Facile in situ copper(ii) mediated C-S bond activation transforming dithiocarbimate to carbamate and thiocarbamate generating Cu(ii) and Cu(i) complexes

Article abstract of DOI:10.1039/c1dt11622c

Facile in situ Cu(ii) mediated transformation of p- tolylsulfonyldithiocarbimate in conjunction with polypyridyl or phosphine ligands into corresponding carbamate and thiocarbamate led to the formation of new copper complexes with varying nuclearities and

Full text of DOI:10.1039/c1dt11622c

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COMMUNICATION
Facile in situ copper(II) mediated C–S bond activation transforming
dithiocarbimate to carbamate and thiocarbamate generating Cu(II) and Cu(I)
complexes†
Kiran Diwan,a Bandana Singh,a Santosh K. Singh,a Michael G. B. Drewb and Nanhai Singh*a
Received 28th August 2011, Accepted 1st November 2011
DOI: 10.1039/c1dt11622c
Facile in situ Cu(II) mediated transformation of p-
tolylsulfonyldithiocarbimate in conjunction with polypyridyl
or phosphine ligands into corresponding carbamate and
thiocarbamate led to the formation of new copper complexes
with varying nuclearities and geometries, via C–S bond
activation of the ligand within identical reaction systems.
Cu(I) complexes and suggest a possible mechanism for ligand
transformation.
Treatment of an aqueous solution of Cu(ClO4)2·6H2O
with aqueous-methanolic solution containing the ligands p-
tolylsulfonyldithiocarbimate (L) and L¢ = phen or bpy or PPh3 in
appropriate molar ratios afforded the formation of greenish blue
needles of 1, blue blocks of 2 and white blocks of 3 (Scheme 1).
In recent years metal mediated C–S bond cleavages within the
ligands have gained much importance because of their growing
synthetic and industrial applications.1 The ligand dianionic 1,1-
aromatic sulfonyldithiocarbimate2 (see ESI†, Scheme S1) exhibits
some remarkable features such as (i) it can provide greater
delocalization beyond the MS2 bond through C N, C–S, and
Ar–SO2–N groups (ii) it has both hard (N,O) and soft (S) donor
atoms available for bonding and (iii) it has a higher negative
charge on the sulphur atoms providing greater electron density
on the central metal atom which are normally not associated
with the other 1,1-dithio ligands.3 These features cause significant
differences in the electronic and structural characteristics and
properties of their complexes.2c–e,4 Furthermore, the incorporation
of sterically demanding phosphines with s-donor and p-acceptor
properties and highly delocalized polypyridine ligands into metal
complexes may cause dramatic changes in chemical reactivity,
structural rearrangement and physical properties of the resulting
complexes.2c–e,4,5
Scheme 1 Synthesis of the complexes.
There are two possible mechanisms for this transformation
detailed in Scheme 2.
When the reaction proceeds via path (a) the p-
tolylsulfonyldithiocarbimate (I) and p-tolylsulfonylthiocynate
(II) are in the same oxidation state and therefore the former is
easily converted to sulfonyl thiocyanate under the influence of
Cu2+ salt (a Lewis acid) which takes away sulphur as copper
sulphide. The resulting thiocyanate being highly electrophilic in
character reacts with methanol to yield thiocarbamate (III). The
intermediate III can also be formed via path (b) which involves
nucleophilic attack of methanol on doubly bonded electrophilic
carbon forming a tetrahedral intermediate IV. The Cu2+ ion takes
away sulphur as CuS, forming intermediate III. This intermediate
III is again activated through a copper–sulphur interaction and
reacts with water to give a tetrahedral intermediate V which is
further converted to a sp2 carbon to give methyl carbamate VI
through the extrusion of sulphur. The lone pair on nitrogen in
III interacts with the Cu+ followed by deprotonation to give the
complex 3 whereas the lone pair on the nitrogen of IV forms 1 and
2 with the Cu2+ ion. In addition to the CuS, the unreacted ligands
such as bpy, phen, PPh3 and p-tolylsulfonyldithiocarbimate are
eliminated as sacrificial ligands and were not recovered. When
PPh3 was used in place of polypyridine ligands, the reaction stops
at III with reduction of Cu(II) to Cu(I) thus transforming it to
thiocarbamate (3). It is worth mentioning that the C–S bond
scission of this ligand does not occur with other metal ions such
as Ni(II), Pd(II), Pt(II) and Cu(I).2c–e
Hence, in view of the emerging prospects of C–S bond acti-
vation, we report herein, Cu(II) mediated in situ transformation
of the ligand p-tolylsulfonyldithiocarbimate in conjunction with
the ligands 2,2¢-bipyridine (bpy), 1,10-phenanthroline (phen) or
triphenylphosphine (PPh3) into their respective carbamate and
thiocarbamate resulting in the formation of three new Cu(II) and
aDepartment of Chemistry, Banaras Hindu University, Varanasi, U.
P., 221005, India. E-mail: nsinghbhu@gmail.com, nsingh@bhu.ac.in;
Fax: +91-542-2368127; Tel: +91-542-2307321*108
bDepartment of Chemistry, University of Reading, Whiteknights, Reading,
RG6 6AD, UK
† Electronic supplementary information (ESI) available: Experimental
procedures including schemes for resonating structures of the ligand,
the IR, 1H, 13C and 31P NMR discussions of the complexes along with
their absorption spectra and photoluminescence spectra as well as variable
temperature magnetic susceptibility studies of complex 2. CCDC reference
numbers 795631, 795632 and 815020. For ESI and crystallographic data
in CIF or other electronic format see DOI: 10.1039/c1dt11622c
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Fig. 1 Structure of monomeric cation [Cu(phen)2(L¢¢)]+ in 1, showing the
numbering schemes and displacement ellipsoids at the 30% probability
˚
level. Selected bond lengths in [A]: Cu(1)–N(11) 2.001(4), Cu(1)–N(22)
2.161(4), Cu(1) –N(31) 1.988(4), Cu(1)–N(42) 2.079(3), Cu(1)–N(51)
2.064(4).
Scheme 2 Plausible mechanisms (a) and (b) for transformation of the
ligand dithiocarbimate to carbamate in 1 and 2 and thiocarbamate in 3.
Although the oxidation state of the copper(II) is unchanged in
1 and 2, it is reduced to copper(I) in 3 due to the presence of the
PPh3 ligand which stabilizes lower oxidation states of transition
metals owing to its p–acceptor character. The choice of the
copper(II) salts seems to be vital because, in the above experiments,
attempts to obtain crystals of complexes with common Cu(II) salts
other than perchlorate such as (NO3 , SO42-, AcO-, Cl-) were
-
not successful. It thus seems to be more appropriate to describe
these “syntheses” as “crystallization techniques”. All synthesized
complexes have been fully characterized by spectroscopy, variable
temperature (2–300 K) magnetic susceptibility (see ESI†) and
X-ray crystallography. 1 and 2 show a broad structured band
in the vis-NIR region, 500–1100 nm. Complex 3 shows strong
photoluminescence emission at around 325 and 475 nm in CH2Cl2
solution at room temperature. Tetranuclear complex 2 exhibits
an antiferromagnetic interaction between the copper atoms via a
metal–metal interaction in the given temperature range.
Fig.
2
The structure of the tetrameric cation [Cu4(m3-OH)2(m2-
OH)2(bpy)4(L¢¢)2]2+ in 2 with ellipsoids at 50% probability. Selected bond
˚
lengths in [A]: Cu(1)–Cu(3) 2.898(1), Cu(2)–Cu(4) 2.912(1), Cu(1)–O(4)
1.900(4), Cu(2)–O(1) 1.926(4), Cu(3)–O(4) 1.929(4), Cu(4)–O(1)
1.918(4), Cu(1)–O(3) 1.977(4), Cu(3)–O(3) 1.962(4), Cu(2)–O(2) 1.973(4),
Cu(4)–O(2) 1.969(4).
The steric considerations demonstrate that the carbamate
group coordinates in a unidentate (N bonded) manner rather
than bidentate (N, O) fashion. Notwithstanding the chemical
transformation of the ligand L, the oxidation state of copper and
the geometry around the metal centre is virtually alike in 1 and 2
despite the fact that 1 is mononuclear whereas 2 is tetranuclear due
to variation in the steric constraints imposed by the polypyridine
ligands.
The structure of complex
1 consists of mononuclear
[Cu(phen)2(L¢¢)]+ [L¢¢ = CH3C6H4SO2NCOOCH3] cation and
-
ClO4 counterion illustrated in Fig. 1 (for structural details see
ESI†).
The crystal structure of 2·5H2O consists of teteranu-
clear [Cu4(m3-OH)2(m2-OH)2(bpy)4(L¢¢)2]2+ cation, two perchlorate
counterions and the lattice water molecules. The structure of the
cation is shown in Fig. 2.
The
unit
cell
of
3
consists
of
mononuclear
[Cu(PPh3)2(CH3C6H4SO2NCSOCH3)] neutral molecules as
shown in Fig. 3 (for details see ESI†).
As is apparent from Fig. 2, the dication is approximately
centrosymmetrical. The copper atoms are five-co-ordinate with
square pyramidal environments as indicated by t values of 0.279,
0.254, 0.218 and 0.240 for Cu(1), Cu(2), Cu(3) and Cu(4) square
pyramidal environments as indicated by tau values of 0.279, 0.254,
0.218 and 0.240 for Cu(1), Cu(2), Cu(3) and Cu(4), respectively
(for details see ESI†).
In conclusion, we have described the first examples
of copper(II) mediated in situ transformation of p-tol-
ylsulfonyldithiocarbimate ligand to p-tolylsulfonylcarbamate and
p-tolylsulfonylthiocarbamate yielding mono- and tetranuclear
Cu(II) and Cu(I) complexes 1, 2 and 3. Interestingly, a change of the
sterically hindered phenanthroline by the relatively less hindered
bipyridine resulted in the formation of 2 rather than 1 whereas in
368 | Dalton Trans., 2012, 41, 367–369
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Notes and references
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Fig.
3 Structure of complex 3, showing the numbering schemes
and displacement ellipsoids at the 30% probability level. Selected
bond lengths in [A] and bond angles (◦): Cu(1)–P(1) 2.231(1),
˚
Cu(1)–P(2) 2.246(1), Cu(1)–N(73) 2.111(3), Cu(1)–S(71) 2.505(1);
P(1)–Cu(1)–P(2) 129.97(4), N(73)–Cu(1)–S(71) 67.10(7), N(73)–Cu(1)–
P(2) 112.2(1), N(73)–Cu(1)–P(1) 113.3(1), S(71)–Cu(1)–P(1) 115.38(5),
S(71)–Cu(1)–P(2) 100.49(3).
3 the electronic properties and steric restrictions of PPh3 reduced
copper(II) to copper(I) forming a Cu(I) thiocarbamate complex.
The utilization of copper(II) perchlorate plays a key role in the
crystallization of the complexes which could not be accomplished
when other copper(II) common salts, NO3 , SO42-, Cl- and OAc-
-
were used. Complex 2 shows weak antiferromagnetic interactions
between the copper(II) centres and 3 exhibited photoluminescence
properties in solution at room temperature. Further investigations
of the scope of these reactions are underway in our laboratory.
We thank the Council of Scientific and Industrial Re-
search (CSIR), New Delhi for funding through Project No.
01(2290)/09/EMR(II), Banaras Hindu University, EPSRC (U.K.)
and the University of Reading for funds for the CCD diffrac-
tometer. Prof. Jun Tao, Xiamen University, Xiamen, P. R. China,
is gratefully acknowledged for performing variable temperature
magnetic susceptibility measurements.
This journal is
The Royal Society of Chemistry 2012
Dalton Trans., 2012, 41, 367–369 | 369
©