C-H...O hydrogen bond-directed ligand exchange reaction: Diastereoselective synthesis of P,S-bridged (μ-alkyne)Co2(CO) 4 complexes

Article abstract of DOI:10.1021/om060656m

Here we describe a nonclassical interaction between a sulfonyl group in a dicobaltcarbonyl-alkyne complex and a methine moiety attached to three heteroatoms (O, P, S) included hi the PuPHOS and CamPHOS ligands. This interaction provides an efficient recognition event that yields up to 86% de in the CO-phosphine exchange reaction. The isomerization mechanism leading to the observed diastereomeric bias was studied computationally at the semiempirical level.

Full text of DOI:10.1021/om060656m

Organometallics 2006, 25, 5795-5799
5795
C-H‚‚‚O Hydrogen Bond-Directed Ligand Exchange Reaction:
Diastereoselective Synthesis of P,S-Bridged (µ-alkyne)Co2(CO)4
Complexes
Jordi Sola`,† Antoni Riera,*,† Xavier Verdaguer,*,† and Miguel A. Maestro‡
Unitat de Recerca en S´ıntesi Asime`trica (URSA-PCB), Institute for Research in Biomedicine (IRB), and
Departament de Qu´ımica Orga`nica, UniVersitat de Barcelona, c/Josep Samitier, 1-5, E-08028 Barcelona,
Spain, and Departamento de Qu´ımica Fundamental, UniVersidade da Corun˜a, E-15071 A Corun˜a, Spain
ReceiVed July 20, 2006
Here we describe a nonclassical interaction between a sulfonyl group in a dicobaltcarbonyl-alkyne
complex and a methine moiety attached to three heteroatoms (O, P, S) included in the PuPHOS and
CamPHOS ligands. This interaction provides an efficient recognition event that yields up to 86% de in
the CO-phosphine exchange reaction. The isomerization mechanism leading to the observed diastereomeric
bias was studied computationally at the semiempirical level.
Scheme 1. Diastereomeric Bridged P, S Complexes in the
Presence of an Amidocarbonyl Hydrogen Bond Acceptor
Introduction
Our group has developed the bidentate P,S ligands PuPHOS
and CamPHOS (Scheme 1). When coordinated to a terminal
alkyne dicobalt complex, these ligands provide two bridged
diastereomeric complexes.1 Upon reaction with norbornadiene,
each diastereomer produces the corresponding Pauson-Khand
adduct in high optical purity.2 Although the utility of these li-
gands has been demonstrated in the synthesis of chiral cyclo-
pentenones,3 they show poor diastereoselectivity in the ligand
exchange process (up to 50% de). To overcome this handicap,
we recently reported a hydrogen bond-directed ligand coordina-
tion procedure.4 The methine moiety attached to three heteroa-
toms (O, P, S) and contained in PuPHOS and CamPHOS ligands
serves as a strong hydrogen bond donor. When an appropriate
hydrogen bond acceptor is placed on the alkyne moiety, a
nonclassical C-H‚‚‚O interaction can take place. Given the
chirality of the ligand, this interaction can be established only
for one of the two possible diastereomers (Scheme 1). Hydrogen
bonds are key forces in molecular recognition and are crucial
for the stability of biological systems. Nonclassical C-H‚‚‚X
(X ) O, N, halogen, π) are important in solvation processes
and in biological and supramolecular chemistry.5 Nevertheless,
applications of weak hydrogen bond interactions in asymmetric
synthesis and catalysis are still scarce.6
PHOS to weaker hydrogen bond acceptor groups. Here we report
on the diastereoselective ligand exchange of P,S ligands with
dicobalthexacarbonyl complexes of a sulfonylacetylene. The
sulfone group is a weaker hydrogen acceptor than amides and
sulfoxides and slightly stronger than ketones and esters.7 Thus,
the sulfone should help to determine the efficiency of this
approach for a variety of widely occurring functional groups.
In this scenario, we sought to extend the generality and scope
of the directed C-H‚‚‚O coordination of PuPHOS and Cam-
Results and Discussion
* Corresponding author. E-mail: xverdaguer@pcb.ub.es.
† Institute for Research in Biomedicine and Dept. Qu´ımica Orga`nica
(UB).
A terminal p-tolylsulfonylacetylene dicobalthexacarbonyl
complex (1) was synthesized following a recently described
procedure.8 Using the sulfone complex 1 we proceeded to study
the ligand exchange reaction with CamPHOS and PuPHOS
(Table 1). The reaction was performed by heating a mixture of
the corresponding borane-protected phosphine and 1 equiv of
1 at 65 °C in the presence of DABCO (1,4-diazabicyclo[2.2.2]-
octane). Thus, borane removal and ligand exchange reaction
occurred in a one-pot process.
‡ Universidade da Corun˜a.
(1) (a) Verdaguer, X.; Moyano, A.; Perica`s, M. A.; Riera, A.; Maestro,
M. A.; Mahia, J. J. Am. Chem. Soc. 2000, 122, 10242-10243. (b)
Verdaguer, X.; Perica`s, M. A. Riera, A.; Maestro, M.A.; Mah´ıa, J.
Organometallics 2003, 22, 1868-1877.
(2) For recent reviews on the Pauson-Khand reaction see: (a) Gibson,
S. E.; Mainolfi, N. Angew. Chem., Int. Ed. 2005, 44, 3022-3027. (b)
Laschat, S.; Becheanu, A.; Bell, T.; Baro, A. Synlett 2005, 17, 2547-2570.
(3) (a) Verdaguer, X.; Lledo´, A.; Lo´pez-Mosquera, C.; Maestro, M. A.;
Perica`s, M. A.; Riera, A. J. Org. Chem. 2004, 69, 8053-8061. (b) Iqbal,
M.; Evans, P.; Lledo´, A.; Verdaguer, X.; Perica`s, M. A.; Riera, A.; Loeffler,
C.; Sinha, A. K.; Mueller, M. J. ChemBioChem 2004, 6, 276-280.
(4) Sola`, J.; Riera, A.; Verdaguer, X.; Maestro, M. A. J. Am. Chem.
Soc. 2005, 127, 13629-13633.
Ligand exchange reaction of 1 with CamPHOS and PuPHOS
provided the corresponding diastereomeric complexes in excel-
(5) IUCr Monographs on Crystallography; Desiraju, G. R., Steiner, T.,
Eds.; Oxford University Press: New York, 2001; Vol IX.
(6) Corey, E. J. Angew. Chem., Int. Ed. 2002, 41, 1650-1667.
(7) Hunter, C. A. Angew. Chem., Int. Ed. 2004, 43, 5310-5324.
(8) Sola`, J.; Riera, A.; Perica`s, M.A.; Verdaguer, X.; Maestro, M.A.
Tetrahedron Lett. 2004, 45, 5387-5390.
10.1021/om060656m CCC: $33.50 © 2006 American Chemical Society
Publication on Web 10/21/2006

5796 Organometallics, Vol. 25, No. 24, 2006
Sola` et al.
Table 1. Ligand Exchange Reaction for Bidentate P,S
Ligands
entry
L
yield (%) time (h) Solvent Xa/Xb de (%)a
1
2
3
4
5
6
CamPHOS
PuPHOS
CamPHOS
PuPHOS
CamPHOS
PuPHOS
96
89
76
88
98
88
16
16
24
24
16
16
toluene
toluene
hexane
hexane
THF
1a/1b
2a/2b
1a/1b
2a/2b
1a/1b
2a/2b
78
86
20
50
0
Figure 2. Evolution of diastereomeric excess of complexes 2a/2b
as determined by H NMR analysis.
1
THF
13
alkynes bearing a bridged ligand may exist in two fluxional
conformations, anti and syn, depending on the relative position
of the alkyne substituent and the bidentate ligand. It is important
to note that the hydrogen bond interaction can occur only in
the syn conformation. For most reported X-ray structures,
repulsive steric effects force the ligand to adopt an anti
conformation away from the alkyne substituent.10 In the present
example, the X-ray structure for 2a demonstrates that, in the
solid state, the weak C-H‚‚‚OdS bond forces the bidentate
ligand and the sulfone group in the alkyne moiety to adopt a
syn conformation. The distances for C(12)-O(6) and H(12)-
O(6) are 3.29(1) and 2.40(1) Å, respectively, and the angles
are θ(C-H‚‚‚O) ) 151.3(5)° and φ(H‚‚‚O-S) ) 112.4(5)°.
These values are consistent with those reported in the literature.11
The H(12)-O(6) distance reported here is larger than that of
the amidocarbonyl contact previously described (2.35 Å).4
1
a As determined by H NMR spectroscopy.
Occurrence of the weak hydrogen bond in solution was
confirmed by 1H NMR in C6D6. The methine (H12) resonance
for major diastereomers 1a and 2a displayed a downfield shift
(≈0.7 ppm) in comparison with minor diastereomers 1b and
2b, which could not provide the stabilizing contact. The
interaction energy for the present C-H‚‚‚O bonding contact was
evaluated at the MP2/6311+G(2d,p) level.12,13 Single-point
calculations on a computing model constructed from the X-ray
structure of 2a yielded an interaction energy of 3.18 kcal/mol.14
The present estimated energy for the methine-sulfone contact
falls in the range of strong C-H‚‚‚O bonds.
Figure 1. ORTEP drawing of 2a with 50% probability ellipsoids.
Only the hydrogen providing the weak hydrogen bond is shown.
lent yields as red solids. The best diastereoselectivity was ob-
tained in toluene (Table 1, entries 1 and 2). The reaction in
hexane provided lower selectivity than toluene (Table 1, entries
3 and 4), probably because of the reduced product solubility in
this solvent even at higher temperatures. Finally, ligand ex-
change reaction in THF provided no selectivity (Table 1, entries
5 and 6). With respect to the ligand moiety, reactions with Pu-
PHOS were slightly more selective than the corresponding reac-
tions with CamPHOS. These experiments indicate that the
C-O‚‚‚H-directed ligand exchange reaction also occurs when
the hydrogen bond acceptor is a sulfone group. The selectivity
decreased dramatically when a polar solvent such as THF was
used (entries 5, 6). This observation suggests that solvent inter-
ference in the weak interaction is responsible for the lower
selectivity.
Study of the Weak C-H‚‚‚OdS Contact. A solid-state
study was performed on the major diastereomeric complexes
1a and 2a. Fortunately, suitable single crystals of 2a could be
grown layering hexane over toluene. X-ray analysis of 2a
(Figure 1) confirmed that a nonclassical hydrogen bonding was
established between the methine (C12, H12) and an oxygen atom
from the sulfone group (O6). Bridged ligands show fluxionality
around the Co-Co axis.9 Dicobalt complexes of terminal
Isomerization of P,S-Bridged (µ-alkyne)Co2(CO)4 Com-
1
plexes. Reaction progress analysis by H NMR indicated that
the diastereoselectivity observed in the ligand exchange process
is the result of a thermodynamic equilibrium. The selectivity
increased to reach, over time, a fixed diastereomeric ratio (Figure
2). Experimentally, the present isomerization process occurs
(10) For X-ray structures of terminal alkyne-dicobalthexacarbonyl
complexes with bridged bidentate ligands, see: (a) Gimbert, Y.; Robert,
F.; Durif, A.; Averbuch, M.-T.; Kann, N.; Greene, A. E. J. Org. Chem.
1999, 64, 3492-3497. (b) Verdaguer, X.; Perica`s, M. A.; Riera, A.; Maestro,
M. A.; Mah´ıa, J. Organometallics 2003, 22, 1868-1877.
(11) (a) Taylor, R.; Kennard O. J. Am. Chem. Soc. 1982, 104, 5063-
5070. (b) Taylor, R.; Kennard, O. Acc. Chem. Res. 1984, 17, 320-326. (c)
Desiraju, G. R. Acc. Chem. Res. 1991, 24, 290-296.
(12) Corrections for the basis set superposition error (BSSE) were
included and were determined through the counterpoise method described
by Boys and Bernardi; see: Boys, S. F.; Bernardi, F. Mol. Phys. 1970, 19,
553-556.
(13) Quantum chemical calculations were performed using the Gaussian
03 program package: Frisch, M. J.; et al. Gaussian 03, revision C.02;
Gaussian, Inc.: Wallingford, CT, 2004.
(14) See Supporting Information for a graphical representation of the
computing model.
(9) Hanson, B. E.; Mancini, J. S. Organometallics 1983, 2, 126-128.

P,S-Bridged (µ-alkyne)Co2CO4 Complexes
Organometallics, Vol. 25, No. 24, 2006 5797
Figure 3. Intermediates and transition states involved in the alkyne-dissociation mechanism for the isomerization of model complex R.
Table 2. Computed Absolute and Relative Energies (DFT)
under nitrogen in the absence of CO; therefore it is not assisted
of the Species Involved in the Alkyne-Dissociation
by external carbon monoxide.15 Under these conditions, two
mechanisms for the interconversion between diastereomeric
Mechanism for the Isomerization of R
compound
absolute ∆Hf (au)
relative ∆Hf (kcal/mol)
complexes Xa and Xb can be envisaged. A first possible
isomerization pathway is the equilibration through partial or total
dissociation of the alkyne moiety from one of the metal centers.
It has been postulated that isomerization of M2C2 tetrahedral
systems occurs through direct rotation of the alkyne with respect
to the metal-metal bond.16 On the basis of the cluster theory,
Jaouen and co-workers proposed a mechanism by which
elongation of one of the metal-carbon bonds provides a but-
terfly-like intermediate.17 This partial dissociation of the alkyne
ligand could evolve to a full dissociation of the alkyne from
one cobalt atom18 or to a formal rotation of the alkyne with
respect to the Co-Co axis. An alternative isomerization pathway
would imply cleavage of the Co-S bond to form a coordina-
tively unsaturated complex. Migration of the pendant phosphine
would then provide the rearranged complex without the as-
sistance of an incoming CO ligand. This is a reasonable scenario
for a P,S ligand such as PuPHOS and CamPHOS, where the
sulfur linkage is much weaker than the phosphorus one.19
To gain further insight into the mechanism of the isomer-
ization process in the absence of CO, we performed a compu-
tational study. Geometries of stationary points and transition
states (TS) were fully optimized and characterized at the semi-
empirical PM3(tm) level, which includes parametrization for
transition metals. Reaction pathways were verified by optimiza-
tion of the perturbed TS structures. Finally, to efficiently com-
pare reaction pathways from the energetic point of view, single-
point calculations were performed at the DFT level using the
functional B3LYP. For this purpose we used the LACVP* basis
set, which includes Hay and Wadt’s effective core potential
(ECP) for cobalt, while all other atoms are described using a
6-31G* basis set.20-22
R
-1719.50628
-1719.48453
-1719.48766
-1719.49333
-1719.49227
-1719.47389
-1719.46910
-1719.49289
-1719.48718
-1719.47200
-1719.48318
0.0
13.6
11.68
8.1
TS-1a
TS-1b
INT-1a
INT-1b
TS-2a
TS-2b
INT-2a
INT-2b
TS-3a
INT-3a
8.8
20.3
23.3
8.4
11.9
21.5
14.5
Using complex R as a computing model (Figure 3), we first
studied the partial dissociation of the alkyne from one cobalt
atom in a butterfly-like mechanism.17 Starting from R, enlarge-
ment of bond (a) and concomitant arrangement of a carbonyl
ligand in a bridging position allowed the location of TS-1a,
which subsequently led to a butterfly-like intermediate, INT-
1a. A pseudosymmetrical reaction pathway was located upon
elongation of bond (b) in R, which allowed us to locate TS-1b
and INT-1b. Absolute and relative energies for all the inter-
mediates are shown in Table 2. From INT-1a, inversion of the
cobalt center attached to phosphorus through a pseudo-
octahedral transition state (TS-2a) led to INT-2a, in which the
alkyne is arranged along the same plane as the cobalt-cobalt
bond. INT-2a is a 1,2-dimetallacyclobutene and represents a
formal half rotation of the alkyne around the Co-Co axis.16
From INT-2a a symmetrical reaction pathway would provide
the isomerized starting material. Alternatively, from INT-1a
another isomerization pathway involving full dissociation of the
alkyne from one of the metal centers was computed. Thus,
elongation of bond (c) in INT-1a permitted us to locate TS-3a.
Moving the alkyne moiety further from the departing metal
center with the concomitant formation of another bridged
carbonyl ligand provided INT-3a, in which the alkyne is bonded
to only one cobalt atom. The intermediate INT-3a would
provide free rotation of the alkyne moiety and could also account
(15) Isomerization of phosphine (µ-alkyne)Co2(CO)5 complexes is a well-
documented process; see: Hay, A.M.; Kerr, W. J.; Kirk, G. G.; Middlemiss,
D. Organometallics 1995, 14, 1986-4988.
(16) Iwashita, Y.; Tamura, F.; Wakmatsu, H. Bull. Chem. Soc. Jpn. 1970,
43, 1520-1523.
(17) Jaouen, G.; Marinetti, A.; Saillard, J.-Y.; Sayer, B. G.; McGlinchey,
M. J. Organometallics 1982, 1, 225-227.
(21) PM3(tm) provides a good reproduction of the structural features of
X-ray crystal data for alkyne-dicobalt carbonyl complexes. However, this
level of theory shows a tendency to saturate vacant coordination sites on
cobalt through bridging carbonyls or through agostic interactions with
hydrogen atoms. See: Verdaguer, X.; Vazquez, J.; Fuster, G.; Bernardes-
Genisson, V.; Greene, A. E.; Moyano, A.; Pericas, M. A.; Riera, A. J. Org.
Chem. 1998, 63, 7037-7052. To validate the significance of intermediates
bearing bridging carbonyls, independent geometry optimization of these
structures was performed at the DFT level.
(18) Partial alkyne dissociation from one metal center was also proposed
in the isomerization of Co2(alkyne)(binap)CO4 complexes; see: Gibson,
S.E.; Kaufmann, K. A. C.; Loch, J. A.; Steed, J. W.; White, A. J. P. Chem
Eur. J. 2005, 11, 2566-2576.
(19) For reviews on hemilabile ligands, see: (a) Braunstein, P.; Naud,
F. Angew. Chem., Int. Ed. 2001, 40, 680-699. (b) Dieguez, M.; Pamies,
O.; Ruiz, A.; Diaz, Y.; Castillon, S.; Claver, C. Coord. Chem. ReV. 2004,
248, 2165-2192.
(20) Spartan ’04; Wavefunction, Inc.; Irvine, CA.
(22) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 270-283.

5798 Organometallics, Vol. 25, No. 24, 2006
Sola` et al.
able for substrates bearing stronger binding ligands (i.e., diphos-
phines) in which partial dissociation of the ligand is unlikely.18
Conclusions
The methine moiety contained in PuPHOS and CamPHOS acts
as a general nonclassical hydrogen bond donor. Intermolecular
contact between this methine and a sulfone hydrogen bond accep-
tor provides a stabilizing interaction that leads to a diastereoselec-
tive coordination of the P,S ligands to a dicobalthexacarbonyl
tosylacetylene complex. The diastereoselectivities observed in
the ligand exchange process, the X-ray analysis of major diastere-
omer 2a, and the quantum mechanical calculations on a model
structure indicate that the present C-H‚‚‚OdS(O)R interaction
is slightly less effective than the reported C-H‚‚‚OdCNR2. This
observation could be attributed to the lower competence of the
sulfone moiety to act as a hydrogen bond acceptor in comparison
with an amidocarbonyl group. These results validate the gener-
ality and efficiency of the present approach as an alternative to
steric repulsion for substrate-ligand recognition in asymmetric
synthesis and catalysis. Finally, here we have performed a
theoretical modeling of the isomerization of P,S-bridged alkyne
dicobaltcarbonyl complexes in the absence of CO. Our studies
indicate that isomerization for P,S hemilabile ligands occurs
via a sulfide-dissociation/phosphine-migration mechanism.
Figure 4. Energy (kcal/mol) profile for the partial and total
dissociation of the alkyne from one cobalt center in model complex
R. Profile corresponds to elongation of bond (a) in R.
Figure 5. Model structures of dimethylsulfide dicobalt-acetylene
complex (R-SMe2) and the same complex without the sulfide ligand
and a vacant coordination site (R-0).
Experimental Section
Dicobalthexacarbonyl Complex of p-Tolylsulfonylethyne, 1.
Solid dicobaltoctacarbonyl (1.42 g, 4.10 mmol) was added to a
solution of p-tolylsulfonyltrimethylsilylethyne (1.0 g, 3.96 mmol)
in diethyl ether (40 mL) under nitrogen. The reaction mixture was
stirred at room temperature until CO evolution ceased. The solvent
was removed in vacuo, and the resulting red residue was solved in
MeOH (120 mL). A KHCO3/K2CO3 aqueous buffer solution (6.2
× 10-3 M, 25 mL) was added, and the resulting mixture was stirred
at 40 °C for 24 h until no intermediate complex could be detected
by TLC. At this stage, 40 mL of water was added and the reaction
mixture was filtered over a pad of silica gel. The product complex
was then eluted with CH2Cl2. The organic phase was dried
(MgSO4), and the solvent was removed in vacuo to yield 1.5 g
(81%) of 1 as an orange crystalline solid. Mp: 155-165 °C (dec).
IR (KBr): νmax 2020, 2033, 2053, 2064, 2083, 2107 cm-1. 1H NMR
(300 MHz, CDCl3): δ 2.43 (s, 3H), 6.24 (s, 1H), 7.34-7.87 (dd,
J ) 8 Hz, 4H) ppm. 13C NMR (75 MHz, CDCl3): δ 21.6, 73.0,
93.3, 127.7, 129.7, 138.1, 144.7, 196.6 (broad, 6CO) ppm. Anal.
Calcd for C15H8Co2O8S: C 38.65, H 1.73, S 6.88. Found: C 38.99,
H 1.95, S 6.87.
Co2(µ-p-CH3-C6H4C2H)(CO)4(µ-C23H27OPS) 1a and 1b. A 100
mg (0.21 mmol) portion of dicobalthexacarbonyl complex of
p-tolylsulfonylethyne, 1, 80 mg (0.20 mmol) of CamPHOS-borane
complex, and 35 mg (0.3 mmol) of DABCO were placed in a
Schlenk flask under an argon atmosphere. Then 4 mL of freshly
distilled toluene was added, and the mixture was heated at 65 °C.
CO was periodically removed from the reaction by means of
vacuum and argon refilling. After 16 h, the solvent was removed
under reduced pressure. Chromatography (80:20 hexanes/EtOAc)
provided 135 mg (85%) of the major (less polar) complex and 18
mg of the minor (more polar) complex as red solids. Data for major
complex 1a: IR (KBr) νmax 2955, 2044, 2018, 1990 cm-1. 1H NMR
(400 MHz, C6D6): δ 0.48 (s, 3H), 0.60 (m, 1H), 0.73 (m, 1H),
0.81 (s, 3H), 0.91 (m, 1H), 1.20-1.40 (m, 3H), 1.58 (m, 1H), 1.87
(s, 3H), 2.92 (dd, J ) 6, 13 Hz, 1H), 3.63 (d, J ) 13 Hz, 1H), 3.89
(m, 1H), 5.70 (m, 1H), 6.81 (d, J ) 8 Hz, 1H), 7.04 (s, 1H), 7.05-
7.25 (m, 6H), 7.74 (m, 2H), 7.86 (m, 2H), 7.92 (d, J ) 8 Hz, 2H)
ppm. 13C NMR (100 MHz, C6D6): δ 20.0, 21.2, 22.6, 27.0, 33.7,
37.2, 40.3 (J ) 10 Hz), 44.8, 45.9, 46.7, 80.0 (J ) 16 Hz), 87.6 (J
for the isomerization of the initial tetrahedral dicobalt-alkyne
cluster R. As shown in Figure 4, the most energetically
demanding step is associated with either ligand inversion at the
cobalt center (TS-2a) or the full dissociation of the alkyne from
one of the cobalt atoms (TS-3a). In this regard, both reaction
pathways are viable from the common butterfly-like intermediate
INT-1a since they exhibit similar energy requirements. Pseu-
dosymmetric reaction pathways resulting from enlargement of
bonds (a) and (b) are almost degenerate (Table 2). Reaction
pathway (b) provides a slightly lower energy barrier for the
formation of the first intermediate, while in the metal inversion
step TS-2, path (a), provides a lower energy barrier. The present
theoretical model (Figure 4) indicates that the isomerization of
P,S-bridged (µ-alkyne)Co2(CO)4 complexes through partial or
total dissociation of the alkyne from one metal center is a
kinetically feasible process under the experimental reaction
conditions tested here, i.e., extended heating (60-70 °C) in
toluene solution in the absence of CO.
The departure of the sulfide ligand leaving a vacant coordina-
tion site on cobalt was also examined computationally. To this
end, the DFT (single point) ∆H°f for model R-SMe2 (Figure
5), the corresponding complex R-0 with a vacant coordination
site, and dimethylsulfide were calculated. This provided a
theoretical ∆H°r ) 15.4 kcal/mol for the departure of SMe2 in
R-SMe2. An analogous energy barrier should be associated with
the cleavage of the Co-S bond in a P,S-bridged complex to
yield a vacant coordination site. At this point, migration of the
loose phosphine ligand may provide an energetically advanta-
geous isomerization pathway. Taking into account the previously
calculated alkyne-dissociation pathway, the present theoretical
study suggests that in the absence of CO a complex containing
a hemilabile ligand isomerizes through the sulfide-dissociation/
phosphine-migration mechanism to preferentially provide the
diastereomeric complex in which the C-H‚‚‚O interaction
occurs. The alternative alkyne-dissociation mechanism cannot
be completely ruled out; however, this mechanism is more prob-

P,S-Bridged (µ-alkyne)Co2CO4 Complexes
Organometallics, Vol. 25, No. 24, 2006 5799
) 3 Hz), 91.3 (J ) 29 Hz), 128.9 (J ) 10 Hz), 129.6, 129.9, 130.3,
131.2 (J ) 2 Hz), 132.2 (J ) 11 Hz), 134.6, 135.1 (J ) 13 Hz),
141.2, 143.1 ppm. 31P NMR (121 MHz, C6D6): δ 52.8 ppm. MS
(FAB-NBA): 764 ([M - CO]+, 5%), 736 ([M - 2CO]+, 12%),
708 ([M - 3CO]+, 24%), 680 ([M - 4CO]+, 100%). HRMS: calcd
for C32H36Co2O3PS2 [M - 4CO + H]+, 681.0507, found 681.0508.
Data for minor complex 1b: IR (KBr) νmax 2925, 2042, 2018, 1987
cm-1. 1H NMR (400 MHz, C6D6): δ 0.40-2.00 (m, 7H), 0.66 (s,
3H), 1.30 (s, 3H), 1.91 (s, 3H), 3.02 (m, 1H), 3.50 (m, 1H), 5.40
(s, 1H), 6.34 (s, 1H), 6.90-6.20 (m, 8H), 4.73 (m, 2H), 7.78 (m,
2H), 8.26 (d, J ) 7 Hz, 2H) ppm. 13C NMR (100 MHz, C6D6): δ
20.0, 21.2, 22.4, 27.0, 33.7, 37.0, 41.2, 44.6, 45.8, 46.7, 73.2, 87.3,
90.9 (J ) 24 Hz), 129.1 (J ) 10 Hz), 129.6, 130.0, 130.3, 121.0,
131.1 (J ) 11 Hz), 134.6, 125.8 (J ) 13 Hz), 140.8, 143.2 ppm.
31P NMR (121 MHz, C6D6): δ 55.6 ppm.
with C6D6, 7H), 7.78-7.94 (m, 5H) ppm. 13C NMR (75 MHz,
C6D6): δ 19.7, 21.2, 21.8, 24.9, 27.4, 30.8, 34.2, 41.0, 49.9 (J )
8 Hz), 50.6, 79.0, 81.3, 86.0, 128.2 (overlap with C6D6) ,128.7 (J
) 9 Hz), 129.4, 130.0, 130.8, 131.3 (J ) 36 Hz), 132.9 (J ) 11
Hz), 134.4, 135.0 (J ) 12 Hz), 141.2, 142.9 ppm. 31P NMR (121
MHz, C6D6): δ -30.6 ppm. HRMS: calcd for C33H38Co2O4PS2
[M - 3CO]+, 711.0613, found 711.0622. Data for minor complex
2b: IR (KBr): νmax 2043, 2014, 1987, 1968 cm-1. 1H NMR (300
MHz, C6D6): δ 0.22-0.30 (m, 2H), 0.65 (s, 3H), 0.68-0.98 (m,
2H), 0.87 (s, 3H), 0.97 (s, 3H), 1.18-1.22 (m, 3H), 1.55-1.74
(m, 1H), 1.91 (s, 3H), 2.89 (m, 1H), 5.21 (s, 1H), 5.86 (m, 1H)
6.92 (d, J ) 8 Hz, 2H), 6.90-7.32 (m, overlap with C6D6, 6H),
7.56 (m, 2H), 7.88 (m, 2H), 8.26 (d, J ) 6 Hz, 2H) ppm. 31P NMR
(121 MHz, C6D6): δ -28.3 ppm.
Co2(µ-p-CH3-C6H4C2H)(CO)4(µ-C23H29OPS) 2a and 2b. A 61
mg (0.13 mmol) sample of dicobalthexacarbonyl complex of
p-tolylsulfonylethyne 1, 50 mg (0.12 mmol) of PuPHOS-borane
complex, and 26 mg (0.24 mmol) of DABCO were placed in a
Schlenk flask under an argon atmosphere. Then 2 mL of freshly
distilled toluene was added, and the mixture was heated at 65 °C.
CO was periodically removed from the reaction by means of
vacuum and argon refilling. After 16 h, the solvent was removed
under reduced pressure. Chromatography (80:20 hexanes/EtOAc)
provided 82 mg (85%) of the major (less polar) complex and 7 mg
(7%) of the minor (more polar) complex as red solids. Data for
Acknowledgment. This work was supported by MEC
(CTQ2005-623). X.V. thanks the DURSI for a “Distincio´ per a
la Promocio´ de la Recerca Universita`ria”. J.S. thanks the DURSI
and Universitat de Barcelona for a fellowship. We thank S.
Olivella for helpful advice on quantum mechanics calculations.
Supporting Information Available: General methods; 1H NMR
spectra for major complexes 1a and 2a; X-ray crystal data with a
complete numbering scheme, atomic distances and angles for 2a;
computing model for hydrogen bond contact and complete ref 19;
Cartesian coordinates for molecular modeling compounds and
transition states. This material is available free of charge via the
Internet at http://pubs.acs.org.
major complex 2a: IR (KBr): νmax 2045, 2014, 1993, 1968 cm-1
.
1H NMR (300 MHz, C6D6): δ 0.22-0.40 (m, 2H), 0.64 (d, J ) 6
Hz, 3H), 0.68-0.98 (m, 2H), 0.98 (s, 3H), 1.08-1.40 (m, 3H),
1.55-1.74 (m, 1H), 1.65 (s, 3H), 1.87 (s, 3H), 3.43 (m, 1H), 5.86
(s, 1H), 6.80 (d, J ) 8 Hz, 2H), 6.90 (s, 1H), 7.00-7.32 (m, overlap
OM060656M