Phenyldiquinolinylarsine as a Nitrogen-Arsenic-Nitrogen Pincer Ligand

Article abstract of DOI:10.1002/ejic.202000542

A nitrogen-arsenic-nitrogen (NAN) ligand, phenyldiquinolinylarsine (pdqa), was newly synthesized by utilizing diiodophenylarsine as a key precursor. The copper(I) halide (CuX, X = Cl, Br, I) and gold(I) chloride (AuCl) complexes of pdqa were synthesized and their structures were analyzed by NMR spectroscopy and X-ray crystallography. [CuX(pdqa)] formed pincer complexes, while only the arsenic atom coordinated to the Au^I ion in [AuCl(pdqa)]. Moreover, it was found that steric strain for forming the pincer coordination was relieved around the arsenic atom, which had less directional coordination, when comparing the structure of a CuI complex with bidentate ligand diphenylquinolinylarsine.

Full text of DOI:10.1002/ejic.202000542

European Journal of Inorganic Chemistry
Accepted Article
Title: Phenyldiquinolinylarsine as a nitrogen-arsenic-nitrogen pincer
ligand
Authors: Hyota Kihara, Susumu Tanaka, Hiroaki Imoto, and Kensuke
Naka
This manuscript has been accepted after peer review and appears as an
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To be cited as: Eur. J. Inorg. Chem. 10.1002/ejic.202000542
Link to VoR: https://doi.org/10.1002/ejic.202000542
01/2020

10.1002/ejic.202000542
European Journal of Inorganic Chemistry
COMMUNICATION
Phenyldiquinolinylarsine as a nitrogen-arsenic-nitrogen pincer
ligand
Hyota Kihara,[a] Susumu Tanaka,[a] Hiroaki Imoto,*[a,b] and Kensuke Naka*[a,b]
[a]
[b]
H. Kihara, Dr. S. Tanaka, Prof. Dr. H. Imoto, Prof. Dr. K. Naka
Faculty of Molecular Chemistry and Engineering, Graduate School of Science and Technology
Kyoto Institute of Technology
Goshokaido-cho, Matsugasaki, Sakyo-ku, Kyoto 606-8585, Japan
E-mail: himoto@kit.ac.jp (H.I.), kenaka@kit.ac.jp (K. N.)
Homepage: http://www.cis.kit.ac.jp/~kenaka/index_eng.html
Materials Innovation Lab, Kyoto Institute of Technology,
Goshokaido-cho, Matsugasaki, Sakyo-ku, Kyoto 606-8585, Japan.
Supporting information for this article is given via a link at the end of the document.
Abstract:
A
nitrogen-arsenic-nitrogen
(NAN)
ligand,
phenyldiquinolinylarsine (pdqa), was newly synthesized by utilizing
diiodophenylarsine as a key precursor. The copper(I) halide (CuX, X
= Cl, Br, I) and gold(I) chloride (AuCl) complexes of pdqa were
synthesized and their structures were analyzed by NMR spectroscopy
and X-ray crystallography. [CuX(pdqa)] formed pincer complexes,
while only the arsenic atom coordinated to the Au(I) ion in
[AuCl(pdqa)]. Moreover, it was found that steric strain for forming the
pincer coordination was relieved around the arsenic atom, which had
less directional coordination, when comparing the structure of a CuI
complex with bidentate ligand diphenylquinolinylarsine.
Development of novel ligands is
a pivotal process for
advancement of coordination chemistry. Numerous backbones
and elements have been investigated for superior catalysts,
luminophores, magnetic materials, redox systems, etc. Pincer
ligands are a particularly important class because the pincer-type
complexes can offer high stability and unique reactivity as
catalysts.[1,2] Nitrogen, phosphorus, and carbon atoms are
typically employed for the three coordination sites. For further
diversity of pincer ligands, various elements, e.g., boron[3] and
silicon,[4] have been introduced so far. Moreover, incorporation of
heavy elements is an emerging strategy toward new class of
pincer ligands.[5-7] Germylenes (Figure 1a)[6] and stannylenes
(Figure 1b),[7] being analogues of carbene, are used for
Figure 1. Representative examples of pincer complexes with heavy element-
containing ligands: (a) germanium, (b) tin, (c, d) arsenic.
Despite their promise, arsenic-containing pincer ligands have
been rarely studied to date. Nishibayashi and coworkers reported
an arsenic-nitrogen-arsenic (ANA) pincer ligand as analogues of
the phosphorus-nitrogen-phosphorus one (Figure 1c).[8]
Ruthenium complexes with the ANA ligand can work as a catalyst
with high selectivity for dehydrogenation transformations.[8b] On
the other hand, arsenic-centered pincer ligands, e.g., nitrogen-
arsenic-nitrogen (NAN) type, are unexplored. Phosphorus
analogues, NPN type, have been employed in coordination
chemistry.[9] For example, a high-spin iron ylide complex is
supported by an NPN ligand to proceed alkylidene-transfer
reactions. The preceding NPN chemistry has motivated us to
explore NAN ligands.[9e] Although Wild and coworkers
synthesized bis(6-methylquinolin-8-yl)phenylarsine and tris(6-
methylquinolin-8-yl)arsine as multidentate arsenic ligands, the
pincer type complexes were not examined.[10]
phosphorus-germanium-phosphorus
and
phosphorus-tin-
phosphorus pincer ligands, respectively. Such carbene-like
species require intramolecular donor-stabilization as represented
by N-heterocyclic carbenes. As more simple ligand design, heavy
element-containing pincer ligands can be constructed by using
group 15 elements such as arsenic, antimony, and bismuth.
Among them, arsenic is favorable because arsenic(III) has
sufficient stability against inversion, oxidation, and decomposition.
In comparison, nitrogen has low inversion barrier to cause
racemization at room temperature, phosphorus is easily oxidized
to be pentavalent state in air, and molecular design for
organoantimony and organobismuth compounds is restricted due
to relatively unstable Sb−C and Bi−C bonds.
The major challenge for the development of NAN ligands lies in
a lack of synthetic methods for organoarsenic compounds.
Traditional As−C bond formation reactions need volatile and toxic
precursors such as arsenic chlorides and hydrides. Serious safety
concerns have kept researchers away from experimental studies
on organoarsenic-related chemistry. To get over the barrier, we
1
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10.1002/ejic.202000542
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have developed various synthetic routes towards organoarsenic
compounds by utilizing non-volatile arsenic precursors.[11] The
key materials are cyclooligoarsines, which can be prepared from
inorganic arsenic compounds.[12] Cleavage of the As−C bonds
generates reactive species, i.e., radicals, electrophiles, and
nucleophiles, in situ. In particular, diiodoarsines (RAsI2) have
given various kinds of functional organoarsenic compounds;
RAsI2 can be easily prepared by only mixing cyclooligoarsine with
iodine (I2).[13] In this work, we newly synthesized a NAN ligand, as
a novel class of pincer ligands (Figure 1d). The copper(I) halide
(CuX, X = Cl, Br, I) and gold(I) chloride (AuCl) complexes of pdqa
were examined to understand the coordination behaviors.
The target NAN ligand pdqa was synthesized according to
Scheme 1. A tetrahydrofuran (THF) solution of diiodophenylarsine
(PhAsI2) was prepared by our reported protocol.[13] 8-
Bromoquinoline was lithiated by sec-butyllithium (s-BuLi), and
then the THF solution of PhAsI2 was added to obtain pdqa. The
chemical structure of pdqa was determined by NMR
spectroscopy, high resolution mass analysis, and X-ray
crystallography.[14] The solid sample of pdqa was quite stable
Figure 2. 1H-NMR spectra (400 MHz in CDCl3) of bare ligand pdqa and
complexes [CuX(pdqa)] (X = Cl, Br, I) and [AuCl(pdqa)]. The protons at 2-, 4-,
5-, and 7-positions of the qunolinyl group are defined as H(a), H(b), H(c), and
H(d).
1
under ambient condition; negligible change was observed in H-
NMR even after storage for more than 2 months. This is because
the arsenic(III) atom has sufficient oxidative resistance. Hydrogen
peroxide (H2O2), not oxygen in air, was necessary (for detail, see
supporting information).
For further study on the structures, the molecular structures
were determined by single crystal X-ray diffraction. Single crystals
suitable for the analysis were obtained for [CuBr(pdqa)],
[CuI(pdqa)], and [AuCl(pdqa)] by recrystallization from
CH2Cl2/methanol or CH2Cl2/hexane at room temperature. Though
the result of [CuBr(pdqa)] indicated the presence of two
conformations, one conformer is discussed here; there is no
significant conformational difference between the two conformers
(for the data of the other, see supporting information). Initially, it
was confirmed that pdqa worked as NAN-pincer and As-
monodentate ligands for Cu(I) and Au(I) ions, respectively. This
result well corresponds to the observation for the NMR
spectroscopy shown above. The Cu(I) ions of [CuX(pdqa)] (X =Br,
I) adopted distorted tetrahedral structures in reference to the
As−Cu−X, N−Cu−X, N−Cu−As, and N−Cu−X bond angles. The
sum of the N−Cu−As and N−Cu−X angles were 402.8° (X = Br)
and 393.5° (X = I), which are far from that of a regular tetrahedron
(ca 438°). This suggests that the tetrahedral metal centers were
planarized by the NAN pincer backbone. On the other hand, the
Au(I) ion possessed a linear coordination form considering the
As−Au−Cl bond angle (174.60(7)°). In addition, it was found that
the As−Cu bond length of [CuBr(pdqa)] (2.374(2) Å) was slightly
shorter than that of [CuI(pdqa)] (2.403(1) Å). This is probably
because the trans-influence of the former is weaker than that of
the latter due to the weaker σ-donation of the bromide ion.
Scheme 1. Synthesis of NAN ligand pdqa.
The NAN ligand pdqa was mixed with CuX (X = Cl, Br, I) and
AuCl to produce complexes [CuX(pdqa)] (Scheme S1) and
[AuCl(pdqa)] (Scheme S2), respectively. NMR spectra were
measured for structural analysis of the obtained complexes in
solutions. We focused on some of the signals due to the quinolinyl
group; the protons at 2-, 4-, 5-, and 7-positions are defined as
H(a), H(b), H(c), and H(d) (Figure 2). The changes of the chemical
shifts after the coordination were dependent on the metals. This
means that coordination forms were different between
[CuX(pdqa)] and [AuCl(pdqa)], considering that the electron
density of the protons near coordination sites should be lowered
to cause downfield shift. The signals due to the pyridine moiety
(H(a) and H(b)) of [CuX(pdqa)] were significantly shifted to
downfield, while those of the [AuCl(pdqa)] showed negligible shift.
On the other hand, the signals due to the benzene ring were
shifted to downfield in all the complexes. These results strongly
support the idea that the Cu(I) ions coordinated to the arsenic and
nitrogen atoms, whereas the Au(I) ion coordinated to only the
arsenic atom. Au(I) ion in general prefers coordinate number of
two, and soft Lewis basicity of arsenic gave strong interaction
between the arsenic atom and gold(I) ion, compared with the
quinoline moieties having harder Lewis basicity.
2
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10.1002/ejic.202000542
European Journal of Inorganic Chemistry
COMMUNICATION
Figure 3. ORTEPs (top: top view, bottom: side view) of (a) [CuBr(pdqa)], (b)
[CuI(pdqa)], and (c) [AuCl(pdqa)]. CPh−As−M bond angles are posted (below).
Hydrogen atoms were omitted for clarity. Thermal ellipsoids are drawn at the
50% probability level. [CuBr(pdqa)] had two conformations, but one conformer
displayed here (for the other, see supporting information).
147.7°) even in the optimized structures under vacuum condition,
suggesting that the distorted structures are not attributed by
molecular packing in the crystalline states. These results mean
that the coordination direction of the arsenic atoms was flexibly
adjusted to form pincer complex structures. In general, a lone pair
at arsenic has a higher degree of s-character when compared to
a lone pair at phosphorus, thus less directional orbital overlap
leading to coordination is possible with arsenic compounds.[16]
The Wiberg bond indices (WBIs)[17] of the As−Cu bonds of
[CuBr(pdqa)] and [CuI(pdqa)] in the crystals were estimated by
natural bond orbital (NBO) analysis using DFT calculations
(B3LYP/6-31G(d) for C, H, N and SDD for As, Cu, Br, I). The
results (WBI = 0.45 ([CuBr(pdqa)]) and 0.43 ([CuBr(pdqa)]))
indecated that the As and Cu had relatively weak orbital overlap,
considering the P−Cu bond of the reported NPN pincer complex[9f]
had WBI of 0.51. We thus assumed that arsenic atom is beneficial
for construction of pincer ligands because it can mitigate structural
strains in the resulting complexes.
To understand the structural features of the arsenic-centered
pincer coordination, bidentate ligand, diphenylquinolinylarsine
(dpqa), was synthesized by nucleophilic substitution of 8-
bromoquinoline with diphenylarsenolithium[18] (Scheme 2), and
then the CuI complex of dpqa was prepared (Scheme S3). The
X-ray crystallography revealed that the CuI complex formed
iodide-bridged dinuclear rhombic structure, and we thus defined
it as [Cu2I2(dpqa)2] (Figures 4a and 4b). This result indicates that
pincer ligand pdqa inhibited cluster formation, which are typically
constructed for CuI complexes,[19] by strict encapsulation of the
Cu(I) center. We focused on the bond angles around the arsenic
atoms (Figures 4c and 4d). The CQu−As−Cu bond angles of
[Cu2I2(dpqa)2] were 94.3(2)° and 95.5(2)°, similar to those of
[CuI(pdqa)] (95.2(1)° and 93.7(1)°). On the other hand, the
CPh−As−Cu bond angles of [Cu2I2(dpqa)2] (118.9(2)-131.6(2)°)
were much smaller than that of [CuI(pdqa)] (155.9(1)°). The large
bond angle of [CuI(pdqa)] strongly supports the idea that the
distortion attributed to the pincer coordination was concentrated
around the arsenic atom, which flexibly mitigated it.
Interestingly, the bond angles around the arsenic atoms were
highly distorted in [CuX(pdqa)] compared with those of
[AuCl(pdqa)]. The carbon atoms attached to the arsenic atoms
are defined as CPh (phenyl group) and CQu (quinolinyl group). The
conformation of the arsenic atom of [AuCl(pdqa)] was a nearly
regular tetrahedron because the bond angles around the arsenic
atom were 101.6(4)°-120.1(3)°. In contrast, the CPh−As−Cu and
CQu−As−Cu bond angles of [CuX(pdqa)] had wide ranges, i.e.,
94.8(2)°-152.9(2)° (X = Br) and 93.7(1)°-155.9(1)° (X = I), while
the CPh−As−CQu and CQu−As−CQu ones were close to 109.5°.
Particularly, CPh−As−Cu bond angles were exceptionally large
(vide infra); 152.9(2)° and 155.9(1)° for [CuBr(pdqa)] and
[CuI(pdqa)], respectively. Actually, in the case of a CuI complex
with an NPN pincer ligand, the corresponding CPh−P−Cu bond
angle was reported to be 130.9(1)°.[9f] In addition, it is notable that
the C−N−Cu bond angles were approximately 120°, suggesting
that the coordination bonds around the nitrogen atoms were not
distorted by the pincer coordination. Density functional theory
(DFT) calculations were conducted to optimize the full-geometry
of [CuBr(pdqa)] and [CuI(pdqa)] (B3LYP/6-31G(d) for C, H, N
and SDD for As, Cu, Br, I).[15] The estimated CPh−As−Cu bond
angles were still large ([CuBr(pdqa)]: 151.2° and [CuI(pdqa)]:
Table 1. Selected bond lengths and angles of [CuBr(pdqa)], [CuI(pdqa)],
and [AuCl(pdqa)].[a]
[CuBr(pdqa)]
2.374(2)
[CuI(pdqa)]
2.403(1)
[AuCl(pdqa)]
2.343(1)
M−As
M−X
Bond
length
[Å]
2.356(2)
2.530(1)
2.298(3)
2.041(5)
2.040(5)
2.056(3)
2.055(3)
Cu−N
-
As−M−X
138.67(4)
143.64(3)
174.60(7)
-
86.6(2)
86.7(2)
86.1(1)
85.3(1)
N−Cu−As
115.1(2)
114.4(2)
111.5(1)
110.6(1)
N−Cu−X
-
N−Cu−N
110.3(2)
152.9(2)
118.9(1)
155.9(1)
-
Scheme 2. Synthesis of bidentate ligand dpqa.
CPh−As−M
115.1(3)
Bond
angle
[°]
94.8(2)
94.8(2)
95.2(1)
93.7(1)
110.0(3)
120.1(3)
CQu−As−M
100.0(3)
104.0(3)
101.0(2)
100.1(2)
105.8(4)
102.6(4)
CPh−As−CQu
CQu−As− CQu
102.1(3)
102.2(2)
101.6(4)
119.8(4)
121.8(4)
122.2(4)
120.7(4)
119.3(3)
123.0(3)
120.0(3)
119.3(3)
C−N−Cu
-
[a] M = Cu or Au, X = Cl, Br, or I.
3
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Figure 4. (a) Chemical structure and (b) ORTEPs of [Cu2I2(dpqa)2]. Hydrogen
atoms were omitted for clarity. Thermal ellipsoids are drawn at the 50%
probability level. Bonds around arsenic atoms are depicted for (c) [CuI(pdqa)]
and (d) [Cu2I2(dpqa)2] based on the results of X-ray crystallography.
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data are provided free of charge by the joint Cambridge Crystallographic
Data Centre and Fachinformationszentrum Karlsruhe Access Structures
service www.ccdc.cam.ac.uk/structures.
In conclusion, we have newly developed NAN-pincer ligand
phenyldiquinolinylarsine (pdqa). The ligand formed pincer
coordination with CuX (X = Cl, Br, I), while only the arsenic atom
was selectively interacted to an Au(I) center for AuCl. It is notable
that the distortion attributed to the pincer coordination was
relieved by the less directional coordination of the arsenic atom.
We are now investigating complexes containing pdqa and related
ligands in catalysis. This new class of pincer ligands, giving
unique structures, will contribute to the advancement of
coordination chemistry.
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Acknowledgements
This work was supported by JSPS KAKENHI, grant Number
19H04577 (Coordination Asymmetry) and
in-Aid for Scientific Research (B)) to HI.
20H02812 (Grant-
[16] W. Levason, G. Reid, Comprehensive Coordination Chemistry II (Eds.: J.
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Keywords: arsenic • pincer ligand • copper halide • gold chloride
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4
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Entry for the Table of Contents
In the present work, phenyldiquinolinylarsine has been synthesized as the first example of a nitrogen-arsenic-nitrogen (NAN) pincer
ligand. It was demonstrated that copper halide complexes of the NAN ligand formed pincer coordination. Importantly, steric strain
attributed to the pincer coordination was relieved by the less directional coordination of the arsenic atom.
5
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