Electrochemical behaviour of amino substituted β-amino α,β-unsaturated ketones: A computational chemistry and experimental study

Article abstract of DOI:10.1016/j.electacta.2018.11.144

The synthesis, identification and electrochemical properties are reported here, for a series of five novel and seven known amino substituted β-amino α,β-unsaturated ketones (bidentate N,O-ligands) of the type CH₃COCHC(NHR)CH₃, where R = H, Ph, CH₂Ph, CH(CH₃)₂, p-CF₃-Ph or p-^tBu-Ph (Series 1), as well as type PhCOCHC(NHR)CH₃, where R = H, Ph, p-NO₂-Ph, 3,5-di-Cl-Ph, 2-CF₃-4-Cl-Ph, and also PhCOCHC(NHPh)CF₃ (Series 2). The cyclic voltammograms measured in CH₃CN, generally exhibit both a chemically and electrochemically irreversible reduction peak between ?1.2 V and ?3.1 V vs FcH/FcH⁺, producing an unstable radical anion, for most of these 1,3-amino ketones. Only ligands PhCOCHC(NHPh)CH₃, PhCOCHC(NHPh)CF₃ and PhCOCHC(NH(p-NO₂-Ph))CH₃, showed reversible electrochemical behaviour, at higher scan rates. Density functional theory (DFT) calculations proved the unpaired spin density in the radical anion to be distributed over the pseudo-aromatic O–C–C–C–N backbone of the 1,3-amino ketones, extending further over the phenyl rings of the phenyl-containing ligands. Various DFT calculated energies, such as the energy of the lowest unoccupied molecular orbital (the orbital into which the electron is added upon reduction), as well as the DFT calculated gas phase adiabatic electron affinities, relate linearly to the experimentally measured reduction potential. These obtained linear relationships confirmed that good communication via conjugation exists, between the R substituent on the amino group and the rest of the 1,3-amino ketone.

Full text of DOI:10.1016/j.electacta.2018.11.144

Accepted Manuscript
Electrochemical behaviour of amino substituted β-amino α,β-unsaturated ketones: A
computational chemistry and experimental study
Tankiso Lawrence Ngake, J.H. Potgieter, Jeanet Conradie
PII:
S0013-4686(18)32627-6
https://doi.org/10.1016/j.electacta.2018.11.144
EA 33145
DOI:
Reference:
To appear in: Electrochimica Acta
Received Date: 28 May 2018
Revised Date: 22 October 2018
Accepted Date: 21 November 2018
Please cite this article as: T.L. Ngake, J.H. Potgieter, J. Conradie, Electrochemical behaviour of amino
substituted β-amino α,β-unsaturated ketones: A computational chemistry and experimental study,
Electrochimica Acta (2018), doi: https://doi.org/10.1016/j.electacta.2018.11.144.
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ACCEPTED MANUSCRIPT
Electrochemicalꢀbehaviourꢀofꢀaminoꢀsubstitutedꢀβ-aminoꢀα,β-
unsaturatedꢀketones:ꢀꢀAꢀcomputationalꢀchemistryꢀandꢀ
experimentalꢀstudyꢀ
1
2,3*
1
Tankiso Lawrence Ngake, J. 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
Research highlights
•
•
•
β-amino α,β-unsaturated ketones produce unstable radical anions upon reduction
Combined electrochemical and DFT study
Relationships between experimental reduction potentials and DFT calculated energies
Abstract
The synthesis, identification and electrochemical properties are reported here, for a series of five
novel and seven known amino substituted β-amino α,β-unsaturated ketones (bidentate N,O-ligands)
t
of the type CH COCHC(NHR)CH , where R = H, Ph, CH Ph, CH(CH ) , p-CF -Ph or p- Bu-Ph
3
3
2
3 2
3
(
Series 1), as well as type PhCOCHC(NHR)CH , where R = H, Ph, p-NO -Ph, 3,5-di-Cl-Ph, 2-CF -
3 2 3
1

4
-Cl-Ph, and also PhCOCHC(NHPh)CF (Series 2 T cyclic voltammograms measured in
ACCEPTED MA N) .US Ch eR IPT
3
CH CN, generally exhibit both a chemically and electrochemically irreversible reduction peak
3
+
between -1.2 V and -3.1 V vs FcH/FcH , producing an unstable radical anion, for most of these 1,3-
amino ketones. Only ligands PhCOCHC(NHPh)CH , PhCOCHC(NHPh)CF and (p-NO -
3
3
2
Ph)COCHC(NHPh)CH , showed reversible electrochemical behaviour, at higher scan rates.
3
Density functional theory (DFT) calculations proved the unpaired spin density in the radical anion
to be distributed over the pseudo-aromatic O-C-C-C-N backbone of the 1,3-amino ketones,
extending further over the phenyl rings of the phenyl-containing ligands. Various DFT calculated
energies, such as the energy of the lowest unoccupied molecular orbital (the orbital into which the
electron is added upon reduction), as well as the DFT calculated gas phase adiabatic electron
affinities, relate linearly to the experimentally measured reduction potential. These obtained linear
relationships confirmed that good communication via conjugation exists, between the R substituent
on the amino group and the rest of the 1,3-amino ketone.
Keywords
bidentate N,O-ligands; 1,3-amino ketones; Reduction potential; DFT
SYNOPSIS TOC
SYNOPSIS TEXT
The electrochemically irreversible reduction potential of a series of twelve amino-substituted β-
amino α,β-unsaturated ketones, relates linearly to various computational chemistry calculated
2

ectron withdrawi ability of each of the twelve amino
energies, thereby describing th eA Ce lC EPTED MAN Un gS CRIPT
substituents.
1
Introduction
β-amino α,β-unsaturated ketones are known as enaminones, since they contain the conjugated
system N-C=C-C=O. Enaminones are easily synthesized by reacting primary or secondary amines
with 1,3-diketone or 3-keto-ester [1]. These reactions result in three different tautomers, namely
enol, ketone and enol-amine [2], but occur predominantly in the carbonyl (keto) form [3].
However, some studies have discovered otherwise, such as the reaction between 2,4-pentanedione
and 2-methoxyethylamine, which resulted in the enol-amine tautomer instead [4]. Enaminones are
used in a number of organic reactions such as alkylation, acylation and Grignard reactions amongst
others. In coordination chemistry, these compounds are well known as N,O-bidentate ligands [5]
and are analogous to β-diketones [6] (with O,O donor atoms). Enaminones have been studied for
different properties and applications. They have been studied for their anticonvulsant activities
such as lipophilicity as well as steric, electronic and hydrogen-bonding effects [7]. Enaminones
react well with metals such as Cu(II) and Ni(II), forming volatile metal complexes which are
monomeric and isostructural in the solid state. When coordinated to platinum metal, these
complexes show significant photophysical properties [8]. Enaminones-metal complexes are mostly
used to prepare metal films, by the chemical vapour deposition (CVP) method. During preparation
of a metal film, enaminones (β-ketoimine) have been proven to decrease the substrate oxidation
process and to improve thermal stability of their complexes, as compared to complexes with β-
diketones [9]. Recent studies have been conducted on Ru(β-ketoiminato) complexes as potential
precursors for chemical vapour deposition, where fluorinated ketoimine ligands were used [10].
Many different enaminones have been synthesized in the past and characterized, also by solid state
crystal structure analyses, yet no electrochemical study on these enaminones could be found. In this
study we therefore present a combined synthetic, electrochemical and computational chemistry
study of five new and seven previously known amino substituted β-amino α,β-unsaturated ketones
(
bidentate N,O-ligands), of the type CH COCHC(NHR)CH , where R = Ph (1), H (2), CH Ph (3),
3 3 2
t
CH(CH ) (7), p- Bu-Ph (8), p-CF -Ph (10) (Series 1), and also of the type PhCOCHC(NHR)CH ,
3
2
3
3
where R = Ph (4), H (5), 2-CF -4-Cl-Ph (9), p-NO -Ph (11), 3,5-di-Cl-Ph (12), and also the ligand
3
2
PhCOCHC(NHPh)CF (6) (Series 2), as shown in Scheme 1.
3
3

ACCEPTED MANUSCRIPT
Scheme 1.
Amino substituted β-amino α,β-unsaturated ketones, 1 – 12 of this study.
Series 1:
t
CH COCHC(NHR)CH (with R = Ph, H, CH Ph, CH(CH ) , p- Bu-Ph, p-CF -Ph), as well as Series 2:
3
3
2
3 2
3
PhCOCHC(NHR)CH (with R = Ph, H, p-NO -Ph, 3,5-di-Cl-Ph, 2-CF -4-Cl-Ph) and also PhCOCHC(NHPh)CF .
3
2
3
3
2
Experimental
2.1 General
Melting points (m.p.) were determined, using an Olympus BX51 system microscope, assembled on
top of a Linkam THMS600 stage and connected to a Linkam TMS94 temperature programmer. The
1
13
H and C spectra of all the products were recorded, either on a 600 MHz Bruker AVANCE II
1
13
spectrometer (at 600.28 and 150.96 MHz respectively for H and C) or on a 300 MHz Bruker
FOURIER NMR spectrometer (at 300.18 MHz and 75.48 MHz respectively), at 25 °C. All the
samples were dissolved in deuterated chloroform. The chemical shifts were reported relative to
SiMe (0.00 ppm). Positive values indicate a downfield shift. UV/vis spectra were recorded on a
4
4

Varian Cary 50 Conc ultra-violet/visible spectrophotometer. FTIR measurements (solid samples)
ACCEPTED MANUSCRIPT
were determined, using a Bruker Tensor 27 IR spectrometer and Pike MIRacle ATR, running OPUS
software (Version 1.1).
2.2 Synthesis of twelve 1,3-amino ketones (1) – (12)
The bidentate N,O-ligands were synthesized, using literature methods as a guide [11], with slight
modifications. The bidentate N,O-ligands were characterized by NMR, IR, UV/vis and melting
points.
2
.2.1 (Z)-4-(phenylamino)pent-3-en-2-one (1)
Ligand 1 was synthesized by dispersing acetyl acetone (12 ml, 116.9 mmol) over Montmorillonite
K-10 (3 g) and slowly adding aniline (14.2 ml, 155.8 mmol) while stirring. The reaction mixture
was orange in colour. The reaction was left stirring overnight, at room temperature. The catalyst
was removed by filtration, and the product was extracted by washing with DCM (4ˣ20 ml) and
dried over MgSO . The solvent was removed on a rotatory evaporator, and then in vacuo to yield
4
orange crystals. Yield 63%. Colour: orange. M.p. 47.6 ºC (reported: 48-50 °C) [21,11]. UV: λmax
-1
3
-1
1
3
24 nm, εmax 40.03 mol dm cm (CH CN). H NMR (300 MHz, CDCl , 25°C): δ 12.39 ppm (s,
3
3
N-H, 1H); 7.28-7.01 ppm (m, aromatic, 5H); 5.12 ppm (s, C-H, 1H); 2.02 ppm (s, CH -CN, 3H);
3
1
3
1
.91 ppm (s, CH -CO, 3H). C NMR (300 MHz, CDCl , 25°C): δ 19.86, 29.19, 97.61, 124.78,
3
3
125.58, 129.10, 138.76, 160.28, 196.18.
2
.2.2 (Z)-4-aminopent-3-en-2-one (2)
Acetyl acetone (8.3 ml, 80.83 mmol) was dispersed over Montmorillonite K-10 (2 g) and ammonia
10 ml, 346.45 mmol) was added slowly while stirring. The reaction formed a white cloud during
(
the addition of ammonia; the white cloud disappeared after a while. The reaction was stirred at
room temperature for 5 hours. The catalyst was removed by filtration and the product was extracted
by washing with DCM (4ˣ20 ml) and dried over MgSO4. The solvent was removed on a rotatory
evaporator, and then under vacuum, yielding yellow crystals (6.4400 g, 64.97 mmol). : Yield 80%.
-
Colour: yellow. M.p. 31 ºC (reported: 30-35 °C) [2,11,21,12]. UV: λmax 291 nm, εmax 12.93 mol
1
3
-1
1
dm cm (CH CN). H NMR (300 MHz, CDCl , 25°C): δ 9.64 ppm (s, N-H, 1H); 4.97 ppm (s,
3
3
C-H, 1H); 1.97 ppm (s, CH -CN, 3H); 1.85 ppm (s, CH -CO, 3H).
3
3
5

2
.2.3 (Z)-4-(benzylamino)pent-3-en-2-one (3)
ACCEPTED MANUSCRIPT
Acetyl acetone (5 ml, 48.69 mmol) was dispersed over Montmorillonite K-10 (2 g) and benzyl
amine (7 ml, 64.09 mmol) was added slowly while stirring. The reaction mixture was light
brown/yellowish in colour. The reaction was stirred at room temperature for 24 hours. The catalyst
was removed by filtration and the product was extracted by washing with DCM (3ˣ20 ml) and dried
over MgSO . The solvent was removed on a rotatory evaporator, yielding 53%. Colour: army
4
-1
3
-1
1
green. Oil. UV: λmax 476 nm, εmax 112.56 mol dm cm (CH CN). H NMR (300 MHz, CDCl ,
3
3
25°C): δ 11.17 ppm (s, N-H, 1H); 7.36-7.23 ppm (m, aromatic, 5H); 5.05 ppm (s, C-H, 1H); 4.45
ppm (d, CH , 2H); 2.01 ppm (s, CH -CN, 3H); 1.88 ppm (s, CH -CO, 3H).
2
3
3
2
.2.4 (Z)-4-phenyl-4-(phenylamino)but-3-en-2-one (4)
Benzoylacetone (409 mg, 2.5217 mmol) was dispersed over Montmorillonite K-10 (2 g) and a
mixture of aniline (0.25 ml, 2.7435 mmol) and diethyl ether (4 ml) was added slowly while stirring.
The reaction was stirred at room temperature for 48 hours. After 24 hours, 5 – 10 ml of diethyl
ether was added. The reaction mixture was filtered to remove the catalyst. The filtrate was washed
with DCM (2ˣ20 ml) and lastly the organic layer was washed with saturated NaCl. The solvent was
removed on a rotatory evaporator, yielding yellow/brownish crystals (0.162 g, 0.6827 mmol). Yield
-1
3
-1
1
2
7%. Colour: brown. M.p. 109 ºC. UV: λmax 357 nm, εmax 35.65 mol dm cm (CH CN). H
3
NMR (300 MHz, CDCl , 25°C): δ 13.10 ppm (s, N-H, 1H); 7.88 ppm (d, aromatic, 2H); 7.42 –
3
7.30 ppm (m, aromatic, 5H); 7.17 – 7.12 ppm (m, aromatic, 3H); 5.86 ppm (s, C-H, 1H); 2.18 ppm
(
s, CH -CO, 3H).
3
2
.2.5 (Z)-4-amino-4-phenylbut-3-en-2-one (5)
Benzoylacetone (1.6214 g, 9.99 mmol) was dispersed over Montmorillonite K-10 (1.4010 g), and
ammonia (0.35 ml, 12.13 mmol) and chloroform (5 ml) were added slowly while stirring. The
reaction was stirred at room temperature for 24 hours. The reaction was filtered to remove the
catalyst. The filtrate was washed with DCM (2ˣ20 ml) and dried over MgSO . The solvent was
4
removed on rotatory evaporator and the product was dried in vacuo. Yield 38%. Colour: orange.
-1
3
-1
1
M.p. 55 ºC. UV: λmax 315 nm, εmax 13.08 mol dm cm (CH CN). H NMR (600 MHz, CDCl ,
3
3
25°C): δ 13.10 ppm (s, N-H, 1H); 7.85 – 7.82 ppm (m, aromatic, 2H); 7.48 – 7.46 ppm (m,
aromatic, 3H); 6.14 ppm (s, C-H, 1H); 2.16 ppm (s, CH -CO, 3H).
3
6

2
.2.6 (Z)-1,1,1-trifluoro-4-phenyl-4-(phenylamino)but-3 2-one (6)
ACCEPTED MANUSCR- eI Pn -T
Benzoyltrifluoroacetone (0.736 g, 3.4049 mmol) was dispersed over Montmorillonite K-10 (1.110
g), and a mixture of aniline (0.4 ml, 4.3896 mmol) and diethyl ether (10 ml) was added. The
mixture was stirred at room temperature for 24 hours. The reaction was filtered to remove the
catalyst. The filtrate was washed with DCM (2ˣ20 ml) and dried over MgSO . The solvent was
4
removed on a rotatory evaporator and the product was dried under vacuum, yielding orange crystals
-1
3
-1
(
0.442 g, 60 %). Colour: orange. M.p. 171 ºC. UV: λmax 325 nm, εmax 17890 mol dm cm
1
(
CH CN), H NMR NMR (600 MHz, CDCl , 25°C): δ 7.88 (d, R-Ph, 2H); 7.56 – 7.41 ppm (m,
3
3
2
2
C=H, R-Ph, 3H); 7.09 ppm (t, C=H, R -Ph, 2H); 6.77-6.63 ppm (m, R -Ph, 3H); 6.50 ppm (s, C-H,
H).
1
2
.2.7 (Z)-4-(isopropylamino)pent-3-en-2-one (7)
Acetyl acetone (4 ml, 22.3 mmol) was dispersed over Montmorillonite K-10 (~2 g), and isopropyl
amine (4 ml, 46.6 mmol) was added. The reaction mixture was stirred at room temperature for 5
hours. The reaction was filtered to remove the catalyst. The filtrate was washed with DCM (2ˣ20
ml) and dried over MgSO . The solvent was removed on a rotatory evaporator. Yield 76%.
4
1
Colour: dark brown oil. H NMR (600 MHz, CDCl , 25°C): δ 10.74 ppm (b, N-H, 1H); 4.85 ppm
3
(
s, C-H, 1H); 3.65 ppm (m, C-H, 1H); 1.91 ppm (s, CH -CN, 3H); 1.88 ppm (s, CH -CO, 3H);
3
3
1
.17-1.15 (d, CH 6H).
3,
2
.2.8 (Z)-4-((4-tert-butyl)phenyl)amino)pent-3-en-2-one (8)
Acetyl acetone (0.5 ml, 2.782 mmol) was dispersed over Montmorillonite K-10 (~0.5-1 g), 4-tert-
butyl aniline (1 ml, 6.279 mmol) and diethyl ether (5 ml) were added. The reaction mixture was
stirred at room temperature for 24 hours. The reaction was filtered to remove the catalyst. The
solvent was removed on a rotatory evaporator and the product was dried in vacuo. Yield 65%.
-1
3
-1
1
Colour: brown. M.p. 59 ºC. UV: λmax 326 nm, εmax 17.11 mol dm cm (CH CN). H NMR (600
3
MHz, CDCl , 25°C): δ 12.44 ppm (b, N-H, 1H); 7.38-7.35 ppm (d, aromatic, 2H); 7.07-7.04 ppm
3
(
d, aromatic, 2H); 5.19 ppm (s, C-H, 1H); 2.10 ppm (s, CH -CN, 3H); 2.01 ppm (s, CH -CO, 3H),
3
3
1
.38 (s, CH 9H).
3,
2
.2.9 (Z)-4-((4-chloro-2-(trifluoromethyl)phenyl)amino)-4-phenylbut-3-en-2-one (9)
Benzoyl acetone (0.549 g, 3.385 mmol) was dispersed over Montmorillonite K-10 catalyst (1.5g)
and a mixture of 4-chloro-2-trifluoromethylaniline (0.6 ml, 4.252 mmol) with 20 ml THF was
7

added dropwise, while stirring.
The reaction was stir room temperature for 48 hours. The
ACCEPTED MAN Ur e dS Ca tR IPT
catalyst was removed by filtration and the solvent was removed on rotary evaporator to yield red
-1
3
-1
1
oil. Yield 84%. Colour: red. UV: λmax 312 nm, εmax 19502 mol dm cm (CH CN), H NMR (300
3
MHz, CDCl , 25°C): δ 8.02-7.99 ppm (d, aromatic, 2H); 7.82-7.80 ppm (d, aromatic, 2H); 7.48-
3
7.36 ppm (m, aromatic, 4H); 6.57-6.54 ppm (d, aromatic, 2H); 6.11 ppm (s, CH, 1H); 2.14 ppm (s,
CH , 3H).
3
2
.2.10 (Z)-4-((4-trifluoromethyl)phenyl)amino)pent-3-en-2-one (10)
Acetyl acetone (0.5 ml, 2.782 mmol) was dispersed over Montmorillonite K-10 (~0.5-1 g), 4-
trifluoromethyl aniline (0.7 ml, 5.574 mmol) and diethyl ether (5 ml) were added and the reaction
mixture was stirred at room temperature for 6 hours. The reaction was filtered to remove the
catalyst. The solvent was removed on a rotatory evaporator and the product was dried in vacuo,
yielding orange crystals. Yield 85%. Colour: orange. M.p. 68 ºC. UV: λmax 333 nm, εmax 19.83
-1
3
-1
1
mol dm cm (CH CN). H NMR (300 MHz, CDCl , 25°C): δ 10.74 ppm (b, N-H, 1H); 4.85 ppm
3
3
(
s, C-H, 1H); 3.65 ppm (m, C-H, 1H); 1.91 ppm (s, CH -CN, 3H); 1,88 ppm (s, CH -CO, 3H);
3
3
1
.17-1.15 (d, CH 6H).
3,
2
.2.11 (Z)-4-((4-nitrophenyl)amino)-4-phenylbut-3-en-2-one (11)
Benzoyl acetone (0.546 g, 3.4 mmol) was dispersed over Montmorillonite K-10 catalyst (1.5g) and
a mixture of 4-nitroaniline (0.552 g, 4 mmol) with 20 ml THF was added dropwise, while stirring.
The reaction was stirred at room temperature for 48 hours. The catalyst was removed by filtration
and the solvent was removed on a rotary evaporator, yielding a yellow powder. (0.459 g). Yield
-1
3
-1
1
8
4%. Colour: yellow. M.p. 115 ºC. UV: λmax 396 nm, εmax 21707 mol dm cm (CH CN). H
3
NMR (300 MHz, CDCl , 25°C): δ 8.21-6.56 ppm (m, aromatic, 9H); 6.13 ppm (s, CH, 1H); 2.16
3
ppm (s, CH , 3H).
3
2
.2.12 (Z)-4-((3,5-dichlorophenyl)amino)-4-phenylbut-3-en-2-one (12)
Benzoyl acetone (0.165 g, 1.017 mmol) was dispersed over Montmorillonite K-10 catalyst (1.5g)
and a mixture of 3.5-dichloroaniline (0.552 g, 3.407 mmol) with 20 ml THF was added dropwise,
while stirring. The reaction was stirred at room temperature for 48 hours. The catalyst was
removed by filtration and the solvent was removed on a rotary evaporator, yielding a dark red oil.
-1
3
-1
1
Yield 93%. Colour: dark red. UV: λmax 359 nm, εmax 29453 mol dm cm (CH CN), H NMR (300
3
MHz, CDCl , 25°C): δ 7.91-7.89 ppm (d, aromatic, 2H); 7.54-7.44 ppm (m, aromatic, 3H); 6.75-
3
8

6
.74 ppm (t, aromatic, 1H); 6.56
H).
ppm (d, aromatic, 2H); 6.20 m (s, CH, 1H); 2.22 ppm (s, CH3,
ACCEPTED MANUSCRI Pp pT
3
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 298 K and the temperature was kept constant to within 0.5 K. Successive experiments
under the same experimental conditions showed that all reduction potentials were reproducible
-3
within 0.010 V. Cyclic voltammetric (CV) measurements were performed on ca. 2 mmol dm (or
saturated) solutions of each of the twelve complexes, dissolved in CH CN as solvent, containing
3
-3
1
00 mmol dm tetrabutylammonium hexafluorophosphate (TBAPF , [NBu ][PF ]) as supporting
6 4 6
electrolyte. Measurements were conducted under a blanket of purified Argon. A three-electrode
cell was used, consisting of a Pt auxiliary electrode, a glassy carbon working electrode (surface
2
area 0.0707 cm ) and a Pt reference electrode [13,14]. The working electrode was polished with 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 CH CN, and dried before each experiment. Scan rates varied between
2
3
-1
+
0
.050 and 1.000 V.s . All experimental potentials were referenced against the FcH/FcH couple
IUPAC [15]). Under our experimental conditions, ferrocene (FcH) exhibited a peak separation of
E = E – E = 0.074 V and i /i = 1.00; where E (or E ) is the anodic (or cathodic) peak
(
∆
p
pa
pc
pc pa
pa
pc
potential, and i (or i ) is the anodic (or cathodic) peak current.
pa
pc
2.4 DFT calculations
Density functional theory (DFT) calculations were performed via the B3LYP (and UB3LYP for
spin unrestricted calculations) functional, as implemented in the Gaussian 09 package [16], using
the triple-ζ basis set 6-311G(d,p). Geometry relaxed (adiabatic) energies of the neutral (N) electron
system were calculated, and also of the corresponding N−1 (reduced) and N+1 (oxidized) electron
systems, in order to determine the following values for each of the complexes: Electron affinity
(EA), ionization potential (IP) and Mulliken electronegativity (χ)by application of the following
formulas [17,18]:
EA(complex) = E(reduced complex) − E(complex)
IP(complex) = E(oxidized complex) − E(complex)
9

χ = (IP + EA)/2
ACCEPTED MANUSCRIPT
The global electrophilicity index (ω), defined by Parr and coworkers as the electrophilic power of
atoms and molecules [19], was calculated by application of the following formula:
2
ω = ꢀ /(2 η)
where ꢀ = −(IP + EA)/2 is the electronic chemical potential (the same as −χ), and η = (IP − EA)
is the chemical hardness of the ground state of atoms and molecules.
3
Results and Discussion
3.1 Synthesis
β-amino α,β-unsaturated ketones are generally prepared by the condensation of β-diketones with
amines, either directly under reflux [4], or by using a catalyst such as an acidic catalyst [20], or
otherwise by using a solid support-catalyst, e.g. silica gel [21], alumina or montmorillonite K-10
[
11]. For this study, a simple and convenient method, slightly modified from the above, was used:
namely by dispersing the relevant primary amines and 1,3-diketones on the solid montmorillonite
K-10 (catalyst) with a solvent. The solvent was added to ensure the reaction mixture was in
solution, especially in those cases where the starting materials are in the solid phase. Scheme 2
indicates the general synthetic route for the preparation of these twelve ligands. Two series of
1
2
ligands were prepared in this study, as shown in Scheme 1. In Series 1, both groups R and R are
1
2
CH groups, while in Series 2, group R is either CH or CF and group R is Ph. The starting
3
3
3
material for synthesizing the six ligands of Series 1 is acetyl acetone, while benzoyl acetone was
used for Series 2 instead, or trifluorobenzoylacetone (for ligand 6). Not only is this method easy
and convenient, but it also results in good yields of up to 93%, without any side products. Another
advantage of this method is the use of less energy than some of the reported methods [4], since most
of these reactions are conducted at room temperature instead of under reflux [4]. The products were
purified either by recrystallization or by sublimation. Reactions with acetyl acetone (Series 1)
produced relatively higher yields than reactions with benzoyl acetone (Series 2).
10

Scheme 2. Synthetic route for the twe
A
lv
C
e a
C
m
E
in
P
o
T
su
E
b
D
stit
M
ute
A
d
N
β-
U
am
S
i
C
no
R
α
I
,β-unsaturated ketones. The solvent used
P
T
in each case is determined by the solubility of the specific reactants.
It is known that the related O,O bidentate ligands, namely β-diketones, can exist in three tautomeric
forms, namely a keto form and two different enol forms [22]. Similarly, these 1,3-amino ketones
from this study can also exist in both the enol and keto forms. The various possible isomers for the
two series of amino substituted β-amino α,β-unsaturated ketones of this study, are illustrated in
Scheme 3.
Scheme 3. Different possible keto and enol isomers for the twelve amino substituted β-amino α,β-unsaturated
ketones of this study.
1
The H NMR resonance of the methine proton (=C-H signal) was found at 4.85 to 5.85 ppm, for the
six ligands of Series 1, while the =C-H signal of Series 2 shifted to lower field at 5.92 – 6.20 ppm.
1
This shift agrees quantitatively with the corresponding H NMR shift for the enol form of the
1
related O,O bidentate β-diketones: where the H NMR shifts of the methine proton are 5.50 and
6.19 ppm for acetyl acetone and benzoyl acetone respectively, which are the starting materials of
1
the related β-diketones. The H NMR signal of the methine proton of ligands 1 – 12 of this study,
was integrated for one proton relative to the other signals of the ligand, indicating that these
1
compounds exist in solution as the enol tautomer (see Scheme 3). The proton H NMR spectra of
ligands 1 – 12 also showed a broad signal at 10 – 13 ppm, consistent with a proton bonded to a
1
3
hetero atom, which again confirms the enol form. Further, the C NMR spectrum of ligand 1
11

indicates the presence of a C=O signal at 196.18 ppm, a a C-N
ACCEPTED MAN Un dS CRIP Ts ignal at 160.23 ppm respectively,
which is also consistent with the enol-NH form (Scheme 3).
-1
The IR spectra of all twelve ligands show a wide strong band between 3000 – 3300 cm , which is
associated with valence ν(N-H) vibrations [23], which only occur in the enol isomer.
Available crystal structures of the ligands of this study, as well as structures related to the ligands of
this study, were all found to be the enol-NH isomer (also containing a carbonyl functional group,
see Scheme 3), while no enol-OH (without carbonyl) or keto structures could be found [24].
Furthermore, the Ph group on the ligands of Series 2 [25], or structures related to the ligands of
Series 2 [26], were all found to have the Ph group orientated on the same side as the carbonyl group
in the isomer, proving them all to be of the type Enol-NH(2), as shown in Scheme 3.
3.2 DFT results
3
.2.1 Geometry and energy
Solid state crystal structure data is available for Ligands 1 (two crystals), 2 (six crystals) and 5
three crystals) [24,25]. Selected bond lengths and angles of the density functional theory (DFT)
(
optimized geometries of Ligands 1, 2 and 5, are compared with the experimental X-ray data in
Table 1. Good agreement between the optimized structural parameters and the experimental crystal
X-ray structure values were obtained. Gas phase DFT optimizations often lead to a slight over-
estimation of bond lengths [27], therefore the small differences obtained here between
experimentally measured and calculated bond lengths and angles, are not significant. In ligand 1
for instance, the reported crystal X-ray structure bond distances for the C2=O1 and N1-C4 bonds in
the two crystals, are 1.244-1.245 Å and 1.352 Å respectively [28,29]. The corresponding values for
optimized (calculated) bond lengths, are 1.244 and 1.357 Å respectively. Consequently, the
insignificant difference of no more than 0.001 Å for C2=O1 and 0.005 Å for N1-C4, is negligible.
Similarly, the experimentally measured and calculated bond lengths of the C2-C3 bond, of the two
experimental crystals for ligand 1, show a difference of no more than 0.016 Å, while the difference
in the C3=C4 bond lengths is 0.018 Å. The latter difference between experimental and calculated
bond lengths of 0.018 Å, is slightly more than the difference in the C3=C4 bond lengths of the two
experimental structures, namely 0.014 Å. Similar to the bond lengths, the indicated experimental
bond angles were also reproduced by the DFT calculations, and were found to be within 1° of the
12

experimental values, again comparable e difference of 0.5° in the experimental bond angles of
ACC Et oP Tt hED MANUSCRIPT
the two structures. In ligand 2, the maximum bond length and angle difference between
experimental and calculated bonds and angles of the enamino-ketone backbone, was found to be
0.05 Å and 2° respectively, for all six experimental crystals listed in Table 1 for ligand 2. These
differences are comparable with the differences between the bond lengths and angles of the six
different experimental values themselves, namely 0.03 Å and 2° respectively. The DFT method
used, therefore is in good agreement with these experimentally obtained structural parameters.
Table 1. Selected bond lengths (Å) and bond angles (°) for ligands 1, 2 and 5, obtained both from experimental
crystallographic data of various crystals, as well as from theoretical DFT calculations.
Bond lengths (Å)
Bond angles (°)
CSD ref. code
Ligand 1
SEHGOC
SEHGOC01
DFT
C2=O1
C2-C3
C3=C4
N1-C4
N1…..O1 O1-C2-C3 C2-C3-C4 N1-C4-C3
1.244(3) 1.424(4) 1.364(4)
1.245(1) 1.427(2) 1.378(2)
1.244
1.352(3) 2.658(3)
1.352(1) 2.650(1)
1.357
123.3(2)
123.6(1)
123.384
124.8(2)
124.3(1)
123.757
120.2(2)
119.71(9)
119.757
1.440
1.382
2.643
Ligand 2
KUBWIO
1.259(2) 1.412(2) 1.385(2)
.254(2) 1.411(2) 1.385(2)
1.253(2) 1.418(3) 1.379(3)
.261(2) 1.406(3) 1.383(3)
1.247(9) 1.414(8) 1.37(1)
1.327(2) 2.679(2)
1.325(2) 2.690(2)
1.321(2) 2.684(2)
1.325(2) 2.694(2)
1.332(9) 2.673(7)
123.2(1)
123.6(1)
122.8(2)
123.2(2)
122.3(6)
124.2(7)
123.413
123.7(1)
123.6(1)
124.1(2)
124.4(2)
123.9(6)
124.9(7)
122.886
122.5(1)
122.9(1)
122.6(2)
122.5(2)
123.5(6)
120.6(7)
122.238
1
KUBWIO01
DUFBEL
1
1
.24(1)
1.39(1)
1.444
1.38(1)
1.375
1.30(1)
1.348
2.669(8)
2.669
DFT
1.239
Ligand 5
PICGAJ
PICGAJ01
PICGAJ02
DFT
1.255(6) 1.406(6) 1.384(6)
1.254(2) 1.407(2) 1.375(2)
1.258(1) 1.411(1) 1.387(1)
1.324(6) 2.711(5)
1.318(2) 2.710(2)
1.320(1) 2.711(1)
123.3(4)
122.9(2)
123.50(9)
122.466
124.6(4)
125.0(2)
123.99(9)
122.653
122.5(4)
122.8(2)
123.0(1)
122.527
1.245
1.442
1.378
1.346
2.648
To determine whether the ADF method (Amsterdam Density Functional program) used, produces
appropriate energy values, the relative energies are listed in Table 2, as obtained by DFT
calculations for the various possible isomers of the twelve amino substituted β-amino α,β-
unsaturated ketones of this study. The results clearly show that the enol-NH isomers always have
the lowest energy, and therefore are the favoured configuration, for both Series 1 and Series 2
ligands. This result agrees with experimental observation, as described in the last paragraph of
section 3.1, hereby also confirming that for ligands in Series 2, the preferred configuration is of the
type Enol-NH(2), where the Ph group is orientated on the same side as the carbonyl group.
13

ACCEPTED MANUSCRIPT
Table 2. Relative energies (in eV) for the various possible isomers of the twelve amino substituted β-amino α,β-
unsaturated ketones. The geometry of the different isomers is given in Scheme 3. All the energies listed are
relative to the favoured enol-NH isomer in the last column, indicated by 0 eV.
Series 1
Enol- Keto Enol- Series 2 Enol-
Enol-
OH(2)
Keto(1)
Keto(2)
Enol- Enol-
NH(1) NH(2)
OH
NH
OH(1)
L1
L2
L3
L7
L8
L10
0.97
1.13
1.03
1.13
0.99
0.91
0.43 0.00
0.53 0.00
0.53 0.01
L4
L5
L6
L9
L11
L12
1.09
1.00
0.99
0.90
0.98
0.99
0.98
0.97
0.91
0.82
0.88
0.89
0.63
0.68
0.68
0.53
0.58
0.58
0.42
0.55
0.48
0.35
0.38
0.37
0.08
0.04
0.00
0.02
0.06
0.07
0.00
0.00
0.01
0.00
0.00
0.00
0.5
0.00
0.43 0.00
0.41 0.00
3
.2.2 Electron distribution
–
The reduction of a molecule involves the addition of an e to the lowest unoccupied molecular
orbital (LUMO) of the molecule. Knowledge of the character of this LUMO provides an
understanding of the location of the reduction centre of the molecule, where the electron is being
added to. The distribution of this added electron, over the radical anion which is formed upon
reduction, can be visualized by the spin density plot of the anion, as determined by DFT. The
LUMOs of the lowest energy isomer (namely the enol-NH isomer) of each of the twelve amino
substituted β-amino α,β-unsaturated ketones of this study, are shown in Figure 1, while spin
density plots of the respective anions of the twelve amino substituted β-amino α,β-unsaturated
ketones (also the enol-NH isomers), are shown in Figure 2. In each case, the LUMOs are
distributed over the entire enamino-ketone (backbone) fragment for all these ligands, extending
further over the phenyl rings of the phenyl-containing ligands, except for ligand 3 where the phenyl
group is not directly connected to the enamino-ketone backbone C(N)=C-C(O).
14

ACCEPTED MANUSCRIPT
Figure 1. B3LYP DFT calculated LUMOs of the lowest energy isomer (enol-NH/enol-NH(2) isomer from Table
3
2
) of all twelve amino substituted β-amino α,β-unsaturated ketones, 1 - 12. A contour of 0.06 e/Å was used for
the MO plots. See Scheme 1 for the structure of the ligands. Colour code of the atoms (online version): F (light
blue), Cl (green), N (blue), C (grey), O (red), H (white).
1
2
The spin density distribution is indicated by the ratio [R : R : enamino-ketone-fragment : R ], as
shown in Figure 2. When only aliphatic groups are attached to the pseudo-aromatic enamino-
ketone fragment (ligands 2, 3 and 7), the spin density on the enamino-ketone fragment is as much as
70 – 90 % of the unpaired electron density. As soon as a phenyl or substituted phenyl group is
2
attached to this pseudo-aromatic enamino-ketone fragment, either in the R or the R position
(Scheme 2), the spin density on this pseudo-aromatic enamino-ketone fragment decreases to ca. 60
%, while rising to about 20% on the relatively electron withdrawing phenyl rings. Exceptions are
t
ligands 8 and 11, containing the electron donating Bu and electron withdrawing NO groups
2
t
respectively. The electron donating Bu group (on ligand 8) leads to an increase in electron density
on the pseudo-aromatic enamino-ketone fragment up to 70%, while the electron withdrawing NO2
group (on ligand 11) leads to a decrease in electron density on the pseudo-aromatic enamino-ketone
fragment down to 34%. This shift in spin density distribution upon varying the substituents,
1
2
indicates that good communication exists between the different R, R and R groups and the
enamino-ketone-fragment.
15

ACCEPTED MANUSCRIPT
Figure 2. B3LYP DFT calculated spin plots of the lowest energy isomer (enol-NH/enol-NH(2) isomer from Table
2
) of each of the anions of the twelve N,O-ligands 1 - 12. See Scheme 1 for the structure of the ligands. A
3
contour of 0.006 e/Å was used for the spin plots. Colour code of the atoms (online version): F (light blue), Cl
(
green), N (blue), C (grey), O (red), H (white).
Since ligand 11 exhibits two electro-active centres, namely the enamino-ketone-backbone as well as
the strongly electron-withdrawing NO group [30] on the ligand, the di-anion of ligand 11 (after the
2
second reduction) was also optimized. This di-anion of 11 was found to be an open shell singlet,
0.16 eV and 0.09 eV lower in energy than the closed shell singlet and the triplet respectively. The
LUMO of ligand 11 before reduction (Figure 1) is 43% on the NO group and only 20% on the
2
enamino-ketone-backbone, indicating that the first reduction site of ligand 11 will be on the NO2
group. This is reversed for the anion of 11 (after the first reduction), where the LUMO instead is
mainly on the enamino-ketone-backbone (33% versus only 17% on the NO group, see Figure 3),
2
suggesting that the locus of a second reduction of ligand 11 will be on the enamino-ketone-
backbone of 11 instead. The locus of the two added electrons of the di-anion of 11 (after the
second reduction) is shown in the spin density plot in Figure 3: one electron is on the NO and the
2
other electron is on the enamino-ketone-backbone of ligand 11. In addition, the HOMO and the
16

HOMO-1 of the di-anion of 11 show the locus of each electr which is added upon the second and
ACCEPTED MANUSC Ro nI PT
the first reduction respectively, see Figure 3.
Figure 3. Selected B3LYP/6-311G(d,p) calculated frontier orbitals and spin plot of the anion (after first
reduction) and di-anion (after second reduction) of ligand 11, containing the electron-withdrawing NO group.
2
3
A contour of 0.06 and 0.006 e/Å was used for the MOs and spin plot respectively. Colour code of the atoms
(
online version): N (blue), C (grey), O (red), H (white).
3.3 CV results
The cyclic voltammograms (CVs) of the twelve amino substituted β-amino α,β-unsaturated
-1
ketones, 1 – 12, obtained at one specific scan rate of 0.100 V s , are all shown on one graph in
Figure 4, with the corresponding CV data summarized in Table 3. Almost all of these amino
substituted β-amino α,β-unsaturated ketones (Ligands 1 – 12) indicated only one reduction process
on their CV graphs, except for ligand 11 (the only ligand containing the electron-withdrawing NO2
substituent), which showed two reduction processes; as can be seen in the top graph of Figure 4.
The first reduction process of the eleven ligands, 1 ─ 10 and 12, is chemically irreversible in each
case. Some ligands however do display some electrochemically reversible redox behaviour, such as
ligand 4 (with two non-substituted phenyl rings), with peak current separation ∆E = 0.064 V, at a
17

-1
scan rate of 0.10 V s , see Figure 5. This re-oxidation pe becomes increasingly visible as the
ACCEPTED MANUSC Ra kIPT
scan rate is increased. The radical anion which forms upon reduction of ligand 4, is therefore stable
enough on the timescale of the cyclic voltammogram (CV), to be re-oxidized. From the spin
2
density plot (Figure 2) of ligand 4, which contains two phenyl groups (on the R and R position),
without any electron withdrawing or donating groups attached to these phenyl groups, it can be seen
that the unpaired electron of the reduced anion of ligand 4 is stabilized by π-conjugation throughout
the ligand, thereby stabilizing it on the time-scale of the CV. The same stabilization also applies to
ligand 6 (which is similar to 4 but with an additional CF group), which also shows
3
electrochemically reversible redox behaviour, with peak current separation ∆E = 0.070 V, at a scan
-1
2
rate of 0.10 V s . Ligand 6 also contains two unsubstituted phenyl groups on the R and R
positions, similar to ligand 4. For ligands 1, 3, 7, 9 and 12 however, a small re-oxidation peak is
-1
only observed at the highest scan rate of 5 V s , as is shown in Figure 6 for ligand 12 as an
example.
Ligand 11 is the only example undergoing two reduction processes, due to the presence of the NO2
substituent: The first (less negative) reduction process of ligand 11 (with E = -1.511 V) is
pc
reversible if the scan direction is reversed just after the first reduction peak, before the second
reduction takes place (Figure 4 top graph). This first reduction wave of ligand 11, is associated
with the reduction of the NO group, which is substituted on this ligand. This assignment of the
2
first reduction is in agreement with previous studies, which also found that the NO groups contain a
2
+
reversible redox couple between -1.0 V to -1.7 V vs FcH/FcH [30]. The DFT study of ligand 11
presented in Section 3.2.2, showed that the locus of the first reduction of ligand 11 is on the NO2
group, while the second reduction is on the enamino-ketone-backbone (NCCCO) of ligand 11. If
the scan direction of the CV of ligand 11 is reversed only after the second NCCCO-based reduction
peak (with E = -1.974 V), then the second peak is reversible, while the first reduction peak now
pc
becomes irreversible at low scan rates. However, the re-oxidation peak of the first reduction
process becomes increasingly visible with increasing scan rate, see Figure 6. The reversible
NCCCO-based reduction of ligand 11 is similar to that observed for ligands 4 (Figure 4) and 6,
which also contain two phenyl groups. The irreversibility of the first reduction peak at low scan
rates (when the scan direction is reversed after the second reduction peak only) implies that the di-
anionic species which forms after the second reduction, is not stable on the timescale of the CV at
low scan rates, but stable enough to be observed at higher scan rates (Figure 6).
18

ACCEPTED MANUSCRIPT
+
Figure 4. Cyclic voltammograms (versus FcH/FcH ) of all twelve amino substituted β-amino α,β-unsaturated
-1
-3
ketones, 1 – 12, at only one selected scan rate of 0.100 V s . Concentrations were normalized to 0.002 mol dm .
-
3
Scans were initiated from -1.0 V in the negative direction. The CVs were measured in 0.100 mol dm
TBAPF /CH CN, on a glassy carbon working electrode at 25 °C. Note the two reduction peaks in the top graph,
6
3
for ligand 11 (with the NO substituent). All other ligands display only one reduction peak.
2
+
-3
Figure 5. Cyclic voltammograms (versus FcH/FcH ) of a 0.003 mol dm solution of one of the amino substituted
β-amino α,β-unsaturated ketones (ligand 4), measured at various scan rates, namely 0.05 (smallest peak
19

-
1
current), 0.10, 0.15, 0.20, 0.25, 0.30, 0.
A
40
C
an
C
d
E
0
P
.5
T
(l
E
ar
D
ges
M
t p
A
ea
N
k
U
cu
S
rr
C
en
RI
PTs . Scans were initiated from -1.0 V in
t)
V
-3
the negative direction. The CVs were measured in 0.100 mol dm TBAPF /CH CN, on a glassy carbon working
6
3
electrode at 25 °C. Ligand 4 (with two unsubstituted phenyl groups) displays some electrochemically reversible
redox behaviour. Note how the re-oxidation peak becomes increasingly visible with increasing scan rate.
+
Figure 6. Cyclic voltammograms (versus FcH/FcH ) of a saturated solution of one of the amino substituted β-
amino α,β-unsaturated ketones (ligand 12), measured at various scan rates of 0.05 (smallest peak current), 0.10,
-1
0
.15, 0.20, 0.25, 0.30, 0.40, 0.5. 1.0 and 5.0 (largest peak current) V s . Scans were initiated from -1.0 V in the
-3
negative direction. The CVs were measured in 0.100 mol dm TBAPF /CH CN, on a glassy carbon working
6
3
-1
electrode at 25 °C. A small re-oxidation peak is only observed at the highest scan rate of 5 V s , indicating some
electrochemically reversible redox behaviour.
20

ACCEPTED MANUSCRIPT
+
-3
Figure 7. Cyclic voltammograms (versus FcH/FcH ) of a 0.002 mol dm solution of one of the amino substituted
β-amino α,β-unsaturated ketones (ligand 11), measured at various scan rates of 0.05 (smallest peak current),
-1
0
.10, 0.15, 0.30, 0.5 and 1.0 (largest peak current) V s . Scans were initiated from -1.0 V in the negative
-
3
direction. The CVs were measured in 0.100 mol dm TBAPF /CH CN, on a glassy carbon working electrode at
6
3
2
5 °C. Note how the re-oxidation peak of the first reduction peak becomes increasingly visible with increasing
scan rate.
1
2
Different substituents R, R and R (Scheme 2) have a different effect on the reduction of the β-
1
2
amino α,β-unsaturated ketones. The reduction potential of Series 1, with R =R =CH , is lower
3
1
2
(
more negative) than that of Series 2 containing a phenyl group, R =CH or CF and R =Ph. In
3 3
Series 1, the ligands with a phenyl group attached to the amino nitrogen (R=Ph), namely ligands 1,
and 10, have a higher reduction potential than the ligands with an alkyl group attached to the
amino nitrogen. In the DFT section it was shown that the LUMO of the phenyl-containing ligands
Series 2) are extended from the enamino-ketone (backbone) fragment to the phenyl rings, thereby
8
(
stabilizing the LUMO through π-conjugation. This implies that an electron can more easily be
added at a higher reduction potential to the lower energy LUMO. Figure 8 gives a schematic
presentation of the influence of phenyl groups attached to ligands 1 – 12, on the reduction potential
of such ligands: The experimental reduction potential of ligands containing a phenyl group directly
attached to the carbonyl group (Series 2), leads to higher reduction potentials (due to stabilization
through π-conjugation via the unsaturated bonds), than if the phenyl is attached to the amino
nitrogen only, without π-conjugation. In Series 1, for ligand 10, containing the strongly electron
withdrawing CF group on position R (R = p-CF -Ph), the reduction potential is higher (-2.526 V vs
3
3
21

+
t
FcH/FcH ) than that of ligand 8,
A
co
C
n
C
ta
E
in
P
in
T
g
E
a
D
n e
M
le
A
ct
N
ro
U
n
S
do
C
n
R
at
I
i
P
ng
T
Bu group on position R (R = p-
t
+
Bu-Ph), with a reduction potential of -2.726 V vs FcH/FcH . Similarly, the reduction potential of
ligands in Series 2 increases (becomes less negative) as the R group becomes more electron
withdrawing: phenyl < 3,5-dichlorophenyl < 4-chloro-2-(trifluoromethyl)phenyl < 4-nitrophenyl.
Figure 8. Schematic representation of the influence of phenyl groups attached to the amino substituted β-amino
α,β-unsaturated ketones, 1 – 12, on the reduction potential of those ligands. X = alkyl group, R = H or one of the
substituted phenyl groups.
3.4 Combining DFT, CV and electronic parameters
A summary of all the calculated, as well as experimentally obtained parameters, is shown in Table
. Figure 9 illustrates the relationships found between the reduction potential E and both the
3
pc
energy of the LUMO, ELUMO (Figure 9 a), and electron affinity, EA (Figure 9 b), for all twelve
amino substituted β-amino α,β-unsaturated ketones 1 – 12 from this study. It is important to obtain
a good linear relation between experimental and calculated results, since this can be used to predict
the reduction potential of further amino substituted β-amino α,β-unsaturated ketones.
The correlation between E and ELUMO, the orbital that the electron is added to upon reduction, is
pc
2
R = 0.95, and correlation between E and EA (associated with the amount of energy released
pc
2
during the reduction process) is R = 0.92, see Figure 9 a and b. Figure 9 also shows the
relationship between E and the DFT calculated electrophilicity index ω (Figure 9 c), as well as the
pc
22

DFT calculated Mulliken electro
electrons) (Figure 9 d) of the ligand, which both follow a similar trend as with EA. To test the
application of the relationship obtained between E and E for these kind of ligands in general,
negativities χ (indicati e ability of the ligand to attract
ACCEPTED MANU So nC Ro fI Pt hT
pc
LUMO
we also included the data obtained for the similar O,O-ligand acetylacetone (Hacac) to Figure 9 a,
indicated by a red cross on the graph. This data for ligand Hacac also fits the trend perfectly.
a)
b)
c)
d)
+
Figure 9. The relationships found between the reduction potentials E (V versus FcH/FcH ) of the twelve amino
pc
substituted β-amino α,β-unsaturated ketones, 1 – 12, and various energies calculated by computational
chemistry, namely: (a) ELUMO, (b) electron affinity (EA), (c) electrophilicity index (ω), and (d) Mulliken group
electronegativities (χ). Data is summarized in Table 3. The additional data obtained for ligand acetylacetone
(
Hacac) is indicated by a red cross in (a).
+
Table 3: Data for the experimental reduction potentials (E vs. FcH/FcH , i normalized for 0.002 M solutions),
pc
pc
calculated LUMO energies (ELUMO), electron affinities (EA), electrophilicity indices (ω) and group Mulliken
electronegativities (χ) for the twelve amino substituted β-amino α,β-unsaturated ketones 1 – 12, as well as for
ligand acetylacetone (Hacac).
Ligand
Epc vs.
ipc
ELUMO
Electron
electrophilicity Mulliken
23

+
FcH/FcH
(µA)
A
/
C
e VC EPTE Da ff
M
ini
A
tie
N
s
USC Ri n
I
d
P
e
T
x
electronegativities
(χ) / eV
3.574
3.577
3.456
3.769
3.841
4.249
3.368
3.455
4.117
3.989
4.472
4.140
4.164
/
V
(EA) / eV
-0.054
-0.717
-0.481
0.480
0.061
1.090
-0.666
-0.053
0.824
(ω) / eV
0.880
0.745
0.758
1.080
0.976
1.429
0.703
0.851
1.287
1.122
1.620
1.315
0.943
L1
L2
L3
L4
L5
L6
L7
L8
-2.673
-3.017
-2.976
-2.370
-2.156
-1.826
-3.094
-2.726
-2.105
-2.526
-1.511
-2.212
-2.575
23.0 -1.309
22.3 -0.747
22.4 -0.883
22.5 -1.688
22.2 -1.437
22.2 -2.256
22.0 -0.762
22.1 -1.337
22.3 -2.125
22.6 -1.690
22.4 -2.653
22.0 -2.051
L9
L10
L11
L12
Hacac
0.443
1.385
0.881
-0.432
-
-1.340
4
Conclusions
Experimental NMR, IR and solid state crystal data all prove that the ligands 1 – 12 preferably exist
in the enol-NH form (which also contains a carbonyl group), and that the Ph group substituted on
the six ligands of Series 2, is orientated on the same side as this carbonyl group. DFT calculations,
comparing the energies of the different enol- and keto-isomers of the ligands, agree with these
experimental findings, proving the enol-NH group to be the most stable with the lowest energy.
The DFT calculations further showed that the LUMO of these ligands is distributed over the
enamino-ketone fragment (backbone), extending further over the phenyl rings of the phenyl-
containing ligands. This implies that upon reduction of the ligand, the extra electron added to the
LUMO, will also be distributed over the enamino-ketone fragment (backbone), also extending
further over the phenyl rings of the phenyl-containing ligands, thereby forming an anion radical;
except in the case of the first reduction of ligand 11, which is NO based. This electron distribution
2
is confirmed by the spin density plots of the reduced anions. Experimentally, these reduced ligands
were found to be unstable, leading to irreversible electrochemical behaviour at low scan rates for
most of these ligands, except for ligands 4, 6 and 11, which instead showed reversible
electrochemical behaviour at higher rates. The experimentally measured reduction potential of each
of the twelve enamino-ketone fragments, depends on the respective substituted R-group, in the
following order of decreasing reduction potential: For Series 1: p-CF -Ph > Ph > C(CH ) >
3
3 3
CH Ph > H > CH(CH ) and for Series 2: p-NO -Ph > 2-CF -4-Cl-Ph > 3,5-di-Cl-Ph > H > Ph.
2
3 2
2
3
The experimental reduction potential of Series 1 is generally lower than that of Series 2, indicating
that a phenyl group attached directly to the carbonyl group (as in Series 2) leads to higher reduction
potentials than a phenyl attached to the amino nitrogen. The experimentally obtained reduction
24

potentials of these ligands relate linearly to their DFT calculated
ACCEPTED MANUSCRIP Te nergies, such as LUMO energies,
electron affinities, electrophilicity indices and Mulliken electronegativities.
Acknowledgements
This work has received support from the South African National Research Foundation Foundation
(Grant Nos: 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
Optimized coordinates of the DFT calculations.
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4
[
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(
similar to Ligand 8 and 10 (GEZSEJ, IXOJOU, IXOJOU01, IXOJOU02, IXOJOU03, IXOJOU04,
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2
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5
6
[
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BIVQUT, BIVRAA, BIVRAA01, BIVREE, HUDVAE, KUXJUI, KUXJUI01, KUXJUI02,
KUXJUI03, NAWKUS, PUKQUI, RALROM, RAWPOW, RAWQEN, VAZPOC, WUHNIX,
ZIJNOX, ZIJNOX01.
2
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reference code SEHGOC).
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11 13
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27