Ligand substitution in 1,2-Os3(CO)10(MeCN) 2 by the diphosphine (PhO)2PN(Me)N(Me)P(OPh)2: X-ray diffraction structure of 1,2-Os3(CO)10[(PhO) 2PN(Me)N(Me)P(OPh)2</

Article abstract of DOI:10.1016/j.molstruc.2011.11.037

The diphosphine 1,2-bis(diphenoxyphosphino)-1,2-dimethylhydrazine, (PhO)₂PN(Me)N(Me)P(OPh)₂, reacts with 1,2-Os ₃(CO)₁₀(MeCN)₂ (1) to afford the corresponding ligand-bridged cluster 1,2-Os₃

Full text of DOI:10.1016/j.molstruc.2011.11.037

Journal of Molecular Structure 1010 (2012) 91–97
Contents lists available at SciVerse ScienceDirect
Journal of Molecular Structure
journal homepage: www.elsevier.com/locate/molstruc
Ligand substitution in 1,2-Os3(CO)10(MeCN)2 by the diphosphine
(PhO)2PN(Me)N(Me)P(OPh)2: X-ray diffraction structure of
1,2-Os3(CO)10[(PhO)2PN(Me)N(Me)P(OPh)2] and DFT investigation of the
isomeric Os3(CO)10[(PhO)2PN(Me)N(Me)P(OPh)2] clusters
⇑
Sean W. Hunt, Vladimir Nesterov, Michael G. Richmond
Department of Chemistry, University of North Texas, Denton, TX 76203, United States
a r t i c l e i n f o
a b s t r a c t
Article history:
The diphosphine 1,2-bis(diphenoxyphosphino)-1,2-dimethylhydrazine, (PhO)2PN(Me)N(Me)P(OPh)2,
reacts with 1,2-Os3(CO)10(MeCN)2 (1) to afford the corresponding ligand-bridged cluster 1,2-Os3(CO)10[(-
PhO)2PN(Me)N(Me)P(OPh)2] (2b) in high yield. The product cluster has been characterized in solution by
IR and NMR spectroscopy, and the solid-state structure determined by X-ray crystallography. Cluster 2b
is thermally robust, and shows no evidence for formation of the corresponding chelated isomer 1,1-
Os3(CO)10[(PhO)2PN(Me)N(Me)P(OPh)2] (2c) in refluxing toluene. The energetics for the bridged and che-
lated clusters have been evaluated by electronic structure calculations, and the thermodynamic prefer-
ence for the ligand-bridged isomer has been confirmed. DFT calculations on the known bridged and
chelated diphosphine isomers of Os3(CO)10(dppe) clusters have also been performed, and these data
are contrasted with clusters 2b and 2c.
Received 28 September 2011
Received in revised form 22 November 2011
Accepted 22 November 2011
Available online 29 November 2011
Keywords:
Metal cluster chemistry
Ligand substitution
Ligand rearrangement
DFT calculations
Ó 2011 Elsevier B.V. All rights reserved.
X-ray crystallography
1. Introduction
Compared with the diphosphine ligands in Eq. (1), the ligands
depicted in Scheme 1, all of which are structurally rigid and
The reactivity of the labile cluster 1,2-Os3(CO)10(MeCN)2 (1)
with a variety of diphosphine ligands (PAP) has been investigated
with respect to the coordination mode adopted by the ancillary
diphosphine ligand at the cluster polyhedron [1–9]. The well-
known diphosphine ligands dppm, dppe, dppp, and dppb, all of
which possess a saturated carbon backbone, react with 1 to give
1,2-Os3(CO)10(PAP) as the principal product of ligand substitution
(Eq. (1) [1–3,5]). These products are stable with respect to intramo-
lecular isomerization to the corresponding chelated species 1,1-
Os3(CO)10(PAP).
possess an unsaturated two-carbon backbone, react with cluster
1 to afford 1,2-Os3(CO)10(PAP) as the kinetic product of ligand sub-
stitution [10–13]. These diphosphine-bridged isomers are unsta-
ble, and they readily transform into the corresponding chelated
isomer in a non-dissociative, unimolecular process that is unaf-
fected by the presence of trapping ligands. A recent DFT study
has provided compelling evidence for a rate-limiting step that in-
volves the merry-go-round exchange of two equatorial CO groups
and one of the phosphine moieties [14]; Eq. (2) illustrates this
particular transformation in the case of the ligand 1,2-(Z)-
bis(diphenylphosphino)ethylene.
Ph2
P
NCMe
Os
Ph2
(CH2)n
PPh2
Os
P
Ph2
Os
Os
P
Ph2P(CH2)nPPh2
where n = 1-4
PPh2
Os
Os
Os
Os
Os
Os
Os
Os
PPh2
NCMe
ð2Þ
ð1Þ
Os
Ph2
P
Os
Os
P
⇑
Corresponding author. Tel.: +1 940 565 3548.
E-mail address: cobalt@unt.edu (M.G. Richmond).
Ph2
0022-2860/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.molstruc.2011.11.037

92
S.W. Hunt et al. / Journal of Molecular Structure 1010 (2012) 91–97
O
O
Ph2P
PPh2
PPh2
Ph2P
Ph2P
PPh2
dppen
O
bpcd
O
dppbz
MeO
O
Ph2P
N
N
PPh2
Ph2P
PPh2
Ph2P
PPh2
O
Fe
fbpcd
bmf
dppq
Scheme 1. Diphosphine ligands that exhibit bridge-to-chelate isomerization in Os3(CO)10(PAP).
In this report we present our results on the reaction between
2.2. Synthesis of 1,2-Os3(CO)10[(PhO)2PN(Me)N(Me)P(OPh)2] (2b)
the bis(phosphanyl)hydrazine ligand 1,2-bis(diphenoxyphosphi-
no)-1,2-dimethylhydrazine and 1,2-Os3(CO)10(MeCN)2 (1). Here
the new cluster 1,2-Os3(CO)10[(PhO)2PN(Me)N(Me)P(OPh)2] (2b)
has been isolated and characterized in solution by a combination
of NMR and IR spectroscopies. 2b represents the first structurally
characterized polynuclear cluster that has possesses this particular
bis(phosphanyl)hydrazine ligand [15]. The bridging of adjacent os-
mium centers in 2b by the ancillary diphosphine ligand has been
established by X-ray crystallography. Cluster 2b is stable in reflux-
ing toluene, and unlike those diphosphine ligands depicted in
Scheme 1, shows no evidence for isomerization to the chelated iso-
mer 2c. The energy difference between the bridged and chelated
isomers of Os3(CO)10[(PhO)2PN(Me)N(Me)P(OPh)2] has been deter-
mined by DFT calculations, with the diphosphine-bridged cluster
computed to be the thermodynamically more stable isomer.
To 0.25 g (0.27 mmol) of cluster 1 in a medium Schlenk tube
was added 50 mL of CH2Cl2 at room temperature, followed by
the dropwise addition of 0.14 g (0.29 mmol) of (PhO)2PN(Me)N(-
Me)P(OPh)2 in 10 mL of CH2Cl2. The solution was stirred overnight,
after which time TLC analysis, using a 2/3 mixture of CH2Cl2/hex-
ane as the eluent, revealed the presence of
a single spot
(Rf = 0.64) corresponding to the diphosphine-bridged cluster. The
solvent was removed under vacuum, and the resulting residue
was purified by column chromatography over silica using a 4:1
mixture of CH2Cl2/hexane as the mobile phase. The isolated prod-
uct was recrystallized from benzene/hexane to afford 0.37 g (93%
yield) of 1,2-Os3(CO)10[(PhO)2PN(Me)N(Me)P(OPh)2]. IR (CH2Cl2):
m
(CO) 2086 (m), 2021 (m), 2005 (vs), 1976 (m), 1945 (m) cmꢁ1
.
3
1H NMR (CDCl3): d 2.80 (6H, d, Me, JPAH = 6.5 Hz), 7.00–7.25
(20H, m, aryl). 31P NMR (CDCl3): d 106.90 (s).
2. Experimental section
2.3. X-ray crystallographic data
2.1. Materials and instrumentation
Single crystals of cluster 2b suitable for X-ray diffraction analy-
sis were grown from a p-xylene/hexane solution at room temper-
ature. The diffraction data were collected on a Bruker SMART
APEX2 CCD-based X-ray diffractometer at 296(2) K. We were un-
able to collect a dataset at low temperature (<223 K) due to exces-
sive cracking of the crystal, which in turn led to difficulty in finding
suitable parameters for the unit cell. Data collection, indexing, and
initial cell refinements were carried out using APEX2 [20], with
frame integration and final cell refinements done using SAINT
[20]. The molecular structure of 2b was determined using direct
methods and Fourier techniques, and the data were refined by
full-matrix least-squares. An absorption correction, including a
face-indexed absorption correction, was applied using the program
SADABS [20]. All non-hydrogen atoms were refined anisotropically.
The crystal contained a highly disordered molecule of p-xylene
that occupies a special position in the cell, and the solvent was
appropriately treated by PLATON/SQUEEZE [21]. This correction
of the X-ray data (47 electron/cell) was close to the required value
for one C8H10 molecule per the full unit cell (58 electron/cell). All
hydrogen atoms in the structure were positioned geometrically
and were refined as riding atoms attached to the carbon atom.
Structure solution, refinement, graphics and generation of publica-
tion materials were performed using the available SHELXTL soft-
ware [22]. The X-ray data and processing parameters are
reported in Table 1, with selected bond distances and angles
quoted in Table 2.
The bis(acetonitrile) cluster 1,2-Os3(CO)10(MeCN)2 (1) was syn-
thesized from Os3(CO)12 [16]; the latter cluster was prepared via
the high-pressure carbonylation of OsO4 in EtOH solvent [17].
The diphosphine (PhO)2PN(Me)N(Me)P(OPh)2 was prepared from
1,2-bis(dichlorophosphino)-1,2-dimethylhydrazine and phenol
[18]. The OsO4 used in our studies was purchased from Engelhard
Chemical Co, while the chemicals Me3NOꢀxH2O, 1,2-dimethylhy-
drazine dihydrochloride, PCl3, and phenol were purchased from Al-
drich Chemical Co. The anhydrous Me3NO employed in our studies
was obtained from Me3NOꢀxH2O, after the waters of hydration
were azeotropically removed under reflux, using benzene as a sol-
vent. 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 sol-
vents were stored in Schlenk storage vessels equipped with high-
vacuum Teflon stopcocks [19]. The CDCl3 NMR solvent was purified
by bulb-to-bulb distillation from P2O5.
The IR spectrum was recorded on a Nicolet 6700 FT-IR spec-
trometer in amalgamated NaCl cells having a 0.1 mm path length.
The 1H NMR spectral data were recorded at 400 MHz or 500 MHz
on Varian VXR-400 and VXR-500 spectrometers, respectively. The
31P NMR spectra were recorded at 202 MHz on the latter spectrom-
eter, and these data were collected in the proton-decoupled mode
and referenced to external H3PO4 (85%), whose chemical shift was
set at d = 0.

S.W. Hunt et al. / Journal of Molecular Structure 1010 (2012) 91–97
93
Table 1
2.4. Computational methodology and modeling details
X-ray crystallographic data and processing parameters for 1,2-Os3(CO)10[(PhO)2PN(-
Me)N(Me)P(OPh)2] (2b).
The reported calculations were performed with the hybrid DFT
functional B3LYP, as implemented by the Gaussian 09 program-
ming package [23]. This functional utilizes the Becke three-param-
eter exchange functional (B3) [24], combined with the correlation
functional of Lee, Yang and Parr (LYP) [25]. The Os atoms were de-
scribed by Stuttgart–Dresden effective core potentials (ecp) and
SDD basis set, while the 6-31G(d0) basis set, as implemented in
the Gaussian 09 program, was employed for the remaining atoms.
All reported geometries were fully optimized, starting from the
X-ray crystallographic structure when available, and evaluated for
the correct number of imaginary frequencies through calculation of
the vibrational frequencies, using the analytical Hessian. Species
with zero imaginary frequencies (positive eigenvalues) correspond
to a stationary point or minimum, whereas an imaginary frequency
(negative eigenvalue) designates a transition state. The transition
state connecting the isomeric clusters Os3(CO)10[(PhO)2PN(Me)N(-
Me)P(OPh)2] and Os3(CO)10(dppe) was also examined by IRC calcu-
lations (both directions) to verify the nature of the reactant and
product in the merry-go-round process. The computed frequencies
were used to make zero-point and thermal corrections to the elec-
tronic energies; the reported potential energies and enthalpies are
quoted in kcal/mol relative to the specified standard [26]. The
geometry-optimized structures have been drawn with the JIMP2
molecular visualization and manipulation program [27].
CCDC entry no.
Cryst system
Space group
a (Å)
b (Å)
c (Å)
729793
Triclinic
Pꢁ1
11.6081(4)
14.2034(5)
15.4030(6)
75.460(1)
74.480(1)
68.036(1)
2236.7(1)
a
(°)
b (°)
c
(°)
V (Å3)
Mol formula
fw
C36H26N2O14Os3P2ꢀ0.5C8H10
1396.21
Formula units per cell (Z)
2
Dcalcd (Mg/m3)
2.073
0.71073
8.637
k (MoK
l
a
), Å
)
(mmꢁ1
Absorption correction
Abs corr factor
Total reflections
Semi-empirical from equivalents
0.6575/0.2783
28314
10002
10002/0/504
0.0270
0.0559
1.018
1.411, ꢁ0.959
Independent reflections
Data/res/parameters
R1a [I ꢂ 2
r(I)]
wR2b (all data)
GOF on F2
D
q(max),
D
q
(min) (e/Å3)
P
P
a
R1 = ||Fo| ꢁ |Fc||/ |Fo|.
P
P
2
2 1=2
R2 ¼ ½wðF2o ꢁ Fc2Þ ꢃ= ½wðF2oÞ ꢃ
.
b
3. Results and discussion
Table 2
Selected bond distances (Å) and angles (°) in 1,2-Os3(CO)10[(PhO)2PN(Me)N(Me)-
P(OPh)2] (2b).
3.1. Synthesis and spectroscopic characterization
Bond distances
Cluster 1 reacts with (PhO)2PN(Me)N(Me)P(OPh)2 in CH2Cl2 at
room temperature to furnish the diphosphine-bridged cluster
1,2-Os3(CO)10[(PhO)2PN(Me)N(Me)P(OPh)2] (2b), as verified by
TLC analysis and spectroscopic monitoring of the reaction solution.
2b was isolated in 93% yield as the sole observable product by col-
umn chromatography over silica gel, followed by recrystallization.
While 2b is relatively stable toward oxygen in the solid state over
the course of several days, solutions containing the product exhib-
ited noticeable decomposition over the same period when exposed
to oxygen.
Os(1)AOs(2)
Os(2)AOs(3)
Os(2)AP(2)
P(2)AN(2)
N(1)AC(35)
P(1)ꢀ ꢀ ꢀP(2)
2.8899(2)
2.9105(3)
2.272(1)
1.673(4)
1.490(5)
3.573(2)
Os(1)AOs(3)
Os(1)AP(1)
P(1)AN(1)
N(1)AN(2)
N(2)AC(36)
2.8756(3)
2.268(1)
1.674(4)
1.430(5)
1.473(6)
Bond angles
P(1)AOs(1)AOs(3)
P(2)AOs(2)AOs(3)
N(1)AP(1)AOs(1)
N(1)AN(2)AP(2)
C(36)AN(2)AP(2)
N(1)AN(2)AC(36)
154.03(3)
163.22(3)
122.7(1)
119.7(3)
119.7(3)
110.4(4)
P(1)AOs(1)AOs(2)
P(2)AOs(2)AOs(1)
N(2)AP(2)AOs(2)
N(2)AN(1)AP(1)
C(35)AN(1)AP(1)
N(2)AN(1)AC(35)
93.51(3)
103.84(3)
120.1(1)
114.8(3)
120.5(3)
117.3(3)
2b was characterized in solution by IR, 1H and 31P NMR spec-
troscopies. The IR spectrum recorded in CH2Cl2 exhibits terminal
Fig. 1. Displacement ellipsoid plot of the molecular structure of the non-hydrogen atoms of 2b at the 30% probability level (left), and the optimized B3LYP structure of 2b
(right). The bond distances specified in the B3LYP structure are reported in angstrom.

94
S.W. Hunt et al. / Journal of Molecular Structure 1010 (2012) 91–97
carbonyl stretching bands at 2086 (m), 2021 (m), 2005 (vs), 1976
(m), and 1945 (m) cmꢁ1, and these frequencies and their relative
intensities are comparable to those data reported for related
diphosphine-bridged Os3 clusters [1–3,7,10–14]. The 1H NMR spec-
trum displayed a doublet for the two methyl groups at d 2.80 and
the twenty aryl hydrogens appeared as an overlapping set of reso-
nances from d 7.00–7.25. The singlet resonance found in the
31P{1H} NMR spectrum at d 106.90 is consistent with the formu-
lated structure, and this confirms that the reaction between 1
and the bis(phospanyl)hydrazine ligand yields 2b as an isomerical-
ly pure product. Coordination of the bis(phosphanyl)hydrazine li-
gand to the triosmium core in 2b leads to a pronounced upfield
31P shift relative to the free ligand (d 138.16) recorded under iden-
tical conditions.
TS2b2c
30.6
29.2
3.2. X-ray diffraction structure of 2b
The unequivocal disposition of the bis(phosphanyl)hydrazine li-
gand relative to the cluster polyhedron in the isolated product was
established by X-ray crystallography. Fig. 1 (left-hand side) shows
the displacement ellipsoid plot of cluster 2b, which confirms the
coordination of the Os(1) and Os(2) centers by the bis(phospha-
nyl)hydrazine ligand. The bridging of adjacent osmium centers in
2b by the bis(phosphanyl)hydrazine auxiliary does not adversely
affect the triangular array of osmium atoms, whose three OsAOs
vectors, which are more or less identical in length, exhibit a mean
distance of 2.892 Å. The mean OsAP bond distance of 2.270 Å is
slightly longer than the mean CoAP distance of 2.156 Å reported
by us for ligand-bridged cluster PhCCo3(CO)7[(MeO)2PN(Me)N(-
Me)P(OMe)2] [15]. While numerous mononuclear compounds that
possess a chelating R2PNR0NR0PR2 ligand (where R = alkyl, aryl, alk-
oxides; R0 = Me, Et) have been prepared [28–31], the coordination
chemistry of these ligands with polynuclear metal clusters remains
largely unexplored. To our knowledge, 2b and PhCCo3(CO)7[(-
MeO)2PN(Me)N(Me)P(OMe)2] represent the only structurally char-
acterized polynuclear clusters containing these ligands, and here
the ancillary bis(phosphanyl)hydrazine ligand serves to bind two
adjacent metal centers. One factor that has hampered a more thor-
ough investigation of the ligand substitution chemistry of
bis(phosphanyl)hydrazine ligands at metal clusters is the facile
fragmentation of the diphosphine ligand that occurs upon coordi-
nation [32].
2c
7.1
6.8
2b
0.0
Fig. 2. Potential energy surface for the merry-go-round isomerization of 2b to 2c.
The geometry-optimized structures for species TS2b2c and 2c are included, and the
energy values are represented as
DE(DH) in kcal/mol with respect to 2b.
internuclear Pꢀ ꢀ ꢀP distance found in the bridged isomer of Os3(-
CO)10(dppq) and Os3(CO)10(dppbz) [13,14]. The ten carbonyl
groups are all linear and exhibit bond distances and angles consis-
tent with their linear nature. The angle subtended by the
Os(1)AP(1)AN(1) [122.7(1)°] and Os(2)AP(2)AN(2) [120.1(1)°]
linkages is unexceptional relative to other phosphine-bridged Os3(-
CO)10(PP) clusters of this genre. The remaining bond distances and
angles are unremarkable and require no comment.
The P(1)AN(1) [1.674(4) Å], P(2)AN(2) [1.673(4) Å], and
N(1)AN(2) [1.430(5) Å] distances are comparable to those dis-
tances in PhCCo3(CO)7[(MeO)2PN(Me)N(Me)P(OMe)2] [15],
W(CO)4[(PhO)2PN(Me)N(Me)P(OPh)2] [28], and CpRuCl[(PhO)2PN(-
Me)N(Me)P(OPh)2] [29]. The N(1) and N(2) atoms are best viewed
as trigonal planar since of the sum of angles about each nitrogen
center is 352.6° and 349.8°, respectively. The internuclear
P(1)ꢀ ꢀ ꢀP(2) distance of 3.573(2) Å in 2b is comparable to the
3.3. Thermal isomerization of 2b to 2c and DFT studies
The isomerization of 2b to the corresponding chelated isomer
1,1-Os3(CO)10[(PhO)2PN(Me)N(Me)P(OPh)2] (2c) was next ex-
plored, as part of our interest in the chemical reactivity of polynu-
clear cluster compounds [33]. It was anticipated that heating 2b
would furnish 2c without complications, based on the facile rear-
rangement of the diphosphine ligand in different Os3(CO)10(PAP)
R2
P
X
R2
P
Os
Os
Os
X
X
X
R2
P
PR2
X
X
Os
Os
Os
Os
Os
Os
P
P
R2
R2
1,1-Os3(CO)10(P-P)
1,2-Os3(CO)10(P-P)
2b (R = OPh; X = NMe)
3b (R = Ph; X = CH2)
2b
3b
Scheme 2. Merry-go-round exchange process for the isomeric clusters 2 and 3.

S.W. Hunt et al. / Journal of Molecular Structure 1010 (2012) 91–97
95
systems already investigated by us. Samples of 2b were found to be
stable in refluxing toluene! The isomerization reaction was not
examined at higher temperatures in order to avoid dissociative loss
of CO from 2b, a process that becomes competitive at elevated
temperature (>110 °C). This deleterious manifold would afford
the unsaturated cluster Os3(CO)9(PAP), which has been shown to
serve as the bifurcation point for ortho metalation and irreversible
degradation of the diphosphine ligand by PAC bond cleavage in re-
lated systems [10,34].
and 90° [37], and this magnitude is in good agreement with the
ground-state energy difference computed for clusters 2b and 2c.
3.4. DFT evaluation of the bridged and chelated isomers of
Os3(CO)10(dppe)
Having established the greater stability of the bridged isomer
2b versus 2c, we wished to probe the isomer preference in a trios-
mium cluster possessing a diphosphine ligand with a different sat-
urated backbone of comparable spacer length. The ligands 1,2-
bis(diphenylphosphino)ethane (dppe) and (PhO)2PN(Me)N(Me)-
P(OPh)2 are analogous since they both contain a two-atom spacer
in the form of an ethano versus a hydrazine-based moiety, respec-
tively. The bridged (3b) and chelated (3c) isomers of Os3(-
CO)10(dppe), which have been synthesized and structurally
characterized several years ago [3,38], provide the ideal pair of
clusters in which to test the generality of bridging versus chelating
isomer stability in this genre of clusters. The solid-state structures
reveal no unusually short inter- or intramolecular contacts, and the
dppe ligand exerts no adverse perturbation on the Os3(CO)10 moi-
ety in each isomer. Thus, any energy difference between the two
isomers may be attributed to effects within ancillary diphosphine
ligand, as modulated by the coordination mode adopted the ligand.
The PES for the concerted isomerization reaction and the geom-
etry-optimized structures for 3b, 3c, and TS3b3c are depicted in
Fig. 3. The ground-state energy difference in the two dppe-substi-
tuted clusters mirrors the trend found in Os3(CO)10[(PhO)2PN(-
Me)N(Me)P(OPh)2], with 3b thermodynamically more stable and
7.3 kcal/mol lower in enthalpy than its chelated counterpart. While
the ground-state energy difference between the isomeric clusters 2
and 3 is comparable, the presence of the dppe ligand destabilizes
the merry-go-round exchange process in TS3b3c 4.0 kcal/mol rela-
tive to TS2b2c. The kinetic stability of the isomeric clusters based
on Os3(CO)10(dppe) is consistent with the earlier experimental
observations of Deeming and coworkers [2].
To gain a greater understanding on the responsible factor(s) for
the unobserved isomerization reaction, the ground-state energies
of 2b and 2c and the energetics for the merry-go-round isomeriza-
tion were investigated by DFT calculations. Scheme 2 illustrates
this bridge-to-chelate isomerization process. The B3LYP-optimized
structures of 2b and 2c were computed. The DFT structure of 2b is
depicted in Fig. 1 alongside the X-ray structure, allowing for a di-
rect comparison. Overall, there is good agreement between the dif-
fraction and DFT structures for the diphosphine-bridged cluster.
The DFT structure computed for 2c is shown in Fig. 2, where it is
confirmed that 2b lies 6.8 kcal/mol lower in enthalpy than 2c.
The absence of 2c on heating is fully consistent with the experi-
mental data and the DFT computed Keq value of ca. 10ꢁ3 in reflux-
ing toluene for the 2b ꢀ 2c transformation. The conversion of 2b to
2c was found to occur in a one-step process through the triply
bridged transition structure TS2b2c. Here the concerted merry-
go-round motion of an equatorially disposed phosphine and two
CO groups represents the lowest energy pathway for the isomeri-
zation [14]. That TS2b2c serves as a transition-state structure
and not a high-energy stationary point on the potential energy sur-
face (PES) for the isomerization was verified by normal mode anal-
ysis, which revealed an imaginary frequency of 61i for TS2b2c.
Subsequent IRC calculations confirmed that TS2b2c connects 2b
directly to 2c. While the barrier for the isomerization is high, it is
the ground-state energy difference between the isomeric clusters
that retards the isomerization.
Our previous studies on the ligand isomerization in
Os3(CO)10(PAP) have employed diphosphines that possessed an
unsaturated backbone, where the vicinal phosphorus centers were
tethered to an etheno- or benzo-based platform. In these particular
systems, the chelated isomer of Os3(CO)10(PAP) has been found to
be thermodynamically more stable than the corresponding bridged
cluster, a trend that is reversed in the case of the (PhO)2PN(Me)N(-
Me)P(OPh)2 ligand in 2b. To our knowledge, only one other docu-
mented case exists where the bridged isomer is preferred over
the chelated isomer, and this involves the cluster Os3(CO)10(dppf)
The isomer preference in Os3(CO)10(dppe) has its origin in the
Pitzer (torsional) and angle strain associated with the different
sized rings that result from dppe coordination to the Os3(CO)10
moiety. Examination of the dppe ligand in the six- and five-
[where
dppf = 1,10-bis(diphenylphosphino)ferrocene]
[35].
Undoubtedly, the isomer preference in Os3(CO)10(PAP) is con-
trolled by the nature of ancillary diphosphine, with the bite angle
of the ligand, the composition of the backbone that tethers the
phosphorus centers, and the substituents on the phosphorus and
backbone chain representing the critical variables in play.
TS3b3c
34.5
33.2
In the case of 2b and 2c, the isomer preference parallels the po-
tential energy profile for internal rotation displayed by hydrazine.
Both molecular mechanics (MM) and ab initio calculations have
shown the vicinal lone-pair orbital interactions between the nitro-
gen centers in hydrazine to be the dominant factor in determining
conformational stability [36,37]. A plot of the potential energy
function versus the LPANANALP dihedral angle (where LP = lone
pair) in free hydrazine reveals two distinct maxima at 0° and
180° and a single minimum at 90°. The PANANAP dihedral angle
in 2b and 2c is ca. 85° and 41°, respectively, which allows the
hydrazine lone-electron pairs in 2b to achieve a near orthogonal
orientation in concert with the preferred conformation found for
hydrazine. The B3LYP/TZVP-computed energy profile for hydrazine
internal rotation computed by Maginn and coworkers reveals an
energy difference of ca. 5 kcal/mol for the conformations at 41°
3c
7.3
7.3
3b
0.0
Fig. 3. Potential energy surface for the merry-go-round isomerization of 3b to 3c.
The geometry-optimized structures for species 3b, 3c and corresponding transition
state TS3b3c are included, and the energy values are represented as
mol with respect to 3b.
DE(DH) in kcal/

96
S.W. Hunt et al. / Journal of Molecular Structure 1010 (2012) 91–97
membered ring that is produced in the bridged and chelated iso-
mers, respectively, reveals a greater number of torsional deriva-
tions in the latter isomer. The strain about the HACAPACaryl
linkages in 3c is particularly pronounced in the case of the one of
the two CH2PPh2 units. Such a scenario is akin to that used to ac-
count for the stability difference between cyclohexane and cyclo-
pentane rings [39].
found,
j.molstruc.2011.11.037.
in
the
online
version,
at
doi:10.1016/
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4. Conclusions
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Acknowledgments
[26] (a) In our calculations on the different osmium clusters reported here,
numerous low-frequency vibrations (10–100 cmꢁ1) were encountered in the
two minima and the connecting merry-go-round transition state. Within the
harmonic oscillator approximation, these frequencies will serve as a source of
error for the temperature-corrected free energy values. Accordingly, we have
avoided using free energies in our discussions in the absence of corroborating
experimental data.;
Financial support from the Robert A. Welch Foundation (Grant
B-1093-MGR) and is much appreciated. NSF support of the NMR
and computational facilities through grants CHE-0840518 and
CHE-0741936 is acknowledged. We also wish to thank Prof. Mi-
chael B. Hall (TAMU) for providing us a copy of his JIMP2 program,
which was used to prepare the geometry-optimized structures re-
ported here.
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Appendix A. Supplementary material
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tion-state structures are available upon request from the author
(MGR). Supplementary data associated with this article can be

S.W. Hunt et al. / Journal of Molecular Structure 1010 (2012) 91–97
97
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