Synthesis, structures, and properties of nickel(II) mixed-ligand complexes containing various β-diketonates and a phosphorus donor bidentate ligand

Article abstract of DOI:10.1246/bcsj.80.963

New mixed-ligand nickel(II) complexes containing β-diketonates and l,2-bis(diphenylphosphino)ethane, [Ni-(dike)(dppe)]X (X = BF₄, CIO₄, and NO₃) were synthesized and characterized. Since the ligand field strength of the phosphorus donor is very strong, the complexes [Ni(dike)-(dppe)]X studied in this work are as stable as the square-planar both in solid state and in solutions.

Full text of DOI:10.1246/bcsj.80.963

Short Articles
Bull. Chem. Soc. Jpn. Vol. 80, No. 5, 963–965 (2007)
963
R2
acac: R1= Me, R2= Me
R1
bzac: R1= Me, R2= Ph
dibm: R1= Ph, R2= Ph
dipm: R1= t-Bu, R2= t-Bu
Ph
Ph
Ph
Ph
Synthesis, Structures, and Properties
of Nickel(II) Mixed-Ligand
P
P
O
O
dppe
dike
Complexes Containing Various
ꢀ-Diketonates and a Phosphorus
Donor Bidentate Ligand
Fig. 1. Ligands dppe and dike.
gated by UV–vis spectra and NMR spectra. For acac complex
1a, 1b, and dibm complex 3, their crystal structures have been
determined by the method of the X-ray single crystal analysis.
Machiko Arakawa,1 Hiroshi Miyamae,2
Results and Discussion
ꢀ1
Synthesis of the Mixed-Ligand Complexes. Only one NiII
mixed-ligand complex containing acac and dppe with the tetra-
phenylborate counter anion, [Ni(acac)(dppe)]BPh4, has been
synthesized in the way showed in Eq. 1 by Favero et al.4
and Yutaka Fukuda
1Department of Chemistry, Faculty of Science, Ochanomizu
University, 2-1-1 Otsuka, Bunkyo-ku, Tokyo 112-8610
dppe
NaBPh4
2Department of Chemistry, Faculty of Science,
Josai University, 1-1 Keyakidai, Sakado 350-0295
[Ni(acac)2] ꢁꢁꢁꢁ! ꢁꢁꢁꢁ! [Ni(acac)(dppe)]BPh4:
ð1Þ
But any other complexes containing different kinds of coun-
ter anions (such as BF4ꢁ, ClO4ꢁ, and NO3ꢁ) have not been re-
ported so far. We have re-tried to synthesize the perchlorate in
the same way,3 and confirmed the presence of mixed-ligand
complex [Ni(acac)(dppe)]ClO4 in the dilute solution by UV–
vis spectra. But after concentration for obtaining crystals at
room temperature, the disproportionation products, the bis-
dppe complex [Ni(dppe)2](ClO4)2 and the bis-acac complex
[Ni(acac)2(Solvent)2] were obtained (see Eq. 2).
Received November 22, 2006; E-mail: fukuda.yutaka@ocha.
ac.jp
New mixed-ligand nickel(II) complexes containing
ꢀ-diketonates and 1,2-bis(diphenylphosphino)ethane, [Ni-
(dike)(dppe)]X (X = BF4, ClO4, and NO3) were synthesized
and characterized. Since the ligand field strength of the phos-
phorus donor is very strong, the complexes [Ni(dike)-
(dppe)]X studied in this work are as stable as the square-
planar both in solid state and in solutions.
2[Ni(acac)(dppe)]ClO4
ꢀ [Ni(acac)2(Solvent)2] þ [Ni(dppe)2](ClO4)2:
ð2Þ
We can conclude that the equilibrium shown as Eq. 2 exists
in the solution and this equilibrium shifts to the right hand side
at room temperature because the solubility of [Ni(dppe)2]-
(ClO4)2 is less than that of [Ni(acac)(dppe)]ClO4 at the tem-
perature. Then, we have succeeded in the isolation of mixed-
ligand complexes with the ClO4 or BF4 anion at low temper-
ature. In detail, as shown in Scheme 1, to a green ethanol
solution of [Ni(dike)(H2O)4]X (dike = acac, bzac, dibm, and
dipm, X = BF4, ClO4, and NO3) in ice bath, an equimolar
amount of dppe solution was added and then color changed
to brown. UV–vis spectra of this brown solution of all com-
plexes have the similar absorption peak at around 425 nm,
which is the characteristic shape of the square-planar NiII com-
plex.1 For comparison, the absorption of the bis-dppe complex
shows a shoulder around 400 nm. The solution was kept in a
freezer for several hours; then the yellow crystals could be ob-
served. Although formations of the square-planar acac-nitrate
complex 1c and bzac-perchlorate complex 2 were confirmed
by the absorption peaks in UV–vis spectra and 31P NMR in
EtOH solutions, it could not be obtained as a crystal but only
as an oily product. In the IR spectra of obtained complexes, the
characteristic bands due to ꢁC=O and ꢁC=C of coordinated ꢀ-
diketonate in the regions of 1600–1400 cmꢁ1 and free anion
around 1100 cmꢁ1 for both perchlorate and tetrafluoroborate
were observed,5 and all complexes were diamagnetic which
imply that they have square-planar structures in solid state.
X-ray Crystal Structures of 1a, 1b, and 3. Figure 2 show
the new crystal structure of complex [Ni(dibm)(dppe)]ClO4
(3). Because the complexes [Ni(acac)(dppe)]BF4 (1a) and
Nickel(II) complexes have mainly two types of structures
depending on ligand field strength and steric properties of
the ligands; one is diamagnetic square-planar structure with
yellow-red-pink colors, the other is paramagnetic octahedral
structure with purple-blue-green colors, so their general struc-
tures can be easily expected from UV–vis spectra and magnet-
ic behaviors.1,2 In previous papers, we reported the chromo-
tropic properties of many mixed-ligand nickel(II) complexes
in solutions.3 Our success in obtained stable mixed-ligand
complexes (ternary system) is due to suitable combinations
of the ligands with different stereo-chemical properties, i.e.,
one ligand is a sterically hindered ligand and the partner is slim
one. The donor set of these complexes is mostly N2O2 such as
[Ni(acac)(tmen)]BPh4.3 If we use the stronger coordinating li-
gand containing phosphorus as the partner ligand of ꢀ-diketo-
nate, it is interesting that whether we can observe or can not
observed the solvatochromic behavior in the new system. In
this paper, we have used 1,2-bis(diphenylphosphino)ethane
(dppe) and four ꢀ-diketonates shown in Fig. 1, to be the
nickel(II) mixed-ligand complexes; Ni(acac)(dppe)BF4 (1a),
Ni(acac)(dppe)ClO4 (1b), Ni(acac)(dppe)NO3 (1c), Ni(bzac)-
(dppe)ClO4 (2), Ni(dibm)(dppe)ClO4 (3), and Ni(dipm)(dppe)-
ClO4 (4). As the previous work related to this type of the sys-
tem, only one mixed-ligand complex, [Ni(acac)(dppe)]BPh4,
was reported with the tetraphenylborate counter anion.4 But
no other complexes with different anions have been reported.
The solution behavior of the obtained complexes was investi-
Published on the web May 14, 2007; doi:10.1246/bcsj.80.963

964 Bull. Chem. Soc. Jpn. Vol. 80, No. 5 (2007)
Ó 2007 The Chemical Society of Japan
cooling
1)Hdike
dppe
[Ni(H O) ]X
2
[Ni(dike)(dppe)]X
(brown)
yellow crystal
[Ni(dike)(H O) ]X
2
6
2
4
2)Et N
3
(bluish-green)
Scheme 1. Synthesis of mixed-ligand complexes [Ni(dike)(dppe)]X.
2000
(c)
1500
(a) x10
(b)
ε
1000
500
0
300
400
500
600
700
800
wavelength / nm
Fig. 2. An ORTEP view of the [Ni(dibm)(dppe)]ClO4 (3).
Perchlorate anion is omitted for clear viewing. Thermal
ellipsoids are at 50% probability.
Fig. 3. UV–vis spectra of (a) [Ni(acac)(tmen)]BPh4/DCE
(ten times "), (b) [Ni(acac)(dppe)]ClO4 (1b)/ACN, and
(c) [Ni(dppe)2](ClO4)2/ACN.
ꢂ
˚
Table 1. Selected Bond Lengths (A) and Angles ( ) for
Complex 3
mixed-ligand complex 1b, dissolving into five solvents with
different donor numbers shown in respective parentheses,
i.e., DCM (0), ACN (14), ACO (17), DMF (27), and DMSO
(30), the yellow color of the complex solutions did not change
any more, in fact UV–vis spectra were the same shape, as
shown in Fig. 3b. Comparing d–d transition spectra of the
dppe complexes containing the different ꢀ-diketonates, we
could not observe any important differences in their spectral
peaks, which mean that there is no substituent effect of ꢀ-di-
ketonates on the ligand field strength of these dike–dppe com-
plexes. From these results, the complexes obtained in this
study, [Ni(dike)(dppe)]X, do not show any solvatochromism
and keep their square-planar structures stably due to the strong
ligand field strength of dppe ligand.
And it is interesting to note that the value of d–d transition
energy of the square-planar complex [Ni(acac)(dppe)]X (427
nm) is higher than that of corresponding tmen complex [Ni-
(acac)(tmen)]BPh4 (486 nm, Fig. 3a). This reflects the fact that
the ligand field strength of dppe is stronger than that of tmen, so
the square-planar structure of the dppe ternary complex is more
stable. Additionally, the intensities at ꢂmax of dppe binary and
ternary complexes are ten times as large as that of the tmen
complex, which is attributed to the intensity stolen from the
strong CT transition in the dppe complex. [Ni(CN)4]2ꢁ and
[Ni(TetraMeen)2]2þ (TetraMeen = 2,3-dimethyl-2,3-diamino-
butane) are known as the complexes which keep their square-
planar structures in any solvent due to their strong ligand field
Ni(1)–O(1)
Ni(1)–O(2)
Ni(1)–P(1)
Ni(1)–P(2)
1.866(2)
1.858(2)
2.1650(11)
2.1645(11)
O(1)–Ni(1)–O(2)
P(1)–Ni(1)–P(2)
P(1)–Ni(1)–O(2)
P(2)–Ni(1)–O(1)
P(1)–Ni(1)–O(1)
P(2)–Ni(1)–O(2)
94.83(11)
85.23(4)
89.79(8)
90.85(8)
170.63(8)
172.87(9)
[Ni(acac)(dppe)]ClO4 (1b) were almost same structures as
[Ni(acac)(dppe)]BPh4,4 we describe only complex 3 in this re-
port. Table 1 lists selected bond distances and angles around
the metal center in complex 3. Two phenyl substituent groups
of dibm have no direct steric influenceꢁto the partner ligand
dppe. For all complexes, the anion BF4 or ClO4ꢁ, does not
coordinate to the central metal ion and the coordination geom-
etry of the nickel(II) is a distorted square-planar geometry in-
volving two oxygen atoms of the ꢀ-diketonate and two phos-
phorus atoms of the dppe ligand. The bite angles of dppe and
Ni–P bond lengths are almost same as the literature.6 On the
other hand, comparing Ni–O bond lengths of Ni–dike in dppe
complexes with those of [Ni(acac)(tmen)]BPh4,7 one can see
˚
that the former bond distances (1.866(2) and 1.858(2) A for
complex 3) are longer than the latter tmen complex (1.848(2)
˚
and 1.840(2) A) due to the strong coordination power of phos-
phorus donors in dppe ligand. It is also noteworthy that the var-
iations in the bond lengths C–O and C–C of the acetylacetonate
chelate ring were observed in all complexes.8
Solution Behavior of the Complexes. Figure 3 shows
UV–vis spectra of the analogous square-planar systems,
[Ni(acac)(tmen)]BPh4 in DCE,3 [Ni(acac)(dppe)]ClO4 (1b)
and [Ni(dppe)2](ClO4)2 in ACN. For the tmen complex, the
spectra depended on the donor strength of the solvents, that
is, structures and colors changed with the donor strength of
the solvents (solvatochromism). However, for the dppe
strength; [Ni(CN)4]2ꢁ: ꢂmax ¼ 357 nm, [Ni(TetraMeen)2]2þ
:
ꢂmax ¼ 434 nm.9,10 The ꢂmax value of [Ni(acac)(dppe)]X in
this work exists in between the values of the complex with
cyano-ligand and that with TetraMeen ligand.
Experimental
The abbreviations of the solvents used are as follows: dichloro-
methane, DCM; 1,2-dichloroethane, DCE; acetone, ACO; aceto-
nitrile, ACN; N,N-dimethylformamide, DMF; and dimethylsulf-

M. Arakawa et al.
Table 2. Crystallographic Data of 1a, 1b, and 3
Bull. Chem. Soc. Jpn. Vol. 80, No. 5 (2007)
965
1a
1b
3
Crystal color
brown
C31H31BF4NiO2P2
643.05
0:80 ꢄ 0:50 ꢄ 0:30
monoclinic
C2=c
25.934(12)
10.261(3)
23.587(8)
92.97(3)
6268.(4)
8
1.363
0.771
27.50
6467
371
0.0611
0.1910
1.016
brown
C31H31ClNiO6P
655.69
0:50 ꢄ 0:50 ꢄ 0:30
monoclinic
C2=c
26.204(11)
10.260(4)
23.543(8)
92.53(3)
6323.(4)
8
1.378
0.840
27.53
6351
371
0.0695
0.2114
1.071
brown
C41H35ClNiO6P2
779.82
0:40 ꢄ 0:35 ꢄ 0:20
monoclinic
P21=n
17.523(4)
10.288(2)
20.011(4)
91.447(8)
3606.4(11)
4
1.436
0.7492
30.05
5488
495
0.0681
0.0804
1.227
Empirical formula
Formula weight
Crystal dimensions/mm3
Crystal system
Space group
˚
a/A
˚
b/A
˚
c/A
ꢀ/ꢂ
˚
V/A
3
Z
Dcalcd/Mg mꢁ3
ꢃ/mmꢁ1
ꢂ
ꢄmax
/
No. of observations
Parameters
R1 (I > 2ꢅðIÞ)
Rw
S (Fit on F2)
oxide, DMSO.
Preparation of dppe Mixed-Ligand Complexes. The com-
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1
ꢃ
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Supporting Information
9
Materials, physical measurements, crystallographic data of 1a
and 1b, and analytical data of all complexes. This material is
available free of charge on the web at: http://www.csj.jp/journals/
bcsj/.