å3
2.60 e−/Å3
−3.21 e−/Å3
0.95 e−/Å3
−0.76 e−/Å3
The quasi-linear Se-I-I fragment resides above the peri-gap at
92.24(1)° to the Se(1)–C(13) bond and 69.23(1)° to the C(13)–
Se(1)–C(1) plane (Fig. 2). The equatorial configuration of the
type B structure reduces the interaction between repulsive
halogen and chalcogen lone-pairs and promotes the existence of
an attractive quasi-linear three-body Br⋯Se–CPh fragment
[169.59(1)°]. This has been classified as an attractive three-
centre four electron type interaction resulting from the delocali-
sation of a halogen lone-pair (G) into the antibonding σ* (Se–C)
orbital.29,30
crowding around the selenium lone-pairs [4 θ = 79.27(1)°; 5 θ =
79.32(1)°; Fig. 2].7 Additional weak non-covalent interactions
exist within the crystal structures of 4 and 5. The CA-cis orien-
tation encourages the association of neighbouring phenyl ring
systems through intramolecular nonbonded π-π stacking.
Rotation around the Se–CPh bonds affords a mutual equatorial-
axial arrangement of the phenyl rings with respect to the
CAcenap–E–CPh planes which promotes an edge-to-face motif,
resulting from weak CH⋯π interactions.37 CH⋯centroid (Cg)
distances are in the range for typical CH⋯π edge-to-face
π-stacking (3.0 Å).38
Conversely, CT adducts 4 and 5 display a notable increase in
molecular distortion compared with donors D2 and D3, respect-
ively,30 but naturally, adduct 4 containing the larger peri-
moieties, exhibits the greatest degree of distortion. In both
derivatives rotation around the Se(1)–C(1) bond aligns the Se–CPh
bond in a twist conformation, thus altering the geometry from
type BA, exhibited by D2 and D3, to type CA-c (Fig. 3).30,36
The geometry of the two acenaphthene ring systems, is domi-
nated by the divergence of the peri-groups within the mean
plane, though tilting of the E–CAcenap bonds is more pronounced
in 4 (splay angles: 4 20.5°, 5 19.0°). Correspondingly, Se⋯E
peri-distances in the pair of CT adducts [4 3.326(3) Å; 5 3.252
(2) Å] are notably longer than the respective donor compounds
[D2 3.1834(10) Å; D3 3.113(4) Å],30 only 12% shorter than the
respective van der Waals radii (cf. D2/D3 16%).30 Adversely, the
hypervalent Se-I-I fragments in both compounds occupy a pos-
ition pointing away from the peri-gap, contrasting with the geo-
metry adopted by 1 and naphthalene analogues of 4 and 5.27
This is indicated by E⋯Se-I angles 128.41° (4) and 129.05° (5)
(cf. 1 Br⋯Se-I 78.24°). Nevertheless, the Se-I vector lies quasi-
perpendicular to the C(13)–Se(1)–C(1) plane due to steric
Further intermolecular CH⋯π short contacts exist between
individual molecules within the molecular structures of 4 and 5.
The π⋯π interactions are illustrated in Fig. 4 and hydrogen bond
data is displayed in Table S3†.
Crystallographic data suggests that dibromine addition pro-
ducts formed from diorganoselenium donor compounds exclu-
sively adopt see-saw geometries, bearing a distinctive linear
Br–Se–Br component.7–9,39 We have recently reported the syn-
thesis of two tribromide salts containing bromoselanyl cations
[R2Se–Br]+, prepared from the naphthalene analogues of D2 and
D3, which do not adopt see-saw geometries and thus contradict
the observed trend.27 The reactivity of D2 and D3 mirrors that of
the corresponding naphthalene donor compounds. Upon treat-
ment with dibromine, the extensive electron donating ability of
selenium affords a strong donor-strong acceptor system which
subsequently weakens the dibromine bond. Partial negative
charge, delocalised on the terminal Br atom encourages donation
to a second dibromine molecule (acceptor).7,8,27 The Se(1)–
Br(1) bond is strengthened to such an extent that the Br(1)–Br(2)
bond is cleaved and tribromide salts 2 and 3 result [R2Se–Br]+⋯
3158 | Dalton Trans., 2012, 41, 3154–3165
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Scheme 1 The syntheses of addition products 1–12, formed from the reactions of selenium and tellurium donors D1–D7 with dibromine and
diiodine.
Fig. 1 The molecular structures of CT spoke adducts 1 and 4 (H atoms omitted for clarity). The structure of 5 (adopting a similar conformation to 4)
is omitted here but can be found in (Fig. S1†).
[Br–Br2]− (Fig. 5).7,8,27 The nature of the chemical bond in
linear trihalide anions has been reviewed and classified by the
Rundle and Pimentel model for electron rich 3c-4e systems and
a charge-transfer model.2,17 As expected, the Br–Br bonds in the
hypervalent quasi-linear tribromide anions of 2 and 3 [2 178.19
(9)°;
3
179.24(5)°] are weaker and subsequently longer
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Fig. 2 Dihedral angle θ, defines the location of the dihalogen molecule
and the geometry of CT spoke adducts formed from dihalogens binding
to sp3 hybridised chalcogen donor atoms.7
[2.50–2.62 Å; Table 3] than the bonding in free bromine
[2.28 Å].35
The acenaphthene bromoselanyl cations of 2 and 3 adopt
axial-axial conformations, aligning the E–CPh bonds perpendicu-
lar to the mean plane, with the phenyl rings assuming a cis-
configuration (type AA-c; Fig. 3).36 As a consequence of the
geometry, the Br(1)–Se(1) bond lies equatorial with respect to
the acenaphthene plane and by virtue forms a quasi-linear Br–
Se⋯E′ three-body fragment [2 171.42°; 3 174.95(1)°], which
dominates the acenaphthene geometry. This can be thought of as
a weak hypervalent G⋯Se-X 3c-4e type interaction, closely
related to the T-shaped 3c-4e interaction, and has been shown to
control the fine structures of compounds.9
Invariably, when large heteroatoms of Group 16 are con-
strained by a rigid organic backbone, such as acenaphthene or
naphthalene, they experience considerable steric hindrance.5,6,20
The repulsive interactions which consequently transpire between
the two peri-functionalities as a result of their sub-van der Waals
contacts, gives a natural preference for the carbon framework to
distort from an ideal geometry.18,19 This is achieved via in-plane
and out-of-plane distortions of the exocyclic bonds and sup-
plementary buckling of the aromatic ring system (angular
strain).5,6,20,21
Fig. 4 Intramolecular (4) edge-to-face π-π stacking as a result of weak,
hydrogen bond type, CH⋯π interactions.
Evidence of weak hypervalent E′⋯Se–Br 3c-4e type inter-
actions operating in 2 and 3 comes from conspicuously
short intramolecular Se⋯E′ peri-separations [2 2.801(3) Å] and
[3 2.740(3) Å] compared with the non-bonding interactions
observed in donors D2 [3.1834(5) Å] and D3 [3.113(4) Å].30
These distances are 26% shorter than the sum of van der Waals
radii for the two interacting atoms and approach the distances for
single electron pair Se–Se/Se–S bonds [2.3639(5) Å; 2.24(1)
Å].27,40 This dramatic contraction is also accompanied by a
natural reduction in molecular distortion, particularly within the
acenaphthene plane where the divergence of exocyclic E–CAcenap
bonds is noticeably less pronounced (splay angles 2 7.3°, 3 5.4°;
cf. D2 16.5°, D3 12.7°).30
It is therefore conceivable to predict that an attractive intramo-
lecular non-covalent interaction exists between the two bulky
chalcogen atoms in both cations to alleviate steric strain. The
linear nature of the Br–Se⋯E′ fragments suggest these are
weakly attractive hypervalent 3c-4e interactions,2,9,17 which are
found to prevail in analogous naphthalene systems.28
Alternatively, strain relief can be accomplished through attrac-
tive intramolecular interactions between peri-substituents due
to the presence of weak or strong bonding, thus relaxing the
geometry of the backbone without the need for molecular
distortion.6,20
To complement these findings, density functional theory
(DFT) calculations were performed for selected compounds of
this study, calling special attention to the extent of chalcogen-
chalcogen binding. Table 7 summarises selected geometrical
Fig. 3 The orientation of the E(phenyl) groups, the quasi-linear arrangements and structural conformations of 1–6.
3160 | Dalton Trans., 2012, 41, 3154–3165 This journal is © The Royal Society of Chemistry 2012

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Fig. 5 The molecular structure of bromoselanyl tribromide salt 2
(H atoms omitted for clarity). The structure of 3 (adopting a similar con-
formation) is omitted here but can be found (Fig. S1†).
Fig. 6 B3LYP-computed Se⋯Se distances and WBIs [in brackets] in
selected acenaphthene derivatives.
parameters for 2, 3 and the corresponding bare cations 2a and
3a, together with the chalcogen-chalcogen Wiberg bond indices
(WBIs).41 The latter are a probe for the extent of covalent
bonding, approaching a value close to one for true single bonds.
In all cases, significant WBIs between ca. 0.36 and 0.26 are
obtained for the Se–Se or Se–S pairs (Table 7). The WBIs
obtained for the Se–Br bonds are between 0.57 and 0.69,
suggesting substantial 3c-4e character in these systems.
compounds [χ(Te) 2.08]32 are known to react with dihalogens
and interhalogens [χ(F) 3.94–χ(I) 2.36]32 to form see-saw or
T-shaped adducts, with no CT XY-adducts known for organic
tellurium donors.9 Tellurium forms strong donor-strong acceptor
systems with the dihalogens, strong enough to cleave the X-X
bond and oxidize tellurium.
−
Even though the orientation of the Br3 moiety in the opti-
Tellurium donors D4–D6 react conventionally with dibromine
and diiodine, affording a series of insertion adducts (6–11) exhi-
biting molecular see-saw geometries analogous to those of corre-
sponding naphthalene adducts (Fig. 7).9,28 In the mixed
chalcogen species D5 and D6, the addition reaction occurs
exclusively at the tellurium site due to the greater donor ability
of Te over Se/S. In each derivative, the linear X-Te-X moiety
[172.7–177.7°] lies axial to the equatorial plane consisting of the
two organo-groups and the lone pair on tellurium. Te–Br bond
distances vary from 2.66–2.70 Å and Te–I distances from
2.89–2.96 Å, with a degree of asymmetry in some instances (cf.
10 Te(1)–I(1) 2.89, Te(1)–I(2) 2.96; Table 3). In each case, the
tellurium atom adopts a distorted trigonal bipyramidal (TBP)
geometry with angles around the central atom in the range
86.1–100.3° (Fig. 8).
The presence of the linear three-body system in 6–11 has no
significant effect on the conformation of the phenyl rings, the
acenaphthene configuration or the degree of molecular distortion
compared with D4–D6.30 Bromo compounds 6 and 7 adopt type
B structures with an equatorial alignment of the Te–CPh bond
with respect to the acenaphthene plane and mixed chalcogen
compounds 8–11 adopt similar BA type structures (Fig. 8).36
In each tellurium derivative, a quasi-linear G⋯Te–CPh align-
ment exists with angles [159.6–176.6°] and intramolecular peri-
distances 17–19% shorter than the sum of van der Waals radii [6
3.2581(19) Å; 7 3.2050(11) Å; 8 3.218(3) Å; 9 3.2729(8) Å; 10
3.141(4) Å; 11 3.2677(18) Å (3.2862(18) Å)].
mised isolated ion pairs differs somewhat from that observed in
the periodic crystals, it is interesting to note that its presence
appears to reinforce chalcogen-chalcogen bonding. This
increased bonding is evidenced by a slight decrease in the
Se⋯X distances (by ca. 0.06 Å to 0.08 Å) and a concomitant
increase in the WBIs (by 0.04 to 0.06, compare data for 2 and
2a, or for 3 and 3a in Table 7). The computed values for the di-
selenium cation 2a are similar to those of the corresponding
naphthalene congeners28 and in between those for the neutral
acenaphthene species with two SePh groups30 and with one26 or
two42 Se–Br moieties (see Fig. 6).
Typically, see-saw or T-shaped molecular geometries are
formed when the electronegativity of the halide is greater
than that of the chalcogen donor species.7–9 In this instance,
nucleophilic halide attack occurs at the chalcogen atom of
the intermediate [R2Eδ+–Xδ−] cation, affording a hypervalent
quasi-linear X-E-X moiety.7–9 Consequently, organotellurium
Table 7 Selected optimised distances [B3LYP/6-31+G* level, in Å]
for 2, 3, together with the Se⋯X bond indices [WBIs, in brackets]. In
italics: experimental data (XRD)a
Compound
d(Se⋯X)
[WBI]
d(Se⋯Br3)
d(X⋯Br3)
2ab
2
2.913
2.829
2.801
2.842
2.778
2.74
[0.304]
[0.364]
[0.341]
[0.256]
[0.295]
[0.277]
—
—
3.812
3.53
—
3.639
3.67
3.466
3.868
—
3.728
3.973
2
3ab
3
Whilst this linear alignment exhibits weak hypervalent 3c-4e
character,26,28,29 the interactions between peri-functionalities are
minimal when compared with adducts 2 and 3. The generation
of the X-Te-X insertion fragment is accompanied by a prominent
downfield shift in the 125Te NMR spectra of 6–11 related to
3
a Including WBIs calculated for the coordinates from X-ray
crystallography; b 2a and 3a are bare cations, i.e. with the Br3−
counteranion deleted.
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Fig. 7 The molecular see-saw geometry adopted in the structures of tellurium insertion adducts 6, 8 and 11 (H atoms omitted for clarity). The struc-
tures of 7, 9 and 10 (adopting conformations similar to 6, 8 and 11, respectively) are omitted here but can be found (Fig. S2†).
Fig. 8 The orientation of the E(phenyl) groups, the quasi-linear arrangements and structural conformations of 7–12.
D4–D6 [δ = 696.0, 689.4, 663.4 ppm, respectively].30 Single
peaks are observed in all six spectra, with signals for bromine
analogues 6 [δ = 918.8 ppm], 8 [δ = 930.6 ppm] and 9 [δ =
921.9 ppm], displaying higher chemical shifts than their iodine
counterparts 7 [δ = 860.6 ppm], 10 [δ = 878.9 ppm] and 11 [δ =
871.0 ppm].
Additional weak non-covalent interactions exist between indi-
vidual molecules within the crystal structures of 6–11; CH⋯π
interactions37 range from 2.69–2.96 Å within the known
CH⋯centroid (Cg) range38 and a number of short CH⋯X
hydrogen bond type interactions are also present (H⋯Br
2.66–2.94 Å; H⋯I 3.03–3.05 Å; see Table S3 †. Further short
intermolecular contacts exist between Te and I atoms of neigh-
bouring I–Te–I units in 10 constructing a planar Te2I2 square
with Te⋯I distances of 3.925(1) Å.
dibromine solution. In order to explore the reason why bromina-
tion of Acenap(TePh)2 results exclusively in the oxygenated
derivative 12, rather than in pure organotellurium bromides,
additional DFT computations were performed. Products that
could have been expected comprise Ax, the analogue of
the known compounds Acenap(TePhBr2)(EPh) (E = S, Se),28
Ay1, a doubly brominated species, and Ay2, the congener of 2
(Fig. 10).
The optimised geometries of these possible products are unre-
markable, with Te⋯Te distances of 3.44 Å [0.15], 3.84 Å [0.01],
and 3.04 Å [0.48] for Ax, Ay1, and Ay2, respectively [WBIs in
brackets]. Ay1 has two unsymmetric Te–Br⋯Te bridges (Te–Br
Generally, when tellurium donors are treated with dihalogen
or interhalogen acceptors, see-saw or T-shaped molecular geo-
metries are observed, characterised by hypervalent X-Te-X frag-
ments.9 In contrast, the di-tellurium donor D7 reacts with a
single equivalent of dibromine to afford the insertion see-saw
adduct 12, exhibiting an unexpected Te–O–Te bridge, two quasi-
linear Br–Te–O fragments and no classical Br–Te–Br moiety
(Fig. 9). Repeating the reaction under standard Schlenk con-
ditions and with a higher loading of dibromine had no effect on
the outcome of the reaction, exclusively affording 12 each time
and indicating the origin of the oxygen atom was the parent
Fig. 9 The molecular structure of tellurium insertion adduct 12 (H
atoms omitted for clarity).
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by dispersion (compare the last two ΔGr(CH2Cl2) values in
Table 8), but with a very low overall driving force. Notwith-
standing the crude nature of some approximations involved
(ideal-gas entropies, continuum solvation model), (12)2 does not
appear to be a deep thermodynamic sink. Its formation according
to reactions (4) and (5) is viable under the experimental con-
ditions, when the other product, HBr, is removed from the equili-
brium mixture through vaporisation.
The formation of the two Br–Te–O fragments is accompanied
by a prominent downfield shift in the125Te NMR spectrum of 12
compared to donor D7 [δ = 585.9 ppm] with a single peak at δ =
961.3 ppm.
The see-saw molecular geometry ensures the central tellurium
atoms occupy similar distorted trigonal bipyramidal (TBP)
environments [84.0–96.5°], with both linear Br–Te–O moieties
[172.5–176.5°] lying axial to the equatorial planes formed by
the tellurium organo-groups and lone pairs. The two Br–Te–O
fragments lie quasi-perpendicular to one another, the oxygen
atom forced into a distorted angular geometry, with a Te–O–Te
angle of 113.11(15)° [116.17(16)°].
Fig. 10 Possible products from the reaction of Acenap(TePh)2 D7 and
dibromine.
and Te⋯Br distances of 2.81 Å and 3.47 Å, respectively), but is
otherwise similar to the known, more symmetric ArBr2Te(μ-Br)2
TeBr2Ar (Ar = p-C6H4OMe).43
Interestingly, the degree of molecular distortion in 12 is com-
parable to donor D7,30 with an intramolecular Te⋯Te peri-
separation of 3.335(1) Å [3.385(1) Å], ∼19% shorter than the
sum of van der Waals radii (cf. D7 3.3674(19) Å; 18%).30 The
large tellurium atoms are accommodated by a significant in-
plane distortion of the exocyclic Te–CAcenap bonds, with a large
angular splay of 19.0° [20.1°]. Conversely, only a small displace-
ment of the peri-atoms is observed from the mean acenaphthene
plane, with Te(1) 0.05(1) Å [0.10(1) Å] and Te(2) 0.14(1) Å
[0.12(1) Å] from the plane respectively. The twist alignment of
the two Te–CPh bonds, cis to the mean acenaphthene plane (type
CC-c; Fig. 8),36 implies there is no phenyl ring overlap and no
π-stacking, however a number of short intermolecular inter-
actions exist between Br and Te atoms of neighbouring Br–Te–O
units (Br⋯Te 3.32–3.68 Å). Br–Te bond lengths 2.77–3.02 Å
are all longer than expected45 and longer than the bond lengths
observed in 6–11, with the Br(3)–Te(3) bond distance in the
second independent molecule significantly longer at 3.024(7) Å.
Te–O bond lengths 1.94–2.05 Å are at the lower end of the range
for known Br–Te–O fragments.45
The structure of the actual product, 12, is more complicated,
as it forms dimers in the crystal, denoted (12)2 in the following.
A monomeric minimum 12 can be optimised, in which the
roughly linear Br–Te–O moieties of the dimer are preserved. A
similar structure with an isosceles BrTe–O–TeBr triangle is
found in (nBu)2BrTe–O–TeBr(nBu)2,44 albeit with a wider Te–
O–Te bond angle, 122.4° (cf. 111.5° and 118.3° in 12 and (12)2,
respectively, mean expt. 114.6°). As expected, little direct Te–Te
bonding is apparent in 12 and (12)2, despite the rather short
Te⋯Te contacts of 3.38 Å and 3.49 Å, respectively (cf. mean
expt. 3.36 Å, all WBIs 0.02).
Selected reaction energies are collected in Table 8, including
corrections for entropy and environment effects. The latter are
assessed through a polarisable continuum modelling a solvent of
medium polarity, dichloromethane (for compatibility with our
previous results for related Se complexes).28 In the following,
we discuss primarily the ΔGr(CH2Cl2)/B3LYP-D3 data (last
column in Table 8). Reaction (1), i.e. formation of Ax, has the
largest computed driving force. A second bromination yielding
Ay1, reaction (2), is much less favourable. Ionic Ay2 is predicted
to be endothermic throughout, consistent with the failure to
observe it experimentally. Assuming that water is the most likely
source for the O atom in 12, reaction (4) is a plausible channel
leading to the latter. Formation of monomeric 12 through this
path is predicted to be unfavourable throughout, by at least
7.8 kcal mol−1. Dimerisation of 12 appears to be mainly driven
Conclusion
The group of selenium and tellurium acenaphthene donors
D1–D7 [Acenap(EPh)(Br) E = Se, Te; Acenap(SePh)(EPh) E =
Se, S; Acenap(TePh)(EPh) E = S, Se, Te] have been reacted
with dihalogens, dibromine and diiodine, to afford a range of
Table 8 Selected reaction energies and free energies [in kcal mol−1] at the B3LYP level, except where otherwise noted
Reaction
ΔEr
ΔEr+ZPEa
ΔGr298 K
ΔGr(CH2Cl2)
ΔGr(CH2Cl2) B3LYP-D3b
(1) Acenap(TePh)2 + Br2 → Ax
(2) Ax + Br2 → Ay1
(3) Ay1 → Ay2
−16
−1.5
15.7
12.5
−6.4
−14.9
−0.8
15.4
8.4
−2.9
10.8
13.6
10.6
5.3
−7.1
5.4
5
8.3
11.7
−18.7
−4.8
7.9
7.8
−1.6
(4) Ax + Br2 + H2O → 12 + 2 HBr
c
(5) 2 12 → (12)2
−6.4
a Including zero-point energy. b Including an empirical dispersion correction. c Including BSSE correction.
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structurally diverse addition products. Selenium donors D1–D3
reacted conventionally with diiodine forming three neutral
charge-transfer (CT) spoke adducts (R2E-X-Y, 10-X-2) 1, 4 and
5, characterised by hypervalent three-body Se-I-I linear frag-
ments. In each case, upon coordination of selenium to the diio-
dine molecule, no significant alteration to the acenaphthene
geometry was observed compared with the respective donor
compounds.
performed by Caroline Horsburgh. Calculations were performed
using the EaStCHEM Research Computing 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 EaStCHEM and the University of
St Andrews for support.
Conversely, when selenium donors D2 and D3 were treated
with dibromine, two tribromide salts containing bromoselanyl
cations [R2Se–Br]+ were afforded, similar to the reactions of cor-
responding naphthalene donors. In both adducts, the geometry
of the bromoselanyl cation is dominated by a quasi-linear Br–
Se⋯E′ three-body fragment, resulting from the cis-orientation of
axial phenyl rings with respect to the mean acenaphthene plane
(type AA-c). The combination of conspicuously short intramole-
cular Se⋯E′ peri-separations (26% shorter than the sum of van
der Waals radii) and angles that approach 180°, implies a weak
hypervalent G⋯Se–X 3c-4e type interaction operates within the
linear three-body systems. Density functional theory (DFT) cal-
culations performed on 2, 3 and their bare cations 2a and 3a,
support this interpretation; WBIs were obtained between ca.
0.26 and 0.36 in all four cases, with computed values for di-sel-
enium cation 2a, similar to the corresponding naphthalene con-
gener and in between those of the neutral donor species D2
[WBI 0.07] and acenaphthene species with one26 [WBI 0.55] or
two52 [WBI 0.60] Se-Br moieties. Further evidence of an attrac-
tive peri-interaction operating in 2 and 3 is the substantial
reduction in molecular distortion observed in the acenaphthene
geometry compared with respective donor compounds.
Treatment of tellurium donors D4–D6 with dibromine and
diiodine afforded a series of typical insertion adducts (6–11)
exhibiting molecular see-saw geometries, distinguished by
quasi-linear hypervalent X–Te–X functionalities. In the mixed
chalcogen species D5 and D6, the addition reaction occurred
exclusively at the tellurium site due to the greater donor ability
of Te over Se/S. Acenaphthene distortion in all six compounds is
comparable with donors D4–D6 and no significant attractive
interaction is observed between the peri-substituents.
Conversely, ditellurium donor D7 reacted with dibromine to
afford an unusual oxygenated derivative, containing a Te–O–Te
bridge and two quasi-linear Br–Te–O fragments. Further reac-
tions performed under standard Schlenk conditions and with a
higher loading of dibromine had no effect on the outcome and
exclusively afforded 12 each time. In order to understand why
the oxygenated derivative is favoured over more conventional
pure organotellurium bromides, additional DFT computations
were performed for possible products of the reaction. Whilst 12
was found to be the least thermodynamically favourable of a
series of potential products, its formation is viable under exper-
imental conditions following the removal of HBr from the
system. Despite the short Te⋯Te contacts in 12, no direct Te–Te
bonding is apparent [WBIs 0.02].
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