The synthesis of the triply bonded tetramethyl complex Re2 (CH3)4-(μ-dppm)2 and its reaction with carbon monoxide to afford Re2-(μ-CH2) 2(CO)4(μ-dppm)2

Article abstract of DOI:10.1016/S0022-328X(03)00101-3

The reaction of Re₂Cl₄(μ-dppm)₂ (dppm=Ph₂PCH₂PPh₂) with CH₃Li in toluene affords Re₂(CH₃) ₄(μ-dppm) ₂ (1) in high yield. This is the first simple alkyl complex of the triply bonded dirhenium(II) core. The crystal structure of 1 (Re-Re = 2.2841(7) A?) shows it to be very closely related to that of Re₂Cl₄(μ-dppm) ₂. Compound 1 reacts with CO to produce the di-μ-methylene complex Re₂(μ-CH₂)₂(CO)₄ (μ-dppm)₂ (2) which is shown to have an edge-sharing bioctahedral structure on the basis of X-ray crystal structure determinations on crystals of composition 2.2THF and 2.1.734CH₂Cl₂. The dirhenium units have Re-Re single bond distances 2.9931(5) and 2.8930(5) A? in these two crystals, and the [Re₂(μ-dppm)₂] units possess chair and boat conformations, respectively.

Full text of DOI:10.1016/S0022-328X(03)00101-3

Journal of Organometallic Chemistry 671 (2003) 166Á171
/
www.elsevier.com/locate/jorganchem
The synthesis of the triply bonded tetramethyl complex Re2(CH3)4-
(m-dppm)2 and its reaction with carbon monoxide to afford Re2-
(m-CH2)2(CO)4(m-dppm)2
Mani Ganesan, Phillip E. Fanwick, Richard A. Walton *
Department of Chemistry, Purdue University, Brown Laboratories, 425 Central Drive, West Lafayette, IN 47907-2018, USA
Received 13 January 2003; received in revised form 5 February 2003; accepted 5 February 2003
Abstract
The reaction of Re2Cl4(m-dppm)2 (dppmꢀ
/
Ph2PCH2PPh2) with CH3Li in toluene affords Re2(CH3)4(m-dppm)2 (1) in high yield.
˚
Reꢀ2.2841(7) A)
This is the first simple alkyl complex of the triply bonded dirhenium(II) core. The crystal structure of 1 (Reꢀ
/
/
shows it to be very closely related to that of Re2Cl4(m-dppm)2. Compound 1 reacts with CO to produce the di-m-methylene complex
Re2(m-CH2)2(CO)4(m-dppm)2 (2) which is shown to have an edge-sharing bioctahedral structure on the basis of X-ray crystal
structure determinations on crystals of composition 2×
distances 2.9931(5) and 2.8930(5) A in these two crystals, and the [Re2(m-dppm)2] units possess chair and boat conformations,
respectively.
/
2THF and 2×
/
1.734CH2Cl2. The dirhenium units have Reꢀ
/
Re single bond
˚
# 2003 Elsevier Science B.V. All rights reserved.
Keywords: Dirhenium(II) complexes; Methyl complexes; m-Methylene complexes; Crystal structures
1. Introduction
we describe the synthesis and characterization of
Ph2PCH2PPh2) and its
reaction with carbon monoxide to afford the unexpected
Re2(CH3)4(m-dppm)2 (dppmꢀ
/
In spite of the extensive array of chemistry that has
been developed for the electron-rich triple bond in
bis-m-methylene complex dirhenium(II) Re2(m-CH2)2-
Re]4ꢁ core
[1], only two simple alkyl complexes have been reported
to date, namely, the compounds Re2Cl2(CH2Si-
(CO)4(m-dppm)2 in which a Reꢀ
/Re single bond is
complexes based upon the dirhenium [Reꢁ
/
present.
Me3)2(PR3)4 (PR3ꢀPMe3 or PMe2Ph) which are
/
formed by degradation of the trinuclear cluster Re3-
Cl3(CH2SiMe3)6 upon its reaction with these phosphines
[2]. In neither case was the product characterized by X-
ray crystallography. Although a few alkyl-containing
complexes of quadruply bonded dirhenium(III) exist,
such as the salt Li2[Re2(CH3)8] [3], several complexes of
the type Re2(CH3)6(PR3)2 [2], and an assortment of
mixed alkyl-acetato complexes [4,5], the dearth of
dirhenium(II) species has led us to examine the possibi-
lity of expanding this chemistry with a view to obtaining
structurally characterizable species. In the present report
2. Experimental
2.1. Starting materials and reaction procedures
The dirhenium complexes Re2Cl4(m-dppm)2 and
Re2Cl6(m-dppm)2 (dppmꢀ
/Ph2PCH2PPh2) were pre-
pared by procedures we have described previously
[6,7], while [(m5-C5H5)2Fe]BF4 was obtained by use of
the standard literature method [8]. A 1.4 M solution of
CH3Li in diethyl ether was purchased from Aldrich
Chemical Co. and carbon monoxide was obtained from
Matheson Gas Products; both were used as received.
Solvents were obtained from commercial sources and
were generally dried and distilled under a dinitrogen
* Corresponding author. Tel.:
4940239.
E-mail address: rawalton@purdue.edu (R.A. Walton).
ꢁ
/
1-765-4945464; fax:
ꢁ/1-765-
0022-328X/03/$ - see front matter # 2003 Elsevier Science B.V. All rights reserved.
doi:10.1016/S0022-328X(03)00101-3

M. Ganesan et al. / Journal of Organometallic Chemistry 671 (2003) 166Á
/171
167
atmosphere before use. All reactions were carried out
under an atmosphere of dinitrogen.
NMR (CDCl3, d): 8.42 (p, 4H, m-CH2), 7.64Á
40H, C6H5), 2.79 (p, 4H, ꢀCH2ꢀ
NMR (CDCl3, d): ꢂ4.4(s). Cyclic voltammogram (0.1
M n-Bu4NPF6ÁCH2Cl2, Pt-bead electrode, nꢀ200 mV
sꢂ1): E1/2
0.36 (Epꢀ100 mV) and Ep,a 0.86 V
0.11 V) versus
/
7.08 (m,
/
/
of dppm). 31P{1H}-
/
2.2. Synthesis of Re2(CH3)4(m-dppm)2 (1)
/
/
ꢀ
/
ꢁ
/
/
ꢀ
/
ꢁ
/
A solution of Re2Cl4(m-dppm)2 (0.50 g, 0.39 mmol) in
60 ml of toluene was treated at room temperature with
an excess of CH3Li (1.3 ml of a 1.4 M solution in Et2O,
1.82 mmol). The resulting red reaction mixture was
stirred for 18 h and then filtered. The solvent was
evaporated from the filtrate to leave a dark red residue
and 1 was obtained as a dark orange needle from a
concentrated solution in toluene; yield 0.44 g (90%).
(with a coupled product wave at Ep,c
ꢀ
/
ꢂ
/
Ag/AgCl.
2.4. Reaction of 2 with [(h5-C5H5)2Fe]BF4
A solution of 2 (0.10 g, 0.07 mmol) in 20 ml of
dichloromethane was treated with a slight stoichiometric
excess of [(h5-C5H5)2Fe]BF4 (0.022 g, 0.081 mmol) and
then stirred for 2 days. The reaction mixture was
centrifuged to remove an insoluble suspension, and the
clear solution layered with hexanes which afforded a few
Anal. Calc. for C57.5H60P4Re2 (i.e. 1×
/
0.5C7H8): C, 55.32;
H, 4.81. Found: C, 55.47; H, 4.87%.
This same product was isolated when Re2Cl6(m-
dppm)2 (0.26 g, 0.18 mmol) was reacted with CH3Li
(1.1 ml, 1.54 mmol) in toluene for 3 days. Work-up was
as described above; yield 0.17 g (74%). The spectro-
scopic and electrochemical properties are identical to
those of the product obtained from Re2Cl4(m-dppm)2.
dark green crystals (ꢀ10 mg). Characterization of this
/
product by NMR spectroscopy and cyclic voltammetry
indicated that it was a mixture and contained both
diamagnetic and paramagnetic materials. Based upon
the X-ray crystallographic characterization of one of the
crystals, it is apparent that one of the components of this
mixture is a salt of the [Re2(m-CH2)2(CO)4(m-dppm)2]ꢁ
cation (see Section 2.5).
IR spectra (Nujol mull, 1600Á
400 cmꢂ1): 1586w,
/
1571w, 1495w, 1482(w), 1435s, 1306w, 1261w, 1186w,
1158w, 1132w, 1090m-s, 1027m-w, 999w, 907m-w,
801w, 764m-w, 750s, 734s, 713m-s, 695vs, 670w,
618vw, 518vs, 494w, 478w, 462vw, 446vw, 429m,
2.5. X-ray crystallography
1
420m-w. H-NMR (CDCl3, d): 8.13Á
/
7.20 (m, ꢀ
of dppm), 2.35 (s, 1.5H,
CH3 of toluene solvent), 0.75 (p, br, 12H, CH3).
31P{1H}-NMR (CDCl3, d): ꢁ
8.2(s). Cyclic voltammo-
gram (0.1 M n-Bu4NPF6ÁCH2Cl2, Pt-bead electrode,
nꢀ 0.14 (Epꢀ90 mV) and Ep,a
200 mV sꢂ1): E1/2
0.59 V versus Ag/AgCl.
/
42H,
C6H5), 5.34 (m, 4H, ꢀ
/
CH2ꢀ
/
Crystals of Re2(CH3)4(m-dppm)2 (1) were grown from
a concentrated solution in toluene, while single crystals
/
of compositions Re2(m-CH2)2(CO)4(m-dppm)2×
2THF) and Re2(m-CH2)2(CO)4(m-dppm)2×1.734CH2Cl2
(2×1.734CH2Cl2) were obtained from THFÁhexane and
dichloromethane, respectively. A crystal of apparent
composition
/
2THF (2×
/
/
/
/
ꢀ
/
ꢂ
/
/
ꢀ
/
/
/
ꢁ
/
[Re2(m-CH2)2(CO)4(m-dppm)2]Cl0.53
-
2.3. Synthesis of Re2(m-CH2)2(CO)4(m-dppm)2 (2)
(BF4)0.47 (3) was selected from the batch of crystals
isolated in Section 2.4. Data collections were performed
A slow stream of CO(g) was bubbled through a
solution of 1 (0.09 g, 0.072 mmol) in 30 ml of toluene for
1 h during which time the reaction solution became
at 150 (9
radiation (lꢀ
diffractometer. Lorentz and polarization corrections
were applied to the data sets. The important crystal-
/
)K with graphite-monochromated MoÁ
/
Ka
˚
/
0.71073 A) on a NoniusÁ
/
Kappa CCD
yellowÁorange in color. The reaction solvent was
/
removed by evaporation and the residue extracted with
10 ml of THF. The extract was filtered and the filtrate
layered with hexanes to afford crystals of composition
lographic data for 1, 2×
/
2THF and 2×
/
1.734CH2Cl2 are
given in Table 1.
The structure of 1 was solved using the structure
solution program PATTY in DIRDIF-92 [9], the structures
2×/2THF as established by X-ray crystallography; yield
0.055 g (54%). The THF molecules were lost upon
drying these crystals under a vacuum. Anal. Calc. for
C56H48O4P4Re2: C, 52.48; H, 3.75. Found: C, 53.18; H,
4.54%. X-ray quality crystals were also obtained from a
concentrated solution of 2 in CH2Cl2 and were found to
of 2×
/
2THF and 3 with the use of PATTY in DIRDIF-99
[10], while that of 2×
/
CH2Cl2 was solved by direct
methods using SIR-97 [11]. The remaining atoms were
located in succeeding difference Fourier syntheses.
Hydrogen atoms were placed in calculated positions
˚
Hꢀ0.95 A
be of composition 2×
IR spectrum (KBr pellet, 2000Á
1915vs, 1903m, 1887s, ꢀ1850sh, 1587w, 1573w, 1484m-
/1.734CH2Cl2.
according to idealized geometries with Cꢀ
/
/
/
400 cmꢂ1): 1985w,
and U(H)ꢀ1.3Ueq(C). They were included in the
/
/
refinement but constrained to ride on the atom to which
they are bonded. An empirical absorption correction
using SCALEPACK [12] was applied in all instances
w, 1434 m-s, 1360w, 1306w, 1187w, 1158vw, 1123vw,
1092m, 1059m-w, 1028w, 1001w, 942vw, 909m-w,br,
1845vw, 780m, 752vw, 736m, 720w, 691m-s, 610vw,
579w, 561w, 522m-w, 504w, 483m, 455w, 419w. 1H-
except 2×
/
2THF. The final refinements were performed
by the use of the program SHELXL-97 [13]. All non-

168
M. Ganesan et al. / Journal of Organometallic Chemistry 671 (2003) 166Á171
/
Table 1
Crystallographic data for crystals of composition Re2(CH3)4(m-dppm)2 (1), Re2(m-CH2)2(CO)4(m-dppm)2 2THF (2×
/
2THF) and Re2(m-CH2)2(CO)4(m-
dppm)2×1.734CH2Cl2 (2×1.734CH2Cl2)
/
/
1
2×
/
2THF
2×1.734CH2Cl2
/
Empirical formula
Formula weight
Space group
C54H56P4Re2
1201.34
Pbcn (No. 60)
11.7245(3)
27.4137(5)
16.2428(8)
90
C64H64O6P4Re2
1425.52
I41/a (No. 88)
31.3724(10)
31.3724(10)
11.3650(2)
90
C57.73H51.47Cl3.47O4P4Re2
1428.57
¯
P1 (No. 2)
/
˚
a (A)
12.5246(4)
13.4041(4)
18.8627(7)
103.9548(14)
97.6111(14)
111.675(2)
2768.4(4)
2
˚
b (A)
˚
c (A)
a (8)
b (8)
90
90
90
90
g (8)
3
V (A )
˚
5220.6(4)
4
1.528
11185.8(9)
8
1.693
Z
rcalcd (g cmꢂ3
)
1.714
4.755
m(MoÁ
R(Fo) a
Rw(Fo2) b
/
Ka) (mmꢂ1
)
4.849
0.058
0.148
Á/
0.039
0.081
0.975
0.053
0.120
0.952
/
/
Goodness-of-fit
1.016
a
Rꢀ
/
a ½½Fo½ꢂ½Fc½½=a ½Fo½ with Fo2 ꢁ2s(Fo2)
Rwꢀ
[a w(½Fo2½ꢂ½Fc2½)2=a w½Fo2½2]1=2
/.
b
/
:/
hydrogen atoms were refined with anisotropic thermal
parameters unless otherwise indicated. The highest peak
compositional disorder between [BF4]ꢂ (0.47) and a
small amount of Clꢂ (0.03). The charge balance for 3 is
satisfactory and is confirmed by the conductivity
properties of these crystals (1:1 electrolyte). The source
of Clꢂ is probably from the CH2Cl2 solvent. Although
the treatment of the anion refinement is not entirely
satisfactory, the identity of the chemically important
dirhenium cation is clearly established. Crystallographic
data for 3 are as follows: formula weight, 1342.00; space
in the final difference Fouriers of 1, 2×
/
2THF, 2×
/
1.734CH2Cl2 and 3 had a height of 1.95, 1.57, 2.41
and 2.10 e Aꢂ3, respectively.
˚
The structure solution and refinement of 1 proceeded
routinely. A small amount of disordered lattice solvent
(presumably toluene) could not be satisfactorily mod-
eled and was removed with the squeeze option in
PLATON [14]. The dirhenium unit is located on a twofold
group, C2/c (No. 15); aꢀ
/
21.3403(4); bꢀ
104.9700(10)8; Vꢀ
1.675 g cmꢂ3; mꢀ
collected 27 877, data with I ꢁ2.0s(I) 6511; R(Fo)ꢀ
0.050; Rw(Fo2)
0.100; Goodness-of-fitꢀ0.930.
/
14.3708(3);
3
˚
˚
cꢀ
/
35.9259(8) A, bꢀ
/
/
10643.7(7) A ;
rotation axis that bisects the Reꢀ
/
Re bond. The dirhe-
Zꢀ/8; rcalcd
ꢀ
/
/
4.802 mmꢂ1, data
nium unit that is present in crystals of 2×
/2THF is located
/
/
on a center of inversion and the asymmetric unit also
contains a full molecule of THF solvent. The second
/
ꢀ
/
/
crystal of 2, of composition 2×
/
1.734CH2Cl2, contains
two independent molecules of CH2Cl2 solvent that were
allowed to refine freely to occupancies of 0.931 and
0.803. The dirhenium unit has no crystallographically
imposed symmetry. In the case of 3, several problems
were encountered in the refinement. The dirhenium
cation itself posed no special problems although three
of the carbon atoms of one of the phenyl rings of a m-
dppm ligand were disordered; the resulting six ‘half’
carbon atoms (with multiplicities of 0.55 or 0.45) were
refined with isotropic thermal parameters. An unidenti-
fied and badly disordered solvent molecule was present
in this crystal but the disorder could not be adequately
modeled. Accordingly, the molecule was removed with
the squeeze option in PLATON [14]. The most significant
problem arose in the modeling and refinement of the
counter anions that were present in the asymmetric unit.
There are two independent anions located at special
positions, one of which refined satisfactorily as Clꢂ at
0.50 occupancy and the other that was treated as a
3. Results and discussion
The reaction of Re2Cl4(m-dppm)2 with CH3Li in
toluene affords the tetramethyl complex Re2(CH3)4(m-
dppm)2 (1) in high yield. Interestingly, this same product
is formed from the reaction of the edge-sharing biocta-
hedral dirhenium(III) complex Re2Cl6(m-dppm)2 with
CH3Li with the use of similar reaction conditions albeit
with a longer reaction time. The latter reaction may
proceed through the intermediacy of an unstable inter-
mediate Re2(CH3)6(m-dppm)2 which reductively elimi-
nates ethane. The characterization of
1
was
straightforward. The H-NMR spectrum (recorded in
CDCl3) has a broad pentet at d ꢁ0.75 that is assigned
to the four methyl groups; in toluene-d8 this resonance is
at d ꢁ 3.6 Hz).
1.14 and is better resolved (3JHꢀP
Compound 1 shows a singlet in its 31P-NMR spectrum
(recorded in CDCl3) at d ꢁ8.2, similar to that reported
1
/
/
ꢀ
/
/

M. Ganesan et al. / Journal of Organometallic Chemistry 671 (2003) 166Á
/171
169
for the Re2Cl4(m-dppm)2 precursor [7]. In the cyclic
voltammogram of 1 (recorded for a solution in 0.1 M n-
Mo2(CH3)4(PR3)4 (PR3ꢀPMe3 or PMe2Ph) have been
/
reported [18] but, as might be expected [19], these
compounds have eclipsed rotational geometries.
The tetramethyl complex 1 reacts in toluene solution
Bu4NPF6ÁCH2Cl2) there are two one-electron redox
/
processes associated with the [Re2]5ꢁ/[Re2]4ꢁ and
[Re2]6ꢁ/[Re2]5ꢁ couples that are characterized by values
with CO at room temperature to give an orangeÁyellow
/
complex 2 identified as the di-m-methylene complex
Re2(m-CH2)2(CO)4(m-dppm)2. The IR spectrum of 2
of E1/2
respectively. For the complex Re2Cl4(m-dppm)2 both
processes are reversible and have E1/2 values of ꢁ0.27
0.80 V versus SCE [7]. The shift in potentials
presumably reflects the effect of replacing Clꢂ by CH3ꢂ;
the latter being a better s-donor which leads to a more
electron-rich dirhenium core that is more easily oxi-
dized.
A single crystal X-ray structure determination of 1
confirms its close structural relationship to Re2Cl4(m-
dppm)2 [15,16]. The asymmetric unit contains one-half
of the dirhenium unit. An ORTEP representation [17] of
the structure of 1 is shown in Fig. 1 along with the
ꢀ
/
ꢂ
/
0.14 and Ep,a
ꢀ
/
ꢁ0.59 V versus Ag/AgCl,
/
shows the presence of n(CO) modes (1920Á1850
/
/
cmꢂ1) that can be assigned to terminal carbonyl ligands
and precludes the presence of bridging carbonyls. The
presence of two equivalent m-CH2 groups in 2 is
evidenced in the 1H-NMR spectrum (recorded in
and ꢁ
/
CDCl3) by a pentet at d ꢁ
/
8.42 (integrating to 4H and
3
with JHꢀP
ꢀ
/
7.8 Hz)). This chemical shift is similar to
those reported for the cis and trans isomers of (h5-
C5Me5)2Rh2(m-CH2)2(CH3)2 (d ꢁ
spectively) [20]. The 31P{1H}-NMR spectrum of 2
(recorded in CDCl3) has a singlet at d ꢂ4.4 that reflects
its symmetrical structure. This is confirmed by X-ray
structure determinations of crystals of compositions 2×
2THF and 2×1.734CH2Cl2 that contain different crystal-
/
8.03 and ꢁ
/
8.10, re-
/
important bond distances and bond angles. The Reꢀ
/
Re
˚
triple bond distance of 2.284(7) A is similar to the values
/
/
reported for different crystalline forms of Re2Cl4(m-
˚
2.250 A) [15,16]. Complex 1, like
lization solvent molecules. Both edge-sharing bioctahe-
dral structures are similar but not identical. The ORTEP
representation [17] of the dirhenium molecule present in
dppm)2 (range 2.231Á
/
Re2Cl4(m-dppm)2, has a staggered rotational geometry,
which is typical of triply-bonded complexes of the type
2×
distances and angles are given in the caption to this
figure. Full details of the structure of 2×1.734CH2Cl2 are
/
2THF is shown in Fig. 2; the important bond
Re2X4(m-dppm)2. The four torsion angles Cl(1)ꢀ
Re(a)ꢀC(1a), C(2)ꢀReꢀRe(a)ꢀC(2a), P(1)ꢀReꢀRe(a)ꢀ
P(2a) and P(2)ꢀReꢀRe(a)ꢀP(1a) average to 42.28; the
/
Reꢀ
/
/
/
/
/
/
/
/
/
/
/
/
average torsion angle in the non-disordered structure of
Re2Cl4(m-dppm)2 is 50.38 [16]. The structures of the
quadruply bonded dimolybdenum(II) complexes
Fig. 2. ORTEP [17] representation of the structure of the dirhenium(II)
complex Re2(m-CH2)2(CO)4(m-dppm)2 as present in crystals of 2×
/
2THF. Thermal ellipsoids are drawn at the 50% probability level
except for the carbon atoms of the phenyl rings which are circles of
arbitrary radius. Unlabeled atoms are related to the equivalent labeled
Fig. 1. ORTEP [17] representation of the structure of the dirhenium(II)
complex Re2(CH3)4(m-dppm)2 (1). Thermal ellipsoids are drawn at the
50% probability level except for the carbon atoms of the phenyl rings
˚
atoms by an inversion center. Selected bond distances (A) and bond
angles (8) are as follows: Reꢀ
C(2) 1.945(7), ReꢀC(11) 2.257(6), Re(a)ꢀ
2.4023(14), ReꢀP(2) 2.4205(14), C(1)ꢀO(1) 1.099(7), C(2)ꢀ
1.099(7); ReꢀC(11)ꢀRe(a) 80.77(18), C(1)ꢀReꢀC(2) 86.7(3), C(1)ꢀ
ReꢀC(11) 79.8(3), C(1)ꢀReꢀC(11a) 178.3(2), C(2)ꢀReꢀC(11)
165.8(2), C(2)ꢀReꢀC(11a) 94.4(2), P(1)ꢀReꢀP(2) 175.33(4), Reꢀ
C(1)ꢀO(1) 172.7(7), ReꢀC(2)ꢀO(2) 175.3(6).
/
Re(a) 2.9931(5), Reꢀ
C(11) 2.361(5), Reꢀ
O(2)
/
C(1) 1.947(7), Reꢀ
/
which are circles of arbitrary radius. This molecule contains a two-fold
˚
/
/
/
P(1)
rotation axis that bisects the Reꢀ
/
Re bond. Selected bond distances (A)
/
/
/
and bond angles (8) are as follows: Reꢀ
2.169(8), ReꢀC(2) 2.154(9), ReꢀP(1) 2.380(2), Reꢀ
C(1)ꢀReꢀC(2) 121.9(4), C(1)ꢀReꢀRe(a) 118.6(3), C(2)ꢀ
119.5(2), P(1)ꢀReꢀP(2) 173.88(8).
/
Re(a) 2.2841(7), Reꢀ
P(2) 2.378(2);
ReꢀRe(a)
/
C(1)
/
/
/
/
/
/
/
/
/
/
/
/
/
/
/
/
/
/
/
/
/
/
/
/
/
/
/
/
/

170
M. Ganesan et al. / Journal of Organometallic Chemistry 671 (2003) 166Á171
/
available as Section 4. The Reꢀ
2THF and 2×1.734CH2Cl2 are 2.9931(5) and 2.8930(5)
A, respectively, values that are consistent with the
presence of ReꢀRe single bonds which in turn accord
with 18-electron counts for the Re atoms in these
compounds. The ReꢀC bond lengths within the [Re(m-
CH2)2Re] units are 2.257(6) and 2.361(5) A for 2×
/
Re bond distances for 2×
/
head methylene carbon atoms of the dppm ligands that
renders the ꢀCH2ꢀ protons equivalent.
/
/
/
˚
In the conversion of 1 to 2 we observed no other
rhenium-containing intermediates or products (as mon-
itored by 1H-NMR in toluene-d8). Furthermore, the
formation of acetone, a possible reaction by-product,
/
/
1
˚
was not detected by H-NMR spectroscopy or GS/MS
/
2THF,
2.243(8) A in the case of
1.734CH2Cl2. Other parameters are similar between
˚
but the initial appearance of a weak singlet at d ꢁ0.18
/
and are in the range 2.148(12)Á
/
1
in the H-NMR spectrum of the reaction solution, that
decreased in intensity with reaction time, could be due to
the formation of CH4 or C2H6 as a by-product. Since H2
was not detected we suggest that the reaction stoichio-
metry may be that shown in Eq. (1).
2×
/
these two structures with one notable exception, namely,
the conformation of the [Re2(m-dppm)2] unit (Fig. 3). In
the case of 2×
/
2THF, which has a crystallographic
inversion center, this conformation is required to be
chair-like (Fig. 3a). For 2×1.734CH2Cl2, which has no
/
Re2(CH3)4(m-dppm)2 ꢁ4CO
crystallographically imposed symmetry, a boat confor-
mation is present (Fig. 3b). These different ring con-
formation, and associated differences in ring strain,
0 Re2(m-CH2)2(CO)4(m-dppm)2 ꢁ2CH4
The CV of a solution of 2 in 0.1 M n-Bu4NPF6Á
CH2Cl2 shows a reversible one-electron oxidation with
an E1/2 value of ꢂ0.14 V versus Ag/AgCl. Treatment of
solution of
in dichloromethane with [(h5-
(1)
/
could explain the variations in Reꢀ/CH2 distances
/
between these two structures. This is only the second
time, as far as we are aware, that such boat and chair
isomeric forms have been found for compounds that
contain a [M2(m-dppm)2] unit. The previous example
was encountered in the case of salts of the unsymme-
trical edge-sharing bioctahedral cation [Re2(m-Br)2(m-
dppm)2Br(CO)(CNXyl)2]ꢁ [21]. This difference in con-
formation presumably accounts (at least in part) for the
a
2
C5H5)2Fe]BF4 afforded the oxidized species [2]ꢁ in
low yield, admixed with other unidentified species.
Although this reaction is obviously complex we were
able to structurally characterize a crystal of composition
close to [Re2(m-CH2)2(CO)4(m-dppm)2]Cl0.5(BF4)0.5 (3).
Full structural details are not reported here since we are
not entirely satisfied with our modeling of the anion;
however this structural information is available as
Section 4. The structure of the edge-sharing bioctahedral
difference found for the Reꢀ
/
Re distance in these two
0.10 A). The NMR spectral properties of
these two crystalline forms are identical. Since the ꢀ
CH2ꢀ groups of the dppm ligands appear as a pentet
in the NMR spectrum (2JHꢀP
4.5 Hz) this is consistent
˚
structures (ꢀ
/
/
cation is similar to that present in the crystal of 2×
1.734CH2Cl2, in which the [Re2(m-dppm)2] unit assumes
a boat conformation. The ReꢀRe distance of 2.8204(4)
/
/
ꢀ
/
/
with only the chair isomer being present in solution or
with the occurrence of rapid inversion about the bridge-
˚
˚
A in 3 is shorter (by 0.07 A) than that present in this
crystal of 2, implying that the electron removed in the
oxidation of 2 to 3 is lost from a non-bonding or weakly
anti-bonding metal based orbital.
4. Supplementary material
Crystallographic data for the structural analysis have
been deposited with the Cambridge Crystallographic
Data Centre, CCDC Nos. 200914Á
/
200917 for com-
pounds 1, 2×2THF, 2×1.734CH2Cl2 and 3, respectively.
/
/
Copies of the information may be obtained free of
charge from The Director, CCDC, 12 Union Road,
Cambridge CB2 1EZ, UK (Fax: ꢁ44-1223-336033;
/
e-mail: deposit@ccdc.ac.uk or www: http://www.
ccdc.cam.ac.uk).
Acknowledgements
Fig. 3. A comparison of the [Re2(m-dppm)2] units present in the
crystals of: (a) 2×
ReÁRe axis to show the chair conformation in (a) and boat
conformation in (b).
/
2THF; and (b) 2×1.734CH2Cl2 as viewed down the
/
We thank the John A. Leighty Endowment Fund for
support of this work.
/

M. Ganesan et al. / Journal of Organometallic Chemistry 671 (2003) 166Á
/171
171
System, Crystallography Laboratory, University of Nijmegen,
Netherlands, 1998.
References
[11] A. Altomare, M.C. Burla, M. Camalli, G.L. Cascarano, C.
Giacovazzo, A. Guagliardi, A.G.G. Moliterni, G. Polidori, R.
Spagna, J. Appl. Cryst. 32 (1999) 115.
[1] See, for example, F.A. Cotton, R.A. Walton, Multiple Bonds
Between Metal Atoms, 2nd ed., Oxford University Press, Oxford,
1993, pp. 69Á111.
/
[12] Z. Otwinowski, W. Minor, Methods Enzymol. 276 (1996)
307.
[13] G.M. Sheldrick, SHELXL-97, A Program for Crystal Structure
[2] P. Edwards, K. Mertis, G. Wilkinson, M.B. Hursthouse, K.M.
Abdul Malik, J. Chem. Soc. Dalton Trans. (1980) 334.
[3] F.A. Cotton, L.D. Gage, K. Mertis, L.W. Shive, G. Wilkinson, J.
Am. Chem. Soc. 98 (1976) 6922.
Refinement, University of Gottingen, Germany, 1997.
¨
[14] P.V.D. Sluis, A.L. Spek, Acta Crystallogr. Sect. A 46 (1990)
194.
[4] R.A. Jones, G. Wilkinson, J. Chem. Soc. Dalton Trans. (1978)
1063.
[15] T.J. Barder, F.A. Cotton, K.R. Dunbar, G.L. Powell, W.
Schwotzer, R.A. Walton, Inorg. Chem. 24 (1985) 2550.
[16] J.K. Bera, P.E. Fanwick, R.A. Walton, Inorg. Chim. Acta 31
(2000) 138.
[5] R.A. Jones, G. Wilkinson, J. Chem. Soc. Dalton Trans. (1979)
472.
[6] A.R. Cutler, D.R. Derringer, P.E. Fanwick, R.A. Walton, J. Am.
Chem. Soc. 110 (1988) 5024.
[17] C.K. Johnson, ORTEP-II, Rep. ORNL-5138, Oak Ridge National
Laboratory, TN, 1976.
[7] T.J. Barder, F.A. Cotton, D. Lewis, W. Schwotzer, S.M. Tetrick,
R.A. Walton, J. Am. Chem. Soc. 106 (1984) 2882.
[8] D.N. Hendrickson, Y.S. Sohn, H.B. Gray, Inorg. Chem. 10 (1971)
1560.
[18] F.A. Cotton, K.J. Wiesinger, Inorg. Chem. 29 (1990) 2594.
[19] See, for example, F.A. Cotton, R.A. Walton, Multiple Bonds
Between Metal Atoms, 2nd ed., Oxford University Press, Oxford,
[9] P.T. Beurskens, G. Admirall, G. Beurskens, W.P. Bosman, S.
Garcia-Granda, R.O. Gould, J.M.M. Smits, C. Smykalla, The
DIRDIF-92 Program System, Technical Report, Crystallography
Laboratory, University of Nijmegen, Netherlands, 1992.
[10] P.T. Beurskens, G. Beurskens, R. de Gelder, S. Garcia-Granda,
R.O. Gould, R. Israel, J.M.M. Smits, The DIRDIF-99 Program
1993, pp. 19Á20.
/
[20] P.M. Maitlis, J. Organomet. Chem. 500 (1995) 239.
[21] (a) W. Wu, P.E. Fanwick, R.A. Walton, J. Chem. Soc. Chem.
Commun. (1997) 755;
(b) W. Wu, P.E. Fanwick, R.A. Walton, Inorg. Chem. 37 (1998)
3122.