Preparation, crystal structures, and spectroscopic and redox properties of nickel(II) complexes containing phosphane-(amine or quinoline)-type hybrid ligands and a nickel(I) complex bearing 8-(diphenylphosphanyl)quinoliney

Article abstract of DOI:10.1002/ejic.200900767

Nickel(II) complexes containing P-N-type bidentate hybrid ligands of 2-aminoethylphosphanes (RR'Pea; RR' = Ph₂ or MePh) or 8-quinolylphosphanes (RR'Pqn), namely [Ni(P-N)₂](BF₄) ₂ [P-N = Ph₂Pea (1), MePhPea (2), Ph₂Pqn (3), or MePhPqn (4)] have been prepared, and their structural, spectroscopic, and electrochemical properties determined. The crystal structure analysis indicates that: the 2-amlnoethylphosphane complexes (1 and 2) have a square-planar coordination geometry around the Ni^II center with a cis(P,P) configuration, whereas the 8-quinolylphosphane complexes (3 and 4) exhibit a severe tetrahedral distortion because of the steric repulsion between the ortho-H atoms in the mutually cis-positioned quinolyl rings, Complexes 1 and 2 maintain their diamagnetic square-planar four-coordinate structure on acetonitrile solution, whereas complexes 3 and 4 show paramagnetic behavior. Spectroscopic and electrochemical measurements suggest that the ligand-field strengths of these four PN-type ligands increase in the order Ph₂Pqn (3) < MePhPqn (4) < Ph₂Pea (1) < MePhPea (2). The Ph ₂Pqn complex 3 is readily reduced by Zn powder to afford the corresponding nickel(I) complex [Ni(Ph₂Pqn)₂]BF ₄ (5), The crystal structure of complex 5 reveals that the Ni ^I ion adopts a distorted tetrahedral coordination geometry with slightly longer (≈ 0.05 A) Ni-P and. Ni-N bond lengths than those in the corresponding Ni^II complex 3. 2010 Wiley-VCH Verlag GmbH & Co. KGaA.

Full text of DOI:10.1002/ejic.200900767

FULL PAPER
DOI: 10.1002/ejic.200900767
Preparation, Crystal Structures, and Spectroscopic and Redox Properties of
Nickel(II) Complexes Containing Phosphane–(Amine or Quinoline)-Type
Hybrid Ligands and a Nickel(I) Complex Bearing
8-(Diphenylphosphanyl)quinoline
Akira Hashimoto,[a] Hiroshi Yamaguchi,[a] Takayoshi Suzuki,*[b] Kazuo Kashiwabara,[a]
Masaaki Kojima,[b] and Hideo D. Takagi*[a]
Keywords: N,P ligands / Nickel / Phosphane ligands / Ligand effects
Nickel(II) complexes containing P–N-type bidentate hybrid
ligands of 2-aminoethylphosphanes (RRЈPea; RRЈ = Ph2 or
MePh) or 8-quinolylphosphanes (RRЈPqn), namely [Ni(P–N)2]-
(BF4)2 [P–N = Ph2Pea (1), MePhPea (2), Ph2Pqn (3), or
MePhPqn (4)] have been prepared, and their structural, spec-
troscopic, and electrochemical properties determined. The
crystal structure analysis indicates that the 2-aminoethyl-
diamagnetic square-planar four-coordinate structure on ace-
tonitrile solution, whereas complexes 3 and 4 show paramag-
netic behavior. Spectroscopic and electrochemical measure-
ments suggest that the ligand-field strengths of these four P–
N-type ligands increase in the order Ph2Pqn (3) Ͻ MePhPqn
(4) Ͻ Ph2Pea (1) Ͻ MePhPea (2). The Ph2Pqn complex 3 is
readily reduced by Zn powder to afford the corresponding
phosphane complexes (1 and 2) have a square-planar coordi- nickel(I) complex [Ni(Ph2Pqn)2]BF4 (5). The crystal structure
nation geometry around the NiII center with a cis(P,P) config-
uration, whereas the 8-quinolylphosphane complexes (3 and
4) exhibit a severe tetrahedral distortion because of the steric
of complex 5 reveals that the NiI ion adopts a distorted tetra-
hedral coordination geometry with slightly longer (≈ 0.05 Å)
Ni–P and Ni–N bond lengths than those in the corresponding
repulsion between the ortho-H atoms in the mutually cis-po- NiII complex 3.
sitioned quinolyl rings. Complexes 1 and 2 maintain their
ethylphosphanes (RRЈPea) or 8-quinolylphosphanes
(RRЈPqn) (Scheme 1), which are P–N-type bidentate li-
Introduction
Transition-metal complexes containing hybrid-type hemi-
labile phosphane ligands[1] have recently attracted a great
deal of interest in applications as wide ranging as effective
chemosensors for small molecules,[2] homogeneous catalysts
for the activation of inert small molecules,[3] anticancer
agents,[4] and so on.[5] In order to develop novel functionali-
ties and catalytic and biological activities for these com-
plexes, it is essential to design the electronic differentia-
tions[1b,1c] of the donor groups as well as the steric require-
ments of the hybrid-type ligands. In this respect, we are
currently focusing our attention on P–N-type hybrid li-
gands bearing an amine or pyridine/quinoline donor group
and the investigation of their coordination behavior.[6–11]
gands that form a five-membered chelate ring.
Scheme 1. P–N-type ligands used in this study.
Several transition-metal complexes with (2-aminoethyl)-
Herein we consider nickel(II) complexes having 2-amino- diphenylphosphane (Ph2PCH2CH2NH2: Ph2Pea) have been
prepared[12–14] since the first reports by Issleib and co-
[a] Graduate School of Science and Research Center for Materials workers.[15] In contrast, complexes of the analogous P–N-
Science, Nagoya University,
type ligand having an asymmetric phosphorus donor atom,
Furocho, Chikusa, Nagoya 464-8602, Japan
namely (2-aminoethyl)methylphenylphosphane (MePhP-
CH2CH2NH2: MePhPea), are limited to those of co-
balt(III),[12c] palladium(II), and platinum(II).[16] Moreover,
studies on the molecular structures and properties of metal
Fax: +81-52-789-5473
E-mail: h.d.takagi@nagoya-u.jp
[b] Department of Chemistry, Faculty of Science, Okayama Uni-
versity,
Tsushima-naka 3-1-1, Okayama 700-8530, Japan
Fax: +81-86-251-7900
complexes
with
8-quinolylphosphanes
are
also
E-mail: suzuki@cc.okayama-u.ac.jp
scarce.[7–9,17,18] As an example, one of the present authors
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejic.200900767.
(T.S.) has reported a novel crystal modification of cis(P,P)-
Eur. J. Inorg. Chem. 2010, 39–47
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39

T. Suzuki, H. D. Takagi et al.
FULL PAPER
[Pd(Ph2Pqn)2]Br2 (yellow blocks vs. orange prisms) and its
chloride and/or tetrafluoroborate salts, which could be re-
lated to a tetrahedral distortion of the coordination plane
of PdII because of the intramolecular steric interaction be-
tween two mutually cis-positioned quinolyl groups.[8] For
the corresponding NiII complexes, including those of the
related P–N-type hybrid ligands,[19] there are very limited
reports on structural determinations and spectroscopic
characterization of the complexes. We have previously de-
scribed the syntheses and structures of [Ni(Me2Pea)2]Cl2,
[Ni(Me2Pea)2][NiCl4] (where Me2Pea was previously ab-
breviated as edmp),[6] and [Ni(Me2Pqn)2](BF4)2.[7] Herein
we report the syntheses, structural analyses, and spectro-
scopic and electrochemical properties of the NiII complexes
[Ni(P–N)2](BF4)2 containing Ph2Pea (1), MePhPea (2),
Ph2Pqn (3), or MePhPqn (4) as the P–N ligand (Scheme 1).
The corresponding NiI complex, [Ni(Ph2Pqn)2]BF4 (5), pre-
pared by reduction of complex 3 with Zn powder, is also
described.
Figure 1. An ORTEP view (50% probability level, hydrogen atoms
omitted) of the cationic moiety in [Ni(Ph2Pea)2](BF4)2 (1).
(MePhPea)2(BF4)2, although its 1H NMR spectrum (in
CD3CN) was indicative of a mixture of two isomers (Figure
S1). These isomers were separated on the basis of their dif-
ferent solubilities in dichloromethane, and the crystal struc-
ture of the less soluble product was determined by X-ray
analysis. As seen from Figure 2, the analyzed complex is
assigned as the racemic (RR/SS) diastereoisomer (abbrevi-
ated as rac-2).
Results and Discussion
Preparation and Crystal Structures of Nickel(II) Complexes
The nickel(II) complexes [Ni(P–N)2](BF4)2 (1–4) were
prepared from Ni(BF4)2·6H2O and a small excess of P–N
ligand in methanol or ethanol following a similar method
to that used to obtain [Ni(Me2Pea or Me2Pqn)2](BF4)2.[6,7]
[Ni(Ph2Pea)2](BF4)2 (1) was obtained analytically pure, and
magnetic susceptibility measurements at room temperature
1
indicated that this complex is diamagnetic. The H NMR
spectrum of 1 (in CD3CN) shows all resonances in the nor-
mal diamagnetic region, although the resonances of the 2-
aminoethyl moieties (PCH2CH2NH2) are remarkably
broad, presumably because of the puckering motion of the
chelate ring. The molecular structure of complex 1 was de-
termined by single-crystal X-ray analysis; a perspective view
of the divalent NiII complex cation in 1 is given in Figure 1.
The coordination geometry around the NiII center was con-
firmed as four-coordinate square planar with a cis(P,P)
configuration. It is known that the cis(P,P) isomer is elec-
tronically favored when two phosphanyl-donor groups hav-
ing a strong trans influence coordinate to a square-planar
metal center, although it sterically disfavored when the phos-
phanyl-donor groups bear bulky substituents on the P
atom.[6] The present result indicates that the electronic ef-
Figure 2. An ORTEP view (50% probability level) of the cationic
moiety in rac-[Ni(MePhPea)2](BF4)2 (rac-2). Note that the illus-
trated complex is assigned as an SS stereoisomer; however, this
¯
compound crystallized in the centrosymmetric space group P1.
Complexes 1 and 2 show an ideal planar coordination
fect, in other words the trans influence, is more important geometry around the Ni atom. The largest deviation of the
for the NiII complexes, and that the steric interaction be- constituent atoms from the ideal plane defined by
tween the phenyl substituents may be reduced efficiently by {Ni1,P1,P2,N1,N11} was only 0.023(2) Å (1 for N11) or
a pair of conformations of two 2-aminoethylphosphane 0.131(3) Å (2 for N11). For both complexes, a pair of con-
chelate rings (see below).
formations of the five-membered 2-aminoethylphosphane
Complex 2, which contains MePhPea, was expected to chelate rings was determined as δδ/λλ (racemic). This con-
have a similar coordination geometry because the related formation, with a pseudo-C2 molecular symmetry, would be
complex [Ni(Me2Pea)2](BF4)2 was also obtained as a the most favorable for reducing the intramolecular steric
cis(P,P) isomer.[6] However, complex 2 can form two pos- interactions in the cis(P,P) isomer, although a different pair
sible diastereoisomers (rac and meso) due to the pair of of ring conformations was observed in the analogous Me2-
asymmetric P atoms. The crude product isolated from the Pea complexes.[6] A weak interaction exists between the
reaction mixture was found to have the composition Ni- metal(II) ion and the chloride in the crystal structures of
40
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Nickel(II) Complexes with Hybrid Phosphane-Type Ligands
[Ni(Me2Pea)2]Cl2 and its PdII and PtII analogues, and the
pair of ring conformations of the Me2Pea chelates was
found to be δλ (meso), with a pseudo-Cs molecular sym-
metry. In contrast, the crystal structure of [Ni(Me2Pea)2]-
[NiCl4] did not exhibit any interaction between the central
NiII ion in the complex and the counter anion, and the
chelate ring conformation was found to be δδ/λλ (racemic),
the same as those of complexes 1 and 2. Thus, these find-
ings may indicate a relationship between the accessibility of
coordination of the fifth ligand and the chelate ring confor-
mation in the NiII triad complexes.
The Ph2Pqn complex 3 afforded orange platelet crystals
when crystallized from acetonitrile/diethyl ether, but appar-
ently more reddish crystals with a dichloromethane solvate
molecule (3·CH2Cl2) when deposited by vapor diffusion of
dichloromethane into an acetonitrile solution. We deter-
mined the crystal structures of both 3 and 3·CH2Cl2 and
found that the molecular structures of the complex cations
are almost identical. A perspective drawing of the complex
cation in 3 is shown in Figure 3 (and that of 3·CH2Cl2 is in
Figure S2). The MePhPqn complex [Ni(MePhPqn)2](BF4)2
(4) was obtained as red platelet crystals, and X-ray struc-
tural analysis (Figure 4) confirmed the product to be the
racemic (RR/SS) diastereoisomer. The coordination plane
around the NiII center is distorted toward a tetrahedral ge-
ometry in both 3 (3·CH2Cl2) and 4. The dihedral angles
between the planes defined by {Ni1,P1,N1} and
{Ni1,P2,N11} are 19.7(3)°, 21.9(3)°, and 27.9(1)° for com-
plexes 3, 3·CH2Cl2, and 4, respectively. This distortion re-
sults from the steric hindrance between the ortho-H atoms
of mutually cis-positioned quinolyl groups (Figure 5), and
Figure 3. An ORTEP view (50% probability level, hydrogen atoms
omitted) of the cationic moiety in [Ni(Ph2Pqn)2](BF4)2 (3).
has previously been observed in the crystal structures of
[7]
[Pd(Me2Pqn)2](BF4)2
and [Pd(Ph2Pqn)2](Cl or Br)2.[8]
Such a tetrahedral distortion of the coordination plane
(around 20°) forces the phenyl groups on the P atoms to
become close enough for appreciable intramolecular stack-
ing interactions (Figure 5, b). However, the crystal structure
of the MePhPqn complex 4 is racemic (RR/SS) rather than
meso (RS), the latter of which must be favorable for such
an intramolecular stacking interaction. Furthermore, as the
P1–Ni1–P2 angle of the Ph2Pqn complex [3: 96.76(8)°;
3·CH2Cl2: 96.54(8)°] is much larger than that of the
MePhPqn complex [4: 93.26(4)°], there is probably no
stacking interaction between these phenyl rings.
Figure 4. An ORTEP view (50% probability level) of the cationic
moiety in [Ni(MePhPqn)2](BF4)2 (4).
Table 1 lists selected bond lengths and angles for com-
plexes 1–4. The Ni–P and Ni–N bond lengths in the 2-
aminoethylphosphane complexes 1 and 2 are similar to
those in [Ni(Me2Pea)2][NiCl4] [av. Ni–P: 2.148(4) Å; av. Ni–
N: 1.977(12) Å]. We expected that the steric requirement of
the phenyl substituent and/or the strong trans influence of
the phosphanyl donor group might change these bond
Figure 5. (a) Side and (b) top views of a space-filling model of the
cationic moiety in [Ni(Ph2Pqn)2](BF4)2 (3).
lengths, but only relatively small differences are observed [Ni(Ph2PCH2py)2][NiCl4] [av. Ni–P: 2.160(3) Å; av. Ni–N:
for the Ni–P and Ni–N bond lengths between all [Ni- 1.956(6) Å].[19] It should also be noted that the Ni–P and
(P–N)2]2+ complexes. On the other hand, the P1–Ni1–P2 Ni–N bond lengths of the 2-aminoethylphosphane and 8-
bond angles are indeed affected by the steric requirement quinolylphosphane complexes (1 and 2 vs. 3 and 4) are
of the phosphane substituents {cf. 1 98.79(2)° Ͼ 2 94.72(3)° rather similar, whereas the spectroscopic and electrochemi-
Ͼ [Ni(Me2Pea)2][NiCl4] 92.2(1)°}. The Ni–P and Ni–N cal properties of these complexes exhibit remarkable differ-
bond lengths in
1
are also similar to those in ences, as discussed below.
Eur. J. Inorg. Chem. 2010, 39–47
© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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41

T. Suzuki, H. D. Takagi et al.
FULL PAPER
Table 1. Selected bond lengths [Å] and angles [°] for complexes 1–4.
1
2
3
3·CH2Cl2
4
Ni1–P1
Ni1–P2
Ni1–N1
Ni1–N11
P1–Ni1–P2
N1–Ni1–N11
P1–Ni1–N1
P2–Ni1–N11
P2–Ni1–N1
P1–Ni1–N11
2.1584(5)
2.1614(5)
1.967(2)
1.959(2)
98.79(2)
89.41(6)
85.73(5)
86.06(5)
175.46(5)
175.08(5)
2.1526(7)
2.1475(8)
1.969(2)
1.970(2)
94.72(3)
91.8(1)
86.57(7)
87.08(8)
177.07(8)
175.58(8)
2.168(2)
2.177(2)
1.954(6)
1.949(5)
96.76(8)
95.0(2)
86.6(2)
84.6(2)
168.0(2)
165.3(2)
2.180(2)
2.191(2)
1.969(4)
1.943(4)
96.54(8)
97.3(2)
83.9(2)
86.0(2)
165.3(2)
165.2(2)
2.162(1)
2.151(1)
1.954(3)
1.977(3)
93.26(4)
99.0(1)
87.4(1)
86.6(1)
162.2(1)
159.3(1)
Spectroscopic Properties and Molecular Structures in
Solution
CD3CN was also observed for the MePhPqn complex 4.
Temperature-dependent H NMR spectra of complex 3 in
1
CD3CN (Figure S3) suggest that it exists as an equilibrium
mixture of diamagnetic and paramagnetic species. The dia-
magnetic species must have a distorted square-planar coor-
dination geometry as in CD3NO2, whereas two possibilities
can be considered for the paramagnetic species. One is the
existence of an acetonitrile adduct having a five-coordinate
square-pyramidal geometry or a six-coordinate octahedral
coordination geometry. We attempted to detect such com-
plexes by ESI-TOF mass spectrometry but were unsuccess-
ful. The other possibility for the observed paramagnetism
is a much stronger tetrahedral distortion of the NiII coordi-
nation geometry. As both diamagnetic and paramagnetic
species were detected at lower temperatures, there should
be a moderately high-energy transition state, which might
be caused by a collision of the phenyl groups arising from
the tetrahedral distortion. This explanation seems to be
plausible, but we are unsure as to why such a tetrahedral
distortion occurs in acetonitrile but not in nitromethane.
As mentioned above, the 1H NMR spectra for the 2-
aminoethylphosphane complexes (1 and 2) in CD3CN were
typical of diamagnetic complexes. This fact suggested that
the square-planar coordination geometry around the NiII
center would be retained in solution. The MePhPea com-
plex 2 afforded two diastereoisomers (rac and meso), which
could be separated by fractional recrystallization. The rac
isomer (rac-2), the structure of which was confirmed by sin-
gle-crystal X-ray analysis, showed the P–CH3 resonance at
δ = 1.05 ppm as a virtual triplet. This virtual coupling
pattern of the P–CH3 group is normal for square-planar
NiII and PdII complexes with a cis(P,P) configuration.[6]
The other isomer (meso-2) gave the corresponding reso-
nance at δ = 1.85 ppm (virtual triplet, Figure S1). The rela-
tively large difference in their chemical shifts could result
from the ring current effect of the phenyl ring in the mutu-
ally cis-positioned MePhP donor groups, as seen in Fig-
ure 2. Both rac-2 and meso-2 showed rather broad reso-
nances for the 2-aminoethyl moieties (NH2CH2CH2P) at
23 °C, similar to the case of complex 1 (see above). At
–16.5 °C, however, each resonance became separated into
two broad resonances (Figure S1), thus indicating that there
is a fast puckering motion of the five-membered chelate
ring at ambient temperature. In addition, it should be noted
that no isomerization was observed between rac-2 and
meso-2 in CD3CN; the relative intensities of the resonances
for these two species were independent of temperature
(from –16.5 to 40 °C) and unchanged on standing for sev-
eral days at ambient temperature. Thus, the square-planar
coordination of 2-aminoethylphosphanes toward NiII with
a cis(P,P) configuration is robust in acetonitrile solution.
The 8-quinolylphosphane complexes (3 and 4) are dia-
1
The H NMR spectrum of the MePhPqn complex 4 in
CD3NO2 (Figure S4) was also indicative of diamagnetism
of the solutes, and the temperature-dependent spectral
change suggested that they were an equilibrium mixture of
two species (rac-4 and meso-4). Two P–CH3 resonances
from the respective species were observed at δ = 1.58 and
2.48 ppm (at 21 °C); this difference in chemical shifts is sim-
ilar to that of rac-2 and meso-2 (see above) and results from
the ring current effect of the mutually cis-positioned
MePhP donor groups. Thus, the species having a higher-
field P–CH3 resonance would be assigned as rac-4 (Fig-
ure 4). The equilibrium fraction of rac-4 (over meso-4) be-
came larger at higher temperature (rac-4/meso-4 = 1.7 at
0 °C and 5.5 at 40 °C; Figure S4), which suggests that the
8-quinolylphosphane complexes 3 and 4 are rather labile
towards structural change in solution, in contrast to the
robustness of 2-aminoethylphosphane complexes 1 and 2.
The UV/Vis absorption spectra of complexes 1 and 2[20]
1
magnetic in the solid state (up to 400 K measured). The H
NMR spectrum of the Ph2Pqn complex 3 in CD3NO2
shows reasonably sharp resonances for the phenyl and quin-
olyl protons at δ = 7.33–8.90 ppm (Figure S3), thus indicat- in acetonitrile are shown in Figure 6. In accordance with
ing a diamagnetic nature in solution. The distorted square- the above 1H NMR study, these spectra are similar in
planar coordination geometry found in the crystal structure pattern to those in nitromethane and the diffuse-reflectance
analysis of 3 therefore seems to be maintained in nitrometh- spectra of the solid samples (Figure S5). Square-planar NiII
ane solution. However, the 1H NMR spectrum of 3 in complexes often exhibit a single, broad unsymmetrical band
CD3CN shows very broad signals in the range δ = –20 to in the region 20000–25000 cm–1, which can be assigned to
30 ppm (Figure S3). A similar paramagnetic behavior in the superposition of the d–d transition bands; the band en-
42
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Nickel(II) Complexes with Hybrid Phosphane-Type Ligands
ergy corresponds to ∆oct (ligand-field strength) of the ligand process of the NiII complexes, except for MePhPea complex
to a first approximation.[21a] The Ph2Pea (1) and MePhPea 2,[20] which gives an irreversible voltammogram at very low
(2) complexes exhibit such an absorption band at 24100 and potential, presumably because of immediate decomposition
25300 cm–1, respectively, with an intensity, ε, of around of the reduced product. As seen in Figure 7 (and Table 2),
400 –1 cm–1 (Figure 6).
the electrochemical behavior of complex 3 shows a small
but clear solvent dependence (∆E1/2 ≈ 20 mV and ∆∆Ep ≈
10 mV). These differences are likely to be caused by the
different molecular structures of the Ph2Pqn complex in
acetonitrile and nitromethane solutions, as described above.
Figure 7. Cyclic voltammograms of [Ni(Ph2Pqn)2](BF4)2 (3) in ace-
tonitrile (black) and nitromethane (gray) at 25 °C. [complex] =
1.0ϫ10–3 , I = 0.1  (TBABF4).
Figure 6. Absorption spectra of 2-aminoethylphosphane complexes
1 (– - – -) and 2 (- - - -) in acetonitrile and those of 8-quinolylphos-
phane complexes 3 (––––) and 4 (– – – –) in nitromethane at room
temperature.
Table 2. Redox potential of Ni complexes (in 0.1  TBABF4 solu-
tion in acetonitrile or nitromethane at 298.2Ϯ0.2 K).[a]
Complex/solvent Assignment
E1/2 [mV] ∆Ep [mV]
As seen above, complexes 3 and 4 show a remarkable
solvent dependence in their structures in solution. In aceto- 1/CH3CN
[Ni(Ph2Pea)2]2+/+
[Ni(MePhPea)2]2+/+
[Ni(Ph2Pqn)2]2+/+
[Ni(Ph2Pqn)2]2+/+
[Ni(MePhPqn)2]2+/+
–1147
–1389
–660
–640
–898
87
59[b]
98
109
98
2/CH3CN[b]
nitrile, complex 3 shows a broad absorption band at
3/CH3CN
19900 cm–1, and complex 4 exhibits two broad shoulders at
3/CH3NO2
around 20500 and 25500 cm–1 (Figure S6). These spectra
are apparently different in pattern from the diffuse-reflec-
tance spectra of the solid samples (Figure S6), although the
spectra in nitromethane and the solid diffuse-reflectance
4/CH3CN
[a] vs. Fc+/Fc. [b] Irreversible process due to decomposition of the
reduced species.
spectra are similar in pattern for both complexes. These ob-
The reduction potentials of these NiII complexes are
1
servations are consistent with the results of the H NMR largely dependent on the type of phosphane ligands. Thus,
measurements above: the distorted square-planar coordina- substitution of methyl for phenyl on a P donor atom in-
tion geometry of complexes 3 and 4 is maintained in nitro- duces an approximately 240 mV negative shift in the redox
methane, whereas in acetonitrile they exist as an equilib- potential, while 8-quinolylphosphane complexes exhibit less
rium mixture with a paramagnetic species (a five- or six- negative (by ca 490 mV) redox potentials than the corre-
coordinate acetonitrile adduct or a tetrahedral complex). sponding 2-aminoethylphosphane complexes. This behavior
The Ph2Pqn (3) and MePhPqn (4) complexes in nitrometh- is consistent with the order of the ligand-field strengths ob-
ane exhibit an absorption band at 21700 cm–1 and a shoul- served by absorption spectroscopy: MePhPea (2) Ͼ Ph2Pea
der absorption at around 22000 cm–1 (ε ≈ 800 –1 cm–1), (1) Ͼ MePhPqn (4) Ͼ Ph2Pqn (3).
respectively (Figure 6). These band energies indicate weaker
ligand-field strengths of 8-quinolylphosphanes than 2-ami-
noethylphosphane [∆oct (cm–1) = 25300 (MePhPea) Ͼ
24100 (Ph2Pea) ϾϾ ca. 22000 (MePhPqn) Ͼ 21700
(Ph2Pqn)].
Preparation and Crystal Structure of Nickel(I) Ph2Pqn
Complex
Reduction of Ph2Pqn complex 3 with Zn powder[22] in
acetonitrile afforded a dark red solution, from which a dark
red precipitate could be isolated on addition of deaerated
water. As the cyclic voltammogram of this product (in ace-
Electrochemical Properties
Figure 7 shows cyclic voltammograms of complex 3 in tonitrile) was almost identical to that of complex 3, it was
acetonitrile and nitromethane (those for the other com- assumed to be the corresponding NiI complex of
plexes are shown in Figure S7). The redox potentials (E1/2 [Ni(Ph2Pqn)2]BF4 (5). The UV/Vis spectrum of 5 in aceto-
vs. Fc+/Fc) and the difference in potential between anodic nitrile shows shoulder absorptions at around 11500 and
and cathodic peaks (∆Ep) are listed in Table 2. These data 14500 cm–1 with intensities of around 200 –1 cm–1 (Figure
are indicative of the quasi-reversible one-electron reduction S8). This spectral pattern is similar to the diffuse-reflectance
Eur. J. Inorg. Chem. 2010, 39–47
© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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43

T. Suzuki, H. D. Takagi et al.
FULL PAPER
spectrum of a solid sample of complex 5, thus indicating a 2.216(1) Å.[27] Compared with these examples, the present
similar molecular structure of the NiI complex cation in the NiI–P bond lengths in complex 5 [av. 2.227(1) Å] seem to
solid state and in acetonitrile.
be normal for NiI phosphane complexes.
The crystal structure of complex 5 was determined by X-
¯
ray analysis. The asymmetric unit (triclinic space group P1,
Z = 2) contains an [Ni(Ph2Pqn)2]+ cation and a BF4– anion.
The structure of the [Ni(Ph2Pqn)2]+ moiety is shown in Fig-
ure 8. The coordination geometry around the NiI center is
distorted tetrahedral; the dihedral angle between the
{Ni1,P1,N1} plane and the {Ni1,P2,N11} plane is 57.6(1)°,
and that between the least-squares quinolyl rings is
61.06(9)°. The P1–Ni1–P2 bond angle is 115.67(4)°. If a
regular tetrahedral geometry around the Ni center is as-
sumed, a large steric hindrance between the phenyl substit-
uents would be expected. Thus, to reduce such a steric inter-
action effectively, the coordination geometry is distorted by
around 30° toward a syn structure.[23] A Jahn–Teller effect
of the d9 tetrahedral complex may be another reason for
this distortion.[21b]
Conclusions
We have characterized four nickel(II) complexes of the
type [Ni(P–N)2](BF4)2, where P–N is a 2-aminoethylphos-
phane or 8-quinolylphosphane ligand. The coordination
geometry of the 2-aminoethylphosphane complexes 1 and 2
is square planar with a cis(P,P) configuration in the solid
state and in acetonitrile solution. It seems that the coordi-
nation bonds of Ph2Pea and MePhPea to a NiII ion are
robust in solution. The diastereoisomers (rac and meso) of
cis(P,P)-[Ni(MePhPea)2](BF4)2 (2) have been separated by
fractional recrystallization and the crystal structure of rac-
2 determined by X-ray analysis.
In contrast, the 8-quinolylphosphanes result in distorted
square-planar NiII complexes (3 and 4) because of the steric
repulsion between the ortho-H atoms in the mutually cis-
positioned quinolyl rings. This distortion makes the σ- and
π-overlaps between NiII 3d and ligand orbitals ineffective,
therefore the ligand-field strengths of the 8-quinolylphos-
phanes must be weaker in these complexes. In agreement
with this conclusion, complexes 3 and 4 exhibit fluxional
behavior in solution. The distorted square-planar geometry
seems to be maintained in nitromethane as the absorption
spectra in nitromethane are similar in pattern to the diffuse-
reflectance spectra of the solid samples. Typical diamag-
1
netic H NMR spectra are also observed in CD3NO2, but
the temperature-dependent 1H NMR spectra of cis(P,P)-
[Ni(MePhPqn)2](BF4)2 (4) indicate that this complex exists
as an equilibrium mixture of rac and meso diastereoisomers.
In acetonitrile, however, these complexes show paramag-
netic behavior, presumably arising either from the coordina-
tion of solvent acetonitrile molecule(s) to form a five- or
six-coordinate complex or the stronger tetrahedral distor-
tion of the coordination geometry.
The weaker ligand-field strengths of 8-quinolylphos-
phanes than 2-aminoethylphosphanes are shown experi-
mentally by the d–d transition energies observed by absorp-
tion spectroscopy: Ph2Pqn Ͻ MePhPqn Ͻ Ph2Pea Ͻ
MePhPea. This order is consistent with that of the NiII/I
redox potential of the complexes. The most readily reduc-
ible Ph2Pqn complex (3) affords the corresponding nickel(I)
complex [Ni(Ph2Pqn)2]BF4 (5). The molecular structure of
this NiI complex, which contains two bidentate phosphane
ligands, is distorted tetrahedral because of the steric repul-
sion caused by the phenyl substituents and/or a Jahn–Teller
effect of the ideally tetrahedral d9 complex.
Figure 8. An ORTEP view (30% probability level, hydrogen atoms
omitted) of the cation in [Ni(Ph2Pqn)2]BF4 (5). Selected bond
lengths [Å] and angles [°]: Ni1–P1 2.230(1), Ni1–P2 2.224(1), Ni1–
N1 2.003(3), Ni1–N11 2.017(3); P1–Ni1–P2 115.67(4), N1–Ni1–
N11 96.48(12), P1–Ni1–N1 86.22(9), P2–Ni1–N11 86.19(9), P1–
Ni2–N11, 134.72(9), P2–Ni1–N1 145.92(9).
The Ni–P and Ni–N bond lengths in complex 5 are
2.224(1)–2.230(1) and 2.003(3)–2.017(3) Å, respectively,
around 0.05 Å longer than the corresponding bonds in the
NiII complex 3. This can be explained by the difference in
ionic radii between Ni+ and Ni2+ [24]
The Ni–P bond
.
lengths in [Ni(PMe3)4]BPh4, where the coordination geome-
try of NiI is also distorted tetrahedral, are reported to be
2.211(3)–2.221(3) Å.[25] The Ni–P bonds in the three-coor-
dinate iminobenzylamide complex [NiI(ArN=CHC6H4N-
ArЈ)(PPh3)] are 2.210(1) and 2.233(2) Å.[26] Several other
NiI complexes containing the bidentate phosphane ligand
tBu2PCH2CH2PtBu2 (dtbpe) have been reported by Hil-
house et al., and the NiI–P bond lengths in the chlorido-
Experimental Section
Materials and Measurements: All solvents used during preparation
of the phosphanes and their complexes were dried with appropriate
bridged
dimer
[(dtbpe)Ni(µ-Cl)]2
are
2.210(1)– drying agents and distilled prior to use. The P–N-type hybrid li-
44
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© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2010, 39–47

Nickel(II) Complexes with Hybrid Phosphane-Type Ligands
1
gands Ph2Pea (previously abbreviated as edpp), MePhPea,[12c]
Ph2Pqn,[28] and MePhPqn[17] were synthesized according to litera-
ture methods.
4.07, N 3.14; found C 57.4, H 3.78, N 3.22. H NMR (300 MHz,
CD3NO2, 21 °C): δ = 7.36 (t, J = 7.5 Hz, 4 H, m-Ph), 7.57 (t, J =
7.5 Hz, 2 H, p-Ph), 7.78 (m, 4 H, o-Ph), 7.94 (m, 2 H, qn), 8.14
(m, 1 H, qn), 8.42 (d, J = 8.1 Hz, 1 H, qn), 8.72 (m, 1 H, qn), 8.88
(d, J = 7.5 Hz, 1 H, qn) ppm. 31P{1H} NMR (152 MHz, CD3NO2,
21 °C): δ = 43.2 (br) ppm. UV/Vis [solvent]: σmax (εmax/–1 cm–1) =
12400 (40), 19900 (160), and 26300sh (ca. 900) [CH3CN]; 21700
(741) cm–1 [CH3NO2]. Crystals suitable for X-ray analysis were ob-
tained from an acetonitrile solution by vapor diffusion of diethyl
ether (orange platelet crystals of 3) or dichloromethane (red platelet
crystals of 3·CH2Cl2).
1H NMR spectra were acquired on a Varian Mercury 300 spec-
trometer. Chemical shifts were referenced to the residual 1H NMR
signals of the deuterated solvents and are reported with respect
to TMS. The 31P{1H} NMR spectra were obtained on the same
spectrometer, using 85% H3PO4 as an external reference for the 31
P
NMR chemical shifts. Absorption spectra were recorded with a
Jasco V-550 spectrophotometer at room temperature. Diffuse-re-
flectance spectra of the solid samples were obtained on the same
spectrophotometer using an integration sphere (Jasco ISN-470).
Cyclic voltammograms were measured at 25.0(2) °C on a BAS-
100B/W electrochemical analyzer in acetonitrile or nitromethane
solution ([complex] = 1.0 m, 0.1  TBABF4). A platinum disk (φ
1.8 mm), a platinum wire, and a Ag/AgNO3 electrode were used as
the working, auxiliary, and reference electrodes, respectively. The
redox potentials of the samples were calibrated with respect to the
redox signal for the ferrocenium/ferrocene couple.
[Ni(MePhPqn)2](BF4)2 (4):
A methanol solution (50 mL) of
Ni(BF4)2·6H2O (4.1 g, 0.012 mol) was added with stirring to an
ethanol solution (100 mL) of (crude) MePhPqn (6.0 g). The mix-
ture was refluxed for 1 h and then cooled to 0 °C. The resulting
yellow precipitate was collected by filtration, washed with ethanol
and diethyl ether, and dried in vacuo; yield 5.93 g (67%).
C32H28B2F8N2NiP2 (734.83): calcd. C 52.3, H 3.84, N 3.81; found
C 52.0, H 3.83, N 3.75. 1H NMR (300 MHz, CD3NO2, 21 °C): δ
= 1.58 (s, Me of rac-4), 2.48 (s, Me of meso-4), 7.19 (t, J = 7.5 Hz,
qn of meso-4), 7.40–7.50 (m, qn of meso-4), 7.72 (t, J = 7.5 Hz, qn
of rac-4), 7.80–8.10 (m, qn of rac-4 and meso-4), 8.44 (d, J =
8.1 Hz, qn of rac-4), 8.78 (m, qn of rac-4), 8.91 (d, J = 8.1 Hz, qn
of rac-4) ppm. 31P{1H} NMR (152 MHz, CD3NO2, 21 °C): δ =
32.2 (br) ppm. UV/Vis [solvent]: σmax (εmax/–1 cm–1) = 12900 (10),
20500sh (ca. 300), and 25500sh (ca. 800) [CH3CN]; 22000sh (ca.
750) cm–1 [CH3NO2]. Crystals suitable for X-ray analysis were ob-
tained from an acetonitrile solution by vapor diffusion of diethyl
ether.
[Ni(Ph2Pea)2](BF4)2 (1): A methanol solution (50 mL) of Ni(BF4)2·
6H2O (1.51 g, 4.44 mmol) was added with stirring to a suspension
of Ph2Pea (2.16 g, 9.42 mmol) in methanol (50 mL). The mixture
was refluxed for 30 min and then cooled to 0 °C. The resulting
yellow precipitate was collected by filtration, washed with methanol
and diethyl ether, and dried in vacuo; yield 1.58 g (51.5%).
C28H32B2F8N2NiP2 (690.83): calcd. C 48.7, H 4.67, N 4.06; found
1
C 48.4, H 4.53, N 4.33. H NMR (300 MHz, CD3CN, 21 °C): δ =
2.58 (br., 2 H, CH2), 2.70 (br., 2 H, CH2), 3.90 (br., 2 H, NH2),
7.33–7.38 (m, 4 H, Ph), 7.48–7.53 (m, 6 H, Ph) ppm. 31P{1H} NMR
(152 MHz, CD3CN, 21 °C): δ = 49.2 (br) ppm. UV/vis [solvent]:
σmax (εmax/–1 cm–1) = 24100 (349) [CH3CN]; 24500 (575) cm–1
[CH3NO2]. Crystals suitable for X-ray analysis were obtained from
an acetonitrile solution by vapor diffusion of diethyl ether.
[Ni(Ph2Pqn)2]BF4 (5): An acetonitrile solution (20 mL) of
3
(223 mg, 0.260 mmol) was stirred over Zn powder (ca. 1 g) at room
temperature for 1 h under argon and then unreacted Zn powder
was filtered off. The filtrate was concentrated under reduced pres-
sure, and deaerated water (50 mL) was added to the concentrate.
The resulting dark red precipitate was collected by filtration,
washed with water and diethyl ether, and dried in vacuo; yield
152 mg (76%). UV/Vis [solvent]: σmax (εmax/mol–1 dm3 cm–1) =
11500sh (ca. 200) and 14500sh (ca. 270) cm–1 [CH3CN]. Crystals
suitable for X-ray analysis were obtained by vapor diffusion of di-
ethyl ether into a nitromethane solution.
[Ni(MePhPea)2](BF4)2 (2): Under an argon atmosphere, 1.01 g of
(crude) MePhPea was added to an ethanol solution (100 mL) of
Ni(BF4)2·6H2O (1.03 g, 3.03 mmol), and the mixture stirred under
reflux for 1 h. The resulting yellow solution was concentrated to
about 10 mL under reduced pressure, and then cooled to 0 °C to
afford the product as a yellow precipitate; yield 0.48 g (28%).
C20H34B2F8N2NiOP2 (2·EtOH, 612.75): calcd. C 39.3, H 5.61, N
Crystallography: X-ray diffraction data for complexes 1–5 were ob-
tained at –73(2) °C on a Rigaku R-axis rapid imaging plate detec-
tor with graphite-monochromated Mo-Kα radiation (λ
1
4.59; found C 39.1, H 5.29, N 4.25. H NMR (300 MHz, CD3CN,
23 °C): δ = 1.05 (virtual t, Me of rac-2), 1.85 (virtual t, Me of meso-
2), 2.0–2.4 (br., CH2 of rac-2 and meso-2), 2.6–3.2 (br., CH2 of rac-
2 and meso-2), 3.75 (br., NH2 of rac-2 and meso-2), 7.13–7.35 (m,
Ph of meso-2), 7.50–7.85 (m, Ph of rac-2) ppm. 31P{1H} NMR
(152 MHz, CD3CN, 23 °C): δ = 38.3 (rac-2) and 39.1 (meso-2) ppm.
=
0.71073 Å). A suitable crystal was mounted with a cryoloop and
flash-cooled in a cold nitrogen stream. Data were processed by the
Process-Auto program package,[29] and absorption corrections were
applied by the empirical method.[30] The structures were solved by
either direct methods using SIR92[31] or by a heavy-atom method
using DIRDIF99-PATTY,[32] and refined on F2 (with all indepen-
dent reflections) using the SHELXL97 program.[33] All non-H
atoms were refined anisotropically, and H atoms were introduced
at the positions calculated theoretically and treated with riding
models. All calculations were carried out using the CrystalStructure
software package.[34] Crystal data are collected in Table 3.
UV/Vis [solvent]: σmax (εmax/–1 cm–1)
=
25300 (347) cm–1
[CH3CN]. This product was found to be a mixture of diastereoiso-
mers (rac and meso). To obtain suitable crystals for the X-ray dif-
fraction analysis, the crude product was mixed with a minimum
amount of dichloromethane and the insoluble precipitate quickly
collected by filtration. The resulting yellow product was recrys-
tallized from an acetonitrile solution by vapor diffusion of diethyl
ether to give yellow columnar crystals of rac-2.
CCDC-743141 (for 1), -743142 (for 2), -743143 (for 3), -743144 (for
3·CH2Cl2), -743145 (for 4), -743146 (for 5) contain the supplemen-
tary crystallographic data for this paper. These data can be ob-
tained free of charge from The Cambridge Crystallographic Data
Centre via www.ccdc.cam.ac.uk/data_request/cif.
[Ni(Ph2Pqn)2](BF4)2 (3): A methanol solution (50 mL) of Ni(BF4)2·
6H2O (830 mg, 2.44 mmol) was added with stirring to a suspension
of Ph2Pqn (1.76 g, 5.62 mmol) in methanol (50 mL). The mixture
was refluxed for 30 min and then cooled to 0 °C. The resulting
reddish-brown precipitate was collected by filtration, washed with
methanol, thf, and diethyl ether, and dried in vacuo; yield 1.17 g
(56.2%). C43H36B2F8N2NiOP2 (3·MeOH, 891.01): calcd. C 57.7, H
Supporting Information (see also the footnote on the first page of
this article): An ORTEP drawing of the cationic moiety in
Eur. J. Inorg. Chem. 2010, 39–47
© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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45

T. Suzuki, H. D. Takagi et al.
FULL PAPER
Table 3. Crystal data for complexes.
Complex
1
2
3
3·CH2Cl2
4
5
Chemical formula
C28H32B2F8-
N2NiP2
690.83
monoclinic
9.3420(3)
9.2833(4)
35.5768(14)
90
94.725(1)
90
3074.9(2)
200(2)
C18H28B2F8-
N2NiP2
566.68
C42H32B2F8-
N2NiP2
858.97
monoclinic
14.121(7)
15.758(10)
17.976(7)
90
94.94(4)
90
3985(4)
200(2)
P21/c
4
1.432
0.638
0.1127
515/8795
0.0886
0.2218
C43H34B2Cl2F8-
N2NiP2
943.89
orthorhombic
15.905(14)
17.949(11)
28.80(2)
90
90
90
8221(11)
200(2)
Pbca
C32H28B2F8-
N2NiP2
734.83
monoclinic
9.1097(5)
20.535(1)
17.912(1)
90
104.597(1)
90
3242.6(3)
200(2)
P21/n
4
1.505
0.770
0.0741
427/7348
0.0686
0.1869
C42H32BF4-
N2NiP2
772.16
M
Crystal system
a [Å]
b [Å]
c [Å]
α [°]
β [°]
triclinic
triclinic
8.5664(5)
10.9473(6)
13.8767(8)
79.256(2)
79.886(2)
79.508(2)
1243.56(12)
200(2)
8.837(2)
11.967(3)
17.554(4)
90.901(5)
99.519(5)
101.362(5)
1792.8(7)
200(2)
γ [°]
V [Å3]
T [K]
¯
P1
2
¯
P1
2
Space group
Z
P21/n
4
1.492
0.806
8
Dcalc [Mgm–3]
µ(Mo-Kα) [mm–1]
Rint
1.497
0.977
0.0151
301/5629
0.0469
0.1418
1.525
0.752
0.2015
542/9102
0.0816
0.0908
1.430
0.686
0.0719
470/8039
0.0605
0.1751
0.0151
Number of param./reflections 416/6767
R1(F2) [Fo2 Ͼ 2σ(Fo )][a]
0.0325
0.0822
2
wR2(F2) (all data)[b]
2
2 2
2 1/2
[a] R1 = Σ||Fο| – |Fc||/ Σ|Fο|. [b] wR2 = [Σw(Fο – Fc ) /ΣwFο
] .
[Ni(Ph2Pqn)2](BF4)2·CH2Cl2 (3·CH2Cl2), 1H NMR spectra, UV/
Vis absorption spectra, and cyclic voltammograms of complexes 1–
4.
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Acknowledgments
We thank Mr. Keita Ariyoshi and Ms. Yuki Yamane (Okayama
University) for 31P NMR and magnetic measurements, respectively.
This work was supported by the Ministry of Education, Culture,
Sports, Science, and Technology, Japan, (Grant-in-Aid for Scien-
tific Research; grant numbers 16550055 and 20550064).
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© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2010, 39–47

Nickel(II) Complexes with Hybrid Phosphane-Type Ligands
[20] We used a crude mixture of rac-2 and meso-2 (ca. 2:1) for the
spectrophotometric and electrochemical measurements in or-
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Received: August 5, 2009
Published Online: November 26, 2009
Eur. J. Inorg. Chem. 2010, 39–47
© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
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