Spectroscopic and computational studies of the ligand redox non-innocence in mono- and binuclear ruthenium vinyl complexes

Article abstract of DOI:10.1021/om1010534

A combination of IR spectroelectrochemistry and DFT calculations has been used to demonstrate that the vinyl ligands in the complexes [Ru(CH=CHC ₆H₄Me-4)Cl(CO)(PMe₃)₃] and [{RuCl(CO)(PMe₃)₃}₂(μ-CH=CHC ₆H₄CH=CH)] are redox noninnocent, and one-electron oxidation results in radical cations that are best described in terms of metal-stabilized organic radicals.

Full text of DOI:10.1021/om1010534

ARTICLE
pubs.acs.org/Organometallics
Spectroscopic and Computational Studies of the Ligand Redox
Non-Innocence in Mono- and Binuclear Ruthenium Vinyl Complexes
Wing Y. Man,† Jian-Long Xia,‡ Neil J. Brown,† Julian D. Farmer,† Dmitry S. Yufit,† Judith A. K. Howard,†
Sheng Hua Liu,‡ and Paul J. Low*,†
†Department of Chemistry, Durham University, South Road, Durham DH1 3LE, U.K.
‡Key Laboratory of Pesticide and Chemical Biology, Ministry of Education, College of Chemistry, Central China Normal University,
Wuhan 430079, People’s Republic of China
S Supporting Information
b
ABSTRACT: A combination of IR spectroelectrochemistry and
DFT calculations has been used to demonstrate that the vinyl
ligands in the complexes [Ru(CHdCHC6H4Me-4)Cl(CO)-
(PMe3)3] and [{RuCl(CO)(PMe3)3}2(μ-CHdCHC6H4CHd
CH)] are redox noninnocent, and one-electron oxidation
results in radical cations that are best described in terms of
metal-stabilized organic radicals.
uch of the intense contemporary interest in the redox
properties of ligand-bridged bi- and polymetallic com-
donating properties of the PMe3 ligands. On the basis of the
electrochemical data and the magnitude of the comproportiona-
tion constant KC, it was concluded that modification of the
bridging ligand through introduction of donor and acceptor
substituents resulted in tuning of the intermetallic electronic
interactions.
M
pounds {LnM(n)}-bridge-{M(n)Ln} arises from the potential
to use such systems as precursors to mixed-valence complexes
{LnM(x)}-bridge-{M(y)Ln}, in which the extent of electronic
interaction and electron exchange rate between the metal centers
can be tuned by variations in metal, supporting ligand, and nature
of the bridge.1-3 Fine levels of control over the electronic
properties in such systems is essential to the development of a
future molecular electronics technology based on metal-contain-
ing molecules.4-8 However, the design of mixed-valence com-
plexes based on the prototypical {LnM(n)}-bridge-{M(n)Ln}
structure can be complicated by the presence of low-lying bridge
states, with oxidation (or reduction) leading to radical systems in
which the hole (or electron) is more or less localized on the
bridge. Distinction of the mixed-valence or bridge-localized state
is best made on spectroscopic grounds by taking advantage of any
redox-state-sensitive spectroscopic “handle” that may be offered
by the metal centers and/or the bridging ligands.
However, as has been noted elsewhere, even when redox
processes in multimetallic ligand-bridged systems can be robustly
attributed to the metal centers, the magnitude of KC is sensitive
to a variety of factors, including solvation and ion-pairing
energies, as well as the synergistic or resonance terms that relate
to the concept of “metal-metal interactions”.1,17-21 To further
explore the suggestion of inversion of the relative order of
oxidation of the metal centers and bridge in these RuCl(CO)-
(PMe3)3-based systems versus the five- and six-coordinate com-
plexes of Winter, we have carried out spectroelectroche-
mical investigations of both [Ru(CHdCHC6H4Me-4)Cl(CO)-
(PMe3)3] (1) and [{RuCl(CO)(PMe3)3}2(μ-CHdCHC6H4
CHdCH)] (2), supported by DFT studies on the related models
[Ru(CHdCHC6H5)Cl(CO)(PH3)3] ([1-H]nþ; n = 0, 1) and
Winter and colleagues have demonstrated the redox noninno-
cence of the vinyl ligand in a variety of mono-,9 bi-,10,11 and
polymetallic12,13 complexes based on the [RuCl(CO)(PR3)2L]
(L = vacant coordination site or redox-innocent 2e donor; PR3 =
PPh3, PPri3) auxiliary through observation of the limited shift in
the ν(CO) band as a function of molecular redox state, among
other methods. The same group has noted how the relative order
of the metal- and bridge-based redox states can be systematically
varied by the choice of supporting ligands at the metal center.14
In the context of the latter report, in a recent study the bimetallic
ruthenium vinyl complexes [{RuCl(CO)(PMe3)3}2(μ-CHd
CHArCHdCH)] (Ar = aryl) were shown to undergo two
sequential one-electron-oxidation processes (Chart 1).15,16 It
was hypothesized that the redox events were metal-centered,
which is reasonable at first inspection, given the strongly electron
[{RuCl(CO)(PH3)3}2(μ-CHdCHC6H4CHdCH)] ([2-H]nþ
;
n = 0-2). Our results are consistent with the description of the
radical cations [1]þ and [2]þ in terms of a vinylaryl localized radical.
’ RESULTS AND DISCUSSION
The complexes 1 and 2 were prepared by the previously
reported method15,21 and characterized by the usual spectro-
scopic methods. The 31P NMR spectra exhibited an AM2 pattern,
clearly demonstrating the meridional configuration of the three
PMe3 ligands, and a single strong ν(CO) band (1, 1921 cm-1
;
Received: November 9, 2010
Published: March 10, 2011
r
2011 American Chemical Society
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2, 1921 cm-1). Single-crystal X-ray diffraction studies confirmed
the expected molecular geometries (Figures 1 and 2 and
Table 1). In each case, the metal centers exhibit a distorted-
octahedral geometry, with the vinyl ligand lying in the plane of
the CO ligand, as expected from orbital considerations.22 The
bond between the metal centers and the phosphine trans to
the vinyl ligand is elongated relative to the bond between the
metal and the mutally trans phosphines, reflecting the greater
trans effect of the vinyl ligand. There is no evidence for a
significant cumulenic contribution to the structure of the
bridiging ligand in 2.
It has been shown elsewhere that monometallic vinyl com-
plexes [Ru(CHdCHC6H4R-4)Cl(CO)(PMe3)3] undergo a sin-
gle, chemically irreversible, one-electron-oxidation process, the
potential of which tracks linearly with the Brown σþ parameter of
the vinyl ligand subsituent, R.21 The chemical reversibility of this
redox event is enhanced by strong donor substituents such as R =
OMe, NH2, NMe2. At room temperature in CH2Cl2/0.1 M
NBu4PF6, the CV of 1 (v = 100 mV/s) displays a single,
Chart 1. Family of Bimetallic Bis(vinyl) Complexes and
Selected Electrochemical Dataa
irreversible oxidation wave at a glassy-carbon electrode (Ep,a
=
0.59 V). At lower temperatures the chemical reversibility of the
redox wave improves (ip,c/ip,a = 0.69 at ca. -50 °C). The redox
event never becomes perfectly chemically reversible, with a
substantial ΔE = |Ep,a - Ep,c| = 210 mV, which is larger than
that of the internal decamethylferrocene standard and indicative
of slow electron transfer. The CV of the bimetallic complex 2 is
characterized by a series of oxidation waves, the first two of which
(E1/2 = 0.13, 0.44 V) become chemically, but not electrochemi-
cally, reversible at ca. -50 °C.
To better assign the character of these redox processes, IR
spectroelectrochemical studies were carried out on both 1 and 2,
using the ν(CO) band as an indicator of the metal oxidation
state. For 1, the ν(CO) band at 1921 cm-1 shifted smoothly to
1974 cm-1 on oxidation to [1]þ in the spectroelectrochemical
cell (Table 2). The 53 cm-1 shift to higher wavenumbers is
somewhat smaller than the ca. 100 cm-1 shift expected for a
metal-based oxidation11 and indicates an appreciable fraction of
the charge lost on oxidation originates from the organic π system.
In support of this proposal, the phenylene ring ν(CC) stretch is
observed to gain intensity (Table 2). Reduction of [1]þ resulted
in a significant recovery of the spectrum of 1, confirming the shift
in ν(CO) to be associated with the electrochemical formation of
[1]þ, although a degree of decomposition was evident by the
reduced intensity of the ν(CO) band observed after reduction,
and a secondary product band at 1944 cm-1 not present in the
original sample. Similarly, stepwise oxidation of the bimetallic
complex 2 caused the ν(CO) band to shift from 1921 (2) to 1934
([2]þ) and 1974 cm-1 ([2]2þ) (Figure 3), with recovery of 2 on
back-reduction accompanied by formation of a decomposition
aConditions: CH2Cl2/0.1 M NBu4PF6; Ag/Agþ (0.01 M AgNO3/0.1 M
NBu4PF6/NCMe) reference, separated by a salt bridge containing
CH2Cl2/0.1 M NBu4PF6 at ambient temperature.15 ΔE = E2 - E1;
KC = exp((ΔE)F/RT).
Figure 1. Plot of a molecule of 1, showing the atom-labeling scheme. Thermal ellipsoids are plotted at the 50% probability level.
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Figure 2. Plot of a molecule of 2, showing the atom-labeling scheme. Thermal ellipsoids are plotted at the 50% probability level.
Table 1. Selected Bond Lengths, Bond Angles, and Torsion Angles from the Crystallographically Determined Structures of 1
and 2, together with Analogous Data from the Computational Models [1-H]nþ (n = 0, 1) and [2-H]nþ (n = 0-2)a
1
1-H
[1-H]þ
2
[2-H]
[2-H]þ
[2-H]2þ
Bond Lengths (Å)
2.3947
2.5091
2.3947
2.009
Ru(1)-P(1)
Ru(1)-P(2)
Ru(1)-P(3)
Ru(1)-C(1)
C(1)-C(2)
C(2)-C(3)
C(3)-C(4)
C(4)-C(5)
C(7)-C(8)
C(6)-C(9)
C(9)-C(11)
C(11)-Ru(2)
Ru(2)-P(4)
Ru(2)-P(5)
Ru(2)-P(6)
2.3633(6)
2.3940(6)
2.3617(6)
2.092(3)
1.311(4)
1.488(4)
1.398(4)
1.413(4)
1.369(4)
1.512(4)
2.3514
2.4350
2.3514
2.107
1.346
1.480
1.408
1.392
1.395
2.3611(11)
2.4064(11)
2.3663(11)
2.112(4)
2.3510
2.4347
2.3508
2.108
1.347
1.477
1.407
1.390
1.390
1.477
1.347
2.108
2.3510
2.4347
2.3508
2.3744
2.4697
2.3744
2.051
1.377
1.436
1.428
1.372
1.372
1.436
1.377
2.051
2.3744
2.4697
2.3743
2.4028
2.5281
2.4039
1.981
1.415
1.401
1.448
1.359
1.359
1.401
1.415
1.981
2.4028
2.5281
2.4039
1.396
1.333(6)
1.431
1.483(6)
1.428
1.399(6)
1.382
1.388(6)
1.383
1.387(6)
1.481(6)
1.327(6)
2.115(4)
2.3597(12)
2.3969(12)
2.3614(13)
Bond Angles (deg)
166.74
P(1)-Ru(1)-P(3)
P(2)-Ru(1)-C(1)
Ru(1)-C(1)-C(2)
C(9)-C(11)-Ru(2)
P(4)-Ru(2)-P(6)
P(5)-Ru(2)-C(11)
168.10(2)
177.55(7)
134.0(2)
161.86
169.74
130.3
161.97(4)
178.27(12)
131.3(3)
161.63
169.76
130.1
166.41
169.72
131.2
166.37
170.20
131.5
169.80
130.8
130.4(3)
130.1
131.2
163.44(4)
178.92(13)
161.63
169.76
166.41
169.72
166.34
170.20
Torsion Angles (deg)
C(10)-Ru(1)-C(1)-C(2)
C(1)-C(2)-C(3)-C(4)
C(7)-C(6)-C(9)-C(11)
C(21)-Ru(2)-C(11)-C(9)
-0.04
0.00
0.00
0.76
0.00
0.01
0.01
0.00
-0.07
-0.15
-0.15
-0.07
0.06
-1.42
-1.42
0.76
a For atom labeling schemes, see Figures 1 and 2.
product (ν(CO) 1950 cm-1). The small changes in ν(CO)
frequency as a function of oxidation state in the bimetallic
complex [2]nþ are not consistent with a metal-localized redox
event. In addition, the phenylene ring ν(CC) stretch remains
silent in all oxidation states of [2]nþ, consistent with a symmetric
electronic structure.
of a phenylene radical cation not present in 2 or [2]2þ and
supporting the notion of considerable phenylene ligand chara-
cter in the orbital supporting the unpaired electron/hole
(Figure 4).23,24 The near-IR spectrum of [2]þ featured a unique
absorption envelope not found for 1, [1]þ, 2, or [2]2þ. Decon-
volution of this absorption band into a sum of Gaussian-shaped
curves reveals two transitions (ν1 = 8400 cm-1, ε = 3200 M-1
The UV-vis spectrum of [2]þ features a vibrationally struc-
tured absorption between 22 000 and 16 000 cm-1, characteristic
cm-1, Δν1/2 = 2200 cm-1; ν2 = 9800 cm-1, ε = 1700 M-1 cm-1
,
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Table 2. Spectroelectrochemically Determined and Calcu-
lated ν(CO)/ν(CdC)ring Vibrational Frequencies (cm-1) for
[1]nþ, [1-H]nþ, [2]nþ, and [2-H]nþ
n
[1]nþ
[1-H]nþ
[2]nþ
[2-H]nþ
0
1
2
1921 s/not obsd 1984 s/1637 vw 1921/not obsd 1984 s/1643 vw
1974 s/1607 m 2015 m/1611 m 1934/not obsd 2001 s/1633 vw
1974/not obsd 2025 s/1645 vw
Figure 5. Near-IR spectrum of [2]þ showing deconvolution into two
Gaussian shaped sub-bands.
In the present case, the observation of two low-energy optical
transitions in [2]þ is also entirely consistent with a three-state
model in which the bridge is appreciably involved in the
stabilization of the charge, although the number of bands alone
cannot distinguish between a bridge-localized or mixed-va-
lence ground state.33,34 However, taken together, the IR ν-
(CO) and UV-vis data are compelling in the assignment of a
bridge-localized ground state to [2]þ.
Figure 3. ν(CO) spectra of [2]nþ (n = 0-2) collected during in situ
oxidation in a spectroelectrochemical cell (CH2Cl2/0.1 M NBu4PF6).
To further explore the electronic structures of [1]nþ (n = 0, 1)
and [2]nþ (n = 0-2), DFT calculations were carried out on
the model systems [Ru(CHdCHC6H5)Cl(CO)(PH3)3] ([1-
H]nþ) and [{RuCl(CO)(PH3)3}2(μ-CHdCHC6H4CHdCH)]
([2-H]nþ) using the B3LYP/3-21G* basis set and functional
found suitable for studies of related acetylide complexes.35 Se-
lected bond lengths and angles from the optimized geometries of
[1-H]nþ and [2-H]nþ are summarized in Table 1, together with
the crystallographically determined data from 1 and 2. Key
vibrational frequencies from the spectroelectrochemical work
and those calculated from the models are given in Table 2. The
small shift in ν(CO) frequency to higher wavenumbers observed
upon oxidation of 1 and 2 in the spectroelectrochemical experi-
ments is reproduced in the computational model systems [1-H]
and [2-H]. Overall, the excellent agreement between the observed
and calculated structural and IR parameters gives considerable
confidence in the accuracy of the models.
Oxidation of [1-H] to [1-H]þ causes a small elongation of the
Ru-P bond lengths of 0.04 Å for Ru-P(1,3) and 0.07 Å for Ru-
P(2). These variations are accompanied by changes in the Ru-C
and C-C bond lengths in the vinyl ligand, which can be
represented by an increased contribution from the cumulenic
tautomer shown in Figure 6. These structural changes are readily
understood when the orbital structures of [1-H] and [1-H]þ are
compared. In the case of [1-H], the HOMO is heavily (84%)
localized on the vinyl ligand, antibonding in character between
Ru and C(1) and between C(2) and C(3), and bonding between
C(1) and C(2) (Figure 7a). In the case of [1-H]þ the R-HOSO
and β-LUSO offer essentially the same orbital characteristics as
the HOMO in [1-H], and a plot of the spin density in [1-H]þ
clearly shows the dominant role of the unsaturated ligand in
supporting the unpaired electron/hole (Figure 7b). The struc-
tural variations noted above are likely due to decreased metal-to-
phosphine back-bonding, and enhanced metal-vinyl π bonding
brought about by depopulation of the HOMO in 1. In essence,
Figure 4. UV-vis-near-IR spectra of [2]nþ (n = 0-2) collected
during in situ oxidation in a spectroelectrochemical cell (CH2Cl2/0.1
M NBu4PF6).
Δν1/2 = 2600 cm-1), although we cannot rule out the presence
of additional bands of much lower intensity (Figure 5). Genuine
mixed valence complexes often exhibit multiple intervalence
charge transfer (IVCT) bands in the near-IR region, arising from
a lifting of degeneracy of the metal d orbitals at the metal donor
site by spin-orbit coupling25,26 or low metal complex local
symmetry imposed by a combination of supporting and bridging
ligand.27,28 IVCT bands can also exhibit vibrational progressions
which can complicate the appearance of these transitions.29 In
addition, interconfigurational (dd) bands can also be found
in the near-IR region and overlap with genuine IVCT
bands,25,30-32 as can transitions to higher-lying bridge states.33
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Figure 6. Oxidation of 1 to [1]þ and valence tautomers.
Figure 8. Plots of (a) the HOMO in 2-H and (b) the spin density in
[2-H]þ. Sum of Mulliken atomic spin densities for key fragments in
[2-H]þ: Ru(1), 0.0923; C(1)HdC(2)H, 0.2461; C6H4, 0.3432; C-
(9)H-C(11)H, 0.2461; Ru(2), 0.0924.
Figure 7. Plots of (a) the HOMO of 1-H and (b) the spin density in
[1-H]þ. Mulliken atomic spin densities for key fragments of [1-H]þ:
Ru, 0.2944; CHdCH, 0.3840; C6H5, 0.3629.
the β-LUSO in [2-H]2þ and is heavily (74%) based on atoms
associated with the bridging ligand, with only 22% of the
character coming from the metal centers.
the vinyl ligand becomes a better π acceptor on oxidation,
leading to an increase in the trans-located Ru-P bond and a
decrease in the metal electron density available for back-bonding
to the phosphine and carbonyl ligands, the latter being reflected
by the small increase in ν(CO) frequency.
In summary, the small shifts of ν(CO) frequencies to higher
wavenumbers observed on oxidation of mono- and bimetallic
vinyl complexes 1 and 2 are not consistent with metal-based
redox processes. DFT studies of model compounds indicate that
the HOMOs of 1 and 2 are vinyl ligand in character, and
consequently the oxidation processes observed in the experi-
mental system can be rationalized in terms of depopulation of
this unsaturated ligand orbital. Given the large contribution of
the bridging ligand to the β-LUSO in [2-H]þ, [2]þ is better
described in terms of a radical cation with a low-lying bridge state
rather than as a mixed-valence compound. The trends in the
electrochemical responses of the related complexes shown in
Chart 1 can therefore be rationalized in terms of the electronic
effect of the substituent groups on the redox properties of the
divinylbenzene moiety. With 2 as a reference, the electron-
donating aryl substituents (CH3, OMe) make oxidation of the
divinyl ligand more facile (in the thermodynamic sense), while
the electron-withdrawing substituents have the anticipated
opposite effect on the first redox potential. The range of first
oxidation potentials spans some 0.47 V. There is less variation in
the second oxidation potentials, which span only 0.28 V, reflect-
ing the diminished inductive effects of the substituents on the
oxidation of the aryl radical cation. The greater KC values
For the bimetallic model complex [2-H], the HOMO is again
largely localized on the unsaturated bridging ligand (86%)
(Figure 8a), and this orbital is sequentially depopulated upon
oxidation to [2-H]þ and [2-H]2þ. This is reflected in the small
increases in the Ru-P bond lengths, with the most significant
elongation associated with those ligands trans to the bridge and
the evolution of a significant degree of quinoidal character in the
structure (Figure 9). A plot of the spin density in [2-H]þ is given
in Figure 8b, with a summation of the Mulliken charges on key
molecular fragments given in the caption. With regard to the
vis-near-IR spectra of [2]þ, TD-DFT calculations with the
model [2-H]þ predict two low-energy transitions, which can
be described as metal to bridge (MLCT) and chloride to bridge
(LLCT) in character.
The computational model of the dication [2-H]2þ only gave a
stable minimum for the singlet state; the triplet state is some 18
kcal mol-1 higher in energy and will not be discussed further
here. An examination of the bond lengths and angles in Table 1
indicate that the structure is best represented in terms of a
quinoidal form (Figure 9). The LUMO of [2-H]2þ offers
essentially the same composition as the HOMO in 2-H and
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refined in riding mode. Disordered atoms of solvent CH2Cl2 molecules
in structure 2 were refined isotropically. Crystallographic data for the
structures have been deposited with the Cambridge Crystallographic
Data Centre as supplementary publications CCDC 809635 and 809636.
Crystal Data for 1: C19H36ClOP3Ru, Mr = 509.91, monoclinic, space
group P21/n, a = 14.3554(5) Å, b = 12.3124(4) Å, c = 14.8419(5) Å, β =
111.26(1)°, U = 2444.7(1) Å3, F(000) = 1056, Z = 4, Dc = 1.385 Mg m-3
,
μ = 0.953 mm-1, 32 201 reflections (1.69 e θ e 30.00°) collected (ω
scan, 0.2°/frame), yielding 7117 unique data (Rmerg = 0.0276), final
wR2(F2) = 0.0933 for all data (226 refined parameters), conventional
R1(F) = 0.0352 for 6539 reflections with I g 2σ, GOF = 1.070.
Crystal Data for 2: C30H62O2Ru2Cl2P6 3CH2Cl2, Mr = 1168.43,
3
monoclinic, space group C2/c, a = 55.2276(11) Å, b = 9.4525(2) Å, c =
21.2707(4) Å, β = 106.95(1)°, U = 10621.8(4) Å3, F(000) = 4768, Z = 8,
Dc = 1.461 Mg m-3, μ = 1.179 mm-1, 63 667 reflections collected (ω
scan, 0.2°/frame), yielding 14 109 unique data (Rmerg = 0.0317), final
wR2(F2) = 0.1277 for all data (485 refined parameters), conventional
R1(F) = 0.0568 for 12 694 reflections with I g 2σ, GOF = 1.015.
Computational Details. All DFT computations were carried out
with the Gaussian 03 package.38 The model geometries of the dinuclear
systems [1-H]nþ and [4-H]nþ (n = 0-2) discussed here were optimized
at the B3LYP/3-21G* level of theory,39-42 to reduce computational
effort, with no symmetry constraints. MOs and frequencies were
computed on these optimized geometries at the same level of theory.
All geometries were identified as minima (no imaginary frequencies).
The MO contributions were generated using the GaussSum package.43
Figure 9. Valence bond representations of the structures of [2-H]nþ
(n = 0-2).
’ ASSOCIATED CONTENT
S
Supporting Information. Text giving the complete ref 38,
b
observed for the complexes bearing donor substituents therefore
reflects the greater thermodynamic stability of the aryl cation
bearing these groups, and not a more significant delocalization of
charge between the metal centers, or intermetallic “communica-
tion”. This family of complexes therefore serves as another
example of the important role spectroscopic studies can play
when coupled with electrochemical measurements in the assess-
ment of the properties and redox behavior of ligand-bridged
bimetallic compounds {LnM}-bridge-{MLn}.
CIF files giving crystallographic data, tables giving Cartesian
coordinates of optimized structures, and a figure giving plots of
the cyclic voltammograms of 1 and 2. This material is available free
of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION
Corresponding Author
*Tel: þ44 (0) 191 334 2114. Fax: þ44 (0) 191 384 4737. E-mail:
p.j.low@durham.ac.uk.
’ EXPERIMENTAL SECTION
’ ACKNOWLEDGMENT
General Conditions. The complexes 1 and 2 were prepared as
described elsewhere.15,21 Electrochemical analyses were carried out
using an EcoChemie Autolab PG-STAT 30 potentiostat, from solutions
in CH2Cl2 containing 0.1 M NBu4PF6 electrolyte, glassy-carbon work-
ing, Pt counter, and Pt pseudoreference electrodes; v = 100 mV s-1. The
data are reported against ferrocene (E(Fc) = 0 V) with the decamethyl-
ferrocene/decamethylferrocenium couple used as an internal reference
for potential measurements (E([Fc*]/[Fc*]þ) = -0.48 V]).36 Spectro-
electrochemical measurements were carried out in an OTTLE cell of
Hartl design,37 from CH2Cl2 solutions containing 0.1 M NBu4PF6
electrolyte. The cell was placed in the sample compartment of a Thermo
6700 FT-IR or a Perkin-Elmer Lambda 900 UV-vis-near-IR spectro-
photometer. Bulk electrolysis was performed with a home-built
potentiostat.
Crystallography. Single-crystal X-ray data were collected on a
Bruker SMART 6000 diffractometer equipped with a Cryostream
(Oxford Cryosystems) nitrogen cooling device at 120 K using graphite-
monochromated Mo KR radiation (Mo KR, λ = 0.710 73 Å). The
structures were solved by direct methods and refined by full-matrix least
squares on F2 for all data using SHELXTL and OLEX2 software. All non-
disordered non-hydrogen atoms were refined with anisotropic displace-
ment parameters; H atoms were placed in calculated positions and
Dr. Mark A. Fox and Dr. Phil A. Schauer are gratefully
acknowledged for helpful discussions and technical assistance.
W.Y.M. holds a scholarship from the Durham Doctoral Training
Account. J.D.F. held a University of Durham Doctoral Fellow-
ship. This work was supported by the EPSRC through a Leader-
ship Fellowship awarded to P.J.L. (No. EP/H005595/1).
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