Heterocyclic substituted methanides as promising alternatives to the ubiquitous nacnac ligand

Article abstract of DOI:10.1039/c4dt01008f

A series of group 13 complexes containing deprotonated bisheterocyclomethanes have been prepared and structurally as well as spectroscopically characterised. In the case of the parent neutral homo-disubstituted bisheterocyclomethanes bis-(benzoxazol-2-yl)-methane (abbreviated as (NCOC₆H₄)₂CH₂) (1) and bis-(benzothiazol-2-yl)-methane (abbreviated as (NCSC₆H₄)₂CH₂) (2), two interesting ligand systems were investigated with respect to their coordination abilities toward group 13 metals. Upon deprotonation of the acidic methylene bridge in each case a β-diketiminate-like structure is formed, where the negative charge becomes delocalised about the whole ligand framework. There is a mesomeric stabilisation between the carbanionic and the amidic resonance structure. The ligands mentioned above were treated with AlMe₃, AlMe₂Cl and GaMe₃to achieve deprotonation of the backbone and coincidental metal complexation. The resulting complexes [Me₂Al{(NCOC₆H₄)₂CH}] (3), [Me₂Al{(NCSC₆H₄)₂CH}] (4), [ClMeAl{(NCOC₆H₄)₂CH}] (5), [ClMeAl{(NCSC₆H₄)₂CH}] (6), [Me₂Ga{(NCOC₆H₄)₂CH}] (7), and [Me₂Ga{(NCSC₆H₄)₂CH}] (8) were structurally characterized in the solid state by X-ray single crystal diffraction and in solution by different (heteronuclear) 1D/2D-NMR spectroscopic techniques. Each of those molecules shows a nearly planar arrangement with the group 13 metal cation coordinated by the two ring nitrogen atoms of the conjugated heterocycles. Furthermore, in this work (NCOC₆H₄)₃C(C₃H₇) (9) could be isolated, which might turn out to be a promising analogue of the omnipresent trisoxazoline ligands in asymmetric catalysis.

Full text of DOI:10.1039/c4dt01008f

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Heterocyclic substituted methanides as promising
alternatives to the ubiquitous nacnac ligand†‡
Cite this: Dalton Trans., 2014, 43,
14432
David-R. Dauer and Dietmar Stalke*
A series of group 13 complexes containing deprotonated bisheterocyclomethanes have been prepared
and structurally as well as spectroscopically characterised. In the case of the parent neutral homo-disub-
stituted bisheterocyclomethanes bis-(benzoxazol-2-yl)-methane (abbreviated as (NCOC6H4)2CH2) (1)
and bis-(benzothiazol-2-yl)-methane (abbreviated as (NCSC6H4)2CH2) (2), two interesting ligand systems
were investigated with respect to their coordination abilities toward group 13 metals. Upon deprotonation of
the acidic methylene bridge in each case a β-diketiminate-like structure is formed, where the negative charge
becomes delocalised about the whole ligand framework. There is a mesomeric stabilisation between the
carbanionic and the amidic resonance structure. The ligands mentioned above were treated with AlMe3,
AlMe2Cl and GaMe3 to achieve deprotonation of the backbone and coincidental metal complexation. The
resulting complexes [Me2Al{(NCOC6H4)2CH}] (3), [Me2Al{(NCSC6H4)2CH}] (4), [ClMeAl{(NCOC6H4)2CH}] (5),
[ClMeAl{(NCSC6H4)2CH}] (6), [Me2Ga{(NCOC6H4)2CH}] (7), and [Me2Ga{(NCSC6H4)2CH}] (8) were structu-
rally characterized in the solid state by X-ray single crystal diffraction and in solution by different (hetero-
nuclear) 1D/2D-NMR spectroscopic techniques. Each of those molecules shows a nearly planar
arrangement with the group 13 metal cation coordinated by the two ring nitrogen atoms of the conju-
gated heterocycles. Furthermore, in this work (NCOC6H4)3C(C3H7) (9) could be isolated, which might turn
out to be a promising analogue of the omnipresent trisoxazoline ligands in asymmetric catalysis.
Received 4th April 2014,
Accepted 23rd May 2014
DOI: 10.1039/c4dt01008f
www.rsc.org/dalton
Therefore, the new designed ligand systems shown in
Scheme 1 should maintain more rigidity and lead to a more
Introduction
Referring to the monoanionic β-diketiminate ligand, the so- planar alignment in the corresponding metal complexes. Due
called nacnac ligand, enabling stabilisation of the main group to this planarity within the six-membered metalla heterocycle
elements in the low oxidation state, new ligand systems should be C3N2M the orbital overlap between the donor atoms and
exploited mimicking those electronic and steric properties. In the the chelated metal atom should be increased, so that a more
case of nacnac substituted systems it is known that the scaffold efficient conjugation and hence delocalisation can be obtained.
tends to deviate from a planar arrangement within metal coordi-
In former publications, we also investigated related com-
nation by means of twisting or bending, and merely the backbone pounds containing pyridyl substituents instead of benzox-
remains in plane. This deviation is caused by the flexible but bulky azole and benzothiazole.2 Several complexes of this
organic substituents at the imine moieties. Although this feature bis-(pyrid-2-yl)-methanide with group 1 and 13 metals could
of the twisting seems to be problematic, it also offers an efficient be synthesised and structurally characterized by X-ray diffrac-
shielding of the coordinated (low-valent) metal cations, which are tion: representing the group 1 complexes [([12]crown-4)2Li]-
sensitive towards reactions with electrophiles or dimerization.1
[Li{(2-NC5H4)2CH}] can be highlighted as a solvent separated
ion pair or [(thf)2Li{(2-NC5H4)2CH}] as a monomeric lithiated
compound both solely yielding Li–N contacts.3,4 Addition-
ally, some group 13 complexes like [Me2Al{(2-NC5H4)2CH}] and
Institut für Anorganische Chemie der Georg-August-Universität Göttingen,
Tammannstr. 4, 37077 Göttingen, Germany.
E-mail: dstalke@chemie.uni-goettingen.de; Fax: +49 551 39 33373;
Tel: +49 551 39 33000
†Dedicated to Professor Frank T. Edelmann on the occasion of his 60th birthday.
‡Electronic supplementary information (ESI) available: Tables of data collection
parameters, bond lengths and angles for compounds 1–9, 2D-NMR spectra of 3,
4 and 8, additional crystal structures of 4, 5 and 7, nine X-ray files in CIF format.
CCDC 995110–995118. For ESI and crystallographic data in CIF or other
electronic format see DOI: 10.1039/c4dt01008f
Scheme 1 Similarities between the nacnac ligand (left) and the
bisheterocyclomethanides (right).
14432 | Dalton Trans., 2014, 43, 14432–14439
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[Me2Ga{(2-NC5H4)2CH}], where formally the (thf)2Li unit is
replaced by a Me2Al or Me2Ga moiety, were prepared.5,6
Moreover, this kind of ligand system was varied by replacing
the bridging CH unit isoelectronically by N,7,8 P9,10 or As.11,12
The corresponding main group metal complexes were
investigated.
In the past decade many main group metal containing
β-diketiminate structures were synthesised and fully
characterized.13–25 The presented aluminium(III) and gallium(III)
complexes 3–8, which are described in the following, can
be compared with many analogue compounds of the
Dipp-substituted nacnac ligand. These complexes embrace
aluminium(III), gallium(III) and indium(III) compounds and, in the
case of Al and Ga, also the reduced species in the oxidation
state +I.26–28 The former range from the alkyl over the hydride
to halide substituted metal centres. The latter, the so-called
metallylenes, are rarely known and offer interesting prospects
as ligands in catalysis, where they might replace N-heterocyclic
carbenes or phosphanes.29,30 Furthermore the dialkyl-
aluminium β-diketiminates show catalytic activities towards
ring-opening polymerisation.31
Scheme 3 Syntheses of the metallated compounds 3–8.
derived from malonic acid and two equivalents of the corres-
ponding ortho-substituted anilines.
The reaction pathway (a) in Scheme 2 shows that the cyclo-
condensation reaction of the phenol derivative is more compli-
cated as in the case of the thiophenol (b). Therefore, the
former malononitrile has to be further activated before it
undergoes a reaction with the phenolic nucleophile. This can
be achieved by generating the derived ethylbisimidate dihydro-
chloride, wherein ammonia as well as ethanol act as leaving
groups.33
The bisheterocyclomethanides show three optional donor
sites: the deprotonated bridging carbanionic center, the ring-
chalcogene (oxygen or sulfur) and -nitrogen atoms. Because
O and S do not share the same good Lewis-donor abilities of
the ring-nitrogen atoms, each of the metallated species shows
a nacnac-like coordination, wherein the metal gets chelated by
the two heteroaromatic nitrogen atom donors. It should be
noted that the presented ligand systems show also analogy to
the popular bis-(oxazolin-2-yl)-methanes, which are used in
asymmetric catalysis. In contrast to 1 and 2 those ligands
consist of saturated not benzannulated heterocycles with chiral
centres, which can transfer chiral information in catalysis.32
The syntheses of the closely related [Me2Al{Dipp2nacnac}]
and [Me2Ga{Dipp2nacnac}] can be achieved by a facile reaction
of nacnacH with AlMe3 and GaMe3, respectively.13,14,31 Refer-
ring to this also LH 1 and 2 can be deprotonated at the acidic
methylene bridge by release of methane and coordinated by
the group 13 metal in a concerted way (Scheme 3). In this
reaction protocol many different metallated compounds are
accessible and can be compared structurally.
In this context the effect of the diverging metal coordi-
nation (M = Al, Ga) and moreover the related substituents at
these metal cations (R = R′ = Me; R = Me, R′ = Cl) on the
bonding situation within the different ligand systems should
be investigated.
Results and discussion
Syntheses
Structural data
In the following part a structural comparison of compounds
1–8, which could be obtained by the earlier mentioned reac-
tion protocols, will be discussed. For a better commensur-
ability also the molecular solid state structures of the parent
ligand systems 1 and 2 have been determined by X-ray single
crystal diffraction and are shown in Fig. 1 and 2.
Exemplarily for the metallated species the crystal structures
of 3, 6 and 8 are depicted in the anisotropic displacement para-
meter plots in Fig. 3–5. Selected (averaged) bond lengths and
angles are listed in Table 1. The metallated species 3–8 have
several intrinsic features in common, which have to be
highlighted:
The herein discussed bisheterocyclomethanes 1 and 2 mimic
the related β-diketiminate ligand. Therefore, formally the
imine substituent in the case of nacnac is fixed to the back-
bone of the ligand by building up an additional five-mem-
bered heterocycle containing either oxygen or sulfur atoms as
additional donor sites.
As depicted in Scheme 2 both ligand systems were syn-
thesised in a cyclocondensation reaction of a suitable linker
(a) In comparison with the deprotonated conjugated ligand
system it is obvious that the free ligand LH shows in both
cases a considerably larger torsion angle concerning the C–N
bonds within the heterocycles.
(b) Upon deprotonation the whole ligand system becomes
nearly planar in each molecule and the metal cations are co-
ordinated exclusively by the two ring-nitrogen donor atoms.
Scheme 2 Cyclocondensation reactions for synthesis of the parent
ligand systems 1 and 2.
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Fig. 1 Molecular structure of (NCOC6H4)2CH2 (1). Anisotropic displace-
ment parameters are depicted at the 50% probability level. Hydrogen
atoms are omitted for clarity except the methylene bridge. Structural
data are given in Tables 1 and 2.
Fig. 4 Molecular structure of [ClMeAl{(NCSC6H4)2CH}] (6). Anisotropic
displacement parameters are depicted at the 50% probability level.
Hydrogen atoms and the positional disorder are omitted for clarity.
Structural data are given in Tables 1 and 2.
Fig. 2 Molecular structure of (NCSC6H4)2CH2 (2). Anisotropic displace-
ment parameters are depicted at the 50% probability level. Hydrogen
atoms are omitted for clarity except the methylene bridge. Structural
data are given in Tables 1 and 2.
Fig. 5 1H, 15N-HMBC-NMR spectrum of 7.
Table 1 Selected bond lengths (Å) and angles (°) for compounds 1–8
1
2
3
4
M–N(av)
Cip–Cbr(av)
N–Cip(av)
—
—
1.9176(20)
1.380(3)
1.346(3)
1.9233(14)
1.390(2)
1.3518(19)
Fig. 3 Molecular structure of [Me2Al{(NCOC6H4)2CH}] (3). Anisotropic
displacement parameters are depicted at the 50% probability level.
Hydrogen atoms are omitted for clarity. Structural data are given in
Tables 1 and 2.
1.4891(15)
1.2864(14)
1.5106(15)
1.2955(16)
N–M–N
Cip–Cbr–Cip
—
—
91.67(9)
119.5(2)
94.79(6)
123.54(14)
111.23(9)
109.59(14)
5
6
7
8
(c) Furthermore, the metals are significantly dislocated
from the ideal C3N2 planes, ranging from 10.93(27) to 29.57
(26) pm, depending on the heteroatom in the backbone of the
ligand.
M–N(av)
Cip–Cbr(av)
N–Cip(av)
1.8930(17)
1.382(2)
1.348(3)
1.894(2)
1.388(3)
1.365(3)
1.996(6)
1.394(9)
1.323(9)
1.9945(13)
1.3909(19)
1.3436(19)
(d) The experimentally determined C–C and C–N distances
for 3–8 span the narrow range of 1.380(3) to 1.394(9) Å and
1.323(9) to 1.365(3) Å, respectively. These facts are an indicator
of an apparent fully conjugated and delocalised ligand charac-
ter, as far as the abovementioned bonds are concerned.
(e) The observed N–M–N bite angle inside the formed
N–M–N
Cip–Cbr–Cip
94.33(11)
120.2(3)
96.84(9)
124.1(2)
89.2(2)
119.7(6)
92.99(8)
124.0(2)
br = bridge; ip = ipso.
The most interesting feature within the series of the
C3N2M metalla heterocycle is the narrowest one (89.2(2)–96.84 obtained structures is the deviation of the metal atom from
(9)°) whereas all the other angles are wider than 120°.
the extended plane, which is made from the N1–C2–C1–C9–N2
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more properly gallium than aluminium and the ligand derived
from 2 vice versa.
Table 2 Folding parameters for 1–8
Metal dist. from Torsion
C3N2 plane (pm) angle (°)
Folding
angle (°)
By comparing the structural values of the parent ligands
with those of the metallated species given in Table 2, it can be
considered that there is a change in hybridisation of the
central bridging carbon atom upon metallation from sp3 to
sp2. As an evidence for this statement the angular sum around
the bridging carbon atom can be taken into account ranging
from 359.9(2) to 360.1(2)°, which displays a nearly ideal tri-
gonal planar coordination geometry for C1 as expected for a
sp2-hybridised carbon atom. In both cases the benzothiazole as
well as the benzoxazole containing ligand, a shortening of the
Compound
(NCOC6H4)2CH2 (1)
(NCSC6H4)2CH2 (2)
—
—
81.3(9)
188.98(24)
1.0(8)
1.2(6)
—
0.6(8)
—
—
—
—
[Me2Al{(NCOC6H4)2CH}] (3)
[Me2Al{(NCSC6H4)2CH}] (4)
29.57(26)
13.79(29)
9.12(8)
1.36(2)
3.69(40)
1.21(2)
3.62(2)
8.90(6)
[ClMeAl{(NCOC6H4)2CH}] (5) 18.06(32)
[ClMeAl{(NCSC6H4)2CH}] (6) 10.93(27)
[Me2Ga{(NCOC6H4)2CH}] (7) 14.34(3)
[Me2Ga{(NCSC6H4)2CH}] (8) 20.61(26)
Cipso–Cbridge bond (starting from 1.5106(15) and 1.4891(15) Å
to approx. 1.39 Å) and a widening of the Cipso–Cbridge–Cipso
angle (from 109.59(14) and 111.23(9)° to approx. 120°) were
observed. Including the corresponding elongated N–Cipso dis-
tances in the deprotonated species 3–8, an efficient delocalisa-
tion of the lone pair is anticipated. As shown in Scheme 4
there are different mesomeric resonance structures which can
be proposed: (a) a carbanionic, (b) an amidic and (c) a comple-
tely delocalised canonical form.
Regarding the abovementioned structural changes, species
(a) has a certain eligibility as in previous studies within
[MeZnCH(2-NC5H4)2]2 the central deprotonated methanide
carbon atom is coordinated by the electropositive zinc atom.4
Nevertheless case (c) seems to explain the bonding situation
best within the compounds 3–8, because the C–C and C–N
bond lengths lie in between a distinct single and double bond
character (C(sp2)–C(sp3): 1.510 Å, C(sp2)vC(sp2): 1.335 Å,
C(sp2)–N(sp3): 1.43 Å, C(sp2)vN(sp2): 1.29 Å).34 Measurements
of 1H, 15N-HMBC-NMR data gave evidence that complexes 3, 4,
7 and 8 show the same structure in solution at room tempera-
ture (at the NMR time scale) and in the solid state. The coordi-
nation geometry concerning the nacnac-like chelating ability
can be proven by the apparent cross peaks, which clearly
display 3J-coupling between the protons of the AlMe2 or GaMe2
unit and the nitrogen donor atoms, exemplarily shown in
Fig. 5 for compound 7.
array. With this M⋯plane distance the folding angle between
both heteroaromatic substituents is associated, which can be
seen as a reason for the different deviations. Various data con-
cerning the M⋯plane distances, the torsion and the folding
angles are listed in Table 2.
To highlight this special aspect the molecular structures of
3 and 8 are shown in Fig. 3 and 6. Both species exhibit the
greatest metal distance from the C3N2 plane (29.57(26) and
20.61(26)°, respectively) and also the greatest folding angle of
the heteroaromates (9.12(8) and 8.90(6)°, respectively). As a
representative of the AlMeCl containing species the molecular
structure of 6 is shown in Fig. 4.
Concerning the series of the synthesised group 13 metal
complexes of the bis-(benzothiazol-2-yl)- and bis-(benzoxazol-
2-yl)-methanides, one has to emphasize that in the case of the
oxygen containing ligand on average the bite angle is signifi-
cantly smaller than in the sulfur analogues (89.2(2)–94.33(11)°
vs. 92.99(8)–96.84(9)°) and the angle around the bridging
carbon atom in the backbone is also smaller (119.5(2)–120.2
(3)° vs. 123.54(14)–124.1(2)°).
The aluminium species show a correlation between two
important structural features: the narrower the N–M–N angle,
the greater the metal distance from the C3N2 plane and also
the greater the folding angle of the heteroaromatic residues.
However, in the case of the gallium complexes this tendency is
the other way round. Because of these observations there are
reasons to believe that the ligand derived from 1 coordinates
Conspicuously within the series from the AlMe2 derivatives
via AlMeCl towards GaMe2 the N–M distance and the N–M–N
angle alter as expected. In comparison with 3 and 4 the N–M
distance decreases in 5 and 6 because of the more electronega-
tive, electron withdrawing chloro substituent in the latter
cases. Concomitantly the N–M–N angles widen.
Fig. 6 Molecular structure of [Me2Ga{(NCSC6H4)2CH}] (8). Anisotropic
displacement parameters are depicted at the 50% probability level.
Hydrogen atoms are omitted for clarity. Structural data are given in
Tables 1 and 2.
Scheme 4 Mesomeric resonance structures of the bisheterocyclo-
methanides.
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Furthermore, 9 shows structural similarities to the class of
scorpionates, which are also tridentate ligands and form facial
coordination geometries upon building metal complexes.36 As
common popular examples the monoanionic trispyrazolyl-
borates and the neutral isoelectronic trispyrazolylmethanes37
have to be mentioned, which are used in bioinorganic chem-
istry to mimic the active sites in metallo proteins in model
complexes.38
Conclusion
In summary, a variety of group 13 metal complexes containing
bisheterocyclomethanides as ligand systems could successfully
be synthesised and structurally evaluated. In each case the
metal cation is coordinated in a distorted tetrahedral fashion
by the ring-nitrogen atoms acting as Lewis-bases and the
ligand became almost perfectly planar. Depending on the con-
sidered combination of the heterocycle (benzoxazole or benzo-
thiazole) and the chelated metal (Al or Ga) some significant
correlations between the folding angle of the scaffold and the
metal distance from the C3N2 plane were observed. The metal
coordination in all cases takes place exclusively by donation of
nitrogen atoms within the heterocyclic moieties. The chalco-
genes did not participate in coordination neither in the solid
Fig. 7 Molecular structure of (NCOC6H4)3C(C3H7) (9). Anisotropic
displacement parameters are depicted at the 50% probability level. Hydro-
gen atoms and the positional disorder are omitted for clarity. Selected bond
lengths and angles C22–C1(av) 151.1(10) pm, C22–C8 150.51(16) pm, C22–
C15 151.20(16) pm, C22–C23 155.04(17) pm; C1–C22–C8(av) 110.0(8)°,
C1–C22–C15(av) 105.3(14)°, C1–C22–C23(av) 109.9(9)°, C8–C22–C15
113.07(10)°, C8–C22–C23 108.69(10)°, C15–C22–C23 109.91(10)°.
Furthermore, the dimethylgallium compounds 7 and 8 state nor in solution as proven in the appropriate crystal struc-
feature increasing N–M bond lengths and smaller bite angles tures and the 1H, 15N-HMBC-NMR experiments.
due to the bigger covalent radius of the coordinated gallium
cation (Al: 1.21 Å, Ga: 1.22 Å).35
These empirical results gave rise to the occurrence of pre-
ferred ligand–metal pairs in presumably the most stable com-
In addition to the above discussed structures 1–8 the plexes: whereas 1 seems to suit gallium more properly, 2 fits
interesting side product 9 could be isolated from the aluminium the most.
synthesis of 1 in moderate yields (29%) and structurally
Moreover, the negative charge arising upon deprotonation
and spectroscopically characterised. In a first attempt to seems to be delocalised about the whole ligand backbone as is
synthesise 1 two equivalents of 2-chlorobenzoxazole were indicated by the observed N–Cipso and Cipso–Cbridge bond
attempted to be lithiated in the 2-position and subsequently lengths. In this context the carbanionic canonical formula can
coupled via addition of one equivalent of diethylcarbonate. be neglected, because the linking carbon atoms show clear
Surprisingly the coupling product
was obtained. It represents a side product of the used
9
shown in Fig.
7
sp2-hybridisation and no involvement in metal coordination.
Referring to the title of this work one can conclude that
nBuLi with 2-chlorobenzoxazole, where the butyl carbanion bisheterocyclomethanides as new ligand systems can definitely
is formally substituted three times by
heterocycle.
a
benzoxazole compete with the popular β-diketiminate ligand and offer
promising opportunities for investigating and exploring a new
Compound 9 crystallizes in the monoclinic space group interesting research area.
C2/c and the central carbon atom C22 shows a nearly ideal
tetrahedral coordination environment (105.3(14)–113.07(10)°).
The involved C–C bond lengths outgoing from C22 to the
benzoxazole moieties lie in a narrow range from 1.5051(16) to
1.5120(16) Å, whereas the C22–C23 distance is significantly
Experimental section
General procedures
longer (1.5504(17) Å). This shortening is expected for an All manipulations were carried out under an atmosphere of N2
average C(sp3)–C(sp3) (1.544 Å) to an average C(sp3)–C(sp2) or Ar by Schlenk techniques. All solvents used within metalla-
(1.510 Å).34
tion reactions were distilled from Na or K before use. The start-
This kind of tripodal, tridentate ligand features interesting ing materials were purchased commercially and used as
coordination abilities because of the presence of soft as well as received. Ethylbisimidate dihydrochloride was prepared
hard donor atoms within the scaffold and can be regarded as according to a literature method.33 1H, 13C, 15N and 27Al NMR
an analogue of trispyrazolylmethanes or even more of trisox- spectroscopic data were recorded on a Bruker Avance 500 MHz,
azoline ligands. The latter ones are known to be valuable a Bruker Avance 400 MHz and a Bruker Avance 300 MHz
auxiliary ligands in asymmetric catalysis.
spectrometer and referenced to the deuterated solvent (thf-d8).
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Elemental analyses (C, H, N and S) were carried out on a Vario reduced to a few milliliters and the resulting concentrated
EL3 at the Mikroanalytisches Labor, Institut für Anorganische solution was stored at −32 °C in a fridge. Overnight crystals
Chemie, University of Göttingen. All EI-MS spectra (70 eV) suitable for X-ray diffraction experiments could be obtained.
were recorded on a Finnigan MAT 95.
The crystals thus formed were filtered, washed twice with pre-
cooled toluene or hexane (0 °C) and finally dried in a vacuum.
The given yields below are just based on the received crystals
Ligand syntheses
Both ligand systems were synthesised by cyclocondensation unless stated otherwise. No further improvement of the yields
reaction of a suitable linker derived from malonic acid and the was applied to the remaining solutions because of impurities
corresponding ortho-substituted anilines.
upon repeated precipitation.
(NCOC6H4)2CH2 (1). 2-Aminophenol (1.99 g, 18.0 mmol,
[Me2Al{(NCOC6H4)2CH}] (3). Colourless crystals were obtained
2.0 eq.) and ethylbisimidate dihydrochloride (2.11 g, 9.1 mmol, in a yield of 46 mg (0.15 mmol, 8%; not optimised). Anal.
1.0 eq.) were dissolved in methanol (50 mL). Then the reaction Calcd for C17H15AlN2O2 (306.29 g mol−1): C, 66.66; H, 4.94;
mixture was heated to reflux for 3 h and, after cooling to rt, N, 9.15. Found: C, 66.65; H, 5.08; N, 9.19; δ1H (300 MHz,
stored at −32 °C in a fridge. The resulting crystalline material thf-d8): 7.42 (ddd, JHH = 7.8, 1.2, 0.6 Hz, 4 H, H5 + H8),
was filtered off, washed subsequently with sat. aq. NaHCO3 7.29 (td, JHH = 7.7, 1.2 Hz, 2 H, H6), 7.19 (dd, JHH = 7.7, 1.4 Hz,
solution (2 × 50 mL) and water (2 × 50 mL) and dried under 2 H, H7), 5.41 (s, 1 H, H10), −0.48 (s, 6 H, H11); δ13C{1H}
reduced pressure. Crystals suitable for X-ray diffraction experi- (75 MHz, thf-d8): 169.42 (s, 2 C, C2), 149.39 (s, 2 C, C9), 137.31
ments could be obtained upon recrystallisation from ethanol. (s, 2 C, C4), 125.81 (s, 2 C, C6), 123.86 (s, 2 C, C7), 113.58
Pale yellow crystals were obtained in a yield of 1.29 g (s, 2 C, C5), 110.67 (s, 2 C, C8), 60.48 (s, 1 C, C10), −10.22
(5.1 mmol, 56%). Anal. Calcd for C15H10N2O2 (250.25 g mol−1): (s, 2 C, C11); δ15N{1H} (30 MHz, thf-d8): −231.31 (s); δ27Al{1H}
C, 71.99; H, 4.03; N, 11.19. Found: C, 71.86; H, 4.03; N, 11.12; (78 MHz, thf-d8): 155.08 (s); EI-MS, m/z (%): 306 (9) [M]+,
δ1H (300 MHz, thf-d8): 7.70–7.62 (m, 2 H, H5), 7.59–7.50 291 (100) [M − Me]+, 276 (10) [M − 2Me]+, 145.5 (10) [M − Me]2+.
(m, 2 H, H8), 7.37–7.27 (m, 4 H, H6 + H7), 4.70 (s, 2 H, H10);
[Me2Al{(NCSC6H4)2CH}] (4). Orange crystals were obtained
δ13C{1H} (75 MHz, thf-d8): 161.46 (s, 2 C, C2), 152.35 (s, 2 C, in a yield of 450 mg (1.3 mmol, 7%, not optimised). Anal.
C9), 142.66 (s, 2 C, C4), 125.89 (s, 2 C, C7), 125.17 (s, 2 C, C6), Calcd for C17H15AlN2S2 (338.41 g mol−1): C, 60.33; H, 4.47;
120.83 (s, 2 C, C5), 111.30 (s, 2 C, C8), 29.58 (s, 1 C, C10); N, 8.28; S, 18.95. Found: C, 60.66; H, 4.52; N, 8.26; S, 18.72;
δ15N{1H} (30 MHz, thf-d8): −133.56 (s); EI-MS, m/z (%): 250 δ1H (500 MHz, thf-d8): 7.68 (ddd, JHH = 7.9, 1.2, 0.5 Hz, 2 H,
(100) [M]+, 132 (25) [M − NCOC6H4]+.
H8), 7.59 (ddd, JHH = 8.2, 1.0, 0.6 Hz, 2 H, H5), 7.39 (ddd,
(NCSC6H4)2CH2 (2). 2-Aminothiophenol (10.0 g, 80.0 mmol, JHH = 8.2, 7.4, 1.3 Hz, 2 H, H6), 7.20 (ddd, JHH = 7.9, 7.4, 1.0 Hz,
2.0 eq.) and malononitrile (2.64 g, 40.0 mmol, 1.0 eq.) were 2 H, H7), 6.05 (s, 1 H, H10), −0.43 (s, 6 H, H11); δ13C{1H}
dissolved in ethanol (40 mL). Then the reaction mixture was (125 MHz, thf–d8): 167.22 (s, 2 C, C2), 149.25 (s, 2 C, C4),
heated to reflux for 6 h and, after cooling to rt, stored at 129.48 (s, 2 C, C9), 127.52 (s, 2 C, C6), 124.01 (s, 2 C, C7),
−32 °C in a fridge. The resulting yellow precipitate was filtered 122.56 (s, 2 C, C8), 116.09 (s, 2 C, C5), 82.56 (s, 1 C, C10),
off, washed with hexane (2 × 50 mL) and dried under reduced −9.54 (s, 2 C, C11); δ15N{1H} (30 MHz, thf-d8): −201.16 (s);
pressure. Crystals suitable for X-ray diffraction experiments δ27Al{1H} (130 MHz, thf-d8): 151.02 (s); EI-MS, m/z (%): 338 (11)
could be obtained upon recrystallisation from ethanol. A yellow [M]+, 323 (100) [M − Me]+, 308 (5) [M − 2Me]+, 161.5 (14)
powder was obtained in a yield of 8.36 g (30.0 mmol, 74%). [M − Me]2+.
Anal. Calcd for C15H10N2S2 (282.37 g mol−1): C, 63.80; H, 3.57;
[ClMeAl{(NCOC6H4)2CH}] (5). A colourless powder was
N, 9.92; S, 22.71. Found: C, 63.80; H, 3.52; N, 9.90; S, 22.63; obtained in a yield of 1.21 g (3.7 mmol, 74%). Anal. Calcd for
δ1H (300 MHz, thf-d8): 7.97 (ddd, JHH = 8.2, 1.2, 0.6 Hz, 2 H, C16H12AlClN2O2 (326.72 g mol−1): C, 58.82; H, 3.70; N, 8.57.
H5), 7.92 (ddd, JHH = 7.9, 1.3, 0.6 Hz, 2 H, H8), 7.45 (ddd, Found: C, 60.13; H, 4.04; N, 8.25 (deviation due to remaining
JHH = 7.9, 7.3, 1.3 Hz, 2 H, H6), 7.36 (ddd, JHH = 8.1, 7.3, toluene); δ1H (500 MHz, thf-d8): 7.51 (dddd, JHH = 17.1, 8.0,
1.3 Hz, 2 H, H7), 4.97 (s, 2 H, H10); δ13C{1H} (75 MHz, thf-d8): 1.1, 0.6 Hz, 4 H, H5 + H8), 7.35 (dd, JHH = 7.7, 1.2 Hz, 2 H, H6),
166.68 (s, 2 C, C2), 154.47 (s, 2 C, C4), 137.04 (s, 2 C, C9), 126.79 7.28–7.24 (m, 2 H, H7), 5.62 (s, 1 H, H10), −0.18 (s, 3 H, H11);
(s, 2 C, C6), 125.94 (s, 2 C, C7), 123.88 (s, 2 C, C5), 122.49 (s, δ13C{1H} (125 MHz, thf-d8): 169.31 (s, 2 C, C2), 149.22 (s, 2 C,
2 C, C8), 39.19 (s, 1 C, C10); δ15N{1H} (30 MHz, thf-d8): −65.02 C9), 136.57 (s, 2 C, C4), 126.12 (s, 2 C, C6), 124.60 (s, 2 C, C7),
(s); EI-MS, m/z (%): 282 (100) [M]+, 148 (10) [M − NCSC6H4]+.
114.08 (s, 2 C, C5), 110.98 (s, 2 C, C8), 61.79 (s, 1 C, C10), −10.18 (s,
1 C, C11); δ15N{1H} (50 MHz, thf-d8): −231.52 (s); δ27Al{1H}
(78 MHz, thf-d8): 133.35 (s); EI-MS, m/z (%): 326 (26) [M]+, 311 (100)
Metallation reactions
To a solution of the corresponding ligand 1 or 2 (1.0 eq.) in [M − Me]+, 291 (4) [M − Cl]+, 155.5 (10) [M − Me]2+.
toluene a slight excess of the pure organometallic reactant
[ClMeAl{(NCSC6H4)2CH}] (6). A yellow powder was obtained
AlMe3, AlMe2Cl or GaMe3 (1.1 eq.) was slowly added at 0 °C. in a yield of 3.33 g (9.3 mmol, 93%). Anal. Calcd for
The reaction mixture was stirred overnight and allowed to C16H12AlClN2S2 (358.84 g mol−1): C, 53.56; H, 3.37; N, 7.81;
warm to rt. In the case of 5 and 6 the resulting precipitate was S, 17.87. Found: C, 53.09; H, 3.32; N, 7.92; S, 17.40;
filtered off, and the reaction mixture of 3 and 4 afforded a δ1H (500 MHz, thf-d8): 7.77 (ddd, JHH = 8.2, 1.0, 0.6 Hz, 2 H, H8),
clear solution. Afterwards the volume of the solution was 7.72 (ddd, JHH = 7.9, 1.2, 0.6 Hz, 2 H, H5), 7.47–7.40
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(m, 2 H, H6), 7.26 (ddd, JHH = 7.9, 7.4, 1.0 Hz, 2 H, H7), 6.26 X-ray crystallographic studies
(s, 1 H, H10), −0.18 (s, 3 H, H11); δ13C{1H} (125 MHz, thf-d8):
Single crystals were selected from a Schlenk flask under an
argon or nitrogen atmosphere and covered with perfluorated
polyether oil on a microscope slide, which was cooled with a
nitrogen gas flow using the X-TEMP2 device.39 An appropriate
crystal was selected using a polarized microscope, mounted on
the tip of a MiTeGen©MicroMount or glass fiber, fixed to a
goniometer head and shock cooled by the crystal cooling device.
The data for 1–9 were collected from shock-cooled crystals
at 100(2) K. The data of 1, 2, 5, 6, 8 and 9 were collected on an
Incoatec Mo Microsource40 and compound 7 was collected on
an Incoatec Ag Microsource each equipped with mirror optics
and an APEX II detector with a D8 goniometer. The data of 3
and 4 were measured on a Bruker TXS-Mo rotating anode with
mirror optics and an APEX II detector with a D8 goniometer.
All diffractometers were equipped with a low-temperature
device and used either MoKα radiation of λ = 71.073 pm or
AgKα radiation of λ = 56.086 pm. The data were integrated with
Saint41 and an empirical absorption correction (Sadabs)42 was
applied. The structures were solved by direct methods (Shelxs-
97) and refined by full-matrix least-squares methods against F2
(Shelxl-97).34,43 All non-hydrogen atoms were refined with ani-
sotropic displacement parameters. The hydrogen atoms were
refined isotropically on calculated positions using a riding
model with their Uiso values constrained to equal 1.5 times the
Ueq of their pivot atoms for terminal sp3 carbon atoms and 1.2
times for all other carbon atoms. Disordered moieties were
refined using bond length restraints and isotropic displace-
ment parameter restraints.
167.55 (s, 2 C, C2), 148.29 (s, 2 C, C4), 129.12 (s, 2 C, C9),
127.80 (s, 2 C, C6), 124.68 (s, 2 C, C7), 122.75 (s, 2 C, C8),
116.69 (s, 2 C, C5), 83.55 (s, 1 C, C10), −9.27 (s, 1 C, C11);
δ15N{1H} (50 MHz, thf-d8): −204.41 (s); δ27Al{1H} (78 MHz,
thf-d8): 128.26 (s); EI-MS, m/z (%): 358 (28) [M]+, 343 (100)
[M − Me]+, 323 (4) [M − Cl]+, 171.5 (18) [M − Me]2+.
[Me2Ga{(NCOC6H4)2CH}] (7). Pale yellow crystals were
obtained in a yield of 0.583 g (1.54 mmol, 31%, not opti-
mised). Anal. Calcd for C17H15GaN2O2 (349.04 g mol−1):
C, 58.50; H, 4.33; N, 8.03. Found: C, 58.45; H, 4.35; N, 8.21;
δ1H (500 MHz, thf-d8): 7.38 (ddd, JHH = 8.0, 1.1, 0.6 Hz, 2 H,
H8), 7.29–7.21 (m, 4 H, H5 + H6), 7.14 (ddd, JHH = 8.0, 7.2,
1.5 Hz, 2 H, H7), 5.25 (s, 1 H, H10), −0.06 (s, 6 H, H11);
δ13C{1H} (75 MHz, thf-d8): 168.63 (s, 2 C, C2), 149.35 (s, 2 C,
C9), 138.41 (s, 2 C, C4), 125.53 (s, 2 C, C6), 123.21 (s, 2 C, C7),
112.95 (s, 2 C, C5), 110.40 (s, 2 C, C8), 59.23 (s, 1 C, C10),
−7.39 (s, 2 C, C11); δ15N{1H} (50 MHz, thf-d8): −229.86 (s);
EI-MS, m/z (%): 348 (11) [M]+, 333 (100) [M − Me]+, 318 (17)
[M − 2Me]+, 166.5 (10) [M − Me]2+, 69 (11) Ga+.
[Me2Ga{(NCSC6H4)2CH}] (8). Yellow crystals were obtained
in a yield of 138 mg (0.36 mmol, 36%, not optimised). Anal.
Calcd for C17H15GaN2S2 (381.16 g mol−1): C, 53.57; H, 3.97;
N, 7.35; S, 16.82. Found: C, 53.75; H, 3.94; N, 7.43; S, 16.74;
δ1H (500 MHz, thf-d8): 7.64 (ddd, JHH = 7.8, 1.2, 0.6 Hz, 2 H,
H8), 7.40 (ddd, JHH = 8.1, 1.1, 0.6 Hz, 2 H, H5), 7.36–7.32
(m, 2 H, H6), 7.14 (ddd, JHH = 7.9, 7.3, 1.2 Hz, 2 H, H7), 5.87
(s, 1 H, H10), −0.02 (s, 6 H, H11); δ13C{1H} (75 MHz, thf-d8):
165.82 (s, 2 C, C2), 149.77 (s, 2 C, C4), 129.57 (s, 2 C, C9),
127.35 (s, 2 C, C6), 123.48 (s, 2 C, C7), 122.40 (s, 2 C, C8),
115.44 (s, 2 C, C5), 81.28 (s, 1 C, C10), −6.71 (s, 2 C, C11);
δ15N{1H} (50 MHz, thf-d8): −199.22 (s); EI-MS, m/z (%): 380 (13)
[M]+, 365 (100) [M − Me]+, 350 (21) [M − 2Me]+, 182.5 (9)
[M − Me]2+, 69 (14) Ga+.
Crystallographic data for the structures reported in this
paper have been deposited with the Cambridge Crystallo-
graphic Data Centre. The CCDC numbers, crystal data and
experimental details for the X-ray measurements are listed in
the ESI.‡
(NCOC6H4)3C(C3H7) (9). At −78 °C to a solution of 2-chloro-
benzoxazol (5.0 g, 3.8 mL, 32.6 mmol, 1.0 eq.) in thf (25 mL) a
solution of nBuLi (2.15 m in hexane, 15.2 mL, 32.6 mmol, 1.0 Acknowledgements
eq.) was slowly added and stirred overnight. After warming at
Thanks to the Danish National Research Foundation DNRF
rt the remaining nBuLi was hydrolysed by addition of water
(120 mL) and the reaction mixture was extracted with DCM
(6 × 50 mL). The combined organic layers were dried over
MgSO4 and the solvent was evaporated under reduced pressure.
After recrystallisation from acetone, light brown crystals were
obtained in a yield of 1.30 g (3.2 mmol, 29% based on 2-chloro-
benzoxazole). Anal. Calcd for C25H19N3O3 (409.4 g mol−1):
C, 73.34; H, 4.68; N, 10.26. Found: C, 72.60; H, 4.55; N, 10.20.
δ1H (500 MHz, thf-d8): 7.75–7.68 (m, 3 H, H5), 7.63–7.54 (m,
3 H, H8), 7.42–7.31 (m, 6 H, H6 + H7), 3.11–3.01 (m, 2 H, H11),
1.71–1.60 (m, 2 H, H12), 1.03 (t, 3JHH = 7.3 Hz, 3 H, H13); δ13C{1H}
(125 MHz, thf-d8): 163.18 (s, 3 C, C2), 152.25 (s, 3 C, C9), 141.92 (s,
3 C, C4), 126.56 (s, 3 C, C6), 125.49 (s, 3 C, C7), 121.35 (s, 3 C, C5),
111.77 (s, 3 C, C8), 51.83 (s, 1 C, C10), 39.96 (s, 1 C, C11), 19.34 (s,
1 C, C12), 14.57 (s, 1 C, C13); δ15N{1H} (50 MHz, thf-d8): −128.85
(s); EI-MS, m/z (%): 409 (7) [M]+, 380 (28) [M − Et]+, 367 (100) [M −
Pr]+, 263 (7) [M − NCOC6H4 − Et]+.
funded Center for Materials Crystallography (CMC) for partial
support and the Land Niedersachsen for providing a fellow-
ship in the GAUSS PhD program.
The authors thank Prof. Dr. Dr. h.c. mult. H. W. Roesky for
the numerous subject-specific discussions and Dr. Regine
Herbst-Irmer for the support in crystallographic issues.
Notes and references
1 W. W. Schoeller, Inorg. Chem., 2011, 50, 2629–2633.
2 For review: F. Baier, Z. Fei, H. Gornitzka, A. Murso,
S. Neufeld, M. Pfeiffer, I. Rüdenauer, A. Steiner, T. Stey and
D. Stalke, J. Organomet. Chem., 2002, 661, 111–127.
3 H. Gornitzka and D. Stalke, Angew. Chem., Int. Ed. Engl.,
1994, 33, 693–695.
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Dalton Transactions
Paper
4 H. Gornitzka, C. Hemmert, G. Bertrand, M. Pfeiffer and 24 N. Kuhn, S. Fuchs and M. Steimann, Z. Anorg. Allg. Chem.,
D. Stalke, Organometallics, 2000, 19, 112–114. 2002, 628, 458–462.
5 For review: L. Mahalakshmi and D. Stalke, Struct. Bonding, 25 C. E. Radzewich, I. A. Guzei and R. F. Jordan, J. Am. Chem.
2002, 103, 85–113. Soc., 1999, 121, 8673–8674.
6 H. Gornitzka and D. Stalke, Organometallics, 1994, 13, 26 C. Cui, H. W. Roesky, H.-G. Schmidt, M. Noltemeyer,
4398–4405.
H. Hao and F. Cimpoesu, Angew. Chem., Int. Ed., 2000, 112,
4444–4446.
7 M. Pfeiffer, F. Baier, T. Stey, D. Leusser, D. Stalke,
B. Engels, D. Moigno and W. Kiefer, J. Mol. Model., 2000, 6, 27 X. Li, X. Cheng, H. Song and C. Cui, Organometallics, 2007,
299–311. 26, 1039–1043.
8 M. Pfeiffer, A. Murso, L. Mahalakshmi, D. Moigno, 28 N. J. Hardman, B. E. Eichler and P. P. Power, Chem.
W. Kiefer and D. Stalke, Eur. J. Inorg. Chem., 2002,
3222–3234.
9 A. Steiner and D. Stalke, J. Chem. Soc., Chem. Commun.,
1993, 444–446.
10 A. Steiner and D. Stalke, Angew. Chem., 1995, 107,
1908–1910.
11 H. Gornitzka and D. Stalke, Eur. J. Inorg. Chem., 1998,
311–317.
Commun., 2000, 1991–1992.
29 M. Asay, C. Jones and M. Driess, Chem. Rev., 2011, 111,
354–396.
30 Y. Mizuhata, T. Sasamori and N. Tokitoh, Chem. Rev., 2009,
109, 3479–3511.
31 D. Li, Y. Peng, C. Geng, K. Liu and D. Kong, Dalton Trans.,
2013, 42, 11295–11303.
32 B. D. Ward and L. H. Gade, Chem. Commun., 2012, 48,
10587–10599.
33 H. B. Ammar, J. Le Nôtre, M. Salem, M. T. Kaddachi and
P. H. Dixneuf, J. Organomet. Chem., 2002, 662, 63–69.
12 A. Steiner and D. Stalke, Organometallics, 1995, 14,
2422–2429.
13 M. Stender, B. E. Eichler, N. J. Hardman, P. P. Power,
J. Prust, M. Noltemeyer and H. W. Roesky, Inorg. Chem., 34 P. Müller, R. Herbst-Irmer, A. L. Spek, T. R. Schneider and
2001, 40, 2794–2799.
14 B. Qian, D. L. Ward and M. R. Smith III, Organometallics,
1998, 17, 3070–3076.
M. R. Sawaya, in Crystal Structure Refinement – A Crystallo-
grapher’s Guide to SHELXL, ed. P. Müller, IUCr Texts on
Crystallography, Oxford University Press, Oxford, UK, 2006,
vol. 8.
15 S. Singh, H.-J. Ahn, A. Stasch, V. Jancik, H. W. Roesky,
A. Pal, M. Biadene, R. Herbst-Irmer, M. Noltemeyer and 35 Cambridge Structural Database, taken from “CSD Elemen-
H.-G. Schmidt, Inorg. Chem., 2006, 45, 1853–1860.
tal Radii”, http://www.ccdc.cam.ac.uk/Lists/ResourceFile-
16 Z. Yang, H. Zhu, X. Ma, J. Chai, H. W. Roesky, C. He,
List/Elemental_Radii.xlsx, last modified: 21st June 2012.
J. Magull, H.-G. Schmidt and M. Noltemeyer, Inorg. Chem., 36 S. Trofimenko, Chem. Rev., 1993, 93, 943–980.
2006, 45, 1823–1827. 37 D. L. Reger, Comments Inorg. Chem., 1997, 21, 1–28.
17 S. Nagendran and H. W. Roesky, Organometallics, 2008, 27, 38 W. Kaim and B. Schwederski, Bioanorganische Chemie –
457–492.
Zur Funktion chemischer Elemente in Lebensprozessen,
B. G. Teubner Verlag, Wiesbaden, 2005.
39 (a) T. Kottke and D. Stalke, J. Appl. Crystallogr., 1993, 26,
615–619; (b) T. Kottke, R. J. Lagow and D. Stalke, J. Appl.
Crystallogr., 1996, 29, 465–468; (c) D. Stalke, Chem. Soc.
Rev., 1998, 27, 171–178.
18 Y. Ding, H. W. Roesky, M. Noltemeyer, H.-G. Schmidt and
P. P. Power, Organometallics, 2001, 20, 1190–1194.
19 A. Jana, H. W. Roesky, C. Schulzke, A. Döring, T. Beck,
A. Pal and R. Herbst-Irmer, Inorg. Chem., 2009, 48,
193–197.
20 Z. Yang, H. Zhu, X. Ma, J. Chai, H. W. Roesky, C. He, 40 T. Schulz, K. Meindl, D. Leusser, D. Stern, J. Graf,
J. Magull, H.-G. Schmidt and M. Noltemeyer, Inorg. Chem.,
2006, 45, 1823–1827.
C. Michaelsen, M. Ruf, G. M. Sheldrick and D. Stalke,
J. Appl. Crystallogr., 2009, 42, 885–891.
21 Y. Cheng, D. J. Doyle, P. B. Hitchcock and M. F. Lappert, 41 SAINT v7.68A in Bruker APEX v2011.9, Bruker AXS Inst. Inc.,
Dalton Trans., 2006, 4449–4460. Madison, USA, 2008.
22 N. Kuhn, J. Fahl, S. Fuchs, M. Steimann, G. Henkel 42 G. M. Sheldrick, SADABS 2008/2, Universität Göttingen,
and A. H. Maulitz, Z. Anorg. Allg. Chem., 1999, 625,
Germany, 2008.
2108–2114.
43 (a) G. M. Sheldrick, Acta Crystallogr., Sect. A: Fundam.
Crystallogr., 2008, 64, 112–122; (b) G. M. Sheldrick, Acta
Crystallogr., Sect. A: Fundam. Crystallogr., 1990, 46, 467–473.
23 N. Kuhn, S. Fuchs and M. Steimann, Eur. J. Inorg. Chem.,
2001, 359–361.
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