Unusual chemical reactivity in the reactions of Re2(CO) 8(μ-H)(μ-η)1,η (see abstract)

Article abstract of DOI:10.1016/j.jorganchem.2003.10.043

The reaction between Re₂(CO)₈(μ-H) (ν-η¹,η²-CH=CHBu) (1) and the diphosphine ligand 2,3-bis(diphenylphosphino)maleic anhydride (bma) proceeds slowly at room temperature to furnish the expected dirhenium compound Re₂(CO)₈(bma) (2) and the zwitterionic compound Re(CO)₄[Re(CO)₄(bma)] (3). Both 2 and 3 have been isolated and characterized in solution by IR and ³¹P NMR spectroscopy, in addition to X-ray crystallography. The solid-state structure of 2 reveals a bridging bma ligand that spans the Re-Re vector and whose Re(CO)₄ moieties exhibit a staggered rotational geometry. 3 consists of a Re(CO)₄ unit that is coordinated to the bma ligand in a chelating fashion by the two PPh₂ moieties, with the other Re(CO)₄ unit being π-bound to the alkene bond of the bma ligand. Thermolysis of Re₂(CO)₈(bma) in the presence of added Ni(cod) ₂ affords the phosphido-bridged complex Re₂(CO)₈[μ-η¹, η¹-C=C(PPH₂C(O)OC(O)](μ-PPh₂) (4), whose identity has been established by solution spectroscopic methods and X-ray diffraction analysis. The redox properties of Re₂(CO)₈(bma) have been examined in detailed by cyclic voltammetry and constant-potential coulometry, where two fully reversible, one-electron reductions (0/1^- and 1^-/2^-) were observed at E_1/2=-0.38 V and E_1/2=-1.29 V in CH₂Cl₂. The reduction behavior is discussed relative to the nature of the LUMO level, which has been determined by extended Hu?ckel MO calculations. The redox chemistry and the LUMO in Re₂(CO) ₈(bma) are contrasted with the known redox chemistry and the LUMO composition of the mononuclear species BrRe(CO)₃(bma) and related bma-substituted complexes. The MO calculations on Re(CO) ₄[Re(CO)₄(bma)] (3) reveal that the HOMO and LUMO levels are formed from the HOMO of the d₈-ML₄ fragment Re(CO)₄^- and its interaction with the ψ₄π* antibonding MO of the bma ligand on the [Re(CO)₄(bma)]⁺ fragment.

Full text of DOI:10.1016/j.jorganchem.2003.10.043

Journal of Organometallic Chemistry 689 (2004) 791–800
www.elsevier.com/locate/jorganchem
Unusual chemical reactivity in the reactions of Re2(CO)8(l-H)
(l-g1,g2-CH@CHBu) with 2,3-bis(diphenylphosphino)maleic
anhydride (bma) and Re2(CO)8(bma) with Ni(cod)2:
X-ray diffraction structures of Re2(CO)8(bma),
zwitterionic Re(CO)4[Re(CO)4(bma)], and
the phosphido-bridged complex
Re2(CO)8[l-g1,g1-
(O)](l-PPh2)
a,
b
b,
*
Simon G. Bott *, Kaiyuan Yang , Michael G. Richmond
a
Department of Chemistry, University of Houston, Houston, TX 77204, USA
Department of Chemistry, University of North Texas, Denton, TX 76203, USA
b
Received 9 July 2002; accepted 22 October 2003
Abstract
The reaction between Re2(CO)8(l-H)(m-g1,g2-CH@CHBu) (1) and the diphosphine ligand 2,3-bis(diphenylphosphino)maleic
anhydride (bma) proceeds slowly at room temperature to furnish the expected dirhenium compound Re2(CO)8(bma) (2) and the
zwitterionic compound Re(CO)4[Re(CO)4(bma)] (3). Both 2 and 3 have been isolated and characterized in solution by IR and 31P
NMR spectroscopy, in addition to X-ray crystallography. The solid-state structure of 2 reveals a bridging bma ligand that spans the
Re–Re vector and whose Re(CO)4 moieties exhibit a staggered rotational geometry. 3 consists of a Re(CO)4 unit that is coordinated
to the bma ligand in a chelating fashion by the two PPh2 moieties, with the other Re(CO)4 unit being p-bound to the alkene bond of
the bma ligand. Thermolysis of Re2(CO)8(bma) in the presence of added Ni(cod)2 affords the phosphido-bridged complex
Re2(CO)8[l-g1,g1-
(O)](l-PPh2) (4), whose identity has been established by solution spectroscopic methods and X-
ray diffraction analysis. The redox properties of Re2(CO)8(bma) have been examined in detailed by cyclic voltammetry and constant-
potential coulometry, where two fully reversible, one-electron reductions (0/1ꢀ and 1ꢀ/2ꢀ) were observed at E1=2 ¼ ꢀ0:38 V and
E1=2 ¼ ꢀ1:29 V in CH2Cl2. The reduction behavior is discussed relative to the nature of the LUMO level, which has been determined
€
by extended Huckel MO calculations. The redox chemistry and the LUMO in Re2(CO)8(bma) are contrasted with the known redox
chemistry and the LUMO composition of the mononuclear species BrRe(CO)3(bma) and related bma-substituted complexes. The
MO calculations on Re(CO)4[Re(CO)4(bma)] (3) reveal that the HOMO and LUMO levels are formed from the HOMO of the d8-
ML4 fragment Re(CO)4ꢀ and its interaction with the w4pꢁ antibonding MO of the bma ligand on the [Re(CO)4(bma)]þ fragment.
Ó 2003 Published by Elsevier B.V.
€
Keywords: Dirhenium compounds; Diphosphine ligand; Redox chemistry; Extended Huckel MO calculations
1. Introduction
enylphosphino)maleic anhydride (bma) and its close
relative 4,5-bis(diphenylphosphino)-4-cyclopenten-1,3-
dione (bpcd) have been duly demonstrated in recent
years [1]. The utility of bma- and bpcd-substituted
complexes to serve as electron reservoirs in chemical and
electrochemical reduction sequences and the ability of
the corresponding 18 þ d complexes to modulate disso-
ciative CO loss in sundry metal-carbonyl compounds
The diverse coordination chemistry and reactivity
exhibited by the diphosphine ligand 2,3-bis(diph-
* Corresponding authors. Tel.: +1-713-743-2771 (S.G. Bott), +1-940-
565-3548 (M.G. Richmond).
E-mail addresses: sbott@uh.edu (S.G. Bott), cobalt@unt.edu
(M.G. Richmond).
0022-328X/$ - see front matter Ó 2003 Published by Elsevier B.V.
doi:10.1016/j.jorganchem.2003.10.043

792
S.G. Bott et al. / Journal of Organometallic Chemistry 689 (2004) 791–800
have been thoroughly explored [2]. One of our main
interests in this genre of diphosphine ligand derives from
our initial observation of facile P–C bond cleavage that
often accompanies the attachment of these ligands to
polynuclear metal clusters, as depicted in Eq. (1) for the
tricobalt cluster PhCCo3(CO)7(bma) [3]. Besides the
facile activation of this ligand at a wide variety of
polynuclear clusters by both C(maleic anhydride)–P and
Ph–P manifolds [4], we have also observed that this
particular ligand is capable of ambulatory migration
about a cluster polyhedron via discernable bridging and
chelating phosphine isomers. The dynamic isomerism
exhibited by the bma ligand, in the documented para-
digms to date, proceeds by an intramolecular path that
does not require CO loss.
NaReO4 and CO, employing the carbonylation proce-
dure of Heinekey [9]. The hexenyl compound
Re2(CO)8(l-H)(l-g1,g2-CH@CHBu) (1) was obtained
from the photochemical reaction of Re2(CO)10 and 1-
hexene, as originally described by Brown [10]. Cp2Co
was prepared according the procedure of King [11]. The
1-hexene required for the synthesis of compound 1 was
passed across a column of activated alumina immedi-
ately before its use. All reaction, NMR, and electro-
chemistry solvents were distilled under argon from a
suitable drying agent and stored in Schlenk storage
vessels [12]. The tetra-n-butylammonium perchlorate
electrolyte (TBAP) was purchased from Johnson Mat-
they Electronics and recrystallized from ethyl acetate/
hexane (1:1), followed by drying for at least 48 h under
high vacuum. All C and H analyses were performed by
Altantic Microlab, Norcross, GA.
The reported infrared data were recorded on a Nic-
olet 20 SXB FT-IR spectrometer in 0.1 mm NaCl cells,
using PC control and OMNIC software, while the 31P
NMR spectra were recorded at 121 MHz on a Varian
300-VXR spectrometer. The reported 31P chemical shifts
are referenced to external H3PO4 (85%), taken to have
d ¼ 0:0. Here positive chemical shifts are to low field of
the external standard.
ð1Þ
Several years ago we published our results on the
synthesis and redox investigation of the rhenium com-
plexes fac-BrRe(CO)3(bma)0=1ꢀ. Here the effects of
electron accession in going from the neutral starting
material to the reduced 18 þ d complex, fac-
BrRe(CO)3(bma)1ꢀ, were fully analyzed by IR spec-
troscopy, X-ray crystallography, and MO calculations
[5]. Given the dearth of structurally characterized rhe-
nium complexes that contain an ancillary bma ligand,
we have embarked on the synthesis and study of other
such rhenium complexes [6]. Herein we present our data
on the reaction between the activated hexeny compound
Re2(CO)8(l-H)(l-g1,g2-CH@CHBu) and bma, which
affords Re2(CO)8(bma) (2) and Re(CO)4[Re(CO)4(bma)]
2.2. Synthesis of Re2(CO)8(bma) and Re(CO)4[Re-
(CO)4(bma)]
To 2.00 g (3.07 mmol) of Re2(CO)10 in an Ace Glass
photochemical reactor under argon flush was added ca.
250 ml of 1-hexene. The solution was irradiated at 254
nm until the conversion to Re2(CO)8(l-H)(l-g1,g2-
CH@CHBu) was judged to be complete by IR spec-
troscopy (ca. 2 h), after which time the solution was
transferred to a large Schlenk flask and the 1-hexene was
removed under vacuum. At this point, 200 ml of CH2Cl2
was added, followed by 1.50 g (3.22 mmol) of bma. The
reaction was protected from room light and stirred for
at least two weeks, during which time the solution ac-
quired a red color. The progress of the reaction was
assessed by periodically monitoring the solution by TLC
and IR analyses. Upon the consumption of the starting
rhenium compound (>95%), the CH2Cl2 solvent was
removed and the crude material was extracted with
petroleum ether until the remaining residue was pale
yellow in color. Since compounds 2 and 3 exhibit sub-
stantial decomposition during chromatographic sepa-
ration over silica gel at room temperature (vide supra),
the desired products were isolated in pure form by
low-temperature ()78 °C) column chromatography.
Re2(CO)8(bma) was obtained from the column using
CH2Cl2 as the eluent, followed by Re(CO)4[Re-
(CO)4(bma)] when the eluent was changed to CH2Cl2/
acetone (9:1). Single crystals of 2 and 3 suitable for
combustion analysis and X-ray diffraction examination
(3). The cleavage of the P–C bond in
added Ni(cod)2 gives the corresponding phosphido-
2 by
bridged dirhenium compound Re2(CO)8[l-g1,g1-
(O)](l-PPh2) (4). Compounds 2–4
have been fully characterized in solution and their mo-
lecular structures established by X-ray crystallography.
2. Experimental
2.1. General methods
The bma ligand used in these studies was synthesized
from 2,3-dichloromaleic anhydride (Aldrich Chemical
Co.) and Ph2PTMS according to the known procedure
[7]. The latter chemical was prepared from Ph2PH and
TMSCl [8], while the Re2(CO)10 was prepared from

S.G. Bott et al. / Journal of Organometallic Chemistry 689 (2004) 791–800
793
were grown from CH2Cl2 solutions containing these
compounds that had been layered with hexane. Yield of
yellow 2: 0.65 g (19.9%). IR (CH2Cl2): m(CO) 2076 (s),
2025 (s), 1985 (vs), 1959 (m), 1926 (s), 1828 (vw, anti-
symm anhydride carbonyl), 1775 (s, symm anhydride
All non-hydrogen atoms in 3 were refined anisotropi-
cally with the exception of the ancillary phenyl carbons
and some of the carbon atoms belonging to the carbonyl
groups and the maleic anhydride moiety. The asym-
metric unit of 4 contains half a molecule of hexane that
resides on an inversion center. Refinement of compound
4 followed similar data reduction procedures, with all
non-hydrogen atoms except for the phenyl and solvent
carbon atoms being refined anisotropically. Refinement
on 2 converged at R ¼ 0:0461 and Rw ¼ 0:0500 for 3382
unique reflections with I > 3rðIÞ, while for 3 refinement
converged at R ¼ 0:0439 and Rw ¼ 0:0529 for 2541 un-
ique reflections with I > 3rðIÞ. The refinement for 4
converged at R ¼ 0:0345 and Rw ¼ 0:0381 for 3163 un-
ique reflections with I > 3rðIÞ.
carbonyl) cmꢀ1 31P NMR (CH2Cl2): d – 9.37. Anal.
.
Calc. (found) for C36H20O11P2Re2: C, 40.68 (40.56); H,
1.90 (1.89). Yield of colorless 3: 0.80 g (24.5%). IR
(CH2Cl2): m(CO) 2108 (m), 2080 (m), 2014 (vs), 1996
(vs), 1962 (vs), 1948 (vs), 1781 (w, antisymm anhydride
carbonyl), 1723 (m, symm anhydride carbonyl) cmꢀ1
.
31P NMR (CH2Cl2): d 39.07. Anal. Calc. (found) for
C36H20O11P2Re2: C, 40.68 (40.56); H, 1.90 (1.95).
2.3. Synthesis of Re2(CO)8[l-g1,g1-
(O)](l-PPh2)
2.5. Extended H€uckel MO calculations
To 0.20 g (0.19 mmol) of Re2(CO)8(bma) in 35 ml of
toluene was added 0.05 g (0.18 mmol) of Ni(cod)2. The
reaction solution was heated at 60–70 °C for less than
1.0 h, followed by TLC analysis in CH2Cl2/petroleum
ether (1:1), which revealed the presence of a yellow spot
ðRf ¼ 0:47Þ and much material that remained at the
origin. The material at the origin showed no propensity
for migration using a wide variety of developing sol-
vents. The yellow band was isolated by chromatography
over silica gel using CH2Cl2/petroleum ether (1:1) as the
eluent. Single crystals of 4 suitable for combustion
analysis and X-ray diffraction examination were grown
from CH2Cl2 solutions containing 4 that had been lay-
ered with hexane. Yield of 4: 30.0 mg (15.0%). IR
(CH2Cl2): m(CO) 2103 (w), 2088 (m), 2039 (m), 2007
(vs), 1976 (vs), 1807 (vw, antisymm anhydride carbonyl),
€
The extended Huckel calculations on Re2(CO)8
(bma), Re(CO)4[Re(CO)4(bma)], and Re2(CO)8[l-
g1,g1-
(O)](l-PPh2) were conducted
with the original program developed by Hoffmann [13],
as modified by Mealli and Proserpio [14], using weighted
HÕijs. The input Z-matrix for the model compounds
Re2(CO)8(H4-bma), Re(CO)4[Re(CO)4(H4-bma)], and
Re2(CO)8[l-g1,g1-
(O)](l-PH2)
was
constructed from the X-ray fractional coordinates of
each compound. Here the phenyl groups were replaced
by hydrogens in order to simplify the calculations, with
ꢀ
the P–H bond lengths assigned a value of 1.42 A [15].
2.6. Cyclic voltammetry data
The electrochemical data (CV and CPC) were re-
corded on a PAR Model 273 potentiostat/galvanostat,
equipped with positive feedback circuitry to compensate
for iR drop. The airtight CV cell used was based on a
three-electrode design, and a platinum disk was em-
ployed as the working and auxiliary electrodes. The
reference electrode utilized a silver wire as a quasi-ref-
erence electrode, and the reported potential data are
referenced to the formal potential of the Cp2Fe/Cp2Feþ
1743 (m, symm anhydride carbonyl) cmꢀ1 31P NMR
.
(CH2Cl2): d 100.01 (JP–P ¼ 19:7 Hz), 10.50 (JP–P ¼ 19:7
Hz). Anal. Calc. (found) for C36H20O11P2Re2: C, 40.68
(40.54); H, 1.90 (1.97).
2.4. X-ray diffraction structures of compounds 2–4
Selected crystals of 2–4 suitable for X-ray diffraction
analysis were grown as described above and were each
sealed inside a Lindemann capillary, followed by
mounting on an Enraf–Nonius CAD-4 diffractometer.
After the cell constants were obtained for all three
samples, intensity data in the range of 2° 6 2h 6 44° were
collected at 298 K and were corrected for Lorentz, po-
larization, and absorption (DIFABS). All structures
were solved by using standard Patterson techniques,
which revealed the positions of the rhenium and phos-
phorus atoms. In the case of 2, all non-hydrogen atoms
were located with difference Fourier maps and full-
matrix least-squares refinement and were refined aniso-
tropically with the exception of the phenyl carbon atoms
and the oxygen atoms associated with carbonyl groups.
(internally added) redox couple, taken to have E1=2
0:307 V [16].
¼
3. Results and discussion
3.1. Synthesis, spectroscopic data, and X-ray diffraction
structures
The reaction of Re2(CO)8(l-H)(l-g1,g2-CH@CHBu)
with bma in CH2Cl2 proceeds slowly to produce the
rhenium compounds Re2(CO)8(bma) (2) and Re(CO)4
[Re(CO)4(bma)] (3). The progress of these reactions was
ascertained by TLC and IR analyses of the reaction

794
S.G. Bott et al. / Journal of Organometallic Chemistry 689 (2004) 791–800
solution, with maximum conversion to compounds 2 and
3 being achieved in ca. two weeks time at room temper-
ature. It is our opinion that the long reaction times re-
quired for the formation of 2 and 3 are due to the low
nucleophilicity of the bma ligand. In contrast, the di-
phosphine ligands dppm, dppe, and (Z)-Ph2PCH@
CHPPh2 undergo essentially quantitative substitution
with 1 to give Re2(CO)8(P–P) within a few hours [17]. The
direct reaction of Re2(CO)10 with bma in the presence of
added Me3NO was also examined as a route to compound
2. However, the reaction of Me3NO with the diphosphine
ligand led to the decomposition of the bma ligand as ev-
idenced by IR and TLC analyses. Compounds 2 and 3
were subsequently isolated by low-temperature column
chromatography over silica gel, albeit with considerable
material loss due to the silica gel-induced hydrolysis of the
anhydride ring in each compound. We have encountered
this behavior before during the chromatographic purifi-
cation of fac-BrRe (CO)3(bma) [5] and other bma-
substituted di- and trimetal compounds [1h,3a,18].
Both 2 and 3 were characterized in solution by stan-
dard methods. The IR spectrum of Re2(CO)8(bma) in
the terminal carbonyl region closely matches that
of the structurally similar compound Re2(CO)8[(Z)-
Ph2PCH@CHPPh2] and other related diphosphine-
substituted derivatives [17]. The m(CO) bands in 2 at 1830
and 1776 cmꢀ1 belong to the vibrationally coupled anti-
symmetric and symmetric anhydride C-O stretches, re-
spectively [19]. These frequencies found for the m(CO)
bands of the bma moiety in 2 are shifted to higher energy
relative to the free ligand and are typical of other bma-
substituted compounds prepared by us. A single 31P res-
onance at d – 9.37 is consistent with equivalent phosphine
groups in solution and is not unlike the chemical shift
found at d – 10.67 for phosphines in Re2(CO)8
[(Z)Ph2PCH@CHPPh2] [20]. The IR spectrum of
Re(CO)4[Re(CO)4(bma)] reveals six prominent m(CO)
bands for the two Re(CO)4 groups, along with bma m(CO)
bands at 1781 and 1723 cmꢀ1, whose shift to lower energy
relative to 2 and free bma signifies the coordination of the
bma bond to one of the Re(CO)4 moieties. The chelation
of the bma ligand to one of the Re(CO)4 centers in 3 gives
rise to the downfield 31P chemical shift at d 39.07 [21].
The photochemical and thermolysis behavior of 2
and 3 was next investigated to probe for possible C–P
bond cleavage reactivity, as documented by us for bma-
substituted Co2, CoRu, and Co3 compounds [1h,3a,18].
Both compounds are stable up to 100 °C in toluene
under argon, and only begin to decompose above this
temperature to unidentified non-carbonyl-containing
material(s). Compound 2 is stable to near-UV irradia-
tion at 366 nm in toluene and does not display P–C
bond activation upon optical excitation unlike other
bma-substituted dinuclear and polynuclear complexes
prepared by our groups [3a,22]. In the case of compound
2, these experiments allow us to rule out the possibility
of Re2(CO)8(bma) serving as a precursor to Re(CO)4
[Re(CO)4(bma)]. No evidence for the formation of 3
from 2 was observed in any of the reactivity studies
carried out. 3 undergoes slow decomposition during
photolysis, as assessed by IR and TLC analyses, and was
not examined any further.
Given the coordination of a Re(CO)4 fragment to the
bma bond in Re(CO)4[Re(CO)4(bma)], we investigated
the reactivity of the free bond in Re2(CO)8(bma) and its
potential to bind unsaturated organometallic fragments
that have a known affinity for alkenes. The reactions
of 2 with Fe(CO)4(THF), W(CO)5(THF), or Ru3
(CO)12 ꢀ n(MeCN)n (where n ¼ 1, 2) were all unsuccessful,
as judged by IR analysis. However, treatment of
Re2(CO)8(bma) with Ni(cod)2 in toluene at 60–70 °C led
to the complete consumption of 2 within minutes. The
central portion of Scheme 1 illustrates the course of this
reaction. TLC analysis of the reaction solution revealed
the presence of a small yellow spot (Rf ¼ 0:5 in CH2Cl2/
petroleum ether) and considerable material at the origin.
Increasing the polarity of the mobile phase did not allow
for the separation/observation of any other compound(s)
from the origin material. The IR spectrum of the crude
reaction solution exhibited an extremely broadened car-
bonyl pattern where the only clearly discernable fre-
quencies were at 2086 and 1986 cmꢀ1. Chromatographic
separation of the yellow spot allowed for the isolation of
the phosphido-bridged complex Re2(CO)8[l-g1,g1-
C@C(PPh2)C(O)OC(O)](l-PPh2) in low yield. The 31P
NMR spectrum of 4 displays two doublet resonances
(JP–P ¼ 19:7 Hz) at 100.01 and 10.50 that are assignable to
the l2-PPh2 and the PPh2(carbocyclic ring) moieties, re-
spectively [23].
The unequivocal identity of 2–4 was established by
X-ray crystallography. Single crystals of each compound
were found to exist as discrete molecules in the unit cell
with no unusually short inter- or intramolecular contacts.
Scheme 1.

S.G. Bott et al. / Journal of Organometallic Chemistry 689 (2004) 791–800
795
ꢀ
ꢀ
Tables 1 and 2 report the X-ray data collection and
processing parameters and selected bond distances and
angles, respectively.
The ORTEP diagram of Re2(CO)8(bma) shown in
Fig. 1 confirms the bridging of the Re–Re vector by the
bma ligand and the staggered arrangement of the equa-
torial groups about the Re–Re bond [24]. The Re(1)–
4.76 A exceeds the normally acceptable 2.90–3.30 A range
that is commonly found for Re–Re single bonds [27].
Coordination of the Re(CO)ꢀ4 moiety to C(11) and C(15)
is accompanied by a sizeable departure from the normally
observed planar environment involving the central Re(1)
atom, the two phosphorus centers, and the maleic anhy-
dride ring. In 3, the observed torsion angles ofca. 135° and
132° for the atoms Re(1)–P(1)–C(11)–C(12) and Re(1)–
P(2)–C(15)–C(14), respectively, are atypical for this di-
phosphine ligand and are the result of coordination of the
Re(CO)ꢀ4 fragment to the C(11) and C(15) centers. The
Re(1)–P(1)–C(11)–Re(2) and Re(1)–P(2)–C(15)–Re(2)
torsion angles are ca. 81° and 86°, respectively. The
ꢀ
Re(2) bond distance of 3.0593(9) A is identical to that
reported for Re2(CO)10 [25] and supports the presenceof a
Re–Re single bond that connects the two square-pyra-
midal Re(CO)4P groups. Coordination of the bma ligand
to the Re2(CO)8 moiety leads to a minor perturbation in
the planarity of the bma core as evidenced by the P(1)–
C(11)–C(15)–P(2) dihedral angle of ca. 13.3. The C(11)–
C(15) carbon–carbon double bond length of 1.32(2) is in
good agreement with the C@C bond distance of simple
alkenes and the analogous bond distance in bma- and
bpcd-substituted complexes prepared by us [26]. The re-
maining bond distances and angles are unexceptional and
do not require comment.
ꢀ
elongation of the C(11)–C(15) bond distance to 1.56(3) A
relative to the free p bond of a bma ligand or a simple
alkene is expected and in keeping with the Duncanson–
Dewar–Chatt model for the bonding between an alkene
and a transition metal [28]. Chelation of the bma ligand to
the Re(1) center gives rise to a bond angle of 83.3(2)° for
P(1)–Re(1)–P(2) atoms. The C(11)–Re(2) [2.26(2)] and
ꢀ
Fig. 2 shows the ORTEP diagram of the zwitterionic
compound Re(CO)4[Re(CO)4(bma)]. Compound 3 may
be conceptualized as arising from the union of the anionic,
16-electron fragment Re(CO)ꢀ4 with the cationic, 18-
electron species Re(CO)4(bma)þ, as facilitated by the free
p bond of the bma ligand in Re(CO)4(bma)þ. Each rhe-
nium is formally six-coordinate and possesses idealized
octahedral geometry, assuming that the p bond bound to
Re(2) functions as a two-coordinate ligand. The non-
bonded rhenium centers are electronically saturated and
possess 18 valence electrons each according to the EAN
rule, and the internuclear Re(1)ꢂ ꢂ ꢂRe(2) bond distance of
C(15)–Re(2) [2.27(2) A] bond distances and the C(11)–
Re(2)–C(15) bond angle of 40.3(7)° are in agreement with
distances and angles reported for several rhenium metal-
locyclopropane complexes [29].
The X-ray structure of Re2(CO)8[l-g1,g1-
(O)](l-PPh2), as the hexane solvate,
confirms the P–C bond cleavage and loss of the Re–
Re bond in Re2(CO)8(bma), as shown in the ORTEP
diagram in Fig. 3. The Re(1)ꢂ ꢂ ꢂRe(2) bond length of
ꢀ
4.36 A precludes any direct bonding interaction be-
tween these centers. The two ReL6 centers display
idealized octahedral geometry and are tethered by a
Table 1
X-ray crystallographic data and processing parameters for the dirhenium compounds 2–4
Compound
2
3
4
Space group
ꢀ
Monoclinic, P21=n
10.1162(8)
27.928(3)
Monoclinic, P21=n
11.888(2)
18.302(2)
16.023(2)
97.67(1)
3462.6(8)
C36H20O11P2Re2
1062.90
4
Monoclinic, C2=c
35.292(3)
10.457(1)
20.976(2)
99.944(7)
7684(1)
C39H27O11P2Re2
1105.99
8
a (A)
ꢀ
b (A)
ꢀ
c (A)
12.5221(9)
95.266(6)
3522.8(6)
b (°)
3
ꢀ
V (A )
Molecular formula
C36H20O11P2Re2
fw
Formula units per cell (Z)
Dcalcd (g/cmꢀ3
k (Mo Ka) (A)
Absorption coefficient (cmꢀ1
1062.90
4
2.004
)
ꢀ
2.039
1.912
0.71073
71.08
0.019
0.71073
72.3
0.031
0.71073
65.21
0.024
)
Rmerge
Abs corr factor
0.88–1.11
4705
4421
0.76–1.51
4634
4398
0.80–1.29
5098
5015
Total reflections
Independent reflections
Data/res/parameters
R
Rw
GOF
Weights
3382/0/285
0.0461
0.0500
1.34
2541/0/330
0.0439
0.0529
3163/0/356
0.0345
0.0381
0.71
0.61
½0:04F 2 þ ðrF Þ ꢃꢀ1
½0:04F 2 þ ðrF Þ ꢃꢀ1
½0:04F 2 þ ðrF Þ ꢃꢀ1
2
2
2

796
S.G. Bott et al. / Journal of Organometallic Chemistry 689 (2004) 791–800
Table 2
ꢀ
Selected bond distances (A) and angles (°) in the dirhenium com-
pounds 2–4a
Re2(CO)8(bma)(2) (bond distances)
Re(1)–Re(2)
Re(2)–P(2)
3.0593(9) Re(1)–P(1)
2.429(4)
1.54(2)
1.55(2)
1.42(2)
1.16(2)
2.426(4)
1.32(2)
1.13(2)
1.38(2)
C(11)–C(12)
C(14)–C(15)
O(13)–C(12)
O(14)–C(14)
C(11)–C(15)
O(12)–C(12)
O(13)–C(14)
Re2(CO)8(bma)(2) (bond angles)
Re(2)–Re(1)–P(1)
Re(1)–Re(2)–P(2)
Re(1)–P(1)–C(11)
P(1)–C(11)–C(15)
95.1(1)
95.89(9)
122.0(5)
132(1)
P(1)–Re(1)–C(1)
P(2)–Re(2)–C(7)
Re(2)–P(2)–C(15)
P(2)–C(15)–C(11)
178.7(5)
177.1(5)
124.0(5)
133(1)
Re(CO)4[Re(CO)4(bma)] (3) (bond distances)
Re(1)–P(1)
2.466(5)
2.26(2)
1.56(3)
1.47(3)
1.39(2)
1.20(2)
Re(1)–P(2)
2.473(6)
2.27(2)
1.51(2)
1.20(3)
1.38(2)
Re(2)–C(11)
C(11)–C(15)
C(14)–C(15)
O(13)–C(14)
O(12)–C(12)
Re(2)–C(15)
C(11)–C(12)
O(14)–C(14)
O(13)–C(12)
Fig. 1. ORTEP drawing of Re2(CO)8(bma) (2) showing the thermal
ellipsoids at the 50% probability level and phenyl carbons as spheres of
arbitrary size.
Re(CO)4[Re(CO)4(bma)] (3) (bond angles)
P(1)–Re(1)–P(2)
C(8)–Re(2)–C(11)
Re(1)–P(1)–C(11)
P(1)–Re(1)–C(3)
83.3(2)
C(7)–Re(2)–C(15)
C(11)–Re(2)–C(15)
Re(1)–P(2)–C(15)
P(2)–Re(1)–C(1)
154.0(8)
40.3(7)
151.2(9)
109.5(6)
171.0(7)
109.3(7)
169.1(6)
Re2(CO)8[l-g1; g2-
tances)
(O)](l-PPh2) (4) (bond dis-
Re(1)–P(1)
Re(2)–P(2)
2.497(3)
2.513(3)
1.17(2)
1.38(2)
1.51(2)
1.54(2)
Re(1)–P(2)
2.533(3)
2.14(1)
1.37(2)
1.18(2)
1.35(2)
Re(2)–C(15)
O(13)–C(12)
O(14)–C(14)
C(11)–C(15)
O(12)–C(12)
O(13)–C(14)
C(11)–C(12)
C(14)–C(15)
Re2(CO)8[l-g1;g2-
gles)
(O)](l-PPh2) (4) (bond an-
P(1)–Re(1)–P(2)
P(2)–Re(1)–C(2)
C(8)–Re(2)–C(15)
P(1)–C(11)–C(15)
Re(2)–C(15)–C(11)
94.4(1)
175.9(4)
178.3(5)
119.9(9)
139(1)
P(1)–Re(1)–C(1)
P(2)–Re(2)–C(5)
Re(1)–P(1)–C(11)
P(1)–C(11)–C(15)
Re(2)–C(15)–C(14)
178.7(4)
177.2(5)
120.0(4)
129(1)
117.7(8)
Fig. 2. ORTEP drawing of Re(CO)4[Re(CO)4(bma)] (3) showing the
thermal ellipsoids at the 50% probability level.
a Numbers in parentheses are estimated standard deviations in the
least significant digits.
3.2. Cyclic voltammetry data and extended H€uckel MO
calculations
bridging phosphido moiety and the three-electron
donor group Ph2P
ꢀ
ꢀ
[2.513(3) A] bond distances fall within acceptable
ranges for Re–P distances in a wide variety of rhe-
nium compounds [30]. The internal bond angles as-
sociated with the atoms Re(1)–P(1)–C(11)–C(15)–
Re(2)–P(2), that are involved in the six-membered ring
that tethers the two rhenium centers, range from a
low of 89.2(3)° [P(2)–Re(2)–C(15)] to a high of 139(1)
[Re(2)–C(15)–C(11)]. The Re(2)–C(15) single-bond
(O). The Re(1)–P(1)
ꢀ
[2.497(3) A], Re(1)–P(2) [2.533(3) A], and Re(2)–P(2)
The redox properties of 2–4 were examined at a
platinum disk electrode in CH2Cl2 and THF solvents
containing 0.25 M TBAP as the supporting electrolyte.
No major differences in the CV data as a function of the
solvents were noticed. Fig. 4(a) shows the CV of
Re2(CO)8(bma) recorded at a scan rate of 0.5 V/s. Here
two diffusion-controlled reductions at E1=2 ¼ ꢀ0:38 and
E1=2 ¼ ꢀ1:29 V, ascribed to the 0/1ꢀ and 1ꢀ/2ꢀ redox
couples, respectively, are observed in CH2Cl2 solvent.
Both reductions are chemically reversible given the lin-
ear plots of the current function (Ip) vs the square root
of the scan rate (m) and the near unity current ratios
(Ipa=Ipc) over the scan rates investigated [32]. The electron
ꢀ
distance of 2.14(1) A is similar to other r-bonded
rhenium compounds [31], while the C(11)–C(15) dou-
ꢀ
ble-bond distance of 1.35(2) A is unaffected by the
activation of the bma ligand.

S.G. Bott et al. / Journal of Organometallic Chemistry 689 (2004) 791–800
797
stoichiometry was ascertained by current calibration
against the known one-electron standard ferrocene [16],
taking into account WaldenÕs rule, and coulometric
measurements (vide infra). The potentials found for the
two reduction waves in 2 are similar to those reported by
us for the mononuclear compound fac-BrRe(CO)3(bma)
[5], and these data are not surprising given the com-
monality of the LUMO in bma-ligated complexes. An
irreversible oxidation at Epa ¼ 1:09 V (m ¼ 0:1 V/s) was
also observed when the sample is scanned out to 1.5 V.
Increasing the scan rate to 1.0 V/s (room temperature)
or lowering the temperature to )60 °C (m ¼ 0:1 V/s)
increased the Ipc=Ipa ratio to 0.55 and 0.75, respectively.
Re2(CO)8(bma) was further investigated by constant-
potential coulometry (CPC) in order to gain more
information concerning the nature of [Re2(CO)8(bma)]ꢀꢀ
and site of electron accession. Re2(CO)8(bma) was re-
duced at room temperature in THF at )0.75 V, and the
total charge passed upon completion of reduction was
calculated to Q ¼ 0:95 C/mol of Re2(CO)8(bma). The
magnitude of Q obtained by the CPC experiment is in
excellent agreement with the one-electron stoichiometry
determined by using CV criteria [32]. The catholyte was
analyzed by IR spectroscopy so that the effect of the
added electron on the carbonyl bands could be explored.
[Re2(CO)8(bma)]ꢀꢀ exhibits m(CO) bands at 2064 (s),
2009 (s), 1969 (vs), 1944 (m), 1937 (m), 1908 (s), 1739
(w), and 1656 (s) cmꢀ1. The carbonyl bands belonging to
the Re2(CO)8 moiety in the radical anion have shifted
from 11 to 20 cmꢀ1 to lower energy relative to the
neutral starting material [33]. The magnitude of these
frequency shifts indicates that little of the added electron
density in [Re2(CO)8(bma)]ꢀꢀ resides on the rhenium
centers and the terminal carbonyl groups. The pre-
dominant locus associated with the reduction is the bma
ligand as revealed by the dramatic shift in the carbonyl
bands of the anhydride residue. Here the m(CO) bands of
the reduced ligand have shifted 91 and 120 cmꢀ1 to
lower energy relative the m(CO) bands at 1830 and 1776
cmꢀ1 for Re2(CO)8(bma) and are entirely consistent
with the reports on sundry other 18 þ d compounds
characterized to date [1,2,5]. Corroboration of the
electron stoichiometry and the site of reduction was also
demonstrated by parallel experiments conducted with
the known one-electron reducing agent Cp2Co. Treat-
ment of [Re2(CO)8(bma)] with cobaltocene (1.0 mol
equiv) in THF at room temperature lead to the imme-
diate formation of [Re2(CO)8(bma)]ꢀꢀ in quantitative
yield as determined by IR spectroscopy. The IR spec-
trum of [Re2(CO)8(bma)][Cp2Co] was identical to that
obtained by bulk electrolytic reduction, removing any
ambiguity as to the identity of the radical anion. Finally,
we attempted to grow single crystals of [Re2(CO)8-
(bma)][Cp2Co] for X-ray diffraction analysis but not
were successful and abandoned that avenue for the
characterization of the radical anion.
Fig. 3. ORTEP drawing of Re2(CO)8[l-g1,g1-
(l-PPh2) ꢂ 1/2C6H14 (4) showing the thermal ellipsoids at the
50% probability level. The solvent molecule has been omitted for
clarity.
(O)]-
Fig. 4. Cathodic scan cyclic voltammogram for Re2(CO)8(bma) (2) ca.
10ꢀ3 M in CH2Cl2 containing 0.25 M TBAP at 500 mV/s at room
temperature (top), and the CACAO drawing for the LUMO of
Re2(CO)8(H4-bma) (bottom).

798
S.G. Bott et al. / Journal of Organometallic Chemistry 689 (2004) 791–800
The redox chemistry of compounds 3 and 4 was briefly
The HOMO and LUMO levels in the zwitterionic
compound Re(CO)4[Re(CO)4(H4-bma)] may be viewed
as arising from the union of the anionic d8-ML4 species
Re(CO)ꢀ4 and the octahedral cation Re(CO)4(H4-bma)þ,
as depicted below. The empty pꢁ acceptor orbital on
Re(CO)4(H4-bma)þ corresponds to the /4 MO associ-
ated with the bma moiety, as discussed above for Re-
2(CO)8(H4-bma). This empty bma orbital has the correct
symmetry for strong overlap with its filled b2 counter-
part on the Re(CO)ꢀ4 fragment, whose nodal properties
are well known and mimic that of its isolobal cousin
CH2 [35]. In-phase and out-of-phase overlap of the
bma-acceptor orbital with the dxz hybrid orbital on the
Re(CO)ꢀ4 fragment leads to the HOMO ()11.94 eV) and
the LUMO ()9.32 eV), respectively, as shown by the
CACAO drawings in Fig. 5. The HOMO and LUMO
both contain essentially equal contributions from the
maleic anhydride moiety and rhenium metal character
from the p-bound Re(CO)ꢀ4 fragment. On the basis of
these data, electron addition to this LUMO by either
electrochemical or chemical methods is expected to
furnish a very unstable radical anion, whose pathway
for decomposition could well involve the loss of the
16-electron species Re(CO)ꢀ4 and the neutral radical
[Re(CO)4(H4-bma)]ꢀ. Independent experiments designed
to test this premise are planned for the future.
examined by CV. Re(CO)4[Re(CO)4(bma)] exhibits an
irreversible reduction at Epc ¼ ꢀ1:85 V in THF containing
0.1 M TBAP at a scan rate of 0.5 V/s, with anodic peaks
detected at Epa ¼ ꢀ1:20 V and Epa ¼ ꢀ0:52 V on the return
scan. Increasing the scan rate up to 5.0 V/s did not im-
prove the irreversible behavior of the reduction wave in
Re(CO)4[Re(CO)4(bma)]. The phosphido-bridged com-
plex Re2(CO)8[l-g1,g1-
(O)](l-PPh2)
displays a single reduction at E1=2 ¼ ꢀ1:21 V in CH2Cl2 in
0.25 M TBAP. The peak-to-peak separation of 124 mV is
slightly larger than that found for internally added fer-
rocene, whose DEp was 75 mV, and suggests a slow elec-
tron transfer (i.e., khet low) for this redox couple.
The orbital composition of the HOMO and LUMO
levels in the model complexes Re2(CO)8(H4-bma),
Re(CO)4[Re(CO)4(H4-bma)],
g1-
and
Re2(CO)8[l-g1,
(O)](l-PH2) was examined by ex-
€
tended Huckel MO calculations in an effort to under-
stand more fully the observed electrochemistry in
compounds 2–4. The HOMO in Re2(CO)8(H4-bma),
which occurs at )11.82 eV, is composed of an in-phase
overlap of d2z orbitals on the rhenium centers that fol-
lows the general bonding pattern typified by the inter-
action of two d7-ML5 fragments [28,34]. The LUMO in
Re2(CO)8(H4-bma) exhibits an orbital energy of )9.98
eV and is located exclusively on the maleic anhydride
residue. Fig. 4(b) shows the CACAO diagram [14] of the
LUMO in Re2(CO)8(H4-bma). The antibonding pꢁ
character displayed by the LUMO Re2(CO)8(H4-bma) is
akin to that of w4 found in simple six-p-electron systems
like maleic anhydride and 1,3,5-hexatriene, and the re-
lated compound fac-BrRe(CO)3(H4-bma) [5]. The pꢁ
character of the LUMO in Re2(CO)8(H4-bma) explains
fully the reductive electrochemistry that is largely lo-
calized on the bma ligand in Re2(CO)8(bma) and is in
keeping with the redox chemistry reported for a wide
variety of bma-substituted complexes [1a–1f,2,5,22].
Fig. 5. CACAO drawings for the HOMO (left) and the LUMO (right) of Re(CO)4[Re(CO)4(H4-bma)].

S.G. Bott et al. / Journal of Organometallic Chemistry 689 (2004) 791–800
799
The HOMO in Re2(CO)8[l-g1,g1-
-
Cyprus Sierrita Corporation for their generous gift of
ammonium perrhenate.
(O)](l-PH2) consists of contributions from both metals
in the form of hybridized dyz orbitals and the p bond of
the maleic anhydride ring, as shown below. The HOMO
occurs at )12.49 eV and clearly exhibits an in-phase dp–
pp bonding interaction involving the maleic anhydride
carbon atom that is sigma-bound to the rhenium center
in compound 4. The LUMO, which occurs at )10.14 eV,
is a maleic anhydride-based orbital whose nodal sym-
metry approximates that of rpsi4, as previously dis-
cussed for Re2(CO)8(H4-bma). Completing the overall
picture in the LUMO is a minor antibonding interaction
present between the pꢁ system of the anhydride ring and
the sigma-bound Re center.
References
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The diphosphine ligand bma reacts with Re2(CO)8(l-
H)(l-g1,g2-CH@CHBu) to give the expected product of
substitution Re2(CO)8(bma) and the zwitterionic com-
pound Re(CO)4[Re(CO)4(bma)]. The structural com-
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previously in reactions between the alkenyl complexes
Re2(CO)8(l-H)(l-g1,g2-alkenyl) and unsaturated di-
phosphine ligands. Both of these dirhenium compounds
are relatively stable, but in the case of Re2(CO)8
(bma) thermolysis with added Ni(cod)2 affords
the phosphido-bridged compound Re2(CO)8[l-g1,g1-
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been deposited with the Cambridge Crystallographic
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220770 for 4. Copies of this information may be ob-
tained free of charge from the Director, CCDC, 12
Union Road, Cambridge, CB2 1EZ UK [Fax:
+44(1223)336-033; email: deposit@ccdc.ac.uk or http://
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Financial support from the Robert A. Welch Foun-
dation (B-1093-MGR) is appreciated. We also thank the
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(2001) 485.

800
S.G. Bott et al. / Journal of Organometallic Chemistry 689 (2004) 791–800
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~
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[33] Cf. the IR data in THF for Re 2(CO)8 (bma) where m(CO) bands
are found at 2075 (s), 2023 (s), 1983 (vs), 1957 (m), 1950 (m, sh),
1927 (s), 1830 (vw, antisymm anhydride carbonyl), 1776 (s, symm
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516 (1996) 65;
anhydride carbonyl) cmꢀ1
.
(b) S.G. Bott, H. Shen, M.G. Richmond, J. Chem. Crystallogr. 28
(1998) 385;
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Crystallogr. 29 (1999) 391;
[35] (a)For a few reports on the bonding between ML4 fragments and
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(d) S.G. Bott, H. Shen, M.G. Richmond, Struct. Chem. 12 (2001)
237.
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