Understanding the selectivity of a moderate oxidation catalyst: Hydrogen abstraction by a fully characterized, activated catalyst, the robust dihydroxo manganese(IV) complex of a bridged cyclam

Article abstract of DOI:10.1021/ja0673229

Distinctive mechanistic pathways account for the remarkable selectivity of a molecularly designed and robust manganese catalyst, Mn(Me₂EBC)Cl₂, whose activated form has been isolated and thoroughly characterized. In keeping with the existence of two activated catalytic functional groups, Mn^IV(OH)₂²⁺ (BDE_OH = 83.0 kcal/mol) and Mn^IV(O)OH⁺ (BDE_OH = 84.3 kcal/mol), two distinctive Polanyi correlations are observed, the latter being over 10-fold more rapid despite their similarity. Remarkably, only relatively weak C-H bonds (≤～82 kcal/mol) suffer hydrogen abstraction by either group, with the stoppage occurring at substrate BDE_CH ≥ catalyst BDE_OH. Earlier studies established a selective Lewis acid mechanism as the dominant oxygen atom insertion pathway for this catalyst. These two pathways (hydrogen abstraction and oxygen atom transfer) dominate the oxidation reactions catalyzed by these systems, accounting for the moderate oxidizing power and associated selectivity of Mn(Me₂EBC)Cl₂ in H₂O₂ oxidations. Copyright

Full text of DOI:10.1021/ja0673229

Published on Web 01/24/2007
Understanding the Selectivity of a Moderate Oxidation Catalyst: Hydrogen
Abstraction by a Fully Characterized, Activated Catalyst, the Robust
Dihydroxo Manganese(IV) Complex of a Bridged Cyclam
†
†
‡
‡
‡
Guochuan Yin, Andrew M. Danby, David Kitko, John D. Carter, William M. Scheper, and
,
†
Daryle H. Busch*
Department of Chemistry, The UniVersity of Kansas, Lawrence, Kansas 66045, and The Procter and Gamble
Company, Cincinnati, Ohio 45202
Received October 12, 2006; E-mail: busch@ku.edu
Hydrogen atom transfer and oxygen transfer are among the most
studied topics in chemistry because they provide promise of
understanding the basic events controlled by biological oxidases
and oxygenases and because these reactions are basic to many
industries, ranging from oil refining to home care products and
pharmaceuticals.1 Studies in this laboratory have produced useful
,2
3
selective oxidation catalysts through design features that (1) favor
4
oxygen transfer via a clean Lewis acid pathway and (2) limit
hydrogen atom transfer to relatively reactive substrates. Here we
characterize the selective hydrogen abstraction with empirically
based BDE calculations and experiments demonstrating two distinct
Polanyi correlations attributable to catalytic species differing only
in their levels of protonation. Whereas those two catalyst species
react at different rates, their thermodynamic oxidizing powers are
very similar (i.e., BDEOH of 83 and 84 kcal/mol). Further, they
both show a remarkable inability to oxidize substrates having
BDECH g BDEOH of the catalyst. The work of decades, beginning
with Bordwell and culminating with Mayer, produced a method,
Figure 1. The X-ray structure of Mn(Me EBC)(OH) 2+.
2 2
Scheme 1
The BDEOH values for the reduced forms of the oxidizing agents
were calculated by the Mayer/Bordwell method (Scheme 1; see
a
based on redox potentials and pK values, for evaluating the bond
dissociation energy (BDE) of OH and related groups ligated to
transition metal centers.5 Sequences of substrates with known
BDECH values have been studied to test and make use of this so-
called Polanyi correlation.5 This report is about a rare successful
5
IV
Supporting Information for details). [Mn (Me
2
EBC)(OH)
2 6 2
](PF )
7c
was synthesized as reported. The pK
a
values were measured in
) 5.87 for Mn (Me EBC)-
2
7b,c
III
aqueous solution by titration (pK
a
b,6
2+
IV
2+
2
(OH)(H O)
and 6.86 for Mn (Me
2
EBC)(OH)
7c
2
), and the
6
application of this relationship in the design of oxidation catalysts.
reduction potential (+0.756 V vs NHE, corrected to aqueous
These new catalysts are manganese complexes with a rigid, cross-
IV
2+
III
media) was measured for the Mn (Me
2
EBC)(OH)
2
2
/Mn (Me -
bridged, cyclam ligand, 4,11-dimethyl-1,4,8,11-tetraazabicyclo-
+
EBC)(OH)
2
couple by cyclic voltammetry. Calculations gave
7
[
6.6.2]hexadecane (abbreviated as Me
2
EBC). The corresponding
surprisingly similar results for the reduced forms of the two
7c
IV
2+
manganese(IV) complex, Mn (Me
2
EBC)(OH)
2
, is doubly unique
III
2+
oxidizing species: Mn (Me
2
EBC)(OH)(H
2
O) , BDEOH ) 83.0
(1) as the first thoroughly characterized monomeric Mn(IV)
III
+
kcal/mol and Mn (Me
2
EBC)(OH)
2
, BDEOH ) 84.3 kcal/mol. The
complex containing a pair of hydroxo ligands (Figure 1), and (2)
it constitutes the activated form of the catalyst (vide infra). Earlier
close similarity in magnitude of these two values indicates that the
IV
IV
hydrogen abstracting abilities of Mn dO and Mn -OH are
studies on catalytic epoxidation of olefins by Mn(Me
2 2
EBC)Cl
thermodynamically very similar, even though the charges on
identified the H adduct of the manganese(IV) complex as the
2
O
2
IV
2+
IV
+
Mn (Me
2
EBC)(OH)
2
2
and Mn (Me EBC)(O)(OH) are different
4
species responsible for the epoxidation. This Lewis acid intermedi-
ate performs clean selective epoxidation reactions, like high valent
and the abstracting functional groups are different.
To learn the significance of the calculated BDEOH values for
water and hydroxide bound to the manganese(III) complex,
hydrogen atom abstraction reactions were performed on a small
synthetic scale by treating solutions containing predominantly
8
early transition metal ions. Further, this work proved that Mn-
(
(
Me
Groves’ rebound mechanism). A remaining issue has been the
EBC)(O)(OH)+ is not capable of oxygen transfer to olefins
2
IV
IV
hydrogen abstracting ability of the Mn dO or Mn -OH groups
in this manganese(IV) complex. Having access to thoroughly
characterized, pure crystalline salts of the activated catalyst
facilitated the determination of BDE values for the Mn dO or
Mn -OH groups and the experimental study of their hydrogen
IV
2+
Mn (Me
2
EBC)(OH)
2
with selected substrates (Table 1). As the
IV
reaction proceeds, the original purple color of Mn gradually
changes to red-brown, revealing the conversion to the corresponding
IV
III
Mn species. As the substrate BDECH value is increased, the
IV
reaction rate decreases dramatically. For example, in neutral 4:1
acetone/water, the second-order rate constant for the reaction
between Mn(IV) and fluorene (BDECH ) 80 kcal/mol) is 2 orders
slower than that of the reaction between Mn(IV) and xanthene
(BDECH ) 75.5 kcal/mol). When the BDECH value exceeds ∼80
kcal/mol, no reaction is observed over periods of weeks (e.g.,
abstracting abilities. These studies show that, for these systems,
the rate of hydrogen abstraction becomes vanishingly slow as the
thermodynamic driving force approaches zero.
†
The University of Kansas.
The Procter and Gamble Company.
‡
1512
9
J. AM. CHEM. SOC. 2007, 129, 1512-1513
10.1021/ja0673229 CCC: $37.00 © 2007 American Chemical Society

C O M M U N I C A T I O N S
Table 1. Hydrogen Abstraction from Selected Substrates by
Manganese(IV) Complex
Information for details). The failure to cross the thermodynamic
barrier strongly suggests an extraordinary selectivity of definable
mechanistic origins. Ordinarily one might expect that a strong
follow-up reaction could drive the reaction even when the equi-
librium for formation of the hydrogen abstraction transition state
a
BDECH
(kcal/mol)
yield
(mmol)
k
2
(M- s-1)
1
substrate
product
xantheneb
1
75.5
76
78
xanthone
benzene
anthracene
anthraquinone
9-fluorenone
0.071
0.16
0.14
0.0069
0.083
0.00
1.8 × 10
8.7 × 10
-3
,4-cyclohexadieneb
-4
3.7 × 10-
4
is unfavorable. In the case of Mn(Me
such a follow-up reaction is mechanistically unavailable. Indeed,
the oxidation of substrates having BDECH > BDEOH for HOMnO
2 2
EBC)Cl , we suggest that
9
,10-dihydro
b
anthracene
fluoreneb
80
82
90
99
2.6 × 10-5
3
b,c
diphenyl methane
5b
(for example) may be driven by energetic follow-up reactions.
b,c
toluene
0.00
0.00
b,c
These results may help in understanding and controlling synthetic,
biomimetic, and biological oxidations.
In summary, this manganese complex is a highly selective
oxidation catalyst because of mechanistic constraints, both in
oxygen atom transfer and hydrogen abstraction. Polanyi correlations
cyclohexane
a
Reactions were run in a wet glovebox at room temperature. b 0.25 mmol
IV
of Mn complex and 0.5 mmol of substrate. Solvent: acetone/water (4:1).
No hydrogen abstraction detected after stirring reactant solution for >19
days.
c
IV
IV
are observed for both groups (Mn -OH and Mn dO), but
Table 2. Pseudo-First-Order Rate Constants for Hydrogen
IV
hydrogen abstractions for the Mn dO group average ∼14 times
a
Abstraction with Manganese Complex at pH 4.0 and 8.4
IV
faster than those for Mn -OH. This illustrates intrinsically different
k
1OH at pH 4.0
k1oxo at pH 8.4
1
oxidizing abilities for distinctly different functional groups when
all else is about equal. Remarkably, the hydrogen abstracting
abilities of both manganese complexes disappear when the thermal
driving force becomes very small (i.e., BDECH for the substrate g
BDEOH for the manganese complex). Although the slopes of the
Polanyi correlations are substantially different, the thermodynami-
cally controlled termination of oxidation is approximately the same
since the BDEOH values are similar for the two oxidizing groups.
While intuitively appealing, the thermodynamic control of hydrogen
-
1
-
substrate
(s
)
(s
)
k
1oxo/k1OH
-
-
4
-3
xanthene
(1.59 ( 0.01) × 10
(5.4 ( 0.3) × 10
(2.45 ( 0.02) × 10
15.4
16.5
15.1
10.6
-5
-4
1
,4-cyclohexadiene
(8.9 ( 0.20) × 10
(6.64 ( 0.06) × 10
5
5
-4
-4
9
,10-dihydroanthracene (4.39 ( 0.09) × 10
fluorene
(2.86 ( 0.06) × 10- (3.02 ( 0.07) × 10
a
Solvent: acetone/water (4:1), initial concentration of Mn(IV), 2 mM,
initial concentration of substrate, 40 mM.
diphenylmethane, toluene, cyclohexane in Table 1). The corre-
sponding Polanyi correlation is observed; see Supporting Informa-
tion for details. The termination of hydrogen abstraction at
diphenylmethane with BDECH ) 82 kcal/mol is remarkable in two
respects. First, it is consistent with the totally independent calculated
9
abstraction described here appears to be rare. Finally, its selective
moderate oxidizing power makes this catalyst valuable for demand-
ing applications.
Acknowledgment. Support by the Procter and Gamble Com-
pany is deeply appreciated, and we also acknowledge the National
Science Foundation Engineering Research Center Grant (EEC-
IV
IV
BDEOH values for Mn - OH (83.0 kcal/mol) or Mn -OH (84.3
2
kcal/mol) ligated to the manganese(III) complex within the 1-2
kcal/mol uncertainties of the BDE values; the reaction stops when
the driving force stops. Second, while extensive earlier studies have
confirmed the Polanyi relationship, termination of the hydrogen
abstraction reaction in the manner reported here is rare.9
0310689) for partial support. GC-MS analysis of reaction products
was performed by R. C. Drake.
Supporting Information Available: Methods used for calculations
of BDE values; experimental procedures for hydrogen abstraction;
kinetic results from hydrogen abstraction studies with different
substrates under various conditions. This material is available free of
charge via the Internet at http://pubs.acs.org.
The existence of two forms of the activated catalyst that differ
only in their degree of protonation at the active catalyst site presents
a unique opportunity to probe intrinsic hydrogen abstraction factors.
Preliminary studies of the pH dependence of the oxidation rates
reveal two distinctive Polanyi correlations for this novel catalyst
system (Table 2). Even with this limited data, it is reasonable to
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IV
IV
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IV
+
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.0 for hydrogen abstraction from 9,10-dihydroanthracene. The
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IV
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IV
IV
hydroxo moieties with the ability to abstract hydrogen atoms from
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substrates.5
,6
IV
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conducted at pH 8.6 with diphenylmethane; see Supporting
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J. AM. CHEM. SOC.
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