Ruthenium(ii) complexes containing functionalised β-diketonate ligands: Developing a ferrocene mimic for biosensing applications

Article abstract of DOI:10.1039/c4dt01459f

Three series of ruthenium complexes with the general formula Ru(bpy) _n(β-diketonato)_3-n (bpy = 2,2′-bipyridine, n = 0, 1, 2) were prepared and investigated using cyclic voltammetry and UV-vis spectroscopy. Variation of both the number and electronic demand of the β-diketonato ligands resulted in well-defined modulation of the potential at which oxidation of the metal centre occurred. The observed potentials were shown to be in good agreement with calculated ligand electrochemical parameters. A novel ruthenium(ii) complex with electrochemical behaviour similar to that of ferrocene was identified. This journal is the Partner Organisations 2014.

Full text of DOI:10.1039/c4dt01459f

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Ruthenium(II) complexes containing functionalised
β-diketonate ligands: developing a ferrocene
mimic for biosensing applications†
Cite this: Dalton Trans., 2014, 43,
12734
Yeng Ying Lee, D. Barney Walker, J. Justin Gooding* and Barbara A. Messerle*
Three series of ruthenium complexes with the general formula Ru(bpy)n(β-diketonato)3−n (bpy = 2,2’-
bipyridine, n = 0, 1, 2) were prepared and investigated using cyclic voltammetry and UV-vis spectroscopy.
Variation of both the number and electronic demand of the β-diketonato ligands resulted in well-defined
modulation of the potential at which oxidation of the metal centre occurred. The observed potentials were
shown to be in good agreement with calculated ligand electrochemical parameters. A novel ruthenium(II)
complex with electrochemical behaviour similar to that of ferrocene was identified.
Received 19th May 2014,
Accepted 20th June 2014
DOI: 10.1039/c4dt01459f
www.rsc.org/dalton
Here we present the synthesis and electrochemical charac-
terisation of three series of ruthenium complexes (of the
Introduction
Ferrocene is well established as an electrochemically active tag general formulae [Ru(bpy)2(β-diketonato)](PF6), [Ru(β-diketo-
for investigating and monitoring biological activity in situ.1–9 nato)3] and [Ru(bpy)(β-diketonato)2], bpy = 2,2′-bipyridine)
This is largely due to the fact that, in addition to its favourable wherein the number and type of ligand is varied in order to
electrochemical properties, the ferrocenyl group can be readily tune the redox potential of the metal centre towards values
functionalised and is considered to be relatively stable in suitable for biosensing applications. We take advantage of the
aqueous, aerobic media. However, the ferricenium ion formed fact that the potential at which oxidation of these ruthenium
following oxidation has been shown to decompose when complexes occurs can be attenuated by varying the substitu-
exposed to chloride ions,10–13 potentially limiting the appli- ents on the β-diketonato ligands and identify several candidates
cation of ferrocene as a redox label in sensors designed for for biosensor integration that have a very similar electrochemi-
long-term analyte monitoring (e.g. implantable devices).
cal profile to that of ferrocene.
The stable and reversible nature of the Ru2+/3+ redox couple
suggests that ruthenium complexes could be an attractive
alternative to ferrocene for biological sensors and several
groups have explored this possibility.14–22 An advantage of
utilising ruthenium species is that the potential at which oxi-
dation of the metal centre occurs can be influenced by the
electronic demand of the ligands occupying the primary
coordination sphere. Large changes in the E1/2 of a Ru2+ centre
can be induced either by introducing anionic ligands23,24 and/
or by changing the number of π-acceptor coordinating species
around the metal.25–30 More subtle ‘tuning’ of the redox poten-
tial has previously been demonstrated by attaching either
electron-withdrawing or electron-donating groups (EWG or EDG)
to the peripheral sites around the coordinating ligands.28,31,32
Results and discussion
Synthesis
Three series of ruthenium complexes with β-diketonato
ligands were prepared (Scheme 1) where the complexes of
Series I each contains a single β-diketonato ligand, Series II,
three β-diketonato ligands and Series III, two β-diketonato
ligands.
Synthesis of Series I [Ru(bpy)2(β-diketonato)](PF6) com-
plexes. Ruthenium complexes 1–7 (Scheme 1a) were prepared
using a method adapted from a literature procedure.33 Com-
pounds 1, 2 and 4 have been prepared previously (see ESI† for
more information).34 All Series I complexes (excluding 4) were
prepared by displacing chloride from Ru(bpy)2Cl2 with selected
β-diketones in the presence of a stoichiometric amount of
tBuOK in a hot EtOH–H2O mixture. The desired product was
precipitated following cooling and the addition of an aqueous
solution of NH4PF6 to the reaction mixture. Complex 4 was pre-
pared by treatment of 1 with N-bromosuccinimide in CH2Cl2.35
School of Chemistry, The University of New South Wales, Sydney, NSW 2052,
Australia. E-mail: b.messerle@unsw.edu.au, justin.gooding@unsw.edu.au;
Fax: +61 2 93854161; Tel: +61 2 93854653
†Electronic supplementary information (ESI) available. See DOI: 10.1039/
c4dt01459f
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Scheme 1 Synthesis of complexes 1–17.
Novel compounds 5, 6 and 7 were prepared in the same way as (Scheme 1b). Complex 8 also underwent facile bromination
compounds 1–4 in 60%, 53% and 27% yields respectively.
with N-bromosuccinimide to give Ru(Br-acac)3 (10) in a 90%
Synthesis of Series II [Ru(β-diketonato)3]complexes. yield.32 Ru(I-acac)3 (11) was synthesised in the same way using
Methods for the direct functionalisation of the acetylacetonate N-iodosuccinimide in 85% yield. Complex 12 was prepared in
(acac) ligand of complex 8 have been described previously36,37 a 25% yield by following a literature procedure.28,31
and these were utilised to expand the family of complexes
Synthesis of Series III [Ru(bpy)(β-diketonato)2]complexes.
investigated in this study.32,38–42 Following the method of Due to the relatively substitution-inert nature of Ru(β-diketo-
Collman et al.,42 nitration of 8 with Cu(NO3)2 in acetic anhy- nato)3 complexes, one approach to the synthesis of the mixed
dride was achieved to give Ru(NO2-acac)3 (9) in 70% yield β-diketonato and bpy complexes is to prepare an intermediate
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that allows clean ligand substitution reactions with bpy.43 Ru-
(β-diketonato)2(MeCN)2 complexes were targeted as MeCN is
sufficiently labile to be readily displaced by bpy. This method
was originally applied to the synthesis of Ru(acac)2(bpy) (14).44
Complex 13 was synthesised in two steps via a bis-aceto-
nitrile (Ru(dbm)2(MeCN)2) (dbm = dibenzoylmethane) inter-
mediate. This intermediate was accessed by the reduction of
complex 12 with activated zinc dust in refluxing EtOH followed
by cooling and the addition of excess MeCN. The mixture was
then refluxed for a further 2 h resulting in a colour change
from bright red to orange.45 Treatment of the intermediate
(Ru(dbm)2(MeCN)2) with an equimolar amount of bpy in
refluxing EtOH gave Ru(dbm)2(bpy) (13) as a dark green solid
in 60% yield. The methyl analogue (R2 = Me) was prepared
using the same procedure (8 was readily converted to 14 in a
60% overall yield). This method also proved successful for the
preparation of nitro-analogue 15 but treatment of the tri-
bromo/iodo ruthenium(III) complexes (10, 11) with elemental
zinc resulted in dehalogenation of the ruthenium species and
instead formed 14 along with other decomposition products.
A second approach to preparing Series III complexes via Ru-
(bpy)(Cl)4 was investigated concurrently. The starting material
was obtained in 85% yield by dissolving RuCl3·3H2O in dilute
HCl, adding 1.2 equiv. of bpy and then leaving the solution for
21 days at room temperature.46 Subsequent treatment of
Ru(bpy)(Cl)4 with activated zinc dust in EtOH–H2O for 15 min
followed by the addition of Na2CO3 and either 1,1,1-trifluoro-
pentane-2,4-dione (tfac) or 1,1,1,5,5,5-hexafluoropentane-2,4-
dione (hfac) followed by refluxing for 1 h resulted in the for-
mation of complexes 16 and 17 in 25% and 33% yields respecti-
vely (Scheme 1c).47 Complex 16 exists as a mixture of three
geometrical isomers in 1 : 1 : 2 ratio of cis- and trans-isomers
due to its asymmetric β-diketonato ligand. This was confirmed
Table 1 E1/2 for Series I Ru(bpy)2(β-diketonato) complexes
[Ru(bpy)2((R2C(O))2CR1))]PF6
E1/2 a/V
1 (R1 = H, R2 = Me)
0.228
0.128
0.181
0.292
0.201
0.167
0.194
2 (R1 = Me, R2 = Me)
3 (R1 = Et, R2 = Me)
4 (R1 = Br, R2 = Me)
5 (R1 = H, R2 = iPr)
6 (R1 = p-MeOBn, R2 = H)
7 (R1 = p-NO2Bn, R2 = H)
a E1/2 vs. FcH+/0 in 0.01 M AgNO3 in MeCN with 0.1 M NBu4PF6,
ν = 0.1 V s−1
.
Series I. Ruthenium(II) complexes 1–7 [Ru(bpy)2(β-diketo-
nato)](PF6). The potential at which oxidation of the ruthenium(II)
complexes occurs in Series I (compounds 1 to 7) were
observed to be subtly influenced by the various substituents
on the β-diketonato ligands. Complex 2 has an electron-donat-
ing methyl group at the methine position (R1
= Me,
Scheme 1a). Consequently, a cathodic shift of the redox poten-
tial of 2 by 100 mV (128 mV vs. 228 mV) was observed when
compared with that of complex 1 (Table 1). A similar cathodic
shift was observed for the redox potential of 3 (R1 = Et)
although in this case the change is less pronounced (ΔE1/2
=
47 mV) in line with the reduced electron donating capacity of
the ethyl groups of 3 compared with the methyl groups of 2.
These observations are in agreement with previous reports49
and can be rationalised as follows: an electron-donating group
(EDG) on the β-diketonato ligand increases the electron density
around the metal centre and consequently destabilises
electrons in the metal d-orbitals. Thus the metal centre is
more readily oxidised resulting in the observed cathodic shift
in the E1/2
.
1
By the same argument, introducing an electron-withdraw-
ing group (EWG) should result in an anodic shift in the E1/2
and this effect was observed for complex 4 (R1 = Br) when
compared with 1 (ΔE1/2 = 64 mV).
by H NMR where the following configurations were observed:
trans-CF3-cis-H-[Ru(tfac)2(bpy)],
cis-CF3-cis-H-[Ru(tfac)2(bpy)]
and cis-CF3-trans-H-[Ru(tfac)2(bpy)]. This is further supported
by the presence of four signals in 19F NMR (see Experimental
section).
i
As expected, complex 5 (R2 = Pr) and complex 1 (R2 = Me)
had almost identical electrochemical profiles (ΔE1/2 = 27 mV).
The potentials at which complexes 6 and 7 were oxidised are
quite similar (167 mV and 194 mV respectively) despite having
functional groups with quite different electronic demands in
the para position of the benzyl group (OMe vs. NO2). This is
not surprising considering that the aromatic ring is not in
direct conjugation with the diketonate portion of the ligand
bound to the ruthenium(II) ion. Consequently a modest catho-
dic shift (ΔE1/2 = 61 mV for 6, ΔE1/2 = 34 mV for 7) is observed
in both cases due to the slight electron donating effect of the
benzylic CH2 group.
Series II and III. Ruthenium(II) complexes 8–17, Ru (β-diketo-
nato)3 vs. [Ru(bpy) (β-diketonato)2](PF6). The potentials at
which all the Series II complexes (8–12) oxidised were all
shifted cathodically compared with those of Series I due to the
presence of three anionic ligands. In addition, as with Series I,
the effect of a strongly EWG (R1 = NO2, 9) results in a large
anodic shift (671 mV) of the potential at which the complex
Whilst this route initially appeared to be a more direct way
of preparing Ru(bpy)(β-diketonato)2 complexes compared with
preforming the Ru(β-diketonato)3 species first (Scheme 1b) our
observation was that the overall yields tended to be signifi-
cantly lower and in some cases the final product did not form
at all. However this method did allow us to furnish sufficient
amounts of complexes 16 and 17 for subsequent electro-
chemical analysis.
Cyclic voltammetry
All complexes in Series I–III were shown to undergo a redox
cycle (assumed to be Ru2+/3+) with a peak separation (ΔE)
between 59 to 95 mV, and ipa/ipc at near unity indicating that
the complexes were both electrochemically and chemically
reversible. The E1/2 of the complexes was independent of the
scan rate (ν) and the redox processes were diffusion-controlled
as ip vs. ν1/2 was found to be linear. Waves were assigned by
comparison to those of analogous metal complexes.48
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Table 2 E1/2 for Series II and III complexes
Table 3 Diffusion coefficients for ruthenium complexes in Series I
and III
Series II
E1/2 a/V
Series I
Do/10−9 m2 s−1
8 (acac)
−1.06
1 (R1 = H, R2 = Me)
1.759
1.489
1.182
1.330
1.561
1.495
1.346
9 (NO2-acac)
10 (Br-acac)
11b (I-acac)
12 (dbm)
−0.389
−0.830
−0.829
−0.906
2 (R1 = Me, R2 = Me)
3 (R1 = Et, R2 = Me)
4 (R1 = Br, R2 = Me)
5 (R1 = H, R2 = iPr)
Series III
E1/2 a/V
6 (R1 = p-MeOBn, R2 = H)
7 (R1 = p-NO2Bn, R2 = H)
13 (dbm)
14 (acac)
15 (NO2-acac)
16 (tfac)
−0.416
−0.478
−0.054
−0.087
0.395
Series III
13 (dbm)
14 (acac)
15 (NO2-acac)
16 (tfac)
0.100
1.354
0.645
1.745
1.369
17 (hfac)
a E1/2 vs. FcH+/0 in 0.01 M AgNO3 in MeCN with 0.1 M NBu4PF6,
ν = 0.1 V s−1 b CV recorded in CH2Cl2 with 0.1 M NBu4PF6 as
supporting electrolyte.
.
17 (hfac)
where ip = current, n = number of electrons transferred, A =
electrode area, D = diffusion coefficient, C = bulk concen-
tration of redox species and ν = scan rate (Table 3).
The electrode area on the glassy carbon electrode, A, was
determined using the known Do of ferrocene in acetonitrile
with 0.1 M NBu4PF6 as a supporting electrolyte (2.24 ×
10−5 cm2 s−1).50
oxidised when compared with 8. The halogenated analogues
of Ru(acac)3 (10 and 11) were also found to have an anodically
shifted E1/2 although the effect was less significant (ΔE1/2
=
230 mV for Br, 231 mV for I). The potential at which complex
12 was oxidised was relatively close to that of 8 despite having
six phenyl rings conjugated with the coordinating oxygen
atoms (Table 2).
Complexes with bulky ligands, and those of higher charge,
would be expected to have smaller Do values as they would be
expected to migrate more slowly in solution.15,51,52 Addition-
ally, changes in electron density around ruthenium brought
on by donor/acceptor groups on β-diketonates will affect the
degree to which counterions will attach to the reduced com-
plexes.53 The structurally simplest complex in Series I, 1, has
the largest Do while the rest of the complexes are between
1.182 to 1.561 × 10−9 m2 s−1. However, the bulkiest complex of
the series did not have the smallest Do. This could be attribu-
ted to the interaction between these charged complexes of
Series I with acetonitrile and the supporting electrolytes.
In Series III, the Do for 13, which contains the bulkiest
ligand, dbm, is about one order of magnitude smaller than
that of 8. In the same way complex 17, with two trifluoromethyl
groups on each β-diketonato ligand, has a smaller Do than 16
which only has one trifluoromethyl group on each ligand.
Complex 15 has a lower Do when compared to 16 and 17,
which can possibly be attributed to its interaction with the
supporting electrolyte as the nitro group, although formally
neutral, has considerable polarisation between its nitrogen
and oxygen atoms.
It might be expected that moving from Series II to Series III
compounds, wherein one β-diketonato ligand is substituted
for a bpy ligand, would result in a set of complexes with an
E1/2 closer to the desired redox window (−0.743 V to +0.067 V
vs. FcH+/0, −0.343 V to +0.467 V vs. Ag/AgCl).
The replacement of a β-diketone in 12 and 8 by bpy to form
13 and 14 raised the E1/2 by 490 mV and 672 mV respectively.
This was attributed to the fact that bpy is a π-acceptor – and
the HOMO–LUMO gap of the complex could be increased and
may result in a more stabilised Ru2+ complex. The stabilising
effect of bpy works in concert with the NO2-acac ligands used
to form 15 resulting in an observed E1/2 of −54 mV (vs. FcH+/0).
In the same way it was possible to anodically shift the potential
at which the ruthenium metal centre is oxidised by replacing
the acac ligands with tri- and hexafluorinated 2,4-diones. A
comparison of the E1/2s of 16 with 14 demonstrates this effect
(ΔE1/2 = 391 mV). The potential at which 17 oxidised was
anodically shifted by 482 mV compared to that of 16. This
difference corresponds to the number of trifluoromethyl sub-
stituents on the ligands: there are four in 17 compared to two
in 16. Of the Series III complexes, 15 and 16 were each shown
to have an E1/2 very close to that of ferrocene (E1/2 = −54 mV
and −87 mV respectively).
Relationships between the nature of the ligands and E1/2
The ruthenocycle formed by the coordination of acac-type
ligands to Ru cations can be considered pseudo-aromatic. As
such, the Hammett constant can be used to correlate the com-
bined electronic effects of ‘para’ and ‘meta’ substituents on the
β-diketonates with the potential at which the metal centres oxi-
dised on the basis that such complexes can be thought of as
having similar geometries and a common redox centre, for
Diffusion coefficient, Do
The diffusion coefficients, Do, of complexes 1–17 in aceto-
nitrile were obtained using the Randles–Sevcik equation:
ip ¼ 2:69 ꢀ 105n3=2ADo1=2Cν1=2
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Table 4 Absorption maxima, λmax, and molar extinction coefficients,
ε for Series I
Complex
λ/nm (ε/103 M−1 cm−1
)
1 (R1 = H, R2 = Me)
2 (R1 = Me, R2 = Me)
3 (R1 = Et, R2 = Me)
4 (R1 = Br, R2 = Me)
5 (R1 = H, R2 = iPr)
6 (R1 = p-MeOBn, R2 = H)
7 (R1 = p-NO2Bn, R2 = H)
247 (28.4), 295 (53.8), 369 (11.3), 515 (9.3),
568sh (6.1)
247 (22.9), 296 (46.5), 374 (10), 521 (7.5),
586sh (5.1)
247 (26.0), 295 (5.9), 373 (11.2), 519 (10.4),
580sh (5.6)
246 (23.8), 294 (53.6), 370 (10.7), 504 (8.3),
558sh (5.5)
247 (25.2), 295 (47.8), 370 (10.2), 516 (7.7),
574sh (5.5)
246 (25.5), 295 (51.1), 376 (11.2), 518 (8.0),
577sh (5.7)
247 (20.7), 295 (42.1), 373 (8.5), 512 (5.9),
575sh (4.1)
Fig. 1 ∑E(L) vs. E1/2 for selected complexes.
which linear free energy relationships can be established.54–56
In this study correlation of the calculated Hammett constants
with the recorded E1/2s is limited by the fact that the contri-
bution of the bpy ligands is not included. Consequently, a
reasonably linear correlation was observed for Series II and III
but not for Series I (see ESI†).
Table 5 Absorption maxima, λmax, and molar extinction coefficients,
ε for Series III complexes
Complex
λ/nm (ε/103 M−1 cm−1
)
13 (dbm)
14 (acac)
246 (28.8), 300 (34.8), 323 (31.2), 485 (9.0), 608 (7.5)
277sh (24.5), 297 (17.5), 411 (8.8), 617 (4.9)
Instead a more comprehensive method for probing the
relationship between the structural and electronic features of a
complex and its redox potential was applied.57 The most
commonly utilised additive model was originally developed by
Lever58 and has proved to be a useful tool for investigating
metal-centred redox processes. The ligand electrochemical
parameter, EL(L), used in this model and based on the Ru2+/3+
redox couple, has been widely applied to examine the corre-
lation between ligands and E1/2 of metal complexes. For a
metal (M) centre bound to multiple varying ligands (Xx, Yy, Zz),
EL(L) is defined as:
15 (NO2-acac)
16 (tfac)
17 (hfac)
249 (14.8), 287 (19.7), 297 (21.0), 408 (8.5), 546 (5.14)
248 (15.0), 287sh (27.5), 294 (25.4), 425 (7.7), 559 (6.7)
243 (6.5), 289 (16.2), 337 (1.6), 509 (5.6)
to oxidise the Ru(II) metal centre of 4 (E1/2 = 292 mV) when
compared with 1 (E1/2 = 228 mV). Overall, the absorbance
spectra of all Series I complexes are dominated by the influ-
ence of the anionic β-diketonato ligand and are consequently
very similar to each other (see Fig. S4† for spectra).61
In contrast to Series I, the UV-vis absorption spectra of com-
plexes in series III are markedly different from each other
(Table 5). The absorptions due to the four phenyl rings
attached to complex 13 are clearly visible in the UV-vis spec-
trum at 246 nm. These additional aromatic groups may also
account for the broad band observed around 300 nm which
could result from a degree of overlapping with the π→π* intra-
ligand transitions of bpy.
The spectrum for complex 15 contains significant distortion
of the band at around 300 nm where no clear maximum is
visible. This could be attributed to an overlap of the nitro
group absorption with the t2g→π* (MLCT) band.62 The strongly
electron-withdrawing nitro group may also account for the sig-
nificantly blue-shifted MLCT maxima observed at 546 nm.
Both 16 and 17 contain strongly electron-withdrawing tri-
fluoromethyl groups on the β-diketonato ligands. In line with
this, the spectrum of complex 16 contains a MLCT band at
559 nm whereas the corresponding peak for 17 appears at
509 nm (Fig. 2 and 3).
X
E
¼ xELðXÞ þ yELðYÞ þ zELðZÞ:
ðLÞ
This formula can be used to predict the E1/2 of complexes
with an octahedral geometry.58
The ∑E(L) was calculated for all complexes in Series I–III
where values for E(L)(L) were available (Table S4). A strongly
linear correlation was established between E(L)(L) and E1/2
of the complexes of Series I–III suggesting that the ligand
contributions are additive (Fig. 1).
Correlation between UV-Vis absorbance and E1/2
UV-vis spectra of complexes 1–7 (Series I) were recorded in
MeCN at a concentration of 0.05 mM and the data is summar-
ised in Table 4. In total, there are two strong absorption bands
assigned to π→π* intraligand transitions in the UV region and
three MLCT bands dπ (Ru2+)→π* (L) (L = bpy, β-diketonate) in
the visible region. For complexes 1–3 the data reported here is
in agreement with literature values.48,59,60 The UV-vis spectrum
for complex 4 (R1 = Br, R2 = Me) shows a slight hypsochromic
shift in the MLCT bands (504 vs. 515 nm, 558 vs. 568 nm)
when compared with those of 1 that could be indicative of an
Conclusions
increased HOMO–LUMO gap. This is in agreement with the In this paper, ruthenium complexes bearing bpy and β-diketo-
electrochemical data as a notably higher potential is required nato ligands were prepared, fully characterised and investi-
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Experimental methods
All manipulations of metal complexes and air sensitive
reagents were performed using either standard Schlenk tech-
niques or in a nitrogen/argon filled Braun glove-box. Reagents
were purchased from Aldrich Chemical Company Inc. or Alfa
Aesar Inc. and were used without further purification unless
otherwise stated. For the purposes of air sensitive manipula-
tions and in the preparation of air sensitive complexes,
pentane, hexane, dichloromethane and tetrahydrofuran were
dispensed from a PuraSolv solvent purification system. Sol-
vents were dried and distilled under an atmosphere of nitro-
gen using standard procedures and stored under nitrogen in
glass ampoules, each fitted with a Youngs© Teflon valve prior
to use. Ethanol (EtOH) was distilled from diethoxymagnesium
and dimethylformamide (DMF) was first dried over molecular
sieves (4 Å) and distilled. Argon (>99.999%) was obtained from
Air Liquide and used as received. Nitrogen gas for Schlenk line
operation comes from in-house liquid nitrogen boil-off.
1H and 13C NMR spectra were recorded on Bruker DPX300,
DMX400, DMX500 and DMX600 spectrometers operating at
300, 400, 500 and 600 MHz (1H) respectively and 75, 100, 125
and 150 MHz (13C) respectively. Unless otherwise stated,
spectra were recorded at 25 °C and chemical shifts (δ) are
quoted in ppm. Coupling constants (J) are quoted in Hz and
Fig. 2 UV-Vis absorbance spectra for complexes 1–7.
have uncertainties of 0.05 Hz for H and 0.5 Hz for 13C. H
and 13C NMR chemical shifts were referenced internally to
residual solvent resonances. Deuterated solvents were pur-
chased from Cambridge Stable Isotopes and used as received.
Microanalyses were carried out at the Campbell Micro-analyti-
cal Laboratory, University of Otago, New Zealand or at the
Research School of Chemistry, the Australian National Univer-
sity, Canberra, Australia. Mass spectra were acquired using a
Thermo LTQ Orbitrap XL located in the Bio-analytical Mass
Spectrometry Facility (BMSF) of the Mark Wainwright Analyti-
cal Centre, UNSW or on a Micromass ZQ (ESI-MS) mass
spectrometer located in the School of Chemistry, UNSW. M is
defined as the molecular weight of the compound of interest
or cationic fragment for cationic metal complexes.
Cyclic voltammetry was performed using an Autolab
PGSTAT 12 potentiostat (Eco Chemie, Netherlands). The
CV data was processed using Nova Windows software.
A conventional three-electrode electrochemical cell comprised
of a working electrode (glassy carbon), a counter electrode
(platinum wire) and a reference electrode (aqueous or non-
aqueous depending on the solvent system of the experiment)
was used for all analysis. The aqueous reference electrode was
a Ag/AgCl (3 M KCl) electrode (CH Instruments, Inc., TX, USA)
while the non-aqueous reference electrode was a Ag/Ag+ elec-
trode (CH Instruments, Inc., TX, USA) which was referenced
against ferrocene. The working electrodes used were glassy
carbon electrodes (CH Instruments, Inc., TX, USA). The
solutions for electrochemical measurements were deoxy-
genated with nitrogen gas for 10 min prior to measurements
and kept under a blanket of nitrogen during the course of
measurements.
1
1
Fig. 3 UV-Vis absorbance spectra for complexes 13–17.
gated using cyclic voltammetry and UV-vis spectroscopy. Three
series of ruthenium complexes were investigated: Series I (1–7)
of the general formula [Ru(bpy)2(β-diketonato)](PF6); Series II
(8–12) of the general formula Ru(β-diketonato)3 and Series III
(13–17) of the general formula Ru(bpy)(β-diketonato)2. In line
with previous studies, varying the substituents tethered to the
β-diketonato ligand attenuated the E1/2 of the complexes. It
was observed that EDG shifted the E1/2 cathodically whereas
EWG shifted the E1/2 anodically. Furthermore the effects were
shown to be cumulative based on the number of β-diketonato
ligands bound to the metal centre. Further correlations were
made between E1/2 of complexes and the ligand electro-
chemical parameter (∑E(L)). The ligand electrochemical para-
meter was shown to have a linear relationship with E1/2 for all
three series.
Of the complexes prepared, the E1/2 of complexes 7, 13, 14,
15 and 16 were demonstrated to have E1/2s within the range
considered suitable for biological sensors (−0.743
V to
+0.067 V vs. FcH+/0). Furthermore, the novel complex 15 and
the previously reported complex 16 were demonstrated each to
have an E1/2 very close to that of ferrocene (E1/2 = −54 mV and
−87 mV vs. FcH+/0, respectively) suggesting that they would
be also good candidates for redox labels in electroactive
biosensors.
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UV-Vis spectra were recorded on Shimadzu UV-2401PC in [Ru(bpy)2(mbpd)][PF6], 6
dry MeCN (5 × 10−4 M) and reported as λmax/nm (ε/M−1 cm−1).
For characterisation of previously reported compounds see
the ESI.† All complexes were isolated as a racemic mixture of
Δ and Λ enantiomers.
[Ru(bpy)2(eacac)][PF6], 3
Prepared as for 3 with 0.111 g (0.503 mmol, 1.2 equiv.) of
mbpd. Yield: 0.20 g, 53%. 1H NMR (400 MHz, DMSO-d6): δ
1.79 (s, 6H), 3.57 (s, 2H), 3.73 (s, 3H), 6.86 (m, 4H), 7.22 (m,
2H), 7.71 (m, 2H), 7.80–7.88 (m, 4H), 8.22 (m, 2H), 8.67 (m,
4H), 8.78 (m, 2H) ppm. 13C NMR (100.64 MHz, DMSO-d6): δ
27.56, 34.70, 54.99, 106.26, 113.76, 123.38, 123.44, 125.57,
126.37, 128.03, 133.26, 134.64, 136.51, 149.74, 152.79, 157.28,
157.36, 158.68, 185.82 ppm. MS (ESI): m/z 633.1420 ([M]+
required 633.1434). Anal. Calcd for C33H31F6N4O3PRu: C,
50.97; H, 4.02, N, 7.20. Found: C, 50.34; H, 4.04; N, 7.24.
Ru(bpy)2Cl2·2H2O (0.406 g, 0.780 mmol, 1 equiv.) was dis-
solved in degassed H2O–EtOH (1 : 1) and heated to 75 °C for
30 minutes. 3-ethyl-2,4-pentanedione (eacac, 0.100 g,
0.780 mmol, 1.5 equiv.) was added to the solution followed by
t-BuOK (0.131 g, 1.17 mmol, 1.5 equiv.). The mixture was then
stirred at 75 °C for 1 h and cooled to room temperature before
NH4PF6 (0.699 g, 4.29 mmol, 5.5 equiv.) was added to precipi-
tate the product. The solid was collected and recrystallised
from CH2Cl2–hexane to give the crude products. The dark
solid was then purified using column chromatography (silica
gel, CH2Cl2–MeCN 4 : 1). Yield: 0.05 g, 15%. 1H NMR
(400 MHz, DMSO-d6): δ 0.92 (t, J = 6.8 Hz, 3H), 1.90 (s, 6H),
2.21 (q, J = 6.8 Hz, 2H), 7.21 (m, 2H), 7.66 (m, 2H), 7.74 (m,
2H), 7.83 (m, 2H), 8.16 (m, 2H), 8.62 (m, 4H), 8.75 (m, 2H)
ppm. 13C NMR (150.90 MHz, DMSO-d6): δ 15.32, 23.51, 27.02,
109.61, 123.34, 123.45, 125.61, 126.37, 134.60, 136.44, 149.77,
152.73, 157.34, 158.76, 184.99 ppm. MS (ESI): m/z 541.1166
([M]+ required 541.1172). Anal. Calcd for C27H27F6N4O2PRu: C,
47.30; H, 3.97; N, 8.17; found: C, 46.93; H, 3.94; N, 7.91.
[Ru(bpy)2(nbpd)][PF6], 7
Prepared as for 3 with 0.113 g (0.480 mmol, 1 equiv.) of nbpd.
Yield: 0.20 g, 27%. 1H NMR (300 MHz, DMSO-d6): δ 1.79 (s,
6H), 3.81 (s, 2H), 7.23 (m, 2H), 7.28 (m, 2H), 7.72 (m, 2H),
7.84–7.88 (m, 4H), 8.16 (m, 2H), 8.23 (m, 2H), 8.65 (m, 2H),
8.71 (m, 2H), 8.78 (m, 2H) ppm. 13C NMR (100.64 MHz,
DMSO-d6): δ 27.74, 35.82, 105.25, 123.42, 123.46, 123.52,
125.61, 126.49, 128.39, 134.73, 136.65, 145.84, 149.77, 150.49,
152.85, 157.29, 158.68, 185.96 ppm. MS (ESI): m/z 648.1188
([M]+ required 648.1179). Anal. Calcd for C32H28F6N5O4PRu: C,
48.49; H, 3.56, N, 8.84. Found: C, 47.85; H, 3.53; N, 8.80.
[Ru(bpy)2(dmhd)][PF6], 5
Ru(acac-NO2)3, 9
Prepared as for 3 with 0.100 g (0.640 mmol, 1.2 equiv.) of 2,6-
dimethyl-3,5-heptanedione (dmhd). Yield: 0.37 g, 60%.
1H NMR (300 MHz, DMSO-d6): δ 0.58 (d, J = 6.8 Hz, 6H), 0.79
(d, J = 6.8 Hz, 6H), 2.25 (qq, J = 6.8, 6.8 Hz, 2H), 5.34 (s, 1H), Acetic anhydride (30 mL) was added to finely ground Cu-
7.23 (ddd, J = 7.3, 7.3, 1.3 Hz, 2H), 7.73 (ddd, J = 7.3, 7.3, (NO3)2·3H2O (2.35 g, 9.73 mmol) to give a light blue suspen-
1.3 Hz, 2H), 7.83–7.90 (m, 4H), 8.16 (ddd, J = 7.3, 7.3, 1.5 Hz, sion. The contents were stirred at 0 °C for 15 min after the
2H), 8.50 (dd, J = 7.3, 1.3 Hz, 2H), 8.64 (dd, J = 7.3, 1.3 Hz, 2H), flask was fitted with a calcium chloride drying tube. 8 (1.20 g,
8.75 (dd, J = 7.3, 1.5 Hz, 2H) ppm. 13C NMR (150.90 MHz, 3.01 mmol) was added to the cold deep blue solution. The
DMSO-d6): δ 19.98, 38.18, 94.27, 123.01, 123.21, 125.40, 126.03, mixture was stirred at 0 °C for 2 h and a further 2 h at room
134.74, 136.35, 149.63, 153.11, 157.46, 158.75, 192.85 ppm. temperature. Ice (100 g), deionised water (100 g) and anhy-
MS (ESI): m/z 569.1547 ([M]+ required 569.1485). Anal. Calcd for drous sodium acetate (2.14 g, 26.1 mmol) were added to the
C29H31F6N4O2PRu: C, 48.81; H, 4.38; N, 7.85. Found: C, 48.15; now reddish brown mixture. The colour immediately turned
H, 4.52; N, 7.70.
greenish blue. The solution was left to stir for 18 h. The contents
12740 | Dalton Trans., 2014, 43, 12734–12742
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were filtered to give bright red powder. The solid was washed
with water to give product as a bright red powder. Yield:
1.12 g, 70%. 1H NMR (400 MHz, CDCl3): δ −3.55 (s, 18H) ppm.
MS (ESI): m/z 556.9813 ([M + Na]+ required 556.9826). Anal.
Calcd for C15H18N3O12Ru: C, 33.78; H, 3.40; N, 7.88. Found:
C, 33.01; H, 3.54; N, 7.65.
Notes and references
1 N. Metzler-Nolte and M. Salmain, in Ferrocenes, John
Wiley & Sons, Ltd, 2008, pp. 499–639, DOI: 10.1002/
9780470985663.ch13.
2 D. R. van Staveren and N. Metzler-Nolte, Chem. Rev., 2004,
104, 5931–5986.
3 M. F. R. Fouda, M. M. Abd-Elzaher, R. A. Abdelsamaia and
A. A. Labib, Appl. Organomet. Chem., 2007, 21, 613–625.
4 M. Kind and C. Woell, Prog. Surf. Sci., 2009, 84, 230–278.
5 I. Willner and E. Katz, Angew. Chem., Int. Ed., 2000, 39,
1181–1218.
Ru(NO2-acac)2(bpy), 15
6 Y. Wan, Y. Su, X. Zhu, G. Liu and C. Fan, Biosens. Bioelec-
tron., 2013, 47, 1–11.
7 S. Martic, M. Labib, P. O. Shipman and H.-B. Kraatz, Dalton
Trans., 2011, 40, 7264–7290.
8 T. Joshi, G. J. Barbante, P. S. Francis, C. F. Hogan,
A. M. Bond, G. Gasser and L. Spiccia, Inorg. Chem., 2012,
51, 3302–3315.
9 S. M. Khor, G. Liu, J. R. Peterson, S. G. Iyengar and
J. J. Gooding, Electroanalysis, 2011, 23, 1797–1804.
10 R. Prins, A. R. Korwagen and A. G. T. G. Kortbeek, J. Organo-
met. Chem., 1972, 39, 335–344.
11 D. Ge and R. Levicky, Chem. Commun., 2010, 46, 7190–
7192.
12 R. Prins, A. R. Korswagen and A. G. T. G. Kortbeek, J. Orga-
nomet. Chem., 1972, 39, 335–344.
13 H. Ju and D. Leech, Phys. Chem. Chem. Phys., 1999, 1, 1549–
1554.
14 S. M. Zakeeruddin, D. M. Fraser, M. K. Nazeeruddin and
M. Grätzel, J. Electroanal. Chem., 1992, 337, 253–283.
15 D. M. Fraser, S. M. Zakeeruddin and M. Grätzel, J. Electro-
anal. Chem., 1993, 359, 125–139.
16 C. Zhang, T. Haruyama, E. Kobatake and M. Aizawa, Anal.
Chim. Acta, 2000, 408, 225–232.
17 S. Warren, T. McCormac and E. Dempsey, Bioelectrochemis-
try, 2005, 67, 23–35.
18 T. V. Federova, A. S. Vilesov, S. A. Kurzeev, E. V. Stepanova,
E. O. Landesman and O. V. Koroleva, Appl. Biochem. Micro-
biol., 2006, 42, 550–557.
19 E. V. Ivanova, V. S. Sergeeva, J. Oni, C. Kurzawa,
A. D. Ryabov and W. Schuhmann, Bioelectrochemistry, 2003,
60, 65–71.
20 I. L. Medintz, D. Farrell, K. Susumu, S. A. Trammell,
J. R. Deschamps, F. M. Brunel, P. E. Dawson and
H. Mattoussi, Anal. Chem., 2009, 81, 4831–4839.
21 M.-Y. Wei, S.-D. Wen, X.-Q. Yang and L.-H. Guo, Biosens.
Bioelectron., 2009, 24, 2909–2914.
(a) Compound 9 (0.574 g, 1.08 mmol) was stirred in EtOH with
activated zinc dust (0.5 g) for 1 h, during which time the
colour changed from bright red to brown. MeCN (5 mL) was
added to the brown mixture and refluxed for 4 h. The mixture
was filtered through a bed of celite on which a brown layer
remained. The crude product was subjected to silica gel
column chromatography to first elute unreacted 9 with CH2Cl2
followed by EtOAc to flush the product out from the column as
an orange fraction. Solvent was removed to give the title
product as an orange solid. Yield: 0.46 g, 92%. 1H NMR
(400 MHz, DMSO-d6): δ 2.09 (s, 6H), 2.11 (s, 6H), 2.73 (s, 6H)
ppm. 13C NMR (100.64 MHz, DMSO-d6): δ 3.83, 26.59, 26.83,
128.46, 139.13, 183.15, 184.52 ppm. MS (ESI): m/z 495.0052
([M + Na]+ 495.0060).
(b) The Ru(acac-NO2)2(MeCN)2 intermediate isolated in (a)
(0.200 g, 0.424 mmol) and bpy (0.0660 g, 0.423 mmol) were
added to a Schlenk flask followed by EtOH (∼15 mL). The reac-
tion was refluxed for 5 h before solvent was removed under
reduced pressure to give a dark brown solid. The solid was
purified by column chromatography (silica gel, MeCN–CH2Cl2
1
1 : 5) to give a dark brown solid. Yield: 0.030 g, 13%. H NMR
(500 MHz, DMSO-d6): δ 1.76 (s, 6H), 2.28 (s, 6H), 7.52 (m, 2H),
7.90 (m, 2H), 8.59 (m, 2H), 8.68 (m, 2H) ppm. 13C NMR
(150.90 MHz, DMSO-d6): 26.57, 27.11, 122.74, 125.21, 128.44,
134.66, 152.07, 159.82, 182.14, 184.49 ppm. MS (ESI): m/z
569.0209 ([M + Na]+ required 569.0217). Anal. Calcd for
C20H20N4O8Ru: C, 44.04; H 3.70; N 10.27. Found: C, 44.17;
H, 3.77; N, 10.04.
22 N. A. Yusof, Asian J. Chem., 2011, 23, 1153–1157.
23 W.-W. Yang, Y.-W. Zhong, S. Yoshikawa, J.-Y. Shao,
S. Masaoka, K. Sakai, J. Yao and M. Haga, Inorg. Chem.,
2012, 51, 890–899.
Acknowledgements
Financial support from the University of New South Wales and
the Australian Research Council (LP100200593) is gratefully 24 R. Cerón-Camacho, S. Hernández, R. L. Lagadec and
acknowledged. We also would like to thank staff of the NMR A. D. Ryabov, Chem. Commun., 2011, 47, 2823–2825.
Facility, Mark Wainwright Analytical Centre, UNSW for their 25 H. Weizman and Y. Tor, J. Am. Chem. Soc., 2002, 124, 1568–
assistance.
1569.
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26 M. Vrábel, M. Hocek, L. Havran, M. Fojta, I. Votruba, 45 T. Kobayashi, Y. Nishina, K. Shimizu and G. P. Satô, Chem.
B. Klepetářá, R. Pohl, L. Rulíšek, L. Zendlová, P. Hobza, Lett., 1988, 1137–1140.
I. Shih, E. Mabery and R. Mackman, Eur. J. Inorg. Chem., 46 R. A. Krause, Inorg. Chim. Acta, 1977, 22, 209–213.
2007, 1752–1769.
27 J. J. Rack, E. S. Krider and T. J. Meade, J. Am. Chem. Soc.,
2000, 122, 6287–6288.
28 A. Endo, M. Kajitani, M. Mukaida, K. Shimizu and
G. P. Sato, Inorg. Chim. Acta, 1988, 150, 25–34.
29 N. L. Frank and T. J. Meade, Inorg. Chem., 2003, 42, 1039–
1044.
47 A. M. El-Hendawy, A. H. Al-Kubaisi and H. A. Al-Madfa,
Polyhedron, 1997, 16, 3039–3045.
48 A. M. El-Hendawy, A. H. Al-Kubaisi and H. A. Al-Madfa,
Polyhedron, 1997, 16, 3039–3045.
49 E. Eskelinen, M. Haukka, T.-J. J. Kinnunen and
T. A. Pakkanen, J. Electroanal. Chem., 2003, 556, 103–
108.
30 M. Haga, T. Matsumura-Inoue, K. Shimizu and G. P. Satô, 50 N. G. Tsierkezos, J. Solution Chem., 2007, 36, 289–302.
J. Chem. Soc., Dalton Trans., 1989, 371–373.
31 L. Wolf, E. Butter and H. Weinelt, Z. Anorg. Allg. Chem.,
1960, 306, 87–93.
51 A. Endo, Y. Hoshino, K. Hirakata, Y. Takeuchi, K. Shimizu,
Y. Furushima, H. Ikeuchi and G. P. Satô, Bull. Chem. Soc.
Jpn., 1989, 62, 709–716.
32 A. Endo, K. Shimizu and G. P. Satô, Chem. Lett., 1985, 1985, 52 H. Ikeuchi, K. Naganuma, M. Ichikawa, H. Ozawa, T. Ino,
581–584.
M. Sato, H. Yonezawa, S. Mukaida, A. Yamamoto and
T. Hashimoto, J. Solution Chem., 2007, 36, 1243–1259.
53 A. Endo, Bull. Chem. Soc. Jpn., 1983, 56, 2733–2738.
33 S. Munery, J. Jaud and J. Bonvoisin, Inorg. Chem. Commun.,
2008, 11, 975–977.
34 M. S. Chan and A. C. Wahl, J. Phys. Chem., 1985, 89, 5829– 54 C. Hansch, A. Leo and R. W. Taft, Chem. Rev., 1991, 91,
5831. 165–195.
35 S. Munery, J. Jaud and J. Bonvoisin, Inorg. Chem. Commun., 55 G. S. Patterson and R. H. Holm, Inorg. Chem., 1972, 11,
2008, 11, 975–977. 2285–2288.
36 J.-H. Olivier, J. Harrowfield and R. Ziessel, Chem. Commun., 56 S. J. Slattery, W. D. Bare, D. L. Jameson and K. A. Goldsby,
2011, 47, 11179–11188.
J. Chem. Soc., Dalton Trans., 1999, 1999, 1347–1352.
57 A. J. L. Pombeiro, Eur. J. Inorg. Chem., 2007, 1473–
1482.
58 A. B. P. Lever, Inorg. Chem., 1990, 29, 1271–1285.
59 A. Wu, J. Masland, R. D. Swartz, W. Kaminsky and
J. M. Mayer, Inorg. Chem., 2007, 46, 11190–11201.
60 G. M. Bryant, J. E. Fergusson and H. K. J. Powell,
Aust. J. Chem., 1971, 24, 257–273.
37 J. P. Collman, Angew. Chem., Int. Ed., 1965, 4, 132–138.
38 J. P. Collman, R. A. Moss, H. Maltz and C. C. Heindel,
J. Am. Chem. Soc., 1961, 83, 531–534.
39 R. W. Kluiber, J. Am. Chem. Soc., 1960, 82, 4839–4842.
40 P. R. Singh and R. Sahai, Aust. J. Chem., 1967, 20, 639–648.
41 P. R. Singh and R. Sahai, Aust. J. Chem., 1967, 20, 649–655.
42 J. P. Collman, R. L. Marshall, W. L. I. Young and
S. D. Goldby, Inorg. Chem., 1962, 1, 704–710.
61 Y. Takahashi, H. Arakawa, H. Sugihara, K. Hara, A. Islam,
R. Katoh, Y. Tachibana and M. Yanagida, Inorg. Chim. Acta,
2000, 310, 169–174.
43 I. R. Baird, S. J. Rettig, B. R. James and K. A. Skov,
Can. J. Chem., 1999, 77, 1821–1833.
44 A. K. Gupta and R. K. Poddar, Indian J. Chem. Sect. A: 62 K. Kuroda, K. Yoshitani, K. Kunigita, Y. Kamiiba and
Inorg., Bio-inorg., Phys., Theor. Anal. Chem., 2000, 39,
457–460.
K. Watanabe, Bull. Chem. Soc. Jpn., 1976, 49, 2445–
2450.
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