Electrochemistry and spectroscopy of substituted [Ru(phen)3]2+ and [Ru(bpy)3]2+ complexes

Article abstract of DOI:10.1016/j.electacta.2019.07.051

The metal-to-ligand charge transfer property of nitrogen-based ruthenium complexes earns it a central place in dye-sensitized solar cell and photo-catalytic H₂O and CO₂ reduction research and applications. Electronic and spectral tuning are conveniently done by altering substituents and ligands. Cyclic voltammograms and UV–visible spectra of a synthesized series of electronically altered phenanthroline and bipyridyl ruthenium complexes (ΔE°' > 1.4 V for Ru^II-III) were obtained and, amongst others, correlated with DFT computed HOMO energies and ionization potentials. A good linear relationship with R² = 0.97 were found for the combined bipyridyl and phenanthrolinato ruthenium series, thereby providing a convenient computational tool for the theoretical prediction of associated redox potentials. TDDFT closely simulates spectral properties of these complexes, where λ_max varies from 420 to 520 nm. The former wavelength is representative of the dione-phenanthroline and the latter of the dinitro-bipyridyl ruthenium complex.

Full text of DOI:10.1016/j.electacta.2019.07.051

Accepted Manuscript
2+
2+
Electrochemistry and spectroscopy of substituted [Ru(phen) ] and [Ru(bpy) ]
complexes
3
3
Deidré van der Westhuizen, Karel G. von Eschwege, Jeanet Conradie
PII:
S0013-4686(19)31376-3
https://doi.org/10.1016/j.electacta.2019.07.051
EA 34540
DOI:
Reference:
To appear in: Electrochimica Acta
Received Date: 24 June 2019
Revised Date: 12 July 2019
Accepted Date: 12 July 2019
Please cite this article as: Deidré. van der Westhuizen, K.G. von Eschwege, J. Conradie,
2+ 2+
Electrochemistry and spectroscopy of substituted [Ru(phen) ] and [Ru(bpy) ] complexes,
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3
Electrochimica Acta (2019), doi: https://doi.org/10.1016/j.electacta.2019.07.051.
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ACCEPTED MANUSCRIPT
Electrochemistry and spectroscopy of substituted
[Ru(phen)3]2+ and [Ru(bpy)3]2+ complexes
Deidré van der Westhuizen, Karel G. von Eschwege* and Jeanet Conradie*
Department of Chemistry, PO Box 339, University of the Free State, Bloemfontein, 9300, South Africa.
*Contact author details: Tel: +27-51-4012923/2194, Fax: +27-51-4017295
e-mail: vEschwKG@ufs.ac.za, ConradJ@ufs.ac.za
SYNOPSIS TOC
SYNOPSIS TEXT
Cyclic voltammograms and UV-visible spectral data of series of [RuII(bipyridyl)3](BF4)2 and
[RuII(phenanthroline)3](BF4)2 complexes.
HIGHLIGHTS
Complexation of ruthenium with phenanthroline and bipyridine derivatives.
Cyclic voltammetry of electronically altered Ru complex series.
DFT studies of electronically altered Ru complex series.
Correlations between experimental and theoretically computed redox data.
Correlations between experimental and theoretically computed spectral data.
Keywords: synthesis, electrochemistry, spectroscopy, catalyst, computational
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Abstract
The metal-to-ligand charge transfer property of nitrogen-based ruthenium complexes earns it a
central place in dye-sensitized solar cell and photo-catalytic H2O and CO2 reduction research and
applications. Electronic and spectral tuning are conveniently done by altering substituents and
ligands. Cyclic voltammograms and UV-visible spectra of a synthesized series of electronically
altered phenanthroline and bipyridyl ruthenium complexes (∆E°' > 1.4 V for RuII-III) were obtained
and, amongst others, correlated with DFT computed HOMO energies and ionization potentials. A
good linear relationship with R2 = 0.97 were found for the combined bipyridyl and phenanthrolinato
ruthenium series, thereby providing a convenient computational tool for the theoretical prediction of
associated redox potentials. TDDFT closely simulates spectral properties of these complexes,
where λmax varies from 420 to 520 nm. The former wavelength is representative of the dione-
phenanthroline and the latter of the dinitro-bipyridyl ruthenium complex.
1. Introduction
The element ruthenium has potential for wide-ranging practical applications, one of which lies
in the modern field of biomedicinal chemistry, as comprehensively described by Mishra in their
book, Ruthenium Chemistry [1]. Traditionally however, as far back as 1966 charge transfer
properties of ruthenium bipyridyl complexes have been of greatest interest amongst researchers [2].
Focus on this class of compound culminated in the Millenium Technology Prize being awarded to
Michael Grätzel in 2000, for his application of it in new dye-sensitized solar cell (DSC) technology
[3,4]. Hagfeldt et al wrote an excellent review on this topic [5], while Zhang et al summarized
recent progress associated with functionalization of interfaces to improve DSC performance [6].
The electron that is localized in the highest occupied molecular orbital (HOMO) of the central
ruthenium atom is excited by solar photons and transferred to the lowest unoccupied molecular
orbital (LUMO) that is spread over surrounding ligands, from where it may be put to practical use
via an external circuit. One such practical application may be towards the photo-catalytic reduction
of H2O to produce hydrogen fuel [7,8], or CO production from CO2, where carbon monoxide serves
as major reagent in Fischer-Tropsch liquid fuel production plants [9].
As to optimally overlap with the solar emission spectrum, it is evident that the absorption
spectra of such catalytic dyes should be chemically tunable, and equally important, have readily
accessible redox potentials. Towards this end an extensive variety of electron-withdrawing or
donating ligands and/or substituents may be employed. In general, electron withdrawing groups
decrease both HOMO and LUMO energies of molecules.
In an effort to lower costs, and speed up research in this field, we embarked on a systematic
research initiative to find the simplest yet accurate way to predetermine redox potentials by
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theoretical means. DFT studies have shown that in most cases HOMO and LUMO energies,
conveniently obtainable from computational output files, linearly correlate almost 99% with
experimental oxidation and reduction potentials respectively; of small molecules [10], bidentate
ligands [11,12,13] and various metal complexes [14,15].
During the investigation we
systematically increased both the size and structural diversity of series of molecules, while
obtaining electrochemical data under similar and thus comparable conditions.
Regardless extensive past research in ruthenium-polypyridyl charge transfer chemistry,
convenient synthesis procedures, accompanied by detailed charaterization data, are not readily
found in literature. An electrochemistry study published more than 40 years ago compared the first
oxidation potentials (vs SCE) of four ruthenium complexes, respectively containing the ligands:
terpyridine (1.28 V), bipyridine (1.35 V), ortho-phenanthroline (1.40 V) and 2,4,6-tripyridyl-s-
triazine (1.52 V) [16].
In this report we now present synthesis, characterization, electrochemical, spectral and
computational DFT data of a series of substituted [Ru(bpy)3]2+ and [Ru(phen)3]2+ complexes.
2. Experimental
2.1. General
Solvents supplied by Merck (AR grade) and reagent grade Sigma-Aldrich syntheses chemicals
were used without further purification. Melting points were determined with an Olympus BX51
system microscope, assembled on top of a Linkam THMS600 stage and connected to a Linkam
TMS94 temperature programmer. UV-visible spectra were recorded on a Varian Cary 50
1
spectrophotometer and H NMR spectra on a 300 MHz Bruker Fourier NMR spectrometer
operating at 25 °C. FTIR measurements (solid samples) were done with a Bruker Tensor 27 IR
spectrometer and Pike MIRacle ATR, equiped with OPUS software (Version 1.1). Cyclic
voltammetric (CV) measurements were recorded on a BAS100B Electrochemical Analyzer linked
to a personal computer, utilizing the BAS100W Version 2.3 software.
2.2. Syntheses.
Only the diamino- and dinitro-bipyridine ligands could not be purchased and was therefore
synthesized, by methods adopted from procedures found in published literature, see Scheme 1.
2.2.1. 2,2’-bipyridine-N,N’-dioxide [17]
2,2’-Bipyridine (10.00 g, 64 mmol), glacial acetic acid (75 mL) and hydrogen peroxide (30 %, 13
mL) were heated together at 75 °C for 3 hours. Hydrogen peroxide (30 %, 9 mL) was added to the
solution and stirred at 75 °C for 19 hours. The solution was cooled to room temperature and acetone
was added to form a white precipitate, which was recrystallized from warm water and an excess of
acetone, yielding fine white crystalline 2,2’-bipyridine-N,N’-dioxide (11.7 g, 97 %).
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2.2.2. 4,4’-dinitro-2,2’-bipyridine-N,N’-dioxide [18]
2,2’-Bipyridine-N,N’-dioxide (7.00 g, 37 mmol) was added to concentrated sulphuric acid (34 mL).
The mixture was cooled in ice and fuming nitric acid (12 mL) was added slowly. The solution was
refluxed (95 – 100 °C) for 20 hours before it was cooled to room temperature. The acidic solution
was poured on ice (–40 °C / which was prepared by adding liquid nitrogen ( 150 mL) to crushed
ice), turning green with liberation of N2O4 fumes. During consequent stirring a bright yellow
precipitate formed. The solution was filtered and the 4,4’-dinitro-2,2’-bipyridine-N,N’-dioxide
precipitate washed with water (3 x 83 mL) and air dried (5.6 g, 54 %).
2.2.3. 4,4’-diamino-2,2’-bipyridine [19]
A mixture of 4,4’-dinitro-2,2’-bipyridine-N,N’-dioxide (1.50 g, 5.4 mmol) and Pd/C (10 %, 1.50 g)
in ethanol (180 mL) was purged with N2 gas. The suspension was heated and refluxed under
nitrogen and after the complex was completely dissolved, hydrazine hydrate (13 mL, 0.4 mol) in
ethanol (45 mL) was added dropwise over a period of 1 hour. The resulting solution was refluxed
for 15 hours, filtered through a celite bed while still hot, and washed with hot ethanol (4 x 45 mL).
After removal of the solvent water (120 mL) was poured on the yellow precipitate and left
overnight at 2 °C. While the yellow precipitate dissolved, a white solid appeared, which was
filtered, washed with cold water and dried at 50 °C to yield 0.57 g (57 %) of the desired 4,4’-
1
diamino-2,2’-bipyridine ligand. M.p. >250 °C. H NMR (300 MHz, DMSO-d6, 25 °C): δ 8.03 (d,
2H), δ 7.54 (d, 2H), δ 6.45 (dd, 2H), δ 6.02 (s, 4H).
2.2.4. 4,4’-dinitro-2,2’bipyridine [20]
4,4’-Dinitro-2,2’-bipyridine-N,N’-dioxide (1.50 g; 5.39 mmol) was refluxed in PCl3 (12.5 mL,
0.145 mol) for 21 hours. The reaction mixture was poured onto ice water (100 mL). The solution
pH was adjusted to 13 with addition of concentrated NaOH (1 M) and extracted exhaustively with
CHCl3. The CHCl3 layers were combined, dried over MgSO4 and evaporated to yield 0.935 g (70
1
%) of the desired yellow dinitro ligand. M.p. > 250 °C. H NMR (300 MHz, DMSO-d6, 25 °C): δ
9.15 (dd, 2H, 4.8, 0.54 Hz), δ 8.96 (dd, 1.74, 2H, 0.48 Hz), δ 8.30 (dd, 2H, 3.09, 2.25 Hz).
2.3. RuII complex derivatives
[Ru(bpy)3]2+, [Ru(phen)3]2+ and its substituted derivatives were complexed by modification of
published methods [21,22,23,24].
General Procedure: Dried RuCl3 (0.20 g, 0.96 mmol) was dissolved in dipropylene glycol (10 mL)
and deionized water (1 mL). The solution was refluxed until the metal salt was dissolved, obtaining
a dark green solution. Bipyridine (0.469 g; 3.0 mmol) was added, resulting in a brown solution.
Ascorbic acid (0.177 g, 1.0 mmol) was then added and the solution refluxed for 20 minutes at 250
°C, the brown colour changing to red. After cooling, the solution was diluted to 40 mL and the pH
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adjusted to 8 by addition of a few drops of NaOH solution (2.5 M). NaBF4 (4.0 g, 36 mmol) was
added and the solution cooled on ice. After vacuum filtration, washing with cold water, and drying,
0.329 g [Ru(bpy)3](BF4)2 product was obtained.
2.3.1. Tris(4,4’-dinitro-2,2’-bipyridine)ruthenium bis(tetrafluoroborate), 1
Yield 70.12 %. M.p. 200 °C. UV-vis: λmax 630 nm, εmax 7888 mol-1 dm3 cm-1 (DMSO).
1H NMR (300 MHz, DMSO-d6, 25 °C): δ 9.15 (d, 3 x 2H, 5.90 Hz), δ 8.97 (d, 3 x 2H, 1.81 Hz), δ
8.30 (dd, 3 x 2H, 1.89, 3.22 Hz).
MS Calcd. ([M]2+], positive mode): m/z 839.6. Found: m/z 840.3.
2.3.2. Tris(5,6-dione-1,10-phenanthroline)ruthenium bis(hexafluorophosphate), 2
(Complex supplied by Sigma-Aldrich.)
M.p. >200 °C. UV-vis: λmax 420 nm, εmax 22149 mol-1 dm3 cm-1 (DMSO).
1H NMR (300 MHz, DMSO-d6, 25 °C): δ 8.79 (d, 3 x 2H), δ 7.96 (d, 3 x 2H), δ 7.26 (dd, 3 x 2H).
2.3.3. Tris(5-nitro-1,10-phenanthroline)ruthenium bis(tetrafluoroborate), 3
Yield 83 %. M.p. 135 °C. UV-vis: λmax 460 nm, εmax 12900 mol-1 dm3 cm-1 (DMSO).
1H NMR (300 MHz, DMSO-d6, 25 °C): δ 9.25 (dd, 3 x 1H, 1.80 Hz, 2.66 Hz), δ 9.21 (dd, 3 x 1H,
1.63 Hz, 2.66 Hz), δ 8.97 (s, 3 x 1H), δ 8.87(dd, 3 x 1H, 1.59, 7.03 Hz), δ 8.74 (dd, 3 x 1H, 1.77,
6.46 Hz), δ 7.87 (m, 3 x 2H).
2.3.4. Tris(4,7-dichloro-1,10-phenanthroline)ruthenium bis(tetrafluoroborate), 4
Yield 54 %. M.p. >200 °C. UV-vis: λmax 460 nm, εmax 16634 mol-1 dm3 cm-1 (DMSO).
1H NMR (300 MHz, DMSO-d6, 25 °C): δ 8.62 (s, 3 x 2H), δ 8.14 (d, 3 x 2H, 5.61 Hz), δ 7.95 (d, 3
x 2H, 7.02 Hz).
2.3.5. Tris(5-chloro-1,10-phenanthroline)ruthenium bis(tetrafluoroborate), 5
Yield 85 %. M.p. >200 °C. UV-vis: λmax 450 nm, εmax 14750 mol-1 dm3 cm-1 (DMSO).
1H NMR (300 MHz, DMSO-d6, 25 °C): δ 8.87 (dd, 3 x 1H, 1.07, 7.38 Hz), δ 8.72 (dd, 3 x 2H,
1.39, 3.47 Hz), δ 8.14 (m, 3 x 2H), δ 7.82 (m, 3 x 2H).
2.3.6. Tris(1,10-phenanthroline)ruthenium bis(tetrafluoroborate), 6
Yield 55 %. M.p. >200 °C. UV-vis: λmax 450 nm, εmax 29220 mol-1 dm3 cm-1 (DMSO).
1H NMR (300 MHz, DMSO-d6, 25 °C): δ 8.78 (dd, 3 x 2H, 0.68, 7.50 Hz), δ 8.39 (s, 3 x 2H), δ
8.08 (dd, 3 x 2H, 0.68, 4.45 Hz), δ 7.77 (dd, 3 x 2H, 3.01, 5.34 Hz).
2.3.7. Tris(2,2’-bipyridine)ruthenium bis(tetrafluoroborate), 7
Yield 46 %. M.p. >200 °C. UV-vis: λmax 455 nm, εmax 16390 mol-1 dm3 cm-1 (DMSO).
1H NMR (300 MHz, DMSO-d6, 25 °C): δ 8.84 (d, 3 x 2H, 8.11 Hz), δ 8.17 (m, 3 x 2H), δ 7.74 (d, 3
x 2H, 0.62, 4.93 Hz), δ 7.53 (m, 3 x 2H).
2.3.8. Tris(4-methyl-1,10-phenanthroline)ruthenium bis(tetrafluoroborate), 8
Yield 19 %. M.p. 185 °C. UV-vis: λmax 450 nm, εmax 23650 mol-1 dm3 cm-1 (DMSO).
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1H NMR (300 MHz, DMSO-d6, 25 °C): δ 8.77 (m, 3 x 1H), δ 8.44 (dd, 3 x 2H, 9.08, 12.62 Hz), δ
8.05 (3, 3 x 1H), δ 7.94 (m, 3 x 1H), δ 7.74 (m, 3 x 1H), δ 7.60 (m, 3 x 1H), δ 2.89 (s, 3 x 3H).
2.3.9. Tris(4,4’-dimethyl-2,2’-bipyridine)ruthenium bis(tetrafluoroborate), 9
Yield 45 %. M.p. >200 °C. UV-vis: λmax 465 nm, εmax 8850 mol-1 dm3 cm-1 (DMSO).
1H NMR (300 MHz, DMSO-d6, 25 °C): δ 8.68 (s, 3 x 2H), δ 7.53 (d, 3 x 2H, 6.16 Hz), δ 7.34 (d, 3
x 2H, 5.67 Hz), δ 1.02 (s, 3 x 6H).
2.3.10. Tris(4,4’-di-tert-butyl-2,2’-bipyridine)ruthenium bis(tetrafluoroborate), 10
Yield 63 %. M.p. 110 ºC. UV-vis: λmax 460 nm, εmax 11530 mol-1 dm3 cm-1 (DMSO).
1H NMR (300 MHz, DMSO-d6, 25 °C): δ 8.84 (s, 3 x 2H), δ 8.60(dd, 3 x 2H, 1.63, 4.37 Hz), δ 7.55
(d, 3 x 2H, 6.17 Hz), δ 1.39 (s, 3 x 18H).
2.3.11. Tris(3,4,7,8-tetramethyl-1,10-phenanthroline)ruthenium bis(tetrafluoroborate), 11
Yield 50 %. M.p. >200 °C. UV-vis: λmax 445 nm, εmax 14810 mol-1 dm3 cm-1 (DMSO).
1H NMR (300 MHz, DMSO-d6, 25 °C): δ 8.47 (s, 3 x 2H), δ 7.67 (s, 3 x 2H), δ 2.77 (s, 3 x 2H), δ
2.21 (s, 3 x 2H).
2.3.12. Tris(4,4’-dimethoxy-2,2’-bipyridine)ruthenium bis(tetrafluoroborate), 12
Yield 68 %. M.p. 179 °C. UV-vis: λmax 480 nm, εmax 13640 mol-1 dm3 cm-1 (DMSO).
1H NMR (300 MHz, DMSO-d6, 25°C): δ 8.44 (d, 3 x 2H, 2.94 Hz), δ 7.52 (d, 3 x 2H, 6.85 Hz), δ
7.14 (dd, 3 x 2H, 2.45, 3.67 Hz), δ 3.99 (s, 3 x 6H).
2.3.13. Tris(4,7-dimethoxy-1,10-phenanthroline)ruthenium bis(tetrafluoroborate), 13
Yield 56 %. M.p. > 200 °C. UV-vis: λmax 450 nm, εmax 21544 mol-1 dm3 cm-1 (DMSO).
1H NMR (300 MHz, DMSO-d6, 25°C): δ 8.35 (s, 3 x 2H), δ 7.93 (d, 3 x 2H, 6.46 Hz), δ 7.25 (d, 3
x 2H, 6.39 Hz), δ 4.10 (s, 3 x 6H).
2.3.14. Tris(4,4’-diamino-2,2’-bipyridine)ruthenium bis(tetrafluoroborate), 14
Yield 42 %. M.p. >200 °C. UV-vis: λmax 460 nm, εmax 9581 mol-1 dm3 cm-1 (DMSO).
1H NMR (300 MHz, DMSO-d6, 25 °C): δ 7.28 (s, 3 x 2H), δ 7.09 (d, 3 x 2H, 6.42 Hz), δ 6.72 (s, 3
x 2H), δ 6.54 (dd, 3 x 4H, 2.13, 4.27 Hz). MS Calcd. ([M]2+], positive mode): m/z 659.2. Found:
m/z 659.3.
2.4. Cyclic Voltammetry
Cyclic voltammetric measurements were done on a BAS100B Electrochemical Analyzer linked
to a personal computer, utilizing BAS100W Version 2.3 software. Measurements were done at 293
K. Successive experiments under the same experimental conditions showed that all formal
reduction and oxidation potentials were reproducible within 0.005 V. Cyclic voltammetric
measurements were performed on 0.005 mol dm-3 solutions of the complex, dissolved in CH3CN,
containing 0.100 mol dm-3 tetrabutylammonium hexafluorophosphate (TBAPF6, [NBu4][PF6]) as
supporting electrolyte. Measurements were conducted under a blanket of purified Argon. A three-
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electrode cell consisting of a Pt auxiliary electrode, a glassy carbon (surface area 3.14 x 10-6 m2)
working electrode and a Pt-wire pseudo reference electrode were used. The working electrode was
polished on a Buhler polishing mat; first with 1 micron and lastly with ¼ micron diamond paste.
Scan rates were between 0.050 and 5.000 V.s-1. All experimental potentials were referenced against
the FcH/FcH+ couple (IUPAC) [25]. Under our experimental conditions, ferrocene (FcH) exhibited
a peak separation of ∆Ep = Epa – Epc = 0.069 V and ipc/ipa = 1.00; where Epa (or Epc) is the anodic (or
cathodic) peak potential, and ipa (or ipc) is the anodic (or cathodic) peak current. E°' (FcH/FcH+) =
0.66(5) V vs SHE in [n(Bu4)N][PF6]/CH3CN [26]
2.5. DFT calculations
Density functional theory (DFT) calculations were performed with the B3LYP (and UB3LYP
for spin unrestricted calculations) functional, as implemented in the Gaussian 09 package [27]. The
triple-ζ 6-311G(d,p) basis set was used for lighter atoms (C, H, N, O), the Stuttgart/Dresden (SDD)
pseudopotential was used to describe the Ru metal electronic core, while the metal valence
electrons were described using the def2-TZVPP basis set [28]. Geometry relaxed (adiabatic)
energies of RuII complexes and corresponding RuI (reduced) and RuIII (oxidized) systems were
calculated to determine electron affinity (EA), ionization potential (IP), electronegativity (χ) and
global electrophilicity index (ω), according to the following formulae:
EA(compound) = E(reduced compound) – E(compound)
IP(compound) = E(oxidized compound) – E(compound)
χ = (IP + EA)/2 and ω = ꢀ2/(2 η),
where ꢀ = −(IP + EA)/2 and η = IP – EA [29,30,31,32].
3. Results and Discussion
For the purpose of expanding the electronic properties of the complex series here under
investigation, a reaction sequence was employed from which both the strongly electron
withdrawing dinitro- and strongly electron donating diamino-bipyridines were synthesized
[17,18,19] , see Scheme 1. At first, the nitrogens of unsubstituted bipyridine were oxidized,
resulting in positive charges on the aromatic rings, which enabled consequent nitration at the ring
para-positions. Catalytic reduction of the nitro groups with palladium and hydrazine converted the
electron-withdrawing species to the electron-donating 4,4’-diamino-2,2’-bipyridine final product.
Synthesis of 4,4’-di(trifluoromethyl)-2,2’-bipyridine was also attempted, but extremely poor yields,
as reported in literature, prevented inclusion in the present series [33].
The dinitro and diamino derivatives, together with four commercially obtained bipyridines and
eight 1,10-phenanthroline variants were then reacted with RuCl3. The high boiling point solvent,
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dipropylene glycol, acts as a weak reducing agent, however, to decrease reaction times for the
reduction of RuIII to RuII in the complex series, strongly reducing ascorbic acid was also added.
These tris-coordinated RuII complexes were readily synthesized and isolated, with yields varying
from 19 to 85 %.
O2N
NO2
Glacial Acetic Acid
H2O2
H2SO4
HNO3
(fum
ing)
70 ꢀ 80 oC
22 h, 97 %
95ꢀ100 oC
20 h, 54 %
N
N
N
N
N
N
O
O
O
O
H2N
NH2
H2N
N
NO2
H2N
O2N
NH2
Dipropylene Glycol
Ascorbic Acid
N
N
RuII
N
RuCl3
+
250 oC
15 min, 42 %
N
N
N
N
N
N
H2N
NH2
NH2
Scheme 1. Synthesis of 4,4`-dinitro-2,2`-bipyridine and the 4,4`-diamino-2,2`-bipyridine ligand and its RuII complex.
As for its electrochemistry, the oxidation of the RuII complex involves removal of an electron
from the highest occupied molecular orbital of the complex, while reduction involves the addition
of an electron to the lowest unoccupied molecular orbital of the complex, see Table 3. The
character of the HOMO and LUMO can therefore be used to identify the oxidation and reduction
centre of the complex. RuII is diamagnetic with spin = 0. Addition or removal of an electron to the
RuII complex will thus result in a paramagnetic complex with spin S = ½ (one unpaired electron).
The cylic voltammograms of unsubstituted ruthenium(II) bipyridyl and phenanthroline
complexes are similar to corresponding iron(II) complex CV’s [15], except for the RuII-III formal
reduction potentials which are 0.2 V higher than corresponding FeII-III potentials. As opposed to the
three low-voltage reversible reductions of the bipyridine complexes, the corresponding two
phenanthroline complex reductions are irreversible, see Fig.1. Interestingly, the bipyridine complex
reductions of both ruthenium(II) and iron(II) occur at exactly the same point, i.e. with first reduction
peak potentials in both cases at precisely –1.78 V. The same is true for both phenanthroline metal
complexes. As for these ligand based reductions, the electronic effect of varying metals within the
same transition metal group therefore appears to be no different. Present studies by us, involving
also the third element in this group, namely osmium, will elaborate on this trend shortly. Moving
sideways to cobalt, however reveals a significant difference; the CoII-III formal reduction potential
is dramatically lower than that of RuII-III in its bipyridine complexes, decreasing by almost 0.9 V to
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ca 0 V [34,35]. The negative shift of the first low-voltage ligand reduction of cobalt is less
dramatic, namely ca 0.22 V, to –2.0 V.
Fig. 1. Full width [Ru(bpy)3]2+ and [Ru(phen)3]2+ cyclic voltammograms illustrating the RuII-III oxidation, and the
reductions of the coordinated ligand. CV’s were obtained (vs FcH/FcH+) in 0.005 mol dm-3 solutions, at a scan rate of
0.100 V s-1 in 0.200 mol dm-3 TBAPF6/CH3CN, on a glassy carbon working electrode at 20 °C. Scans were initiated in
the positive direction.
The stacked cyclic voltammograms in Fig. 2 illustrate the electronic effect on RuII-III formal
reduction potentials under similar conditions, which were brought about by the systematic changing
of substituents. The cyclic voltammograms of six bipyridyl and eight phenanthroline ruthenium
complexes referenced against the redox potential of ferrocene and obtained at 0.100 V s-1 scan rate
are presented, with corresponding data summarized in Table 1. (CV’s at various scan rates are
available in the associated Data in Brief publication.)
A large over-all range of more than 1.4 V is represented by this series. As expected, the
dinitro-bipyridyl complex 1 was found most resistant against oxidation (E°' = 1.448 V), while the
bpy-(NH2)2 complex 14 oxidizes most readily (E°' = 0.006 V). The two amine substituents have a
dramatic effect, with E°' dropping substantially by 0.507 V from the second lowest value of 0.513
V, namely that of bpy-(OMe)2 complex 13. Unsubstituted 1,10-phenanthroline and bipyridine are
electronically very similar with respect to their ruthenium(II-III) transitions; both E°' values lie
within 0.014 V from each other. Typically all the aliphatic, alkoxy and amine substituents – as
electron-donating groups – cause negative shifts in redox potentials, while the electron-withdrawing
Cl and NO2 groups resist oxidation.
The RuII-III redox potential of the mononitro-substituted bipyridyl complex a was obtained from
a set of data reported by Weiner et al [36], see Table 1. For comparative reasons the reported
potential in CH3CN vs SCE had to be converted to a potential vs FeH/FcH+, where E(FcH/FcH+) =
0.42 V vs SCE (in 0.2 M[NBu4]/CH3CN has been used [37]. The reported E°' = 1.29 V vs SCE or
0.87 V vs FcH/FcH+ of the unsubstituted [Ru(bpy)3]2+ complex of this series corresponds to our
value of 0.883 V (complex 7). Addition of a single NO2 substituent to bipyridine resulted in a
positive shift of 0.237 V (complex a, reported 1.54 V vs SCE or 1.120 V vs FcH/FcH+), and a
further 0.328 V in the dinitro-substituted complex 1 of this study (1.448 V vs FcH/FcH+).
9

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Fig. 2. Cyclic voltammograms (vs FcH/FcH+) of 0.005 mol dm-3 (or saturated) solutions of complexes 1 – 14, at 0.100
V s-1 scan rates, in 0.200 mol dm-3 TBAPF6/CH3CN, on a glassy carbon working electrode at 21 °C. Scans were
initiated in the positive direction.
Ruthenium catalysts and dyes – of which the present series is part of – dominate wide-spread
and extensive electro- and photocatalytic research in previously mentioned sections of the global
sustainable energy drive. It speaks for itself that computational simulations at the onset of research
projects would save significantly in time and cost, with the aim of thereby first establishing the
most suitable molecular configurations for specific applications. For this reason, reporting
convenient and best theoretical redox potential and spectral simulations of the electronically altered
dye series is to be beneficial.
At the basis of ruthenium’s relevance to the above applications is its directional photo-induced
metal-to-ligand charge transfer property which is characteristic in its polypyridyl complexes.
Orbital renderings, as presented in Table 2, clearly illustrate the HOMO orbitals in both its
bipyridyl and phenanthroline complexes being fully metal-centered. During photochemical
excitation the electron preciding in the HOMO gets excited to the LUMO, which is spread out
homogenously over all three unsubstituted ligands. Substituent groups however, occasionally alter
the LUMO electron density distribution: this is seen only in the tertiary-butyl derivative of the
present series, where the electron density is located on the bipyridyl rings of only two of the three
ligands.
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Table 1. Experimental formal reduction potentials (E°') arranged in decreasing order, and corresponding theoretical E°'
calculated via linear correlation of experimental E°' with DFT computed HOMO energies (from Fig.3), of the RuII
complex series, [Ru(X)3]2+
.
Nr RuII Complex Ligands, X
E°'
E°'
(exp.)
(DFT)
1
2
a
3
4
5
bpy-(NO2)2
phen-dione
bpy-4NO2
phen-5NO2
phen-(Cl)2
phen-5Cl
1.448
1.240
1.120a
1.085
1.031
0.990
1.427
1.211
1.200
1.098
0.969
0.963
6
7
8
9
phen-H
bpy-H
0.897
0.883
0.807
0.730
0.898
0.945
0.811
0.762
phen-4Me
bpy-(Me)2
bpy-(tBu)2
10
11
0.727
0.671
0.547
0.513
0.006
0.649
0.632
0.597
0.541
outlier
phen-
3,4,7,8Me
12 bpy-(OMe)2
phen-
13
(OMe)2
14 bpy-(NH2)2
11

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Table 2. DFT-calculated HOMO and LUMO plots of unsubstituted [Ru(phen)3]2+ and [Ru(bpy)3]2+ complexes and the
tertiary-butyl bipyridyl derivative. A contour of 0.05 e/Å3 was used for the orbital plots. Color code of atoms (online
version): Ru (red), C (black), N (purple), H (light blue).
complex
HOMO
LUMO
[Ru(phen)3]2+
[Ru(bpy)3]2+
[Ru(bpy-(tBu)2)3]2+
The relative ease with which molecules are excited by either photochemical or electrochemical
means is reflected in its ionization potential (IP), i.e. the energy required to remove the valence
electron. IP is calculated from the difference in computed energies of the neutral compound and its
singly oxidized form. Electron affinity, on the other hand, reflects the reduction process. Figure 3
shows correlations between experimentally measured formal reduction potentials of the entire RuII
complex series and DFT-computed (a) HOMO energies, (b) ionization potentials and (c)
electronegativities, χ, see corresponding data in Table 3. All three these computational descriptors
linearly correlate with measured CV data. The only complex (14) that did not follow the present
trend closely, was that of the strongly electron-donating diamine bipyridyl, and was therefore
omitted from the linear trendlines shown in Fig. 3.
It should be noted that simultaneous correlation of both the bipyridine and phenanthroline
complexes via a single linear equation as seen here for ruthenium, is not the norm. Iron bipyridine
12

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and phenanthroline complexes required separate equations with significantly different y-axis
intercepts [15], as was the case for several other studies referred to earlier.
As previously indicated regarding the other molecular species of our former investigations
[10,15], EHOMO once again also here gave the highest R2 value of 0.97, followed by 0.96 and 0.95
for IP and χ respectively. As opposed to other computed descriptors EHOMO is conveniently
obtained directly from initial geometry optimization output files. This best correlation therefore
renders additional calculations, as required for all other descriptors, redundant.
Fig. 3. Experimental oxidation potentials, E°' (V vs FcH/FcH+), of the ruthenium polypyridine series 1-13 & a,
correlated with computed (a) EHOMO, (b) ionization potential (IP), and (c) electronegativities (χ). Complex 14 (E°' =
0.006 V) was treated as an outlier.
Applying the linear equation,
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y = –0.3306x – 2.7437,
where y = E°' and x = EHOMO (see Fig. 3 top),
to computed data of related but untested ruthenium complexes is therefore anticipated to provide a
close estimate of expected experimental redox potentials. The redox potential for the present series
of ruthenium-polypyridine complexes are in this manner computed accurately to within 0.001 to
0.080 V of experimentally measured formal reduction potentials, see Table 1. Computing the redox
potential of a dye molecule that is not synthesized would for instance require the calculation of E°'
of the RuII(5NH2-phen)3 complex. With the computed EHOMO = –10.491 eV, E°' is predicted to be
0.725 V – a value that is comparable to E°' of the tertiary-butyl bipyridyl ruthenium complex 10
(see
Table 1).
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Table 3
Electrochemical data (V vs. FcH/FcH+), calculated HOMO and LUMO energy (EHOMO and ELUMO), calculated and experimental HOMO-LUMO gaps (latter obtained
from redox potentials), electron affinity (EA), ionization potential (IP), hardness (η), electrochemical potential (µ), electrophilicity index (ω) and electronegativity
(χ) of complexes 1 – 14 & a.
ω = (ꢁ)2 / (2η)
(eV)
E°
’ Ru(IIꢀIII)
(V)
E°
’ ligand
(V)
EHOMO
(eV)
ELUMO
(eV)
EHOMOꢀLUMO
(eV) DFT
EHOMOꢀLUMO
(eV) exp.
EA
(eV)
IP
(eV)
η = (IP ꢀ EA)
(eV)
ꢁ = ꢀ½ (IP +
EA)
χ = ½ (IP + EA)
(eV)
1
2
a
3
4
5
6
7
8
9
Ru(bpy-(NO2)2)3
Ru(phen-dione)3
Ru(bpy-4NO2)3
Ru(phen-5NO2)3
Ru(phen-(Cl)2)3
Ru(phen-5Cl)3
Ru(phen)3
1.455
1.240
1.120
1.085
1.031
0.990
0.897
0.883
0.807
0.730
0.727
0.671
0.547
0.513
0.006
-
-
-
-
-
-12.617
-11.962
-11.930
-11.621
-11.229
-11.211
-11.014
-11.158
-10.752
-10.603
-10.263
-10.211
-10.104
-9.935
-9.326
-9.082
-8.651
-8.194
-7.820
-7.724
-7.520
-7.725
-7.264
-7.254
-6.892
-6.696
-6.868
-6.498
-6.386
3.291
2.880
3.279
3.427
3.409
3.487
3.494
3.433
3.488
3.349
3.371
3.515
3.236
3.437
3.090
-
8.38
8.08
7.66
7.27
6.88
6.72
6.49
6.66
6.24
6.27
5.99
5.76
5.93
5.85
5.46
13.40
12.81
12.76
12.45
11.98
12.03
11.88
12.07
11.57
11.43
10.99
10.97
10.88
10.66
10.29
5.021
4.736
5.100
5.177
5.102
5.309
5.391
5.411
5.329
5.159
5.008
5.211
4.950
4.804
4.826
-10.887
-10.443
-10.209
-9.858
-9.427
-9.378
-9.181
-9.363
-8.909
-8.846
-8.491
-8.364
-8.401
-8.254
-7.876
11.80
11.51
10.22
9.39
8.71
8.28
7.82
8.10
7.45
7.58
7.20
6.71
7.13
7.09
6.43
10.89
10.44
10.21
9.86
9.43
9.38
9.18
9.36
8.91
8.85
8.49
8.36
8.40
8.25
7.88
-
-
2.618
-
-1.628
-1.783
-1.742
-1.868
-1.850
-1.839
-2.042
-1.989
-
-
2.680
2.625
2.675
2.580
2.566
2.713
2.536
-
Ru(bpy)3
Ru(phen-4Me)3
Ru(bpy-(Me)2)3
10 Ru(bpy-(tBu)2)3
11 Ru(phen-Me4)3
12 Ru(bpy-(OMe)2)3
13 Ru(phen-(OMe)2)3
14 Ru(bpy-(NH2)2)3
-
-9.476
-
15

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Fig. 4. Selected B3LYP TDDFT calculated oscillators (red) and experimental UV-vis spectra (blue) for:
TOP: (a) Ru(bpy)3]2+, (b) [Ru(bpy-(OMe)2)3]2+, and BOTTOM: (c) [Ru(phen)3]2+ and (d) [Ru(phen-5Cl)3]2+.
Gas phase time-dependant density functional theory (TDDFT) calculations in general gave
reasonably good simulations of the entire series here under investigation, see Fig.4 and Supporting
Information. The broad absorption band of the phenanthroline complex that peaks around 450 nm
is seen to be attributable to the approximate ten lowest energy transitions lying in close proximity,
which involves transitions from mainly the top three HOMOs to the top six LUMOs. The
calculated strongest oscillators of eg. [Ru(phen)3]2+ lies at 394 and 422 nm, with corresponding
experimental absorption maxima about 30 nm red-shifted, to 425 and 450 nm respectively.
Electronic transitions of the bipyridyl complexes occur at energies that correspond closely to that of
the phenanthroline complexes, with DFT HOMO-LUMO gap energies mostly lying just below -3.5
eV, i.e. ca 450 nm (see Table 3).
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4. Conclusions
Electronically altered ruthenium phenanthroline and bipyridyl complexes may all conveniently
be synthesized in the same way, and at relatively high yields. Cyclic voltammetry studies, on going
from the complex with the strongly electron-donating bpy-(NH2)2 ligand to the strongly electron-
withdrawing bpy-(NO2)2 derivative, show a positive shift of more than 1.4 V for the RuII-III
reversible redox wave.
Calculated EHOMO values with experimental E°' provides a linear close correlation with which
97 % accurate predictions of both substituted bipyridyl- and phenanthrolinato-ruthenium oxidation
potentials may be made. TDDFT calculations of these photo-active dyes can be simulated mostly
within 30 nm of the experimental absorbance maxima. Molecular orbital renderings clearly indicate
the propensity for intramolecular charge transfer within each compound.
Acknowledgements
This work received support from the South African National Research Foundation (Grants
113327 and 96111) and the Central Research Fund of the University of the Free State,
Bloemfontein. The High Performance Computing facility of the UFS is acknowledged for
computer time.
Supporting Information
Infrared and UV-visible Spectra and TDDFT Oscillators.
Optimized Cartesian Coordinates of the DFT Calculations.
HOMO & LUMO Molecular Orbital Renderings.
Linear Correlations between E°' and computed HOMO and LUMO energy, Ionization Potential,
Electron Affinity, Electronegativity, Global Electrophilicity Index and Electrochemical Potential of
all complexes.
17

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