Journal of Nuclear and Radiochemical Sciences, Vol. 6, No. 3, pp. 161-164, 2005

A Kinetic Study on Ligand Substitution Reaction of N-Cyclohexyl-2-pyrrolidone for
Triphenylphosphine in Trichlorooxorhenium(V) Complex
Nobuyoshi Koshino, Sanae Usui, Yoshio Iwai,† Tamás R. Varga,‡ Shubhamoy Chowdhury,
Attila C. Bényei, § Munetaka Iwamura,† and Yasuhisa Ikeda*
Research Laboratory for Nuclear Reactors, Tokyo Institute of Technology, 2-12-1-N1-34 O-okayama, Meguro-ku,
Tokyo 152-8550, Japan
Received: July 31, 2005; In Final Form: September 30, 2005
The crystal structure of the ReOCl 3 (PPh 3 )(NCP) (PPh 3 = triphenylphosphine and NCP = N-cyclohexyl-2pyrrolidone) complex was determined by a single-crystal X-ray diffraction method. It was found that NCP coordinates to the trans-position of the oxo group, Re = O, and that the Re(V) center is in a distorted octahedral
geometry. The equilibrium reaction of ReOCl3(PPh3)2 + NCP ↔ ReOCl3(PPh3)(NCP) + PPh3 was examined, andits equilibrium constant was determined as 0.33 ± 0.1 in CH2Cl2 at 25 ˚C. Kinetic studies were carried out for theforward and backward reactions, and revealed that the rate determining steps for the both reactions are dissociationof the coordinated ligand from the Re center. Taking into account of geometrical isomers of the ReOCl3 (PPh3) 2complex, a reaction mechanism was proposed.

1. Introduction

2. Experimental

The wide use of 99mTc as diagnostic agents and the recent
development of 186Re and 188Re radiopharmaceuticals have promoted studies for Tc and Re coordination chemistries.1, 2 It is
known that 186Re and 188Re decay by β -ray emission as well as
γ -ray emission. Therefore, 186Re and 188Re compounds should
be used as diagnostic and therapeutic medicines.3 From thispoint of view, a number of Re complexes have been synthesized.4
In syntheses, ReOCl3 (PPh 3) 2 (PPh 3 = triphenylphosphine) isoften used as a starting material.5, 6 Its molecular structure insolid state has been reported as a mer-trans geometry.7 However,
it is also subjected that this kind of Re complexes have geometrical isomers, i.e. mer-cis and fac-cis.8 The ligand substitution
behavior of this complex is not well understood so far. We havepreviously found out that N-cyclohexyl-2-pyrrolidone (NCP) is apreferable ligand for metal-oxo ions such as the uranyl ion.9−11
Crystallographic studies of uranyl complexes have clarified thatNCP coordinates through amide O atom to the uranyl ion. It
was also found that donicity of NCP is comparable to N,Ndimethylformamide (DMF) and dimethyl sulfoxide.11 It is often
observed that Re(V) complexes prefer soft donor atoms.
Therefore, it would be interesting to compare the reactivity ofO-donor and S-donor ligands. We have selected tetramethylthiourea (TMTU) as a S-donor ligand representative. In thisstudy, we have examined the substitution reactions of NCP orTMTU (Figure 1) for PPh3 in the ReOCl3(PPh3)2 complex.

Materials and Instruments: All chemicals used in thisstudy were of reagent grade and used without further purification. The ReOCl3 (PPh3) 2 complex was synthesized by using
the literature method.12 The equilibrium reaction and kinetic
studies were performed in dehydrated CH2Cl2 by using an UVvis spectrophotometer (Shimadzu UV-3150). NMR spectra
were measured by a Jeol GXR 270 NMR spectrometer.
Synthesis and X-ray diffraction of ReOCl3 (PPh3)(NCP):
A green crystalline complex of ReOCl3 (PPh3)(NCP) was synthesized by mixing ReOCl3(PPh3)2 with NCP and grown in themixture with diethyl ether. The molecular structure was determined by an X-ray diffraction method. Intensity measurements
were carried out on a Rigaku RAXIS RAPID diffractometerwith Mo-Kα radiation (λ = 0.71075 Å). Crystal data collection
and final refinement parameters are summarized in Table 1.
The structures were solved by a heavy-atom Patterson method,and expanded using Fourier techniques. The non-hydrogenatoms were refined using anisotropic temperature factors.
Hydrogen atoms were introduced at calculated positions usingOSNN

N

NCP

TMTU

Figure 1. Chemical formulas of NCP and TMTU.
*Corresponding author. E-mail: yikeda@nr.titech.ac.jp. FAX:
+81-3-5734-3061.
†Seikei University, Japan. ‡Szent Istvan Univeristy, Hungary.
§University of Debrecen, Hungary.

TABLE 1: Crystallographic Data of ReOCl3 (PPh3)(NCP)
Complex
Empirical formula
Formula weight
Crystal system
Space groupa (Å)
b (Å)
c (Å)
β (˚)
V (Å3)
Z
Temperature (K)Residuals: R1a , wR 2b
Goodness of fitab

R1 = Σ||Fo| – |Fc||/Σ|Fo|.
wR 2 = {Σ[w(Fo2 – Fc2) 2]/Σw(Fo2) 2}1/2 .

© 2005 The Japan Society of Nuclear and Radiochemical Sciences
Published on Web 12/28/2005

C28H32NO2PCl3Re
738.11
monoclinic
P21/c
13.2336(8)10.228(1)20.752(2)95.487(4)2796.0(4)4113 + 1
0.020, 0.051
1.04

162 J. Nucl. Radiochem. Sci., Vol. 6, No. 3, 2005

Koshino[NCP] / mmol dm0Absorbance1.2-30.80.4

85.50.0300

350400450500550600

Wavelength / nm
Figure 3. Spectral changes in the reaction of ReOCl3 (PPh3) 2 with
NCP in CH 2Cl2 at 25 ˚C. [ReOCl3 (PPh3) 2] 0 = 0.94 mmol dm−3 and[PPh3]T = 10.2 mmol dm−3.

Figure 2. Molecular structure of ReOCl3(PPh3)(NCP) complex.

the riding model. All calculations were performed using theCrystal Structure crystallographic software package.13
3. Results and Discussion
Molecular structure of ReOCl3 (PPh3)(NCP): The obtained
molecular structure of ReOCl3(PPh3)(NCP) is shown in Figure 2
and selected bond lengths and angles are listed in Table 2. It was
found that NCP coordinates to Re(V) center on the trans-positionof Re1-O1 bond, and Re(V) center is in a distorted octahedral
geometry.14 The geometry around Re(V) center is almost identical to those of ReOCl3(PPh3)(DMF) and ReOCl3(PPh3)(OPPh3)(OPPh 3 = triphenylphosphine oxide).15, 16 This is consistent
with “the self-consistency rule”, where trans position to oxogroup tends to be occupied by a σ -donor ligand.17 It is alsoworthy of note that the bond length of Re1-Cl2, where the Cl
atom coordinates to the Re(V) center in the trans site to PPh3,
is the shortest of the Re-Cl bonds in three complexes.
TABLE 2: Selected Bond Lengths and Bond Angles of
ReOCl3 (PPh3)L Complexes
L
Bond length / Å
Re1–O1
Re1–O2
Re1–Cl1
Re1–Cl2
Re1–Cl3
Re1–P1
Bond angle / ˚
O1–Re1–O2
Cl1–Re1–Cl3
Cl2–Re1–P1aNCP

DMF a

OPPh3 b1.671(2)2.100(2)2.383(1)2.362(1)2.392(1)2.487(1)1.664(4)2.133(4)2.372(2)2.368(2)2.373(2)2.500(1)1.669(4)2.082(4)2.373(2)2.361(2)2.384(2)2.506(2)168.34(9)168.21(3)165.32(3)170.8(2)167.40(6)166.02(5)171.35(18)169.74 (6)171.52 (6)Ref. 15, b Ref. 16.

Equilibrium constant for the reaction between ReOCl3(PPh3)2and NCP: As mentioned above, ReOCl3(PPh3)(NCP) is formedby mixing ReOCl3- (PPh3) 2 with NCP in CH2Cl2. We noticed
that this reaction is reversible as follows.
ReOCl3(PPh3)2 + NCP

forwardbackward

ReOCl3(PPh3)(NCP) + PPh(1)3The equilibrium (1) was examined in CH2Cl2 at 25 ˚C. Figure
3 shows each spectrum measured by adding NCP to a solutionof ReOCl3 (PPh3) 2. From the absorbance changes, the equilib-rium constant of (1) was determined to be 0.33 ± 0.1 in CH2Cl2
at 25 ˚C. This indicates that donicity of NCP to the ReO2+ moiety is comparable to that of PPh3. The relatively high donicityof NCP is also observed in our previous work on the complex
formation of UO22+ ion with NCP.11
We also used TMTU as an unidentate ligand, and found thatthe reaction is not reversible in this case. This finding indicatesthat TMTU strongly coordinates to the Re(V) center. SomeRe(V) complexes with TMTU have been reported.18, 19 We measured 31P NMR spectrum of a mixture of ReOCl3 (PPh3) 2 with
TMTU in CD2Cl2, and observed two peaks at − 23.7 and − 2.3
ppm relative to the peak of 85% H3PO4. Those peaks should be
assigned to the coordinated and free PPh3, respectively. We have
not obtained the crystal structure of the product yet. However, it
is likely that the product with a formula of ReOCl3(PPh3)(TMTU)
is formed by the reaction (2). The 31P chemical shifts of several
compounds are listed in Table 3.
ReOCl3(PPh3)2 + TMTU

ReOCl3(PPh3)(TMTU) + PPh3 (2)TABLE 3: 31P chemical Shifts of Phosphorus Compounds
Compound
PPh3
ReOCl3 (PPh3) 2
ReOCl3 (PPh3)(NCP)
ReOCl3 (PPh3)(TMTU) c
ReOCl3 (PPh3)(OPPh3)
OPPh3aδ / ppm a–2.3–15.8 b–16.6
–23.7
–30.8 (PPh3) b
52.5 (OPPh3) b
30.5 b

In CD2Cl2 vs. external 85% H3PO4; b Ref. 7; c Plausible formula.

Kinetics of the reaction of ReOCl 3 (PPh 3 ) 2 with L (L =NCP and TMTU): The kinetic studies of the reactions ofReOCl3(PPh3)2 with L were carried out spectrophotometricallyin CH2Cl2 at various temperatures. The pseudo first-order rate
constants (k obs) were measured by following the absorbancechanges at 416 nm after mixing a solution of ReOCl3 (PPh3) 2with excess L ([Re] << [L]). Each kinetic trace was nicely fittedby the first-order equation.20 It was found that the kobs values are
independent of the concentrations of L. The plots of kobs values
against [NCP] is shown in Figure 4. This result indicates thatthe rate-determining step (rds) of the forward reaction is ligand
dissociation from the Re(V) center, and the dissociation groupwould be one of PPh3 ligands in ReOCl3(PPh3)2.
ReOCl3(PPh3)2

rds

ReOCl3(PPh3) + PPh3

(3)

We assume that the PPh3 dissociation reaction (3) involves

J. Nucl. Radiochem. Sci., Vol. 6, No. 3, 2005 163

A Kinetic Study on Ligand Substitution Reaction

0.4

0.1225 °C30 °C
0.3

kobs / s−1

kobs / s−1

0.08

25 °C
0.04

20 °C

0.2

20 °C

15 °C

0.1

10 °C

15 °C
0.00

000.01

0.02

0.04

0.03

24

[PPh3] / mmol dm[NCP] / mol dm−3
Figure 4. Plots of kobs against [NCP] for the forward reaction of (1).
[ReOCl3(PPh3) 2] = 0.1 mmol dm–3 in CH2Cl2.

mer-trans and mer-cis isomerization of ReOCl3 (PPh3) 2. From
recent 31P NMR study, we have tentatively obtained evidence
showing that such isomerization would exist in CD 2 Cl 2 . 21
Furthermore, it is also reported that mer-trans and mer-cis
ReOCl3(PPh3)2 complexes have been isolated.22

Scheme 2OOClPh3PReCl
mer-trans Cl

PPh3

Ph3P

ClPh3PReKiso

Cl
PPh3

PPh3

Cl
mer-transReCl

ClClkdPh3P

rdsClRe

Ph3P

ClCl

PPh3
mer-cisKiso kd
1 + Kiso- PPh3ReO
Cl+ NCP Ph3P

Cl- NCP

ReClClCl
NCP(4)

and if Kiso is fairly smaller than 1, eq 4 can give eq 5.21
kobs = Kisokd

mer-cisO

Cl

When reaction (3) proceeds in Scheme 1, the rate constant isexpressed by eq 4,kobs =ClOOPh3PClRe+PPh3

O6−3Figure 5. Plots of k obs against [PPh3] for the backward reaction of(1). [ReOCl3(PPh3)(NCP)] = 0.1 mmol dm–3 in CH2Cl2.

Scheme 1Cl0(5)From the Eyring plot, the activation parameters were obtainedas ∆H‡ = 86.4 ± 0.1 kJ mol–1 and ∆S‡ = 21 ± 1 J K–1 mol–1. We
also investigated the forward reaction (1) by using TMTU as anentering ligand at various temperatures, and confirmed that thekobs values at each temperature are independent of [TMTU] andare almost the same as those in the reaction with NCP (Weobtained ∆H‡ and ∆S‡ as 81 ± 1 kJ mol–1 and 2 ± 3 J K–1 mol–1for TMTU reaction).
Kinetics of the reaction of ReOCl 3 (PPh 3 )(NCP) with
PPh3 : The backward reaction of (1) was also examined by theabsorbance changes at 416 nm after mixing a solution ofReOCl3(PPh3)(NCP) with PPh3 under the condition of [Re] <<[PPh 3]. Figure 5 shows that the rate constants (k obs) are notaffected by [PPh3]. Therefore, it is also proposed that ligand
dissociation should be the rate-determining step in the backward
reaction. From the Eyring plot, the values of ∆H‡ and ∆S‡ wereobtained as 90 ± 2 kJ mol–1 and 48 ± 5 J K–1 mol–1, respectively.
Reaction mechanism of reaction (1): From the kinetic
data, it is presumed that the rate-determining steps of both
directions of reaction (1) are dissociation of the leaving ligand(PPh3 from ReOCl3 (PPh3) 2 in the forward reaction, and NCPfrom ReOCl3(PPh3)(NCP) in the backward reaction).
Considering this finding and geometrical aspect of Re complexes, we propose a mechanism of the ligand substitution
reaction of ReOCl3(PPh3)2 with NCP as shown in Scheme 2.

4. Summary
The results of present study are summarized as follows. TheReOCl3 (PPh 3) 2 complex reacts with NCP in CH 2Cl 2 to form
ReOCl3(PPh3)(NCP). The resulting ReOCl3(PPh3)(NCP) complex has a distorted octahedral geometry and NCP coordinates
to the trans position of Re = O group. The equilibrium constant of ReOCl3(PPh3)2 + NCP ↔ ReOCl3(PPh3)(NCP) + PPh3
was determined as 0.33 ± 0.1 in CH 2Cl 2 at 25 ˚C. The ratedetermining steps of forward and back-ward reactions in theabove equilibrium are proposed to be the dissociation of PPh3and NCP coordinated to the trans position of the oxo group in
Re = O moiety.
Acknowledgement. This research was partly supported by
the Ministry of Education, Culture, Sports, Science andTechnology of Japan. S. C. is thankful for Invitation fellowship
programs for research in Japan (JSPS).
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