Selective mono-and bis-Au(III) metalations of 5,10,15,20,25,30-Hexaaryl- [26]hexaphyrins(1.1.1.1.1.1)

Article abstract of DOI:10.1246/cl.2013.22

Selective mono- and bis-Au(III) metalations of [26]hexaphyrins( 1.1.1.1.1.1) were achieved in a 4:1 mixture of CH₂Cl₂ and methanol by using Na[AuCl₄] as a Au(III) ion source and Ag ₃PO₄ and Ag₂CO₃ as Au(III)-activating reagents.

Full text of DOI:10.1246/cl.2013.22

doi:10.1246/cl.2013.22
22
Published on the web December 22, 2012
Selective Mono- and Bis-Au(III) Metalations
of 5,10,15,20,25,30-Hexaaryl-[26]hexaphyrins(1.1.1.1.1.1)
Koji Naoda, Hirotaka Mori, and Atsuhiro Osuka*
Department of Chemistry, Graduate School of Science, Kyoto University, Sakyo-ku, Kyoto 606-8502
(Received October 12, 2012; CL-121049; E-mail: osuka@kuchem.kyoto-u.ac.jp)
Selective mono- and bis-Au(III) metalations of [26]hexa-
phyrins(1.1.1.1.1.1) were achieved in a 4:1 mixture of CH2Cl2
and methanol by using Na[AuCl4] as a Au(III) ion source and
Ag3PO4 and Ag2CO3 as Au(III)-activating reagents.
In recent years, the chemistry of expanded porphyrins has
witnessed a considerable upsurge,1 displaying their attractive
properties such as multi-metal coordination,2,3 anion sensing,4
unprecedented skeletal rearrangements,2b,2c,5 large nonlinear
optical properties,6 and versatile oxidation states including
stable radical species.7 Moreover, expanded porphyrins can
serve as an effective platform for the realization of Möbius
aromatic and antiaromatic systems.8-10 Among many metal
complexes of expanded porphyrins,3 mono- and bis-Au(III)
complexes of hexaphyrins are useful molecules since they are
held in rigidly rectangular shapes, and become aromatic and
antiaromatic depending upon the number of ³ electrons in the
conjugated circuits.11 Mono-Au(III) complexes can be used for
the preparation of hetero-bis-metal complexes such as Au(III)-
Ag(III), Au(III)-Cu(III), Au(III)-Rh(I), and Au(III)-Rh(III)
complexes.11,12 Intriguingly, antiaromatic Au(III) complexes of
[28]hexaphyrins are stable enough to allow their full character-
ization, including the evaluation of the aromatic versus
antiaromatic influence on their optical and electrochemical
properties.11 By taking advantage of these properties, researchers
have revealed the two-photon absorption cross sections of
aromatic [26]hexaphyrin complexes to be approximately 4-5
times larger than those of the corresponding antiaromatic
[28]hexaphyrin complexes.11b
We previously reported that the mono- and bis-Au(III)
complexes 2a and 3a were prepared in 16% and 14% yields,
respectively, through the reaction of 1a with Na[AuCl4] in the
presence of NaOAc using a 4:1 mixture of CH2Cl2 and methanol
as a solvent for 3 days (Scheme 1 and Table 1, Entry 1).11a This
mixed solvent was used to assure certain solubilities of 1a and
Na[AuCl4]. Later, we found that the addition of AgOTf
accelerated the Au(III) metalation significantly to yield 2a and
3a in 9% and 39% yields after only 1 h (Table 1, Entry 2).11b
The silver salt added was considered to activate the Au(III)
cation by capturing chloride anions. This method allowed rapid
metalation but at the same time tended to cause the oxidative
decompositions of 1a-3a, probably owing to the highly
activated Au(III) cation, which was suggested by the result
with a longer reaction time (Table 1, Entry 3). It was envisioned
that the efficiency and/or selectivity of Au(III) metalations of
[26]hexaphyrins might be improved by the addition of other
silver salts.
Scheme 1. Au(III) metalation of hexaphyrin 1a.
Table 1. Effect of Ag salts on the metalation of hexaphyrin 1a
Yielda/%
Entry Ag salt (X)
Time/h
1a
2a
3a
1
2
3
4
5
6
7
8
9
none
72
1
15b
15b
®
®
®
®
77
®
6
16b
14b
39b
35
22
15
26
®
7
AgOTf (40)
AgOTf (40)
AgBF4 (40)
AgSbF6 (40)
AgClO4 (40)
AgOAc (40)
Ag2SO4 (20)
Ag2O (100)
Ag3PO4 (13.3)
Ag2CO3 (20)
9b
®
24
>72
72
24
24
72
72
24
24
®
®
®
®
®
®
34
trace
®
6
42
10
11
3
32
b
a 1H NMR yields except for Entries 1 and 2. Isolated yields.
AgBF4, AgSbF6, AgClO4, AgOAc, Ag2SO4, and Ag2O (Table 1,
Entries 4-9), we found that the addition of Ag3PO4 led to the
selective formation of the mono-Au(III) complex 2a as a rare
case (Table 1, Entry 10), and the addition of Ag2CO3 gave an
improved yield of the bis-Au(III) complex 3a with better
material balance (Entry 11). After extensive optimization ex-
periments, we employed 10 equiv of Na[AuCl4] and 50 equiv of
Ag3PO4 for mono-Au(III) metalation (protocol A) and 20 equiv
of Na[AuCl4] and 100 equiv of Ag2CO3 for bis-Au(III) metal-
ation (protocol B) for better results.
In the course of this study, solvent effects on the Au(III)
metalation of hexaphyrin 1a were also examined. Almost no
Au(III) metalation was observed in acetone, ethyl acetate, THF,
nitromethane, DMF, or DMSO. In acetonitrile, the metalation
with 20 equiv of Na[AuCl4] and 100 equiv of Ag2CO3 produced
3a in 58% yield. However, the metalation with 20 equiv of
Na[AuCl4] and 50 equiv of Ag3PO4 resulted in the nonselective
formation of 2a and 3a in 11% and 33% yields, respectively, and
[26]hexaphyrins are usually only poorly soluble in acetonitrile.
Then, we applied the selective Au(III) metalation methods
(protocols A and B) to the hexaphyrins 1b-1e (Scheme 2). In all
the hexaphyrin substrates examined, protocol A afforded the
mono-Au(III) complexes 2b-2e and protocol B provided the
With this idea in mind, we examined the effect of various
Ag salts in the Au(III) metalation of hexaphyrin 1a (Scheme 1).
While notable improvements were not observed for additions of
Chem. Lett. 2013, 42, 22-24
© 2013 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/

23
Scheme 3. The structure of 3d¤.
Scheme 2. Au(III) metalation of hexaphyrins.
Table 2. Au(III) metalation of hexaphyrins
Entry
Hexaphyrin
Protocol
Time/h
Yielda/%
1
2
3
4
5
6
7
8
9
1a
1b
1c
1d
1e
1a
1b
1c
1d
1e
b
A
A
A
A
A
B
B
B
B
B
18
7
3
24
24
24
10
10
9
2a; 50
2b; 33
2c; 19
2d; 37
2e; 68
3a; 51
3b; 36
3c; 5
3d; 37b
3e; 47
10
17
aIsolated yield. 1:2 mixture of 3d and 3d¤.
bis-Au(III) complexes 3b-3e selectively, without significant
contamination with undesired complexes (Table 2). The ob-
served selective formation of either mono- or bis-Au(III)
complexes was advantageous, making the separation steps quite
easy. The roles of the anions in the Ag salts are not clear at
present but may tune the reactivity of the Au(III) species through
complexation. With Na[AuCl4]/AgOTf, the reactivity of the
Au(III) cation is high, accelerating the complexation but causing
3¹
2¹
serious oxidative damage. Since the PO4 and CO3 anions are
Figure 1. X-ray crystal structures of (a) 2b and (b) 3e. Top:
top view. Bottom: side view. The thermal ellipsoids are scaled
to 50% probability. Solvent molecules, 2,4,6-trifluorophenyl
groups (bottom in (a)), and pentafluorophenyl groups are
omitted for clarity.
¹
more basic than the TfO anion, the reactivities of the Au(III)
species from Na[AuCl4]/Ag3PO4 and Na[AuCl4]/Ag2-
CO3 may be attenuated through the interactions with the anions.
PO4 is more basic than CO3 , which may lead to a suitable
reactivity of mono-Au(III) metalation for PO4 and of bis-
3¹
2¹
3¹
2¹
Au(III) metalation for CO3
.
metalation of 1d, two isomers of 3d and 3d¤ (Scheme 3) were
obtained in a 1:2 ratio, which implies the occurrence of a
caterpillar-motion-like macrocyclic ring rotation13 during the
bis-Au(III) metalation, while such isomerization was not
observed for the corresponding mono-Au(III) metalation.
The structures of 2b, 2c, 2d, 3b, 3c, 3d, 3d¤, and 3e were
confirmed by X-ray crystallographic analyses performed on their
single crystals (SI).14,15 Of these compounds, the structures of 2b
and 3e are shown in Figure 1. Complex 2b exhibits a twisted
As shown in Table 2, there is a tendency that the yields of
the Au(III) complexes 2 and 3 are better for hexaphyrin
substrates bearing more electron-withdrawing meso-aryl sub-
stituents. This tendency may indicate that electron-withdrawing
meso-aryl substituents suppress the undesired oxidative decom-
positions. High yields of 2e and 3e (Table 2, Entries 9 and 10)
may reflect the effective steric protection of bulky 9-anthryl
groups against oxidation at the meso-positions. In the bis-Au(III)
Chem. Lett. 2013, 42, 22-24
© 2013 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/

24
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Figure 2. Absorption spectra of 2e and 3e in CH2Cl2.
structure, in which the metalated half is planar but the other free-
base half is distorted owing to repulsion between the inner
pyrrolic ¢-protons. On the other hand, X-ray analysis of 3e
reveals a remarkably planar structure with two anthryl groups at
1
the shorter side, in accordance with the H NMR data. It also
8
9
displays a crystal-packing structure with an infinite molecular
network aided by the C-H and ³ interactions of the anthryl
groups with the ³ surfaces of the hexaphyrins (SI).15 Other
Au(III) metal complexes also show similar crystal structures
(SI).15
Figure 2 shows the absorption spectra of 2e and 3e in
CH2Cl2. Other Au(III) metal complexes also show similar
absorption spectra (SI).15 The mono-Au(III) hexaphyrin 2e
exhibits a Soret-like band at 613 nm, and Q-like bands at 782
and 1088 nm, and the bis-Au(III) hexaphyrin 3e displays split
and intensified Soret-like bands at 671 and 689 nm, and Q-like
bands at 828 and 1189 nm, both of which are red-shifted
compared to those of 2e.
In summary, we have developed simple protocols allowing
the selective preparations of mono- and bis-Au(III) hexaphyrin
complexes by using Ag3PO4 and Ag2CO3, respectively. Further
applications of these Au(III) complexes are currently under
investigation in our laboratory and will be reported elsewhere.
References and Notes
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14 CCDC 905507 (2b), 905508 (3b), 905504 (2c), 905511 (3c),
905505 (2d), 905510 (3d), 905509 (3d¤), and 905506 (3e)
contain the supplementary crystallographic data for this
paper. These data can be obtained free of charge from
The Cambridge Crystallographic Data Centre via http://
www.ccdc.cam.ac.uk/data_request/cif.
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2
15 Supporting Information is available electronically on the
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index.html.
Chem. Lett. 2013, 42, 22-24
© 2013 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/