Elemental Sulfur and Sulfur-Rich Compounds I

199 Pages·2003·2.675 MB·English

by  Ralf Steudel, Bodo Eckert (auth.), Ralf Steudel (eds.)

TopCurrChem(2003)230:1–79
DOI10.1007/b12110
Solid Sulfur Allotropes
RalfSteudel1·BodoEckert2
1Institutf(cid:2)rChemie,Sekr.C2,TechnischeUniversit(cid:3)tBerlin,10623Berlin,Germany
E-mail:[email protected]
2FachbereichPhysik,Universit(cid:3)tKaiserslautern,67663Kaiserslautern,Germany
E-mail:[email protected]
Abstract Sulfuristheelementwiththelargestnumberofsolidallotropes.Mostofthesecon-
sistofunbranchedcyclicmoleculeswithringsizesrangingfrom6to20.Inaddition,poly-
mericallotropesareknownwhicharebelievedtoconsistofchainsinarandomcoilorheli-
cal conformation. Furthermore, several high-pressure allotropes have been characterized.
Inthischapterthepreparation,crystalstructures,physicalpropertiesandanalysisofthese
allotropesarediscussed.AbinitioMOcalculationsrevealedtheexistenceofisomericsulfur
ringswithpartlyratherunusualstructuresathightemperatures.
Keywords Sulfurhomocycles·Sulfurchains·Polymerization·Physicalproperties·
High-pressureallotropes·Crystalstructures
1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
2 AllotropesatAmbientPressure. . . . . . . . . . . . . . . . . . . . 4
2.1 Preparation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
2.1.1 AllotropesConsistingofCyclicMolecules . . . . . . . . . . . . . 4
2.1.1.1 PreparationofS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
6
2.1.1.2 PreparationofS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
7
2.1.1.3 PreparationofPureS . . . . . . . . . . . . . . . . . . . . . . . . . . 6
8
2.1.1.4 PreparationofS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
9
2.1.1.5 PreparationofS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
10
2.1.1.6 PreparationofS ·S . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
6 10
2.1.1.7 PreparationofS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
11
2.1.1.8 PreparationofS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
12
2.1.1.9 PreparationofS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
13
2.1.1.10 PreparationofS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
14
2.1.1.11 PreparationofS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
15
2.1.1.12 PreparationofS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
18
2.1.1.13 PreparationofS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
20
2.1.2 AllotropesConsistingofLongSulfurChains
(PolymericSulfur:S ,S andS ). . . . . . . . . . . . . . . . . . . 14
m y w
2.2 MolecularandCrystalStructures . . . . . . . . . . . . . . . . . . . 16
2.2.1 AllotropesConsistingofCyclicMolecules . . . . . . . . . . . . . 17
2.2.1.1 RhombohedralS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
6
2.2.1.2 AllotropesofS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
7
2.2.1.3 AllotropesofS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
8
2.2.1.3.1 Orthorhombica-S . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
8
(cid:1)Springer-VerlagBerlinHeidelberg2003
2 RalfSteudel·BodoEckert
2.2.1.3.2 Monoclinicb-S . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
8
2.2.1.3.3 Monoclinicg-S . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
8
2.2.1.4 AllotropesofS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
9
2.2.1.5 MonoclinicS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
10
2.2.1.6 TheCompoundS ·S . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
6 10
2.2.1.7 OrthorhombicS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32
11
2.2.1.8 OrthorhombicS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33
12
2.2.1.9 MonoclinicS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
13
2.2.1.10 TriclinicS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36
14
2.2.1.11 SolidS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37
15
2.2.1.12 AllotropesofS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37
18
2.2.1.13 OrthorhombicS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38
20
2.2.2 IsomersoftheSulfurHomocycles . . . . . . . . . . . . . . . . . . 39
2.2.3 AllotropesConsistingofLongChains . . . . . . . . . . . . . . . . 40
2.2.3.1 FibrousSulfur(S ) . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43
y
2.2.3.2 2ndFibrousandLaminarSulfur(S andS ) . . . . . . . . . . 45
w1 w2
2.2.3.3 PolymericSulfurinTa P S . . . . . . . . . . . . . . . . . . . . . . 49
4 4 29
2.2.4 ConcludingRemarks . . . . . . . . . . . . . . . . . . . . . . . . . . . 50
2.3 PhysicalProperties . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52
2.3.1 MeltingPoints . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52
2.3.2 ThermalBehavior . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53
2.3.3 Solubilities. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55
2.3.4 Densities. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56
2.3.5 PhotochemicalBehavior. . . . . . . . . . . . . . . . . . . . . . . . . 57
2.4 Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59
3 High-PressureAllotropes. . . . . . . . . . . . . . . . . . . . . . . . 59
3.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59
3.2 TriplePointsintheVicinityoftheMeltingCurve. . . . . . . . . 61
3.3 High-PressureStructures . . . . . . . . . . . . . . . . . . . . . . . . 62
3.3.1 General. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62
3.3.2 Photo-InducedStructuralChanges(p<20GPa). . . . . . . . . . 63
3.3.3 High-PressureHigh-TemperaturePhases(p<20GPa,T>300K) 67
3.3.4 HighPressurePhasesabove20GPa . . . . . . . . . . . . . . . . . 68
3.4 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72
ListofAbbreviations
DAC Diamondanvilcell
DSC Differentialscanningcalorimetry
MD Moleculardynamics
S Polymericsulfurusuallypreparedfromquenchedliquidsulfur
m
STP Standardtemperatureandpressureconditions
SolidSulfurAllotropes 3
1
Introduction
An allotrope of a chemical element is defined as a solid phase (of the pure
element)whichdiffersby itscrystalstructureandthereforeby itsX-raydif-
fraction pattern from the other allotropes of that element. This definition
can be extended to microcrystalline and amorphous phases which may be
characterized either by their diffraction pattern or by suitable molecular
spectra.
No other element forms more solid allotropes than sulfur. At present,
about30wellcharacterizedsulfurallotropesareknown.Thesecanbedivid-
ed into ambient pressure allotropes and high-pressure allotropes depending
ontheconditionsduringpreparation.Whilethemolecularandcrystalstruc-
tures of the ambient pressure allotropes are known in most cases, this does
not apply to all of the high-pressure forms. Therefore, in the following the
twogroupsaredescribedinseparatesectionsofthischapter.
The allotropesprepared at ambientpressurecan alsobe groupedby their
molecular structures depending on whether homocyclic rings or chains of
indefinite length arethe constituents of the particular phase. At present, the
following20crystallinephasesconsistingofringsareknown:
S ;S (a,b,g,d);S (a,b,g);S (a,b);S ;S ·S ;S ;S ;S ;S ;S ;endo-
6 7 8 9 10 6 10 11 12 13 14 15
S ;exo-S ;S
18 18 20
The Greek letters given in parentheses indicate different phases of the
sametypeofmolecules.Examplesareorthorhombica-S andmonoclinicb-
8
S which contain molecules of the same size and the same conformation
8
(D symmetry) but in different packing patterns inthe unit cells. However,
4d
endo-S (formerly a-S ) and exo-S (formerly b-S ) consist of molecules
18 18 18 18
of the same size but in different conformations. The allotrope S ·S is aun-
6 10
ique case among the many allotropes of the non-metallic elements in so far
as it consists of two different molecules of the same element in a stoichio-
metric ratio. Inaddition,the solvate S ·CS has beenstructurallycharacter-
12 2
ized.
The sulfur allotropes consisting of chains are less well characterized and
their nomenclaturehaschangedintimecausingsomeconfusionintheliter-
ature. At least three ambient pressure polymeric forms are known, termed
as“fibroussulfur”ory-sulfur(S ),“secondfibroussulfur”orw1-sulfurand
y
“laminar sulfur” or w2-sulfur (S and S ). These allotropes arecrystalline,
w1 w2
while polymeric insoluble sulfur is usually obtained in a microcrystalline
(randomcoil)stateandisoftencalledm-sulfurorS .Thesepolymericforms
m
seem to consist, inprincipal, of the same type of helical molecules. In addi-
tion to long chains the polymeric allotropes are likely to contain also large
sulfurringsindifferingconcentrations.
Another way to indicate the polymeric nature of sulfur chains is to use
thesymbolS ;thissymbolwillbereservedfor thepolymericsulfurpresent
1
in liquid sulfur and most probably consisting of very large rings (S R) and
1
4 RalfSteudel·BodoEckert
diradicalic chains (S C) which have no end groups while the endgroups in
1
thechain-likecomponentsofS andS aremostprobablySHorOH.
m w
At high pressures, sulfur undergoes several phase transitions towards
close-packing. In the low pressure regime (<20 GPa), the phase transitions
observedbyRamanandX-raystudiesarecomplicatedduetophoto-induced
transformations which have to be attributed to the pressure-tuned red-shift
of theopticaledgeofsulfur.Athigher pressures(around90GPa),metalliza-
tionandsuperconductingstateshavebeenobserved.
The chemistry of elemental sulfur has been reviewed before [1–7]. In the
older literature there are many claims of doubtful sulfur allotropes which
have never been characterized properly and which most probably do not
consistofpuresulfurorwhichareidenticaltothewellknownallotropesbut
withadifferenthabitusofthecrystals.Thesematerialswillnotbediscussed
here[8].
2
Allotropes at Ambient Pressure
2.1
Preparation
2.1.1
AllotropesConsistingofCyclicMolecules
In the following, convenient methods for the preparation of the homocyclic
sulfur allotropes will be described. Which method to use depends on the
amount of material needed, on the skills of the experimentalist and on the
chemicals and equipment available. Therefore, several alternative prepara-
tionproceduresareprovided.
Metastable sulfur allotropes are light-sensitive and should be protected
fromdirectexposuretosun-lightorother intense illumination.Thesemate-
rials are also very sensitive towards nucleophiles including alkaline glass
surfaces. Therefore, pure and dry solvents should be used andthe glassware
should be treated with concentrated hydrochloric acid followed by rinsing
withwateranddryinginanovenpriortouse.
2.1.1.1
PreparationofS
6
cyclo-Hexasulfur S forms orange-colored rhombohedral crystals whichmay
6
bepreparedbyavarietyofmethods:
1. Historically,S wasfirstpreparedfromthetwoinexpensivechemicalssodi-
6
um thiosulfate and hydrochloric acid [9] which, according to more recent
results,yieldamixtureofmainlyS ,S ,andS [10]:
6 7 8
SolidSulfurAllotropes 5
Na S O þ2HClðaqÞ!1=nS þSO þ2NaClþH O ð1Þ
2 2 3 n 2 2
The sulfur rings are extracted from the aqueous reaction mixture by tolu-
ene from which S as the major product (69 mol%) crystallizes as orange
6
crystals on cooling to (cid:4)20 (cid:5)C. However, the evolution of large quantities of
poisonousSO gasmakesthispreparationsomewhatunpleasant.
2
2. A more convenient but also slightly more expensive method to prepare S
6
uses the thermal instability of diiododisulfane which is generated in situ
from simple chemicals [11]. Commercial dichlorodisulfane (“sulfur-
monochloride”), dissolved in CS , is stirredwith aqueous potassium or so-
2
diumiodideat20(cid:5)Cfor15minwhereuponiodineandelementalsulfurare
formed. The latter is composed of mainly S and S with small concentra-
6 8
tionsoflargereven-memberedrings[12]ofwhichS ,S andS havebeen
12 18 20
isolatedfromthismixture:
S Cl þ2KI!S I þ2KCl ð2Þ
2 2 2 2
nS I !S þI ð3Þ
2 2 2n 2
The iodine is reduced by reaction with stoichiometric amounts of aqueous
sodium thiosulfate before the sulfur rings are separated by fractional pre-
cipitation with pentane and recrystallization from CS (yield of S : 36%)
2 6
[11]. The formation ofS is likely toproceedviathe intermediates S I and
6 4 2
S I with subsequent ring closure by intramolecular elimination of I . The
6 2 2
larger rings probably result from the intermolecular reaction of the diio-
dosulfanes to give sulfur-rich homologs such as S I and S I which then
8 2 12 2
undergo ring closure. The thiosulfate solution contains sodium iodide and,
after all thiosulfate has been oxidized, may be used again for another reac-
tionwithS Cl [11].
2 2
3. Titanocene pentasulfide Cp TiS (Cp=h5-C H ) is commercially available
2 5 5 5
butcaneasily bepreparedfromCp TiCl andaqueoussodiumorammoni-
2 2
um polysulfide solution [13]. Using chloroform as a solvent a yield of 88%
was obtained [14]. The organometallic pentasulfide forms dark-red air-sta-
blecrystalssolubleinseveralorganicsolvents.Themoleculescontainasix-
membered metallacycle in a chair conformation [15]. The pentasulfide re-
actswithmanyS-Clcompoundsat0–20(cid:5)Casasulfur transferreagentwith
formation of Cp TiCl . For example, with SCl the two rings S and S are
2 2 2 6 12
formed[16]:
Cp TiS þSCl !S þCp TiCl ð4Þ
2 5 2 6 2 2
2Cp TiS þ2SCl !S þ2Cp TiCl ð5Þ
2 5 2 12 2 2
Commercial“sulfurdichloride”isamixtureofSCl ,S Cl ,andCl whichare
2 2 2 2
inequilibriumwitheachother[17].Therefore,thismixtureneedstobedis-
tilledtoobtainpureSCl immediately prior touse.Duetotheir verydiffer-
2
6 RalfSteudel·BodoEckert
ent solubilities in CS (see below) [18], the reaction products Cp TiCl , S
2 2 2 6
andS caneasilybeseparated.Yields:87%S ,11%S [16].
12 6 12
2.1.1.2
PreparationofS
7
1. SmallamountsofS arebestpreparedfromtitanocenepentasulfide(seethe
7
preparation of S above) by reaction with dichlorodisulfane S Cl (“sulfur-
6 2 2
monochloride”)inCS at0(cid:5)C[16]:
2
Cp TiS þS Cl !S þCp TiCl ð6Þ
2 5 2 2 7 2 2
This reaction proceeds quantitatively, but the isolated yield of S (23%) is
7
lower owing to its high solubility. Since S rapidly decomposes at 20 (cid:5)C, it
7
needs to be handledwith cooling and should be stored attemperatures be-
low(cid:4)50(cid:5)C.
2. Liquid sulfur contains at all temperatures several percent of S besides the
7
main constituentS ;inaddition,rings of other sizes and,at higher tempera-
8
tures, polymeric sulfur S are present [19]. After quenching of the melt at
1
low temperatures it is possible to separate the main components and to iso-
late S in 0.7% yield; see below under “Preparation of S , S , and S from
7 12 18 20
S ”.
8
DependingonthecrystallizationconditionsS isobtainedaseither thea,
7
b, g, or d allotrope [20, 21] which are all very well soluble in CS , CH Cl ,
2 2 2
toluene, and cyclo-alkanes. a-S is obtained on rapid cooling of solutions in
7
CS , CH Cl , or toluene and forms intense-yellow needle-shaped crystals of
2 2 2
m.p. 38.5 (cid:5)C which aredisordered. b-S was obtained as a powder from d-S
7 7
bystorageat25(cid:5)Cfor10min.d-S crystallizesfromCS solutionsat(cid:4)78(cid:5)C
7 2
and forms block-shaped, tetragonal-bipyramidal and sarcophagus-like crys-
tals. g-S was obtained from a solution in CH Cl containing small amounts
7 2 2
oftetracyanoetheneat(cid:4)25(cid:5)C[20].
Regardless of the allotropic modification, solid S decomposes at 20 (cid:5)C
7
completelywithintendaysbutcanbestoredat(cid:4)78(cid:5)Cforlonger periodsof
time without decomposition. The first signs of the decomposition products
S and S (polymeric sulfur) can be detected already after 30 min at 20 (cid:5)C
8 m
[20].InCS solutionS isquitestable.
2 7
2.1.1.3
PreparationofPureS
8
cyclo-Octasulfur crystallizes at ambient pressure either as orthorhombic a-
S , monoclinic b-S or monoclinic g-S . Commercially available sulfur sam-
8 8 8
plesusuallyconsistofmixturesofa-S withsomeS andtracesofS [22].It
8 m 7
is this S content which causes the bright-yellow color of most commercial
7
SolidSulfurAllotropes 7
sulfur samples while pure a-S is greenish-yellow. Sulfur samples from vol-
8
canic areas sometimes also contain traces of S but in addition minute con-
7
centrations of selenium may be present (as determined by neutron activa-
tion analysis),most probablyas S Se heterocycles [23].To removethese im-
7
purities the material is dissolved in toluene or CH Cl , and after filtration
2 2
the solution is cooled to (cid:4)50 (cid:5)C. Carbon disulfide is not a good solvent for
this purpose since traces of it tend to remain inthe product. However, even
after this treatment most sulfur samples still contain traces of carbon com-
poundswhichcanbestbetestedforbycarefullyheatingthesulfurinaclean
test tube for 2–3 min to the boiling point (445 (cid:5)C) avoiding ignition of the
vapor! After cooling of the sample to room temperature black spots will be
seen on the walls of the glass and the color of the sulfur itself may have
changed to darker hues or even to black, caused by the formed carbon-sul-
fur polymer. The organic impurities can be removed by heating the sulfur
for 10 h to 300 (cid:5)C (with addition of 1% magnesium oxide) followed by re-
fluxing for 1 h [24] which causes these impurities to decompose to H S and
2
CS which both escape; in addition, a black precipitate is formed which
2
looks like carbon-black but is in fact a sulfur-richpolymer. After slow cool-
ing to 125 (cid:5)C and decantation from the black sludge the liquid is filtered
through glass-wool. If necessary, this procedure is repeated several times.
An improved method uses an immersed electrical heater to keep the sulfur
boiling[25].Thepurifiedliquidsulfuristhendistilledinavacuumresulting
in a bright-yellow, odorless product. Commercial “high-purity sulfur”
(99.999%)oftenstillcontainsorganicimpuritiessincethepurityclaimedon
thelabelappliestothemetalcontentonly.Manycontradictoryreportsabout
the physical properties of elemental sulfur possibly can be explained by the
differing purityof the samples investigated, especially but not exclusively in
the older literature. S can also be highly purified by zone melting (carbon
8
contentthen<2.4(cid:6)10(cid:4)4%)[26].
From mostsolvents S crystallizesasorthorhombica-S .Monoclinic b-S
8 8 8
is stable above 96 (cid:5)C and is usually obtained by cooling liquid sulfur slowly
below the triple-point temperature of 115 (cid:5)C. At 25 (cid:5)C crystals of b-S con-
8
verttopolycrystallinea-S inlessthan1hbutarestableforseveralweeksat
8
temperatures below (cid:4)20 (cid:5)C [27]. g-S is metastable at all temperatures and
8
occasionally crystallizes by chance, for example from ethanolic solutions of
ammonium polysulfide [28], by decomposition of copperethylxanthate [29]
or in the preparation of bis(dialkylthiophosphoryl)disulfane [30]. Surpris-
ingly, g-S occurs also naturally as the mineral rosickyite. Furthermore, g-S
8 8
is a component of stretched “plastic sulfur” which is obtained byquenching
liquid sulfur from 350 (cid:5)C to 20 (cid:5)C (in cold water) and stretching the fibers
obtained in the direction of their axes. According to an X-ray diffraction
study, this “fibrous” sulfur consists of helical polymeric sulfur chains (S ,
w
see below) which formpockets filledwith S molecules as the monoclinic g-
8
allotrope[31].
8 RalfSteudel·BodoEckert
2.1.1.4
PreparationofS
9
Inprinciple, there is onlyone methodtoprepare S , andthat is the reaction
9
of titanocene pentasulfide with either S Cl [32] or S (SCN) [33]. The need-
4 2 4 2
eddichlorotetrasulfaneS Cl canbemostconvenientlypreparedbycarefully
4 2
controlledchlorinationofcyclo-S inCCl at20(cid:5)C[33]:
6 4
S þCl !Cl(cid:1)S (cid:1)Cl ð7Þ
6 2 6
Cl(cid:1)S (cid:1)ClþCl !S Cl þS Cl ð8Þ
6 2 4 2 2 2
The solvent and the S Cl are distilled off from the mixture and the resi-
2 2
dueisusedforthepreparationofS :
9
Cp TiS þS Cl !S þCp TiCl ð9Þ
2 5 4 2 9 2 2
S wasobtainedin30%yield[32].
9
However,sinceS Cl isanoilyliquidwhichowingtoitsinstabilitycannot
4 2
be purified by distillation and consequently always contains small amounts
ofotherdichlorosulfanes,itisrecommendedtoconvertittoS (SCN) [iden-
4 2
ticaltoS (CN) ].Dicyanohexasulfaneconsistsofchain-likemoleculeswhich
6 2
form an odorless solid (m.p. 35 (cid:5)C) that can be easily recrystallized for pu-
rificationalthoughitfairly rapidlypolymerizesatroomtemperature[33]:
S Cl þHgðSCNÞ !S ðSCNÞ þHgCl ð10Þ
4 2 2 4 2 2
This reactiontakes place at 0 (cid:5)C in CS ; based on the starting material S
2 6
the yield of S (SCN) is 27%. This product reacts in CS solution at 20 (cid:5)C
4 2 2
withtitanocenepentasulfidetoS in18%isolatedyield:
9
Cp TiS þS ðSCNÞ !S þCp TiðSCNÞ ð11Þ
2 5 4 2 9 2 2
Depending on the conditions, S crystallizes as either a- or b-S the Ra-
9 9
man spectra of which are very similar but not identical. a-S forms intense
9
yellowneedle-shapedmonocliniccrystalsofmeltingpoint63(cid:5)C[33].
2.1.1.5
PreparationofS
10
cyclo-DecasulfurS canbepreparedaccordingtoseveraldifferentmethods:
10
1. If several grams are needed, the sulfur transfer method is most convenient
[16]:
2Cp TiS þ2SO Cl !S þ2SO þCp TiCl ð12Þ
2 5 2 2 10 2 2 2
The reagents titanocene pentasulfide and sulfurylchloride are mixed at
(cid:4)78 (cid:5)C in CS and the mixture is allowed towarm up to 0 (cid:5)C with stirring.
2
YieldofS :35%.S formsintenseyellowcrystalswhichslowlydecompose
10 10
at room temperature to S with partial polymerization to S . The reaction
8 m
SolidSulfurAllotropes 9
mechanism for the formation of S will be explained below (see “Prepara-
10
tionofS ”).
15
2. IfonlysmallamountsofS areneededandS orS areavailable,theoxida-
10 6 7
tion of either one withtrifluoroperoxoacetic acid provides S in a reaction
10
of unknown mechanism. The intermediates S O or S O decompose at5 (cid:5)C
6 2 7
in CS or CH Cl solution within several days to give S , some insoluble
2 2 2 10
sulfuraswellasSO :
2
2S þ4CF CO H!2½S O (cid:2)þ4CF CO H ð13Þ
6 3 3 6 2 3 2
2½S O (cid:2)!S þ2SO ð14Þ
6 2 10 2
2S þ2CF CO H!2S Oþ2CF CO H ð15Þ
7 3 3 7 3 2
2S O!S þ3S þSO ð16Þ
7 10 m 2
Since the homocyclic oxides do not have to be isolated, the solution of S
6
or S after addition of the peroxoacid (prepared from H O and trifluo-
7 2 2
roaceticacidanhydrideinCH Cl )issimplykeptintherefrigeratoruntilS
2 2 10
has formedwhich is then crystallized bycooling andpurified by recrystalli-
zation[34,35].
2.1.1.6
PreparationofS ·S
6 10
When S and S are dissolved together in CS and the solution is cooled,
6 10 2
then, under special concentration conditions, a stoichiometric well ordered
solid solution of the two components crystallizes as orange-yellow opaque
crystalsofm.p.92(cid:5)C[34].ThestructureofS ·S consistsofalternating lay-
6 10
ers of S and S molecules in their usual conformations of D respectively
6 10 3d
D symmetry [35]. In liquid solutions the molecular mass of S ·S was de-
2 6 10
terminedas258corresponding to8atomsper moleculeindicatingcomplete
dissociation [34]. This is the only example of an allotrope of a chemical ele-
mentconsistingofmoleculesofdifferentsizes.
2.1.1.7
PreparationofS
11
The sulfur transfer reactionusing titanocenepentasulfideanddichloropoly-
sulfanes S Cl is very versatile and has made it possible to prepare S after
n 2 11
thenecessaryS Cl becameaccessibleinsufficientpurity:
6 2
Cp TiS þS Cl !S þCp TiCl ð17Þ
2 5 6 2 11 2 2
The reaction is carried out in CS at 0 (cid:5)C and provides pure S
2 11
(m.p. 74 (cid:5)C) in 7% yield as yellow crystals [36]. The precursor S Cl is best
6 2
preparedbycarefullycontrolledchlorinationofcyclo-S withelementalchlo-
6
10 RalfSteudel·BodoEckert
rine at 0–20 (cid:5)C in a CS /CCl mixture; see Eq. (7). In the solid state the S
2 4 11
moleculesareofapproximateC symmetry[37,38].
2
2.1.1.8
PreparationofS
12
Thermodynamically,S isthesecondmoststablesulfurringafterS .There-
12 8
fore,S isformedinmanychemicalreactionsinwhichelementalsulfur isa
12
product. In addition, S is a component of liquid sulfur at all temperatures.
12
ThesameholdsforS andS whichareoftenformedtogetherwithS :
18 20 12
1. The preparation of S from titanocene pentasulfide and SCl has been de-
12 2
scribedaboveunder“PreparationofS ”:
6
2Cp TiS þ2SCl !S þCp TiCl ð18Þ
2 5 2 12 2 2
2. PreparationofS fromS Cl andapolysulfanemixtureH S :sulfanesH S
12 2 2 2 x 2 n
and dichlorosulfanes S Cl react with each other with elimination of HCl
n 2
forming new S-S bonds. Since pure sulfanes with more than two sulfur
atoms are difficult to prepare, this synthesis uses a mixture of sulfanes,
called“crudesulfaneoil”,whichcaneasilybepreparedfromaqueoussodi-
umpolysulfideandconcentratedhydrochloricacidat0(cid:5)C[39,40]:
Na S þ2HCl!H S þ2NaCl ð19Þ
2 4 2 4
Since the aqueous sodium polysulfide contains already several polysulfide
anions in equilibrium and since the acidification results in some intercon-
versionreactions,asulfanemixtureH S isobtainedrather thanpureH S .
2 x 2 4
This mixture nevertheless reacts in dry CS /Et O mixture at 20 (cid:5)C with
2 2
dichlorodisulfane, besidesother products,toS whichhas been isolatedin
12
4%yieldbyextractionwithCS andfractionalcrystallization[41]:
2
2S Cl þ2H S !S þ4HCl ð20Þ
2 2 2 4 12
Evidently, this reaction proceeds in several steps with H-S -Cl and
6
H-S -Claslikely intermediates.
12
3. Preparation of S , S and S from liquid sulfur: liquid sulfur after equili-
12 18 20
bration contains sulfur homocycles of all sizes [19] and some of these can
be isolated by quenching, extraction, fractional precipitation and crystalli-
zation depending on their differing solubilities. Commercial elemental sul-
fur (several hundred gram) is heated electrically to about 200 (cid:5)C for 5–
10 min or longer and is then allowed to cool to 140–160 (cid:5)C within ca.
15min.Assoonasthemelthas becomelessviscous,it ispouredinasthin
a stream as possible into liquid nitrogen in order to quench the equilibri-
um. The boiling nitrogen ruptures the melt into small pieces resulting in a
yellow powder. The liquid nitrogen is decanted off this powder which is
thenextractedwithCS at20(cid:5)C(solution“A”).Asmallamountofpolymer-
2
ic sulfur remains undissolved and is filtered off. The yellow solution is