A new stable monomeric lead(ii) dithiolate Pb(SCH2CH 2NMe2)2: An interplay between a dynamic flip-flop process in solution and conformational isomerism in the solid-state

Article abstract of DOI:10.1039/c0dt00783h

The structure and dynamic behaviour of monomeric lead(ii) dithiolate Pb(SCH₂CH₂NMe₂)₂ are studied by multi-nuclear NMR, IR and Raman spectroscopy in solution, as well as variable-temperature Raman spectroscopy and X-ray diffraction analysis in the solid-state, revealing an unusual dynamic flip-flop process in solution and reversible conformational isomerism of the chelated five-membered rings in the solid-state.

Full text of DOI:10.1039/c0dt00783h

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A new stable monomeric lead(II) dithiolate Pb(SCH CH NMe ) : an
2
2
2 2
interplay between a dynamic “flip-flop” process in solution and
conformational isomerism in the solid-state†
a
a
b
a
a
Victor N. Khrustalev,* Rinat R. Aysin, Irina V. Borisova, Alexander S. Peregudov, Larissa A. Leites and
b
Nikolai N. Zemlyansky
Received 3rd July 2010, Accepted 27th July 2010
DOI: 10.1039/c0dt00783h
The structure and dynamic behaviour of monomeric lead(II)
dithiolate Pb(SCH
2
CH
2
NMe
2
2
) are studied by multi-nuclear
NMR, IR and Raman spectroscopy in solution, as well as
variable-temperature Raman spectroscopy and X-ray diffrac-
tion analysis in the solid-state, revealing an unusual dynamic
Scheme 1
Complex 4 is a colourless substance quite stable both in the
solid-state and in solution at room temperature under air. Only
slow decomposition to elemental lead occurs in solution upon
storage on a timescale of a few weeks. Compound 4 is highly
soluble in pyridine and toluene and slightly soluble in ethanol and
hexane.
“flip-flop” process in solution and reversible conformational
isomerism of the chelated five-membered rings in the solid-
state.
Previously, we have shown that the b-dimethylaminoethoxy
group is a suitable substituent for the stabilisation of divalent
1
In the H NMR spectrum of 4 recorded at room temperature in a
1
Group 14 compounds in the monomeric form. Furthermore,
toluene-d
8
solution, the SCH
2
CH N group signals appear as sharp
2
the M(OCH
2
CH
2
NMe
2
) (M = Ge (1), Sn (2)) species have
2
symmetrical multiplets (pseudotriplets) at 2.78 and 3.62 ppm and
those of the methyl groups at the nitrogen atoms appear as a sharp
singlet at 2.04 ppm. The C NMR spectrum contains three sharp
signals at 29.0, 45.6 and 68.2 ppm corresponding to the SCH
and CH N groups, respectively. The Pb NMR spectrum
exhibits a wide signal at 2766 ppm (Dn1/2 = 630 Hz), the respective
value of the chemical shift is rather close to those of substantially
broadened Pb signals (Dn1/2 = 17–18 kHz) observed for related
1a,2
demonstrated interesting structural and chemical properties.
Therefore, we are interested in the synthesis of analogous lead(II)
derivatives bearing the b-dimethylaminoethoxy or related ligands.
Recently, we reported the synthesis and structure of the ho-
1
3
2
,
2
07
NMe
2
2
moleptic lead(II) complex Pb(OCH
found to be a coordination polymer. Herein we report the
preparation and characterisation of its monomeric thio-analogue
2
CH
2
3
NMe
2
) (3), which was
2
2
07
Pb(SCH
2
CH
2
NMe
2
)
2
(4). As the germanium and tin congeners
5
lead(II) dithiolates (2623–2647 ppm).
In the H NMR spectrum of 4 recorded at 203 K, all the proton
1
and 2 display a dynamic “flip-flop” process in solution, the
1
analogous behaviour of 4 is of significant interest.
Complex 4 was obtained by the reaction of PbO with
1
3
signals are broad. In the C NMR spectrum of 4 at 203 K, the
carbon signals of the SCH andCH N groups are also wide (Dn1/2
6 and 50 Hz, respectively), whereas the singlet of the NMe groups
is significantly broadened (Dn1/2 = 173 Hz) (Fig. 1). Similarly to
and 2, these results could be explained by the presence of a
2
2
=
HSCH
2
CH
2
NMe
2
·HCl and NaOH, according to the procedure
4
2
4
described earlier for Pb(SCH
2
CH
2
NH
2
)
2
(5) (Scheme 1).‡
1
a
A. N. Nesmeyanov Institute of Organoelement Compounds RAS, Vavilov St.,
dynamic “flip-flop” process in solution of 4.
The characteristic NMR shifts observed for 4 are close to
those for 1 and 2. The symmetrical structure of the latter
molecules, with both five-membered rings adopting the envelope
conformations and the carbon atoms adjacent to nitrogens being
out-of-plane (rings A, conformer AA), was established by the
X-ray analysis (Fig. 2a). Based on these data as well as the solution
2
8, Moscow, 119991, Russian Federation. E-mail: vkh@xray.ineos.ac.ru;
Fax: +7 (499) 135 5085; Tel: +7 (499) 135 9343
b
1a
A. V. Topchiev Institute of Petrochemical Synthesis RAS, Leninsky Prosp.,
2
9, Moscow, 119991, Russian Federation
†
Electronic supplementary information (ESI) available: Full crystal data,
NMR and IR spectra and details of the DFT calculations. CCDC reference
numbers 764409 and 764410. For ESI and crystallographic data in CIF or
other electronic format, see DOI: 10.1039/c0dt00783h
13
Fig. 1 Signals of the SCH
2 2 2
CH NMe moiety in the C NMR spectrum of 4 at 203 K. The signal at 26.9 ppm belongs to a cyclohexane impurity.
9
480 | Dalton Trans., 2010, 39, 9480–9483
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Fig. 2 Crystal structures of (a) 1 (M = Ge) and 2 (M = Sn) at 110 K (conformer AA), (b) 4 at 100 K (conformer AB) and (c) 4 at 290 K; hydrogen atoms
are omitted for clarity; the open lines indicate the N→M coordination bonds and the dashed lines indicate the alternative minor conformation of the
five-membered ring.
6
6
Raman spectra of 1 , 2 and 4 (see below), it can be supposed that
all the molecules in question preserve the structure AA in solution.
Surprisingly, the single-crystal X-ray structural analysis of 4 at
1
00 K has established its asymmetrical structure, with both five-
membered rings being in the envelope conformations, but differing
in the positions of the capping-carbon atoms (rings A and B,
conformer AB; in the ring B, the carbon atom adjacent to sulfur
deviates from planarity) (Fig. 2b). Notably, the molecule 5 has a
structure, in which both five-membered chelated rings adopt the
distorted twist conformations due to the N–H ◊ ◊ ◊ S intermolecular
4
hydrogen bonds.
Theoretical calculations at the DFT level using the GAUS-
SIAN03 program reveal a very small difference in energy
-
1
(
<0.5 kcal mol ) between the symmetrical AA and asymmetrical
Fig. 3 Crystal packing of 4 at 100 K showing the infinite chains along the
b axis; hydrogen atoms are omitted for clarity; the dashed lines indicate
weak intermolecular Pb ◊ ◊ ◊ S interactions.
AB conformers of 4, the transformation barrier being as small as
-
1
ca. 4 kcal mol . Furthermore, the relative stabilities of the two
conformers depend on the choice of the functional (see ESI†).
Compound 4 is chiral and crystallises in the orthorhombic
s
space group P2
1
2
1
2
1
, with one monomer in the crystallographically
contribution from the n (N–C) stretching coordinate. Its lowered
independent part of the unit cell. The Pb atom is coordinated by
two S atoms (covalent bonds) and two N atoms (dative bonds),
giving a coordination number of four and a distorted trigonal
bipyramidal configuration of the central atom (the S–Pb–S bond
value, close to those observed for 1 and 2, is diagnostic for
6
the M←N coordination.
The most salient feature of the spectrum is a doublet
-
1
885/897 cm . Analogous doublets in this spectral region were
observed previously when conformational equilibrium existed due
◦
angle is 93.73(2) ). The N atoms occupy the apical positions and
are tilted away from the lead lone pair (the N→Pb←N bond angle
to a hindered rotation about the C–C bond within the CH
2
–
◦
7
is 156.33(8) ). The angle between the S–Pb–S and the N→Pb←N
CH
2
fragment. Obviously, the components of this doublet in the
◦
˚
planes is 83.16(5) . The Pb–S (2.6283(8) and 2.6370(8) A) and
spectrum of 4 correspond to two different rings A and B. The
˚
Pb←N (2.607(3) and 2.623(2) A) bond distances are close to
results of normal coordinate analysis (NCA) for the conformers
˚
-1
those found in 5 (2.633(3), 2.639(3) and 2.550(8), 2.626(9) A,
AA and AB of 4 confirm the difference of ~10 cm between the
respectively).
frequency values of this mode, which corresponds mostly to CH
2
-
1
Interestingly, crystal packing of 4 at 100 K shows the presence
of short Pb ◊ ◊ ◊ S intermolecular contacts between the Pb atom of
one molecule and the S atom of the ring B from a neighbouring
molecule, with a distance of 3.9827(7) A. Due to these contacts, in
the crystal at 100 K the molecules 4 form infinite chains along the
b axis (Fig. 3).
group rocking. The intense band at ~662 cm corresponds to
the n(C–S) stretch. According to NCA, its frequency does not
depend on the ring conformation. As can be seen in Fig. 4,
this line is a doublet at a lower temperature (93–250 K) and
becomes a singlet at a higher temperature. This fact could be
explained by the emergence of the above-mentioned weak Pb ◊ ◊ ◊ S
interactions at low temperatures, which induces the Pb–S bond
nonequivalence. The latter seems to relax upon thermal expansion
of the crystal. The modes of mixed origin with frequencies of
˚
Taking into account the assumed symmetrical AA structure of
4
in solution and its asymmetrical AB structure in the crystal at
1
00 K, we explored the temperature dependence of its Raman
-
1
and IR spectra over the interval of 93–303 K. The Raman results
are presented in Fig. 4, the IR data are similar (see ESI†). The
band at ~761 cm belongs to a normal mode with a significant
~350 and ~515 cm appeared conformationally sensitive, which is
1
-
reflected by their doubling (343/364 and 510/522 cm ). It is very
important to note that the intensity ratio in the doublets observed
-
1
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“flip-flop” process, common for 1, 2 and 4, thus could be as follows.
In the rigid structure of these molecules, two methyl groups of the
NMe fragment are nonequivalent, one of them being closer to
2
the stereochemically active electron lone pair of the metal atom
than the other. It is evident that the singlet observed in the NMR
spectra is a result of the exchange of these groups. The exchange
can be envisaged as an overturn of the CH
2
NMe fragment with
2
the cleavage of M←N coordination and the inversion of nitrogen
in the transition state. Calculated values of the activation barrier
-
1
8
to this process (12.1 kcal mol for 4) are very reasonable.
Thus, the variable-temperature Raman and IR spectra confirm
the AB structure of 4 in the crystal at low temperatures and point
to some conformational changes on heating.
With this in mind, we carried out the X-ray diffraction study of
the same single crystal of 4 at 290§ and 333 K (see ESI†). These
experiments revealed the coexistence of the two conformers AB
and AA in ratios of ~7 : 3 and ~6 : 4, respectively (Fig. 2c). Thus,
the vibrational spectroscopy and X-ray diffraction data indicate
that the rotational isomerism about the C–C bond takes place in
the solid 4.
It is notable that the unit cell parameters of the crystal change
upon heating anisotropically (from 100 to 290 K: Da = 0.0916(5),
˚
Db = 0.2948(5) and Dc = 0.1318(5) A) with the maximum dilatation
of the b parameter. The latter leads to an increase in the Pb ◊ ◊ ◊ S
˚
intermolecular distance up to 4.212(2) A. This “extra” space
facilitates the conformational transition from AB into AA upon
heating. A similar “defreezing” of the conformational degree of
freedom was observed for the solid polymer [Et
2
Si]
n
after an
Fig. 4 Temperature evolution of the Raman spectrum of solid 4.
9
analogous anisotropic increase in the unit cell parameters.
Both conformational transitions, i.e., symmetrical conformer
AA (solution) ꢀ mixture of the symmetrical AA and asymmetrical
AB conformers (250–333 K, solid) ꢀ asymmetrical conformer AB
varies with temperature, whereas only single lines in these regions
6
were observed in the spectra of solid 1 and 2.
Remarkably, in the Raman spectrum of a pyridine solution of
, the 343/364, 510/522 and 885/897 doublets, observed for solid
, appear as singlets (Fig. 5) which testifies the existence of only
one symmetrical conformer in the solution and is consistent with
the NMR spectroscopy data discussed above.
(93–250 K, solid) described for 4, are reversible and reproducible
4
4
as has been confirmed by several independent runs with different
samples. The DSC data for solid 4 did not exhibit any abrupt
changes within the interval of 223–298 K. Consequently, there is
no phase transition in the temperature range studied.
In conclusion, we have prepared and comprehensively struc-
turally characterised a new stable monomeric lead(II) dithiolate
4
displaying not only the unusual dynamic “flip-flop” process in
solution, but also the conformational isomerism in the solid-state.
In solution, contrary to the conventional “flip-flop” process, two
different enantiomers of 4 do not coexist in equilibrium, and the
Pb←N coordination bonds cleave only in the transition state.
In solid 4, temperature-dependent Raman and IR spectra (93–
3
3
03 K) as well as the X-ray diffraction study at 100, 290 and
33 K reveal the reversible rotational isomerism about the C–
C bond within the five-membered rings. DFT calculations show
the very small difference in energy between the symmetrical AA
and asymmetrical AB conformers of 4 observed in the solid-state.
Nevertheless, NMR and Raman data suggest the symmetrical
AA structure of 4 in solution. A more thorough investigation of
chemical properties of the new stable monomeric lead(II) dithiolate
4 is currently underway.
We thank the Russian Foundation for Basic Research (V. N. K.,
R. R. A. and L. A. L. grant no. 10-03-01115) and the Russian
Academy of Sciences in the framework of the program “Theoreti-
cal and experimental study of chemical bonding and mechanisms
of chemical reactions and processes” (R. R. A. and L. A. L.)
Fig. 5 Raman spectrum of pyridine solution of 4 (*- pyridine lines).
Quite unexpectedly, the solution Raman spectrum of 4 revealed
an unconventional mechanism of the “flip-flop” process for this
compound. The conventional mechanism includes the exchange
of the coordinated positions of the two ligands, with the open
form of one of them composing the intermediate state and different
8
enantiomers occuring in equilibrium. However, this process is not
realised in solution of 4 as evidenced by the presence of only one
-
1 6
line in the region of 700–800 cm . A plausible mechanism of the
9
482 | Dalton Trans., 2010, 39, 9480–9483
This journal is © The Royal Society of Chemistry 2010

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for partial financial support of this work. We are indebted to
Dr M. Buzin for the DSC measurements and Dr S. S. Bukalov for
his help in obtaining temperature-dependent Raman spectra.
§ Crystal data for 4 at 100 K: C
1 1 1
space group P2 2 2 , a = 10.3938(5) A, b = 10.7759(5) A, c = 11.4619(6) A,
3 -3 -1
8
H
20
N
2
˚
S
2
Pb (M
r
= 415.57), orthorhombic,
˚
˚
˚
V = 1283.76(11) A , Z = 4, r = 2.150 g cm , m = 13.432 mm , Bruker
c
SMART APEX-II CCD diffractometer, graphite monochromator, l(Mo-
˚
Ka) = 0.71073 A. A total of 16634 reflections were collected in the q
◦
range of 2.72–30.61 with 3889 reflections being unique (Rint = 0.0284).
An analytical absorption correction was applied based on the indicing
of crystal faces. Least-squares refinement on 122 parameters converged
Notes and references
‡
All manipulations were carried out under a purified argon atmosphere
normally with R
1
(I > 2s(I)) = 0.0185, wR
.936. CCDC 764410. Crystal data for 4 at 290 K: C
15.57), orthorhombic, space group P2
2
(all data) = 0.0379, GOF =
using the standard Schlenk technique. The commercially available solvents
were purified by conventional methods and distilled immediately prior to
0
4
1
8
H
20
N
2
S
2
Pb (M
r
=
˚
1
2
1
2
1
, a = 10.4854(8) A, b =
use. NMR spectra were recorded on Bruker AVANCE-400 and AVANCE-
˚
˚
˚
3
-3
1.0707(8) A, c = 11.5937(8) A, V = 1345.80(17) A , Z = 4, r
c
= 2.051 g cm ,
1
6
1
00 NMR spectrometers at 400.1 and 600.2 MHz ( H), 100.6 and
-1
m = 12.812 mm , Bruker SMART APEX-II CCD diffractometer, graphite
13
207
50.9 MHz ( C), and 83.71 MHz ( Pb) in toluene-d
8
. The chemical
˚
monochromator, l(Mo-Ka) = 0.71073 A. A total of 17566 reflections were
1
13
shifts for H and C NMR are indirectly referenced to TMS via the solvent
signals. The assignment of the signals in H and C NMR spectra has been
performed by the use of 2D COSY, HMQC and HMBC techniques on the
◦
collected in the q range of 2.68–30.55 with 4077 reflections being unique
1
13
(Rint = 0.0384). An analytical absorption correction was applied based on
the indicing of crystal faces. Least-squares refinement on 118 parameters
converged normally with R (I > 2s(I)) = 0.0297, wR (all data) = 0.0613,
GOF = 1.006. CCDC 764409.
207
basis of the Bruker program standard library. The chemical shift for Pb
NMR was measured with lead(II) diacetate as an external standard. The
1
2
1
accuracy of chemical shift measurements is ±0.01 ppm ( H), ±0.05 ppm
1
3
207
(
C) and ±0.2 ppm ( Pb). The Raman spectra were recorded with a
1 (a) N. N. Zemlyansky, I. V. Borisova, V. N. Khrustalev, Yu. A. Ustynyuk,
M. S. Nechaev, V. V. Lunin, J. Barrau and G. Rima, Organometallics,
2003, 22, 1675; (b) N. N. Zemlyansky, I. V. Borisova, V. N. Khrustalev,
M. Yu. Antipin, Yu. A. Ustynyuk, M. S. Nechaev and V. V. Lunin,
Organometallics, 2003, 22, 5441.
2 V. N. Khrustalev, I. A. Portnyagin, M. S. Nechaev, S. S. Bukalov and
L. A. Leites, Dalton Trans., 2007, 3489.
3 V. N. Khrustalev, I. V. Glukhov, I. V. Borisova and N. N. Zemlyansky,
Acta Cryst., 2008, A64, C412.
4 H. Fleischer and D. Schollmeyer, Inorg. Chem., 2004, 43, 5529.
5 G. G. Briand, A. D. Smith, G. Schatte, A. J. Rossini and R. W. Schurko,
Inorg. Chem., 2007, 46, 8625.
6 R. R. Aysin, L. A. Leites, S. S. Bukalov, V. N. Khrustalev, I. V. Borisova,
N. N. Zemlyansky, A. Yu. Smirnov and M. S. Nechaev, Russ. Chem.
Bull., 2010, accepted.
7 (a) N. G. Mirkin and S. Krimm, J. Phys. Chem., 1993, 97, 13887; (b) L. A.
Leites, S. S. Bukalov, A. V. Zabula, D. V. Lyubetsky, I. V. Krylova and
M. P. Egorov, Russ. Chem. Bull., 2004, 53, 33.
8 P. Jutzi, S. Keitemeyer, B. Neumann, A. Stammler and H.-G. Stammler,
Organometallics, 2001, 20, 42.
9 L. A. Leites, S. S. Bukalov, M. V. Gerasimov, I. I. Dubovik and V. S.
Papkov,, Polym. Sci., 1996, 38A, 924.
LabRAM Jobin–Yvon laser Raman spectrometer, with the exciting He–Ne
laser line of 632.8 nm. Temperature measurements were carried out using
a temperature cell Linkam THMS600. The IR spectrum was recorded
on a spectrophotometer Carl Zeiss Specord M82 for thin film prepared
by evaporation of a toluene solution of 4 onto CsI plates. The chemical
analysis was performed using a Carlo Erba EA1108 CHNS–O analyser.
Pb(SCH
2
CH
2
NMe
2
)
2
. A solution of HSCH
0.0 mmol) in 1 M NaOH (20 mL) was added to a stirred suspension
2
CH
2
NMe
2
·HCl (2.83 g,
2
of PbO (2.23 g, 10.0 mmol) in ethanol (100 mL). The mixture was refluxed
◦
for 2 h and kept at 20 C overnight. Then, the precipitate was separated
by filtration and all volatiles were removed in vacuo. The crude product
was recrystallised from hexane to afford 3.76 g (91%) of a colourless solid.
◦
Mp 110 C (from hexane). (Found: C, 23.06; H, 4.87; N, 6.67; Pb, 49.13;
S, 14.95. Calcd. for C
8
H
20
N
2
PbS
(400 MHz; toluene-d
.78 (4H, m, CH N), 3.62 (4H, m, CH
98 K) 29.0 (CH S), 45.6 (Me N), 68.2 (CH
; 298 K) 2766. IR nmax/cm- (film on CsI) 342, 362, 420, 512, 526,
64 (C–S), 758, 886, 896, 944, 994, 1030, 1038, 1054, 1098, 1124, 1058,
214, 1244, 1256, 1288, 1362, 1400, 1426, 1454, 1470. For solution and
2
: C, 23.12; H, 4.85; N, 6.74; Pb, 49.86; S,
; Me Si; 298 K) 2.04 (12H, s, NMe ),
S). d (100 MHz; toluene-d ; Me Si;
N). dPb (84 MHz; toluene-d
1
2
2
5.43%). d
H
8
4
2
2
2
C
8
4
2
2
2
8
;
1
Pb(OAc)
2
6
1
solid Raman Dn, see Figs. 4 and 5.
This journal is © The Royal Society of Chemistry 2010
Dalton Trans., 2010, 39, 9480–9483 | 9483