Oxalato complexes of Re(V). Synthesis and structural characterization of [ReO(OCH3)(ox)(L)] (L=bipy, dppe and dppee)

Article abstract of DOI:10.1016/S0020-1693(01)00651-X

Three novel neutral complexes containing the [ReO]³⁺ core have been prepared starting from K₂ReBr₆. Complexes with general formula [ReO(OCH₃)(ox)(L)] (ox = oxalate; L = 2,2′-bipyridine (bipy), 1,2-bis(diphenylphosphino)ethane (dppe) or cis-1,2-bis(diphenylphosphino)ethylene (dppee)) have been obtained and characterized. The molecular structures of [ReO(OCH₃)(ox)(bipy)] and [ReO(OCH₃)(ox)(dppe)] have been determined of single-crystal X-ray analysis. The structure of the complexes consists of a six-coordinated rhenium atom bonded to two trans-oxygen atoms, one of the [ReO]³⁺ core and one from a methoxy group. One oxalate ligand and a neutral diamine or diphosphine make the equatorial plane.

Full text of DOI:10.1016/S0020-1693(01)00651-X

www.elsevier.com/locate/ica
Inorganica Chimica Acta 325 (2001) 203–207
Note
Oxalato complexes of Re(V). Synthesis and structural
characterization of [ReO(OCH3)(ox)(L)] (L=bipy, dppe and
dppee)
Rau´l Chiozzone a, Ricardo Gonza´lez a, Carlos Kremer a,*, Giovanni De Munno b,
Juan Faus c
a Ca´tedra de Qu´ımica Inorga´nica, Facultad de Qu´ımica, CC 1157 Monte6ideo, Uruguay
b Dipartimento di Chimica, Uni6ersita` degli Studi della Calabria, 87030 Arca6acata di Rende, Cosenza, Italy
c Departamento de Qu´ımica Inorga´nica, Facultad de Qu´ımica de la Uni6ersidad deValencia, Dr. Moliner 50, 46100 Burjassot, Valencia, Spain
Received 20 April 2001; accepted 27 August 2001
Abstract
Three novel neutral complexes containing the [ReO]3+ core have been prepared starting from K2ReBr6. Complexes with general
formula [ReO(OCH3)(ox)(L)] (ox=oxalate; L=2,2%-bipyridine (bipy), 1,2-bis(diphenylphosphino)ethane (dppe) or cis-1,2-bis-
(diphenylphosphino)ethylene (dppee)) have been obtained and characterized. The molecular structures of [ReO(OCH3)(ox)(bipy)]
and [ReO(OCH3)(ox)(dppe)] have been determined of single-crystal X-ray analysis. The structure of the complexes consists of a
six-coordinated rhenium atom bonded to two trans-oxygen atoms, one of the [ReO]3+ core and one from a methoxy group. One
oxalate ligand and a neutral diamine or diphosphine make the equatorial plane. © 2001 Elsevier Science B.V. All rights reserved.
Keywords: Rhenium; Re(V) complexes; Oxalate; Crystal structures; Oxo complexes
(Asf4=tetraphenylarsonium cation) was prepared and
characterized by X-ray diffraction [6]. No other reports
can be found in the literature with this simple ligand.
The situation is not very different with the congener
Tc. The tris oxalato compound [Tc(ox)3]2− [7], the
dimer [(ox)Tc(m-O)2Tc(ox)]4− [8] analogous to the
above Re(IV) compound and also a neutral Tc(II)
complex [Tc(ox)(dppe)2] [9] have been fully
characterized.
No Re(V) or Tc(V) oxalato complexes have been
reported. Re(V) is normally stabilized by oxo groups,
forming very stable cores like [ReO2]+ and [ReO]3+
[10,11]. It is assumed that the major factor of the
formation of these cores is the lowering of the high
positive formal charge by bonding with a hard base.
Then, the coordination sphere is usually completed by
soft bases like amines or phosphines. This seems to be
the reason why oxalato complexes of Re(V) or Tc(V)
appears so difficult to obtain.
1. Introduction
The interest in the coordination chemistry of rhenium
has been increased due to the expanding role in thera-
peutic applications in Nuclear Medicine [1]. Taking into
account the success of the 186Re(Sn)-HEDP as a thera-
peutic agent for the palliation of bone pain, new Re
complexes are been currently designed and evaluated
for the treatment of cancers and other therapies [2].
Although different kinds of ligands have been used
and assayed to stabilize Re complexes, oxalato com-
plexes have been only scarcely studied. The first report
was of a brown solid obtained by reaction of ReO2 with
oxalic acid [3,4]. This diamagnetic solid was later iden-
tified as the dimer K4[(ox)Re(m-O)2Re(ox)]·3H2O (ox=
oxalate)
[5].
Recently,
(Asf4)2[ReIVCl4(ox)]
* Corresponding author. Fax: +598-2-924 1906.
E-mail address: ckremer@bilbo.edu.uy (C. Kremer).
0020-1693/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved.
PII: S0020-1693(01)00651-X

204
R. Chiozzone et al. / Inorganica Chimica Acta 325 (2001) 203–207
As a part of our investigation to develop new Re-ox-
C, 32.7; H, 2.3; N, 5.9; O, 20.1. Found: C, 32.6; H, 2.3;
N, 5.9; O, 19.6%.
alato compounds, we have prepared neutral complexes
with the general formula [ReO(OCH3)(ox)(L)] starting
from K2ReIVBr6; L represents a didentate ligand like
2.3.3. [ReO(OCH3)(ox)(dppe)] (2)
2,2%-bipyridine
(bipy),
1,2-bis(diphenylphosphino)-
Dppe (37 mg, 0.093 mmol) was added to solution A.
The mixture was allowed to react in a water bath at
40 °C for 30 min. The solution turned dark orange.
Then, the solution was kept at r.t. for 20 h. Dark
orange crystals were collected by filtration and washed
with methanol (5×2 ml), DMF (2×1 ml) and acetone
(3×1 ml). Yield: 45–50%. Some of the crystals were
suitable for X-ray diffraction studies. IR (cm−1): bands
associated to the oxalato ligand appear at 1713 (vs),
1682 (vs), 1344 (vs) and 796 (vs); 1436 (wPꢀf, s), 1123
ethane (dppe) or cis-1,2-bis(diphenylphosphino)-
ethylene (dppee). The general structure of these com-
plexes shows a [ReO]3+ core with a methoxy group in
a trans position and the oxalate anion occupying two
equatorial sites. The other didentate ligand completes a
distorted octahedral coordination geometry around the
metal ion.
2. Experimental
(wOꢀCH
,
s), 947 (wReꢀO
,
vs). Anal. Calc. for
3
C29H27O6P2Re: C, 48.4; H, 3.8; O, 13.3. Found: C, 47.9;
H, 3.8; O, 13.3%.
2.1. Materials
All chemicals were of reagent grade and used as
received. K2ReBr6 was prepared using the previously
reported method [12].
2.3.4. [ReO(OCH3)(ox)(dppee)] (3)
Dppee (37 mg, 0.096 mmol) was added to solution A.
The mixture was allowed to react in a water bath at
60 °C for 20 min. The solution turned brown. Brown
crystals were obtained and filtered upon slow evapora-
tion of the solvent and washed with CH2Cl2 (5×2 ml).
Yield: 30–35%. IR (cm−1): bands associated to the
oxalato ligand appear at 1715 (vs), 1683 (vs), 1347 (vs)
and 798 (vs); 1436 (wPꢀf, s); 1118 (wOꢀCH , s), 946 (wReꢀO
vs). Anal. Calc. For C29H25O6P2Re: C, 48.5; H, 3.5; O,
13.4. Found: C, 48.1; H, 3.4; O, 13.6%.
2.2. Physical measurements
IR spectra were recorded as KBr pellets on a Bomen
MB FT-IR spectrophotometer and UV–Vis spectra on
a Spectronic 300 instrument. Elemental analysis was
accomplished on a Carlo Erba model 1108 elemental
analyzer.
,
3
2.3. Synthesis of the complexes
2.4. Crystallographic measurements and structure
determinations
2.3.1. Solution A
A mixture of K2ReBr6 (70 mg, 0.094 mmol),
H2C2O4·2H2O (100 mg, 0.793 mmol), tris(hydrox-
ymethyl)aminomethane (96 mg, 0.793 mmol) and 4 ml
of methanol was stirred in a water bath at 60 °C. After
30 min, a deep green solution was obtained. A white
solid was also formed, which was filtered off and dis-
carded. The green solution was used to prepare the
following compounds.
Crystals of dimensions 0.28×0.22×0.15 (1) and
0.30×0.35×0.38 mm (2) were mounted on a Bruker
R3m/V automatic four-circle diffractometer and used
for data collection. Diffraction data were collected at
r.t. by using graphite monochromated Mo-Ka: radia-
,
tion (u=0.71073 A) with the ꢀ–2q scan method. The
unit cell parameters were determined from least-squares
refinement of the setting angles of 25 reflections in the
2q range of 15–30°. Information concerning crystallo-
graphic data collection and structure refinements is
summarized in Table 1. Examination of two standard
reflections, monitored after every 98 reflections, showed
no sign of crystal deterioration. Lorentz-polarization
and absorption corrections were applied to the intensity
data [13]. The maximum and minimum transmission
factors were 0.267 and 0.086 for 1, 0.264 and 0.185 for
2. 3373 (1) and 5422 (2) reflections were measured in
the 2q range 3–54° with index ranges 05h510, 05
k524 and −115l510 (1), 05h512, 05k522
and −l75l517 (2).
2.3.2. [ReO(OCH3)(ox)(bipy)] (1)
Bipy (15 mg, 0.096 mmol) was added to solution A.
The mixture was allowed to react in a water bath at
45 °C for a few minutes, until the imine was dissolved
and the solution turned to dark brown. Then, the
solution was kept at room temperature (r.t.) for 20 h.
Dark green crystals were collected by filtration and
washed with DMF (2×1 ml), water (5×1 ml) and
acetone (3×1 ml). Yield: 65–70%. Some of the crystals
were suitable for X-ray diffraction studies. IR (cm−1):
bands associated to the oxalato ligand appear at 1723
(vs), 1680 (vs) and 1334 (s); 1111 (wOꢀCH , s), 955 (wReꢀO
s); bands associated to the aromatic rings appear at 855
(s), 811(s) and 726 (s). Anal. Calc. for C13H11N2O6Re:
,
The structures were solved by standard Patterson
methods and subsequently completed by Fourier recy-
cling. All non-hydrogen atoms were refined anisotropi-
3

R. Chiozzone et al. / Inorganica Chimica Acta 325 (2001) 203–207
205
Table 3
cally. The hydrogen atoms were set in calculated posi-
,
Selected bond distances (A) and bond angles (°) for compound 2
tions and refined as riding atoms with a common fixed
isotropic thermal parameter. The refinement was per-
formed on F2 using 3023 for 1 and 5005 for 2. Models
reached convergence with values of the R and Rw
indices listed in Table 1. Solutions and refinements were
performed with the SHELXTL PLUS system [14]. The
Bond distances
Re(1)ꢀO(5)
Re(1)ꢀO(6)
Re(1)ꢀO(1)
Re(1)ꢀO(2)
Re(1)ꢀP(1)
Re(1)ꢀP(2)
1.691(5)
1.875(4)
2.094(4)
2.082(4)
2.432(2)
2.427(1)
Table 1
Bond angles
Crystal data and structural refinement parameters for compounds 1
and 2
O(5)ꢀRe(1)ꢀO(6)
O(1)ꢀRe(1)ꢀO(2)
O(1)ꢀC(1)ꢀC(2)
O(2)ꢀC(2)ꢀC(1)
O(5)ꢀRe(1)ꢀO(1)
O(5)ꢀRe(1)ꢀO(2)
O(6)ꢀRe(1)ꢀO(1)
O(6)ꢀRe(1)ꢀO(2)
O(5)ꢀRe(1)ꢀP(1)
O(5)ꢀRe(1)ꢀP(2)
O(6)ꢀRe(l)ꢀP(1)
O(6)ꢀRe(1)ꢀP(2)
174.2(2)
79.0(1)
115.9(4)
114.7(4)
95.3(2)
96.6(2)
87.9(2)
88.8(2)
90.7(1)
1
2
Chemical formula
C13H11N2O6Re
C
29H27O6P2Re
Formula weight
Crystal system
Space group
477.44
Monoclinic
P21/n
719.65
Monoclinic
P21/n
88.65(13)
90.74(14)
88.34(16)
Unit cell dimensions
,
a (A)
8.411(2)
19.176(6)
8.705(2)
101.00(2)
1378.2(6)
4
2.301
904
88.49
10.350(2)
18.762(3)
14.709(3)
99.27(1)
2819.0(9)
4
1.696
1416
44.65
,
b (A)
,
c (A)
i (°)
V (A )
graphical manipulations were performed using the XP
utility of the SHELXTL PLUS system. Main interatomic
bond distances and angles are listed in Table 2 (1) and
Table 3 (2).
3
,
Z
Dcalc (g cm−3
F(000)
)
v (cm−1
)
Reflections, unique/observed
3023/2670
0.0378
0.1229
1.068
5005/4334
0.0333
0.0994
0.868
a
R
Rw
3. Results and discussion
b
c
S
The reaction of K2ReBr6 with an excess of oxalic
acid in the presence of equivalent amounts of a base
Details in common: T=25 °C.
a R=S(ꢀꢀFoꢀ−ꢀFcꢀꢀ)/SꢀFoꢀ.
like tris(hydroxymethyl)aminomethane, produces
a
b Rw=[S w(Fo 2−Fc 2)2/S w(Fo 2)2]1/2
.
green solution characterized by a strong absorption at
640 nm. This solution is stable for periods up to 2 days,
then it turns to brown and finally ReO4− is evident in
solution. This decomposition product can be precipi-
tated as Asf4ReO4 and identified by IR spectroscopy.
Addition of bulky cations (like Asf4+) to the green
solution only yields small amounts of a green powder
which could not been fully characterized. The IR spec-
trum of this solid shows the presence of coordinated
oxalate (1707, 1670 and 788 cm−1) and Asf4+ (1439,
1081, 997, 741 and 688 cm−1). A strong band at 971
cm−1 reveals the presence of a [ReO]3+ core. The
position of this signal is similar with those seen for
monooxo Re(V) complexes [15]. The characteristic
band of the ꢀOCH3 group (around 1100 cm−1) is not
detected. Electron microprobe chemical analysis on the
solid shows a 1:1 rhenium–arsenic ratio. Magnetic
measurements indicate that this solid is diamagnetic, as
expected for a Re(V) complex [16]. In conclusion, dur-
ing this first stage of the synthesis, the Re(IV) atom of
the precursor seems to be oxidized to Re(V) forming an
anionic Re(V) oxo complex containing oxalate in the
coordination sphere. The formula Asf4[ReO(ox)2] is
consistent with all the experimental data.
c Goodness-of-fit=[S w(Fo 2−Fc 2)2/(No−Np)]1/2
.
Table 2
,
Selected bond distances (A) and bond angles (°) for compound 1
Bond distances
Re(1)ꢀO(5)
Re(1)ꢀO(6)
Re(1)ꢀO(1)
Re(1)ꢀO(2)
Re(1)ꢀN(l)
Re(1)ꢀN(2)
1.699(5)
1.886(5)
2.057(6)
2.037(5)
2.126(6)
2.131(5)
Bond angles
O(5)ꢀRe(1)ꢀO(6)
O(1)ꢀRe(1)ꢀO(2)
O(1)ꢀC(11)ꢀC(12)
O(2)ꢀC(12)ꢀC(11)
O(5)ꢀRe(1)ꢀO(1)
O(5)ꢀRe(1)ꢀO(2)
O(6)ꢀRe(1)ꢀO(1)
O(6)ꢀRe(1)ꢀO(2)
O(5)ꢀRe(1)ꢀN(1)
O(5)ꢀRe(1)ꢀN(2)
O(6)ꢀRe(1)ꢀN(1)
O(6)ꢀRe(1)ꢀN(2)
Re(1)ꢀO(6)ꢀC(13)
170.1(2)
80.3(2)
114.8(8)
114.4(6)
97.1(3)
98.1(2)
89.4(2)
90.3(2)
89.0(2)
88.6(3)
82.3(2)
84.7(2)
146.9(5)

206
R. Chiozzone et al. / Inorganica Chimica Acta 325 (2001) 203–207
uninegative oxygen trans to the oxo group [15,17].
When a didentate ligand L is added to the green
Anionic charge of the methoxy and oxalate groups
produces the elongation of the ReꢀO distance com-
pared with the expected mean value [18]. In addition,
the presence of a negative charged equatorial ligand
originates an important distortion in the geometry.
High electron density of the oxalate causes the apical
groups to be pushed away towards the neutral ligand.
The atoms that constitute the equatorial plane
around Re(1) in 1 [O(1), O(2), N(1), N(2)] and 2 [O(1),
O(2), P(l), P(2)] deviate from planarity, the largest
solution, solids with composition [ReO(OCH3)(ox)(L)]
are obtained. This composition is consistent with the
elemental analysis. IR spectra of complexes 1–3, show
a very intense band around 950 cm−1 due to the
presence of a [ReO]3+ core. Moreover, this position is
in line with those previously reported for other trans-
[OꢀReꢀOCH3] complexes [15]. In addition, characteris-
tic bands of the oxalate and methoxy groups are
evident (see Section 2).
Single crystals were obtained for compounds 1 and 2.
Both structures contain discrete, monomeric, neutral
monooxorhenium(V) units. Perspective drawings of the
complexes are shown in Figs. 1 and 2.
The rhenium atom is in a distorted octahedral envi-
ronment with apical positions occupied by the oxo
group O(5) and the methoxy group O(6). The equato-
rial plane of the octahedron is formed by the oxalate
and the neutral ligand (bipy or dppe). Oxygen atoms
are not aligned with the metal. OꢀReꢀO angles are
170.1(2) (1) and 174.2(2)° (2). The short ReꢀO(5) dis-
tance is consistent with other Re(V) complexes with an
,
deviation being 0.013(6) (1) and 0.070(4) A (2) for O(1).
,
The Re(1) atom is 0.721(1) (1) and 0.112(1)(2) A out of
this plane. The values of the dihedral angle between this
mean plane and that of the oxalate are 3.6(2)° (1) and
4.9(1)° (2). The corresponding value with bipy in 1 is
only 1.3(2)°.
The Re(1) atom constitutes, with N(1), C(5), C(6)
and N(2) atoms in 1, an almost planar ring. On the
contrary, the corresponding ring containing the P(1),
C(4), C(5) and P(2) atoms in 2 is very distorted from
the planarity.
The pyridyl rings of the bipy ligand in 1 are planar as
expected, the maximum deviation from the mean planes
,
being 0.017(8) A at the C(2) atom. The ligand as a
whole is also planar, the dihedral angle between the two
pyridyl rings being of 2.7(2)°. The values of the angle
subtended at the metal atom by the chelating bipy
(average 76.2(2)°) is significantly reduced with respect
to the ideal value of 90°. The carbonꢀcarbon and
carbonꢀnitrogen bond distances within the bipy ligand
are in agreement with those reported for the coordi-
nated bipy molecule.
The phosphorus atoms in 2 show the expected tetra-
hedral geometry with angles in the range 104.3(3)–
115.6(2)°.
Fig. 1. Perspective drawing of complex 1 showing the atom number-
ing. Thermal ellipsoids are plotted at 30% probability level.
4. Conclusions
The reaction of K2ReBr6 with oxalic acid produces a
new kind of mixed-ligand Re(V) complexes. As far as
we know, these are the first examples of Re(V) com-
plexes with oxalate. Usually, Re(V) oxo-methoxy com-
plexes are prepared starting from other Re(V)
precursors: [ReOCl3(PPh3)2], [ReOCl4]−, [ReO2I-
(PPh3)2], etc. According to our results, [ReIVBr6]2− can
be a good alternative for complexes with such core.
The basic structural framework for these complexes
is provided by the rhenium-oxo-methoxy core and the
oxalate ligand. The two remaining positions can be
occupied by a neutral didentate coligand. These results
open the possibility to prepare and study a new series
of neutral complexes by changing the coligand.
Fig. 2. Perspective drawing of complex 2 showing the atom number-
ing. Thermal ellipsoids are plotted at 30% probability level.

R. Chiozzone et al. / Inorganica Chimica Acta 325 (2001) 203–207
207
5. Supplementary material
References
[1] K. Hashimoto, K. Yoshihara, Top. Curr. Chem. 176 (1995) 275.
[2] W.A. Volkert, T.J. Hoffman, Chem. Rev. 99 (1999) 2269.
[3] D. Sen, P. Ray, J. Indian Chem. Soc. 30 (1953) 171.
[4] B. Jezowska-Trzebiatowska, S. Wajda, Bull. Acad. Polon. Sci.
Ser. Sci. Chim. 6 (1958) 217.
[5] T. Lis, Acta Crystallogr., Sect. B 31 (1975) 1594.
[6] R. Chiozzone, R. Gonza´lez, C. Kremer, G. De Munno, J.
Cano, F. Lloret, M. Julve, J. Faus, Inorg. Chem. 38 (1999)
4745.
Crystallographic data for the structural analyses and
complete atom coordinates, thermal parameters and
bond lengths and angles have been deposited with the
Cambridge Crystallographic Data Centre, CCDC no.
167907 and 167908 for compounds 1 and 2. Copies of
this information may be obtained free of charge from
The Director, CCDC, 12 Union Road, Cambridge CB2
1EZ, UK (fax: +44-1223-336-033: e-mail: deposit@
ccdc.cam.ac.uk or www: http://www.ccdc.cam.ac.uk).
[7] S.F. Colmanet, G.A. Williams, M.F. Mackay, J. Chem. Soc.,
Dalton Trans. (1987) 2305.
[8] R. Alberto, G. Anderegg, A. Albinati, Inorg. Chim. Acta 178
(1990) 125.
[9] S. Seifert, R. Muenze, P. Leibnitz, G. Reck, J. Stach, Inorg.
Chim. Acta 193 (1992) 167.
Acknowledgements
[10] B. Johannsen, H. Spies, Top. Curr. Chem. 176 (1995) 77.
[11] G. Roushias, Chem. Rev. 74 (1974) 531.
[12] C.L. Rulfs, R.J. Meyer, J. Am. Chem. Soc. 77 (1955) 4505.
[13] A.C.T. North, D.C. Phillips, F.S. Mathews, Acta Crystallogr.,
Sect. A 24 (1968) 351.
[14] SHELXL PLUS, version 4.11/V Bruker Analytical X-ray Instru-
ments Inc., Madison WI, 1990.
[15] C. Melia´n, C. Kremer, L. Suescun, A. Mombru´, R. Mariezcur-
rena, E. Kremer, Inorg. Chim. Acta 306 (2000) 70.
[16] J.H. Beard, J. Casey, R.K. Murmann, Inorg. Chem. 4 (1965)
797.
This research received financial support from the
Programa de Desarrollo de Ciencias Ba´sicas
(PEDECIBA), Comisio´n Sectorial de Investigacio´n Ci-
ent´ıfica (CSIC, project number 205) and CONICYT
(project number 6053). R.C and R.G. are indebted to
the European Union for a grant (ALFA programme
ALR/B7-3011/94.04-5.0273(9)). Financial support from
the Spanish DGICYT through Project PB97-1397, the
Training and Mobility Research Program from the
European Union TMR Contract ERBFMRXCT-
980181 and the Italian Ministero dell’ Universita` e della
Ricerca Scientifica e Tecnologica are also gratefully
acknowledged.
[17] C. Kremer, M. Rivero, E. Kremer, L. Suescun, A. Mombru´, S.
Dom´ınguez, A. Mederos, S. Midollini, A. Castin˜eiras, Inorg.
Chim. Acta 291 (1999) 47.
[18] J.M. Mayer, Inorg. Chem. 27 (1988) 3899.