Journal of Nuclear and Radiochemical Sciences, Vol. 6, No. 3, pp. 157-159, 2005

X-ray Structure and Electrochemical Properties of a Tpen-Bridged Hetero-Binuclear
Complex fac-, fac-[ReVIIO3 (µ -tpen)ReI(CO) 3](PF6)2 where Tpen Is
N, N, N', N'-Tetrakis(2-pyridylmethyl)ethylenediamine
Toshihiro Tabeya, Masaaki Abe,* Ayako Mitani, Kiyoshi Tsuge, and Yoichi Sasaki*
Division of Chemistry, Graduate School of Science, Hokkaido University, Sapporo 060-0810, Japan
Received: July 31, 2005; In Final Form: September 30, 2005
The molecular structure and electrochemical properties of the title complex, 1, are reported. Single-crystal X-ray
diffraction study reveals that complex 1 consists of two distinct metal centers, fac-{ReO3}+ and fac-{Re(CO)3}+,
which are bridged by tpen in a κ 3 –κ 3 coordination mode with a minimal metal–metal interactions (Re···Re =7.660(1) Å). Complex 1 undergoes irreversible reduction and oxidation in 0.1 M n-Bu4NPF6 –CH3CN, which areassociated with the two metal centers, fac-{ReO3} and fac-{Re(CO)3}+.

1. Introduction
N,N,N',N'-tetrakis(2-pyridylmethyl)ethylenediamine (hereafter abbreviated as tpen) affords a versatile array of coordination compounds.1 It usually coordinates to a metal center as ahexadentate ligand, while some other coordination modes, inwhich the number of coordinating atoms is limited to five orless, have been also described.2
We have recently reported synthesis and characterizations of aseven-coordinated rhenium-oxo complex [ReVO(tpen)]3+ and theone-electron-reduced derivative [ReIVO(tpen)] 2+, in which tpen
coordinates to the {ReV/IVO} with the hexadentate mode,1 as well
as a rhenium-trioxo complex [ReO3 (tpenH)] 2+ (tpenH is a protonated tpen) which exhibits an unusual κ 3-coordination mode.3
We have found that the dangling bis(2-pyridylmethyl)amine
arm in the [ReO3 (tpenH)] 2+ complex can coordinate, upon
deprotonation, to a single metal center to afford mixed-metal
dinuclear complexes. Thus, the reaction of the [ReO3(tpenH)] 2+complex with [Re(CO) 5Br] in refluxing CH 3CN afforded athermally stable dinuclear complex formulated as [ReO3 ( µ tpenH)Re(CO) 3](ClO 4) 2.3 The fac-{Re(CO) 3}+ coordination
unit adds an additional interest in this dinuclear tpen complex
because of its widespread current interest and development ofchemistry associated with this structural motif, including
ligand-substitution reactions,4, 5 photoluminescence,6 supramolecular assembly formation,7, 8 self-assembled monolayer formation on solid supports,9 photochemical catalysis of CO2,10, 11
and therapeutic radiopharmaceuticals.12
In this paper, we describe X-ray structure and electrochemical properties of a PF6 − salt of the ReVIIO3 –ReI (CO)3 complex,fac-,fac-[ReO3(µ-tpen)Re(CO)3](PF6)2 (1). This work provides
the first structurally-determined example of tpen complexes, inwhich the tpen bridges two unequivalent metal centers without
the support of any additional ligands. Spectroscopic characterizations of 1, including infrared, UV-Vis, and 1H NMR spectroscopy, have been reported earlier.3
2. Experimental Section
Materials. The ligand tpen was prepared as described in
Reference 13. CH3CN for electrochemical measurements wasdistilled from CaH2 under Ar. n-Bu4NPF6 used as a supporting
electrolyte was recrystallized twice from ethanol. All the other*Cor responding author. E-mail: mabe@sci.hokudai.ac.jp;
ysasaki@sci.hokudai.ac.jp. FAX: +81-11-706-3447.

chemicals were used without further purifications. Complex[ReO3(tpenH)](ClO4)2 was prepared as reported previously.3
Synthesis fac-, fac-[ReO3 ( µ -tpen)Re(CO) 3](PF6) 2 (1). To
a CH3CN solution (20 mL) of [ReO3 (tpenH)](ClO4) 23 (97 mg,0.11 mmol) was added a CH3CN solution (5 mL) of [Re(CO)5Br](47 mg, 0.11 mmol), and the solution was refluxed for 2.5 h.
After cooling, the solvent was reduced by a rotary evaporator
to ca. 10 mL and filtered. After 4 days, white precipitate thus
appeared (30 mg), formulated as [ReO3(µ-tpen)Re(CO)3](ClO4)2,
was collected by filtration. The PF6 − salt 1 was obtained bymetathesis between the ClO4 − salt and NaPF6 in CH3CN/H2O.
Diffusion of diethyl ether into a CH3CN solution of 1 afforded
single crystals 1·(CH3CN) 0.85(H2O) 0.15 suitable for X-ray diffraction study. Yield, 21 mg (15%, based on [ReO3(tpenH)](ClO4)2).
Anal. Calcd. for 1·(CH3CN) 0.85(H2O) 0.15 {C29H 28F12N6O6P 2Re2 ·(CH3CN) 0.85 (H 2O) 0.15}: C, 29.35; H, 2.47; N, 7.64. Found: C,29.45; H, 2.52; N, 7.81. 1H NMR spectrum of 1 (in CD3CN) was
essentially identical to that of [ReO3 ( µ-tpen)Re(CO)3](ClO4) 2
reported previously.3
Physical Measurements. 1H NMR spectra were recorded
on a JNM-EX270 spectrometer. Infrared spectra were obtainedon a JASCO FT/IR-660Plus V spectrophotometer with the KBrmethod. Cyclic voltammetry was performed under Ar at room
temperature with a Hokuto Denko HZ-3000 electrochemicalsystem. The working electrode, counter electrode, and reference electrode were platinum (φ = 3 mm), platinum coil, andAg/AgCl, respectively. The electrolyte solution was 0.1 M nBu4NPF6 –CH3CN and the sample concentration was 1.0 mM.
Under our experimental conditions, the half-wave potential (E1/2)of the ferrocene/ferricinium couple was 0.45 V vs. Ag/AgCl.
Elemental analysis was performed at the Center for Instrumental
Analysis, Hokkaido University.
X-ray Diffraction Study. Structural data of a colorless needle-like single crystal (0.250 × 0.025 × 0.012 mm3) were collected on a Rigaku model AFC-7R diffractometer with aMercury CCD area detector using graphite-monochromated
Mo Kα radiation (λ = 0.71073 Å) at 153 K and processed using
the Crystal Clear software program.14 Final cell parameters
were obtained from a least-squares analysis of reflections of I> 10σ(I). The crystal structure was solved by direct methods
and expanded using Fourier and difference Fourier techniques.
All calculations were performed using the teXsan crystallographic software package.15 It appeared that the structuralsimilarity of the fac-{Re(CO) 3} and fac-{ReO3} caused thepartial mixing of two sites in 1; a model without disordered
atoms showed the considerably large densities in D-Fourier© 2005 The Japan Society of Nuclear and Radiochemical Sciences
Published on Web 12/28/2005

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

Tabeyamap and the addition of disordered {ReO3} and {Re(CO) 3}
units appeared to improve R values. A 85% : 15% mixing ofthe two was admitted in the final structure. Isotropic andanisotropic displacement parameters were applied for the atoms
with 0.15 occupancy and for all other non-hydrogen atoms,respectively. Atomic coordinates and displacement parameters
were refined by a full-matrix least-squares method. The hydrogen atoms were placed at the calculated positions. Further
crystallographic data are provided in Table 1.
The X-ray crystallographic file in CIF format for 1 has been
deposited at the Cambridge Crystallographic Data Centre, 12Union Road, Cambridge CB2 1EZ, UK, and copies can beobtained on request, free of charge, by quoting the publication
citation and the deposition number CCDC 276472.
TABLE 1: Crystallographic Data of 1·(CH3CN) 0.85 (H2O) 0.15
empirical formulafw
crystal system
space group
T, Ka, Åb, Åc, Åβ, degV, Å3Z
dcalcd, g cm−3
no. of unique reflectionsµ, mmR1, wR2ba

C30.7H30.85N6.85O6.15Re2P2F121256.52
monoclinic
P21/a153
17.219(3)12.648(2)19.591(3)110.191(1)4004(1)4
2.099139
6.231
0.055, 0.120

R1 = Σ||Fo| – |Fc|/Σ|Fo|, wR2 = {Σ[w(|Fo|2 – |Fc|2) 2]/Σ[w(|Fo|2) 2]}1/2,w = {σ 2 (Fo2) + [0.05(max(Fo2, 0) + 2Fc2)/3] 2}−1.

3. Results and Discussion
In our previous study,3 synthesis and spectroscopic properties of a ClO4 − salt of fac-, fac-[ReO3 (µ-tpen)Re(CO)3] 2+ wereestablished. We have found in this study that replacement of
the counter anion from ClO4 − to PF6 − provides single crystals
suitable for X-ray diffraction study (see below). The PF6 − saltof fac-,fac-[ReO3 ( µ-tpen)Re(CO) 3] 2+, 1, gives a satisfactory
result of elemental analysis and spectroscopic features that areessentially identical to those of the ClO4 − salt.
X-ray Structure. The molecular structure of the cationic
portion of 1 is presented in Figure 1, and selected interatomic
distances and angles are collected in Table 2.17 The X-rayanalysis unequivocally presents dinuclear structure of 1, in
which two heterometallic centers fac-{ReI (CO) 3}+ and fac-

O1O5N2

N3C1O2

Re(1)–C(1)Re(1)–C(2)Re(1)–C(3)Re(1)–N(1)Re(1)–N(2)Re(1)–N(3)Re(1) ··· Re(2)1.942(9)1.897(8)1.916(7)2.194(7)2.148(6)2.138(6)7.660(1)Re(2)–O(4)Re(2)–O(5)Re(2)–O(6)Re(2)–N(4)Re(2)–N(5)Re(2)–N(6)1.694(7)1.714(6)1.716(6)2.339(7)2.264(8)2.272(7)N(1)–Re(1)–C(1)N(1)–Re(1)–C(2)N(1)–Re(1)–C(3)N(2)–Re(1)–C(1)N(2)–Re(1)–C(2)N(2)–Re(1)–C(3)N(3)–Re(1)–C(1)N(3)–Re(1)–C(2)N(3)–Re(1)–C(3)C(1)–Re(1)–C(2)C(1)–Re(1)–C(3)C(2)–Re(1)–C(3)Re(1)–C(1)–O(1)Re(1)–C(2)–O(2)Re(1)–C(3)–O(3)171.6(3)97.4(3)98.1(3)94.9(3)176.7(3)91.7(3)94.1(3)96.5(3)173.4(3)88.1(3)88.3(3)89.7(3)175.9(9)172.5(7)177.0(7)O(4)–Re(2)–O(5)O(4)–Re(2)–O(6)O(4)–Re(2)–N(4)O(4)–Re(2)–N(5)O(4)–Re(2)–N(6)O(5)–Re(2)–O(6)O(5)–Re(2)–N(4)O(5)–Re(2)–N(5)O(5)–Re(2)–N(6)O(6)–Re(2)–N(4)O(6)–Re(2)–N(5)O(6)–Re(2)–N(6)107.2(3)107.2(3)156.7(3)88.8(3)88.1(3)105.8(3)85.3(3)155.4(3)87.9(3)87.4(3)86.4(3)154.7(3){ReVIIO3}+ are bridged by tpen with a κ 3 – κ 3 coordination
mode. The Re(1) ··· Re(2) separation was 7.660(1) Å. This is
the first example of structurally-determined tpen-complexes inwhich heterometallic groups are bridged by a single tpen without the support of any additional ligands.
Each metal center is coordinated by a tridentate bis(2pyridylmethyl)amine group of tpen, which completes the distorted octahedral geometry around the rhenium centers with a“C3N3” donor set for Re(1) and an “O3N3” donor set for Re(2).
The fac-{Re I (CO) 3} + and fac-{ReVII O 3} + groups show no
unusual structural features as compared to the corresponding
derivatives in literatures.18−21 The Re–Namine distances are
somewhat longer than the Re–Npy distances for both Re centers,
resulted from the sp3 - and sp2 -hybridized nitrogen donors,respectively. The structure of 1 also illustrates that the coordination of the fac-{ReI(CO)3}+ core to the fac-[ReO3 (tpenH)] 2+complex does not give significant structural perturbation to thefac-{ReVIIO3}+ core which is present in the opposite end.
Cyclic Voltammetry. Cyclic voltammogram of 1 (1 mM) in
the potential range between –2.00 and +2.20 V vs. Ag/AgCl ina 0.1 M n-Bu4NPF6 –CH3CN solution is shown in Figure 2.

O6N1N4Re1C2TABLE 2: Selected Interatomic Distances (Å) and Angles(deg) for 1·(CH3CN) 0.85 (H2O) 0.15

C3O3Re2
N6O4N5Figure 1. A n ORT EP d rawing of the cationic por tion of1·(CH3CN) 0.85(H2O) 0.15 at 50% probability level. Hydrogen atoms areomitted for clarity.

Figure 2. Cyclic voltammogram (CV) of 1 in 0.1 M n-Bu4NPF6 –CH3CN at a scan rate of 0.1 V/s with a process labeling scheme. The
inset shows the scan rate dependence (0.1, 0.5, 1.0, and 2.0 V/s) ofCVs associated with the first redox process.

J. Nucl. Radiochem. Sci., Vol. 6, No. 3, 2005 159

X-ray Structure and Electrochemical Properties

On a potential scan from the rest potential (+0.18 V) to thenegative direction, two well-defined cathodic peaks, denoted as
A and B, are observed at Ep values of –0.72 and –1.71 V. The
peak A is irreversible at this scan rate, but at much faster scanrates (inset) a return wave which is denoted as C can be observed
at –0.65 V (at 2.0 V/s) when the potential is switched at –0.85 V
back to the positive direction, accompanying with a shift ofpeak A to the negative direction (∆ = 0.03 V upon increase in
the scan rate from 0.10 to 2.0 V/s). We attribute the first quasireversible couple to the {ReO3}+ -based redox process, because
the parent [ReO3(tpen)]+ complex22 displays an irreversible peakat a closely similar potential (–0.70 V) under identical conditions. The second irreversible process can be ascribed to thereduction of either the {Re(CO)3}+ core, electrochemically-generated {ReO3}0 core, or an undefined species which is generated
upon reduction of 1 during the potential scan.
On a potential scan to the positive direction, an anodic shoulder at +2.02 V, D, and a return peak at +1.73 V with much smaller
intensity, E, are observed. This redox wave corresponds most
likely to a one-electron oxidation of the {Re(CO) 3}+ core to{Re(CO)3}2+ .5
4. Conclusion
In this paper, we have described a structurally-determined
new coordination type of tpen, which bridges two different
metal centers (fac-{Re(CO)3}+ and fac-{ReO3}+) with a κ 3–κ 3coordination mode. The bond distances and angles of the fac{ReVIIO3N3}+ center appears to be maintained upon introductionof the fac-{ReI(CO)3}+ into the dangling arm of the parent complex [ReO3 (tpenH)] 2+. Cyclic voltammetric study reveals thatcomplex 1 undergoes irreversible reduction and oxidation which
are associated with two unequivalent metal centers involved. In
summary, chemical and structural characters of the individual
metal centers in 1, with extreme oxidation states stabilized by
two different coordinating ligands (CO and O2−), are essentially
unaffected by linking with tpen due most likely to the long-distance, electronically-unconjugated nature of tpen. This characteristics in turn may promise future studies on precise design ofgeometrical/electronic structures and chemical reactivities of
tpen-supported Re and Tc complexes yet prepared.23 Since thefac-{ReI (CO)3}+ complexes with a d6 electronic configuration
are, in general, emissive, we are currently studying photoluminescence properties of 1 in the solid state and in solution. Our
ongoing study further includes exploration of synthetic ability of[ReO3 (tpenH)] 2+ for generating higher-nuclearity clusters byincorporating not only a single metal ion, but also a “metal-oxometal” fragment or a metal-metal bonded dimer, which will bereported in due course.
Acknowledgments. The authors thank financial support
from Grant-in-Aid on Priority Areas of “Metal-AssembledComplexes” (No. 10149102), “Reaction Control of DynamicComplexes” (No. 15036201), and “Chemistry of Coordination
Space” (No. 17036002) from the Ministry of Education, Culture,Sports, Science and Technology, Japan.
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