Isomerization of the diphosphine ligand 3,4-bis(diphenylphosphino)-5- methoxy-2(5H)-furanone (bmf) at a triosmium cluster and P-C bond cleavage in the unsaturated cluster 1,1-Os3(CO)9(bmf): Synthesis and X-ray diffraction structures of the isomeric Os3(CO) 10(bmf) clusters and HOs3(CO)8(μ-C 6H4)

Article abstract of DOI:10.1016/j.poly.2010.07.003

The labile cluster 1,2-Os₃(CO)₁₀(MeCN)₂ (1) reacts with the chiral diphosphine ligand 3,4-bis(diphenylphosphino)-5- methoxy-2(5H)-furanone (bmf) to furnish 1,2-Os₃(CO)₁₀(bmf) (2a) in high yield. Heating cluster 2a over the temperature range 358-383 K under CO leads to isomerization of the bmf ligand and formation of the diphosphine-chelated cluster 1,1-Os₃(CO)₁₀(bmf) (2b) and an equilibrium mixture consisting of 2a and 2b in a 15:85 ratio. Extended thermolysis of an equilibrium mixture of Os₃(CO)₁₀(bmf) is accompanied by CO loss and ortho-metalation of an aryl ring to afford an inseparable mixture of three diastereomeric hydride clusters HOs ₃(CO)₉(C₂₉H₂₃O₃P ₂) (3a-c). Thermolysis of HOs₃(CO)₉(C ₂₉H₂₃O₃P₂) (3a-c) in refluxing toluene leads to P-C bond cleavage and formation of the benzyne-substituted clusters HOs₃(CO)₈(μ-C₆H₄)(μ- C₂₃H₁₉O₃P₂) (4a,b) as a 4:1 mixture of diastereomers. The unequivocal identity of the major benzyne-substituted cluster has been determined by X-ray diffraction analysis, where the activation of one of the phenyl groups situated α to the furanone carbonyl group in the bmf ligand has been established. The isomerization and activation of the bmf ligand are contrasted with other Os₃(CO)₁₀(diphosphine) derivatives prepared by our groups.

Full text of DOI:10.1016/j.poly.2010.07.003

Polyhedron 29 (2010) 2814–2821
Contents lists available at ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
Isomerization of the diphosphine ligand 3,4-bis(diphenylphosphino)-5-methoxy-
2(5H)-furanone (bmf) at a triosmium cluster and P–C bond cleavage
in the unsaturated cluster 1,1-Os3(CO)9(bmf): Synthesis and X-ray diffraction
structures of the isomeric Os3(CO)10(bmf) clusters and HOs3(CO)8(
-PhPC@C(Ph2P)CH(OMe)OC(O)]
l-C6H4)
[l
Srikanth Kandala a, Li Yang a, Charles F. Campana b, Vladimir Nesterov a, Xiaoping Wang c,
Michael G. Richmond a,
*
a Department of Chemistry, University of North Texas, Denton, TX 76203, USA
b Bruker AXS Inc., 5465 East Cheryl Parkway, Madison, WI 53711-5373, USA
c Neutron Scattering Science Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
The labile cluster 1,2-Os3(CO)10(MeCN)2 (1) reacts with the chiral diphosphine ligand 3,4-bis(diphenyl-
phosphino)-5-methoxy-2(5H)-furanone (bmf) to furnish 1,2-Os3(CO)10(bmf) (2a) in high yield. Heating
cluster 2a over the temperature range 358–383 K under CO leads to isomerization of the bmf ligand
and formation of the diphosphine-chelated cluster 1,1-Os3(CO)10(bmf) (2b) and an equilibrium mixture
Received 14 April 2010
Accepted 1 July 2010
Available online 7 July 2010
consisting of 2a and 2b in
a 15:85 ratio. Extended thermolysis of an equilibrium mixture of
Keywords:
Os3(CO)10(bmf) is accompanied by CO loss and ortho-metalation of an aryl ring to afford an inseparable
mixture of three diastereomeric hydride clusters HOs3(CO)9(C29H23O3P2) (3a–c). Thermolysis of HOs3
(CO)9(C29H23O3P2) (3a–c) in refluxing toluene leads to P–C bond cleavage and formation of the ben-
zyne-substituted clusters HOs3(CO)8(
The unequivocal identity of the major benzyne-substituted cluster has been determined by X-ray diffrac-
tion analysis, where the activation of one of the phenyl groups situated to the furanone carbonyl group
Osmium clusters
Ligand substitution
Diphosphine ligand
P–C bond-activation
Crystallography
l-C6H4)(l-C23H19O3P2) (4a,b) as a 4:1 mixture of diastereomers.
a
in the bmf ligand has been established. The isomerization and activation of the bmf ligand are contrasted
with other Os3(CO)10(diphosphine) derivatives prepared by our groups.
Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction
1,3-dione (fbpcd) [5], whose structures are shown in Scheme 1, to af-
ford the bridging cluster 1,2-Os3(CO)10(P–P) as the kinetically con-
Numerous studies dealing with the substitution chemistry of the
lightly-activated cluster 1,2-Os3(CO)10(MeCN)2 with a wide variety
of diphosphine ligands have been published [1]. The formal
displacement of the MeCN ligands typically proceeds with the
formation of either the bridging or chelating diphosphine-
substituted cluster, where the observed substitution product has
been assumed to be the thermodynamically more stable isomeric
form of the cluster. We have recently demonstrated that 1,2-
Os3(CO)10 (MeCN)2 undergoes reaction with the rigid, unsaturated
diphosphine ligands (Z)-Ph2PCH@CHPPh2 (dppen) [2], 4,5-
bis(diphenylphosphino)-4-cyclopentene-1,3-dione (bpcd) [3],
2,3-bis(diphenyl-phosphino)-N-p-tolylmaleimide (bmi) [4], and
2-(ferrocenylidene)- 4,5-bis(diphenylphosphino)-4-cyclopentene-
trolled product of substitution. These particular 1,2-Os3(CO)10(P–
P) clusters are not stable and readily undergo isomerization to the
diphosphine-chelated clusters 1,1-Os3(CO)10(P–P) when heated at
elevated temperature [6]. In the case of the dppen ligand, the rear-
rangement of the diphosphine ligand about the cluster polyhedron
has been studied by our groups, and convincing kinetic evidence
has been presented for a reversible, non-dissociative unimolecular
process involving a species where one of the phosphorus groups is
simultaneously bound to two osmium metals, as depicted below
in Eq. (1) [2]. The ability of a phosphine ligand to bind simulta-
neously to two osmium centers in the triply bridged species Os3(-
CO)8(
ubiquitous CO ligand, whose coordinative flexibility is well-estab-
lished [7]. While we could not eliminate Os3(CO)8( 2-CO)2(l2
l2-CO)2(l2,g
1-dppen) parallels the fluxional behavior of the
l
, -
g1
dppen) as a short-lived intermediate or a transition state in the
reversible isomerization of the dppen ligand about the triosmium
* Corresponding author. Tel.: +1 940 565 3548; fax: +1 940 565 4318.
E-mail address: cobalt@unt.edu (M.G. Richmond).
0277-5387/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.poly.2010.07.003

S. Kandala et al. / Polyhedron 29 (2010) 2814–2821
2815
O
N
O
Ph2P
O
O
O
Ph2P
PPh2
PPh2
Ph2P
Ph2P
Ph2P
PPh2
O
Fe
dppen
bpcd
bmi
fbpcd
Scheme 1.
cluster in our earlier report, recently completed DFT calculations
indicate that Os3(CO)8 2-CO)2(l2
1-dppen) serves as the transi-
tion state connecting the ground-state clusters 1,2-Os3(CO)10(dp-
characterization of the isomeric clusters 1,2- and 1,1-Os3(CO)10
(bmf), including our data on the thermal and photochemical reac-
tivity of these new clusters.
(l
,g
Ph2
P
Ph2
Os
Os
Os
Ph2
P
P
ð1Þ
PPh2
Os
Os
Os
Os
Os
Os
P
Ph2
P
Ph2
pen) and 1,1-Os3(CO)10(dppen) [8].
In the case of the triosmium cluster 1,2-Os3(CO)10(bpcd), the
bridge-to-chelate isomerization of the bpcd ligand is irreversible
in nature, proceeding quantitatively to 1,1-Os3(CO)10(bpcd) [3].
Thermolysis of 1,1-Os3(CO)10(bpcd) leads to CO loss, followed by
the activation of one of the ancillary aryl groups through an
ortho-metalation process. The resulting hydride cluster HOs3
MeO
O
O
bmf
Ph2P
PPh2
(CO)9[l-PhP(C6H4)C@C(PPh2)C(O)CH2C(O)] quantitatively regener-
ates 1,1-Os3(CO)10(bpcd) when heated under CO, which establishes
the reversible nature associated with the reactivity of the C–H
bond in terms of oxidative cleavage and reductive coupling se-
quences. Detailed kinetic and isotopic substitution studies on the
reductive coupling step have confirmed that C–H bond formation
exhibits an inverse equilibrium isotope effect (EIE). These data sig-
2. Experimental
2.1. General methods
The starting cluster 1,2-Os3(CO)10(MeCN)2 was prepared from
Os3(CO)12 Me3NO, and MeCN [12], with the parent cluster
,
nal the involvement of the transient pi complex Os3(CO)9[l-PhP(l-
Os3(CO)12 prepared from OsO4 and CO [13]. The bmf ligand was
synthesized from 3,4-dichloro-5-hydroxy-2(5H)-furanone and
Ph2PSiMe3 via the procedure described by Fenske and Becher
[14]. The OsO4 was purchased from Engelhard Chemical Co, and
the chemicals Me3NOꢀxH2O, Me3SiCl, and mucochloric acid were
purchased from Aldrich Chemical Co. The anhydrous Me3NO em-
ployed in our studies was obtained from Me3NOꢀxH2O, after the
waters of hydration were azeotropically removed under reflux
using benzene as a solvent. All reaction solvents were distilled
from a suitable drying agent under argon or obtained from an Inno-
vative Technology (IT) solvent purification system; when not in
use, purified solvents were stored in Schlenk storage vessels
equipped with high-vacuum Teflon stopcocks [15]. The photo-
chemical studies were conducted with GE Blacklight bulbs having
a maximum output at 366 20 nm and a photon flux of ca.
1 ꢁ 10ꢂ6 Einstein/min. Combustion analyses were performed by
Atlantic Microlab, Norcross, GA.
C6H5)C@C(PPh2)C(O)CH2C(O)], whose formation precedes the
generation of the coordinatively unsaturated cluster 1,1-Os3
(CO)9(bpcd). The right-hand portion of Scheme 2 illustrates the
pertinent aspects of the observed ortho-metalation reaction. In
the absence of a scavenging ligand (i.e., added CO), the unsaturated
cluster 1,1-Os3(CO)9(bpcd) reacts with one of the ancillary aryl
rings of the bpcd ligand via an irreversible P–C bond-activation
to afford ultimately the benzyne-substituted cluster HOs3(CO)8
(l3-C6H4)[l2
,g
1-PPhC@C(PPh2)C(O)CH2C(O)].
To date all of our ligands employed in the above osmium cluster
studies have utilized symmetrical diphosphine ligands. The prepa-
ration of a 1,1-Os3(CO)10(P–P) cluster bearing a chiral diphosphine
would allow us to probe regio- and stereochemical aspects related
to the ortho-metalation process [9]. The chiral ligand 3,4-
bis(diphenylphosphino)-5-methoxy-2(5H)-furanone (bmf), whose
structure is depicted below, is a close relative to bpcd and is ideally
suited as an auxiliary ligand for the investigation of fundamental
bond-activation processes at a metal cluster. In fact, the bmf ligand
has already been employed by us in studies involving alkyne func-
tionalization and P–C(aryl) bond cleavage at polynuclear com-
pounds [10,11]. Herein we report the synthesis and structural
2.2. Instrumentation
The IR spectra were recorded on a Nicolet 6700 FT-IR spectrom-
eter in sealed 0.1 mm NaCl cells, while the quoted 1H NMR data

2816
S. Kandala et al. / Polyhedron 29 (2010) 2814–2821
Os
Ph2
P
O
1,1-Os3(CO)10(bpcd)
Os
Os
P
Ph2
O
+CO
-CO
Os
Ph2
O
P
unsaturated cluster
Os3(CO)9(bpcd)
Os
Os
P
Ph2
O
-CO
reductive
coupling
oxidative
cleavage
irreversible
Os
H
Os
H
Os
Os
Os
Os
PPh2
Ph
P
P
O
PPh2
Ph
O
O
O
HOs3(CO)9[µ-PhP(C6H4)C=C(PPh2)C(O)CH2C(O)]
HOs3(CO)8(µ-C6H4)[µ-PhPC=C(PPh2)C(O)CH2C(O)]
Scheme 2.
3
was recorded at either 200 or 500 MHz on Varian Gemini and Var-
ian VXR-500 spectrometers, respectively. The 31P NMR spectra
were recorded at 121 and 201 MHz on Varian 300-VXR and Varian
VXR-500 spectrometers, respectively. The reported 31P chemical
shift data were recorded in the proton-decoupled mode and are
referenced to external H3PO4 (85%), whose chemical shift was set
at d = 0.
ꢂ5.26 (d, JP–P = 8.0 Hz). Anal. Calc. for C39H24Os3O13P2: C, 35.14;
H, 1.81. Found: C, 34.77; H, 1.82%.
2.4. Isomerization of 1,2-Os3(CO)10(bmf) (2a) to an equilibrium
mixture of 2a and 1,1-Os3(CO)10(bmf) (2b)
A large Schlenk tube was charged with 0.20 g (0.15 mmol) of 2a
and 20 mL of toluene, and the solution was saturated with CO for
15 min. The reaction vessel was then sealed and heated at ca.
90 °C for a period of 72 h, after which time the solvent was
removed under vacuum and the residue was separated by column
chromatography over silica gel using CH2Cl2/hexane (3:2) to give
0.16 g (0.12 mmol) of a 15:85 equilibrium mixture containing clus-
ters 2a and 2b, respectively, as assessed by 1H and 31P NMR spec-
2.3. Synthesis of 1,2-Os3(CO)10(bmf) (2a)
To a 250 mL Schlenk flask containing 25 mL of CH2Cl2 and 0.25 g
(0.27 mmol) of 1,2-Os3(CO)10(MeCN)2 was added 0.15 g (0.30
mmol) of bmf under argon flush, after which the solution was stir-
red for 2 h at room temperature. TLC examination using CH2Cl2/
hexane (3:2) as the eluent confirmed the consumption of the start-
ing cluster and the presence of a large dark orange spot (Rf = 0.45),
in addition to unreacted bmf and a small amount of material that
remained at the origin of the TLC plate. The solvent was removed
under vacuum, and the crude residue separated by column chro-
matography over silica gel using the aforementioned mobile phase
to give cluster 2a, which was subsequently recrystallized from
benzene/CH2Cl2 to furnish 2a in 60% yield (0.22 g). IR (CH2Cl2):
troscopy. Spectroscopic data for 2b: IR (CH2Cl2):
2046 (vs), 2014 (vs) 2005 (vs), 1991 (m), 1977 (m) 1771 (s, fura-
none) cmꢂ1 1H NMR (C6D6): d 2.79 (s, 3H), 4.83 (s, 1H), 6.86–
m(CO) 2095 (vs),
.
7.29 (m, 17H, aromatic), 7.70 (dd, 1H, J = 8.0, 12.5 Hz), 7.87 (dd,
1H, J = 7.0, 11.5 Hz), 8.00 (dd, 1H, J = 8.5, 12.5 Hz). 31P NMR
3
3
(C6D6): d 13.26 (d, JP–P = 8.4 Hz), 17.22 (d, JP–P = 8.4 Hz).
2.5. Photolysis of 2a,b and formation of the hydride clusters
HOs3(CO)9(C29H23O3P2)(3a–c)
m
(CO) 2090 (vs), 2042 (s), 2011 (vs) 2007 (vs), 1976 (m) 1772 (s,
furanone) cmꢂ1 1H NMR (C6D6): d 2.22 (s, 3H), 4.40 (s, 1H),
.
6.92–7.20 (m, 17H, aryl), 7.36 (m, 1H, aryl), 7.62 (m, 1H, aryl),
About 0.20 g (0.15 mmol) of a thermally equilibrated mixture of
the clusters 1,2-Os3(CO)10(bmf) and 1,1-Os3(CO)10(bmf) was
3
7.97 (m, 1H, aryl). 31P NMR (C6D6): d ꢂ18.12 (d, JP–P = 8.0 Hz),

S. Kandala et al. / Polyhedron 29 (2010) 2814–2821
2817
charged to a large Schlenk tube, followed by 40 mL of CH2Cl2 via
syringe. The solution was then irradiated at 366 nm for 7 days at
room temperature. The CO that accompanies the formation of the
hydride clusters was periodically removed by multiple freeze–
pump–thaw degas cycles, allowing the reaction to be driven to
completion. The sample was also briefly heated at 60 °C several
times during the photolysis reaction to help drive the bridging
isomer to the chelating isomer, the latter which undergoes
photochemical reaction to afford the hydride products 3a–c. At
the end of 7 days, the solvent was removed under vacuum and
the crude mixture of hydride clusters were separated by column
chromatography over silica gel using CH2Cl2/hexane (3:2) to give
an inseparable, ternary mixture of the hydride clusters HOs3(CO)9
9.20 (19H, benzyne and aryl groups). 31P NMR (C6D6): d ꢂ70.31,
(d, phosphido, JP–P = 8.0 Hz), ꢂ66.17 (d, phosphido, JP–P = 6.8 Hz),
7.08 (d, phosphine, JP–P = 6.8 Hz), 7.98 (d, phosphine, JP–P
=
8.0 Hz). Anal. Calc. for C37H24O11Os3P2ꢀC6H6: C, 38.11; H, 2.23.
Found: C, 37.93; H, 2.41%.
2.7. X-ray crystallography
Single crystals of 2a, as the benzene solvate, suitable for X-ray
diffraction analysis were grown from a dichloromethane solution
of 2a that had been layered with benzene, while single crystals
of 2b were grown from a 15:85 equilibrium mixture containing
2a,b in toluene and hexane at 5 °C. X-ray quality crystals of the
benzyne-substituted cluster 4a were obtained through repeated
recrystallization of a mixture of 4a,b using hexane and toluene at
room temperature. The X-ray diffraction data for 2a were collected
in the variable scan speed mode at 295(1) K on a Bruker SMART APEX
II CCD-based diffractometer. The X-ray diffraction for 2b and 4a
were collected in the variable scan speed mode at 100(2) K on Bru-
ker SMART APEX CCD- and APEX II CCD-based diffractometers, respec-
tively. The frames were integrated with the available SAINT and APEX
II software packages using a narrow-frame algorithm [16,17], and
each structure was solved and refined using the available SHELXTL
program package. An absorption correction was applied to all three
clusters using the program SADABS [18], and the molecular struc-
tures were checked using PLATON [19]. The bridging hydride ligand
spanning the Os(1)–Os(2) vector in 4a was located during refine-
ment, while all other hydrogen atoms were assigned calculated
positions and allowed to ride on the attached carbon atom. In
the case of cluster 2b, the diffraction data revealed a disorder
among the six osmium atoms Os(1)–Os(6) and the 10 CO groups
associated with the two Os3(CO)10 units. The bmf ligand is not af-
fected in the two structures and the diffraction data give rise to a
‘‘Star of David” disorder involving two Os3(CO)10 units, with each
cluster containing a single bmf ligand. The occupancy factors for
1,1-Os3(CO)10(bmf) (chelating isomer, 2b) have been refined to
0.85 and those of 1,2-Os3(CO)10(bmf) (bridging isomer, 2a) refined
to 0.15. The clusters 2a and 2b each contained one disordered
(l
-C29H23O3P2) in an overall yield of 90% yield (0.18 g). 1H NMR
(C6D6): d ꢂ15.92 (t, 2-H, JP–H = 12.4 Hz), ꢂ15.99 (dd, 2-H, JP–H
12.4, 14.1 Hz), ꢂ16.17 (dd, 2-H, JP–H = 12.4, 14.0 Hz), 2.70, 2.71,
l
l
=
l
2.75 (s, 3H, MeO groups), 4.41, 4.78, 4.93 (s, 1H, methine groups),
6.85–8.77 (m, 19H, aromatic). 31P (C6D6): d 13.63 (s), 17.03 (bs),
23.73 (bs), 24.40 (s). Anal. Calc. for C38H24Os3O12P2: C, 34.97; H,
1.85. Found: C, 34.61; H, 1.86%.
2.6. Synthesis of HOs3(CO)9(l3-C6H4) [l-C23H19O3P2](4a,b)
About 0.10 g (0.07 mmol) of a cluster mixture of 1,2-Os3(CO)10
(bmf) and 1,1-Os3(CO)10(bmf) was charged to a Carius tube, fol-
lowed by 10 mL of toluene. The reaction tube was sealed and then
heated at 110 °C in a thermostated bath. The progress of reaction
was monitored by 31P NMR spectroscopy through the periodic re-
moval of aliquots for NMR analysis. Upon consumption of the start-
ing cluster mixture of 2a,b, the solvent was removed under
vacuum and the residue was separated by column chromatography
over silica gel using CH2Cl2/hexane (3:2). Recrystallization of the
crude product from a mixture of benzene/hexane afforded 90 mg
(95% yield) of 4a,b. IR (CH2Cl2):
(vs), 2004 (vs), 1988 (m), 1770 (s, furanone) cmꢂ1. 1H NMR (CDCl3):
d ꢂ16.60 (major, dd, 2-H, JP–H = 14.0, 5.5 Hz), ꢂ16.50 (minor, dd,
2-H, JP–H = 12.5, 7.5 Hz), 3.55 (minor, s, MeO), 3.61 (major, s,
m(CO) 2074 (vs), 2040 (vs), 2028
l
l
MeO), 5.87 (major, s, methine), 6.26 (major, s, methine), d 6.70–
Table 1
X-ray crystallographic data and processing parameters for the triosmium clusters 2a, 2b, and 4a.
Compound
2a
2b
4a
CCDC entry No.
Crystal system
Space group
a (Å)
b (Å)
c (Å)
730 527
triclinic
P1
9.4668(4)
12.3057(6)
20.776(1)
75.473(2)
79.960(2)
68.052(1)
2164.4(2)
730 529
monoclinic
P21/c
9.3489(6)
19.736(1)
21.182(1)
730 528
triclinic
P1
ꢀ
ꢀ
10.3986(5)
11.3464(5)
17.0559(7)
74.683(1)
72.592(1)
84.108(1)
1851.4(1)
C37H24O11Os3P2
1277.10
a
(°)
b (°)
92.153(3)
c
(°)
V (Å3)
3905.4(4)
C39H24O13Os3P2
1333.12
4
Molecular formula
Formula weight
Formula units per cell (Z)
Dcalc (Mg/m3)
C42H27O13Os3P2
1372.18
2
2
2.105
2.267
2.291
k (Mo K
a
) (Å)
0.71073
8.921
0.6640/0.2685
20 403
0.71073
9.884
0.3342/0.2957
29 023
0.71073
10.416
0.5738/0.2422
23 179
Absorption coefficient (mmꢂ1
Absorption correction factor
Total reflections
)
Independent reflections
9515
7132
8182
Data/restraints/parameters
9515/204/542
0.0242
0.0582
1.030
2.164/ꢂ0.685
7132/107/586
0.0741
0.1629
1.183
5.509/ꢂ1.551
8182/0/483
0.0174
0.0433
1.049
1.529/ꢂ0.962
a
R1 (I P 2
r
(I)]
b
wR2
Goodness-of-fit (GOF) on F2
D
qmax
,
D
qmin (e/Å3)
a
R1
R2 = {
=
R
R
||Fo| ꢂ |Fc||/
R|Fo|.
b
[w(F2o ꢂ F2c)2/
R
[w(F2o)2]}1/2
.

2818
S. Kandala et al. / Polyhedron 29 (2010) 2814–2821
Table 2
terized in solution by IR and NMR spectroscopy. The observation of
a pair of doublets at d ꢂ18.12 and ꢂ5.26 in the 31P NMR spectrum
of 2a is consistent with the two inequivalent phosphine groups in
the bmf ligand and the presence of a single isomer in solution. The
1H NMR spectrum of 2a recorded in C6D6 showed two singlets at d
2.22 and 4.40 for the methoxy and methine groups, respectively,
associated with the furanone platform, along with 20 aromatic
hydrogens appearing at d 6.92–7.20 (m, 17H), 7.36 (m, 1H), 7.62
(m, 1H), and 7.97 (m, 1H). Scheme 3 summarizes the course of
the ligand substitution reaction between 1 and bmf.
Selected bond distances (Å) and angles (°) for the triosmium clusters 2a, 2b, and 4a.
Cluster 2a
Bond distances
Os(1)–Os(2)
Os(2)–Os(3)
Os(2)–P(2)
P(2)–C(2)
O(1)–C(4)
O(3)–C(5)
C(2)–C(3)
P(1)ꢀ ꢀ ꢀP(2)
2.8611(3)
2.8742(3)
2.322(1)
1.841(4)
1.443(5)
1.443(6)
1.333(5)
3.831(4)
Os(1)–Os(3)
Os(1)–P(1)
P(1)–C(3)
O(1)–C(1)
O(3)–C(4)
C(1)–C(2)
C(3)–C(4)
2.8776 (2)
2.322(1)
1.843(4)
1.346(6)
1.365(5)
1.511(6)
1.524(5)
The bridging nature of the bmf ligand in 2a was unequivocally
established by X-ray diffraction analysis. The molecular structure
of 2a, as the benzene solvate, is shown in Fig. 1, where the bridging
disposition of the bmf ligand and its equatorial coordination to the
Os(1) and Os(2) centers are confirmed. The Os–Os bond distances
range from 2.8611(3) Å [Os(1)–Os(2)] to 2.8776(2) Å [Os(1)–
Os(3)] and display a mean distance of 2.871 Å. The Os–Os bond
distances in 2a are in good agreement with those Os–Os bond dis-
tances in the parent cluster Os3(CO)12 and numerous diphosphine-
bridged clusters Os3(CO)10(P–P) [1c–e,20,21]. The two Os–P bonds
display distances of 2.322(1) Å [Os(1)–P(1)] and 2.322(1) Å [Os(2)–
P(2)], while the 10 terminal carbonyl groups exhibit distances and
angles consistent with their linear nature. Coordination of the bmf
ligand across the Os(1)–Os(2) vector is accompanied by a signifi-
cant ‘‘stretching” of the bmf ligand and an overall ground-state
destabilization of cluster 2a relative to the chelating isomer 2b
(vide infra). Here the internuclear P(1)ꢀ ꢀ ꢀP(2) distance of
3.831(4) Å found in 2a is significantly longer (>0.55 Å) than the
corresponding P(1)ꢀ ꢀ ꢀP(2) distance found in cluster 2b and the
P(1)ꢀ ꢀ ꢀP(2) distance in the free diphosphine ligands (Z)-
Ph2PCH@CHPPh2 and dppbz [22,23]. The remaining bond distances
and angles are unexceptional and require no comment.
Bond angles
P(1)–Os(1)–Os(2)
P(2)–Os(2)–Os(1)
C(3)–P(1)–Os(1)
C(3)–C(2)–P(2)
C(4)–C(3)–P(1)
97.07(3)
104.09(3)
118.2(1)
132.4(3)
118.4(3)
P(1)–Os(1)–Os(3)
P(2)–Os(2)–Os(3)
C(2)–P(2)–Os(2)
C(2)–C(3)–P(1)
C(1)–C(2)–P(2)
154.82(3)
160.44(3)
119.7(1)
132.9(3)
119.5(3)
Cluster 2b
Bond distances
Os(4)–Os(6)
Os(5)–Os(6)
Os(4)–P(2)
P(2)–C(1)
O(2)–C(4)
O(3)–C(5)
C(2)–C(3)
P(1)ꢀ ꢀ ꢀP(2)
2.8905(9)
2.8814(9)
2.303(4)
1.83(2)
1.42(2)
1.46(2)
Os(4)–Os(5)
Os(4)–P(1)
P(1)–C(2)
O(2)–C(3)
O(3)–C(4)
C(1)–C(2)
C(1)–C(4)
2.9020(9)
2.298(4)
1.87(2)
1.39(2)
1.32(2)
1.35(2)
1.50(2)
1.47(2)
3.22(1)
Bond angles
P(1)–Os(4)–Os(5)
P(2)–Os(4)–Os(5)
C(2)–P(1)–Os(4)
C(2)–C(1)–P(2)
P(1)–Os(4)–P(2)
106.1(1)
163.4(1)
104.3(5)
123(1)
P(1)–Os(4)–Os(6)
P(2)–Os(4)–Os(6)
C(1)–P(2)–Os(4)
C(1)–C(2)–P(1)
163.5(1)
106.6(1)
103.7(5)
118(1)
88.8(2)
Cluster 4a
Bond distances
Os(1)–Os(2)
Os(1)–P(2)
2.9305(2)
2.3454(8)
2.3889(8)
1.81(4)
2.372(3)
2.143(3)
1.511(4)
3.8137(3)
Os(2)–Os(3)
Os(1)–P(1)
Os(1)–C(14)
Os(2)–C(14)
Os(2)–H(1)
C(9)–C(12)
C(12)–C(13)
P(1)ꢀ ꢀ ꢀP(2)
2.8227(2)
3.2. Isomerization of 2a to an equilibrium mixture of 2a/2b
2.3789(8)
2.158(3)
2.310(3)
1.89(4)
1.500(4)
1.330(4)
3.229(1)
Os(3)–P(2)
Os(1)–H(1)
Os(2)–C(15)
Os(3)–C(15)
C(10)–C(13)
Os(1)ꢀ ꢀ ꢀOs(3)
The thermal stability of 2a was next probed by NMR spectros-
copy in toluene-d8 at 85 °C. Samples of 2a that were heated under
1 atm CO, whose presence retards CO loss and formation of the hy-
dride-bridged clusters 3a–c, were accompanied by a decrease in
the intensity of the initial methoxy and methine singlets of 2a
and the growth of two new singlets at d 2.85 and 4.80 correspond-
ing to the methoxy and C(5) methine groups of the chelating iso-
mer 2b. The spectral changes in the 31P NMR spectrum of 2a also
corroborate the isomerization of the bmf ligand about the trios-
mium cluster. Here a new pair of 31P doublets was observed at d
13.26 and 17.22, coming at the expense of 2a, and whose large nu-
clear deshielding is consistent with the formation of the bmf-che-
lated cluster 2b [24]. Continued heating of the 2a,b mixture over
the course of several days afforded an equilibrium value of 15:85
in favor of the chelating isomer 2b, as determined by both 1H
and 31P NMR spectroscopy. The Keq value for the isomerization of
2a to 2b was found to be constant over the temperature range of
85–110 °C with a magnitude of 5.7 in toluene solvent under
1 atm CO. The magnitude of the Keq value found for 2a,b is close
to the Keq value of 6.9 determined for the reversible isomerization
of the dppen ligand at Os3(CO)10(dppen) at 373 K [3,25]. Finally, a
preparative isomerization reaction employing 2a was also exam-
ined by TLC, and the single TLC spot exhibited by the isomeric mix-
ture of 2a,b negated any possible chromatographic separation of
these two clusters.
Bond angles
Os(3)–Os(2)–Os(1)
C(14)–Os(1)–P(1)
C(2)–Os(1)–Os(2)
P(1)–Os(1)–Os(2)
C(5)–Os(2)–Os(1)
Os(1)–P(1)–Os(3)
83.017(6)
81.51(8)
92.2(1)
83.66(2)
117.2(1)
106.24(3)
C(14)–Os(1)–P(2)
P(1)–Os(1)–P(2)
P(2)–Os(1)–Os(2)
C(14)–Os(2)–C(15)
C(3)–Os(2)–Os(1)
165.23(9)
86.23(3)
119.26(2)
36.1(1)
89(1)
phenyl group involving the atoms C(24)–C(29) and C(111)–C(116),
respectively. These phenyl groups were refined using an appropri-
ate disorder model. Table 1 summarizes the pertinent X-ray data
collection and processing parameters, with Table 2 providing se-
lected bond distances and angles for the triosmium clusters.
3. Discussion
3.1. Synthesis, spectroscopic data, and solid-state structure for 2a
The reaction between the activated cluster 1,2-Os3(CO)10
(MeCN)2 (1) and the unsaturated diphosphine ligand bmf proceeds
rapidly at room temperature to furnish the ligand-bridged cluster
1,2-Os3(CO)10(bmf) (2a) as the major product. Trace amounts of
The molecular structure of 2b was established by X-ray diffrac-
tion analysis (Fig. 1), and interestingly enough, the two indepen-
dent structures found in the solid state consisted of an isomeric
mixture of clusters 2a,b whose ratio coincidently is identical with
the Keq value determined by NMR spectroscopy. The observed
solid-state disorder is best described as that arising from a ‘‘Star
the minor product Os3(CO)11(g
1-bmf) have been observed in this
reaction and are attributed to the mono(acetonitrile) derivative
Os3(CO)11(MeCN) that often accompanies the synthesis of 1. 2a
was purified by column chromatography over silica gel and charac-

S. Kandala et al. / Polyhedron 29 (2010) 2814–2821
2819
OCH3
Ph2
NCMe
Os
P
O
Os
Os
Ph2
P
OCH3
O
bmf
Os
O
Os
Os
-2MeCN
PPh2
Os
Os
Os
P
Ph2
O
NCMe
cluster 1
cluster 2a
cluster 2b
Scheme 3.
Fig. 1. Molecular structures and numbering schemes of 2a (left) and 2b (right) at the 30% and 50% probability level, respectively. The benzene solvent in the former structure
has been omitted for clarity and hydrogen atoms drawn as spheres of arbitrary radii.
of David” involving two locally disordered Os3(CO)10 moieties rel-
ative to the bmf ligand [26]. The bmf ligand in 2b coordinates the
Os(1) atom and resides in the equatorial plane defined by the three
osmium atoms. The Os–Os bond distances range from 2.9020(9) Å
[Os(4)–Os(5)] to 2.8814(9) Å [Os(5)–Os(6)] and the two Os–P
bonds exhibit distances of 2.303(4) Å [Os(4)–P(2)] and 2.298(4) Å
[Os(4)–P(1)]. The observed bond angle of 88.8(2)° for the P(1)–
Os(4)–P(2) atoms and the internuclear P(1)ꢀ ꢀ ꢀP(2) distance of
3.22(1) Å mirror those values reported by us for other bmf-substi-
tuted cluster compounds [10,11,27].
values for these bridging hydrides are reported in the experimental
section. Three sets of methoxy (d 2.70, 2.71, 2.75) and methine
(d 4.41, 4.78, 4.93) singlets are also observed and are consistent
with the formation of the three new hydride clusters. The 19 aryl
hydrogens belonging to 3a–c appear as a complex set of overlap-
ping resonances from d 6.85–8.77. Unfortunately, the extensive
overlap of these aryl hydrogens precluded any definitive spectral
assignments. The 31P NMR spectrum showed four new resonances
at d 13.63, 17.03, 23.73, and 24.40, of which the two resonances
centered at d 17.03 and 23.73 actually derive from the overlap of
two of the three new hydride species. Careful integration of these
resonances relative to the remaining pair of 31P resonances pro-
vided additional support for this premise. Assignment of the likely
hydride-based products is problematic given the presence of two
different PPh2 groups and the C-5 chiral center, which give rise
to a maximum of eight theoretical diastereomers [28]. Depicted
below are the likely candidates that arise from the ortho-metala-
tion that takes place upon CO loss in 2b, the nature of which draws
from our earlier work where the crystallographic structure of the
hydride cluster obtained from the ortho-metalation of the diphos-
phine ligand 2-(ferrocenylidene)-4,5-bis(diphenylphosphino)-
4-cyclopenten-1,3-dione (fbpcd) in 1,1-Os3(CO)10(fbpcd) was
determined [5]. There it was shown that the phenyl group involved
in the ortho-metalation process must occupy an axial site relative
the metallic frame. Suffice it to say, the three hydride diastereo-
3.3. Thermal and photochemical preparation of the hydride clusters
HOs3(CO)9(C29H23O3P2) (3a–c) from 2a,b
The thermal and photochemical reactivity of the equilibrium
mixture of clusters 2a,b was next examined due to our previous re-
ports of efficient CO loss and C–H bond-activation of an ancillary
aryl ligand in numerous diphosphine-chelated Os3(CO)10(P–P)
clusters. Controlled thermolysis of 2a,b in a sealed NMR tube in
benzene-d6 at 353 K in the absence of added CO furnished a ternary
mixture of hydride diastereomers through the loss of CO and rapid
ortho-metalation of one of the aryl groups attached to the bmf li-
gand. 1H NMR analysis of the reaction revealed the growth of three
hydride resonances at d ꢂ15.92, ꢂ15.99, and ꢂ16.17 in roughly
equal amounts for clusters 3a–c. The splitting patterns and JP–H

2820
S. Kandala et al. / Polyhedron 29 (2010) 2814–2821
mers resulting from the thermolysis of 2a,b indicate a modicum of
selectivity for the ortho-metalation that occurs at the putative
intermediate 1,1-Os3(CO)9(bmf).
respectively. The nineteen hydrogens associated with the benzyne
and aryl moieties were observed from d 6.70–9.20.
The cleavage of one of the phenyl groups from the bmf ligand
and stereochemical disposition of the bmf ligand relative to the tri-
osmium frame were unequivocally established by X-ray crystallog-
raphy in the case of the major diastereomer, which we have
arbitrarily labeled as 4a for discussion purposes. While difficult
and time consuming, 4a could be isolated, albeit with considerable
material loss, through repeated recrystallization [31]. The molecu-
lar structure of 4a is shown in Fig. 2, where the presence of a face-
capping benzyne ligand and phosphido moiety is confirmed. 4a
contains 50eꢂ, which is two electrons in excess of the electron-pre-
cise count of 48 valence electrons for saturated triangular Os3 clus-
ters, accounting for the observed polyhedral opening of the metallic
frame across the initial Os(1)–Os(3) bond. The two Os–Os vectors in
4a display distances of 2.8227(2) Å [Os(2)–Os(3)] and 2.9305(2) Å
[Os(1)–Os(2)], with the longer of the two bond distances serving
as the site for the ancillary hydride H(1) that resides on the metallic
face opposite the benzyne ligand. The observed lengthening of the
hydride-bridged Os(1)–Os(2) vector is in keeping with the trend
found in numerous hydride-bridged clusters [32]. The internuclear
Os(1)ꢀ ꢀ ꢀOs(3) distance of 3.8137(3) Å far exceeds the van der Waals
radii of the two metal atoms and precludes any direct bonding. The
P(1) phosphido moiety and the three osmium atoms give rise to a
near planar butterfly arrangement of atoms based on a rp value
of 0.092 Å. The origin of the benzyne ligand, which is defined by
the atoms C(14)–C(19), is traced to the P(1) atom that spans the
Os(1) and Os(3) centers. The benzyne ligand functions as a typical
Os
Os
Os
P
Os
P
PPh2
MeO
PPh2
Os
Os
O
H
OCH3
H
O
O
Ph
Ph
O
possible diastereomeric hydride clusters
Examination of preparative thermolysis reactions using 2a,b by TLC
showed a single spot for 3a–c that prevented any physical separa-
tion of this ternary product mixture. The hydride clusters 3a–c
are sensitive to the presence of CO, and this particular reaction
may be reversed through the addition of CO and continued heating,
as previously demonstrated by us for related diphosphine-chelated
clusters HOs3(CO)9[l-(Ph2P)HC@CH{PPh(C6H4)}] and HOs3(CO)9[l-
(Ph2P)C@C{PPh(C6H4)}C(O)CH2C(O)] [2,3]. Control reactions involv-
ing samples of 3a–c that were heated in benzene at 358 K under CO
(1 atm) regenerated the equilibrium mixture of 2a,b in quantitative
yield.
Photolysis of 2a,b at room temperature using 366 nm light pro-
ceeds selectively with the consumption of the chelating isomer 2b
to furnish the hydride clusters 3a–c. In fact, photolysis of 2a,b with
periodic removal of the liberated CO ultimately furnished 2a (15%)
and 3a–c (85%). The observed reactivity data, namely diastereose-
lective ortho-metalation of 2b, parallel the isomer dependency
exhibited by the bridging and chelating isomers of Os3(CO)10[(Z)-
dppen] [2]. The clusters 2a,b could be driven to 3a–c in essentially
quantitative yield through photolysis with intermittent heating of
the sample at 358 K, followed by resumption of irradiation. Utiliza-
tion of this irradiation/thermolysis protocol ultimately allowed the
equilibrium mixture of 2a and 2b to be driven completely to the
hydride clusters 3a–c.
four-electron, face-capping ligand by way of two Os–C
[Os(1)–C(14) 2.158(3) Å and (Os(3)–C(15) 2.143(3) Å] and an Os–
bond defined by the Os(2)–C(14) [2.310(3) Å] and Os(2)–C(15)
[2.372(3) Å] vectors. The methoxy group bound to C(10) of the fura-
none ring is distally oriented away from the cluster core and the
C(26)–C(31) phenyl carbons. The stereoselective binding of the
bmf ligand to polynuclear clusters is strongly influenced by the
steric environment about the cluster polyhedron, and we have
r bonds
C
p
3.4. Preparation of the benzyne-substituted clusters 4a,b and X-ray
diffraction structure of HOs3(CO)8(l-C6H4)(l-C23H19O3P2)
The thermal stability of the hydride clusters 3a–c was next
explored for their propensity to undergo P–C(aryl) bond cleavage
of an aryl group from the diphosphine ligand, leading to the forma-
tion of a benzyne-substituted cluster. Degradation of a phosphine
ligand via this manifold represents one of the chief modes by
which a phosphine-modified catalyst is rendered catalytically inac-
tive [29]. Heating samples of 2a,b above 383 K in toluene in a Cari-
us tube led to the consumption of 3a–c and the appearance of a
new spot by TLC ascribed to the phosphido-bridged clusters 4a,b.
The 31P NMR data for the crude reaction mixture also supported
the presence two phosphido-bridged clusters based on the pres-
ence of two pairs of new resonances recorded at d ꢂ70.31 and
7.98 (major) and d ꢂ66.17 and 7.08 (minor) in a 4:1 ratio. The up-
field 31P resonance in these products is consistent with the pres-
ence of a bridging phosphido moiety that spans non-bonded
osmium centers [30]. The two products could not be separated
by column chromatography and were consequently isolated as a
mixture of diastereomers. The 1H NMR spectrum of 4a,b revealed
a pair of hydrides at ꢂ16.60 (major) and ꢂ16.50 (minor), along
with resonances at d 3.55 (minor) and 3.61 (major) and d 5.87 (ma-
jor) and 6.26 (major) for the methoxy and methine hydrogens,
Fig. 2. Molecular structure and numbering scheme of 4a at the 50% probability
level, with hydrogen atoms drawn as spheres of arbitrary radii.

S. Kandala et al. / Polyhedron 29 (2010) 2814–2821
2821
[4] W.H. Watson, B. Poola, M.G. Richmond, J. Organomet. Chem. 691 (2006) 4676.
[5] W.H. Watson, B. Poola, M.G. Richmond, Polyhedron 26 (2007) 3585.
[6] For a Related Report that Describes the Bridge-to-Chelate Isomerization of a
Thiophosphine at a Triosmium Cluster, See: R. Persson, M. Monari, R. Gobetto,
A. Russo, S. Aime, M.J. Calhorda, E. Nordlander, Organometallics 20 (2001)
4150.
documented this phenomenon in the reaction of bmf with the clus-
ter compounds PhCCo2NiCp(CO)6, PhCCo2MoCp(CO)8, and Ru6(l6
C) (CO)17 [11,33]. The bond distances and angles exhibited by the
benzyne ligand in 4a are similar to those distances and angles found
in other benzyne-substituted osmium clusters [34]. The remaining
bond distances and angles are unexceptional and require no
comment.
-
[7] (a) F.A. Cotton, Inorg. Chem. 6 (1966) 1083;
(b) R.D. Adams, F.A. Cotton, in: L.M. Jackman, F.A. Cotton (Eds.), Dynamic
Nuclear Magnetic Resonance Spectroscopy, Academic Press, New York, 1975
(Chapter 12).
[8] Unpublished results.
4. Conclusions
[9] For a Report on the Reaction of the Optically Active Ligand (R)-BINAP with
Os3(CO)12ꢂn(MeCN)n (where n = 0, 2), See: A.J. Deeming, M. Stchedroff, J. Chem.
Soc., Dalton Trans. (1998) 3819.
Substitution of the MeCN ligands in 1,2-Os3(CO)10(MeCN)2 by
the chiral diphosphine bmf affords the ligand-bridged cluster 1,2-
Os3(CO)10(bmf) as the kinetic product of ligand substitution. The
bmf ligand in 1,2-Os3(CO)10(bmf) is coordinatively flexible and
readily isomerizes under CO (1 atm) over the temperature range
of 358–373 K to furnish an equilibrium mixture of 1,2- and 1,1-
Os3(CO)10(bmf) in favor of the latter isomer (Keq = 5.7). The iso-
meric decacarbonyls undergo CO loss upon thermolysis or optical
excitation using near-UV light to afford a mixture of hydride clus-
ters (3a–c) through ortho metalation of one of the ancillary phenyl
groups. Thermolysis of 3a–c at elevated temperatures leads, ulti-
mately, to the release of one of the aryl groups and the formation
of a pair of benzyne-substituted Os3 clusters, which have been
characterized spectroscopically in solution and by X-ray diffraction
analysis in the case of one of the diastereomers. Future studies will
probe the coordination chemistry of different chiral ligands at Os3
and other metal clusters, and we hope to achieve high regio- and
diastereoselectivity in the activation of the ancillary C–H and P–C
bonds of the chiral auxiliary, which then can serve as a starting
point for the more elaborate functionalization of the initial ligand
system.
[10] K. Yang, S.G. Bott, M.G. Richmond, Organometallics 14 (1995) 4977.
[11] S.G. Bott, K. Yang, M.G. Richmond, J. Organomet. Chem. 691 (2006) 20. 3771.
[12] J.N. Nicholls, M.D. Vargas, Inorg. Synth. 26 (1989) 289.
[13] S.R. Drake, P.A. Loveday, Inorg. Synth. 28 (1990) 230.
[14] (a) D.T. Mowry, J. Am. Chem. Soc. 72 (1950) 2535;
(b) D. Fenske, H.J. Becher, Chem. Ber. 107 (1974) 117;
(c) D. Fenske, H.J. Becher, Chem. Ber. 108 (1975) 2115.
[15] D.F. Shriver, The Manipulation of Air-Sensitive Compounds, McGraw-Hill, New
York, 1969.
[16] SAINT, Version 6.02, Bruker Advanced Analytical X-ray Systems, Inc., Madison,
WI, 1997–1999.
[17] APEX2 Version 2.14, Bruker Advanced Analytical X-ray Systems, Inc., Madison,
WI, 2007.
[18] Bruker SADABS, Bruker AXS Inc., Madison, WI, 2007.
[19] A.L. Spek, PLATON – A Multipurpose Crystallographic Tool, Utrecht University,
Utrecht, The Netherlands, 2006.
[20] (a) E.R. Corey, L.F. Dahl, Inorg. Chem. 1 (1962) 521;
(b) M.R. Churchill, B.G. DeBoer, Inorg. Chem. 16 (1977) 878.
[21] (a) J.A. Clucas, R.H. Dawson, P.A. Dolby, M.M. Harding, K. Pearson, A.K. Smith, J.
Organomet. Chem. 311 (1986) 153;
(b) W.H. Watson, B. Poola, M.G. Richmond, J. Chem. Crystallogr. 36 (2006)
123;
(c) V. Nesterov, B. Poola, X. Wang, M.G. Richmond, J. Organomet. Chem. 692
(2007) 1806;
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Chem. Crystallogr. 29 (1999) 813.
[22] (a) A Search of the Cambridge Structural Database (CSD Version 5.30)
Revealed Two Structures for the Free Diphosphine Ligand (Z)-
Ph2PCH@CHPPh2 having Internuclear P(1)ꢀ ꢀ ꢀP(2) Distances of 3.279 Å (Ref. a)
and 3.260 Å (Ref. b). See: S.J. Berners-Price, L.A. Colquhoun, P.C. Healy, K.A.
Byriel, J.V. Hanna, J. Chem. Soc., Dalton Trans. (1992) 3357;
(b) P.G. Jones, M.C. Gimeno, Z. Kristallogr. 209 (1994) 688.
[23] W. Levason, G. Reid, M. Webster, Acta Crystallogr. C62 (2006) 440.
[24] P.E. Garrou, Chem. Rev. 81 (1981) 229.
[25] Preliminary Kinetic Measurements on the Isomerization of 2a to 2b indicate a
Reversible Equilibrium between the Bridging and Chelating forms of the
Cluster (Unpublished Results). The Full Details of these Kinetic Studies will be
addressed in a Separate Publication.
5. Supplementary data
CCDC 730527, 730529, and 730528 contains the supplementary
crystallographic data for 2a, 2b, and 4a. These data can be obtained
free of charge via http://www.ccdc.cam.ac.uk/conts/retriev-
ing.html, or from the Cambridge Crystallographic Data Centre, 12
Union Road, Cambridge CB2 1EZ, UK; fax: (+44) 1223-336-033;
or e-mail: deposit@ccdc.cam.ac.uk.
[26] (a) For Other Structural Reports that Display a ‘‘Star of David” Disorder, See: D.
Braga, F. Grepioni, Chem. Soc. Rev. 29 (2000) 229;
(b) L.J. Farrugia, J. Chem. Soc., Dalton Trans. (1997) 1783;
(c) J.K. Ruff, R.P. White Jr., L.F. Dahl, J. Am. Chem. Soc. 93 (1971) 2159.
[27] S.G. Bott, K. Yang, M.G. Richmond, J. Chem. Crystallogr. 35 (2005) 709.
[28] A Maximum of Eight Diastereomers is Predicted from the Ortho-metalation
Process. Here the Activation of Four Different Aryl Groups, Coupled with the
Proximal and Distal Stereochemistry of the C-5 Methoxy/Methine Groups and
the Orientation of the Furanone Ring Relative to the Triosmium Frame, must
be taken into Account. Accordingly, a Total of Sixteen Stereoisomers are thus
Predicted from the Act of Ortho Metalation When the Enantiomers of these
Diastereomers are taken into Account.
[29] G. Rothenberg, Catalysis: Concepts and Green Applications, Wiley-VCH,
Weinheim, DE, 2008.
[30] A.J. Carty, S.A. MacLaughlin, D. Nucciarone, in: J.G. Verkade, L.D. Quin (Eds.),
Phosphorus-31 NMR Spectroscopy in Stereochemical Analysis, VCH, New York,
1987.
Acknowledgments
Financial support from the Robert A. Welch Foundation (Grant
B-1093-MGR) is greatly appreciated. X. Wang acknowledges the
support by the US Department of Energy, Office of Science, under
Contract No. DE-AC05-00OR22725 managed by UT Battelle, LLC.
Prof. Arnold Rheingold is thanked for his hospitality and the use
of his X-ray diffractometer that was used to collect the X-ray data
on the isomeric clusters 2a,b during the 2004 ACS-PRF summer
school on Crystallography for Organic Chemists.
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