Synthesis and electrochemical studies of octahedral nickel β-diketonate complexes

Article abstract of DOI:10.1016/j.ica.2008.03.014

The reaction of [Ni(tmhd)₂] and [Ni(dbm)₂] with N-donor chelating ligands in dichloromethane and acetone, respectively, yields the complexes [Ni(tmhd)₂(L-L)] (L-L = 2,2′-bpy 1, phen 2 and dmae 3) and [Ni(dbm)₂(L

Full text of DOI:10.1016/j.ica.2008.03.014

Inorganica Chimica Acta 362 (2009) 78–82
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Inorganica Chimica Acta
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Synthesis and electrochemical studies of octahedral nickel b-diketonate complexes
a,
b
b
c
Phimphaka Harding a, , David J. Harding , Wasinee Phonsri , Saowanit Saithong , Hirihattaya Phetmung
*
*
a Molecular Technology Unit Cell, Department of Chemistry, Walailak University, Thasala, Nakhon Si Thammarat 80161, Thailand
b Department of Chemistry, Faculty of Science, Prince of Songkhla University, Hat Yai, Songkhla 90112, Thailand
c Department of Chemistry, Faculty of Science, Taksin University, Songkhla 90000, Thailand
a r t i c l e i n f o
a b s t r a c t
Article history:
The reaction of [Ni(tmhd)2] and [Ni(dbm)2] with N-donor chelating ligands in dichloromethane and ace-
tone, respectively, yields the complexes [Ni(tmhd)2(L–L)] (L–L = 2,20-bpy 1, phen 2 and dmae 3) and
[Ni(dbm)2(L–L)] (L–L = 2,20-bpy 4, phen 5, dmae 6). UV–Vis spectroscopy shows very strong bands in
the UV region consistent with ligand centred p ? p* transitions. The electrochemical studies of 1–6 reveal
oxidation to Ni(III). The [Ni(tmhd)2(L–L)] 1–3 are more easily oxidized by ca. 300 mV and are quasi-
reversible whereas for the [Ni(dbm)2(L–L)] series only complex 6 shows significant reversibility. X-ray
crystallographic studies have been conducted in the case of [Ni(dbm)2(phen)] 5 and [Ni(dbm)2(dmae)]
6. The structures both show that the nickel metal centre is octahedral with an O4N2 coordination envi-
ronment. In the structures the b-diketonate ligands exhibit a cis-arrangement, with the metal displaced
out of the planar chelate ring.
Received 29 November 2007
Received in revised form 28 February 2008
Accepted 5 March 2008
Available online 18 March 2008
Keywords:
Ni(II) and Ni(III)
b-Diketonates
Cyclic voltammetry
Crystal structure
Octahedral
Ó 2008 Elsevier B.V. All rights reserved.
1. Introduction
almost completely unexplored [9,10]. Thus, in this paper we report
the synthesis, structural and electrochemical characterization of a
Metal b-diketonates, [M(b-diketonate)2] (b-diketonate = acetyl-
acetonate (acac), hexafluoroacetylacetonate (hfac) 1,3-diphenyl-
propane-1,3-dionate (dbm), 2,2,6,6-tetramethylheptane-3,5-dionate
(tmhd); M = Mn, Co, Ni, Cu and Zn) have been known for many
years and have been extensively studied [1]. These investigations
have revealed that in the presence of additional ligands the metal
will frequently expand its coordination number from 4 to 6
(Scheme 1) [2]. In the case of divalent nickel this change in coordi-
nation number is also accompanied by a spin change from low-
spin to high-spin [3].
The resulting adducts have the advantage of additional func-
tionality to tune the metal complexes for a particular application.
For instance, simple alkyl diamines (e.g. ethylenediamine) are of-
ten used to synthesize volatile precursors suitable for the prepara-
tion of metal oxide thin films by chemical vapour deposition [4],
while rigid ligands such as imino-pyridines have been used for ba-
sic physical studies [3]. More recently, chelating organic radicals
have been utilized as ligands for the preparation of single molecule
magnets [5–8]. However, despite this wealth of chemistry much of
the research still focuses on the acac and hfac ligands with compar-
atively little research into the more substituted b-diketonate com-
plexes. Furthermore, the redox chemistry of such systems remains
range of nickel(II) b-diketonate complexes, [Ni(tmhd)2(L–L)] and
[Ni(dbm)2(L–L)] when L–L = 2,20-bipyridine (2,20-bpy), 1,10-phe-
nanthroline (phen) and 2-dimethylaminoethylamine (dmae).
2. Experimental
2.1. General
All reactions were conducted in air using HPLC grade solvents.
[Ni(tmhd)2] and [Ni(dbm)2] were prepared by literature methods
[1,11]. Although [Ni(tmhd)2(2,20-bpy)] has been reported previ-
ously [2] no explict synthesis or characterization data was reported
and is included for completeness. All other chemicals were pur-
chased from Fluka Chemical Company and used as received. Infra-
red spectra (as KBr discs) were recorded on a Perkin–Elmer
spectrum one infrared spectrophotometer in the range 400–
4000 cmꢀ1. Electronic spectra were recorded in CH2Cl2 or DMSO
on a Unicam UV300 UV–Vis spectrometer. Elemental analyses
were carried out on a Eurovector EA3000 analyzer. ESI-MS were
carried out on a Bruker Daltonics 7.0 T Apex 4 FTICR Mass Spec-
trometer. Electrochemical studies were carried out using a palms-
ensPC Vs 2.11 potentiostat in conjunction with three electrode cell.
The auxiliary electrode was a platinum rod and working electrode
was a platinum disc (2.0 mm diameter). The reference was Ag–
AgCl electrode. Solution were 5 ꢁ 10ꢀ4 mol dmꢀ3 in the test com-
* Corresponding authors. Tel.: +66 75672100; fax: +66 75672004 (P. Harding);
tel.: +66 75672094; fax: +66 75672004 (D.J. Harding).
E-mail addresses: kphimpha@wu.ac.th (P. Harding), hdavid@wu.ac.th (D.J.
Harding).
pound and 0.1 mol dmꢀ3 in ½NBunꢂ[PF6] as the supporting
4
electrolyte.
0020-1693/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2008.03.014

P. Harding et al. / Inorganica Chimica Acta 362 (2009) 78–82
79
C, 72.7; H, 4.6; N, 4.7%. Electrospray ionization (ESI) mass data,
found (Calc.): m/z 437.08 (437.13) [Mꢀdbm]+.
2.6. Synthesis of [Ni(dbm)2 (phen)] 5
[Ni(dbm)2] (0.11 g, 0.20 mmol) was dissolved in acetone
(20 ml). The lime green solution was stirred for 15 min and phen
(0.02 g, 0.1 mmol) was added. The green solution was stirred for
3.5 h. This resulted in formation of a green precipitate which was
collected by filtration and dried in air, yield 0.11 g (80%). Anal. Calc.
for C42H30N2NiO4 ꢃ 0.2CH2Cl2: C, 72.2; H, 4.4; N, 4.0. Found: C, 72.4;
H, 4.3; N, 4.3%. Electrospray ionization (ESI) mass data, found
(Calc.): m/z 461.08 (461.15) [Mꢀdbm]+.
Scheme 1. Formation of M(acac)2 adducts with chelating ligands.
2.7. Synthesis of [Ni(dbm)2(dmae)] 6
2.2. Synthesis of [Ni(tmhd)2(2,20-bpy)] 1
[Ni(dbm)2] (0.11 g, 0.20 mmol) was dissolved in acetone
(15 ml). The lime green solution was stirred for 15 min and then
dmae (0.022 ml, 0.20 mmol) was added. The dark green solution
was stirred for 3.5 h, filtered through Celite and evaporated to
small volume. n-Hexane (10 ml) was added and the mixture left
at room temperature for 1.5 h resulting in dark green microcrystals
isolated by filtration, yield 0.10 g (84%). Anal. Calc. for
C34H34N2NiO4: C, 68.8; H, 5.8; N, 4.7. Found: C, 68.6; H, 5.8; N,
4.7%. Electrospray ionization (ESI) mass data, found (Calc.): m/z
369.11 (369.10) [Mꢀdbm]+.
[Ni(tmhd)2] (0.14 g, 0.30 mmol) was dissolved in dichlorometh-
ane (15 ml) giving a pink purple solution and stirred for 10 min
2,20-bpy (0.05 g, 0.30 mmol) was added and the solution stirred
for 2 h. The resulting green solution was filtered through Celite
and evaporated to small volume. n-Hexane (10 ml) was added giv-
ing a dark green precipitate. The mixture was left at room temper-
ature for 45 min and the green microcrystals collected using a
Hirsch funnel and washed with n-hexane (5 ꢁ 2 ml), yield 0.14 g
(75%). Anal. Calc. for C32H46N2NiO4: C, 66.1; H, 8.0; N, 4.8. Found:
C, 66.2; H, 7.9; N, 5.3%. Electrospray ionization (ESI) mass data,
found (Calc.): m/z 397.14 (397.15) [Mꢀtmhd]+.
3. Results and discussion
2.3. Synthesis of [Ni(tmhd)2(phen)] 2
The reaction of [Ni(tmhd)2] or [Ni(dbm)2] with three chelating
ligands, 2,20-bpy, phen and dmae in CH2Cl2 or acetone affords
green or blue solids of the octahedral complexes [Ni(tmhd)2(L–
L)] (L–L = 2,20-bpy 1, phen 2 and dmae 3) and [Ni(dbm)2(L–L)] (L–
L = 2,20-bpy 4, phen 5 and dmae 6), see Table 1.
[Ni(tmhd)2] (0.14 g, 0.30 mmol) was dissolved in dichlorometh-
ane (15 ml). The pink-purple solution was stirred for 15 min and
then phen (0.06 g, 0.3 mmol) was added, and stirring continued
for 1 h. The resulting green solution was filtered through Celite
and evaporated to small volume. n-Hexane (10 ml) was added
and the mixture left at room temperature for 30 min. The resulting
green solid was collected using a Hirsch funnel and washed with n-
hexane (5 ꢁ 2 ml), yield 0.11 g (64%). Anal. Calc. for C34H46N2NiO4:
C, 67.4; H, 7.7; N, 4.6. Found: C, 66.7; H, 7.4; N, 5.2%. Electrospray
ionization (ESI) mass data, found (Calc.): m/z 421.14 (421.17)
[Mꢀtmhd]+.
. . .
IR spectroscopy of complexes 1–6 shows C•O stretches be-
tween 1582 and 1595 cmꢀ1 indicating that the b-diketonate li-
gands adopt a chelating coordination mode [12]. The position of
the bands are similar to those reported for [Ni(acac)(en)2]+ and
[Ni(acac)(bpy)2]+ [13]. In addition, the C–H stretches for the tmhd
ligand around 2955 cmꢀ1 while Ar-H stretches for the dbm ligand
are observed between 3055 and 3062 cmꢀ1. Strong bands at
around 3364 and 3352 cmꢀ1 for 3 and 6, respectively, are consis-
tent with NH stretches for the amino groups present in these com-
pounds. In all cases the IR spectra are significantly different from
the starting materials confirming the presence of the chelating li-
gand in the metal coordination sphere.
2.4. Synthesis of [Ni(tmhd)2(dmae)] 3
[Ni(tmhd)2] (0.09 g, 0.20 mmol) was dissolved in dichlorometh-
ane (15 ml) giving a pink-purple solution and stirred for 10 min
dmae was added (0.02 ml, 0.2 mmol) and the green-blue solution
stirred for 1.5 h. The resulting green-blue solution was filtered
through Celite and evaporated to small volume. n-Hexane
(10 ml) was added and the mixture left at room temperature. After
24 h a pastel blue solid appeared, which was isolated by filtration,
yield 0.08 g (78%). Anal. Calc. for C26H50N2NiO4: C, 60.7; H, 9.8; N,
5.5. Found: C, 60.7; H, 9.9; N, 5.8%. Electrospray ionization (ESI)
mass data, found (Calc.): m/z 513.31 (513.38) [M]+, 329.18
(329.12) [Mꢀtmhd]+.
UV–Vis spectroscopy shows very strong bands in the UV region
consistent with ligand centred p ? p* transitions. In all cases no d–
d transitions were observed presumably due to their low intensity.
The NMR spectra of 1–6 are broad and featureless providing little
or no structural information and consistent with paramagnetic
octahedral Ni(II).
Electrochemical studies conducted in CH2Cl2 between 1.8 V
versus Ag–AgCl electrode at room temperature using a three-elec-
trode configuration reveal one-electron oxidation waves to Ni(III)
(Table 2). A representative example of 3 is given in Fig. 1. The
waves are quasi-reversible in the case of 1–3, 6 and irreversible
for 4 and 5. The tmhd complexes are easier to oxidize by ca.
300 mV suggesting that the remote groups on the b-diketonate li-
gand can significantly affect the electron density on the metal.
Interestingly, the bpy and phen ligands show virtually identical
peak potentials and are more difficult to oxidize by 110 mV com-
pared with 3 and 6 suggesting that the dmae ligand is a signifi-
cantly better net donor. Further comparisons are somewhat
difficult as the only previously reported Ni b-diketonate adducts
2.5. Synthesis of [Ni(dbm)2 (2,20-bpy)] 4
[Ni(dbm)2] (0.11 g, 0.20 mmol) was dissolved in acetone
(15 ml).The lime green solution was stirred for 15 min and then
(2,20-bpy) (0.03 g, 0.20 mmol) was added. The dark green solution
was stirred for 2.5 h, filtered through Celite and evaporated to
small volume. n-Hexane (5 ml) was added, and the solution fil-
tered to give a green solid which was dried in air, yield 0.11 g
(89%). Anal. Calc. for C40H30N2NiO4: C, 72.6; H, 4.6; N, 4.2. Found:

80
P. Harding et al. / Inorganica Chimica Acta 362 (2009) 78–82
Table 1
Analytical, IR and UV–Vis spectroscopic data of [Ni(acacR)2(L–L)]
Complex
Yield (%)
Colour
Analysisa (%)
C
mA–B/cmꢀ1
mN–H/cmꢀ1
kmax/nm (e)b
H
N
mC O/cmꢀ1
. . .
•
1
2
3
4
5c
6
75
64
78
89
80
84
light green
leaf green
pastel blue
lime green
lime green
dark green
66.2(66.1)
66.7(67.4)
60.7(60.8)
72.7(72.6)
72.4(72.2)
68.6(68.8)
7.9(8.0)
7.4(7.7)
9.9(9.8)
4.6(4.6)
4.3(4.4)
5.8(5.8)
5.3(4.8)
5.2(4.6)
5.8(5.5)
4.7(4.2)
4.3(4.0)
5.0(4.7)
–
–
3364
–
–
1591
1582
1584
1595
1595
1595
269 (46500)
246 (22940), 292 (29340)
306 (20090)
362 (31650)
292 (30520), 360 (35120)
362 (25740)
3352
a
Calculated values are in parenthesis.
Complexes 1–3 measured in CH2Cl2, complexes 4–6 measured in DMSO.
Calculated as a 0.2 CH2Cl2 solvate.
b
c
Table 2
Electrochemical data of [Ni(acacR)2(L–L)]
Complex
Eopx=V
Erped=V
DE/mV
1
2
3
0.87
0.87
0.79
0.76
0.75
0.66
110
120
130
4
5
6
1.17
1.19
1.08
–
–
0.89
–
–
190
All measurements were performed at 298 K, in dried and degassed CH2Cl2 0.1 M
½NBun4ꢂ[PF6] solution; scan rate 100 mV sꢀ1
.
Fig. 2. ORTEP (50% probability ellipsoids) of [Ni(dbm)2(phen)] 5.
Fig. 1. Cyclic voltammogram of 3 in CH2Cl2 at 100 mV/s. Potentials were measured
vs Ag–AgCl.
are [Ni(hfac)2(2-pyBN)] and [Ni(hfac)2(Me3PO)] (2-pyBN = N-tert-
butyl-a-(2-pyridyl)nitrone; Me3PO = 2,5,5-trimethyl-1-pyrroline-
N-oxide) with oxidation peak potentials at 1.8 and 1.36 V,
respectively [9]. The presence of electron withdrawing hfac groups
means that the complexes are considerably more difficult to oxi-
dize. Attempts have been made to oxidize complexes 1–3 but to
date we have been unable to identify the decomposition products
from the oxidation and thus the nature of the oxidized species re-
mains unknown.
Crystals of 5 and 6 were obtained by slow diffusion of n-hexane
into a concentrated solution of the complex in CH2Cl2. The struc-
tures and crystallographic data are presented in Figs. 2 and 3 and
Table 3, respectively.
The structures both show that the nickel metal centre is octahe-
dral with an O4N2 coordination environment. The Ni–O bond
lengths are almost identical differing only by 0.02 Å. The bond
lengths are similar to those reported for [Ni(dbm)2(en)]
{2.009(6), 2.060(7)} [14] and trans-[Ni(dbm)2(4-Phpy)2] {2.033(1),
2.019(1)} [15]. In contrast, the Ni–N bond lengths in 6 are different
by 0.09 Å suggesting either that the NH2 group is a better donor
than the NMe2 group or that the greater steric hindrance in the
case of the NMe2 group distances it from the Ni centre. The later
conclusion is supported by the Ni–N bond lengths in
Fig. 3. ORTEP (50% probability ellipsoids) of [Ni(dbm)2(dmae)] 6.
[Ni(acac)2(tmeda)] and [Ni(tmhd)2(tmeda)] (tmeda = tetramethyl-
ethylenediamine) which are 2.152(2), 2.1655(17) [16] and
2.163(2), 2.166(3) [2b], respectively. However, asymmetric binding

P. Harding et al. / Inorganica Chimica Acta 362 (2009) 78–82
81
Table 3
Bond lengths and angles for 5 and 6
Complex
Bond lengths (Å)
Bond angles (°)
5
Ni–O(1)
Ni–O(2)
Ni–N(1)
C(13)–O(1)
C(21)–O(2)
C(13)–C(20)
C(20)–C(21)
2.0351(12)
2.0413(11)
2.0929(13)
1.2755(18)
1.2675(18)
1.405(2)
O(1)–Ni–O(2)
O(2)–Ni–O(2A)
O(1)–Ni–O(2A)
O(1)–Ni–N(1)
O(1)–Ni–N(1A)
O(2)–Ni–N(1)
O(1)–Ni–O(1A)
N(1)–Ni–N(1A)
88.08(5)
101.05(7)
83.03(4)
98.94(5)
91.89(5)
90.18(7)
166.01(6)
78.64(7)
1.406(2)
6
Ni–O(1)
Ni–O(2)
Ni–O(3)
Ni–O(4)
Ni–N(1)
Ni–N(2)
2.0304(15)
2.0210(17)
2.0365(17)
2.0364(15)
2.166(2)
O(1)–Ni–O(2)
O(3)–Ni–O(4)
N(1)–Ni–N(2)
O(1)–Ni–N(1)
O(1)–Ni–N(2)
O(1)–Ni–O(3)
O(1)–Ni–O(4)
O(2)–Ni–O(4)
O(2)–Ni–O(3)
O(2)–Ni–N(2)
O(4)–Ni–N(2)
88.77(6)
88.76(6)
82.84(8)
92.25(7)
92.44(7)
87.06(6)
175.77(7)
90.70(7)
90.78(7)
174.87(8)
88.45(7)
2.073(2)
Fig. 4. Mercury plot of [Ni(dbm)2(dmae)] 6 showing the H-bonding interactions
within the dimeric unit. Hydrogen atoms not involved in the H-bonding interac-
tions have been removed for clarity.
of the symmetric ligand ethylenediamine is also observed in
[Ni(dbm)2(en)] {Ni–N: 2.081(8), 2.116(8)} [14]. As expected the
phenanthroline group is planar and exhibits a smaller bite angle
(78.6° cf. 82.9°) than the less rigid dimethylaminoethylamine li-
gand. The dbm groups are essentially symmetric and planar sug-
gesting that the negative charge is completely delocalized over
the b-diketonate framework. However, the dbm ligand binds to
the metal in a slightly angular fashion such that the nickel centre
is raised above the plane of the dbm ligand. This effect is more pro-
nounced in complex 5 than in 6 perhaps because of the larger steric
bulk of the phenanthroline ligand. However, it is also possible the
lower temperature at which the structure of 5 was performed
might result in closer packing and therefore more deformation of
the structure than that of 6 which was done at higher temperature
(see Table 4).
A further point of interest in the structure of 6 is the presence of
two sets of intermolecular H-bonding interactions between the
amino protons of the dmae ligand and the coordinated O atoms
of the neighbouring dbm ligands which create discrete dimers
within the structure (see Fig. 4). The two interactions, N2–
H2Aꢃ ꢃ ꢃO3 {2.270(4) Å; 155.30°} and N2–H2Bꢃ ꢃ ꢃO1 {2.561(4) Å;
123.60°} are different in length by 0.29 Å suggesting that the for-
mer interaction is significantly stronger than the later [17]. In addi-
tion, there are intramolecular interactions between the methyl H
atoms and the dbm O atoms {O1ꢃ ꢃ ꢃH31B, 2.519(5) Å; O4ꢃ ꢃ ꢃH32B,
2.478(5) Å}.
4. Conclusions
In conclusion, we have successfully synthesized a range of new
b-diketonate Ni(II) adducts. Characterization of the complexes
shows them to be the octahedral nickel(II) complexes
[Ni(tmhd)2(L–L)] and [Ni(dbm)2(L–L)] (L–L = 2,20-bipy, phen and
dmae), a fact later confirmed by X-ray structural studies. Electro-
chemical studies of all these complexes reveal either irreversible
or quasi-reversible oxidation to Ni(III) depending upon the nature
of the b-diketonate or the chelating ligand.
Acknowledgements
We gratefully acknowledge the Institute of Research and Devel-
opment, Walailak University (Grant No.: 5/2549) for funding this
research. We also acknowledge the Royal Golden Jubilee Scholar-
ship for a postgraduate scholarship (S.S.) and the Development
and Promotion of Science and Technology Talents project for an
undergraduate scholarship (W.P.). The School of Chemistry, Uni-
versity of Bristol, UK is thanked for element analyses and MS
services.
Table 4
Crystallographic data for 5 and 6
Complex
5
6
Chemical formula
Mr
C42H30N2NiO4
685.37
C34H34N2NiO4
593.34
Crystal System
Space group
a (Å)
b (Å)
c (Å)
a (°)
b (°)
monoclinic
C2/c
20.840(4)
17.240(3)
9.4599(19)
90
107.23(3)
90
3246.2(11)
4
1.402
0.646
173(2)
18237
3732 (0.0195)
0.0327, 0.0895
0.0368, 0.0988
triclinic
P1
ꢀ
Appendix A. Supplementary material
11.5162(10)
12.1220(10)
12.6727(11)
77.0400(10)
70.2630(10)
68.7460(10)
1541.4(2)
2
1.278
0.668
283(2)
13638
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.ica.2008.03.014.
c (°)
V (Å3)
Z
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Dcalc (g cmꢀ3
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.
a

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