N2O activation and oxidation reactivity from a non-heme iron pyrrole platform

Article abstract of DOI:10.1021/ja076842g

We report the design, syntheses, and oxidation reactivity of a new family of non-heme iron pyrrole complexes. The most reactive congener is capable of activating N₂O, an appealing yet challenging oxidant, for oxygen atom transfer. The first generation tpa^PhFe complex reacts with oxygen atom donors to generate a ligand hydroxylated species by intramolecular C-H activation and oxygenation. The second-generation tpa^MesFe system expands on this reactivity to intermolecular hydrogen atom abstraction chemistry through N₂O activation. The observation of arene C-H hydroxylation and the consumption of external hydrogen atom donors implicates a potent metal-centered oxidant, likely an iron(IV)-oxo species, along these reaction pathways. This work establishes a new coordination platform for high-valent iron reactivity. Copyright

Full text of DOI:10.1021/ja076842g

Published on Web 11/16/2007
N2O Activation and Oxidation Reactivity from a Non-Heme Iron Pyrrole
Platform
W. Hill Harman and Christopher J. Chang*
Department of Chemistry, UniVersity of California, Berkeley, California 94720, and Chemical Sciences DiVision,
Lawrence Berkeley National Laboratory, Berkeley, California 94720
Received September 10, 2007; E-mail: chrischang@berkeley.edu
Scheme 1. Synthesis of [tpaRFe]- Complexes
Iron occupies a central place in chemistry and biology owing in
large part to its rich catalytic oxidation reactivity within heme
porphyrin and non-heme environments.1 In particular, the impor-
tance of high-valent mononuclear iron intermediates in biocatalytic
transformations has stimulated interest in creating synthetic systems
to probe aspects of their bonding and reactivity as well as expanding
this chemistry to new applications.2-4 We have initiated a program
aimed at creating hybrid systems that incorporate key attributes of
both heme and non-heme ancillary ligands, namely the η1-pyrrole
donors in hemes that can support iron in multiple oxidation states
coupled with the flexibility of varying coordination geometries in
non-heme frameworks. Because the majority of natural systems
support iron-based oxidants in 4-fold symmetry,1 we envisioned
that exploring other geometries could allow access to high-valent
iron centers with new and potentially interesting electronic,
magnetic, and reactivity properties. In this regard, the activation
Figure 1. Proposed pathway of oxygen atom transfer to 5 involving a
putative Fe(IV)-oxo intermediate with subsequent hydrogen atom abstrac-
of nitrous oxide provides a salient challenge for this general strategy.
N2O is an appealing oxidant because of its ease of handling and
benign byproducts, but its kinetic stability5 and poor properties as
a ligand6 render it a difficult molecule to activate for further
transformations.7 We now report a new 3-fold symmetric, non-
heme iron pyrrole platform and its ability to activate N2O and other
oxygen-atom transfer reagents for intra- and intermolecular oxida-
tion reactions. This work provides a rare example of oxygen transfer
from N2O to iron.8
tion.
acetonitrile, is observed in solution or in the solid state. However,
treatment of 2 with the inner-sphere oxidant Me3NO in a THF
solution results in a rapid color change from bright yellow to dark
brown. Subsequent workup and crystallization affords a paramag-
netic, NMR-silent species in 23% yield. This product has been
identified by single-crystal X-ray structural analysis, high-resolution
ESI mass spectrometry, and elemental analyses as the iron(III)-
phenoxide complex [tpaPh2PhOFe]- (3) that results from intramo-
lecular aryl hydroxylation (Scheme 2). Given the kinetic barriers
to arene C-H hydroxylation and literature precedent for related
reactions mediated by synthetic iron-oxo species,11b the facile
generation of this oxygenated product suggests the intermediacy
of a potent iron-centered oxidant.
Encouraged by the ability of our non-heme iron-pyrrole platform
to effect intramolecular oxidation chemistry, we sought to promote
intermolecular oxidation reactivity by providing greater access to
the iron center and removing the proximate arene C-H bonds. Both
of these goals can be met through the synthesis of the tris-mesityl
tris(pyrrolylmethyl)amine platform, H3tpaMes (4), obtained through
a triple Mannich reaction starting from 2-mesitylpyrrole.13 The
synthesis of K[tpaMesFe] (5) proceeds in an analogous manner to
its phenyl congener and its solid-state structure displays a similar
trigonal monopyramidal core. Unlike the phenyl congener, 5 reacts
rapidly with small Lewis bases, including CO, acetonitrile and tert-
butylisocyanide, to form trigonal bipyramidal adducts in solution.
We suggest that the observed differential reactivity between the
phenyl and mesityl tpaR systems is due to the ability of the ortho
methyl groups in the latter to gear the pendant aryl and pyrrolide
rings toward a more orthogonal disposition (Scheme 2), providing
greater overall access to the metal site.
The reduced π-donation of pyrrole donors compared to hard
amides, combined with the ability to introduce R-substituents on
pendant pyrroles, provides an attractive electronic environment for
unsaturated, 3-fold symmetric compounds with mid-to-late first-
row transition metals. Inspired by R-free group 4 tris(pyrrolylm-
ethyl)amine and group 5 tris(indolylmethyl)amine complexes
reported by Odom9 and Parkin,10 respectively, we prepared the new
triply phenyl substituted tris(pyrrolylmethyl)amine derivative, H3-
tpaPh (1), through the convergent Mannich reaction of 2-phenylpyr-
role, formaldehyde, and ammonium chloride (Scheme 1). This first-
generation design employs phenyl groups both as steric pickets and
potential traps for high-valent electrophilic intermediates.11
Iron insertion into the H3tpaPh ligand proceeds smoothly via
deprotonation with NaH followed by salt metathesis with FeCl2 to
generate Na[tpaPhFe] (2) in 82% yield. The solid-state structure of
the anionic component of 2 reveals a relatively rare example of
iron(II) in a four-coordinate trigonal monopyramidal geometry.12
Fe-N bond lengths for the equatorial η1-pyrrolide (2.010(6) Å)
and apical amine (2.174(9) Å) donors are typical, and the metal
center is raised by ca. 0.26 Å out of the trigonal plane defined by
the three pyrrolide ligands. The solution Evans’ method measure-
ment is consistent with high-spin Fe(II) (S ) 2). With the flanking
phenyl groups able to attain a nearly coplanar arrangement with
the pyrrolide donors (Scheme 2), no evidence for appreciable
coordination of simple Lewis bases, including CO, pyridine, and
Treatment of iron(II) complex 5 with pyridine-N-oxide in
dimethoxyethane (DME) furnishes a paramagnetic, brown-colored
(λmax at 400 nm) species in 58% yield. Single-crystal X-ray
9
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J. AM. CHEM. SOC. 2007, 129, 15128-15129
10.1021/ja076842g CCC: $37.00 © 2007 American Chemical Society

C O M M U N I C A T I O N S
Scheme 2. Oxygen Transfer to Generate Iron(III)-Phenoxide and -Hydroxide Complexes on Non-heme Tris-Pyrrole Platforms
2000, 39, 5306-5317. (e) Rohde, J.-U.; In, J.-H.; Lim, M. H.; Brennessel,
W. W.; Bukowski, M. R.; Stubna, A.; Muenck, E.; Nam, W.; Que, L., Jr.
Science 2003, 299, 1037-1039. (f) Lim, M. H.; Rohde, J.-U.; Stubna,
A.; Bukowski, M. R.; Costas, M.; Ho, R. Y. N.; Munck, E.; Nam, W.;
Que, L., Jr. Proc. Natl. Acad. Sci. U.S.A. 2003, 100, 3665-3670. (g)
Klinker, E. J.; Kaizer, J.; Brennessel, W. W.; Woodrum, N. L.; Cramer,
C. J.; Que, L., Jr. Angew. Chem., Int. Ed. 2005, 44, 3690-3694. (h)
Martinho, M.; Banse, F.; Bartoli, J.-F.; Mattioli, T. A.; Battioni, P.; Horner,
O.; Bourcier, S.; Girerd, J.-J. Inorg. Chem. 2005, 44, 9592-9596.
(3) Selected iron-nitrido complexes: (a) Betley, T. A.; Peters, J. C. J. Am.
Chem. Soc. 2004, 126, 6252-6254. (b) Berry, J. F.; Bill, E.; Bothe, E.;
George, S. D.; Mienert, B.; Neese, F.; Wieghardt, K. Science 2006, 312,
1937-1941.
structural analysis, high-resolution mass spectrometry, and elemental
analyses establish this product as the terminal iron(III)-hydroxide
complex [tpaMesFe(OH)]- (6) with no evidence for intramolecular
ligand oxidation (Scheme 2). With the oxidative reactivity and
coordinative unsaturation of 5 established, we next examined the
activation of N2O. Exposure of 5 to N2O generates the same
terminal hydroxide in 23% recrystallized yield. The use of two-
electron oxygen-atom donors for this chemistry suggests a high-
valent Fe(IV)-oxo intermediate along the reaction pathway that
undergoes hydrogen atom abstraction from solvent (Figure 1). To
test this proposal, we performed oxidation reactions in the presence
of diphenylhydrazine (DPH) or cyclohexadiene (CHD) as external
hydrogen-atom sources. Indeed, when the reaction between 5 and
pyridine-N-oxide is carried out in the presence of DPH, azobenzene
is produced in 92% isolated yield. Analogous oxidations in the
presence of CHD produce benzene, although the yields are variable.
Taken together, the foregoing reactivity studies establish that a
putative non-heme oxoferryl species is viable and suggest an Fe-
(II)/Fe(IV) manifold for the observed non-heme iron pyrrole
reactivity.
(4) Selected iron-imido complexes: (a) Verma, A. K.; Nazif, T. N.; Achim,
C.; Lee, S. C. J. Am. Chem. Soc. 2000, 122, 11013-11014. (b) Brown,
S. D.; Betley, T. A.; Peters, J. C. J. Am. Chem. Soc. 2003, 125, 322-
323. (c) Lucas, R. L.; Powell, D. R.; Borovik, A. S. J. Am. Chem. Soc.
2005, 127, 11596-11597. (d) Thomas, C. M.; Mankad, N. P.; Peters, J.
C. J. Am. Chem. Soc. 2006, 128, 4956-4957. (e) Bart, S. C.; Lobkovsky,
E.; Bill, E.; Chirik, P. J. J. Am. Chem. Soc. 2006, 128, 5302-5303. (f)
Eckert, N. A.; Vaddadi, S.; Stoian, S.; Lachicotte, R. J.; Cundari, T. R.;
Holland, P. L. Angew. Chem., Int. Ed. 2006, 45, 6868-6871. (g) Klinker,
E. J.; Jackson, T. A.; Jensen, M. P.; Stubna, A.; Juhasz, G.; Bominaar, E.
L.; Mu¨nck, E.; Que, L., Jr. Angew. Chem., Int. Ed. 2006, 45, 7394-7397.
(5) (a) Banks, R. G. S.; Henderson, R. J.; Pratt, J. M. Chem. Commun. 1967,
387-388. (b) Banks, R. G. S.; Henderson, R. J.; Pratt, J. M. J. Chem.
Soc. (A) 1968, 2886-2889.
(6) Bottomley, F.; Lin, I. J. B.; Mukaida, M. J. Am. Chem. Soc. 1980, 102,
5238-5242.
(7) (a) Armor, J. N.; Taube, H. J. Am. Chem. Soc. 1971, 93, 6476-6480. (b)
Vaughan, G. A.; Rupert, P. B.; Hillhouse, G. L. J. Am. Chem. Soc. 1987,
109, 5538-5539. (c) Vaughan, G. A.; Hillhouse, G. L.; Lum, R. T.;
Buchwald, S. L.; Rheingold, A. L. J. Am. Chem. Soc. 1988, 110, 7215-
7217. (d) Smith, M. R., III; Matsunaga, P. T.; Andersen, R. A. J. Am.
Chem. Soc. 1993, 115, 7049-7050. (e) Matsunaga, P. T.; Hillhouse, G.
L.; Rheingold, A. L. J. Am. Chem. Soc. 1993, 115, 2075-2077. (f) Groves,
J. T.; Roman, J. S. J. Am. Chem. Soc. 1995, 117, 5594-5595. (g) Koo,
K.; Hillhouse, G. L.; Rheingold, A. L. Organometallics 1995, 14, 456-
460. (h) Laplaza, C. E.; Odom, A. L.; Davis, W. M.; Cummins, C. C.;
Protasiewicz, J. D. J. Am. Chem. Soc. 1995, 117, 4999-5000. (i) Kaplan,
A. W.; Bergman, R. G. Organometallics 1998, 17, 5072-5085. (j)
Pamplin, C. B.; Ma, E. S. F.; Safari, N.; Rettig, S. J.; James, B. R. J. Am.
Chem. Soc. 2001, 123, 8596-8597. (k) Lee, J. H.; Pink, M.; Tomaszewski,
J.; Fan, H.; Caulton, K. G. J. Am. Chem. Soc. 2007, 129, 8706-8707.
(8) For Fe-zeolite catalyzed oxidations with N2O, see: Kharitonov, A. S.;
Sheveleva, G. A.; Panov, G. I.; Sobolev, V. I.; Paukshtis, Y. A.;
Romannikov, V. N. Appl. Catal., A 1993, 98, 33-43.
(9) Shi, Y.; Cao, C.; Odom, A. L. Inorg. Chem. 2004, 43, 275-281.
(10) Tanski, J. M.; Parkin, G. Inorg. Chem. 2003, 42, 264-266.
(11) (a) Cruse, R. W.; Kaderli, S.; Meyer, C. J.; Zuberbuehler, A. D.; Karlin,
K. D. J. Am. Chem. Soc. 1988, 110, 5020-5024. (b) Jensen, M. P.; Lange,
S. J.; Mehn, M. P.; Que, E. L.; Que, L., Jr. J. Am. Chem. Soc. 2003, 125,
2113-2128. (c) Yamashita, M.; Furutachi, H.; Tosha, T.; Fujinami, S.;
Saito, W.; Maeda, Y.; Takahashi, K.; Tanaka, K.; Kitagawa, T.; Suzuki,
M. J. Am. Chem. Soc. 2007, 129, 2-3.
To close, we have presented a new non-heme iron pyrrole
platform and its ability to activate N2O for intra- and intermolecular
oxidation reactions. This work represents a rare homogeneous
system capable of mediating oxidative O-atom transfer from N2O.
Given the demonstrated potency of the putative oxoferryl interme-
diate, opportunities for stoichiometric or catalytic oxidations with
N2O are promising. In addition to probing spectroscopic and
structural features of the active species generated in these oxidation
reactions, we are exploring new pyrrole platforms to expand the
intermolecular reactivity of these systems.
Acknowledgment. We thank the University of California,
Berkeley, the Dreyfus, Beckman, Packard, and Sloan Foundations,
and the LBNL Chemical Sciences Division for funding this work.
W.H.H. was also supported by an Arkema graduate fellowship.
Supporting Information Available: Synthetic and experimental
details. This material is available free of charge via the Internet at http://
pubs.acs.org.
(12) (a) Cummins, C. C.; Lee, J.; Schrock, R. R.; Davis, W. M. Angew. Chem.,
Int. Ed. Engl. 1992, 31, 1501-1503. (b) Ray, M.; Golombek, A.; Hendrich,
M. P.; Young, V. G., Jr.; Borovik, A. S. J. Am. Chem. Soc. 1996, 118,
6084-6085. (c) Ray, M.; Hammes, B.; Yap, G. P. A.; Rheingold, A. L.;
Borovik, A. S. Inorg. Chem. 1998, 37, 1527-1532.
(13) While this manuscript was in press, an alternative synthesis of the mesityl
tpa ligand appeared: Wampler, K. M.; Schrock, R. R. Inorg. Chem. 2007,
46, 8463-8465.
References
(1) Groves, J. T. J. Inorg. Biochem. 2006, 100, 434-447.
(2) Selected iron-oxo complexes: (a) Que, L. Acc. Chem. Res. 2007, 40, 493-
500. (b) Nam, W. Acc. Chem. Res. 2007, 40, 522-531. (c) MacBeth, C.
E.; Golombek, A. P.; Young, V. G., Jr.; Yang, C.; Kuczera, K.; Hendrich,
M. P.; Borovik, A. S. Science 2000, 289, 938-941. (d) Grapperhaus, C.
A.; Mienert, B.; Bill, E.; Weyhermueller, T.; Wieghardt, K. Inorg. Chem.
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