Syntheses and electronic and optical properties of complexes of the bis(2,2′-bipyrazyl)ruthenium unit

Article abstract of DOI:10.1016/j.poly.2015.04.028

Seven new complexes of the form cis-[Ru^II(bpz)₂(L-L)]ⁿ⁺ (bpz = 2,2′-bipyrazyl: n = 2; L-L = 4,4′-bis(tert-butyl)-2,2′-bipyridyl, 4,4′-diphenyl-2,2′-bipyridyl, 4,4′-dichloro-2,2′-bipyridyl, 4,4′-diamino-2,2′-bipy

Full text of DOI:10.1016/j.poly.2015.04.028

Accepted Manuscript
Syntheses and electronic and optical properties of complexes of the bis(2,2′-
bipyrazyl)ruthenium unit
Benjamin J. Coe, Martyn K. Peers, Nigel S. Scrutton
PII:
S0277-5387(15)00225-9
DOI:
http://dx.doi.org/10.1016/j.poly.2015.04.028
POLY 11292
Reference:
Polyhedron
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Accepted Date:
25 February 2015
23 April 2015
Please cite this article as: B.J. Coe, M.K. Peers, N.S. Scrutton, Syntheses and electronic and optical properties of
complexes of the bis(2,2′-bipyrazyl)ruthenium unit, Polyhedron (2015), doi: http://dx.doi.org/10.1016/j.poly.
2015.04.028
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1
Syntheses and electronic and optical properties of complexes
of the bis(2,2′-bipyrazyl)ruthenium unit
Benjamin J. Coe a,*, Martyn K. Peers b, Nigel S. Scrutton b
a
School of Chemistry, The University of Manchester, Oxford Road, Manchester M13 9PL,
UK
b Manchester Institute of Biotechnology, Faculty of Life Sciences, The University of
Manchester, 131 Princess Street, Manchester M1 7DN, UK
* Corresponding author. E-mail: b.coe@manchester.ac.uk; Fax +44 (0)161 275 4598; Tel +44
(0)161 275 4601.

2
ABSTRACT
Seven new complexes of the form cis-[RuII(bpz)2(L–L)]n+ (bpz = 2,2′-bipyrazyl: n = 2; L–L =
4,4′-bis(tert-butyl)-2,2′-bipyridyl, 4,4′-diphenyl-2,2′-bipyridyl, 4,4′-dichloro-2,2′-bipyridyl,
4,4′-diamino-2,2′-bipyridyl,
4,4′-bis(trifluoromethyl)-2,2′-bipyridyl,
4,4′-
bis(methoxycarbonyl)-2,2′-bipyridyl: n = 4; L–L = N′′,N′′′-dimethyl-4,4′:2′,2′′:4′′,4′′′-
–
quaterpyridinium) are prepared and isolated as their PF6 and Cl– salts. Improved methods for
synthesising bpz and 4,4′-bis(trifluoromethyl)-2,2′-bipyridyl are described also.
1
Characterisation involves various techniques including H NMR spectroscopy and mass
spectrometry. The new compounds are studied alongside the known species where n = 2 and
L–L = 2,2′-bipyridyl, 4,4′-dimethyl-2,2′-bipyridyl or 2,2′-bipyrimidine. Their UV–vis spectra
display intense intraligand π → π* absorptions, and also metal-to-ligand charge-transfer
(MLCT) bands with two resolved maxima in the visible region. Red-shifts in the MLCT
bands occur as the electron-donating strength of L–L increases. Cyclic voltammograms show
reversible RuIII/II oxidation waves, and several ligand-based reductions that are also mostly
reversible. The variations in the redox potentials correlate with changes in the MLCT
energies. Time-dependent density functional theory calculations give relatively good
correlations with the experimental UV–Vis spectra for selected complexes when using the
M06 functional and basis sets Def2-QZVP (on Ru) and Def2-SVP (on all other atoms) in
acetonitrile. The lowest energy visible absorption band is confirmed to be due to RuII → bpz
MLCT, while further such transitions occur along with MLCT to L–L at higher energies.
Keywords: ruthenium complexes, 2,2′-bipyrazyl, UV–Vis spectroscopy, electrochemistry,
density functional theory

3
1. Introduction
Ruthenium occupies a central position among the d-block transition metals. It shows
an amazingly rich coordination and organometallic chemistry, forming complexes with every
kind of ligand imaginable [1,2]. Comparatively high stability and relative ease of synthesis
have allowed this area to flourish. Beyond primary scientific value, such compounds are
practically useful in many important fields including catalysis [3–5] and biology/medicine [6–
8]. Impinging on both of these areas, and of special current interest are technologies that
exploit photophysical/chemical properties [9–11]. These include photoredox catalysis [12–
14], cellular imaging [15–17], organic light-emitting diodes (OLEDs) [18–20], and dye-
sensitized solar cells (DSSCs) [21–23]. All of these topics involve complexes of chelating
polypyridyl ligands, especially 2,2′-bipyridyl (bpy), with Ru in the divalent oxidation state.
Such compounds have fascinating electronic absorption, emission and electron/energy-
transfer properties, based on low energy metal-to-ligand charge-transfer (MLCT) excited
states. Tuning the properties of these states by changing ligand structures is a mature area [9–
11].
While the complex [RuII(bpy)3]2+ and related species with bpy-based ligands have
been studied very extensively, analogous complexes of 2,2′-bipyrazyl (bpz) have received
somewhat less attention. [RuII(bpz)3]2+ was first reported in 1980 [24], its apparently
favourable photoredox properties when compared with [RuII(bpy)3]2+ indicating substantial
promise. Subsequent studies include examples focusing on basic synthesis and physical
characterisation [25–31], and also reports of relatively more complicated structures and/or a
focus on applications. The latter incorporate polymetallic complexes [32,33], DNA
photocleavage [34], DSSCs [35,36], electron-transfer probes for enzymes [37,38], catalysis
[39], and dyads for proton-coupled electron transfer studies [40]. Here we report a new and
improved procedure for synthesising bpz, and the preparation of a series of new complexes
containing the {RuII(bpz)2}2+ moiety. Experimental measurements are accompanied by
theoretical studies in order to develop a thorough knowledge and understanding of the new
complexes.

4
2. Experimental
2.1. Materials, procedures and physical measurements
The compounds 4,4′-dinitro-2,2′-bipyridyl [41], 4,4′-dichloro-2,2′-bipyridyl [41],
4,4′-diamino-2,2′-bipyridyl [42], 4,4′:2′,2′′:4′′,4′′′-quaterpyridyl (qpy) [43], N′′,N′′′-dimethyl-
4,4′:2′,2′′:4′′,4′′′-quaterpyridinium hexafluorophosphate ([Me2qpy2+](PF6)2) [44], cis-
RuII(bpz)2Cl2 [26] and [RuII(bpz)3]Cl2 [24] were prepared according to published methods. All
other reagents and solvents were used as supplied from Fluorochem, Sigma-Aldrich, Alfa-
Aesar or Fisher Scientific. Products were dried at room temperature overnight in a vacuum
desiccator (CaSO4) or by direct attachment to a high-vacuum line for several hours prior to
characterisation.
1H NMR spectra were recorded on a Bruker AV-400 spectrometer, with all shifts
referenced to residual solvent signals and quoted with respect to TMS. The AA′BB′ patterns
of pyridyl or phenyl rings are reported as simple doublets, with ‘J values’ referring to the two
most intense peaks. Elemental analyses were performed by the Microanalytical Laboratory,
University of Manchester. ESI and GC mass spectrometry were recorded respectively on a
Waters SQD2 or Agilent 5975C spectrometer. UV–Vis absorption spectra were obtained by
using a Shimadzu UV-2401 PC spectrophotometer. Cyclic voltammetric measurements were
performed by using an Ivium CompactStat. A single-compartment BASi VC-2 cell was used
with a silver/silver chloride reference electrode (3 M NaCl, saturated AgCl) separated by a
salt bridge from a 2 mm disc Pt working electrode and Pt wire auxiliary electrode. The
ferrocene/ferrocenium couple was used as an internal standard. Acetonitrile was used as
supplied from Fisher Scientific (HPLC grade), and [NBun ]PF6 (Sigma Aldrich,
4
electrochemical grade) was used as the supporting electrolyte. Solutions containing ca. 10–3
M analyte (0.1 M [NBun ]PF6) were deaerated by purging with dried N2. E1/2 values were
4
calculated from (Epa + Epc)/2 at a scan rate of 100 mV s–1.

5
2.2. Syntheses
2.2.1. 2,2′-Bipyrazyl, bpz
2-Iodopyrazine (5.00 g, 24.3 mmol), PdII(OAc)2 (34 mg, 0.151 mmol), K2CO3 (3.4 g,
24.6 mmol) and poly(ethylene glycol) (Mw 4000, 24.0 g) were combined in an argon-purged
flask. The mixture was gradually heated to 120 °C and the temperature maintained for 48 h
with stirring. The mixture was cooled to ca. 80 °C and warm water (30 mL) was added to
prevent solidification. On cooling to room temperature, further water (100 mL) was added
and the suspension exhaustively extracted with ethyl acetate. The combined extracts were
washed once with saturated aqueous Na2S2O3 and then thrice with brine. The organic layer
was dried over MgSO4 and solvent removed under vacuum. The residue was triturated with n-
pentane and the white crystalline solid filtered off, washed with n-pentane and dried. Yield:
1.35 g (70%). δ (400 MHz, CDCl3) 9.60 (2H, d, J = 1.1 Hz), 8.68–8.66 (4H). GC-MS: m/z =
H
158 ([M]+). Anal. Calc. (%) for C8H6N4: C, 60.8; H, 3.8; N, 35.4. Found: C, 60.4; H, 3.4; N,
35.8.
2.2.2. 4,4′-Bis(trifluoromethyl)-2,2′-bipyridyl
This compound was prepared in a manner similar to bpz by using 2-bromo-4-
(trifluoromethyl)pyridine (2.25 g, 9.96 mmol) in place of 2-iodopyrazine, PdII(OAc)2 (56 mg,
0.249 mmol), K2CO3 (1.38 g, 9.99 mmol) and poly(ethylene glycol) (10.0 g). After
evaporation of the dried ethyl acetate extracts, the residue was purified by column
chromatography on silica gel eluting with dichloromethane. A white crystalline solid was
obtained. Yield: 811 mg (56%). δ (400 MHz, CDCl3) 8.89 (2H, dt, J = 5.0, 0.7 Hz), 8.73
H
(2H, dt, J = 1.6, 0.8 Hz), 7.59 (2H, ddd, J = 5.0, 1.7, 0.7 Hz). GC-MS: m/z = 291.9 ([M]+).
Anal. Calc. (%) for C12H6F6N2: C, 49.3; H, 2.1; N, 9.6. Found: C, 49.3; H, 2.1; N, 9.8.
2.2.3. [RuII(bpz)2(bpy)](PF6)2 (1)
cis-RuII(bpz)2Cl2 (100 mg, 0.205 mmol) and bpy (64 mg, 0.410 mmol) were combined
in an argon-purged flask. An argon-sparged mixture of 2-methoxyethanol (7 mL) and water (3

6
mL) was added and the mixture heated at reflux for 24 h. After cooling to room temperature,
the solvents were removed under vacuum and a small amount of water was added. The
suspension was filtered, and an excess of solid NH4PF6 was added to the filtrate to precipitate
the crude product. The solid was filtered off and purified by column chromatography on silica
gel eluting with 0.1 M NH4PF6 in acetonitrile. The main orange fraction was evaporated to
dryness and the product washed extensively with ice-cold water, then dried to give an orange
solid. Yield: 95 mg (56%). δ (400 MHz, CD3CN) 9.75 (4H, t, J = 1.2 Hz), 8.59 (4H, dd, J =
H
3.2, 2.2 Hz), 8.53 (2H, d, J = 8.0 Hz), 8.15 (2H, td, J = 7.9, 1.5 Hz), 7.85 (2H, dd, J = 3.2, 1.2
Hz), 7.78 (2H, dd, J = 3.2, 1.2 Hz), 7.69 (2H, ddd, J = 5.6, 1.4, 0.7 Hz), 7.46 (2H, ddd, J =
7.7, 5.7, 1.3 Hz). ES-MS: m/z = 718 ([M – PF6]+), 287 ([M – 2PF6]2+). Anal. Calc. (%) for
C26H20F12N10P2Ru•H2O: C, 35.4; H, 2.5; N, 15.9. Found: C, 35.5; H, 2.1; N, 15.7.
2.2.4. [RuII(bpz)2(4,4′-Me2bpy)](PF6)2 (2)
This compound was prepared and purified in a manner similar to 1 by using 4,4′-
dimethyl-2,2′-bipyridyl (76 mg, 0.413 mmol) in place of bpy to give a dark red solid. Yield:
91 mg (50%). δ (400 MHz, CD3CN) 9.74 (4H, dd, J = 2.7, 1.2 Hz), 8.62 (4H, dd, J = 9.9, 3.2
H
Hz), 8.38 (2H, dd, J = 1.0, 0.6 Hz), 7.84 (2H, dd, J = 3.2, 1.2 Hz), 7.77 (2H, dd, J = 3.3, 1.3
Hz), 7.48 (2H, d, J = 5.9 Hz), 7.30–7.28 (2H), 2.55 (6H, s). ES-MS: m/z = 746 ([M – PF6]+),
301 ([M – 2PF6]2+). Anal. Calc. (%) for C28H24F12N10P2Ru: C, 37.7; H, 2.7; N, 15.7. Found:
C, 38.0; H, 2.7; N, 15.4.
2.2.5. [RuII(bpz)2(4,4′-tBu2bpy)](PF6)2 (3)
This compound was prepared and purified in a manner similar to 1 by using 4,4′-
bis(tert-butyl)-2,2′-bipyridyl (110 mg, 0.410 mmol) in place of bpy to give a dark red solid.
Yield: 112 mg (55%). δ (400 MHz, CD3CN) 9.75 (4H, dd, J = 3.6, 1.2 Hz), 8.62 (2H, d, J =
H
3.2 Hz), 8.58 (2H, d, J = 3.2 Hz), 8.49 (2H, d, J = 2.4 Hz), 7.84 (2H, dd, J = 3.2, 1.2 Hz), 7.77
(2H, dd, J = 3.2, 1.2 Hz), 7.54 (2H, d, J = 6.0 Hz), 7.42 (2H, dd, J = 6.1, 2.1 Hz), 1.41 (18H,
s). ES-MS: m/z = 831 ([M – PF6]+), 343 ([M – 2PF6]2+). Anal. Calc. (%) for
C34H36F12N10P2Ru•H2O: C, 41.1; H, 3.9; N, 14.1. Found; C, 41.4; H, 3.4; N, 13.9.

7
2.2.6. [RuII(bpz)2(4,4′-Ph2bpy)](PF6)2 (4)
This compound was prepared and purified in a manner similar to 1 by using 4,4′-
diphenyl-2,2′-bipyridyl (126 mg, 0.409 mmol) in place of bpy to give an orange solid. Yield:
141 mg (65%). δ (400 MHz, CD3CN) 9.77 (4H, t, J = 1.4 Hz), 8.93 (2H, t, J = 1.2 Hz), 8.62
H
(4H, dd, J = 4.3, 3.3 Hz), 7.95−7.93 (4H), 7.90 (4H, ddd, J = 5.6, 3.3, 1.2 Hz), 7.72 (4H, d, J
= 1.2 Hz), 7.64−7.60 (6H). ES-MS: m/z = 870 ([M – PF6]+), 363 ([M – 2PF6]2+). Anal. Calc.
(%) for C38H28F12N10P2Ru•2H2O: C, 43.4; H, 3.1; N, 13.3. Found: C, 43.2; H, 2.6; N, 13.3.
2.2.7. [RuII(bpz)2(4,4′-Cl2bpy)](PF6)2 (5)
This compound was prepared and purified in a manner similar to 1 by using 4,4′-
dichloro-2,2′-bipyridyl (92 mg, 0.409 mmol) in place of bpy to give an orange solid. Yield:
97 mg (51%). δ (400 MHz, CD3CN) 9.75 (4H, dd, J = 4.8, 1.1 Hz), 8.63 (4H, t, J = 2.3 Hz),
H
8.59 (2H, d, J = 3.2 Hz), 7.83 (4H, ddd, J = 7.0, 3.2, 1.2 Hz), 7.61 (2H, d, J = 6.1 Hz), 7.54
(2H, dd, J = 6.2, 2.2 Hz). ES-MS: m/z = 787 ([M – PF6]+), 321 ([M – 2PF6]2+). Anal. Calc.
(%) for C26H18Cl2F12N10P2Ru: C, 33.5; H, 1.9; N, 15.0. Found: C, 33.4; H, 1.8; N, 14.6.
2.2.8. [RuII(bpz)2(4,4′-(NH2)2bpy)](PF6)2 (6)
This compound was prepared and purified in a manner similar to 1 by using 4,4′-
diamino-2,2′-bipyridyl (76 mg, 0.408 mmol) in place of bpy to give a dark red-brown solid.
Yield: 103 mg (54%). δ (400 MHz, CD3CN) 9.71 (2H, d, J = 1.3 Hz), 9.69 (2H, d, J = 1.3
H
Hz), 8.62 (2H, d, J = 3.3 Hz), 8.50 (2H, d, J = 3.3 Hz), 7.96 (2H, dd, J = 3.3, 1.3 Hz), 7.80
(2H, dd, J = 3.3, 1.3 Hz), 7.40 (2H, d, J = 2.5 Hz), 6.92 (2H, d, J = 6.5 Hz), 6.53 (2H, dd, J =
6.5, 2.5 Hz), 5.72 (4H, s). ES-MS: m/z = 748 ([M – PF6]+), 302 ([M – 2PF6]2+). Anal. Calc.
(%) for C26H22F12N12P2Ru•2H2O: C, 33.6; H, 2.8; N, 18.1. Found: C, 33.7; H, 2.5; N, 18.0.
2.2.9. [RuII(bpz)2(bpym)](PF6)2 (7)
This compound was prepared and purified in a manner similar to 1 by using 2,2′-
bipyrimidine (65 mg, 0.411 mmol) in place of bpy to give a bright orange solid. Yield: 77 mg

8
(43%). δ (400 MHz, CD3CN) 9.76 (4H, dd, J = 3.3, 1.2 Hz), 9.18 (2H, dd, J = 4.8, 2.0 Hz),
H
8.65 (2H, d, J = 3.2 Hz), 8.60 (2H, d, J = 3.2 Hz), 8.02 (2H, dd, J = 5.8, 2.0 Hz), 7.96 (2H, dd,
J = 3.2, 1.1 Hz), 7.84 (2H, dd, J = 3.2, 1.1 Hz), 7.60 (2H, dd, J = 5.7, 4.9 Hz). ES-MS: m/z =
720 ([M – PF6]+), 288 ([M – 2PF6]2+). Anal. Calc. (%) for C24H18F12N12P2Ru•H2O: C, 32.6;
H, 2.3; N, 19.0. Found: C, 32.8; H, 2.2; N, 18.9.
2.2.10. [RuII(bpz)2(Me2qpy2+)](PF6)4 (8)
This compound was prepared in a manner similar to 1 by using [Me2qpy2+](PF6)2 (194
mg, 0.308 mmol) in place of bpy. Purification was effected by using Sephadex-CM C-25 with
an eluent of water/acetone (5:3) and a progressively increasing concentration of NaCl (0.025–
0.125 M). The product was eluted as a red band; partially evaporating the solvents under
vaccum and adding an excess of solid NH4PF6 gave a precipitate which was filtered off,
washed with water and dried to give a bright orange solid. Yield: 126 mg (45%). δ (400
H
MHz, CD3CN) 9.80 (4H, s), 9.15 (2H, d, J = 1.7 Hz), 8.83 (4H, d, J = 6.7 Hz), 8.65 (4H, dd, J
= 3.2, 2.4 Hz), 8.49 (4H, d, J = 7.0 Hz), 7.98 (2H, d, J = 6.0 Hz), 7.90–7.85 (6H), 4.39 (6H,
s). ES-MS: m/z = 1192 ([M – PF6]+), 524 ([M – 2PF6]2+), 302 ([M – 3PF6]3+), 189 ([M –
4PF6]4+). Anal. Calc. (%) for C38H32F24N12P4Ru•H2O: C, 33.7; H, 2.5; N, 12.4. Found: C,
33.7; H, 2.4; N, 12.2.
2.2.11. [RuII(bpz)2(4,4′-(CF3)2bpy)](PF6)2 (9)
A solution of cis-RuII(bpz)2Cl2 (100 mg, 0.205 mmol) and AgNO3 (77 mg, 0.453 mol)
in water (25 mL) was heated at reflux for 48 h. After cooling to room temperature, the
mixture was filtered through Celite to remove AgCl, and the filtrate was evaporated to
dryness. The residue was dissolved in DMF (15 mL) and the solution purged with argon for
15 min. 4,4′-bis(trifluoromethyl)-2,2′-bipyridyl (121 mg, 0.414 mmol) was added and the
mixture heated at 100 °C for 24 h under argon. After cooling to room temperature, the
solution was evaporated under vacuum to a small volume and diethyl ether (150 mL) added.
The precipitate was filtered off and dissolved in a minimum of cold water to which solid
NH4PF6 was added. The solid was filtered off and purified by column chromatography as for

9
1 to give an orange solid. Yield: 73 mg (35%). δ (400 MHz, CD3CN) 9.77 (4H, d, J = 1.2
H
Hz), 8.96 (2H, t, J = 3.2 Hz), 8.63 (4H, d, J = 3.2 Hz), 7.97 (2H, d, J = 5.9 Hz), 7.84 (2H, dd,
J = 3.2, 1.2 Hz), 7.80 (2H, dd, J = 3.2, 1.2 Hz), 7.75 (2H, dd, J = 6.0, 1.4 Hz). δ (400 MHz,
F
CD3CN) –63.89 (6H, s), –71.36 (12H, d, J = 707 Hz). ES-MS: m/z = 854 ([M – PF6]+), 355
([M – 2PF6]2+). Anal. Calc. (%) for C28H18F18N10P2Ru•H2O: C, 33.1; H, 2.0; N, 13.8. Found:
C, 32.7; H, 1.5; N, 13.7.
2.2.12. [RuII(bpz)2(4,4′-(CO2Me)2bpy)](PF6)2 (10)
This compound was prepared and purified in a manner similar to 9 by using 4,4′-
bis(methoxycarbonyl)-2,2′-bipyridyl (112 mg, 0.411 mmol) in place of 4,4′-
bis(trifluoromethyl)-2,2′-bipyridyl to give an orange solid. Yield: 98 mg (48%). δ (400
H
MHz, CD3CN) 10.02 (4H, t, J = 1.5 Hz), 9.25 (2H, d, J = 1.1 Hz), 8.72 (2H, d, J = 3.2 Hz),
8.69 (2H, d, J = 3.2 Hz), 8.07 (2H, dd, J = 5.8, 0.5 Hz), 8.05–8.02 (4H), 7.94 (2H, dd, J = 3.2,
1.2 Hz), 3.35 (6H, s). ES-MS: m/z = 835 ([M – PF6]+), 345 ([M – 2PF6]2+). Anal. Calc (%) for
C30H24F12N10O4P2Ru•H2O: C, 36.1; H, 2.6; N, 14.0. Found: C, 36.0; H, 2.3; N, 13.9.
2.3. Theoretical calculations
Geometry optimisation (in the gas phase only) and subsequent time-dependant density
functional theory (TD-DFT) calculations were carried out by using the Gaussian 09 suite of
programs [45]. A range of calculations was performed with the functionals BP86 [46,47],
B3LYP [48], PBE1PBE [49] and M06 [50] with the basis set Def2-QZVP [51] on Ru and
Def2-SVP [51] on all other atoms. The M06 functional allowed the most accurate modeling
of the experimental data when including a CPCM [52,53] solvent model of acetonitrile during
TD-DFT calculations. Using this approach, the first 100 excited singlet states were calculated
and simulated UV–Vis spectra in the range of 200–800 nm were convoluted with Gaussian
curves of FWHM of 3000 cm–1 by using GaussSum [54].

10
3. Results and discussion
Fig. 1. Chemical structures of the RuII complex salts investigated.
3.1. Syntheses
The bpz proligand was synthesised from 2-iodopyrazine, by adapting the Pd-catalysed
homocoupling method of Wang et al. [55]. Previous syntheses from 2-pyrazinecarboxylate
with Cu [25,56] or from 2-chloropyrazine with Ni [57] or Pd [58] catalysts give isolated
yields well below the 70% achieved here. The compound 4,4′-bis(trifluoromethyl)-2,2′-
bipyridyl was prepared also via the Wang method from 2-bromo-4-trifluoromethylpyridine,
again giving higher isolated yields when compared with published methods using 2-chloro-4-
trifluoromethylpyridine with Ni catalysis [59–61].
The complex salts 1, 2 and 7 (Fig. 1) have been studied previously on a number of
occasions. The original preparation of 1 [27] involved the precursor RuIVCl4(bpy); while
subsequent related reports have mentioned using instead cis-RuII(bpz)2Cl2 [62–64], details are
lacking. The latter complex is significantly less reactive towards chloride ligand substitutions
when compared with cis-RuII(bpy)2Cl2, so the use of a relatively high-boiling solvent mixture
is required to give reasonable yields of ca. 45–65%. 1–7 were purified by using column
chromatography on silica gel, while Sephadex-CM C-25 was used for 8 in order to remove
uncoordinated Me2qpy2+. For 9, using Ag(I) to abstract the chloride ligands is necessary to
give satisfactory results, probably due to the decreased basicity of the incoming bpy-based
ligand. The same method was applied for 10, because that used for 1 leads to coordination but
also trans-esterification by 2-methoxyethanol of the 4,4′-bis(methoxycarbonyl)-2,2′-bipyridyl

11
ligand. The identities and purities of all the compounds prepared are confirmed by diagnostic
1H NMR spectra, together with mass spectra and CHN elemental analyses. Most of the
complex salts retain ca. 1–2 equivalents of water, as is typically observed for such
1
compounds. Portions of representative H NMR spectra for the complex salts 1, 3 and 9 are
shown in the Supplementary Information (Fig. S1).
Fig. 2. UV–Vis absorption spectra of the complex salts 1 (green), 3 (blue), 6 (red) and 9
(gold) in acetonitrile at 293 K; (a) full spectra; (b) expansion of the low energy region.

12
3.2. Electronic spectroscopy
UV–Vis absorption spectra were recorded for 1–10 in acetonitrile, and the results are
listed in Table 1. Representative spectra of 1, 3, 6 and 9 are shown in Fig. 2.
Each complex salt shows two broad, overlapping bands in the visible region, assigned
to transitions of MLCT character. The λ
value for the well-defined lowest energy band
max
shows a clear trend of increasing as the co-ligand (L–L) becomes more electron-donating, in
the order 4,4′-(NH2)2bpy > 4,4′-Me2bpy = 4,4′-tBu2bpy > 4,4′-Ph2bpy ≥ bpy > 4,4′-Cl2bpy ≥
Me2qpy2+ ≥ bpym ≥ 4,4′-(CO2Me)2bpy ≥ 4,4′-(CF3)2bpy > bpz. Within the bpy-containing
series, the energy of this band decreases by ca. 0.3 eV on replacing the most strongly
electron-withdrawing -CF3 with the most donating -NH2 groups. The higher energy band
shows a similar shifting pattern, but its position is less clearly defined and the decrease
between the two extremes is less pronounced at ca. 0.16 eV. Anion metathesis allowed
isolation of the Cl– salts (see the Supplementary Information for details), and their spectra
were measured in water (Supplementary Information, Table S1). In most cases, very slight
blue-shifts are observed on moving from acetonitrile to water, but the overall trend in λ
max
values remains essentially unchanged.
Assignment of the separate low energy MLCT transitions to specific ligands is non-
trivial. Resonance Raman spectroscopic measurements on heteroleptic complexes such as
[RuII(bpz)2(bpy)]2+ indicate that the transitions to bpz occur at lower energies than those to
bpy or substituted bpy ligands [63, 65, 66]. This conclusion agrees with the expected relative
stabilisation of the π*-orbitals of bpz. The red shift of the lowest energy visible band
maximum observed here on increasing the electron-donating ability of L–L is consistent with
destabilisation of the Ru-based HOMO, while the bpz-based LUMO remains essentially
unchanged. The position of the higher energy band depends on changes in both the HOMO
and L–L-based LUMO. However, given that both of the homoleptic complexes [RuII(bpz)3]2+
and [RuII(bpy)3]2+ also show two overlapping MLCT bands in this region, this analysis is
probably somewhat over-simplified. The band assignments are best addressed by using TD-
DFT calculations (see below).

13
Additional absorptions in the region ca. 320–400 nm are probably also due to MLCT
transitions primarily, while the more intense bands to higher energies in the UV region are
attributable to π → π* and n → π* intraligand transitions.
3.3. Electrochemistry
1–10 were studied by using cyclic voltammetry in acetonitrile, and the results are
included in Table 1. Representative voltammograms for 1, 3, 6 and 9 are shown in Fig. 3. All
potentials are quoted with respect to the Ag–AgCl reference electrode.
Fig. 3. Representative cyclic voltammograms of the complex salts 1 (green), 3 (blue), 6 (red)
and 9 (gold) at 293 K in acetonitrile (Pt disc working electrode, scan rate = 100 mV s–1). The
single-headed arrow indicates the direction of the initial scans.

14
In each case, a reversible RuIII/II wave is observed with E1/2 in the range 1.43–1.92 V.
All of these values are lower than that found for [RuII(bpz)3]2+ under the same conditions,
showing that the Ru centre is more electron-rich when even 4,4′-(CF3)2bpy replaces bpz. E1/2
decreases as L–L becomes more electron-donating, with the order following essentially the
same sequence as noted for λmax of the lowest energy MLCT band, i.e. excluding L–L = bpz,
E1/2 is highest when L–L is 4,4′-(CF3)2bpy (in 9) and lowest with 4,4′-(NH2)2bpy (in 6). The
total difference between these extremes is ca. 0.5 V.
Each of 1–10 also shows at least three reversible, ligand-based reduction processes,
excepting 6 for which the third process is irreversible. 8 is a special case and discussed
separately below. In every other case, the first two reductions can be assigned to the bpz
ligands, and their E1/2 values increase slightly as L–L becomes less electron-donating and the
bpz ligands become progressively easier to reduce. The extreme values for the first two
reductions are –0.76/–1.06 V for 6 and –0.69/–0.88 V for 9, and slightly higher again for
[RuII(bpz)3][PF6]2. Hence, the second wave is more sensitive to the nature of L–L. The third
reduction wave is assigned to L–L, its E1/2 or Epc value increasing as this coligand becomes
more electron deficient.
For 8, the reductive region shows overlapping waves that were resolved by using
differential pulse voltammetry. The closely related compound [RuII(bpy)2(Me2qpy2+)][PF6]4
shows the first two reductions as overlapped waves with E1/2 values of –0.62 and –0.73 V,
then further waves at –1.19, –1.42 and –1.59 V [43]. The waves at –0.58 and –0.69 V in 8 are
therefore attributable to reduction of the pyridinum units in Me2qpy2+, while the next one at –
0.82 V and the irreversible process at Epc = –1.04 V likely correspond with the bpz-based
reductions.

15
Table 1
UV–Vis absorption and electrochemical data for the complex salts [RuII(bpz)2(L–L)][PF6]n in
acetonitrile.a
Emax
E1/2, V vs, Ag–AgCl
(∆Ep, mV)c
λmax, nm
(ε, 103 M–1 cm–1)b
Compound (L–L)
[RuII(bpz)3][PF6]2
Assignment
(eV)
RuIII/II
Ligand-based
239 (19.6)
292 (53.3)
338sh (15.7)
414sh (10.5)
441 (13.0)
241 (22.3)
254sh (20.0)
295 (49.3)
342sh (11.5)
378 (5.7)
5.18
4.25
3.67
3.00
2.81
5.15
4.88
4.20
3.63
3.28
2.95
2.68
5.15
4.19
3.32
2.84
2.64
5.96
5.17
4.19
3.59
3.32
2.94
2.64
5.10
4.73
4.16
3.20
2.92
2.67
5.77
4.29
3.65
2.88
2.72
4.77
4.09
3.45
2.84
2.48
5.08
4.73
4.19
3.65
3.00
2.75
4.90
4.16
2.92
2.74
5.15
4.86
4.23
2.01 (90)
−0.66 (70)
−0.83 (70)
−1.10 (90)
−1.74 (90)
π → π*
π → π*
d → π*
d → π*
d → π*
π → π*
π → π*
π → π*
d → π*
d → π*
d → π*
d → π*
π → π*
π → π*
d → π*
d → π*
d → π*
π → π*
π → π*
π → π*
d → π*
d → π*
d → π*
d → π*
π → π*
π → π*
π → π*
d → π*
d → π*
d → π*
π → π*
π → π*
d → π*
d → π*
d → π*
π → π*
π → π*
d → π*
d → π*
d → π*
π → π*
π → π*
π → π*
d → π*
d → π*
d → π*
π → π*
π → π*
d → π*
d → π*
π → π*
π → π*
π → π*
1 (bpy)
1.79 (80)
1.72 (80)
−0.73 (80)
−0.96 (70)
−1.50 (70)
420 (9.2)
464 (11.7)
241 (21.2)
296 (45.6)
374sh (11.0)
422 (8.4)
470 (11.0)
208 (39.3)
240 (22.7)
296 (51.2)
345sh (11.8)
374sh (7.4)
422 (9.5)
470 (12.1)
243 (37.2)
262 (39.3)
298 (80.2)
388 (9.5)
425 (15.4)
465 (13.8)
215 (42.8)
289 (54.2)
340sh (13.3)
430sh (10.5)
455 (11.3)
260 (48.1)
303 (49.1)
359 (12.4)
436 (7.8)
499 (11.9)
244 (41.7)
262 (30.6)
296 (42.1)
340sh (15.1)
413sh (9.3)
451 (12.6)
253 (54.5)
298 (50.0)
425sh (17.1)
453 (21.3)
241 (20.9)
255sh (18.4)
293 (60.1)
−0.75 (70)
−0.99 (60)
−1.58 (70)
2 (4,4′-Me2bpy)
3 (4,4′-tBu2bpy)
1.72 (100) −0.75 (70)
−0.98 (60)
−1.58 (70)
1.74 (90)
−0.73 (70)
−0.96 (70)
−1.43 (70)
−1.83 (70)
4 (4,4′-Ph2bpy)
1.82 (100) −0.73 (80)
−0.95 (70)
5 (4,4′-Cl2bpy)
6 (4,4′-(NH2)2bpy)
7 (bpym)
−1.27 (70)
1.43 (80)
1.91 (90)
−0.76 (60)
−1.06 (70)
−1.89d
−0.71 (70)
−0.92 (60)
−1.17 (70)
−1.80 (100)
8 (Me2qpy2+)
1.85 (90)
−0.58 (70)e
−0.69 (70)e
−0.82 (60)
−1.04d
1.92 (120) −0.69 (70)
−0.88 (60)
9 (4,4′-(CF3)2bpy)
−1.13 (70)

16
344sh (11.6)
417sh (10.6)
447 (13.3)
220 (29.9)
298 (65.1)
343sh (15.6)
418sh (13.9)
449 (17.0)
3.60
2.97
2.77
5.64
4.16
3.62
2.97
2.76
d → π*
d → π*
d → π*
π → π*
π → π*
d → π*
d → π*
d → π*
1.89 (90)
−0.69 (70)
−0.89 (60)
−1.15 (70)
−1.65 (70)
10 (4,4′-(CO2Me)2bpy)
a n = 2 for all except 8, where n = 4.
b Solutions ca. 10−5–10−4 M.
c Solutions ca. 10−3 M in analyte and 0.1 M in [NBun ]PF6 with a 2 mm disc Pt working electrode with
4
a scan rate of 100 mV s−1. Fc/Fc+ internal reference, E1/2 = 0.44 V, ∆Ep = 70–90 mV.
d Epc value for an irreversible process.
e Strongly overlapping waves resolved by using differential pulse voltammetry (potential increment =
2 mV; amplitude = 50 mV; pulse width = 0.01 s).
3.4. Computations
In order to rationalise their electronic structures and optical properties, DFT and TD-
DFT calculations have been performed on [RuII(bpz)3]2+ and the complexes in salts 1, 3, 6, 8
and 9 (denoted 1′, etc.) by using Gaussian 09 [45]. [RuII(bpz)3]2+ has been subjected to such
calculations previously, with the B3LYP [67–69] or B3PW91 [70] functionals. Having
experimented with various methods, we found that the M06 functional gives the best
prediction of UV–Vis absorption spectra in acetonitrile. Also, the Ru–N distances derived
from this level of theory for [RuII(bpz)3]2+ (2.072–2.074 Å) are shorter than those predicted
using other methods and closer to the average X-ray crystallographic value of 2.05 Å [71].
Selected calculated geometric parameters are in the Supplementary Information (Table S8).
Selected TD-DFT-calculated electronic transitions are presented in Table 2, and
simulated UV–Vis spectra are shown together with the experimental ones in Fig. 4. The
corresponding results for [RuII(bpz)3]2+ (can be found in the Supplementary Information
(Table S9 and Fig. S2). Selected MOs for 1′ and 8′ are shown in Figs. 5 and 6, while further
contour surface diagrams for these and the other complexes can be found in the
Supplementary Information (Figs. S3–S8). The tetracationic complex 8′ shows behaviour
significantly different from that of the dicationic complexes, so merits separate discussion.

17
Table 2
Selected data obtained from TD-DFT calculations on the complexes [RuII(bpz)2(L–L)]n+
(M06/Def2-QZVP/Def2-SVP) in acetonitrile (CPCM).a
b
Complex
(L–L)
fos
Major contributions (%)
λ
∆E
(nm) (eV)
455
425
2.73
2.92
0.14
0.08
1′ (bpy)
HOMO−2 → LUMO (69)
−1
HOMO → LUMO+1 (25)
HOMO−2 → LUMO+1 (71)
HOMO−1 → LUMO (10)
−1
HOMO → LUMO+2 (10)
HOMO−1 → LUMO+2 (78)
HOMO → LUMO+5 (14)
383
352
337
459
428
375
356
344
484
3.24
3.52
3.68
2.70
2.90
3.31
3.48
3.60
2.56
0.07
0.04
0.06
0.14
0.08
0.10
0.03
0.05
0.13
−1
HOMO → LUMO+5 (10)
HOMO → LUMO+4 (87)
HOMO−2 → LUMO+3 (74)
−1
HOMO → LUMO+5 (11)
3′(4,4′-tBu2bpy)
6′(4,4′-(NH2)2bpy)
8′(Me2qpy2+)
HOMO → LUMO (72)
−2
HOMO−1 → LUMO+1 (24)
−2
HOMO → LUMO+1 (73)
−1
HOMO → LUMO (10)
HOMO−1 → LUMO+2 (77)
HOMO → LUMO+5 (19)
−1
HOMO → LUMO+5 (12)
HOMO → LUMO+4 (85)
−1
HOMO → LUMO+5 (72)
HOMO → LUMO+4 (10)
HOMO−2 → LUMO (79)
−1
HOMO → LUMO+1 (13)
444
381
2.79
3.25
0.06
0.04
−2
HOMO → LUMO+1 (75)
HOMO−1 → LUMO+2 (65)
HOMO → LUMO+4 (26)
366
364
3.39
3.41
0.10
0.06
−1
HOMO → LUMO+4 (35)
HOMO → LUMO+5 (56)
−1
HOMO → LUMO+3 (34)
−1
HOMO → LUMO+5 (39)
HOMO → LUMO+4 (10)
HOMO−1 → LUMO (43)
475
449
2.61
2.76
0.07
0.19
−1
HOMO → LUMO+1 (53)
HOMO−2 → LUMO+3 (23)
HOMO−1 → LUMO (42)
−1
HOMO → LUMO+1 (24)
HOMO−2 → LUMO+1 (68)
HOMO−1 → LUMO+3 (23)
440
418
393
2.82
2.97
3.16
0.13
0.10
0.13
−2
HOMO → LUMO+3 (27)
HOMO → LUMO+2 (53)
HOMO−1 → LUMO+2 (86)

18
HOMO−1 → LUMO+3 (11)
−2
380
319
3.27
3.89
0.07
0.28
HOMO → LUMO+2 (62)
−2
HOMO → LUMO+3 (21)
HOMO−5 → LUMO (47)
−4
HOMO → LUMO (14)
−2
HOMO → LUMO+5 (16)
HOMO−2 → LUMO (75)
−1
HOMO → LUMO+1 (20)
444
426
345
339
332
331
2.79
2.91
3.59
3.66
3.74
3.74
0.12
0.14
0.03
0.03
0.02
0.02
9′(4,4′-(CF3)2bpy)
−2
HOMO → LUMO+1 (50)
HOMO−1 → LUMO+2 (38)
HOMO → LUMO+4 (71)
HOMO → LUMO+9 (19)
HOMO−1 → LUMO+3 (78)
HOMO → LUMO+5 (12)
−1
HOMO → LUMO+4 (14)
HOMO → LUMO+6 (58)
−1
HOMO → LUMO+5 (56)
HOMO → LUMO+4 (10)
HOMO → LUMO+7 (14)
n = 2 for all except 8′, where n = 4. Only the main transitions within each absorption band
a
above 300 nm are included.
b Oscillator strength.
The calculated spectra match the experimental ones relatively well (Fig. 4), but do not
predict the two maxima observed in the visible region. The major orbital contributions for the
lowest energy transition of significant intensity are the same for each of the dicationic
complexes 1′, 3′, 6′ and 9′; largely HOMO−2 → LUMO with some HOMO−1 → LUMO+1.
Both HOMO−2 and HOMO−1 are primarily metal-based (Fig. 5); the former is derived from
2
2
the dxy and dxz orbitals, while the latter has mostly dx −y character with a variable dyz
contribution. However, HOMO–1 also features a significant contribution from L–L in the
amino derivative 6′. The LUMO and LUMO+1 comprise predominantly equal contributions
from the π*-orbitals of the two bpz ligands. Thus, the lowest energy transitions have largely
RuII → bpz MLCT character, as expected (see above). Also, their calculated energies
reproduce closely the trend observed in the measured Emax values for the lowest energy band
for 1, 3, 6 and 9.

19
Fig. 4. Experimental (green) and TD-DFT calculated (blue dashed) UV–Vis absorption
–
spectra of (a) 1′, (b) 6′, (c) 8′ and (d) 9′ in acetonitrile. The experimental data is for the PF6
salts and plotted against the ε-axes, with the calculated spectra scaled to allow for comparison
of the main absorption bands. Individual calculated vertical transitions (red) are plotted
against the fos-axes
For complex 1′, the next lowest energy transition of significant intensity (at 425 nm)
also has mainly RuII → bpz MLCT character, but some involvement of LUMO+2 corresponds
with MLCT towards the bpy ligand (Fig. 5). LUMO+2 is also predominantly located on L–L
for 3′ and 9′, while for 6′ it is spread approximately evenly across all three ligands. The only
other dicationic complex for which LUMO+2 is involved significantly in the second lowest
energy transition is 9′, due to the stabilising effect of the electron-withdrawing -CF3
substituents.

20
Fig. 5. M06/Def2-QZVP/Def2-SVP-derived contour surface diagrams of the HOMO and
MOs involved in the transitions of 1′ at 455 and 425 nm (isosurface value 0.03 au).
A number of significantly intense transitions are predicted in the region 300–400 nm
for 1′, 3′, 6′ and 9′. These involve various combinations of metal-based donor orbitals with
LUMO+2 or higher energy acceptor orbitals. The LUMO+3, LUMO+4 and LUMO+5 are
almost exclusively spread over the bpz ligands in 1′ and 3′, but in the case of 6′ and 9′,
LUMO+3 is located substantially on L–L, and LUMO+5 also features some contribution from
this ligand in 6′. Therefore, these relatively high energy transitions have a mixture of RuII →
bpz and RuII → L–L MLCT character.
The intense band at ca. 300 nm is in most cases due to bpz-based π → π* transitions
primarily, involving orbitals like HOMO–4 and HOMO–5 in 1′ and 3′, which have very
similar energies. In the case of 9′, π → π* transitions located on L–L have energies similar to
the bpz-based ones, due to the relative electron deficiency of the 4,4′-(CF3)2bpy ligand.

21
For the tetracationic complex 8′, the lowest energy transition (at 475 nm) corresponds
with mostly HOMO−1 → LUMO+1 and HOMO−1 → LUMO. As for the dicationic
complexes, HOMO–1 is still Ru-based, and LUMO+1 is spread over both bpz ligands, but the
LUMO is in this case located largely on the Me2qpy2+ ligand (Fig. 6). Three further main
transitions are predicted above 400 nm, involving HOMO, HOMO–1 and HOMO–2 with
LUMO, LUMO+1, LUMO+2 and LUMO+3. The donor orbitals are all Ru-based, while
LUMO+2 is Me2qpy2+-based and LUMO+3 involves again the bpz ligands. Therefore, the
visible absorption band of this complex is attributable to a combination of RuII → bpz and
RuII → Me2qpy2+ MLCT. The same general conclusion can be drawn regarding the main
transitions predicted for 8′ in the 300–400 nm region.
Fig. 6. M06/Def2-QZVP/Def2-SVP-derived contour surface diagrams of the two lowest
energy Me2qpy2+-based MOs in 8′ (isosurface value 0.03 au).
The low energy shoulder on the first UV band for 8′ is due to a transition at 319 nm,
mostly HOMO–5 → LUMO which corresponds with π → π* within the Me2qpy2+ ligand. As
for the other complexes, a number of primarily ligand-based transitions are also predicted at
higher energies.

22
The calculated relative orbital energies are depicted in the Supplementary Information
(Fig. S9). For the dicationic complexes, the predicted HOMO-LUMO gap follows the same
trend as observed for Emax of the lowest energy MLCT band, while the relative size of this
energy gap seems somewhat underestimated for 8′. This apparent anomaly is explained by the
fact that the lowest energy transition for the latter complex has mixed RuII → bpz and RuII →
Me2qpy2+ character. The trends in the energies for 1′, 3′, 6′, 8′ and 9′ are largely in agreement
with the cyclic voltammetric data. For example, the predicted HOMO energy increases by 0.5
eV on moving from 9′ to 6′, while the corresponding decrease in E1/2[RuIII/II] is 0.49 V (Table
1). Also, the predicted location of the LUMO and LUMO+1 on the bpz ligands is consistent
with the assignment of the first two reduction waves for 1, 3, 6 and 9. In contrast, in 8′ the
LUMO is Me2qpy2+-based and initial reduction involves this ligand rather than bpz.
4. Conclusions
Complexes of cis-{RuII(bpz)2}2+ with a bpy-based coligand L–L have been
–
1
synthesised and characterised as their PF6 and Cl– salts by using H NMR spectroscopy and
other techniques. The UV–Vis spectra of these complex salts show intense intraligand π → π*
absorptions and low energy MLCT bands with two maxima. The lowest energy MLCT band
red-shifts as the electron-donating strength of L–L increases, with an energy difference of ca.
0.3 eV between the complex salts containing 4,4′-(CF3)2bpy and 4,4′-(NH2)2bpy. Cyclic
–
voltammetry with the PF6 salts reveals reversible RuIII/II oxidation waves and multiple,
mostly reversible ligand-based reductions. The trend shown by the MLCT energies correlates
with the measured reduction potentials. TD-DFT calculations at the M06/Def2-QZVP/Def2-
SVP level of theory with an acetonitrile solvent continuum give relatively good agreement
with the experimental UV–Vis spectra. The assignment of the lowest energy visible
absorption band as being due to RuII → bpz MLCT is confirmed, while further such
transitions occur together with RuII → L–L MLCT at higher energies.
Acknowledgments

23
We thank the BBSRC for support in the form of a PhD studentship (MKP). NSS is
supported by an EPSRC Established Career Fellowship (grant EP/J020192/1) and was funded
by a Royal Society Wolfson Merit Award. We are grateful to Dr Octavia A. Blackburn for
advice concerning TD-DFT calculations.
Appendix A. Supplementary data
Supplementary data associated with this article can be found, in the online version, at
http://dx.doi.org/10.1016/j.poly.2015...

24
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