A New Conformation with an Extraordinarily Long, 3.04 ? Two-Electron, Six-Center Bond Observed for the π-[TCNE]22- Dimer in [NMe4]2[TCNE]2 (TCNE=Tetracyanoethylene)

Article abstract of DOI:10.1002/chem.201501963

[NMe₄]₂[TCNE]₂ (TCNE=tetracyanoethenide) formed from the reaction of TCNE and (NMe₄)CN in MeCN has ν_CN IR absorptions at 2195, 2191, 2172, and 2156 cm^-1 and a ν_CC absorption at 1383 cm^-1 that are characteristic of reduced TCNE. The TCNEs have an average central C-C distance of 1.423 ? that is also characteristic of reduced TCNE. The reduced TCNE forms a previously unknown non-eclipsed, centrosymmetric π-[TCNE]₂^2- dimer with nominal C₂ symmetry, 12 sub van der Waals interatomic contacts <3.3 ?, a central intradimer separation of 3.039(3) ?, and comparable intradimer C?N distances of 3.050(3) and 2.984(3) ?. The two pairs of central C?C atoms form a ≯C-C?C-C of 112.6° that is substantially greater than the 0° observed for the eclipsed D_2h π-[TCNE]₂^2- dimer possessing a two-electron, four-center (2e^-/4c) bond with two C?C components from a molecular orbital (MO) analysis. A MO study combining CAS(2,2)/MRMP2/cc-pVTZ and atoms-in-molecules (AIM) calculations indicates that the non-eclipsed, C₂ π-[TCNE]₂^2- dimer exhibits a new type of a long, intradimer bond involving one strong C?C and two weak C?N components, that is, a 2e^-/6c bond. The C₂ π-[TCNE]₂^2- conformer has a singlet, diamagnetic ground state with a thermally populated triplet excited state with J/k_B=1000 K (700 cm^-1; 86.8 meV; 2.00 kcalmol^-1; H=-2JS_a·S_b); at the CAS(2,2)/MBMP2 level the triplet is computed to be 9.0 kcalmol^-1 higher in energy than the closed-shell singlet ground state. The results from CAS(2,2)/NEVPT2/cc-pVTZ calculations indicate that the C₂ and D_2h conformers have two different local metastable minima with the C₂ conformer being 1.3 kcalmol^-1 less stable. The different natures of the C₂ and D_2h conformers are also noted from the results of valence bond (VB) qualitative diagram that shows a 10e^-/6c bond with one C?C and two C?N bonding components for the C₂ conformer as compared to the 6e^-/4c bond for the D_2h conformer with two C?C bonding components. C₂ or D_2h? That is the question: The C₂ conformation of π-[TCNE]₂^2- observed in [NMe₄]₂[TCNE] (see figure) differs significantly from the eclipsed, D_2h conformation reported for about 30 other cations. The C₂ conformation is computed to be 1.3 kcalmol^-1 less stable than the D_2h conformation, and both conformations are minima in their potential energy surfaces.

Full text of DOI:10.1002/chem.201501963

DOI: 10.1002/chem.201501963
Full Paper
&
Electronic Structure
A New Conformation With an Extraordinarily Long, 3.04 Š Two-
2À
Electron, Six-Center Bond Observed for the p-[TCNE] Dimer in
2
[
NMe ] [TCNE] (TCNE=Tetracyanoethylene)
4
2
2
[a]
[b]
[a]
[c]
Adora G. Graham, Fernando Mota, Endrit Shurdha, Arnold L. Rheingold,
Juan J. Novoa,* and Joel S. Miller*
[b]
[a]
th
Dedicated to Professor Jack Simons on the occasion of his 70 Birthday
2À
Abstract:
[NMe ] [TCNE]
2
(TCNE=tetracyanoethenide)
C2 p-[TCNE]2 dimer exhibits a new type of a long, intra-
4
2
formed from the reaction of TCNE and (NMe )CN in MeCN
dimer bond involving one strong C···C and two weak C···N
4
À1
À
2À
has nCN IR absorptions at 2195, 2191, 2172, and 2156 cm
components, that is, a 2e /6c bond. The C p-[TCNE] con-
2
2
À1
and a nCC absorption at 1383 cm that are characteristic of
former has a singlet, diamagnetic ground state with a ther-
reduced TCNE. The TCNEs have an average central CÀC dis-
tance of 1.423 Š that is also characteristic of reduced TCNE.
The reduced TCNE forms a previously unknown non-
mally populated triplet excited state with J/k =1000 K
B
À1
À1
(700 cm ; 86.8 meV; 2.00 kcalmol ; H=À2JS ·S ); at the
a
b
CAS(2,2)/MBMP2 level the triplet is computed to be 9.0 kcal
2À
À1
eclipsed, centrosymmetric p-[TCNE]2 dimer with nominal
C2 symmetry, 12 sub van der Waals interatomic contacts
mol higher in energy than the closed-shell singlet ground
state. The results from CAS(2,2)/NEVPT2/cc-pVTZ calculations
<
3.3 Š, a central intradimer separation of 3.039(3) Š, and
comparable intradimer C···N distances of 3.050(3) and
.984(3) Š. The two pairs of central C···C atoms form a
aCÀC···CÀC of 112.68 that is substantially greater than the
8 observed for the eclipsed D p-[TCNE] dimer possess-
indicate that the C and D conformers have two different
2
2h
local metastable minima with the C2 conformer being
À1
2
1.3 kcalmol less stable. The different natures of the C and
2
D2h conformers are also noted from the results of valence
2À
À
0
bond (VB) qualitative diagram that shows a 10e /6c bond
2
h
2
À
ing a two-electron, four-center (2e /4c) bond with two C···C
components from a molecular orbital (MO) analysis. A MO
study combining CAS(2,2)/MRMP2/cc-pVTZ and atoms-in-
molecules (AIM) calculations indicates that the non-eclipsed,
with one C···C and two C···N bonding components for the C2
À
conformer as compared to the 6e /4c bond for the D2h
conformer with two C···C bonding components.
[
2]
Introduction
mark description of covalent bonding. Organic chemistry has
been exceptionally well served by this two-electron, two-
À
The concept and implications of chemical bonding are core
center (2e /2c) bonding description and the concept of
[1]
aspects of chemistry that continue to attract attention even
as we approach the centennial anniversary of G. N. Lewis’ land-
resonance. The study of electron-deficient boranes
[
3]
expanded bonding concepts to more than two centers, but
À
2
e /2c bonding suffices for the vast majority of CÀC bonds.
2À
[
a] A. G. Graham, Dr. E. Shurdha, Prof. J. S. Miller
Department of Chemistry, University of Utah
Salt Lake City, UT 84112-0850 (USA)
The structure of D2 eclipsed p-[TCNE]2 (TCNE=tetracyano-
h
ethylene), Figure 1a, possesses a substantial sub van der Waals
À
intradimer separation of about 2.9 Š, and a 2e /4c CÀC molec-
E-mail: jsmiller@chem.utah.edu
[
4]
ular orbital (MO) bonding description is invoked. Albeit com-
puted to be metastable, the isolated dimers share many of the
[
b] Dr. F. Mota, Prof. J. J. Novoa
Departamento de Química Física & IQTCUB
Facultad de Química, Universidad de Barcelona
Av. Diagonal, 645, 08028-Barcelona (Spain)
E-mail: juan.novoa@ub.edu
À
[4–6]
features of a conventional 2e /2c covalent bond,
including
a bond critical point from an atoms-in-molecules (AIM) analy-
[
5,7]
sis,
although this is not always an accepted description of
[
c] Prof. A. L. Rheingold
a bond as both false positive and false negative examples
Department of Chemistry and Biochemistry
University of California, San Diego, Urey Hall 5128,code 0358
[
8]
À
have been reported. This 2e /4c CÀC bonding is a new type
[
4,5]
9
500 Gilman Drive, La Jolla, CA 92093-0358 (USA)
of bond, first termed a long, multicenter bond. Alternatively,
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201501963. It contains Ortep drawings,
HOMO and LUMO and optimum geometries of D2h and C p-[TCNE] , and
estimated energy gain induced by the overlap of two p orbitals of p sym-
metry placed each in one atom.
À
a valence bond (VB) study later described this as a 6e /4c
bond with three electrons placed in the p orbitals of the two
2
À
z
2
2
[
9]
central C atoms. This allows two equivalent resonance forms,
À
each with a 2e /2c bond between two of the overlapping cen-
Chem. Eur. J. 2015, 21, 13240 – 13245
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Results and Discussion
The reaction of TCNE and
[
23]
(
NMe )CN
in MeCN forms
4
[
24]
a purple crystalline solid with
nCN absorptions at 2195, 2191,
À1
2
172, and 2156 cm that differ
from the three absorptions at
À1
2
191, 2172, and 2162 cm that
are typical of the eclipsed D p-
2h
2
À [5]
[
TCNE]2
.
Nonetheless, they
are in the range attributed to
[
25]
monoreduced TCNE. However,
À1
the absorption at 1383 cm is
À1
assigned to n , but it is 20 cm
CC
higher in frequency than is typi-
cally observed for D2h p-
2À
[
TCNE]2 . In accord with the in-
dication that the reduced TCNE
is neither the above D2h p-
Figure 1. Perspective view (a) and a description of the electronic structure requiring two VB determinants having
À
À
2À
2À
a 2e /2c bonds (b) and one determinants having two 3e /2c bonds (c) for D2h p-[TCNE]
2
, as well as perspective
[TCNE]2
nor the s-dimer,
2
À
2À [26]
(
[
d) and top (e) views of the noneclipsed C
NMe [TCNE]
p-[TCNE]
2 2
(the atoms with yellow surfaces are below) present in
[
C (CN) ] ,
its structure was
4
8
4
]
2
2
.
elucidated, Figure 1d,e and Fig-
ure S1 in the Supporting Infor-
[
27]
tral C atoms of each fragment (Figure 1b), and a third resonant
mation. The TCNEs have average CÀCN and central CÀC and
distances of 1.417 and1.423 Š, respectively, in accord with D
À
form in which two 3e /2c bonds form among the central C
2h
À
2À
atoms (Figure 1c). This 6e /4c VB description is numerically
p-[TCNE]2 . In sharp contrast, however, this new reduced
[
10]
[11]
equivalent to the results from a CASSCF(6,4) MO study,
and also gives nearly identical orbital occupation numbers for
TCNE forms dimers with local C symmetry with twelve <3.3 Š
2
intradimer separations ranging from 2.947 to 3.168 Š, that
average 3.048Æ0.075 Š, Figure 1d,e. The central, intradimer
CC distance is 3.039(3) Š, while the two shortest intradimer
C···N distances are 3.050(3) and 2.984(3) Š. The nitrile groups
are slightly bent away from the nominal TCNE plane by about
4.58; thus, the central C atoms are slightly pyramidalized, that
[12–14]
the SOMO ÆSOMOs than does a CASSCF(2,2) study,
and
are in agreement with the results from RASSCF computations
using very large active spaces of near-full configuration inter-
[12]
action (CI) quality for the dimer p orbitals.
2
À
The long, intradimer bond found for p-[TCNE]2 is proto-
3
2
typical of a growing number of organic compounds that are
is, mixing some sp into the dominant sp character. This is in
[15–20]
best described by multicenter bonding.
To date there are
accord with an average out-of-the plane bending of 5.08 for
2
À [4,5]
about 30 structural determinations of the eclipsed, D p-
D2h p-[TCNE]2
.
Furthermore, the 112.68 aCÀC···CÀC dihe-
2h
2
À
[
TCNE]2 dimer (Figure 1a) with cations that range from small
dral angle is substantially greater than 08 for the eclipsed D
2h
+
[4,5,21]
2À
with significant electrostatic interactions, for example, K ,
to distant bulky cations with reduced electrostatic interactions,
p-[TCNE]2 . Except for different orientation, the geometrical
2À
features are similar to those for D2h p-[TCNE]2 , thus suggest-
+
[22]
for example, [Cr(C H ) ] . Geometrically, all examples (~30)
ing that a long, intradimer bond is also present for the non-
6
6 2
2À
2À
of D2h p-[TCNE]2 are essentially identical with eclipsed, re-
duced TCNE moieties with a 2.895Æ0.047 Š intradimer separa-
tion, the CÀCN moieties bend away from the nominal TCNE
plane by on average 5.08 (thus, the central C atoms are slightly
eclipsed C p-[TCNE]2 conformer. Hence, it is a new type of
2
long, multicenter bond with an intradimer CC distance of
3.039 Š; 5% longer than that observed for the eclipsed D p-
2
h
2À
[TCNE]2 conformer (Figure 1a).
2
pyramidal and deviate from sp character), and a aCÀC···CÀC
Further support for the presence of a long, intradimer bond
[
4,5]
2À
dihedral angle between the central CC bonds of 08 is found.
in C2 p-[TCNE]2 comes from magnetic measurements. The
As part of a project aimed at enhancing our knowledge of
unusually long multicenter CC bonds, and bonding in general,
magnetic susceptibility, c(T), of [NMe ] [TCNE] was measured
4
2
2
[
28]
above 5 K in a 3 kOe applied magnetic field, Figure 2.
[NMe ] [TCNE] exhibits a diamagnetic response in accord with
2À
the systematic study of the physical properties of p-[TCNE]2
4 2
2
has led to several examples possessing the aforementioned
eclipsed geometry. [NMe ] [TCNE] reported herein is anoma-
its closed-shell, singlet ground state as occurs for the eclipsed
2
À [4,5]
D2h p-[TCNE]2
,
and is inconsistent with the presence of S=
4
2
2
2À
À
2À
lous due to it lacking the robust eclipsed D2h p-[TCNE]2
1/2 [TCNE]C . However, for D p-[TCNE]2 above about 250 K,
2h
structure, and it is the first example with local C symmetry.
c(T) increases suggestive of thermal population of the triplet
state. The c(T) data were fitted to the Bleaney–Bower’s expres-
sion [Eq. (1); for H=À2JS ·S ] for coupled spins, in which N =
2
a
b
A
Avogadro’s constant, k =Boltzmann’s constant, g=LandØ g-
B
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2
À
Table 1. Shortest inter-fragment C···C distance (Š) for the D2h and C
2
p-[TCNE]
2
conformers and computed orbital occupation number for the SOMOÆ
À
SOMOs (nbÀSOMO and nabÀSOMO in e ). Also given is the interaction energy, Eint, relative to the four ionic fragments, Eint(4), the interaction energy relative to
two radical anions, Eint(2), and the energetic components of the latter [that is, exchange-repulsion, Eer, electrostatic, Eel, dispersion, Edisp, and bonding, Ebond
:
[
31]
E
er +Eel +Edisp +Ebond =Eint(2)]. The difference between the singlet, E(S), and triplet, E(T), total energy is computed as ESÀT =E(S)ÀE(T). All energies were cal-
[
32]
À1
culated at the MRMP2 level (as described in the Supporting Information) and are given in kcalmol
.
r
CC [Š]
Method
n
b
n
ab
Eint(4)
E
int(2)
E
er
E
el
E
disp
E
bond
E
ST
[a]
[a]
[b]
[a]
[a]
[b]
[c]
[c]
D
2h
2.89
CAS(2,2)/MRMP2
CAS(2,2)/NEVPT2
CAS(2,2)/NEVPT2
CAS(2,2)/MRMP2
CAS(2,2)/NEVPT2
CAS(2,2)/NEVPT2
1.63
1.63
1.81
1.54
1.54
1.67
0.37
0.37
0.19
0.46
0.46
0.33
À200.6
43.0
39.3
36.4
44.3
40.1
39.6
20.1
60.8
À25.8
À21.0
À12.1
À12.1
À14.1
2
2
.89
.63
À15.3
[e]
[d]
[d]
C
2
3.04
À161.4
17.3
54.1
À8.0
À8.0
À9.0
3
2
.04
.82
À10.6
[e]
[
a] Geometry from the crystal. [b] Geometry optimized using CASSCF(2,2)/NEVPT2 calculations. [c] These values respectively become 1.70 and 0.32 using
a CASSCF(6,4) calculation and 1.81 and 0.23 using a CASSCF(22,20) calculation. [d] These values respectively become 1.60 and 0.41 using a CASSCF(6,4) cal-
culation. [e] When the optimum geometry of the triplet is computationally searched, starting from the experimental structure, the computed energy of the
À
dimer exceeds that of the two [TCNE]C fragments and no minimum is found.
value, m =Bohr magneton, S=spin quantum number (i.e., 1/
B
2
), and q is the intradimer coupling for the thermal population
of the triplet state. (The second term of Equation (1) accounts
[
29]
for unpaired spin impurities arising from defects sites. ) The
data can be fit to Equation (1) with 1=0.01, q=0 K, and J/k =
B
À1
À1
1
000 K (700 cm ; 86.8 meV; 2.00 kcalmol ), Figure 2.
4 2 2
Figure 2. c(T) (”) for [NMe ] [TCNE] and the fit (c) of the data to Equa-
tion (1).
2
2 2
B
2
N g m
N g m SðS þ 1Þ
A
B
A
c ¼ ð1 À 1Þ
þ 1
ð1Þ
2
kB T
J
3k
T
B
k ðT À qÞð3 þ e Þ
B
2
2
À
The electronic structure of C p-[TCNE] was computation-
2
ally characterized by CAS(2,2)/MRMP2 and CAS(2,2)/NEVPT2
computations on the observed structure, using the cc-pVTZ
basis set, Table 1, Figure 3. A MO analysis reveals that the C p-
2
2À
[5]
[
TCNE]2 , akin to the D2h conformer, has a non-negligible
overlap between the SOMOs providing the type of MO dia-
gram found for conventional covalent bonds. Thus, the ground
electronic state is due to 1) the overlap of the two [TCNE]CÀ
fragments orbitals yielding a bonding and antibonding combi-
nation, and 2) double occupation of each of the bonding and
antibonding combinations for the doubly occupied orbitals
and the SOMO+SOMO combination of the fragments. Thus,
[30]
Figure 3. a) Side and b) top views of the HOMO, and c) side and d) top
2
À
views of the LUMO for C
2
p-[TCNE]
2
(cutoff of 0.02 atomic units). e) 3D and
f) contour view of the [TCNE]CÀ SOMO, and g) three bond critical points iden-
tified from an AIM analysis of the closed shell singlet electron density for
[NMe ] [TCNE] .
4 2 2
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the ground state should be a diamagnetic, closed-shell singlet
[5]
similar to the D2h conformer, as observed. The first excited
singlet state is computed to be open-shell and results from ex-
citing one electron from the SOMO+SOMO combination to
the SOMOÀSOMO combination. According to the results from
the CAS(2,2)/MRMP2 calculations (Table 1), the triplet states for
À1
the C2 and D2h conformers are 9.0 and 14.1 kcalmol ,
respectively, above their closed-shell singlet ground states.
This multireferent character of the wavefunction describing
these systems is manifested in the occupation number of the
[
33]
bonding and antibonding combinations of the SOMOs. For
the C conformer the computed orbital occupation numbers
Figure 4. Potential energy curves for the D and C conformations of the p-
2
2h
2
À
2À
[TCNE]
2
dimer computed at the CAS(2,2)/MRMP2 level (see Figure 1d,e for
are 1.6 and 0.4 e , respectively, which differ slightly from the
À
the geometries).
values of 1.8 and 0.2 e , respectively, found in the D confor-
2h
[
5]
mer (Table 1). Both sets of numbers significantly differ from
that expected for a monoreferent
closed-shell singlet state (i.e., 2
À
and 0 e , respectively). The C p-
2
2À
[
TCNE]2 structure is also pre-
dicted to have a closed-shell sin-
À1
glet ground state 9.0 kcalmol
below the lowest lying triplet
state at the MBMP2 level, and is
À1
about 70% of the 14.1 kcalmol
2À
computed for D2h p-[TCNE] by
2
[
5]
the same method.
Both conformations have
a similar computed interaction
energy (E ) and stability with re-
int
spect to dissociation into two
TCNE]CÀ fragments (Table 1), but
[
the D2h conformer is slightly
more stable than the C confor-
2
À1
mer (by 1.3 and 0.8 kcalmol
at the CAS(2,2)/MRMP2 and
[34]
CAS(2,2)/NEVPT2 levels,
re-
spectively), and the results from
the latter methodology have dif-
ferent local metastable minima
in their potential energy surface
[35]
(
Figures 4 and 5).
Hence,
a
polymorph possessing the
more prevalent D2h conformer
might be observed, perhaps
from a different solvent; howev- Figure 5. Shape of the potential energy surface E(d
x
,d
y
) for a) the D2h and b) the C
2
conformations around the ex-
perimental geometry indicating c) the d
x
(red) and d) the d
y
(green) directions located at the center of the surface.
er, this has yet to be noted.
2À
The lower molecules are fixed and the upper ones move as indicated by the green (y axis) and red (x axis). In
both surfaces the (0,0) point corresponds to the experimental geometry, that is, the dimer when the upper mole-
cule is not displaced. c) and d): Movement along x in c) displaces the upper molecule along the central CÀC bond
The C2 and D2h p-[TCNE]2
conformers share the same
metastable character with re- versus the fixed molecule; thus, decreasing the energy. Movement along y induces a lateral motion of the mole-
cule downward in d) and the energy increases, but then is slightly reduced (red region) when the two nitrile
groups overlap. The curves were computed when the two fragments were at their experimental geometry.
spect to its dissociation into
two [TCNE]CÀ fragments[5] (i.e.,
Eint(2)>0,
Table 1;
Figure 4).
2
À
However, although moderately repulsive, once formed the
dimers do not dissociate due to the barriers with respect to
dissociation, Figure 4. (These barriers were computationally
identified to originate from the competition between the sta-
Note that computationally both p-[TCNE]2
conformers
become energetically stable only when the cations are includ-
+
À
ed, as the sum of the attractive cation ···anion interactions
+
+
À
exceed the sum of the repulsive cation ···cation and anion
[
36]
À
[5]
bilizing Ebond and the destabilizing E components of E (2).
)
···anion interactions (that is, E (4)<0, Table 1).
int
el
int
Chem. Eur. J. 2015, 21, 13240 – 13245
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The nature of Eint for the C2
and D2h conformers was also
quantitatively compared by
looking at the relative strength
of the Eint components, that is,
E , E , Edisp, and Ebond, Table 1. Eel
er
el
dominates for both conforma-
tions and destabilizes E , while
int
for both conformers Edisp ꢀ40%
jE j is stabilizing. For the
el
closed-shell single state, the sta-
bilizing Ebond is approximately
equal to Edisp/2. Nonetheless, de-
spite this small relative weight
of the bonding component, it is
the key component indicating
the energetic preference for the
singlet state. Note, that the
energy differences in the com-
À
Figure 6. Qualitative VB illustration of a) the C(central)···C(central) and b) the two weaker C(central)···N 3e /2c
2 2
bonding components for C p-[TCNE] .
2
À
ponents of Eint between the C and D conformers are not con-
The different intradimer bonding in the C and D conform-
2 2h
2
2h
À1
stant. Thus, while E is 6.7 kcalmol more repulsive for the D2h
ers are also noted from a qualitative VB analysis. For the C2
conformation the electronic distribution in each [TCNE]CÀ frag-
el
with respect to the C conformer (due to the eclipsed orienta-
2
tion of the two interacting fragments in the D2h conformer),
ment (Figures 1b,c and 6) and the type of bonds that they
À1
À
À
À
the Edisp and Ebond components are also 4.8 and 4.1 kcalmol
allow (i.e., 1e /2c, 2e /2c, and 3e /2c types, Figure S2 in the
À
more stable for the D2h conformer.
Supporting Information) indicates that some [TCNE]C fragment
À
The aforementioned MO results show that the C conformer
resonant forms allow the simultaneous presence of a 3e /2c
2
has the same electronic structure and E (2) interaction energy
C(central)···C(central) bonding component (Figure 6a), and two
int
À
as the D2h conformer. Hence, in accord with all the criteria
3e /2c C(central)···N bonding components for the C conformer
2
used before to determine the presence of an intradimer bond
(Figure 6b) in agreement with the MO analysis. Thus, at the VB
À
for the D2 conformer, an intradimer bond is also present for C2
level the C conformer has a 10e /6c bond with one central
h
2
2À
p-[TCNE]2
.
C···C and two weaker, lateral C···N bonding components.
The differing structures, however, lead to very important dif-
ferences in their electronic properties. Their MO diagrams are
similar with an identical occupation of the fragment SOMO+
SOMO and SOMOÀSOMOs, and a predominantly closed-shell
singlet ground state. However, due to their different geome-
tries they have different bonding overlaps that impact their im-
Conclusion
2
À
The C conformation of p-[TCNE]2 observed in [NMe ] [TCNE]
2
4 2
differs significantly from the eclipsed, D2h conformation report-
ed for about 30 other cations. The C conformation is comput-
2
À1
portant electronic properties. The C conformer has three short
ed to be 1.3 kcalmol less stable than the D2h conformation,
2
intradimer contacts: one C(central)···C(central) and two C(cen-
and both conformations are minima in their potential energy
surfaces. Both conformations have a long, multicenter CÀC
bond between its two [TCNE]CÀ fragments and the interaction
tral)···N. A detailed analysis of the SOMO+SOMO combinations
for the C (Figure 3a-d,f) and D2h (Figure S1 in the Supporting
2
Information) conformers show regions in which the two
SOMOs simultaneously maximize their in-phase overlap, with
minimal out-of-phase overlap; thus, maximizing the net over-
lap, and the bonding energy component, Ebond. Also, the elec-
trostatic repulsion is minimized (Table 1) creating a local mini-
mum.
energy (E ) is dominated by the electrostatic component (E ).
int
el
However, due to their different geometries, the nature of the
intradimer bond in the C and D conformers differs, being
2
2h
À
À
2e /6c and 2e /4c in a MO description for the C and D con-
2
2h
formations, respectively. In VB terms the D2 conformer has
h
À
À
a 6e /4c bond, whereas for the C conformer has a 10e /6c
2
The MO diagrams and the orbital occupation numbers
bond, and both VB descriptions are equivalent to their respec-
tive MO description.
(
Table 1) indicate that the HOMO involves two electrons, one
À
À
from each fragment SOMO. Thus, in the MO methodology the
Besides the repulsive (radical anion) ···(radical anion) inter-
À
2À
long bond in the C conformer is a 2e /6c, bond, and thus, dif-
actions, the p-[TCNE]2 conformational preference is expected
2
À
[5]
fers from the 2e /4c description for the D conformer. The
to be strongly affected by the size and shape of the adjacent
2h
AIM analysis of the electron density computed at the CAS(2,2)/
MRMP2 level indicates that three intradimer bond critical
points (bcps) are located in the three aforementioned regions
cations. These are accounted for by Eel and E , while smaller
er
cations have a larger E (i.e., alkali cations possess significantly
el
stronger electrostatic interactions than bulkier alkylammonium
cations). Furthermore, cations with different shapes will lead to
anisotropies in Eel and Eer that may lead to new cation–p-
[
5]
for the C conformer, and two for the D2h conformer, Fig-
2
ure S2c in the Supporting Information.
Chem. Eur. J. 2015, 21, 13240 – 13245
www.chemeurj.org
13244
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper
2
À
[
TCNE]2 structural arrangements that minimize void space.
[22] J. S. Miller, D. M. O’Hare, A. Charkraborty, A. J. Epstein, J. Am. Chem. Soc.
1989, 111, 7853.
+
+
Thus, cations larger than Cs but smaller than [NMe ] , espe-
4
[
23] [NMe
2.532 g; 15.73 mmol) in MeCN (ca. 20 mL) and [NEt
25.82 mmol) dissolved in MeCN (ca. 15 mL). The [NEt ]CN was slowly
4
]CN was synthesized by the reaction of a slurry of [NMe
4 4
][BF ]
cially with different shapes and hydrogen bonding, may lead
to new structural conformations and new bonding features for
(
4
]CN (2.586 g;
4
2À
p-[TCNE]2 and their likely associated long bonds.
added into the [NMe
4 4
][BF ] solution, and after stirring overnight a white
powder formed that was collected by filtration and washed with MeCN
(
ca. 15 mL). Yield: 1.366 g (87%); IR (KBr, ): n=3020 (s; CH), 2053 (m;
À1
CN), 1483 (s; CC), 1408 (w), 1400 (w), 959 (s), 950 cm (s).
Acknowledgements
[
24] [NMe
TCNE (1,117 mg; 8.727 mmol) dissolved in MeCN (6 mL) was added
dropwise to an [NMe ]CN (0.874 g; 8.460 mmol) slurry in MeCN (ca.
4
][TCNE] was synthesized by means of the following reaction.
J.S.M., E.S., and A.G.G. appreciate the continued support by
the U. S. Department of Energy Division of Material Science
4
22 mL) in a Schlenk flask with an airtight syringe, and the solution im-
mediately turned dark. The solution was placed under vacuum for
about 5 min to remove any remaining cyanogen from the solution. The
solution was transferred back to the dry-box and concentrated in
vacuo to near dryness. A minimal amount MeCN was added to dissolve
any remaining solids and the solution was filtered through dry Celite to
(
No. DE-FG03-93ER45504) for the chemical synthesis as well as
spectroscopic and magnetic studies. F.M. and J.J.N. provided
the computational studies, and thank MINECO for funding
(
MAT2011-25972, using Feder funds) and the Catalan Autono-
remove any unreacted [NMe
4
]CN. Crystallization of the product was
mous Government (2014SGR1422) for funding. F.M. and J.J.N.
also thank the allocation of computer time provided by CESCA
and BSC. All authors contributed to the writing of the manu-
script.
achieved via Et O vapor diffusion, and the product was filtered and
2
washed with minimal (ca. 2 to 3 mL) fresh MeCN. The final product was
dried under vacuum with minimal heating for about 30 min .Yield
646 mg (37%).
[25] J. S. Miller, Angew. Chem. Int. Ed. 2006, 45, 2508; Angew. Chem. 2006,
118, 2570.
Keywords: ab initio calculations
cyanocarbon · electronic structure · long multicenter bond
·
bond energy
·
[26] J. Zhang, L. M. Liable-Sands, A. L. Rheingold, R. E. Del Sesto, D. C.
Gordon, B. M. Burkhart, J. S. Miller, Chem. Commun. 1998, 1385; K. I. Po-
khodnya, M. Bonner, A. G. DiPasquale, A. L. Rheingold, J. S. Miller, Chem.
Eur. J. 2008, 14, 714.
[
[
[
1] S. K. Ritter, Chem. Eng. News 2013, 91(36), 28.
2] G. N. Lewis, J. Am. Chem. Soc. 1916, 38, 762.
À1
[
27] [NMe
4
][TCNE]: C20
H
24
N
10, M
w
=404.49 gmol , monoclinic, space group:
Cc, a=6.5401(2), b=18.9278(6), c=18.4588(6) Š, b=96.4410(10)8, V=
3] See, for example: W. N. Lipscomb, Boron Hydrides, Benjamin, New York,
963. Chapter 2.
4] J. J. Novoa, P. Lafuente, R. E. Del Sesto, J. S. Miller, Angew. Chem. Int. Ed.
001, 40, 2540; Angew. Chem. 2001, 113, 2608.
5] R. E. Del Sesto, J. S. Miller, J. J. Novoa, P. Lafuente, Chem. Eur. J. 2002, 8,
894.
3
À3
2
270.59(12) Š , Z=4, T=100(2) K, 1calcd =1.183 gcm , R1=0.0348,
1
wR2=0.0941. CCDC 1051797 contains the supplementary crystallo-
graphic data. These data can be obtained free of charge by The Cam-
bridge Crystallographic Data Centre.
[
[
2
À6
À1
[
28] The diamagnetic correction is À132.5”10 emumol ; E. J. Brandon,
D. K. Rittenberg, A. M. Arif, J. S. Miller, Inorg. Chem. 1998, 37, 3376.
29] O. Kahn, Molecular magnetism, VCH, New York, 1993, p. 107.
30] When there is near degeneration between the orbitals, the electronic
ground state sometimes does not obey the aufbau principle.
31] This is the expression of the interaction energy of two interacting radi-
cals, and is an extension of IMPT method for closed-shell systems (C.
Hayes, A. J. Stone, Mol. Phys. 1984, 53, 83); for a detailed account on
how each component is estimated, see reference [17].
32] The CAS(2,2)/NEVPT2 multireferent method is an alternative implemen-
tation of the CAS(2,2)/MRMP2 method, and was selected due to its abil-
ity to perform geometry optimizations, while allowing a proper descrip-
tion of the dimer (R. W. A. Havenith, P. R. Taylor, C. Angeli, R. Cimiraglia,
K. Ruud, J. Chem. Phys. 2004, 120, 4619).
33] The occupation number of an orbital for a given method and basis set
is the eigenvalue of the first order density matrix for that orbital (P.-O.
Lçwdin, Phys. Rev. 1955, 97, 1474); by construction the CAS(n,m) and
CAS(n,m)/MRMP2 methods have the same occupation numbers as both
are based on the CAS(n,m) wavefunction.
34] The CAS(2,2)/NEVPT2 multireferent method is an approximate imple-
mentation of the CAS(2,2)/MRMP2 method, and was selected due to its
ability to perform geometry optimizations while allowing a proper de-
scription of the dimer.
4
[
[
[
6] J. S. Miller, J. J. Novoa, Acc. Chem. Res. 2007, 40, 189.
[
[
7] R. F. W. Bader, Atoms in Molecules, Clarendon, Oxford, 1990.
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H. Willner, Angew. Chem. Int. Ed. 2009, 48, 2592; Angew. Chem. 2009,
[
1
21, 2629. J. Cioslowski, S. T. Mixon, J. Am. Chem. Soc. 1992, 114, 4382.
[
9] B. Braida, K. Hendrickx, D. Domin, J. P. Dinnocenzo, P. C. Hiberty, J.
Chem. Theory Comput. 2013, 9, 2276.
[
[
10] a) S. Shaik, New J. Chem. 2007, 31, 2015; b) K. Hirao, H. Nakano, K. Na-
kayama, M. Dupuis, J. Chem. Phys. 1996, 105, 9227.
11] A complete active space that contains six electrons and four orbitals.
The orbitals result from the bonding and antibonding combinations of
[
À
the HOMO and SOMOs of each [TCNE]C fragment, that have three elec-
trons per fragment.
[
[
[
[
12] M. Capdevila-Cortada, J. Ribas-Arino, J. J. Novoa, J. Chem. Theory
Comput. 2014, 10, 650.
13] The active space contains two electrons and the SOMO+SOMO and
SOMOÀSOMOs.
14] The numerical accuracy of CASSCF computations can be improved by
including a substantial dynamical correlation, for example, running
CAS(2,2)/MRMP2 calculations. MRMP2 is similar to the CASPT2 method,
but has a different reference Hamiltonian; K. Hirao, Chem. Phys. Lett.
[
[
1
992, 190, 374.
15] I. Garcia-Yoldi, J. S. Miller, J. J. Novoa, Phys. Chem. Chem. Phys. 2008, 10,
106.
35] Although various DFT functionals fail to reproduce the existence of
[
2
a minimum for the C conformation, a CAS(2,2)/NEVPT2 full geometry
4
optimization confirms the minimum energy nature of the two confor-
mations, as that the optimum geometry close to the observed structure
is obtained (see the Supporting Information).
[
[
[
16] I. Garcia-Yoldi, J. S. Miller, J. J. Novoa, J. Phys. Chem. A 2009, 113, 484.
17] F. Mota, J. S. Miller, J. J. Novoa, J. Am. Chem. Soc. 2009, 131, 7699.
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Am. Chem. Soc. 2009, 131, 9070.
[
36] J. Jakowski, J. Simons, J. Am. Chem. Soc. 2003, 125, 16089.
[
[
19] I. Garcia-Yoldi, J. S. Miller, J. J. Novoa, J. Phys. Chem. A 2009, 113, 7124.
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J. S. Miller, Chem. Eur. J. 2011, 17, 9326.
[
21] J. Casado, S. R. Gonzµlez, F. J. Ramírez, J. T. L. Navarrete, S. H. Lapidus,
P. W. Stephens, H.-L. Vo, J. S. Miller, F. Mota, J. J. Novoa, Angew. Chem.
Int. Ed. 2013, 52, 6421; Angew. Chem. 2013, 125, 6549.
Received: May 19, 2015
Published online on July 29, 2015
Chem. Eur. J. 2015, 21, 13240 – 13245
www.chemeurj.org
13245
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim