Copper(n) complexes of hydroquinone-containing Schiff bases. towards a structural model for copper amine oxidases

Article abstract of DOI:10.1039/b001320j

Complexation of the preformed ligand 2,5-dihydroxy-Ar-{pyridin-2-ylmethyl}-benzylideneamine (HL1) with hydrated Cu(BF₄₂₎ afforded [{Cu(u-L')}2][BF4]j 1. The crystal structure of l-MeNO2 shows a dimer of near-planar copper(n) ions, with a bridging apical BF₄^- anion. Variable temperature susceptibility measurements showed the copper(n) ions in 1 to be moderately antiferromagnetically coupled. The complexes [CuL2]X (X- = C1O4 2, NO3 3, CP 4 or NCS5) and [CuL3]ClO4 (6; HL2 = A-{pyridin-2-ylmethyl}-A ^f'-{2,5-dihydroxybenzylidene}-l,2-diaminoethane, HL3 = A{pyridin-2-ylmethyl}-Ar'-{2,4,5-trihydroxybenzylidene}-l,2-diaminoethane) have been prepared by template condensation of Apyridin-ylmethylH^--diaminoethane with the appropriate benzaldehyde derivative and copper salt. The single crystal structure of 2 shows a near-planar four-co-ordinate copper(n) centre, with a non-co-ordinated C1O4^- anion. The chelate ligand backbone is disordered over two orientations, which correspond to different patterns of intermolecular hydrogen bonding in the lattice. UV/vis and EPR data in dmf solution suggest that 2-6 all undergo solvolysis to form an identical [CuL(dmf)Jt (x = 0-2) species in solution. Cyclic voltammograms of HL1 and 1-6 are complex, and demonstrate rapid acid-catalysed decomposition of the benzoquinonecarbaldimine ligand oxidation products. The Royal Society of Chemistry 2000.

Full text of DOI:10.1039/b001320j

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Copper(II) complexes of hydroquinone-containing Schiff bases.
Towards a structural model for copper amine oxidases
Peiyi Li,a Nayan K. Solanki,a Helmut Ehrenberg,ab Neil Feeder,a John E. Davies,a
Jeremy M. Rawsona and Malcolm A. Halcrow*c
a Department of Chemistry, University of Cambridge, Lensfield Road, Cambridge,
UK CB2 1EW
b Interdisciplinary Research Centre in Superconductivity, University of Cambridge,
Madingley Road, Cambridge, UK CB3 0HE
c School of Chemistry, University of Leeds, Woodhouse Lane, Leeds, UK LS2 9JT.
E-mail: M.A.Halcrow@chem.leeds.ac.uk
Received 17th February 2000, Accepted 31st March 2000
Published on the Web 17th April 2000
Complexation of the preformed ligand 2,5-dihydroxy-N-{pyridin-2-ylmethyl}-benzylideneamine (HL1) with hydrated
Cu(BF4)2 afforded [{Cu(µ-L1)}2][BF4]2 1. The crystal structure of 1ؒMeNO2 shows a dimer of near-planar copper()
ions, with a bridging apical BF4Ϫ anion. Variable temperature susceptibility measurements showed the copper() ions
in 1 to be moderately antiferromagnetically coupled. The complexes [CuL2]X (XϪ = ClO4Ϫ 2, NO3Ϫ 3, ClϪ 4 or NCSϪ
5) and [CuL3]ClO4 (6; HL2 = N-{pyridin-2-ylmethyl}-NЈ-{2,5-dihydroxybenzylidene}-1,2-diaminoethane, HL3 =
N-{pyridin-2-ylmethyl}-NЈ-{2,4,5-trihydroxybenzylidene}-1,2-diaminoethane) have been prepared by template
condensation of N-(pyridin-2-ylmethyl)-1,2-diaminoethane with the appropriate benzaldehyde derivative and copper
salt. The single crystal structure of 2 shows a near-planar four-co-ordinate copper() centre, with a non-co-ordinated
ClO4Ϫ anion. The chelate ligand backbone is disordered over two orientations, which correspond to different patterns
of intermolecular hydrogen bonding in the lattice. UV/vis and EPR data in dmf solution suggest that 2–6 all undergo
solvolysis to form an identical [CuL(dmf)x]ϩ (x = 0–2) species in solution. Cyclic voltammograms of HL1 and 1–6 are
complex, and demonstrate rapid acid-catalysed decomposition of the benzoquinonecarbaldimine ligand oxidation
products.
Introduction
Copper-containing amine oxidases (CAOs) catalyse the aerobic
oxidation of primary amines, eqn. (1), and are ubiquitous in
RCH2NH2 ϩ O2 ϩ H2O → RCHO ϩ NH3 ϩ H2O2 (1)
nature, having been isolated from organisms as diverse as
bacteria, yeast, plants and mammals, including humans.1 From
EXAFS,2 EPR,3 NMR,4 mutagenesis5 and crystallographic6
Fig. 1 Views of the TPQ-on (a) and TPQ-off (b) forms of the active
data it is known that CAOs contain a mononuclear copper
centre with a [Cu(His)3(OH2)n]ϩ/2ϩ (n = 0 or 2) co-ordination
sphere, together with the cofactor topaquinone (TPQ, 2,4,5-
trihydroxyphenylalaninequinone).7 The TPQ residue is very
mobile within the active site, and has been observed crystal-
lographically to be either co-ordinated to the copper ion
(Fig. 1a) or to lie 3–5 Å from it (Fig. 1b). The latter structural
form represents the resting state of a catalytically active CAO
molecule.
site of copper amine oxidase. For simplicity, only the methylene
carbon atom of the cofactor is shown.
one),11 and may represent a pathway for TOPA→Cu electron
transfer during catalysis. However, recent kinetic data have
thrown the role of copper as an electron acceptor in the CAO
oxidative half-reaction into some doubt.9 Little model chem-
istry that could shed light on these questions has been published
to date.12–14
The amine oxidation chemistry is performed by the TPQ
residue, which is concomitantly reduced to an amino-TOPA
derivative (TOPA = 2,4,5-trihydroxyphenylaniline) by a well
characterised mechanism.8 The role of the copper ion in reoxi-
dising the reduced co-factor back to TPQ is less clear.9 There is
crystallographic evidence that O2 binds close to the copper ion,
although the observed Cu ؒ ؒ ؒ O2 distance of 2.9 Å is too long to
be called a ‘proper’ co-ordinative bond.10 A catalytically com-
petent temperature- or cyanide-dependent valence tautomerism
equilibrium has also been characterised in substrate-reduced
CAO(eqn.(2);TPSQ = 2,4,5-trihydroxyphenylalaninesemiquin-
We have recently described a synthetic, spectroscopic and
electrochemical study of a series of salen-analogue Schiff
base complexes derived from 2,5-dihydroxybenzaldehyde.15
This study showed that Schiff base formation was potentially a
facile route to the incorporation of a hydroquinone moiety into
a polydentate ligand. As a complement to our continuing work
on galactose oxidase model chemistry,16 we have, therefore,
embarked on a program to prepare new CuII/quinone or CuII/
hydroquinone complexes to attempt to address the role of
copper in CAO catalysis. Our target compounds for this work
are mononuclear four-co-ordinate copper(II) complexes bearing
one hydroquinonate moiety, which have potential for copper-
and ligand-centred redox chemistry that mimics that undergone
CuII/TOPA
DOI: 10.1039/b001320j
CuI/TPSQ
(2)
J. Chem. Soc., Dalton Trans., 2000, 1559–1565
This journal is © The Royal Society of Chemistry 2000
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Table 1 Selected bond lengths (Å) and angles (Њ) for [{Cu(µ-L1)}2]-
[BF4]2ؒMeNO2 (1ؒMeNO2). Primed atoms are related to their unprimed
equivalents by the relation 1 Ϫ x, 1 Ϫ y, 2 Ϫ z
by CAO. This paper describes our initial efforts towards this
end.
Cu(1) ؒ ؒ ؒ Cu(2)
Cu(1)–N(3)
Cu(1)–N(11)
Cu(1)–O(19)
Cu(1)–O(37)
Cu(1) ؒ ؒ ؒ F(42)
2.9852(16)
1.987(7)
1.949(7)
1.933(5)
1.970(5)
2.558(5)
Cu(2)–O(19)
Cu(2)–N(21)
Cu(2)–N(29)
Cu(2)–O(37)
Cu(2) ؒ ؒ ؒ O(38Ј)
Cu(2) ؒ ؒ ؒ F(40)
1.976(5)
1.968(7)
1.937(7)
1.935(5)
2.736(7)
2.869(7)
Results and discussion
Complexes of tridentate Schiff base ligands
Reaction of 2-(2-aminoethyl)pyridine (Scheme 1) with 1 molar
equivalent of 2,5-dihydroxybenzaldehyde in refluxing MeOH
afforded a yellow solution. Evaporation of this solution to dry-
ness yielded a deep yellow oil, from which a mustard-coloured
solid HL1 could be isolated in near-quantitive yield following
trituration with the minimum volume of Et2O. Complexation
of hydrated Cu(BF4)2 with 1 equivalent of HL1 in MeNO2
afforded a deep brown solution, which deposited an oily solid
following concentration and layering with Et2O; recrystallis-
ation of this crude product from MeNO2–Et2O afforded a dark
brown crystalline material. Upon drying, this solid analysed
reproducibly as [CuL1]BF4ؒ½H2O. However, FAB mass spec-
trometry showed a strong peak at m/z = 607, assignable to the
fragment [63Cu2(L1)2 Ϫ H]ϩ. Hence, this complex was assigned
a dimeric formulation [{Cu(µ-L1)}2][BF4]2 1, which was sub-
sequently confirmed by crystallography (see below).
N(3)–Cu(1)–N(11)
N(3)–Cu(1)–O(19)
N(3)–Cu(1)–O(37)
N(3)–Cu(1)–F(42)
N(11)–Cu(1)–O(19)
95.8(3)
163.3(3)
96.1(3)
83.2(2)
93.7(3)
O(19)–Cu(2)–F(40)
N(21)–Cu(2)–N(29)
N(21)–Cu(2)–O(37) 159.5(3)
N(21)–Cu(2)–O(38Ј) 104.0(2)
N(21)–Cu(2)–F(40)
N(29)–Cu(2)–O(37)
N(29)–Cu(2)–O(38Ј)
N(29)–Cu(2)–F(40)
O(37)–Cu(2)–O(38Ј)
O(37)–Cu(2)–F(40)
105.6(2)
95.7(3)
79.5(2)
93.4(3)
85.0(2)
82.0(2)
95.1(2)
83.6(2)
N(11)–Cu(1)–O(37) 166.7(3)
N(11)–Cu(1)–F(42)
O(19)–Cu(1)–O(37)
O(19)–Cu(1)–F(42)
O(37)–Cu(1)–F(42)
O(19)–Cu(2)–N(21)
O(19)–Cu(2)–N(29) 165.9(3)
O(19)–Cu(2)–O(37)
O(19)–Cu(2)–O(38Ј)
95.8(2
76.3(2)
82.2(2)
91.5(2)
97.3(3)
O(38Ј)–Cu(2)–F(40) 166.82(17)
Cu(1)–O(19)–Cu(2)
Cu(1)–O(37)–Cu(2)
99.6(2)
99.7(2)
76.1(2)
86.7(2)
Fig. 2 View of the complex cation in the structure of [{Cu(µ-L1)}2]-
[BF4]2ؒMeNO2, showing the atom numbering scheme employed.
Primed atoms are related to unprimed atoms by the relation 1 Ϫ x,
1 Ϫ y, 2 Ϫ z. For clarity, all C-bound H atoms have been omitted.
Scheme 1 Ligands referred to in this study.
In addition to the above reaction, a large number of com-
plexations were attempted of other copper() salts by HL1, and
by other tridentate ligands derived by condensation of 2,5-
dihydroxybenzaldehyde or 2,4,5-trihydroxybenzaldehyde with
2-aminomethylpyridine, N,N-dimethyl-1,2-diaminoethane or
histamine (imidazole-4-ethanamine). In all cases, brown oils
were obtained from the reaction mixtures that could not be
further purified. From some reactions carried out in MeCN,
colourless crystals of [Cu(NCMe)4]BF4 could be isolated from
the brown oily residue. Attempted template syntheses of com-
plexes using 2,5-dihydroxybenzaldehyde, the appropriate chelat-
ing diamine and hydrated Cu(BF4)2, Cu(ClO4)2 or CuCl2 were
similarly unsuccessful. We suggest that the intractability of this
system originates from partial reduction of the copper content
of the reaction mixtures to CuI (see above), the reducing agent
probably being 2,5-dihydroxybenzaldehyde.
squares basal planes of the two Cu ions, [Cu(1), N(3), N(11),
O(19), O(37)] and [Cu(2), O(19), N(21), N(29), O(37)], forming
a dihedral angle of 40.2(3)Њ. The Cu(1) ؒ ؒ ؒ Cu(2) distance
is 2.985(2) Å, while the bridging Cu–O–Cu angles are
Cu(1)–O(19)–Cu(2) 99.6(2) and Cu(1)–O(37)–Cu(2) 99.7(2)Њ.
Although there are some deviations from trigonality, the
sums of angles at O(19) and O(37) are 359.1(7) and 360.0(7)Њ
respectively, showing that both bridging O atoms are almost
perfectly planar.
In addition to the bridging phenoxide donors the copper ions
are bridged by weak axial interactions to one BF4Ϫ anion, with
Cu(1) ؒ ؒ ؒ F(42) 2.558(5) and Cu(2) ؒ ؒ ؒ F(40) 2.869(7) Å. There
is also a weak intermolecular axial interaction to Cu(2) from
a 5-hydroxyl group on one [L1]Ϫ ligand of a neighbouring
molecule (related by 1 Ϫ x, 1 Ϫ y, 2 Ϫ z), with Cu(2) ؒ ؒ ؒ O(38Ј)
2.736(7) Å. Hence, Cu(1) adopts a square pyramidal geometry,
while the geometry at Cu(2) is better described as a distorted
octahedron (Fig. 2). The hydroxyl group O(20)–H(20) is hydro-
Single crystals of formula [{Cu(µ-L1)}2][BF4]2ؒMeNO2
(1ؒMeNO2) were grown from MeNO2–Et2O by vapour diffu-
sion. The structure contains one dimeric molecule lying on a
general position (Fig. 2, Table 1). The two copper ions adopt
tetragonal geometries, with typical Cu–N and Cu–O dis-
tances17 and an approximately planar arrangement of basal
donors, the trans-N–Cu–O angles within the basal planes of the
two ions being 159.5(3)–166.7(3)Њ (Table 1). The basal [N2Cu-
(µ-O)2CuN2] unit of the complex is not planar, the least
Ϫ
gen bonded to the second (disordered) BF4 anion, with the
following metric parameters: O(20) ؒ ؒ ؒ F(45Љ) 2.961(10), O(20)–
H(20) ؒ ؒ ؒ F(45Љ) 135Њ; O(20) ؒ ؒ ؒ F(48AЉ) 2.847(14) Å, O(20)–
H(20) ؒ ؒ ؒ F(48AЉ) 153Њ; and O(20) ؒ ؒ ؒ F(48BЉ) 2.739(18) Å,
O(20)–H(20) ؒ ؒ ؒ F(48BЉ) 163Њ. For these interactions, primed
atoms are related to unprimed atoms by 2 Ϫ x, Ϫy, 1 Ϫ z, while
F(48A) and F(48)B are different disorder orientations of the
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macrocyclic nature of 1, which removes Thompson’s proposed
additional antiferromagnetic superexchange pathway for
macrocyclic Schiff base complexes via a conjugated 2,6-bis-
(carbaldimino)phenoxy fragment.18 Alternatively, the bent,
rather than planar, Cu2O2 bridge shown by 1 (see above) might
also be expected to make J less negative.20 Incorporation of a
Weiss constant into the equation to allow for interdimer inter-
actions did not noticeably improve the refinement. This is
consistent with the crystal structure of 1ؒCH3NO2, in which the
shortest potential intermolecular superexchange pathway is via
the apical Cu(1) ؒ ؒ ؒ O(38Ј) interaction (Figs. 2 and 3). This is 7
bonds in length, and does not involve the dxy magnetic orbital
at copper. Intermolecular superexchange would therefore be
expected to be negligible.
Fig. 3 Partial packing diagram of the structure of [{Cu(µ-L1)}]2-
[BF4]2ؒMeNO2 (1ؒMeNO2), showing the ‘dimer of chains’ packing
motif. For clarity, all C-bound H atoms, and the non-co-ordinated
anion and solvent molecule, have been omitted.
In the powder complex 1 is EPR-silent at 293 and 77 K. The
X-band EPR spectrum in MeCN at 77 K exhibits only a very
weak isotropic signal at <g> = 2.08, with no hyperfine coupling
being apparent. No half-field absorption that would be indic-
ative of population of a triplet spin state21 was observed.
These data imply that the major species present in MeCN
solutions of 1 is the intact antiferromagnetically coupled
[{Cu(µ-L1)}2]2ϩ dication, with only a small amount of solvolysis
product [Cu(L1)(MeCN)x]BF4 (x = 1–3) being formed. The
visible spectrum of 1 in MeCN is dominated by O→Cu LMCT
absorptions,22 which appear as shoulders at λmax = 390 and 475
nm. The d–d bands from the copper() ions23 also occur as
shoulders, near 610 and 900 nm.
Fig. 4 Plots of χm vs. T (circles) and χmT vs. T (squares) for complex 1.
The solid lines represent best fits of the data by the Bleaney–Bowers
equation; see text for details and fitting parameters.
same F atom (see Experimental section). There are no close
contacts between the complex cation and the CH3NO2 solvent
molecule.
Complexes of tetradentate Schiff base ligands
Since the copper() complex chemistry of [L1]Ϫ and its tri-
dentate analogues is barely tractable and complicated by dimer-
isation processes, we decided to examine the co-ordination
chemistry of related tetradentate imine ligands derived from
In addition to the above intermolecular interaction, the com-
plex molecules are linked into chains by intermolecular hydro-
gen bonds between 5-hydroxyl groups of molecules related by
1 ϩ x, Ϫ1 ϩ y, Ϫ1 ϩ z, the O(20) ؒ ؒ ؒ O(38ٞ) distance being
2.731(9) Å and O(20) ؒ ؒ ؒ H(38ٞ)–O(38ٞ) 172Њ. There are also
intermolecular π–π-stacking interactions between phenol rings
of the complexes: C(13)–C(18) and C(13Љ)–C(18Љ) [2 Ϫ x, Ϫy,
1 Ϫ z], which are separated by 3.3 Å and whose centroids are
offset by 1.3 Å; and C(31)–C(36) and C(31Ј)–C(36Ј) [1 Ϫ x,
1 Ϫ y, 2 Ϫ z], which are separated by 3.2 Å and are offset by 1.6
Å. In both cases the stacked aromatic rings are strictly coplanar
by symmetry. The effects of these interactions are to link the
complex cations into ‘pairs of chains’ in the crystal lattice
(Fig. 3), the molecules associating into 1-dimensional chains
by hydrogen bonding, and each chain within the dimer being
linked by weak axial Cu ؒ ؒ ؒ O and π–π-stacking interactions.
Adjacent ‘pairs of chains’ in the lattice interact by van der
Waals contacts only.
N-(pyridin-2-ylmethyl)-1,2-diaminoethane as
a means of
obtaining mononuclear CuII/hydroquinone complexes.
Surprisingly, there is only one previous report of a Schiff
base derived from this diamine,24 although the closely related
complex [CuCl(paip)] (Hpaip = N-{2-hydroxybenzaldimino}-
NЈ-{pyridin-2-ylmethyl}-1,3-diaminopropane) did appear
during the course of this work.25 Attempts to prepare HL2 and
HL3 by reaction of N-(pyridin-2-ylmethyl)-1,2-diaminoethane
with an equimolar amount of the appropriate benzaldehyde
derivative in refluxing MeOH or MeCN afforded in all cases
crude residues following removal of the solvent, which con-
tained predominantly the imidazoline derivatives NC5H4CH2-
NC2H4NHCHR (R = C6H3{OH}2-2,5-X-4; X = H or OH) by
1H NMR. Therefore, template syntheses of complexes contain-
ing the [CuL]ϩ (LϪ = [L2]Ϫ or [L3]Ϫ) moiety were explored.
Reaction of N-(pyridin-2-ylmethyl)-1,2-diaminoethane with
2,5-dihydroxybenzaldehyde and Cu(ClO4)2ؒ6H2O, Cu(NO3)2ؒ
2.5H2O, CuCl or CuSCN in refluxing MeOH under N2 affords
green solutions, from which green or brown solids precipitate
in moderate-to-high yields following cooling and concentration
of the solutions. The solubilities of these products varied
markedly depending on the anion used; however, all the com-
plexes could be recrystallised from an appropriate solvent (see
Experimental section). The recrystallised compounds analysed
cleanly as [CuL2]X (XϪ = ClO4Ϫ 2, NO3Ϫ 3, ClϪ 4 or NCSϪ 5), a
conclusion confirmed by FAB mass spectrometry which, for all
compounds, exhibited an intense peak at m/z = 332, corre-
sponding to [Cu(L2 Ϫ H)]ϩ. The complex [CuL3]ClO4 6 was
also prepared, by an analogous template synthesis using Cu-
(ClO4)2ؒ6H2O and 2,4,5-trihydroxybenzaldehyde; this product
showed an intense peak from the fragment [CuL3]ϩ by FAB
mass spectrometry, at m/z = 349. It is noteworthy that, in those
reactions where copper() template salts were used, their oxid-
ation to CuII proceeded cleanly during the template synthesis
despite the anaerobic conditions used; the nature of the
oxidising agent in these reactions is unclear.
Variable susceptibility data on dried, powdered crystals of
complex 1 were collected between 300 and 10 K (Fig. 4). The
χmT vs. T curve is typical of a strongly antiferromagnetically
coupled system, as expected for a diphenoxo-bridged CuII
2
dimer.18 The data were analysed using the Bleaney–Bowers
equation, with corrections for a mononuclear impurity and
temperature-independent paramagnetism, according to the
H = –2J(S1ؒS2) spin Hamiltonian, eqn. (3).19 A reasonable fit of
χm =
2Ng2β2
Ϫ2J
Ng2β2
2kT
Ϫ1
ͫ
3 ϩ exp ͩ ͪͬ (1 Ϫ ρ) ϩ
ρ ϩ 2Nα
(3)
kT
kT
χmT was obtained for J = –285 cm–1 and ρ = 0.09 (Fig. 4).
During the refinements g converged consistently to values of
2.0–2.1, and so was fixed at 2.1 for the final fit; Nα was also fixed
at 60 × 10Ϫ6 cm3 molϪ1. This J value is somewhat lower than
would be expected from the correlation of J with the Cu–O–Cu
bridging bond angle (‘α’) for macrocyclic [Cu2(µ-phenoxide)2]2ϩ
compounds published by Thompson and co-workers, which
predicts J ≈ Ϫ360 cmϪ1 for α = 99.7Њ.18 This may reflect the non-
J. Chem. Soc., Dalton Trans., 2000, 1559–1565
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Table 2 Selected bond lengths (Å) and angles (Њ) for [CuL2]ClO4 (2)
Cu(1)–N(2)
Cu(1)–N(9A)
Cu(1)–N(9B)
1.967(5)
2.010(7)
2.075(16)
Cu(1)–N(12)
Cu(1)–O(20)
1.915(5)
1.870(4)
N(2)–Cu(1)–N(9A)
N(2)–Cu(1)–N(9B)
N(2)–Cu(1)–N(12)
N(2)–Cu(1)–O(20)
N(9A)–Cu(1)–N(12)
82.3(2)
85.0(3)
167.5(2)
96.29(18)
86.2(2)
N(9A)–Cu(1)–O(20) 166.0(3)
N(9B)–Cu(1)–N(12) 82.5(3)
N(9B)–Cu(1)–O(20) 167.6(5)
N(12)–Cu(1)–O(20) 96.02(18)
Fig. 6 Partial packing diagram of [CuL2]ClO4 2, showing the various
intermolecular hydrogen bonding interactions in the lattice. Details as
for Fig. 5. For clarity, the following simplifications have been made:
only the major disorder orientation of atoms C(3)–C(8), i.e. C(3A)–
C(8A), is shown; and, except for those interactions where the central
molecule is the hydrogen-bond donor, only the donor/acceptor atom
and those atoms immediately connected to it are given for each
interaction.
Fig. 5 View of the asymmetric unit in the structure of [CuL2]ClO4 2,
showing the atom numbering scheme employed. Both the major (solid
bonds) and minor (hollow bonds) disorder orientations for the cation
and anion are shown. Primed atoms are related to unprimed atoms by
the relation by 1 Ϫ x, Ϫ½ ϩ y, ½ Ϫ z. For clarity, all C-bound H atoms
have been omitted.
may reflect the presence of two different cation conformations
in the crystal. Three disordered O atoms of the anion are posi-
tioned to form hydrogen bonds to the 5-hydroxyl group of a
neighbouring ligand related by 1 Ϫ x, Ϫ½ ϩ y, ½ Ϫ z, with:
O(23) ؒ ؒ ؒ O(21ٞ) 2.918(14) Å, O(23) ؒ ؒ ؒ H(21ٞ) ؒ ؒ ؒ O(21ٞ) 175Њ;
O(30) ؒ ؒ ؒ O(28ٞ) 3.14(3) Å, O(30) ؒ ؒ ؒ H(21ٞ) ؒ ؒ ؒ O(21ٞ) 124Њ;
and O(30) ؒ ؒ ؒ O(21ٞ) 2.81(3) Å; O(30) ؒ ؒ ؒ H(21ٞ) ؒ ؒ ؒ O(21ٞ)
171Њ. There is also a hydrogen bond from O(24) to the amino
moiety of a neighbouring cation in the major ligand disorder
orientation (see above).
The IR spectroscopy of complex 5 as a Nujol mull shows an
intense thiocyanate N–C stretch at 2043 cm–1, consistent with
an N-bound NCSϪ ligand.26 Presumably, this compound exists
as a neutral square pyramidal [Cu(NCS)(L2)] complex, with an
apical NCSϪ ligand (cf. [CuCl(paip)]25). For 2 and 6, however,
the single broad Cl–O vibration centred at 1070 cmϪ1 in the IR
spectrum is consistent with a four-co-ordinate copper() com-
plex containing a non-co-ordinated anion.26 This suggestion
was subsequently confirmed crystallographically (see below). It
is unclear whether apical anion co-ordination occurs in the
solid for 3 or 4.
A dataset was also collected from weakly diffracting crystals
of [CuL3]ClO4 [6; a = 7.416(4), b = 10.752(5), c = 11.281(5) Å,
α = 105.36(2), β = 94.01(2), γ = 99.52(2)Њ, triclinic, space group
¯
P1]. An initial refinement demonstrated that the co-ordination
sphere at copper in this compound is indistinguishable from
that of 2, with a near-planar four-co-ordinate copper() centre
Ϫ
and non-co-ordinated ClO4 anion. However, this structure
X-Ray quality crystals of [CuL2]ClO4 2 were grown from
MeCN–Et2O. A view of the complex molecule is shown in Fig.
5, while selected metric parameters are given in Table 2. The
structure is complicated by disorder of the NC5H4CH2NHC2H4
portion of the ligand backbone in a 70 (C3A–C10A):30 (C3B–
C10B) occupancy ratio (Fig. 5). The co-ordination geometries
at copper in the two disorder orientations are indistinguishable,
the Cu(1)–N(9A) and Cu(1)–N(9B) bond lengths, and the
angles X–Cu(1)–N(9A) and X–Cu(1)–N(9B) [X = N(2), N(12)
or O(20)], all being identical to within 3σ. The copper ion sits
in a near-square planar environment, with unexceptional Cu–N
and Cu–O distances and trans angles within the donor plane of
166.0(3)–167.6(5)Њ. In particular, the metric parameters at
copper are essentially identical to those of the literature com-
pound [CuCl(paip)], which exhibits a very long apical Cu ؒ ؒ ؒ Cl
distance.25
also suffers from the ligand disorder described above which,
given the inferior quality of the data, could not satisfactorily be
resolved.
Since 3–5, in particular, are only poorly soluble, to enable
comparisons between these compounds all solution measure-
ments on 2–6 were performed in dmf. Complexes 2–5 exhibit
nearly identical UV/vis spectra at 298 K, showing a d–d band
with λmax = 555 1 nm (εmax = 190–220 MϪ1 cmϪ1) and O→Cu
LMCT absorption at 410 nm (4100–4500 MϪ1 cmϪ1). Similar
peaks were observed for 6, at λmax = 540 (sh) and 388 nm
(7100 MϪ1 cmϪ1), respectively. All of the above complexes gave
identical EPR spectra in dmf at 77 K, with g|| = 2.19–2.20,
g⊥ = 2.05 0.01, A||{63,65Cu} = 195 2 G at 77 K. These spectra
1
2
2
are typical of square planar {dx
}
or {dxy}1 copper()
Ϫ y
complexes,21,23 while the ratio g|| :A||{63,65Cu, cmϪ1} = 110 1 cm
from these spectra is appropriate for a near-planar four-co-
ordinate copper() centre.27 We conclude that, in solution, 2
and 6 have identical co-ordination gometries at copper, and
that, for 3–5, substantial anion dissociation occurs in dmf to
afford tetragonal [Cu(L2)(dmf)x]ϩ (x = 0–2) as the dominant
copper-containing species. As powdered solids, these complexes
all exhibit isotropic signals at <g> = 2.07–2.09.
The ligand disorder in the structure is related to inter-
molecular hydrogen bonding of the ligand NH group in the
crystal lattice (Fig. 6). In the major orientation, H(9A) forms a
hydrogen bond to O(20Ј) of an adjacent molecule related by the
operation Ϫx, Ϫy, 1 Ϫ z, with N(9A) ؒ ؒ ؒ O(20Ј) 3.106(9) Å and
N(9A)–H(9A) ؒ ؒ ؒ O(20Ј) 137Њ. There are also weaker hydro-
gen bonds to the major disorder orientation (see below) of a
neighbouring anion also related by Ϫx, Ϫy, 1 Ϫ z, with
N(9A) ؒ ؒ ؒ O(24Ј) 3.520(16) Å, N(9A)–H(9A) ؒ ؒ ؒ O(24Ј) 140Њ.
In the minor form, H(9B) is now hydrogen bonded to O(21Љ)
related by x, ½ Ϫ y, ½ ϩ z, with N(9B) ؒ ؒ ؒ O(21Љ) 3.388(18) Å
and N(9B)–H(9B) ؒ ؒ ؒ O(21Љ) 135Њ. The increased number of
hydrogen bonds in the former orientation is consistent with its
Electrochemistry
Cyclic voltammograms of the ligands and complexes were run
in dmf–0.1 M NBun PF6 or MeCN–0.1 M NBun PF6 at 293 K.
4
4
Unless otherwise stated, all potentials quoted refer to meas-
urements run at a scan rate (‘ν’) of 100 mV sϪ1 and against an
internal ferrocene–ferrocenium standard. The results of these
experiments are summarised in Table 3. The nomenclature used
to describe the voltammetric processes undergone by these
Ϫ
being more populated in the crystal. The ClO4 anion is also
disordered over two orientations which are best modelled with a
70:30 occupancy ratio. This suggests that the anion disorder
1562
J. Chem. Soc., Dalton Trans., 2000, 1559–1565

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Table 3 Cyclic voltammetric data for the complexes in this study (dmf–0.1 M NBun PF6 or MeCN–0.1 M NBun PF6, 298 K, ν = 100 mV sϪ1). All
4
potentials quoted vs. the ferrocene–ferrocenium couple. The nomenclature of ligand-centred processes corresp4onds to that we have previously
employed in ref. 15
P1
Epa/V
P2
Epc/V
P3
Epc/V
CII–CuI
E1/2/V (∆Ep/mV)
CuI–Cu0
Epc/V
Compound
Solvent
HL1
MeCN
MeCN
dmf
MeCN
dmf
dmf
dmf
ϩ0.20
ϩ0.44
ϩ0.30
ϩ0.53
ϩ0.23
ϩ0.22
ϩ0.22
ϩ0.10
Ϫ0.42
Ϫ0.29
—
—
—
—
—
—
Ϫ1.14a
—
—
—
Ϫ1.03
—
—
—
—
—
—
1 [{Cu(µ-L1)}2](BF4)2
2 [CuL2]ClO4
Ϫ0.56b
Ϫ0.96
Ϫ0.72
Ϫ0.99
Ϫ0.93
Ϫ1.08
Ϫ0.94
Ϫ1.34 (200)
Ϫ1.25b
3 [CuL2]NO3
4 [CuL2]Cl
Ϫ1.31 (240)
Ϫ1.33 (220)
Ϫ1.39 (90)
Ϫ1.36b
5 [Cu(NCS)(L2)]
6 [CuL3]ClO4
dmf
a Has an associated daughter oxidation, at Epa = ϩ0.83 V (corresponding to process P4 in ref. 15). b Irreversible process, Epc value quoted (see text).
complexes follows our earlier electrochemical study of other
hydroquinone-containing Schiff base complexes.15
of the return peak varies markedly depending on the anion
present. This suggests that close association of the anions with
the copper() centres may occur following reduction at the elec-
trode surface. No CuI–Cu0 reduction was detected for 2–6. Of
all of 2–6, only 2 was sufficiently soluble in MeCN to allow a
cyclic voltammogram to be run. In this solvent, 2 shows an
identical pattern of peaks as in dmf, showing that the ligand-
based EC chemistry undergone by 2 is not solvent-dependent.
The potentials of all these processes are slightly more positive
in MeCN than their positions in dmf, however (Table 3).
The CV of HL1 in MeCN shows a single irreversible oxid-
ation (‘P1’) at Epa = ϩ0.20 V which exhibits two irreversible
daughter processes: a weak irreversible peak (‘P2’) at Epc
=
Ϫ0.42 V; and a stronger, broad quasi-reversible couple (‘P3/P4’)
at E1/2 = Ϫ0.99 V (∆Ep = 310 mV). This pattern of daughter
peaks, and their potentials, are all very similar to those we have
previously reported for H2L4.15 By analogy with this earlier
study, we therefore assign P2 to the reduction of the protonated
initial benzoquinonecarbaldimine product,28 and the P3/P4
couple to the product of an electrochemical step–chemical step
(EC) reaction resulting in decomposition of the benzoquinone-
carbaldimine moiety. This decomposition reaction is probably
catalysed by protons liberated during the initial oxidation.15
Complex 1 under the same conditions shows two irreversible
ligand-based oxidation waves at Epa = ϩ0.44 and ϩ0.81 V, with
an approximately 1:2 intensity ratio that does not vary with
scan rate. Two weak irreversible daughter peaks are associated
with these oxidations, one of which can probably be assigned to
the P2 process on the basis of its potential. This is different
behaviour to that of [CuL4], which shows a single P1 oxidation
and a similar pattern of P2–P4 daughter waves as observed for
H2L4.15 We tentatively assign these oxidations for 1 to oxidation
of [Cu(L1)(MeCN)x]ϩ and [{Cu(µ-L1)}2]2ϩ, respectively, since
the bridging hydroquinonate groups in the latter species would
be expected to have a more positive oxidation potential. This is
also consistent with the EPR spectrum of this compound in
MeCN, which demonstrated the presence of a minor mono-
nuclear solvolysis product (see above). In addition to these
ligand-based peaks, two irreversible metal-based processes
Concluding remarks
Using a tetradentate N3O Schiff base ligand system, we have
succeeded in preparing tetragonal copper() complexes
containing, in one case, a 2,4,5-trihydroxybenzene moiety. In
principle, this latter compound (6) could be a good model to
examine TOPA→Cu electron transfer in substrate-reduced
CAO. However, the oxidative electrochemistry of the 2,5-
dihydroxybenzaldimine and 2,4,5-trihydroxybenzaldimine sub-
stituents is complex, leading to decomposition of the resultant
1,4-benzoquinonecarbaldimine products. The products of these
reactions have yet to be identified, which is something we are
pursuing. In any case, the instability of our ligands to oxidation
makes them extremely limited as CAO models, for which chem-
ically reversible ligand-based redox is a prerequisite. In order to
achieve this we are examining other ways to incorporate a 2,4,5-
trihydroxyphenylate or hydroxybenzoquinonate moiety into a
copper complex, aside from Schiff base condensation. We shall
report these results in due course.
were detected for 1: an irreversible CuII–CuI reduction at Epc
=
Experimental
Ϫ0.56 V (a shoulder on P1 near Epa = ϩ0.25 V was tentatively
assigned to a CuI–CuII daughter reoxidation); and a CuI–Cu0
peak at Epc = Ϫ1.03 V, with an associated copper-desorption
spike near Ϫ0.6 V.
Unless otherwise stated, all manipulations were carried out
in air using commercial grade solvents. 2,5-Dihydroxybenz-
aldehyde (Aldrich), 2,4,5-trihydroxybenzaldehyde (Lancaster)
and all copper salts (Avocado) were used as supplied, while
N-(pyridin-2-ylmethyl)-1,2-diaminoethane was prepared by the
literature procedure.29
The voltammetric behaviour of complexes 2–6 in dmf is
much closer to that exhibited by [CuL4].15 The complexes
exhibit two irreversible oxidations; a 2-electron P1 oxidation at
Epa = 0.10–0.30 V, and a weaker shoulder on the solvent front at
Epa ≈ 0.7–0.8 V. There is one irreversible daughter reduction
associated with the first oxidation, at a potential corresponding
to P3 of Epc = Ϫ0.94 to Ϫ1.08 V. Upon addition of the hindered
base 2,6-dimethylpyridine to the samples, the P3 daughter
product and (for 2, 3 and 6) second oxidation decrease signifi-
cantly in intensity, while a P2 daughter oxidation appears at
Epc = Ϫ0.4 V. This behaviour is consistent with the same acid-
catalysed ECE behaviour observed for HL1, H2L4 and [CuL4].15
In addition to the above ligand-based processes, complexes
Syntheses
2,5-Dihydroxy-N-{pyridin-2-ylmethyl}benzylideneamine
(HL1). To a solution of 2,5-dihydroxybenzaldehyde (3.5 g,
28.8 × 10Ϫ3 mol) in MeOH (150 cm3) was added 2-(2-amino-
ethyl)pyridine (4.0 g, 28.8 × 10Ϫ3 mol). Following a 3 h reflux,
the solvent was removed in vacuo to afford a yellow oil. Tritur-
ation with Et2O afforded a yellow solid which was recrystallised
from MeOH. Yield 5.7 g, 82% (Found: C, 69.2; H, 5.8; N, 11.6.
Calc. for C7H7NO: C, 69.4; H, 5.8; N, 11.6%). mp 145–147 ЊC.
EI mass spectrum: m/z 242, Mϩ; 123, [C6H3{OH}2CH2]ϩ and
106, [NC5H4C2H4]ϩ. NMR spectra ({CD3}2SO, 298 K): 1H,
δ 12.50 (br, 1H, Ph OH5), 8.98 (br, 1H, Ph OH2), 8.51 (d, 4.8 Hz,
₁
2–6 show a quasi-reversible one-electron reduction at E =
₂

Ϫ1.31 to Ϫ1.39 V, with Ipa :Ipc ≈ 0.7, assignable to a CuII–CuI
process. Although the forward peak in this process is almost
invariant at Epc = Ϫ1.39 to Ϫ1.44 V, the position and broadness
J. Chem. Soc., Dalton Trans., 2000, 1559–1565
1563

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1H, py H6), 8.41 (s, 1H, N᎐CH), 7.69 (dd, 7.7 and 7.4 Hz, 1H,
Table 4 Experimental details for the single crystal structure deter-
᎐
py H4), 7.28 (d, 7.7 Hz, 1H, py H2), 7.22 (ddd, 7.4, 4.8 and 0.9
Hz, 1H, py H5), 6.75 (m, 2H, Ph H4 and H6), 6.68 (d, 9.6 Hz,
1H, py H3), 3.96 (t, 6.9 Hz, 2H, CH2) and 3.08 (t, 6.9 Hz, 2H,
CH2). UV/vis (MeCN): λmax = 208 nm (εmax = 19 200 MϪ1 cmϪ1),
231 (23 000) 256 (10 400), 345 (4200).
minations
[{Cu(µ-L1)}2][BF4]2ؒMeNO2
1ؒMeNO2
[CuL2]ClO4 2
Formula
Mr
C29H29B2Cu2F8N5O6
844.27
C15H16ClCuN3O6
433.30
[{Cu(ꢀ-L1)}2][BF4]2 1. Reaction of HL1 (0.30 g, 1.2 × 10Ϫ3
mol) and Cu(BF4)2ؒxH2O (0.39 g, 1.2 × 10Ϫ3 mol) in MeNO2
(25 cm3) at room temperature rapidly afforded a dark brown
solution. This solution was filtered and concentrated to ca. 5
cm3. Vapour diffusion of Et2O afforded dark brown crystals.
Yield 0.34 g, 69% (Found: C, 41.7; H, 3.4; N, 7.0. Calc. for
C28H26B2Cu2F8N4O4.H2O: C, 42.0; H, 3.5; N, 7.0%). FAB mass
spectrum: m/z 607, [63Cu2(L1)2 Ϫ H]ϩ and 304, [63CuL1]ϩ. UV/
vis (MeCN): λmax = 207 nm (εmax = 47100 MϪ1 cmϪ1), 225
(36 800) 255 (19 100), 326 (8800), 390 (sh), 475 (sh), 610 (sh)
and 900 (sh). IR spectrum (Nujol): 1633s, 1613m, 1283s,
Crystal class
Space group
a/Å
b/Å
c/Å
α/Њ
β/Њ
Triclinic
P1
Monoclinic
P21/c
7.683(1)
11.404(1)
19.242(1)
—
99.56(1)
—
1662.5(3)
4
¯
14.605(4)
12.551(3)
11.021(3)
106.74(2)
75.19(3)
71.34(2)
1692.9(8)
2
γ/Њ
V/Å3
Z
µ(Mo-Kα)/mmϪ1
T/K
1.350
150(2)
4617
1.514
180(2)
11232
Measured
reflections
Independent
reflections
Rint
1161m, 950–1150vs, 813s, 769m and 525m cmϪ1
.
4407
2891
[CuL2]ClO4 2. A mixture of N-(pyridin-2-ylmethyl)-1,2-
diaminoethane (0.15 g, 1.0 × 10Ϫ3 mol), 2,5-dihydroxybenz-
aldehyde (0.14 g, 1.0 × 10Ϫ3 mol) and Cu(ClO4)2ؒ6H2O (0.37 g,
1.0 × 10Ϫ3 mol) in MeOH (25 cm3) was refluxed under N2 for
3 h. The resultant yellow-green solution was then concentrated
under vacuum, yielding a green-brown solid which was
recrystallised from MeCN. Yield 0.19 g, 44% (Found: C, 41.4;
H, 3.7; N, 9.5. Calc. for C15H16ClCuN3O6: C, 41.6; H, 3.7; N,
9.7%). FAB mass spectrum: m/z 432, [63Cu(L2)35ClO4]ϩ and 332,
[63Cu(L2 Ϫ H)]ϩ. UV/vis (dmf): λmax = 410 nm (εmax = 4500 MϪ1
cmϪ1) and 556 (220). IR spectrum (Nujol): 3376m, 3265m,
1634s, 1610m, 1540s, 1301s, 1285s, 1229m, 950–1150vs, 846m,
0.050
0.067
0.183
0.076
0.064
0.145
R(F)
wR(F2)
1.0 × 10Ϫ3 mol) in MeOH (25 cm3) was refluxed under N2 for
3 h. The resultant yellow-green solution was then concentrated
under vacuum and the green-brown solid isolated by filtration.
The product was recrystallised from a 1:1 MeCN:MeOH mix-
ture. Yield 0.42 g, 93% (Found: C, 40.0; H, 3.5; N, 9.1. Calc. for
C15H16ClCuN3O7: C, 40.1; H, 3.6; N, 9.3%). FAB mass spec-
trum: m/z 349, [63CuL3]ϩ. UV/vis (dmf): λmax = 388 nm (εmax
=
7100 MϪ1 cmϪ1) and 540 (sh). IR spectrum (Nujol): 3373s,
3242s, 1631s, 1611m, 1552s, 1513s, 1434s, 1343m, 1282s, 1250s,
950–1150vs, 876s, 786s, 763s, 721m, 658m, 621s, 600m and
815s, 767s, 722m, 659m, 625s, 575m and 417m cmϪ1
.
[Cu(NO3)(L2)] 3. Method as for complex 2, using Cu(NO3)2ؒ
H2O (0.23 g, 1.0 × 10Ϫ3 mol). The product was a green-brown
solid which could be recrystallised from MeCN. Yield 0.36 g,
91% (Found: C, 45.3; H, 4.0; N, 13.9. Calc. for C15H16CuN4O5:
C, 45.5; H, 4.1; N, 14.1%). FAB mass spectrum: m/z 332,
[63Cu(L2 Ϫ H)]ϩ. UV/vis (dmf): λmax = 410 nm (εmax = 4500 MϪ1
cmϪ1) and 555 (220). IR spectrum (Nujol): 3239m, 3156m,
1639s, 1609m, 1538s, 1230s, 1148s, 1120m, 1092m, 1052m,
1034m, 1008m, 941m, 884m, 836m, 813s, 757s, 724m, 656m and
406m cmϪ1
.
Single crystal structure determinations
Crystals of complexes 1ؒMeNO2 and 2 were grown by vapour
diffusion of Et2O into solutions of the complexes in MeNO2
and MeCN, respectively. Experimental details from the struc-
ture determinations are given in Table 4. All structures were
solved by direct methods (SHELXTL Plus30) and refined by full
matrix least squares on F2 (SHELXL 9331 or 9732).
416m cmϪ1
.
[CuCl(L2)] 4. Method as for complex 2, using CuCl (0.10 g,
1.0 × 10Ϫ3 mol). The product was a green solid which could be
recrystallised from MeOH. Yield 0.20 g, 54% (Found: C, 47.4;
H, 4.5; N, 10.9. Calc. for C15H16ClCuN3O2ؒ½H2O: C, 47.6; H,
4.5; N, 11.1%). FAB mass spectrum: m/z 332, [63Cu(L2 Ϫ H)]ϩ.
UV/vis (dmf): λmax = 410 nm (εmax = 4100 MϪ1 cmϪ1) and 556
(190). IR spectrum (Nujol): 3190m, 1635s, 1608m, 1540s, 1302s,
1279s, 1227m, 1145s, 1098m, 1052m, 1033m, 1025m, 836m,
[{Cu(ꢀ-L1)}2][BF4]2ؒMeNO2 (1ؒMeNO2). During refinement
the BF4Ϫ anion that does not interact with the Cu was found to
be disordered by rotation about one B–F bond. This disorder
was modelled over 3 orientations with a 0.5:0.3:0.2 occupancy
ratio, using B–F and F ؒ ؒ ؒ F distances restrained to values of
1.36(2) and 2.22(2) Å. All fully occupied non-H atoms were
refined anisotropically, and all H atoms were placed in calcu-
lated positions.
819s, 754s, 724m, 656m and 418m cmϪ1
.
[CuL2]ClO4 2. Following an initial refinement using an
ordered chelate ligand, abnormally high thermal parameters at
atoms in the NC5H4C2H4NHCH2 portion of the ligand were
indicative of disorder. The atoms C(3)–C(10) were therefore
modelled over two orientations: C(3A)–C(10A) with an occu-
pancy of 0.7; and C(3B)–C(10B) with an occupancy of 0.3. All
equivalent bond lengths within the two orientations were
restrained to be equal with an esd of 0.02 Å, and equivalent 1,3-
interatomic distances were restrained to be equal with an esd of
0.01 Å. Disorder of the O atoms of the ClO4Ϫ anion was mod-
elled using 2 orientations O(23)–O(26) and O(27)–O(30) with a
0.7:0.3 occupancy ratio, using Cl–O and O ؒ ؒ ؒ O distances
restrained to be the same value with an esd of 0.03 Å. All non-
H atoms with occupancies > 0.5 were refined anisotropically,
and all H atoms were placed in calculated positions.
[Cu(NCS)(L2)] 5. Method as for complex 2, using CuNCS
(0.12 g, 1.0 × 10Ϫ3 mol). The product was a brown solid which
was recrystallised from dmf–Et2O–hexanes. Yield 0.30 g, 76%
(Found: C, 48.9; H, 4.2; N, 14.2. Calc. for C16H16CuN4O2S: C,
49.0; H, 4.1; N, 14.3%). FAB mass spectrum: m/z 332, [63Cu-
(L2 Ϫ H)]ϩ. UV/vis (dmf): λmax = 409 nm (εmax = 4500 MϪ1
cmϪ1) and 557 (220). IR spectrum (Nujol): 3240m, 2043s, 1634s,
1615m, 1557s, 1294s, 1280s, 1218s, 1116m, 1091m, 1047m,
1032m, 996m, 940s, 859m, 830m, 814s, 769s, 724m, 656m,
573m, 533m, 471m and 416m cmϪ1
.
[CuL3]ClO4 6. A mixture of N-(pyridin-2-ylmethyl)-1,2-
diaminoethane (0.15 g, 1.0 × 10Ϫ3 mol), 2,4,5-trihydroxybenz-
aldehyde (0.17 g, 1.0 × 10Ϫ3 mol) and Cu(ClO4)2ؒ6H2O (0.37 g,
1564
J. Chem. Soc., Dalton Trans., 2000, 1559–1565

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H. C. Freeman, J. M. Guss, I. Harvey, M. A. McGuirl, M. J. C.
CCDC reference number 186/1916.
Wilce and V. M. Zubak, Structure, 1996, 4, 943; M. C. J. Wilce,
D. M. Dooley, H. C. Freeman, J. M. Guss, H. Matsunami, W. S.
McIntire, C. E. Ruggiero, K. Tanizawa and H. Yamaguchi,
Biochemistry, 1997, 36, 16116; R. Li, J. P. Klinman and F. S.
Mathews, Structure, 1998, 6, 293.
See http://www.rsc.org/suppdata/dt/b0/b001320j/ for crys-
tallographic files in .cif format.
Other measurements
7 S. M. Janes, D. Mu, D. Wemmer, A. J. Smith, S. Kaur, D. Maltby,
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Infrared spectra were obtained as Nujol mulls pressed between
KBr windows, between 400 and 4000 cmϪ1 using a Perkin-
Elmer Paragon 1000 spectrophotometer, UV/visible spectra
with a Perkin-Elmer Lambda 12 spectrophotometer operating
between 200 and 1100 nm, in 1 cm quartz cells, NMR spectra
on a Bruker DPX250 spectrometer operating at 250.1 (1H) and
62.9 MHz (13C) and mass spectra on a Kratos MS890 spec-
trometer, the fast atom bombardment spectra employing
a 3-nitrobenzyl alcohol matrix. CHN microanalyses were
performed by the University of Cambridge Department of
Chemistry microanalytical service. X-Band EPR spectra were
obtained using a Bruker ER200D spectrometer. Melting points
are uncorrected. Variable temperature magnetic susceptibility
measurements were obtained using a Quantum Design SQUID
magnetometer operating at 1000 G. A diamagnetic correction
for the sample was estimated from Pascal’s constants;19
a
diamagnetic correction for the sample holder was also applied.
Observed and calculated data were refined using SIGMA-
PLOT.33
13 J. Rall, M. Wanner, M. Albrecht, F. M. Hornung and W. Kaim,
Chem. Eur. J., 1999, 5, 2802.
14 E. Waldhör, B. Schwederski and W. Kaim, J. Chem. Soc., Perkin
Trans. 2, 1993, 2109.
All electrochemical measurements were carried out using an
Autolab PGSTAT20 voltammetric analyser, in dmf or MeCN
containing 0.1 M NBun PF6 as supporting electrolyte. Cyclic
15 E. H. Charles, L. M. L. Chia, J. Rothery, E. L. Watson, E. J. L.
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16 M. A. Halcrow, L. M. L. Chia, X. Liu, E. J. L. McInnes, L. J.
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4
voltammetric experiments involved the use of a double
platinum working/counter electrode and a silver wire reference
electrode; all potentials quoted are referenced to an internal
ferrocene–ferrocenium standard and were obtained at a scan
rate of 100 mV sϪ1. The number of electrons involved in a given
voltammetric process was determined by comparison of the
peak height with that of the FeII–FeIII couple shown by an
equimolar amount of ferrocene.
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The authors gratefully acknowledge funding by The Royal
Society (M. A. H.), the EPSRC and I.C.I. R&T Division
(N. K. S.), the Committee of Vice Chancellors and Principles
(P. L.), the European Union (Marie Curie Fellowship to H. E.),
the University of Cambridge and the University of Leeds.
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