Tris(β-ketoiminato)ruthenium(III) complexes: Electrochemical and computational chemistry study

Article abstract of DOI:10.1016/j.electacta.2019.134635

The electronic and electrochemical properties are reported here for the first time for a series of five tris(β-ketoiminato)ruthenium(III) complexes. Since the β-ketoiminato ligand is unsymmetrical, both fac and mer isomers are theoretically possible for t

Full text of DOI:10.1016/j.electacta.2019.134635

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Tris(β-ketoiminato)ruthenium(III) complexes: Electrochemical and computational
chemistry study
Tankiso Lawrence Ngake, Johannes H. Potgieter, Jeanet Conradie
PII:
S0013-4686(19)31494-X
DOI:
https://doi.org/10.1016/j.electacta.2019.134635
EA 134635
Reference:
To appear in: Electrochimica Acta
Received Date: 27 April 2019
Revised Date: 29 July 2019
Accepted Date: 1 August 2019
Please cite this article as: T.L. Ngake, J.H. Potgieter, J. Conradie, Tris(β-ketoiminato)ruthenium(III)
complexes: Electrochemical and computational chemistry study, Electrochimica Acta (2019), doi: https://
doi.org/10.1016/j.electacta.2019.134635.
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Tris(β-ketoiminato)ruthenium(III)ꢀcomplexes:ꢀꢀ
Electrochemicalꢀandꢀcomputationalꢀchemistryꢀstudyꢀ
1
2,3*
1
Tankiso Lawrence Ngake, Johannes. H. Potgieter and Jeanet Conradie *
1. Department of Chemistry, University of the Free State, P.O. Box 339, Bloemfontein, 9300, South
Africa
2. Division of Chemistry and Environmental Science, Manchester Metropolitan University,
Manchester, M1 5GD, UK
3. School of Chemical and Metallurgical Engineering, University of the Witwatersrand, Private Bag
X3, Wits, 2050, South Africa
Contact author details:
Name: Jeanet Conradie, Tel: ++27-51-4012194, Fax: ++27-4017295, email: conradj@ufs.ac.za
SYNOPSIS TOC
SYNOPSIS TEXT
Electrochemical and computational chemistry study of the fac and mer isomers of tris(β-
ketoiminato)ruthenium(III) complexes:
1

Research highlights
III/IV
III/II
II
•
•
•
Ru
Separate Ru
DFT calculations provide understanding of the locus of the observed redox couples
, Ru
redox couples, as well as ligand based reduction of Ru complex
III/IV
redox couples for fac and mer isomers
Abstract
The electronic and electrochemical properties are reported here for the first time for a series of five
tris(β-ketoiminato)ruthenium(III) complexes. Since the β-ketoiminato ligand is unsymmetrical,
both fac and mer isomers are theoretically possible for these octahedral complexes. Density
functional theory calculations show that for complexes containing an H on the imino position, both
the fac and mer are energetically possible, while for complexes with a Ph on the imino position, the
mer isomer is energetically favoured, due to the steric hindrance caused by the Ph group in the fac
III/IV
III/II
isomer. Electrochemistry, utilizing cyclic voltammetry, showed Ru
oxidation, Ru
reduction,
II
III/IV
III/II
as well as ligand based reduction of the Ru complex. Different Ru
and Ru
redox couples
were observed for the different fac and mer isomers of the tris(β-ketoiminato)ruthenium(III)
complexes.
Keywords
bidentate N,O ligands; β-ketoimine; ruthenium(III); Redox potential; DFT
1
Introduction
Ruthenium complexes containing N,N, N,O or O,O bidentate ligands have been extensively
1
studied before, for example in their use as potential anti-cancer agents [ ]. The N,O bidentate
2
ligand-containing Ru-complexes have been studied for their photophysical properties [ ] and for
3
their use in the catalytic dehydrogenation of benzoyl amine as co-oxidant [ ]. The N,N bidentate
ligand-containing Ru-complexes have been studied for artificial photosynthesis purposes and
4
excited state properties [ ]. Furthermore, Ru-complexes containing these N,N bidentate ligands
5
such as (2,2’-bipyridine, 1-10-phenanthroline) are used as dye-sensitizers for solar cells [ ]. On the
other hand, metal complexes containing O,O bidentate ligands (such as acetyl acetone) are used in
6
7
catalysis [ ] and as electrolyte in non-aqueous redox flow batteries [ ]. All these complexes have
attracted a lot of interest because of their favourable photophysical and electrochemical properties,
which can be systematically fine-tuned to achieve optimal customized material characteristics. In
2

the event where these complexes are studied for their catalytic properties, it is of importance
especially for industries, to study the factors that influence the activity of a catalyst towards
oxidation, reduction, substitution or oxidative addition, in order to develop more effective, cheaper
8
and more earth-abundant catalysts [ ]. As yet not much attention has been given to the
electrochemical properties of the ruthenium complexes containing N,O bidentate ligands (N,O-
BID), therefore it is of great importance to also study electrochemical properties of such complexes.
It has been previously shown that complexes with nitrogen as donor atom preferably coordinate
9
with metals above those complexes with oxygen as donor atom [ ]. It therefore will be interesting
to study the electronic influence of both the ligand substituents as well as the two donor atoms N
and O of the bidentate ligands, N,O-BID, on the metal they are coordinated to (metal-N,O-BID
complex), comparing them with the related metal-O,O-BID complexes.
In this study we thus present an electrochemical and computational chemistry study of five
novel tris(β-ketoiminato)ruthenium(III) complexes, containing amino substituted β-amino α,β-
unsaturated ketones (bidentate N,O-ligands), see Scheme 1. The electronic influence of the phenyl
group (on the metal complex) in different positions on the ligand are highlighted. These complexes
have two geometrical isomers, meridian (mer) and facial (fac), see Scheme 1.
Scheme 1. Structure of the β-ketoimine ligands L1 – L5, as well as the fac and mer isomers of the corresponding
1
2
tris(β-ketoiminato)ruthenium(III) complexes 1 – 5 of this study (with the two groups R and R on the ligand
being either CH or Ph). Series 1 exists of complexes 2, 3 and 5, containing ligands L2, L3 or L5 with group R″ =
3
H, while Series 2 exists of complexes 1 and 4, containing ligands L1 or L4 with group R″ = Ph (where the group
on the N atom is R″).
3

2
Experimental
2.1 General
UV/vis spectra were recorded on a Varian Cary 50 Conc ultra-violet/visible
spectrophotometer. MS was recorded on an ABSCIEX 4000QTRAP or Bruker Microflex LRF20
spectrophotometer. Proton NMR were recorded on a 300 MHz Bruker FOURIER NMR
1
spectrometer operating at 25°C, H frequency = 300.18 MHz. Chemical shifts are reported relative
to tetramethylsilane Si(CH ) at 0.00 ppm. Solid state Fourier transform infrared measurements
3
4
were performed on a Thermo Scientific Nicolet iS50 Attenuated Total Reflectance Fourier
transform infrared (ATR-FTIR) spectrometer using the iS50 ATR option running OMNIC software
(Version 9.2).
2.2 Synthesis
The five bidentate N,O-ligands were synthesized and characterized as reported for ligands 1, 2, 4
and 5 [10,11]. The synthesis of ligand 3 is described below. The five ruthenium complexes of the
corresponding ligands were synthesized by using literature methods of related complexes as a guide
[
22,12].
2
.2.1 Ligand 3 (L3)
,3-diphenylpropane-1,3-dione (2.438 g, 10.9 mmol) was dispersed over Montmorillonite K-10
1
(2 g) and DCM (20 ml) was added. While stirring, a mixture of ammonia solution (0.35 ml, 20.5
mmol) and DCM (10 ml) was added. The reaction was stirred at room temperature for 24 hours.
The catalyst was removed by filtration and the solvent was firstly removed on a rotatory evaporator,
1
then under vacuum, yielding a red powder (1.687 g). Yield 69%. H NMR (300 MHz, CDCl ,
3
25°C): δ 8.03 – 7.49 ppm (m, Ph, 10H); 6.89 ppm (s, C-H, 1H); 5.32 ppm (s, NH, 1H).
2
.2.2 [Ru(L1) ] (1)
3
A mixture of L1 (1.951 g, 11.1 mmol) in THF (25 ml) was added dropwise, while stirring, to a
mixture of sodium hydride (0.38 g, 15.8 mmol) and THF (25 ml). The reaction mixture was stirred
at room temperature for about 30 minutes. The unreacted sodium hydride was removed by
4

filtration. The filtrate was added to a mixture of [Ru(COD)Cl ] , (0.82 g, 2.93 mmol) in THF (25
2
2
ml) and the mixture was refluxed for 48 hours. The solution was washed with water to eliminate
any formed NaCl. The solvent was removed by rotatory evaporator and the resultant crystals were
+
placed under vacuum to yield dark green crystals (0.507 g). Yield 62%. MS Calcd. ([M] positive
-1
3
-1
mode): m/z 624.75. MS Found: m/z 625.3. UV: λmax 325 nm, εmax 54847 mol dm cm . Elemental
analysis calculated for RuC N H O (element, %): C, 63.55; H, 5.82; N, 6.74; obtained: C, 63.22;
33
3
36
3
H, 5.49; N, 6.74.
2
.2.3 [Ru(L2) ] (2)
3
.
3
RuCl 3H O (100 mg, 0.4 mmol) was dissolved in 10 ml of ethanol. The mixture was refluxed
2
for 15-20 minutes until the black colour turned green. A mixture of L2 (0.2970 g, 3 mmol) in 20 ml
ethanol was added and the whole reaction mixture was refluxed for 3 hours. The solvent was
removed by rotatory evaporator to obtain a dark blue solid (0.1907 g) Yield 64.2%. MS Calcd. m/z
-
395.44. Found: m/z 395.36. UV: λmax 567, 354 and 279 nm, εmax 2164, 8063 and 12597 mol
1
3
-1
dm cm . Elemental analysis calculated for RuC N H O (element, %): C, 45.56; H, 6.12; N,
13
3
24
3
10.63; obtained: C, 45.66; H, 5.75; N, 11.61.
2
.2.4 [Ru(L4) ] (4)
3
.
A mixture of RuCl 3H O (100 mg, 0.48 mmol), L4 (0.4556 g, 1.92 mmol) and KOH (215 mg,
3
2
3.84 mmol) in methanol (30 ml) was stirred for 24 hours at room temperature. The solvent was
removed on the rotatory evaporator. To the dark green solid was added ~10 ml of DCM and the
solution was filtered and the solvent was removed from the filtrate. The product was purified by
column chromatography on silica (column). The product was eluted by 8:2 DCM:Hexane. Yield:
+
0
.1604 g (35%), MS Calcd. ([M] positive mode): m/z 810.95. Found: m/z 810.2. UV: λ 355 nm,
max
-1
3
-1
εmax 21189 mol dm cm . Elemental analysis calculated for RuC N H O (element, %): C, 71.18;
48
3
42
3
H, 5.23; N, 5.19; obtained: C, 70.47; H, 6.18; N, 4.72.
2
.2.5 [Ru(L5) ] (5)
3
.
A mixture of RuCl 3H O (100 mg, 0.48 mmol), L5 (0.314 g, 1.95 mmol) and KOH (215 mg,
3
2
3.84 mmol) in methanol (30 ml) was stirred for 24 hours at room temperature. The solvent was
removed on the rotatory evaporator. To the dark green solid was added ~10 ml of DCM and the
solution was filtered and the solvent was removed from the filtrate to yield dark blue solid. Yield:
+
0
.1934 g (62%), MS Calcd. ([M] positive mode): m/z 582.66. Found: m/z 582.5. UV: λmax 305 and
5

-1
3
-1
2
45 nm, εmax 15629 and 21009 mol dm cm . Elemental analysis calculated for RuC N H O
30 3 30 3
(element, %): C, 61.95; H, 5.20; N, 7.22; obtained: C, 62.26; H, 5.64; N, 7.74.
2
.2.6 [Ru(L3) ] (3)
3
A mixture of L3 (0.784 g, 3.5 mmol) in THF (25 ml) was added dropwise, while stirring, to a
mixture of sodium hydride (0.134 g, 5.58 mmol) and THF (25 ml). The reaction mixture was
stirred at room temperature for about 30 minutes. The unreacted sodium hydride was removed by
filtration. The filtrate was added to a mixture of [Ru(COD)Cl ] (0.275 g, 0.98 mmol) in THF (25
2
x
ml) and the mixture was refluxed for 48 hours. The solution was washed with water to eliminate
any NaCl formed. The solvent was removed by rotatory evaporator and the resultant powder was
+
placed under vacuum to yield a dark brown powder (0.45 g) Yield 58%. MS Calcd. ([M] positive
-
mode): m/z 768.87. Found: m/z 769.2. UV: λmax 324 and 251 nm, εmax 23688 and 30961 mol
1
3
-1
dm cm . Elemental analysis calculated for RuC N H O (element, %): C, 70.39; H, 4.73; N,
45
3
36
3
5.47; obtained: C, 70.07; H, 4.03; N, 5.00.
2.3 Cyclic Voltammetry
Cyclic voltammetric (CV) measurements were conducted on a BAS100B Electrochemical
Analyzer linked to a personal computer, utilizing the BAS100W Version 2.3 software.
Measurements were done at 293 K and the temperature was kept constant within 0.5 K. Successive
experiments under the same experimental conditions showed that all formal reduction and oxidation
potentials were reproducible within 0.005 V. Cyclic voltammetric (CV) measurements were
-3
performed either on a concentration of 2 mmol dm or on saturated solutions of each of the five
-3
complexes, dissolved in CH CN as solvent, containing 0.2 mol.dm tetrabutylammonium
3
hexafluorophosphate (TBAPF , [NBu ][PF ]) as supporting electrolyte. Measurements were
6
4
6
conducted under a blanket of purified Argon. A three-electrode cell was used, consisting of a Pt
2
auxiliary electrode, a glassy carbon working electrode (surface area 0.0707 cm ) and a Pt reference
electrode [13,14]. The working electrode was polished first by a 3 ꢀm, followed by 1 ꢀm Diapat
diamond paste on an abrasive cloth (in a figure-of-eight motion), rinsed with EtOH, H O and
2
-1
CH CN, and dried before each experiment. Scan rates varied between 0.050 and 5.000 V.s . All
3
experimental potentials were referenced against the redox couple of ferrocene (FcH) (IUPAC [15]).
+
Either ferrocene or decamethyl ferrocene (Fc*, -0.508 V vs FcH/FcH ) was used as internal
standard. Under these experimental conditions, ferrocene (FcH) exhibited a peak separation of ∆Ep
=
E – E = 0.069 V and ratio i /i = 1.00; where E (or E ) is the anodic (or cathodic) peak
pa pc pc pa pa pc
potential, and i (or i ) is the anodic (or cathodic) peak current.
pa
pc
6

2.4 DFT calculations
Density functional theory (DFT) calculations were performed via the B3LYP functional (and
UB3LYP for spin unrestricted calculations), as implemented in the Gaussian 09 package [16], using
the triple-ζ basis set 6-311G(d,p) for the lighter atoms (C, H, N, O), whereas the Stuttgart/Dresden
(SDD) pseudopotential was used to describe the Ru electronic core, while the metal valence
electrons were described using the def2-TZVPP basis set [17]. Calculations were done in the gas
phase, as well as the experimental solvent of the electrochemical study, CH CN with ε = 37.5. The
3
solvation model density (SMD) [18] polarizable continuum model (PCM) that was used, which
solves the non-homogeneous Poisson equation by applying an integral equation formalism variant
(
IEF-PCM) [19].
2.5 Mass Spectrometry
ESI-MS spectra (complexes 1, 3, 4, 5) (electrospray-ionization mass spectrometry) were
collected on an ABSCIEX 4000QTRAP ion-trap mass spectrometer. The dried sample was
dissolved in 1mL acetonitrile and further diluted 1000x before being infused into a Sciex
4000QTRAP hybrid triple quadrupole ion-trap mass spectrometer, at 10 uL/min. During infusion a
Q1 scan was performed between 100 and 800 Da in, while ramping the declustering potential
between 0 and 400 V. The scan was performed in both positive and negative ionisation mode. The
ionisation voltage was set at 5500V in positive mode and -4500V in negative mode, with a 10 psi
curtain gas setting and 20 psi ionisation gas (GS1) setting.
MALDI-TOF-MS spectra (complex 2) (matrix assisted laser desorption/ionization time-of-
flight mass spectrometry) were collected by a Bruker Microflex LRF20 in the negative reflection
mode, using the minimum laser power required to observe signals.
3
Results and Discussion
3.1 Synthesis
7

Scheme 2: Synthetic route for the tris(β-ketoiminato)ruthenium(III) complexes 1 – 5. The ruthenium salt
.
[
Ru(COD)Cl ] as used in the synthesis of complexes [Ru(L1) ] and [Ru(L3) ], while RuCl 3H O was used in the
2
2
3
3
3
2
synthesis of complexes [Ru(L2) ], [Ru(L4) ] and [Ru(L5) ], as shown on the left. The solvents and reaction
3
3
3
1
2
’’
conditions for each synthesis are shown on the right. The R , R and R groups are indicated in Scheme 1.
The β-amino α,β-unsaturated ketoimine ligands L1 – L5 (Scheme 1), containing both N and
O donor atoms, were synthesized as described in literature [10,11]. Scheme 2 indicates the general
synthetic route for the preparation of the five tris(β-ketoiminato)ruthenium(III) complexes 1 – 5 of
this study. Complexes 1 – 5 were prepared by either refluxing a mixture of a relevant ruthenium
.
3
salt ([Ru(COD)Cl ] / RuCl 3H O) with the corresponding ketoimine ligands [12], or by stirring a
2
2
2
.
3
mixture of RuCl 3H O and the relevant ketoimine ligand at room temperature [22]. The colour of
2
each of the five complexes in the solid state appear black until dissolved in a solvent. When
dissolved in a solvent, complexes [Ru(L1) ] and [Ru(L2) ] are dark blue, [Ru(L4) ] appears green,
3
3
3
[
Ru(L5) ] appears a very dark purple colour, while [Ru(L3) ] appears brown. The complexes are
3 3
stable in air, soluble in most organic solvents and are non-hygroscopic.
3.2 DFT results
3
.2.1 Molecular Geometry
Both the fac and the mer isomers of all five synthesized tris(β-ketoiminato)ruthenium(III)
complexes 1 – 5, as well as the mer isomer only of complex 6 obtained from literature (Figure 1
1
2
with R″ = CH , R = R = CF ), of which solid state crystal data was available from previous
3
3
research [12], were optimized by DFT, with the resulting coordinates provided in the Supporting
Information. Complex 6 is the available complex related to the complexes 1 – 5 of this study of
which XRD data is available. Selected bond lengths and angles of the B3LYP density functional
theory (DFT) optimized geometries of the six complexes studied, were summarized and compared
with the existing XRD data of complex 6 [12] in Table 1. For complex 6, the bonds involving
ruthenium, Ru-N and Ru-O were accurately calculated within 0.03 Å of the provided experimental
values. The O-Ru-N angles were calculated within 1.5° of the experimental values from literature;
see Figure 1 for a root mean square (RMS) overlay of the calculated and experimental structures of
complex 6. The calculated bond lengths and angles for complex 6 were generally slightly larger
than the experimental values, as often observed when comparing gas phase calculated structures
2
0
with solid state experimental structures [ ]. It was also observed that the calculated Ru-N and Ru-
O bond lengths (and the O-Ru-N angles) of complexes 1 – 5 fall into a small range of 1.99-2.14 Å
8

(for Ru-N) and 1.99-2.08 Å (for Ru-O) (and 88-92° for O-Ru-N) respectively, with insignificant
differences between the geometrical parameters of the fac and mer isomers. The three substituents
1
2
R″, R and R on the β-ketoiminato ligands had little effect on the bond distance between the metal
and the ligand.
Figure 1: Top: Structure of [Ru(L6) ] complex 6 (mer isomer) from literature, that was previously solved by
3
1
2
XRD (CSD reference GAGROW [12]), (with the two ligand groups R = R = CF and the group on the N donor
3
atom R″ = CH ). Bottom: RMS overlay (RMSD = 0.22) of the experimental (black) and B3LYP calculated (red)
3
structure of complex 6 (H and F omitted for clarity).
Table 1: Selected bond lengths (Å) and bond angles (°) for the mer and fac isomers of complexes Ru(L1) to
3
Ru(L6) as obtained from the theoretical B3LYP DFT calculations, as well as experimental crystallographic data
3
only of the Ru(L6) complex shown in Figure 1.
3
[
Ru(L1)3]
Calculated
Mer Fac
[Ru(L2)3]
[Ru(L4)3]
[Ru(L5)3]
Mer Fac
[Ru(L3)3]
Mer Fac
[Ru(L6)3]
Exp
Mer Fac
Mer Fac
Mer
Mer
Ru-N1
Ru-N2
Ru-N3
Ru-O1
Ru-O2
Ru-O3
O1-Ru-N1 89.4
O2-Ru-N2 90.8
O3-Ru-N3 90.6
2.084 2.137 2.022 2.000 2.095 2.096 2.033 1.995 2.016 1.990 2.112
2.125 2.102 2.031 2.016 2.121 2.107 2.019 2.028 2.027 2.015 2.063
2.069 2.078 2.068 2.038 2.085 2.087 2.085 2.021 2.060 2.025 2.064
2.039 2.026 2.045 2.042 2.036 2.057 2.061 2.040 2.044 2.038 2.014
2.046 2.043 2.020 2.078 2.030 2.019 2.044 2.036 2.012 2.078 2.001
2.012 2.029 2.024 2.040 2.010 2.029 2.019 2.078 2.018 2.034 2.020
2.090
2.037
2.042
2.015
1.984
2.013
92.1
92.0
89.4
89.5
90.0
88.2
88.9
91.1
88.5
87.8
90.7
91.0
90.3
89.7
89.6
92.1
90.1
89.0
88.1
90.9
87.9
88.5
89.9
88.9
87.8
90.8
87.7
88.3
90.6
91.1
91.1
91.7
90.7
9

3
.2.2 Energy
Results obtained in the previous section showed that the B3LYP DFT method provided a reliable
calculated structure for complexes 1 – 5, when compared to existing experimental crystallographic
data from a similar compound (6 obtained from literature). In order to determine whether the
electronic energy of these complexes also was accurate, the possible spin states, S = ½, 3/2 and 5/2,
5
were calculated for the d complex 2 using two different functionals, see Table 2. The results were
in agreement with experimental observation [21,22], clearly showing that the neutral tris(β-
ketoiminato)ruthenium(III) complexes 1 – 5 of this study were all low-spin with S = ½.
Table 2: Relative electronic energy (eV) for complex Ru(L2) , calculated by the different DFT functionals, as
3
indicated. Lowest energies are indicated in bolt font.
Isomer S
∆E (eV)
B3LYP
0.02
B3LYP-D3
0.00
2.38
3.80
0.06
OLYP
0.00
1.70
2.85
0.01
1.41
2.71
fac
½
3
5
½
/2 1.93
/2 3.27
0.00
mer
3
5
/2 1.59
/2 3.09
2.07
3.65
The relative energies and populations according to the Boltzmann equation of the fac and mer
isomers of complexes 1 – 6 were listed in Table 3. The results showed that for complexes 2, 3 and
5
(Series 1, with R″ = H), both the fac and mer isomer could exist, with fac the main isomer in
solution. However, for complexes 1 and 4 (Series 2, with R″ = Ph), as well as complex 6 from
literature (with R″ = CH ), the mer isomer was favoured. The mer isomers of the latter were less
3
affected by steric hindrance caused by the Ph or CH groups (on the N donor atom of each ligands),
3
when compared to their fac isomers; see Figure 2. This result was also in agreement with the fact
that only the mer isomer of complex 6 was isolated in the solid state [12].
1
0

Table 3: Relative energies (eV) and population (%) for complexes 1 – 6 in the two ligand series, obtained from
theoretical DFT calculations in both the gas and solvent phase, using CH CN as solvent.
3
Gas phase
Gas phase
B3LYP-D3
Solvent
CH CN)
B3LYP
Erel
Solvent (CH CN)
3
(
3
B3LYP
Erel
B3LYP-D3
%
Erel
%
%
Erel
%
Series 1 with R″ = H
[
[
[
[
[
[
Ru(L2) ] - fac 0.02
32.71
67.29
50.55
49.45
78.33
21.67
0.02
0.00
0.01
0.00
0.00
0.04
33.85 0.00
90.81 0.00
9.19 0.06
90.82 0.00
9.18 0.04
96.69 0.00
90.13
9.87
83.67
16.33
96.50
3.50
3
Ru(L2) ] - mer 0.00
66.15 0.06
43.14 0.00
56.86 0.06
83.44 0.00
16.56 0.09
3
Ru(L5) ] - fac
0.00
3
Ru(L5) ] - mer 0.00
3
Ru(L3) ] - fac
0.00
3
Ru(L3) ] - mer 0.03
3.31
0.09
3
Series 2 with R″ = Ph or CH3
[
[
[
[
[
[
Ru(L1) ] - fac
0.23
0.01
99.99
0.07
99.93
0.00
100.00 0.00
0.12
0.00
0.22
0.00
0.35
0.88
99.12 0.00
0.02 0.19
99.98 0.00
0.00 0.20
100.00 0.00
0.21
0.03
0.09
2.74
97.26
0.03
99.97
0.01
99.99
3
Ru(L1) ] - mer 0.00
99.97 0.00
0.05 0.21
99.95 0.00
0.00 0.24
100.00 0.00
3
Ru(L4) ] - fac
0.19
3
Ru(L4) ] - mer 0.00
3
Ru(L6) ] - fac
0.29
3
Ru(L6) ] - mer 0.00
3
fac isomer
mer isomer
Figure 2: Top: B3LYP geometry of the fac (left) and mer (right) isomer of complex 1, highlighting the higher
steric hindrance caused by the Ph group (on the N donor atom of each ligand) in the fac isomer (top left) than in
the mer isomer. Bottom: Spin density plot of the fac (left) and mer (right) isomer of complex 1. Contour = 0.004
-
3
eÅ .
3
.2.3 Electronic structure
5
Selected frontier molecular orbitals of complex 2 were shown in Figure 3. The neutral d
Ru(III)-complex 2 with S = ½ has three alpha and two beta Ru-d based molecular orbitals (MOs).
1
1

Both the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital
(
(
LUMO) of complex 2 were of Ru-d character, while the LUMO+1 was β-ketoiminato ligand based
see Figure 4). The spin density plot of complexes 1 – 5 showed the locus of the unpaired electron
-
on each complex. The spin distribution indicated that about 0.80 e was distributed over the Ru-
metal for all the complexes, with the rest of the electron density distributed over both donor atoms
(N and O) and the central carbon between (C-N) and (C-O) on the β-ketoiminato ligands (see Table
S1 and Figure S3). For the complexes of Series 2 where R″ = Ph (namely [Ru(L1) ] and
3
[
Ru(L4) ]), the spin was additionally distributed over two of the three Ph groups on the R″ positions
3
on the N donor atom (Figure S3). The Mulliken spin population on Ru of ca. 0.8 was nearing value
1
(Table S1), which is consistent with a low-spin Ru(III) centre.
During the reduction of the neutral Ru(III) complex, an electron is added to the complex. This
added electron could either go into the β-LUMO resulting in q = -1 and S = 0, or into the α-LUMO
overall LUMO+1), resulting in two unpaired electrons for the reduced complex with q = -1 and
(
spin = 1. DFT calculations for 2 showed that the complex with spin = 0 had a lower energy
compared to the complex with spin = 1 (with ΔE = 1 – 1.5 eV for complexes 1 – 5), therefore the
electron added upon reduction of complex 2 was added to the β-LUMO. Since the β-LUMO is Ru-
III/II
d based, the first reduction of complex 2 was therefore proven to be metal based, i.e. Ru
reduction.
In contrast, during oxidation, an electron is removed from the molecule. This electron could
either be removed from the β-HOMO, resulting in q = 1 and spin = 1, or from the α-HOMO
(overall HOMO-1) resulting in q = 1 and spin = 0 (no unpaired electrons). DFT calculations for 1 -
5
showed that the complex with spin = 1 had the lowest energy compared to the complex with spin
=
0, with ΔE = 0.2 – 0.3 eV, implying that the reduced complex was diamagnetic with S = 0. Since
the β-HOMO is Ru-d based, the first oxidation of complex 2 was therefore proven to be metal
III/IV
based, i.e. Ru
oxidation.
A second reduction would involve an added electron, either to the LUMO+1 of the neutral
complex, or to the LUMO of the reduced complex. These MOs did not have Ru-d character and
were ligand based, see Figure 4. Thus the second reduction of complex 2 was proven to be ligand
based.
1
2

alpha 86 HOMO-4
alpha 87 HOMO-2
alpha 88 HOMO-1
alpha 93 LUMO+9
beta 86 HOMO-3
beta 87 HOMO
beta 88 LUMO
beta 93 LUMO+11
Figure 3: Selected B3LYP calculated MOs of the fac isomer of the tris(β-ketoiminato)ruthenium(III) complex 2
-3
of this study. The contour used for the MO plots is 0.08 eÅ .
alpha LUMO+1 of 2 alpha LUMO of 2−
Figure 4: B3LYP calculated LUMO+1 of the fac isomer of complex 2 and LUMO of the anion of 2. The contour
-3
used for the MO plots is 0.07 eÅ .
Similarly to complex 2, the LUMO and the HOMO of complexes 1, 3 – 5 were also proven to
be predominantly Ru-d based, however extending further by small amounts also to the two donor
atoms N and O, as well as to the central carbon between (C-N) and (C=O) on the ligand (see Figure
S1).
Table 4 shows both the HOMO and LUMO energies for the mer and fac isomers of complexes
1
– 5. The results in Table 4 showed that in all five complexes both HOMO and LUMO energies of
the fac isomer were lower (more negative) than those of the mer isomer, except for complex
Ru(L3) ] B3LYP-D3 gas phase calculations where the order reversed, though the gas phase
[
3
energies of the HOMOs of the two isomers differ only with only 0.003 eV. This means that the fac
isomer was reduced more readily than the mer isomer (since it is easier to accept an electron in a
1
3

lower energy LUMO). The mer isomer, however, was oxidized more easily than the fac isomer,
since it is easier to remove an electron from a higher energy HOMO.
1
4

Table 4: DFT calculated HOMO and LUMO energies (eV) for the two possible isomers fac and mer of the five Ru(III) complexes 1 – 5 in the two ligand series.
Gas
Gas
Solvent (CH CN)
Solvent (CH CN)
3
3
B3LYP
B3LYP-D3
B3LYP
B3LYP-D3
EHOMO (eV) ELUMO (eV)
EHOMO (eV) ELUMO (eV) EHOMO (eV) EHOMO (eV)
EHOMO (eV) EHOMO (eV)
Series 1 with R″ = H
[
[
[
[
[
[
Ru(L2) ] - fac
-4.703
-2.219
-2.173
-2.412
-2.387
-2.529
-2.515
-4.721
-4.670
-4.840
-4.813
-4.898
-4.840
-2.179
-2.137
-2.343
-2.327
-2.463
-2.466
-5.064
-5.049
-5.152
-5.137
-5.245
-5.224
-2.604
-2.552
-2.751
-2.704
-2.911
-2.864
-5.074
-5.049
-5.157
-5.137
-5.223
-5.183
-2.559
-2.506
-2.695
-2.638
-2.845
-2.811
3
Ru(L2) ] - mer -4.666
3
Ru(L5) ] - fac
-4.827
3
Ru(L5) ] - mer -4.812
3
Ru(L3) ] - fac
-4.897
3
Ru(L3) ] - mer -4.860
3
Series 2 with R″ = Ph
[
[
[
[
Ru(L1) ] - fac
-4.876
-2.516
-2.379
-2.630
-2.524
-4.843
-4.747
-4.922
-4.831
-2.541
-2.362
-2.635
-2.475
-5.216
-5.105
-5.283
-5.195
-2.849
-2.681
-2.953
-2.816
-5.167
-5.041
-5.229
-5.105
-2.866
-2.635
-2.952
-2.733
3
Ru(L1) ] - mer -4.784
3
Ru(L4) ] - fac
-4.964
3
Ru(L4) ] - mer -4.899
3
15

3.3 CV results
The redox behaviour of the five ruthenium(III) complexes 1 – 5 in this research were studied
under the conditions discussed in section 2.3. Selected electrochemical data was summarized in
Table 5, and representative voltammograms were shown in Figure 5 (for Series 1) and Figure 6 (for
Series 2). All five these complexes showed two to three redox couples in the solvent window
possible for CH CN. The first redox couple on the anodic side, and the first redox couple on the
3
cathodic side, were assigned respectively to the oxidation of Ru(III) to Ru(IV), and the reduction of
Ru(III) to Ru(II). This assignment was in agreement with the DFT study presented in Section 3.2.3,
and also in qualitative agreement with the redox behaviour of a related tris(β-
2
3
diketonato)ruthenium(III) complex [Ru(acac) ] from literature [ ], that also exhibited a Ru(III)/(IV)
3
oxidation on the anodic side, and a Ru(III)/(II) reduction on the cathodic side. For complexes 3, 4
+
and 5 a second redox process was observed below 1.9 V vs FcH/FcH ; see Figure 6 for complex 4
as an example. This second redox process was assigned to the reduction of the coordinated β-
ketoimine ligands of the tris(β-ketoiminato)ruthenium(III) complexes of this study (see section
3.2.3 and Figure 4). This ligand based reduction of complexes 1 and 2 was not observed in the
solvent window possible for CH CN.
3
+
The Ru(III)/(IV) oxidation on the anodic side, between -0.04 to 0.70 V vs FcH/FcH , consisted
of two oxidation peaks with two associated reduction peaks, which were assigned in accordance to
the DFT results presented in Section 3.2.3 above, namely as the oxidation of firstly the mer isomer
and secondly the fac isomer of each complex. The first Ru(III)/(IV) redox couple, which was
assigned to the mer isomer, was reversible with ꢁE = 0.061 - 0.079 V, while for the second
p
oxidation wave which was associated with the fac isomer, the ꢁE value was between 0.061 - 0.077
p
V. The Ru(III)/(IV) oxidation occurred at the potential range of -0.04 V to 0.40 V for the mer
isomers, and at 0.12 V to 0.67 V for the fac isomers. It was observed that the Ru(III)/(IV) oxidation
1
occurred at increasingly higher oxidation potentials, as the amount of aromatic Ph groups at the R
2
and R position increased from 0 (complex 2) to 1 (complex 5) to two Ph groups (complex 3) in the
Series 1 complexes; see Figure 5. The same trend was observed for the Series 2 complexes, when
1
2
2
comparing complex 1 (no Ph group at R or R ) with complex 4 (one Ph group at R ); see Figure 6.
+
The Ru(III)/(II) reduction on the cathodic side between -1.52 V to -0.89 V vs FcH/FcH ,
showed a different behaviour for the different complexes. For Series 1 complexes (Figure 5), the
following was obtained: Complex 2 showed one reversible Ru(III)/(II) redox couple between -
1
6

1
.358 V and -1.426 V (with ΔE = 0.068 V), which was assigned to the reduction of the closely
p
overlapping fac and mer isomers of complex 2. Complex 5 on the other hand showed two
Ru(III)/(II) redox couples between -1.232 V and -1.006 V, which were assigned to the reduction of
the fac and the mer isomer of 5 respectively. Complex 3 again showed one Ru(III)/(II) redox
couple between -0.864 V and -0.925 V (with ΔE = 0.061 V), which also was assigned to the
p
reduction of the closely overlapping fac and mer isomers of 3.
For the Series 2 complexes (Figure 6), a very small reduction peak was obtained at -1.243 V for
the fac isomer of complex 1, and a larger reduction peak at -1.552 V (with the corresponding
oxidation peak at -1.486 V) for the mer isomer of complex 1. The observed small reduction peak
for the fac isomer was consistent with the DFT study above, which showed that due to steric
hindrance, complex 1 existed mainly in the mer form. For complex 4 also of Series 2, the reversible
(
ΔE = 0.067 V) reduction wave between -1.437 V and -1.370 V, was assigned to the reduction of
p
the fac and mer isomer closely overlapping. Only one Ru(III)/(II) reduction peak was observed for
-1
complex 4, at all scan rates from 0.05 up to 5.00 V s ; see Figure S5.
The fac-mer equilibrium observed here thus seemed to be slow and did not change with time or
scan rate. The experimental assignment of redox peaks of the different isomers was previously
made possible by observance of the slow fac-mer and keto-enol equilibrium kinetics of related
chromium-carbene complexes [24] and ruthenocene-containing β-diketones [25] respectively. For
the fac and mer isomers of the chromium-carbene complexes, different peaks ca. 0.2 V apart, were
observed for the oxidation of the fac and mer isomers, similarly to the difference between the fac
and mer oxidation observed for 1 – 5. For some chromium-carbene complexes, the reduction of
both fac and mer isomers could be distinguished, while for other chromium-carbene complexes only
one reduction peak was observed, since in the latter case the reduction peaks of the fac and mer
isomers were too near to each other be distinguished [25]. This is similar to the redox behaviour
that was obtained here for complexes 1 and 5 (two reduction peaks for the fac and mer isomers
respectively) and for complexes 2, 3 and 4 (one reduction peak only for both the fac and mer
isomer, closely overlapping).
The same trend that was observed for the Ru(III)/(IV) oxidation, was also observed for the
Ru(III)/(II) reduction, namely that the reduction occurred at an increasingly higher potential, as the
1
2
amount of aromatic Ph groups at the R and R positions increased from 0 (complex 2) to 1
complex 5) to two Ph groups (complex 3) in the Series 1 complexes (Figure 5). Similarly, the
reduction also occurred at an increasingly higher potential, as the amount of aromatic Ph groups at
(
1
2
the R and R positions increased from 0 (complex 4) to 1 Ph group (complex 1) in the Series 1
complexes (Figure 6).
1
7

The potential difference between the two successive redox processes (Ru(III)/Ru(II) and
Ru(III)/Ru(IV)) of complexes 1 – 5, was 1.3 V to 1.6 V. This result was in line with previous
studies of Ru(III) complexes, which showed that the average potential difference between two
successive redox processes (Ru(III)/Ru(II) and Ru(III)/Ru(IV)) was 1.2 V to 1.7 V [26,27].
It was further observed that both the Ru(III)/Ru(II) reduction and the Ru(III)/Ru(IV) oxidation
-
of complexes 1 and 2 (containing N,O-bidentate ligands of the type (CH C(NR″)CHCOCH ) ),
3
3
+
occurred at a lower potential than the Ru(III)/Ru(II) reduction (-1.15 V vs Fc/Fc ) and the
+
Ru(III)/Ru(IV) oxidation (0.602 V vs Fc/Fc ) of the related [Rh(acac) ] compound from literature
3
-
(
containing the O,O-bidentate ligand acac = acetylacetonato = (CH COCHCOCH ) ) [23]. This
3
3
observation also agrees with literature results obtained for [Ru(O,O-BID) (N,O-BID) ] (n = 1 or
3
-n
n
2), where it was observed that when one of the two O,O bidentate ligands was substituted with a
-
N,O bidentate ligand from [Ru(acac) ] (acac = acetylacetonato = (CH COCHCOCH ) ), both the
3
3
3
Ru(III)/Ru(II) reduction and the Ru(III)/Ru(IV) oxidation occurred at a lower potential [22].
Table 5: The summary of the cyclic voltammetric data of the Ru(III) complexes.
Oxidation Ru(III)/(IV)
Reduction Ru(III)/(II)
0
0
Epc
Epa
ꢁE
E '
Epc
Epa
ꢁE
E '
Series 1
[
[
[
[
[
[
Ru(L2) ] - fac
0.082
-0.078 -0.003
0.641
0.369
0.363
0.144
0.159
0.077
0.075
0.062
0.061
0.063
0.062
0.121
-0.041
0.672
0.400
0.395
0.175
-1.426
-0.925
-1.358
-0.864
0.068 -1.392
0.061 -0.895
3
Ru(L2) ] - mer
3
Ru(L3) ] - fac
0.703
0.430
0.426
0.206
3
Ru(L3) ] - mer
3
Ru(L5) ] - fac
-1.069
-1.232
-1.006
-1.165
0.063 -1.038
0.067 -1.199
3
Ru(L5) ] - mer
3
Series 2
[
[
[
[
Ru(L1) ] - fac
0.215
0.032
-
0.276
0.097
0.508
0.137
0.061
0.065
0.246
0.065
-1.243
-1.552
-1.437
-1.172
-1.486
-1.370
0.071 -1.208
0.066 -1.519
0.067 -1.404
3
Ru(L1) ] - mer
3
Ru(L4) ] - fac
3
Ru(L4) ] - mer
0.058
0.079
0.098
3
1
8

+
Figure 5: Cyclic voltammograms (versus FcH/FcH ) of Series 1, tris(β-ketoiminato)ruthenium(III) complexes 2,
-1
5
and 3 of this study, at a scan rate of 0.100 V s . Decamethyl ferrocene (Fc*) was used as internal standard
-3
reference. Scans were initiated from ca. -0.3 V in the positive direction. The CVs were measured in 0.2 mol dm
TBAPF /CH CN, on a glassy carbon working electrode at 25 °C.
6
3
+
Figure 6: Cyclic voltammograms (versus FcH/FcH ) of Series 2, of tris(β-ketoiminato)ruthenium(III) complexes
-1
1
and 4 of this study, at a scan rate of 0.100 V s . Decamethyl ferrocene (Fc*) was used as internal standard
-3
reference. Scans were initiated from ca. -0.3 V in the positive direction. The CVs were measured in 0.2 mol dm
TBAPF /CH CN, on a glassy carbon working electrode at 25 °C.
6
3
1
9

4
Conclusions
The DFT calculations showed that tris(β-ketoiminato)ruthenium(III) complexes 2, 5 and 3
mainly favour the fac isomer, while complexes 1 and 4 favour the mer isomer in solution, due to
steric hindrance of the Ph group on the R″ position. Moreover, the spin density plots of complexes
1
– 5 showed that 80% of the electron distribution was distributed over the metal, while the rest was
distributed over the donor atoms N and O, spreading over the central carbon between (C-N) and (C-
O) on the β-ketoiminato ligand. DFT calculations also showed that the first reduction and the first
oxidation was Rh-d-metal based. Electrochemistry results showed that oxidation of Ru(III) to
+
Ru(IV) occurred between -0.04 V to 0.70 V vs FcH/FcH , and reduction of Ru(III) to Ru(II)
+
between -0.90 V to -1.52 V vs FcH/FcH . The experimental results also showed that two oxidation
peaks occurred, due to the two isomers, mer and fac. The DFT results showed that the mer isomer
will be oxidised first, while the fac isomer will be reduced first. Lastly, the experimental results
(Cyclic Voltammetry) showed that both the oxidation (Ru(III) to Ru(IV)) and reduction (Ru(III) to
1
2
Ru(II)) potentials increased with an increase in the amount of aromatic Ph groups at the R and R
positions, for both ligand series 1 and 2.
Acknowledgements
This work has received support from the South African National Research Foundation (Grant Nos:
13327 and 96111) and the Central Research Fund of the University of the Free State,
1
Bloemfontein. The High Performance Computing facility of the UFS is acknowledged for
computer time.
Supporting Information
Optimized coordinates of the DFT calculations, additional figures and tables.
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