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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