Salicylaldimines: Formation via Ring Contraction and Synthesis of Mono- And Heterobimetallic Alkali Metal Heterocubanes

Article abstract of DOI:10.1021/acs.inorgchem.0c02920

The formation of salicylaldimine derivatives via ring contraction as byproducts in 2-aminotropone syntheses has been investigated. Salicylaldiminate (SAI) complexes of the alkali metals Li-K have been synthesized and transformed into heterobimetallic complexes. Important findings include an unusual double heterocubane structure of the homometallic sodium SAI, an unprecedented ligand-induced E/Z isomerization of the aldimine functional group in the homometallic potassium SAI, and the first example of a structurally authenticated mixed-metal SAI based on s-block central atoms. Rapid equilibria have been shown to play a crucial role in the solution phase chemistry of mixed-metal SAIs. Analytical techniques applied in this work include (heteronuclear) NMR spectroscopy, VT- and DOSY NMR spectroscopy, high-resolution mass spectrometry, single-crystal X-ray diffraction analysis, and DFT calculations.

Full text of DOI:10.1021/acs.inorgchem.0c02920

pubs.acs.org/IC
Article
Salicylaldimines: Formation via Ring Contraction and Synthesis of
Mono- and Heterobimetallic Alkali Metal Heterocubanes
̈
Anna Hanft, Malte Jurgensen, Laura Wolz, Krzysztof Radacki, and Crispin Lichtenberg*
Cite This: Inorg. Chem. 2020, 59, 17678−17688
Read Online
sı
*
Metrics & More
Article Recommendations
Supporting Information
ACCESS
ABSTRACT: The formation of salicylaldimine derivatives via ring
contraction as byproducts in 2-aminotropone syntheses has been
investigated. Salicylaldiminate (SAI) complexes of the alkali metals
Li−K have been synthesized and transformed into heterobimetallic
complexes. Important findings include an unusual double hetero-
cubane structure of the homometallic sodium SAI, an unprece-
dented ligand-induced E/Z isomerization of the aldimine functional
group in the homometallic potassium SAI, and the first example of a
structurally authenticated mixed-metal SAI based on s-block central
atoms. Rapid equilibria have been shown to play a crucial role in the
solution phase chemistry of mixed-metal SAIs. Analytical techniques
applied in this work include (heteronuclear) NMR spectroscopy,
VT- and DOSY NMR spectroscopy, high-resolution mass
spectrometry, single-crystal X-ray diffraction analysis, and DFT calculations.
Salicylaldimines of type D have a long-standing history as
ligands in coordination chemistry and catalysis.10 For example,
transition metal salicylaldiminate (SAI) complexes show
antimicrobial activity11 and have been exploited as catalysts12
for the polymerization of ethylene13 and cyclic esters,14 in olefin
metathesis,15 and in hydroamination reactions.9b
INTRODUCTION
■
Aminotroponiminates (ATIs) are a well-established ligand
family1 with a rich coordination chemistry,2 a multi-faceted
redox chemistry,3 ligand cooperativity,4 and a variety of catalytic
applications (e.g., alkyne oligomerization,4a olefin hydroamina-
tion,5 and polymerization of cyclic esters2d). A range of
approaches have been developed for the generation of various
types of ATI ligands.1a,d The synthetic route to ATIs with aryl
and alkyl substituents at nitrogen (without excessive steric bulk)
and without further substituents in the ATI backbone is robust,
high-yielding, and probably the one that has been applied most
frequently (Scheme 1a).1a,d,6
The first step of this reaction (1 → A) reflects a substitution
reaction transforming the tropone into an aminotropone. Ring
contraction reactions may compete with this transformation,
when additional substituents are present in the tropone
backbone,7 when transition metal complexes are present,7h or
when high reaction temperatures in combination with bulky
amines are employed (Scheme 1b).8 The nature of the leaving
group X and the substituents in the tropone backbone as well as
the nature of the amine employed as a nucleophile can have a
dramatic impact on the chemo- and regioselectivity of these
reactions.7e By using tropones with bromo substituents in the
backbone, for instance, a substitution-rearrangement pathway
has been addressed deliberately in order to generate bromo-
substituted salen-type ligands for hydroamination catalysis
(Scheme 1c).9 In the widely applied standard protocol for the
synthesis of 2-aminotropones A, however, such rearrangements
have not been reported to date.1a,d,6
Main group complexes and especially alkali metal derivatives
of SAIs are less explored. They have been utilized as SAI transfer
reagents in the synthesis of transition metal complexes,13b,16 as
bases in carboxylation reactions,17 and as catalysts in ring
opening polymerizations (ROP) of cyclic esters.14c,18 In the last
type of reactions, the ligand architecture and the coordination
chemistry of the metal complexes have been shown to critically
influence the catalytic performance of SAI complexes and have
thus been studied in some detail. In the solid state, alkali metal
SAIs may aggregate to give dinuclear,18e,f,l tetranuclear, and
hexanuclear species, corresponding to planar, cu-
bic,13b,14c,17,18b,e,l and double cubic19 core structures, respec-
tively (Scheme 2a). These structural features may be
rationalized in analogy with the ring stacking principle that has
originally been proposed for the analysis of the solid state
structures of lithium compounds.20
Received: October 1, 2020
Published: November 23, 2020
https://dx.doi.org/10.1021/acs.inorgchem.0c02920
© 2020 American Chemical Society
Inorg. Chem. 2020, 59, 17678−17688
17678

Inorganic Chemistry
pubs.acs.org/IC
Article
2b).18g,21,22 In olefin polymerization, the heterobimetallic
species show enhanced catalytic activities or improved
molecular weight control compared to their monometallic
counterparts.21c,22 Heterobimetallic complexes with both metals
being main group metals have so far only been realized in a series
of alkali metal aluminate complexes (Scheme 2c).23 The
intriguing cubic and double cubic structural motifs found in
alkali metal SAI complexes (Scheme 2a) have so far not been
authenticated for heterobimetallic SAI species.
Scheme 1. (a) Synthetic Route to Aminotroponimines (H-
ATIs). (b) Substitution Reaction of Tropone Derivatives
with Amines, Followed by Ring Contraction to Give
Salicylaldimines (H-SAIs). (c) Example of a Double
Substitution at a Brominated Tropone Derivative, Followed
a
by a Ring Contraction
We herein report the formation of salicylaldimines during the
synthesis of unsubstituted 2-aminotropones B (Scheme 1a) and
factors influencing the chemoselectivity of this reaction. Mono-
and heterobimetallic alkali metal SAI complexes were
synthesized, and their structure in the solid state and in solution
was investigated.
RESULTS AND DISCUSSION
■
Salicylaldimines. In the standard protocol for the synthesis
of 2-aminotropones B, the tosyl-oxy-tropone 1 can simply be
stirred in an excess of amine, if the amine is sufficiently
nucleophilic, liquid at room temperature, and affordable in the
required quantities (Scheme 1, route 1). Alternatively,
stoichiometric amounts of an amine are used in the presence
of a base, if the amine is valuable and/or solid at room
temperature (route 2).1a,d,6,24 Many primary alkylamines fulfill
the requirements for the more facile route 1, isopropylamine
being one of the archetypical examples. Consequently, the
reaction of 1 to give 2 is one of the most common reactions in
the preparation of aminotropones and aminotroponimines
(Scheme 3a, left).25 We were therefore surprised to detect a
Scheme 3. Isolation of Ring Contraction Products 3a and 5
a
from Reactions of 1 with H2NiPr and H2NFc, Respectively
a
OTs = tosylate.
Scheme 2. (a) Schematic Presentation of Structural Motifs
Reported for Alkali Metal SAI Complexes. (b, c) Examples of
a
Heterobimetallic SAI Complexes
a
Tos = SO2-p-(C6H4Me); Fc = ferrocenyl; results from conditions A
in part (b) have been reported in ref 3b.
side product in this frequently used synthetic procedure,
whichto the best of our knowledgehad never been
identified in any literature reports. This species was obtained
in small, but reproducible, quantities and could be isolated by
column chromatography. Elemental analysis and high-resolu-
tion mass spectrometry indicated the composition C10H13N1O1;
i.e., it is an isomer of compound 2 (for details, see the
Experimental Section). This left the possibilities of a ring
contraction to give literature-known 3a26 or a cine-substitution
pathway27 to give the unknown seven-membered ring 3b.
a
R = alkyl, aryl; Z = substituent(s) in the aromatic backbone.
Heterobimetallic SAI complexes are rare and comprise
transition metal complexes linked to alkali metals by additional
donor substituents in the ligand periphery (e.g., Scheme
17679
https://dx.doi.org/10.1021/acs.inorgchem.0c02920
Inorg. Chem. 2020, 59, 17678−17688

Inorganic Chemistry
pubs.acs.org/IC
Article
Evaluation of the NMR data of the sample strongly suggested
the formation of 3a,26 which was verified by transformation of 3a
into metalated derivatives and their detailed analysis (vide infra).
The unprecedented observation of a ring contraction in a
standard protocol for the synthesis of 2-aminotropones
prompted us to perform a short survey on the factors influencing
the selectivity of these reactions. As a result, reaction of 1 with
only 1 equiv of isopropylamine at extended reaction times of 7 d
and in the absence of an additional base gave selectivities of
>50% toward 3a at moderate overall conversions (for details, see
the Supporting Information). It should be noted that, under the
applied reaction conditions, isolated 2 did not rearrange to give
compound 3a (for details, see the Supporting Information).
In the light of these findings on the occurrence of ring
contractions during the synthesis of 2-aminotropones via route
1, ferrocenylamine (H2NFc) was also investigated in this
reaction as a typical representative of a less nucleophilic and
more valuable primary amine (route 2). In this case, the product
of a ring contraction (compound 528) was obtained in low yield,
only when the reaction was performed in the absence of an
external base (Scheme 3b; for details, see the Experimental
Section and, for NMR spectroscopic reaction monitoring, see
the Supporting Information).
These results show that the ring contraction reactions
identified here only play a minor role under standard conditions
for the synthesis of aminotropones, but may become reaction
pathways of significant importance when the reaction conditions
are slightly modified. This is an important point to keep in mind
(i) for the synthesis of yet unknown aminotropones (and ATIs
derived thereof) and (ii) for the exploitation of synthetic
strategies that deliberately address such ring contraction
reactions in order to generate salicylaldimines that may be
more difficult to synthesize via more established synthetic
routes.9
For the potassium SAI crown ether complex 8-crown, NMR
spectroscopy revealed the presence of a second signal set of
minor intensity (3% upon solvation of the sample). This was
ascribed to an E/Z isomerism of the aldimine functional group
with the main species being the E-isomer.30 This is supported by
analysis of the 1H1H-NOESY NMR spectrum of this compound:
a cross signal between the CH moiety of the isopropyl group and
the aromatic CH group in the 3-position is absent for the major
isomer, but present for the minor isomer. A trans configuration
of the Caryl−Caldimine group was tentatively assigned to both
isomers based on cross signals between the aldimine CH group
and the ethylene groups of the crown ether.
For aldimines in general, E/Z isomerism is a well-known
phenomenon, with the isomer distribution depending on
electronic and steric factors as well as solvent effects.31
Transformations between the two isomers can be triggered
thermally or photochemically.32 However, E/Z isomerism has
only rarely been discussed for salicylaldiminate complexes and
not been unambiguously identified.33 Most importantly, the
isomerism was not observed for compounds 6−8; in other
words, it can be triggered by addition of the strong chelating
ligand 18-crown-6. This is most likely due to a switch of the SAI
ligand from a chelating coordination mode in the absence of 18-
crown-6 to a κ1O coordination mode in the presence of 18-
crown-6 (vide infra). In agreement with these findings,
irradiating solutions of 8-crown with a mercury vapor lamp
for 2 h increased the amount of the Z-isomer to 11%. Storing the
same sample at ambient light for 2 d gave back the initial 3% of
the Z-isomer. Heating the sample to 60 °C for 2 h or shielding
from ambient light for 7 d resulted in formation of the E-isomer,
exclusively. No decomposition of the sample was observed
during the above-mentioned manipulations.
Single-crystal X-ray analysis of the lithium complex 6 revealed
the literature-known Li4SAI4 structural motif,13b although single
crystals were grown under various conditions, including the use
of polar solvents such as pyridine.
Alkali Metal Salicylaldiminates (SAIs). The synthesis of
alkali metal salicylaldiminates (SAIs) was targeted in order to
explore their potential in the generation of mixed-metal
compounds. Thus, compound 3a was reacted with alkali metal
bases MHMDS (Scheme 4; M = Li−K; HMDS =
Slow diffusion of pentane into a THF solution of the sodium
compound 7 yielded crystals suitable for single-crystal X-ray
diffraction analysis. Compound 7 crystallized as a hexanuclear
species with a Na6O6 core structure, which is built of two face-
fused Na4O4 heterocubanes (triclinic space group P1 with Z = 2;
Figure 1). The structure can be thought of as being composed of
three Na2SAI2 units, which are equivalent to two outer faces and
one inner face of the double heterocubane. The outer face units
are linked to the inner face unit only through Na−O interactions
(not through Na−N bonding). The SAI ligands adopt chelating
coordination modes with a sodium atom in the N/O binding
pocket. At the same time, the oxygen atoms of the ligands that
are part of the outer face units bridge three sodium atoms, while
each of the oxygen atoms that is part of the central Na2SAI2 unit
bridges four sodium atoms. As a result, the sodium atoms of the
outer faces are four-coordinate and adopt a strongly distorted
tetrahedral coordination geometry (O−Na(1/2/4/5)−O/N,
81.4−154.1°).34 The inner face sodium atoms are five-
coordinate and adopt a slightly distorted square pyramidal
coordination geometry (Na3, τ = 0.06, O3 in the apical position;
Na6, τ = 0.05, O6 in the apical position). The four-coordinate
sodium atoms (Na1, Na2, Na4, and Na5) show short Na−O and
Na−N bond lengths (Na−O, 2.23−2.38 Å; Na−N, 2.32−2.36
Å) as compared to the five-coordinate sodium atoms Na3 and
Na6 (Na−O, 2.33−2.46 Å; Na−N, 2.44 Å).
Scheme 4. Synthesis of Alkali Metal Salicylaldiminate
a
Complexes 6, 7, 8, and 8-crown
a
∗: Traces of THF (0.02 equiv) and toluene (0.04 equiv) were
detected.
hexamethyldisilazide). Isolation and characterization of the
literature-known lithium species 613b confirmed the structural
assignment of starting material 3a obtained from the ring
contraction reaction of 1 (cf. Scheme 3a). The sodium and the
potassium compounds 7 and 8 were isolated as colorless solids
in good yields. NMR spectroscopic analysis of 6−8 in pyridine-
d5 revealed the expected signal patterns for the isopropyl, the
aldimine, and the 1,2-substituted aryl functional groups with
clearly distinct chemical shifts for each of the complexes.29
While heterocubane structures of sodium SAIs have been
reported in several instances,14c,17,18b,e,35 double heterocubane
17680
https://dx.doi.org/10.1021/acs.inorgchem.0c02920
Inorg. Chem. 2020, 59, 17678−17688

Inorganic Chemistry
pubs.acs.org/IC
Article
Figure 2. Molecular structure of [K(SAI)(18-crown-6)] 8-crown in the
solid state. All atoms except the ethylene units of the crown ether, which
are shown as wire frame, are represented by displacement ellipsoids at
the 50% probability level. Hydrogen atoms are omitted for clarity.
Selected bond lengths (Å) and angles (deg): K1−O1, 2.501(3); K1−
O2, 2.847(3); K1−O3, 2.841(2); K1−O4, 2.874(3); K1−O5,
2.808(3); K1−O6, 2.863(3); K1−O7, 2.866(3); O1−C1, 1.282(4);
C2−C7, 1.458(5); N1−C7, 1.271(4); C1−C2, 1.428(5); C2−C3,
1.406(5); C3−C4, 1.379(5); C4−C5, 1.394(5); C5−C6, 1.364(5);
C1−C6, 1.430(5); O(1−7)−K1−O(1−7), 58.35(7)−161.49(7); C1−
O1−K1, 164.1(2).
Figure 1. Molecular structure of [Na6(SAI)6] 7 in the solid state (left)
and schematic presentation (right). Heteroatoms are represented by
displacement ellipsoids at the 50% probability level; carbon atoms are
shown as wireframe. Hydrogen atoms and a lattice bound solvent
molecule are omitted for clarity. Selected bond lengths (Å) and angles
(deg): Na1−N1, 2.3305(13); Na1−O1, 2.2284(11); Na1−O2,
2.2844(11); Na1−O6, 2.3835(11); Na2−N2, 2.355(3); Na2−O1,
2.2773(12); Na2−O2, 2.2375(11); Na2−O3, 2.3178(11); Na3−N3,
2.4416(13); Na3−O2, 2.3659(11); Na3−O3, 2.3252(11); Na3−O4,
2.4019(11); Na3−O6, 2.4324(11); Na4−N4, 2.3181(13); Na4−O3,
2.3636(11); Na4−O4, 2.2287(13); Na4−O5, 2.2799(12); Na5−N5,
2.3371(19); Na5−O4, 2.2852(12); Na5−O5, 2.2364(11); Na5−O6,
2.3276(11); Na6−N6, 2.437(2); Na6−O1, 2.3688(13); Na6−O3,
2.4621(11); Na6−O5, 2.3382(11); Na6−O6, 2.3289(11); O1−C1,
1.3019(18); O2−C11, 1.3067(17); O3−C21, 1.3176(17); O4−C31,
1.3044(17); O5−C41, 1.3061(17); O6−C51, 1.3211(18); O(1−6)−
Na(1/2/4/5)−O(1−6), 88.98(4)−96.84(4); O(1−6)−Na(1/2/4/
5)−N(1/2/4/5), 81.44(8)−154.13(7); O(1−6)−Na(3/6)−O(1−6),
84.84(4)−169.95(4); O(1−6)−Na(3/6)−N(5/6), 76.61(4)−
166.82(7).
(18-crown-6)], where the iPr group of 8-crown is formally
substituted by a Ph group (K−O, 2.62 Å; C−O−K, 130.9°).18h
This was ascribed to the electron-donating character of the iPr
group in 8-crown.
Mixed-Metal Salicylaldiminates (SAIs). Mixed-metal
compounds, in general,38 and mixed-metal compounds of s-
block elements, in particular, have been shown to display a
complex coordination chemistry in solution and in the solid
state, with high potential for applications such as metalation
reactions,39 cyclization reactions,40 (reversible) de-aromatiza-
tion reactions,41 olefin oligomerization30 as well as the
polymerization of olefins30,42 and cyclic esters.2d,42b We aimed
to evaluate the potential of simple SAI ligands without additional
functional groups for the synthesis of heterobimetallic
complexes. Thus, two series of reactions were performed with
compounds 6−8: (i) reactions of two alkali metal SAIs in a 1:1
stoichiometry (Table 1, entries 1−3) and (ii) reactions of the
structural motifs are extremely rare for sodium SAIs.36 In fact,
only one such example has been reported in the literature, but
contains additional thioether functional groups, which leads to
larger coordination numbers of the sodium atoms (CN = 5−6)
and larger Na−O bond lengths of up to 2.62 Å (as compared to
an upper limit of 2.46 Å for the Na−O bond lengths in 7).19
Thus, the structural analysis of 7 proves that the unusual double
heterocubane structural motif can be realized for sodium SAIs
without the aid of additional chelating functional groups.37
Potassium SAI complexes have been reported to be mono-,
di-, or tetranuclear in the solid state.18e,f,h Mononuclear species
can be obtained, for instance, by complexation of the potassium
atom with 18-crown-6. Since our attempts to obtain single
crystals of compound 8 were unsuccessful under various
conditions, crystallization in the presence of 18-crown-6 was
performed. Slow diffusion of pentane into a THF solution
containing 8 and 1 equiv of 18-crown-6 led to crystals suitable
for single-crystal X-ray diffraction analysis. 8-crown crystallized
in the monoclinic space group P21 with Z = 2 (Figure 2). In
accordance with the literature, a mononuclear complex is
formed, and the coordination sphere of the potassium atom is
saturated by the crown ether and the oxygen atom of the SAI
ligand, which coordinates in a κ1O fashion.
Table 1. Synthesis of Heterobimetallic Complexes 9−13
a
Isolated yield, conversion quantitative.
The Caryl−Caldimine group shows a trans configuration, which
minimizes steric repulsion between the iPr substituent and the
K(18-crown-6) moiety. The CN double bond shows an E-
configuration. These findings confirm the structure suggested
for the main isomer of 8-crown in solution (vide supra). The
K1−O1 bond length of 2.50 Å is significantly smaller, and the
C1−O1−K1 angle of 164.1° is much larger than the
corresponding values in the closely related species [K(SAIPh)-
lithium compound 6 and the sodium compound 7 in 1:3, and 3:1
stoichiometries (entries 4 and 5). The products 9−13 of these
reactions were isolated as colorless solids. In all cases, NMR
spectroscopic analysis (1H, 13C; 7Li, 23Na where appropriate) at
23 °C revealed one set of well-resolved signals with the expected
multiplicities and with chemical shifts, which did not correspond
to the weighted average of the chemical shifts recorded for the
pure monometallic complexes. The NMR spectra of all
17681
https://dx.doi.org/10.1021/acs.inorgchem.0c02920
Inorg. Chem. 2020, 59, 17678−17688

Inorganic Chemistry
pubs.acs.org/IC
Article
compounds 9−13 remained unchanged, when solutions of these
complexes were kept under inert conditions for several days.
Slow evaporation of the solvent of a THF solution of complex
9 with its Li/Na = 1:1 stoichiometry led to the formation of
crystals suitable for single-crystal X-ray diffraction analysis
(orthorhombic space group Pna21 with Z = 4). Unexpectedly,
the structural analysis revealed a complex with a Li/Na = 3:1
stoichiometry, [Li3(SAI)3Na(SAI)(thf)2] (12·(thf)2).43 Com-
pound 12·(thf)2 shows a heterocubane core structure (Figure
3), confirming the formation of the first mixed alkali metal SAI
complex.
The isolation of 12·(thf)2 from a solution of compound 9
raised the question of dynamic exchange reactions in such
heterobimetallic species. In order to gain first insights into the
aggregation behavior of and potential interconversion between
these compounds, DFT calculations were performed (for
details, see the Experimental Section and Supporting Informa-
tion). According to these studies, the addition of 2 equiv of THF
to 9 and 12, respectively, is exothermal and exergonic in both
cases (Scheme 5a). The dissociations of tetranuclear species 9·
(thf)2 and 12·(thf)2 into dinuclear species are thermoneutral
(for 9·(thf)2) and exothermic (for 12·(thf)2) as well as slightly
endergonic in both cases (Scheme 5b). Thus, these trans-
formations represent a potential entry into rearrangement
reactions of this type of heterocubanes. The formation of
“lithium-rich” compound 12·(thf)2 from compound 9·(thf)2
requires the appearance of one or more “sodium-rich”
byproducts. The formation of isolable homometallic sodium
complex 7 fulfills this requirement and results in the reaction
equation given in Scheme 5c. This reaction is marginally
endothermic and exergonic, demonstrating that such rearrange-
ment reactions are thermodynamically feasible.
Scheme 5. Thermodynamic Data for Reactions of
Heterocubanes [Li2Na2(SAI)4] (9) and [Li3Na1(SAI)4] (12)
a
As Determined by DFT Calculations
Figure 3. Molecular structure of [Li3Na(SAI)4(thf)2] (12·(thf)2) in the
solid state. Heteroatoms are represented by displacement ellipsoids at
the 50% probability level; carbon atoms are shown as wireframe.
Hydrogen atoms are omitted for clarity. Selected bond lengths (Å) and
angles (deg): Na1−O2, 2.545(2); Na1−O3, 2.469(2); Na1−O4,
2.3270(19); Na1−O5, 2.400(2); Na1−O6, 2.453(14); Na1−N4,
2.514(2); Li1−O1, 1.943(5); Li1−O2, 1.962(4); Li1−O4, 1.974(4);
Li1−N1, 2.037(4); Li2−O1, 2.029(4); Li2−O2, 1.930(5); Li2−O3,
1.914(4); Li2−N2, 2.030(5); Li3−O1, 2.005(4); Li3−O3, 1.995(5);
Li3−O4, 2.020(4); Li3−N3, 2.049(4); O1−C1, 1.310(3); O2−C11,
1.298(3); O3−C21, 1.306(3); O4−C31, 1.316(3); N1−C7, 1.282(3);
N2−C17, 1.277(4); N3−C27, 1.279(4); N4−C37, 1.275(4); O(2−
6)−Na1−(O2−6), 74.67(6)−169.0(3); O(2−6)−Na1−N4,
76.88(7)−152.84(7); O(1−4)−Li(1−3)−O(1−4), 92.21(19)−
104.6(2); O(1−4)−Li(1−3)−N(1−3), 91.77(18)−135.4(2).
The coordination geometries are distorted tetrahedral for
lithium atoms and distorted octahedral for the sodium center
with its two thf ligands. The structure of 12·(thf)2 is related to
that of the parent homometallic lithium complex 613b by
exchange of one Li atom for one [Na(thf)2] unit. This leads to a
significant distortion of the M4O4 cube in 12·(thf)2 due to the
larger Na−O bond lengths. In addition, the incorporation of a
sodium atom into the heterocubane also increases the Li···Li
interatomic distances to 2.68−2.72 Å, as compared to 2.55−2.69
Å in homometallic 6.13b The Li−O (1.91−2.03 Å) and Li−N
bond lengths (2.03−2.05 Å) in 12·(thf)2 are only marginally
larger than those in 6 (Li−O, 1.89−2.00 Å; Li−N, 1.97−2.00
Å).13b The Na1−OSAI (2.33−2.55 Å) and Na1−N4 (2.51 Å)
bonds in 12·(thf)2 are longer than those in homometallic 7 (CN
= 4: Na−O, 2.23−2.38 Å; Na−N, 2.32−2.36 Å; CN = 5: Na−O,
2.33−2.46 Å; Na−N, 2.44 Å), which was ascribed to the higher
coordination number of the sodium atom in the heterobimetallic
species.
a
[Li] = Li(SAI); [Na] = Na(SAI); L = thf.
The potential of compound 9 to be susceptible to dynamic
exchange reactions in solution motivated more detailed
investigations of the solution behavior of lithium/sodium
mixed-metal SAI complexes (all of which were performed in
THF). Low-temperature NMR spectroscopy on compound 9
revealed one sharp set of signals even at −100 °C, suggesting the
exchange indicated by the isolation of 12·(thf)2 from solutions
of 9 and by DFT calculations (vide supra) to be fast on the time
scale of the NMR spectroscopic experiment (Figure S23). In
contrast, the mixed-metal complexes 12 and 13 revealed
significant broadening of all resonances in THF solution at
17682
https://dx.doi.org/10.1021/acs.inorgchem.0c02920
Inorg. Chem. 2020, 59, 17678−17688

Inorganic Chemistry
pubs.acs.org/IC
Article
Table 2. Investigation of Compounds 6, 7, 9, 12, and 13 by DOSY NMR Spectroscopy in THF-d8 at Room Temperature: Diffusion
Coefficients (D) and Molecular Masses (M) Using THF-d7 as Internal Standard and Assuming That Complexes Behave as
Compact Spheres
examples of species at different degrees of aggregation that may be formed from the parent complex in solution
#
Cpd. dinuclear species (Mcalcd [g·mol−1]) tetranuclear species (Mcalcd [g·mol−1]) hexanuclear species (Mcalcd [g·mol−1]) log(Dx,norm
)
M
exp [g·mol−1
]
1
2
3
4
5
6
7
9
12
13
Li2(SAI)2(thf)2 (483)
Na2(SAI)2(thf)2 (515)
Li2(SAI)2(thf)2 (483)
Li2(SAI)2(thf)2 (483)
Na2(SAI)2(thf)2 (515)
Li4(SAI)4 (677)
Na4(SAI)4 (741)
Li2Na2(SAI)4(thf)2 (853)
Li3Na(SAI)4(thf)2 (837)
LiNa3(SAI)4(thf)3 (941)
−9.0878
−9.1422
−9.0949
−9.1087
−9.1443
526
678
544
580
685
Na6(SAI)6 (1111)
Na6(SAI)6 (1111)
Na6(SAI)6 (1111)
Na6(SAI)6 (1111)
−40 °C. Using the CHaldimine resonance as a diagnostic tool, at
least two (compound 12)44 and three (compound 13) different
species were present under these conditions (Figures S24 and
S25).33a This suggests that these compounds exist as mixtures of
rapidly interconverting aggregates in solution, leading to an
averaged set of sharp resonances in their NMR spectra recorded
at room temperature. The homometallic complexes 6 and 7 each
showed one set of signals at −100 °C, which were significantly
broadened in the case of the sodium compound.
In order to gather information on the (average) degree of
aggregation of these compounds in solution, complexes 6, 7, 9,
12, and 13 were subjected to DOSY NMR spectroscopic
experiments in THF at room temperature. In each case, one set
of signals was detected in the diffusion dimension. Using an
external calibration curve with normalized diffusion coeffi-
cients,45 the diffusion coefficients were translated into molecular
weights (Table 2). The molecular weights determined with this
method range between 526 and 685 g·mol−1. In agreement with
the results from DFT calculations and VT NMR spectroscopy,
this rules out the exclusive presence of tetranuclear or even
hexanuclear species. It is rather in line with the presence of rapid
equilibria between dinuclear (solvated) species and tetranuclear
(solvated) species (or a hexanuclear species in cases where the
homometallic sodium species may also be formed).
High-resolution mass spectrometric analyses of 6, 7, 9, 12,
and 13 confirmed the presence of alkali metal SAI species with
degrees of aggregation ranging from one to five and multiple
different species being detected in each individual sample (for
details, see the Supporting Information). This is in contrast to
alkali metal SAIs with additional substituents in ortho- and para-
positions (relative to the oxygen atom), which have been
suggested to be dinuclear based on ESI mass spectrometry.18f
It should be noted that mixed-metal compounds with Li:Na
ratios of 1:3 in heterocubane structural motifs have been
reported when a guanidinate ligand was used instead of SAI
ligands,46 suggesting that modifications of the ligand framework
will allow access to a wide variety of s-block mixed-metal
heterocubane complexes.
isomerization of the aldimine functional group can be triggered
by addition of 18-crown-6 as a strong chelating ligand. Using
alkali metal SAIs as starting materials (Li−K), the potential of
simple SAI ligands to support the formation of mixed-metal
compounds was demonstrated, and the first example of a mixed-
metal SAI complex of s-block metals was structurally
authenticated. These species exhibit a dynamic solution
behavior: investigations by (VT) NMR and DOSY NMR
spectroscopy, as well as single-crystal X-ray analysis, mass
spectrometry, and DFT calculations, demonstrate that these
compounds engage in rapid equilibria in solution at ambient
temperature, which involve different degrees of aggregation
(most likely between two and six). These findings in the growing
domain of higher aggregate mixed-metal compounds will
contribute to the understanding of complex equilibria in
solution and to the deliberate design of well-defined compounds
in this field of research.
EXPERIMENTAL SECTION
■
All air- and moisture-sensitive manipulations were carried out using
standard vacuum-line Schlenk or glovebox techniques in an atmosphere
of purified argon. Solvents were degassed and purified according to
standard laboratory procedures. NMR spectra were recorded on Bruker
instruments operating at 200, 300, 400, or 500 MHz with respect to 1H.
1H and 13C NMR chemical shifts are reported relative to SiMe4 using
1
the residual H and 13C chemical shifts of the solvent as a secondary
standard. 7Li NMR and 23Na NMR chemical shifts are reported relative
to 1 M LiCl in D2O and 1 M NaCl in D2O. 15N NMR chemical shifts are
reported relative to CH3NO2 (90% in CDCl3) and were determined by
two-dimensional 1H−15N correlation NMR spectroscopic experiments.
If not otherwise noted, NMR spectra were recorded at 23 °C. UV/vis
spectra were recorded with a Mettler Toledo UV5 UV Vis spectrometer
in THF solution. IR spectra were measured with a Bruker Alpha-P FT-
IR instrument, and spectra of neat solid material were recorded in all
cases. Elemental analyses were performed on a Leco or a Carlo Erba
instrument. For mass spectrometric analyses, an Exactive plus
instrument (Thermo Scientific) was used (ionization methods:
LIFDI, ESI, ASAP). Single crystals suitable for X-ray diffraction were
coated with perfluorinated polyether oil in a glovebox, transferred to a
nylon loop, and then transferred to the goniometer of a diffractometer
equipped with a molybdenum (λ = 0.71073 Å) or copper (λ = 1.5418
Å) X-ray tube. The structures were solved using the intrinsic phasing
method, completed by Fourier synthesis, and refined by full-matrix
least-squares procedures. CCDC 2034962−2034964 contain the
crystallographic information for this work. For an example of a general
method for the synthesis of salicylaldimines, see ref 26a.
CONCLUSION
■
It has been demonstrated that, in reactions that are commonly
employed for the synthesis of aminotroponiminate ligand
systems, salicylaldimines are formed in minor amounts via ring
contraction. Modification of the reaction conditions can
significantly increase the selectivity toward ring contraction,
turning it into a major reaction pathway. Metalation of
salicylaldimines gave the corresponding alkali metal salicylaldi-
minates (SAIs). Structural analysis of the sodium complex
revealed the first example of an unsupported double
heterocubane motif in this family of compounds. Investigation
of the potassium compound uncovered that a reversible E/Z
Computational Details. DFT calculations were performed with
the Gaussian program using the 6-31G(d,p)47 (H, Li, C, N, O) and the
6-311G(d,p)48 (Na) basis sets and the B3LYP functional.49 The D3
version of Grimme’s dispersion model with the original D3 damping
function was applied.50 Frequency analyses of the reported structures
showed no imaginary frequencies. Thermodynamic parameters were
calculated at a temperature of 298.15 K and a pressure of 1.00 atm.
Further details are given in the Supporting Information. Cartesian
17683
https://dx.doi.org/10.1021/acs.inorgchem.0c02920
Inorg. Chem. 2020, 59, 17678−17688

Inorganic Chemistry
pubs.acs.org/IC
Article
coordinates of optimized structures are provided in .xyz format
(Supporting Information).
1H NMR (400 MHz, THF-d8): δ = 1.06 (d, 6H, 3JHH = 6.4 Hz, CH3),
1.76−1.79 (m, n × 4H, β-THF), 3.26 (sept, 1H, 3JHH = 6.4 Hz, CHiPr),
3.60−3.63 (m, n × 4H, α-THF), 6.19 (dd, 1H, 3JHH = 8.2 Hz, 3JHH = 6.2
Hz, 4-H), 6.46 (d, 1H, 3JHH = 8.3 Hz, 6-H), 6.89−6.93 (m, 1H, 5-H),
Compounds 2 and 3a.6a Isopropylamine (27.8 g, 470 mmol, 40.3
mL) was cooled to 0 °C. 2-Tosyloxytropone (10.0 g, 36.2 mmol) was
added in small portions while stirring. The mixture was allowed to warm
to room temperature and was stirred at room temperature overnight.
All volatiles were removed under reduced pressure, and the crude
product was purified by column chromatography (silica; EtOAc:Pen-
tane 1:4, 3 vol % NEt3). The product 2 was obtained as a yellow
crystalline solid. Yield: 5.19 g, 31.8 mmol, 88%. Analytical data were in
agreement with the literature.6a
7.03 (dd, 1H, 3JHH = 7.7 Hz, 4JHH = 1.7 Hz, 3-H), 8.19 (s, 1H, HAldimine
)
ppm. 1H NMR (400 MHz, pyridine-d5): δ = 1.17 (d, 6H, 3JHH = 6.4 Hz,
CH3), 1.60−1.64 (m, n × 4H, β-THF), 3.28 (sept, 1H, 3JHH = 6.3 Hz,
CHiPr), 3.65−3.68 (m, n × 4H, α-THF), 6.54 (dd, 1H, 3JHH = 8.4 Hz,
3JHH = 7.2 Hz, 4-H), 6.96 (d, 1H, 3JHH = 8.4 Hz, 6-H), 7.24−7.28 (m,
1H, 5-H), 7.03 (dd, 1H, 3JHH = 7.6 Hz, 4JHH = 2.1 Hz, 3-H), 8.55 (s, 1H,
HAldimine) ppm. 13C NMR (100 MHz, THF-d8): δ = 24.59 (s, CH3),
63.36 (s, CHiPr), 110.64 (s, 4-C), 123.09 (s, 6-C), 123.86 (s, 2-C),
131.91 (s, 5-C), 135.54 (s, 3-C), 164.64 (s, CAldimine), 172.71 (s, 1-C)
ppm. 23Na NMR (106 MHz, THF-d8): δ = 5.93 ppm.
During column chromatography, compound 3a was isolated as a
brown oil. Yield: 183 mg, 1.1 mmol, 3%.
1H NMR (300 MHz, CDCl3): δ = 1.29 (d, 6H, 3JHH = 6.4 Hz, CH3),
3.55 (sept, 1H, 3JHH = 6.4 Hz, CHiPr), 6.86 (dt, 1H, 4JHH = 1.1 Hz, 3JHH
Elemental analysis: calcd (%) for C10H12NaNO·(C4H8O)0.1 (192.41
g/mol): C 64.92, H 6.71, N 7.21; found: C 64.85, H 6.84, N 7.00.
Compound 8. 2-Isopropyliminomethylphenol (3a) (60 mg, 0.37
mmol) and potassium HMDS (74 mg, 0.37 mmol) were dissolved in
THF (1 mL). Immediately, a white precipitate was formed. Pentane (6
mL) was added, and the precipitate was isolated by filtration. The white
solid was washed with pentane (4 × 5 mL) and dried in vacuo. The
product was obtained as a white solid. The amount of THF in the
isolated compound has to be checked individually for every batch.
Yield: 60 mg, 0.29 mmol (with n = 0.1 equiv of THF), 78%.
= 7.4 Hz, 4-H), 6.95 (td, 1H, 3JHH = 8.3 Hz, 6-H), 7.23 (dd, 1H, 4JHH
=
3
1.7 Hz, JHH = 7.6 Hz, 3-H), 7.27−7.31 (m, 1H, 5-H), 8.35 (s, 1H,
HAldimine), 13.68 (s, 1H, OH) ppm. 13C NMR (76 MHz, CDCl3): δ =
24.31 (s, CH3), 60.14 (s, CHiPr), 117.13 (s, 6-C), 118.52 (s, 4-C),
118.96 (s, 2-C), 131.18 (s, 3-C), 132.09 (s, 5-C), 161.43 (s, 1-C),
162.12 (s, CAldimine) ppm. Elemental analysis: calcd (%) for C10H13NO
(163.22 g/mol): C 73.59, H 8.03, N 8.58; found: C 73.69, H 8.32, N
8.64. ASAP-MS, positive mode (%): calcd for [C10H13NO + H+]+, m/z
= 164.1070; found: m/z = 164.1064.
1H NMR (400 MHz, pyridine-d5): δ = 1.24 (d, 6H, 3JHH = 6.4 Hz,
CH3), 1.61−1.64 (m, n × 4 H, β-THF), 3.29−3.35 (sept, 1H, 3JHH = 6.1
Hz, CHiPr), 3.65−3.68 (m, n × 4 H, α-THF), 6.50 (dd, 1H, 3JHH = 7.9
Hz, 3JHH = 6.2 Hz, 4-H), 7.01 (d, 1H, 3JHH = 8.0 Hz, 6-H), 7.25−7.30
Compounds 4 and 5. (In contrast to the literature protocol for the
synthesis of 4, no NEt3 was used here).3b
Ethanol (40 mL) was added to a mixture of 2-tosyloxytropone (3.03
g, 10.9 mmol) and aminoferrocene (2.20 g, 10.9 mmol). The reaction
mixture was heated under reflux for 4 h. All volatiles were removed in
vacuo. The crude reaction product was purified by column
chromatography. The product 4 was obtained as a dark orange solid.
Yield: 1.27 g, 4.16 mmol, 38%. Analytical data were in agreement with
the literature.3b
(m, 1H, 5-H), 8.00 (d, 1H, 3JHH = 7.5 Hz, 3-H) 9.04 (s, 1H, HAldimine
)
ppm. 13C NMR (100 MHz, pyridine-d5): δ = 25.73 (s, CH3), 26.26 (s,
β-THF), 63.29 (s, CHiPr), 68.29 (s, α-THF), 109.09 (s, 4-C), 123.78 (s,
overlay with signal of pyridine-d5, 6-C),124.95 (s, 2-C), 132.17 (s, 3-C),
132.74 (s, 5-C), 162.03 (s, CAldimine), 174.76 (s, 1-C) ppm. Elemental
analysis: calcd (%) for C10H12KNO·(C4H8O)0.1 (208.52 g/mol): C
59.90, H 6.19, N 6.72; found: C 59.71, H 6.12, N 6.72.
During column chromatography, compound 5 was isolated as a red
solid. Yield: 140 mg, 457 μmol, 4%.
Compound 8-crown. 2-Isopropyliminomethylphenol (3a) (50
mg, 0.31 mmol) and potassium HMDS (61 mg, 0.31 mmol) were
dissolved in THF (3 mL). Immediately, a white precipitate was formed.
18-crown-6 was added, and the resulting light yellow solution was
layered with pentane (1 mL). After 24 h, a beige solid had precipitated,
was isolated by filtration, washed with pentane (4 × 2 mL), and dried in
vacuo. The product was obtained as a beige solid. Yield: 131 mg, 0.28
mmol, 92%.
1H NMR (200 MHz, CDCl3): δ = 4.19 (s, 5H, C5H5), 4.29 (t,
2H,3JHH = 1.9 Hz, 3,4-(C5H4NHR)), 4.60 (t, 2H,3JHH = 1.9 Hz, 2,5-
(C5H4NHR)), 6.87−6.95 (dd, 3JHH = 8.1 Hz, 3JHH = 6.8 Hz, 1H, 4-H),
3
6.99 (d, 1H, JHH = 8.1 Hz, 6-H), 7.29−7.39 (m, 2H, 3-H, 5-H
(overlapping)), 8.63 (s, 1H, HAldimine), 13.37 (s, 1H, OH) ppm. 13C
NMR (50 MHz, CDCl3): δ = 62.60 (s, 2,5-(C5H4NHR)), 67.73 (s, 3,4-
(C5H4NHR)), 70.00 (s, C5H5), 102.37 (s, 1-(C5H4NHR), 117.31 (s, 6-
C), 119.10 (s, 4-C), 119.85 (s, 2-C), 131.16 (s, 3-C), 132.19 (s, 5-C),
160.20 (s, 1-C), 160.93 (s, CAldimine) ppm. ESI, positive mode (%):
calcd for [C17H16FeNO + H+]+, m/z = 306.0576; found: m/z =
306.0578.
Signals of the E-isomer (97%): 1H NMR (500 MHz, pyridine-d5): δ
= 1.34 (d, 6H, 3JHH = 6.3 Hz, CH3), 3.48 (s, 24H, CH2 crown), 3.60−
3.68 (sept, 1H, 3JHH = 6.3 Hz, CHiPr), 6.41−6.44 (m, 1H, 4-H), 7.05−
7.08 (m, 1H, 6-H), 7.34−7.38 (m, 1H, 5-H), 8.50 (m, 1H, 3-H) 9.74 (s,
1H, HAldimine) ppm. 13C NMR (126 MHz, pyridine-d5): δ = 26.10 (s,
CH3), 62.73 (s, CHiPr), 70.71 (s, CH2 crown), 107.31 (s, 4-C),124.50
(s, 6-C), 125.21 (s, 2-C),128.38 (s, 3-C), 132.58 (s, 5-C), 160.75 (s,
CAldimine), 176.25 (s, 1-C) ppm.
Compound 6.13b 2-Isopropyliminomethylphenol (3a) (115 mg,
0.71 mmol) and lithium HMDS (117 mg, 0.71 mmol) were dissolved in
THF (1 mL) and stirred for 10 min. All volatiles were removed in vacuo,
and the remaining white solid was washed with a mixture of toluene and
pentane (1:1, 2 × 4 mL) and dried in vacuo. The product was obtained
as a white solid. Yield: 88 mg, 0.52 mmol, 74%.
Signals of the Z-isomer (3%): 1H NMR (500 MHz, pyridine-d5): δ =
1.36 (d, 6H, 3JHH = 6.1 Hz, CH3), 3.48 (s, 24H, CH2 crown), 4.76−4.83
(sept, 1H, 3JHH = 6.1 Hz, CHiPr), 6.39−6.42 (m, 1H, 4-H), 7.05−7.08
(m, 1H, 6-H), 7.35−7.40 (m, 1H, 5-H), 7.58 (m, 1H, 3-H (overlapping
with signal of pyridine-d5)) 9.38 (s, 1H, HAldimine) ppm. 13C NMR (126
MHz, pyridine-d5): δ = 25.79 (s, CH3), 51.02 (s, CHiPr), 68.29 (s, CH2
crown), 105.93 (s, 4-C),124.17 (s, overlay with signal of pyridine-d5, 6-
C), 124.64 (s, 2-C), 130.08 (s, 3-C),131.90 (s, 5-C), 161.84 (s,
CAldimine), 175.98 (s, 1-C) ppm.
1H NMR (400 MHz, pyridine-d5): δ = 1.17 (d, 6H, 3JHH = 6.4 Hz,
CH3), 3.25−3.34 (sept, 1H, 3JHH = 6.3 Hz, CHiPr), 6.67 (dd, 1H, 3JHH
=
7.8 Hz, 3JHH = 6.5 Hz, 4-H), 7.15 (d, 1H, 3JHH = 8.5 Hz, 6-H), 7.40−
7.43 (m, 2H, 3-H, 5-H), 8.28 (s, 1H, HAldimine) ppm. 13C NMR (100
MHz, pyridine-d5): δ = 25.36 (s, CH3), 62.98 (s, CHiPr), 112.01 (s, 4-
C), 123.53 (s, 6-C), 123.77 (s, 2-C), 133.35 (s, 5-C), 136.47 (s, 3-C),
7
165.17 (s, CAldimine), 171.59 (s, 1-C) ppm. Li NMR (155.7 MHz,
pyridine-d5): δ = 2.64 ppm. Further analytical data were in agreement
Elemental analysis: calcd (%) for C23H36KNO7 (465.63 g/mol): C
56.75, H 7.79, N 3.01; found: C 56.93, H 8.00, N 2.94.
with the literature.13b
Compound 7. 2-Isopropyliminomethylphenol (3a) (100 mg, 0.61
mmol) and sodium HMDS (112 mg, 0.61 mmol) were dissolved in
THF (1 mL) and stirred for 10 min. All volatiles were removed in vacuo,
and the remaining white solid was washed with a mixture of toluene and
pentane (1:1, 2 × 4 mL) and dried in vacuo. The product was obtained
as a white solid. The amount of THF in the isolated compound has to be
checked individually for every batch. Yield: 90 mg, 0.49 mmol (with n =
0.02 equiv of THF and m = 0.04 equiv of toluene), 79%.
Compound 9. A solution of 6 (8.7 mg, 51 μmol) in THF (1 mL)
was added to a solution of 7·(thf)0.14 (10 mg, 51 μmol) in THF (1 mL).
All volatiles were removed in vacuo. The product was obtained as a
white solid, which was washed with pentane (2 × 2 mL) and dried in
vacuo.
Yield: 11 mg, 31 μmol, 61%.
1H NMR (400 MHz, pyridine-d5): δ = 1.17 (d, 12H, 3JHH = 6.4 Hz,
CH3), 3.26−3.32 (sept, 2H, 3JHH = 6.3 Hz, CHiPr), 6.55 (dd, 2H, 3JHH
=
17684
https://dx.doi.org/10.1021/acs.inorgchem.0c02920
Inorg. Chem. 2020, 59, 17678−17688

Inorganic Chemistry
pubs.acs.org/IC
Article
8.6 Hz, 3JHH = 7.0 Hz, 4-H), 6.85 (d, 2H, 3JHH = 8.1 Hz, 6-H), 7.19 (m,
overlay with signal of pyridine-d5, 2H,5-H), 7.35 (d, 2H, 3JHH = 7.6 Hz,
3-H) 8.27 (s, 2H, HAldimine) ppm. 13C NMR (100 MHz, pyridine-d5): δ
= 25.28 (s, CH3), 63.10 (s, CHiPr), 110.89 (s, 4-C), 123.79 (s, overlay
with signals of pyridine-d5, 6-C),124.03 (s, overlay with signals of
pyridine-d5, 2-C), 132.70 (s, 5-C), 136.03 (s, overlay with signals of
pyridine-d5, 3-C), 164.51 (s, CAldimine), 172.66 (s, 1-C) ppm. 7Li NMR
(156 MHz, pyridine-d5): δ = 2.78 (s) ppm. 23Na NMR (106 MHz,
pyridine-d5): δ = 3.79 (br. s) ppm.
vacuo. The amount of THF in the isolated compound has to be checked
individually for every batch. Yield: 32 mg, 43 μmol (with n = 0.06 equiv
of THF and 0.13 equiv of pentane), 73%.
1H NMR (400 MHz, pyridine-d5): δ = 1.17 (d, 24H, 3JHH = 6.4 Hz,
CH3), 1.61−1.64 (m, n × 4 H, β-THF), 3.24−3.33 (sept, 4H, 3JHH = 6.3
Hz, CHiPr), 3.65−3.68 (m, n × 4 H, α-THF), 6.52−6.56 (dd, 4H, 3JHH
=
8.6 Hz, 3JHH = 7.0 Hz, 4-H), 6.90 (d, 4H, 3JHH = 7.8 Hz, 6-H), 7.22−
7.26 (m, overlay with signals of pyridine-d5, 5-H,), 7.44 (d, 4H, 3JHH
=
7.7 Hz, 3-H) 8.39 (s, 4H, HAldimine) ppm. 13C NMR (100 MHz,
pyridine-d5): δ = 25.33 (s, CH3), 26.26 (s, β-THF), 63.20 (s, CHiPr),
68.29 (s, α-THF), 110.59 (s, 4-C), 124.13 (s, 6-C),124.24 (s, overlay
with signals of pyridine-d5, 2-C), 132.66 (s, 5-C), 135.8 (s, overlay with
signals of pyridine-d5, 3-C), 164.51 (s, CAldimine), 173.29 (s, 1-C) ppm.
7Li NMR (156 MHz, pyridine-d5): δ = 2.80 (s) ppm. 23Na NMR (106
MHz, pyridine-d5): δ = 5.68 (br. s) ppm. Elemental analysis: calcd (%)
for C40H48LiNa3N4O4·(C4H8O)0.1 (731.97 g/mol): C 66.29, H 6.72, N
7.65; found: C 66.34, H 6.87, N 7.36.
Elemental analysis: calcd (%) for C20H24LiNaN2O2 (354.35 g/mol):
C 67.79, H 6.83, N 7.91; found: C 67.81, H 6.99, N 7.87.
Compound 10. A solution of 6 (8 mg, 48 μmol) in THF (2 mL)
was added to a solution of 8·(thf)0.08 (10 mg, 48 μmol) in THF (2 mL).
All volatiles were removed in vacuo. The product was obtained as a
white solid, which was washed with pentane (2 × 2 mL) and dried in
vacuo. Yield: 17 mg, 46 μmol, 96%.
1H NMR (400 MHz, pyridine-d5): δ = 1.22 (d, 12H, 3JHH = 6.3 Hz,
CH3), 3.24−3.34 (sept, 2H, 3JHH = 6.3 Hz, CHiPr), 6.54 (dd, 2H, 3JHH
=
7.5 Hz, 3JHH = 5.8 Hz, 4-H), 6.91 (d, 2H, 3JHH = 8.4 Hz, 6-H), 7.25 (dd,
overlay with signal of pyridine-d5, 2H, 3JHH = 10.2 Hz, 3JHH = 6.9 Hz 5-
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.inorgchem.0c02920.
■
sı
H), 7.39 (d, 2H, 3JHH = 7.5 Hz, 3-H) 8.32 (s, 2H, HAldimine) ppm. 13
C
NMR(100 MHz, pyridine-d5): δ = 25.29 (s, CH3), 63.47 (s, CHiPr),
110.46 (s, 4-C), 123.85 (s, overlay with signals of pyridine-d5, 6-
C),123.87 (s, overlay with signals of pyridine-d5, 2-C), 132.83 (s, 5-C),
135.71 (s, overlay with signals of pyridine-d5, 3-C), 164.24 (s, CAldimine),
172.89 (s, 1-C) ppm. 7Li NMR (156 MHz, pyridine-d5): δ = 3.99 (s)
ppm. Elemental analysis: calcd (%) for C20H24KLiN2O2 (370.46 g/
mol): C 64.84, H 6.53, N 7.56; found: C 64.66, H 6.56, N 7.38.
Compound 11. A solution of 7·(thf)0.02(toluene)0.04 (9.0 mg, 48
μmol) in THF (1 mL) was added to a solution of 8·(thf)0.08 (10 mg, 48
μmol) in THF (1 mL). All volatiles were removed in vacuo. The product
was obtained as a white solid, which was washed with pentane (2 × 2
mL) and dried in vacuo. The amount of THF in the isolated compound
has to be checked individually for every batch. Yield: 15 mg, 38 μmol
(with n = 0.15 equiv of THF), 79%.
Experimental and computational details, as well as NMR
spectra (PDF)
Coordinates of optimized structures obtained from DFT
calculations (XYZ)
Accession Codes
CCDC 2034962−2034964 contain the supplementary crystallo-
graphic 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 Cam-
bridge Crystallographic Data Centre, 12 Union Road, Cam-
bridge CB2 1EZ, UK; fax: +44 1223 336033.
1H NMR (400.1 MHz, pyridine-d5): δ = 1.18 (d, 12H, 3JHH = 6.4 Hz,
CH3), 1.61−1.64 (m, n × 4 H, β-THF), 3.23−3.32 (sept, 2H, 3JHH = 6.3
Hz, CHiPr), 3.65−3.68 (m, n × 4 H, α-THF), 6.53 (dd, 2H, 3JHH = 7.8
Hz, 3JHH = 6.6 Hz, 4-H), 6.93 (d, 2H, 3JHH = 8.4 Hz, 6-H), 7.19 (dd,
overlay with signal of pyridine-d5, 2H, 3JHH = 8.7 Hz, 3JHH = 6.8 Hz 5-
H), 7.60 (d, overlay with signal of pyridine-d5, 2H, 3JHH = 7.7 Hz, 3-H)
8.59 (s, 2H, HAldimine) ppm. 13C NMR (100 MHz, pyridine-d5): δ =
25.41 (s, CH3), 26.26 (s, β-THF), 63.41 (s, CHiPr), 68.29 (s, α-THF),
110.46 (s, 4-C), 123.94 (s, overlay with signals of pyridine-d5, 6-
C),124.49 (s, 2-C), 132.74 (s, 3-C), 134.94 (s, 5-C), 163.93 (s,
CAldimine), 173.68 (s, 1-C) ppm. 23Na NMR (106 MHz, pyridine-d5): δ
= 8.79 (br. s) ppm. Elemental analysis: calcd (%) for C20H24KNaN2O2·
(C4H8O)0.08 (392.28 g/mol): C 62.23, H 6.35, N 7.12; found: C 62.23,
H 6.39, N 6.95.
AUTHOR INFORMATION
Corresponding Author
■
Crispin Lichtenberg − Department of Inorganic Chemistry,
Julius-Maximilians Universität, Würzburg, 97074 Würzburg,
Germany; orcid.org/0000-0002-0176-0939;
Email: crispin.lichtenberg@uni-wuerzburg.de
Authors
Anna Hanft − Department of Inorganic Chemistry, Julius-
Maximilians Universität, Würzburg, 97074 Würzburg,
Germany
Compound 12. Compound 6 (30 mg, 177 μmol) was added to a
solution of compound 7·(toluene)0.13 (12 mg, 59 μmol) in THF (2
mL). All volatiles were removed in vacuo. The product was obtained as a
white solid, which was washed with pentane (2 × 2 mL) and dried in
vacuo. Yield: 24 mg, 35 μmol, 59%.
Malte Jurgensen − Department of Inorganic Chemistry, Julius-
̈
Maximilians Universität, Würzburg, 97074 Würzburg,
Germany
Laura Wolz − Department of Inorganic Chemistry, Julius-
Maximilians Universität, Würzburg, 97074 Würzburg,
Germany
Krzysztof Radacki − Department of Inorganic Chemistry,
Julius-Maximilians Universität, Würzburg, 97074 Würzburg,
Germany
1H NMR (400 MHz, pyridine-d5): δ = 1.17 (d, 24H, 3JHH = 6.0 Hz,
CH3), 3.25−3.34 (sept, 4H, 3JHH = 6.2 Hz, CHiPr), 6.61 (m, 4H, 4-H),
6.96 (br. s, 4H, 6-H), 7.29 (br. m, 4H,5-H), 7.36 (d, 4H, 3JHH = 7.4 Hz,
3-H) 8.26 (s, 4H, HAldimine) ppm. 13C NMR (100 MHz, pyridine-d5): δ
= 25.28 (s, CH3), 63.01 (s, CHiPr), 111.45 (s, 4-C), 123.74 (s, overlay
with signals of pyridine-d5, 6-C),123.91 (s, overlay with signals of
pyridine-d5, 2-C), 132.96 (s, 5-C), 136.24 (s, overlay with signals of
pyridine-d5, 3-C), 164.83 (s, CAldimine), 172.09 (s, 1-C) ppm. 7Li NMR
(156 MHz, pyridine-d5): δ = 2.66 (s) ppm. 23Na NMR (106 MHz,
pyridine-d5): δ = 3.12 (br. s) ppm.
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.inorgchem.0c02920
Author Contributions
Elemental analysis: calcd (%) for C40H48Li3NaN4O4 (692.66 g/
mol): C 69.36, H 6.99, N 8.09; found: C 69.67, H 7.20, N 7.99.
Compound 13. Compound 6 (10 mg, 59 μmol) was added to a
solution of compound 7·(toluene)0.13 (33 mg, 177 μmol) in THF (2
mL). All volatiles were removed in vacuo. The product was obtained as a
white solid, which was washed with pentane (3 × 5 mL) and dried in
The manuscript was written through contributions of all
authors. All authors have given approval to the final version of
the manuscript.
Notes
The authors declare no competing financial interest.
17685
https://dx.doi.org/10.1021/acs.inorgchem.0c02920
Inorg. Chem. 2020, 59, 17678−17688

Inorganic Chemistry
pubs.acs.org/IC
Article
Torres, M. R.; Elguero, J. Solid-state structure and tautomerism of 2-
aminotroponimines studied by X-ray crystallography and multinuclear
NMR spectroscopy. Eur. J. Org. Chem. 2004, 2004, 4452−4466.
(c) Dochnahl, M.; Lohnwitz, K.; Pissarek, J. W.; Biyikal, M.; Schulz, S.
R.; Schon, S.; Meyer, N.; Roesky, P. W.; Blechert, S. Intramolecular
hydroamination with homogeneous zinc catalysts: evaluation of
substituent effects in N,N’-disubstituted aminotroponiminate zinc
complexes. Chem. - Eur. J. 2007, 13, 6654−66.
(7) (a) Nozoe, T.; Kitahara, Y.; Masamune, S.; Yamaguchi, S.
Tropone and Its Derivatives. IV Rearrangement Reactions of 2, 4, 7-
Tribromotropone-1 by Alkaline Reagents. Proc. Jpn. Acad. 1952, 28,
85−88. (b) Nozoe, T.; Seto, S.; Takeda, H.; Morosawa, S.; Matsumoto,
K. Tropone and Its Derivatives. (V) On 2-Aminotropones. (2). Proc.
Jpn. Acad. 1952, 28, 192−197. (c) Nozoe, T.; Seto, S.; Matsumura, S.
Tropone and Its Derivatives. VIII On the Reaction of Halotropones.
Proc. Jpn. Acad. 1952, 28, 483−487. (d) Pauson, P. L. Tropones and
Tropolones. Chem. Rev. 1955, 55, 9−136. (e) Mukai, T. Reaction of 2-
Chloro-3-phenyltropone and 2-Chloro-7-phenyltropone with Ammo-
ACKNOWLEDGMENTS
■
Generous financial support by the Fonds der Chemischen
Industrie (Liebig fellowship to C.L.) and the DFG is gratefully
acknowledged. C.L. thanks Prof. Holger Braunschweig for
continuous support.
REFERENCES
■
(1) (a) Dias, H. V. R.; Wang, Z.; Jin, W. Aminotroponiminato
complexes of silicon, germanium, tin and lead. Coord. Chem. Rev. 1998,
176, 67−86. (b) Roesky, P. W. The co-ordination chemistry of
aminotroponiminates. Chem. Soc. Rev. 2000, 29, 335−345. (c) Jenter,
J.; Luhl, A.; Roesky, P. W.; Blechert, S. Aminotroponiminate zinc
̈
complexes as catalysts for the intramolecular hydroamination. J.
Organomet. Chem. 2011, 696, 406−418. (d) Hanft, A.; Lichtenberg,
C. New Perspectives for Aminotroponiminates: Coordination Chem-
istry, Redox Behavior, Cooperativity, and Catalysis. Eur. J. Inorg. Chem.
2018, 2018, 3361−3373.
̂
nia and Amines. Bull. Chem. Soc. Jpn. 1959, 32, 272−279. (f) Ito, S.;
(2) (a) Kirin, V.; Roesky, P. W. Aminotroponiminate and a Related
Diimine as Ligands for Tungsten Complexes. Z. Anorg. Allg. Chem.
2004, 630, 466−469. (b) Pittracher, M.; Frisch, U.; Kopacka, H.;
Tsunetsugu, J.; Kanno, T.; Sugiyama, H.; Takeshita, H. Syntheses and
reactions of deuterated troponoids. Tetrahedron Lett. 1965, 6, 3659−
3663. (g) Nozoe, T.; Mukai, T.; Sakai, K. Nitration of 2-chlorotropone.
Various types of rearrangement of 2-chloro-7-nitrotropone. Tetrahe-
dron Lett. 1965, 6, 1041−1047. (h) Kikuchi, K. The Reaction of
Tropones with Alkali and Amines in the Presence of the Copper
Ammine Complex and Potassium Ferricyanide. Bull. Chem. Soc. Jpn.
1967, 40, 385−388. (i) Toda, T. The Structures of the Products from
the Reaction of 3-Bromotropolone and 7-Bromohinokitiol with Caustic
Alkali. Bull. Chem. Soc. Jpn. 1967, 40, 588−590. (j) Biggi, G.; Del Cima,
F.; Pietra, F. Reactivity of pseudoaromatic compounds. X. Normal vs.
abnormal nucleophilic substitutions on cycloheptatrienones carrying a
mobile.alpha. substituent. Rationalization. J. Am. Chem. Soc. 1973, 95,
7101−7107. (k) Biggi, G.; De Hoog, A. J.; Del Cima, F.; Pietra, F.
Reactivity of pseudoaromatic compounds. XI. Base-induced ring
contractions of cycloheptatrienones carrying a mobile.alpha. sub-
stituent. Available pathways and competition with substitution
reactions. J. Am. Chem. Soc. 1973, 95, 7108−7113. (l) Shindo, K.;
Ishikawa, S.; Nozoe, T. Cyclohepta[b][1,4]benzothiazines and their
diazine analogs. 2. Formation and properties of cyclohepta[b]-
quinoxalines. Bull. Chem. Soc. Jpn. 1989, 62, 1158−66. (m) Nozoe,
T.; Okai, H.; Wakabayashi, H.; Ishikawa, S. Cyclohepta[b][1,4]-
benzoxazines. 3. The reaction of 6-bromocyclohepta[b][1,4]-
benzoxazine with o-aminophenol. Bull. Chem. Soc. Jpn. 1989, 62,
2307−14.
Wurst, K.; Muller, T.; Oehninger, L.; Ott, I.; Wuttke, E.; Scheerer, S.;
̈
Winter, R. F.; Bildstein, B. π-Complexes of Tropolone and Its N-
Derivatives: Ambidentate [O,O]/[N,O]/[N,N]-Cycloheptatrienyl
Pentamethylcyclopentadienyl Ruthenium Sandwich Complexes. Orga-
nometallics 2014, 33, 1630−1643. (c) Lichtenberg, C. Amino-
troponiminates: Alkali Metal Compounds Reveal Unprecedented
Coordination Modes. Organometallics 2016, 35, 894−902. (d) Hanft,
A.; Jurgensen, M.; Bertermann, R.; Lichtenberg, C. Sodium Amino-
̈
troponiminates: Ligand-Induced Disproportionation, Mixed-Metal
Compounds, and Exceptional Activity in Polymerization Catalysis.
ChemCatChem 2018, 10, 4018−4027. (e) Hanft, A.; Lichtenberg, C.
Rationalizing the Effect of Ligand Substitution Patterns on Coordina-
tion and Reactivity of Alkali Metal Aminotroponiminates. Organo-
metallics 2018, 37, 1781−1787.
(3) (a) Lichtenberg, C.; Krummenacher, I. Aminotroponiminates as
tunable, redox-active ligands: reversible single electron transfer and
reductive dimerisation. Chem. Commun. 2016, 52, 10044−10047.
(b) Hanft, A.; Lichtenberg, C. Aminotroponiminates: ligand-centred,
reversible redox events under oxidative conditions in sodium and
bismuth complexes. Dalton Trans. 2018, 47, 10578−10589. (c) Hanft,
A.; Krummenacher, I.; Lichtenberg, C. Alkali-Metal Aminotroponimi-
nates: Selectivities and Equilibria in Reversible Radical Coupling of
Delocalized π-Electron Systems. Chem. - Eur. J. 2019, 25, 11883−
11891.
(8) Hicks, F. A.; Brookhart, M. Synthesis of 2-Anilinotropones via
Palladium-Catalyzed Amination of 2-Triflatotropone. Org. Lett. 2000,
2, 219−221.
(4) (a) Korolev, A. V.; Ihara, E.; Guzei, I. A.; Young, V. G.; Jordan, R.
F. Cationic Aluminum Alkyl Complexes Incorporating Amino-
troponiminate Ligands. J. Am. Chem. Soc. 2001, 123, 8291−8309.
(b) Korolev, A. V.; Delpech, F.; Dagorne, S.; Guzei, I. A.; Jordan, R. F.
Main-Group-Metal Chlorobenzene Complexes. Organometallics 2001,
20, 3367−3369. (c) Delpech, F.; Guzei, I. A.; Jordan, R. F. Cationic
Indium Alkyl Complexes Incorporating Aminotroponiminate Ligands.
Organometallics 2002, 21, 1167−1176.
(9) (a) Hussein, L.; Purkait, N.; Biyikal, M.; Tausch, E.; Roesky, P. W.;
Blechert, S. Highly enantioselective hydroamination to six-membered
rings by heterobimetallic catalysts. Chem. Commun. 2014, 50, 3862−
̈
3864. (b) Biyikal, M.; Lohnwitz, K.; Roesky, P. W.; Blechert, S.
Preparation and Catalytic Performance of Novel Dimeric Tetranuclear
Zinc Complexes in Hydroamination of Alkenes at Room Temperature.
Synlett 2008, 2008, 3106−3110.
̈
̈
(5) (a) Dochnahl, M.; Lohnwitz, K.; Luhl, A.; Pissarek, J.-W.; Biyikal,
M.; Roesky, P. W.; Blechert, S. Functionalized Aminotroponiminate
Zinc Complexes as Catalysts for the Intramolecular Hydroamination of
Alkenes. Organometallics 2010, 29, 2637−2645. (b) Zulys, A.;
(10) (a) Schiff, H. Eine neue Reihe organischer Diamine. Justus Liebigs
Ann. Chem. 1866, 140, 92−137. (b) Holm, R. H.; Everett, G. W. Metal
Complexes of Schiff Bases and β-Ketoamines. In Progress in Inorganic
Chemistry; John Wiley Sons, Inc.: Hoboken, NJ, 1966; pp 83−214.
(11) Abu-Dief, A. M.; Mohamed, I. M. A. A review on versatile
applications of transition metal complexes incorporating Schiff bases.
Beni-Suef Univ. J. Basic Appl. Sci. 2015, 4, 119−133.
̈
Dochnahl, M.; Hollmann, D.; Lohnwitz, K.; Herrmann, J.-S.; Roesky,
P. W.; Blechert, S. Intramolecular Hydroamination of Functionalized
Alkenes and Alkynes with a Homogenous Zinc Catalyst. Angew. Chem.,
Int. Ed. 2005, 44, 7794−7798. (c) Dochnahl, M.; Lohnwitz, K.;
Pissarek, J.-W.; Biyikal, M.; Schulz, S. R.; Schon, S.; Meyer, N.; Roesky,
P. W.; Blechert, S. Intramolecular Hydroamination with Homogeneous
Zinc Catalysts: Evaluation of Substituent Effects in N,N′-Disubstituted
Aminotroponiminate Zinc Complexes. Chem. - Eur. J. 2007, 13, 6654−
6666.
(6) (a) Dias, H. V. R.; Jin, W.; Ratcliff, R. E. Aluminum Derivatives of
N-Isopropyl-2-(isopropylamino)troponimine. Inorg. Chem. 1995, 34,
6100−5. (b) Claramunt, R. M.; Sanz, D.; Perez-Torralba, M.; Pinilla, E.;
̈
̈
(12) Gupta, K. C.; Sutar, A. K. Catalytic activities of Schiff base
transition metal complexes. Coord. Chem. Rev. 2008, 252, 1420−1450.
̈
(13) (a) Parssinen, A.; Luhtanen, T.; Klinga, M.; Pakkanen, T.;
̈
Leskela, M.; Repo, T. Alkylphenyl-Substituted Bis(salicylaldiminato)
Titanium Catalysts in Ethene Polymerization. Organometallics 2007,
̈
26, 3690−3698. (b) Strauch, J.; Warren, T. H.; Erker, G.; Frohlich, R.;
Saarenketo, P. Formation and structural properties of salicylaldiminato
17686
https://dx.doi.org/10.1021/acs.inorgchem.0c02920
Inorg. Chem. 2020, 59, 17678−17688

Inorganic Chemistry
pubs.acs.org/IC
Article
complexes of zirconium and titanium. Inorg. Chim. Acta 2000, 300−
302, 810−821.
Effect of the M···F interactions in the polymerization control. J.
Organomet. Chem. 2019, 898, 120854. (m) García-Valle, F. M.; Cuenca,
T.; Mosquera, M. E. G.; Milione, S.; Cano, J. Ring-Opening
Polymerization (ROP) of cyclic esters by a versatile aluminum
Diphenoxyimine Complex: From polylactide to random copolymers.
Eur. Polym. J. 2020, 125, 109527. (n) Gao, J.; Zhu, D.; Zhang, W.;
Solan, G. A.; Ma, Y.; Sun, W.-H. Recent progress in the application of
group 1, 2 & 13 metal complexes as catalysts for the ring opening
polymerization of cyclic esters. Inorg. Chem. Front. 2019, 6, 2619−2652.
(19) Song, X.; Wang, Z.; Zhao, J.; Hor, T. S. A. Sodium cubane and
double-cubane aggregates of hybridised salicylaldimines and their
transmetallation to nickel for catalytic ethylene oligomerisation. Chem.
Commun. 2013, 49, 4992−4994.
(20) (a) Barr, D.; Clegg, W.; Mulvey, R. E.; Snaith, R.; Wade, K.
Bonding implications of interatomic distances and ligand orientations
in the iminolithium hexamers [LiN=C(Ph)But]6 and [LiN=C(Ph)-
NMe2]6: a stacked-ring approach to these and related oligomeric
organolithium systems. J. Chem. Soc., Chem. Commun. 1986, 295−297.
(b) Armstrong, D. R.; Barr, D.; Snaith, R.; Clegg, W.; Mulvey, R. E.;
Wade, K.; Reed, D. The ring-stacking principle in organolithium
chemistry: its development through the isolation and crystal structures
of hexameric iminolithium clusters (RR′C=NLi)6 (R′= Ph, R = But or
Me2N; R = R′= Me2N or But). J. Chem. Soc., Dalton Trans. 1987, 1071−
1081. (c) Barr, D.; Clegg, W.; Mulvey, R. E.; Snaith, R. Synthesis and
crystal structure of the mixed alkali metal imide Li4‑xNa2+x[N=C(Ph)-
But]6: three (metal−nitrogen)2 ring dimers in a triple-layered stack. J.
Chem. Soc., Chem. Commun. 1989, 57−58.
(21) (a) Meally, S. T.; Taylor, S. M.; Brechin, E. K.; Piligkos, S.; Jones,
L. F. Homo- and heterometallic planes, chains and cubanes. Dalton
Trans. 2013, 42, 10315−10325. (b) Upadhyay, A.; Das, C.; Meera, S.
N.; Langley, S. K.; Murray, K. S.; Shanmugam, M. Synthesis and
magnetic properties of a 1-D helical chain derived from a Nickel-
Sodium Schiff base complex. J. Chem. Sci. 2014, 126, 1443−1449.
(c) Cai, Z.; Xiao, D.; Do, L. H. Fine-Tuning Nickel Phenoxyimine
Olefin Polymerization Catalysts: Performance Boosting by Alkali
Cations. J. Am. Chem. Soc. 2015, 137, 15501−15510. (d) Griffiths, K.;
Escuer, A.; Kostakis, G. E. Topological insights in polynuclear Ni/Na
coordination clusters derived from a schiff base ligand. Struct. Chem.
2016, 27, 1703−1714.
(22) Roitershtein, D. M.; Minashina, K. I.; Minyaev, M. E.; Ananyev, I.
V.; Lyssenko, K. A.; Tavtorkin, A. N.; Nifant’ev, I. E. Different
coordination modes of trans-2-{[(2-methoxyphenyl)imino]methyl}-
phenoxide in rare-earth complexes: influence of the metal cation radius
and the number of ligands on steric congestion and ligand coordination
modes. Acta Crystallogr., Sect. C: Struct. Chem. 2018, 74, 1105−1115.
(23) García-Valle, F. M.; Tabernero, V.; Cuenca, T.; Cano, J.;
Mosquera, M. E. G. Schiff-base -ate derivatives with main group metals:
generation of a tripodal aluminate metalloligand. Dalton Trans. 2018,
47, 6499−6506.
(14) (a) Routaray, A.; Nath, N.; Maharana, T.; Sahoo, P. K.; Das, J. P.;
Sutar, A. K. Salicylaldimine Copper(II) complex catalyst: Pioneer for
ring opening Polymerization of Lactide. J. Chem. Sci. 2016, 128, 883−
891. (b) Clowes, L.; Walton, M.; Redshaw, C.; Chao, Y.; Walton, A.;
Elo, P.; Sumerin, V.; Hughes, D. L. Vanadium(iii) phenoxyimine
complexes for ethylene or ε-caprolactone polymerization: mononuclear
versus binuclear pre-catalysts. Catal. Sci. Technol. 2013, 3, 152−160.
(c) Pastor, M. F.; Whitehorne, T. J. J.; Oguadinma, P. O.; Schaper, F.
Zinc complexes of chiral ligands obtained from methylbenzylamine.
Inorg. Chem. Commun. 2011, 14, 1737−1741.
(15) Chang, S.; Jones, L.; Wang, C.; Henling, L. M.; Grubbs, R. H.
Synthesis and Characterization of New Ruthenium-Based Olefin
Metathesis Catalysts Coordinated with Bidentate Schiff-Base Ligands.
Organometallics 1998, 17, 3460−3465.
̈
(16) Al-Shboul, T. M. A.; Ziemann, S.; Gorls, H.; Jazzazi, T. M. A.;
Krieck, S.; Westerhausen, M. Synthesis of Dipotassium 2,2′-Bis(2-
oxidobenzylideneamino)-4,4′-dimethyl-1,1′-biphenyl Derivatives and
Use as Ligand Transfer Reagent. Eur. J. Inorg. Chem. 2018, 2018, 1563−
1570.
̈
(17) Fischer, R.; Gorls, H.; Walther, D. Natriumkomplexe mit
Salicylideniminen: Synthesen, Strukturen, CO2-Aufnahme und Car-
boxylierung von 2-Fluorpropiophenon bzw. Estronmethylether. Z.
Anorg. Allg. Chem. 2004, 630, 1387−1394.
(18) (a) Liu, J.; Iwasa, N.; Nomura, K. Synthesis of Al complexes
containing phenoxy-imine ligands and their use as the catalyst
precursors for efficient living ring-opening polymerisation of ε-
caprolactone. Dalton Trans. 2008, 3978−3988. (b) Lu, W.-Y.; Hsiao,
M.-W.; Hsu, S. C. N.; Peng, W.-T.; Chang, Y.-J.; Tsou, Y.-C.; Wu, T.-Y.;
Lai, Y.-C.; Chen, Y.; Chen, H.-Y. Synthesis, characterization and
catalytic activity of lithium and sodium iminophenoxide complexes
towards ring-opening polymerization of l-lactide. Dalton Trans. 2012,
41, 3659−3667. (c) Gallaway, J. B. L.; McRae, J. R. K.; Decken, A.;
Shaver, M. P. Ring-opening polymerization of rac-lactide and ε-
caprolactone using zinc and calcium salicylaldiminato complexes. Can.
J. Chem. 2012, 90, 419−426. (d) Yi, W.; Ma, H. Magnesium complexes
containing biphenyl-based tridentate imino-phenolate ligands for ring-
opening polymerization of rac-lactide and α-methyltrimethylene
carbonate. Dalton Trans. 2014, 43, 5200−5210. (e) García-Valle, F.
M.; Estivill, R.; Gallegos, C.; Cuenca, T.; Mosquera, M. E. G.;
Tabernero, V.; Cano, J. Metal and Ligand-Substituent Effects in the
Immortal Polymerization of rac-Lactide with Li, Na, and K Phenoxo-
imine Complexes. Organometallics 2015, 34, 477−487. (f) Ghosh, S.;
Chakraborty, D.; Varghese, B. Group 1 salts of the imino(phenoxide)
scaffold: Synthesis, structural characterization and studies as catalysts
towards the bulk ring opening polymerization of lactides. Eur. Polym. J.
2015, 62, 51−65. (g) Lee, C.-L.; Lin, Y.-F.; Jiang, M.-T.; Lu, W.-Y.;
Vandavasi, J. K.; Wang, L.-F.; Lai, Y.-C.; Chiang, M. Y.; Chen, H.-Y.
Improvement in Aluminum Complexes Bearing Schiff Bases in Ring-
Opening Polymerization of ε-Caprolactone: A Five-Membered-Ring
System. Organometallics 2017, 36, 1936−1945. (h) Wu, B.-B.; Tian, L.-
L.; Wang, Z.-X. Ring-opening polymerization of rac-lactide catalyzed by
crown ether complexes of sodium and potassium iminophenoxides.
RSC Adv. 2017, 7, 24055−24063. (i) Fuoco, T.; Pappalardo, D.
Aluminum Alkyl Complexes Bearing Salicylaldiminato Ligands:
Versatile Initiators in the Ring-Opening Polymerization of Cyclic
Esters. Catalysts 2017, 7, 64. (j) García-Valle, F. M.; Tabernero, V.;
Cuenca, T.; Mosquera, M. E. G.; Cano, J.; Milione, S. Biodegradable
PHB from rac-β-Butyrolactone: Highly Controlled ROP Mediated by a
Pentacoordinated Aluminum Complex. Organometallics 2018, 37,
837−840. (k) Huang, T.-W.; Su, R.-R.; Lin, Y.-C.; Lai, H.-Y.; Yang,
C.-Y.; Senadi, G. C.; Lai, Y.-C.; Chiang, M. Y.; Chen, H.-Y.
Improvement in aluminum complexes bearing a Schiff base in ring-
opening polymerization of ε-caprolactone: the synergy of the N,S-Schiff
base in a five-membered ring aluminum system. Dalton Trans. 2018, 47,
(24) It may be noted that problems have been encountered, when
using route 2 toward the synthesis of compounds bearing two 2-
aminotropolone units: Hanft, A.; Lichtenberg, C. Dimerization of 2-
[(2-((2-Aminophenyl)thio)phenyl)amino]-cyclohepta-2,4,6-trien-1-
one through Hydrogen Bonding. Z. Kristallogr. - New Cryst. Struct.
2020, 235, 963−966.
(25) For instance, a SciFinder search on compounds featuring an
aminotroponiminate motif with at least one isopropyl group at a
nitrogen atom gives more than 350 hits.
(26) (a) Torzilli, M. A.; Colquhoun, S.; Kim, J.; Beer, R. H. Structural
and 1H NMR spectroscopic characterization of bis(N-
isopropylsalicylaldiminato)iron(II). Polyhedron 2002, 21, 705−713.
(b) Tripathi, S. M.; Tandon, J. P. Acetoxyboron derivatives of N-
substituted salicylaldimines. J. Inorg. Nucl. Chem. 1978, 40, 983−985.
(27) (a) Hobson, J. D.; Malpass, J. R. The chemistry of azidotropones.
Part II. 3- and 4-Azidotropones and related compounds. J. Chem. Soc. C
́
1969, 1499−1503. (b) Iriarte, J.; Camargo, C.; Crabbe, P. Reaction of
15565−15573. (l) García-Valle, F. M.; Munoz, M. T.; Cuenca, T.;
̃
Milione, S.; Mosquera, M. E. G.; Cano, J. Fluorinated alkali metal
catalysts for the Ring-Opening Polymerization (ROP) of rac-lactide.
6-bromobenzocyclohepten-5-one with amines. J. Chem. Soc., Perkin
Trans. 1 1980, 2077−2080.
17687
https://dx.doi.org/10.1021/acs.inorgchem.0c02920
Inorg. Chem. 2020, 59, 17678−17688

Inorganic Chemistry
pubs.acs.org/IC
Article
(28) Bracci, M.; Ercolani, C.; Floris, B.; Bassetti, M.; Chiesi-Villa, A.;
Guastini, C. Molecular and electronic structure of the complexes
formed by the Schiff base N-(o-hydroxybenzylidene)ferroceneamine
with Co, Ni, Cu, and Zn. J. Chem. Soc., Dalton Trans. 1990, 1357−1363.
(29) In solutions of the strong donor pyridine, compounds 6−8 are
likely to exist as solvates.
Amide Bases. Angew. Chem., Int. Ed. 2011, 50, 9794−9824. (d) Mulvey,
R. E. Avant-Garde Metalating Agents: Structural Basis of Alkali-Metal-
Mediated Metalation. Acc. Chem. Res. 2009, 42, 743−755. (e) Mulvey,
R. E.; Mongin, F.; Uchiyama, M.; Kondo, Y. Deprotonative Metalation
Using Ate Compounds: Synergy, Synthesis, and Structure Building.
Angew. Chem., Int. Ed. 2007, 46, 3802−3824. (f) Mulvey, R. E. s-Block
metal inverse crowns: synthetic and structural synergism in mixed alkali
metal−magnesium (or zinc) amide chemistry. Chem. Commun. 2001,
1049−1056. (g) Benrath, P.; Kaiser, M.; Limbach, T.; Mondeshki, M.;
Klett, J. Combining Neopentyllithium with Potassium tert-Butoxide:
Formation of an Alkane-Soluble Lochmann−Schlosser Superbase.
Angew. Chem., Int. Ed. 2016, 55, 10886−10889. (h) Mulvey, R. E.;
Robertson, S. D. Synthetically Important Alkali-Metal Utility Amides:
Lithium, Sodium, and Potassium Hexamethyldisilazides, Diisopropy-
lamides, and Tetramethylpiperidides. Angew. Chem., Int. Ed. 2013, 52,
11470−11487. (i) Robertson, S. D.; Uzelac, M.; Mulvey, R. E. Alkali-
Metal-Mediated Synergistic Effects in Polar Main Group Organo-
metallic Chemistry. Chem. Rev. 2019, 119, 8332−8405.
(30) Lichtenberg, C.; Spaniol, T. P.; Peckermann, I.; Hanusa, T. P.;
Okuda, J. Cationic, Neutral, and Anionic Allyl Magnesium Com-
pounds: Unprecedented Ligand Conformations and Reactivity Toward
Unsaturated Hydrocarbons. J. Am. Chem. Soc. 2013, 135, 811−821.
(31) (a) Bjørgo, J.; Boyd, D. R.; Watson, C. G.; Jennings, W. B.; Jerina,
D. M. E−Z-isomerism in aldimines. J. Chem. Soc., Perkin Trans. 2 1974,
̈
1081−1084. (b) Paetzold, R.; Reichenbacher, M.; Appenroth, K. Die
Kohlenstoff-Stickstoff-Doppelbindung: Spektren, Struktur, thermische
und photochemische E/Z-Isomerisierung. Z. Chem. 1981, 21, 421−
430. (c) Zhao, L.; Liu, J.; Zhou, P. New Insight into the
Photoisomerization Process of the Salicylidene Methylamine under
Vacuum. J. Phys. Chem. A 2016, 120, 7419−7426.
́
́
́
(40) Fairley, M.; Davin, L.; Hernan-Gomez, A.; García-Alvarez, J.;
O’Hara, C. T.; Hevia, E. s-Block cooperative catalysis: alkali metal
magnesiate-catalysed cyclisation of alkynols. Chem. Sci. 2019, 10,
5821−5831.
(32) (a) Yeh, H. J. C.; Ziffer, H.; Jerina, D. M.; Boyd, D. R.
Stereochemical dependence of the sign and magnitude of coupling
constants on geometry in nitrogen-15 (E)- and (Z)-aldimines. J. Am.
Chem. Soc. 1973, 95, 2741−2743. (b) Kessler, H. Thermal isomer-
ization about double bonds: Rotation and inversion. Tetrahedron 1974,
30, 1861−1870. (c) Osamura, Y.; Yamabe, S.; Nishimoto, K. MO Study
of the photochemical behavior of the imine bond. Int. J. Quantum Chem.
1980, 18, 457−462.
(41) Lichtenberg, C.; Spaniol, T. P.; Perrin, L.; Maron, L.; Okuda, J.
Reversible 1,4-Insertion of Pyridine Into a Highly Polar Metal−Carbon
Bond: Effect of the Second Metal. Chem. - Eur. J. 2012, 18, 6448−6452.
́
(42) (a) Deffieux, A.; Shcheglova, L.; Barabanova, A.; Marechal, J. M.;
̈
(33) (a) Gungor, O.; Gurkan, P. Synthesis and characterization of
̈ ̈
̈
Carlotti, S. Aluminate and Magnesiate Complexes as Propagating
Species in the Anionic Polymerization of Styrene and Dienes.
Macromol. Symp. 2004, 215, 17−28. (b) Honeyman, G. W.;
Kennedy, A. R.; Mulvey, R. E.; Sherrington, D. C. Synthesis, Structure,
and Methyl Methacrylate Polymerization Activity of a Mixed π-
Ferrocene π-Toluene Complex of Potassium Tris-
(hexamethyldisilazide)magnesiate. Organometallics 2004, 23, 1197−
1199.
higher amino acid Schiff bases, as monosodium salts and neutral forms.
Investigation of the intramolecular hydrogen bonding in all Schiff bases,
antibacterial and antifungal activities of neutral forms. J. Mol. Struct.
2014, 1074, 62−70. (b) Hayvali, Z.; Yardimci, D. Synthesis and
spectroscopic characterization of asymmetric Schiff bases derived from
4′-formylbenzo-15-crown-5 containing recognition sites for alkali and
transition metal guest cations. Transition Met. Chem. 2008, 33, 421−
429.
1
(43) The H NMR spectrum of the crystalline material, which was
obtained in small amounts, was identical with that of compound 12
(and thus clearly different from those of 9).
(44) Two main and two minor resonances were detected for the
(34) Additionally, weak Na···C interactions between the outer face
sodium atoms and the ipso-carbon atoms of the ligands coordinated to
the inner face sodium atoms may be discussed based on distance criteria
(Na···C, 2.70−2.84 Å; sum of the van der Waals radii: 3.97 Å).
(35) (a) García-Valle, F. M.; Tabernero, V.; Cuenca, T.; Mosquera, M.
E. G.; Cano, J. Intramolecular C−F Activation in Schiff-Base Alkali
Metal Complexes. Organometallics 2019, 38, 894−904. (b) Solari, E.;
De Angelis, S.; Floriani, C.; Chiesi-Villa, A.; Rizzoli, C. Cubane
structure of sodium derivatives of tetradentate Schiff bases. J. Chem.
Soc., Dalton Trans. 1991, 2471−2476.
(36) The aryl oxide [Na(1-O,2-(CH2NMe2)-C6H4]4 is structurally
related to 7. It also forms a double heterocubane in the solid state.
Contrary to 7, however, outer and inner faces of the double
heterocubane are linked through Na-O and Na-N interactions and
additional Na-arene interactions are present: Hogerheide, M. P.;
Ringelberg, S. N.; Janssen, M. D.; Boersma, J.; Spek, A. L.; van Koten, G.
Influence of Intramolecular Coordination on the Aggregation of
Sodium Phenolate Complexes. X-ray Structures of
[NaOC6H4(CH2NMe2)-2]6 and [Na(OC6H2(CH2NMe2)2-2,6-Me-
4)(HOC6H2(CH2NMe2)2-2,6-Me-4)]2. Inorg. Chem. 1996, 35,
1195−1200.
aldimine protons (see the Supporting Information).
(45) Neufeld, R.; Stalke, D. Accurate molecular weight determination
of small molecules via DOSY-NMR by using external calibration curves
with normalized diffusion coefficients. Chem. Sci. 2015, 6, 3354−3364.
(46) Clegg, W.; Mulvey, R. E.; Snaith, R.; Toogood, G. E.; Wade, K.
The synthesis and X-ray structural characterisation of the first mixed
alkali metal organonitrogen molecular cluster LiNa3[O=P-
(NMe2)3]3[N=C(NMe2)2]4. J. Chem. Soc., Chem. Commun. 1986,
1740−1742.
(47) Hariharan, P. C.; Pople, J. A. The influence of polarization
functions on molecular orbital hydrogenation energies. Theor. Chim.
Acta 1973, 28, 213−222.
(48) (a) McLean, A. D.; Chandler, G. S. Contracted Gaussian basis
sets for molecular calculations. I. Second row atoms, Z=11−18. J. Chem.
Phys. 1980, 72, 5639−5648. (b) Krishnan, R.; Binkley, J. S.; Seeger, R.;
Pople, J. A. Self-consistent molecular orbital methods. XX. A basis set
for correlated wave functions. J. Chem. Phys. 1980, 72, 650−654.
(49) Becke, A. D. Density-functional thermochemistry. III. The role of
exact exchange. J. Chem. Phys. 1993, 98, 5648−5652.
(37) Thus, the solid state structure of compound 7 can be put into
context with the ring stacking principle that has originally been
proposed for the description of lithium compounds (cf. ref 20).
(38) Campos, J. Bimetallic cooperation across the periodic table. Nat.
Rev. Chem. 2020. DOI: 10.1038/s41570-020-00226-5.
(50) Grimme, S.; Antony, J.; Ehrlich, S.; Krieg, H. A consistent and
accurate ab initio parametrization of density functional dispersion
correction (DFT-D) for the 94 elements H-Pu. J. Chem. Phys. 2010,
132, 154104.
(39) (a) Martínez-Martínez, A. J.; Kennedy, A. R.; Mulvey, R. E.;
O’Hara, C. T. Directed ortho-meta′- and meta-meta′-dimetalations: A
template base approach to deprotonation. Science 2014, 346, 834−837.
(b) Mulvey, R. E. Modern Ate Chemistry: Applications of Synergic
Mixed Alkali-Metal-Magnesium or -Zinc Reagents in Synthesis and
Structure Building. Organometallics 2006, 25, 1060−1075. (c) Haag, B.;
Mosrin, M.; Ila, H.; Malakhov, V.; Knochel, P. Regio- and Chemo-
selective Metalation of Arenes and Heteroarenes Using Hindered Metal
17688
https://dx.doi.org/10.1021/acs.inorgchem.0c02920
Inorg. Chem. 2020, 59, 17678−17688