Introducing NacNac-Like Bis(4,6-isopropylbenzoxazol-2-yl)methanide in s-Block Metal Coordination

Article abstract of DOI:10.1021/acs.inorgchem.7b02276

Within this work, the field of bulky methanides in metal coordination is exceeded by the synthesis of the versatile and promising bis(4,6-isopropylbenzoxazol-2-yl)methane (7) ligand platform. As an enhancement in this class of ligands, isopropyl (iPr) substituents as steric-demanding groups have been successfully introduced in proximity to the coordination pocket, mimicking the shielding abilities of the ubiquitous NacNac ligand scaffold to improve the steric protection of a coordinated s-block metal cation. A percent buried volume (% V_bur) calculation as well as an electronic structure analysis shades light onto the shielding and electronic abilities of the ligand in comparison to other selected methanides and diketiminates. Upon deprotonation with a variety of different group 1 and 2 metalation agents, a row of novel s-block metal complexes of the parent deprotonated monoanionic ligand 7 was obtained and structurally, as well as spectroscopically, characterized. In particular, in this context, the alkali-metal precursor complexes [Li(THF)₂{(4,6-iPr-NCOC₆H₂)₂CH}] (8) and [K{μ-(4,6-iPr-NCOC₆H₂)₂CH}]_∞ (9) as well as the alkaline-earth-metal compounds [MgCl(THF)₂{(4,6-iPr-NCOC₆H₂)₂CH}] (10) and [M(THF)_n{(4,6-iPr-NCOC₆H₂)₂CH}₂] [M = Mg, n = 0 (11); M = Ca, n = 1 (12); M = Sr, n = 1 (13); M = Ba, n = 1 (14)] were successfully synthesized. Especially, the latter four exhibit interesting trends in the solid state as well as in solution within the metal series.

Full text of DOI:10.1021/acs.inorgchem.7b02276

Article
Cite This: Inorg. Chem. XXXX, XXX, XXX-XXX
pubs.acs.org/IC
Introducing NacNac-Like Bis(4,6-isopropylbenzoxazol-2-
yl)methanide in s‑Block Metal Coordination
Ingo Koehne, Nico Graw, Thorsten Teuteberg, Regine Herbst-Irmer, and Dietmar Stalke*
†
†
‡
†
,†
†
Institut fu
̈
r Anorganische Chemie, Georg-August-Universitat
Institut fur Physikalische Chemie, Georg-August-Universitat
S Supporting Information
̈
Go
̈
ttingen, Tammannstraße 4, 37077 Go
̈
ttingen, Germany
‡
̈
̈
Gottingen, Tammannstraße 6, 37077 Go
̈
̈
ttingen, Germany
*
ABSTRACT: Within this work, the field of bulky methanides in
metal coordination is exceeded by the synthesis of the versatile
and promising bis(4,6-isopropylbenzoxazol-2-yl)methane (7)
ligand platform. As an enhancement in this class of ligands,
isopropyl (iPr) substituents as steric-demanding groups have been
successfully introduced in proximity to the coordination pocket,
mimicking the shielding abilities of the ubiquitous NacNac ligand
scaffold to improve the steric protection of a coordinated s-block
metal cation. A percent buried volume (% V ) calculation as well
bur
as an electronic structure analysis shades light onto the shielding
and electronic abilities of the ligand in comparison to other
selected methanides and diketiminates. Upon deprotonation with
a variety of different group 1 and 2 metalation agents, a row of
novel s-block metal complexes of the parent deprotonated
monoanionic ligand 7 was obtained and structurally, as well as spectroscopically, characterized. In particular, in this context,
the alkali-metal precursor complexes [Li(THF) {(4,6-iPr-NCOC H ) CH}] (8) and [K{μ-(4,6-iPr-NCOC H ) CH}] (9) as
2
6
2 2
6
2 2
∞
well as the alkaline-earth-metal compounds [MgCl(THF) {(4,6-iPr-NCOC H ) CH}] (10) and [M(THF) {(4,6-iPr-
2
6
2 2
n
NCOC H ) CH} ] [M = Mg, n = 0 (11); M = Ca, n = 1 (12); M = Sr, n = 1 (13); M = Ba, n = 1 (14)] were successfully
6
2 2
2
synthesized. Especially, the latter four exhibit interesting trends in the solid state as well as in solution within the metal series.
INTRODUCTION
Chart 1. β-Diketiminate-Related Bis(heterocyclo)methane,
alkane, -borane, or -amine Derivatives (A−C)
■
-
The highly modular design of the β-diketiminate or NacNac
ligand platform and the versatile applicability of its transition-
and main-group-metal complexes in catalytic transforma-
1
−7
tions
as well as its ability to stabilize low-oxidation-state
8
−14
main-group-metal cations
has fired the search for other
promising NacNac-like ligand scaffolds. In terms of mimicking
the formation of a six-membered metallaheterocycle upon
chelation of a metal ion by two imine nitrogen atoms, the bis(2-
pyridyl)methane ligand platform and its nitrogen-, phospho-
rus-, and arsenic-bridged homologues were extensively
1
5−24
on neutral BOX ligands, a couple of monoanionic oxazolinato
studied.
oxazoline residues, the class of chiral bis(oxazol-2-yl)methane
BOX) ligands is accessed (A in Chart 1). Interestingly, when
By substitution of the 2-pyridyl side arms with
30−38
structures are known.
As a highlight, it could also be
shown that BOX ligands promote copper-mediated dioxygen
(
3
9
activation in a biomimetic hemocyanin fashion. With
at least one substituent at the bridging methylene moiety is a
hydrogen atom, a tautomerism between the corresponding
diimine and the (E)-iminoenamine tautomer of A was
observed, showing that the latter is favored when the backbone
bis(indaoxazol-2-yl)methane and its derivatives (B in Chart
1
), also chiral ligand platforms comprising an extended ring
system were accessed, promoting metal-mediated catalytic
applications.
40−42
25
In the field of ligand systems that display
spacer carries a phenyl substituent. Hence, on the basis of A,
several different ligand species bearing numerous different
substituents (e.g. Me, iPr, tBu, Cy, Ph) in close proximity to the
coordination pocket have been synthesized and quickly risen to
high-performance ligand scaffolds, finding widespread applica-
tion in a multitude of metal-mediated catalytic transforma-
an extended π system within the ligand periphery, our group
was able to establish bis(benzoxazol-2-yl)methanides and their
higher benzothiazole homologues as promising platforms in
Received: September 4, 2017
2
6−29
tions.
Although the majority of these complexes are based
©
XXXX American Chemical Society
A
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Article
4
3−45
group 1, 2, and 13 chemistry (C in Chart 1).
Furthermore,
6
Scheme 2. Syntheses of Compounds 5 and 6 (top) and 7
(bottom)
4
the corresponding nitrogen-bridged derivatives as well as the
bis(1-methylbenzimidazol-2-yl)methanide and asymmetrically
substituted congeners were extensively investigated with
respect to their metal coordination.
47
So far, bis(heterocyclo)methanes and their higher amine
homologues with H, Me, or OMe groups at the C4 position
close to the coordination pocket mimicked only marginally the
steric demand provided by the NacNac ligand. Most recently,
our group was able to initially introduce bulky tert-butyl (tBu)
48
substituents at this key position. As an extension, we herein
present our efforts to implement iPr groups at the pole position
as well. In the following paragraph, the seven-step synthesis and
its behavior in the metal coordination of bis(4,6-isopropylben-
zoxazol-2-yl)methane (7) are discussed in detail.
−
1
asymmetric (ν
̃
= 1525 cm ) NO stretching vibrations, which
2
−1
are in good agreement with the values of 1366 and 1518 cm
found for the tBu-substituted derivative. A subsequent
reduction of the nitro function in 5 was carried out using
dihydrogen and palladium on charcoal (Pd/C) as the
hydrogenation catalyst, yielding the desired 3,5-diisopropyl-2-
aminophenol (6) in an excellent yield of 90%. In the H NMR
spectrum [tetrahydrofuran (THF)-d ], a characteristic broad
singlet at 3.82 ppm with an integral of two can be assigned to
the NH group. Again, distinctive vibrational bands at 3307 and
3376 cm , respectively, arising from the symmetric and
asymmetric NH stretching vibrations, are in tune with values
RESULTS AND DISCUSSION
Because the symmetrically substituted 3,5-diisopropylphenol
4) is 10 times as expensive as its corresponding tBu-
■
(
1
8
substituted derivative, ligand synthesis was started by first
generating 4 in a four-step procedure following a protocol by
Diemer and co-workers, which was modified and improved
2
−1
4
9
(
Scheme 1). Starting from 2,6-diisopropylaniline, in the first
2
−1
of 3316 and 3415 cm found for the corresponding tBu-
substituted derivative. As a last step, the desired ligand system 7
was generated by the reaction of 2 equiv of 6 and the well-
Scheme 1. Syntheses of Compounds 1−4
established ethylbisimidate dihydrochloride C -linker unit
3
44,52
(
Scheme 2, bottom).
Practically pure 7 was obtained in a
yield of 43% by simple extraction of the reaction crude with
pentane and sonication. Besides resonances for the aromatic
protons at 7.23 and 7.07 ppm, respectively, the H NMR
1
spectrum (THF-d ) of 7 additionally comprises a distinct
8
singlet at 4.62 ppm due to the two methylene bridge protons.
Furthermore, the characteristic pattern of two septets at 3.46
and 2.99 ppm and two doublets at 1.38 and 1.26 ppm,
respectively, can be assigned to the chemically inequivalent iPr
substituents at the corresponding C4 and C6 positions.
Interestingly, despite bearing four methyl groups less than its
tBu-substituted congener accompanied by a supposedly
decreased +I effect, the H NMR signals in 7 are less
downfield-shifted, suggesting an enhanced electron density
residing at these positions.
step a bromine substituent was introduced at the para position
of the aniline derivative, forming 4-bromo-2,6-diisopropylani-
1
50
line (1) in quantitative yield. A subsequent diazotization
readily gave 1-bromo-3,5-diisopropylbenzene (2) in a high yield
of 93%. In the next step, subsequent borylation with
tributylborate of in situ generated (3,5-diisopropylphenyl)-
lithium gave a mixture of boronic acid 3 (14%) and
cyclotriboroxane 3a (86%) in an appreciable yield of 60%.
Finally, anion oxidation was carried out through the addition of
hydrogen peroxide to a solution of 3/3a in aqueous sodium
hydroxide to facilitate the formation of reactive hydro-
When n-butyllithium (nBuLi) was added to a solution of 7 in
THF at −60 °C, the corresponding 4-fold-coordinated lithium
precursor complex [Li(THF) {(4,6-iPr-NCOC H ) CH}] (8)
2
6
2 2
7
was obtained in a good yield of 69% (Scheme 3). 8 shows a Li
NMR signal (THF-d ) at 2.07 ppm. Additionally, in contrast to
8
1
the free uncharged ligand upon deprotonation in the H NMR
spectrum (THF-d ) of 8, the resonances intrinsic to the
8
5
1
aromatic protons experience a significant upfield shift to 6.84
and 6.82 ppm. This observation is in tune with an increased
peroxide. After protonation of the formed phenolate with
aqueous hydrogen chloride and a subsequent straightforward
workup, 4 was obtained in an almost quantitative yield of 95%.
Just like as was shown for the corresponding tBu-substituted
Scheme 3. Synthesis of Compound 8
4
8
derivative, upon the addition of 1 equiv of nitric acid to 4, the
mono-ortho-substituted 3,5-diisopropyl-2-nitrophenol (5) was
isolated in an appreciable yield of 46% (Scheme 2, top). The
presence of a doublet at 6.89 ppm and a doublet of doublets at
6
.83 ppm with an integral of one for each of the aromatic
1
protons in the H NMR spectrum (acetone-d ) of 5 indicates
6
the formation of an asymmetric product and thus a successful
mono-ortho-nitration. The IR spectrum of 5 exhibits character-
−1
̃
istic vibrational bands for the symmetric (ν = 1365 cm ) and
B
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electron density at these positions due to a successful charge
delocalization from the C N moiety toward the C perimeter
3
2
6
in the ligand periphery. This is also accompanied by a
deshielding and simultaneous downfield shift of the corre-
sponding methylene bridge signal to 4.72 ppm. Furthermore, 8
crystallizes in the non-centrosymmetric monoclinic space group
Cc (Figure 1). The solid-state structure reveals only slightly
Figure 1. Molecular structure of 8. Anisotropic displacement
parameters are depicted at the 50% probability level. Hydrogen
atoms are omitted for clarity except for that at the bridging methylene
position. Structural data are given in Tables 1, 2, and S1).
+
distorted tetrahedral coordination around the central Li cation.
With a dislocation between 0.00(3) and 0.07(5) Å, the lithium
ion is almost ideally placed within the chelating C N plane.
3
2
Moreover, a butterfly-like folding angle in the range of only
2
.2(10)−2.3(6)° between the two benzoxazole moieties
indicates an almost planar ligand framework. As can be seen
in Table 1, in comparison to the closely related compound
48
[
Li(THF){(4,6-tBu-NCOC H ) CH}] (D), the N−M dis-
6
2 2
tances of 2.03(1) and 1.96(2) Å in 8 are elongated, most likely
caused by an electronically more depleted ligand periphery and
an increased coordination number of 4 due to an additional
THF donor. The same trend can be derived for the
corresponding N−M−N bite angle between 91.7(8) and
9
9.7(6)° and a considerable widening of the Cipso−C −C
1
ipso
angle from 121.7(3)° in D to 119(2)−126.8(1)° in 8. Another
r e l a t e d l i t h i u m c o m p l e x , [ L i (T H F ) { ( 1 - M e -
2
4
7
NCNC H ) CH}], has N−M bond lengths comparable to
6
4 2
those of 8 [1.979(3) and 1.974(3) Å, respectively] and reduced
N−M−N and C −C −C angles [96.85(1) and
ipso
1
ipso
1
24.28(2)°] due to less steric crowding. Furthermore, the
general structural features of 8 are also in good agreement with
those for comparable β-diketiminate-based lithium compounds
5
3,54
from the literature.
The addition of compound 7 to a suspension of potassium
hydride in THF led to the formation of a novel potassium
species in a yield of 71% (Scheme 4). Recrystallization of [K{μ-
(
4,6-iPr-NCOC H ) CH}] (9) only afforded small needle-
6 2 2 ∞
shaped crystals not suitable for single-crystal X-ray diffraction
experiments. Nevertheless, in light of other bis(benzoxazol-2-
yl)methanide-based potassium compounds like [K{(4-
4
5
MeNCOC H ) CH}(μ-K(4-MeNCOC H ) CH)(THF)]
6
3
2
6
3
2
∞
4
8
and [K{μ-(4,6-tBu-NCOC H ) CH}] (E) forming poly-
6
2 2
∞
meric strands in the solid state, this coordination polymer is
1
assumed for 9 as well. Because the H NMR spectrum of 9
(
THF-d ; see the Supporting Information, SI) solely exhibits
8
resonances of the monoanionic ligand 7 and no additionally
attached THF molecules, a donor-base free polymeric structure
like that in the above-mentioned closely related tBu-substituted
derivative E is most likely. In particular, in comparison to E in
C
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Inorganic Chemistry
Article
Table 2. Additional Selected Bond Lengths [Å] and Angles [deg] for 8 and 10−14
N−Cipso
1.331(8), 1.33(1), 1.32(2)
C1−Cipso
folding angle
2.3(6), 2.2(10)
8
1
1.41(1), 1.39(1), 1.39(2), 1.37(2)
0
1.325(5), 1.341(5), 1.336(5), 1.341(4), 1.335(5), 1.336(5), 1.397(5), 1.389(5), 1.392(5), 1.391(5), 1.392(5), 1.383(5), 1.99(9), 15.72(6), 4.27(8),
1.332(5), 1.330(4)
1.390(5), 1.397(4)
13.35(5),
11
12
13
14
1.340(2), 1.341(2), 1.336(2), 1.339(2)
1.323(3), 1.334(3)
1.391(2), 1.386(2), 1.386(2), 1.389(2)
1.399(3), 1.385(3)
13.34(3), 12.09(4), 10.76(3)
3.90(3)
1.336(3), 1.328(3), 1.332(3), 1.329(3)
1.332(2), 1.325(2)
1.388(4), 1.397(4), 1.390(4), 1.393(4)
1.399(2), 1.398(2)
1.98(5), 8.24(4)
8.01(6)
Scheme 4. Synthesis of Compound 9
1
the H NMR spectrum of 9, the signals for the aromatic
protons experience a stronger upfield shift (6.78 and 6.73 ppm
versus 7.01 and 6.97 ppm in E), while the resonance for the
bridging CH moiety (4.61 ppm compared to 4.56 ppm in E) is
slightly more downfield-shifted, again indicating an enhanced
charge redistribution toward the benzene perimeter than in its
tBu-substituted congener.
Figure 2. Molecular structure of 10. Anisotropic displacement
parameters are depicted at the 50% probability level. Hydrogen
atoms are omitted for clarity except for the one at the bridging
methylene position. Three additional molecules of 10 and four lattice
hexane molecules also present in the asymmetric unit are omitted as
well. Structural data are given in Tables 1, 2, and S1).
A very slow addition of 7 to a solution of 2 equiv of the
55
Hauser base (iPr) NMgCl gave the derived magnesium halide
2
compound [MgCl(THF) {(4,6-iPr-NCOC H ) CH}] (10) in
2
6
2 2
an appreciable yield of 56% (Scheme 5). Again, the intrinsic
Scheme 5. Synthesis of Compound 10
Figure 3. Superimposition of the molecular structures of 10 (red) and
F (blue). Hydrogen atoms and the corresponding iPr/tBu substituents
are omitted for clarity.
feature of the deprotonated iPr-substituted ligand system 7 to
show an enhanced dislocation of the negative charge
throughout the ligand framework is reflected in the chemical
blue) shows an in-plane arrangement with respect to the
chelating C N plane. In contrast, the cation in 10 (red)
3
2
1
shifts found in the H NMR spectrum (C D ) of 10. In detail,
exhibits a significant dislocation between 0.412(5) and 0.547(4)
Å from this moiety in contrast to the vanishingly low 0.010(2)
Å in F (blue). This can most likely be ascribed to the reduced
steric demand of the iPr substituents in 10 diminishing the
kinetic barrier for the Mg2+ cation to leave an in-planar
arrangement, while in the case of F, the cation is locked
between the inwardly pointing methyl groups of the tBu
substituents bound at the C4 position (also see Figure 4, vide
infra). As a result of a decreased steric repulsion, the N−M
distances in 10 cover the narrow range of 2.098(3)−2.114(3) Å
and are considerably shortened with respect to the correspond-
ing values of 2.179(2) and 2.160(2) Å found in F. The same
6
6
48
regarding the closely related tBu-substituted compound F, the
two resonances for the aromatic protons (7.11 and 7.02 ppm
versus 7.42 and 7.28 ppm in F) and the signal arising from the
bridging CH fragment (5.59 relative to 5.62 ppm in F)
experience a stronger electronic shielding effect expressed by a
more pronounced upfield shift.
10 was refined as a 4-fold non-merohedral twin (see the SI).
The asymmetric unit contains 4 equiv of non-crystallographic
C -symmetric 10 and additionally four lattice hexane molecules
2
v
2
+
(
Figure 2). The central Mg cation displays 5-fold trigonal-
bipyramidal coordination (τ parameter of 0.77). As can be
56
5
seen from Table 1 and the superposition plot in Figure 3, the
trend can be derived for the correlating N−M−N and C
−
ipso
Mg2 ion in [MgCl(THF) {(4,6-tBu-NCOC H ) CH}] (F;
+
C −C angles. In 10, they range from 93.8(1) to 94.4(1)°
2
6
2 2
1
ipso
D
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Proceeding with the bis(4,6-tert-butylbenzoxazol-2-yl)-
methanide-based structure F, the steric map of this complex
reveals sufficient shielding of the MgCl unit, which is almost
perfectly positioned between the four inwardly pointing methyl
groups visualized by the red bulges in all four quadrants (Figure
4
, top, middle). In this array, an enhanced charge delocalization
between the metal ion and the ligand framework is assured.
The tBu substituents additionally offer a fair shielding on top
and underneath via gentle wrapping around the MgCl moiety,
resulting in the highest % Vbur value of 53.5% of all of the
considered structures.
Despite the fact that the coordination pocket of the chiral
BOX ligand in G is characterized by SW and NE grooves, the
tBu substituents, which correlate with the red bulges that
protrude into the NW and SE, provide sufficient shielding in
these directions, still resulting in a % Vbur of 51.8 that is only
slightly smaller than that in 10 (Figure 4, top, right).
Interestingly, the iPr- and tBu-substituted NacNac ligands in
H and I give rise to almost identical topographic steric maps
and % Vbur values of 48.8 and 49.6, respectively, which lie
between those of the bis(benzoxazol-2-yl)methanide ligand
systems in 10 and F (Figure 4, bottom, left and middle).
The molecular structures of J, comprising the popular
Dipp
NacNac ligand, and complex 10 share some common
structural features: Despite different tetrahedral coordination,
the metal in J shows a similar dislocation of 0.534 Å from the
chelating C N plane as well as comparable N−Mg−N bite
angle and N−Mg distance (see Table 1). J exhibits a
coordination pocket that almost covers the entire area, but
with angles of 73.3 and 69.9°, the parent Dipp substituents are
considerably twisted away from a perpendicular alignment with
Figure 4. % Vbur and topographic steric maps of compounds 10 and
F−J. For clarity, only the relevant metal ion and the considered
dicoordinate ligand are depicted. The orientation of the ligand as well
as the notation of the quadrants as NE (northeast), SE (southeast),
SW (southwest), and NW (northwest) for 10 is valid for all systems
and steric maps. For structures that show several molecules in the
asymmetric unit or more than one molecule of the considered ligand
attached to the metal ion, averaged % Vbur values are given.
3
2
respect to the chelating C N plane. This is visualized by the
3
2
strong bulges in the SW and SE and rather depleted NW and
NE regions in the steric map of this structure (Figure 4,
bottom, right). The % Vbur value was calculated as 53.1, which
is slightly smaller than that in methanide F.
and from 124.0(3) to 125.2(3)°, respectively, while being
significantly widened in F [99.83(6) and 128.9(2)°].
To gain a firm estimation of the steric demand and the
offered shielding capabilities of each ligand system toward the
coordinated magnesium dication, percent buried volume (%
Vbur) and topographic steric map calculations were carried out
for the monoanionic ligand platforms in compound 10, the
closely related tBu-substituted F, and the bis(oxazol-2-
In conclusion, in light of the iPr substituents in 10 bending
away from the coordination pocket, the shielding abilities of the
iPr-substituted bis(benzoxazol-2-yl)methanide in 10 with %
Vbur of 45.6 are still good and comparable to those of the iPr-
and tBu-substituted NacNac systems in complexes H and I.
The tBu-substituted bis(benzoxazol-2-yl)methanide in F and
the Dipp-substituted NacNac ligand in J exhibit with % Vbur
values of 53.5 and 53.1, respectively, the best shielding
capabilities within the considered series, being most pro-
nounced in the methanide F. Nevertheless, both bis-
(benzoxazol-2-yl)methanide ligands provide sufficient shielding
to the cation, preventing the complexes from aggregating.
Regarding the coordination pockets in the MgCl complexes,
the size offered by the tBu-substituted bis(benzoxazol-2-yl)
ligand seems to be the most suitable. Currently, we try to
introduce even bigger substituents at the C4 pole position that
provide sufficient steric shielding in all three dimensions.
In order to gain some insight into the electronic structure
and related donating properties, we carried out an electronic
structure analysis. In a related study, this tool was successfully
used to clarify the structural description of 2-picolyllithium,
which turned out to be better described as a lithium amide than
tBu
57
yl)methanide-derived species [ZnMe( BOX)] (G). Because
of the lack of metal complexes comprising the monoanionic
iPr
BOX, this ligand derivative is not considered. For
comparison, also the closely related iPr- and tBu-substituted
R
NacNac ligands in [Mg( NacNac) ] [R = iPr (H) and tBu
2
5
8
(
I)] and the popular Dipp-substituted congener in [MgCl-
Dipp
59
(
THF)( NacNac)] (J) are considered (Figure 4). The
calculations as well as generation of topographical steric maps
were performed with the help of the SambVca 2 web tool (for
60−62
computational details, see the SI).
The steric map of 10 reveals good shielding properties (%
Vbur = 45.6) of the coordination pocket within this system
(Figure 4, top, left). Another feature also responsible for the
dislocation of the metal from the chelating plane is revealed
when a closer look is taken at the CH hydrogen atoms of the
C4 iPr groups pointing in toward the coordination pocket (red
bulges in the SW and SE quadrants). Because they already
63
as a carbanion. Hence, the lithium complexes 8 and D as well
2+
tBu
64
Dipp
claim an in-planar arrangement to the C N scaffold, the Mg
as [Li(THF)( NacNac)] (K), [Li(THF)( NacNac)]
65
3
2
Dipp
ion is forced to leave this plane to evade steric congestion,
resulting in an upwardly aligned magnesium chloride (MgCl)
bond and dislocation of the metal ion.
(L), and amino-functionalized [Li(Et O)(N- NacNac)]
2
66
(M) were taken into consideration. The latter is based on a
highly electron-rich so-called N-NacNac ligand system that
E
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bears NMe groups at the β-carbon atoms that was reported
In all cases, however, the lone pair on the bridging γ-carbon
atom is assigned an occupation number of only about 1.3
electrons, and second-order perturbation theory indicates
2
only very recently. For these systems, natural population
analysis (NPA) charges were calculated and natural bond
orbital (NBO) as well as natural resonance theory (NRT)
analyses were carried out. All calculations were performed at
the BP86-D3/def2-TZVP level of theory. Some calculations
were repeated at the B3LYP-D3/def2-TZVP level of theory to
ensure consistency of the results, and only minor differences
could be observed (see the SI for details).
strong delocalization toward the N/N′−C
anti bonds.
ipso
Thus, as one would expect, the electronic structure is strongly
delocalized, somewhere between a carbanionic diimino and an
iminoamide structure. While, in principle, the perturbation
interaction energies provide useful insight into the donating
properties, an interpretation based on a single Lewis
representation is not meaningful in this case.
NPA only reveals minor differences between the systems (see
Chart 2). As expected, in all cases, the lithium is almost entirely
Because the single carbanionic structure does not represent
the calculated electronic structure very well, NRT analyses were
performed to estimate the influence of other resonance
structures. Although, for all five systems, the carbanionic
diimino structure is found to have the largest contribution, it
only contributes to a small extent for the NacNac-based
structures K (21.5%), L (12.3%), and M (9.2%) and even less
for D and 8 (8.3% and 7.7%, respectively). Hence, a large
number of small contributions from several resonance
structures has to be taken into account. The summation and
renormalization of contributions for all structures featuring a
lone pair at γ-carbon or one of the coordinating nitrogen atoms
N and N′ lead to much more comparable results (see Table 3).
Chart 2. Calculated NPA Charges for the Selected Lithium
Compounds 8, D, and K−M at the BP86-D3/def2-TZVP
Level
Table 3. Fraction of NRT Resonance Structures Exhibiting a
Carbanionic Diimino (Lone Pair at the Bridging γ-Carbon)
or an Iminoamide (Two Lone Pairs at N or N′) Structure for
8, D, and K−M
ionic, and both the coordinating nitrogen atoms N and N′ as
well as the bridging γ-carbon atom are assigned with significant
negative charges. The two bis(benzoxazol-2-yl)methanide
systems 8 and D show only slightly different charge
distributions, therefore exhibiting similar donating properties.
Still, a stronger asymmetric charge distribution, e.g., at the
coordinating nitrogen atoms, in 8 (−0.609 and −0.631 e to
The NacNac-based compounds K and L show more
contribution from carbanionic structures (24.6% and 19.6%,
Dipp
respectively) than the electron-rich N- NacNac-derived M
(17.4%) and the bis(benzoxazol-2-yl)methanide systems 8 and
D (17.9% and 17.2%, respectively) and slightly less from amidic
structures. While the summation revealed all five systems to be
significantly more amidic than carbanionic, it seems reasonable
to assign M, 8, and D a somewhat more amidic character
compared to K and L. This also indicates that the electron-rich
bis(benzoxazol-2-yl)methanide systems could act as better
donor systems than the “normal” NacNac ones and even the
−
0.640 and −0.651 e in D) is found. Accompanied by
considerably deviating M−N and N−C distances in 8 (see
ipso
Tables 1 and 2), these features indicate a less pronounced
delocalized iminoamide ligand description than that in D or the
highly symmetric NacNac species K and L. Interestingly, a
comparable asymmetric charge distribution was calculated for
the N-NacNac-based species M. An averaged charge of −0.401
e residing at the nitrogen atoms in its ligand periphery fits quite
well to the averaged values of −0.384 and −0.378 e calculated
for the oxygen atoms in 8 and D, indicating an enhanced
conjugation throughout the oxygen atoms in the methanides. In
general, slightly larger charges of average −0.64 e (K), −0.66 e
Dipp
electron-rich N- NacNac systems.
The treatment of 7 with the alkaline-earth metal
hexamethyldisilazanes [M(THF) {N(SiMe ) } ] (M = Mg,
2
3 2 2
Ca, Sr, and Ba) gave the corresponding homoleptic compounds
[M(THF) {(4,6-iPr-NCOC H ) CH} ] [M = Mg, n = 0 (11);
n
6
2 2
2
(
L), and −0.71 e (M) at the coordinating nitrogen atoms
M = Ca, n = 1 (12); M = Sr, n = 1 (13); M = Ba, n = 1 (14)] in
yields of 40−71% (Scheme 6). 11 displays a 4-fold distorted
indicate slightly enhanced donating properties for the NacNac
systems.
tetrahedral MgN arrangement made up by four ring nitrogen
4
Standard NBO analysis for all systems results in a
carbanionic diimino structure as the reference Lewis
representation.
atoms of two monoanionic ligands 7 (Figure 5). The related
higher homologues 12−14 all show an isotype 5-fold
coordination with one additionally attached THF donor
F
DOI: 10.1021/acs.inorgchem.7b02276
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Scheme 6. Syntheses of Compounds 11−14
pronounced metal−π interaction of the cation to the non-
directed π density of the delocalized aromatic system.
Furthermore, in 11−14 also the N−M distances increase
along the row Mg < Ca < Sr < Ba (average values: Mg 2.045 Å;
Ca 2.424 Å; Sr 2.569 Å; Ba 2.733 Å), while the corresponding
N−M−N bite angles decrease with cation size: Mg > Ca > Sr >
Ba [average values: Mg 95.0°; Ca 80.81(6)°; Sr 75.2°; Ba
7
0.83(4)°]. These observations are in tune with increasing
2+
2+
2+
ionic radii from Mg (0.66 Å) over Ca (1.00 Å) to Sr (1.18
Å) and Ba2+ (1.35 Å). Interestingly, in comparison to the
magnesium complex, an elongation of 0.69 Å of the M−N
distance in the barium complex fits quite well to the 0.69 Å
difference regarding their ionic radii. Moreover, a cross-
validation for a given set of bis(4-methylbenzoxazol-2-yl)-
methanide-based complexes [Mg{(4-MeNCOC H ) CH} ]
67
molecule to suit the coordination sphere. With τ parameters of
5
0
.46 (12), 0.48 (13), and 0.37 (14), all three structures adopt a
6
3
2
2
distorted square-pyramidal-based coordination of the dication,
being most pronounced in the barium derivative. Against all
expectations, the ionic radius of 1.35 Å for the Ba ion still is
45
45
(
N) and [Ca(THF) {(4-MeNCOC H ) CH} ] (O) and
2 6 2
3 2
Dipp
2
+
related NacNac-derived structures [M( NacNac) ] [M = Sr
(
2
68
P) and Ba (Q)] reveals the same trends with increasing
not large enough to accommodate a second THF donor
molecule. All four compounds show an exclusive κ2
cation radii for the N−M distances and N−M−N bite angles
like for those in 11−14 (see Table 1). Here even the
coordination by the imine ring nitrogen atoms and a similar
puckering of the six-membered metallaheterocycle. With
butterfly folding angles between 10.76(3) and 13.34(3)°, this
feature is most prominent in the tetrahedral structure 11, while
the square-pyramidal-based species 12−14 exhibit angles
between 1.98(5) and 8.24(4)° and show enhanced planarity
due to the present change in the geometry and an accompanied
decrease in steric crowding. The opposite trend is observed for
metal dislocation from the chelating C N plane, which
dislocation of the cations from the chelating C N plane follows
3
2
a similar tendency, additionally emphasizing the close relation-
ship between both ligand platforms. The same is valid among
related iPr- and tBu-substituted homoleptic β-diketiminate
iPr/tBu
58,69
complexes [M(
NacNac) ] (M = Mg, Ca, Sr, and Ba).
2
2
While 11−14 exhibit solely κ coordination from the imine
nitrogen donors to the metal ions, different motifs are reported
for the related NacNac structures. While the magnesium
complexes in both NacNac structures show κ coordination as
well, in association with the tBu-substituted NacNac ligand, the
heavier alkaline-earth metals (Ca, Sr, and Ba) preferably adopt
3
2
R
2
gradually increases along the line Mg [0.018(2)/0.072(2) Å]
Ca [1.182(3) Å] < Sr [1.247(4)/1.130(4) Å] < Ba [1.281(2)
Å]. Because the C N moiety provides both σ- and π-electron
<
3
2
5
density and the polarizability and softness of the dications rise
η -binding modes consistent with the above-mentioned
2
+
2+
with increasing ionic radii from Mg to Ba , the increasing
out-of-plane arrangement also indicates a more and more
preference of those cations for nondirected π density. In the
2
iPr-substituted systems, the calcium compound maintains κ
Figure 5. Molecular structures of 11−14. Anisotropic displacement parameters are depicted at the 50% probability level. Hydrogen atoms are
omitted for clarity except for those at the bridging methylene positions. Additionally, a lattice hexane molecule also present in the asymmetric unit of
1
1 is omitted as well. Structural data are given in Tables 1, 2, and S1).
G
DOI: 10.1021/acs.inorgchem.7b02276
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
1
13
1
Chart 3. H (left) and C{ H} (right) NMR Shifts of the Homoleptic Complexes: [(THF) M{(4,6-iPr-NCOC H ) CH} ] [M =
n
6
2 2
2
a
Mg, n = 0 (11); M = Ca, n = 1 (12); M = Sr, n = 1 (13); M = Ba, n = 1 (14)]
a
13
1
Measurements were performed in THF-d , except for the C{ H} (C D ) of 12 because of the poor solubility of pure crystalline 12 in THF-d .
8
6
6
8
The stacks of numbers represent the values for the corresponding magnesium, (calcium), [strontium], and {barium} compounds.
coordination and the strontium complex η5 coordination,
respectively, due to decreased steric crowding. Only the barium
species steps out of line, now exhibiting an unprecedented
compounds adopt a C -symmetric geometry in solution,
2v
resulting in one set of chemically and magnetically equivalent
heteroaromatic side arms as well as ligand molecules in each
structure. As summarized in Chart 3, some general trends
2
+
mixed coordination pattern to two Ba cations.
Despite exhibiting a less pronounced out-of-plane arrange-
ment, also the structurally characterized less bulky solvated
alkaline-earth metal aminoiminophosphoranates [M-
2+
2+
within the Mg to Ba series can be derived from the NMR
1
data. Concerning the H NMR shifts for the C−H bridging
position, a continuous decline along the line Mg (5.39 ppm) >
Ca (5.08 ppm) > Sr (5.02 ppm) > Ba (4.85 ppm) can be
reported. Because this resonance is highly indicative for the
corresponding electron density accumulated in the perpendic-
ular p orbital being part of the aromatic system, a stronger
downfield shift descending from Mg2+ to Ba directly
correlates with an enhanced negative charge at this position.
The same trend can be deduced for the aromatic proton at the
C7 position, while all remaining protons in the ligand
periphery, even the iPr methyl groups, exhibit a small but
steady upfield shift. The carbon atoms involved in the six-
membered metallaheterocycle show a comparable tendency like
the proton at the bridging moiety. As was also stated in a
related study on homoleptic alkaline-earth-metal NacNac
complexes, a steady decrease in the chemical shifts in the row
Mg > Ca > Sr > Ba is related to an increasing ionicity
accompanied by an enhanced charge delocalization through the
7
0
(
THF) {(NSiMe ) PPh } ] (M = Be−Ba; n = 0−2) and
n
3 2
2 2
benzamidinates [ML {(NSiMe ) CPh} ] (M = Mg−Ba; L =
n
3 2
2
n
71−74
THF, benzonitrile, and 1,2-dimethoxyethane)
show the
same trends for metal dislocation with increasing cation size as
in 11−14. In the phosphoranates, it increases gradually from
2+
.098 Å for Be2 to 0.557 Å for Ba , while for the latter, values
+
2+
0
2
+
2+
between 0.054 Å for Mg and 0.816 Å for Ba are reported,
indicating a less populated and therefore less attractive π
density in both ligand systems. Again, the above-mentioned
comparable influences on the N−M distances and N−M−N
bite angles are observed as well. However, in comparison to
1
1−14, considerably elongated N−M distances are reported
because of increased steric demand within the narrowed N−P−
N and N−C−N coordination pockets. Interestingly, the group
2
diazasulfinates [M(THF) {(NSiMe ) SPh} ] (M = Ca−Ba),
2
3 2
2
which bear an electron-withdrawing phenyl substituent at the
ligand backbone, show no metal dislocation from the chelating
plane. With an electron-donating substituent at this position in
the corresponding triazasulfites [M(THF) {(NSiMe ) SN-
68
ligand system. Interestingly, concerning the ligand backbone,
n
3 2
only the chemical shifts of the quaternary C4 and C6 ring
carbon atoms seem to be affected by a cation change. On the
one hand, for the C6 carbon, again a decreasing chemical shift
on the way from Mg (144.4 ppm) to Ba (142.8 ppm) can be
reported, while on the other hand, for the corresponding C4
carbon, the opposite trend is observed (Mg 136.9 ppm to Ba
(
SiMe ) } ] (M = Mg, Sr, n = 0−1), again an increasing
3
2 2
dislocation of the metal with increasing ionic radius is
observed.
7
5
1
Additionally, structures 11−14 were studied by H and
1
3
1
C{ H} NMR spectroscopy. All of the recorded spectra show a
distinctive pattern of chemical shifts arising from the
monoanionic ligand scaffold and, in 12−14, from an addition-
ally attached THF donor molecule. In particular, regarding the
1
40.6 ppm). Because the charge accumulation increases with
2+
2+
2+
increasing ionic character along the array Mg < Ca < Sr
<
2+
1
Ba , this observation is in tune with the negative charge being
more and more pushed into the benzene perimeter and
obviously preferably accumulated at the C6 position. The
unique tendencies in the NMR shifts observed within the series
of discussed homoleptic compounds 11−14 emphasize this
method to be indicative to charge distribution analysis.
H NMR spectra of the discussed complexes, the resonances
arising from the ligand molecules show a ratio of
4
:4:2:4:4:24:24 for the aromatic protons (4:4), the bridging
CH moiety (2), and the corresponding CH (4:4) and CH3
24:24) moieties of the iPr substituents at the C4 and C6
positions, respectively. This observation indicates that all four
(
H
DOI: 10.1021/acs.inorgchem.7b02276
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
CONCLUSIONS
tional details for NPA charge calculations and NBO/
NRT analyses, and crystallographic data (PDF)
■
Within this work, the successful seven-step synthesis of a novel
bulky ligand system 7 comprising iPr groups at the pole
positions could be presented. Compound 7 widens the range of
rare examples of bis(benzheterocyclo-2-yl)methanes bearing
substituents close to the 5-fold coordination pocket that
provides a certain NacNac-like shielding to a coordinate metal
cation, which was affirmed by % Vbur calculations. On the way
to precursor complexes for subsequent salt metathesis
reactions, deprotonation of the parent ligand system with
nBuLi and KH afforded the derived alkali compounds 8 and 9.
Subsequent computational electronic structure analyses (NPA,
NBO, and NRT) revealed deprotonated 7 to exhibit a slightly
less pronounced delocalized iminoamide structure but also
slightly enhanced donor capabilities over related “normal”
NacNac and electron-rich N-NacNac systems. Additionally,
with the heteroleptic MgCl complex 10, a promising platform
for subsequent reduction attempts to generate a potentially
Accession Codes
CCDC 1572042−1572047 contain the supplementary crystal-
lographic data for this paper. These data can be obtained free of
charge via www.ccdc.cam.ac.uk/data_request/cif, or by email-
ing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
Corresponding Author
E-mail: dstalke@chemie.uni-goettingen.de. Webpage: www.
uni-goettingen.de/de/53448.html.
■
*
ORCID
Dietmar Stalke: 0000-0003-4392-5751
Notes
+
low-oxidation-state Mg species is provided. Reactions of 7 with
the corresponding alkaline-earth-metal hexamethyldisilazanes
The authors declare no competing financial interest.
[
M(THF) {N(SiMe ) } ] (M = Mg, Ca, Sr, and Ba) yielded
2 3 2 2
ACKNOWLEDGMENTS
■
the correlated homoleptic complexes 11−14. As was also
proposed for their β-diketiminate congeners, the latter might be
seen as potential aspirants for future applications in chemical-
vapor-deposition film growth or as precursors for the synthesis
Thanks go to the Danish National Research Foundation
DNRF93) funded Centre for Materials Crystallography
CMC) for partial support and the federal state of Lower
Saxony, Germany, for providing fellowships in the GAUSS and
CaSuS Ph.D. programs. The work group of Prof. Dr. Ricardo
Mata is duly thanked for providing computational resources for
the electronic structure calculations.
(
(
6
8,69,76
of catalytically active heteroleptic compounds.
All
synthesized complexes were intensively studied by NMR
spectroscopy and/or single-crystal X-ray diffraction analysis,
showing interesting trends and features in the solid state as well
as in solution. In particular, especially the latter four exhibit
distinctive trends with increasing cation size from Mg to Ba :
the N−M distances and metal dislocation from the chelating
C N plane increase along the array Mg < Ca < Sr < Ba, while
the opposite trend is observed for the N−M−N bite angles. All
NMR chemical shifts of the six-membered metallaheterocycle
decline along the row Mg > Ca > Sr > Ba, indicating an
increasing amount of electron density residing within this
perimeter. Additionally, this is accompanied by an increasing
ionic character of the complexes and a resulting enhanced
charge delocalization throughout the ligand framework.
2
+
2+
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Inorganic Chemistry
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