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electrophilic Cu center.
The relative acidity of the monoꢀprotonated species 3 and 3
responsible for dramatic differences in function between globins
60 and peroxidases. In the latter, aspartate residues hydrogen bond
iPr
17
appears to be between the acidity of HBF •Et O and formic acid,
as the latter acid protonates 1 and 1 completely, but formic acid
to the Fe bound histidine NH and create a stronger donor with
4
2
iPr
18ꢀ20
anionic character.
Thus, we have a biomimetic approach to
5
gives incomplete protonation (see the SI). Therefore, we
improve electrophilic catalysis. Our catalyst activates a CꢀH bond
in cyclohexane, and activity increases under acidic conditions.
iPr
anticipate that the pK values for 3 and 3 lie between ꢀ0.4 and
a
iPr
3
.77 in water. Similarly, the relative acidity of 4 and 4 is less 65 Thus we have shown that the electronic tunability of Ttz can lead
than HBF •Et O, and a pK value for 4/4 is expected to fall in
iPr
to switchable catalysts that are turned on or off with pH as the
chemical stimulus to change function in situ, to potentially lead to
greener, smarter, versatile catalysts with changeable properties.
We thank NSF CAREER (CHEꢀ0846383 to E.T.P.), 3M
4
2
a
this same range. Preliminary titration data for 1 that is first
doubly protonated (with HCl) and then doubly deprotonated (with
1
1
2
2
3
3
4
4
5
5
0
5
0
5
0
5
0
5
0
5
KOH) in mixed media (CH CN/water : 90/10 by volume) gives
3
an observed apparent pK value of 3.6 for 4. Although two 70 (NTFG 5286067 to N.L.) for financial support; Juan Urbano and
a
protons are removed from 4 to give 1, only one equivalence point
is observed in the data, so presumably an averaged pKa is
measured in these experiments, and the acidities of 4 and 3 are
similar and both near 3.6 in water. The air and moisture
sensitivity of 2 precluded a similar measurement.
The electronic tunability of Ttz has implications for enhancing
the performance of electrophilic catalysts. Studies by Pérez et al.
Pedro J. Pérez for helpful discussions; Tim Wade for MS; and
Mukesh Kumar for synthesis of 5.
Notes and references
iPr
a
Department of Chemistry, Drexel University, 3141 Chestnut St.,
75
Philadelphia, PA 19104, United States. Tel: 1-215-895-2666; E-mail:
b
have used halogenation of Tp complexes (Tp'Cu(NCCH )) for Cꢀ
Department of Chemistry, University of Michigan, Ann Arbor, MI,
3
United States. Tel: 1-734-615-367; E-mail: lehnertn@umich.edu
† Electronic Supplementary Information (ESI) available: [experimental
and computational details]. See DOI: 10.1039/b000000x/
H activation by a mechanism that involves formation of
Tp'Cu(carbene) complexes that insert into strong CꢀH bonds.
Electrophilic CꢀH activation occurs by donating σ CꢀH electron
density into the empty p orbital on the carbene. Carbene
electophilicity is enhanced by a metal that backꢀbonds less
readily. Importantly, as shown in this work, protonation of Ttz
11
8
8
9
9
0
5
0
5
1. A. Caballero, E. DespagnetꢀAyoub, M. Mar DíazꢀRequejo, A. Díazꢀ
Rodríguez, M. E. GonzálezꢀNúñez, R. Mello, B. K. Muñoz, W.ꢀS.
Ojo, G. Asensio, M. Etienne and P. J. Pérez, Science, 2011, 332, 835ꢀ
838.
complexes can be used to decrease π backꢀbonding and enhance
2
.
F. E. Jernigan, III, N. A. Sieracki, M. T. Taylor, A. S. Jenkins, S. E.
Engel, B. W. Rowe, F. A. Jove, G. P. A. Yap, E. T. Papish and G. M.
Ferrence, Inorg. Chem., 2007, 46, 360ꢀ362.
tBu,Me
coꢀligand electrophilicity. We used (Ttz
)Cu(NCCH ) (5) for
3
insertion into ethyl diazoacetate (EDA), followed by CꢀH
activation of cyclohexane to yield ethyl 2ꢀcyclohexylacetate (5
was present at 2 mol % relative to EDA; cyclohexane was the
solvent). The yield of the CꢀH activation product increased upon
3
4
5
.
.
.
M. Kumar, N. A. Dixon, A. C. Merkle, M. Zeller, N. Lehnert and E.
T. Papish, Inorg. Chem., 2012, 51, 7004ꢀ7006.
M. Kumar, E. T. Papish, M. Zeller and A. D. Hunter, Dalton Trans.,
2011, 40, 7517ꢀ7533.
S. N. Oseback, S. W. Shim, M. Kumar, S. M. Greer, S. R. Gardner,
K. M. Lemar, P. R. DeGregory, E. T. Papish, D. L. Tierney, M. Zeller
and G. P. A. Yap, Dalton Trans., 2012, 41, 2774ꢀ2787.
E. T. Papish, T. M. Donahue, K. R. Wells and G. P. A. Yap, Dalton
Trans., 2008, 2923ꢀ2925.
+
+
adding 1H or 2H to preꢀcatalyst (5), from 10% to 20% or 40%,
respectively. The observed yields are similar to literature
1
1
reports, but the new result is that protonation enhances
catalysis. Further kinetic studies will be pursued. The enhanced
6
7
.
.
R. Alsfasser and H. Vahrenkamp, Chem. Ber., 1993, 126, 695ꢀ701.
activity is attributed to a more electrophilic copperꢀcenter upon
tBu,Me
8. M. Kumar, E. T. Papish, M. Zeller and A. D. Hunter, Dalton Trans.,
2010, 39, 59ꢀ61.
protonation of (Ttz
). A control study shows that the acid
Thus,
1
1
1
1
1
1
00
alone does not lead to CꢀH activation products.
9
.
K. Malek, G. Schroeder and L. M. Proniewicz, Vib. Spectrosc., 2007,
44, 19ꢀ29.
electrophilic catalysts can be enabled without lengthy syntheses
of halogenated Tp ligands. Tandem reaction sequences can be
envisioned in which the catalyst is (de)activated in situ with pH
as the chemical stimulus.
In conclusion, Ttz ligands can be tuned electronically in ways
that Tp ligands cannot. The change in donor properties is gradual
1
0. F. Billes, H. Endrédi and G. Keresztury, Journal of Molecular
Structure: THEOCHEM, 2000, 530, 183ꢀ200.
05 11. A. Caballero, M. M. DíazꢀRequejo, T. R. Belderraín, M. C. Nicasio,
S. Trofimenko and P. J. Pérez, Organometallics, 2003, 22, 4145ꢀ
4150.
1
2. C. S. Letko, T. B. Rauchfuss, X. Zhou and D. L. Gray, Inorg. Chem.,
2012, 51, 4511ꢀ4520.
+
(
with increasing H ) and reversible; these features appear to be
10 13. H. V. R. Dias and H.ꢀL. Lu, Inorg. Chem., 1995, 34, 5380ꢀ5382.
4. X. Kou and H. V. R. Dias, Dalton Trans., 2009, 7529ꢀ7536.
15. X. Kou, J. Wu, T. R. Cundari and H. V. R. Dias, Dalton Trans., 2009,
15ꢀ917.
ꢀ
1
unprecedented. Only one report details a similar ꢀν of 26 cm
for monoꢀdeprotonation of [HC(6ꢀMeꢀ2ꢀpy) (6ꢀOHꢀ2ꢀpy)]CuCO
where py = pyridine), but here both the starting material (ν
1
2
1
2
9
(
CO
ꢀ
1
ꢀ1
16. K. Fujisawa, T. Ono, Y. Ishikawa, N. Amir, Y. Miyashita, K.
= 2088 cm ) and the product (ν
= 2062 cm ) are relatively
CO
15
Okamoto and N. Lehnert, Inorg. Chem., 2006, 45, 1698ꢀ1713.
electron rich. The extremely electrophilic Cu ions in LCuCO
were previously obtained by chemical synthesis to create
1
1
7. I. Bertini, H. B. Gray, E. I. Stiefel and J. S. Valentine, Biological
Inorganic Chemistry, University Science Books, Sausalito, CA, 2007.
8. S.ꢀi. Ozaki, M. P. Roach, T. Matsui and Y. Watanabe, Acc. Chem.
Res., 2001, 34, 818ꢀ825.
ꢀ
1 13ꢀ15
fluorinated Tp ligands (νCO
=
2137 cm )
or
ꢀ
1 16
tris(pyrazolyl)methane ligands (νCO = 2107 cm ). In our work,
a relatively electron rich CuCO (2086 cm ) can be transformed to
an electron poor CuCO (2117 cm ) in situ by changing the
concentration of H . This is a biomimetic modulation of donor
20 19. C. Hu, B. C. Noll, P. M. B. Piccoli, A. J. Schultz, C. E. Schulz and
W. R. Scheidt, J. Am. Chem. Soc., 2008, 130, 3127ꢀ3136.
20. C. Hu, C. D. Sulok, F. Paulat, N. Lehnert, A. I. Twigg, M. P.
Hendrich, C. E. Schulz and W. R. Scheidt, J. Am. Chem. Soc., 2010,
ꢀ
1
ꢀ
1
+
1
32, 3737ꢀ3750.
properties, similar to how changing amino acid residues is
25
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