Rare antimony(iii) imidazole selone complexes: Steric controlled structural and bonding aspects

Article abstract of DOI:10.1039/d0dt02999h

Novel antimony(iii) imidazole selone complexes in a super crowded environment are reported for the first time. The super bulky selone antimony complexes, [{IPr?Se}(SbCl3)2] (1) and [{IPr?Se}(SbBr3)2] (2), were isolated from the reactions between IPr?Se (IPr?Se = [1,3-bis(2,6-diphenylmethylphenyl)imidazole selone]) and suitable antimony(iii) halides. 1 and 2 are dinuclear complexes with a Sb:Se ratio of 1:0.5 with an unusual coordination mode of selone. The molecules 1 and 2 consist of both Menshutkin-type Sb?πaryl interactions and a Sb-Se coordination bond. However, the reaction between antimony(iii) halides and [(IPaul)Se] ([(IPaul)Se] = [1,3-bis(2,4-methyl-6-diphenyl phenyl)imidazole selone]) with a spatially defined steric impact gave the dinuclear complex [{(IPaul)Se}(SbCl3)]2 (3) and the mononuclear complex [{(IPaul)Se}(SbBr3)] (4) without Menshutkin-type interactions. The Sb:Se ratio in 3 and 4 is 1:1. Interestingly, the Menshutkin-type interaction was absent in 3 and 4 due to the efficient coordinating ability of the ligand [(IPaul)Se] with the Sb(iii) center compared to that of the super bulky ligand IPr?Se. The thermal property of these antimony selone complexes was also investigated. Density functional theory (DFT) calculations were carried out on the model systems [L(SbCl3)2] (1A), [L(SbCl3)] (1B), [L′(SbCl3)2] (1C), and [L′(SbCl3)] (1D), where L = [1,3-bis(2,6-diisopropyl-4-methyl phenyl)imidazole selone] and L′ = [1,3-bis(phenyl)imidazole selone], to understand the nature of orbitals and bonding situations. The computed metrical parameters of 1A are in good agreement with the experimental values. Natural population analysis of the model system reveals that the natural charge and total population of antimony(iii) are comparable. The unequal interaction between selenium and antimony obtained using Wiberg bond indices (WBIs) is fully consistent with the findings of the single-crystal X-ray studies.

Full text of DOI:10.1039/d0dt02999h

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DOI: 10.1039/D0DT02999H
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ARTICLE
Rare Antimony(III) Imidazole Selone Complexes: Steric Controlled
Structural and Bonding Aspects
Muneshwar Nandeshwar,a, Mannem Adinarayana,a, Katam Srinivas,a Kavitha Velappanb and
Received 00th January 20xx,
Accepted 00th January 20xx
Ganesan Prabusankar*a
DOI: 10.1039/x0xx00000x
Novel antimony(III) imidazole selone complexes under a super crowded environment are reported for the first time. The
super bulky selone antimony complexes, [{IPr*Se}(SbCl3)2] (1) and [IPr*Se(SbBr3)2] (2), were isolated from the reactions
between IPr*Se, (IPr*Se = [1,3-bis(2,6-diphenylmethylphenyl)imidazole selone]), and suitable antimony(III) halides. 1 and 2
are dinuclear complexes with Sb:Se ratio of 1:0.5. with an unusual coordination mode of selone. The molecules 1 and 2
consists of both Menshutkin-type Sb···πaryl interactions as well as the Sb-Se coordination bond. Whereas, the reaction
between antimony(III) halides and, [(IPaul)Se], ([(IPaul)Se] = [1,3-bis(2,4-methyl-6-diphenyl phenyl)imidazole selone]) with
the spatially-defined steric impact gave dinuclear complex [{(IPaul)Se}(SbCl3)]2 (3) and mononuclear complex
[{(IPaul)Se}(SbBr3)] (4) without Menshutkin-type of interactions. The Sb:Se ratio in 3 and 4 is 1:1. Interestingly, the
Menshutkin-type of interaction was absent in 3 and 4 due to the efficient coordinating ability of ligand [(IPaul)Se] with Sb(III)
center compared to the super bulky ligand IPr*Se. The thermal property of these antimony selone complexes was also
investigated. Density Functional Theory (DFT) calculations were carried out on model systems [L(SbCl3)2] (1A), [L(SbCl3)] (1B),
[L’(SbCl3)2] (1C), and [L’(SbCl3)] (1D), L = [1,3-bis(2,6-diisopropyl-4-methyl phenyl)imidazole selone] and L’ = [1,3-
bis(phenyl)imidazole selone], to understand the nature of orbitals and bonding situations. The computed metrical
parameters of 1A are in good agreement with the experimental values. Natural Population Analysis of the model system
reveals that the natural charge and total population of antimony(III) are comparable. The unequal interaction between
selenium and antimony obtained using Wiberg Bond Indices (WBIs) is fully consistent with the single-crystal X-ray studies.
www.rsc.org/
probed the chemistry of a variety of bismuth(III) imidazole
chalcogenone complexes, ultimately developing potential
catalysts for the acylative cleavage of cyclic ethers,11 multiple
C–C bond formation reactions12 and C-S cross-coupling
reactions.13 The C-S cross-coupling efficiency of the bismuth-
imidazole selone complex is comparable with that of zinc-
imidazole selone complexes.10e,10f
Depending on the steric nature of imidazole selone ligands,
three different coordination modes were reported with
bismuth(III) (Scheme 1). As a result, mono, di, tri, and
polynuclear bismuth(III) imidazole chalcogenone complexes
were isolated. Recently the first polynuclear coordination chain
Introduction
The use of bulky aryl-substituted N-heterocyclic carbene (NHC)
to kinetically stabilize unprecedented chemical functionalities
with a p-block element is a breakthrough of the contemporary
main group organometallic chemistry.1-3 Therefore, special
attention has been put on developing the NHC analogs of
neutral -donor ligands to realize the p-block elements with
hitherto unknown coordination number or oxidation state with
bonding situations.4-7 For example, the urea derivatives of N-
heterocyclic chalcogenones of the type (NHC)E (E = O, S, Se,
Te) have been considered as NHC analogs and successfully
incorporated as monodentate, bidentate, and even polydentate
ligands with transition metals and main group metals.7
Several transition metal imidazole chalcogenone complexes
have been isolated and structurally characterized. The catalytic
application of some of these species has also been
demonstrated successfully.7-10 However, similar chemistry with
the main group metal is relatively sluggish. Lately, we have
of
imidazole-1-methyl-3-(2-isopropyl)-selone
bismuth
trichloride was isolated using the sterically less congested
ligand.14 In contrast, the related chemistry of antimony(III) with
imidazole selone is rare despite these imidazole selones showed
interesting coordination ability with several bismuth(III) salts.11-
15
a. Department of Chemistry, Indian Institute of Technology Hyderabad, INDIA-502
285, E-mail: prabu@chy.iith.ac.in
b. DAV-IITH, Indian Institute of Technology Hyderabad, INDIA-502 285
 Equal Contributions
Electronic Supplementary Information (ESI) available: [details of any supplementary
information available should be included here]. See DOI: 10.1039/x0xx00000x
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 1
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ARTICLE
Journal Name
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DOI: 10.1039/D0DT02999H
N
N
N
N
R
N
N
R
R
R
R
R
Se
Se
SbX3
Se
Me
N
N
Me
Me
N
N
Me
Toluene
CH2Cl2
24 h, 100 o
Bi
Bi
Bi
Bi
X
X
X
X
X
X
X
X
C
X
Se
X
Se
X
X
X
2
n
Known
Sb
Sb
X
IPr*Se
X
X
X
X
N
N
N
N
X = Cl (1); Br (2)
R
R
R
R
Scheme 2 Synthesis of 1 and 2.
Se
Se
Sb
Sb
Sb
X
X
X
X
X
To understand the influence of steric hindrance at Sb(III), a
similar type of ligand with the spatially-defined steric impact such as
[(IPaul)Se] was subjected under similar studies. The ligand [(IPaul)Se]
was prepared from the reaction between 1,3-bis(2-diphenyl methyl-
4,6-dimethyl phenyl)-imidazolium chloride and selenium powder in
the presence of potassium carbonate. The new antimony(III) selone
complexes [{(IPaul)Se}(SbCl3)]2 (3) and [{(IPaul)Se}(SbBr3)] (4) were
synthesized from the direct reaction between the corresponding
antimony halides and an equimolar quantity of [(IPaul)Se] in toluene
(Scheme 3). The resultant complexes 3 and 4 were isolated as a white
precipitate with good yield. These complexes were insoluble in
dichloromethane, chloroform, and hexane, while sparingly soluble in
MeOH, CH3CN, DMSO, and DMF. The antimony complexes 1-4 were
characterized by multinuclear NMR spectra, FTIR spectroscopy,
elemental analysis, and thermogravimetric analysis.
X
X
X
2
X
Present Work
Scheme 1 Known group 15 complexes of imidazole selone.
Herein we report the imidazole-selone antimony(III) complexes
with unique coordination properties. In our current approach, we
have explored the influence of the steric demand of the N-
heterocyclic selenoureas on the antimony(III) salts. For this purpose,
we have utilized the super bulky ligand, IPr*Se (IPr*Se = [1,3-bis(2,6-
diphenylmethylphenyl)imidazole selone]) and the ligand with
spatially-defined steric impact, [(IPaul)Se] ([(IPaul)Se] = [1,3-bis(2,4-
methyl-6-diphenyl phenyl)imidazole selone]). Initially, IPr*Se has
been studied by Nolan on Au16a; by Nahra and Roesky on Cu, Ag, and
Au.16b,c Thus, we report the synthesis and characterization of first
super
bulky
antimony(III)-imidazole
selone
complexes,
[{IPr*Se}(SbX3)2] [X = Cl (1) and X = Br (2)] and [{(IPaul)Se}(SbCl3)]2 (3)
and [{(IPaul)Se}(SbBr3)] (4), where the cyclic seleno urea ligands
depict an unusual steric controlled coordination mode in a discrete
molecule.
Me
Me
N
N
Me
Me
Sb
Se
Cl
Cl
SbCl3
Results and discussion
Cl
Cl
Cl
Toluene
24 h, 100 o
C
Synthesis and characterization of 1, 2, 3 and 4
Cl
Sb
Me
We speculated that the use of a very bulky N, N’-disubstituted cyclic
seleno urea ligand could increase the steric pressure on the metal
center and hence prefer its discrete molecular formation during the
coordination process. In addition, bulky N-heterocyclic selenourea
ligand could influence the electron donating approach towards the
antimony(III) center and thus suppress the coordination number of
the metal. Nevertheless, steric congestion about the antimony(III)
center could be prejudicial for the coordination of incoming
substrates. Thus, the use of a super bulky yet flexible cyclic
selenourea ligand could be a solution. Therefore, the ligand IPr*Se
was synthesized from the reaction between 1,3-bis(2,6-diphenyl
methyl-4-methyl phenyl)-imidazolium chloride, [IPr*.HCl] and
selenium powder in the presence of K2CO3 base. The discrete
antimony imidazole selones 1 and 2 were isolated in good yield from
Se
Me
Me
N
N
Me
Me
Me
N
N
Se
(3)
Me
Me
[(IPaul)Se)]
Me
Me
N
N
Me
Me
Sb
SbBr3
Se
Toluene
24 h, 100 o
C
Br
Br
(4)
Br
the direct reaction between IPr*Se and corresponding antimony Scheme 3 Synthesis of 3 and 4.
halide salts (Scheme 2).
1
In H NMR spectrum of 1, the chemical shift value for Im-CH is
considerably upfield shifted compared to that of 2. The 1H NMR
chemical shift value of CHPh2 in 1 and 2 are upfield shifted compared
to that of IPr*Se. Due to poor solubility of 1 and 2, attempts to get
well resolved 13C NMR was not possible. Indeed, the 13C NMR
2 | J. Name., 2012, 00, 1-3
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1
chemical shift value of C=Se(Sb) was not detected. In H NMR, the
Im-CH and CHPh2 chemical shift values of 3 and 4 were considerably
upfield shifted compared to that of [(IPaul)Se]. The 13C NMR chemical
shift value of C=Se for 3 (δ 157) and 4 (δ 157) showed upfield shift (δ
~2) compared to that of [(IPaul)Se] (δ 159 and 158).
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DOI: 10.1039/D0DT02999H
The solid-state structures of 1, 2, 3, and 4 were unambiguously
determined by the single-crystal X-ray diffraction technique. 1 and 2
crystallized in the monoclinic space group, C1 (Fig. 1). Though solid-
state structures of 1 and 2 appear to be isostructural, the structural
parameters of these two molecules are different. 1 and 2 are
dinuclear antimony(III) complexes, where two SbX3 moieties are
connected by IPr*Se. Similar such coordination mode of (NHC)E is
common in polymeric14 or dimeric coordination11,12,14 structure but
not in the discrete molecules.
The Sb(III) centers in 1 and 2 are penta-coordinated. The
geometry of each metal center can be described as aryl capped see-
saw. The coordination environment of the metal center is
accomplished by one selone, one aryl, and three halides. The discrete
penta coordinated (X3)Sb(III)-selone complexes (as a single molecule
without forming a polymeric network or dimeric species) 1 and 2 with
Sb:Se ratio of 1:0.5 have never been isolated and structurally
characterized.14,15 This solution-based access to the modulation
selenium vs. antimony could potentially benefit in the nanostructure
preparation.17
Fig. 1 (ia) Molecular structure of 1 with anisotropic displacement parameters
depicted at the 50 % probability level. The hydrogen atoms have been omitted for
the clarity. (ib) Molecular structure of 1 with Menshutkin-type interactions. The
hydrogen atoms have been omitted for the clarity. (iia) Molecular structure of 2
with anisotropic displacement parameters depicted at the 50 % probability level.
The hydrogen atoms have been omitted for the clarity. (IIb) Molecular structure
of 2 with Menshutkin-type interactions. The hydrogen atoms and solvent
molecule dichloromethane have been omitted for clarity. Selected bond lengths
(Å) and angles (o) for 1: Sb(1)-Se(1) 3.271, Sb(2)-Se(1) 3.284, C(1)-Se(1) 1.894(13),
Sb(1)-Cl(1) 2.377(6), Sb(1)-Cl(2) 2.345(4), Sb(1)-Cl(3) 2.336(7), Sb(2)-Cl(4) 2.382(5),
Sb(2)-Cl(5) 2.336(5), Sb(2)-Cl(6) 2.309(7), Sb(1)····C(41), 3.554(1); Sb(1)····C(42),
3.606(1); Sb(1)····C(40), 3.786(1); Sb(2)····C(5), 3.611(1); Sb(2)····C(6), 3.419(1);
Sb(2)····C(7), 3.487(1); Sb(2)····C(8), 3.726(1), Sb(1)-Se(1)-Sb(2) 130.71, Sb(1)-C(1)-
Se(1) 114.81, C(1)-Se(1)-Sb(2) 114.39. Selected bond lengths (Å) and angles (o) for
2: Sb(1)-Se(1) 3.287(2), Sb(2)-Se(1) 3.298(2), C(1)-Se(1) 1.833(6), Sb(1)-Br(1)
2.504(13), Sb(1)-Br(2) 2.486(16), Sb(1)-Br(3) 2.542(16), Sb(2)-Br(4) 2.542(15),
Sb(2)-Br(5) 2.474(17), Sb(2)-Br(6) 2.512(14), Sb(1)····C(40), 3.687(1); Sb(1)····C(41),
3.475(1); Sb(1)····C(42), 3.556(1); Sb(1)····C(65), 3.312(1); Sb(2)····C(5), 3.614(1);
Sb(2)····C(6), 3.444(1); Sb(2)····C(7), 3.555(1); Sb(2)····C(17), 3.476(1), Sb(1)-Se(1)-
Sb(2) 131.07, C(1)-Se(1)-Sb(1) 113.45, C(1)-Se(1)-Sb(2) 115.45.
This journal is © The Royal Society of Chemistry 20xx
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Moreover, the super bulky nature of IPr*Se appears to affect the
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coordination position of Sb(III)X3 to adopt a position that maintains
both Se-Sb and Menshutkin-type Sb···πaryl contacts. This trend is
apparent in both 1 and 2 complexes. The Sb···πaryl interaction found
in 1 and 2 is similar to that of SbCl3···toluene complex through
electrostatic and dispersion interactions.18a The Sb(III)····C(aryl)
interactions are in the range of 3.3 Å to 3.8 Å and well within the
range of the Sb(III)····C(aryl) interactions reported in the literature.18
As shown in figure 1, each antimony atom is positioned endo relative
to the aromatic moiety, allowing for the stereochemically active
antimony lone pairs to be directed towards the ligand.
DOI: 10.1039/D0DT02999H
Fig. 2 (i) Space-filling model and molecular packing (view along an axis) of 1.
(ii) Space-filling model and molecular packing (view along c axis) of 2 has been
omitted for clarity.
The Se-Sb bond distances in 1 (av. 3.278 Å) and 2 (av. 3.293
Å) are comparable. The Se-Sb bond distances in 1 and 2 are
Fig. 3. (i) Molecular structure of 3 with anisotropic displacement parameters depicted at
about 0.4 Å longer than that of [{(Me2N)(NH2)CSe}2SbCl3] (2.871 the 50 % probability level. The hydrogen atoms have been omitted for clarity. (ii)
Å),19 while Se-Sb bond distances in 1 and 2 are well within the
Molecular association of 3. The hydrogen atoms have been omitted for clarity. (iii) The
solid-state packing of 3, view along c axis. Selected bond lengths (Å) and angles (o) for 3:
sum of van der walls radii of Sb and Se (rw = rwSb 2.06 Å + rwSe
Sb(1)-Se(1) 2.933(4), C(1)-Se(1) 1.847(3), Sb(1)-Cl(1) 2.3594(8), Sb(1)-Cl(2) 2.5225(9),
Sb(1)-Cl(3) 2.367(9), C(1)-Se(1)-Sb(1) 106.74(8).
1.90 Å = 3.96 Å).20
The super bulky nature of ligand favors the formation of the
discrete molecule and prevents the polymerization. As depicted in
figure 2, the metal centers in 1 and 2 are completely wrapped-up by
super bulky ligand. Thus the sterically protecting organic moiety
prevents the poly coordinated mode of IPr*Se or interaction
between Sb(III)X3 units. The molecular packing of 1 and 2 are
distinctly different.
Interestingly 3 is a dinuclear Sb(III) selone complex (Figure
3). Due to the decrease in the steric hindrance of ligand
[(IPaul)Se] at the ortho position compared to IPr*Se, complex 3
exist as a dimer with the absence of Menshutkin-type
interactions and shortening of Sb-Se bond distance. Thus both
steric hindrance, as well as the facile coordination of selenium,
disfavoured the Menshutkin-type interactions. Complex 3 has a
bifacial square pyramid dinuclear structure. The two antimony
centers in 3 are weakly connected through two bridging
chloride ligands. These dimers are arranged in a columnar
structure in the solid-state packing (Figure 3iii). The Sb(1)-Cl
bond distances are not comparable in 3. In Sb(1)-Cl(2)(µ2)-
Sb(1)’, one of the Sb(1)-Cl(2)(µ2) bond distance is much shorter
(2.5225(9) Å) than that of the Sb(1)’-Cl(2)(µ2) (3.220(9) Å).
However, the Sb(1)’-Cl(2)(µ2) bond distance is well within the
sum (3.81 Å) of van der Waals radius of antimony (2.06 Å) and
chlorine (1.75 Å).20 The Sb(1)-Cl(1) bond distance is slightly
4 | J. Name., 2012, 00, 1-3
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shorter than the Sb(1)-Cl(3) bond distance. The C=Se bond atom spans the axial sites. The Sb-Br bond distance in the axial
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length of selone coordinating to the antimony atom is typical position is considerably longer than tDhOatI: 1o0f.10t3h9e/De0DqTu0a2t9o9r9iaHl
for the imidazole selone ligand. The distance (4.279(3) Å) positions. Notably, the ligand arrangement in this complex 4 is
between two antimony centers is significantly longer than a very different compared to 1 and 2 due to the electronic and
typical Sb–Sb single-bond length found in CAAC-SbSb-CAAC steric properties of the ligands concerned. Our attempts to
[CAAC = cyclic alkyl-(2,6-diisopropylphenyl amino)carbene] isolate the single crystals of 3 and 4 under different solvent
(2.8125(10) Å),21 suggesting no bonding interaction between conditions to understand the role of solvents towards the solid
two antimony centers.
packing and Menshutkin-type interactions were hampered
because of the limited solvent choices. Moreover, the solution-
state NMR study is not in favor of the Menshutkin-type
interactions present in the solid-state structures of 1 and 2.
Therefore, we presume that the coordination environment and
structural features of 1-4 in the solid-state could be different
compared to the solution state.
Furthermore, 1 was chosen as a representative example to
understand the structural and bonding properties by Density
Functional Theory (DFT) calculation at the B3LYP/Def2TZVP
level of theory using the 6-31++G** basis set (See ESI). Our
initial attempts to optimize the reported complexes 1 or 3 were
not successful due to the super bulky substituents at aryl ring.
Therefore, the non-interacting aryl groups were eliminated
from the representative complex by keeping the optimal steric
hindrance. The antimony selone complexes, [L(SbCl3)2] (1A),
[L(SbCl3)] (1B), [L’(SbCl3)2] (1C), and [L’(SbCl3)] (1D), L = [1,3-
bis(2,6-diisopropyl-4-methyl phenyl)imidazole selone] and L’ =
[1,3-bis(phenyl)imidazole selone], were computed using DFT
calculation (Scheme 4).
N
N
(1)
Se
Sb
Cl
Sb
Cl
Cl
Cl
Cl
Cl
Fig. 4. (i) Molecular structure of 4 with anisotropic displacement parameters depicted at
the 50 % probability level. The hydrogen atoms have been omitted for the clarity. (ii)
Solid-state packing of 4, view along “a” axis. Selected bond lengths (Å) and angles (o) for
4: Sb(1)-Se(1) 2.9227(5), C(1)-Se(1) 1.861(3), Sb(1)-Br(1) 2.5352(5), Sb(1)-Br(2) 2.7393(5),
Sb(1)-Br(3) 2.5278(5), C(1)-Se(1)-Sb(1) 108.78(11).
N
N
N
N
The solid-state structure of 4 is shown in figure 4.
Interestingly, complex 4 is a mononuclear species without
any Menshutkin-type interactions. The origin of this difference
could be a spatially-defined steric impact of [(IPaul)Se], as it
turns the SbBr3 moiety further away from the phenyl ring plane.
The bond lengths of the C–Se bond in 4 is similar to that of
3. The Sb-Se bond distance is comparable with 3, while it is
about 0.354 Å shorter than that of 2. Thus, the shortening of Sb-
Se bond distances and the disappearance of Menshutkin-type
interactions can be attributed to the facile contribution of
selone. Unlike 3, the intermolecular Sb····Br interactions are
absent in 4 (Figure 4ii). However, molecule 4 depicts the
intermolecular C-H····Br interactions. The Sb(Se)Br3 core of 4
adopts a see-saw geometry. The electron pair geometry of
[(:)Sb(Se)Br3] in 4 can be described as trigonal bipyramidal in
which both bromide ligands and a lone pair electron occupy
equatorial positions, while the selone ligand and a bromine
Se
Cl
Se
Sb
Cl
Sb
Cl
Sb
Cl
Cl
Cl
Cl
Cl
Cl
(1B)
(1A)
N
N
N
N
Se
Se
Sb
Cl
Sb
Cl
Sb
Cl
Cl
Cl
Cl
Cl
(1C)
Cl
Cl
(1D)
Scheme 4. Density Functional Theory (DFT) calculations have been performed on
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the model systems 1A-1D at the B3LYP/Def2TZVP level of theory using the 6-
selenium decreased to 1.77614 (Orbital contribution: s (4.19%), p
22.84(95.79%), d 0.00(0.02%)). The vacantDoOrbI:i1ta0l.1o03fV9ai/enDwt0iAmDrtTioc0lne2yO99nhl9ianHes
significant occupancy 0.58150 (Orbital contribution: s (1.45%), p
67.66(98.24%), d 0.14(0.21%), f 0.07(0.10%)). Therefore, the unusual
coordination mode of IPr*Se seems to be the most desired option
than the monodentate coordination mode. As shown in figure 5a, the
orbital that features the most antimony lone pair character in 1A is
the HOMO-6. The lone pair orbital of Sb(III) has more s-character and
less p-character (Fig. 5b). The orbital occupancy of lone pair on Sb(1)
is 1.99500 with 90.56% s-character and 9.43% p-character, while the
orbital occupancy of lone pair on Sb(2) is with 88.67% s-character and
11.33% p-character.
31++G** basis set.
The input metrics for the geometry optimization were generated
from the X-ray crystallographic data set for 1. The computed metrical
parameters of 1A and 1B are in good agreement with the
experimental values, while the optimized structures of 1C and 1D are
no in agreement with experimental results. The HOMO energy of 1A
is considerably lower than that of 1B. The HOMO–LUMO gap of 1A is
(4.16 eV) considerably lower than that of 1B (4.33 eV). Hence, these
observation supports the formation of 1 and 2 under the bulky
environment.
Besides, we have analyzed the electronic structure of
complex 1A with the atoms in molecules (AIM) method (Fig. 6).
The atoms-in-molecule (AIM) analysis was performed using the
AIMALL software package (See ESI). The contour line diagrams
of the Laplacian distribution ∇2ρ(r) of 1A represents in the
plane, which comprises the antimony atom with a chlorine
atom and selenium atom of the imidazole selone ligand.
Notably, there is a bond path between the antimony atom and
selenium atom. The positive ∇2ρ(r), provides a local charge
depletion. The total charge density, ρ(r) is dominated by the
electrostatic attraction of the electrons for the positively
charged antimony nuclei. The maxima occur at the nuclear sites,
and ρ(r) decays in a nearly spherical manner away from the
nuclei. Topological properties of the electron density computed
at Bond Critical Points (BCP’s) for Sb-Se complex computed at
the B3LYP level. Bond ellipticity (), measures the extent to
which density is preferentially accumulated in a given plane
containing the bond path. Large  values indicate structural
instability and vice versa. Both Sb1-Se1 and Sb2-Se1 bonds
show  values, which are reasonably small, indicating the stable
character of binding and probably weak involvement of d-
orbitals in bonding. Thus, the Sb-Se bonding interaction can be
considered as the closed-shell (electrostatic) interactions.
Fig. 5. (a) HOMO-6 of 5 features the most antimony lone pair character; (b)
The nature of lone pair of electrons in antimony. The lone pair electrons of
Sb(III) are with more s-character and less p-character.
Fig. 6. Contour line diagrams of the Laplacian distribution ∇2ρ(r) of 1A. The
Laplacian of rho, ∇2(ρ), provides a measure of the local charge concentration
or depletion. If the value is positive values mean there is local charge
depletion (blue) whereas if it is negative values, mean there is local charge
concentration (red).
The electrons in lone pair orbital on selenium delocalize into the
vacant orbital of antimony. The vacant d-orbital of antimony(III) is
not involved in the bonding, while only the p-orbitals are involved in
the bonding. As a result, the occupancy of lone pair orbital on
6 | J. Name., 2012, 00, 1-3
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antimony center in scheme 6 (II and III). The coordination mode of
View Article Online
IPr*Se in 1 and 2 can be attributed to the interactions between
DOI: 10.1039/D0DT02999H
occupied py and pz orbitals on the Se atom and empty p orbital on
Sb(III) (Scheme 6(II)), while one of the occupied (py or pz) orbital on
the Se atom and empty p orbital on Sb(III) may contribute to the
coordination mode of 3 and 4 (Scheme 6(III)).23
N
N
C1
Se1
N
N
N
N
N
N
N
N
Ar
Ar
Ar
Ar
Ar
Ar
Ar
Ar
Se
Sb2
Sb1
Se
Se
I-L
Se
Cl
Cl
III-L
II-L
Cl
Cl
Cl
Cl
Resonance Hybrid
Y
Natural Charges
C1 : 0.25064
Se1 : -0.22536
Sb1 : 1.28597
Sb2 : 1.31865
Bond Order
Sb1-Se1 : 0.2146
Sb2-Se1 : 0.0478
Se1-C1 : 1.2505
Ar
Ar

N
N
Se
X
(I)
Se
py
N
N
Ar
Ar
Y
Total Population
X
Y
X
C1 : 5.74936; Se1 : 34.22536
Sb1 : 49.71403; Sb2 : 49.68135
X
Ar
Sb

X
Ar
Sb

N
X
X
N
X
X
Se
py
Scheme 5. The calculated NPA (natural population analysis) charges and WBIs (Wiberg
Bond Indices) bond orders of 1A.
X
X
Se
N
N
X
Ar
Sb
py
Ar
Natural Population Analysis (NPA) of 1A indicates that
antimony(III) centers cooperatively participate in bonding in
complexes 1 and 2 (Scheme 5). The NPA calculations identify a
positive charge at antimony(III) centers and carbon center (selone),
while negative charge at selenium. The negative charge on selenium
is mainly a result of σ-type interactions inducing a drift of electron
density from carbon to selenium. Therefore, the selenium atom can
be considered more nucleophilic due to the greater electron density
found on the atom. The positive charge on Sb(1) is slightly lower than
that of Sb(2). According to NPA, the charge on antimony is close to
+1.3. The nature of bonding (unequal) between Se(1)-Sb(1) and
Se(1)-Sb(2) obtained from Wiberg Bond Indices (WBIs) on 1A is fully
consistent with the single-crystal X-ray studies on 1 and 2 (vide
supra). The calculated bond order using WBIs of Se(1)-Sb(1) is about
four-fold higher than that of Se(1)-Sb(1). However, the calculated
core, total, and Rydberg populations using NPA of both Sb(1) and
Sb(2) are comparable. Similarly, the electronic configuration of Sb(1)
and Sb(2) are comparable.
(II)
(III)
Scheme 6. The plausible resonance structures and the interacting orbitals of the ligand
in the antimony(III) complexes.
Experimental
Materials and methods
All manipulations were carried out under argon atmosphere
in a glove box or using standard Schlenk techniques. The
solvents were purchased from commercial sources and freshly
distilled under argon atmosphere before use. Unless otherwise
stated, the chemicals were purchased from commercial
sources. 1,3-bis(2,6-diphenyl methyl-4-methyl phenyl)-
imidazolium chloride (IPr*.HCl),24 IPr*Se16 and 1,3-bis(2,4-
methyl-6-diphenyl phenyl) imidazolium chloride, and
[(IPaul)Se]13 were synthesized as reported. SbCl3 and SbBr3 were
purchased from Sigma Aldrich and used as received. Elemental
analyses were performed by the Euro EA-300 elemental
analyzer. FT-IR measurements were carried out on a Bruker
Alpha-P Fourier transform spectrometer. NMR spectra were
recorded on Bruker Ultra shield 400 spectrometers at 25 C.
Chemical shifts are given relative to TMS and were referenced
to the solvent resonances as internal standards. The solubility
of samples 1-4 in CDCl3 or DMSO-d6 was not appreciable for the
NMR experiments. The reported NMR spectra were recorded
using diluted samples of 1-4 (about 7-8 mg sample was
dissolved in 0.8 mL of NMR solvent at 50 oC -60 oC) in the
shortest possible time as the sample gets precipitated within
few minutes. Thus, possible NMR data has been reported.
The suitable single crystals of 1 and 2 for the X-ray analysis
were obtained from the saturated solutions of toluene and
dichloromethane at room temperature. The suitable single
crystals of 3 and 4 for the X-ray analysis were obtained from the
saturated solutions of toluene at room temperature. The
Moreover, as per the single-crystal analysis, the carbon-selenium
bond length in IPr*Se is not in a typical carbon single bond selenium
in nature. Upon coordination, the C=Se bond length (slightly)
increases in 1, 2, 3, and 4, compared to the corresponding ligands.
However, the observed carbon-selenium bond distance in these
complexes is comparable with that of selenourea (1.86 Å).22 The
observed carbon-selenium bond distances in 1, 2, 3 and 4 are
considerably shorter than the carbon-selenium single bond distance
(1.98 Å) as well as the sum of van der Waals radius of carbon and
selenium ( 1.70 + 1.90 = 3.60 Å).20 Therefore decrease in the C−N
bond lengths and elongation in the carbon-selenium bond length
suggest a delocalization of the lone pair on the amines. Based on this
observation as well as the above calculation, the plausible resonance
structures and the interacting orbitals of the ligand in the
antimony(III) complexes are depicted in scheme 6. The carbon-
selenium double bond character in 1-4 is partially last due to the
resonance form reported in scheme 6 (I-L to III-L). Thus, we have
summarized the possible orbital interactions between selenium and
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100 °C for 24 hours then slowly brought to room temperature and 4
suitable crystals of 1-4 were selected and mounted on an
Oxford SuperNova, Dual, Cu at zero, Eos diffractometer. The
crystal was kept at 293 K or 298 K during data collection. Using
Olex2, the structure was solved with the olex2.solve the
structure solution program using Charge Flipping and refined it
with the olex2.refine refinement package using Gauss-Newton
minimization.25 The crystallographic related issues are
addressed in the ESI. Absorption corrections were performed
based on multi-scans. Non-hydrogen atoms were
anisotropically refined. Hydrogen atoms were included in the
refinement in the calculated positions riding on their carrier
atoms.
View Article Online
drops of dichloromethane was added. The colorless crystals of 2
DOI: 10.1039/D0DT02999H
were obtained within 72 h. Yield: 82% (Based on SbBr3). M.p: 275-
276 oC. CHN analysis for C60H58Br6Cl2N2Sb2Se (1800.03): Calcd. C,
46.71; H, 3.25; N, 1.56; Found C, 46.6; H, 3.2; N, 1.6. 1H NMR (400
MHz, CDCl3), δ: 7.31-7.23 (13H, m, Ar-H), 7.19-7.12 (19H, m, Ar-H),
6.92 (4H, s, Ar-H), 6.77-6.76 (8H, m, Ar-H), 5.41 (2H, s, NCH2N), 5.14
(4H, s, CHPh2), 2.29 (6H, s, CH3). 13C NMR (100 MHz, CDCl3), δ: 142.8
(CAr), 142.6 (CAr), 141.6 (CAr), 141.1 (CAr), 130.8 (CAr), 130.0 (CHAr),
129.5(CHAr), 129.1 (CHAr), 128.8 (CHAr), 128.5(CHAr), 127.1 (CHAr),
126.9 (CHAr), 126.7 (CHAr), 124.1 (NCH2N), 52.0(CPh2), 22.2 (CH3). The
C=Se carbon signal was not detected in 13C NMR spectrum due to the
poor solubility of 2 in CDCl3 or DMSO-d6. FT-IR (cm-1, neat): 3026(m),
2921(bw), 1596(s), 1491(s), 1446(s), 1408(m), 1331(s), 1263(m),
1218(bw), 1151(bw), 916(m), 842(bw), 758(s), 700(s).
Calculations
All the calculations were done using the Gaussian 16 suite of
program.26 The input files were generated using GaussView,
Version 6.27 The structural parameters of the 1 complex was
adopted from X-ray crystal structure data. The [L(SbCl3)2] (1A),
[L(SbCl3)] (1B), [L’(SbCl3)2] (1C), and [L’(SbCl3)] (1D), L = [1,3-
bis(2,6-diisopropyl-4-methyl phenyl)imidazole selone] and L’ =
[1,3-bis(phenyl)imidazole selone], complexes were optimized at
the B3LYP/Def2TZVP level of theory using the 6-31++G** basis
set. The geometry obtained at the B3LYP level of theory is found
to be in good agreement with the experimental data.
Vibrational frequency calculations were carried out to ensure
the structure is minima on the potential energy surface. The
atoms-in-molecule (AIM) analysis was performed Using the
AIMALL software package,28 with B3LYP/Def2TZVP wave
functions to find topological features of the electron
density. AIM analysis is established on Bader’s theory.29 Natural
Population Analysis (NPA) was used to find atom charges, and
Wiberg Bond Indices (WBIs) were used to find out bond orders.
Both were obtained from Natural Bond Orbital (NBO) analysis
within Gaussian09.
Synthesis of 3
To a solution of [(IPaul)Se] (100 mg, 0.145 mmol) in toluene (5
mL), SbCl3 (33 mg, 0.145 mmol) was added and the reaction mixture
was stirred for 24 h at 100 °C and then slowly brought to the room
temperature. Colorless crystals were obtained within 2-3 days. Yield
79% (based on SbCl3). M.p.: 180–183 °C. CHN analysis for
C52H48N2SeCl3Sb (MW: 1008.11) (%): Calcd. C, 61.96; H, 4.80; N, 2.78.
Found C, 61.9; H, 4.8; N, 2.7. 1H NMR (400 MHz, DMSO-d6) δ: 7.15–
7.31 (18H, m, ArH), 6.76–6.87 (5H, m, ArH), 6.57 (2H, s, N(CH)2N),
6.28 (1H, s, ArH), 5.41 (2H, s, CHPh2), 2.26 (3H, s, CH3), 1.99 (3H, s,
CH3). 13C NMR (100 MHz, DMSO-d6) δ: 157.2 (C–Se), 143.4, 141.8
(ArC), 140.5 (ArC), 138.5 (ArC), 137.3 (ArC), 135.8 (ArC), 134.0 (ArC),
129.7 (ArC), 129.4 (ArC), 129.1 (ArC), 128.8 (ArC), 128.5 (ArC), 128.3
(ArC), 128.1 (ArC), 128.0 (ArC), 126.4 (ArC), 125.2 (ImC), 121.2 (ImC),
50.9 (CHPh2), 21.0 (CH3), 17.8 (CH3). FT-IR (cm-1, neat): 3136(bw),
3024(bw), 2909(bw), 1596(s), 1550(m), 1477(bw), 1446(m),
1346(bw), 1229(m), 1166(bw), 1077(m), 1029(s), 971(bw), 854(m),
738(s), 694 (s), 627(m).
Synthesis of 4
4 was synthesized as reported for 3 using [(IPaul)Se] (100 mg,
0.145 mmol) and SbBr3 (52 mg, 0.145 mmol). Colorless crystals were
obtained within 2-3 days. Yield 81% (based on SbBr3). M.p.: 170-173
°C. CHN analysis for C52H48N2SeBr3Sb (MW: 1141.38) (%): Calcd. C,
Synthesis of 1
To a stirred solution of IPr*Se (100 mg, 0.1 mmol) in toluene (5
mL), antimony(III) chloride (68 mg, 0.3 mmol) was added. The
reaction mixture temperature was maintained at 100 °C for 24 hours
1
54.72; H, 4.24; N, 2.45. Found C, 54.6; H, 4.1; N, 2.6. H NMR (400
then slowly brought to room temperature and
4 drops of
MHz, DMSO-d6) δ: 7.33–7.14 (18H, m, ArH), 6.87–6.76 (5H, m, ArH),
6.22–6.55 (2H, s, N(CH)2N), 5.42 (2H, s, CH(Ph2), 2.29 (3H, s, CH3), 1.9
(3H, s, CH3). 13C NMR (100 MHz, DMSO-d6) δ: 157.2 (C–Se), 143.4
(ArC), 141.8 (ArC), 140.5 (ArC), 138.5 (ArC), 137.3 (ArC), 135.8 (ArC),
134.0 (ArC), 129.7 (ArC), 129.4 (ArC), 129.2 (ArC), 128.8 (ArC), 128.3
(ArC), 128.1 (ArC), 128.0 (ArC), 126.4 (ArC), 125.2, (ArC), 121.3 (ImC),
50.9 (CHPh2), 21.0 (CH3), 17.8 (CH3). FT-IR (cm-1, neat): 3137(bw),
3020(m), 2914(m), 2060(bw), 1960(m), 1752(bw), 1592(s), 1477(bw),
1343(bw), 1235(m), 1164(m), 1076(m), 1028(s), 925(m), 856(s),
694(s), 624(m), 569(m).
dichloromethane was added. The colorless crystals of 1 were
obtained within 72 h. Yield: 83% (based on SbCl3). M.p: 283-284 oC.
CHN analysis for C69H56Cl6N2Sb2Se (1448.39): Calcd. C, 57.22; H, 3.90;
N, 1.93; Found C, 57.2; H, 3.8; N, 1.9. 1H NMR (400 MHz, CDCl3), δ:
7.31-7.20 (22H, m, Ph-Ar), 7.11-7.10 (12H, m, Ar-H), 6.90 (4H, S, Ar-
H), 6.77-6.75 (8H, m, Ar-H), 5.34 (2H, s, N(CH)2N), 5.18 (4H, s,
CH(Ph2), 2.29 (6H, s, CH3). 13C NMR (100 MHz, CDCl3); δ: 142.7 (CAr),
141.4 (CAr), 141.0 (CAr), 137.5 (CAr), 130.9 (CHAr), 129.5 (CHAr), 129.1
(CHAr), 128.8 (CHAr), 128.5 (CHAr), 127.2 (CHAr), 126.7 (CHAr), 52.0
(CHPh2), 22.2 (CH3). The C=Se carbon signal was not detected in 13C
NMR spectrum due to the poor solubility of 1 in CDCl3 or DMSO-d6.
FT-IR (cm-1, neat): 2885(bw), 1595(s), 1489(bw), 1445(s), 1406(m),
1330(s), 1263(m), 1152(bw), 1113(m), 1075(m), 1028(s), 914(m),
853(m), 763(m), 698(s).
Conclusions
In summary, the sterically congested imidazole selone coordinated
Sb(III) complexes 1-4 are reported. To the best of our knowledge, 1-
4 are the first antimony-imidazole selone complexes reported in the
literature. We showed that Sb(III) chloride or Sb(III) bromide could
exist in mono capped see-saw geometry without forming the
coordination polymer in 1 and 2. In addition, the imidazole selone
depicts the unusual coordination mode without creating the
Synthesis of 2
Antimony bromide (109 mg, 0.3 mmol) was added to a stirred
solution of IPr*Se (100 mg, 0.1 mmol) in toluene (5 mL).
Subsequently, the reaction mixture temperature was maintained at
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8
(a) W.-G. Jia, L.-L. Gao, Z.-B. Wang, L.-Y. Sun, and Y.-F. Han,
polymeric network along with Menshutkin-type Sb···πaryl
interactions. This coordination strategy contrasts with known
IPr*Se ligand coordinated metal derivatives.16 Furthermore, we
have demonstrated that the sterically less crowded cyclic seleno
urea [(IPaul)Se] can effectively coordinate with antimony salts
to yield the dinuclear complex 3 and mononuclear complex 4,
where the Menshutkin-type interaction is absent. Interestingly,
complex 3 is subtly different from the structure of 4 due to the
nature of halides. The sterically more crowded complexes 1 and
2 depicted the long bond distances between Se to Sb in the
presence of Menshutkin-type interactions. In contrast, the
sterically less crowded complexes 3 and 4 showed the short
bond Se to Sb distances (compared to 1 and 2) in the absence
of Menshutkin-type interactions. The calculated charge and total
population of antimony using NPA are comparable. The calculated
bond order using WBIs supports unequal selenium and antimony
interactions in 1 and 2. Therefore both the steric and electronic
properties play a critical role in the coordination mode of
imidazole selone, coordination nature, and geometry of Sb(III).
View Article Online
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Conflicts of interest
There are no conflicts to declare.
Acknowledgments
GP gratefully acknowledges DST-SERB (EMR/2017/001211) for
financial support. VK thank DST-WOS-A (WOS-A/CS-65/2016)
for fellowship. MN thank CSIR-JRF for the fellowship. MA thank
CSIR-SRF for the fellowship.
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2
3
4
15 Coordination chemistry of thiazole-2-chalcogenone and
imidazole-chalcogenone are entirely different. Thiazole-2-
chalcogenone-antimony(III) and their related chemistry are
not relevant to the current discussion. Literature related to
thiazole-2-chalcogenone-antimony(III) complexes: (a) I. I.
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DOI: 10.1039/D0DT02999H
ToC
Rare antimony(III) selone complexes under a super crowed environment have been synthesized
and characterized.