Synthesis of mononuclear and dinuclear ruthenium(II) tris(heteroleptic) complexes via photosubstitution in bis(carbonyl) precursors

Abstract

A novel, and quite general, approach for the preparation of tris(heteroleptic) ruthenium(II) complexes is reported. Using this method, which is based on photosubstitution of carbonyl ligands in precursors such as [Ru(bpy)(CO)₂Cl₂] and [Ru(bpy)(Me₂bpy)(CO)₂](PF₆)₂, mononuclear and dinuclear Ru(II) tris(heteroleptic) polypyridyl complexes containing the bridging ligands 3,5-bis(pyridin-2-yl)-1,2,4-triazole (Hbpt) and 3,5-bis(pyrazin-2-yl)-1,2,4-triazole (Hbpzt) have been prepared. The complexes obtained were purified by column chromatography and characterized by HPLC, mass spectrometry, ¹H NMR, absorption and emission spectroscopy and by electrochemical methods. The X-ray structures of the compounds [Ru(bpy)(Me₂bpy)(bpt)](PF₆)·0.5C₄H₁₀O [1·0.5C₄H₁₀O], [Ru(bpy)(Me₂bpy)(bpzt)](PF₆)·H₂O (2·H₂O) and [Ru(bpy)(Me₂bpy)(CH₃CN)₂](PF₆)₂·C₄H₁₀O (6·C₄H₁₀O) are reported. The synthesis and characterisation of the dinuclear analogues of 1 and 2, [{Ru(bpy)(Me₂bpy)}₂bpt](PF₆)₃·2H₂O (3) and [{Ru(bpy)(Me₂bpy)}₂bpzt](PF₆)₃ (4), are also described.

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Introduction

The photophysical and photochemical properties of dinuclear Ru(II) metal complexes have received considerable attention for their potential to facilitate light-induced functions including energy and charge transfer.^1–4 Ligand composition has been used widely to fine-tune the electronic and redox properties of such compounds^5–9 for example to manipulate intercomponent interactions^4,9 or to introduce specific functionalities to allow surface binding.¹⁰ Of particular interest would be compounds of the type shown in Fig. 1 where D is an electron donor and A an electron acceptor. With such compounds detailed studies of photoinduced electron transfer in dinuclear systems could be carried out as a function of the bridge. However, the synthesis of trisheteroleptic compounds of this type, especially with triazole type ligands, is by no means straightforward. Therefore detailed studies on the preparation of such complexes have been carried out in our laboratories.

Fig. 1 Prototype dinuclear acceptor–donor complex.

It is just over 20 years since the first tris-heteroleptic Ru(II) polypyridyl complex was reported¹¹ and the various ensuing synthetic routes produced only a relatively small number of complexes.^12–15 However, the introduction of methods based on the chemical decarbonylation of a [Ru(L)(L′)(CO)₂]²⁺ precursor or on the sequential addition of ligands to [Ru(DMSO)₄Cl₂] has seen a growing series of these complexes synthesised.^16–21 A full and comprehensive review of the various synthetic routes to these complexes has recently been published.²² In this report we describe a new synthetic route based on the photochemical, rather than chemical, elimination of carbonyl ligands in ruthenium bis-carbonyl complexes, which we have successfully used to the prepare two dinuclear tris(heteroleptic) Ru(II) complexes containing the bridging ligands Hbpt and Hbpzt (for structures, see Fig. 2).

Fig. 2 1,2,4-Triazole-based bridging ligands used in this study.

This synthetic approach opens the way for the synthesis of compounds such as those outlined in Fig. 1 and hence the detailed investigation of photoinduced electron transfer processes in dinuclear acceptor–donor compounds of this type.

Experimental

Materials

All materials used were of reagent grade or better. Bpy (Aldrich) and Me₂bpy (Fluka) were used as received. All solvents were HPLC grade or better. [Ru(bpy)(CO)₂Cl₂]²³ and Hbpt and Hbpzt^9,24 were synthesised according to previously reported procedures.

Synthetic procedures

[Ru(bpy)(MeCN)₂Cl₂]/[Ru(bpy)(MeCN)₃Cl]Cl. [Ru(bpy)(CO)₂Cl₂] (0.5 g, 1.3 mmol) was dissolved in 250 cm³ dry MeCN and placed in a custom built immersion well. The solution was sparged with Ar for 15 min before photolysis with a 400 W medium pressure Hg vapour lamp commenced. After 1 h the solution was reduced in vacuo yielding a dark red product. Yield (0.42 g).

For further details see text.

[Ru(bpy)(Me₂bpy)Cl₂]. [Ru(bpy)(MeCN)₂Cl₂]/[Ru(bpy)(MeCN)₃Cl]Cl, (0.4 g) and Me₂bpy (0.2 g, 1.1 mmol) were refluxed in dry acetone (30 cm³) for 15 h. The solution was filtered hot. The purple precipitate collected was washed with cold acetone and diethyl ether. Yield 0.16 g, 0.3 mmol. ¹H NMR (d₆-DMSO, 298 K); δ 9.99 (d), 9.76 (d), 8.61 (d), 8.50 (s), 8.45 (d), 8.35 (s), 8.03 (t), 7.76 (t), 7.63 (t), 7.61 (d), 7.54 (d), 7.29 (d), 7.10 (t), 6.94 (d), 2.62 (s), 2.34 (s). Elemental analysis for C₂₂H₂₀Cl₂N₄Ru: Calc.: C 51.57, H 3.93, N 10.93. Found: C 51.90, H 3.90, N 10.99%. UV-Vis (CH₃CN): λ_max 554 and 378 nm. Mass spectrometry (CH₃CN): m/z 513 ([MH]⁺).

[Ru(bpy)(Me₂bpy)(bpt)](PF₆)·H₂O (1). Hbpt (0.11 g, 0.5 mmol) was dissolved in hot EtOH–H₂O (80 : 20, 50 cm³). The Hbpt solution was brought to reflux and [Ru(bpy)(Me₂bpy)Cl₂] (0.20 g, 0.4 mmol) was added in four portions over the course of 2 h. Upon addition of the final portion, the solution was heated at reflux for a further 3 h. After cooling and filtering the reaction solution was reduced and the product was columned on a silica column using MeCN–H₂O (80:20) with 0.05 M KNO₃ mobile phase. The product eluted as the second band and was precipitated as the PF₆ salt. The product was further purified by column chromatography on an alumina column using MeCN as mobile phase. Yield 0.24 g, 0.3 mmol, 75%. Elemental analysis for C₃₄H₃₀F₆N₉OPRu: Calc.: C 49.40, H 3.66, N 15.25. Found: C 49.71, H 3.37, N 15.48%. Mass spectrometry (CH₃CN): m/z 664 ([M − PF₆]⁺).

[Ru(bpy)(Me₂bpy)(bpzt)](PF₆)·2H₂O (2). As for 1 except Hbpzt (0.11 g, 0.5 mmol) and [Ru(bpy)(Me₂bpy)Cl₂] (0.20 g, 0.4 mmol) were used. Yield 0.19 g, 0.23 mmol, 58%. Elemental analysis for C₃₂H₃₀F₆N₁₁O₂PRu: Calc.: C 45.39, H 3.57, N 18.20. Found: C 45.16, H 3.69, N 18.59%. Mass spectrometry (CH₃CN): m/z 666 ([M − PF₆]⁺).

[{Ru(bpy)(Me₂bpy)}₂bpt](PF₆)₃·2H₂O (3). Hbpt (0.08 g, 0.36 mmol) was dissolved in EtOH–H₂O (80/20, 20 cm³) and heated at reflux. [Ru(bpy)(Me₂bpy)Cl₂] (0.41 g, 0.81 mmol) was added in one portion and the reaction continued for 24 h. The solution was reduced and purified by column chromatography on silica using a 0.1 M KNO₃ in MeCN–H₂O mobile phase. The second band (main band) was collected, reduced to dryness and re-dissolved in H₂O. A conc. aqueous NH₄PF₆ solution (1 cm³) was added. The precipitate was filtered off and dried in vacuo. The compound was further purified by column chromatography on alumina with MeCN as mobile phase and recrystallised from acetone–water. Yield 0.29 g, 0.19 mmol, 53%. Elemental analysis for C₅₆H₄₈F₁₈N₁₃P₃Ru₂: Calc.: C 43.67, H 3.14, N 11.82. Found: C 43.52, H 3.08, N 11.52%. Mass spectrometry (CH₃CN): m/z 1396 (M − PF₆)⁺.

[{Ru(bpy)(Me₂bpy)}₂bpzt](PF₆)₃ (4). As for 3 except Hbpzt (0.08 g, 0.36 mmol) and [Ru(bpy)(Me₂bpy)Cl₂] (0.42 g, 0.83 mmol) in EtOH–H₂O (80/20, 20 cm³) for 24 h. Yield 0.29 g, 0.19 mmol, 53%. Elemental analysis for C₅₄H₄₆F₁₈N₁₅P₃Ru₂: Calc.: C 42.06, H 3.01, N 13.62. Found: C 42.52, H 3.08, N 13.32%. Mass spectrometry (CH₃CN): m/z 1398 (M).

[Ru(bpy)(Me₂bpy)(CO)₂](PF₆)₂ (5). [Ru(Me₂bpy)(CO)₂Cl₂] (1.0 g, 2.4 mmol) and bpy (0.57 g, 3.7 mmol) were heated at reflux in EtOH–H₂O (2 : 1, 50 cm³) for 5 h. The yellow solution was then reduced by rotary evaporation and the remaining solid dissolved in H₂O, filtered and added to a saturated aqueous solution of NH₄PF₆. The slightly yellow precipitate was filtered, washed with H₂O and allowed dry under vacuum. Yield 1.55 g, 1.97 mmol, 82%. ¹H NMR (d₆-DMSO, 298 K); δ 9.24 (d), 9.06 (d), 7.93 (d), 8.80 (d), 8.79 (s), 8.68 (s), 8.56 (t), 8.33 (t), 8.02 (t), 7.87 (d), 7.63 (t), 7.47 (d), 7.44 (d), 7.28 (d), 2.76 (s, 3H), 2.54 (s, 3H). IR (MeCN): 2099 and 2047 cm⁻¹.

[Ru(bpy)(Me₂bpy)(CH₃CN)₂](PF₆)₂ (6). [Ru(bpy)(Me₂bpy)(CO)₂](PF₆)₂ (0.5 g, 0.64 mmol) was dissolved in HPLC grade MeCN (Ar purged) and irradiated with light using a 400 W medium pressure Hg lamp with continuous stirring and constant purge of Ar. After 1 h (as determined by absence of v_(CO) stretching bands) the solvent was removed to leave a red solid. Yield 0.49 g, 0.6 mmol, 94%. ¹H NMR (d₆-DMSO, 298 K); δ 9.38 (d), 9.19 (d), 8.81 (d), 8.70 (s), 8.67 (d), 8.56 (s), 8.37 (t), 8.05 (t), 7.93 (t), 7.78 (d), 7.59 (d), 7.40 (t), 7.39 (d), 7.22 (d), 2.69 (s, 3H), 2.47 (s, 3H), 2.46 (s, 3H), 2.44 (s, 3H). Elemental analysis for C₂₆H₂₆N₆P₂F₁₂Ru: Calc.: C 38.39, H 3.22, N 10.33. Found: C 38.18, H 2.91, N 9.90%.

Physical measurements

¹H NMR spectra were performed on a Bruker Avance 400 NMR Spectrometer in CD₃CN with TMS as reference. Free induction decay (FID) profiles were processed using XWIN-NMR software package. Mass spectra were recorded with a Bruker-EsquireLC_00050 electrospray ionisation mass spectrometer at positive polarity with cap-exit voltage of 167 V. Each spectrum was recorded by summation of 20 scans.

HPLC measurements were performed on a JVA analytical HPLC system consisting of a Varian Prostar HPLC pump using a HiChrom Partisil P10SCX-3095 cation exchange column and Varian Prostar photodiode array and 280 nm detection. A 20 µl injection loop delivered the sample to the column using 0.08 M LiClO₄ in MeCN–H₂O (80/20) mobile phase at a flow rate of 1.8 cm³ min⁻¹.

Infrared spectra were measured using a Perkin Elmer 2000 FTIR spectrometer. UV-vis absorption spectra were recorded in acetonitrile on a Shimadzu 3100 UV-Vis/NIR instrument with 1-cm quartz cells. Emission spectra were recorded at 298 K in MeCN using a Perkin-Elmer LS50B luminescence spectrometer equipped with a red sensitive Hamamatsu R928 detector. At 77 K, measurements were carried out in ethanol–methanol (4 : 1 v/v). Lifetime measurements were performed on an Edinburgh Analytical Instruments single photon counter with a T setting, using a lamp (nF900, in a nitrogen setting), monochromators (J-yA models), with a single photon photomultiplier detection system (model S 300), an MCA card (Norland N5000) and PC interface (Cd900 serial). Data correlation and manipulation was carried out using the program F900, Version 5.13. Samples were de-aerated for 15 min using Ar prior to analysis.

DPV electrochemical measurements were carried out using a CH Instruments CHI Version 2.07 software controlled potentiostat (CH Instruments Memphis 660). Solutions of the complex (typically 1 mM) to be tested were made up in a 0.1 M solution of TBABF₄ (Aldrich) in dry MeCN. The solution was purged with Ar (10 min) and an Ar atmosphere was maintained throughout the experiment. A three compartment cell housed a platinum disc (working, 2 mm diameter), platinum wire (counter) and a Ag/Ag⁺ (acetonitrile + 10 mM AgNO₃ + 0.1 M TBABF₄) half-cell (reference). The instrument was calibrated before and after each session using the Fc/Fc⁺ couple.

Elemental analyses were carried out by the Microanalytical Department, University College Dublin.

Crystals suitable for X-ray studies of 1, 2 and 6 were grown by vapour diffusion of diethyl ether into an acetonitrile solution of the respective complex. For the three crystal structure determinations, the X-ray intensity data were collected on either a Bruker SMART CCD diffractometer (1·0.5C₄H₁₀O) or a Nonius KappaCCD diffractometer (2·H₂O and 6·C₄H₁₀O). Data were corrected for Lorentz and polarization effects, but not for absorption effects in the case of 2·H₂O and 6·C₄H₁₀O.^25–28 The structures were solved by direct methods (SHELXS²⁹) and refined by full-matrix least-squares techniques against F_o² (SHELXL-97³⁰). The hydrogen atoms were included at calculated positions with fixed thermal parameters except for the hydrogen atoms of the H₂O molecule in 2 which could not be located. All non-hydrogen atoms were refined anisotropically. XP (SIEMENS Analytical X-ray Instruments, Inc.) was used for structure representations.

CCDC reference numbers 272204 (1·0.5C₄H₁₀O), 273899 (2·H₂O) and 273900 (6·C₄H₁₀O).

For crystallographic data in CIF or other electronic format see DOI: 10.1039/b510751b

Results and discussion

Synthetic procedures

Synthesis of mononuclear and dinuclear Ru(II) tris(heteroleptic) complexes containing 1,2,4-triazole bridging ligands was successfully accomplished by using the product obtained by irradiating [Ru(bpy)(CO)₂Cl₂] in the presence of dry MeCN as precursor. The overall synthetic route is shown in Scheme 1.

Scheme 1 The synthetic route used to prepare the tris(heteroleptic) complexes 1–4.

A second method based on the photolysis of precursors of the type [Ru(L1)(L2)(CO)₂]²⁺ was considered but found less attractive (see below). Infrared spectroscopy was used to follow the course of the reaction by monitoring the loss of CO (see Fig. 3).

Fig. 3 Photolysis of [Ru(bpy)(CO)₂Cl₂] in MeCN.

Initially the two ν_CO stretching bands at 2064 and 2001 cm⁻¹ indicative for the presence of [Ru(bpy)(CO)₂Cl₂] disappeared rapidly and were replaced by one band at 1969 cm⁻¹, indicative of a monocarbonyl species as suggested by Eskelinen et al.³¹ This band gradually disappears. ¹H NMR of the product obtained (see Fig. S1, ESI†) shows eight aromatic peaks each integrating to one and four aromatic peaks (shown by arrows) each integrating to a half, suggesting the formation of a mixture containing the symmetric [Ru(bpy)(MeCN)₂Cl₂] with the asymmetric [Ru(bpy)(MeCN)₃Cl]Cl as the major species. This is confirmed by electrochemistry where the mixture shows two reversible oxidation potentials at E_1/2 = 0.14 and 0.74 V (Ag/Ag⁺) in a 1 : 4 ratio which correspond to [Ru(bpy)(MeCN)₂Cl₂] and [Ru(bpy)(MeCN)₃Cl]Cl respectively (see Fig. S1, ESI†). The assignment of the redox features is based on the reasonable assumption that replacement of a Cl⁻ with a weaker σ-donor ligand such as MeCN shifts the oxidation potential of the metal centre to a more positive value.

Although the photolysis product was found to be a mixture of two species, pure [Ru(bpy)(Me₂bpy)Cl₂] was obtained after refluxing the mixture with Me₂bpy in dry acetone for 15 h. Only the cis-dichloride species was observed and both the CHN and ¹H NMR data indicate that no water of crystallisation was present. The ¹H NMR spectrum of the dichloride is shown in the ESI, Fig. S2.† Reacting [Ru(bpy)(Me₂bpy)Cl₂] with excess Hbpt or Hbpzt in aqueous EtOH at reflux produced the tris(heteroleptic) mononuclear complexes [Ru(bpy)(Me₂bpy)(bpt)](PF₆)·H₂O (1) and [Ru(bpy)(Me₂bpy)(bpzt)](PF₆)·2H₂O (2), respectively.

The dinuclear complexes [{Ru(bpy)(Me₂bpy)}₂bpt](PF₆)₃·2H₂O (3) and [{Ru(bpy)(Me₂bpy)}₂bpzt](PF₆)₃ (4) were obtained by reacting a greater than two-fold excess of the dichloride precursor, the 1 : 4 mixture of [Ru(bpy)(MeCN)₂Cl₂] and [Ru(bpy)(MeCN)₃Cl]Cl, with the bridging ligand, Hbpt or Hbpzt. It was also possible to synthesise 3 and 4 by reacting 1 and 2 with excess [Ru(bpy)(Me₂bpy)Cl₂].

In all of these reactions HPLC analysis of the crude products showed that the product complex, 1–4, was contaminated by a small trace of the corresponding dinuclear or mononuclear species, 3, 4, 1 and 2, respectively. Pure samples of 1–4 were readily obtained by chromatography on alumina: afterwards only one peak was observed in the HPLC analysis.

The second method that was considered was based on the photolysis of compounds such as [Ru(bpy)(Me₂bpy)(CO)₂]²⁺ (5). This compound was obtained by reacting [Ru(Me₂bpy)(CO)₂Cl₂] with excess bpy. Photolysis of 5 proved to be very efficient so this method is a viable alternative for chemical decarbonylation. The progress of the reaction was followed with infrared spectroscopy. The spectra show that the bands at 2099 and 2047 cm⁻¹ observed for [Ru(Me₂bpy)(bpy)(CO)₂]²⁺ gradually disappear and are replaced by a single band at 2012 cm⁻¹, indicative of an intermediate monocarbonyl species, which disappears upon further irradiation. After removal of the solvent, the ¹H NMR and elemental analysis data on the product (see Experimental section) were consistent with the formation of the desired intermediate complex, [Ru(Me₂bpy)(bpy)(CH₃CN)₂]²⁺6. This was confirmed by X-ray analysis (see below). Although some tris(heteroleptic) complexes were isolated by reacting this bis(acetonitrile) intermediate with an appropriate chelating ligand, several problems were identified with regard to the generality of this approach, such as the availability of bicarbonyl starting materials. For example, in our hands only the starting materials [Ru(bpy)₂(CO)₂]²⁺ and [Ru(Me₂bpy)(bpy)(CO)₂]²⁺ were obtained in reasonable yields. Against this background it was decided that the photo induced decarbonylation of [Ru(L)(CO)₂Cl₂] should be focussed on as the preferred synthetic route and the second route, via5, was not investigated any further.

X-Ray crystallography

The X-ray data for 1·0.5C₄H₁₀O, 2·H₂O and 6·C₄H₁₀O are given in Table 1.

Table 1 Crystallographic data of [Ru(bpy)(Me₂bpy)(bpt)](PF₆)·0.5C₄H₁₀O (1·0.5C₄H₁₀O), [Ru(bpy)(Me₂bpy)(bpzt)](PF₆)·H₂O (2·H₂O) and [Ru(bpy)(Me₂bpy)(CH₃CN)₂](PF₆)₂·C₄H₁₀O (6·C₄H₁₀O)

	1·0.5C4H10O	2·H2O	6·C4H10O
Empirical formula	C36H33N9RuPF6O0.5	C32H28N9RuPF6O	C30H36N6ORuP2F12
M r	845.75	828.69	887.66
Colour	Red	Brown	Red–brown
Crystal source	MeCN–diethyl ether	MeCN–diethyl ether	MeCN–diethyl ether
T/K	200(2)	183(2)	183(2)
Crystal size/mm	0.42 × 0.24 × 0.20	0.09 × 0.07 × 0.06	0.03 × 0.03 × 0.02
Space group	P21/c	P21/c	P21/c
a/Å	13.95240(10)	16.2156(4)	11.1155(3)
b/Å	12.24230(10)	13.8572(4)	18.1355(5)
c/Å	23.4412(10)	14.8161(4)	18.3079(7)
β/°	95.9090(10)	92.117(2)°	95.932(1)
V/Å^3	3982.70(8)	3326.95(16)	3670.8(2)
D c/g cm^−3	1.411	1.654	1.606
Z	4	4	4
F(000)	1716	1672	1792
Radiation	Mo-Kα	Mo-Kα	Mo-Kα
Abs. coeff., µ/mm^−1	0.501	0.60	0.61
Abs. corr., T(min, max)	0.40, 0.93	None	None
2θ limits/°	1.47–26.38	2.35–27.44	2.25–27.48
Number of reflections	22545	13029	14401
Number of unique reflections	8079	7514	8418
Number of parameters	509	469	469
R1 (observed data)	0.059	0.049	0.056
wR2 (observed data)	0.1060	0.125	0.092
Goodness of fit	1.001	1.011	0.955

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Selected bond lengths and angles are summarised in Table 2. The geometrical properties of the bis(acetonitrile) complex 6·C₄H₁₀O are in agreement with those of [Ru(bpy)₂(CH₃CN)₂](PF₆)₂ reported by Heeg et al.³² In both cases an octahedral coordination mode is observed with the bipyridine ligands exhibiting acute bite-angles, in this case 78.70(14)° for the Me₂bpy ligand and 78.74(14)° for the bpy ligand (See Fig. 4).

Table 2 Selected bond lengths (Å) and angles (°) for [Ru(bpy)(Me₂bpy)(CH₃CN)₂](PF₆)₂·C₄H₁₀O (1·0.5C₄H₁₀O), [Ru(bpy)(Me₂bpy)(bpt)](PF₆)·0.5C₄H₁₀O) (2·H₂O) and [Ru(bpy)(Me₂bpy)(bpzt)](PF₆)·H₂O (6·C₄H₁₀O)

	6·C4H10O	1·0.5C4H10O	2·H2O
a N7 substituted for N6 in [Ru(bpy)(Me2bpy)(CH3CN)2](PF6)2 for comparision.
Ru–N1	2.067(3)	2.059(4)	2.061(3)
Ru–N2	2.046(3)	2.054(4)	2.050(3)
Ru–N3	2.043(3)	2.064(4)	2.070(3)
Ru–N4	2.067(3)	2.065(4)	2.055(3)
Ru–N5	2.032(4)	2.105(4)	2.069(3)
Ru–N7	—	2.048(4)	2.046(3)
Ru–N6	2.037(4)	—	—
N7(6)–Ru–N3	89.66(13)	174.82(14)	173.99(12)
N2–Ru–N5	89.88(13)	171.10(15)	171.61(11)
N4–Ru–N1	171.14(13)	173.35(15)	175.05(11)
N7(6)–Ru–N5	90.35(13)	77.82(15)	78.03(11)
N7(6)–Ru–N4	88.80(13)	98.24(16)	97.68(11)
N7(6)–Ru–N1	96.65(14)	87.73(15)	86.40(11)
N7(6)–Ru–N2	175.34(14)	93.81(15)	93.65(11)
N4–Ru–N5	97.30(14)	86.71(15)	84.23(12)
N5–Ru–N3	176.04(14)	97.85(15)	96.75(11)
N1–Ru–N5	89.67(13)	97.46(15)	99.39(12)
N4–Ru–N3	78.74(14)	78.56(16)	78.69(12)
N2–Ru–N4	95.79(14)	97.59(15)	98.11(11)
N1–Ru–N3	94.26(13)	95.69(16)	97.46(12)
N2–Ru–N3	90.43(13)	90.66(16)	91.61(11)
N2–Ru–N1	78.70(14)	79.01(15)	78.78(11)

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Fig. 4 Molecular structure of [Ru(bpy)(Me₂bpy)(CH₃CN)₂]²⁺, the cation of 6·C₄H₁₀O. Hydrogen atoms, anions and solvent have been omitted for clarity.

Both bidentate ligands exhibit similar the bond lengths between the metal and chelating nitrogen atoms. Ru–N bond lengths for bpy are (Ru–N3) 2.043(3) and (Ru–N4) 2.067(3) Å whereas those for Me₂bpy are (Ru–N1) 2.067(3) and (Ru–N2) 2.046(3) Å. In each case the shorter bond length is that trans to a MeCN ligand as a consequence of the bipyridyl ligands being stronger π-acids than MeCN ligands. The presence of a methyl group on the 4- and 4′-position of one of the bipy ligands has little effect on the Ru–N bond lengths to that ligand. Ru–N bond lengths for the two MeCN ligands are 2.032(4) and 2.037(4) Å respectively.

The crystal structures of 1·0.5C₄H₁₀O and 2·H₂O unambiguously confirm the presence of three different bis-chelating ligands around the metal centre (Fig. 5 and 6).

Fig. 5 Molecular structure of [Ru(bpy)(Me₂bpy)(bpt)]⁺, the cation of 1·0.5C₄H₁₀O. The hydrogen atoms, counter ion and solvent have been omitted for clarity.

Fig. 6 Molecular structure of [Ru(bpy)(Me₂bpy)(bpzt)]⁺, the cation of 2·H₂O. The hydrogen atoms have been omitted for clarity.

It is clear from these structures that the ruthenium centre binds to the three ligands in an octahedral fashion and via N(7) of the triazole ring. Importantly, for both compounds the triazole nitrogen is trans to the bpy ring. One PF₆⁻ cation is present in each case, confirming that the triazole ring deprotonates upon coordination.

The bond lengths and angles observed in the structures of 1 and 2 are typical of those found for other Ru(II) polypyridyl complexes containing triazoles.^9,33 The Ru–bpy and Me₂bpy bond lengths are in the range of 2.055–2.070 Å with their respective bite angles in the range of 78.56–79.01°. The biggest difference between the two complexes is the metal–triazole bond lengths. Although the Ru–N7(triazole) distances are approximately identical (2.048(4) Å for bpt⁻ and 2.046(3) Å for bpzt⁻), a significant difference exists between the two Ru–N5 bond lengths. In the case of [Ru(bpy)(Me₂bpy)(bpt)]⁺ this distance is 2.105(4) Å. For the pyrazine analogue this distance is more in line with the other polypyridyl ligands at 2.069(3) Å. This observation is in accordance with the properties of the two types of triazole ligands as has been described previously.³⁴ Specifically, bpt⁻ is a ligand with strong σ-donating and weak π-accepting properties. This weak π-accepting property leads to an increase in the Ru–N(5) bond length. On the other hand, bpzt⁻ is a ligand that combines both the strong σ-donating capabilities of the triazole and the strong π-accepting properties of the pyrazine. Therefore, while in [Ru(bpy)(Me₂bpy)(bpzt)]⁺ the Ru–N7(triazole) distance remains almost identical to that in [Ru(bpy)(Me₂bpy)(bpt)]⁺, the Ru–N5 bond is shorter by 0.036 Å.

In [Ru(bpy)(Me₂bpy)(bpzt)]⁺ a strong H-bond interaction is exhibited in the solid state as depicted in Fig. 6. The water (O1W) of crystallisation acts as H-bond donor to a pyrazine nitrogen (N6–O1W 2.815 Å) of the cationic complex and a fluorine (F3–O1W 2.856 Å) of the PF₆⁻ anion.

Spectroscopic and electrochemical characterisation

¹H NMR spectroscopy. Complexes 1–6 showed satisfactory elemental analysis and mass spectrometry data. However, for 1–4 complicated ¹H NMR spectra suggest the presence of isomers. A typical ¹H NMR spectrum of 1 illustrating the 2.4–2.7 ppm region outlining the methyl resonances is shown in Fig. 7.

Fig. 7 ¹H NMR of aliphatic region of two samples of [Ru(bpy)(Me₂bpy)(bpt)]⁺ in d₃-MeCN. Sample (a) is that of the product after column chromatography on alumina. Sample (b) is the recrystallised sample used for the X-ray analysis.

For the mononuclear complexes, both N2′ and N4′ are available for binding but previous studies have shown that if a large substituent is present at C5′ on the triazole ring the N2′ binding mode is preferred for steric reasons and this is indeed observed (see above). However, the asymmetrical nature of the bridging ligands still allows for two positional isomers as shown in Fig. 8.

Fig. 8 Two positional isomers of [Ru(bpy)(Me₂bpy)(bpt)]⁺. One isomer has the triazole ring trans to a Me₂bpy ring (left) whereas the other has the triazole ring trans to a bpy ring (right).

In one instance the triazole ring is trans to a Me₂bpy ring, whereas in the other it is trans to a bpy ring. Fig. 7(a) shows that initially two isomers are formed, while the recrystallised sample used for the X-ray analysis contains a single isomer (Fig. 7(b)). As the X-ray data show that in the crystal the triazole nitrogen is trans to the bpy ring. This indicates that the resonances observed at 2.42 and 2.44 ppm can be attributed to this particular isomer. The ¹H NMR spectrum for this isomer in the aromatic range is shown in Fig. S3 (ESI†). Due to the complexity of the spectrum no efforts were made to assign the various peaks. It is proposed for the second isomer shown in Fig. 7(a) the triazole is trans to the Me₂bpy ligand.

Electrochemical and electronic properties. Compounds 1–4 were fully characterised by differential pulse voltammetry, cyclic voltammetry and absorption and emission spectroscopy. The data obtained for the various redox processes, referenced against Fc/Fc⁺ given in Table 3 together with those reported for the equivalent [Ru(bpy)₂-based bpt and bpzt equivalents; [Ru(bpy)₂(bpt)]⁺, (7), [Ru(bpy)₂(bpzt)]⁺, (8) [{Ru(bpy)₂}₂(bpt)]³⁺, (9) and [{Ru(bpy)₂}₂(bpzt)]³⁺(10).

Table 3 Characterisation of complexes 1–4 by absorption, emission and electrochemical studies

		Emission, λmax/nm (τ/µs)		E 1/2 ^c/V (vs. Fc/Fc^+)
Complex	Absorption,^aλmax/nm (10^−4ε/M^−1 cm^−1)	298 K^a	77 K^b	Ox	Red
a In MeCN, deaerated solutions. b In EtOH–MeOH (4 : 1, v/v) deaerated solutions. c In MeCN (0.1 M solution of TBABF4). Potentials vs. Fc/Fc^+ represent one-electron transfer unless noted otherwise. d Measured in proprionitrile–butyronitrile (4 : 5). e Measured in aerated solutions. Data for compounds 7–10 obtained from references 9 and 34.
1	475 (1.10)	686 (0.37)	612 (3.0)	0.42	−1.90, −2.16
2	445 (1.45)	647 (0.51)	600 (8.1)	0.64	−1.80, −2.02
3	452 (2.18)	645 (0.08)	606 (3.8)	0.62, 0.92	−1.81 (2e), −2.11 (2e)
4	451 (2.32)	666 (0.09)	602 (7.8)	0.79, 1.05	−1.66, −1.77, −1.91, −2.25
7	475 (1.13)	678 (0.16)	628 (2.8)^d	0.48	−1.85, −2.10
8	453 (1.42)	662 (0.10)	610 (5.0)^d	0.60	−1.78, −2.00
9	453 (2.26)	648 (0.10)^e	608 (3.6)^d	0.66, 0.96	−1.78, −2.00, −2.05
10	449 (2.65)	690 (0.10)^e	610 (6.4)^d	0.78, 1.08	−1.70, −1.76, −1.94, −2.07

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Both mononuclear complexes 1 and 2 exhibit a one-electron reversible oxidation wave and two one-electron reversible reduction waves. Metal-based oxidation waves for 1 and 2 are found at 0.42 and 0.64 V, respectively, similar to those reported for 7 and 8. Two one-electron oxidation waves separated by about 300 mV are observed at 0.62 and 0.92 V for 3 and 0.79 and 1.05 V for 4. The complexes 1–4 exhibit intense metal to ligand charge transfer (MLCT) absorption bands in the visible part of the spectrum (see Table 3). The MLCT maximum for the compounds is red shifted compared to [Ru(bpy)₃]²⁺ as the σ-donating ability of the triazole ligand raises the HOMO energy level on the metal thus lowering the HOMO–LUMO energy difference.⁹ When the triazole negative charge is shared between two metal centres, as in the case of 3, the relative HOMO level is lowered, thus raising the energy of λ_max from 475 nm (mononuclear) to 452 nm (dinuclear). The same principles can be used to rationalise the shift in emission spectra from 686 nm for 1 to 606 nm for 3. Overall the electronic and electrochemical properties of the compounds are very similar to those reported for the bis(bpy) analogues 7–10. Small variations in the energy of the electronic transitions are obtained which can be attributed to the slight differences in the electronic properties of the polypyridyl ligands employed.

Conclusion

A new synthetic procedure for the preparation of tris(heteroleptic) complexes containing a triazole bridging ligand has been developed and successfully utilised to produce both mononuclear and dinuclear Ru(II) polypyridyl complexes. An alternative photochemical method based on [BzRuCl₂]₂ as starting material²¹ was recently reported. Our strategy is similar to that of Freedman et al. but a very different starting material is used for the photolysis step. The reaction scheme is general enough to bring a large number of tris(heteroleptic) complexes within synthetic reach. As outlined in the introduction the ultimate aim of this work is to investigate dinuclear complexes containing acceptor and donor moieties as shown in Fig. 1. In the present study the polypyridyl ligands were chosen to facilitate the development of the method and to be able to address the formation of isomers (i.e. the use of Me₂bpy). Therefore, the complexes produced have properties which are very similar to the Ru bis(bpy) analogues previously reported. At present work is in progress on the use of different polypyridyl ligands such as biq and dpp to investigate whether the excited state in these dinuclear compounds can be located specifically on one of the polypyridyl ligands. In addition, the synthesis the type of compounds outlined in Fig. 1 is in progress. Overall, the synthetic procedure utilised here will allow for more in-depth studies of the interaction between different metal centres in these dinuclear compounds by systematically altering the nature of the polypyridyl ligands within the metal coordination sphere.

Acknowledgements

We gratefully acknowledge Enterprise Ireland and the Electricity Supply Board of Ireland (ESB) for their financial support of this work. H. G. and S. R. acknowledge the DFG (RA1017/1 and SFB 436) and S. B. the Marsden Fund (Royal Society of New Zealand) for financial support. We thank T. Groutso and Professor G. R. Clark (University of Auckland) for the X-ray data collection on 1·0.5C₄H₁₀O.

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Footnote

† Electronic supplementary information (ESI) available: Fig. S1: ¹H NMR (CD₃CN) and DPV of the photolysis mixture obtained after irradiating [Ru(bpy)(CO)₂Cl₂] in MeCN for 1 h. Fig. S2: ¹H NMR of [Ru(bpy)(Me₂bpy)Cl₂] in d₆-DMSO. Fig. S3: ¹H NMR of the aromatic region of the crystal sample of [Ru(bpy)(Me₂bpy)(bpt)]⁺ in d₃-MeCN. See DOI: 10.1039/b510751b

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