Air-stable, dinuclear and tetranuclear σ,π-acetylide gold(I) complexes and their catalytic implications

Article abstract of DOI:10.1002/chem.201301623

Two for one gold: Factors governing the formation of isolable digold(I) σ,π-acetylide complexes are given (see scheme), indicating the general tendency of phosphine-Au^I precatalysts to form this type of complexes, which are involved as reaction

Full text of DOI:10.1002/chem.201301623

COMMUNICATION
DOI: 10.1002/chem.201301623
Air-Stable, Dinuclear and Tetranuclear s,p-Acetylide Gold(I) Complexes
and Their Catalytic Implications
Abdessamad Grirrane,[a] Hermenegildo Garcia,*[a] Avelino Corma,*[a] and
Eleuterio ꢀlvarez[b]
The possibility that s,p-digold
alkynide complexes play
a
key role as active species
in the cycloadddition of al-
kynes[1–9] has recently been put
forward.[10] Although precedents
for the formation of digold–ace-
tylide complexes[11–17] and gem-
diaurated species[18–20] can be
found in the literature, we were
among the first to isolate fluxio-
nal complexes with two AuI
atoms of bulky biphenylphos-
ꢀ
phines connected to the C C
triple bond of phenylacetylene
through the s and p bonds,[21]
supporting their involvement
in the catalytic intermolecular
[2+2] alkyne–alkene cycloaddi-
tion reaction (Scheme 1).[21]
Simultaneously, Widenhoefer
and co-workers succeeded in
isolating dinuclear gold(I) s,p-
Scheme 1. Intermolecular [2+2] cycloaddition of the alkyne–alkene reaction catalyzed by bulky AuI phos-
phines.
arylacetylide complexes with bulky, 1,3-bis(2,6-diisopropyl-
phenyl)imidazol-2-ylidene as the co-ligand (Scheme 1).[22]
Herein, we will show the general tendency of phosphine AuI
precatalysts to form digold s,p-acetylide complexes under
the conditions in which AuI catalysis is carried out.[23] The
influence of the nature of the terminal alkyne and the bulki-
ness of the AuI ligand on the stability of the digold–alkyne
complexes and the potential implications for catalysis of
these digold–alkynide complexes will also be presented. To
address this point and due to the interest in imines in organ-
ic synthesis and industrial chemistry,[24–27] we have selected
the AuI-catalyzed coupling of amines with terminal alkynes
with the goal of isolating the real active species formed
during the catalytic process.[28–42]
The reaction of hepta-1,6-diyne (1) with gold com-
plexes 2[43] or 3[44] was initially studied (see Scheme 2). Slow
addition of n-hexane to solutions containing 1 and com-
plexes 2 or 3 gave, after 12 h at À308C, colorless crystals of
the air-stable dicationic tetragold complexes 4 and 5, respec-
tively, in good yields with respect to the initial AuI com-
plexes 2 and 3 (76 and 62%; see the Experimental Section
in the Supporting Information). The structures of the stable
dicationic tetragold complexes 4 and 5 could be resolved by
X-ray crystal structure analysis (see Scheme 2 and atom
coordinates in Tables S1 and S2 and Figures S7 and S11 in
the Supporting Information). The solid-state structures of
complexes 4 and 5 correspond to the space groups P21/n and
[a] Dr. A. Grirrane, Prof. Dr. H. Garcia, Prof. Dr. A. Corma
Instituto Universitario de Tecnologꢀa Quꢀmica CSIC-UPV
Universidad Politꢁcnica de Valencia
Av. De los Naranjos s/n, 46022 Valencia (Spain)
Fax : (+34)963877809
E-mail: hgarcia@qim.upv.es
acorma@itq.upv.es
[b] Dr. E. ꢂlvarez
Instituto de Investigaciones Quꢀmicas CSIC-US
Departamento de Quꢀmica Inorgꢃnica
Av. Amꢁrico Vespucio 49, 41092 Sevilla (Spain)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201301623. It contains details of
the preparation, isolation, and characterization in full of new gold
compounds 4, 5, 8, 9, 13, and 14, together with the experimental pro-
cedures, additional spectroscopic data, TEM images, and kinetic data
of compounds 10, 15, 16, 17, 18, and 19 and CIF files for compounds
4, 5, 8, 9, 13, and 14.
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porting Information,). Thus, upon addition of complexes 2
or 3 to hepta-1,6-diyne 1, 31P NMR spectroscopy shows the
appearance of a new peak at 61.84 or 38.83 ppm, corre-
sponding to compounds 4 and 5, respectively. These peaks
are different from those recorded for the original complexes
2 (57.49 ppm) and 3 (33.29 ppm) in CD2Cl2 and indicate that
the transformation of 2 and 3 into 4 and 5 takes place spon-
taneously in the presence of diyne 1 (Scheme 2). It is inter-
esting to note that dicationic tetragold complexes 4 and 5
were obtained even in the presence of a large excess of 1
(8 equiv with respect to 2 or 3), for which possible mono- or
di-adducts of hepta-1,6-diyne with the AuI complexes would
be favored versus the isolated tetragold complexes.
1
Similarly to the 31P NMR studies, H NMR spectroscopy
also shows new signals for methyl groups of complex 4 at
1.369 and 1.315 instead of those corresponding to the initial
complex 2 (1.38 and 1.33 ppm) and for complex 5 at 0.90,
1.21, and 1.38 instead of those corresponding to the initial
complex 3 (0.87, 1.19, and 1.27 ppm). Further evidence for
the transformation of monogold(I) complexes 2 and 3 into 4
and 5 was obtained from liquid-phase 13C and 19F NMR
spectroscopy (see Figures S3 and S10 in the Supporting In-
formation) and solid-state 31P and 19F NMR spectroscopy
(see Figures S4 and S5 in the Supporting Information).
Compared to isolation of tetragold complexes 4 and 5 by
starting from AuI complexes with bulky biphenyl phosphine
ligands L1 and L2 and aliphatic hepta-1,6-diyne, the behavior
of the triphenylphosphine AuI complex with phenylacety-
lene 6 is significantly different. It should be noted that bi-
phenylphosphine AuI complexes 2 and 3 have been previ-
ously reported[21] to react with phenylacetylene (6) to form
ꢀ
Scheme 2. Synthesis and structures of dicationic tetragold complexes 4
and 5. Ellipsoids are drawn at the 30% probability level (X-ray single-
crystal data and ORTEP diagrams for 4 and 5 are given in Tables S1 and
S2 and Figures S7 and S11 in the Supporting Information). The structures
of L1 and L2 are given in Scheme 1. Note that formation of two s LAu
complexes releases two molecules of acid.
cationic s and p digold complexes with the C C bond in 6
(see Scheme 1). Therefore, complexes 2 and 3 behave simi-
larly with phenylacetylene (6) and aliphatic hepta-1,6-diyne
(1), leading to the corresponding s,p-digold complexes. We
were interested in determining whether a triphenylphos-
phine AuI complex (7)[46] behaves similarly to AuI complexes
2 and 3 with phenylacetylene (6).
C2/c, respectively. Dicationic tetranuclear complexes 4 and
5 consist of an h1,h2,h1’,h2’-propanediacetylide ligand acting
as a bridge between four fluxional (phosphine)AuI moieties.
The distance between the two proximal Au atoms of each
For this propose, we mixed triphenylphosphine AuI com-
plex 7 and phenylacetylene 6 (see Scheme 3). Interestingly,
in this case, no evidence for the formation of a digold com-
plex between 6 and 7 was obtained. Moreover, the data col-
lected indicates completely different behavior of triphenyl-
phosphine AuI complex 7 than the bulky biphenylphosphine
AuI complexes 2 and 3. Thus, slow addition of n-hexane to
the mixture of 6 and 7 in dichloromethane allowed collec-
tion, after 4 or 12 h at À308C, of two kinds of air-stable crys-
tal, the structures of which could be determined by X-ray
crystal structure analysis (Scheme 3). As can be seen in
Scheme 3, the solid was formed from a mixture of neutral
dicoordinate AuI complex 8, with triphenylphosphine and
phenylacetylide ligands coordinated through a s bond, and
complex 9, which corresponds to a cationic AuI complex
with two triphenylphosphine ligands with a bistriflurome-
thylsulphonylamide counteranion. Both AuI intermediates
were isolated as pure crystals suitable for X-ray crystal
structure analysis (see Figures S12 and S13 and Tables S3
ꢀ
[Au1(s)C CAu2(p)] fragment in complexes 4 and 5 is sig-
nificantly longer than the 2.88 ꢅ value for bulk Au metal.
À
Nonetheless the Au Au distance in our complexes (4 and 5)
is still in the range in which weak “aurophilic” interactions
occur.[45] We propose that the Au1 Au2 contact [3.2983 (4)
À
and 3.652 ꢅ (5)], the short distance between each cationic
AuI and each distal phenyl ring of the biphenyl ligand,[21]
and the interaction with the triple bond in acetylene all con-
tribute to the stability of these unprecedented organometal-
lic compounds, 4 and 5.
1H, 13C, 31P, and 19F NMR spectroscopy of solutions in
CD2Cl2 provide evidence showing that under the reaction
conditions the starting complexes 2 and 3 are completely
converted into the corresponding dicationic tetragold com-
plexes 4 and 5 (see Figures S1–S3 and S8–S10 in the Sup-
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Dinuclear and Tetranuclear s,p-Acetylide Gold(I) Complexes
COMMUNICATION
peaks at 599.4 and 721.5 amu that correspond to the expect-
ꢀ
ed mass of the neutral gold complex PhC CAuPPh3 (8) ion-
ized with a potassium atom and the cationic [Au
A
fragment of gold complex 9 (see Figure S18 in the Support-
ing Information). For pure complex 9, we succeeded in re-
cording the positive 721.5 amu signal for [Au
(PPh3)2]+ and
ACHTUNGTRENNUNG
the negative 279.7 amu signal for [N
AHCTUNGTRENNUNG
ure S19 in the Supporting Information). It is interesting to
note that, as well as the previously mentioned peaks corre-
sponding to complex 9, we were able to detect, in positive-
mode MS, an extra, weak peak, the value (1019.6 amu) of
which matches that expected for the cationic digold complex
ꢀ
[PhC CACHTNUGRTNEUNG
(AuPPh3)2]+ (10), formed very early in the reaction,
but never isolated due its low stability (see Figures S20 and
S52 in the Supporting Information for the MALDI-TOF-MS
experiment), providing support for the formation of unsta-
ꢀ
ble digold s and p complexes with the C C bond in 6 that
quickly decompose to form 8, 9, and Au nanoparticles (NPs;
see Scheme 3).
Importantly, ESI-MS and 31P NMR spectroscopy show no
peaks or signals besides those corresponding to complexes
8, 9, and 10, suggesting that they were the only species pres-
ent. Transformation of 8 into 9 requires a PPh3/Au stoichi-
ometry of 2:1, which is double than that of the starting com-
plex 7 and indicates that up to one half on the total number
of gold atoms must be forming a different species. Since
Au NPs and clusters are frequently observed in gold cataly-
sis,[49–52] we speculated that the gold atoms that have lost
PPh3 ligands and are not present in the form of complex 9
could have evolved into Au NPs. To support this hypothesis,
we recorded the optical spectra of the solution upon react-
ing 6 and 7 in CH2Cl2; this mixture was also characterized
by transmission electron microscopy (TEM). As can be seen
in Scheme 3, an absorption band appears for this solution at
520–650 nm, which is characteristic of the surface plasmon
band of Au NPs;[53] the TEM image shows the presence of
two kinds of crystal, squares and needles, corresponding to
complexes 8 and 9, respectively, together with a homoge-
nous distribution of Au NPs with diameters of 8–25 nm (see
Figure S61 in the Supporting Information). This lends sup-
port to the theory of the spontaneous generation of Au NPs
concomitant with the formation of the cationic complex
Scheme 3. Synthesis and molecular structure of monogold neutral and
cationic complexes 8 and 9 from decomposition of digold complex 10.
Complexes 8 and 9 could be resolved by X-ray single-crystal analysis; el-
lipsoids are shown at the 30% probability level. The plot shows the UV/
Vis absorption spectra recorded in CH2Cl2 at room temperature of
a) complex 7 and b) the reaction mixture of phenylacetylene 6 and 7
after 1 h. The absorption band peaking at 560 nm corresponds to the for-
mation of Au NPs. The TEM image of the reaction mixture of phenylace-
tylene 6 and gold complex 7 is shown.
and S4 in the Supporting Information for atom coordinates).
Crystals of neutral [AuACHTUNTRGNEUNG(PPh3)(phenylethynyl)] (8) have al-
ready been obtained[47,48] by a different synthetic method
and the crystal structure has been determined previously.
However, our analysis of the new X-ray diffraction data for
the gold compound (8; crystallized from toluene) resulted in
significantly different unit-cell parameters.
Liquid-phase 31P NMR spectra recorded for complexes 8
and 9 show the presence of two characteristic peaks at 36.23
and 45.02 ppm, instead of at 30.47 ppm (starting complex 7;
see Figures S14 and S15 in the Supporting Information). At
short contact times (15 min) between 6 and 7, complex 8 is
the main complex formed, accompanied by residual complex
7 (see Figure S16 in the Supporting Information). Over time
(1 h), complex 7 completely disappears, giving complexes 8
and 9 (see Figure S17 in the Supporting Information). If the
reaction mixture is allowed to evolve for longer (3 h), the
transformation of complex 8 into 9 was observed, and cati-
onic AuI complex 9 becomes the predominant species (see
Scheme 3 and Figure S14 in the Supporting Information).
The formation of monoaurate complex 8 from AuI com-
plex 7 and the conversion of complex 8 into 9 (see
Scheme 3) can be followed easily by ESI-MS (see Fig-
ures S18–S20 in the Supporting Information). ESI-MS of a
solution recorded immediately after dissolving compounds 6
and 7 in CH2Cl2/methanol (1:1) shows two positive MS
{[AuI(PPh3)2]+[N(SO2CF3)2]À} (9) from AuIPPh3 (7) in the
ACTHNUTRGENNUG CAHTUNGTRENNUGN
ꢀ
presence of PhC CH (6).
To gain an understanding of the implications of digold(I)–
alkynide complexes in catalysis, we selected the hydroami-
nation of terminal alkynes by using complex 2 as a precata-
1
lyst and followed the reaction by H and 31P NMR spectro-
scopy. In the first step, gold complex 2 (3 mol%) was added
to 1-octyne (11). 31P NMR spectroscopy of this mixture
shows a new peak at 62.64 ppm, which is different from the
original complex 2 (57.49 ppm) in CD2Cl2, that disappeared
completely over time (30 min), indicating the spontaneous
transformation of 2 into a new compound when alkyne 11 is
present (see Figure S21 in the Supporting Information). In
view of the previous isolation of digold complexes, we an-
ticipated the formation of a digold–1-octyne adduct and this
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H. Garcia, A. Corma et al.
assumption was firmly supported by an independent experi-
ized (see the Experimental Section and Figures S24–S29 for
13 and S30–S34 for 14 in the Supporting Information,).
Scheme 4 summarizes the isolation of complexes 13 and 14,
which were characterized by X-ray crystal structure analysis
(see Figure S29 for 13 and S34 for 14 and Tables S5 and S6
in the Supporting Information for atom coordinates), and
their subsequent reactivity with 12 and 11, respectively, to
ment (see below). In a second step, aniline 12 was added to
1
the mixture and changes were followed by H and 31P NMR
spectroscopy. 31P NMR spectroscopy after 1 h shows a new
peak at 57.95 ppm, along with the peak at 62.64 ppm formed
in the first step (see Figure S22 in the Supporting Informa-
tion). The peak at 62.64 ppm was gradually consumed over
time (48 h; see Figure S23 in the Supporting Information).
The variations in the 31P NMR spectra are compatible with
the generation of a new species, 13, by reaction of catalyst 2
with an alkyne; species 13 then reacts with aniline.
1
obtain 15, which was characterized in situ by H, 31P, and
13C NMR spectroscopy (see Figures S35–S37 in the Support-
ing Information) and MALDI-TOF-MS, showing its ability
to lose imine ligand 16 due the weakness of the gold(I)–ni-
trogen interactions[54] (see Figure S38 in the Supporting In-
formation). When intermediate complex 15 was submitted
to hydrolysis, it decomposed to amine–AuI complex 14 and
the corresponding ketone (see Scheme 4). Although this in-
termediate (15) could not be characterized by single-crystal
XRD, its presence with very high purity upon addition of 2,
11, and 12 was supported by MALDI-TOF-MS and 31P, 13C,
and 1H NMR spectroscopy. Furthermore, 31P, 13C, and
1H NMR spectroscopic data for 15 agree with the peaks ob-
served under the reaction conditions of 11 and 12 promoted
by 2.
To elucidate the nature of the intermediates, we per-
formed two independent experiments mixing AuI complex 2
with either 1-octyne (11) or aniline (12; see Scheme 4 and
Besides X-ray crystal structure analysis, cationic digold(I)
complex 13 and cationic gold(I) amine complex 14 were
also characterized by NMR spectroscopy, ESI-MS, and com-
bustion analysis (see the Experimental Section and Support-
ing Information). ESI-MS of a solution obtained after dis-
solving complex 13 in CH2Cl2/CH3CN (1:1) shows mainly a
positive MS peak at 1099.5 amu, attributable to the cationic
digold complex [C48H67Au2P2]+ (see Figure S28 in the Sup-
porting Information). Complex 14 dissolved in CH2Cl2/
MeOH (0.01:1) exhibits a positive MS peak at 588.2 amu, as
expected for the cationic gold complex [C26H34AuNP] + (see
Figure S33 in the Supporting Information).
1
Similarly, H, 13C, and 31P NMR spectroscopy in CD2Cl2
provide evidence that under the reaction conditions the
starting complex 2 is completely converted to the corre-
sponding cationic digold and cationic monogold complexes
13 and 14, depending on the reagent added (see Figur-
es S24–S27 and S30–S32 in the Supporting Information).
31P NMR spectroscopy shows new peaks at 62.63 and
59.25 ppm for complexes 13 and 14, respectively, upon addi-
tion of complex 2 (57.49 ppm) to 1-octyne (11) and aniline
(12), indicating that the transformation of 2 into 13 and 14
takes place spontaneously in the presence of 11 and 12, re-
spectively (Scheme 4). It is interesting to note that cationic
digold complex 13 is obtained even in the presence of a
large excess of 11 (4 equiv with respect to 2), for which pos-
sible monoadducts of 1-octyne (11) with the AuI complex 2
should be favored with respect to the isolated digold com-
Scheme 4. Synthesis and molecular structure obtained by X-ray single-
crystal analysis of diaurated–alkynide complex 13 and cationic amine
gold complex 14 (see Scheme 1 for L1; ellipsoids are drawn at the 30%
probability level) and their subsequent reactivity with 12 and 11, respec-
tively, to obtain 15. Complex 15 can also be obtained in a single step
upon mixing gold complex 2 with 11, and 12. Gold imine complex 15 can
be hydrolysed to 14 and the corresponding ketone. Dashed lines propose
the catalytic reaction mechanism of the formation of imine 16 in the
presence of 11 and 12, with complex 2 used as the catalyst. See the Sup-
porting Information for reaction times, yields of products, and reaction
conditions.
the Experimental Section). Interestingly, in the first experi-
ment, evidence for the formation of a digold complex 13 be-
tween 2 and 11 was obtained. The data collected in the
second experiment indicates that the initial biphenylphos-
phine AuI complex 2 is transformed into a new amine–AuI
complex (14) by replacement of acetonitrile with aniline
(12). Complexes 13 and 14 were successfully isolated in
good yields with respect to the initial AuI complex 2 (ca. 74
and 89% for 13 and 14, respectively), and fully character-
1
plex 13. Similarly to 31P NMR spectroscopy, H NMR spec-
troscopy shows new signals of methyl groups for complex 14
at 1.337 and 1.283 ppm instead of the peaks at 1.38 and
1.33 ppm, which correspond to the initial complex 2 (see
Figure S30 in the Supporting Information). Further evidence
showing the transformation of monogold(I) complex 2 into
13 and 14 was obtained in liquid-phase 13C NMR spectrosco-
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Dinuclear and Tetranuclear s,p-Acetylide Gold(I) Complexes
COMMUNICATION
py (see Figures S26, S27, and S32 in the Supporting Informa-
tion).
phenylacetylene 6 by aniline 12, with 2, 3, or 7 as the cata-
lyst, was obtained with high Markovnikov regioselectivity
(see Figures S53–S60 in the Supporting Information for ki-
When the intermolecular hydroamination reaction of 1-
octyne with aniline was carried out by using complex 2 as
the catalyst under typical reaction conditions, 31P NMR
spectroscopy initially shows the peak at 62.64 ppm, charac-
teristic of 13, and gradual growth of a new peak at
57.93 ppm, attributable to complex 15, in which the imine is
coordinated to a AuI complex (see Scheme 4). Evidence of
the intermediacy of 15 was obtained by 1H, 31P, and
13C NMR spectroscopy (see Figures S35–S37 in the Support-
ing Information) and by MALDI-TOF-MS (see Figure S38
in the Supporting Information) of the reaction mixture. Re-
lease of N-(2-octylidene)aniline (16)[30] as the final product
by ligand exchange with 1-octyne or aniline would lead to
gold complexes 13 or 14, respectively, ready for the next cat-
alytic cycle. Only the Markovnikov product is formed (>
1
netic H and 31P NMR study).
In summary, in this study, we have been able to isolate
single crystals of a series of cationic digold and dicationic
ꢀ
tetragold complexes involving terminal C C groups as s
and p ligands, showing the general tendency of phosphine
AuI precatalysts to form this type of complex. When the
phosphine ligand is not a sufficiently bulky and stabilizing
agent, decomposition of the digold(I)–alkynide complex
occurs with the formation of monogold complexes and
Au NPs. 31P NMR spectroscopy and MALDI-TOF mass
spectrometry of the catalytic hydroamination reaction of ter-
minal alkynes shows that digold–alkynide complexes are
formed under the reaction conditions and their distribution
varies depending upon the reaction mixture. Overall, our
study provides a detailed description of the structures of the
AuI species during alkyne activation, showing the prevalence
of cationic digold complexes as intermediates.
1
99%). Imine 16 was isolated and characterized by H NMR
spectroscopy and GC-MS (see Figures S39 and S40 in the
Supporting Information).
An alternative catalytic experiment was carried out, in
which 15 mol% of gold complex 2 was added to aniline (12)
at RT and, after 1 h, the mixture was analyzed by 31P NMR
spectroscopy, showing the peak at 59.26 ppm characteristic
of complex 14 instead of the peak at 57.53 ppm for complex
2 (see Figures S41 and S42 in the Supporting Information).
At this time, an excess of 1-octyne (11, 2 equiv) with respect
to the starting aniline (12) was added at RT and, after 1 h,
the 31P NMR spectrum was recorded again. It was observed
that complex 14 (59.26 ppm) was completely consumed and
complex 13 was formed (62.64 ppm), together with an NMR
peak at 57.98 ppm, attributable to complex 15 (see
Scheme 4 and Figure S43 in the Supporting Information).
Over time (12 h), the peak corresponding to 15 was con-
sumed with the concomitant formation of N-(2-octylidene)a-
niline (16). The final gold species was 13 due to the condi-
tions for this experiment, with an excess of alkyne versus
amine (see Figure S44 in the Supporting Information); thus,
31P NMR spectroscopy conclusively shows that starting com-
plex 2, added as a precatalyst, is completely and consecu-
tively converted under the conditions of the hydroamination
of a terminal alkyne into the corresponding complexes 14,
13, and 15 and finally 13 depending upon the composition of
the reaction mixture and the relative concentrations of the
potential AuI ligands.
Experimental Section
X-Ray crystal structure analysis for 4, 5, 8, 9, 13, and 14: CCDC-921288
(4), CCDC-921289 (5), CCDC-921290 (8), CCDC-921291 (9), CCDC-
921292 (13), and CCDC-921293 (14) contain the supplementary crystallo-
graphic data for this paper. These data can be obtained free of charge
from The Cambridge Crystallographic Data Centre via www.ccdc.cam.a-
c.uk/data_request/cif.
Acknowledgements
Financial support by the Spanish Ministry of Economy and Competitive-
ness (Severo Ochoa and CTQ2012-36351) and Generalidad Valenciana
(Prometeo 2012/014) is gratefully acknowledged.
Keywords: gold · gold complexes · gold nanoparticles ·
homogeneous catalysis · hydroamination reactions
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spectroscopy and GC-MS (see Figures S45, S46, and S47 in
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and 13C NMR spectroscopy (see Figures S48–S50 for 18 in
the Supporting Information) and by MALDI-TOF-MS (see
Figure S51 for 18 and S52 for 19 in the Supporting Informa-
tion), whereas the imine, 17, formed by hydroamination of
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H. Garcia, A. Corma et al.
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Received: April 27, 2013
Published online: August 9, 2013
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