N-Triphenylboryl- and N,N′-bis(triphenylboryl)benzo-2,1,3- telluradiazole

Article abstract of DOI:10.1039/b904713a

Being isoelectronic and isostructural analogues of N-alkyl-substituted telluradiazolyl cations, the adducts of triphenylborane with benzo-2,1,3-telluradiazole provide an electrically neutral point of reference with which the properties of the heterocyclic

Full text of DOI:10.1039/b904713a

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N-Triphenylboryl- and N,N0-bis(triphenylboryl)benzo-2,1,3-telluradiazolew
Anthony F. Cozzolino, Alex D. Bain, Stephanie Hanhan and Ignacio Vargas-Baca*
Received (in Berkeley, CA, USA) 12th March 2009, Accepted 12th May 2009
First published as an Advance Article on the web 9th June 2009
DOI: 10.1039/b904713a
Being isoelectronic and isostructural analogues of N-alkyl-
substituted telluradiazolyl cations, the adducts of triphenylborane
with benzo-2,1,3-telluradiazole provide an electrically neutral
point of reference with which the properties of the heterocyclic
ions can be compared.
The crystal structure of 2 (Fig. 1) demonstrated attachment
of the triphenylborane moiety to each nitrogen atom of the
benzo-2,1,3-telluradiazole molecule. The structure approxi-
mates a C2 symmetry, although it sits in a general position.
Two of the phenyl rings hinder access to the chalcogen,
preventing association to any Lewis bases. This would be the
first example of a C2N2TeII ring that does not appear to be
associated to another heterocycle molecule, solvent, anion or
donor atom in the solid state. However, the role of the
aromatic ring might not be innocent. The N–B–C bond angle
is significantly smaller for the rings next to the tellurium atom.
These rings exhibit short distances from their centroids to Te
(3.1188(2) and 3.1311(2) A). Similar Te–aryl interactions have
been examined by Zukerman-Schpector and Haiduc,6 but in 2
the distances from Te to the centroids of the rings are
significantly below the value reported for the shortest contact
of this type (3.482 A).7 On closer inspection, the chalcogen is
in contact (2.801(2) A) with an atom of one ring while for the
other ring the shortest distance (2.7925(2) A) is to the centroid
of a C–C bond. All these distances are shorter than the sum of
Te and C van der Waals’ radii (3.76 A).
Mono- and di-cations derived by formal N-alkylation of
1,2,5-chalcogenadiazolyl heterocycles are receiving increasing
attention due to their intriguing structural and chemical
properties. The dialkylated systems are regarded as a family
of structural analogues of the N-heterocyclic carbene (NHC)
imidazol-2-ylidene;1,2 of which the Se member has been
described as a ‘‘chalcogen dication sequestered by a chelating
diimine’’.2 In spite of coulombic repulsion, the monoalkylated
selenium3 and tellurium4 rings can form the dimers that are
characteristic of the crystal structures of the parent hetero-
cyles5 with even shorter secondary-bonding distances. The
existence of such aggregates in solution has been proposed
as an explanation for the singular redox behaviour of the
N-methyl benzotelluradiazolylium cation, as compared to its
lighter congeners, which prevented the observation of the
corresponding neutral free radical by EPR.4
The Te–N bond lengths of 2 are comparable to those
observed in the ribbon polymer of 1 (2.003(7) A)8 and dimers
of 4,7-dibromobenzo-2,1,3-telluradiazole (1.988(8) unsolvated,
2.001(5) A DMSO-solvated).8 The DFT optimized geometry of
2 reproduces all the major structural features.
Compared to the parent neutral heterocycles, the study of
the cations is probably complicated by electrostatic inter-
actions, counterion binding and an enhanced sensitivity
to water and nucleophiles in general. Therefore we chose to
prepare neutral isoelectronic analogues by reacting benzo-
2,1,3-telluradiazole (1) with Lewis acids. Both the 2 : 1 (2)
and 1 : 1 (3) triphenylborane adducts were isolated when using
the appropriate stoichiometry of reactants in toluene. The
products were isolated in micro-crystalline form and fully
characterized.w X-Ray diffraction quality crystals of 2 were
grown from a 1 : 1 mixture of bromobenzene and acetonitrile
while crystals of 3 were grown from toluene.z
The crystal structure of 3 (Fig. 2), modelled with 1.4%
twinning resulting from 1801 rotation about c, confirmed the
binding of one borane molecule to each heterocycle. Because
Department of Chemistry, McMaster University, 1280 Main St. W.,
Hamilton, ON, Canada. E-mail: vargas@chemistry.mcmaster.ca;
Fax: (905) 522-2509; Tel: (905) 525-9140 x23497
w Electronic supplementary information (ESI) available: Details of the
synthetic procedures, the spectroscopic data (including VT NMR),
and the DFT calculations. CCDC 720234 and 720235. For ESI and
crystallographic data in CIF or other electronic format see DOI:
10.1039/b904713a
Fig. 1 ORTEP (50% probability) of the asymmetric unit in the
crystal of 2. Hydrogen atoms and disordered atoms are omitted.
Selected distances (A) and angles (1): Te1–N1 1.996(2), Te1–N2
2.006(2), N1–B1 1.629(3), N2–B2 1.646(3); N1–Te1–N2 81.79(7),
Te1–N1–B1 116.0(1), Te1–N2–B2 115.1(1), N1–B1–C11 100.2(2),
N1–B1–C21 108.8(2), N1–B1–C31 108.0(2), N2–B2–C41 101.2(2),
N2–B2–C51 108.6(2), N2–B2–C61 108.8(2).
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Fig. 2 ORTEP (50% probability) of the asymmetric unit in the
crystal of 32. Hydrogen atoms are omitted. Selected distances(A)
and angles (1): Te1–N1 1.977(8), Te1–N2 2.056(7), Te2–N3 1.998(9),
Te2–N4 2.048(8), N2–B1 1.65(1), N4–B2 1.62(1); N1–Te1–N2 82.3(3),
N3–Te2–N4 82.5(3), Te1–N2–B1 118.1(6), Te2–N4–B2 119.2(6),
N2–B1–C7 108.6(8), N2–B1–C13 107.9(7), N2–B1–C19 102.1(7),
N4–B2–C31 108.7(8), N4–B2–C37 107.8(8), N4–B2–C43 102.8(8).
Fig. 3 Resonances of the ortho (4,7) protons in the benzo moiety of 32
in d8-toluene solution at different temperatures.
in this 1 : 1 adduct one of the nitrogen atoms and the
chalcogen atom are capable of secondary bonding, the
[Te–N]2 supramolecular synthon renders a dimeric structure.
Although 32 is centrosymmetric, its placement in a general
position duplicates the metrical parameters. In this case the
Te–N supramolecular contacts (2.591(9) and 2.580(8) A)
are ca. 0.1 A shorter than those in the 4,7-dibromobenzo-
2,1,3-telluradiazole dimer8 (2.697(8) A) but almost 0.2 A
longer than in the analogous structure of the N-methylbenzo-
telluradiazolylium cation (2.471(3) A).4 As in those cases,
within each heterocycle, the Te–N bond opposite to the
secondary interaction is significantly longer than the other.
The structure of 32 also displays short distances from the
The net stabilization of the Te–N intermolecular inter-
actions in 32 is therefore primarily the result of the charge
redistribution.
1 + BPh3 - 3; DH = ꢁ125.1 kJ molꢁ1
3 + BPh3 - 2; DH = ꢁ123.5 kJ molꢁ1
12 + 2 BPh3 - 32; DH = ꢁ259.8 kJ molꢁ1
1 + 1 - 12; DH = ꢁ67.5 kJ molꢁ1
(1)
(2)
(3)
(4)
(5)
3 + 3 - 32; DH = ꢁ75.9 kJ molꢁ1
At room temperature, the 1H NMR spectrum of 2 in solution
displayed two resonances for the protons of the benzo-
telluradiazole moiety, as expected. The corresponding bands
in the spectrum of 32 appeared as very broad peaks in
CD3C6D5, C6D6 and CD2Cl2; the presence of a dynamic
exchange process was confirmed by VT experiments in
d8-toluene (Fig. 3).w While two resonances were clearly defined
above 340 K, three bands were resolved below 250 K, the
fourth was obscured by other aromatic resonances. Line shape
analysis for the exchange of the protons in positions 4 and 7 of
benzo-2,1,3-telluradiazole gave a concentration independent
activation energy of 83 ꢂ 9 kJ molꢁ1. Although the magnitude
of this activation barrier is comparable to the supramolecular
association energy calculated for 3, the high-temperature
1H NMR spectrum implies that the dynamic process includes
fast shift of the BPh3 moiety between the two nitrogen atoms.
Such behaviour is observed in the 1 : 1 adduct of pyrazine and
BPh3, for which VT experiments gave an activation barrier of
78 ꢂ 8 kJ molꢁ1 and DFT calculations estimated an associa-
tion energy of 102.1 kJ molꢁ1. These observations suggest that
heavy atoms to the aromatic rings (dTe–centroid(C–C)
=
2.9157(7), 2.9487(7) A; dTe–centroid(Ph) = 3.4108(7), 3.4179(8) A)
and the N–B–C angles are smaller for the phenyl ring that
associates with the Te atom. The DFT optimized geometry of
32 reproduces all the major structural features.
Because the shorter secondary bonding distances suggest
that the supramolecular interactions might be stronger in the
borane adducts than in the parent heterocycles, the association
energies were estimated from the DFT total bonding energies
by considering the reaction enthalpies for eqn (1)–(5); the
numbers include contributions from structural relaxation but
not BSSE or ZPE, which are very small in closely related
systems.8 The average B–N bond energy from eqn (1) to (2)
(126.2 kJ molꢁ1) is within the range of ab initio calculated
binding energies for a number of simple Lewis base-borane
adducts (59.8–230.8 kJ molꢁ1).9,10 The enthalpies of reactions
(4) and (5) indicate that the secondary bonding interactions
are indeed stronger in the borane adduct. This enhancement of
binding energy was analyzed considering three contributions:
the Pauli (steric) repulsive contribution that arises from the
interaction of completely occupied orbitals, the electrostatic
contribution that results from the local dipole moments,
and the interaction of empty and occupied orbitals. The
Pauli repulsion and the electrostatic interaction became more
stabilizing by 57.6 and 25.4 kJ molꢁ1, respectively, while the
1
the VT H NMR spectrum of 32 does not reflect the simple
dissociation of the dimer and that dissociation of the B–N
bond is the most important, if not the only, contribution to the
activation barrier.
The 125Te resonance shifted to lower frequencies upon
attachment of each BPh3 molecule; from 2401 ppm in 1y to
2203 ppm in 32, and 2188 ppm in 2. A similar trend was
orbital interaction became less stabilizing by 74.5 kJ molꢁ1
.
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Table 1 Hirshfeld and VDD (in parentheses) atomic/group charges (a.u.) for 1, BPh3 and their adducts calculated using PW91 with the TZP
basis set
1
12
0.36 (0.30)
ꢁ0.20 (ꢁ0.19)
BPh3
3
32
0.47 (0.36)
2
Te
N
N (B)
N (SBI)
C6H4
B
Pha
Phb
Phc
0.37 (0.32)
ꢁ0.23 (ꢁ0.23)
0.47 (0.39)
ꢁ0.23 (ꢁ0.23)
0.58 (0.63)
ꢁ0.08 (ꢁ0.08)
ꢁ0.09 (ꢁ0.09)
ꢁ0.19 (ꢁ0.19)
0.19 (0.27)
0.01 (ꢁ0.05)
ꢁ0.11 (ꢁ0.03)
ꢁ0.14 (ꢁ0.14)
ꢁ0.14 (ꢁ0.13)
ꢁ0.09 (ꢁ0.10)
ꢁ0.24 (ꢁ0.24)
0.09 (0.13)
0.08 (0.14)
0.19 (0.23)
0.01 (ꢁ0.05)
ꢁ0.08 (0.01)
ꢁ0.14 (ꢁ0.14)
ꢁ0.14 (ꢁ0.13)
0.25 (0.28)
0.01 (0.03)
0.13 (0.09)
ꢁ0.04 (ꢁ0.03)
ꢁ0.04 (ꢁ0.03)
ꢁ0.04 (ꢁ0.03)
ꢁ0.06 (ꢁ0.08)
ꢁ0.14 (ꢁ0.15)
ꢁ0.14 (ꢁ0.15)
observed for the resonances of the benzo protons, extra-
polated to 306.3 K. Atomic charges were calculated in order
to examine the influence of the Lewis acid on the electron
density as a possible explanation for this trend. Mulliken and
Natural Population (NPA) analyses, Hirschfeld and Atoms-
In-Molecules (AIM) charges, and the Voronoi Deformation
Densities (VDD) were evaluated under the PW91 GGA and
the B3LYP hybrid exchange–correlation functionals with the
DZ, DZP, TZP and TZ2P basis sets and the ZORA relati-
vistic correction. As it is often the case,11,12 the Mulliken,
AIM and NPA charges were inconsistent across basis sets
and functionals, precluding a meaningful interpretation.
Density-based charges (Hirschfeld and VDD) were less
dependent on the basis set and functional; Table 1 summarizes
these results. While the dimerization of 1 and 3 causes
minimal changes to the charges, formation of the B–N bond
has a more significant influence. The most significant increases
of positive charge are located at the tellurium atom and
the benzo moiety. The positive charge on boron decreases
and the phenyl rings acquire a more negative charge; this
change is less pronounced on the aromatic ring which is in
contact with the chalcogen. Consequently, the observed
changes of 1H and 125Te chemical shifts upon borane coordina-
tion are probably due to depletion of p electron density on the
benzo ring and the anisotropic shielding of tellurium by the
phenyl rings.
Academic Research Computing Network (SHARCNET:
http://www.sharcnet.ca).
Notes and references
z Crystal data at 286(2) K for 2: C42H34B2N2Te1, M = 715.03 g molꢁ1
,
P21/c, a = 13.0123(7), b = 15.2679(6), c = 17.828(1) A, b =
106.256(4)1, V = 3400.3(3) A3, Z = 4, Dc = 1.399 g cmꢁ3, m =
0.908 mmꢁ1; 517 parameters were refined with 250 restraints using
41888 reflections to give R = 0.0393, Rw = 0.0712 and Rint = 0.0000.
CCDC 720234. Crystal data at 296(2) K for 3: C24H19B1N2Te1, M =
473.82 g molꢁ1, P21/n, a = 8.0389(6), b = 32.245(3), c = 15.754(1) A,
b = 90.755(1)1, V = 4083.2(6) A3, Z = 8, Dc = 1.542 g cmꢁ3, m =
1.468 mmꢁ1; 506 parameters were refined with 0 restraints using
8396 unique reflections to give R = 0.0713 and Rw = 0.1508 and
Rint = 0.1341. CCDC 720235.
y Because a different 125Te NMR chemical shift was recently published
for 1,4 the value reported here, and elsewhere,8 was verified by
acquiring the spectrum using various spectral widths and different
centre frequencies while locking to d6-DMSO at 303 K. The resonance
was consistently found at 158.1689 MHz (2401 ppm vs. an actual
standard of neat Me2Te measured at 157.7900 MHz, cf. the literature
value of 157.7899 MHz13).
1 H. M. Tuononen, R. Roesler, J. L. Dutton and P. J. Ragogna,
Inorg. Chem., 2007, 46, 10693–10706.
2 J. L. Dutton, H. M. Tuononen, M. C. Jennings and
P. J. Ragogna, J. Am. Chem. Soc., 2006, 128, 12624–12625.
3 (a) J. L. Dutton, J. J. Tindale, M. C. Jennings and P. J. Ragogna,
Chem. Commun., 2006, 2474–2476; (b) C. E. Bacon, D. J. Eisler,
R. L. Melen and J. M. Rawson, Chem. Commun., 2008, 4924–4926;
(c) J. L. Dutton, A. Sutrisno, R. W. Schurko and P. J. Ragogna,
Dalton Trans., 2008, 3470–3477.
While these investigations showed that attachment of a
Lewis acid to one of the nitrogen atoms of the telluradiazole
ring results in stronger secondary bonding interactions
(assessed from experimental distances and calculated
dimerization energies), there was no direct evidence of
the existence of dimers in solution. On the other hand, the
4 M. Risto, R. W. Reed, C. M. Robertson, R. Oilunkaniemi,
R. S. Laitinen and R. T. Oakley, Chem. Commun., 2008, 3278–3280.
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6 J. Zukerman-Schpector and I. Haiduc, CrystEngComm, 2002, 4,
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7 E. S. Lang, U. Abram and J. Strahle, Z. Anorg. Allg. Chem., 1997,
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structures of
2 and 32 invite further research into
8 A. F. Cozzolino, J. F. Britten and I. Vargas-Baca, Cryst. Growth
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9 V. Jonas, G. Frenking and M. T. Reetz, J. Am. Chem. Soc., 1994,
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10 G. Leroy, M. Sana and C. Wilante, Theor. Chem. Acc., 1993, 85,
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the use of Lewis acid coordination as a means to control the
association of chalcogenadiazoles and related heterocycles
and to introduce functionality to their supramolecular
structures.
We gratefully acknowledge the financial support of the
Natural Sciences and Engineering Research Council of
Canada for the Discovery Grant (IVB) and the PGSD
Scholarship which supported these investigations. This work
was made possible by the facilities of the Shared Hierarchical
11 C. F. Guerra, J. W. Handgraaf, E. J. Baerends and
F. M. Bickelhaupt, J. Comput. Chem., 2004, 25, 189–210.
12 I. Vargas-Baca, M. Findlater, A. Powell, K. V. Vasudevan and
A. H. Cowley, Dalton Trans., 2008, 6421–6426.
13 R. K. Harris, E. D. Becker, S. M. C. De Menezes, R. Goodfellow
and P. Granger, Pure Appl. Chem., 2001, 73, 1795–1818.
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