Bimetallo-radical carbon-hydrogen bond activation of methanol and methane

Article abstract of DOI:10.1021/ja034494m

Carbon-hydrogen bond cleavage reactions of CH₃OH and CH₄ by a dirhodium(II) diporphyrin complex with a m-xylyl tether (·Rh(m-xylyl)Rh·(1)) are reported. Kinetic-mechanistic studies show that the substrate reactions are bimolecular and occur through the use of two Rh(II) centers in the molecular unit of 1. Second-order rate constants (T = 296 K) for the reactions of 1 with methanol (k(CH₃OH) = 1.45 × 10^-2 M^-1 s^-1) and methane (k(CH₄) = 0.105 M^-1 s^-1) show aclear kinetic preference for the methane activation process. The methanol and methane reactions with 1 have large kinetic isotope effects (k(CH₃OH)/k(CD₃OD) = 9.7 ± 0.8, k(CH₄)/k(CD₄) = 10.8 ± 1.0, T = 296 K), consistent with a rate-limiting step of C-H bond homolysis through a linear transition state. Activation parameters for reaction of 1 with methanol (ΔH = 15.6 ± 1.0 kcal mol^-1; ΔS = -14 ± 5 cal K^-1 mol^-1) and methane (ΔH = 9.8 ± 0.5 kcal mol^-1; ΔS = -30 ± 3 cal K^-1 mol^-1) are reported. Copyright

Full text of DOI:10.1021/ja034494m

Published on Web 04/04/2003
Bimetallo-Radical Carbon-Hydrogen Bond Activation of Methanol and
Methane
Weihong Cui, X. Peter Zhang, and Bradford B. Wayland*
Department of Chemistry, UniVersity of PennsylVania, Philadelphia, PennsylVania 19104-6323
Received February 4, 2003; E-mail: wayland@sas.upenn.edu
Development of effective strategies for selective hydrocarbon
activation and functionalization continues as a set of premier issues
in chemical reactivity and transformation.1-5 The limited informa-
tion available on metallo-radical activation of hydrocarbons indi-
cates a reaction specificity for saturated hydrocarbons over that of
aromatics, and among hydrocarbons there is a selectivity preference
for methane.6,7 Metallo-radical (M‚) reactions with substrates such
as hydrocarbons6,7 and hydrogen,8-10 where atom abstractions are
highly endothermic, often proceed by a concerted interaction of
two metallo-radicals with the substrate through a four-centered
transition state.6-10 The structure for this type of transition state
Figure 1. Illustration of the dirhodium(II) diporphyrin bimetallo-radical
complex.
provides an opportunity to obtain kinetic selectivity based on the
substrate size and shape and is responsible for metallo-radicals
manifesting unusual selectivities for C-H bond activation.6,7
Reactions that utilize this type of pathway have relatively low
activation enthalpies because of extensive bond making in the
transition state, but they generally occur slowly because of the
kinetic disadvantage for termolecular processes and the large
activation entropies associated with organizing three particles in
the transition state.6-11 One example is (tetramesitylporphyrinato)-
rhodium(II) ((TMP)Rh‚), which manifests relatively small activation
enthalpy (∆Hq ) 7.1 kcal mol-1) and large unfavorable activation
entropy (∆Sq ) -40 cal K-1 mol-1) in the reaction with CH4 to
form (TMP)Rh-CH3 and (TMP)Rh-H.6,7 A potential approach for
obtaining improved kinetics while retaining the unusual selectivity
is to incorporate two metallo-radical centers into a single molecular
unit capable of attaining a bimolecular reaction pathway.12-14 This
article reports on carbon-hydrogen bond homolysis reactions of
CH3OH and CH4 with a dirhodium(II) diporphyrin bimetallo-radical
complex that illustrate the efficacy of this strategy. These C-H
bond reactions occur by a pathway that is first order in bimetallo-
radical with reduced activation entropy and kinetic preference for
CH4 over CH3OH.
Figure 2. Time evolution of the 1H NMR for the Rh-CH2OH unit of the
diamagnetic products of reaction 1 with CH3OH (0.11 M): (a) t ) 3 h (δ
) -1.611 ppm, JH-H ) 8.2 Hz, J103Rh-H ) 3.4 Hz); (b) t ) 4 days; (c)
t ) 20 days (δ ) -1.602 ppm, 3JH-H ) 8.2 Hz, 2J103Rh-H ) 3.4 Hz) (spectra
were enhanced with Gussian multiplication; 2 ) HOCH2-Rh(m-xylyl)-
Rh-H; 3 ) HOCH2-Rh(m-xylyl)Rh-CH2OH).
3
2
OH diamagnetic species produced by reaction of CH3OH with 1 is
illustrated in Figure 2. A C-H bond reaction with a substrate like
CH3OH that involves the intermolecular use of RhII‚ sites from
different molecules of 1 would produce two diamagnetic complexes
containing a Rh-CH2OH unit (HOCH2-Rh(m-xylyl)Rh-H (2) and
HOCH2-Rh(m-xylyl)Rh-CH2OH (3)), but an intramolecular use
of two RhII‚ centers produces a single diamagnetic species contain-
ing the Rh-CH2OH unit (HOCH2-Rh(m-xylyl)Rh-H). Time
The diporphyrin ligand and the dirhodium(II) bimetallo-radical
complex selected for C-H bond reaction studies are illustrated in
Figure 1. Steric demands of the porphyrin units that prohibit
intermolecular and intramolecular Rh(II)-Rh(II) bonding and a
tether unit that has an appropriate size and flexibility to permit the
two metal centers to reach the transition state for an intramolecular
substrate reaction are essential design features that have been
successfully incorporated into the diporphyrin ligand.14 An ad-
1
evolution for the H NMR of the Rh-CH2OH fragment in the
reaction of 1 with CH3OH illustrates the relatively fast formation
of a single diamagnetic species containing a Rh-CH2OH unit and
then a very slow evolution of a second type of diamagnetic Rh-
CH2OH species. The second species formed (Figure 2, species 3)
is shown to be identical to an authentic sample of HOCH2-Rh-
(m-xylyl)Rh-CH2OH (3) produced by directed synthesis from
sequential reaction of 1 with H2 and CH2O in benzene. H-Rh(m-
xylyl)Rh-CH2OH (2) thus forms as the exclusive initial diamag-
netic species (Figure 2, species 2), and a much slower subsequent
process results in the appearance and growth of 3. This sequence
of events is explained by a fast initial intramolecular reaction that
exclusively forms 2, followed by much slower intermolecular
processes that result in reductive elimination of H2 and formation
of 3. The most important conclusion is that an intramolecular
1
ditional important feature of this biporphyrin is that the H NMR
of the methylene groups of the m-xylene tether is sensitive to the
chemical environment and is well separated from all other
resonances, which provides a useful observable for identifying
solution species and facilitating kinetic-mechanistic studies.
Methanol (0.01-0.12 M) is observed to react with 1 (∼2.0 ×
10-3 M) exclusively by C-H bond cleavage to form Rh-CH2OH
and Rh-H units. Time evolution of the 1H NMR for the Rh-CH2-
9
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J. AM. CHEM. SOC. 2003, 125, 4994-4995
10.1021/ja034494m CCC: $25.00 © 2003 American Chemical Society

C O M M U N I C A T I O N S
is found to be substantially larger than the value for methanol
(k(CH4)/k(CH3OH) ) 7.2, T ) 296 K). Large kinetic isotope effects
are observed at 296 K for the reactions of 1 with methanol (k(CH3-
OH)/k(CD3OD) ) 9.7 ( 0.8) (Figure 3A) and methane (k(CH4)/
k(CD4) ) 10.8 ( 1.0) (Figure 4A), which are consistent with a
rate-determining step involving C-H bond homolysis through a
near linear transition state.
Temperature dependence of the rate constants for the C-H
activation reactions are used in deriving activation parameters for
reaction of 1 with methanol (∆Hq ) 15.6 ( 1.0 kcal mol-1; ∆Sq )
-14 ( 5 cal K-1 mol-1) (Figure 3B) and methane (∆Hq ) 9.8 (
0.5 kcal mol-1; ∆Sq ) -30 ( 3 cal K-1 mol-1) (Figure 4B).
The primary initial objectives for designing bimetallo-radical
species were to test the strategy to obtain rate enhancements for
metallo-radical reactions by converting from a termolecular process
to a bimolecular reaction with reduced activation entropy while
retaining selectivity. These objectives have been realized in the
reaction of ‚Rh(m-xylyl)Rh‚ with methane, which occurs as a
bimolecular process with reduced activation entropy as compared
to the methane reaction of (TMP)Rh‚.6,7a Realization of additional
rate enhancements and substrate selectivity in this system requires
reducing the activation enthalpy through designing bimetallo-radical
structures that more closely emulate the transition state.
Figure 3. (A) Changes in the [‚Rh(m-xylyl)Rh‚] ([1]i ) 2.5 × 10-3 M) in
C6D6 at 296 K with time in the reaction of (a) CH3OH (0.067 M); k(CH3-
OH) ) 1.45 × 10-2 M-1 s-1 and (b) CD3OD (0.050 M); k(CD3OD) )
1.50 × 10-3 M-1 s-1. (B) Determination of the activation parameters for
reaction of ‚Rh(m-xylyl)Rh‚ with CH3OH.
Acknowledgment. This research was supported by the Depart-
ment of Energy, Division of Chemical Sciences, Office of the
Science through grant DE-FG02-86ER-13615.
Supporting Information Available: Experimental procedures, data
analysis, and1H NMR for compounds (PDF). This material is available
free of charge via the Internet at http://pubs.acs.org.
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Figure 4. (A) Changes in the [‚Rh(m-xylyl)Rh‚] ([1]i ) 1.6 × 10-3 M) in
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