Nmr studies of Ru3(CO)10(PMe2Ph)2 and Ru3(CO)10(PPh3)2 and their H2 addition products: Detection of new isomers with complex dynamic behavior

Article abstract of DOI:10.1021/ja004245n

The clusters Ru₃(CO)₁₀L₂, where L = PMe₂Ph or PPh₃, are shown by NMR spectroscopy to exist in solution in at least three isomeric forms, one with both phosphines in the equatorial plane on the same ruthenium center and the others with phosphines in the equatorial plane on different ruthenium centers. Isomer interconversion for Ru₃(CO)₁₀(PMe₂Ph)₂ is highly solvent dependent, with ΔH? decreasing and ΔS? becoming more negative as the polarity of the solvent increases. The stabilities of the isomers and their rates of interconversion depend on the phosphine ligand. A mechanism that accounts for isomer interchange involving Ru - Ru bond heterolysis is suggested. The products of the reaction of Ru₃(CO)₁₀L₂ with hydrogen have been monitored by NMR spectroscopy via normal and para hydrogen-enhanced methods. Two hydrogen addition products are observed with each containing one bridging and one terminal hydride ligand. EXSY spectroscopy reveals that both intra- and interisomer hydride exchange occurs on the NMR time scale. On the basis of the evidence available, mechanisms for hydride interchange involving Ru - Ru bond heterolysis and CO loss are proposed.

Full text of DOI:10.1021/ja004245n

9760
J. Am. Chem. Soc. 2001, 123, 9760-9768
NMR Studies of Ru3(CO)10(PMe2Ph)2 and Ru3(CO)10(PPh3)2 and
Their H2 Addition Products: Detection of New Isomers with
Complex Dynamic Behavior
Damir Blazina,† Simon B. Duckett,*,† Paul J. Dyson,*,† Brian F. G. Johnson,*,‡
Joost A. B. Lohman,§ and Christopher J. Sleigh†
Contribution from the Department of Chemistry, UniVersity of York, Heslington, York YO10 5DD, U.K.,
UniVersity Chemistry Laboratory, Lensfield Road, Cambridge CB2 1EW, U.K., and Bruker UK Limited,
Banner Lane, CoVentry CV4 9GH, U.K.
ReceiVed December 12, 2000. ReVised Manuscript ReceiVed June 6, 2001
Abstract: The clusters Ru3(CO)10L2, where L ) PMe2Ph or PPh3, are shown by NMR spectroscopy to exist
in solution in at least three isomeric forms, one with both phosphines in the equatorial plane on the same
ruthenium center and the others with phosphines in the equatorial plane on different ruthenium centers. Isomer
interconversion for Ru3(CO)10(PMe2Ph)2 is highly solvent dependent, with ∆H‡ decreasing and ∆S‡ becoming
more negative as the polarity of the solvent increases. The stabilities of the isomers and their rates of
interconversion depend on the phosphine ligand. A mechanism that accounts for isomer interchange involving
Ru-Ru bond heterolysis is suggested. The products of the reaction of Ru3(CO)10L2 with hydrogen have been
monitored by NMR spectroscopy via normal and para hydrogen-enhanced methods. Two hydrogen addition
products are observed with each containing one bridging and one terminal hydride ligand. EXSY spectroscopy
reveals that both intra- and interisomer hydride exchange occurs on the NMR time scale. On the basis of the
evidence available, mechanisms for hydride interchange involving Ru-Ru bond heterolysis and CO loss are
proposed.
Introduction
tions.4,5 The X-ray crystal structures of several phosphine
derivatives have been reported for ruthenium and osmium
clusters of this type,4-7 and both conformational and structural
isomers have been observed with the osmium clusters.8,9 In these
structures, the phosphine ligands are restricted to equatorial
locations. While X-ray crystallography remains the main tool
for elucidating the structure of clusters, their stereochemical
nonrigid behavior is studied most readily by NMR spectros-
copy.10
Since transition metal carbonyl clusters represent an inter-
mediate state between mononuclear complexes and colloids/
nanoparticles they have been widely studied over the last 30
years.1 In particular, organic substrates are able to adopt
multicenter bonding modes with clusters that profoundly influ-
ence their subsequent reactivity.2 Recently, this has enabled
organic transformations to be completed that are not possible
by mononuclear homogeneous catalysts.3 The ability of a species
to access a degree of electronic unsaturation remains a key step
in such processes. While homogeneous mononuclear complexes
tend to achieve this by ligand dissociation, clusters may employ
other processes, including fragmentation, polyhedral transforma-
tions, and the activation of multiple metal-metal bonds.
Triruthenium dodecacarbonyl and its phosphine (L) deriva-
tives Ru3(CO)12-nLn (where n ) 1-3) have been shown to
catalyze numerous hydrogenation and hydroformylation reac-
The substitution chemistry of Ru3(CO)12-xLx has been studied
by a number of groups.11-16 Simple substitution has been
(4) Gervasio, G.; Giordano, R.; Marabello, D.; Sappa, E. J. Organomet.
Chem. 1999, 588, 83.
(5) (a) Fontal, B.; Reyes, M.; Sua´rez, T.; Bellandi, F.; D´ıaz, J. C. J.
Mol. Catal. A. 1999, 149, 75. (b) Fontal, B.; Reyes, M.; Sua´rez, T.; Bellandi,
F.; Ruiz, N. J. Mol. Catal. A. 1999, 149, 87.
(6) Bruce, M. I.; Matisons, J. G.; Nicholson, B. K. J. Organomet. Chem.
1983, 247, 321.
(7) (a) Bruce, M. I.; Skelton, B. W.; White, A. H.; Zaitseva, N. N. Aust.
J. Chem. 1997, 50, 163. (b) Bruce, M. I.; Liddell, M. J.; Hughes, C. A.;
Patrick, J. M.; Skelton, B. W.; White, A. H. J. Organomet. Chem. 1988,
347, 181.
† University of York.
‡ University Chemistry Laboratory.
§ Bruker UK Ltd.
(1) For example, see: (a) Transition Metal Clusters; Johnson, B. F. G.,
Ed.; Wiley: Chichester, U.K., 1980. (b) Catalysis by Di- and Polynuclear
Metal Cluster Complexes; Adams R. D., Cotton F. A., Eds.; Wiley-VCH:
New York, 1998. (c) Metal Clusters in Chemistry; Braunstein, P., Oro, L.
A., Raithby, P. R., Eds.; Wiley-VCH: New York, 1999. (d) Dyson, P. J.;
McIndoe, J. S. Transition Metal Carbonyl Cluster Chemistry; Gordon and
Breach: Amsterdam, 2000.
(2) Adams, R. D. In The Chemistry of Metal Cluster Complexes; Shriver,
D. F., Kaesz, H. D., Adams, R. D., Eds.; Wiley-VCH: New York, 1990.
(3) For example, see: (a) Kondo, T.; Akazome, M.; Tsuji, Y.; Watanabe,
Y. J. Org. Chem. 1990, 55, 1286. (b) Chatani, N.; Ie, Y.; Kakiuchi, F.;
Murai, S. J. Org. Chem. 1997, 62, 2604. (c) Ishii, Y.; Chatani, N.; Kakiuchi,
F.; Murai, S. Organometallics 1997, 16, 3615. (d) Chatani, N.; Ishii, Y.;
Ie, Y.; Kakituchi, F.; Murai, S. J. Org. Chem. 1998, 63, 5129.
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M. J. Organometallics 1998, 17, 5267.
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Organomet. Chem. 1999, 584, 174.
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Koshevoy, I. O.; Podkorytov, I. S.; Smawfield, D. J.; Tunik, S. P.; Whyman,
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10.1021/ja004245n CCC: $20.00 © 2001 American Chemical Society
Published on Web 09/12/2001

Ru3(CO)10(PMe2Ph)2 and Ru3(CO)10(PPh3)2
J. Am. Chem. Soc., Vol. 123, No. 40, 2001 9761
proposed to occur via dissociative pathways, but competing
formation of mononuclear products such as Ru(CO)4L and Ru-
(CO)3L2 results in highly complex reaction systems. These
studies have involved kinetic measurements and the rationaliza-
tion of activation parameter data. For example, the substitution
of PPh3 in Ru3(CO)9(PPh3)3 has been shown to follow a pathway
that involves reversible PPh3 loss with competing CO loss and
homolytic Ru-Ru bond fission.11 The latter process leads to
fragmentation and the initial formation of mono- and binuclear
products. Consistently low values of ∆S‡ for the dissociative
route have been explained by invoking reorganization of the
46-electron intermediate Ru3(CO)10L such that an 18-electron
count is obtained at each metal center.12 It has also been
proposed that these three pathways could be preceded by
common, and reversible, fission of a single Ru-Ru bond.11
Alternative mechanisms for CO substitution in M3(CO)12-xLx
(where M ) Fe, Ru, Os and x ) 0, 1, 2) have been suggested,
which invoke both heterolytic fission of a M-M bond13 and
merry-go-round exchange.14,15 Similar mechanisms have been
proposed for MM′(CO)10 (where M ) M′ ) Mn, Re; M ) Mn;
and M′ ) Re)16 as well as for Ir4(CO)12-nLn (n ) 0-3).17,18
The reactivity of such clusters toward hydrogen and their
catalytic activity has also been investigated.1,19 One high-
sensitivity NMR technique that has recently been developed to
probe the role that metal hydride complexes play in catalytic
transformations involves the use of para hydrogen (p-H2). This
phenomenon, initially termed the PASADENA effect,20 now
usually called para hydrogen-induced polarization (PHIP), has
been extensively reviewed.21 Notable achievements include the
demonstration that the non-Boltzmann population generated by
PHIP enhances scalar-coupled 31P and 13C heteronuclei by cross-
relaxation22 and cross-polarization,23 the development of selec-
tive excitation of polarization by Bargon et al. to remove the
need for π/4 pulse excitation,24,25 and the use of 2D methods to
facilitate the rapid indirect observation of insensitive nuclei.26,27
Although applications of p-H2 to cluster chemistry are much
(15) Adams, H.; Agustinho, C. M.; Mann, B. E.; Smith, S. J. Organomet.
Chem. 2000, 607, 175.
(16) Johnson, B. F. G. J. Organomet. Chem. 1991, 415, 109.
(17) Johnson, B. F. G.; Roberts, Y. Inorg. Chim. Acta 1993, 205, 175.
(18) (a) Mann, B. E.; Pickup, B. T.; Smith, A. K. J. Chem. Soc., Dalton
Trans. 1989, 889. (b) Mann, B. E.; Vargas, M. D.; Khadar, R. J. Chem.
Soc., Dalton Trans. 1992, 1725.
(19) (a) Cabeza, J. A.; Fernandez-Colinas, J. M.; Llamazares, A.; Riega,
V. Organometallics 1993, 12, 4141. (b) Castiglioni, M.; Deabate, S.;
Giordano, R.; King, P. J.; Knox, S. A. R.; Sappa, E. J. Organomet. Chem.
1998, 571, 251. (c) Gervasio, G.; Giordano, R.; Marabello, D.; Sappa, E.
J. Organomet. Chem. 1999, 588, 83. (d) Su¨ss-Fink, G.; Meister, G. AdV.
Organomet. Chem. 1993, 35, 41.
(20) Bowers, C. R.; Weitekamp, D. P. J. Am. Chem. Soc. 1987, 109,
5541.
(21) (a) Bowers, C. R.; Jones, D. H.; Kurur, N. D.; Labinger, J. A.;
Pravica, M. G.; Weitekamp, D. P. AdV. Magn. Reson. 1990, 14, 269. (b)
Natterer, J.; Bargon, J. Prog. Nucl. Magn. Reson. Spectrosc. 1997, 31, 293.
(c) Duckett, S. B.; Sleigh, C. J. Prog. Nucl. Magn. Reson. Spectrosc. 1999,
34, 71.
less common, Aime et al. have demonstrated that H2Os3(CO)10,
a species with magnetically equivalent hydrides, can be
enhanced.28 The enhanced hydride signal arises via the involve-
ment of an intermediate with inequivalent hydrides. Studies
involving Ru3(CO)11(NCMe) yield an enhanced emission signal
for molecular hydrogen that indicated the reversible interaction
of p-H2 with the Ru3 cluster containing inequivalent hydrides.29
We previously used p-H2 to detect new H2 addition products
of Ru3(CO)9(PPh3)3.30 Here we describe studies on the fluxional
behavior of Ru3(CO)10(L)2, where L ) PMe2Ph or PPh3, and
the detection and characterization of their hydrogen addition
products.
Experimental Section
General Methods and Chemicals. All reactions were carried out
under nitrogen using glovebox, high-vacuum, or Schlenk line tech-
niques. Subsequent purifications were carried out without precautions
to exclude air. Triphenylphosphine and dimethylphenylphosphine
(Aldrich), Ru3(CO)12 (Strem Chemicals), hydrogen (99.99%, BOC),
and 13CO (99.9%, BP) were used as received. Tetrahydrofuran (THF)
was dried over sodium prior to use. The clusters Ru3(CO)10(PMe2Ph)2
1 and Ru3(CO)10(PPh3)2 2 were prepared according to literature
methods31 and purified on a silica column with 80:20 hexane/
dichloromethane as the eluent. The mixed phosphine cluster Ru3(CO)10-
(PMe2Ph)(PPh3) was prepared from Ru3(CO)11(PPh3) by reaction with
PMe2Ph, in a manner analogous to that used by Farrar and Lunniss for
triiron clusters.32 Initial characterization was based on IR spectroscopy
(νCO, recorded on a Mattson Unicam Research Series FT-IR instrument)
and on FAB mass spectroscopy (performed on a VG Autospec
instrument) with observations in excellent agreement with the literature
data. Full characterization was achieved using NMR spectroscopy (see
later). Labeling of Ru3(CO)10(PMe2Ph)2 with 13CO was achieved by
stirring a solution of 50 mg of the cluster in THF overnight under an
atmosphere of 13CO. This procedure was repeated three times to ensure
essentially complete 13CO labeling.
All NMR solvents (Apollo Scientific) were dried using appropriate
methods and degassed prior to use. The NMR measurements were made
on NMR tubes fitted with J. Young Teflon valves, and solvents were
added by vacuum transfer on a high-vacuum line. For the PHIP
experiments, hydrogen enriched in the para spin state was prepared by
cooling H2 to 77 K over a paramagnetic catalyst (Fe2O3) as described
previously.33 An atmosphere of H2 equivalent to ∼3 atm pressure at
298 K was first introduced into an NMR tube cooled in liquid nitrogen.
Samples were thawed immediately prior to introduction into the NMR
spectrometer. All NMR spectra were recorded on Bruker DRX-400
1
spectrometers with H at 400.13 MHz, 31P at 161.9 MHz, and 13C at
1
100 MHz, respectively. H NMR chemical shifts are reported in ppm
1
relative to residual H signals in the deuterated solvents (benzene-d5,
δ 7.13; toluene-d7, δ 2.13; CHCl3, δ 7.27; THF-d7, δ 1.73; ni-
tromethane-d2, δ 4.33), 31P NMR in ppm downfield of an external 85%
solution of phosphoric acid, and 13C NMR relative to benzene-d6, δ
128.0, and toluene-d8, δ 21.3. Normal and modified COSY,27 HMQC,34
and EXSY35 pulse sequences were used as previously described.
Dynamic Methods for Complexes 1 and 2. The 31P EXSY spectra
were analyzed according to literature methods.36 The rate of phosphine
(22) (a) Eisenschmid, T. C.; McDonald, J.; Eisenberg, R.; Lawler, R. G.
J. Am. Chem. Soc. 1989, 111, 7267. (b) Barkemeyer, J.; Haake, M.; Bargon,
J. J. Am. Chem. Soc. 1995, 117, 2927.
(23) (a) Duckett, S. B.; Newell, C. L.; Eisenberg, R. J. Am. Chem. Soc.
1993, 115, 1156. (b) Haake, M.; Natterer, J.; Bargon, J. J. Am. Chem. Soc.
1996, 118, 8688.
(28) Aime, S.; Gobetto, R.; Canet, D. J. Am. Chem. Soc. 1998, 120,
6770.
(29) Aime, S.; Dastru, W.; Gobetto, R.; Russo, A.; Viale, A.; Canet, D.;
J. Phys. Chem. A. 1999, 103, 9702.
(30) Sleigh, C. J.; Duckett, S. B.; Mawby, R. J.; Lowe, J. P. Chem.
Commun. 1999, 1223.
(24) Barkemeyer, J.; Bargon, J.; Sengstschmid, H.; Freeman, R. J. Magn.
Reson. Ser. A 1996, 120, 129.
(25) Sengstschmid, H.; Freeman, R.; Barkemeyer, J.; Bargon, J. J. Magn.
Reson. Ser. A 1996, 120, 249.
(31) (a) Bruce, M. I.; Kehoe, D. C.; Matisons, J. G.; Nicholson, B. K.;
Reiger, H. R.; Williams, M. L. J. Chem. Soc. Chem. Commun. 1982, 442.
(b) Bruce, M. I.; Matisons, J. G.; Nicholson, B. K. J. Organomet. Chem.
1983, 247, 321.
(26) (a) Duckett, S. B.; Barlow, G. K.; Partridge, M. G.; Messerle, B.
A. J. Chem. Soc., Dalton. Trans. 1995, 3427. (b) Duckett, S. B.; Mawby,
R. J.; Partridge, M. G. J. Chem. Soc. Chem. Commun. 1996, 383. (c) Millar,
S. P.; Jang, M.; Lachicotte, R. J.; Eisenberg, R. Inorg. Chim. Acta 1998,
270, 363.
(27) Messerle, B. A.; Sleigh, C. J.; Partridge, M. G.; Duckett, S. B. J.
Chem. Soc., Dalton Trans. 1999, 1429.
(32) Farrar, D. H.; Lunniss, J. A. J. Chem. Soc., Dalton Trans. 1987,
1249.
(33) Duckett, S. B.; Newell, C. L.; Eisenberg, R. J. Am. Chem. Soc.
1994, 116, 10548.
(34) (a) Aue, W. P.; Bartholdi, E.; Ernst, R. R. J. Chem. Phys. 1976, 64,
2229. (b) Nagayama, K.; Kumar, A.; Wu¨thrich, K.; Ernst, R. R. J. Magn.
Reson. 1980, 40, 321.

9762 J. Am. Chem. Soc., Vol. 123, No. 40, 2001
Blazina et al.
exchange, k/s-1, was determined according to eq 1, where tm represents
1
I + 1
k ) ln
(1)
(
)
tm I - 1
the mixing time and I the ratio of intensities of the diagonal and
exchange cross-peaks. Suitable plots enable kforward and kreverse to be
determined from the gradient. This analysis is normally used for simple
two-site exchange but has been used here to determine kforward and kreverse
even though the populations of the two sites are not the same. The
apparent relaxation times of the 31P centers were found to be identical
(T1 ) 0.28 s) within experimental error.
At high temperatures, the process became too fast to be monitored
by the EXSY route. Quantitative 31P spectra were then acquired and
line-shape fitting, performed using the gNMR program version 4.0.1
(Cherwell Scientific), yielded the rate constants. The experiment was
repeated over a range of temperatures, and the activation parameters
∆H‡ and ∆S‡ were determined by nonlinear regression using SPSS for
Windows version 9.0.
Dynamic Methods for Complex 3. To monitor hydride ligand
exchange in 3, a series of 2D gradient-assisted EXSY spectra were
recorded.27 In a typical run, data were collected at selected temperatures
for a series of mixing times τ (usually 25, 50, 75, 200, 400, 600, and
800 ms). The sum of the integrals of the diagonal peak areas for a
given hydride location and the cross-peak areas arising due to exchange
were then normalized to 100%. The rate of site exchange was then
determined by simulation; the change in intensity of the signal at a
given hydride position (100 when τ ) 0) was modeled as a function
of τ. This was carried out using Microsoft Excel 97. The peak intensities
were calculated for 0.01 s intervals from 0.01 s to τmax and those for
each experimentally observed time were then compared with the
experimental values via a minimized linear least-squares difference
analysis.37 The associated rate constants were varied until the sum of
the squares of the difference between measured and simulated points
was minimized. Rate constants obtained in this way were multiplied
by a factor of 2 to take into account the analysis method.38
Kinetic Model. To model the hydride exchange process, the
following assumptions were made: the rate of hydride exchange within
an isomer is the same for both terminal f bridge and bridge f terminal
processes. Likewise, for isomer exchange, the rate of transfer of both
hydrides must be the same. The magnitude of the following rate
constants were obtained by simulation of the EXSY data: (i) intra-
molecular hydride exchange within 3a, i.e., 3ab f 3at and reverse;
(ii) intramolecular hydride exchange within 3b, i.e., 3bb f 3bt and
reverse; (iii) intermolecular hydride exchange from 3a to 3b, keeping
original hydride position, i.e., 3ab f 3bb and 3at f 3bt; (iv)
intermolecular hydride exchange from 3a to 3b with hydride position
interchange, i.e., 3ab f 3bt and 3at f 3bb; (v) the reverse of processes
iii and iv.
Figure 1. Structures of Ru3(CO)10(PR3)2 with two equatorial phosphine
ligands (PR3 ) PMe2Ph or PPh3). Arrangement a′ corresponds to the
solid-state structure found in the literature.7
into two equally intense signals at δ 1.55 and -4.77 and 189
K. These two signals coalesced at 208 K, with line shape
analysis indicating that the associated rate constant for ligand
interchange is 90 s-1
.
In the case of 2, the corresponding 31P NMR spectrum at
301 K contains a broad singlet at δ 36.5, while at 253 K two
resonances are detected (δ 36.1 and 37.9) with relative intensity
1:0.63 (peak coalescence at 265 K). Upon cooling to 189 K,
the signal at δ 36.1 splits again into two equally intense signals
at δ 42.7 and 36.7. This information suggests that 2 also exists
as a set of rapidly interconverting isomers. Since literature
evidence indicates that axial phosphine arrangements are
unprecedented,39 the four structures shown in Figure 1 might
be expected (form a′ matches that reported in the solid state).7,31
To learn more about 1 and 2, the mixed phosphine cluster
Ru3(CO)10(PMe2Ph)(PPh3) was prepared. 31P NMR investigation
of this sample indicated the presence of a 1:1:3 ratio of 1:2:
Ru3(CO)10(PMe2Ph)(PPh3) at 296 K, of which the signals for
Ru3(CO)10(PMe2Ph)(PPh3) corresponded to a pair of coupled
resonances at δ -1.5 (d, J(PP) 9.8 Hz, PMe2Ph, 275 K) and 36.9
(d, J(PP) 9.8 Hz, PPh3) and two further singlets at δ -0.86 and
38.5. Since the coupled signals are connected by an NOE
interaction in the appropriate NMR experiment at 203 K the
associated phosphine ligands must be located on the same Ru
center (structure b from Figure 1). Furthermore, both the two
additional resonances split into equally intense signals at δ 0.42
and -4.88 (PMe2Ph) and δ 40.67 and 40.91 (PPh3), respectively,
at 189 K. Unfortunately, since structural forms a′, a′′, and a′′′
contain just two distinct trans phosphine arrangements, namely,
R3P-Ru-Ru-CO and R3P-Ru-Ru-PR3, only two 31P signals
might be expected for these three isomers.40 The spectral features
described here are therefore consistent with the observation of
structure b and at least two of the remaining three isomers. It
is also worth noting that while very rapid interisomer exchange
is observed, interspecies exchange between 1, 2, and Ru3(CO)10-
(PMe2Ph)(PPh3) was not seen.
Results and Discussion
Characterization of Ru3(CO)10(PMe2Ph)2 and Ru3(CO)10-
(PPh3)2. Clusters Ru3(CO)10(PMe2Ph)2 1 and Ru3(CO)10(PPh3)2
2, prepared using literature methods, produced mass spectra and
IR data that matched those expected.31 1H NMR data for 1 and
2 were also found to agree with those reported previously.31
Surprisingly, the 31P{1H} NMR spectrum of 1 in toluene-d8 at
301 K consisted of two singlets at δ -0.95 and -2.3 in the
ratio 1:0.67. These signals coalesce at 344 K. While these data
confirm the identity of 1 as Ru3(CO)10(PMe2Ph)2, they indicate
the presence of two phosphine environments and hence more
than one structural form. While the resonance at δ -2.3 was
unaffected by cooling below 301 K, the peak at δ -0.95 split
Ligand Exchange Pathways in Ru3(CO)10(PMe2Ph)2 and
Ru3(CO)10(PPh3)2. The change in appearance of the 31P NMR
signals of 1 as a function of temperature revealed that inter-
conversion occurs between the isomers with phosphine ligands
on different ruthenium centers (1a′, 1a′′, 1a′′′).41 However, these
(35) Bax, A.; Griffey, R. H.; Hawkins, B. L. J. Magn. Reson. 1983, 55,
301.
(36) Bodenhausen, G.; Ernst, R. R. J. Am. Chem. Soc. 1982, 104, 1304.
(37) Jones, W. D.; Rosini, G. P.; Maguire, J. A. Organometallics 1999,
18, 1754.
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(39) Deeming, A. J. In ComprehensiVe Organometallic Chemistry II;
Abel, E. W., Stone, F. G. A., Wilkinson, G., Eds.; Pergamon Press: Oxford,
U.K., 1995; Vol. 7.
2660.
(40) Tolman, C. A. Chem. ReV. 1977, 77, 313.

Ru3(CO)10(PMe2Ph)2 and Ru3(CO)10(PPh3)2
J. Am. Chem. Soc., Vol. 123, No. 40, 2001 9763
Table 1. Typical Rate and Equilibrium Constants for the Slower
Dynamic Processes in 1 and 2 in CDCl3 as Determined by NMR
Methods
would result. These values are consistent with the findings of
Chen and Poe¨, where ∆S‡ for dissociative CO loss in triruthe-
nium clusters is positive and ∆H‡ far in excess of those found
here.12
temp (K)
Further support for a nondissociative pathway comes from
the fact that when the process was studied in CDCl3 at 285 K
neither excess CO (1 atm) nor excess PMe2Ph (∼120-fold
excess) had any effect on the observed exchange rate constants.
On moving to toluene-d8, excess CO again had no effect,
whereas added PMe2Ph led to the rapid formation of Ru3(CO)9-
(PMe2Ph)3. Fortunately, the corresponding 31P{1H} spectrum
contained separate signals for 1a and 1b, with line shapes and
relative intensities identical to those observed in the absence of
phosphine. Thus, the addition of neither CO nor phosphine
affects the exchange pathway in toluene. This is consistent with
the failure to observe interspecies exchange when the mixture
of 1, 2, and Ru3(CO)10(PMe2Ph)(PPh3) was studied.
223 228 238 244
285
289
296
301
k1af1b (s-1
)
)
15.9 22.8 31.1 39.3
11.0 15.3 20.1 24.9
1.88 1.92 1.97 2.02
k
1bf1a (s-1
K1af1b
k
k
2af2b (s-1
2bf2a (s-1
)
)
0.8 2.6 8.4 16.9
0.6 1.9 9.3 17.9
K2af2b 1.09 1.01 0.97 0.90
processes were too fast to be accurately monitored. Exchange
between the signals of 1a and 1b has been monitored by 2D
31P-EXSY spectroscopy35 and line shape analysis.36 To clarify
the mechanism of exchange, this process was monitored in four
solvents, toluene-d8, CDCl3, THF-d8, and nitromethane-d3. 31
P
peak integrations from quantitative measurements were used
to obtained K, the equilibrium constant, and the equilibrium
parameters ∆HL and ∆SL via a Van’t Hoff analysis. Interest-
ingly, the equilibrium position at 301 K showed a slight solvent
dependence, with the integrated peak ratios of 1a to 1b changing
from 1.54 in toluene-d8 through 1.96 in THF-d8 and 2.08 in
CDCl3 to 2.26 in nitromethane-d3. Similar NMR methods have
been used to explore the dynamic behavior of 2. Selected rate
and equilibrium constants for the dynamic behavior of 1 and 2
are listed in Table 1, while the activation and equilibrium
parameters for interconversion are given in Table 2.
The free energy difference between the averaged forms of
1a and isomer 1b obtained from these data is only 1.03 kJ mol-1
at 301 K in toluene-d8. This information is summarized in the
reaction coordinate versus free energy plot illustrated in Figure
2. Furthermore, the slight solvent dependence of the equilibrium
position (1.54 in toluene-d8, 1.96 in THF-d8, 2.08 in CDCl3,
and 2.26 in nitromethane-d3), and the change in ∆HL of only 5
kJ mol-1, suggest that the relative stability of these species is
only slightly affected by the solvent.
In contrast, the effect of the solvent on the rate constant for
isomer interchange is dramatic. Overall, the associated entropy
of activation for the 1a f 1b process changes from -25 J K-1
mol-1 in toluene through -64 J K-1 mol-1 in THF-d8 and -85
J K-1 mol-1 in CDCl3 to -105 J K-1 mol-1 in nitromethane-
d3. The reverse process, 1b f 1a, shows a similar trend. These
data suggest that the solvent participates in the exchange process,
a hypothesis that is supported by the associated decrease in ∆H‡.
The total change in bond energy up to the rate-limiting step for
this interconversion is therefore clearly reduced by the introduc-
tion of a bonding interaction with the solvent, and initial ligand
loss can be excluded since a large positive entropy of activation
Several options still exist for the exchange pathway. However,
since the phosphine ligands are restricted to nonaxial sites, their
presence eliminates the merry-go-round option.39,42 Mechanisms
involving initial Ru-Ru bond cleavage, via homolytic and
heterolytic steps, remain. Homolytic fission, is however, very
unlikely because of the dramatic solvent participation43 and the
fact that p-H2 enhancements are observed for the corresponding
H2 addition products described later. Heterolytic activation
would lead to the generation of an “ionic pair” with one 16-,
and two 18-, electron ruthenium centers. This process has
already been suggested to account for the substitution chemistry
of a wide variety of systems containing metal-metal bonds.43
The stability of the transition state in such a transformation
would be highly solvent dependent. We conclude that the most
likely exchange pathway therefore involves such a process, as
illustrated in Scheme 1. After the rate-determining step, further
ligand rearrangement is necessary, with stepwise movement of
the phosphine, followed by CO, or concerted movement of both
ligands necessary to re-form 1. The kinetic and thermodynamic
parameters described here provide no information for these steps.
The equilibrium constant connecting structures 2a and 2b,
where L ) PPh3, is of the same order of magnitude as those
found for 1. Furthermore, the values obtained for the difference
in isomer free energy and the associated activation enthalpies
are similar. This indicates that there is little steric cost to placing
two triphenylphosphine ligands on the same ruthenium center
in an equatorial arrangement. There is, however, a notable
difference in the effect of solvent on the entropy of activation
for isomer interconversion since the values are now essentially
zero. This requires significantly reduced levels of solvent
participation in rate-determining Ru-Ru bond heterolysis. Since
PMe2Ph is the better electron donor and hence more able to
Table 2. Thermodynamic and Kinetic Data for Slower Isomer Interchange in 1 and 2
process
variable
toluene-d8
THF-d8
CDCl3
CD3NO2
1a f 1b
(equilibrium)
∆HØ (kJ mol-1
)
5 ( 1
20 ( 5
2.3 ( 0.3
1 ( 1
3.2 ( 0.3
15 ( 1
7 ( 1
19 ( 4
∆SØ (J K-1 mol-1
)
∆GØ301 (kJ mol-1
∆H‡ (kJ mol-1
∆S‡ (J K-1 mol-1
∆H‡ (kJ mol-1
∆S‡ (J K-1 mol-1
∆HØ (kJ mol-1
∆SØ (J K-1 mol-1
∆GØ301 (kJ mol-1
∆H‡ (kJ mol-1
∆S‡ (J K-1 mol-1
∆H‡ (kJ mol-1
∆S‡ (J K-1 mol-1
)
1.03 ( 0.05
56 ( 7
1.33 ( 0.03
46 ( 3
1.76 ( 0.05
39 ( 3
2.04 ( 0.04
34 ( 3
1a f 1b
1b f 1a
)
)
-25 ( 24
54 ( 4
-64 ( 10
38 ( 4
-86 ( 13
-85 ( 10
34 ( 3
-105 ( 10
30 ( 2
-114 ( 6
)
)
-29 ( 12
8 ( 1
-104 ( 11
4 ( 1
2a f 2b
(equilibrium)
)
)
)
30 ( 6
18 ( 4
0.72 ( 0.02
49 ( 3
1.23 ( 0.06
54 ( 3
2a f 2b
2b f 2a
)
)
-14 ( 13
46 ( 2
0 ( 11
)
52 ( 2
)
-35 ( 8
-8 ( 8

9764 J. Am. Chem. Soc., Vol. 123, No. 40, 2001
Blazina et al.
Figure 2. Free energy vs reaction coordinate diagram for 1 and 3 in toluene-d8 at 301 K. The free energies of activation for interchange of 1a41
and 1b are included.
Scheme 1. Proposed Mechanism for Isomer Exchange from
1a′ to 1b
Figure 3. 1H NMR spectrum showing the enhanced hydride resonances
obtained for 3 when it is generated from the reaction of 1 with p-H2 in
benzene-d6 at 296 K.
signals were connected in a similar way. Careful examination
of the hydride resonance at δ -18.15 (3ab, where the label 3a
indicates the complex, and the b that it is in a bridging position)
revealed, in addition to a hydride-hydride coupling of -6.8
stabilize the developing positive charge, this observation is the
opposite to that expected on an electronic basis and most likely
signifies a steric effect.
Reactions of Ru3(CO)10(PMe2Ph)2 and Ru3(CO)10(PPh3)2
Hz, the presence of extra couplings to two different phosphorus
centers (JPH values 13.8 and 18.2 Hz).44 However, the partner
that resonates at δ -11.10 (3at, t indicates terminal) only
coupled to a single 31P center with a JPH coupling of 12.5 Hz
with Hydrogen. When a sample containing 1 in toluene-d8
being indicative of a cis hydride-phosphine arrangement.30 This
under 3 atm of p-H2 is monitored by 1H NMR spectroscopy at
suggests that 3a contains the (PMe2Ph)(H)Ru(µ-H)Ru-
296 K, the resulting spectrum (see Figure 3) contains four
(PMe2Ph) subunit as shown in Figure 4. Both sets of hydride
enhanced hydride resonances at δ -10.37, -11.10, -18.15, and
resonances for isomer 3b are split into triplets by two apparently
-18.61 (the NMR data for these products are listed in Table
equivalent 31P nuclei. Consequently, the two phosphine ligands
3). The chemical shifts of these hydride resonances suggest that
are inequivalent and located on the same Ru center in a
the first two arise from terminal hydride ligands and the second
(PMe2Ph)2(H)Ru(µ-H)Ru arrangement with the 1H-31P coupling
two arise from bridging hydride ligands.30 Since the signals at
δ -11.10 and -18.15 are connected in a COSY experiment,
they originate from the same product; the δ -10.37 and -18.61
of 16.4 assigned to the terminal hydride and the 13.0 Hz
coupling to the bridging hydride as determined via suitable
coupling-selective HMQC observations. In view of fact that we
see apparently equivalent 31P couplings from two phosphine
(41) Throughout the text, the use of the symbol a to identify a compound
(e.g., in 1a or 2a) refers to the time-averaged NMR signal observed at higher
centers to each hydride ligand, this situation requires careful
temperatures for isomers a′, a′′, and a′′′. See Figure 1 for structures.
examination.30 If the phosphine ligands are exchanging positions,
(42) McAuliffe, C. A.; Levason, W. In Phosphine, Arsine and Stilbene
Complexes of the Transition Elements; Elsevier Scientific Publishing Co.:
Amsterdam, 1979.
the spectra would contain the average of the two distinct JPH
(43) Johnson, B. F. G.; Roberts, Y. V. Polyhedron 1993, 12, 977.
(44) Aime, S.; Gobetto, R.; Valls, E. Inorg. Chim. Acta 1998, 275, 521.

Ru3(CO)10(PMe2Ph)2 and Ru3(CO)10(PPh3)2
J. Am. Chem. Soc., Vol. 123, No. 40, 2001 9765
Table 3. NMR Data for Hydrogen Addition Products 3 (toluene-d8 at 296 K) and 4 (toluene-d8, 253 K) (See Figure 6 for Positions of PA and
PB)
compd
1H
31P
13C
3a
(3at) -11.10, dd (JPAH ) 12.5 Hz,
JHH ) -6.8 Hz)
14.4, s (PA), -4.8, s (PB)
201.5, d (JCC ) 5.6 Hz, JCC ) 5.6 Hz),
206.5, d (JCC ) 5.6 Hz)
(3ab) -18.15, ddd (JPBH ) 13.8 Hz,
JPAH ) 18.2 Hz, JHH ) -6.8 Hz)
(3bt) -10.37, td (JPH ) 16.4 Hz,
JHH ) -4.5 Hz)
197.0, d (JCC ) 1.4 Hz), 204.0, t (JCC ) 1.4 Hz)
3b
-0.6, s
202.0, s
(3bb) -18.61, td (JPH ) 13.0 Hz,
JHH ) -4.5 Hz)
194.5, t (JCC ) 2.7 Hz), 204.5, t (JCC ) 2.7 Hz)
4a
4b
(4at) -10.11, d (JPH ) 11.0 Hz)
(4ab) -17.47, dd (JPBH ) 10.2,
JPAH ) 20.5 Hz)
44.2, s (PA), 31.2, s (PB)
(4bt) -11.19, unknown multiplicity
(4bb) -17.37, unknown multiplicity
To characterize more fully the hydrogen addition products,
the reaction of p-H2 with a benzene-d6 solution of 1-13C was
examined. The enhanced hydride resonances now showed visible
couplings to 13C when compared with signals obtained from a
sample with 13CO present at natural levels (see Figure 5a and
b). For the low-field hydride resonances, the 1H-13C coupling
of 12.5 Hz (obtained by simulation) is indicative of a trans 1H-
1
13CO arrangement.46 A series of H-13C HMQC spectra were
1
Figure 4. Structures of (a) 3a and (b) 3b as revealed by H, 31P, and
recorded in order to locate the associated carbonyl carbon
signals. A typical spectrum is illustrated in Figure 5c. Spectra
were optimized for each 13C resonance by changing the
associated delays to account for the different values of cis and
trans 2J(13C-1H) and changing the spectral center to match the
desired resonance’s chemical shift (see Table 3). For example,
13C NMR data.
couplings. For 3a, the JPH value corresponding to the Ru based
(bridging-hydride) - (terminal phosphine) connection is 18.2
Hz, while for 3b the time-averaged coupling is 13.0 Hz. Since
the time-averaged coupling is less than 18.2 Hz, the 18.2-Hz
coupling must arise from a trans-(P)Ru(µ-H) arrangement since
this yields the largest JPH value. The chemical shifts for the
when a J(13C-1H) value of 12.5 Hz was employed, strong
2
connections were visible to peaks at δ 201.5 from 3at and δ
197.0 from 3ab. For a value of 2.5 Hz, corresponding to a cis
coupling, connections from 3at appear to a peak at δ 206.5 and
from 3ab to a peak at δ 204.0.
1
phosphine ligands of 3a and 3b were located using a 2D H-
31P HMQC experiment. While two 31P signals were found for
3a at δ 14.4 and -4.8, only one was located for 3b at δ -0.6
(toluene-d8). From the structures in Figure 4, 3b should contain
a pair of inequivalent phosphines located on a single ruthenium
center, which are always magnetically distinct in the context
of their interaction with the hydride ligands. Since close
examination of the 31P signal for 3b failed to split the δ -0.6
resonance and the hydride resonances are first order in appear-
ance, we can reliably deduce that the two phosphines interchange
their positions rapidly at 296 K.
It was possible to assign structures to 3a and 3b by assuming
they have 48-valence electrons. For example, the hydride
resonance due to 3bt-13C is split by a 12.5 Hz coupling to one
carbonyl carbon. The 1H-13C{31P} HMQC located the associ-
ated 13C resonance as a singlet at δ 202.0 and since it does not
couple to any other carbonyl group on that metal center it is
unique. The hydride and the carbonyl must therefore adopt axial
positions with respect to each other, with the two phosphine
ligands located at the remaining two sites in the equatorial plane.
The associated bridging hydride 3bb was found to interact with
four carbonyls on the second Ru center as evidenced by the
two sets of 13C resonances that are each split into triplets due
to couplings to the two additional 13CO ligands. These signals
arise from pairs of apparently equivalent axial and equatorial
carbonyl ligands and the structure 3b shown in Figure 4 is
indicated. A similar solid-state structure has been reported for
substituted H2Os3(CO)10L clusters.49
By using a similar process, the structure of 3a was assigned
(see Table 3 and Figure 4). In brief, the hydride resonance 3at-
13C shows an extra 12.5 Hz splitting due to a trans carbonyl
carbon which resonates at δ 201.5 and is split into a doublet by
coupling to a second 13CO ligand (located as a doublet at δ
206.5). The latter carbonyl is located in the equatorial plane
and the remaining position is occupied by the phosphine. Similar
connections from the bridging hydride 3ab to the ligands on
From these data, it is not unreasonable to assume that 1a
and 1b have the same basic structure as that of their hydrogen
adducts, i.e., one phosphine ligand on each of two separate Ru
atoms for 1a and two phosphine ligands on the same Ru for
1b. To explore this further, 1 was labeled with 13CO, viz. 1-13C,
and analyzed by 13C NMR spectroscopy. The 13C NMR
spectrum of the precursor 1-13C, in toluene-d8, contained only
three signals due to carbonyl ligands at δ 196.2, 207.7, and
211.5. These signals were still substantially broadened due to
dynamic effects and possessed no fine structure that could be
attributable to 31P splittings. This result is not surprising
considering the widely reported carbonyl fluxionality of M3-
(CO)12 clusters and their derivatives, where M ) Fe,45 Ru,46 or
Os.47 Analogous behavior has also been reported for the
carbonyl ligands of H2Os3(CO)10 and its derivatives.48 Rapid
interchange of the phosphine ligands in 3b is therefore not totally
unexpected.
(47) (a) Deeming, A. J.; Donovan-Mtunzi, S.; Kabir, S. E. J. Organomet.
Chem. 1985, 281, C43. (b) Deeming, A. J.; Donovan-Mtunzi, S.; Kabir, S.
E.; Manning, P. J. J. Chem. Soc., Dalton Trans. 1985, 1037.
(48) Aime, S.; Osella, D.; Milone, L.; Rosenberg, A. J. Organomet.
Chem. 1981, 213, 207.
(45) (a) Mann, B. E. J. Chem. Soc., Dalton Trans. 1997, 1457. (b)
Johnson, B. F. G. J. Chem. Soc., Dalton Trans. 1997, 1473.
(46) Aime, S.; Dastru, W.; Gobetto, R.; Viale, A. Organometallics 1998,
17, 3182.
(49) Keister, J. B.; Shapley, J. R. Inorg. Chem. 1982, 21, 3304.

9766 J. Am. Chem. Soc., Vol. 123, No. 40, 2001
Blazina et al.
Figure 6. 2D 1H{31P-decoupled} EXSY spectrum (positive contours)
of a sample of 3 with p-H2 in toluene-d8 at 301 K for a mixing time of
0.4 s. Exchange peaks connecting the hydride resonance of 3ab to 3at
(#), 3bt (%), and 3bb (&), visible with a mixing time of 0.4s, are
indicated.
Reaction of 2 with p-H2 in toluene-d8 enabled the corre-
sponding hydrogen addition products 4 to be detected. However,
the exchange between the hydrides and free hydrogen was too
slow for PHIP enhancements to be observed at 253 K, and
normal signals were seen. Pairs of terminal and bridging
hydrides were observed (4at, 4ab, 4bt, 4bb), suggesting that
products analogous to 3a and 3b are formed. While the data
for 4a clearly demonstrate that it contains one phosphine ligand
on each of two Ru atoms, as well as a terminal and bridging
hydride, the signals for 4b were much weaker and their
multiplicities could not be determined. The spectral data for 4
are summarized in Table 3. Above 265 K, the exchange rate
for hydrogen addition/reductive elimination is such that small
PHIP enhancements are first observed, and at even higher
temperatures the associated resonances become extremely broad.
Upon removal of the H2 atmosphere 3 and 4 disappear over
a period of ∼30 s. When the [H2] (toluene-d8, 301 K) present
in solution in a sample of 1 was increased by a factor of 2.15,
the ratios of 3a to 1a and 3b to 1b changed dramatically. The
ratio ([1a][H2])/([3a][CO]) was determined to be (2.3 ( 0.6)
× 10-4 for both [H2]. Similarly, the corresponding ratio for the
reaction 1b + H2f 3b + CO was determined as (9.7 ( 1) ×
10-5. Assuming that these ratios correspond to equilibrium
constants, the corresponding free energy differences for 1a/3a
and 1b/3b are 21 ( 1 and 23 ( 1 kJ mol-1, respectively. These
free energy differences are illustrated in Figure 2.
Figure 5. p-H2 enhanced NMR spectra of 3 in benzene-d6 at 296 K:
(a) 1H{31P} spectrum of an unlabeled sample, (b) 1H{31P} spectrum of
a 13C-labeled sample revealing enhanced peak separations due to 1H-
13C couplings, and (c) selected cross-peaks (absolute value display)
and projections showing hydride and carbonyl chemical shift connec-
1
tions in a typical H-13C{31P-decoupled} HMQC spectrum.
Ligand Exchange Pathways in (H)2Ru3(CO)9(PMe2Ph)2
and (H)2Ru3(CO)9(PPh3)2. EXSY spectroscopy has been used
to follow the dynamic behavior of the hydride ligands in 3a
and 3b, and a typical spectrum is shown in Figure 6. The
observation of positive exchange cross-peaks between all the
hydride resonances of 3a and 3b at longer mixing times indicates
that all the hydride ligands are able to exchange positions.
Exchange with free H2 is not observed on this time scale.
EXSY spectra were recorded for a variety of mixing times,
temperatures. and solvents and the resultant data analyzed
according to the procedure outlined in the Experimental
Section.37 For example, when EXSY spectra were recorded at
301 K for 3, the raw data shown in Table 4 were obtained. The
data presented illustrate the correlation between measured and
calculated peak areas for exchange of 3at into 3ab and 3bt and
3bb, as illustrated in Figure 7a. The rate constants obtained for
the second Ru center reveal the presence of one equatorial and
two axial carbonyls. Collectively, these data support the
structures for 3a and 3b shown in Figure 4.
We note that the carbonyl and phosphine ligands in the
equatorial plane of 3b either have coincident chemical shifts or
rapidly interchange their positions while maintaining their
location on the same ruthenium center. Since the bridging
hydride resonance in 3b appears as a triplet rather than a second-
order multiplet, chemical shift coincidence can be excluded and
averaging of the inequivalent cis and trans JP-H couplings must
occur. It is possible that a similar process facilitates interchange
within 1a. Furthermore, since the intensity of the cross-peaks
observed in the HMQC spectra is controlled by the delay used
for coherence transfer, exchange between equatorial and axial
sites, if present, must occur on a much slower time scale.

Ru3(CO)10(PMe2Ph)2 and Ru3(CO)10(PPh3)2
J. Am. Chem. Soc., Vol. 123, No. 40, 2001 9767
Table 4. EXSY Data for Hydride Interchange from Location 3at
into Locations 3ab, 3bt, and 3bb at 301 Ka
Table 5. Activation Parameters for Identified Hydride Exchange
Processes in Species 3a and 3b
2
∑
process
toluene-d8
CDCl3
measured/calculated
mixing time
mix (s)
[measd -
calcd]
3ab f 3at and 3at f 3ab
τ
[3at]
[3ab]
[3bt]
[3bb]
0/0
0/1.8
0/2.5
0/3.1
∆H‡ (kJ mol-1
)
30 ( 8
-122 ( 26
49 ( 4
-57 ( 14
∆S‡ (J K-1 mol-1
3ab f 3bb and 3at f 3bt
∆H‡ (kJ mol-1
∆S‡ (J K-1 mol-1
3ab f 3bt and 3at f 3bb
∆H‡ (kJ mol-1
∆S‡ (J K-1 mol-1
)
0
100/100
0/0
0/0
0
10
29
27
41
26
8
0.05
0.075
0.1
65.3/65.0 34.7/32.2 0/0.9
62.0/57.5 38.0/38.6 0/1.5
55.5/52.9 44.5/42.0 0/2.0
)
150 ( 12
242 ( 37
)
0.3
0.5
43.4/44.0 39.2/43.9 7.3/5.7 10.0/6.5
37.4/41.8 44.0/41.8 9.7 /8.0 8.9/8.5
)
51 ( 8
-76 ( 24
0.6
38.8/41.0 42.6/41.1 9/8.7
9.6/9.2
)
minimized value:
141
2
∑
all τmix [measd - calcd]
On a slower time scale, interisomer interchange occurs, with
3at moving into the terminal position 3bt at a rate of 0.30 s-1
and into the bridging position 3bb at a rate of 0.84 s-1. The
reverse processes are much faster, in keeping with the domi-
nance of isomer 3a. At 301 K, the rate constant for interchange
of the precursor 1a into 1b is 62.1 s-1. This process therefore
occurs on a much faster time scale than intraisomer hydride
interchange in 3a.
a tabulated data include measured and calculated peak areas for
specified mixing times, together with indicators of goodness of fit.
The rate of hydride exchange within 3a was unaffected
(within experimental error) by CO although the observed rate
constants for 3at f 3bt and 3ab f 3bb were both dramatically
reduced. Unfortunately no data could be collected for the effect
of phosphine since immediate conversion to Ru3(CO)9-
(PMe2Ph)3 occurred.30
The bridge T terminal exchange process in 3a has a relatively
small enthalpy of activation (∆H‡ ) 30 kJ mol-1 in toluene-
d8) and a substantially negative entropy term (∆S‡ ) -122 J
K-1 mol-1). As such, solvent participation in this step is sizable.
A concerted motion of hydride and carbonyl ligands has been
proposed to account for the hydride interchange in H2Os3(CO)11
and its derivatives.49,50 However, such motion is unlikely here
since the solvent effect would be minimal. Homolysis of a
ruthenium-ruthenium bond is also unlikely since this would
quench the p-H2 enhancement.51 It is therefore conceivable that
the mechanism involves rate-determining heterolysis of the
hydride-bridged Ru-Ru bond, with concurrent formation of a
16-electron metal dihydride cation and an 18-electron anion,
followed by reversible formation of an η2-H2 ligand, rotation,
and closure (see Scheme 2a). For this process, the low-polarity
solvent toluene promotes exchange when compared to the more
polar CDCl3. Rate-limiting solvent displacement from the
solvated intermediate is consistent with the data. The role of
η2-H2 ligands in the oxidative addition of H2 is well estab-
lished.52
Figure 7. (a) Simulated (solid line) and experimental data ((, b, 9,
2) for the exchange processes involving site 3at in toluene-d8 at 301
K and (b) schematic diagram for complex 3 showing the exchange
rate constants at 301 K. (The ligands on the third ruthenium center are
omitted for clarity.)
these processes at 301 K are shown in Figure 7b. Repeating
this procedure over the temperature range 273-312 K enabled
the activation parameters ∆H‡ and ∆S‡ to be obtained for these
pathways.
By microscopic reversibility,53 the same rationale can be
extrapolated to postulate a pathway for hydrogen addition to
1a. Thus, oxidative addition of H2 occurs to the 16-electron
center produced after heterolytic scission of the Ru-Ru bond.
Subsequent CO loss, and reestablishment of the Ru-Ru bond,
as shown in Scheme 2b, leads to the final product.
The interisomer exchange process where the hydrides of the
major and minor isomers remain in the same relative positions
(i.e., 3at f 3bt and 3ab f 3bb) yielded ∆H‡ ) 150 kJ mol-1
and ∆S‡ ) 242 J K-1 mol-1 in toluene-d8. The large positive
entropy term suggests that this process involves ligand dis-
sociation. This conclusion is supported by the dramatic reduction
in observed rate for this process when CO is added to the system.
It should be noted that although a large temperature range
was used, not all the processes could be followed over the full
range. For example, while the intramolecular hydride exchange
for the major species 3a could always be reliably followed,
exchange between the isomers was too slow at the lower
temperatures, and too fast at the higher temperatures, to be
monitored accurately. Furthermore, since the minor isomer 3b
could only be detected readily at high temperatures, little data
were available for processes starting from 3b. These limitations
constrained the available rate data and consequently widened
the confidence limits of the associated activation parameters.
The activation parameters for processes i, iii, and iv were
determined reliably and are presented in Table 5.
(50) Aime, S.; Osella, D.; Milone, L.; Rosenberg, E. J. Organomet. Chem.
1981, 213, 207.
(51) Eisenberg, R. Acc. Chem. Res. 1991, 24, 110.
(52) (a) Heinecke, D. M.; Oldhem, W. J. Chem. ReV. 1993, 93, 913. (b)
Jessop, P. G.; Morris, R. H. Coord. Chem. ReV. 1992, 121, 155.
(53) Jones, W. D.; Feher, F. J. Acc. Chem. Res. 1989, 22, 91.
It is noteworthy that intraisomer exchange in 3a corresponds
to the fastest process, with the terminal hydride 3at moving
into position 3ab at a rate of 18.2 s-1 in toluene-d8 at 301 K.

9768 J. Am. Chem. Soc., Vol. 123, No. 40, 2001
Blazina et al.
Scheme 2. (a) Proposed Hydride Exchange Mechanism for
the Process 3at f 3ab and (b) Proposed Mechanism of
Hydrogen Addition to 1a′′
stage, we are unable to speculate further about the exact nature
of this process. No rate data are presented for interconversions
involving 4 because these products could not be detected with
sufficient signal strengths to facilitate their analysis.
Conclusions
In this paper, we have shown that clusters Ru3(CO)10-
(PMe2Ph)2 1 and Ru3(CO)10(PPh3)2 2 exist in solution in at least
three distinct isomeric forms. The minor isomer contains both
phosphines on the same ruthenium center in the equatorial plane,
and the others have phosphines in the equatorial plane on
different ruthenium centers. The isomers were shown to
interconvert by EXSY spectroscopy and line shape analysis.
High levels of solvent participation and associated change in
activation parameters indicate that phosphine movement between
ruthenium centers proceeds via initial heterolysis of a Ru-Ru
bond with the intermediates being stabilized by the solvent. The
effect of the solvent is found to be much greater for the
PMe2Ph-substituted cluster than for the PPh3 analogue, an effect
that can be attributed to the bulk of the PPh3 ligand preventing
the solvent from interacting with the Ru-Ru bond during
heterolysis.
Both these clusters have been shown to react with hydrogen.
NMR spectroscopy, in conjunction with para hydrogen-induced
polarization, has allowed the determination of the structures of
the two products of hydrogen addition to 1 as H(µ-H)-
Ru3(CO)9(PMe2Ph)2 3 (see Figure 4). Hydrogen addition is
suggested to proceed via oxidative addition to the 16-electron
center formed after the heterolysis of the Ru-Ru bond, followed
by loss of CO and closure of the Ru-Ru bond. Removal of the
H2 atmosphere results in rapid re-formation of the precursor
clusters. These four species are connected via a suitable free
energy plot (see Figure 2), with the Ru0 precursors being the
most stable.
The two isomers of the hydrogen addition product have been
shown to interconvert, and the kinetic data suggest that
intramolecular hydride exchange within the major isomer, which
contains a (PMe2Ph)(H)Ru(µ-H)Ru(PMe2Ph) arrangement, pro-
ceeds via Ru-Ru bond heterolysis followed by formation of
an η2-H2 ligand, rotation about the Ru-H2 bond, and closure.
Two slower processes, comprising exchange between the above
isomer and one containing a (PMe2Ph)2(H)Ru(µ-H)Ru arrange-
ment have been detected and are shown to involve CO loss.
CO ligand loss can occur directly from 3a or from the
intermediate formed after Ru-Ru bond heterolysis. Subsequent
steps, including phosphine migration, are necessary to complete
the exchange. For example, if the carbonyl ligand illustrated in
boldface type in Scheme 2a dissociates, transfer of the phosphine
from the remote ruthenium center into the vacant coordination
site and recoordination of CO complete this process.
The exchange from the major to the minor isomer with
interchanging hydride positions (i.e., 3at f 3bb and 3ab f
3bt) is the last process for which meaningful kinetic data could
be extracted. The kinetic analysis required that this intercon-
version be distinct from that which is achieved by the product
of the first two processes. This second, slower and direct route
yielded values for ∆H‡ of 51 kJ mol-1 and for ∆S‡ of -76 J
K-1 mol-1 in toluene-d8. Given the negative entropy of
activation, it is surprising that added CO also inhibited this
process. Clearly, if both the isomer interchange pathways
involve a common intermediate formed by CO loss, the rate-
determining step for the conversion 3at f 3bb is significantly
different from that involved in conversion 3at to 3bt. At this
Acknowledgment. D.B. acknowledges financial support
from the ORS Award Scheme. S.B.D. is grateful to the
University of York, the EPSRC, the JREI scheme, and Bruker
UK (sponsorship and spectrometer) for financial support. P.J.D.
thanks the Royal Society for a University Research Fellowship.
Discussions with Prof. R. Eisenberg (NATO support), Prof. R.
N. Perutz, and Dr. R. J. Mawby are gratefully acknowledged.
Supporting Information Available: Additional details as
indicated in text. This material is available free of charge via
the Internet at http://pubs.acs.org.
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