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

Abstract

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.

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Introduction

Ferrocene is well established as an electrochemically active tag for investigating and monitoring biological activity in situ.^1–9 This is largely due to the fact that, in addition to its favourable electrochemical properties, the ferrocenyl group can be readily functionalised and is considered to be relatively stable in aqueous, aerobic media. However, the ferricenium ion formed following oxidation has been shown to decompose when exposed to chloride ions,^10–13 potentially limiting the application of ferrocene as a redox label in sensors designed for long-term analyte monitoring (e.g. implantable devices).

The stable and reversible nature of the Ru^2+/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 oxidation of the metal centre occurs can be influenced by the electronic demand of the ligands occupying the primary coordination sphere. Large changes in the E_1/2 of a Ru²⁺ centre can be induced either by introducing anionic ligands^23,24 and/or by changing the number of π-acceptor coordinating species around the metal.^25–30 More subtle ‘tuning’ of the redox potential 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

Here we present the synthesis and electrochemical characterisation of three series of ruthenium complexes (of the general formulae [Ru(bpy)₂(β-diketonato)](PF₆), [Ru(β-diketonato)₃] and [Ru(bpy)(β-diketonato)₂], bpy = 2,2′-bipyridine) wherein the number and type of ligand is varied in order to tune the redox potential of the metal centre towards values suitable for biosensing applications. We take advantage of the fact that the potential at which oxidation of these ruthenium complexes occurs can be attenuated by varying the substituents on the β-diketonato ligands and identify several candidates for biosensor integration that have a very similar electrochemical profile to that of ferrocene.

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.

Scheme 1 Synthesis of complexes 1–17.

Synthesis of Series I [Ru(bpy)₂(β-diketonato)](PF₆) complexes. Ruthenium complexes 1–7 (Scheme 1a) were prepared using a method adapted from a literature procedure.³³ Compounds 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 prepared by treatment of 1 with N-bromosuccinimide in CH₂Cl₂.³⁵ Novel compounds 5, 6 and 7 were prepared in the same way as compounds 1–4 in 60%, 53% and 27% yields respectively.

Synthesis of Series II [Ru(β-diketonato)₃]complexes. Methods for the direct functionalisation of the acetylacetonate (acac) ligand of complex 8 have been described previously^36,37 and these were utilised to expand the family of complexes investigated in this study.^32,38–42 Following the method of Collman et al.,⁴² nitration of 8 with Cu(NO₃)₂ in acetic anhydride was achieved to give Ru(NO₂-acac)₃ (9) in 70% yield (Scheme 1b). Complex 8 also underwent facile bromination with N-bromosuccinimide to give Ru(Br-acac)₃ (10) in a 90% yield.³² Ru(I-acac)₃ (11) was synthesised in the same way using N-iodosuccinimide in 85% yield_. Complex 12 was prepared in a 25% yield by following a literature procedure.^28,31

Synthesis of Series III [Ru(bpy)(β-diketonato)₂]complexes. Due to the relatively substitution-inert nature of Ru(β-diketonato)₃ complexes, one approach to the synthesis of the mixed β-diketonato and bpy complexes is to prepare an intermediate 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-acetonitrile (Ru(dbm)₂(MeCN)₂) (dbm = dibenzoylmethane) intermediate. 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-trifluoropentane-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 formation of complexes 16 and 17 in 25% and 33% yields respectively (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 by ¹H 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).

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 significantly 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 electrochemical analysis.

Cyclic voltammetry

All complexes in Series I–III were shown to undergo a redox cycle (assumed to be Ru^2+/3+) with a peak separation (ΔE) between 59 to 95 mV, and i_pa/i_pc at near unity indicating that the complexes were both electrochemically and chemically reversible. The E_1/2 of the complexes was independent of the scan rate (ν) and the redox processes were diffusion-controlled as i_pvs. ν^1/2 was found to be linear. Waves were assigned by comparison to those of analogous metal complexes.⁴⁸

Series I. Ruthenium(II) complexes 1–7 [Ru(bpy)₂(β-diketonato)](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-donating methyl group at the methine position (R¹ = Me, Scheme 1a). Consequently, a cathodic shift of the redox potential 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 (ΔE_1/2 = 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 E_1/2.

Table 1 E _1/2 for Series I Ru(bpy)₂(β-diketonato) complexes

[Ru(bpy)2((R^2C(O))2CR^1))]PF6	E 1/2 ^a/V
a E 1/2 vs. FcH^+/0 in 0.01 M AgNO3 in MeCN with 0.1 M NBu4PF6, ν = 0.1 V s^−1.
1 (R^1 = H, R^2 = Me)	0.228
2 (R^1 = Me, R^2 = Me)	0.128
3 (R^1 = Et, R^2 = Me)	0.181
4 (R^1 = Br, R^2 = Me)	0.292
5 (R^1 = H, R^2 = ^iPr)	0.201
6 (R^1 = p-MeOBn, R^2 = H)	0.167
7 (R^1 = p-NO2Bn, R^2 = H)	0.194

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By the same argument, introducing an electron-withdrawing group (EWG) should result in an anodic shift in the E_1/2 and this effect was observed for complex 4 (R¹ = Br) when compared with 1 (ΔE_1/2 = 64 mV).

As expected, complex 5 (R² = ⁱPr) and complex 1 (R² = Me) had almost identical electrochemical profiles (ΔE_1/2 = 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 cathodic shift (ΔE_1/2 = 61 mV for 6, ΔE_1/2 = 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-diketonato)₃vs. [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 oxidised when compared with 8. The halogenated analogues of Ru(acac)₃ (10 and 11) were also found to have an anodically shifted E_1/2 although the effect was less significant (ΔE_1/2 = 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).

Table 2 E _1/2 for Series II and III complexes

Series II	E 1/2 ^a/V
8 (acac)	−1.06
9 (NO2-acac)	−0.389
10 (Br-acac)	−0.830
11 (I-acac)	−0.829
12 (dbm)	−0.906

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Series III	E 1/2 ^a/V
a E 1/2 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.
13 (dbm)	−0.416
14 (acac)	−0.478
15 (NO2-acac)	−0.054
16 (tfac)	−0.087
17 (hfac)	0.395

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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 E_1/2 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 E_1/2 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 E_1/2 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 E_1/2s of 16 with 14 demonstrates this effect (ΔE_1/2 = 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 substituents 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 E_1/2 very close to that of ferrocene (E_1/2 = −54 mV and −87 mV respectively).

Diffusion coefficient, D_o

The diffusion coefficients, D_o, of complexes 1–17 in acetonitrile were obtained using the Randles–Sevcik equation:

i _p = 2.69 × 10⁵n^3/2AD_o^1/2Cν^1/2 (1)

where i_p = current, n = number of electrons transferred, A = electrode area, D = diffusion coefficient, C = bulk concentration of redox species and ν = scan rate (Table 3).

Table 3 Diffusion coefficients for ruthenium complexes in Series I and III

Series I	D o/10^−9 m^2 s^−1
1 (R^1 = H, R^2 = Me)	1.759
2 (R^1 = Me, R^2 = Me)	1.489
3 (R^1 = Et, R^2 = Me)	1.182
4 (R^1 = Br, R^2 = Me)	1.330
5 (R^1 = H, R^2 = ^iPr)	1.561
6 (R^1 = p-MeOBn, R^2 = H)	1.495
7 (R^1 = p-NO2Bn, R^2 = H)	1.346

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Series III
13 (dbm)	0.100
14 (acac)	1.354
15 (NO2-acac)	0.645
16 (tfac)	1.745
17 (hfac)	1.369

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The electrode area on the glassy carbon electrode, A, was determined using the known D_o of ferrocene in acetonitrile with 0.1 M NBu₄PF₆ as a supporting electrolyte (2.24 × 10⁻⁵ cm² s⁻¹).⁵⁰

Complexes with bulky ligands, and those of higher charge, would be expected to have smaller D_o values as they would be expected to migrate more slowly in solution.^15,51,52 Additionally, 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 complexes.⁵³ The structurally simplest complex in Series I, 1, has the largest D_o 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 D_o. This could be attributed to the interaction between these charged complexes of Series I with acetonitrile and the supporting electrolytes.

In Series III, the D_o 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 D_o than 16 which only has one trifluoromethyl group on each ligand. Complex 15 has a lower D_o 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.

Relationships between the nature of the ligands and E_1/2

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 combined electronic effects of ‘para’ and ‘meta’ substituents on the β-diketonates with the potential at which the metal centres oxidised on the basis that such complexes can be thought of as having similar geometries and a common redox centre, for which linear free energy relationships can be established.^54–56 In this study correlation of the calculated Hammett constants with the recorded E_1/2s is limited by the fact that the contribution 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†).

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, E_L(L), used in this model and based on the Ru^2+/3+ redox couple, has been widely applied to examine the correlation between ligands and E_1/2 of metal complexes. For a metal (M) centre bound to multiple varying ligands (X_x, Y_y, Z_z), E_L(L) is defined as:

∑E_(L) = xE_L(X) + yE_L(Y) + zE_L(Z). (2)

This formula can be used to predict the E_1/2 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 E_1/2 of the complexes of Series I–III suggesting that the ligand contributions are additive (Fig. 1).

Fig. 1 ∑E_(L)vs. E_1/2 for selected complexes.

Correlation between UV-Vis absorbance and E_1/2

UV-vis spectra of complexes 1–7 (Series I) were recorded in MeCN at a concentration of 0.05 mM and the data is summarised 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 increased HOMO–LUMO gap. This is in agreement with the electrochemical data as a notably higher potential is required to oxidise the Ru(II) metal centre of 4 (E_1/2 = 292 mV) when compared with 1 (E_1/2 = 228 mV). Overall, the absorbance spectra of all Series I complexes are dominated by the influence of the anionic β-diketonato ligand and are consequently very similar to each other (see Fig. S4† for spectra).⁶¹

Table 4 Absorption maxima, λ_max, and molar extinction coefficients, ε for Series I

Complex	λ/nm (ε/10^3 M^−1 cm^−1)
1 (R^1 = H, R^2 = Me)	247 (28.4), 295 (53.8), 369 (11.3), 515 (9.3), 568sh (6.1)
2 (R^1 = Me, R^2 = Me)	247 (22.9), 296 (46.5), 374 (10), 521 (7.5), 586sh (5.1)
3 (R^1 = Et, R^2 = Me)	247 (26.0), 295 (5.9), 373 (11.2), 519 (10.4), 580sh (5.6)
4 (R^1 = Br, R^2 = Me)	246 (23.8), 294 (53.6), 370 (10.7), 504 (8.3), 558sh (5.5)
5 (R^1 = H, R^2 = ^iPr)	247 (25.2), 295 (47.8), 370 (10.2), 516 (7.7), 574sh (5.5)
6 (R^1 = p-MeOBn, R^2 = H)	246 (25.5), 295 (51.1), 376 (11.2), 518 (8.0), 577sh (5.7)
7 (R^1 = p-NO2Bn, R^2 = H)	247 (20.7), 295 (42.1), 373 (8.5), 512 (5.9), 575sh (4.1)

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In contrast to Series I, the UV-vis absorption spectra of complexes 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 spectrum 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* intraligand transitions of bpy.

Table 5 Absorption maxima, λ_max, and molar extinction coefficients, ε for Series III complexes

Complex	λ/nm (ε/10^3 M^−1 cm^−1)
13 (dbm)	246 (28.8), 300 (34.8), 323 (31.2), 485 (9.0), 608 (7.5)
14 (acac)	277sh (24.5), 297 (17.5), 411 (8.8), 617 (4.9)
15 (NO2-acac)	249 (14.8), 287 (19.7), 297 (21.0), 408 (8.5), 546 (5.14)
16 (tfac)	248 (15.0), 287sh (27.5), 294 (25.4), 425 (7.7), 559 (6.7)
17 (hfac)	243 (6.5), 289 (16.2), 337 (1.6), 509 (5.6)

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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 t_2g→π* (MLCT) band.⁶² The strongly electron-withdrawing nitro group may also account for the significantly blue-shifted MLCT maxima observed at 546 nm.

Both 16 and 17 contain strongly electron-withdrawing trifluoromethyl 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).

Fig. 2 UV-Vis absorbance spectra for complexes 1–7.

Fig. 3 UV-Vis absorbance spectra for complexes 13–17.

Conclusions

In this paper, ruthenium complexes bearing bpy and β-diketonato ligands were prepared, fully characterised and investigated 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 E_1/2 of the complexes. It was observed that EDG shifted the E_1/2 cathodically whereas EWG shifted the E_1/2 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 E_1/2 of complexes and the ligand electrochemical parameter (∑E_(L)). The ligand electrochemical parameter was shown to have a linear relationship with E_1/2 for all three series.

Of the complexes prepared, the E_1/2 of complexes 7, 13, 14, 15 and 16 were demonstrated to have E_1/2s 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 E_1/2 very close to that of ferrocene (E_1/2 = −54 mV and −87 mV vs. FcH^+/0, respectively) suggesting that they would be also good candidates for redox labels in electroactive biosensors.

Experimental methods

All manipulations of metal complexes and air sensitive reagents were performed using either standard Schlenk techniques 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 manipulations and in the preparation of air sensitive complexes, pentane, hexane, dichloromethane and tetrahydrofuran were dispensed from a PuraSolv solvent purification system. Solvents were dried and distilled under an atmosphere of nitrogen 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 have uncertainties of ±0.05 Hz for ¹H and ±0.5 Hz for ¹³C. ¹H and ¹³C NMR chemical shifts were referenced internally to residual solvent resonances. Deuterated solvents were purchased from Cambridge Stable Isotopes and used as received. Microanalyses were carried out at the Campbell Micro-analytical Laboratory, University of Otago, New Zealand or at the Research School of Chemistry, the Australian National University, 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 Analytical 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⁺ electrode (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 deoxygenated with nitrogen gas for 10 min prior to measurements and kept under a blanket of nitrogen during the course of measurements.

UV-Vis spectra were recorded on Shimadzu UV-2401PC in dry MeCN (5 × 10⁻⁴ M) and reported as λ_max/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

Ru(bpy)₂Cl₂·2H₂O (0.406 g, 0.780 mmol, 1 equiv.) was dissolved 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 precipitate 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(dmhd)][PF₆], 5

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), 7.23 (ddd, J = 7.3, 7.3, 1.3 Hz, 2H), 7.73 (ddd, J = 7.3, 7.3, 1.3 Hz, 2H), 7.83–7.90 (m, 4H), 8.16 (ddd, J = 7.3, 7.3, 1.5 Hz, 2H), 8.50 (dd, J = 7.3, 1.3 Hz, 2H), 8.64 (dd, J = 7.3, 1.3 Hz, 2H), 8.75 (dd, J = 7.3, 1.5 Hz, 2H) ppm. ¹³C NMR (150.90 MHz, DMSO-d₆): δ 19.98, 38.18, 94.27, 123.01, 123.21, 125.40, 126.03, 134.74, 136.35, 149.63, 153.11, 157.46, 158.75, 192.85 ppm. MS (ESI): m/z 569.1547 ([M]⁺ required 569.1485). Anal. Calcd for C₂₉H₃₁F₆N₄O₂PRu: C, 48.81; H, 4.38; N, 7.85. Found: C, 48.15; H, 4.52; N, 7.70.

[Ru(bpy)₂(mbpd)][PF₆], 6

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)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(acac-NO₂)₃, 9

Acetic anhydride (30 mL) was added to finely ground Cu(NO₃)₂·3H₂O (2.35 g, 9.73 mmol) to give a light blue suspension. The contents were stirred at 0 °C for 15 min after the flask was fitted with a calcium chloride drying tube. 8 (1.20 g, 3.01 mmol) was added to the cold deep blue solution. The mixture was stirred at 0 °C for 2 h and a further 2 h at room temperature. Ice (100 g), deionised water (100 g) and anhydrous sodium acetate (2.14 g, 26.1 mmol) were added to the now reddish brown mixture. The colour immediately turned greenish blue. The solution was left to stir for 18 h. The contents 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.

Ru(NO₂-acac)₂(bpy), 15

(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 reaction 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 : 5) to give a dark brown solid. Yield: 0.030 g, 13%. ¹H 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.

Acknowledgements

Financial support from the University of New South Wales and the Australian Research Council (LP100200593) is gratefully acknowledged. We also would like to thank staff of the NMR Facility, Mark Wainwright Analytical Centre, UNSW for their assistance.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4dt01459f

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