Versatile coordinating abilities of acyclic N4 and N2P2 ligand frameworks in conjunction with Sn[N(SiMe3)2]2

Article abstract of DOI:10.1039/c9dt00617f

The adaptability of three acyclic tetradentate ligands with the-CHR-CHR-(R = H or alkyl substituent) linker in the backbone: bis(α-iminopyridine) L1 and the reduced form L2 and diaminodiphosphine L3 to stabilize various stannylenes has been explored. The reaction of L1 with two equivalents of Sn[N(SiMe₃)₂]₂ led to the stabilization of a bisstannylene 1 through the ene-Amide transformation of L1. The reaction of bisstannylene 1 with B(C₆F₅)₃ and silver trifluoromethane sulfonate led to the formation of ligand stabilized Sn(ii) dications 2 and 3 respectively. A mixture of Sn(ii) dication 3 and a Sn(ii) monocation 4 has been obtained from the reaction between 1 and trimethylsilyl trifluoromethane sulfonate. A 1:1 stoichiometric reaction between L3 and Sn[N(SiMe₃)₂]₂ led to the isolation of a dimeric monostannylene 5 having a step-like structure with a Sn₂N₂ central ring. The reaction of L2 with Sn[N(SiMe₃)₂]₂ underwent an electron transfer reaction ultimately leading to bis(α-iminopyridine) isolation.

Full text of DOI:10.1039/c9dt00617f

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DOI: 10.1039/C9DT00617F
Journal Name
ARTICLE
Versatile coordinating abilities of acyclic N4 and N2P2 ligand
frameworks in conjunction with Sn[N(SiMe3)2]2
Ravindra K. Raut a, Padmini Sahoo a, Dipti Chimnapurea and Moumita Majumdar*a
Received 00th January 20xx,
Accepted 00th January 20xx
The adaptability of three acyclic tetradentate ligands with –CHR-CHR- (R = H or alkyl substituent) linker in the backbone:
bis(-iminopyridine) L1 and the reduced form L2 and diaminodiphosphine L3 to stabilize various stannylenes have been
explored. The reaction of L1 with two equivalents of Sn[N(SiMe3)2]2 led to the stabilization of a bisstannylene 1 through ene-
amide transformation of L1. Reaction of bisstannylene 1 with B(C6F5)3 and silver trifluoromethane sulfonate led to the
formation of ligand stabilized Sn(II) dications 2 and 3 respectively. A mixture of Sn(II) dication 3 and a Sn(II) monocation 4
have been obtained from the reaction between 1 and trimethylsilyl trifluoromethane sulfonate. A 1:1 stoichiometric reaction
between L3 and Sn[N(SiMe3)2]2 led to the isolation of a dimeric monostannylene 5 having a step-like structure with a Sn2N2
central ring. The reaction of L2 with Sn[N(SiMe3)2]2 underwent an electron transfer reaction ultimately leading to bis(-
iminopyridine) isolation.
DOI: 10.1039/x0xx00000x
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Introduction
Acyclic tetradentate ligands wherein the two bidentate
coordinating fragments are tethered by –CH2-CH2- bridges have
been widely used in stabilizing a variety of transition metals and
lanthanide complexes. The flexibility at the alkyl backbone
empowers the ligand denticity to vary from stabilizing
mononuclear complexes to bimetallic complexes. Recent
examples are the bis(Schiff-base) type ligand bis(-
iminopyridine) crafting bimetallic Cu(I) complexes,1 polynuclear
Fe(III) clusters,2 dinuclear Ln(III) complexes with single-molecule
magnet behaviour,3 luminescent Zn(II)/Hg(II) complexes,4 Co(II)
coordination polymer5 etc. Although there are plentiful
examples of the utilization of these ligand types in transition
metal chemistry, they have been less frequently known in low-
valent main group chemistry. With the increasing success of
main-group ligand systems in transition metal catalysts and
homogeneous catalytic transformations,6 the stabilization of
main-group elements within newer ligand frameworks stands
appropriate.
Scheme 1. Ligands
L and L1-L3.
bis(chlorogermyliumylidene).8 Worth mentioning,
bisimino ligand possessing –CH2-CH2- backbone has been used
a
bulky
to stabilize a boron dication and a dinuclear boron(II) dicationic
complex.9 Furthermore, the ligand
L
being redox-active,
underwent reductive cyclization upon reduction of the
stabilized bis(chlorogermyliumylidene)8 and can also be easily
reduced to give the bisamine L2. The methylimine proton in
L
We have successfully explored the potential of such bis(
iminopyridine) ligand in Group 14 E(II) (E = Ge, Sn) cationic
chemistry. The four N donor sites in the bis( -iminopyridine)
(Scheme 1) cumulatively stabilizes nucleophilic Ge(II)
-
L
and L1 can potentially undergo deprotonation in the presence
of a base to generate the ene-amide.10 Therefore, the versatile
binding possibilities make these much coveted ligands (L, L1 and
L2) to further explore the stabilization of various low-valent
main-group compounds.

L
a
dicationic center as the only example known so far.7 The
torsional amplitude of the –CH2-CH2- linker enables the
bifunctional modality of the same ligand to stabilize the first
The tetradentate ligands with a variety of different donor
groups N2X2 (X = P, O, S) are intriguing as they potentially
impose on the metals to discriminate between binding sites.11
In recent times, a variety of PNNP tetradentate ligand
frameworks have gained popularity in stabilizing pincer-type N-
a.Department of Chemistry, Indian Institute of Science Education and Research,
Pune-411008, Maharashtra, India.
†
Electronic Supplementary Information (ESI) available: All relevant spectra,
crystallographic detail, and DFT calculations. CCDC 1896341 ( ), 1913314 ( ),
1913312 ( ), 1913313 ( ), 1896344 ( ). For the ESI and crystallographic data in the
1
2
3
4
5
CIF or other electronic format see DOI: 10.1039/x0xx00000x
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Scheme 2. Syntheses of compound 1.
Heterocyclic tetrylene (PNNP)E (E = Ge, Sn).12 Such phosphine
functionalized germylene and stannyene pincer-type ligands
with a rigid or flexible backbone have emerged as versatile
frameworks appropriate for metal coordination and catalytic
applications.13
Very recently, we have employed the diaminodiphosphine14 or
diiminodiphosphine15 (PNNP) asbifunctional ligands to host two
[:GeCl]+
units,
bis(chlorogermyliumylidene).16 The steric and electronic effects
at the centre forges the stabilization of the
leading
to
the
formation
of
Figure 1. Molecular structure of 1 in the solid state (thermal
ellipsoids at 30%, H atoms are omitted for clarity). Selected bond
lengths [Å] and angles []:Sn1-N1 2.126(5), Sn1-N3 2.191(5), Sn2-N4
2.178(6), Sn2-N6 2.140(5), N2-Sn1-N3 73.1(2), N4-Sn2-N3 71.7(2).
P
bis(chlorogermyliumylidene) exclusively.16 Such flexible PNNP
ligands have been previously reported to stabilize both
dinuclear and mononuclear transition metal complexes as
efficient catalysts.17 Thus, appropriate utilization of this multi-
faceted N2P2 ligand in main-group chemistry is certainly a
worthwhile study.
analysis were grown at -40 C from the hexane solution. Alternately,
compound 1 has also been synthesized in good yield of 85% under
solvent-free conditions by heating the two precursors L1 and
Sn[N(SiMe3)2]2 taken in a 1:2 ratio at 60C. Notably, very small
amounts of metallic tin formation was observed under both the
reaction conditions which was removed by filtration from the
toluene solution. Compound 1 has been characterized using hetero-
nuclear NMR techniques (Figures S8-S11, ESI†). The 1H NMR
spectrum shows the clear disappearance of the –CH3 proton in L1 and
the presence of the terminal H2C=C protons at 4.87 and 4.43 ppm.
The corresponding 13C NMR spectrum shows peak at 82.13 ppm for
the terminal alkenyl carbon. A singlet resonance at -47 ppm appears
in the 119Sn{1H} NMR spectrum of compound 1 in C6D6, which falls
within the range of reported stannylene chemical shift values.18,12
UV/Vis spectra of compound 1 in tetrahydrofuran solvent exhibits
the longest wavelength absorption max = 401 nm ( = 2797 M-1cm-1)
(Figures S30-31, ESI†). Worth mentioning, reacting L1 and
Sn[N(SiMe3)2]2 in 1:1 ratio also led to the isolation of the
bisstannylene 1 as the sole product. There are two literature
precedence in main-group chemistry where the redox-active
iminopyridine based ligands form ene-amide complexes with Al(III)19
and Sn(II)18. In the case of Sn(II), the 2,6-diiminopyridine ligand (2,6-
In this contribution, we have chosen L1-L3 (Scheme 1) as the
three competent ligand frameworks with tetradentate N4 and
N2P2 donor sites and studied their coordinating abilities in
combination with Sn[N(SiMe3)2]2. The facile cleavage of Sn-N
bond makes Sn[N(SiMe3)2]2 as the appropriate precursor for a
variety of stannylene compounds. The formation of
bis(stannylene) by methylimine deprotonation of L1, the
conversion of L2 to in the presence of Sn[N(SiMe3)2]2, and the
a
L
formation of dimeric PSnP pincer-type ligand with free P
pockets through transamination reaction from L3 have been
discussed. Notably, the hitherto unknown conversion of the
bis(stannylene) to a L1 stabilized Sn(II) dication in the presence
of Lewis acids has been reported herewith.
Results and Discussion
Syntheses and Characterizations
Ligand L1 has been synthesized involving the simple Schiff-base
condensation following modified literature procedure.4 Reduction of
L using NaBH4 gave L2, and reduction of the corresponding
diiminodiphosphine with LiAlH4 led to L3 in acceptable yields (Figures
S1-S7, ESI†).
[ArN=C(Me)]2(NC5H3) (Ar
=
C6H3-2,6-iPr2)) undergo ene-amide
formation only on one side leading to the stabilization of the
heteroleptic monostannylene.18
The bisstannylene compound 1 (Scheme 2) was obtained from a
reaction between L1 and Sn[N(SiMe3)2]2 taken in a 1:2 ratio in
toluene for 2 days at room temperature. Deprotonation of the two
methylimines occur to generate the ene-amide stabilized
bisstannylene 1 with concomitant elimination of NH(SiMe3)2 which
was removed under reduced pressure. Compound 1 was obtained in
82% yield as an orange precipitate by the addition of small amounts
of pentane. Orange coloured single crystals of 1 suitable for X-ray
The reactivity of bisstannylene 1 with Lewis acids have been studied.
Bisstannylene 1 was reacted with two equivalents of B(C6F5)3 at room
temperature in toluene to afford the zwitterionic adduct 2 (Scheme
3) with the elimination of one equivalent of Sn[N(SiMe3)2]2. The
colourless crystals of the zwitterionic 2 was obtained in a yield of 48%
by layering with pentane. Compound 2 has been characterized by 1H,
19F and 11B NMR spectroscopy in CDCl3 (Figures S12-14, ESI†). Two
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Scheme 3. Reactivity study of compound 1.
peaks at -12.3 and -13.3 ppm appear in the 11B spectrum, revealing
the two different environments around the B centers within the
molecule even in solution state at the NMR time scale. The 19F NMR
spectrum shows peaks that corroborates with the borate group. The
characteristic –CH2B(C6F5)3 proton peak appears at 3.85 and 3.24
ppm in the 1H NMR spectrum of 2. 119Sn and 13C NMR were not
obtained owing to the poor solubility of the zwitterionic compound
2. Precedence to zwitterionic adduct formation with B(C6F5)3 have
been reported in the case of -ketiiminate stabilized silylene and
germylene.20 To the best of our knowledge, this is the first example
of a stannylene exhibiting such a dipolar behaviour ultimately leading
to the conversion to a Sn(II) dication.
The reaction of bisstannylene 1 with silver trifluoromethane
sulfonate in tetrahydrofuran at room temperature led to the
isolation of crystals of L1 stabilized Sn(II) dication 3 in 40% yield.
Presumably, the solvent acts as the proton source. Compound 3 has
been characterized using hetero-nuclear NMR techniques in CD3CN
(Figures S15-S18, ESI†). The 1H NMR spectrum of 3 appears
considerably downfield shifted compared to the free ligand L1,
indicating the coordination of the Sn(II) dication to L1 binding sites.
The 19F NMR peak at -79.29 ppm reflects the presence of the triflate
anion. The 119Sn NMR spectrum displays a peak at -596 ppm, which
is considerably downfield shifted compared to the literature
reported chemical shifts of [Sn(C7H8)3][B(C6F5)4]2 (-1468 ppm)21 and
[Sn(CH3CN)6][Al(ORF)4]2 (RF = C(CF3)3) ( - 1490 ppm)22 in CD3CN. This
suggests that compound 3 has a lower net coordination number in
CD3CN. Crystals of stannylene-silver coordination complex23 has not
been obtained from this reaction.
Figure 2. Molecular structure of 2 in the solid state (thermal
ellipsoids at 30%, H atoms are omitted for clarity). Selected bond
lengths [Å]: Sn1-N1 2.329(3), Sn1-N2 2.274(3), Sn1-N3 2.273(3), Sn1-
N4 2.328(3), B1-C7 1.718(5), B2-C14 1.702(5).
Sn[N(SiMe3)2]2.24 The transamination reaction was revisited in this
contribution. In an effort to prepare bisstannylene by
transamination, we reacted L2 with two equivalents of
Sn[N(SiMe3)2]2 in tetrahydrofuran at room temperature (Scheme 4).
Immediate intense red colouration along with precipitation was
observed. Single crystals grown from ethereal solvent post filtration
showed the generation of bis(-iminopyridine) L in good amounts
(crystallization yield = 70%) after 4 days (Figures S26-S27, ESI†).
Essentially, ligand L2 being redox non-innocent triggers the
conversion of the in situ generated stannylene to L and Sn(0) along
with the generation of NH(SiMe3)2. Low temperature 119Sn NMR
study in THF-d8 of the reaction between L2 and Sn[N(SiMe3)2]2
reveals the initial formation of stannylene via transamination.
Subsequently, raising the reaction mixture to room temperature led
to the disappearance of the peaks in the 119Sn NMR spectra (Figure
On the other hand, reaction of 1 with two equivalents of
trimethylsilyl trifluoromethane sulfonate in toluene at room
temperature led to a mixture of products. Layering dichloromethane
solution of the reaction mixture with pentane led to co-crystallization
of compounds 3 and a Sn(II) monocation 4 in minor amount (Scheme
3 and Figure S19, ESI†). The electrophilic attack of the trimethysilyl
cation on the ene-amido nitrogen led to the formation of
heteroleptic Sn(II) monocation 4.
Figure 3. Molecular structure of 3 in the solid state (thermal
ellipsoids at 30%, H atoms and triflate counter anions are omitted for
clarity). Selected bond lengths [Å]: Sn1-N1 2.350(7), Sn1-N2 2.344(8),
Sn1-N3 2.301(7), Sn1-N4 2.345(7).
A handful of bisstannylenes have been reported by Hahn et. al.
synthesized by a transamination reaction between a tetraamine and
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Scheme 4. Transamination reaction between Sn[N(SiMe3)2]2 and L2
and L3.
1
S29, ESI†) due to the formation of metallic tin precipitate. H NMR
spectrum of the reaction mixture post-workup recorded at room
temperature after 4 days shows majorly the formation of L along
with small amounts of L2 and stannylene (Figure S28, ESI†).
Figure 4. Molecular structure of 5 in the solid state (thermal
ellipsoids at 30%, H atoms are omitted for clarity). Selected bond
lengths [Å] and angles []: Sn1-N1 2.088(3), Sn1-N2 2.235(3), Sn1-N2’
2.299(4), N1-Sn1-N2 80.30(13).
Analogous observation has been reported in the case of
a
heteroleptic bisstannylene which eventually led to the isolation of
2,6-diiminopyridine stabilized stannylone.18
mentioning, the Sn(II) center in the half unit of 5 has been
electronically saturated by the N donor from the other half resulting
in a dimeric structure at room temperature. This is unlike the case of
phosphine functionalized pincer-type stannylene, where the two P
appendages coordinate to the Sn(II) center to stabilize the
monomer.12 Presumably in our case, the presence of the methylene
bridge along with the phosphine bulk deters the P → Sn coordination
and favours stablilization through dimer 5 formation.
Compound 5 was synthesized by a transamination reaction between
L3 and Sn[N(SiMe3)2]2 taken in 1:1 ratio in toluene solvent at room
temperature (Scheme 4). Small amounts of metallic tin formed was
removed by filtration. Colourless single crystals of 5 appropriate for
X-ray diffraction were obtained from the filtrate in good yield of 80%.
Compound 5 was characterized in its solution state by NMR
spectroscopy in Tol-d8 (Figures S20-S22, ESI†). Room temperature 1H
NMR spectrum in Tol-d8 shows two broad doublets for the two –
CH2Ar at 4.61 ppm and 4.25 ppm and two peaks for the –CH2-CH2-
protons at 3.49 ppm and 2.99 ppm corresponding to the dimer 5.
Upon lowering the temperature, these characteristic peaks split
further, due to the restricted rotation of the phosphine appendages
in the dimer under low temperature conditions (Figure S23, ESI†).
The possible free rotations of the phosphine appendages upon rising
the temperature causes the peaks to merge as observed in the NMR
spectra (Figure S24, ESI†). The further merging of the two peaks each
assigned for –CH2Ar and –CH2-CH2- protons provides the diagnostic
feature identifying the possible formation of the monomer under
high temperature conditions (Figure S24, ESI†). Notably, the NMR
peak intensity for the free ligand L3 increases under higher
temperature conditions, hinting the instability of the monomer
formed. The 31P{1H} NMR spectrum shows resonances of only one
singlet resonance at -15.14 ppm with 119Sn satellites having a weak
coupling constant of 165 Hz. Correspondingly a triplet has been
observed in the 119Sn{1H} NMR at 46 ppm with a weak SnP coupling
constant of 173 Hz.12 Variable temperature 31P{1H} NMR spectra also
echoes the phenomenon observed in the proton NMR of 5 (Figure
S25, ESI†).
Crystal Structure and Analyses
Compound 1 crystallizes in the monoclinic crystal system with P21/n
space group (Figure 1 and Table S1, ESI†). The ligand L1 backbone
represents the transformed features with bis(ene-amide)
functionality. Each of the two stannylene centers have been
stabilized involving two Sn-N covalent bonds of parameters (Å): Sn1-
N1 = 2.126(5), Sn1-N3 = 2.191(5), Sn2-N4 = 2.178(6), Sn2-N6 =
2.140(5). The two strong pyridyl N → Sn donor-acceptor bonds of
lengths Sn1-N2 = 2.254(6) Å and Sn2-N5 = 2.336(6) Å further stabilize
the respective stannylene centers. Both the covalent and coordinate
bond parameters in compound 1 agree well with those reported in
the literature.18 While the five-membered ring stabilizing Sn1 is
almost planar, Sn2 is raised by 0.72 Å above the N5-C16-C14-N4
coordinating plane. The exocyclic C=C bonds of the five-membered
rings stabilizing the two stannylene centers exhibit bond lengths: C6-
C7 = 1.352(10) Å and C14-C15 = 1.372(10) Å. Concurrently, the C-N
bonds show bond lengths: C6-N3 = 1.386(9) Å and C14-N4 = 1.365(8)
Å. The N2-Sn1-N3 and the N4-Sn2-N5 bond angles within the five-
membered rings are 73.1(2) and 71.7(2) respectively. The two Sn
centers are separated by 3.540(8) Å and are canted by a Sn1-N3-N4-
Sn2 torsional angle of 90.77(18) due to the flexible cyclohexyl linker
The dimer 5 has been obtained as the only product even when excess
of Sn[N(SiMe3)2]2 has been taken in the reaction mixture. Worth
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between the two coordinating fragments. Such bisstannylene
geometries serve as appropriate chelating ligands in transition metal
chemistry.24
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Compound 2 crystallized in the monoclinic crystal system with P21/n
space group as observed in the single crystal X-ray analysis (Figure 2
and Table S2, ESI†). The unit cell shows the presence of three toluene
solvent molecules in the crystal lattice. At the core of the molecular
structure, the four N donor sites coordinate to the Sn(II) dicationic
center with average Sn-Nim (im = imino) distance of 2.27 Å and
average Sn-Npy (py = pyridyl) distance of 2.33 Å. Similar to our earlier
reports,7,25 the four N atoms form a mildly distorted basal
coordinating plane (fold angle N3-N2-N1-N4 is approximately
11.65(1)) with the Sn(II) center being perpendicularly disposed by
1.21 Å. The overall structure appears pyramidal with the sum of
bond angle around Sn being 295.7. This is the second example of a
Sn(II) dication after our first report made in early 2019,25 where the
dicationic center is not encapsulated.26 In the periphery of the
molecular structure 2, the two flanking bulky B(C6F5)3 substituents on
the methylene carbon are placed at an dihedral angle B1-C7C14-B2
of approximately 134.8(4), with B1B2 separated by a distance of
7.93(5) Å. The average B-Cmethylene bond distance is 1.71 Å, indicating
that they are single bonds.20
Figure 5. Relevant contour plots of 1’ at an isovalue of 0.04 au.
stannylenes dimerize through N → Sn donor-acceptor interaction to
form a central Sn2N2 four-membered planar ring. The covalent and
coordinate Sn1-N2 and Sn1-N2’ bond distances in the central
rhomboid are 2.235(3) Å and 2.299(4) Å respectively. The Sn1-N1
covalent bond distance is 2.088(3) Å. Notably, in contrast to N1, N2
acts as a μ2 bridging donor to the two stannylene centers from the
two monomers. Within the four-membered ring the internal N2-Sn1-
N2 and Sn1-N2-Sn1 angles are 81.61(14) and 98.39(14)
respectively. The four pendant phosphine groups in the dimeric
structure 5 are away from the stannylene sites, the closest Sn1P1
distance being 3.785(12) Å. Nonetheless, the presence of flexible
methylene in the appendages allow for structural rearrangement
and phosphine coordinating sites available for further
metallation.12,13
The structural formulation of 3 was unequivocally confirmed from
single crystal X-ray diffraction studies. Compound 3 crystallized in the
triclinic crystal system with P-1 space group (Figure 3, Table S3, ESI†).
The structural parameters of 3 are similar to our earlier reported
Sn(II) dication.25 Amongst the four coordinating nitrogen donor sites
to the Sn(II) dication center, the two Sn-Nim (imino-N atoms) bonds
are shorter with bond distances Sn1-N2 2.343(8) Å and Sn1-N3 2.301
(7) Å, while the Sn-Npy (pyridyl-N atoms) are slightly longer with Sn1-
N1 2.350 (7) Å and Sn1-N4 2.345 (7) Å bond lengths. As in the case of
compound 2 and our earlier report,7,25 the four nitrogen atoms form
a slightly distorted basal coordinating plane (fold angle N2-N3-N4-N1
is approximately 13.0(3)) with the Sn(II) center being displaced
perpendicularly by 1.33 Å. The resultant overall dome-shaped or
pyramidal structure has a sum of bond angle 285.14 at Sn center.
The triflate anion despite it’s known weakly coordinating nature, is
excluded from the coordination sphere of Sn(II) dicationic center, the
closest approach Sn1-O2 distance being 3.014(8) Å.26 It is apparent
from the crystal structures of 2 and 3 that the presence of bulky boryl
substituents makes 2 with a comparatively flattened pyramid.
DFT Calculations
In order to elucidate the electronic features, DFT calculations were
carried out for compounds 1 and 5 at the B3LYP level, using 6-
31G(d,p) as the basis set for C, H, N, Si, P and LANL2DZ for Sn.28
Compounds 1 and 5 have been optimized and the optimized
geometries 1’ and 5’ satisfactorily replicates the key metrical
parameters (see ESI†). The filled frontier orbitals HOMO to HOMO-2
of 1’ show the maximum contributions from the lone pairs on the
two stannylene centers (Figure 5). The LUMO is ligand-centered and
reveal the anti-bonding interaction of the terminal C=C (Figure 5).
The vacant p-orbital on Sn(II) centers are the energetically high-lying
LUMO+4 and LUMO+5 orbitals (Figure 5). The Wiberg bond indices
from NBO analysis clearly help to differentiate between the covalent
and dative bonds in this molecule. The average WBI values of Sn-Nene-
Compound 4 crystallized in the monoclinic crystal system with P21/c
space group (Figure S31 and Table S4, ESI†). The molecular structure
reveals the stabilization of a Sn(II) monocation covalently bonded to
the ene-amido nitrogen and electronically saturated by the two
pyridyl nitrogens within the unsymmetrical N4 ligand framework
(Figure S31, ESI†). The Sn-Namido bond length is 2.095(6) Å and the
Sn-Npy bond lengths are 2.323(5) Å (Sn1-N1) and 2.311(5) Å (Sn1-N4)
bonds. The Sn(II) center is pyramidalised in 4 with the the closest
approach Sn1-O1 distance of the triflate anion being 3.080(5) Å.26
and Sn-Namide covalent bonds are 0.35 and 0.45 respectively,
amide
while the value for donor-acceptor Sn-Npy is comparatively low being
0.23. The average WBI for the C-C bond in the ene-amide is 1.62,
reflecting their double bond character. There exists no covalent
bonding between the two Sn centers as confirmed from the very low
WBI value of 0.18. The Mulliken charges on the stannylene centers
are +0.91. The simulated UV/Vis spectrum of 1’ by TD-DFT
calculations shows the λmax at 454 nm (HOMO → LUMO+1, f =
Compound 5 crystallized in the centrosymmetric P-1 space group as
observed in the X-ray analysis (Table S5, ESI†). The solid-state
structure shows a dimer of the monostannylene unit (Figure 4). The
molecular unit involves a three edge-bridged rings giving rise to a
step like structure.27 The two puckered five-membered diamido-
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Journal Name
General Remarks. All manipulations were carried VioewutArtuicnledOenrlinae
protective atmosphere of argon applying standard
DOI: 10.1039/C9DSTc0h0l6e1n7Fk
techniques or in a dry box. Tetrahydrofuran and toluene were
refluxed over sodium/benzophenone. Methanol was dried with
magnesium cake and stored over 3Å molecular sieves. All the
solvents were distilled and stored under argon and degassed before
use. Benzene-d6 and toluene-d8 were purchased from Sigma Aldrich
(Sigma Aldrich Co., St. Louis, MO, USA) and dried over potassium.
Acetonitrile-d3 and tetrahydrofuran-d8 was purchased from Sigma
1
Aldrich and used as it is. All chemicals were used as purchased. H,
13C{1H}, and 29Si{1H} NMR spectra were referenced to external SiMe4
using the residual signals of the deuterated solvent (1H) or the
solvent itself (13C). 31P{1H} NMR was referenced to external 85%
H3PO4. 119Sn NMR was referenced to SnCl4 as the external standard.
11B NMR was referenced to BF3.OEt2 as the external standard. 19F
NMR was referenced to C6H5CF3 as the external standard. NMR
spectra were recorded on Bruker AVANCE III HD ASCEND 9.4
Tesla/400 MHz and Jeol 9.4 Tesla/400 MHz spectrometer. Solution
phase UV/Vis spectra were acquired using a Thermo-Scientific
Evolution 300 spectrometer using quartz cells with a path length of 1
cm. Melting points were determined under argon in closed NMR
tubes and are uncorrected. Elemental analyses were performed on
Elementar vario EL analyzer. Single crystal data were collected on
Bruker SMART APEX four-circle diffractometer equipped with a
CMOS photon 100 detector (Bruker Systems Inc.) with a Cu K
radiation (1.5418 Å).
Figure 6. Relevant contour plots of 5’ at an isovalue of 0.02 au.
0.0477), corresponding to the n(Sn) → π*(L1) transition (see ESI†),
which is close by the experimentally obtained value.
The HOMO-1 to HOMO-3 orbitals of 5’ reveal the major contributions
from the Sn lone pairs. The vacant p-orbital on the Sn centers are
depicted in LUMO along with P-C σ*-π* conjugation and majorly in
LUMO+4 (Figure 6). The WBI value for the single bonds N1-Sn1 is 0.48
and N2-Sn1 is 0.32. The difference obviously arises from the bridging
donor nature of N2 in contrast to N1 and correlates well with the
experimental findings. The Sn1-N2’ bond between the two
monomeric units in 5’ has a WBI value of 0.29. The dissociation
energy of the dimer has been calculated to be 17.3 kcal/mol (see
ESI†), implying the favourable dimer formation at room
temperature. Furthermore, the dissociation energies have been
calculated in toluene, tetrahydrofuran and acetonitrile solvent
medium using the polarization continumm model (see ESI†).
Although apparently the dissociation energy of the dimer increases
with the decrease in dielectric constant of the medium, the change
is small (within 0.5 kcal/mol), which indicates that electrostatic
interactions do not predominate. Therefore, the formation of the
dimer 5 may involve partial covalent interaction.
Synthesis of Ligand L1: 6.48 mL (57.7 mmol) of 2-acetyl pyridine was
added to solution of 3.47 mL (28.8 mmol) of 1,2-cyclohexyldiamine
in 50 mL of dry methanol and reaction mixture was set to reflux for
8 h. MeOH was evaporated from reaction mixture under reduced
vacuum. The residue was dissolved in minimum amount of diethyl
ether, and flask kept aside to get colourless crystals of ligand L1 with
the yield of 71% (6.56 g). 1H NMR (400 MHz, CDCl3, TMS ) δ 8.48 (m,
2H, Pyr-H); 7.91 (m, 2H, Pyr-H); 7.58 (m, 2H, Pyr-H); 7.18 (m, 2H, Pyr-
H); 3.89 (m, , 2H, -CH- Cyclohexyl); 2.35 (s, 6H, -CH3); 1.94-1.45 (m,
8H, -CH2-CH2-cyclohexyl) ppm. 13C{1H} NMR (101 MHz, CDCl3, TMS) δ
164.66 (C-CH3); 158.39(C-Pyr); 148.14(C-Pyr); 136.31(C-Pyr);
123.83(C-Pyr); 120.99(C-Pyr); 65.70 (-CH- Cyclohexyl); 31.58 (-CH2-
CH2- cyclohexyl); 24.64(-CH2-CH2-cyclohexyl); 14.44 (-CH3) ppm.
Elemental Analysis: Calcd. for C20H24N4: C, 74.97; H, 7.55; N, 17.48.
Found: C, 75.09; H, 7.63; N, 17.61.
Conclusions
To summarize, the adaptability of the tetradentate ligands L1-L3
possessing –CH2-CH2- backbone linker along with the readily
cleavable Sn-N bond in Sn[N(SiMe3)2]2 have been utilized to stabilize
stannylenes. While L1 stabilizes a bisstannylene involving ene-amide
transformation of the ligand, L3 stabilizes a monostannylene through
the straightforward transamination reaction. However, the
monostannylene stabilizes in a dimeric form with four P coordinating
Synthesis of Ligand L2: NaBH4 (0.511 g, 13.51 mmol) was added
portionwise in ligand L (0.9 g, 3.38mmol) in 20 mL of dry MeOH at
room temperature under inert atmosphere. The reaction mixture
was stirred for 4h at room temperature. Solvent was evaporated and
sites available for further metallation. The bisstannylene has been residue was quenched with water. Product was extracted in
observed to undergo the unprecedented conversion to ligand
stabilized tin(II) dication by reaction with Lewis acids. The
complexation of the bisstannylene and the dimeric monostannylene
with transition metals targeting metal clusters for photophysical
studies and efficient catalysis are being considered as our current
research goals.
dichloromethane, dried over sodium sulphate. Solvent was removed
under vacuum yielding 0.81g (90%) pale yellow viscous liquid L2. 1H
NMR (400 MHz, C6D6, TMS) δ 8.42 (td, J = 1.6, 4.8 Hz, 2H, Pyr-H); 7.07-
7.03 (m, 4H, Pyr-H); 6.55 (m, 2H, Pyr-H); 3.85-3.77 (m, 2H, -CH-CH3 );
2.50-2.43 (m, 4H, -CH2-CH2 ); 1.86 (bs, 2H, -NH-); 1.31 (d, J = 8, 6H, -
CH3)ppm. 13C{1H} NMR (101 MHz, C6D6, TMS) δ 165.65 (C-Pyr);
149.24(C-Pyr); 135.69(C-Pyr); 121.30(C-Pyr); 120.58(C-Pyr); 59.68(-
HC-CH3); 47.85 (-CH2-CH2-); 23.09 (-HC-CH3) ppm. Elemental Analysis:
Experimental
6 | J. Name., 2012, 00, 1-3
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ARTICLE
Calcd. for C16H22N4: C, 71.08; H, 8.20; N, 20.72. Found: C, 71.25; H, reaction mixture was concentrated and layered with pentane to get
View Article Online
DOI: 10.1039/C9DT00617F
8.35; N, 20.81.
colourless crystals of compound 2 with the yield of 48% (0.040 g).
1
Characterization of 2: H NMR (400 MHz, CDCl3, TMS) δ 8.87 (d, J =
Synthesis of Ligand L3: Corresponding diiminodiphosphine ligand (1
g, 1.65 mmol) and LiAlH4 (0.282 g, 7.44 mmol) were taken in 30 mL
of diethyl ether and stirred at room temperature for 24 hours. The
mixture was quenched with water and extracted with DCM. The
organic layer was separated and dried over anhydrous Na2SO4. The
solvent was evaporated under vacuum to obtain a pale yellow sticky
solid yielding 0.58 g (57.62%) of L3. 1H NMR (400 MHz, CDCl3, TMS )
δ 7.45 (ddd, J=7 Hz, 4.8Hz, 0.8 Hz, 2H, HC-2); 7.34-7.24 (m, 22 H, Ar-
H); 7.16 (dt, J = 7.6 Hz, 0.8 Hz, 2H, HC-4); 6.90 (ddd, J = 7.6Hz, 4.4Hz,
1.2 Hz, 2H, HC-5); 3.92 (s, 4H, Ar-CH2-NH); 2.50 (s, 4H, -CH2-CH2-);
1.55 (br s, 2H, NH) ppm. 13C NMR (101 MHz, CDCl3, TMS) δ 144.84(d,
C-1, JP-C = 23.81 Hz); 136.94 (d, C-6, JP-C=10.15 Hz), 135.81(d, Ar-Cipso,
JP-C = 13.75 Hz); 134.06 (d, ArCo, JP-C = 19.81 Hz); 133.66 (C-5); 129.17
4.16 Hz, 1H, Pyr-H); 8.59 (d, J = 5.24 Hz, 1H, Pyr-H); 8.27 (d, J = 8.08
Hz, 1H, Pyr-H); 8.19 (t, J = 7.88 Hz, 1H, Pyr-H); 7.98 (t, J = 8 Hz, 1H,
Pyr-H); 7.89 (t, J = 5.2 Hz, 1H, Pyr-H); 7.80 (t, J = 6 Hz, 1H, Pyr-H); 7.64
(d, J = 5.2 Hz, 1H, Pyr-H); 3.84 (m, 3H, Cy-H & CH2); 3.57 (t, J = 8 Hz,
1H, Cy-H); 3.27 (s, 2H, CH2); 2.57 (d, 2H, Cy-H); 1.29 (m, 1H, Cy-H);
2.05 (s, 1H, Cy-H); 1.95 (d, 1H, Cy-H); 1.28 (m, 2H, Cy-H): 0.91 (t, J = 8
Hz, 2H, Cy-H) ppm. 19F{1H} NMR (377 MHz, CDCl3, TFT) δ -164.21 (t, J
= 19.36 Hz, m-F); -163.99 (t, J = 19.28 Hz, m-F); -159.31 (t, J = 20.64
Hz, p-F): -159.06 (t, J = 20.37 Hz, p-F); -129.88 (bs, o-F); -130.77 (bs,
o-F)ppm. 11B{1H} NMR (128.43 MHz, CDCl3, BF3.OEt2) δ -13.3 & -12.3
ppm. Elemental Analysis: Calcd. for C56H22B2F30N4Sn: C, 46.03; H,
1.52; N, 3.83. Found: C, 46.50; H, 1.32; N, 3.95.
(d, Ar-Cm, JP-C = 5.48); 129.02 (Ar-Cp); 128.79 (C-3); 128.67 (d, C-2, JP- Synthesis of 3: AgOTf (0.14g, 0.54 mmol) was added to a
C = 6.96 Hz); 127.23 (C-4); 52.41 (d, N-CH2, JP-C = 21.16 Hz); 48.68 (- tetrahydrofuran (20 mL) solution of comp. 1 (0.2g, 0.27 mmol) at
CH2-CH2-) ppm. 31P NMR ( 162 MHz, CDCl3, H3PO4) δ = -15.42 ppm. room temperature. Colour of the reaction mixture instantly changed
Elemental Analysis: Calcd. for C40H38N2P2: C, 78.93; H, 6.29; N, 4.60. from yellow to orange. Reaction mixture was stirred for 1 h at room
Found: C, 78.91; H, 6.43; N, 4.55.
temperature and then it was concentrated, followed by layering with
pentane to get colourless crystals of comp. 3 with crystallization yield
of 40% (0.067g).
Synthesis of 1: Method A Toluene (10 mL) was added to a the
mixture of ligand L1 (0.4 g, 1.25 mmol) and Sn[N(SiMe3)2]2 (1.099 g,
2.5 mmol) and stirred it for 48 h at room temperature. All the
volatiles were evaporated under vacuum to get green solid. The
green solid was washed with 4-5 mL of pentane to get yellow
coloured residue. The yellow residue was further dissolved into
diethyl ether and filtered to remove small amount of metallic tin.
Solvent was removed under vacuum to get yellow coloured powder
82% (0.87 g) (Decomp. 165 – 167 °C).
Characterization of 3: 1H NMR (400 MHz, CD3CN, TMS) δ 9.16 (s, 2H,
Pyr-H); 8.41 (dt, J = 8, 1.6 Hz, 2H, Pyr-H); 8.31 (td, J = 8, 1Hz, 2H, Pyr-
H); 8.02 (s, Pyr-H); 4.56 (s, 2H, Cy-H); 2.69 (s, 6H, -CH3); 2.47 (s, 2H,
Cy-H); 2.04 (m, 4H, Cy-H): 1.66 (t, 2H, Cy-H) ppm. 13C{1H} NMR (101
MHz, CD3CN, TMS) 150.57 (CH3-C); 148.33 (Pyr-Co); 143.29 (Pyr- Co);
129.41 (Pyr-Cp); 126.93 (Pyr-Cm); 125.74 (Pyr-Cm);122.55, 119.37
(CF3SO3); 66.64 (Cy-CH-); 31.06 (Cy-CH2); 24.61 (Cy-CH2); 17.64 (CCH3)
ppm. 19F{1H} NMR (377 MHz, CD3CN, TFT) δ -79.34 (TMSOTf- F) ppm
119Sn{1H} NMR (149.74 MHz, C6D6) δ -595.80 ppm Elemental Analysis:
Calcd. for C22H24F6N4O6S2Sn: C, 35.84; H, 3.28; N, 7.60. Found: C,
35.98; H, 3.50; N, 7.91.
Method B Ligand L1 (0.4 g, 1.25 mmol) and Sn[N(SiMe3)2]2 (1.09 g,
2.5 mmol) was heated in a schlenk tube at 60°C for 1 h. The residue
was washed with small amount of pentane to get orange solid. The
solid was dissolved in toluene and filtered to remove small amount
of metallic tin. Solvent was removed under vacuum to get orange
yellow solid in 85% (0.93 g).
Characterization of 1: 1H NMR (400 MHz, C6D6, TMS) δ 7.66 (ddd, J =
5.36, 1.6, 0.84 Hz, 2H, Pyr-H); 7.43 (d, J = 8.48 Hz, 2H, Pyr-H); 6.72 (td,
J = 7.4, 1.68 Hz, 2H, Pyr-H); 6.22 (td, J = 0.96, 7.2 Hz, 2H, Pyr-H); 4.87
(d, J = 1.52 Hz, 2H, C=CH2); 4.43 (d, J = 1.52 Hz, 2H, C=CH2); 4.15 (d, J
= 8 Hz, 2H, -CH-CH- cyclohexyl); 2.97 (d, 2H, J = 12 Hz, -CH-CH-
cyclohexyl); 1.83-1.51 (m, 6H, -CH2-CH2- cyclohexyl): 0.34 (s, 36H,
SiMe3-H) ppm. 13C{1H} NMR (101 MHz, C6D6, TMS) δ 158.79 (C=CH2);
151.91 (Pyr-C); 142.88 (Pyr-C); 137.96 (Pyr-C); 122.36 (Pyr-C); 121.69
(Pyr-C); 82.13 C=CH2); 63.24(-CH-CH- cyclohexyl); 31.07 (-CH2-CH2-
cyclohexyl); 25.71 (-CH2-CH2- cyclohexyl); 6.76 (-Si(CH3) 3)ppm.
29Si{1H} NMR (79.53 MHz, C6D6) δ -1.30 ppm. 119Sn{1H} NMR (149.74
Reaction of
1 with TMSOTf: Trimethylsilyl trifluoromethane
sulphonate (41 µL, 0.228 mmol) was added to toluene solution of
compound 1 (0.1 g, 0.114 mmol) and reaction mixture was stirred for
1 h at room temperature. Solvent was removed by evaporation and
the residue was dissolved in DCM. Reaction mixture was filtered and
filtrate was concentrated. Concentrated DCM solution was layered
with pentane to get colourless crystals of compound 3 and 4.
Synthesis of 5: Toluene (20 mL) was added to mixture of ligand L3
(0.3g, 0.49 mmol) and Sn[N(SiMe3)2]2 (0.22g, 0.49 mmol). The orange
reaction mixture was stirred for 12h at room temperature. The pale
yellow reaction mixture was filtered and filtrate was concentrated
and kept for crystallization to get colourless crystals of compound 5
1
(Decomp. 128-130 °C) with the yield of 0.28g (80%). H NMR (400
MHz, C6D6)
δ -47.05 ppm. Elemental Analysis: Calcd. for
MHz, Tol-d8, TMS) δ 7.31-7.39 (m, 8H, Ar-H); 7.02-7.13 (m, 18H, Ar-
H); 6.89 (t, J = 8 Hz, 2H, Ar-H); 4.61 (d, J= 16 Hz, 2H, -CH2Ar); 4.25 (bs,
2H, -CH2Ar); 3.49 (bs, 2H, -CH2-); 2.99 (bs, 2H, -CH2-) ppm. 31P{1H}
NMR (162 MHz, Tol-d8) δ -16.54 (Jp-Sn= 165.55 Hz) ppm 119Sn{1H}
NMR (149.27 MHz, Tol-d8) δ 46.12 (t, JSn-P = 173.20 Hz )ppm
C32H58N6Si4Sn2: C, 43.84; H, 6.67; N, 9.59. Found: C, 43.89; H, 6.61; N,
9.45.
Synthesis of 2: B(C6F5)3 (0.058 g, 0.114 mmol) was added to toluene
solution of compound 1 (0.05 g, 0.057 mmol) and the pale yellow
reaction mixture was stirred for 1 h at room temperature. The
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Journal Name
11478; D.Gallego, A. Brück, E. Irran, F. Meier, F. Kaupp and M.
Driess, J. Am. Chem. Soc. 2013, 135, 1D5O6I:1170;.1H03V.9ieR/wCeA9nrDt,icTYl0e.0O6n–1li7nPFe.
Zhou, Y. Bai, C. Cui and M. Driess, Chem. Eur. J. 2017, 23, 5663;
Y. –P. Zhou, M. Karni, S. Yao, Y. Apeloig and M. Driess, Angew.
Chem. Int. Ed. 2016, 55, 15096; T. T. Metsӓnen, D. Gallego, T.
Elemental Analysis: Calcd. for C80H72N4P4Sn2: C, 66.23; H, 5.00; N,
3.86. Found: C, 66.19; H, 5.20; N, 3.95.
VT NMR for 5: Compound 5 was dissolved in Tol-d8 and NMR (1H and
31P) were recorded at temperature ranges from 223 K to 368 K with
difference of 20 K.
Szilvási, M. Driess and M. Oestreich, Chem. Sci. 2015, 6, 7143.
7
8
R. K. Raut and M. Majumdar, Chem. Commun., 2017, 53, 1467.
M. Majumdar, R. K. Raut, P. Sahoo and V. Kumar, Chem.
Commun., 2018, 54, 10839.
D. Franz, T. Szilvási, A. Pӧthig, F. Deiser and S. Inoue, Chem.
Eur. J., 2018, 24, 4283.
Deprotonation of L2 by Sn[N(SiMe3)2]3 leading to L: Tetrahydrofuran
was added to mixture of ligand L2 (0.2 g, 0.74 mmol) and
Sn[N(SiMe3)2]2 (0.65 g, 1.48 mmol). Immediate colour change with
instant precipitate is occurred in the reaction mixture. The reaction
mixture was stirred it for 24h at room temperature. Reaction mixture
was filtered and solvent was evaporated to get red coloured solid.
Solid was dissolved in diethyl ether and kept for crystallization to get
70% (0.14 g) of L. 1H NMR (400 MHz, CDCl3, TMS) δ 8.60 (ddd, J = 1.2,
2.0, 4.8 Hz, 2H, Pyr-H); 8.09 (td, J = 0.8, 8 Hz, 2H, Pyr-H); 7.71 (td, J =
1.6, 7.6 Hz, 2H, Pyr-H); 7.29 (ddd, J = 1.2, 5.2, 7.2 Hz, 2H, Pyr-H); 4.00
9
10 B. M. Lindley, P. T. Wolczanski, T. R. Cundari and E. B.
Lobkovsky, Organometallics, 2015, 34, 4656.
11 D. Nartop, W. Clegg, R. W. Harrington, R. A. Henderson and C.
Y. Wills, Dalton Trans., 2014, 43, 3372.
12 S. Bestgen, N. H. Rees and J. M. Goicoechea, Organometallics,
2018, 37, 4147; L. Alvarez-Rodriguez, J. Brugos, J. A. Cabeza,
P. Garcia-Alvarez, E. Perez-Carreno and D. Polo, Chem.
Commun. 2017, 53, 893; J. Brugos, J. A. Cabeza, P. Garcia-
Álvarez, E. Pérez-Carreño and D. Polo, Dalton Trans., 2018, 47
,
4534.
(s , 4H, -CH2 -CH2 -); 2.45 (s, 6H, -CH3) ppm. 13C{1H} NMR (101 MHz, 13 L. Álvarez-Rodríguez, J. Brugos, J. A. Cabeza, P. García-Álvarez
and E. Pérez-Carreno, Chem. Eur J. 2017, 23, 15107; K. M.
Krebs, S. Freitag, J. J. Maudrich, H. Schubert, P. Sirsch and L.
Wesemann, Dalton Trans. 2018, 47, 83; J. Takaya, K. Miyama,
C. Zhu and N. Iwasawa, Chem. Commun. 2017, 53, 3982.
14 R. M. Stoop, S. Bachmann, M. Valentini and A. Mezzetti,
Organometallics, 2000, 19, 4117; C. Sui-Seng, F. N. Haque, A.
Hadzovic, A.-M. Pütz, V. Reuss, N. Meyer, A. J. Lough, M. Z. De-
Iuliis and R. H. Morris, Inorg. Chem., 2009, 48, 735.
CDCl3, TMS) δ 167.60 (C-CH3); 157.82 (Pyr-C); 148.33 (Pyr-C); 136.37
(Pyr-C); 124.11 (Pyr-C); 120.98 (Pyr-C); 53.62 (-CH2 -CH2 -); 14.48 (-
CH3)ppm. Elemental Analysis: Calcd. for C16H18N4: C, 72.15; H, 6.81;
N, 21.04. Found: C, 72.30; H, 6.95; N, 21.22.
NMR scale reaction: In a NMR tube, THF-d8 was added to mixture of
ligand L2 (20 mg, 0.07 mmol) and Sn[N(SiMe3)2]2 (65 mg, 0.14 mmol)
at -50C and shaken well. NMR spectra were recorded at different
temperatures.
15 H. Zhang, C. –B. Yang, Y. –Y. Li, Z. –R. Donga, J. –X. Gao, H.
Nakamura, K. Murata, T. Ikariya, Chem. Commun., 2003, 142;
W. Wong, Li Zhang, Y. Chen, W. Wong, W. Wong, F. Xue and
T. C. W. Mak, J. Chem. Soc., Dalton Trans., 2000, 1397.
16 P. Sahoo, R. K. Raut, D. Maurya, V. Kumar, P. Rani, M.
Majumdar, Dalton Trans., DOI: 10.1039/c9dt00109c
Conflicts of interest
17 N. Meyer, A. J. Lough, R.H. Morris, Chem. Eur. J., 2009, 15
,
“There are no conflicts to declare”.
5605; C. Sui-Seng, F. Freutel, A. J. Lough, R. H. Morris, Angew.
Chem. Int. Ed., 2008, 47, 940; J. F. Sonnenberg and R. H.
Morris, ACS Catal., 2013, 3, 1092.
18 J. Flock, A. Suljanovic, A. Torvisco, W. Schoefberger, B. Gerke,
Acknowledgements
R. Pӧttgen, R. C. Fischer and M. Flock, Chem. Eur. J., 2013, 19
15504.
,
The authors thank the Department of Science and Technology
(DST), India (EMR/2015/001135) and DST Nano-mission 19 T. W. Myers and L. A. Berben, Inorg. Chem., 2012, 51, 1480.
20 M. Driess, S. Yao, M. Brym, C. van Wüllen, Angew. Chem. Int.
Ed., 2006, 45, 6730; A. Jana, I. Objartel, H. W. Roesky, D.
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21 Schäfer, F. Winter, W. Saak, D. Haase, R. Pöttgen, T. Müller,
Chem. Eur. J., 2011, 17, 10979.
Thematic Unit, India (SR/NM/TP-13/2016) for their financial
support. The authors thank Dr. Kumar Vanka (NCL Pune, India)
for his valuable insights in the computational study.
22 M. Schleep, C. Hettich, D. Kratzert, H. Scherer, I. Krossing,
Chem. Commun., 2017, 53, 10914.
Notes and references
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1
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