Synthesis, characterization and crystal structures of rhenium(V) complexes with diphosphines

Article abstract of DOI:10.1016/S0020-1693(99)00272-8

Two cationic complexes containing a Re^V(O)₂⁺ core have been prepared by substitution on the precursor [Re(O)₂(PPh₃)₂I]. Careful control of reaction conditions allowed the synthesis of [Re(O)₂L₂]I (L=bis(1,2-diphenylylphosphino)ethane (dppe), bis(1,3-diphenylphosphino)propane (dppp)) compounds in high yield. Complexes were characterized by UV-Vis and IR spectrophotometry, elemental analyses, FAB, ¹H, ¹³C{¹H} and ³¹P{¹H} NMR spectra and high-performance liquid chromatography. Four compounds have also been characterized by single-crystal X-ray analyses. Cations consist of a six-coordinated rhenium atom bonded to two trans-oxo groups. The phosphines lie on the equatorial plane, almost perpendicular to the Re^V(O)₂⁺ core. The average value for the Re-O bond is 1.77 A, similar to that in other Re(V) dioxo complexes. Diphosphine ligands form five- and six-membered rings in which λδ conformation is adopted. The unexpected [Re(O)(OCH₃)(dppp)(Cl)₂]·CH₃OH has been additionally characterized from X-ray data. It is also an octahedral complex with a methoxy ligand trans to an oxo group. Species are very stable towards redox processes and can be protonated in acidic media (70:30 dioxane:water, v/v) to yield [Re(OH)₂L₂]³⁺. The protonation constants of the diphosphine ligands are also obtained for the first time.

Full text of DOI:10.1016/S0020-1693(99)00272-8

www.elsevier.nl/locate/ica
Inorganica Chimica Acta 294 (1999) 47–55
Synthesis, characterization and crystal structures of rhenium(V)
complexes with diphosphines
Carlos Kremer a, Mario Rivero a, Eduardo Kremer a, Leopoldo Suescun b,
b
Alvaro W. Mombru´ , Rau´l Mariezcurrena b, Sixto Dom´ınguez c, Alfredo Mederos c,*,
Stefano Midollini d, Alfonso Castin˜eiras e
a Ca´tedra de Qu´ımica Inorga´nica, Facultad de Qu´ımica, CC 1157, Monte6ideo, Uruguay
b Laboratorio de Cristalograf´ıa, Ca´tedra de F´ısica, Facultad de Qu´ımica, CC 1157, Monte6ideo, Uruguay
c Departamento de Qu´ımica Inorga´nica, Uni6ersidad de La Laguna, 38200 La Laguna, Tenerife, Canary Islands, Spain
d Istituto per lo Studio della Stereochimica ed Energetica dei Composti di Coordinazione, Via J. Nardi 39, 50132 Florence, Italy
e Departamento de Qu´ımica Inorga´nica, Facultad de Farmacia, Uni6ersidad de Santiago de Compostela, Campus Sur,
15706 Santiago de Compostela, Galicia, Spain
Received 6 April 1999; accepted 16 June 1999
Abstract
+
Two cationic complexes containing a ReV(O)2 core have been prepared by substitution on the precursor [Re(O)2(PPh3)2I].
Careful control of reaction conditions allowed the synthesis of [Re(O)2L2]I (L=bis(1,2-diphenylylphosphino)ethane (dppe),
bis(1,3-diphenylphosphino)propane (dppp)) compounds in high yield. Complexes were characterized by UV–Vis and IR
1
spectrophotometry, elemental analyses, FAB, H, 13C{1H} and 31P{1H} NMR spectra and high-performance liquid chromatogra-
phy. Four compounds have also been characterized by single-crystal X-ray analyses. Cations consist of a six-coordinated rhenium
+
atom bonded to two trans-oxo groups. The phosphines lie on the equatorial plane, almost perpendicular to the ReV(O)2 core.
,
The average value for the ReꢀO bond is 1.77 A, similar to that in other Re(V) dioxo complexes. Diphosphine ligands form five-
and six-membered rings in which ld conformation is adopted. The unexpected [Re(O)(OCH3)(dppp)(Cl)2]·CH3OH has been
additionally characterized from X-ray data. It is also an octahedral complex with a methoxy ligand trans to an oxo group. Species
are very stable towards redox processes and can be protonated in acidic media (70:30 dioxane:water, v/v) to yield [Re(OH)2L2]3+
.
The protonation constants of the diphosphine ligands are also obtained for the first time. © 1999 Elsevier Science S.A. All rights
reserved.
Keywords: Crystal structures; Rhenium complexes; Oxo complexes; Diphosphine complexes
radionuclide for therapeutic radiopharmaceuticals [4–
6].
1. Introduction
As far as the relevant chemistry is concerned, rheni-
um(V) and technetium(V) chemistry is governed by oxo
species. It is supposed that the major factor of the
formation of these complexes is the lowering of the
high positive formal charge on metal(V) [7,8]. Hence,
negatively charged ancillary ligands favor monooxospe-
cies (with [M(O)]3+ core) while neutral ligands, such as
amines or phosphines, favor the formation of bis-
oxometal(V) species containing a [M(O)2]+ core. With
this in mind, [M(O)2Ln]+ complexes (L=amine, phos-
phine, arsine; n=1, 2, 4) have been extensively studied
[2,9–17].
Rhenium(V) chemistry is of great interest, not only
because of promising applications of trans-Re(V) dioxo
complexes in photophysics [1–3]. Taking into account
the strong resemblance between second- and third-row
congeners, Re complexes permit non-radioactive model
studies for 99mTc radiopharmaceuticals. Moreover, the
recent development of 188W/188Re generators attracts
attention in connection with the utilization of this
* Corresponding author. Tel.: +34-922-318 458; fax: +34-922-
318 461.
E-mail address: amederos@ull.es (A. Mederos)
0020-1693/99/$ - see front matter © 1999 Elsevier Science S.A. All rights reserved.
PII: S0020-1693(99)00272-8

48
C. Kremer et al. / Inorganica Chimica Acta 294 (1999) 47–55
Recently, interest in Tc(V) with bidentate phosphine
analyzer Carlo Erba model 1108. Iodine was analyzed
by titration with AgNO3. Fast atom bombardment
(FAB) was obtained from a 3-nitrobenzylalcohol ma-
trix using a Kratos MS50TC spectrometer connected to
a DS90 system.
ligands has been renewed due to the development of the
radiopharmaceutical 99mTc-tetrofosmin [18]. This is a
Tc(V) dioxo complex with bis(2-ethyoxyethyl)-
phosphinoethane. In this case also, the study of
analogous diphosphine complexes of Re will provide
very useful information for developing and predicting
in vivo behavior of similar new Tc radiopharmaceuti-
cals. For this study, we have chosen the simplest com-
plexes of this group, i.e. [Re(O)2L2]+ (L=1,2-bis-
(diphenylphosphino)ethane (dppe); 1,3-bis(diphenyl-
phosphino)propane (dppp)). Preparation of these com-
plexes has already been reported [2,19,20]. However,
they have not been fully studied even though the struc-
ture of [Re(O)2(dppe)2]ReO4 has been elucidated [21].
In this work we prepared and characterized
[Re(O)2L2]I (L=dppe, dppp) compounds. In addition,
the related methoxo–oxo compound [ReO(OCH3)-
(dppp)Cl2] was prepared for the first time. With these
results, it is possible to compare the effect of changing
chelate ring size on the stability of the molecule. Addi-
tionally, these structures provide useful information
when compared with the similar [Re(O)2L2]+ (L=1,2-
diaminoethane (en) and trimethylenediamine (tn)).
2.3. HPLC analyses
Chromatograms were obtained with a HPLC system
(E. Merck) equipped with a variable wavelength moni-
tor and a PRP-X200 (10 mm) cation exchange column
(4×250 mm). A 0.02 M solution of NaCl in methanol
was used as mobile phase with 1.5 ml min−1 flow.
Detection was accomplished at 255 nm.
2.4. Potentiometric measurements
The potentiometric titrations were carried out in
mixed solvent 70:30 dioxane:water (v/v) solutions at
25°C and ionic strength I=0.1 mol dm−3 in KCl. The
sealed 100 cm3 thermostated double-walled glass reac-
tion vessel was fitted with a Radiometer G202B glass
electrode, a Radiometer K711 calomel reference elec-
trode, and inert argon atmosphere inlet and outlet
tubes, a magnetic stirrer and a titrant (aqueous 0.05
mol dm−3 HCl) inlet. Readings of potentials were
obtained with a Radiometer type PHM-85 potentiome-
ter. The titrant was delivered through an immersed
capillary tip from a Crison Microbur 2031 piston buret,
and the temperature was maintained at 25.090.1°C by
circulating water from a refrigerated constant-tempera-
ture bath. The cell constants E° and the liquid junction
potentials J were determined according to the methods
of Sille´n [24] and Liberti and Light [25] by means of a
least-squares program before each experiment. The
autodissociation constant of the solvent pKw was 16.3,
in good agreement with reported data [26]. Measure-
ments were taken of phosphine ligands alone (to com-
pare basicity of the P atoms in the mixed solvent) at the
concentrations CL=1.0 and 2.0 mol dm−3, and the
complexes at CC=1.2 and 2.1 mol dm−3. The experi-
mental potentiometric data were analyzed by means of
the SUPERQUAD program [27] (Table 5).
2. Experimental
2.1. Materials
All chemicals were of reagent grade and used as
received. HPLC grade methanol and all other solvents
(reagent grade) were used without further purification.
Violet microcrystals of [Re(O)2(PPh3)2I] were obtained
by using the reported method [22] except for
NH4[ReO4], which was used in place of H[ReO4].
[Re(O)Cl3(PPh3)2] was obtained by using the reported
method [23].
2.2. Physical measurements
Electronic absorption spectra were obtained using a
Spectronic 3000 spectrophotometer. IR spectra were
recorded as KBr pellets on a Bomen MB FT-IR spec-
trophotometer. 1H and 13C{1H} NMR spectra were
recorded on a Varian VXR-300 spectrometer operating
at 299.94 and 75.42 MHz, respectively, or on a Bruker
AC200 operating at 200.13 and 50.32 MHz. Peak posi-
tions (in ppm) are relative to tetramethylsilane and
were calibrated against the residual solvent resonance
(1H) or the deuterated solvent multiplet (13C). 31P{1H}
NMR spectra were recorded on the same instruments
operating at 121.42 or 81.01 MHz, respectively. Chemi-
cal shifts (in ppm) were measured relative to external
85% H3PO4 with downfield values taken as positive.
Elemental analysis was accomplished with an elemental
2.5. Cyclic 6oltammetry
Cyclic voltammograms in methanol were obtained on
a Metrohm glassy-carbon working electrode versus Ag/
AgCl (methanol, saturated in LiCl). A Pt wire was used
as the auxiliary electrode. The solutions were purged
with nitrogen before recording the voltammograms.
Ionic strength was kept constant at 0.5 M LiCl. Scans
were carried out at rates between 50 and 500 mV
min−1. All experiments were performed at 25.09
0.1°C. A PGP 201 potentiostat (Radiometer, Copen-
hagen) was employed as electrochemical controller.

C. Kremer et al. / Inorganica Chimica Acta 294 (1999) 47–55
49
Data were processed by Voltamaster 1 software. All
2.6.5. Perchlorate salts
potentials were later referred to the standard hydrogen
electrode (SHE).
In order to avoid interference from the iodide in the
voltammograms, the perchlorate salts were obtained by
adding a large excess of LiClO4 to a methanolic solu-
tion of the complexes. Yield 65–70%.
2.6. Preparation of the complexes
Caution! Perchlorate salts of [Re(O2)(amine)2]ClO4
have been reported as hazardous materials [28]. During
development of this work, no problems were detected
with these perchlorates. Nevertheless, experiments with
them were carried out with small amounts only and
handled with care.
2.6.1. [Re(O)2(dppe)2]I (1)
[Re(O)2(PPh3)2I] (0.30 g, 0.345 mmol) and 1,2-
diphosphinoethane (1.5 g, 3.76 mmol) were stirred in
dichloromethane (20 ml) for 10 min. A yellow solution
was obtained and the solvent was completely evapo-
rated by heating in a water bath (80°C). From this
crude material, the complex was extracted with
methanol (4×10 ml). The pale-yellow solution was
filtered (through paper) and the solvent was evaporated
again. The yellow solid was washed with ether (4×5
ml); yield 60–65%. Even when this solid presented a
high purity (see below), it could be recrystallized from
methanol; yield 70%. The complex is soluble in acetone,
dichloromethane, chloroform, 2-propanol and acetoni-
trile and virtually insoluble in water and diethyl ether.
Suitable crystals for X-ray diffraction were obtained by
slow diffusion of a methanolic solution into diethyl
ether. IR: 786.3 cm−1 was(OꢁReꢁO). UV–Vis
(methanol; u, nm (m, M−1 cm−1)): 205 (174 600), 264
(65 300). Selected NMR data (CD2Cl2, 22°C). 1H
(299.94 MHz): l 2.8 (m, 4H, CH2); 13C{1H} (75.42
MHz): l 32 (m, CH2); 31P{1H} (121.42 MHz): l 9.16 (s)
ppm. Anal. Calc. for C52H48IO2P4Re: C, 54.69; H, 4.24;
I, 11.11. Found: C, 54.27; H, 4.26; I, 11.0%.
2.6.6. [Re(O)2(dppp)2](ClO4)x(I)1−x·H2O·CH3OH
(x=0.585(4)) (4)
Single crystals suitable for X-ray crystallography
were grown by slow evaporation at room temperature
(r.t.) of a methanolic solution of [Re(O)2(dppp)2]I with
an excess of LiClO4.
2.6.7. [Re(O)(OCH3)(dppp)(Cl)2]·CH3OH (5)
Violet single crystals suitable for X-ray crystallogra-
phy were obtained by slow evaporation at r.t. of a
50:50 methanol:ether (v/v) solution of [Re(O)2(dppp)2]I
with an excess of LiClO4. In order to obtain large
amounts of this compound, [Re(O)Cl3(PPh3)2] was as-
sayed as starting material. [Re(O)Cl3(PPh3)2] (0.30 g,
0.42 mmol) and 1,3-diphosphinopropane (0.175 g, 0.42
mmol) were refluxed in methanol (20 ml) for 1 h. A
yellow solution was obtained and after 1 day at r.t., a
violet solid appeared. The solid was collected by filtra-
tion and washed with ether (4×5 ml); yield 30–50%.
The complex could be recrystallized from acetonitrile;
yield 80–90%. The complex is soluble in hot acetoni-
trile and dichloromethane. IR: 909.1 cm−1 was(Re(O).
UV–Vis (acetonitrile; u, nm (m, M−1 cm−1)): 234
(46 600), 502 (35). Selected NMR data (CD2Cl2, 22°C).
1H (200.13 MHz): l 2.31 (t, 3H, 4JPꢀH=1 Hz,
ReꢀOꢀCH3), 3.50–2.60 (m, 4H, CH2ꢀCH2ꢀCH2), 3.25–
3.43 (m, 2H, CH2ꢀCH2ꢀCH2); 13C{1H} (50.32 MHz): l
21.6 (s, CH2ꢀCH2ꢀCH2), 30.6 (m, CH2ꢀCH2ꢀCH2),
57.85 (s, CH3ꢀOꢀRe); 31P{1H} (81.01 MHz): l 15.22
(s). Anal. Calc. for C28H29Cl2O2P2Re: C, 46.93; H, 4.08.
Found: C, 47.05, H, 4.57%.
2.6.2. [Re(O)2(dppe)2]PF6·2CH3OH (2)
To obtain crystals for X-ray diffraction, a large
excess of NaPF6 was added to a solution of 1 in 90:10
methanol–dichloromethane (v/v). The solvent was al-
lowed to evaporate slowly in open air.
2.6.3. [Re(O)2(dppp)2]I
As above. Yield 55%. The complex is soluble in
dichloromethane and chloroform, partially soluble in
methanol and acetonitrile, slightly soluble in ethanol
and 2-propanol and insoluble in water and diethyl
ether. IR: 784.8 cm−1 was(OꢁReꢁO). UV–Vis
(methanol; u, nm (m, M−1 cm−1)): 204 (164 300), 265
(55 600). Selected NMR data (CD2Cl2, 22°C). 1H
(299.94 MHz): l 1.84 (m, 2H, CH2CH2CH2), 2.78 (m,
4H, CH2CH2CH2); 13C{1H} (75.42 MHz): l 19.6 (m,
CH2CH2CH2), 27.2 (m, CH2CH2CH2); 31P{1H} (121.42
2.7. Crystallographic measurements and structure
determinations
MHz):
l
−31.9 (s) ppm. Anal. Calc. for
X-ray data collection for all compounds was per-
formed at r.t. on a Rigaku AFC7S diffractometer using
Mo Ka radiation. Important crystallographic data are
collected in Table 1. The cell parameters were refined
with 25 centered reflections. The intensity was moni-
tored every 150 measurements with three standard
reflections. Lorentz, polarization and absorption cor-
rections were applied. Structure determination for com-
C54H52IO2P4Re: C, 55.43; H, 4.48; I, 10.75. Found: C,
55.97; H, 4.52; I, 10.9%.
2.6.4. [Re(O)2(dppp)2]PF6·CH3OH (3)
This compound was synthesized by the same proce-
dure used for the preparation of complex 2 except that
[Re(O)2(dppp)2]I was used in place of 1.

50
C. Kremer et al. / Inorganica Chimica Acta 294 (1999) 47–55
−3
,
pounds 1–3 was carried out by direct methods. The
other two structures were solved by the heavy atom
method, locating the Re atoms from a Patterson map.
In all cases, the other non-hydrogen atoms were located
by successive DF maps. Refinement was anisotropic for
all non-hydrogen atoms, except for the methanol
molecule in 3 and 5. The hydrogen atoms belonging to
phenyl and methylene groups were calculated at ideal-
refinement led to the values −2.441 and 2.461 e A
as minimum and maximum, respectively.
,
2.7.2. [Re(O)2(dppe)2]PF6·2CH3OH (2)
A yellow prismatic crystal with dimensions 0.4×
0.35×0.2 mm was used. Intensities were measured for
6424 reflections up to a maximum value of q=27.62°,
R
int=0.0843, and 05h512, 05k521 and −225
,
ized positions (0.93 and 0.97 A, respectively). The hy-
l521. No disorder was observed in this structure.
−3
drogen atoms corresponding to methyl groups in the
,
Residual peaks in the range 3.071 to −2.804 e A
,
methanol molecules were also calculated (0.96 A). The
remained after the final refinement.
distances corresponding to other hydrogen atoms were
left free to refine. Structure determination was achieved
using SHELXS86 [29] and refinement was done using
SHELXL97 [30]. Drawings were performed using ZORTEP
[31]. Absorption correction was performed by C-scan
[32] and no extinction correction was applied for all five
compounds. Other crystallographic data are summa-
rized in the supplementary crystallographic data.
2.7.3. [Re(O)2(dppp)2]PF6·CH3OH (3)
A yellow prismatic crystal with dimensions 0.35×
0.20×0.20 mm was used. Intensities were measured for
12 534 reflections up to a maximum value of q=27.5°,
Rint=0.0300, and 05h518, 05k524 and −255
l525. After the final refinement residual peaks −3.226
−3
,
and 1.024 e A
remained unexplained.
2.7.1. [Re(O)2(dppe)2]I (1)
2.7.4. [Re(O)2(dppp)2](ClO4)x(I)1−x·H2O·CH3OH
(x=0.586(4)) (4)
A yellow prismatic crystal with dimensions 0.4×
0.2×0.2 mm was used. Intensities were measured for
12 324 reflections up to a maximum value of q=27.5°,
A yellow prismatic crystal with dimensions 0.2×
0.1×0.1 mm was used. Intensities were measured for
6074 reflections up to a maximum value of q=27.50°,
Rint=0.0358, and 05h515, −165k516 and
−125l511. The remanent electron density after
Rint=0.0211, and 05h518, 05k524 and −255
Table 1
Crystal data and structural refinement parameters for structures [Re(O)2(dppe)2]I (1), [Re(O)2(dppe)2]PF6·2CH3OH (2), [Re(O)2(dppp)2]PF6·
CH3OH (3), [Re(O2)(dppp)2](ClO4)x(I)1−x·H2O·CH3OH (4), and [Re(O)(OCH3)(dppp)Cl2]CH3OH (5) a
1
2
3
4
5
Formula weight
Space group
Unit cell dimensions
1141.88
triclinic, P1
1224.04
1220.05
1202.11
762.62
(
monoclinic, P21/c monoclinic, P21/n monoclinic, P21/n orthorhombic, Pnma
,
a (A)
11.950(4)
12.860(4)
9.627(3)
104.33(2)
108.59(2)
105.72(3)
1256.3(7)
1
9.516(9)
16.552(7)
17.251(9)
90
104.21(6)
90
2634(3)
2
1.543
13.904(4)
18.978(3)
19.879(4)
90
91.41(2)
90
5244(2)
4
1.545
13.948(6)
18.813(4)
19.659(5)
90
90.91(3)
90
5158(3)
4
1.548
17.113(5)
21.3461(12)
8.174(2)
90
90
90
2985.8(11)
4
1.697
4.39
1512
,
b (A)
,
c (A)
h (°)
i (°)
k (°)
V (A )
3
,
Z
Dcalc. (g cm−3
)
1.509
3.20
564
v (mm−1
F(000)
)
2.52
1232
2.53
2456
2.81
2415
q Range (°)
Data collected
2.07–27.50
6074
4988
0.0727
0.2074
0.720–1.000
0.1238
6.8114
2.21–27.62
6424
4139
0.0518
0.1336
0.575–1.000
0.1068
0
2.05–27.50
12 534
7854
0.0605
0.1579
0.895–1.000
0.0713
24.3259
2.07–27.50
12 324
7849
0.0483
0.1191
0.798–1.000
0.0470
26.9698
2.38–27.49
4437
2944
0.0360
0.0995
0.230–1.000
0.0595
6.0381
No. of observed data [I]2|(I)]
b
R1
c
wR2
Transmission factors range
d
s1
s2
d
a
,
In all cases, temperature was 293(2) K and radiation Mo Ka, u=0.71069 A.
b R1=SꢀFo−Fcꢀ/SFc.
c wR2=[S[w(Fo2−Fc2)2]/S[w(Fo2)2]1/2
.
d w=1/[|2(Fo)2+(s1)2+s2P]; P=[max(Fo)2+2Fo2]/3.

C. Kremer et al. / Inorganica Chimica Acta 294 (1999) 47–55
51
l525. The iodide and perchlorate anions occupied the
same crystallographic position in different asymmetric
units producing a partial substitution. The refinement
was performed balancing their occupancies. Residual
−3
,
peaks within −3.576/3.257 e A
remained near the
Re atom. This high residual electron density can be
explained by the absorption correction performed by
psi-scans for non-regular crystal shapes.
2.7.5. [Re(O)(OCH3)(dppp)(Cl)2]·CH3OH (5)
A violet prismatic crystal with dimensions 0.4×
0.4×0.1 mm was used. Intensities were measured for
4437 reflections up to a maximum value of q=27.49°,
Rint=0.0156, and −105h522, −205k527 and
−75l510. From 3525 unique reflections, 2944 ob-
served reflections (I/|(I)]2) were used for structure
−3
,
determination. Residual peaks within 92.30 e A
remained in the final difference Fourier map near the
Re atom. The atoms of CH3OH were disordered and
refined anisotropically with an occupancy factor of 0.5.
3. Results and discussion
Fig. 1. ORTEP view of complex 1 with displacement ellipsoids drawn
at 30% probability level. H atoms are omitted for clarity.
Dioxo rhenium(V) complexes with general formula
[Re(O)2(diphosphine)2]I can be readily prepared by us-
ing the strategy suggested by Brewer and Gray [33].
Yields are usually high (greater than 70%) and pure
compounds are obtained. This fact can be easily
checked by HPLC. Using a cation-exchange column
(see Section 2), complexes appear as a two-peaks
profile. One peak corresponds to the iodide counterion
(retention time: 1.6 min), while the second corresponds
to the cationic complex (retention times: 5.4 min
(Reꢀdppe) or 6.7 min (Reꢀdppp)).
rial plane of the octahedron. ReꢀP distances (between
,
2.483 and 2.602 A) are similar to those previously
reported for other Re(V) complexes [2,16,38,39]. On the
other hand, they are significantly longer than ReꢀN
distances in the Reꢀen and Reꢀtn complexes that do
,
not exceed 2.20 A [40], as a consequence of changing N
Compositions (by elemental analysis) of the solids fit
with the expected formula. This is confirmed by the
FAB MS spectrum, in which molecular ion peaks ap-
pear at m/z=1016 (Reꢀdppe) and 1042 (Reꢀdppp).
Doubly and triply charged ions, previously reported for
en and tn analogous complexes [17] were not detected.
The most prominent peaks were present at m/z=617
(Reꢀdppe) and 629 (Reꢀdppp), corresponding to the
loss of one ligand (M+−dppe or M+−dppp).
The structures of [Re(O)2(dppe)2]I (1), [Re(O)2-
(dppe)2]PF6·2CH3OH (2), [Re(O)2(dppp)2]PF6·CH3OH
(3), [Re(O)2(dppp)2](ClO4)x(I)1−x·H2O·CH3OH (4) and
[Re(O)(OCH3)(dppp)(Cl)2]·CH3OH (5) were deter-
mined. ORTEP drawings of the complexes are shown in
Figs. 1–3, while selected distances and angles are listed
in Tables 2–4.
Cations 1–4 consist of squashed octahedrons around
the Re(V) ion. Oxygen atoms occupy the trans position,
almost linear with the Re atom. ReꢀO distances range
,
from 1.768 to 1.782 A, typical of such complexes
Fig. 2. ORTEP view of the [Re(O)2(dppp)2]+ (3) cationic complex
(50% probability ellipsoids; H atoms are omitted for clarity).
[15,17,34–37]. Diphosphine ligands lie on the equato-

52
C. Kremer et al. / Inorganica Chimica Acta 294 (1999) 47–55
Fig. 3. ORTEP view of the complex [Re(O)(OCH3)(dppp)(Cl)2]·CH3OH (5) (30% probability ellipsoids; H atoms are omitted for clarity).
to P with a larger covalent radius. Taking into account
average ReꢀP bond lengths and PꢀReꢀP angles, the
distances between P atoms belonging to the same ring
irregularities. The O(1)ꢀReꢀO(2%) angle is 175.5(2)°, i.e.
O(1) is slightly bent towards the dppp ligand giving an
O(1)ꢀReꢀP(1) angle of 87.58(13)°. ReꢀO distances are
markedly different as a consequence of the bond char-
,
(bite size) can be calculated as 3.82 (dppe) and 3.72 A
(dppp). This represents a much more open ring than
those of Reꢀen and Reꢀtn analogs [40]. Consequently,
steric crowding in the complexes is greatly reduced,
allowing the accommodation of eight phenyl groups per
Re atom.
,
acter. The ReꢀO(1) distance is 1.702(5) A, which is
shorter than those for compounds 1–4. This length is
comparable with that expected for Re(V) monooxo
complexes [37], showing a certain triple bond character.
The five- and six-membered rings formed between
Re, P and C atoms offer the possibility of having
molecules in different conformations. In the four
cations, a ld conformation is adopted. As previously
studied by molecular mechanics and semi-empirical cal-
culations [40], this is the most stable conformation. The
presence of the phenyl groups seems to stabilize the ld
conformation (with eclipsed substitutes on phosphorus
atoms) in comparison with ll or dd conformations.
Together with crystals of compound 4, violet crystals
were observed. X-ray diffraction structure determina-
tion proved them to be [Re(O)(OCH3)(dppp)(Cl)2]·
CH3OH (5). The appearance of this compound seems
to be the result of a redox process in which the perchlo-
rate anion is reduced to chloride. Then, in a second
step, one dppe ligand is substituted by two chlorides.
One of the oxo groups is lost, a methoxy group arising
from the crystallization solvent occupying the trans
position. Compound 5 also shows an octahedral ar-
rangement around the Re atom but with significant
Table 2
,
Comparison of selected bond lengths (A) and bond angles (°) for the
complexes [Re(O)2(dppe)2]I (1) and [Re(O)2(dppe)2]PF6·2CH3OH
(2) a
1
2
ReꢀO(1)
ReꢀP(1)
ReꢀP(2)
1.772(4)
2.487(3)
2.483(3)
1.780(4)
2.513(2)
2.487(2)
O(1)ꢀReꢀO(1%)
O(1)ꢀReꢀP(1)
O(1)ꢀReꢀP(2)
O(1)ꢀReꢀP(1%)
O(1)ꢀReꢀP(2%)
P(1)ꢀReꢀP(2)
P(1)ꢀReꢀP(1%)
P(2)ꢀReꢀP(2%)
P(1)ꢀC(1)ꢀC(2)
P(2)ꢀC(2)ꢀC(1)
180.0
180.0
84.4(2)
84.8(2)
95.6(2)
95.2(2)
79.4(1)
180.0
180.0
113.5(7)
110.6(7)
94.50(14)
94.87(13)
85.50(14)
85.13(13)
80.50(6)
180.0
180.0
113.3(4)
110.8(4)
a The atoms labeled (%) are related by the symmetry operation
−x+1, −y, −z+1 (in (1)) and −x, −y+1, −z (in (2)).

C. Kremer et al. / Inorganica Chimica Acta 294 (1999) 47–55
53
Table 3
shorter than the ReꢀCl distance for a chlorine atom
,
Comparison of selected bond lengths (A) and bond angles (°) for the
complexes [Re(O)2(dppp)2]PF6·CH3OH (3) and [Re(O)2(dppp)2]
(ClO4)x(I)1−x (4)
,
trans to an oxo group (2.44 A) [46]. Based on the
structure of this new compound, the synthesis from
[Re(O)(Cl)3(PPh3)2] was performed.
3
4
The methanol solvent molecules are connected to oxo
groups in compounds 2, 4 and 5 by hydrogen bonds of
ReꢀO(1)
ReꢀO(2)
ReꢀP(1)
ReꢀP(2)
ReꢀP(4)
ReꢀP(3)
1.768(6)
1.754(5)
2.568(2)
2.541(2)
2.517(2)
2.517(2)
1.782(5)
1.748(4)
2.5209(18)
2.519(2)
2.5331(18)
2.602(2)
,
O···O mean length 2.77(2) A. In compound 4 there is
evidence of the formation of hydrogen bonding connec-
tions between the water molecule and the perchlorate
,
anion with an O···O distance of 2.75(1) A and the water
molecule and the iodide anion with an I···O distance of
,
3.76(1) A.
O(1)ꢀReꢀO(2)
O(1)ꢀReꢀP(1)
O(1)ꢀReꢀP(2)
O(1)ꢀReꢀP(4)
O(1)ꢀReꢀP(3)
O(2)ꢀReꢀP(1)
O(2)ꢀReꢀP(2)
O(2)ꢀReꢀP(4)
O(2)ꢀReꢀP(3)
P(1)ꢀReꢀP(2)
P(4)ꢀReꢀP(3)
P(1)ꢀReꢀP(4)
P(2)ꢀReꢀP(3)
P(1)ꢀC(1)ꢀC(2)
P(2)ꢀC(3)ꢀC(2)
P(4)ꢀC(6)ꢀC(5)
P(3)ꢀC(4)ꢀC(5)
C(1)ꢀC(2)ꢀC(3)
C(4)ꢀC(5)ꢀC(6)
177.0(3)
95.9(2)
87.5(2)
89.1(2)
85.6(2)
177.5(2)
85.62(17)
88.8(2)
87.99(17)
96.1(2)
The IR spectra of the iodide salts show a very strong
and sharp absorption at 786.3 cm−1 (Reꢀdppe) and
784.8 (Reꢀdppp) cm−1, characteristic of the linear
[OꢁReꢁO]+ core. These vibrations are in the expected
range (775–835 cm−1) for the asymmetric Re(O)2
stretching [47]. However, the ReꢀO stretching frequency
lies in the lower part of this range, showing a reduction
in the bond order. This phenomenon should be inter-
preted in terms of the fact that phosphorus donor
atoms are part of a conjugated system which could
increase ReꢀP bond order. In addition, typical signals
of the ligands are also found in the complexes [48,49].
For dppe: 3052.0 (CH stretching of the phenyl groups),
2905.3 (CH2 stretching), 1484.9 (CH2 bending), 1435.2
(P-phenyl group stretching), 1102.6 (CꢀC stretching),
743.7 and 693.0 cm−1 (out of plane CH bending).
Analogously for dppp: 3052.1, 2922.8, 1484.3, 1436.1,
1099.4, 741.9 and 697.7 cm−1. The IR spectrum of 5
shows characteristic absorption of the [Re(O]3+ core at
909.9 cm−1. The presence of ReꢀOCH3 is detected by a
weak vibration at 787.9 cm−1. Both bands are similar
to others previously reported [50,51]. Also, a strong
w(OꢀCH3) vibration at 1109.5 cm−1 is detected.
81.09(17)
92.13(18)
93.91(17)
94.66(18)
84.53(7)
86.49(7)
174.99(7)
172.83(7)
113.4(7)
114.7(6)
114.7(6)
115.7(6)
119.4(8)
115.3(8)
94.58(15)
93.69(15)
91.72(15)
81.41(15)
86.41(6)
84.27(6)
173.27(6)
175.09(6)
115.2(5)
114.7(5)
116.1(5)
113.7(6)
114.3(7)
116.9(6)
,
On the other hand, ReꢀO(2) is 1.856(4) A. This is in
line with the reported distance in [Re(O)(acac){h2-
B(pz)4}(OMe)] (acac=acetylacetonate; pz=pyrazolyl)
,
of 1.896 A [41] in which the methoxy group is coordi-
nated to the Re(V) trans to an oxo group. However, it
is different from the distance reported for a methanol
Electronic absorption spectra of the cations are
mainly determined by the strong absorptions of the
ligands. Typical bands of the [Re(O)2]+ core seem to be
shadowed by the aromatic system of phosphines.
,
ligand coordinated to Re(V) center, 2.38 A [42]. ReꢀCl
,
distances in compound 5 are 2.450(1) A. They are
similar to others reported for cis dichloro complexes of
Re(V), ranging from 2.33 to 2.40 A [43–46] and slightly
,
The 31P{1H} spectra of the complexes [Re(O)2-
(dppe)2]I and [Re(O)2(dppp)2]I, with a singlet resonance
at 9.16 and −31.9 ppm, respectively, are consistent
with an octahedral geometry in which the two chelate
ligands are in trans position to each other on the
equatorial plane of the coordination polyhedron. 1H
and 13C{1H} spectra (see Section 2) are in agreement.
The values of the 31P chemical shifts appear in line with
those previously reported for other rhenium(V) octahe-
dral diphosphine complexes [52]. The 31P{1H} spectrum
of complex 5, with a singlet resonance at −15.22 ppm,
indicates that the two phosphorus atoms are equivalent
Table 4
,
Selected bond lengths (A) and bond angles (°) for the complex
[Re(O)(OCH3)(dppp)(Cl)2]·CH3OH (5) a
5
ReꢀO(1)
ReꢀO(2)
ReꢀP(1)
1.702(5)
1.856(4)
2.4431(11)
O(1)ꢀReꢀO(2)
O(1)ꢀReꢀP(1)
O(2)ꢀReꢀP(1)
P(1)ꢀReꢀP(1%)
P(1)ꢀC(1)ꢀC(2)
C(1)ꢀC(2)ꢀC(1%)
175.5(2)
87.58(13)
89.39(10)
94.49(5)
113.8(4)
113.1(6)
1
and in trans position to the chlorine atoms. H and
13C{1H} spectra are consistent with such a geometry,
1
the H absorption of ReꢀOꢀCH3 being a triplet, due to
a The atoms labeled (%) are related by the symmetry operation x,
−y+1/2, z.
the long-range coupling with the two phosphorus
atoms.

54
C. Kremer et al. / Inorganica Chimica Acta 294 (1999) 47–55
Table 5
Protonation constants of dppe, dppp, [Re(O)2(dppe)2]+ and [Re(O)2(dppp)2]+ (70:30 dioxane:water (v/v), 25°C, I=0.1 mol dm−3 KCl)
2
Compound
Number of experimental
pH range
log i12

|(E)
points/number of titrations
Dppe
Dppp
60/2
68/2
94/2
91/2
2.0–3.1
1.8–3.3
1.7–3.5
1.7–3.5
3.5590.05
3.5290.03
3.990.1
16.0
25.2
14.1
14.6
0.28
0.30
0.94
1.31
[Re(O)2(dppe)2]+
[Re(O)2(dppp)2]+
3.6990.07
Potentiometric titrations were performed in 70:30
dioxane:water (v/v) in order to reach sufficient solubil-
ity of the complexes. For [Re(O)2(amine)2]+ species in
aqueous solution, experiments previously reported
showed successive protonation of the oxo groups
[17,53]. Hence, [Re(O)2(amine)2]+, [Re(O)(OH)-
(amine)2]2+ and [Re(OH)2(amine)2]3+ species exist in
solution. In this case the result is different. Both proto-
nations are simultaneous, without a significant presence
of the monoprotonated species. For both complexes,
the processes involved in the titrations are:
six-membered rings and protonation reactions and re-
dox changes seem to be out of reach for biological
systems. The stability of Re(V) dioxo complexes with
P-containing ligands offers the possibility to develop
new therapeutic radiopharmaceuticals with the
[ReV(O)2]+ core.
5. Supplementary material
Complete listings of atomic coordinates, anisotropic
thermal parameters, complete bond distances and an-
gles, hydrogen coordinates, as well as a listing of calcu-
lated and observed structure factors are available from
the authors on request.
[Re(O2(phosphine)2]++2H+ ?[Re(OH)2(phosphine)2]3+
The protonation constant i11 is rejected, but i12 is well
defined 3.9(1) for Reꢀdppe and 3.69(7) for Reꢀdppp.
It is interesting to compare these values with those
corresponding to the free ligands. Also, a simultaneous
protonation of both phosphine groups is detected with
i12 values near 3.5 (see Table 5). Assuming that P atom
basicity is reduced if coordinated, it is reasonable to
postulate protonation of the complexes on the oxo
groups as in the case of amine analogues.
Cyclic voltammograms of the complexes in methanol
do not show signals in the studied range of potentials
(−1000 to 1400 mV versus SHE). Re(V) dioxo com-
plexes with amines show electronic processes in this
range [15,36,53–55]. Although it is difficult to compare
the results because experiments were performed under
different conditions, it seems clear that phosphine com-
plexes are much more stable than amine analogs.
This fact is also reflected in the good stability of
solutions of the cations.
Acknowledgements
This work was partially supported by the Consejo
Nacional de Investigaciones Cient´ıficas y Te´cnicas
(CONICYT), project number 305 and PEDECIBA,
Uruguay. Financial support from Action APC 94-0031
(DGICYT, Spain) and the Human Capital and Mobil-
ity Program (Network on Metals and Environmental
Problems, E.C.) through grant ERBCRHX-CT94-0632
is also gratefully acknowledged.
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