An unusual compound containing μ-O-bridged Re26+ cores in very different coordination environments

Article abstract of DOI:10.1016/S0020-1693(02)00739-9

The reaction of Re₂(O₂CC₆H ₅)₄Cl₂ with PMe₃ in ethanol in the presence of traces of air or a small amount of H₂O₂ affords the unusual compound [Cl(PMe₃)₃Re(μ-O ₂CC₆H₅)Re(O)]₂(μ-O)₂ (1). The crystal structure of this new rhenium 'dimer of dimers' type compound has been established by X-ray crystallography. In this centrosymmetric molecule each dirhenium unit is supported by one benzoate ligand and further ligated by Cl, PMe₃ and O-ligands; the two dinuclear units are united by two μ-O atoms. Interestingly, the rhenium centers have rarely observed distinctly different coordination environments within the dimers, namely ReClOP₃-ReO₄, which can be formally considered as Re(II)-Re(IV) or Re(I)-Re(V).

Full text of DOI:10.1016/S0020-1693(02)00739-9

Inorganica Chimica Acta 334 (2002) 67Á70
/
www.elsevier.com/locate/ica
An unusual compound containing m-O-bridged Re26ꢀ cores in very
different coordination environments
F. Albert Cotton a,*, Evgeny V. Dikarev b, Marina A. Petrukhina b
a Department of Chemistry, Laboratory for Molecular Structure and Bonding, Texas A&M University, P.O. Box 30012, College Station, TX 77842-
3012, USA
b Department of Chemistry, University at Albany, State University of New York, Albany, NY 12222, USA
Received 11 September 2001; accepted 12 November 2001
Abstract
The reaction of Re2(O2CC6H5)4Cl2 with PMe3 in ethanol in the presence of traces of air or a small amount of H2O2 affords the
unusual compound [Cl(PMe3)3Re(m-O2CC6H5)Re(O)]2(m-O)2 (1). The crystal structure of this new rhenium ‘dimer of dimers’ type
compound has been established by X-ray crystallography. In this centrosymmetric molecule each dirhenium unit is supported by one
benzoate ligand and further ligated by Cl, PMe3 and O-ligands; the two dinuclear units are united by two m-O atoms. Interestingly,
the rhenium centers have rarely observed distinctly different coordination environments within the dimers, namely ReClOP3ꢀ
/
ReO4,
which can be formally considered as Re(II)ꢀRe(IV) or Re(I)ꢀRe(V). # 2002 Published by Elsevier Science B.V.
/
/
Keywords: Rhenium complexes; Mixed-valent compounds; Phosphine complexes; Crystal structures
1. Introduction
obtained reproducibly and its yield was optimized by
introducing H2O2 into the system before the crystal-
lization procedure. We report here this new complex
because it shows unusual structural features rarely
observed.
While the vast majority of the compounds containing
5ꢀ
the Re26ꢀ, Re2
rhenium centers, that is both are Re(III), both are
and Re2
cores have equivalent
4ꢀ
5ꢀ
Re(II), or the charge distribution over a Re2
is
symmetrical [1], a few examples of mixed-valence multi-
ply bonded dirhenium complexes, which are formally
the products of intramolecular disproportionation reac-
tions, have been reported [2]. A unique example of
isomerization in which an electron-rich triple bond
converts to formally mixed-valence charge-separated
2. Experimental
The starting material Re2(O2CC6H5)4Cl2 was synthe-
sized as previously described [4]; and all operations were
carried out under a nitrogen atmosphere using standard
Schlenk techniques.
species in which no Reꢀ
/
Re bond is present has been
recently discovered [3].
In this work, we unintentionally isolated an oxygen
bridged dirhenium complex when studied transforma-
tion of Re2(O2CC6H5)4Cl2 in the presence of mono-
dentate phosphine, PMe3, in ethanol. We believe the
origin of oxygen was the traces of air penetrating the
tube during long-term crystal growth. Compound 1 was
2.1. Preparation of [Cl(PMe3)3Re(m-
O2CC6H5)Re(O)]2(O)2 (1)
(1a) A weighed amount, 0.180 g (0.2 mmol), of
Re2(O2CC6H5)4Cl2 was suspended in 10 ml of EtOH,
1.0 ml of PMe3 was added, and the mixture was refluxed
for 4 h. All volatiles were then removed to leave a dark-
brown residue, which was washed with hexanes, dried
overnight under vacuum, and then dissolved in 10 ml of
CH2Cl2. The dark-brown CH2Cl2 solution was filtered
* Corresponding author. Tel.: ꢀ1-409-845 4432; fax: ꢀ1-409-845
9351.
E-mail address: cotton@tamu.edu (F.A. Cotton).
0020-1693/02/$ - see front matter # 2002 Published by Elsevier Science B.V.
PII: S 0 0 2 0 - 1 6 9 3 ( 0 2 ) 0 0 7 3 9 - 9

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F.A. Cotton et al. / Inorganica Chimica Acta 334 (2002) 67Á
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Table 1
Crystallographic data for [Cl(PMe3)3Re(m-O2CC6H5)Re(O)]2(m-O)2 (1)
and layered with 20 ml of hexanes. Dark-brown crystals
of 1 appeared in 3 weeks. Yield: approximately 5%.
(1b) A weighed amount, 0.180 g (0.2 mmol), of
Re2(O2CC6H5)4Cl2 was suspended in 10 ml of EtOH,
1.0 ml of PMe3 was added and the mixture was stirred
for 3 h at room temperature (r.t.); then 0.1 ml of 30%
H2O2 was added. The stirring was continued for two
more hours, and all volatiles were then removed to leave
a brown residue. This was treated as above to give the
same crystalline product 1. Yield: approximately 20%. A
few dark-purple crystals of a known compound [5],
[Re2Cl4(PMe3)4][ReO4], were found along with crystals
of 1.
1
Empirical formula
Formula weight
Temperature (8C)
Crystal system
Space group
C32H64Cl2O8P6Re4
1578.35
ꢂ60
monoclinic
P21/c
15.397(2)
˚
a (A)
˚
b (A)
14.109(4)
11.7547(8)
˚
c (A)
b (8)
98.14(1)
2527.8(8)
3
V (A )
˚
Z
2
2.074
9.881
Dcalc (g cmꢂ3
)
m (mmꢂ1
Radiation (l, A)
Unique data
)
2.2. X-ray crystallography
˚
Mo Ka (0.71073)
3212
Single crystals of compound 1 were obtained as
described above. The X-ray diffraction experiment was
carried out on a Nonius FAST diffractometer with an
Observed data [I ꢀ2s(I)]
Parameters refined
Final R indices [I ꢀ2s(I)]
R indices (all data)
2880
235
b
b
R1ꢃ0.0471 a, wR2ꢃ0.1102
R1ꢃ0.0548 a, wR2ꢃ0.1185
area detector using Mo Ka radiation. A greenꢀ
/
black
a
R1ꢃa jjFojꢂjFcjj/ajFoj.
block crystal with dimensions 0.15ꢁ0.10ꢁ0.07 mm
/
/
wR2ꢃ[a [w(Fo2ꢂFc2)2]/a [w(Fo2)2]]1/2
.
b
was mounted on the tip of a quartz fiber with silicone
grease, and the setup was quickly placed in the cold N2
stream at ꢂ60 8C of a low temperature controller. Fifty
/
Table 2
˚
Selected bond lengths (A) and angles (8) in 1
reflections were used in cell indexing and 242 reflections
in cell refinement. The data were corrected for Lp by the
MADNES program [6]. Reflection profiles were fitted and
values of F2 and s(F2) for each reflection were obtained
by the program PROCOR [7]. All calculations were done
on a DEC Alpha running VMS. The coordinates of
rhenium atoms were found in direct E maps using the
structure solution program SHELXTL [8]. The positions
of the remaining atoms were located after an alternating
series of least-squares cycles and difference Fourier
maps [9]. Anisotropic displacement parameters were
assigned to all non-hydrogen atoms. Hydrogen atoms
were included in the structure factor calculations at
idealized positions. Crystallographic data for 1 are
summarized in Table 1; selected bond distances and
angles are given in Table 2.
Bond lengths
Re(1)ꢀRe(2)
Re(2)ꢀ ꢀ ꢀRe(2A)
Re(1)ꢀO(1)
Re(2)ꢀO(2)
Re(2)ꢀO(4)
Re(2)ꢀO(4A)
Re(2)ꢀO(3)
Re(1)ꢀP(1)
Re(1)ꢀP(2)
Re(1)ꢀP(3)
Re(1)ꢀCl(1)
2.3396(8)
3.007(1)
2.191(9)
2.096(9)
1.904(9)
1.976(8)
1.720(9)
2.382(4)
2.431(4)
2.439(4)
2.516(3)
Bond angles
Re(1)ꢀRe(2)ꢀRe(2A)
Re(2)ꢀO(4)ꢀRe(2A)
P(1)ꢀRe(1)ꢀP(2)
P(1)ꢀRe(1)ꢀP(3)
P(2)ꢀRe(1)ꢀP(3)
P(1)ꢀRe(1)ꢀCl(1)
P(2)ꢀRe(1)ꢀCl(1)
P(3)ꢀRe(1)ꢀCl(1)
O(3)ꢀRe(2)ꢀO(4)
O(3)ꢀRe(2)ꢀO(4A)
O(4)ꢀRe(2)ꢀO(4A)
O(3)ꢀRe(2)ꢀO(2)
O(4)ꢀRe(2)ꢀO(2)
120.09(3)
101.6(4)
94.3(1)
93.3(1)
160.6(1)
94.0(1)
80.4(1)
81.3(1)
103.2(4)
131.4(4)
78.4(4)
87.3(4)
79.9(4)
3. Results and discussion
We believe that the transformations resulting in the
isolation of 1 can be described by the following
equation:
2Re2(O2CC6H5)4Cl2 ꢀ14PMe3 ꢀ4C2H5OH
ꢀ2O20[Re2(O2CC6H5)(PMe3)3OCl]2O2
ꢀ4CH3COHꢀ8[HPMe3]ꢀ ꢀ6C6H5COOꢂ
ꢀ2Clꢂ
presence of traces of air led to the formation of 1.
Actually two forces were working in opposite directions
in the above-mentioned reaction: ethanol was acting as a
reducing agent and oxygen as an oxidizing agent. This
caused an intramolecular disproportionation reaction
affording an overall Re(III) oxidation state in 1, the
Compound 1 was first obtained in very small yield
from the reaction of Re2(O2CC6H5)4Cl2 with PMe3 in
ethanol after some work-up procedures. Probably the

F.A. Cotton et al. / Inorganica Chimica Acta 334 (2002) 67Á
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69
˚
2.096(9) A) and a
same as in the starting material. We have not optimized
the yield, but it has been shown that the yield can be
improved by adding H2O2 to the reaction mixture.
However, the latter reaction also afforded a few crystals
of a known compound [5], [Re2Cl4(PMe3)4][ReO4],
which represents an overall oxidation state Re(IV).
The latter is an interesting example of a complete
disproportionation process. Most likely, compound 1
a benzoate oxygen atom (Reꢀ
/
Oꢃ
/
typical terminal, doubly-bonded oxygen atom (Reꢁ
/
Oꢃ
/
˚
1.720(9) A).
The overall oxidation number of each Re2 unit is 6ꢀ
/.
˚
Re(2) bond length, 2.3396(8) A, is
However, the Re(1)ꢀ
/
considerably longer than the Reꢀ/Re distances in
6ꢀ
symmetrical Re2
compounds, which are typically in
˚
2.25 A. Clearly, in the present case we
the range 2.22Á
/
ꢂ
are not dealing with a homopolar Reꢀ
/
Re quadruple
is an intermediate way to the ReO4 anion confirming
bond, as, for example, in Re2Cl82ꢂ, Re2(O2CR)4Cl2 or
Re2Cl6(PR3)2. As a crude estimate of the oxidation
states of Re(1) and Re(2) we might chose numbers
the fact that oxidation of the dirhenium unit proceeds
not symmetrically, but occurs preferentially at one end
of the dimetal molecule.
between ꢀ
/
1/ꢀ
/
5 and ꢀ
/
2/ꢀ
/
4.
The
tetranuclear
molecule
[Cl(PMe3)3Re(m-
Comparisons may be made with a few of the prior
O2CC6H5)Re(O)]2(m-O)2 (1) is centosymmetric (Fig. 1).
At the center, there is a rhombic Re2(m-O)2 unit in which
each oxo-bridge is unsymmetrical, with Reꢀ
of 1.904(9) and 1.976(8) A. The Re(2)ꢀ
nꢀ
examples of unsymmetrical Re2
(EtO)2Cl2ReReCl2(PPh3)2 [2a], can be simply described
as a Re(IV)ꢀRe(II) molecule, although the actual charge
distribution is probably less extreme than ꢀ4/ꢀ
because of the tendency of the EtO ligands to be p-
donors and the PPh3 ligands to be p-acceptors. The Reꢀ
compounds.
/O distances
˚
/
/
Re(2A) distance
˚
/
/
2
across this rhombus is 3.007(1) A, which indicates that
Re(2A) bonding.
While each half of the molecule consists of a pair of
there is no Re(2)ꢀ
/
/
˚
Re distance in this molecule, 2.231(1) A, is within the
strongly bonded rhenium atoms, as is typical of
nꢀ
6ꢀ
range for symmetrical Re2
5ꢀ
compounds. The first
compound, Re2Cl5(CH3SCH2CH2-
compounds with Re2
(nꢃ4, 5, 6) cores, and the
/
reported Re2
rotational conformation is eclipsed, the similarity to
most other such compounds goes no further, since the
two rhenium atoms are in very different environments
and oxidation states. The outer rhenium atoms, Re(1)
and Re(1A), are each surrounded by a set of four
SCH3)2 [10], and the analogous Re2Cl5(C2H5SCH2-
CH2SC2H5)2 [11], also have a very large formal differ-
ence in the oxidation states of the two rhenium atoms
(ꢀ
/
1/ꢀ
/
4) based on the very different ligand sets around
the two rhenium atoms. Here the Reꢀ
about 2.27 A, partly owing to the fact that there is only a
triple bond since the rotational conformation is stag-
/
Re bonds are
equatorial ligands, comprising three PMe3 molecules
˚
˚
(mean Reꢀ
/
Pꢃ
Oꢃ
chlorine atom (Reꢀ
/
2.417(4) A) and one benzoate oxygen
˚
atom (Reꢀ
/
/
2.191(1) A). In addition there is an axial
˚
Clꢃ2.516(3) A). The inner rhenium
gered. In the case of Re2O3Cl2(dmpm)2 [2b], a ReO3 unit
˚
Re bond (2.4705(5) A) to
/
/
is joined by an unbridged Reꢀ
/
atoms, Re(2) and Re(2A), are each surrounded by four
a 6-coordinate ReCl2(dmpm)2 moiety, and the formal
oxygen atoms. In addition to the two m-O atoms, there is
oxidation states are ꢀ6 and ꢀ2. There are also several
/
/
6ꢀ
examples of unsymmetrical Mo2 compounds with the
metal atoms in disparate oxidation states [12].
Interestingly, compound 1 represents a very rare
example of a Reꢀ/Re bonded molecule having three
phosphine groups bounded to one rhenium atom.
Another unusual rhenium compound, (Me3SiCH2)3-
(O)Re(m-O)Re(PMe3)4Re(O)2(CH2SiMe3), has been
crystallographicaly characterized [13]. In this trinuclear
molecule the central rhenium atom carries four PMe3
and is bound to another rhenium atom by a short Reꢀ
/
˚
Re bond (2.381(1) A), and to a third one by an
asymmetric oxygen bridge. This results in the formal
oxidation states VI and V for the outer rhenium atoms,
while the central rhenium atom is formally Re(I).
We close by noting that in this laboratory [14], we
have also obtained and structurally characterized1 the
1
Fig. 1. The molecular structure of [Cl(PMe3)3Re(m-O2CC6H5)-
Re(O)]2(m-O)2 (1). Displacement ellipsoids are shown at the 35%
probability level. All carbon and oxygen atoms are shown as
arbitrarily sized circles. Hydrogen atoms are omitted for clarity.
[Cl(PEt3)2Re(m-O2CC6H5)2Re(X)]2(m-O): Re4Cl2P4O11C52H82,
F.W.ꢃ1822.83, space group P21/n, aꢃ12.811(3) , bꢃ10.966(2) ,
cꢃ22.801(5) A, bꢃ102.92(2)8, Zꢃ2. Further details may be
obtained from the corresponding author.
˚

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F.A. Cotton et al. / Inorganica Chimica Acta 334 (2002) 67Á70
/
centrosymmetric molecule [Cl(PEt3)2Re(m-O2CC6H5)2-
Re(X)]2(O) from a reaction of Re2(O2CC6H5)4Cl2 with
the less basic and more sterically bulky phosphine
References
[1] F.A. Cotton, R.A. Walton, Multiple Bonds Between Metal
Atoms, 2nd ed., 2nd ed., Oxford University Press, Oxford, UK,
1993.
ligand, PEt3, in ethanol. The Reꢀ
/
Re distance is
2.298(1) A, but the assignment of oxidation states
cannot be made since it is uncertain whether XꢃO or
˚
[2] (a) A.R. Chakravarty, F.A. Cotton, A.R. Cutler, R.A. Walton,
Inorg. Chem. 25 (1986) 3619;
/
(b) I. Ara, P.E. Fanwick, R.A. Walton, Inorg. Chem. 31 (1992)
3211.
OH, and the preparation has not been reproduced.
[3] W. Wu, P.E. Fanwick, R.A. Walton, J. Am. Chem. Soc. 118
(1996) 13091.
[4] M.J. Bennett, W.K. Bratton, F.A. Cotton, W.R. Robinson,
Inorg. Chem. 7 (1968) 1570.
4. Supplementary material
[5] (a) F.A. Cotton, J.G. Jennings, A.C. Price, K. Vidyasagar, Inorg.
Chem. 29 (1990) 4138;
(b) F.A. Cotton, E.V. Dikarev, M.A. Petrukhina, Inorg. Chem. 37
(1998) 1949.
Crystallographic data (excluding structure factors)
have been deposited with the Cambridge Crystallo-
graphic Data Centre, CCDC No. 163481 for compound
1. Copies of this information may be obtained free of
charge from The Director, CCDC, 12 Union Road,
[6] J. Pflugrath, A. Messerschmitt, MADNES, Munich Area Detector
(New EEC) System, version EEC 11/9/89, with enhancements by
EnrafÁNonius Corp., Delft, The Netherlands. A description of
/
MADNES appears in: A. Messerschmitt, J. Pflugrath, J. Appl.
Crystallogr. 20 (1987) 306.
Cambridge, CB2 1EZ, UK (fax: ꢀ44-1223-336-033; e-
/
[7] (a) W. Kabsch, J. Appl. Crystallogr. 21 (1988) 67;
(b) W. Kabsch, J. Appl. Crystallogr. 21 (1988) 916.
[8] SHELXTL V.5, Siemens Industrial Automation Inc., Madison, WI,
1994.
mail: deposit@ccdc.cam.ac.uk or www: http://
www.ccdc.cam.ac.uk).
[9] G.M. Sheldrick, in: H.D. Flack, L. Parkanyi, K. Simon (Eds.),
Crystallographic Computing 6, Oxford University Press, Oxford,
UK, 1993, p. 111.
[10] M.J. Bennet, F.A. Cotton, R.A. Walton, Proc. R. Soc. London,
Ser. A 303 (1968) 175.
Acknowledgements
[11] B.J. Heyen, G.L. Powell, Polyhedron 7 (1988) 1207.
[12] D. Lawton, R. Mason, J. Am. Chem. Soc. 87 (1965) 921.
[13] K.W. Chiu, W.-K. Wong, G. Wilkinson, A.M.R. Galas, M.B.
Hursthouse, Polyhedron 1 (1982) 31.
We thank the National Science Foundation for
support of this work.
[14] Dr. J. Lu, F.A. Cotton, unpublished results.