Efficient formation of organoiridium macrocycles via C-H activation directed self-assembly

Article abstract of DOI:10.1039/b923335k

Versatile and efficient procedures for the construction and modification of organometallic macrocycles with half-sandwich Ir corners via C-H activation and self-assembly have been developed.

Full text of DOI:10.1039/b923335k

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Efficient formation of organoiridium macrocycles via C–H activation
directed self-assemblyw
Ying-Feng Han, Hao Li, Lin-Hong Weng and Guo-Xin Jin*
Received 9th November 2009, Accepted 11th March 2010
First published as an Advance Article on the web 13th April 2010
DOI: 10.1039/b923335k
Versatile and efficient procedures for the construction and
modification of organometallic macrocycles with half-sandwich
Ir corners via C–H activation and self-assembly have been
developed.
The construction of new nanoscopic macrocycles and cages
with interesting structural features and technologically useful
functions have been topics of intense study with considerable
potential.1–3 Synthetic routes including stepwise and self-
assembly strategies have been developed by many groups,
and such structures are also used to build up organometallic
Scheme 1 Synthesis of tetranuclear complexes 1a–d.
half-sandwich molecular rectangles, prisms and cages,4 how-
ever, a simple one-pot approach for preparing molecular
[Cp*4Ir4(m-pyrazine)2(m-L1)2](OTf)4 (1a) was obtained simply
rectangles from half-sandwich metal corners and two different
by extracting with CH2Cl2.w X-Ray structural analysis revealed,
to our surprise, that four half-sandwich cyclometalated iridium
rigid or semi-rigid bifunctional ligands is a challenge. Aromatic
C–H activation mediated by transition metals is an important
process in many well-known organic reactions.5 Although
ortho-metallation is a fairly common and expectable pheno-
menon in iridium chemistry, and iridium complexes have been
extensively used in the study of the mechanisms of C–H
activation,6 using this process to construct nanoscopic macro-
cycles has hardly been explored.7
corners are formed.w
As shown in Fig. 1 (left), the cation of 1a indeed adopts a
rectangular structure with dimensions of 6.96 ꢀ 6.98 A
according to Irꢁ ꢁ ꢁIr non-bonding distances. The Ir–NCHQN
,
Ir–CAr bond lengths and the CAr–Ir–NCHQN chelate bite angle
are similar in related Cp*Ir cyclometallated complexes.6
The isolation of the tetranuclear macrocycles generally
formulated as [Cp*4Ir4(m-pyrazine)2(m-L)2](OTf)4 (L = L2 (1b),
L3 (1c), L4 (1d)) demonstrated the compatibility of this
reaction with other terephthal-bis-imine ligands L2–L4. The
products have been fully characterized by NMR, elemental
analyses, and the formation of half-sandwich cyclometalated
iridium corners were confirmed by X-ray structural analyses of
1b and 1c (Fig. 1 (right)).z
Motivated by interest in supramolecular chemistry with
organometallic half-sandwich complexes,8 we have initiated
a new one-pot approach for preparing heterotopic organo-
metallic macrocycles. In this contribution, we report an efficient
one-pot method for synthesizing molecular macrocycles of
half-sandwich iridium complexes via C–H activation directed
muticomponent self-assembly under very mild conditions.
Also an approach for postsynthetic modification (PSM)9
of organometallic macrocycles through dimethylacetylene
dicarboxylate (DMAD) insertion has been employed.
With the intent of exploring the scope of our synthetic
approach toward different organometallic macrocycle systems
and to demonstrate the regioselectivity in the C–H activation
As shown in Scheme 1, [Cp*IrCl2]2 was first treated with
pyrazine in dichloromethane at room temperature to afford
the m-pyrazine bridged dinuclear complexes,8a to which
4 equiv. of AgOTf (Tf = O2SCF3) was then added. After
the mixture was stirred at room temperature for 2 h, sodium
acetate and terephthal-bis-imine ligand L1 were added and
keep stirring for additional 12 h. A tetranuclear macrocycle
Shanghai Key Laboratory of Molecular Catalysis and Innovative
Material, Department of Chemistry, Fudan University, 200433,
Shanghai, P. R. China. E-mail: gxjin@fudan.edu.cn;
Fig. 1 Left: complex of 1a in stick mode. Triflate anions, solvent
molecules and hydrogen atoms are omitted for clarity. Right: the cores
of 1a–c in stick mode illustrating the formation of five-membered
iridium(III) rings. Triflate anions, solvent molecules, Cp* rings, the
substitutions of nitrogen atoms and hydrogen atoms are omitted for
clarity. Carbon atoms are shown as dark gray, nitrogen as blue, and
iridium as green.
Fax: (+86)-21-65643776
w Electronic supplementary information (ESI) available: Experimental
details and crystal structure determination. CCDC 744136 (1a),
CCDC 744137 (1b), CCDC 744138 (1c), CCDC 744142 (2a), CCDC
744133 (3a), CCDC 744134 (3b), CCDC 744140 (4a) and CCDC
744141 (4b). For ESI and crystallographic data in CIF or other
electronic format see DOI: 10.1039/b923335k
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Scheme 3 Stepwise formation of 1a–d.
ligands L1–L4 with [Cp*IrCl2]2, and then reaction with pyrazine
in the presence of AgOTf (Scheme 3).11 While the electron-
withdrawing substituent (p-Cl) inhibits C–H activation,
electron-donating substituents (p-OMe or p-Me) favour C–H
Scheme 2 Synthesis of tetranuclear complexes 2a and 2b.
1
activation. The H NMR spectra of products 3a–d in CDCl3
exhibited similar two singlets in a 1 : 1 intensity ratio due to the
imine and the central phenyl ring, respectively, indicative of
highly symmetric structures. The structures of 3a and 3b
containing two five-membered cyclometalated rings was con-
firmed by X-ray structure analyses.w An ORTEP view of the
molecular structure of 3a is shown in Fig. 3, with selected
bond lengths and angles listed. It revealed that the complex
has a dimeric structure containing two five-membered cyclo-
metalated rings. The M–N, M–C bond lengths and the
C–M–N chelate bite angles are similar to related Cp*Ir
cyclometallated complexes.6e,f The reactions of N,N0-bis-
benzylidenebenzene-1,4-diamines ligands L5 or L6 with
[Cp*IrCl2]2 leads to the formation of binuclear iridium-based
‘‘organometallic clip’’ linear moieties 4a,b in high yields.
Detailed structures of 4a,b were established by X-ray single
crystal analyses,z and as shown in Fig. 4, both imine moieties
are coordinated to Cp*Ir fragments by a C–H activation
reaction in ortho-position with respect to the imine group.
Analogously, organoiridium macrocycles 2a,b which contain
Cp*Ir-based five-membered metallacycles and pyrazine ligands,
were prepared by direct reactions of 4a,b with pyrazine in the
presence of AgOTf, respectively (Scheme 4).
Fig. 2 Left: side views of 2a in stick mode. Right: top view of the 2a
in stick mode. Triflate anions in space-filling mode. Two triflate
anions, solvent molecules and hydrogen atoms are omitted for clarity.
Carbon atoms are shown as dark gray, nitrogen as blue, sulfur as
yellow, fluorine as pink and iridium as green.
step. N,N0-bisbenzylidenebenzene-1,4-diamine ligands L5 or
L6 were introduced to these reactions. Treatment of L5 or L6
with the previously described iridium-catalyzed C–H activa-
tion conditions resulted in the formation of 2a and 2b in 68
and 71% yields, respectively (Scheme 2). All of imine moieties
are coordinated to Cp*Ir fragments by a C–H activation
reaction in ortho-position with respect to imine groups. The
structure of 2a, which contains Cp*Ir-based five-membered
metallacycles and pyrazine ligands was established by X-ray
structure analysis (Fig. 2).w The complex cation of 2a has a
rectangular cavity with dimensions of 8.4 ꢀ 7.0 A according to
Irꢁ ꢁ ꢁIr separations. Interestingly, in the solid state the macro-
cycle exhibits an arrangement in which the Cp* rings exist in
cis conformation. A remarkable feature of the structure is that
two of the four triflate anions are located above and below the
macrocycle accommodated by the cavity in different ways.
On the basis of these observations, we hypothesized that the
multi-component self-assembly and C–H activation can be
accomplished in the process. The mechanism of aromatic C–H
activation at [Cp*Ir] fragments has been studied using density
functional theory by Davies and Macgregor,6e,h and although
the computed C–H activation barriers are not consistent with
the reactivity patterns of experimental studies, a range of
carboxylates and triflates were found to effect cyclometallation.
As further experimental evidence, it is worth noting that the
C–H activation in our system can be carried out without
additional sodium acetate, indicating that triflate can promote
C–H activation.10
PSM is an important methodology due to its potential for
accessing advanced materials.9 Taking into account that some
functionalized alkynes, such as DMAD, can be employed to
expand metallacycles through DMAD insertion into metal–
aryl of cyclometalated compounds6f PSM of our metallacycles
was attempted by reacting DMAD with the tetranuclear
cyclometalated complexes 1a and 1b at room temperature.
In these cases, only one single alkyne insertion in each Ir–aryl
Organoiridium macrocycles 1a–d could also be prepared by an
alternative ‘‘C–H activation before self-assembly’’ stepwise syn-
thetic pathway, by first creating the stable bimetallic edges 3a–d
using two-site aromatic C–H activation in double-Schiff-base
Fig. 3 ORTEP view of 3a (ellipsoids at the 30% probability level).
Hydrogen atoms have been omitted for clarity. Selected bond lengths
(A) and angles (1): Ir(1)–C(2) 2.029(4), Ir(1)–N(1) 2.102(4);
C(2)–Ir(1)–N(1) 77.54(15), C(4)–N(1)–Ir(1) 115.8(3).
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Notes and references
z Two of the four triflate anions in complexes 1a, 2a and some
unknown solvents in complexes 1a, 1c, 2a, 4a, are strongly disordered
and cannot be refined properly, as a result, the SQUEEZE algorithm
was used to omit all of these disordered fragments. Details of the
crystallographic analyses are provided in the ESIw.
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Hydrogen atoms have been omitted for clarity. Selected bond lengths
(A) and angles (1): Ir(1)–C(1) 2.030(6), Ir(1)–N(1) 2.092(4), N(1)–C(7)
1.292(7); C(1)–Ir(1)–N(1) 78.1(2).
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Scheme 5 Postsynthetic modification of 1a,b by alkyne insertion.
has been determined by 1H NMR spectra and elemental
analyses of 5a,b (Scheme 5).
In summary, we have developed an efficient procedure for
the construction of a series of organometallic macrocycles via
C–H activation directed muticomponent self-assembly. The
procedure avoided the separation problems and product loss
that occurred in previous stepwise-formation procedures, and
the desired organometallic macrocycles were readily obtained
in good yields. The dimensions of these discrete organometallic
macrocycles could be expanded through the insertion of
unsaturated molecules into the five-membered cyclometalation
corners. This synthetic approach might be efficiently applied to
the construction and PSM of novel multi-metallic discrete
macrocycles or cages via C–H activation directed self-assembly
under very mild conditions.
This work was supported by the National Science Foundation
of China (20721063, 20771028), Shanghai Leading Academic
Discipline Project (B108), Shanghai Science and Technology
Committee (08DZ2270500, 08DJ1400103) and the National
Basic Research Program of China (2009CB825300).
9 Z. Q. Wang and S. M. Cohen, Chem. Soc. Rev., 2009, 38,
13159.
10 When the mixture was stirred at 55 1C for 12 h in the absence of
sodium acetate, the products could be obtained in moderate
yields.
11 See ESIw for additional details.
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This journal is The Royal Society of Chemistry 2010
3558 | Chem. Commun., 2010, 46, 3556–3558