Diphosphine isomerization and C-H and P-C bond cleavage reactivity in the triosmium cluster Os3(CO)10(bpcd)

Article abstract of DOI:10.1021/om050839t

The coordination and reactivity of the diphosphine ligand 4,5-bis(diphenylphosphino)-4-cyclopentene1,3-dione (bpcd) with Os ₃(CO)₁₀(MeCN)₂ (1) has been explored. The initial substitution product 1,2-Os₃(CO)₁₀(bpcd) (2b) undergoes a nondissociative, intramolecular isomerization to furnish the bpcdchelated cluster 1,1-Os₃(CO)₁₀(bpcd) (2c) over the temperature range of 323-343 K. The isomerization reaction is unaffected by trapping ligands, yielding the activation parameters ΔH^? = 25.0(0.7) kcal/mol and ΔS^? = -2(2) eu. Thermolysis of 2c in refluxing toluene gives the hydrido cluster HOs₃(CO) ₉/μ-(PPh₂)C=C{PPh(C₆H₄)}C(O) CH₂C(O)] (3) and the benzyne cluster HOs₃(CO) ₈(μ₃-C₆H₄)[μ₂, η¹-PPhC= C(PPh₂)C(O)CH2C(O)] (4). Time - concentration profiles obtained from sealed-tube NMR experiments starting with either 2c or 3 suggest that both clusters are in equilibrium with the unsaturated cluster 1,1-Os₃(CO)₉(bpcd) and that the latter cluster serves as the precursor to the benzyne-substituted cluster 4. The product composition in these reactions is extremely sensitive to CO, with the putative cluster 1,1-Os₃(CO)₉(bpcd) being effectively scavenged by CO to regenerate 2c. Photolysis of cluster 2c using near-UV light affords 3 as the sole product. These new clusters have been fully characterized in solution by IR and NMR spectroscopy, and the molecular structures of clusters 2b,c, and 4 have been determined by X-ray crystallography. Reversible C-H bond formation in cluster 3 is demonstrated by ligand trapping studies to give 1,1-Os ₃(CO)₉L(bpcd) (where L = CO, phosphine) via the unsaturated intermediate 1,1-Os₃(CO)₉(bpcd). The kinetics for reductive coupling in HOs₃(CO)₉[μ-(PPh ₂)C=C{PPh(C₆H₄)}C(O)CH₂C-(O)] and DOs₃(CO)₉[M-(PPh₂-d₁₀)C=C{P(Ph- d₅)(C₆D₄)}C(O)CH₂C(O)] in the presence of PPh₃ give rise to a k_H/k_D value of 0.88, a value that supports the existence of a preequilibrium involving the hydride (deuteride) cluster and a transient arene-bound Os₃ species that precedes the rate-limiting formation of 1,1-Os₃(CO) ₉(bpcd). Strong proof for the proposed hydride (deuteride)/arene preequilibrium has been obtained from photochemical studies employing the isotopically labeled cluster 1,1-Os₃(CO)₁₀(bpcd-d _4,ortho), whose bpcd phenyl groups each contain one ortho hydrogen and deuterium atom. Generation of 1,1-Os₃(CO)₉-(bpcd- d_4,ortho) at 0°C gives rise to a 55:45 mixture of the corresponding hydride and deuteride clusters, respectively, from which a normal KIE of 1.22 is computed for oxidative coupling of the C-H(D) bond in the ortho metalation step. Photolysis of 1,1-Os₃(CO)10(bpcd-d _4,ortho) at elevated temperature and thermolysis of the low-temperature photolysis hydride/deuteride mixture afford an equilibrium mixture of hydride (67%) and deuteride (33%), yielding a K_eq value of 0.49, which in conjunction with the k_H/k_D ratio from the C-H(D) ortho-metalation step allows us to establish a k_H/k _D value of 0.60 for the reductive coupling from the participant hydride/deuteride clusters. These data, which represent the first isotope study on ortho metalation in a polynuclear system, are discussed relative to published work on benzene activation at mononuclear rhodium systems. UV - vis kinetic data on the transformation 3 → 4 provide activation parameters consistent with the rate-limiting formation of the unsaturated cluster 1,1-Os ₃(CO)₉(bpcd), preceding the irreversible P-C cleavage manifold. The ortho metalation of the bpcd ligand in 3 and formation of the benzvne moietv 4 are discussed relative to liaand degradation reactions in this genre of cluster.

Full text of DOI:10.1021/om050839t

930
Organometallics 2006, 25, 930-945
Diphosphine Isomerization and C-H and P-C Bond Cleavage
Reactivity in the Triosmium Cluster Os3(CO)10(bpcd): Kinetic and
Isotope Data for Reversible Ortho Metalation and X-ray Structures
of the Bridging and Chelating Isomers of Os3(CO)10(bpcd) and the
Benzyne-Substituted Cluster
HOs3(CO)8(µ3-C6H4)[µ2,η1-PPhCdC(PPh2)C(O)CH2C(O)]†
William H. Watson*
Department of Chemistry, Texas Christian UniVersity, Fort Worth, Texas 76129
Guanmin Wu and Michael G. Richmond*
Department of Chemistry, UniVersity of North Texas, Denton, Texas 76203
ReceiVed September 29, 2005
The coordination and reactivity of the diphosphine ligand 4,5-bis(diphenylphosphino)-4-cyclopentene-
1,3-dione (bpcd) with Os3(CO)10(MeCN)2 (1) has been explored. The initial substitution product 1,2-
Os3(CO)10(bpcd) (2b) undergoes a nondissociative, intramolecular isomerization to furnish the bpcd-
chelated cluster 1,1-Os3(CO)10(bpcd) (2c) over the temperature range of 323-343 K. The isomerization
reaction is unaffected by trapping ligands, yielding the activation parameters ∆Hq ) 25.0(0.7) kcal/mol
and ∆Sq ) -2(2) eu. Thermolysis of 2c in refluxing toluene gives the hydrido cluster HOs3(CO)9[µ-
(PPh2)CdC{PPh(C6H4)}C(O)CH2C(O)] (3) and the benzyne cluster HOs3(CO)8(µ3-C6H4)[µ2,η1-PPhCd
C(PPh2)C(O)CH2C(O)] (4). Time-concentration profiles obtained from sealed-tube NMR experiments
starting with either 2c or 3 suggest that both clusters are in equilibrium with the unsaturated cluster
1,1-Os3(CO)9(bpcd) and that the latter cluster serves as the precursor to the benzyne-substituted cluster 4.
The product composition in these reactions is extremely sensitive to CO, with the putative cluster 1,1-
Os3(CO)9(bpcd) being effectively scavenged by CO to regenerate 2c. Photolysis of cluster 2c using near-
UV light affords 3 as the sole product. These new clusters have been fully characterized in solution by
IR and NMR spectroscopy, and the molecular structures of clusters 2b,c, and 4 have been determined by
X-ray crystallography. Reversible C-H bond formation in cluster 3 is demonstrated by ligand trapping
studies to give 1,1-Os3(CO)9L(bpcd) (where L ) CO, phosphine) via the unsaturated intermediate 1,1-
Os3(CO)9(bpcd). The kinetics for reductive coupling in HOs3(CO)9[µ-(PPh2)CdC{PPh(C6H4)}C(O)CH2C-
(O)] and DOs3(CO)9[µ-(PPh2-d10)CdC{P(Ph-d5)(C6D4)}C(O)CH2C(O)] in the presence of PPh3 give rise
to a kH/kD value of 0.88, a value that supports the existence of a preequilibrium involving the hydride
(deuteride) cluster and a transient arene-bound Os3 species that precedes the rate-limiting formation of 1,1-
Os3(CO)9(bpcd). Strong proof for the proposed hydride (deuteride)/arene preequilibrium has been obtained
from photochemical studies employing the isotopically labeled cluster 1,1-Os3(CO)10(bpcd-d4,ortho), whose
bpcd phenyl groups each contain one ortho hydrogen and deuterium atom. Generation of 1,1-Os3(CO)9-
(bpcd-d4,ortho) at 0 °C gives rise to a 55:45 mixture of the corresponding hydride and deuteride clusters, re-
spectively, from which a normal KIE of 1.22 is computed for oxidative coupling of the C-H(D) bond in
the ortho metalation step. Photolysis of 1,1-Os3(CO)10(bpcd-d4,ortho) at elevated temperature and thermolysis
of the low-temperature photolysis hydride/deuteride mixture afford an equilibrium mixture of hydride
(67%) and deuteride (33%), yielding a Keq value of 0.49, which in conjunction with the kH/kD ratio from
the C-H(D) ortho-metalation step allows us to establish a kH/kD value of 0.60 for the reductive coupling
from the participant hydride/deuteride clusters. These data, which represent the first isotope study on ortho
metalation in a polynuclear system, are discussed relative to published work on benzene activation at mono-
nuclear rhodium systems. UV-vis kinetic data on the transformation 3 f 4 provide activation parameters
consistent with the rate-limiting formation of the unsaturated cluster 1,1-Os3(CO)9(bpcd), preceding the
irreversible P-C cleavage manifold. The ortho metalation of the bpcd ligand in 3 and formation of the
benzyne moiety 4 are discussed relative to ligand degradation reactions in this genre of cluster.
well-documented phenomenon. Mono- and bidentate phosphine
ligands continue to receive extensive attention with respect to
Heck, Suzuki, and Sonogashira carbon-carbon bond forming
reactions,1 asymmetric catalysis and transformations,2 and
construction of dimensionally defined supramolecular com-
plexes.3 Of concern to researchers in these fields is the long-
term integrity of the ancillary phosphine ligand(s) that is bound
to the metal template during catalytic reactions and synthetic
Introduction
The use of ancillary phosphine ligands for the stabilization
and reactivity modification of organometallic compounds is a
* To whom correspondence should be addressed. Tel: 817-257-7195
(W.H.W.); 940-565-3548 (M.G.R.). E-mail: w.watson@tcu.edu (W.H.W.);
cobalt@unt.edu (M.G.R.).
† Dedicated to Prof. Jay K. Kochi for his seminal work on electron-
transfer reactions and mechanistic inorganic/organometallic chemistry on
the occasion of his 79th birthday.
10.1021/om050839t CCC: $33.50 © 2006 American Chemical Society
Publication on Web 01/17/2006

The Triosmium Cluster Os3(CO)10(bpcd)
Organometallics, Vol. 25, No. 4, 2006 931
manipulations, as deleterious cyclometalation of the alkyl
substituents, ortho metalation of the aryl substituents,4 and P-C
bond cleavage reactions can lead to diminished catalytic
efficiency and serve as an entry point for the complete
destruction of the phosphine ligand.5
Numerous studies of phosphine-substituted polynuclear clus-
ters have provided unambiguous evidence and conclusively
dispelled the early assumptions concerning the inert and
spectator nature of such ligands.6,7 The relative ease and outcome
associated with the stepwise activation and bond cleavage
reactions exhibited by cluster-bound phosphines have largely
been elucidated by solution spectroscopic and crystallographic
methods.8 In the case of the dppm-substituted clusters Ru3-
(CO)10(dppm) and Os3(CO)10(dppm), valuable insight into the
reactivity of the dppm ligand as a result of multisite activation
at the cluster polyhedron through ortho metalation of an aryl
group(s) and P-C bond activation has been achieved.9 Mild
thermolysis of the triruthenium cluster promotes ortho metalation
at an aryl ring on each phosphorus atom with concomitant loss
of benzene to initially furnish the phosphido-bridged cluster Ru3-
(CO)9[µ-PhPCH2PPh(C6H4)], which upon subsequent reaction
with adventitious CO, followed by reductive coupling, gives
the cyclic diphosphine-substituted cluster Ru3(CO)10[µ-PhP-
(CH2)(C6H4)PPh].10 A slightly different outcome is found for
the dppm ligand during the thermolysis of Os3(CO)10(dppm).
Here the ortho metalation of one of the aryl groups is triggered
by the formal loss of CO to ultimately yield the unsaturated
cluster HOs3(CO)8[Ph2PCH2PPh(C6H4)].11 The kinetic stabiliza-
tion of HOs3(CO)8[Ph2PCH2PPh(C6H4)] can be traced to the
capping of one of the Os3 faces by the two phosphorus atoms
and an edge-bridging aryl carbon, the latter resulting from the
ortho-metalation sequence. This particular triosmium cluster has
proven to be a pivotal precursor for the synthesis of a variety
of osmium-substituted clusters under mild conditions because
of the highly reversible ortho metalation, which in turns opens
up an accessible coordination site within the cluster.12 Scheme
1 illustrates the course of the dppm activation observed in these
two clusters.
(1) (a) Miyaura, N.; Suzuki, A. Chem. ReV. 1995, 95, 2457. (b) Heck,
R. F. Org. React. 1982, 27, 345. (c) Sonogashira, K. In Metal-Catalyzed
Cross-Coupling Reactions; Diederich, F., Stang, P. J., Eds.; Wiley-VCH:
New York, 1998; pp 203-229.
The impetus for this report is based, in part, on our earlier
studies, where the reactions of the diphosphine ligand 4,5-bis-
(diphenylphosphino)-4-cyclopentene-1,3-dione (bpcd) with the
clusters Fe3(CO)12 and Ru3(CO)12 were investigated. Whereas
complete cluster fragmentation and formation of the simple
mononuclear complex Fe(CO)3(bpcd) was found in the reaction
with the iron cluster,13 reaction of the ruthenium cluster
produced the bpcd-chelated cluster Ru3(CO)10(bpcd), which was
found to be unstable at ambient temperature, decomposing to
Ru3(CO)12 and the donor-acceptor complex Ru2(CO)6(bpcd).14
Emerging from this work was the unexpected ease by which
Ru2(CO)6(bpcd) furnished the phosphido compound Ru2(CO)6-
[µ-CdC(PPh2)C(O)CH2C(O)](µ2-PPh2) upon near-UV optical
excitation through a transient zwitterionic biradical intermediate.
Wishing to complete our diphosphine substitution studies with
all three group 8 trimetal clusters, we have explored the
reactivity of the bpcd ligand with Os3(CO)10(MeCN)2. Herein
we present our results on the reaction of the activated osmium
cluster with bpcd that initially gives the ligand-bridged cluster
1,2-Os3(CO)10(bpcd) (2b). The stepwise conversion of cluster
2b to the chelated cluster 1,1-Os3(CO)10(bpcd) (2c) and then to
the hydrido cluster HOs3(CO)9[µ-(PPh2)CdC{PPh(C6H4)}C(O)-
CH2C(O)] (3) and the benzyne cluster HOs3(CO)8(µ3-C6H4)-
[µ2,η1-PPhCdC(PPh2)C(O)CH2C(O)] (4) has been established,
with the unsaturated cluster 1,1-Os3(CO)9(bpcd) functioning as
the bifurcation point for clusters 3 and 4. All new clusters have
(2) (a) Akotsi, O. M.; Metera, K.; Reid, R. D.; McDonald, R.; Bergens,
S. H. Chirality 2000, 12, 514. (b) Noyori, R. AdV. Synth. Catal. 2003, 345,
15. (c) Cre´py, K. V. L.; Imamoto, T. AdV. Synth. Catal. 2003, 345, 79.
(3) Das, N.; Arif, A. M.; Stang, P. J.; Sieger, M.; Sarkar, B.; Kaim, W.;
Fiedler, J. Inorg. Chem. 2005, 44, 5798 and references therein.
(4) By the commonly adopted terminology, an activation of a phosphorus-
bound alkyl group by an intramolecular C-H bond oxidative addition is
referred to as a cyclometalation, while the analogous activation of a
phosphorus-bound aryl group at the ortho position relative to the phosphorus-
substituted carbon atom is termed an ortho metalation: Collman, J. P.;
Hegedus, L. S.; Norton, J. R.; Finke, R. G. Principles and Applications of
Organotransition Metal Chemistry; University Science Books: Mill Valley,
CA, 1987.
(5) (a) Dubois, R. A.; Garrou, P. E.; Lavin, K. D.; Allcock, H. R.
Organometallics 1984, 3, 649. (b) Abatjoglou, A. G.; Bryant, D. R.
Organometallics 1984, 3, 932. (c) Garrou, P. E. Chem. ReV. 1985, 85, 171.
(d) Garrou, P. E.; Dubois, R. A.; Jung, C. W. CHEMTECH 1985, 15, 123.
(e) Dubois, R. A.; Garrou, P. E. Organometallics 1986, 5, 466. (f) Hermann,
W. A.; Brossmer, C.; O¨ fele, K.; Beller, M.; Fischer, H. J. Mol. Catal. A
1995, 103, 133. (g) Goodson, F. E.; Wallow, T. I.; Novak, B. M. J. Am.
Chem. Soc. 1997, 119, 12441.
(6) (a) Gainsford, G. J.; Guss, J. M.; Ireland, P. R.; Mason, R.; Bradford,
C. W.; Nyholm, R. S. J. Organomet. Chem. 1972, 40, C70. (b) Bruce, M.
I.; Shaw, G.; Stone, F. G. A. J. Chem. Soc., Dalton Trans. 1972, 2094. (c)
Bradford, C. W.; Nyholm, R. S. J. Chem. Soc., Dalton Trans. 1973, 529.
(d) Deeming, A. J.; Kimber, R. E.; Underhill, M. J. Chem. Soc., Dalton
Trans. 1973, 2589. (e) Deeming, A. J.; Underhill, M. J. Chem. Soc., Dalton
Trans. 1973, 2727.
(7) For several, more recent reports on the activation of cluster-bound
phosphine (PR3) ligands, see: (a) Sakakura, T.; Kobayashi, T.; Hayashi,
T.; Kawabata, Y.; Tanaka, M.; Ogata, I. J. Organomet. Chem. 1984, 267,
171. (b) Deeming, A. J.; Smith, M. B. J. Chem. Soc., Dalton Trans. 1993,
3383. (c) Cullen, W. R.; Rettig, S. J.; Zheng, T. C. Organometallics 1993,
12, 688. (d) Cabeza, J. A.; Franco, R. J.; Llamazares, A.; Riera, V.; Pe´rez-
Carren˜o, E.; Van der Maelen, J. F. Organometallics 1994, 13, 55. (e) Chen,
G.; Deng, M.; Lee, C. K.; Leong, W. K. Organometallics 2002, 21, 1227.
(f) Sa´nchez-Cabrera, G.; Zuno-Cruz, F. J.; Rosales-Hoz, M. J.; Bakhmutov,
V. I. J. Organomet. Chem. 2002, 660, 153. (g) Diz, E. L.; Neels, A.;
Stoeckli-Evans, Su¨ss-Fink, G. Inorg. Chem. Commun. 2002, 5, 414. (h)
Adams, R. D.; Captain, B.; Fu, W.; Smith, M. D. J. Organomet. Chem.
2002, 651, 124.
(10) (a) Lugan, N.; Bonnet, J.-J.; Ibers, J. A. J. Am. Chem. Soc. 1985,
107, 4484. (b) Lugan, N.; Bonnet, J.-J.; Ibers, J. A. Organometallics 1988,
7, 1538.
(11) (a) Clucas, J. A.; Foster, D. F.; Harding, M. M.; Smith, A. K. J.
Chem. Soc., Chem. Commun. 1984, 949. (b) Clucas, J. A.; Harding, M.
M.; Smith, A. K. J. Chem. Soc., Chem. Commun. 1985, 1280.
(12) For some representative examples, see: (a) Brown, M. P.; Dolby,
P. A.; Harding, M. M.; Mathews, A. J.; Smith, A. K. J. Chem. Soc., Dalton
Trans. 1993, 1671. (b) Brown, M. P.; Dolby, P. A.; Harding, M. M.;
Mathews, A. J.; Smith, A. K.; Osella, D.; Arbrun, M.; Gobetto, R.; Raithby,
P. R.; Zanello, P. J. Chem. Soc., Dalton Trans. 1993, 827. (c) Azam, K.
A.; Hursthouse, M. B.; Islam, M. R.; Kabir, S. E.; Malik, K. M. A.; Miah,
R.; Sudbrake, C.; Vahrenkamp, H. J. Chem. Soc., Dalton Trans. 1998, 1097.
(d) Abedin, S. M. T.; Azam, K. A.; Hursthouse, M. B.; Kabir, S. E.; Malik,
K. M. A.; Mottalib, M. A.; Rosenberg, E. J. Cluster Sci. 2001, 12, 5. (e)
Kabir, S. E.; Miah, M. A.; Sarker, N. C.; Hossain, G. M. G.; Hardcastle,
K. I.; Rokhsana, D.; Rosenberg, E. J. Organomet. Chem. 2005, 690, 3044.
(f) Kabir, S. E.; Miah, M. A.; Sarker, N. C.; Hossain, G. M. G.; Hardcastle,
K. I.; Nordlander, E.; Rosenberg, E. Organometallics 2005, 24, 3315.
(13) Unpublished work.
(8) In comparison to the extensive number of solution and structural
reports that have helped to shape our current knowledge on the reaction
pathways favored in phosphine activation reactions at metal clusters, fewer
kinetic studies on such activations exist. Here the vast majority of reports
involve a rate-limiting dissociative CO loss as a prelude to phosphine
activation. See: (a) Yang, K.; Smith, J. M.; Bott, S. G.; Richmond, M. G.
Organometallics 1993, 12, 4779. (b) Bott, S. G.; Yang, K.; Talafuse, K.
A.; Richmond, M. G. Organometallics 2003, 22, 1383. (c) Bott, S. G.; Yang,
K.; Richmond, M. G. J. Organomet. Chem. 2005, 690, 3067.
(9) For cyclometalation reactivity at the CH2 moiety in dppm in selected
clusters, see: (a) Lavigne, G.; de Bonneval, B. In Catalysis by Di- and
Polynuclear Metal Cluster Compounds; Adams, R. D., Cotton, F. A., Eds.;
Wiley-VCH: New York, 1998; Chapter 2. (b) Lavigne, G. In The Chemistry
of Metal Cluster Complexes; Shriver, D. F., Kaesz, H. D., Adams, R. D.,
Eds.; VCH: New York, 1990; Chapter 5.
(14) Shen, H.; Bott, S. G.; Richmond, M. G. Organometallics 1995, 14,
4625.

932 Organometallics, Vol. 25, No. 4, 2006
Watson et al.
Scheme 1
OMNIC software. The 1H and 13C NMR spectra were recorded at
200 and 50 MHz, respectively, on a Varian Gemini 200 spectrom-
eter, and the 31P NMR spectra were recorded at 121 MHz on a
Varian 300-VXR spectrometer. The reported 31P chemical shifts
are referenced to external H3PO4 (85%), taken to have δ 0.0. Here
positive chemical shifts are to low field of the external standard.
All 13C and 31P NMR spectra were run in the proton-decoupled
mode. The high-resolution FAB mass spectra were obtained at the
Mass Spectrometry facility at the University of California at San
Diego using 3-nitrobenzyl alcohol as the sample matrix and
polypropylene glycol as a reference.
been isolated and fully characterized in solution; in addition,
X-ray analyses have been performed for the clusters 2b,c, and
4. The ortho-metalation reaction that gives 3 is shown to be
reversible and has been probed using deuterated bpcd isoto-
pomers. Kinetic and thermodynamic isotope data are presented
that support the existence of an intermediate aryl π complex in
the ortho-metalation reaction.
Experimental Section
General Methods. The Os3(CO)10(MeCN)2 used in our studies
was prepared from Os3(CO)12,15 which in turn was obtained from
the high-pressure carbonylation of OsO4.16 The bpcd ligand used
in these studies was synthesized from 4,5-dichloro-4-cyclopentene-
1,3-dione and Ph2PTMS according to the known procedure.17 PPh3-
Preparation of 1,2-Os3(CO)10(bpcd). To the activated cluster
Os3(CO)10(MeCN)2, prepared from 0.80 g (0.88 mmol) of Os3(CO)12
and 0.14 g (1.92 mmol) of Me3NO, was added 100 mL of benzene,
followed by 0.41 g (0.89 mmol) of bpcd. The solution was stirred
for 4.0 h at room temperature, after which time TLC analysis (Rf
) 0.77 in CH2Cl2) confirmed the presence of a green spot belonging
to the desired product. The bpcd-bridged cluster was subsequently
isolated by column chromatography over silica gel using CH2Cl2/
hexane (2:3) as the eluent and recrystallized from CH2Cl2/hexane
(1:1) to give spectroscopically pure 1,2-Os3(CO)10(bpcd) in 48%
yield (0.60 g). IR (CH2Cl2): ν(CO) 2091 (m), 2030 (m, sh), 2008
(vs, b), 1975 (m, b), 1958 (m, b), 1761 (w, sym dione carbonyl),
d15 was prepared from bromobenzene-d5 and PCl3 according to the
published procedure,18 and PPh3-d3,ortho was synthesized by the
method of Noyce et al.19 1,2-Bromochlorobenzene and Me3NO‚
2H2O were purchased from Aldrich Chemical Co., with the former
used as received and the latter used after the water was removed
by azeotropic distillation from benzene. The 13CO (>99%) em-
ployed in the enrichment of Os3(CO)12 and the C6D6 (>99.5%)
and D2O (>99.8%) used in the preparation of the isotopically
substituted bpcd ligands were all purchased from Isotec, Inc., and
used directly as received. All reaction and NMR solvents were
distilled under argon from a suitable drying agent and stored in
Schlenk storage vessels,20 and with the exception of chromato-
graphic separations, all reactions were conducted under argon,
employing standard Schlenk techniques. Routine photolyses were
conducted with GE blacklight bulbs, having a maximum output of
366 ( 20 nm and a photon flux of ca. 1 × 10-6 einstein/min, while
the low-temperature NMR tube photochemical experiments were
carried out with a 200 W Oriel Hg(Xe) arc lamp. The reported
quantum yield for the conversion 2c f 3 was determined by
ferrioxalate actinometry.21 All C and H analyses were performed
by Atlantic Microlab, Norcross, GA.
1
1724 (m, antisym dione carbonyl) cm-1. H NMR (CDCl3; 298
K): δ 7.25-7.70 (m, 20H, aryl), 3.06 (s, 2H, dione CH2). 13C NMR
(C7D8; 233 K): δ 193.07 (s, 4C, axial), 184.78 (s, 2C, axial), 178.17
(s, 2C, equatorial), 173.12 (s, 2C, equatorial). 31P NMR (CDCl3,
298 K): δ -17.55 (s).
Isomerization of 1,2-Os3(CO)10(bpcd) to 1,1-Os3(CO)10(bpcd).
In a large Schlenk tube was charged 0.56 g (0.43 mmol) of 1,2-
Os3(CO)10(bpcd) and 40 mL of benzene by syringe, after which
the solution was saturated with CO for several minutes in order to
suppress the formation of cluster 3 (vide infra). At this point the
vessel was sealed and heated for 20 h at 50-60 °C, at which time
the solution gradually changed from green to brown. TLC examina-
tion of the crude reaction solution showed that the chelating isomer
possessed the same Rf value as the bridging isomer in all solvents
explored, rendering this mode of analysis useless. The solvent was
next removed under vacuum and the crude product was purified
by chromatography, as described above, to give 0.42 g (74%) of
1,1-Os3(CO)10(bpcd) as a brown solid. Recrystallization from CH2-
Cl2/hexane furnished the analytical sample. IR (CH2Cl2): ν(CO)
2096 (m), 2047 (vs), 2010 (vs, b), 1992 (m), 1977 (m), 1964 (w,
b), 1925 (m, b), 1749 (w, sym dione carbonyl), 1717 (m, antisym
dione carbonyl) cm-1. 1H NMR (CDCl3; 298 K): δ 7.25-7.70 (m,
20H, aryl), 3.74 (s, 2H, dione CH2). 13C NMR (C7D8; 233 K): δ
198.59 (t, 2C, axial, JP-C ) 8 Hz), 185.03 (s, 4C, axial), 177.02 (s,
2C, equatorial), 171.02 (s, 2C, equatorial). 31P NMR (CDCl3, 298
The reported infrared data were recorded on a Nicolet 20 SXB
FT-IR spectrometer in 0.1 mm NaCl cells, using PC control and
(15) Nicholls, J. N.; Vargas, M. D. Inorg. Synth. 1989, 26, 289.
(16) Drake, S. R.; Loveday, P. A. Inorg. Synth. 1990, 28, 230.
(17) (a) Fenske, D.; Becher, H. Chem. Ber. 1974, 107, 117. (b) Fenske,
D. Chem. Ber. 1979, 112, 363.
(18) Bianco, V. D.; Doronzo, S. Inorg. Synth. 1976, 16, 164.
(19) Noyce, D. S.; Kittle, P. A.; Banitt, E. H. J. Org. Chem. 1968, 33,
1500.
(20) Shriver, D. F. The Manipulation of Air-SensitiVe Compounds;
McGraw-Hill: New York, 1969.
(21) (a) Calvert, J. G.; Pitts, J. N. Photochemistry; Wiley: New York,
1966. (b) Parker, C. A. Proc. R. Soc. London, Ser. A 1953, 220, 104. (c)
Hatchard, C. G.; Parker, C. A. Proc. R. Soc. London, Ser. A 1956, 235,
518.

The Triosmium Cluster Os3(CO)10(bpcd)
Organometallics, Vol. 25, No. 4, 2006 933
K): δ 17.97 (s). Anal. Calcd (found) for C39H22O12Os3P2: C, 35.62
(35.56); H, 1.69 (1.74).
2 h with warming to room temperature. The solvent, unreacted
TMSCl, and C6D5TMS were removed under vacuum, and the
residue containing (C6D5)2PTMS was dissolved in 5.0 mL of Et2O,
transferred to a clean Schlenk vessel, and cooled to 0 °C. To this
solution was added 0.33 g (2.00 mmol) of 4,5-dichloro-4-cyclo-
pentene-1,3-dione dissolved in 20 mL of Et2O and 3.0 mL of THF
dropwise over the course of 0.5 h by the means of a pressure-
equilibrated addition funnel, with stirring continued overnight. The
solvents were removed the following day, and the crude ligand was
isolated by column chromatography over silica gel using a 1:1
mixture of CH2Cl2 and hexanes to furnish 0.31 g (32% based on
dione consumed) of bpcd-d20 as a yellow solid. 1H NMR (CDCl3;
298 K): δ 2.92 (s, 2H, dione). 31P NMR (CDCl3, 298 K): δ -22.76
(s). FAB-MS (m/z): 484.2344 (calcd for C29H2D20O2P2 484.2345).
Preparation of HOs3(CO)9[µ-(PPh2)CdC{PPh(C6H4)}C(O)-
CH2C(O)]. To a Schlenk vessel was added Os3(CO)10(MeCN)2,
prepared from 0.25 g (0.28 mmol) of Os3(CO)12 and 60 mg (0.80
mmol) of Me3NO in MeCN, 0.13 g (0.28 mmol) of bpcd, and 50
mL of benzene. The reaction solution was stirred for 3 days at ca.
45-50 °C with periodic removal of the liberated CO by argon
purge. TLC inspection of the reaction solution showed two very
close moving spots in CH2Cl2 assigned to the hydride cluster HOs3-
(CO)9[µ-(PPh2)CdC{PPh(C6H4)}C(O)CH2C(O)] (Rf ) 0.80) and
the chelating cluster 2c (Rf ) 0.77). The presence of these two
clusters was also verified by IR and NMR analyses, the latter
method indicating a 7:1 ratio of 2c to 3. Both clusters were subjected
to an initial purification by column chromatography over silica gel
using a 2:3 mixture of CH2Cl2/hexane. The cluster mixture
comprised of 2c and 3 was dissolved in ca. 40 mL of CH2Cl2 and
irradiated between two parallel blacklight bulbs for 7 days at room
temperature. During this time the CO was periodically removed
by argon purge. The crude reaction mixture was filtered through
silica gel using CH2Cl2 to afford spectroscopically pure HOs3(CO)9-
[µ-(PPh2)CdC{PPh(C6H4)}C(O)CH2C(O)] as a brown solid in 23%
yield (82 mg). The analytical sample was obtained from the slow
evaporation of a benzene solution containing cluster 3. IR (CH2-
Cl2): ν(CO) 2084 (s), 2042 (s), 2019 (vs), 2003 (m), 1751 (w,
Preparation of 1,1-Os3(CO)10(bpcd-d20) and DOs3(CO)9[µ-
(PPh2-d10)CdC{P(Ph-d5)(C6D4)}C(O)CH2C(O)]. Since the overall
procedure employed for the synthesis of 1,1-Os3(CO)10(bpcd-d20)
and DOs3(CO)9[µ-(PPh2-d10)CdC{P(Ph-d5)(C6D4)}C(O)CH2C(O)]
is identical with that described for 1,1-Os3(CO)10(bpcd) and HOs3-
(CO)9[µ-(PPh2)CdC{PPh(C6H4)}C(O)CH2C(O)], only the high-
lights will be given. To Os3(CO)10(MeCN)2, prepared from 0.28 g
(0.31 mmol) of Os3(CO)12 and 53 mg (0.71 mmol) of Me3NO in
MeCN, in 50 mL of CH2Cl2 was added 0.15 g (0.31 mmol) of
bpcd-d20 in 15 mL of CH2Cl2 by cannula, after which the reaction
mixture was stirred for 6 h at 40 °C to produce a mixture of 1,2-
and 1,1-Os3(CO)10(bpcd-d20). Both clusters were isolated by chro-
matography, dissolved in CO-saturated CH2Cl2, and heated at ca.
45-50 °C for 3 days. At this point the solution contained the desired
cluster 1,1-Os3(CO)10(bpcd-d20), which was obtained in 58% yield
(0.24 g) after chromatographic separation from the trace amount
of unreacted bpcd-d20. The entire amount of 1,1-Os3(CO)10(bpcd-
d20) was next dissolved in 12 mL of a 1:2 mixture of toluene-d8
and benzene-d6 in a small Schlenk tube and then irradiated for 10
1
sym dione carbonyl), 1719 (m, antisym dione carbonyl) cm-1. H
NMR (CDCl3; 298 K): δ 6.90-8.30 (m, 19H, aryl), 3.54 (s, 2H,
1
dione CH2), -16.28 (t, 1H, JP-H ) 13 Hz). H NMR (C6D6; 298
K): δ 6.85-8.50 (m, 19H, aryl), 2.07 (AB quartet, 2H, dione CH2,
JH-H ) 22 Hz), -15.91 (t, 1H, JP-H ) 13 Hz). 13C NMR (C7D8;
223 K): δ 185.32-184.00 (m, 3C), 179.02 (s, 1C), 178.24 (d, 1C,
JP-C ) 9 Hz), 176.88 (s, broad, 1C), 176.09 (s, broad, 1C), 175.31
(s, 1C), 173.79 (s, 1C). 31P NMR (CDCl3, 298 K): δ 26.71 (d,
JP-P ) 15 Hz), 19.18 (d, JP-P ) 15 Hz). Anal. Calcd (found) for
C38H22O11Os3P2‚0.5C6H6: C, 37.13 (37.07); H, 1.90 (2.08).
1
days at room temperature with monitoring by H NMR spectros-
copy. The reaction was driven to completion by periodically
releasing the liberated CO by either freeze-pump-thaw degas
cycles or by argon purge. When all of the 1,1-Os3(CO)10(bpcd-
d20) had been consumed, the irradiation was terminated and the
solvents were removed under vacuum, affording 0.20 g (83%) of
DOs3(CO)9[µ-(PPh2-d10)CdC{P(Ph-d5)(C6D4)}C(O)CH2C(O)] after
chromatographic separation (note: there was no loss of deuterium
Preparation of HOs3(CO)8(µ3-C6H4)[µ2,η1-PPhCdC(PPh2)C-
(O)CH2C(O)]. A 0.16 g (0.12 mmol) portion of 1,1-Os3(CO)10-
(bpcd) and 0.70 mL of toluene-d8 were charged into a screw-capped
NMR tube and heated at 110 °C overnight, with monitoring by 1H
NMR spectroscopy. NMR analysis showed the complete consump-
tion of the cluster 2c and formation of the hydride cluster 3 and
the desired benzyne cluster 4. The solvent was stripped under
vacuum, and the crude residue was purified by column chroma-
tography using a 1:1 mixture of CH2Cl2 and hexane. Cluster 4 was
isolated as a brown solid in 40% yield (62 mg). The combustion
sample of 4 was obtained by slow evaporation of a benzene solution
containing 4. IR (CH2Cl2): ν(CO) 2077 (s), 2042 (vs), 2030 (m),
1999 (m), 1990 (m), 1977 (m), 1748 (w, sym dione carbonyl), 1715
1
after chromatography over silica gel, on the basis of the H NMR
data). 1,1-Os3(CO)10(bpcd-d20) data are as follows. IR (CH2Cl2):
ν(CO) 2096 (m), 2047 (vs), 2011 (vs, b), 1992 (m), 1977 (m), 1963
(w, b), 1926 (m, b), 1749 (w, sym dione carbonyl), 1717 (m,
1
antisym dione carbonyl) cm-1. H NMR (CDCl3; 298 K): δ 3.73
(s, 2H, dione). 31P NMR (CDCl3, 298 K): δ 17.03 (s). DOs3(CO)9-
[µ-(PPh2-d10)CdC{P(Ph-d5)(C6D4)}C(O)CH2C(O)] data are as fol-
1
lows. H NMR (CDCl3; 298 K): δ 3.55 (s, 2H, dione). 31P NMR
1
(m, antisym dione carbonyl) cm-1. H NMR (CDCl3; 298 K): δ
(CDCl3, 298 K): δ 26.21 (d, JP-P ) 14 Hz), 18.92 (d, JP-P ) 14
Hz).
6.50-9.20 (m, 19H, aryl and benzyne), 3.66 (AB quartet, 2H, dione
CH2, JH-H ) 22 Hz), -16.68 (dd, 1H, JP-H ) 14, 7 Hz). 13C NMR
(C7D8; 223 K): δ 184.41 (d, 1C, JP-C ) 7 Hz), 182.42 (s, 1C),
180.86 (d, 1C, JP-C ) 74 Hz), 179.98 (d, 1C, JP-C ) 74 Hz), 175.64
(s, broad, 1C), 175.10 (d, 1C, JP-C ) 7 Hz), 174.70 (s, 1C), 171.44
Preparation of bpcd-d4,ortho. To a THF solution (150 mL)
22
containing 1.40 g (5.30 mmol) of PPh3-d3,ortho was added 0.62 g
(16.0 mmol) of freshly cut potassium. The reaction mixture was
stirred overnight at room temperature and then filtered under argon
to remove the excess potassium. The filtered solution of [P(Ph-
(s, 1C). 31P NMR (CDCl3, 298 K): δ 10.96 (d, phosphine, JP-P
)
12 Hz), -72.18 (d, phosphido, JP-P ) 12 Hz). Anal. Calcd (found)
for C37H22O10Os3P2‚0.5C6H6: C, 37.01 (36.58); H, 1.94 (1.90).
dortho)2][K] and [Ph-dortho][K] was cooled to 0 °C with an ice/water
bath and treated with 3.5 mL (27.0 mmol) of trimethylsilyl chloride
(TMSCl) dropwise via syringe, followed by warming of the reaction
solution to room temperature and continued stirring for an additional
2 h. All volatiles were removed under vacuum, and the crude (Ph-
Preparation of bpcd-d20. To 1.30 g (4.70 mmol) of P(C6D5)3
in a Schlenk flask was added 150 mL of THF via cannula, followed
by 0.55 g (14.1 mmol) of freshly cut potassium. The reaction
mixture was stirred overnight at room temperature and filtered
through a sintered-glass funnel under argon to remove the unreacted
potassium. The filtrate containing the [P(C6D5)2][K] and [C6D5]-
[K] was next cooled to 0 °C with an ice/water bath and treated
with 2.5 mL (18.8 mmol) of trimethylsilyl chloride (TMSCl)
dropwise via syringe to furnish (C6D5)2PTMS and C6D5TMS. The
reaction mixture was stirred for 1 h at 0 °C and for an additional
dortho)2PTMS was dissolved in 5.0 mL of Et2O and cooled to 0 °C.
To this ether solution was added 0.26 g (1.60 mmol) of 4,5-dichloro-
(22) The purity of the PPh3-d3,ortho employed in the synthesis of bpcd-
d4,ortho was ascertained by high-resolution FAB mass spectrometry. Here
the PPh3-d3,ortho sample exhibited an m/z value of 265.1099 (calcd for
C18H12D3P 265.1094).

934 Organometallics, Vol. 25, No. 4, 2006
Watson et al.
4-cyclopentene-1,3-dione dissolved in 20 mL of Et2O and 3.0 mL
of THF, as described above for the synthesis of bpcd-d20. The
solution was stirred overnight with warming and the bpcd-d4,ortho
was isolated by column chromatography over silica gel using a
1:1 mixture of CH2Cl2 and hexanes as the eluent to give 0.16 g
(22%) of bpcd-d4,ortho as a yellow solid. 1H NMR (CDCl3; 298 K):
δ 7.15-7.45 (m, 16 H, aryl), 2.93 (s, 2H, dione). 31P NMR (CDCl3,
298 K): δ -22.48 (s). FAB-MS (m/z): 468.1341 (calcd for
C29H2D20O2P2 468.1341).
SHELXTL program package.25 The molecular structure was
checked by using PLATON,26 and all non-hydrogen atoms were
refined anisotropically, with the hydrogen atoms assigned calculated
positions and allowed to ride on the attached heavy atom. The
refinement for 2b converged at R ) 0.0477 and Rw ) 0.0782 for
10 128 independent reflections with I > 2σ(I) and for 2c at R )
0.0249 and Rw ) 0.0582 for 5120 independent reflections with I
> 2σ(I). The benzyne-substituted cluster 4 afforded convergence
values of R ) 0.0780 and Rw ) 0.1643 for 9724 independent
reflections with I > 2σ(I).
Preparation of 1,1-Os3(CO)10(bpcd-d4ortho). To Os3(CO)10-
(MeCN)2, prepared in situ from 0.25 g (0.28 mmol) of Os3(CO)12
and 53 mg (0.71 mmol) of Me3NO in MeCN, in 50 mL of CH2Cl2
was added 0.16 g (0.34 mmol) of bpcd-d4,ortho in 15 mL of CH2-
Cl2, after which time the reaction mixture was stirred for 6 h at 40
°C. The mixture of 1,2- and 1,1-Os3(CO)10(bpcd-d4,ortho) was then
dissolved in 40 mL of CO-saturated CH2Cl2 and heated in a closed
Schlenk vessel for 2 days at ca. 55 °C. The desired chelating isomer
of 1,1-Os3(CO)10(bpcd-d4,ortho) was isolated by chromatography to
furnish 0.22 g (48% yield based on bpcd-d4,ortho consumed) of 1,1-
Os3(CO)10(bpcd-d4,ortho) as a brown solid. IR (CH2Cl2): ν(CO) 2096
(m), 2047 (vs), 2010 (vs, b), 1992 (m), 1977 (m), 1964 (w, b),
1926 (m, b), 1749 (w, sym dione carbonyl), 1717 (m, antisym dione
Kinetic Studies. All UV-vis studies were carried out in the
specified solvent at a cluster concentration of ca. 10-4 M using 1.0
cm quartz UV-visible cells that were equipped with a high-vacuum
Teflon stopcock to facilitate handling on the vacuum line and
glovebox transfers. Stock solutions of all clusters, when not in use,
were stored in the dark under argon or CO. The Hewlett-Packard
8452A diode array spectrometer employed in our studies was
configured with a variable-temperature cell holder and was con-
nected to a VWR constant-temperature circulator, which regulated
1
the reaction temperature to within (0.5 K. The H NMR kinetic
studies were conducted in 5 mm NMR tubes that possessed a J.
Young valve for easy evacuation or the admission of atmospheric
pressure of argon or CO gas or in NMR tubes configured for
attachment to the vacuum line, which after three freeze-pump-
thaw degassing cycles were flame-sealed. All NMR studies
employed either benzene-d6 or toluene-d8 as a solvent with a cluster
concentration of ca. 10-2 M. The NMR samples were heated in
the same VWR temperature bath and quenched in an external ice
bath immediately before NMR analysis.
1
carbonyl) cm-1. H NMR (CDCl3; 298 K): δ 7.35-7.70 (m, 16
H, aryl), 3.75 (s, 2H, dione). 31P NMR (CDCl3, 298 K): δ 17.32.
Determination of the KIE for C-H(D) Oxidative Coupling
and the EIE in 1,1-Os3(CO)10(bpcd-d4,ortho). (a) KIE Experiment.
A 0.020 g (0.015 mmol) portion of 1,1-Os3(CO)10(bpcd-d4,ortho) in
0.50 mL of toluene-d8 was added to a 5 mm NMR tube equipped
with a J. Young value. The solution was freeze-pump-thaw
degassed three times and then cooled to 0 °C in a quartz Dewar,
after which the sample was irradiated with a 200 W Oriel Hg(Xe)
arc lamp until ca. 5% of the starting cluster remained. The amount
of hydride, HOs3(CO)9[µ-(PPh2-d2,ortho)CdC{P(Ph-dortho)(C6H3D)}-
C(O)CH2C(O)], versus the amount of deuteride, DOs3(CO)9[µ-
(PPh2-d2,ortho)CdC{P(Ph-dortho)(C6H4)}C(O)CH2C(O)], was then
determined by measuring the area under the hydride resonance and
the methylene group associated with the bpcd ring of both
isotopomers.23 Under these conditions the amount of hydride to
deuteride cluster was found to be 55:45.
The UV-vis kinetics for the isomerization 2b f 2c were
monitored by following the increase of the 364 nm absorbance band
as a function of time typically for 3-4 half-lives, while the extent
of the reaction involving cluster 3 in the presence of phosphine
traps and the reaction leading to the benzyne-substituted cluster 4
were determined by following the absorbance decrease in the 396
nm band of 3 for at least 3 half-lives. The rate constants quoted
for these reactions were determined by nonlinear regression analysis
using the single-exponential function27
A(t) ) A∞ + ∆Ae-kt
(b) EIE Experiment. These experiments were set up analo-
gously, using either benzene-d6 or toluene-d8 as the solvent, and
the samples were irradiated with two GE blacklights at an ambient
temperature of ca. 40 °C. Under these conditions the amount of
hydride to deuteride cluster was found to be 67:33.
The quoted activation parameters were calculated from plots of
ln(k/T) versus T-1 28
, with the error limits representing deviation of
the data points about the least-squares line of the Eyring plot.
X-ray Structural Determinations. Single crystals of the cluster
1,2-Os3(CO)10(bpcd) (2b), as the m-xylene solvate, were grown by
slow diffusion of m-xylene into a CH2Cl2 solution containing 2b
at 5 °C. The X-ray-quality crystals of 1,1-Os3(CO)10(bpcd) (2c)
were obtained from a CH2Cl2 solution containing the cluster that
had been layered with hexane, while single crystals of HOs3(CO)8-
(µ3-C6H4)[µ2,η1-PPhCdC(PPh2)C(O)CH2C(O)] (4), as the toluene
solvate, were grown from toluene and hexane. The tiny satellite
crystals attached to 4 were too small to remove and consequently
contributed to some of the observed reflections, lowering slightly
the final agreement factor. All X-ray data were collected on a Bruker
SMART 1000 CCD-based diffractometer at 300(2) K for cluster
2c and 213(2) K for clusters 2b and 4. The frames were integrated
with the available SAINT software package using a narrow-frame
algorithm,24 and the structure was solved and refined using the
Results and Discussion
Syntheses, X-ray Diffraction Structures of 1,2-Os3(CO)10-
(bpcd) and 1,1-Os3(CO)10(bpcd), and Isomerization Kinetics.
Exploratory reactions between Os3(CO)12 and bpcd were carried
out in refluxing toluene. A complicated mixture composed of
at least four distinct bpcd-derived products was confirmed when
the reaction was monitored by 1H NMR spectroscopy. The low
selectivity and complicated nature of the reactions are not
surprising and can be attributed to the severe conditions
necessary to effect the activation of the parent cluster.29
cleaner reaction was achieved when the labile cluster Os3(CO)10-
A
(25) SHELXTL, Version 5.1; Bruker Analytical X-ray Systems, Inc.,
Madison, WI, 1998.
(26) Spek, A. L. PLATON-A Multipurpose Crystallographic Tool;
Utrecht University, Utrecht, The Netherlands, 2001.
(27) All rate calculations were performed with the aid of the commercially
available programs Origin6.0 and KaleidaGraph. Here the initial (A0) and
final (A∞) absorbances and the rate constant were floated to give the quoted
least-squares value for the first-order rate constant k.
1
(23) A H spin-lattice (T1) relaxation study on cluster 3 in benzene-d6
was conducted, and the T1 values for the hydride (1.9 s) and the methylene
(0.3 s) groups were determined by using the standard inversion-recovery
pulse sequence. Accurate integration intensities, free from saturation effects,
were thus obtained for our KIE and EIE experiments with Os3(CO)10(bpcd-
d
4,ortho) by employing an acquisition delay of 10 s (>5T1).
(28) Carpenter, B. K. Determination of Organic Reaction Mechanisms;
Wiley-Interscience: New York, 1984.
(24) SAINT, Version 6.02; Bruker Analytical X-ray Systems, Inc.,
Madison, WI, 1997-1999.
(29) Deeming, A. J. AdV. Organomet. Chem. 1986, 26, 1.

The Triosmium Cluster Os3(CO)10(bpcd)
Organometallics, Vol. 25, No. 4, 2006 935
Figure 1. Thermal ellipsoid plots of 1,2-Os3(CO)10(bpcd) (2b, left) and 1,1-Os3(CO)10(bpcd) (2c, right) showing thermal ellipsoids at the
50% probability level.
Table 1. X-ray Crystallographic Data and Processing Parameters for the Triosmium Clusters 2b,c and 4
2b
2c
4
CCDC entry no.
277346
277347
277348
space group
a, Å
b, Å
c, Å
monoclinic, P21/c
14.031(4)
30.89(1)
monoclinic, P21/n
11.028(1)
19.913(3)
18.382(3)
100.107(2)
3915(2)
C39H22Os3O12P2
1315.11
4
monoclinic, P21/c
22.350(6)
11.511(3)
17.383(5)
111.281(5)
4167(2)
C44H29Os3O10P2
1350.21
4
11.881(4)
115.05(2)
4665(3)
â, deg
V, Å3
a
mol formula
fw
C39H22Os3O12P2
1411.19
4
2.009
formula units per cell (Z)
D
calcd (Mg/m3)
2.231
2.152
λ(Mo KR), Å
abs coeff (mm-1
Rmerge
0.710 73
8.279
0.0998
0.710 73
9.857
0.0596
0.710 73
9.260
0.2122
)
abs cor factor
total no. of rflns
0.9751-0.4934
37 461
0.7019-0.1409
22 472
1.000-0.515
34 779
no. of indep rflns
10 128
5120
9724
no. of data/restraints/params
R
10 128/0/537
0.0477
5120/0/506
0.0249
9724/0/534
0.0780
Rw
0.0782
0.923
0.0582
0.966
0.1643
0.993
GOF on F2
weights
[0.04F2 + (σF)2]-1
1.779, -0.891
[0.04F2 + (σF)2]-1
1.298, -1.127
[0.04F2 + (σF)2]-1
7.498, -2.449
largest diff in peak and hole (e/Å3)
a Disordered m-xylene is also present.
(MeCN)2 (1) was employed as the starting osmium cluster,
allowing us to establish a definitive course of reaction in a
methodical fashion.
lography, as seen in the thermal ellipsoid plot displayed in Figure
1. Tables 1 and 2 summarize the pertinent X-ray data. The Os-
Os bond distances range from 2.8535(8) Å (Os(2)-Os(3)) to
2.8633(8) Å (Os(1)-Os(3)) and exhibit an average distance of
2.857 Å, consistent with those distances in other structurally
characterized phosphine-substituted Os3 derivatives.32 The mean
length of 2.320 Å for the Os-P bonds and the D3 twisting of
the axial carbonyl groups in 2b are typical for this genre of
cluster.33,34 The bridging of the Os(1) and Os(2) centers by the
bpcd ligand leads to ground-state destabilization in 2b that
Treatment of an equimolar mixture of 1 and bpcd in benzene
gives rise to a rapid reaction and formation of the bpcd-
substituted cluster Os3(CO)10(bpcd), as assessed by TLC and
IR analyses that confirmed the consumption of cluster 1 but
not the coordination mode adopted by the bpcd ligand. The
bridging of adjacent osmium centers by the bpcd ligand was
established by 31P NMR and X-ray diffraction analysis. The
31P NMR spectrum of 1,2-Os3(CO)10(bpcd) (2b) exhibits a high-
field singlet at δ -17.55, in agreement with signals for the
related derivative 1,2-Os3(CO)10[(Z)-Ph2PCHdCHPPh2]30 and
other triosmium clusters containing saturated diphosphine
ligands.31 The bridging nature of the bpcd ligand and the overall
molecular structure of 2b were established by X-ray crystal-
(31) (a) Deeming, A. J.; Donovan-Mtunzi, S.; Kabir, S. E. J. Organomet.
Chem. 1987, 333, 253. (b) Deeming, A. J.; Donovan-Mtunzi, S.; Hardcastle,
K. I.; Kabir, S. E.; Henrick, K.; McPartlin, M. J. Chem. Soc., Dalton Trans.
1988, 579.
(32) (a) Hansen, V. M.; Ma, A. K.; Biradha, K.; Pomeroy, R.; Zaworotko,
M. J. Organometallics 1998, 17, 5267. (b) Churchill, M. R.; DeBoer, B.
G. Inorg. Chem. 1977, 16, 878. (c) Constable, E. C.; Johnson, B. F. G.;
Khan, F. K.; Lewis, J.; Raithby, P. R.; Mikulcik, P. J. Organomet. Chem.
1991, 403, 15. (d) Biradha, K.; Hansen, V. M.; Leong, W. K.; Pomeroy, R.
K.; Zaworotko, M. J. J. Cluster Sci. 2000, 11, 285.
(30) Watson, W. H.; Wu, G.; Richmond, M. G. Organometallics 2005,
24, 5431.

936 Organometallics, Vol. 25, No. 4, 2006
Watson et al.
Table 2. Selected Bond Distances (Å) and Angles (deg) in
the Triosmium Clusters 2b,c and 4a
chelating isomer 2c (vide infra). The P(1)-C(2)-C(3) and
P(2)-C(3)-C(2) bond angles of 131.3(7) and 130.8(7)° as-
sociated with the CdC π bond of the bpcd moiety are ca. 10°
larger than the analogous linkages in (Z)-Ph2PCHdCHPPh2.35
Heating cluster 2b at ambient temperature (<70 °C) leads to
bpcd isomerization and formation of the chelated cluster 1,1-
Os3(CO)10(bpcd) (2c), as shown in eq 1. The extent of the
Cluster 2b
Bond Distances
Os(1)-Os(2)
Os(2)-Os(3)
Os(2)-P(2)
C(1)-C(2)
C(3)-C(4)
C(2)-C(3)
2.855(2)
2.8535(8)
2.322(2)
1.52(1)
1.51(1)
1.36(1)
Os(1)-Os(3)
Os(1)-P(1)
P(1)‚‚‚P(2)
C(1)-C(5)
C(4)-C(5)
2.8633(8)
2.318(2)
3.797(7)
1.54(1)
1.49(1)
Bond Angles
P(1)-Os(1)-Os(2)
P(2)-Os(2)-Os(1)
C(2)-P(1)-Os(1)
C(3)-C(2)-P(1)
C(31)-Os(1)-Os(2)
C(31)-Os(1)-Os(3)
C(35)-Os(2)-Os(3)
C(35)-Os(2)-Os(1)
C(39)-Os(3)-Os(2)
C(39)-Os(3)-Os(1)
103.09(6)
97.13(6)
118.6(3)
131.3(7)
93.5(3)
77.5(3)
76.7(3)
94.9(3)
98.0(3)
77.0(3)
P(1)-Os(1)-Os(3)
P(2)-Os(2)-Os(3)
C(3)-P(2)-Os(2)
C(2)-C(3)-P(2)
C(32)-Os(1)-Os(2)
C(32)-Os(1)-Os(3)
C(33)-Os(2)-Os(3)
C(33)-Os(2)-Os(1)
C(36)-Os(3)-Os(2)
C(36)-Os(3)-Os(1)
159.62(6)
154.38(6)
114.4(3)
130.8(7)
81.3(3)
96.1(3)
97.7(3)
79.3(3)
76.2(3)
reaction was easily assessed by 1H and 31P NMR spectroscopy
due to the downfield shift of the methylene singlet belonging
to the dione ring and the phosphorus groups of the product.
Here the initial methylene resonance (in CDCl3) at δ 3.06 in
cluster 2b is replaced by a new signal in the 1H NMR spectrum
at δ 3.74 for 2c, with the associated phenyl resonances remaining
invariant in terms of their general chemical shift information.
The 31P NMR spectrum is accompanied by a similar transfor-
mation insomuch as a new 31P resonance at δ 17.97 is observed,
whose large nuclear deshielding is consistent with a chelating
bpcd ligand.36 The isomeric clusters display the same Rf value
on silica gel, and the IR data are virtually indistinguishable given
their idealized C2V symmetry, negating the use of TLC analysis
and IR spectroscopy as probes for the study of this reaction.
The molecular structure of cluster 2c was ascertained by X-ray
crystallography. Figure 1 confirms the chelation of the bpcd
ligand to the Os(1) atom in 2c. The mean Os-Os and Os-P
bond lengths of 2.9074 and 2.299 Å, respectively, are unre-
markable with respect to those in 2b and other phosphine-
substituted osmium clusters, while the 10 ancillary carbonyls
are all linear in nature. The axial CO groups exhibit only a slight
twist or canting relative to the corresponding axial CO groups
in cluster 2b. The nonbonding P(1)‚‚‚P(2) bond distance of
3.169(4) Å in 2c is ca. 0.63 Å shorter than the internuclear
P(1)‚‚‚P(2) separation in 2b and is consistent with the inter-
nuclear P‚‚‚P distance displayed by mono- and polynuclear
compounds containing a chelating bpcd ligand.37 The experi-
mentally determined angles of 105.8(2), 106.0(2), 120.9(5), and
118.7(5)° found for the Os(1)-P(1)-C(1), Os(1)-P(2)-C(2),
P(1)-C(1)-C(2), and P(2)-C(2)-C(1) linkages, respectively,
show minimal deviation from the idealized bond angles of 109°
for phosphorus and 120° for carbon atoms and reinforce the
fact that cluster 2c is thermodynamically more stable than 2b.
The kinetics for the conversion of cluster 2b to cluster 2c
were next studied because of the small but growing number of
reports on nondissociative phosphine isomerization reactions at
97.2(3)
Cluster 2c
Bond Distances
Os(1)-Os(2)
Os(2)-Os(3)
Os(1)-P(2)
C(1)-C(2)
C(2)-C(3)
C(4)-C(5)
2.9143(5)
Os(1)-Os(3)
Os(1)-P(1)
P(1)‚‚‚P(2)
C(1)-C(5)
C(3)-C(4)
2.9092(5)
2.295(2)
3.169(4)
1.519(9)
1.509(9)
2.8988(6)
2.303(2)
1.340(8)
1.516(9)
1.503(9)
Bond Angles
P(1)-Os(1)-Os(3)
P(2)-Os(1)-Os(2)
C(2)-P(1)-Os(1)
C(2)-C(1)-P(2)
P(1)-Os(1)-P(2)
C(31)-Os(1)-Os(3)
C(31)-Os(1)-Os(2)
C(35)-Os(2)-Os(3)
C(35)-Os(2)-Os(1)
C(37)-Os(3)-Os(2)
C(37)-Os(3)-Os(1)
108.68(4)
104.32(4)
105.8(2)
118.7(5)
87.15(6)
88.9(2)
93.3(2)
88.5(2)
88.5(2)
90.5(2)
P(1)-Os(1)-Os(2)
P(2)-Os(1)-Os(3)
C(1)-P(2)-Os(1)
C(1)-C(2)-P(1)
C(30)-Os(1)-Os(3)
C(30)-Os(1)-Os(2)
C(32)-Os(2)-Os(3)
C(32)-Os(2)-Os(1)
C(39)-Os(3)-Os(2)
C(39)-Os(3)-Os(1)
167.82(4)
163.97(4)
106.0(2)
120.9(5)
88.1(2)
87.6(2)
90.6(2)
86.5(2)
87.0(2)
87.8(2)
89.2(2)
Cluster 4
Bond Distances
2.918(1)
Os(1)-Os(2)
Os(1)‚‚‚Os(3)
Os(1)-P(2)
P(1)‚‚‚P(2)
Os(2)-Os(3)
Os(1)-P(1)
Os(3)-P(2)
Os(1)-C(24)
Os(2)-C(25)
C(1)-C(5)
2.818(1)
2.350(3)
2.374(3)
2.17(1)
2.33(2)
1.38(2)
1.52(2)
1.52(2)
1.44(2)
1.32(2)
1.40(2)
3.809(1)
2.373(4)
3.238(4)
2.31(1)
2.16(1)
1.50(2)
1.49(2)
1.39(2)
1.47(2)
1.40(2)
Os(2)-C(24)
Os(3)-C(25)
C(1)-C(2)
C(2)-C(3)
C(3)-C(4)
C(4)-C(5)
C(24)-C(25)
C(25)-C(26)
C(27)-C(29)
C(24)-C(29)
C(26)-C(27)
C(28)-C(29)
Bond Angles
Os(3)-Os(2)-Os(1)
Os(1)-P(2)-Os(3)
C(1)-P(2)-Os(3)
C(1)-C(5)-P(1)
Os(1)-C(24)-Os(2)
C(25)-C(24)-Os(1)
83.18(2)
P(1)-Os(1)-P(2)
C(5)-P(1)-Os(1)
C(5)-C(1)-P(2)
C(24)-Os(2)-C(25)
Os(3)-C(25)-Os(2)
C(24)-C(25)-Os(3)
86.6(2)
106.5(4)
121(1)
34.9(5)
77.6(4)
125(1)
106.7(1)
114.7(4)
120(1)
81.2(4)
123(1)
(34) (a) Alex, R. F.; Einstein, F. W. B.; Jones, R. H.; Pomeroy, R. K.
Inorg. Chem. 1987, 26, 3175. (b) Bruce, M. I.; Liddell, M. J.; Hughes, C.
A.; Skelton, B. H.; White, A. H. J. Organomet. Chem. 1988, 347, 157. (c)
Bruce, M. I.; Liddell, M. J.; Hughes, C. A.; Patrick, J. M.; Skelton, B. H.;
White, A. H. J. Organomet. Chem. 1988, 347, 181. (d) Bruce, M. I.; Liddell,
M. J.; Skawkataly, O. B.; Hughes, C. A.; Skelton, B. H.; White, A. H. J.
Organomet. Chem. 1988, 347, 207.
a Numbers in parentheses are estimated standard deviations in the least
significant digits.
(35) Berners, S. J.; Colquhoun, L. A.; Healy, P. C.; Byriel, K. A.; Hanna,
J. V. J. Chem. Soc., Dalton Trans. 1992, 3357.
(36) (a) Garrou, P. E. Chem. ReV. 1981, 81, 229. (b) Richmond, M. G.;
Kochi, J. K. Inorg. Chem. 1987, 6, 254.
(37) (a) Shen, H.; Williams, T. J.; Bott, S. G.; Richmond, M. G. J.
Organomet. Chem. 1995, 505, 1. (b) Bott, S. G.; Shen, H.; Richmond, M.
G. Struct. Chem. 2001, 12, 225. (c) Shen, H.; Bott, S. G.; Richmond, M.
G. Inorg. Chim. Acta 1996, 241, 71.
manifests itself in an internuclear P(1)‚‚‚P(2) distance of 3.797-
(7) Å, which is ca. 0.60 Å longer than the corresponding distance
reported for the free ligand (Z)-Ph2PCHdCHPPh2 and the
(33) Orpen, A. G.; Brammer, L.; Allen, F. K.; Kennard, O.; Watson, D.
G.; Taylor, R. J. Chem. Soc., Dalton Trans. 1989, S1.

The Triosmium Cluster Os3(CO)10(bpcd)
Organometallics, Vol. 25, No. 4, 2006 937
Figure 2. UV-vis spectral changes for 2b f 2c recorded at 323 K in benzene (left) and the absorbance versus time curve for the experimental
data (9) and the least-squares fit (s) of the first-order rate constant k (right).
Table 3. Experimental Rate Constants for the Isomerization
of 1,2-Os3(CO)10(bpcd) (2b) to 1,1-Os3(CO)10(bpcd) (2c)a
companied by the growth of a methylene singlet at δ 2.45
belonging to cluster 2c. The NMR data conclusively showed
that the isomerization furnished 2c in quantitative yield, unlike
the isomerization found for the related cluster Os3(CO)10[(Z)-
Ph2PCHdCHPPh2], where a reversible equilibrium that favored
the chelating isomer was found (Keq ) 6.9 for chelate/bridge
clusters over the temperature range of 358-382 K).30 Treatment
of the NMR data by traditional ln[2b] versus time plots or by
nonlinear regression analysis afforded first-order rate constants
that were in excellent agreement with each other and with those
data obtained from the UV-vis studies. Moreover, plots of the
total cluster concentration (2b and 2c) versus time displayed
smooth exponential decay and growth curves for the consump-
tion and formation of 2b and 2c, respectively. Inspection of
the data in Table 3 reveals that the isomerization is unaffected
by added phosphine, phosphite, and CO trapping ligands. The
invariant rates in the presence of added ligands and the Eyring
activation parameters (UV-vis, ∆Hq ) 25.0(0.7) kcal/mol and
∆Sq ) -2(2) eu; NMR, ∆Hq ) 23.9(0.4) kcal/mol and ∆Sq )
-4(1) eu) strongly support a nondissociative, rate-limiting
unimolecular rearrangement of the bpcd ligand.
These kinetic data undeniably support an intramolecular
migration or transit of one of the phosphorus atoms across an
Os-Os bond to afford a diphosphine-chelated osmium center,
as opposed to a process involving the dissociative release of
one of the PPh2 moieties of the bpcd ligand. This latter scenario
would lead to the formation of an unsaturated cluster with an
η1-bpcd ligand that could be scavenged by any trapping ligand
present; however, the expected substituted clusters Os3(CO)10L-
(η1-bpcd) (L ) CO, PR3, P(OEt)3) have not been observed.39,40
Given the clean conversion of 2b to 2c, the two most likely
isomerization processes that are consistent with the kinetic data
and activation parameters are shown in Scheme 2. A pairwise
exchange of a CO ligand with an adjacent phosphorus atom,
through the intermediacy of the doubly bridged cluster Os3-
(CO)9(µ-CO)(µ-bpcd), gives rise to cluster 2c. Alternatively,
temp (K)
104k (s-1
)
trapping ligand
method
323.0
323.0
328.0
328.0
328.0
333.0
333.0
333.0
338.0
338.0
338.0
338.0
338.0
338.0
343.0
343.0
0.58 ( 0.01
0.51 ( 0.01
0.78 ( 0.03
1.02 ( 0.07
0.87 ( 0.01
1.91 ( 0.06
1.74 ( 0.01
1.60 ( 0.02
3.04 ( 0.23
2.81 ( 0.05
2.82 ( 0.05
3.18 ( 0.08
2.67 ( 0.01
2.87 ( 0.06
5.82 ( 0.09
4.62 ( 0.06
UV-vis
NMR
UV-vis
UV-visb
NMR
UV-vis
UV-vis
NMR
UV-vis
NMR
UV-vis
UV-vis
NMR
6.8 atm of CO
25 equiv of PPh3
25 equiv of PPh3
1.0 atm of CO
10 equiv of PPh3
10 equiv of P(OEt)3
NMR
UV-vis
NMR
a The UV-vis kinetic data were collected in benzene using ca. 10-4
M
1,2-Os3(CO)10(bpcd) by following the increase in the absorbance of the
368 nm band, and the NMR kinetics were conducted in benzene-d6 with
1,2-Os3(CO)10(bpcd) at an initial concentration of ca. 10-2 M in the presence
of an internal standard (either p-dimethoxybenzene or tert-butylbenzene).
The extent of the isomerization was followed by monitoring the decrease
in intensity of the methylene singlet of the bridging isomer at δ 1.78.
b Experiment conducted in a Fischer-Porter tube under constant CO
pressure, where aliquots were removed and analyzed by UV-vis spectros-
copy.
di- and polynuclear compounds.8a,c,30,38 The isomerization rates
for 2b f 2c were conveniently measured by both UV-vis and
1H NMR spectroscopy over the temperature range of 323-343
K, with the reaction rates being reported in Table 3. The UV-
vis-derived rates were obtained by following the increase in
the absorbance of the 364 nm band belonging to cluster 2c.
Figure 2 shows the UV-vis spectral changes for the thermolysis
of 2b in benzene solution at 323 K, where multiple, well-defined
isosbestic points accompany the reaction. The fit of the least-
squares regression curve and the absorbance data displayed in
Figure 2 underscore the fact that the isomerization is well-
behaved and is free from kinetic complications. Complimentary
1H NMR kinetic studies were also conducted in benzene-d6 by
monitoring the decrease in the methylene moiety of 2b at δ
1.78. The complete consumption of this resonance was ac-
(39) This assertion concerning the formation of an unsaturated intermedi-
ate as a steady-state species is valid, provided that the rate of ring closure
(to either the bridged or chelated cluster) is slow relative to the rate of the
ligand trapping reaction. For some related examples, see: (a) Glueck, D.
S.; Bergman, R. G. Organometallics 1991, 10, 1479. (b) Mao, F.; Tyler,
D. R.; Keszler, D. J. Am. Chem. Soc. 1989, 111, 130. (c) Jones, W. D.;
Libertini, E. Inorg. Chem. 1986, 25, 1794.
(38) (a) Ohki, Y.; Suzuki, H. Angew. Chem., Int. Ed. 2002, 41, 2994.
(b) Adams, R. D.; Captain, B.; Fu, W.; Pellechia, P. J. Inorg. Chem. 2003,
3111. (c) See also: Persson, R.; Monari, M.; Gobetto, R.; Russo, A.; Aime,
S.; Calhorda, M. J.; Nordlander, E. Organometallics 2001, 20, 4150.
(40) We note that an independently prepared sample of Os3(CO)11(η1-
bpcd) was found to be stable toward ring closure to Os3(CO)10(bpcd) under
conditions identical with those employed in our isomerization reaction.

938 Organometallics, Vol. 25, No. 4, 2006
Watson et al.
Figure 3. Plots of the cluster distribution of 2c, 3, and 4 versus time from the thermolysis starting from cluster 2c (left) and cluster 3
(right) in benzene-d6 at 90 °C. The extent of the reaction was determined by 1H NMR analysis using the methylene group from each cluster
species.
Scheme 2
the merry-go-round migration involving two equatorially dis-
posed CO ligands and a phosphorus atom can also furnish the
cluster 2b upon bridge-terminal opening of the three bridging
participants.
bpcd migration to cluster 2c will meet with failure.43,44 Of
importance to this body of work is the fact that the bpcd ligand
isomerization is facile and gives cluster 2c as the platform for
the ortho metalation described below.
Thermal and Photochemical Activation of 1,1-Os3(CO)10-
(bpcd) and Characterization of HOs3(CO)9[µ-(PPh2)CdC-
{PPh(C6H4)}C(O)CH2C(O)] and HOs3(CO)8(µ3-C6H4)[µ2,η1-
PPhCdC(PPh2)C(O)CH2C(O)]. Thermolysis of cluster 2c at
elevated temperatures provided evidence for the activation of
Distinction between these two exchange processes was sought
through examination of the VT 13C NMR spectra of cluster 2b.
The slow-exchange 13C NMR spectral data for 13CO-enriched
2b (ca. 20%) in toluene-d8 recorded at 233 K confirm the
presence of four terminal carbonyl resonances at δ 193.07,
184.78, 178.17, and 173.12 in a 2:1:1:1 integral ratio, in
excellent agreement with the structure of 2b. Heating 2b from
233 to 273 K leads to the disappearance of the CO groups at
the bpcd-bridged osmium atoms (δ 193.07 and 178.17), due to
an in-plane merry-go-round scrambling of these groups.30,31b,41
Continued heating to 358 K promotes the turnstile scrambling
of the remaining axial and equatorial CO groups at the Os-
(CO)4 center. The rate of CO scrambling at coalescence in 2b
can be conservatively estimated as ca. 1500 and 600 s-1 for
the in-plane merry-go-round and turnstile processes, respec-
tively.42 Consequently, any attempt to use the 13C NMR data
from 2b to elucidate the isomerization mechanism for the slower
1
the bpcd ligand, as shown in Figure 3 (left) for the H NMR
distribution data from a sealed-tube thermolysis conducted at
90 °C in toluene-d8.45 Here a rapid production of the hydrido
cluster HOs3(CO)9[µ-(PPh2)CdC{PPh(C6H4)}C(O)CH2C(O)]
(3, red boxes) is observed, followed by the slower formation
of the benzyne-substituted cluster HOs3(CO)8(µ3-C6H4)[µ2,η1-
(42) The quoted rate constants for CO scrambling at the coalescence
temperature were estimated by use of the equation kcoalescence ) (π(δν))/
(21/2): Sanders, J. K. M.; Hunter, B. K. Modern NMR Spectroscopy; Oxford
University Press: New York, 1993.
(43) This situation is akin to assigning the locus of CO dissociation in
a metal carbonyl cluster when the rate of carbonyl scrambling about the
cluster polyhedron exceeds the rate of dissociative CO loss by several orders
of magnitude. See: Richmond, M. G.; Kochi, J. K. Inorg. Chem. 1986, 25,
1334.
(41) For analogous CO scrambling pathways in other phosphine-
substituted Os3 clusters, see: (a) Deeming, A. J.; Donovan-Mtunzi, S,; Kabir,
S. E. J. Organomet. Chem. 1985, 281, C43. (b) Alex, R. F.; Pomeroy, R.
K. Organometallics 1987, 6, 2437. (c) Johnson, B. F. G.; Lewis, J.; Reichert,
B. E.; Schorpp, K. T. J. Chem. Soc., Dalton Trans. 1976, 1403.
(44) The VT 13C NMR behavior of cluster 2c has also been studied in
toluene-d8 solution, with comparable CO fluxionality, vis-a`-vis 2b, observed.
(45) The small amount of cluster 3 in the thermolysis starting with cluster
2c does not adversely affect the outcome of the reaction.

The Triosmium Cluster Os3(CO)10(bpcd)
Organometallics, Vol. 25, No. 4, 2006 939
PPhCdC(PPh2)C(O)CH2C(O)] (4, green boxes). Heating a
sealed NMR tube containing cluster 3 under analogous condi-
tions furnished 4 as the major product along with 2c, as shown
in Figure 3 (right). Two important points emerging from these
thermolysis experiments are that cluster 2c serves as the direct
precursor for the formation of the hydride cluster 3 and that
this latter cluster can be converted back to 2c through a
reversible ortho-metalation sequence. Carrying out the ther-
molyses at 110 °C with purging of the liberated CO furnishes
cluster 4 as the dominant product in solution. The two new
clusters, whose structures are depicted below, were subsequently
isolated by column chromatography and fully characterized in
solution; in addition, X-ray diffraction analysis was used in the
case of 4.
Figure 4. Thermal ellipsoid plot of HOs3(CO)8(µ3-C6H4)[µ2,η1-
PPhCdC(PPh2)C(O)CH2C(O)] (4) showing thermal ellipsoids at
the 50% probability level.
diffraction structure (Figure 4). Cluster 4 contains 50 valence
electrons, which is two electrons in excess of the electron-precise
count of 48 valence electrons found for numerous triangular
Os3 clusters, accounting for the observed polyhedral opening
of the metallic frame.49 Despite our inability to locate the lone
bridging hydride group during data refinement, the significantly
different Os(1)-Os(2) (2.919(1) Å) and Os(2)-Os(3) (2.818-
(1) Å) bond distances allow us to confidently assign the bridging
hydride group to the longer Os(1)-Os(2) vector. The Os(1)‚‚
‚Os(3) internuclear distance of 3.809(1) Å clearly precludes any
direct bonding interaction between these osmium atoms. The
benzyne ligand is composed of the atoms C(24)-C(29) and
functions as a typical four-electron, face-capping ligand, where
the two Os-C σ bonds (Os(1)-C(24) ) 2.17(1) Å and Os-
(3)-C(25) ) 2.16(1) Å) and the Os-C π bonds (Os(2)-C(24)
) 2.31(1) Å and Os(2)-C(25) ) 2.33(2) Å) are similar in
distance to those in other benzyne-substituted osmium clusters.12f,50
The phosphido ligand, which is defined by the P(2) atom, is
equatorially situated and serves to tether the nonbonded Os(1)
and Os(3) atoms. The distorted four-membered ring defined by
the Os(1)-Os(2)-Os(3)-P(2) atoms is nearly planar on the
basis of σp ) 0.08 Å. The phosphorus atom P(1) behaves as a
normal phosphine ligand and is bound in an η1 fashion to the
Os(1) center. The Os-P distances range from 2.350(3) Å (Os-
(1)-P(1)) to 2.374(3) Å (Os(3)-P(2)), with a mean distance
of 2.366 Å.
Definitive support for the ortho metalation of an aryl group
in 3 comes from the H NMR spectrum, where a high-field
triplet at δ -15.91 is observed in benzene-d6 solvent. The small
P-H coupling of 13 Hz indicates that the bridging hydride group
1
J
is situated mutually cis to the two inequivalent phosphorus
centers. The diastereotopic methylene hydrogens belonging to
the bpcd residue are solvent sensitive and appear as an AB
quartet centered at δ 2.07 in benzene-d6 and as a singlet at δ
3.54 in CDCl3. The 31P NMR spectrum of 3 shows a pair of
inequivalent phosphorus resonances at δ 26.70 and 19.18, and
the nine terminal carbonyl groups found in a 13CO-enriched
sample of the cluster at 223 K are fully consistent with the
proposed structure.46 The ortho metalation of the ancillary bpcd
ligand in 3 is a phenomenon that has been seen in related
triosmium clusters containing the diphosphine ligands dppm,
dppe, and dppp.11a,47
The 1H NMR data for the benzyne-substituted cluster 4
include resonances at δ -16.88 and 3.66 for the bridging
hydride and the methylene group associated with the dione ring,
respectively. A 1H COSY spectrum of 4 revealed that the four
hydrogens belonging to the benzyne moiety exist as an ABXY
spin system that is buried within the aromatic hydrogens of the
other three phenyl groups. The 31P NMR spectrum shows two
resonances at δ 10.96 and -72.18, with the latter readily
assignable to a phosphido moiety that spans two nonbonded
osmium centers.48 The eight distinct carbonyl resonances found
in the 13C NMR spectrum of a 13CO-enriched sample of 4
(Experimental Section) are in full agreement with the proposed
structure. Unequivocal support for the presence of a benzyne
moiety and an opened cluster core in 4 is derived from the X-ray
With the identities of the two product clusters established
from the thermolysis of 2c, we next explored the photochemical
activation of 2c. Irradiation of 2c in either benzene or CH2Cl2
solvent with 366 nm light leads to CO loss and clean conversion
to 3, as verified by NMR and UV-vis analyses. A quantum
efficiency (Φ) of 0.02 was found for this ortho metalation at
(49) Cluster 4, with its seven skeletal electron pairs (SEP), may also be
viewed as possessing a three-vertex hypho architecture when treated within
the framework of polyhedral skeletal electron pair (PSEP) theory. See:
Mingos, D. M. P.; Wales, D. J. Introduction to Cluster Chemistry; Prentice
Hall: Englewood Cliffs, NJ, 1990.
(50) (a) Goudsmit, R. J.; Johnson, B. F. G.; Lewis, J.; Raithby, P. R.;
Rosales, M. J. J. Chem. Soc., Dalton Trans. 1983, 2257. (b) Chen, G.; Deng,
M.; Lee, C. K.; Leong, W. K. Organometallics 2002, 21, 1227. (c) Leong,
W. K.; Chen, G. Organometallics 2001, 20, 2280. (d) Tay, C. T.; Leong,
W. K. J. Organomet. Chem. 2001, 625, 231.
(46) The chelation of the ortho-metalated bpcd ligand in 3 via axial and
equatorial sites at the osmium center is strengthened by a recent X-ray
structure of the related hydride-bridged cluster HOs3(CO)9[µ-PhP(C6H4)Cd
CC(O)C(dCHFc)C(O)PPh2]. Unpublished results.
(47) Deeming, A. J.; Hardcastle, K. I.; Kabir, S. E. J. Chem. Soc., Dalton
Trans. 1988, 827.
(48) Carty, A. J.; MacLaughlin, S. A.; Nucciarone, D. In Phosphorus-
31 NMR Spectroscopy in Stereochemical Analysis; Verkade, J. G., Quin,
L. D., Eds.; VCH: New York, 1987.

940 Organometallics, Vol. 25, No. 4, 2006
Watson et al.
Figure 5. UV-vis spectral changes for the reaction of 3 in the presence of PPh3 (25 equiv) recorded at 331 K in toluene (left) and the
absorbance versus time curve for the experimental data (9) for the decay of 3 and the least-squares fit (s) of the first-order rate constant
kH (right).
Table 4. Experimental Rate Constants for the Conversion of
18 °C in benzene, with the formation of cluster 3 being retarded
HOs3(CO)9[µ-(PPh2)CdC{PPh(C6H4)}C(O)CH2C(O)] (3) to
when the irradiation was conducted under 1 atm of CO. The
1,1-Os3(CO)9(P)(bpcd) and of
observed CO inhibition supports the generation of the coordi-
natively unsaturated cluster 1,1-Os3(CO)9(bpcd) upon optical
excitation. That CO is lost from 2c is of interest, as the related
clusters Os3(CO)10(R-diimine) are resistant to dissociative CO
loss on excitation, producing spectroscopically observable
biradical and zwitterionic species from Os-Os bond cleavage
reactions, depending on the polarity of the solvent employed.51,52
No evidence for the activation of the benzene or CH2Cl2 solvents
was observed in our photolysis experiments when the reaction
DOs3(CO)9[µ-(PPh2-d10)CdC{P(Ph-d5)(C6D4)}C(O)CH2C(O)]
to 1,1-Os3(CO)9(P)(bpcd-d20)a
temp
(K)
104kH
104kD
b
b,c
d
(s-1
)
(s-1
)
trapping ligand
kH/kD
331.0
338.0
346.0
353.0
353.0
353.0
353.0
359.0
0.13 ( 0.03
0.30 ( 0.02
0.74 ( 0.06
1.66 ( 0.01
1.62 ( 0.01
1.62 ( 0.02
1.64 ( 0.04
3.47 ( 0.08
0.15 ( 0.01
0.33 ( 0.04
0.90 ( 0.08
25 equiv of PPh3
25 equiv of PPh3
25 equiv of PPh3
10 equiv of PPh3
10 equiv of P(OEt)3
25 equiv of PPh3
25 equiv of PCy3
25 equiv of PPh3
0.87
0.91
0.82
1.78 ( 0.01
3.99 ( 0.07
0.91
0.87
1
solutions were monitored by H and 31P NMR spectroscopy.
Proof of Reversible Ortho Metalation Through Ligand
Trapping of the Unsaturated Intermediate 1,1-Os3(CO)9-
(bpcd). The observation of minor amounts of 2c from the
thermolysis of 3 (Figure 3) signaled that the ortho metalation
of the aryl ligand is reversible, and as such, we wished to study
this reaction in more detail, given its relevance to hydrocarbon
activation. Cluster 3 was found to react readily with added CO
(1 atm) at 75 °C in a variety of solvents to furnish 2c in
a The UV-vis kinetic data were collected in toluene using ca. 10-4
M
HOs3(CO)9[µ-(PPh2)CdC{PPh(C6H4)}C(O)CH2C(O)] (3) and DOs3(CO)9[µ-
(PPh2-d10)CdC{P(Ph-d5)(C6D4)}C(O)CH2C(O)] by following the decrease
in the absorbance of the 396 nm band. b The quoted rate constants kH and
k
D represent the rates of reaction for the reductive coupling of the hydride
cluster 3 and its deutride isotopomer, respectively. c The Eyring activation
parameters were determined as ∆Hq ) 26.8(0.8) kcal/mol and ∆Sq ) 0(2)
eu. d The average of the five kH/kD values is 0.88 ( 0.05.
1
quantitative yield when monitored by H NMR and UV-vis
plot (left) for the reaction with PPh3. The UV-vis changes for
this reaction are clean, and the isosbestic points observed at
383 and 452 nm support the formation of 1,1-Os3(CO)9(PPh3)-
(bpcd) as the major product.54 The first-order rate constants
quoted in Table 4 were obtained from nonlinear regression
analysis. Examination of entries 4-7 in Table 4 confirms that
the rate of the reaction is independent of added ligand, allowing
us to eliminate from consideration a rate-limiting bimolecular
process involving the cluster and ligand. The activation param-
eters for these data are ∆Hq ) 26.8(0.5) kcal/mol and ∆Sq )
0(2) eu.
spectroscopy. The carbonylation proceeded cleanly, without the
presence of spectroscopically observable intermediates. Having
established the reversible C-H bond formation in the reaction
of 3 with added CO, we next turned our attention to the kinetic
trapping of the postulated unsaturated intermediate 1,1-Os3-
(CO)9(bpcd) with donor ligands other than CO.53 The reaction
of the hydride cluster 3 with excess trapping ligand (>10 equiv)
was investigated in toluene solution over the temperature range
of 331-359 K. The progress of each reaction was followed by
monitoring the decrease in the absorbance of the 396 nm band
for 3, as illustrated in Figure 5 for the absorbance versus time
In keeping with the current body of knowledge on alkane
and arene activation by mononuclear complexes, the demon-
strated reductive C-H bond coupling in 3 is expected to proceed
by way of an intermediate π-complex, prior to the rate-limiting
(51) (a) van Outersterp, J. W. M.; Oostenbrink, M. T. G.; Nieuwenhuis,
H. A.; Stufkens, D. J.; Hartl, F. Inorg. Chem. 1995, 34, 6312. (b) Bakker,
M. J.; Hartl, F.; Stufkens, D. J.; Jina, O. S.; Sun, X.-Z.; George, M. W.
Organometallics 2000, 19, 4310. (c) Vergeer, F. W.; Kleverlann, C. J.;
Stufkens, D. J. Inorg. Chim. Acta 2002, 327, 126. (d) Vergeer, F. W.;
Kleverlaan, C. J.; Matousek, P.; Towrie, M.; Stufkens, D. J.; Hartl, F. Inorg.
Chem. 2005, 44, 1319.
(52) For Os3(CO)12 photochemistry involving CO loss, see: Bentsen, J.
G.; Wrighton, M. S. J. Am. Chem. Soc. 1987, 109, 4518 and references
therein.
(53) Preliminary studies on the kinetic trapping of the unsaturated cluster
1,1-Os3(CO)9(bpcd), as generated from cluster 3, in the presence of CO
(low pressure) show saturation kinetics arising from the competitive capture
of 1,1-Os3(CO)9(bpcd) by the arene π bond and CO. Unpublished results.
(54) A preparative-scale reaction between cluster 3 and excess PPh3 (1:
10 mole ratio) has been carried out and 1,1-Os3(CO)9(PPh3)(bpcd) isolated
by column chromatography. On the basis of 1H and 31P NMR data, the
major stereoisomer present (>90%) found in solution contains a chelating
bpcd ligand and one equatorially bound PPh3 ligand that is attached to an
adjacent osmium atom. 31P NMR (C6D6, 298 K): δ 16.62 (1P, d, bpcd,
JP-P ) 12 Hz), 15.47 (1P, d, bpcd, JP-P ) 12 Hz), -6.64 (s, 1P, PPh3). 1H
NMR (C6D6, 298 K): δ 6.85-8.00 (35 H, multiplet), 2.43 (2H, s,
methylene).

The Triosmium Cluster Os3(CO)10(bpcd)
Organometallics, Vol. 25, No. 4, 2006 941
Scheme 3
Scheme 4
formation of the unsaturated cluster 1,1-Os3(CO)9(bpcd).55 Such
aspects of our reaction could readily be probed through parallel
kinetic studies employing an isotopically substituted bpcd ligand
where all the aryl hydrogens have been replaced by deuterium
atoms. To this end, we have synthesized the diphosphine ligand
bpcd-d20 (eq 2) and have used this ligand in the preparation of
constants (kD) for the consumption of 3-d20 reported in Table
4. The observed inverse isotope effect of 0.88 ( 0.05 (average
value for all five runs) for the reductive C-H bond coupling is
best interpreted as arising from an equilibrium isotope effect
(EIE) that is coupled with a slower, isotope-insensitive step (i.e.,
K‚k). In the present study, such a scenario is easily rationalized
within a multistep manifold that involves a rapid and reversible
preequilibrium step between the hydride (deuteride) cluster and
an arene π complex (krc/koc) that precedes the rate-limiting
dissociation of the arene π bond from the osmium center (kdiss).56
Scheme 4 illustrates this stepwise progression from the hydride
cluster 3 to the phosphine-substituted cluster 1,1-Os3(CO)9P-
(bpcd). While many paradigms provide unequivocal evidence
for the involvement of σ-alkane and π-arene intermediates
through equilibrium isotope effects in reductive coupling
reactions at mononuclear complexes,55,57,58 to our knowledge,
the data reported here represent the first such isotope work
concerning the ortho metalation of an aryl moiety at a poly-
nuclear system.59
the deuteride cluster DOs3(CO)9[µ-(PPh2-d10)CdC{P(Ph-d5)-
(C6D4)}C(O)CH2C(O)] (3-d20) and kinetic trapping studies of
the latter by PPh3, as shown in Scheme 3.
The reaction of the cluster 3-d20 and excess PPh3 (25 equiv)
was investigated in toluene solution by UV-vis spectroscopy,
as described for 3-d0 (vide supra), with the first-order rate
Thermodynamic and Kinetic Ortho Metalation in 1,1-Os3-
(CO)9(bpcd) and H/D Scrambling in the π-Arene Intermedi-
ate. Apart from the inverse isotope effect experimentally
determined from the ligand trapping studies employing 3-d0 and
3-d20, intramolecular H/D scrambling between the bridging
hydride or deuteride of cluster 3 and an appropriately labeled
ortho-metalated aryl moiety (C6D4 for the hydride and C6H4
for the deuteride) would also furnish corroborating evidence
for our intermediate π complex. Replacement of the bridging
hydride in 3-d0 by deuterium or the bridging deuteride in 3-d20
(55) (a) Jones, W. D.; Feher, F. J. Acc. Chem. Res. 1989, 22, 91. (b)
Arndtsen, B. A.; Bergman, R. G.; Mobley, T. A.; Peterson, T. H. Acc. Chem.
Res. 1995, 28, 154. (c) Jones, W. D. Acc. Chem. Res. 2003, 36, 140. (d)
Churchill, D. G.; Janak, K. E.; Wittenberg, J. S.; Parkin, G. J. Am. Chem.
Soc. 2003, 125, 1403. (e) Jones, W. D. Inorg. Chem. 2005, 44, 4475. (f)
Lersch, M.; Tilset, M. Chem. ReV. 2005, 105, 2471.

942 Organometallics, Vol. 25, No. 4, 2006
Watson et al.
Scheme 5
by hydrogen would give isotopically substituted clusters (shown
above) that could be used to probe H/D scrambling in 3.
Unfortunately, attempts to deprotonate the hydride ligand in 3-d0
using BuLi or Et3N, followed by acidification with acetic acid-d
or D2O, did not furnish the desired deuteride cluster DOs3(CO)9-
[µ-(PPh2)CdC{PPh(C6H4)}C(O)CH2C(O)] (left-hand cluster
depicted in Scheme 5) but led to deuterium incorporation into
the methylene group of the dione ring, along with substantial
decomposition of the starting cluster. Analogous methodologies
have been utilized by many researchers to demonstrate isotopic
exchange via a transient π-arene complex and σ-alkane complex.
Two classic examples that demonstrate the involvement of a
π-arene and σ-alkane intermediate in H/D scrambling sequences
are the mononuclear complexes Cp*Rh(PMe3)H(C6D5)57d and
Cp*Rh(PMe3)D(13CH2CH3),60 respectively.
Since the hydride deprotonation/acidification route employing
cluster 3 could not be used to synthesize a suitable probe cluster
for the H/D scrambling study, an isotopically substituted
alternative was sought, one where the ancillary bpcd ligand
contains one ortho hydrogen and deuterium relative to the
phosphorus atom. This was achieved by starting with the
isotopically substituted phosphine PPh3-d3,ortho and preparing the
diphosphine ligand bpcd-d4,ortho through the standard protocols
outlined in eq 3. The bpcd-d4,ortho ligand was then used in the
hydride and deuteride clusters depicted in the upper portion of
Scheme 6. On the basis of our earlier trapping studies with
cluster 3, the partitioning of the π complex between the hydride
and deuteride clusters is predicted to be rapid at ambient
temperatures relative to the rate-limiting dissociation of the
coordinated aryl ligand from the cluster, since this would
regenerate the high-energy unsaturated cluster. The important
ortho-metalation equilibria may be treated in terms of the relative
concentrations of the product hydride/deuteride clusters, where
Keq ) [deuteride]/[hydride]. Alternatively, Keq may be expressed
as a function of the individual rate constants for the forward
and reverse reactions so that Keq ) [k(H)RCk(D)OC]/[k(D)RCk-
(H)OC], which may also be rearranged in terms of the reductive
coupling and oxidative coupling kinetic isotope effects: Keq
)
[k(H)RC/k(D)RC]/[k(H)OC/k(D)OC]. As pointed out by Jones,
extraction of two of the three related isotope effects allows for
a complete understanding of the preequilibrium isotope contri-
butions to our ortho-metalation reaction.55c
Irradiation of a sealed NMR tube containing 1,1-Os3(CO)10-
(bpcd-d4,ortho) in toluene-d8 at 0 °C allowed the kinetic selectivity
for the ortho metalation to be established. Here we found a 55:
45 mixture of the corresponding hydride and deuteride clusters.61
This product ratio affords k(H)OC/k(D)OC ) 1.22 ( 0.03 for
the C-H(D) bond activation for the ortho-metalation step. Our
observation of a small normal KIE is on par with the value of
1.4 found for the oxidative cleavage of 1,3,5-C6H3D3 by Cp*Rh-
(PMe3).57d Heating kinetically generated samples of our cluster
over the temperature range of 40-60 °C led to the equilibration
of the hydride and deuteride clusters and a final isomer
composition of 67:33 in favor of the hydride cluster with Keq
) 0.49 ( 0.04.62 Irradiation of 1,1-Os3(CO)10(bpcd-d4,ortho) at
(57) For some representative examples, see: (a) Parkin, G.; Bercaw, J.
E. Organometallics 1989, 8, 1172. (b) Buchanan, J. M.; Stryker, J. M.;
Bergman, R. B. J. Am. Chem. Soc. 1986, 108, 1537. (c) Janowicz, A. H.;
Bergman, R. B. J. Am. Chem. Soc. 1983, 105, 3929. (d) Jones, W. D.;
Feher, F. J. J. Am. Chem. Soc. 1986, 108, 4814. (e) Jensen, M. P.; Wick,
D. D.; Reinartz, S.; White, P. S.; Templeton, J. L.; Goldberg, K. I. J. Am.
Chem. Soc. 2003, 125, 8614. (f) Wick, D. D.; Reynolds, K. A.; Jones, W.
D. J. Am. Chem. Soc. 1999, 121, 3974. (g) Bullock, R. M.; Headford, C. E.
L.; Hennessy, K. M.; Kegley, S. E.; Norton, J. R. J. Am. Chem. Soc. 1989,
111, 3897.
synthesis of 1,1-Os3(CO)10(bpcd-d4,ortho), which in turn can be
used to generate the unsaturated cluster 1,1-Os3(CO)9(bpcd-
d4,ortho) through photochemical activation.
(58) On the basis of the totality of the available data for reductive
coupling processes not proceeding via free-radical paths, we neglect a single-
step reaction having a product-like or late transition state and invoke a
multistep sequence for the formation of the aryl C-H bond in cluster 3.
(59) For isotope studies involving hydride/deuteride fluxionality and
related C-H bond activation reactions at Ru3 and Os3 clusters, see: (a)
Rosenberg, E. Polyhedron 1989, 8, 383. (b) Calvert, R. B.; Shapley, J. R.
J. Am. Chem. Soc. 1977, 99, 5225. (c) Duggan, T. P.; Golden, M. J.; Keister,
J. B. Organometallics 1990, 9, 1656.
Near-UV irradiation of 1,1-Os3(CO)10(bpcd-d4,ortho) is ex-
pected to lead to CO loss and formation of 1,1-Os3(CO)9(bpcd-
d4,ortho), as illustrated in the lower portion of Scheme 6.
Coordination of one of the aryl groups to an osmium center
adjacent to the bpcd-chelated osmium center furnishes a labile
π complex that is in equilibrium with the ortho-metalated
(56) No kinetic distinction between a π-arene or an agostic interaction
involving the ortho C-H(D) bond can be made on the basis of the data at
hand. However, given the preponderance of structural and solution examples
of η2-bound arene complexes, we will treat the intermediate in our ortho-
metalation reaction similarly. See: (a) Cheng, T.-Y.; Szalda, D. J.; Bullock,
R. M. Chem. Commun. 1999, 1629. (b) Johansson, L.; Tilset, M.; Labinger,
J. A.; Bercaw, J. E. J. Am. Chem. Soc. 2000, 122, 10846. (c) Reinartz, S.;
White, P. S.; Brookhart, M.; Templeton, J. L. J. Am. Chem. Soc. 2001,
123, 12724.
(60) Periana, R. A.; Bergman, R. G. J. Am. Chem. Soc. 1986, 108, 7332.
(61) The presence of the bridging deuteride ligand in DOs3(CO)9[µ-(PPh2-
d
2,ortho)CdC{P(Ph-dortho)(C6H4)}C(O)CH2C(O)] was also verified by 2H
NMR analysis, which revealed a broadened, high-field resonance at ca. -16
ppm.
(62) The difference in the EIE values (0.88 vs 0.49) obtained from the
two different ligands (bpcd-d20 vs bpcd-d4,ortho) presumably reflects the effect
of isotopic substitution at the R, â, and γ aryl C-H(D) bonds (relative to
the ortho-metalated carbon) between the two ligands.

The Triosmium Cluster Os3(CO)10(bpcd)
Organometallics, Vol. 25, No. 4, 2006 943
Scheme 6
40 °C in benzene-d6 gave a Keq value identical with that obtained
from the thermally equilibrated cluster samples. That the hydride
cluster is thermodynamically favored over the corresponding
deuteride cluster is understood within the tenets of the Born-
Oppenheimer approximation, where changes in the vibrational
energies of the participant bonds control the direction of the
hydride/deuteride equilibrium in this ortho-metalation reaction.63
The smaller zero-point energy difference between the bridging
osmium-hydride (-deuteride) bonds vis-a´-vis the fully formed
arene ortho C-H(D) bonds in the ortho-metalated aryl ring
guarantees that the hydride-bridged cluster with its stronger
arene C-D bond will dominate the equilibrium and favor the
hydride-bridged cluster HOs3(CO)9[µ-(PPh2-d2,ortho)CdC{P(Ph-
serve to liberate the benzyne ligand. However, such a path would
violate the principle of microscopic reversibility that requires
the regeneration of the coordinatively unsaturated cluster 1,1-
Os3(CO)9(bpcd), which in turn must function as a precursor for
the production of cluster 4. The kinetics for the conversion of
3 to 4 were studied over the temperature range 363-383 K in
toluene solvent by following the decrease in the 396 nm
absorbance band belonging to 3. Regression analysis provided
the first-order rate constants that are quoted in Table 5, from
which the values of ∆Hq ) 29.2(1.1) kcal/mol and ∆Sq ) 4(3)
eu were determined by Eyring analysis. The activation data are
in excellent agreement with those data obtained from the ligand
trapping studies employing 3, as expected. Under these higher
temperature conditions 1,1-Os3(CO)9(bpcd), once formed, can
undergo an irreversible P-C bond activation and ultimately
furnish the benzyne cluster 4. The exact nature of the aryl group
formed upon P-C bond cleavage cannot be determined on the
basis of the present data, but the generation of a cluster-bound
η1-Ph moiety would not be unreasonable, as depicted below
for Os3(CO)9(η1-Ph)[µ-(PPh2)CdC(PPh)C(O)CH2C(O)].65 Sub-
dortho)(C6H3D)}C(O)CH2C(O)]. From these data we find that
the reductive coupling step exhibits an inverse KIE with k(H)RC
/
k(D)RC ) 0.60. The gross characteristics for the reversible ortho
metalation in 1,1-Os3(CO)10(bpcd) are remarkably similar to the
activation of benzene by Cp*Rh(PMe3)H2.57d Taken collectively,
our data on the multistep conversion of the hydride clusters 3-d0
and 3-d20 to 1,1-Os3(CO)9L(bpcd) in the presence of trapping
ligands are best described by the reaction coordinate depicted
in Figure 6.64
Kinetics for the Conversion of HOs3(CO)9[µ-(PPh2)Cd
C{PPh(C6H4)}C(O)CH2C(O)] (3) to HOs3(CO)8(µ3-C6H4)-
[µ2,η1-PPhCdC(PPh2)C(O)CH2C(O)] (4). The relationship of
cluster 3 to cluster 4 was examined next. Conceptually, an
attractive route for conversion of 3 to 4 would involve a P-C
bond cleavage of the activated aryl group in 3, as this would
(63) (a) Bullock, R. M.; Bender, B. R. Isotope Methods in Homogeneous
Catalysis. In Encyclopedia of Catalysis; Horva´th, I. T., Ed.; Wiley: New
York, 2003. (b) Wolfsberg, M. Acc. Chem. Res. 1972, 7, 225.

944 Organometallics, Vol. 25, No. 4, 2006
Watson et al.
Figure 6. Reaction coordinate for the multistep conversion of cluster 3 to 1,1-Os3(CO)9L(bpcd) in the presence of trapping ligands.
Table 5. Experimental Rate Constants for the Conversion of
sequent ortho metalation of the η1-Ph moiety could then give
HOs3(CO)9[µ-(PPh2)CdC{PPh(C6H4)}C(O)CH2C(O)] (3) to
the coordinated benzyne ligand found in 4.
HOs3(CO)8(µ3-C6H4)[µ2,η1-PPhCdC(PPh2)C(O)CH2C(O)]
(4)a
104k (s-1
)
temp (K)
104k (s-1
)
Conclusions
temp (K)
363.0
368.0
373.0
1.31 ( 0.04
2.21 ( 0.03
3.58 ( 0.04
378.0
383.0
7.10 ( 0.10
11.0 ( 0.3
The reaction between the diphosphine ligand bpcd and Os3-
(CO)10(MeCN)2 has been investigated and found to give 1,2-
Os3(CO)10(bpcd) as the kinetic product of ligand substitution.
Nondissociative ligand isomerization to the chelated cluster 1,1-
Os3(CO)10(bpcd) occurs upon heating, followed by the loss of
CO and formation of the hydride cluster HOs3(CO)9[µ-(PPh2)Cd
C{PPh(C6H4)}C(O)CH2C(O)] through an ortho-metalation se-
quence. The ortho-metalation reaction is reversible and proceeds
by way of a transient π complex. The individual kinetic isotope
effects associated with the preequilibrium step have been
a The UV-vis kinetic data were collected in toluene using ca. 10-4
M
HOs3(CO)9[µ-(PPh2)CdC{PPh(C6H4)}C(O)CH2C(O)] (3) by following the
decrease in the absorbance of the 396 nm band.
determined, and the role of the unsaturated cluster 1,1-Os3(CO)9-
(bpcd) as a common precursor for the formation of both the
hydride cluster 3 and the benzyne-substituted cluster 4 is
confirmed. In the present study, the benzyne ligand is derived
from an irreversible P-C bond cleavage reaction, followed by
an ortho metalation of the cluster-bound phenyl moiety. Future
work will concentrate on the mechanistic examination of other
cluster-mediated ligand degradation reactions, and the influence
that the metal cluster has on the magnitude of the isotope
contributions for the ortho-metalation step will be probed.
(64) Technically, the observed reductive coupling step requires a short-
lived terminal Os-H(D) ligand prior to the formation of the coordinated π
complex on the basis of microscopic reversibility. VT 1H NMR analysis of
3-d0 showed no evidence for hydride fluxionality over the temperature range
298-340 K, as only a sharp high-field triplet at δ -15.91 (in benzene-d6)
was observed. In terms of the reaction coordinate depicted in Figure 6, we
show the reductive coupling arising directly from the bridging clusters to
the corresponding π complex directly for simplicity. It is acknowledged
that the situation is more complex with the small equilibrium isotope effect
expected between the bridging hydride (deuteride) clusters and the terminal
hydride (deuteride) isomers that furnish the π complex.
Acknowledgment. Financial support from the Robert A.
Welch Foundation (Grant Nos. P-0074 to W.H.W. and B-1093
to M.G.R.) is greatly appreciated. We also thank Prof. V. J.
Shiner, Jr., for helpful comments concerning our isotope data.
(65) For structurally characterized clusters possessing an η1-phenyl
moiety, see: (a) Bruce, M. I.; Humphrey, P. A.; Skelton, B. W.; White, A.
H. J. Organomet. Chem. 1996, 526, 85. (b) Deeming, A. J.; Smith, M. B.
J. Chem. Soc., Dalton Trans. 1993, 3383. (c) de Araujo, M. H.; Vargas,
M. D.; Braga, D.; Grepioni, F. Polyhedron 1998, 17, 2865. (d) Briard, P.;
Cabeza, J. A.; Llamazares, L.; Riera, V. Organometallics 1993, 12, 1006.
(e) Chen, G.; Deng, M.; Lee, C. K.; Leong, W. K. Organometallics 2002,
21, 1227. (f) See also ref 50c.
Supporting Information Available: Tables and CIF files giving
crystallographic data for 2b,c and 4. This material is available free
of charge via the Internet at http://pubs.acs.org. Crystallographic

The Triosmium Cluster Os3(CO)10(bpcd)
Organometallics, Vol. 25, No. 4, 2006 945
data for the structural analyses have also been deposited with the
Cambridge Crystallographic Data Center: CCDC No. 277346 for
2b, 277347 for 2c, and 277348 for 4. Copies of this information
may be obtained free of charge from the Director, CCDC, 12 Union
Road, Cambridge, CB2 1EZ U.K. (fax, +44(1223)336-033; e-mail,
deposit@ccdc.ac.uk; web, http://www:ccdc.cam.ac.uk).
OM050839T