Synthesis and Coordination Chemistry of 3,4-Ethylene-Bridged 1,1,2,5-Tetrasubstituted Biguanides

Article abstract of DOI:10.1021/acs.inorgchem.9b03093

The synthesis of 3,4-ethylene-bridged 1,1,2,5-tetrasubstituted biguanides is reported, which are accessible by three alternative routes. Exemplary molecular structures of the ligand and an observed side product have been elucidated by X-ray diffraction an

Full text of DOI:10.1021/acs.inorgchem.9b03093

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Synthesis and Coordination Chemistry of 3,4-Ethylene-Bridged
1,1,2,5-Tetrasubstituted Biguanides
̈
Maximilian Dehmel, Valentin Vass, Lukas Prock, Helmar Gorls, and Robert Kretschmer*
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ABSTRACT: The synthesis of 3,4-ethylene-bridged 1,1,2,5-tetrasubstituted
biguanides is reported, which are accessible by three alternative routes. Exemplary
molecular structures of the ligand and an observed side product have been
elucidated by X-ray diffraction analysis. Mono- and dinuclear complexes of the
biguanide in both its neutral and monoanionic forms were obtained, including
examples of aluminum, copper, magnesium, potassium, tin, and zinc, indicating a
versatile coordination behavior, as evidenced by means of single-crystal X-ray
diffraction analysis.
substituted derivatives, limits their applicability.6 The synthesis
of 1,1,2,4,5,5- or 1,2,3,4,5,5-hexasubstituted biguanides (III),
for example, originating from carbodiimides is limited in terms
of the substitution pattern and requires multiple steps.7
However, a high degree of substitution is relevant in order
to reduce the coordinational freedom, aiming to address the
various requirements of a metal complex arising from the
different areas of application such as catalysis, material design,
and medicinal chemistry.
INTRODUCTION
■
Biguanides I (Figure 1), which were already synthesized in
1879,1 find numerous applications not only as ligands but also
Due to our interest in polynuclear metal complexes, we
became interested in bis(guanidines)8 and developed a
synthetic protocol to obtain these from readily available
bis(thioureas).9 However, when ethylene-bridged bis-
(thioureas) bearing terminal alkyl substituents were applied,
we did not obtain the desired bis(guanidines). Instead, C−N
bond scission takes place, yielding an isothiocyanate along with
an N-substituted 4,5-dihydro-1H-imidazol-2-amine. However,
when ethylene-bridged bis(thioureas) bearing bulky terminal
aryl groups were used, 3,4-ethylene-bridged 1,1,2,5-tetrasub-
stituted biguanides were obtained in good yields along with the
respective bis(guanidines) (vide infra). Inspired by these
results, we set out to explore the synthetic access and
coordination chemistry of this new ligand class, aiming for a
comparison with the more classical biguanides. In comparison
to the 1,2,3,4,5,5-hexasubstituted biguanides III, the five-
membered heterocycle within IV/IV′ gives rise to a more rigid
ligand framework, which consequently will affect the
coordination capabilities. However, the biguanides IV/IV′
Figure 1. (a) The parent biguanide I, (b) the type 2 diabetes drug
metformin II, and (c) 1,2,3,4,5,5-hexasubstituted biguanides. (d) The
tautomeric forms of the 3,4-ethylene-bridged 1,1,2,5-tetrasubstituted
biguanides presented in this work.
as important compounds in their own right: e.g., for the
treatment of filariasis, hyperglycemia, influenza, and malaria,2
with metformin (II) being the most prominent example.
Surprisingly, in the case of the parent biguanide the wrong
tautomer is regularly represented in the scientific literature.3
While related metal complexes attracted quite some interest in
the middle of the last century, reports faded after an initial
period of intense research.4 Recently, however, biguanide
complexes have experienced a revitalization, as they have the
potential to be large-spectrum antimicrobial and anticancer
agents or catalysts for organic transformations.5 However, the
synthetic access to biguanides, especially to the more highly
Received: October 22, 2019
https://dx.doi.org/10.1021/acs.inorgchem.9b03093
© XXXX American Chemical Society
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Scheme 1. Synthetic Routes toward 3,4-Ethylene-Bridged 1,1,2,5-Tetrasubstituted Biguanides IV/IV′ from Readily Available
Ethylene-Bridged Bis(thioureas) 1
Table 1. Synthesis of Biguanides IV/IV′ from Bis(carbodiimides) 4
entry
R
amine
yield (%)
entry
R
amine
piperidine
azepane
diethylamine
diisopropylamine
LDA
yield (%)
1
2
3
4
5
6
Dipp
pyrrolidine
pyrrolidine
pyrrolidine
pyrrolidine
pyrrolidine
morpholine
61 (IVa′)
62 (IVb′)
a
41 (IVc)
58 (IVd)
42 (IVe′)
6
7
8
9
10
11
Dipp
Dipp
Dipp
Dipp
Dipp
tBu
35 (IVf′)
37 (IVg′)
nr
DMP
phenyl
isopropyl
tBu
nr
18 (IVh′)
36 (IVi)
Dipp
LDA
a
The protocol led to an intractable reaction mixture.
still offer multiple binding sites, and so we investigated the
formation of certain coordination isomers depending on the
metal applied in order to understand the coordination
behavior. Furthermore, the substituents R and R′ can be
varied independently, which allows for a fine tuning of the
electronic properties of the ligand or for the introduction of
suitable anchor and bridging groups in order to attach the
ligand on surfaces or to connect several complexes, aiming for
molecular assemblies.
adopting procedures regularly used for the synthesis of
carbodiimides12 allowed for the isolation of alkyl- and aryl-
substituted bis(carbodiimides) 4 in yields ranging from 23 to
63%. Bis(carbodiimides) bearing rather bulky terminal groups
were accessible using cyanuric chloride as a desulfurization
agent, while derivatives of 4 containing less bulky terminal
groups, i.e., ethyl and phenyl, respectively, could only be
obtained in low yields (23% and 28%, respectively) by using
PPh3/Br2. The diminished yields in the latter case most likely
arise from subsequent reactions such as polymerization and
cyclopolymerization, which is a known behavior of poly-
(carbodiimides).11a,13
Treatment of the bis(carbodiimides) 4 with an excess of
pyrrolidine allows for the isolation of the desired biguanides IV
and IV′, respectively, in yields ranging from 35% to 62%
(Table 1). It is worth noting that intramolecular reactions of
protic groups and carbodimides have already been observed:
for example, in the formation of 2-amino-2-oxazolines.14
Although the protocol tolerates terminal alkyl and 2,6-
disubstituted aryl groups, the reaction of the phenyl-
substituted bis(thiourea) gives rise to an intractable reaction
mixture. With respect to the nucleophile, the reaction is limited
to cyclic secondary amines, which is in line with the decreased
nucleophilicity of acyclic secondary amines.15 However, using
the corresponding lithium amides is a suitable protocol to
obtain the related biguanides (Table 1, entries 10 and 11).
Using morpholine illustrates that bridging groups are readily
installed in the backbone, as mentioned in the Introduction.
In order to elucidate which of the two tautomers, i.e., IV or
IV′ (Scheme 1) exists in solution, multidimensional NMR
experiments have been conducted using IVa′ and IVi,
RESULTS AND DISCUSSION
■
Proligand Synthesis and Structure. We investigated
three different approaches toward the biguanides IV or the
respective tautomer IV′ (Scheme 1), all originating from the
bis(thioureas) 1, which are readily available from ethylenedi-
amine and the respective isothiocyanates. Route i is based on
the formation of the respective S-methylated bis(isothioureas)
2, for which the alkyl and aryl derivatives (R = tBu, Dipp) are
conveniently available by applying established protocols.10 In
the case of the terminal tert-butyl group, the reaction with
pyrrolidine does not yield the desired biguanide. Instead, the
N,N′-substituted S-methyl(4,5-dihydro-1H-imidazolyl-1)-
isothiourea 3 is formed, which, to the best of our knowledge,
has yet not been reported in the literature. However, if 2,6-
diisopropylphenyl (Dipp) is used as the terminal substituent,
the biguanide IVa′ is formed in 37% yield.
Route ii involves the synthesis of the bis(carbodiimides) 4,
which have only limited precedence in the literature,11 and the
necessary distillative purification causes unwanted side
reactions (such as polymerization reactions) for their heavier
weight derivatives due to their higher boiling points. However,
B
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Figure 2. Possible reaction pathway and potential energy surface (Gibbs free energies at the B3LYP-D3/TZVP level of theory) for the reaction of
the bis(carbodiimide) 4d with pyrrolidine.
Scheme 2. Possible Reaction Pathways Originating from the Bis(thioureas) 1 and Various Conditions
respectively. Notably, for the known open chain biguanides no
reports about the relationship between tautomer stability and
substitution pattern can be found.16 The N−H resonances
exist as monomers or dimers formed through hydrogen
bonding, diffusion ordered NMR spectroscopy (DOSY)
experiments have been conducted using dioxane as an internal
reference. The experimentally obtained hydrodynamic radii of
IVa′ (4.1 Å) and IVi (5.0 Å), respectively, are in good
agreement with the calculated values (4.0−5.1 and 4.7−6.7 Å,
respectively)17 of the monomeric species.
DFT calculations at the B3LYP-D3/TZVP level of theory
were performed to obtain mechanistic insights and to reveal
the origin of the preferred formation of the biguanides IV/IV′
over the formation of bis(guanidines) 5. On the basis of the
Gibbs free energies and as shown in Figure 2, formation of the
biguanide IVd is favored by about 14.3 kJ mol−1 over the
related bis(guanidine). Notably, the also conceivable tautomer
IVd′ is 38.9 kJ mol−1 higher in energy, in line with the
experimental observation.
Initially, addition of pyrrolidine to 4d favors the formation of
a terminal rather than a lateral N−H bond, yielding the
intermediate Int1 via TS1; addition to the lateral CN bond
via transition state TS1′ is 35.1 kJ mol−1 higher in energy.
From Int1, the reaction pathway branches out in three
different reaction channels, which are discussed separately in
the following. Addition of a second pyrrolidine molecule to
1
appear in the H NMR spectrum as broad singlets at 3.82
(IVa′) and 3.83 (IVi) ppm. In the 1H,1H-COSY NMR spectra
in both cases only one cross peak was observed for these
3
resonances (Figures S47 and S70, respectively). For IVa′, J
coupling with the triplet resonance at 3.36 ppm, which is
assigned to the CH2 group of the five-membered ring, was
4
observed, while for IVi J coupling to the singlet resonance at
1.20 ppm accounting for the tert-butyl group is observed.
1
Furthermore, in the H,13C-HMBC NMR spectrum of IVa′, a
cross peak between the N−H and 13C methylene resonances at
3
47.2 ppm accounts for a J coupling, while such a peak is
absent in the case of IVi. Hence, evidence is given that in the
case of IVa′ the tautomer IV′, in which the proton is bound to
the endocyclic nitrogen atom of the five-membered ring, is
present in solution, while the data obtained for IVi carrying
terminal tert-butyl groups are in favor of tautomer IV. This
illustrates nicely the crucial effect of the terminal substituent,
which is readily explained by the positive inductive effect of the
alkyl group causing an increase in the basicity of the exocyclic
nitrogen atom. In order to distinguish whether the biguanides
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a
Table 2. Reactions of Bis(thioureas) 1 with Secondary Amines and Lead(II) Oxide
yield (%)
entry
Ar
amine
IV′
5
7
9
1
2
3
4
5
6
7
8
9
Dipp
DMP
Dipp
Dipp
Dipp
Dipp
2-CH3-C6H4
4-CH3-C6H4
4-NO2-C6H4
4-F-C6H4
pyrrolidine
pyrrolidine
morpholine
piperidine
azepane
diethylamine
pyrrolidine
pyrrolidine
pyrrolidine
pyrrolidine
60 (39) IVa′
70 (46) IVb′
30 (23) IVe′
90 (42) IVf′
60 (37) IVg′
20
5
15
30
50
10
20
60 (44)
99 (57)
99 (57)
75 (43)
c
b
10
c
c
a
b
1
Crude yields were obtained from H NMR data, and isolated yields are given in parentheses. In addition to compound 8h, unreacted starting
c
material was observed in the crude reaction mixture. The protocol led to an intractable reaction mixture.
Int1 proceeds via TS2, forming the respective bis(guanidine)
5. Again, the proton transfer can occur either to the terminal or
to the lateral nitrogen atom of the remaining carbodiimide
unit, but in contrast to TS1, the respective transition states
TS2 and TS2′ differ in energy by only 1.0 kJ mol−1. However,
thermodynamically, formation of 5 containing two terminal
N−H bonds is favored by 11.6 kJ mol−1 over the formation of
5′ containing a lateral and a terminal N−H bond.
of pyrrolidine at 100 °C, leading to the isolation of 9 in 92%
yield. Consequently, the desired formation of 4 competes with
the generation of 6 and 9, respectively.
Replacing the alkyl groups by electron-withdrawing aryl
substituents should facilitate the desulfurization and thus
suppress the formation of the mixed carbodiimide thiourea
species 6. Performing the reaction with ethylene-1,2-bis(2,6-
diisopropylphenylthiourea) and ethylene-1,2-bis(2,6-dimethyl-
phenylthiourea) allowed for the isolation of the desired
biguanides IVa′ and IVb′ in 39% and 46% yield, respectively.
However, the formation comes along with the generation of
the imidazolidyl carbothioamide 7a (R = Dipp) and of the
asymmetric thiourea 9b (R = 2,6-DMP, R′ = (CH2)4),
respectively (Table 2). The protocol works with a variety of
cyclic secondary amines, but the 2,6-substitution pattern of the
terminal aryl groups appears to be most relevant, as the
respective bis(thioureas) 1j−l, bearing a 2-methylphenyl, 4-
methylphenyl, or 4-nitrophenyl group, yield the N,N′,N′-
trisubstituted thioureas 9 instead of the biguanide as the main
product. Notably, the concentration is also crucial to the
relative ratio of IVa′ to 9a: in low concentrations (0.05 mol/L
of 1g) 9a was not observed in the crude product, while at a
concentration of 0.16 mol/L, 9a amounts to about 40% of the
crude reaction mixture. Despite the fact that the reaction
proceeds with various cyclic secondary amines, using acyclic
diethylamine does not yield the desired biguanides but the
imidazolidyl carbothioamide 7a (R = Dipp) is formed as the
major product (44% isolated yield) instead. Hence, applying
bulky aryl groups stabilizes the species 7, which was not
detectable in the case of terminal tert-butyl groups.
For the biguanide formation, two pathways need to be
considered: here, the nucleophilic attack of the lateral nitrogen
atom of the guanidine unit to the electrophilic carbon atom of
the carbodiimide unit and the proton transfer can occur in
either a concerted reaction via TS3 or by a subsequential
process en route, TS4 → Int2 → TS5. However, and with
respect to Int1, TS3 (58.7 kJ mol−1 relative to Int1) is much
lower in energy in comparison to TS2 (159.6 kJ mol−1 relative
to Int1) and TS5 (95.9 kJ mol−1 relative to Int1).
Consequently, the sequence TS1 → Int1 → TS3 → IVd
belongs to the energetically preferred pathway for biguanide
formation on route i. It is worth noting that the transition state
TS5′, associated with the direct formation of the energetically
less preferred tautomer IVd′, is 34.2 kJ mol−1 more demanding
in comparison to TS5.
Route iii, finally, is based on our previous work concerning
the synthesis of bis(guanidines),9 where we observed that the
treatment of ethylene-1,2-bis(3-tert-butylthiourea) with PbO
and pyrrolidine did not yield the desired bis(guanidine) but
instead generated N-(tert-butyl)-4,5-dihydro-1H-imidazolyl-2-
amine (8, R = C(CH3)3). We assumed that the formation of 8
is most likely driven by the intermediary formation of the
mixed carbodiimide thiourea species 6, which undergoes an
intramolecular cyclization, yielding the imidazolidyl carbo-
thioamide 7 (Scheme 2). Subsequent C−N bond breakage
gives rise to 8 and the intermediarily formed isothiocyanate
reacts with an excess of pyrrolidine by generating the
asymmetric thiourea 9. Please note that symmetric N,N′-
disubstituted thioureas are readily converted to the corre-
sponding N,N′,N′-trisubstituted thioureas upon reaction with
an excess of aliphatic or aromatic secondary amines.18 Indeed,
in a control experiment, 1g was allowed to react with 20 equiv
In summary, the most relevant and sterically demanding
biguanide IVa′ can be obtained by three different routes, all
originating from the readily available bis(thiourea) 1g, from
which the one-pot synthesis (route iii) is most preferred due to
the facile workup. Route ii is the most versatile, as it also
tolerates terminal alkyl substituents and allows the installation
of acyclic secondary amines by applying the respective lithium
amides.
Compounds IVa′, IVb′, and 7a were obtained as single
crystals suitable for X-ray diffraction studies, and their
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Figure 3. Solid-state structures (hydrogen atoms and for 7a an acetonitrile molecule are omitted for the sake of clarity) with selected bond lengths
(Å) and angles (deg) of IVa′ and 7a: (a) IVa′, C1−N1 1.3742(15), C4−N4 1.2778(15), C1−N5 1.2765(15), C5−N3−C8 111.33(10); (b) 7a,
C1−N1 1.347(3), C4−N3 1.339(3), N3−H3 0.91(3), C1−N4 1.283(3), C4−S1 1.673(2), N3−C4−S1 123.90(18), N2−C4−S1 121.24(18).
Scheme 3. Synthesis of the Mononuclear Tin(II) and Zinc(II) Complexes V and VI, Respectively, and of the Diinuclear
Copper(I) Complex VII Starting from the Protio Ligand IVa′
Figure 4. Relative Gibbs free energies (given in kJ mol−1) of the isomers of (a) V and (b) VI and a hypothetical MgCl2 complex at the B3LYP/
def2SVP level of theory.
molecular structures are given in Figure 3 and Figure S1. As
both IVa′ and IVb′ have similar structural features, further
discussions are based solely on compound IVa′, for which also
metal derivatives have been obtained (vide infra). However,
the molecular structure in the solid state and selected bond
lengths and angles of IVb′ are given in Figure S1. The C1−N5
and C4−N4 bond lengths with values of 1.2765(15) and
1.27788(15) Å, respectively, are indicative of CN double
bonds. The C5−N3−C8 bond angle (111.33(10)°) of the
pyrrolidine ring is more obtuse in comparison to free
pyrrolidine (105.2(35)°).19
E
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Figure 5. Solid-state structures (hydrogen atoms and in the case of V and VI residual toluene molecules are omitted for the sake of clarity) with
selected bond lengths (Å) and angles (deg) of V−VII: (a) V, Sn1−N1 2.2216(17), Sn1−Cl1 2.4841(6), Sn1−Cl2 2.4647(5), C1−N1 1.314(3),
C4−N4 1.285(3), C1−N5 1.337(2), N5−H5 0.86(3), N1−Sn1−Cl1 86.66(5), Cl1−Sn1−Cl2 94.88(2); (b) VI, Zn1−Cl1 2.2004(4), Zn1−Cl2
2.2135(4), Zn1−N5 1.9970(12), Zn1−N3 2.1455(11), C1−N1 1.3502(18), C1−N5 1.2968(18), C4−N3 1.4416(17), Cl1−Zn1−Cl2
115.016(16), N3−Zn1−N5 88.89(4); (c) VII, Cu1−Cl1 2.0929(7), Cu1−N5 1.872(2), Cu2−Cl2 2.0796(9), Cu2−N4 1.885(2), C1−N1
1.341(3), N5−Cu1−Cl1 174.72(6), N4−Cu2−Cl2 173.06(7).
Compound 7a carries two nitrogen-based protons at N1 and
N3, respectively. This causes the formation of an intra-
molecular N3−H3···N4 hydrogen bond, generating a pseudo-
bicyclic framework. Consequently, the C1−N1 bond in 7a
(1.347(3) Å) is significantly shorter in comparison to that in
individual protons within the complex, although the
resonances remained comparably broad (Figures S81−S84).
It is worth noting that the limited solubility of VI in solvents
other than CDCl3 impedes the acquisition of spectra at
elevated temperature.
1
IVa′ (1.3742(15) Å). The H NMR spectrum of 7a gives a
Aiming to obtain insight into the different binding scenarios,
we performed DFT calculations at the B3LYP/def2SVP level
of theory for the conceivable isomers of V, VI, and a
hypothetical MgCl2 complex of IVa′ (Figure 4). In the case of
the tin(II) complex, V belongs to the most stable isomer and
V′, V′′, and V′′′ are higher in energy by 20.6, 51.8, and 83.0 kJ
mol−1, respectively. Steric demands are likely to account for
this significant energy difference, as in the case of V′ and V′′
the interaction of the SnCl2 fragment with the bulky Dipp
groups is more pronounced in comparison to V. In V′′′, finally,
the tin center adopts a seesaw geometry, which is less favored
due to obvious electronic reasons. The relative energies of the
isomers of both the zinc(II) and the respective hypothetical
magnesium(II) complex (Figure 4b) indicate an opposite
behavior, as VI′′ is the least favored isomer. The isomers VI
and VI′ with M being Mg and Zn differ only slightly in energy,
and in line with the experiment, VI is the most stable isomer in
the case of zinc, while for magnesium VI′ is slightly lower in
energy by 1.3 kJ mol−1. This indicates that even relatively
“cheap” basis sets allow for a prediction of the coordination
geometry, which is of value for further application of the
biguanides IV/IV′ as ligands.
pattern of two methine septets and four methyl doublets for
the Dipp groups, which is indicative of a plane of symmetry
within the molecule, and the two N−H resonances appear as
broad singlets at 4.34 and 13.43 ppm, respectively.
Complexation Behavior. Having the related protio
ligands in hand, we set out to investigate their coordination
capabilities of both the neutral and monoanionic forms and
chose biguanide IVa′ as a reference compound. Due to the
anti-infective properties and large antimicrobial spectrum of
biguanide complexes of the middle and late 3d metals,5a we
chose copper and zinc as examples along with tin as a
representative of a main-group element. Under anaerobic
conditions IVa′ readily forms the complexes V−VII upon
treatment with SnCl2, ZnCl2, and CuCl, respectively (Scheme
3). Their molecular structures in the solid state are given in
Figure 5. Addition of SnCl2 to IVa′ induces a proton transfer
from N1 to N5, giving rise to an intramolecular N4···H5−N5
hydrogen bond along with the formation of a pseudobicyclic
ring system. SnCl2 coordinates to the endocyclic N1 atom
occupying the apex of the trigonal pyramid. The formation of
only one dative N−Sn bond instead of formation of a chelate
was also observed in the case of an acenaphthene-1,2-diimine,
for which, however, a substantially longer coordinative Sn−N
bond (2.4700(7) Å) in comparison to V (2.2216(17) Å) has
been reported. In the case of VI, the zinc atom is not bound to
both imine moieties but instead to the nitrogen atoms of the
amino (N3) and imino function (N5). Consequently, the Zn−
N bond lengths of 1.9970(12) Å (Zn1−N5) and 2.1455(11) Å
(Zn1−N3) resemble those observed for aminoimine zinc
complexes.20 This stands in contrast to the well-known
NacNac ligand, for which the related unfavorable β-diimine
form is stabilized in the coordination sphere of divalent metals
In the case of copper(I), the dinuclear complex VII is
obtained in which the two CuCl units are each κ1-bound to N4
and N5, respectively. It is worth noting that VII is formed
independently from the stoichiometry of IVa′ to CuCl and
using a 1:1 ratio yields the dinuclear complex VII along with
unreacted protio ligand. The overall structure of VII is
reminiscent of the dinuclear copper(I) methanetrisamidine
complex reported by the Schulz group23 and hence features
similar structural characteristics. Each CuCl molecule is κN1-
coordinated by one of the two imino groups, while the N−H
bond at N1 remains intact. The Cu−N bond lengths (Cu1−
N5 1.872(2) Å, Cu2−N4 1.885(2) Å) and the N−Cu−Cl
bond angles (N5−Cu1−Cl1 174.72(6)°, N4−Cu2−Cl2
173.06(7)°) are comparable to those observed by Schulz and
co-workers.23 Coordination of the Lewis acidic CuCl
molecules causes a flux of electron density from the nitrogen
atoms to the copper ions; therefore, the C1−N5 (1.310(3) Å)
and C4−N4 (1.301(4) Å) bonds are elongated in comparison
21
such as NiBr2 and ZnCl2.22
While the 1H NMR spectrum of V gives sharp signals, which
indicate Cs symmetry on the NMR time scale, broadened
resonances attributed to slow conformational exchange at
room temperature were observed for compound VI. By
acquisition of the spectra at 233 K the majority of the
resonances were resolved and allowed their assignment to
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Scheme 4. Synthesis of the Potassium, Magnesium, Aluminum, and Zinc Complexes VIII−XIII Originating from the Protio
Ligand IVa′
Figure 6. Solid-state structures (hydrogen atoms are omitted for the sake of clarity) with selected bond lengths (Å) and angles (deg): (a) VIII,
K1B−N1A 2.733(5), K1B−N3B 3.113(5), K1B−N5B 2.666(5), K1B−RING 2.938(5), N3B−K1B−N5B 61.90(14), N1A−K1B−N5B
105.23(16); (b) IX, Mg1−N1 2.0787(17), Mg1−N5 2.1437(17), Mg1−N6 2.0791(18), Mg1−N10 2.1592(15), N1−Mg1−N5 64.80(6), N6−
Mg1−N10 64.66(6).
to the protio ligand IVa′ (1.2765(15) and 1.2778(15) Å,
respectively). Notably, copper complexes of the more tradi-
tional and less substituted biguanide ligands are also known
and regularly incorporate copper(II) centers chelated by two
biguanides.5a,24 Obviously, the bulky Dipp groups of IVa′
would inhibit the formation of a comparable mononuclear
copper complex.
and provide well-resolved resonances. The 1H NMR spectrum
includes two sets of two doublets for the methyl groups of each
Dipp substituent, originating from a hindered rotation about
the Caryl−N bonds. In addition, the NH resonance appears at
1
lower field (8.89 ppm) in the H NMR in comparison to the
protio ligand IVa′. Overall, the three complexes reported
before nicely illustrate that each tautomer, i.e., IVa and IVa′,
can be stabilized by selecting the proper metal.
The 1H and 13C NMR spectra of VII in CDCl3 are
consistent with the solid-state structure on the NMR time scale
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Figure 7. Solid-state structures (hydrogen atoms are omitted for the sake of clarity) with selected bond lengths (Å) and angles (deg): (a) X, Al1−
N4 1.9210(12), Al1−N5 1.8590(12), C1−N1 1.2756(19), C1−N5 1.3490(18), N4−Al1−N5 95.69(5), H1−Al1−H2 111.8(10);( b) XI, Al1−N4
1.9477(11), Al1−N5 1.8966(11), C1−N1 1.2847(16), C1−N5 1.3567(16), N4−Al1−N5 94.23(5), C33−Al1−C34 120.26(6); (c) XII, Al1−N4
1.9460(11), Al1−N5 1.9385(11), C1−N1 1.3194(15), C1−N5 1.3415(15), Al2−N1 1.9937(11), N4−Al1−N5 93.91(4), C33−Al1−C34
122.31(6).
In addition to complexes V−VII, in which the neutral protio
ligand remains present, deprotonation is readily achieved using
potassium bis(trimethylsilyl)amide (KHMDS), di-n-butylmag-
nesium, aluminum hydride trimethylamine complex, trimethy-
laluminum, and diethylzinc, giving rise to manifold coordina-
tion modes (Scheme 4 and Figures 6−8). The elements have
center of the C6 perimeter (2.938 Å) is in the expected
range.27
The H NMR spectrum of a solution of VIII in d8-THF
1
measured at room temperature contains well-resolved and
assignable signals (Figure S87), while certain resonances
appeared broad in the respective 13C spectrum. Upon cooling
to 223 K, the 13C resonances became more sharp, while in the
1
case of the H NMR spectrum a temperature dependence of
the chemical shift, line broadening, and the eventual splitting
of certain signals was observed (Figures S92−S94). With
1
respect to the latter, the H resonances of the methylene
groups of the pyrrolidine ring, for example, now appear as four
broad singlets in comparison to two singlets at room
temperature, indicating the inhibition of ring inversion upon
cooling.
In case of compound IX, magnesium does not form a six-
membered chelate ring but instead coordination originates
from the nitrogen atoms N1 and N5 of two ligands, forming a
spiro[3.3] compound in which the magnesium atom fuses two
four-membered rings and reaches a coordination number of 5
upon complexation of a THF molecule. The formation of IX is
in line with the propensity of heteroleptic alkaline-earth-metal
complexes of the general formula LMX to undergo a Schlenk-
like solution redistribution to the respective homoleptic
compounds L2M along with MX2. Such a coordination mode
has previously been observed for amidinate28 and guanidi-
nate29 complexes, and the magnesium−nitrogen bond lengths
of 2.0787(17) Å (Mg1−N1) to 2.1592(15) Å (Mg1−N10)
and N−Mg−N bond angles of 64.66(6)° (N6−Mg1−N10) to
64.80(6)° (N1−Mg1−N5) are in accordance with those
reported before.
Due to the rotational dynamics of IX at room temperature, it
was not possible to derive 1H and 13C spectra that allowed for
an assignment of the signals and additional NMR experiments
were conducted at 223 and 334 K (Figures S102−S111).
Across the investigated temperature range, a pronounced
temperature dependence of the chemical shifts was observed.
On acquisition of the 1H NMR spectrum at 223 K, the rate of
exchange was sufficiently reduced; thus, the majority of
resonances were resolved. However, the slow exchange caused
sufficiently more complex spectra. At 343 K an expected fast
exchange caused a 1H NMR spectrum containing well-resolved
but averaged resonances, which could be assigned to individual
protons of complex IX.
Figure 8. Solid-state structure (hydrogen atoms and residual pentane
molecule are omitted for the sake of clarity) with selected bond
lengths (Å) and angles (deg) of XIII: Zn1−N1 1.945(2), Zn1−N6
1.995(2), Zn2−N8 1.962(2), Zn2−N10 1.987(2), Zn1−C65
1.988(3), Zn2−C67 1.955(3), N1−Zn1−N6 119.07(10), N8−
Zn2−N10 95.11(8).
been chosen because of the potential application of the related
product. The potassium species may act as a precursor for
transmetalation or salt metathesis. However, the aluminum,
magnesium, and zinc complexes are of potential interest for
catalytic applications such as the Meerwein−Ponndorf−Verley
reaction, as reported before for aluminum biguanide complex-
es.5f
The potassium species VIII forms a 1D coordination
polymer network25 in the solid state, in which the anionic
ligand provides four donor sites (Figure 6a and Figure S2).
Each potassium coordinates in a κ2 mode to the nitrogen
atoms N3 and N5 of one ligand molecule and in a κ1 and η6
fashion to both the nitrogen atom N1 and the phenyl ring of
the Dipp group bound to N5. So far, there has been only one
report about such a κN2-κN1,η6-binding behavior, which was
observed for a multidentate nitrogen−sulfur ligand that,
however, does not give rise to a coordination polymer but
forms a dimer in the solid state.26 The potassium bond lengths
are in the range of 2.666(5) Å (K1B−N5B) to 3.113(5) Å
(K1B−N3B), and the distance of the potassium ion to the
Reaction of the protio ligand IVa with 1 equiv of aluminum
hydride trimethylamine complex or trimethylaluminum readily
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affords the mononuclear species X and XI in 51% and 71%
yields, respectively (Scheme 4). Notably, in case of compound
XI the reaction conditions are crucial, as the reaction with 1
equiv of trimethylaluminum at room temperature yields the
dinuclear species XII along with unreacted starting material. In
X and XI, the biguanide chelates the aluminum atom, forming
a nonplanar six-membered AlC2N3 ring, similar to those
complexes obtained with NacNac and closely related ligands.30
Their NMR spectroscopic data as well as the characteristics of
their solid-state structures differ to quite some extent, as
delocalization cannot be achieved within X and XI. Common
number of 3 is obtained. The Zn−C bond lengths of 1.955(3)
and 1.988(3) Å for Zn2−C67 and Zn1−C65, respectively, are
comparable to those obtained for the related NacNac
derivatives.32
CONCLUSION
■
In summary, the synthesis of a new class of electron-rich N,N-
bidentate ligands, i.e. 3,4-ethylene-bridged 1,1,2,5-tetrasubsti-
tuted biguanides, is reported, incorporating three synthetic
methodologies originating from readily available
bis(thiourea)s. Although the most relevant biguanide IVa′ is
obtained by each of the three routes, the one-pot synthesis
(route iii) is preferred over the others due to the facile workup.
However, the most versatile route ii also tolerates terminal
alkyl substituents and allows the installation of acyclic
secondary amines to the backbone of the ligand. The protio
ligand IVa′ readily forms complexes upon reaction with SnCl2,
ZnCl2, or CuCl, in which the proton resides either on the
endocyclic nitrogen atom N1 (Cu, Zn) or on the acyclic N5
position (Sn). Deprotonation of the protio ligand by using
potassium bis(trimethylsilyl)amide (KHMDS), di-n-butylmag-
nesium, aluminum hydride trimethylamine complex, trimethy-
laluminum, and diethylzinc is readily achieved. In the case of
potassium a 1D coordination polymer is obtained, which might
serve as a suitable precursor in subsequent transmetalation
reactions. While the Schlenk equilibrium favors the formation
of a homoleptic magnesium complex, with AlH3·N(CH3)3 and
trimethylaluminum mono- and dinuclear heteroleptic com-
plexes are obtained, which will be further tested in catalytic
applications. Finally, diethylzinc gives rise to a heterodinuclear
zinc complex incorporating two ligand molecules. Although the
coordination chemistry does not seem trivial, preliminary DFT
calculations indicate that a prediction of the coordination
geometry is feasible.
1
features observed in the respective H NMR spectra include
four doublets for the methyl groups and two septets for the
methine groups of the Dipp ligand, indicating an averaged Cs
symmetry on the NMR time scale. In the case of XI, further
evidence is given by one singlet at −0.93 ppm related to the
Al(CH3)2 methyl group. In the solid state, the Al1−N5 bonds
(1.8590(12) and 1.8966(11) Å for X and XI, respectively) are
shorter in comparison to the Al1−N4 bonds (1.9210(12) and
1.9477(11) Å for X and XI, respectively) due to the covalent
character of the former. The longer Al−N bonds and the hence
more acute N4−Al−N5 bond angle of 94.23(5)° for XI in
comparison to X (95.69(5)°) accounts for the reduced
electron-withdrawing character of the methyl versus the
hydride substituent. Short C1−N1 and C4−N4 bond lengths
are in agreement with localized carbon−nitrogen double
bonds. For X, the presence of an AlH2 group was established
by IR and 1H NMR spectroscopy: the asymmetric and
symmetric Al−H stretches occur at 1802 and 1833 cm−1, and
the mean value of 1823 cm−1 is blue-shifted in comparison to
the respective complexes incorporating the NacNac ligand or
its electron-rich derivative with mean values of 1814 and 1817
cm−1, respectively.31
Upon coordination of an additional Al(CH3)3 molecule, the
dinuclear species XII is formed, which resembles structural
features of the dinuclear bicyclic aluminum derivatives
obtained by the Schulz group from the reaction of propylene-
and butylene-bridged bis(carbodiimides) with trimethyl
aluminum at elevated temperatures.11b In comparison with
XI, the Al1−N4 bond remains unaffected within the
experimental error but the bonds associated with N1, C1,
and N5 are elongated due to the Lewis acidic nature of Al2.
The 1H NMR spectrum of XII contains distinct resonances for
the methyl groups of the bridging (Al1) and terminal (Al2)
aluminum centers at −0.93 and −1.36 ppm, respectively.
While the latter appears as a sharp singlet, indicating free
rotation about the Al2−C and Al2−N1 bonds, the former is
broad due to hindered rotation about the Al1−C bonds, which
is in contrast with the results obtained for XI, where one sharp
singlet at −0.93 ppm was observed. In the 27Al NMR spectra of
the compounds X−XII, no signals were detectable.
EXPERIMENTAL SECTION
Further details on the synthesis of the protio ligands are given in the
Supporting Information.
■
General Procedure for the Synthesis of Bis(carbodiimides)
4. A suspension of the bis(thiourea) 1 (3.0 mmol) and triethylamine
(6.00 g, 60 mmol) in 25 mL of dichloromethane was cooled with an
ice bath and treated with cyanuric chloride (2.22 g, 12 mmol). After
15 min 50 mL of petroleum ether was added to the yellow-orange
mixture and stirring was continued for an additional 5 min. The solids
were filtered off, and the gray residue was washed with 25 mL of
petroleum ether. The combined filtrates were concentrated in vacuo,
and the resulting mixture was suspended in 15 mL of n-pentane and
filtered over Celite. The filter cake was washed with 5 mL of n-
pentane, and the combined filtrates were concentrated in vacuo,
giving the desired bis(carbodiimides) 4 as oily liquids pure enough for
subsequent reactions.
4a. Yellow oil, 63%. 1H NMR (300 MHz, CDCl3): δ 1.26 (d, 3JHH
3
= 6.8 Hz, 24H, CH3) 3.37 (sept, JHH = 6.8 Hz, 4H, CHCH3), 3.61
Finally, the protio ligand IVa was reacted with diethylzinc at
−78 and 90 °C, yielding in both cases colorless crystals after a
(br, 4H, N(CH2)2N), 7.12 (br, 6H, CHarom). 13C{1H} NMR (75
MHz, CDCl3): δ 23.3 (CHCH3), 29.1 (CHCH3), 47.7 (N(CH2)2N),
123.2 (m-CHarom), 125.0 (p-CHarom), 132.9 (NCN), 133.8 (o-Carom),
142.4 (i-Carom). IR (cm−1): ν(NCN) 2130, ν(CN) 1659.
1
short workup. The H NMR spectra are identical for both
fractions and have a complex pattern of resonances, whose
origin has been elucidated by means of single-crystal X-ray
diffraction (Figure 8). While one zinc atom (Zn2) is chelated
by the two Dipp-substituted nitrogen atoms N8 and N10,
forming a nonplanar six-membered ZnC2N3 ring, similarly to
compounds X and XI, a second zinc (Zn1) is acting as a bridge
to a second deprotonated ligand molecule in a nonchelating
fashion. Overall, the dinuclear complex XIII consisting of two
anionic ligand molecules and two zinc ions with a coordination
General Procedure for the Synthesis of Biguanides IV from
Bis(carbodiimides) 4. A mixture of the bis(carbodiimide) 4 (1.5
mmol) and pyrrolidine (2.13 g, 30.0 mmol) in 40 mL of acetonitrile
was stirred at 80 °C for 16 h. After it was cooled to room temperature,
the solution was concentrated in vacuo. The residue was dissolved in
hot acetonitrile, cooled to room temperature, and stored at −28 °C,
causing crystallization of the desired biguanide.
General Procedure for the Synthesis of Biguanides IV/IV′
from Bis(thioureas) 1. A mixture of the bis(thiourea) 1 (1.0 mmol),
I
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1.42 g (20.0 mmol) of pyrrolidine, and 0.45 g (2.1 mmol) of PbO in
40 mL of toluene was stirred at 100 °C for 16 h. After the mixture was
cooled to room temperature, the solids were filtered off and washed
with toluene and the combined filtrates were concentrated in vacuo.
The obtained residue was dissolved in hot acetonitrile, and the
desired product was crystallized at −28 °C.
(CHCH3), 39.7 (CNCH2CH2NCH), 49.6 (CH2(CH2)2CH2), 53.5
(CNCH2CH2NH), 123.6 (CHarom), 124.8 (CHarom), 126.7 (CHarom),
137.5 (Carom), 139.0 (Carom), 142.5 (Carom), 143.4 (Carom), 150.0
(NpyrrolidineCNDipp), 157.6 (NCNDipp), 1H NMR (400 MHz,
3
CDCl3, −40 °C (233 K)): δ 1.04 (d, JHH = 6.8 Hz, 6H, CHCH3),
3
3
1.12 (d, JHH = 6.8 Hz, 6H, CHCH3), 1.21 (d, JHH = 6.8 Hz, 6H,
1
3
CHCH3), 1.23 (d, JHH = 6.8 Hz, 6H, CHCH3), 1.65 (br, 4H,
IVa′. Colorless blocks, 39%, H NMR (300 MHz, CDCl3): δ 1.17
CH2(CH2)2CH2), 3.18 (br, 4H, CHCH3), 3.32 (t, 3JHH = 8.2 Hz, 2H,
CNCH2CH2NH), 3.49 (br, 4H, CH2(CH2)2CH2), 4.06 (t, 3JHH = 8.2
Hz, 2H, CNCH2CH2NH), 4.86 (br, 1H, NH), 7.08−7.28 (m, 6H,
CHarom). 13C{1H} NMR (101 MHz, CDCl3, −40 °C (233 K)): δ 23.2
(CHCH3), 24.8 (CHCH3), 25.3 (CH2(CH2)2CH2), 25.4 (CHCH3),
26.2 (CHCH3), 27.9 (CHCH3), 28.1 (CHCH3), 39.6
(CNCH2CH2NH), 49.4 (CH2(CH2)2CH2), 50.2 (CNCH2CH2NH),
123.7 (CHarom), 124.2 (CHarom), 126.4 (CHarom), 138.1 (o-Carom),
138.8 (o-Carom), 142.0 (i-Carom), 143.1 (i-Carom), 149.9
(NpyrrolidineCNDipp), 157.0 (NCNDipp), Anal. Calcd for
C32H47Cl2N5Zn·0.7C7H8: C, 63.09; H, 7.55; N, 9.97. Found: C,
63.05; H, 7.52; N, 9.89.
3
3
(d, JHH = 6.8 Hz, 6H, CHCH3), 1.19 (d, JHH = 6.8 Hz, 6H,
3
3
CHCH3), 1.22 (d, JHH = 6.8 Hz, 6H, CHCH3), 1.24 (d, JHH = 6.8
Hz, 6H, CHCH3), 1.65 (br, 4H, CH2(CH2)2CH2), 3.19 (sept, 3JHH
=
=
6.8 Hz, 4H, CHCH3), 3.25 (br, 4H, CH2(CH2)2CH2), 3.36 (t, 3JHH
7.1 Hz, 2H, NCH2CH2NHC), 3.83 (br, 1H, NCH2CH2NHC), 3.95
3
3
(t, JHH = 7.1 Hz, 2H, NCH2CH2NHC), 6.94 (t, JHH = 7.2 Hz, 1H,
p-CHarom), 6.98 (t, 3JHH = 7.2 Hz, 1H, p-CHarom), 7.04 (d, 3JHH = 7.2
Hz, 2H, m-CHarom), 7.11 (d, 3JHH = 7.1 Hz, 2H, m-CHarom). 13C{1H}
NMR (75 MHz, CDCl3): δ 22.5 (CHCH3), 23.6 (CHCH3), 23.7
(CHCH3), 23.8 (CHCH3), 25.3 (CH2(CH2)2CH2), 28.1 (CHCH3),
28.2 (CHCH3), 40.0 (NCH2CH2NHC), 47.2 (NCH2CH2NHC),
48.0 (CH2(CH2)2CH2), 121.2 (p-CHarom), 122.1 (m-CHarom), 122.4
(p-CHarom), 123.1 (m-CHarom), 138.4 (o-Carom), 140.2 (o-Carom), 144.5
(i-Carom), 145.3 (i-Carom), 145.6 (NpyrrolidineCNDipp), 151.4
(NHCNDipp). IR (cm−1): ν(NH) 3427, ν(CH3) 2956, ν(CN) =
1672, ν(CCarom) 1639, 1585, and 1483. HR-ESI-MS: calcd for
C32H47N5 [M + H]+ 502.3910; found 502.3903. Anal. Calcd for
C32H47N5: C, 76.60; H, 9.44; N, 13.96. Found: C, 76.68; H, 9.49; N,
13.85.
VII. A mixture of 0.50 g (1.0 mmol) of (E)-N-(2,6-diisopropyl-
phenyl)-1-((E)-((2,6-diisopropylphenyl)imino)(pyrrolidin-1-yl)-
methyl)imidazolidinyl-2-imine and 0.20 g (2.0 mmol) of CuCl in 10
mL of tetrahydrofuran was stirred at room temperature for 16 h. The
mixture was filtered off, and the filtrate was concentrated in vacuo.
The residue was dissolved in toluene, and the desired product VII was
crystallized in the form of colorless blocks at room temperature.
Colorless crystals, 0.37 g, 0.5 mmol, 53%. 1H NMR (400 MHz,
Synthesis of the Metal Complexes V−XIII. V. A mixture of 0.50
g (1.0 mmol) of (E)-N-(2,6-diisopropylphenyl)-1-((E)-((2,6-
diisopropylphenyl)imino)(pyrrolidin-1-yl)methyl)imidazolidinyl-2-
imine and 0.21 g (1.1 mmol) of SnCl2 in 10 mL of tetrahydrofuran
was stirred at room temperature for 16 h. The mixture was filtered off,
and the filtrate was concentrated in vacuo. The residue was dissolved
in toluene, and the desired product V was crystallized in the form of
colorless blocks at room temperature. Colorless crystals, 0.43 g, 0.6
3
3
CDCl3): δ 1.10 (d, JHH = 6.8 Hz, 6H, CHCH3), 1.12 (d, JHH = 6.8
Hz, 6H, CHCH3), 1.25 (d, 3JHH = 6.8 Hz, 6H, CHCH3), 1.38 (d, 3JHH
= 6.8 Hz, 6H, CHCH3), 1.77 (quint, JHH = 3.3 Hz, 4H,
CH2(CH2)2CH2), 3.09 (sept, JHH = 6.8 Hz, 4H, CHCH3), 3.13
(br, 4H, CH2(CH2)2CH2), 3.70 (t, JHH = 8.2 Hz, 2H,
3
3
3
3
NCH2CH2NH), 3.92 (t, JHH = 8.2 Hz, 2H, NCH2CH2NH), 7.00−
7.07 (m, 3H, CHarom), 7.26 (d, 3JHH = 7.7 Hz, 2H, m-CHarom), 7.45 (t,
3JHH = 7.7 Hz, 1H, p-CHarom), 8.89 (br, 1H, NH). 13C{1H} NMR
(101 MHz, CDCl3): δ 22.7 (CHCH3), 23.6 (CHCH3), 24.4
(CHCH3), 24.6 (CHCH3), 25.4 (CH2(CH2)2CH2), 28.5 (CHCH3),
29.0 (CHCH3), 49.0 (CH2(CH2)2CH2), 49.3 (NCH2CH2NH), 50.0
(NCH2CH2NH), 122.8 (m-CHarom), 123.4 (p-CHarom), 125.0 (m-
CHarom), 129.7 (p-CHarom), 130.8 (i-Carom), 138.7 (o-Carom), 142.0 (i-
1
3
mmol, 62%. H NMR (400 MHz, CDCl3): δ 1.08 (d, JHH = 6.8 Hz,
3
3
6H, CHCH3), 1.09 (d, JHH = 6.8 Hz, 6H, CHCH3), 1.25 (d, JHH
6.8 Hz, 6H, CHCH3), 1.39 (d, JHH = 6.8 Hz, 6H, CHCH3), 1.77
=
3
3
(quint, JHH = 3.3 Hz, 4H, CH2(CH2)2CH2), 3.04−3.17 (m, 4H,
3
CH2(CH2)2CH2), 3.11 (sept, JHH = 6.8 Hz, 4H, CHCH3), 3.94 (t,
3JHH = 8.2 Hz, 2H, NCH2CH2NC), 4.19 (t, JHH = 8.2 Hz, 2H,
3
3
C
arom), 145.3 (NpyrrolidineCNDipp), 145.6 (o-Carom), 161.4 (N
NCH2CH2NC), 7.02−7.08 (m, 3H, CHarom), 7.27 (d, JHH = 7.7
3
CNDipp). Anal. Calcd for C32H47Cl2Cu2N5·0.18C7H8: C, 55.63; H,
6.81; N, 9.81. Found: C, 55.59; H, 6.90; N, 9.99.
Hz, 2H, m-CHarom), 7.34 (t, JHH = 7.7 Hz, 1H, p-CHarom), 9.70 (br,
1H, NH). 13C{1H} NMR (101 MHz, CDCl3): δ 22.7 (CHCH3), 23.1
(CHCH3), 24.7 (CHCH3), 24.9 (CHCH3), 25.5 (CH2(CH2)2CH2),
28.5 (CHCH3), 28.9 (CHCH3), 45.7 (CNCH2CH2NC), 49.1
(CH2(CH2)2CH2), 50.1 (CNCH2CH2NC), 122.9 (m-CHarom),
123.8 (p-CHarom), 126.0 (m-CHarom), 130.4 (p-CHarom), 131.7 (i-
VIII. A mixture of 0.50 g (1.0 mmol) of (E)-N-(2,6-
diisopropylphenyl)-1-((E)-((2,6-diisopropylphenyl)imino)-
(pyrrolidin-1-yl)methyl)imidazolidinyl-2-imine and 0.20 g (1.0
mmol) of potassium bis(trimethylsilyl)amide in 20 mL of toluene
was stirred at 60 °C for 16 h. After a few minutes a precipitate could
be observed. The suspension was filtered off, and the precipitate was
dried in vacuo. The residue was dissolved in a toluene/diethyl ether
mixture, and the desired product VIII was crystallized in the form of
colorless blocks at room temperature. Colorless crystals, 0.38 g, 0.7
C
arom), 139.0 (o-Carom), 141.6 (o-Carom), 145.7 (NpyrrolidineCNDipp),
146.9 (i-Carom), 160.7 (NCNDipp), 119Sn NMR (199 MHz,
CDCl3): δ −178.7. Anal. Calcd for C32H47Cl2N5Sn: C, 55.59; H,
6.85; N, 10.13. Found: C, 55.80, H, 6.95, N, 10.12.
VI. A 0.50 g portion (1.0 mmol) of(E)-N-(2,6-diisopropylphenyl)-
1-((E)-((2,6-diisopropylphenyl)imino)(pyrrolidin-1-yl)methyl)-
imidazolidinyl-2-imine was dissolved in 20 mL of tetrahydrofuran.
Also, 0.14 g (1 mmol) of ZnCl2 was dissolved in another Schlenk.
Then the solution of the biguanide-based ligand system was added to
the ZnCl2 solution. Immediately after the addition a white suspension
could be observed. The reaction mixture was stirred for 16 h. The
mixture was concentrated in vacuo. The white solid was taken up in
hot toluene (100 mL). The product VI was crystallized in the form of
colorless blocks at room temperature after a few minutes. Colorless
1
mmol, 71%. H NMR (400 MHz, thf-d8, room temperature): δ 1.09
3
3
(d, JHH = 6.8 Hz, 6H, CHCH3), 1.12 (d, JHH = 6.8 Hz, 6H,
3
3
CHCH3), 1.21 (d, JHH = 6.8 Hz, 6H, CHCH3), 1.23 (d, JHH = 6.8
Hz, 6H, CHCH3), 1.56 (br, 4H, CH2(CH2)2CH2), 3.20−3.35 (br,
3
4H, CH2(CH2)2CH2), 3.23 (t, JHH = 7.1 Hz, 2H, CNCH2CH2N
3
3
C), 3.35 (sept, JHH = 6.8 Hz, 4H, CHCH3), 3.62 (t, JHH = 7.1 Hz,
2H, CNCH2CH2NC), 6.67−6.72 (m, 2H, p-CHarom), 6.90 (d, 3JHH
3
= 7.5 Hz, 2H, m-CHarom), 6.95 (d, JHH = 7.5 Hz, 2H, m-CHarom).
13C{1H} NMR (101 MHz, thf-d8, room temperature): δ 22.7
(CHCH3), 23.6 (CHCH3), 24.3 (CHCH3), 24.4 (CHCH3), 26.0
(CH2(CH2)2CH2), 28.5 (CHCH3), 48.5 (CH2(CH2)2CH2), 50.0
(CNCH2CH2NC), 51.0 (CNCH2CH2NC), 118.3 (p-CHarom),
120.3 (p-CHarom), 122.2 (m-CHarom), 122.4 (m-CHarom), 138.5 (o-
1
crystals, 0.56 g, 0.9 mmol, 88%. H NMR (400 MHz, CDCl3, room
3
temperature): δ 1.15 (br, 6H, CHCH3), 1.23 (d, JHH = 6.8 Hz, 6H,
3
3
CHCH3), 1.27 (d, JHH = 6.8 Hz, 6H, CHCH3), 1.29 (d, JHH = 6.8
Hz, 6H, CHCH3), 1.73 (br,4H, CH2(CH2)2CH2), 2.94 (br, 4H,
CHCH3), 3.35 (br, 2H, CNCH2CH2NH), 3.54 (br, 4H,
CH2(CH2)2CH2), 4.09 (br, 2H, CNCH2CH2NH), 4.77 (br, 1H,
NH), 7.13−7.28 (m, 6H, CHarom). 13C{1H} NMR (101 MHz, CDCl3,
room temperature): δ 23.4 (CHCH3), 24.1 (CH2(CH2)2CH2), 24.6
(CHCH3), 25.1 (CHCH3), 26.2 (CHCH3), 28.0 (CHCH3), 28.4
C
arom), 143.0 (o-Carom), 148.6 (i-Carom), 150.8 (NpyrrolidineCNDipp),
1
156.3 (i-Carom), 163.7 (NCNDipp). H NMR (400 MHz, thf-d8,
3
−50 °C (223 K)): δ 1.08 (d, JHH = 6.8 Hz, 6H, CHCH3), 1.12 (d,
3JHH = 6.8 Hz, 6H, CHCH3), 1.20 (d, JHH = 6.8 Hz, 6H, CHCH3),
3
1.22 (d, 3JHH = 6.8 Hz, 6H, CHCH3), 1.42 (br, 2H, CH2(CH2)2CH2),
J
https://dx.doi.org/10.1021/acs.inorgchem.9b03093
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
pubs.acs.org/IC
Article
3
1.66 (br, 2H, CH2(CH2)2CH2), 2.80 (br, 2H, CH2(CH2)2CH2), 3.21
(br, 2H, CH2(CH2)2CH2), 3.33 (br, 2H, CNCH2CH2NC), 3.35
(br, 8H, NCH2(CH2)2CH2), 1.49 (d, JHH = 6.8 Hz, 12H, CHCH3),
3
1.58 (quin, JHH = 3.2 Hz, 4H, O(CH2)2(CH2)2), 2.66 (br, 8H,
3
(sept, JHH = 6.8 Hz, 4H, CHCH3), 3.57 (br, 2H, CNCH2CH2N
NCH2(CH2)2CH2), 2.93 (sept, 3JHH = 6.8 Hz, 4H, CHCH3), 3.02 (t,
3
3
C), 6.71 (br, 2H, p-CHarom), 6.90 (d, JHH = 7.5 Hz, 2H, m-CHarom),
3JHH = 7.5 Hz, CNCH2CH2NC), 3.44 (sept, JHH = 6.8 Hz, 4H,
3
6.95 (d, JHH = 7.5 Hz, 2H, m-CHarom). 13C{1H} NMR (101 MHz,
CHCH3), 3.89 (br, 4H, O(CH2)2(CH2)2), 3.92 (t, 3JHH = 7.5 Hz, 4H,
CNCH2CH2NC), 6.85−7.06 (m, 12H, CHarom), 13C{1H} NMR
(101 MHz, tol-d8, 70 °C): δ 21.9 (CHCH3), 22.7 (CHCH3), 25.1
(CH2(CH2)2CH2), 25.1 (CHCH3), 25.6 (O(CH2)2(CH2)2), 26.6
(CHCH3), 28.0 (CHCH3), 29.0 (CHCH3), 45.4 (CNCH2CH2N
C), 47.1 (CH2(CH2)2CH2), 54.7 (CNCH2CH2NC), 69.3 (O-
(CH2)2(CH2)2), 122.1 (m-CHarom), 122.1 (p-CHarom), 122.8
(CHarom), 138.5 (o-Carom), 142.0 (o-Carom), 144.7 (i-Carom), 145.5 (i-
thf-d8, −50 °C (223 K)): δ 22.7 (CHCH3), 23.5 (CHCH3), 24.3
(CHCH3), 24.5 (CHCH3), 26.1 (CH2(CH2)2CH2), 28.5 (CHCH3),
48.5 (CH2(CH2)2CH2), 50.0 (CNCH2CH2NC), 51.0
(CNCH2CH2NC), 118.3 (p-CHarom), 120.2 (p-CHarom), 122.2
(m-CHarom), 138.2 (o-Carom), 142.7 (o-Carom), 148.6 (i-Carom), 151.3
(NpyrrolidineCNDipp), 156.0 (i-Carom), 163.6 (NCNDipp). Anal.
Calcd for C32H46KN5: C, 71.20; H, 8.59; N, 12.97. Found: C, 71.03;
H, 8.69; N, 13.27.
C
arom), 145.8 (NpyrrolidineCNDipp), 167.1 (NNDipp). Anal. Calcd
IX. A 1.00 g portion (2.0 mmol) of (E)-N-(2,6-diisopropylphenyl)-
1-((E)-((2,6-diisopropylphenyl)imino)(pyrrolidin-1-yl)methyl)-
imidazolidinyl-2-imine was dissolved in 15 mL of toluene and 5 mL of
tetrahydrofuran. Then a di-n-butylmagnesium solution (1 mL, 1
mmol, 1 M in heptane) was added at −78 °C. During stirring for 16 h
the mixture was warmed room temperature. Then the mixture was
heated to 50 °C and it was stirred for 1 h. The mixture was filtered off,
and the filtrate was concentrated in vacuo. The residue was dissolved
in a toluene/pentane/tetrahydrofuran mixture, and the product IX
was crystallized in the form of colorless blocks at room temperature.
Colorless crystals, 0.72 g, 0.7 mmol, 66%. 1H NMR (400 MHz, C6D6,
room temperature): δ 1.23−1.25 (m, 48H, CHCH3), 0.96−1.90 (br,
for C68H100MgN10O·0.5C4H8O: C, 74.14; H, 9.24; N, 12.38. Found:
C, 73.78; H, 9.18; N, 12.72.
X. A mixture of 0.50 g (1.0 mmol) of (E)-N-(2,6-diisopropyl-
phenyl)-1-((E)-((2,6-diisopropylphenyl)imino)(pyrrolidin-1-yl)-
methyl)imidazolidinyl-2-imine and 0.09 g (1.0 mmol) of AlH3·NMe3
in 8 mL of tetrahydrofuran was stirred at room temperature for 16 h.
The mixture was filtered off, and the filtrate was concentrated in
vacuo. The residue was dissolved in toluene, and the desired product
X was crystallized in the form of colorless blocks at room temperature.
Colorless crystals, 0.27 g, 0.5 mmol, 51%. 1H NMR (300 MHz,
3
3
CDCl3): δ 1.22 (d, JHH = 6.8 Hz, 6H, CHCH3), 1.23 (d, JHH = 6.8
Hz, 6H, CHCH3), 1.27 (d, 3JHH = 6.8 Hz, 6H, CHCH3), 1.28 (d, 3JHH
3
3
8H, NCH2(CH2)2CH2), 1.49 (quint, JHH = 7.4 Hz, 4 H,
= 6.8 Hz, 6H, CHCH3), 1.70 (quin, JHH = 3.2 Hz, 4H,
CH2(CH2)2CH2), 3.02 (quin, JHH = 3.2 Hz, 4H, CH2(CH2)2CH2),
3.33 (sept, JHH = 6.8 Hz, 2H, CHCH3), 3.41 (sept, JHH = 6.8 Hz,
3
O(CH2)2(CH2)2), 2.97 (br, 16H, CH2(CH2)2CH2, CHCH3,
3
3
3
CNCH2CH2NC), 3.50 (sept, JHH = 6.8 Hz, 4H, CHCH3), 3.89
3
(br, 4H, O(CH2)2(CH2)2), 3.97 (br, 4H, CNCH2CH2NC), 7.00−
7.16 (m, 12H, CHarom). 13C{1H} NMR (101 MHz, C6D6, room
2H, CHCH3), 3.56 (t, JHH3 = 7.1 Hz, 2H, CNCH2CH2NC), 3.72
(br, 2H, AlH2), 3.97 (t, JHH = 7.1 Hz, 2H, CNCH2CH2NC),
7.12−7.27 (m, 6H, CHarom). 13C{1H} NMR (75 MHz, CDCl3): δ
23.5 (CHCH3), 24.7 (CHCH3), 25.5 (CH2(CH2)2CH2), 26.7
(CHCH3), 28.3 (CHCH3), 28.5 (CHCH3), 49.2 (CNCH2CH2N
C), 50.2 (CH2(CH2)2CH2), 51.1 (CNCH2CH2NC), 123.9 (m-
CHarom), 124.7 (m-CHarom), 126.1 (p-CHarom), 127.4 (p-CHarom),
137.9 (i-Carom), 139.0 (i-Carom), 144.2 (o-Carom), 145.6 (o-Carom),
155.1 (NpyrrolidineCNDipp), 156.9 (NCNDipp); 27Al NMR (104
MHz, CDCl3): δ no signal. IR (cm−1): ν(CH3) 2962, ν(AlH2) 1833
and 1802, ν(CN) 1628. Anal. Calcd for C32H48N5Al: C, 72.55; H,
9.13; N, 13.22. Found: C, 72.79, H, 9.10, N, 13.26.
temperature):
δ 21.7 (CHCH3), 22.8 (CHCH3), 25.1
(NCH2(CH2)2CH2), 25.3 (CHCH3), 25.4 (O(CH2)2(CH2)2), 26.8
(CHCH3), 27.9 (CHCH3), 29.0 (CHCH3), 45.1 (CNCH2CH2N
C), 54.6 (CNCH2CH2NC), 69.3 (O(CH2)2(CH2)2), 121.8
(CHarom), 122.0 (CHarom), 144.6 (Carom), 145.4 (Carom), 145.8
(NpyrrolidineCNDipp), 166.6 (NCNDipp), 1H NMR (400 MHz,
tol-d8, room temperature): δ 1.16−1.18 (m, 48H, CHCH3), 0.96−
3
1.90 (br, 8H, NCH2(CH2)2CH2), 1.69 (quint, JHH = 7.4 Hz, 4 H,
O(CH2)2(CH2)2), 2.94 (br, 16H, CH2(CH2)2CH2, CHCH3,
3
CNCH2CH2NC), 3.43 (sept, JHH = 6.8 Hz, 4H, CHCH3), 3.87
(br, 4H, O(CH2)2(CH2)2), 3.92 (br, 4H, CNCH2CH2NC), 6.90−
7.06 (m, 12H, CHarom). 13C{1H} NMR (101 MHz, tol-d8, room
XI. A 0.50 g portion (1.0 mmol) of (E)-N-(2,6-diisopropylphenyl)-
1-((E)-((2,6-diisopropylphenyl)imino)(pyrrolidin-1-yl)methyl)-
imidazolidinyl-2-imine was dissolved in 20 mL of toluene. The
solution was heated to 90 °C. At this temperature a trimethylalumi-
num solution (0.5 mL, 1.0 mmol, 2 M in toluene) was added
dropwise. The mixture was stirred at 90 °C for 16 h. Then the
reaction mixture was warmed to room temperature. After that the
mixture was filtered off and the filtrate was concentrated in vacuo. The
residue was dissolved in a toluene/pentane mixture, and the product
XI was crystallized in the form of colorless blocks at room
temperature. Colorless crystals, 0.40 g, 0.7 mmol, 71%. 1H NMR
temperature):
δ 21.7 (CHCH3), 22.8 (CHCH3), 25.1
(NCH2(CH2)2CH2), 25.3 (CHCH3), 25.5 (O(CH2)2(CH2)2), 26.8
(CHCH3), 27.9 (CHCH3), 29.0 (CHCH3), 45.1 (CNCH2CH2N
C), 54.6 (CNCH2CH2NC), 69.3 (O(CH2)2(CH2)2), 121.9
(CHarom), 122.0 (CHarom), 144.6 (Carom), 145.4 (Carom), 145.8
(NpyrrolidineCNDipp), 166.7 (NCNDipp), 1H NMR (400 MHz,
tol-d8, −50 °C): δ 0.89 (br, 2H, CH2(CH2)2CH2), 0.98−1.47 (m,
52H, CHCH3, NCH2(CH2)2CH2), OCH2(CH2)2CH2),), 1.55 (br,
2H, NCH2(CH2)2CH2), 1.96 (d, 3JHH = 6.8 Hz, 6H, CHCH3,), 2.25−
3
3
2.32 (m, 2H, NCH2(CH2)2CH2), 2.56 (t, JHH = 9.4 Hz, 2H,
(400 MHz, CDCl3): δ −0.93 (s, 6H, Al(CH3)2), 1.19 (d, JHH = 6.8
Hz, 6H, CHCH3), 1.21 (d, 3JHH = 6.8 Hz, 6H, CHCH3), 1.24 (d, 3JHH
CNCH2CH2NC), 2.81−3.00 (m, 6H, CHCH3, CNCH2CH2NC,
NCH2(CH2)2CH2), 3.35−3.43 (m, 8H, CHCH3, NCH2(CH2)2CH2),
3.65−3.72 (m, 2H, OCH2(CH2)2CH2), 3.78−3.84 (m, 4H,
CNCH2CH2NC, OCH2(CH2)2CH2), 4.04−4.10 (m, 2H,
CNCH2CH2NC), 6.94−7.21 (m, 12H, CHarom). 13C{1H} NMR
(101 MHz, tol-d8, −50 °C): δ 21.2 (CHCH3), 21.5 (CHCH3), 22.7
(CHCH3), 22.9 (CHCH3), 25.0 (CH2(CH2)2CH2), 25.2 (CHCH3),
25.3 (CH2(CH2)2CH2), 25.4 (CH2(CH2)2CH2), 25.6 (CHCH3),
26.1 (CHCH3), 27.5 (CHCH3), 27.7 (CHCH3), 28.2 (CHCH3), 28.9
(CHCH3), 29.3 (CHCH3), 44.5 (CNCH2CH2NC), 45.5
(NCH2 (CH2 )2 CH2 ), 47.8 (NCH2 (CH2 )2 CH2 ), 54.6
(CNCH2CH2NC), 68.9 (O(CH2)2(CH2)2), 120.8 (m-CHarom),
121.5 (m-CHarom), 121.9 (m-CHarom), 122.3 (m-CHarom), 123.8 (p-
CHarom), 124.3 (p-CHarom), 137.2 (o-Carom), 139.3 (o-Carom), 139.8 (o-
3
= 6.8 Hz, 6H, CHCH3), 1.25 (d, JHH = 6.8 Hz, 6H, CHCH3), 1.73
3
3
(quin, JHH = 3.4 Hz, 4H, CH2(CH2)23CH2), 2.99 (quin, JHH = 3.4
Hz, 4H, CH2(CH2)2CH2), 3.31 (sept, JHH = 6.8 Hz, 2H, CHCH3),
3
3
3.34 (sept, JHH = 6.8 Hz, 2H, CHCH3), 3.48 (t, JHH = 7.1 Hz, 2H,
CNCH2CH2NC), 3.87 (t, 3JHH = 7.1 Hz, 2H, CNCH2CH2NC),
7.09−7.25 (m, 6H, CHarom). 13C{1H} NMR (101 MHz, CDl3): δ
−10.0 (Al(CH3)2), 23.7 (CHCH3), 24.3 (CHCH3), 25.1 (CHCH3),
25.4 (CH2(CH2)2CH2), 26.3 (CHCH3), 28.0 (CHCH3), 28.8
(CHCH3), 49.2 (CNCH2CH2NC), 49.9 (CH2(CH2)2CH2), 51.1
(CNCH2CH2NC), 123.6 (m-CHarom), 124.4 (m-CHarom), 125.0 (p-
CHarom), 127.1 (p-CHarom), 137.9 (i-Carom), 141.4 (i-Carom), 144.1 (o-
C
arom), 144.7 (o-Carom), 153.3 (NpyrrolidineCNDipp), 158.5 (N
CNDipp), 27Al NMR (104 MHz, CDCl3): δ no signal. Anal. Calcd
for C34H52N5Al: C, 73.21; H, 9.40; N, 12.56. Found: C, 73.21, H,
9.44, N, 12.51.
C
arom), 142.3 (o-Carom), 144.4 (i-Carom), 145.1 (i-Carom), 145.9
(NpyrrolidineCNDipp), 166.2 (N = CNDipp), 1H NMR (400 MHz,
tol-d8, 70 °C): δ 1.12 (d, 3JHH = 6.8 Hz, 12H, CHCH3), 1.13 (d, 3JHH
= 6.8 Hz, 12H, CHCH3), 1.18 (d, 3JHH = 6.8 Hz, 12H, CHCH3), 1.25
XII. A 0.50 g portion (1.0 mmol) of (E)-N-(2,6-diisopropylphenyl)-
1-((E)-((2,6-diisopropylphenyl)imino)(pyrrolidin-1-yl)methyl)-
K
https://dx.doi.org/10.1021/acs.inorgchem.9b03093
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
pubs.acs.org/IC
Article
imidazolidinyl-2-imine was dissolved in 20 mL of toluene. Then a
trimethylaluminum solution (1 mL, 2 mmol, 2 M in toluene) was
added at room temperature. The mixture was stirred for 16 h. The
mixture was filtered off, and the filtrate was concentrated in vacuo.
The residue was dissolved in toluene, and the product XII was
crystallized in the form of colorless blocks at room temperature.
Colorless crystals, 0.34 g, 0.5 mmol, 54%. 1H NMR (400 MHz,
CDCl3): δ −1.36 (s, 9H, Al(CH3)3), −0.93 (br, 6H, Al(CH3)2), 1.10
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.inorgchem.9b03093.
■
sı
Additional figures, experimental and computational
details, crystallographic data, NMR and IR spectra, and
XYZ coordinates (PDF)
3
3
(d, JHH = 6.8 Hz, 6H, CHCH3), 1.22 (d, JHH = 6.8 Hz, 6H,
Accession Codes
3
3
CHCH3), 1.27 (d, JHH = 6.8 Hz, 6H, CHCH3), 1.32 (d, JHH = 6.8
CCDC 1539253, 1812648, and 1954639−1954648 contain the
supplementary crystallographic data for this paper. These data
can be obtained free of charge via www.ccdc.cam.ac.uk/
data_request/cif, or by emailing data_request@ccdc.cam.ac.
uk, or by contacting The Cambridge Crystallographic Data
Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44
1223 336033.
Hz, 6H, CHCH3), 1.80 (br, 4H, CH2(CH2)2CH2), 3.02 (br, 8H,
3
CHCH3,CH2(CH2)2CH2), 3.69 (t, JHH
= 7.1 Hz, 2H,
3
CNCH2CH2NC) 3.83 (t, JHH = 7.1 Hz, 2H, CNCH2CH2N
3
C), 7.15 (m, 5H, CHarom), 7.26 (t, JHH = 7.6 Hz, 1H, p-CHarom).
13C{1H} NMR (101 MHz, CDl3): δ −10.2 (Al(CH3)2), −6.1
(Al(CH3)3), 23.5 (CHCH3), 23.6 (CHCH3), 24.7 (CHCH3), 25.5
(CH2(CH2)2CH2), 25.8 (CHCH3), 28.2 (CHCH3), 30.0 (CHCH3),
49.1 (CNCH2CH2NC), 49.4 (CNCH2CH2NC), 50.2
(CH2(CH2)2CH2), 124.7 (m-CHarom), 125.1 (m-CHarom), 126.0 (p-
CHarom), 128.0 (p-CHarom), 136.5 (i-Carom), 139.2 (i-Carom), 142.9 (o-
AUTHOR INFORMATION
Corresponding Author
■
Robert Kretschmer − Institute of Inorganic and Analytical
Chemistry (IAAC) and Jena Center for Soft Matter (JCSM),
Friedrich Schiller University Jena, 07743 Jena, Germany;
orcid.org/0000-0002-7731-3748;
C
arom), 143.6 (o-Carom), 152.4 (NpyrrolidineCNDipp), 160.9 (N
CNDipp). 27Al NMR (104 MHz, CDCl3): δ no signal. Anal. Calcd
for C37H61AlN5: C, 70.55; H, 9.76; N, 11.12. Found: C, 70.84, H,
9.81, N, 11.28.
XIII. A 0.50 g portion (1.0 mmol) of (E)-N-(2,6-diisopropylphen-
yl)-1-((E)-((2,6-diisopropylphenyl)imino)(pyrrolidin-1-yl)methyl)-
imidazolidinyl-2-imine was dissolved in 20 mL of toluene. Then a
diethylzinc solution (1.2 mL, 1.2 mmol, 1 M in hexane) was added at
−78 °C. During stirring for 16 h the mixture was warmed to room
temperature. The mixture was filtered off, and the filtrate was
concentrated in vacuo. The residue was taken up in pentane. The
product XIII was crystallized in the form of colorless blocks at room
temperature. Colorless crystals, 0.31 g, 0.3 mmol, 52%. 1H NMR (400
MHz, CDCl3): δ −0.08 (quart, 3JHH = 8.1 Hz, 2H, ZnCH2CH3), 0.13
Email: robert.kretschmer@uni-jena.de
Authors
Maximilian Dehmel − Institute of Inorganic and Analytical
Chemistry (IAAC), Friedrich Schiller University Jena, 07743
Jena, Germany
Valentin Vass − University of Regensburg, Institute of Inorganic
Chemistry, 93053 Regensburg, Germany
Lukas Prock − University of Regensburg, Institute of Inorganic
Chemistry, 93053 Regensburg, Germany
3
3
̈
(quart, JHH = 8.1 Hz, 2H, ZnCH2CH3), 0.82 (t, JHH = 8.1 Hz, 2H,
Helmar Gorls − Institute of Inorganic and Analytical Chemistry
3
ZnCH2CH3),), 0.98 (quint, JHH = 3.3 Hz, 4H, CH2(CH2)2CH2),
(IAAC), Friedrich Schiller University Jena, 07743 Jena,
Germany
1.09 (t, 3JHH = 8.1 Hz, 2H, ZnCH2CH3), 1.12 (d, 3JHH = 6.8 Hz, 6H,
3
CHCH3), 1.19 (d, JHH = 6.8 Hz, 6H, CHCH3), 1.35−1.40 (m, 4H,
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.inorgchem.9b03093
CH2(CH2)2CH2), 1.27 (d, 3JHH = 6.8 Hz, 6H, CHCH3), 1.29 (d, 3JHH
3
= 6.8 Hz, 6H, CHCH3), 1.39 (d, JHH = 6.8 Hz, 6H, CHCH3), 1.42
3
3
(d, JHH = 6.8 Hz, 6H, CHCH3), 1.43 (d, JHH = 6.8 Hz, 6H,
Author Contributions
3
3
CHCH3), 1.29 (d, JHH = 6.8 Hz, 6H, CHCH3), 2.55 (quint, JHH
3.3 Hz, 4H, CH2(CH2)2CH2), 3.07 (t, JHH = 7.1 Hz, 2H,
=
The manuscript was written through contributions of all
authors. All authors have given approval to the final version of
the manuscript.
3
3
CNCH2CH2NC), 3.21 (sept, JHH = 6.8 Hz, 2H, CHCH3), 3.31
(t, 3JHH = 7.1 Hz, 2H, CNCH2CH2NC), 3.45 (sept, 3JHH = 6.8 Hz,
3
Funding
2H, CHCH3), 3.48−3.54 (m, 4H, CH2(CH2)2CH2), 3.51 (t, JHH
=
3
The project was financially supported by the Fonds der
Chemischen Industrie, the Deutsche Forschungsgemeinschaft
(DFG, KR4782/3-1), the University of Regensburg, and the
Friedrich Schiller University Jena.
7.1 Hz, 2H, CNCH2CH2NC), 3.53 (sept, JHH = 6.8 Hz, 2H,
CHCH3), 3.74 (sept, 3JHH = 6.8 Hz, 2H, CHCH3), 3.90 (t, 3JHH = 7.1
Hz, 2H, CNCH2CH2NC), 7.00−7.28 (m, 12H, CHarom). 13C{1H}
NMR (101 MHz, CDCl3): δ −1.5 (ZnCH2CH3), −0.3 (ZnCH2CH3),
11.6 (ZnCH2CH3), 12.3 (ZnCH2CH3), 22.8 (CHCH3), 23.1
(CHCH3), 23.7 (CHCH3), 24.2 (CHCH3), 24.4 (CHCH3), 24.6
(CHCH3), 24.9 (CHCH3), 25.1 (CHCH3), 25.3 (CH2(CH2)2CH2),
25.6 (CH2(CH2)2CH2), 28.5 (CHCH3), 28.5 (CHCH3), 28.8
(CHCH3), 29.0 (CHCH3), 45.3 (CNCH2CH2NC), 48.4
(CH 2 ( C H2 )2 CH 2 ) , 4 9 . 0 ( C N CH 2 C H 2 N  C) , 49 . 2
(CH 2 ( C H2 )2 CH 2 ) , 5 0 . 3 ( C N C H 2 CH 2 N  C) , 51 . 4
(CNCH2CH2NC), 121.9 (m-CHarom), 122.8 (m-CHarom), 123.5
(m-CHarom), 124.2 (m-CHarom), 125.0 (p-CHarom), 126.1 (p-CHarom),
126.7 (p-CHarom), 126.8 (p-CHarom), 138.5 (o-Carom), 140.8 (o-Carom),
141.5 (o-Carom), 142.0 (i-Carom), 143.9 (o-Carom), 145.0 (i-Carom),
146.5 (i-Carom), 146.7 (NpyrrolidineCNDipp), 149.4 (i-Carom), 154.9
(NpyrrolidineCNDipp), 158.6 (NCNDipp), 162.1 (N = CNDipp);
Anal. Calcd for C68H102N10Zn2: C, 68.61; H, 8.64; N, 11.77. Found:
C, 68.34, H, 8.59, N, 11.62.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work is dedicated to T. Don Tilley on the occasion of his
65th birthday. Thanks are due to the Rechenzentrum of the
Friedrich Schiller University for the allocation of computer
time and Professor Manfred Scheer for generous support.
Furthermore, helpful discussions with the NMR department of
the Friedrich Schiller University are gratefully acknowledged.
ABBREVIATIONS
Dipp, 2,6-diisopropylphenyl; DMP, 2,6-dimethylphenyl; Nac-
Nac, β-diiminate
■
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