New osmium cluster compounds containing the heterocyclic ligand 2,3-bis-(diphenylphosphino)quinoxaline (dppq): Ligand isomerization and crystal structures of dppq, the isomeric clusters Os3(CO)10(dppq), and HOs3(CO)9[μ-2,3-PhP(η1-C 6H4)(Ph2P)quinoxaline]

Article abstract of DOI:10.1016/j.jorganchem.2011.01.019

Treatment of the labile cluster 1,2-Os₃(CO)₁₀(MeCN) ₂ (1) with the diphosphine ligand 2,3-bis(diphenylphosphino) quinoxaline (dppq) at room temperature affords 1,2-Os₃(CO) ₁₀(dppq) (2b) as the kinetic product of ligand substitution in 84% yield. 2b isomerizes to the thermodynamically more stable dppq-chelated cluster 1,1-Os₃(CO)₁₀(dppq) (2c) as the sole observable product under CO at temperatures below 358 K. The kinetics for the conversion of 2b → 2c have been investigated by NMR spectroscopy in CDCl₃ over the temperature range 323-353 K, and the reaction was found to exhibit a rate law that is first order in 2b. The calculated activation parameters [ΔH^≠ = 25.4(4) kcal/mol; ΔS^≠ = -3(1) eu] support an intramolecular isomerization scenario, one that involves the migration of phosphine and CO groups about the cluster polyhedron. The disposition of the dppq ligand in the isomeric Os₃(CO) ₁₀(dppq) clusters has been established by X-ray crystallography and ³¹P NMR spectroscopy. Photolysis of 2c at 366 nm leads to CO loss and ortho metalation of one of the aryl groups on the Ph₂P moiety to furnish the hydride cluster HOs₃(CO)₉[μ- PhP(η¹-C₆H₄)(Ph₂P)quinoxaline] (3). The isomerization behavior exhibited by 2b follows that of related diphosphine-substituted Os₃ clusters prepared by us.

Full text of DOI:10.1016/j.jorganchem.2011.01.019

Journal of Organometallic Chemistry 696 (2011) 1432e1440
Contents lists available at ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
New osmium cluster compounds containing the heterocyclic ligand 2,3-bis-
(diphenylphosphino)quinoxaline (dppq): Ligand isomerization and crystal
structures of dppq, the isomeric clusters Os3(CO)10(dppq), and
HOs3(CO)9[m-2,3-PhP(h
1-C6H4)(Ph2P)quinoxaline]
,
Sean W. Hunt a, Li Yang a, Xiaoping Wang b, Michael G. Richmond a
*
a Department of Chemistry, University of North Texas, Denton, TX 76203, United States
b Neutron Scattering Science Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831, United States
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 27 December 2010
Accepted 11 January 2011
Treatment of the labile cluster 1,2-Os3(CO)10(MeCN)2 (1) with the diphosphine ligand 2,3-bis(diphe-
nylphosphino)quinoxaline (dppq) at room temperature affords 1,2-Os3(CO)10(dppq) (2b) as the kinetic
product of ligand substitution in 84% yield. 2b isomerizes to the thermodynamically more stable dppq-
chelated cluster 1,1-Os3(CO)10(dppq) (2c) as the sole observable product under CO at temperatures below
358 K. The kinetics for the conversion of 2b / 2c have been investigated by NMR spectroscopy in CDCl3
over the temperature range 323e353 K, and the reaction was found to exhibit a rate law that is first order
Keywords:
Osmium clusters
Diphosphine isomerization
PeC bond activation
Crystallography
in 2b. The calculated activation parameters [
D
Hs ¼ 25.4(4) kcal/mol;
D
Ss ¼ ꢀ3(1) eu] support an
intramolecular isomerization scenario, one that involves the migration of phosphine and CO groups
about the cluster polyhedron. The disposition of the dppq ligand in the isomeric Os3(CO)10(dppq) clusters
has been established by X-ray crystallography and 31P NMR spectroscopy. Photolysis of 2c at 366 nm
leads to CO loss and ortho metalation of one of the aryl groups on the Ph2P moiety to furnish the hydride
Redox properties
cluster HOs3(CO)9[
follows that of related diphosphine-substituted Os3 clusters prepared by us.
Ó 2011 Elsevier B.V. All rights reserved.
m-PhP(h
1-C6H4)(Ph2P)quinoxaline] (3). The isomerization behavior exhibited by 2b
1. Introduction
the chelating diphosphine isomer serving as the thermodynami-
cally more stable form of the cluster.
The reactivity of a wide variety of ligands at the activated cluster
1,2-Os3(CO)10(MeCN)2 (1) has been intensely explored over the
years. Of the many different classes of ligands examined, diphos-
phine ligands have received extensive attention. These ligands have
been thoroughly investigated with respect to 1) the coordination
mode adopted by the ancillary diphosphine at the cluster poly-
hedron, 2) metal-directed bond activation of the ligand as a route to
unusual phosphido- and phosphinidene-capped clusters, and 3)
the dynamical properties associated with the fluxional behavior of
the ancillary CO and diphosphine ligands [1]. In the case of vicinal
diphosphine ligands that possess a rigid backbone, the formal
displacement of the MeCN ligands in 1 typically proceeds with the
formation of the bridging and/or chelating diphosphine isomers of
Os3(CO)10(PeP). The stereochemistry exhibited by the decacarbonyl
product depends upon the nature of the diphosphine ligand, with
The reaction between 1 and the diphosphine ligands
(Z)-Ph2PCH ¼ CHPPh2 (dppen) [2], 4,5-bis(diphenylphosphino)-
4-cyclopentene-1,3-dione (bpcd) [3], 2,3-bis(diphenylphos-
phino)-N-p-tolylmaleimide (bmi) [4], 2-(ferrocenylidene)-4,5-bis
(diphenylphosphino)-4-cyclopentene-1,3-dione (fbpcd) [5], 3,4-
bis(diphenylphosphino)-5-methoxy-2(5H)-furanone (bmf), and
1,2-bis(diphenylphosphino)benzene (dppbz) has been explored
by us, as part of our interest in the coordinative flexibility and
ligand activation of these and related ligands at polynuclear
clusters. The structures of these particular diphosphine ligands
are depicted in Scheme 1. All six pnictogen ligands react with 1 to
furnish initially the corresponding bridged clusters 1,2-
Os3(CO)10(PeP), which are not stable and readily isomerize to the
chelated 1,1-Os3(CO)10(PeP) isomers upon heating [6]. These
isomerizations are unaffected by trapping ligands, and each
reaction displays a rate law that is first order in starting cluster.
Mechanistically speaking,
a non-dissociative, unimolecular
process involving the migration of phosphorus and CO groups
about the cluster polyhedron is in keeping with the kinetic data
* Corresponding author. Tel.: þ1 940 565 3548; fax: þ1 940 565 4318.
E-mail address: cobalt@unt.edu (M.G. Richmond).
0022-328X/$ e see front matter Ó 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2011.01.019

S.W. Hunt et al. / Journal of Organometallic Chemistry 696 (2011) 1432e1440
1433
CO [13]. The dppq ligand employed in our studies was synthesized
from 2,3-dichloroquinoxaline and Ph2PH, as described in detail
below. The OsO4 was purchased from Engelhard Chemical Co., and
the chemicals Me3NO$xH2O, 2,3-dichloroquinoxaline, Ph2PH, and
BuLi (2.5 M in hexanes) were purchased from Aldrich Chemical Co.
The anhydrous Me3NO employed in our studies was obtained from
Me3NO$xH2O, after the waters of hydration were azeotropically
removed under reflux using benzene as a solvent. The molarity of
the BuLi was checked immediately before each reaction by titration
against diphenylacetic acid [14]. All reaction solvents were distilled
from a suitable drying agent under argon or obtained from an
Innovative Technology (IT) solvent purification system. When not
in use all 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. Combustion analyses were
performed by Atlantic Microlab, Norcross, GA.
O
O
N
O
O
Ph2P
PPh2
Ph2P
PPh2
Ph2P
PPh2
dppen
O
bpcd
O
bmi
MeO
O
Ph2P
Ph2P
PPh2
Ph2P
PPh2
Ph2P
O
Fe
fbpcd
bmf
dppbz
2.2. Instrumentation
Scheme 1.
The IR spectra were recorded on a Nicolet 6700 FT-IR spec-
trometer in amalgamated NaCl cells having a 0.1 mm path length,
while the quoted 1H NMR data were recorded at 400 MHz or
500 MHz on Varian VXR-400 and VXR-500 spectrometers, respec-
tively. The 31P NMR spectra were recorded at 202 MHz on the latter
spectrometer, and these data were collected in the proton-decou-
pled mode and are referenced to external H3PO4 (85%), whose
and the Eyring activation parameters. DFT calculations on the
rearrangement of bridged-to-chelated isomers have verified
a merry-go-round exchange of one phosphine and two CO groups
[7]. Here the concerted permutation of the three migrating
ligands parallels the exchange scheme originally postulated by
Cotton over forty years ago for CO migration about the CoeCo
vectors in Co4(CO)12 [8].
chemical shift was set at
d
¼ 0.
The rigid diphosphine ligand 2,3-bis(tert-butylmethylphos-
phino)quinoxaline (Quinox-P), which was first prepared by Ima-
moto et al. [9], has been employed as a chiral auxiliary in
asymmetric hydrogenation and CeC bond forming reactions [10].
The redox properties of several d6 and d8 mononuclear Quinox-P-
substituted compounds have been recently investigated by Kaim
et al., and the electron-withdrawing properties of the quinoxaline
platform verified by spectroelectrochemical and epr measurements
[11]. The observation of site-localized radical anions on the qui-
noxaline heterocycle confirms the ability of this diphosphine to
function as an electron reservoir in electron-transfer reactions.
Given the current interest in the Quinox-P ligand, coupled with
the absence of any reports on the reactivity of this genre of diphos-
phine with polynuclear compounds, we wished to explore the
substitution chemistry of the Quniox-P analogue 2,3-bis(diphenyl-
phosphino)quinoxaline (dppq). The Ph2P-substituted derivative
more closely matches those ligand systems already explored by us,
and this commonality will facilitate a reactivity comparison of the
different ligand systems as a function of the platform that tethers the
Ph2P moieties. Herein we report our results on the reaction between
Os3(CO)10(MeCN)2 and dppq, which affords the bridged and chelated
clusters Os3(CO)10(dppq) (2b and 2c) depending upon the reaction
conditions. Near-UV photolysis of 2c leads to CO loss and rapid ortho
metalation of one of the ancillary phenyl groups to furnish the cor-
2.3. Synthesis of the diphosphine ligand dppq
To a 500 mL Schlenk flask containing ca. 125 mL of hexane and
1.0 g (5.6 mmol) of Ph2PH at ꢀ78 ꢂC was added 2.4 mL (5.8 mmol) of
2.4 M BuLi dropwise, with stirring continued for additional 1 h at this
temperature. The slurry was allowed to warm to room temperature,
after which the hexane and the unreacted BuLi were cannulated away
from the precipitated Ph2PLi. The phosphide was dissolved in ca.
25 mL ofTHFandthesolutionwascooled to atꢀ78 ꢂC, after which the
phosphide solution was added dropwise to 0.52 g (2.6 mmol) of 2,3-
dichloroquinoxaline in 100 mL of THF at the same temperature. Upon
completion of the addition phase, stirring was continued overnight
with warming to room temperature. The yellow solution was
concentrated to dryness and then taken up in ca. 20 mL CH2Cl2, fol-
lowed by extraction with 3 ꢃ 10 mL portions of water, and drying of
the organic layer over MgSO4. The desired product was purified by
column chromatography over silica gel using CH2Cl2 as the eluent.
Recrystallization of the crude product from CH2Cl2/hexane at 5 ꢂC
gave dppq as a hygroscopic yellow solid in 85% yield (1.1 g). 1H NMR
(CDCl3):
d 7.20e7.29 (m, 12H, aryl), 7.30e7.35 (m, 8H, aryl), 7.67 (m,
0
0
2H, quinoxaline ring, JAB ¼ 8.5 Hz, JAB ¼ 1.3 Hz, JAA ¼ 0.7 Hz), 7.93 (m,
1
0
0
2H, quinoxaline ring, JAB ¼ 8.5 Hz, JAB ¼ 1.3 Hz, JBB ¼ 6.8 Hz). H NMR
(C6D6): 7.03 (bm,14H, aryl meta and para, quinoxaline), 7.51 (m, 8H,
aryl ortho), 7.78 (bm, 2H, quinoxaline). 31P NMR (CDCl3):
e 10.72 (s).
31P NMR (C6D6):
e 9.55. Anal. Calcd (found) for C32H24N2P2: C, 77.10
d
d
responding hydride cluster HOs3(CO)9[m-PhP(h
1-C6H4)(Ph2P)qui-
d
noxaline] (3). The redox properties of the dppq ligand and clusters 2b
and 2c have also been examined by cyclic voltammetry, and these
data are discussed with respect to the HOMO and LUMO levels
computed by DFT calculations.
(76.07); H, 4.85 (4.85).
2.4. Synthesis of 1,2-Os3(CO)10(dppq) (2b) from 1,2-
Os3(CO)10(MeCN)2 and dppq
2. Experimental
To a Schlenk tube under argon was charged 0.15 g (0.16 mmol) of
1,2-Os3(CO)10(MeCN)2, followed by 50 mL of CH2Cl2 via syringe. To
this solution was added, in one portion, 88 mg (0.17 mmol) of dppq
and the reaction was stirred at room temperature for 1 h. TLC
analysis using CH2Cl2/hexane (4:1) confirmed the consumption of
the starting cluster and the presence of a new spot attributed to the
2.1. General methods
Cluster 1 was prepared from Os3(CO)12, Me3NO, and MeCN [12],
while the parent cluster Os3(CO)12 was synthesized from OsO4 and

1434
S.W. Hunt et al. / Journal of Organometallic Chemistry 696 (2011) 1432e1440
desired product (Rf ¼ 0.93). The solvent was removed under
vacuum and the residue chromatographed over silica gel using the
aforementioned mobile phase. The crude product was then
recrystallized from a 1:1 mixture of hexane/benzene to afford 0.18 g
immediately subjected to three freezeepumpethaw degas cycles
prior to the admission of CO (1 atm). The NMR samples were
heated in an external VWR constant temperature bath at the
desired temperature, whose value is assumed to be accurate
within ꢁ0.5 K of the quoted temperature. The relative molar ratio
of the bridging and chelating isomers of 2 was calculated based on
the integral value associated with the 31P singlet of each isomer,
and the first-order rate constants were determined by graphical
analysis by measuring the consumption of the bridging isomer as
a function of time. In the case of the 1H NMR experiment, changes
in the cluster composition were determined by integrating the
different quinoxaline hydrogens of 2b relative to a known amount
of ferrocene, which was employed as an internal standard. The
activation parameters for the isomerization reaction were calcu-
lated from a plot of ln(k/T) versus Tꢀ1 [17], with the error limits
representing the deviation of the data points about the least-
squares line of the Eyring plot.
(84% yield) of 2b as a redeorange solid. IR (CH2Cl2):
2024 (m), 2009 (vs) 1973 (m), 1953 (m) cmꢀ1 1H NMR (CDCl3):
7.13e7.18 (m, 8H, aryl), 7.19e7.24 (m, 8H, aryl), 7.28e7.32 (m, 4H,
aryl), 7.76 (m, 4H, quinoxaline ring). 1H NMR (C6D6):
6.86e7.02
n(CO) 2089 (s),
.
d
d
(m, 14H, aryl meta and para, quinoxaline), 7.32 (m, 8H, aryl ortho),
0
7.63 (m, 2H, quinoxaline ring, JAB ¼ 8.4 Hz, JAB ¼ 1.4 Hz,
JBB ¼ 7.0 Hz) [16]. 31P NMR (CDCl3):
d
4.73 (s). 31P NMR (C6D6):
0
d
4.41 (s). Anal. Calcd (found) for C42H24N2O10Os3P2$benzene: C,
40.39 (39.96); H, 2.12 (2.64).
2.5. Isomerization of 2b to 1,1-Os3(CO)10(dppq) (2c) under CO
To a small Schlenk tube was added 0.10 g (0.074 mmol) of 2b and
25 mL of toluene, after which the solution was saturated with CO and
the vessel sealed. The reaction was heated overnight at 70 ꢂC in
a thermostated bath and then analyzed by TLC analysis, which
revealed the presence of an orange spot at Rf ¼ 0.91 using a 4:1
mixture of CH2Cl2/hexane. Cluster2cwasisolated, asdescribed above
for 2b, and recrystallized in hexane/benzene to afford 2c in 90% yield
2.8. Electrochemical data
The reported cyclic voltammetry (CV) data were recorded on
a PAR Model 273 potentiostat/galvanostat, equipped with positive
feedback circuitry to compensate for iR drop. The CVs were recor-
ded under oxygen- and moisture-free conditions in a homemade
three-electrode cell. A platinum disk was utilized as the working
and auxiliary electrode, and the reference electrode utilized a silver
wire as a quasi-reference electrode, with the reported potential
data standardized against the formal potential of Cp2Fe/Cp2Feþ
redox couple (external), taken to have E1/2 ¼ 0.31 V [18].
(90 mg). IR (CH2Cl2):
1989 (m), 1976 (m), 1959 (m), 1923 (w) cmꢀ1
7.30e7.38 (m, 12H, aryl meta and para), 7.44e7.48 (m, 8H, aryl
n
(CO) 2093 (s), 2068 (m), 2044 (vs), 2008 (b, vs),
.
1H NMR (CDCl3):
d
0
ortho), 7.95 (m, 2H, quinoxaline ring, JAB ¼ 8.4 Hz, JAB ¼ 1.2 Hz,
0
0
JAA ¼ 0.6 Hz), 8.22 (m, 2H, quinoxaline ring, JAB ¼ 8.4 Hz, JAB ¼ 1.2 Hz,
1
0
JBB ¼ 6.7 Hz). H NMR (C6D6):
d 6.94 (bm, 4H, para), 7.02e7.09 (bm,
10H, meta and dppq), 7.68e7.76 (bm, 10H, ortho and dppq). 31P NMR
(CDCl3): 13.20(s). Anal. Calcd (found) for
12.81 (s). 31P NMR (C6D6):
C42H24N2O10Os3P2$1/2benzene: C, 38.90 (38.31); H, 1.94 (2.58).
d
d
2.9. X-ray crystallography
Single crystals of dppq, 2b, and 2c$CH2Cl2 suitable for diffraction
analysis were grown from CH2Cl2/hexane at 5 ꢂC, while the X-ray
quality crystals of 3$2CDCl3 were obtained by the slow evaporation
of CDCl3 from an NMR tubing containing the cluster 3. The X-ray
data were collected on an APEX II CCD-based diffractometer at 100
(2) K. The frames were integrated with the available APEX2 soft-
ware package using a narrow-frame algorithm [19], and the
structures were solved and refined using the SHELXTL program
package [20]. Each molecular structure was checked using PLATON
[21], and all non-hydrogen atoms were refined anisotropically
unless otherwise noted. Absorption corrections were applied to all
four compounds using SADABS [22]. The location of the bridging
hydride ligand in 3$2CDCl3 was determined with the aid of XHY-
DEX [23], and the hydride group was allowed to ride on the
attached Os(1) and Os(2) atoms with appropriate distance
restraints. The disorder found in the solvent molecule in 2c$CH2Cl2
was refined accordingly with distance and similarity restraints. All
hydrogen atoms were assigned calculated positions and allowed to
ride on the attached carbon atom during data reduction. The X-ray
data and processing parameters for the four compounds are
reported in Table 1, with selected bond distances and angles for
clusters 2 and 3 quoted in Table 2.
2.6. Synthesis of HOs3(CO)9[m-2,3-PhP(h
1-C6H4)(Ph2P)quinoxaline]
(3) from 2c
0.10 g (0.074 mmol) of 2c in a small Schlenk tube was treated
with 20 mL of toluene and the solution was then subjected to
three freezeepumpethaw degas cycles. The vessel was sealed and
irradiated (366 nm) at room temperature for a period of 3 days.
The CO that accompanies the formation of 3 was periodically
removed by additional freezeepumpethaw degas cycles, in order
to drive the reaction to completion. Following photolysis, the
reaction solvent was concentrated under vacuum and the residue
was purified by column chromatography, followed by recrystalli-
zation using hexane/benzene to give 3 in 88% yield (86 mg) as
a yelloweorange solid. IR (CH2Cl2):
2016 (vs), 2001 (m), 1986 (sh, m), 1979 (m), 1965 (m) cmꢀ1
NMR (CDCl3):
n
(CO) 2082 (vs), 2041 (vs),
.
1H
d
e 16.26 (t, m2-H, JP-H ¼ 13 Hz), 6.97e7.06 (m, 2H),
7.14e7.19 (m, 2H), 7.26e7.29 (m, 3H), 7.36e7.45 (m, 3H), 7.52e7.57
(m, 3H), 7.59e7.64 (m, 2H), 7.73e7.78 (m, 2H), 7.80e7.86 (m, 2H),
8.07e8.12 (m, 2H), 8.15 (bd, 1H, J ¼ 8.0 Hz), 8.29 (bt, 1H, J ¼ 7.5 Hz).
1H NMR (C6D6):
d
e 15.84 (t, m2-H, JP-H ¼ 13 Hz), 6.90e7.04 (m,
12H), 7.08 (bt, 1H, J ¼ 7.5 Hz), 7.37e7.42 (m, 2H), 7.53 (d, 1H,
J ¼ 5.0 Hz), 7.59e7.65 (m, 3H), 7.91e7.96 (m, 2H), 8.39 (bm, 1H),
8.70 (t, 1H, J ¼ 7.5 Hz). 31P NMR (CDCl3):
d
14.34, (d, JP-P ¼ 24 Hz),
13.60, (d, JP-P ¼ 24 Hz),
2.10. Computational methodology
18.95 (d, JP-P ¼ 24 Hz). 31P NMR (C6D6):
d
17.58 (d, JP-P ¼ 24 Hz). Anal. Calcd (found) for C37H24O11Os3P2: C,
The DFT calculations reported here were performed with the
Gassian09 package of programs [24]. The calculations were carried
out with the B3LYP functional, which utilizes the Becke three-
parameter exchange functional (B3) [25] combined with the
correlation functional of Lee, Yang, and Parr (LYP) [26]. The osmium
atoms were described by StuttgarteDresden effective core poten-
tials (ecp) and the SDD basis set, while the 6e31G(d0) basis set, as
implemented in the Gaussian09 program suite, was employed for
37.27 (37.10); H, 1.83 (1.92).
2.7. Kinetic measurements
The conversion of 2b / 2c was investigated by either 31P or 1H
NMR spectroscopy in 5 mm NMR tubes that were equipped with
a
J-Young valve. Each sample was freshly prepared and

S.W. Hunt et al. / Journal of Organometallic Chemistry 696 (2011) 1432e1440
1435
Table 1
X-ray crystallographic data and processing parameters for the ligand dppq and the triosmium clusters 2b, 2c$CH2Cl2, and 3$2CDCl3.
Compound
dppq
2b
2c
3
CCDC entry no.
Cryst syst
730322
Triclinic
P ꢀ 1
730323
Triclinic
P ꢀ 1
730320
Triclinic
P ꢀ 1
730321
Triclinic
P ꢀ 1
Space group
ꢀ
a, A
9.373(2)
10.087(2)
14.860(3)
74.249(2)
75.552(2)
72.217(2)
1266.1(4)
C32H24N2P2
498.47
12.114(1)
12.973(1)
13.706(1)
89.681(1)
69.009(1)
86.667(1)
2007.2(3)
C42H24N2O10Os3P2
1349.17
2
10.4813(5)
12.4726(6)
17.7436(9)
94.388(1)
106.205(1)
94.191(1)
2210.1(2)
C43H26Cl2N2O10Os3P2
1434.10
10.972(2)
12.140(2)
18.592(3)
83.210(2)
75.021(1)
79.369(1)
2344.8(6)
ꢀ
b, A
ꢀ
c, A
a
, deg
, deg
b
g
, deg
3
ꢀ
V, A
Mol formula
fw
C43H24Cl6D2N2O9Os3P2
1561.91
2
2.212
0.71073
8.575
Formula units per cell (Z)
2
2
Dcalcd (Mg/m3)
1.308
0.71073
0.196
0.9442/0.9404
5641
2.232
0.71073
9.614
0.2615/0.1339
24,370
2.155
0.71073
8.855
0.4755/0.1490
27,231
ꢀ
l
(Mo Ka), A
Absorption coeff (mmꢀ1
Abs corr factor
)
0.6641/0.4840
27,210
Total reflections
Independent reflections
Data/res/parameters
5641
5641/0/326
0.0535
0.1328
1.133
8665
8665/0/532
0.0169
0.0452
1.004
9401
9401/10/569
0.0241
0.0651
1.050
9185
9185/2/590
0.0367
0.0515
0.996
R1a (I ꢄ 2
s(I)]
wR2b
GOF on F2
3
ꢀ
Dr(max), Dr(min) (e/A )
0.856/ꢀ0.614
0.955/ꢀ1.487
2.636/ꢀ1.824
1.147/ꢀ0.882
a
R1 ¼ SkFoj ꢀ jFck/SjFoj.
b
½
R2 ¼ {S[w(F2o ꢀ Fc2)2/S[w(F2o)2]}
.
the remaining atoms. The geometry-optimized structures reported
here represent minima based on zero imaginary frequencies
(positive eigenvalues), as established by frequency calculations
using the analytical Hessian.
Table 2
ꢂ
ꢀ
Selected bond distances (A) and angles ( ) for the triosmium clusters 2b, 2c$CH2Cl2,
and 3$2CDCl3.
3. Discussion
Cluster 2b
Bond distances
Os(1)eP(1)
Os(1)eOs(3)
Os(2)eOs(3)
P(1)/P(2)
2.3403(8)
2.9097(3)
2.8638(3)
3.621(1)
Os(1)eOs(2)
Os(2)eP(2)
C(11)eC(12)
2.8813(3)
2.3286(8)
1.446(4)
3.1. Synthesis and X-ray structure of dppq
The synthesis of dppq has recently been reported by Glueck
et al. through the CuCl-catalyzed coupling of 2,3-dichlor-
oquinoxaline with Ph2PH [27,28]. Alternatively, the reaction
between 2,3-dichloroquinoxaline and Ph2PLi, the latter which is
prepared in situ from BuLi and Ph2PH, also furnishes the desired
diphosphine in high yield as a relatively air-stable, yellow solid. Our
procedure makes use of the facile addition-elimination reaction of
the nucleophilic Ph2Pꢀ anion at the chlorine-activated C2 and C3
centers of the parent heterocycle [29]. The 1H and 31P NMR spec-
troscopic properties of dppq recorded in CDCl3 and C6D6 are
consistent with the formulated structure. The solid-state structure
of dppq was crystallographically determined, and the thermal
ellipsoid plot of the molecular structure is shown in Fig. 1, whose
caption also contains selected bond distances and angles for dppq.
The diphosphine ligand was also examined by DFTcalculations, and
the geometry-optimized B3LYP structure of dppq is depicted in the
right-hand side of Fig. 1. The optimized structure computed for
dppq, and with it the basic disposition of the aryl groups associated
with the P(1) and P(2) moieties, shows an excellent correspondence
with the solid-state structure. This uniformity between experiment
and theory is important in terms of the orbital properties to be
discussed in connection with the cyclic voltammetric behavior of
dppq and the dppq-substituted clusters 2b and 2c.
Bond angles
P(1)eOs(1)eOs(2)
P(2)eOs(2)eOs(3)
C(11)eP(1)eOs(1)
103.13(2)
154.21(2)
115.35(9)
P(1)eOs(1)eOs(3)
P(2)eOs(2)eOs(1)
C(12)eP(2)eOs(2)
159.18(2)
93.67(2)
113.0(1)
Cluster 2c
Bond distances
Os(1)eP(2)
Os(1)eOs(3)
Os(2)eOs(3)
P(1)/P(2)
2.299(1)
2.9116(3)
2.8983(3)
3.132(1)
Os(1)eP(1)
Os(1)eOs(2)
C(11)eC(12)
2.299(1)
2.9237(3)
1.426(6)
Bond angles
P(2)eOs(1)eP(1)
P(1)eOs(1)eOs(2)
C(12)eP(2)eOs(1)
C(11)eC(12)eP(2)
85.90(4)
108.60(3)
106.5(1)
117.6(3)
P(2)eOs(1)eOs(2)
C(11)eP(1)eOs(1)
C(12)eC(11)eP(1)
163.57(3)
105.3(1)
116.8(3)
Cluster 3
Bond distances
Os(1)eOs(3)
Os(2)eOs(3)
Os(2)eH(1)
Os(1)eP(2)
C(10)eC(11)
2.9300(5)
2.8720(5)
1.71(6)
2.367(2)
1.419(9)
Os(1)eOs(2)
Os(1)eH(1)
Os(1)eP(1)
Os(2)eC(37)
2.9683(5)
1.79(6)
2.310(2)
2.192(8)
Bond angles
C(1)eOs(1)eP(2)
P(1)eOs(1)eP(2)
P(1)eOs(1)eOs(2)
C(4)eOs(2)eOs(3)
C(8)eOs(3)eOs(2)
91.6(2)
85.86(6)
115.42(5)
84.7(2)
C(2)eOs(1)eP(2)
C(1)eOs(1)eOs(3)
C(5)eOs(2)eC(37)
C(3)eOs(2)eOs(1)
C(9)eOs(3)eOs(1)
171.7(2)
88.0(2)
174.3(3)
115.9(2)
98.2(2)
3.2. Synthesis, spectroscopic data, and solid-state structure for 2b
Treatment of 1,2-Os3(CO)10(MeCN)2 (1) with dppq in CH2Cl2 at
room temperature leads to the diphosphine-bridged cluster 2b as
100.5(2)

1436
S.W. Hunt et al. / Journal of Organometallic Chemistry 696 (2011) 1432e1440
ꢀ
Fig. 1. Thermal ellipsoid plot of the molecular structure of dppq at the 50% probability level (left) and the optimized B3LYP structure of dppq (right). Selected bond distances (A) and
angles (deg) for the X-ray structure: P(1)eC(1) ¼ 1.850(2), P(2)eC(2) ¼ 1.854(2), N(1)eC(1) ¼ 1.310(3), N(1)eC(8) ¼ 1.378(3), N(2)eC(2) ¼ 1.324(3), N(2)eC(3) ¼ 1.373(3), C(1)eC
(2) ¼ 1.437(3), P(1)/P(2) ¼ 3.193(2), N(1)eC(1)eP(1) ¼ 119.5(2), C(2)eC(1)eP(1) ¼ 118.1(2), N(2)eC(2)eP(2) ¼ 121.0(2), C(1)eC(2)eP(2) ¼ 117.5(2). The bond distances specified in
the B3LYP structure are reported in angstrom.
the sole observable product in 84% isolated yield. 2b was charac-
a significant “stretching” of the dppq ligand and an overall ground-
state destabilization of cluster 2b relative to the chelating isomer
terized in solution by IR and NMR spectroscopy, and the recorded
high-field singlet in the 31P NMR spectrum at
d
4.41 in C6D6 is in full
2c. Here the internuclear P(1)/P(2) distance of 3.621(1) A found in
ꢀ
ꢀ
agreement with the bridging of adjacent osmium atoms by the
phosphine atoms of the dppq ligand. The formation of 2b as the
kinetically favored substitution product follows the trend displayed
by the rigid diphosphine ligands depicted in Scheme 1 and their
reaction with cluster 1. The molecular structure of 2b, which is
shown in Fig. 2, was unequivocally established by X-ray diffraction
analysis. The bridging disposition of the dppq ligand and its
equatorial coordination to the Os(1) and Os(2) centers are
2b is significantly longer (ca. >0.40 A) than the corresponding
P(1)/P(2) distance found in free dppq and the chelated cluster 2c.
This perturbation serves as the driving force for the isomerization
of 2b to 2c (vide supra), paralleling the isomerization behavior of
the diphosphine ligands depicted in Scheme 1. Moreover, the
theoretical underpinning for the isomerization of the PeP ligand in
Os(CO)10(PeP) clusters has been thoroughly examined by us as
a function of the diphosphine ligand [7]. The B3LYP optimized
structure of 2b is depicted in Fig. 2, and apart from the enhanced D3
twist of the axial CO groups relative to the metallic plane and the
slightly elongated internuclear P(1)/P(2), it is in qualitative
agreement with the crystallographic structure.
ꢀ
confirmed. The OseOs bond distances range from 2.8638(3) A [Os
ꢀ
(2)eOs(3)] to 2.9097(3) A [Os(1)eOs(3)] and are in good agreement
with those OseOs bond distances in the parent cluster Os3(CO)12
and numerous diphosphine-bridged clusters Os3(CO)10(PeP)
ꢀ
[2e5,30,31]. The two OseP bonds display distances of 2.3403(8) A
ꢀ
[Os(1)eP(1)] and 2.3286(8) A [Os(2)eP(2)], while the ten terminal
3.3. Isomerization kinetics of 2b to 2c
carbonyl groups exhibit distances and angles consistent with their
linear nature. The quinoxaline platform that serves to tether the
vicinal Ph2P moieties is tipped up out of the plane defined by three
osmium atoms based on a fold angle of 54.35(5)ꢂ. Coordination of
the dppq ligand across the Os(1)eOs(2) vector is accompanied by
Numerous reports now exist on the non-dissociative, bridge-to-
chelate isomerization of the ancillary diphosphine in Os(CO)10(PeP)
clusters [2e5,7,32]. Naturally, this led us to examine the thermal
stability of 2b in order to establish the influence of the dppq ligand on
Fig. 2. Thermal ellipsoid plot of the molecular structure of 2b (left; at the 50% probability level) and the optimized B3LYP structure of 2b (right). The bond distances specified in the
B3LYP structure are reported in angstrom.

S.W. Hunt et al. / Journal of Organometallic Chemistry 696 (2011) 1432e1440
1437
N
Ph2
P
Os
Os
Os
H
N
Ph2
h
υ⎯or Δ
P
-CO
PPh2
N
Δ
Os
Os
Os
Os
Os
Os
PPh2
+CO
P
N
P
Ph2
Ph
N
1,2-Os3(CO)10(dppq)
2b
1,1-Os3(CO)10(dppq)
2c
N
Os3(CO)10[μ-2,3-PhP(η1-C6H4)(Ph2P)quinoxaline]
3
Scheme 2.
the isomerization reaction. When heated under CO, 2b isomerizes
cleanly to 2c and this confirms that the latter cluster is the thermo-
dynamically more stable form of Os(CO)10(dppq). The addition of CO
suppresses the formation of the hydride cluster 3, but CO addition has
been shown not to affect the equilibrium for the bridged-to-chelated
ligand isomerization in other decacarbonyl clusters Os3(CO)10(PeP)
examined by our group [2e5,7,32]. Scheme 2 illustrates this isom-
erization and the fate of the unsaturated intermediate,
Os3(CO)9(dppq) (not shown), that forms from decarbonylation of 2c
and which functions as the precursor to the hydride cluster 3.
Cluster 2c was isolated by column chromatography, character-
ized in solution by traditional spectroscopic methods, and the
molecular structure determined by X-ray diffraction analysis. The
linkages display values that are in excellent agreement with the
idealized hybridization state of the subtended atom. The observed
angle of 85.90(4)ꢂ for the P(1)eOs(1)eP(2) atoms is in keeping with
the chelating nature of the ancillary dppq ligand. The DFT structure
computed for 2c that is depicted in Fig. 3 shows a good corre-
spondence with the diffraction structure. Here the computed bond
distances and angles, and the orientation of the phenyl groups at
the P(1) and P(2) atoms closely match the crystallographic values
and disposition of the ancillary ligands. The DFT calculations on the
isomeric Os3(CO)10(dppq) clusters confirm the greater stability of
dppq-chelated species vis-à-vis the bridging isomer 2b. Here 2c is
computed to have a 4.8 kcal/mol lower free energy than 2b, which
translates to a Keq > 3300 in favor of 2c [34]; this finding is in
keeping with our recent calculations on a series of diphosphine-
substituted Os3(CO)10(PeP) clusters [7].
singlet recorded at d
13.20 in C6D6 in the 31P NMR spectrum of 2c is
in full agreement with a triosmium cluster possessing a chelating
dppq ligand. The large nuclear deshielding in the 31P resonance that
accompanies the bridge-to-chelate isomerization is a diagnostic
feature of such a reaction [33]. Fig. 3 shows the thermal ellipsoid
plot of the molecular structure of 2c, as the CH2Cl2 solvate, and
confirms the attendant isomerization of the dppq ligand upon
The 2b / 2c isomerization was investigated kinetically by NMR
spectroscopy under CO, over the temperature range 323e353 K.
The first-order rate constants are given in Table 3. The progress of
these reactions was easily monitored by following the concentra-
tion changes in either the 31P singlet or 1H resonances associated
with 2b as a function of time. The Eyring plot of these data is shown
ꢀ
heating. The OseOs bond distances range from 2.8983(3) A [Os(2)e
Os(3)] to 2.9237(3) A [Os(1)eOs(2)]. The internuclear P(1)/P(2)
in Fig. 4, and the Eyring activation parameters [
D
Hs ¼ 25.4(4) kcal/
ꢀ
distance of 3.132(1) A has undergone a significant contraction,
mol;
D
Ss ¼ ‑3(1) eu] parallel those data reported by us for related
ꢀ
relative to that distance found in 2b, and the angles associated with
the Os(1)eP(1)eC(11) [105.3(1)ꢂ], Os(1)eP(2)eC(12) [106.5(1)ꢂ], C
(12)eC(11)eP(1) [116.8(3)ꢂ], and C(11)eC(12)eP(2) [117.6(3)ꢂ]
ligand isomerizations (Table 4), whose intramolecular isomeriza-
tion involves the “merry-go-round” migration of phosphine and CO
groups about the cluster polyhedron.
Fig. 3. Thermal ellipsoid plot of the molecular structure of 2c (left; at the 50% probability level) and the optimized B3LYP structure of 2c (right). The CH2Cl2 solvent molecule in the
X-ray diffraction structure has been omitted for clarity, and the bond distances specified in the B3LYP structure are reported in angstrom.

1438
S.W. Hunt et al. / Journal of Organometallic Chemistry 696 (2011) 1432e1440
Table 3
experiment establishes the reversible nature of the ortho metal-
ation of the ancillary dppq ligand.
Experimental rate constants for the isomerization of 1,2-Os3(CO)10(dppq) to 1,1-
Os3(CO)10(dppq) under CO.a
3 was isolated in 88% yield after purification by column chro-
matography, followed by recrystallization. The bridging hydride
Entry
Temp (K)
106k (sꢀ1
)
recorded at
of inequivalent doublets recorded at
d
ꢀ15.84 in the 1H NMR spectrum (in C6D6) and the pair
1
2
3
4
323.0
333.0
343.0
353.0
2.25(2)
8.9(1)
d
13.60 and 17.58 in the 31P
26.2(4)b
73(2)
NMR spectrum are in keeping with the formulated structure of 3.
Fig. 5 shows the thermal ellipsoid plot of the molecular structure of
3, as the bis-CDCl3 solvate, and confirms the ortho metalation of one
of the aryl rings of the dppq ligand. 3 contains 48e and is elec-
tronically saturated, assuming that the ortho-metalated dppq
ligand functions as a 5e donor group. The OseOs bond lengths range
a
The 31P NMR kinetic data were collected in CDCl3 solvent using a ca. 10ꢀ2
M
solution of starting cluster by following the change in the 31P resonances for the
bridging and chelating isomers of Os3(CO)10(dppq). All NMR reactions were per-
formed under 1 atm CO.
b
1H NMR reaction conducted in CDCl3 using a ca. 10ꢀ2 M of starting cluster in the
ꢀ
ꢀ
presence of Cp2Fe (internal standard).
from 2.8720(5) A [Os(2)eOs(3)] to 2.9683(5) A [Os(1)eOs(2)], with
the latter OseOs vector serving as the locus of the bridging hydride
ligand [36]. The stereochemical disposition of the C(3)O(3) and P(1)
groups about the cluster polyhedron further supports our conten-
tion concerning the location of the hydride. Here the Os(1)eOs(2)e
C(3) and Os(2)eOs(1)eP(1) angles of 115.9(2) and 115.42(5)ꢂ,
respectively, are significantly expanded from the idealized value of
ca. 90ꢂ typically found for equatorial substituents at triangular
clusters due to the presence of the ancillary hydride [37,38]. Both
phosphine atoms are coordinated to the Os(1) center and display
axial [P(2)] and equatorial [P(1)] dispositions. The Os(2)eC(37)
-15
-16
-17
-18
-19
ꢀ
bond distance of 2.192(8) A is comparable to the OseC bond
distance found in the triosmium clusters H2Os3(CO)9( -C6H4PPh),
m
HOs3(CO)9(PPh3)( -SbPh2)( -C6H4), and Os3(CO)9[m-PPh(C6H4)
m
m
CH2PPh], each of which possesses an activated aryl moiety [39].
3.5. Redox behavior and MO properties of the HOMO and LUMO
levels in dppq and clusters 2b and 2c
0.0028
0.0029
0.0030
1/T, K-1
0.0031
The redox properties of the dppq ligand and the isomeric
clusters Os3(CO)10(dppq) were examined by cyclic voltammetry
(CV) in CH2Cl2 containing 0.2 M TBAP as the supporting electrolyte.
The CV of dppq recorded over the potential range of 1.4 V to ꢀ1.6 V
and at a scan rate of 250 mV/s revealed a diffusion-controlled,
one-electron reduction at E1/2 ¼ ꢀ1.66 V [40]. Employing the
electrochemical assignments of Kaim for the R,R-2,3-bis(tert-
butylmethylphosphino)quinoxaline ligand, the site of the 0/1ꢀ
redox couple in dppq is readily ascribed to quinoxaline platform
[11]. A single, irreversible oxidation at Eap ¼ 1.03 was also found,
and this wave remained irreversible under all conditions explored
(scan rates up to 1.0 V/s and temperatures down to 243 K). The
Fig. 4. Eyring plot for the conversion of 2b / 2c over the temperature range of
323e353 K.
3.4. Reversible ortho metalation in 2c and formation of
HOs3(CO)9[m-2,3-PhP(h
1-C6H4)(Ph2P)quinoxaline] (3)
The reactivity of 2c was next examined as part of our ongoing
interest in ligand activation processes associated with cluster-
coordinated ligands [1f,2e5,7,32,35]. 2c is stable to CO loss at
temperatures below 333 K, but is sensitive to near-UV irradiation
(366 nm). Photolysis of a toluene solution containing 2c leads to CO
loss and formation of 3. The hydride-bridged cluster 3 was obtained
in high yield and without complications by simply purging the
reaction solution during photolysis, using nitrogen or argon to
remove the liberated CO. Alternatively, CO removal through several
freezeepumpedegas cycles during the course of the irradiation
also furnishes 3 in comparable yield. This particular reaction is
particularly sensitive to extraneous CO, and treatment of 3 with
added CO at 333 K regenerates 2c in quantitative yield. This latter
Table 4
Summary of the Eyring activation data for the bridge-to-chelate isomerization of
different diphosphine ligands in the triosmium clusters Os3(CO)10(PeP).
Ligand
D
Hs (kcal/mol)
D
Ss (eu)
Ref
dppena
bpcd
bmi
fbpcd
dppbz
bmfa
26.5(6)
25.0(7)
24.0(1)
23.1(3)
21.6(3)
26.9(7)
25.4(4)
ꢀ5(2)
ꢀ2(2)
ꢀ5(1)
ꢀ7(1)
ꢀ11(1)
6(1)
[2]
[3]
[4]
[5]
[7]
Unpublished
This work
dppq
ꢀ3(1)
a
The isomerization is reversible and only the activation parameters for the
forward portion of the reaction are reported.
Fig. 5. Thermal ellipsoid plot of the molecular structure of 3 at the 50% probability
level. The CDCl3 solvent molecules have been omitted for clarity.

S.W. Hunt et al. / Journal of Organometallic Chemistry 696 (2011) 1432e1440
1439
Fig. 6. Contour plots of the HOMO (left) and LUMO (right) for dppq.
Fig. 7. Contour plots of the HOMO (left) and LUMO (right) for 2b.
orbital composition of the HOMO and LUMO levels in dppq was
addressed through DFT analysis, using the optimized structure for
bonding orbitals [41]. Accordingly, the oxidation in 2b is ascribed to
an oxidation of an OseOs bonding orbital, one with little or no dppq
character. Our redox assignments for 2b were validated by DFT
analysis of the HOMO and LUMO levels, whose contour plots are
depicted in Fig. 7.
Isomerization of the dppq ligand in 2b does not lead to signifi-
cant changes in the redox properties of 2c. Apart from the minor
potential differences in the reversible reduction (E1/2 ¼ ‑1.62) and
the irreversible oxidation (Eap ¼ 0.34), the recorded CV of 2c is
unexceptional relative to that found for 2b; the DFT computed
HOMO and LUMO levels for 2c (not shown) follow suit and warrant
no further comment.
dppq. As seen in the contour diagrams in Fig. 6, the
p* LUMO is
localized almost exclusively on the heterocyclic platform, while
the principal contribution to the HOMO is best viewed as origi-
nating from the out-of-phase combination involving the lone-
electron pair orbital at each phosphorus atom.
The CV of 2b was recorded in CH2Cl2 under conditions similar to
those described above for dppq. A reversible, one-electron reduc-
tion (E1/2 ¼ ꢀ1.72 V) and an irreversible, multielectron oxidation
(Eap ¼ 0.52) were found for 2b. While electron accession at the
heterocyclic ring of the bridging dppq ligand accounts for the 0/1ꢀ
redox process, the observed cathode shift in the Eap wave is incon-
sistent with an dppq-based oxidation. Coordination of the dppq
ligand to the Os3 frame in 2b is expected to an anodic shift in the
redox potential of the energy of the out-of-phase lone-electron pair
(i.e., the HOMO of the free ligand), as demonstrated by Kaim for
different mononuclear Quinox-P-substituted derivatives [11].
Phosphine substitution at metal clusters has been shown to raise
the energy of metal-based orbitals, especially metalemetal
4. Conclusions
Replacement of the MeCN ligands in 1,2-Os3(CO)10(MeCN)2 by
the diphosphine ligand dppq furnishes 1,2-Os3(CO)10(dppq) as
the kinetic product of ligand substitution. The dppq ligand in
1,2-Os3(CO)10(dppq) is coordinatively flexible and readily isomer-
izes under CO (1 atm) over the temperature range of 323e353 K to

1440
S.W. Hunt et al. / Journal of Organometallic Chemistry 696 (2011) 1432e1440
[11] A.K. Das, E. Bulak, B. Sarkar, F. Lissner, T. Schleid, M. Niemeyer, J. Fiedler,
W. Kaim, Organometallics 27 (2008) 218.
furnish 1,1-Os3(CO)10(dppq) in quantitative yield. The ground-state
energy difference between the kinetic and thermodynamic forms of
Os3(CO)10(dppq) has been evaluated by DFT calculations, and the
computed energy difference is in concert with the experimental
data. The redox properties of the free ligand and the decacarbonyl
products were investigated by cyclic voltammetry, and the LUMO in
all three compounds was found to be localized on the quinoxaline
platform, as verified by DFT calculations. Future studies will probe
the electrochemical behavior of different Os3(CO)10(P‑P) clusters
that contain a redox-active PeP auxiliary, one whose potential is
tunable as a function of the ancillary substituents and heterocyclic
architecture.
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[16] We have arbitrarily assigned the shielded portion of quinoxaline 1H spin
system as belonging to the AA0 hydrogens. The JAA coupling constant was
0
computed to be 0.8 Hz.
[17] B.K. Carpenter, Determination of Organic Reaction Mechanisms. Wiley-
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Financial support from the Robert A. Welch Foundation (Grant
B-1093-MGR) is greatly appreciated, and X. Wang acknowledges
support by the U.S. Department of Energy, Office of Science, under
Contract No. DE-AC05-00OR22725 managed by UT Battelle, LLC.
NSF support of the NMR and computational facilities at UNT
through grants CHE-0840518 and CHE-0741936 is acknowledged.
We also wish to thank Prof. Michael B. Hall (TAMU) for providing us
a copy of his JIMP2 program, which was used to prepare the
geometry-optimized structures reported here, and Dr. David A.
Hrovat (Center for Advanced Scientific Computing and Modeling,
UNT) for his assistance and guidance with computational aspects
for this work.
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Appendix A. Supporting information
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CCDC 730322, 730323, 730320, and 730321 contain the
supplementary crystallographic data for the ligand dppq and the
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