Synthesis, structure and reactivity of β-chalcocyclohexenals: Dichalcogenides and chalcogenides

Article abstract of DOI:10.1016/j.jorganchem.2016.04.019

The present work describes the synthesis and characterization of a series of organochalcogen compounds derived from β-chlorocyclohexenal (27a)/β-bromocyclohexenal (27b) which are stabilized by E...O (E = S, Se, Te) intramolecular secondary bonding interaction (IM-SBI). Di-(2-formylcyclohex-1-ene)sulfide (21) was prepared by treating 27b with disodium sulfide. Di-(2-formylcyclohex-1-ene)diselenide (28) was obtained by reacting 27a with disodium diselenide. The reaction always produced a mixture of di-(2-formylcyclohex-1-ene)diselenide (28) and di-(2-formylcyclohex-1-ene)selenide (29). Attempts to synthesize di-(2-formylcyclohex-1-ene)ditelluride 30 by the reaction of 27b with disodium ditelluride afforded a mixture of monotellurides; 3,4,5,6,7,8-hexahydro-2H-9-telluraanthracene-1-carbaldehyde (26), di-(2-formylcyclohex-1-ene)telluride (22) and 9-hydroxy-2,3,5,6,7,8,9,9a-octahydro-1H-telluroxanthene-4-carbaldehyde (31). Reactions of 28 with halogenating reagents afforded the corresponding organylselenenyl halides; selenenyl chloride 35, selenenyl bromide 36 and selenenyl iodide 37. Tellurides 26 and 22 were used as ligands for metal complexation reactions.

Full text of DOI:10.1016/j.jorganchem.2016.04.019

Accepted Manuscript
Synthesis, structure and reactivity of β-chalcocyclohexenals: Dichalcogenides and
chalcogenides
Poonam R. Prasad, Harkesh B. Singh, Ray J. Butcher
PII:
S0022-328X(16)30163-2
10.1016/j.jorganchem.2016.04.019
JOM 19475
DOI:
Reference:
To appear in: Journal of Organometallic Chemistry
Received Date: 3 February 2016
Revised Date: 8 April 2016
Accepted Date: 14 April 2016
Please cite this article as: P.R. Prasad, H.B. Singh, R.J. Butcher, Synthesis, structure and reactivity of β-
chalcocyclohexenals: Dichalcogenides and chalcogenides, Journal of Organometallic Chemistry (2016),
doi: 10.1016/j.jorganchem.2016.04.019.
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ACCEPTED MANUSCRIPT
Synthesis, Structure and Reactivity of
β-Chalcocyclohexenals:
Dichalcogenides and Chalcogenides
Poonam R. Prasad,a Harkesh B. Singh*a and Ray J. Butcherb
aDepartment of Chemistry, Indian Institute of Technology Bombay, Powai,
Mumbai 400076, India bDepartment of Chemistry, Howard university
Washington DC, 20059, USA. Email; chhbsia@chem.iitb.ac.in
Synopsis
Due to presence of secondary bonding interaction in alicyclic system substrate leads to
the isolation of dichalcogenide and monochalcogenides with the reaction of Na2E2/Na2E
(E = S, Se, Te). The complexation reaction of monochalcogenides with Pd(II) and Pt(II)
afforded the exclusive metal complexes without any cleavage.

ACCEPTED MANUSCRIPT
Synthesis, Structure and Reactivity of
β-Chalcocyclohexenals:
Dichalcogenides and Chalcogenides
Poonam R. Prasad,a Harkesh B. Singh*a and Ray J. Butcherb
aDepartment of Chemistry, Indian Institute of Technology Bombay, Powai,
Mumbai 400076, India
bDepartment of Chemistry, Howard university
Washington DC, 20059, USA
Email; chhbsia@chem.iitb.ac.in
Entry for the Table of Contents
Synthesis, Structure and
Reactivity of β-Chalcocyclohexenals:
Dichalcogenides and Chalcogenides
−−−−−−−−−−−−−−−
Poonam R. Prasad,a Harkesh B.
Singh*a and Ray J. Butcherb
Keywords:
β-Chalcocyclohexenals
Dichalcogenides
The organochalcogen derivatives
derived from alicyclic substrates, β-
bromocyclohexenal/β-
Chalcogenides
chlorocyclohexenal, show stronger
Intramolecular secondary bonding
SeꢀꢀꢀO/TeꢀꢀꢀO
intramolecular
interaction compared with the
corresponding aromatic analogues.
1

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Abstract: The present work describes the synthesis and characterization of a series of
organochalcogen
compounds
derived
from
β-chlorocyclohexenal
(27a)/β-
bromocyclohexenal (27b) which are stabilized by EꢀꢀꢀO (E = S, Se, Te) intramolecular
secondary bonding interaction (IM-SBI). Di-(2-formylcyclohex-1-ene)sulfide (21) was
prepared by treating 27b with disodium sulfide. Di-(2-formylcyclohex-1-ene)diselenide
(28) was obtained by reacting 27a with disodium diselenide. The reaction always
produced a mixture of di-(2-formylcyclohex-1-ene)diselenide (28) and di-(2-
formylcyclohex-1-ene)selenide (29). Attempts to synthesize di-(2-formylcyclohex-1-
ene)ditelluride 30 by the reaction of 27b with disodium ditelluride afforded a mixture of
monotellurides; 3,4,5,6,7,8-hexahydro-2H-9-telluraanthracene-1-carbaldehyde (26), di-
(2-formylcyclohex-1-ene)telluride (22) and 9-hydroxy-2,3,5,6,7,8,9,9a-octahydro-1H-
telluroxanthene-4-carbaldehyde (31). Reactions of 28 with halogenating reagents
afforded the corresponding organylselenenyl halides; selenenyl chloride 35, selenenyl
bromide 36 and selenenyl iodide 37. Tellurides 26 and 22 were used as ligands for metal
complexation reactions.
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Introduction
The chemistry of organochalcogen derivatives having intramolecular Eꢀ·ꢀX secondary
bonding interactions (where E = Se Te; X = N, O) has attracted considerable current
interest owing to its applications in various fields such as; (a) in the isolation of
novel/unstable organochalcogen compounds[1] (b) in organic synthesis[2] (c) ligand
chemistry[3] and (d) enzyme mimetic.[4] For example, Wirth and co-workers[5] have
reported that due to the presence of intramolecular SeꢀꢀꢀO interaction, an asymmetric
product was formed in the electrophilic selenenylation reaction of alkenes using
asymmetric selenium reagent 1. Goldstein and Burling et al.[6] have demonstrated that the
molecular structure of biologically active selenazofurin was controlled by intramolecular
SeꢀꢀꢀO interaction. Diorganyl diselenides having weak intramolecular SeꢀO interactions
are considered to be important class of selenoenzyme Glutathione Peroxidase (GPx)
mimics.[7] Wirth et al.[7a] were first to report the GPx-like activity of a new class of
diselenides 1 containing an oxygen atom in close proximity to the selenium. Recently,
…
several other related derivatives such as chalcogenides and their halides having oxygen
donating group such as formyl (2-7),[8] hydroxyl (8-10),[9] nitro (11-14),[10] and amide
(15-16)11 at ortho-position (Chart 1) have been reported and extensively studied. Also the
O···Se IM-SBI has been probed by single crystal X-ray,[9, 12] NMR[13] and theoretical
studies.[14]
Chart 1. Arylchalcogen derivatives with ortho-oxygen donating atom
Compared to the arylchalcogen derivatives stabilized by IM-SBI with oxygen, the alkyl
analogues and in particular, the alicyclic analogues have not been systematically
investigated.
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Some examples of related β-aryltellurovinyl aldehydes and ketones have been prepared
by nucleophilic addition reaction of aryltellurolate anion (ArTe-) to a triple bond of α-
acetylenic aldehyde and ketones.[15] Detty and coworkers have made a significant
contribution in the area of β-chlorotellurenyl ketones 17-20.[16] Concerning chalcogen
derivatives of alicyclic substrates, though the synthesis of di-(2-formylcyclohex-1-
ene)sulfide 21[17] has been reported, there is no report about the intramolecular interaction
and structural aspects. Minkin and co-workers have explored the chemistry of di-(2-
formylcyclohex-1-ene)tellurides 22-23 and their derivatives.[14, 18] Mollier and coworkers
have studied related 3,4,5,6,7,8-hexahydro-2H-9-chalcaanthracene-1-carbaldehydes 24-
26.[19]
Chart 2. Alkylchalcogen derivatives with heteroatom (e. g. O) donating atom
The coordination chemistry of transition metal ions, in particular platinum group
metal ions Pd(II) and Pt(II), with heavier organochalcogen (Se, Te) ligands has attracted
considerable current interest.[3b-d, 20] These complexes have been used as catalysts in C-
C/C-E (E = S, Se,Te) bond formation. The complexation of Pd(II) and Pt(II) with heavier
chalcogen ligands not only leads to; (i) the formation of coordinate complexes but also
(ii) to the cleavage of the E-C bond or (iii) even complete decomposition of complexes
under metallation. The cleavage of the C-E bond is mostly observed for the Te ligands
which are stabilized by IM-SBI. For example, McWhinnie and coworkers[21] have
observed dealkylation of 1,6-bis-2-butyltellurophenyl-2,5-diazahexa-1,5-diene upon
complexation reaction with PtCl2. A similar result has been also reported by Kemmitt and
co-workers.[22] 2-(2-Pyridyl)(p-ethoxyphenyl-)tellurium, reacted with HgCl2 at room
temperature to afford cleaved C-Te bond,[23] while the reaction with Pd(II)Cl2 and
Pt(II)Cl2 led to isolation of very complex systems.[24] Singh and coworkers have reported
a facile C-Te bond cleavage/transmetalation in the reaction of the 22-membered
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[25]
telluraaza Schiff base macrocycle with HgCl2
and Pt(COD)Cl2.[26] They have also
observed the facile cleavage of the C-Te bond in bis[2-(4,4-dimethyl-2-
oxazolinyl)phenyl]telluride, bis[2-(((R)-methylbenzylamino)methinyl)-phenyl]telluride
1,1’-(tellurobis(2,1-phenyllene))bis(N,N-dimethylmethanamine)
and
22-membered
azamacrocyclic on reaction with HgCl2, Pd(C6H5CN)Cl2 / Pd(COD)Cl2 and PtCl2
27]
respectively.[26b,
An
analogous
reaction
of
bis[2-(4,4-dimethyl-2-
oxazolinyl)phenyl]telluride with a Pd(II) chloride afforded a yellow-orange solid, which
was insoluble in common organic solvents, presumably due to formation of a polymeric
complex and further characterization could not be carried out.[8e] Kaur et al. reported the
cleavage of C-Te and Te-Te bond in bis[2-((dimethylamino)methyl)phenyl]telluride and
ditelluride with PdII ions.[28] The cleavage of C-Te bond in these cases is facilitated by the
strong NꢀꢀꢀTe intramolecular interaction, which involves donation of a nitrogen lone pair
to the σ* orbital of the trans C-Te bond. In continuation of our work on intramolecularly
coordinated organochalcogens, we extend this work to the alicyclic system, β-chloro-
/bromocyclohexenal and report the synthesis and isolation of the first structurally
characterized alicyclic diselenide and selenides and their derivatives like halides
stabilized by intramolecular interactions and compare the structural and reactivity with
the much studied aryl analogues. Along with this we also present stable metal complexes
of Pd(II) and Pt(II) of di-(2-formylcyclohex-1-ene)telluride (22) and 3,4,5,6,7,8-
hexahydro-2H-9-telluraanthracene-1-carbaldehyde (26). The C-Te bond remains intact in
these complexes.
Results and discussion
Synthesis
The precursor, β-chlorocyclohexenal 27a was synthesized by treating cyclohexanone
with phosphoryl chloride in N,N-dimethylformamide by following the literature
procedure.[29] β-Chlorocyclohexenal 27a is not indefinitely stable even on storing at low
temperature (-13 °C). After long periods of time (in a refrigerator), it suddenly
decomposes more or less violently with the evolution of hydrogen chloride and formation
of a black tar. Incautious distillation can also lead to a similar result. Therefore, we chose
another substitute, β-bromocyclohexenal, which is more stable and quite easy to
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ACCEPTED MANUSCRIPT
handle.[30] For better yields of organochalcogens, it is best to prepare this compound
freshly before use. The key chalcogen derivative, di-(2-formylcyclohex-1-ene)diselenide
28 was obtained by the disodium diselenide route[31] (Scheme 1). The reaction always
gave a mixture of 28 and di-(2-formylcyclohex-1-ene)selenide 29 and it was difficult to
separate the mixture by fractional crystallization. These could be separated by column
chromatography with very slow elution using petroleum ether and ethyl acetate (0-2%) as
the mobile phase. The first fraction isolated was 28 as a pale yellow solid and the second
fraction was 29 as a yellow solid. Hence the yield of the desired product, di-(2-
formylcyclohex-1-ene)diselenide (28), was very low. Syper et al.[8c] have also obtained a
mixture of bis(o-formylphenyl)diselenide (5) and bis(o-formylphenyl)selenide (3) along
with other side products during the preparation of bis(o-formylphenyl)diselenide (5) from
o-bromobenzaldehyde. As expected, β-chlorocyclohexenal 27a is much more reactive
than the aromatic analogue, o-chlorobenzaldehyde. Due to the higher reactivity of 27a,
the preparation of diselenide 28 required very mild condition (0 ⁰C) and short time (30
min) compared with the formation of bis(o-formylphenyl)diselenide which occurred
under vigorous reaction condition {12 h room temperature and further reflux for 6 h
containing hexamethylphosphoramide (HMPA)}.
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Scheme 1. Synthesis of alicyclic organochalcogens
Attempts to synthesize 30, the tellurium analogue of 28 by the reaction of disodium
ditelluride with 27b (Scheme 1), yielded a mixture of products; di-(2-formylcyclohex-1-
ene)telluride
22,[18]
its precursor 9-hydroxy-2,3,5,6,7,8,9,9a-octahydro-1H-telluroxanthene-4-
3,4,5,6,7,8-hexahydro-2H-9-telluraanthracene-1-carbaldehyde
(26),[19,
32]
carbaldehyde (31) and the fourth fraction {a mixture of 32, 33 and 34[33] (identified by
125Te NMR and mass spectrometry)}, instead of the desired ditelluride.
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The products were separated by column chromatography using petroleum ether and ethyl
acetate (5%) as an eluent. The first fraction isolated was 26 (3.04 g; 29%) as dark red
liquid and the second fraction was telluride 22 (2.01g; 12%) as light yellow solid. The
third fraction afforded telluride 31 (2.44 g; 22%) as deep yellow solid. The fourth fraction
was as a pale white solid (combined yield 3.23 g) as a mixture of 32, 33 and 34 (vide
infra). It was only soluble in methanol and DMSO and could not be purified. The
products 32, 33 and 34, presumably, formed due to the aerial oxidation of reactive
telluride 22 (Scheme 2). A similar observation has been reported by Selvakumar et al. in
the reaction of the n-butyltellurolate and 2-bromo-5-tert-butyl-isophthalic acid dimethyl
ester.[34] The preparation of compounds 22[18] and 26[19, 32] has been reported in literature
using slightly different methods. To improve the yield of telluride 22, the reaction 27b
was carried out with Na2Te. The reaction yielded the same mixture of four products; 22
(0.82; 18%), 26(1.26 g; 14%), 31(1.11 g; 21%) and the fourth fraction (combined yield
1.40 g) with only the ratio of products formed being different. In both the cases the ratio
of the fourth fraction i.e. the oxidized products was highest. The presence of the mixture
of oxidized products 32, 33 and 34 was evident from the 125Te NMR spectrum. The 125Te
NMR chemical shifts for the compounds 32, 33 and 34 were observed at 1214 (32),
1184(33), 1083(34)[33] ppm respectively. It was further confirmed by mass spectrometry.
The mass spectrum of the fourth fraction showed three prominent m/z peaks at 381, 393
and 377 corresponding to 32, 33 and 34 respectively. The plausible mechanism for the
formation of 22, 26, and 31 is given in Scheme 2. The formation of intermediates A, B
and C could occur due to Na2Te/Na2Te2 facilitated deprotonation of 22 as indicated in
Scheme 2. Alternatively, NaOH, resulting from air exposed Na metal surface while
cutting and transferring into the reaction flask, can act as base for the intramolecular
cyclization and elimination reaction. The areal oxidation of 22, presumably, occurs
during the workup in open atmosphere. In contrast, the aryl analogue of 22, i.e., bis(o-
formylphenyl)telluride (4) is quite stable and posed no such problems in workup.[8e] To
compare the structural features among monochalcogenides, di-(2-formylcyclohex-1-
ene)sulfide[17] (21) was also prepared. It was obtained by treating 27b with disodium
sulfide.
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Na
O
O
ONa
O
H
O
Na2Te or
Na2Te2
Na2Te or
Na2Te2,THF
H
H
H
H
Te
Te
O
Te
O
-NaHTe or
-NaHTe2
Br
- NaBr
H
H
27b
A
B
22
H2O
Na2Te or
Na2Te2
-NaHTe or
-NaHTe2
[O]
air
31
O
Na
COOH
COOH
Intramolecular
cyclization
NaO
O
Te
+
Te
Te
O
O
26
-Na2O
Te
COOH
COOH
O
34
32
33
H
O
C
fourth fraction
Scheme 2. Plausible mechanism for the formation of 26, 31, 32, 33 and 34
The selenenyl halides 35-37 were obtained by reacting 28 with different
halogenating reagents. The selenenyl chloride 35 was obtained as a pale white solid by
reacting 28 with a stoichiometric amount of sulfuryl chloride. The bromo- and iodo-
derivatives 36-37 were synthesized by a similar method using stoichiometric amounts of
bromine and iodine respectively (Scheme 3).
Scheme 3. Synthesis of selenenyl halides
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Complexation with Pd(II) and Pt(II)
Tellurides 22 and 26 were used as ligands for the complexation studies. The reactions of
22 with Pd(C6H5CN)2Cl2 and PtCl2 afforded the corresponding mononuclear complexes
38 as yellow solid and 39 as a red solid (Scheme 4). These were recrystallized from a
mixture of dichloromethane/hexane (4:1).
Scheme 4. Synthesis of 38 and 39
Interestingly, the reaction of 26 with Pd(C6H5CN)2Cl2 in CH2Cl2 afforded Pd(II) complex
40 as a dimer (vide infra) whereas PtCl2 yielded 41 as a monomer (Scheme 5).
Scheme 5. Synthesis of 40 and 41
GPx-like activity
Diselenide 28 was evaluated for its GPx-like antioxidant catalytic activity.[33] It is a more
efficient catalyst in comparison with ebselen, (2-phenyl-1,2-benzisoselenazol-3(2H)-one)
and bis(o-formylphenyl)diselenide. However di-(2-formylcyclohexenyl)selenide (29)
does not show any significant GPx-like activity.
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Spectroscopic studies
(i) FT-IR: IR-spectroscopy is a wonderful experimental tool to study the relative strength
of EꢀꢀꢀO interaction in carbonyl group coordinated organochalcogen compounds.
Generally, the presence of weak to strong EꢀꢀꢀO interaction involves the mixed
contribution of c and its resonance form c’ for the ground state electronic structure of
carbonyl substituted aryl/alkyl chalcogen compounds (Scheme 6). As we can see from the
scheme, contributing form c’ has reduced C=O double bond character. It is in turn
reflected in IR stretching frequency. Compounds with strong C=OꢀꢀꢀE interaction have
lower carbonyl stretching frequency (νC=O) (Table 1).
Scheme 6. Possible structures of cyclohexenal based chalcogen compounds
For example, diselenide 28 has lower νC=O (1655 cm-1) than that observed for the
monoselenide 29 (1663 cm-1). This indicates either absence or presence of very weak
SeꢀꢀꢀO interaction in compound 29. In accordance with this prediction, X-ray structural
analysis indicated the absence of SeꢀꢀꢀO interaction in the latter system (vide infra). In
general, the observation of split or doubled νC=O indicates the distinct orientation of the
two carbonyl groups in diorganyl dichalcogenides/monochalcogenides (3-5). Hence, the
observation of doubled νC=O (1669 and 1645 cm-1) for telluride 22 could be due to the two
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distinct orientations of the carbonyl groups with respect to the tellurium center. The
stretching frequency at 1645 cm-1 corresponds to the intramolecularly coordinated
carbonyl, whereas the peak at 1669 cm-1 corresponds to the stretching of non-
coordinating carbonyl (supported by single crystal X-ray crystallography).[35] The
plausible reason for the presence of TeꢀꢀꢀO interaction in telluride 22 could be the
increased polarizability of its valance shell electrons in comparison to lower electron
polarizability of its lighter congeners sulfur and selenium. In aryl systems (2-5), there is
a significant reduction of υC=O stretching frequency of the carbonyl group involved in
secondary interaction while going from S to Se system. Although the stretching
frequency of C=O involved in secondary bonding in Te system 4 is relatively unaffected
in comparison to selenium system 3, the generally observed trend from X-ray analysis
and computation is that TeꢀꢀꢀO interaction is much stronger than the SeꢀꢀꢀO interaction.[1j,
9]
Therefore, good acceptors Te-C (σ*) or Se-Se (σ*) orbitals facilitate adaptation of c-
like structures (c or c’, e. g. 28 and 22) and poor acceptors S-C/Se-C (σ*) orbitals
facilitate the adaptation of its structural isomer i-like structure (e.g 21 and 29). A single
νC=O frequency was observed for the metal complex 38 (1675 cm-1). The reason for the
observation of the single νC=O for the carbonyl groups of 38 is unclear. Two distinct νC=O,
1669 and 1678 cm-1, have been observed for the metal complex 39. It indicates the
presence of slightly different carbonyl groups in compound 39. The observation of two
different TeꢀꢀꢀO distances from X-ray analysis (vide infra) further corroborates with IR
analysis. It is interesting to note that there is a significant increase of νC=O 1675 and 1678
cm-1 for 38 and 39 respectively from the ligand νC=O (1669 and 1645 cm-1). This could
presumably be due to retraction of the flow of electron density to the carbonyl group via
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σ
,β
-unsaturated carbonyl π-framework. It could occur as a consequence Te-M σ-bonding
interaction. In other words, due to the Te-M σ-bonding, the extent of conjugation is
slightly reduced in the -unsaturated carbonyl π-framework of the coordinated ligand
σ,β
(22) in comparison to its unligated form. The longer O···Te distances (2.688(4) and
2.940(6) Å) (vide infra) observed for 38 in comparison to the corresponding distance
observed in free ligand 22 (2.662 Å)[35] further supports the IR analysis.
Table 1. 1H, 77Se NMR chemical shifts and IR stretching frequencies
Compound
Entry
1H
NMR
CHO
(ppm)
10.41
10.20
9.99
77Se/125Te
NMR
FT-IR
υC=O
(cm-1)
Compound
Entry
1H
NMR
CHO
(ppm)
10.33
10.28
10.27
77Se/125Te
NMR
FT-IR υC=O
(cm-1)
(ppm)
(ppm)
-
1663
1663
1669,
1645
1655
2[8a]
3[8c]
4[8e]
-
1674
21
29
22
370
764
393
686
1652, 1681
1654,1694
9.97
490
5[8d]
10.20
468
1665, 1692
28
(ii) 1H, 77Se and 125Te NMR studies
Interestingly, in 1H NMR spectra of 22 and 28, the aldehydic peaks are upfield shifted as
compared with their aromatic analogues 4[8e] and 5[8d] (Table1) respectively. In selenenyl
halides also, the formyl protons are shifted to the upfield region (9.56; 35, 9.43; 36, 9.08;
37) as compared with the aromatic analogues 7a (10.27 ppm) and 7b (10.12 ppm) (Table
2).[8f] Further the coordination of tellurium with Pd/Pt in complexes 38-41, leads to an
upfield shift of the aldehydic protons compared to the ligands 22 (9.99 ppm) and 26 (9.77
ppm). The 1H NMR spectrum of the mixture of three compounds (32, 33 and 34), did not
exhibit any peak in the region of aldehydic proton. Only broad peaks were observed in
the region of 2.48-1.55 ppm corresponding to the alicyclic protons.
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Table 2. 1H, 77Se NMR chemical shifts of selenenyl halides
Compound
Entry
1H NMR
CHO
(δppm)
9.56
77Se NMR
Compound
Entry
1H NMR
CHO
77Se NMR
(δppm)
(δppm)
(δppm)
1266
1234
1040
7a[8f, g]
7b[8f, g]
-
10.27
10.12
-
1114
1030
-
35
36
37
9.43
9.08
For compound 28, the 77Se NMR chemical shift was observed at 490 ppm, which is
significantly downfield shifted with respect to 5 (468 ppm) (Table 1).[8d] The relative
downfield shift indicated the presence of stronger intramolecular SeꢀꢀꢀO interaction in 28
compared to 5. This was confirmed by X-ray structure (vide infra). However, the 77Se
NMR spectrum of 29 shows peak at 370 ppm which is significantly upfield as compared
with 393 ppm for 3.[8c] This indicated the absence of SeꢀꢀꢀO intramolecular interaction in
29 (vide infra). In the case of 22,125Te signal[18] is observed at 764 ppm which is quite
downfield shifted with respect to the 125Te peak observed for the 4 (686 ppm)[8e]. This
again confirms the presence of stronger intramolecular TeꢀꢀꢀO interaction in 22. The 77Se
chemical shift of 35 (1266 ppm) is significantly downfield from that reported for 2-
formylphenylselenenyl chloride 7a (1114 ppm).[8f, g] Similarly, the chemical shift of
selenenyl bromide 36 (1234 ppm) is much higher than those reported for 2-
g]
formylphenylselenenyl bromide 7b (1030 ppm)[8f,
and methyl 2-
(bromoselanyl)benzoate (1042 ppm).[36]
In the cases of 38 and 39, the 125Te NMR peaks appeared at 837 and 818 ppm
respectively which are expectedly in deshielded region as compared with 22 (764 ppm).
In the cases of metal complexes 40 and 41, 125Te peaks are observed at 685 and 573 ppm
respectively and are upfield shifted with respect to the 125Te peak observed for the ligand
26 (834 ppm). This is probably due to the back donation of electrons from metal to the Te
centers.[37]
Mass spectrometric studies
The HRMS of compounds showed peaks at 273.0924 [M+Na]+(21), 378.9735 [M+H]+
(28), 299.0557 [M+H]+ (29), which are in good agreement with the corresponding
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calculated values of 273.0925 [M+Na]+(21), 378.9715 (28) and 299.0550 (29)
respectively. The mass spectrum of the mixture of three products showed peaks at m/z
381 [M+H]+ (32), 393 [M]+ (33) and 377 [M+H]+ (34). The mass spectrum of 39 showed
molecular ion peak at m/z 958 [M+H]+. In the case of 40, the molecular ion peak could
not be observed. The base peak observed at m/z 330 corresponds to the ligand
[C14H16OTe]+ and shows very low intensity peak at m/z 487 corresponding to the
[C14H18O2TeClPd]+. The Pt(II) complex of 41 shows a very low intensity peak at m/z 944
[M+Na]+.
X-ray crystallographic studies
Molecular structure of 21
The molecular structure of compound 21 is depicted in Figure 1 along with significant
bond lengths and bond angles. The geometry around the sulfur atom is V-shaped with the
bond angle (<C8-S1-C7) being 100.93(6)°. This is close to that observed for related
aromatic derivative bis[2-(4,4-dimethyl-2-oxazolinyl)phenyl]sulfide (103.8(2)°).[38]
Interestingly, the oxygen atoms of both the formyl groups are positioned away from the
sulfur center, whereas in the case of bis[2-(4,4-dimethyl-2-oxazolinyl)phenyl]sulfide, one
of the oxazolinyl ring was intramolecularly coordinated through its nitrogen atom to S.
The S-C bond distances of 1.776(1) S1-C8 and 1.777(1) Å S1-C7 compare well with the
S-C bond distance of 1.774(4)
oxazolinyl)phenyl]sulfide.[38]
Å
reported for bis[2-(4,4-dimethyl-2-
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Figure 1. Molecular structure of compound 21. Selected bond lengths (Å) and bond
angles (°): S1-C8 1.776(1), S1-C7 1.777(1), O1-C1 1.209(2), O2-C15 1.213(2), C8-S1-
C1 100.93(6).
Interestingly, in molecule 21, instead of SꢀꢀꢀO intramolecular interaction, a short C-HꢀꢀꢀS
intramolecular interaction was observed. The short interatomic distances between sulfur
and hydrogens (C-HꢀꢀꢀS) are 2.685(3) and 2.661(4) Å with bond angles 105.3(1)° and
106.6(1)°. These are significantly shorter than the sum of the bond distance of van der
Waals radii of sulfur and hydrogen atoms (2.89 Å).[39] The presence of a weak C-HꢀꢀꢀS
intramolecular interaction was further confirmed by NBO (Natural Bond Orbital analysis)
and AIM (Atoms in Molecules) analysis (vide infra). The packing diagram of 21 shows
two weak C-HꢀꢀꢀO-intermolecular interactions (Figure S64). The bond distances are
O2ꢀꢀꢀH11B and O1ꢀꢀꢀH13A 2.4601(1) and 2.588(1) Å respectively which are less than the
sum of the van der Waals radii O2+H1B (2.72 Å). The bond angle <C11-H11B-O2 is
168.24°.
Molecular structure of 29
The molecular structure of 29 (Figure 2) is similar to that of the corresponding sulfur
compound 21. The geometry around the selenium atom is V-shaped with ˂C(1b)-Se-
C(1a) bond angle of 98.9(11)° which, as expected, is slightly less than that of 21 (C-S-C
100.93(6)°) but more than that of 22 (C-Te-C 95.8(3)°).[35] The C1A-Se-C1B bond angle
in 29 is close to that reported for the structure of 3 (97.8°)[8b]. Similar to compound 21,
both the oxygens of formyl group in 29 are trans (or anti) to the selenium center. This is
in contrast to the
aromatic compound 3[8b] and alicyclic di-(2-
formylcyclohexenyl)telluride 22[35] where one of the oxygen atoms lies in the plane
thereby forming a five-membered heterocyclic ring via intramolecular EꢀꢀꢀO coordination.
Theoretically, one can predict three possible (syn, syn), (syn, anti), (anti, anti) isomers for
di-(2-formylcyclohexenyl) chalcogenides (vide infra). However, from the basic principles
of organic chemistry one would anticipate that (anti, anti) configuration is the most stable
configuration due to the higher stability of trans-
α,β-unsaturated carbonyl systems with
respect to cis- -unsaturated carbonyl systems. It is indeed found for the sulfur and
α,β
selenium systems, 21 and 29 respectively. The finding of (syn, anti) configuration for
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compound 22 with a relatively strong TeꢀꢀꢀO interaction could be due to the highly
polarizable nature of the valence shell electrons of tellurium center in its low-valent state.
It appears that stabilization energy resulting from TeꢀꢀꢀO interaction contributes to the
reorganization energy required for rearranging from (anti, anti) to (syn, anti) orientation.
Figure 2. Molecular structure of compound 29. Selected bond lengths (Å) and bond
angles (°): Se-C1A 1.946(1), Se-C1B 1.927(3), C1B- Se-C1A 98.9(1), C7-H9ꢀꢀSe 2.723,
C14-H18ꢀꢀꢀSe 2.763.
Similar to the sulfur analogue 21, in molecule 29, a short C-HꢀꢀSe intramolecular
interaction was observed. The short interatomic distances between selenium and
hydrogens (C-HꢀꢀSe) are 2.763Å and 2.723 Å which are significantly shorter than the
reported for diselenocin[40] (2.92 Å) and are in good agreement with reported alicyclic
bis(2-hydroxymethylcyclohex-1-ene)selenide (2.710 and 2.704 Å).[33] The C-HꢀꢀSe bond
angles for 29 are 108.4⁰ and 110.618⁰ which are close to C-HꢀꢀSe bond angles of bis(2-
hydroxymethylcyclohex-1-ene)selenide (110.8° and 112.4°) and diselenocin (101.7° and
107.0°). The presence of C-HꢀꢀꢀSe intramolecular interaction was further proved by NBO
and AIM (vide infra).
Computational studies
To gain more information about the intramolecular EꢀꢀꢀH (E = S, Se) interaction, density
functional theory calculations were carried out using Gaussian09 suite of programs.[41]
The geometry of 21 (Table S1) and 29 (Figure 3, Table S2) was optimized at B3LYP/6-
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31+g(d) basis set. The NBO[8f] and AIM[42] calculations were carried out using 6-
311+g** basis set. The optimized bond distances and bond angles are in good agreement
with the experimental values. The calculated bond distance for SꢀꢀꢀH distances (2.651 Å)
and SeꢀꢀꢀH distance (2.705 Å) are close to the experimental values (2.685(3) and 2.661(4)
Å; 21 and 2.763 Å and 2.723Å); 29. Further, the NBO analysis with second-order
perturbation method reveals that a stabilizing orbital interaction is operating between one
of the E (S, Se) lone pairs with the C-H σ* orbital of the formyl group. The NBO second-
order perturbation energies for C-HꢀꢀꢀS (21) and C-HꢀꢀꢀSe (29) are 0.79 kcal mol-1 for (EC-
HꢀꢀꢀS) and 1.22 and 1.24 kcal mol-1 (EC-HꢀꢀꢀSe) respectively. The latter is in good agreement
with those reported for bis(o-formylphenyl)diselenide.[43] Apart from this we have studied
the relative energies among three conformers of 29, i.e., (29a(syn, anti), 29b(syn, syn)
and 29c(anti, anti)) in detail. Computed relative energies of the three most important
conformers are given in Figure 3. The relative energies of the conformers are in the order
of 29b(-3094.1853760 Hartree) > 29a (-3094.1945698 Hartree) > 29c (-3094.1956416
Hartree). Based on these energies we can infer that 29c (anti, anti) when hydrogens,
instead of oxygen are in syn configuration with respect to the selenium is energetically
more favorable.
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29b
29a
29c
Figure 3. The B3LYP/6-31+g(d) optimized geometries of 29
The presence of C-HꢀꢀꢀE interaction was further confirmed by AIM. On carrying out AIM
analysis of 21 and 29, the bond critical points were located in between C-HꢀꢀꢀE which
confirms the presence of C-HꢀꢀꢀE short interaction (Figure S71). The values of electron
density (ρ) at the bond critical point (bcp) for C-HꢀꢀE (E = S, Se) were 0.0142 (21) and
0.0144 and 0.0143 a.u (29). The negative values of total energy density H at bcp observed
for E···H-C interaction support the contribution of covalent character and in corollary
with orbital interaction energy (EC-HꢀꢀꢀE) predicted by the NBO analysis (Table 3).
Although Se···H-C interaction is predominantly covalent, the observation of positive
2
value of Laplacian ( ρ) at the bcp (Table 3) implies the fact that the considerable ionic
contribution to the C-H···E interaction cannot be ruled out.
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Table 3. Summary of quantum chemical calculations on 21 and 29
2ρ C-HꢀꢀꢀE
0.0474
0.0435, 0.0435 -.0014, -0.0014
Compound
EC-HꢀꢀꢀE kcal mol-1 ρC-HꢀꢀꢀE a.u.
H
0.79
0.0142
0.0144, 0.0143
-0.0016
21
29
1.22, 1.24
AIM calculations were carried out at the B3LYP/6-311+g** level.
Molecular structure of 28
Compound 28 crystallizes in P-1 space group with two molecules in the asymmetric unit
Out of the two molecules, one has a transoid geometry (C25-Se4-Se3-C15, 102.84°) and
the other has a cisoid geometry (C1-Se2-Se1-C9, -84-02°). Interestingly, in the case of
aromatic analogue 5,[44] both the molecules in the asymmetric unit have cisoid geometry
with the torsion angles of -86.28⁰ and -86.32⁰. The geometry around each selenium atom
is approximately T-shaped (cisoid conformer, Figure 4). The Se-Se distance [Se1-Se2
2.357(2) Å] is comparable to the values of Se-Se bond distances reported for related
diselenides.[44-45]
Figure 4. Molecular structure of compound 28. Selected bond lengths (Å) and bond
angles (°): Se1ꢀꢀꢀO4 2.657(1), Se2ꢀꢀꢀO1 2.637(1), Se1-Se 2.357(2), C1-Se2-Se1-C9, -
84.02.
The SeꢀꢀꢀO distances Se1ꢀꢀꢀO4 2.657(1) and Se2ꢀꢀꢀO1 2.637(1) Å), are much longer than
the sum of the covalent radii (1.89 Å) of selenium and oxygen,[46] however, these
distances are much shorter than the sum of their van der Waals radii (3.42 Å).[39] These
SeꢀꢀꢀO distances are significantly shorter than those observed for 5[44] (2.720-2.751 Å) and
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3[8b] (2.806 Å). The packing diagram shows two types of a weak C-HꢀꢀꢀO hydrogen
bonding (Figure S65). The HꢀꢀꢀO bond distances of H4BꢀꢀꢀO1 and H7BꢀꢀꢀO4 are 2.500(1)
Å and 2.544(1) Å respectively with bond angles C7B-H7BꢀꢀꢀO4 (177.4°) and C4B-
H4BꢀꢀꢀO1 (162.2°) and eventually form the two dimensional network.
Molecular structure of 31
The coordination geometry around the tellurium atom is T-shaped in which the tellurium
is directly bonded to two carbon atoms and intramolecularly coordinated to the oxygen
atom of formyl group with bond angle of O-Te-C14A 165.7(5)° (Figure 5). The TeꢀꢀꢀO
bond distance { 2.720(6) Å } is comparatively longer than the observed for 26 {2.591(5)
Å}[32] and 22 (2.662 Å).[35] This is probably due to high conjugation and pseudo-aromatic
character in 26 which is lacking in 31.
Figure 5. Molecular structure of compound 31. Selected bond lengths (Å) and bond
angles (°): Te-O1 2.720(6), Te-C1 2.099(6), Te-C1 2.049(2), C7-O1 1.196(1), C8A-O2
1.421(2), O-Te-C14A 165.7(5), C1-Te-C14A 94.87(4).
The two-dimensional packing diagram shows O-HꢀꢀꢀO intermolecular hydrogen bonding
with O1ꢀꢀꢀH2 bond distance of 2.052(2) Å which is much shorter than the sum of their
van der Waals radii (2.72 Å) (Figure S66).
Molecular structure of 36
Molecules of 36 crystallize in triclinic crystal system with P-1 space group with two
molecules present in the asymmetric unit (Figure 6). The asymmetric unit contains two
crystallographically independent molecules (A and B) with slight differences in bond
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distances and bond angles. The coordination geometry around selenium (Se1A) or Se1B
is T-shaped. The intramolecular SeꢀꢀꢀO bond distances are 2.111(1) Å (Se1AꢀꢀꢀO1A) and
2.116(1) Å (Se1BꢀꢀꢀO1B). These SeꢀꢀꢀO bond distances are much shorter than the sum of
the van der Waals radii (3.42 Å) and are slightly longer than the sum of the covalent radii
(1.89 Å) of selenium and oxygen atom.[46] These SeꢀꢀꢀO distances are much shorter than
the SeꢀꢀꢀO distances of 2.305(2) Å and 2.420(2) Å reported for 7b and 42, respectively.
The Se-C bond distances are 1.87 Å (Se1A-C1A 1.898(1), Se1B-C1B 1.873(2)), are
shorter than those reported for [2-(4,4-dimethyl-2-oxazolinyl)phenyl]selenenyl bromide
(1.926(4) Å).[47] The Se-Br bond distances of 2.487(3) and 2.509(3) Å in 36 are slightly
elongated as compared to that reported for 7b (2.403(4) Å),[8h] and 42[45] (2.371(9) Å).
This strong SeꢀꢀꢀO interaction is also reflected in O1B-Se1B-Br1B bond angle of
178.8(3)° which is close to the linear arrangement as compared with the reported value of
OꢀꢀꢀSe-Br 166.31(7)° for selenenyl bromide 42.[45]
Although the molecule is monomeric, it is further stabilized by a weak intermolecular
Se1BꢀꢀꢀBr1A interaction with another molecule present in the asymmetric unit (Figure 6).
The intermolecular Se1BꢀꢀꢀBr1A bond distance 3.654(3) Å is about 0.1 Å higher than that
reported for [2-(4,4-dimethyl-2-oxazolinyl)phefnyl]selenenyl bromide (3.556(9) Å)[47]
and of slightly lower (~ 0.08 Å) than the sum of the van der Waals radii of selenium and
bromine atom (3.73 Å).[39]
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Figure 6. Molecular structure of 36 showing the two molecules of asymmetric unit along
with intermolecular SeꢀꢀꢀBr interaction. Selected bond distances (Å) and bond angles(°):
Br1A-Se1A 2.487(3), Se1B-Br1B 2.509(3), Br1AꢀꢀꢀSe1B 3.654(3), Se1A-C1A 1.898(1),
Se1B-C1B 1.873(2), Se1A-O1A 2.111(1), Se1B-O1B 2.116(1), Se1A-Br1A-Se1B
171.69(1).
Iwaoka and coworkes[8f, g] have shown a linear correlation between the relative SeꢀꢀꢀO
distance and the logarithm of the n(O)→σ*Se orbital interaction energy (Figure S69).
Using the correlation plot, an interaction energy of 70.25 kcal mol-1 is estimated for 36.
This is the highest interaction energy reported for any selenenyl bromide.
Molecular structure of 39
The molecular structure of 39 reveals that Pt is bonded to the two chlorine atoms and two
alicyclic units via tellurium atoms (Figure 7). Interestingly, both the oxygen atoms of the
formyl groups are intramolecularly coordinated to the tellurium atoms with TeꢀꢀꢀO bond
distances of 2.688(4) and 2.940(6) Å.
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Figure 7. Molecular structure of compound 39. Selected bond lengths (Å) and bond
angles (°): Te-C1B 2.129(6), Te-C1A 2.171(6), Pt-Cl 2.319(1), Pt-Te 2.584(3), Te-Pt-Te
180.0, C1B-Te-C1A 96.0(2).
Thus, the tellurium atoms behave simultaneously as Lewis acid as well as Lewis base.
The Pt-Te bond distance observed, 2.584(4) is in fair agreement with reported Pt-Te bond
distances of 2.518(6) Å[48] and 2.528(5) Å.[49] The packing diagram of 39 shows
intermolecular hydrogen bonding between formyl oxygen of one molecule and formyl
hydrogen of the other molecule (Figure S68). The distance between H7BA and O1A is
2.477(4) Å and bond angle <C7B-H7BA-O1A is 149.7(1)°. The molecular structure of
compound 38 is similar to the molecular structure of 39, but good quality of single crystal
suitable for X-ray analysis could not be obtained (Please see supporting information,
Figure S67).
Molecular structure of 40
Complex 40 (Figure 8) crystallizes as a dimer and is associated with two molecules of
dichloromethane. The geometry around palladium is distorted square planar in which Pd
is surrounded with three chlorine atoms and one tellurium atom with bond angle of Cl-
Pd-Cl 178.75(7)°. The geometry around tellurium is distorted tetrahedral in which
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tellurium is bonded with two carbon atoms and intramolecularly coordinated with oxygen
atom. The Pd-Te and Pd-Cl (terminal) bond lengths in 40 are 2.516(1) Å and 2.291(2) Å)
respectively which are slightly shorter than the Pd-Te (2.546(6) Å) and Pd-Cl (2.360(2))
bond distance of related Pd2[2-Me2NCH2C6H4Te]2Cl2.[28] The Pd-Te, Pd–Cl1(ꢁ-Cl) and
Pd–Cl (terminal) bond distances are 2.516(1), 2.405(2) and 2.291(2) Å, respectively
which are in good agreement with those reported for analogous complexes of palladium
with Pd2(ꢁ-Cl)2Cl2(TeMes2)2[20b] ((Pd-Te (2.506(14) Å), Pd–Cl1(ꢁ-Cl)2.403(4) and Pd-Cl
(2.290(4) Å), bis(trimethylsilylmethyl)tellane[50] (Pd-Te (2.591(5) Å) and Pd-Cl (2.30(1)
Å) and N-morpholine51 (Pd–Te 2.505(6) Å, Pd(1)–Cl(1) 2.357(2) Å, Pd(1)–Cl(2)
2.288(2) Å).
Figure 8. Molecular structure of compound 40. Selected bond lengths (Å) and bond
angles (°): Te1ꢀꢀꢀO1 2.652(7), Te1-C8 2.111(9), Te1-C14 2.119(8), Te1-Pd1 2.516(1),
Pd1-Cl2 2.291(2), Pd1-Cl1 2.329(2), C8-Te1-C14 95.6(3), C8-Te1-Pd1 105.5(2), Cl2-
Pd1-Cl1 178.75(7).
Molecular structure of 41
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The platinum complex, 41, crystallizes as a monomer with two molecules of CHCl3
solvent (Figure 9). Although the % R factor for crystal data refinement is high (14.5%),
the X-ray crystallographically observed elemental composition of the compound 41
indicating coordination of two molecules of 26 to PtCl2, confirms to the elemental
composition measured in elemental analysis. The observation of m/z corresponding to
[41+Na]+ in mass spectral analysis also indicated the coordination of two molecules of 26
to PtCl2 (vide supra). The X-ray structure indicated that the geometry around platinum is
square planar in which both of the chlorine atoms are in cis configuration. All the atoms,
PtTe2Cl2, involved in complex formation are in the same plane confirming its square
planar geometry. However, due to the bulkiness of the ligand 26, Te1-Pt-Te2 angle is
slightly obtuse (96.43(6)°), which in turn affects the other angles within plane. A similar
cis configuration was observed in [Pt(TeMes)2(dppp)].3C6H6 complex.[52] The bond
distance between Te and oxygen is 2.644(7) Å which is relatively longer than that found
in the corresponding ligand 26 (TeꢀꢀꢀO; 2.591(5) Å). The Pt–Te bond lengths in 41 are
2.542(2) and 2.551(3)
Å
are slightly longer than the reported for
[Pt(TeMes)2(dppp)].3C6H6 (2.6394(3)Å).[52]
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(41a)
(41b)
Figure 9. Molecular structure of compound 41. (41a) one unit; (41b) two unit with
CHCl3 solvent); Selected bond lengths (Å) and bond angles (°): Te1ꢀꢀꢀO1B 2.644(7), Pt2-
Te3 2.542(2), Pt2-Te4 2.551(3), Pt1-Cl1 2.311(6), Pt1-Cl2 2.338(7), Pt1-Te1 2.538(19),
Pt1-Te2 2.541(19), Cl1-Pt1-Te2 172.75(18), Cl2-Pt1-Te1 175.5(2), Cl2-Pt1-Te2
82.74(18),Te1-Pt1-Te2 96.43(6), Cl1-Pt1-Cl2 90.4(2)
For all the four complexes 38-41 there was no cleavage of C-Te/Te-Pd bond observed
even in the presence of strong intramolecular TeꢀꢀꢀO interaction. This could be due to
stronger Te-C bond present in alicyclic systems.
Conclusion
Di-(2-formylcyclohex-1-ene)sulfide, 21 and di-(2-formylcyclohex-1-ene)selenide 29 do
not exhibit any intramolecular EꢀꢀꢀO interaction. In both the chalcogenides, the formyl
groups are in anti configuration with respect to the chalcogen (S/Se) atoms. The
structural comparison of monochalcogenides (21, 22 and 29) indicates that, for
compounds 21 and 29 the prevalence of (anti, anti) configuration dominates over (syn,
anti) configuration with the intramolecular EꢀꢀꢀO (E = S, Se) interactions. Whereas the
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latter configuration predominates over the former in tellurium system 22 due to
propensity of low-valent tellurium to form a relatively stronger TeꢀꢀꢀO interaction in
relation to its lighter congeners (S and Se). From the current results and their comparison
with the aromatic analogues, however, it is observed that the strength of SeꢀꢀꢀO
intramolecular interactions in alicyclic diselenide and their halide derivatives is greater
than the aromatic counterparts. This is probably due to the incomplete delocalization of
the lone pair of electrons present on the oxygen. Compound 31, an intermediate in the
formation of 26, has been isolated. The formation of a dimer via intermolecular
interaction in 36 is unusual. Monotellurides 22 and 26 afforded complexes of Pd(II) and
Pt(II) without cleavage or transmetallation occurring during metallation.
Experimental section
All the organochalcogen compounds were synthesized under nitrogen or argon
atmosphere using standard Schlenk line techniques. Solvent were purified and dried by
standard procedures and were freshly distilled prior to use.[53] All the chemicals used
were reagent grade and were used as received. Melting points were recorded in capillary
1
tubes. The NMR spectra were recorded in CDCl3 solvent. The H (400 MHz/500 MHz),
13C (100/125 MHz), 77Se (76.4/95.4 MHz) and 125Te (126.3/157.8 MHz) spectra were
recorded on a Varian Mercury plus or Bruker Avance spectrometer. Chemical shifts cited
were referenced with respect to TMS for (1H and 13C) as internal standard and Me2Se (for
77Se), Me2Te (for 125Te) as external standards. Elemental analysis was performed on
Carlo-Erba model 1106 and Eager 300 EA112 elemental analyzers. The IR spectra were
recorded in the range 400-4000 cm-1 by using KBr pellets for solid samples on a Thermo
Nicolet Avatar 320 FT-IR spectrometer. Mass spectral (MS) studies were completed by
using a QTOF Micro mass spectrometer with electrospray ionization (ESI) mode
analysis. In the case of isotopic patterns, the value is given for the most intense
peak. In column chromatography, silica gel was used as a stationary phase whereas
petroleum ether (60-80°C) and ethyl acetate were used as mobile phase.
28