Exclusive C-C oxidative addition in a rhodium thiophosphoryl pincer complex and computational evidence for an ν3-C-C-H agostic intermediate

Article abstract of DOI:10.1021/om201205y

The room-temperature reaction between the Rh(I) precursor [Rh(COE) ₂(acetone)₂]BF₄ (COE = cyclooctene) and a new thiophosphoryl-based SCS pincer ligand leads to oxidative addition of an sp ²-sp³ C-C b

Full text of DOI:10.1021/om201205y

Article
pubs.acs.org/Organometallics
Exclusive C−C Oxidative Addition in a Rhodium Thiophosphoryl
Pincer Complex and Computational Evidence for an η3-C−C−H
Agostic Intermediate
Michael Montag,†,∥ Irena Efremenko,†,∥ Yael Diskin-Posner,‡ Yehoshoa Ben-David,† Jan M. L. Martin,†,§
and David Milstein*,†
†Department of Organic Chemistry, Weizmann Institute of Science, Rehovot 76000, Israel
‡Department of Chemical Research Support, Weizmann Institute of Science, Rehovot 76000, Israel
S
* Supporting Information
ABSTRACT: The room-temperature reaction between the Rh(I)
precursor [Rh(COE)2(acetone)2]BF4 (COE = cyclooctene) and a new
thiophosphoryl-based SCS pincer ligand leads to oxidative addition of an
sp2−sp3 C−C bond as the only observed outcome, despite the presence of
accessible sp3 C−H bonds. A DFT study reveals that the chemistry of the
SCS system is controlled by π repulsion between occupied rhodium d
orbitals and the lone-pair electrons on the two sulfur atoms. This
repulsion gives rise to the thermodynamic selectivity for C−C over C−H
cleavage, as it is attributed to the higher electronegativity of a methyl
versus hydride ligand, thereby allowing more effective release of excessive
π electron density. It is also demonstrated that the observed C−C and
unobserved C−H cleavage pathways originate from a common
intermediate that features a novel η3-C−C−H agostic interaction. The
COE ligand is shown to play an important role by greatly stabilizing this intermediate, making it the only available entry point to
both reaction pathways.
Scheme 1. Exclusive C−C Oxidative Addition upon Reaction
of the Thiophosphoryl SCS Ligand with a Rh(I) Precursor
INTRODUCTION
■
Selective activation of nonstrained carbon−carbon single bonds
by transition metals is one of the fundamental challenges of
modern organometallic chemistry.1,2 Mechanistic insights
regarding such reactions have been obtained with pincer
ligands,3,4 which provide a highly controlled steric and
electronic environment for metal centers. These studies have
primarily involved phosphorus-based pincer ligands, wherein
one ligand arm bears a strongly coordinating phosphorus donor
group (typically phosphine), and the second arm bears a donor
group of varying lability, such as phosphine (PCP), amine
(PCN), or ether (PCO).3 Much less attention has been
devoted to other pincer systems,5 even though a diverse range
of pincer ligands is available. One type of ligands that has not
̀
been explored vis-a-vis C−C activation is the thiophosphoryl-
based pincers.6 These ligands feature soft sulfur donors that are
suitable for binding late transition metals and are typically air-
stable, in contrast to the commonly employed phosphorus
donor ligands.
Herein, we describe the reaction of the new thiophosphoryl
SCS-type pincer ligand 1 (Scheme 1) with the cationic Rh(I)
precursor [Rh(COE)2(acetone)2]BF4 (COE = cyclooctene),
which resulted in exclusive oxidative addition of a strong sp2−
sp3 C−C bond. The observed preference of the SCS system
toward cleavage of this bond, rather than the more numerous
and accessible sp3 C−H bonds of the Ar−CH3 moiety, was
probed computationally. This revealed the root cause of the
selectivity to be significant Rh−S π repulsion, and the fact that
this repulsion can be better reduced by the methyl ligand of the
C−C cleavage product than by the less electronegative hydride
ligand of the C−H cleavage product. The resulting thermody-
Received: December 2, 2011
Published: December 28, 2011
© 2011 American Chemical Society
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namic selectivity for C−C cleavage is further enhanced by high
reversibility of the kinetically favored C−H cleavage reaction.
Intriguingly, the reaction pathways for C−C and C−H cleavage
were found to share a common intermediate featuring a
hitherto unreported η3-C−C−H agostic interaction.
Complex 2 was the only observed reaction product in
acetone, and its gradual formation over several hours was not
accompanied by C−H cleavage products. Of particular
significance was the absence of sp3 C−H cleavage product 3
(Scheme 1), the analogues of which had been observed in
previous pincer systems.4c,d,h,i,k,n,q Moreover, no C−H cleavage
products were detected when the solution containing 2 was
heated at 60 °C for 12 h. We have previously shown that
phosphine-based pincer ligands that are structurally analogous
to 1 may undergo concurrent or consecutive sp3 C−H and
sp2−sp3 C−C bond activations upon reaction with various
transition-metal precursors.4c,d,h,i,k,n,q In one notable case, the
reaction could be directed toward either C−C or C−H
oxidative addition simply by changing the solvent.4i In that
instance, when the reaction was carried out in THF, exclusive
C−C cleavage was observed, whereas in the presence of
acetonitrile only C−H cleavage occurred. In light of these
results, we repeated the reaction of ligand 1 with the Rh(I)
precursor in solvents of varying donicity, that is, THF (THF-
d8/CD2Cl2, 1:1),10 methanol (CD3OD/CD2Cl2, 1:1),11 and
acetonitrile (18 equiv of CD3CN in acetone-d6).12 Never-
theless, only C−C cleavage was observed in all cases,
supporting the conclusion that the thiophosphoryl SCS system
is exclusively selective toward C−C bond cleavage.
Exclusive C−C cleavage has been previously observed in
rhodium complexes of PCN13 and POCOP14 (phosphinite-
based) pincer ligands. The differences in selectivity between
these systems and earlier PCP complexes, which exhibited both
C−C and C−H activation, were attributed to differences in
ligand arm length. It was suggested that the amine and
phosphinite arms draw the metal ion closer to the C−C bond
than do the longer phosphine arms of the PCP ligands, thereby
favoring C−C over C−H activation. Nevertheless, the present
results are not in line with this explanation, since the
thiophosphoryl arms of the SCS ligand are longer than those
of previously reported PCP ligands,15 and yet the SCS system
exhibits exclusive C−C cleavage.
RESULTS AND DISCUSSION
■
As shown in Scheme 1, when an acetone solution of
[Rh(COE)2(acetone)2]BF4 was treated with 1 equiv of ligand
17 at room temperature, a facile C−C cleavage reaction ensued,
resulting in the gradual formation of complex 2 over several
hours.8 The reaction product was isolated in high yield (∼90%)
and characterized by solution NMR techniques, as well as
single-crystal X-ray crystallography. The 31P{1H} NMR
spectrum of 2 in acetone-d6 exhibits a sharp singlet at 80.63
ppm, and its 1H NMR spectrum features a broad singlet at 1.37
ppm, which was assigned to the methyl ligand. The latter also
gives rise to a doublet at −6.77 ppm (1JRhC = 26.2 Hz) in the
1
13C{1H} NMR spectrum. Both the H and the 13C{1H} NMR
spectra of 2 are consistent with a meridionally coordinated
pincer ligand and an axial methyl ligand (overall Cs molecular
symmetry), as corroborated by crystallographic analysis (see
below). The 19F{1H} NMR spectrum features a sharp singlet at
−152.51 ppm, indicative of a noncoordinated BF4− counterion.
An attempt to crystallize complex 2 from acetone yielded
crystals of its adduct with diacetone alcohol (4-hydroxy-4-
methyl-2-pentanone), complex 2a. The solid-state structure of
this complex (Figure 1) exhibits a rhodium atom situated in an
To probe the energetic and electronic basis for the observed
selectivity of the SCS system, we undertook a density functional
theory (DFT) study of the mechanisms of both sp2−sp3 C−C
and sp3 C−H oxidative addition, using model structures as
shown in Scheme 2. The mechanisms were examined in the
absence of ancillary ligands, as well as in the presence of the
experimentally relevant, electron-donating ligands CH2O
(acetone model) and BF4−, and the electron-withdrawing
ligand cis-2-butene (cyclooctene model). The computed
reaction profiles for C−C and C−H cleavage are displayed in
Figure 2. The C−C cleavage product, 2′, is thermodynamically
stable for all of the examined ancillary ligands, and even in their
absence. By contrast, formation of the C−H cleavage product,
3′, is considerably less favorable than 2′ and is even endergonic
in the absence of an ancillary ligand or with CH2O. This clearly
indicates that C−C oxidative addition is thermodynamically
preferred for the SCS system, in agreement with the
experimental results, as well as previous computational
investigations of C−C versus C−H cleavage in pincer
systems.4n,16
Figure 1. ORTEP drawing (50% probability level) of complex 2a, the
adduct of complex 2 with diacetone alcohol (4-hydroxy-4-methyl-2-
pentanone). All hydrogen atoms (except H1) and BF4− were omitted
for clarity. Selected bond distances (Å) and angles (deg) for complex
2: Rh1−C1, 1.971(2); Rh1−C19, 2.031(2); Rh1−O1, 2.2336(18);
Rh1−O2, 2.2612(18); Rh1−S1, 2.3579(6); Rh1−S2, 2.3730(6); C1−
Rh1−C19, 85.88(9); C1−Rh1−S1, 91.86(6); C1−Rh1−S2, 92.03(6);
C1−Rh1−O1, 177.49(8); C1−Rh1−O2, 99.45(8).
octahedral environment that is defined by a meridionally
coordinated SCS ligand, an axial methyl moiety, and the
chelating ketol. The BF4− counterion is outer-sphere (not
shown in Figure 1), in agreement with the solution 19F NMR
spectrum, but does interact with the complex via H-bonding to
the OH group of the ketol. The ketol ligand was not detected
by solution NMR techniques, nor implicated by elemental
analysis of a bulk sample of the complex. It might have been
produced in situ by rhodium-promoted aldol condensation of
two acetone molecules.9
In addition to the reaction products, two precleaved
intermediates were also located as minima on the potential
energy surface (PES). The first structure, complex 4′, exhibits
no bonding interactions between the metal and the adjacent
methyl moiety, but features η2 arene−metal coordination, as
previously found for an analogous PCN−Rh(I) pincer
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systems have found η1-arene, η2-arene, and sp3 C−H agostic
species.4n,16 Generally speaking, the vast majority of agostic
interactions reported to date involve η2-bound C−H bonds,18
whereas C−C agostic interactions have seldom been
observed.4p,19 It is important to emphasize that the η3-C−C−
H agostic interaction in 5′ differs markedly from the well-
documented β-agostic interaction,20 despite their structural
resemblance. In the former, all three atoms are anchored to the
metal center through noncovalent dative bonding, whereas in
the latter the alkyl group features a fully fledged M−C covalent
bond. The fact that the η3-C−C−H agostic interaction has not
been previously reported should be attributed not only to the
unique characteristics of the SCS system but also to the
relatively large polarization consistent basis set applied in the
present calculations. It has been previously demonstrated that
polarization functions have conceptual importance in represent-
ing weak electron back-donation effects.21 The relevance of η3-
C−C−H agostic interactions to other pincer systems will be
addressed in a future report.
Scheme 2. Model Structures Used for the DFT Examination
of Oxidative Addition of sp2−sp3 C−C and sp3 C−H Bonds
in the SCS System
Electronic structure analysis reveals that strong π repulsion
between occupied S p and Rh d orbitals is a key factor that
determines the stability and reactivity of the SCS−Rh
complexes. Thus, charge decomposition analysis (CDA)
indicates that arene → Rh electron donation, which is the
dominant component of the η1-arene interaction (Scheme 3a),
system.16c In the latter case, preference for the η2 structure was
attributed to the short length of the amine arm, such that the
metal center is constrained near the arene ring and prevented
from effectively interacting with the methyl moiety. However,
this geometrical argument does not apply in the SCS case, as
this pincer ligand exhibits much longer arms.
Scheme 3. Key Electronic Interaction in an η1-Arene
Structure (a) and Its Stabilization by Electron Transfer to
the Arene Ring, Leading to an η2-Arene Structure (b) as in
Intermediate 4′
The second reaction intermediate, complex 5′, is located
along the reaction pathways connecting 4′ with products 2′ and
3′.17 Interestingly, this intermediate features a novel η3-C−C−
H agostic interaction, wherein the metal center interacts
simultaneously with the sp2−sp3 C−C and sp3 C−H bonds. To
the best of our knowledge, such an intermediate has not been
previously found for the oxidative addition of either C−C or
C−H bonds. Instead, earlier DFT investigations of pincer
is energetically unfavorable in the electron-rich SCS system.
Consequently, the system adopts the η2-arene configuration
(Scheme 3b) that characterizes 4′ and enables partial release of
Figure 2. Computed reaction profiles for the oxidative addition of sp2−sp3 C−C and sp3 C−H bonds in the SCS system as a function of ancillary
ligand (L). For L = vacant site, 5′ and TSC−H could not be located on the potential energy surface (PES). For L = cis-2-butene, 4′ could not be
located on the PES. Energies are referenced to the reactants, [Rh(cis-2-butene)2(CH2O)2]+ + 1′.
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excessive π electron density via Rh → arene back-donation into
high-lying empty arene π* orbitals. Agostic complex 5′ is
characterized by a further increase in the C−C−H → Rh
electron donation (Scheme 4a). The overall interaction
transition state and its energy barrier is lower. It should
nevertheless be noted that once 5′ is formed the intrinsic
kinetic barriers to both C−C and C−H cleavage are highly
accessible at room temperature (ΔG‡298 ≤ 14.3 kcal mol−1),
such that both reactions are very facile.
Scheme 4. Key Electronic Interaction in the η3-C−C−H
Agostic Intermediate 5′ (a) and Its Stabilization by cis-2-
Butene through Metal-to-Olefin Electron Transfer (b)
It is important to stress that the behavior of the SCS system
is dominated by its olefin adduct. This was shown above to be
the only attainable entry point to C−C and C−H cleavage, and
it is therefore the olefin adduct that determines the selectivity
of the system. In this case, the extremely low barrier for C−H
cleavage, and the fact that 3′ and 5′ are essentially isergonic,
renders C−H oxidative addition highly reversible. Therefore,
complex 2′ is obtained as the only reaction product, in
agreement with the experimental findings.23 It is important to
emphasize that the above kinetic considerations pertain only to
the selectivity of the SCS system, whereas the apparent rate of
C−C cleavage is probably determined by processes that
precede the formation of complexes 4′ and 5′, for example,
solvent or olefin detachment from the Rh(I) precursor.
Finally, electron transfer from rhodium to the sp2−sp3 C−C
or sp3 C−H bond ultimately leads to their dissociation, thereby
affording complexes 2′ and 3′, respectively. Hence, it is this
charge transfer that determines the thermodynamic balance of
the overall processes. CDA shows that preference for the C−C
cleavage product arises from the significantly higher electro-
negativity of the methyl ligand in 2′ relative to the hydride in 3′,
which renders the Rh → C−C electron transfer more effective
than Rh → C−H (with total donations of 0.49 and 0.37
electrons, respectively). Consequently, bonding in the Ar−Rh−
CH3 fragment is 12.5 kcal mol−1 stronger than in the ArCH2−
Rh−H fragment.
between the Ar−CH3 fragment of 5′ and the remainder of
this complex, as found by CDA, is 5.9 kcal mol−1 stronger than
in η2-arene complex 4′.22 However, increased Rh−S π repulsion
ultimately renders complex 5′ a few kcal mol−1 less stable than
4′, and both intermediates are thermodynamically unstable with
respect to the reactants, as well as the C−C and C−H cleavage
products. It should be noted that, in the absence of ancillary
ligands, complex 5′ could not be located on the PES, whereas 4′
constitutes a direct precursor for both 2′ and 3′. In the presence
of ancillary ligands, 5′ becomes the immediate precursor for 2′
and 3′, thereby constituting a direct entry point to both C−C
and C−H cleavage.
Dramatic changes in the electronic and energetic properties
of the SCS system take place when the electron-withdrawing
olefin, cis-2-butene, is introduced as an ancillary ligand,
modeling the experimentally employed cyclooctene. In this
case, the Rh → arene electron donation becomes less
important, and consequently complex 4′ could not be located
on the PES. By contrast, the η3-C−C−H agostic interaction is
strongly reinforced by Rh → olefin π back-bonding, which
facilitates electron donation from the occupied σC−C and σC−H
orbitals of the Ar−CH3 moiety to the metal (Scheme 4b). This
stabilizes complex 5′ to an extraordinary extent, making it
thermodynamically stable and isergonic with the C−H cleavage
product 3′. Thus, the olefin adduct of 5′ is the most prevalent
reaction intermediate within the SCS system, and is the only
feasible gateway to C−C and C−H cleavage in this system.
The transition state for C−H cleavage (TSC−H) is lower in
energy than the corresponding one for C−C cleavage (TSC−C),
regardless of the ancillary ligand, thereby making C−H
oxidative addition kinetically favored. This is in agreement
with both experimental and computational results for
previously reported pincer systems.4c,h,i,k,n,q,16 From the
electronic structure perspective, the transition states of both
C−C and C−H cleavage are characterized by further
strengthening of the interaction between the metal center
and the Ar−CH3 fragment (by 13.3 and 11.9 kcal mol−1,
respectively), because of an increase in both electron donation
and back-donation. Nevertheless, despite the high electron
density on rhodium, it is a poor electron donor, because of the
low energy of its occupied orbitals. Consequently, the buildup
of electron charge at the metal center increases the Rh−S π
repulsion. Because the C−C → Rh donation is larger than C−
H → Rh (0.15 vs 0.06 electrons, respectively), the resulting
Rh−S repulsion is stronger in the former case, thus leading to a
late transition state and higher activation barrier for C−C
cleavage. Conversely, C−H cleavage occurs through an early
CONCLUSION
■
In summary, we have shown that the new thiophosphoryl-based
SCS pincer ligand 1 reacts with the Rh(I) precursor
[Rh(COE)2(acetone)2]BF4 to undergo sp2−sp3 C−C bond
cleavage as the only observed outcome, even though sp3 C−H
bonds are also available for activation. A DFT study
demonstrated that the chemistry of the SCS system is
controlled by significant repulsion between occupied rhodium
d orbitals and the lone-pair electrons on the two sulfur atoms.
The selectivity for C−C cleavage was shown to be
thermodynamic, and this was linked to the higher electro-
negativity of the resulting methyl ligand relative to the hydride
ligand of the C−H cleavage product, thus allowing for more
effective release of excessive π electron density. The selectivity
is enhanced by the high reversibility of the kinetically favored
C−H cleavage reaction. It was also shown that the C−C and
C−H cleavage pathways originate from a common intermediate
that features a novel η3-C−C−H agostic interaction. The COE
ligand was found to play a significant role in the present SCS
system, by greatly stabilizing the agostic intermediate and
making it the only available entry point to both C−C and C−H
cleavage.
EXPERIMENTAL SECTION
■
General Procedures. All experiments were carried out under an
atmosphere of purified nitrogen in an MBraun MB 150B-G glovebox,
or under purified argon in an MBraun Unilab glovebox. The complex
[Rh(COE)2(acetone)2]BF4 was prepared according to a literature
procedure.24 All solvents were reagent grade or better. All non-
deuterated solvents were refluxed over sodium/benzophenone ketyl
and distilled under argon. Deuterated solvents were used as received
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2
and were degassed with argon and kept in the glovebox over 3 or 4 Å
molecular sieves (except for acetone, which was dried with Drierite).
Commercially available reagents were used as received. Crystal
structures were drawn using the program ORTEP-3.25
PCH(CH3)2), 17.76 (d, JPC = 1.5 Hz, PCH(CH3)2). Assignment of
the 13C{1H} NMR signals was confirmed by 13C DEPT 135.
Elemental analysis: Found: C, 58.35%; H, 8.88%. Calcd for
C19H34P2S2: C, 58.73%; H, 8.82%.
Analysis. NMR spectra (1H, 13C, 19F, and 31P) were recorded using
Bruker Avance-400 and Bruker Avance-500 NMR spectrometers. All
Synthesis of Complex 2. To a solution of 90.5 mg (0.172 mmol)
of [Rh(COE)2(acetone)2]BF4 in 1.6 mL of acetone was added a
solution of 67.2 mg (0.173 mmol) of ligand 1 in 1.9 mL of acetone,
and the resulting solution was stirred at room temperature for 22 h,
during which its color changed from orange to dark brown. The
solution was then concentrated under vacuum to 1.1 mL and added to
17 mL of pentane, with stirring. The liquid phase was then decanted,
and residual solvent was removed from the product under vacuum.
The resulting solid was then crushed to powder and washed with 6 mL
of pentane. Removal of residual solvent under vacuum yielded 98.9 mg
(0.155 mmol, 90.4% yield) of the product as a dark brown powder.
1
measurements were done at 20 °C, unless noted otherwise. H and
13C NMR chemical shifts are reported in parts per million relative to
tetramethylsilane. 1H NMR chemical shifts are referenced to the
residual hydrogen signal of the deuterated solvent, and the 13C NMR
chemical shifts are referenced to the 13C signal(s) of the deuterated
solvent. 19F NMR chemical shifts are reported in parts per million
relative to CFCl3 and referenced to an external solution of C6F6 in
CDCl3. 31P NMR chemical shifts are reported in parts per million
relative to H3PO4 and referenced to an external 85% solution of
phosphoric acid in D2O. Abbreviations used in the description of
NMR data are as follows: Ar, aryl; br, broad; s, singlet; d, doublet; t,
triplet; q, quartet; m, multiplet. Electrospray (ES) mass spectrometry
was performed at the Chemical Analysis Laboratory (Department of
Chemical Research Support), Weizmann Institute of Science, using
Micromass Platform LCZ 4000 (Micromass, Manchester, U.K.) with a
cone voltage of 43 V, an extractor voltage of 4 V, and a desolvation
temperature of 150 °C. Elemental analysis was performed at H. Kolbe
31P{1H} NMR (202 MHz, acetone-d6): 80.63 (bs). H NMR (500
1
MHz, acetone-d6): 7.56 (m, 3JHH = 7.7 Hz, 2H, Ar-H), 7.22 (m, 3JHH
=
2
3
7.7 Hz, 1H, Ar-H), 2.84 (m, JPH = 13.9 Hz, JHH = 6.9 Hz, 2H,
PCH(CH3)2), 2.67 (m, JPH = 14.2 Hz, JHH = 7.1 Hz, 2H,
2
3
3
PCH(CH3)2), 1.37 (br s, 3H, Rh-CH3), 1.31 (dd, JPH = 17.7 Hz,
3JHH = 6.9 Hz, 6H, PCH(CH3)2), 1.23 (dd, 3JPH = 17.9 Hz, 3JHH = 6.9
3
3
Hz, 6H, PCH(CH3)2), 1.17 (dd, JPH = 17.8 Hz, JHH = 7.0 Hz, 6H,
3
3
PCH(CH3)2), 1.15 (dd, JPH = 17.6 Hz, JHH = 7.0 Hz, 6H,
PCH(CH3)2). 13C{1H} NMR (126 MHz, acetone-d6): 178.48 (bm,
Mikroanalytisches Laboratorium, Mulheim an der Ruhr, Germany.
̈
Synthesis of Ligand 1. This new ligand was prepared in analogy
1
3
Cipso), 145.78 (dd, JPC = 92.2 Hz, JPC = 11.9 Hz, Aror1tho), 134.35 (m,
to a literature procedure.7
3
Armeta), 121.07 (t, JPC = 11.8 Hz, Arpara), 29.96 (d, JPC = 44.7 Hz,
Step 1: Synthesis of the Bisphosphine Precursor 2,6-Bis-
(diisopropylphosphino)toluene. A 1 L Schlenk-type flask equipped
with a magnetic stirring bar was loaded, in the nitrogen glovebox, with
5.00 g (20.0 mmol) of 2,6-dibromotoluene, 5.66 g (47.9 mmol) of
diisopropylphosphine, 0.20 g (1.1 mmol) of PdCl2, 6.40 g (65.2
mmol) of KOAc, and 75 mL of DMF. The flask was then removed
from the glovebox, and the mixture was stirred at 130 °C for 2 h,
under argon, after which it was cooled to room temperature. A 400 mL
portion of degassed water was then added. The resulting mixture was
extracted with 2 × 400 mL of diethyl ether, under nitrogen, and the
combined organic phases were dried with Na2SO4. The mixture was
then passed through a sintered-glass filter, and the resulting clear
solution was reintroduced into the glovebox and placed under high
vacuum to remove the solvent. The resulting solid residue was washed
with cold pentane and again placed under vacuum to remove the
solvent. This yielded 4.00 g (12.9 mmol, 61.6% yield) of the product
as a cream-colored solid. 31P{1H} NMR (162 MHz, CDCl3): −3.74
(s). 1H NMR (400 MHz, CDCl3): 7.32 (d, 3JHH = 7.6 Hz, 2H, Ar-H),
7.12 (t, 3JHH = 7.5 Hz, 1H, Ar-H), 2.85 (s, 3H, Ar-CH3), 2.04 (m, 3JHH
1
PCH(CH3)2), 27.59 (d, JPC = 43.8 Hz, PCH(CH3)2), 17.24 (s,
PCH(CH3)2), 16.99 (s, PCH(CH3)2), 16.28 (s, PCH(CH3)2), 15.77
(s, PCH(CH3)2), −6.77 (d, 1JRhC = 26.2 Hz, Rh-CH3). Assignment of
the 13C{1H} NMR signals was confirmed by 13C−1H heteronuclear
correlation. 19F{1H} NMR (376 MHz, acetone-d6): −152.51 (s, free
BF4). 31P{1H} NMR (202 MHz, CD3OD): 79.00 (s). H NMR (500
1
3
3
MHz, CD3OD): 7.37 (m, JHH = 7.6 Hz, 2H, Ar-H), 7.11 (m, JHH
=
3
7.6 Hz, 1H, Ar-H), 2.64 (m, JHH = 7.0 Hz, 2H, PCH(CH3)2), 2.58
(m, 3JHH = 7.0 Hz, 2H, PCH(CH3)2), 1.29 (dd, 3JPH = 17.6 Hz, 3JHH
=
3
3
6.9 Hz, 12H, PCH(CH3)2), 1.18 (dd, JPH = 17.9 Hz, JHH = 7.2 Hz,
2
6H, PCH(CH3)2), 1.16 (d, JRhH = 2.1 Hz, 3H, Rh-CH3), 1.16 (dd,
3JPH = 17.8 Hz, 3JHH = 7.1 Hz, 6H, PCH(CH3)2). 13C{1H} NMR (126
1
2
MHz, CD3OD): 180.75 (dt, JRhC = 39.9 Hz, JPC = 17.9 Hz, Cipso),
145.36 (ddd, 1JPC = 93.2 Hz, 3JPC = 12.3 Hz, J = 2.0 Hz, Arortho), 134.37
(m, Armeta), 120.76 (t, 3JPC = 11.9 Hz, Arpara), 29.85 (d, 1JPC = 44.7 Hz,
1
PCH(CH3)2), 29.34 (d, JPC = 43.5 Hz, PCH(CH3)2), 17.52 (s,
PCH(CH3)2), 17.34 (s, PCH(CH3)2), 16.33 (s, PCH(CH3)2), 16.07
(s, PCH(CH3)2), −8.78 (d, 1JRhC = 25.7 Hz, Rh-CH3). Assignment of
the 13C{1H} NMR signals was confirmed by 13C−1H heteronuclear
correlation. 19F{1H} NMR (376 MHz, CD3OD): −155.59 (s, free
3
= 6.9 Hz, 4H, PCH(CH3)2), 1.08 (dd, JPH = 14.8 Hz, 3JHH = 7.0 Hz,
3
3
12H, PCH(CH3)2), 0.86 (dd, JPH = 11.7 Hz, JHH = 6.9 Hz, 12H,
BF4). 31P{1H} NMR (202 MHz, CD3OD, −70 °C): 78.84 (s). H
1
PCH(CH3)2). 13C{1H} NMR (126 MHz, CDCl3): 151.06 (t, JPC
=
2
NMR (500 MHz, CD3OD, −70 °C): 7.43 (m, 3JHH = 7.7 Hz, 2H, Ar-
24.0 Hz, Ar), 134.85 (d, 1JPC = 14.1 Hz, CAr2-P), 132.68 (s, Ar), 124.29
(s, Ar), 24.06 (m, PCH(CH3)2), 21.75 (t, JPC = 27.8 Hz, CAr-CH3),
20.17 (m, PCH(CH3)2), 19.15 (m, PCH(CH3)2). Assignment of the
13C{1H} NMR signals was confirmed by 13C−1H heteronuclear
correlation. Elemental analysis: Found: C, 70.24%; H, 10.52%. Calcd
for C19H34P2: C, 70.34%; H, 10.56%.
3
3
H), 7.12 (m, JHH = 7.7 Hz, 1H, Ar-H), 2.65 (m, JHH = 7.0 Hz, 4H,
PCH(CH3)2), 1.30−1.12 (m, 24H, PCH(CH3)2), 1.04 (d, 2JRhH = 2.3
Hz, 3H, Rh-CH3). Selected 13C{1H} NMR (126 MHz, CD3OD, −70
3
°C): 134.40 (m, Armeta), 120.69 (t, JPC = 11.8 Hz, Arpara), 29.77 (d,
1JPC = 44.8 Hz, PCH(CH3)2), 28.46 (d, 1JPC = 44.5 Hz, PCH(CH3)2),
17.34 (s, PCH(CH3)2), 17.13 (s, PCH(CH3)2), 16.22 (s, PCH-
(CH3)2), 15.64 (s, PCH(CH3)2), −8.38 (d, 1JRhC = 25.4 Hz, Rh-CH3).
Assignment of the 13C{1H} NMR signals was confirmed by 13C−1H
heteronuclear correlation. 31P{1H} NMR (162 MHz, CD3CN): 87.55
Step 2: Sulfurization of 2,6-Bis((diisopropylphosphino)toluene. A
solution of 327.0 mg (1.01 mmol) of 2,6-bis(diisopropylphosphino)-
toluene in 3.2 mL of THF was added to a suspension of 64.5 mg (2.01
mmol) of elemental sulfur in 5.9 mL of THF, and the resulting clear
solution was stirred at room temperature for 21 h. The solvent was
then removed under vacuum overnight to yield 382.7 mg (0.98 mmol,
97.7% yield) of ligand 1 as a white solid. 31P{1H} NMR (162 MHz,
1
3
(s). H NMR (400 MHz, CD3CN): 7.34 (m, JHH = 7.7 Hz, 2H, Ar-
3
3
H), 7.13 (m, JHH = 7.7 Hz, 1H, Ar-H), 2.58 (m, JHH = 7.0 Hz, 2H,
3
PCH(CH3)2), 2.51 (m, JHH = 7.0 Hz, 2H, PCH(CH3)2), 1.23 (dd,
3JPH = 17.8 Hz, 3JHH = 6.9 Hz, 6H, PCH(CH3)2), 1.22 (dd, 3JPH = 17.7
1
acetone-d6): 76.10 (s). H NMR (400 MHz, acetone-d6): 8.69 (m,
3JHH = 7.9 Hz, 2H, Ar-H), 7.54 (m, 3JHH = 7.9 Hz, 1H, Ar-H), 3.04 (s,
Hz, 3JHH = 6.9 Hz, 6H, PCH(CH3)2), 1.13 (dd, 3JPH = 17.8 Hz, 3JHH
=
3
3
2
3
7.0 Hz, 6H, PCH(CH3)2), 1.07 (dd, JPH = 17.9 Hz, JHH = 7.0 Hz,
3H, Ar-CH3), 2.91 (m, JPH = 13.6 Hz, JHH = 6.8 Hz, 4H,
6H, PCH(CH3)2), 0.90 (d, JRhH = 2.4 Hz, 3H, Rh-CH32). 13C{1H}
2
3
3
PCH(CH3)2), 1.31 (dd, JPH = 17.3 Hz, JHH = 6.7 Hz, 6H,
1
3
3
NMR (101 MHz, CD3CN): 184.47 (dt, JRhC = 35.5 Hz, JPC = 20.8
PCH(CH3)2), 0.83 (dd, JPH = 17.9 Hz, JHH = 7.0 Hz, 6H,
1
3
PCH(CH3)2). 13C{1H} NMR (101 MHz, acetone-d6)1: 143.37 (bm,
Hz, Cipso), 142.44 (ddd, JPC = 92.2 Hz, JPC = 13.5 Hz, J = 2.1 Hz,
Arortho), 134.12 (m, Armeta), 121.15 (t, 3JPC = 11.7 Hz, Arpara), 30.21 (d,
1JPC = 44.3 Hz, PCH(CH3)2), 28.04 (d, 1JPC = 43.7 Hz, PCH(CH3)2),
17.25 (s, PCH(CH3)2), 17.02 (s, PCH(CH3)2), 16.18 (s, PCH-
2
CAr-Me), 139.78 (d, JPC = 10.4 Hz, Ar), 131.71 (dd, JPC = 61.8 Hz,
3JPC = 8.0 Hz, CAr-P), 125.84 (d, 2JPC = 11.8 Hz, Ar), 29.53 (d, JPC
=
1
3
49.9 Hz, PCH(CH3)2), 22.41 (t, JPC = 2.4 Hz, Ar-CH3), 18.35 (s,
509
dx.doi.org/10.1021/om201205y | Organometallics 2012, 31, 505−512

Organometallics
Article
(CH3)2), 15.91 (s, PCH(CH3)2), −1.66 (d, 1JRhC = 21.9 Hz, Rh-CH3).
7.6 Hz, 2H, Ar-H), 7.13 (m, 3JHH = 7.6 Hz, 1H, Ar-H), 2.64 (m, 2JPH
=
2
3
19F{1H} NMR (376 MHz, CD3CN): −153.28 (s, free BF4). Elemental
3JHH = 6.9 Hz, 2H, PCH(CH3)2), 2.46 (m, JPH = JHH = 7.0 Hz, 2H,
analysis (acetone adduct): Found: C, 41.58%; H, 6.36%. Calcd for
C22H40BF4OP2RhS2: C, 41.52%; H, 6.34%.
PCH(CH3)2), 1.31 (br s, 3H, Rh-CH3; overlaps with isopropyl
3
3
signals), 1.23 (dd, JPH = 17.7 Hz, JHH = 6.9 Hz, 12H, PCH(CH3)2),
3
3
1.13 (dd, JPH = 18.0 Hz, JHH = 7.1 Hz, 6H, PCH(CH3)2), 1.10 (dd,
X-ray Structural Analysis of Complex 2. Complex 2 was
crystallized at −20 °C from an acetone solution overlaid with diethyl
ether. CCDC deposition number: 745236. Crystal data:
C25H46O2P2RhS2 + BF4, colorless needle, 0.12 × 0.10 × 0.08 mm3,
monoclinic, P21/c, a = 8.8444(6) Å, b = 21.2018(14) Å, c =
17.7823(12) Å, β = 101.268(2)°, from 27° of data, T = 100(2) K, V =
3270.2(4) Å3, Z = 4, fw = 694.40 g mol−1, Dc = 1.410 g cm−3, μ =
0.791 mm−1. Data collection and processing: Bruker APEX-II
KappaCCD diffractometer, Mo Kα (λ = 0.71073 Å), graphite
monochromator, equipped with Miracol optics. 38418 reflections
collected, −12 ≤ h ≤ 9, −30 ≤ k ≤ 26, −25 ≤ l ≤ 25, 2θmax = 61.1°,
(R-int = 0.0438). The data were processed with SAINT.26 Solution
and refinement: Structure was solved by direct methods with SHELXS
and refined with SHELXL-97 using the full-matrix least-squares
method based on F2.27 There are 350 parameters with no restraints,
final R1 = 0.0389 (based on F2) for data with I > 2σ(I) and R1 = 0.0563
for 10 079 reflections, goodness of fit on F2 = 1.070, largest electron
density peak = 1.493 e Å−3.
In Situ Synthesis of Complex 2 in Acetone-d6 and NMR
Monitoring of the Reaction Progress. To a solution of 7.0 mg
(0.013 mmol) of [Rh(COE)2(acetone)2]BF4 in 0.2 mL of acetone-d6
was added a solution of 5.2 mg (0.013 mmol) of ligand 1 in 0.5 mL of
acetone-d6. The resulting solution was stirred manually at room
temperature for a few seconds and then loaded into an NMR tube.
The sample was then placed in the NMR spectrometer, and 1H NMR
spectra were recorded at room temperature every 5 min. Gradual
formation of complex 2 was observed, whereas no C−H cleavage
products were detected. See above for NMR data of complex 2 in
acetone-d6.
3JPH = 17.7 Hz, JHH = 7.0 Hz, 6H, PCH(CH3)2). Assignment of the
3
1H NMR signals was confirmed by 13C−1H heteronuclear correlation.
Selected 13C{1H} NMR (126 MHz, THF-d8:CD2Cl2): −9.74 (bm, Rh-
CH3).
In Situ Synthesis of Complex 2 in CD3CN/Acetone-d6 and
NMR Monitoring of the Reaction Progress. To a solution of 5.8
mg (0.015 mmol) of ligand 1 in 0.3 mL of acetone-d6 was added 12
mg (0.272 mmol) of CD3CN. The resulting solution was then mixed
at room temperature with a solution containing 7.9 mg (0.015 mmol)
of [Rh(COE)2(acetone)2]BF4 in 0.5 mL of acetone-d6. The afforded
solution was then loaded into an NMR tube and placed in an NMR
1
spectrometer that was preheated to 60 °C. H NMR spectra of the
solution were then recorded at 60 °C every 10−15 min. Gradual
formation of complex 2 was observed, whereas no C−H cleavage
products were detected. The following NMR data for complex 2 were
collected after 1 h. 31P{1H} NMR (202 MHz, CD3CN:acetone-d6, 60
°C): 87.42 (s). 1H NMR (500 MHz, CD3CN:acetone-d6, 60 °C): 7.52
(m, 3JHH = 7.7 Hz, 2H, Ar-H), 7.20 (m, 3JHH = 7.7 Hz, 1H, Ar-H), 2.74
3
3
(m, JHH = 7.0 Hz, 2H, PCH(CH3)2), 2.64 (m, JHH = 7.0 Hz, 2H,
3
3
PCH(CH3)2), 1.31 (dd, JPH = 13.3 Hz, JHH = 7.3 Hz, 6H,
PCH(CH3)2), 1.28 (dd, JPH = 13.5 Hz, JHH = 7.2 Hz, 12H,
PCH(CH3)2), 1.19 (dd, JPH = 11.1 Hz, JHH = 7.2 Hz, 6H,
PCH(CH3)2), 1.15 (dd, JPH = 11.1 Hz, JHH = 7.2 Hz, 12H,
3
3
3
3
3
3
2
PCH(CH3)2), 1.11 (d, JRhH = 2.4 Hz, 3H, Rh-CH3).
Computational Methods. All calculations were carried out using
the Gaussian 03 software package.28 Geometry optimizations and
evaluation of harmonic frequencies were performed at the DFT level,29
using the PBE030 hybrid density functional in conjunction with the
PC-1 basis set. The latter consists of the SDD basis set31 with an
added f function for rhodium (exponent 1.062, the geometric mean of
the 2f exponents given by Martin and Sundermann32), together with
Jensen’s “polarization consistent” pc-1 basis set for the remaining
elements.33 This combination is of double-ζ plus polarization quality.
The model structures used for the calculations featured CH3 groups
instead of the iPr substituents on the phosphorus atoms. Test
calculations using the full experimental structures afforded results that
are qualitatively similar to the model systems. The accuracy of the
computational method in predicting the geometries of complexes was
validated by calculating the geometry of complex 2, for which the
crystal structure is known. All structures were fully optimized in the
gas phase and characterized as minima or transition states by
calculating the harmonic vibrational frequencies. The reaction
pathways for C−C and C−H cleavage were traced from the
corresponding transition states to the products and to the reactants
using the intrinsic reaction coordinate (IRC) method.34 The energetic
data are presented as free energies (ΔG) at 298.15 K and include
corrections for solvation and dispersion (see below).
Bulk solvent effects of the experimental acetone and methanol
media have been taken into account via the self-consistent reaction
field (SCRF) method, using the continuum solvation model COSMO
(conductor-like screening model) as it is implemented in Gaussian
03.35 In this model, the solvent is represented by an infinite dielectric
medium characterized by the relative dielectric constant of the bulk
solvent (ε = 20.7 for acetone), and the effective cavity occupied by the
solute in the solvent is calculated on the basis of the united atom
(UA0) topological model radii. Gas-phase optimized geometries were
used in single-point calculations at the COSMO level.
In Situ Synthesis of Complex 2 in CD3OD/CD2Cl2 and NMR
Monitoring of the Reaction Progress. To a solution of 7.3 mg
(0.014 mmol) of [Rh(COE)2(acetone)2]BF4 in 0.3 mL of CD3OD
was added a solution of 5.4 mg (0.014 mmol) of ligand 1 in 0.3 mL of
CD2Cl2. The resulting solution was stirred manually at room
temperature for a few seconds and then loaded into an NMR tube.
The sample was then placed in the NMR spectrometer, and 1H NMR
spectra were recorded at room temperature every 5 min. Gradual
formation of complex 2 was observed, whereas no C−H cleavage
products were detected. The following NMR data for complex 2 were
collected after 1.5 h. 31P{1H} NMR (162 MHz, CD3OD:CD2Cl2):
78.94 (s). 1H NMR (400 MHz, CD3OD:CD2Cl2): 7.24 (m, 3JHH = 7.7
Hz, 2H, Ar-H), 7.09 (m, 3JHH = 7.7 Hz, 1H, Ar-H), 2.55 (m, 3JHH = 7.0
3
Hz, 2H, PCH(CH3)2), 2.48 (m, JHH = 7.0 Hz, 2H, PCH(CH3)2),
1.31−1.24 (m, 12H, PCH(CH3)2), 1.21 (s, 3H, Rh-CH3; overlaps with
3
3
isopropyl signals), 1.17 (dd, JPH = 17.8 Hz, JHH = 7.1 Hz, 6H,
3
3
PCH(CH3)2), 1.17 (dd, JPH = 17.6 Hz, JHH = 7.1 Hz, 12H,
1
PCH(CH3)2). Assignment of the H NMR signals was confirmed by
13C−1H heteronuclear correlation. Selected 13C{1H} NMR (126 MHz,
1
CD3OD:CD2Cl2): −8.74 (d, JRhC = 26.0 Hz, Rh-CH3).
In Situ Synthesis of Complex 2 in THF-d8/CD2Cl2 and NMR
Monitoring of the Reaction Progress. To a suspension of 10.5 mg
(0.015 mmol) of [Rh(COE)2Cl]2 in 0.45 mL of THF was added a
solution of 5.7 mg (0.029 mmol) of AgBF4 in 0.45 mL of THF, and
the resulting mixture was stirred in the dark, at room temperature, for
30 min. This mixture was then filtered via a cotton pad to afford a clear
orange solution. The solvent was then removed under vacuum, and the
resulting residue was dissolved in 0.3 mL of THF-d8. To this solution
was then added a solution of 11.3 mg (0.029 mmol) of ligand 1 in 0.3
mL of CD2Cl2, and the resulting solution was stirred manually at room
temperature for a few seconds and then loaded into an NMR tube.
The sample was then placed in the NMR spectrometer, and 1H NMR
spectra were recorded at room temperature every 5 min. Gradual
formation of complex 2 was observed, whereas no C−H cleavage
products were detected. The following NMR data for complex 2 were
collected after 3.5 h. 31P{1H} NMR (162 MHz, THF-d8:CD2Cl2):
Dispersion interactions within the computed structures were also
taken into account. These weak interactions are usually poorly
described by DFT methods, but can amount to 10−20 kcal mol−1 for
dissociation and atomization energies of large systems.36 In the present
work, these interactions were included by adding an empirical
dispersion correction term, as was proposed by Grimme and co-
workers,36a−c with a value of s6 = 0.7. This value was suggested by
Karton et al. for the PBE0 functional.36d For the problem at hand, the
1
3
77.45 (bs). H NMR (400 MHz, THF-d8:CD2Cl2): 7.35 (m, JHH
=
510
dx.doi.org/10.1021/om201205y | Organometallics 2012, 31, 505−512

Organometallics
Article
largest effect of the dispersion correction was to increase the
interaction energy between 4′ and cis-2-butene with formation of 5′
from 20.1 to 26.0 kcal mol−1.
(3) Reviews on C−C activation in pincer systems: (a) van der
Boom, M. E.; Milstein, D. Chem. Rev. 2003, 103, 1759−1792.
(b) Milstein, D. Pure Appl. Chem. 2003, 75, 445−460. (c) Rybtchinski,
B.; Milstein, D. ACS Symp. Ser. 2004, 885, 70−85. (d) Rybtchinski, B.;
Milstein, D. In The Chemistry of Pincer Compounds; Morales-Morales,
D., Jensen, C. M., Eds.; Elsevier: Amsterdam, 2007; pp 87−105.
(e) Albrecht, M.; Lindner, M. M. Dalton Trans. 2011, 40, 8733−8744.
(4) C−C bond cleavage has been studied extensively in our group
using pincer ligands with phosphorus donor groups. For further
details, see: (a) Gozin, M.; Weisman, A.; Ben-David, Y.; Milstein, D.
Nature 1993, 364, 699−701. (b) Gozin, M.; Aizenberg, M.; Liou, S.-Y.;
Weisman, A.; Ben-David, Y.; Milstein, D. Nature 1994, 370, 42−44.
(c) Liou, S.-Y.; Gozin, M.; Milstein, D. J. Am. Chem. Soc. 1995, 117,
9774−9775. (d) Rybtchinski, B.; Vigalok, A.; Ben-David, Y.; Milstein,
D. J. Am. Chem. Soc. 1996, 118, 12406−12415. (e) Gandelman, M.;
Vigalok, A.; Shimon, L. J. W.; Milstein, D. Organometallics 1997, 16,
3981−3986. (f) Liou, S.-Y.; van der Boom, M. E.; Milstein, D. Chem.
Commun. 1998, 6, 687−688. (g) van der Boom, M. E.; Ben-David, Y.;
Milstein, D. Chem. Commun. 1998, 8, 917−918. (h) van der Boom, M.
E.; Liou, S.-Y.; Ben-David, Y.; Gozin, M.; Milstein, D. J. Am. Chem. Soc.
1998, 120, 13415−13421. (i) Rybtchinski, B.; Milstein, D. J. Am.
Chem. Soc. 1999, 121, 4528−4529. (j) van der Boom, M. E.; Ben-
David, Y.; Milstein, D. J. Am. Chem. Soc. 1999, 121, 6652−6656.
(k) van der Boom, M. E.; Kraatz, H.-B.; Hassner, L.; Ben-David, Y.;
Milstein, D. Organometallics 1999, 18, 3873−3884. (l) Cohen, R.; van
der Boom, M. E.; Shimon, L. J. W.; Rozenberg, H.; Milstein, D. J. Am.
Chem. Soc. 2000, 122, 7723−7734. (m) Gandelman, M.; Vigalok, A.;
Konstantinovski, L.; Milstein, D. J. Am. Chem. Soc. 2000, 122, 9848−
9849. (n) Rybtchinski, B.; Oevers, S.; Montag, M.; Vigalok, A.;
Rozenberg, H.; Martin, J. M. L.; Milstein, D. J. Am. Chem. Soc. 2001,
123, 9064−9077. (o) Gauvin, R. M.; Rozenberg, H.; Shimon, L. J. W.;
Milstein, D. Organometallics 2001, 20, 1719−1724. (p) Gandelman,
M.; Shimon, L. J. W.; Milstein, D. Chem.Eur. J. 2003, 9, 4295−4300.
(q) van der Boom, M. E.; Liou, S.-Y.; Shimon, L. J. W.; Ben-David, Y.;
Milstein, D. Inorg. Chim. Acta 2004, 357, 4015−4023. (r) Salem, H.;
Ben-David, Y.; Shimon, L. J. W.; Milstein, D. Organometallics 2006, 25,
2292−2300. (s) Gauvin, R. M.; Rozenberg, H.; Shimon, L. J. W.; Ben-
David, Y.; Milstein, D. Chem.Eur. J. 2007, 13, 1382−1393. (t) Frech,
C. M.; Shimon, L. J. W.; Milstein, D. Chem.Eur. J. 2007, 13, 7501−
7509.
Charge decomposition analysis (CDA)37,38 was applied for the
quantification of electron transfers at critical points along the potential
energy surfaces of the SCS system.
ASSOCIATED CONTENT
* Supporting Information
■
S
Crystallographic data (CIF) and optimized geometries for
computed structures (xyz coordinates). This material is
available free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*E-mail: david.milstein@weizmann.ac.il.
■
Present Address
§Department of Chemistry, University of North Texas, Denton,
TX 76203, USA.
Author Contributions
∥These authors contributed equally.
ACKNOWLEDGMENTS
■
This research was supported by the European Research Council
under the FP7 framework (ERC No 246837); by the Israel
Science Foundation; by the MINERVA Foundation; and by the
Kimmel Center for Molecular design. D.M. holds the Israel
Matz Professorial Chair of Organic Chemistry. I.E. gratefully
acknowledges financial support from the Ministry of Immigrant
Absorption, State of Israel.
REFERENCES
■
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́
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(7) The new ligand 1 was preparedin analogy to a literature
procedure. See: Kanbara, T.; Yamamoto, T. J. Organomet. Chem. 2003,
688, 15−19 . Detailsregarding the present synthetic methodolgy
aregiven in the Experimental Section.
(8) About 65% conversion was observed within 1 h at room
temperature for initial concentrations [1]0 = [[Rh(COE)2(acetone)2]
BF4]0 ≈ 20 mM. The degree of conversion was estimated on the basis
1
of H NMR signal integration, using the arene hydrogen signals.
(9) A complex of rhodiumwith an adventitious ketol ligand has been
previously reported byour group. See:
Rybtchinski, B.;
Konstantinovski, L.; Shimon, L. J. W.; Vigalok, A.; Milstein, D.
Chem.Eur. J. 2000, 6, 3287−3292.
(10) Complex 2 is sparingly soluble in neat THF, precluding NMR
monitoring of the reaction in this solvent, and therefore, dichloro-
methane was added to increase solubility. The reaction was carried out
at room temperature and monitored periodically by 1H NMR
spectroscopy.
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dx.doi.org/10.1021/om201205y | Organometallics 2012, 31, 505−512

Organometallics
Article
(11) Ligand 1 is sparingly soluble in neat methanol, and therefore,
the stability of the olefin adduct of 2′ is comparable to that of its BF4−
and CH2O adducts. Because, in the experimental system, the
concentration of the solvent acetone is much higher than that of the
olefin, the latter is easily displaced by the solvent, leading to the
acetone adduct.
dichloromethane was added to increase solubility. The reaction was
1
carried out at room temperature and monitored periodically by H
NMR spectroscopy.
(12) No reaction was observed when acetonitrile was used as a neat
solvent, and it was, therefore, added in a smaller amount to an acetone
solution containing [Rh(COE)2(acetone)2]BF4 and 1. Even under
these conditions, the reaction was prohibitively slow at room
temperature, and the solution was, therefore, heated at 60 °C and
monitored periodically by 1H NMR spectroscopy. At this temperature,
20% conversion was obtained within 1 h (for initial concentrations
[1]0 = [[Rh(COE)2(acetone)2]BF4]0 ≈ 20 mM).
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(22) The energy of interaction between the fragments of the complex
was calculated within the CDA scheme for their geometries and
interfragment distances adopted in the complex. The calculation does
not involve the relaxation energy of the fragments caused by
coordination.
(23) It should be noted that, in the experimental system, complex 2 is
not observed as the adduct of COE, but rather of acetone. Unlike 5′,
512
dx.doi.org/10.1021/om201205y | Organometallics 2012, 31, 505−512