Steric crowding effect of 1,3-diketonates on the structures and the solution behaviors of nickel(II) mixed-ligand complexes

Article abstract of DOI:10.1246/bcsj.20090322

Seven nickel(II) mixed-ligand complexes were synthesized: Ni(dike)(diam)X, where dike is a 1,3-diketonate such as 3,5-heptanedionate (dedk) or 2,6-dimethyl-3,5-heptanedionate (dmhd), diam is an N-alkylated ethylenediamine such as N,N,N′,N′-etramethylethylenediamine (tmen) or 1,2-dipiperidinoethane (dipe), and X is BPh₄ ^- or NO ₃ ^-. The crystal structures were analyzed: each nitrate complex has a six-coordinated octahedral structure with a bidentate-coordinated nitrate, and each tetraphenylborate complex has a four-coordinated square-planar structure. The ligand field strength of [Ni(dike)(diam)]BPh₄ in the solid state increases in the order: [Ni(dmhd)(dipe)]BPh₄ < [Ni(dedk)(dipe)]BPh₄ < [Ni(dmhd)(tmen)]BPh₄ = [Ni(dedk)(tmen)]BPh₄. In solution, the change of the color depending on the donor/acceptor ability of the solvent, i.e., solvatochromism was observed. When dimethyl sulfoxide was added to a solution of [Ni(dike)(diam)]BPh₄ in inert 1,2-dichloroethane, the adduct formation constants are in the order: [Ni(dmhd)(dipe)]-BPh₄ < [Ni(dedk)(dipe)]BPh₄ [Ni(dmhd)(tmen)]BPh₄ = [Ni(dedk)(tmen)]BPh₄, reflecting the difference in steric hindrance of the ligands rather than the difference in ligand field strength. The steric crowding effect of 1,3-diketonates was observed in the solid state and solution when bulky dipe is used as diamine.

Full text of DOI:10.1246/bcsj.20090322

790
Bull. Chem. Soc. Jpn. Vol. 83, No. 7, 790–798 (2010)
© 2010 The Chemical Society of Japan
Steric Crowding Effect of 1,3-Diketonates on the Structures and
the Solution Behaviors of Nickel(II) Mixed-Ligand Complexes
Machiko Arakawa-Itoh,1 Masayo Shibata,1 Wolfgang Linert,2 and Yutaka Fukuda*1
1Department of Chemistry, Faculty of Science, Ochanomizu University, 2-1-1 Otsuka, Bunkyo-ku, Tokyo 112-8610
2Institute of Applied Synthetic Chemistry, Technical University of Vienna, Getreidemarkt 9, A-1060 Vienna, Austria
Received December 7, 2009; E-mail: fukuda.yutaka@gmail.com
Seven nickel(II) mixed-ligand complexes were synthesized: Ni(dike)(diam)X, where dike is a 1,3-diketonate such as
3,5-heptanedionate (dedk) or 2,6-dimethyl-3,5-heptanedionate (dmhd), diam is an N-alkylated ethylenediamine such as
¹
¹
N,N,N¤,N¤-tetramethylethylenediamine (tmen) or 1,2-dipiperidinoethane (dipe), and X is BPh4 or NO3 . The crystal
structures were analyzed: each nitrate complex has a six-coordinated octahedral structure with a bidentate-coordinated
nitrate, and each tetraphenylborate complex has a four-coordinated square-planar structure. The ligand field strength
of [Ni(dike)(diam)]BPh4 in the solid state increases in the order: [Ni(dmhd)(dipe)]BPh4 < [Ni(dedk)(dipe)]BPh4 <
[Ni(dmhd)(tmen)]BPh4 = [Ni(dedk)(tmen)]BPh4. In solution, the change of the color depending on the donor/acceptor
ability of the solvent, i.e., solvatochromism was observed. When dimethyl sulfoxide was added to a solution of
[Ni(dike)(diam)]BPh4 in inert 1,2-dichloroethane, the adduct formation constants are in the order: [Ni(dmhd)(dipe)]-
BPh4 < [Ni(dedk)(dipe)]BPh4 ¹ [Ni(dmhd)(tmen)]BPh4 = [Ni(dedk)(tmen)]BPh4, reflecting the difference in steric
hindrance of the ligands rather than the difference in ligand field strength. The steric crowding effect of 1,3-diketonates
was observed in the solid state and solution when bulky dipe is used as diamine.
Some 3d transition-metal complexes have a color due to a
diam has been studied in previous work.2,4 So far, the steric
effect of dike has been considered rather small since substituent
groups on dike protrude away from the nickel(II) center. Even
the steric hindrance of the bulky tert-butyl substituents on dipm
(=dipivaloylmethanate) is obscured by its strong electron-
releasing property. In order to know the “pure” steric effect
of dike on properties of the mixed-ligand nickel(II) complex,
we used two 1,3-diketonate ligands here: 3,5-heptanedionate
(dedk) and 2,6-dimethyl-3,5-heptanedionate (dmhd) shown
in Figure 1. Since the Hammett’s ·m values of ethyl- and
isopropyl groups are the same (·m = ¹0.08),8 these ligands can
be considered to have the same electron-releasing property, and
the steric crowding of dmhd is larger than that of dedk. Hence
properties of the complexes having the same conditions of the
nickel(II) center except for dike should reflect the steric
hindrance of dike. For diamine ligands, at first, most common
N,N,N¤,N¤-tetramethylethylenediamine (tmen, Figure 1) was
d-d transition which depends on the coordination structure. For
example, nickel(II) can form four-, five-, and six-coordinated
complexes and exhibit a variety of colors. We have synthesized
a series of mixed-ligand nickel(II) complexes with 1,3-
diketonate (dike) and N-alkylated ethylenediamine (diam)
ligands, Ni(dike)(diam)X where X is a mono-anion.1-4 We
adopted the mixed-ligand system of the sterically hindered
diam and the slim dike to obtain a stable complex and to
control the environment of the nickel(II) center finely. This
type of complex shows remarkable solvatochromism due to an
equilibrium among the octahedral (Oh) and square-planar (Sp)
structures. When X is non-coordinating tetraphenylborate ion,
the structure remains square-planar in inert solvent such as 1,2-
dichloroethane. On the other hand, in solvent with strong donor
ability, such as dimethyl sulfoxide, two solvent molecules
coordinate to nickel(II) and the Sp ꢀ Oh equilibrium shifts to
the right. In solvent with medium donor ability such as acetone,
two species coexist. In our research, Gutmann’s donor number
(DN) is used for the parameter of the donor ability of the
solvent, and acceptor number (AN) is used for that of the
acceptor ability of the solvent.5 Quantitative approaches have
been also performed in this mixed-ligand nickel(II) system by
determining the equilibrium constant,3,6,7 which reflects the
degree of the interaction between the complex and the solvent
molecules.
O
O
O
O
Hdmhd
Hdedk
Solvatochromic behavior of Ni(dike)(diam)X is affected by
the steric crowding around the nickel(II) center and the ligand
field stabilization energy. Larger steric hindrance and/or
stronger ligand field strength of the complex more stabilizes
the square-planar structure. The effect of steric hindrance of
N
N
N
N
dipe
tmen
Figure 1. Structures of ligands used in this study.
Published on the web June 30, 2010; doi:10.1246/bcsj.20090322

M. Arakawa-Itoh et al.
Bull. Chem. Soc. Jpn. Vol. 83, No. 7 (2010)
791
Table 1. Absorption Spectra of the Nitrate Complexes
in Solid State and in Various Solventsa) (-max/nm,
used, which allows us to easily compare with many analogous
complexes Ni(dike)(tmen)X.1 Bulky and rigid 1,2-dipiperi-
dinoethane (dipe, Figure 1) was then adopted because the dipe
ligand is known to make the solvatochromism of the complex
more remarkable than that of tmen complex due to the steric
effect of the bulky piperidine rings.2
¹1
¹3
¾
max/dm3 mol¹1 cm
in Parentheses, Concn: 8 © 10
¹3
mol dm
)
Complex
Solid DCE
NM
dmso
[Ni(acac)(tmen)(NO3)]b) 1034 1045 (10.1) 1040 (8.5) 1059 (8.0)
605 614 (19.3) 615 (17.5) 634 (6.3)
482 (6.5)
In the view of these molecular designs, seven solvatochro-
mic complexes are newly obtained: [Ni(dedk)(tmen)(NO3)]
(1a), [Ni(dmhd)(tmen)(NO3)] (2a), [Ni(dedk)(dipe)(NO3)]
(3a), [Ni(dedk)(tmen)]BPh4 (1b), [Ni(dmhd)(tmen)]BPh4
(2b), [Ni(dedk)(dipe)]BPh4 (3b), and [Ni(dmhd)(dipe)]BPh4
(4b). The crystal structures of all complexes except for 2b
have been determined by single-crystal X-ray analysis. The
properties in solution of the complexes were investigated by
UV-vis spectroscopy. For the tetraphenylborate complexes 1b-
4b, the equilibrium constants between the square-planar and
adduct species have been determined by the spectral changes
associated with the addition of a strong donor solvent molecule.
Especially, five-coordinated species have been observed as
equilibrium mixtures in the case of dipe complexes 3b and 4b.
These results are discussed from the viewpoint of the steric
crowding effects of the dike ligand.
1a
1036 1041 (11.3) 1038 (10.1) 1056 (9.1)
617 613 (22.3) 615 (21.1) 635 (7.4)
488 (11.9)
2a
1034 1038 (16.1) 1034 (9.6) 1052 (9.9)
612 611 (31.1) 613 (20.0) 631 (7.5)
487 (10.3)
[Ni(acac)(dipe)(NO3)]c) 1020 1081 (9.1) 1070 (9.6) 1103 (7.3)
620 628 (19.0) 630 (20.1) 656 (6.0)
491 (6.75)
3a
1066 1063
621 626
1050
624
500 (shd))
1192
661
a) Donor number and acceptor number of the solvent: DCE,
0 and 17; NM, 2.7 and 21; dmso, 30 and 19. b) Taken from
Ref. 1. c) Taken from Ref. 2. d) sh: shoulder.
Results and Discussion
Preparation of Mixed-Ligand Complexes.
From a
each complex shows two bands with different intensities
(¯1 < ¯2), indicating the octahedral structure with bidentate
nitrate anion.2,3,9 Comparison of the peak position (-max) of ¯1
(Table 1) indicates that the ligand field strength of the nitrate
complexes ordered as follows:
stoichiometric mixture of components in an ethanol solution
(this mixture is denoted “ethanol solution [A]”), the nitrate
complexes are obtained. Since the nitrate complexes are very
soluble in ethanol, 1,2-dichloroethane, and also diethyl ether,
it seems that concentration of the solution with cooling is
better to obtain crystals. In an attempt to synthesize [Ni(dmhd)-
(dipe)(NO3)], formation of this complex in ethanol was
confirmed by the UV-vis spectrum showing two bands (¯1
and ¯2 from longer wavelength) with different intensities
(¯1 < ¯2), which is characteristic for the octahedral structure of
3d8 electron metal with bidentate nitrate anion.2,3,9 However, it
could not be obtained on concentration of the solution; instead,
the bis-dmhd complex [Ni(dmhd)2(dipe)] was precipitated as
powder. Elemental analysis data of 3a did not agree with the
calculated values because of contamination by a colorless
impurity which could not be removed by recrystallization.
Therefore the data of 3a were used for rough comparison.
Fortunately, a single crystal of 3a was found in the first
precipitate from the synthetic solution.
Tetraphenylborate complexes precipitated together with
NaNO3 after the addition of NaBPh4 to the “ethanol solution
[A]”. [Ni(dike)(diam)]BPh4 could be separated from NaNO3 by
dissolving in 1,2-dichloroethane. In previous reports, anion
exchange was performed in 1,2-dichloroethane dissolving the
nitrate complex. Tetraphenylborate complexes are usually
insoluble to ethanol, so the method in this report is also useful
when the nitrate complex cannot be obtained as powder,
such as the dedk-dipe complex. The crystal of [Ni(dmhd)-
(tmen)]BPh4 (2b) contained solvent molecules and gradually
degraded due to loss of the solvent molecules even at 173 K,
preventing X-ray analysis.
½NiðdedkÞðdipeÞðNO3Þꢀ ð3aÞ < ½NiðdedkÞðtmenÞðNO3Þꢀ ð1aÞ
¼ ½NiðdmhdÞðtmenÞðNO3Þꢀ ð2aÞ
ð1Þ
Since the difference between [1a, 2a] and 3a arises from the
diamine ligands (the ligand field strength: tmen > dipe), no
steric crowding effect of the dike ligands is observed in the
solid state of the nitrate complexes. On the other hand,
complexes 1b-4b are diamagnetic and have one strong band
around 490-520 nm in the solid-state reflectance spectra, hence
they have a square-planar structure.9 Comparison of the -max
values (Table 2) shows that the order of the ligand field
strengths of the tetraphenylborate complexes in solid state is
the following:
½NiðdmhdÞðdipeÞꢀBPh4 ð4bÞ < ½NiðdedkÞðdipeÞꢀBPh4 ð3bÞ
< ½NiðdedkÞðtmenÞꢀBPh4 ð1bÞ
¼ ½NiðdmhdÞðtmenÞꢀBPh4 ð2bÞð2Þ
The difference between [1b, 2b] and 3b is due to the diamine
ligands as observed in the nitrate complexes. For tmen
complexes 1b and 2b, the same -max values imply no steric
crowding effect of the dike ligands. For dipe complexes 3b and
4b, the differences in -max by 14 nm and in the color were
observed. The steric repulsion between dipe and dmhd ligands
might be larger than that between dipe and dedk.
In the IR spectra, there are weak but important bands due
¹
to the coordination mode of NO3 in the region of 1700-
¹
Properties of Complexes in Solid State. Analytical data
are shown in the experimental section. Complexes 1a, 2a, and
3a are paramagnetic. The solid-state reflectance spectrum of
1800 cm¹1, where one band due to the free NO3 ion
appears. This band splits into two on coordination with a
small splitting (ΔNO = 20-25 cm¹1) for a monodentate or a
3

792
Bull. Chem. Soc. Jpn. Vol. 83, No. 7 (2010)
Steric Crowding Effect of 1,3-Diketonates
Table 2. Absorption Spectra of the Tetraphenylborate Complexes in Solid State and in Various Solventsa)
¹1
¹3
(-max/nm, ¾max/dm3 mol¹1 cm in Parentheses, Concn: 8 © 10¹3 mol dm
)
Complex
Solid
DCE
ACN
ACO
dmso
b)
[Ni(acac)(tmen)]BF4
491c)
488 (150.0)
594 (9.5)
993 (9.2)
475 (23.2)
617 (6.3)
1048 (6.4)
491 (46.9)
624 (sh)
1050 (7.5)
490 (45.1)
610 (sh)
1045 (10.6)
496 (22.6)
647 (6.9)
1025 (3.2)
499 (67.3)
659 (9.9)
1040 (5.9)
499 (73.7)
664 (10.3)
1036 (5.6)
633 (6.13)
1055 (8.3)
1b
2b
490
493
®
489 (154)
489 (158)
496 (89)
592 (10.4)
997 (9.9)
635 (6.8)
1055 (9.1)
597 (17.6)
992 (11.9)
630 (8.8)
1057 (9.6)
b)
[Ni(acac)(dipe)]BPh4
622 (4.2)
1026 (3.3)
661 (4.05)
>1100
3b
4b
505
519
499 (199)
499 (203)
490 (sh)c)
612 (10.9)
1016 (8.4)
490 (sh)c)
615 (11.3)
1012 (8.9)
658 (5.7)
1098 (6.6)
653 0.7)
1095 9)
a) Donor number and acceptor number of the solvent: ACN (acetonitrile), 14 and 19; ACO (acetone), 17 and 13.
b) Taken from Ref. 6. c) Showing data of [Ni(acac)(tmen)]BPh4 in Ref. 1.
Table 3. Selected Bond Distances/¡ and Angles/° of
Nitrate Complexesa)
1aa)
2a
3a
Ni(1)-O(1)
Ni(1)-O(2)
Ni(1)-N(1)
Ni(1)-N(2)
Ni(1)-O(3)
Ni(1)-O(4)
2.005(2)
1.985(2)
2.147(2)
2.102(2)
2.163(2)
2.140(2)
2.008(3)
1.982(3)
2.147(3)
2.081(4)
2.132(3)
2.127(3)
1.978(5)
1.990(4)
2.098(7)
2.112(5)
2.126(4)
2.199(5)
O(1)-Ni(1)-O(2)
N(1)-Ni(1)-N(2)
O(3)-Ni(1)-O(4)
O(1)-Ni(1)-N(1)
O(2)-Ni(1)-O(3)
N(2)-Ni(1)-O(4)
90.82(10)
84.80(9)
59.64(8)
176.69(9)
156.81(9)
160.00(9)
90.67(11)
85.50(14)
60.29(12)
179.55(13)
163.19(13)
158.52(15)
90.36(19)
87.0(2)
59.69(16)
179.3(2)
162.66(18)
158.43(16)
Figure 2. An ORTEP drawing of 1a. Hydrogen atoms are
a) Only one data of two independent molecules are shown.
omitted for clarity. Only one of two independent molecules
is shown. Displacement ellipsoids are drawn at 30%
probability.
this type of complexes [Ni(dike)(diam)(NO3)] or [Ni(dike)2-
(diam)]13 and in contrast to usual coordination compounds,
which have rather high melting points or show decomposition
at higher temperature.
larger (>25 cm¹1) for a bidentate.2,10 In the spectra of 1a, 2a,
and 3a, there are two bands with ΔNO = 49-51 cm¹1 indicating
X-ray Crystal Structures. Nitrate Complexes: Figure 2
shows the molecular structure of 1a. The selected bond
distances and angles of 1a-3a are listed in Table 3. For each
complex, the central nickel(II) ion is coordinated by two
oxygen atoms of dike, two nitrogen atoms of diam and two
oxygen atoms of nitrate to give a six-coordinated octahedral
structure. The small bite angles of NO3 result in distorted
structures. For complex 1a, two independent molecules are
observed in the crystal but no remarkable difference in the bond
length or angles was observed. The Ni-donor bond distances
are almost same in tmen complex 1a and 2a. The Ni-donor
bond distances in 3a are similar to those in [Ni(acac)(dipe)-
(NO3)] (acac = acetylacetonate).2 Therefore, in octahedral
3
¹
that the NO3 is bidentate. It is interesting to note that the
difference between ¯C=O and ¯C=C of dike (Δdike) is also
characteristic.1,5,11,12 This difference varies greatly from a
square-planar structure to an octahedral one. As the Ni-O(dike)
bond becomes weaker, the neighboring C=O bond becomes
stronger and the C=C bond becomes weaker, therefore Δdike
increases. The structures could be confirmed from this view-
point: the Δdike values observed in the octahedral complexes 1a,
2a, and 3a are larger than those of the corresponding square-
planar complexes 1b, 2b, and 3b.
In TG-DTA, the nitrate complexes have a melting point
below 100 °C. This thermal property is characteristic for

M. Arakawa-Itoh et al.
Bull. Chem. Soc. Jpn. Vol. 83, No. 7 (2010)
793
ε
Figure 3. An ORTEP drawing of 1b. Hydrogen atoms
and counter anion are omitted for clarity. Displacement
ellipsoids are drawn at 30% probability.
Figure 4. UV-vis spectra of [Ni(dedk)(tmen)(NO3)] (1a) in
various solvents.
but also by the acceptor ability of the solvent.1 The steric
effects on the solvatochromism have been investigated by UV-
vis spectroscopy. Spectral data for the present complexes and
the analogous acac complexes in three solvents with different
donor or accepter numbers are summarized in Table 1. The
solvatochromic behavior of 3a is not discussed because of its
poor purity.
Table 4. Selected Bond Distances/¡ and Angles/° of
Tetraphenylborate Complexes
1b
3b
4b
Ni(1)-O(1)
Ni(1)-O(2)
Ni(1)-N(1)
Ni(1)-N(2)
1.8345(16)
1.8409(15)
1.9450(18)
1.9404(19)
1.842(4)
1.836(4)
1.944(4)
1.951(4)
1.848(4)
1.839(4)
1.967(5)
1.962(5)
Figure 4 shows the UV-vis spectra of 1a in 1,2-dichloro-
ethane (DCE), nitromethane (NM), and dimethyl sulfoxide
(dmso). In 1,2-dichloroethane, the spectrum has two bands
whose intensities are different (¯1 < ¯2). This means that the
O(1)-Ni(1)-O(2)
N(1)-Ni(1)-N(2)
O(1)-Ni(1)-N(2)
O(2)-Ni(1)-N(1)
95.20(7)
87.82(8)
176.17(6)
176.40(6)
94.27(18)
89.1(2)
173.4(2)
172.6(2)
94.47(19)
89.3(2)
170.2(2)
171.3(2)
¹
complex preserves an octahedral structure with the NO3 ion
coordinated (denoted by Oh1 structure) the same as in the solid
state. In nitromethane with acceptor ability, an additional weak
absorption band due to the square-planar structure appears
at 482 nm, while two bands corresponding to the octahedral
species still remain. Nitromethane with poor donor ability does
not coordinate to the nickel(II) ion. Therefore, the behavior of
1a in nitromethane can be interpreted in terms of a shift of the
equilibrium between square-planar (Sp) and Oh1 species. The
relative abundances of the square-planar species, xSp, in
nitromethane were almost the same for 1a (8%) and 2a (7%).14
In a strong donor solvent, dimethyl sulfoxide, the spectrum
is quite consistent with the typical spectrum of [Ni(dike)-
(diam)(dmso)2]+.1-3,6,7 It is thought that two molecules
structures, no substituent effect of dike was observed because
of the long Ni-donor bond distance.
Tetraphenylborate Complexes:
Figure 3 shows the
molecular structure of 1b. The selected bond distances and
angles of 1b, 3b, and 4b are listed in Table 4. For each
complex, the central nickel(II) ion is coordinated by two
oxygen atoms of dike and two nitrogen atoms of diam to give
a four-coordinated square-planar structure. The bond distances
of tmen complex 1b are comparable to those of [Ni(acac)-
(tmen)]BPh4.12 For dipe complexes, the bond distances,
particularly the Ni-N distances, in dmhd complex 4b are
longer than those in dedk complex 3b due to the steric
repulsion between dipe and dmhd. This result agrees with that
of the solid reflectance spectra. The tetraphenylborate com-
plexes with the four-coordinated square-planar structures have
shorter Ni-donor bond distances than those of the octahedral
nitrate complexes, which is explained by Gutmann’s bond
variation rules.5 In the solid state, the steric repulsion between
bulky dipe and dike differs between dedk and dmhd, and this
difference causes observable color change and the difference in
bond distances, although such difference was not observed in
the tmen complexes.
¹
of dimethyl sulfoxide replaced the NO3 and coordinated
(denoted by Oh2 structure). These data suggest that there are
equilibrium mixtures among three structures; Sp, Oh1, and Oh2
in these solutions illustrated in Scheme 1. The peak positions
of 1a and 2a in the three solvents are almost the same as those
of [Ni(acac)(tmen)(NO3)],1 which means that the solvato-
chromism of these complexes is explained in terms of the same
type of solute-solute-solvent interaction. For tmen complexes
[Ni(dike)(tmen)(NO3)], the steric crowding effect of dike was
not observed in solution.
Solvatochromism of the Complexes in Solution. Nitrate
Complexes: In the mixed-ligand complex system studied
here, the major species is affected not only by the donor ability
Tetraphenylborate Complexes:
showed solvatochromism due to the “solute-solvent interac-
tion.” Spectral data for these complexes and analogous acac
Complexes 1b-4b

794
Bull. Chem. Soc. Jpn. Vol. 83, No. 7 (2010)
Steric Crowding Effect of 1,3-Diketonates
+
because of its linear and slim shape. Therefore the acetonitrile
can easily coordinate to the nickel(II) center.1,3 In addition, the
nitrogen donor atom in acetonitrile causes blue-shift of the two
bands of the octahedral species compared with those observed
in other solvents with oxygen donor atoms.1,3,12 For other
complexes 2b, 3b, and 4b, the major components in each
solvent evaluated from the UV-vis spectrum are same as that of
1b. The ligand field strengths of the square-planar complexes
are ordered on the basis of the -max values in 1,2-dichloro-
ethane as follows:
O
N
O
-
Ni
NO3
N
solv
solv
Strong donor
solvent
Oh2
O
N
O
Strong donor
solvent
Ni
N
O
N
O
O
Oh1
½NiðdedkÞðdipeÞꢀBPh4 ð3bÞ ¼ ½NiðdmhdÞðdipeÞꢀBPh4 ð4bÞ
< ½NiðdedkÞðtmenÞꢀBPh4 ð1bÞ
Inert solvent
+
Weak donor
and strong acceptor
solvent
N
O
-
¼ ½NiðdmhdÞðtmenÞꢀBPh4 ð2bÞ ð3Þ
Ni
NO3
N
O
The difference between [1b, 2b] and [3b, 4b] is due to the
diamine ligands as observed in the solid state (eq 2). Interest-
ingly, the difference between 3b and 4b observed in the solid
state disappeared, that is, their ligand field strengths are the
same in 1,2-dichloroethane.
Sp
Scheme 1.
Adduct Formation with dmso: When dimethyl sulfoxide
was added to the tetraphenylborate complex in an inert solvent,
1,2-dichloroethane, the Sp species forms a five- and six-
coordinated complexes as represented by eq 4.
ε
½NiðdikeÞðdiamÞꢀþ + dmso ꢀ ½NiðdikeÞðdiamÞðdmsoÞꢀþ
½NiðdikeÞðdiamÞðdmsoÞꢀþ + dmso
ꢀ ½NiðdikeÞðdiamÞðdmsoÞ2ꢀþ ð4Þ
K1=mol dmꢁ3 ¼ C
K2=mol dmꢁ3 ¼ C
=ðC
Cdmso
Þ
Cdmso
½NiðdikeÞðdiamÞðdmsoÞꢀ
½NiðdikeÞðdiamÞꢀ
=ðC
Þ
½NiðdikeÞðdiamÞðdmsoÞ2ꢀ
½NiðdikeÞðdiamÞðdmsoÞꢀ
ε
log ¢ ¼ log K1 ꢂ K2 ¼ log K1 þ log K2
ð5Þ
The spectral changes associated with the addition of
dimethyl sulfoxide are shown in Figure 6 (for 1b) and
Figure 7 (for 3b). The variation in the spectra for 2b and 4b
are similar to those for 1b and 3b, respectively. In the cases of
1b and 2b (Figure 6), the absorbance at 489 nm corresponding
to the square-planar species decreases and those at 635 and
1055 nm corresponding to the octahedral species increase
with the addition of dimethyl sulfoxide. For dipe complexes 3b
and 4b (Figure 7), absorptions attributed to a five-coordinated
intermediate were observed around 1400, 900, 650, and
400 nm at the beginning of the addition of dimethyl sulfoxide
and their intensities decreased on further addition of dimethyl
sulfoxide. The calculated spectrum of the five-coordinated
intermediate for complex 3b (Figure S5) agrees well with that
of [Ni(Me6tren)Cl]Cl where Me6tren is 2,2¤,2¤¤-tri(N,N-di-
methylamino)triethylamine with the five-coordinated trigonal-
bipyramidal structure.15 The primary coordination of dimethyl
sulfoxide gives the five-coordinated trigonal-bipyramidal
structure and then the cis-octahedral structure is favored to
form by coordination of second dimethyl sulfoxide molecule
as shown in Scheme 2. This result confirms the reaction
scheme proposed by Linert et al.6 for the first time by
spectroscopy. It is notable that the five-coordinated structure
with one solvent molecule is stable enough to be captured
during titration. In the series of Ni(dike)(diam)X, so far the
coordination of the two solvent molecules has been observed
to form the octahedral complex (Oh2) and the spectral pattern
due to only the square-planar and octahedral species are
Figure 5. UV-vis spectra of [Ni(dedk)(tmen)]BPh4 (1b) in
various solvents.
complexes are summarized in Table 2. Figure 5 shows the
UV-vis spectra and color variations of 1b in 1,2-dichloro-
ethane, acetonitrile (ACN), acetone (ACO), and dimethyl
sulfoxide. In 1,2-dichloroethane, the solution is red, and
the spectrum shows a strong absorption band at 489 nm
(¾ = 158 dm3 mol¹1 cm¹1), indicating that 1b has a square-
planar stereochemistry. On the other hand, the solution is green
in dimethyl sulfoxide, showing two relatively weak bands at
1055 and 635 nm (¾ = 9.1 and 6.8 dm3 mol¹1 cm¹1, respec-
tively). This means that two molecules of dimethyl sulfoxide
are coordinated to the nickel(II) center to form an octahedral
geometry, i.e., the equilibrium of Sp ꢀ Oh is shifted to the
right. In solvent with a medium donor ability, such as acetone
(ACO), the spectrum shows a band at 491 nm and a relatively
weak band at 1050 nm and shoulder around 625 nm, indicating
that both square-planar and octahedral species coexist in the
solution. The spectrum of 1b in acetonitrile (ACN) shows only
the bands due to the octahedral species, which is not consistent
with the expectations for DN (ACO > ACN). The acetonitrile
molecule is not affected by the steric hindrance of the complex

M. Arakawa-Itoh et al.
Bull. Chem. Soc. Jpn. Vol. 83, No. 7 (2010)
795
O
O
N
solv
solv
N
O
N
O
O
Ni
Ni
Ni
N
solv
N
O
N
solv
solv
square-planar
trigonal-bipyramidal
cis-species
Scheme 2.
Table 5. Equilibrium Constants (300 « 0.5 K)
Complex log K1
log K2
K1/K2 log ¢
ε
1b
2b
3b
4b
1.4 « 0.1
1.3 « 0.2
2.72 « 0.02
2.498 « 0.004 1.34 « 0.01 14.4
2.8 « 0.1
2.9 « 0.2
1.23 « 0.06 30.9
0.040 4.29 « 0.03
0.025 4.25 « 0.04
3.95 « 0.06
3.84 « 0.01
log K1: ½NiðdedkÞðtmenÞꢀBPh4 ð1bÞ
¼ ½NiðdmhdÞðtmenÞꢀBPh4 ð2bÞ
ꢃ ½NiðdmhdÞðdipeÞꢀBPh4 ð4bÞ
< ½NiðdedkÞðdipeÞꢀBPh4 ð3bÞ
log K2: ½NiðdedkÞðdipeÞꢀBPh4 ð3bÞ
¼ ½NiðdmhdÞðdipeÞꢀBPh4 ð4bÞ
< ½NiðdedkÞðtmenÞꢀBPh4 ð1bÞ
¼ ½NiðdmhdÞðtmenÞꢀBPh4 ð2bÞ
log ¢: ½NiðdedkÞðdipeÞꢀBPh4 ð3bÞ
¼ ½NiðdmhdÞðdipeÞꢀBPh4 ð4bÞ
< ½NiðdedkÞðtmenÞꢀBPh4 ð1bÞ
¼ ½NiðdmhdÞðtmenÞꢀBPh4 ð2bÞ
Figure 6. Spectral change of [Ni(dedk)(tmen)]BPh4 (1b) in
1,2-dichloroethane on addition of dmso at 300 K. Concen-
¹3
trations of complex are 6.6 © 10¹3-5.9 © 10¹3 mol dm
ð6Þ
and concentrations of dmso are 0-0.035 mol dm¹3 from the
top to the bottom.
ð7Þ
ð8Þ
The order of log K1 values means that the primary coordination
of dimethyl sulfoxide to the dipe complexes 3b and 4b occurs
more easily than that to the corresponding tmen complexes 1b
and 2b. This is rationalized in terms of the difference of the
ligand field strengths shown in eq 3. No difference was
observed between 1b and 2b because there is no steric effect of
dike as mentioned above. On the other hand, log K1 of 4b is
smaller than that of 3b, indicating that the square-planar
structure of 4b is more stable than that of 3b. Since the ligand
field strengths of the two complexes in 1,2-dichloroethane
are the same as shown in eq 3, the difference in log K1 arises
from the difference in the steric crowding effect of dike.
The nickel(II) center is shielded from the solvent by bulky
piperidine rings in dipe and isopropyl groups in dmhd.
The log K2 values are usually much larger than the log K1
values in the coordination of dimethyl sulfoxide to [Ni(dike)-
(diam)]BPh4 including [Ni(acac)(dipe)]BPh4.6 The log K2
values of 1b and 2b are also larger than their log K1 values,
hence only the absorptions due to the square-planar and
octahedral species were observed in the titration spectra as
shown above. In dipe complexes, inverse results were obtained:
log K1 > log K2. The secondary coordination of dimethyl
sulfoxide to the nickel(II) center is inhibited by the existing
ligands (including one dmso molecule). The log K2 values of
3b and 4b are almost the same. The steric hindrances of dike
may not affect the change from five-coordinated structure to
octahedral structure due to long Ni-donor bond lengths. This
ε
Figure 7. Spectral change of [Ni(dedk)(dipe)]BPh4 (3b) in
1,2-dichloroethane on addition of dmso at 300 K. Concen-
¹3
trations of complex are 5.1 © 10¹3-4.1 © 10¹3 mol dm
and concentrations of dmso are 0-0.057 mol dm¹3 from the
top to the bottom.
observed as shown in Figure 6.3,6,16 Even when a halide or
pseudohalide anion is used for counter anion X, no five-
coordinated species were obtained in solid states, and the
five-coordinated structures formed in solutions are unstable
and rapidly disproportionate to [Ni(dike)2(diam)] and [Ni-
(diam)X2].17
Comparison of Equilibrium Constants: The equilibrium
constants defined in eq 5 were determined in 1,2-dichloro-
ethane at 300 K and are listed in Table 5. The orders of
equilibrium constants are summarized in eqs 6-8.

796
Bull. Chem. Soc. Jpn. Vol. 83, No. 7 (2010)
Steric Crowding Effect of 1,3-Diketonates
¹1
assumption is supported by similarity of the calculated spectra
of the five-coordinated species derived from 3b and 4b. The
higher stability of the five-coordinated structure in 3b (K1/K2:
3b > 4b) is due to the difference of the relative stabilities of the
square-planar structures.
IR bands/cm¹1: ¯C=O + ¯C=C 1597, 1515 cm (Δdike = 82
cm¹1); combination bands of bidentate NO3 1765, 1717 cm
(ΔNO = 48 cm¹1). Mp/°C: 71. FAB+ MS: 302 [M + H ¹
¹
¹1
3
NO3]+ (calcd 302). ®eff/BM: 3.23.
[Ni(dmhd)(tmen)(NO3)] (2a):
Blue. Anal. Found: C,
The order of the log ¢ values conflicts with the expectation
from the ligand field strengths in 1,2-dichloroethane (eq 3). In
45.42; H, 8.05; N, 10.36%. Calcd for C15H31N3NiO5: C,
45.94; H, 7.97; N, 10.72%. Solid reflection -max/nm: 1034,
¹
¹
¹1
the acac complexes Ni(acac)(tmen/dipe)X (X = NO3 , BF4 ),
the ligand field strength of the dipe complex is smaller than
that of the tmen complex and the octahedral structure of the
dipe complex is more stable than that of the tmen complex in
solvatochromism.2,6 The bulkiness and rigidity of dipe is
effective, but the effect of the ligand field strength when the
dike is acac, which is weaker and less bulky donor than dedk or
dmhd. In the cases of the present dedk or dmhd complex, the
steric effect of dipe and dike is stronger than the effect of the
ligand field strengths. Therefore the reactions into the octahe-
dral structure are unfavorable in 3b and 4b due to the steric
effect in spite of their weak ligand field strengths as compared
with 1b and 2b.
612. Selected IR bands/cm¹1: ¯C=O + ¯C=C 1601, 1531 cm
¹
(Δdike = 70 cm¹1); combination bands of bidentate NO3 1770,
¹1
1719 cm (ΔNO = 51 cm¹1). Mp/°C: 77. FAB+ MS: 330
3
[M + H ¹ NO3]+ (calcd. 330). ®eff/BM: 3.21.
[Ni(dedk)(dipe)(NO3)] (3a): Pale Blue. Anal. Found: C,
49.19; H, 7.82; N, 9.23%. Calcd for C19H35N3NiO5: C, 51.38;
H, 7.94; N, 9.46%. Solid reflection -max/nm: 1066, 621.
¹1
Selected IR bands/cm¹1: ¯C=O + ¯C=C 1599, 1519 cm
¹
(Δdike = 79 cm¹1); combination bands of bidentate NO3
1774, 1725 cm (ΔNO = 49 cm¹1). Mp/°C: 93. FAB+ MS:
¹1
3
381 [M ¹ NO3]+ (calcd 381). ®eff/BM: 3.39.
Synthesis of Tetraphenylborate Complexes. The com-
plexes were prepared by anion-exchange from nitrate com-
plexes. Sodium tetraphenylborate (8 mmol) in ethanol was
added to the “ethanol solution [A].” After stirring the solution
for twenty minutes, the red-violet precipitate was collected and
recrystallized from 1,2-dichloroethane to give crystals. The
crystal was used for X-ray structure analysis except for
[Ni(dmhd)(tmen)]BPh4 (2b). Yield 52-68%.
Experimental
Materials.
All chemicals were commercially available
from Wako Pure Chemical Industries, Ltd., Kanto Chemical
Co., and Sigma-Aldrich Co. and were used without further
purification. Solvents for spectral measurements were “spectro-
grade.”
[Ni(dedk)(tmen)]BPh4 (1b): Bright red. Anal. Found: C,
71.19; H, 7.69; N, 4.63%. Calcd for C37H47BN2NiO2: C, 71.53;
Physical Measurements. Elemental analyses (C, H, N)
were measured on a Perkin-Elmer 2400 CHN analyzer. IR
spectra were obtained as KBr pellets on a Perkin-Elmer FT-IR
SPECTRUM 2000. Melting points were obtained by thermal
analyses (TG-DTA), which were carried out using a Shimadzu
thermal analyzer (DTG-50). Mass spectra were obtained on a
JEOL JMS-700 Mstation in the fast atom bombardment (FAB)
mode using ultramark as standard and 3-nitrobenzyl alcohol
(NBA) as matrix, in which the tetraphenylborate complexes are
decomposed during measurements. Magnetic data were mea-
sured on a Shimadzu Torsion Magnetometer MB-100 at room
temperature and the effective magnetic moments (®eff) were
calculated with correcting »diamagnetic. UV-vis spectra were
obtained on a Shimadzu UV-3100PC Spectrophotometer using
a 1 cm quartz cell. Solid reflectance spectra were recorded on
the Shimadzu UV-3100PC Spectrophotometer using an inte-
gration sphere and barium sulfate as reference.
Synthesis of Nitrate Complexes. To an ethanol solution
(20 mL) of Ni(NO3)2¢6H2O (5 mmol), Hdike (5 mmol), and
Et3N (5 mmol) were added, followed by the addition of diam
(5 mmol) to give a deep green solution (this mixture is denoted
“ethanol solution [A]”). After concentration of the solution
with cooling, a blue-green powdery or crystalline solid was
obtained. The powder was recrystallized from 1,2-dichloro-
ethane. Yield 50-60%. Although the formation of [Ni(dmhd)-
(dipe)(NO3)] was confirmed by the absorption peaks in UV-vis
spectra in the ethanol solution, concentration of this solution
gave not the desired nitrate complex but powder of a bis-dmhd
complex [Ni(dmhd)2(dipe)].
H, 7.63; N, 4.51%. Solid reflection -max/nm: 490. Selected IR
¹1
bands/cm
: ¯
C=O + ¯C=C 1566, 1527 (Δdike = 39 cm¹1).
FAB+ MS: 302 [M + H ¹ BPh4]+ (calcd 302). Diamagnetic.
[Ni(dmhd)(tmen)]BPh4 (2b): Bright red. Anal. Found: C,
72.66; H, 8.27; N, 4.63%. Calcd for C39H51BN2NiO2: C, 72.14;
H, 7.92; N, 4.32%. Solid reflection -max/nm: 493. Selected
IR bands/cm¹1: ¯C=O + ¯C=C 1580, 1532 (Δdike = 48 cm¹1).
FAB+ MS 330 [M + H ¹ BPh4]+ (calcd 330). Diamagnetic.
[Ni(dedk)(dipe)]BPh4 (3b): Reddish pink. Anal. Found: C,
73.50; H, 8.04; N, 4.08%. Calcd for C43H55BN2NiO2: C, 73.63;
H, 7.90; N, 3.99%. Solid reflection -max/nm: 505. Selected
IR bands/cm¹1: ¯C=O + ¯C=C 1565, 1525 (Δdike = 40 cm¹1).
FAB+ MS: 381 [M ¹ BPh4]+ (calcd 381). Diamagnetic.
[Ni(dmhd)(dipe)]BPh4 (4b):
Violet. Anal. Found: C,
73.88; H, 8.30; N, 3.90%. Calcd for C45H59BN2NiO2: C, 74.09;
H, 8.15; N, 3.84%. Solid reflection -max/nm: 519. Selected
IR bands/cm¹1: ¯C=O + ¯C=C 1561, 1535 (Δdike = 26 cm¹1).
FAB+ MS: 409 [M ¹ BPh4]+ (calcd 409). Diamagnetic.
X-ray Crystallography. X-ray data of complexes were
collected on a Mac Science M03XHF four-circle diffractom-
eter with graphite-monochromatized Mo K¡ radiation (- =
0.71073 ¡) at 298 K. The structures were solved by a direct
method using SIR92,18 and refined by full-matrix least-square
techniques with SHELXL97.19 All non-hydrogen atoms were
refined with anisotropic thermal parameters, and hydrogen
atoms were included in calculated positions and refined with
isotropic thermal parameters. C(1) and C(15)-C(19) in 3a, C(7)
in 3b, and C(1), C(2), C(8), and C(9) in 4b were disordered and
located in two positions. All calculations were performed using
maXus20 and WinGX.21 Crystallographic data and refinement
parameters are listed in Table 6.
[Ni(dedk)(tmen)(NO3)] (1a): Blue. Anal. Found: C, 42.81;
H, 7.56; N, 11.57%. Calcd for C13H27N3NiO5: C, 42.89; H,
7.48; N, 11.54%. Solid reflection -max/nm: 1036, 617. Selected

Table 6. Crystallographic Data
[Ni(dedk)(tmen)(NO3)] [Ni(dmhd)(tmen)(NO3)] [Ni(dedk)(dipe)(NO3)] [Ni(dedk)(tmen)]BPh4 [Ni(dedk)(dipe)]BPh4 [Ni(dmhd)(dipe)]BPh4
1a
2a
3a
1b
3b
4b
Formula
Molecular weight
Crystal system
Space group
a/¡
b/¡
c/¡
¡/°
¢/°
C13H27N3NiO5
364.07
Monoclinic
P21/c
15.760(7)
14.074(5)
17.338(6)
90
109.23(3)
90
3631(2)
8
C15H31N3NiO5
392.12
Monoclinic
P21/c
7.588(5)
14.863(5)
18.445(3)
90
92.16(3)
90
2078.8(15)
4
C19H35N3NiO5
444.19
C37H47BN2NiO2
621.27
C43H55BN2NiO2
701.39
Monoclinic
Cc
C45H59BN2NiO2
729.43
Monoclinic
P21/c
Triclinic
P1
Triclinic
P1
8.483(5)
10.281(3)
13.948(4)
77.43(2)
89.39(4)
75.30(3)
1147.1(8)
2
9.998(5)
12.783(15)
14.330(6)
79.85(6)
80.03(4)
73.03(6)
1701(2)
2
12.384(7)
18.450(6)
17.185(4)
90
96.30(3)
90
3903(3)
4
1.194
9.792(5)
22.682(6)
18.390(3)
90
92.63(2)
90
4080(2)
4
1.188
£/°
V/¡3
Z
¹3
D
calcd/g cm
1.332
0.75 © 0.75 © 0.35
1.092
1.253
0.80 © 0.70 © 0.50
0.959
1.286
0.50 © 0.15 © 0.13
0.877
1.207
0.75 © 0.50 © 0.50
0.601
Crystal size/mm3
0.75 © 0.25 © 0.13
0.534
0.30 © 0.25 © 0.20
0.513
¹1
®/mm
Total reflections
7046
4087
4091
8031
4309
6219
Unique reflections
Observed reflections
6805
6503
0.0500
0.1287
1.043
3802
3552
0.0608
0.1648
0.996
3816
3233
0.0790
0.1886
7683
7486
0.0429
0.1202
4309
3962
0.0498
0.1154
1.006
6030
5096
0.0897
0.1714
1.013
2
R1 [F02 > 2·(F0 )]
wR2 (all data)
Goodness of fit
1.006
1.083
Reflection/Parameter ratio
17.10
17.44
12.43
19.75
9.53
13.28

798
Bull. Chem. Soc. Jpn. Vol. 83, No. 7 (2010)
Steric Crowding Effect of 1,3-Diketonates
1091.
7
A 2006, 64, 1058.
Crystal data have been submitted to CCDC; the document
numbers are 723549 (1a), 723551 (2a), 723552 (3a), 723550
(1b), 723553 (3b), and 723554 (4b). 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,
U.K. (Fax: +44 1223 336033).
A. A. Soliman, A. Taha, W. Linert, Spectrochim. Acta, Part
8
L. P. Hammett, J. Am. Chem. Soc. 1937, 59, 96; Daigakuin-
kogi Yuki-kagaku I, ed. by R. Noyori, Tokyo Kagaku Dojin, 1999.
9
A. B. P. Lever, Inorganic Electronic Spectroscopy, Studies
in Physical and Theoretical Chemistry 33, Elsevier, Amsterdam,
1984.
Equilibrium Constants. The equilibrium constants defined
by eq 5 were determined by spectrophotometric titration at
300 K. A 1,2-dichloroethane solution of dimethyl sulfoxide
was successively added to a 1,2-dichloroethane solution
(6 © 10¹3 mol dm¹3) of the complex [Ni(dike)(diam)]BPh4
and the UV-vis spectra were recorded. The obtained series of
spectra were fitted to a two-step reaction scheme using
SPECFIT program to evaluate the equilibrium constants.22
The component spectrum of the five-coordinated species was
also obtained in the curve-fitting process.
10 A. B. P. Lever, E. Mantovani, B. S. Ramaswamy, Can. J.
Chem. 1971, 49, 1957.
11 K. Nakamoto, Infrared and Raman Spectra of Inorganic
and Coordination Compounds, 3rd ed., John Wiley & Sons, New
York, 1978.
12 M. Arakawa, N. Suzuki, S. Kishi, M. Hasegawa, K. Satoh,
E. Horn, Y. Fukuda, Bull. Chem. Soc. Jpn. 2008, 81, 127.
13 Y. Saito, T. Takeuchi, Y. Fukuda, K. Sone, Bull. Chem. Soc.
Jpn. 1981, 54, 196.
14 The xSp values were estimated from the equation, xSp
=
o
(¾Sp/¾ o) © 100, where ¾
is the molar absorption coefficient
Sp
Sp
Authors are grateful to Dr. Yukie Mori of Ochanomizu
Univ., for her scientific advice.
in 100% of the square-planar structure, which is assumed equal
to ¾
value of [Ni(dike)(diam)]BPh4 in 1,2-dichloroethane, and
max
¾
is the molar absorption coefficient at -max of Sp species in
Sp
Supporting Information
nitromethane.
15 M. Ciampolini, N. Nardi, G. P. Speroni, Coord. Chem. Rev.
1966, 1, 222; R. J. West, S. F. Lincoln Inorg. Chem. 1973, 12, 494.
16 K. F. Purcell, J. C. Kotz, An Introduction to Inorganic
Chemistry, Saunders, Tokyo, 1982.
17 N. Hoshino, Y. Fukuda, K. Sone, Bull. Chem. Soc. Jpn.
1981, 54, 420; K. Miyamoto, M. Sakamoto, C. Tanaka, E. Horn, Y.
Fukuda, Bull. Chem. Soc. Jpn. 2005, 78, 1061.
18 A. Altomare, G. Cascarano, C. Giacovazzo, A. Guagliardi,
M. C. Burla, G. Polidori, M. Camalli, J. Appl. Crystallogr. 1994,
27, 435.
ORTEP drawings of the X-ray structure of 2a, 3a, 3b, and
4b are shown in Figures S1, S2, S3, and S4, respectively.
Figure S5 is the calculated spectra of the 5-coordinated
complex. These materials are available free of charge on the
web at http://www.csj.jp/journals/bcsj/.
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