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Elemental SulfurЧA Challenge to Theory and Practice.

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Elemental Sulfur-A
Challenge to Theory and PracticeC**]
By Max Schmidt“]
Surprisingly little is still known about many aspects of the system “elemental sulfur”,
in spite of its being one of the longest known and common “chemical substances” and
today available cheaply and in great purity. This paper summarizes some of the newer
investigations that have increased our understanding of this system.
1. Introduction
Four years ago the answers to a world-wide questionnaire
that had been circulated to academic research Institutes
were published‘’]. Though certainly incomplete, these
showed that at least 2250 scientists were then engaged
in sulfur chemistry in no fewer than 275 laboratories.
Problems concerning sulfur chemistry arise in all branches
of chemistry -from inorganic chemistry through biochemistry, organic chemistry, physical chemistry and
industrial chemistry to purely theoretical chemistry and
finally also in environmental pollution.
From the many known and still unknown sulfur compounds our School is at present interested particularly
in those molecules in which at least two sulfur atoms
are directly linked to one another, because we consider
the nature of the sulfur-sulfur bond to constitute an important, still unsolved problem[*’.
The apparently simplest chemical compound containing
such sulfur-sulfur bonds in the molecule is, of course,
elemental sulfur itself, io which indeed this progress report
must be largely restricted.
We can follow the history of sulfur back to the days
of Sodom and Gomorrahr3]. Nowadays, sulfur is a very
cheap material which is available industrially with unusual
simplicity, in a purity that is exceptionally high for an
element, and in enormous quantities. The present annual
world consumption of sulfur amounts to ca. 40 million
tons. That, so far, only a negligibly small proportion of
this production is used directly as the element is mainly
because we still know and understand far too little about
the properties of sulfur to be able to change them to
meet selected practical ends. One typical example of this
is that scientists are still in dispute about the “melting
point” of sulfur (see Section 2).
2. Elemental Sulfur
2.1. General
What actually is sulfur? Is that a real question? Certainly
not if we are content with the answer, trivial today, that
p] Prof. M. Schmidt
Institut fur Anorganische Chemie der Universitat
87 Wurzburg, Landwehr (Germany)
Based on a Lecture to the General Meeting of the Gesellschaft
Deutscher Chemiker in Karlsruhe on September 17, 1971, and to the
Regional Meeting at Leverkusen on May 10, 1972.
Angew. Chem. internat. Edit.
1 Vol. I 2 (1973) 1 No. 6
it has the simplest stoichiometric composition conceivable,
merely a series of atoms of mass number 16. This answer,
however, clarifies only the centuries-long arguments about
the “composition” of sulfur, whether this was a combination of one “fiery” principle with one “acid” principle,
a matter that was very intimately connected with the development of chemistry from ancient times through alchemy
to the present dayC41. Nevertheless, this answer is of no
further help if we want to know more about sulfur than
that it is an element. Actually, this element presents an
extremely complicated chemical system which is still far
from fully understood. According to the external conditions, that system comprises sulfur molecules each containing from at least two atoms to several hundred thousand,
mixed together in great variety:
S,(X = 2 to
= 106!)
We are dealing here with true dynamic equilibrium mixtures that change their molecular composition on dissolution and by formation of new chemical bonds.
In this connection let us note yet again a fundamental
misunderstanding that is often injected at the beginning
of chemistry teaching, namely that there is a difference
in principle between chemical elements on the one hand
and chemical compounds on the other. There is in fact
no essential difference between compounds composed of
atoms all of the same kind and those composed of atoms
of different kinds. Except for the noble gases, all the elements are chemical compounds whose properties at any
one time depend greatly on the nature of the bonding
and/or on the molecular size in the “modification” under
consideration. Consider, for instance, the pairs graphitediamond, white phosphorus-black
phosphorus, or
ozone-molecular oxygen. In these cases chemical reactions change the properties-reactions whose study, with
the resultant understanding and ability to influence the
properties, have been neglected for a surprisingly long
time. The interaction between atoms-whether
they are
the same or different plays no decisive role-is the main
problem in chemistry, the questions namely: how d o atoms
react with one another by means of their electron shells?
why d o they d o this? and how?
If we consider the system “elemental sulfur” from this
viewpoint it immediately becomes clear that the apparently
simple question asked at the beginning of this Section
still cannot be answered at all completely. The main reason
for this is the surprisingly well developed tendency and
ability of sulfur to combine with itself to form chains
or rings; only carbon is superior in this respect. Sulfur
atoms have six outer electrons, of which the two unpaired
3 p electrons are used for covalent bonding to neighboring
atoms (Fig. 1). It follows that the chains cannot be collinear
but must be zig-zag.
j l l j l
been misused to denote mixtures of largely unknown composition, hardly a single name has been used consistently
for one material, and the same name has not seldom
been used for different allotropic forms and for mixture^^^!
Most of this is better forgotten. Let us stick to the relatively
few facts that are really firmly based. The most important
fact is that, under normal conditions, only one compound
of sulfur atoms is thermodynamically stable, namely the
eight-membered cyclooctasulfur, S8 (Fig. 3); for reasons
mentioned above, this is not planar but has a crown form;
it crystallizes in rhombs, which at 954°C pass into monoclinic crystals.
1 3 388
Fig. 1. Electronic configuration of a sulfur atom in the ground state;
two-dimensional representation of a three-dimensional sulfur helix.
Such zig-zag chains are not planar because each sulfur
atom has two further electron pairs which prevent free
rotation around the sulfur-sulfur bond and thus produce
a dihedral angle of ~ 1 1 0 0 ” By
. dihedral angle is meant
the angle formed by the planes each containing any two
neighboring three out of four atoms in a section of the
chain (cf. Fig. 2). According to whether we proceed upwards
or downwards when passing from the third to the fourth
atom in the first plane, we have the beginning of a righthanded or a left-handed helix:
Fig. 3. Structure of cyclooctasulfur; S-S
p- 108”. dihedral angle 7 ~ 9 8 ” .
Fig. 2. Dihedral angle (7) between any three neighboring of four sulfur
atoms, which begin a right- or a left-handed helix.
Sulfur chains (and, of course, also sulfanes, i. e. compounds
containing chains of sulfur atoms) thus have helical configurations. A helix with zero translation leads to a nonplanar ring.
2.2. Well Known Sulfur Modifications
We shall now recapitulate briefly what textbooks usually
teach about the system “elemental sulfur”, this being often
coupled with a general exposition of the concept of allotropy-an
exposition that is only too frequently an
example of how to kill any interest in the subject. To
make matters worse, nowhere else in chemistry does one
find such an incurably confused nomenclature as for sulfur:
as if it were not enough that literally almost the whole
Greek alphabet plus very honorable generic names have
distance -2.06A, bond angle
It is this,,cyclooctasulfur that we normally mean when
we speak of sulfur. It is yellow-‘sulfur-yellow’-but
at room temperature; at low temperatures it becomes snowwhite. The color, or colors, of sulfur will be discussed
At z 119°Csolid cyclooctasulfur passes into a pale-yellow
mobile melt which, remarkably, solidifies again only at
a temperature 6 5 ° C lower than the melting point. This
phenomenon, investigated so frequently and yet of old
so unsuccessfully, has been ascribed to the difference
between the “ideal” and the “natural” melting point of
sulfur. Yet, precisely expressed, this means only that S,
has not got a reversible melting point, but that at or
below the melting point it decomposes partly to form
other molecules which depress the solidification temperature.
Further heating of the sulfur melt leads at zl16o”C to
a peculiar phenomenon which we remember as a lecture
experiment in our first term: the melt suddenly becomes
very viscous and plastic; within a very narrow temperature
range the viscosity increases by a factor of 2000! Further
Angew. Chem. internat. Edir. / Vol. I 2 (1973) / N o . 6
heating slowly decreases the viscosity again, until at the
boiling point of 444°C the melt is again mobile, during
which processes the color changes from pale yellow to
deep red; sulfur can thus be not only white or yellow
but also deep red! Finally, sulfur vapor is said to contain
various molecules each containing an even number of
atoms, namely S,, S,, S, and S,.
If the temperature change is then slowly reversed, sulfur
passes from the gas phase by way of the melt with its
remarkable anomaly (now naturally in the reverse order)
until at room temperature one has again cyclooctasulfur.
If the melt is suddenly chilled from above 160”C, the
famous plastic sulfur is obtained-a
still imprecisely
understood mixture of components some soluble and
some insoluble in carbon disulfide.
Indeed, this “rolling up” of sulfur chains to rings is the
reason why all compounds composed of sulfur atoms are
converted finally into S,[81.
Even quite feeble illumination converts S,, either in solution or in the solid state, into S,. For this change a direct
reaction 4 S,+3 S, is logically excluded; the six-membered
rings that are breaking up must first form longer helical
molecules, which then unwind so as to yield eight-membered rings in the manner illustrated above.
Current opinion is that exactly the opposite takes place,
namely stepwise formation of sulfur chains during formation of S, (admittedly in rather poor yield) when thiosulfates are decomposed by mineral acids at low temperature,
a reaction which was discovered already in the preceding
Somewhat “outside” this sulfur system lies cyclohexasulfur,
S h (Fig. 4), known for 80 years as a barely considered
laboratory curiosity:
s-s-so~’- + sZo3’S ~ S O ~ ’+
Fig. 4. Structure of cyclohexasulfur; S-S
p-102“, dihedral angle y-75“.
2.06 A. bond angle
Its chair-shaped molecules form shiny orange-red crystals.
This S6, together with s,, were until recently the only
properly defined compounds containing only sulfur atoms.
S, shows considerable ring strain in that it polymerizes
already at z5@-60°C.
Chemically it reacts with both
nucleophiles and electrophiles in redox reactions that are
up to 1OOOO times faster than those of S,!L61 Further,
it is extremely sensitive to light. This is perhaps the place
to mention the peculiar but generally disregarded fact
that all sulfur-sulfur bonds are in principle sensitive to
light, although this is quantitatively much dependent on
the “strain” in the bonding. Even the most stable form
of sulfur, S,, polymerizes, for instance in benzene solution
merely under the action of strong sunlight. The polymeric
and chemically very reactive “photosulfur” thus formed
depolymerizes rapidly in the dark, re-forming the eightmembered ring[’]. The mechanism proposed for this production of stable eight-membered rings from the longer
chain molecules is that thermal movements and the preferred geometry of S, cause the terminal atom of a chain
toattack always the eighth atom, with simultaneous formation of one sulfur-sulfur bond and cleavage of another.
Angew. Chem. internat. Edit. J Vol. 12 (1973) J N o . 6
s-s-s-so~’- + so3’-
- s-s-s-s-s-s-so~~- + so3’-
ss +
In this reaction, polysulfanemonosulfonic acids (discovered
earlier by us[91) are formed starting from monosulfanemonosulfonic acid (thiosulfuric acid), the
increasing until it is long enough for the terminal sulfur
atom to attack the other end of the chain, which it does
by ring closure and extrusion ofa sulfite ionlsl. This equilibrium reaction can be reversed by a sufficient excess of
sulfite: there then occurs the familiar stepwise nucleophilic
“sulfite degradation” of sulfur that leads to thiosulfatel’!
That in this special case the cyclohexasulfur can be separated from the main product S, is due to the very different
solubility relations of the two modifications[”].
2.3. New Sulfur Modifications from
Sulfanes and Chlorosulfanes
Since the nature of this modification, known previously
as “Engel’s sulfur” or “Aten’s sulfur”, was recognized, there
has been no dearth of attempts to prepare sulfur rings
other than S8 and S,; prior to our work, they were all
unsuccessful. Also, there has been no lack of theoretical
attempts to calculate and predict the stability or instability
of hypothetical sulfur rings[’ ll.
In view of this rather unsatisfactory situation we asked
ourselves whether it might be possible to build new thermodynamically unstable compounds containing only atoms
of one kind, i.e. new modifications of an element, by
scientifically planned, kinetically controlled syntheses. This
object would be the more easily achieved the fewer reaction
steps were needed to combine precisely defined molecular
units into the desired ring. Our first successes with this
principle were when we used a two-stage redox reaction
between sulfanes and chlorosulfanes:
- +2
+ C1-Sy-C1
H-S -S y -C 1
In an open-chain sulfane the sulfur chain as a whole has
the oxidation state - 2, and in a chlorosulfane correspondingly +2. The overall reaction (6) is formally very simple,
but for various reasons it is experimentally extremely difficult to realize it in such a way that pure sulfur is formed
and not long-chain sulfanes or chlorosulfanes. It is essential
to use the dilution principle, so that the unsymmetrically
substituted intermediate product with a mean oxidation
state +O has the opportunity to split off hydrogen chloride
when it assumes a suitable geometric orientation [reaction
light on the composition of sulfur melts, and their earlier
interpretation thus requires reinvestigation, as will be
briefly mentioned again below. As a small curiosity it
may be mentioned here that small amounts of S,, crystallize out if saturated benzene solutions of s6 are left for
a short time in the light[61.
A rather stable cyclooctadecasulfur, S,,, which decomposes
at 126“C,and a cycloicosasulfur, S20,with a decomposition
temperature of 121“C could also be obtained from sulfanes
and chlorosulfanes[161. Their interesting structures (Figs.
6 and 7) have been elucidated quite recently[’6a.16bl.Bond
distances, bond angles and dihedral angles are similar
to those in S, and SlZ.
We studied this procedure for the case of cyclohexasulfur
[reaction (9)], whose properties were known. After an
unstable modification of sulfur had thus for the first time
become preparatively available in a pure form, we tackled
the synthesis of new rings. From suitable components
(x + y in the general overall equation = 12) we then obtained
the new sulfur modification cyclododecasulfur, S, Jl21.
Contrary to Pauling’s prediction[’ 31, this compound which
crystallizes in pale yellow needles is disconcertingly stable;
it “melts” appreciably higher than any other known form
of sulfur, namely at 148“C,although admittedly not reversibly but with decomposition. X-ray structure analysis
showed the highly symmetrical form of the twelve-membered ring (Fig. S), in which the atoms are arranged in
three planes-six in the middle plane and three each symmetrically above and below
Fig. 6. Structure of cyclooctadecasulfur; S-S
angle p-106”, dihedral angle y - 8 5 ” .
2.06 A, bond
Fig. 7. Structure of cycIoicosasulfur; S-S
p~106’,dihedralangle y - 8 5 ” .
Fig. 5. Structure of cyclododecasulfur.
Like S,, cyclododecasulfur S, forms “regular” solutions
in carbon disulfide, but its solubility therein is more than
150 times lower than that of S,[’OI. This unexpected solubility relation made it possible to separate the very stable
new form from the large excess of S, and t h s also to
look for it in “normal” sulfur. And in fact cyclododecasulfur
could be isolated from cooled sulfur melts, pre-heating
to z i20”C being sufficient[”1. This finding throws a new
distance ~2.04
A, bond angle
Our optimism about obtaining further relatively small
sulfur rings by the procedure described was sadly damped
by unexpected experimental difficulties. For example, it
is almost impossible to carry out reactions with 100%-pure,
homogeneous sulfanes, for the extremely unstable compounds of this class readily form equilibrium mixtures
of various chain lengths, and these necessarily lead to
various cyclic molecules, some of which are liquid at room
temperature. Sulfur that is liquid at room temperature!
an interesting phenomenon indeed, but once again only
an indefinite mixture like the long-known melts. Because
of the marked sensitivity of the strained rings to heat,
light, or indeed even rough surfaces, all attempts at separation, e. g. chromatographically, have so far failed except
when particularly favorable solubility relations prevailed,
as between S6, S,, SI2,S I B , and Szo.
Angew. Chem. internat. Edit.
Vol. 12 ( 1 9 7 3 ) / No. 6
3. Sulfur Rings Containing Hetero Atoms
3.1. Rings Containing Metal Atoms
Out of this rather unhopeful position we found a way
that brought us a few steps forward, namely by substituting
for the sulfane a much more stable sulfur compound, of
exactly defined composition, as starting material for the
redox reaction that affords the sulfur rings.
A few years ago, when studying organometallic sulfur compounds, we synthesized a chair-form six-membered ring
comprising five sulfur atoms and one titanium atom, where
the metal atom carried also two n-bonded cyclopentadienyl
ligands" '1. This bis(n-cyc1opentadienyl)titanium pentasulfide (Fig. 8) is the prototype of a class of compound containing sulfur rings with a transition-metal atom as hetero
atom; such compounds are under intensive study at the
present time.
Fig. 8. Structure of bis(rr-cyclopentadienyl)titanium(lV) pentasulfide
It is interesting that at about the same time Katz and
Jones['81 showed by X-ray structure analysis that this group
includes a compound with the somewhat unusual stoichiometric composition (NH4),PtSI5, known for almost 70
years but subjected to hardly any notice. In the PtS:;
ion the octahedrally coordinated Pt atom is the hetero
atom in three chair-form six-membered rings, each of which
contains five sulfur atoms whilst the Pt atom is common
to all three rings-a structure that is hardly a daily occurrence in sulfur chemistry (see Fig. 9).
Fig. 9. Structure of the ion PtS:;.
These remarkable metal-containing sulfur rings will not
be discussed in detail here. However, one important quesAngew Chem. rnternat. Edit. j Vot. 12 (1973) J N o . 6
tion about their chemistry must be asked although it is
not yet possible to give a convincing answer; it is this:
Why, in these compounds, are ring sizes favored and stable
that in wholly sulfur rings are very unstable or incapable
of existence, as for example the six-membered sulfur rings
mentioned above and-for molybdenum['91-a five-membered ring containing four sulfur atoms and one metal
atom? Consider our titanium compound (above): even
conditions that we should expect to favor formation of
other ring sizes always lead to formation of only the sixmembered ring, which is apparently extremely favored;
the permanganate-colored crystals melt only at 201 "C,
indicating a thermal stability of sulfur chains that would
have been inconceivable to a sulfur chemist a short time
3.2. Rings Containing Methylene Groups
The question about ring size and stability applies not
only to wholly sulfur rings but also to sulfur heterocycles
other than those with transition-metal atoms. Two
examples will suffice: As a model substance for cyclooctasulfur minus the sulfur-sulfur bonds we have built up
a tetrameric thioformaldehyde[20]in which each second
sulfur atom of the S, ring is replaced by an isoelectronic
-CH2group. In this series, the resulting 1,3,5,7-tetrathiacyclooctane, melting at 48 "C, is appreciably less stable
than the well-known 1,3,5-trithiane which contains a sixmembered ring; the former is readily converted irreversibly
into a crystalline high polymer that melts undecomposed
at 245 "C and corresponds to the unstable polymeric sulfur.
The powerful influence of the methylene group as "hetero
atom" on the size of the more stable cyclic sulfur compounds has also been clearly shown in recent work by
Fehe'r et aZ.'z'l
3.3. Rings Containing Selenium Atoms
What d o we observe if we insert sulfur's closest relative,
namely a selenium atom, as hetero atom between sulfur
atoms? Unlike sulfur, cyclooctaselenium is very unstable,
changing spontaneously into the thermodynamically
stable, high-polymeric selenium with a helical conformation. Why? We are not yet sure. What is the position
with compounds containing both sulfur and selenium?
At what stoichiometric composition d o the stability relations between the eight-membered ring and the chain
polymer "tip over"? Isolated attempts at an answer by
melting the elements together in various stoichiometric
proportions and mass-spectroscopic study of the gas phase
above such mixtures d o not lead us further here. We
should bear in mind that, even for the (still hypothetical)
eight-membered ring system comprising S,Se to SSe,, the
conceivable different structural arrangements of these so
closely related atoms can lead to more than 40 different
isomers; we can then see clearly that only carefully planned
syntheses, if any, will provide any possibility of isolating
definite compounds as products. We have not made too
remarkable progress in such investigations. Nevertheless,
we now know three of the seven possible "stoichiomers"
containing an eight-membered ring, namely S,Se, S6Se,
and S,Se,. As to the question about the influence of the
hetero atoms on molecular size, it is significant that we
can now say the following with certainty: In the sulfurselenium system the sulfur is dominant up to a stoichiometry of at least 62: 38 atom-% of sulfur to selenium. For,
as in the wholly sulfur system, the (not yet realized) sixmembered ring is certainly considerably less stable than
the polymeric helix, which in turn changes again, spontaneously, into the thermodynamically most stable eightmembered ring.
Monoselenacyclooctasulfur,S,Se, which presents no structural problem, was prepared by Cooper and Culka[221from
dichloroheptasulfane and hydrogen selenide by our synthetic method for chalcogen rings:
+ H,Se
+ S,Se
We ourselves wanted to assess the possibility of existence
of a six-membered ring of four sulfur and two selenium
atoms by the reaction:
+ 2H,Se
+ S,Se,
but we had no success. Under the mild conditions sufficing
for synthesis of cyclohexasulfur we always obtained as
intermediate only a polymer of the same stoichiometry,
pentadienyl)titanium(Iv) pentasulfide reacts quantitatively
with dichloromonosulfane, thus:
it affords di(n-cyclopentadieny1)titanium dichloride and
sulfur, the latter being separable into 87% of cyclohexasulfur ( n= 1) and 11% of cyclododecasulfur (n = 2)Lz4I.This
gave us our first chance of synthesizing a ring with an
odd number of members, by treating the titanium compound with dichlorodisulfane, S,Cl,, thus:
(C,H,)2TiS5 + S,CI,
+ S,
Theoretical predictions made relatively small, odd-membered sulfur rings such as S, appear unlikely; if we exclude
a planar ring with a uniform dihedral angle of 180”,which
is impossible on bond-theoretical ground, we find that
a seven-membered ring with equal bond lengths, bond
angles and dihedral angles cannot be constructed. In spite
of this, reaction according to eq. (15) occurs even under
quite mild conditions[24! Cycloheptasulfur crystallizes in
yellow needles. It melts reversibly at 39°C. X-ray structure
analysis[251has disclosed definite differences between the
various dihedral angles. In the chair form of the seven-membered ring (Fig. 10) four atoms lie almost exactly in one
plane. This therefore presents an element modification
This helix contained throughout its length a series of two
sulfur atoms and one selenium atom. It very rapidly undergoes the reaction
yielding the new eight-membered rings S$e, and S6Se,
in the expected proportion 2:1 by the “rolling up”
mechanism described in Section 2.2 for pure polymeric
sulfur. Diselenacyclooctasulfur,S,Se,, forms lemon-yellow
crystals melting at 124°C; and triselenacyclooctasulfur,
S,Se,, forms magnificent orange-red crystals melting at
120°C; the difference in their solubilities is sufficient for
them to be separable on a preparative scale and they
are notably stable[23!
4. New Modifications of Sulfur from
Metal-containing Sulfur Rings
After this excursion into the field of sulfur molecules containing hetero atoms we return to the desired rings containing atoms all of the same type. There we have got a
little further by substituting the very stable titanium-sulfur
six-membered ring for the very labile sulfanes in the redox
reaction with chlorosulfanes described above. Bis(n-cyclo450
Fig. 10. Perspective view of a cycloheptasulfur molecule and its x y
projection (bond lengths in A).
in which identical atoms exist in differing electronic environments and thus have different energy contents. The
ring strain is apparent in, inter &a, a notable sensitivity
to light and heat: S, polymerizes even at ca. 45°C; wherefore cycloheptasulfur can be kept for a long period only
in the dark and the cold; even then it is converted slowlyto about 50% in a year-into a new insoluble but crystalline form of sulfur, the crystal structure of which has
not yet been studied.
We have succeeded in preparing also a second ring containing an odd number of sulfur atoms, namely cyclononasulfur, from the titanium-sulfur ring and dichlorotetrasul-
Anyew. Ckem. internat. Edir.
Vol. 12 ( 1 9 7 3 ) 1 No. 6
+s CI
(C5H,)2TiCIZ S ,
Since the reactivity of chlorosulfanes decreases markedly
with increasing chain length, S,CI, can no longer cleave
the titanium-sulfur bond. We therefore made use of a
trick: adding a small amount of hydrogen chloride cleaves
the original ring, giving the sulfane derivative with five
sulfur atoms, which, on reaction with S,CI,, regenerates
HCl and builds up a chain of nine sulfur atoms; and
this cyclizes to the expanded nine-membered ring on elimination of the titanium dichloride complex. Its stability
is about the same as that of s6.
Very recently we have been successful in an analogous
reaction with S6CI2,thus:
+ S6C12
+ S,,
In this way we obtained cycloundecasulfur as the third
compound with an odd-membered sulfur
new modification of sulfur can also be kept only in the
dark and under cooling.
Are there also rings smaller than Sg? We d o not yet
know. From our experiments it appears anyway that if
a pure cyclopentasulfur can be prepared at all it will be
very unstable. Bis(n-cyclopentadieny1)molybdenumtetrasulfide which, as mentioned above, contains a five-membered ring of four sulfur and one molybdenum atom, reacts
with SCI, thus:
The dichloro complex is formed together with elemental
sulfur that is liquid at room temperature. The pronounced
tendency of this sulfur to polymerize has so far prevented
its characterization. The mass spectrum of a fresh sample
did show a strong S: line, but along with mass lines
of larger sulfur ions. We d o not yet know, however, whether
the latter were formed directly during the synthesis or
as a result of polymerization(L71.The question of the existence of a five-membered sulfur ring is thus still open.
Next we may ask a question that has much interest from
a bond-theoretical aspect: Are there well-defined sulfur
rings with an oxidized sulfur atom of oxidation state +6
in the ring, i.e. with a sulfonyl group? According to our
experiments to date we must answer this question in the
negative. We may cite one example: if our compound
with the titanium-sulfur ring is treated, not with SCI,,
but with sulfonyl chloride, the formation of a six-membered
cyclic sulfone is conceivable:
1/2 Slo
however, apparently owing to the instability of a
-S-SO,-Sgroup, sulfur dioxide is split off, as in
Angew. Chem. internat. Edit. J Vol. 12 (1973)
No. 6
In this connection the result of recent experiments by
Steudel is of particular interest. He succeeded in isolating
cyclooctasulfur monooxide from the product of reaction
of “crude sulfane” with thionyl chloride‘
+ S0Cl2 +
+S80 + ?
This is the first sulfur-ring compound containing an oxygen
atom bonded to it.
It is important also that reaction (20) does not give an
isolable five-membered sulfur ring, but instead a further
new sulfur modification, c y c l ~ d e c a s u l f u r [whose
~ ~ ~ , crystals
resemble those of all the other new cyclic sulfurs, except
S,,, S,, and Szo, in having little stability towards light
or heat. Thus we now have the compounds S,, S,, S,,
S9, Slo, S l l , S12, S I Eand S,, available for comparative
study-a task that is still far from complete. The definite
proof of their mere existence, however, throws up several
questions about interpretation of the whole sulfur system
to which I shall return briefly below. But first I shall
insert a few remarks about charged sulfur molecules, some
of which have aroused great interest in the last few years.
5. Charged Sulfur Molecules
The neutral sulfur molecules described in the preceding
Sections change their nature in characteristic manners
when either reduced to oxidation state -2 or oxidized
to oxidation state +2.
It has been known for a very long time that cyclooctasulfur
is reduced by sulfide or hydrogen sulfide ions to polysulfide
ions S:-, these naturally being partly protonated in water.
The nucleophilic attack of S2- ions on the electrophilically
acting S, ring is the rate-determining step in that reaction:
The ring is broken to give the open-chain nonasulfide
ion. Such open-chain polysulfide ions are rapidly degraded
further (=further reduced) to shorter chains[29], for
example thus:
Such negatively charged sulfur ions can also be easily
obtained free from water, for example by reduction of
elemental sulfur by non-noble metals. They are all openchain, doubly negatively charged fragments of a sulfur
For an account of very recent interesting studies on the
colored radical anions S; see Ref.[30a1.
In the reverse process, however, namely oxidation, the
S, ring may under certain circumstances remain intact,
though with a change of conformation.
45 1
As early as 1804[311it was known that sulfur dissolves
in oleum with formation of colored compounds-according to the conditions red, yellow or blue. The best known
of these “compounds”, the blue “S,O,”, has been in the
literature for nearly 100 years[321 and is mentioned in
many textbooks even nowadays. S2[331and recently the
radical ions (X,S-SX,)+[34J or Sl[351
have also been made
responsible for the colors. It is now regarded as proved
that the various colors should be considered as due to
the cations S f : , S;+ and S:+, and I must describe these
A solution of sulfur in oleum must contain other species
in addition to these diamagnetic ions, for such solutions
are paramagnetic, as has been known for 16
The nature of these paramagnetic ions has not yet been
finally decided. S:, S l , S: and S,+ have been discussed[37, the favorite in the most recent investigations
being S:.
Thediamagnetic ions have been studied mainly by GilZespie
et u Z . ’ ~ ~ ] who succeeded in oxidizing elemental sulfur to
the red cations Si: in various r e a c t i o n ~ [ ~ ~ - ~ ~ ] :
+ 3AsF,
+ 3SbF,
2 % + SZ06Fz
+ AsF,
+ SbF,
+ 2S03F-
The deep blue S;+ ion was obtained by further oxidation,
S8 + 3AsF,
+ 5SbF5
0:s = s:0
“s -.s:
11 :I
:s ‘2
s= s” 0
:s.-s: 0
I: I1
..s’s: 0
These charged sulfur molecules will keep research workers
busy for some time yet and may be expected to provide
many further interesting results. They will also exert an
influence on our ideas about elemental sulfur itself, a subject
to which I must now return.
6. Sulfur Vapor
First a few words about the gas phase above sulfur melts:
This contains all possible fragments from S,, and in addition traces of S, and S101301,as proved by mass spectrometry and contrary to statements in the textbooks mentioned
Blue S,, i. e. blue sulfur, whose paramagnetism corresponds
to that of the oxygen molecule, was obtained in a pure
state by Meyer out of the gas phase under certain conditions, or better by photolysis of S2C12, and after being
frozen in a matrix was very exactly chara~terized[“~J.
results from Ginsberg et
should be noted here
as of especial interest: these workers stabilized S,-analogously to 02-by n-bonding it to rhodium or iridium
in phosphane complexes at room temperature (Fig. 12).
+ AsF,
S8(Sb2F,,), + SbF,
X-ray crystal structure d e t e r m i n a t i ~ n cshowed
that in
this case the eight-membered ring remained intact, but
that its conformation was changed from e m - e m (crown
form of cyclooctasulfur) to em-endo (Fig. 11).
Fig. 12. Structure of the complex of S , with iridium
Fig. 1 I . Structure of the Sgl+ ion.
The average S-S separation (2.04 A) in the doubly positively charged ring is identical with that in the neutral
The pale yellow S:+ ion is obtained from S, and Sz06F2
in liquid sulfur dioxide or from S, and SbF, at 150°C.
The compounds S4(SbF6), and S,(SO,F), are diamagnetic.
Spectroscopic findings[44] provide compelling reasons for
believing that, like Se:+ which has been defined by X-ray
structure investigation~[~~J,
the cation S:+ exists as a planar
“Huckel aromatic compound:
But what d o we know about the nature of the other
components of the gas phase? The mass, i. e. the number
of atoms, of the higher aggregates of S3 tells us nothing
at all. It seems an impermissibly crude simplification to
regard the problem as solved by determining the masses
of the aggregates of sulfur atoms that occur as positive
ions in a mass spectrometer. Many problems merely begin
at that stage! For example, is S 3 a ring, a diradical chain,
a trithioozone or sulfur dithiooxide?
.. .\..
Angew. Ckem. internat. Edit. 1 Vol. I 2 (1973)
NO. 6
In this connection there is the interesting new observation
that S, and S4, obtained by photolysis of S3CIzand S,CIz,
respectively, in an organic glass correspond exactly in
their absorption and thus in their colors-intense green
and dark red, respectively-to
the colors of hot sulfur
melts and hot sulfur vapor. They d o indeed cause the
color of the melt and vapor‘461.
To go a step further, what is the structure of S4, which
we know in the gas phase? Is it a ring, a diradical, a
bipolar compound or a chain with double bonds? Recent
HMO calculations[461ascribed a completely unexpected,
especial stability to a branched structure, namely sulfur
... \$
s - s
...?. / \.?...
.. \.s.:-
This mathematical result certainly sets a challenge to the
preparative chemist.
And we know absolutely nothing about the aggregates
s,, s6, S,, s8.S9, s,,, S l 1 , and S 1 2 beyond that they
have been shown to exist in the gas phase.
7. Sulfur Melts
We return now to the melts, which are surely the most
complicated part of sulfur chemistry and about which
we are still largely in the
After many unsuccessful
attempts to interpret their singular phenomena, the hypothesis of Tobolsky and Eisenberg, which has a most polished
is nowadays generally
O n this hypothesis a sulfur melt represents a reversible
monomer-polymer equilibrium with cyclooctasulfur as the
“monomer” which is via catenaoctasulfur in thermal
equilibrium with high-polymeric plycatenasulfur, thus:
This interpretation can, however, no longer be maintained
unchanged. The existence, in the melt, of sulfur compounds
other than multiples of S, is proved unambiguously. The
problem is thus open again and is the subject of renewed
intensive theoretical study. According to Semylen’s more
recent calculations[511, even at 120”C, i. e. barely above
the melting point, the mobile, pale yellow S, melt contains
8.2wt.-% of larger rings containing an average of 13.8
atoms per ring. The “melting point” of sulfur is thus clearly
a decomposition point and, by the theory of “regular solutions”, we have estimated a true melting point of z 133“C
for S8[l0l.At 120°C the still mobile melt already contains
almost 20% of rings with an average of 17.6 atoms per
ring, and at 160°C there should be 30% of larger rings.
Angew. Chem. internat. Edit.
1 Vol. 12 (1973) 1 No. 6
What happens at the critical temperature where at a single
stroke the melt suddenly becomes so viscous that the
flask can be turned upside down without anything spilling
out? We must admit that we d o not yet know. PresumabIy
we are then dealing with giant macromolecules. But why
are they formed so suddenly and what is their structure?
Are they rings or chains, and, if chains, are they diradicals
or polar chains or d o they contain fluxional double bonds?
The problem of their conformation seems beyond hope
when we remember that each time a chain is lengthened
by one atom there are two equally justifiable alternatives
for the new dihedral angle; thus simple calculation shows
that there are 2(n-3)(!j possibilities for each chain (n= the
number ofchain members), which gives about 130 possibilities for even 10 atoms and 9OOO for 20 atoms; the theoretically possible conformations for molecules containing
several hundred thousand atoms, such as may well exist
in sulfur melts at z 200”C, run into astronomical figures.
Why d o we nevertheless wish to clarify the situation?
Well, because a knowledge of what happens in sulfur melts
would enable us to influence their behavior and thus to
multiply the practical applications of this substance that
is so cheap and available in such huge quantities.
On continued heating, sulfur melts slowly become more
mobile again and an increasingly deeper red until finally
almost black-red. Because of thermal cracking of the macromolecules into smaller fragments the viscosity decreases
up to the boiling point of 444°C. The composition of
the melt at this temperature is quite unknown. According
to investigations by Meyer, the color becomes continuously
more intense because, from the temperature of liquid air
at which sulfur is snow-white, u p t o about 200”C, there
is a steady red shift of the absorption edge and thus of
the absorption in the visible region, and this is caused
by increasing thermal population of excited states of S8
and later also by absorption due to higher polymers. The
much stronger red shift of the absorption edge and thus
the faster increase in color that finally occur at the higher
absorption maxima appear at 4100
and 5300A-are to be ascribed to formation of S, and
S4 molecules which are present in a concentration of = 2%
in hot polymeric sulfur and also cause the very intense
color of the
The longer radical chains, analogous
to the isoelectronic alkanediyl radicals, should be almost
black. I cannot quite reconcile myself to the view that
chilled, insoluble sulfur is again pale because the chain
ends are saturated by impurities, for example hydrogen
atoms, although that cannot be excluded-polymeric
alkanes are, after all, colorless. However, cycloalkanes are
also colorless! It seems thus more probable that “insoluble”
sulfur consists of cyclic molecules which, as we know from
the solubility relations of cyclododecasulfur, need not be
all too large.
The proof that there are very reactive molecules such
as S3 and S4 in sulfur melts throws a new light also
on the very interesting reactions of hot sulfur with hydrocarbons, but I cannot go into that subject here.
A few words more about the much discussed “critical
temperature” of sulfur.’Our view is that, in principle, it
cannot be determined. The prolonged dispute about the
exact polymerization temperature, often treated as a matter
of the purity of the sulfur, is irrelevant today, as a few
simple experimental results will show. First, however, an
interesting interpretation of the conditions in sulfur melts
should be mentioned, which was suggested several years
ago but has been largely negle~ted‘~’].Wiewiorowski et
a / . interpreted the conditions in sulfur melts on the &asis
that diradical sulfur chains form charge-transfer complexes
with still unbroken S8 rings as chain endings. Can we
then imagine that, at the “critical temperature”, the concentration of the eight-membered rings still present in the
equilibrium mixture no longer suffices to stabilize the
relatively short chains, so that these unite to give the
macromolecules observed and thus cause the sudden
increase in viscosity?
We are now familiar with highly viscous and therefore
polymeric sulfur at temperatures as low as ~ 4 5 ° C - t h i s
is the pale yellow polymerizate of cycloheptasulfur.
Between 60 and 80°C, &, s,, S,,, and s,, form a similar
polymerizate which on further heating to above z 120°C
becomes mobile again and then once more plastic before,
at 444 “C, it is once more less viscous and boils. However,
we are also able to alter the “critical temperature” of
“ordinary” sulfur by design: adding a mere ~ 2 of
% s6
to sulfur melts at = 150°C lowers the polymerization temperature by z1O”C-a phenomenon that lasts for ca.
20 minutes. And the opposite is also possible: we can
prevent the polymerization of sulfur to highly viscous compounds up to at least 200°C-i.e. retain sulfur melts in
a mobile state-by adding as little as 1-2% of thermally
stable,cyclic sulfur compounds such as trithiane or trimeric
thioacetaldehyde. Do these rings take over the stabilizing
role of the unstable S, rings in formation of complexes
by sulfur chains? Anyway, they can be recovered from
the sulfur unchanged and q~antitatively[’~?
At this point,
however, I must break off speculation and thus also the
discussion of liquid sulfur.
8. General Concluding Remarks
It is to be expected that the sulfur problem will continue
to excite interest for a long time yet, even if no further
groups of atoms are unrolled. Alone the future necessity,
for prevention of environmental pollution, to remove sulfur
dioxide from the combustion products of fossil fuels and,
preferably, for practical reasons, to convert it into elemental
will have its effect on the price of sulfur: the
world price of sulfur, which has varied for so long, will
then settle down into a “valley” which will perhaps be
deeper than has previously been assumed. This foreseeable
development should encourage us to make a virtue of
necessity ! If by solving the environmental SO, problem
we make sulfur into a new cheap material, what large
quantities will be available! The following few, arbitrarily
selected, potential uses, which we can only sketch here,
are based on serious experiments in different laboratories:
sulfur as a filler to modify organic polymers, as a covering
for slow-acting fertilizers, as a material for marking streets,
as an additive to improve the quality of bituminous material in road construction; and, finally, in the building industry-potentially a particularly important application-as
a surprisingly stable mortar for conventional building
stone, but also as a construction unit on its own if filled
with sand or made into a foam; etc. The idea of a “front
parlor” made of sulfur is certainly unusual, but that is
not intended; we should think rather of the need that
so often arises for urgent manufacture of prefabricated
buildings units for disaster areas: heated sand, mixed with
sulfur, would be ready for immediate use.
In this connection a final word on flammability. Several
recent investigations have shown that adding a few percent
of certain materials, such as styrene and maleic acid,
reduces the flammability of sulfur drastically-sufficiently
for practical use-and that conversely, even though it
sounds like a paradox, adding sulfur to polystyrene or
polyurethane foams considerably increases their fire-resistan~e[~’].
The more we learn how to modify the mechanical
and chemical properties of the system “elemental sulfur”,
the more varied will be its future applications as a raw
material; and these modifications will be the more successful the more knowledge and understanding we have of
Insofar as this report treats the results of our own School,
thanks are due t o the dedicated and skilled work of the
following collaborators: Frau B. Jutzi, Dr. H . D. Block,
Frau Dr. B. Block, Dr. G. Knippschild, Dr. H . Kopf, and
Dr. E . Wilhelm, who has been engaged for the longest time
and most intensively with elemental suyur. Grateful thanks
are oflered for financial support from Buyer AG, the Fonds
der Chemischen Industrie, the Deutsche Forschungsgemeinschaft and the Sulphur Institute WashingtonlLondon.
Received: July 31, 1972 [A 940 IE]
German version: Angew. Chem. 85,474 (1973)
[I] Current Academic Research in Sulphur Chemistry 1968. The Sulphur
Institute, Washington, D. C. and London, December 1968; Current
Academic Research in Sulphur Chemistry, North America 1969. The
Sulphur Institute, Washington, D. C. and London, September 1969.
[2] M . Schmidt, Oesterr. Chem. Zt. 64, 236 (1963).
131 E.9.: Genesis 19, Verse 24.
[a] E.g.r F. A. v. Wasserberg: Chemische Abhandlung vom Schwefel.
J. P Krauss, Wien 1788.
[5] J . Donohue and B. Meyer in B. Meyerr Elemental Sulfur. Wiley-lnterscience, New York 1965, p. 1.
[6] G. Knippschild, Dissertation, Universitat Wurzburg 1968.
[7] M . Schmidt and D . Eichelsdorfer, Z. Anorg. Allg. Chem. 330, 122
( 1964).
[8] H . D. Block, Dissertation, Universitat Wurzburg 1969.
[9] M . Schmidt, Angew. Chem. 73, 394 (1961).
[101 M. Schmidt and H. D. Block, 2.Anorg. Allg. Chem. 385, 1 19 ( I 97 1 ).
[I I] F. Tuinstra: Structural Aspects of the Allotropy of Sulphur and
the other Divalent Elements. Uitgeverij Waltman, Delft 1967.
[12] M . Schmidt and E. Wilhrlm, Angew. Chem. 78, I020 (1966); Angew.
Chem. internat. Edit. 5, 964 (1966).
[131 L. Pauling, Proc. Nat. Acad. Sci. U. S. 35, 495 ( I 949).
[I41 A. Kutoglu and E . Hellner, Angew. Chem. 78, 1021 (1966); Angew.
Chem. internat. Edit. 5,965 (1966).
[I51 M . Schmidt and H . D. Block, Angew. Chem. 79,944 (1967); Angew.
Chem. internat. Edit. 6 , 955 (1967).
[I61 M . Schmidt and E . Wilhelm, unpublished work.
[16a] 7: Debaerdemaeker and A . Kutoglu, Naturwiss 60, 49 (1973).
[16b] M . Schmidt, E. Wilhelm, T Debaerdemaeker, and A . Kutoglu, Naturwiss., in press.
1171 H . KOpL B. Block, and M . Schmidt, Chem. Ber. 101, 272 (1968).
[I81 P. E . Jones and L. K a t z , Chem. Commun. 1967, 842.
1191 H . KOpJ Angew. Chem. 81, 332 (1969); Angew. Chem. internat.
Edit. 8, 375 (1969).
Angew. Chem. internat. Edzt.
Vol. 12 ( I 973) 1 No. 6
[20] M. Schmidt, K . Blijrfner,P. Kochendorfer,and H . Ruf, 2. Naturforsch.
21 b, 622 (1966).
[37] M. Stillings, M . C. R. Symons, and J . G . Wilkinson, J. Chem. SOC.
A 1971,3201.
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[38] M. C. R. Symons and J . G. Wilkinson, Nature Phys. Sci. 236, 127
[39] R. J . Gillespie and J . Passmore, Accounts Chem. Res. 4,413 (1971).
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[41] R . J . Gillespie, J . Passmore, P. K . Ummat, and 0.C. Vaidya, Inorg.
Chem, 10, 1327 (1971).
1421 J . Barr, R. J . Gillespie, and P. K . Ummat, Chem. Commun. 1970,
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10, 2781 (1971).
[44] P. J . Stevens, Chem. Commun. 1969, 1496.
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2319 (1971).
[46] B. Meyer, Advan. Chem. Ser. 110, 53 (1972).
[47] A. P. Ginsberg and W E. Lindsell, Chem. Commun. 1971, 232.
[48] W D. Bonds and J . A. ibers, J. Amer. Chem. SOC.94, 3413 (1972).
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[53] M. Schmidt and B. Jutzi, unpublished work.
1541 M. Schmidt, Int. J. Sulfur Chem. Part B, 7, 91 (1972)
1551 For potential applications of elemental sulfur see, e. g. the quarterly
Sulphur lnst. J.; also H . L. Fike, Advan. Chem. Ser. 110, 208 (1972).
[22] R. Cooper and J .
I/. Culka,
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Chem. 80,660 (1968); Angew. Chem. internat. Edit. 7, 632 (1968).
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[27] M. Schmidt and E. Wilhelm, unpublished work
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Hydrazidinyl Radicals :
1,2,4,5Tetraazapentenyls, Verdazyls, and Tetrazolinyls
By Franz A.
Dedicated to Professor Theodor Wieland on the occasion of his 60th birthday
This report reviews the preparation of hydrazidinyls and their chemical and physical properties.
1. Introduction
and in many cases even exceed the stability of
the radical standard diphenylpi~rylhydrazyl[~~.
x radicals with allylic structure are often notable for their
great stability, especially when the allylic system leads
to an extensive, symmetrical delocalization of the unpaired
electron. An historically interesting[**]example for such
a stable ally1 radical is (2,2'-biphenyldiyl)phenylallyl[11.
Kuhn and Trischmann[21 discovered a remarkably stable
class of allylic radicals in the verdazyls ( 3 ) . These radicals
are much more stable than the corresponding hydrazyls
. /
y] Doz. Dr. F. A. Neugebauer
Max-Planck-Institut fur Medizmische Forschung
Abteilung Molekulare Physik
69 Heidelberg, Jahnstrasse 29 (Germany)
[**I The paper in ref. [I] was first submitted to J. Amer. Chem. Soc.
for publication on June 9, 1932, but not accepted until April 10, 1957.
The paper was rejected in 1932 because the general opinion was that
the properties of the compound were incompatible with the structure
of a carbon radical. The unequivocal proof of the structure with the
ESR method was not possible until 25 years later.
Angew. Chem. internat. Edit. 1 Vol. 12 (1973)
1 No. 6
The discovery of the verdazyls has prompted the examination of other radicals having the same basic structure
((2) and ( 4 ) ) . We shall refer to this basic structural element
as the hydrazidinyl system, in analogy to the nomenclature
of the amidines and hydrazidines. The various representatives of this class are then hydrazidinyls.
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