134
Chemistry 1983
1. Intramolecular Electron Transfer
It has occurred to many that in trying to arrive at a basic understanding of
electron transfer processes, it would be a great advantage if the reactions could
be studied in an intramolecular mode rather than, as is commonly done, in the
bimolecular or (intermolecular) mode, particularly if the geometrical relation
between the two metal centers were unambiguosly defined. Such systems had
been encountered in studies of “induced” electron transfer: (73) when a
powerful
ing coordinated organic radical can undergo intramolecular electron transfer,
the oxidation of the ligand to the carboxylic acid being completed by Co(III).
In some cases, intramolecular electron transfer can be intercepted by reaction
of the radical with the external oxidant, but at best, only relative rates were
obtained for these systems. In an elaboration of this kind of approach, in which
pulse radiolysis is used to convert the organic ligand to a radical - usually by
reduction - intramolecular transfer rates can be measured (74). These results
are important in their own right, but they do not substitute for experiments
in which metal-to-metal transfer is studied.
A strategy for dealing with the metal-to-metal case was devised (75a), which
depends on the special properties of the Co(III)/Co(II) and Ru(III)/Ru(II)
couples. The principle is the following: when a molecule (76) such as
which has both metal centers in the oxidized state, is treated with an external
reducing agent, Ru(III) is reduced more rapidly than Co(III). This is a direct
result of the differences in electronic structure,
πd
6
and
for Co(III) and
Ru(III) respectively, the former requiring much more in the way of reorga-
nization energy because the incoming electron is anti-bonding. In a subsequent
step, Ru(II) reduces Co(II) by an intramolecular process, at least if the solution
of the binuclear complex is sufficiently dilute.
The first method (75a) devised to produce the [
III,III] molecule is rather
ingenious, but it involves many steps, and it has the disadvantage that SO
4
2-
rather than NH
3
is trans to the pyridine on Ru(III). Schiffer (77) has greatly
simplified the preparative procedure by taking advantage of chemistry deve-
loped by Sargeson and co-workers (78) and has studied intramolecular electron
cules with the ortho and meta isomers as the bridging ligands. Quite indepen-
dently of our work, Haim et al. (79) h ave done experiments similar to those
described, but with Fe(CN)
5
H
2
O
3-
as the reducing agent. Substitution on
F e ( C N )
5
H
2
O
3-
takes place readily so that the simple mixing procedure
attempted by Roberts (75b) often can be used with this particular kind of
reducing agent.
H. Taube
135
A point of interest in all of these studies is to learn how the rate of reaction
responds to changes in the structure of the bridging group. In the bipyridine
case, the coupling between the pyridine rings has been variously modified (76);
since the immediate environment about each metal is left unaltered, the driving
force for the reaction is but little affected, and changes in rate can then be
attributed to changes in electronic coupling. The results obtained in studies of
this kind are outlined in a recent review article by Haim (80). Here I will only
mention an extension of this kind of strategy to a system of biochemical
interest. Gray et al (81) and Isied et al (82) have succeeded in placing
(NH
3
)
5
Ru on cytochrome C at a position remote from the porphyrin (Ru(III)-
Fe(III) separation 15Å). Different pulse methods were used by the two groups
to reduce Ru(III) preferentially over Fe(III), and though the results of the two
studies differ somewhat
respectively), it is clear that
the general strategy is effective.
2. Robust Mixed Valence Molecules
The resurgence of interest (83) in the properties of mixed valence compounds
can be traced to review articles (84, 85, 86) which appeared in 1967, and to the
first deliberate synthesis of a robust mixed valence molecule, the species shown
below, which is commonly referred to as the Creutz-Taube (87) ion
(Quite independently of our work, Cowan and Kaufman (88) prepared a
molecule based on the ferricinium/ferrocene couple.) Peter Ford and I first
produced the Creutz-Taube ion in 1967. In undertaking its preparation, we
were motivated by simple curiosity rather than by questions which might arise
from a deep understanding of the issues raised by the properties of the mixed
valence compounds. The fully reduced ([II,II]) state is readily prepared by
direct substitution using pyrazine and (NH
3
)
5
RuOH
2
2+
, and in undertaking
the project, we were taking advantage of our knowledge of affinities and rates of
substitution for both oxidation states of ruthenium. Complexes with Ru(II)
attached to heterocyclic nitrogen show very strong absorption in the visible
region of the spectrum
(89) and on observing that the quality of the
color was not significantly altered when the [II,II] species is half oxidized, we
did not pursue the matter further. Fortunately Carol Creutz took up the subject
again. The electrochemical results which she obtained about June 1968 showed
that the mixed valence state is very stable relative to the isovalent, and this
suggested to us that electronic coupling in the mixed valence species is strong.
By now, the review papers by Hush (85) and Robin & Day (86) had appeared,
and taking their content to heart, we felt certain that an intervalence band must
exist, which Carol Creutz then located in the near infra red region
= 1570
nm) where it does not affect the color (heretofore this region of the spectrum
had been little investigated by chemists). Intervalence absorption corresponds
to using the energy of a photon to transfer an electron from the reduced to the
oxidized metal center, subject to the Franck-Condon restriction. Intervalence