Design, Synthesis, and Structural Analysis of Divalent NI Compounds and Identification of a New Electron-Donating Ligand

Article abstract of DOI:10.1002/chem.201503618

The dative-bond representation (L→E) in compounds with main group elements (E) has triggered extensive debate in the recent past. The scope and limits of this nonclassical coordination bond warrant comprehensive exploration. Particularly compounds with (L

Full text of DOI:10.1002/chem.201503618

DOI: 10.1002/chem.201503618
Full Paper
&
Divalent Nitrogen(I) Compounds
Design, Synthesis, and Structural Analysis of Divalent NI
Compounds and Identification of a New Electron-Donating Ligand
Prasad V. Bharatam,*[a] Minhajul Arfeen,[a] Neha Patel,[a] Priyanka Jain,[a] Sonam Bhatia,[a]
Asit K. Chakraborti,*[a] Sadhika Khullar,[b] Vijay Gupta,[b] and Sanjay K. Mandal*[b]
!
Abstract: The dative-bond representation (L!E) in com-
pounds with main group elements (E) has triggered exten-
sive debate in the recent past. The scope and limits of this
nonclassical coordination bond warrant comprehensive ex-
ing to the (L!N L’)+ class carrying this unconventional
ligand were synthesized. Quantum chemical and X-ray dif-
fraction analyses showed that the electronic and geometric
parameters are consistent with those of already reported di-
valent NI compounds. The molecular orbital analysis, geo-
metric parameters, and spectral data clearly support the L!
!
ploration. Particularly compounds with (L!N L’)+ arrange-
ment are of special interest because of their therapeutic im-
portance. This work reports the design and synthesis of
novel chemical species with the general structural formula
!
N and N L’ interactions in these species. The newly identi-
fied ligand has the properties of a reactive carbene and high
!
(L!N L’)+ carrying the unusual ligand cyclohexa-2,5-
nucleophilicity.
diene-4-(diaminomethynyl)-1-ylidene. Four species belong-
Introduction
salt forms) and lead compounds (e.g., WR99210, guanylthiour-
ea derivatives) have been shown to have divalent NI character
!
Electron-accepting nitrogen centers, as in (L!N L’)+ provide
exciting new opportunities for chemical exploration.[1,2] Such
nitrogen centers accept electron density from electron-donat-
ing ligands such as N-heterocyclic carbenes (NHCs) and diami-
!
nocarbenes (I-III in Scheme 1). Compounds with (L!N L’)+
electronic structure carry two lone pairs of electrons at the
central nitrogen atom and show low nucleophilicity.[1] Such
compounds are considered to be divalent NI compounds,[1j] in
comparison to the reported divalent C0 compounds.[3,4] Frenk-
ing and co-workers reported divalent NI character at the cen-
tral nitrogen atom in N5+, N(CO)2+, N(PPh3)2+, N(NHC)2+, and
so on.[1e] Novel electronic character at the central nitrogen
atom due to donor–acceptor interactions were described earli-
+
+ [1l–n]
er in N5 and N(CO)2
.
Many of the known drug molecules
(e.g., metformin, proguanil, famotidine, and ebrotidine in their
[a] Prof. Dr. P. V. Bharatam, M. Arfeen, N. Patel, P. Jain, S. Bhatia,
Prof. Dr. A. K. Chakraborti
Department of Medicinal Chemistry
National Institute of Pharmaceutical Education and Research (NIPER)
Sector 67, S.A.S. Nagar - 160 062, Punjab (India)
E-mail: pvbharatam@niper.ac.in
akchakraborti@niper.ac.in
[b] Dr. S. Khullar, V. Gupta, Prof. Dr. S. K. Mandal
Department of Chemical Sciences
Indian Institute of Science Education and Research (IISER), Mohali
Sector 81, S.A.S. Nagar - 140 308, Punjab (India)
E-mail: sanjaymandal@iisermohali.ac.in
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201503618. It contains synthetic procedures
of all intermediates and final compounds with their characterization details
(1H NMR, 13C NMR, IR and HRMS), X-ray crystallographic details, computa-
tional analysis details, and coordinates of the optimized geometries.
Scheme 1. Representative structures of the compounds with possible L!N
dative bond.
Chem. Eur. J. 2016, 22, 1088 – 1096
1088
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper
in the protonated state.[1] Some of these species were found to
be suitable for phase-transfer catalysis.[2a,e] Species IV and V
with hidden divalent NI character, with the general structural
representation L!NR, show super basic properties.[5] They can
!
be utilized to generate the (L!N L’)+ species, owing to their
high basicity. Species VI, VII, and VIII are related systems with
L!N dative bonds involving an electron-accepting nitrogen
center.[6] These species are very closely related to the (L!
!
P
L’)+ systems (IX), which have been known for many years,
but the L!P coordination-bond character was only recently
!
highlighted.[7,8] Related (L!As L’)+ systems X have also been
reported.[9] Many other systems with possible L!E interaction
(E=BH,[10,1e] Si,[11] Ge,[12] Sn[13]) are also known.
Recently, Himmel et al. proposed “a case for fewer arrows”
and suggested that the dative-bond representation should be
used judiciously in compounds with main group elements.[14]
However, Frenking supported “a case for more arrows” and
suggested that there is a scope for identifying many examples
with such out-of-the-box ideas, which provide opportunities
for refining concepts in chemical bonding.[15] In the wake of
such healthy scientific discussion, it is important to generate
Scheme 3. Reagents and conditions (yield). a) p-TsOH, iPrOH, reflux, 5 h
(80%); b) CH3I, TEA, acetone, 428C, 12 h (60%); c) P4S10, ethanol, RT, 10 h
(65%); d) CH3I, reflux, 1 h (reaction mixture was directly used in the next
step), e) NH4OAC, MeOH, reflux, 1.5 h (74%). See Supporting Information for
the chemical characterization data and details.
!
more compounds belonging to the (L!N L’)+ class and ex-
plore the scope and limits of this unorthodox concept. The
work reported herein originated from the quantum chemical
design of compounds 1–4 followed by their synthesis and
X-ray crystallographic characterization. The highlight of the
work is the identification of novel electron-donating ligand
cyclohexa-2,5-diene-4-(diaminomethynyl)-1-ylidene (A, see
Scheme 5) which is shown to donate electron density in new
species 1–4 (Scheme 2) carrying a ligand-stabilized nitrogen
cation center.
acetate salts by similar synthetic procedures. The chloride and
bromide salts of 1–4 were obtained by treating the corre-
sponding acetate salts with methanolic HCl and HBr solution
in glacial acetic acid, respectively. The formation of the chloride
and bromide salts was confirmed by the absence of the
methyl group and carbonyl group (of the acetate ion) in the
NMR spectra. The synthesized compounds were found to be
nonhygroscopic and stable at room temperature, both in solu-
tion and in the solid state (m.p. 230–2508C), similar to the
report on I.[2c] Attempts to obtain the neutral form of 1 by
treating 12 with methanolic ammonia resulted in formation of
the nitrile derivative 10, in place of the desired product.
Scheme 2. Structures of the compounds reported in this work.
Compounds 1–4 have C9 chemical shifts of 148.07, 157.54,
146.83, and 161.53 ppm, respectively. The C10 chemical shifts
of 155.65, 156.24, 151.18, and 151.78 ppm for 1–4, respective-
ly, are comparable to the quantum chemically estimated
values. These values are quite similar to the d(13C) values noted
for the electron-donating carbon centers in carbodicarbenes
Results and Discussion
Synthesis
The synthesis of the desired compound 1 (Scheme 3) began
with the preparation of 7 by treating commercially available 2-
chlorobenzimidazole with methyl iodide in the presence of po-
tassium carbonate in DMF. Reaction of 7 with 4-aminobenzoni-
trile (8) in the presence of p-TsOH and iPrOH afforded the cor-
responding nitrile derivative 9.[16] The N-methylation of 9 with
methyl iodide in the presence of triethylamine gave 10, which
when treated with hydrogen sulfide gas[17a] (or phosphorus
pentasulfide)[17b] produced the thioamide derivative 11. The
thioamide derivative was then methylated with methyl iodide,
followed by treatment with ammonium acetate to obtain the
acetate salt of 1 (overall yield 12%). Compound 2 (acetate salt)
was synthesized in a similar way from 2-chlorobenzothiazole.
Compounds 3 and 4 were also obtained as white crystalline
(144.8 ppm)[4 h] and
160 ppm).[2b,c,e,4g,5]
divalent
NI compounds
(145–
Description of the structures
Crystals of the acetate salt of 1 were obtained by the vapor-
diffusion method by employing a saturated solution in metha-
nol and using toluene as the nonpolar solvent. The acetate salt
of 2 did not form good-quality crystals, and hence crystals of
the corresponding tosylate salt (2a) were obtained after treat-
ing the acetate salt of 2 with p-TsOH by using the vapor-diffu-
sion method. The crystals of 3 and 4, which were actually
found to be in the corresponding tautomeric states 3t and 4t
(Scheme 4) in the solid state, were obtained by slow evapora-
Chem. Eur. J. 2016, 22, 1088 – 1096
www.chemeurj.org
1089
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper
Scheme 4. Structures of the designed and synthesized compounds 1-4 with traditional Lewis representation, their isomers 1i–4i and tautomers 3t and 4t,
and the corresponding neutral species 5 and 6 and their isomers.
Figure 1. Molecular structures of the cations in 1, 2a, 3t, and 4t (Scheme 4) in the solid state, obtained by X-ray diffraction (thermal ellipsoids set at 30%
probability) with bond lengths [Š] and angles [8]. Details of the structures with counterions as well as the supramolecular interactions are provided in the
Supporting Information.
tion as acetate salts. Figure 1 shows the solid-state structures
of the final compounds. The crystallographic parameters, de-
tails of data collection and structure refinement for all com-
pounds are summarized in Table 2. In 1–4, the counterion is lo-
cated near the diaminocarbene group with formation of two
hydrogen bonds (see Supporting Information).
analysis are 1.309 and 1.339 Š (Table 1), respectively. The N2À
C10 bond length increased to 1.380 Š when the counterion
was included (acetate ion near the C(NH2)2 unit) in the quan-
tum chemical calculations (Table 1), which makes the compari-
son with the X-ray data more realistic. The observed CÀN bond
lengths are quite comparable to the C!N coordination bond
lengths (1.33 Š) reported in divalent NI compounds (see Ta-
bles S5–S7, Supporting Information). The six-membered ring in
1 adopts a quinoidal structure, with two shorter bonds oppo-
The N2ÀC9 and N2ÀC10 bond lengths in 1 are 1.308(3) and
1.379(2) Š, respectively, in the crystal structure (Table 1). These
CÀN bond length values in 1 according to quantum chemical
Chem. Eur. J. 2016, 22, 1088 – 1096
www.chemeurj.org
1090
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper
Table 1. Comparison of the theoretical and experimental geometrical parameters of 1–4. All the bond lengths are in Š and bond angles are in degree (o).
Geometric
parameters
1
2a
X-ray[d]
2
M06
3t
X-ray[d]
4t
X-ray[d]
I (R=iPr)[2b]
X-ray[d]
I (R=H)[1i]
M06
X-ray[d]
M06[b]
M06
M06[b]
M06
M06
N2ÀC9
1.308(2)
1.379(2)
1.379(2)
1.377(2)
1.407(2)
1.393(3)
1.399(2)
1.402(3)
1.470(2)
120.8(2)
40.3
1.297
1.380
1.390
1.388
1.417
1.406
1.408
1.415
1.479
126.2
29.4
1.309
1.339
1.368
1.364
1.424
1.411
1.414
1.422
1.427
125.4
44.9
1.286(3)
1.399(3)
1.394(3)
1.387(3)
1.410(4)
1.406(4)
1.405(4)
1.410(4)
1.485(3)
124.4(2)
3.9
1.273
1.383
1.382
1.379
1.401
1.395
1.397
1.400
1.470
123.3
7.4
1.290
1.353
1.372
1.368
1.415
1.406
1.409
1.415
1.435
127.0
17.8
1.368(4)
1.384(4)
1.374(5)
1.370(5)
1.388(4)
1.385(6)
1.393(4)
1.388(6)
1.474(5)
127.7(3)
À11.1
1.386
1.369
1.373
1.370
1.407
1.404
1.406
1.409
1.439
128.4
0.2
1.362(4)
1.391(4)
1.386(5)
1.368(5)
1.387(5)
1.383(5)
1.387(5)
1.397(5)
1.456(5)
128.6(3)
À4.0
1.386
1.370
1.374
1.370
1.407
1.404
1.406
1.409
1.440
132.5
0.5
1.33
1.33
NA[c]
NA
NA
NA
NA
NA
NA
124.9
44.7
1.32
1.32
NA
NA
NA
NA
NA
NA
NA
N2ÀC10
C11ÀC12
C14ÀC15
C10ÀC11
C12ÀC13
C13ÀC14
C15ÀC10
C13ÀC16
C9-N2-C10
X[a]-C9-N2-C10
123.41
23.3
[a] X=N3 (1, 3), X=S1 (2, 2a), X=N1 (4). [b] Geometric parameters of 1 and 2 after including acetate and tosylate ions, respectively, in the calculations.
[c] NA=not available. [d] The X-ray structural data are of acetate (1, 3t, and 4t) and tosylate (2a) salts, respectively.
hence, 1 and 2a cannot be considered to be N+ ana-
logues of allene. This is further supported by the ob-
Table 2. Crystal structure data and refinement parameters for 1, 2a, 3t and 4t.
servation that N2ÀC9 and N2ÀC10 bond rotational
1
2a
3t
4t
barriers are very low (Table 1). The N2ÀC9 and N2À
C10 bond lengths and C9-N2-C10 bond angle clearly
suggest that the CÀN interactions in 1 and 2a can be
considered to be dative bonds, as indicated by the
quantum chemical results (see below). The geometric
parameters obtained from the crystal structures of 3
and 4 are closely comparable to those of 3t and 4t
according to quantum chemical calculations
(Scheme 4). This is supported by the fact that the
N2ÀC9 and N2ÀC10 bonds in these structures are
longer in comparison to those in 1 and 2. Also, the
C9ÀN3 and C9ÀN1 (3t and 4t, respectively) bond
lengths are shorter (ꢀ1.31 Š) in comparison to that
in 1 and 2 (1.38 Š). The observation of the tautomers
3t and 4t in the crystal structures can be rationalized
in terms of the smaller energy differences between
the tautomeric pairs (3 vs. 3t and 4 vs. 4t). Though 3
and 4 are marginally more stable than their tauto-
mers, crystal packing forces and intermolecular inter-
actions facilitate crystallization of the tautomers 3t
and 4t.
formula
counter ion
T [K]
C18H21N5O2
acetate
100
339.4
monoclinic
P21/c
11.4638(12)
16.846(2)
9.8297(11)
90
103.637(7)
90
1844.8(4)
4
1.222
C44H44N8O6S4
tosylate
100
909.11
triclinic
C16H19N5O3
acetate
100
329.36
monoclinic
C2/c
21.234(2)
11.4976(10)
16.605(3)
90
123.031(4)
90
C16H16N4O2S
acetate
100
328.39
orthorhombic
Pbca
12.0202(8)
12.8635(9)
21.1231(14)
90
90
90
3266.1(4)
8
1.336
M
crystal system
space group
a [Š]
b [Š]
c [Š]
¯
P1
11.320(5)
12.476(6)
16.848(8)
68.775(4)
85.070(5)
84.955(5)
2205.7(18)
2
a [8]
b [8]
g [8]
V [Š3]
3398.7(7)
8
1.287
Z
1calcd [gcmÀ3
]
1.369
0.273
m [mmÀ1
Tmax/Tmin
]
0.83
0.092
0.213
0.7452/0.6648 0.684/0.745
21417 19250
2599 (0.0533) 6756 (0.0276) 1646 (0.0628) 1483 (0.1187)
0.7452/0.7029 0.7452/0.6618
24599 21619
total reflns
unique reflns (Rint
restraints/parameters 0/229
)
9/581
1.045
0/226
1.085
0/215
1.012
GOF
1.114
R1, wR2 ([I>2s(I)])
R1, wR2 (all data)
1max./1min. [eŠ
0.0468, 0.1389 0.0367, 01122 0.0642, 0.1266 0.0551, 0.1034
0.0607, 0.1523 0.0420, 0.1184 0.1697/0.2071 0.1399, 0.1341
0.257/À0.321 1.371/À0/359 0.350/À0.254 0.184/À0.234
À3
]
site to each other (C11ÀC12 and C14ÀC15 with distances of
about 1.38 Š each) and four longer bonds (ꢀ1.40 Š for C10À
C11, C12ÀC13, C13ÀC14, and C10ÀC15; Table 1).
Quantum chemical analysis
Quantum chemical calculations with the B3LYP and M06 meth-
ods were performed on 1–4 and their isomers (Scheme 4) to
estimate the relative energies. The discussion herein is based
on M06 data. 1–4 are global minima on the respective poten-
tial-energy surfaces. 1,7-Hydrogen shift is possible in 1–4, but
the resulting isomers 1i–4i are about 14–18 kcalmolÀ1 less
stable, so that such a shift is not easy (Table 3). Compounds 3
and 4 can also undergo 1,3-hydrogen shift, and the resulting
tautomers 3t and 4t are marginally less stable by only about
2–3 kcalmolÀ1, so that 1,3-H shift is quite feasible in 3 and 4.
Calculations were performed on the corresponding neutral
analogues 5 and 6, in which the global minimum-energy iso-
mers are in the aminic state rather than the iminic state (5t
The geometrical data of 2a, which are also as per expecta-
tion, mainly demonstrate the quinoidal character of the six-
membered ring and are comparable to the data obtained from
the quantum chemical analysis. The N2ÀC10 bonds in 1 and
2a are due to the donation of electron density from the C10
center to the nitrogen center. Hence, these bonds are longer
than typical C=N double bonds (1.28 Š) and much shorter than
typical CÀN single bonds (1.42–1.47 Š). The C9-N2-C10 bond
angles in 1 (120.88) and 2a (124.48) are quite comparable to
those of the reported divalent NI compounds I–III (ꢀ1268) and
are comparable to the quantum chemically calculated values
(Table 1). These bond angles are much smaller than 1808;
Chem. Eur. J. 2016, 22, 1088 – 1096
www.chemeurj.org
1091
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper
HOMO is a p-type orbital at the central nitrogen
atom in all cases (1–4). It is involved in antibonding
interaction with the p orbitals of the two ligands L
and L’ in 1–4. This is clearly evident from the absence
of orbital contribution from carbon centers C9 and
C10 (Figure 2). Hence, in the HOMO of 1–4, the p or-
bital is more localized on the central nitrogen in
comparison to that of carbones.[4l] The HOMOÀ4 is
a s-type orbital (sp2 character at the central nitrogen
atom) carrying a lone pair of electrons. These elec-
tron lone pairs are significantly occupied with about
1.75e in the s-type lone pair and about 1.40e in the
p-type lone pair; the eigenvalues associated with
these molecular orbitals are listed in Table 3. The
HOMO–LUMO gaps in 1–4 are sufficiently large
(ꢀ3.5 eV) to indicate stability of these species. The
electron localization function (ELF) values at the N2
center in 1–4 are in the range of 2.90–3.13e with
a bean-shaped isosurface indicating accumulation of
excess electron density. An NBO charge analysis indi-
cated that the N2 center in all these systems carries
a negative charge of up to about À0.6e, confirming
the charge accumulation at this center. Similarly, the
other electronic parameters estimated for 1–4 are
quite comparable to those of I (R=H, Table 3).
Table 3. Comparison of various parameters calculated for 1–4 at the M06/6-311+ +
G(d,p) level of theory. See Supporting Information for further details.
Parameter
Symbol
1
2
3
4
I (R=H)
1,7-H shift energy difference[a]
1,3-H shift energy difference[a]
energy of p-type lone pair[b]
energy of s type lone pair[b]
DE(HOMOÀLUMO)[b]
DE1
DE2
À14.2 À18.0 À14.8 À13.9 À5.0
NA[e] NA
À2.6 À3.4 NA
EHOMO
EHOMOÀ4
DE3
1st PA
Edep
À7.4 À7.4 À8.7 À8.9 À8.5
À9.4 À9.3 À11.1 À11.1 À11.0
3.7
3.4
3.5
3.5
5.0
proton affinity[a]
159.7 154.6 167.2 166.5 118.5
249.5 246.1 245.7 238.7 233.8
À21.9 À15.1 À22.9 À20.1 À18.6
À38.6 À28.4 À35.1 À31.1 À28.2
À34.5 À29.3 À32.9 À32.1 À26.5
À1.0 À2.4 À7.2 À5.0 À6.7
deprotonation energy[a]
complexation energy of BH3
complexation energy of AlCl3
complexation energy of AuCl[a]
[a]
EBH
3
[a]
EAlCl
3
EAuCl
[a]
complexation energy of (BH3)2
complexation energy of (AlCl3)2
complexation energy of (AuCl)2
decomp energy[a]
E
ðBH3 Þ2
[a]
[a]
E
0.02
À6.5 À15.2 À12.6 À13.7
ðAlCl3Þ2
E
À21.2 À21.3 À22.4 À20.0 À16.0
ðAuClÞ2
De
402.5 403.7 402.0 403.4 355.0
n-inversion barrier[a]
N2–C9 rotational barrier[a]
N2–C10 rotational barrier[a]
ELF value[c]
ENi
9.0
1.6
4.6
3.13
10.0
6.1
8.3
10.0
4.4
3.6
6.4
3.7
2.3
2.98
14.4
3.5
3.5
ERB1
ERB2
V(N2)
Q(N2)
LP s (N2) 1.72
LP p (N2) 1.46
fÀ(N2)
2.97
3.08
3.26
charge at N2[c]
À0.61 À0.59 À0.60 À0.57 À0.63
LP occupancy (s)[c,d]
LP occupancy (p)[c,d]
Fukui function
1.75
1.41
1.75
1.42
1.73
1.40
1.74
1.48
À0.18 À0.18 À0.18 À0.17 À0.20
Wiberg bond index
N2ÀC10
WBI
WBI
1.30
1.37
1.22
1.48
1.25
1.42
1.22
1.50
1.33
1.33
N2ÀC9
[a] Values are given in in kcalmolÀ1, [b] eV, and [c] electrons. [d] Enforced NBO analysis
The proton affinities of cationic species 1–4, which
are in the range of 154–167 kcalmolÀ1, are much
larger than that of I (R=H, 118.5 kcalmolÀ1). These
values are low in comparison to those of divalent C0
was used to locate two lone pairs at the N2 center. [e] NA: not applicable.
and 6t are less stable by 1.2 and 5.0 kcalmolÀ1 with respect to
5 and 6, respectively). The isomers with C(NH2)2 unit (5i and
6i) are less stable (ꢀ11.2 and 13.6 kcalmolÀ1, respectively). The
comparison of the geometric features and electronic features
of 1–4 with those of reported[1,2] divalent NI compounds sug-
gests that these compounds can be regarded as species with
systems due to the presence of positive charge on 1–4. How-
ever, proton affinities are higher when divalent NI character is
found in neutral analogues.[1g] The proton affinities of 5i and
6i (to give 3t and 4t, respectively) are 244.05 and 246.8 kcal
molÀ1, respectively. The nucleophilicity at the central nitrogen
can be estimated for 1–4 in terms of complexation energy
with BH3, AlCl3, and AuCl. Table 3 shows that these complexa-
tion energies are sufficiently high to indicate a reasonable
amount of nucleophilicity in 1–4. Complexation with two
metal-centered Lewis acids (BH3, AlCl3, and AuCl) is also possi-
ble, albeit with highly reduced complexation energies. The de-
composition energy of these species, as per the hypothetical
!
the (L!N L’)+ structure in which the central nitrogen cation
is flanked by two ligands (see below). This clearly establishes
that in systems 1–4, the preferred structure has possible L!N
interactions, but tautomeric alternatives can be accessible by
1,3-H shift when R=H.
Molecular orbital analysis (Figure 2) showed that the central
nitrogen atom in 1–4 has two lone pairs of electrons. The
!
reaction (L!N L’)+!L+L’ +N+ (X3P), is about 402–404 kcal
Figure 2. HOMO (p-type) and the HOMOÀ4 (s-type) molecular orbitals of 1–4 at the M06/6-311+ +G(d,p) level of theory.
Chem. Eur. J. 2016, 22, 1088 – 1096 www.chemeurj.org ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
1092

Full Paper
molÀ1. The higher bond decomposition energies for com-
pounds 1–4 compared to I can be attributed to the stronger
donor–acceptor interactions, which are due to the strong bind-
ing properties of L’.[1e] The bending potentials of 1–4 (estimat-
ed for a hypothetical linear structure, C9-N2-C10 1808, with
two negative frequencies) are 6.4–10.0 kcalmolÀ1.
NHC (pyridyl-4-ylidene (B) in 4-pyridones).[19c] The heterocyclic
species B, C, D, and E (Scheme 5) are known as remote
NHCs[19] with sufficient nucleophilicity, which form complexes
with transition metals as well as main group elements,[19c] but
they are not stable carbenes. Following these examples, and
the observed geometrical parameters, ligand A can also be
considered to be a ligand capable of donating electron density
to the N+ center (as in 1 and 2). Quantum chemical analysis
suggested that the nucleophilic characteristics, that is, proton
affinity (300.3 kcalmolÀ1), complexation energy with Lewis
acids BH3 (64.3 kcalmolÀ1) and AuCl (86.7 kcalmolÀ1), as well as
the Fukui function and the local nuleophilicity at the electron-
donating carbon center (À0.54 and À3.08, respectively), are
sufficiently high. Thus, A can indeed be considered to be
a newly identified electron-donating ligand.
The N2ÀC9 and N2ÀC10 rotational barriers in 1–4 are in the
range of about 1-8 kcalmolÀ1 and thus indicate that the p
character is very minor, even though the bond lengths are
very short. The low rotational barriers and N-inversion barriers
indicate that systems 1–4 have sufficient dynamism. Wiberg
bond indices for the N2ÀC9 and N2ÀC10 bonds in 1–4 on the
order of 1.22–1.50 indicate a lack of double-bond character
(for N-methylisoproylimine, which has CÀN double and single
bonds, the Wiberg bond indices were found to be 1.86 and
1.02, respectively), although the bond lengths are quite short,
consistent with the coordination character. Taken together, the
quantum chemical analysis discussed above clearly supports
divalent NI character at the N2 center in 1–4.
Conclusion
Compounds containing electron-accepting nitrogen cation
centers are important in medicinal chemistry and in phase-
transfer catalysis. The bonding situation at the central nitrogen
atom in these systems is quite intriguing. To explore this novel
bonding environment, four new compounds 1--4 with the
The new nucleophilic ligand
On the basis of the above discussion, it can be inferred that in
compounds 1 and 2 the central nitrogen atom is flanked by an
N-heterocyclic carbene (NHC) on one side and cyclohexa-2,5-
diene-4-(diaminomethynyl)-1-ylidene (A) ligand on the other
side (Scheme 5). Ligand A may be regarded as a unique car-
!
general formula (L!N L’)+ were synthesized. The desired
compounds were crystallized by evaporation/vapor-diffusion
methods in their salt forms (acetate/tosylate). X-ray diffraction
!
studies showed that 1 and 2 crystallized in a (L!N L’)+
structural arrangement, whereas 3 and 4 are crystallized in an
energetically accessible tautomeric form. The geometrical pa-
rameters of the species clearly establish that compounds
1 and 2 have divalent NI character. The C!N bonds in 1 and 2
are longer than a typical CÀN double bond and much shorter
than a typical CÀN single bond. The bond lengths found in
this work and in the literature indicate that the C!N coordina-
tion bond length varies in the range of 1.31 to 1.38 Š. The C-
N-C bond angles in 1 and 2 are about 121 and 1248, which are
far from 1808, in which case N-heteroallene-like character is ex-
pected.
The electronic structures of 1–4, their isomers, and tauto-
mers were analyzed by DFT (M06 and B3LYP) methods with 6-
311+ +G(d,p) basis set. Compounds 1–4 are characterized by
the following features 1) global minima on their respective po-
tential surfaces, 2) high isomerization energy (1,7-H shift in 1–
4), but low tautomerization energy (1,3-H shift in 3 and 4),
3) low proton affinity but sufficient nucleophilicity, 4) excess
electron density and excess negative charge accumulation at
the central nitrogen atom, although formal positive charge is
assigned to this center, 5) two lone pairs of electrons at the
central nitrogen atom, 6) small CÀN rotational barriers (2–8 kcal
molÀ1), and 7) small N-inversion barriers (6–10 kcalmolÀ1). All
these factors in combination with the geometrical features
clearly establish that 1–4 should be regarded as divalent NI
compounds. In 1–4, the functional unit, cyclohexa-2,5-diene-4-
(diaminomethynyl)-1-ylidene, has been shown to donate elec-
tron density to the central nitrogen atom, and thus may be
considered to be a new electron-donating ligand.
Scheme 5. Structure of ligand A and representative structures of the remote
carbenes reported in the literature: pyridyl-4-ylidene (B), pyridyl-3-ylidene
(C), N-methyl isoquinoline-4-ylidene (D) 4-pyrazolylidene (E), bis(diisopropy-
lamino)cyclopropenylidene (F), and NHC (G).
bene with a six membered ring without any heteroatom. Such
“cyclic non-NHCs” are quite rare, and the only known example
is ligand F[2b,3b,18] (Scheme 5). The nucleophilic character in
ligand A is triggered by electron donation from the diamino-
carbene moiety, that is, C(NH2)2. The ligand A in 1 and 2 can
be considered to be a novel species with electron-donating ca-
pacity. The electron-donating property of A is not clearly evi-
dent and appears to be “nonsensical”,[14] but it can be mani-
fested in the presence of an appropriate electron-accepting
center. (In the complexes between A and BH3 as well as be-
tween A and AuCl, quinoidal character and complete planarity
in A are observed; see Supporting Information.) A similar elec-
tron-donating property was noticed recently in the remote
Chem. Eur. J. 2016, 22, 1088 – 1096
www.chemeurj.org
1093
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper
3346, 3208, 3001, 1704, 1610, 1557, 1479, 1462, 1410, 1259, 1178,
1046, 1012, 841, 741, 650 cmÀ1; HRMS (ESI-TOF): calculated for
[M+]: 252.1243; found: 252.1254.
Experimental Section
General information
Compound 4·OAc: White solid; m.p. > 2008C; 219 mg, 67.00%
yield; 1H NMR (400 MHz, CD3SOCD3): d=8.01 (d, J=8.92 Hz, 2H),
7.85 (d, J=6.92, 2H) 7.76 (d J=8.52, 1H), 7.69 (d, J=7.64 Hz, 1H),
7.39 (t, J=8.12 Hz, 1H), 7.24 (t, J=7.84, 1H), 1.76 (s, 3H); 13C NMR
(100 MHz, CD3SOCD3): d=178.68, 166.08, 161.53, 151.78, 145.94,
130.46, 128.88, 125.85, 122.98, 120.56, 120.39, 119.70, 117.48, 29.41,
All starting materials were purchased from Sigma-Aldrich/Alfa
Aesar and used without any additional purification. All reactions
were carried out in flame-dried glassware under N2. All the com-
pounds (intermediates and final compounds) were purified by
column chromatography with silica gel (100–200 mesh) as station-
1
ary phase and hexane/ethyl acetate as mobile phase. H NMR spec-
~
22.54 ppm; IR (neat): nmax =3461, 3230, 2927, 1679, 1609, 1563,
tra (400 MHz) were recorded with a Bruker Avance III 400 NMR
1
1494, 1440, 1408, 1340, 1278, 1212, 1129, 1020, 920, 842, 754, 659;
HRMS (ESI-TOF): calculated for [M+] 269.0861; found 269.0909.
spectrometer. H chemical shifts are expressed in parts per million
(ppm, d scale) and are referenced to residual proton of the solvent
(CDCl3, d=7.26, CD3OD, d=3.31 and CD3SOCD3, d=2.49). Cou-
pling constants J are reported in hertz. 13C NMR spectra (100 MHz)
were recorded with a Bruker Avance III 400 NMR spectrometer and
were fully decoupled by broadband decoupling. Chemical shifts
are reported in ppm referenced to the center line at 77.0, 49.0, and
39.7 ppm of CDCl3, CD3OD and CD3SOCD3, respectively. High-reso-
lution ESI-TOF mass spectra were taken with a Bruker maXis instru-
ment.
Single-crystal X-ray data collection and refinement
Initial crystal evaluation and data collection were performed with
a Kappa APEX II diffractometer equipped with a CCD detector
(with the crystal-to-detector distance fixed at 60 mm) and sealed-
tube monochromated MoKa radiation by using the program
APEX2.[20] By using the program SAINT[20] for the integration of the
data, reflection profiles were fitted, and values of F2 and s(F2) for
each reflection were obtained. Data were also corrected for Lorent-
zian and polarization effects. The subroutine XPREP[20] was used for
the processing of data, which included determination of the space
group, application of an absorption correction (SADABS),[20] merg-
ing of data, and generation of files necessary for solution and re-
finement. The crystal structures were solved and refined with
SHELX97.[21] In each case, the space group was chosen on the basis
of systematic absences and confirmed by successful refinement of
the structure. Several full-matrix, least-squares/difference Fourier
cycles were performed, which located the remainder of the non-
hydrogen atoms. All non-hydrogen atoms were refined with aniso-
tropic displacement parameters except where mentioned. Wher-
ever possible the positions of the hydrogen atoms observed in the
difference Fourier map were used and refined isotropically; all
other hydrogen atoms were placed in ideal positions and refined
as riding atoms with individual isotropic displacement parameters.
All figures were drawn with MERCURY 3.0[22] and hydrogen-bond-
ing parameters were generated with PLATON.[23] The final position-
al and thermal parameters of the non-hydrogen atoms for all struc-
tures are listed in the CIF files (Supporting Information).
CCDC 1035519 (1), 1409616 (2a), 1035520 (3t), and 1035521 (4t)
contain the supplementary crystallographic data for this paper.
These data are provided free of charge by The Cambridge Crystal-
lographic Data Centre.
Synthesis
Preparation of compound 1: Compound 12 (470 mg, 1 mmol) was
dissolved in methanol followed by addition of ammonium acetate
(3 equiv). The mixture was heated under reflux for 3 h, cooled, and
concentrated under vacuum. The final compound was precipitated
as the acetate salt by addition of acetone to the reaction mixture.
Similarly compounds 2, 3, and 4 were prepared.
Characterization of 1·OAc: White solid; m.p. > 2008C, 148 mg,
44% yield; 1H NMR (400 MHz, CD3SOCD3): d=7.70 (d, J=8.68 Hz,
2H), 7.21–7.18 (m, 2H), 7.13–7.10 (m, 2H), 6.92 (d, J=8.68 Hz, 2H),
3.26 (s, 6H), 1.72 (s, 3H); d=176.86, 165.47, 155.65, 148.07, 132.34,
128.99, 121.89, 121.60, 118.31, 108.31, 30.77, 25.30 ppm; IR (neat):
~
nmax =3244, 2974, 1682, 1622, 1571, 1536, 1472, 1417, 1396, 1380,
1249, 1192, 1159, 1097, 1004, 942, 838, 739, 722 cmÀ1; HRMS (ESI-
TOF): calcd for [M+]: 280.1560; found: 280.1562.
Compound 2·OAc: White solid; m.p. > 2008C, 107.9 mg, 32%
yield; 1H NMR (400 MHz, CD3SOCD3): d=7.82 (d, J=8.52 Hz, 2H),
7.56 (d, J=7.52 Hz, 1H), 7.36 (t, J=7.24 Hz, 1H), 7.28 (d, J=
7.80 Hz, 1H), 7.21 (d, J=8.52 Hz, 2H), 7.12 (t, J=7.16 Hz, 1H), 3.57
(s, 3H), 1.72 (s, 3H); 13C NMR (100 MHz, CD3SOCD3): d=177.02,
165.82, 157.34, 155.70, 140.40, 129.79, 127.30, 124.08, 122.97,
~
122.72, 121.94, 121.29, 110.68, 31.04, 25.30 ppm; IR (neat): nmax
=
2926, 2297, 1680, 1624, 1570, 1477, 1408, 1338, 1220, 1183, 1129,
1058, 1032, 902, 852, 811, 742 cmÀ1; HRMS (ESI-TOF): calcd for [M+
]: 283.1017; found: 283.1015.
Methods of calculation
Compound 2·OTs: Brown solid; m.p. > 2008C; 413 mg, 98% yield;
1H NMR (400 MHz, CD3SOCD3): d=9.21 (brs, 2H), 8.82 (brs, 2H),
7.85 (d, J=8.60 Hz, 2H), 7.59 (d, J=7.16 Hz, 1H), 7.50 (d, J=
8.04 Hz, 3H), 7.38 (t, J=8.20 Hz, 1H), 7.31 (d, J=7.56 Hz, 1H), 7.26
( d, J=8.68 Hz, 2H), 7.16–7.12 (m, 3H), 3.59 (s, 3H), 2.29 (s, 3H);
13C NMR (100 MHz, CD3SOCD3): d=165.29, 157.54, 156.24, 145.90,
140.34, 138.29, 130.45, 128.60, 127.38, 125.95, 123.01, 122.85,
The quantum chemical calculations were carried out with the
Gaussian 09 suite[24] of programs on a cluster computer with Intel
octacore processors. Complete geometry optimizations were per-
formed on all the structures without any symmetry constraints at
B3LYP/6-311+ +G(d,p) and M06/-6-311+ +G(d,p) levels of theory.
A scaling factor[25] of 0.9877 was employed on the estimated zero-
point vibrational energies obtained from the frequency calcula-
tions. Partial atomic charges were calculated with the Natural Bond
Orbital (NBO) method.[26] NBO analysis was carried out with the en-
forced option to obtain clear plots of the molecular orbitals. NMR
calculations were performed by using the GIAO method. Electron
localization function (ELF) analysis was carried out by using the
Multiwfn program.[27] Formulas adopted to estimate various param-
eters listed in Table 3 are given in the Supporting Information.
~
122.47, 122.08, 121.25, 110.83, 31.12, 21.25 ppm; IR (neat): nmax
=
2924, 2854, 1575, 1464, 1180, 1124, 1035, 1011, 686, 568 cmÀ1
;
HRMS (ESI-TOF): calcd for [M+] 283.1017; found: 283.1030.
Compound 3·OAc: White solid; m.p. > 2008C; 230 mg, 74% yield;
1H NMR (400 MHz, CD3SOCD3): d=7.99 (d, J=8.64 Hz, 2H), 7.81 (d,
J=872 Hz, 2H), 7.33 (brs, 2H), 7.03–7.01 (m, 2H), 1.82 (s, 3H);
13C NMR (100 MHz, CD3SOCD3): d=176.86, 165.32, 151.18, 146.85,
~
129.55, 120.63, 118.76, 116.54, 31.15, 24.99 ppm; IR (neat): nmax
=
Chem. Eur. J. 2016, 22, 1088 – 1096
www.chemeurj.org
1094
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper
J. 2012, 18, 3621–3630; b) H. Bruns, M. Patil, J. Carreras, A. Vazquez, W.
Thiel, R. Goddard, M. Alcarazo, Angew. Chem. Int. Ed. 2010, 49, 3680–
3683; Angew. Chem. 2010, 122, 3762–3766; c) Ref. [5b] provides clear
evidence for L!N-R compounds. Ref. [5a] reported the generation of
several compounds with possible L!NR arrangement, but did not ex-
plicitly claim L!N coordination-bond character.
Acknowledgements
The authors thank Department of Science and Technology
(DST), Govt. of India, New Delhi, India for financial support. The
X-ray facility at IISER Mohali is acknowledged; V.G. thanks Uni-
versity Grant Commission (UGC), Govt. of India, New Delhi,
India for financial assistance. The authors thank NIPER, S.A.S.
Nagar for infrastructural facilities.
[6] a) N. Holzmann, D. Dange, C. Jones, G. Frenking, Angew. Chem. Int. Ed.
2013, 52, 3004–3008; Angew. Chem. 2013, 125, 3078–3082; b) D. J. D.
Wilson, S. A. Couchman, J. L. Dutton, Inorg. Chem. 2012, 51, 7657–
7668; c) M. Reinmuth, C. Neuhäuser, P. Walter, M. Enders, E. Kaifer, H.-J.
Himmel, Eur. J. Inorg. Chem. 2011, 83–90.
[7] a) B. D. Ellis, C. L. B. Macdonald, Coord. Chem. Rev. 2007, 251, 936–973;
b) B. D. Ellis, C. A. Dyker, A. Decken, C. L. Macdonald, Chem. Commun.
2005, 1965–1967; c) B. D. Ellis, M. Carlesimo, C. L. B. Macdonald, Chem.
Commun. 2003, 1946–1947; d) M. Driess, H. Ackermann, J. Aust, K.
Merz, C. v. Wüllen, Angew. Chem. Int. Ed. 2002, 41, 450–453; Angew.
Chem. 2002, 114, 467–470; e) R. J. Barnham, R. M. K. Deng, K. B. Dillon,
A. E. Goeta, J. A. K. Howard, H. Puschmann, Heteroat. Chem. 2001, 12,
501–510; f) R. O. Day, A. Willhalm, J. M. Holmes, R. R. Holmes, A.
Schmidpeter, Angew. Chem. Int. Ed. Engl. 1985, 24, 764–765; Angew.
Chem. 1985, 97, 775–777; g) A. H. Cowley, R. A. Kemp, Chem. Rev. 1985,
85, 367–382; h) A. Schmidpeter, S. Lochschmidt, A. Willhalm, Angew.
Chem. Int. Ed. Engl. 1983, 22, 545–546; Angew. Chem. 1983, 95, 561–
562; i) A. Schmidpeter, W. Gebler, F. Zwaschka, W. S. Sheldrick, Angew.
Chem. Int. Ed. Engl. 1980, 19, 722–723; Angew. Chem. 1980, 92, 767–
768; j) K. Dimroth, Fortschr. Chem. Forsch. 1973, 38, 1–147; k) K. Dim-
roth, P. Hoffmann, Angew. Chem. Int. Ed. Engl. 1964, 3, 384–384; Angew.
Chem. 1964, 76, 433–434.
[8] a) N. Holzmann, G. Frenking, Naturforscher 2014, 69, 385–395; b) M. Al-
carazo, K. Radkowski, G. Mehler, R. Goddard, A. Furstner, Chem.
Commun. 2013, 49, 3140–3142; c) Y. Wang, Y. Xie, P. Wei, H. F. Schaefer,
P. v. R. Schleyer, G. H. Robinson, J. Am. Chem. Soc. 2013, 135, 19139–
19142; d) O. Back, B. Donnadieu, P. Parameswaran, G. Frenking, G. Ber-
trand, Nat. Chem. 2010, 2, 369–373; e) O. Back, G. Kuchenbeiser, B.
Donnadieu, G. Bertrand, Angew. Chem. Int. Ed. 2009, 48, 5530–5533;
Angew. Chem. 2009, 121, 5638–5641; f) O. Back, G. Kuchenbeiser, B.
Donnadieu, G. Bertrand, Angew. Chem. 2009, 121, 5638–5641; g) Y.
Wang, Y. Xie, P. Wei, R. B. King, H. F. Schaefer III, P. v. R. Schleyer, G. H.
Robinson, J. Am. Chem. Soc. 2008, 130, 14970–14971; h) J. D. Masuda,
W. W. Schoeller, B. Donnadieu, G. Bertrand, Angew. Chem. Int. Ed. 2007,
46, 7052–7055; Angew. Chem. 2007, 119, 7182–7185.
Keywords: carbene ligands · density functional calculations ·
ligand design · nitrogen
[1] a) S. Bhatia, P. V. Bharatam, J. Org. Chem. 2014, 79, 4852–4862; b) M.
Arfeen, D. S. Patel, S. Abbat, N. Taxak, P. V. Bharatam, J. Comput. Chem.
2014, 35, 2047–2055; c) L. Adane, S. Bhagat, M. Arfeen, S. Bhatia, R. Sir-
awaraporn, W. Sirawaraporn, A. K. Chakraborti, P. V. Bharatam, Bioorg.
Med. Chem. Lett. 2014, 24, 613–617; d) S. Bhatia, Y. J. Malkhede, P. V.
Bharatam, J. Comput. Chem. 2013, 34, 1577–1588; e) M. A. Celik, R.
Sure, S. Klein, R. Kinjo, G. Bertrand, G. Frenking, Chem. Eur. J. 2012, 18,
5676–5692; f) S. Bhatia, C. Bagul, Y. Kasetti, D. S. Patel, P. V. Bharatam, J.
Phys. Chem. A 2012, 116, 9071–9079; g) D. S. Patel, P. V. Bharatam, J.
Phys. Chem. A 2011, 115, 7645–7655; h) D. S. Patel, P. V. Bharatam, J.
Org. Chem. 2011, 76, 2558–2567; i) A. Mehdi, L. Adane, D. S. Patel, P. V.
Bharatam, J. Comput. Chem. 2010, 31, 1259–1267; j) D. S. Patel, P. V.
Bharatam, Chem. Commun. 2009, 1064–1066; k) P. V. Bharatam, D. S.
Patel, P. Iqbal, J. Med. Chem. 2005, 48, 7615–7622; l) I. S. K. Kerkines, A.
Papakondylis, A. Mavridis, J. Phys. Chem. A 2002, 106, 4435–4442; m) A.
Vij, W. W. Wilson, V. Vij, F. S. Tham, A. J. Sheehy, K. O. Christe, J. Am.
Chem. Soc. 2001, 123, 6308; n) K. O. Christe, W. W. Wilson, J. A. Sheehy,
J. A. Boatz, Angew. Chem. Int. Ed. 1999, 38, 2004; Angew. Chem. 1999,
111, 2112.
[2] a) R. Mirabdolbaghi, T. Dudding, T. Stamatatos, Org. Lett. 2014, 16,
2790–2793; b) A. Kozma, G. Gopakumar, C. Fares, W. Thiel, M. Alcarazo,
ˇ
Chem. Eur. J. 2013, 19, 3542–3546; c) R. A. Kunetskiy, I. Císarovµ, D.
ˇ
Saman, I. M. Lyapkalo, Chem. Eur. J. 2009, 15, 9477–9485; d) I. Bernhar-
di, T. Drews, K. Seppelt, Angew. Chem. Int. Ed. 1999, 38, 2232–2233;
Angew. Chem. 1999, 111, 2370–2372; e) T. Ma, X. Fu, C. W. Kee, L. Zong,
Y. Pan, K. W. Huang, C. H. Tan, J. Am. Chem. Soc. 2011, 133, 2828–2831;
f) Ref. [2b] provides the experimental evidence for divalent NI character
in compounds containing carbene-stabilized N-centered cations.
[9] a) M. Y. Abraham, Y. Wang, Y. Xie, R. J. Gilliard, Jr., P. Wei, B. J. Vaccaro,
M. K. Johnson, H. F. Schaefer III, P. v. R. Schleyer, G. H. Robinson, J. Am.
Chem. Soc. 2013, 135, 2486–2488; b) M. Y. Abraham, Y. Wang, Y. Xie, P.
Wei, H. F. Schaefer III, P. v. R. Schleyer, G. H. Robinson, Chem. Eur. J. 2010,
16, 432–435; c) B. D. Ellis, C. L. B. Macdonald, Inorg. Chem. 2004, 43,
5981–5986.
!
Refs. [2c–e] reported the synthesis of (L!N L)+ species, but did not
explore the possibility of divalent NI character in these species. Ref. [2a]
explored the electronic characteristics of carbene-stabilized NI-centered
cations and argued against divalent NI character.
[10] a) S. De, P. Parameswaran, Dalton Trans. 2013, 42, 4650–4656; b) H.
Braunschweig, R. D. Dewhurst, K. Hammond, J. Mies, K. Radacki, A.
Vargas, Science 2012, 336, 1420–1422; c) R. Kinjo, B. Donnadieu, M. A.
Celik, G. Frenking, G. Bertrand, Science 2011, 333, 610–613.
[11] a) Y. Xiong, S. Yao, S. Inoue, J. D. Epping, M. Driess, Angew. Chem. 2013,
125, 7287–7291; b) N. Takagi, R. Tonner, G. Frenking, Chem. Eur. J. 2012,
18, 1772–1780; c) P. Parameswaran, G. Frenking, Chem. Eur. J. 2009, 15,
8807–8816; d) N. Takagi, T. Shimizu, G. Frenking, Chem. Eur. J. 2009, 15,
3448–3456; e) N. Takagi, T. Shimizu, G. Frenking, Chem. Eur. J. 2009, 15,
8593–8604; f) Y. Wang, Y. Xie, P. Wei, R. B. King, H. F. Schaefer III, P.
von R. Schleyer, G. H. Robinson, Science 2008, 321, 1069.
[12] a) T. Chu, L. Belding, A. van der Est, T. Dudding, I. Korobkov, G. I. Niko-
nov, Angew. Chem. Int. Ed. 2014, 53, 2711–2715; Angew. Chem. 2014,
126, 2749–2753; b) Y. Xiong, S. Yao, S. Inoue, J. D. Epping, M. Driess,
Angew. Chem. Int. Ed. 2013, 52, 7147–7150; Angew. Chem. 2013, 125,
7287–7291; c) Y. Xiong, S. Yao, G. Tan, S. Inoue, M. Driess, J. Am. Chem.
Soc. 2013, 135, 5004–5007; d) A. Sidiropoulos, C. Jones, A. Stasch, S.
Klein, G. Frenking, Angew. Chem. Int. Ed. 2009, 48, 9701–9704; Angew.
Chem. 2009, 121, 9881–9884.
[3] a) G. Frenking, F. Tonner, (Eds.: G. Frenking, S. Shaik), Wiley-VCH, Wein-
heim, 2014, pp. 71–112; b) G. Frenking, R. Tonner, Wiley Interdiscip. Re-
v. :Comput. Mol. Sci. 2011, 1, 869–878.
[4] a) S. R. Barua, W. D. Allen, E. Kraka, P. Jerabek, R. Sure, G. Frenking,
Chem. Eur. J. 2013, 19, 15941–15954; b) C. Esterhuysen, G. Frenking,
Chem. Eur. J. 2011, 17, 9944–9956; c) S. Klein, R. Tonner, G. Frenking,
Chem. Eur. J. 2010, 16, 10160–10170; d) R. Tonner, G. Frenking, Pure
Appl. Chem. 2009, 81, 597–614; e) M. Alcarazo, C. W. Lehmann, A.
Anoop, W. Thiel, A. Fürstner, Nat. chem. 2009, 1, 295–301; f) C. A. Dyker,
G. Bertrand, Nat. Chem. 2009, 1, 265–266; g) V. Lavallo, C. A. Dyker, B.
Donnadieu, G. Bertrand, Angew. Chem. Int. Ed. 2008, 47, 5411–5414;
Angew. Chem. 2008, 120, 5491–5494; h) C. A. Dyker, V. Lavallo, B. Don-
nadieu, G. Bertrand, Angew. Chem. Int. Ed. 2008, 47, 3206–3209; Angew.
Chem. 2008, 120, 3250–3253; i) O. Kaufhold, F. E. Hahn, Angew. Chem.
Int. Ed. 2008, 47, 4057–4061; Angew. Chem. 2008, 120, 4122–4126; j) R.
Tonner, G. Frenking, Chem. Eur. J. 2008, 14, 3260–3272; k) R. Tonner, G.
Frenking, Chem. Eur. J. 2008, 14, 3273–3289; l) R. Tonner, G. Frenking,
Angew. Chem. Int. Ed. 2007, 46, 8695–8698; Angew. Chem. 2007, 119,
8850–8853; m) R. Tonner, F. Oxler, B. Neumuller, W. Petz, G. Frenking,
Angew. Chem. Int. Ed. 2006, 45, 8038–8042; Angew. Chem. 2006, 118,
8206–8211.
[13] C. Jones, A. Sidiropoulos, N. Holzmann, G. Frenking, A. Stasch, Chem.
Commun. 2012, 48, 9855–9857.
ˇ
ˇ
[14] a) D. Himmel, I. Krossing, A. Schnepf, Angew. Chem. Int. Ed. 2014, 53,
6047–6048; Angew. Chem. 2014, 126, 6159–6160; b) D. Himmel, I.
[5] a) R. A. Kunetskiy, S. M. Polyakova, J. Vavrík, I. Císarovµ, J. Saame, E. R.
Nerut, I. Koppel, I. A. Koppel, A. Kütt, I. Leito, I. M. Lyapkalo, Chem. Eur.
Chem. Eur. J. 2016, 22, 1088 – 1096
www.chemeurj.org
1095
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper
Krossing, A. Schnepf, Angew. Chem. Int. Ed. 2014, 53, 370–374; Angew.
Chem. 2014, 126, 378–382.
[15] G. Frenking, Angew. Chem. Int. Ed. 2014, 53, 6040–6046; Angew. Chem.
2014, 126, 6152–6158.
[16] M. Schnürch, B. Waldner, K. Hilber, M. D. Mihovilovic, Bioorg. Med. Chem.
Lett. 2011, 21, 2149–2154.
[17] a) K. Dabak, Turk. J. Chem. 2002, 26, 547–550; b) C. G. Cummings, N. T.
Ross, W. P. Katt, A. D. Hamilton, Org. Lett. 2009, 11, 25–28.
[18] a) L. E. Johnson, D. B. Dupre, J. Phys. Chem. A 2007, 111, 11066–11073;
b) L. E. Johnson, D. B. Dupre, J. Phys. Chem. A 2008, 112, 7448–7454;
c) W. W. Schoeller, G. D. Frey, G. Bertrand, Chem. Eur. J. 2008, 14, 4711–
4718.
[19] a) O. Schuster, L. Yang, H. G. Raubenheimer, M. Albrecht, Chem. Rev.
2009, 109, 3445–3478; b) R. H. Crabtree, Coord. Chem. Rev. 2013, 257,
755–766; c) E. Stander-Grobler, O. Schuster, G. Heydenrych, S. Cronje, E.
Tosh, M. Albrecht, G. Frenking, H. G. Raubenheimer, Organometallics
2010, 29, 5821–5833; d) H. G. Raubenheimer, S. Cronje, Dalton Trans.
2008, 1265–1272; e) T. Koizumi, T. Tomon, K. Tanaka, J. Organomet.
Chem. 2005, 690, 1258–1264; f) S. K. Schneider, G. R. Julius, C. Loschen,
H. G. Raubenheimer, G. Frenking, W. A. Herrmann, Dalton Trans. 2006,
1226–1233; g) G. Heydenrych, M. von Hopffgarten, E. Stander, O. Schus-
ter, H. G. Raubenheimer, G. Frenking, Eur. J. Inorg. Chem. 2009, 2009,
1892–1904; h) A. K. Guha, C. Das, A. K. Phukan, J. Organomet. Chem.
2011, 696, 586–593; i) Y. Han, H. V. Huynh, Chem. Commun. 2007,
1089–1091; j) S. K. Schneider, P. Roembke, G. R. Julius, C. Loschen, H. G.
Raubenheimer, G. Frenking, W. A. Herrmann, Eur. J. Inorg. Chem. 2005,
2005, 2973–2977; k) O. Schuster, H. G. Raubenheimer, Inorg. Chem.
2006, 45, 7997–7999.
[20] APEX2, SADABS and SAINT, Bruker AXS Inc, Madison, WI, USA, 2008.
[21] G. M. Sheldrick, Acta Crystallogr. Sect. A 2008, 64, 112–122.
[22] C. F. Macrae, I. J. Bruno, J. A. Chisholm, P. R. Edginton, P. McCabe, E. Pi-
docck, L. Rodriguez-Monge, T. Taylor, J. Van de Streek, P. A. Wood, J.
Appl. Crystallogr. 2008, 41, 466–470.
[23] A. L. Spek, PLATON, Version 1.62, University of Utrecht, 1999.
[24] M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M. A. Robb, J. R.
Cheeseman, G. Scalmani, V. Barone, B. Mennucci, G. A. Petersson, H. Na-
katsuji, M. Caricato, X. Li, H. P. Hratchian, A. F. Izmaylov, J. Bloino, G.
Zheng, J. L. Sonnenberg, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Ha-
segawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, T. Vreven,
J. A., Jr., Montgomery, J. E. Peralta, F. Ogliaro, M. Bearpark, J. J. Heyd, E.
Brothers, K. N. Kudin, V. N. Staroverov, R. Kobayashi, J. Normand, K. Ra-
ghavachari, A. Rendell, J. C. Burant, S. S. Iyengar, J. Tomasi, M. Cossi, N.
Rega, N. J. Millam, M. Klene, J. E. Knox, J. B. Cross, V. Bakken, C. Adamo,
J. Jaramillo, R. Gomperts, R. E. Stratmann, O. Yazyev, A. J. Austin, R.
Cammi, C. Pomelli, J. W. Ochterski, R. L. Martin, K. Morokuma, V. G. Zakr-
zewski, G. A. Voth, P. Salvador, J. J. Dannenberg, S. Dapprich, A. D. Dan-
iels, Ö. Farkas, J. B. Foresman, J. V. Ortiz, J. Cioslowski, D. J. Fox, Gaussian
09, RevB.01, Gaussian, Inc., Wallingford CT, 2010.
[25] M. P. Andersson, P. Uvdal, J. Phys. Chem. A 2005, 109, 2937–2941.
[26] A. E. Reed, L. A. Curtiss, F. Weinhold, Chem. Rev. 1988, 88, 899–926.
[27] a) A. D. Becke, K. E. Edgecombe, J. Chem. Phys. 1990, 92, 5397; b) T. Lu,
F. Chen, J. Comput. Chem. 2012, 33, 580–592.
Received: September 9, 2015
Published online on November 30, 2015
Chem. Eur. J. 2016, 22, 1088 – 1096
www.chemeurj.org
1096
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim