Comparison of steric and electronic requirements for C-C and C-H bond activation. Chelating vs nonchelating case

Article abstract of DOI:10.1021/ja016126t

C-H bond activation was observed in a novel PCO ligand 1 (C₆H(CH₃)₃(CH₂OCH₃) (CH₂P(t-Bu)₂)) at room temperature in THF, acetone, and methanol upon reaction with the cationic rhod

Full text of DOI:10.1021/ja016126t

9064
J. Am. Chem. Soc. 2001, 123, 9064-9077
Comparison of Steric and Electronic Requirements for C-C and
C-H Bond Activation. Chelating vs Nonchelating Case
Boris Rybtchinski,† Stephan Oevers,† Michael Montag,† Arkadi Vigalok,†
Haim Rozenberg,‡ Jan M. L. Martin,*,† and David Milstein*,†
Contribution from the Department of Organic Chemistry and Chemical SerVices Unit,
Weizmann Institute of Science, 76100 RehoVot, Israel
ReceiVed May 2, 2001
Abstract: C-H bond activation was observed in a novel PCO ligand 1 (C6H(CH3)3(CH2OCH3)(CH2P(t-Bu)2))
at room temperature in THF, acetone, and methanol upon reaction with the cationic rhodium precursor, [Rh-
(coe)2(solv)n]BF4 (solv ) solvent; coe ) cyclooctene). The products in acetone (complexes 3a and 3b) and
methanol (complexes 4a and 4b) were fully characterized spectroscopically. Two products were formed in
each case, namely those containing uncoordinated (3a and 4a) and coordinated (3b and 4b) methoxy arms,
respectively. Upon heating of the C-H activation products in methanol at 70 °C, C-C bond activation takes
place. Solvent evaporation under vacuum at room temperature for 3-4 days also results in C-C activation.
The C-C activation product, ((CH3)Rh(C6H(CH3)2(CH2OCH3)(CH2P(t-Bu)2)BF4), was characterized by X-ray
-
crystallography, which revealed a square pyramidal geometry with the BF4 anion coordinated to the metal.
Comparison to the structurally similar and isoelectronic nonchelating Rh-PC complex system and computational
studies provide insight into the reaction mechanism. The reaction mechanism was studied computationally by
means of a two-layer ONIOM model, using both the B3LYP and mPW1K exchange-correlation functionals
and a variety of basis sets. Polarization functions significantly affect relative energetics, and the mPW1K
profile appears to be more reliable than its B3LYP counterpart. The calculations reveal that the electronic
requirements for both C-C and C-H activation are essentially the same (14e intermediates are the key ones).
On the other hand, the steric requirements differ significantly, and chelation appears to play an important role
in C-C bond activation.
Introduction
accessible sterically than C-C bonds, activation of the former
is normally kinetically more favorable. The higher degree of
C-C bond directionality (sp3-sp3) compared to that of C-H
(sp3-s) was also suggested to significantly contribute to the
higher kinetic barriers for C-C bond activation.5 An additional
thermodynamic driving force, such as strain relief or aromati-
zation, is generally required for C-C activation to take place.1
Nonetheless, it is possible to design a C-C activation system
in such a way that the reaction is driven by formation of strong
bonds and/or volatile products (such as CH4 or C2H4). We have
demonstrated that the PCP (i.e., symmetric chelated system with
two phosphine arms) and PCN (i.e., asymmetric chelated system
with one amino and one phosphino arm) ligand systems are
extremely efficient in the activation of strong, unstrained,
nonactivated C-C bonds, which are situated between the two
chelating arms of the ligand.6,7 Theoretical calculations,8 and a
kinetic study of a direct, single-step insertion of a neutral Rh(I)
complex into a strong C-C bond,9 reveal that the reaction
mechanism involves a nonpolar three-centered transition state.
Understanding the factors which control C-C vs C-H activa-
tion is of prime importance. Here we report on an experimental
and theoretical study of competitive carbon-carbon and carbon-
hydrogen bond activation processes in a novel PCO (i.e.,
asymmetric chelated system with one phosphine and one
Carbon-carbon and carbon-hydrogen bond activation by
transition metal complexes are topics of much current interest.
Metal-promoted activation of C-C and C-H bonds in ho-
mogeneous media under mild conditions may lead to the design
of new selective processes for the utilization of hydrocarbons.1,2
Many examples of both intra- and intermolecular C-H bond
activation are known, and mechanistic studies of these processes
have revealed that agostic intermediates and σ-alkane complexes
can be involved.2 Coordinative unsaturation is an important
requirement for C-H bond cleavage.3 Theoretical calculations
are in line with these findings and provide further insight into
the details of the reaction mechanism.4
Examples of carbon-carbon bond activation in solution are
scarce relative to those of carbon-hydrogen bond activation,
since, in general, kinetic and thermodynamic factors favor C-H
over C-C activation. Since C-H bonds are generally more
* To whom correspondence should be addressed. E-mail: comartin@
wicc.weizmann.ac.il, david.milstein@weizmann.ac.il.
† Department of Organic Chemistry.
‡ Chemical Services Unit.
(1) For recent reviews on C-C bond activation, see: (a) Rybtchinski,
B.; Milstein, D. Angew. Chem., Int. Ed. 1999, 38, 870. (b) Murakami, M.;
Ito, Y. In Topics in Organometallic Chemistry; Murai, S., Ed.; Springer-
Verlag: Berlin Heidelberg, 1999; Vol. 3, p 97.
(2) (a) Hall, C.; Perutz, R. N. Chem. ReV. 1996, 96, 3125, (b) Shilov, A.
E.; Shul’pin, G. B. Chem. ReV. 1997, 97, 2879.
(5) (a) Low J. J.; Goddard, W. A., III. J. Am. Chem. Soc. 1984, 106,
10779. (b) Blomberg, M. R. A.; Siegbahn, P. E. M.; Nagashima, U.;
Wennerberg, J. J. Am. Chem. Soc. 1991, 113, 424. (c) Siegbahn, P. E. M.
Blomberg, M. R. A. J. Am. Chem. Soc. 1992, 114, 10548.
(3) Milstein, D. Acc. Chem. Res. 1984, 17, 221.
(4) For the review see: Niu, S.; Hall, M. B. Chem. ReV. 2000, 100, 353.
See also: Saillard, J.-Y.; Hoffman R. J. Am. Chem. Soc. 1984, 106, 2006.
10.1021/ja016126t CCC: $20.00 © 2001 American Chemical Society
Published on Web 08/25/2001

(Non)Chelating CC Vs CH ActiVation
J. Am. Chem. Soc., Vol. 123, No. 37, 2001 9065
methoxy arm) ligand system involving a cationic Rh(I) complex.
Our studies clearly show that the methoxy moiety, although
being a relatively weak ligand, plays a critical role in the C-C
bond activation process: this demonstrates that chelation of the
methoxy group to the metal center is fundamental to the
chemistry of C-C activation in this system. Coordinated solvent
molecules also play an important role in the activation processes.
Remarkably, we show that the reaction can be controlled not
only by the coordinating ability of the solvent, but merely by
the presence or absence of the solVent in the reaction. In the
presence of a coordinating solvent (methanol) only C-H bond
activation takes place, while C-C bond activation can be
selectively achieved simply by solvent evaporation. Theoretical
calculations have provided important insight into the reaction
mechanism. It was found that cationic 14-electron Rh(I)
complexes are key intermediates in both C-H and C-C
activation and that the chelating effect facilitates C-C activation
both kinetically and thermodynamically. A comparison of the
transition states for C-C and C-H activation indicates that
specific steric requirements are important for achieving metal
insertion into the C-C bond. Another important finding is that
coordination of solvent molecules (methanol or ether) to a
cationic Rh(I) metal center can significantly lower the kinetic
barrier of C-H as well as C-C activation.
Scheme 1
momethyl-3-methoxymethyl-2,4,6-trimethylbenzene. This com-
pound was then reacted with di-tert-butylphosphine to yield the
corresponding phosphonium salt. Treatment of the salt with
sodium acetate yielded the novel PCO phosphine 1, which was
purified by chromatography on a silica column.
Carbon-Hydrogen Bond Activation. Cationic rhodium(I)
precursors, [Rh(coe)2(solv)n]BF4 (solv ) solvent; coe ) cy-
clooctene), were obtained by chloride abstraction from [Rh-
(coe)2Cl]2 according to a known procedure.12 When [Rh(coe)2-
(solv)n]BF4 was reacted at room temperature with the ligand 1
in THF, acetone, or methanol, quantitative formation of the
C-H activation products 2, 3, and 4, respectively, was observed
within 1 h (Scheme 2).
Experimental Results
Ligand Design and Synthesis. In our quest for a better
understanding of the factors which are important for C-C bond
activation, we planned to study the significance of the chelating
effect. The novel PCO-type ligand 1 was designed to create a
C-C activating system in which one of the chelating moieties
is labile. The mono-chelating PC system studied by us appeared
to be inactive in C-C bond activation.10 The PCO ligand system
mimics the electron density environment and general structure
of the mono-chelating PC system, but it potentially enables both
mono- and bis-chelating binding modes. Thus, the PCO ligand
system may reveal the impact of the chelate effect as a
controlling factor in C-C vs C-H bond activation.
Ligand 1, 1-methoxymethyl-3-(di-tert-butylphosphino)methyl-
2,4,6-trimethylbenzene, was synthesized (Scheme 1) from 1,3-
bis(bromomethyl)-2,4,6-trimethylbenzene, which was prepared
according to a literature procedure.11 Reaction of the dibromide
with NaOMe produced the mono-methoxy intermediate, 1-bro-
Complexes 3 and 4 are stable, while 2 gradually decomposes
in THF. 2, 3, and 4 immediately decompose when dissolved in
noncoordinating solvents, such as benzene. NMR spectra of a
THF solution of 2 indicate the formation of a C-H activation
product, which gives rise to a broad doublet at 125.5 ppm (1JRhP
) 193.9 Hz) in 31P{1H} NMR; the hydride signal appears as a
broad signal at -27.0 ppm in 1H NMR, more detailed
characterization being impeded by decomposition. In acetone
or methanol, two different C-H activation products are formed,
namely an “open-arm” (noncoordinated methoxy) and a “closed-
arm” (coordinated methoxy) species. In acetone-d6, the 31P{1H}
NMR spectrum exhibits two signals: a doublet at 130.4 ppm
(1JRhP ) 192.2 Hz) and a doublet at 126.5 ppm (1JRhP ) 191.5
1
Hz), with an integrated peak area ratio of 4.4. In the H NMR
spectrum, two signals due to the hydride ligand bonded to
rhodium appear: a broad signal at -24.70 ppm and a doublet
of doublets at -24.89 ppm (1JRhH ) 27.2 Hz, 2JPH ) 18.1 Hz).
The methylene hydrogens of the methoxy arm, which is
coordinated to the metal, are diastereotopic and appear as an
AB quartet, the splitting pattern being typical of a sidearm
methylene involved in chelation in a bis-chelating pincer system,
such as PCP and PCN.7 Thus, the ratio between the “open-
arm” and “closed-arm” structures (complexes 3a and 3b,
respectively) is most clearly evaluated from the 1H NMR
spectrum by observing the ratio between the AB quartets
comprising two doublets at 5.0 and 4.6 ppm (2JHH ) 12.5 Hz),
and representing the “closed-arm” form, complex 3bsand the
singlet at 4.4 ppm, which represents the “open-arm” form,
complex 3a. The observed ratio of 3b:3a ) 4.4 indicates that
the “closed-arm” form 3b prevails in acetone. The main spectral
features of complex 4 (in methanol) are similar to those of 3
(in acetone). In methanol, the “open-arm” complex prevails,
resulting in a ratio of 4a:4b ) 1.5, in agreement with theoretical
calculations (vide infra).
(6) (a) Gozin, M.; Weisman, A.; Ben-David, Y.; Milstein, D. Nature
1993, 364, 699. (b) Gozin, M.; Aizenberg, M.; Liou, Sh.-Y.; Weisman, A.;
Ben-David, Y.; Milstein, D. Nature 1994, 370, 42. (c) Liou, Sh.-Y.; Gozin,
M.; Milstein, D. J. Am. Chem. Soc. 1995, 117, 9774. (d) Liou, Sh.-Y.; Gozin,
M.; Milstein, D. J. Chem. Soc., Chem. Commun. 1995, 1965. (e) van der
Boom, M. E.; Kraatz, H.-B.; Ben-David, Y.; Milstein, D. J. Chem. Soc.,
Chem. Commun. 1996, 2167. (f) van der Boom, M. E.; Liou, Sh.-Y.; Ben-
David, Y.; Milstein, D. J. Am. Chem. Soc. 1998, 120, 13415. (g) Liou,
Sh.-Y.; van der Boom, M. E.; Milstein, D. Chem. Commun. 1998, 687. (h)
van der Boom, M. E.; Ben-David, Y.; Milstein, D. Chem. Commun. 1998,
917. (i) van der Boom, M. E.; Kraatz, H.-B.; Hassner, L.; Ben-David, Y.;
Milstein, D. Organometallics 1999, 18, 3873. (j) Cohen, R.; van der Boom,
M. E.; Shimon, L. J. W.; Rozenberg, H.; Milstein, D. J. Am. Chem. Soc.
2000, 122, 7723. (k) Rybtchinski, B.; Milstein, D. J. Am. Chem. Soc. 1999,
121, 4528. (l) Gauvin, R. M.; Rozenberg, H.; Shimon, L. J. W.; Milstein,
D. Organometallics 2001, 20, 1719.
(7) (a) Rybtchinski, B.; Vigalok, A.; Ben-David, Y.; Milstein, D. J. Am.
Chem. Soc. 1996, 118, 12406. (b) Gandelman, M.; Vigalok, A.; Shimon,
L. J. W.; Milstein, D. Organometallics 1997, 16, 3981.
(8) Sundermann, A.; Uzan, O.; Milstein, D.; Martin, J. M. L. J. Am.
Chem. Soc. 2000, 122, 7095.
(9) Gandelman, M.; Vigalok, A.; Milstein, D. J. Am. Chem. Soc. 2000,
117, 9774.
(10) Rybtchinski, B.; Konstantinovsky, L.; Shimon, L. J. W.; Vigalok,
A.; Milstein D. Chem. Eur. J. 2000, 17, 3287.
Two “open-arm” C-H activation products are in principle
possible, resulting from the activation of the two different methyl
(11) van der Made, A. W.; van der Made, R. H. J. Org. Chem. 1993,
58, 1262.

9066 J. Am. Chem. Soc., Vol. 123, No. 37, 2001
Rybtchinski et al.
Scheme 2
groups: one structure in which the methoxy arm is located at
the ortho position to the methylene bridge, and the other in
which it is located at the para position to the bridge. In the
former case the “open-arm” form is expected to be in equilib-
rium with the “closed-arm” one, because the methoxy arm is
in close proximity to the rhodium center. On the other hand,
the latter structure has the methoxy arm on the opposite side of
the aromatic ring. Hence, for this “open-arm” form to be in
equilibrium with the other “open-arm” speciesslet alone the
“closed-arm” formsit is necessary for the methylene bridge to
be completely detached from the metal center (via reductive
elimination). Only then can the aromatic ring rotate about the
Ar-CH2P bond, permitting the methoxy moiety to come within
coordination distance.
The identity of 4a in methanol was determined by spin
saturation transfer (SST) experiments, performed at room
temperature. When the hydride ligand of 4a was irradiated, no
spin saturation transfer was detected. On the other hand, it was
found by SST that the hydride ligand of the “open-arm” form
exchanges with one of the methylene bridge hydrogens,10 while
no exchange takes place with the other methyl groups of the
PCO ligand. This means that no full detachment of the C-H
activated methyls occurs in both systems. Furthermore, as shown
by irradiating the methylene hydrogens of the methoxy arm,
no hydrogen exchange appears to take place between the
methoxy arms of the “open-arm” and “closed-arm” forms. This
indicates that there is no equilibrium between the two species
on the time scale of the experiment. Thus, once C-H activation
takes place, the products 4a and 4b do not interconvert. It is
therefore concluded that the “open-arm” structure consistent with
the above observations is 4a, in which the methoxy arm is
located para to the methylene bridge. The ratio between the
“open-arm” and “closed-arm” forms is likely to be determined
by the competition between the methoxy arm and the solvent
with respect to coordination to the metal center and by the barrier
for C-H activation with these species. Complex 4a, with its
methoxy arm in the para position, very closely resemblessboth
in structure and in behaviorscomplex 5 which was recently
reported by us.10
SST experiments performed on a methanol solution of
complex 5 showed results very similar to those obtained for
complex 4a. Exchange between the hydride ligand and the
methylene bridge hydrogens of complex 5 was found to occur
at room temperature, whereas exchange with the other methyl
group occurred only at higher temperatures (above 35 °C in
methanol). This was attributed to agostic bonding between the
metal and C-H bonds of the methyl group formed upon
reductive elimination, preventing complete detachment of this
group at room temperature. Thus, C-H activation of the two
methyl groups ortho to the phosphine arm in complex 4 to
produce compounds 4a,b, which are stable at room temperature
and do not interconvert, is expected, on the grounds of the
similarity of complex 4 to complex 5.
Carbon-Carbon Bond Activation. When a solution of the
C-H activation products 4 in methanol is heated to about 70
°C, the complexes are transformed into the C-C activation
product 6. The process is rather rapid, with 50% conversion
taking place within 15 min (determined by 31P{1H} NMR).
However, prolonged heating results in considerable decomposi-
tion, such that the reaction is not quantitative (85% yield by
31P{1H} NMR). Formation of the C-C activation products very
likely takes place also in acetone and THF after heating, but
the reaction gives rise to many decomposition products, and
appropriate characterization could not be accomplished. The 31P-
{1H} NMR spectrum of 6 in methanol exhibits only one signal,
a doublet at 90.2 ppm (1JRhP ) 190.8 Hz), in contrast to the
(12) Schrock, R. R.; Osborn, J. A. J. Am. Chem. Soc. 1971, 93, 3089.

(Non)Chelating CC Vs CH ActiVation
J. Am. Chem. Soc., Vol. 123, No. 37, 2001 9067
two low-field signals of the C-H activation product mixture.
1
The H NMR spectrum shows a doublet at 1.02 ppm (2JRhH
)
2.3 Hz), which corresponds to the hydrogen atoms of the methyl
bound to rhodium. This moiety also gives rise to a doublet of
doublets in the 13C{1H} NMR spectrum, located at -4.55 ppm
2
(1JRhC ) 32.0 Hz, JPC ) 8.3 Hz). The same spectrum also
shows a doublet of doublets at 157.02 ppm (1JRhC ) 35.7 Hz,
2JPC ) 4.7 Hz), which corresponds to the aromatic carbon atom
to which the metal is bound (the ipso carbon atom). The
difference between complexes 4 and 6 is also highlighted by
electrospray mass spectrometry. Although 4 and 6 exhibit a
molecular cation peak at practically the same location, with m/e
425.37 (no solvent or BF4 is present), their fragmentation pattern
is markedly different. The spectrum of 6 shows several
significant peaks, the most notable of which is at m/e 411.35,
with over 50% intensity, indicating loss of a methyl group. In
the mass spectrum of 4, on the other hand, the molecular peak
is the only significant one. The structure of complex 6 was
confirmed by an X-ray structural study (vide infra). The fact
that heating converts the C-H activation complex 4 into the
C-C cleavage product 6, and that continued heating increases
the ratio of C-C to C-H activation products, clearly indicates
that the C-C activation product is the thermodynamically more
stable of the two complexes. On the other hand, the fact that
reaction between [Rh(coe)2(solv)n]BF4 and ligand 1 yields only
C-H activation products at room temperaturesin all solvents
usedsimplies that C-H activation is kinetically favored over
C-C activation. Thus, in the Rh-PCO system selective
formation of kinetic C-H activation products takes place at
room temperature, while selective rhodium insertion into a C-C
bond can be achieved under mild heating. Significantly, whereas
C-H activation of the two methyl groups is observed, only the
C-C bond between the chelating arms is activated, demonstrat-
ing that the chelating methoxy ligand is both essential and
sufficient for C-C activation in this system.
Figure 1. ORTEP plot of the X-ray structure of the closed-arm CC
activation product with BF4 counterion, 6.
-
site, with a Rh-CH3 bond length of 2.02 Å. The aromatic ring
is linked to the rhodium center via the ipso carbon atom, with
a Rh-CAr bond length of 1.95 Å. The chelating sidearms of
the PCO ligand are coordinated to the metal atom through
oxygen and phosphorus, with bond lengths of 2.15 Å for Rh-O
and 2.22 Å for Rh-P. Finally, the position trans to the ipso
carbon is occupied by BF4-, which is coordinated to the rhodium
atom via one of the fluorine atoms, F(3). The length of the Rh-
F(3) bond is 2.27 Å. Furthermore, the existence of this bond is
supported by the fact that the B-F(3) bond is longer than the
other B-F bonds: 1.44 vs 1.36-1.38 Å, respectively. To the
best of our knowledge, no crystal structures containing Rh-
FBF3 bonds have been reported.13 Such a bond between the
-
rhodium atom and the weakly coordinating anionic ligand BF4
is not common.13,14
Comparison of the PCO and PC Ligand Systems. Impor-
tant insight into the mode of reactivity of the PCO-Rh system
is obtained by a comparison to the PC complex 5 (vide supra).
As mentioned above, the latter complex is very similar sterically
and electronically to the “open-arm” form of the PCO complex.
However, the PC system differs markedly from the PCO
regarding bond activation. In the PC system, only C-H bond
activation takes place, resulting in complex 5; this process is
reversible. In striking contrast with the PCO system, the C-H
activation product 5 is stable toward evaporation of the solvent
and is not transformed into a C-C activation product. Further-
more, no C-C bond activation was observed upon heating of
a solution of 5 in THF, acetone, or methanol. The exclusive
activation of the C-C bond situated between the phosphine and
methoxy arm and the absence of C-C activation in the PC
system suggest that chelation is crucial for C-C bond actiVa-
tion. Significantly, the methoxy group, although weakly coor-
dinating in comparison to phosphines and amines, promotes
C-C bond activation when it is involved in chelation. Appar-
ently, the chelate effect influences both the kinetics and
thermodynamics of the process. An important kinetic factor is
the bonding of the metal center by the two ligand arms, which
brings the metal into close proximity to the C-C bond to be
cleaved. The resulting C-C activation product is then signifi-
cantly stabilized by the presence of two five-membered chelate
rings, creating a thermodynamic driving force for the reaction.
An attempt to isolate the C-H activation products led to a
very intriguing observation: removal of the solvent from a
solution of the C-H activation complexes 2-4 under vacuum
for several days resulted in conversion of the C-H to the C-C
activation product. This result, which has not been observed
elsewhere so far, suggests that the system is very sensitive to
the presence of a solvent. Our X-ray structural studies and
computational results suggest that this is due to a special effect
-
of BF4 anion coordination, which takes place upon solvent
evaporation (vide infra). An interesting result supporting the
effect of solvent removal was obtained in the presence of
ethylene glycol. As in the case of methanol, heating an acetone
solution of the C-H activation products at 70-80 °C in the
presence of ethylene glycol is required in order to achieve C-C
activation. However, upon evaporation, C-C bond activation
does not take place. Ethylene glycol was observed to remain
coordinated to the metal center after the evaporation under high
vacuum, most probably due to its chelation to the metal. It is
known that ethylene glycol can be a chelating ligand for cationic
rhodium centers.10
X-ray Crystallographic Study of Complex 6. Crystals of
complex 6 suitable for X-ray analysis were grown from an ether
extract of the dry complex 6 at room temperature. The rhodium
atom is centered at the base of a slightly distorted square
pyramid, with the methyl group at the apex (Figure 1). The
C(1)-Rh-X bond angles are close to 90° (X ) C(3), O(4),
F(3) or P(2)): C(1)-Rh-C(3) ) 88.71°, C(1)-Rh-O(4) )
92.93°, C(1)-Rh-F(3) ) 89.92°, C(1)-Rh-P(2) ) 97.29°).
The methyl group is situated trans to the empty coordination
(13) For an example of a Rh-FBF3 bond characterized in solution, see:
Sutherland, B. R.; Cowie, M. Inorg. Chem. 1984, 23, 1290.
(14) Beck, W.; Su¨nkel, K. Chem. ReV. 1988, 88, 1405-1421.

9068 J. Am. Chem. Soc., Vol. 123, No. 37, 2001
Rybtchinski et al.
Scheme 3
Further insight regarding these factors was obtained from the
computational study of the Rh-PCO system.
at the “bottom of the well” (∆Ee) at all three levels of theory
haven been given in Figures 2, 3, and 4, while ∆H298 and ∆G298
profiles at our best level of theory (mPW1K/lanl2dz+p) have
been given in Figures 5, 6, and 7, respectively.
Computational Results
The Intermediate Structures. The reaction of the PCO
ligand with the mononuclear cationic Rh(I) olefin complex
results in ligand substitution which takes place prior to the bond
activation. Of the four possible entry channels A, A′, B, and B′
which could act as common intermediates, we found the Rh(I)
complexes A and A′ to be more stable than their agostic
counterparts B and B′. The local minima A and A′ are common
intermediates, from which both C-H and C-C activation are
possible. In the “closed-arm” intermediate A′, a tricoordinate
(T-shaped) rhodium is found, with the phosphine and methoxy
ligands acting as the two “arms” of the T. The complex is
slightly stabilized by a weak η1 interaction with the ipso carbon
of the phenyl ring. In the “open-arm” intermediate A, only the
phosphine “arm” is present, but the rhodium is η2 bound to the
ring, specifically to the ipso carbon and the ortho carbon on
the phosphorus side. Some weak interaction, in addition, exists
with the facing ortho and meta carbons, but both the Wiberg
bond orders and an electron density plot (see Supporting
To understand the reactivity of the PCO system with the
cationic rhodium(I) precursor, which at room temperature
selectively cleaves the C-H bond in the “open-arm” and
“closed-arm” systems and leads exclusively to C-C cleavage
in the “closed-arm” system in the absence of solvent or upon
heating, we explored a variety of reaction pathways.
Possible Reaction Pathways without Solvent Ligands. For
a reaction pathway involving the C-H and C-C products, we
found 26 stationary points on the energy hypersurface (Scheme
3). Computed barrier heights, reaction energies, and ligand
stabilization energies at all levels of theory considered by us
are given in Tables 1, 2, and 3, respectively. Our discussion
here will be in terms of the ONIOM(mPW1K/lanl2dz+p:
mPW1K/lanl2dz) values, which we deem to be the most reliable.
(See below for a comparison of the different levels of theory.)
Unless stated otherwise, we will be referring to free energies at
room temperature (∆G298). For easy reference, reaction profiles

(Non)Chelating CC Vs CH ActiVation
J. Am. Chem. Soc., Vol. 123, No. 37, 2001 9069
Table 1. Computed Reaction Barrier Heights (kcal‚mol-1
)
B3LYP/lan2dz
B3LYP/lanl2dz+p
mPW1K/lanl2dz+p
a
∆E#
∆E#
∆H#
∆G#
∆E#
∆E#
∆G#
e
0
298
298
e
e
298
Pathways without MeOH Spectator Ligands
P
CCfA
35.5
26.9
1.7
35.6
25.2
1.6
13.5
9.7
35.0
27.7
14.4
8.3
25.7
15.4
4.3
35.2
25.1
1.1
13.0
9.2
34.5
27.7
14.0
7.5
36.5
25.3
2.7
14.1
11.0
36.1
27.3
14.4
9.8
31.0
25.6
1.9
26.5
27.5
Af PCC
Af A′
30.9
0.6
29.3
1.6
A′fA
13.9
10.9
35.1
28.6
16.9
10.5
26.5
16.1
4.2
8.0
5.3
5.5
A′fP′CC
P′CCfA′
11.3
32.3
24.0
16.0
11.2
24.4
15.6
3.5
12.3
28.1
19.4
18.5
9.8
12.4
29.1
18.1
16.1
9.1
P
CHfA
AfPCH
A′fP′CH
P′CHf A′
AfB
25.5
15.1
4.0
25.8
15.6
5.1
19.8
16.5
4.7
19.0
16.1
5.7
BfA
B′fA′
A′fB′
24.0
2.3
22.9
2.1
22.4
1.7
23.2
2.8
18.8
2.4
18.9
2.5
18.1
3.0
Pathways with One MeOH Spectator Ligand
P
CCfA
32.6
24.5
3.4
32.4
23.1
3.7
12.2
9.3
31.1
25.6
9.8
32.0
22.9
3.0
33.2
23.5
5.7
12.7
10.4
32.6
24.6
10.2
6.7
29.7
26.2
3.1
25.4
30.4
2.7
26.1
29.4
5.0
Af PCC
Af AA′
AA′fA
A′fP′CC
C′CCfA′
12.6
10.5
30.7
26.3
12.0
8.1
11.7
8.8
30.4
26.1
9.3
5.3
18.0
6.8
5.5
5.6
11.0
28.5
23.9
15.0
10.2
17.8
12.5
25.1
19.4
16.9
10.8
14.7
12.4
27.0
17.7
15.1
9.3
P
CHfA
AfPCH
A′fP′CH
P′CHfA′
6.0
18.3
18.7
18.9
14.9
Pathways with Two MeOH Spectator Ligands
P
CCfA
Af PCC
CHfA
AfPCH
27.2
18.2
18.4
4.1
27.2
16.8
17.9
2.0
26.7
16.8
17.8
1.7
28.4
17.4
18.0
2.3
24.5
17.3
16.1
3.9
21.3
20.2
13.5
5.5
22.5
19.4
13.1
3.7
P
a Using the additivity approximation ∆G#298[mPW1K] ≈ ∆E# [mPW1K/lanl2dz+p] + ∆G#298[B3LYP/lanl2dz] - ∆E# [B3LYP/lanl2dz].
e
e
Table 2. Computed Reaction Energies with Respect to the Relevant Intermediates (kcal‚mol-1
)
B3LYP/lan2dz
∆E0 ∆H298
Without MeOH Spectator Ligands
B3LYP/ lanl2dz+p
∆Ee
mPW1K/lanl2dz+p
∆Ee
∆G298
∆Ee
∆G298
PCC-A
PCH-A
B-A
-8.6
-11.7
11.8
-12.2
-24.2
-16.1
21.7
-10.4
-13.3
11.1
-10.1
-13.6
11.1
-11.9
-25.3
-18.0
20.7
-11.2
-12.8
10.5
-11.5
-25.1
-16.0
20.4
-5.4
-8.0
12.0
-6.2
-21.0
-13.2
16.5
4.3
-0.9
11.8
1.8
-2.0
10.4
A-A′
-11.9
-25.3
-17.4
20.9
-4.6
-15.8
-9.9
16.4
-4.0
-16.7
-9.9
15.1
P′CC-A′
P′CH-A′
B′-A′
With One MeOH Spectator Ligand
PCC-A + 1MeOH
PCH-A + 1MeOH
B-A + 1MeOH
A-A′ + 1MeOH
P′CH-A′ + 1MeOH
P′CC-A′ +1MeOH
B′-A′ + 1MeOH
-8.1
-14.4
6.1
-9.2
-10.5
-20.2
13.3
-9.3
-15.8
6.5
-9.1
-16.7
6.2
-8.7
-12.6
-21.7
12.8
-9.7
-14.5
6.9
-7.1
-12.2
-22.3
12.5
-3.4
-8.9
7.9
-3.7
-7.6
-17.4
9.9
5.0
-2.5
10.3
-2.8
-4.0
-12.5
11.2
3.3
-2.6
11.0
-0.6
-5.6
-14.6
10.3
-8.5
-12.3
-21.8
13.0
With Two MeOH Spectator Ligands
PCC-A + 2MeOH
-8.9
-14.2
-15.4
-10.4
-16.0
-12.9
-9.9
-16.1
-13.4
-11.0
-15.7
-1.2
-7.2
-12.2
-8.8
-1.0
-8.0
-8.8
-3.1
-9.4
5.4
PCH-A + 2MeOH
A(MeOH)2 f A′(MeOH) + MeOH
a Using the additivity approximation: ∆G#298[mPW1K] ≈ ∆E# [mPW1K/lanl2dz+p] + ∆G#298[B3LYP/lanl2dz] - ∆E# [B3LYP/lanl2dz].
e
e
Information) reveal that the interaction with the phenyl ring
predominantly has η2 rather than η4 character. A is found to be
4.0 kcal‚mol-1 less stable than A′ because of missing stabiliza-
tion by the methoxy coordination. Perspective views of struc-
tures A and A′, with and without added MeOH ligands (vide
infra), can be found in Figure 8.
η1 and η2 coordination in the PCO system are greatly favored
over the formation of an agostic bond. The C-H agostic
intermediates B and B′ are destabilized by 10.4 kcal‚mol-1 for
the open-arm system and 15.1 kcal‚mol-1 for the closed-arm
system. In both cases, aside from the C-H agostic interaction,
only the phosphine “arm” is attached to the rhodium. Since no
interaction between the methoxy group and the rhodium could
be found in either case, both C-H agostic intermediates are
almost isoergonic (∆∆G ) 0.6 kcal‚mol-1). Apparently, the
methoxy “arm” in B′ is too short to permit an optimal interaction
of the chelated rhodium atom with the C-H bonds of the methyl
group. Because the C-OMe as well as the Rh-O bonds are
shorter than the corresponding bonds in the phosphine “arm”,
the rhodium is forced to be close to the phenyl ring, and
therefore the tricoordinate (T-shape) η1 complex is favored in
intermediate B′. Intermediate B favors the η2 coordination for

9070 J. Am. Chem. Soc., Vol. 123, No. 37, 2001
Rybtchinski et al.
Table 3. Computed Association Energies (kcal‚mol-1) for One and Two MeOH Spectator Ligands
B3LYP/lanl2dz
B3LYP/lan2dz+p
mPW1K/lanl2dz+p
∆Ee
∆E0
∆H298
∆G298
∆Ee
∆Ee
∆G298
First Association Energies
TS-APCC + 1MeOH
TS-APCC + 1MeOH
A + 1MeOH
-25.1
-28.1
-25.7
-30.6
-28.4
-31.4
-18.7
-23.1
-22.7
-25.1
-17.2
-24.0
-33.4
-22.7
-26.0
-23.9
-28.5
-26.4
-28.5
-17.0
-20.9
-20.5
-22.8
-15.4
-21.8
-30.4
-22.8
-26.0
-23.7
-28.5
-26.9
-28.7
-16.9
-21.0
-20.6
-22.7
-15.2
-21.8
-30.1
-11.8
-15.1
-13.3
-17.6
-14.9
-16.9
-6.1
-19.5
-20.8
-21.5
-22.5
-22.4
-25.7
-15.5
-19.3
-19.0
-20.0
-13.3
-20.2
-27.9
-22.4
-23.5
-23.1
-24.7
-24.7
-24.6
-17.9
-21.0
-21.2
-20.3
-15.2
-21.0
-29.0
-9.1
-10.5
-10.7
-11.7
-11.2
-10.1
-5.2
TS-APCH + 1MeOH
PCH + 1MeOH
B + 1MeOH
P′CC + 1MeOH
TS-P′P′CC + 1MeOH
A′ + 1MeOH
-9.5
-7.4
-8.9
-7.3
TS-A′P′CH + 1MeOH
P′CH + 1MeOH
-12.0
-5.1
-7.2
-3.1
TS-AA′ + 1MeOH
B′ + 1MeOH
-10.2
-19.6
-7.3
-15.1
Second Association Energies
PCC + 2MeOH
-16.3
-21.7
-15.4
-23.3
-15.3
-14.0
-19.2
-12.9
-20.8
-13.1
-14.2
-19.5
-13.4
-21.0
-12.7
-2.4
-7.2
-1.2
-9.0
-2.4
-12.5
-17.7
-8.8
-19.9
-12.1
-14.9
-19.0
-8.8
-20.3
-14.3
-1.0
-4.6
5.4
-6.0
-1.4
TS-APCC + 2MeOH
A + 2MeOH
TS-APCH + 2MeOH
PCH + 2MeOH
a Using the additivity approximation ∆G#298[mPW1K] ≈ ∆E# [mPW1K/lanl2dz+p] + ∆G#298[B3LYP/lanl2dz] - ∆E# [B3LYP/lanl2dz].
e
e
Figure 2. B3LYP/lanl2dz computed ∆Ee reaction profiles for compet-
ing closed-arm and open-arm C-C and C-H activation routes with
and without MeOH spectator ligands.
Figure 4. mPW1K/lanl2dz+p computed ∆Ee reaction profiles for
competing closed-arm and open-arm C-C and C-H activation routes
with and without MeOH spectator ligands.
Figure 3. B3LYP/lanl2dz+p computed ∆Ee reaction profiles for
competing closed-arm and open-arm C-C and C-H activation routes
with and without MeOH spectator ligands.
Figure 5. mPW1K/lanl2dz+p computed ∆H298 reaction profiles for
competing closed-arm and open-arm C-C and C-H activation routes
with and without MeOH spectator ligands.
electronic reasons. More electron density can thus be donated
to the rhodium than by a C-H agostic bond.
There are now two potential pathways from here to the C-C
and C-H activated products. One could go directly from the
η2 and (T-shape) η1 complexes A and A′ to their respective
activated product pairs {PCC, PCH} and {P′CC, P′CH}; this route
shall be termed the “direct pathway”. The other alternative
proceeds via the above-mentioned C-H agostic intermediates
B and B′; we shall call this the “agostic pathway”. The first
barriers on the agostic pathways are the ones for the formation
of B and B′, and they are found to be 16.1 kcal‚mol-1 for B
and 18.2 kcal‚mol-1 for B′. As the barriers for the back-reactions

(Non)Chelating CC Vs CH ActiVation
J. Am. Chem. Soc., Vol. 123, No. 37, 2001 9071
Figure 6. mPW1K/lanl2dz+p computed ∆G298 reaction profiles for
competing closed-arm and open-arm C-C and C-H activation routes
with and without MeOH spectator ligands.
Figure 7. mPW1K/lanl2dz+p computed ∆G298 reaction profiles for
competing closed-arm and open-arm C-C and C-H activation routes
with one and two MeOH spectator ligands.
Figure 8. Perspective view of computed structures of the intermediates.
are by far smaller (5.7 and 3.1 kcal‚mol-1 for the open-arm
and closed-arm systems, respectively), the direct pathways are
naturally favored.
At this point, our calculations explain why no C-C activation
is seen for the “open-arm” system; in addition, for the “closed-
arm” system, formation of the kinetic C-H product at room
temperature and the exclusive formation of the C-C product
upon heating can be rationalized. Note that barrier heights for
the back-reaction of the C-H products to intermediates A and
A′ stand at 18.8 and 19.1 kcal‚mol-1, respectively, low enough
for C-H formation to be reversible at room temperature. The
experimentally observed formation of the “open-arm” system
C-H product, however, is in discord with our computational
predictions, according to which P′CH and P′CC are 11.9 and 18.7
kcal‚mol-1 lower in energy, respectively, than PCH. We have
however, thus far, taken into account neither solvent nor
counterion.
Influence of Methanol on the Reaction Profile. The
experimental results suggest that the solvent plays a major role
in the C-H and C-C bond activation processes. The reactions
have been carried out in methanol. Therefore, to a first
approximation, we added one methanol “spectator ligand” to
each structure and reoptimized all stationary points on the
reaction profile (direct pathway) (Scheme 5).
All structures are stabilized by the addition of one methanol
molecule. This leads to association energies ranging from 3.1
kcal‚mol-1 for P′CH to 11.7 kcal‚mol-1 for TS-APCH (see Table
3). As expected, for the closed-arm system the optimized
structures for the 14e intermediate A′ is T-shaped (discounting
the fairly weak ring interaction), and for the Rh(III) complexes
the C-H as well as the C-C products are square pyramidal
(Figure 8). For the closed-arm system, the addition of one
methanol molecule does change almost none of both barriers,
The η2 complexes are connected on the energy hypersurface
via the transition states for C-C activation (TS-APCH, TS-
A′P′CC), their counterparts for C-H activation (TS-APCH, TS-
A′P′CH), and the transition state (TS-AA′) for interconversion
between A and A′. The outcome of our calculations is that, with
no solvent molecule coordinated to the rhodium center, C-C
activation cannot occur at room temperature with the “open-
arm” system due to the large barrier height of 29.3 kcal‚mol-1
.
The barrier of the C-H activation is 16.1 kcal‚mol-1; i.e., the
activation process is feasible at room temperature. Compared
to the C-H activation barrier calculated for the “open-arm”
system, the barrier appears to be almost twice as high. This is
in disagreement with the observed product ratio in C-H
activation. The C-H and C-C products are nearly isoergonic
(∆∆G ) 0.2 kcal‚mol-1).
For the “closed-arm” system, the barrier for C-H activation,
9.1 kcal‚mol-1, is 3.3 kcal‚mol-1 lower than that for C-C
activation, 12.4 kcal‚mol-1. Both are much lower than the
corresponding barriers for the “open-arm” system. Consequently,
among the “closed-arm” system products (P′CH, P′CC), the C-H
product is kinetically favored and the C-C product is thermo-
dynamically favored, being 6.8 kcal‚mol-1 more stable than the
C-H product. Comparison of the barrier heights for the direct
and agostic pathways indicates that, considering the above-
mentioned rapid back reaction, only the direct pathway will be
involved in the reaction. Therefore, the reaction mechanism to
be investigated is simplified to the one in Scheme 4.

9072 J. Am. Chem. Soc., Vol. 123, No. 37, 2001
Rybtchinski et al.
Scheme 4
Scheme 5
the C-H barrier increases slightly by 0.2 kcal‚mol-1, and the
C-C barrier remains unchanged. The C-H product is thus
kinetically favored by 3.1 kcal‚mol-1 over the thermodynamic
C-C product, which is more stable than the C-H product by
9.0 kcal‚mol-1. Therefore, in solution one would expect the
kinetic C-H product at room temperature and the thermody-
namic C-C product at higher temperature, in agreement with
experiment.
does coordinate a second methanol molecule. Interestingly, this
leads to a much greater change of the energy profile than the
addition of the first solvent molecule.
At the ONIOM(B3LYP/LANL2DZ:HF/LANL1MB) level,
the addition of the second methanol molecule to the joint
intermediate A leads to an enthalpy gain of 13.4 kcal‚mol-1
but is found to be just barely exergonic (1.2 kcal‚mol-1) because
of the entropy loss in the reaction A(MeOH) + MeOH f
A(MeOH)2. In contrast, the second methanol molecule has a
dramatic effect on the stability of the C-H transition state, and
the enthalpy gain of 21.0 kcal‚mol-1 is large enough that the
In the “open-arm” system, intermediate A again shows an
η2 interaction with the phenyl ring. It forms a T-shaped 14-
electron complex, with the methanol molecule being trans to
the η2 bond and cis to the phosphine arm. The addition of a
addition would be exergonic by 9.0 kcal‚mol-1
.
solvent molecule stabilizes the complex by 10.7 kcal‚mol-1
,
At the ONIOM(mPW1K/LANL2DZ+p:mPW1K/LANL2DZ)
level, however, we find that A(MeOH) + MeOH f A(MeOH)2
is endergonic by 5.4 kcal‚mol-1. This peculiar result can be
ascribed in part to the use of nonoptimum ONIOM(B3LYP/
LANL2DZ:HF/LANL1MB) reference geometries, as well as to
the use of frequencies and rotational constants at this latter level
of theory for the statistical thermodynamics. (The rigid rotor-
harmonic oscillator approximation is itself somewhat suspect
for systems as nonrigid as those studied here, and complete error
cancellation between all species involved cannot be relied upon.)
Unfortunately, reoptimization of all structures at the higher level
of theory would be beyond our computational resources.
Be that as it may, at all levels of theory we find the barrier
from A(MeOH)2 to the C-H product to be exceedingly low
(2-4 kcal‚mol-1), and the rate-determining step for formation
of the open-arm C-H product from the closed-arm intermediate
A′ would be the A-A′ interconversion.
whereas the association energy for intermediate A′ is just 7.3
kcal‚mol-1. As a result, both intermediates are nearly isoergonic
(∆∆G ) 0.8 kcal‚mol-1), and the respective interconversion
barriers of 5.0 and 5.5 kcal‚mol-1 are reasonably low. For the
“open-arm” system, the addition of one solvent molecule
decreases the C-H activation barrier by 1.0 kcal‚mol-1, while
the C-C activation barrier is basically unchanged. The C-H
activation barrier of 15.1 kcal‚mol-1 appears to be still high
compared to the corresponding barrier in the “closed-arm”
system. The computational results therefore do not reflect the
amount of the C-H product (PCH) formed at room temperature.
In contrast, it is easily seen why no open-arm C-C product is
formed: it is 5.9 kcal‚mol-1 less stable than its C-H counter-
part, and in fact 3.4 kcal‚mol-1 less stable than the joint
intermediate A.
We next explored the effect of addition of second methanol
molecule. Attempts to optimize structures for the “closed-arm”
systems with a second methanol molecule added invariably led
to the latter being evicted or to a structure with the free energy
of formation being positive. The “open-arm” system, however,
Considering the comparable stabilities of the open-arm and
closed-arm CH products, as well as the fairly low 9.3 kcal‚mol-1
C-H activation barrier for the closed-arm system, the two

(Non)Chelating CC Vs CH ActiVation
J. Am. Chem. Soc., Vol. 123, No. 37, 2001 9073
Table 4. Computed and Observed (X-ray) Values for Selected Geometrical Parameters of the Closed-Arm CC Activation Product without
MeOH Ligandsa
Distances (Å) and Angles (deg)
P′CC
P′CC
B3LYP/
LANL2dz+p
(nonCH)
P′CC-BF4
B3LYP/
LANL2dz+p
(nonCH)
ONIOM(B3LYP/
LANL2DZ:
HF/LANL1MB)
P′CC
B3LYP/
LANL2DZ
P′CC-BF4
X-ray structure
Rh-P
2.351
2.153
2.027
1.974
2.379
2.145
2.030
1.972
2.305
2.187
2.028
1.967
2.287
2.199
2.042
1.979
2.286
84.4
2.2169(4)
2.1543(13)
2.0220(18)
1.9527(18)
2.2664(12)
83.46(5)
Rh-O
Rh-CMe
Rh-CPh
Rh-F
P-Rh-CPh
O-Rh-CPh
CMe-Rh-CPh
85.6
81.6
97.1
85.6
81.9
96.9
85.8
81.3
97.0
80.1
81.17(6)
86.8
88.71(8)
a Calculated structures are presented both with and without the BF4- ligand. The suffix “+p(N,P,O,F)” indicates addition of a d-type polarization
function(C3H8), and diacetylene (HCCCCH); the methyl radical (•CH3) to B, O, F, and P atoms, and of an f-type polarization function to the metal
(see text).
reactions can be regarded as competitive at room temperature,
explaining why both closed-arm-and open-arm C-H products
are found experimentally. The reverse reaction barriers are
significant but still low enough that, upon heating, the thermo-
dynamically favored C-C product can be reached.
Since the methoxy arm is an ether-type ligand, it was of
interest to study the effect of an ether solvent on the reaction
profile. Very similar results were obtained when we considered
the “closed-arm” channel with a single dimethyl ether spectator
ligand instead of the methanol molecule.
As expected, mPW1K has a tendency to increase barrier
heights; it is seen to increase the gap between CC and CH
activation barriers compared to B3LYP. Upon comparing the
mPW1K/lanl2dz+p and B3LYP/lanl2dz surfaces, perhaps the
most striking change seen is the leveling of the path between
the two intermediates A and A′.
Comparison of Computed and X-ray Structures for
Closed-Arm CC Product. A comparison of computed and
observed structures is given in Table 4. Agreement between
the computed ONIOM(B3LYP/LANL2DZ:HF/LANL1MB) and
the X-ray structure is rather poor at first sight: large discrep-
ancies exist in particular for the CMe-Rh-Cipso angle (more
than 8°) and for the Rh-P distance (0.13 Å). This is not due to
the ONIOM approximation: only comparatively minor changes
are seen in the computed geometry upon reoptimizing the
structure at the B3LYP/LANL2DZ level. A rather drastic change
in the Rh-P distance is seen upon introducing polarization
functions: in all probability, the remaining gap would be closed-
arm if we added additional high-exponent d functions to
phosphorus.17 Yet the CMe-Rh-Cipso angle problem noted
above remains: it disappears upon introducing the BF4- ligand
counterion. The latter also causes noticeable changes in the bond
distances for the other ligands. Remaining discrepancies between
the best computed and the X-ray structures can be ascribed
primarily to basis set incompleteness (however, even our rather
modest best basis set strained our computational resources to
the limit) and secondarily to intrinsic differences between the
averaged crystal and bottom-of-the-well gas-phase structures.
We finally considered the possibility of adding yet an
additional solvent molecule. While we did find local minima
for the open-arm PCH(MeOH)3 and PCC(MeOH)3 products as
well as for the closed-arm PCC(MeOH)2 product, all of these
are endergonic at room temperature. (For the closed-arm
PCH(MeOH)2 product, we could only find a minimum in which
the -OMe arm had opened up; this structure exhibits an internal
hydrogen bond, the strength of which is expected to be
overestimated by more than a factor of 2 because of basis set
superposition error.15) This structure is found to be exergonic
by 5.3 kcal‚mol-1, which would probably cease to be the case
if larger basis sets were used. Although it was not experimentally
observed, we considered a structure of the closed-arm C-H
product with three MeOH molecules with the methoxy arm
being detached and pointing up. We found a local minimum,
but the structure is endergonic at room temperature. The bottom
line of these latter “numerical experiments” is that, at room
temperature, we can safely rule out structures with more than
one coordinated MeOH in the closed-arm case or more than
two coordinated MeOH in the open-arm case.
Comparison of Results at Different Levels of Theory. A
comparison of relative energies can be found in Table 1. As
expected, expanding the basis set from LANL2DZ to
LANL2DZ+P reduces the ligand addition energies of the MeOH
molecules, because of a reduction in basis set superposition error
(BSSE). Some of the relative energies are affected by as much
as 10 kcal‚mol-1 upon addition of polarization functions; the
most chemically significant change seen is for the energy
difference between the closed-arm (A′) and open-arm (A)
intermediates, which is reduced from -12.2 to -6.2 kcal‚mol-1
(favoring A′ in both cases). Working in even larger basis sets
(such as the recently proposed SDB-cc-pVTZ basis set16) was
not possible because of CPU time constraints, but we expect
further basis set expansion to affect the energy profile only
quantitatively, not qualitatively.
-
Effect of BF4 on the Relative Stability of the CC and
CH Closed-Arm Products. Given the fairly pronounced effect
-
of the BF4 ligand on the closed-arm CC product geometry,
we considered the question of whether its coordination, upon
evaporation, might affect the relative stability of the closed-
arm CC and CH products.
Since fully optimized B3LYP/LANL2DZ+p(hetero) struc-
tures are already available for the CC product with and without
the BF4- ligand, we carried out the corresponding calculations
for the CH product as well. The results are somewhat surprising.
At the B3LYP/LANL2DZ+p(hetero) level, without BF4-, the
CC structure is 8.1 kcal‚mol-1 more stable than the CH structure.
This value does not differ significantly from 8.1 kcal‚mol-1 at
the ONIOM(B3LYP/LANL2DZ:HF/LANL1MB) level, 8.8
kcal‚mol-1 at the ONIOM(B3LYP/ LANL2DZ+p:B3LYP/
LANL2DZ) level from an ONIOM(B3LYP/LANL2DZ:HF/
LANL1MB) reference geometry, or 5.9 kcal‚mol-1 at the
(15) E.g.: Evdokimov, A. G.; Kalb (Gilboa), A. J.; Koetzle, T.; Klooster,
W.; Martin, J. M. L. J. Phys. Chem. A 1999, 103, 744.
(16) Martin, J. M. L.; Sundermann, A. J. Chem. Phys. 2001, 114, 3408.
(17) Martin, J. M. L.; Uzan, O. Chem. Phys. Lett. 1998, 282, 16.

9074 J. Am. Chem. Soc., Vol. 123, No. 37, 2001
Rybtchinski et al.
ONIOM(B3LYP/LANL2DZ+p:B3LYP/LANL2DZ) level from
in the case of neutral rhodium-PCP systems with bulky
substituents (iPr, tBu), the three-coordinate reaction intermediates
are stabilized by an agostic interaction with one of the C-H
bonds.8 In the case of less bulky substituents, coordination to
the aromatic ring can play a role in the stabilization of the
intermediates.8,18 An analysis of our computational results with
regard to the lowest energy reactive intermediates leads to a
number of important observations.
the same reference geometry.
-
Introducing the additional BF4 ligand, however, markedly
favors the CC structure over the CH structure. As a result, the
P′CC-BF4 complex ends up being 14.8 kcal‚mol-1 more stable
than the P′CH-BF4 complex. This marked additional stabilization
provides an additional driving force for the formation of the
CC product upon solvent evaporation.
(A) The Chelating (Closed-Arm) Case (PCO Ligand with
One Bound MeOH Molecule). C-H and C-C bond activation
proceed through the common intermediate (Figure 8) with a
weak η1 ring-metal bond, similar to the one found in the case
of PCP systems.8,18 A bond order analysis (Rh-C bond order
0.2) indicates that the stabilizing effect of such a bond is
relatively low (the intermediate can be viewed as an essentially
14e species, vide infra). Thus, we suggest that the η1 binding
is a consequence of a rigid bis-chelating structure of the
intermediate bringing the metal into close proximity with the
aromatic ring and imposing a metal-ring interaction. It can play
a role in promoting C-C bond activation since it brings the
metal center closer to the C-C bond.18 However, it does not
discriminate significantly between C-H and C-C bonds in the
process of activation, as evident from the similar kinetic barriers
for both reactions. This weak metal-ring interaction can be
viewed as being synergetic with the chelating ligand binding
and assisting both C-H and C-C activation.
(B) The Nonchelating (Open-Arm) Case (PCO Ligand
with One and Two Bound MeOH Molecules: A(MeOH)1
and A(MeOH)2). C-C and C-H bond activation in the open-
arm case can start from the intermediate A(MeOH)1, with one
MeOH molecule and a genuine η2 metal-ring interaction or
from an intermediate with two methanol molecules A(MeOH)2,
stabilized by an agostic interaction (Figure 8). In the former
case, the intermediate can bind an additional molecule of
methanol, with concomitant breaking of the metal-ring bond
to give a 14e species (vide supra). Although A(MeOH)2 is
calculated to be about 5 kcal‚mol-1 higher in energy than
A(MeOH)1, it may actually be comparable to A(MeOH)1 if
solvent effects and larger basis set geometry optimization were
applied. In this instance, the lowest energy transition state is
reached directly from the corresponding intermediate, simplify-
ing the picture of the reaction profile. In any case, coordination
of two MeOH molecules seems essential to create the lowest
energy 14e channel for both C-H and C-C bond actiVation.
In both chelating and nonchelating cases, metal-ring interac-
tions seem to be an auxiliary stabilizing factor and can be
involved to a minor extent in lowering the activation barrier of
C-C activation in the closed-arm case.
Our calculations show that direct bond activation processes
are preferable energetically over involvement of C-H agostic
species both in the absence and in the presence of the solvent
molecules. Yet, local minima on the energy surface can be found
for the C-H agostic species, which can also lead to C-C and
C-H activation. Since in the case of the agostic intermediates
the methoxy arm is detached from the metal, it seems that it is
not relevant to C-C bond activation. On the other hand, in the
case of C-H activation, one of the intermediates, A(MeOH)2,
was found to be stabilized by an agostic interaction. It should
be noted that agostic interactions, similar to those found in the
PCO open-arm case, were suggested to play an important role
in the observed C-H activation/elimination reactivity and
selectivity in the PC type complex 5 (which is almost identical
to 3a-4a).
-
The better coordination of the BF4 anion to the CC ligand
is probably best ascribed to steric effects. In the CC case, the
complex with BF4 comfortably achieves a nearly perfect square
pyramidal geometry, while in the CH case, the BF4 ligand has
to choose between having the highly electronegative fluorine
either cis to the hydride ligand (surely not favorable) or trans
to the ligand, and being subject to sterical hindrance from the
tertiary butyl groups.
Discussion
Our theoretical calculations reveal the general tendency of
the “open-arm” system to prefer C-H over C-C activation, in
agreement with the experimental results. It appears that Rh(I)
14e species are significant for both C-C and C-H activation,
the calculated barriers being the lowest for the 14e situation.
Both thermodynamics and kinetics favor C-H over C-C
activation in the “open-arm” PCO case. In the “closed-arm”
system our calculations show that C-H activation is kinetically
preferred over C-C activation, while the C-C activation
product is thermodynamically more stable.
Reactive Intermediates: Aromatic Ring Binding and
Agostic Interactions. Theoretical calculations demonstrate that
(18) Hall, M. B.; Cao, Z. Organometallics 2000, 19, 3338.

(Non)Chelating CC Vs CH ActiVation
J. Am. Chem. Soc., Vol. 123, No. 37, 2001 9075
C-H and C-C Bond Activation by a Cationic Rh(I)
Complex: Electronic Requirements. C-H bond activation by
cationic Rh(I) complexes has not been studied extensively, and
computational studies of such systems have not been reported
to the best of our knowledge. Computational studies on C-H
bond oxidative addition were performed for d10 ML2, d8 MLn
(n ) 3, 4; CpML and TpML being special cases), and d6 MLn
species.4 In the cases of three- and four-coordinate d8 RhLn,
which are the most relevant to our studies, it was found that
T-shaped 14e intermediates ML3 are the most reactive in C-H
activation. The whole range of RhL1-4 intermediates and
transition states is possible in the case of the PCO open-arm
system. Interestingly, our computational results suggest that the
behavior of cationic Rh(I) is essentially comparable to that of
its neutral counterpart: 14e intermediates are most active in
both the “open-arm” and “closed-arm” systems. The fact that
the metal center coordinates solvent molecules in order to
achieve the 14e configuration demonstrates the general impor-
tance of this electronic structure for the C-H bond activation.
Importantly, relatively poor electron donating ligands such as
methanol molecules are sufficient to provide the electron density
necessary for C-H bond activation, most probably due to the
cationic nature of the metal center, which can coordinate the
hard solvent molecules better than a neutral center. It appears
that the electron density of the [R3PRh(MeOH)2]+ fragment,
which is probably lower compared to most of the known
examples of d8 intermediates in C-H activation, is adequate
for the C-H activation process.
Figure 9. Perspective view of selected structures of selected transition
states.
in the nonchelating case. This reflects the significance of the
specific steric requirements for C-C bond activation, which
can be imposed by chelation.
Although the open-arm system favors C-H activation both
kinetically and thermodynamically, the relatively small differ-
ence in the stability of the C-H and C-C products and the
reasonable kinetic barrier required for C-C bond activation
suggest that the C-H vs C-C reaction aptitudes can be reversed
by an appropriate system design. Relevant studies are currently
under way in our group.
Significantly, the same electronic requirements are necessary
for C-C bond activation. Our computational results show that
14e intermediates are involved in the lowest energy pathways
for C-C bond oxidative addition.
Sterics of C-C vs C-H Bond Activation: Comparison
of Transition States. According to the computational results,
both C-C and C-H bond oxidative addition in the PCO-Rh
system proceed through three-centered transition states, similar
to the neutral PCP and PCN systems.
A comparison of the transition states for C-C and C-H bond
activation (Figure 9) reveals a striking difference between the
open-arm and closed-arm cases. Whereas in the closed-arm
system the transition states for both C-C and C-H activation
have similar geometries (TS-A′P′CC and TS-A′P′CH), in the
open-arm case their geometries differ significantly (TS-APCC
and TS-APCH). In TS-A′P′CC and TS-A′P′CH, the metal center
is situated in close proximity to both the C-H and C-C bonds,
which is reflected in similar barriers of activation. C-H
activation in the open-arm case proceeds through TS-APCC, in
which a metal center is not situated in close proximity to the
C-C bond. For the C-C bond to be cleaved (TS-APCH), the
metal must approach it in a mode similar to the closed-arm
system, where it is already held in a proximal position. Thus,
in terms of the reaction kinetics, the outer bond, C-H, is more
available for activation, while the inner C-C can stay intact in
the open-arm system. Since C-C activation in the open-arm
system is disfavored thermodynamically as well, the C-H
product is the only possible one in the open-arm system. In the
closed-arm case, chelation results in a geometry which favors
both C-C and C-H oxidative addition, due to the proximity
of the metal to these bonds. It should be noted that the
experimental ratio of the closed-arm and open-arm C-H
activation products in methanol and the calculated kinetic
barriers suggest that there is no significant preference for C-H
activation in the closed-arm case as compared with the open-
arm one. On the other hand, C-C bond activation is unfavorable
Conclusions
We have demonstrated both carbon-hydrogen and carbon-
carbon bond activation in a novel PCO ligand system. The
importance of the chelate effect for C-C bond cleavage was
clearly demonstrated. The computational study of the system
provides important insights into the reaction mechanism.
Theoretical calculations demonstrate that the steric requirements
for C-C bond activation are more restricting than for the C-H
one, and therefore chelation is much more important for the
cleavage of the C-C bond. On the other hand, the electronic
requirements for both C-C and C-H are very similar, and 14e
intermediates play a key role in both activation processes. We
also note that coordinating solvent molecules significantly affect
the reaction profiles, and much more so for the open-arm than
for the closed-arm system.
Notably, in an unprecedented observation, conversion of a
C-H activated product complex into a C-C activated one was
achieved solely by solvent evaporation. Theoretical calculations
suggest that this is largely due to the special effect of BF4
coordination upon solvent evaporation.
-
Experimental Methods
All experiments with metal complexes and the phosphine ligand were
carried out under an atmosphere of purified nitrogen in an MBraun
MB 150B-G glovebox. The complex [Rh(coe)2Cl]2 was prepared
according to a literature procedure.19

9076 J. Am. Chem. Soc., Vol. 123, No. 37, 2001
Rybtchinski et al.
1H, 13C, and 31P NMR spectra were recorded at 400, 100, and 162
MHz, respectively, at 295 K, using a Bruker Avance-400 NMR
spectrometer. 1H NMR and 13C{1H} NMR chemical shifts are reported
in ppm downfield from tetramethylsilane. 1H NMR chemical shifts are
referenced to the residual hydrogen signal of the deuterated solvents,
and in 13C{1H} NMR the 13C signal of the deuterated solvents was
used as a reference. 31P NMR chemical shifts are reported in ppm
downfield from 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; dist, distorted; s, singlet; d,
doublet; m, multiplet. Electrospray (ES) mass spectrometry was
performed using a MicroMass LCZ Detector 4000 with CV ) 43 V,
temperature ) 150 °C, and EE ) 4.2 V. Elemental analysis was
performed at the Hebrew University of Jerusalem.
integration is given only for 3b (major product)): 6.88 (s, Ar-H, 3a),
2
6.77 (s, 1H, Ar-H, 3b), 4.97 (d, JHH ) 12.5 Hz, 1H, Ar-CH2-O,
2
left part of AB-system, 3b), 4.61 (d, JHH ) 12.7 Hz, 1H, Ar-CH2-
O, right part of AB-system, 3b), 4.38 (s, Ar-CH2-O, 3a), 3.27 (s,
2
2
3H, O-CH3, 3b), 3.06 (dd, JHH ) 8.8 Hz, JPH ) 3.7 Hz, 1H, Ar-
CH2-P, left part of AB-system, 3b), 2.59 (br m, 4H, overlapping signals
of Ar-CH2-P and Ar-CH2-Rh, 3b), 2.42 (s, Ar-CH3, 3a), 2.36 (s,
3H, Ar-CH3, 3b), 2.34 (s, 3H, Ar-CH3, 3b), 2.28 (s, Ar-CH3, 3a),
1.62 (d, 3JPH ) 12.5 Hz, 9H, P-C(CH3)3, 3b), 1.48 (br s, P-C(CH3)3
3
of 3b), 1.07 (d, JPH ) 13.1 Hz, 9H, P-C(CH3)3, 3b), -24.70 (br m,
Rh-H, 3a), -24.89 (dd, 1JRhH ) 27.2 Hz, 2JPH ) 18.1 Hz, 1H, Rh-H,
1
3b). Selected 13C{1H} NMR (acetone-d6): 8.85 (d, JRhC ) 25.1 Hz,
Ar-CH2-Rh, 3b).
Complex 4 (4a:4b ) 1.5:1). 31P{1H} NMR (methanol-d4): 126.8
1
1
1
Synthesis of C6H(CH3)3(CH2OCH3)(CH2Br). A freshly prepared
methanol solution of NaOMe (0.53 g, 9.81 mmol) was added dropwise
to a THF solution of 1,3-bis(bromomethyl)-2,4,6-trimethylbenzene11
(3.00 g, 9.81 mmol) at 0 °C. The solution was stirred for 2 h at room
temperature, and the solvent was evaporated under vacuum. This yielded
the monomethoxy compound, 1-bromomethyl-3-methoxymethyl-2,4,6-
trimethylbenzene, which was purified by column chromatography (SiO2,
hexane:THF) to give 1.34 g (53.3% yield) of the pure product as a
white solid.
(d, JRhP ) 195.8 Hz, 4b), 126.3 (d, JRhP ) 194.8 Hz, 4a). H NMR
(methanol-d4, integration relates to 4a and 4b separately): 6.95 (s, 1H,
Ar-H, 4b), 6.76 (s, 1H, Ar-H, 4a), 4.61 (s, 2H, Ar-CH2-O, 4a),
4.44 (dd, J ) 9.1 Hz, J ) 9.0 Hz, 2H, Ar-CH2-O, 4b), 3.46 (s, 3H,
O-CH3, 4a), 3.34 (d, J ) 3.1 Hz, 3H, O-CH3, 4a), 2.83 (m, 2H,
Ar-CH2-Rh, 4b), 2.75 (m, 2H, Ar-CH2-Rh, 4a), 2.46 (s, 3H, Ar-
CH3, 4b), 2.42 (s, 3H, Ar-CH3, 4b), 2.37 (s, 3H, Ar-CH3, 4b), 2.32
(s, 3H, Ar-CH3, 4a), 2.31 (s, 3H, Ar-CH3, 4a), 2.29 (s, 3H, Ar-
CH3, 4a), 1.43 (d, 3JPH ) 12.5 Hz, 9H, P-C(CH3)3, 4b), 1.35 (d, 3JPH
1H NMR (CDCl3): 6.87 (s, 1H, Ar-H), 4.57 (s, 2H, Ar-CH2-Br),
4.44 (s, 2H, Ar-CH2-O), 3.39 (s, 3H, O-CH3), 2.41 (s, 3H, Ar-
CH3), 2.35 (s, 3H, Ar-CH3), 2.34 (s, 3H, Ar-CH3).
3
) 12.5 Hz, 9H, P-C(CH3)3, 4a), 1.17 (d, JPH ) 12.5 Hz, 9H,
P-C(CH3)3, 4b), 1.23 (d, 3JPH ) 13.1 Hz, 9H, P-C(CH3)3, 4a), -25.28
1
2
(dd, JRhH ) 27.4 Hz, JPH ) 21.2 Hz, 1H, Rh-H, 4a), -25.49 (dd,
1JRhH ) 28.0 Hz, JPH ) 20.9 Hz, 1H, Rh-H, 4b). ES-MS (methanol
2
Synthesis of C6H(CH3)3(CH2OCH3)(CH2P(t-Bu)2) (1). HP(t-Bu)2
(0.890 g, 6.09 mmol) was added to a solution of C6H(CH3)3(CH2-
OCH3)(CH2Br) (1.30 g, 5.06 mmol) in acetone. The resulting solution
was refluxed for 30 min, followed by solvent evaporation under vacuum.
The residue was washed with ether, and the resulting phosphonium
salt was dissolved in degassed water, treated with NaOAc, and extracted
with ether. Final purification of the phosphine 1 was accomplished by
column chromatography (SiO2, hexane:THF) to afford 1.14 g (70%
yield) of the PCO ligand 1, 1-methoxymethyl-3-(di-tert-butylphos-
phino)methyl-2,4,6-trimethylbenzene, as a colorless oil.
solution): M+ found 425.31, calcd for C20H35OPRh 425.37 (no solvent
molecules or BF4 are present); M- found 86.95, calcd for BF4 86.8.
Reaction of the C-H Activation Products 2-4 with Ethylene
Glycol. Ethylene glycol was added in a large excess (10 equiv) to a
solution of complexes 2-4 in THF, methanol, or acetone. The solution
was left at room temperature for 1 h, and then the solvent was evap-
orated under vacuum. The resulting residue was washed with pentane
and ether to remove excess of ethylene glycol and dried under vacuum.
1
31P{1H} NMR (methanol-d4): 127.1 (dd, JRhP ) 193.5 Hz), 126.7
31P{1H} NMR (benzene-d6): 26.38 (s). 1H NMR (benzene-d6): 6.80
(s, 1H, Ar-H), 4.35 (s, 2H, Ar-CH2-O), 3.15 (s, 3H, O-CH3), 2.81
(dd, 1JRhP ) 194.2 Hz). Selected 1H NMR signals (methanol-d4): 6.95
(s, 1H, Ar-H), 6.75 (s, 1H, Ar-H), 4.84 (s, 2H, Ar-CH2-O), 4.62 (s,
2H, Ar-CH2-O), 4.42 (d, J ) 1.6 Hz, 2H, Ar-CH2-O), 3.64 (s, 4H,
O-CH2CH2-O), 3.45 (s, 3H, O-CH3), 3.35 (s, 3H, O-CH3), -25.2
(dd, 1JRhH ) 39.2 Hz, 2JPH ) 27.6 Hz, Rh-H), -25.2 (d, 2JPH ) 27.1
Hz, Rh-H).
2
(d, JPH ) 2.0 Hz, 2H, Ar-CH2-P), 2.65 (s, 3H, Ar-CH3), 2.47 (s,
3
3H, Ar-CH3), 2.27 (s, 3H, Ar-CH3), 1.08 (d, JPH ) 10.3 Hz, 18H,
2 P-C(CH3)3). 13C{1H} NMR (benzene-d6): 137.05 (s, Ar), 136.07
(s, Ar), 136.05 (d, JPC ) 11.0 Hz, Ar), 134.76 (s, Ar), 133.34 (s, Ar),
130.8 (d, JPC ) 9.0 Hz, Ar), 69.46 (s, Ar-CH2-O), 57.45 (dd, J )
10.3 Hz, J ) 4.6 Hz, O-CH3), 32.32 (d, 1JPC ) 26.2 Hz, 2 P-C(CH3)3),
Formation of the C-C Activation Product 6. (a) Solutions of
complexes 2-4 in THF, methanol, or acetone were dried under vacuum
at room temperature for 3 days and redissolved in methanol to form
the C-C activation product 6 (70-80% yield). (b) Heating of a
methanol solution of 4 at 70 °C for 1 h leads to formation of 6 in 85%
yield (31P{1H}-NMR). Further heating leads to significant decomposi-
tion. Heating in acetone or THF also results in formation of C-C
activation products, but the reactions are not clean enough to character-
ize the products.
2
1
29.95 (d, JPC ) 13.0 Hz, 2 P-C(CH3)3), 24.17 (d, JPC ) 29.7 Hz,
Ar-CH2-P), 22.31 (d, J ) 8.4 Hz, Ar-CH3), 19.77 (d, J ) 6.1 Hz,
Ar-CH3), 17.73 (br s, Ar-CH3). 1H and 13C{1H} NMR signal
assignment was confirmed by 13C-1H correlation and 13C DEPT.
Reaction of [Rh(coe)2(solv)n]BF4 with the PCO Ligand 1. Forma-
tion of C-H Activation Products 2-4. A THF (3 mL) solution of
AgBF4 (12 mg, 0.062 mmol) was added to a THF solution (4 mL) of
[Rh(coe)2Cl]2 (22 mg, 0.031 mmol). Immediate precipitation of AgCl
took place, and the precipitate was removed by filtration after the
mixture was allowed to stand at room temperature for 15 min. The
resulting clear orange solution was evaporated under vacuum to yield
[Rh(coe)2(solv)n]BF4. This was then redissolved in THF, methanol, or
acetone to yield an orange solution in every solvent. To each of these
solutions was added a solution of 20 mg (0.062 mmol) of the PCO
ligand in the corresponding solvent. The color of the THF solution
changed to brown after a few minutes, whereas in methanol or acetone
there was no noticeable color change. The solution was kept at room
temperature for 1 h, resulting in formation of the C-H activation
products 3 and 4. Complex 2 was not sufficiently stable for spectro-
scopic characterization. Complexes 3 (in acetone) and 4 (in methanol)
exist in solution as a mixture of open-arm (3a, 4a) and closed-arm
(3b, 4b) forms. In the closed-arm form, the methoxy sidearm is
coordinated to the rhodium center. In the open-arm form this arm is
not coordinated and is replaced by a solvent molecule.
31P{1H} NMR (methanol-d4): 90.2 (d, 1JRhP ) 190.8 Hz). 1H NMR
(methanol-d4): 6.55 (s, 1H, Ar-H), 5.13 (d, 2JHH ) 12.9 Hz, 1H, Ar-
2
CH2-O, left part of AB-system), 5.00 (d, JHH ) 12.8 Hz, 1H, Ar-
CH2-O, right part of AB-system), 4.88 (s, 3H, O-CH3), 3.82 (br d,
2JPH ) 1.1 Hz, 2H, Ar-CH2-P), 2.25 (s, 3H, Ar-CH3), 2.11 (s, 3H,
3
3
Ar-CH3), 1.38 (d, JPH ) 13.5 Hz, 9H, P-C(CH3)3), 1.22 (d, JPH
)
2
12.8 Hz, 9H, P-C(CH3)3), 1.02 (d, JRhH ) 2.3 Hz, 3H, Rh-CH3).
13C{1H} NMR (methanol-d4): 157.02 (dd, 1JRhC ) 35.7 Hz, 2JPC ) 4.7
Hz, ipso CAr-Rh), 144.50 (dd, 2J ) 8.3 Hz, 2J ) 3.5 Hz, Ar), 140.25
(s, Ar), 133.21 (d, 2JRhC ) 15.9 Hz, Ar), 130.06 (s, Ar), 128.19 (s, Ar),
1
82.52 (s, Ar-CH2-O), 68.83 (s, O-CH3), 38.77 (d, JPC ) 21.9 Hz,
P-C(CH3)3), 36.92 (dd, 1JPC ) 19.5 Hz, 2JRhC ) 2.8 Hz, P-C(CH3)3),
1
2
31.68 (dd, JPC ) 31.0 Hz, JRhC ) 3.9 Hz, Ar-CH2-P), 31.07 (d,
2JPC ) 2.5 Hz, P-C(CH3)3), 29.74 (d, JPC ) 2.1 Hz, P-C(CH3)3),
2
1
20.94 (s, Ar-CH3), 19.16 (s, Ar-CH3), -4.55 (dd, JRhC ) 32.0 Hz,
2JPC ) 8.3 Hz, Rh-CH3). ES-MS (methanol solution): m/e 425.37,
M+ calcd for C20H35OPRh 425.37 (no solvent molecules are present).
ES-MS: M- found 86.96, calcd for BF4 86.80.
Complex 3 (3a:3b ) 1:4.4). 31P{1H} NMR (acetone-d6): 130.4 (d,
1
1JRhP ) 192.2 Hz, 3b), 126.5 (d, JRhP ) 191.5 Hz, 3a). 1H NMR
Elemental Analysis (after prolonged evaporation). Calcd for C20H35-
BF4OPRh (no solvent molecules present): C, 46.90; H, 6.89. Found:
C, 47.73; H, 7.81.
(acetone-d6, only selected signals of 3a (minor product) are given,
(19) Herde, J. L.; Senoff, C. V. Inorg. Nucl. Chem. Lett. 1971, 7, 1029.

(Non)Chelating CC Vs CH ActiVation
J. Am. Chem. Soc., Vol. 123, No. 37, 2001 9077
X-ray Structural Analysis of Complex 6. Complex 6 was crystal-
lized by dissolving the solid in boiling diethyl ether, followed by cooling
the solution to room temperature. Allowing the ether solution to stand
at room temperature overnight yielded orange prismatic crystals.
Crystal Data: C20H35OPRh + BF4, orange prisms, 0.3 × 0.2 × 0.2
mm3, monoclinic, P21/c (No. 14), a ) 13.8280(3) Å, b ) 11.8580(3)
Å, c ) 15.0660(4) Å, â ) 114.7050(15)°, from 10° of data, T ) 120
K, V ) 2211.84(9) Å3, Z ) 4, fw ) 512.17, Dc ) 1.538 Mg/m3, µ )
MOLDEN.30 In cases where this approach failed to converge, we
likewise used analytical second derivatives at every step.
Where necessary, the Grid ) UltraFine combination, i.e., a pruned
(99 590) grid in the integration and gradient steps and a pruned (50 194)
grid in the CPKS (coupled perturbed Kohn-Sham) steps, was used as
recommended in ref 31. While an overly coarse grid convergence
obviously cannot cause false symmetry breaking here (of the sort
previously noted for the PCP system8), we did encounter a number of
cases for which no structure with all real frequencies could be obtained
unless we switched to the “ultrafine” grid combination.
Zero-point and RRHO (rigid rotor-harmonic oscillator) thermal
corrections were obtained from the unscaled computed frequencies.
Where necessary to resolve ambiguities about the nature of a
transition state, intrinsic reaction coordinate (IRC32) calculations were
carried out.
0.886 mm-1
.
Data Collection and Treatment: Nonius KappaCCD diffractometer,
MoKR (λ ) 0.71073 Å), graphite monochromator, 17 604 reflections
collected, 0 < h e 18, 0 e k e 16, -20 e l e 18, frame scan width
) 2.0°, scan speed 1° per 20 s, typical peak mosaicity ) 0.6°, 5630
independent reflections (Rint ) 0.028). Data were processed with Denzo-
Scalepack.
The energetics for our final profile were validated by single-
point energy calculations, using the ONIOM(B3LYP/LANL2DZ:HF/
LANL1MB) reference geometries, at two higher levels of theory. The
first is ONIOM(B3LYP/LANL2DZ+P:B3LYP/LANL2DZ), where the
“+P” stands for the addition of polarization functions with exponents
taken from ref 33. The second is ONIOM(mPW1K/LANL2DZ+P:
mPW1K/LANL2DZ), in which the very recent mPW1K (modified34
Perdew-Wang 199135 for Kinetics) exchange-correlation functional of
Truhlar and co-workers36 was employed. This functional was very
recently shown37 to yield more reliable reaction barrier heights than
other exchange-correlation functionals.
Solution and Refinement. The structure was solved by direct
methods with SHELXS-97, followed by full matrix least-squares
refinement based on F2 with SHELXL-97 (279 parameters with no
restraints). Idealized hydrogens were placed and refined in a riding
mode. The final R-factors are R1 ) 0.025 (based on F2) for data with
I > 2σ(I), and R1 ) 0.030 on all 5630 reflections. The goodness-of-fit
on F2 is 1.046, and the largest electron density equals 1.011 e/Å3.
Computational Methods
All calculations were carried out using the Gaussian 98 program
system20 running on Compaq ES40 and XP1000 computers in our
research group, as well as on the (Israel) Inter-University Computing
Center (IUCC) SGI Origin 2000.
To assist with interpretation, natural bond orbital (NBO) analyses38
with Wiberg bond orders39 were carried out.
As in our previous studies on the PCP8 and PCN21 systems, the initial
survey of the potential surface was carried out by a two-layer ONIOM22
approach, in which only the tert-butyl groups on the phosphorus were
placed in the outer layer, and the remainder of the system was placed
in the inner layer. The inner layer was treated using the B3LYP23
exchange-correlation functional with the Los Alamos National Labora-
tory double-ú (LANL2DZ) basis set-relativistic effective core potential
(RECP) combination.24 (As is customary, first-row atoms in this
approach were treated by means of the Dunning-Hay valence double-
Acknowledgment. D.M. is the Israel Matz Professor of
Organic Chemistry and J.M.L.M. the incumbent of the Helen
and Milton A. Kimmelman Career Development Chair. S.O.
acknowledges a postdoctoral fellowship from the Minerva
Foundation, Munich, Germany. This research was supported by
the Tashtiyot program of the Ministry of Science (Israel), by
the Israel Science Foundation, and by the US-Israel Binational
Science Foundation, Jerusalem, Israel. We gratefully acknow-
ledge support from the Helen and Martin Kimmel Center for
Molecular Design.
ú
25 basis set.) The tert-butyl substituents in the outer layer were treated
at the Hartree-Fock level with the LANL1MB basis set-RECP
combination.24
Supporting Information Available: Cartesian coordinates
for all computed ONIOM(B3LYP:LANL2DZ:HF/LANL1B)
structures in XMol (.xyz) format; total energies of all computed
structures; crystallographic data in plain text and CIF (Crystal-
lographic Information File) format; and B3LYP/LANL2DZ
electron density plots (color GIF format) of A(MeOH), TS-
APCHMeOH, TS-APCH(MeOH)2, TS-A′P′CC(MeOH), and TS-
A′P′CH(MeOH) are available on the World Wide Web at http://
theochem.weizmann.ac.il/web/papers/pco.html. This material is
also available free of charge via the Internet at http://pubs.acs.org.
Geometry optimizations for minima were carried out using the
standard Schlegel algorithm26 in redundant internal coordinates until
in the neighborhood of the solution and then continued using analy-
tical second derivatives.27 Optimizations for transition states were
carried out by means of the QST3 approach,28 with an initial guess
for the transition state being generated from either linear synchronous
transit (LST29) or from manual manipulation of the geometry using
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