Preparation, crystal structures, and behavior in solution of cobalt(III) complexes containing 2-cyanoethylphosphines: Trans-[Co(Me2dtc) 2{P(CH2CH2CN)nPh3-n}] BF4 (n = 1-3; Me<sub

Article abstract of DOI:10.1246/bcsj.20120148

A series of cobalt(III) complexes containing 2-cyanoethylphosphines, [Co(Me₂dtc)₂{P(CH₂CH₂CN) _nPh_3-n}₂]BF₄{Me₂dtc ^-: N,N-dimethyldithiocarbamate;

Full text of DOI:10.1246/bcsj.20120148

1160 Bull. Chem. Soc. Jpn. Vol. 85, No. 10, 1160-1166 (2012)
© 2012 The Chemical Society of Japan
Preparation, Crystal Structures, and Behavior in Solution of
Cobalt(III) Complexes Containing 2-Cyanoethylphosphines:
trans-[Co(Me2dtc)2{P(CH2CH2CN)nPh3¹n}]BF4
¹
(n = 1-3; Me2dtc : N,N-Dimethyldithiocarbamate)
Keiko Kihara,1 Takayoshi Suzuki,*1 Masakazu Kita,2 Yukinari Sunatsuki,1
Masaaki Kojima,1 and Hideo D. Takagi3
1Department of Chemistry, Graduate School of Natural Science and Technology,
Okayama University, 3-1-1 Tsushima-naka, Okayama 700-8530
2Graduate School of Education, Okayama University, 3-1-1 Tsushima-naka, Okayama 700-8530
3Research Center for Materials Science, Nagoya University, Nagoya, Aichi 464-8602
Received May 24, 2012; E-mail: suzuki@okayama-u.ac.jp
A series of cobalt(III) complexes containing 2-cyanoethylphosphines, [Co(Me2dtc)2{P(CH2CH2CN)nPh3¹n}2]BF4
¹
{Me2dtc : N,N-dimethyldithiocarbamate; n = 1 (1), 2 (2), and 3 (3)} have been prepared, and their molecular structures in
the crystals and in solution have been characterized. X-ray analysis revealed that all complexes isolated as single-crystals
were the trans-isomers. The Co-P bond lengths for the 2-cyanoethylphosphines in complexes 1-3 are almost invariant
to the number of 2-cyanoethyl substituent groups. Unlike related complexes with other monodentate P-donor ligands,
complexes 1-3 in solution showed dissociation and isomerization equilibriums, which were achieved immediately
after dissolving the crystals. These characteristic features of the complexes would result from the weak ·-donor and ³-
acceptor abilities of 2-cyanoethylphosphines. Furthermore, mixing of complex 1 and NaN3 in acetonitrile gave cis-
[Co(Me2dtc)2(N3){P(CH2CH2CN)Ph2}] quantitatively, and refluxing the mixture resulted in the phosphine-tetrazolate
complex, [Co(Me2dtc)2(Ph2PCH2CH2CN4-¬2P,N1)] (4).
In general, basicities of phosphines having a 2-cyano-
ethyl substituent are relatively low, as compared to those of
the sterically similar ethyl-substituted analogs, owing to the
strong electron-withdrawing properties of a terminal cyano
group. For example, the pKa values of the conjugated acids of
P(CH2CH2CN)Ph2, P(CH2CH2CN)2Ph, and P(CH2CH2CN)3 are
2.27, 1.82, and 1.36, respectively, which are remarkably smaller
than those of PEtnPh3¹n (Table 1).1-4 The »d values4 represent-
ing the ·-donicity (»d is smaller in number for a stronger
·-donor) of the 2-cyanoethylphosphines are larger than those of
the corresponding ethylphosphines, and comparable to those of
P(OMe)nPh3¹n having a methoxy substituent. However, unlike
P(OMe)nPh3¹n, the phosphines of P(CH2CH2CN)nPh3¹n are not
Table 1. Stereoelectronic Parameters and pKa Values of P-
Donor Ligandsa)
b)
P-Ligand
ª/° pKa
»
»
d
Ear
³
p
PPh3
PEtPh2
PEt2Ph
PEt3
PMePh2
PMe2Ph
PMe3
P(OMe)Ph2
P(OMe)2Ph
P(OMe)3
P(OCH2)3CEt
PHPh2
145 2.73
140 4.9
136 6.25
132 8.69
136 4.57
122 6.5
118 8.65
13.25 13.25 2.7
0
0
0
0
0
0
0
11.30 11.1
2.3
1.1
0
9.30
6.30
8.6
6.3
12.10 12.10 2.2
10.60 10.60
8.55 8.55
14.8
120 (2.64) 19.45 16.4
1
0
132 (2.69) 16.3
2.3 0.9
1.7 1.9
107 2.60
101 1.74
126 0.03
24.10 17.9
31.20 20.0
17.35 14.5
16.28c) 14.5
19.31c) 15.8
22.35 17
1
0.2
2
2
1
2.8
5
1.2
0.4
0.8
1.2
good ³-acceptors. The ³ value5 (which is larger in number
p
for a better ³-acceptor) of P(CH2CH2CN)3 is 1.2, which is
remarkably smaller than that of P(OMe)3 (2.8) and comparable
to that of PPh2H (1.2). Therefore, 2-cyanoethylphosphines are
neither strong ·-donor nor ³-acceptor ligands, and their transi-
tion-metal complexes are thus limited,6 as compared to a huge
number of complexes with other phosphines or phosphites.
In previous studies,6-11 we have prepared and characterized
a series of cobalt(III) complexes bearing various monodentate
P(CH2CH2CN)Ph2 140 2.27
P(CH2CH2CN)2Ph 136 1.82
P(CH2CH2CN)3
132 1.36
0
a) Values are taken from refs. 1 and 4. b) Values of the
conjugated acids: [HPRnPh3¹n]+. c) These data were not
reported in the original papers, so we obtained these values
from the data of PPh3 and P(CH2CH2CN)3 by assuming the
additivity of the substituents.
¹
P-donating ligands: [Co(Me2dtc)2(P-ligand)2]+ (Me2dtc : N,N-
dimethyldithiocarbamate), in order to elucidate the donor
properties of P-ligands in the cobalt(III) complexes. It was
revealed that the ligand-field strengths, the coordination bond
lengths, and the strengths of trans influence of the P-ligands
were largely dependent on the electronic and steric effects of
their substituents. Also, the thermodynamic stability of the
geometric (trans/cis) isomers are dependent on the nature of P-
Published on the web October 10, 2012; doi:10.1246/bcsj.20120148

K. Kihara et al.
Bull. Chem. Soc. Jpn. Vol. 85, No. 10 (2012) 1161
ligands; the sterically demanding ligands prefer the mutually
trans configuration, while the P-ligands having a strong trans
influence usually sit in mutually cis positions. In some cases,
it was possible to prepare thermodynamically unstable (but
kinetically favorable) trans-isomers with sterically less de-
manding P-ligands, and reversible thermal and photochemical
cis-trans isomerizations were observed. In this study we
have prepared the analogous P(CH2CH2CN)nPh3¹n complexes,
[Co(Me2dtc)2{P(CH2CH2CN)nPh3¹n}2]BF4 {n = 1 (1), 2 (2),
and 3 (3)}, and investigated their structures in the solid
states and in solution, in order to compare their structures
and properties to those of the corresponding PMenPh3¹n and
P(OMe)nPh3¹n complexes. In addition, a reaction of complex
[Co(Me2dtc)2(Ph2PCH2CH2CN4)] (4). Under an atmos-
phere of dinitrogen a mixture of complex 1 (0.12 g, 0.14 mmol)
and NaN3 (0.028 g, 0.44 mmol) in acetonitrile (20 mL) was
refluxed for 26 h. After cooling to room temperature,
the mixture was allowed to stand in a refrigerator (¹20 °C)
overnight. The resulting dark brown precipitate was collected
by filtration, and the crude product was dissolved in dichloro-
methane. To the filtered solution was diffused diethyl ether
vapor in a closed vessel, affording dark brown crystals of 4.
Yield: 0.061 g (73%). Anal. Found: C, 41.74; H, 4.14; N,
13.59%. Calcd for 4¢0.5CH2Cl2, C21.5H27ClCoN6PS4: C,
41.44; H, 4.38; N, 13.49%. 1H NMR (CDCl3, 20 °C, 300
MHz): ¤ 2.50-3.06 (m, 4H, -CH2CH2-), 2.65 (s, 3H, NCH3),
2.74 (s, 3H, NCH3), 3.09 (s, 3H, NCH3), 3.26 (s, 3H, NCH3),
7.19-7.89 (m, 10H, Ph).
¹
1 with N3 to form a 5-(2-diphenylphosphinoethyl)tetrazolato
complex was attempted.
Measurements. Proton NMR spectra were acquired on
a Varian Mercury 300 spectrometer. The chemical shifts were
referenced to the residual H NMR signals of the deuterated
solvents and are reported versus TMS. Absorption spectra
were recorded on a Jasco V-550 spectrophotometer at room
temperature.
Experimental
1
Materials. The ligand, P(CH2CH2CN)Ph2 was prepared by
a literature method,12 while P(CH2CH2CN)2Ph and P(CH2CH2-
CN)3 were purchased from Strem Chemicals Inc. These
phosphines were handled under an atmosphere of dinitrogen
using a standard Schlenk technique until such time that air
stable cobalt(III) complexes were formed. The triphenylphos-
phine complex, trans-[Co(Me2dtc)2(PPh3)2]BF4, was synthe-
sized by a method reported previously.11 Other chemicals and
solvents used in preparation of the phosphine and complexes
were used as received.
Crystallography. The X-ray diffraction data of compounds
1-4 were obtained at ¹80(2) °C on a Rigaku R-axis rapid
imaging plate detector with a graphite-monochromated Mo K¡
radiation (- = 0.71073 ¡). A suitable crystal was mounted
with a cryoloop and flash-cooled by cold nitrogen stream. Data
were processed by the Process-Auto program package,13 and
absorption corrections were applied by the empirical method.14
The structures were solved by the direct method using
SIR2004,15 and refined on F2 (with all independent reflections)
using SHELXL97 program.16 All non-H atoms were refined
anisotropically. Hydrogen atoms were introduced at the posi-
tions calculated theoretically and treated with riding models.
All calculations were carried out using CrystalStructure soft-
ware package.17
Crystal data are collected in Table 2. Crystallographic
data have been deposited with Cambridge Crystallographic
Data Centre: Deposition numbers CCDC-860724 to 860727
for compounds 1-4, respectively. Copies of the data can be
obtained free of charge via http://www.ccdc.cam.ac.uk/conts/
retrieving.html (or from the Cambridge Crystallographic Data
Centre, 12, Union Road, Cambridge, CB2 1EZ, U.K., Fax:
+44 1223 336033; e-mail: deposit@ccdc.acm.ac.uk).
trans-[Co(Me2dtc)2{P(CH2CH2CN)Ph2}2]BF4 (1).
A
mixture of Co(BF4)2¢6H2O (0.70 g, 2.0 mmol) and P(CH2CH2-
CN)Ph2 (1.2 g, 5.0 mmol) in ethanol (30 mL) was stirred at
room temperature for 1 h. To this mixture was added a solution
of tetramethylthiuram disulfide (0.49 g, 2.0 mmol) in a mixture
of ethanol and dichloromethane (1:1, 80 mL) dropwise with
stirring for 3 h. After complete addition of disulfide solution,
the mixture was further stirred for 30 min, and the resulting
greenish brown precipitate was collected by filtration. The
crude product was dissolved in acetonitrile, and vapor diffusion
of diethyl ether into the filtered solution gave dark brown
platelet crystals. Yield: 0.79 g (45%). Anal. Found: C, 49.97;
H, 4.66; N, 6.48%. Calcd for 1, C36H40BCoF4N4P2S4: C, 50.01;
H, 4.30; N, 6.48%.
trans-[Co(Me2dtc)2{P(CH2CH2CN)2Ph}2]BF4 (2). Meth-
od A: Dark brown block crystals were obtained by a method
similar to the above, using P(CH2CH2CN)2Ph (0.88 g, 4.0
mmol) instead of P(CH2CH2CN)Ph2. Yield: 0.66 g (39%).
Anal. Found: C, 44.03; H, 4.29; N, 10.20%. Calcd for 2,
C30H38BCoF4N6P2S4: C, 44.02; H, 4.68; N, 10.27%.
Method B: A mixture of trans-[Co(Me2dtc)2(PPh3)2]BF4
(0.072 g, 0.078 mmol) and P(CH2CH2CN)2Ph (0.040 g, 0.18
mmol) in acetonitrile (5 mL) was stirred in an ice bath for 1 h.
Then, the solution was concentrated by flow of dinitrogen gas,
and vapor diffusion of diethyl ether into the concentrate gave
dark brown crystals of 2.
trans-[Co(Me2dtc)2{P(CH2CH2CN)3}2]BF4 (3). Greenish
brown microcrystals were obtained by a method similar
to the above Method A, using P(CH2CH2CN)3 (0.81 g, 4.2
mmol) instead of P(CH2CH2CN)Ph2. Yield: 1.1 g (70%). Anal.
Found: C, 38.68; H, 4.60; N, 15.58%. Calcd for 3¢CH3CN,
C26H39BCoF4N9P2S4: C, 38.38; H, 4.83; N, 15.49%.
Results and Discussion
Preparation of 2-Cyanoethylphosphine Complexes. In
our previous studies,7-11 the mixed-ligand complexes contain-
ing dithiocarbamate and monodentate P-donor ligand, [Co-
(Me2dtc)2(P-ligand)2]+, were prepared by two methods. One
is oxidation of an ethanolic mixture of [Co(H2O)6]2+ and
P-ligand (1:2 molar ratio) by tetramethylthiuram disulfide,
and the other is substitution of PPh3 in trans-[Co(Me2dtc)2-
(PPh3)2]+ by a sterically less demanding P-ligand. The former
method gave a thermodynamically stable isomer: cis-isomer
of the PMe3, PEt3, PHPh2, P(OCH2)3CEt, and P(OMe)nPh3¹n
complexes, but trans-isomer of the PPh3 complex. The latter
ligand-substitution method afforded the kinetically favorable
trans-isomer for the above-mentioned P-ligands. In the cases of
PHPh2,8 P(OMe)nPh3¹n,9 and P(OCH2)3CEt10 complexes ther-

1162 Bull. Chem. Soc. Jpn. Vol. 85, No. 10 (2012)
Table 2. Crystallographic Data for Complexes 1-4
CoIII Complexes of 2-Cyanoethylphosphines
Complex/P-Liganda)
¹
1/P(CH2CH2CN)Ph2 2/P(CH2CH2CN)2Ph 3/P(CH2CH2CN)3
4/Ph2PCH2CH2CN4
Chemical formula
Formula weight
C36H40BCoF4N4P2S4 C30H38BCoF4N6P2S4 C24H36BCoF4N8P2S4
C21H26CoN6PS4
580.62
864.66
818.59
772.53
T/K
193(2)
193(2)
193(2)
193(2)
Crystal color and shape
Size of specimen/mm
Crystal system
Space group, Z
brown, plate
0.2 © 0.1 © 0.1
triclinic
brown, block
0.3 © 0.3 © 0.2
triclinic
brown, plate
0.5 © 0.3 © 0.15
monoclinic
P21/n, 4
10.2776(3)
22.2481(7)
15.7395(4)
90
103.5039(8)
90
3499.5(2)
1.466
brown, plate
0.3 © 0.2 © 0.05
monoclinic
Cc, 8
30.267(3)
8.4744(8)
21.297(2)
90
91.046(3)
90
5461.7(9)
1.412
ꢀ
ꢀ
P1, 2
P1, 1
a/¡
b/¡
c/¡
¡/°
¢/°
£/°
9.3127(5)
9.6394(6)
22.181(1)
98.915(3)
99.475(2)
91.511(3)
1937.5(2)
1.482
8.747(2)
10.073(2)
11.746(2)
94.802(5)
110.533(6)
101.659(6)
935.5(3)
1.453
U/¡3
¹3
Dcalcd/Mg m
¹1
®(Mo K¡)/mm
7.936
8.185
8.717
10.143
Rint
0.025
0.019
0.058
0.057
No. reflns/params.
8870/477
0.033
0.101
4253/216
0.037
0.109
8013/402
0.049
0.145
6177/303
0.044
0.120
2
R1 (F2: Fo2 > 2·(Fo ))
wR2 (F2: all data)
a) For complexes 1-3: trans-[Co(Me2dtc)2(P-ligand)2]BF4. Complex 4: [Co(Me2dtc)2(P-ligand)].
¹
mal trans-to-cis and photochemical cis-to-trans isomerizations
were also observed.
inversion centers, and a BF4 anion. In both complex cations
(Figures 1a and S1), one of the phenyl groups (C10-C15
in Figure 1a) was nearly perpendicular, while the other
phenyl group was almost parallel, to the [Co(Me2dtc)2] equa-
In this study we have prepared 2-cyanoethylphosphine
complexes, [Co(Me2dtc)2{P(CH2CH2CN)nPh3¹n}2]+, by the
above two methods. Addition of Me2NC(S)S-SC(S)NMe2 into
a mixture of [Co(H2O)6](BF4)2 and P(CH2CH2CN)nPh3¹n
in ethanol/dichloromethane afforded a greenish brown pre-
cipitate, which was recrystallized from acetonitrile by vapor
diffusion of diethyl ether to deposit dark brown single crystals
of [Co(Me2dtc)2{P(CH2CH2CN)nPh3¹n}2]BF4 (1-3). The same
products were obtained by ligand-substitution from trans-
[Co(Me2dtc)2(PPh3)2]BF4 and P(CH2CH2CN)nPh3¹n in aceto-
nitrile, followed by addition of diethyl ether into the reaction
mixture. As seen in the following paragraphs, the crystal
structures of 1-3 prepared by both methods were revealed to
be the trans-isomers. We expected that the oxidation method
would give the cis-isomers as the previous P-ligand complexes,
but only the trans-isomers were isolated for the complexes
with 2-cyanoethylphosphines. Moreover, when compounds 1-3
were dissolved in acetonitrile, equilibriums based on the disso-
ciation of 2-cyanoethylphosphines and the trans/cis isomer-
ization were rapidly achieved (vide infra). Thus, the lower
solubility of the trans-isomers of 1-3 resulted in preferable
crystallization of the isomers.
torial plane. In previous studies for trans-[Co(Me2dtc)2{PHPh2,
PMePh2, or P(OMe)Ph2}2]BF4
7-9
a similar orientation of
phenyl groups was a key factor for reducing the intramolecular
steric hinderance between the P-ligand and [Co(Me2dtc)2]
moiety, which is the reason for the unusually long Co-P bonds
in trans-[Co(Me2dtc)2(PPh3)2]BF4.7 In compound 1, the Co1-
P1 and Co2-P2 bond lengths are 2.2920(4) and 2.2945(4) ¡,
respectively, which are comparable to those in the analogous
PMePh2 and P(OMe)Ph2 complexes (2.303(1) and 2.291(3) ¡,
respectively).
The P(CH2CH2CN)2Ph complex salt, 2, crystallized in a
triclinic space group P1 with Z = 1, having a crystallographic
ꢀ
inversion center at Co1. The molecular structure of the complex
cation in 2 is shown in Figure 1b. The phenyl group is oriented
in a parallel fashion to the [Co(Me2dtc)2] plane; this orientation
is also observed in the corresponding PMe2Ph and P(OMe)2Ph
complexes.7,9 The Co1-P1 bond length in 2 is 2.3025(5) ¡, and
those in the PMe2Ph and P(OMe)2Ph complexes are 2.2843(8)
and 2.266(1) ¡, respectively.
The compound, trans-[Co(Me2dtc)2{P(CH2CH2CN)3}2]BF4
(3), crystallized in a monoclinic space group of P21/n with
Z = 4. As seen in Figure 1c, one of the 2-cyanoethyl groups
(C12-C11-N4) showed a characteristic orientation, as com-
pared to the other groups, although the orientations did not alter
the Co-P bond lengths so much: Co1-P1 2.3011(8) vs. Co1-P2
2.2985(7) ¡. The average Co-P bond length in trans-[Co-
Crystal Structures. The crystal structures of a series of
compounds, trans-[Co(Me2dtc)2{P(CH2CH2CN)nPh3¹n}2]BF4
(1-3), were determined by X-ray diffraction. It was revealed
that all complex cations in 1-3 were the trans-isomers in the
crystals (Figures 1 and S1).
The compound, trans-[Co(Me2dtc)2{P(CH2CH2CN)Ph2}2]-
7
ꢀ
BF4 (1) crystallized in a triclinic space group P1 with Z = 2. In
(Me2dtc)2(PMe3)2]BF4 is 2.287(1) ¡, and that in trans-[Co-
(Me2dtc)2{P(OMe)3}2]BF4 is 2.241(1) ¡. Thus, in the series
of P(CH2CH2CN)nPh3¹n and PMenPh3¹n complexes the Co-P
9
the asymmetric unit there are two halves of the complex cation,
in which the Co1 and Co2 atoms are located at crystallographic

K. Kihara et al.
Bull. Chem. Soc. Jpn. Vol. 85, No. 10 (2012) 1163
(a)
(b)
(c)
Figure 1. ORTEPs (30% probability level, H atoms omitted for clarity) with atom-numbering schemes of (a) one of the
crystallographically independent complex cations in trans-[Co(Me2dtc)2{P(CH2CH2CN)Ph2}2]BF4 (1), (b) the cationic part in
trans-[Co(Me2dtc)2{P(CH2CH2CN)2Ph}2]BF4 (2), and (c) that in trans-[Co(Me2dtc)2{P(CH2CH2CN)3}2]BF4 (3).
(a)
(b)
Figure 2. Correlation between the (averaged) Co-P bond lengths of trans-[Co(Me2dtc)2(P-ligand)2]+ in the crystals of the BF4 or
PF6 salts against (a) Tolman’s cone angle or (b) electronic parameters of P-ligands.
bond lengths were not altered significantly, while they changed
gradually in the series of P(OMe)3¹nPhn complexes. In
Figure 2 the averaged Co-P bond lengths are plotted against
the Tolman’s cone angle, ª, and electronic parameters, », of the
P-ligands, respectively. These plots indicated that the Co-P
coordination bond lengths are roughly related to the steric
factors (cone angles) of the P-ligands, but apparently there is no
simple relation to the electronic properties.
largely dependent on the kind of P-ligands. The 1H NMR
spectra were also diagnostic for the geometrical structure by the
number of Me2dtc resonances observed, owing to the molecu-
lar symmetry. In the cases of the P(OMe)nPh3¹n and PHPh2
complexes, reversible (photochemical and thermal) cis- and
trans-isomerizations were observed by H NMR spectroscopy,
as to measure the rate constants for the slow isomerization
reactions.8,9
1
Behavior in Solution.
In previous studies concerning
The present 2-cyanoethylphosphine complexes, 1-3, showed
a different behavior in solution from the above-mentioned
complexes; a remarkable concentration-dependency in the UV-
vis spectra was observed. As seen in Figure 3 for complex 3
(for complexes 1 and 2, see: Figure S2), the absorption bands
trans- and cis-[Co(Me2dtc)2(P-ligand)2]+ complexes, we have
also investigated their molecular structures in solution. For the
tertiary phosphines (PMenPh3¹n), both isomers are stable in
solution; no isomerization was observed at ambient conditions.7
¹1
In their UV-vis absorption spectra, the cis-isomers showed two
d-d bands (¾ µ 103 M¹1 cm¹1) around 18000 and 23000 cm
around 25000 and 29000 cm became larger with the higher
¹1
,
concentration. There are two pseudo-isosbestic points at 21900
and 30500 cm¹1. When an excess amount of P(CH2CH2CN)3
was added, two absorption bands became more distinct,
indicating the formation of trans-[Co(Me2dtc)2{P(CH2CH2-
CN)3}2]+ in solution. Thus, it is obvious that complexes 1-3
exhibit dissociation equilibrium of the 2-cyanoethylphos-
while the trans-isomers exhibited two intense charge-transfer
bands (¾ µ 104 M¹1 cm¹1) in the range of 22000-28000 and
¹1
¹1
28000-30000 cm as well as a d-d band (¾ µ 300 M¹1 cm
)
around 15000 cm¹1. Here, it should be noted that the lowest-
energy CT band (22000-28000 cm¹1) of the trans-isomer was

1164 Bull. Chem. Soc. Jpn. Vol. 85, No. 10 (2012)
CoIII Complexes of 2-Cyanoethylphosphines
(a)
(b)
Figure 3. Absorption spectral change of acetonitrile solu-
tion of compound 3 depended on the concentration: (light
¹3
blue) 7.2 © 10¹5, (blue) 1.4 © 10¹3, (purple) 2.9 © 10
and (red) 1.4 © 10¹3 M + excess free ligand.
,
phines. This easy dissociation is indicative of a weak coor-
dination ability of P(CH2CH2CN)nPh3¹n to a CoIII ion. The
lowest-energy CT bands of complexes 1-3 (in the presence
of excess phosphines) were little shifted by the phosphines:
(c)
¹1
23700, 24000, and 25000 cm for the P(CH2CH2CN)Ph2,
P(CH2CH2CN)2Ph, and P(CH2CH2CN)3 complexes, respec-
tively. The corresponding band of trans-[Co(Me2dtc)2-
(PPh3)2]BF4 was observed at 23080 cm¹1.7 For the PMenPh3¹n
and P(OMe)nPh3¹n series of complexes, the CT band was
¹1
varied in the range of 23080-27380 and 23080-27760 cm
,
Figure 4. 1H NMR spectra (in the region for the Me2dtc
signals only) of compounds 1 (a), 2 (b), and 3 (c) dissolved
in CD3CN (1.0 © 10¹2 M). Signals marked by A, B, and
C would be due to the species of trans-[Co(Me2dtc)2-
(P-ligand)(CD3CN)]+, cis-[Co(Me2dtc)2(P-ligand)(CD3-
CN)]+, and cis-[Co(Me2dtc)2(CD3CN)2]+, respectively.
respectively.7,9 The little shift in the present P(CH2CH2CN)n-
Ph3¹n complexes is consistent with a small variation of the ·-
donicity and ³-acidity among the 2-cyanoethylphosphines.
The 1H NMR spectra of complexes 1-3 in CD3CN
(Figure 4) indicated that, not only the dissociation, but also
the isomerization equilibrium exist in solution. The predom-
inant species gave only one (singlet for 1 and doublet for 2
and 3) Me2dtc resonance (marked by A in Figure 4), whose
chemical shift was largely shifted in the order of 1: ¤ 2.67 < 2:
¤ 2.95 < 3: ¤ 3.29. Because this resonance (of 2 and 3) was
observed as a doublet, presumably owing to coupling to a
31P nucleus, the species would be assigned as trans-[Co-
(Me2dtc)2{P(CH2CH2CN)nPh3¹n}(CD3CN)]+. The other main
species gave four Me2dtc resonances (marked by B); sug-
gesting a formation of the cis-isomer: cis-[Co(Me2dtc)2-
{P(CH2CH2CN)nPh3¹n}(CD3CN)]+. In the spectrum of 1, the
third species giving two singlet resonances for Me2dtc (C)
was also detected, which would be either cis-[Co(Me2dtc)2-
{P(CH2CH2CN)Ph2}2]+ or cis-[Co(Me2dtc)2(CD3CN)2]+. The
latter species was more plausible, because a larger steric
demand of P(CH2CH2CN)Ph2 than the others would make
the formation of the former bis(P-ligand) complex unfavorable.
We have also measured the temperature- and concentration-
dependency of the 1H NMR spectra (Figures S3 and S4),
in order to confirm the equilibrium is reached rapidly in
solution. At lower temperature and at higher concentration,
the trans-(P-ligand)(CD3CN) isomer was more favorable than
the cis-isomer.
Conversion of Complex 1 to 5-(2-Diphenylphosphino-
ethyl)tetrazolato Complex 4.
Erbe and Beck reported
preparation of PdII, PtII, RhI, and IrI complexes containing 5-
¹
(2-diphenylphosphinoethyl)tetrazolate, Ph2PCH2CH2CN4 , by
a reaction of P(CH2CH2CN)Ph2 with the azido ligand in the
square-planar precursor complexes.18 However, they did not
characterize the molecular structures of the complexes unam-
biguously. Moreover, to our best knowledge, no other reports
concerning transition-metal complexes with Ph2PCH2CH2CN4
have appeared to date.
¹
In this study we have attempted to react the P(CH2CH2CN)-
Ph2 complex, 1, with an excess amount of NaN3 in refluxing
acetonitrile. This reaction afforded a dark brown crystalline
product of [Co(Me2dtc)2(Ph2PCH2CH2CN4)] (4) in ca. 70%
yield (based on Co). The molecular structure of 4 was con-
firmed by single-crystal X-ray analysis as shown in Figure 5.
The ligand, 5-(2-diphenylphosphinoethyl)tetrazolate (Ph2-
¹
PCH2CH2CN4 ) coordinates to a CoIII center, forming a six-
membered chelate ring. Because of the planarity of the tetra-
zolate ring, the chelate ring has a gauche conformation. The
Co1-P1 bond in 4 is 2.2547(7) ¡, which is shorter than that in

K. Kihara et al.
Bull. Chem. Soc. Jpn. Vol. 85, No. 10 (2012) 1165
¹
complex 1, but relatively longer than that in [Co(Me2dtc)2-
{NH2(CH2)3PPh(CH2OH)}]BPh4 (2.195(2) ¡).19 The Co1-S2
bond (2.2951(7) ¡) is slightly longer than the mutually trans
Co-S bonds (Co1-S1: 2.2686(6) and Co1-S4: 2.2660(6) ¡),
indicating moderate trans influence of the Ph2P- donor group.
The molecular structure of complex 4 in CDCl3 was also
ited four Me2dtc resonances consistent with the molecular
structure of [Co(Me2dtc)2(Ph2PCH2CH2CN4-¬2P,N1)].
In order to examine the conversion reaction from complex
1 to 4 in detail, we have measured the H NMR spectra of
1
the reaction mixture. Figure 6 showed the spectral change of
complex 1 in CD3CN and the mixture with an excess amount of
NaN3 (immediately after mixing and after two hours stirring
at room temperature), together with the spectrum of the product
4 (in CDCl3). These spectra indicated that the intermediate
species, which is probably cis-[Co(Me2dtc)2(N3){P(CH2CH2-
CN)Ph2}], was quantitatively formed in solution, and the 1,3-
dipolar cycloaddition between the coordinated azido and 2-
cyanoethylphosphine groups took place to give phosphino-
ethyltetrazolato complex (Scheme 1). In this reaction one
equivalent of P(CH2CH2CN)Ph2 was dissociated from the CoIII
center, and the dissociated phosphine did not react with azide
anion, but was recovered from the filtrate of the reaction
mixture.
1
confirmed by the H NMR spectrum (Figure 6d), which exhib-
Conclusion
In this study three new cobalt(III) complexes contain-
ing 2-cyanoethylphosphines, [Co(Me2dtc)2{P(CH2CH2CN)n-
Ph3¹n}2]BF4 (1-3), have been prepared and their molecular
structures in the crystals and in solution have been charac-
terized. In contrast to the related P-ligand (phosphine/phos-
Figure 5. ORTEP (50% probability level, hydrogen atoms
are omitted for clarity) with atom-numbering scheme of
[Co(Me2dtc)2(Ph2PCH2CH2CN4-¬2P,N1)] (4).
(d)
(c)
(b)
(a)
Figure 6. 1H NMR spectra of (a) complex 1 in CD3CN, (b) immediately after the addition of excess NaN3 to the solution, and
(c) after stirring the mixture at room temperature for 2 h. The resonance marked with * is of impurity of [Co(Me2dtc)3]. The
1H NMR spectrum of complex 4 in CDCl3 (d) is also shown.
Scheme 1. A schematic drawing of possible intermediate in the reaction of complex 1 with NaN3 to give complex 4.

1166 Bull. Chem. Soc. Jpn. Vol. 85, No. 10 (2012)
CoIII Complexes of 2-Cyanoethylphosphines
phite) complexes reported previously,7-11 these 2-cyanoethyl-
phosphine complexes exist as an equilibrium mixture of ligand
dissociation and isomerization, which were attained immedi-
ately after dissolving. The easy dissociation of P(CH2CH2-
CN)nPh3¹n from the CoIII center is consistent with the inher-
ent weak donor properties (·-donicity and ³-acidity) of
2-cyanoethylphosphines. The rapid equilibriums in solution
would be a reason for the selective crystallization of trans-
[Co(Me2dtc)2{P(CH2CH2CN)nPh3¹n}2]BF4, which would have
lower solubility than the cis-isomer. For the other P-ligand
complexes, the cis-isomers are thermodynamically more stable
and can be isolated as crystals; however, we could not isolate
the cis-isomers of 2-cyanoethylphosphine complexes 1-3.
The Co-P bond lengths in trans-[Co(Me2dtc)2{P(CH2CH2-
CN)nPh3¹n}2]BF4 (1-3) and the lowest-energy LMCT transi-
tion energy of the complexes are little affected by the numbers
of 2-cyanoethyl substituents. These facts are also consistent
with little difference in their ·-donicity and ³-acidity among
the 2-cyanoethylphosphines.1-4
3
S. Joerg, R. S. Drago, J. Sales, Organometallics 1998, 17,
589, and references therein.
4
D. Woska, A. Prock, W. P. Giering, Organometallics 2000,
19, 4629, and references therein.
5
A. L. Fernandez, C. Reyes, A. Prock, W. P. Giering,
J. Chem. Soc., Perkin Trans. 2 2000, 1033.
For example: Md. N. I. Khan, C. King, J. P. Fackler, Jr.,
6
R. E. P. Winpenny, Inorg. Chem. 1993, 32, 2502; M. S. Holt, J. H.
Nelson, Inorg. Chem. 1986, 25, 1316; D. S. Barratt, A. Hosseiny,
C. A. McAuliffe, C. Stacey, J. Chem. Soc., Dalton Trans. 1985,
135; N. A. Bell, M. Goldstein, L. A. March, I. W. Nowell,
J. Chem. Soc., Dalton Trans. 1984, 1621; R. A. Walton, R.
Whyman, J. Chem. Soc. A 1968, 1394.
7
T. Suzuki, S. Kashiwamura, K. Kashiwabara, Bull. Chem.
Soc. Jpn. 2001, 74, 2349.
8
S. Iwatsuki, T. Suzuki, A. Hasegawa, S. Funahashi, K.
Kashiwabara, H. D. Takagi, J. Chem. Soc., Dalton Trans. 2002,
3593; T. Suzuki, S. Iwatsuki, H. D. Takagi, K. Kashiwabara,
Chem. Lett. 2001, 1068.
9
S. Iwatsuki, S. Kashiwamura, K. Kashiwabara, T. Suzuki,
H. D. Takagi, Dalton Trans. 2003, 2280.
10 H. Matsui, M. Kita, K. Kashiwabara, J. Fujita, Bull. Chem.
Soc. Jpn. 1993, 66, 1140.
11 M. Kita, A. Okuyama, K. Kashiwabara, J. Fujita, Bull.
Chem. Soc. Jpn. 1990, 63, 1994.
12 H. Dibowski, F. P. Schmidtchen, Tetrahedron 1995, 51,
2325.
13 Process-Auto, Automatic Data Acquisition and Processing
Package for Imaging Plate Diffractometer, Rigaku Co. Ltd.,
Akishima, Tokyo, Japan, 1998.
14 T. Higashi, ABSCOR, Empirical Absorption Correction
Based on Fourier Series Approximation, Rigaku Corporation,
Tokyo, Japan, 1995.
15 M. C. Burla, R. Caliandro, M. Camalli, B. Carrozzini, G. L.
Cascarano, L. De Caro, C. Giacovazzo, G. Polidori, R. Spagna,
J. Appl. Crystallogr. 2005, 38, 381.
16 G. M. Sheldrick, Acta Crystallogr., Sect. A 2008, 64, 112.
17 CrystalStructure ver. 3.7.0. Crystal Structure Analysis
Package, Rigaku and Rigaku/MSC, Akishima, Tokyo, Japan and
The Woodlands, TX, USA, 2000-2005.
When an excess of NaN3 was added to an acetonitrile
solution of complex 1, cis-[Co(Me2dtc)2(N3){P(CH2CH2CN)-
Ph2}] was formed quantitatively, and it was converted to
[Co(Me2dtc)2(Ph2PCH2CH2CN4)] (4) on refluxing the reaction
mixture. The cycloaddition reaction between 2-cyanoethyl and
the coordinated azido moieties gave a six-membered chelate
¹
ligand of Ph2PCH2CH2CN4 . To our best knowledge, com-
plex 4 is the first example of 5-(2-diphenylphosphinoethyl)-
tetrazolato complex whose molecular and crystal structure was
confirmed by X-ray analysis.
Supporting Information
An ORTEP of two crystallographically independent com-
plex cations in 1, concentration-dependent absorption spectra
1
of 1 and 2 in acetonitrile, and temperature-dependent H NMR
spectra of 1-3 in CD3CN. This material is available free of
charge on the Web at: http://www.csj.jp/journals/bcsj/.
References
18 J. Erbe, W. Beck, Chem. Ber. 1983, 116, 3867.
19 K. Kashiwabara, M. Watanabe, M. Kita, T. Suzuki, Bull.
Chem. Soc. Jpn. 1996, 69, 1947.
1
2
C. A. Tolman, Chem. Rev. 1977, 77, 313.
P. B. Dias, M. E. M. de Piedade, J. A. M. Simões, Coord.
Chem. Rev. 1994, 135-136, 737.