Equilibrium of Low- and High-Spin States of Ni(II) Complexes Controlled by the Donor Ability of the Bidentate Ligands

Article abstract of DOI:10.1021/ic035486+

Low-spin nickel(II) complexes containing bidentate ligands with modulated nitrogen donor ability, Py(Bz)₂ or MePy-(Bz)₂ (Py(Bz) ₂ = N,N-bis(benzyl)-N-[(2-pyridyl)methyl]amine, MePy(Bz)₂ = N,N-bis(benzyl)-N-[(6-methyl-2-pyridyl)-methyl]amine), and a β-diketonate derivative, tBuacacH (tBuacacH = 2,2,6,6-tetramethyl-3,5-heptanedione), represented as [Ni(Py(Bz) ₂)(tBuacac)](PF₆) (1) and [NI(MePy(Bz) ₂)(tBuacac)](PF₆) (2) have been synthesized. In addition, the corresponding high-spin nickel(II) complexes having a nitrate ion, [Ni(Py(Bz)₂)(tBuacac)(NO₃)] (3) and [Ni(MePy(Bz) ₂)(tBuacac)(NO₃)] (4), have also been synthesized for comparison. Complexes 1 and 2 have tetracoordinate low-spin square-planar structures, whereas the coordination environment of the nickel ion in 4 is a hexacoordinate high-spin octahedral geometry. The absorption spectra of low-spin complexes 1 and 2 in a noncoordinating solvent, dichloromethane (CH₂Cl₂), display the characteristic absorption bands at 500 and 540 nm, respectively. On the other hand, the spectra of a CH ₂Cl₂ solution of high-spin complexes 3 and 4 exhibit the absorption bands centered at 610 and 620 nm, respectively. The absorption spectra of 1 and 2 in N,N-dimethylformamide (DMF), being a coordinating solvent, are quite different from those in CH₂Cl₂, which are nearly the same as those of 3 and 4 in CH₂Cl₂. This result indicates that the structures of 1 and 2 are converted from a low-spin square-planar to a high-spin octahedral configuration by the coordination of two DMF molecules to the nickel ion. Moreover, complex 1 shows thermochromic behavior resulting from the equilibrium between low-spin square-planar and high-spin octahedral structures in acetone, while complex 2 exists only as a high-spin octahedral configuration in acetone at any temperature. Such drastic differences in the binding constants and thermochromic properties can be ascribed to the enhancement of the acidity of the nickel ion of 2 by the steric effect of the o-methyl group in the MePy(Bz)₂ ligand in 2, which weakens the Ni-N(pyridine) bond length compared with that of the nonsubstituted Py(Bz)₂ ligand in 1.

Full text of DOI:10.1021/ic035486+

Inorg. Chem. 2004, 43, 3024−3030
Equilibrium of Low- and High-Spin States of Ni(II) Complexes Controlled
by the Donor Ability of the Bidentate Ligands
Hideki Ohtsu and Koji Tanaka*
Institute for Molecular Science, CREST, Japan Science and Technology Agency (JST),
38 Nishigonaka, Myodaiji, Okazaki, Aichi 444-8585, Japan
Received December 26, 2003
Low-spin nickel(II) complexes containing bidentate ligands with modulated nitrogen donor ability, Py(Bz)2 or MePy-
(Bz)2 (Py(Bz)2 ) N,N-bis(benzyl)-N-[(2-pyridyl)methyl]amine, MePy(Bz)2 ) N,N-bis(benzyl)-N-[(6-methyl-2-pyridyl)-
methyl]amine), and a â-diketonate derivative, tBuacacH (tBuacacH ) 2,2,6,6-tetramethyl-3,5-heptanedione),
represented as [Ni(Py(Bz)2)(tBuacac)](PF ) (1) and [Ni(MePy(Bz)2)(tBuacac)](PF ) (2) have been synthesized. In
6
6
addition, the corresponding high-spin nickel(II) complexes having a nitrate ion, [Ni(Py(Bz)2)(tBuacac)(NO )] (3) and
3
[Ni(MePy(Bz)2)(tBuacac)(NO )] (4), have also been synthesized for comparison. Complexes 1 and 2 have
3
tetracoordinate low-spin square-planar structures, whereas the coordination environment of the nickel ion in 4 is a
hexacoordinate high-spin octahedral geometry. The absorption spectra of low-spin complexes 1 and 2 in a
noncoordinating solvent, dichloromethane (CH Cl2), display the characteristic absorption bands at 500 and 540
2
nm, respectively. On the other hand, the spectra of a CH Cl2 solution of high-spin complexes 3 and 4 exhibit the
2
absorption bands centered at 610 and 620 nm, respectively. The absorption spectra of 1 and 2 in
N,N-dimethylformamide (DMF), being a coordinating solvent, are quite different from those in CH Cl2, which are
2
nearly the same as those of 3 and 4 in CH Cl2. This result indicates that the structures of 1 and 2 are converted
2
from a low-spin square-planar to a high-spin octahedral configuration by the coordination of two DMF molecules
to the nickel ion. Moreover, complex 1 shows thermochromic behavior resulting from the equilibrium between
low-spin square-planar and high-spin octahedral structures in acetone, while complex 2 exists only as a high-spin
octahedral configuration in acetone at any temperature. Such drastic differences in the binding constants and
thermochromic properties can be ascribed to the enhancement of the acidity of the nickel ion of 2 by the steric
effect of the o-methyl group in the MePy(Bz)2 ligand in 2, which weakens the Ni−N(pyridine) bond length compared
with that of the nonsubstituted Py(Bz)2 ligand in 1.
Introduction
ions can be controlled by external perturbation in the
viewpoint of applications to molecular memories and
switches.1-4,7 In the case of nickel(II) complexes, in par-
ticular, the transformation of the nickel(II) spin state from a
low-spin (S ) 0) state with a square-planar structure to a
high-spin (S ) 1) state with an octahedral geometry is
triggered by the coordination of two solvent molecules to
the nickel ion, which depends on the temperature and the
solvent.1,2 Such phenomena are known as thermochromism
and solVatochromism, respectively.
Control of the spin states of metal ions has attracted much
attention in inorganic1-4 and biological chemistry.5,6 It has
been of special interest to clarify how the spin states of metal
* To whom correspondence should be addressed. E-mail: ktanaka@
ims.ac.jp.
(1) Bloomquist, D. R.; Willett, R. D. Coord. Chem. ReV. 1982, 47, 125-
164.
(2) Sone, K.; Fukuda, Y. Inorganic Thermochromism; Springer: Berlin,
1987.
(3) Toftlund, H. Coord. Chem. ReV. 1989, 94, 67-128.
(4) (a) Gu¨tlich, P.; Hauser, A.; Spiering, H. Angew. Chem., Int. Ed. Engl.
1994, 33, 2024-2054. (b) Gu¨tlich, P.; Garcia, Y.; Goodwion, H. A.
Chem. Soc. ReV. 2000, 29, 419-427. (c) Gu¨tlich, P.; Garcia, Y.;
Woike, T. Coord. Chem. ReV. 2001, 219-221, 839-879.
(5) (a) Maltempo, M. M. J. Chem. Phys. 1974, 61, 2540-2547. (b)
Maltempo, M. M.; Moss, T. H.; Cusanovich, M. A. Biochim. Biophys.
Acta 1974, 342, 290-305.
(6) Fujii, S.; Yoshimura, T.; Kamada, H.; Yamaguchi, K.; Suzuki, S.;
Shidara, S.; Takakuwa, S. Biochim. Biophys. Acta 1995, 1251, 161-
169.
(7) (a) Kahn, O.; Launay, J. P. Chemtronics 1988, 3, 140-151. (b) Kahn,
O.; Kro¨ber, J.; Jay, C. AdV. Mater. (Weinheim, Ger.) 1992, 4, 718-
728.
3024 Inorganic Chemistry, Vol. 43, No. 9, 2004
10.1021/ic035486+ CCC: $27.50 © 2004 American Chemical Society
Published on Web 04/09/2004

Spin State Equilibrium of Ni(II) Complexes
Synthesis. All ligands used in this study were prepared according
to the following procedures, and the structures of the products were
confirmed by the analytical data (Vide infra).
N,N-Bis(benzyl)-N-[(2-pyridyl)methyl]amine (Py(Bz)2) and
N,N-bis(benzyl)-N-[(6-methyl-2-pyridyl)methyl]amine) (MePy-
(Bz)2). These ligands used in this study were prepared as described
previously.14
[Ni(Py(Bz)2)(tBuacac)](PF6) (1). To a methanol solution of Py-
(Bz)2 (0.288 g, 1.0 mmol) and tBuacacH (0.184 g, 1.0 mmol) was
added Ni(ClO4)2‚6H2O (0.366 g, 1.0 mmol) in methanol, and a few
drops of triethylamine were then added to the resulting solution.
The reaction mixture was stirred for 1 h at room temperature and
concentrated to dryness under reduced pressure. The residue was
dissolved in a dichloromethane solution, and ammonium hexafluo-
rophosphate (0.326 g, 2.0 mmol) was added to the solution. After
the mixture was stirred overnight at room temperature, insoluble
material was removed by filtration. Addition of diethyl ether to
the filtrate gradually gave red powder that was collected by filtration
and recrystallized from dichloromethane/diethyl ether. Yield: 0.304
g (45.0%). Anal. Calcd for C31H39O2N2P1F6Ni1: C, 55.14; H, 5.82;
N, 4.15. Found: C, 55.02; H, 5.74; N, 4.13. Electrospray ionization
(ESI) mass data: m/z 529 [M - PF6]+.
[Ni(MePy(Bz)2)(tBuacac)](PF6) (2). This complex was prepared
in the same manner as that for the synthesis of 1 using the MePy-
(Bz)2 ligand instead of Py(Bz)2. Yield: 0.197 g (28.6%). Anal.
Calcd for C32H41O2.5N2P1F6Ni1: C, 55.04; H, 6.06; N, 4.01.
Found: C, 55.21; H, 5.95; N, 4.07. ESI mass data: m/z 543 [M -
PF6]+.
[Ni(Py(Bz)2)(tBuacac)(NO3)] (3). Ni(NO3)2‚6H2O (0.291 g, 1.0
mmol) in acetonitrile was added to an acetonitrile solution contain-
ing Py(Bz)2 (0.288 g, 1.0 mmol) and tBuacacH (0.184 g, 1.0 mmol),
and a few drops of triethylamine were then added to the resulting
solution. The reaction mixture was stirred for 1 h at room
temperature and concentrated to dryness under reduced pressure.
The residue was dissolved in a dichloromethane solution and poured
into diethyl ether to give pale blue powder that was collected by
filtration and recrystallized from dichloromethane/diethyl ether.
Yield: 0.291 g (49.2%). Anal. Calcd for C31H40O5.5N3Ni1: C, 61.92;
H, 6.70; N, 6.99. Found: C, 62.08; H, 6.71; N, 7.23. Fast atom
bombardment (FAB) mass data: m/z 529 [M - NO3]+.
A number of nickel(II) complexes showing thermochromic
and solvatochromic behaviors have been synthesized to
explain the phenomena in terms of geometrical factors such
as changes in coordination geometry.8-13 Among those
nickel(II) complexes with thermochromic and solvatochromic
behaviors, however, only a few complexes have been
structurally established.13 To the best our knowledge, more-
over, there has so far been no attempt to control the
thermochromism and the solvatochromism, taking advantage
of the donor ability of the ligands.
We report herein the synthesis, the X-ray crystal structure
determination, and the electronic spectroscopic investigations
of low-spin square-planar nickel(II) complexes containing
bidentate ligands with modulated nitrogen donor ability, Py-
(Bz)2 or MePy(Bz)2 (Py(Bz)2 ) N,N-bis(benzyl)-N-[(2-
pyridyl)methyl]amine, MePy(Bz)2 ) N,N-bis(benzyl)-N-[(6-
methyl-2-pyridyl)methyl]amine),14 and a â-diketonate deriv-
ative, tBuacacH (tBuacacH ) 2,2,6,6-tetramethyl-3,5-hep-
tanedione), represented as [Ni(Py(Bz)2)(tBuacac)](PF6) (1)
and [Ni(MePy(Bz)2)(tBuacac)](PF6) (2). In addition, the
corresponding high-spin octahedral nickel(II) complexes
having a nitrate ion, [Ni(Py(Bz)2)(tBuacac)(NO3)] (3) and
[Ni(MePy(Bz)2)(tBuacac)(NO3)] (4), have also been syn-
thesized and the spectroscopic properties are compared with
those of the low-spin complexes. The drastic difference in
the solvatochromic and thermochromic behaviors accompa-
nied by the change of spin states from S ) 0 to S ) 1 is
observed by the comparison of the low-spin square-planar
nickel(II) complexes bearing Py(Bz)2 and MePy(Bz)2 ligands.
[Ni(MePy(Bz)2)(tBuacac)(NO3)] (4). This complex was prepared
in the same manner as that for the synthesis of 3 using the MePy-
(Bz)2 ligand instead of Py(Bz)2. Yield: 0.306 g (50.5%). Anal.
Calcd for C32H42O5.5N3Ni1: C, 62.46; H, 6.88; N, 6.83. Found: C,
62.48; H, 6.88; N, 6.88. FAB mass data: m/z 543 [M - NO3]+.
X-ray Structure Determination. The syntheses of complexes
1, 2, and 4 afforded well-shaped crystals suitable for X-ray
diffraction study. The crystals were mounted on a glass capillary.
The X-ray experiments were carried out on a Rigaku/MSC mercury
CCD diffractometer equipped with a Rigaku GNNP low-temper-
ature device. Data collections were performed at 173 K under cold
nitrogen gas using graphite monochromated Mo KR radiation (λ
) 0.71070 Å) and processed with Crystal Clear.16 The structure
was solved by the direct method17 and expanded by Fourier
techniques.18 The non-hydrogen atoms were refined anisotropically
by full-matrix least-squares calculations. Each refinement was
Experimental Section
Materials. All chemicals used for the synthesis of the ligands
and complexes were commercial products of the highest available
purities and were further purified by the standard methods.15
Solvents were also purified by standard methods before use.15
(8) (a) Ihara, Y.; Tsuchiya, R. Bull. Chem. Soc. Jpn. 1980, 53, 1614-
1617. (b) Ihara, Y.; Izumi, E.; Uehara, A.; Tsuchiya, R.; Nakagawa,
S.; Kyuno, E. Bull. Chem. Soc. Jpn. 1982, 55, 1028-1032. (c) Ihara,
Y.; Tsuchiya, R. Bull. Chem. Soc. Jpn. 1984, 57, 2829-2831. (d) Ihara,
Y. Bull. Chem. Soc. Jpn. 1985, 58, 3248-3251. (e) Ihara, Y.; Wada,
A.; Fukuda, Y.; Sone, K. Bull. Chem. Soc. Jpn. 1986, 59, 2309-
2315. (f) Ihara, Y.; Fukuda, Y.; Sone, K. Inorg. Chem. 1987, 26,
3745-3750.
(9) Fukuda, Y.; Sone, K. J. Inorg. Nucl. Chem. 1972, 34, 2315-2328.
(10) Vitiello, J. D.; Billo, E. J. Inorg. Chem. 1980, 19, 3477-3481.
(11) (a) Hay, R. W.; Bembi, R.; Sommerville, W. Inorg. Chim. Acta 1982,
59, 147-153. (b) Hay, R. W.; Jeragh, B.; Ferguson, G.; Kaitner, B.;
Ruhl, B. L. J. Chem. Soc., Dalton Trans. 1982, 1531-1539.
(12) Laskar, I. R.; Maji, T. K.; Das, D.; Lu, T.-H.; Wong, W.-T.; Okamoto,
K.; Chaudhuri, N. R. Polyhedron 2001, 20, 2073-2082.
(13) Flamini, A.; Fares, V.; Pifferi, A. Eur. J. Inorg. Chem. 2000, 537-
544.
(15) Perrin, D. D.; Armarego, W. L. F. Purification of Laboratory
Chemicals; Butterworth-Heinemann: Oxford, England, 1988.
(16) Crystal Clear; software package; Rigaku and Molecular Structure
Corporation: 1999.
(17) SIR92: Altomare, A.; Cascarano, G.; Giacovazzo, C.; Guagliardi, A.;
Burla, M. C.; Polidori, G.; Camalli, M. J. Appl. Crystallogr. 1994,
27, 435-436.
(14) Shimazaki, Y.; Nogami, T.; Tani, F.; Odani, A.; Yamauchi, O. Angew.
Chem., Int. Ed. Engl. 2000, 40, 3859-3862.
Inorganic Chemistry, Vol. 43, No. 9, 2004 3025

Ohtsu and Tanaka
Table 1. Crystal Data for Complexes 1, 2, and 4
1
2
4
formula
formula weight
color
C31H39N2O2Ni1P1F6
675.33
red
C32H41N2O2Ni1P1F6
689.35
purple
C32H41N3O5Ni1
606.39
light blue
crystal size/mm
crystal system
space group
a/Å
b/Å
c/Å
0.30 × 0.10 × 0.10
triclinic
P1h (No. 2)
10.8749(5)
11.9336(5)
14.6196(5)
71.93(1)
84.55(2)
63.03(1)
1605.2(2)
2
0.20 × 0.20 × 0.20
monoclinic
C2/c (No. 15)
21.82(1)
11.160(6)
27.91(1)
0.30 × 0.15 × 0.15
monoclinic
P21 (No. 4)
9.774(2)
9.712(2)
16.132(3)
R/deg
â/deg
104.536(5)
96.583(9)
γ/deg
V/Å3
6579(5)
8
1521.2(4)
2
Z
T/K
173
1.397
173
1.392
173
1.324
Dc/(g cm-3
radiation
µ/cm-1
)
Mo KR (λ ) 0.71070 Å)
7.19
704.00
55.0
7043
6978
388
1.88
Mo KR (λ ) 0.71070 Å)
7.04
Mo KR (λ ) 0.71070 Å)
6.82
F(000)/e
2θmax/deg
2880.00
55.0
7815
7392
397
1.74
0.062
0.078
644.00
55.0
3568
3562
371
1.28
0.034
0.048
no. of reflctns measd
no. of observations
no. of variables
GOF
Ra (I > 2.00σ(I))
Rwa (I > 2.00σ(I))
0.069
0.087
2
a R ) ∑(|Fo| - |Fc|)/∑|Fo|; Rw ) [∑w(|Fo| - |Fc|)2/∑w|Fo| ]1/2; w ) 1/σ2(Fo).
continued until all shifts were smaller than one-third of the standard
deviations of the parameters involved. Atomic scattering factors
were taken from the literature.19 All hydrogen atoms were located
at the calculated positions, and they were assigned a fixed
displacement and constrained to ideal geometry with C-H ) 0.95
Å. All the calculations were performed by using the teXsan
crystallographic software program package.20 Summaries of the
fundamental crystal data and experimental parameters for the
structure determination of complexes 1, 2, and 4 are given in Table
1.
Instrumental Measurements. Electronic spectra were measured
with a Hewlett-Packard 8453 diode array spectrophotometer with
a Unisoku thermostated cell holder designed for low-temperature
experiments. NMR measurements were performed with a JEOL
GX-500 (500 MHz) NMR spectrometer. ESI mass spectra were
obtained with a Shimadzu LCMS-2010 liquid chromatograph mass
spectrometer. FAB mass spectra were recorded with a JEOL JMS-
GCmateII spectrometer. Cold-spray ionization (CSI) mass spectra
were taken on a JEOL JMS-T100CS spectrometer attached to a
syringe pump apparatus (Harvard Apparatus model 22). Sample
solutions were delivered to the sprayer through a fused silica
capillary (100 µm diameter) at 10 µL/s. The sprayer was held at a
potential of 2.0 kV, and compressed N2 which controlled the
temperature at 248 K was employed to assist liquid nebulization.
The orifice potential was maintained at 30 V. The positive ion CSI
mass spectra were measured in the range m/z ) 100-1000.
Variable-temperature magnetic susceptibility data were measured
on polycrystalline samples of complexes 1, 2, 3, and 4 using a
Quantum Design MPMS-7 SQUID susceptometer in the temper-
ature range 4-300 K with an applied field of 5.0 kG. The
polycrystalline sample was embedded in Parafilm to prevent
torquing of the crystallites in an external field. A diamagnetic
correction, estimated from Pascal’s constants,21 was added from
the experimental susceptibilities to give the molar paramagnetic
susceptibilities. Elemental analyses were carried out at the Research
Center for Molecular-scale Nanoscience, Institute for Molecular
Science.
Results and Discussion
Crystal Structures. ORTEP views of low-spin nickel (II)
complexes 1 and 2 are shown in Figure 1 (parts a and b,
respectively), and that of the corresponding high-spin nickel-
(II) complex 4 containing a nitrate ion is shown in Figure 2.
Selected bond distances and angles of complexes 1, 2, and
4 are summarized in Tables 2, 3, and 4, respectively.
The nickel ions in 1 and 2 have tetracoordinate structures
formed by the pyridine nitrogen, the tertiary amine nitrogen,
and two oxygens from tBuacac. The bond lengths of 1 and
2 from the nickel ion to each of the donor nitrogens and
oxygens are Ni-N(1), 1.893(3) and 1.936(2) Å; Ni-N(2),
1.944(3) and 1.949(3) Å; Ni-O(1), 1.842(3) and 1.844(2)
Å; and Ni-O(2), 1.821(3) and 1.845(2) Å, respectively. It
should be noted that the Ni-N(1) distance in 2 (1.936(2)
Å) is significantly longer than that in 1 (1.893(3) Å). Such
a difference in the Ni-N(pyridine) distance may be ascribed
to the steric effect of the o-methyl group22 of the MePy-
(Bz)2 ligand in 2, which leads to weaker coordination to the
nickel ion as compared to the case of the nonsubstituted Py-
(Bz)2 ligand in 1. This is consistent with the higher value of
the binding constant with DMF in 2 than in 1, as discussed
later. The dihedral angles between the Ni-N(1)-N(2) plane
and the Ni-O(1)-O(2) one in 1 and 2 are 12.4 and 19.9°,
(18) DIRDIF94: Beurskens, P. T.; Admiraal, G.; Beurskens, G.; Bosman,
W. P.; de Gelder, R.; Israel, R.; Smits, J. M. M. The DIRDIF94
program system; Crystallography Laboratory, University of Nij-
megen: The Netherlands, 1994.
(19) International Tables for X-ray Crystallography; Kynoch Press: Bir-
mingham, U.K., 1974; Vol. IV.
(20) teXsan; crystal structure analysis package; Molecular Structure
Corporation: 1985 and 1999.
(21) Mabbs, F. E.; Marchin, D. J. Magnetisms and Transition Metal
Complexes; Chapman and Hall: London, 1975.
3026 Inorganic Chemistry, Vol. 43, No. 9, 2004

Spin State Equilibrium of Ni(II) Complexes
Table 2. Selected Bond Distances and Angles of Complex 1
Bond Distances (Å)
Ni-N(1)
Ni-N(2)
1.893(3)
1.944(3)
Ni-O(1)
Ni-O(2)
1.842(2)
1.821(3)
Bond Angles (deg)
N(1)-Ni-N(2)
N(1)-Ni-O(1)
N(1)-Ni-O(2)
86.0(1)
169.6(1)
90.2(1)
N(2)-Ni-O(1)
N(2)-Ni-O(2)
O(1)-Ni-O(2)
89.9(1)
170.5(1)
95.3(1)
Table 3. Selected Bond Distances and Angles of Complex 2
Bond Distances (Å)
Ni-N(1)
Ni-N(2)
1.936(2)
1.949(3)
Ni-O(1)
Ni-O(2)
1.844(2)
1.845(2)
Bond Angles (deg)
N(1)-Ni-N(2)
N(1)-Ni-O(1)
N(1)-Ni-O(2)
85.7(1)
165.5(1)
94.38(10)
N(2)-Ni-O(1)
N(2)-Ni-O(2)
O(1)-Ni-O(2)
88.39(10)
164.3(1)
94.87(9)
Table 4. Selected Bond Distances and Angles of Complex 4
Bond Distances (Å)
Ni-N(1)
Ni-N(2)
Ni-O(1)
2.086(3)
2.184(2)
1.983(2)
Ni-O(2)
Ni-O(3)
Ni-O(4)
1.979(2)
2.120(2)
2.182(3)
Bond Angles (deg)
N(1)-Ni-N(2)
N(1)-Ni-O(1)
N(1)-Ni-O(2)
N(1)-Ni-O(3)
N(1)-Ni-O(4)
N(2)-Ni-O(1)
N(2)-Ni-O(2)
N(2)-Ni-O(3)
80.9(1)
177.23(9)
90.7(1)
90.70(9)
93.75(10)
96.67(9)
103.20(10)
92.55(9)
N(2)-Ni-O(4)
O(1)-Ni-O(2)
O(1)-Ni-O(3)
O(1)-Ni-O(4)
O(2)-Ni-O(3)
O(2)-Ni-O(4)
O(3)-Ni-O(4)
152.34(10)
91.10(8)
88.1(1)
87.8(1)
164.22(9)
103.99(9)
60.23(9)
The transformation from low-spin square-planar complexes
1 and 2 to the corresponding high-spin octahedral complexes
was demonstrated by the 1:1 adduct formation with a nitrate
ion affording 3 and 4, respectively (see the Experimental
Section), and well-shaped crystals suitable for X-ray structure
determination were obtained in the case of 4. The coordina-
tion geometry of the nickel ion in 4 is an octahedral structure
with two nitrogens of the pyridine and the tertiary amine,
two oxygens from tBuacac, and two oxygens of the nitrate
ion bonded in the bidentate manner, as shown in Figure 2.
The bond lengths and angles around the nickel coordination
sphere of 4 (see Table 4) are essentially the same as those
of the reported complex [Ni(dipe)(acac)(NO3)].23,24 Thus, the
structure of 3 is also quite similar to those of 4 and [Ni-
(dipe)(acac)(NO3)],23,24 with respect to the coordination
geometry. In addition, the effective magnetic moments of
3, 4, and [Ni(dipe)(acac)(NO3)]23,24 were 3.20, 3.02, and 3.23
µB, respectively,23,25 indicating that the spin states of the
nickel ions in these complexes are high-spin states (S ) 1).
Absorption Spectra in Noncoordinating and Coordi-
nating Solvents. Parts a and b of Figure 3 show the
Figure 1. ORTEP views of (a) [Ni(Py(Bz)2)(tBuacac)](PF6) (1) and (b)
[Ni(MePy(Bz)2)(tBuacac)](PF6) (2). The hydrogen atoms and the counter-
anion are omitted for clarity. Thermal ellipsoids are drawn at the 50%
probability level.
(22) (a) Nagao, H.; Komeda, N.; Mukaida, M.; Suzuki, M.; Tanaka, K.
Inorg. Chem. 1996, 35, 6809-6815. (b) Shimazaki, Y.; Huth, S.;
Hirota, S.; Yamauchi, O. Bull. Chem. Soc. Jpn. 2000, 73, 1187-1195.
(c) Ohtsu, H.; Shimazaki, Y.; Odani, A.; Yamauchi, O.; Itoh, S.;
Fukuzumi, S. J. Am. Chem. Soc. 2000, 122, 5733-5741. (d) Ohtsu,
H.; Itoh, S.; Nagatomo, S.; Kitagawa, T.; Ogo, S.; Watanabe, Y.;
Fukuzumi, S. Inorg. Chem. 2001, 40, 3200-3207. (e) Ohtsu, H.;
Fukuzumi, S. Chem. Lett. 2001, 920-921.
Figure 2. ORTEP view of [Ni(MePy(Bz)2)(tBuacac)(NO3)] (4). The
hydrogen atoms are omitted for clarity. Thermal ellipsoids are drawn at
the 50% probability level.
respectively. Thus, the coordination environments of 1 and
2 are slightly distorted square-planar structures, which are
consistent with the magnetic susceptibilities of 1 and 2 that
were diamagnetic (S ) 0).
(23) Fukuda, Y.; Fujita, C.; Miyamae, H.; Nakagawa, H.; Sone, K. Bull.
Chem. Soc. Jpn. 1989, 62, 745-752.
(24) dipe ) 1,2-dipiperidinoethane, acac ) acetylacetonate.
Inorganic Chemistry, Vol. 43, No. 9, 2004 3027

Ohtsu and Tanaka
Figure 5. Absorption spectral change observed upon addition of various
amounts of DMF into a CH2Cl2 solution of [Ni(Py(Bz)2)(tBuacac)](PF6)
(1: 10.0 mM) at 298 K. Inset: plot of (A0 - A)/(A - A∞) vs [DMF]2.
Figure 3. Absorption spectra of (a) [Ni(Py(Bz)2)(tBuacac)](PF6) (1: 10.0
mM) and (b) [Ni(MePy(Bz)2)(tBuacac)](PF6) (2: 10.0 mM) in CH2Cl2 at
298 K.
bands due to 3A2g f 3T1g (F) transitions.26 Such a difference
in the absorption maxima between 1 and 2 or between 3
and 4 may be caused by the steric effect of the o-methyl
group of the pyridine moiety. In addition, the reflectance
spectra of 1, 2, 3, and 4 in the solid states showed the same
bands, indicating that the structures of 1, 2, 3, and 4 are
maintained in the solution states.
The absorption spectra of 1 and 2 completely change when
a coordinating solvent, N,N-dimethylformamide (DMF), has
been used instead of a noncoordinating solvent, CH2Cl2. The
3A2g f 3T1g (F) transition bands of 1 and 2 in DMF appeared
at λmax (ꢀ/(M-1 cm-1)) ) 630 (20) and 645 (20) nm,
respectively, which are quite close to those of 3 and 4 in
CH2Cl2, as shown in Figure 4. This result indicates that the
structures of 1 and 2 are transformed from a low-spin square-
planar to a high-spin octahedral configuration because of the
coordination of two DMF molecules to the tetradentate
square-planar nickel ion. Formation of the 1:2 DMF adducts
has been further confirmed by the CSI mass spectrum of 1
in DMF, which exhibited a signal at m/z ) 677. The observed
mass and isotope patterns correspond to the ion [Ni(Py(Bz)2)-
(tBuacac)(DMF)2]+.
As expected from the solvatochromic properties of 1 and
2, the spectral changes can be observed upon gradual addition
of DMF into a CH2Cl2 solution of 1 and 2, as shown in
Figures 5 and 6. The absorption bands at 500 (for 1) and
540 nm (for 2) characteristic of the nickel(II) low-spin
square-planar complexes decrease with increasing the con-
centration of added DMF, accompanied by increases in the
absorption bands at 615 and 635 nm, respectively, which
belong to the corresponding high-spin octahedral DMF
adducts. Such spectral changes are reasonably associated with
the 1:2 complex formation between 1 or 2 and DMF, as
shown in Scheme 1.27 The binding constants (Kb) for the
1:2 adducts with DMF expressed by eq 1 are calculated by
using the equation
Figure 4. Absorption spectra of (a) [Ni(Py(Bz)2)(tBuacac)(NO3)] (3: 10.0
mM) and (b) [Ni(MePy(Bz)2)(tBuacac)(NO3)] (4: 10.0 mM) in CH2Cl2 at
298 K.
absorption spectra of 1 and 2, respectively, in dichlo-
romethane (CH2Cl2), which is a noncoordinating solvent. An
intense absorption band at 500 nm (ꢀ ) 120 M-1 cm-1) of
1 (Figure 3a) can be assigned to the d-d transition (1A1g f
1A2g),26 which is characteristic of low-spin square-planar
nickel(II) complexes. This is consistent with the results of
X-ray crystal structure determination and magnetic suscep-
tibility. The absorption spectrum of a CH2Cl2 solution of 2
(Figure 3b) displays the 1A1g f 1A2g transition band26 at 540
nm (ꢀ ) 120 M-1 cm-1) which is red-shifted compared to
that of 1 (500 nm, ꢀ ) 120 M-1 cm-1).
The absorption spectra of the corresponding high-spin
complexes 3 and 4 having a hexacoordinate octahedral
structure in CH2Cl2 are shown in parts a and b of Figure 4,
respectively. The spectra of 3 and 4 exhibit prominent bands
centered at 610 and 620 nm (ꢀ ) 20 M-1 cm-1 in both
complexes), respectively, and the absorption coefficient of
4 is slightly red-shifted compared to that of 3. These
characteristic bands at ∼600 nm are assigned to the d-d
Kb ) [Ni(L)(tBuacac)(DMF)2]/[Ni(L)(tBuacac)] [DMF]2
(25) The effective magnetic moments of 3 and 4 are independent of the
measured temperature range (4-300 K).
(26) Lever, A. B. P. Inorganic Electronic Spectroscopy, 2nd ed.; Elsevier
Science: Amsterdam, The Netherlands, 1984.
(1)
L ) Py(Bz)2 or MePy(Bz)2
3028 Inorganic Chemistry, Vol. 43, No. 9, 2004

Spin State Equilibrium of Ni(II) Complexes
Scheme 1
Scheme 2
(A0 - A)/(A - A∞) ) 0.01Kb[DMF]2. The plots of
(A0 - A)/(A - A∞) versus [DMF]2 give straight lines pass-
ing through the origin, as depicted in the insets of Figures 5
and 6. The Kb values are determined as 0.20 and 930 M-1
for 1 and 2, respectively, from the slopes of the plots. The
significantly larger binding constant of 2 compared with that
of 1 indicates that the o-methyl group of the MePy(Bz)2
ligand greatly enhances the acidity of the nickel ion. This is
consistent with the longer Ni-N(pyridine) distance in 2
(1.936(2) Å) than in 1 (1.893(3) Å), derived from the crystal
structures (Vide supra). Such a drastic difference of Kb values
in 1 and 2 reflects the thermal behavior of both complexes
as described in the next section.
ture-dependent change between the low-spin square-planar
and high-spin octahedral structures. Reliable magnetic
susceptibility of 1 in acetone, however, was not obtained
because of the low solubility of the complex at low
temperature. The spectral change depending on the temper-
ature can be reasonably explained by the shift of the
equilibrium to the right with decreasing temperature (Scheme
2).
Indeed, the absorption spectrum of 1 in acetone at 183 K
is quite close to that in the presence of DMF in CH2Cl2
(Figure 5). On the other hand, the absorption spectrum of 2
in acetone displayed only the 630 nm band arising from the
high-spin octahedral configuration at room temperature, and
no spectral change was observed upon cooling the solution
down to 183 K.
In contrast to the case of 1, the lack of thermochromism
in an acetone solution of 2 apparently results from too strong
an affinity of the nickel ion for acetone. The distinct
difference in the thermochromic behavior between 1 and 2
is associated with the binding constants of 1 and 2 with DMF,
Kb ) 0.20 and 930 M-1, respectively (Vide supra). Thus,
the o-methyl group of the pyridine moiety in 2 plays the
key role in developing thermochromism as well as solvato-
Thermochromic Properties. We observed that complexes
1 and 2 display very different thermal behaviors in acetone,
which is a weaker coordinating solvent as compared to DMF.
The absorption spectrum of 1 in acetone at room temperature
is quite close to that in CH2Cl2 (see Figure 3a) and exhibits
1
1
the A1g f A2g transition band26 at 500 nm. Upon cooling
an acetone solution of 1 down to 183 K, however, the color
gradually changes from red (λmax ) 500 nm) toward pale
blue (λmax ) 610 nm), as shown in Figure 7. The remarkable
thermochromism is associated with the reversible tempera-
Figure 6. Absorption spectral change observed upon addition of various
amounts of DMF into a CH2Cl2 solution of [Ni(MePy(Bz)2)(tBuacac)](PF6)
(2: 10.0 mM) at 298 K. Inset: plot of (A0 - A)/(A - A∞) vs [DMF]2.
Figure 7. Absorption spectral change observed by decreasing the
temperature from 298 to 183 K in an acetone solution of [Ni(Py(Bz)2)-
(tBuacac)](PF6) (1: 10.0 mM).
Inorganic Chemistry, Vol. 43, No. 9, 2004 3029

Ohtsu and Tanaka
chromism by weakening the Ni-N(pyridine) bond at a
certain level compared with the case of the nonsubstituted
Py(Bz)2 ligand in 1.
indicates that the steric effect of the o-methyl group of the
MePy(Bz)2 ligand greatly enhances the affinity of the central
Ni(II) ion toward coordinating molecules. As a result, an
acetone solution of 2 exists as octahedral high-spin [Ni-
(MePy(Bz)2)(tBuacac)(acetone)2]+ in a wide range of tem-
peratures. On the other hand, complex 1 maintains the low-
spin square-planar geometry ([Ni(Py(Bz)2)(tBuacac)]+) in
acetone at room temperature, but the molecular structure of
1 gradually changes to octahedral high-spin [Ni(Py(Bz)2)-
(tBuacac)(acetone)2]+ with decreasing temperature. Thus,
control of the spin states in nickel(II) complexes can be
accomplished by fine-tuning of the donor abilities of the
ligands. The present results will provide valuable information
for the development of molecular memories and switches.1-4,7
Conclusions
The low-spin square-planar nickel(II) complexes [Ni(Py-
(Bz)2)(tBuacac)](PF6) (1) and [Ni(MePy(Bz)2)(tBuacac)]-
(PF6) (2), having bidentate ligands with modulated nitrogen
donor ability and â-diketonate derivatives, exhibit the unique
solvatochromism as well as the thermochromism induced by
the change of the spin states of the nickel(II) ion from the
square-planar low-spin (S ) 0) to the octahedral high-spin
(S ) 1) states. The binding constants of 1 and 2 for the 1:2
adduct formation with DMF are determined as 0.20 and 930
M-1, respectively, by the titration upon addition of DMF
into a CH2Cl2 solution of 1 and 2. The significantly larger
binding constant of 2 compared with that of 1 clearly
Acknowledgment. We are grateful to JEOL Ltd. for CSI
mass measurements.
Supporting Information Available: X-ray crystallographic file
in CIF format for complexes 1, 2, and 4. This material is available
free of charge via the Internet at http://pubs.acs.org.
(27) The octahedral diaqua-complex [Ni(en)2(H2O)2]2+ (en ) ethylenedi-
amine) favors the cis configuration as compared to the trans one in
the solution state: Farago, M. E.; James, J. M.; Trew, V. C. G. J.
Chem. Soc. A 1967, 820-824. In the case of the DMF adducts with
1 and 2, the type of coordination has yet to be determined.
IC035486+
3030 Inorganic Chemistry, Vol. 43, No. 9, 2004