Can Remote N-Heterocyclic Carbenes Coordinate with Main Group Elements? Synthesis, Structure, and Quantum Chemical Analysis of N+-Centered Complexes

Article abstract of DOI:10.1002/chem.201705999

Remote N-heterocyclic carbenes (rNHCs), such as N-methyl-4-pyridylidene, are known to form coordination complexes with TMs. Herein, it is established that rNHCs can also coordinate to the N⁺ centre. Synthesis of some novel divalent N^I complexes with the general formula (rNHC)→N⁺←(NHC) and (rNHC)→N⁺←(rNHC) was achieved, and X-ray diffraction studies supported the coordination bond character between the rNHCs and the N⁺ centre. Quantum chemical analysis established the presence of divalent N^I character at the central nitrogen in these systems.

Full text of DOI:10.1002/chem.201705999

A Journal of
Accepted Article
Title: Can Remote N-Heterocyclic Carbenes Coordinate with Main
Group Elements? Synthesis, Structure and Quantum Chemical
Analysis of N+ Centre Complexes
Authors: Neha Patel, Minhajul Arfeen, Radhika Sood, Sadhika Khullar,
Asit K. Chakraborti, Sanjay Mandal, and Prasad V. Bharatam
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To be cited as: Chem. Eur. J. 10.1002/chem.201705999
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1
Can Remote N-Heterocyclic Carbenes Coordinate with Main Group
Elements? Synthesis, Structure and Quantum Chemical Analysis
of N+ Centre Complexes
Neha Patel,[a] Minhajul Arfeen,[a] Radhika Sood,[a] Sadhika Khullar,[b] Asit K. Chakraborti,[a] Sanjay K.
Mandal[c] and Prasad V. Bharatam*[a]
Abstract: Remote N-heterocyclic carbenes (rNHCs) like N-methyl-4-
pyridylidene are known to form coordination complexes with transition
metals. In this article, it is established that rNHCs can also coordinate
to the N+ centre. Synthesis of some novel divalent NI complexes with
the general formula (rNHC)N+(NHC) and (rNHC)N+(rNHC) is
reported. X-ray diffraction studies support the coordination bond
character between rNHC and N+ centre. Quantum chemical analysis
established the presence of divalent NI character at the central
nitrogen in these systems.
Figure 1. Representative examples of rNHCs reported in literature.
The coordination chemistry of NHCs with transition metals
is very well known.[6] In the recent past, the coordination chemistry
of NHCs with main group elements was also established.[7] NHC
Introduction
complexes with the main group elements, as in
(NHC)→E←(NHC) (E = BH,[8] C,[7b, 9] Si,[10] Ge,[11] N [12],[13] and P
The remote N-heterocyclic carbenes (rNHCs) are N-
heterocyclic carbenes (NHCs) in which the nitrogen atom is not
adjacent to the carbenic carbon.[1] rNHCs possess unique
[14]; (I, Figure 2)), were designed and synthesized. Crystal
structures and the electronic structure analyses of these systems
clearly established the (NHC)E coordination bonds. Similarly,
properties and exhibit higher nucleophilicity in comparison to that
the coordination chemistry of NHCs with diatomic species (E—E :
of NHCs.[1b] Several rNHCs such as 4-pyridylidene (A),[2] 3-
B2,[15] Si2,[16] Ge2,[17] N2,[18] P2,[19] As2,[20] N-P[21]) as in NHC→E—
pyridylidene (B),[2] quinolylidene (C-F),[2b-d,
9-acridinylidene
3]
E←NHC complexes (J, Figure 2) were also reported. A few
compounds with this general formula were obtained and their
structures were established. Compounds with the general formula
LN—R (K)[22] and LN+L′ (L and M),[13a-g] also are of special
interest (because of their medicinal importance) and such
compounds are labelled as divalent NI compounds (Figure 2).
(G)[2c] and 4-pyrazolylidene (H)[4] were studied (Figure 1). Till date,
none of the rNHCs could be isolated as independent species.
However, the transition metal (TM) complexes of these species
containing (rNHC)TM coordination bonds were obtained by
generating rNHCs in situ.[2-5] These transition metal (TM)
complexes were studied for their stability, electron donating
properties and organometallic catalysis.[2-5] These (rNHC)TM
complexes have also been used in C-C bond formation reactions
such as Suzuki-Miyaura coupling and Mizoroki-Heck reaction.[2a,
2b, 2e, 3b, 4b]
The experimental and quantum chemical analysis
established that the (rNHC)TM interaction is stronger than the
(NHC)TM interaction, which has a consequence on the higher
catalytic activity of the (rNHC)TM complexes.3b
[a]
N. Patel, Dr. M. Arfeen, Radhika Sood, Prof. Dr. A.K. Chakraborti
and Prof. Dr. P.V. Bharatam,
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
Dr. S. Khullar
Department of Chemistry
Figure 2. General schematic representation of compounds with donor-
acceptor interactions within main group elements (I and J) with some important
examples of species with the general formula LN-R (K) and (LNL′)+ (L
and M).
[b]
[c]
D.A.V. University,
Jalandhar-Pathankot National Highway,
Jalandhar-144012, Punjab, India
Prof. Dr. S.K. Mandal
Department of Chemistry,
Though there are reservations regarding the use of arrows
in main group chemistry,[23] the dative bond representation
between carbon and main group elements (LE) renewed the
discussion on chemical bonding and is offering innovative
opportunities for chemical research.[24],[3a] Many new compounds
Indian Institute of Science Education and Research (IISER)
Sector 81, S.A.S. Nagar – 140 308, Punjab, India
Supporting Information for this article is given via a link at the end of the
document.
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10.1002/chem.201705999
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2
with the general formula (LNL′)+ were designed and
synthesized, in which L is an NHC/CCC.[13a],[12d] Several chiral
pentanidium salts which exhibit phase transfer catalytic activity,
can also be considered as divalent NI compounds.[12c, 25],[12a, 26]
Till now, none of the reports showed the possibility of rNHCs
as donor ligands to main group elements (rNHCE). Since the
compounds with an rNHCN+ interaction may act as potential
candidates for organic catalysis and can also carry potential
biological activity, it is important to generate compounds with such
interactions and evaluate their bonding properties. To address
this, experimental (synthesis and X-ray diffraction) and quantum
chemical studies have been taken up on compounds 1-8, which
carry the (rNHCN+) interaction (Figure 3) and the results are
presented in this article.
The treatment of IIIa with 4-aminopyridine resulted in the
formation of 1a. The anion metathesis by the treatment of 1a with
KPF6 in water afforded the corresponding hexafluorophosphate
salt 1b. Similarly, 2b was obtained from the starting compound 2-
mercaptobenzothiazole. Scheme
2
shows the synthetic
procedures for compounds 3-7. Compound IIIb was subjected to
salt metathesis to give IV, which was then coupled with various
N-substituted 4-aminopyridine derivatives (V-IX, see Supporting
Information for further details), in the presence of potassium
carbonate in refluxing acetonitrile to give compounds 3-7 as
hexafluorophosphate salts.
Figure 3. Divalent NI compounds with 4-pyridylidene group, reported in this
work.
Scheme 2. General scheme for the synthesis of compounds 3-7. Reagents and
conditions: (a) KPF6, water, rt (b) N-substituted 4-aminopyridine derivatives (V-
IX, see Supporting Information for further details), K2CO3, acetonitrile, reflux, 5-
7 h (23% - 62%).
Results and Discussion
1. Synthesis
Synthetic procedure for compound 1 and 2 is depicted in
Scheme 1. The treatment of 2-mercaptobenzimidazole, Ia with
MeI in the presence of K2CO3 in DMF afforded N-methyl(2-
methylmercapto) benzimidazole IIa, which upon treatment with
Me2SO4 produced the salt IIIa.
The starting material for the synthesis of 8 was 4,4′-
dipyridylamine (X), prepared by Pd2(dba)3-catalysed Buchwald-
Hartwig coupling of the hydrochloride salt of 4-bromopyridine with
4-aminopyridine.[27] Treatment of X with Me2SO4 in the presence
of K2CO3 in MeCN followed by anion metathesis with KPF6 in
water led to 8b (Scheme 3). The C2 carbon (Figure 4) in 1b and
2b appeared at 151.6 and 162.1 ppm, respectively, in the 13C
NMR spectra. The C4 carbon of all the compounds (1-8)
appeared at ~162 ppm. In the 13C NMR spectrum of 8b, both the
C4 and C4, appeared at 161.9 ppm. It can be concluded from the
above observations that the C4 carbon in all the compounds (1b-
8b) moved downfield by ~ 5.6 ppm in comparison to that of 4-
aminopyridine, which confirms the existence of the C4 carbon as
an electron donor centre and N-methylpyridylidene as an rNHC.
Scheme 3. Scheme for the synthesis of compound 8. Reagents and conditions:
(a) 4-aminopyridine, Pd2(dba)3, K3PO4, K-OtBu', Xantphos, 1,4-Dioxane, 100°C,
12 h (38%), (b) Me2SO4, acetonitrile, reflux, 8 h (99%), (c) KPF6, water, rt.
Scheme 1. General scheme for the synthesis of compounds 1 and 2. Reagents
and conditions: (a) CH3I, DMF, K2CO3, reflux, 1 h (98%), (b) Me2SO4, acetonitrile,
reflux, 8 h (90%), (c) 4-aminopyridine, K2CO3, reflux, 3 h (65%), (d) KPF6, water,
rt.
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2. Solid state structure analysis
with allenes and are expected to have a linear structure. On the
contrary, 1, 2 and 8 are highly bent (with C2/C4-N2-C4 angle
122.8 (1) 123.5 (2) and 130.9 (8)). The origin of these acute
angles can be attributed to the presence of excess electron
density at the central nitrogen atom as observed in all LN+L
systems.7
To understand the solid state structural features of these
species, compound from each category, containing (i)
benzimidazoleN+rNHC (1), (ii) benzothiazoleN+rNHC (2)
and rNHCN+rNHC (8) were crystallized.[28] The crystals for
the hexafluorophosphate salts of 1, 2 and 8 were obtained using
the vapour diffusion method in methanol/toluene.
The compounds 1 and 8 appeared as rectangular (light
yellow) transparent crystals, while compound 2 was obtained as
golden yellow rectangular crystals. The compound 8 was found to
have a molecule of HPF6 crystallized with it in the asymmetric unit.
Figure 4 shows the solid state structures of 1, 2 and 8. An
important observation is that the C2/C4-N2 and C4-N2 bonds
were in the order of 1.320-1.380 Å and these values are quite
comparable to the CN coordination bond lengths reported in the
literature.9a
Table 1. Comparison of the geometrical parameters of 1, 2 and 8 obtained
theoretically (quantum chemical calculations at M06/def2TZVPP level of
theory) and experimentally by single crystal X-ray crystallography. All bond
lengths are given in Angstrom (Å) and all angles are given in degree ().
Geometrical
parameters
1
2
8
M06
X-ray
M06
X-ray M06
X-ray
N2-C2‡
1.317 1.324(4)
1.318 1.352(3)
1.425 1.417(4)
1.357 1.360(4)
1.351 1.343(4)
1.359 1.347(4)
1.351 1.353(4)
1.424 1.406(4)
1.361 1.363(4)
1.353 1.358(3)
126.2 122.8(2)
1.300 1.282(7) 1.319 1.386(10)
1.331 1.379(7) 1.318 1.379(11)
1.417 1.398(7) 1.424 1.400(09)
1.360 1.353(9) 1.357 1.355(10)
1.349 1.328(8) 1.352 1.352(09)
1.357 1.352(7) 1.359 1.334(12)
1.354 1.358(9) 1.352 1.361(14)
1.418 1.386(8) 1.424 1.387(10)
N2-C4
C4-C3
C3-C2
C2-N1
N1-C6
C6-C5
C5-C4
N3*-C2
N1-C2‡
C2‡-N2-C4
C2‡-N2-C4-C3
1.758 1.750(6)
1.350 1.355(8)
-
-
-
-
127.7 123.5(5) 128.1
24.4 40.6 -30.9
130.9(6)
13.4
15.8
-26.5
‡In compound 8, C4 is present in place of C2. *In compound 2, S1 is present
in place of N3.
In a previous report, N-methylpyridylidene was shown to
coordinate to transition metal, i.e. [(N-methylpyridylidene)-
Ni(PPh3)2Cl].(BF4).[2b] The geometrical parameters of the ligand in
the above transition metal complex are compared with the
geometrical parameters of N-methylpyridylidene unit of
compound 1. In both the cases, the quinoidal character is clearly
present in the N-methylpyridylidene unit. Considering that the
rNHC unit in [(N-methylpyridylidene)-Ni(PPh3)2Cl].(BF4) is treated
as donor carbene, the N-methylpyridylidene unit may also be
treated as donor ligand in compound 1.
Figure 4. Solid state structures of 1, 2 and 8, (thermal ellipsoids set 30%
probability) with bond lengths in Angstrom (Å) and angles in degree ().
Quantum chemical data obtained using M06/def2TZVPP optimization is given
in italics.
The geometrical data within the N-methylpyridylidene ring
points out a change in the electronic character of the pyridine ring
from a fully delocalized system to a partially localized (quinoidal)
system, confirming the rNHC character of this ring in these
compounds.[29] These C-N+-C systems should be isostructural
Figure 5. Comparison of geometrical parameters of N-methylpyridylidene units
of 1 (italics) and [(N-methylpyridylidene)-Ni(PPh3)2Cl].(BF4) complex, obtained
from solid state structure analysis.
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3. Quantum chemical analysis
character, which is also supported by calculated Wiberg bond
index. The molecular orbital visualization also indicated the
absence of the  bond character between the central nitrogen and
carbenic carbons (C2/C4 and C4). The C2/C4-N2-C4
linearization enthalpies are small (6-9 kcal/mol). The
complexation enthalpies with BH3, AuCl and AlCl3 are quite
comparable to that of the already reported divalent NI
compounds.9
Quantum chemical analysis was performed on 1, 2 and 8
using B3LYP and M06 methods, employing def2TZVPP as the
basis set. These species are global minima on the respective
potential energy surfaces. The optimized geometries of 1, 2 and
8 provided similar geometrical features as that of the X-ray
diffraction data of these species (Figure 4, Table 1). Minor
deviations between the calculated and the experimental bond
lengths of N2-C2/C4 and N2-C4' bonds in compounds 1, 2 and 8
can be attributed to the absence of the counter ion in the
calculated geometries, which was also observed in our previous
report.[13a]
The molecular orbital analysis clearly established the
presence of two lone pairs of electrons on the central nitrogen
atom. The HOMO is a π type orbital with an antibonding
interaction between the p orbitals of the central nitrogen and the
p orbitals of C2/C4 and C4 atoms (Figure 6, for compound 8).
Other electronic parameters, such as ELF (Electron Localization
Function) analysis, partial charges and lone pair occupancy
calculations at central nitrogen atom (Table 1), confirm that the N+
atom carries excess electron density. This excess electron
density is accumulated due to the acceptance of electron density
from the ligands, and it is responsible for the non-linear
arrangement of these systems (in comparison to isoelectronic
hetero allenes). The C2/C4-N2 and C4-N2 rotational barriers are
< 9 kcal/mol, indicating the absence of C=N double bond
Figure 6. HOMO (-type) and the HOMO-1 (σ-type) molecular orbitals of 8
generated at the M06/def2TZVPP level of theory (Isosurface value 0.04).
The proton affinities (PA) of these systems were calculated
up to 168 kcal/mol. The PA of these species are low in
comparison to the first PA values of corresponding carbones,[30]
but are equal to the second PA values of carbones, due to the
presence of positive charge on these molecules. However, these
compounds have been reported for the protonation and to form
(LNHL)2+ systems (Table 2).[12d, 26]
Table 2. The electronic and energy parameters obtained from the optimized structures of the cationic species 1, 2 and 8, using M06/def2TZVPP level of
theory. Full details of these results and the corresponding data obtained using B3LYP/ def2TZVPP level are available in the Supporting Information.
8
Symbol
1
2
Energy parameters
Energy of π type LP[a]
EHOMO
-9.38
-11.60
159.13
-9.42
-11.48
153.18
-10.56
-22.88
-26.24
411.32
8.14
-9.53
-10.36*
167.78
-20.55
-25.10
-32.37
435.90
5.35
Energy of σ type LP[a]
Proton affinity[b]
EHOMO-3
HPA
[b]
Enthalpy of complexation with BH3
Hcomp (BH3)
Hcomp (AlCl3)
Hcomp (AuCl)
De
-22.01
-31.19
[b]
Enthalpy of complexation with AlCl3
Enthalpy of complexation with AuCl[b]
Bond dissociation Energy[b]
Enthalpy of N-Linearization[b]
Rotational barrier
-31.20
412.05
7.43
HNl
C2/C4-N2 rotational barrier[b]
C4-N2 rotational barrier[b]
HRB1
HRB2
0.56
6.26
4.47
8.37
2.42
2.42
Electronic parameters
ELF analysis[c]
3.19
3.00
3.00
푁v(N)
QN2
Charge at N2[c]
-0.554
1.71
-0.543
1.76
-0.488
1.72
LP occupancy (σ) [c,d]
LP occupancy (π) [c,d]
Global nucleophilicity value
Local nucleophilicity value
Fukui function
LPN2σ
LPN2π
1.45
1.40
1.39
(N)
1.58
1.53
1.42
-
(NN2
)
-0.23
-0.15
-0.24
-0.16
-0.34
-0.24
-
(fN2
)
Bond parameters
Wiberg bond index
C2/C4-N2
WBI
WBI
1.31
1.39
1.37
1.38
1.38
1.38
C4-N2
Values given [a]in eV, [b]in kcal/mol, [c]in electrons, [d]NBO analysis is used for locating two lone pairs at N2 atomic centre. For compound 8, the σ-type lone
pair was found in HOMO-1.
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10.1002/chem.201705999
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The isomerization study of the isomers (1i, 2i and 8i) of
compounds 1, 2 and 8 was also performed. The corresponding
isomers were found to be clearly less stable on the respective
potential energy surfaces (Scheme 4).
Many resonance structures may be considered for
compounds 1, 2 and 8 (see Supporting Information for the details),
in which the traditional bonding environment is considered across
C-N bonds. However, in our study, the electronic structure
analysis, 13C NMR study and the crystal structure analysis,
revealed that electronic and molecular structures of 1-8 can be
best explained better with the help of CN coordination bonds.
Conclusions
In conclusion, this work is the first report showing the
possibility of rNHCE coordination chemistry, which established
that rNHC can also donate to N+ centres similar to NHC. The
significant outcome is that the divalent NI compounds synthesized
in this work (i) show CN coordination bond lengths in the range
of 1.30-1.38 Å, (ii) bear two lone pairs on the central nitrogen atom,
(iii) are associated with excess electron density at the N+ centre,
(iv) are global minima on the respective potential energy surfaces.
All these properties confirm the CN coordination bond
characteristics. These compounds can be further explored for
their reactivity and catalytic properties, which is ongoing in our
laboratory and will be reported in the near future work.
Scheme 4. Isomerization enthalpies (H) in 1,
M06/def2TZVPP level of theories (The corresponding data calculated at
B3LYP/def2TZVPP level has been provided in Supporting Information).
2 and 8 calculated at
Tautomerization study was performed on the corresponding
non-methylated analogues of 1, 2 and 8, i.e. 9, 10 and 11,
respectively, to understand the most stable tautomeric form of
these species (Scheme 5). 1-3 H and 1-5 H shifts are possible in
compounds 9 and 10, resulting in the respective tautomers T1 and
T2. Among all three tautomeric forms of 9 and 10, the structures
with divalent NI character are the most stable structures. In
compound 11, only the 1-5 H shift is possible to give tautomer 11T,
which is unstable by ~5 kcal/mol in comparison to 11. Overall, it
can be concluded that the structures 9, 10 and 11 are the
preferred structures over their tautomers, and can be labelled as
rNHCN+NHC/rNHC complexes.
Experimental Section
1. Methodology
1.1. Experiential methods for synthesis
The starting materials were purchased and used further without any
additional purification. All reactions were carried out in flame-dried
glassware under N2. The compounds (intermediates and final compounds)
were purified by column chromatography using silica gel (100-200 mesh)
as stationary phase and hexane-ethyl acetate as mobile phase. Proton
nuclear magnetic resonance spectra (1H NMR) were recorded at 400 MHz.
Scheme 5. Tautomerization enthalpies (H) in non-methylated analogs of 1 (i), 2 (ii) and 8 (iii) calculated at M06/def2TZVPP level of theories, showing the 1-3H
and 1-5H shifts (The corresponding data calculated at B3LYP/def2TZVPP level has been provided in Supporting Information).
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Proton chemical shifts are expressed in parts per million (ppm, δ scale)
and are referenced to residual proton in the NMR solvent (CDCl3, δ 7.26,
CD3OD, δ 3.31 and CD3SOCD3, δ 2.49). The following abbreviations were
used to describe peak patterns when appropriate: br = broad, s = singlet,
d = doublet, t = triplet, q = quartet, quin. = quintet, m = multiplet, sext =
sextet. Coupling constants, J, were reported in Hertz unit (Hz). Carbon 13
nuclear magnetic resonance spectroscopy (13C NMR) was recorded on
100 MHz and was fully decoupled by broad band decoupling. Chemical
shifts were reported in ppm referenced to the centre line at a 77.0, 49.0
and 39.7 ppm of CDCl3, CD3OD and CD3SOCD3 respectively. High
resolution mass spectra were taken using ESI-TOF method.
to give the desired compound. The compound 2b was prepared by the
similar procedure (Scheme 1).
Compound 1b: Golden yellow solid, mp 214 °C; 329 mg, 65% yield; 1H
NMR (400 MHz, CD3SOCD3): δ = 8.08 (d, J = 7.24 Hz, 2H), 7.60 (dd, J =
3.16, 2.72 Hz, 2H), 7.38 (dd, J = 3.12, 2.76 Hz, 2H), 6.82 (d, J = 7.28 Hz,
2H), 3.88 (s, 6H) 3.50 (s, 3H); 13C NMR (100 MHz, CD3SOCD3): δ = 161.10,
151.59, 142.91, 130.88, 123.42, 115.04, 110.43, 44.03, 30.37; IR
(neat): ν˜=3090, 2952, 1932, 1659, 1557, 1468, 1421, 1256, 1206, 837,
+
751, 556; HRMS (ESI-TOF) m/z: [M]+ Calcd for C15H17N4 253.1448;
Found 253.1451.
Compound 2b: Golden yellow solid, mp 208 °C; 332 mg, 65% yield; 1H
NMR (400 MHz, CD3SOCD3): δ = 8.60 (d, J = 6.92, 2H), 7.84 (d, J = 7.72
Hz, 1H), 7.60-7.56 (m, 3H), 7.52 (t, J = 8.16 Hz, 1H), 7.34 (t, J = 7.38 Hz,
1H), 4.13 (s, 3H), 3.73 (s, 3H); 13C NMR (100 MHz, CD3SOCD3): δ =
162.18, 162.06, 146.10, 139.30, 128.06, 124.64, 123.49, 121.65, 118.46,
112.66, 46.30, 32.04; IR (neat): ν˜= 3395, 3151, 3090, 2918, 1651, 1568,
1347, 1197, 826, 755, 557; HRMS (ESI-TOF) m/z: [M]+ Calcd for
C14H14N3S+ 256.0903; Found 256.0922.
1.2. Computational methods
The geometry optimization of the cations 1, 2 and 8, along with their
nonmethylated analogues 9, 10, 11 respectively and their tautomers as
well as isomers, have been carried out using Gaussian09 suite of
programs.[31] The Density Functional (DFT) method B3LYP[32] was utilized
to carry out geometry optimization using def2TZVPP.[33] The calculations
were repeated with M06/def2TZVPP[34] method to evaluate the importance
of dispersion factors on the potential energy (PE) surface. The zero point
vibrational frequency calculations were performed for all the optimized
species, to characterize them as either minima or transition state
structures. In case of AuCl complexes, the Au is treated with associated
effective core potentials (ECP). The Natural Bond Orbital (NBO) charge
analysis[35] was employed to estimate partial atomic charges for all the
optimized geometries. The nucleophilicity index (N), was estimated by
Preparation of 8b: The compound X (100 mg, 0.58 mmol) was dissolved
in acetonitrile and potassium carbonate (243 mg, 1.74 mmol, 3 equiv.) was
added DMS (146 mg, 0.11 mL, 2 equiv) in the presence of acetonitrile (5
mL). The resulting reaction mixture was then refluxed until the completion
of the reaction (TLC). The mixture containing 8a was dried under vacuum.
Similar method was used to exchange the counter anion. KPF6 (324 mg,
1.74 mmol, 3 equiv) was added taking water as solvent to give white
precipitate of 8b. The obtained precipitate was then collected by filtration
and washed with water to give the desired compound (Scheme 3).
using the formula reported recently by Domingo et al.[36] N = EHOMO(Nu)
̶
EHOMO(TCE); where tetracyanoethylene (TCE) is chosen as the reference.
The local nucleophilicity index is calculated using Nk- = N * Fk-; where Fk-
is Fukui function at the atomic centre.[37] ELF (Electron Localization
Function)calculations[38] were carried out for the cations 1, 2 and 8 species
to estimate the population of the basins (푁v(N)) at central coordinating
nitrogen. The 푁v(N) values were obtained using Multiwfn version 3.2.[39]
Bond dissociation energy (De) was calculated fragmenting the molecule in
the following fragments[40] (for 1, as example):
Compound 8b: White solid, mp 210 °C; 60 mg, 74% yield; 1H NMR (400
MHz, CD3SOCD3): δ = 8.02 (d, J = 6.68 Hz, 4H), 6.91 (d, J = 7.40 Hz, 4H),
3.86 (s, 6H); 13C NMR (100 MHz, CD3SOCD3): δ =162.00, 143.35, 115.61,
44.52; IR (neat): ν˜= 3401, 1629, 1505, 1444, 1395, 1199, 1032, 826, 742,
+
718; HRMS (ESI-TOF) m/z: [M]+ Calcd for C12H14N3 200.1182; Found
200.1182.
Acknowledgements
The authors thank Council of Scientific & Industrial Research
(CSIR), New Delhi, India for Senior Research Fellowship and
Department of Science & Technology (DST) Govt. of India, New
Delhi, India for the financial support. The X-ray facility availed at
IISER Mohali is acknowledged.
The enthalpies of N-linearization (also called as ‘Bending potential’) was
calculated by taking the differences of enthalpy values between the
geometries of compounds 1, 2 and 8, which are global minima and their
corresponding linear geometries (in which the C2/C4-N2-C4 angle is
180).
Keywords: Remote N-heterocyclic carbene
•
Divalent NI
compounds • Donor-acceptor interactions • Chemical bonding
analysis • Quantum chemical analysis.
Supporting Information
Electronic Supporting Information (ESI) is available, provided
NMR spectra (1H NMR, 13C NMR) and HRMS data of all the final
compounds and the intermediates. X-ray crystallographic details,
computational analysis details and the coordinates of the
optimized geometries are also provided.
Wiberg Bond Index[41] across C-N bonds in 1, 2 and 8 and species was
calculated using NBO analysis.
2. Experimental procedures for compounds 1b, 2b and 8b
Preparation of 1b: To the magnetically stirred mixture of IIIa (386 mg, 2
mmol) and K2CO3 (830 mg, 6 mmol, 3 equiv) in MeCN was added 4-
aminopyridine (226 mg, 2.4 mmol, 1.2 equiv) and the resultant mixture was
stirred at 90˚C until the reaction was complete (TLC). The mixture
containing 1a was dried under vacuum. KPF6 (1116 mg, 6 mmol, 3 equiv)
was added taking water as solvent to give white precipitate of 1b. The
obtained precipitate was then collected by filtration and washed with water
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Entry for the Table of Contents
FULL PAPER
Remote N-heterocyclic carbenes
(rNHCs) like N-methyl-4-pyridylidene
are known to form coordination
complexes with transition metals. In
this article, it is established that
rNHCs can also coordinate to the N+
centre. Synthesis of some novel
divalent NI complexes with the
general formula (rNHC)N(NHC)+
Neha Patel, Minhajul Arfeen,
Radhika Sood, Sadhika Khullar,
Asit K. Chakraborti, Sanjay K.
Mandal and Prasad V. Bharatam*
Page No. – Page No.
Title:
Can
Remote
N-
Heterocyclic
Carbenes
and
(rNHC)N(rNHC)+
is
Coordinate with Main Group
Elements? Synthesis, Structure
and Quantum Chemical Analysis
of N+ centre complexes
reported. X-ray diffraction studies
support the coordination bond
character between rNHC and N+
centre. Quantum chemical analysis
established the divalent NI character
at the central nitrogen in these
This article is protected by copyright. All rights reserved.