Mononuclear Pseudostannatranes Possessing Unsymmetrical [4.4.3.01,5]Tridecane Cage: Experimental and Theoretical Aspects of Reverse Kocheshkov Reaction in Phenyl Pseudostannatrane

Article abstract of DOI:10.1021/acs.inorgchem.0c01202

The synthetic protocols, structural aspects, and spectroscopic aspects of mononuclear pseudostannatranes possessing a [4.4.3.01,5]tridecane cage have been reported. A tripodal ligand N(CH2CH2OH){CH2(2-t-Bu-4-Me-C6H2OH)}2 (H3L) having unsymmetrical arms wa

Full text of DOI:10.1021/acs.inorgchem.0c01202

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Article
Mononuclear Pseudostannatranes Possessing Unsymmetrical
[4.4.3.01,5]Tridecane Cage: Experimental and Theoretical Aspects of
Reverse Kocheshkov Reaction in Phenyl Pseudostannatrane
Keshav Kumar, Neha Srivastav, Mayank Khera, Neetu Goel, Raghubir Singh,* and Varinder Kaur*
Cite This: Inorg. Chem. 2020, 59, 13098−13108
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ABSTRACT: The synthetic protocols, structural aspects, and spectro-
scopic aspects of mononuclear pseudostannatranes possessing a
[4.4.3.01,5]tridecane cage have been reported. A tripodal ligand
N(CH2CH2OH){CH2(2-t-Bu-4-Me-C6H2OH)}2 (H3L) having unsym-
metrical arms was reacted with n-butyltrichlorostannane, phenyl-
trichlorostannane, and tin tetrachloride under different solvent systems
to obtain pseudostannatranes (1−3). The reaction of n-butyltrichlor-
ostannane and the ligand in CH3OH/Na/THF yielded an aqua
complex of pseudostannatrane [LSnBu(H2O)] (1a), which was
crystallized as its acetone solvate (i.e 1a·Me2CO). However, the same
reactants yielded methanol complex [LSnBu(CH3OH)] (1b) when the
reaction was carried out in the NaOCH3/C2H5OH system. Similarly,
the reaction of phenyltrichlorostannane and the ligand under these
solvent systems yielded pseudostannatranes, i.e., an aqua complex
[LSnPh(H2O)] (2a) and a methanol complex [LSnPh(CH3OH)] (2b) (where 2a was crystallized as 2a·Me2CO). The reaction of tin
tetrachloride and the ligand in the Et3N/THF system resulted in the formation of pseudostannatrane [LHSnCl2] (3). A similar
product was isolated as its triethylamine solvate (3·NEt3) due to the disproportion reaction when PhSnCl3 was reacted with the
ligand in the Et3N/C6H5CH3 system, which demonstrates the first report on the reverse Kocheshkov reaction in pseudostannatranes.
The experimental findings on the formation of 3·NEt3 due to the reverse Kocheshkov reaction have been corroborated with 119Sn
NMR spectroscopy and density functional calculations that provide insightful information about the underlying details of the
reaction route.
cages with oligomeric structures formed via Sn−O bridges and
a symmetrical [4.4.4.01,6]tetradecane cage as a mononuclear
entity.3,4,29 To the best of our knowledge, pseudostannatranes
possessing a [4.4.3.01,5]tridecane cage as mononuclear entities
have not been reported yet.
The interest in stannatrane-like molecules is driven by the
fact that they are a prerequisite for breakthrough applications
influencing basic organic synthesis, polymerization reactions,
and polyurethane formation; therefore they are essential for
the chemical and medicinal industries.30−34 The biggest
obstacle for the exploration of stannatranes in multiple
applications is the oligomerization of mononuclear entities
during reaction via reactive groups/atoms present in the
ligating system.6,17,29 Therefore, it is desirable to engineer new
INTRODUCTION
■
Over recent years, there has been growing interest in the
synthesis and structural investigation of tin(IV) tricyclic
compounds having N → Sn transannular interactions.1−8
Among the fused organotin tricycles, molecules having five-
membered rings with a [3.3.3.01,5]undecane cage are termed
“stannatranes,” while molecules containing rings other than
five-membered arms are named “pseudostannatranes.”2,9−12
These structures are distinguished by their cage-like structure
inclusive of right- or left-handed (Δ and Λ) propeller type
geometry and an intramolecular N → Sn bond.13−15 Various
stannatranes with general formula N(CH2CH2X)3SnR (where
X = O, S, NMe, CH2; R = alkyl/aryl) and N{CH2C(O)X}3SnR
(where X = O or NMe) have been reported in the past.2,16−22
Their popularity has led to the syntheses of organic
compounds and drug carbapenem from N(CH2CH2CH2)3SnR
due to selective reactivity of the Sn−R bond.23−28 In addition,
purely inorganic (lacking Sn−C bond) stannatranes are also
known and have been recently reported by Jurkschat et
al.6−8,15,29 On the contrary, known pseudostannatranes have
unsymmetrical [4.3.3.01,5]dodecane and [4.4.3.01,5]tridecane
Received: April 24, 2020
Published: September 9, 2020
https://dx.doi.org/10.1021/acs.inorgchem.0c01202
© 2020 American Chemical Society
Inorg. Chem. 2020, 59, 13098−13108
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Article
investigated by performing geometry optimizations followed by
frequency calculations within the formalism of density functional
theory (DFT). Hybrid exchange-correlation functional B3LYP in
conjunction with/6-31+G (d,p) and LanL2DZ as implemented in the
Gaussian 09 package is the employed level of theory.48−51
skeletons of tin(IV) with sufficient architectural patterns to
suppress oligomerization.
In our previous attempts to obtain pseudostannatrane from a
tripodal unsymmetrical ligand, we obtained oligomeric
structures in a solid as well as in a solution phase due to the
flexibility of one of the arms of the ligand which provided space
for μ-oxo bridges.4 Herein, we utilized a heteropolydentate
ligand N(CH2CH2OH){CH2(2-t-Bu-4-Me-C6H2OH)}2 (H3L)
with a bulky substituent in the phenolic ring to obtain
pseudostannatranes. The H3L is previously reported to obtain
dinuclear complexes of iron and vanadium, Fe2L2 and V2O2L2,
respectively (where L is deprotonated ligand).35−37 Although
the structure of its analog, i.e., N(CH2CH2OMe){CH2(2-t-Bu-
4-Me-C6H2OH)}2 (having variation at the alcoholic arm), has
been investigated,38 structural aspects of the H3L, however,
were not elucidated in detail. In the present work, the H3L has
been isolated in crystalline form, and its structure is confirmed
by spectroscopic studies and single-crystal X-ray crystallog-
raphy. It is reacted with tin precursors in varying solvent
systems to obtain pseudostannatranes. The structures of
pseudostannatranes are elucidated by elemental studies,
spectroscopic studies, spectrometric studies, and single-crystal
X-ray crystallography. The compounds are found to be
mononuclear [4.4.3.01,5]tridecane cages with hexacoordination
at the Sn center. It is expected that tert-butyl groups present in
the ligating system provide steric crowding around central
metal and facilitate the formation of mononuclear entities. The
formation of distinct products (i.e., 2a/b and 3·NEt3) by the
reaction of PhSnCl3 and a ligand under different experimental
conditions exemplifies the reverse Kocheshkov reaction. The
mechanistic route for exceptional findings in the formation of
3·NEt3 is justified based on spectroscopic studies of the
reaction mixture at different intervals, computational studies,
and literature reports. Previously, the reverse Kocheshkov
reaction is reported in organostannanes, alkyl and aryl tin
species, and organotin(IV) complexes; however, the formation
3·NEt3 is the first report on the reverse Kocheshkov reaction in
the family of stannatranes.39−44
Syntheses. H3L: N(CH2CH2OH){CH2(2-t-Bu-4-Me-C6H2OH)}2. The
ligand H3L was synthesized by Mannich condensation reaction.
Briefly, starting reagents 2-(t-butyl)-4-methylphenol (3.28 g, 20.00
mmol), aqueous formaldehyde (37%, 1.63 g, 20.00 mmol), and 2-
aminoethanol (0.61 g, 10.00 mmol) were heated at reflux in methanol
(30 mL) for 24 h to afford a clear solution. The solvent was
evaporated, and the resulting oil was dissolved in toluene. The
contents were left to stand at room temperature, which gave crystals
suitable for X-ray crystallography. Yield: 85% (3.51 g, 8.50 mmol).
Melting point: 75 °C. Elemental analysis calculated for C26H39NO3:
C, 75.50; H, 9.50; N, 3.39. Found: C, 75.30; H, 9.58; N, 3.31. FT-IR
(cm−1): 1476 (CC, phenyl ring), 1605 (CC, phenyl ring), 2864,
1
2913, 2952 (C−H), 3391 br, 3581 (O−H). H NMR (400 MHz,
CDCl3): δ (ppm) 1.38 (s, 18H10−12,22−24), 2.22 (s, 6H8,20), 2.68 (t,
2H25, 3J(1H−1H) = 5.2 Hz), 3.70 (s, 4H1,13), 3.79 (t, 2H26,
4
3J(1H−1H) = 5.2 Hz), 6.71 (d, 2H3,15, J(1H−1H) = 1.6 Hz), 6.99
4
(d, 2H5,17, J(1H−1H) = 1.6 Hz), 8.10 (br, 1H, CH2OH). 13C NMR
(100 MHz, CDCl3): δ (ppm) 20.7 (C8,20), 29.6 (C10−12,22−24), 34.6
(C9,21), 53.2 (C25), 57.4 (C1,13), 61.2 (C26), 122.4 (C2,14), 127.3
(C5,17), 127.7 (C3,15), 128.8 (C4,16), 136.8 (C6,18), 152.8 (C7,19). MS:
m/z 414.30 [M + H]+.
(1a·Me2CO): [(N(CH2CH2O){CH2(2-t-Bu-4-Me-C6H2O)}2)Sn(n-Bu)-
(H2O)]·(Me2CO). To the ligand H3L (0.83 g, 2.00 mmol), freshly
prepared sodium methoxide solution (0.14 g, 6.00 mmol of sodium
metal in 10 mL of methanol) was added. The solution was heated at
reflux for 1 h and diluted with 20 mL of THF. The resultant solution
was transferred dropwise to a solution of n-butyltin trichloride (0.33
mL, 2.00 mmol) in THF (20 mL) and heated at reflux for 1 h. The
reaction mixture was filtered to separate sodium chloride, and the
filtrate was evaporated under a vacuum to remove the solvent. The
solid obtained was dissolved in dichloromethane (10 mL) and filtered
again to remove solid impurity (if any). The contents were evaporated
to dryness in vacuo to afford a solid. The resulting solid was dissolved
in acetone, which, after slow evaporation, resulted in colorless blocked
crystals of acetone solvate of the aqua complex of pseudostannatrane
1 (1a·Me2CO). Melting point: 221 °C. Yield: 84% (0.99 g, 1.68
mmol). Elemental analysis calculated for C33H53NO5Sn: C, 59.83; H,
8.06; N, 2.11. Found: C, 59.67; H, 7.97; N, 2.04. FT-IR ν (cm−1):
440 (Sn−N), 496 (Sn−O), 520 (Sn−C), 1466 (CC, phenyl ring),
1609 (CC, phenyl ring), 1705 (CO, acetone), 2868, 2913, 2954
EXPERIMENTAL SECTION
■
1
(C−H), 3292 (O−H). H NMR (400 MHz, DMSO-d6): δ (ppm)
Materials. Synthesis of all the compounds was carried out using
the Schlenk technique under dry nitrogen. Commercially purchased
solvents were dried and stored under nitrogen. Triethylamine (CDH)
was distilled over KOH pellets before use. Other chemicals, 2-(t-
butyl)-4-methylphenol (Acros), formaldehyde (Aldrich), ethanol-
amine (Acros), n-butyltrichlorostannane (Acros), phenyltrichloros-
tannane (Aldrich), and tin tetrachloride (Aldrich), were used without
any further purification.
0.92 (t, 3H30
, = 7.2 Hz), 1.30−1.38 (m,
3J(1H−1H)
22H10−12,22−24,27,29), 1.76−1.84 (m, 2H28), 2.14 (s, 6H8,20), 2.67 (t,
2H25, 3J(1H−1H) = 5.6 Hz), 3.00 (t, 2H26, 3J(1H−1H) = 5.6 Hz), 3.60
2
2
(d, 2H1,13, J(1H−1H) = 12.0 Hz), 4.34 (d, 2H1′,13′, J(1H−1H) =
12.0 Hz), 6.66 (d, 2H3,15, J(1H−1H) = 1.8 Hz), 6.86 (d, 2H5,17
,
4
4J(1H−1H) = 1.8 Hz). 13C NMR (100 MHz, DMSO-d6): δ (ppm)
13.6 (C30), 20.4 (C27), 22.7 (C28), 26.6 (C8,20), 27.4 (C29), 29.4
(C10−12,22−24), 34.1 (C9,21), 48.2 (C1,13), 59.3 (C25), 61.9 (C26), 122.4
(C2,14), 122.9 (C5,17), 126.9 (C3,15), 128.6 (C4,16), 137.4 (C6,18), 159.5
(C7,19). 119Sn NMR (149 MHz, DMSO-d6): δ (ppm) −476. MS: m/z
414.31 [L+H]+, 588.26 [M + H]+.
Physical Measurements. A Thermo Scientific Nicolet IS50 FT-
IR spectrometer was used to record the FT-IR spectra in the solid
state. Mass spectra were recorded with a Xevo G2-XS QTOF
spectrometer and VG Analytical (70-S) Spectrometer. A Flash-2000
organic elemental analyzer was used for C, H, and N elemental
microanalyses. Solution NMR spectra were recorded at 25 °C on a
Bruker Avance II FT NMR (AL 400 MHz) or Bruker Avance II FT
NMR (AL 500 MHz) or Jeol JNM ECS400 (400 MHz) spectrometer
(1H, 13C, 119Sn). Chemical shifts are reported in parts per million
1b: [(N(CH2CH2O){CH2(2-t-Bu-4-Me-C6H2O)}2)Sn(n-Bu)(MeOH)]).
The ligand H3L (0.41 g, 1.00 mmol) and sodium methoxide (0.16
g, 3.00 mmol) were dissolved in 30 mL of ethanol with stirring for 15
min. A solution of n-butyltin trichloride (0.16 mL, 1.00 mmol) in 10
mL of ethanol was added dropwise over a period of 20 min at room
temperature. The mixture was stirred for 6 h. The solvent was
removed in a vacuum, and dichloromethane (10 mL) was added to
dissolve the residue. The solid impurities were filtered and the filtrate
was reduced to 1/2 volumes. Then, 5 mL of ethanol was added to the
concentrated solution and left to stand for a few days, which yielded
crystals of pseudostannatrane 1b as colorless blocks. Melting point:
218 °C. Yield: 88% (0.51 g, 0.88 mmol). Elemental analysis calculated
for C31H49NO4Sn: C, 60.21; H, 7.99; N, 2.26. Found: C, 60.04; H,
1
relative to tetramethylsilane (TMS) for H and 13C and tetramethyl-
stannane for 119Sn NMR. A Rigaku XTA Lab SuperNova, single
source (Mo Kα, λ = 0.71073) at offset/far, HyPix3000 diffractometer
was used to collect crystallographic data. The crystal was kept at
293(2) K during data collection. The structure was solved using
Olex2 with the ShelXT structure solution program using intrinsic
phasing and refined with the ShelXL refinement package using least
squares minimization.45−47 The reaction mechanism has been
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a
Scheme 1. Reaction Scheme to Obtain Pseudostannatranes (1a, 1b, 2a, 2b, 3, and 3·Et3N)
a
The numbers on the structural skeletons refer to the assignment of the NMR data.
7.84; N, 2.17. FT-IR ν (cm−1): 441 (Sn−N), 496 (Sn−O), 519 (Sn−
C), 1467 (CC, phenyl ring), 1605 (CC, phenyl ring), 2872,
2913, 2953 (C−H), 3224 (O−H). 1H NMR (400 MHz, DMSO-d6):
δ (ppm) 0.94 (t, 3H30, 3J(1H−1H) = 7.6 Hz), 1.28−1.41 (m,
22H10−12,22−24,27,29), 1.79−1.87 (m, 2H28), 2.15 (s, 6H8,20), 2.69 (br,
mL, 1.00 mmol) in the presence of sodium methoxide (0.16 g, 3.00
mmol). Melting point: 239 °C. Yield: 89% (0.54 g, 0.89 mmol).
Elemental analysis calculated for C33H45NO4Sn: C, 62.08; H, 7.10; N,
2.19. Found: C, 61.91; H, 6.96; N, 2.04. FT-IR ν (cm−1): 447 (Sn−
N), 504 (Sn−O), 539 (Sn−C), 1465 (CC, phenyl ring), 1613
1
2H25), 3.03 (br, 2H26), 3.44 (br, 3H, CH3OH), 3.60 (d, 2H1,13
,
(CC, phenyl ring), 2859, 2900, 2945 (C−H), 3201 (O−H). H
NMR (400 MHz, DMSO-d6): 1.27 (s, 18H10−12,22−24), 2.16 (s,
2
2J(1H−1H) = 8.0 Hz), 4.35 (d, 2H1′,13′, J(1H−1H) = 8.0 Hz), 6.67
2
6H8,20), 2.86 (s, 2H25), 3.40 (s, 2H26), 3.79 (d, 2H1,13, J(1H−1H) =
(s, 2H3,15), 6.88 (s, 2H5,17). 13C NMR (100 MHz, DMSO-d6): δ
(ppm) 13.6 (C30), 20.4 (C27), 22.1 (C28), 26.6 (C8,20), 27.4 (C29),
29.4 (C10−12,22−24), 34.1 (C9,21), 55.8 (C25), 59.3 (C1,13), 62.0 (C26),
62.6 (CH3OH), 122.3 (C2,14), 122.9 (C5,17), 126.9 (C3,15), 128.6
(C4,16), 137.5 (C6,18), 159.4 (C7,19). 119Sn NMR (149 MHz, DMSO-
d6): δ (ppm) −477. MS, m/z 415.31 [L + 2H]+, 588.20 [M + H]+.
(2a·Me2CO): [(N(CH2CH2O){CH2(2-t-Bu-4-Me-C6H2O)}2)Sn(Ph)-
(H2O)].(Me2CO). The ligand H3L (0.83 g, 2.00 mmol) and phenyltin
trichloride (0.33 mL, 2.00 mmol) were reacted in the same way as
mentioned above for pseudostannatrane 1a·Me2CO. Melting point:
243 °C. Yield: 92% (1.12 g, 1.84 mmol). Elemental analysis calculated
for C35H49NO5Sn: C, 61.60; H, 7.24; N, 2.05. Found: C, 61.44; H,
7.12; N, 1.96. FT-IR ν (cm−1): 452 (Sn−N), 501 (Sn−O), 534 (Sn−
C), 1465 (CC, phenyl ring), 1609 (CC, phenyl ring), 1701
(CO, acetone), 2876, 2913, 2949 (C−H), 3304 (O−H). 1H NMR
(400 MHz, DMSO-d6): 1.26 (s, 18H10−12,22−24), 2.09 (s, 6H8,20), 2.86
11.0 Hz), 4.14 (s, 3H, CH3OH), 4.32 (d, 2H1′,13′, 2J(1H−1H) = 11.0
Hz), 6.71 (d, 2H3,15
,
4J(1H−1H) = 1.4 Hz), 6.89 (d, 2H5,17
,
,
4J(1H−1H) = 1.4 Hz), 7.30−7.38 (m, 3H29−31), 7.99 (d, 2H28,32
3
3
3J(1H−1H) = 7.6 Hz, J(1H−117Sn) = 87.6, J(1H−119Sn) = 100.4).
13C NMR (100 MHz, DMSO-d6): δ (ppm) 20.4 (C8,20), 29.5
(C10−12,22−24), 34.1 (C9,21), 48.5 (CH3OH), 59.1 (C25), 61.7 (C1,13),
62.0 (C26) 122.5 (C2,14), 123.4 (C5,17), 127.0 (C30), 127.2 (C29,31),
128.1 (C3,15), 128.6 (C4,16), 136.2 (C28,32), 137.9 (C6,18), 147.1 (C27),
159.2 (C7,19). 119Sn NMR (149 MHz, DMSO-d6): δ (ppm) −531.
MS: m/z 414.31 [L+H]+, 530.19 [M−C6H5]+, 608.20 [M + H]+.
3: [(N(CH2CH2OH){CH2(2-t-Bu-4-Me-C6H2O)}2)SnCl2]. The mixture
of ligand H3L (0.41 g, 1.00 mmol) and triethylamine (0.30 g, 3.00
mmol) in THF (30 mL) was stirred for about 10 min in an ice bath.
Then, a solution of tin tetrachloride (0.12 mL, 1.00 mmol) in THF
(10 mL) was added dropwise over a period of 20 min under ice cold
conditions. The mixture was stirred for 5 h at room temperature and
filtered to remove the salt (triethylammonium chloride). The filtrate
was reduced under a vacuum and kept for crystallization. It yielded a
powdered pseudostannatrane 3 after few days. Melting point: 206−
209 °C. Yield: 89% (crude, 0.53 g, 0.89 mmol). Elemental analysis
calculated for C26H37Cl2NO3Sn: C, 51.94; H, 6.20; N, 2.33. Found:
C, 51.85; H, 6.14; N, 2.29. FT-IR ν (cm−1): 457 (Sn−N), 503 (Sn−
O), 1464 (CC, phenyl ring), 1611 (CC, phenyl ring), 2865,
2917, 2951 (C−H) 3385 (O−H). 1H NMR (400 MHz, CDCl3): 1.29
2
(s, 2H25), 3.20 (s, 2H26), 3.78 (d, 2H1,13, J(1H−1H) = 11 Hz), 4.31
2
(d, 2H1′,13′, J(1H−1H) = 11 Hz), 6.70 (s, 2H3,15), 6.87 (s, 2H5,17),
7.31−7.37 (m, 3H29−31), 7.97 (d, 2H28,32
,
3J(1H−1H) = 6.4 Hz,
3J(1H−117Sn) = 85.6, J(1H−119Sn) = 98.4). 13C NMR (100 MHz,
DMSO-d6): δ (ppm) 20.4 (C8,20), 29.5 (C10−12,22−24), 34.1 (C9,21),
59.1 (C25), 61.6 (C1,13), 61.7 (C26), 123.4 (C2,14), 126.6 (C5,17), 126.9
(C30), 127.2 (C29,31), 128.6 (C3,15), 129.0 (C4,16), 136.2 (C28,32),
136.4 (C6,18), 137.9 (C27), 159.2 (C7,19). 119Sn NMR (149 MHz,
DMSO-d6): δ (ppm) −532. MS: m/z 414.31 [L+H]+, 608.23 [M +
H]+, 686.25 [M+C6H5+2H]+.
3
3
(s, 18H10−12,22−24), 2.15 (s, 6H8,20), 2.85 (t, 2H25, J(1H−1H) = 5.0
2b: [(N(CH2CH2O){CH2(2-t-Bu-4-Me-C6H2O)}2)Sn(Ph)(MeOH)]. A
similar procedure was adopted to that of pseudostannatrane 1b for the
reaction of H3L (0.41 g, 1.00 mmol) and phenyltin trichloride (0.16
Hz), 3.36 (t, 2H26, 3J(1H−1H) = 5.0 Hz), 3.45 (d, 2H1,13, 2J(1H−1H)
2
= 10 Hz), 4.86 (d, 2H1′,13′, J(1H−1H) = 10 Hz), 6.54 (s, 2H3,15),
7.02 (s, 2H5,17). 13C NMR (100 MHz, CDCl3): δ (ppm) 20.7 (C8,20),
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29.9 (C10−12,22−24), 34.7 (C9,21), 46.6 (C25), 56.3 (C1,13), 68.1 (C26),
119.8 (C2,14), 126.2 (C5,17), 128.3 (C3,15), 129.7 (C4,16), 139.3 (C6,18),
158.4 (C7,19). 119Sn NMR (149 MHz, CDCl3): δ (ppm) −565, −569,
−625, −669. MS: m/z 414.31 [L+H]+, 530.18 [M-2Cl−H]+, 608.20
[M-2Cl+C6H5].
theoretical calculations performed on the reaction of phenyl-
tintrichloride and a ligand suggested the thermodynamically
favorable formation of 2b as it is an exothermic reaction and is
accompanied by the release of 49.2 kcal/mol of energy.
Overall, both the experimental and theoretical findings (i.e.,
use of strong base and exothermic reaction profile) explain why
2b did not further undergo redistribution for the exchange of
Ph and Cl groups and is isolated as a phenyl-pseudostanna-
trane (with Ph group retained on the Sn center).
3·NEt3: [(N(CH2CH2OH){CH2(2-t-Bu-4-Me-C6H2O)}2)SnCl2]·NEt3.
The ligand H3L (0.41 g, 1.00 mmol) and triethylamine (0.30 g,
3.00 mmol) were dissolved in toluene (30 mL) and stirred for 10 min
in an ice bath. A solution of phenyltin trichloride (0.16 mL, 1.00
mmol) in toluene (10 mL) was added dropwise over a period of 20
min. The ice bath was removed after complete addition, and the
mixture was stirred for 5 h at room temperature. The
triethylammonium chloride formed was removed by filtration, and
the filtrate was reduced under a vacuum. The slow evaporation of
concentrated filtrate yielded crystals of pseudostannatrane 3·NEt3.
Melting point: 212 °C. Yield: 49% (0.29 g, 0.49 mmol). Elemental
analysis calculated for C26H37Cl2NO3Sn: C, 51.94; H, 6.20; N, 2.33.
Found: C, 51.87; H, 6.16; N, 2.31. FT-IR ν (cm−1): 447 (Sn−N), 502
(Sn−O), 1471 (CC, phenyl ring), 1613 (CC, phenyl ring),
2865, 2908, 2954 (C−H), 3395 (O−H). 1H NMR (400 MHz,
CDCl3): 1.32 (s, 18H10−12,22−24), 2.15 (s, 6H8,20), 2.94 (t, 2H25,
In the case of 3, the reaction of the ligand with Et3N
generated a partially deprotonated ligand HL2−. The partial
deprotonation is observed only for the cases where Et3N (a
weak base) was used as a base and complete deprotonation
was observed in the case of sodium methoxide (a strong base).
It is supported by the X-ray crystallographic data of
pseudostannatranes and a parallel experiment where SnCl4
and H3L were reacted in the presence of another strong base
(NaH) to investigate the deprotonation of ligand (see section
S1 in the Supporting Information). The HL2− ligand
coordinated a Sn atom via N, two deprotonated bisphenolic-
O atoms, and a protonated ethanolic-O atom. The remaining
positions were occupied by two chloro groups to form a
distorted octahedral geometry. In most of the previous reports,
Sn acquires hexacoordination by coordinating reactive species
present in the reaction mixture or via additional reactive
groups of the ligand.2,3 A similar trend is observed in the cases
of 1a, 1b, 2a, and 2b where hexacoordination is acquired by
coordinating a solvent molecule (methanol/water) in the sixth
position. This type of behavior was also observed in one of our
earlier reports on pseudostannatrane [NEt3][(N{CH2(2-Me-4-
Me-C6H2O)}3)SnCl2] which was isolated as an ionic product.3
In that case, despite abstraction of all three protons from the
tripodal ligand (as all the arms were phenolic), Cl− occupied
the sixth position, resulting in the formation of triethylammo-
nium salt of anionic pseudostannatrane. However, in the
formation of pseudostannatrane 3, triethylamine, being a weak
base, could not deprotonate the ethanolic arm. Therefore,
untrapped Cl− ions acted as a nucleophile and occupied one of
the octahedral positions. This satisfied the covalency of Sn and
eliminated the need for deprotonation of the ethanolic arm.
Thus, the ethanolic arm coordinated with Sn just like the
coordination of protonated solvents in the case of
pseudostannatrane 1 and 2. The coordination of the
protonated ethanolic arm was also evidenced by the single-
crystal X-ray diffraction of 3·Et3N, which was a reverse
Kocheshkov product. It had a similar structure as 3 but was
isolated as triethylamine solvate due to the noncovalent
interaction of triethylamine with ethanolic OH.
3
3J(1H−1H) = 5.4 Hz), 3.42 (t, 2H26, J(1H−1H) = 5.4 Hz), 3.90 (d,
2
2
2H1′,13′, J(1H−1H) = 12.4 Hz), 4.91 (d, 2H1′,13′, J(1H−1H) = 12.4
Hz), 6.56 (s, 2H3,15), 6.96 (s, 2H5,17). 13C NMR (100 MHz, CDCl3):
δ (ppm) 25.6 (C8,20), 30.1 (C10−12,22−24), 34.5 (C9,21), 46.3 (C25),
56.6 (C1,13), 68.1 (C26), 120.1 (C2,14), 129.9 (C5,17), 130.4 (C3,15),
130.9 (C4,16), 140.2 (C6,18), 151.6 (C7,19). 119Sn NMR (149 MHz,
CDCl3): δ (ppm) −570. MS: m/z 414.31 [L+H]+, 530.18 [M-2Cl−
H]+, 608.20 [M-2Cl+C6H5].
RESULT AND DISCUSSION
■
Syntheses. The tripodal ligand N(CH2CH2OH){CH2(2-t-
Bu-4-Me-C6H2OH)}2 (H3L) was obtained from ethanolamine,
formaldehyde, and 2-(tert-butyl)-4-methylphenol via the
Mannich condensation reaction. The reaction of H3L with n-
butyltintrichloride and phenyltintrichloride in THF/Na/
CH3OH yielded aqua complexes of pseudostannatranes
[LSnBu(H2O)] (1a) and [LSnPh(H2O)] (2a) as their acetone
solvates 1a·Me2CO and 1b·Me2CO, respectively. However,
after varying the solvent system to CH3ONa/ethanol, the
respective pseudostannatranes were obtained as their methanol
complexes [LSnBu(MeOH)] (2a) and [LSnPh(MeOH)] (2b;
see Scheme 1). In contrast, the reaction of H3L with tin
tetrachloride in THF/Et3N yielded pseudostannatrane 3
[LHSnCl2] as a major product; however its crystals were not
obtained. Interestingly, in previous attempts to synthesize
pseudostannatrane 2 by the reaction of H3L and phenyl-
tintrichloride in a toluene/Et3N system, crystals of triethyl-
amine solvate of [LHSnCl2] (i.e., 3·Et3N) were obtained
instead of the formation of expected pseudostannatrane
[LSnPh]. The formation of the unexpected product
[LHSnCl2·NEt3] can be justified by facile reverse Kocheshkov
reactions of organochlorostannanes.39−44
The phenomenon of the reverse Kocheshkov reaction and
formation of 3·NEt3 was supported by carrying DFT
calculations (see Scheme 2). The molecular species involved
in the reaction mechanism has been optimized by performing
density functional (B3LYP/6-31G + (d,p)/LanL2DZ) calcu-
lations as instigated in Gaussian 09 without imposing any
symmetry restrictions. The frequency calculations determined
stationary points and first-order saddle points. The zero-point
vibrational energy ZPVE results from the vibrational motion of
molecular systems at 0 K; it was calculated as a sum of
contributions from all i vibrational modes (harmonic oscillator
model) of the molecular system. The minimum energy
mechanistic route was attained for the reaction, and transition
state (TS) optimization was done according to the TS Berny
algorithm.52 The change in Gibbs free energy (ΔG) was
As discussed above, the reaction of H3L with BuSnCl3 and
PhSnCl3 in the presence of Na/MeOH yielded expected
pseudostannatranes 1a and 2a (or 1b/2b in the presence of
NaOMe). This is quite obvious because a strong base
deprotonated H3L at the initial stages of the reaction to
form the L3− anion, which coordinated the tin center to form
expected products rather than a reverse Kocheshkov reaction.
Moreover, reverse Kocheshkov reactions always involve the
redistribution of substituents between two organochlorotin
derivatives, and in the present case, all the Sn−Cl bonds are
broken due to complete deprotonation of the ligand, which
limits the scope of the redistribution reaction. In addition, the
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In step III, the four-membered cyclic transition state cleaved
in such a way that the phenyl group of the stannatrane cage
and the chloro group of PhSnCl3 replaced their respective
positions. Simultaneous bond formation of alcoholic oxygen
and the Sn center along with ring cleavage resulted in the
formation of the actual product 3·NEt3 and the byproduct
Ph2SnCl2. The overall Gibbs free energy change for the
disproportionation from 3′·NEt3 to 3·NEt3 was found to be
−15.9 kcal/mol. The exothermic energy change seems to be
the driving force behind the disproportionation in the reported
pseudostannatrane.
Scheme 2. Proposed Mechanism for Disproportionation in
Phenyl Pseudostannatrane
The proposed mechanism was supported by monitoring the
progress of the reaction spectroscopically, which evidenced the
formation of 3′·NEt3 and Ph2SnCl2 during the reaction. For
this, aliquots of the reaction mixture were taken at different
time intervals (i.e., 0, 0.5, 1, 2, 3, and 4 h) and kept under an
inert atmosphere at −10 °C. The 119Sn NMR spectrum of each
aliquot was recorded in CDCl3 and compared with the
spectrum of pure PhSnCl3 and final product (as references;
Figure 2). The 119Sn NMR spectrum of pure PhSnCl3
(spectrum “a”) showed a signal at −62 ppm. Spectrum b,
recorded for the first sample collected immediately after the
addition of PhSnCl3 to the stirred mixture of ligand and NEt3
(time = 0 h), showed a signal of unreacted PhSnCl3 (at −62
ppm) and a new signal at −533 ppm, suggesting the formation
of new species in the reaction mixture. The signal at −533 ppm
had a similar chemical shift as in the case of the 2a (−532
ppm) and the 2b (−531 ppm), suggesting the formation of
phenyl pseudostannatrane (3′·NEt3). Spectrum c, recorded
after half an hour of stirring, showed the appearance of two
additional signals at −36 ppm and −570 ppm, which could be
calculated according to the equation given as ΔG = ΔGProduct
ΔGReactant
−
.
The mechanism was sketched by supposing that the reaction
of H3L and PhSnCl3 in NEt3/toluene should yield phenyl-
pseudostannatrane (i.e., 3′·NEt3) instead of 3·NEt3 (step I).
Just like 3·NEt3, it should have a deprotonated ethanolic −OH
and a −Cl group (on Sn center) due to partial deprotonation
of the ligand by a weak NEt3 base. The presence of −Cl
provided a means to interact with another PhSnCl3 molecule
and forms a high lying transition state (TS) which is analogous
to the one documented in the literature.42 The transition state
reported by Nechaev et al. had a four-membered cyclic
transition state positioned at an energy difference of 24.8 kcal/
mol from the precursor, and the energy released during the
overall disproportionation reaction was reported to be 7.4
kcal/mol. In the present case, the cleavage of the coordinate
bond between alcoholic oxygen and the Sn center generated a
vacancy to incorporate the bond with the chloro group of
PhSnCl3. This led to the formation of a four-membered cyclic
transition state constructed by two distinct Sn centers, a chloro
group and a C atom of the phenyl group (see in step II). The
Gibbs free energy change (ΔG-TS) for the formation of TS
was found to be 46.6 kcal/mol (see Figure 1).
53
assigned to the Ph2SnCl2 and 3·NEt3, respectively. Both the
signals were retained in spectra d, e, and f (recorded at 1, 2,
and 3 h), confirming the presence of four tin species in the
reaction mixture (i.e PhSnCl3, Ph2SnCl2, 3′·NEt3, and 3·
NEt3). After 4 h (spectrum g), the signals at −62 ppm and
−533 ppm disappeared owing to the complete consumption of
the PhSnCl3 and the intermediate 3′·NEt3, and signals of the
final products Ph2SnCl2 and 3′·NEt3 remained in the reaction
mixture. The 119Sn NMR spectroscopic study of the reaction
mixture at different stages of the reaction evidenced the
formation of the intermediate and its conversion into the
obtained product due to the reverse Kocheshkov reaction.
Spectroscopic Studies. The FT-IR spectrum of H3L
showed vibrational bands at 3391 and 3581 cm−1 due to −OH
groups; however the broad vibrational bands observed in the
case of pseudostannatranes 1a, 1b, 2a, and 2b at 3292, 3224,
3304, and 3201 cm−1, respectively, supported the coordination
of solvents. In the case of 3 and 3·Et3N, the vibrational band
corresponding to the ethanolic arm was retained at 3385 and
3395 cm−1, respectively, confirming the coordination of this
arm without deprotonation. The formation of the tricyclic cage
was supported by the presence of the vibrational bands at 496/
496, 501/504, and 501/502 cm−1 due to Sn−O and at 440/
441, 452/447, and 451/448 cm−1 due to the Sn ← N bond for
pseudostannatranes 1a/b, 2a/b, and 3/3·NEt3, respectively. In
addition, Sn−C vibrational band in pseudostannatrane 1a/b and
2
a/b was observed at 520/519 and 534/539 cm−1, respectively.
The H NMR spectrum of H3L consisted of two triplets at
1
2.68 and 3.79 ppm with 3J(1H−1H) = 5.2 Hz due to a
−CH2CH2− chain along with singlets at 1.38 and 2.22 ppm
due to tert-butyl and methyl substituents, respectively. The
signal corresponding to one hydroxyl proton appeared at 8.10
Figure 1. Energy profile for the disproportionation in the phenyl
pseudostannatrane.
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Figure 2. Comparison of 119Sn NMR spectra of reaction mixture at different time intervals (in CDCl3). (a) PhSnCl3 as a reference, (b) 0 h, (c) 0.5
h, (d) 1 h, (e) 2 h, (f) 3 h, (g) 4 h, and (h) isolated product 3·NEt3 as a reference.
Table 1. X-ray Crystal Data of 1−3: Selected Bond Lengths (Å) and Angles (deg)
a
bond lengths [Å]
1a
1b
2a
2b
3·NEt3
Sn1−O1
Sn1−O2
Sn1−O3
Sn1−O4
Sn1−N1
Sn1−C27
2.0577(15)
2.0424(14)
2.0429(15)
2.2802(15)
2.2730(18)
2.125(2)
2.043(2)
2.057(2)
2.334(3)
2.032(2)
2.271(3)
2.121(4)
2.034(3)
2.035(3)
2.043(3)
2.262(4)
2.266(4)
2.135(5)
2.030(4)
2.116(3)
2.444(3)
2.080(3)
2.207(4)
2.002(3)
2.016(4)
2.031(4)
2.046(4)
2.266(5)
bond angles (deg)
O1−Sn1−O4
O1−Sn1−O2
O1−Sn1−O3
O2−Sn1−O3
O2−Sn1−O4
O3−Sn1−O4
O1−Sn1−N1
O2−Sn1−N1
O3−Sn1−N1
O4−Sn1−N1
C27−Sn1−N1
81.51(6)
96.46(6)
157.60(7)
98.21(6)
171.00(6)
81.36(6)
79.13(6)
86.46(6)
84.97(6)
84.55(6)
174.78(8)
98.71(10)
95.99(10)
170.86(9)
80.00(10)
157.64(11)
82.92(10)
86.81(9)
79.12(10)
84.37(10)
84.95(9)
171.84(12)
98.74(13)
95.06(14)
158.91(14)
83.06(14)
81.13(15)
87.29(13)
84.74(13)
80.06(14)
84.95(14)
174.33(15)
154.18(15)
95.24(13)
82.11(13)
171.10(12)
105.47(13)
75.06(13)
85.12(15)
92.62(13)
78.71(13)
78.79(14)
171.30(14)
92.54(18)
166.84(18)
94.00(18)
-
-
88.08(18)
86.97(16)
80.89(16)
172.85(14)
a
Some additional parameters of 3·NEt3. Bond lengths: Sn1−Cl1, 2.4822(16); Sn1−Cl2, 2.3619(17); Bond angles: Cl2−Sn1−Cl1, 90.38(7); O1−
Sn1−Cl1, 87.24(13); O1−Sn1−Cl2, 93.73(15); O2−Sn1−Cl1, 178.36(13); O2−Sn1−Cl2, 91.26(12); O3−Sn1−Cl1, 85.89(13); O3−Sn1−Cl2,
97.52(13); N1−Sn1−Cl1, 91.39(12); N1−Sn1−Cl2, 177.53(13)
1
ppm as a broad signal. The H NMR data of pseudostanna-
tranes 1a and 2a were found almost similar to 1b and 2b;
therefore the data discussed below correspond to 1a and 2a.
The formation of pseudostannatrane 1a was confirmed by the
appearance of signals in the region 0.92−1.87 ppm due to the
n-butyl group in addition to the signals of the ligating skeleton.
The diastereotopic protons at positions 1 and 13 appeared as
doublets at 3.60 and 4.31 ppm due to geminal coupling (with
2J(1H−1H) = 12.0 Hz). Similarly, the formation of
pseudostannatrane 2a was confirmed by the appearance of
additional signals in the aromatic region, i.e., a multiplet within
7.31−7.97 ppm and a doublet at 7.97 ppm due to the phenyl
substituent. The binding of phenyl with a Sn atom was
observed as satellites around the H28,32 doublet with
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and 13C NMR investigation, the hydrogen bond interactions
observed in the solid-state structure, however, were absent in
the solution phase permitting the free rotation of arms.
Therefore, methylene protons H1 and H13 appeared as a singlet
that is otherwise restrictive based on crystal structure and
would have led to diastereotopic protons as seen in the case of
pseudostannatranes. The free rotation of these arms also led to
some disorders in its solid-state structure. The ethanolic arm
was found to be split, and as a consequence, an alcoholic −OH
did not find any acceptor for hydrogen bonding. This type of
disorder and important structural parameters (bond lengths
and bond angles) of H3L are consistent with the reported
structures of the related ligand.38
3J(1H−117Sn) = 85.6 Hz and 3J(1H−119Sn) = 98.4 Hz. The 1H
NMR of pseudostannatrane 3 consisted of the signals of the
ligating cage. It was not isolated in crystalline form (obtained
as a powder); however, the signals of the major product
correspond to NMR signals of pseudostannatrane 3·NEt3. The
spectrum of pseudostannatrane 3·NEt3 had all the signals of
the ligating skeleton including two doublets due to
diastereotopic protons, which indicated the coordination of
the ligating system with Sn (as these signals appear as a singlet
in free ligand).
The 13C NMR of pseudostannatranes showed characteristic
signals of all carbon atoms with the signals corresponding to n-
butyl groups in the region 13.6−27.4 ppm and to phenyl in the
regions 126.9−136.2 ppm and 127.0−136.2 ppm for 1a/b and
The mononuclear pseudostannatrane 1a was crystallized as
an acetone solvate 1a·Me2CO in the triclinic crystal system
2
a/b, respectively. The 119Sn NMR signal of pseudostannatranes
̅
with the P1 space group. The crystal structure showed a
1a/b and 2a/b appeared at −476/−477 and −532/−531 ppm,
respectively. The pseudostannatrane 3·NEt3 showed a signal at
−570 ppm, whereas the spectrum of pseudostannatrane 3
consisted of four signals at −565, −569, −625, and −669 ppm,
suggesting the formation of multiple species in the solution
phase. The mass spectra of pseudostannatranes 1a/b and 2a/b
exhibited molecular ion peaks at m/z 588.26/588.20 and
608.23/608.20, respectively, along with the fragmentation peak
due to the ligand at m/z 414.31. In contrast, 3/3·NEt3
exhibited a peak at m/z 530.18 which corresponded to the
[LSn] fragment formed after the loss of the two chloride ions.
Single Crystal X-ray Diffraction Studies. All the
pseudostannatranes were obtained in crystalline forms (except
3), and their structures were elucidated by X-ray crystallog-
raphy. Selected bond lengths and bond angles for pseudos-
tannatranes 1a,b, 2a,b, and 3·NEt3 are presented in Table 1 and
their crystallographic data and structure refinement parameters
are listed in Table S1 (see caption of Figure 3 for selected
bond lengths and bond angles of ligand).
[4.4.3.01,5]tridecane cage with distorted octahedral geometry
around Sn (Figure 4). The coordination sphere of Sn consisted
The ligand H3L was crystallized in the Pbca space group of
the orthorhombic crystal system. Some intramolecular hydro-
gen bond contacts were found between O3−H3···N1 (2.011
Å) and O2−H2···O1B (2.063 Å; shown in Figure 3). Although
Figure 4. ORTEP presentation of 1a·Me2CO with the partial
numbering scheme (thermal ellipsoids were drawn at 30% probability,
and hydrogen atoms have been omitted for clarity).
1
the solid-state structure of H3L is in agreement with the H
of three oxygen atoms of the tripodal tetradentate ligand and a
water molecule at a square planar position. The water molecule
could have arrived from the moisture in the air during the
crystallization process.
The remaining sites were occupied by a nitrogen atom and
n-butyl chain. The water molecule was further bonded to the
acetone molecule via noncovalent interactions and stabilized
the crystal lattice. Despite the unsymmetrical podands and
different trans substituents, the three Sn−O bonds formed by
L3− were essentially similar (Sn1−O1, 2.0577(15); Sn1−O2,
2.0424(14); Sn1−O3, 2.0429(15) Å). The Sn1−O4
(2.2802(15) Å) bond length was found to be different from
the rest as it originated from the coordinated water molecule.
The Sn−C and transannular Sn−N bond lengths were found
to be 2.125(2) and 2.2730(18), respectively. The sum of the
equatorial O−Sn−O angles is 357.5(2)°, and the Sn atom
occupied a position slightly out of the plane toward the
exocyclic n-butyl group.
Figure 3. ORTEP presentation of H3L with the partial numbering
scheme (thermal ellipsoids were drawn at 30% probability and
hydrogen atoms have been omitted for clarity). [Some bond lengths
(in Å): O1A−C1A, 1.390(20); O1B−C1B, 1.465(18); O2−C9,
1.372(2); O3−C21, 1.367(3); N1−C3, 1.477(3); N1−C2, 1.486(3);
N1−C15, 1.464(3). Bond angles (in deg): C3−N1−C2, 110.1(2);
C2−N1−C15, 112.3(2); C3−N1−C15, 111.3(2); O1A−C1A−C2,
115.3(12); O1B−C1B−C2, 107.0(10); O2−C9−C4, 119.9(2); O2−
C9−C8, 118.6(2).]
Pseudostannatrane 1b was also crystallized in the triclinic
crystal system with the P1 space group. Its structural features
̅
were similar to 1a except for the coordination of methanol with
the Sn instead of a water molecule. The sum of the equatorial
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O−Sn−O angles is 357.6(4)°, and the Sn atom was seen
slightly tilted toward the n-butyl group (Figure 5). The three
Figure 7. ORTEP presentation of 2b with the partial numbering
scheme (thermal ellipsoids were drawn at 30% probability and
hydrogen atoms have been omitted for clarity).
direct bond between Sn and methanol (instead of water) at the
equatorial position. The deviation of octahedral geometry was
confirmed by the sum of all the equatorial bond angles equal to
357.9(5)°.
Single-crystal X-ray diffraction analysis of 3·NEt3 revealed
the formation of a [4.4.3.01,5]tridecane cage, which was
cocrystallized with a triethylamine molecule (Figure 8).
Figure 5. ORTEP presentation of 1b with the partial numbering
scheme (thermal ellipsoids were drawn at 30% probability and
hydrogen atoms have been omitted for clarity).
Sn−O bond lengths originating from the ligand coordination
are comparable (Sn1−O1:2.043(2), Sn1−O2:2.057(2), and
Sn1−O4:2.032(2) Å), whereas Sn1−O3 was found to be
slightly longer (i.e., 2.334(3) Å). The Sn−C and transannular
Sn−N bond lengths were found to be the same as in 1a.
The pseudostannatrane 2a was crystallized as its acetone
̅
solvate 2a·Me2CO in the P1 space group of the triclinic system.
It consisted of a similar cage to that of 1a and 1b with
octahedral geometry around the tin. The tripodal tetradentate
cage occupied three equatorial and an axial position, while the
phenyl ring and water occupied the rest of the positions
(Figure 6). The sum of all the equatorial angles was found to
be 358.0(6)° with comparable Sn−O bond lengths.
Unlike the other pseudostannatranes, 2b was crystallized in
the P21/n space group of the monoclinic crystal system (Figure
7). Its structure was the same as 2a, however, it involved a
Figure 8. ORTEP presentation of 3·NEt3 with the partial numbering
scheme (thermal ellipsoids were drawn at 30% probability, whereas
hydrogen atoms have been omitted for clarity).
It featured hexacoordinated Sn in which the ligand was
coordinated in such a way that its N atom and one of the
oxygen atoms are situated trans to the Cl groups. Despite the
different trans donors, the three Sn−O bond lengths were
found to be practically the same (Sn−O1 2.016(4), Sn−O2
2.031(4), and Sn−O3 2.046(4) Å). The Sn−Cl bond lengths
were more responsive to their distinct trans-donor environ-
ment and are found to be Sn−Cl1 = 2.4822(16) and Sn−Cl2 =
2.3619(17) Å.
All of the six-membered fused rings in pseudostannatranes
1−3 assumed twist-boat conformations, whereas the five-
membered fused rings acquired a uniform enveloped
Figure 6. ORTEP presentation of 2a·Me2CO with the partial
numbering scheme (thermal ellipsoids were drawn at the 30%
probability, and hydrogen atoms have been omitted for clarity).
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conformation and made the molecules chiral in terms of Δ and
Λ stereochemistry. Pseudostannatranes 1a/b and 2a/b each
contained only one crystallographically independent molecule
but as a pair of enantiomers in their unit cell, Δ /Λ-1a, Δ /Λ-
1b, Δ /Λ-2a, and Δ /Λ-2b, whereas the unit cell of
pseudostannatrane 3·NEt3 consisted of only one crystallo-
graphic independent molecule with clockwise (Δ-3·NEt3)
orientation of the propellers when viewed along the Z−Sn−N
axis. All of the pseudostannatrane cages showed distorted
octahedral geometry. One of the efficient criteria of the
“goodness” of the octahedral geometry is the difference
(Δ∑(ϑ)°) of equatorial and axial angles, which is 0° (4 ×
90° to 4 × 90°) for the ideal octahedron.15,54 The geometric
goodness Δ∑(ϑ)° of the octahedral configuration of the tin
atoms in pseudostannatranes 1−3 lies in the range between
13.22° and 26.22° and depicts a strong distortion from the
containing crystallographic data and structure refine-
ment parameters (PDF)
Accession Codes
CCDC 1950219 (H3L), 1950224 (1a), 1950229 (1b), 1950240
(2a), 1950241 (2b) and 1950262 (3·NEt3) 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.
AUTHOR INFORMATION
Corresponding Authors
■
Raghubir Singh − Department of Chemistry, DAV College,
Chandigarh 160011, India; Email: raghu_chem2006@
yahoo.com
ideal geometry (Table 2). In pseudostannatranes 1a/b and 2a/b
,
Varinder Kaur − Department of Chemistry, Panjab University,
Chandigarh 160014, India; orcid.org/0000-0002-4585-
0426; Email: var_ka04@yahoo.co.in, var_ka04@pu.ac.in
Table 2. Geometrical Goodness of the Octahedral
Geometry Δ∑(ϑ)° and Distances Δ(E(Oeq/Cleq)−Sn) for
Pseudostannatranes 1a/b, 2a/b, and 3·NEt3
pseudostannatrane
|Δ∑(ϑ)°| (deg)
Δ(E(Oeq/Cleq)−Sn) (Å)
Authors
Keshav Kumar − Department of Chemistry, Panjab University,
Chandigarh 160014, India; orcid.org/0000-0001-7001-
0034
Neha Srivastav − Department of Chemistry, Panjab University,
Chandigarh 160014, India
1a
1b
2a
2b
26.22
25.02
23.51
24.63
13.22
0.222
0.220
0.202
0.207
0.116
3·NEt3
Mayank Khera − Department of Chemistry, Panjab University,
Chandigarh 160014, India
Neetu Goel − Department of Chemistry, Panjab University,
Chandigarh 160014, India; orcid.org/0000-0001-5345-
0783
the equatorial positions were occupied by O1−O4, whereas
they were taken up by O1−O3 and Cl1 in pseudostannatrane
3·NEt3. The displacement of the tin atom was found to be in
the direction of the exocyclic ligand from the mean equatorial
plane E(Oeq/Cleq) of oxygen/chlorine atoms (Table 2).
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.inorgchem.0c01202
CONCLUSION
■
Notes
The pseudostannatranes 1−3 were obtained as mononuclear
entities with hexacoordination at the Sn atom. The ligating
system possessing bulky substituents certainly disfavored the
formation of oligomeric tin(IV) compounds for which the
oligomerization is pretty much common possibly due to
weaker steric constraints. The formation of the octahedral tin
coordination sphere is assisted by the coordination of solvent
in the case of pseudostanntranes 1a, 1b, 2a, and 2b and
monodentate Cl− ligands in the case of 3 and 3·Et3N. The
partial deprotonation of the ligand resulted in the retention of
the Sn−Cl bond, which provides a means for the reverse
Kocheshkov reaction and yielded a different product 3·Et3N
instead of 2. The partial deprotonation is observed only for the
cases where Et3N (a weak base) was used as a base and
complete deprotonation was observed in the case of sodium
methoxide (a strong base). The experimental findings have
been rationalized by exhaustive density functional calculations
that provide conclusive evidence for the results presented in
the study.
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
The authors are thankful to DST, New Delhi [EMR/2016/
006530] for providing financial support. Dr. Raghubir Singh
■
̈
and Dr. Varinder Kaur dedicate this work to Dr. Jorg Wagler,
Institut fur Anorganische Chemie, Technische Universitat
̈
̈
Bergakademie, Freiberg−09596 Freiberg, Germany for his
continuous support.
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ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.inorgchem.0c01202.
■
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