Photo-induced ligand substitution at a remote site via electron transfer in a porphyrin-appended rhenium carbonyl supermolecule

Article abstract of DOI:10.1039/b200127f

The photochemical and electrochemical properties of a Zn-porphyrin appended rhenium(I) tricarbonyl bipyridine 3-Me-pyridine complex have been investigated; visible-light sensitisation of electron transfer results in ligand substitution at a site remote from the chromophore.

Full text of DOI:10.1039/b200127f

Photo-induced ligand substitution at a remote site via electron transfer in
a porphyrin-appended rhenium carbonyl supermolecule†
Anders Gabrielsson,a Frantis˘ek Hartl,b John R. Lindsay Smitha and Robin N. Perutz*a
a Department of Chemistry, University of York, York, UK YO10 5DD. E-mail: rnp1@york.ac.uk
b Institute of Molecular Chemistry, University of Amsterdam, Nieuwe Achtergracht, 166, NL-1018 WV
Amsterdam, The Netherlands. E-mail: hartl@science.uva.nl
Received (in Cambridge, UK) 3rd January 2002, Accepted 6th March 2002
First published as an Advance Article on the web 2nd April 2002
The photochemical and electrochemical properties of a Zn-
porphyrin appended rhenium( ) tricarbonyl bipyridine
3-Me-pyridine complex have been investigated; visible-light
sensitisation of electron transfer results in ligand substitu-
tion at a site remote from the chromophore.
reactions show 18-electron products only, photolysis in the
presence of a trace of free 3-Me-pyridine shows characteristic
IR features of the radical 1· (Table 1). Photolysis of 1+ with
Et3N and excess 3-Me-pyridine in PrCN provides evidence for
a rhenium bipyridine-based radical, either 1· or 4· by EPR (g =
2.003, peak-to-peak separation = 6.9 mT) and IR spectroscopy,
as well as the cationic solvento complex 4+ (Table 1).9,10
The excitation in the visible region must occur on absorption
by the porphyrin macrocycle that acts as a sensitiser towards
visible light; product formation depends on electron transfer
from triethylamine. In order to test this hypothesis, we
I
Molecular dyads consisting of a photosensitiser and an electron
acceptor have been used in conjunction with an external
electron donor as mimics of photosynthesis.1 Complexes of the
type [ReI(CO)3(bpy)L]n (n = 0, +1) have been studied both for
reduction of carbon dioxide, and as components of supramo-
lecular assemblies.2–7 We show that a metalloporphyrin cova-
lently linked to a [ReI(CO)3(bpy)L] unit sensitises the rhenium
moiety towards photo-reduction. Low energy irradiation results
in expulsion of a ligand at a site remote from the chromophore.
Electrochemical studies are consistent with reaction via electron
transfer.
Compound 1+OTf2 was synthesised from compound 2 by
removing the axial bromide ligand with AgOTf in the presence
of 3-Me-pyridine in refluxing THF.† The synthesis of com-
pound 2 has been described elsewhere.2
Fig. 1 (a) IR spectra recorded during the photolysis of 1+ (1.9 mM) with
Et3N (72 mM) in THF after 0, 5, 15, 25, 35 and 45 min (a trace of 2 is present
at 2020 cm21). (b) IR spectroelectrochemistry of the reduction of 1+ (THF,
[NBu4][PF6], OTTLE cell, 293 K).
Complexes 1+ and 2 are stable towards photolysis in THF.
Their UV-Vis spectra are completely dominated by the
porphyrin absorption bands. The MLCT band of the Re-
(CO)3(bpy) unit which is normally found at ca. 400 nm is
masked by the porphyrin Soret band. Quenching of steady state
fluorescence provides evidence of excited state interaction in 1+
and 2.† On photolysis of complex 1+ with long-wavelength light
(l > 495 nm) in the presence of triethylamine, the IR spectrum
reveals that a new metal carbonyl complex is formed.
Comparison with [Re(CO)3(bpy)(THF)]+ shows that the axial
ligand is displaced by the solvent to yield the THF complex 3+
(Fig. 1(a), Table 1).8 When the photoreaction was repeated in
the presence of excess bromide and triethylamine, complex 2
was formed quantitatively. While the IR spectra from these
Table 1 CO stretching frequencies of complexes under study
Complex
Solvent
n(CO)/cm21
1+
1·
1·
THF
PrCN
THF
THF
THF
THF
PrCN
PrCN
THF
THF
2031, 1927br
2010, 1903, 1893
2011, 1907, 1893
2020, 1920, 1897
1998, 1888, 1868
2016, 1914, 1892
2038, ~ 1835
2010, 1903br, 1893bra
2031, 1927br
2010, 1905, 1892
2
2·2
3+
4+
4·
6+
6·
† Electronic supplementary information (ESI) available: energy balance,
synthesis and characterisation of 1+OTf2 and fluorescence spectra of 1+, 2
and 5. See http://www.rsc.org/suppdata/cc/b2/b200127f/
a T = 223 K.
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CHEM. COMMUN., 2002, 950–951
This journal is © The Royal Society of Chemistry 2002

synthesised 6+ as an analogue without the porphyrin unit.
Indeed, photolysis of complex 6+ in THF with visible light (l
> 495 nm) in the presence of triethylamine yields no prod-
ucts.
Table 2 Reduction potentials
Complex
Solvent E1/2(I)/V (vs. Fc/Fc+) Ref
1+
THF
THF
THF
THF
MeCN
THF
THF
21.44a
21.75c
21.45
21.65
21.47e
21.69
21.83
b
b
b
b
2
6+
[Re(CO)3(dmb)(3-Me-Py)]+d
[Re(CO)3(bpy)(3-Me-Py)]+
[Re(CO)3(bpy)(THF)]+
[Re(CO)3(bpy)Br]
11
8
b 8
a Partly chemically irreversible. b This work. c Totally irreversible. d dmb =
4,4A-dimethyl-2,2A-bipyridine. e Conversion using Fc/Fc+ = 380 mV vs.
SCE in MeCN ([Et4N][ClO4] electrolyte).
The electron-transfer mechanism was also tested by cyclic
voltammetry together with UV-Vis and IR spectroelectrochem-
ical studies. UV-Vis spectra show that the reversible oxidation
of 1+ and 2 is based on the porphyrin macrocycle and occurs at
the same potential as for 5 and Zn(TPP) (380 mV vs. Fc/Fc+ in
THF) (Fig. 2).
chemically irreversible. At room temperature in THF, the
reduced species 1· readily undergoes ligand substitution with
other nucleophiles. On reduction of 1+ in the presence of
bromide ions, the 3-Me-pyridine is displaced by bromide and
the radical anion 2·2 formed is reoxidised (Tables 1 and 2) to
compound 2 quantitatively.
The rhenium-free analogue 5 is effectively photo-reduced by
triethylamine to the porphyrin radical anion 5·2, confirming
that Et3N can transfer an electron to the excited state of this
porphyrin. The driving force for intramolecular electron transfer
is 20.25 eV for 1+ and ca. 0.06 eV for 2.†
In spite of the similarities between 1+ and 2 noted above,
there are great differences in their photochemical and electro-
chemical properties. Remarkably, compound 2 does not
undergo photochemical reduction under the same conditions.
Complex 2 is reduced at more negative potential than 1+ (Table
2) and the cyclic voltammogram shows an irreversible reduction
wave in THF (Fig. 2). Nevertheless, IR spectroelectrochemistry
of 2 demonstrates that the axial ligand is displaced by THF
during the reduction and subsequent reoxidation giving the
cationic THF complex 3+. The differences between 1+ and 2 are
not yet fully understood.
In conclusion, the introduction of the porphyrin into the
design of 1+ induces substitution at a remote site as a
consequence of photo-induced electron transfer. This is one of
the principles of supramolecular photochemistry described by
Balzani and Scandola but has not previously been observed in a
porphyrin metal carbonyl supermolecule.1
Fig. 2 Cyclic voltamograms of 1+, 2 and 5 in THF ([NBu4][PF6], 300 K,
scan rate 100 mV s21 and Fc/Fc+ as internal standard).
IR spectroelectrochemistry indicates that the reduction
occurs on the rhenium bipyridine site for both 1+ and 2
(compare spin distribution for [Re(CO)3(bpy)L]0).9,10 The
reduction waves for 1+ and 2 in Fig. 2 are notably different from
simple models such as 6+, [Re(CO)3(bpy)Br] and from 5. For 1+
the reduction in THF is partly chemically reversible while for
the model 6+ it is reversible. IR spectroelectrochemistry allows
identification of the reduction products thanks to the extensive
literature on [ReI(CO)3(bpy)L]n complexes.8,10–12 During re-
duction of 1+ in an OTTLE cell, three new IR bands grow in at
2016, 1914 and 1892 cm21 (Fig 1(b)); since they are identical to
those in the IR spectra of [Re(CO)3(bpy)(THF)]+ and the
photoproduct (Fig. 1(a)), they are assigned to 3+.8 The radical 1·
is expected at lower frequencies and reduction at low tem-
perature (223 K) in butyronitrile gives bands at 2010, 1903 and
1893 cm21 consistent with a radical (Table 1).10 Thus reduction
of 1+ occurs at the rhenium bipyridine site whereas reduction of
5 necessarily occurs at the porphyrin. No ligand substitution
occurs without reduction; hence the radical 1· must be the
species that undergoes substitution. Electrode potentials listed
in Table 2 reveal that, at the potential where 1· is formed,
[Re(CO)3(bpy)(THF)]· is reoxidised to the corresponding
cation. The latter should be a good model for 3·/3+. Substitution
should therefore occur catalytically [eqns. (1), (2) and (3)].
We thank Mr T. Mahabiersing and Dr A. Whitwood for help
with the experiments and COST Action D14, project D14/
0001/19 for funding.
Notes and references
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A. L. Rheingold, Coord. Chem. Rev., 1998, 171, 221.
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K. C. Gordon, Chem. Commun., 2000, 747.
6 V. W. W. Yam, Chem. Commun., 2001, 789.
7 S. S. Sun, E. Robson, N. Dunwoody, A. S. Silva, I. M. Brinn and A. J.
Lees, Chem. Commun., 2000, 201.
8 G. J. Stor, F. Hartl, J. W. M. van Outersterp and D. J. Stufkens,
Organometallics, 1995, 14, 1115.
9 W. Kaim and S. Kohlmann, Inorg. Chem., 1990, 29, 2909.
10 J. W. M. van Outersterp, F. Hartl and D. J. Stufkens, Organometallics,
1995, 14, 3303.
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(1)
(2)
(3)
The labile nature of the axial ligand upon reduction explains
why the first reduction in the cyclic voltammogram is partly
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