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Polymorphism of lead(II) benzenethiolate: a noncentrosymmetricnew allotropic form of Pb(SPh)2†

Michel Giffard,*a Nicolas Mercier,a Benoıt Gravoueille,a Emilie Ripaud,a Jerome Lucb and Bouchta Sahraouib

Received 28th February 2008, Accepted 16th April 2008

First published as an Advance Article on the web 13th May 2008

DOI: 10.1039/b803434f

Depending upon its conditions of crystallization, lead(II) benzene-

thiolate can exist in two forms: a low-temperature centrosymmetric

phase a-Pb(SPh)2 which can be converted by heating into the

noncentrosymmetric, 2nd order NLO strongly active and room

temperature metastable b-Pb(SPh)2 phase, thus affording an

example of transition towards noncentrosymmetry induced by a rise

of temperature.

The present work originates from the search for active compounds

for second-order nonlinear optics (NLO).1 Let us recall that only

materials which possess a macroscopic noncentrosymmetric struc-

tural arrangement can display such an activity; then the magnitude of

the optical response will be correlated with the importance of the

polarizability of the compound. In this regard, due to the presence of

a polarizable sulfur atom attached to a conjugated ring, aromatic

thiolates are good candidates for NLO properties2 and it can be

hoped that their association with transition or heavy metals cations in

arenethiolates M(SAr)x or arenethiolato complexes [M(SAr)x]y� still

lead to the enhancement of these properties.3 We will be interested

here in the compound Pb(SPh)2 (lead(II) benzenethiolate).

The crystal structure of Pb(SPh)2 was determined by A. D. Rae

et al.4 and found to be centrosymmetric. This result, however, was in

apparent contradiction with until now unpublished experiments,

performed in our laboratory, in which some samples of Pb(SPh)2

indeed displayed 2nd order NLO activity.

Trying to understand the reasons of this apparent discrepancy, we

repeated, at first, the experiments performed by Rae et al.:4‡ the

metathesis reaction between stoichiometric amounts of lead(II)

acetate and benzenethiol, conducted in a refluxing ethanol/water

mixture, led to the quick precipitation of Pb(SPh)2 in the form of

a microcrystalline powder5 (yield: 98%); in a second step, this powder

could be recrystallized by prolonged contact (3 days at room

temperature), without stirring, with a methanolic solution of benze-

nethiol and sodium benzenethiolate. The mechanism of this recrys-

tallization involves dissolution of the microcrystalline powder of

Pb(SPh)2 through formation of a soluble complex (probably Na+,

aUniversite d’Angers, CNRS, Laboratoire de Chimie et IngenierieMoleculaire d’Angers, UMR 6200, 2, boulevard Lavoisier, Angers,49045, France. E-mail: michel.giffard@univ-angers.fr; Fax: +33241735405; Tel: +33 241735404bLaboratoire des Proprietes Optiques des Materiaux et Applications,UMR-CNRS 6136, Universite d’Angers, 2, boulevard Lavoisier, Angers,49045, France

† Electronic supplementary information (ESI) available: Syntheticprocedures; crystallographic data parameters; DSC analysis; SHGexperiments. CCDC reference numbers 670193 and 684203. For ESIand crystallographic data in CIF or other electronic format see DOI:10.1039/b803434f

968 | CrystEngComm, 2008, 10, 968–971

[Pb(SPh)3]�)4,6 and then its slow and partial recrystallization in the

form of platelets suitable for single-crystal X-ray analysis (recrystal-

lization yield: 66%):

PbðSPhÞ2

ðsolidÞ þ Naþ; PhS�

ðsolutionÞ %Naþ; ½PbðSPh3Þ��

ðsolutionÞ (1)

The results of this X-ray analysis were in agreement with those of

Rae et al.:4x the material obtained in this way, which we will call

a-phase, belongs to the centrosymmetric Pmcn space group.

Furthermore, the X-ray powder diffractograms of several samples of

a-phase could be fully indexed using the parameters of this

centrosymmetric Pmcn unit cell, indicating the homogeneity of these

batches (Fig. 1).

On the contrary, the diffractogamm of the initally precipitated

microcrystalline powder of Pb(SPh)2, recorded before the recrystal-

lization step, cannot be satisfactorily indexed using the parameters of

this Pmcn a-phase (Fig. 1); we will see that this is due to the fact that

unrecrystallized Pb(SPh)2 exists as a different allotropic form

(b-phase).

However, this initial powder of the b-phase was not convenient for

single-crystal structure determination; the way to obtain suitable

crystals of the b-phase came from examination of the behaviour of

both phases by differential scanning calorimetry (DSC). As can be

seen in Fig. 2, the unrecrystallized powder (b-phase) displayed only

one endothermic peak at 201 �C, which corresponds to its melting

point, but the recrystallized centrosymmetric a-phase exhibited,

before melting, another endothermic accident at 146 �C. The most

obvious explanation for this latter peak appeared to assign it to

a transition from a-phase to b-phase; furthermore, in the conditions

used, this transition was not reversible on cooling.

Consequently, we heated overnight, in a sealed tube under

nitrogen, a sample of a-phase at 175 �C, i.e. between the phase

transition and the melting point, and then cooled it rapidly down to

room temperature. This allowed in situ transformation of the crystals

while conserving their size and integrity.

The single-crystal X-ray analysis of the b-phase sample thus

obtained did indicate that it had become different from a-phase,

belonging now to the noncentrosymmetric P21ca space group.

Furthermore, the powder X-ray diffractograms of both this b-phase

sample and of the b-phase powder obtained by the initial metathesis

reaction, without any further transformation, could be fully indexed

using the parameters of this P21ca cell (Fig. 1), showing the

crystallographic identity and the homogeneity of these samples.‡xThe structures of the two phases are compared in Fig. 3 and 4.

From the point of view of chemical bonding, both of them contains

lead atoms linked by strong covalent bonds to three sulfur atoms,

leading to extended 1D polymers in the c direction (Fig 3). However,

taking into account the Pb/S secondary bonds, especially the one

This journal is ª The Royal Society of Chemistry 2008

Fig. 1 Theoretical (calculated from single crystals XRD data) and experimental XRPD patterns of a-Pb(SPh)2 and b-Pb(SPh)2. b-Pb(SPh)2 can be

prepared either by reaction in solution (exp1), or by heating a sample of the a-phase at 175 �C (exp2).

Fig. 2 Results of DSC experiments carried out on samples of

a-Pb(SPh)2 (two endothermic peaks, lower curve) and b-Pb(SPh)2 (upper

curve).

Fig. 3 View of the structures along b. (a) a-Phase, one coordination

polymer along c is drawn without disorder (right) showing the primary

bonding around Pb, Pb–S1A: 2.556(2) A, Pb–S1A#1: 2.730(2) A, Pb–

S1B#2: 2.805(3) A, and the Pb–S intra-polymer secondary bonding, Pb/S1B#3: 3.444(1)A, #1: x, 0.5 � y, 0.5 + z; #2: 1.5 � x, y, z; #3: 1.5 � x,

0.5 � y, �0.5 + z. (b) b-Phase, Pb–S1: 2.648(2) A, Pb–S2: 2.660(2) A,

Pb–S2#1: 2.861(2) A and Pb/S1#2: 3.474(4) A, #1: x, 1 � y, �0.5 + z;

#2: x, 1 � y, 0.5 + z.

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which occurs between polymeric chains, the structures may be better

described as layered (Fig. 4). In the structure of the a-phase, the

apparently disordered chains are, as obviously deduced from chem-

ical considerations of bond distances and bond angles, the result of

a statistical disorder of two well-ordered chains (Fig. 3a). Then, two

consecutive chains in layers can display the same orientation (Fig. 4a

left, acentric situation) or different orientations (Fig. 4a right,

symmetrical situation), the interchain Pb/S secondary bond

distance being of 3.448(1) A in both cases. We may also anticipate

that the whole layers are built from either one kind of component, or

both. Anyway, the transformation of the a-phase by heating leads to

the well ordered structure of b-Pb(SPh)2 in which the polymer layout

in the layers corresponds to the acentric situation (Fig. 4b). The most

striking differences between both structures are, on one hand, the

relative position of two consecutive layers, as depicted in Fig. 4 and,

on the other hand, the length of the Pb/S interchain bond distance

which increases from 3.448(1) A (a-phase) to 3.704(3) A (b-phase).

This relative bonding’s deficit in the b-phase is in good accordance

with its lower compactness (Vb-cell ¼ 1190 A3; Va-cell ¼ 1159 A3);

especially a difference of 4.5% is observed for the b parameter which

corresponds to the direction of the interchain Pb/S contacts

(bb-cell ¼ 6.0119 A; ba-cell ¼ 5.7437 A).

This journal is ª The Royal Society of Chemistry 2008

The 2nd order NLO abilities of the two forms of Pb(SPh)2 were

evaluated through the second harmonic generation (SHG) technique

according to the Kurtz and Perry powder method,7 using a near

infrared laser irradiation (lu ¼ 1064 nm). The results were in full

agreement with the structural characteristics of both phases: the

centrosymmetric a-phase was inactive but the noncentrosymmetric

b-phase afforded, indeed, an intense SHG signal (l2u ¼ 532 nm),

CrystEngComm, 2008, 10, 968–971 | 969

Fig. 4 View of the structures along c. (a) a-Phase, disorder not shown,

two consecutive 1D polymers in the bc plane belong either to the same

component (left) or to the two different components (right, centrosym-

metric situation), inter-polymer Pb–S interactions: Pb–S1B#4 and Pb#5–

S1B#6: 3.448(1) A, #4: x, 1 + y, z; #5: �0.5 + x, 1 � y, �z + 1; #6: 1 � x,

�y, �z + 1. (b) b-Phase, inter-polymer Pb–S interaction: Pb#3–S1:

3.704(3) A, #3: x, 1 + y, z.

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reaching a value of 20% relative to the reference, which was the very

active 3-methyl-4-nitropyridine-1-oxide (POM)1; this means, for

example, that the NLO efficiency of b-Pb(SPh)2 is approximately the

same as the one of commonly used commercial NLO material, such

as potassium titanyl phosphate (KTP).8

The observed phenomena can be interpreted as follows: at room

temperature, the centrosymmetric a-phase is thermodynamically

more stable than the noncentrosymmetric b-phase, however, the

formation of the b-phase is kinetically favoured; that is why this

metastable b-phase quickly precipitates during the synthesis of

Pb(SPh)2 by the metathesis reaction and then can be converted into

an a-phase during the slow recrystallization step. At temperatures

higher than 146 �C, the order of thermodynamic stabilities is inverted,

which explains the possibility of converting a-phase into b-phase by

heating, the observed apparent irreversibility of this process being

again due to kinetic factors (negligible speed of the reverse reaction).

Now, concerning the thermodynamic reasons for this inversion of the

stability’s order upon heating, it can be seen from DSC that the

conversion of the a-phase into a b-phase is endothermic (Da/bH> 0)

and thus favoured by an increase of temperature according to Le

Chatelier’s principle; this also means that the entropy change for this

transition is positive (Da/bS > 0) so that the free energy of transition

Da/bG decreases with temperature, in algebraic value.9 The endo-

thermic nature of Da/bH can be explained by the fact that, on the

whole, the Pb–S bonding is weaker in the b-phase than in thea-phase:

using the bond-valence model,10 we calculated that VPb, the total

bond valence of lead in a-Pb(SPh)2 is 2.28, the three primary Pb–S

bonds contributing by a value of 2.10 to this total and the two weak

secondary Pb.S bonds accounting for the remaining 0.18, while in

970 | CrystEngComm, 2008, 10, 968–971

b-Pb(SPh)2 the corresponding figures are only 2.07, 1.94 and 0.13,

respectively.{ On the other hand, the fact that the entropy of the

b-phase is greater than the one of the a-phase may seem, at first sight,

somewhat paradoxical since the a-phase appears crystallographically

disordered while the b-phase is not; however, it is in good accordance

with the greater molar volume of the b-phase, this lower compactness

of the b-phase being also, as quoted above, due to the relative

weakness of bonding and especially of the interchain Pb.S contacts.

To conclude, the noncentrosymmetric b-phase of Pb(SPh)2 is

rather simply obtained and may find applications in NLO but also in

other fields, such as ferroelectricity,11 in which noncentrosymmetric

materials are also required. Furthermore, this example also invokes

great interest from the point of view of crystal engineering: although

polymorphism is a widespread phenomenon, the conditions neces-

sary to its occurrence are far from being fully rationalized by general

rules12; thus we observe here a transition from a centrosymmetric

low-temperature form to a noncentrosymmetric high-temperature

phase, this phenomenon is not unprecedented13 even though the rise

of temperature generally favours transitions towards centrosymmetry

and increase of symmetr,y in general, so that the reverse behaviour is

much more frequent.11,14

Notes and references

‡ It is emphasised that all the experiments reported here were performedat least two times and found to be reproducible unless specifically stated.Full experimental details are given in the Electronic SupplementaryInformation (ESI) file.

x These authors describe the structure of a-Pb(SPh)2, at first, in a disor-dered orthorhombic Pmcn cell with parameters a¼ 27.03 A, b¼ 5.734 A,c ¼ 7.4387 A; then a detailed analysis of substructure patterns leads themto a 4-fold larger and partially ordered monoclinic C1121/d cell withparameters a0 ¼ 2a¼ 54.06 A, b0 ¼ 2b¼ 11.468 A, c0 ¼ c¼ 7.4387 A, g¼90.0�4.4 As for us, we only solved the structure of a-Pb(SPh)2 in the smalldisordered Pmcn cell.Crystal data for a-Pb(SPh)2: C12H10PbS2, M ¼ 425.54, orthorhombic,a ¼ 27.0798(13) A, b ¼ 5.7437(3) A, c ¼ 7.452(3) A, V ¼ 1159.0(4) A3,space group Pmcn, Z ¼ 4, calculated density 2.439 g cm�3, crystaldimensions (mm): 0.20 � 0.12 � 0.08, T ¼ 293 K, m ¼ 14.88 mm�1,2qmax ¼ 58�, 11520 measured reflections of which 1558 were unique(R(int) ¼ 0.063) and 987 had I/s(I) > 2. The refinements of positions andanisotropic thermal motion parameters of the non-H atoms, converge toR(F) ¼ 0.036 (987 reflections, 79 parameters), wR2(F2) ¼ 0.063 (all data),GOF on F2 1.03, Drmax ¼ 1.07 e A�3.Crystal data for b-Pb(SPh)2: C12H10PbS2, M ¼ 425.54, orthorhombic,a ¼ 27.1492(11) A, b ¼ 6.0119(2) A, c ¼ 7.2935(3) A, V ¼ 1190.43(8) A3,space group P21ca, Z ¼ 4, calculated density 2.374, crystal dimensions(mm): 0.20 � 0.12 � 0.10, T¼ 293 K, m¼ 14.48 mm�1, 2qmax ¼ 55�, 10365measured reflections of which 2205 were unique (R(int) ¼ 0.045) and 1822had I/s(I) > 2. The refinements of positions and anisotropic thermalmotion parameters of the non-H atoms, converge to R(F) ¼ 0.027 (1822reflections, 136 parameters), wR2(F2) ¼ 0.045 (all data), GOF on F2 1.03,Drmax ¼ 0.72 e A�3.

For both crystals, data were collected on a Nonus KAPPA CCDdiffractometer, graphite-monochromated Mo Ka radiation (l ¼ 0.71073A). The intensities were corrected for Lorentz-polarization effects, as wellas for absorption effect (multi-scan method). The structure were solvedby direct methods, and refined by full-matrix least-squares routinesagainst F2 using the Shelxl97 package. The hydrogen atoms were treatedwith a riding model. CCDC 684203 and 670193 contain the supplemen-tary crystallographic data for the a- and b- phases respectively.

{ The formula used was VPb ¼P

exp[(RPbS – ri)/0.37] where RPbS ¼2.55 A is the bond valence parameter for the lead/sulfur atomic pair10b

and the ri terms are the various Pb–S bond distances listed in Fig. 3 and 4.

1 D. S. Chemla, and J. Zyss, Nonlinear Optical Properties of OrganicMolecules and Crystals, Academic Press, Boston, 1987;S. R. Marder and J. W. Perry, Adv. Mater., 1993, 5, 804.

This journal is ª The Royal Society of Chemistry 2008

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K. Singh, K. Lahiri, S. Kotha, R. Boese and G. R. Desiraju,Angew. Chem., Int. Ed., 2004, 43, 1149; (b) J. L. Atwood,L. J. Barbour, G. O. Lloyd and P. K. Thallapally, Chem. Commun.,2004, 922; (c) L. Perez-Garcia and D. B. Amabilino, Chem. Soc.Rev., 2007, 36, 941.

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