Enhanced reactivity of a biomimetic iron(II) α-keto acid complex through immobilization on functionalized gold nanoparticles

Article abstract of DOI:10.1002/anie.201305994

Immobilized biomimetic iron complex: An iron(II) benzoylformate complex of a thiol-appended N₄ ligand immobilized on gold nanoparticles activates dioxygen to carry out oxidative decarboxylation of benzoylformic acid to benzoic acid catalytically.

Full text of DOI:10.1002/anie.201305994

.
Angewandte
Communications
DOI: 10.1002/anie.201305994
Surface Functionalization
Enhanced Reactivity of a Biomimetic Iron(II) a-Keto Acid Complex
through Immobilization on Functionalized Gold Nanoparticles**
Debobrata Sheet, Partha Halder, and Tapan Kanti Paine*
Immobilization of transition-metal complexes by surface
functionalization of gold nanoparticles (AuNPs) has recently
attracted the attention for several applications.[1] Thiol-
protected AuNPs[2] are stable and soluble in organic solvents.
Therefore, immobilization of metal complexes on AuNPs
permits reactions in common organic solvents and also
induces properties of the metal complex to the NP.[3] AuNPs
functionalized by thiol-appended transition metal complexes
are expected to find applications as immobilized catalysts to
bridge between homogeneous and heterogeneous catalysis.
The high surface area of a nanocatalyst increases the contact
between the reactant and catalyst dramatically. These cata-
lysts are easy to synthesize through desired surface modifi-
cation and can be characterized by different analytical and
spectroscopic techniques. Moreover, the catalyst can easily be
separated from the reaction mixture. Several reports are now
available where immobilization of metal catalysts on AuNPs
has been shown to increase the catalytic reactivity.[4]
Improved catalytic activities of transition-metal catalysts
through surface functionalization of AuNPs have inspired us
to initiate a project on immobilization of biomimetic iron
complexes. In this direction, we have investigated the
dioxygen reactivity of nonheme iron complexes on function-
alized AuNPs. Dioxygen-activating a-ketoglutarate (a-KG)-
dependent oxygenases that carry out versatile biological
reactions are well-studied in biology and biomimetic chemis-
try.[5] The iron(II) centers of the enzymes activate dioxygen,
upon binding of a bidentate a-KG and the substrate, to form
reported to react with O2 for days to undergo stoichiometric
decarboxylation of the coordinated a-keto acids.[8a] More-
over, they often form m-oxo diiron(III) complex at the end of
the decarboxylation reaction, leading to the deactivation of
the complex.[8a] Such deactivation reaction may be avoided
with a properly immobilized complex on the surface of
AuNPs and is expected to exhibit improved/catalytic reac-
tivity upon immobilization. As an outcome of our investiga-
tion, we report herein an enhanced reactivity of a biomimetic
iron(II)–benzoylformate complex of a thiol-appended N4
ligand (TPASH, Scheme 1) towards dioxygen upon immobi-
Scheme 1. Ligands used in this study. AIBN=azobis(isobutyronitrile).
an iron–oxygen intermediate.[6] The heterolytic O O bond
À
cleavage of the intermediate results in formation of an
iron(IV)–oxo oxidant to affect substrate oxidations.
Iron(IV)–oxo intermediates have been trapped and charac-
terized in studies with several a-ketoglutarate-dependent
enzymes.[7] In biomimetic chemistry, a number of iron(II)–a-
keto acid complexes have been reported as functional models
of a-ketoglutarate-dependent oxygenases.[8] The model com-
plexes exhibit versatile reactivity towards dioxygen and, in
some cases, iron(IV)–oxo intermediates have been inter-
cepted in the decarboxylation of a-keto acids. Model iron(II)–
a-keto acid complexes of N4 donor ligands have been
lization on functionalized AuNPs. The reactivity of the
immobilized complex is compared with that of a related
iron(II)–benzoylformate complex supported by a tetradentate
nitrogen-donor ligand, TPA-O-allyl (Scheme 1).
The tetradentate ligand TPA-O-allyl was synthesized
from [6-(hydroxymethyl)-2-pyridylmethyl]bis(2-pyridylme-
thyl)amine (TPAOH)[9] according to Scheme 1. The
iron(II)–a-keto acid complex [(TPA-O-allyl)2Fe2(BF)2]-
(ClO4)2 (1) (where BF = monoanionic benzoylformate) was
isolated from the reaction of TPA-O-allyl, Fe(ClO4)2·xH2O,
and NaBF in methanol (see the Experimental Section in the
Supporting Information). The complex displays a paramag-
1
netically shifted H NMR spectrum in CDCl3 indicating the
[*] D. Sheet, Dr. P. Halder, Dr. T. K. Paine
Department of Inorganic Chemistry
high-spin nature of the iron(II) center (Supporting Informa-
tion, Figure S1). The ESI-MS (in positive-ion mode, CH3CN)
of the complex shows an ion peak at m/z = 565.2 with the
isotope distribution pattern calculated for the [(TPA-O-
Allyl)Fe(BF)]+ ion (Figure 1). The solid-state structure of
the complex cation, however, displays a symmetrical dinu-
clear complex in which each iron(II) center is coordinated by
a ligand and two carboxylate oxygen donors from two
Indian Association for the Cultivation of Science
2A & 2B Raja S. C. Mullick Road, Jadavpur, Kolkata-700032 (India)
E-mail: ictkp@iacs.res.in
[**] T.K.P. acknowledges the DST, Govt. of India (Project SR/S1/IC-51/
2010) for financial support. D.S. thanks CSIR, India, for a fellowship.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201305994.
13314
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2013, 52, 13314 –13318

Angewandte
Chemie
derived from the elemental analysis data. The XPS data for
C8Au reveals the presence of gold, carbon, and sulfur, and the
XPS signature of Au4f doublet with the peak-to-peak
distance of 3.7 eV indicates the gold to be in Au0 state
(Figure S8).[2a,10]
The TPASH-immobilized NPs, TPASH@C8Au, were
isolated by place-exchange reaction after incubation of
TPASH (2 equiv) and C8Au (1 equiv) for 36 h at 258C in
dichloromethane (Scheme 2).[11] Elemental analysis data of
TPASH@C8Au indicate an Au/S molar ratio of 2.78:1. The
experimental C/N and C/H ratio clearly illustrate the
immobilization of TPASH. The surface coverage of the NP
with TPASH was estimated by an 1H NMR integration using
1,4-benzoquinone as an internal standard (Figure S9). An
estimated 0.002 mmol of TPASH were immobilized on 10 mg
of TPASH@C8Au with an effective molar ratio of TPASH:-
C8Au being 1:9. On the basis of these results, a composition
of (Au)37(C8)9(TPASH) has been formulated for TPASH@-
C8Au.
Figure 1. ESI-MS spectra (positive-ion mode, CH3CN) of 1 (left) and
the final oxidized species after the reaction of 1 with dioxygen at
295 K. The substrates (S) and their oxidized products (Sox) are shown
at the bottom. The values at the parenthesis indicate the oxidation
product yields.
benzoylformates (Figure S2). The iron–nitrogen and iron–
oxygen bond distances are typical of high-spin iron(II)
complexes (Table S1). Two benzoylformates bridge between
the two iron centers through the carboxylate groups. The
optical spectrum of complex 1 in dichloromethane displays
a broad MLCT band (FeII to p* of the keto group) between
450 nm and 620 nm, suggesting a bidentate chelation of the a-
keto acid (BF) to the iron(II) center by one keto oxygen and
the carboxylate oxygen in solution. A lower extinction
coefficient of the MLCT band may be attributed to the
existence of an equilibrium between iron(II)–BF complexes
with different binding modes (bidentate, monodentate, and
bridging) of BF in solution.[8a] Upon exposure of a dichloro-
methane solution of the complex to dioxygen the MLCT band
gradually decreases (Figure S3) over a period of 24 h,
indicating oxidation of the complex. The final reaction
solution exhibits an ion peak at m/z = 537.2 attributable to
the [(TPA-O-allyl)Fe(benzoate)]+ ion (Figure 1). Time-de-
Scheme 2. Functionalization of gold nanoparticles with N4 ligand and
iron complexes.
The functionalization of AuNPs by iron(II) complex was
made by the reaction of TPASH@C8Au with iron(II)
perchlorate in dichloromethane (Scheme 2). After immobili-
zation of iron, the NPs became soluble in acetonitrile,
indicating the formation of charged species. The metal ion
was easily incorporated owing to strong chelating ability of
the pyridine nitrogen atoms of the ligand with the iron center.
The reaction of immobilized iron(II) complex with NaBF in
dichloromethane–methanol afforded [(TPASH)Fe(BF)]@-
C8Au (2) (Scheme 2).
The functionalization of AuNPs by iron complex is
supported by the IR spectrum of [(TPASH)Fe]@C8Au,
which exhibits a peak owing to perchlorate counterions at
1116 cmÀ1. Additionally, in 2, two peaks at 1602 and 1640 cmÀ1
indicate the binding of BF with the iron(II) center (Fig-
ure S10). In the optical spectra, all the NP exhibit broad
surface plasmon band at around 520 nm (Figure 2).[2d,12] After
complexation with iron(II), a slight shift in the surface
plasmon band of TPASH@C8Au is observed with the
appearance of a shoulder between 365–382 nm. Upon binding
of BF to the iron center a new shoulder appears at 304 nm
along with broad shoulder at 375 nm (Figure 2, inset). The
band at 368 nm could be attributed to the MLCT band from
1
pendent H NMR data also support the oxidative decarbox-
ylation of BF to benzoic acid slowly over 24 h (Figure S4).
The GC-MS spectrum of methyl ester of benzoic acid exhibits
ion peak at m/z = 136. When the reaction is carried out with
18O2, the ion peak is shifted to m/z = 138, confirming
incorporation of one oxygen atom from molecular oxygen
into benzoic acid (Figure S5). An active iron–oxo oxidant is
intercepted by external substrates such as thioanisole, dime-
thylsulfoxide, dimethyl sulfide, benzyl alcohol, and 9,10-
dihydroanthracene (Figure 1; Supporting Information, Figur-
es S6,S7).
For immobilization of the iron(II)–benzoylformate com-
plex on AuNPs, an alkanethiol terminated in the tripodal N4
ligand (TPASH) was synthesized (Scheme 1). Octanethiol-
capped NP, C8Au (1–5 nm) were isolated according to the
procedure reported by Brust et al.[2a,b] by the reaction of
gold(III) chloride and alkanethiol using a phase-transfer
catalyst and subsequent reduction with sodium borohydride.
Elemental analysis data of C8Au exhibits an Au/S molar ratio
of 2.52:1, and the calculated C/H and C/S ratio indicate the
capping of octanethiol on gold surface within experimental
uncertainties. A composition of (Au)37(C8)14 for the cluster is
Angew. Chem. Int. Ed. 2013, 52, 13314 –13318
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org 13315

.
Angewandte
Communications
Figure 2. Absorption spectra of functionalized gold nanoparticles in
dichloromethane at 295 K.
Figure 3. Time-dependent 1H NMR spectra (500 MHz, CDCl3, 295 K)
during the reaction of 2 with O2.
FeII to pyridine typically observed in FeII–TPA complexes.
The binding mode of BF to the iron center in 2 could not be
analyzed by optical spectroscopy owing to a high molar
extinction coefficent of the SP band as compared to the FeII-
to-keto charge transfer band. The 1H NMR spectrum of
C8Au exhibits broad peaks at 0.88 and 1.27 ppm. The peaks
corresponding to TPASH are broadened upon immobiliza-
tion on C8Au (Figure S9). A paramagnetically shifted
1H NMR spectrum is observed for [(TPASH)Fe]@C8Au,
with a large downfield shift of the Py (b,b’) protons (Fig-
ure S11). The appearance of paramagnetically shifted broad
peaks at 10–17 ppm for 2 suggests the binding of BF to the
iron center of [(TPASH)Fe]@C8Au. Thermogravimetric data
also support the incorporation of iron–BF complex on the
of decarboxylation of BF to benzoic acid for the immobilized
complex (2) is found to be 8 times faster than 1, about 20 times
faster than [(TPA)FeII(BF)]+, and 50 times faster than
[(6Me3TPA)FeII(BF)]+.[8a] It has been reported that NPs
protected by thiols are unreactive towards oxygen.[15]
A
number of control experiments were performed to establish
the inertness of NP towards dioxygen. No decarboxylation of
BF could be observed when 1) C8Au (25 mg) was reacted
with NaBF (0.005 mmol) in dichloromethane and acetone for
16 h; 2) C8Au (25 mg) was treated with NEt3 (0.005 mmol)
and HBF (0.005 mmol) in dichloromethane and acetone for
16 h; and 3) C8Au (25 mg), NaBF (0.005 mmol), and Fe-
(ClO4)2·xH2O (0.005 mmol) were reacted together for 16 h.
On the basis of the above results, it may safely be concluded
that the dramatic increase in the rate of BF decarboxylation is
a result of immobilization of the complex on AuNPs, which
provides large surface area and the self-assembly providing
spherical curvature with open access of the active iron center
for faster reactivity.
To intercept the active iron–oxygen species formed upon
decarboxylation of 2, some external reagents were used as
indirect probes. Thioanisole, dimethyl sulfoxide, benzyl
alcohol, and 9,10-dihydroanthracene could intercept the
active iron–oxo oxidant, affording thioanisole oxide (20%),
dimethyl sulfone (60%), benzaldehyde (30%), and anthra-
cene (30% and a small amount of anthraquinone), respec-
tively, in the reaction of 2 with dioxygen in acetone
(Scheme 3; Supporting Information, Figure S17). With all of
these substrates, the yield of benzoic acid is found to be
quantitative. The GC-MS spectrum exhibits ion peak of
thioanisole oxide at m/z = 140, which is shifted two mass unit
higher in the presence of 18O2 (Figure S18). A labeling
experiment further confirmed incorporation of another
oxygen atom into benzoic acid (Figure S19).
À
surface of NP with the Au S bonds being stable up to 2008C
(Figure S12). The XPS signature of 2, consisting of a Au4f
doublet with a peak-to-peak distance of 3.7 eV, indicates that
gold is in the Au0 state (Figure S13).
PXRD data of the NP exhibit patterns similar to that of
fcc Au (Figure S14). The broad patterns and the disappear-
ance of (200) plane reveal that all the NPs are less than 5 nm
in diameter.[10,13] HRTEM images reveal the polycrystalline
nature of the AuNP (Figure S15 and S16). The attachment of
thiol unit to the gold was further characterized by EDAX,
which supports the presence of C, N, S, and Au in the
functionalized NPs. The histogram indicates that the max-
imum number of particles are in the range of 2–5 nm for
C8Au (Figure S16a).[2d] The particle core size remains almost
unaltered and well dispersed without any sign of agglomer-
ation upon immobilization of TPASH (Figure S15a). The
HRTEM images reveal that the interparticle distances
increase upon introduction of iron(II) ions (Figure S15b).
The NPs are stabilized upon coordination with the metal ion,
and the resulting charge prevents agglomeration.[14] Interest-
ingly, binding of BF to the immobilized iron complex does not
affect the particle size; rather the NPs are further stabilized,
as depicted from TEM images (Figure S16b).
[(TPASH)Fe(BF)]@C8Au (2) reacts with dioxygen to
undergo oxidative decarboxylation of the coordinated ben-
zoylformate (BF) to afford benzoic acid as the only product.
The high valent oxo species, formed on the AuNP surface,
is thus capable of transferring oxo atom to substrate to carry
out oxidation reactions (Scheme 3). An IR spectrum of the
NP isolated after oxidation exhibits a similar spectral feature
1
Time-dependent H NMR spectra reveal a quantitative for-
mation of benzoic acid in 2 h (Figure 3). Interestingly, the rate
13316 www.angewandte.org
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2013, 52, 13314 –13318

Angewandte
Chemie
The reactivity remains stoichiometric even in the presence of
higher concentration of complex 1. The immobilized complex
therefore proved to be a catalyst showing a highest TON of 6
using dioxygen as the terminal oxidant. The TON obtained
with the immobilized complex is an outcome of the barrier
caused by the functionalized AuNPs for the formation of any
inactive dimeric iron complex. GC-MS analysis after 12 h
under catalytic conditions suggests the formation of dioctyl
disulfide in about 15% yield indicating the leaching of
adsorbed thiolates. Leaching of the gold surface is also
evident from the TEM image after catalysis (Figure S24).
In conclusion, we have reported the synthesis and
characterization of a surface-immobilized nonheme iron
complex. The reactivity of the NP functionalized with
terminated N4 ligand is enhanced considerably compared to
its non-immobilized analogue. Moreover, the immobilized
complex exhibits catalytic reactivity in the presence of excess
substrates. The increased reactivity can be accounted for the
surface properties providing suitable orientation and larger
surface area that favor enhanced interaction. The confine-
ment of the complex may also exert cooperativity. The proof-
of-concept system reported herein show potential and would
provide useful insight in developing a bioinspired immobi-
lized catalyst. Place-exchange with a long-chain alkanethiol
with metal complexes terminated at a shorter chain is
expected to create a protective environment at the catalytic
metal center to provide selectivity for biomimetic oxidative
transformations. Further tuning of the immobilized complex
to increase the catalytic efficiency is presently being pursued
in our laboratory.
Scheme 3. Oxidative decarboxylation of benzoylformic acid by an iron
complex on functionalized gold nanoparticles.
as in 2 (Figure S20). The SP band at 520 nm for 2 remains
almost similar, suggesting that oxidation does not really affect
the AuNP surface (Figure S21). GC-MS analysis immediately
after oxidation suggests about 5% leaching of the adsorbed
thiolates. The TEM image suggests the recycled NPs to be
within 5 nm without any agglomeration (Figure S22). The
XRD pattern (Figure S23) after oxidation is similar to the
XRD pattern of 2. All of these results strongly support the
robustness of the biomimetic complex on functionalized
AuNPs and the immobilized complex can return into the cycle
for catalytic conversion.
Received: July 10, 2013
Revised: September 29, 2013
Published online: October 18, 2013
Preliminary studies on the catalytic reactivity were carried
out with 2 (10 mg, 0.002 mmol) in the presence of excess
NaBF and thioanisole in CH2Cl2/CH3CN mixture or in
acetone at room temperature (Table 1). Complex 2 was
found to catalyze the oxo-atom transfer to thioanisole in the
presence of excess BF, which put back the dioxygen activating
species into the catalytic cycle (Scheme 3). It is important to
mention here that complex 1 does not exhibit any catalytic
reactivity under similar experimental conditions (Table 1).
Keywords: a-keto acids · catalysis · dioxygen ·
.
gold nanoparticles · iron
[1] a) J. D. E. T. Wilton-Ely, Dalton Trans. 2008, 25; b) S. Roy, M. A.
Pericꢀs, Org. Biomol. Chem. 2009, 7, 2669; c) E. K. Beloglaz-
kina, A. G. Majouga, R. B. Romashkina, N. V. Zyk, N. S. Zefirov,
Russ. Chem. Rev. 2012, 81, 65.
[2] a) M. Brust, M. Walker, D. Bethell, D. J. Schiffrin, R. Whyman, J.
Chem. Soc. Chem. Commun. 1994, 801; b) M. Brust, J. Fink, D.
Bethell, D. J. Schiffrin, C. Kiely, J. Chem. Soc. Chem. Commun.
1995, 1655; c) A. C. Templeton, W. P. Wuelfing, R. W. Murray,
Acc. Chem. Res. 2000, 33, 27; d) M.-C. Daniel, D. Astruc, Chem.
Rev. 2004, 104, 293.
[3] a) P. D. Beer, D. P. Cormode, J. J. Davis, Chem. Commun. 2004,
414; b) D. Samanta, N. Faure, F. Rondelez, A. Sarkar, Chem.
Commun. 2003, 1186; c) M. Alvaro, C. Aprile, B. Ferrer, F.
Sastre, H. Garcꢁa, Dalton Trans. 2009, 7437; d) C. R. Mayer, G.
Cucchiaro, J. Jullien, F. Dumur, J. Marrot, E. Dumas, F.
Sꢂcheresse, Eur. J. Inorg. Chem. 2008, 3614; e) S. Wang, W.-S.
Sim, Langmuir 2006, 22, 7861; f) T. B. Norsten, B. L. Frankamp,
V. M. Rotello, Nano Lett. 2002, 2, 1345; g) P. Pramod, P. K.
Sudeep, K. G. Thomas, P. V. Kamat, J. Phys. Chem. B 2006, 110,
20737.
Table 1: Catalytic reactivity of 2.
Complex (amount) Solvent
NaBF [equiv] t [h] TON[d]
2[a] (10 mg)
2[a] (10 mg)
2[a] (10 mg)
2[a] (10 mg)
2[a] (10 mg)
2[a] (10 mg)
1[b] (1.3 mg)
1[b] (1.3 mg)
1[b] (1.3 mg)
1[c] (13 mg)
acetone
acetone
acetone
acetone
CH2Cl2/CH3CNN
CH2Cl2/CH3CN
CH3CN
CH3CN
CH3CN
3
5
10
20
5
10
5
10
5
8
8
8
8
8
8
8
24
48
48
2
4
5
6
3
5
0
0
0
0
CH3CN
5
[a] Reaction conditions: 10 mg (0.002 mmol) catalyst, 100 equiv
(0.2 mmol) of substrate (thioanisole). [b] Conditions: 1.3 mg
(0.002 mmol) complex, 100 equiv (0.2 mmol) of substrate (thioanisole).
[c] Conditions: 13 mg (0.02 mmol) complex, 100 equiv (2 mmol) of
substrate (thioanisole). [d] The TON was calculated on the basis of the
percentage of benzoic acid formed from excess NaBF used.
[4] a) M. Bartz, J. Kꢃther, R. Seshadri, W. Tremel, Angew. Chem.
1998, 110, 2646; Angew. Chem. Int. Ed. 1998, 37, 2466; b) H. Li,
Y.-Y. Luk, M. Mrksich, Langmuir 1999, 15, 4957; c) K. Mar-
Angew. Chem. Int. Ed. 2013, 52, 13314 –13318
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org 13317

.
Angewandte
Communications
ubayashi, S. Takizawa, T. Kawakusu, T. Arai, H. Sasai, Org. Lett.
2003, 5, 4409; d) T. Belser, M. Stçhr, A. Pfaltz, J. Am. Chem. Soc.
2005, 127, 8720; e) R. Bonomi, F. Selvestrel, V. Lombardo, C.
Sissi, S. Polizzi, F. Mancin, U. Tonellato, P. Scrimin, J. Am. Chem.
Soc. 2008, 130, 15744.
Chem. 1995, 34, 2265; c) M. P. Mehn, K. Fujisawa, E. L. Hegg, L.
Que, Jr., J. Am. Chem. Soc. 2003, 125, 7828; d) A. Mukherjee,
M. A. Cranswick, M. Chakrabarti, T. K. Paine, K. Fujisawa, E.
Mꢃnck, L. Que, Jr., Inorg. Chem. 2010, 49, 3618; e) T. K. Paine,
H. Zheng, L. Que, Jr., Inorg. Chem. 2005, 44, 474; f) O. Das, S.
Chatterjee, T. K. Paine, J. Biol. Inorg. Chem. 2013, 18, 401.
[9] Z. He, P. J. Chaimungkalanont, D. C. Craig, S. B. Colbran, J.
Chem. Soc. Dalton Trans. 2000, 1419.
[5] a) M. Costas, M. P. Mehn, M. P. Jensen, L. Que, Jr., Chem. Rev.
2004, 104, 939; b) R. P. Hausinger, Crit. Rev. Biochem. Mol. Biol.
2004, 39, 21.
[6] J. M. Elkins, M. J. Ryle, I. J. Clifton, J. C. D. Hotopp, J. S. Lloyd,
N. I. Burzlaff, J. E. Baldwin, R. P. Hausinger, P. L. Roach,
Biochemistry 2002, 41, 5185.
[7] a) A. R. McDonald, L. Que, Jr., Coord. Chem. Rev. 2013, 257,
414; b) C. Krebs, D. G. Fujimori, C. T. Walsh, J. M. Bollinger, Jr.,
[10] D. V. Leff, L. Brandt, J. R. Heath, Langmuir 1996, 12, 4723.
[11] a) M. J. Hostetler, S. J. Green, J. J. Stokes, R. W. Murray, J. Am.
Chem. Soc. 1996, 118, 4212; b) M. J. Hostetler, A. C. Templeton,
R. W. Murray, Langmuir 1999, 15, 3782; c) R. S. Ingram, M. J.
Hostetler, R. W. Murray, J. Am. Chem. Soc. 1997, 119, 9175.
[12] a) P. Mulvaney, Langmuir 1996, 12, 788; b) A. C. Templeton, J. J.
Pietron, R. W. Murray, P. Mulvaney, J. Phys. Chem. B 2000, 104,
564.
[13] D. V. Leff, P. C. Ohara, J. R. Heath, W. M. Gelbart, J. Phys.
Chem. 1995, 99, 7036.
[14] a) M. Montalti, L. Prodi, N. Zaccheroni, M. Beltrame, T.
Morotti, S. Quici, New J. Chem. 2007, 31, 102; b) F. S. Nunes,
L. D. S. Bonifꢄcio, K. Araki, H. E. Toma, Inorg. Chem. 2006, 45,
94.
´
Acc. Chem. Res. 2007, 40, 484; c) D. P. Galonic, E. W. Barr, C. T.
Walsh, J. M. Bollinger, Jr., C. Krebs, Nat. Chem. Biol. 2007, 3,
113; d) S. Sinnecker, N. Svensen, E. W. Barr, S. Ye, J. M.
Bollinger, Jr., F. Neese, C. Krebs, J. Am. Chem. Soc. 2007, 129,
6168; e) M. L. Matthews, C. M. Krest, E. W. Barr, F. H. Vaillan-
court, C. T. Walsh, M. T. Green, C. Krebs, J. M. Bollinger, Jr.,
´
Biochemistry 2009, 48, 4331; f) D. Galonic Fujimori, E. W. Barr,
M. L. Matthews, G. M. Koch, J. R. Yonce, C. T. Walsh, J. M.
Bollinger, Jr., C. Krebs, P. J. Riggs-Gelasco, J. Am. Chem. Soc.
2007, 129, 13408.
[15] P. Ionita, M. Conte, B. C. Gilbert, V. Chechik, Org. Biomol.
Chem. 2007, 5, 3504.
[8] a) Y.-M. Chiou, L. Que, Jr., J. Am. Chem. Soc. 1995, 117, 3999;
b) E. H. Ha, R. Y. N. Ho, J. F. Kisiel, J. S. Valentine, Inorg.
13318 www.angewandte.org
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2013, 52, 13314 –13318