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Inorganica Chimica Acta 361 (2008) 601–612

Synthesis and characterisation of bis(2,2 0-bipyridine)-(4-carboxy-4 0-(pyrid-2-ylmethylamido)2,2 0-bipyridine)-ruthenium(II) di(hexafluorophosphate): Comparison of

spectroelectrochemical properties with related complexes

Pauline Pearson, Alan M. Bond, Glen B. Deacon, Craig Forsyth, Leone Spiccia *

School of Chemistry, Monash University, Clayton, Vic. 3800, Australia

Received 7 February 2007; accepted 17 March 2007Available online 27 March 2007

Dedicated to Professor Dr. Michael Gratzel.

Abstract

The new complex, [RuII(bpy)2(4-HCOO-4 0-pyCH2 NHCO-bpy)](PF6)2 Æ 3H2O (1), where 4-HCOO-4 0-pyCH2NHCO-bpy is 4-(car-boxylic acid)-4 0-pyrid-2-ylmethylamido-2,2 0-bipyridine, has been synthesised from [Ru(bpy)2(H2dcbpy)](PF6)2 (H2dcbpy is 4,4 0-(dicar-boxylic acid)-2,2 0-bipyridine) and characterised by elemental analysis and spectroscopic methods. An X-ray crystal structuredetermination of the trihydrate of the [Ru(bpy)2(H2dcbpy)](PF6)2 precursor is reported, since it represented a different solvate to an exist-ing structure. The structure shows a distorted octahedral arrangement of the ligands around the ruthenium(II) centre and is consistentwith the carboxyl groups being protonated. A comparative study of the electrochemical and photophysical properties of [RuII(bpy)2(4-HCOO-4 0-pyCH2NHCO-bpy)]2+ (1), [Ru(bpy)2(H2dcbpy)]2+ (2), [Ru(bpy)3]2+ (3), [Ru(bpy)2Cl2] (4) and [Ru(bpy)2Cl2]+ (5) was thenundertaken to determine their variation upon changing the ligands occupying two of the six ruthenium(II) coordination sites. The ruthe-nium(II) complexes exhibit intense ligand centred (LC) transition bands in the UV region, and broad MLCT bands in the visible region.The ruthenium(III) complex, 5, displayed overlapping LC bands in the UV region and a LMCT band in the visible. 1, 2 and 3 werefound, via cyclic voltammetry at a glassy carbon electrode, to exhibit very positive reversible formal potentials of 996, 992 and893 mV (versus Fc/Fc+) respectively for the Ru(III)/Ru(II) half-cell reaction. As expected the reversible potential derived from oxidationof 4 (�77 mV (versus Fc/Fc+)) was in excellent agreement with that found via reduction of 5 (�84 mV (versus Fc/Fc+)). Spectroelect-rochemical experiments in an optically transparent thin-layer electrochemical cell configuration allowed UV–Vis spectra of the Ru(III)redox state to be obtained for 1, 2, 3 and 4 and also confirmed that 5 was the product of oxidative bulk electrolysis of 4. These spect-rochemical measurements also confirmed that the oxidation of all Ru(II) complexes and reduction of the corresponding Ru(III) complexare fully reversible in both the chemical and electrochemical senses.Crown Copyright � 2007 Published by Elsevier B.V. All rights reserved.

Keywords: Ruthenium(II) polypyridine complexes; Synthesis; Characterisation; Spectroelectrochemical studies

1. Introduction

Since the discovery of the photoluminescence oftris(bipyridine)ruthenium(II) in the 1950’s [1] much efforthas been made to incorporate [Ru(L)3]2+ type complexes

0020-1693/$ - see front matter Crown Copyright � 2007 Published by Elsevie

doi:10.1016/j.ica.2007.03.031

* Corresponding author. Fax: +61 3 9905 4597.E-mail address: [email protected] (L. Spiccia).

into practical devices [2] that, for example, harvest solarenergy. The development of dye sensitised solar cell(DSSC) by the Gratzel group represents a landmarkachievement in this field [3]. This technology has attractedthe attention of numerous research groups worldwide. Amajor impetus for new synthetic investigations in this fieldhas been the desire to ‘‘tune’’ the properties of the ruthe-nium dyes through systematic variation of the ligands. In

r B.V. All rights reserved.

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602 P. Pearson et al. / Inorganica Chimica Acta 361 (2008) 601–612

the DSSC area, the goal has been a ruthenium(II) complexwith a very positive reversible potential, broad visibleabsorption and carboxylate moieties for convenient attach-ment to the TiO2 substrate (for recent reviews see Refs.[4,5]). However, for other applications of ruthenium photo-active complexes, such as photodynamic therapy [6,7], abroad visible absorption and the presence of carboxylategroups is not as essential.

In this study, a new complex, [RuII (bpy)2(4-HCOO-4 0-pyCH2NHCO-bpy)]2+ (1), was generated from the reactionof an amine (2-aminomethylpyridine) with one of the car-boxylic acid groups on [RuII(bpy)2(H2dcbpy)]2+ (2). Thepresence of the remaining carboxylic acid on complex 1

allows for further functionalisation or attachment to vari-ous substrates.

Although electrochemical and spectrochemical datahave been previously reported for [Ru(bpy)3]2+ (3) [8,9],[Ru(bpy)2(H2dcbpy)]2+ (2) [10–12] and [Ru(bpy)2Cl2] (4)[13–15], an electrochemical study of each complex wasundertaken in order to compare their behaviour with thatof 1 under identical experimental conditions. In addition,single crystals of [RuIII(bpy)2Cl2]Cl (5) were unexpectedlyobtained from a synthetic procedure that used 4 as a start-ing material. This allowed us to compare the electrochem-ical and spectroelectrochemical behaviour of the RuII andRuIII states of [Ru(bpy)2Cl2]0/+ when the complex is ini-tially present in either oxidation state (see Fig. 1).

N

N

O

RuI I

HO

O

HO

NN

NN

N

NRuI I

NN

NN

N

NRuI I

NN

N

NRuI I I

NN

ClCl

ClCl

2+2+

+

32

4 5

N

N

O

RuI I

HO

O

HN

NN

NN

2+

1

N

Fig. 1. Structures of [RuII(bpy)2(4-HCOO-40-pyCH2NHCO-bpy)]2+ (1),[Ru(bpy)2(H2dcbpy)]2+ (2), [Ru(bpy)3]2+ (3), [Ru(bpy)2Cl2] (4), [Ru-(bpy)2Cl2]+ (5).

2. Experimental

2.1. Materials

TBAPF6 was purified by reaction with KPF6 in acetone.This procedure precipitated any iodide present as KI. Themixture was filtered, the solid filtrate evaporated to dry-ness, and the resulting powder recrystallised from ethanol.All other materials were of reagent or analytical reagentgrade and used as received from commercial suppliers. Sol-vents used for synthetic procedures were of reagent gradeand generally used without further drying. HPLC gradeacetonitrile was used for UV–Vis, electrochemical andspectroelectrochemical measurements.

2.2. Instrumentation

Infrared spectra were recorded on a Perkin–Elmer 1600FTIR spectrophotometer as potassium bromide disks at aresolution of 8 cm�1, or as Nujol mulls between sodiumchloride plates at a resolution of 4 cm�1. 1H nuclear mag-netic resonance (NMR) spectra were recorded on a BrukerDPX300 spectrometer operating at 300 MHz. Chemicalshift values are reported in ppm, and are referenced againstthe residual solvent protons (2.50 ppm for d6-DMSO).Asterisks (*) are used to denote any peaks which weredue to overlapping signals. Mass spectra were recordedon a Biomass Platform II mass spectrometer using an elec-trospray ionization source. Peaks are listed according tothe most intense signal (containing 102Ru) within an isoto-pic distribution pattern. Assignments to ions are corre-spondingly based upon the mass of highest relativeabundance as calculated from the natural isotopic distribu-tion of constituent elements. Elemental microanalyses wereperformed by the Campbell Microanalytical Service, Dun-edin, New Zealand. UV–Vis spectra were recorded on aVarian Cary 5 NIR/Vis/UV spectrophotometer using a1 mm quartz cuvette containing an optically transparentthin-layer platinum electrode. The samples were preparedin CH3CN with a supporting electrolyte present (0.1 MTBAPF6). Optically transparent thin-layer electrolysis(OTTLE) experiments were carried out using an EG&GPAR model 273 potentiostat. The OTTLE experimentswere performed with approximately 2 mL of solution con-taining 0.1–0.5 mM of the analyte in CH3CN.

All voltammograms were acquired using a Bio Analyti-cal Systems (BAS100W) computer controlled electroana-lytical workstation. Solution phase electrochemicalexperiments were carried out at room temperature(20 ± 2 �C). These solutions were thoroughly purged ofoxygen by degassing with nitrogen that had been pre-satu-rated with the relevant solvent. A single compartment cellwith a conventional three-electrode arrangement was used.A glassy carbon (GC) macro disk (d = 3.0 mm) electrodewas used as the working electrode. The counter electrodeconsisted of platinum gauze. A silver wire in the supportingelectrolyte solution (0.1 M TBAPF6) separated from the

P. Pearson et al. / Inorganica Chimica Acta 361 (2008) 601–612 603

electroactive solution by a porous frit was used as a refer-ence electrode. Rotating disk electrode experiments used aGC disk electrode (d = 3 mm) which was rotated by a var-iable speed rotator (Metrohm 628-10). Reversible poten-tials (E0

r ) were calculated from the average of oxidationand reduction peak potentials ððEox

p þ Eredp Þ=2Þ over a typi-

cal scan rate range of 50 mV/s to 5 V/s. Ferrocene (Fc) or,where the Fc/Fc+ redox couple was in the same region asthe electroactive species, cobaltacinium hexafluorophos-phate ([CoCp2](PF6)) were used as internal references. Allpotentials are quoted against the Fc/Fc+ redox couple withan error of ±5 mV.

2.3. Preparation of ruthenium complexes

[RuII(bpy)2Cl2] was prepared according to the literaturemethod [16]. [RuII(bpy)3](PF6)2 was prepared by dissolving[RuII(bpy)3]Cl2 (115 mg, 0.18 mmol) in water (10 mL) fol-lowed by treatment with KPF6(aq) (0.1 M, 5 mL, 0.5 mmol)instantly resulting in a bright orange suspension. The prod-uct was collected by filtration and washed with ether[RuII(bpy)3](PF6)2 (107 mg, 70% yield). [RuIII(bpy)2Cl2]Clwas generated as the by-product of the synthesis of [RuII(b-py)2(Hcpp)]2+, where Hcpp = 4-carboxy-2-(2-pyridyl)-pyrimidine [17] and its composition established bycomparison with published data [18].

2.3.1. [RuII(bpy)2(H2dcbpy)] (PF6)2 (2)

2.3.1.1. Method 1: [RuII(bpy)2(H2dcbpy)] (PF6)2 Æ 1.5H2O

(2 Æ 1.5H2O). A low yielding synthesis produced singlecrystals suitable for X-ray crystallography as follows:[RuII(bpy)2Cl2] (0.46 g, 0.95 mmol) and H2dcbpy (0.25 g,1.0 mmol) were refluxed for 5 h in degassed 2-methoxyeth-anol (25 mL), then stirred at room temperature under N2

overnight. The reaction mixture was filtered through Celiteand the filtrate treated with aqueous KPF6 (0.1 M, 15 mL)resulting in a small amount of black precipitate, which wasremoved by filtration. The filtrate was then diluted withwater (to 250 mL) and the 2-methoxyethanol was removedby azeotropic distillation. The pH was adjusted to 1 usingHCl (2 M) resulting in a mixture of dark red crystals([RuII(bpy)2(H2dcbpy)] (PF6)2 Æ 3H2O (2 Æ 3H2O), yield80 mg, 9%) and a brown microcrystalline precipitate. Asample of the red crystals was prepared for analysis bywashing the mixture with water (to dissolve the brown pre-cipitate) and filtering off the red crystals. Anal. Calc. forC32H27F12N6O5.5P2Ru ([RuII(bpy)2(H2dcbpy)](PF6)2 Æ 1.5-H2O): C, 39.4; H, 2.8; N, 8.6. Found: C, 39.0; H, 2.8; N,8.5%. Infrared spectrum (KBr disk) cm�1: 1736m masym-(COO), 1637w, 1606w, 1558w, 1547w, 1467w, 1445w,1406w, 1314w, 1264m, 1231m, 1124w, 1024w, 842vs,765m, 558m. 1H NMR spectrum (d6-DMSO, 300 MHz):7.53 (p*, 3J4,5 � 7 Hz, 4H, bpy-H5,H5 0), 7.73 (dd(br),3J5,6 5.4 Hz, 4H, bpy-H6,H6 0), 7.87 (dd, 3J5,6 5.7, 4J3,5

1.7 Hz, 2H, H2dcbpy-H5), 7.93 (d, 3J5,6 5.7 Hz, 2H,H2dcbpy-H6), 8.18 (2xtd, 3J4,5 7.9, 3J4,3 4.0, 4J4,6 1.7 Hz,4H, bpy-H4,H4 0), 8.83 (d(br), 3J3,4 � 8 Hz, 4H, bpy-

H3,H3 0), 9.21 (s(br), 2H, H2dcbpy-H3). Electrospray mass

spectrum (MeOH): m/z 351 (100%, Na2[RuII-(bpy)2(dcbpy)]2+), 507 (25%, Na[RuII(bpy)2Cl2]+). UV–

Vis spectrum (CH3CN (0.1 M TBAPF6)): 245 nm(2.6 · 104 M�1 cm�1), 288 nm (5.8 · 104 M�1 cm�1),356 nm (8.7 · 103 M�1 cm�1), 465 nm (1.4 · 104 M�1

cm�1).

2.3.1.2. Method 2: [RuII(bpy)2(H2dcbpy)](PF6)2 (2). [RuII-(bpy)2Cl2] (1.1 g, 2.2 mmol), H2dcbpy (0.76 g, 3.1 mmol)and NaHCO3 (1.5 g, 18 mmol) were refluxed for 8 h in adegassed methanol:water (4:1, 50 mL) mixture under N2.The reaction mixture was stored at 0 �C overnight and fil-tered to remove precipitate. The pH of the filtrate wasadjusted to 4 with conc. H2SO4 and filtered to remove anunwanted precipitate. The filtrate was treated with aqueousKPF6 (0.1 M, 70 mL) and cooled in an ice bath to precip-itate [RuII(bpy)2(H2dcbpy)](PF6)2 (yield 0.90 g, 44%). After4 days red needle-like crystals (0.26 g) had formed in the fil-trate and additional precipitate (0.17 g) was obtained byreducing the volume of the filtrate under vacuum. Totalyield: 1.33 g, 64%. Anal. Calc. for C32H24F12N6O4P2Ru([RuII(bpy)2(H2dcbpy)](PF6)2): C, 40.6; H, 2.6; N, 8.9.Found: C, 41.2; H, 3.2; N, 8.9%. Infrared spectrum (KBrdisk) cm�1: 1716m masym(COO), 1606m, 1544w, 1467m,1447m, 1404w, 1368m, 1316m, 1268m, 1234m, 1122w,1024w, 840vs, 765m, 681w, 558m. 1H NMR spectrum (d6-DMSO, 300 MHz): 7.52 (2xddd, 3J4,5 7.4, 3J5,6 5.7, 4J3,5

1.1 Hz, 4H, bpy-H5,H5 0), 7.73 (dd(br), 3J5,6 � 4.8 Hz,4H, bpy-H6,H6 0), 7.84 (dd, 3J5,6 5.8, 4J3,5 1.4 Hz, 2H,H2dcbpy-H5), 7.87 (d, 3J5,6 5.7 Hz, 2H, H2dcbpy-H6),8.18 (2xtd, 3J4,5 � 3J3,4 7.9, 4J4,6 1.4 Hz, 4H, bpy-H4,H4 0), 8.83 (d(br), 3J3,4 7.3 Hz, 4H, bpy-H3,H3 0), 9.20(s(br), 2H, H2dcbpy-H3). Electrospray mass spectrum

(MeOH): m/z 351 (100%, Na2[RuII(bpy)2- (dcbpy)]2+).UV–Vis spectrum (CH3CN (0.1 M TBAPF6)): 245 nm(2.6 · 104 M�1 cm�1), 288 nm (5.8 · 104 M�1 cm�1),356 nm (8.7 · 103 M�1 cm�1), 465 nm (1.4 ·104 M�1 cm�1).

2.3.2. [RuII(bpy)2(4-HCOO-4 0-pyCH2NHCO-

bpy)](PF6)2 Æ 3H2O (1)

A mixture of 2 (98 mg, 0.10 mmol) and SOCl2 (20 mL)was refluxed under N2 for 2.5 h resulting in a dark redmixture. The excess SOCl2 was removed by distillation.The residue was dissolved in dry acetonitrile (2 mL) andadded dropwise to a solution of 2-aminomethylpyridine(0.025 mL, 0.24 mmol) and triethylamine (0.15 mL,1.07 mmol) in dry acetonitrile (5 mL) causing white smoketo form above the mixture. The mixture was refluxed for3 h, then stirred at room temperature overnight underN2. The reaction mixture was filtered, and the solidobtained dissolved in water and treated with aqueousKPF6 (0.1 M, 10 mL) to precipitate the product. Theprecipitate of [RuII(bpy)2(4-HCOO-4 0-pyCH2NHCO-bpy)](PF6)2 (yield 31 mg, 30%) was collected by filtration.Anal. Calc. for C38H36F12N8O6P2Ru ([RuII(bpy)2-

Table 1Crystal data for 2 Æ 3H2O

Chemical formula C32H30F12N6O7P2RuFormula weight (g mol�1) 1001.62Crystals system monoclinicSpace group P2(1)/ca (A) 11.149(2)b (A) 24.858(5)c (A) 13.537(3)a (�) 90b (�) 94.10(3)c (�) 90V (A3) 3742.0(13)Z 4Dcalc (g cm�3) 1.764F(000) 1976l(Mo Ka) (mm�1) 0.620k(Mo Ka) (A) 0.710732hmax (�) 50T (K) 123(2)Nt 35550N(Rint) 6558 (0.0815)Residuals, R; Rw 0.0771, 0.2104Maximum, minimum residual peak in final map

(e�/A3)1.639, �0.770


(4-HCOO-4 0-(pyCH2NHCO)-bpy)](PF6)2 Æ 3H2O): C, 41.8;H, 3.3; N, 10.3. Found: C, 41.6; H, 3.4; N, 9.8%. Infrared

spectrum (KBr disk) cm�1: 3424m(br), 1738w, 1730w,1704w, 1656m, 1626m, 1561m, 1544m, 1525m, 1463m,1434m, 1384m, 1318w, 1245w, 1165w, 1106w, 1023w,843vs, 766s, 559s. 1H NMR spectrum (d6-DMSO): 4.66(2H, d, 3JCH2,NH 5.8 Hz, L-H5), 7.32 (1H, ddd(br), 3J4,5

7.0,3J5,6 4.9 Hz, L-H8), 7.41 (1H, d, 3J3,4 7.7 Hz, L-H6),7.53 (4H, p*, J � 7 Hz, bpy-H5), 7.77 (5H, m, bpy-H6 + L-H7), 7.91 (2H, dd, 3J5,6 6.0, 4J3,5 1.8 Hz L-H2),7.94 (2H, d, 3J5,6 5.3 Hz, L-H1), 8.19 (4H, tdd(br), 3J3,4

7.3, bpy-H4), 8.53 (1H, ddd, 3J5,6 � 5,5J3,6 � 0.9 Hz, L-H9) 8.84 (4H, d(br), bpy-H3), 9.26 (2H, s(br), L-H3) 9.70(1H, t, 3JCH2,NH 6.0 Hz, L-H4). Electrospray MassSpectrum (H2O): m/z 419 (100%, [RuII(bpy)2((pyCH2NH-CO)2bpy)]2+), 374 (30%, [RuII(bpy)2(4-HCOO-4 0-(pyCH2NHCO)-bpy)]2+). UV–Vis spectrum (CH3CN;0.1 M TBAPF6): 249 nm (sh) (3.0 · 104 M�1 cm�1),254 nm (3.1 · 104 M�1 cm�1), 287 nm (5.1 · 104

M�1 cm�1), 306 nm (sh) (3.1 · 104 M�1 cm�1), 358 nm(1.0 · 104 M�1 cm�1), 470 nm (br) (1.3 · 104 M�1 cm�1).

12

34

567

89

12

3

Numbering system for 1H NMR(pyridine rings appear equivalent)

N

N

HO

O

HN

O

N

2.4. Crystal structure determination

Intensity data for [RuII(bpy)2(H2dcbpy)](PF6)2 Æ 3H2O(2 Æ 3H2O) was measured at 123 K on a Nonius KappaCCD fitted with graphite-monochromated Mo Ka radia-tion (0.71073 A). The data were collected to a maximum2h value of 50� (due to a lack of high angle reflections)and processed using the Nonius software. Crystal parame-ters and details of the data collection for 2 Æ 3H2O are sum-marised in Table 1.

The structure was solved by direct methods andexpanded using standard Fourier routines in the SHELX-97software package [19,20]. All hydrogens were placed inidealised positions and all the non-hydrogen atoms wererefined anisotropically, except for the following: the PF6

anions display significant thermal motion possibly indica-tive of disorder. At the limit of the current data, only theF atoms at P2 could be successfully modelled as havingtwo components, but even here the geometry of each disor-dered anion was restrained and high residual thermalparameters suggested even further unresolved disorder

was present. Hydrogen atoms were not located on the lat-tice water molecules and the CO2 groups were not includedin the model.

3. Results and discussion

3.1. Synthesis and characterisation

3.1.1. Synthesis of [RuII(bpy)2(4-HCOO-4 0-pyCH2NHCO-

bpy)](PF6)2 (1)

Our initial aim was to convert [RuII(bpy)2(H2dcb-py)](PF6)2 (2) to the diamide complex [RuII(bpy)2-((pyCH2NHCO)2bpy)](PF6)2. An adaptation of a literatureprocedure [21] was employed, where 2 was first convertedto an acid chloride intermediate prior to addition of theprimary amine 2-aminomethylpyridine. However, despiteusing 2.5 equivalents of the amine, the main productobtained following work-up was the monoamide complex,[RuII(bpy)2(4-CO2H-4 0-pyCH2NHCO-bpy)](PF6)2 Æ 3H2O(1), in 30% yield (Scheme 1).

3.1.2. Characterisation of [RuII(bpy)2(4-HCOO-4 0-pyCH2NHCO-bpy)](PF6)2 (1)

Elemental analysis confirmed the formulation of 1 as[RuII(bpy)2(4-HCOO-4 0-pyCH2NHCO-bpy)](PF6)2 Æ 3H2O.The IR spectra confirm the amide bond formation in 1. Theamide N–H bending band at 1544 cm�1 and C@O band at1656 cm�1 are consistent with the amide bands in the freeligand (presumably the N–H stretching band at�3300 cm�1 is obscured by the broad m(OH) band due towater in the KBr). Weak bands at �1730 cm�1 are attrib-utable to the carboxylate C@O band. The C@C and

N

N

O

ClRu(bpy)2

2+

NH2

CH3CNNE(i)

(ii)

t3

N

N

O

Ru(bpy)22+

HO

O

Cl

N

O

HNN

30% yield

N2

H O2

, Δ 3h

SO

ClClN

N

HORu(bpy)2

2+

N2Δ 2h

O

HO

(2)

O

(1)

Scheme 1. Synthesis of [RuII(bpy)2(4-HCOO-40-pyCH2NHCO-bpy)]-(PF6)2 (1).


C@N stretching bands of the pyridyl rings appear in therange 1390–1650 cm�1.

The aromatic (C–H) region of 1H NMR spectra of 1

was complex. However, valuable information wasobtained from the amide and methyl proton signals. Theamide NH signal appears as a triplet at 9.70 ppm, welloutside the aromatic region. This signal is shifted down-field in the ruthenium complex, 1, relative to the primaryamine NH2 signal in free 2-aminomethylpyridine (singletat 2.08 ppm). In addition, the methylene proton signal(4.66 ppm in 1) is shifted downfield compared with free2-aminomethylpyridine (3.94 ppm, singlet, 300 mHz, 30%in DMSO [22]) and has changed multiplicity to a doublet

[Ru(bpy)2Cl2]H2dcbpy

1/ CH3OCH2CH2

N2 Δ 5h2/ KPF6 (aq)

3/ HCl

(4)

[Ru(bpy)2(H2dcbpy)]( PF6)2

64% Yield

(2)

H2 dcbpy

1/ NaHCO3 MeOH:H2O (4:1)N2 Δ 8h

2/ KPF6 (aq)

3/ H2SO4

Scheme 2. Syntheses of [RuI

in 1. By comparison of the integrations, it was obviousthat only one amide bond formed, i.e., the ratio of theNH signal to any bpy-H signal is 1:4. Interestingly, thetwo pyridine rings on the unsymmetrical ligand appearto be almost chemically equivalent from the NMR, i.e.,their proton signals occur at almost the same chemicalshifts. The 1H NMR spectra also revealed a minor prod-uct which had similar chemical shifts and multiplicity to1. The electrospray mass spectroscopy revealed that theminor product was the diamide, [RuII(bpy)2-((pyCH2NHCO)2- bpy)](PF6)2, and the integration fromthe 1H NMR indicated that it was present in approxi-mately 10% (CHN analysis, however, was as expectedfor 1).

The main peak in the electrospray mass spectrum of 1

was observed at m/z 419, which corresponded to [RuII(b-py)2((pyCH2NHCO)2bpy)]2+. The peak corresponding tothe monoamide, [RuII(bpy)2(4-HCOO-4 0-pyCH2NHCO-bpy)]2+, was also observed at m/z 374 (30%). As the ratioof peak heights in a mass spectrum do not correlate directlyto the ratio of the products present, it was not surprisingthat the minor product, [RuII(bpy)2((pyCH2NH-CO)2bpy)]2+, gave the largest peak.

3.1.3. Synthesis of [RuII(bpy)2(H2dcbpy)](PF6)2 (2)

While there has been much interest in ruthenium com-plexes of H2dcbpy as components in DSSCs (as the carbox-ylate groups provide a convenient point of attachment tothe TiO2 semiconductor), the focus has been on complexesof the type [RuII(H2dcbpy)2X2] (where X is generally a hal-ogen or pseudo-halide). As a result, although [RuII-(bpy)2(H2dcbpy)]2+ (2) has previously been reported [10,23–26], it has generally been used as a precursor to more com-plex structures for a variety of applications, and it has notbeen studied in detail (e.g., Sprintschnik et al. were the firstto prepare this complex, as an intermediate to long chainester derivatives [27], but only reported the elementalanalyses).

OH

9% Yield

(2 .1.5H2O)

[Ru(bpy)2(H2dcbpy)](PF6)2

I(bpy)2(H2dcbpy)](PF6)2.


Two methods were employed to synthesise 2 from[RuII(bpy)2Cl2] (4) (Scheme 2). The first method was lowyielding but produced single crystals that were used in anX-ray structure determination. The product crystallisedas a mixture of dark red crystals and a microcrystallinebrown precipitate, with the target compound in only 9%yield. Due to the small quantity of the brown microcrystal-line product, it was not characterised, but was most likelyan analogous complex with H2dcbpy partially or fullydeprotonated.

To increase the yield and avoid complicated proceduresfor isolating the product, an adaptation of Terpetschnig’smethod [24] was employed (Scheme 2) resulting in the pre-cipitation of 2 in 44% yield. Additional product wasobtained from the filtrate by reducing the volume. Theseadditional fractions appeared to have different degrees ofprotonation of H2dcbpy, but this did not present a problemas the product can be converted to the fully protonatedform by further treatment with acid. To overcome theproblems associated with the acid–base equilibrium ofH2dcbpy others have used the diethyl ester of H2dcbpy tofirst make [RuII(bpy)2((EtO2C)2-bpy)]2+, followed byhydrolysis of the ester groups to generate [RuII-(bpy)2(H2dcbpy)]2+. However, this method requires moresteps and in the end gives a similar yield of 60% [26].

3.1.4. Characterisation of [RuII(bpy)2(H2dcbpy)](PF6)2

(2)

For both 2 Æ 1.5H2O and 2, the analytical data indicatedthat the H2dcbpy ligand is protonated and the elementalanalyses indicated that their composition differed only inthe degree of solvation. The slightly higher than expectedpercentage of carbon and hydrogen for 2 could be due topartial deprotonation of a carboxylate group. In contrastto the bulk samples, the crystal structure showed the pres-ence of three water molecules per [RuII(bpy)2(H2dcb-py)](PF6)2. This discrepancy suggests that the solidsample lost solvent prior to microanalysis.

The infrared spectra of 2 Æ 1.5H2O and 2 display asym-metric carboxylate stretching bands masym(COO) at1737 cm�1 and 1716 cm�1, respectively. Both bands areindicative of a protonated carboxylate group on theH2dcbpy ligand. If the ligand was partially deprotonated,masym(COO) would be expected at �1600 cm�1 [25]. How-ever, C@C and C@N stretching bands of the bipyridylligands also appear near 1600 cm�1 making it difficult todetermine if the H2dcbpy ligand is fully or only partiallyprotonated on this basis. Aside from the small variationin the masym(COO) bands and a band at 1368 cm�1 in 2,the IR spectra of 2 Æ 1.5H2O and 2 are in very goodagreement.

The 1H NMR spectrum of 2 would be expected to show11 individual aromatic proton signals. In practice, how-ever, the signals overlap so that the two distinct pyridineenvironments on the bpy ligands appear nearly equivalent.The degree of this overlap, and hence the apparent multi-plicity, varied a little between the spectra of 2 Æ 1.5H2O

and 2 due to a dependence on the sample concentration.However, the two spectra are in good agreement with eachother and with the spectrum in CD3CN reported by Kellyet al. [26] who did not assign the peaks or coupling con-stants. The only significant difference is in the positionand separation of the H2dcbpy-H5 and H2dcbpy-H6 sig-nals. In the spectrum of 2 Æ 1.5H2O the H2dcbpy-H5 andH2dcbpy-H6 signals are further downfield and have agreater separation, 0.06 ppm than in the spectrum of 2

(0.03 ppm).Positive ion electrospray mass spectroscopy verified the

cation component of 2 Æ 1.5H2O and 2 as [RuII(bpy)2-(H2dcbpy)]2+. The degree of protonation of the H2dcbpyligand could not be determined from the mass spectroscopyas sodium ions were captured by the carboxylate groups (asis common in ESMS) resulting in the charged species{Na2[RuII(bpy)2(dcbpy)]2+} which appeared at m/z 351for both 2 Æ 1.5H2O and 2. The mass spectrum of bulk2 Æ 1.5H2O, from the low yielding synthesis, showed evi-dence of residual starting material, [RuII(bpy)2Cl2], with apeak at m/z 507 corresponding to {Na[RuII(bpy)2Cl2]+}.

3.1.5. Crystal structure determination of

[RuII(bpy)2(H2dcbpy)](PF6)2 Æ 3H2O (2 Æ 3H2O)

The crystal structure of [RuII(bpy)2(H2dcbpy)]-(PF6)2 Æ CH3CN (2.CH3CN) was recently reported by Cas-par et al. [28]. However, the single crystals produced in thisstudy, 2 Æ 3H2O, gave a different unit cell due to the differ-ence in solvation, and as a consequence, an X-ray crystalstructure determination of 2 Æ 3H2O was undertaken. Crys-tals of 2 Æ 3H2O were found to be monoclinic and crystallo-graphic data were refined in space group P2(1)/c. Casparet al. [28] reported a monoclinic unit cell with space groupC2/c for 2 Æ CH3CN. Although a relatively low precisionstructure was obtained 2 Æ 3H2O, due to a high degree ofthermal motion in the PF6

� anions and water molecules,the key structural features of the complex cation were welldefined and comparable to those reported for 2 Æ CH3CN.An ORTEP representation is shown in Fig. 2 and the rele-vant bond lengths and angles are shown in Tables 2 and 3.

Few ruthenium(II) complexes of H2dcbpy have beencharacterised by single crystal X-ray crystallography.These include [RuII(H2dcbpy)Cl3(NO)] [29], [RuII(H2dcb-py)2(NCS)2] [30], [RuII(H2dcbpy)3]Cl2[31] and [RuII-(H2dcbpy)2(dcbpy2�)] [31]. In addition, the structure of acomplex containing the related ligand 3,3 0-dicarboxy-2,2 0-bipyridine, [RuII(bpy)2(3,3 0-H2dcbpy)](PF6)2 Æ 4H2O [32],has been reported. The coordination geometry of the RuII

centre in the complex 2 Æ 3H2O may be described as dis-torted octahedral. All the Ru–N distances for 2 Æ 3H2Oand 2 Æ CH3CN were within the uncertainty limits of eachdetermination. The Ru–N distances for the bpy ligandswere also consistent with those reported for [RuII-(bpy)3](PF6)2 (2.056(2) A) [33]. Likewise, the Ru–N bondlengths of the H2dcbpy ligands in both structures were con-sistent with reported values for the structures of[RuII(H2dcbpy)3]Cl2 Æ 2.75H2O and [RuII(H2dcbpy)2-

Fig. 2. ORTEP representation of the cationic unit [RuII-(bpy)2(H2dcbpy)]2+ in 2 Æ 3H2O.

Table 2Ruthenium environment in structure of 2 Æ 3H2O and corresponding Ru–N distances for 2 Æ CH3CN [28]a

r N(21) N(31) N(41) N(51) N(61)

2 Æ 3H2O 2 Æ CH3CN

N(11) 2.057(6) 2.060(6) 78.5(3) 172.4(3) 96.6(2) 96.5(3) 87.7(3)N(21) 2.039(6) 2.047(6) 95.9(3) 86.0(2) 174.1(2) 97.4(3)N(31) 2.062(7) 2.054(6) 77.8(3) 89.5(2) 98.1(3)N(41) 2.077(6) 2.052(6) 97.7(2) 175.0(3)N(51) 2.053(6) 2.053(6) 79.3(3)N(61) 2.054(6) 2.056(6)

a r is the Ru–N distance (A); other entries in the matrix are the anglesubtended at the ruthenium by the relevant atoms at the head of the rowand column.

Table 3CO bond lengths (in A) in 2 Æ 3H2O, cf. 2 Æ CH3CN [28]

2 Æ 3H2O 2 Æ CH3CN

C(17)–O(1) 1.24(1) C(1)–O(1) 1.24(1)C(17)–O(2) 1.27(1) C(1)–O(2) 1.27(1)C(27)–O(3) 1.32(1) C(2)–O(3) 1.31(1)C(27)–O(4) 1.24(1) C(2)–O(4) 1.19(1)

Fig. 3. Crystal packing diagram for [RuII(bpy)2(H2dcbpy)](PF6)2 Æ 3H2O(2 Æ 3H2O).


(dcbpy2�)] Æ 6H2O (2.043(2)–2.072(2) A) [31]. The N–Ru–N‘bite’ angle for the H2dcbpy ligand in 2 Æ 3H2O (78.5(3)�)was consistent with reported values for [RuII(H2dcbpy)3]-Cl2 Æ 2.75H2O and [RuII(H2dcbpy)2(dcbpy2�)] Æ 6H2O(78.44(8)–79.20(8)�) [31] and for the bpy ligands were con-sistent with those in [RuII(bpy)3](PF6)2 (78.7(1)�) [33].

The presence of two PF6� anions per Ru indicated that

both carboxylates were protonated, but these protonscould not be located. As may have been expected, the C–O bond lengths for each carboxylic acid in 2 Æ 3H2O were

similar to those of 2 Æ CH3CN (Table 3). In the reportedstructure of [RuII(H2dcbpy)2(dcbpy2�)], the correspondingpairs of C–O bond lengths for each carboxyl group on boththe H2dcbpy and dcbpy2� ligands [31] are the same as thoseof 2 Æ 3H2O or 2 Æ CH3CN.

In 2 Æ CH3CN, dicationic subunits are held together bytwo hydrogen bonds of equal distances (O� � �O 0 = 2.60 A).Further hydrogen bonding (O� � �O 0 = 2.43 A) betweenthese dicationic moieties resulted in a double chained 1Dpolymer [28]. The structure of 2 Æ 3H2O was of insufficientlyquality for crystal packing features to be ascertained. How-ever, some qualitative observations can be made (Fig. 3).The [RuII(bpy)2(H2dcbpy)]2+ cations associated with eachother in pairs, surrounded by the PF6

� anions and solventwater molecules. There were no significant H-bonding orp–p interactions between the cation pairs. All three watermolecules were poorly defined, but appeared to be in a posi-tion to participate in H-bonding with the carboxylic acids.The (O� � �O) separations were between 2.53 and 2.74 A.

3.2. Absorption spectra

The UV–Vis spectra of [Ru(bpy)2L]n+ in CH3CN areshown in Fig. 4 (see also Table 4 for a summary of data).All four ruthenium(II) complexes display two intensebands in the ultraviolet region at approximately 240 nmand 290 nm. Replacement of one bpy ligand with twohalides results in a slight shift to lower energy and adecrease in the intensity of the 290 nm band. The band at290 nm has been assigned to a p! p* ligand centred(LC) transition [9,34]. There is some controversy over theassignment of the band at 244 nm. Juris et al. assigned itas an MLCT transition, while others have assigned it toa second LC transition [34,35]. In light of the appearanceof corresponding weak bands in the RuIII spectra of eachcomplex (e.g. the weak band at �250 nm in the spectraof [RuIII(bpy)2Cl2]+ (Fig. 10) cf. the band at 241 nm in[RuII(bpy)2Cl2]), and with reference to a simplified molecu-lar orbital diagram (Fig. 5), the assignment to a LC transi-tion seems most logical.

0

5

0

5

0

5

0

5

200 300 400 500 600 7000

5

(2)

(3)

Wavelength (nm)

(4)

(1)

ε (M

-1cm

-1/1

04)

(5)

Fig. 4. UV–Vis spectra of [RuII(bpy)2(4-HCOO-4 0-pyCH2NHCO-bpy)]2+

(1), [Ru(bpy)2(H2dcbpy)]2+ (2), [Ru(bpy)3]2+ (3), [Ru(bpy)2Cl2] (4) and[Ru(bpy)2Cl2]+ (5) in CH3CN (0.1M TBAPF6).

Table 4Summary of UV–Vis spectral data for complex 1–5, recorded inacetonitrile and the reversible formal potentials, E0

f , values obtained fromcyclic voltammetry in acetonitrile (0.1 M Bu4NPF6)

Complex E0f

RuIII/IIaAbsorption spectrab

UV Visible

[RuII(bpy)2(4-HCOO-40-pyCH2NHCO-bpy)]2+

996 254* (3.1), 287(5.1), 306(sh) (3.1)

358 (1.0),473* (1.3)

[RuII(bpy)2(H2dcbpy)]2+ 992 245 (2.6), 288 (5.8) 356 (0.87),465* (1.4)

[RuII(bpy)3]2+ 893 244 (2.6), 287 (6.5) 451* (1.6)[RuII(bpy)2Cl2] �77 241 (2.1), 297 (5.0) 379 (1.1),

552 (1.1)[RuIII(bpy)2Cl2]+ �84 300 (1.9), 311 (1.9) 380 (0.47)

a (mV) vs. Fc/Fc+, error ±5 mV.b kmax (nm), (e (·104 M�1 cm�1)).* Denotes overlapping bands.

πM

πL

σL

σM*

πL*

filledorbitals

empty orbitals

MLC

T

MC LC

L

L

MLC

T

MC LC

πMa1 (d)

πMe (d)

πL* a2(ψ)

πL* e (ψ)

a b

Fig. 5. (a) Simplified molecular orbital diagram for [Ru(L)3]2+ complexesin octahedral symmetry showing the three types of electronic transitions;(b) detailed representation of the MLCT transition in D3 symmetry [9].


In the visible region, the lowest energy bands for theRuII complexes appear between 450 nm and 550 nm andhave been assigned to a MLCT (d! p*) transition [9].This assignment can be rationalised in terms of the simpli-fied molecular orbital diagram shown in Fig. 5a for[RuII(L)3]2+ in octahedral symmetry. Three types of tran-

sitions (MLCT, metal centred (MC) and LMCT) areknown to appear in the UV–Vis region as shown. Thehighest occupied molecular orbital (HOMO) is a pM pre-dominantly localised on the metal and the lowest unoccu-pied molecular orbital (LUMO) is p�L predominantlylocalised on the ligand, resulting in the lowest energy tran-sition being MLCT. From the more detailed picture of theHOMO and LUMO (Fig. 5b) it is clear that a number oftransitions contribute to the broad MLCT band, as seenin Fig. 4.

A number of publications have described the pH depen-dence of the absorption spectra of related RuII-bpy carbox-ylate complexes in aqueous solutions [11,12,23,27]. ThepKa1 and pKa2 values of 2 have been reported as 2.85and 1.75 in aqueous solution [11], but these have limitedrelevance in non-aqueous solutions. For the sake of sim-plicity we have assumed that the carboxylate groups are100% protonated and the spectrum shown in Fig. 4 is ingood agreement with that of the acetonitrile solution of 2

reported by Aranyos et al. [36].The chloride ligands in [RuII(bpy)2Cl2] resulted in a shift

to lower energy of the MLCT band (relative to thetris(‘bpy’) complexes), presumably due to an increase inthe M! L p-interactions. The decrease in intensity of thisband, and the LC transition at �300 nm, for the chloridecomplex supports their assignment as involving the bipyri-dine ligands. [RuII(bpy)2Cl2] also displayed a secondintense band in the visible region at 379 nm which has pre-viously been assigned to both LMCT (p! e�g) [35] andMLCT (d! p*) [34] transitions.

The ruthenium(III) complex, [RuIII(bpy)2Cl2]+, dis-played overlapping bands in the near UV region at 300and 311 nm, which is less than half the intensity of theLC transition (at �300 nm) in [RuII(bpy)2Cl2]. Bryantet al. have suggested this splitting is due to either vibra-tional effects or to the splitting of the p orbitals of theligand by the symmetry of the metal orbitals [37]. As forthe RuII complexes, this band has been assigned to LC(p! p*) transitions of the bipyridine ligands [37]. The vis-ible region contains a single broad charge transfer band at380 nm which has previously been assigned to LMCT(Cl� ! t2) [37].


3.3. Electrochemistry

3.3.1. Cyclic voltammetry

In applications, such as in dye sensitized cells and otherssummarized in Section 1, the oxidation of ruthenium(II) toruthenium(III) is the key feature of interest. As such, onlythe electrochemistry related to Ru(II/III) redox process isdiscussed herein. However, it should be noted that undervoltammetric conditions, all the complexes also undergoligand based reduction processes at potentials significantlymore negative than for the metal based process. A detailedinvestigation of these types of reduction processes for thecomplex [Ru(bpy)3]2+ has been reported by Bard et al. [8].

Under conditions of cyclic voltammetry at a glassy car-bon electrode, all [RuII(bpy)2L]n+ complexes exhibit a well-defined mono-electronic reversible metal based(RuII

M RuIII) couple in agreement with previous reportsfor these types of complex [8,9,13,15,38] (see Fig. 6 andTable 4). In all cases, the presence of an ideal diffusion con-trolled response was confirmed by plotting the square rootof the scan rate (50–1000 mV s�1) versus peak current andobserving the expected linear relationship [39]. The revers-ible formal potential (E0

f – calculated as the average of theoxidative and reductive peak potentials ððEox

p þ Eredp Þ=2Þ)

for the Ru(III/II) half-cell reaction for the homoleptic com-plex 3 was 893 ± 5 mV (versus Fc/Fc+). The substitution ofone bpy ligand with a H2dcbpy ligand, as in 2, resulted in apositive shift of �100 mV in E0

f . Conversion of one of thecarboxylates on 2 to a pyridylmethyl-amide (complex 1)produced the same E0

f value. In contrast, the replacementof the third polypyridine ligand by two chlorides (complex4) gave rise to a significant decrease of E0

f (RuIII/II)(�1000 mV more negative). As can be gleaned from Table4, the sequence of E0

f (RuIII/II) for this group of [RuII-(bpy)2L]2+ complexes was:

-600 -400 -200 0 200 400 600 800 1000 1200 1400

0.1

E (mV)

(5)

(3)

(2)

I /(C

Oυ1/

2 ) (A

s M

-1V

-1)

(4)

(1)

Fig. 6. Cyclic voltammograms of [RuII(bpy)2(4-HCOO-4 0-pyCH2NHCO-bpy)]2+ (1), [Ru(bpy)2(H2dcbpy)]2+ (2), [Ru(bpy)3]2+ (3), [Ru(bpy)2Cl2](4), [Ru(bpy)2Cl2]+ (5) (in CH3CN (0.1 M TBAPF6)). Co is the bulkconcentration of the analyte (M), t is the scan rate (V s�1).

L ¼ Cl2 � bpy < H2dcbpy

� 4-HCOO-40-pyCH2NHCO-bpy:

As expected, the E0f (RuIII/II) value calculated from cyclic

voltammograms for reduction of [RuIII(bpy)2Cl2]+ wasessentially identical to the E0

f (RuIII/II) calculated from oxi-dation of [RuII(bpy)2Cl2].

For a Nernstian (reversible) system, the number of elec-trons can be determined from the shape of the cyclic vol-tammogram using Eq. (1), where Ep is the peak potentialand Ep/2 is the half peak potential, measured at Ip/2 [40]

jEp � Ep=2j ¼ 2:20RTnF¼ 55:6=n mV at 20 �C: ð1Þ

The number of electrons involved in the RuIII/II redox pro-cess was calculated via use of Eq. (1) to be 0.9 ± 0.1 foreach complex, confirming that the [RuII/III(bpy)2L]n+ redoxcouples are mono-electronic and reversible over the scanrate range of 50–1000 mV s�1. However, a minor level ofnon-ideality was detected for 1 in the form of a smallpre-wave (Fig. 6), which could be due to the presence ofnon-protonated material of a complex with the diamide li-gand as shown by NMR spectroscopy and MS analysis tobe present at small concentrations (vide supra).

RDE voltammograms for the RuIIIM RuII redox pro-

cess for 3, 4 and 5 showed classically reversible behaviourproducing a sigmoidal shaped curve with E1=2 ¼ E0

f . Inthese cases a plot of the limiting current, iL, versus thesquare root of the angular velocity, x1/2 (over the range1000 P N P 3000 rpm, where x = 2pN), was linear asexpected for a mass transport controlled process. Figs. 7aand b demonstrate the distinct difference in sign of currentbetween the oxidation process (RuII! RuIII) occurring for4 and the reduction process (RuIII! RuII) for 5. In thecase of 1 and 2 (Figs. 7c and d) RDE voltammogramsexhibited a greater level of hysteresis between the currenton the forward and reverse scans, and a truly potentialindependent limiting current regime was not achieved.These voltammograms suggested either the presence of sur-face interaction between the glassy carbon electrode andcarboxylate groups or a dependence on the acid–basechemistry of the carboxylic acid group. Furthermore, 1

and 2 exhibited a small pre-wave, analogous to thatdetected for 1 under conditions of cyclic voltammetry(see above discussion).

3.3.2. OTTLE

In order to further probe the metal based (RuIII/RuII)redox process, Optically Transparent Thin-Layer Electrol-ysis (OTTLE) experiments were conducted for each com-plex. For the RuII complexes a constant potential200 mV positive of the E0

f value (determined from cyclicvoltammetry) was applied to a solution containing thecomplex and the extent of oxidation to RuIII was moni-tored by UV–Vis spectrophotometry. In all cases, isosbesticpoints were obtained, suggesting that only two speciesare involved in the metal based oxidation process, i.e.

-400 -200 0 200

0

10

20

30

40

50

500 rpm 1000 rpm 1500 rpm 2000 rpm 2500 rpm 3000 rpmI x

10-6

(A

)

E (mV) v. Fc/Fc+-750 -500 -250 0 250 500

-30

-20

-10

0

1000 rpm 1500 rpm 2000 rpm 2500 rpm 3000 rpm

I x 1

0-6 (

A)

E (mV) v. Fc/Fc+

600 800 1000 1200-5

0

5

10

15

20

25

30 1000 rpm 1500 rpm 2000 rpm 2500 rpm 3000 rpm

I x 1

0-6 (

A)

E (mV) v. Fc/Fc+

600 800 1000 1200

0

4

8

12

16 1000 rpm 1500 rpm 2000 rpm 2500 rpm 3000 rpm

I x 1

0-6 (

A)

E (mV) v. Fc/Fc+

a

c d

b

Fig. 7. RDE voltammograms of (a) 4 (0.59 mM), (b) 5 (0.35 mM), (c) 1 (0.11 mM) and (d) 2 (0.23 mM) in CH3CN (0.1 M TBAPF6), 20 mV/s.


[RuII(bpy)2(L)]n+ and [RuIII(bpy)2(L)](n+1)+. After 1 h, theconstant potential was switched to a value 200 mV lesspositive than E0

f in order to reduce the complex back toits original oxidation state. This procedure always resultedin a reversal of the spectral changes until a spectrum closelymatching that of the original sample was obtained.

The OTTLE spectra obtained during the course of oxi-dation of 1 are shown in Fig. 8. The rapid disappearance of

200 300 400 500 600 7000.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

Abs

orba

nce

Wavelength (nm)

Fig. 8. Spectroelectrochemical oxidation of 1 (0.22 mM in CH3CN (0.1 MTBAPF6) in an OTTLE cell, Eapplied = 1250 mV vs. Fc/Fc+, t = 0–15 min).

bands at 287, 358 and 473 nm was accompanied by thegrowth of bands at 256 and 314 nm. After approximately15 min the spectral changes were essentially complete. Isos-bestic points were observed at 340, 301 and 271 nm. Theabsorption spectrum of the electrogenerated RuIII productdisplayed two bands in the ultraviolet region at 256 nm and314 nm (which has a shoulder) and a very weak band in thevisible region at 423 nm. The UV absorptions are due tointraligand (p! p*) transitions of the bipyridine ligand[37]. The weak band in the visible region is attributableto a LMCT transitions (p! t2) [37].

In the case of 1, 2 and 3 with very positive E0f values,

slow reversal of the RuIII electrogenerated complex backthe RuII state was detected when the potential was nolonger applied. This is attributed to reduction with adven-titious water present in acetonitrile

4½RuIIIðbpyÞ2L23þ þ 2H2O! ½RuIIðbpyÞ2L22þ þO2 þ 4Hþ:

Following complete reduction of the electrochemicallyformed [RuIII(bpy)2(4-HCOO-40-pyCH2NHCO-bpy)]+, thespectrum obtained matched closely to that of the originalsample of 1 (Fig. 9).

The spectra of [RuII(bpy)2Cl2] (4) obtain in an OTTLEcell are of particular interest as they can be compared withthat of [RuIII(bpy)2Cl2]Cl (5) (the expected product ofoxidation). In this case, oxidation of 4 led to the disappear-ance of the bands at 297 nm and 552 nm and was accompa-

200 300 400 500 600 7000.0

0.2

0.4

0.6

0.8

0-5 min

Abs

orba

nce

Wavelength (nm)

Fig. 10. Oxidation of 4 (0.16 mM in CH3CN (0.1 M TBAPF6) using anOTTLE cell, Eapplied = 300 mV vs. Fc/Fc+).

200 300 400 500 600 700

0.0

0.3

0.6

0.9

1.2

1.5 initial sample of 1 after complete oxidation followed by complete reduction

Abs

orba

nce

Wavelength (nm)

Fig. 9. Comparison of initial and final spectra obtained after oxidative(Eapplied = 1250 mV vs. Fc/Fc+, t = 0–60 min) and reductive (Eapplied =700 mV vs. Fc/Fc+, t = 60–75 min) Spectroelectrochemical experimentson 1 in an OTTLE (0.22 mM 1 in CH3CN (0.1 M TBAPF6)).

200 300 400 500 600 700

0.3

0.6

0.9

1.2

1.5

last scan after complete oxidation

Abs

orba

nce

Wavelength (nm)

Fig. 11. Reduction of product of oxidation of 4 (0.16 mM in CH3CN(0.1 M TBAPF6) in an OTTLE cell, Eapplied = �300 mV vs. Fc/Fc+,t = 60–120 min).


nied by the growth of bands at 300 and 311 nm indicativeof the formation of [RuIII(bpy)2Cl2]+ (Fig. 10). Isosbesticpoints were observed at 332, 301, 278, 247 and 234 nm.

After 5 min of oxidative electrolysis the spectral changeswere essentially complete. Only small changes consistentwith evaporation of solvent were detected in the remaining55 min. Comparison of the final spectrum obtained withthat prepared from a sample of 5 confirmed that [RuIII(b-py)2Cl2]+ is the product of the oxidative bulk electrolysis inessentially 100% yield.

In the case of reduction of [RuIII(bpy)2Cl2]+ formedelectrochemically, the reappearance of bands at 297 nmand 552 nm is consistent with regeneration of [RuII-(bpy)2Cl2] (Fig. 11). As expected, OTTLE spectra obtainedduring the course of reduction of a prepared solution of 5

were essentially identical to those in Fig. 11. Oxidation ofthe electrogenerated species regenerated 5, as determinedby almost identical spectra to those shown in Fig. 10. Isos-

bestic points were observed at 330, 302, 278 and 251 nm forthe both the reduction and oxidation processes.

4. Conclusion

In summary, we have synthesized the novel complex[RuII(bpy)2(4-HCOO-4 0-pyCH2NHCO-bpy)]2+ and com-pared its electrochemical and photophysical properties toa group of related complexes. Our results show that con-version one –COOH in 2 to an amide (–CONHCH2py) in1 did not significantly change either the absorption spec-trum or the reversible redox potential of the complex.The breadth and intensity of the absorption bands in thevisible region were similar for the amide complex 1 andthe parent dicarboxylate complex 2. The metal basedreversible redox potentials for 2 and 1 were both approxi-mately 100 mV more positive than the homoleptic complex3. The high oxidation potential for oxidation and broadvisible absorption for 1 render it an ideal candidate foruse biomedical applications, involving electrochemical orphotochemical detection methods, and therapeutic applica-tions. In this context, the free carboxylic acid group allowsfor further functionalisation as required to attach the com-plex to various substrates and biomolecules.

5. Supplementary material

CCDC 634137 contains the supplementary crystallo-graphic data for 2 Æ 3H2O. These data can be obtained freeof charge via http://www.ccdc.cam.ac.uk/conts/retriev-ing.html, or from the Cambridge Crystallographic DataCentre, 12 Union Road, Cambridge CB2 1EZ, UK; fax:(+44) 1223-336-033; or e-mail: [email protected].

Acknowledgements

The authors acknowledge financial support provided bythe Australian Research Council (ARC) (Australian Centre

http://www.ccdc.cam.ac.uk/conts/retrieving.html

http://www.ccdc.cam.ac.uk/conts/retrieving.html


for Electromaterials Science), ARC Discovery Grants andthe ARC Federation Fellowship Scheme (A.M.B.). P.P.was the recipient of an Australian Postgraduate Award.

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