A dibenz[a,c]phenazine-supported N-heterocyclic carbene and its rhodium and iridium complexes

Article abstract of DOI:10.1016/j.jorganchem.2013.09.034

A new polycyclic N-heterocyclic carbene featuring a fused dibenz[a,c]phenazine moiety was generated in situ from the corresponding tetrafluoroborate salt. The synthesis and NMR data of its corresponding precursors, its sulfur adduct and dimer are reported

Full text of DOI:10.1016/j.jorganchem.2013.09.034

Journal of Organometallic Chemistry 749 (2014) 134e141
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Journal of Organometallic Chemistry
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A dibenz[a,c]phenazine-supported N-heterocyclic carbene and its
rhodium and iridium complexes
,
a
a
a
Daniela Tapu a , Zachary McCarty , Lauren Hutchinson , Christopher Ghattas ,
*
Mahatab Chowdhury a, John Salerno a, Donald VanDerveer b
a Department of Chemistry and Biochemistry, Kennesaw State University, 1000 Chastain Road, Kennesaw, GA 30144, USA
b X-ray Crystallography Laboratory, Department of Chemistry, Clemson University, Clemson, SC 29634, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
A new polycyclic N-heterocyclic carbene featuring a fused dibenz[a,c]phenazine moiety was generated in
situ from the corresponding tetrafluoroborate salt. The synthesis and NMR data of its corresponding
precursors, its sulfur adduct and dimer are reported. Complexes of type [MCl(COD)(1)] and [MCl(CO)2(1)]
(M ¼ Rh and Ir, 1 ¼ 1,3-dibutyldibenzo[a,c]phenazino[11,12-d]imidazol-2-ylidene) were prepared and
characterized using spectroscopic and crystallographic methods. The electron-releasing capacity of this
new carbene was investigated by evaluation of its corresponding IrCl(COD) and IrCl(CO)2 complexes by IR
spectroscopy (nav) and cyclic voltammetry (E1/2). These studies revealed that the electron donicity of this
ligand is comparable to that of the previously reported naphthoquinone-annulated imidazolin-2-ylidene.
Some preliminary studies of the photophysical properties and catalytic activity of these metal complexes
are reported.
Received 21 August 2013
Received in revised form
9 September 2013
Accepted 12 September 2013
Keywords:
Fused N-hererocyclic carbene
Dibenz[a,c]phenazine
Rhodium
Iridium
Ó 2013 Elsevier B.V. All rights reserved.
Catalytic activity
Hydrosilylation
1. Introduction
numerous examples of bicyclic 4,5-annulated imidazol-2-ylidenes
have been studied [27,28], reports of more extended aromatic
Since the isolation and crystallographic characterization of the
first stable N-heterocyclic carbene (NHC) in early 1991, carbene
chemistry has become a very prolific field of research [1]. Owing
to their unique electronic and steric properties, NHCs have
emerged as a powerful class of carbon-based ligands rivaling the
leading status of phosphines as ligands in homogeneous catalysis
[2e18]. It was recognized early on that expanding on the struc-
tural diversity of NHCs may extend the ability to fine-tune their
electronic and steric properties, which is of prime importance for
the design of catalysts with enhanced activities. One approach
that has been used to modify the electronic properties of these
carbenes is annulation of the typical 5-membered ring parent
system (Fig. 1, I) with aromatic or nonaromatic carbo- and het-
erocycles. It has been shown that annulation at C4eC5 can have a
significant impact on the stability and the electronic nature of
NHCs [19e21]. With a few exceptions [22e26], the majority of
systems (tri- and polycyclic) are scarce, despite their proven
utility in catalysis, polymer chemistry and materials science
[19,27,29e37]. Among these carbenes, only three of them incor-
porate a fused polycyclic heteroaromatic moiety (Fig. 1, IIeIV))
[33,38,39].
This contribution reports our investigation toward the syn-
thesis and reactivity of a new polycyclic heteroaromatic carbene:
1,3-dibutyldibenzo[a,c]phenazino[11,12-d]imidazol-2-ylidene (1).
Our interest in the development of this new carbene stems from
its potential application as a fluorescent ligand in homogeneous
catalysis [40]. Dibenzo[a,c]phenazine (DBP) and its derivatives are
a class of aromatic compounds with interesting photophysical
properties [41e46]. Fusion of such a ring to an imidazolin-2-
ylidene provides a promising framework in which the carbene
center is a component of an electron-rich, extended aromatic
system. This should have a significant effect on the donor prop-
erties of the ligand. In addition, the carbene center is tightly
coupled to the fluorescent DBP moiety which should lead to
useful photophysical properties of the metal complexes that
incorporate 1. Preliminary studies of the catalytic performance of
these metal complexes in hydrosilylation of acetophenone are
also reported.
4,5-annulated imidazol-2-ylidenes reported to date contain a
p-
system fused to the 5-membered parent ring [27]. Although
* Corresponding author. Tel.: þ1 678 797 2259; fax: þ1 770 423 6744.
E-mail address: dtapu@kennesaw.edu (D. Tapu).
0022-328X/$ e see front matter Ó 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.jorganchem.2013.09.034

D. Tapu et al. / Journal of Organometallic Chemistry 749 (2014) 134e141
135
Fig. 1. Examples of NHCs featuring fused polycyclic heteroaromatic moieties.
2. Results and discussion
Carbene 1 was found to decompose upon concentration, which
precluded its isolation. Our attempt to generate the free carbene
through deprotonation of 6 with potassium t-butoxide or NaH/
DMSO(cat) mixture in THF resulted, upon evaporation of the sol-
vent, in a complex mixture of products, one of which was isolated
and identified as being the enetetramine 8, whose N2C carbon
2.1. Chemical synthesis
2.1.1. Synthesis of the carbene precursor
The most common method used to generate NHCs is the
deprotonation of the appropriate imidazolium salt precursors [1]. As
summarized in Scheme 1, five steps provided access to imidazolium
salt 6 from the commercially available 5-nitrobenzimidazole 2. First,
benzimidazole 2 was nitrated with a mixture of HNO3/H2SO4
following a known procedure [47]. Subsequent alkylation with
excess n-butyl iodide in the presence of sodium hydroxide yielded
1,3-dibutyl-6-dinitrobenzimidazolium iodide 3. Reduction of the
nitro groups with SnCl2 in methanol afforded 4. Iodide salt 4 was
converted to the tetrafluoroborate salt 5 in order to circumvent
potential halide exchange and any subsequent separation problems
in reactions involving metal chlorides. The 13C and 1H NMR spectra
of 5 are consistent with the proposed structure. In the 1H NMR
spectrum, the characteristic imidazolium proton appears at
signal appears at
d 155.21 ppm [48]. However, in an NMR experi-
ment treatment of a dilute solution of 6 with NaH/DMSO(cat) in dry
d8-THF led to immediate disappearance of the acidic proton and the
formation of the free carbene 1 as the major product. This carbene
was stable enough to be analyzed by 13C NMR spectroscopy [49].
However, all attempts to observe the chemical shift of the carbenic
carbon were unsuccessful. Treatment of this NMR solution with
elemental sulfur after 24 h led to the formation of the corre-
sponding sulfur adduct 7 as the major product.
2.1.3. Synthesis of metal complexes
Subsequent efforts were focused on the coordination of the
carbene to various transition metals. Rhodium(I) and iridium(I)
complexes 9 and 10 were prepared by in situ deprotonation of the
imidazolium salt 6 with potassium t-butoxide in the presence of
[MCl(COD)]2 (M ¼ Rh, Ir) at room temperature (Scheme 3). Both
complexes are air and moisture stable. No decomposition was
observed upon heating the solids under vacuum at 100 ꢀC for 5
days. They are soluble and stable in THF and halogenated solvents
(CH2Cl2, CHCl3) at room temperature. Their structures were
confirmed by analytical and spectroscopic techniques. In the 13C
NMR spectra of the Rh complex 9, the carbenoid carbon appears as
d
9.13 ppm in d6-DMSO, shifted upfield by 1.11 ppm in comparison
with that of 3. This shift reflects the electronic properties of the two
substituents on the benzene ring (amino vs. nitro). The 13C NMR
signal of C2 appears at d 137.51 ppm (d6-DMSO),11.04 ppm upfield in
comparison with the corresponding signal for 3. Condensation of 5
with 9,10-phenanthrenedione afforded the desired imidazolium
salt 6 in excellent yields. The acidic proton display a signal at
d
10.17 ppm which suggests that the newly formed dibenzoqui-
noxaline fragment withdraws electrons from the imidazolium ring
as stronglyas the two nitro groups in 3. The 13C NMR signal of C2 of 6
a doublet at
d
208.2 ppm (1JRh-C ¼ 50.4 Hz), falling in the range
appears at
d
148.31 ppm in d6-DMSO.
previously observed for related Rh complexes of saturated five and
six-membered NHCs [50]. The carbons in the COD double bonds of
9 are each coupled with the rhodium atom differently (1JRh-C ¼ 6.35,
14.28 Hz), which is consistent with their placement trans to
different groups. For the iridium complex 10, the carbenoid carbon
2.1.2. Generation of carbene 1
The imidazolium salt 6 could be deprotonated with NaH and a
catalytic amount of DMSO in THF at room temperature, leading to
the neutral carbene 1, which could be trapped by addition of
elemental sulfur to give the thiourea 7 in good yield (Scheme 2). In
the 13C NMR spectrum of the thiourea 7, the signal for the N2CS
occurs as a singlet at
d 201.9 ppm.
Metal-bound carbonyls are useful spectroscopic handles for
measuring the electron density at the coordinated metal centers.
Many [IrCl(CO)2] and [RhCl(CO)2] complexes supported by NHCs
carbon atom appears at a typical chemical shift of
d 174.29 ppm.
Scheme 1. Synthesis of imidazolium salt 6.

136
D. Tapu et al. / Journal of Organometallic Chemistry 749 (2014) 134e141
Scheme 4. Synthesis of iridium complex 11.
and þ0.959 V (
D
E1/2 ¼126 mV) that are attributed to MI/II couples, and
these values are higher than those reported for other non-annulated
NHC-supported [MCl(COD)] complexes (E ¼ 0.648e0.920 V) [59],
½
but comparable to the value reported by Bielawski for the [IrCl(COD)]
complex of the naphtoquinone-annulated NHC (E ¼ 0.95 V) [19,56].
½
The IrI/II couple of 11 was shifted beyond the solvent window and
could not be observed [56,60]. The collective IR spectroscopy and
electrochemical data suggest that the annulationwith the DBP moiety
of the parent 5-membered parent ring resulted in an attenuated
ligand donicity relative to that of the non-annulated counterparts, but
comparable to that of the naphtoquinone-annulated NHC reported by
Bielawski.
Scheme 2. Synthesis and trapping of carbene 1.
have been prepared and used to determine and compare the
electron donating abilities of new NHCs to those of known ligands
[51]. To access the [IrCl(CO)2] complex of our carbene, carbon
monoxide was bubbled through a CH2Cl2 solution of 10, resulting in
a clean conversion to complex 11 (Scheme 4). The complex is air
and moisture stable and readily soluble in halogenated solvents.
2.3. Structural studies
Complex 10 gave X-ray quality crystals upon slow diffusion of
methanol into a saturated toluene solution. Selected bond lengths
and angles are given in Table 2. Iridium complex 10 crystallizes in
the monoclinic space group P21/n. As shown in the ORTEP plot
(Fig. 2), complex 10 adopts a square planar coordination geometry
around the iridium. Relative to the [IrCl(COD)] plane, ligand 1 is
rotated by 87.5(4)ꢀ. The IreC2 and IreCl lengths of 2.008(5) and
2.2. Assessment of the ligand donating properties of carbene 1
Several distinctly different methods were developed to quantify
the electronic properties of NHCs [52]. Initially, the ligand-donating
ability of carbene 1 was investigated by analyzing the carbonyl com-
plex 11 using IR spectroscopy. Complex 11 exhibited trans (2078 cm-1)
and cis (1982 cmꢁ1) carbonyl stretching frequencies consistent with
those observed in other known NHC-supported [IrCl(CO)2] complexes
[51,53e55]. The average value nav ¼ 2030 cmꢁ1 was used to calculate
the Tolmanelectronic parameter (TEP¼ 2055.4 cmꢁ1) of 1 [51]. Onthe
basis of this calculated TEP value, carbene 1 appears to beas strong of a
donor as a previously reported naphtoquinone-annulated NHC
(TEP ¼ 2055.4 cmꢁ1) [19,56]; however, it is a weaker donor than 1,3-
dimesitylimidazol-2-ylidene (IMes) (TEP ¼ 2050.7 cmꢁ1) and its
ꢀ
2.3640(15) A compare closely with those measured in other
[IrCl(COD)(NHC)] complexes, with typical values from 1.99 to
ꢀ
ꢀ
2.091 A and 2.335 to 2.39 A, respectively [2,51,59,61e64]. As ex-
pected, the average distance between iridium and the olefinic
ꢀ
carbons trans to 1 [rav(IreC34(35) ¼ 2.189 (5) A] is slightly larger
than the average distance between iridium and the double bond
ꢀ
carbons cis to 1 [rav(IreC38(39) ¼ 2.117 (5) A]. This indicates a
stronger trans-influence of the carbene and a weaker iridium-(C]
C) bond for the carbons trans to C2 than trans to chloride.
saturated
analog
1,3-dimesitylimidazolin-2-ylidene
(SIMes)
(TEP ¼ 2053) [51,54]. While TEPs are commonly used to analyze the
electron donicity of NHCs, the measurement is indirect, as it relies
upon changes in the stretching frequency of the coordinated carbonyl
groups [57,58]. Voltammetry was shown to be a superior approach for
the determination of the electron donating properties of NHCs
because the metal center is directly bonded to the NHC ligand [59].
The results of the cyclic and differential pulse voltammetry mea-
surements are summarized in Table 1. Complexes 9 and 10 exhibit
Single crystals of 11 were grown from a saturated dichloro-
methane solution by vapor diffusion with methanol at room tem-
perature. Complex 11 crystallizes in the triclinic P-1 group with
square planar coordination geometry around the iridium as
depicted in Fig. 3. Selected bonds and angles for 11 are given in
Table 2. The asymmetric unit contains two unique molecules. The
ꢀ
IreC2 distance of 2.065(6) A and the iridium-carbonyl and CeO
ꢀ
bond lengths measured cis (1.833(9) and 1.024(11) A, respectively)
ꢀ
quasi-reversible redox processes at þ0.948 V ( E1/2 ¼ 142 mV)
D
and trans (1.878(7) and 1.158(9) A) relative to the carbene moiety
are within the ranges reported for other NHC-supported [IrCl(CO)2]
complexes [51]. The plane of the carbene is almost perpendicular to
Table 1
Summary of electrochemical data.a
E1/2 (V)
DE (mV)
Epa (V)
Epc (V)
9
10
0.948
0.962
143
127
1.020
1.026
0.877
0.899
a
The electrochemical data were obtained in CH2Cl2 in the presence of 0.1 M
[Bu4N][PF6] electrolyte with decamethylferrocene (Fc*) as an internal standard
(100 mV sꢁ1 scan rate), referenced to SCE.
Scheme 3. Synthesis of rhodium and iridium complexes 9 and 10.

D. Tapu et al. / Journal of Organometallic Chemistry 749 (2014) 134e141
137
Table 2
ꢀ
Selected bond lengths (A) and angles (deg) for 10 and 11.
10
11a
MeC2
MeX
MeC34
MeC35
MeC38
MeC39
C34eO1
C35eO2
C2eN1
C2eN3
C4eC25
C25eN1
C4eN3
MeC2eN1
MeC2eN3
N1eC2eN3
Cl1eMeC2
Cl1eMeC34
Cl1eMeC35
Cl1eMeC38
Cl1eMeC39
2.008(5)
2.3640(15)
2.194(5)
2.185(5)
2.133(5)
2.102(5)
e
2.065(6)
2.358(2)
1.878(7)
1.833(9)
e
e
1.158(9)
1.024(11)
1.341(8)
1.362(9)
1.440(9)
1.387(8)
1.353(8)
126.4(5)
127.4(5)
106.2(5)
89.26(17)
89.9(2)
177.8(3)
e
e
1.366(6)
1.368(7)
1.410(7)
1.390(6)
1.390(6)
126.1(4)
127.5(4)
105.7(4)
87.31(15)
92.30(18)
89.46(16)
163.18(15)
157.35(16)
e
a
Structural data for one of the two unique molecules in the asymmetric unit.
the iridium coordination plane (interplanar angle 89.7(45)ꢀ). In the
crystal, the molecules’ hexacyclic planes are in face-to-face contact
ꢀ
with one another, with an interplanar distance of about 3.4 A as
depicted in Fig. 3 (bottom).
The buried volume (% Vbur ¼ 26.4) of 1 was calculated using the
Fig. 3. (Top) Molecular structure of complex 11 showing 50% probability ellipsoids.
Hydrogen atoms have been omitted for clarity. (Bottom) Lattice view of the molecular
network showing the parallel arrangement of the molecules.
crystallographic information files and the software developed by
ꢀ
ꢀ
Cavallo and co-workers (r ¼ 3.5 A, dIr-C2 ¼ 2.008(5) A) [65,66]. This
value is comparable with the values found for IMes (% Vbur ¼ 26)
and SIMes (TEP ¼ % Vbur ¼ 27) [51].
39 nm of the absorption maxima to DBP (Table 3). These shifts are
consistent with the more delocalized
relative to DBP.
p-systems of 6, 9 and 10
2.4. UVeVis and fluorescence studies
In CH2Cl2, 6, 9 and 10 exhibit fluorescence emissions in the
region 400e650 nm (Fig. 5). With excitation at 394 nm, imida-
zolium salt 6 exhibits emission over three-fold stronger than DBP,
while the emission intensity for 9 and 10 are 73% and 44% of that
Further investigation of the spectroscopic properties of the
imidazolium salt 6 and metal complexes 9 and 10 revealed key
room temperature characteristics that are summarized in Table 3.
All compounds absorb UVeVis light strongly at
l < 500 nm (Fig. 4,
of DBP. The quantum efficiencies (F) of 6, 9 and 10 are 0.02,
log ( abs) w 4.83e4.89 at lmax). A comparison of the absorption
3
1.4 ꢂ 10ꢁ3 and 1.8 ꢂ 10ꢁ3, respectively, as determined by inte-
gration of their emission spectra and comparison to the emission
spectrum of riboflavin Refs. [67,68]. The metal complexes 9 and 10
exhibit reduced fluorescence in comparison with imidazolium salt
6, consistent with literature precedent for related NHC-supported
[IrCl(COD)] complexes [35]. The fluorescence quenching is likely
spectra of 6, 9 and 10 with the DBP spectrum reveals a close sim-
ilarity of their spectral features. They all display complex absorp-
tion spectra with two major regions: one between 240 and 325 nm
and the other between 340 and 500 nm, corresponding to spin-
1
allowed optical transitions of the
(pep*) type. The most notice-
able differences among them are the bathochromic shifts of up to
Table 3
Room-temperature UVeVis and fluorescence spectroscopic properties of 6, 9, 10 and
DBP.a
Compound labs (nm)b lem (nm) [relative Intensity]c
[log
Fd
3 ]
6
256 [4.89], 496 [1.00]
0.02
414 [4.36]
9
293 [4.83], 405 [0.033] 427 [0.230] 447 [0.172] 1.4 ꢂ 10ꢁ3
424 [4.71]
10
DBP
292 [4.86], 406 [0.035] 428 [0.138] 446 [0.105] 1.8 ꢂ 10ꢁ3
431 [4.71]
254 [4.89], 406 [0.266] 422 [0.313] 446 [0.239] 3.7 ꢂ 10ꢁ3
393 [4.33]
a
Data obtained for CH2Cl2 solutions (2.5 ꢂ 10ꢁ6 mol lꢁ1) under ambient condi-
tions. See also Supporting information.
b
lmax indicated in bold.
Excitation at 394 nm; emission maximum indicated in bold.
c
Fig. 2. Molecular structure of complex 10 showing 50% probability ellipsoids.
Hydrogen atoms have been omitted for clarity. There is some disorder in both butyl
chains.
d
Quantum efficiencies
¼ 0.23) [67,68].
(F) were determined relative to riboflavin in water
(
F

138
D. Tapu et al. / Journal of Organometallic Chemistry 749 (2014) 134e141
Table 4
Hydrosilylation of acetophenone derivatives 12 with complex 9 and 10.a
Entry
Catalyst
R
Time (h)
NMR yield 13
NMR yield 14
1b
9
10
9
10
9
10
9
10
9
10
9
10
9
10
9
10
9
H
H
H
H
3
3
3
3
6
2
4
2
3
80%
98%
80%
90%
88%
98%
76%
100%
83%
100%
30%
88%
83%
92%
92
11%
2%
10%
2%
5%
2%
13%
0%
8%
0%
26%
3%
10%
1%
8%
2%
13%
2%
2b
3c
4c
5c
4-CH3
4-CH3
2-OMe
2-OMe
4-OMe
4-OMe
2-Cl
2-Cl
4-F
6c
7c
Fig. 4. Normalized UVeVis absorption spectra of solutions of 6, 9, and 10 and DBP in
CH2Cl2 (2.5 ꢂ 10ꢁ6 mol lꢁ1) at 22 ꢀC.
8c
9c
10c
11c
12c
13c
14c
15c
16c
17c
18c
1
10
10
5
5
5
5
8
22
caused by the heavy atom effect [69]. The presence of the chlorine
atoms could promote
a non-radiative decay or intersystem
4-F
crossover to a non-emissive triplet excited state. The electronic
structure of DBP is strongly affected by the annulation of the
imidazolium moiety as evidenced not only by the difference in
appearance and the relative intensities of the emission bands of 6
and DBP, but also by the difference in their lmax values, where
lmax of 6 is bathochromically shifted by as much as 74 nm relative
to DBP (lmax ¼ 496 and 422 nm, respectively). On the other hand,
the emission spectra of 9 and 10 resemble the emission spectrum
of DBP quite well, emission maxima being red shifted by only 5
and 6 nm relative to DBP (Table 3). These results are promising
and suggest that the incorporation of 1 into catalytically relevant
metal complexes could facilitate the monitoring of the state of a
catalyst during a catalytic process by fluorescence spectroscopy
[40].
4-Cl
4-Cl
4-Br
4-Br
98
80%
97%
10
a
Initial concentration: 0.083 M of 12. Yields calculated by 1H NMR spectroscopy
as averages of two runs using 1,3,5-trimethoxybenzene as internal standard.
b
In dichloromethane-d2.
In benzene-d6.
c
were carried out at room temperature in the presence of 3 equiv-
alents of Ph2SiH2 and an initial 0.083 M concentration of 12. The
reactions were monitored by 1H NMR spectroscopy. The yield of
silyl ether 13 was calculated by comparing the integration values of
the CH3 doublet and the CH quartet with that of the OCH3 singlet of
the internal standard, 1,3,5-trimethohybenzene. The yield of 14 was
calculated by comparing the integration values of the two vinylic
CH doublets with that of the OCH3 singlet of the internal standard.
The results are summarized in Table 4. Hydrosilylation of aceto-
phenone (R ¼ H) was found to proceed with comparable yields in
both benzene-d6 and dichloromethane-d2 (entries 1e4). The
identity of the metal proved to have a significant effect on the
outcome of the reaction, with the iridium complex being more
active than the rhodium complex in all studied reactions. In the
presence of iridium complex 10, 4- and 2-methoxyacetophenones
were converted quantitatively into 13 within 1e2 h.
2.5. Catalytic studies
The catalytic hydrosilylation of acetophenone with diphenylsi-
lane was chosen as a model reaction to evaluate the catalytic po-
tential of complexes 9 and 10. For comparative purposes the
catalyst loading was fixed at 2.5 mol%. All catalytic experiments
3. Conclusion
In summary, a novel dibenz[a,c]phenazine-fused imidazol-2-
ylidene was generated in situ by deprotonation at room tempera-
ture of the corresponding imidazolium tetrafluoroborate 6. Salt 6
was obtained by a five-step synthesis from commercial starting
materials. Details on the chemistry of this ligand with respect to its
ability to support catalytically relevant metal complexes were
provided. A series of metal complexes that incorporate 1 have been
synthesized, fully characterized, and their solid state molecular
structures have been determined by X-ray diffraction studies. The
buried volume of 1 was calculated as being % Vbur ¼ 26.4, compa-
rable with that found for IMes and SIMes. Electrochemical and IR
spectroscopic analyses of complexes 9e11 revealed that carbene 1
is among the weakest
s-donors in the unsaturated series of
Arduengo-type carbenes, and that its donicity is comparable to that
displayed by the previously reported naphthoquinone-annulated
imidazolin-2-ylidene. The UVeVis absorption and emission
Fig. 5. Normalized emission spectra of CH2Cl2 solutions (2.5 ꢂ 10ꢁ6 mol lꢁ1) of 6, 9,
and 10 and DBP at 22 ꢀC after excitation at 394 nm.

D. Tapu et al. / Journal of Organometallic Chemistry 749 (2014) 134e141
139
spectra of 6, 9 and 10 were determined and compared to that of
DBP. The results show that the dibenz[a,c]phenazine fused NHCs
are suitable ligands for rhodium and iridium catalysts in the
hydrosilylation of acetophenones. Further studies of these new
fluorescent compounds and their analogs with respect to their
possible use as fluorescent catalysts are underway.
small portions. The reaction mixture became colorless within mi-
nutes and the heating was continued for 24 h. The volatiles were
removed and the residue was basified with 2 M aqueous sodium
hydroxide. The product was extracted into CH2Cl2. After drying
with MgSO4 and evaporation of the volatiles, pure 4 was obtained
as an off-white solid (4.74 g, 91.3% yield). 1H NMR (d6-DMSO,
300 MHz):
d 9.15 (s, 1H, C2H), 6.84 (s, 2H, ArH), 5.21 (s, 4H, NH2),
4. Experimental section
4.22 (t, 4H, J ¼ 7.1 Hz, NCH2), 1.78 (p, 4H, J ¼ 7.3 Hz, NCH2CH2), 1.26
(sextet, 4 H, J ¼ 7.3 Hz, CH2CH3),0.89 (t, 6H, J ¼ 7.3 Hz, CH3). 13C NMR
4.1. General procedures
(d6-DMSO, 75.5 MHz): d 137.17 (NCN), 135.59 (ArC), 123.92 (ArC),
94.15 (ArC), 45.87 (NCH2), 30.27 (NCH2CH2), 19.13 (CH2CH3), 13.40
(CH3). Anal. Calcd. for C15H25N4I (388.29): C, 46.39; H, 6.48; N,
14.42. Found: C, 46.25; H, 6.52; N, 14.46.
All solvents were reagent grade, except THF which was dried
and distilled prior to use. Reactions with air- or moisture-sensitive
compounds were conducted under a nitrogen atmosphere using a
glove box or Schlenk techniques. Reagents were purchased from
commercial sources and used as supplied. NMR spectra were
recorded on a Bruker DPX 300 (1H, 300 MHz; 13C, 75.5 MHz).
Chemical shifts are described in parts per million downfield
shifted from SiMe4. UVevisible absorption and fluorescence
emission spectra were acquired using 2.5 ꢂ 10ꢁ6 M solutions in
CH2Cl2 under ambient conditions on a Cary 4000 UVeVis spec-
4.2.3. Synthesis of 1,3-dibutyl-5,6-diaminobenzimidazolium
tetrafluoroborate (5)
Imidazolium salt 4 (4.74 g, 12.2 mmol) was dissolved in meth-
anol (100 mL). An aqueous solution of Pb(BF4)2 (2.7 mL, 50% wt.,
6.1 mmol) was added to form a yellow precipitate immediately. The
mixture was stirred at room temperature for 2 h and filtered
through Celite. The filtrate was evaporated to give the product 5
trophotometer and
a
PTI QuantaMaster spectrofluorimeter,
(4.09 g, 96.2% yield). 1H NMR (d6-DMSO, 300 MHz):
d 9.13 (s, 1H,
respectively. Room temperature quantum yields were determined
C2H), 6.83 (s, 2H, ArH), 5.21 (s, 4H, NH2), 4.22 (t, 4H, J ¼ 7.1 Hz,
NCH2), 1.78 (p, 4H, J ¼ 7.3 Hz, NCH2CH2), 1.26 (sextet, 4 H, J ¼ 7.3 Hz,
CH2CH3),0.89 (t, 6H, J ¼ 7.3 Hz, CH3) 13C NMR (d6-DMSO, 75.5 MHz):
relative to a 2.5 ꢂ 10ꢁ6 M solution of riboflavin in water
(
F
¼ 0.230) [67,68], as described in Supplementary information.
Electrochemical experiments were conducted on an Epsilon
Electrochemical Workstation from BAsi using a three electrode cell
under an atmosphere of nitrogen at room temperature. The cell
was equipped with platinum working and counter-electrodes, as
well as a Ag/AgCl reference electrode containing aqueous 3 M KCl.
Measurements were performed in dry CH2Cl2 with 0.1 M [tetra-n-
butylammonium][PF6] as the electrolyte and decamethylferrocene
[Fc*] as the internal standard. All potentials were determined at a
100 mV sꢁ1 scan-rate and referenced to SCE by shifting [Fc*]0/þ
to ꢁ0.057 V [70].
d 137.51 (NCN), 136.06 (ArC), 124.36 (ArC), 94.65 (ArC), 46.28
(NCH2), 30.68 (NCH2CH2), 19.54 (CH2CH3), 13.81 (CH3). Anal. Calcd.
for C15H25N4BF4 (348.19): C, 51.74; H, 7.23; N,16.09. Found: C, 51.58;
H, 7.46; N, 15.99.
4.2.4. Synthesis of 1,3-dibutyldibenzo[a,c]phenazino[11,12-d]
imidazolium tetrafluoroborate (6)
A mixture of 9,10-phenanthrenequinone (0.770 g, 3.73 mmol)
and 5 (1.30 g, 3.73 mmol) in ethanol (90 mL) was heated for 24 h at
95 ꢀC. The reaction mixture was filtered and dried in vacuo to obtain
pure 6 (1.82 g, 94% yield). 1H NMR (d6-DMSO, 300 MHz):
d 10.17 (s,
4.2. Synthesis
1H, C2H), 9.19 (d, 2H, J ¼ 7.9 Hz, ArH), 9.00 (s, 2H, ArH), 8.75 (d, 2H,
J ¼ 8.0 Hz, ArH), 7.91 (t, 2H, J ¼ 7.5 Hz, ArH), 7.82 (t, 2H, J ¼ 7.5 Hz,
ArH), 4.66 (t, 4H, J ¼ 7.3 Hz, NCH2) 2.04 (p, 4H, J ¼ 7.4 Hz, NCH2CH2),
4.2.1. Synthesis of 1,3-dibutyl-5,6-dinitrobenzimidazolium iodide
(3)
1.48 (sextet, 4H, J ¼ 7.4 Hz, CH2CH3), 1.00 (t, 6H, J ¼ 7.3 Hz, CH3). 13
C
To
a
solution of 5,6-dinitro-benzimidazole [47] (6.00 g,
NMR (d6-DMSO, 75.5 MHz): d 148.31 (NCN), 142.52 (ArC), 138.49
28.8 mmol) in 120 mL acetonitrile were added 4.75 mL of a 6.25 M
aqueous NaOH solution. The reaction mixture was stirred at room
temperature for 30 min. After addition of butyl iodide (14.4 mL,
126 mmol), the reaction was stirred at room temperature for
another 25 min. The temperature was increased to 90 ꢀC, and the
reaction mixture was stirred at this temperature for 10 days. The
reaction progress was monitored by 1H NMR spectroscopy. The
solvent was then removed, and the residue was taken up in CH2Cl2
and filtered. The volatiles were removed from the filtrate and the
solid residue was further purified by trituration with ethyl acetate
to give 3 after filtration (11.51 g, 89% yield). 1H NMR (d6-DMSO,
(ArC), 133.10 (ArC), 131.89 (ArC), 131.59 (ArC), 129.01 (ArC), 128.56
(ArC), 125.77 (ArC), 123.86 (ArC), 112.21 (ArC), 47.01 (NCH2), 30.45
(NCH2CH2), 19.29 (CH2CH3), 13.56 (CH3). Anal. Calcd. for
C29H29N4BF4 (348.19): C, 66.93; H, 5.61; N,10.76. Found: C, 66.70; H,
5.49; N, 10.85.
4.2.5. Synthesis of (1,3-dibutyldibenzo[a,c]phenazino[11,12-d]
imidazole-2-thione (7)
To a mixture of imidazolium salt 6 (0.100 g, 0.19 mmol), NaH
(10 mg, 0.4 mmol), and S8 (10 mg, 0.31 mmol) was added under
inert conditions anhydrous THF (6 mL) and a catalytic amount of
DMSO. The reaction mixture was stirred at room temperature for
12 h. The mixture was evaporated and the residue was purified by
flash chromatography (silica, 7:4 hexane:CH2Cl2 mixture) to give 7
as a yellow solid (0.084 g, 94.3% yield). 1H NMR (CDCl3, 300 MHz):
300 MHz):
d 10.24 (s, 1H, C2H), 9.24 (s, 2H, ArH), 4.56 (t, 4H,
J ¼ 7.3 Hz, NCH2), 1.88 (p, 4H, J ¼ 7.4 Hz, NCH2CH2), 1.35 (sextet, 4H,
J ¼ 7.4 Hz, CH2CH3), 0.92 (t, 6H, J ¼ 7.43 Hz (CH3). 13C NMR (d6-
DMSO, 75.5 MHz):
d 148.55 (NCN), 140.53 (ArC), 132.58 (ArC),
113.24 (ArC), 47.69 (NCH2), 30.60 (NCH2CH2), 19.04 (CH2CH3), 13.45
(CH3). Anal. Calcd. for C15H21N4O4I (448.25): C, 40.19; H, 4.72; N,
12.49. Found: C, 40.09; H, 4.74; N, 12.48.
d
9.29 (d, 2H, J ¼ 7.8 Hz, ArH), 8.54 (d, 2H, J ¼ 7.4 Hz, ArH), 7.75 (s,
2H, ArH), 7.76 (m, 4H, ArH), 4.35 (t, 4H, J ¼ 7.3 Hz, NCH2), 1.90
(pentet, 4H, J ¼ 7.6 Hz, NCH2CH2), 1.53 (sextet, 4H, J ¼ 7.4 Hz,
CH2CH3), 1.04 (t, 6H, J ¼ 7.3 Hz, CH3). 13C NMR (CDCl3, 75.5 MHz):
4.2.2. Synthesis of 1,3-dibutyl-5,6-diaminobenzimidazolium iodide
(4)
Compound 3 (6.00 g, 13.4 mmol) was dissolved into methanol
(190 mL) under nitrogen to give an orange solution. This solution
was heated at 95 ꢀC and SnCl2 (41.5 g, 184 mmol) was added in
d 174.29 (N2CS), 141.21 (ArC), 139.47 (ArC), 135.61 (ArC), 131.84
(ArC), 130.34 (ArC), 130.20 (ArC), 128.04 (ArC), 126.02 (ArC), 123.13
(ArC),104.94 (ArC), 45.13 (NCH2), 29.86 (NCH2CH2), 20.53 (CH2CH3),
14.10 (CH3). Anal. Calcd. for C29H28N4S (464.64): C, 74.96; H, 6.07; N,
12.05. Found: C, 74.74; H, 6.22; N, 12.06.

140
D. Tapu et al. / Journal of Organometallic Chemistry 749 (2014) 134e141
4.2.6. Synthesis of enetetramine 8
4.2.9. Synthesis of (1,3-dibutyldibenzo[a,c]phenazino[11,12-d]
imidazolin-2-ylidene)iridium(CO)2 chloride (11)
To imidazolium salt 6 (0.050 g, 0.096 mmol) in anhydrous THF
(5 mL), a suspension of KOt-Bu (13 mg 0.12 mmol) in THF (2 mL)
was added dropwise under inert conditions. The reaction mixture
was stirred at room temperature for 2 h. The mixture was evapo-
rated and the residue, which showed five different spots by TLC,
was purified by flash chromatography (silica, CH2Cl2) to give 8 as a
Complex 10 (75 mg, 0.097 mmol) was dissolved in CH2Cl2
(10 mL) and placed under an atmosphere of CO(g) for 2 h. The sol-
vent was removed and the resulting solid was triturated with
hexane to remove residual COD. The remaining solid was dried in
vacuo to yield 11 as a light yellow powder (66 mg, 96% yield). 1H
yellow solid (0.011 g, 27% yield). 1H NMR (CDCl3, 300 MHz):
d
9.34
NMR (CDCl3, 300 MHz)
d
9.28 (d, 2H, J ¼ 7.8 Hz, ArH), 8.50 (d, 2H,
(d, 4H, J ¼ 6.9 Hz, ArH), 8.58 (d, 4H, J ¼ 7.1 Hz, ArH), 7.76 (m, 8H,
ArH), 7.64 (s, 4H, ArH), 4.00 (t, 8H, J ¼ 7.3 Hz, NCH2), 1.87 (pentet,
8H, J ¼ 7.5 Hz, NCH2CH2), 1.49 (sextet, 8H, J ¼ 7.5 Hz, CH2CH3), 1.03
J ¼ 7.8 Hz, ArH), 8.10 (s, 2H, ArH), 7.81 (t, 2H, J ¼ 7.2 Hz, ArH), 7.73 (t,
2H, J ¼ 7.2 Hz, ArH), 4.81 (dd, 1H, J ¼ 9.6, 6.3 Hz, NCH2), 4.87 (dd, 1H,
J ¼ 9.6, 6.4 Hz, NCH2) 4.64 (dd, 1H, J ¼ 9.6, 5.7 Hz, NCH2) 4.59 (dd,
1H, J ¼ 9.6, 5.7 Hz, NCH2) 2.10 (m, 4H, NCH2CH2), 1.60 (sextet, 4H,
J ¼ 7.1 Hz, NCH2CH3), 1.09 (t, 6H, J ¼ 7.35 Hz, CH3) 13C NMR (CDCl3,
(t, 12H, J ¼ 7.3 Hz, CH3). 13C NMR (CDCl3, 75.5 MHz):
d 155.21 (NCN),
140.39 (ArC), 139.78 (ArC), 134.51 (ArC), 131.74 (ArC), 130.63 (ArC),
129.83 (ArC), 127.98 (ArC), 125.84 (ArC), 123.12 (ArC), 103.74 (ArC),
41.65 (NCH2), 30.30 (NCH2CH2), 20.41 (CH2CH3), 13.99 (CH3). Anal.
Calcd. for C58H8H.56CH2Cl2 (950.05): C, 74.58; H, 6.15; N, 11.74.
Found: C, 74.22; H, 6.31; N, 11.43.
75.5 MHz)
d 191.01 (NCN) 180.65 (CO), 167.67 (CO), 142.11 (ArC),
137.85 (ArC), 135.47 (ArC), 131.64 (ArC), 130.34 (ArC), 129.38 (ArC),
127.63 (ArC), 125.84 (ArC), 122.62 (ArC), 108.03 (ArC), 48.88 (NCH2),
30.69 (NCH2CH2), 19.91 (CH2CH3), 13.45 (CH3). IR: nCO (cmꢁ1): 2078,
1982. Anal. Calcd. for C31H28N4O2IrCl (716.25): C, 51.98; H, 3.94; N,
7.82. Found: C, 51.81; H, 3.87; N, 7.73.
4.2.7. Synthesis of (1,3-dibutyldibenzo[a,c]phenazino[11,12-d]
imidazolin-2-ylidene)rhodium(1,5-cyclooctadiene) chloride (9)
To a mixture of imidazolium salt 6 (0.30 g, 0.57 mmol) and
[RhCl(COD)]2 (0.15 g, 0.30 mmol) in anhydrous THF (20 mL), a
suspension of KOt-Bu (86 mg, 0.76 mmol) in THF (6 mL) was
added dropwise under inert conditions. The reaction mixture was
stirred at room temperature for 12 h. The mixture was filtered
and the resulting solid was extracted with CH2Cl2 to remove
KBF4. The organic extract was evaporated to give 0.23 g of pure
product. Additional product was recovered from the initial
filtrate upon evaporation. The resulting solid was dissolved in a
small amount of CH2Cl2 and precipitated with hexane. The total
4.3. Catalytic studies (Table 4)
Rhodium or indium complexes 9 and 10 (1.0
mmol) and 1,3,5-
trimethoxybenzene (internal standard, 6.9 mg, 41.4
mmol) were
weighted into an NMR tube. Deuterated benzene (0.48 mL) was
added and the tube was inverted until the metal complex dissolved.
Diphenylsilane (21
mL, 0.112 mmol) and one of the acetophenones
11 (40
m
mol) were then added. The reaction was monitored by 1H
NMR spectroscopy. The yield of 13 was calculated by comparing the
integration values of the CH3 doublet and the CH quartet with that
of the OCH3 singlet of the internal standard, 1,3,5-
trimethohybenzene. The yield of 14 was calculated by comparing
the integration values of the two vinylic CH doublets with that of
the OCH3 singlet of the internal standard.
yield was 0.33 g (77% yield). 1H NMR (CDCl3, 300 MHz):
d 9.27 (d,
2H, J ¼ 7.8 Hz, ArH), 8.48 (d, 2H, J ¼ 7.4 Hz, ArH), 7.95 (s, 2H, ArH),
7.73 (m, 4H, ArH), 5.27 (bs, 2H, CHCOD), 5.07e4.90 (m, 4H NCH2),
3.52 (bs, 2H, CHCOD), 2.58e2.42 [m, 4H, (CH2)COD], 2.34 (m, 2H,
NCH2CH2), 1.96e2.16 [m, 4H (CH2)COD and 2H NCH2CH2], 1.68
(sextet, 4H, J ¼ 7.3 Hz, CH2CH3), 1.18 (t, 6H, J ¼ 7.3 Hz, CH3). 13C
Acknowledgments
NMR (CDCl3, 75.5 MHz):
d
208.2 (d, JRh-C ¼ 50.4 Hz), 142.20 (ArC),
138.32 (ArC), 137.16 (ArC), 132.12 (ArC), 130.56 (ArC), 130.32 (ArC),
128.17 (ArC), 126.25 (ArC), 123.21 (ArC), 106.58 (ArC), 101.75 (d,
JRh-C ¼ 6.35 Hz, CHCOD) 69.36 (d, JRh-C ¼ 14.28 Hz, CHCOD) 49.46
(NCH2), 33.20 (NCH2CH2), 31.44 ((CH2)COD), 29.01 ((CH2)COD),
20.90 (CH2CH3), 14.15 (CH3). Anal. Calcd. for C37H40N4RhCl
(679.1023): C, 65.43; H, 5.93; N, 8.25. Found: C, 65.27; H, 5.97; N,
8.28.
Acknowledgment is made to the Donors of the American
Chemical Society Petroleum Research Fund for support of this
research (Grant # 50126-UNI1). This work has been partially sup-
ported by internal grants from Kennesaw State University (Mentor
Protégé grant and 2011 KSU Foundation prize for publication). We
would like to thank Huggins Msimanga for invaluable electro-
chemical assistance and John Haseltine and Chris Alexander for
their insightful suggestions.
4.2.8. Synthesis of (1,3-dibutyldibenzo[a,c]phenazino[11,12-d]
imidazolin-2-ylidene)iridium(1,5-cyclooctadiene) chloride (10)
To a mixture of imidazolium salt 6 (0.30 g, 0.57 mmol) and
[IrCl(COD)]2 (0.193 g, 0.287 mmol) in anhydrous THF (30 mL), a
suspension of KOt-Bu (73 mg, 0.85 mmol) in dry THF (10 mL) was
added dropwise under inert conditions. The reaction mixture was
stirred at room temperature overnight. The mixture was then
evaporated and the residue was purified by flash chromatography
(silica, CH2Cl2, Rf ¼ 0.34) to give 10 as a yellow solid (0.35 g, 80%
Appendix A. Supplementary material
CCDC 899151 and 899152 contain the supplementary crystal-
lographic data for this paper. These data can be obtained free of
charge from The Cambridge Crystallographic Data Centre via www.
ccdc.cam.ac.uk/data_request/cif.
Appendix B. Supplementary data
yield). 1H NMR (CDCl3, 300 MHz):
d
9.37 (d, 2H, J ¼ 7.6 Hz, ArH), 8.56
(d, 2H, J ¼ 7.8 Hz, ArH), 7.78 (m, 4H, ArH), 4.99e4.77 (m, 4H NCH2
and 2H CHCOD), 3.13 (bs, 2H, CHCOD), 2.37e2.22 and 2.08e1.81 [two
multiplets, 6H each, (CH2)COD and NCH2CH2], 1.67 (sextet, 4H,
J ¼ 7.3 Hz, CH2CH3), 1.16 (t, 6H, J ¼ 7.3 Hz, CH3). 13C NMR (CDCl3,
Supplementary data related to this article can be found at http://
dx.doi.org/10.1016/j.jorganchem.2013.09.034.
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90.5 MHz): d 201.99 (NCN), 142.04 (ArC), 138.47 (ArC), 137.57 (ArC),
132.06 (ArC), 130.49 (ArC), 130.32 (ArC), 128.14 (ArC), 126.22 (ArC),
123.17 (ArC), 106.59 (ArC), 88.90 (CHCOD), 53.32(CHCOD), 49.02
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