Immobilisation of electroactive macrocyclic complexes within titania films

Article abstract of DOI:10.1039/b505917h

The 4-carboxyphenyl-appended macrocyclic ligand trans-6,13-dimethyl-6-((4- carboxybenzyl)amino)-11,4,8,11-tetraazacyclotetradecane-6-amine (HL ¹⁰) has been synthesised and complexed with Co^III. The mononuclear complexes [Co(HL¹⁰)(CN)]²⁺ and [CoL ¹⁰(OH)]⁺ have been prepared and the crystal structures of their perchlorate salts are presented, where the ligand is bound in a pentadentate mode in each case while the 4-carboxybenzyl-substituted pendent amine remains free from the metal. The cyano-bridged dinuclear complex [CoL ¹⁰-μ-NC-Fe(CN)₅]^2- was also prepared and chemisorbed on titania-coated ITO conducting glass. The adsorbed complex is electrochemically active and cyclic voltammetry of the modified ITO working electrode in both water and MeCN solution was undertaken with simultaneous optical spectroscopy. This experiment demonstrates that reversible electrochemical oxidation of the Fe^II centre is coupled with rapid changes in the optical absorbance of the film. The Royal Society of Chemistry 2005.

Full text of DOI:10.1039/b505917h

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F U L L P A P E R
Immobilisation of electroactive macrocyclic complexes within
titania films
a
a
a
b
Paul V. Bernhardt,* Nathan L. Kilah, Andrew P. Meacham, Paul Meredith and
b
Robert Vogel
a
Department of Chemistry, University of Queensland, Brisbane 4072, Australia
School of Physical Sciences, University of Queensland, Brisbane 4072, Australia
b
Received 27th April 2005, Accepted 20th June 2005
First published as an Advance Article on the web 1st July 2005
The 4-carboxyphenyl-appended macrocyclic ligand trans-6,13-dimethyl-6-((4-carboxybenzyl)amino)-1,4,8,11-
1
0
III
tetraazacyclotetradecane-6-amine (HL ) has been synthesised and complexed with Co . The mononuclear
1
0
2+
10
+
complexes [Co(HL )(CN)] and [CoL (OH)] have been prepared and the crystal structures of their perchlorate
salts are presented, where the ligand is bound in a pentadentate mode in each case while the 4-carboxybenzyl-
1
0
substituted pendent amine remains free from the metal. The cyano-bridged dinuclear complex [CoL -l-NC-
2
−
Fe(CN)
5
]
was also prepared and chemisorbed on titania-coated ITO conducting glass. The adsorbed complex is
electrochemically active and cyclic voltammetry of the modified ITO working electrode in both water and MeCN
solution was undertaken with simultaneous optical spectroscopy. This experiment demonstrates that reversible
II
electrochemical oxidation of the Fe centre is coupled with rapid changes in the optical absorbance of the film.
Introduction
reactive that they could be coupled with surface lysine residues
on the protein bovine serum albumin to afford the corresponding
Cu-labelled proteins.
Macromonocyclic tetraamines bearing potentially coordinating
appended functional groups (pendent arms) represent a versatile
class of multidentate ligand. Substituents attached either to the
ring N-donors or the C-atom framework expand the coordinat-
ing ability of the ligand from typically tri- or tetradentate to
penta- and hexadentate thus enabling the partial or complete
encapsulation of octahedral metal ions. Primary amines and
carboxylic acids are the more common examples of pendent
arms that have been introduced to the macrocyclic periphery.
Although functionalisation of the ring secondary amines is a di-
rect route to N-functionalised macrocycles, selectivity is an issue,
with partial protection of the N-donors being necessary unless
complete alkylation is desired. An additional drawback with this
approach is that the ensuing sterically crowded tertiary amines
are poorer ligands than their secondary amine precursors and
this can lead to complications such as slow complexation kinet-
ics, mixtures of N-based isomers and lower formation constants.
Our efforts in the area of functionalised macrocyclic chemistry
have focussed exclusively on attaching substituents to the C-
Carboxylic acids also exhibit a high affinity for metal
oxides. Here we have turned our attention to the grafting of
III
macrocyclic Co complexes onto nanocrystalline TiO using
2
1
0
the new benzoic acid functionalised macrocycle L . Titania
exists in three phases; rutile, brookite (which rapidly converts to
11
rutile at low temperatures) and the semiconductor anatase. On
the surface of titania there are Ti atoms that require additional
O-atoms to complete their preferred coordination sphere.
Oxygen-containing functional groups such as carboxylic acids
are strongly chemisorbed and the result is a new Ti–OCOR
12,13
bond.
The binding mode e.g. mono-, bidentate, bridging
etc., varies with the temperature of chemisorption as indicated
by FT-IR spectroscopy through a decrease in the asymmetric
m¯ OCO stretching band for the monodentate coordinated acid and
a subsequent increase in the degree of bidentate coordination
◦
14,15
above 100 C.
Nanostructured anatase may be supported on a conducting
glass substrate and incorporated into an electrical circuit for
a variety of applications. Dye-sensitised photoelectrochemical
cells comprising Ru polypyridine dyes as the sensitiser anchored
1
1
atom framework by using ligands such as the hexamine L
in this case in its trans isomeric form) or the pentaamine
(
2
16
analogue L (Chart). Both of these ligands are easily synthesised
to titania were first reported by Gr a¨ tzel and co-workers and
2–4
in large quantities. The exocyclic amines in these ligands may
serve two purposes. With respect to metal ion complexation, the
primary amines of L and L expand the coordinating ability of
the ligand to hexadentate or pentadentate, respectively, relative
to the tetradentate mode of an unsubstituted cyclam ligand.
On the other hand, these exocyclic primary amines are ideal
this has been a very active area of research over the last 20 or
so years. The Ru dyes are attached to TiO through carboxylic
2
1
2
acid functional groups appended to the polypyridine ligand thus
ensuring high quantum efficiency for photoelectron injection
into the conduction band of titania and then into the electrical
circuit of the device. A similar principle has been applied by
Bignozzi et al. in the covalent attachment of photoactive and
3
5
targets for the attachment of redox active centres (ferrocene, L ),
8
6
fluorophores (anthracene, L ) and metal binding groups (crown
electrochromic transition metal containing dyes to TiO
2
via
9
7
ethers, L ) as shown in the Chart. In studies to date, we have
functional groups such as phosphonic, boronic and carboxylic
7–10
17–19
found that functionalisation of the exocyclic amines renders
them incapable of coordination, presumably due to steric effects.
acids.
Inclusion of acid functionality has facilitated the
chemisorption of electrochromic viologen dyes to the surface
9
20–23
That is, ligands such as L behave effectively as pentadentates
of titania.
Both photoelectrochemical and electrochromic
when complexed with octahedral metal ions (in the same way
as L ), with the four macrocyclic secondary amines plus the
exocyclic primary amine coordinated.
applications require the formation of optically transparent films
of the dye in addition to effective electronic communication
between the dye and the conducting surface, which is usually
2
An earlier investigation of this chemistry involved the attach-
ment of both aromatic and aliphatic carboxylic acids to the
conducting glass e.g. indium-doped SnO applied as a thin film
2
onto a glass slide. Here we demonstrate that macrocyclic ligands
2
9
II
10
mono-amino-substituted cyclam L . The Cu complexes of the
such as L provide an ideal scaffold for the covalent attachment
5
7
carboxylic acid functionalised ligands L –L were sufficiently
of tightly bound transition metal ions onto titania.
2
5 0 8
D a l t o n T r a n s . , 2 0 0 5 , 2 5 0 8 – 2 5 1 5
T h i s j o u r n a l i s © T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 5

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Chart 1
3
Experimental
off and the solution was diluted to 10 dm and loaded onto
a Sephadex C-25 cation exchange column (60 cm × 3.5 cm,
Safety note
+
Na form). Four distinct bands were obtained in the order
given below using various concentrations of aqueous NaClO
solution as eluent. All compounds were isolated as perchlorate
4
The release of HCN gas requires carefully controlled conditions
allowing adequate ventilation to avoid inhalation and personal
protective equipment to prevent absorption through the skin.
Perchlorate salts are potentially explosive and should only be
handled in small quantities and should never be heated in the
solid state or scraped from a sintered glass frit.
salts (caution!) by concentrating the eluate to small volume
3
(
ca. 40 cm ) which afforded crystals upon standing at room
temperature.
Band 1 (0.05 mol dm NaClO
−
3
4
, pale yellow, minor): trans-
1
[
CoL (CN)
Band 2 (0.05 mol dm NaClO
2
]ClO
4
(kmax 411 nm).
, yellow, major): trans-
(kmax 430 nm). The eluate pH was adjusted
−
3
4
Syntheses
1
0
[
CoL (CN)](ClO
4 2
)
The parent ligand trans-6,13-dimethyl-1,4,8,11-tetraazacyclo-
to 3 resulting in precipitation of X-ray quality crystals of
1
24
10
tetradecane-6,13-diamine (L ) was prepared as described.
Na [Co(CO O was synthesised according to a published
]·3H
procedure.
[Co(HL )(CN)](ClO ) ·H O. Anal. Found: C, 35.68; H, 5.51; N,
4
2
2
3
3
5
)
3
2
13.86 Calc. for CoC H N Cl O ·2H O: C, 35.40; H, 5.66; N,
3 −1 −1
2 max
2
1
36
7
2
10
2
2
13.76. Electronic spectrum (H O): k /nm (e/dm mol cm )
1
4
30 (87). H NMR (D
3
2
O-TSP) 1.14 (s, 3H, CH
) 3.77 (s, 2H CH
.59, 7.68 (q, 4H, p-substitution). C NMR (D O-TSP) 20.97,
3
), 1.24 (s, 3H,
1
0
(
a) Synthesis of crude ligand HL . In a well ventilated fume
1
CH ), 2.15–3.59 (m, 16H, macrocyclic CH
2
2
),
hood, L ·6HCl (4.77 g, 10 mmol) was dissolved in EtOH :
13
7
2
1
2
3
H
2
O (300 cm , 9 : 1 v/v) by raising the apparent pH to 5.5
3.64, 47.63, 54.09, 55.41, 58.87, 60.24, 62.88, 66.83, 72.46,
31.30, 132.27, 138.23 147.05, 178.02.
Band 3 (0.2 mol dm NaClO
CoL (CN)](ClO
Band 4 (0.5 mol dm NaClO
max 326, 447 nm).
with sodium hydroxide solution. Sodium cyanoborohydride
(
(
1.88 g, 30 mmol) was added and the pH readjusted to 5.5
caution!). 4-Carboxybenzaldehyde (1.50 g, 10 mmol) was added
−3
4
, yellow, minor): trans-
1
[
4
)
2
(kmax 434 nm).
with stirring and the pH was maintained at 5.5 for 3 h
through the periodic addition of dil. HCl. The reaction mixture
−3
1
4
, yellow, minor): [CoL ](ClO
4 3
)
(
k
4
10
was left to stir overnight, resulting in a mixture of L , L
The compounds isolated from bands 1 and 3 are reported
and 4-hydroxymethylbenzoic acid which coprecipitated as their
sodium/hydrogen salt. Purification of this mixture was carried
out by complexation and column chromatography as described
below.
26
elsewhere and the compound from band 4 has also been
2
characterised previously.
1
0
(
c) trans-[CoL (OH)]ClO
4
·5H
2
O. In a well ventilated fume
hood, the crude reaction mixture from part (a) was acidified
with HCl (1 mol dm ) to pH 3 with stirring and then
1
0
−3
(
b) [Co(HL )(CN)](ClO
4
)
2
.
The suspension from part (a)
was evaporated to dryness (caution!—contains residual cyanide)
then resuspended in water (400 cm ) in a fume hood. The pH
aerated vigorously to purge the solution of HCN gas (caution!).
Additional aliquots of acid were added to maintain a pH of 3 and
removal of HCN was judged to be complete when no further rise
3
was raised to 8 and Na
added with stirring. This mixture was heated to ca. 60 C and left
3
[Co(CO
3
)
3
]·3H
2
O (3.62 g, 10 mmol) was
◦
in pH was observed. Na
3
[Co(CO
3
)
3
]·3H
2
O (3.62 g, 10 mmol) was
◦
to stir overnight. A black precipitate of cobalt oxide was filtered
added and the mixture was stirred at 60 C overnight, filtered
D a l t o n T r a n s . , 2 0 0 5 , 2 5 0 8 – 2 5 1 5
2 5 0 9

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3
and the filtrate was diluted to 10 dm . Sephadex C-25 column
of a transparent ITO electrode through a drag coating procedure
28
chromatography (as above) yielded four bands.
Band 1 (0.05 mol dm NaClO , pale yellow, minor): trans-
derived from a literature method. The film was left overnight to
cure until dry and was then heated in a furnace. The temperature
−
3
4
1
−1
◦
[
[
[
CoL (CN)
2
]ClO
4
.
was increased at a rate of 1 min to a temperature of 450 C for
removal of the surfactant and annealing of the P25 particles.
−
3
Band 2 (0.05 mol dm NaClO
4
, yellow, minor): trans-
1
0
CoL (CN)](ClO
4
)
2
.
−
3
Band 3 (0.2 mol dm NaClO
4
, orange, major): trans-
)] . This desired fraction was concentrated to
approximately 100 cm and the pH was adjusted to 13 resulting
in precipitation of X-ray quality crystals of [CoL (OH)]ClO
O. NMR spectra were recorded of the more soluble aqua
complex, generated in situ through the addition of trifluoroacetic
10
]− on titania films
Chemisorption of [Co(HL )-l-NC-Fe(CN)
5
1
0
2+
CoL (OH
2
3
A P25 titanium oxide film on ITO prepared as above was heated
◦
◦
1
0
in a furnace to 400 C for 2 h and allowed to cool to 100 C. Two
different adsorption methods were found to be successful.
4
·
5H
2
−
3
10
(
i) Aqueous method: A ca. 500 lmol dm solution of [CoL -
2
−
l-NC-Fe(CN)
5
]
in water was acidified to pH 3.5 with HCl
3
acid. Anal. Found: C, 38.77; H, 6.75; N, 13.19 Calc. for
−
solution (0.01 mol dm ) and the slide was placed in the solution
whilst hot and allowed to soak overnight. The resulting film was
removed from the solution, washed with methanol and air dried.
CoC20
H
34
N
6
ClO
spectrum (H O): kmax/nm (e/dm mol cm ) 480 (105), 339
163) H NMR (D O-TSP) 1.14 (s, 3H, CH ), 1.24 (s, 3H, CH ),
.15–3.59 (m, 16H, macrocyclic CH ) 3.77 (s, 2H CH ), 7.58, 7.60
O-TSP) 18.89,
2.33, 46.21, 53.57, 58.10, 58.44, 60.56, 67.62, 70.97, 129.86,
7
·3H
2
O: C, 38.68; H, 6.82; N, 13.53. Electronic
3
−1
−1
2
1
(
2
3
3
(
ii) Nonaqueous method: A TiO coated ITO glass slide was
2
2
(
2
3
2
−
3
10
2−
5
1
soaked in a 5 mmol dm solution of [CoL -l-NC-Fe(CN) ]
q, 4H, p-disubstituted phenyl). C NMR (D
2
in MeOH : EtOH (1 : 1) overnight. The slide was removed from
the solution and washed well with MeOH then EtOH.
2
1
31.48, 146.52.
Band 4 (0.5 mol dm NaClO
max 326, 447 nm).
−
3
1
4
, yellow, minor): [CoL ](ClO
)
4 3
(
k
Physical methods
III
10
II
10
Electronic spectra were recorded on a Perkin-Elmer Lambda
(
d) Na[Co (HL )-l-NC-Fe (CN)
5
]·6H
2
O. trans-[CoL -
4
0 spectrophotometer and infrared spectra were recorded on
(
(
OH)]ClO
4
·5H
2
O (0.6 g, 1.1 mmol) was dissolved in water
[Fe(CN) ] (0.4 g, 1.1 mmol) was added
3
a Perkin-Elmer 1600 Series FTIR spectrometer. NMR spectra
300 cm ) at pH 7. K
4
6
1
were measured on a Bruker AC400 at 400 MHz for
H
and the reaction mixture darkened after a few minutes. The
solution was heated to 60 C and left to stir overnight. pH was
readjusted to 7 and the solution was diluted to approximately
0 dm . This solution was sorbed onto a Sephadex DEAE A-25
anion exchange (45 cm × 4 cm, ClO
NaClO
Band 1 (red, 0.05 mol dm NaClO
most probably the neutral trinuclear complex Co L -l-NC-
-l-CN-Co L but this was not present in sufficient
quantity to enable its isolation as a solid.
◦
13
spectra and 100 MHz for C spectra. Samples were mea-
sured as D O solutions and were referenced to the methyl
2
3
resonance of sodium(trimethylsilyl)propionate. Electrochemical
measurements were undertaken with a BAS100B/W poten-
tiostat utilising a Pt wire counter and Ag/AgCl reference
electrode (+196 mV vs. NHE). The working electrode was either
glassy carbon (BAS C2 cell stand) or mercury drop working
electrode (EG&G PARC Model 303 SMDE) or a small piece of
optically transparent indium-doped tin oxide (ITO). Solutions
1
−
4
form) and eluted with
4
.
−
3
4
) eluted rapidly and is
III 10
II
III 10
Fe (CN)
4
−
3
−3
−
3
contained approximately 1 mmol dm analyte and 0.1 mol dm
NaClO as supporting electrolyte in H O. X-ray photoelectron
Band 2 (red, 0.2 mol dm NaClO
4
). After concentration
of this band to ca. 20 cm EtOH was added to afford a co-
precipitate of the complex and NaClO . This was recrystallised
3
4
2
spectroscopy (XPS) data were acquired using a Kratos Axis
ULTRA X-ray Photoelectron Spectrometer incorporating a
4
by vapour diffusion of MeOH into a concentrated aqueous
solution of the crude product. Anal. Found: C, 39.25; H,
.81; N, 21.08 Calc. for C26
H, 6.09; N, 21.16%. Electronic spectrum (H
e/dm mol cm ) 509 (585), 440 (451), 321 (504). H NMR
1
65 mm hemispherical electron energy analyser and utilising
incident monochromatic Al X-rays (1486.6 eV) at 150 W (15 kV,
0 mA). Survey (wide) scans were taken at analyser pass energy
of 160 eV and multiplex (narrow) high-resolution scans at 20 eV.
5
H
36CoFeN12NaO
2
·6H
2
O C, 39.31;
O): kmax/nm
1
2
3
−1
−1
1
(
(
1
−
9
Base pressure in the analysis chamber was 1.0 × 10 Torr and
D
2
O-TSP) 1.26 (s, 3H, CH
6H, macrocyclic CH ) 4.03 (s, 2H CH
substituted). C NMR (D O-TSP) 20.96, 23.47, 48.34, 54.55,
4.90, 59.27, 60.51, 62.05, 68.46, 72.41, 132.09, 132.80, 138.29,
3
), 1.31 (s, 3H, CH
3
), 2.17–3.72 (m,
), 7.87, 8.07 (q, 4H, p-
−
8
during sample analysis was of 10 Torr in magnitude.
2
2
1
3
2
5
1
(
Crystallography
77.66, 178.11, 180.84, 193.46. Infrared (KBr disc): m¯ /cm− 2048
1
Cell constants were determined by a least squares fit to the
setting parameters of 25 independent reflections measured on
an Enraf-Nonius CAD4 four-circle diffractometer employing
graphite-monochromated Mo-Ka radiation (0.71073) and op-
erating in the x–2h scan mode. Data reduction and empirical
equatorial CN), 2069 (axial CN), 2117 (l-CN).
III
10
III
3
(
e) [Co (HL )-l-NC-Fe (CN)
aliquot of band 2 directly from the chromatography column
from the previous synthesis was treated with K (0.1 g).
The solution gradually changed from maroon to yellow over the
period of a couple of hours. On standing, yellow microcrystals
of the compound deposited which were filtered off and air
dried. Anal. Found: C, 37.89; H, 5.99; N, 20.42. Calc. for
5
]·9H
2
O. A ca. 50 cm
2
S
2
O
8
29
absorption corrections (w-scans) were performed with the
30
WINGX package. Structures were solved and refined with the
31
SHELX suite of programs. All non hydrogen atoms with the
exception of disordered perchlorate oxygen atoms were modelled
with anisotropic thermal parameters. Hydrogen atoms were
modelled for all water molecules where visible in the difference
map. All other H atoms were included at estimated positions
using a riding model. Drawings of all molecules were prepared
C
26
H
36CoFeN12
O
2
·9H
tronic spectrum (H O): kmax/nm (e/dm mol cm ) 435 (1550),
10 (sh, 1200), 320 (sh, 1800). Infrared (KBr disc): m¯ /cm 2113
equatorial CN), 2120 (axial CN), 2170 (l-CN).
2
O C, 37.83; H, 6.59; N, 20.36%. Elec-
3
−1
−1
2
−
1
4
(
32
with ORTEP3.
Preparation of P25 titania films
Crystal data
Titanium dioxide films were prepared through suspension of
P25 colloidal titania particles (12 g) in a pH 3–4 solution of dil.
1
0
[Co(HL )(CN)](ClO
4
)
2
·H
2
O.
C
21
H
38Cl
2
CoN
7
O
11
,
M
=
3
¯
nitric acid combined with 1 cm of cetyltrimethylammonium
694.41, triclinic, space group P1 (No. 2), a = 9.366(1), b =
27
˚
chloride (CTAC) surfactant. This suspension was ground with
a mortar and pestle and finally stirred to a consistent paste. A
thin layer of titanium dioxide was laid onto the conducting side
13.012(2), c = 13.069(1) A, a = 106.045(9), b = 105.90(1), c =
◦
3
˚
93.07(1) , U = 1457.6(3) A , T = 293 K, Z = 2, l(Mo-Ka) =
−
1
8.41 cm , 5460 reflections measured, 5119 unique (Rint 0.0304)
2
5 1 0
D a l t o n T r a n s . , 2 0 0 5 , 2 5 0 8 – 2 5 1 5

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1
0
which were used in all calculations, R
1
= 0.0437 (for 3890 obs.
Table 1 Solvatochromism of MMCT Transition of [CoL -l-NC-
2
−
Fe(CN)
5
]
data, I > 2r(I)), wR
2
= 0.1258 (all data).
1
0
m¯ MMCT/cm−1
46,47
[
CoL (OH)]ClO
4
·5H
2
O.
C
20
H
46ClCoN
6
O12, M = 657.01 g.
Solvent
Colour
AN
−
1
mol , monoclinic, space group P2
1
/c (No. 14), a = 9.235(3), b =
◦
3
˚
˚
H
MeOH
Ethylene glycol
DMSO
2
O
19720
18110
17990
16430
Maroon–red
Purple
Purple
54.8
2
2
5
4.023(3), c = 13.845(2) A, b = 109.01(2) , U = 2904(1) A , T =
−
1
41.3
42.8
19.3
93 K, Z = 4, l(Mo-Ka) = 7.51 cm , 5441 reflections measured,
105 unique (Rint 0.0292) which were used in all calculations,
Blue
R
1
= 0.0684 (for 3606 obs. data, I > 2r(I)), wR
data).
CCDC reference numbers 270349 and 270350.
2
= 0.2381 (all
charge which diminishes the electrostatic contribution to the
driving force of the reaction. Reactions leading to cyano-bridged
complexes of this type may proceed by direct substitution on
the Co centre by ferricyanide (without reduction) but require
good leaving groups such as chloro or aqua in the site of
substitution on the Co precursor. In this case the cyano ligand
is too tightly bound to Co and cannot be displaced by the
ferrocyanide anion. Although [CoL (CN)] is an unsuitable
See http://dx.doi.org/10.1039/b505917h for crystallographic
data in CIF or other electronic format.
III
Results and discussion
III
Synthesis
III
1
The parent ligand, L , is of great use in synthetic coordination
10
+
chemistry due to the ease of direct functionalisation and also
in the chemical stability of its transition metal complexes.
Selective reductive alkylation is a particularly versatile procedure
employing suitable aldehydes in reaction with the pendent
amines, and this can be achieved through a combination of
optimised pH conditions and appropriate reductants.
The degree of substitution can be controlled through selective
protonation of the pendent amine (pK
10
2−
precursor to [CoL -l-NC-Fe(CN)
5
] , it offers a potential route
10
2−
to the linkage isomer [CoL -l-CN-Fe(CN)
5
] . We are currently
exploring this possibility with Co complexes of L in reaction
with pentacyanoferrate(II) precursors.
III
2
7,33
Crystal structures
34
10
a
values of 5.5 and 6.3).
The X-ray crystal structure of [Co(HL )(CN)](ClO
4
)
2
·H
2
O
1
The first five stepwise protonation constants of L (expressed
reveals all molecules on general positions with one perchlorate
anion rotationally disordered about a Cl–O axis. A view of
the complex cation is shown in Fig. 1 where pentadentate
coordination of the macrocycle is apparent with the substituted
exocyclic amine remaining uncoordinated. The six-coordinate
geometry of the complex is completed by a C-bound cyano
ligand trans to the pendent amine. The Co–N bond lengths
34
as pK
a
values) are 11.0, 9.9, 6.3, 5.5 and 2.9, The third
and fourth protonation steps occur at the pendent amines as
35
shown by crystallography. Using this information, one of the
two primary amines is fully protonated at pH 5.5, and hence
protected from reaction with an aldehyde. Thus pH control
then provides a high yielding route toward monosubstituted
1
0
III
products (HL ), as desired here, while raising the pH above 5.5
(Table 1) are typical of pentadentate coordinated Co complexes
4
1
2
3,7,26,38,41,42
generates a greater proportion of disubstituted product (H
2
L ).
of L and L and their derivatives.
The cyano ligand
Reduction of the imine intermediate can be achieved selectively
with sodium cyanoborohydride, which reacts only slowly with
exhibits a significant trans influence on the Co–N5 bond length,
which is notably longer than the other four Co–N bonds.
This has been seen in other related pendent amino-macrocyclic
36
aldehydes at this pH. Reaction conditions were optimised to
utilise a three-fold excess of reducing agent, increasing the rate
of secondary amine formation in preference to intramolecular
attack upon the imine by an adjacent macrocyclic amine. This
latter unwanted reaction leads to formation of a five membered
1
2 7,26,42
cyanocobalt(III) complexes of L and L .
The hydroxyl
proton attached to O2 was located from difference maps and
the disparity in the C21–O1 and C21–O2 bond lengths supports
this assignment.
37
imidazolidine ring.
1
0
The ligand HL will inevitably exist in solution as a zwitterion
so extraction of the ligand from the crude reaction mixture was
1
0
III
not possible. Instead, purification of L as its Co complex was
our goal and this was relatively straightforward using cation
1
0
exchange chromatography. Complexes of L could be easily
1
2,26
separated from known complexes of the parent ligand L .
The presence of cyanide in the reaction mixture results from
decomposition of cyanoborohydride and this highly competitive
ligand is scavenged by the cobalt ion resulting in both mono- and
dicyano complexes. Formation of these cyano complexes can be
reduced to trace amounts by acidification of the reaction mixture
and purging with air (to drive off HCN) prior to complexation
with cobalt. This enabled the more synthetically useful hydroxo
1
0
+
10
2+
and aqua complexes [CoL (OH)] and [CoL (OH )] to be
2
obtained as precursors to dinuclear cyano-bridged species (see
below).
1
0
2−
Synthesis of the dinuclear complex [CoL -l-NC-Fe(CN)
5
]
was also facile and followed well established synthetic
1
0
2+
Fig. 1 View of [Co(HL )(CN)] (30% ellipsoids). Selected bond
38–40
methods
that benefit (thermodynamically and kinetically)
lengths ( A˚ ) Co(1)–C(13) 1.893(3), Co(1)–N(1) 1.957(3), Co(1)–N(2)
.947(3), Co(1)–N(3) 1.958(2), Co(1)–N(4) 1.958(3), Co(1)–N(5)
1.979(3), C(21)–O(1) 1.203(5), C(21)–O(2) 1.327(5).
from the electrostatic attraction, ion pair formation and outer
1
1
0
2+
sphere electron transfer between the reactants ([CoL (OH
2
)]
] ), which generates a substitution labile Co com-
plex as an intermediate which is attacked by ferricyanide. The
4
−
II
and [Fe(CN)
6
The secondary amine N-donors adopt the trans-I (RSRS)
configuration where all H-atoms lie on the same side of the
macrocycle. In this conformation, H-bonding between the non-
coordinated amine (N6) and the adjacent secondary amine H-
atoms is enabled, although these interactions are relatively weak;
1
0
+
4−
cyano complex [CoL (CN)] failed to react with [Fe(CN)
6
]
at pH 8. This is most likely related to the cathodically shifted
III/II
Co
redox potential relative to the aqua complex, which
is too negative to be reduced by ferrocyanide and the lower
D a l t o n T r a n s . , 2 0 0 5 , 2 5 0 8 – 2 5 1 5
2 5 1 1

View Article Online
3
8,40
a consequence of constraints of the chelate ring to which the
pendent amine is attached. There are stronger intermolecular
H-bonds involving the complex, perchlorate anions and water
molecule.
other than water.
In this case, the aromatic substituent on
III
10
II
−
5
[Co (HL )-l-NC-Fe (CN)
]
renders it partially soluble in
hydroxylic organic solvents such as ethylene glycol or methanol
and also the aprotic solvent DMSO. The electronic spectra mea-
sured in these solvents are shown in Fig. 3, where pronounced
solvatochromism of the MMCT transition is seen. The energy
1
0
The crystal structure of [CoL (OH)]ClO
4
·5H
2
O (Fig. 2) was
also determined and in this case the carboxylate form of the
macrocyclic ligand is evident. Of note is an intramolecular
H-bond between the hydroxo ligand and the non-coordinated
1
0
2−
5
of the MMCT transition of [CoL -l-NC-Fe(CN)
]
decreases
with decreasing solvent polarity (Table 1) from ca. 510 nm (red
in water) → ca. 600 nm (blue in DMSO). As the complex was
only partially soluble in MeOH and DMSO, saturated solutions
were prepared then filtered so all of the absorbances have been
normalised in the absence of known concentrations. The energies
of the localised d–d transitions at ∼440 (Co) and 325 nm (Fe)
were insensitive to solvent.
˚
amine (O1–H · · · N6). This draws the pendent amine ∼0.5 A
˚
closer to the metal; N6 · · · O1 3.21 A in the hydroxo complex cf.
1
0
˚
N6 · · · C13 3.73 A in [Co(HL )(CN)](ClO
4
) . The configuration
2
of the secondary amine donors is identical to that seen in the
1
0
cyano analogue. In contrast to [Co(HL )(CN)](ClO
4
)
2
·H
2
O, in
the absence of a trans influence from the hydroxo ligand the Co–
N5 bond length is similar to the other Co–N coordinate bonds.
III
10
Fig. 3 Solvatochromism of the MMCT transition of [Co (HL )-
II
−
l-NC-Fe (CN) ] .
5
Interactions between the solvent and solute are defined based
on the electron donating or accepting ability of the solvent, and
this has been quantified by Guttmann in terms of solvent donor
number (DN) and acceptor number (AN). Solvatochromism
1
0
+
Fig. 2 View of [CoL (OH)] (30% ellipsoids). Selected bond lengths
A˚ ) Co(1)–O(1) 1.910(3), Co(1)–N(1) 1.949(4), Co(1)–N(2) 1.950(4),
(
Co(1)–N(3) 1.943(4), Co(1)–N(4) 1.954(4), Co(1)–N(5) 1.967(4),
C(20)–O(2) 1.265(7), C(20)–O(3) 1.257(7).
1
0
2−
of [CoL -l-NC-Fe(CN)
5
]
is complicated by the fact that
there are terminal cyano ligands as electron donors and amine
48,49
protons as electron acceptors present.
Although there are
Electronic spectroscopy and solvatochromism
only four data pairs, the energy of the MMCT transition is
proportional to the solvent acceptor and follow the empirical
The mononuclear cobalt(III) complexes were found to exhibit
electronic maxima corresponding to previously characterised
49
−1
−1
formula m¯ MMCT = A + B·AN, giving 14556 cm and B = 89 cm
−
1
AN (correlation coefficient 0.95). The value of B reflects the
7
cyano and hydroxo pentaamine cobalt(III) complexes. The
polarity and asymmetry of the molecule and this magnitude
1
0
2−
electronic spectrum of [CoL -l-NC-Fe(CN)
5
]
exhibited a
48,49
is typical of Robin–Day Class II mixed valence complexes.
number of features characteristic of similar mixed valence
Positive solvatochromism leads to an increase in the energy of
the MMCT transition with an increase in the electron acceptor
38–40,43
complexes we have studied.
The most prominent feature
is the metal-to-metal charge transfer (MMCT) transition (ca.
46
properties of the solvent. In simple terms, the Lewis basicity of
the terminal cyano N-atoms will be greater in the ground state
II
III
5
10 nm, Fe → Co ) in addition to d–d transitions characteristic
) ca. 325 nm) and
) ca. 440 nm) chromophores. The higher
1
1
of the individual ferrocyanide ( T1g ← A1g (O
h
II
III
(
Fe –CN) than the excited state (Fe –CN), due to the inductive
III
1
1
Co N
6
( T1g ← A1g (O
h
effect of the metal. Therefore, solvents with a greater capacity for
electron acceptance (from the terminal cyano ligands) stabilise
III
1
1
energy Co transition ( T2g ← A1g (O
h
)) was obscured by the
)) band.
1
1
greater intensity iron(II) ( T1g ← A1g (O
h
III
II
II
the ground state (Co –Fe ) more than the excited state (Co –
III 10
II
2−
5
Oxidation of the red [Co L -l-NC-Fe (CN)
]
in aqueous
III
Fe ). By contrast, the poor electron acceptor DMSO cannot
solution with potassium peroxydisulfate results in the yellow
solvate the terminal cyano ligands in either oxidation state.
Electron donating solvents will interact with the amine
protons and similarly this effect will be more significant for the
III
10
III
Co (HL )-l-NC-Fe (CN)
5
complex. Mechanistic studies of
44,45
this type of oxidation reaction have been reported elsewhere.
In our studies with dinuclear complexes of this type,
3
8,40,45
we
III
II
Co –NH groups than Co –NH due to greater polarisation of
the N–H bonds in the ground state. However, all of the solvents
examined here have rather similar DN values (all bear at least
one O-atom donor) so these effects are less important across the
present series.
have observed that the fully oxidised dinuclear complexes are
poorly soluble in water due to their charge neutrality and readily
III
II
III
III
isolated as a consequence. The Co –Fe → Co –Fe oxidation
reaction takes place over a period of minutes to hours depending
2
−
on how much S
2
O
8
is added, and thus may be conveniently
monitored spectrophotometrically. Observed spectral changes
include the loss of the MMCT transition and the emergence of a
typical ferricyanide absorption band (ca. 400–420 nm) coupled
with loss of the MMCT transition at 510 nm that is characteristic
Electrochemistry
Cyclic voltammograms were obtained for mononuclear and
dinuclear complexes of L . The complex [CoL (OH
conjugate acid of the structurally characterised [CoL (OH)] )
1
0
10
2+
2
10
)] (the
38,40,43
+
of this family of cyano bridged compounds.
All previously isolated cyano-bridged dinuclear complexes
from this family have been virtually insoluble in any pure solvent
was examined at pH 7 on a hanging mercury drop electrode
III/II
and exhibited a reversible Co
redox potential at −587 mV
2
5 1 2
D a l t o n T r a n s . , 2 0 0 5 , 2 5 0 8 – 2 5 1 5

View Article Online
1
0
+
vs. Ag/AgCl. The hydroxo analogue [CoL (OH)] (dominant
at pH 12) displayed a redox potential of −707 mV vs. Ag/AgCl,
a cathodic shift that is consistent with the electrostatic effect of
deprotonation of the aqua ligand.1
required for successful chemisorption of carboxylic acids to
titania. Successful chemisorption of [CoL -l-NC-Fe(CN) ]
was achieved from acidified aqueous solution (pH 3.5) and a 1 :
1 mixture of MeOH : EtOH. Previous studies on the binding
1
0
2−
5
0
2−
The dinuclear complex [CoL -l-NC-Fe(CN)
5
]
exhibits
redox couples
couple at 411 mV vs.
of carboxylic acids to titania have utilised the protonated (acid)
III/II
III/I
15,50,51
waves corresponding to the Co
(
and Fe
form of the molecule.
There was no apparent leaching of
III/II
Fig. 4). A totally reversible Fe
complex from either film soaked in MeOH, but soaking the films
in water at pH 7 led to significant loss of complex as a result
of hydroxide ion attack on the Ti–O(carboxylate) bonds. More
detailed kinetic studies of this process and an optimal method
for chemisorption and stabilisation of the adsorbed complex are
underway.
Ag/AgCl was identified using a glassy carbon working electrode
which is similar to that found previously with similar Co –Fe
III
II
38,40
complexes from this family.
These values are ca. 200 mV more
illustrating the electrostatic influence
3
-/4−
positive than [Fe(CN)
6
]
III 10 2+
of the positively charged {Co L } moiety. Quasi-reversible
III/II
Co
waves were observed on glassy carbon. This voltammetric
X-ray photoelectron spectroscopy was utilised to characterise
1
0
behaviour appears to be due to both sluggish electron transfer
and lability of the fully reduced Co –Fe complex. Evidence
for the latter comes from the emergence of new Fe
on successive cycles that are consistent with the formation
within the diffusion layer) of equal amounts of the trinuclear
-l-CN-CoL ] (Fe
230 mV), as reported before for
These additional Fe
are absent in the initial anodic sweep thus reduction of the
titania films that included the dinuclear complex [CoL -l-
II
II
2−
NC-Fe(CN)
5
]
chemisorbed using the aqueous method (i)
III/II
10
waves
in the Experimental section. The spectrum of [CoL -l-NC-
2
−
Fe(CN)
5
]
bound to titania revealed a number of elements,
3/2
(
which were charge corrected to titanium(IV) 2p (TiO
2
, 458.6
eV) and rounded to the nearest electron volt. Observed peaks
1
0
10
III/II
52
derivative [CoL -l-NC-Fe(CN)
and free ferrocyanide (Fe
other members of this series.
4
600 mV)
III/II
1/2
3/2
include: cobalt(III) 797 eV (2p ), 782 eV (2p ); iron(II) 721 eV
3
8,40
III/II
1/2
3/2
waves
(2p ), 708 eV (2p ); oxygen 530 eV (1s); titanium(IV) 464 eV
1/2 3/2
(2p ), 456.8 eV (2p ); nitrogen 400 eV (1s) and carbon 285 eV
III
Co centre is necessary for this rearrangement to take place.
(1s).
III/II
10
The Co
redox potential was found at −685 mV, which is
Chemisorption of [Co(HL )CN](ClO
4
2
) onto P25 titania
1
0
2+
ca. 150 mV more negative than that seen in [CoL (OH
2
)]
films was unsuccessful when experimental procedures success-
10
−
5
and attributable to the influence of the coordinated negatively
charged ferrocyanide group.
ful for [Co(HL )-l-NC-Fe(CN) ] were employed. The film
retained no colour and XPS showed no evidence of complex
on the surface of the film. Given the similarities between the two
complexes, this result suggests that charge is an important factor
in facilitating chemisorption of the complex. In other words, the
1
0
2+
cation [Co(HL )CN] will be repelled by the positively charged
titania surface (at pH 3.5) whereas the negatively charged
1
0
−
5
[
Co(HL )-l-NC-Fe(CN) ] complex will be attracted initially
and eventually covalently bound.
Spectroelectrochemistry of titania-adsorbed
1
0
2−
5
[
CoL -l-NC)Fe(CN) ]
1
0
2−
5
Leaching of [CoL -l-NC)Fe(CN)
]
from the titania film
was rapid in neutral aqueous solution so electrochemistry
of the adsorbed complex was conducted at pH 4 where no
leaching of complex was apparent over the duration of the
III/II
III/II
experiment. The Fe
couple was observed but the Co
wave was obscured by a large cathodic wave that is most
probably proton reduction. For technical reasons a silver wire
pseudo reference electrode was used in addition to a Pt wire
counter electrode so that all electrodes could be contained
within a 1 cm spectrophotometer cuvette. Separate experiments
conducted in larger electrochemical cells (without the need for
simultaneous optical absorbance measurement) enabled the use
of an Ag/AgCl reference electrode and the actual potentials
shown in Fig. 5 have been corrected accordingly.
1
0
2−
5
Fig. 4 Cyclic voltammogram of [CoL -l-NC-Fe(CN)
]
(aqueous
−
1
solution, sweep rate 100 mV s ). Arrow shows initial direction of sweep.
Chemisorption to titania
The main objective of this work was to develop carboxylic acid
functionalised macrocyclic complexes that could be bound (irre-
versibly) to optically transparent conducting surfaces. As proof
1
0
Unlike the solution electrochemistry of [CoL -l-NC-
2
−
Fe(CN)
5
]
(Fig. 4) the titania-immobilised complex exhibits
1
0
III/II
of concept, we chose the dinuclear complex [Co(HL )-l-NC-
more than one Fe
wave (Fig. 5, left). This electrochemical
−
Fe(CN)
5
] as the adsorbate. This compound, like other mixed
complexity may be a consequence of various Ti–O–COR link-
ages (monodentate/bidentate) and/or different conformations
of the redox active molecule. Rearrangement to form other
oligomeric complexes on the surface cannot be ruled out either.
The peak current was found to be proportional to scan rate
which is consistent with voltammetric responses of a surface
valence complexes from its family, exhibits a prominent visible
MMCT transition that is sensitive to redox reactions at either
metal centre. Also, the electrochemistry of the ferri/ferrocyanide
moiety is well behaved and these two properties may be exam-
ined simultaneously in a spectroelectrochemical experiment to
examine various properties of the film including electrochemical
communication between the complex and the electrode, optical
switching time and the concentration of adsorbate within the
film.
The coupling between a carboxylic acid and titania requires
the TiO surface to bear a high concentration of hydroxyl groups
that enable the elimination of water upon formation of the Ti–
O–COR bond. The isoelectric point of titania is in the range
pH 5.5 to 6.5 so on this basis, acidic aqueous solutions are
53
confined molecule. As expected from the bistability of this
redox couple (both forms isolated as stable solid compounds)
III/II
the electrochemistry is totally reversible and the Fe
redox
potential of the immobilised complex (Fig. 5, left) is essentially
the same as that determined in solution (Fig. 4). In concert
with voltammetry of the surface confined complex, the spectral
changes of the optically transparent working electrode were
monitored as a function of time (Fig. 5, right). As mentioned
previously, the most obvious spectral changes upon oxidation
2
D a l t o n T r a n s . , 2 0 0 5 , 2 5 0 8 – 2 5 1 5
2 5 1 3

View Article Online
10
2−
5
Fig. 5 Left: Cyclic voltammogram of [CoL -l-NC-Fe(CN)
]
chemisorbed on a TiO
2
modified ITO working electrode (6 cycles shown) in aqueous
solution (pH 4). Right: Simultaneous absorbance changes of optically transparent working electrode at 520 nm as a function of time (note 16 s
periodicity).
will be the loss of the visible MMCT band (ca. 520 nm) of the
III
II
Co –Fe complex with a concomitant rise in absorption from
the ferrocyanide chromophore (400–420 nm). The cathodic and
anodic sweeps were 800 mV which equates to a cycle of 16 s at
−
1
the sweep rate of 100 mV s (Fig. 5, right).
The redox-linked absorbance changes at 520 nm are rapid and
−
1
not rate limiting at 100 mV s . There is a baseline absorbance
drift over the 100 s period shown (ca. 6 cycles). The baseline
absorbance (∼1.44 units on the scale shown) is due to the
titania film and the changes over the course of the experiment
perhaps are related to variations in hydration of the porous
titania structure driven by the redox reaction of the complex.
The amount of electroactive complex can be determined
directly by integrating the current under the voltammetric curves
(
Fig. 5, left) with respect to time. This gave a value in the
conservative range 100–200 pmol. The greatest uncertainty in
this number emerges from an accurate background current
subtraction. The optical absorbance changes (ca. 0.002) are di-
rectly proportional to the amount of electroactive complex. The
1
0
2−
Fig. 6 Cyclic voltammogram of [Co(L -l-NC-Fe(CN)
5
]
chemi-
sorbed on a TiO modified ITO working electrode in MeCN (Ag wire
2
pseudo reference, 9 cycles shown).
3
−1
−1
extinction coefficient of the complex is ca. 500 dm mol cm
−
2
3
−1
−1
(
5 × 10 dm mol lm ) and the titania film thickness (optical
Conclusions
path length) is estimated to be ca. 20 lm on the basis that (i)
1
0
the thickness of the tape used in the drag-coating procedure
The 4-carboxyphenyl-appended cyclam derivative HL has been
synthesised and it has been shown to be an effective pentadentate
ligand for Co where the substituted amine remains free
2
7
(
∼25 lm) will be an upper bound ) and (ii) there will be
III
some shrinkage upon dehydration of the film. Assuming that
the complex is evenly distributed within the film we arrive at
a concentration of electroactive complex within the film of ca.
1
0
from the metal. The cyano-bridged complex [CoL -l-NC-
2
−
Fe(CN)5] was prepared and chemisorbed on titania-coated
ITO conducting glass. In this form, the adsorbed complex
was electrochemically active and cyclic voltammetry of the
modified ITO working electrode was performed with simulta-
neous optical spectroscopy. This experiment demonstrated that
electrochemical switching was rapid. Although the magnitudes
of the absorbance changes in these experiments are too small
to be of practical use in electrochromic devices, other more
intensely coloured dyes comprising a coordinated macrocyclic
−
3
2
mmol dm . Coupled with the charge calculation above, this
leads to an estimation of the volume of the electroactive film of
−
9
−3
ca. 50–100 × 10 dm .
A similar experiment was conducted on the film prepared in
non-aqueous solution (nonaqueous method (ii) in Experimental
section) using MeCN as the solvent for electrochemistry. Under
these conditions variations due to hydrolysis of the Ti–OCOR
linkages and the level of hydration of the film during the
course of the experiment are eliminated. A silver wire pseudo
reference was again used and in this case the potentials have
not been calibrated. The most important difference between
1
0
ligand such as HL may be envisaged that would lead to
significant absorbance changes upon oxidation or reduction.
This is work that is currently underway in our group.
this experiment (Fig. 6) and that conducted in water (Fig. 5,
III/II
left) is the greater homogeneity of the Fe
voltammogram.
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amplitude of the change was ca. 0.003 absorbance units. Similar
charge calculations as presented above lead to ca. 300 pm of
electroactive complex. Given the approximations made this is
not considered significantly different to the calculations made
with the films examined in water.
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