Reactivity studies on FeIII-(O22-)-Cu II compounds: Influence of the ligand architecture and copper ligand denticity

Article abstract of DOI:10.1021/ic700363k

Heme-Cu/O₂ adducts are of interest in the elucidation of the fundamental metal-O₂ chemistry occurring in heme-Cu enzymes which effect reductive O-O cleavage of dioxygen to water. In this report, the chemistry of four heme-peroxo-copper [Fe^III-(O₂ ^2-)-Cu^II]⁺ complexes (1-4), varying in their ligand architecture, copper-ligand denticity, or both and thus their structures and physical properties are compared in their reactivity toward CO, PPh ₃, acids, cobaltocene, and phenols. In 1 and 2, the copper(II) ligand is N₄-tetradentate, and the peroxo unit is bound side-on to iron(III) and end-on to the copper(II). In contrast, 3 and 4 contain a N ₃-tridentate copper(II) ligand, and the peroxo unit is bound side-on to both metal ions. CO displaces the peroxo ligand from 2-4 to form reduced CO-Fe^II and CO-Cu^I species. PPh₃ reacts with 3 and 4 displacing the peroxide ligand from copper, forming (porphyrinate)Fe^III-superoxide plus Cu^I-PPh₃ species. Complex 2 does not react with PPh₃, and surprisingly, 1 reacts neither with PPh₃ nor CO, exhibiting remarkable stability toward these reagents. The behavior of 1 and 2 compared to that of 3 and 4 correlates with the different denticity of the copper ligand (tetra vs tridentate). Complexes 1-4 react with HCl releasing H₂O₂, demonstrating the basic character of the peroxide ligand. Cobaltocene causes the two-electron reduction of 1-4 giving the corresponding μ-oxo [Fe ^III-(O^2-)-Cu^II]⁺ complexes, in contrast to the findings for other heme-peroxo-copper species of different design. With t-butyl-substituted phenols, no reaction occurs with 1-4. The results described here emphasize how ligand design and variations influence and control not only the structure and physical properties but also the reactivity patterns for heme-Cu/O₂ adducts. Implications for future investigations of protonated heme/Cu-peroxo complexes, low-spin analogues, and ultimately O-O cleavage chemistry are discussed.

Full text of DOI:10.1021/ic700363k

Inorg. Chem. 2007, 46, 6382−6394
2-
Reactivity Studies on FeIII
−(O2
)−
CuII Compounds: Influence of the
Ligand Architecture and Copper Ligand Denticity
Eduardo E. Chufa´n,§ Biplab Mondal,§ Thirumanavelan Gandhi,§ Eunsuk Kim,§ Nick D. Rubie,‡
Pierre Moe
nne-Loccoz,‡ and Kenneth D. Karlin*,§
1
Department of Chemistry, Johns Hopkins UniVersity, Baltimore, Maryland, Maryland 21218, and
Department of EnVironmental & Biomolecular Systems, OGI School of Science & Engineering,
Oregon Health & Science UniVersity, BeaVerton, Oregon 97006
Received February 26, 2007
Heme
−
Cu/O2 adducts are of interest in the elucidation of the fundamental metal
−
O2 chemistry occurring in heme
−
Cu enzymes which effect reductive O
−
O cleavage of dioxygen to water. In this report, the chemistry of four heme
−
2-
peroxo
−
copper [FeIII
−(O2
)−
CuII]+ complexes (1
−4), varying in their ligand architecture, copper-ligand denticity, or
both and thus their structures and physical properties are compared in their reactivity toward CO, PPh3, acids,
cobaltocene, and phenols. In 1 and 2, the copper(II) ligand is N4-tetradentate, and the peroxo unit is bound side-on
to iron(III) and end-on to the copper(II). In contrast, 3 and 4 contain a N3-tridentate copper(II) ligand, and the
peroxo unit is bound side-on to both metal ions. CO “displaces” the peroxo ligand from 2
−4 to form reduced
CO
−
FeII and CO CuI species. PPh3 reacts with 3 and 4 displacing the peroxide ligand from copper, forming
−
(porphyrinate)FeIII-superoxide plus CuI-PPh3 species. Complex 2 does not react with PPh3, and surprisingly, 1
reacts neither with PPh3 nor CO, exhibiting remarkable stability toward these reagents. The behavior of 1 and 2
compared to that of 3 and 4 correlates with the different denticity of the copper ligand (tetra vs tridentate). Complexes
1
−
4 react with HCl releasing H2O2, demonstrating the basic character of the peroxide ligand. Cobaltocene causes
-
(O2
the two-electron reduction of 1
−4 giving the corresponding
µ
-oxo [FeIII
−
)
−
CuII]+ complexes, in contrast to the
findings for other heme-peroxo-copper species of different design. With t-butyl-substituted phenols, no reaction
occurs with 1 4. The results described here emphasize how ligand design and variations influence and control not
only the structure and physical properties but also the reactivity patterns for heme Cu/O2 adducts. Implications for
peroxo complexes, low-spin analogues, and ultimately O O cleavage
−
−
future investigations of protonated heme/Cu
chemistry are discussed.
−
−
Introduction
reactivity patterns for a series of heme-peroxo-copper
complexes, FeIII-(O22-)-CuII(ligand), which possess varying
structures depending on the nature of the ligand on copper.
Our interest in this series comes as part of our program whose
goal is to elucidate the underlying chemistry occurring at
the active site of heme-copper oxidases.4-6 There has been
an emphasis on dioxygen reactivity to unravel the chemistry
of O2 binding in the presence of both heme and copper
complexes, given a previously established separate heme-
The chemistry at metalloprotein active sites cannot be
disconnected from the inherent fundamental chemistry as-
sociated with the metal ion and its ligand environment.1-3
Furthermore, the relationship between structure and reactivity
for a given chemical system is part of the basic knowledge
required to obtain a deep understanding of chemical function.
In this report, we describe systematic studies concerning
7,8
9-11
* To whom correspondence should be addressed. E-mail: karlin@jhu.edu.
§ Johns Hopkins University.
O2 and copper-O2
chemistries.
‡ Oregon Health & Science University.
(4) Chufan, E. E.; Puiu, S. C.; Karlin, K. D. Acc. Chem. Res. 2007, in
press.
(1) Lee, Y.; Karlin, K. D. Highlights of Copper Protein Active-Site
Structure/Reactivity and Synthetic Model Studies; In Concepts and
Models in Bioinorganic Chemistry; Metzler-Nolte, N., Kraatz, H.-B.,
Eds.; Wiley-VCH: New York, 2006; pp 363-395.
(2) Karlin, K. D. Science 1993, 261, 701-708.
(3) Holm, R. H.; Solomon, E. I. Chem. ReV. 2004, 104, 347-348.
(5) Kim, E.; Chufa´n, E. E.; Kamaraj, K.; Karlin, K. D. Chem. ReV. 2004,
104, 1077-1133.
(6) Collman, J. P.; Boulatov, R.; Sunderland, C. J.; Fu, L. Chem. ReV.
2004, 104, 561-588.
(7) Momenteau, M.; Reed, C. A. Chem. ReV. 1994, 94, 659-698.
6382 Inorganic Chemistry, Vol. 46, No. 16, 2007
10.1021/ic700363k CCC: $37.00
© 2007 American Chemical Society
Published on Web 07/06/2007

FeIII-(O22-)-CuII Compounds
(O22-)-CuII(TMPA)]+ (1) {F8 ) tetrakis(2,6-difluorophenyl)-
porphyrinate}, the copper ion is coordinated by a tripodal
tetradentate tris(2-pyridylmethyl)amine (TMPA).18 The Fe·
‚‚Cu distance is 3.7 Å (EXAFS determination).19 The peroxo
unit is bound side-on to the high-spin iron(III) (thus overall
five-coordinate with the iron out of the porphryinate plane)
and end-on to the copper(II), thus in an overall µ-η2:η1
coordination mode,19 νO-O ) 808 cm-1 (resonance Raman
spectroscopy).18 This complex is structurally analogous to
that synthesized and crystallographically characterized by
Naruta and co-workers.20 In [(6L)FeIII-(O22-)-CuII]+ (2), the
tetradentate TMPA ligand is covalently appended to the
periphery of a 2,6-difluorophenyl-substituted porphyrin and
νO-O ) 788 cm-1.21,22 A second group of complexes are
rather different in aspects of their physical properties. In
[(F8)FeIII-(O22-)-CuII(AN)]+ (3) {AN ) 3,3′-iminobis(N,N-
dimethyl-propylamine} (vide infra) and [(2L)FeIII-(O22-)-
CuII]+ (4)23,24 (Chart 1), the copper(II) ion is coordinated by
a tridentate ligand; the difference between the two is that
the latter possesses a linker between the heme and copper
chelate, see Chart 1. This different environment for copper
in both of these complexes, compared to that of 1 and 2, is
reflected in the O-O stretching frequency which is consider-
ably lowered: νO-O for 3 is 756 cm-1 (vide infra) and that
for 4 is 747 cm-1.23,24 This lower νO-O reflects a different
mode of coordination, side-on bound to both iron and copper
ions, thus in an overall µ-η2:η2-peroxo ligation.4,24,25
Chart 1
Heme-copper oxidases include cytochrome c oxidase
(CcO), which is the terminal enzyme of the respiratory chain
of mitochondria and many aerobic bacteria. It catalyzes the
reduction of molecular oxygen to water, a process that
requires four electrons and four protons. The reaction is
coupled to the pumping of four additional protons across
the mitochondrial or bacterial membrane.5,12 In the mecha-
nism of O2-reduction by CcO, a key step is the O-O
reductive cleavage of the bound dioxygen moiety.13 A well-
established early enzyme intermediate is a FeIII-(O2 )‚‚‚CuI
species (A); however, a transient peroxo complex, formally
FeIII-(O22-)‚‚‚CuII, possibly bridged between the two metals,
is often considered to form subsequently.14-16
Probing the reactivity of a complex or series of compounds
by subjecting them to a series of reagents is one good way
to elucidate structure-reactivity (functional) relationships.
Such approaches and systematic investigations have been
carried out with O2-derived complexes, for example “clas-
sical” M-peroxo complexes (M ) Pt, Rh, ...),26 copper-
dioxygen complexes (as µ-peroxo-dicopper(II) species),
which turn out to either have a nucleophilic or electrophilic
character (depending on the ligand),27,28 η2-peroxo-iron(III)
-
By reaction of O2 with either a 1:1 mixture of reduced
hemes (i.e., FeII) and copper(I) complexes or with FeII‚‚‚CuI
complexes employing binucleating ligands (i.e., a porphyrin
with appended/tethered tridentate or tetradentate ligand for
copper), a series of (porphyrinate)FeIII-(O22-)-CuII(ligand)
complexes have been generated and characterized; they all
possess a high-spin iron(III) ion (Chart 1).4,17 In [(F8)FeIII-
(18) Ghiladi, R. A.; Hatwell, K. R.; Karlin, K. D.; Huang, H.-w.; Moe¨nne-
Loccoz, P.; Krebs, C.; Huynh, B. H.; Marzilli, L. A.; Cotter, R. J.;
Kaderli, S.; Zuberbu¨hler, A. D. J. Am. Chem. Soc. 2001, 123, 6183-
6184.
(19) del Rio, D.; Sarangi, R.; Chufa´n, E. E.; Karlin, K. D.; Hedman, B.;
Hodgson, K. O.; Solomon, E. I. J. Am. Chem. Soc. 2005, 127, 11969-
11978.
(20) Chishiro, T.; Shimazaki, Y.; Tani, F.; Tachi, Y.; Naruta, Y.; Karasawa,
S.; Hayami, S.; Maeda, Y. Angew. Chem., Int. Ed. 2003, 42, 2788-
2791.
(21) Ghiladi, R. A.; Huang, H. W.; Moe¨nne-Loccoz, P.; Stasser, J.;
Blackburn, N. J.; Woods, A. S.; Cotter, R. J.; Incarvito, C. D.;
Rheingold, A. L.; Karlin, K. D. J. Biol. Inorg. Chem. 2005, 10, 63-
77.
(22) Ghiladi, R. A.; Ju, T. D.; Lee, D.-H.; Moe¨nne-Loccoz, P.; Kaderli,
S.; Neuhold, Y.-M.; Zuberbu¨hler, A. D.; Woods, A. S.; Cotter, R. J.;
Karlin, K. D. J. Am. Chem. Soc. 1999, 121, 9885-9886.
(23) Kim, E.; Helton, M. E.; Lu, S.; Moe¨nne-Loccoz, P.; Incarvito, C. D.;
Rheingold, A. L.; Kaderli, S.; Zuberbu¨hler, A. D.; Karlin, K. D. Inorg.
Chem. 2005, 44, 7014-7029.
(24) Kim, E.; Shearer, J.; Lu, S.; Moe¨nne-Loccoz, P.; Helton, M. E.;
Kaderli, S.; Zuberbu¨hler, A. D.; Karlin, K. D. J. Am. Chem. Soc. 2004,
126, 12716-12717.
(25) Karlin, K. D.; Kim, E. Chem. Lett. 2004, 33, 1226-1231.
(26) Sheldon, R. A.; Kochi, J. K. Metal-Catalyzed Oxidations of Organic
Compounds; Academic Press: New York, 1981.
(8) Traylor, T. G.; Traylor, P. S. Reaction of Dioxygen and Its Reduced
Forms with Heme Proteins and Model Porphyrin Complexes. In ActiVe
Oxygen: ActiVe Oxygen in Biochemistry; Valentine, J. S., Foote, C.
S., Greenberg, A., Liebman, J. F., Eds.; Chapman & Hall: New York,
1995; pp 84-187.
(9) Quant Hatcher, L.; Karlin, K. D. J. Biol. Inorg. Chem. 2004, 9, 669-
683.
(10) Lewis, E. A.; Tolman, W. B. Chem. ReV. 2004, 104, 1047-1076.
(11) Mirica, L. M.; Ottenwaelder, X.; Stack, T. D. P. Chem. ReV. 2004,
104, 1013-1045.
(12) Ferguson-Miller, S.; Babcock, G. T. Chem. ReV. 1996, 96, 2889-
2907.
(13) Ogura, T.; Kitagawa, T. Biochim. Biophys. Acta 2004, 1655, 290-
297.
(14) Proshlyakov, D. A.; Pressler, M. A.; Babcock, G. T. Proc. Natl. Acad.
Sci. U.S.A. 1998, 95, 8020-8025.
(15) Blomberg, M. R. A.; Siegbahn, P. E. M.; Wikstro¨m, M. Inorg. Chem.
2003, 42, 5231-5243.
(16) Blomberg, M. R. A.; Siegbahn, P. E. M. Biochim. Biophys. Acta 2006,
1757, 969-980.
(17) Ghiladi, R. A.; Chufan, E. E.; del Rio, D.; Solomon, E. I.; Krebs, C.;
Huynh, B. H.; Huang, H.-w.; Moenne-Loccoz, P.; Kaderli, S.;
Honecker, M.; Zuberbuhler, A. D.; Marzilli, L.; Cotter, R. J.; Karlin,
K. D. Inorg. Chem. 2007, 46, 3889-3902.
(27) Hatcher, L. Q.; Karlin, K. D. AdV. Inorg. Chem. 2006, 58, 131-184.
(28) Paul, P. P.; Tyekla´r, Z.; Jacobson, R. R.; Karlin, K. D. J. Am. Chem.
Soc. 1991, 113, 5322-5332.
Inorganic Chemistry, Vol. 46, No. 16, 2007 6383

Chufa´n et al.
hemes,29 and others.30 In the discussion that follows, the four
peroxo complexes, 1-4 (Chart 1), are compared in their
reactivity toward five reagents: (i) carbon monoxide (CO),
(ii) triphenylphosphine (PPh3), (iii) HCl or other acids as
proton sources, (iv) cobaltocene (Co(Cp)2) as an electron
donor, and (v) t-butyl-substituted phenols as potential proton/
electron donors. The molecules CO and PPh3 help to probe
the environment of the heme-copper assembly, possibly
displacing the peroxide, perhaps as CO displaces O2 (bound
as superoxide) in oxy-hemoglobin or oxy-myoglobin to
regenerate the reduced (FeII) enzyme.6,31 PPh3 may also cause
a displacement or possibly be oxygenated. Protonation of
reductively cleaved O2-derived species is obviously of
importance in CcO chemistry whose function is not only
“dioxygen activation” but also proton translocation;32 thus,
determination of the reactivity of protons with complexes
1-4 is of interest. Because the enzymatic reduction of O2
(to water) requires 2 more electrons to go beyond the peroxo
stage found in complexes 1-4, the addition of cobaltocene
was deemed to be of interest for study. Further, Co(Cp)2
has been used by Collman and co-workers33-36 to chemically
probe the overall oxidation state of heme-peroxo-copper
complexes. Reactions of 1-4 with phenols complement such
reduction chemistry.
Figure 1. UV-vis spectra of reduced species, 1:1 mixture of (F8)FeII and
[CuI(AN)](B(C6F5)4) (red trace, λmax ) 422, 542 nm), and of µ-peroxo
complex [(F8)FeIII-(O22-)-CuII(AN)]+ (3) (blue trace, λmax ) 418, 478, 538,
561 nm) in THF at -80 °C. The inset shows the R-region of the spectra.
Scheme 1
Results and Discussion
Complex [(F8)FeIII-(O22-)-CuII(AN)]+ (3). This heter-
obinuclear heme-peroxo-copper complex was employed
in the present studies because it is easier and cheaper to
generate than a previously well-studied system employing
NMR characterization of the µ-peroxo product 3 shows a
downfield-shifted pyrrole resonance at 96 ppm (spectrum not
shown) with a small amount of the µ-oxo decompostion
product [(F8)FeIII-O-CuII(AN)]+. The persistent formation of
10-20% of µ-oxo species at NMR scale is not observed
however at lower concentration (e.g., at Soret and Q-band
UV-vis scale, where distinctive and readily seen changes
from the peroxo complex occurs; see Supporting Information
(Figures S1) for the µ-oxo UV-vis spectrum). The 96 ppm
pyrrole chemical shift position found for peroxo complex 3
(and confirmed by 2H NMR studies using the pyrrole-
[(F8)FeIII-(O22-)CuII(LMe N)]+,37 where LMe N is the tridentate
donor ligand N,N-bis{2-[2-(N,N-4-dimethylamino)pyridyl]-
ethyl}methylamine. However, 3 has not been previously
described so we report its characteristics here. Peroxo
complex 3 can be generated by bubbling O2 at -80 °C
through an equimolar solution of (F8)FeII and [CuI(AN)]-
(B(C6F5)4) in THF, pre-placed in a Schlenk flask, cuvette
assembly, or NMR tube in an inert atmosphere glove box
(See Experimental Section). The UV-vis changes which
occur during this reaction are given in Scheme 1, and the
2
2
1
actual spectra are shown in Figure 1. Low-temperature H
deuterated (d8-F8)FeII analogue) excludes the possibility that
2-
this complex is (i) the µ-peroxo complex [(F8)FeIII]2-(O2
)
(29) Selke, M.; Sisemore, M. F.; Valentine, J. S. J. Am. Chem. Soc. 1996,
118, 2008-2012.
(17.5 ppm, -80 °C), (ii) an iron-superoxide, that is, (F8)-
(30) Groves, J. T.; Gross, Z.; Stern, M. K. Inorg. Chem. 1994, 33, 5065-
-
FeIII-(O2 ) (8.9 ppm, -80 °C), (iii) the ferryl species (F8)-
5072.
FeIVdO (3.5 ppm, -80 °C), or (iv) (F8)FeIII-OH (∼135
ppm, -80 °C).38 In fact, the 96 ppm pyrrole resonance in 3
is in the range expected for a high-spin FeIII (d5, S ) 5/2)
system which is strongly antiferromagnetically coupled to
CuII ion (d9, S ) 1/2), giving an overall S ) 2 spin system.5
Further, resonance Raman spectroscopy (rR) (Figure 2)
reveals a dominant peroxide O-O stretching vibration at 756
cm-1 (∆18O2 ) -48 cm-1). Another O-O vibration at 746
cm-1 (∆18O2 ) -48 cm-1) is observed as a shoulder to the
major component at 756 cm-1 and probably represents a
(31) Walker, F. A.; Simonis, U. Iron Porphyrin Chemistry. In Encyclopedia
of Inorganic Chemistry, 2nd ed.; King, R. B., Ed.; John Wiley & Sons
Ltd.: New York, 2005; Vol. IV, pp 2390-2521.
(32) Wikstrom, M.; Verkhovsky, M. I. Biochim. Biophys. Acta 2006, 1757,
1047-1051.
(33) Collman, J. P.; Herrmann, P. C.; Boitrel, B.; Zhang, X.; Eberspacher,
T. A.; Fu, L.; Wang, J.; Rousseau, D. L.; Williams, E. R. J. Am. Chem.
Soc. 1994, 116, 9783-9784.
(34) Collman, J. P.; Fu, L.; Herrmann, P. C.; Zhang, X. Science 1997, 275,
949-951.
(35) Collman, J. P. Inorg. Chem. 1997, 36, 5145-5155.
(36) Collman, J. P.; Fu, L.; Herrmann, P. C.; Wang, Z.; Rapta, M.; Bro¨ring,
M.; Schwenninger, R.; Boitrel, B. Angew. Chem., Int. Ed. 1998, 37,
3397-3400.
(37) Kim, E.; Helton, M. E.; Wasser, I. M.; Karlin, K. D.; Lu, S.; Huang,
H.-w.; Moe¨nne-Loccoz, P.; Incarvito, C. D.; Rheingold, A. L.;
Honecker, M.; Kaderli, S.; Zuberbu¨hler, A. D. Proc. Natl. Acad. Sci.
U.S.A. 2003, 100, 3623-3628.
(38) Ghiladi, R. A.; Kretzer, R. M.; Guzei, I.; Rheingold, A. L.; Neuhold,
Y.-M.; Hatwell, K. R.; Zuberbu¨hler, A. D.; Karlin, K. D. Inorg. Chem.
2001, 40, 5754-5767.
6384 Inorganic Chemistry, Vol. 46, No. 16, 2007

FeIII-(O22-)-CuII Compounds
40 °C (depending on the solvent) solutions of either the 1:1
mixture of reduced (F8)FeII and CuI(ligand) complexes (1
and 3) or the heterobinuclear complexes [(6L)FeII‚‚‚CuI]+ (2)
or [(2L)FeII‚‚‚CuI]+ (4) (Chart 1). Previous studies, along with
current observations on the new complex 3, show that these
FeIII-(O22-)-CuII complexes fully form and that this process
is irreversible in that application of a vacuum or bubbling
with Ar to the low-temperature complex solutions will not
cause O2 release.17,21,23 The reactivity studies were carried
out at low-temperatures because the peroxo complexes are
thermally unstable, transforming cleanly to the corresponding
µ-oxo species (i.e., FeIII-O-CuII,40-42 for all but complex
4). Prior to the addition of reagents for reactivity investiga-
tions, excess O2 was removed through the application of
vacuum/Ar purges.
Reactions with CO. Carbon monoxide has been exten-
sively used in biological studies on metalloproteins and, in
particular, on heme/Cu oxidases, as a probe or surrogate for
active-site dioxygen binding.5 For example, CO binds to the
heme in reduced cytochrome c oxidase, and many biochemi-
cal/biophysical investigations employ the heme-CO adduct
to initiate laser photoejection of carbon monoxide to probe
binding to CuB (with subsequent deligation and rebinding
to FeII), perturbation/motions/proton movements in the
heme-Cu surrounding, or both.43-46 Carbon monoxide is
known to give CO2 in a reaction with reductively activated
(with O2) CcO, and the nature of the enzyme intermediates
involved have recently been probed using resonance Raman
spectroscopy.47 Furthermore, CO is well-known to strongly
bind to five-coordinated ferrous heme proteins, as well as
in small molecule synthetic analogues.31,48 Rather than an
examination of reduced complex/CO interactions (for which
we separately have information),49 the reaction of 1-4 with
CO may reveal if peroxide can be displaced (as O2), that is,
if CO is thermodynamically a better ligand (than O2) for the
reduced heme or copper center. In addition, insights into how
copper and iron work synergistically in these heme-copper
environments may be revealed. Indeed, we find that 1 does
not react with CO, while for 2-4, the peroxo group is
displaced via redox reactions.
Figure 2. Resonance Raman spectra of [(F8)FeIII-(O22-)-CuII(AN)]+ (3),
formed by the oxygenation of an equimolar solution of (F8)FeII and [CuI-
(AN)](B(C6F5)4) in THF with 16O2 (black) and 18O2 (red). The difference
spectrum (16O2 minus 18O2) is also shown (green). All spectra were obtained
at 90 K with 413 nm excitation.
minor conformer of 3 where the Cu(AN) ligand moiety
resides at a different orientation with respect to the peroxo
O-O axis. This interpretation is consistent with our previous
analysis of [(F8)FeIII-(O22-)CuII(LMe N)]+, where two con-
2
formers were also detected by rR spectroscopy.37 The minor
component observed in the rR spectra of 3 appears to increase
in intensity during the course of these experiments, suggest-
ing that laser illumination might promote isomerization of
the peroxo complex. Exposure of [CuI(AN)]+ to O2 in THF
is known to form a mixture of µ-η2:η2-peroxo complex
[{CuII(AN)}2(O2)]2+ and bis-µ-oxo complex [{CuIII(AN)}2-
(O)2]2+ with characteristic νO-O at 721 cm-1 (683 cm-1 for
18O2) and νCu-O at 608 cm-1 (580 cm-1 for 18O2), respec-
tively.39 The 413 nm rR spectra provide no evidence for such
species in the presence of (F8)FeII, since the 413 nm
excitation is expected to strongly favor porphyrin related
vibrations at the expense of putative Cu-only complexes. This
rR information about Cu-only complexes by itself cannot
categorically rule these out; however, the single Soret band
and pyrrole signal found for [(F8)FeIII-(O22-)-CuII(AN)]+
precludes formation of the Cu-only CuII-(O22-)-CuII or bis-
µ-oxo CuIII-(O)2-CuIII species to the extent of 0-5% total.
Following the spectroscopic analysis and arguments de-
scribed previously for [(2L)FeIII-(O22-)-CuII]+ (4) and [(F8)-
FeIII-(O22-)CuII(LMe2N)]+,24,25 the relatively low ν(O-O)
frequencies observed in 3 support a µ-η2:η2-peroxo ligation
to both iron and copper, as shown in Chart 1.
(40) Karlin, K. D.; Nanthakumar, A.; Fox, S.; Murthy, N. N.; Ravi, N.;
Huynh, B. H.; Orosz, R. D.; Day, E. P. J. Am. Chem. Soc. 1994, 116,
4753-4763.
(41) Ju, T. D.; Ghiladi, R. A.; Lee, D.-H.; van Strijdonck, G. P. F.; Woods,
A. S.; Cotter, R. J.; Young, J. V. G.; Karlin, K. D. Inorg. Chem. 1999,
38, 2244-2245.
(42) The X-ray structure of [(F8)FeIII-O-CuII(AN)]+ will be described
elsewhere.
(43) Yoshikawa, S.; Shinzawa-Itoh, K.; Nakashima, R.; Yaono, R.;
Yamashita, E.; Inoue, N.; Yao, M.; Jei-Fei, M.; Libeu, C. P.;
Mizushima, T.; Yamaguchi, H.; Tomizaki, T.; Tsukihara, T. Science
1998, 280, 1723-1729.
(44) Koutsoupakis, C.; Pinakoulaki, E.; Stavrakis, S.; Daskalakis, V.;
Varotsis, C. Biochim. Biophys. Acta 2004, 1655, 347.
(45) Einarsdottir, O.; Szundi, I. Biochim. Biophys. Acta 2004, 1655, 263.
(46) McMahon, B. H.; Fabian, M.; Tomson, F.; Causgrove, T. P.; Bailey,
J. A.; Rein, F. N.; Dyer, R. B.; Palmer, G.; Gennis, R. B.; Woodruff,
W. H. Biochim. Biophys. Acta 2004, 1655, 321.
General Approach and Conditions for Reactivity Stud-
ies. All FeIII-(O22-)-CuII complexes (1-4) under study were
generated by bubbling excess dioxygen through - 80 or -
(47) Kim, Y.; Shinzawa-Itoh, K.; Yoshikawa, S.; Kitagawa, T. J. Am. Chem.
Soc. 2001, 123, 757-758.
(48) Silvernail, N. J.; Roth, A.; Schulz, C. E.; Noll, B. C.; Scheidt, W. R.
J. Am. Chem. Soc. 2005, 127, 14422-14433.
(39) Liang, H.-C.; Zhang, C. X.; Henson, M. J.; Sommer, R. D.; Hatwell,
K. R.; Kaderli, S.; Zuberbuehler, A. D.; Rheingold, A. L.; Solomon,
E. I.; Karlin, K. D. J. Am. Chem. Soc. 2002, 124, 4170-4171.
(49) Kretzer, R. M.; Ghiladi, R. A.; Lebeau, E. L.; Liang, H.-C.; Karlin,
K. D. Inorg. Chem. 2003, 42, 3016-3025.
Inorganic Chemistry, Vol. 46, No. 16, 2007 6385

Chufa´n et al.
CO observed for [(6L)FeIII-(O22-)-CuII]+ (2) (vide supra),
which possesses the similar tetradentate TMPA Cu chelate,
this result suggests that the bonding at the metal-peroxo
core in 2 is weaker than that in 1. This may be because of
6
the nature of the binucleating L ligand in 2, with linker to
one pyridyl group of the TMPA moiety, which leads to ligand
constraints translating to distortions from optimal coordina-
tion geometry.41 In fact, in detailed CO binding studies to
reduced complex [(6L)FeII‚‚‚CuI]+, we previously published
that the ligand constraints lead to the major species [(solvent)-
(6L)FeII-CO‚‚‚CuI-CO]+ possessing tridentate chelation
within the CuI-CO moiety (see diagram below), as deduced
from νC-O IR data in comparison to known complexes.49 In
other words, 2 behaves more like it has tridentate chelation
to copper.
Figure 3. UV-vis spectra monitoring the reaction of [(6L)FeIII-(O22-)-
CuII]+ (2) (red, λmax ) 560 nm) with CO in CH3CN at -40 °C, producing
[(6L)FeII-CO‚‚‚CuI-CO]+ (blue, λmax ) 538 nm). Spectra shown in black
are generated during the transformation, the first obtained immediately and
the second after 5 min of bubbling CO. Inset: IR spectrum of [(6L)FeII-
CO‚‚‚CuI-CO]+ in CH3CN; the complete IR spectrum is given in
Supporting Information.
Scheme 2
Along with the CO reactions seen with 2 and 4, [(F8)-
FeIII-(O22-)-CuII(AN)]+ (3)/CO gives corresponding chem-
istry (Scheme 2 and Figure S3 (UV-vis monitoring)).
Bubbling CO through solutions of 3 at -80 °C leads to the
formation of a UV-vis spectrum50 typical of a heme-
carbonyl complex.49,52 We also characterized the reaction
products (Scheme 2) by IR spectroscopy; the expected
heme-carbonyl is formed (ν(C-O) ) 1982 cm-1),49,52 along
with [(AN)CuI-CO]+ (ν(C-O) ) 2077 cm-1), and the latter
result was confirmed by comparison with the CO adduct
formed starting with authentic [(AN)CuI]+.50 This finding is
interesting in regards to the tridentate (AN)Cu chelate present
in 3, again by comparison to having a tetradentate TMPA
chelate in 1. The results suggest that the metal-peroxo core
bonding in 1 (known to have µ-η2:η1 peroxo ligation, Chart
1) is stronger than would be found in 3 with side-on side-on
µ-η2:η2 peroxo bonding. This seems surprising to us, and
we propose that the favorable CO reactivity with 3 reflects
stronger CO bonding in the resulting complex [(AN)CuI-
CO]+ compared to that in [(TMPA)CuI-CO]+. In support
of this supposition, it is qualitatively very difficult to remove
(by vacuum purge, Ar bubbling, or both) the CO in [(AN)-
CuI-CO]+ or other tridentate ligand CuI-complexes,53,54
while [(TMPA)CuI-CO]+ readily loses carbon monoxide.49
Further investigations which can provide thermodynamic data
on heme-peroxo-copper and ligand-CuI-CO complex
stabilities would be needed to obtain further insights.
[(6L)FeIII-(O22-)-CuII]+ (2) was found to react with CO
to form the bis(carbon monoxide) adduct [(solvent)(6L)FeII-
CO‚‚‚CuI-CO]+ (Scheme 2); as shown earlier, the heme-
CO moiety is also solvent bound leading to an overall six-
coordinate low-spin complex.49 When CO was bubbled
through an acetonitrile solution of 2 at - 40 °C, the Q-band
absorption at 560 nm, corresponding to 2, blue-shifted to
538 nm (Figure 3), which is the Q-band position indepen-
dently determined for [(CH3CN)(6L)FeII-CO‚‚‚CuI-CO]+
via CO exposure to the reduced complex [(6L)FeII‚‚‚CuI]+.49
Solution IR spectra (in CH3CN) confirmed that CO is bound
to both metals (Figure S2),50 as evidenced by the presence
of two ν(C-O) stretching bands, at 2094 cm-1 (CuI-CO)
and 1979 cm-1 (FeII-CO), values typical for copper-
carbonyl5,51 and reduced heme-CO5,31,49 coordination com-
plexes and metalloproteins. The same behavior was previ-
ously observed and described for [(2L)FeIII-(O22-)-CuII]+
(4), and evolution of O2 was detected (Scheme 2).23
Surprisingly, there is no reaction between [(F8)FeIII-
(O22-)-CuII(TMPA)]+ (1) and CO (after vigorous bubbling
for 2-5 min). By comparison to the positive reaction with
(51) Rondelez, Y.; Se´neque, O.; Rager, M.-N.; Duprat, A. F.; Reinaud, O.
Chem.sEur. J. 2000, 6, 4218-4226.
(52) Thompson, D. W.; Kretzer, R. M.; Lebeau, E. L.; Scaltrito, D. V.;
Ghiladi, R. A.; Lam, K.-C.; Rheingold, A. L.; Karlin, K. D.; Meyer,
G. J. Inorg. Chem. 2003, 42, 5211-5218.
(50) See Supporting Information.
6386 Inorganic Chemistry, Vol. 46, No. 16, 2007

FeIII-(O22-)-CuII Compounds
Reactions with PPh3. Phosphines such as PPh3 are also
interesting probes of metal-peroxo complexes because, as
is similar to carbon monoxide, they bind well to low-valent
(reduced) metal ions such as Fe(II)55 and copper(I).56-61 In
contrast to CO, PPh3 may in some cases be readily oxygen-
ated to give OdPPh3. However, it is already known that
mononuclear side-on bound peroxo-iron(III)-porphyrin
complexes do not oxidize PPh3;29 thus, we wondered if now
when the peroxide ligand also coordinates copper(II) ion, it
may be capable of phosphine oxidation. Indeed, with
differently coordinated peroxo-dicopper(II) complexes, PPh3
can either lead to displacement of the peroxo ligand (as O2),
accompanied by ligand-CuI-PPh3 formation, or alterna-
tively undergo oxidation.28,57-61
Figure 4. UV-vis spectra for the reaction of [(F8)FeIII-(O22-)-CuII(AN)]+
(3) (red, λmax ) 419 (Soret), 538, 561 nm) with 5 equiv of PPh3 in THF at
-80 °C. The product is (F8)FeIII-(O2-) (green, λmax ) 417 (Soret), 531
nm), identified by comparison with an authentic sample.37,38
Interestingly, [(F8)FeIII-(O22-)-CuII(AN)]+ (3) reacts with
PPh3 to displace the peroxo ligand from copper, but not from
iron, thus with formally a partial reduction reaction. The
products are the iron-superoxo species (solvent)(F8TPP)-
Scheme 3
-
FeIII-(O2 ) (UV-vis identification, λmax ) 417 and 531
nm)37,38 and the reduced CuI-phosphine adduct [CuI(AN)-
(PPh3)]+ (Scheme 3 and Figure 4). Tridentate N3- and
tetradentate N4-ligand-CuI complexes readily form PPh3
adducts.57,58,61 Direct evidence for the presence of [CuI(AN)-
(PPh3)]+ was not straightforward; however 31P NMR spec-
troscopic interrogation of the product solutions showed only
the presence of PPh3 (and no OdPPh3).62 The same behavior
was previously observed and documented for PPh3 reaction
with [(2L)FeIII-(O22-)-CuII]+ (4),23 see Scheme 3.
Unexpectedly, with complexes possessing the TMPA
tetradentate ligand on copper, [(F8)FeIII-(O22-)-CuII(TMPA)]+
(1) and [(6L)FeIII-(O22-)-CuII]+ (2), there is no reaction toward
PPh3. There are no UV-vis changes, and 31P NMR spec-
troscopy confirms that no PPh3 oxidation has occurred (see
Experimental Section). [CuI(TMPA)(PPh3)]+ is a known
compound with a described X-ray structure, and PPh3
displaces O2 from the well-known µ-1,2-peroxo dicopper-
(II) complex [(TMPA)CuII-(O22-)-CuII(TMPA)]2+.27,28,61 How-
ever, the observations here suggest the overall peroxo-metal
core (i.e., FeIII-(O22-)-CuII) bonding in 1 and 2 is stronger
than that in [(TMPA)CuII-(O22-)-CuII(TMPA)]2+, despite
the similar end-on binding to copper(II). Surprisingly (to us),
3 and 4 with their side-on η2-peroxo-Cu ligation (i.e., two-
bonds to Cu(II)) readily undergo reactions with PPh3
(Scheme 3). More insights are needed.63
Overall, with respect to CO and PPh3 binding, which
compete for the peroxo iron or copper ligation, complex
[(F8)FeIII-(O22-)-CuII(TMPA)]+ (1) appears to be the most
stable heme-peroxo-copper complex of the four being
studied.
(53) Karlin, K. D.; Haka, M. S.; Cruse, R. W.; Meyer, G. J.; Farooq, A.;
Gultneh, Y.; Hayes, J. C.; Zubieta, J. J. Am. Chem. Soc. 1988, 110,
1196-1207.
(54) Zhang, C. X.; Liang, H.-C.; Kim, E.-i.; Shearer, J.; Helton, M. E.;
Kim, E.; Kaderli, S.; Incarvito, C. D.; Zuberbu¨hler, A. D.; Rheingold,
A. L.; Karlin, K. D. J. Am. Chem. Soc. 2003, 125, 634-635.
(55) Simonneaux, G. Coord. Chem. ReV. 1997, 165, 447-474.
(56) Jardine, F. H. AdV. Inorg. Chem. Radiochem. 1975, 17, 115-163.
(57) Karlin, K. D.; Cruse, R. W.; Gultneh, Y.; Farooq, A.; Hayes, J. C.;
Zubieta, J. J. Am. Chem. Soc. 1987, 109, 2668-2679.
(58) Karlin, K. D.; Ghosh, P.; Cruse, R. W.; Farooq, A.; Gultneh, Y.;
Jacobson, R. R.; Blackburn, N. J.; Strange, R. W.; Zubieta, J. J. Am.
Chem. Soc. 1988, 110, 6769-6780.
Reactions with Acids. Protonation studies of peroxo-
bridged iron-copper synthetic models of heme-copper
oxidases are of particular importance because the O-O
cleavage process is assisted by 1 or 2 equiv of protons in
(59) Karlin, K. D.; Gan, Q.-F.; Farooq, A.; Liu, S.; Zubieta, J. Inorg. Chim.
Acta 1989, 165, 37-39.
(60) Karlin, K. D.; Tyekla´r, Z.; Farooq, A.; Haka, M. S.; Ghosh, P.; Cruse,
R. W.; Gultneh, Y.; Hayes, J. C.; Toscano, P. J.; Zubieta, J. Inorg.
Chem. 1992, 31, 1436-1451.
(61) Tyekla´r, Z.; Jacobson, R. R.; Wei, N.; Murthy, N. N.; Zubieta, J.;
Karlin, K. D. J. Am. Chem. Soc. 1993, 115, 2677-2689.
(62) The triphenylphosphine added to complex 3 either remains free in
solution or coordinated as [CuI(AN)(PPh3)]+, which leads to some
observed line broadening of the 31P-NMR resonance of PPh3, as we
have previously described in detail for the chemistry of complex 4,
see ref 23.
(63) A reviewer suggested perhaps the lack of reactivity of 1 and 2 with
PPh3 could be caused by sterics. We see this as being unlikely because,
as already noted in the text, PPh3 reacts readily with the very bulky
complexes 3 and 4 and with the peroxodicopper(II) complex with
TMPA ligand. We tried a reaction of 1 with PMe3 (1 and then 2 equiv),
and even with one equivalent, an immediate reaction leading to the
bisphosphine heme-iron(II) compound (UV-vis monitoring) occurs,
as confirmed by separate control experiments. Thus, a totally different
chemistry occurs with PMe3, undoubtedly because its very strong
Lewis basicity and reducing capacity.
Inorganic Chemistry, Vol. 46, No. 16, 2007 6387

Chufa´n et al.
Figure 5. UV-vis spectra for the reaction of [(F8)FeIII-(O22-)-CuII-
(TMPA)]+ (1) (red, λmax ) 558 nm) with 2 equiv of HCl in CH3CN at
-40 °C. The brown spectrum of the final product is dominated by the
absorption features distinctive to (F8)FeIII-Cl (λmax ) 503, 567, 640 nm),
also identified by comparison with an authentic sample.40
the key enzymatic O-O cleavage step, A f ferryl (FeIVd
O‚‚‚CuII-OH- species) (see Introduction and some further
comments below).14-16 A hydroperoxo transient intermediate
(following electron-transfer reduction of A and subsequent
protonation) has also been at times invoked, and generation
of such a species by protonation of complexes 1-4 would
be of great fundamental interest.15,64 Furthermore, protonation
studies on synthetic models may afford insights relating to
the intricate CcO proton membrane translocation mechanism,
which is coupled to the O2-reductive cleavage.32,65
The basic nature of the peroxo ligand in all four FeIII-
peroxo-CuII compounds under study (1-4) is revealed in
the reaction with Brønsted acids. For example, the addition
of two or more equivalents of HCl to a solution of 1 at -40
°C cleanly gives (F8)FeIII-Cl, as revealed by formation of its
distinctive UV-vis spectrum (see “titration” in MeCN
solvent, with isosbestic behavior, Figure 5). There is no
evidence for any hydroperoxo compound intermediate (but
see discussion below).
Figure 6. Reaction of [(F8)FeIII-(O22-)-CuII(TMPA)]+ (1) with HCl in
CD3CN at -40 °C, followed by 1H NMR spectroscopy (only the 6-17 nm
“diamagnetic” region is shown): red asterisk, phenyl hydrogens of 1; green
asterisk, phenyl hydrogens of (F8)FeIII-Cl; blue asterisk, 4-pyridyl hydro-
gens of [CuII(TMPA)Cl]+. See text for further explanation.
HCl, the changes in the spectra indicate the reaction is
complete (Figure 6).68 The results show that no hydroperoxo
species is detected under these conditions.
1
Low-temperature H NMR spectroscopy was also em-
ployed to provide more insights and hopefully unveil
information concerning other reaction products. Figure 6
reveals the clean transformation of the [(F8)FeIII-(O22-)-CuII-
(TMPA)]+ (1) to two mononuclear products, again (F8)FeIII-
Cl, along with [CuII(TMPA)Cl]+, upon titration of 1 with
HCl in CD3CN at -40 °C (also, see Experimental Section).
With 1 equiv of HCl, the signals corresponding to the two
meta and one para phenyl hydrogens of the peroxo complex
1 (red asterisk) diminish in intensity, while new resonances
arise, attributed to the meta and para phenyl protons of (F8)-
FeIII-Cl (green asterisks). In addition, the 4-pyridyl hydrogen
resonance assignable to [CuII(TMPA)Cl]+ now appears (blue
asterisk).66 These assignments were confirmed by obtaining
spectra of authentic materials under the same conditions of
solvent and temperature.67 After the addition of 2 equiv of
Hydrogen peroxide as a coproduct was not detected by
NMR spectroscopy (a peak may be expected at ∼9.5 ppm).69
However, H2O2 was quantitatively determined spectropho-
tometrically as a peroxotitanyl species.70,71 After the addition
of 10 equiv of HCl (an excess, to optimize this analytical
procedure), H2O2 was found to be produced in a 70% yield
from [(F8)FeIII-(O22-)-CuII(TMPA)]+ (1) (See Experimental
Section).
1
To support the results found by H NMR spectroscopy,
the reaction of [(F8)FeIII-(O22-)-CuII(TMPA)]+ (1) with HCl
was also investigated by EPR spectroscopy and ESI-MS.
When studied in CH3CN at -40 °C, aliquots from the
reaction solution were frozen, and EPR spectra were
(68) A downfield broad absorption assigned to the pyrrole hydrogens of 1
(see ref 17) decreases in intensity as the new peak at 100 ppm (due to
the high-spin (F8)FeIII-Cl complex) grows (data not shown). In addition,
3- and 5-pyridyl hydrogens at ∼35 ppm (broad), due to CuII(TMPA)
(see ref 66), grow (data not shown).
(69) Shearer, J.; Scarrow, R. C.; Kovacs, J. A. J. Am. Chem. Soc. 2002,
124, 11709-11717.
(70) Eisenberg, G. M. Ind. Eng. Chem.sAnal. Ed. 1943, 15, 327-328.
(71) Chaudhuri, P.; Hess, M.; Mueller, J.; Hildenbrand, K.; Bill, E.;
Weyhermueller, T.; Wieghardt, K. J. Am. Chem. Soc. 1999, 121,
9599-9610.
(64) Collman, J. P.; Decre´au, R. A.; Sunderland, C. J. Chem. Commun.
2006, 3894-3896.
(65) Fox, S.; Nanthakumar, A.; Wikstro¨m, M.; Karlin, K. D.; Blackburn,
N. J. J. Am. Chem. Soc. 1996, 118, 24-34.
(66) Nanthakumar, A.; Fox, S.; Murthy, N. N.; Karlin, K. D. J. Am. Chem.
Soc. 1997, 119, 3898-3906.
(67) [CuII(TMPA)(CH3CN)]2+ was used as the reference compound for
1
the H NMR assignment.
6388 Inorganic Chemistry, Vol. 46, No. 16, 2007

FeIII-(O22-)-CuII Compounds
Figure 8. UV-vis spectra for the reaction of [(F8)FeIII-(O22-)-CuII-
(TMPA)]+ (1) (red, λmax ) 558 nm) with tetrafluoroboric acid (HBF4) in
CH3CN at -40 °C. The orange spectrum is assigned to (F8)FeIII-F (λmax
) 478, 535, 595 nm), while the brown spectrum (final product) corresponds
to (F8)FeIII-(BF4) (λmax ) 509, ∼580, ∼640 nm).73 See text for further
discussion.
Figure 7. Reaction of [(F8)FeIII-(O22-)-CuII(TMPA)]+ (1) with HCl in
CH3CN at -40 °C, followed by EPR spectroscopy. Spectrum of 1 (red,
“silent” but with a small CuII(TMPA) impurity, upper spectrum) and after
the addition of 1 equiv of HCl (blue, middle spectrum) showing the spectra
associated with the FeIII-Cl and CuII-Cl complexes. The lower spectrum
(black) is associated with addition of 2 equiv of HCl, and small distortions
in the upfield copper(II) signal occur when excess HCl is present, see
Supporting Information: frequency, 9.192 GHz; temperature, 77 K; time
constant, 10.24 ms; sweep width, 4200 G. See text for further explanations.
addition of 2 equiv of tert-butylammonium chloride (as a
source of chloride) to 1 does not yield the expected (F8)-
FeIII-Cl (only ∼ 20% conversion to the µ-oxo analogue
[(F8)FeIII-O-CuII(TMPA)]+ is observed, UV-vis monitoring),
revealing that the reaction driving force is not only chloride
but also the proton.
recorded. As shown in Figure 7, the original 1 is EPR silent
because of the strong antiferromagnetic coupling between
the high-spin iron(III) and copper(II) ions.19 After the
addition of 1 equiv of HCl, a spectrum with two major
components is obtained: one occurs at g ) 5.8, and the other,
a copper(II) signal, occurs near g ) 2. The signal in the
low-field region of the spectrum is attributed to the perpen-
dicular contribution of the high-spin (F8)FeIII-Cl (with the
expected low-intensity parallel signal at g ≈ 2.00 being
overlapped by the strong signal of the copper(II) complex).
To gain more insights on the effect of the counterion, we
also carried out protonation experiments with [(F8)FeIII-
(O22-)-CuII(TMPA)]+ (1) using triflic (HOTf) and tetrafluo-
roboric acids. The reaction of 1 with HOTf acid in CH3CN
at -40 °C yields the compound identified as (F8)FeIII-OTf
(Figure S9)50 because triflate is also a good ligand for iron-
(III).73 Surprisingly, we did not observe the expected Cu
products, [CuII(TMPA)(X)]y+ (X ) OTf- or CH3CN), but
ESI-MS interrogation revealed that the only product is [CuII-
(TMPA)(CN)]+ (m/z ) 379.36, confirmed by simulation,
Figure S10);50 this complex has been previously synthesized
and characterized by X-ray diffraction.74 Further investigation
of the nature of this reaction will be carried out and reported
elsewhere. We surmise that the solvent acetonitrile is the
cyanide source following its oxidation, possibly by a copper-
hydroperoxo species which forms upon protonation of 1; we
have separately documented such a reaction effected by a
hydroperoxo-dicopper(II) complex.75
The result of the reaction of 1 with HBF4 was quite
different. After the addition of two equiv of HBF4, we
observed a new UV-vis spectrum (Figure 8), assigned to
(F8)FeIII-F [λmax (CH3CN) ) 478, 530, 595 nm], because
of its similarity to the UV-vis spectrum of the analogue
(OEP)FeIII-F [λmax (CH2Cl2) ) 485, 510, 600 nm].76 We
considered the possibility that a fluoride-bridged complex
formed, that is, [(F8)FeIII-F-CuII(TMPA)]2+, which would
be an analogue to the known compound [(OEP)FeIII-F-
CuII(bnpy2)(CH3CN)]+ {OEP ) octaethylporphyrinate; bnpy
In fact, the higher-field copper(II) signal is distinctive (g
||
) 2.08, g ) 2.18, A ) 91 × 10-4 cm-1; a “reverse” axial
EPR signal (g < g ) that is expected for trigonal-bypyra-
||
midal complexes with the TMPA ligand, namely, [CuII-
(TMPA)(X)]y+ (X ) CH3CN or Cl-, Figure 7)).72 The
addition of the second equivalent of HCl completes the
reaction as shown by the development of more intense signals
for the chloride complexes of both iron and copper. ESI-
MS investigation supports the EPR conclusions concerning
the copper complex product; a prominent signal at m/z )
388.38 and simulation of the signal envelope around this
corresponds to [CuII(TMPA)Cl]+ (Figure S7).50
We also carried out the protonation of [(F8)FeIII-(O22-)-
CuII(TMPA)]+ (1) in CH2Cl2 to corroborate the above
findings; we were concerned if CH3CN as solvent and known
ligand for iron and copper (in oxidized or reduced states)
might influence the chemistry. This was not the case because
dissolution of solid peroxo complex 1 (see Experimental
Section) in CH2Cl2 and monitoring of the HCl reactivity by
UV-vis spectroscopy gave the same results (see Supporting
Information, Figure S8).
From the observations thus far described, it is clear that
the protonation reactions are strongly influenced by the
presence of the chloride counterion, with its high affinity
for iron(III) and copper(II) ions; consequently, (F8)FeIII-Cl
and [CuII(TMPA)Cl]+ are formed (vide supra). However, the
(72) Lucchese, B.; Humphreys, K. J.; Lee, D.-H.; Incarvito, C. D.; Sommer,
R. D.; Rheingold, A. L.; Karlin, K. D. Inorg. Chem. 2004, 43, 5987-
5998.
(73) Boersma, A. D.; Goff, H. M. Inorg. Chem. 1982, 21, 581-586.
(74) Corsi, D. M.; Murthy, N. N.; Young, J. V. G.; Karlin, K. D. Inorg.
Chem. 1999, 38, 848-858.
(75) Li, L.; Sarjeant, A. A. N.; Karlin, K. D. Inorg. Chem. 2006, 45, 7160-
7172.
(76) Lee, S. C.; Holm, R. H. Inorg. Chem. 1993, 32, 4745-4753.
Inorganic Chemistry, Vol. 46, No. 16, 2007 6389

Chufa´n et al.
) N,N-bis(2-(2-pyridylethyl))benzylamine}.76 However, this
can be ruled out because EPR spectroscopy (Figure S11)50
reveals distinctive signals for both uncoupled heme (i.e.,
mononuclear FeIII high-spin) and copper complexes; EPR
spectra of a heme-F-CuII coupled system show distinctively
different resonances.77,78 As would be expected, the addition
of excess HBF4 yields the mononuclear (F8)FeIII-(BF4)
complex (Figure 8).73 The overall results with HBF4 addition
Scheme 4
-
to heme-peroxo-copper complex 1 show that BF4 as
counterion acts as a reactive source of fluoride (as is well-
known),76 and this is responsible for part of observed
chemistry. As concerns the copper complex product in the
HBF4 + 1 reaction, an analogous result to that seen with
triflic acid was observed. The only product detected, by ESI-
MS, was the same cyanide complex [CuII(TMPA)(CN)]+
(m/z ) 379.46),50 and we suggest similar chemistry (vide
supra) is occurring here.
Scheme 5
To summarize, the counterions chloride, triflate, and
tetrafluoroborate (by itself or providing fluoride) are playing
key roles in the reactions of heme-peroxo-copper complex
of [(F8)FeIII-(O22-)-CuII(TMPA)]+ (1) with HCl, HOTf, and
HBF4, respectively. The HCl reactions are quite clean, giving
chloride-metal products, but for the other acids with weak
ligand counterions, additional chemistry occurs. As men-
tioned, HBF4 causes the (F8)FeIII-F complex to form. With
either HBF4 or HOTf, some new chemistry occurs leading
to cyanide production and ready formation of [CuII(TMPA)-
(CN)]+. In the probe for production of hydrogen peroxide
by HCl reaction with 1, this occurs in a near stoichiometric
fashion. However, with HOTf and HBF4, we could not
confirm any H2O2 formation with the titanyl oxo-sulfate
reagent because of spectroscopic interference with aqueous
acetonitrile soluble heme complexes or other chemical
reactions leading to new strongly absorbing species. In fact,
we would not expect hydrogen peroxide to be left over
because it would be used up in the CH3CN oxidation and
cyanide formation.75
As is relevant to the discussion above concerning proto-
nation of [(F8)FeIII-(O22-)-CuII(TMPA)]+ (1), we mention
here preliminary observations with [H(Et2O)2][B(C6F5)4]79
as our acid source, with its non-coordinating perfluorinated
tetraarylborate counteranion. Reaction of 1 with 1 equiv of
[H(Et2O)2][B(C6F5)4] at - 40 °C in CH3CN indeed leads to
a new UV-vis spectrum (see Supporting Information, Figure
S13). In parallel, reaction of 1 with one equiv of trimethyl-
oxonium tetrafluoroborate (as a proton analogue methylating
reagent) gave essentially the same UV-vis spectrum. We
speculate that the new species (based on UV-vis criterion)
are hydro- or methyl-peroxo complexes FeIII-(µ--O2R)-
CuII or FeIII-(-O2R)‚‚‚CuII, R ) H or Me. Future investiga-
tions will focus on further characterization of these interesting
and potentially important species.
We now very briefly summarize the similar results
observed for the other complexes 2-4 (Scheme 4). They all
react with HCl to give a (porphyrinate)FeIII-Cl moiety (as
judged by UV-vis or 1H NMR spectroscopies, see Experi-
mental Section and Supporting Information, Figures S14-
S16) liberating hydrogen peroxide, in 80% yield from 2 and
77% yield from 3. The yield in the reaction of 4 with HCl
was not determined.
Reactions with Cobaltocene. The full reductive cleavage
of dioxygen to water requires four electron equivalents.
However, in our present CcO model complexes 1-4,
dioxygen has been reduced only by two-electrons to the
peroxo level and is coordinated by a high-spin iron(III) and
copper(II) ion. In this section, we report the reactivity of
peroxo compounds 1-4 toward cobaltocene (CoCp2), a
strong one-electron outer-sphere reductant.80 Can the addition
of one or two more reducing equivalents lead to O-O
reductive cleavage and what will occur? Part of the reason
for examining this reaction is the prior work of Collman and
co-workers33-36 where they showed that heme-peroxo-
copper complexes with added Co(Cp)2 lead to fully reduced
FeII/CuI compounds (Scheme 5). We wished to compare and
contrast such behavior with our own series of complexes.
In fact, the reaction of 1 and 2 with cobaltocene has been
previously described in earlier reports.21,81 As shown in
Scheme 6, the addition of two or more equiv of CoCp2 to a
solution of 1 or 2 yields the corresponding structurally
characterized µ-oxo-bridged compounds [(F8)FeIII-O-CuII-
(TMPA)]+ 40 and [(6L)FeIII-O-CuII]+,41 respectively. Here, we
add to this information and report on the other cases. For
the reaction with 1, cobaltocinium cation ([CoIIICp2]+,
Scheme 6) was directly observable by UV-vis spectroscopy,
(77) Oganesyan, V. S.; Butler, C. S.; Watmough, N. J.; Greenwood, C.;
Thomson, A. J.; Cheesman, M. R. J. Am. Chem. Soc. 1998, 120, 4232-
4233.
(78) Watmough, N. J.; Cheesman, M. R.; Gennis, R. B.; Greenwood, C.;
Thomson, A. J. FEBS Lett. 1993, 319, 151-154.
(79) Jutzi, P.; Muller, C.; Stammler, A.; Stammler, H. G. Organometallics
2000, 19, 1442-1444.
(80) Connelly, N. G.; Geiger, W. E. Chem. ReV. 1996, 96, 877-910.
(81) Chufa´n, E. E.; Karlin, K. D. J. Am. Chem. Soc. 2003, 125, 16160-
16161.
6390 Inorganic Chemistry, Vol. 46, No. 16, 2007

FeIII-(O22-)-CuII Compounds
complexes relative to our series of compounds;5,35 this would
prevent formation of µ-oxo heme-Cu complexes requiring
an Fe‚‚‚Cu separation of e3.6 Å.5 We find pervasive
formation of µ-oxo FeIII-O-CuII(ligand) complexes, that is,
from “acid-base” reactions or from thermal transformation
products of heme-peroxo-copper(II) compounds.5 The
(porphyrinate)FeIII-O-CuII entity seems to be a “sink” for
heme/Cu/O2 chemistry in the same way that the µ-oxo
(porphyrinate)FeIII-O-FeIII(porphyrinate) compounds are
the sink for heme/O2 chemistry.8,84,85
Scheme 6
Reactions with t-Butyl-Substituted Phenols. Phenol
addition to heme-Cu/O2 adducts is inspired by reactivity
observed for the “mixed-valent” CcO enzyme form, wherein
a tyrosine, as part of a histidine-tyrosine binuclear center
cofactor (the His is bound to CuB), is thought be a H-atom
donor.5 When either 2,4-tBu2-phenol or 2,4,6-tBu3-phenol
as good hydrogen atom (H+ + e-) donors are added to 1-4
(Chart 1) solutions at low-temperature, no reactions occur.
These conclusions are based on monitoring by UV-vis and
EPR (to follow possible phenoxyl radical formation) spec-
troscopies.
λmax ) 262 nm81 (data not shown).35,82 In the reaction of
[(F8)FeIII-(O22-)-CuII(AN)]+ (3) with two or more equivalents
of CoCp2, a similar result occurs, Scheme 6; the complete
clean conversion to the corresponding oxo-bridged complex
[(F8)FeIII-O-CuII(AN)]+ takes place in THF at -80 °C, as
monitored by UV-vis spectroscopy (Supporting Information,
Figures S1 and S17).83 Reaction of excess CoCp2 with [(2L)-
FeIII-(O22-)-CuII]+ (4) gives a related result (Scheme 6), in
that an overall two-electron reduction occurs, giving a
mixture of µ-oxo and iron(III) hydroxide complexes within
the 2L framework, [(2L)FeIII-O(H)‚‚‚CuII]2+, based on
distinctive UV-vis spectra known for the FeIII-OH and
µ-oxo [FeIII-O-CuII]+ moieties.65 The 2L binucleating ligand
appears not to be well set up to stabilize an FeIII-O-CuII
moiety, and it is very sensitive to solvent water/protons.23
Thus, in all cases for peroxo complexes 1-4, reaction even
with excess CoCp2 leads only to two-electron reduction
chemistry, and either oxo or hydroxo (for 4) stabilization of
iron(III) and Cu(II) precludes further reduction to iron(II)
and Cu(I). This contrasts strongly with the results of Collman
and co-workers, as mentioned above. A number of factors
may be in play. Collman’s peroxo complexes are low-spin
because of the presence of a tethered axial ligand (pyridyl
or imidazolyl) “base” (B, Scheme 5), which is part of the
sophisticated superstructured binucleating ligand design, or
in certain cases, an excess of an imidazole derivative was
added. Thus, the low-spin nature, which would favor
reduction to iron(II) (d6, low-spin) may facilitate the full
reduction observed. Another factor could be the likely
elongated (by ligand design) Fe‚‚‚Cu separation in Collman’s
Collman and co-workers64 recently reported that an ex-
ogenously added phenol can be added to a heme-superoxo-
-
copper(I) (i.e., with FeIII(O2 )‚‚‚CuI moiety), resulting in
O-O reductive cleavage; this heme-Cu complex possesses
an internal (i.e., appended) imidazole axial base for iron,
presumably rendering the system to be low-spin.88 It is
notable that for non-heme iron FeIII-OOR complexes, low-
spin rather than high-spin species are amenable to reductive
O-O cleavage chemistry as a consequence of weakened
peroxide O-O bonds and strengthened Fe-O bonds.86,87
Summary and Conclusions
The results described in this paper provide interesting
insights into the nature of the peroxo moiety in FeIII-(O22-)-
CuII complexes 1-4 (Chart 1) as revealed by reactivity
studies, with CO, PPh3, acids, cobaltocene, and phenol.
The reactions with carbon monoxide and triphenylphos-
phine in part reflect the fact that all of these complexes are
in fact dioxygen adducts derived from FeII/CuI/O2 reactions,
since CO or PPh3 can cause the release of O2 or its (formal)
reduction to the superoxide level now bound to the heme
(Schemes 2 and 3). The reversibility/irreversibility of O2
binding is not the reason for the different CO and PPh3
reactivity found for the 1-4 peroxo complexes because, as
was stated at the beginning, the formation of all 1-4 peroxo
complexes is irreversible. The driving force would appear
to be the formation of very stable low-valent CuI-CO or
CuI-PPh3 fragment, Schemes 2 and 3. In copper-dioxygen
chemistry with closely related N3 tridentate or N4 tetradentate
chelates, the same phenomenon is observed, namely,
[LCuII-(O22-)CuIIL]2+ complexes react with either CO or
(82) Scheme 6 indicates water is produced in the reactions of peroxo
complexes. While H2O was not directly detected, our conclusions about
the products and stoichiometry of reaction follow from (1) the
observation that cobaltocinium cation is formed along with the Fe-
oxo-Cu species, suggesting oxide ion would also be a product, and
(2), Collman (ref. 35) also suggests an oxide forms in such reactions.
We suggest that the two protons would come from solvent water
molecules, with the “leftover” hydroxide anions acting as counterions
for the 2 equiv of cobaltocinium produced.
(84) Kurtz, D. M., Jr. Chem. ReV. 1990, 90, 585-606.
(85) Balch, A. L. Inorg. Chim. Acta 1992, 198-200, 297-307.
(86) Lehnert, N.; Ho, R. Y. N.; Que, L., Jr.; Solomon, E. I. J. Am. Chem.
Soc. 2001, 123, 12802-12816.
(87) Ho, R. Y. N.; Roelfes, G.; Feringa, B. L.; Que, L., Jr. J. Am. Chem.
Soc. 1999, 121, 264-265.
(83) The UV-vis features of [(F8)FeIII-O-CuII(AN)]+ are shown in Figures
S1 and S17; a confirming X-ray structure has been obtained and will
be reported elsewhere.
Inorganic Chemistry, Vol. 46, No. 16, 2007 6391

Chufa´n et al.
PPh3 to give 2 LCuI-L′ (L′ ) CO or PPh3) + O2.28,57-61
Remarkably, one complex does not at all react with CO or
PPh3, [(F8)FeIII-(O22-)-CuII(TMPA)]+ (1). In summary, there
are clear differences in FeIII-(O22-)-CuII reactions with CO
or PPh3 that reveal the dioxygen origin of the peroxo group
in 2-4 and reflect variations in the detailed nature of the
peroxo structure and ligand architecture.
The reactions of 1-4 with hydrochloric acid lead to good
yields of hydrogen peroxide. The procedures used to
demonstrate this may be generally useful as a chemical rather
than spectroscopic tool to provide evidence that O2 adducts
have a peroxidic nature.81 The other point is that the chloride
present, as an excellent FeIII and CuII ligand, facilitates release
of peroxide and H2O2 production. Thus, future use of acids
with non-coordinating counteranions (e.g., H(Et2O)2B(C6F5)4)
may lead to the study of interesting and potentially important
hydroperoxo heme-copper assemblies.
The reactions of cobaltocene with 1-4 are quite different
than that observed for heme-copper/O2 adducts studied by
Collman and co-workers. For those, complete reduction to
FeII/CuI species occurred, but in our case, only partial
reduction occurs; the peroxide is reduced and either FeIII-
oxo-CuII or FeIII-hydroxo‚‚‚CuII products form. The origin
of the difference most likely lies in the fact that 1-4 are
high-spin entities not possessing any ligating heme axial
“base” (pyridine or imidazole).
These observations are probably also related to our
findings on the lack of reactivity of peroxo complexes 1-4
even with rather easily oxidizable tbutyl-substituted phenols.
Thus, reductive (protonation) of heme-peroxo-copper
complexes would seem to require heme-copper assemblies
with axial “base” ligands (Vide supra), and this is a direction
for future in-depth studies of the O-O cleavage process,88
of broad fundamental interest and of course relevant to CcO
function.
around 9.5 GHz, with sample temperature maintained at 77 K. EPR
spectra were referenced to 2,2diphenyl-1-picrylhydrazyl (g )
2.0036). IR spectra were obtained at room temperature using a
Mattson Galaxy 4030 series FT-IR spectrophotometer. Mass
spectrometry (ESI in the positive ion mode) was measured on a
Thermo-Finnigan LCQ Deca.
Syntheses. The reduced complexes (F8)FeII‚H2O,38,89 [CuI(TMPA)-
(CH3CN)](ClO4),61 [CuI(AN)](B(C6F5)4),39 [(6L)FeII‚‚‚CuI](B(C6F5)4),21
and [(2L)FeII‚‚‚CuI](B(C6F5)4)23 were synthesized according to
published procedures.
Generation of [(F8)FeIII-(O22-)-CuII(AN)]+ (3) and Thermal
Decomposition. Equimolar amounts of (F8)FeII‚H2O and [CuI(AN)]-
(B(C6F5)4) were dissolved in 10 mL deoxygenated THF in glovebox
and transferred to a modified low-temperature cuvette assembly
equipped with a Schlenk-type sidearm.53 After the mixture was
cooled to 193 K, UV-vis spectra of the reduced mixture were taken
[λmax ) 422 (Soret), 542 nm, see Figure 1]. Then, generation of
the peroxo adduct 3 [λmax ) 418 (Soret), 478, 538, and 561 nm]
was accomplished by bubbling dioxygen through the solution.
Complex 3 underwent thermal decomposition (with 1/2 mole of
O2 liberated from each mole of 3, determined spectrophotometrically
using a pyrogallol solution) to the corresponding µ-oxo complex
[(F8)FeIII-O-CuII(AN)]+ with the characteristic red-shifted Soret
band at 440 nm and broad 557 nm absorption (Figure S1). This
disproportionation chemistry is observed with the other heme-
peroxo-copper complexes 1 and 2, and perspectives on these
interesting transformations are discussed elsewhere.4,17,21 This µ-oxo
product is thermally stable, but in presence of moisture, it
decomposes to the corresponding µ-hydroxo [(F8)FeIII-(OH-)-CuII-
(AN)]2+, with absorptions at 410 and 572 nm.
Resonance Raman Studies on [(F8)FeIII-(O22-)-CuII(AN)]+ (3).
In the glovebox, 5 mM solutions of an equimolar mixture of (F8)-
FeII‚H2O and [(AN)CuI]+ in THF were prepared and transferred to
NMR tubes (∼0.5 mL solution in each tube) and capped with tight-
fitting septa. The sample tubes were placed in a cold bath (dry
ice/acetone) for ∼15 min and oxygenated using 16O2 and 18O2 (18O2
gas was purchased from ICON, Summit, NJ, and 16O2 gas was
purchased from BOC gases, Murray Hill, NJ). The labeled gases
were cooled in dry ice for 5 min and injected through the solution
by using a Hamilton gastight syringe. The oxygenated samples were
set in a cold bath for 10 min, after which the sample tubes were
frozen in liquid N2 and sealed by flame. Resonance Raman spectra
were collected on a McPherson 2061/ 207 spectrograph in a 0.67
m configuration equipped with a Kaiser Optical supernotch filter
to minimize interferences from the Rayleigh scattering. Raman
scattered light, gathered in a backscattering geometry, was dispersed
using a 2400 gr/mm holographic grating and analyzed using a liquid
nitrogen-cooled Princeton Instruments LN-1100PB CCD detector.
A Coherent Innova 302 krypton ion laser was used as the source
for the 413 nm excitation. The samples were placed in a custom
glass Dewar equipped with a copper coldfinger cooled with liquid
nitrogen to maintain the sample temperature at ∼90 K. A custom
sample spinning was employed to minimize sample degradation
upon laser illumination. Additionally, the laser power was kept
below 10 mW, and individual spectra were compared to confirm
the integrity of the samples during the data acquisition. Vibration
Experimental Section
Materials and Methods. All reagents and solvents were
purchased from commercial sources and were of reagent quality
unless otherwise noted. Acetonitrile (CH3CN) and heptane were
distilled from calcium hydride; tetrahydrofuran (THF) was distilled
from sodium/benzophenone under argon, and methylene chloride
(CH2Cl2) was purified over an activated alumina column. Prepara-
tion and handling of air-sensitive compounds were performed under
an argon atmosphere using standard Schlenk techniques or in a
MBraun Labmaster 130 inert atmosphere (<1 ppm O2, <1 ppm
H2O) glovebox filled with nitrogen. Deoxygenation of solvents was
effected either by repeated freeze/pump/thaw cycles or by bubbling
with argon for 30-45 min.
Low-temperature UV-vis spectra were recorded on a Hewlett-
Packard Model 8453A diode array spectrometer with HP Chem-
station software; the instrument was equipped with a variable-
temperature Dewar and cuvette assembly as described elsewhere.53
NMR spectra were measured on a Varian XL-400 NMR instrument
at ambient or at low temperatures. All spectra were recorded in 5
mm o.d. NMR tubes, and chemical shifts δ (ppm) were referenced
either to an internal standard (Me4Si) or to residual solvent peaks.
Electron paramagnetic resonance (EPR) spectra were obtained in
frozen solutions with 4 mm o.d. quartz tubes in a Bruker EMX
spectrometer operating at X-band using microwave frequencies
(88) We have also described related chemistry. While 3 does not react with
2,4,-tBu2-phenol, if dicylohexylimidazole is first added to form a low-
spin adduct, this then reacts with the phenol to give O-O cleavage
and a (porphryinate)FeIVdO (ferryl) product, plus the bis-phenol
3,3′,5,5′-tetra-tert-butyl-2,2′-dihydroxybiphenyl. Details will be re-
ported elsewhere.
(89) Kopf, M.-A.; Neuhold, Y.-M.; Zuberbu¨hler, A. D.; Karlin, K. D. Inorg.
Chem. 1999, 38, 3093-3102.
6392 Inorganic Chemistry, Vol. 46, No. 16, 2007

FeIII-(O22-)-CuII Compounds
frequencies were calibrated relative to an internal standard and are
as described above) was then added. The UV-vis spectra of the
original peroxo complex 3 and the product of the reaction were
recorded (Figure 4). In a separate experiment, the oxidation of PPh3
was investigated by 31P NMR spectroscopy. The low-temperature
reaction was carried out in a Schlenk flask and then, at room
temperature, the solvent was removed, and the solid residue was
taken with acetone-d6. The 31P NMR spectrum show no significant
peak for OdPPh3.
Reaction of [(F8)FeIII-(O22-)-CuII(TMPA)]+ (1) with HCl. (a)
UV-vis Analysis in CH3CN. The peroxo compound 1 was
generated in situ in CH3CN at -40 °C as described above, and
then 0.5, 1.0, 1.5, and 2.0 equiv of HCl (50, 100, 150, and 200 µL,
respectively, of HCl stock solution prepared with 100 mg of a
hydrogen chloride 1.0 M solution in diethyl ether, Aldrich, in 10
mL of CH3CN) were added, and the respective spectra recorded at
- 40 °C (see Figure 5). Finally excess HCl was added, but there
was no further change in the UV-vis spectrum.
accurate to (1 cm-1
.
Generation of [(F8)FeIII-(O22-)-CuII(TMPA)]+ (1) for Reac-
tions Monitored by UV-vis Spectroscopy. In a typical experi-
ment, a stock solution of equimolar amounts of (F8)FeII‚H2O (20
mg) and [CuI(TMPA)(CH3CN)](ClO4) (12 mg) in 19.1 mL of
deoxygenated CH3CN (15 g) was prepared in glovebox. Then, 4.1
mL (3.20 g) of stock solution was diluted to 12.7 mL (10 g) and
transferred to a modified low-temperature cuvette assembly equipped
with a Schlenk-type sidearm.53 After the mixture was cooled to
233 K, generation of the peroxo adduct 1 was carried out by
bubbling the chilled solution with dioxygen. Removal of excess
O2 was performed by application of 3-5 vacuum/Ar cycles.
Generation of [(6L)FeIII-(O22-)-CuII]+ (2) for Reactions Moni-
tored by UV-vis Spectroscopy. In a typical experiment, 9 mg of
[(6L)FeII‚‚‚CuI][B(C6F5)4] was dissolved in ∼20 g of deoxygenated
CH3CN, THF, or acetone in a glovebox and transferred to a
modified low-temperature cuvette assembly equipped with a
Schlenk-type sidearm.53 After the mixture was cooled to 233 or
193 K (depending of the solvent), generation of the peroxo adduct
2 was carried out by bubbling the chilled solution with dioxygen.
Removal of excess O2 is performed by application of 3-5 vacuum/
Ar cycles.
(b) UV-vis Analysis in CH2Cl2. Fifteen milligrams of solid 1
(isolated as reported in ref 19) was dissolved in 25 mL of precooled
CH2Cl2 (0.46 mM), and 7 mL of that solution was transferred to a
cuvette assembly with sidearm stopcock. One and two equivalents
of HCl (50 and 100 µL, respectively, of HCl stock solution prepared
with 472 mg hydrogen chloride 1.0 M solution in diethyl ether,
Aldrich, in 10 mL CH2Cl2) were added, and the respective spectra
were recorded at -80 °C (see Figure S5). Finally, the solution was
warmed to RT, and then the spectrum of the decomposition product
was recorded at -80 °C.
Generation of [(F8)FeIII-(O22-)-CuII(AN)]+ (3) and [(2L)FeIII-
(O22-)-CuII]+ (4) for Reactions Monitored by UV-vis Spec-
troscopy. Complexes 3 and 4 were generated in a similar way to
that described for 1 and 2, respectively (see above).
Reaction of [(6L)FeIII-(O22-)-CuII]+ (2) with CO. (a) UV-vis
Analysis. The peroxo compound 2 was generated in situ in CH3-
CN at -40 °C as described above, and its UV-vis spectrum was
recorded. CO was bubbled through the solution for ∼10 s, using a
long needle, and then the UV-vis spectrum of the product was
taken. The product of the reaction, [(6L)FeII-CO‚‚‚CuI-CO]+, was
proven to be stable at room temperature.
(b) IR Analysis of the Product. After the UV-vis experiment
(see above), an aliquot of the solution at room temperature was
transferred to a SPECAC cell and its IR spectrum was recorded
(see Figure 3 and S2).
Reaction of [(F8)FeIII-(O22-)-CuII(AN)]+ (3) with CO. (a) UV-
vis Analysis. The peroxo compound 3 was generated in situ in
THF at -80 °C as described above, and its UV-vis spectrum was
recorded (Figure S3). CO was bubbled through the solution for
∼10 s, and then the UV-vis spectrum of the product was taken.
At least one of the products of the reaction, (F8)FeII-CO (which
dominates the UV-vis features of the spectrum) proved to be stable
at room temperature.
(b) IR Analysis of the Product. After the UV-vis experiment,
an aliquot of the solution at room temperature was transferred to a
SPECAC cell, and its IR spectrum was recorded (see Figure S4).
Investigations on [(F8)FeIII-(O22-)-CuII(TMPA)]+ (1) with
PPh3. In a UV-vis cuvette assembly, 10.2 mL (8 g) of a 0.4 mM
solution of [(F8)FeIII-(O22-)-CuII(TMPA)]+ (1) was generated in
CH3CN at -40 °C ; then 5 equiv of PPh3 was added (176 µL of a
115 mM fresh stock solution prepared with 150.5 mg of PPh3 in 5
mL of acetonitrile). The cuvette was kept in a freezer at -32 °C
(with another cuvette containing only the peroxo compound 1 as a
control), and there was no change in the original UV-vis spectrum
of the peroxo compound 1 for at least 5 days. Furthermore, the
oxidation of PPh3 was tested by 31P NMR spectroscopy, and the
result was negative, confirming that the peroxo compound 1 is stable
before PPh3.
1
(c) H NMR Analysis in CD3CN. In the glovebox, 20 mg of
(F8)FeII‚H2O and 12 mg of [CuI(TMPA)(CH3CN)]ClO4 were
dissolved in 5 mL of deoxygenated CD3CN (4.9 mM), and the
mixture was transferred to a Schlenk flask capped with a septum.
Outside the glovebox, the solution was cooled to -40 °C and
bubbled with dioxygen to generate the peroxo compound 1. Then
0.6 mL of the solution (containing 0.0029 mmol of 1) was
transferred to a screw-capped NMR tube, and the 1H NMR spectrum
was recorded at -40 °C. One equivalent of HCl (50 µL, 0.0029
mmol, CH3CN solution prepared from diethyl ether 1.0 M solution
of hydrogen chloride, Aldrich) was then introduced via syringe;
1
the solution was shaked quickly, and the H NMR spectrum was
recorded at -40 °C. This process of adding 1 equiv of HCl and
recording spectrum was repeated to complete the reaction.
(d) EPR Analysis in CH3CN. In the glovebox, 20 mg of (F8)-
FeII‚H2O and 12 mg of [CuI(TMPA)(CH3CN)]ClO4 were dissolved
in 10.2 mL (8 g) of CH3CN, and the mixture was transferred to a
25 mL Schlenk flask equipped with a stir-bar and capped with a
septum. Outside the glovebox, the solution was cooled to -40 °C
and bubbled with dioxygen to generate the peroxo compound 1.
Then, 0.30 mL of the solution was transferred to an EPR tube and
the EPR spectrum was recorded at 77 K (conc ∼2 mM). One
equivalent of HCl (48.5 µL 1.0 M diethyl ether solution of hydrogen
chloride, Aldrich) was then added to the mother solution, and a
new aliquot of 0.30 mL was taken; then, the EPR spectrum was
recorded at 77 K. This process of adding 1 equiv of HCl and
recording spectrum was repeated to complete the reaction.
(e) Quantitative Determination of H2O2 by the Peroxotitanyl
Method. Equimolar amounts of (F8)FeII‚H2O (24.9 mg) and
[CuI(TMPA)(CH3CN)]ClO4 (14.8 mg) were dissolved in 10 mL
of CH3CN (conc ) 3 mM) in the glovebox. The solution was
brought out of the glovebox and immersed in a cold bath at - 40
°C, and dioxygen was then bubbled through the solution to generate
the peroxo compound 1. Excess O2 was removed by several
vacuum/Ar cycles; then 10 equiv of HCl (0.3 mL of 1.0 M diethyl
ether solution of hydrogen chloride, Aldrich) was added, and the
dark red solution turned to dark green. The solution was removed
from the cold bath, followed by addition of 20 mL of distilled water
Reaction of [(F8)FeIII-(O22-)-CuII(AN)]+ (3) with PPh3. In a
UV-vis cuvette assembly, a solution of [(F8)FeIII-(O22-)-CuII(AN)]+
(3) was generated in THF at -80 °C as described above; a
concentrated THF solution containing 5 equiv of PPh3 (prepared
Inorganic Chemistry, Vol. 46, No. 16, 2007 6393

Chufa´n et al.
(final volume ) 30.3 mL). A suspension is formed because of the
insolubility of (F8)FeIII-Cl in the solvent mixture. H2O2 was
determined spectrophotometrically (at 408 nm) upon addition of 1
mL of titanyl reagent (titanium(IV) oxysulfate in sulfuric acid,
Riedel de Hae¨n) to a 10 mL suspension, followed by centrifugation.
A calibration curve was made in the same solvent mixture of the
reaction (CH3CN/Ether/H2O). The yield of H2O2 was determined
to be 70% on the basis of two experiments.
Reaction of [(F8)FeIII-(O22-)-CuII(TMPA)]+ (1) with HOTf
(triflic acid), HBF4, [H(Et2O)2][B(C6F5)4], and [(CH3)3O](BF4)
(Trimethyloxonium Tetrafluoroborate). The peroxo compound
1 was generated in situ in CH3CN at -40 °C as described above;
then HOTf (trifluoromethanesulfonic acid) in different amounts
(from an acetonitrile stock solution prepared form a previously
distilled acid, Aldrich ampule) were added with syringe, and the
respective spectra were recorded at -40 °C (see Figure S9). Finally
excess acid was added but there was no further change in the UV-
vis spectrum. In the same manner, the experiments with HBF4
(using an acetonitrile stock solution prepared form Aldrich reagent)
(Figure 8), [H(Et2O)2][B(C6F5)4] (acetonitrile solution of the acid
prepared following a modified procedure of that published by P.
Jutzi et al.)79 (Figure S13), and [(CH3)3O](BF4) (acetonitrile solution
prepared from Aldrich reagent) were carried out.
Reaction of [(F8)FeIII-(O22-)-CuII(AN)]+ (3) with CoCp2 by
UV-vis Spectroscopy. To a solution of 3 generated as described
above [λmax ) 477(sh), 538, 561(sh) nm; THF, 193 K] (see Figure
S17) was added a THF solution of CoCp2 in small aliquots. The
addition of cobaltocene produced an immediate change in the UV-
vis spectrum corresponding to a loss of the peroxo complex 3 and
formation of the oxo product [(F8)FeIII-O-CuII(AN)]+ [λmax ) 557
nm]. Full formation of the oxo product was achieved after the
addition of 2 equiv of CoCp2, while excess CoCp2 did not produce
any further change in the UV-vis spectrum.
Reaction of [(2L)FeIII-(O22-)-CuII]+ (4) with CoCp2 by UV-
vis Spectroscopy. The reaction was carried out in the same manner
as that for complex 3 (see above).
Acknowledgment. We thank the National Institutes of
Health (GM60353, K.D.K.; GM34468, T. M. Loehr and
P.M-L.) and Universidad Nacional de San Luis-Argentina
(E.E.C) for research support.
Supporting Information Available: UV-vis spectrum of
µ-oxo complex [(F8)FeIII-O-CuII(AN)]+ (Figure S1), IR spectrum
of [(6L)FeII-CO‚‚‚CuI-CO]+ (Figure S2), UV-vis spectra moni-
toring the reaction of [(F8)FeIII-(O22-)-CuII(AN)]+ (3) with CO
(Figure S3), IR spectrum of the products [(THF)(F8)FeII(CO)] and
[(AN)CuI-CO]+ generated upon CO reaction of peroxo complex
3 (Figure S4), IR spectrum of authentic [(AN)CuI-CO]+ (Figure
S5), small distortion at the EPR upfield copper(II) signal of [CuII-
(TMPA)Cl]+ upon addition of excess HCl (Figure S6), Positive
ion electrospray (ESI) mass spectrum of the reaction product of
[(F8)FeIII-(O22-)-CuII(TMPA)]+ (1) with 2 equiv of HCl in CH3-
CN at -40 °C with calculated positive ESI mass spectrum of the
cation [CuII(TMPA)Cl]+ (Figure S7), UV-vis spectra for the
reaction of [(F8)FeIII-(O22-)-CuII(TMPA)]+ (1) with HCl in CH2-
Cl2 (Figure S8), UV-vis spectra for the reaction of [(F8)FeIII-(O22-)-
CuII(TMPA)]+ (1) with triflic acid (HOTf) (Figure S9), positive
ion electrospray (ESI) mass spectrum of the reaction product of
[(F8)FeIII-(O22-)-CuII(TMPA)]+ (1) with 2 equiv of triflic acid
(HOTf) in CH3CN at -40 °C with calculated positive ESI mass
spectrum of the cation [CuII(TMPA)CN]+ (Figure S10), X-band
EPR spectrum of the reaction product of [(F8)FeIII-(O22-)-CuII-
(TMPA)]+ (1) with 2 equiv of HBF4 in CH3CN at -40 °C (Figure
S11), positive ion electrospray (ESI) mass spectrum of the reaction
product of [(F8)FeIII-(O22-)-CuII(TMPA)]+ (1) with 2 equiv of HBF4
in CH3CN at -40 °C (Figure S12), UV-vis spectra for the reaction
of [(F8)FeIII-(O22-)-CuII(TMPA)]+ (1) with 1 equiv of [H(Et2O)2]-
[B(C6F5)4] and for the reaction of [(F8)FeIII-(O22-)-CuII(TMPA)]+
(1) with 1 equiv of (CH3)3OBF4 (Figure S13), UV-vis spectra for
the reaction of [(6L)FeIII-(O22-)-CuII]+ (2) with HCl (Figure S14),
UV-vis spectra for the reaction of [(F8)FeIII-(O22-)-CuII(AN)]+ (3)
with HCl (Figure S15), UV-vis spectra for the reaction of [(2L)-
FeIII-(O22-)-CuII]+ (4) with HCl (Figure S16), and reduction of [(F8)-
FeIII-(O22-)-CuII(AN)]+ (3) to [(F8)FeIII-O-CuII(AN)]+ by addition
of 2 equiv of cobaltocene monitored by UV-vis spectroscopy
(Figure S17). This material is available free of charge via the
Internet at http://pubs.acs.org.
Reaction of [(6L)FeIII-(O22-)-CuII]+ (2) with HCl. (a) UV-
vis Analysis in Acetone. The peroxo compound 2 was generated
in situ in acetone at -80 °C as described above; then 2 equiv and
excess HCl (taken from hydrogen chloride 1.0 M solution in diethyl
ether, Aldrich) were added, and the respective spectra were recorded
at -40 °C (see Figure S14).
1
(b) Analysis of Residue by H NMR. After the reaction was
UV-vis monitored (see above), the solvent was removed by
1
vacuum, and the solid residue taken with acetone-d6. H NMR
spectrum of the residue showed the paramagnetic pyrrole signal at
85 ppm, characteristic of [(6L)FeIII-Cl‚‚‚CuII-Cl]+.
(c) Quantitative Determination of H2O2 by the Peroxotitanyl
Method. The yield of H2O2 (80%) generated upon acidification of
2 was determined in a similar manner to that described for 1 (see
above).
Reaction of [(F8)FeIII-(O22-)-CuII(AN)]+ (3) with HCl. (a)
UV-vis Analysis. The peroxo compound 3 was generated in situ
in THF at -80 °C as described above; then 1, 2, and 4 equiv of
HCl (taken from hydrogen chloride 1.0 M solution in diethyl ether,
Aldrich) were added, and the respective spectra were recorded at
-80 °C (see Figure S15).
1
(b) Analysis of Residue by H NMR. After the reaction had
been UV-vis monitored (see above), the solvent was removed by
1
vacuum, and the solid residue taken with chloroform-d. H NMR
spectrum of the residue showed the paramagnetic pyrrole signal at
82 ppm, characteristic for (F8)FeIII-Cl.
(c) Quantitative Determination of H2O2 by the Peroxotitanyl
Method. The yield of H2O2 (77%) generated upon acidification of
3 was determined in a similar manner to that described for 1 (see
above).
Reaction of [(2L)FeIII-(O22-)-CuII]+ (4) with HCl. (a) UV-
vis Analysis. The peroxo compound 4 was generated in situ in
THF at -80 °C as described above (at Soret band scale); then 1,
2, and excess equiv of HCl (taken from hydrogen chloride 1.0 M
solution in diethyl ether, Aldrich) were added, and the respective
spectra were recorded at -80 °C (see Figure S16).
IC700363K
6394 Inorganic Chemistry, Vol. 46, No. 16, 2007