Synthesis and reactivity of N -aminotroponiminatogermylenepyrrole and its derivatives

Article abstract of DOI:10.1021/om4003158

Through the reaction of the aminotroponiminatogermylene monochloride complex [(Buⁱ₂ATI)GeCl] (1) with sodium pyrrolide, the stable N-germylene pyrrole complex [(Buⁱ₂ATI)GeNC ₄H₄] (2) has been isolated. The reaction of compound 2 with thiophenol and selenophenol afforded the first germylene thio- and selenophenoxide complexes [(Buⁱ₂ATI)GeSPh] (3) and [(Buⁱ₂ATI)GeSePh] (4) through the substitution of the pyrrole moiety (NC₄H₄) with an EPh moiety (E = S (3), Se (4)), respectively. Interestingly, the chalcogenide derivatives of compound 2, such as the N-germathioacylpyrrole complex [(Buⁱ₂ATI)Ge(S) NC₄H₄] (5) and N-germaselenoacylpyrrole complex [(Bu ⁱ₂ATI)Ge(Se)NC₄H₄] (6), also underwent the aforementioned substitution reaction with thiophenol and selenophenol, resulting in the first examples of germa thioester complexes ([(Buⁱ₂ATI)Ge(S)SPh] (7) and [(Buⁱ ₂ATI)Ge(Se)SPh] (8)) and germa selenoester complexes ([(Bu ⁱ₂ATI)Ge(S)SePh] (9) and [(Buⁱ ₂ATI)Ge(Se)SePh] (10)), respectively. All the novel germanium compounds 3-10 have been unequivocally characterized through multinuclear NMR spectroscopy along with the germylene complex 2. Further, compounds 3-5 and 7-10 were characterized through single-crystal X-ray diffraction studies. The Ge^II-S and Ge^II-Se bond lengths in compounds 3 and 4 are 2.367(1) and 2.511(1) A, respectively. The average Ge^IV-S and Ge^IV-Se bond lengths in germa thioester (7 and 8) and germa selenoester (9 and 10) complexes are 2.241(1) and 2.362(1) A, respectively.

Full text of DOI:10.1021/om4003158

Article
pubs.acs.org/Organometallics
Synthesis and Reactivity of N‑Aminotroponiminatogermylenepyrrole
and Its Derivatives
Surendar Karwasara, Mahendra Kumar Sharma, Rupali Tripathi, and Selvarajan Nagendran*
Department of Chemistry, Indian Institute of Technology Delhi, Hauz Khas, New Delhi 110 016, India
S
* Supporting Information
ABSTRACT: Through the reaction of the amino-
troponiminatogermylene monochloride complex [(Bui2ATI)-
GeCl] (1) with sodium pyrrolide, the stable N-germylene
pyrrole complex [(Bui2ATI)GeNC4H4] (2) has been isolated.
The reaction of compound 2 with thiophenol and
selenophenol afforded the first germylene thio- and
selenophenoxide complexes [(Bui2ATI)GeSPh] (3) and
[(Bui2ATI)GeSePh] (4) through the substitution of the
pyrrole moiety (NC4H4) with an EPh moiety (E = S (3), Se
(4)), respectively. Interestingly, the chalcogenide derivatives of compound 2, such as the N-germathioacylpyrrole complex
[(Bui2ATI)Ge(S)NC4H4] (5) and N-germaselenoacylpyrrole complex [(Bui2ATI)Ge(Se)NC4H4] (6), also underwent the
aforementioned substitution reaction with thiophenol and selenophenol, resulting in the first examples of germa thioester
complexes ([(Bui2ATI)Ge(S)SPh] (7) and [(Bui2ATI)Ge(Se)SPh] (8)) and germa selenoester complexes ([(Bui2ATI)Ge(S)-
SePh] (9) and [(Bui2ATI)Ge(Se)SePh] (10)), respectively. All the novel germanium compounds 3−10 have been unequivocally
characterized through multinuclear NMR spectroscopy along with the germylene complex 2. Further, compounds 3−5 and 7−
10 were characterized through single-crystal X-ray diffraction studies. The GeII−S and GeII−Se bond lengths in compounds 3
and 4 are 2.367(1) and 2.511(1) Å, respectively. The average GeIV−S and GeIV−Se bond lengths in germa thioester (7 and 8)
and germa selenoester (9 and 10) complexes are 2.241(1) and 2.362(1) Å, respectively.
important non-halo-functionalized germylenes LGeX′ (X′ =
OH, OR, NR2, CCR, alkyl/aryl, etc.). Roesky and co-workers
carried out the reaction of the germylene monochloride
complex LGeCl (L = HC{CMeN(2,6-Pri2C6H3)}2) with
water in the presence of an N-heterocyclic carbene and
INTRODUCTION
■
Compounds of the type LGeX (L = monoanionic ligand; X = F,
Cl, Br, I) are generally called halo-functionalized germylenes,
due to the presence of a reactive Ge(II)−X bond apart from the
lone pair of electrons on the germanium center.1 Since the first
isolation of LGeI (L = HC(COCH3)2),2 several halo-
functionalized germylenes have been synthesized and charac-
terized.3 Among them, the chloro-functionalized germylenes
(germylene monochlorides) have been widely used and studied
because of their easy accessibility. They can show additional
reactivity in comparison to the non-functionalized germylenes
(L′Ge) (L′ = dianionic ligand) due to the possibility of
eliminating or substituting the chloride attached to the
germanium(II) center.4 Jones and Krossing’s quasi-one-
coordinate amido-germylene monocation5 [(2,6,4-
(HCPh2)2MeC6H2)(Me3Si)N−Ge]+[Al(OC(CF3)3)4]− and
Driess’s potassium salt of germacyclopentadienide6 [HC-
(CMe)2N(2,6-Pri2C6H3)Ge]−[K(Et2O)2]+ can be cited as
examples of unusual germanium compounds7 obtained from
chloro-functionalized germylenes through the elimination of
chloride ion. The former compound was isolated from the
reaction of RGeCl (R = Me3SiNAr, Ar = 2,6,4-
(HCPh2)2MeC6H2) with Ag[Al{OC(CF3)3}4], and the latter
was obtained as one of the products during the reduction of
LGeCl (L = HC{CMeN(2,6-Pri2C6H3)}2) with potassium.5,6
The substitution reaction on germylene monochlorides is also a
very important reaction, because it leads generally to several
obtained the hydroxy-functionalized germylene LGeOH.8
A
reaction of the germylene monochloride complex [N(SiMe3)-
C(Ph)C(SiMe3)(C5H4N-2)]GeCl with NaCp (Cp = cyclo-
pentadienyl) performed by Leung’s group afforded the Cp-
substituted germylene [N(SiMe3)C(Ph)C(SiMe3)(C5H4N-
2)]GeCp.9 Thus, the substitution reactions can be carried out
on germylene monochlorides using either suitable reagents with
an acidic proton in the presence of a base or the metal salts of
such reagents. Nevertheless, if the aforementioned substitution
reactions can be carried out on non-halo-functionalized
germylenes, the advantage would be the possibility to use
directly reagents with an acidic proton (without the need for a
base or for conversion of the reagents into their metal salts).
Such an idea has been pursued with amino-functionalized
germylenes containing the GeII−NR2 moiety (R = acyclic
substituent). Thus, the reaction of the amino-functionalized
germylene ArGeN(Pri)2 (Ar = 2,6-(Et2NCH2)2C6H3) with
2,4,6-trimethylphenol resulted in the germylene aryloxide
ArGe[OC6H2(CH3)3] through the cleavage of the GeII−N
Received: April 12, 2013
Published: July 8, 2013
© 2013 American Chemical Society
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Scheme 1. Synthesis of N-Germylene Pyrrole Complex 2 from Aminotroponiminatogermylene Monochloride 1
bond.10 However, for cases where the N atom is a part of an
Scheme 2. Synthesis of Aminotroponiminatogermylene
Thiophenoxide and Selenophenoxide Complexes 3 and 4
from N-Germylenepyrrole Complex 2
aromatic ring, the possibility of substituting such an N-
heterocyclic ring (with a suitable group) by cleaving the GeII−
Nring bond has not yet been explored. Further, the feasibility of
substituting the NR2 group/N-heterocycle attached to a
Ge(IV) center formally double bonded to an E atom (E = S,
Se) has also never been studied. To address these issues and to
expand the scope of the substitution reactions on non-halo-
functionalized germylenes, we report here the pyrrole-function-
alized germylene complex [(Bui2ATI)GeNC4H4] (2) stabilized
by a bulky aminotroponiminate ligand and substitution
reactions on it to result in the first germylene thiophenoxide,
[(Bui2ATI)GeSPh] (3), and selenophenoxide, [(Bui2ATI)-
GeSePh] (4), complexes. Further, we also demonstrate the
feasibility of carrying out the same substitution reaction on
chalcogenide derivatives (N-germathioacylpyrrole complex
[(Bui2ATI)Ge(S)NC4H4] (5) and N-germaselenoacylpyrrole
complex [(Bui2ATI)Ge(Se)NC4H4] (6)) of compound 2 and
isolating the germa thioester and selenoester complexes
([(Bui2ATI)Ge(E)SPh] (E = S (7), Se (8)) and [(Bui2ATI)-
Ge(E)SePh] (E = S (9), Se (10)), respectively). Compounds 7
and 8 and 9 and 10 are the first examples of germa thioester
and selenoester complexes, respectively. The synthesis of germa
ester complexes 9 and 10 gains further importance due to the
absence of any heavier group 14 selenoester complexes.
important to mention here that such metallylene thio and
seleno alkoxide/aryloxide complexes of other group 14
elements are rare. The only known example of a group 14
metallylene thio-alkoxide/aryloxide is the stannylene thiophen-
oxide12 [4-But-2,6-{P(O)(OPri)2}2C6H2]SnSPh, and there is
no example of a group 14 metallylene seleno alkoxide/
aryloxide. Interestingly, the germylene thio- and selenophen-
oxide complexes [(Bui2ATI)GeSPh] (3) and [(Bui2ATI)-
GeSePh] (4) are also obtainable quantitatively through the
reaction of germylene monochloride complex 1 with the
lithium salts of thiophenol and selenophenol (obtained by
reacting thiophenol and selenophenol with n-BuLi), respec-
tively.
In these reactions, the feasibility of substituting the pyrrole
moiety (NC4H4) in compound 2 with an E′Ph group has been
witnessed. With this success, the possibility of carrying out the
aforementioned substitution reactions on compounds of the
type LGe(E)NC4H4 (L = monoanionic ligand; E = S, Se) that
contain a pyrrole moiety attached to the germanium(IV) center
was checked. To obtain the germanium(IV) compounds
[(Bui2ATI)Ge(E)NC4H4] (E = S (5), Se (6)) required for
this study, the oxidative addition reaction of compound 2 was
carried out with elemental sulfur and selenium, respectively.
Interestingly, in the reaction of N-germaacylpyrrole complexes
5 and 6 with a slight excess of thiophenol, the substitution
reaction occurred to afford the first examples of germa thioester
complexes 7 and 8 ((Bui2ATI)Ge(E)SPh; E = S (7), Se (8)) in
about 94% and 96% yields, respectively (Scheme 3). The only
known heavier group 14 thioester complex, [{PhC(NBut)2}Si-
(S)SBut], was obtained when amidinatosilyldichlorothio-tert-
butoxide [{PhC(NBut)2}SiCl2SBut] was reduced with potas-
sium.13 The aforementioned substitution of the pyrrole moiety
in complexes 5 and 6 was observed with selenophenol also.
Thus, the reaction of selenophenol with complexes 5 and 6 in a
1.1:1 ratio resulted in the first germa selenoester complexes
(Bui2ATI)Ge(S)SePh (9) and (Bui2ATI)Ge(Se)SePh (10) in
excellent yields (95% and 92%), respectively (Scheme 3).
Further, it has been found that the germa ester complexes 7−
10 can also be obtained through the following two conventional
RESULTS AND DISCUSSION
■
Synthesis and Spectra. The desired germylene complex 2
with a pyrrole substituent was synthesized using the amino-
troponiminatogermylene monochloride 1 isolated by us.4a
Accordingly, the reaction of 1 equiv of germylene mono-
chloride complex 1 with 1.2 equiv of sodium pyrrolide at 0 °C
in THF afforded the N-germylenepyrrole complex 2 in a
quantitative yield (Scheme 1). The other known germylene
complex with a pyrrole moiety is Ge(NC4H3COR)2 (R =
Me2N, Me, H). It contains a pyrrole derivative as a ligand and
was obtained by reacting germylenediamide Ge[N(TMS)2]2
with the pyrrole-based ligand HNC4H3COR.11
To test the feasibility of substitution reactions on a
heterocycle-functionalized germylene, compound 2 was reacted
with various reagents with acidic protons. Eventually, it was
identified that such reactivity on compound 2 is possible cleanly
with thio- and selenophenol. Thus, the reaction of 1 equiv of
the N-germylenepyrrole complex 2 with a slight excess of
thiophenol in toluene at room temperature led to the cleavage
of the GeII−Nring bond and resulted in the first germylene
thiophenoxide complex [(Bui2ATI)GeSPh] (3) with an
excellent yield of 99% (Scheme 2). Similarly, the reaction of
compound 2 with selenophenol also occurred smoothly at 0 °C
and afforded the first germylene selenophenoxide complex
[(Bui2ATI)GeSePh] (4) in 97% yield (Scheme 2). It is
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between the selenium resonances seen in tetracoordinate
germanium(IV) complexes II−V (Table 1) with polar Ge
Table 1. 77Se NMR Chemical Shifts of Compounds I−V, 4,
6, and 8−10
Scheme 3. Synthesis of Aminotroponiminatogerma
Thioester and Selenoester Complexes 7−10 from N-
Germaacylpyrrole Complexes 5 and 6
δ(GeSe) δ(Ge−Se)
entry
compd
(ppm)
(ppm)
ref
a
1
2
(Tbt)(Tip)GeSe (I)
940.60
4j
b
[N(SiMe3)C(Ph)C(SiMe3)
−266.21
4d
(C5H4N-2)]Ge(Se)Cl (II)
a
3
{HC[CMeN(2,6-Pri2C6H3)]2}
−439.80
4c
Ge(Se)OH (III)
c
c
4
5
6
(Bui2ATI)Ge(Se)Cl (IV)
(But2ATI)Ge(Se)OBut (V)
(Bui2ATI)GeSePh (4)
−213.13
−77.76
4a
4b
synthetic routes in about quantitative yields: (a) an oxidative
addition reaction of compounds 3 and 4 with elemental sulfur
and selenium and (b) reaction of germa acid chloride
complexes (Bui2ATI)Ge(S)Cl and (Bui2ATI)Ge(Se)Cl with
the lithium thio- and selenophenoxides.
Compounds 2−10 are all stable at room temperature under a
dry N2 atmosphere. Compounds 2 and 3 are freely soluble in
most of the organic solvents, whereas compound 4 is soluble in
hexane, toluene, and THF but decomposes in dichloromethane
and chloroform. Compounds 5−8 are soluble in toluene,
dichloromethane, chloroform, and THF, whereas compounds 9
and 10 decompose in dichloromethane and chloroform but are
freely soluble in toluene and THF. Further, it has been
observed that compound 7 in chloroform and dichloromethane
starts to decompose after a period of about 2 h. Compounds 2,
3 and 4, and 5−10 are orange, red, and yellow solids,
respectively.
Compounds 2−10 have been characterized in solution by
multinuclear (1H, 13C, and 77Se) NMR spectroscopy. In the 1H
NMR spectrum of compound 2, methyl protons of isobutyl
groups appeared as two doublets (0.89 and 0.97 ppm). Its
methylene protons were observed as two doublets of doublets
(3.22 and 3.37 ppm) due to their diastereotopic nature,
whereas methine protons were observed as a multiplet (2.05−
2.19 ppm). All these resonances are shifted upfield if a
comparison is made with respect to the isobutyl groups of
germylene monochloride complex 1.4a The aforementioned
splitting pattern of the isobutyl groups in compound 2 was also
observed for the same groups in compounds 3−10. Two
singlets were seen at 6.17 and 6.59 ppm for the pyrrole protons
of compound 2, although the latter resonance overlaps with the
signals due to seven-membered-ring protons (6.52−6.68 ppm).
The pyrrole ring protons of compounds 5 (6.23 and 6.76 ppm)
and 6 (6.23 and 6.76 ppm) also appeared as two singlets but
without the aforementioned overlap with seven-membered-ring
resonances (5, 7.01−7.59 ppm; 6, 7.00−7.58 ppm). In
compounds 7−10, the ten protons of phenyl and seven-
membered rings were seen in the range 6.02−7.32 ppm. In the
13C NMR spectra of compounds 2−10, methyl, methylene, and
methine carbons were observed as two, one, and one singlet,
respectively. Two singlets were seen for the pyrrole carbons of
compounds 2 (108.17, 123.34 ppm), 5 (110.62, 123.06 ppm),
and 6 (110.50, 123.08 ppm). As anticipated, eight singlet
resonances were observed for the phenyl and seven-membered-
ring carbons of compounds 3, 4, 7, and 8. In compounds 9 and
10, only six and seven singlets were seen, respectively, as the
rest merged into the resonance for C6D6. In the 77Se NMR
spectra of compounds 6, 8, and 10, the resonance for the
selenium center in the GeSe bond appeared at −436.60,
−253.86, and −165.49 ppm, respectively. These values are
a
329.74
this
work
c
c
7
(Bui2ATI)Ge(Se)NC4H4 (6)
(Bui2ATI)Ge(Se)SPh (8)
(Bui2ATI)Ge(S)SePh (9)
(Bui2ATI)Ge(Se)SePh (10)
−436.60
−253.86
this
work
8
this
work
a
a
9
257.76
288.15
this
work
a
10
−165.49
this
work
a
b
c
In C6D6. In THF-d8. In CDCl3.
Se bonds. Nevertheless, the tricoordinate germanium(IV)
compound (Tbt)(Tip)GeSe (I; Tbt = 2,4,6-tris(bis-
(trimethylsilyl)methyl)phenyl; Tip = 2,4,6-triisopropylphenyl)
with an electronically unperturbed GeSe bond shows a
singlet resonance at 940.60 ppm (Table 1). The singlet
resonances due to the Ge−SePh moiety in compounds 4, 9,
and 10 were observed at 329.74, 257.76, and 288.15 ppm in
their 77Se NMR spectra, respectively.
X-ray Crystal Structures of Compounds 3−5 and 7−
10. The molecular structure of compounds 3−5 and 7−10
were further confirmed by single-crystal X-ray diffraction
analysis. The crystallographic data for these compounds are
summarized in Table S1 (see the Supporting Information).
Compounds 3 and 4 crystallized in the monoclinic space group
P21/n. In the molecular structure of germylene complexes 3
(Figure 1) and 4 (Figure S1; see the Supporting Information),
the germanium atom is bonded to two nitrogen atoms and one
sulfur/selenium (sulfur (3), selenium (4)) atom in a distorted-
trigonal-pyramidal geometry. The sums of the bond angles
around the germanium center in compounds 3 and 4 are
274.8(2) and 275.4(4)°, respectively. The orientations of the
SPh (in compound 3) and SePh (in compound 4) moieties
with respect to the fused five- and seven-membered rings are
different; in the former it is away, and in the latter it is above.
The dihedral angles between the five- and seven-membered
ring mean planes in compounds 3 and 4 are 4.0(1) and 2.7(1)°,
respectively, and these values hint at the nearly planar nature of
the fused rings in these compounds. The average Ge−N bond
lengths in compounds 3 and 4 are 1.958(4) and 1.950(2) Å,
respectively. Further, the Ge−S and Ge−Se bond lengths are
2.367(1) and 2.511(1) Å in compounds 3 and 4, respectively.
The average S−Ge−N (97.9(2)°) and Se−Ge−N (97.9(7)°)
bond angles in compounds 3 and 4 are almost equal.
Compounds 5, 9, 7, 8, and 10 crystallized in the monoclinic
P21/n, monoclinic P21/n, monoclinic C2/c, monoclinic C2/c,
and triclinic P1 space groups, respectively. In all of these
compounds, the geometry around the germanium center is
distorted tetrahedral. In compound 5 (Figure 2), the Ge−
̅
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between the mean planes of five- and seven-membered rings is
greater than the same angle found in germylene complexes 3
and 4 and reveals a greater puckering in the fused rings of
compound 5. The C2N2Ge five-membered ring is almost
perpendicular to the pyrrole ring, and the dihedral angle
between the mean planes of these rings is 87.6(9)° (average).
The GeS bond length in N-germaacyl pyrrole complex 5
(average 2.067(1) Å) is very close to that in the germa thioacid
chloride complex [(Bui2ATI)Ge(S)Cl] (2.065(1) Å).4a The
length of the GeS bond in compounds 7 (2.071(1) Å, Figure
S2 (see the Supporting Information)) and 9 (2.075(1) Å,
Figure 3) is comparable to the length of the same bond present
Figure 1. Molecular structure of (Bui2ATI)GeSPh (3). All of the
hydrogen atoms are omitted for clarity, and thermal ellipsoids are
drawn at the 40% probability level. Selected bond lengths (Å) and
angles (deg): Ge(1)−S(1) = 2.367(1), Ge(1)−N(1) = 1.962(4),
Ge(1)−N(2) = 1.953(4); S(1)−Ge(1)−N(1) = 97.3(1), S(1)−
Ge(1)−N(2) = 98.5(1), N(1)−Ge(1)−N(2) = 79.0(2), Ge(1)−
S(1)−C(16) = 96.4(2).
Figure 3. Molecular structure of (Bui2ATI)Ge(S)SePh (9). All of the
hydrogen atoms are omitted for clarity, and thermal ellipsoids are
drawn at the 30% probability level. Selected bond lengths (Å) and
angles (deg): Ge(1)−S(1) = 2.075(1), Ge(1)−Se(1) = 2.356(1),
Ge(1)−N(1) = 1.886(3), Ge(1)−N(2) = 1.884(4); S(1)−Ge(1)−
Se(1) = 111.4(1), S(1)−Ge(1)−N(1) = 122.4(1), S(1)−Ge(1)−N(2)
= 120.9(1), N(1)−Ge(1)−N(2) = 84.7(1), Ge(1)−Se(1)−C(16) =
99.9(1).
Figure 2. Molecular structure of (Bui2ATI)Ge(S)NC4H4 (5). All of
the hydrogen atoms are omitted for clarity, and thermal ellipsoids are
drawn at the 40% probability level. One of the two molecules present
in the asymmetric unit is shown here. Selected bond lengths (Å) and
angles (deg): molecule 1, Ge(1)−S(1) = 2.069(1), Ge(1)−N(1) =
1.874(2), Ge(1)−N(2) = 1.884(2), Ge(1)−N(3) = 1.867(2), S(1)−
Ge(1)−N(3) = 116.2(1), S(1)−Ge(1)−N(1) = 125.1(1), S(1)−
Ge(1)−N(2) = 118.2(1), N(1)−Ge(1)−N(2) = 84.9(1); molecule 2,
Ge(2)−S(2) = 2.064(1), Ge(2)−N(4) = 1.879(2), Ge(2)−N(5) =
1.878(2), Ge(2)−N(6) = 1.873(2), S(2)−Ge(2)−N(6) = 115.2(1),
S(2)−Ge(2)−N(4) = 123.7(1), S(2)−Ge(2)−N(5) = 120.9(1),
N(4)−Ge(2)−N(5) = 84.8(1).
in germanium complexes of the type LGe(S)X (Table S2 (see
the Supporting Information)). The length of the GeSe bond
in compounds 8 (2.202(1) Å, Figure 4) and 10 (2.205(1) Å,
Figure S3 (see the Supporting Information)) is almost the same
and it matches with the corresponding bond length in
germanium(IV) complexes of general formula LGe(Se)X
(Table S2 (see the Supporting Information)). The aforemen-
tioned comparisons suggest that the GeE bond (E = S (5, 7,
9), Se (8, 10)) in these compounds is polarized and can be
considered as a formal double bond.4a,e,f,m−p Nevertheless,
these polar GeS (in compounds 5, 7, and 9) and GeSe (in
compounds 8 and 10) bonds are longer than the electronically
unperturbed GeE (E = S, Se) bond present in the kinetically
stabilized compounds Tbt(Tip)GeS (2.049(3) Å)4j and
Tbt(Tip)GeSe (2.180(2) Å),4j respectively. The length of
the Ge−S bond in compounds 7 and 8 (2.240(1) and 2.242(1)
Å, respectively) is comparable to the length of the Ge−S single
bond (2.242(4) Å) present in the β-diketiminato germa
thioacid LGe(S)SH.3d The Ge−Se single bond lengths in
compounds 9 and 10 are 2.356(1) and 2.367(1) Å, respectively.
As there is no reported compound that has a Ge(E)Se (E = S,
Se) moiety, the aforementioned bond lengths have been
Npyrrole bond length (average 1.870(2) Å) is almost equal to the
average Ge−Nligand bond length (1.878(2) Å). As anticipated,
the latter average is shorter than the corresponding value seen
in the germylene complexes 3 (1.958(4) Å) and 4 (1.950(2) Å)
due to an increase in the formal oxidation state of the
germanium center from +2 (in 3 and 4) to +4 (in 5). In
compound 5, the average S−Ge−Nligand bond angle
(122.0(7)°) is greater than the S−Ge−Npyrrole bond angle
(average 115.7(8)°). The dihedral angle (average 5.9(7)°)
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conventional procedures. Hexane, THF, and toluene were further
dried over potassium. CDCl3 and C6D6 were dried over 4 Å molecular
sieves and potassium, respectively. Sulfur, selenium, thiophenol, and
selenophenol were purchased from Aldrich and used without any
further purification. Compound 1 and sodium pyrrolide were
synthesized according to the literature procedures.4a,15,16 Melting
points were recorded using an Ambassador melting point apparatus by
sealing the samples in glass capillaries, and the reported melting points
are uncorrected. Elemental analyses were carried out on a Perkin-
Elmer CHN analyzer. Multinuclear (1H, 13C, and 77Se) NMR
spectroscopic studies were carried out on a 300 MHz Bruker Topspin
NMR spectrometer using dry CDCl3 or C6D6. The chemical shifts δ
are reported in ppm and are referenced internally with respect to the
residual solvent (1H NMR) and solvent (13C NMR) resonances.17 In
the case of 77Se NMR spectroscopic studies, Me2Se was used as an
external reference.
Synthesis of (Bui2ATI)GeNC4H4 (2). To a solution of compound
1 (2.40 g, 7.07 mmol) in THF (40 mL) was added sodium pyrrolide
(0.76 g, 8.48 mmol) at 0 °C; the mixture was then warmed to room
temperature and stirred for 12 h. All the volatiles were then removed
under reduced pressure, the resultant residue was extracted into
hexane, and the extract was filtered using a G4 frit containing Celite.
Removal of hexane from the filtrate under reduced pressure afforded
an analytically pure sample of compound 2 as an orange solid. Yield:
2.60 g (7.02 mmol, 99%). Mp: 93 °C. Anal. Calcd for C19H27GeN3 (M
= 370.08): C, 61.66; H, 7.35; N, 11.35. Found: C, 61.42; H, 7.18; N,
Figure 4. Molecular structure of (Bui2ATI)Ge(Se)SPh (8). One
solvent molecule (THF) and all of the hydrogen atoms are omitted for
clarity, and thermal ellipsoids are drawn at the 40% probability level.
Selected bond lengths (Å) and angles (deg): Ge(1)−Se(1) =
2.202(1), Ge(1)−S(1) = 2.242(1), Ge(1)−N(1) = 1.882(1),
Ge(1)−N(2) = 1.892(1); Se(1)−Ge(1)−S(1) = 109.5(1), Se(1)−
Ge(1)−N(1) = 119.4(1), Se(1)−Ge(1)−N(2) = 127.7(1), N(1)−
Ge(1)−N(2) = 84.2(1), Ge(1)−S(1)−C(16) = 102.8(1).
compared with those of the germanium compound [C6H11NC-
(Me)NC6H11]2Ge[SeC6H5]2, which has GeIV−Se single bonds
(average 2.474(2) Å),14 and are found to be quite shorter. The
SGe−S and SeGe−S bond angles in compounds 7 and 8
are 110.0(1) and 109.5(1)°, respectively. These are shorter
than the SGe−Se (111.4(1)°) and SeGe−Se (114.5(1)°)
bond angles in compounds 9 and 10, respectively. As in
compound 4, the phenyl group in compounds 7−10 lies above
the fused rings. The dihedral angles between the five- and
seven-membered rings in compounds 7, 8, and 10 are 9.7(1),
10.2(1), and 9.3(2)°, respectively, and these values are close to
that seen in compound 5 (vide supra). Nevertheless, the
dihedral angle between the aforementioned rings in compound
9 is 4.2(2)° and close to the same angle found in compound 3
(vide supra).
1
3
11.05. H NMR (300 MHz, CDCl3): δ 0.89 (d, JHH = 6.6 Hz, 6H,
3
CH(CH3)2), 0.97 (d, JHH = 6.3 Hz, 6H, CH(CH3)2), 2.05−2.19 (m,
2H, CH(CH3)2), 3.22 (dd, JHH = 13.8, 8.1 Hz, 2H, CH2), 3.37 (dd,
JHH = 13.8, 6.0 Hz, 2H, CH2), 6.17 (s, 2H, CHpyrrole), 6.59 (s, 2H,
CHpyrrole), 6.52−6.68 (m, 3H, CHring), 7.14−7.21 (m, 2H, CHring).
13C{1H} NMR (75 MHz, CDCl3): δ 20.70 (CH(CH3)2), 21.06
(CH(CH3)2), 27.60 (CH(CH3)2), 53.93 (CH2), 108.09 (pyrrole),
114.30 (tp), 121.10 (tp), 123.26 (pyrrole), 136.85 (tp), 161.23 (tp)
(tp = seven-membered tropylium ring).
Synthesis of (Bui2ATI)GeSPh (3). To a solution of compound 2
(0.90 g, 2.43 mmol) in toluene (15 mL) was added thiophenol (0.26
mL, 2.55 mmol) at room temperature, and the mixture was stirred for
8 h. Removal of the volatiles from the resultant solution under reduced
pressure resulted in an analytically pure sample of compound 3 as a
red solid. Single crystals of compound 3, suitable for X-ray diffraction
studies, were obtained from its hexane solution by the slow
evaporation method at room temperature. Yield: 0.97 g (2.36 mmol,
99%). Mp: 81 °C. Anal. Calcd for C21H28GeN2S (M = 413.13): C,
CONCLUSION
■
1
61.05; H, 6.83; N, 6.78. Found: C, 60.71; H, 6.64; N, 6.45. H NMR
We have demonstrated for the first time the possibility of
substituting the pyrrole moiety in a pyrrole-functionalized
germylene with SPh and SePh groups that involves GeII−Nring
bond cleavage. Thus, by reacting the N-germylenepyrrole
complex 2 with thiophenol and selenophenol, the first examples
of germylene thiophenoxide (3) and selenophenoxide (4)
complexes have been obtained, respectively. Further, we have
found that the aforementioned substitution of the pyrrole
moiety is possible even when it is bonded to the Ge(IV) center
in LGe(E)NC4H4 compounds that have a polar GeE bond.
Accordingly, when N-germaacylpyrrole complexes with a polar
GeE bond (E = S (5), Se (6)) were reacted with thiophenol
and selenophenol, the first examples of germa thioester and
selenoester complexes (7−10) were obtained. Preparation of
the silicon analogues of compounds 2, 5, and 6 and
examination of their amenability to similar substitution
reactions are being pursued currently in our laboratory.
3
(300 MHz, CDCl3): δ 1.02 (d, JHH = 1.5 Hz, 6H, CH(CH3)2), 1.04
3
(d, JHH = 1.2 Hz, 6H, CH(CH3)2), 2.12−2.26 (m, 2H, CH(CH3)2),
3.33 (dd, JHH = 13.5, 5.7 Hz, 2H, CH2), 3.42 (dd, JHH = 13.5, 8.4 Hz,
2H, CH2), 6.31−6.38 (m, 3H, CHring), 6.84−7.00 (m, 5H, CHring),
7.12 (d, 3JHH = 6.9 Hz, 2H, CHring). 13C{1H} NMR (75 MHz, CDCl3):
δ 21.19 (CH(CH3)2), 21.42 (CH(CH3)2), 27.58 (CH(CH3)2), 54.03
(CH2), 114.58 (tp), 121.02 (tp), 124.98 (Ph), 127.87 (Ph), 134.47
(tp), 136.68 (Ph), 138.07 (Ph), 161.14 (tp).
Synthesis of (Bui2ATI)GeSePh (4). To a solution of compound 2
(0.80 g, 2.16 mmol) in toluene (15 mL) was added selenophenol (0.24
mL, 2.26 mmol) at 0 °C, and the mixture was stirred for 7 h at room
temperature. Removal of the volatiles under reduced pressure resulted
in an analytically pure sample of compound 4 as a red solid. Single
crystals of compound 4 were obtained by cooling its hexane solution at
−40 °C. Yield: 0.96 g (2.10 mmol, 97%). Mp: 71 °C. Anal. Calcd for
C21H28GeN2Se (M = 460.02): C, 54.82; H, 6.13; N, 6.09. Found: C,
54.70; H, 5.82; N, 5.78. 1H NMR (300 MHz, C6D6): δ 0.77 (d, 3JHH
=
3
6.6 Hz, 6H, CH(CH3)2), 0.88 (d, JHH = 6.6 Hz, 6H, CH(CH3)2),
1.97−2.11 (m, 2H, CH(CH3)2), 3.08 (dd, JHH = 13.5, 5.4 Hz, 2H,
CH2), 3.38 (dd, JHH = 13.5, 8.4 Hz, 2H, CH2), 5.95−6.03 (m, 3H,
EXPERIMENTAL SECTION
■
3
CHring), 6.50 (t, JHH = 9.6 Hz, 2H, CHring), 6.86−6.88 (m, 3H,
The synthesis and handling of air- and moisture-sensitive compounds
were performed under a dry dinitrogen atmosphere using either
standard Schlenk or glovebox techniques (Jacomex (GP Concept)-T2
workstation). Solvents for synthesis were dried by following the
CHring), 7.63−7.66 (m, 2H, CHring). 13C{1H} NMR (75 MHz, C6D6):
δ 20.98 (CH(CH3)2), 21.29 (CH(CH3)2), 27.69 (CH(CH3)2), 54.15
(CH2), 114.90 (tp), 120.60 (tp), 125.68 (Ph), 128.25 (Ph), 132.56
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Organometallics
Article
(Ph), 136.66 (tp), 137.14 (Ph), 161.32 (tp). 77Se{1H} NMR (57
MHz, C6D6): δ 329.74 (Ge−Se).
50.93; H, 5.81; N, 5.58. 1H NMR (300 MHz, CDCl3): δ 1.06 (d, 3JHH
= 6.6 Hz, 6H, CH(CH3)2), 1.10 (d, JHH = 6.6 Hz, 6H, CH(CH3)2),
3
Synthesis of (Bui2ATI)Ge(S)NC4H4 (5). To a solution of
compound 2 (1.00 g, 2.70 mmol) in toluene (20 mL) was added
elemental sulfur (0.08 g, 2.70 mmol) at room temperature, and the
mixture was stirred for 4 h. Removal of the volatiles under reduced
pressure led to the formation of a yellow solid. The solid was washed
with cold hexane (5 mL) and dried to give an analytically pure sample
of compound 5 as a yellow solid. Keeping a saturated solution of
compound 5 in THF at −40 °C afforded single crystals suitable for X-
ray diffraction studies. Yield: 1.02 g (2.54 mmol, 93.6%). Mp: 108 °C.
Anal. Calcd for C19H27GeN3S (M = 402.12): C, 56.75; H, 6.77; N,
10.45. Found: C, 56.59; H, 6.50; N, 10.16. 1H NMR (300 MHz,
CDCl3): δ 0.78 (d, 3JHH = 6.6 Hz, 6H, CH(CH3)2), 0.93 (d, 3JHH = 6.6
2.32−2.46 (m, 2H, CH(CH3)2), 3.50 (dd, JHH = 13.8, 6.3 Hz, 2H,
CH2), 3.62 (dd, JHH = 13.8, 8.1 Hz, 2H, CH2), 6.71−6.81 (m, 3H,
CHring), 6.88−6.94 (m, 3H, CHring), 7.11−7.14 (m, 2H, CHring), 7.25−
7.32 (m, 2H, CHring). 13C{1H} NMR (75 MHz, CDCl3): δ 21.21
(CH(CH3)2), 21.55 (CH(CH3)2), 27.78 (CH(CH3)2), 53.25 (CH2),
116.23 (tp), 125.53 (tp), 127.49 (Ph), 128.39 (Ph), 131.81 (Ph),
134.89 (Ph), 138.08 (tp), 157.53 (tp). 77Se{1H} NMR (57 MHz,
CDCl3): δ −253.83 (GeSe).
Synthesis of (Bui2ATI)Ge(S)SePh (9). To a solution of compound
5 (0.30 g, 0.74 mmol) in toluene (15 mL) was added selenophenol
(0.08 mL, 0.78 mmol) at room temperature, and the mixture was
stirred for 10 h. All of the volatiles were then removed under reduced
pressure; the resultant residue was washed with hexane (2 mL) and
dried under reduced pressure to give an analytically pure sample of
compound 9 as a yellow solid. Slow diffusion of hexane into a solution
of 9 in toluene at room temperature afforded single crystals suitable for
X-ray diffraction studies. Yield: 0.35 g (0.71 mmol, 95%). Mp: 96 °C.
Anal. Calcd for C21H28GeN2SSe (M = 492.09): C, 51.25; H, 5.73; N,
5.69. Found: C, 51.06; H, 5.88; N, 5.65. 1H NMR (300 MHz, C6D6): δ
Hz, 6H, CH(CH3)2), 2.00−2.14 (m, 2H, CH(CH3)2), 3.42 (dd, JHH
=
14.1, 7.8 Hz, 2H, CH2), 3.70 (dd, JHH = 13.8, 6.9 Hz, 2H, CH2), 6.22
(s, 2H, CHpyrrole), 6.76 (s, 2H, CHpyrrole), 7.01−7.08 (m, 1H, CHring),
3
7.16 (d, JHH = 11.1 Hz, 2H, CHring), 7.52−7.58 (m, 2H, CHring).
13C{1H} NMR (75 MHz, CDCl3): δ 20.38 (CH(CH3)2), 21.07
(CH(CH3)2), 28.03 (CH(CH3)2), 53.77 (CH2), 110.62 (pyrrole),
117.34 (tp), 123.06 (pyrrole), 126.54 (tp), 139.09 (tp), 157.46 (tp).
Synthesis of (Bui2ATI)Ge(Se)NC4H4 (6). To a solution of
compound 2 (1.00 g, 2.70 mmol) in toluene (20 mL) was added
selenium powder (0.21 g, 2.70 mmol) at room temperature, and the
mixture was stirred for 6 h. Then, the reaction mixture was filtered
through a G4 frit. Removal of the volatiles from the filtrate under
reduced pressure resulted in a solid residue. The residue was washed
with cold hexane (5 mL) and dried under reduced pressure to yield an
analytically pure sample of compound 6 as a yellow solid. Yield: 1.05 g
(2.34 mmol, 86.5%). Mp: 149 °C. Anal. Calcd for C19H27GeN3Se (M
= 449.04): C, 50.82; H, 6.06; N, 9.36. Found: C, 50.57; H, 5.94; N,
3
3
0.79 (d, JHH = 6.6 Hz, 6H, CH(CH3)2), 1.06 (d, JHH = 6.6 Hz, 6H,
CH(CH3)2), 2.26−2.40 (m, 2H, CH(CH3)2), 3.22 (dd, JHH = 13.8, 6.3
Hz, 2H, CH2), 3.60 (dd, JHH = 13.8, 8.7 Hz, 2H, CH2), 6.04 (t, 3JHH
=
9.3 Hz, 1H, CHring), 6.14 (d, 3JHH = 11.1 Hz, 2H, CHring), 6.46 (t, 3JHH
= 10.2 Hz, 2H, CHring), 6.60−6.65 (m, 3H, CHring), 7.28 (d, 3JHH = 6.3
Hz, 2H, CHring). 13C{1H} NMR (75 MHz, C6D6): δ 20.96
(CH(CH3)2), 21.43 (CH(CH3)2), 27.81 (CH(CH3)2), 53.14 (CH2),
115.40 (tp), 124.00 (tp), 127.30 (Ph), 136.84 (Ph), 137.39 (tp),
157.21 (tp). 77Se{1H} NMR (57 MHz, C6D6): δ 257.76 (Ge−Se).
Synthesis of (Bui2ATI)Ge(Se)SePh (10). To a solution of
compound 6 (0.32 g, 0.71 mmol) in toluene (15 mL) was added
selenophenol (0.08 mL, 0.75 mmol) at room temperature, and the
mixture was stirred for 10 h. Removal of volatiles under reduced
pressure resulted in a crude product which was washed with hexane (3
mL) and dried under reduced pressure to give an analytically pure
sample of compound 10 as a yellow solid. Single crystals of compound
10 were obtained by employing the same method used for compound
9. Yield: 0.33 g (0.61 mmol, 92%). Mp: 105 °C. Anal. Calcd for
C21H28GeN2Se2 (M = 538.98): C, 46.79; H, 5.24; N, 5.20. Found: C,
1
3
9.06. H NMR (300 MHz, CDCl3): δ 0.76 (d, JHH = 6.6 Hz, 6H,
3
CH(CH3)2), 0.93 (d, JHH = 6.6 Hz, 6H, CH(CH3)2), 2.00−2.13 (m,
2H, CH(CH3)2), 3.42 (dd, JHH = 14.1, 7.8 Hz, 2H, CH2), 3.71 (dd,
JHH = 14.1, 6.9 Hz, 2H, CH2), 6.23 (s, 2H, CHpyrrole), 6.76 (s, 2H,
3
CHpyrrole), 7.00−7.04 (m, 1H, CHring), 7.14 (d, JHH = 11.4 Hz, 2H,
CHring), 7.51−7.58 (m, 2H, CHring). 13C{1H} NMR (75 MHz,
CDCl3): δ 20.34 (CH(CH3)2), 21.13 (CH(CH3)2), 28.05 (CH-
(CH3)2), 53.71 (CH2), 110.50 (pyrrole), 117.40 (tp), 123.08
(pyrrole), 126.58 (tp), 138.94 (tp), 157.53 (tp). 77Se{1H} NMR (57
MHz, CDCl3): δ −436.60 (GeSe).
46.66; H, 5.10; N, 5.03. 1H NMR (300 MHz, C6D6): δ 0.81 (d, 3JHH
=
3
Synthesis of (Bui2ATI)Ge(S)SPh (7). To a solution of compound
5 (0.50 g, 1.24 mmol) in toluene (30 mL) was added thiophenol (0.14
mL, 1.36 mmol) at room temperature, and the mixture was stirred for
12 h. All the volatiles were then removed under reduced pressure, the
resultant residue was washed with hexane (5 mL), and dried under
reduced pressure to result in an analytically pure sample of compound
7 as a yellow solid. Keeping a saturated solution of compound 7 in
THF at −40 °C afforded single crystals suitable for X-ray diffraction
studies. Yield: 0.52 g (1.16 mmol, 94%). Mp: 145 °C. Anal. Calcd for
C21H28GeN2S2 (M = 445.23): C, 56.65; H, 6.34; N, 6.29. Found: C,
56.81; H, 6.32; N, 6.10. 1H NMR (300 MHz, CDCl3): δ 1.03 (d, 3JHH
6.6 Hz, 6H, CH(CH3)2), 1.06 (d, JHH = 6.6 Hz, 6H, CH(CH3)2),
2.30−2.44 (m, 2H, CH(CH3)2), 3.26 (dd, JHH = 13.8, 6.3 Hz, 2H,
CH2), 3.61 (dd, JHH = 13.5, 8.4 Hz, 2H, CH2), 6.05−6.12 (m, 1H,
3
CHring), 6.17 (d, JHH = 11.1 Hz, 2H, CHring), 6.48−6.54 (m, 2H,
3
CHring), 6.60−6.68 (m, 3H, CHring), 7.28 (d, JHH = 6.3 Hz, 2H,
CHring). 13C{1H} NMR (75 MHz, C6D6): δ 20.98 (CH(CH3)2), 21.57
(CH(CH3)2), 27.85 (CH(CH3)2), 53.03 (CH2), 115.70 (tp), 124.34
(tp), 127.43 (Ph), 128.50 (Ph), 136.77 (Ph), 137.39 (tp), 157.38 (tp).
77Se{1H} NMR (57 MHz, C6D6): δ −165.49 (GeSe), 288.15 (Ge−
Se).
X-ray Data Collection for Compounds 3−5 and 7−10. Single
crystals of the title compounds were each coated with a layer of
cryoprotectant and then mounted on a glass fiber. The data for these
compounds were collected on a Bruker SMART APEX CCD
diffractometer with a three-axis goniometer.18 SAINT and SADABS
were used for data integration and empirical absorption correction,
respectively.19 Using the crystallographic software SHELXTL, the
structures of these compounds were solved by direct methods and
refined by full matrix least squares on F2.20 All of the non-hydrogen
atoms were anisotropically refined. The positions of the hydrogen
atoms were calculated using a riding model and refined isotropically.
Important crystallographic details pertaining to compounds 3−5 and
7−10 are provided in Table S1 (see the Supporting Information).
3
= 6.6 Hz, 6H, CH(CH3)2), 1.08 (d, JHH = 6.6 Hz, 6H, CH(CH3)2),
2.28−2.41 (m, 2H, CH(CH3)2), 3.47 (dd, JHH = 13.5, 6.3 Hz, 2H,
CH2), 3.59 (dd, JHH = 13.5, 8.1 Hz, 2H, CH2), 6.70−6.80 (m, 3H,
CHring), 6.90 (b, 3H, CHring), 7.10 (b, 2H, CHring), 7.22−7.30 (m, 2H,
CHring). 13C{1H} NMR (75 MHz, CDCl3): δ 21.22 (CH(CH3)2),
21.42 (CH(CH3)2), 27.79 (CH(CH3)2), 53.39 (CH2), 116.10 (tp),
125.46 (tp), 127.45 (Ph), 128.36 (Ph), 131.16 (Ph), 134.96 (Ph),
138.21 (tp), 157.42 (tp).
Synthesis of (Bui2ATI)Ge(Se)SPh (8). To a solution of compound
6 (0.50 g, 1.11 mmol) in toluene (20 mL) was added thiophenol (0.12
mL, 1.22 mmol) at room temperature, and the mixture was stirred for
12 h. After removal of the volatiles under reduced pressure, the residue
obtained was washed with hexane (5 mL) and dried under reduced
pressure to result in an analytically pure sample of compound 8 as a
yellow solid. Single crystals of compound 8, suitable for X-ray
diffraction studies, were obtained from its saturated solution in THF at
−40 °C. Yield: 0.52 g (1.05 mmol, 96%). Mp: 101 °C. Anal. Calcd for
C21H28GeN2SSe (M = 492.13): C, 51.25; H, 5.73; N, 5.69. Found: C,
ASSOCIATED CONTENT
■
S
* Supporting Information
Crystallographic data for compounds 3−5 and 7−10 as CIF
files, crystal data and structure refinement parameters for
3835
dx.doi.org/10.1021/om4003158 | Organometallics 2013, 32, 3830−3836

Organometallics
Article
Rima, G.; Gornitzka, H.; Miqueu, K.; Barrau, J. Organometallics 2003,
compounds 3−5 and 7−10 (Table S1), GeE (E = S, Se)
bond lengths in germanium(IV) compounds (Table S2), and
molecular structure of (Bui2ATI)GeSePh (4), (Bui2ATI)Ge-
(S)SPh (7), and (Bui2ATI)Ge(Se)SePh (10) (Figures S1−S3,
respectively). This material is available free of charge via the
Internet at http://pubs.acs.org.
22, 1106. (p) Ding, Y.; Ma, Q.; Roesky, H. W.; Uson
M.; Schmidt, H.-G. Dalton Trans. 2003, 1094.
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́
, I.; Noltemeyer,
Keller, S.; Pottgen, R.; Krossing, I.; Jones, C. Angew. Chem., Int. Ed.
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2012, 51, 9557.
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2008, 130, 9640.
AUTHOR INFORMATION
Corresponding Author
*S.N.: tel, +91-11-2659 1523; fax, +91-11-2658 1102; e-mail,
sisn@chemistry.iitd.ac.in.
(7) (a) Li, J.; Hermann, M.; Frenking, G.; Jones, C. Angew. Chem.,
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Driess, M. Chem. Commun. 2012, 48, 12198. (c) Singh, A. P.; Roesky,
H. W.; Carl, E.; Stalke, D.; Demers, J.-P.; Lange, A. J. Am. Chem. Soc.
2012, 134, 4998. (d) Choong, S. L.; Schenk, C.; Stasch, A.; Dange, D.;
Jones, C. Chem. Commun. 2012, 48, 2504. (e) Katir, N.; Matioszek, D.;
Ladeira, S.; Escudie, J.; Castel, A. Angew. Chem., Int. Ed. 2011, 50,
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■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This paper is dedicated to Prof. Mitsuo Kira on the occasion of
his 70th birthday. S.K. thanks the Council of Scientific and
Industrial Research (CSIR)-New Delhi, India, for research
fellowships (JRF and SRF). S.N. thanks the Department of
Science and Technology (DST)-New Delhi, India, for financial
support (SR/S1/IC-23/2008). Further, S.N. thanks the DST-
FIST for providing financial support to the Department of
Chemistry, IIT Delhi, New Delhi, India, for establishing the
single-crystal X-ray diffractometer and ESI-MS facilities.
(10) Bibal, C.; Mazier
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