1,4-Alkyl migration associated with simultaneous S-C bond cleavage and N-C bond formation in platinum complexes of 2-aminothioethers. Characterization of intramolecular interligand charge transfer phenomenon

Article abstract of DOI:10.1039/b918336a

Chemical reactions of Pt(pap)Cl₂ [pap = 2-(phenylazo)pyridine] with the N,S-donor atom ligands, 2-alkylthioanilines (HL, (H₂NSR), where R = Me, -CH₂Ph, -CH₂-CHCH₂) in acetonitrile solvent under alkaline conditions yielded mixed chelate donor-acceptor complexes, [Pt(pap)(HNSR)]⁺ (1⁺, R = Me), [Pt(pap)(HNS)] (2, HNS = 2-amidothiophenolate, R = Me, -CH₂Ph, -CH₂-CHCH₂) and [Pt(pap)(RNS)] (RNS = 2-(N-alkyl) amidothiophenolate 3 and 4, R = -CH₂Ph, -CH₂-CHCH ₂). Unusual types of 1,4-alkyl group migration associated with simultaneous S-C bond cleavage and N-C bond formation were observed for R = -CH₂Ph, -CH₂-CHCH₂. However, for R = Me, only S-C bond cleavage occurred producing compound 2 at a higher temperature. Under identical experimental conditions the reaction of Pt(bpy)Cl₂ [bpy = 2,2′-bipyridine] with 2-methylthioaniline afforded [Pt(bpy)(HNSR)] ⁺ (5⁺, R = Me) as the only product with no S-C bond activation. The complexes have been characterized by ¹H NMR, UV-vis-NIR, ESI-MS, EPR and cyclic voltammetry studies. Single-crystal X-ray structures of the complexes, [1][OTf] (OTf = trifluoromethanesulfonate), 2, 3, 4 and [5][OTf] are reported. The Pt-pap complexes (1⁺ and 2-4) showed intense interligand charge transfer (ILCT) transition in the NIR-region (>800 nm). This band in the Pt-bpy analogue, [5]⁺ shifted to a much higher energy, at 545 nm. The cationic complex, [1][OTf] displayed two reversible responses at -0.21 and -0.94 V along with two irreversible anodic responses at 0.48 and 0.90 V. The molecular compounds, 2, 3 and 4 showed two reversible waves near -0.60 and -1.30 V and an irreversible anodic response near 0.5 V. The response at the anodic potential is presumably due to oxidation of 2-amidothioether/2-amidothiophenolate ligand while the reversible responses at cathodic potentials are due to successive reductions of the azo chromophore of the coordinated pap ligand. The redox processes are characterized by EPR and spectroelectrochemistry. Density-functional theory calculations were employed to confirm the structural features and to support the spectral and redox properties of the complexes. The Royal Society of Chemistry 2010.

Full text of DOI:10.1039/b918336a

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1,4-Alkyl migration associated with simultaneous S–C bond cleavage and
N–C bond formation in platinum complexes of 2-aminothioethers.
Characterization of intramolecular interligand charge transfer phenomenon†
Sutanuva Mandal,a Nandadulal Paul,a Priyabrata Banerjee,a Tapan K. Mondalb and Sreebrata Goswami*a
Received 4th September 2009, Accepted 22nd December 2009
First published as an Advance Article on the web 2nd February 2010
DOI: 10.1039/b918336a
Chemical reactions of Pt(pap)Cl2 [pap = 2-(phenylazo)pyridine] with the N,S-donor atom ligands,
=
2-alkylthioanilines (HL, (H2N∧SR), where R = Me, -CH2Ph, -CH2-CH CH2) in acetonitrile solvent
under alkaline conditions yielded mixed chelate donor–acceptor complexes, [Pt(pap)(HN∧SR)]+ (1+,
=
R = Me), [Pt(pap)(HN∧S)] (2, HN∧S = 2-amidothiophenolate, R = Me, -CH2Ph, -CH2-CH CH2)
and [Pt(pap)(RN∧S)] (RN∧S = 2-(N-alkyl)amidothiophenolate 3 and 4, R = -CH2Ph,
=
-CH2-CH CH2). Unusual types of 1,4-alkyl group migration associated with simultaneous S–C bond
cleavage and N–C bond formation were observed for R = -CH2Ph, -CH2-CH CH2. However, for R =
=
Me, only S–C bond cleavage occurred producing compound 2 at a higher temperature. Under identical
experimental conditions the reaction of Pt(bpy)Cl2 [bpy = 2,2¢-bipyridine] with 2-methylthioaniline
afforded [Pt(bpy)(HN∧SR)]+ (5+, R = Me) as the only product with no S–C bond activation. The
complexes have been characterized by 1H NMR, UV-vis-NIR, ESI-MS, EPR and cyclic voltammetry
studies. Single-crystal X-ray structures of the complexes, [1][OTf] (OTf = trifluoromethanesulfonate), 2,
3, 4 and [5][OTf] are reported. The Pt-pap complexes (1+ and 2–4) showed intense interligand charge
transfer (ILCT) transition in the NIR-region (>800 nm). This band in the Pt-bpy analogue, [5]+ shifted
to a much higher energy, at 545 nm. The cationic complex, [1][OTf] displayed two reversible responses
at -0.21 and -0.94 V along with two irreversible anodic responses at 0.48 and 0.90 V. The molecular
compounds, 2, 3 and 4 showed two reversible waves near -0.60 and -1.30 V and an irreversible anodic
response near 0.5 V. The response at the anodic potential is presumably due to oxidation of
2-amidothioether/2-amidothiophenolate ligand while the reversible responses at cathodic potentials
are due to successive reductions of the azo chromophore of the coordinated pap ligand. The redox
processes are characterized by EPR and spectroelectrochemistry. Density-functional theory
calculations were employed to confirm the structural features and to support the spectral and redox
properties of the complexes.
Introduction
In this work we have studied the reaction of Pt(pap)Cl2 [pap =
2-(phenylazo)pyridine] and 2-alkylthioanilines (Chart 1) in an
attempt to prepare mixed ligand donor–acceptor platinum com-
plexes of pap and the aminothioether ligands. Over the past
three decades a vast literature has built up around mixed ligand
complexes of platinum metal, particularly Pt(diimine)(dithiolate)
systems.1 The Pt atom in these examples links the ligands that
exhibit donor–acceptor qualities to each other and the low energy
charge transfer transitions characterize2 these complexes. We
Chart 1
aDepartment of Inorganic Chemistry, Indian Association for the Cultivation
of Science, Kolkata, 700 032, India. E-mail: icsg@iacs.res.in; Fax: +91-33-
24732805
have used two redox non-innocent ligands of opposite character
because of the following reasons. The azoaromatic ligand (pap)
was chosen for its potent p-acceptor ability3 and its superior
acceptor ability over a diimine ligand like 2,2¢-bipyridine is now
well established. Such Pt-complexes containing pap as the acceptor
ligand may be anticipated to constitute charge transfer molecules
with absorption in the long wavelength (red) region. Redox active
molecules that absorb intensely in the red region attract interest as
functional materials.4 Selection of the 2-alkylthioaniline as a donor
was originated from our recent interest in quinone related ligand
bDepartment of Chemistry, Jadavpur University, Kolkata, 700032, India
† Electronic supplementary information (ESI) available: X-ray crystallo-
graphic files in CIF format for [1][OTf], 1, 3, 4 and [5][OTf]; figures of ESI-
MS and 1H NMR of [1]Cl, 2, 3, 4 and [5]Cl; ORTEP and atom numbering
scheme of [1][OTf], 2, 3, 4 and [5][OTf]; DFT and TD DFT calculations
of [1]Cl, 2, 3 and [5][OTf]; EPR spectra of electrogenerated 1, 2∑-, 3∑-, 4∑-
in CH3CN/0.1 M Et4NClO4. CCDC reference numbers 746833–746837.
For ESI and crystallographic data in CIF or other electronic format see
DOI: 10.1039/b918336a
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systems.5 In the present context it is worth noting that coordina-
tion of the 2-amidothiophenolate ligand and its redox partners
has been elaborated over recent years and N-substituted 2-
amidothiophenolate ligands have been the focus of several studies.6
In comparison, the corresponding coordination chemistry of 2-
aminothioether ligands has not been addressed7 systematically.
Moreover, we recently have noted8 some unexpected chemical
reactions of the coordinated aminothioether ligands in their
ruthenium complexes. Similar chemical reactions via activation of
the S–C bond were envisaged in the pap containing Pt-complexes
of the aminothioether ligand. In passing we also wish to note that
in the active site of Galactose Oxidase enzyme, a single Cu(II) is
coordinatively bound to thioether modified tyrosyl radical.9
The reference chemical reactions have resulted in an unprece-
dented type of 1,4-migration of the alkyl group from sulfur
(thioether function) to nitrogen (anilido nitrogen function) in
the coordinated 2-amidothioether ligands. This transformation
is associated with simultaneous S–C bond rupture and N–C
bond formation reactions. The S–C bond rupture processes are
important in the context of developing mechanistic knowledge to
desulfurization technology10 and to biological pathways such as
alkyl transfer11 etc. Two principal pathways viz. homolytic and het-
erolytic bond cleavage reactions are discussed in the literature.11,12
The chemical reactions, studied herein, are followed by isolation
and complete characterization of the products. Attempts have been
made to establish the plausible pathway of the reference chemical
transformations in our systems.
ioethers are studied in this work. X-ray structure along with
the chemical reactions of the starting molecular compound,
Pt(pap)Cl2 were reported recently.3a,14 Characteristic dp(Pt)–
p*(pap) interaction has been noted in this compound which is
reflected in the elongation of the N–N and the shortening of the
Pt–N(azo) distances.3a
A. Chemical reaction of Pt(pap)Cl2 and 2-methylthioaniline
The reaction of Pt(pap)Cl2 and 2-methylthioaniline in the presence
of NEt3 (base) proceeds smoothly at room temperature producing
the cationic mixed chelate, [Pt(pap)(HN∧SMe)]Cl, ([1]Cl). The
reaction involves substitution of two chlorides by the deprotonated
2-methylthioaniline. The above reaction in refluxing conditions,
however, produced a molecular complex, [Pt(pap)(HN∧S)] (2,
H2N∧SH = aminothiophenol) along with the above cationic
complex indicating that the compound [1]Cl undergoes rupture of
the S–C bond at a higher temperature. Accordingly, the pre-formed
pure compound [1]Cl, when boiled in acetonitrile, produced
compound 2 quantitatively in about two hours. The chemical
reactions are summarized in Scheme 1.
Interestingly the above demethylation reaction [1]Cl → 2 was
observed instantaneously even at a room temperature when the
CH3CN solution of [1]Cl was reacted with an aqueous solution
of KI. A similar transformation was also noted in the reaction
between [1]Cl and excess of aqueous KBr or KCl, though the
rate of reactions in the latter cases are considerably slower. These
results signify that the coordinated sulfur atom in [1]Cl has induced
significant electrophilic character on the adjacent alkyl carbon
atom. This is not unexpected since a strong p-acidic nature of
the co-ligand pap imparts electron deficiency at the coordinated
S-atom in the reference compound making the S–C bond strongly
polar. In this situation the S–C bond rupture may be envisaged
primarily via heterolytic cleavage producing a methyl cation.
The electrospray ionization mass spectral data (ESI-MS) of the
compounds [1]Cl and 2 has provided strong support in favor
of the formulation of the complexes. Complex [1]Cl showed an
intense peak at m/z 516 amu along with a peak at 502 amu. The
molecular complex 2, on the other hand, showed a single peak at
m/z 502 amu. The additional peak at m/z 502 amu in the case of
The platinum complexes, reported herein, represent a rare type
of molecule13 that features intense intramolecular charge transfer
(e > 15000 M-1 cm-1) bands above 800 nm. The lowest energy
charge transfer transitions in these complexes are primarily inter-
ligand CT which may be tuned by changing the nature of the lig-
ands. For example, the related compound viz. [Pt(bpy)(HN∧SR)]+
(bpy = 2,2¢-bipyridine and HN∧SR = 2-methylthioanilide) ab-
sorbs at a much shorter wavelength, near 540 nm.
Result and discussion
Reactions of the molecular complex Pt(pap)Cl2 [pap = 2-
(phenylazo)pyridine] with three different S-substituted aminoth-
Scheme 1
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pure [1]Cl is due to the formation of 2 via demethylation under
mass spectral conditions. The observed spectra of [1]Cl and [H2]+
exactly corroborated with their simulated spectra (Fig. S1 and
S2†).
The diamagnetic compound [1]Cl showed a resolved 1H NMR
spectrum in methanol-d4, however, a few peaks were broad.
The protons of the neutral pap ligand resonate in the low field
region (d, 9.54–7.69) with respect to the aromatic protons of 2-
methylthioanilide ligand, d, 7.50–6.52. The NH proton and methyl
proton resonances appeared at d, 4.57 and 2.20, respectively.
1
The molecular compound 2 showed a highly resolved H NMR
1
spectrum in CDCl3. The H NMR spectra of compounds [1]Cl
and 2 are submitted as ESI (Fig. S3 and S4†). The most notable
observation in the spectrum of compound 2 is the disappearance
of the methyl resonance. In this complex the pyridyl and phenyl
protons of the pap ligand resonate in the regions, d, 9.01–
7.50 and d, 7.82–7.31, respectively. The aromatic protons of 2-
methylthioanilide ligand experienced resonances in the relatively
high field region, d, 7.45–6.64.
Fig. 2 Molecular view of [Pt(pap)(HN∧S)], 2.
X-ray structure solution of the compound [1][OTf] (OTf =
trifluoromethanesulfonate) reconfirms its formulation as well as
its geometry. The central platinum atom in it is coordinated to
a neutral pap and deprotonated 2-methylthioaniline ligands in a
square planar geometry (Fig. 1). The complex as a whole is mono-
cationic and the crystallographic asymmetric unit contains one
OTf- as the counter anion. The Pt–N(azo) and Pt–N(py) distances
nature of the deprotonated aminothiophenolate function. The
C–N and C–S bond lengths in compound 2 are 1.364(8) and
˚
1.738(7) A respectively, which are in good accordance with the
dianionic catecholate type coordination of the 2-amidothiopheno-
late ligand.15
B. Reactions of Pt(pap)Cl2 with 2-benzylthioaniline and
2-allylthioaniline
˚
are notably different, viz. 1.9645(19) and 2.020(2) A, respectively.
The shortness of the Pt–N(azo) bond in the compound may be
attributed to dp–pp* interaction between Pt(t2) and the low lying
p*(azo) molecular orbital of pap.
To have further insight into the above S–C bond cleavage reaction,
two similar reactions of Pt(pap)Cl2 with 2-benzylthioaniline and
2-allylthioaniline were planned. Since we have argued in favor of
the heterolytic bond cleavage in the previous reaction (Scheme 1)
and considering the unstable nature of the methyl cation, we now
chose the benzyl and the allyl substituted thioethers since it is well
established that both benzyl and allyl cations are far more stable
than a methyl cation.
(i) 2-benzylthioaniline. In this case two products were isolated
in nearly identical yield. One of these is the molecular com-
pound 2 obtained due to debenzylation and more interestingly,
a second molecular compound 3 containing the deprotonated
2-(N-benzyl)aminothiphenol was also obtained in 35% yield.
The formation of compound 3 from the above reaction involves
debenzylation of the thioether function and N-benzylation of the
coordinated amido nitrogen atom. This reaction was also followed
in the presence of a radical scavenger, TEMPO (TEMPO = 2,2,6,6-
tetramethylpiperidinoxyl). Addition of TEMPO in a 1 : 1 molar
proportion (with respect to 2-benzylthioaniline) to the reaction
mixture, however, did not affect the yields or rate of reaction. Thus
the possibility of the formation of benzyl radical via the homolytic
S–C bond cleavage in these dealkylation reactions is excluded.
We therefore propose here that heterolytic bond cleavage of the
S–C bond is the primary step of the above reaction. The benzyl
cation, thus formed, trapped by the coordinated amido nitrogen
of the intermediate compound, 2 in alkaline conditions produced
compound 3.
Fig. 1 Molecular view of the compound, [Pt(pap)(HN∧SMe)]+ in
[1][OTf].
Analysis of the X-ray structure of compound 2 (Fig. 2)
has revealed the demethylation reaction as noted in Scheme 1.
Consequently, the platinum atom in it is coordinated to a dianionic
2-amidothiophenolato ligand and a pap ligand. The bond lengths,
˚
Pt–N(py) and Pt–N(azo) are 2.045(3) and 1.968(5) A respectively
are different as in the cationic complex, [1][OTf]. Also, the
C(ph)–S bond (1.738(7) A) is shorter than that of a similar bond
In order to have theoretical support for the above chemical
reaction, the net charge populations on the sulfur and nitrogen
atoms (Fig. 3) of the 2-amidothiophenolate ligand in 2 were
˚
˚
in the compound [1][OTf] (1.771(2) A) signifying the anionic
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Fig. 3 Mulliken spin density view of 2.
computed by the Gaussian03 program package. Clearly the
nitrogen centre is more nucleophilic than the sulfur centre. This in
turn correlates the observed N-alkylation reaction.
Scheme 2
˚
˚
We also performed a similar reaction in the presence of an
excess of KI. The reaction yielded the anticipated compound 2 as
the only product. The formation of [Ph-CH2]+ (m/z, 91 amu) in
the reaction mixture was identified by the analysis of the ESI-MS
spectrum of the reaction mixture. In this case strongly nucleophilic
iodide anion traps the benzyl cation and prevents further attack
of the coordinated anilido nitrogen centre. To prove it further,
the pre-formed compound 2 was reacted with the salt, N,N,N -
triethyl(N-benzyl)ammonium bromide, a potential source of the
benzyl cation. The reaction yielded 3 confirming our proposition.
Compound 3 is a molecular compound and was characterized
by ESI-MS, 1H NMR and elemental analyses. For example, the 1H
NMR spectrum of 3 in CDCl3 is complex due to the presence of
overlapping protons of the phenyl moieties of the two coordinated
ligands. However, the four pyridyl proton resonances appeared in
the lower field region (d, 8.56–7.47) were identified. The aliphatic
methylene protons of the benzyl group resonate at d, 4.18. The
ESI-MS and 1H NMR spectra of 3 are submitted as ESI (Fig. S5
and S6†).
N(azo) and Pt–N(py) distances are 1.961(8) A and 2.071(8) A
respectively. The X-ray structures of the two compounds are shown
in Fig. 4 and 5, respectively. Important bond parameters are
collected in Table 1.
(ii) 2-Allylthioaniline. The reaction of Pt(pap)Cl2 and 2-
allylthioaniline in acetonitrile proceeded similarly producing the
deallylated compound, 2 and the N-allylated complex, 4. The latter
was characterized fully spectroscopically (Fig. S7 and S8†). Its
single crystal structure determination by X-ray diffraction finally
confirms the chemical transformations that are summarized in
Scheme 2.
Fig. 4 Molecular view of [Pt(pap)(RN∧S)], 3.
Analysis of the X-ray structures of compounds 3 and 4 has
confirmed 1,4-migration of alkyl groups in the above reactions.
The platinum atom in 3 is coordinated to a dianionic 2-(N-
benzyl)amidothiophenolate ligand and a pap ligand. The Pt–
C. Chemical reaction of Pt(bpy)Cl2 and 2-methylthioaniline
To examine the effect of the ancillary ligand, pap in the above
S–C bond activation reactions, we planned a chemical reaction
16
˚
N(azo) and Pt–N(py) distances are 1.969(8) and 2.055(8) A
of Pt(bpy)Cl2 with 2-methylthioaniline under similar reaction
respectively.
conditions. The reaction produced a cationic mixed chelate,
[Pt(bpy)(HN∧SMe)]Cl ([5]Cl) after 3 h stirring in acetonitrile in al-
most quantitative yield. The reaction involves simple substitution
of two labile Cl atoms by the deprotonated 2-methylthioaniline.
Similarly, the coordination sites of the Pt atom in compound 4
were satisfied by a dianionic 2-(N-allyl)amidothiophenolate ligand
and a neutral pap ligand making the complex neutral. The Pt–
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˚
gap in the compound [1][OTf] (1.75 eV) as compared to that in the
compound [5][OTf] (1.92 eV). Such p-back donation is no doubt
responsible for facilitating the S–C bond cleavage in all the above
pap containing compounds.
Table 1 Selected experimental and calculated bond lengths (A) and
angles (◦) of [1][OTf], 2, 3 and [5][OTf]
[1][OTf]
2
3
Parameter
Exp.
Calc. Exp
Calc. Exp
Calc.
The formulation of the complex, [5]Cl was also established by
electrospray ionization mass spectrometry (ESI-MS) spectral data.
The positive ion ESI mass spectrum of the complex was recorded
in acetonitrile solvent and showed two peaks at m/z 489 amu
and 475 amu. The moderately intense peak at m/z 475 amu in
[5]Cl is due to the formation of the demethylated compound
indicating S–C bond rupture in mass spectral conditions. The
observed isotropic distributions of the above two peaks exactly
corroborated with their simulated patterns (Fig. S9†). The dia-
magnetic compound [5]Cl showed resolved 1H NMR spectrum in
methanol-d4. The protons of the neutral bpy ligand resonate in
the low field region (d, 9.17–7.70) with respect to the aromatic
protons of 2-methylthioanilide ligand at d, 7.16–6.37. The NH
proton and methyl proton resonances appeared at d, 4.90 and 2.84,
respectively. The spectrum of the compound [5]Cl is submitted as
ESI (Fig. S10†).
The X-ray structure of the salt [5][OTf] (OTf = trifluo-
romethanesulfonate) was solved, which also confirms its formu-
lation as well as geometry. The central platinum atom in the
compound is coordinated to a neutral bpy and a deprotonated
2-methylthioaniline ligand in a square planar geometry (Fig. 6).
The complex as a whole is monocationic and the crystallographic
asymmetric unit contains one OTf - as the counter anion and water
molecules as solvate. The average Pt–N(bpy) and Pt–N(anilido)
Pt(1)–S(1)
Pt(1)–N(1)
Pt(1)–N(3)
Pt(1)–N(4)
N(2)–N(3)
S(1)–C(17)
N(4)–C(12)
S(1)–C(18)
S(1)–Pt(1)–N(3) 99.56(6)
S(1)–Pt(1)–N(4) 84.34(8)
N(1)–Pt(1)–N(3) 77.70(8)
N(1)–Pt(1)–N(4) 98.68(9)
2.2783(7) 2.323 2.2707(16) 2.329 2.242(3) 2.313
2.020(2)
1.9645(19) 2.028 1.968(5)
1.941(2)
1.292(3)
1.771(2)
1.376(3)
1.817(3)
2.056 2.045(5)
2.061 2.055(8) 2.097
2.004 1.969(8) 2.009
1.980 1.981(8) 2.023
1.313 1.295(11) 1.308
1.744 1.721(11) 1.738
1.366 1.366(13) 1.383
1.981 1.988(5)
1.289 1.315(7)
1.777 1.738(7)
1.375 1.364(8)
1.831
x
x
x
x
101.8 99.79(15) 100.9 96.3(2)
82.38 83.54(15) 82.63 84.0(2)
77.04 77.61(19) 77.07 77.0(3)
97.1
83.4
76.2
98.69 99.03(19) 99.17 102.4(3) 103.1
Parameter
[5][OTf]
Exp.
Calc.
2.316
2.066
2.050
2.001
1.832
1.770
1.372
99.54
Pt(1)–S(1)
Pt(1)–N(1)
Pt(1)–N(2)
Pt(1)–N(3)
S(1)–C(17)
S(1)–C(16)
N(3)–C(11)
2.259(9)
2.036(3)
2.025(3)
1.981(3)
1.814(4)
1.774(4)
1.355(5)
S(1)–Pt(1)–N(2) 97.38(8)
S(1)–Pt(1)–N(3) 84.43(10) 82.18
N(1)–Pt(1)–N(2) 80.08(12) 78.99
N(1)–Pt(1)–N(3) 98.12(13) 99.32
˚
distances are 2.030(3) and 1.981(3) A respectively.
Fig. 5 Molecular view of [Pt(pap)(RN∧S)], 4.
Fig. 6 Molecular view of [Pt(bpy)(HN∧SMe)]+ in [5][OTf].
However, the cationic mixed chelate [5]Cl is chemically inert and
does not undergo any change in refluxing conditions even in the
presence of KI. This result is notable in the present context and
confirms the active role of pap in the S–C bond activation process
(vide supra).
To investigate the reasons for the different chemical behavior,
we compared the relevant structural data of the two analogous
compounds, [1][OTf] and [5][OTf]. Notably, the two Pt–N(py)
bond lengths of the complex [5][OTf] are comparable with that of
Pt–N(py) of pap in the compound [1][OTf] but are significantly
longer than the Pt–N(azo) bond length. There has been very
strong dp(Pt)–pp*(azo) overlap in the pap containing complex.
This effect has also been reflected in the HOMO–LUMO energy
D. UV-vis spectra and DFT
All the pap-containing compounds exhibited low-energy absorp-
tion spectra with broad allowed band(s) near 800 nm and above
(Table 2, Fig. 7). For example, the molecular compounds, 2, 3
and 4 showed a transition near 800 nm which was associated with
a shoulder near 700 nm. The low energy part of the spectra of
the cationic complexes, [1][OTf] and [5][OTf] on the other hand,
consisted of a single intense transition. The assignment of these
transitions was guided by the results from DFT calculations. The
molecular structures of all the complexes were fully optimized
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Fig. 7 UV-vis-NIR absorption spectra of the compounds, [1][OTf] (red), 2 (green), 3 (orange), 4 (blue) and [5][OTf] (pink) in 10-4M acetonitrile solution.
Table 2 UV-vis-NIR spectral dataa in CH3CN solvent of all reported
compounds
contributions of metal and ligand orbitals to the corresponding
MOs are submitted as ESI (Fig. S11–S14, Tables S5–S8†).
The calculated excitation energies using TD-DFT for all these
Compound
lmax a(e)
complexes are collected in Tables S9–S12.† The low energy bands
for compounds [1][OTf], 2 and 3 appeared in the range, 850–
800 nm. The experimental transitions compared well with the
calculated values for HOMO/HOMO-1 → LUMO transitions.
This band in the bpy analogue, [5][OTf] shifted to a much higher
frequency, at 545 nm. The lowest energy transition in all the cases
can be described as interligand charge transfer. In the complex,
[5][OTf] the acceptor orbitals lie at higher energies resulting in
a significant blue shift of the interligand transition. The mid-
intensity peak in [1][OTf] with a transition maximum close to
382 nm may be assigned to the HOMO-2 → LUMO transition.
Similarly compounds 2 and 3 exhibited low intensity peaks near
425 nm which are assigned to HOMO-2 → LUMO and HOMO-
3 → LUMO transitions. Several other transitions were observed
between 400 to 300 nm.
[1][OTf]
815(10040), 375(7445), 340(7200), 275(8520)
825(19960), 700b, 425(5020), 390(5755), 315(15650),
240(28680),
2
3
855(16575), 700b, 425(4560), 390(5410), 325(12460),
240(24850)
4
846(18581), 700b, 422(5167), 384(6027), 319(14239),
240(31525)
545(2984), 352(3077), 314(14564), 246(19704),
216(23274)
[5][OTf]
a Wavelength in nm, molar extinction coefficients in M-1 cm-1. b Shoulder.
at the B3LYP level of theory. The calculated bond lengths and
angles compared well with the crystallographically established
metrical parameters (see ESI, Tables S1–S4†). These two sets of
numbers are in good agreement with as the bond lengths and
angles are predicted within 2 pm and 1–4◦ of the experimental
values, respectively. However, the Pt–S and Pt–N bond lengths are
overestimated (Pt–S, ~0.05–0.06 A, Pt–N(azo), ~0.035–0.06 A, Pt–
N(pyridyl), ~0.015–0.04 A and Pt–N(amine), ~0.01–0.04 A). The
azo N–N bond length is slightly underestimated by about 0.002–
E. Cyclic voltammetry and EPR
Redox properties of all the reference complexes were studied by
cyclic voltammetry (CV) in acetonitrile solvent (0.1 M TEAP),
with a platinum working electrode. The potentials are referenced
to the saturated Ag/AgCl electrode and the data are collected
in Table 3. Segmented voltammograms (cathodic sweep) of the
two representative compounds are displayed in Fig. 8 and their
full range (+1.6 to -1.6 V) voltammograms are submitted as ESI
(Fig. S15†).
The complex [1][OTf] displayed four electrochemical responses
at 0.94, 0.48, -0.21 and -0.94 V. The two responses at ca-
thodic potentials are reversible while the oxidative responses
are irreversible. The nature of the voltammograms of the
˚
˚
˚
˚
˚
0.020 A. This difference arises most likely from crystallographic
packing and such interactions are not modeled in the calculations.
Analyses of the MOs of the complexes ([1][OTf], 2, 3 and
[5][OTf]) reveal that the highest occupied MOs are centered on the
2-amidothioether or 2-amidothiophenolate ligands. The lowest
unoccupied MOs are also of ligand character and localized over
the p* orbital of the pap or the bpy ligand. The electron density
distribution of the HOMO, HOMO-1 and LUMO and the relative
2722 | Dalton Trans., 2010, 39, 2717–2726
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Table 3 Cyclic voltammetric dataa
participation to the SOMO, the second electron reduced species is
however EPR silent. Similarly, the one electron reduced complexes
2∑-, 3∑- and 4∑- showed an EPR signal with g values of 2.005, 2.008
and 2.005 with metal hyperfine coupling constants of 12.6 and 6.4;
11.9 and 4.9; 12.4 mT respectively. EPR spectra of all the above
one electron reduced complexes are submitted as ESI (Fig. S16†).
Notably the color of the solutions of the above complexes faded
considerably upon reduction. This is not unexpected since the
acceptor character of the pap ligand diminishes upon reduction
resulting in the disappearance of the above interligand transitions.
Oxidation E2/1
bV (DEp, mV)
Compound
Reduction -E2/1 bV (DEp, mV)
[1][OTf]
-0.21(90), -0.94(90)
-0.68(80), -1.35(80)
-0.71(90), -1.38(86)
-0.68(80), -1.35(82)
0.48c, 0.94c
0.50c
2
3
4
0.54c
0.54c
a In acetonitrile solution supporting electrolyte Et4NClO4, reference elec-
trode Ag/AgCl. b E1/2 = 0.5 (Epa + Epc) where Epa and Epc are anodic and
cathodic peak potentials respectively, DEp = Epa - Epc, scan rate 50 mV s-1.
c Irreversible.
F. Conclusion
In this work we have disclosed unusual examples of 1,4-migration
of alkyl groups associated with simultaneous S–C bond cleavage
and N–C bond formation in the platinum complexes of 2-
amidothioether ligands. Design of the chemical reactions with
chosen substrates has been used to understand the course of
these reactions. The reference chemical reactions are promoted
by the presence of the coligand, 2-(phenylazo)pyridine. Strong
dp(Pt)–pp*(azo) interactions in these complexes activate the S–C
bond, which is the driving force for the above transformations.
For comparison, the 2,2¢-bipyridine analogue failed to promote
the above organic transformations. The products of the reactions
are fully characterized using spectral studies and X-ray structure
determination. The complex molecules containing pap showed
allowed charge transfer in the near IR-region (>800 nm), and are
assigned to intramolecular interligand transitions. The low energy
transition in the bpy-analogue shifts blue and appear in the visible
region (near 545 nm). A parallel trend in the reduction potentials
and the energies of LUMOs in the pap- and bpy- complexes are
also noted.
Fig. 8 Cyclic voltammograms of the compound [1][OTf] (blue) and 2
(pink) in CH3CN/0.1 M Et4NClO4.
Experimental section
S-dealkylated/N-alkylated compounds 2, 3 and 4 is nearly
identical. They exhibit one irreversible anodic response between
0.50 and 0.54 V and two reversible cathodic responses near
-0.60 and -1.30 V. The one-electron redox response has been
confirmed by comparison of the current height with the redox
response of the ferrocene/ferrocenium couple under identical
experimental conditions (for irreversible waves) and by constant-
potential electrolysis for the reversible waves. Irreversible responses
at the anodic potentials can be assigned to the oxidation of the
donor ligands, 2-amidothiophenolate or 2-amidothioether while
the cathodic waves formally correspond to the reduction of the azo
chromophore of the pap ligand. These results are fully supported
by the FMOs which clearly indicate that the LUMO and HOMO of
the complexes [1][OTf], 2 and 3 are primarily ligand centered. The
irreversible nature of the oxidative responses may be attributed to
rapid ligand substitution and complex degradation upon electron
transfer.
Materials
The starting Pt-salt, K2PtCl4 was collected from Arora-Matthey,
Kolkata. The ligand 2-methylthioaniline was purchased from
Sigma-Aldrich. The three other ligands, 2-benzylthioaniline, 2-
allylthioaniline and pap [2-(phenylazo)pyridine] were prepared17,18
as before. The compound Pt(pap)Cl23a,14 and tetraethylammonium
perchlorate (TEAP)19 were prepared and crystallized following
published procedures. Caution! perchlorate salts have to be han-
dled with care and appropriate safety precautions. Solvents and
chemicals used for syntheses were of analytical grade and used as
received.
Instrumentation
UV-vis-NIR absorption spectra were recorded either on a Perkin-
Elmer Lambda 950 UV/vis spectrophotometer or on a J&M
TIDAS instrument. The IR spectra were recorded with a Perkin-
Elmer 783 spectrophotometer. Cyclic voltammetry was carried out
in 0.1 M TEAP solutions using a three-electrode configuration
(platinum working electrode, Pt counter electrode, Ag/AgCl
reference electrode) and a PC-controlled PAR model 273A elec-
To have an insight into the nature of the electronic levels
associated with the reversible redox processes, electrogenerated
complexes were studied by EPR and UV-vis-NIR spectroelec-
trochemistry in CH3CN/0.1 M Et4NClO4 at 120 K. The one
electron reduced complex [1], generated by exhaustive electrolysis
of [1][OTf] at -0.40 V, showed an EPR signal with strong hyperfine
trochemistry system. The E 1 for the ferrocenium-ferrocene couple
under our experimental conditions was 0.39 V. EPR spectra in the
X band were recorded with a JEOL JES-FA200 spectrometer. A
2
1
coupling13b constants of 7.3 and 2.9 mT (for 195Pt, I =
,
2
natural abundance = 33%) at g = 2.001 confirming some metal
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Perkin-Elmer 240C elemental analyzer was used to collect micro
analytical data (C, H, N). 1H NMR spectra were taken on a Bruker
Avance DPX 300 spectrometer, and SiMe4 was used as the internal
standard. ESI mass spectra were recorded on a micro mass Q-TOF
mass spectrometer (serial No. YA 263). A Perkin-Elmer 240C
elemental analyzer was used to collect microanalytical data (C, H,
N).
(iv) 2-Allylthioaniline. Another similar reaction of Pt(pap)Cl2
with 2-allylthioaniline yielded the compound [Pt(pap)(RN∧S)], 4
(RN∧S = 2-(N-allyl)amidothiophenolate) along with compound
2. Reaction conditions as well as purification of the products
are similar to that described above in section (iii). The yield and
characterization data of the compound 4 are as follows.
Yield: 35%. ESI-MS, m/z: 543 amu [H4]+. Anal. Calcd for
C20H19N4PtS: C, 44.28; H, 3.50; N, 10.33 Found: C, 44.24; H,
-1
=
=
3.45; N, 10.27. IR (KBr disk, cm ): 1599 (C N), 1294 (N N).
Syntheses
Syntheses of the complexes were carried out by the reaction of
neutral Pt(pap)Cl2 and Pt(bpy)Cl2 with aminothioether in the
presence of NEt3 in acetonitrile solvent.
b. Chemical reaction of [Pt(bpy)Cl2] with 2-methylthioaniline
A mixture of 47 mg (0.11 mmol) Pt(bpy)Cl2 and 16 mg (0.11 mmol)
2-methylthioaniline (H2N∧SMe) in 30 ml of acetonitrile in the
presence of 2 drops of NEt3 was stirred for 3 h at room temperature.
The yellow solution became violet during this period. The crude
mass obtained by evaporation of the solvent was dissolved in a
minimum volume of methanol and was loaded on to a prepara-
tive alumina TLC plate for purification. Chloroform–methanol
solvent mixture was used as the eluent. A broad violet zone
was collected. Evaporation of the solvent yielded the compound
[Pt(pap)(HN∧SMe)]Cl, ([5]Cl). It was finally crystallized from
methanol containing an excess of sodium trifluoromethanesul-
fonate (NaOTf). A highly crystalline compound, [5][OTf] was
obtained in 90% yield. Anal. Calcd for C18H16N3PtS2F3O3: C,
a. Chemical reactions of [Pt(pap)Cl2] with 2-alkylthioanilines
(i) 2-methylthioaniline. To a solution of 50 mg (0.11 mmol)
Pt(pap)Cl2 in 30 ml of acetonitrile, 16 mg (0.11 mmol) 2-
methylthioaniline (H2N∧SMe) was added followed by the addition
of 2 drops of NEt3. The color of the solution changed from dark
red to intense green instantaneously. The mixture was then stirred
for 30 min at room temperature. The crude compound obtained
after evaporation of the solvent was purified on a preparative
alumina TLC plate. Toluene–methanol solvent mixture was used
as the eluent. The compound [Pt(pap)(HN∧SMe)]Cl, ([1]Cl) was
separated on a TLC plate as a major green band. The residue
obtained by solvent evaporation was crystallized from methanol
containing excess of sodium trifluoromethanesulfonate (NaOTf).
Highly crystalline [1][OTf] was obtained in 90% yield. Anal. Calcd
33.85; H, 2.51; N, 6.58. Found: C, 33.79; H, 2.55; N, 6.63. IR
-1
=
=
(KBr disk, cm ): 1608 (C N), 1319 (N N). ESI-MS: m/z 489
amu, [5]+.
for C19H17N4PtS2F3O3: C, 34.28; H, 2.56; N, 8.42. Found: C, 34.23;
-1
Computational details
=
=
H, 2.51; N, 8.49. IR (KBr disk, cm ): 1600 (C N), 1319 (N N).
ESI-MS: m/z 516 amu, [1]+.
Full geometry optimizations were carried out using the density
functional theory method at the (R)B3LYP level.20 All elements
except platinum were assigned using the 6-31G(d) basis set. The
SDD basis set with effective core potential was employed for
the platinum atom.21 The vibrational frequency calculations were
performed to ensure that the optimized geometries represented
the local minima and there were only positive eigen values.
All calculations were performed with the Gaussian03 program
package.22 Natural bond orbital analyses were performed using the
NBO 3.1 module of Gaussian03.23 Vertical electronic excitations
based on B3LYP optimized geometries were computed using the
time-dependent density functional theory (TD-DFT) formalism24
in methanol using a conductor-like polarizable continuum model
(CPCM).25 GaussSum26 was used to calculate the fractional
contributions of various groups to each molecular orbital.
(ii) Conversion of [Pt(pap)(HN∧SMe)]Cl, ([1]Cl) to
[Pt(pap)(HN∧S)],
2
(HN∧S
=
2-amidothiophenolate). The
compound 2 was obtained almost quantitatively by boiling an
acetonitrile solution of the compound [1]Cl or by treating the
same with aqueous KI solution at room temperature. Anal. Calcd
for C17H14N4PtS: C, 40.72; H, 2.80; N, 11.20. Found: C, 40.78; H,
-1
=
=
3.2; N, 11.14. IR (KBr disk, cm ): 1600 (C N), 1292 (N N).
ESI-MS: m/z 502 amu, [H2]+.
(iii) 2-Benzylthioaniline. A solution of 50 mg (0.11 mmol)
Pt(pap)Cl2 in 30 ml acetonitrile was reacted with 24 mg
(0.11 mmol) 2-benzylthioaniline in the presence of 2 drops of
NEt3. The color of the solution immediately changed from dark
red to intense green at room temperature. The mixture was stirred
for 1 h. The crude mass obtained by evaporation of the solvent was
dissolved in a minimum volume of dichloromethane and loaded
on to a preparative silica-gel TLC plate for purification. Toluene
was used as the eluent. Two green bands were obtained. The
first green zone upon evaporation yielded the compound 2. The
second green zone produced a new compound [Pt(pap)(RN∧S)],
3 (RN∧S = 2-(N-benzyl)amidothiophenolate). The solvent was
evaporated and the residue was crystallized by slow evaporation
of a dichloromethane–methanol solution mixture. Yield: 35%.
ESI-MS, m/z: 593 amu [H3]+. Anal. Calcd for C24H20N4PtS: C,
Crystallography
Crystallographic data for the compounds [1][OTf], 2, 3, 4 and
[5][OTf] are collected in Table 4 and their ORTEP and atom
numbering scheme are submitted as ESI (Fig. S17–S21†). Suitable
X-ray quality crystals of the cationic compounds were obtained
from slow evaporation of their methanolic solutions. Suitable crys-
tals of the molecular compounds on the other hand were obtained
by slow evaporation of dichloromethane–methanol solutions of
the compounds. All data were collected on a Bruker SMART
APEX diffractometer, equipped with graphite monochromated
48.65; H, 3.38; N, 9.46 Found: C, 48.61; H, 3.46; N, 9.38. IR (KBr
-1
=
=
disk, cm ): 1599 (C N), 1296 (N N).
2724 | Dalton Trans., 2010, 39, 2717–2726
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Table 4 Crystallographic data for [1][OTf], 2, 3, 4 and [5][OTf]
Parameter
[1][OTf]
2
3
4
[5][OTf]
Empirical formula
Formula weight
Crystal System
Space group
C19H17N4PtS2O3F3
665.59
Triclinic
¯
P1
9.4100(5)
10.5974(6)
11.4765(6)
84.0250(10)
78.3860(10)
72.4940(10)
1067.94(10)
2
C17H14N4PtS
501.47
Monoclinic
C24H20N4PtS
592.60
Monoclinic
C20H18N4PtS
541.53
Triclinic
¯
P1
12.476(3)
18.2405(14)
18.3912(14)
118.647(10)
97.985(10)
93.079(10)
3602.3(10)
8
C36H32N3PtS4F6O7
654.55
Orthorhombic
P21/n
P21/c
Pbcn
˚
a/A
6.8962(6)
22.833(2)
10.0507(9)
90
92.612(4)
90
1581.0(2)
4
2.107
0.07 ¥ 0.15 ¥ 0.24
1.8–29.5
1.02
25683
4350
11.4495(8)
19.8920(13)
27.4519(19)
90
98.806(2)
90
6178.6(7)
12
1.908
0.08 ¥ 0.16 ¥ 0.18
1.3–27.6
1.04
41931
13010
9.2882(3)
20.4170(5)
21.1399(6)
90.00
90.00
90.00
4008.9(2)
8
2.142
0.10 ¥ 0.10 ¥ 0.10
1.9–29.7
1.01
˚
b/A
˚
c/A
a (◦)
b (◦)
g (◦)
3
˚
V/A
Z
D
caled/g cm-3
2.070
1.997
Cryst dimens/mm3
q range for data collection (◦)
GOF on F2
0.07 ¥ 0.17 ¥ 0.20
1.8–25.0
1.02
12737
3765
0.10 ¥ 0.14 ¥ 0.25
1.3–25.0
1.02
26097
12595
Reflections collected
Unique reflection
49301
5633
Final R indices [I > 2s(I)]
R1 = 0.0167
wR2= 0.0418
293
R1 = 0.0398
wR2= 0.0888
293
R1 = 0.0381
wR2= 0.0798
293
R1 = 0.0291
wR2= 0.0756
293
R1 = 0.0285
wR2= 0.0593
293
T/K
˚
Hissler, I. E. McGarrah, W. B. Connick, D. K. Geiger, S. D. Cummings
and R. Eisenberg, Coord. Chem. Rev., 2000, 208, 115; (d) W. B. Connick,
D. Geiger and R. Eisenberg, Inorg. Chem., 1999, 38, 3264.
Mo-Ka radiation (l = 0.71073 A), and were corrected for Lorentz-
polarization effects. [1][OTf]: A total of 12737 reflections were
collected, of which 3765 were unique (Rint = 0.022), satisfying
the (I > 2s(I)) criterion, and were used in subsequent analysis.
2: A total of 25683 reflections were collected, of which 4350 were
unique (Rint = 0.085). 3: A total of 41931 reflections were collected,
of which 13010 were unique (Rint = 0.097). 4: A total of 26097
2 (a) E. Shikhova, E. O. Danilov, S. Kinayyigit, I. E. Pomestchenko, A. D.
Tregubov, F. Camerel, P. Retailleau, R. Ziessel and F. N. Castellano,
Inorg. Chem., 2007, 46, 3038; (b) I. Eryazici, C. N. Moorefield and G. R.
Newkome, Chem. Rev., 2008, 108, 1834; (c) X.-J. Liu, J.-K. Feng, J.
Meng, Q.-J. Pan, A.-M. Ren, X. Zhou and H.-X. Zhang, Eur. J. Inorg.
Chem., 2005, 1856; (d) X. Zhou, H.-X. Zhang, Q.-J. Pan, B.-H. Xia
and A.-C. Tang, J. Phys. Chem. A, 2005, 109, 8809; (e) F. Hua, S.
Kinayyigit, J. R. Cable and F. N. Castellano, Inorg. Chem., 2006, 45,
4304; (f) A. M. E. Geary, J. L. Yellowlees, A. L. Jack, D. H. I. Oswald,
S. Parsons, N. Hirata, R. J. Durrant and N. Robertson, Inorg. Chem.,
2005, 44, 242.
3 See, for example: (a) M. Panda, S. Das, G. Mostafa, A. Castin˜eiras and
S. Goswami, Dalton Trans., 2005, 1249; (b) M. Panda, C. Das, G.-H.
Lee, S.-M. Peng and S. Goswami, Dalton Trans., 2004, 2655; (c) A. K.
Deb, M. Kakoti and S. Goswami, J. Chem. Soc., Dalton Trans., 1991,
3249; (d) A. K. Deb, S. Choudhury and S. Goswami, Polyhedron, 1990,
9, 2251; (e) M. N. Ackermann, C. R. Barton, C. J. Deodene, E. M.
Specht, S. C. Keill, W. E. Schreiber and H. Kim, Inorg. Chem., 1989,
28, 397; (f) S. Wolfgang, T. C. Strekas, H. D. Gafney, A. R. Krause
and K. Krause, Inorg. Chem., 1984, 23, 2650; (g) S. Goswami, A. R.
Chakravarty and A. Chakravorty, Inorg. Chem., 1981, 20, 2246.
4 (a) M. K. C. Chan, C.-H. Tao, H.-L. Tam, N. Zhu, W.-W. V. Yam
and K.-W. Cheah, Inorg. Chem., 2009, 48, 2855; (b) N. J. Wheate, R. I.
Taleb, A. M. Krause-Heuer, R. L. Cook, S. Wang, V. J. Higgins and
J. R. Aldrich-Wright, Dalton Trans., 2007, 5055; (c) J. R. Mishur, C.
Zheng, M. Thomas, M. T. Gilbert, N. Rathindra and N. R. Bose, Inorg.
Chem., 2008, 47, 7972; (d) H.-S. Lo, S.-K. Yip, K. M.-C. Wong, N. Zhu
and V. W.-W. Yam, Organometallics, 2006, 25, 3537; (e) K. M.-C. Wong,
W.-S. Tang, X.-X. Lu, N. Zhu and V. W.-W. Yam, Inorg. Chem., 2005,
44, 1492; (f) C. Das, A. Saha, C.-H. Hung, G.-H. Lee, S.-M. Peng and
S. Goswami, Inorg. Chem., 2003, 42, 198.
5 (a) S. Chatterjee, P. Singh, J. Fiedler, R. Bakova´, S. Za´lisˇ, W. Kaim
and S. Goswami, Dalton Trans., 2009, 7778; (b) K. N. Mitra, S.-M.
Peng and S. Goswami, Chem. Commun., 1998, 1685; (c) K. N. Mitra,
S. Choudhury, A. Castin˜eiras and S. Goswami, J. Chem. Soc., Dalton
Trans., 1998, 2901; (d) K. N. Mitra, P. Majumdar, S.-M. Peng, A.
Castin˜eiras and S. Goswami, Chem. Commun., 1997, 1267; (e) K. N.
Mitra and S. Goswami, Chem. Commun., 1997, 49; (f) A. Saha, C. Das,
K. N. Mitra, S.-M. Peng, G. H. Lee and S. Goswami, Polyhedron, 2002,
21, 97; (g) K. N. Mitra and S. Goswami, Inorg. Chem., 1997, 36, 1322.
6 (a) N. Roy, S. Stephen, E. Bill, T. Weyhermu¨ller and K. Wieghardt,
Inorg. Chem., 2008, 47, 10911; (b) D. Herebian, E. Bothe, E. Bill, T.
Weyhermu¨ller and K. Wieghardt, J. Am. Chem. Soc., 2001, 123, 10012.
7 M. D. Roundhill, Inorg. Chem., 1980, 19, 557.
reflections were collected, of which 12595 were unique (Rint
=
0.031). [5][OTf]: A total of 49301 reflections were collected, of
which 5633 were unique (Rint = 0.074).
The structures were solved by employing the SHELXS-97
program package27a and were refined by full-matrix least-squares
based on F2 (SHELXL-97).27b All hydrogen atoms were added in
calculated positions.
EPR spectral studies
The one electron reduced complexes [1], 2∑-, 3∑- and 4∑-, generated
by exhaustive electrolysis of [1][OTf] at -0.40 V in CH3CN/0.1 M
Et4NClO4, were immediately dipped into liquid nitrogen and the
resulting frozen solutions were used for the EPR measurements
at 120 K. The applied potential for the complexes 2, 3, and 4 is
-0.80 V.
Acknowledgements
Financial support received from the Council of Scientific and In-
dustrial Research (CSIR), New Delhi (Project 01/2358/09/EMR-
II) is gratefully acknowledged. Crystallography was performed at
the DST-funded National Single Crystal Diffractometer Facility
at the Department of Inorganic Chemistry, IACS. S.M. thanks the
Council of Scientific and Industrial Research for her fellowship.
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2726 | Dalton Trans., 2010, 39, 2717–2726
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