Bis(4-benzhydryl-benzoxazol-2-yl)methane – from a Bulky NacNac Alternative to a Trianion in Alkali Metal Complexes

Article abstract of DOI:10.1002/chem.202100616

A novel sterically demanding bis(4-benzhydryl-benzoxazol-2-yl)methane ligand 6 (^4?BzhH2BoxCH₂) was gained in a straightforward six-step synthesis. Starting from this ligand monomeric [M(^4-BzhH2BoxCH)] (M=Na (7), K (8₁)) and dimeric [{M(^4-BzhH2BoxCH)}₂] (M=K (8₂), Rb (9), Cs (10)) alkali metal complexes were synthesised by deprotonation. Abstraction of the potassium ion of 8 by reaction with 18-crown-6 resulted in the solvent separated ion pair [{(THF)₂K@(18-crown-6)}{bis(4-benzhydryl-benzoxazol-2-yl)methanide}] (11), including the energetically favoured monoanionic (E,E)-(^4-BzhH2BoxCH) ligand. Further reaction of ^4?BzhH2BoxCH₂ with three equivalents KH and two equivalents 18-crown-6 yielded polymeric [{(THF)₂K@(18-crown-6)}{K@(18-crown-6)K(^4-BzhBoxCH)}]_n (n→∞) (12) containing a trianionic ligand. The neutral ligand and herein reported alkali complexes were characterised by single X-ray analyses identifying the latter as a promising precursor for low-valent main group complexes.

Full text of DOI:10.1002/chem.202100616

Full Paper
doi.org/10.1002/chem.202100616
Chemistry—A European Journal
Bis(4-benzhydryl-benzoxazol-2-yl)methane – from a Bulky
NacNac Alternative to a Trianion in Alkali Metal Complexes
Johannes Kretsch,[a] Anne-Kathrin Kreyenschmidt,[a] Timo Schillmöller,[a] Märt Lõkov,[b]
Regine Herbst-Irmer,[a] Ivo Leito,[b] and Dietmar Stalke*[a]
Abstract: A novel sterically demanding bis(4-benzhydryl-
benzoxazol-2-yl)methane ligand 6 (4À BzhH2BoxCH2) was gained
in a straightforward six-step synthesis. Starting from this
ligand monomeric [M(4-BzhH2BoxCH)] (M=Na (7), K (81)) and
dimeric [{M(4-BzhH2BoxCH)}2] (M=K (82), Rb (9), Cs (10)) alkali
metal complexes were synthesised by deprotonation. Ab-
straction of the potassium ion of 8 by reaction with 18-crown-
6 resulted in the solvent separated ion pair [{(THF)2K@(18-
crown-6)}{bis(4-benzhydryl-benzoxazol-2-yl)methanide}] (11),
including the energetically favoured monoanionic (E,E)-(4-
BzhH2BoxCH) ligand. Further reaction of 4À BzhH2BoxCH2 with
three equivalents KH and two equivalents 18-crown-6 yielded
polymeric
[{(THF)2K@(18-crown-6)}{K@(18-crown-6)K(4-
BzhBoxCH)}]n (n!1) (12) containing a trianionic ligand. The
neutral ligand and herein reported alkali complexes were
characterised by single X-ray analyses identifying the latter as
a promising precursor for low-valent main group complexes.
Introduction
benzannulated oxazoline or thiazole sidearms.[12] Due to this
variation of the bridging backbone and therefore connected
residues, multiple symmetrical and asymmetrical ligand plat-
forms were developed (Scheme 1).[13] Although additional
chalcogene donor sites (O or S) as well as an optional donating
bridging unit (N, P or CH) are available for coordination, in most
cases, the oxygen or sulfur do not share the same good Lewis-
donor abilities of the ring nitrogen atoms. Hence a six-
membered NacNac-like coordination motif (C3N2M, M=cationic
metal fragment) is maintained. In recent years the focus has
been on symmetrical bis(benzoxazole-2-yl)methanides I to IV in
Scheme 1 that can be straightforwardly synthesised by cyclo-
condensation reaction of two equivalents of corresponding 2-
aminophenol derivatives and one equivalent of the C3-linker
unit ethylbisimidate dihydrochloride derived from malonic
acid.[14] In general, the synthesised bis(benzoxal-2-yl)methanes
are utilised as neutral or as monoanionic ligands.[12a,15] Deproto-
Over the last decades, monoanionic bidentate N,N’-ligands like
β-diketiminates[1] or amidinates[2] gained interest due to their
relative ease of synthesis and versatility with regards to steric
and electronic properties. Based on these ligands a great variety
of interesting transition metal,[1b,3] main group[4] and
lanthanide[5] compounds were synthesised in the last two
decades. For example, in the field of main group chemistry,
catalytically active β-diketiminate-derived group 13 metal
complexes were synthesised.[6] Further remarkable results were
the synthesis of low-oxidation-state magnesium(I)[7] and alumi-
nium(I)[8] complexes, which were facilitated by bulky β-
diketiminate ligands e.g. DippNacNac (Dipp=2,6-diisopropyl-
phenyl). These findings initiated research into other ligand
platforms, which mimic the coordination abilities of the
ubiquitous NacNac ligand, in particular the k2-N,N-coordination
of the two imine nitrogen atoms to a metal ion to form six-
membered metallaheterocycles. Starting from the bis(2-pyridyl)
methane ligand and its isoelectronically N,[9] P[10] and As[11]
bridged derivatives, the 2-pyridyl moieties were exchanged for
[a] J. Kretsch, A.-K. Kreyenschmidt, T. Schillmöller, Dr. R. Herbst-Irmer,
Prof. Dr. D. Stalke
Institut für Anorganische Chemie, Georg-August-Universität Göttingen
Tammannstraße 4, 37077 Göttingen (Germany)
E-mail: dstalke@chemie.uni-goettingen.de
[b] Dr. M. Lõkov, Prof. Dr. I. Leito
Institute of Chemistry, University of Tartu
Ravila 14a, 50411 Tartu (Estonia)
Supporting information for this article is available on the WWW under
https://doi.org/10.1002/chem.202100616
Scheme 1. Symmetrical bis(heterocyclo)amine (A), -phosphane (B), and
-methane (C+D) ligands. Bis(benzoxazol-2-yl)methanes C (I, II) are (un)
substituted in ortho-position to imine group whereas D is iPr (III) or tBu (IV)
substituted in ortho- and para-position. Asymmetrical and unsubstituted bis
(heterocylo)methanes (E).
© 2021 The Authors. Chemistry - A European Journal published by Wiley-
VCH GmbH. This is an open access article under the terms of the Creative
Commons Attribution License, which permits use, distribution and re-
production in any medium, provided the original work is properly cited.
Chem. Eur. J. 2021, 27, 1–9
1
© 2021 The Authors. Chemistry - A European Journal published by Wiley-VCH GmbH
��
These are not the final page numbers!

Full Paper
doi.org/10.1002/chem.202100616
Chemistry—A European Journal
nation is accomplished with a rather strong Lewis base e.g. n-
butyllithium or potassium hydride.[16] Furthermore, these alkali
metal complexes can be used as precursors in salt elimination
reactions for metal complexes e.g. [X2Al(IÀ )][17] (X=Cl, I). Despite
i
t
of the steric demand provided by Pr- or Bu-residues at C4-
position close to the coordination pocket in III or IV, mimicking
the bulkiness provided by NacNac ligands, low-oxidation or
low-valent Group 2 or 13 elements could so far not be
synthesised. Therefore, we decided to introduce bulkier
benzhydryl groups at the ligand scaffold inspired by com-
pounds containing 2,6-dibenzhydrylphenyl residues e.g. [:MH-
(L)][18] (M=Ge or Sn, (L)=-N(Ar)(SiiPr3), Ar=4-iPr-C6H2Bzh2) or
[M(ArN)2CN=CtBu2][19] (Ar=4-tBu-C6H2Bzh2). In the following
paragraph, the six-step synthesis of bis(4-benzhydryl-benzox-
azol-2-yl)methane (6, 4-BzhH2BoxCH2) and its behaviour in alkali
metal coordination are discussed.
Figure 1. Molecular structure of bis(4-benzhydryl-benzoxazol-2-yl)methane
(6, 4-BzhH2BoxCH2). Anisotropic displacement parameters are depicted at the
50% probability level. All hydrogen atoms are omitted for clarity except for
the bridging (methylene) and benzylic ones. Selected bond lengths [Å] and
°
bond angles [ ]: N1À C7 1.289(2), N2À C9 1.291(2), C7À C8 1.491(2), C8À C9
°
°
1.497(2), C7À C8À C9 115.71(14), N1/2À C7À C8À C9=103.5(2) /À 26.38(9) ; O1/
°
°
2À C7À C8À C9=31.47(10) /À 71.05(19) .
[16b]
°
°
O1/O2-C17-C1-C2=23.05(10) /À 46.69(19) ).
Moreover, the
solid-state structure of 4-BzhH2BoxCH2 (6) reveals intermolecular
hydrogen bonds, in which one of the acidic methylene hydro-
gen atom interacts with the imine nitrogen atom (N1···H8’A
2.49 Å) of an adjacent molecule and vice versa. In contrast,
single crystal structure determination of ligand II (N1···H8’A
2.39 Å, N1···H8’’B 2.63 Å) and III (N1···H8’A 2.58 Å, N1···H8’’B
2.64 Å) exhibit intermolecular CÀ H···N hydrogen bonds of both
methylene hydrogen atoms. In addition, an interaction of the
less acidic benzylic hydrogen atoms (pKa =33)[23] in 6 with the
carbon atom of the benzoxazol-2-yl (C9) unit of a neighbouring
molecule is observed (C9···H16’ 2.81 Å). The acidity of the
methylene protons correlates with the substitution or rather the
electronic properties of related benzoxazole moieties.[13] UV/Vis
spectrophotometric titration experiments[24] in acetonitrile un-
veiled a pKa value of 26.61(6), supporting that 4-BzhH2BoxCH2 is a
somewhat stronger acid than I (26.89(3)), II (27.59(3)) and III
(28.85(3)) but slightly weaker than bis(benzo-thiazol-2-yl)
methane (26.14(3)).[13]
Results and Discussion
Ligand synthesis and characterisation
To obtain the sterically demanding bis(4-benzhydryl-benzoxa-
zol-2-yl)methane (6, 4-BzhH2BoxCH2), 2-amino-3-benzhydrylphenol
was first synthesised according to the synthetic protocols of
Quaranta et al.[20] that were slightly modified (for details of
intermediates 1–5 see Supporting Information). Thereafter, two
equivalents of 2-amino-3-benzhydrylphenol and one equivalent
of ethyl cyanoacetimidate hydrochloride C3-linker unit were
reacted by cyclocondensation reaction.[14,16a,b,21] For this purpose,
°
starting materials were heated in MeOH under reflux (85 C) for
at least 3 d. Ligand 6 and side product 2-(4-benzhydrylbenzox-
azol-2-yl) acetamide (6a) precipitated and were separated by
filtration. Further work-up (see Supporting Information) and
purification by column chromatography on silica gel (THF/
hexane 3:2; Rf (6)=0.9; Rf (6a)=0.48) led to 4-BzhH2BoxCH2 (6) in
decent yields (YLD(6): 36%; overall YLD: 13% ; YLD(6a): 13%).
All starting materials and 4-BzhH2BoxCH2 ligand (6) were analysed
by NMR spectroscopy, mass spectrometry, and elemental
analysis. Crystals of 6 suitable for single XRD experiments were
grown out of a saturated ethyl acetate solution by slow cooling
or by vapor diffusion of pentane. It crystallises in the monoclinic
space group P21/c with one molecule in the asymmetric unit
(Figure 1). The bridging carbon atom C8 is coordinated like
other symmetrically substituted bisheterocyclo methanes[12a,21,22]
in a distorted tetrahedral fashion with a slightly increased C7-
Alkali metal complexes of 4-BzhH2BoxCH2
4-BzhH2BoxCH2 (6) was reacted with alkali metal-based deprotona-
tion or reduction agents to better understand its properties and
establish an appropriate precursor for salt elimination reactions.
All reactions were carried out in toluene at ambient temper-
ature, whereas reaction times and input equivalents of alkali
metal (hydrides) were adjusted to the reaction requirements
(Scheme 2). In the case of bis(4-benzhydryl-benzoxazol-2-yl)
methanide} sodium ([Na(4-BzhH2BoxCH)]) (7), starting material 6
and neat sodium metal were vigorously stirred until all metal
was consumed (5 d). After all volatiles had been removed under
reduced pressure, 7 was isolated as a white powder in excellent
yields (98%). Crystals suitable for single crystal XRD experiments
°
°
C8-C9 angle of 115.71(14) (related ligands: II=110.79(12) to
[12a,21]
°
I=111.23(9) ).
Furthermore, the two benzoxazole moieties
°
are less twisted 63.25(4) relative to each other compared to
related unsubstituted I=89.34(13) [12a] and 4-methyl-substituted
°
[21]
°
II=81.14(4)
which exhibit an almost orthogonal alignment
of the heteroaromatic planes.[12a,21] This is attributed to the
bulkier benzhydryl groups. These groups cause wider torsion
angles between the C3-linker unit and the nitrogen or oxygen
were grown from a saturated toluene solution at À 28 C.
°
Donor-free 7 crystallises in the monoclinic space group P21/n
with one molecule in the asymmetric unit (Figure 2). The solid-
state structure of monomeric [Na(4-BzhH2BoxCH)] (7) exhibits a
distorted k2-N,N’-coordination of the sodium ion by the imine
ring nitrogen atoms. A negligible dislocation of the cation from
°
°
atoms (N1/2-C7-C8-C9=103.5(2) / À 26.38(9) ; O1/2-C7-C8-C9=
t
°
°
31.47(10) /À 71.05(19) ), more similar to 4,6- butyl-substituted III
°
°
°
(twist=76.50(7) ,
N1/N2-C17-C1-C2=À 22.06(9) /133.20(18) ,
Chem. Eur. J. 2021, 27, 1–9
www.chemeurj.org
2
© 2021 The Authors. Chemistry - A European Journal published by Wiley-VCH GmbH
��
These are not the final page numbers!

Full Paper
doi.org/10.1002/chem.202100616
Chemistry—A European Journal
around Na+ ion π-arene, interactions are formed to one phenyl
group of the benzhydryl groups each.[28] This way the benzyl
hydrogen atoms are facing away from the metal. The Na···CAr
distances ranging from 2.8305(18) (Cortho) to 3.0414(18) Å (Cipso
)
indicate a dihaptic arene coordination.[29] 1H NMR spectra of 6
and deprotonated species 7 show a significant deshielding in
toluene and simultaneous downfield shift of the methanide
bridge from δ(À H2CÀ )=3.85 ppm to δ(À HCÀ )=5.38 ppm
whereas benzylic protons experience a significant upfield shift
from δ(À HCPh2)=6.41 ppm to 5.27 ppm. Furthermore, arene
protons (Ph and NCOC6H3) are slightly upfield shifted in sodium
complex 7 compared to ligand 6. Elemental analysis and mass
spectrometry (LIFDI, toluene) confirmed the synthesis of bulk
[Na(4-BzhH2BoxCH)] (7).
Scheme 2. Synthesis of monomeric complexes 7 to 10 were carried out in
toluene and ambient temperature under the following conditions (yiel-
d=YLD): a) 1.04 eq. Na, 5 d, YLD: 98%, b) 1.15 eq. KH, 1 d, YLD: 89%, c)
1.10 eq. Rb, 1 d, YLD: 76%, d) 1.10 eq. Cs, 6 h, YLD: 74%.
The reaction of ligand 6 with potassium hydride (Scheme 2)
resulted in two different modifications of [K(4-BzhH2BoxCH)]n with
n=1 (81) and 2 (82), (YLD: 89%). Colourless crystals were grown
°
from a saturated toluene solution at À 30 C after 1 d. Single
crystal XRD experiments revealed a monomeric as well as a
dimeric species in the solid state. Monomeric [(toluene)K(4-
BzhH2BoxCH)] crystallises in the triclinic space group P1 with one
molecule and half a toluene molecule in the asymmetric unit
(Figure 3). Compound 81 consists of a distorted k2-N,N’-coordi-
nated potassium cation, which features additional weak K···π-
arene interactions to one toluene molecule and one phenyl
group of the benzhydryl moieties, respectively. In comparison
with the sodium complex 6, a fairly prominent deviation of the
cation from the C3N2 plane of 1.571(3) Å is present. Due to the
larger radius of potassium, longer KÀ N (K1À N1 2.7768(18) Å;
Figure 2. Molecular structure of {bis(4-benzhydryl-benzoxazol-2-yl)
methanide} sodium (7, [Na(4-BzhH2BoxCH)]). Anisotropic displacement parame-
ters are depicted at the 50% probability level. All hydrogen atoms are
omitted for clarity except for the bridging (methylene) and benzylic ones.
°
K1À N2 2.6982(18) Å) and a less acute N1À K1À N2 of 70.73(5) are
observed. This is accompanied by an increasing butterfly
°
folding angle to 5.54(8) . Furthermore, potassium phenyl carbon
distances suggest that the cation is η2-(K1À C30 3.467(2) Å,
K1À C31 3.224(2) Å) and η3-coordinated (K1À C23 3.314(2) Å,
K1À C27 3.503(2) Å, K1À C28 3.236(2) Å) by one phenyl residue of
each benzhydryl group (A search in the CCDC version 5.41
the C3N2-plane of 0.117(2) Å and a minor butterfly folding angle
°
of 4.82(6) between the two benzoxazol-2-yl moieties is
observed (Table 1). Measured nitrogen sodium bond lengths of
7 (Na1À N1 2.3392(16) Å; Na1À N2 2.3456(15) Å) are similar to the
Na1À N1 2.358(6) Å bond of the six-membered sodium β-
diketiminate complex [(THF)2NaL][25] (L={N(SiMe3)C(Ph)}2CH) but
are slightly shorter than neutral five-membered TMEDA based
[[{Na{N(SiMe3)(Dipp)}(TMEDA)}2] (Na1À N2 2.4726(17) Å; Na1À N3
2.461(2) Å)[26] or four-membered guadinate complex [[Na{(iPrNC
(NAr)(HNiPr)}(THF)]2 (Ar=2,6-Me2C6H3) (Na1À N1 2.453(1) Å,
Na1À N2 2.558(2) Å).[27] To complete the coordination sphere
(Aug. 2020)[30] for structures with
K and phenyl groups
excluding η6-coordinated ones provided a range of short
contacts between K and Cortho of 2.77 to 3.76 Å).[29c,30] In addition,
a toluene molecule is capping the cation in a η3-fashion
(K1À C43 3.344(2) Å, K1À C44 3.098(2) Å, K1À C45 3.152(2) Å)
which prevents the formation of a coordination polymer,
frequently observed for related potassium complexes based on
I–IV[16a,b,d] or HDippNacNac.[31]
Table 1. Selected bond lengths, distances, and angles of complex 7 to 10.
(Z,Z)-(4-BzhH2BoxCH)
(E,E)-(4-BzhH2BoxCH)
7
81
82
9
10
82
9
10
MÀ N [Å]
2.3392(16),
2.3456(15)
86.53(5)
2.7768(18),
2.6982(18)
70.73(5)
2.812(2) to
3.015(2)
67.57(7),
64.07(6)
1.877(4)
2.282(3)
23.22(9)
2.924(5) to
3.063(4)
65.15(13),
63.34(12)
2.036(7),
2.303(7)
3.050(3) to
3.285(3)
62.76(8) to
63.52(8)
2.302(5) to
2.430(5)
20.65(14),
25.24(13)
MÀ N [Å]
2.775(2),
2.812(2)
3.170(3) to
3.493(3)
2.18(3),
0.85(5)
16.75(8)
2.905(5),
2.948(5)
3.297(6) to
3.633(6)
2.196(9),
0.699(13)
15.55(14)
3.033(3),
3.202(3)
2.603(10) to
1.252(19)
2.60(5) to
1.16(8)
°
NÀ MÀ N [ ]
M···C3N2 [Å]
Folding
M···C3N2 [Å]
M···HC2N [Å]
Folding
0.117(2)
4.82(6)
1.571(3)
5.54(8)
23.63(19)
17.85(11),
15.22(10)
°
°
angle [ ]
angle [ ]
Chem. Eur. J. 2021, 27, 1–9
www.chemeurj.org
3
© 2021 The Authors. Chemistry - A European Journal published by Wiley-VCH GmbH
��
These are not the final page numbers!

Full Paper
doi.org/10.1002/chem.202100616
Chemistry—A European Journal
caesium metal (Scheme 2). All three compounds show a
monoanionic (E,E)-bis(4-benzhydryl-benzoxazol-2-yl)methanide}
moiety coordinating two metal ions in k1,η2-fashion (N1À N2À C9;
N1À N2À C7) by the NacNac-like C3N2 unit (Figure 4A),
respectively.[32] In addition, a single phenyl group of each
benzhydryl substituent forms an additional η6- or η2-
coordination.[28] A second deprotonated (Z,Z)-isomer of 6 (Fig-
ure 4B) bridges both metals in an η3-fashion (C49À C48À N3;
N4À C50À C49). The coordination sphere of the cations is again
satiated by π-arene interactions of the adjacent phenyl groups
(η4 and η2).[28] Moreover, close interactions of the potassium ion
and a suitable arranged methanide group in dimeric 82
culminate in rather short potassium hydrogen distances
(K1À H49 2.89(3) Å, K2À H49 2.77(3) Å) and an elongated CÀ H
Figure 3. Molecular structure of monomeric {bis(4-benzhydryl-benzoxazol-2-
yl)methanide} potassium (81, [K(4-BzhH2BoxCH)]). Anisotropic displacement
parameters are depicted at the 50% probability level. All hydrogen atoms
are omitted for clarity except for the bridging (methylene) and benzylic
ones.
bond to 1.01(3) Å.[33] The KÀ H49-C49 angle of 102(2) for K1 and
°
°
127(2) for K2 also witness these interactions, because they are
similar to e.g. [{K(THF)4}2Ln{(μ-H)2BC8H14}4] (Ln=Eu, Yb) (KÀ H
[34]
°
2.64(4) to 3.37(3) Å, KÀ H-C 117.8 to 120.7 ) or [K(Ar)2] [M
The second species derived from 8 is the dimeric donor-
base free 82 in the monoclinic space group P21/c. The three
toluene lattice molecules in the asymmetric unit do not
coordinate to the metal. Isostructural molecular structures were
observed for the rubidium (9) (YLD: 76%) and caesium (10)
(YLD: 74%) complexes (Figure 4, Table 1). They were synthes-
ised via facile deprotonation of ligand 6 with neat rubidium and
{N(SiMe3)2}3] (M=Mg, Ar=benzene, toluene, p-xylene; M=Zn,
Ar=o-xylene) (KÀ H 2.81 to 3.07 Å, KÀ H-C 85 to 145 ). The
[35]
°
solid-state structures of 9 and 10 also display interactions of
metal ions and methanide bridges, but due to the increased
residual electron density for the heavier homologues the
determination of the freely refined hydrogen atom positions is
not that reliable. Rising effective ionic radii,[36] increasing polar-
izability and softness of the cations are encountered moving
down the alkali metal group on account of the shielding
effect.[28a,37]
These features (Table 1) cause rising metal nitrogen bonds
for deprotonated (Z,Z)- and (E,E)-(4-BzhH2BoxCH) (82: 2.775(2) to
3.015(2) Å; 9: 2.905(5) to 3.063(4) Å; 10: 3.033(3) to 3.285(3) Å),
°
°
°
more acute average NÀ MÀ N angles (82: 65.8 ; 9: 64.3 ; 10: 62.6 )
as well as a growing dislocation of the metal from the C3N2
plane (82: 1.877(4) to 2.282(3) Å; 9: 2.036(7) to 2.303(7) Å; 10:
2.302(5) to 2.430(5) Å) as well as from the À HC2N unit (82:
0.85(5) to 2.18(3) Å; 9: 0.699(13) to 2.196(9) Å; 10: 1.16(8) to
2.60(5) Å). Interestingly, the detected butterfly folding angles in
4-BzhH2
both isomers (Z,Z)-(4-BzhH2BoxCH)�23 ; (E,E)-(
BoxCH)�16 )
°
°
are negligibly affected while coordinating the alkali metal ions.
Additionally, all alkali metal complexes 82–10 were studied by
1H and 13C{1H} NMR spectroscopy in [D8]THF. Recorded H NMR
1
spectra show a distinctive pattern of chemical shifts arising
from the C2v symmetry of monoanionic bis(benzoxazol-2-yl)
methanide scaffold and the benzylic bound protons, whereas
phenylic protons of the benzhydryl groups could not be clearly
assigned due to their peak overlap. Deprotonation of 6 results
in an accumulation of electron density in the benzene periphery
of both benzoxazol-2-yl moieties that again entails a general
shielding of corresponding protons and an upfield shift.
1
Concerning the H NMR shifts for the CÀ H bridging position, a
minute but continuous decline along
K (4.66 ppm)>Rb
(4.65 ppm)>Cs (4.64 ppm) complexes is noticed. The opposite
is found for para positioned protons (3-, 13-H) and benzylic
protons. These protons exhibit a small but noticeable downfield
shift (δ(3-, 13-H)=6.33 ppm to 6.43 ppm; δ(16-, 29-H)=
6.03 ppm to 6.17 ppm) which is in tune with the negative
charge being restored in the phenylic benzhydryl groups
Figure 4. Molecular structure of dimeric {bis(4-benzhydryl-benzoxazol-2-yl)
methanide} alkali metal complexes ([{M(4-BzhH2BoxCH)}2], M=K (82), Rb (9), Cs
(10)). Anisotropic displacement parameters are depicted at the 50%
probability level. All hydrogen atoms are omitted for clarity. Superposition of
82 -10 showing NacNac-like C3N2 unit of (E,E)- (4-BzhH2BoxCH) (A) as well as
twisted (E,E)-(4-BzhH2BoxCH) (B) and their coordinating alkali metal ion,
respectively.
Chem. Eur. J. 2021, 27, 1–9
www.chemeurj.org
4
© 2021 The Authors. Chemistry - A European Journal published by Wiley-VCH GmbH
��
These are not the final page numbers!

Full Paper
doi.org/10.1002/chem.202100616
Chemistry—A European Journal
yielding a growing metal arene interaction. Remaining benzox-
azol-2-yl protons in ortho- and meta-position shifts (4-,12- and
5-, 11-H) do not seem to be influenced by varying alkali cations.
Although 1H NMR spectra of 8–10 ([D8]THF) and sodium
complex 7 ([D8]toluene) were measured in different solvents,
benzylic protons (16-, 29-H) display a remarkable change of the
18-crown-6 was added to the solution. Within seconds a strong
blue fluorescence (λmax =454 nm, λex =350 nm, detail see
Supporting Information) of the dark red solution becomes
entrenched. The reaction mixture was further stirred for 1 d at
ambient temperature, volatiles were removed in vacuum and
4-
the solvent separated ion pair [(THF)2K(18-crown-6)]+
[
average chemical shifts ( δ=0.82 ppm). The 87Rb NMR spec-
BzhH2BoxCH]À (11) was isolated as a reddish solid in excellent
yields (95%). Crystals suitable for single XRD experiments were
grown by vapour diffusion of pentane to a saturated THF
solution of 11 at ambient temperature after 3 d. The colourless
crystals in the triclinic space group P1 contain one bis(4-
benzhydryl-benzoxazol-2-yl)methanide anion and two halve
(THF)2K(18-crown-6)} cations in the asymmetric unit (Figure 5).
In contrast to [(L)K(18-crown-6)] (L=IÀ , IIIÀ )[16c,d,38] that develop
contact ion pairs in the solid, complex 11 was found to form a
solvent separated ion pair. Expectedly, the (Z,Z)-(4-BzhH2BoxCH)
isomer seems to be preferred over the (Z,E/E,Z)- or (E,E)-(4-
BzhH2BoxCH) isomers in the solid-state. In similar manner the
formation of a (Z,Z)-isomer accompanied by only an oxygen
atom coordination at the metal is found in [(IIIÀ )K(18-crown-
6)·H2O].[16c,38] The molecular structure of (Z,Z)-(4-BzhH2BoxCH) (11)
~
trum of 9 from THF solution shows two singlets. Most probably
[{Rb(4-BzhH2BoxCH)}2] rearranges to give a THF-solvated rubidium
cation [Rb(THF)n] (δ=À 1.69 ppm) and
a bis(4-benzhydryl-
benzoxazol-2-yl)methanide} based anion [Rb(4-BzhH2BoxCH)2]À
(δ=À 254.69 ppm). The 133Cs NMR spectrum of 10 displays only
one signal at δ=À 31.12 ppm. Additionally, elemental analysis
and mass spectrometry (LIFDI, THF) confirmed the synthesis of
heavier alkali complexes 8–10.
On the basis of reported (Z,Z)- and (E,E)-isomers of complex
8–10 and based on former gained knowledge of sterically less
demanding bis(benzoxazol-2-yl)methanide potassium complex-
es[16a,b] and their related 18-crown-6 derivatives,[16c,d,38] we were
curious whether the reaction of 8 with 18-crown-6 would prefer
the formation of a (Z,Z), (E,E) or (Z,E) isomeric species. Therefore
bis(4-benzhydryl-benzoxazol-2-yl)methane was first deproto-
nated with an excess of potassium hydride in THF at ambient
temperature (Scheme 3). After the reaction mixture had been
stirred for 1 d, excess of hydride was removed by filtration and
°
displays a butterfly folding angle of 5.03(9) , which is almost
consistent to the folding angles observed in monomeric species
°
°
of 81 (5.54(8) ) as well as 7 (4.82(6) ). This obtuse-angled
benzoxazol-2-zyl moieties and the bond length of C3N2 unit
indicate a fully conjugated system extended throughout the
virtually planar ligand.[39] Based on these findings, the Gibbs
~
free energy differences G of the monoanionic (E,E)-, (Z,E/E,Z)-
or (E,E)-(4À BzhBoxCH) isomers were calculated by density func-
tional theory at the BP86/def2-SVP level (see details in
Supporting Information). Calculated Gibbs free energy differ-
~
ences is for all configurational isomers G<14 kcal/mol. Hence
in solution most likely the permanent rotation around the
methanide linker, accompanied by rearrangement of the
isomers (Scheme 3), is ubiquitous. Moreover, the synthesis of
[{(THF)2K(18-crown-6)}(4-BzhH2BoxCH)] (11) was confirmed by mass
spectrometry (ESI[À ],THF) and elemental analysis.
Since Brown or Buncel and Menon ascertained that
triphenylmethane is deprotonated in THF by potassium hydride
when DMF[40] or 18-crown-6[41] is added,[42] we wanted to
examine whether deprotonation of one or both benzylic
functions can be accomplished via reaction of one or two
equivalents of hydride and crown ether, respectively, to a
solution of 8. However, the reactions of different amounts of KH
and 18-crown-6 all yielded the same complex [{(THF)2K(18-
crown-6)}{K2(4-BzhBoxCH)}]n (n!1) 12 as very sensitive dark
Scheme 3. Reaction of monomeric complexes 8 to solvent separated ion
[(THF)2K(18-crown-6)]+[4-BzhH2BoxCH]À (11) via addition of one equivalent 18-
crown-6. In solution permanent rearrangement of the (Z,Z)-, (Z,E/E,Z)-, and
(E,E)-(4-BzhH2BoxCH) isomers is ubiquitous.
°
purple crystals by storing THF solutions at À 30 C. The crystal
selection and mounting was challenging due to the dark colour
and high sensitivity of compound 12. Thus, a moderate single
crystal XRD data set could be collected after many attempts.
Furthermore, the synthesis of 12 was improved by using ligand
6 and the exact stoichiometric quantities (4-BzhH2BoxCH2:KH:18-
crown-6=1:3:2) (Scheme 4) that are based on the crystal
structure. Immediately after adding the starting materials to a
solution of 6 in THF, a solid blue fluorescence of the dark red
solution was observed. The mixture was stirred at least for 1 d,
whereas a red solid precipitated that was separated from the
Figure 5. Molecular structure of {bis(4-benzhydryl-benzoxazol-2-yl)
methanide} anion from 11, [(THF)2K(18-crown-6)]+[4-BzhH2BoxCH]À . Anisotropic
displacement parameters are depicted at the 50% probability level. All
hydrogen atoms are omitted for clarity except for the bridging (methylene)
°
and benzylic ones. Selected bond lengths [Å] and bond angles [ ]: N1À C7
1.329(2), N2À C9 1.327(2), C7À C8 1.390(2), C8À C9 1.389(2), C9À C8À C7
131.34(15).
Chem. Eur. J. 2021, 27, 1–9
www.chemeurj.org
5
© 2021 The Authors. Chemistry - A European Journal published by Wiley-VCH GmbH
��
These are not the final page numbers!

Full Paper
doi.org/10.1002/chem.202100616
Chemistry—A European Journal
in the plane of the trianion are in good agreement with the
unsolvated k2-N,N-[K(DippNacNac)][31] complex (N1À K1 2.710(3) Å,
N2À K1 2.652(2) Å). The bond lengths to flanking Cipso-atoms are
basically shortened (1.426(4) to 1.478(4) Å) compared to
HCbzhÀ Cipso of complexes 6–11 (1.508(8) Å to 1.538(6) Å) because
of the deprotonation of the two benzylic positions (Cbzh
)
(benzhydryl group). The two anionic C9-atoms are essentially in
plane with the bound Cipso-atoms (C2, C10, C16) similar to
previously
studied
potassium
trityl
complexes
[KCPh3(THF)(PMDTA)],[30] [KCPh3(PMDTA)][43] or [KCPh3(L)]n (L=
diglyme, THF).[43] Angles around C9 deviate slightly from the
Scheme 4. Optimised synthesis of polymeric [{(THF)2K(18-crown-6)}{K(18-
crown-6)K(4-BzhBoxCH)}]n (n!1) (12) by addition of 3.33 equivalents of KH
and two equivalents of 18-crown-6.
typical 120 of sp2-hybridised carbon atoms (for details see
°
Supporting Information), while the steric constraints prevent
the phenyl rings from being coplanar leading to a propeller-like
arrangement. Within this arrangement, angles φ of the phenyl
groups with respect to C2À C10À C16 plane (C3-plane) are φ=
supernatant solution by decantation. The precipitate was
washed with pentane, dried under reduced pressure, and stored
°
°
32.38(17) (C10 to C15), and 49.68(10) (C1 to C6), whereas the
smallest angle and shortest bond (C9À C16 1.426(4) Å) involves
the phenyl system to which the cation is coordinated. These
parameters correspond to an increased π-electron-delocalisa-
tion that is correlated with the overlap of the C3-plane
(C2À C10À C16) and coordinating phenyl ring (C16 to C21) which
is dependent on cos(φ)2 function[30,44] of the corresponding
plane angles. Although the phenyl group is twisted (φ=
°
at À 30 C for further syntheses or analyses (YLD: 95%).
°
Obtained THF solution was cooled to À 30 C for crystallisation.
Complex 12 crystallises in the monoclinic space group P2/n
with half a formula unit in the asymmetric unit (Figure 6). The
coordination polymer comprises solvent-separated potassium
ion (K3) solely coordinated by the oxygen atoms of 18-crown-6
and two THF molecules. This is omitted in Figure 6 for clarity. A
second potassium ion (K2) is η2-coordinated (C4À K2 3.329(3) Å,
C5À K2 3.245(8) Å) to the aryl moieties of two adjacent (E,E)-
trianions and also coordinated by an equatorial crown ether
molecule. The η2-coordination provides the link in the poly-
meric structure. Finally, a third potassium ion in the coordina-
tion pocket is surrounded by two nitrogen atoms in k2-fashion
(NÀ K1 2.686(3) Å) and a phenyl ring of each benzhydryl moiety
in asymmetrically η6-fashion (CPhÀ K1: 2.930(3) Å to 3.291(3) Å),
respectively. Determined nitrogen potassium distances and the
perfect planarity of the ligand scaffold with the potassium ion
°
16.4(2) ) the overlap amounts to about 92%, that is a more
significant overlap than observed for [KCPh3(THF)(PMDTA)][30]
°
(φ=17.7 ,�90%). Further direct analyses of 12 were challeng-
ing due to its low solubility in most solvents (toluene, benzene,
or THF). To find further evidence for the synthesis of trifold
deprotonated anion (4-BzhBoxCH), suspensions of the dark red
precipitate (12) were once again protonated (excess�20 eq.
1
2
H2O or D2O) in small scale (NMR experiment). These H and D
NMR experiments (for details see Supporting Information), as
well as mass spectrometry (ESI[À ] HRMS, THF), confirmed the
previous synthesis of 12.
Conclusion
Within this work, the six-step synthesis and characterisation of a
novel
(
bulky
bis(benzoxazol-2-yl)methane
ligand
6
4À BzhH2BoxCH2) comprising benzhydryl groups at both C4-
positions (ortho imine positions) in spatial proximity to the
coordination pocket was presented. To get a better knowledge
of its properties and find a possible precursor complex for
subsequent salt metathesis reactions, 6 was deprotonated with
alkali metal bases. Obtained products were analysed by NMR
spectroscopy, mass spectrometry as well as single crystal XRD
experiments. Crystals grown from toluene solutions unveiled
monomeric [M(4-BzhH2BoxCH)] (M=Na (7), K (81)) and dimeric
[{M(4-BzhH2BoxCH)}2] (M=K (82), Rb (9), Cs (10)) species in solid
state. Latter alkali metal complexes display distorted k2-N,N-
coordinated (Z,Z)- and an (E,E)-(4-BzhH2BoxCH) configurational
isomers, which display various polyhaptic metal arene inter-
actions. Furthermore, potassium ion sequestration of 8 by 18-
crown-6 resulted in [{(THF)2K(18-crown-6)}(4-BzhH2BoxCH)] (11) a
solvent separated ion pair containing energetically favoured
Figure 6. Molecular structure of [{(THF)2K@(18-crown-6)}{K@(18-crown-6)
K-(4-BzhBoxCH)}]n (n!1) (12). Anisotropic displacement parameters are
depicted at the 50% probability level. All hydrogen atoms are omitted for
clarity except for the bridging (methylene) one. K1À N1 2.686(3), K1À C16
2.930(3), K1À C17 3.084(3), K1À C18 3.255(3), K1À C19 3.291(3), K1À C20
3.154(3), K1À C21 2.993(3), N1À C7 1.324(4), C7À C8 1.393(4), C2À C9 1.478(4),
C9À C10 1.459(5), C9À C16 1.426(4), C4À K2 3.329(3) Å, C5À K2 3.245(8) Å,
N1À K1À N1A 72.78(11), C1À C2À C3 114.2(3), C6À C5À C4 115.1(4),
C15À C10À C11 115.7(3), C17À C16À C21 114.4(3).
Chem. Eur. J. 2021, 27, 1–9
www.chemeurj.org
6
© 2021 The Authors. Chemistry - A European Journal published by Wiley-VCH GmbH
��
These are not the final page numbers!

Full Paper
doi.org/10.1002/chem.202100616
Chemistry—A European Journal
monoanionic (E,E)-(4-BzhH2BoxCH) ligand. Additionally, reaction of
4-BzhH2BoxCH2 with three equivalents KH and two equivalents 18-
crown-6 yielded polymeric [{(THF)2K@(18-crown-6)}{K@(18-
crown-6)K(4-BzhBoxCH)}]n (n!1) (12) featuring a remarkable
Research Council grant (PRG690). Open access funding enabled
and organized by Projekt DEAL.
trianionic ligand. The single crystal X-ray diffraction experiment Conflict of Interest
of 12 revealed one of the potassium ions to be η2-coordinated
by the benzoxazol-2-yl scaffold of two adjacent (E,E)-ligands
plus a crown ether molecule. A second potassium ion is
surrounded by two nitrogen atoms in k2-fashion as well as one
phenyl ring of each benzhydryl moiety in a symmetrically η6-
fashion, respectively. Future work with regard to the mono-
The authors declare no conflict of interest.
Keywords: Ligand design · Metalation · Methanides · NacNac ·
Potassium · s-Block chemistry
(
4-BzhH2BoxCH) and trianionic (4-BzhBoxCH) ligand will focus on the
[1] a) L. Bourget-Merle, M. F. Lappert, J. R. Severn, Chem. Rev. 2002, 102,
3031–3066; b) Y.-C. Tsai, Coord. Chem. Rev. 2012, 256, 722–758.
[2] F. T. Edelmann, Advances in the Coordination Chemistry of Amidinate and
Guanidinate Ligands; Elsevier, Burlington, 2008, p. 184–339.
[3] R. L. Webster, Dalton Trans. 2017, 46, 4483–4498.
[4] M. Asay, C. Jones, M. Driess, Chem. Rev. 2011, 111, 354–396.
[5] a) F. T. Edelmann, D. M. M. Freckmann, H. Schumann, Chem. Rev. 2002,
102, 1851–1896; b) H. C. Aspinall, Chem. Rev. 2002, 102, 1807–1850.
[6] a) P. P. Power, Nature 2010, 463, 171–177; b) Z. Yang, M. Zhong, X. Ma,
S. De, C. Anusha, P. Parameswaran, H. W. Roesky, Angew. Chem. Int. Ed.
2015, 54, 10225–10229; Angew. Chem. 2015, 127, 10363–10367; c) Z.
Yang, M. Zhong, X. Ma, K. Nijesh, S. De, P. Parameswaran, H. W. Roesky,
J. Am. Chem. Soc. 2016, 138, 2548–2551.
coordination of other metal, i.e. those from Group 13 or
lanthanides. The benzhydryl groups attached to the benzox-
azol-2-yl scaffold seem to exhibit similar qualities to the
triphenylmethane, which is able to stabilise carbanionic, radical
or carbonium ionic species owing to the extensive π-delocalisa-
tion. With this in mind, future research might focus on the
synthesis of 4-BzhH2BoxCH2 based radical and carbonium ion
compounds.
[7] S. P. Green, C. Jones, A. Stasch, Science 2007, 318, 1754–1757.
[8] a) C. Cui, H. W. Roesky, H. Hao, H.-G. Schmidt, M. Noltemeyer, Angew.
Chem. Int. Ed. 2000, 39, 1815–1817; Angew. Chem. 2000, 112, 4274–
4276.
[9] a) M. Pfeiffer, F. Baier, T. Stey, D. Leusser, D. Stalke, B. Engels, D. Moigno,
W. Kiefer, J. Mol. Model. 2000, 6, 299–311; b) M. Pfeiffer, A. Murso, L.
Mahalakshmi, D. Moigno, W. Kiefer, D. Stalke, Eur. J. Inorg. Chem. 2002,
2002, 3222–3234.
[10] A. Steiner, D. Stalke, Chem. Commun. 1993, 444–446.
[11] a) H. Gornitzka, D. Stalke, Eur. J. Inorg. Chem. 1998, 3, 311–317; b) A.
Steiner, D. Stalke, Organometallics 1995, 14, 2422–2429.
[12] a) D.-R. Dauer, D. Stalke, Dalton Trans. 2014, 43, 14432–14439; b) D.-R.
Dauer, M. Flügge, R. Herbst-Irmer, D. Stalke, Dalton Trans. 2016, 45,
6136–6148.
Experimental section
General procedures: All reactions were carried out under an
atmosphere of N2 and Ar by Schlenk techniques. Solvents were
distilled from Na, K or NaK before used for synthesis. Starting
materials were purchased commercially and used without further
purification. 1D (1H, 13C, 87Rb, 133Cs) and 2D (1H,13C HSQC,
1H,13C and 1H,15N HMBC) NMR spectroscopic data were recorded
on a Bruker Ascend 500 MHz, 400 MHz, and Avance 300 MHz
spectrometer and referenced to the deuterated solvent (benzene-
d6, thf- d8, toluene- d8).[45] Deuterated benzene and toluene were
°
dried over K (65 C), THF was pre-dried with LiAlH4 and stored over
activated molecular sieve (3 Å) and stored in an argon dry box.
Elemental analyses (C, H, N) were performed on a Vario EL3 at the
Mikroanalytische Labor, Institut für Anorganische Chemie, Univer-
sity of Göttingen. LIFDI-MS spectra were measured on a Jeol
AccuTOF spectrometer and ESI (HR-MS) measurements were
performed on Bruker maXis spectrometer. All pKa measurements in
acetonitrile were carried out as in reference [13]. Shock-cooled
crystals were selected from a Schlenk flask under argon atmosphere
using the X-TEMP2 device.[46] The data were collected with a Mo- (6,
81, 9, 10, 11) or Ag- (7, 82, 12) IμS microfocus source.[47] All data
[13] J. Kretsch, I. Koehne, M. Lõkov, I. Leito, D. Stalke, Eur. J. Inorg. Chem.
2019, 2019, 3258–3264.
[14] H. Ben Ammar, J. Le Nôtre, M. Salem, M. T. Kaddachi, P. H. Dixneuf, J.
Organomet. Chem. 2002, 662, 63–69.
[15] a) R. R. Trenholm, D. E. Ryan, Anal. Chim. Acta 1965, 32, 317–322;
b) M. T. Ramos, C. Avendano, J. Elguero, F. Florencio, J. Sanz-Aparicio,
Inorg. Chim. Acta 1990, 174, 169–173; c) H. A. Elagab, H. G. Alt, Inorg.
Chim. Acta 2015, 437, 26–35; d) A. Hijazi, J.-P. Djukic, L. Allouche, A.
de Cian, M. Pfeffer, X.-F. Le Goff, L. Ricard, Organometallics 2007, 26,
4180–4196.
[16] a) I. Koehne, N. Graw, T. Teuteberg, R. Herbst-Irmer, D. Stalke Inorg,
Chem. 2017, 56, 14968–14978; b) I. Koehne, S. Bachmann, T. Niklas, R.
Herbst-Irmer, D. Stalke, Chem. Eur. J. 2017, 23, 13141–13149; c) I.
Koehne, S. Bachmann, R. Herbst-Irmer, D. Stalke, Angew. Chem. Int. Ed.
2017, 56, 15141–15145; Angew. Chem. 2017, 129, 15337–15342; d) J.
Kretsch, A. Kreyenschmidt, R. Herbst-Irmer, D. Stalke, Dalton Trans.
2018, 47, 12606–12612.
[17] J. Kretsch, R. Herbst-Irmer, D. Stalke, Z. Anorg. Allg. Chem. 2018, 644,
657–660.
[18] T. J. Hadlington, M. Hermann, G. Frenking, C. Jones, J. Am. Chem. Soc.
2014, 136, 3028–3031.
[19] A. K. Maity, S. Fortier, L. Griego, A. J. Metta-Magaña, Inorg. Chem. 2014,
53, 8155–8164.
were integrated with SAINT,[48] and
a multiscan absorption
correction (SADABS[49]) was applied. For structure 6 a 3λ correction
was applied.[50] The structures were solved by direct methods
(SHELXT[51]) and refined on F2 using the full-matrix least-squares
methods of SHELXL[52] within the SHELXLE GUI.[53] The Crystallo-
graphic Information Files (CIF) can be obtained free of charge from
the Cambridge Crystallographic Data Centre via www.ccdc.cam.a-
c.uk/ structures/ using the reference numbers 2031907 (for 6),
2031908 (for 7), 2031909 (for 81), 2031910 (for 82), 2031911 (for 9),
2031912 (for 10), 2031913 (for 11), 2031914 (for 12).
[20] L. Quaranta, O. Corminboeuf, P. Renaud, Org. Lett. 2002, 4, 39–42.
[21] D.-R. Dauer, M. Flügge, R. Herbst-Irmer, D. Stalke, Dalton Trans. 2016, 45,
6149–6158.
[22] a) I. Koehne, R. Herbst-Irmer, D. Stalke, Eur. J. Inorg. Chem. 2017, 2017,
3322–3326; b) H. Gornitzka, D. Stalke, Angew. Chem. Int. Ed. 1994, 33,
693–695; Angew. Chem. 1994, 106, 695–698; c) H. Gornitzka, D. Stalke,
Organometallics 1994, 13, 4398–4405.
[23] R. Breslow, W. Chu, J. Am. Chem. Soc. 1970, 92, 2165.
[24] a) I. Kaljurand, A. Kütt, L. Sooväli, T. Rodima, V. Mäemets, I. Leito, I. A.
Koppel, J. Org. Chem. 2005, 70, 1019–1028; b) A. Kütt, I. Leito, I.
Acknowledgements
We thank the Danish National Research Foundation (DNRF93)
funded Center for Materials Crystallography (CMC) for partial
support. The work at UT was supported by the Estonian
Chem. Eur. J. 2021, 27, 1–9
www.chemeurj.org
7
© 2021 The Authors. Chemistry - A European Journal published by Wiley-VCH GmbH
��
These are not the final page numbers!

Full Paper
doi.org/10.1002/chem.202100616
Chemistry—A European Journal
Kaljurand, L. Sooväli, V. M. Vlasov, L. M. Yagupolskii, I. A. Koppel, J. Org.
Chem. 2006, 71, 2829–2838; c) A. Kütt, S. Tshepelevitsh, J. Saame, M.
Lõkov, I. Kaljurand, S. Selberg, I. Leito, Eur. J. Org. Chem. 2021, 9, 1407–
1419.
[37] T. L. Brown, H. E. LeMay Jr, B. E. Bursten, C. J. Murphy, P. M. Woodward,
Chemistry: The central science; Prentice Hall, Upper Saddle River, NJ,
2003, p. 254–259.
[38] I. Koehne, S. Bachmann, R. Herbst-Irmer, D. Stalke, Angew. Chem. Int. Ed.
2017, 56, 15141–15145; Angew. Chem. 2017, 129, 15337–15342.
[39] a) P. Rademacher in Strukturen organischer Moleküle, Physikalische
Organische Chemie, Wiley-VCH, Weinheim, 1987, p. 55–88; b) P. Müller,
R. Herbst-Irmer, A. L. Spek, T. R. Schneider, M. R. Sawaya in Crytal
Structure Refinement – A Crystallographer’s Guide to SHELXL, Oxford
University Press, New York, 2006, p. 1–202.
[25] Y. Cheng, P. B. Hitchcock, M. F. Lappert, M. Zhou, Chem. Commun. 2005,
752–754.
[26] M. Á. Fuentes, A. Zabala, A. R. Kennedy, R. E. Mulvey, Chem. Eur. J. 2016,
22, 14968–14978.
[27] M. de Tullio, A. Hernán-Gómez, Z. Livingstone, W. Clegg, A. R. Kennedy,
R. W. Harrington, A. Antiñolo, A. Martínez, F. Carrillo-Hermosilla, E. Hevia,
Chem. Eur. J. 2016, 22, 17646–17656.
[40] C. A. Brown, J. Org. Chem. 1974, 39, 3913–3918.
[28] a) T. X. Gentner, R. E. Mulvey, Angew. Chem. Int. Ed. 2020, 59, 2–18;
Angew. Chem. 2020, 132, 2–20; b) J. M. Gil-Negrete, E. Hevia, Chem. Sci.
2021, 12, 1982–1992.
[29] a) C. Lambert, P. von Ragué Schleyer, Angew. Chem. Int. Ed. 1994, 33,
1129–1140; Angew. Chem. 1994, 106, 1187–1199; b) G. Rabe, H. W.
Roesky, D. Stalke, F. Pauer, G. M. Sheldrick, J. Organomet. Chem. 1991,
403, 11–19; c) U. Pieper, D. Stalke, Organometallics 1993, 12, 1201–1206;
d) F. Antolini, P. B. Hitchcock, M. F. Lappert, P. Merle, Chem. Commun.
2000, 1301–1302; e) S.-D. Bai, J.-P. Guo, D.-S. Liu, Dalton Trans. 2006,
2244–2250.
[41] E. Buncel, B. Menon, Chem. Commun. 1976, 648–649.
[42] R. E. Gawley, X. Zhang, Q. Wang in Encyclopedia of reagents for organic
synthesis: potassium hydride; Wiley, Chichester, 1995, p. 1–3.
[43] H. Viebrock, T. Panther, U. Behrens, E. Weiss, J. Organomet. Chem. 1995,
491, 19–25.
[44] a) R. A. Bartlett, H. V. R. Dias, P. P. Power, J. Organomet. Chem. 1988, 341,
1–9; b) A. C. Benniston, A. Harriman, Chem. Soc. Rev. 2006, 35, 169–179;
c) R. M. Williams, Photochem. Photobiol. Sci. 2010, 9, 1018–1026.
[45] Virtuelles Labor II, “http://www.stalke.chemie.uni-goettingen.de/vir-
tuelles labor/nmr/de.html”.
[30] D. Hoffmann, W. Bauer, P. v R Schleyer, U. Pieper, D. Stalke, Organo-
[46] a) T. Kottke, D. Stalke, J. Appl. Crystallogr. 1993, 26, 615–619; b) D. Stalke,
metallics 1993, 12, 1193–1200.
Chem. Soc. Rev. 1998, 27, 171–178.
[31] A. G. Avent, M. R. Crimmin, M. S. Hill, P. B. Hitchcock, J. Organomet.
Chem. 2006, 691, 1242–1250.
[47] T. Schulz, K. Meindl, D. Leusser, D. Stern, J. Graf, C. Michaelsen, M. Ruf,
G. M. Sheldrick, D. Stalke, J. Appl. Crystallogr. 2009, 42, 885–891.
[48] Bruker AXS Inc, SAINT, Madison, 2016.
[49] L. Krause, R. Herbst-Irmer, G. M. Sheldrick, D. Stalke, J. Appl. Crystallogr.
2015, 48, 3–10.
[32] S. Brooker, F. T. Edelmann, T. Kottke, H. W. Roesky, G. M. Sheldrick, D.
Stalke, K. H. Whitmire, Chem. Commun. 1991, 325, 144–146.
[33] a) W. Scherer, V. Herz, A. Brück, C. Hauf, F. Reiner, S. Altmannshofer, D.
Leusser, D. Stalke, Angew. Chem. Int. Ed. 2011, 50, 2845–2849; Angew.
Chem. 2011, 123, 2897–2902; b) W. Scherer, A. C. Dunbar, J. E. Barquera-
Lozada, D. Schmitz, G. Eickerling, D. Kratzert, D. Stalke, A. Lanza, P.
Macchi, N. P. M. Casati, J. Ebad-Allah, C. Kuntscher, Angew. Chem. Int. Ed.
2015, 54, 2505–2509; Angew. Chem. 2015, 127, 2535–2539.
[34] X. Chen, S. Lim, C. E. Plecnik, S. Liu, B. Du, E. A. Meyers, S. G. Shore, Inorg.
Chem. 2005, 44, 6052–6061.
[50] L. Krause, R. Herbst-Irmer, D. Stalke, J. Appl. Crystallogr. 2015, 48, 1907–
1913.
[51] G. M. Sheldrick, Acta Crystallogr. Sect. A 2015, 71, 3–8.
[52] G. M. Sheldrick, Acta Crystallogr. Sect. C 2015, 71, 3–8.
[53] C. B. Hübschle, G. M. Sheldrick, B. Dittrich, J. Appl. Crystallogr. 2011, 44,
1281–1284.
[35] a) I. J. Bruno, J. C. Cole, P. R. Edgington, M. Kessler, C. F. Macrae, P.
McCabe, J. Pearson, R. Taylor, Acta Crystallogr. Sect. B 2002, 58, 389–397;
b) G. C. Forbes, A. R. Kennedy, R. E. Mulvey, B. A. Roberts, R. B. Rowlings,
Organometallics 2002, 21, 5115–5121.
Manuscript received: February 17, 2021
Version of record online: ■■■, ■■■■
[36] R. Shannon, Acta Crystallogr. 1976, 32, 751–767.
Chem. Eur. J. 2021, 27, 1–9
www.chemeurj.org
8
© 2021 The Authors. Chemistry - A European Journal published by Wiley-VCH GmbH
��
These are not the final page numbers!

FULL PAPER
Bis(4-benzhydryl-benzoxazol-2-yl)
methane (4À BzhH2BoxCH2) provides a
versatile platform for monomeric
ligands in [M(4-BzhH2BoxCH)] (M=Na, K)
and dimeric [{M(4-BzhH2BoxCH)}2] (M=K,
Rb, Cs) alkali metal complexes. Even
more remarkable the reaction of
4À BzhH2BoxCH2 with three equivalents
KH and two equivalents 18-crown-6
yields polymeric [{(THF)2K@(18-crown-
6)}{K@(18-crown-6)K(4-BzhH2BoxCH)}]n
(n!1), containing a trianionic ligand.
While the monoanions only provide
N,N-chelation like NacNac the latter
trianion potentially provides C,N,N,C-
chelation and should be another
promising ligand platform.
J. Kretsch, A.-K. Kreyenschmidt, T.
Schillmöller, Dr. M. Lõkov, Dr. R. Herbst-
Irmer, Prof. Dr. I. Leito, Prof. Dr. D.
Stalke*
1 – 9
Bis(4-benzhydryl-benzoxazol-2-yl)
methane – from a Bulky NacNac Al-
ternative to a Trianion in Alkali
Metal Complexes