Syntheses and structures of pyrazolylmethane complexes of rhenium(iii), (iv) and (v)

Article abstract of DOI:10.1039/b617549j

Reaction of [ReOCl₃(PPh₃)₂] with HCpz ₃ (pz = pyrazole) in dichloromethane leads to the formation of a new Re(iv) complex [ReCl₃(HCpz₃)]X (XCl, [ReO₄]) with loss of the rhenium-oxo group. We also report a convenient, high-yield synthetic route to complexes of the type [ReOX_n(L)] ^{(3 - n)+} (XCl, Br, n = 2, 3) by the reaction of bis(pyrazolylmethane) and bis(pyrazolylacetate) ligands with [ReOCl₃(PPh₃) ₂]. Dinuclear complexes containing the ORe-O-ReO group were also isolated and structurally characterised. We have also investigated the reactions of these ligands with diazenide precursors and isolated and characterised complexes of the type [ReCl_x(N₂Ph) (L)(PPh₃)] (x = 1,2). The potential applications of these complexes as radiopharmaeuticals is discussed. The Royal Society of Chemistry.

Full text of DOI:10.1039/b617549j

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PAPER
www.rsc.org/dalton | Dalton Transactions
Syntheses and structures of pyrazolylmethane complexes of rhenium(III), (IV)
and (V)
Andrew R. Cowley, Jonathan R. Dilworth* and Maria Salichou
Received 1st December 2006, Accepted 7th February 2007
First published as an Advance Article on the web 6th March 2007
DOI: 10.1039/b617549j
Reaction of [ReOCl3(PPh3)2] with HCpz3 (pz = pyrazole) in dichloromethane leads to the formation of
=
a new Re(IV) complex [ReCl3(HCpz3)]X (X Cl, [ReO4]) with loss of the rhenium-oxo group. We also
report a convenient, high-yield synthetic route to complexes of the type [ReOXn(L)](3 − n)+ (X Cl, Br,
=
n = 2, 3) by the reaction of bis(pyrazolylmethane) and bis(pyrazolylacetate) ligands with
=
=
[ReOCl3(PPh3)2]. Dinuclear complexes containing the O Re–O–Re O group were also isolated and
structurally characterised. We have also investigated the reactions of these ligands with diazenide
precursors and isolated and characterised complexes of the type [ReClx(N2Ph) (L)(PPh3)] (x = 1,2). The
potential applications of these complexes as radiopharmaeuticals is discussed.
been isolated.18 There are a few isolated references to complexes
Introduction
of rhenium with heteroscorpionate ligands including the neutral
species [ReO3O2CCHpz2] prepared from reaction of the so-called
heteroscorpionate ligand with perrhenic acid.18 During the course
of this work rhenium complexes containing the O2CCHpz2 and
CH2pz2 ligands were reported.19 However not all the complexes
have been fully identified and there is some uncertainty as to
their structures. We have investigated the chemistry of a range
of tripodal pyrazole-based ligands (Fig. 1) with rhenium, and
we present a much more convenient method to prepare the
rhenium(V) pyrazolato complexes using [ReOCl3(PPh3)2], and
the synthesis of new Re(III) diazenide complexes by reacting
[ReCl2(N2Ph)(NCMe)(PPh3)2] with ligands HL4 and L7. The main
objective of this work is a systematic study of the chemistry of
pyrazolyl-ligands with rhenium in medium/high oxidation states
in the context of possible radiopharmaceutical applications, with
full characterisation of the products and a study of their redox and
HPLC behaviour.
There is much interest in the development of new radiotherapeutic
cancer agents based on rhenium.1 There are two b-emitting
isotopes of Re, 186Re and 188Re which have suitable nuclear
properties for therapeutic applications. 188Re is readily available
from a generator by decay of 188W and is obtained as a very dilute
solution of [ReO4]−.2,3 This can provide a useful source of sterilising
b radiation if it can be targeted in vivo. Such targeting can be
achieved by the incorporation of the metal atom in a coordination
complex involving a bifunctional ligand. Ideally the ligand must
form a highly stable complex and possess a point of further
functionalisation that can allow the attachment of biologically
active molecules4 to provide specificity and selectivity in vivo.5–8
The addition of a fluorescent probe to the complex would allow
the biological targeting of the complex in living cells in vitro.
The majority of complexes that have been explored in this
context to date have involved the use of linear tetradentate ligands
with the Re(V) oxo-core giving generally neutral square pyramidal
complexes of the general type [ReOL].9 Tetradentate ligands with
a tripodal framework have been relatively much less explored.
Pyrazolylborate ligands are of potential use with rhenium and
complexes of the type [ReOX2HBpz3] (X2 = Cl2, Br2, 1,2-
C6H4O2 1,2-C6H4S2 etc.,10–14 pz = pyrazole) have been investigated
extensively. Other complexes reported include the hexahydride
[ReH6HBpz3].15 However degradation of the pyrazolylborate com-
plexes is a frequent complicating side reaction when protic solvents
are employed.16 Pyrazolylmethane complexes are an attractive
alternative due to the higher stability of the C–N bond over the
B–N bond, but there have been comparatively few reports of their
complexes with rhenium. A series of Re(I) carbonyl complexes
of the type [ReCl(CO)3HCpz3] and [Re(CO)3HCpz3]+17 together
with analogous dimeric complexes of a binucleating variant of the
HCpz3 ligand 1,4-C6H4(HOCH2Cpz3)2 and [ReO3HCpz3]+ have
Fig. 1 Pyrazole-based ligands investigated.
Results and discussion
Synthesis of ligands
The structures of the ligands used are summarised in Fig. 1.
Ligands L1 to L8 were prepared by the literature routes or minor
modifications thereof.20,21 The synthesis of HL6 and other related
ligands will be described elsewhere.22
Central Chemistry Department, University of Oxford, 12 Mansfield Road,
Oxford, UK OX1 3TA. E-mail: jon.dilworth@chem.ox.ac.uk
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Complexes of the type [ReOCl2L1–2]Cl, (1), (2); and
[Re2O3Cl4(L1)2] (3)
showed a strong multi component m(Re–O–Re) band. The results
suggest that there is slow formation of the oxo bridged dimer
on standing in solution (Scheme 1) probably due to the presence
of adventitious water. The amounts formed in the fresh reaction
mixture are too small to be detected by HPLC.
The 1H NMR spectrum of complex 3 showed a complex series of
peaks in the region for the pyrazolyl protons, consistent with one of
the pyrazolyl groups being uncoordinated. Evidently the reaction
to give the complexes of the type [ReOCl2L1–2]+ is accompanied by
the slow formation of the dimeric species.
Single crystals of 3 suitable for an X-ray structure determination
were obtained from the reaction solution on standing for 3 days.
An ORTEP representation of the structure appears in Fig. 3.
Complex 3 contains the well known linear Re2O3 unit comprising
a bridging oxo-group with a trans oxo-group on each rhenium.25
Each rhenium is pseudo octahedral with the trispyrazolylmethane
ligands functioning in a bidentate manner.
[ReOCl3(PPh3)2] is conveniently made from perrhenate in virtually
quantitative yield and reacts rapidly with a range of HC(pzR2)3
type ligands (R2 = H2, L1; 3,5-Me2, L2) in the presence of
potassium tertiary butoxide in thf under reflux to give turquoise
blue complexes [ReOCl2L1]X 1 (Scheme 1) and [ReOCl2L2]X, 2
(X = Cl,) in isolated yields of 78%. The precise role of the added
butoxide in these reactions is unclear but in its absence brown
solutions in thf are formed. Infrared and ESMS(+) data on these
complexes were consistent with the formation of the previously
reported species formulated as [ReCl2(OPPh3)(HC(PzR2)3)].23
1
However the H NMR for complex 1 in d6-dmso differs from
that reported previously23 in CDCl3, showing three resonances for
protons 3-pz, 4-pz and 5-pz, implying that the three pyrazolyl rings
are magnetically equivalent. This suggests that some exchange
process may be occurring, or that the inequivalence generated by
the different groups trans to the pyrazole nitrogens is too small to
detect. The complexes 1 and 2 showed a characteristic IR band
−1
=
at about 990 cm , assigned to the Re O stretching frequency.
The HPLC traces of isolated complexes 1 and 2 (isocratic elution
with CH3CN–H2O, 80 : 20) show predominantly single species
with retention times of 1.38 and 1.46 min, respectively, Fig. 2.
Clearly the products of these reactions are highly dependent on
the reaction solvent used and carefully controlled conditions are
required to prevent formation of inseparable mixtures.24
Fig. 3 ORTEP view of the dimeric structure of 3; thermal ellipsoids are
drawn at the 40% level.
As expected it is the pyrazole trans to the labilising oxo group
that is replaced by the bridging oxo group. Previous reports of
this complex had suggested that it was a chloro-bridged dimer.23
This type of bidentate coordination has been observed previously
for tetrakis(pyrazolylborate) ligands.26 The pyrazolyl groups are
arranged in an anti configuration to minimise steric interactions.
The bond distances and angles are similar to those found for other
complexes of the general type [Re2O3Cl4L2] where L is a neutral
bidentate ligand.25
Scheme 1
Synthesis of [ReCl3L1]X (X = Cl, [ReO4]), 4
The reaction of [ReOCl3(PPh3)2] with L1 in CH2Cl2 under
reflux (Scheme 2) illustrates that the chemistry of the
tris(pyrazolylmethane) ligands with Re(V) oxo precursors is far
from straightforward. After 2 h a brown solution was obtained
which on treatment with diethyl ether gave a brown-green precipi-
tate. ESMS(+) of this solid showed two peaks, at m/z = 470.9871
(100%), ([M–Cl]+. C10H10Cl2N6Re requires 470.9901) and at m/z =
749.0844 (60%) ([M + OPPh3–Cl]+. C28H25Cl2N6OPRe requires
749.0762). The product showed only very broad signals in the 1H
NMR spectrum in CDCl3 consistent with the presence of a para-
magnetic Re(IV) species. The IR spectrum showed a strong band at
917 cm−1 characteristic of the perrhenate anion. Recrystallisation
gave brown crystals unambiguously identified as the Re(IV) cation
Fig. 2 HPLC traces for compounds 1 (
and 1.46 min respectively, with isocratic elution CH3CN–H2O, 80 : 20.
) and 2 (-·-), in dmso at 1.38
On standing the solution of the reaction of L1 with
[ReOCl3(PPh3)2] in thf deposited a dark blue complex 3 (Scheme 1)
in an isolated yield of 2–5% which showed an IR band at 980 cm−1
−1
=
assigned to m(Re O). In the range 720–765 cm the product
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by an X-ray structure determination (see below). We assume that
[ReCl3L1]+ is generated in the mass spectrometer giving rise to the
[M–Cl]+ ion observed. The ESMS spectrum of the recrystallised
product shows peaks at m/z = 470.9775 (100%), ([M–Cl]+.
C10H10Cl2N6Re requires 470.9901) as well as m/z = 505.9508 (1%),
(M+. C10H10Cl3N6Re requires 505.9569). The other species ob-
served in the mass spectrum of the crude product has been reported
as the product of the reaction of complex 1 with triphenylphos-
phine, but was not fully characterised.23 Whatever its structure, it
is removed in the recrystallisation process. Interestingly the same
cationic Re(IV) complex was obtained when L3 was reacted under
the same conditions and the hydroxymethyl group is eliminated
during the reaction. The formation of the Re(IV) cation requires
disproportionation/oxidation to occur during the reaction, but
the formation of the perrhenate anion is well documented in the
chemistry of Re(V) oxo-complexes with non-reducing ligands.
Suitable crystals of 4 for an X-ray structure determination were
obtained by slow evaporation from CHCl3. An ORTEP represen-
tation of the structure appears in Fig. 4 and Table 1 details selected
bond lengths and angles of the two crystallographically distinct
molecules. The asymmetric unit contains two crystallographically
distinct molecules of the cation, two molecules of the perrhenate
anion, and four molecules of the solvent (CHCl3). There are a
number of H-bonding interactions between the hydrogens on the
apical carbons and pyrazole rings and the perrhenate anion. The
significance of H-bonding interactions in the solid state structures
of pyrazolylmethane complexes has been discussed previously.23
The geometry around the Re atom is distorted octahedral and the
bond lengths and angles are similar to those reported for other
Re complexes of L1.17,23 The bond distances and angles in the two
independent molecules are very similar.
Scheme 2
The elemental analysis of the brown product was consistently
low in C, H, N for the chloride salt, but higher than required for
the perrhenate salt and the C, H, N ratios confirmed the presence
of L1. This is consistent with the presence of both [ReO4]− and
Cl− anions. Investigation of another crystal from the same batch
as that of the perrhenate salt above showed a chloride anion.
HPLC studies were carried out to monitor the reaction that
leads to the loss of the rhenium-oxo group, Fig. 5. Samples of
the reaction mixture were collected after 10, 30, and 60 min and
examined by HPLC. In the first 10 min there are two major species
present with retention times of 1.60 and 1.85 min in a ratio of
1 : 6. After 30 min the ratio of the two peaks changes to 1 : 1
whereas 60 min later formation of a single species with retention
time 1.60 min is observed. This peak corresponds to the isolated
Re(IV) cation as confirmed by HPLC measurements on the isolated
product and shows that it is the major product of the reaction
despite the evident complexity. We were unable to identify the
intermediate species with a retention time of 1.85 min.
Fig. 5 HPLC studies of the reaction mixture leading to the formation of
the cationic complex [ReCl3L1]+ 4 in thf at 1.60 min with isocratic elution
CH3CN–H2O 80 : 20. 10 min (· · ·), 30 min (---), 60 min (
). Small
Fig. 4 ORTEP view of the Re(IV) cation [ReCl3L1]+ 4; thermal ellipsoids
at 40% probability.
impurity peaks are caused by the reaction solvent (thf).
◦
˚
Table 1 Selected bond lengths (A) and angles ( ) for the two independent
Complexes of the type [ReOX2L4–6] (HL4, X = Cl), 5a; (HL4, X =
Br), 5b; (HL5, X = Cl), 6; (HL6, X = Cl), 7a; (HL6, X = Br), 7b
molecules of [ReCl3L1]+
Bond lengths and angles
1
2
The synthesis of the complex [ReOCl2(L4)] 5a from [ReOCl4]−
has previously been reported.16 We now report a much more
convenient method leading to complex 5a from [ReOCl3(PPh3)2]
Re–N(1)
Re–N(3)
Re–N(5)
Re–Cl(1)
Re–Cl(2)
Re–Cl(3)
N–Re–Nav
Cl–Re–Clav
2.098(7)
2.116(7)
2.108(7)
2.306(2)
2.290(2)
2.292(2)
2.108(6)
2.119(6)
2.113(6)
2.308(2)
2.296(2)
2.285(2)
t
by reaction with HL4 and BuOK in thf in 2 h under reflux. The
complex is isolated as a turquoise blue solid in yields of 72–
75%. The spectroscopic properties of the complex are identical
to those previously described. Reaction of [ReOX3(PPh3)2] (X =
Cl, Br) with HL5 and HL6 gave products 6, 7a and 7b of the same
82.5(3)
93.62(8)
83.0(3)
93.81(8)
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stoichiometry as for HL4. The HPLC retention times in Fig. 6 for
5a 1.61 min, 6 2.20 min, 7a 5.76 min, show how substituents on
the pyrazoles modulate the lipophilicity of the complexes. HPLC
studies of the reaction solutions show that the complexes are
formed in very high yields.
◦
˚
Table 2 Selected bond distances (A) and bond angles ( ) for complexes
5a and 7a
Bond lengths and angles
5aa
7aa
Re–Cl(1)
2.3480(13)
2.3335(12)
Re–Cl(2)
Re–O(1)
2.3415(14)
1.660(4)
2.3432(12)
1.666(3)
Re–N(1)
Re–N(2)
2.089(4)
2.098(3)
2.159(4)
2.146(6)
Re–O(2)
2.092(3)
2.069(3)
N(1)–Re–N(2)
N(1)–Re–Cl(1)
N(2)–Re–Cl(2)
88.50(17)
89.14(12)
88.78(13)
85.83(15)
90.34(11)
93.31(11)
a The numbering for complex 5a differs from that of 7a but comparable
bond distances and angles have been selected.
The only significant variation seen in the bond distances is the
lengthening of the Re–N bond distances as the steric bulk of the
ligands in 7a forces the pyrazole ligands further from the rhenium.
This is accompanied by a decrease in the N–Re–N angles. There
is also as expected an increase in the N–Re–Cl angles, again to
accommodate the larger aryl substituted pyrazole ligands.
Fig. 6 HPLC traces for complexes 5a (
at 1.61, 2.20 and 5.76 min respectively, with isocratic elution CH3CN–H2O
80 : 20.
), 6 (· · ·), 7a (---) in CH3CN
Complexes of the type [ReOCl3L7–8], 8; 9; and [Re2O3Cl4(L7)2], 10
Crystals of 7a suitable for an X-ray structure determination were
grown by slow evaporation of a solution in thf. This structure was
investigated to assess the steric impact of the bulky substituents
on the pyrazole rings and whether the methoxy groups interacted
with the rhenium. An ORTEP representation of the structure is
shown below in Fig. 7.
[ReOCl3(PPh3)2] reacts with L7 under reflux for 2 h in THF to
give the previously reported19 (made from [ReOCl4]−) neutral
turquoise blue complex 8 in 74% yield (Scheme 3). However, beside
the monomeric Re(V) oxo-complexes ESMS(+) also indicated the
presence of small amounts of a dimeric species. The HPLC of
8 shows a single peak with a retention time of 1.6 min (Fig. 8).
The dimeric species was not present in sufficient amounts to be
detected in the reaction mixture by HPLC and as with complex 3
it is formed slowly on standing the reaction mixtures in air.
Scheme 3
Fig. 7 ORTEP representation of the structure of complex 7a; thermal
ellipsoids at 40% probability.
The coordination about the rhenium atom is in the event directly
comparable with that of complex 5a16 with pseudooctahedral
geometry about the metal. The methoxy groups are oriented so
that there are no bonding interactions with the rhenium and
the aryl pyrazole substituents are twisted to minimise steric
interactions between aromatic C–H hydrogens. Table 2 below
makes a comparison between the structures of complexes 5a16
and 7a.
Fig. 8 HPLC trace for complex 8 in dmso at 1.6 min, with isocratic elution
with CH3CN–H2O 80 : 20.
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◦
˚
Table 3 Selected bond lengths (A) and angles ( ) for complexes 3 and 10
Bond lengths and angles
3
10
2.4025(11)
1.9182(3)
1.692(5)
2.098(4)
90.26(6)
88.46(3)
95.14(12)
90.03f(11)
85.85(11)
90.51(16)
123.7(3)
Re(1)–Cl(1)
Re(1)–O(1)
Re(1)–O(2)
Re(1)–N(1)
Cl(1)–Re(1)–Cl(2)
Cl(1)–Re(1)–O(1)
Cl(1)–Re(1)–O(2)
Cl(1)–Re(1)–N(1)
O(1)–Re(1)–N(1)
O(2)–Re(1)–N(1)
Re(1)–N(1)–N(2)
2.396(2)
1.9242(3)
1.683(6)
2.105(7)
90.62(8)
88.22(6)
95.0(2)
90.3(2)
87.68(18)
90.9(3)
Scheme 4
123.4(5)
and 4-pz protons dominating at 7.5-7.1 ppm. In the IR spectra
(KBr disc) of the complexes the bands assigned to m(NN) of
the diazenide ligands are observed in the 1650–1560 cm−1 range.
The UV/vis spectra of the complexes remain unchanged in dmso
solution over 24 h, suggesting the complexes are stable in solution.
The HPLC traces for complexes 11 and 12 confirm the formation
of single species with retention times of 4 and 4.2 min respectively,
Fig. 10. The minor peaks correspond to the solvent (dmso) used
to dissolve the complexes.
By slow evaporation of a solution of 8 from CH3CN we were
able to isolate two types of blue crystals. The pale blue crystals
corresponded to the monomer 8, whereas crystals of the dimer
10 were a dark blue colour. Crystals of 10 suitable for an X-ray
structure determination confirmed its formulation as a dimer. For
complex 10 selected bond lengths and angles are given in Table 3
with those for dimer 3 above included for comparison. As expected
the two structures are very similar. An ORTEP representation is
shown in Fig. 9. The dimer has been previously been reported but
not structurally characterised.19
Complexes of the type [ReCl(NNPh)L4(PPh3)], 11;
[ReCl2(NNPh)L7(PPh3)], 12
Complexes with diazenide ligands are readily available from
pertechnetate for radiopharmaceutical applications but optimal
co-ligands have yet to be established. We have therefore investi-
gated the potential use of pyrazole-based ligands. The diazenido
complex [ReCl2(N2Ph)(NCMe)(PPh3)2] reacts with HL4 and L7 in
t
MeOH in the presence of BuOK to give the orange complexes
[ReCl(N2Ph)L4(PPh3)] (11) and [ReCl2(N2Ph)L7(PPh3)] (12) re-
spectively in good yields (77–78%) (Scheme 4).
The ESMS(+) spectra show peaks at m/z = 819.0914 (M+ + K.
C32H27ClKN6O2PRe requires 819.0816) for 11 and m/z = 772.0672
(M+ + K. C31H28Cl2KN6PRe requires 811.0685) for 12. Both have
appropriate isotope distribution patterns. 1H NMR spectra for 11
and 12 are complex, with peaks corresponding to PPh3, Ph–N2
Fig. 10 HPLC traces for complexes 11 (
at 4.0 and 4.2 min respectively; dmso 1.75 min, with isocratic elution
CH3CN–H2O, 80 : 20.
) and 12 (· · ·) in dmso
Fig. 9 ORTEP view of the dimeric complex 10; thermal ellipsoids are drawn at the 40% probability level.
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Electrochemistry
Conclusions
The cyclic voltammograms of complexes 1 and 5–12 were recorded
at room temperature in dmso and for complex 4 in CH3CN
A simple synthetic route to the rhenium (V) complexes of the type
[ReOCl2(L1–8)]n+ (n = 0,1) has been established by reaction of L1
and HL4 ligands with [ReOCl3(PPh3)2] in thf in the presence of
potassium tertiary butoxide. There appears to be a labile equi-
librium involving dissociation of the pyrazolyl group trans to the
oxo-group and the monomeric complexes are slowly converted to
dimeric complexes with bidentate tris(pyrazolylmethane) ligands
and an Re2O3 core on standing in solution. The reaction of
the rhenium-oxo precursor with L1 in CH2Cl2 in the absence of
base leads to loss of the oxo group from the rhenium starting
material and to the formation of a Re(IV) complex [ReCl3L1]+. The
formation of these Re(IV) cationic complexes, which is interestingly
observed in the reaction with L3 with the loss of the HOCH2 group,
appears to be accompanied by the formation of perrhenate anion
in varying amounts. These complex equilibria with rhenium in
high oxidation states strongly suggest that this class of ligand is
not appropriate for use in a radiopharmaceutical context with
solutions, at a glassy carbon working electrode with 0.2 M [Bun N]
4
[BF4] as the supporting electrolyte at a scan rate of 400 mV s−1.
Potentials are quoted relative to the ferrocene–ferrocenium couple
taken as 0.38 V in CH3CN. Complexes 1 and 5–10 exhibit
an irreversible one electron ReV/ReVI oxidation near 1.2 V (vs
SCE) and several other irreversible reduction processes at rather
negative potentials, suggesting that both the oxidised Re(VI) and
reduced Re(IV) species are not stable in solution. The potential
values observed show that the complexes are unlikely to undergo
reduction in vivo.. Complex 4 shows a one electron reversible
(ip vs (scan rate)1/2 straight line) oxidation process at E1/2
=
+ 0.017 V vs Fe/Fe+, (E1/2 = + 0.397 V vs SCE) corresponding to
the formation of a Re(V) complex, Fig. 11a. This was confirmed to
be an oxidation process by linear sweep voltammetry in a stirred
solution. On reduction only an irreversible process is observed
at ca. −0.9 V. The failure to observe a reduction to Re(III)
but oxidation to Re(V) is surprising and suggests that the car-
boxypyrazolylmethane ligand is more effective at stabilising higher
oxidation states. This electrochemical behaviour contrasts with
that of the very recently reported complex formulated as [ReCl3L1]
which was quoted as having a single irreversible oxidation process
at ca. +1.2 V.24 Cyclic voltammetry measurements on 11 and 12
in dmso (Fig. 11b) show that the mono-diazenide species undergo
a fully reversible oxidation assigned to the Re(III)/Re(IV) couple
at 0.39 and 0.41 V (vs SCE) respectively. The precursor diazenido
complexes show analogous reversible oxidation processes.
=
Re O cores. However, HPLC studies of the reactions of the
bis(pyrazolyl)acetates shows that single complexes are formed in
high yield in solution. HPLC studies of the isolated complexes
also demonstrate their high purity and give information on the
lipophilicities of the compounds. UV/vis studies of the neutral
complexes also confirm that they are stable in solution for at
least 24 h. In view of the relatively facile derivatisation of these
ligands at the capping carbon, these complexes show some promise
as the basis of future rhenium radiopharmaceuticals with both
oxo- and diazenido-cores. The aryl-substituted pyrazolylmethane
complexes are also strongly fluoresecent and the optical properties
of these complexes and studies of their cellular uptake will be
reported elsewhere.
Experimental
Precursors
The L1,20 and L3,20 ligands used were prepared using stan-
dard literature methods. The ligand HL6 was prepared by a
new route to be described elsewhere.22 [ReOCl3(PPh3)2] and
[ReCl2(NNPh)(MeCN)(PPh3)2] were prepared according to the
literature.27,28 Reactions were carried out under nitrogen using
reagent grade solvents.
Physical measurements
Elemental analyses were performed by the microanalysis service
of the department at the University of Oxford. NMR spectra
were recorded on either a Varian Mercury VX300 spectrometer,
1
(1H 300 MHz, 13C{ H} at 75.5 MHz) or a Varian Unity 500 MHz
1
spectrometer, (1H 499.9 MHz, 13C{ H} at 125.7 MHz) using the
residual solvent signal as an internal reference. Mass spectra were
recorded on a Micromass LCT time of flight mass spectrometer
using positive ion electrospray (ES+), solid probe electron impact
(EI) or field ionisation (FI+) techniques. Where possible accurate
masses are reported to four decimal places using tetraoctylammo-
nium bromide (466.5352 Da) as an internal reference. UV/vis
spectra were recorded on a Cintra 10 UV/vis spectrometer.
High performance liquid chromatography (HPLC) was conducted
using a Gilson HPLC machine with a Hamilton PRP-1 reverse
Fig. 11 Cyclic voltammograms of complexes (a) 4 in CH3CN solution,
(b) 11 (
) and 12 ( · · · ) in dmso, recorded at a scan rate of 400 mV s−1
.
Scale relative to Ag wire pseudoreference electrode.
1626 | Dalton Trans., 2007, 1621–1629
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phase column and UV/vis detection at 254 nm. Retention times,
Rt/min, using a water–acetonitrile gradient elution method are
presented for all compounds. For the cyclic voltammetry tests
a CH Instruments Electrochemical Analyser was used. Cyclic
voltammograms of 0.1 M solutions of complexes in 10 mL of
dmso or CH3CN with tetrabutylammonium tetrafluoroborate as
support electrolyte were recorded using a glassy carbon stationary
working electrode, a platinum wire counter/auxiliary electrode
and a silver/silver ion pseudoreference electrode at scan rates
of 100–600 mV s−1. Nitrogen was bubbled through the solutions
before the electrochemical test to remove oxygen. Ferrocene was
used as an internal reference for which the one-electron redox
process occurs at E1/2 = +0.53 V (dmf) vs. SCE. All electrochemical
measurements were carried out at 25 ◦C.
was stirred for 10 min at ambient temperature. [ReOCl3(PPh3)2]
(0.600 g, 0.72 mmol) was added and the mixture was heated under
reflux for 2 h. The precipitated turquoise blue product was filtered
off, washed with thf (2 × 50 mL) and dried in vacuo (74%). Suitable
crystals for X-ray structure analysis were obtained from a solution
in CH3CN by slow evaporation. (Found: C, 19.50; H, 1.53; N,
11.38. C8H7Cl3N4O3Re requires C, 20.14; H, 1.41; N, 11.92%);
UV-vis (dmso, 1 mM) 345 nm, 680 nm; IR mmax/cm−1 (Nujol)
=
=
=
3119 (w), (CH), 1719 (s), 1703s (C O), 1513 (m) (C C + C N),
=
1462 (s), 1380 (m), 1287 (m), 1082 (w), 1004 (w), 940s (Re O),
761 (m); dH (300 MHz; ppm; d6-dmso; Me4Si) 8.72 (d, 2H, 3-pz),
8.45 (d, 2H, 5-pz), 7.02 (t, 2H, 4-pz), 8.09 (s, 1H, CH); d13C{1H}
=
(75.5 MHz; ppm; d6-dmso; Me4Si) 160.3 (C O), 148.1 (C-3pz),
139.5 (C-5pz), 106.4 (C-4pz), 71.3 (CH); (ESMS)(+) 502.9051 ([M
+ K]+. C8H7Cl2KN4O3Re requires 502.9090); HPLC, 5a (dissolved
in CH3CN), (isocratic elution CH3CN–H2O, 80 : 20) 1.61 min.
Synthesis of complexes
[ReOBr2L4)], 5b. Prepared in an analogous manner to 5a in
75% yield. (Found: C, 17.12; H, 1.16; N, 9.88. C8H7Br2N4O3Re
requires C, 17.37; H, 1.28; N, 10.13%); IR mmax/cm−1 (KBr) cm−1
3418 (b), 3151 (w), 3118 (s), 3021 (w), 2989 (w), 2368 (w), 1712 (s),
1520 (m), 1458 (m), 1412 (s), 1288 (s), 1254 (s), 1108 (w), 1074 (s),
1000 (s), 938 (s), 858 (s), 780 (s), 756 (s), 683 (w), 650 (s), 598 (m),
554 (w), 420 (s); dH (300 MHz; ppm; d6-dmso; Me4Si) 8.69 (2H,
d, 3-pz), 8.52 (2H, d, 5-pz), 8.03 (1H, s, CH), 7.01 (2H, t, 4-pz);
[ReOCl2L1]X (X = Cl), 1. A solution of L1 (0.025 g, 0.12 mmol)
and potassium tert-butoxide (0.013 g, 0.12 mmol) in thf (30 mL)
was stirred for 10 min at room temperature. [ReOCl3(PPh3)2]
(0.100 g, 0.12 mmol) was added and the mixture was heated under
reflux for 2 h. During this time the colour of the brown-green sus-
pension changed to turquoise blue. The precipitated blue product
was filtered off, washed with Et2O (2 × 30 mL) and dried in vacuo
(78%) (Found: C, 22.90; H, 2.12; N, 16.19. C10H10N6OCl3Re
requires C, 22.97; H, 1.93; N, 16.08%); IR mmax/cm−1 (KBr) 1319
(m), 1307 (m), 1234 (m), 1103 (m) 1073 (s), 1038 (w), 995 (m,
=
d13C{1H} (75.5 MHz; ppm; d6-dmso; Me4Si) 160.2 (C O), 148.4
(C-3pz), 138.3 (C-5pz), 111.5 (C-4pz), 71.4 (CH); m/z (ESMS)(+)
590.8346 ([M + K]+. C8H7Br2KN4O3Re requires 590.8079); HPLC,
5b (dissolved in dmso), (gradient elution CH3CN–H2O) 10.65 min.
=
Re O), 950 (w), 821 (m), 765 (s), 722 (s), 633 (m); dH (300 MHz;
ppm; d6-dmso; Me4Si) 8.72 (3H, d, pz-5), 8.45 (3H, d, pz-3),
8.08 (1H, s, CH), 7.02 (3H, t, pz-4); d13C{1H} (75.5 MHz; ppm;
d6-dmso; Me4Si) 148.2 (C-3pz), 139.4 (C-5pz), 111.6 (C-4pz),
71.2 (CH); m/z (ESMS)(+) 486.9452 ([(M–Cl]+. C10H10N6OCl2Re
requires 486.9851); HPLC, 1 (dissolved in dmso), (gradient elution
CH3CN/H2O) 9.7 min.
[ReOCl2L5], 6. Prepared in an analogous manner to 5a in 74%
yield. (Found: C, 49.76; H, 2.91; N, 7.22. C32H23Cl2N4O3Re re-
quires C, 50.00; H, 3.02; N, 7.29%); m/z (ESMS)(+) 807.0893 ([M
+ K]+. C32H23Cl2N4O3Re requires 807.0342); HPLC, 6 (dissolved
in CH3CN), (isocratic elution CH3CN–H2O, 80 : 20) 2.20 min.
ReOCl2(L2)]X (X = Cl), 2. Prepared in an analogous manner
to 1.
[ReOCl2L6], 7a. Prepared in an analogous manner to 5a in
72% yield. (Found: C, 48.31; H, 3.40; N, 6.15. C36H31Cl2N4O3Re
requires C, 48.65; H, 3.52; N, 6.30%); dH (300 MHz; ppm; d6-dmso;
Me4Si) 7.51 (2H, dd, o-Ph) 7.47-7.45 (2H, m, p-Ph), 7.45-7.42
(2H, m, m-Ph), 7.40 (2H, t), 7.17 (2H, d), 7.15 (2H, d), 7.1 (2H,
d), 7.02 (1H, s), 6.97–6.87 (4H, m), 3.8 (6H, s, CH3), 3.7 (6H, s,
CH3); d13C{1H} (75.5 MHz; ppm; d6-dmso; Me4Si) 159.1 (Cquat),
156.8 (Cquat), 155.5 (Cquat), 145.6 (Cquat), 131.2 (Cquat), 131.1
(CH), 128.5 (CH), 128.3 (CH), 120.6 (Cquat), 119.1 (CH), 117.8
(CH), 112.9 (Cquat), 112.5 (CH), 111.2 (CH), 110.7 (CH), 67.8
(CH), 55.3 (CH3), 55.1 (CH3); m/z (ESMS)(+) 927.0798 ([M +
K]+. C36H31Cl2KN4O7Re requires 927.0764); HPLC, 7a (dissolved
in dmso), (gradient elution CH3CN–H2O) 17.8 min, (dissolved in
CH3CN), (isocratic elution CH3CN–H2O, 80 : 20) 5.8 min.
[Re2O3Cl4(L1)2], 3. Complex 3 formed when 1 dissolved in
CH3CN (1 mL). The pale blue filtrate was allowed to stand at
room temperature. Single crystals suitable for X-ray structure
determination were obtained by slow evaporation of a solution
in thf.
[ReCl3L1]X (X = Cl, [ReO4]−), 4. A solution of L1 (0.077 g,
0.36 mmol) and [ReOCl3(PPh3)2] (0.300 g, 0.36 mmol) was heated
under reflux in CH2Cl2 (50 mL) for 2 h. During this time the
colour of the brown-green suspension changed to dark brown.
The precipitated product was filtered off, washed with Et2O (2 ×
50 mL) and dried in vacuo (87%). Suitable crystals for X-ray
structure analysis were obtained from a solution in CHCl3 by
slow evaporation. UV-vis (CH2Cl2; 1 mM) 380 nm; IR mmax/cm−1
(KBr): 1464 (s), 1386 (s), 1094 (m), 880 (m), 752 (m), 721 (m); m/z
(ESMS)(+) 470.9871 ([M–Cl]+. C10H10Cl2N6Re requires 470.9902),
100 (%), 749.0844 (M+–Cl + OPPh3), (60), 263.0984 (PPh3) (30);
505.9508 (1%), (M+. C10H10Cl3N6Re requires 505.9569). HPLC, 4
(reaction mixture in thf), (isocratic elution CH3CN–H2O, 80 : 20)
1.60 min.
[ReOBr2L6], 7b. Prepared in an analogous manner to 5a in
75% yield. (Found: C, 44.42; H, 3.31; N, 5.88. C36H31Br2N4O7Re
requires C, 44.23; H, 3.20; N, 5.73%); UV-vis (dmso; 1 mM)
684 nm; IR mmax/cm−1 (KBr) 3406 (b), 3138 (w), 3062 (w), 2940
(w), 2832 (w), 2366 (w), 2334 (w), 2044 (w), 1911 (w), 1732 (s),
=
=
1610 (s, C N), 1583 (s, C N), 1556 (w), 1516 (w), 1490 (s), 1474
(s), 1440 (s), 1370 (s), 1228 (w), 1253 (s), 1198 (w), 1182 (w),
1168 (w), 1124 (s), 1087 (m), 1052 (m), 1024 (s), 1002 (w), 948 (m,
[ReOCl2L4], 5a. A solution of HL4 (0.138 g, 0.72 mmol) and
potassium tert-butoxide (0.081 g, 0.72 mmol) in thf (50 mL)
Re O), 908 (m), 860 (w), 796 (m), 758 (s), 730 (s), 694 (m), 582 (w),
=
548 (s), 444 (w); dH (300 MHz; ppm, d6-dmso; Me4Si) 7.82 (2H,
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dd, o-Ph), 7.64-7.53 (2H, m, p-Ph), 6.80 (2H, m, m-Ph), 7.0 (2H, s,
4-pz), 6.66 (1H, s, CH), 6.64 (d, 2H), 6.62 (d, 2H), 6.54 (s), 6.42
(s), 3.8 (3H, s, CH3), 3.7 (3H, s, CH3); d13C{1H} (75.5 MHz; ppm;
d6-dmso; Me4Si) 165.9 (COO), 159.2–155.7 (o-Ph), 147.5–141.6
(5-pz), 133.8–131.2 (p-Ph), 129.5–128.4 (o-Ph), 121.9–119.9 (m-
Ph), 118.9–117.7 (C–Ph), 113.7–111.4 (m-Ph), 108.7 (4-pz), 68.2
(CH), 56.1 (CH3), 55.1 (CH3); m/z (ESMS)(+) 999.0072 ([M +
Na]+. C36H31Br2N4O7Re requires 999.0015); HPLC, 7b (dissolved
in dmso), (gradient elution CH3CN–H2O) 18.5 min.
10.91. C31H28Cl2N6PRe requires C, 48.19; H, 3.65; N, 10.88%);
UV-vis (dmso; 1 mM) 458 nm; IR mmax/cm−1 (KBr) cm−1 3452
(b), 3122 (w), 3056 (w), 3014 (w), 2372 (w), 2238 (w), 1646 (s,
mNN), 1614 (m), 1564 (s, mNN), 1488 (s), 1436 (s), 1404 (m), 1339
(w), 1279 (s), 1226 (m), 1164 (m), 1100 (s), 1068 (w), 992 (m),
922 (s), 762 (s), 698 (s), 604 (w), 525 (s), 432 (w); dH (300 MHz;
ppm; d6-dmso; Me4Si) 6.6 (4H, NNC6H4), 7.2 (18H, meta- and
para-ArH PPh3), 7.6 (12H, m ortho-ArHPPh3), 6.9 (d, 2H, 3-pz),
6.8 (d, 2H, 5-pz), 6.4 (s, 2H, CH2), 6.2 (dd, 2H, 4-pz); d13C{1H}
(75.5 MHz; ppm; d6-dmso; Me4Si) 143.7 (C-3pz), 135.1 (PPh3),
134.6 (PPh3), 134.1 (PPh3), 130.3 (PPh3), 129.3 (PhNN), 128.4
(PhNN), 124.6 (PhNN), 121.0 (C-5pz), 108.9 (C-4pz), 72.8 (CH);
d31P{1H} (ppm; d6-dmso; H3PO4) 4.0; m/z (ESMS)(+) 811.0672
(M. C31H28Cl2N6PRe requires 811.0685); HPLC, 12 (dissolved in
dmso), (isocratic elution CH3CN–H2O, 80 : 20) 4.2 min.
[ReOCl3L7], 8. Prepared in an analogous manner to 5a in 74%
yield. (Found: C, 18.78; H, 1.72; N, 12.14. C7H8Cl3N4ORe requires
C, 18.41; H, 1.77; N, 12.27%); UV-vis (dmso; 1 mM) 363 nm,
534 nm; IR mmax/cm−1 (KBr) cm−1 1614 (w), 1462 (s), 1384 (s),
=
1278 (s), 1101 (m), 1093 (s), 954 (s, Re O), 838, (w), 758 (s),
706 (m); UV-vis 363 nm and 534 nm; dH (300 MHz; ppm; d6-
dmso; Me4Si) 8.68 (2H, d, 3-pz), 8.47 (2H, d, 5-pz), 7.93 (2H, s,
CH2), 7.03 (2H, t, 4-pz); d13C{1H} (75.5 MHz; ppm; d6-dmso; Me4Si)
147.8 (C-3pz), 138.1 (C-5pz), 110.6 (C-4pz), 70.86 (CH); (m/z
(ESMS)(+) 455.9351 (M+. C7H8Cl3N4ORe requires 455.9321);
HPLC, 8 (dissolved in dmso), (isocratic elution CH3CN–H2O,
80 : 20) 1.7 min, (gradient elution CH3CN–H2O) 9.73 min.
X-Ray structure determinations
Single crystals of 3, 4, 7a and 10 were mounted on glass fibres using
perfluoropolyether oil and cooled rapidly to 150 K in a stream of
cold N2 using an Oxford Cryosystems Cryostream unit. Diffrac-
tion data were measured using an Enraf-Nonius KappaCCD
diffractometer (graphite-monochromated Mo Ka radiation, k =
[ReOCl3L8], 9. Prepared in an analogous procedure to that
used for 5a in 73% yield. (Found: C, 25.79; H, 3.22; N, 11.01.
C11H16Cl3N4ORe requires C, 25.76; H, 3.14; N, 10.92%); dH
(300 MHz; ppm; d6-dmso; Me4Si) 3.70 (s, 6H); 3.77 (s, 6H); 6.41
(s, 1H); 6.89 (t, 2H); 7.07–7.10 (m, 2H); 7.14–1.16, (m, 2H); 7.06,
(s, 2H); 7.43 (dd, 2H); 7.61 (d, 2H); m/z (ESMS)(+) 511.9971 (M+.
C11H16Cl3N4ORe requires 511.9947); HPLC 9 (dissolved in dmso),
(gradient elution CH3CN–H2O) 9.72 min.
˚
0.71073 A). Intensity data were processed using the DENZO-
SMN package.29 The structures were solved using the direct-
methods program SIR92,30 which located all non-hydrogen atoms.
Subsequent full matrix least squares refinement was carried
out using the CRYSTALS program suite.31 Coordinates and
anisotropic thermal parameters of all non-hydrogen atoms were
refined. Hydrogen atoms were positioned geometrically after each
cycle of refinement. A 3-term Chebychev polynomial weighting
scheme was applied. ORTEP-3 was employed to represent the
structures.32
[Re2O3Cl4(L7)2], 10. Complex 10 formed when compound 8
was dissolved in CH3CN (1 mL). Single crystals suitable for X-ray
structure determination were obtained by slow evaporation of the
CH3CN solution.
CCDC reference numbers 629316–629319.
For crystallographic data in CIF or other electronic format see
DOI: 10.1039/b617549j
[ReCl(N2Ph)L4(PPh3)], 11. In a round bottom flask (250 mL),
potassium tert-butoxide (0.036 g, 0.32 mmol), HL4 (0.062 g,
0.32 mmol) and [ReCl2(N2Ph)(NCMe)(PPh3)2] (0.300 g,
0.32 mmol) were heated under reflux in MeOH (60 mL) for 4 h. The
orange precipitate formed was filtered off and washed with copious
amounts of MeOH and dried (78%). (Found: C, 49.12; H, 3.21; N,
10.56. C32H27ClN6O2PRe requires C, 49.26; H, 3.49; N, 10.77%);
UV-vis (dmso; 1 mM) 450 nm; IR mmax/cm−1 (KBr) 3454 (b),
3116 (w), 3057 (w), 2989 (w), 2368 (w), 2338 (w), 1978 (w), 1702 (s),
1678 (s, mNN), 1622 (s), 1562 (s, mNN), 1486 (s), 1466 (s), 1404 (s),
1306 (m), 1288 (m), 1243 (m), 1187 (s), 1166 (m), 1096 (s), 1066 (w),
1027 (w), 996 (s,), 944 (m), 861 (m), 762 (s), 692 (s), 600 (w), 525 (s),
436 (w); dH (300 MHz; ppm; d6-dmso; Me4Si) 7.8–7.5 (m, 14H, Ph),
7.4 (d, 2H, 5-pz), 7.3–7.1 (m, 6H, Ph), 7.0 (d, 2H, 3-pz), 6.4 (s, 1H,
CH), 6.2 (dd, 2H, 4-pz); d13C{1H} (75.5 MHz; ppm; d6-dmso; Me4Si)
Crystal structure determination of complex 3
Single crystals of [Re2O3Cl4(L1)2] 3 were obtained from a thf
solution by slow evaporation in air.
Crystal data. C20H20Cl4N12O3Re2, M = 990.67, T = 150 K,
˚
k = 0.71073 A, monoclinic, P 21/n, a = 9.7300(3), b = 13.4343(5),
◦
3
˚
˚
c = 10.8514(4) A, b = 95.9521(16) , cell volume = 1410.80(9) A ,
Z = 2, q = 2.332 Mg m−3, l = 9 mm−1, 10 485 reflections measured,
3329 unique, (Rint = 0.076, R = 0.0312, Rw = 0.0311).
The complex is located on a site with a crystallographic centre of
inversion, and consequently the Re–O–Re bridge is exactly linear.
Crystal structure determination of complex 4
=
163.3 (C O), 146.5 (PPh3), 143.1 (PPh3), 136.8 (PPh3), 135.2 (C-
Single crystals of [ReCl3L1]X (X = Cl, [ReO4]) 4 were obtained
3pz), 134.4 (PPh3), 130.1 (PhNN), 129.6 (PhNN), 128.3 (PhNN),
125.4 (PhNN), 121.7 (C-5pz), 109.2 (C-4pz), 73.4 (CH); d31P{1H}
(ppm; d6-dmso; H3PO4) 10.0; m/z (ESMS)(+) 819.0914 ([M +
K]+. C32H27ClKN6O2PRe requires 819.0816); HPLC 11 (dissolved
in dmso), (isocratic elution CH3CN–H2O, 80 : 20) 4.0 min.
from chloroform by slow evaporation.
Crystal data. C12H12Cl9N6O4Re2, M = 995.74, T = 150 K, k =
˚
0.71073 A, monoclinic, P 21/c, a = 15.7338(2), b = 15.8808(2),
◦
3
˚
˚
c = 22.0122(3) A, b = 06.5516(7) , cell volume = 5272.18(12) A ,
Z = 8, q = 2.509 Mg m−3, l = 10.12 mm−1, 52 740 reflections
measured, 12 405 unique, (Rint = 0.075, R = 0.0415, Rw = 0.0489).
[ReCl2(N2Ph)L7(PPh3)], 12. Prepared in an analogous proce-
dure to that used for 11 (77%). (Found: C, 48.35; H, 3.72; N,
1628 | Dalton Trans., 2007, 1621–1629
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A difference Fourier map showed there were a number of peaks
of apparent residual electron density, but these could not be mean-
ingfully assigned, and their positions suggested they were artefact
arising from the presence of highly scattering atoms in the unit cell.
The asymmetric unit contains two crystallographically distinct
molecules of the cation, two molecules of the perrhenate anion
and four molecules of solvent chloroform. The cations are
related by a non-crystallographic translation as are the anions.
There are a number of interactions between the anion oxygen
atoms and the methane hydrogens, indicative of weak hydrogen
bonding.
References
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2001, 57, 1135–1137.
Crystal structure determination of complex 7a
Single crystals of [ReOCl2L7]X (X = Cl) 7a were derived from a
thf solution by slow evaporation.
Crystal data. C36H31Cl2N4O7Re, M = 888.77, T = 150 K, k =
˚
0.71073 A, monoclinic, P 21/n, a = 11.7127(2), b = 16.3000(3),
◦
3
˚
˚
c = 18.0939(3) A, b = 98.4918(7) , cell volume = 3416.56(10) A ,
Z = 4, q = 1.728 Mg m−3, l = 3.769 mm−1, 30 866 reflections
measured, 8039 unique, (Rint = 0.056, R = 0.0324, Rw = 0.0369).
Crystal structure determination of complex 10
18 N. Burzlaff and I. Hegelmann, Inorg. Chim. Acta, 2002, 329, 147.
19 M. Porchia, F. Tisato, F. Refosco, C. Bolzati, M. Cavazza-Ceccato, G.
Bandoli and A. Dolmella, Inorg. Chem., 2005, 44(13), 4766.
20 D. L. Reger, T. C. Grattan, K. J. Brown, C. A. Little, J. J. S. Lamba,
A. L. Rheingold and R. D. Sommer, J. Organomet. Chem., 2000, 607,
120.
21 A. S. Potapov and A. I. Khlebnikov, Polyhedron, 2006, 25, 2683–2690.
22 R. Adlington, J. E. Baldwin, J. R. Dilworth, A. Karpov and M.
Salichou, unpublished work.
23 S. B. Seymore and S. N. Brown, Inorg. Chem., 2000, 39, 325–332.
24 E. C. B. Alegria, L. M. D. R. S. Martins, M. Haukka and A. J. L.
Pombeiro, Dalton Trans., 2006, 4954–4961.
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N. N. Carvalho and A. J. L. Pombeiro, Inorg. Chim. Acta, 1998, 271,
65–74.
Single crystals of [Re2O3Cl4(L7)2] 10 were obtained from an
acetonitrile solution by slow evaporation.
Crystal data. C18H22Cl4N10O3Re2, M = 940.65, T = 150 K, k =
˚
0.71073 A, monoclinic, C 2/m, a = 15.2625(5), b = 10.7896(4), c =
◦
3
˚
˚
8.5400(3) A, b = 95.1189(17) , cell volume = 1400.74(9) A , Z =
2, q = 2.23 Mg m−3, l = 9.056 mm−1, 4815 reflections measured,
1671 unique, (Rint=0.037, R = 0.0261, Rw = 0.0290).
The Re complex is located on a site of crystallographic symmetry
2/m (D2d) and as a consequence the Re–O–Re bridge is obligately
linear. The molecule of MeCN solvent is also located on the mirror
plane, accounting for the observed 2 : 1 stoichiometry. One of the
methylene hydrogens projects towards the midpoint of the two Cl
ligands of the second molecule of complex. The C · · · Cl distances
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ꢀ
˚
are relatively short (C(4) · · · Cl(1 ) = 3.415(6) A, suggesting the
29 Z. Otwinowski and W. Monor, Processing of X-ray diffraction data
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presence of a bifurcated C–H · · · Cl hydrogen bonding interaction.
Acknowledgements
We are indebted to Herrman Starck GmBH for their generous gift
of rhenium metal.
This journal is
The Royal Society of Chemistry 2007
Dalton Trans., 2007, 1621–1629 | 1629
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