Evidence that gold(III) porphyrins are not electrochemically inert: Facile generation of gold(II) 5,10,15,20-tetrakis(3,5-di-tert-butylphenyl)porphyrin

Article abstract of DOI:10.1039/b109795d

Gold(III) porphyrin 1 is shown to undergo reduction at the central metal ion to give the first known gold(II) porphyrin overturning the long held assumption that reduction of such complexes only occurs at the macrocycle.

Full text of DOI:10.1039/b109795d

Evidence that gold(III) porphyrins are not electrochemically inert: facile
generation of gold(II
)
5,10,15,20-tetrakis(3,5-di-tert-butylphenyl)porphyrin
Karl M. Kadish,*ab Wenbo E,a Zhongping Ou,a Jianguo Shao,a Paul J. Sintic,b Kei Ohkubo,c Shunichi
Fukuzumi*c and Maxwell J. Crossley*b
a Department of Chemistry, University of Houston, Houston, Texas TX 77204-5003, USA
b School of Chemistry, The University of Sydney NSW 2006, Australia.
E-mail: m.crossley@chem.usyd.edu.au
c
Department of Material and Life Science, Graduate School of Engineering, Osaka University, CREST,
Japan Science and Technology Corporation (JST), Suita Osaka 565-0871, Japan
Received (in Cambridge, UK) 26th October 2001, Accepted 3rd January 2002
First published as an Advance Article on the web 29th January 2002
Gold(III) porphyrin 1 is shown to undergo reduction at the
central metal ion to give the first known gold(II) porphyrin
overturning the long held assumption that reduction of such
complexes only occurs at the macrocycle.
Three reversible one-electron reductions are seen for
(P)AuPF6 1 in a variety of nonaqueous solvents including
pyridine (E1/2 = 20.52, 21.08 and 21.76 V), THF (E1/2
=
20.40, 21.10 and 21.77 V) and benzonitrile (E1/2 = 20.56,
21.09 and 21.81 V vs. SCE). The first electrode reaction
involves the central metal ion and the next two, the conjugated
porphyrin macrocycle. Evidence for assignment of a Au(III)/
Au(II) process in the first reduction is given on the basis of
electrochemistry coupled with UV-visible and ESR spectros-
copy.
Gold(III) porphyrins have been used as acceptors in porphyrin
dyads1,2 and triads3 due to their ability to be easily reduced,
either chemically or photochemically. The products of the first
and second one-electron reductions are extremely stable in
nonaqueous media but the singly reduced complexes can be
chemically converted to a phlorin after disproportionation or
further reduction in the presence of a proton source.4 Early
electrochemical experiments on (TPP)Au[AuCl4] and two other
gold porphyrins in DMSO showed the compounds to undergo
three one-electron reductions5 but the authors concluded, on the
basis of electrochemical criteria and spectroelectrochemical
data, that Au(III) was electrochemically inert when incorporated
into a porphyrin macrocycle. This was also the conclusion based
on theoretical calculations and electrochemical experiments
involving the reduction of (TPP)AuCl in dichloromethane6 and
has since been assumed to be true for all other subsequently
investigated gold(III) complexes.7
Cyclic voltammograms of (P)AuPF6 1 and (P)CuII in
benzonitrile are illustrated in Fig. 1 while UV-visible spectra
obtained upon reduction of the two compounds in a thin-layer
cell are shown in Fig. 2. Two one-electron reductions are
expected and observed for [5,10,15,20-tetrakis(3,5-di-tert-
butylphenyl)porphyrinato]copper(II), (P)CuII, consistent with
the stepwise formation of a porphyrin p-anion radical and
dianion. In contrast, three reversible one-electron reductions are
seen for (P)AuPF6 1 and indicate that one of the three processes
must involve the central metal ion. As shown below, this
reaction occurs prior to formation of a Au(II) porphyrin p-anion
radical and dianion at more negative potentials.
The first and second reversible half-wave potentials for
reduction of gold porphyrins in functionalised multiporphyrin
systems have sometimes been reported1,3,8 but little attention
has been paid to a third reduction or to the site of electron
transfer itself which was invariably assumed but not proven to
involve the porphyrin macrocycle. This is discussed in the
present communication in which we report on the electro-
chemistry and spectral characterisations (UV-visible and ESR)
of neutral and reduced hexafluorophosphate[5,10,15,20-tetra-
kis(3,5-di-tert-butylphenyl)porphyrinato]gold(III), represented
as (P)AuPF6 1. Our studies were carried out in a variety of
Virtually identical UV-visible spectra are seen for the first
reduction product of (P)AuPF6 1 (lmax = 423 and 537 nm) and
neutral (P)CuII (lmax = 422 and 543 nm). Identical spectroelec-
trochemical results are also seen upon the second reduction of
(P)AuPF6 1 and the first reduction of (P)CuII to its porphyrin p-
anion radical form (see Fig. 2b and c). Similar spectral changes
are seen in pyridine and dichloromethane and provide un-
equivocal evidence for the site of electron transfer in (P)AuPF6
1.
Singly reduced (P)AuPF6 1 is characteristic of a neutral
(P)MII complex rather than a porphyrin p-anion radical. Its UV-
visible spectrum in DMF, pyridine, dichloromethane or benzo-
nonaqueous solvents and show that gold(III) porphyrins are not
electrochemically inert as was previously claimed but can be
rapidly and reversibly converted to a Au(II) form of the
compound rather than to a Au(III) porphyrin p-anion radical.
Fig. 1 Cyclic voltammograms of (a) (P)CuII and (b) (P)AuPF6 1 in
benzonitrile containing 0.1 M TBAP.
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CHEM. COMMUN., 2002, 356–357
This journal is © The Royal Society of Chemistry 2002

of the metal-centred radical, agrees with the reported value
(2.065
0.005) of Au(II) phthalocyanine.13 A hyperfine
interaction with 197Au (I = 3/2, A = 27 G) is observed in
benzonitrile.14 Thus, the broad signal is clearly assigned to the
Au(II) species. Although both the Au(II) porphyrin and the
Au(III) porphyrin p-anion radical are evident in the frozen
solution, the latter species is negligible judging from the large
difference in their linewidths.15 The initial formation of a
transient porphyrin p-anion radical prior to the conversion to a
stable Au(II) derivative cannot be ruled out. This point needs to
be thoroughly investigated in the case of donor–acceptor
complexes containing gold porphyrins where the site of
reduction may vary with minor changes in the macrocycle,
solvent or associated counterions and may thus have strong
implications in interpretation of the photophysical data. In
particular, reduction of a Au(III) porphyrin complex to a Au(II
)
porphyrin will result in loss of the associated ligand/counterion
which might impede back electron transfer, as might inter-
conversion between a Au(II) porphyrin and Au(III) porphyrin p-
anion radical forms.
In summary, the UV-visible, electrochemical and ESR data
are self-consistent and clearly indicate that the investigated
Au(III) porphyrin undergoes an initial metal-centred reduction
followed by formation of a porphyrin p-anion radical and
dianion at more negative potentials. Ongoing studies in our
laboratories suggest that this is a general phenomenon for a
wide range of gold(III) porphyrins.
The support of the Robert A. Welch Foundation (K. M. K.) is
gratefully acknowledged. The Australian Research Council is
thanked for a research grant (to M. J. C.) and the Australian
government for a Postgraduate Award (to P. J. S.).
Fig. 2 Thin-layer UV-visible spectra before and after reduction of (P)AuPF6
1 and (P)CuII in benzonitrile containing 0.2 M TBAP.
nitrile has a well-defined Soret band at 420–423 nm and one
major visible band at 534–537 nm. Similar shaped UV-visible
spectra are also seen for other M(II) complexes having the same
tert-butyl macrocycle, with three examples being given for
(P)ZnII (lmax 430 and 560 nm), (P)NiII (lmax 421 and 531 nm)
and (P)PdII (lmax 422 and 526 nm) in benzonitrile containing
0.1 M TBAP.9
Notes and references
1 A. M. Brun, A. Harriman, V. Heitz and J.-P. Sauvage, J. Am. Chem.
Soc., 1991, 113, 8657.
2 K. Kilså, J. Kajanus, A. N. Macpherson, J. Mårtenson and B. Albinsson,
J. Am. Chem. Soc., 2001, 123, 3069.
3 L. Flamigni, I. M. Dixon, J.-P. Collin and J.-P. Sauvage, Chem.
Commun., 2000, 2479.
4 Z. Abou-Gamra, A. Harriman and P. Neta, J. Chem. Soc., Faraday
Trans. 2, 1986, 82, 2337.
5 M. E. Jamin and R. T. Iwamoto, Inorg. Chim. Acta, 1978, 27, 135.
6 A. Antipas, D. Dolphin, M. Gouterman and E. C. Johnson, J. Am. Chem.
Soc., 1978, 100, 7705.
The formation of a Au(II) species rather than a Au(III
)
porphyrin p-anion radical upon reduction was also confirmed
by ESR spectra obtained after the chemical reduction of
(P)AuPF6 1 with one equivalent of naphthalene radical anion in
DMSO,10 pyridine or benzonitrile.11 These spectra are shown in
Fig. 3.
The broad signal at g = 2.06 is quite different from the sharp
signal at g = 2.006 which is assigned to the Au(III) porphyrin p-
anion radical.12 The large g-value (2.06), which is characteristic
7 J. K. M. Sanders, N. Bampos, Z. Clyde-Watson, S. L. Darling, J. C.
Hawley, H.-J. Kim, C. C. Mak and S. J. Webb, in The Porphyrin
Handbook, ed. K. M. Kadish, K. M. Smith and R. Guilard, Academic
Press, San Diego, 2000, vol. 3, p. 1.
8 E. K. L. Yeow, P. J. Sintic, N. M. Cabral, J. N. H. Reek, M. J. Crossley
and K. P. Ghiggino, Phys. Chem. Chem. Phys., 2000, 2, 4281.
9 M. J. Crossley, P. J. Sintic, R. Walton, W. E. Z. Ou, J. Shao and K. M.
Kadish, in preparation.
10 In DMSO, the generated Au(II) species becomes insoluble. This resulted
in disappearance of the Soret band which had been used as criteria for
formation of the p-anion radical in the case of a related [(TPP)Au]+
complex.5
11 The reduction of 1 can also be achieved using tetramethylsemiquinone
anion radical in benzonitrile. The electron transfer rate was too fast to be
followed spectrometrically.
12 Only the isotropic g-value was determined because of the apparent
isotropic signal due to the broad linewidth. The broad signal has
precluded determination of the coordination symmetry.
13 A. MacCragh and W. S. Koski, J. Am. Chem. Soc., 1965, 87, 2496.
14 The
phthalocyanine (A = 22 G) but smaller than the value of solvated Au2+
ion (A 48 G). This indicates that the spin density is partially
A value is larger than the corresponding value of Au(II)
=
delocalized on the ligand. See: F. G. Herring, G. Hwang, K. C. Lee, F.
Mistry, P. S. Phillips, H. Willner and F. Aubke, J. Am. Chem. Soc., 1992,
114, 1271.
15 Although only the ESR signal due to the p-anion radical is observable
Fig. 3 ESR spectra of singly reduced (P)AuPF6 1 (1.0 mM) generated by the
reduction with one equivalent of naphthalene radical anion (1.0 mM) in
deaerated (a) DMSO, (b) pyridine and (c) benzonitrile at 2160 °C.
in solution at 25 °C because of the fast relaxation time of the Au(II)
species, the amount of the p-anion radical is still negligible as compared
to the initial concentration of (P)AuPF6 1.
CHEM. COMMUN., 2002, 356–357
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