Aromatic and antiaromatic gold(III) hexaphyrins with multiple gold-carbon bonds

Article abstract of DOI:10.1021/ja051662x

Au(III) metalation of hexakis(pentafluorophenyl) [26]hexaphyrin led to formation of aromatic mono-Au(III) hexaphyrin and bis-Au(III) hexaphyrin, in which the inner pyrrolic β-protons are activated to form gold-carbon bonds, hence accommodating Au(III) ion with a NNCC core in a square planar manner. Two-electron reductions of these complexes with NaBH₄ provided the corresponding [28]hexaphyrin complexes which exhibit distinct paratropic ring currents. Copyright

Full text of DOI:10.1021/ja051662x

Published on Web 05/13/2005
Aromatic and Antiaromatic Gold(III) Hexaphyrins with Multiple Gold-Carbon
Bonds
Shigeki Mori and Atsuhiro Osuka*
Department of Chemistry, Graduate School of Science, Kyoto UniVersity, Sakyo-ku, Kyoto 606-8502, Japan
Received March 15, 2005; E-mail: osuka@kuchem.kyoto-u.ac.jp
Porphyrin with an 18π-electronic aromatic network is an
ubiquitous active component of many naturally occurring pigments
that has been extensively studied for more than a century. In recent
years, expanded porphyrins that are homologues of porphyrin have
emerged as a new class of conjugated pyrrolic macrocycles because
of their intriguing optical, electrochemical, and coordination proper-
ties that are difficult to find in porphyrins.1 Expanded porphyrins
are also interesting from the viewpoint of the annulene chemistry,
since the number of π electrons in the conjugated circuit usually
determines the aromaticity of the macrocycles. Among these, meso-
hexakis(pentafluorophenyl)-substituted [26]hexaphyrin(1.1.1.1.1.1)
12 is an attractive aromatic molecule in light of its planar and
Figure 1. X-ray crystal structures of 2 (A) and 3 (B). The thermal ellipsoids
rectangular shape and strong aromaticity with a 26π conjugated
circuit. In addition, two inverted pyrroles in 1 are particularly
intriguing in view of C-H bond activation, as judged from the
recent reports on the formation of organometallic species during
the metalations of N-confused porphyrins (NCP),3 carbaporphy-
rinoids,4 and benziporphyrins.5 A square planar arrangement of the
internal two pyrrolic nitrogens and two â-carbons of the inverted
pyrroles encourages its potential as an effective ligand with twin
CCNN cores such as doubly N-confused porphyrins (N2CP).6 Such
metalation of 1 is quite attractive but, to the best of our knowledge,
has not been achieved thus far.7
are scaled to the 50% probability level. In side views, meso-penta-
fluorophenyl substituents are omitted for clarity.
Chart 1. Structures of Hexaphyrins Studied in This Contribution
In this communication, we report Au(III) metalation of [26]-
hexaphyrin 1. A solution of 1 and 10 equiv of NaAuCl4 in a 4:1
mixture of CH2Cl2 and methanol was stirred in the presence of
sodium acetate at room temperature for 3 days. Separation over a
silica gel column provided mono-Au(III) complex 2 and bis-Au-
(III) complex 3 in 16 and 14% yields, respectively, along with
recovery of 1 (15%). Au(III) metalation proceeded slowly, but either
longer reaction time or higher reaction temperature decreased yields
of 2 and 3 with concomitant increase of byproducts. Positive-mode
high-resolution electrospray ionization mass spectroscopy (HR-ESI-
TOF-MS) revealed the molecular weights of 2 and 3 at m/z )
1655.0290 and 1848.9744; calcd for C66H12F30N6Au, 1655.0304
([M + H]+), and for C66H9F30N6Au2, 1848.9735 ([M + H]+),
respectively. Structures of 2 and 3 have been revealed by single-
crystal X-ray diffraction analysis (Figure 1).8,9 Curiously, in both
cases, the inner â-protons are activated to form Au-C bonds, and
Au(III) ions are coordinated similarly with two pyrrolic nitrogen
atoms and two pyrrolic â-carbon atoms within a flat [26]hexaphyrin
macrocycle. Au-N bond lengths are 2.081(6) and 2.085(6) Å and
Au-C bond lengths are 1.976(8) and 2.012(8) Å in 2 and Au-N
bond lengths are 2.099(6), 2.100(5), 2.105(6), and 2.113(6) Å, and
Au-C bond lengths are 2.011(6), 2.018(7), 2.025(7), and 2.037(6)
Å in 3. The mono-Au(III) complex 2 exhibits a planar but twisted
structure with a large mean plane deviation of 0.520 Å. Deviation
from planarity is larger in the free base part compared with the
Au(III) complexed part, which is attributable to the steric congestion
between the inner pyrrolic â-protons. Thus, the complex 3 takes a
more planar but overall bent structure, since bis Au(III) metalation
eliminates such steric congestion.
Diatropic ring currents of the [26]hexaphyrins 2 and 3 are evident
1
from their H NMR spectra, in which the outer â-protons appear
in a downfield region at 9.51, 9.36, 9.35, and 9.03 ppm, and the
inner â-protons and inner NH protons appear in an upfield region
at -2.93 and -2.08 ppm in 2, and the outer â-protons appear at
9.66 and 9.51 ppm in 3. These data indicate that 2 and 3 retain
strong aromaticity like that of 1. The absorption spectra of 2 and
3 both show strong Soret-like bands and small Q-band-like bands,
indicating their aromatic nature. Both the Soret-like bands and
Q-band-like (0,0) bands are red-shifted in order of 1 < 2 < 3
(Figure 2).
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C O M M U N I C A T I O N S
tetrafluoroborate as an electrolyte. The [26]hexaphyrin 1 undergoes
two reversible one-electron reductions at -0.52 and -0.85 V, and
the complexes 2 and 3 undergo two one-electron reductions at
-0.41 and -0.80 V, and -0.33 and -0.71 V, respectively. These
data indicate that the Au(III) metalation lowers the energy level of
the LUMO orbital of hexaphyrin. The mixed complex 6 exhibits
reduction waves at -0.35 and -0.72 V, which are similar to those
of 3. On the other hand, the [28]hexaphyrins 4 and 5 undergo
oxidations at -0.05 and 0.30 V and at -0.03 and 0.35 V,
respectively.
In summary, Au(III) metalation of 1 led to the formation of all-
in-plane Au(III) complexes 2 and 3 via the C-H bond activation
of the inner â-protons. Two-electron reduction of the aromatic
complexes 2 and 3 provided the antiaromatic complexes 4 and 5
that exhibit distinct paratropic ring currents. The complex 2 serves
as a nice platform for construction of a mixed bis-metal complex.
Our efforts are directed toward this possibility using a variety of
transition metals.
Figure 2. Absorption spectra of 1 (red), 2 (blue), 3 (green), and 5 (black)
in CH2Cl2.
In the next step, 2 and 3 were reduced with NaBH4 to give
complexes 4 and 5 both in quantitative yields without demetalation,
indicating inertness of the Au-C bonds and coordinated Au(III)
ion under the reduction conditions. The complexes 4 and 5 show
respectively the parent ion peaks at m/z ) 1655.0322, calcd for
C66H12F30N6Au ([M - H]-), 1655.0315; and at m/z ) 1848.9763,
calcd for C66H9F30N6Au2 ([M - H]-), 1848.9746. Both 4 and 5
exhibit the absorption spectra consisting of ill-defined Soret-like
bands without Q-band-like bands, indicating nonaromaticity (Figure
2 and Supporting Information, SI). Consistent with the Hu¨ckel’s
rule, the 1H NMR spectrum of the [28]hexaphyrin 4 reveals a
paratropic ring current, exhibiting the inner â-protons and NH
proton at 19.39 and 24.57 ppm, while exhibiting the outer â-protons
as four doublets at 5.02, 4.92, 4.32, and 4.07 ppm and the outer
Acknowledgment. This work is supported by the CREST
project of the Japanese Science and Technology Agency (JST).
Supporting Information Available: Synthetic procedures and
spectral data of complexes 2-6 including absorption spectra, high-
1
resolution ESI-TOF mass spectra, and H NMR spectra. CIF files for
the X-ray structural analysis for 2 and 3. This material is available
free of charge via the Internet at http://pubs.acs.org.
References
1
NH protons at 4.10 ppm. Similarly, the H NMR spectrum of 5
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exhibits the outer â-protons as a pair of doublets at 3.16 and 3.10
ppm and the outer NH protons as a broad singlet at 1.81 ppm,
respectively. These data clearly indicate the antiaromatic nature for
4 and 5. Of particular interest is a large change in the chemical
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The complex 2 still possesses a rigid N2CP-type vacant cavity
that may be used for metalation of a different metal ion. This
possibility was tested by treating a solution of 2 in CH2Cl2 with 10
equiv of CF3COOAg in the presence of sodium acetate at room
temperature for 12 h, which gave Au(III)-Ag(III) mixed metalated
complex 6 in 58% yield. The complex 6 exhibits the parent ion
peak at m/z ) 1760.9174, calcd for C66H9F30N6AuAg ([M + H]+),
1760.9132. The absorption spectrum of 6 is similar to those of 2
and 3 (SI), except for split Soret-like bands at 654 and 672 nm.
The 1H NMR spectrum of the diamagnetic complex 6 exhibits four
sharp doublets at 9.76, 9.63, 9.60, and 9.51 ppm due to the outer
â-protons, showing that the complex 6 is an aromatic Ag(III)
organometallic derivative, as have been reported for modified
porphyrinoid macrocycles including NCP, carbaporphyrinoids, and
benziporphyrins.3-6 Thus, it is concluded that the CCNN core in 2
serves as a trianionic ligand also for Ag ion.
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19.626(11) Å, â ) 90.29(2)°, V ) 5504(5) Å3, Dc ) 1.997 g/cm3, Z ) 4,
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C74H8N6F30Au2Cl2, Mw ) 2015.70, ortho-
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Å, c ) 28.196(10) Å, V ) 13654(9) Å3, Dc ) 1.961 g/cm3, Z ) 8, R )
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The electrochemical properties were studied by cyclic voltam-
metry in CH2Cl2 versus Fc/Fc ion using tetrabutylammonium
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