ChemComm
Communication
that changing the identity of the axial ligand X trans to the oxo
group of [FeIV(O)(TMC)(X)] complexes affects the reactivity of the
oxoiron(IV) species for H-atom abstraction, and that the reactivity
increases as the axial ligand becomes more electron-donating.16b
Taken together, these results demonstrate the important role of
amido ligation in governing the rate of H-atom abstraction by
oxoiron(IV) species.
In summary, we presented the synthesis of the first oxoiron(IV)
complex with monoamido ligation. While this oxoiron(IV) species
is more stabilised than oxoiron(IV) species supported by neutral
polypyridine ligands in terms of its reduction potential, it is
capable of mediating C–H activation at rates comparable to those
of such previously reported oxoiron(IV) species. This study
demonstrates the significant role of amido ligation in the promo-
tion of H-atom abstraction from alkanes by oxoiron(IV) complexes
and suggests that oxoiron(IV) species of BLM should contribute to
the oxidation of the sugar moiety of DNA.
Fig. 4 Plots of log k2 for [FeIV(O)(dpaq)]+ (circles), [FeIV(O)(Bn-TPEN)]2+
(squares) and [FeIV(O)(N4Py)]2+ (triangles) as a function of the C–H BDEs of
various substrates. Data for [FeIV(O)(Bn-TPEN)]2+ and [FeIV(O)(N4Py)]2+
were taken from ref. 13.
0
[FeIV(O)(N4Py)]2+ (27).13 The existence of a large KIE clearly
indicates that hydrogen-atom abstraction is involved in the
rate-determining step of the C–H bond activation of ethylbenzene
by [FeIV(O)(dpaq)]+. In addition to ethylbenzene, oxidation of
triphenylmethane, cumene, toluene and 2,3-dimethylbutane
proceeded smoothly (Fig. S10 and Table S3, ESI†). Notably, the
Notes and references
1 (a) H. Umezawa, K. Maeda, T. Takeuchi and Y. Okami, J. Antibiot.,
1966, 19, 200; (b) B. A. Chabner and D. L. Longo, Cancer Chemotherapy
and Biotherapy: Principles and Practice, 1996.
2 J. Stubbe and J. W. Kozarich, Chem. Rev., 1987, 87, 1107.
3 (a) M. S. Chow, L. V. Liu and E. I. Solomon, Proc. Natl. Acad. Sci.
U. S. A., 2008, 105, 13241; (b) D. Kumar, H. Hirao, S. Shaik and
P. M. Kozlowski, J. Am. Chem. Soc., 2006, 128, 16148; (c) A. Decker,
M. S. Chow, J. N. Kemsley, N. Lehnert and E. I. Solomon, J. Am.
Chem. Soc., 2006, 128, 4719.
4 (a) R. J. Guajardo, S. E. Hudson, S. J. Brown and P. K. Mascharak,
J. Am. Chem. Soc., 1993, 115, 7971; (b) A. Kittaka, Y. Sugano,
M. Otsuka, M. Ohno, Y. Sugiura and H. Umezawa, Tetrahedron Lett.,
1986, 27, 3631; (c) J. M. Rowland, M. Olmstead and P. K. Mascharak,
Inorg. Chem., 2001, 40, 2810; (d) M. R. Bukowski, S. Zhu,
K. D. Koehntop, W. W. Brennessel and L. Que Jr., J. Biol. Inorg.
Chem., 2004, 9, 39.
5 Y. Hitomi, K. Arakawa, T. Funabiki and M. Kodera, Angew. Chem.,
Int. Ed., 2012, 51, 3448.
6 V. V. Zhdankin, A. Y. Koposov, D. N. Litvinov, M. J. Ferguson,
R. McDonald, T. Luu and R. R. Tykwinski, J. Org. Chem., 2005,
70, 6484.
7 J. England, G. J. Britovsek, N. Rabadia and A. J. White, Inorg. Chem.,
2007, 46, 3752.
8 S. O. Kim, C. V. Sastri, M. S. Seo, J. Kim and W. Nam, J. Am. Chem.
Soc., 2005, 127, 4178.
0
plot of the log k2 values (the second order rate constants normal-
ised on a per hydrogen atom basis) against the corresponding
bond dissociation energies (BDEs) of these external substrates is
linear (Fig. 4). When these data are plotted together with those
obtained for [FeIV(O)(Bn-TPEN)]2+ and [FeIV(O)(N4Py)]2+,13 the line
for [FeIV(O)(dpaq)]+ has nearly the same slope as those for the
other complexes and is located in the middle. These results
clearly demonstrate that the oxoiron(IV) species with monoamido
ligation has a C–H bond activation capability similar to that of
oxoiron(IV) species supported by neutral ligands.
To gain further insight into the above-mentioned reactivity
order, cyclic voltammetric measurements for [FeIV(O)(dpaq)]+
were performed in MeCN at 25 1C. In the voltammogram the
anodic and cathodic peaks were widely separated (ꢀ394 mV and
440 mV vs. Fc+/Fc) (Fig. S11, ESI†). Such a large separation was
recently reported for [FeIV(O)(Bn-TPEN)]2+ and [FeIV(O)(N4Py)]2+.15
9 DFT calculation supports our wavenumber assignment. See the
right-hand of Fig. S3 (ESI†).
However, the reduction potential of [FeIV(O)(dpaq)]+ (47 mV vs. 10 A. R. McDonald and L. Que Jr., Coord. Chem. Rev., 2013, 257, 414.
Fc+/Fc) was more negative by about 92 and 72 mV than those
11 M. Martinho, F. Banse, J. F. Bartoli, T. A. Mattioli, P. Battioni,
O. Horner, S. Bourcier and J. J. Girerd, Inorg. Chem., 2005, 44, 9592.
12 T. A. Jackson, J. U. Rohde, M. S. Seo, C. V. Sastri, R. DeHont,
A. Stubna, T. Ohta, T. Kitagawa, E. Munck, W. Nam and L. Que Jr.,
J. Am. Chem. Soc., 2008, 130, 12394.
13 J. Kaizer, E. J. Klinker, N. Y. Oh, J. U. Rohde, W. J. Song, A. Stubna,
J. Kim, E. Munck, W. Nam and L. Que Jr., J. Am. Chem. Soc., 2004,
126, 472.
14 J. U. Rohde and L. Que Jr., Angew. Chem., Int. Ed., 2005, 44, 2255.
15 Y. M. Lee, H. Kotani, T. Suenobu, W. Nam and S. Fukuzumi, J. Am.
Chem. Soc., 2008, 130, 434.
reported for [FeIV(O)(Bn-TPEN)]2+ and [FeIV(O)(N4Py)]2+, respec-
tively, clearly indicating that [FeIV(O)(Bn-TPEN)]2+ and [FeIV(O)-
(N4Py)]2+ are better oxidants than [FeIV(O)(dpaq)]+ with respect
to their reduction potentials ([FeIV(O)(Bn-TPEN)]2+ 4 [FeIV(O)-
(N4Py)]2+ c [FeIV(O)(dpaq)]+). It should be noted that the
reduction potential of [FeIV(O)(TMC)(MeCN)]+ was reported to
be 20 mV vs. Fc+/Fc,15 which is slightly more negative (by 27 mV)
than that of [FeIV(O)(dpaq)]+, but the ability of [FeIV(O)(TMC)- 16 (a) S. Hong, H. So, H. Yoon, K. B. Cho, Y. M. Lee, S. Fukuzumi and
(MeCN)]+ to carry out H-atom abstraction is significantly lower
W. Nam, Dalton Trans., 2013, 42, 7842; (b) C. V. Sastri, J. Lee, K. Oh,
Y. J. Lee, T. A. Jackson, K. Ray, H. Hirao, W. Shin, J. A. Halfen, J. Kim,
L. Que Jr., S. Shaik and W. Nam, Proc. Natl. Acad. Sci. U. S. A., 2007,
104, 19181.
than that of [FeIV(O)(dpaq)]+.16 Therefore, we conclude that the
amido ligation in [FeIV(O)(dpaq)]+ not only thermodynamically
stabilises the iron(IV) state, but also increases the basicity of the
oxo group and hence increases the ability of [FeIV(O)(dpaq)]+ to
carry out C–H activation.17 Sastri and co-workers demonstrated
17 (a) J. M. Mayer, Acc. Chem. Res., 2011, 44, 36; (b) M. T. Green, Curr.
Opin. Chem. Biol., 2009, 13, 84; (c) T. H. Yosca, J. Rittle, C. M. Krest,
E. L. Onderko, A. Silakov, J. C. Calixto, R. K. Behan and M. T. Green,
Science, 2013, 342, 825.
This journal is ©The Royal Society of Chemistry 2014
Chem. Commun., 2014, 50, 7485--7487 | 7487