Templated C-C and C-N Bond Formation Facilitated by a Molybdenum(VI) Metal Center

Article abstract of DOI:10.1021/acs.inorgchem.5b02347

Preparation of molybdenum dioxido complexes with novel iminophenolate ligands bearing pendant secondary amide functionalities led to unprecedented C-C and C-N coupling reactions of two α-iminoamides upon coordination. The diastereoselective cyclization to asymmetric imidazolidines occurs at the metal center in two consecutive steps via a monocoupled intermediate. A meaningful mechanism is proposed on the basis of full characterization of intermediate and final molybdenum-containing products by spectroscopic means and by single-crystal X-ray diffraction analyses. This process constitutes the first example of a diastereoselective self-cyclization of two α-iminoamides.

Full text of DOI:10.1021/acs.inorgchem.5b02347

Article
pubs.acs.org/IC
Templated C−C and C−N Bond Formation Facilitated by a
Molybdenum(VI) Metal Center
Niklas Zwettler, Antoine Dupe, Jorg A. Schachner, Ferdinand Belaj, and Nadia C. Mosch-Zanetti*
́
̈
̈
Institute of Chemistry, Inorganic Chemistry, University of Graz, Schubertstrasse 1, 8010 Graz, Austria
S
* Supporting Information
ABSTRACT: Preparation of molybdenum dioxido complexes with
novel iminophenolate ligands bearing pendant secondary amide
functionalities led to unprecedented C−C and C−N coupling
reactions of two α-iminoamides upon coordination. The diaster-
eoselective cyclization to asymmetric imidazolidines occurs at the
metal center in two consecutive steps via a monocoupled intermediate.
A meaningful mechanism is proposed on the basis of full
characterization of intermediate and final molybdenum-containing
products by spectroscopic means and by single-crystal X-ray
diffraction analyses. This process constitutes the first example of a diastereoselective self-cyclization of two α-iminoamides.
INTRODUCTION
■
Mononuclear dioxidomolybdenum(VI) complexes with a large
variety of ligands have been prepared and extensively
investigated due to their reactivity in oxygen atom transfer
(OAT) reactions. They have proven useful, both as biomimetic
models for mononuclear molybdoenzymes and also as highly
active oxygenation catalysts of, e.g., olefins using oxidants such
as hydrogen peroxide or tert-butyl hydroperoxide.1−3 In both
cases, various mechanistic studies showed hydrogen bonding to
be of crucial importance for the reactivity. Intermolecular H
bonding between the incoming oxidant and the oxido ligands of
the molybdenum complex is involved in catalytic epoxidation
reactions,4 while for biomimetic OAT reactions, the work of
Borovik highlighted the influence of intramolecular H bonding
between the ligand and the oxido group in the stabilization of
highly unusual iron and manganese complexes.5 In our group,
we previously reported Mo complexes coordinated by
phenolate ligands bearing donor atoms or a hydroxyl group
in the side chain in order to influence the hydrogen bonding
and thereby gain information on the OAT process. When Mo
dioxido complexes substituted by bidentate Schiff-base ligands
with donor sites were used in epoxidation reactions, a difference
in reactivity was observed depending on the nature of the
donor atom of the ligand, hence the ability to form weak or
strong hydrogen bonds.2 More recently, we used Mo and W
complexes bearing tetradentate bisphenolate ligands which are
able to form intramolecular H bonding, but here no influence
on the epoxidation activity could be observed.3
Figure 1. Synthesized iminophenolate ligands HL1−HL3 and their
derived complexes of the type [MoO2L2], showing the desired
intramolecular hydrogen bonding.
upon coordination at an electron poor, high oxidation state
metal center (MoVI, d0), involving C−C and C−N bond
formation.
The coupling behavior of the here investigated ligands is
reminiscent of the Huisgen-type [2 + 3] cycloaddition between
an azomethine ylide and a dipolarophile.6 Whereas α-
iminoesters are extensively used to form dipoles which can
undergo cycloaddition reactions with a broad scope of
dipolarophiles as well as intermolecular self-cyclization,7−9
reports of cycloaddition reactions of α-iminoamides are scarce
and require activated alkenes as reaction partners.10,11
According to Grigg and co-workers the acidity of the α-
protons has a crucial influence as demonstrated by introducing
adjacent electron withdrawing groups (EWG) thereby increas-
ing reactivity.8,12 The lower acidity of the amide α-protons in α-
iminoamides compared to structurally related carboxylic acid
esters12 is likely the reason why hardly any cycloadditions with
α-iminoamides are described in the literature. Within this
report, a diastereoselective cyclization of the latter at a metal
Inspired by the tripodal amidoamine ligands reported by
Borovik, we envisioned to introduce similar amide function-
alities in our previously used iminophenolate ligands and
investigated their coordination behavior toward the molybde-
num dioxido core (Figure 1). Herein, we thus report the
synthesis of bidentate iminophenolate ligands bearing amide
functionalities and their unexpected reactivity as 1,3-dipoles
Received: October 12, 2015
© XXXX American Chemical Society
A
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template to form imidazolidine coordinated complexes is
presented, and a stepwise reaction pathway via a single C−C
coupled intermediate of two ligand molecules is demonstrated.
Full characterization of intermediate and final complexes
including single-crystal X-ray diffraction analyses is provided,
allowing us to propose a meaningful mechanism for the
cyclization sequence at the metal center.
Scheme 2. Synthesis of Complexes 1−4 Coordinated by C−
C and C−N Coupling Products
RESULTS AND DISCUSSION
Ligand Synthesis. Ligands HL1−HL3 were prepared
according to Scheme 1. In the first step, 2-chloroacetyl chloride
■
Scheme 1. Three-Step Synthesis of the Iminophenolate
Amide Ligands HL1−HL3
where L1C−C and L2C−C denote dianionic adducts of two HL1/
HL2 ligands, confirms these structures, as resonances for two
sets of ligands, but only one resonance for a CHN proton are
assignable. Coordination of the imine moiety to the electro-
philic metal center supposedly led to the activation of the imine
carbon enabling nucleophilic attack by the activated and
therefore easily deprotonated α-carbon of another coordinated
ligand moiety (vide infra).17,18 To test if different conditions
would lead to the desired [MoO2L2] species, a reaction
screening was performed, varying solvent and temperature.
Whereas the temperature only affected the reaction time, the
solvent had a large influence on the reaction product.
Anhydrous toluene and acetonitrile led selectively to complexes
1 and 2, but analogous reactions in dry MeOH yielded
compounds 3 and 4 as the only products as shown in Scheme
2.
was reacted with 2 equiv of tert-butyl or phenyl amine,
following a published route.13 For the subsequent conversion to
the previously described amines A1 and A2,14,15 a simple
approach, by reaction of N-substituted 2-chloroacetamides with
a large excess of aqueous ammonia (35%) in a pressure reactor,
was used. In the last step, the resulting primary amines were
condensed with the respective aldehydes to the desired
iminophenolates.
Ligands HL1−HL3 were characterized via 1H and 13C NMR
and FT-IR spectroscopy as well as EI-MS. The difference in the
shift of the amide NH proton is noteworthy, depending on the
substituent (6.27 ppm in HL1 and 6.26 ppm in HL3, tert-butyl
amide vs 8.42 ppm in HL2, phenyl amide).
Their HSQC 2D NMR spectra feature resonances assignable
to three aliphatic CH protons and, unexpectedly, the absence of
an imine proton resonance. Single-crystal X-ray diffraction
analyses of 3 and 4, respectively, identified the products to be
unusual mononuclear dioxidomolybdenum compounds coordi-
nated in a tetradentate fashion by dianionic phenolate−
imidazolidine moieties, resulting from C−C and C−N coupling
([MoO2(L1Cyc)] (3) and [MoO2(L2Cyc)] (4), where L1Cyc and
L2Cyc denote dianionic imidazolidine moieties, respectively).
Complex Synthesis. For the synthesis of disubstituted
dioxidomolybdenum(VI) complexes of the type [MoO2L2],
well-established synthetic protocols using the [MoO2(acac)2]
precursor were investigated.16 Thus, reaction of 2 equiv of HL1
and HL2, respectively, with [MoO2(acac)2] in dry MeCN
under inert conditions led instead of the targeted [MoO2L2]
complexes to compounds 1 and 2 in good yields (Scheme 2).
Whereas initial 1H NMR data suggested the selective formation
of the desired disubstituted complexes, HSQC 2D NMR
experiments pointed toward a different species with two
distinct CH moieties, indicating the formation of a new C−C
bond within the synthesis. Unambiguous elucidation of the
nature of the obtained complexes 1 and 2 allowed a single-
crystal X-ray diffraction analysis of 1, revealing a mononuclear
dioxidomolybdenum complex coordinated by a tetradentate,
dianionic adduct, which had formed by C−C coupling of two
ligand molecules (Scheme 2). Extended NMR spectroscopy of
compounds [MoO2(L1C−C)] (1) and [MoO2(L2C−C)] (2),
Whereas such cyclizations are known for α-iminoesters,8,19−21
a
similar reactivity has not been described for the corresponding
amides. Only N-(2-pyridylmethyl)imines were found to
undergo cycloaddition at Cu(II) or Cd(II) centers leading to
imidazolidine coordinated compounds.17,22 However, such
compounds are known to undergo intermolecular self-
cyclization at room temperature in the absence of metal,19,20
a reaction not observed with the herein discussed molecules
(vide infra). A reactivity related to the cyclization step disclosed
herein has been previously reported to occur in the reaction of
the dioxidomolybdenum salicylate precursor [MoO2(sal)2] and
L-ornithine monohydrochloride.23
B
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Scheme 3. Synthesis of Complex 5 and Formation of Minor Product 6
Scheme 4. Proposed Reaction Pathway of the C−C Coupling and Subsequent Cyclization at the [MoO2]2+ Metal Center
In proton NMR spectra of crude [MoO2(L2C−C)] (2),
resonances assignable to small quantities of [MoO2(L2Cyc)] (4)
were noticed, pointing to a synthetic dependence between the
two structural types. To confirm this hypothesis, complexes 1
and 2 were dissolved in dry methanol and stirred for 5 h at 50
°C under inert conditions. Proton NMR spectroscopy showed
quantitative conversion of 1 into 3 and 2 into 4, respectively.
This contrasts with previous observations where addition of an
α-iminoamide to a dipolarophile catalyzed by LiBr/NEt3 led to
a mixture of the heterocycle and a Michael-type adduct in a
competitive rather than sequential fashion.11,24
adduct of HL3 and one acac− ligand, and complex 6 as
[MoO2(L3C−C)], a structural analogue to 1. Steric hindrance is
the plausible explanation for the formation of 6 in only trace
amounts (Scheme 3).
Depending on the nature of the carbonyl derivative,
autocyclization of dipolar compounds related to HL1−HL3
at elevated temperatures was reported.9,19,20 To ensure that the
observed reaction is in fact metal-dependent, blank experiments
where HL1−HL3 were stirred in dry methanol with and
without addition of catalytic amounts of trifluoroacetic acid for
24 h at 60 °C were carried out. Proton NMR spectra of the
residues showed only HL1−HL3 accompanied by decom-
position products (mainly aldehyde), but no coupled species.
Complexes 1 and 3 are very soluble; complexes 4 and 5 are
moderately soluble, and complex 2 is only slightly soluble in
polar solvents (e.g., CHCl3, CH2Cl2, MeCN). All complexes are
practically insoluble in nonpolar solvents and prone to
decomposition upon prolonged exposure to DMSO as well as
water-containing solvents. Compounds 1−5 were characterized
All described yields for complexes 1−4 correspond to
diastereopure compounds and were obtained with dry solvents
in an N2 atmosphere; however, the products are also accessible
under ambient conditions, albeit with significantly lower yields.
Furthermore, by reacting another common molybdenum
precursor, [MoO2Cl2], with 2 equiv of HL1 and 2 equiv of
2,6-lutidine as HCl scavenger in dry acetonitrile, mixtures of 1
and 3 were obtained. It is interesting to note that the exchange
of the amide tert-butyl substituent (HL1) by an electron
withdrawing phenyl substituent (HL2) does not affect the
ligand’s reactivity, despite the significant impact on the acidity
of the NH proton (6.27 ppm in HL1 vs 8.42 ppm in HL2 in
1
via H and 13C NMR and FT-IR spectroscopy, by elemental
analyses, and with the exception of 2 by single-crystal X-ray
diffraction analyses (vide infra). Furthermore, HSQC spectra of
complexes 1−5 were recorded.
1
their H NMR spectra).
Proposed Reaction Mechanism. A reaction mechanism is
proposed for the formation of complexes of structural types 1
and 3, respectively, starting from [MoO2(acac)2] (Scheme 4).
The first step involves the substitution of one monoanionic
acetylacetonate ligand (acac−) by the incoming iminopheno-
late. The electrophilic metal center activates one keto carbon in
the remaining acac− or the imine carbon in a second molecule
of coordinated iminophenolate. It also renders the amide-α-
CH2 group acidic so that a deprotonation and subsequent
nucleophilic attack on the activated keto or imine carbon is
facilitated, leading to adducts of either two α-iminoamide
moieties (1, 2, and 6) or one α-iminoamide with one acac−
ligand (5). Cyclization to the five-membered imidazolidine ring
Analogous reactions employing the sterically more demand-
ing ligand HL3 led to mixtures of several species and a
significant amount of decomposed ligand since HL3 is very
sensitive toward moisture. Nevertheless, careful work-up of the
reaction of [MoO2(acac)2] and HL3 allowed the isolation of
product 5 in low yield (Scheme 3). Recrystallization from
acetonitrile afforded the compound as yellow crystals. Addi-
tionally, in the crystalline batch we observed a small quantity of
bright orange crystals. Both the yellow (5) and the orange (6)
crystals proved to be suitable for single-crystal X-ray diffraction
analyses identifying complex 5 as a complex of the formula
[MoO2(L3ac)], with L3ac being a dianionic C−C coupled
C
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Figure 2. Molecular views (50% probability level) of 1 (top left), 5 (top right), and 6 (bottom); H atoms (except involved in hydrogen bonding or
bound to atoms affected by coupling) as well as solvent molecules are omitted for clarity reasons. Intramolecular hydrogen bonds to oxido groups are
indicated by dashed lines; the open bond in 5 depicts an unusual long Mo−L contact. The tert-butyl substituents on the phenyl rings in 6 are drawn
with small spherical atoms and line bonds for clarity reasons.
Figure 3. Molecular views (50% probability level) of 3 (left) and 4 (right); H atoms (except imidazolidine ring protons) as well as solvent molecules
are omitted for clarity reasons. Intramolecular hydrogen bonds to oxido groups are indicated by dashed lines; open bonds depict unusual long Mo−L
contacts.
occurs via a nucleophilic attack of the amine at the activated
imine carbon. This second step only takes place in methanol,
with the protic solvent facilitating the associated proton
transfer. With HL3, the cyclization is presumably prevented
by steric hindrance leading only to mono C−C coupled adducts
5 or 6.
Molecular Structures. Molecular structures of C−C
coupled complexes 1, 5, and 6, as well as imidazolidine
D
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Table 1. Selected Bond Lengths [Å] and Angles [deg] for Complexes 1 and 3−6
1
3
4
5
6
Mo1−O1
Mo1−O2
Mo1−O21
Mo1−O41
Mo1−O34
Mo1−N12
Mo1−N32
O1−Mo1−N12
O2−Mo1−O41
O21−Mo1−N32
1.7107(12)
1.7225(13)
1.9344(12)
2.0507(12)
1.7054(8)
1.7116(8)
1.9573(8)
1.9513(9)
2.2108(8)
1.7170(17)
1.7082(17)
1.9440(17)
1.9395(18)
2.2329(16)
1.714(2)
1.722(2)
1.920(2)
1.927(2)
2.314(2)
2.280(2)
1.7062(15)
1.7346(15)
1.9155(14)
2.0067(14)
a
2.3182(14)
2.2364(14)
166.73(6)
159.65(6)
155.13(5)
2.3173(17)
2.2261(18)
170.54(7)
156.94(7)
155.43(6)
2.4407(9)
2.4239(19)
158.23(9)
95.59(4)
75.15(3)
95.95(8)
74.66(7)
a
O32 in complex 5.
complexes 3 and 4, were determined by single-crystal X-ray
diffraction analysis. For 3, two different modifications were
observed, dependent on the co-crystallizing solvent. Herein, the
more exact measurement is discussed in detail; data
corresponding to the other modification can be found in the
electronic Supporting Information (SI). Molecular views of 1,
5, and 6 are given in Figure 2, and of 3 and 4 in Figure 3.
Important bond lengths and angles for complexes 1 and 3−6
are given in Table 1; full crystallographic details such as
intramolecular hydrogen bonding, structure refinement, and
experimental details are provided within the SI.
each other, whereas in their related imidazolidine compounds 3
and 4 they are arranged trans. This flexibility of the ligand
framework is unusual due to the tetradentate coordination
motif. A comparison of the MoO bond lengths of the herein
reported complexes, especially for the structurally related
complexes 3/4 and 1/6, points to only minor influence of
hydrogen bonding in the present case. Generally, all observed
molybdenum oxido bond lengths are within expected ranges
(Table 1).25
CONCLUSIONS
■
In all complexes, the molybdenum atoms are coordinated in
a distorted octahedral fashion by the tetradentate ligand and
two terminal oxido ligands. In complex 1, an intramolecular H-
bond between an amide hydrogen and an oxido group is
present (N15···O2 2.868(2)Å). The occurrence of such a
hydrogen bond is interesting, providing ample reasoning for the
choice of the ligands. During complexation a C−C bond
between C13 and C31 is formed, bridging two ligand moieties
in an asymmetric fashion. The saturated rings in complexes 3
and 4 adopt an envelope conformation in which the N32 atom
is bent out of plane (C11−N12−C13−C31-0.50(11)°, 3) and
(C11−N12−C13−C31-1.4(2)°, 4), respectively. The saturated
N32 atoms of the imidazolidine rings coordinate to the Mo
centers via rather long Mo−N contacts of 2.4407(9) Å (3) and
2.4239(19) Å (4), whereas the N12 atoms are involved in
intramolecular hydrogen bonding to O2, (N12···O2
2.9903(13)Å, 3) and (N12···O2 2.984(3)Å, 4). In complex 5,
the oxygen atoms of the acac− derived ligand are inequivalent
(C32−O32 1.429(3) Å, O32−Mo1 1.9270(19) Å vs C34−O34
1.234(3) Å, O34−Mo1 2.314(2) Å) due to a C−C bond
formation between C13 and C32. Similar to complex 1, an
intramolecular hydrogen bond between N15 and O2 can be
observed (N15···O2 2.952(3)Å) in 5. Compounds 1, 5, and 6
contain three chiral carbon centers. As expected, they
crystallized in a racemic fashion. However, it is interesting to
note that, upon comparing the molecular structures of the two
adducts 1 and 6 (Figure 2), two different racemates are formed.
This is due to the different orientation of the amide side chain
at C13, leading to an overall structure of SRR/RSS in 1 and
RRR/SSS in 6 with respect to C13, C31, and N32. Thus, an
intramolecular hydrogen bond between N15 and O2 as
observed in 1 is not possible in 6. This difference is presumably
caused by the larger steric demand in 6. In contrast, the
substituents at the imidazolidine rings in complexes 3 and 4
(Figure 3) show the same configuration with respect to each
other. Another noteworthy observation is that in complexes 1
and 2 the phenolate oxygens O21 and O41 are oriented cis to
The synthesis of three new iminophenolate ligands bearing tert-
butyl and phenyl amide functionalities is reported. They react
with [MoO2(acac)2] to form five dioxidomolybdenum(VI)
complexes 1−5, coordinated by tetradentate ligands including
imidazolidine heterocycles, arising from unprecedented metal-
mediated C−C coupling and subsequent cyclization reactions.
Complexes 1 and 2 are coordinated by dianionic adducts of two
HL1/HL2 moieties, 3 and 4 by the corresponding cyclized
imidazolidine moieties, and 5 by a dianionic C−C coupled
adduct of HL3 with acetyl acetonate. Whereas Huisgen-type [2
+ 3] cycloaddition reactions between azomethine ylides and
dipolarophiles are widespread in the literature, intermolecular
self-cyclizations of dipoles bearing amide functionalities have
not been described. Here, we therefore report the first
observation of a diastereoselective self-cyclization reaction at
an electron poor metal center (MoVI, d0) yielding complexes
employing a coordinated asymmetric imidazolidine moiety.
Mono C−C coupled complexes were found to be intermediate
products to the final imidazolidine complexes. All moderately
air- and moisture-sensitive complexes 1−5 were fully
characterized by spectroscopic means and, except 2, via
single-crystal X-ray diffraction analysis. Intramolecular H-
bonds could be observed in the molecular structures of 1, 3,
4, and 5, although their influence on the molybdenum oxido
bond lengths is negligible. Additionally, a reaction mechanism
for the cyclization reaction at the metal center with the single
C−C coupled adduct complexes acting as intermediates is
proposed. The novel reactivity at the Mo(VI) metal center
could allow for the development of new catalytic systems
promoting the stereoselective synthesis of imidazolidine
heterocycles from a broad scope of 1,3-dipole precursors
including α-iminoamides. Further studies thereof are on the
way.
EXPERIMENTAL SECTION
■
General. Unless specified otherwise, experiments were performed
under inert conditions using standard Schlenk equipment. Commer-
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cially available chemicals were purchased from Sigma-Aldrich and used
as received. No further purification or drying operations have been
performed. The metal precursor [MoO2(acac)2] was synthesized
according to known procedures.26 Solvents were purified via a Pure-
Solv MD-4-EN solvent purification system from Innovative Technol-
ogy, Inc. Methanol was refluxed over activated magnesium for at least
24 h and then distilled prior to use. Amine syntheses have been carried
out in a Parr Instrument 4560 minireactor coupled to a Parr
Instrument 4840 temperature controller. The 1H and 13C NMR
spectra were recorded on a Bruker Optics instrument at 300/75 MHz.
Peaks are denoted as singlet (s), broad singlet (bs), doublet (d),
doublet of doublets (dd), triplet (t), pseudodoublet (“d”),
pseudotriplet (“t”), and multiplet (m). Used solvents and peak
assignment are mentioned at the specific data sets. 13C NMR spectra
recorded in CD3CN have been referenced to the deuteromethyl-
carbon. Due to insolubility, meaningful 13C NMR data for complex 2
could not be obtained; HSQC data is given instead. Electron impact
mass spectroscopy (EI-MS) measurements have been performed with
an Agilent 5973 MSD mass spectrometer with push rod. Peaks are
denoted as cationic mass peaks, and the unit is the according ion’s
mass/charge ratio. Mass spectroscopy reveals no useful data for
complexes 3−5, likely due to decomposition during measurement. Gas
chromatography mass spectroscopy (GC−MS) measurements have
been performed with an Agilent 7890 A gas chromatograph (column
type, Agilent 19091J-433), coupled to an Agilent 5975 C mass
spectrometer. Samples for infrared spectroscopy were measured on a
Bruker Optics ALPHA FT-IR spectrometer. IR bands are reported
with wavenumber (cm−1) and intensities (s, strong; m, medium; w,
weak). All elemental analyses were measured at the University of
Vienna, microanalytical laboratory.
purification (yield, calculated for A1: 59%). Nonetheless, pure A1
was obtained after isocratic column chromatography of a crude
mixture (1.00 g) with CHCl3/MeOH/NH3·H2O(35%) (50/20/1) as
eluent. Product loaded fractions have been identified via TLC using
KMnO4 stain. Evaporation of the solvent in vacuo gave A1 as yellow
oil (88%, 0.88 g). NMR data in CDCl3 is inconsistent with literature
and denoted subsequently.14 1H and 13C NMR shifts in CD3CN are
given for comparison reasons. 1H NMR (300 MHz, CDCl3, 25 °C) δ:
7.06 (bs, 1H, NH), 3.23 (s, 2H, CH2), 1.71 (s, 2H, NH2), 1.35 (s, 9H,
tBu) ppm. 13C NMR (75 MHz, CDCl3, 25 °C) δ: 171.95 (CO),
1
50.56 (q-tBu), 45.45 (CH2), 28.88 (tBu) ppm. H NMR (300 MHz,
CD3CN, 25 °C) δ: 7.12 (bs, 1H, NH), 3.06 (s, 2H, CH2), 1.96 (s, 2H,
NH2), 1.30 (s, 9H, tBu) ppm. 13C NMR (75 MHz, CD3CN, 25 °C) δ:
172.95, 50.76 (q-tBu), 45.84 (CH2), 28.96 (tBu) ppm.
Synthesis of 2-Amino-N-phenylacetamide (A2). For the synthesis
of A2, 2-chloro-N-phenylacetamide (5.00 g, 29.5 mmol) was
suspended in aqueous ammonia (200 mL, 35% sol. in H2O). The
suspension was subsequently stirred in an autoclave (5 atm, 60 °C) for
3 h. The resulting clear aqueous solution was extracted four times with
dichloromethane (100 mL each). The combined organic fractions
were washed with brine, dried over MgSO4, and evaporated in vacuo.
The crude product contained the desired product A2 accompanied by
minor impurities of aniline. Pure A2 was obtained via heating in vacuo
(10−4 atm, 60 °C) for 4 h as a light brown solid (70%, 3.10 g).
Analytical data are consistent with the literature.15 1H and 13C NMR
1
shifts in CD3CN are given for comparison reasons. H NMR (300
MHz, CD3CN, 25 °C) δ: 9.33 (bs, 1H, NH), 7.61 (“d”, 2H, ArH),
7.32 (“t”, 2H, ArH), 7.08 (“t”, 1H, ArH), 3.34 (s, 2H, CH2), 1.87 (bs,
2H, NH2) ppm. 13C NMR (75 MHz, CD3CN, 25 °C) δ: 172.62 (C
O), 139.53, 129.79, 124.52, 120.17 (Ar), 45.99 (CH2) ppm.
X-ray Diffraction Analyses. Single-crystal X-ray diffraction
analyses were measured on a BRUKER-AXS SMART APEX II
diffractometer equipped with a CCD detector. All measurements were
performed using monochromatized Mo Kα radiation from an Incoatec
microfocus sealed tube at 100 K (Table S1). Absorption corrections
were performed semiempirically from equivalents. Structures were
solved by direct methods (SHELXS-97)27 and refined by full-matrix
least-squares techniques against F2 (SHELXL-2014/6).27 CCDC
1420762−1420766 and 1422193 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.
ac.uk/data_request/cif. Full experimental details for single-crystal X-
ray diffraction analyses of all compounds are provided in the SI.
Synthesis of Primary Amines. N-(tert-Butyl)-2-chloroacetamide
and 2-chloro-N-phenylacetamide were synthesized in 84% and 88%
yield, respectively, following a published procedure.13 In contrast to
literature procedure, N-(tert-butyl)-2-chloroacetamide was isolated as
white crystalline needles instead of an oily product. Analytical data are
Ligand Synthesis. All ligands are stable toward air but sensitive
toward moisture. They can be stored in a desiccator over P2O5 for
several weeks without decomposition.
Synthesis of (E)-N-(tert-Butyl)-2-((2-hydroxybenzylidene)-
amino)acetamide (HL1). For the synthesis of HL1, 1 equiv of A1
(500 mg, 3.84 mmol) was mixed with 1 equiv of salicylic aldehyde
(469 mg, 3.84 mmol) in a round flask. The yellow, initially liquid
mixture was subsequently stirred at room temperature for 2 h,
whereupon it solidified. Reaction completion was monitored via GC−
MS. HL1 was isolated after washing with cold pentane and drying in
vacuo as a microcrystalline citron yellow solid (93%, 763 mg). Mp 111
°C. 1H NMR (300 MHz, CD3CN, 25 °C) δ: 13.00 (bs, 1H, OH), 8.42
(s, CHN) 7.40−7.33 (m, 2H, ArH), 6.92 (“t”, 2H, ArH), 6.27 (bs,
1H, NH), 4.14 (s, 2H, CH2), 1.32 (s, 9H, tBu) ppm. 13C NMR (75
MHz, CD3CN, 25 °C) δ: 169.51 (CO), 168.55 (CN), 161.80
(Ar-OH), 133.62, 132.97, 119.89, 119.79, 117.56 (Ar), 63.40 (CH2),
51.78 (q-tBu), 28.81(tBu) ppm. IR (ATR, cm−1): ν
̃
3329 (m), 1639
(s), 1637 (s), 1542 (s), 1282 (m), 1280 (m), 884 (m), 758 (s), 580
consistent with the literature;13,28 here, H and 13C NMR shifts in
1
(m). EI-MS: m/z 234.2 [M+].
CD3CN are given for comparison reasons.
Synthesis of (E)-2-((2−Hydroxybenzylidene)amino)-N-phe-
nylacetamide (HL2). For the synthesis of HL2, 1 equiv of A2 (1202
mg, 8.00 mmol) was dissolved in acetonitrile (25 mL) in a round flask.
Subsequently 1 equiv of salicylic aldehyde (978 mg, 8.00 mmol) was
added, and the reaction mixture stirred at room temperature for 3 h,
whereupon the initially colorless solution turned pale yellow. Reaction
completion was determined via TLC. The reaction solution was dried
over MgSO4, and the solvent was evaporated in vacuo to give HL2 as a
yellow solid (96%, 1953 mg). Mp 134 °C. 1H NMR (300 MHz,
CD3CN, 25 °C) δ: 12.94 (s, 1H, OH), 8.51 (s, 1H, CHN), 8.42 (bs,
1H, NH), 7.58 (“d”, 2H, ArH), 7.44−7.30 (m, 4H, ArH), 7.11 (t, 1H,
ArH), 6.96−6.92 (m, 2H, ArH), 4.41 (s, 2H, CH2) ppm. 13C NMR
(75 MHz, CD3CN, 25 °C) δ: 170.27 (CO), 168.14 (CN), 161.80
(Ar-OH), 139.39, 133.77, 133.11, 129.79, 125.03, 120.91, 119.95,
1
N-(tert-Butyl)-2-chloroacetamide. H NMR (300 MHz, CD3CN,
25 °C) δ: 6.51 (bs, 1H, NH), 3.89 (s, 2H, CH2), 1.31 (s, 9H, tBu)
ppm. 13C NMR (75 MHz, CD3CN, 25 °C) δ: 166.18 (CO), 52.07
(q-tBu), 44.25 (CH2), 28.60 (tBu) ppm.
1
2-Chloro-N-phenylacetamide. H NMR (300 MHz, CD3CN, 25
°C) δ: 8.59 (bs, 1H, NH), 7.56 (“d”, 2H, ArH), 7.35 (“t”, 2H, ArH),
7.14 (“t”, 1H, ArH), 4.16 (s, 2H, CH2) ppm. 13C NMR (75 MHz,
CD3CN, 25 °C) δ: 165.66 (CO), 138.99, 129.86, 125.44, 120.98
(Ar), 44.33 (CH2) ppm.
Synthesis of 2-Amino-N-(tert-butyl)acetamide (A1). For the
synthesis of A1, N-(tert-butyl)-2-chloroacetamide (5.00 g, 33 mmol)
was suspended in aqueous ammonia (200 mL, 35% sol. in H2O). The
suspension was subsequently stirred in an autoclave (5 atm, 60 °C) for
3 h. The resulting clear aqueous solution was extracted four times with
dichloromethane (100 mL each). The combined organic fractions
were washed with brine, dried over MgSO4, and evaporated in vacuo.
The crude product, which contained a mixture of primary (2-amino-N-
(tert-butyl)acetamide) and secondary (2,2′-azanediyl-bis(N-(tert-
butyl)acetamide)) amine in a ratio of approximately 90:10 (GC−
MS), could be used for subsequent syntheses without further
119.83, 117.61 (Ar), 63.38 (CH2) ppm. IR (ATR, cm−1): ν
̃
2981 (w),
1673 (w), 1626 (m), 1595 (m), 1538 (s), 1445 (m), 751 (s), 695 (m).
EI-MS: m/z 254.1 [M+].
Synthesis of (E)-N-(tert-Butyl)-2-((2-hydroxybenzylidene)-
amino)acetamide (HL3). For the synthesis of HL3, 1 equiv of A1
(500 mg, 3.84 mmol) was added to a solution of 1.04 equiv of 3,5-di-
tert-butyl-2-hydroxybenzaldehyde (937 mg, 4.00 mmol) in MeOH (5
F
DOI: 10.1021/acs.inorgchem.5b02347
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
mL) in a round flask. The solution, which turned dark yellow
immediately, was stirred at room temperature for 1 h. After a few
minutes a precipitate started to form. Reaction completion was
monitored via GC−MS. After filtration and a wash with a small
portion of cold MeOH, HL3 was isolated as a fluffy, light yellow solid
(92%, 1227 mg). Mp 155 °C. 1H NMR (300 MHz, CD3CN, 25 °C) δ:
13.64 (s, 1H, OH), 8.41 (s, 1H, CHN), 7.43 (d, 1H, ArH), 7.25 (d,
1H, ArH), 6.26 (bs, 1H, NH), 4.13 (s, 2H, CH2), 1.42 (s, 9H, tBu),
1.32 (s, 9H, tBu), 1.30 (s, 9H, tBu) ppm. 13C NMR (75 MHz,
CD3CN, 25 °C) δ: 170.52 (CO), 168.62 (CN), 158.76 (Ar-OH),
141.41, 137.28, 128.23, 127.64, 119.08 (Ar), 63.13 (CH2), 51.77,
35.66, 34.82 (q-tBu), 31.67, 29.65, 28.82 (tBu) ppm. IR (ATR, cm−1):
(m), 495 (m). Anal. Calcd for C30H26MoN4O6·0.3CH2Cl2: C, 55.14;
H, 4.06; N, 8.49. Found: C, 54.97; H, 3.90; N, 8.65.
Synthesis of [MoO2(L1Cyc)] (3). For the synthesis of 3, 1 equiv of
[MoO2(acac)2] (49 mg, 0.15 mmol) was suspended in dry MeOH (5
mL) in a Schlenk tube under N2 atmosphere. Subsequently, 2 equiv of
HL1 (70 mg, 0.30 mmol) was added, and the yellow reaction mixture
stirred at 50 °C for 2 h. Complete ligand consumption was monitored
via TLC, whereupon the yellow solution was filtered through a glass
frit packed with Celite, concentrated in vacuo, layered with heptane,
and stored at −25 °C for 24 h. The residual precipitate was filtered off,
washed with cold heptane, and dried in vacuo to give 3 as a yellow
crystalline solid (51%, 45 mg). Crystals suitable for single-crystal X-ray
diffraction analysis were obtained via recrystallization from MeOH
layered with heptane or CH3CN. Alternatively, 3 was obtained in
excellent yield starting with 1. Therefore 1 (100 mg, 0.17 mmol) was
suspended in dry MeOH (10 mL) and stirred for 5 h at 50 °C under
N2 atmosphere. The resulting yellow solution was filtered through a
glass frit packed with Celite and subsequently evaporated in vacuo to
obtain pure 3 as a yellow powder (97%, 97 mg). Mp 230 °C
̃
ν 3271 (m), 2960 (s), 1651 (s), 1634 (s), 1556 (s), 1439 (s), 1391
(m), 1360 (s), 1273 (s), 1224 (s), 1172 (m), 713 (w), 586 (w). EI-
MS: m/z 346.3 [M+].
Complex Syntheses. All complexes are moderately sensitive
toward air and sensitive toward moisture. They can be stored in a N2-
filled glovebox for several weeks without decomposition: L1C−C and
L2C−C denote dianionic adducts of two HL1/HL2 ligands, L1Cyc and
L2Cyc denote dianionic cyclized imidazolidine moieties, and L3ac
denotes the dianionic C−C coupled adduct of HL3 with acetyl
acetonate.
1
(decomp). H NMR (300 MHz, CD3CN, 25 °C) δ: 7.38−7.18 (m,
6H, 4 ArH, 2 NH), 6.98−6.92 (m, 1H, ArH), 6.88−6.74 (m, 3H,
ArH), 4.82 (d, 1H, CH), 4.28−4.20 (m, 1H, NH), 4.11−4.08 (m, 1H,
CH), 4.01−3.95 (m, 1H, CH), 3.58−3.36 (m, 2H, CH2), 1.35 (s, 9H,
tBu), 0.97 (s, 9H, tBu) ppm. 13C NMR (75 MHz, CD3CN, 25 °C) δ:
173.12, 171.38 (RCONH), 164.28, 162.73 (Ar-O), 132.76, 132.10,
130.96, 130.81, 123.89, 122.38, 121.68, 121.42, 119.99, 119.82 (Ar),
82.97, 74.98, 64.94 (CH), 56.50 (CH2), 54.20, 51.25 (q-tBu), 28.81,
28.05 (tBu) ppm. IR (ATR, cm−1): ν 1626 (m), 1263 (m), 1236 (m),
1017 (m), 924 (s), 889 (MoO, s), 875 (MoO, s), 752 (s), 635
(s), 441 (s). EI-MS: m/z 578.4 [M+ − H2O]. Anal. Calcd for
C26H34MoN4O6: C, 52.53; H, 5.76; N, 9.42. Found: C, 52.37; H, 5.51;
N, 9.26.
Synthesis of [MoO2(L1C−C)] (1). For the synthesis of 1, 1 equiv of
[MoO2(acac)2] (489 mg, 1.50 mmol) was suspended in dry CH3CN
(10 mL) in a Schlenk tube under N2 atmosphere. Subsequently, 2
equiv of HL1 (703 mg, 3.00 mmol) was added, and the yellow to
orange reaction mixture stirred at 60 °C for 3 h. Complete ligand
consumption was monitored via TLC, whereupon the orange reaction
mixture was concentrated in vacuo and stored at −25 °C for 24 h. The
precipitate was filtered off and washed with cold pentane.
Subsequently, the orange solid was dissolved in a minimal amount
of dry CH2Cl2, and white residue was removed via filtration through a
glass frit packed with Celite. Evaporation of the solvent in vacuo gave 1
as an orange powder (77%, 683 mg). Crystals suitable for single-crystal
X-ray diffraction analysis were obtained via recrystallization from
CH2Cl2. Mp 243 °C (decomp). 1H NMR (300 MHz, CD3CN, 25 °C)
δ: 8.17 (s, CHN), 7.55 (“t”, 1H, ArH), 7.36 (“d”, 1H, ArH), 7.14−
6.93 (m, 4H, ArH), 6.92 (bs, 1H, NH), 6.70 (bs, 1H, NH), 6.67 (“t”,
1H, ArH), 6.46 (“d”, 1H, ArH), 6.16 (bs, 1H, NH), 4.67 (d, 1H, CH),
4.32 (s, 1H, CH), 3.53−3.27 (m, 2H, CH2), 1.32 (s, 9H, tBu), 1.29 (s,
9H, tBu) ppm. 13C NMR (75 MHz, CD3CN, 25 °C) δ: 167.88 (CH
N), 167.32, 166.66 (RCONH), 161.93, 160.66 (Ar−O), 137.09,
136.22, 131.15, 130.29, 122.40, 121.97, 121.90, 120.70, 119.21 (Ar),
118.29 (Ar, concealed by solvent peak, clarified via HSQC), 80.60
(CH), 67.46 (CH), 56.68 (CH2), 52.88, 52.28 (q-tBu), 28.80, 28.70
Synthesis of [MoO2(L2Cyc)] (4). For the synthesis of 4, 1 equiv of
[MoO2(acac)2] (98 mg, 0.30 mmol) as well as 2 equiv of HL2 (153
mg, 0.60 mmol) were suspended in dry MeOH (3 mL) in a Schlenk
tube under N2 atmosphere. Subsequently, the yellow reaction mixture
was stirred at 50 °C for 3 h whereupon a yellow solid started to
precipitate from the solution. Complete ligand consumption was
monitored via TLC, whereupon the precipitate was filtered off and
washed twice with a small portion of cold dry methanol to obtain 4 as
a yellow powder (66%, 126 mg). Crystals suitable for single-crystal X-
ray diffraction analysis have been obtained via recrystallization from a
CH3CN solution containing OPMe3 as additive. Analogous to the
procedure described for the synthesis of 3, 4 can be obtained in high
yield via transformation of 2 in methanol. Therefore, 2 (100 mg, 0.17
mmol) was suspended in dry MeOH (10 mL) and stirred for 5 h at 50
°C under N2 atmosphere. The resulting yellow reaction mixture was
filtered and the precipitate dried in vacuo to obtain pure 4 as a yellow
(tBu) ppm. IR (ATR, cm−1): ν
̃
1668 (s), 1541 (m), 1449 (m), 1285
(s), 1271 (m), 1215 (m), 914 (MoO, s), 880 (MoO, s), 758 (s),
639 (m), 614 (m). EI-MS: m/z 580.4 [M+ − O]. Anal. Calcd for
C26H34MoN4O6·0.4CH2Cl2: C, 50.45; H, 5.58; N, 8.91. Found: C,
50.36; H, 5.48; N, 9.16.
1
powder (95%, 95 mg). Mp 248 °C (decomp). H NMR (300 MHz,
CD3CN, 25 °C) δ: 9.64 (bs, 1H, NH), 9.05 (bs, 1H, NH), 7.68 (“d”,
2H, ArH), 7.56 (dd, 1H, ArH), 7.47 (dd, 1H, ArH), 7.40−7.12 (m,
8H, ArH), 7.06−6.95 (m, 3H, ArH), 6.93−6.83 (m, 3H, ArH), 5.07
(d, 1H, CH), 4.50−4.30 (m, 3H, 2 CH, NH), 3.88−3.59 (m, 2H,
CH2) ppm. 13C NMR (75 MHz, CD3CN, 25 °C) δ: 173.22, 170.65
(RCONH), 164.30, 162.71 (Ar-O), 139.16, 135.74, 133.11, 132.33,
131.21, 131.09, 130.04, 129.87, 127.90, 125.08, 123.49, 123.17, 122.21,
121.95, 121.76, 120.80, 120.09, 119.81 (Ar), 82.50, 74.79, 64.90 (CH),
56.47 (CH2) ppm. IR (ATR, cm−1): ν 1633 (m), 1598 (m), 1529 (m),
1446 (m), 1264 (m), 1232 (m), 915 (MoO, s), 894 (MoO, s),
868 (s), 833 (m), 740 (s), 688 (m), 636 (m), 493 (m), 443 (m). Anal.
Calcd for C30H26MoN4O6: C, 56.79; H, 4.13; N, 8.83. Found: C,
56.07; H, 4.10; N, 8.30.
Synthesis of [MoO2(L2C−C)] (2). For the synthesis of 2, 1 equiv of
[MoO2(acac)2] (489 mg, 1.50 mmol) as well as 2 equiv of HL2 (763
mg, 3.00 mmol) were suspended in dry toluene (10 mL) in a Schlenk
tube under N2 atmosphere. Subsequently, the reaction mixture was
stirred at 60 °C for 1.5 h, whereupon an orange solid started to
precipitate from the initially yellow solution. Complete ligand
consumption was monitored via TLC. The precipitate was filtered
off and washed twice with a small portion of cold dry CH2Cl2 to give 2
as an orange powder (75%, 718 mg). Mp 209 °C (decomp). 1H NMR
(300 MHz, CD3CN, 25 °C) δ: 8.73 (bs, 1H, NH), 8.31 (s, CHN),
8.29 (bs, 1H, NH), 7.60−7.52 (m, 5H, ArH), 7.42−7.31 (m, 5H,
ArH), 7.23−6.92 (m, 7H, 6 ArH, 1 NH), 6.73 (“t”, 1H, ArH), 6.52
(“d”, 1H, ArH), 4.93 (d, 1H, CH), 4.64 (s, 1H, CH), 3.82−3.59 (m,
2H, CH2) ppm. 13C NMR (75 MHz, CD3CN, 25 °C, HSQC, q-C
obscured) δ: 167.75 (CHN), 136.20, 136.08, 135.29, 129.82,
128.95, 128.90, 128.88, 128.83, 124.88, 124.21, 120.98, 120.96, 119.78,
119.75, 119.60, 119.46, 118.30, 117.88 (Ar), 79.41 (CH), 65.81 (CH),
55.19 (CH2) ppm. IR (ATR, cm−1): ν 1598 (m), 1442 (m), 1268 (s),
1247 (m), 907 (MoO, s), 876 (MoO, s), 751 (s), 689 (m), 640
Synthesis of [MoO2(L3ac)] (5). For the synthesis of 5, 1 equiv of
[MoO2(acac)2] (100 mg, 0.30 mmol) as well as 1 equiv of HL3 (104
mg, 0.30 mmol) were dissolved in dry CH3CN (5 mL) in a Schlenk
tube under N2 atmosphere. Subsequently, the initially yellow to orange
reaction solution was stirred at room temperature for 24 h
accompanied by a darkening to deep brown. The reaction solution
was then filtered through a glass frit packed with Celite and the solvent
evaporated in vacuo. The brown solids were dissolved in dry Et2O (5
G
DOI: 10.1021/acs.inorgchem.5b02347
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
́
mL), and the solution was allowed to stand at ambient temperature.
The initially brown solution turned orange gradually, and white solids
started to precipitate. After concentration of the orange liquid layer to
a minimum, the precipitate was filtered off and washed twice with a
small portion of cold dry Et2O to give 5 as a pale yellow, crystalline
solid (26%, 43 mg). Crystals suitable for single-crystal X-ray diffraction
analysis have been obtained via recrystallization from CH3CN. Mp 222
(c) Herbert, M.; Montilla, F.; Alvarez, E.; Galindo, A. Dalton Trans.
2012, 41, 6942−6956. (d) Morlot, J.; Uyttebroeck, N.; Agustin, D.;
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S. L.; Umbarkar, S. B.; Dongare, M. K.; Vanka, K. Chem. - Eur. J. 2013,
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Kuczera, K.; Hendrich, M. P.; Borovik, A. S. Science 2000, 289, 938−
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Yachandra, V. K.; Tolman, W. B.; Yano, J.; Hendrich, M. P.; Borovik,
A. S. J. Am. Chem. Soc. 2012, 134, 1996−1999. (d) Cook, S. A.;
Borovik, A. S. Acc. Chem. Res. 2015, 48, 2407−2414. (e) Cook, S. A.;
Hill, E. A.; Borovik, A. S. Biochemistry 2015, 54, 4167−4180.
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1
°C (decomp). H NMR (300 MHz, CD3CN, 25 °C) δ: 8.52 (s, 1H,
CHN), 7.68 (d, 1H, ArH), 7.42 (d, 1H, ArH), 7.16 (bs, 1H, NH),
4.39 (s, 1H, CH), 3.15−2.98 (m, 2H, CH2), 2.15 (s, 3H, CH3), 1.44
(s, 9H, tBu), 1.39 (s, 3H, CH3), 1.32 (s, 9H, tBu), 1.27 (s, 9H, tBu)
ppm. 13C NMR (75 MHz, CD3CN, 25 °C) δ: 215.49 (RCOR),
169.47 (RCONH), 168.81 (CHN), 159.09 (Ar-O), 143.86,
140.07, 132.47, 129.76, 121.38 (Ar), 85.51 (CH), 83.78 (C−O), 53.29
(CH2), 52.61, 36.10, 35.04 (q-tBu), 31.60 (CH3), 31.44, 29.88, 28.66
(tBu), 26.02 (CH3) ppm. IR (ATR, cm−1): ν 1661 (m), 1626 (m), 916
(MoO, s), 895 (MoO, s), 874 (s), 857 (s), 572 (m). Anal. Calcd
for C26H40MoN2O6: C, 54.54; H, 7.04; N, 4.89. Found: C, 54.41; H,
6.95; N, 4.90.
(9) Grigg, R.; Kemp, J.; Sheldrick, G.; Trotter, J. J. Chem. Soc., Chem.
Commun. 1978, 109.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.5b02347.
■
́
(10) (a) Gonzalez-Esguevillas, M.; Adrio, J.; Carretero, J. C. Chem.
S
Commun. 2012, 48, 2149−2151. (b) Adrio, J.; Carretero, J. C. Chem.
Commun. 2014, 50, 12434−12446.
(11) Tsuge, O.; Kanemasa, S.; Yoshioka, M. J. Org. Chem. 1988, 53,
1384−1391.
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4331. (b) Bordwell, F. G.; Fried, H. E. J. Org. Chem. 1991, 56, 4218−
4223.
NMR spectra and full crystallographic details (PDF)
Crystallographic data (CIF)
(13) Lowe, J. A., III; Hageman, D. L.; Drozda, S. E.; McLean, S.;
Bryce, D. K.; Crawford, R. T.; Zorn, S.; Morrone, J.; Bordner, J. J. Med.
Chem. 1994, 37, 3789−3811.
AUTHOR INFORMATION
Corresponding Author
*E-mail: nadia.moesch@uni-graz.at.
Author Contributions
The manuscript was written through contributions of all
authors. All authors have given approval to the final version of
the manuscript.
■
(14) Friebolin, W.; Jannack, B.; Wenzel, N.; Furrer, J.; Oeser, T.;
Sanchez, C. P.; Lanzer, M.; Yardley, V.; Becker, K.; Davioud-Charvet,
E. J. Med. Chem. 2008, 51, 1260−1277.
(15) Chen, X.; Wang, J.; Cui, J.; Xu, Z.; Peng, X. Tetrahedron 2011,
67, 67.
(16) Schachner, J. A.; Traar, P.; Sala, C.; Melcher, M.; Harum, B. N.;
Sax, A. F.; Volpe, M.; Belaj, F.; Mosch-Zanetti, N. C. Inorg. Chem.
̈
Notes
2012, 51, 7642−7649.
The authors declare no competing financial interest.
(17) Ou, Y.-J.; Zheng, Z.-P.; Hong, X.-J.; Wan, L.-T.; Wei, L.-M.; Lin,
X.-M.; Cai, Y.-P. Cryst. Growth Des. 2014, 14, 5339−5343.
(18) Bluhm, M. E.; Ciesielski, M.; Gorls, H.; Walter, O.; Doring, M.
ACKNOWLEDGMENTS
Financial support by the Austrian Science Fund (FWF) (Grant
P26264) and NAWI Graz is gratefully acknowledged.
■
̈
̈
Inorg. Chem. 2003, 42, 8878−8885.
(19) Grigg, R.; Gunaratne, H.; Sridharan, V.; Thianpatanagul, S.;
Tute, M. Tetrahedron Lett. 1983, 24, 4363−4366.
(20) Grigg, R.; Donegan, G.; Gunaratne, H.; Kennedy, D. A.;
Malone, J. F.; Sridharan, V.; Thianpatanagul, S. Tetrahedron 1989, 45,
1723−1746.
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DOI: 10.1021/acs.inorgchem.5b02347
Inorg. Chem. XXXX, XXX, XXX−XXX