Development of unique dianionic Ir(III) CCC pincer complexes with a favourable spirocyclic NHC framework

Article abstract of DOI:10.1007/s11426-020-9875-6

A new type of dianionic Ir(III) CCC pincer complexes (SNHC-Ir, 1a-1c) is successfully designed and synthesized by developing a one-step methodology, which involves an initial coordination of Ir(I) with the NHC and subsequent metallation of double sp2C- H bonds. This method is considerably useful over those reported by using strong coordination ligand or carbonic anion exchange, and would provide an alternative efficient template of organometallics synthesis. Experimental and density functional theory (DFT) calculation results indicate that the spirocyclic framework is a favourable factor for the facile formation and stabilization of these complexes. Primary investigation shows that chloride 1b can well catalyze homo and hetero addition of styrene derivatives and remote olefin isomerization, which represents the first catalytic application of the dianionic CCC pincer complexes.

Full text of DOI:10.1007/s11426-020-9875-6

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Development of unique dianionic Ir(III) CCC pincer complexes
with a favourable spirocyclic NHC framework
Zhi-Bo Yan1†, Kun-Long Dai1†, Bin-Miao Yang1, Zi-Hao Li2, Yong-Qiang Tu1,2*
,
Fu-Min Zhang1, Xiao-Ming Zhang1, Meng Peng1, Qi-Long Chen1 & Ze-Ran Jing1
1State Key Laboratory of Applied Organic Chemistry & College of Chemistry and Chemical Engineering,
Lanzhou University, Lanzhou 730000, China;
2School of Chemistry and Chemical Engineering, Frontiers Science Center for Transformative Molecules,
Shanghai Jiao Tong University, Shanghai 200240, China
Received September 7, 2020; accepted September 14, 2020; published online October 21, 2020
A new type of dianionic Ir(III) CCC pincer complexes (SNHC-Ir, 1a–1c) is successfully designed and synthesized by developing
a one-step methodology, which involves an initial coordination of Ir(I) with the NHC and subsequent metallation of double sp2C–
H bonds. This method is considerably useful over those reported by using strong coordination ligand or carbonic anion exchange,
and would provide an alternative efficient template of organometallics synthesis. Experimental and density functional theory
(DFT) calculation results indicate that the spirocyclic framework is a favourable factor for the facile formation and stabilization
of these complexes. Primary investigation shows that chloride 1b can well catalyze homo and hetero addition of styrene
derivatives and remote olefin isomerization, which represents the first catalytic application of the dianionic CCC pincer
complexes.
spirocyclic NHC framework, dianionic CCC pincer complexes, iridium-catalyzed, C–H activation
Citation:
Yan ZB, Dai KL, Yang BM, Li ZH, Tu YQ, Zhang FM, Zhang XM, Peng M, Chen QL, Jing ZR. Development of unique dianionic Ir(III) CCC pincer
complexes with a favourable spirocyclic NHC framework. Sci China Chem, 2020, 63, https://doi.org/10.1007/s11426-020-9875-6
Over the past decades, a number of pincer complexes have
been explored due to their novel reactivities and potential
applications in catalytic chemistry [1]. For a certain central
metal atom (M) of a pincer complex, its chemical reactivity
largely relies on the chelation mode, the nature of three co-
ordination atoms (W and 2×Z) around M and ligand back-
bone [1a,2a]. Among the pincer complexes developed so far,
most members fall into the category of type A (Figure 1),
which have two or three heteroatomic (N, O, P) coordination
sites [2]. Hitherto, however, only sporadic examples have
been reported to possess all three carbon centers as co-
ordination atoms (‘CCC’ pincer complexes), and even fewer
contained two or more carbon-M covalent bonds (di- or
multi-anionic complexes) [3]. An earlier example of dia-
nionic CCC pincer complexes was reported by Cavell and
co-workers in 2005 [4], which requires the strong electron
donating ligands and the remove of the cod ligand to facil-
itate C–H bond metallation (type B). To our best knowledge,
the catalytic activities of these dianionic CCC pincer com-
plexes have not been investigated.
The potential versatile reactivities of CCC pincer com-
plexes, especially those with two or more carbon-M covalent
bonds have spurred our research interests. Initially, we syn-
−
thesized the BF4 salt with the typical N,N-diphenyl 1,3-
hexahydro-pyrimidine and 1,3-imidazole carbene, but failed
to obtain the expected pincer complexes after treatment with
several metal species [Ir(cod)Cl]2, [Rh(cod)Cl]2, Pd(cod)Cl2
†These authors contributed equally to this work.
*Corresponding author (email: tuyq@lzu.edu.cn)
© Science China Press and Springer-Verlag GmbH Germany, part of Springer Nature 2020
chem.scichina.com link.springer.com

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Yan et al. Sci China Chem
Figure 1 Representative cyclometalated pincer complexes (color online).
or Ni(cod)2 using a general procedure [5]. In most cases, only
the mono NHC-ligated products were obtained.
In connection with our long-standing interest in spirocyclic
frameworks [6], we envisioned that the presence of such a
structural moiety with a rigid quaternary carbon center in
NHC ligand would probably exert spatial constraints to fa-
cilitate the formation of a stabilized dianionic CCC pincer
complex [7]. Consequently, a six- membered cyclic carbene
BF4− salt S2 with a spirocyclic skeleton was prepared (Figure
1(c)). While S2 was alkalized with lithium hexamethyldisi-
lazide (LiHMDS) followed by treatment with transition-
metal salts [Rh(cod)Cl]2, Au(Me2-S)Cl, or CuCl, the reaction
still gave the simple NHC-ligated complex [5]. Pleasingly,
when [Ir(cod)Cl]2 was used, an expected dianionic CCC
pincer complex 1a was isolated in about 20% yield. This
complex was possibly formed through a tandem NHC→M
coordination and subsequent double consecutive sp2C–H
metallation. Thereafter, we have made a great effort to op-
timize and extend this preparation process. Herein, we pre-
sent our synthetic procedures of dianionic Ir(III) CCC pincer
complexes 1a–1c and investigation of their primary proper-
ties.
The racemic cis,cis-spirodiamine S1 was treated with
NH4BF4 in CH(OEt)3 in the presence of formic acid (0.1
equiv.) over 4 h at 120 °C, affording the carbene BF4− salt S2
in 95% yield. Subsequently, S2 was alkalized and treated
with [Ir(cod)Cl]2 at room temperature would afford the ex-
pected pincer complex 1a in 91% yield after purification by
flash column chromatography. The other two derivatives 1b
and 1c were easily obtained by treatment of a solution of 1a
with K2CO3 in CHCl3 or 1b with AgOTf in CH2Cl2, re-
spectively (Scheme 1). It was notable that complex 1a was
Scheme 1 Synthesis of pincer complexes 1a–1c (color online).
bench stable for at least two weeks. In comparison, 1b and 1c
were much more stable than 1a and did not decompose even
at their melting points in air. The structures of 1a–1c were
confirmed by X-ray diffraction analysis (Section 3 in Sup-
porting Information online).
As shown in the crystal structure of 1a (Figure 2), the bond
length (1.967(8) Å) of Ir(1)–C(1) was within the normal
range of NHC–Ir coordination bonds [8]. And the bond
lengths (2.118(8) and 2.123(9) Å) of Ir(1)–C(12) and Ir(1)–C
(22), respectively, indicated the formation of two sp2C–Ir
covalent bonds [9]. These evidences confirmed the presence
of the central CCC pincer chelation mode. Furthermore, the
hydrogen nuclear magnetic resonance (1H NMR, singlet at
−10.58 ppm), high-resolution mass spectra (HRMS, m/z:
639.2327, calcd. for [C30H35N2Ir+Na]+), and infrared spectra
(IR, ν 2,085 cm−1) also suggested the existence of Ir–H in 1a.
Of note, instead of a typical chair conformation, the hex-
ahydro-pyrimidine NHC moiety showed an unusual envel-

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Yan et al. Sci China Chem
tallation (TS2-3) (energy barriers of 31.8, 33.7 and 85.2 kcal/
mol, respectively) of the key C–H activation were calculated,
but these high energy barriers did not match with our mild
experimental condition. A more rational intermediate A5
with one Ir(I)-cod coordinate bond dissociated was then
proposed, after which an oxidative addition of Ir(I) to
sp2C–H would occur with a reasonable energy barrier
(23.0 kcal/mol) through TS2, affording the Ir(III) inter-
mediate A6. Elimination of HCl from A6 and spontaneous
recombination of the cod ligand would give intermediate A8.
Finally, the second sp2C–H metallation took place through
TS3 to afford the product 1a. During the process (A4-1a),
the overall energy barrier was 24.0 kcal/mol, which agreed
well with our experimental facts. And the HCl produced
above would irreversibly react with HMDS to drive the
equilibrium forwards [12]. Of note, M06-2X functional was
chose to explain the abnormal results (energy of A8 was
similar to 1a) caused by the use of low accuracy B3LYP
functional, and finding the energy of 1a was 3.9 kcal/mol
below than A8. As comparison, higher energy barriers of the
simple six-membered-ring (TS2′, 25.4 kcal/mol) and five-
membered-ring (TS2″, 32.2 kcal/mol) NHC precursors
without spirocyclic moiety were calculated by the same
method, which also supported the unsuccessful preparation
of their Ir(III) complexes (for computational details, see
section 5 in Supporting Information online).
Figure 2 X-ray crystal structure of 1a. Selected bond lengths (Å) and
angles (°): Ir(1)–C(1) 1.967(8), Ir(1)–C(12) 2.118(8), Ir(1)–C(22) 2.123(9),
Ir(1)–H(1) 1.80(7), C(2)–C(3)–C(4) 112.7°(8) (color online).
ope conformation with five atoms (C2, N1, C1, N2, C4) in a
plane and the quaternary C(3) out of it. In addition, the X-ray
structure of spiro[4.4]nonane-1,6-diamine moiety in 1a
showed that the angle (112.7°) of ∠C(2)–C(3)–C(4) was
comparable to that (117.5°) of a free spirocyclic diol mole-
cule [10].
Computational studies were next carried out to insight into
the possible formation mechanism of 1a (Figure 3). The
calculation using density functional theory (DFT) suggested
that the reaction was initiated by generation of a spirocyclic
NHC from S2 under basic condition, which was coordinated
with [Ir(cod)Cl]2 to give the Ir(I) intermediate A4. Subse-
quently, the common transition states [11] such as oxidative
addition (TS2-1 and TS2-2) and base-promoted C–H me-
To develop the chemical reactivity of above complexes, we
have hopefully tested their catalytic activity toward inert
C–H activation and subsequent transformations, though
many conversions mediated by Ir catalysts have been re-
ported [13], they generally involved the Ir(III)/Ir(I) pro-
cesses. We hoped to achieve the unusual Ir(III)/Ir(V)
mechanism, which would be further verified in our later
period work. The first was toward the selective sp2C–H ac-
Figure 3 Energy profiles for the formation of complex 1a from S2. Bond distances were given in Å (color online).

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Yan et al. Sci China Chem
tivation/addition of styrene derivatives. Such a direct con-
version did not require hetero-atomic induction, thus is of
step- and atom-economy and environment benign [14]. Im-
portantly, some precursors of anti-cancer pharmaceuticals,
such as toremifene and tamoxifen [15], could be prepared by
means of this straightforward transformation. Though sev-
eral catalytic systems based on Ni, Pd and Ru complexes
have been employed [16], more effective catalysts are still in
high demand.
formation online), a low catalyst loading of 1b (0.5 mol%–
1.0 mol%) combined with AgOTf (5.0 mol%) in ether sol-
vents (except entry 4) was found to enable the homo or
hetero addition in moderate to high yields (64%–92%) and
excellent regioselectivities. It should be noted that 1c was
also highly effective in catalyzing the reaction, and the
precipitation of AgCl did not influence the experimental
results according to our tests.
As summarized in Table 1, homo addition of styrene de-
rivatives 2a–2d were successfully conducted, providing the
desired products 4aa–4dd in high yields (entries 1–4) re-
Hence, after screening of reaction conditions under the
catalysis of complexes 1a–1c (Table S21, Supporting In-
a)
Table 1 1b-catalyzed head-to-tail addition of styrenes
b)
c)
Entry
1
2
2a
2b
3
Time (h)
16
4
Conversion (%)
>99
Yield (%)
92
−
4aa
2
3
−
−
16
16
4bb
4cc
>99
95
83
89
2c
4 d)
−
24
8
4dd
4ea
95
85
83
2d
5
>99
2e
2f
3a
3a
3b
3c
6
7
8
8
8
8
4fa
4eb
4ec
>99
>99
>99
75
88
64
2e
2e
a) Conditions: i) for entries 1–4, 2 (1.0 mmol), 1b (0.5 mol%) and AgOTf (5.0 mol%) in tetrahydrofuran (THF, 0.5 mL) at 80 °C for 16–24 h; ii) for entries
5–8, 2(0.5 mmol), 3 (1.25 mmol), 1b (1.0 mol%) and AgOTf (5.0 mol%) in dioxane (0.5 mL) at 100 °C for 8 h. b) Crude products were analyzed by LC-MS.
c) Isolated yields. d) 0.5 mL 1,2-dichloroethane (DCE) was used for 2d only.

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Yan et al. Sci China Chem
gardless of the electronic properties of the aryl substrates.
Specifically, when an inert EWG (p-CF3) 2d was used, the
reaction still proceeded in DCE (85% yield, entry 4). Next,
the more challenging hetero addition with different styrenes
2 and 3 also proved feasible. To achieve a highly re-
gioselective hetero head-to-tail addition and avoid undesired
self-addition, sterically hindered 1,1-substituted styrene
diarylethenes 3 should be used in excess equivalents. In these
cases, hetero addition products were generally obtained in
moderate to good yields and high regioselectivities (entries
5–8). Of note, when a highly reactive EDG-(MeO) sub-
stituted substrate 3c was used, the reaction still gave 4ec as a
dominant product, albeit in moderate 64% yield along with
minor homo addition product of 3c (entry 8).
trolled isomerization of olefins (Table 2). This transforma-
tion can provide access to some hard-to-obtain olefin
derivatives. Several metal (Rh, Ru, Pd and Co) catalysts have
been reported [17], but most required external hydrogen
sources and are often confronted with selective issues, such
as regio- and E/Z selectivity. As expected, combination of a
low loading of 1b (0.5 mol%–5.0 mol%) with AgPF6
(1.0 mol%–10.0 mol%) was demonstrated to enable the
isomerization of a diverse range of olefins (Table S22),
generating single regioselective products in high yields
(except entry 5) and high E/Z ratios of 97:3–99:1 (except
entry 7). The remarkable features of this protocol included:
(1) Broad substrate scope, including not only a series of
terminal olefins with a distal phenyl group (entries 1–5), but
also the phenyl allylcarbonate ester (entry 6) and the phenyl
The second investigation was performed toward the con-
a)
Table 2 1b-Catalyzed isomerization of various 1-olefins
b)
c)
Entry
1
5
1b (mol%)
Time (h)
6
6
Yield (%)
92
E/Z
0.5
98:2
98:2
98:2
99:1
97:3
98:2
85:15
5a
6a
2
3
2.5
2.5
2.5
5.0
2.5
0.5
20
20
20
48
24
2
94
92
82
65
96
95
6b
5b
5c
5d
5e
5f
6c
4
6d
5 d)
6e
6f
6
7
5g
6g
a) Unless otherwise specified, reactions were conducted with 5 (0.5 mmol), 1b (0.5 mol%–5.0 mol%), AgPF6 (1.0 mol%–10.0 mol%) in acetone-d6
1
1
(1.0 mL) at r.t.; b) isolated yields; c) determined by H NMR; d) yield determined by H NMR using CH2Br2 as internal standard.

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Yan et al. Sci China Chem
5
6
Yang BM, Xiang K, Tu YQ, Zhang SH, Yang DT, Wang SH, Zhang
FM. Chem Commun, 2014, 50: 7163–7165
allyl ether (entry 7); (2) notably, a remote C=C bond mi-
gration crossing nine methylenes intervals still gave high E/Z
ratio of 97:3 and 65% yield (entry 5). Moreover, due to the
high performance of this reaction, acetone-d6 could be di-
rectly used as the solvent to obtain NMR spectra without
purification of the reaction system.
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In conclusion, we have designed and successfully syn-
thesized a new class of unique dianionic Ir(III) CCC pincer
complexes (SNHC-Ir) by developing a concise template
method under mild conditions. The experimental and DFT
calculation results proved that the spirocyclic skeleton was a
favourable structure element for the formation and stabili-
zation of this type of complex. The catalytic effect of
chloride 1b toward the styrene addition and remote olefin
isomerization was just a preliminary testing result, and a
further verification of unusual Ir(III)/Ir(V) mechanism dur-
ing these processes is ongoing. In addition, more spirocyclic
complexes with other frameworks, and more significant
properties would be developed with our further research.
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Acknowledgements This work was supported by the National Natural
Science Foundation of China (21502080, 21772076, 21871117, 21772071,
21702136, 91956203), the “111” Program of Ministry of Education of
China, the Major Project (2018ZX09711001-005-002) of Ministry of Sci-
ence and Technology of China, and the Science and Technology Commis-
sion of Shanghai Municipality (19IC1430100). This work was supported by
the High-Performance Computing Center of Shanghai Jiao Tong University.
Also, we thank Prof. Qing-Wei Zhang from the University of Science and
Technology of China for his helpful discussion.
Conflict of interest The authors declare no conflict of interest.
Supporting information The supporting information is available online at
http://chem.scichina.com and http://link.springer.com/journal/11426. The
supporting materials are published as submitted, without typesetting or
editing. The responsibility for scientific accuracy and content remains en-
tirely with the authors.
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