Bis(4-methylbenzoxazol-2-yl)methanide in s-Block Metal Coordination

Article abstract of DOI:10.1002/ejic.201700501

On the basis of the bis(4-methylbenzoxazol-2-yl)methanide ligand three new alkali or alkaline earth metal compounds were synthesized, and their solid-state structures are discussed in detail. On the way to new alkaline earth metal species a promising potassium precursor complex [K{(4-MeNCOC₆H₃)₂CH}{η⁵-K(4-MeNCOC₆H₃)₂CH}(THF)]_∞ (2) and a homoleptic magnesium [Mg{(4-MeNCOC₆H₃)₂CH}₂] (3) and calcium complex [Ca{(4-MeNCOC₆H₃)₂CH}₂(THF)₂]·THF (4) were obtained.

Full text of DOI:10.1002/ejic.201700501

A Journal of
Accepted Article
Title: Bis(4-methylbenzoxalzol-2-yl)methanide in s-Block Metal
Coordination
Authors: Dietmar Stalke, Regine Herbst-Irmer, and Ingo Koehne
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To be cited as: Eur. J. Inorg. Chem. 10.1002/ejic.201700501
Link to VoR: http://dx.doi.org/10.1002/ejic.201700501

10.1002/ejic.201700501
European Journal of Inorganic Chemistry
FULL PAPER
Bis(4-methylbenzoxazol-2-yl)methanide in s-Block Metal
Coordination
Ingo Koehne, Regine Herbst-Irmer and Dietmar Stalke*[a]
Abstract: On the basis of the bis(4-methylbenzoxazol-2-yl)methanide
ligand three new alkali or alkaline earth metal compounds could be
synthesized and their solid state structures are discussed in detail. On
the way to new alkaline earth metal species a promising potassium
precursor complex [K{(4-MeNCOC6H3)2CH} (η5-K(4-MeNCOC6H3)2-
CH)(THF)]∞ (2) and a homoleptic magnesium [Mg{(4-MeNCOC6H3)2-
CH}2] (3) and calcium complex [Ca{(4-MeNCOC6H3)2CH}2(THF)2] ·
THF (4) could be obtained.
We embarked to extend the main group chemistry, fired by the
ubiquitous β-diketiminate ligand[7-14] and its derivatives, to related
bisheterocyclo methane and amine ligand systems. Starting from
dipyridyl methane and methanides[15-18] (A in Scheme 1) we
extended the ligand class to the dipyridyl amides and
phosphanides (B).[19-24] More recently a variety of Al(III), Ga(III),
In(III) and Li(I) complexes containing the bis(benzoxazol-2-yl)-
and bis(benzothiazol-2-yl)methanide[25-26] (C) or -amide[27] (D)
ligands could be obtained. Currently this chemistry could also be
extended to bis(benzimidazol-2-yl)methanide (C) and asymmetric
species (E) of the abovementioned ligand systems.[28]
Introduction
In this context especially the bis(4-methylbenzoxazol-2-
yl)methane ligand system 1 seems to be most suitable for the
generation of low valent main group compounds. 1 is synthesized
in a double cyclocondensation reaction starting from 3-methyl-2-
aminophenol and ethyl-bisimidate dihydrochloride as the C3-linker
unit according to literature procedures (Scheme 2).[26, 29] Upon
metallation its complexes show the least deviation of the metal
cation from the chelating ligand plane, indicating the best charge
distribution within the aromatic system, advantageous for
example for subsequently reduced low oxidation state species.
Furthermore the methyl substituents offer a certain amount of
shielding to the metal cation while not hampering further
substitution reactions.[26] Following up on those investigations we
herein present our efforts to exceed this chemistry also to the
early main group elements, in particular alkaline earth metals.
Methylene bridged heteroaromatics like the bis(benzothiazol-2-
yl)methane[1-2] ligand system and its related bis-benzoxazol and
bis-benzimidazol derivatives are well established since many
years. The latter were also extensively studied concerning the
charge demands of the corresponding heteroaryl substituents in
the derived deprotonated forms.[3-5] Despite the fact, that
bisheterocyclo methane ligands could act as a suitable platform
for transition metal complexes because of their ability to behave
as both, neutral and monoanionic ligands[4], they attracted only
little attention in the field of main group coordination chemistry in
the last three decades.[6]
Scheme 2. Synthesis of bis(4-methylbenzoxazol-2-yl)methane 1.
Results and Discussion
Scheme 1. β-diketiminate related methylene and amine bridged symmetric and
asymmetric heteroaromatic ligand systems.
When 1 is added to a suspension of KH in THF (Scheme 3) the
corresponding four-fold coordinated potassium precursor
complex 2 can be obtained in good yields of 73%. Subsequent
transmetallation (Scheme 4) gives the related alkaline earth metal
complexes. 2 crystalizes in the orthorhombic space group Pbca
containing two molecules in the asymmetric unit. They form
infinite zig-zag strands (see ESI). The central potassium cation
shows a distorted tetrahedral coordination by two ring nitrogen
atoms of a monoanionic methanide ligand, a THF molecule and
an additional η5-coordination to the central C3N2 plane of a second
ligand (Figure 1).
[a]
M. Sc. I. Köhne, Dr. R. Herbst-Irmer, Prof. Dr. D. Stalke
Institut für Anorganische Chemie
Georg-August-Universität Göttingen
Tammannstr. 4, 37077 Göttingen (Germany)
E-mail: dstalke@chemie.uni-goettingen.de
Supporting information for this article is given via a link at the end of
the document.((Please delete this text if not appropriate))
1
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10.1002/ejic.201700501
European Journal of Inorganic Chemistry
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Table 1. Selected averaged bond lengths [Å] and angles [°] for 1-4 and related
NacNac species II-IV.
N–M
Cipso–C1–Cipso
N-M-N
M–plane distance
1
--
110.79(12)
--
--
2
2.795
2.057
2.472
2.111
2.379
2.712
125.0
123.3
124.9
--
67.6
94.0
78.6
92.97
83.09
68.89
1.33
0.07
1.34
0.907
1.291
1.222
3
4
II
III
IV
--
Scheme 3. Synthesis of compound 2.
--
In comparison to the free uncharged ligand 1[26] (1.487 Å) the
averaged C1-Cipso distance in 2 (1.393 Å) decreases due to the
Contrary to expectations, adding ligand 1 to a solution of a slight
excess of iPrMgCl in THF at -20 °C does not give the heteroleptic
chloride complex of the parent ligand system as the product but
the disubstituted homoleptic species 3 (Scheme 4). 3 crystalizes
in the monoclinic space group P21/n with one molecule in the
asymmetric unit (Figure 2). The central magnesium(II) cation
adopts a slightly distorted tetrahedral coordination sphere
resulting in the two planar ligand molecules arranged almost
perfectly perpendicular (85.42(4) °). Additionally, in contrast to 2,
4 and the reference NacNac structures II-IV the cation in 3 shares
both C3N2 planes with only a marginal av. deviation of 0.07 Å. This
is also reflected by the shortest averaged Mg-N distance of 2.057
Å.
formation of
a conjugated π-system and the change in
hybridization of the bridging carbon atom from sp3 to sp2 upon
deprotonation. The opposite is true for the averaged N-Cipso
distance (1.332 Å) which gets longer in comparison to 1 (1.291 Å).
Furthermore, to facilitate the additional η5-coordination to a
second ligand molecule the potassium atom shows a strong
dislocation (1.33(3) Å) from the chelating C3N2 plane that fits quite
well to a related homoleptic β-diketiminate barium complex
[Ba(NacNac)2][30] IV, exhibiting almost the same dislocation of
1.222(1) Å. Further structural features like an averaged M-N
distance (2.795 Å (M = K) to 2.712 Å (M = Ba)) and an averaged
N-M-N bite angle (67.6° to 68.89°) are also in good agreement.
These observations are not surprising according to the similar
effective ionic radii of both cations (K+ = 137 pm to
Ba2+ = 135 pm[31]). The η6-coordination of the potassium atom in
[(PMDETA)(THF)KPyCPh2], (Py = pyridyl, PMDETA = (Me2NCH2-
CH2)2NMe) gives a slightly longer K-N distance of 2.809(5) Å.[32]
Scheme 4. Synthesis of compounds 3 and 4.
Figure 1. Excerpt of the polymeric structure of [K{(4-MeNCOC6H3)2CH}(η5-K(4-
MeNCOC6H3)2CH)(THF)]∞ (2). Anisotropic displacement parameters are
depicted at the 50% probability level. Hydrogen atoms are omitted for clarity
except for those at the bridging methanide position. Structural data are given in
Tables 1 and 2, respectively. Additional selected averaged bond lengths [Å]:
C1-Cipso 1.394(3), N-Cipso 1.332(2), O-Cipso 1.395(2), K-OTHF 2.672(2).
Comparing 3 to the related homoleptic structure [Mg(NacNac)2][30]
II, again similar structural features (Mg-N distance: 2.0565 Å to
2.379 Å and N-Mg-N bite angle of 94.06 ° and 92.97 °) can be
found. Interestingly, in the hexa coordinated [Mg{(pz*)3C}2] (pz*
2
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10.1002/ejic.201700501
European Journal of Inorganic Chemistry
FULL PAPER
deshielding per methylene bridge proton. As expected, upon
deprotonation this signal experiences a significant down-field shift
in 3 and 4 showing resonances of 5.05 and 5.07 ppm, respectively.
Furthermore, at 3.62 and 1.77 ppm resonances of the attached
THF molecules in 2 and 4 can be found. Interestingly, the
chemical shift of the methyl groups of 2.41 ppm in 2 is in-between
those of 2.62 and 1.91 ppm found in 3 and 4. The 13C NMR
spectra of 2-4 revealed resonances of 57.0, 60.4 and 59.0 ppm,
respectively for the bridging carbon atom.
Figure 2. Molecular structure of [Mg{(4-MeNCOC6H3)2CH}2] (3). Anisotropic
displacement parameters are depicted at the 50% probability level. Hydrogen
atoms are omitted for clarity except for those at the bridging methanide position.
Structural data are given in Tables 1 and 2, respectively. Additional selected
averaged bond lengths [Å]: C1-Cipso 1.392(2), N-Cipso 1.341(2), O-Cipso 1.376(2).
= 3,5-dimethylpyrazolyl), the Mg-N bond path of 2.1973 Å is only
slightly longer.[33]
Figure 3. Molecular structure of [Ca{(4-MeNCOC6H3)2CH}2(THF)2] · THF (4).
Anisotropic displacement parameters are depicted at the 50% probability level.
A co-crystallized THF molecule and hydrogen atoms are omitted for clarity
except for those at the bridging methanide position. Structural data are given in
Tables 1 and 2, respectively. Additional selected averaged bond lengths [Å]:
C1-Cipso 1.388(3), N-Cipso 1.333(3), O-Cipso 1.390(2), Ca-OTHF 2.386(2).
The reactions of 1 with calcium hexamethyldisilazane, as well as
of the potassium precursor complex 2 with CaI2 beads, both give
the homoleptic calcium compound 4 (Scheme 4). 4 contains one
molecule per asymmetric unit and is depicted in Figure 3. The
central calcium cation shows a distorted octahedral coordination
made up from two ligands and two THF molecules. Steric
crowding causes the average dislocation of 1.34 Å from the C3N2
plane. The two ligands show an almost perfect parallel
arrangement with a twisting angle of only 7.25(9) °. The values for
the Ca-N distance of 2.472 Å and the N-Ca-N bite angle of 78.6 °
are half way between those of compound 2 and 3, because the
effective ionic radius of Ca2+ of 100 pm is located between those
of Mg2+ (72 pm) and K+ (137 pm). They are in good agreement
Conclusions
In this work the successful syntheses and solid-state structure
determination of three metallated bis(4-methylbenzoxazol-2-yl)-
methanides 2–4 could be presented. With the potassium complex
2 a promising precursor complex for subsequent transmetallation
reactions could be introduced. Moreover, two new homoleptic
alkaline earth metal complexes 3 and 4 could be obtained.
Currently, ongoing efforts are made to synthesize more bulky
bis(benzoxazol-2-yl)methane ligand systems bearing sterically
demanding substituents at the four position to facilitate the
formation of heteroleptic alkaline earth metal halide complexes for
subsequent reduction to give the corresponding low oxidation
state species, not yet realized in the current endeavor.
with those found for
a related homoleptic β-diketiminate
compound [Ca(NacNac)2] III. Some general trends for 2-4 can be
deduced, predominantly caused by the increasing size of the
cation: On the one hand the N-M distances and Cipso–C1–Cipso
angles are increasing while on the other hand the N-M-N bite
angle is decreasing. In contrast, due to different coordination
modes no such trend can be determined for the deviation of the
metal ions from the C3N2 plane.
Compounds 2-4 were additionally characterized by 1H and
13C{1H} NMR spectroscopy, LIFDI-MS and elemental analysis. In
the 1H NMR spectra, for the aromatic protons all compounds
display resonances within a range of 7.07 and 6.55 ppm. Counter-
intuitively, in comparison to the chemical shift of 4.70 ppm[25]
found for the methylene-bridging moiety in 1, upon deprotonation
this resonance experiences a slight up-field shift (4.65 ppm) in 2.
This observation is most likely due to the additionally present η5-
coordination of the potassium cation decreasing the net electronic
Experimental Section
Procedures: All manipulations were carried out under an inert argon
atmosphere by using Schlenk techniques[34] or were handled in an argon
dry box. All solvents used within metallation reactions were distilled from
Na or K before use. The starting materials were purchased commercially
and used as received if not stated otherwise. [Ca{N(SiMe3)2}2] x 2 THF
was synthesized according to literature procedures.[35] 1H and 13C
3
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10.1002/ejic.201700501
European Journal of Inorganic Chemistry
FULL PAPER
spectroscopic data were recorded on
a
Bruker Avance 300 MHz
reduced pressure. Recrystallization following the above mentioned
spectrometer and referenced to the residual proton signal of the
deuterated solvent (THF-d8).[36] Deuterated solvents were dried over
activated molecular sieve (3Å) and were additionally stored in an argon dry
box. Elemental analyses (C, H and N) were carried out on a Vario EL3 at
the Mikroanalytisches Labor, Institut für Anorganische Chemie, University
of Göttingen. All LIFDI-MS spectra (70 eV) were recorded on a Jeol
AccuTOF spectrometer.
method yielded 4 in an amount of 212 mg (261 µmol, 66%).
1H NMR (300 MHz, [D8]THF, r.t.): δ=6.97 (m, 4H, H7), 6.73 (t,
3J(H,H)=7.8 Hz, 4H, H6), 6.55 (m, 4H, H5), 5.07 (s, 2H, H10), 3.62 (m, 8H,
THF), 1.91 (s, 12H, Me), 1.77 (m, 8H, THF); 13C NMR (75 MHz, [D8]THF,
r.t.): δ=170.2 (C2), 149.6 (C8), 142.8 (C3), 125.4 (C5), 124.8 (C4), 121.3
(C6), 106.3 (C7), 59.0 (C10), 17.1 (Me); LIFDI-MS: m/z (%): 594.1 (100)
[M]; elemental analysis calcd (%) for C42H42CaN4O6: C 68.27, H 5.73, N
7.58; found: C 67.73, H 5.51, N 7.69.
[K{(4-MeNCOC6H3)2CH}(η5-K(4-MeNCOC6H3)2CH) (THF)]∞ (2). KH
(48.0 mg, 1.19 mmol, 1.10 eq.) was suspended in THF (10 mL) and 1
(300 mg, 1.08 mmol, 1.00 eq.) dissolved in THF (10 mL) was slowly drop
wise added. A red suspension forms upon gas evolution. The mixture was
left stirring at r.t. overnight. The resulting orange-brownish suspension was
filtered via cannula and the solvent was removed under reduced pressure.
The residue was washed with pentane (3 x 10 mL) and dried. For analysis
the residue was recrystallized by slow evaporation of pentane into a sat.
THF solution at r.t.. Colorless needle shaped crystals were obtained in a
yield of 306 mg (788 µmol, 73%).
Crystallographic details
Shock cooled crystals were selected from a Schlenk flask under
argon atmosphere using the X-TEMP2 device.[37-39] The data were
collected on an IµS microfocus diffractometer.[40] All data were
integrated with SAINT.[41] A multi-scan absorption correction
(SADABS)[42] and a 3λ correction were applied.[43] The structures
were solved by direct methods (SHELXT)[44] and refined on F2
using the full-matrix least-squares methods of SHELXL[45] within
the SHELXLE GUI.[46] More details about the crystallographic data
and the refinement can be found in the supporting information.
These data can also be obtained free of charge from the
1H NMR (300 MHz, [D8]THF, r.t.): δ=6.85 (m, 2H, H7), 6.71 (m, 2H, H5),
6.60 (m, 2H, H6), 4.65 (s, 1H, H10), 3.62 (m, 4H, THF) 2.41 (s, 6H, Me),
1.77 (m, 4H, THF); 13C NMR (75 MHz, [D8]THF, r.t.): δ=170.3 (C2), 149.9
(C8), 146.5 (C3), 123.9 (C5), 122.5 (C4), 118.5 (C6), 105.6 (C7), 57.0
(C10), 17.5 (Me); LIFDI-MS: m/z (%): 316.1 (100) [M–2THF], 632.1 (40)
2[M–2THF], 671.1 (12) {2[M–2THF] + K+}; elemental analysis calcd (%) for
C21H21KN2O3: C 64.92, H 5.45, N 7.21; found: C 64.71, H 5.31, N 7.38.
Cambridge
Crystallographic
Data
Centre
via
www.ccdc.cam.ac.uk/ data-request/cif.
[Mg{(4-MeNCOC6H3)2CH}2] (3). iPrMgCl (2 M in THF, 1.00 mL, 1.98 mmol,
1.11 eq.) was diluted in THF (10 mL) and cooled to -20 °C. 1 (500 mg,
1.80 mmol, 1.00 eq.) dissolved in THF (10 mL) was then slowly drop wise
added. After completed addition the cooling bath was removed and the
mixture was left stirring at r.t. overnight. The solvent was removed under
reduced pressure and the residue was extracted with toluene (3 x 10 mL)
and filtered via cannula. The volume of the toluene extract was reduced to
a minimum and a formed precipitate was again dissolved by gentle heating.
From this solution colorless prism shaped crystals were obtained at r.t. in
a yield of 405 mg (702 µmol, 39%).
Table 2. Crystal structure data for 2-4.
2
3
4 · THF
Formula
MW, gmol−1
CCDC no.
Space group
a, Å
C21H21KN2O3
388.50
1536931
Pbca
C34H26MgN4O4
578.90
C46H50CaN4O7
810.98
1536929
P21/n
15369230
Pbca
7.103(2)
28.522(2)
38.061(3)
90
11.336(2)
15.002(2)
16.777(2)
93.10(2)
2849.0(7)
4
18.297(2)
15.994(2)
27.504(3)
90
1H NMR (300 MHz, [D8]THF, r.t.): δ=7.07 (m, 4H, H7), 6.92 (m, 8H, H5 +
H6), 5.05 (s, 2H, H10), 2.69 (s, 12H, Me); 13C NMR (75 MHz, [D8]THF, r.t.):
δ=169.9 (C2), 149.1 (C8), 140.8 (C3), 125.6 (C5), 124.6 (C4), 121.6 (C6),
108.6 (C7), 60.4 (C10), 19.2 (Me); LIFDI-MS: m/z (%): 278.1 (20) [(4-
MeNCOC6H3)2CH], 578.1 (100) [M]; elemental analysis calcd (%) for
C34H26MgN4O4: C 70.54, H 4.53, N 9.68; found: C 69.95, H 4.31, N 9.29.
b, Å
c, Å
β,°
V, Å3
7711.(2)
16
8048.8(16)
8
Z
Abs. coeff., mm-1
Theta max, °
Refl. meas.
Refl. uniq.
Rint
0.299
0.110
0.214
[Ca{(4-MeNCOC6H3)2CH}2(THF)2] x THF (4).
25.352
61122
30.042
26.369
58293
50606
Method A: 2 (290 mg, 747 µmol, 1.00 eq.) and anhydrous CaI2 beads
(241 mg, 821 µmol, 1.10 eq.) were suspended in THF (15 mL). The
suspension was heated under reflux at 75 °C overnight. The solvent was
removed under reduced pressure. The residue was washed with pentane
(2 x 10 mL) and extracted with toluene (3 x 10 mL). The solvent from the
toluene extract was removed and the crude product was recrystallized by
slow evaporation of pentane into a sat. THF solution at -30 °C. From this
solution colorless prism shaped crystals were obtained in a yield of 255 mg
(314 µmol, 42%).
7074
8328
8215
0.0659
7074/0/492
0.0366
0.0891
0.234/-0.263
0.0466
0.0634
Data/restr./para.
R1 [I>2σ(I)]a
wR2 (all)b
Δρfin, e Å−3
8328/0/392
0.0398
8215/11/535
0.0441
0.1034
0.1230
0.416/-0.301
0.599/-0.280
w(Fo2  F2)2
Fo  Fc
R1 

c
wR 2 
w(Fo2)2
Fo


Method B: Ca{N(SiMe3)2}2 x 2 THF (200 mg, 396 µmol, 1.00 eq.) was
suspended in THF (10 mL). Then
1 (110 mg, 396 µmol, 1.00 eq.)
dissolved in THF (5 mL) was drop wise added. The mixture was left stirring
at r.t. for 2 h and was then heated under reflux at 75 °C overnight. The
formed suspension was filtered via cannula and the solvent removed under
4
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10.1002/ejic.201700501
European Journal of Inorganic Chemistry
FULL PAPER
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This article is protected by copyright. All rights reserved.

10.1002/ejic.201700501
European Journal of Inorganic Chemistry
FULL PAPER
FULL PAPER
Ingo Koehne, Regine Herbst-
Irmer, Dietmar Stalke*
New s-Block Complexes derived
from heteroaromatic substituted
methanides! They are based on the
versatile bis-(4-methylbenzoxazol-2-
yl)methane ligand. Metallation of the
bridging methylene moiety with Group
1 and 2 organometallics gives the
N,N-chelating anions comprising
further O-donor centres in the
periphery of the complex. Structural
features are compared to the
Page No. – Page No.
Bis(4-methylbenzoxazol-2-
yl)methanide in s-Block
Metal Coordination
omnipresent monoanionic NacNac
ligand.
6
This article is protected by copyright. All rights reserved.