Synthesis and application of new dipyrido-annulated N-heterocyclic carbene with phosphorus substituents

Article abstract of DOI:10.1246/cl.131074

A new dipyrido-annulated N-heterocyclic carbene with phosphorus substituents at the 4 and 8 positions and the corresponding Rh(I) and Rh(III) complexes prepared from the carbene are reported. The X-ray structure of the Rh(I) complex 11 suggested the presence of strong interactions between the phosphorus atoms and Rh(I), in contrast to the previously reported sulfur-substituted system. The new carbene ligand shows promise as a useful carbene-centered pincer ligand with a fixed scaffold for other metal complexes.

Full text of DOI:10.1246/cl.131074

CL-131074
Received: November 15, 2013 | Accepted: December 14, 2013 | Web Released: December 20, 2013
Synthesis and Application of New Dipyrido-annulated N-Heterocyclic Carbene
with Phosphorus Substituents
Shin-ichi Fuku-en, Junki Yamamoto, Satoshi Kojima, and Yohsuke Yamamoto*
Department of Chemistry, Graduate School of Science, Hiroshima University,
1-3-1 Kagamiyama, Higashi-Hiroshima, Hiroshima 739-8526
(E-mail: yyama@sci.hiroshima-u.ac.jp)
A new dipyrido-annulated N-heterocyclic carbene with
rus substituents at the 4 and 8 positions, thereby providing the
first carbene-centered phosphorus-flanking tridentate ligand with
a rigid backbone. RhI and RhIII derivatives are also reported.
Imidazolium salt 10, the precursor of carbene 1b, was
prepared as shown in Scheme 2. Electrophilic addition of
Ph2PCl to 4, generated from 3, followed by methylation afforded
9. Because crude 9 was sufficiently pure after workup, it was
used for the next step without further purification. To our
surprise, desulfurization did not proceed at all by the use of the
procedure (1.1 equiv of MeMgBr in THF) successful for 5a.
Reaction with MeLi, t-BuLi, and LiAlH4 were equally unsat-
isfactory, resulting in either no reaction or a complex mixture.
However, NaBH4 in MeOH afforded the expected 10 in 59%
yield under the optimized conditions after treatment with
saturated aqueous NH4I (Scheme 2).9 Although 10 gradually
underwent oxidation under aerobic conditions, it was thermally
stable and could be purified by recrystallization from acetone.
The reaction of 10 with LiHMDS in deuterated solvents
such as THF-d8 or C6D6 resulted in a color change from a yellow
suspension to a clear brown solution, suggesting the formation
phosphorus substituents at the 4 and 8 positions and the
corresponding Rh(I) and Rh(III) complexes prepared from the
carbene are reported. The X-ray structure of the Rh(I) complex
11 suggested the presence of strong interactions between the
phosphorus atoms and Rh(I), in contrast to the previously
reported sulfur-substituted system. The new carbene ligand
shows promise as a useful carbene-centered pincer ligand with a
fixed scaffold for other metal complexes.
The versatility and efficiency of N-heterocyclic carbenes
(NHCs) as ligands for transition-metal catalysts and low-valent
main-group element species have helped establish them as
indispensable entities.1,2 Thus, it is no surprise that the chemistry
of pincer ligands with carbene centers has become a growing
research field.3-6 Among these tridentate ligands, however, there
was no precedence of those with a rigid backbone until our
recently disclosed 1a (R = SPh).5b Our method for introducing
SPh groups to the ortho positions involved the sequence shown
in Scheme 1 starting from previously known 2.7 The carbene 1a
could be generated by treating the precursor 6a with a base such
as NaHMDS (HMDS: hexamethyldisilazane), and the corre-
sponding Rh complexes were prepared thereof. It is interesting
to note that the ligand could serve either as a monodentate 7 or a
tridentate 8 based on the oxidation state of the central Rh moiety.
Phosphorus-based ligands have been most widely utilized for
late-transition metals; therefore, for wider application as pincer
ligands for transition metals, phosphorus-substituted carbene
ligands are highly desirable.8 Herein, we report on the
preparation of a new dipyrido-annulated NHC 1b with phospho-
1
of carbene 1b. This was confirmed by H NMR measurements
that revealed that the imidazolium proton of 10 at 8.73 ppm
in CDCl3 was absent. The poor solubility of 1b hampered
13C NMR measurements and the carbenic 13C chemical shift
could not be determined. The trapping of in situ generated 1b
with [Rh(cod)Cl]2 provided indirect support for the generation
of 1b by affording 11, a species gradually oxidized under
aerobic conditions. Sequential one-pot treatment of 11 with I2
afforded the expected RhIII complex [Rh(1b)I3] (12) as an air-
stable species (Scheme 3).
The identity of 11 was established by X-ray diffraction
analysis (Figure 1). In stark contrast to the corresponding RhI
complex 7 of the sulfur series, the 1,5-cyclooctadiene (COD)
Br–
S
H
S
Li
Li
1) t-BuOK
2) S8
N
N
N
N
N
N
t-BuLi
I–
PPh2
I–
PPh2
Me
N
S
H
t-Bu
t-Bu
t-Bu
t-Bu
t-Bu
t-Bu
Ph2P
Ph2P
3
1) Ph2PCl
2) MeI
1) NaBH4 / MeOH
2) NH4l aq.
2
4
N
N
N
I–
SPh
Me
N
I–
SPh
4
S
H
PhS
PhS
t-Bu
t-Bu
t-Bu
t-Bu
1) MeMgBr
2) NH4Cl aq.
1) PhSSPh
2) MeI
9
10
N
N
N
t-Bu
t-Bu
t-Bu
t-Bu
Scheme 2. Preparation of phosphorus-substituted imidazolium
salt 10.
5
6
I
l
I
l
R
R
I
I
Rh
Rh
SPh
SPh
N
PhS
PhS
N
I
l
Rh
Rh
N
N
N
N
PPh2
PPh2
Ph2P
Ph2P
t-Bu
t-Bu
[Rh(cod)Cl]2
I2
LiHMDS
N
N
N
N
1b
t-Bu
t-Bu
10
t-Bu
t-Bu
1a ; R = SPh
1b ; R = PPh2
7
8
t-Bu
t-Bu
t-Bu
t-Bu
11
12
Scheme 1. Procedure for the introduction of substituents at the
1 and 8 positions.
Scheme 3. Generation of the carbene and introduction of Rh.
468 | Chem. Lett. 2014, 43, 468–470 | doi:10.1246/cl.131074
© 2014 The Chemical Society of Japan

Figure 2. ORTEP drawing of 12 with the thermal ellipsoids
shown at the 50% probability level. All hydrogen atoms and the
solvent molecule (dichloromethane) are omitted for clarity.
Selected bond lengths (¡) and angles (deg): Rh1-I1 =
2.6574(7), Rh1-I2 = 2.7240(7), Rh1-I3 = 2.6981(7), Rh1-
P1 = 2.379(1), Rh1-P2 = 2.396(1), Rh1-C5 = 1.904(4). P1-
C4-N1 = 109.3(3), P2-C10-N2 = 109.4(3).
Figure 1. ORTEP drawing of 11 with the thermal ellipsoids
shown at the 50% probability level. All hydrogen atoms and the
solvent molecule (THF) are omitted for clarity. Selected bond
lengths (¡) and angles (deg): Rh1-I1 = 2.6913(14), Rh1-P1 =
2.293(2), Rh1-P2 = 2.273(2), Rh1-C5 = 1.905(6), P1-C4-
N1 = 107.6(5), P2-C10-N2 = 108.0(5).
moiety is used up, thereby indicating that the coordination
scheme has completely changed to incorporate the flanking
phosphine substituents, whereas the sulfur substituents in 7 were
innocent bystanders. This significant difference should be due
to the stronger coordinating ability of the phosphorus atom
compared with the corresponding sulfur atom. This alteration is
also reflected in the distances between the phosphorus atoms and
RhI in 11 that were 2.273(2) and 2.293(2) ¡, while the
corresponding distances between the sulfur atoms and RhI in 7
were as long as 3.056(3) ¡ on average. The distances of 11
are comparable with the P-Rh distances of analogous flexible
tridentate systems, an o-phenylene-bridged NHC diphosphine
[(i-Pr2PCPi-Pr2)RhICl] complex 13a8z with values of 2.2910(13)
and 2.3042(14) ¡, and a 2,3-dihydroperimidine-cored NHC
diphosphine [(Ph2PCPPh2)RhICl] complex 13b8ab with values of
2.247(1) and 2.2545(1) ¡. The 31P NMR of 11 shows one sharp
signal, suggesting that the complex is symmetric in solution, as
one might expect. The low-field chemical shift of 49.5 ppm is
also indicative of the strong coordination of the phosphorus
atoms, and the coupling constant of 1JRh-P = 157 Hz is similar to
that of 13b (¤P = 22.9, 1JRh-P = 153 Hz), suggesting that the
interatomic interaction is approximately the same, regardless of
whether the phosphorus atoms are fixed or not.
As for the structure of 12 (Figure 2), the distances between
the phosphorus atoms and RhIII were 2.379(1) and 2.396(1) ¡.
Here again, the distances are comparable with those of an
analogous flexible tridentate system, an ethyl-bridged NHC
diphosphine [(Ph2PCPPh2)RhIIICl3] complex (13c)8k with values
of 2.3714(18) and 2.3589(19) ¡. Although the distances of 12
are similar to those between the sulfur atoms and RhIII in 8
(2.386(3) ¡), the interaction between P and Rh in 12 should be
stronger based on the difference in the covalent radii between S
(1.04 ¡) and P (1.10 ¡).10 Furthermore, the Rh-I2 distance
(2.7240(7) ¡) in 12 is slightly longer than that of 8 (2.712(2) ¡);
other bonds around the Rh atom show similar elongations. In
addition, the deviation from the ideal value of 120° of the P1-
C4-N1 (109.3(3)°) and P2-C10-N2 (109.4(3)°) angles in 12 are
larger than those of the corresponding angles (111.5(6)°) of 8,
which is consistent with the discussion on coordination strength
between S and P. The coupling constant of 1JRh-P = 92 Hz is also
1
similar to that of 13c (¤P = 1.7, JRh-P = 89 Hz), again suggest-
ing comparable interaction. The resemblance in bond lengths
(from X-ray structures) and the implicated similarity of
coordination strength (from NMR) for both RhI and RhIII
(which can be considered as thermodynamically stable model
species of a catalytic cycle involving this ligand system) with
the corresponding flexible tridentate systems is an indication that
although our ligand system is rigid, it may perhaps retain
sufficient adjustable nature required for efficient catalysis.6
In conclusion, we have developed a method for the
preparation of a new dipyrido-annulated NHC carbene with
phosphorus substituents at the 4 and 8 positions. The corre-
sponding Rh complexes from the carbene are also reported,
where the ligand serves as a tridentate for both RhI and RhIII, in
contrast to our previously reported sulfur-substituted complexes.
The features of the complexes suggest that our new ligand is
promising for transition-metal catalysis. The introduction of
other transition metals and low-valent main-group elements, in
addition to the preparation of other ortho-substituted derivatives,
is in progress.11
This work was supported by a Grant-in-Aid for Science
Research on Innovative Areas (No. 24109002, Stimuli-respon-
sive Chemical Species) from the Ministry of Education, Culture,
Sports, Science and Technology, Japan. Fuku-en, S.-i. acknowl-
edges a JSPS Fellowship for Young Scientists. We thank Dr.
Kajiya and Ms. Amimoto of Natural Science Center for Basic
Research and Development (N-BARD), Hiroshima University
for high-resolution mass spectrometry measurements.
Chem. Lett. 2014, 43, 468–470 | doi:10.1246/cl.131074
© 2014 The Chemical Society of Japan | 469

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470 | Chem. Lett. 2014, 43, 468–470 | doi:10.1246/cl.131074
© 2014 The Chemical Society of Japan