Synthesis, crystal structure, and chromotropic properties of mixed-ligand nickel(II) complexes with 1,3-diketonate and P-N bidentate ligands

Article abstract of DOI:10.1246/bcsj.81.127

Mixed-ligand nickel(II) complexes Ni(acac)(dmap)X, [Ni(acac)(dmap)]BPh ₄ (1), [Ni(acac)(dmap)]BF₄ (2), and [Ni(acac)(dmap)N03] (3) (where acac = acetylacetonate and dmap = l-dimethylamino-2- diphenylphosphinoethane) have been obtaine

Full text of DOI:10.1246/bcsj.81.127

Ó 2008 The Chemical Society of Japan
Bull. Chem. Soc. Jpn. Vol. 81, No. 1, 127–135 (2008)
127
Synthesis, Crystal Structure, and Chromotropic
Properties of Mixed-Ligand Nickel(II) Complexes
with 1,3-Diketonate and P–N Bidentate Ligands
Machiko Arakawa,1 Noriyuki Suzuki,2 Shinobu Kishi,3 Miki Hasegawa,3
ꢀ1
Keiichi Satoh,4 Ernst Horn,5 and Yutaka Fukuda
1Department of Chemistry, Faculty of Science, Ochanomizu University, 2-1-1 Otsuka, Bunkyo-ku, Tokyo 112-8610
2RIKEN, Wako 351-0198
3Department of Chemistry and Biological Science, College of Science and Engineering, Aoyama Gakuin University,
5-10-1 Fuchinobe, Sagamihara 229-8558
4Department of Chemistry, Faculty of Science, Niigata University, 8050 Igarashi-ninomachi, Niigata 950-2181
5Department of Chemistry, Faculty of Science, Rikkyo University,
3-34-1 Nishi-Ikebukuro, Toshima-ku, Tokyo 171-8501
Received June 25, 2007; E-mail: fukuda.yutaka@ocha.ac.jp
Mixed-ligand nickel(II) complexes Ni(acac)(dmap)X, [Ni(acac)(dmap)]BPh4 (1), [Ni(acac)(dmap)]BF4 (2), and
[Ni(acac)(dmap)NO3] (3) (where acac = acetylacetonate and dmap = 1-dimethylamino-2-diphenylphosphinoethane)
have been obtained for the first time and their X-ray single-crystal structures have also been determined. This series
of complexes show different structures, hence colors, in the solid state depending on the coordination strength of the
anion Xꢁ: the complexes with tetraphenylborate and tetrafluoroborate having no coordination ability are square-planar
and orange, while on the contrary, the coordinating nitrate ligand produces an octahedral and pale blue complex. In solu-
tion, two types of solvatochromism due to solute–solvent interaction and solute–solute–solvent interaction have been
observed and these spectral behaviors are attributed to the changes in their geometries due to the donor/acceptor proper-
ties of the solvents. The thermochromic behavior of complex 1 has also been studied with temperature variation UV–vis
spectra in various solvents. The behavior of these solvato- and thermo-chromisms of these complexes is due to the
intermediate ligand field strength created by the dmap and the acac ligands.
Many of transition-metal complexes show a wide variety of
colors due to the d–d transition and CT transition. These colors
are sometimes reversibly changed by external stimuli such
as light,1 solvent,2,3 temperature,2–5 pressure,6 and electrons.7
These chromotropic materials have potential applications in-
cluding a temperature indicator, an ion sensor, a material de-
tector, etc., and the study of such functional materials has at-
tracted attention in the fields of solution chemistry, structural
chemistry, spectrochemistry, and supramolecular chemistry.
It has been our interest to synthesize such chromotropic ma-
terials, in particular solvatochromic and thermochromic mate-
rials, and we have succeeded in obtaining stable mixed-ligand
complexes which show chromotropic behaviors, by carefully
selecting suitable combinations of two different chelating
ligands, i.e., sterically hindered type and slim type, thereby
controlling the coordination environment surrounding the cen-
tral metal. What was advantageous to us was that the resulting
complexes are all very soluble in organic solvents, due to the
hydrophobic alkyl substituent groups on the sterically hindered
ligands. Therefore, we could observe their chromotropic be-
havior in various solvents easily and have reported many
chromotropic mixed-ligand nickel(II) complexes.2 The donor
set of the chromotropic complexes obtained so far is mostly
N2O2 such as [Ni(acac)(tmen)]BPh4 (tmen = N,N,N0,N0-tetra-
methylethylenediamine).2 Different from the nitrogen donor,
the phosphorus donor has a vacant d-orbital and the phospho-
rus–metal bond is stronger than the nitrogen–metal bond due
to the ꢀ-back donation. Although soft phosphorus ligands usu-
ally make stable complexes with a soft acceptor such as palla-
dium(II) or platinum(II) as explained by the HSAB rule, some
complexes with a medium hard acceptor nickel(II) have been
also reported.8,9 It is of interest to understand the formation
and structure of new nickel(II) mixed-ligand complexes con-
taining phosphorus donor instead of nitrogen donor. As report-
ed previously, the mixed-ligand complex [Ni(acac)(dppe)]BF4
containing 1,2-bis(diphenylphosphino)ethane (dppe) as a di-
phosphine bidentate ligand, keeps its stable square-planar
structure even in strong donor solvents without showing any
solvatochromic properties.10 In this paper, we replaced dppe
with a P–N bidentate ligand, 1-dimethylamino-2-diphenyl-
phosphinoehtane (dmap, Fig. 1), and obtained the following
new chromotropic complexes: [Ni(acac)(dmap)]BPh4 (1), [Ni-
(acac)(dmap)]BF4 (2), and [Ni(acac)(dmap)NO3] (3). The
crystal structures have been determined by single-crystal
X-ray analysis. The solution behaviors of the complexes ob-
tained were investigated by UV–vis spectroscopy, NMR spec-
Published on the web January 10, 2008; doi:10.1246/bcsj.81.127

128 Bull. Chem. Soc. Jpn. Vol. 81, No. 1 (2008)
Chromotropism of Nickel(II) Complexes
BuOK (0.562 g, 5.01 mmol) was added, upon which the color
changed to orange. After stirring for fifteen minutes at room tem-
Me
Me
Ph
Ph
Me
Me
P
N
perature, Me2NCH2CH2Cl HCl (0.361 g, 2.51 mmol) was added
ꢄ
O
O
to the solution and refluxed at 75 ꢂC for 6 h. The orange suspen-
sion became white as the reaction proceeded. Solvent was re-
moved in vacuo and the reaction was stopped by adding an aque-
ous saturated NH4Cl. The mixture was extracted with diethyl ether
(20 mL ꢅ 3) and dried with MgSO4. After removing the solvent,
yellow-brown oil was obtained. Yield 60–76%. 31P{1H} NMR
(CDCl3): ꢂ ꢁ20:75 (s). 1H NMR (CDCl3): ꢂ 7.43–7.30 (m, aro-
matic 10H), 2.41–2.37 (t, methylene 2H), 2.26–2.22 (m, methyl-
ene 2H and methyl 6H).
dmap
Hacac
Fig. 1. Hacac ligand and P–N bidentate ligand dmap.
Table 1. Donor Numbers (DN) and Acceptor Numbers
(AN) Used in This Report
Solvent
Abbreviation
DN
AN
1,2-Dichloroethane
Nitromethane
Benzonitrile
DCE
NM
0
2.7
12
14
15
17
20
22
27
30
33
17
21
16
19
19
13
38
37
16
19
14
Synthesis of [Ni(acac)(dmap)]BPh4 (1). The complex was
prepared by the literature method.9 dmap (0.257 g, 1 mmol) in di-
ethyl ether (ca. 5 mL) was added to an ethanol solution (50 mL) of
[Ni(acac)2(H2O)2] (0.293 g, 1 mmol), followed by the addition of
NaBPh4 (0.513 g, 1.5 mmol) in ethanol (10 mL), and the orange
product precipitated. This was recrystallized from a acetone–
ethanol mixture (1:1). Yield 62%. Orange. Anal. Found: C, 73.54;
H, 6.52; N, 1.87%. Calcd for C45H47NBNiO2P: C, 73.60; H, 6.45;
N, 1.91%. 31P{1H} NMR (CD3NO2): ꢂ 28.81 (s). 1H NMR (CD3-
NO2): ꢂ 8.06–7.60 (m, aromatic 10H in dmap), 7.34–6.82 (m,
aromatic 20H in BPh4ꢁ), 5.68 (s, methine 1H in acac), 2.59 (br,
10H in dmap), 2.01 (s, methyl 6H in acac). Solid reflection ꢃmax
(nm): 464. Selected IR bands (cmꢁ1): ꢄC=O þ ꢄC=C 1565, 1527
(ꢀacac ¼ 38 cmꢁ1). FABþ MS: 415, Mþ ꢁ BPh4. Diamagnetic.
BzCN
ACN
TMS
ACO
EtOH
BuOH
DMF
DMSO
PY
Acetonitrile
Tetramethylenesulfoxide
Acetone
Ethanol
n-Butanol
N,N-Dimethylformamide
Dimethylsulfoxide
Pyridine
troscopy, and molar conductivity measurements. Using UV–
vis spectra of complex 2 in different donor solvents at different
temperatures, the ln K, ꢀHꢂ, and ꢀSꢂ values for equilibrium
between the square-planar and octahedral species have been
determined. These results were compared with those previ-
ously reported for the diamine and diphosphine derivatives,
i.e., the Ni(acac)(diamine)X and Ni(acac)(diphosphine)X
analogs.2,10
Synthesis of [Ni(acac)(dmap)]BF4 (2).
Hacac (0.100 g,
1 mmol), Et3N (0.101 g, 1 mmol), and dmap (0.257 g, 1 mmol) in
diethyl ether (ca. 5 mL) were added in this order to a 20 mL of
ethanol solution of Ni(BF4)2 6H2O (0.340 g, 1 mmol) in an ice
ꢄ
bath. After stirring the deep red solution for 20 min, orange crys-
tals were obtained. Vapor diffusion of diethyl ether into the ace-
tone solution gave orange single crystals suitable for X-ray analy-
sis. Yield 53%. Orange. Anal. Found: C, 49.76; H, 5.51; N, 2.69%.
Calcd for C21H27NBF4NiO2P: C, 50.25; H, 5.42; N, 2.79%.
31P{1H} NMR (CD3NO2): ꢂ 29.51 (s). 1H NMR (CD3NO2): ꢂ
8.06–7.62 (m, aromatic 10H in dmap), 5.69 (s, methine 1H in
acac), 2.60 (br, 10H in dmap), 2.06 (s, methyl 6H in acac). Solid
Experimental
Materials. Starting materials for synthesizing the dmap, and
metal salts and 1,3-diketonates were commercially available and
were used without further purification. ‘‘Spectro-grade’’ solvents
were used for spectral measurements. The abbreviation of the
solvents with donor numbers (DN) and acceptor numbers (AN)
are listed in Table 1.11
reflection ꢃmax (nm): 463. Selected IR bands (cmꢁ1): ꢄC=O
þ
ꢄC=C 1575, 1530 (ꢀacac ¼ 45 cmꢁ1). FABþ MS: 415, Mþ ꢁ BF4.
Diamagnetic.
Physical Measurements. Elemental analyses (C, H, and N)
were measured on a Perkin-Elmer 2400 II CHN analyzer. IR
spectra were obtained as KBr pellets on a Perkin-Elmer FT-IR
SPECTRUM 2000. Mass spectra were obtained on a JEOL
JMS-700 Mstation in the Fast Atom Bombardment (FAB) mode
using PEG-600 as standard and 2-nitrophenyl octyl ether (NPOE)
as matrix. Magnetic data were measured on a Shimadzu Torsion
Magnetometer MB-100 at room temperature and the effective
magnetic moments (ꢁeff) were calculated with correcting Pascal’s
constants.12 NMR spectra were obtained on a JEOL JNM-AL400
and referenced to PPh3 (ꢂ ꢁ6:00) for the 31P nucleus. Electric
conductance of the solution was measured with a Conductivity
Outfit Model AOC-10 (Denki-Kagaku-Keiki Co., Ltd.) at 25 ꢃ
0:1 ꢂC. UV–vis spectra were obtained on a UV-3100PC Shimadzu
Spectrophotometer using a 1 cm quartz cell. Temperature varia-
tion spectra were measured on UV-3101PC Shimadzu Spectro-
photometer and Shimadzu TCC-260 temperature controller using
1 cm glass cell which was sealed under argon gas.
Synthesis of [Ni(acac)(dmap)NO3] (3). This complex was
prepared by the same method used for complex 2, but starting
from Ni(NO3)2 6H2O. Yield 51%. Pale blue. Anal. Found: C,
ꢄ
52.25; H, 5.80; N, 5.70%. Calcd for C21H27N2NiO5P: C, 52.87;
H, 5.70; N, 5.87%. Solid reflection ꢃmax (nm): 971, 609. Selected
IR bands (cmꢁ1): ꢄC=O þ ꢄC=C 1593, 1519 (ꢀacac ¼ 74 cmꢁ1);
combination bands of bidentate NO3 1764, 1718 (ꢀNO ¼ 46
3
cmꢁ1). FABþ MS: 415, Mþ ꢁ NO3. ꢁeff (BM): 3.22.
X-ray Crystallography. X-ray data of complex 1 was collect-
ed on a Rigaku RAXIS CS imaging plate area detector with
˚
graphite-monochromatized Mo Kꢅ radiation (ꢃ ¼ 0:71073 A) at
100 K. A total of 88 oscillation images were collected, and the in-
tensities were corrected for Lorentz and polarization effects. The
structure was solved by heavy-atom Patterson methods14 and ex-
panded using Fourier techniques.15 The unit cell contains two
crystallographically independent molecules which have almost
same structure. All non-hydrogen atoms were refined with aniso-
tropic thermal parameters. Hydrogen atoms were included in cal-
culated positions and refined with isotropic thermal parameters.
All calculations were performed using the CrystalStructure16
crystallographic software package except for refinement, which
Synthesis of dmap. 1-Dimethylamino-2-diphenylphosphino-
ethane (dmap) was prepared under N2 and dehydration condition
by the literature13 method with a slight modification. To a solution
of PPh2H (0.389 g, 2.09 mmol) in tetrahydrofuran (65 mL), tert-

M. Arakawa et al.
Table 2. Crystallographic Data
Bull. Chem. Soc. Jpn. Vol. 81, No. 1 (2008)
129
1
2
3
[Ni(acac)(tmen)]BPh4
Crystal color
orange
C45H47BNNiO2P
734.36
orange
blue
red
C35H43BN2NiO2
593.26
Empirical formula
Formula weight
Crystal dimensions/mm3
Crystal system
C21H27BF4NNiO2P C21H27N2NiO5P
501.94 477.13
0:60 ꢅ 0:40 ꢅ 0:30 0:55 ꢅ 0:50 ꢅ 0:30
0:29 ꢅ 0:15 ꢅ 0:10 0:50 ꢅ 0:20 ꢅ 0:20
triclinic
ꢁ
P1
monoclinic
P21=c
14.71(2)
12.243(11)
14.87(2)
90
116.20(7)
90
2403(4)
4
1.387
0.921
27.50
5304
389
0.0516
0.1556
1.060 (Fit on F2)
monoclinic
P21=c
monoclinic
P21=c
16.855(4)
12.111(4)
18.324(9)
90
122.10(5)
90
3169(2)
4
1.244
0.645
27.50
5634
543
0.0645
0.1733
1.083 (Fit on F2)
Space group
˚
a/A
˚
9.7805(12)
21.554(3)
21.618(4)
119.337(4)
99.184(4)
94.174(4)
3861.0(10)
4
1.263
0.5821
27.50
17046
16.667(2)
7.9761(7)
17.251(2)
90
91.260(2)
90
2292.7(4)
4
1.382
0.949
28.46
3659
352
0.0602
0.0605
0.224 (Fit on F)
b/A
˚
c/A
ꢅ/ꢂ
ꢇ/ꢂ
ꢈ/ꢂ
V/A
Z
3
˚
Dcalcd/Mg mꢁ3
ꢁ/mmꢁ1
ꢂ
ꢆmax
No. of observations
/
Parameters
R1 (I > 2ꢉ)
Rw
921
0.0563
0.1367
1.037 (Fit on F2)
S
was performed using full-matrix least-square techniques
SHELXS-97.17
solv
[Ni(acac)(dmap)]+
Sp - orange
[Ni(acac)(dmap)(solv)2]+
Oh - blue-green
X-ray data of complex 2 and [Ni(acac)(tmen)]BPh4 as the ref-
erence data were collected on a Mac Science M03XHF four-circle
diffractometer with graphite-monochromatized Mo Kꢅ radiation
heat
˚
(ꢃ ¼ 0:71073 A) at 298 K. The unit cell parameters were deter-
Ni
mined by a least-square method of 22 reflections (31:54ꢂ < 2ꢆ <
35:00ꢂ for complex 2, 10:33ꢂ < 2ꢆ < 23:54ꢂ for [Ni(acac)-
(tmen)]BPh4). A semi-empirical absorption correction was ap-
plied using psi-scans. The intensity data were collected by the
! ꢁ 2ꢆ scan technique and the intensities were corrected for
Lorentz and polarization effects. The structure was solved by the
direct method SIR92,18 and were refined by full-matrix least-
square techniques SHELXS-97.17 All non-hydrogen atoms were
refined with anisotropic thermal parameters. Hydrogen atoms
were included in calculated positions and refined with isotropic
thermal parameters. All diagrams and calculations were per-
formed using maXus (Bruker Nonius, Delft & MacScience,
Japan).
Ni
Scheme 1. Equilibrium between square-planar (Sp) and
octahedral (Oh) structure for complex 1 and 2.
662908 ([Ni(acac)(tmen)]BPh4). Copies of this information may
be obtained free of charge via www.ccdc.cam.ac.uk/data request/
cif, by e-mailing: data request@ccdc.cam.ac.uk, or contacting
CCDC, 12, Union Road, Cambridge, CB2 1EZ, UK [Fax: +44
1223 336033].
Equilibrium Constants. For solvatochromic and thermochro-
mic behavior of the complex 2, a quantitative approach was per-
formed with ln K, ꢀHo, and ꢀSo values for Scheme 1. Tempera-
ture-variation UV–vis spectra were measured in four donor sol-
vents, ACN, ACO, EtOH, and BuOH, in which two structures (Sp
and Oh) coexisted at room temperature. In ACN solution, there
are three absorptions; the band at 465 nm attributes to the square-
planar structure and the weak bands at 600 and 945 nm attribute to
the octahedral structure. As the temperature decreased, the inten-
sity at 465 nm decreased, while the intensity of other bands in-
creased with the isosbestic point. Since the "Oh at 465 nm is much
smaller than "Sp, the equilibrium constant of Scheme 1 is given
X-ray data of complex 3 was collected on a Bruker Smart
APEX CCD diffractometer with graphite-monochromatized
˚
Mo Kꢅ radiation (ꢃ ¼ 0:71073 A) at 296 K. The unit cell param-
eters were determined by a least-square method of 3348 reflec-
tions (4:719ꢂ < 2ꢆ < 47:325ꢂ). The intensity data were collected
by the ! ꢁ 2ꢆ scan technique, the intensities were corrected for
Lorentz and polarization effects, and an absorption correction
was applied using SADABS.19 The structure was solved by the di-
rect method SIR92,18 and expanded using Fourier techniques.15
All non-hydrogen atoms were refined with anisotropic thermal
parameters. Hydrogen atoms were included in calculated positions
and refined with isotropic thermal parameters. All calculations
were performed using the teXsan20 crystallographic software
package of Molecular Structure Corporation.
o
by the Eq. 1 where ASp is the absorbance in 100% of the
square-planar structure and ASp is the absorbance measured at
any temperature.
o
K ¼ [Oh]/[Sp] ¼ ðASp ꢁ ASpÞ=ASp
:
ð1Þ
Crystallographic data and refinement parameters are listed in
Table 2. Crystal data have been submitted to CCDC; the docu-
ment numbers are 287901 (1), 288106 (2), 662907 (3), and
In the linear ln K ꢁ 1=T plot, ꢀHo and ꢀSo values were calculat-
ed from the slope and intercept, respectively.5

130 Bull. Chem. Soc. Jpn. Vol. 81, No. 1 (2008)
Chromotropism of Nickel(II) Complexes
Fig. 3. An ORTEP drawing of [Ni(acac)(tmen)]BPh4. The
ꢁ
BPh4 ion is not shown for clarity. Displacement ellip-
soids are drawn with 50% probability.
Fig. 2. An ORTEP drawing of [Ni(acac)(dmap)]BF4 (2).
ꢁ
The BF4 ion is not shown for clarity. Displacement ellip-
soids are drawn with 50% probability.
Fig. 4. Molecular (a) and packing (b) structures of [Ni(acac)(dmap)(NO3)] (3). Displacement ellipsoids are drawn with 30% prob-
ability. There is a ꢀ-stuck interaction between two planes: Ni–acac–Ph (pink) and Ph (green).
Figures 2, 3, and 4 show the molecular structures of com-
plex 2, [Ni(acac)(tmen)]BPh4, and complex 3, respectively.
Results and Discussion
Properties of the Complexes in Solid State and Their
Crystal Structures. Complexes 1 and 2 are diamagnetic
and have one strong band at 464 nm in the solid-state reflec-
tance spectra, which indicate that they have a square-planar
structure of Ni2þ: d8 electron metal.21 On the other hand, com-
plex 3 is paramagnetic (ꢁeff ¼ 3:22 BM) and has two bands at
around 971 and 609 nm in the solid-state reflectance spectrum,
hence it has an octahedral structure. In the IR spectra, there are
weak but important bands due to the coordination mode of
The selected bond lengths and angles are listed in Table 3.
For complex 2, the central NiII is coordinated by two oxygen
atoms of the acac ligand, and one phosphorus and one nitrogen
of the dmap ligand to give a four-coordinated square-planar
structure. The two Ni–O(acac) bond lengths are different; the
Ni–O(trans to P) distance is longer than the Ni–O(trans to N)
distance because of the stronger trans influence originated by
the phosphorus atom. The former is comparable to the corre-
sponding Ni–O(acac) distances found in [Ni(acac)(dppe)]-
BF4.10 Similarly, the latter is comparable to those in [Ni(acac)-
(tmen)]BPh4. The bite angle of dmap ligand in complex 2,
P–Ni–N = 87.60(10)ꢂ, lies between the values of dppe in
[Ni(acac)(dppe)]BF4, P–Ni–P = 86.87(4)ꢂ, and tmen in [Ni-
(acac)(tmen)]BPh4, N–Ni–N = 87.97(11)ꢂ. From these re-
sults, the dmap complex is expected to have an intermediate
ligand field strength of the dppe complex and the tmen com-
plex. This is supported by solution behavior given later. Com-
plex 1 also shows a similar structural property to complex 2.
As stated above, the octahedral structure of complex 3 was
confirmed by crystallographic analysis. The small bite angle of
NO3 results in a distorted structure; O(1)–Ni(1)–O(4) =
163.1(2)ꢂ and N(2)–Ni(1)–O(3) = 161.2(2)ꢂ. The Ni–O(acac)
and Ni–O(NO3) bond lengths are similar to those previously
reported for the diamine analogues,23 which indicates that
ꢁ
NO3 in region of 1700–1800 cmꢁ1, where one band appears
ꢁ
due to the free NO3 ion. This band splits into two on coordi-
nation; when the splitting is small (ꢀNO ¼ 20 ꢁ 25 cmꢁ1) the
3
NO3ꢁꢁ is a monodentate, but when it is larger than 25 cmꢁ1, the
NO3 is a bidentate.22 In complex 3, there are two bands
(ꢀNO ¼ 46 cmꢁ1), indicating that the NO3 is a bidentate.
ꢁ
3
It is interesting to note that the differences between ꢄC=O
and ꢄC=C of acac ligand (ꢀacac) are also characteristic.2,21–24
This difference varies greatly from a square-planar structure
to an octahedral one. As the Ni–O(acac) bonds become weak-
er, the C=O bonds become stronger and the C=C bonds be-
come weaker, therefore ꢀacac value increases. We could con-
firm their structures from this viewpoint: the ꢀacac observed
in the octahedral complex 3 is larger than those of the
square-planar complexes 1 and 2.

M. Arakawa et al.
Bull. Chem. Soc. Jpn. Vol. 81, No. 1 (2008)
131
ꢂ
˚
Table 3. Selected Bond Lengths (A) and Angles ( )
in the case of complexes 1 and 2, while the ‘‘solute–solute–sol-
vent interaction’’ operates in the case of complex 3. In Table 4,
spectral data for these complexes in various solvents with dif-
ferent donor number are summarized. Complex 1 showed a
solvatochromic behavior similar to that of complex 2, even
though its behaviors in EtOH and BuOH could not be mea-
sured because complex 1 is not soluble in these solvents.
Figure 5 shows the UV–vis spectra and color variations of
complex 2 in DCE, ACN, and DMSO. In DCE, the solution
is orange, and the spectrum shows a strong absorption band
at 463 nm (" ¼ 560 dm3 molꢁ1 cmꢁ1), which means that the
complex 2 has a square-planar stereochemistry according to
the shift of the equilibrium shown in Scheme 1 to the left
side. On the other hand, the solution is blue-green in the strong
donor, DMSO, showing two relatively weak bands at 1059 and
643 nm (" ¼ 13:2 and 7.50 dm3 molꢁ1 cmꢁ1, respectively).
This means that two DMSO molecules are bound to the Ni2þ
center to form an octahedral geometry due to the shift of the
equilibrium in Scheme 1 to the right. In solvent with a medium
donor strength, such as ACN, the spectrum shows a strong
band at 465 nm and a relatively weak band around 950 nm, in-
dicating that both square-planar and octahedral species coexist
in solution and the equilibrium position depends on the tem-
perature. The spectrum of complex 2 in ACN shows only
one band attributable to the octahedral species because the
other band near 600 nm seems to be hidden in a strong band
attributed to the square-planar species. This is supported by
the spectrum in the same N-donor pyridine whose ꢃmax are
971 and 588 nm. The temperature-variation spectra shown in
Fig. 8 also agree with a shift of the equilibrium in Scheme 1
(see in thermochromic section). The ln K values given by
Eq. 1 vs. DN at room temperature (300 K) are plotted in
Fig. 6. It shows that the equilibrium in Scheme 1 shifts to
the right (octahedral structure) as the DN of the solvent in-
creases. The major species in ACO is a square-planar structure
for complex 2, while both square-planar and octahedral species
exist in ACO with each species given at a comparable concen-
tration in the case of the tmen analog, [Ni(acac)(tmen)]BPh4.2
These indicate that the square-planar species is much more sta-
ble in the dmap complex 2 due to the strong phosphorus donor
and the higher steric hindrance at axial site of Ni2þ in compar-
ison with the tmen complex. As already reported, the corre-
sponding dppe complex has very strong ligand field strength,
therefore only the square-planar species are observed in strong
donor solvents without any solvatochromic behavior.10
1a)
Ni(1)–O(1)
Ni(1)–O(2)
Ni(1)–P(1)
Ni(1)–N(1)
O(1)–C(18)
O(2)–C(20)
C(18)–C(19)
C(19)–C(20)
1.8790(17)
1.842(2)
2.1617(7)
1.969(3)
1.286(3)
1.298(4)
1.393(5)
1.381(3)
O(1)–Ni(1)–O(2)
P(1)–Ni(1)–N(1)
O(1)–Ni(1)–N(1)
O(1)–Ni(1)–P(1)
O(2)–Ni(1)–N(1)
O(2)–Ni(1)–P(1)
96.03(9)
87.78(6)
89.08(9)
175.79(8)
174.84(9)
87.15(6)
2
Ni(1)–O(1)
Ni(1)–O(2)
Ni(1)–P(1)
Ni(1)–N(1)
O(1)–C(18)
O(2)–C(20)
C(18)–C(19)
C(19)–C(20)
1.866(3)
1.837(2)
2.161(3)
1.960(2)
1.271(3)
1.297(3)
1.382(4)
1.370(5)
O(1)–Ni(1)–O(2)
P(1)–Ni(1)–N(1)
O(1)–Ni(1)–N(1)
O(1)–Ni(1)–P(1)
O(2)–Ni(1)–N(1)
O(2)–Ni(1)–P(1)
95.10(11)
87.60(10)
89.51(11)
173.68(7)
175.04(9)
87.99(9)
[Ni(acac)(tmen)]BPh4
Ni(1)–O(1)
Ni(1)–O(2)
Ni(1)–N(1)
Ni(1)–N(2)
O(1)–C(8)
O(2)–C(10)
C(8)–C(9)
C(9)–C(10)
1.840(2)
O(1)–Ni(1)–O(2)
N(1)–Ni(1)–N(2)
O(1)–Ni(1)–N(1)
O(1)–Ni(1)–N(2)
O(2)–Ni(1)–N(1)
O(2)–Ni(1)–N(2)
94.67(10)
87.97(11)
87.71(11)
175.39(11)
177.41(10)
89.68(11)
1.847(2)
1.937(3)
1.946(3)
1.265(4)
1.276(4)
1.388(5)
1.367(5)
3
Ni(1)–O(1)
Ni(1)–O(2)
Ni(1)–P(1)
Ni(1)–N(2)
O(1)–C(1)
O(2)–C(3)
C(1)–C(2)
C(2)–C(3)
Ni(1)–O(3)
Ni(1)–O(4)
1.976(4)
1.986(4)
2.426(1)
2.142(4)
1.263(7)
1.247(7)
1.396(8)
1.400(8)
2.168(4)
2.126(4)
O(1)–Ni(1)–O(2)
P(1)–Ni(1)–N(2)
O(3)–Ni(1)–O(4)
O(1)–Ni(1)–O(4)
O(2)–Ni(1)–P(1)
O(3)–Ni(1)–N(2)
91.9(2)
85.1(1)
59.8(2)
163.1(2)
175.3(1)
161.2(2)
a) Only the data for one of two independent molecules are
shown.
In some solvents such as ACN and EtOH, complex 2 exists
in two forms at room temperature, and the equilibrium in
Scheme 1 shifts depending on the temperature: the solution
turns orange at high temperature and green at low temperature.
The temperature-variation UV–vis spectra in four solvents
(ACN, ACO, EtOH, and BuOH) were measured in the con-
the trans influence of the phosphorus donor is not obvious in
this octahedral structure. It is interesting that one phenyl ring
of dmap (C(10)–C(15)) is coplanar to the Ni–acac ring within
the molecule (pink plane in Fig. 4b), and the other phenyl ring
of dmap (C(16)–C(21), green plane in Fig. 4b) forms a ꢀ-stack
with an adjacent Ni–acac ring (shortest distance C(3)–
centration range 0.0019–0.0021 mol dmꢁ3, as exemplified in
ꢄ
˚
Fig. 7a. As the temperature decreases, the absorbance at 465
nm corresponding to the square-planar species decreases, and
that at 600 nm corresponding to the octahedral species increas-
es at the same time. The ln K is plotted versus the inverse of
temperature (see Fig. 7b). The ꢀHo and ꢀSo values were cal-
culated from the slope and intercept of the plot (see Table 5
and Fig. 8). For determining the solvent donor ability, the
C(20) = 3.635 A). The Ni–O(acac), Ni–P, and Ni–N distances
in the six-coordinated complex 3 are respectively longer than
those in the four-coordinated complexes 1 and 2, in accordance
with the first bond variation rule.11
Solvato- and Thermochromism of the Complexes in
Solution. All complexes showed solvatochromism and
thermochromism. The ‘‘solute–solvent interaction’’ operates

Table 4. ꢃmax (nm) and " (dm3 molꢁ1 cmꢁ1) in Parenthesis for Complexes in Various Solvents with Different Donor Numbera)
Complex
DCE
NM
BzCN
ACN
TMS
ACO
EtOH
—
BuOH
—
DMF
DMSO
PY
1
462 (491)
463 (444)
464 (294)
600b)
950 (14.7)c)
462 (237)
600b)
942 (15.9)
462 (219)
630b)
1050 (3.43)c)
463 (446)
623 (10.4)
1030 (18.7)
636 (8.05)
1059 (14.4)
587 (9.50)
975 (7.71)
2
3
463 (560)
463 (414)
465 (316)
600b)
465 (217)
600b)
945 (13.2)
462 (203)
630b)
464 (435)
462 (183)
650 (5.21)
1056 (4.93)
462 (94.3)
636 (5.58)
1046 (7.22)
621 (10.7)
1032 (18.8)
643 (7.50)
1059 (13.2)
588 (9.00)
971 (7.29)
950 (7.77)c)
1070 (2.98)c)
460b)
603 (26.4)
1008 (24.9)
463 (213)
600b)
1001 (12.7)
460b)
604 (21.8)
1003 (20.7)
463 (65.9)
601 (20.4)
1004 (20.2)
461 (164)
600b)
1017 (7.79)
460b)
604 (24.7)
1010 (24.9)
462 (116)
605 (9.14)
1022 (9.91)
461 (63.5)
607 (13.0)
1021 (13.8)
623 (9.66)
1025 (16.7)
650 (7.24)
1068 (12.5)
587 (10.0)
956 (7.24)
a) Concentration of the complex is 0.00525 mol dmꢁ3. b) Shoulder. c) Broad.
ε
(DCE,ACN) / dm3mol−1cm−1
Absorbance
lnK
l nK
c m m o l
( D ε M S O ) / d m
1 −
1 −
3

M. Arakawa et al.
Bull. Chem. Soc. Jpn. Vol. 81, No. 1 (2008)
133
Table 5. Equilibrium Constants at 300 K, Enthalpies
(kJ molꢁ1) and Entropies (J molꢁ1 Kꢁ1
100
80
60
40
20
0
250
200
150
100
50
)
(a)
(b)
(c)
(d)
(b)
(c)
ε
Solvent
K
ꢀHo
ꢀSo
ACN
ACO
EtOH
BuOH
1.587
0.2898
2.064
4.944
ꢁ21:25
ꢁ22:64
ꢁ24:02
ꢁ24:32
ꢁ67:35
ꢁ103:3
ꢁ69:29
ꢁ77:21
25
24
(a)
(d)
EtOH
BuOH
0
ε
400
600
800
1000
1200
1400
Wavelength / nm
23
ACO
Fig. 9. UV–vis spectra of [Ni(acac)(dmap)NO3] (3); (a)
DCE, (b) NM, (c) ACN, and (d) DMSO. The peak at
1190 nm is due to noise from apparatus.
o
H
∆
22
21
20
Table 6. Molar Conductivity (ohmꢁ1 cm2 molꢁ1 at 25 ꢃ
0:1 ꢂC)a)
ACN
Complex
2b)
3
DCE
NM
ACN
DMSO
22.3
1.34
85.3
26.6
131
29.7
20.1
18.8
12
14
16
18
20
22
24
DN
a) Concentration of the complex is 0.00525 mol dmꢁ3. b) Tak-
en as a 1:1 electrolyte reference value.
Fig. 8. ꢁꢀHo–DN plot of [Ni(acac)(dmap)]BF4 (2).
definition of the Gutmann’s donor number (DN) is the ꢁꢀHo
value of an addition reaction in which a donor such as solvent
coordinates to the trigonal-bipyramidal SbCl5 changing to an
octahedral geometry.2 The same tendency was observed in
ꢁꢀHo versus DN plot obtained in this work. Furthermore,
the K value determined for ACO solution at 298 K is less than
that expected from its DN in Fig. 6, which is due to large con-
tribution of a negative ꢀSo value. It is considered that at room
temperature, an acetone molecule is difficult to coordinate to
the complex because of its medium donor ability and the steric
hindrance of two methyl groups compared with the other three
solvents, but at low temperature, the rotation of methyl groups
is decreased and the partial negative charge of oxygen atom is
strengthened by intermolecular interaction, hence, the acetone
molecule coordinates to the complex more easily.
weaker bands corresponding to the octahedral species are also
observed, as observed for the DCE solution (see Fig. 9). Thus,
the weak donor NM does not coordinate to the Ni2þ ion.
Therefore, the behavior of the complex in NM can be interpret-
ed in terms of a shift of the equilibrium between Sp and Oh1
species. In strong donor DMSO, the molar conductivity is
almost the same as that of a 1:1 electrolyte and the spectrum
is quite consistent with that of [Ni(acac)(dmap)]BF4 in DMSO
([Ni(acac)(dmap)(DMSO)2]þ) (Fig. 5c). It is considered that
ꢁ
two solvent molecules replaced the NO3 from the complex
and coordinated (Oh2 structure). In ACN which has intermedi-
ate donor/acceptor properties, the molar conductivity shows
that about 25 percent of the complex dissolved undergoes
ꢁ
NO3 dissociation. However, the relative abundance ratio of
the square-planar complex, estimated from the UV–vis spec-
trum, is much lower than 25 percent (ca. 10%) and the two
bands attributable to the octahedral species correspond to nei-
ther those of complex 3 in DCE (Fig. 9a, Oh1 structure) nor
those of complex 2 in ACN (Fig. 5b, Oh2 structure). These data
suggest that there is an equilibrium among three structures; Sp,
Oh1, and Oh2 in ACN solution. A possible reaction scheme of
three complexes is illustrated in Fig. 10. For mixed-ligand
nickel(II) systems containing the 1,3-diketonate and the di-
amine, their crystal structures with two solvent molecules show
exclusively cis octahedral geometries.24,25 However, in this
study, there is a possibility of co-existence of cis and trans iso-
mers in solution, and this point is remained as an open question.
Finally, when we compare the UV–vis spectra of the three
square-planar nickel(II) complexes [Ni(acac)(L)]þ (L = dmap
in this work, dppe10 and tmen2), it is noteworthy that the d–d
transition band attributed to the square-planar structure shifts
to higher energy with the increase in the number of the P-
The solvatochromism of complex 3 depends not only on the
donor property, but also on the acceptor property of the sol-
ꢁ
vent. In the strong acceptor solvent, the weak donor NO3
easily dissociates and is solvated by the solvent. In the strong
ꢁ
donor solvent, coordination of the solvent and NO3 to the
metal ion compete with each other. The structures of complex
3 in various solvents were investigated by UV–vis spectrosco-
py (Table 4 and Fig. 9) and molar conductivity measurements
(Table 6). In DCE, the UV–vis spectrum has two weak bands
indicating an octahedral structure and the molar conductivity is
much lower than that of the 1:1 electrolyte. This means thꢁat the
complex preserves an octahedral structure with the NO3 ion
coordinated (Oh1 structure) in DCE. In solvents with strong
acceptor ability, such as NM, the molar conductivity indicates
that about 30 percent of the complex dissolved undergoes
ꢁ
NO3 dissociation. In the UV–vis spectrum of the NM solu-
tion, one strong absorption band appears at 463 nm, while

134 Bull. Chem. Soc. Jpn. Vol. 81, No. 1 (2008)
Chromotropism of Nickel(II) Complexes
This work partly was supported by the Sasakawa Scientific
Research Grant from The Japan Science Society (M.A.). The
authors thank Prof. Yoichi Ishii, Chuo University, for his help
with elemental analysis.
O
P
Ni
X
O
N
strong donor
solv
weak donor
X = BF4, BPh4
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800
Wavelength / nm
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