Synthesis, structure, and thermochemistry of adduct formation between N-heterocyclic carbenes and isocyanates or mesitylnitrile oxide

Article abstract of DOI:10.1007/s11224-013-0305-2

Reaction of N-heterocyclic carbenes (NHCs) with isocyanates yields stable zwitterionic imidates/amidates in toluene solution. These compounds were fully characterized and the crystal structures of several species were determined by X-ray crystallography.

Full text of DOI:10.1007/s11224-013-0305-2

Struct Chem
DOI 10.1007/s11224-013-0305-2
ORIGINAL RESEARCH
Synthesis, structure, and thermochemistry of adduct formation
between N-heterocyclic carbenes and isocyanates or mesitylnitrile
oxide
•
Manuel Temprado Subhojit Majumdar
•
•
Xiaochen Cai Burjor Captain Carl D. Hoff
•
Received: 20 June 2013 / Accepted: 27 June 2013
Ó Springer Science+Business Media New York 2013
Abstract Reaction of N-heterocyclic carbenes (NHCs)
with isocyanates yields stable zwitterionic imidates/ami-
dates in toluene solution. These compounds were fully
characterized and the crystal structures of several species
were determined by X-ray crystallography. The thermo-
chemistry of binding of these and related species was
studied by solution calorimetry. Comparison is made of the
enthalpies of binding of NHC to isocyanates (RNCO) and
isomeric nitrile oxides (RCNO) as well as CO2. DFT cal-
culations were performed to additionally assess the nature
of bonding in these compounds.
fields of chemistry due to their important applications as
ligands in organometallic complexes [2–6], and as organic
catalysts [7–11]. By far, the most used carbenes nowadays are
N-heterocyclic carbenes (NHCs) derived from imidazole. The
stability of NHCs arises from a combination of electronic
effects, p-donation by the lone pairs of the N atoms into the
empty pp orbitalofthecarbene, andstericeffectsincorporating
bulky substituents in the nitrogens. NHCs are strong r-donors
and their application as powerful nucleophilic reagents has
been documented in several important organic catalyzed
transformations [7–15]. It is well known that addition of NHCs
to electrophilic reagents leads to formation of zwitterionic
adducts [16–18]. In this regard, Louie and co-workers have
synthesized and structurally characterized a series of zwitter-
ionicimidazoliumcarboxylatesbyreactionofNHCswithCO2
[19, 20]. Analogously, the synthesis and X-ray structures of a
series of imidazol(in)ium dithiocarboxylates [21] and thio-
carboxylates [22] havealsobeenreportedby Delaudeet al. We
have recently reported [23] that addition of solid MesCNO
(Mes = mesityl) to a toluene solution of NHC resulted in the
immediate formation of the corresponding NHC/MesCNO
adduct. Moreover, Severin and co-workers [24] have also
reported characterization of stable covalent adducts of NHCs
with N2O. Likewise, isothiocyanate adducts of NHC are also
known [16–18]. Furthermore, NHCs have been reported as
catalysts for the trimerization of isocyanates to form the cor-
responding isocyanurates in THF solution [14]. Until recently,
it was accepted in the literature that addition of isocyanates to
NHCs does not usually stop at the zwitterionic stage. Delaude
statedthatisolationoftheadductswasprovenimpossible,even
at low temperatures in the presence of an excess of NHCs [18].
TheonlystructuraldataforbindingofanisocyanatetoanNHC
the authors could find was the report of Schmidt et al. [25]
where an indazoliumamidate obtained by trapping of the
corresponding NHC with an isocyanate was structurally
Keywords Carbenes ꢀ NHC ꢀ Zwitterions ꢀ Betaines ꢀ
Thermochemistry
Introduction
The isolation and characterization of the first stable crystalline
carbene was performed by Arduengo et al. [1]. Since then,
these species have generated great interest in several different
Special Issue in honor of Prof. Maria Victoria Roux.
Electronic supplementary material The online version of this
article (doi:10.1007/s11224-013-0305-2) contains supplementary
material, which is available to authorized users.
M. Temprado (&)
Department of Physical Chemistry, Universidad de Alcala, Ctra.
´
Madrid-Barcelona Km. 33,600, 28871 Madrid, Spain
e-mail: manuel.temprado@uah.es
S. Majumdar ꢀ X. Cai ꢀ B. Captain ꢀ C. D. Hoff (&)
Department of Chemistry, University of Miami, 1301 Memorial
Drive, Coral Gables, FL 33146, USA
e-mail: c.hoff@miami.edu
123

Struct Chem
characterized and the recent report of Ongand co-workers [26]
where the structure of an amine-linked NHC which binds
isocyanates at both the NHC and amine functionality is
determined. However, the authors have not been able to find
reports of the structure of simple adducts between NHCs and
isocyanates. Surprisingly, we were able to isolate and char-
acterize the pale yellow RNCO adducts of IPr, SIPr, and IMes
(R = PhNCO, p-F3CC6H4NCO, Adamantyl; IPr = 1,3-bis(diiso-
propyl)phenylimidazol-2-ylidene, SIPr = 1,3-bis(diisopro-
pyl)phenylimidazolin-2-ylidene, IMes = 1,3-bismesityli-
midazol-2-ylidene; Fig. 1) by addition of stoichiometric
amounts of solid RNCO to a toluene solution of the corre-
sponding NHC. These adducts presumably correspond to the
active species in the catalyzed trimerization [10], but they
can be isolated in toluene solution in the absence of excess
PhNCO, a finding not previously reported in spite of their use
as catalysts [14]. This work reports the isolation and char-
acterization of stable adducts of NHC and isocyanates in
toluene solution and the comparison of the structure and
thermochemistry of their formation with the corresponding
MesCNO adducts recently reported by us [23]. In addition,
thermochemical studies of NHC adducts of CO2 [19, 20] are
made for comparative purposes.
Experimental
General considerations
Unless stated otherwise, all operations were performed in a
Vacuum Atmospheres dry box under an atmosphere of
purified argon. Toluene was dried and deoxygenated by dis-
tillation from sodium benzophenone ketyl under argon into
flame dried glassware. Deuterated solvents were purchased
Fig. 1 Structures of carbenes and adducts (Dipp = 2,6-diisopropyl-
phenyl, Mes = mesityl, R0 = Ph, p-F3CC6H4, Adamantyl)
measurements were made using a Setaram C-80 Calvet
microcalorimeter.
˚
from Aldrich and stored over molecular sieves 4 A in the
glove box. NHCs were purchased from Strem Chemical and
used as-received. Mesityl nitrile oxide (MesCNO)1 and SIPr/
MesCNO [23] were prepared and recrystallized according to
Preparation of the NHC/MesCNO and NHC/RNCO
adducts
1
the literature. H NMR spectra were recorded on a Bruker
Avance 400 mHz spectrometer at room temperature. 1H
NMR chemical shifts are reported in parts per million (ppm)
with respect to the protio impurities referenced at 7.16 ppm
for C6D6 and 2.09 ppm for toluene-d8. FTIR spectra were
obtained using a Perkin Elmer Spectrum 400 FTIR Spec-
trometer in CaF2 solution cells obtained from Harrick Sci-
entific or for solids on a Perkin Elmer Universal ATR
Sampling Accessory. Mass spectra were run on a Bruker
BioFlex IV Maldi-TOF Mass Spectrometer. Calorimetric
In the glove box, 0.1657 g of solid MesCNO (1.03 mmol,
1 eq) was added to a solution of 0.400 g IPr (1.03 mmol,
1 eq) in 10 mL toluene and the mixture was shaken to
dissolve the MesCNO. The solution turned red-orange and
some solid began to precipitate out. It was left undisturbed
in the glove box overnight. Evaporation of toluene led to
isolation of the pure red-orange adduct in near quantitative
yield. Similar procedures were used for other adducts.
Adducts were fully characterized by 1H NMR, IR, UV–vis,
MS, and in some cases X-ray structure determination. In
the case of IPr/MesCNO, elemental analysis was also
performed. NMR data, Mass Spec., and NMR data for all
the complexes are summarized below. Structural data are
reported in a later section.
1
For a description see supporting information files in Ref. [27] which
presents a modified preparation from the original (Ref. [28]).
123

Struct Chem
1150, 1121, 1079, 1061, 1037 cm-1. UV–vis (toluene, 20 °C):
kmax = 563 nm. Anal. Calcd. for C37H47N3O: C, 80.83; H, 8.62;
N, 7.64. Found: C, 79.48; H, 8.50; N, 7.01.
IPr/PhNCO
The light yellow adduct showed a strong peak in the Maldi-
TOF at m/z = 508.34 corresponding to MH? (for
C34H41N3O m/z (calc) = 507.32). 1H NMR (400 MHz,
C6D6): d = 7.66 (d, 2H, o-Ph of PhNCO), 7.20(t, 2H, p-Ar
of IPr), 7.13 (m, 2H, m-Ph of PhNCO), 7.06 (d, 4H, m-Ar
of IPr), 6.79 (t, 1H, p-Ph of PhNCO), 6.14 (s, 2H, vinylic H
IMes/MesCNO
The red adduct showed a strong peak in the Maldi-TOF at
m/z = 438.3 corresponding to MH? (for C29H31N3O
m/z (calc) = 437.25). 1H NMR (400 MHz, C6D6):
d = 6.53 (d, 4H, m-Ar of IMes), 6.42 (s, 2H, m-Ar of
MesCNO), 5.72 (s, 2H, vinylic H of IMes), 2.08 (s, 3H, p-
CH3 of MesCNO), 2.03 (s, 18H, o-CH3 of IMes, o-CH3 of
MesCNO), 1.98 (s, 6H, p-CH3 of IMes) ppm. IR: 2912.1,
i
of IPr), 2.70 (septet, 4H, CH of Pr), 1.38 (d, 12H, CH3 of
iPr), 1.08 (d, 12H, CH3 of iPr) ppm. IR: of the solid: 3164,
3136, 3097, 3074, 2962, 2930, 2870, 1708, 1613, 1581,
1557, 1476, 1446, 1385, 1362, 1333, 1310(sh), 1230,
1239(sh), 1207, 1182, 1089(sh), 1062, 1022 cm-1
.
1610.6, 1505.3, 1447.1, 1330.6, 1284.3, 1229.3(sh) cm-1
.
SIPr/PhNCO and SIPr/F3CC6H4NCO
Solution calorimetric measurements
The light yellow adduct showed a strong peak in the Maldi-
TOF at m/z = 510.35 corresponding to MH? (for
C34H43N3O m/z (calc) = 509.34). 1H NMR (400 MHz,
C6D6): d = 7.62 (d, 2H, o-Ph of PhNCO), 7.09 (t, 2H, p-Ar
of SIPr), 6.99 (d, 4H, m-Ar of SIPr), 6.80 (m, 2H, m-Ph of
PhNCO), 6.75 (t, 1H, p-Ph of PhNCO), 3.38 (s, 4H, CH2 of
A representative experimental procedure is described; for
other data see Table 1. In the glove box a solution of
0.0294 g SIPr (0.0756 mmol) was dissolved in 2 mL
C6D6 and loaded in the outer mixing chamber of the
Calvet solution cell.
A solution of 0.0190 g SIPr
i
SIPr), 3.28 (septet, 4H, CH of Pr), 1.55 (d, 12H, CH3 of
(0.1179 mmol) was dissolved in 2 mL C6D6 and loaded in
the inner mixing chamber of the Calvet solution cell. The
calorimeter cell was sealed, taken from the glove box, and
loaded into the Setaram C-80 calorimeter. Following
temperature equilibration, the reaction was initiated and
the calorimeter rotated to achieve mixing. Following
return to baseline the calorimeter cell was taken into the
glove box, opened and 1 mL of the solution loaded into an
NMR tube which confirmed complete conversion of SIPr
(limiting reagent) to the SIPr/MesCNO complex. The heat
of this reaction was computed by integration of the ther-
mogram and yielded DH = -29.0 kcal/mol. The reaction
was repeated four times to give a final average value of
DH = -29.9 1.3 kcal/mol. In some cases SIPr was
used as the limiting reagent, and in some cases MesCNO
was used as the limiting reagent and the values were the
same within experimental error.
iPr), 1.19 (d, 12H, CH3 of iPr) ppm. IR: of the solid: 2964,
2929, 2868, 1702, 1662, 1621, 1596, 1586, 1547, 1499,
1481, 1464, 1447, 1384, 1309, 1281, 1259, 1085(sh), 1052,
1021 cm-1
.
For SIPr/F3CC6H4NCO analogous procedures were
followed as for SIPr/PhNCO, and it was characterized for
comparative purposes based solely on its crystal structure
reported in a later section and detailed in SI.
IPr/AdNCO
The light yellow adduct showed a strong peak in the Maldi-
TOF at m/z = 566.41 corresponding to MH? (for
C38H51N3O m/z (calc) = 565.40). 1H NMR (400 MHz,
C6D6): d = 7.21 (t, 2H, p-Ar of IPr), 7.09 (d, 4H, m-Ar of
IPr), 6.11 (s, 2H, vinylic H of IPr), 2.81 (septet, 4H, CH of
iPr), 1.75 and 1.62 (d, 6H, CH2 in AdNCO), 1.52 (d, 12H,
i
i
CH3 of Pr), 1.09 (d, 12H, CH3 of Pr) ppm.
Displacement of AdNCO from SIPr/AdNCO by CO2
IPr/MesCNO
In the glove box a 1/1 mol ratio solution was prepared
containing 10 mg AdNCO and 22 mg SIPr in 2 mL of
C6D6. To two NMR tubes fitted with rubber septa, 1 mL of
this solution was added, the tubes sealed and taken from
the glove box. To one of the tubes 4 mL CO2 (gas) was
added utilizing a Hamilton gas tight syringe. After
approximately 3 min, a white precipitate of SIPr/CO2
appeared in this tube. An NMR spectrum was run con-
firming disappearance of the peaks due to SIPr/AdNCO
and appearance of weak bands due to the sparingly soluble
SIPr/CO2 and strong bands due to free AdNCO at 1.68,
The red adduct showed a strong peak in the Maldi-TOF at
m/z = 550.38 corresponding to MH? (for C37H47N3O
1
m/z (calc) = 549.37). H NMR (400 MHz, C6D6): d = 7.08
(t, 2H, p-Ar of IPr), 6.93 (d, 4H, m-ArofIPr),6.51(s,2H,m-Ar of
MesCNO), 6.22 (s, 2H, vinylic H of IPr), 3.02 (septet, 4H, CH of
iPr), 2.07 (s, 3H, p-CH3 of MesCNO), 1.97 (s, 6H, o-CH3 of
MesCNO), 1.23 (d, 12H, CH3 of iPr), 1.02 (d, 12H, CH3 of iPr)
ppm. IR: 3168, 2964, 2930, 2870, 1567, 1495, 1464, 1456, 1446,
1428(sh), 1385, 1361, 1328, 1235, 1221, 1203, 1182(sh), 1173,
123

Struct Chem
Table 1 Selected computed geometrical parameters and enthalpies of binding
a
C–C (A)
Torsional angleb (8)
DHcalc (kcal/mol)c
DHexpt (kcal/mol)
˚
Adduct
IPr/MesCNO
1.445, 1.415c
1.420d, 1.399c
1.487c
1.521c
1.521, 1.528c
1.522c
27, 30c
26d, 25c
89c
90c
90, 90c
52c
-29.3
-32.8
-12.9
-18.8
-19.1
-18.1
-27.0 0.6
-29.9 1.3
SIPr/MesCNO
IPr/PhNCO
-25.1 0.4
-22.6 0.8
SIPr/PhNCO
IPr/CO2
SIPr/CO2
IPr/AdNCO
SIPr/AdNCO
a
1.510e
1.535e
89e
59e
-19.7 2.5f
-17.3 2.5f
-13.4 0.4
-14.5 0.4
1.524
50
C–C distance linking the NHC to RCNO, RNCO or CO2. Unless stated otherwise, bond distances were obtained by X-ray crystallography.
Dihedral angle between the NCN (NHC) and XYZ (NCO for RNCO, CCN for RCNO and OCO for CO2) planes. Unless stated otherwise
b
dihedral angles were obtained by X-ray crystallography. c Calculated values at the M05-2X/6-311G(d,p) level; for the SIPr/MesCNO and SIPr/
PhNCO systems two minima were computed, see text for additional discussion. Value obtained from Ref. [23]. Value obtained from Ref.
[20]. f Based on enthalpy of substitution by PhNCO of the NHC/CO2 adduct in CH2Cl2 solution as described in text. Due to the differing solvent
conditions, an estimated uncertainty of 2.5 kcal/mol is assigned
d
e
1.56, and 1.28 ppm as confirmed by direct comparison to
an authentic sample.
frame integration algorithm [29]. Corrections for Lorentz
and polarization effects were also applied with SAINT?.
An empirical absorption correction based on the multiple
measurement of equivalent reflections was applied using
the program SADABS. All structures were solved by a
combination of direct methods and difference Fourier
syntheses, and refined by full-matrix least-squares on F2,
by using the SHELXTL software package [30, 31]. Crystal
data, data collection parameters, and results of the analyses
are listed in Table S1.
Attempted displacement of CO2 from SIPr/CO2
by AdNCO
In the glove box, 10 mg SIPr/CO2 and 4.6 mg AdNCO
(& 1/1 molar ratio) were dissolved in 0.7 mL CDCl3 and
transferred to an NMR tube which was sealed and taken out
of the glove box. The NMR spectrum showed the starting
materials SIPR/CO2 and AdNCO and no new peaks grew
in. The solution remained colorless and conversion to SIPr/
AdNCO was not observed.
Colorless single crystals of SIPr/PhNCO suitable for
X-ray diffraction analyses obtained by evaporation of
solutions of toluene 25 °C, crystallized in the orthorhombic
crystal system. The systematic absences in the intensity
data were consistent with either space group Pnma or space
group Pna21. The former space group was chosen initially
for structure solution tests. This space group was confirmed
by the successful solution and refinement of the structure.
With Z = 4, the molecule has crystallographic mirror
symmetry.
Displacement of CO2 from SIPr/CO2 by PhNCO
In the glove box, approximately 20 mg SIPr/CO2 was
added to an NMR tube. To this was added 1.5 equivalents
of PhNCO dissolved in 1 mL C6D6. This resulted in for-
mation of a white slurry which slowly dissolved with
evolution of gas bubbles. After approximately 30 min an
NMR spectrum was run and showed quantitative conver-
sion to SIPr/PhNCO whose preparation is described above.
Colorless single crystals of SIPr/p-F3CC6H4NCO suit-
able for X-ray diffraction analyses obtained by evapora-
tion of solvent from toluene at 25 °C, crystallized in the
orthorhombic crystal system. The systematic absences in
the intensity data indicated two possible space groups,
Pnm21 or Pmmn. The structure could only be solved in the
former space group. With Z = 2, the molecule has crys-
tallographic mirror symmetry. The CF3 group is disor-
dered over two orientations and was refined in the ratio
50:50.
Crystallographic analyses
Each crystal was glued onto the end of a thin glass fiber.
X-ray intensity data were measured by using a Bruker
SMART APEX2 CCD-based diffractometer using Mo Ka
˚
Colorless single crystals of IPr/AdNCO suitable for
X-ray diffraction analyses obtained by evaporation of
radiation (k = 0.71073 A) [29]. The raw data frames were
integrated with the SAINT? program by using a narrow-
123

Struct Chem
solvent from a benzene solution at 25 °C, crystallized in
the monoclinic crystal system. The systematic absences in
the intensity data were consistent with the unique space
group P21/c. One molecule of benzene cocrystallized
with the complex and is present in the asymmetric crystal
unit.
Results
Synthetic and structural studies
All NHCs studied (Scheme 1) were found to react rapidly
and cleanly at room temperature in toluene solution with
isocyanates to produce the corresponding NHC/RNCO
adducts as confirmed by NMR spectroscopy. 1H NMR
spectra show that these adducts are stable for more than 2
days at room temperature in toluene solution. This behavior
is in contrast to that observed in THF solution in which
case the catalytic trimerization of isocyanates to form the
corresponding isocyanurates is observed [14].
Red single crystals of IPr/MesCNO suitable for X-ray
diffraction analyses were obtained by slow evaporation of
benzene. The systematic absences in the intensity data
were consistent with the unique space group P21/n. All
non-hydrogen atoms were refined with anisotropic dis-
placement parameters. Hydrogen atoms were placed in
geometrically idealized positions and included as standard
riding atoms during the least-squares refinements. Half a
molecule of benzene, and one water molecule cocrystal-
lized with the complex and is present in the asymmetric
crystal unit. The presence of water presumably occurs from
advantageous water slowly condensing into the system
during the long time needed to grow crystals. Reasonable
peak positions for the H atoms on the water molecules were
located in the difference map and then refined with geo-
metric restraints.
Suitable crystals for X-ray analysis of SIPr/PhNCO,
SIPr/p-F3CC6H4NCO, IPr/AdNCO (Ad = adamantyl), IPr/
MesCNO, and IMes/MesCNO were grown by dissolving
crude samples of the adducts in toluene or benzene, fol-
lowed by slow evaporation of the solutions in an open vial
at room temperature in the glove box. The solid state
structure of adducts SIPr/PhNCO, IPr/AdNCO, and IPr/
MesCNO are shown in Figs. 2, 3, and 4, respectively. The
structures of SIPr/p-F3CC6H4NCO and IMes/MesCNO, as
well as full crystallographic data are available in the
Supplemental Material. The crystal structure of SIPr/
MesCNO has been previously reported [23].
Yellow single crystals of IMes/MesCNO suitable for
X-ray diffraction analyses obtained by evaporation of
solutions of toluene at 25 °C, crystallized in the mono-
clinic crystal system. The systematic absences in the
intensity data were consistent with the unique space group
P21/n. One water molecule cocrystallized with the com-
plex and is present in the asymmetric crystal unit. The
presence of water presumably occurs from advantageous
water slowly condensing into the system during the long
time needed to grow crystals. Reasonable peak positions
for the H atoms on the water molecules were located in
the difference map and then refined with geometric
restraints.
During crystal growth of IPr/MesCNO, and IMes/Mes-
CNO, adventitious H2O slowly entered and was present in
the unit cell. It is not known how this lattice water affects
the primary structure of the adduct. There is a similarity in
the bond distances and angles for the two new structures to
the previously reported SIPr/MesCNO which did not con-
tain water in the unit cell. This is reflected in the C–C
distance linking the NHC to MesCNO and also the dihedral
angle between the NHC NCN plane and the CCN plane of
MesCNO where the distances and angles are: SIPr/Mes-
˚
˚
CNO (1.42 A, 26.1°); IPr/MesCNOꢀH2Oꢀ1/2CH6 (1.45 A,
˚
Computational details
26.5°); IMes/MesCNOꢀH2O (1.45 A, 24.2°). The dihedral
angle and the C–C distance in the structures is
˚
Electronic structure calculations were carried out using the
M05-2X [32] density functional with the 6-311G(d,p) basis
set as implemented in the Gaussian 09 suite of programs
[33]. Minima optimizations were performed by computing
analytical energy gradients. The obtained stationary points
were characterized by performing energy second deriva-
tives, confirming them as minima by the absence of neg-
ative eigenvalues of the Hessian matrix of the energy.
Computed electronic energies were corrected for zero-
point energy, and thermal energy to obtain the corre-
sponding H0 values. To derive binding energies, the basis
set superposition error (BSSE) was computed using coun-
terpoise calculations [34, 35]. Finally, time-dependent
(TD) DFT calculations were carried out to compute the
electronic transition for all molecules.
25.4° 1.2° and 1.44 0.02 A, respectively, for the three
structures in spite of the inclusion of water in the unit cell
of two of the three structures. These values are also similar
to the ones reported recently by Bielawski and co-workers
[36] for several NHC adducts of 2,6-dimethoxybenzonitrile
N-oxide.
Louie and co-workers [20] have shown that for NHC
adducts of CO2 a wide range of dihedral angles are
observed in the crystal structure and that the torsional angle
was dependent upon the steric bulk of the N-substituent in
the imidazolium ring. Thus while the CO2 adduct of IPr
had an angle of &90o that of IMe was &29°. A correlation
was found in thermogravimetric analysis (TGA) between
the decarboxylation temperature and the torsion angle
which decreased with increasing torsional angle. While the
123

Struct Chem
Scheme 1 Possible resonance
structures for NHC adducts of
MesCNO (top) and RNCO
(bottom). For both isomers, a
zwitterionic structure can be
formulated. For MesCNO
formation of an N=O bond
allows a stable resonance
structure with a C=C bond as in
the Wanzlich equilibrium
Fig. 3 An ORTEP showing the molecular structure of IPr/AdNCO,
˚
with thermal ellipsoids set at 30 % probability. Selected distances (A)
and angles (°): C1–C30 = 1.524(4), C30–O1 = 1.247(4), C30–
N3 = 1.293(4), C1–N1 = 1.342(4), C30–C1–N1 = 127.3(3), C30–
C1–N2 = 126.2(2), N1–C1–N2 = 106.4(3). The angle between the
N3–C30–O1 plane and the N1–C1–N2 plane = 49.5°
Fig. 2 Thermal ellipsoid plot (40 % probability) of SIPr/PhNCO.
˚
Selected distances (A) and angles (°): C1–C8 = 1.521(4), C8–N2 =
1.326(2), C10–N2 = 1.451(2), C1–O1 = 1.242(3), C1–N1 =
1.282(3), C1–C8–N2 = 123.88(11), C1–C8–N2* = 123.88(11),
N2*–C8–N2 = 111.5(2). The angle between the N1–C1–O1 plane
and the N2–C8–N2* plane = 90.0°
found in the MesCNO adducts. A summary of these
structural parameters is shown in Table 1.
most common reported structure of NHC betaines appears
to be one in which the NHC and the other moiety are
perpendicular to each other, that may reflect in part that
X-ray quality crystals may be more easily obtained in the
orthogonal configuration and thus for those complexes for
which structures can be determined, it is more common to
succeed in obtaining crystals with a 90° torsional angle.
The computational studies reported here (see below)
appear to show little energy dependence upon the torsional
angle in which the central C atom is bound to the NHC as
observed here for SIPr/PhNCO (Fig. 2) and SIPr/
p-F3CC6H4NCO (see Supplementary Material) which dis-
played a symmetry plane and were crystallographically
exactly perpendicular. That was not the case, however, for
the IPr/AdNCO adduct in which a dihedral angle between
the NCN (NHC) and NCO (RNCO) planes is 49.5°.
Moreover, the C–C distance linking the NHC to all iso-
Solution calorimetric studies
The enthalpies of binding between the IPr or SIPr and
MesCNO, PhNCO, or AdNCO in toluene solution were
measured by solution calorimetry. The values derived are
collected in Table 1. Data for formation of the PhNCO and
MesCNO adducts are based on the enthalpy of reaction as
measured with all species in toluene solution for formation
of the adducts as shown in Eq. 1 for PhNCO and MesCNO
in Eq. 2.
RNCOðtol solnÞ þ NHCðtol solnÞ ! NHC=RNCOðtol solnÞ ð1Þ
MesCNOðtol solnÞ þ NHCðtol solnÞ ! NHC=MesCNOðtol solnÞ
ð2Þ
For comparative purposes we have measured the
enthalpy of reaction of the IPr/CO2 and SIPr/CO2 with
˚
cyanates studied of 1.52 A is significantly longer than that
123

Struct Chem
resulted in precipitation of the sparingly soluble IPr/CO2
carboxylate and formation of free AdNCO as confirmed by
NMR spectroscopy. As a further check, addition of
AdNCO to a CDCl3 solution of SIPr/CO2 resulted in no
reaction–in contrast to the displacement that occurs readily
as shown in Eq. 3 where PhNCO is capable of displacing
CO2 from its NHC adduct. Thus the binding of CO2 is
intermediate between PhNCO and AdNCO in terms of
enthalpy of formation.
Computational studies
In order to further analyze the structural and energetic
differences observed in these adducts, DFT computational
studies were undertaken. The first question addressed was
the reason for the structural differences observed between
the MesCNO and RNCO adducts. As shown in Fig. 5, the
computed structures at the M05-2X/6-311G(d,p) level for
PhNCO perpendicular to the plane of the SIPr ring and for
MesCNO approaching coplanarity are computed to be the
most stable in agreement to the solid state structures
determined by X-ray crystallography of both adducts. In
the case of PhNCO, two different minima with a N–C–C–N
dihedral angle between the NHC and isocyanate moieties
of 90.0° or 52.4° were located with only a 1.0 kcal/mol
difference between the two conformations. The structure of
the minimum with a dihedral angle of 52.4° is similar to
that obtained for the IPr/AdNCO adduct in this work by
X-ray crystallography (49.5°). In contrast, for MesCNO,
the difference between the two structures with an almost
perpendicular arrangement and the one approaching co-
planarity is nearly 20 kcal/mol. In fact, the orthogonal
structure shown in Fig. 5 corresponds to a transition state
connecting two equivalent minima with a dihedral angle of
25°.
Fig. 4 An ORTEP showing the molecular structure of IPr/MesCNO,
˚
with thermal ellipsoids set at 40 % probability. Selected distances (A)
and angles (°): C1–C11 = 1.445(4), N2–C11 = 1.353(3), N1–
C1 = 1.323(3), O1–N1 = 1.295(3), C1–C2 = 1.494(4), N2–C11–
C1 = 127.5(2), N3–C11–C1 = 126.5(2), N2–C11–N3 = 106.0(2).
The angle between the N1–C1–C2 plane and the N2–C11–N3
plane = 26.5°
PhNCO to form the IPr/PhNCO or SIPr/PhNCO adduct.
Due to low solubility of the NHC/CO2 adducts in toluene it
was not possible to measure the enthalpy of reaction in this
solvent. However, in CH2Cl2 it was found that reaction 3
occurred rapidly and cleanly at 30 °C.
NHC=CO
solnÞ þ PhNCO
2ðCH2Cl2
ðCH2Cl2 solnÞ
! NHC=PhNCOðCH Cl solnÞ þ CO2ðCH Cl
ð3Þ
solnÞ
2
2
2
2
DHIPr ¼ ꢁ5:4 ꢂ 0:2 kcal=mol
DHSIPr ¼ ꢁ5:3 ꢂ 0:3 kcal=mol
An additional indication of p-bonding in the MesCNO
adducts is their color. The dramatic differences in color
between the adducts are also of interest, in particular the
fact that monomeric nitroso compounds are often blue in
color [37]. Reasonably good computation of the UV–vis
spectra of these adducts could be obtained at the TD-M05-
2X/6-311G(d,p) level of theory. Experimental and com-
Data for the CO2 adducts must be viewed with some
caution due to the more polar nature of CH2Cl2 versus
toluene as a solvent. However, for Eq. 3 solvation energies
may to some extent cancel for the polar adducts. For both
IPr and SIPr the CO2 adducts are &5.3 kcal/mol less stable
than the PhNCO adducts.
Thermochemical data for reaction of AdNCO with NHC
(see Eq. 1) which is 11.7 and 8.1 kcal/mol less exothermic
for IPr and SIPr, respectively, than the corresponding
reaction of NHC with PhNCO lead to the prediction that
while RNCO can replace CO2 in these adducts when
R = Ph (reaction 3), that situation should be reversed for
R = Ad and CO2 should replace the isocyanate in this case
as shown in reaction 4.
puted data are: SIPr/MesCNO:
k
max(exp) = 605 nm;
max(calc) = 714 nm; IPr/MesCNO: kmax(exp) = 563 nm;
max(calc) = 640 nm. For IPr/PhNCO and SIPr/PhNCO no
k
k
absorptions in the visible region were located. Although
calculations overestimate the value of kmax, they predict
qualitatively the shift in the adducts.
IR data also provide additional insight in the nature of
the adducts. The IR of monomeric nitroso compounds often
displays mNO near 1,550 cm-1 whereas dimeric nitroso
compounds which have a similar structure to the zwitter-
ionic resonance structures shown in Scheme 1 for SIPr/
NHC=AdNCO þ CO2 ! NHC=CO2 þ AdNCO
ð4Þ
This prediction was confirmed experimentally in that
addition of CO2 to a toluene solution of IPr/AdNCO
123

Struct Chem
Fig. 5 Computed structures
and energies of conformational
change for selected
conformations of adducts SIPr/
PhNCO (top) and SIPr/
MesCNO (bottom)
MesCNO show a band near 1,250 cm-1 [37]. The IR
of a C=C bond provided an approximately coplanar
structure can be achieved. In SIPr/PhNCO, the plane con-
taining PhNCO is perpendicular to the plane of the NHC
implying no p-bonding interaction between the Ccarbene and
PhNCO since the relevant p orbitals are orthogonal. The
zwitterionic structure would have free rotation about the
C–C bond and to reduce steric repulsion would be expected
to adopt the orthogonal structure displayed. While in the
solid state, the C–C distances between the NHC and the
isocyanate fragments of the PhNCO and AdNCO adducts
are comparable, the dihedral angles between both moieties
are markedly different for the PhNCO (90°, Fig. 2) and the
AdNCO (49.5°, Fig. 3). A dihedral angle of 59.0° between
the imidazolinium and the carboxylate groups has been
previously observed for the SIPr/CO2 adduct [20].
spectrum of IPr/MesCNO displays a band at 1,567 cm-1
.
When compared to frequency calculations performed at the
M05-2X/6-311G(d,p) level, this frequency can be assigned
as being predominantly an NO stretch though it is coupled
to other modes of vibration.
Discussion
One goal of this work was to evaluate the relative stability
of the NHC adducts we recently reported for nitrile oxides
(RCNO) [23] with their isomeric isocyanate counterparts
(RNCO). Assuming that the NHC will coordinate to the C
atom preferentially, this gives two different bonding
topologies. For RNCO (isoelectronic with CO2), the bond
formed between the NHC and the C atom will be at the
central atom, whereas for RCNO, the bond will be at a
terminal site of the XYO moiety (For RCNO; X = C,
Y = N; For RNCO: X = N, Y = C). As shown in
Scheme 1 for MesCNO, two structural formulations can be
made in simple bonding theory—a zwitterionic form as
well as one involving a C=C bond in a Wanzlich equilib-
rium-type structure [38–40].
Due to the fact that the zwitterionic formulation in
RNCO is further stabilized by amidate (species A in
Scheme 1)/imidate (species B in Scheme 1) resonance
structures, it seems likely that for the ionic structure
binding to the central rather than a terminal atom might
have an advantage due to this added stability. Furthermore,
a through-space attractive attraction between the electronic
deficient carbenoid carbon and the electron rich O and N
atoms (Scheme 1) would confer an additional stability to
the RNCO adducts [22, 41].
The simple analysis in Scheme 1 shows that bonding to
the central atom as in PhNCO and CO2 will yield primarily
a zwitterionic formulation of the adduct, whereas for
MesCNO where the bonding C is in a terminal position an
additional driving force is provided by potential formation
The situation with MesCNO is less clear since the
dihedral angle between the imidazolinium and the nitrile
oxide fragment in the adducts is 25.4° 1.2°. The tor-
sional angle is controlled by steric and electronic effects as
123

Struct Chem
well as crystal packing forces. The importance of elec-
tronic effects is clear since CO2, which is smaller than
MesCNO, nevertheless has a larger dihedral angle. It seems
likely that steric factors prevent the MesCNO adducts from
adopting a planar structure. Nonetheless, partial p bonding
between the carbenoid C of the NHC and the C atom of
MesCNO might be indicated. In fact, the C–C bond length
Moreover, in the PhNCO case, the negative charge in the
zwitterionic structure can be delocalized into the phenyl
ring since the PhNCO moiety is planar (see Figs. 2, 5). In
addition, as stated previously, the dihedral angle between
the NHC and the isocyanate moieties is markedly different
for the PhNCO (90°) and AdNCO (49.5°) adducts. There-
fore, the former can benefit from a through-space attractive
interaction as discussed previously more markedly than the
latter. Furthermore, it should be expected that the steric
hindrance imposed by the bulky substituents in the imi-
dazoli(in)um nitrogens destabilize the adduct more pro-
nouncedly for the bulkier AdNCO as compared to PhNCO.
˚
of 1.44 0.02 A for the MesCNO adducts supports more
double bond character than for the isocyanate adducts
˚
(C–C = 1.52 A) in agreement with a more favorable
delocalization of the p-electronic cloud in the more planar
compounds. Computational studies also confirm the exis-
tence of p-bonding in the MesCNO adducts where a dif-
ference of nearly 20 kcal/mol is computed between the two
structures shown in Fig. 5 as the plane of the NHC is
rotated away from that of the MesCNO plane. In contrast,
for PhNCO the two conformers shown in Fig. 5 differ only
by 1 kcal/mol. Additional evidence of the different nature
in both kind of adducts is their color, whereas nitrile oxide
adducts exhibit high intensity bands in the visible spectra
the isocyanate adducts are pale yellow. The presence of a
band at 1,567 cm-1 assigned to a NO stretch in the typical
range for nitroso compounds in the IR spectrum of IPr/
MesCNO also support the formulation of these species as
nitrosoethylene compounds (resonance structure on the left
for the SIPr/MesCNO adduct in Scheme 1).
Conclusions
This work reports the synthesis, isolation, and character-
ization of stable isocyanate adducts of IPr and SIPr. The
X-ray structures of zwitterionic adducts of these commer-
cial NHC with isocyanates are reported here for the first
time. This has been proven possible due to the enhanced
stability of the adducts in toluene solution as opposed to
that in THF, a solvent in which the trimerization of iso-
cyanates to form the corresponding isocyanurates in pres-
ence of NHC was observed [14] indicating the essential
role that the polar solvent plays in this catalytic
transformation.
While differences occur in the apparent preference for IPr
versus SIPr in the adducts, these are close to the overlap of
experimental error and at this time detailed discussion of
them is not warranted. Additional studies may allow further
refinement, but at this time we are assigning an average value
for the enthalpy of binding of IPr and SIPr to the adducts and
note that the enthalpies of binding (kcal/mol) are in order:
MesCNO (-28.5 1.5) [ PhNCO (-23.9 1.3) [ CO2
(-18.5 1.2) [ AdNCO (-14.0 0.6).
The position of the CO2 adduct as being intermediate
between PhNCO and AdNCO is confirmed in that dis-
placement of CO2 by PhNCO is spontaneous as shown in
Eq. 3, but displacement of AdNCO by CO2 is spontaneous
as shown in Eq. 4. The greater stability of the PhNCO
adduct compared to the CO2 adduct is in keeping with the
fact that the NHC/CO2 adduct can be used as an isocyanate
oligomerization catalyst precursor [14] since the CO2 is
readily displaced by ArNCO. The average value reported
here for PhNCO (-23.9) is surprisingly close to that
derived for binding of PhNCS to IMes by Bielawski and
co-workers [42] of -23 kcal/mol.
Addition of IPr or SIPr to PhNCO or MesCNO give two
different kind of adducts which can be described as zwit-
terions (PhNCO) or conjugated neutral species (MesCNO).
Simple bonding pictures as well as detailed computations
show that changing the binding site from the center of the
conjugated system (ArNCO and OCO) to a terminal site
(ArCNO) appears to enhance the role of p bonding
between the two C atoms in a Wanzlich-like C=C bond
formation. This form is stabilized by concurrent formation
of an N=O p bond and the adducts approximate being
conjugated nitrosoethylene compounds. In spite of the
added source of stability provided by the partial p bonding
between the NHC and MesCNO, the ArCNO adducts are
only &5 kcal/mol more stable than the ArNCO adducts
and &10 kcal/mol more stable than the CO2 adducts. In
part this may be due to an increased stability of the zwit-
terionic formulation of the central bonded adducts due to
imidate/amidate resonance stabilization and by further
delocalization of the negative charge in the phenyl ring in
the case of PhNCO, a factor not possible in the MesCNO
adducts since the mesityl and the CNO moieties are almost
perpendicular (see Figs. 4, 5). Moreover, a through-space
attractive electrostatic interaction between the electronic
deficient carbenoid carbon and the electron rich N and O
atoms of the RNCO moiety may also confer additional
stability to the zwitterionic isocyanate adducts, partially
The &10 kcal/mol difference in stability of the PhNCO
and AdNCO adducts is striking and is attributed to both
steric and electronic effects. The more electron withdraw-
ing nature of Ph compared to Ad group will increase the
positive charge on the C atom of RNCO and increase the
favorability of forming the zwitterionic structure.
123

Struct Chem
20. Van Ausdall BR, Glass JL, Wiggins KM, Aarif AM, Louie J
(2009) J Org Chem 74:7935
21. Delaude L, Demonceau A, Wouters J (2009) Eur J Inorg Chem
1882
22. Hans M, Wouters J, Demonceau A, Delaude L (2011) Eur J Org
Chem 7083
compensating for the loss of C=C bonding. Further studies
on related species are in progress to better understand the
nature of these adducts and to further explore their
reactivity.
23. Cai X, Majumdar S, Fortman GC, Frutos LM, Temprado M,
Clough CR, Cummins CC, Germain ME, Palluccio T, Rybak-
Akimova EV, Captain B, Hoff CD (2011) Inorg Chem 50:9620
24. Tskhovrebov AG, Solari E, Wodrich MD, Scopelliti R, Severin K
(2012) Angew Chem Int Ed 51:232
25. Schmidt A, Habeck T, Lindner AS, Snovydovych B, Namyslo JC,
Adam A, Gjikaj M (2007) J Org Chem 72:2236
26. Li CY, Kuo YY, Tsai JH, Yap GPA, Ong TG (2011) Chem Asian
J 6:1520
Acknowledgments Support of this work by the National Science
Foundation Grant CHE 0615743 (C. D. H.) and by the Spanish
Ministry of Economy and Competitiveness (MINECO) under
CTQ2012-36966 (M. T.) is gratefully acknowledged. The authors
thank Professor Janis Louie at the University of Utah for helpful
discussions.
27. Barybin MV, Diaconescu PL, Cummins CC (2001) Inorg Chem
40:2892
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