Bent and twisted: The electronic structure of 2-azapropenylium ions obtained by guanidine oxidation

Article abstract of DOI:10.1039/c6ra04494h

New bis-2-azapropenylium ions are obtained by oxidation of guanidino-substituted aromatic compounds. The dications 1²⁺ (1 = 1,4-bis-tetramethylguanidinobenzene) and 2²⁺ (2 = 1,4-bis-guanidinobenzene) exhibit unexpected bent-twisted structures, that differ from the known 2-azapropenylium ion structures (linear-orthogonal 2-azaallenium ion or bent-planar 2-azaallylium ion), and resemble the structures of carbodicarbenes. Their electronic structure was analysed by a combination of experiments and quantum chemical calculations. The results indicate that the bent-planar structure is disfavoured for steric reasons, while the allene-type linear-orthogonal structure is also disfavoured due to the electron-donating groups that support the bent form. For these reasons a bent-twisted structure is adopted, in which the nitrogen atom is engaged with two different orbitals in π-bonding with the carbon π-system and in a weaker π-interaction with the CN₂ group of the guanidino system.

Full text of DOI:10.1039/c6ra04494h

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Bent and twisted: the electronic structure of 2-
azapropenylium ions obtained by guanidine
oxidation†
Cite this: RSC Adv., 2016, 6, 39323
*
Julius Hornung, Olaf Hubner, Elisabeth Kaifer and Hans-Jorg Himmel
¨
¨
New bis-2-azapropenylium ions are obtained by oxidation of guanidino-substituted aromatic compounds.
The dications 12+ (1 ¼ 1,4-bis-tetramethylguanidinobenzene) and 22+ (2 ¼ 1,4-bis-guanidinobenzene)
exhibit unexpected bent-twisted structures, that differ from the known 2-azapropenylium ion structures
(linear-orthogonal 2-azaallenium ion or bent-planar 2-azaallylium ion), and resemble the structures of
carbodicarbenes. Their electronic structure was analysed by a combination of experiments and quantum
chemical calculations. The results indicate that the bent-planar structure is disfavoured for steric
reasons, while the allene-type linear-orthogonal structure is also disfavoured due to the electron-
donating groups that support the bent form. For these reasons a bent-twisted structure is adopted, in
which the nitrogen atom is engaged with two different orbitals in p-bonding with the carbon p-system
and in a weaker p-interaction with the CN2 group of the guanidino system.
Received 19th February 2016
Accepted 12th April 2016
DOI: 10.1039/c6ra04494h
www.rsc.org/advances
electron sharing bonds and a nitrogen anion is at least equally
justied or even more justied because it assigns the negative
1. Introduction
charge to the electronegative nitrogen atom and positive
charges to the carbon atoms. In the case of carbodicarbenes,
bent structures with twisted N–C–N planes of the benzimida-
zoline framework were found (see the example in Scheme 1),5a
but the bent-planar structures are only slightly higher in energy
(ca. 14 kJ molꢀ1 for the carbodicarbene in Scheme 1 calculated
at the BP86/TZ2P level of theory). According to MP2/6-31+G*
calculations, the bent-planar form of [N{C(NH2)2}2]+ is ca. 16 kJ
molꢀ1 favoured over a bent-orthogonal form (90ꢁ dihedral angle
between the NC2 plane and CN2 plane).2 The small energy
2-Azapropenylium ions exhibit interesting chemistry, and are
known to exist either in the linear orthogonal 2-azaallenium
or valence isomeric bent-planar 2-azaallylium form (see
Scheme 1).1 According to quantum chemical MP2/6-31G*//4-
31G calculations on the parent compound [N(CH2)2]+ (R1 ¼ R2
¼ H), the bent-planar 2-azaallylium form is 176 kJ molꢀ1 higher
in energy than the linear orthogonal 2-azaallenium form. Later,
a slightly smaller energy difference of 168 kJ molꢀ1 was found by
MP2/6-31+G* calculations.2 Aliphatic or aromatic groups favour
the 2-azaallenium form. On the other hand, electron donor
groups (e.g. OEt) stabilize the 2-azaallylium form. Hence the
compounds [N{C(OEt)(p-Tol)}2]+ (R1 ¼ OEt, R2 ¼ p-Tol) and [N
{C(NH2)2}2]+ (R1 ¼ R2 ¼ NH2)3 adopt the bent-planar 2-azaally-
lium valence isomeric structure.
In recent work by Bharatam et al., [N{C(NH2)2}2]+ and related
compounds were described as 2-azadicarbene cations, +N()
L)2, with two lone-pairs on nitrogen (see Lewis structure in
Scheme 1).2,4 This description is motivated by comparison with
the electronic structure of some isoelectronic bent allenes
(carbodicarbenes).5 However, we note that the description with
two dative bonds – thus two carbene lone pairs bonding to
a nitrogen cation – is arbitrary and a description with two
¨
Anorganisch-Chemisches Institut, Ruprecht-Karls-Universitat Heidelberg, Im
Scheme 1 Lewis structures of linear orthogonal 2-azaallenium and
bent-planar 2-azaallylium ions, 2-azadicarbene cations (e.g. R ¼ H or
Me), and the questionable description as 2-azadicerbene cation,
“+N()L)2”, as well as the carbodicarbene description, C(NHC)2 (NHC
¼ N-heterocyclic carbene).
Neuenheimer Feld 270, 69120 Heidelberg, Germany. E-mail: hans-jorg.himmel@aci.
uni-heidelberg.de
† Electronic supplementary information (ESI) available. CCDC 1454697 and
1454698. For ESI and crystallographic data in CIF or other electronic format see
DOI: 10.1039/c6ra04494h
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differences between these structures are in line with a fairly low Then 14.5 mL of a 1.01 mM solution (14.6 mmol, 2.7 eq.) of 2-
double-bond character.
chloro-N,N,N0,N0-tetramethyl formamidiniumchloride in aceto-
In this work, the electronic structure of the dications 12+ and nitrile and 11 mL triethylamine were added. The reaction
2
2+ (see Scheme 2), which are formed by two-electron oxidation mixture was stirred at 0 ꢁC for 5 h before the ice bath was
of the neutral compounds 1 (1,4-bis-tetramethylguanidino- removed. Solvent and volatile components were removed in
benzene) and 2 (1,4-bisguanidino-benzene), is analysed.
vacuo and the crude product was dissolved in dry CH2Cl2. The
solution was extracted with aqueous HCl (10 w%) for three
times. The combined aqueous fractions were washed with
CH2Cl2 before raising pH with aqueous NaOH-solution to about
pH 12. The resulting precipitate was extracted three times with
toluene. The combined organic fractions were dried over
Na2SO4 before the solvent was removed in vacuo. The pale-
yellow coloured solid was sublimated to afford the product as
a colorless solid in a yield of 0.597 g (1.9 mmol, 35%). Slow
solvent evaporation from a concentrated Et2O solution afforded
crystals suitable for a structural analysis with X-ray diffraction.
1H NMR (CDCl3, 399.89 MHz): d ¼ 6.56 (s, 4H), 2.80–2.53 (brs
with two maxima, 24H) ppm. 13C NMR (CDCl3, 100.55 MHz): d ¼
159.5, 144.8, 122.0, 39.7 ppm. IR (CsI): ~n ¼ 3010 (w), 2938 (m),
2917 (m), 2880 (m), 2804 (w), 1577 (vs), 1506 (m), 1488 (vs), 1464
(vs), 1438 (vs), 1421 (vs), 1410 (m), 1398 (m), 1374 (vs), 1268 (w),
1251 (m), 1233 (m), 1206 (vs), 1141 (vs), 1133 (vs), 1104 (w), 1096
(m), 1065 (m), 1055 (w), 1018 (vs), 923 (m), 843 (vs), 824 (m), 751
(m), 685 (m), 530 (m) cmꢀ1. UV/Vis (CH3CN): lmax (3 in L (mol
cm)ꢀ1) ¼ 300 (1.7 ꢂ 104), 220 (1.4 ꢂ 104) nm. MS (HR-ESI+): m/z
(calculated) ¼ 305.2447 (MH+) (305.2454). Elemental analysis
for C16H28N6: calcd C 63.12, H 9.27, N 27.61; found C 62.89, H
9.27, N 27.74. Crystal data for C16H28N6: Mr ¼ 304.440, 0.30 ꢂ
2. Experimental
All synthetic work was carried out under dry argon atmosphere
using standard Schlenk techniques. The organic solvents were
dried with an MBraun SPS 800 purication system and degassed
prior to their use. 1,4-Diaminobenzene dihydrochloride was
purchased from Sigma-Aldrich (stated purity >99.0%) and used
without further purication. UV/Vis spectra were taken with
a Cary 5000 spectrometer. An EG&G Princeton 273 apparatus
with an SCE reference electrode was used for CV measurements.
Bn4NPF6 (electrochemical grade (>99.0), Fluka ) was employed
as supporting electrolyte. NMR spectra were measured with
a Bruker Avance II 400 spectrometer. IR spectra were taken with
a Biorad Excalibur FTS 3000. Elemental analysis was carried out
at the Microanalytical Laboratory of the University of Heidel-
berg. HR-ESI mass spectra were recorded with a Bruker Apex-Qe
hybrid 9.4T FT-ICR spectrometer.
2-Chloro-N,N,N0,N0-tetramethyl formamidinium chloride
Under inert atmosphere 45 mL dry chloroform, 3.6 (30.0 mmol,
1.0 eq.) tetramethyl urea and 12.9 (150 mmol, 5.0 eq.) oxalyl-
chloride were added into a three times ame dried Schlenk ask
equipped with a reux condenser. The reaction mixture was
heated to reux for 22 h before volatile components were
evaporated in vacuo. The crude product was washed with diethyl
ether (3 ꢂ 20 mL). Aer drying in vacuo, the clean product was
obtained as a colourless solid in a yield of 5.00 g (29.2 mmol,
97%). CH3CN solutions of the compound were stored at ꢀ20 ꢁC.
1H NMR (CD2Cl2, 200 MHz): d ¼ 3.50 (s) ppm.
3
ꢀ
0.25 ꢂ 0.15 mm , triclinic space group P1, a ¼ 6.912(1), b ¼
ꢁ
ꢁ
˚
8.356(2), c ¼ 8.844(2) A, a ¼ 110.90(3) , b ¼ 97.94(3) , g ¼
111.50(3) , V ¼ 422.20(15) A , Z ¼ 1, dcalc ¼ 1.197 g cmꢀ3, Mo-Ka
3
ꢁ
˚
˚
radiation (graphite-monochromated, l ¼ 0.71073 A, T ¼ 100 K),
qrange 5.2 to 60.1ꢁ. Reections measd 4364, indep. 2451, Rint
0.0293, nal R indices [I > 2s(I)]: R1 ¼ 0.0409, wR2 ¼ 0.128.
¼
1(PF6)2
170.36 mg (0.674 mmol, 1.95 eq.) silver hexauorophosphate
and 100 mg (0.345 mmol, 1.0 eq.) of 1 were dissolved in 3 mL
acetonitrile. The reaction mixture was stirred at room temper-
ature for a period of 2 h. Then it was ltered and the ltrate
collected. The solvent was partially removed and the ask was
1,4-Bis-tetramethylguanidino-benzene, 1
A solution of 1.0 g (5,5 mmol, 1.0 eq.) 1,4-diaminobenzene
dihydrochloride in 12 mL acetonitrile and was cooled to 0 C.
ꢁ
ꢁ
stored at 5 C overnight. Yellow coloured crystals formed and
were collected. The solvent was again partially removed and new
crystals were obtained. By repeating this procedure twice, the
product was obtained as a yellow coloured crystalline solid in
a total yield of 59 mg (0.099 mmol, 29%). 1H NMR (CD3CN,
399.89 MHz): d ¼ 7.05 (s, 4H), 3.07 (s, 24H) ppm. 13C NMR
(CD3CN, 100.55 MHz): d ¼ 165.0, 163.8, 135.0, 42.4 ppm. IR
(CsI): ~n ¼ 2963 (w), 2931 (w), 1637 (m), 1609 (m), 1420 (m) 1408
(w), 1335 (vw), 1288 (w), 1229 (w), 1168 (w), 1114 (w), 1069 (w),
1025 (w), 837 (vs), 775 (vw), 558 (m) cmꢀ1. UV/Vis (CH3CN): lmax
(3 in L (mol cm)ꢀ1) ¼ 287 (3.9 ꢂ 104), 229 (2.1 ꢂ 104) nm. MS
Scheme
2
Lewis structure of
1
(1,4-bis-tetramethylguanidino-
+
(HR-ESI+): m/z (calculated)
¼
449.20200 (C16H28N6PF6
)
benzene) and 2 (1,4-bis-guanidino-benzene) and their two-electron
oxidation to new bis-2-azapropenylium dications. For 1, the oxidation
was carried out on a preparative scale using AgPF6 as oxidizing
reagent.
(449.2012). Elemental analysis for C16H28F12N6P2: calcd C 32.33,
H 4.75, N 14.14; found C 32.43, H 4.89, N 14.41. Crystal data for
C
20H34F12N8P2: Mr ¼ 676.490, 0.35 ꢂ 0.25 ꢂ 0.15 mm3,
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monoclinic space group P21/c, a ¼ 9.000(2), b ¼ 1.4045(3), c ¼
3
ꢁ
˚
˚
1.1779(2) A, b ¼ 92.38(3) , V ¼ 1487.6(5) A , Z ¼ 2, dcalc ¼ 1.510 g
cmꢀ3, Mo-Ka radiation (graphite-monochromated, l ¼ 0.71073
A, T ¼ 100 K), qrange 4.5 to 58.3ꢁ. Reections measd 7879, indep.
˚
3998, Rint ¼ 0.0229, nal R indices [I > 2s(I)]: R1 ¼ 0.055, wR2 ¼
0.159.
X-ray crystallographic study
Suitable crystals were taken directly out of the mother liquor,
immersed in peruorinated polyether oil and xed on top of
a cryo loop. Measurements were made with a Nonius-Kappa
CCD diffractometer with low-temperature unit using graphite-
monochromated Mo-Ka radiation. The temperature was set to
100 K. The data collected were processed using the standard
Nonius soware.6 All calculations were performed using the
SHELXT-PLUS soware package. Structures were solved by
direct methods with the SHELXS-97 program and rened with
the SHELXL-97 program.7,8 Graphical handling of the structural
data during solution and renement was performed with
XPMA.9 Atomic coordinates and anisotropic thermal parame-
ters of non-hydrogen atoms were rened by full-matrix least-
squares calculations. CCDC-1454698 (1), and -1454697
(1(PF6)2) contain the supplementary crystallographic data for
this paper.†
Fig. 1 CV curves of 1 in CH2Cl2 and THF (scan speed 100 mV sꢀ1
potentials given versus Fc/Fc+ (Fc ¼ ferrocene, extern calibration)).
,
ꢀ0.22 V (see ESI† for a comparison with the redox potential of
related electron donors).
Then, 1 was chemically oxidized to the dication 12+ by reac-
tion with AgPF6 in CH3CN solution. Yellow crystals of the clean
salt 1(PF6)2 were grown at a temperature of 5 ꢁC in a crystal yield
of 29%. In the 1H NMR spectra recorded for CH3CN solutions of
the salt 1(PF6)2 (see ESI†), all 24 CH3 protons have equal
chemical shi (d ¼ 3.08 ppm), and the coalescence point is even
not reached upon cooling to ꢀ40ꢁ. Inversion or rotation
processes within the guanidino groups are responsible for this
dynamic behavior. Rotation is obviously facilitated in 12+
compared to 1, since oxidation weakens the CN double bonds of
the guanidino groups. In the case of neutral guanidines, an
inversion process of the guanidino group rather than rotation
about the C]N guanidine double bond was found to be
responsible for the equivalence of the chemical shis at higher
temperatures15 (see also the discussion for imines16).
Details of the quantum chemical calculations
Density functional theory (DFT) computations were performed
using the TURBOMOLE10 program. For all calculations the
B3LYP functional11 and def2-TZVP basis set12 were used. The
two electron coulomb integrals are evaluated by means of the
resolution-of-the-identity (RI) approximation13 using the corre-
sponding auxiliary basis set (def2-TZVP).12a Besides full struc-
ture optimizations, further structure optimizations were
performed in D2h and C2h symmetries, to force the molecules to
adopt linear orthogonal and bent-planar structures.
The experimentally derived solid-state structures of 1 and
1(PF6)2 from single-crystal X-ray diffraction analysis are
compared in Fig. 2 and selected structural parameters are given
3. Results and discussion
Compound 1 was synthesized in two steps. Tetramethylurea was
rst reacted (“activated”) with oxalyl chloride to give the more
in Table 1. As expected, the C]N bond length (N1–C4) increases
2+
˚
˚
upon oxidation (from 1.297(2) A in 1 to 1.405(2) A in 1 ), while
all other C–N bond distances decrease. The central guanidino C
atom (C4) approaches the C6 ring plane upon oxidation. Hence
the torsional angle C2–C1N1–C4 decreases from 43.1ꢁ in 1 to
4.2ꢁ in 12+. The C1–N1–C4 angle measures 121.6(1)ꢁ in 1 and
remains almost unchanged upon oxidation to 12+ (119.7(1)ꢁ).
The bent structure clearly shows that the compound could not
be described as a bis-2-azaallenium dication. The most
remarkable feature of the 12+ structure is the almost orthogonal
positions of the C1–N1–C4 plane and the N2/N3–C4–N1 plane.
The torsional angle C1–N1–C4–N2 increases from 27.1ꢁ in 1 to
90.5ꢁ in 12+. Hence, the structure also could not be described as
a classical bis-2-azaallylium dication.
electrophilic
2-chloro-1,10,3,30-tetramethylformamidinium
ꢁ
chloride, and subsequently reacted at 0 C in CH3CN solution
with commercially available 1,4-diaminobenzene dihydro-
chloride in the presence of NEt3. The crude product was puried
and nally sublimed to obtain the clean colourless compound 1
in 35% yield.14 Crystals were grown from concentrated Et2O
solutions by slow solvent evaporation. Unfortunately, attempts
to synthesize 2 failed so far, since deprotonation of the
protonated precursor [2H2]2+ initiated decomposition.
The cyclic voltammetry curves (CV, see Fig. 1) recorded for 1
show a two-electron redox event at E1/2 ¼ ꢀ0.18 V (Eox ¼ ꢀ0.09
V) vs. Fc/Fc+ (Fc ¼ ferrocene) in CH2Cl2 solution. In THF solu-
tion, the redox potential slightly shis and the separation
Table 1 compares some calculated structural parameters
from density-functional theory (DFT) calculations at the B3LYP/
def2-TZVP level of theory with the experimentally derived ones.
between oxidation and reduction half-waves increases (Eox
¼
ꢀ0.14 V, E1/2 ¼ ꢀ0.26 V vs. Fc/Fc+), but still two-electrons are
involved in the redox process. In CH3CN, the E1/2 value shis to
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Fig. 2 Structures of (a) 1 and (b) 1(PF6)2. Hydrogen atoms omitted for clarity. Ellipsoids drawn at the 50% probability level. Selected bond
distances (in A˚) and angles (in ꢁ): for 1: N1–C1 1.4087(15), N1–C4 1.2975(16), N2–C4 1.3794(16), N3–C4 1.3843(16), C1–C2 1.4006(17), C1–C3
1.4040(18), C2–C30 1.3886(17), dieder angle C2–C1–C3, N2–C–N3 85.3. For 1(PF6)2: N1–C1 1.292(2), N1–C4 1.405(2), N2–C4 1.326(2), N3–C4
1.323(2), C1–C2 1.460(2), C1–C3 1.461(2), C2–C30 1.334(3), tetrahedral angle N2–C4–N3, C2–C1–C3 60ꢁ.
Table 1 Comparison between selected exp. determined and calculated (B3LYP/def2-TZVP) structural parameters (bond distances in A˚, bond
angles in ꢁ) for neutral and oxidized 1 and calculated data for 2 and 22+
Parameter
1 exp.
1 calcd
1(PF6)2
12+ calcd
2
22+
d(C1–C2)
1.401(2)
1.389(2)
1.409(2)
1.297(2)
1.379(2)
1.384(2)
121.6(1)
43.1(1)
1.40
1.39
1.40
1.29
1.40
1.40
124.4
44.5
26.8
1.460(3)
1.334(3)
1.292(2)
1.405(2)
1.323(3)
1.326(3)
119.7(1)
4.19(2)
1.462
1.340
1.284
1.380
1.335
1.335
128.1
1.7
1.399
1.389
1.405
1.278
1.384
1.388
121.1
68.0
1.468
1.340
1.287
1.362
1.325
1.325
128.3
0
d(C2–C30)
d(C1–N1)
d(N1–C4)
d(C4–N2)
d(C4–N3)
:(C1–N1–C4)
:(C3–C1–N1–C4)
:(C1–N1–C4–N2)
27.1 (1)
90.5(2)
94.5
10.6
92.5
It can be seen that the general level of agreement is excellent compounds.17 The Lewis structures of the three examples 3
(see also Fig. S1 of ESI†). In particular, the remarkable increase (1,2,4,5-tetrakis(dimethylamino)-benzene), 4 (1,4,5,8-tetrakis-
of the torsional angle C1–N1–C4–N2 is accurately reproduced by (tetramethylguanidino)-naphthalene) and 5 (1,2,4,5-tetrakis-
the calculations (calculated increase from 26.8ꢁ in 1 to 92.6ꢁ in (tetramethylguanidino)-pyridine) are sketched in Scheme 3.
12+ versus experimentally determined increase from 27.1ꢁ to
Table 2 lists some experimentally determined structural
90.5ꢁ). Moreover, the experimentally measured UV/Vis spectrum parameters for the oxidized GFAs 3, 4 and 5, with the values for
recorded for 12+ in a CH3CN solution tallies with the electronic the neutral compounds given in parenthesis.18–20 As expected, in
transitions calculated with TD-DFT for the energy minimum all cases the C–N bond between the central ring and the N of the
structure (see Fig. S2 of ESI†). This result indicates that the guanidino groups decreases and the imino bond distance
solid-state structure is also preserved in solution.
increases. The central C atom of the guanidino groups is dis-
The quantum chemical calculations for compound 2 gave placed from the aromatic ring plane in the neutral compounds
similar results. The torsional angle C1–N1–C4–N2 is 10.6ꢁ in 2 (see torsional angle C3–C1–Ng–Cg), but almost in the ring plane
and increases to 91.6ꢁ in 22+. A population analysis (see ESI†) upon oxidation. This change upon oxidation comes not unex-
shows that the loss of the electron density in 1 upon oxidation pected as it should favour p-bonding, and is similar to the
concerns all atoms, but the nitrogen atom N1 is mostly affected. changes upon oxidation found for 1 and 2. The C1–Ng–Cg angle
The dications 12+ and 22+ thus have structures that differ from is next to 120ꢁ, in similarity to the angle around the central N
the linear orthogonal 2-azaallenium ion and also from atom in bent-planar 2-azyallylium ions. However, in clear
its valence isomeric bent-planar 2-azaallylium form (see difference to the structures of bent-planar 2-azaallylium ions
Scheme 1).
ions, the torsional angle C1–Ng–Cg–Na is large, approaching in
The structural peculiarities found in 12+ and 22+ are also some cases 90ꢁ. In similarity to the results with 1 and 2,
present (at least to some extent) in 1,2,4,5-tetrakis- oxidation of the neutral GFAs leads to a drastic increase of the
(tetramethylguanidino)-benzene and related tetrakisguanidine C1–Ng–Cg–Na torsional angle.
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Scheme 3 (a) Lewis structures for the tetrakis-guanidine compounds 3 (1,2,4,5-tetrakis(tetramethylguanidino)-benzene), 4 (1,4,5,8-tetrakis(-
tetramethylguanidino)-naphthalene) and 5 (1,2,4,5-tetrakis(tetramethylguanidino)-pyridine). (b) Atom notations used in the discussion and in
Table 2. Please note that Ng ¼ N1 and Cg ¼ C4 in the case of the bisguanidines 1 and 2.
Table 2 Selected experimentally determined structural parameters (bond distances in A˚, bond angles in ꢁ) for dicationic tetrakis-guanidine
compounds, with values of the respective neutral molecule given in parenthesis (data from ref. 17–19). Italic numbers are mean values if there is
no significant deviation. Numbers separated by comma represent two different values for the parameter within one guanidino group. Numbers
for different guanidino groups are separated by a slash
3(I3)2
4(I3)2
4(PF6)2
5Br2
Ng–C1
1.30/1.36
(1.42)
1.33/1.35
(1.41)
1.32/1.35
(1.41)
1.31
(1.41)
Ng–Cg
1.37/1.34
(1.29)
1.35/1.31
(1.28)
1.35/1.33
(1.28)
1.37
(1.30)
Na–Cg
1.33, 1.35/1.35
(1.39)
1.34/1.35, 1.37
(1.39)
1.34/1.35, 1.37
(1.39)
1.33/1.33, 1.35
(1.38)
C3–C1–Ng–Cg
C1–Ng–Cg–Na
4.2/24.6
16.5/32.9
(54.2/59.0)
77.9/47.5
(23.4)
18.2/35.8
(54.2/59.0)
70.4/43.7
(23.4)
6.7/11.2
(48.2/53.5)
83.32/77.74
(31.4/23.5)
(58.4/62.7)
90.8/40.1
(27.2/22.3)
To obtain more information about the special structures of to the C6 ring plane, realizing an 2-azaallenium system) is
these oxidized guanidines and special 2-azapropenylium ions, a fourth order saddle point structure with an energy higher by
we calculated the structures of 12+ and 22+ with restrictions 74 kJ molꢀ1 than the minimum structure. Thus, also the 2-
imposed by symmetry. The xyz les and electron energies for all azaallenium structure is not favoured. The D2h linear-planar
structures are collected in the ESI.† In the following discussion, structure (which allows for a full conjugation but forces the
we use again the atom numbering from Fig. 2. For 22+ the C2h C1–N1–C4 angle to be 180ꢁ) is also a fourth order saddle point
bent-orthogonal structure is the lowest-energy structure and higher by 191 kJ molꢀ1, not unexpected (Fig. 4).
a minimum on the potential energy surface (Fig. 3). The bent-
The minimum structure of 12+ with methyl substituted
planar C2h structure (which allows a full conjugation of the guanidino units has Ci symmetry only, because in the Ci case
carbon p-system with the guanidino groups) is a second order the methyl groups are allowed to take a favourable conforma-
saddle point structure with an energy higher by 72 kJ molꢀ1
.
tion. By imposing higher symmetry, the methyl groups are
Obviously, the extended conjugation is not favoured. The D2h forced to take unfavourable conformations that correspond to
linear-orthogonal structure (in which the C1–N1–C4 angle is higher order saddle point structures. However, the minimum
forced to be 180ꢁ but the guanidino group remains orthogonal structure is similar to the C2h bent-orthogonal structure, which
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12+ (bent-planar C2h), the C1–N1–C4 angle amounts to 155ꢁ
whereas for 22+ (bent-planar C2h) it amounts to 132ꢁ. Similar to
22+, also for 12+ the D2h structures have energies higher than the
C2h planar structure: the D2h linear-orthogonal structure is
higher by 97 kJ molꢀ1 (eighth order saddle point structure) and
the D2h linear-planar structure by 213 kJ molꢀ1 (tenth order
saddle point structure).
In summary, the calculations clearly conrm the bent-
twisted structures as the minimum structures of 12+ and 22+.
A population analysis of 12+ indicates that the charge is effec-
tively delocalized in the bent-twisted minimum structure. The
calculations further show that the bent-planar structure (espe-
cially for 12+) is heavily disfavoured over the bent-twisted
structure due to steric strain. The linear-orthogonal structure
is also much higher in energy, in line with the results obtained
for other 2-azapropenylium ions with electron-donating groups
that stabilize the bent form.
To obtain an understanding of the electronic situation of the
C–N–C moiety, we recapitulate the possible bonding situations
referred to in the introduction. The (linear) azaallenium is
characterised by two double bonds and no lone pair on the
nitrogen. The (bent-planar) azaallylium is characterised by two
bonds with partial double bond character and one lone pair.
The last situation with twisted residuals (be it formulated as 2-
azadicarbene or not) is characterised by two single bonds and
two lone pairs on the nitrogen. However, none of the cases
directly applies to the present systems. The structure of the
dications 12+ and 22+ is not symmetrical. The bond distances
clearly indicate a double bond between the C atom of the ring
and the central guanidino N atom and a single bond between
the central guanidine N atom and the guanidino C atom.
Hence, the system is denitely not an azaallenium. If the
structure of the C–N–C moiety was planar, the system clearly
would correspond to one of the mesomeric forms of the azaal-
lylium. However, in the unsymmetric case with a localised
single bond it is clear that there is no large barrier for the
rotation around the bond and the system is able to readily adopt
a structure with a twisted orientation of the two sides, thereby
showing also characteristics of the third bonding situation. In
the twisted orientation, the p-system of the C(NR2)2 group is
able to interact with the lone pair of the nitrogen atom even if
there is no formation of a real second double bond. The
Fig. 3 Relative energies DE of the calculated structures for 22+. SP
means saddle point, and the number in parenthesis denotes the saddle
point order (number of imaginary frequencies). Nitrogen atoms drawn
in black.
Fig. 4 Relative energies DE of the calculated structures for 12+. SP inspection of the orbitals give evidence that in the present
means saddle point, and the number in parenthesis denotes the saddle
point order (number of imaginary frequencies). Nitrogen atoms drawn
in black.
systems there indeed is some interaction between the C(NR2)2
p-system and the N lone pair (Fig. S4, ESI†). This, besides steric
repulsion, is likely the reason for the moiety to take a twisted
conformation.
has an energy higher by only 37 kJ molꢀ1 and is a fourth order
4. Conclusions
saddle point structure. The C2h bent-planar structure (allowing
full conjugation) has an energy higher by 202 kJ molꢀ1 than the
In this work, the electronic structure of special 2-azapropeny-
C2h bent-orthogonal structure and is an eighth order saddle
lium ions, obtained by oxidation of the para-bisguanidino-
point structure. This value is much higher than the corre-
benzenes 1 and 2, was analysed. The experimentally derived
sponding value for 22+ of 72 kJ molꢀ1 and reects the steric
solid state structure of 12+ shows that a bent-twisted structure is
hindrance due to the methyl groups that does not allow the C1–
adopted, in contrast to the linear-orthogonal (2-azaallenium) or
bent-planar (2-azaallylium) structures reported previously for 2-
N1–C4 angle to take a value more close to 120ꢁ (see the theo-
retical work on steric strain in substituted allyl cations21). For
azapropenylium ions. Quantum chemical calculations on the
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isolated dications 12+ and 22+ conrmed that the bent-twisted
(bent-orthogonal) structures are lowest-energy stationary
points on the potential energy surface. Linear-orthogonal and
bent-planar structures are higher in energy. The bent-planar
structure is disfavoured by steric strain (especially for 12+),
and the linear-orthogonal structure is disfavoured due to the
electron-donating groups that support the bent form. In the
bent-twisted minimum structures, the central nitrogen atoms of
the 2-aza-propenylium systems are engaged in p-bonding with
the central C6 ring, and there is also some interaction of the N
lone pair with the guanidino CN2 group. Concerning the
7 (a) G. M. Sheldrick, SHELXS-97, Program for Crystal Structure
¨
Solution, University of Gottingen, Germany, 1997, http://
shelx.uni-ac.gwdg.de/SHELX/index.php; (b) G. M. Sheldrick.
SHELXL-97, Program for Crystal Structure Renement, http://
shelx.uni-ac.gwdg.de/SHELX/index.php.
8 International Tables for X-Ray Crystallography, ed. J. A. Ibers
and W. C. Hamilton, Kynoch Press, Birmingham, vol. 4,
1974.
9 L. Zsolnai and G. Huttner. XPMA, University of Heidelberg,
Germany, 1994; http://www.uni-heidelberg.de/institute/
fak12/AC/huttner/soware/soware.html.
¨
¨
¨
description of the systems as azacarbenes, we note that such 10 (a) R. Ahlrichs, M. Bar, M. Haser, H. Horn and C. Kolmel,
a formulation in principle is always possible, but it relies on the
(arbitrary) assignment of the two electrons of a bond to a single
Chem. Phys. Lett., 1989, 162, 165–169; (b) R. Ahlrichs, J.
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atom. We think that such a description is justied only if it is 11 (a) P. J. Stephens, F. J. Devlin, C. F. Chabalowski and
based on observations that support this description as would be
the nding to readily undergo something like exchange of the
groups or dissociation to the separated systems. But so far we
found no evidence for this. We are currently studying the
consequences of the special structures on the reactivity.
M. J. Frisch, J. Phys. Chem., 1994, 98, 11623–11627; (b)
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