N-Heterocyclic carbene bonding to cobalt porphyrin complexes

Article abstract of DOI:10.1016/j.ica.2011.08.039

N-Heterocyclic carbene (NHC) coordination to a cobalt(III) center embedded in a porphyrin scaffold has been accomplished by decarboxylation from N,N′-dimethylimidazolium-2-carboxylate in the presence of Co(TPP)Cl (TPP = 5,10,15,20-tetraphenylporphyrin). The distal chloride ligand in the resulting complex Co(NHC)(TPP)Cl was successfully substituted with imidazoles and alcohols. Single crystal X-ray diffraction of the latter complexes Co(NHC)(TPP)(ROH) (R = Me, Et) revealed a pronounced ruffling of the porphyrin macrocycle due to the two ortho methyl groups in the carbene ligand and because of the relatively short distance between the cobalt center and the carbene ligand. Spectroscopic investigations support a substantial porphyrin dearomatization upon NHC bonding.

Full text of DOI:10.1016/j.ica.2011.08.039

Inorganica Chimica Acta 380 (2012) 90–95
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N-Heterocyclic carbene bonding to cobalt porphyrin complexes
⇑
Martin Albrecht , Pathik Maji, Christina Häusl, Angèle Monney, Helge Müller-Bunz
School of Chemistry and Chemical Biology, University College Dublin, Belfield, Dublin 4, Ireland
a r t i c l e i n f o
a b s t r a c t
Article history:
Available online 5 September 2011
N-Heterocyclic carbene (NHC) coordination to a cobalt(III) center embedded in a porphyrin scaffold has
been accomplished by decarboxylation from N,N0-dimethylimidazolium-2-carboxylate in the presence of
Co(TPP)Cl (TPP = 5,10,15,20-tetraphenylporphyrin). The distal chloride ligand in the resulting complex
Co(NHC)(TPP)Cl was successfully substituted with imidazoles and alcohols. Single crystal X-ray diffrac-
tion of the latter complexes Co(NHC)(TPP)(ROH) (R = Me, Et) revealed a pronounced ruffling of the por-
phyrin macrocycle due to the two ortho methyl groups in the carbene ligand and because of the
relatively short distance between the cobalt center and the carbene ligand. Spectroscopic investigations
support a substantial porphyrin dearomatization upon NHC bonding.
Young Investigator Award Special Issue
Keywords:
Cobalt
N-Heterocyclic carbene
Porphyrin
Imidazole
Ó 2011 Elsevier B.V. All rights reserved.
Steric strain
Ligand substitution
1. Introduction
across the entire periodic table, including actinides and lantha-
nides [8], (alkali) earth metals [9], main group metals [10], and
The development of N-heterocyclic carbenes as ligands for tran-
sition metal chemistry has undoubtedly been one of the most signif-
icant advances in modern inorganic and organometallic chemistry
[1]. Even though NHCs are formally neutral donor ligands and have
indeed initially been compared to phosphines [2], they are distinctly
differentfrom classical coordination ligandsthat bind the metal cen-
all types of d-block transition metals [11]. Despite this unique
diversity, the organometallic chemistry of NHC ligands has been
largely dominated by platinum group metal complexes (in partic-
ular Ru, Rh, Ir, and Pd) [12], and by coinage metal complexes
[13]. The 3rd row homologues of the platinum group metals have
been investigated much less [14], and in particular NHC cobalt
complexes have been rare [15]. The low popularity may origin, in
parts, from the fact that a number of classical synthetic methods
are less applicable to 3rd row transition metals (e.g. transmetala-
tion from NHC silver intermediates, oxidative addition routes). Per-
haps equally relevant may be the coordinative lability of cobalt in
terms of redox processes and ligand substitutions. Indeed, only a
handful of NHC cobalt complexes have been reported and most
of them are sensitive and require careful handling [15].
We reasoned that the lability of the complexes may be reduced
upon immobilizing the cobalt center in a rigid scaffold. Porphyrins
appear particularly attractive for this purpose [16], as they stabi-
lize the diamagnetic cobalt(III) center and prevent undesired re-
dox-processes at the metal [17]. In addition, coordination of a
NHC leaves one labile coordination site opposite to the carbene,
which should eventually allow the strong trans effect of the NHC
ligand to be exploited. Here we report on the synthesis and struc-
tural characterization of air-stable NHC cobalt complexes.
ter via a heteroatom-centered lone pair or a
pbond [3]. Coordination
ligandsengagein whatis often referredtoas a dativebond totheme-
tal center, indicating a largely ionic contribution and hence a kinetic
lability of the bond, which may be detrimental to some (catalytic)
processes (ligand loss, complex decomposition), yet beneficial or
even essential for other reaction pathways (cyclometalation, tran-
sient and reversible formation of coordinatively unsaturated and
catalytically competent species). As a consequence, the metal–li-
gand bonding in these complexes is often thermodynamically con-
trolled, allowing simple and powerful principles to be established
such as the hard–soft–acid–base concept [4].
In contrast, the bonding between a NHC and a metal center
comprises a much higher covalent contribution [5]. Metal bonding
is thus, generally, under kinetic control, and the NHC–metal bond
is kinetically inert, i.e. reversible ligand dissociation does typically
[6] not occur [7]. While this bonding situation has clear implica-
tions, e.g. on supramolecular chemistry and catalytic cycles, it also
indicates that NHCs may form complexes with a significantly lar-
ger number of metals than for example soft phosphines, or hard
alkoxides. Indeed, NHC complexes have been reported with metals
2. Results and discussion
The air-stable NHC cobalt complex 2 was successfully prepared
by reacting the imidazolium carboxylate 1 [18] in the presence of
equimolar amounts of meso-tetraphenylporphyrin cobalt(III)
⇑
Corresponding author. Tel.: +353 1716 2504; fax: +353 1716 2501.
E-mail address: martin.albrecht@ucd.ie (M. Albrecht).
0020-1693/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2011.08.039

M. Albrecht et al. / Inorganica Chimica Acta 380 (2012) 90–95
91
BF4
Cl
Ph
Ph
O
N
– O
N
N
N
N
AgBF4
Co(TPP)Cl
Ph
Ph
N
N
N
N
Co
Co
N
Ph
Ph
+
Ph
N
Ph
N
N
N
1
2
3
ROH
BF4
H
R
N
O
Ph
Ph
N
N
Co
N
Ph
Ph
N
N
4 a (R = Me)
b (R = Et)
Scheme 1. Synthesis of NHC porphyrin cobalt(III) complexes.
chloride (TPP–CoCl; Scheme 1). Complex formation was indicated
by the intense purple color, which is reflected in the UV–Vis spec-
trum by the Soret band (kmax 434 nm) and two absorption maxima
for the Q band (kmax 560 and 595 nm). A split of the Q band into
two resolved absorptions has been noted previously when using
sterically demanding 2-substituted imidazoles in related TPP–Co
complexes [19]. Structural evidence in solution was obtained by
NMR spectroscopy. Bonding of the NHC ligand to the cobalt center
was indicated by the presence of two singlets in 1:3 ratio at dH 5.06
and ꢀ0.63 ppm, attributed to the heterocyclic protons and the N–
CH3 wingtip groups, respectively. The extraordinary high-field shift
shift from 8.83 to 8.95 ppm upon chloride abstraction. These
observations are in agreement with a more electropositive cobalt
center in complexes 3 and 4 than in 2 and hence a more pro-
nounced
r donation of the TPP ligand to the metal center, which
depletes electron density in the TPP fragment and hence reduces
the ring current. In the spectra of all complexes 2–4, the TPP ligand
appeared as a C4-symmetric unit. No desymmetrization due to
NHC coordination was noted, suggesting that ligand rotation about
the CNHC–Co bond is relatively fast in solution [20].
Complexes 4a and 4b were analyzed by single crystal X-ray dif-
fraction. The ORTEP plots of the formally cationic complexes
(Fig. 1) reveal an octahedral cobalt center with the TPP ligand occu-
pying the equatorial position and the NHC ligand and ROH coordi-
nating through oxygen on the axial positions. Crystals of 4b
contained two crystallographically independent cations in the unit
cell. The metric data around the cobalt center are identical in both
complexes within standard deviations. Analysis of the global struc-
ture reveals in both complexes a staggered orientation of the NHC
ligand with respect to the equatorial Co–NTPP bonds (Fig. 2a), sug-
of these signals (
D
d approximately ꢀ2 and ꢀ5 ppm, respectively)
when compared to other complexes containing the N,N0-dimethy-
limidazolylidene (IMe) ligand [18] is obviously a direct conse-
quence of the porphyrin ring current and thus a clear indication
for carbene bonding to the TPP–Co unit. The integral ratio between
the TPP signals and the NHC ligand are commensurate with a sin-
gle carbene bound to the TPP–Co unit. The 13C NMR chemical shifts
were less sensitive to the ring current effects and the resonances
for both the C4/C5 and the N–CH3 carbons are not unusual (dC
123.7 and 32.5, respectively). Complex 2 was also the exclusive
product when performing the reaction in the presence of a large
excess of 1, suggesting that in contrast to imidazole coordination
[19], bonding of a second NHC ligand to the cobalt porphyrin core
is unfavorable. Complex 2 is air-stable in solution and in the solid
state for weeks, hence contrasting the stability of various other
NHC cobalt complexes [15].
gesting little metal–NHC
p bond interactions [21]. As a conse-
quence of this arrangement, two phenyl groups are pushed
towards the ROH side of the porphyrin ring, while the other two
phenyl groups are twisted towards the empty space on the NHC
side, resulting in a pronounced saddle conformation of the porphy-
rin with a superimposed ruffling to alleviate the steric interactions.
The distinct orientation of the NHC ligand as well as the distortion
in the porphyrin macrocycle renders the complex essentially (but
not crystallographically) C2-symmetric. This specific structural
arrangement is, however, not preserved in solution and rotation
of the NHC ligand appears to be fast (see above).
On an atomic level, it is worth noting that the Co–CNHC bond
distance is within expectation (Co–C 1.93 Å) and does not seem
to be elongated due to the rigid porphyrin skeleton (Table 1). It
is, however, significantly shorter than typical Co–N bonds of prox-
imal pyridine or imidazole ligands, which are around 2.20 Å [22].
The closer distance in the NHC complexes may reflect the higher
covalent contribution of the cobalt–ligand bond and may provide
a rationale for the substantial distortion in the porphyrin skeleton.
All CNHC–Co–N bond angles are slightly but consistently larger than
90° (average angle is 92.5 1.0° in 4a and 4b), while the O–Co–N
bond angles are all smaller than 90° (average angle is 87.5 2.0°
in both structures), thus reflecting considerable distortion. Hence,
The chloride ligand in complex 2 was readily abstracted by
AgBF4 (Scheme 1). Under dry conditions (CHCl3 solution), complex
3 was the only detectable product. Complex 3 features either a
penta-coordinated cobalt(III) center, or an octahedral geometry
ꢀ
including weak coordination of the BF4 anion. Elemental analysis
data do not suggest coordination of a small ligand, e.g. adventitious
water. Complex 3 is sensitive to Lewis bases and afforded the octa-
hedral complexes 4a and 4b in the presence of MeOH and EtOH,
respectively. The 1H NMR resonances of the NHC ligand were diag-
nostic for confirming the effective replacement of the chloride an-
ion by a weaker bound ligand. Thus, the NCH3 wingtip resonance is
shifted downfield by 0.12 ( 1) ppm. Likewise, the heterocyclic
CNHC–H frequencies are considerably less shielded in complexes 3
and 4 and appear in the 5.35–5.38 ppm range (cf 5.06 in 2). A sim-
ilar effect was noted for the heterocyclic porphyrin protons, which

92
M. Albrecht et al. / Inorganica Chimica Acta 380 (2012) 90–95
ꢀ
Fig. 1. Ortep representations (50% ellipsoids, hydrogen atoms, non-coordinating BF4 anion and co-crystallized solvent molecules omitted), of 4a (a; only one of two
disordered conformations of one phenyl group shown) and 4b (b; only one of the two independent molecules in the unit cell shown).
Fig. 2. Pluton drawing of the top and side-view of the complex cation of 4a, visualizing (a) the staggered orientation of the NHC ligand with respect to the equatorial Co–NTPP
bonds, and (b) the saddle conformation due to repulsive interactions between two phenyl groups and the N–CH3 groups of the NHC ligand.
Table 1
R1
Ph
R2
BF4
Selected bond lengths (Å) and angles (°) for complexes 4a and 4b.
N
4a
4b
4a
4b
N
Co–CNHC
Co–O
Co–N1
Co–N2
Co–N3
Co–N4
1.930(3)
2.059(2)
1.933(3)
1.918(2)
1.934(3)
1.923(3)
1.933(4)
2.067(3)
1.923(3)
1.914(3)
1.935(3)
1.911(3)
CNHC–Co–O
176.95(13)
92.52(12)
91.82(13)
91.87(13)
93.94(12)
89.28(10)
85.75(10)
86.34(10)
88.48(10)
177.38(15)
91.55(14)
94.34(15)
91.91(15)
92.17(15)
88.86(13)
88.24(13)
86.69(11)
85.25(12)
CNHC–Co–N1
CNHC–Co–N2
CNHC–Co–N3
CNHC–Co–N4
N
N
imidazole
Ph
N
N
Co
3
Ph
O–Co–N1
O–Co–N2
O–Co–N3
O–Co–N4
Ph
N
N
5 a (R1 = H, R2 = Me)
b (R1 = Me, R2 = H)
the steric demand due to the two methyl wingtip groups at the
metal center appears to be alleviated by a slight move of the cobalt
center out of the least square plane of the four porphyrin nitrogens
towards the NHC ligand (0.085 and 0.084 Å for 4a and 4b, respec-
tively) [23]. The ruffled structure is caused by the significant devi-
ation of the meso carbons out of this least square plane by 0.70–
0.87 Å in both structures.
The specific bonding features presumably prevent the coordina-
tion of another NHC ligand to complexes 2 or 3. Likewise, we have
not been successful to coordinate a larger NHC ligand, e.g. with
butyl wingtip groups, to the cobalt center in Co(TPP)Cl. However,
bonding of less strongly coordinating ligands like alcohols
(complexes 4a and 4b) is obviously feasible. Moreover, imidazole
Scheme 2. Imidazole coordination to NHC porphyrin cobalt(III) complexes.
coordination was demonstrated by the isolation of complexes 5
upon reacting complex 3 with one equivalent of 1,2-dimethylimid-
azole or 2,4-dimethylimidazole and subsequent purification by
preparative thin layer chromatography (Scheme 2). Imidazole
and NHC coordination in complexes 5a and 5b was supported by
the diagnostic resonances in their 1H NMR spectra. Specifically,
the NHC protons were shifted to higher field compared to com-
plexes 3 and 4 and are nearly identical to those of the chloride
complex 2 (for 5a, viz 5.03 and ꢀ0.64 ppm) or even further shifted
to 4.84 and ꢀ0.67 ppm for the CNHC–H and N–CH3 groups in 5b,

M. Albrecht et al. / Inorganica Chimica Acta 380 (2012) 90–95
93
R1
Ph
R2
6 than in the carbene complexes 5. The weaker ring current expo-
sure of the imidazole ligands in complexes 5 may be a direct conse-
quence of the stronger trans influence of the NHC ligand as
compared to the imidazole. In addition, a stronger porphyrin defor-
mation accompanied by a partial reduction of the aromatic charac-
ter may be more significant in the NHC complexes due to the
presence of two ortho substituents in the NHC ligand (as opposed
to only one in the C2-substituted imidazoles) paired with the rela-
tively short Co–CNHC bond.
Cl
N
N
N
N
imidazole
Ph
N
N
Co
Co(TPP)Cl
Ph
Ph
N
N
R1
R2
6 a (R1 = H, R2 = Me)
3. Conclusions
b (R1 = Me, R2 = H)
New porphyrin cobalt(III) complexes comprising a proximal
N-heterocyclic carbene (NHC) ligand were synthesized and fully
characterized. The NHC bonding has significant steric and elec-
tronic implications. In particular, the presence of substituents on
both ortho positioned nitrogens paired with the strong Co–CNHC
bond imparts a substantial distortion of the porphyrin macrocycle,
which may be exploited for the labilization of otherwise tightly
bound ligands in distal position. Based on the critical role of metal-
loporphyrin derivatives as cofactors in a number of biological pro-
cesses and since metalloporphyrins have shown promising
catalytic activity in a variety of synthetic oxidation processes, it
will be interesting to investigate the catalytic scope of these car-
bene cobalt complexes. Preliminary experiments suggest useful
activity in the oxidation of olefins and a full account on these stud-
ies will be reported in due course.
Scheme 3. Twofold imidazole coordination to porphyrin cobalt(III) complexes.
Table 2
Crystal data and structure refinement for 4a and 4b.
4a
4b
Empirical formula
Formula weight
C
50H40BCoF4N6O ꢁ 2CHCl3 C51H42BCoN6OF4 ꢁ 2CH2Cl2
1125.36
1070.50
T (K)
100(2)
100(2)
Crystal system
Space group
orthorhombic
P2(1)2(1)2(1) (#19)
monoclinic
P2(1) (#4)
Unit cell dimensions
a (Å)
b (Å)
c (Å)
b (°)
V (Å3)
Z
16.5323(2)
17.4695(2)
17.8208(2)
90
5146.84(10)
4
16.2566(3)
17.8133(3)
17.3187(3)
91.219(1)
5014.08(15)
4
4. Experimental
q
l
(gcmꢀ3
)
1.452
0.705
1.418
0.616
(mmꢀ1
)
4.1. General
Crystal size
0.27 ꢁ 0.19 ꢁ 0.11
0.32 ꢁ 0.18 ꢁ 0.15
(mm3)
Independent
reflections
27576, 8167
(Rint = 0.0317)
65184, 20471
(Rint = 0.0331)
Complex Co(TPP)Cl [25] was prepared according to published
procedures. Solvents were dried by passage through solvent purifi-
cation columns (CH2Cl2) or by distillation from P2O5 (CHCl3), Mg/I2
(MeOH), or CaH2 (MeCN). All other reagents are commercially
available and were used as received. Unless otherwise stated,
NMR spectra were recorded at 30 °C on Varian spectrometers oper-
ating at 300 or 400 MHz (1H NMR) and 75 or 100 MHz (13C{1H}
NMR), respectively. Chemical shifts (d in ppm, coupling constants
J in Hz) were referenced to residual solvent resonances. Assign-
ments are based on homo- and heteronuclear shift correlation
spectroscopy. Elemental analyses were performed by the microan-
alytical laboratory of University College Dublin, Ireland.
(Rint
Max., min.
transmission
Restraints/
parameters
)
0.964, 0.924
21/749
0.957, 0.916
1/1266
Goodness-of-fit
1.079
1.027
(GOF) on F2
R [I > 2r(I)]
R1 = 0.0409, wR2 = 0.1011
R1 = 0.0468, wR2 = 0.1032
0.014(14)
R1 = 0.0529, wR2 = 0.1463
R1 = 0.0631, wR2 = 0.1515
ꢀ0.017(12)
R (all data)
Absolute
structure
parametera
Largest difference 0.755 and ꢀ0.662
in peak and
1.162 and ꢀ0.721
hole (eÅ3)
4.2. Complex 2
a
See Ref. [28].
A solution of TPPCoCl (0.200 g, 0.283 mmol) in dry MeCN
(20 mL) was stirred with N,N0-dimethylimidazolium-2-carboxylate
(0.040 g, 0.283 mmol) for 12 h at room temperature. All volatiles
were removed under reduced pressure and the residue was puri-
fied by column chromatography (SiO2; CHCl3). The last fraction
was collected and dried in vacuo to give the title product as a pur-
ple solid. Yield 0.18 g, 79%. 1H NMR (CDCl3): d 8.83 (s, 8H, Hpyr),
7.87 (s, 8H, HPh), 7.65 (s, 12H, HPh), 5.06 (s, 2H, HNHC), ꢀ0.63 (s,
6H, NCH3). 13C{1H} NMR (CDCl3): d 143.81 (Cpyr), 139.88 (CPh),
134.38 (Cpyr), 133.61, 128.04, 127.09 (3 ꢁ CPh), 123.70 (CNHC),
120.25 (C–Ph), 32.54 (NCH3), carbene carbon not resolved. Anal.
Calc. for C49H36N6CoCl (803.4) ꢁ H2O: C, 71.66; H, 4.66; N, 10.23.
Found: C, 71.99; H, 4.23; N, 10.02%.
respectively. In addition, signals due to the bound imidazole ligand
were observed in the expected integral ratio and diagnostically
shifted to high field due to the porphyrin ring current.
A potential disproportionation reaction was excluded based on
analytical data of complexes 6 comprising two imidazole ligands
bound to the Co(TPP) unit (Scheme 3). These complexes were
obtained according to previously published methods by using two
molequiv. imidazole [19,24]. NMR signal integration was in full
agreement with the presence of two imidazole ligands at the cobalt
center. The chemical shifts for complexes 6a and 6b were distinctly
different from those recorded for the corresponding mixed-ligand
complexes 5. Specifically, the signals due to the meso-phenyl groups
collapse into a single broad resonance in complexes 5 while the
ortho protons are distinctly different from the meta- and para-posi-
4.3. Complex 3
tioned protons in complexes 6 (d
onances for the protons to the cobalt-bound nitrogen (i.e. C4–H in
a, C5–H in b) are located at higher field in bis(imidazole) complexes
D ca. 0.1 ppm). In addition, the res-
a
A solution of 2 (0.200 g, 0.249 mmol) in dry CHCl3 (15 mL) was
stirred with AgBF4 (0.048 g, 0.25 mmol) for 2 h at room temperature.

94
M. Albrecht et al. / Inorganica Chimica Acta 380 (2012) 90–95
The mixture was filtered through Celite. All volatiles were removed
from the filtrate under reduced pressure and the residue was
purified by preparative thin layer chromatography (SiO2; CHCl3).
Yield 0.178 g, 84%. 1H NMR (CDCl3): d 8.92 (s, 8H, Hpyr), 7.85 (s, 8H,
room temperature for 16 h. All volatiles were removed under re-
duced pressure and the residue was purified by preparative thin
layer chromatography (SiO2; CHCl3). Yield 0.94 g, 93%. 1H NMR
(CDCl3): d 8.98 (s, 8H, Hpyr), 7.79 (s, 8H, HPh), 7.67 (s, 12H, HPh),
4.71 (s, 2H, Him), 2.07 (s, 6H, NCH3), ꢀ0.32 (s, 2H, Him), ꢀ2.20 (s,
6H, C–CH3). Anal. Calc. for C54H44ClCoN8 (899.37): C, 72.11; H,
4.93; N, 12.46. Found: C, 72.01; H, 4.75; N, 12.16%.
HPh), 7.70 (s, 12H, HPh), 5.38 (s, 2H, HNHC), ꢀ0.52 (s, 6H, NCH3).
13C{1H} NMR (CDCl3): d 146.00 (Cpyr), 138.67 (CPh), 134.77 (Cpyr),
132.77, 128.88, 127.72 (3 ꢁ CPh), 126.91 (CNHC), 123.14 (C–Ph),
32.86 (NCH3), carbene carbon not resolved. Anal. Calc. for
C
49H36BCoF4N6 (854.59): C, 68.87; H, 4.25; N, 9.83. Found: C,
4.9. Complex 6b
69.09; H, 4.21; N, 9.69%.
Complex 6b was obtained as described for 6a from TPPCoCl
(0.188 g, 0.27 mmol) and 2,4-dimethylimidazole (0.051 g,
0.53 mmol). Yield 0.210 g, 88%. 1H NMR (CDCl3): d 9.46 (s, 2H,
4.4. Complex 4a
A solution of 3 (0.100 g, 0.12 mmol) in dry CHCl3 (20 mL) and
MeOH (0.02 ml, 0.4 mmol) was stirred for 1 h at room temperature.
All volatiles were removed under reduced pressure. The crude
product (0.095 g, 92%) was recrystallized from CHCl3/pentane. 1H
NMR (CDCl3): d 8.95 (s, 8H, Hpyr), 7.86 (s, 8H, HPh), 7.71 (s, 12H,
HPh), 5.35 (s, 2H, HNHC), 3.78 (s, 1H, OH), 1.25 (s, 3H, OCH3),
ꢀ0.51 (s, 6H, NCH3). 13C{1H} NMR (CDCl3): d 146.06 (Cpyr), 138.56
(CPh), 135.02 (Cpyr), 132.87, 129.02, 127.78 (3 ꢁ CPh), 126.65 (CNHC),
123.07 (C–Ph), 36.11 (OCH3), 32.86 (NCH3), carbene carbon not re-
solved. Anal. Calc. for C50H40N6CoOBF4 (886.63) ꢁ 2CHCl3: C, 55.50;
H, 3.76; N, 7.47. Found: C, 55.19; H, 3.52; N, 7.26%.
Nim–H), 8.77 (s, 8H, Hpyr), 7.64 (s, 8H, HPh), 7.55 (s, 12H, HPh),
0.20 (s, 6H, C–CH3), ꢀ0.83 (s, 2H, Him), ꢀ2.38 (s, 6H, C–CH3).
13C{1H} NMR (CDCl3): d 142.00 (Cpyr), 141.68 (Cim–Me), 139.84
(CPh), 134.52 (Cpyr), 133.68, 127.71, 126.76 (3 ꢁ CPh), 120.80 (Cim
–
Me), 118.65 (Cim–H), 118.64 (CPh), 7.93, 6.44 (2 ꢁ CH3). Anal. Calc.
for C54H44ClCoN8 (899.37): C, 72.11; H, 4.93; N, 12.46. Found: C,
71.98; H, 4.90; N, 12.29%.
4.10. Crystallographic details
Crystal data were collected using an Oxford Diffraction Super-
Nova A diffractometer fitted with an Atlas detector. Crystals were
4.5. Complex 4b
measured with monochromated Mo K
a radiation (0.71073 Å). A
twice redundant dataset was collected, assuming that the Friedel
pairs are not equivalent. An analytical absorption correction based
on the shape of the crystal was performed [26]. The structures
were solved by direct methods using SHELXS-97 and refined by full
matrix least-squares on F2 for all data using SHELXL-97 [27]. Hydro-
gen atoms were added at calculated positions and refined using a
riding model. Their isotropic temperature factors were fixed to
1.2 times (1.5 times for methyl groups) the equivalent isotropic
displacement parameters of the carbon atom the H-atom is at-
tached to. Anisotropic thermal displacement parameters were used
for all non-hydrogen atoms. For complex 4a SAME and SADI
restraints were used to get corresponding disorder parts into
similar shapes. Further crystallographic details are compiled in
Table 2.
Complex 4b was obtained according to the same procedure as de-
scribed for 4a, starting from 3 (0.100 g, 0.12 mmol) and EtOH
(0.02 ml, 0.4 mmol) in dry CH2Cl2 (20 mL) and after recrystallization
from CH2Cl2/pentane. Yield 0.98 g, 93%. 1H NMR (CDCl3): d 8.96 (s,
8H, Hpyr), 7.86 (s, 8H, HPh), 7.71 (s, 12H, HPh), 5.37 (s, 2H, HNHC),
3.83 (s, H, OH), 1.25 (br, 2H, OCH2), 0.87 (t, 3H, CH3CH2OH), ꢀ0.50
(s, 6H, NCH3). Anal. Calc. for C51H42BCoF4N6O (900.66) ꢁ 2CH2Cl2:
C, 59.46; H, 4.33; N, 7.85. Found: C, 59.02; H, 4.11; N, 7.63%.
4.6. Complex 5a
A solution of 3 (0.100 g, 0.12 mmol) in dry CHCl3 (20 mL) was
stirred with 1,2-dimethylimidazole (0.011 g, 0.12 mmol) for 2 h at
room temperature. All volatiles were removed under reduced pres-
sure and the residue was purified by preparative thin layer chroma-
tography (SiO2; CHCl3). Yield 0.095 g, 85%. 1H NMR (CDCl3): d 8.86 (s,
8H, Hpyr), 7.64 (br, 20H, HPh), 5.03 (s, 2H, HNHC), 4.62 (s, 1H, Him), 2.08
(s, 3H, Nim–CH3), ꢀ0.21 (s, 1H, Him), ꢀ0.64 (s, 6H, NNHC–CH3), ꢀ2.20
(s, 3H, Cim–CH3). 13C{1H} NMR (CDCl3): d 142.17 (Cpyr), 141.15 (Cim),
139.16 (CPh), 135.08 (Cpyr), 134.70 (Cim–Me), 133.81 (Cim–Me),
133.34, 128.13, 127.12 (3 ꢁ CPh), 122.15 (CNHC), 121.35 (C–Ph),
32.53 (NCH3), 8.33, 6.03 (Nim–CH3 and Cim–CH3), carbene carbon
not resolved. Anal. Calc. for C54H44BCoF4N8 (950.72): C, 68.22; H,
4.66; N, 11.79. Found: C, 68.43; H, 4.21; N, 11.47%.
Acknowledgements
This work has been financially supported by the European Re-
search Council through a Starting Grant, and by the Swiss National
Science Foundation.
Appendix A. Supplementary material
CCDC 828899 and 828900 contain the supplementary crystallo-
graphic data for complexes 4a and 4b. These data can be obtained
free of charge from The Cambridge Crystallographic Data Centre
via www.ccdc.cam.ac.uk/data_request/cif. Supplementary data
associated with this article can be found, in the online version, at
doi:10.1016/j.ica.2011.08.039.
4.7. Complex 5b
According to the procedure described for 5a, complex 5b was
obtained from 3 (0.100 g, 0.12 mmol) and 2,4-dimethylimidazole
(0.011 g, 0.12 mmol). Yield 0.97 g, 87%. 1H NMR (CDCl3): d 8.81
(s, 8H, Hpyr), 8.03 (s, H, Nim–H), 7.62 (s, 20H, HPh), 4.84 (s, 2H, HNHC),
0.61 (s, 3H, Cim–CH3), ꢀ0.52 (s, 1H, Him), ꢀ0.67 (s, 6H, NCH3), ꢀ2.15
(s, 3H, Cim–CH3). Anal. Calc. for C54H44BCoF4N8 (950.72): C, 68.22;
H, 4.66; N, 11.79. Found: C, 68.34; H, 4.54; N, 11.45%.
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