Table 2 Selected bond lengths (Å) and angles (Њ) for (Me2N)3PSI2
Table 4 Selected bond lengths (Å) and angles (Њ) for [{(Me2N)3-
PS}2S]2ϩ 2[Br3]Ϫ
I(1)–I(2)
I(1)–S(1)
2.856(1)
2.705(3)
P(1)–S(1)
2.014(4)
118.4(4)
Br(1)–Br(2)
Br(2)–Br(3)
Br(4)–Br(5)
Br(5)–Br(6)
2.506(1)
2.538(1)
2.550(1)
2.521(1)
S(1)–S(2)
S(1)–P(1)
S(2)–S(3)
S(3)–P(2)
2.032(3)
2.119(3)
2.053(3)
2.079(3)
S(1)–I(1)–I(2)
N(2)–P(1)–N(1)
177.98(6)
102.6(3)
N(2)–P(1)–S(1)
Br(1)–Br(2)–Br(3)
Br(6)–Br(5)–Br(4)
S(2)–S(1)–P(1)
176.51(5)
177.38(5)
104.2(1)
S(1)–S(2)–S(3)
S(2)–S(3)–P(2)
104.7(1)
100.7(1)
Table 3 Selected bond lengths (Å) and angles (Њ) for Ph3PS(I0.89
Br0.11)Br
-
S(1)–I(1)
I(1)–Br(2)
2.656(1)
2.6832(6)
P(1)–S(1)
2.007(1)
S(1)–I(1)–Br(2)
175.13(2)
P(1)–S(1)–I(1)
107.63(5)
δ 49.2.16 Elemental analysis of this solid confirms its identity as
Ph3PBr2 [Found (Calc.): C, 51.5 (51.2); H, 3.7 (3.6); Br, 37.8
(37.9)%]. Concentration of the resultant filtrate produced some
pale yellow crystals, one of which was selected for analysis by
single crystal X-ray diffraction. A unit cell determination of
this material revealed it to be S8. Clearly, reaction of Ph3PS with
dibromine results in the cleavage of the phosphorus–sulfur
bond, eqn. (2).
N2, ca. 2 d
(2)
8 Ph3PS ϩ 8 Br2
8 Ph3PBr2 ϩ S8
Et2O, RT
This result is in direct contrast to the analogous reaction
of triphenylphosphine selenide with dibromine13 which, in sol-
vents of low relative permittivity, produces the T-shaped adduct
Ph3PSeBr2, analogous to (Me2N)3PSeBr2 and (C6H11)3PSeBr2
previously described. However, reaction of Ph3PSe with Br2 in
solvents of higher relative permittivity, e.g. CH2Cl2, produces
the unusual dinuclear complex [Ph3PSeBrSePPh3]Brؒ2CH2Cl2,
thus illustrating that the phosphorus–sulfur bond is more sus-
ceptible to cleavage upon reaction with dihalogens than the
phosphorus–selenide bond.
Fig. 3 The crystal structure of [{(Me2N)3PS}2S][Br3]2.
In a final attempt crystallographically to characterise the elu-
sive R3PSBr2, we decided to treat (Me2N)3PS with Br2 in diethyl
ether solution. This tertiary phosphine sulfide was chosen since
it contains a very basic parent tertiary phosphine and we have
previously reported that the analogous compound, (Me2N)3-
PSe, reacts with dibromine to produce (Me2N)3PSeBr2 quanti-
tatively.12 In the reaction of (Me2N)3PS with Br2, after ca. 2 d
a large quantity of yellow powder was produced which was
isolated by standard Schlenk techniques. Recrystallisation of
the product from diethyl ether solution (dichloromethane was
avoided since the use of this solvent may have resulted in
cleavage of the phosphorus–sulfur bond) at 50 ЊC produced, on
standing at room temperature for ca. 5 d, a small crop of
yellow-orange crystals which we assumed to be (Me2N)3PSBr2.
The crystals were plunged into an inert oil under anaerobic
conditions and examined under the microscope. From these,
one was chosen for analysis by single crystal X-ray diffraction.
Surprisingly, the material proved to be the unusual ionic com-
pound [{(Me2N)3PS}2S]2ϩ 2[Br3]Ϫ, Fig. 3, and not the expected
1:1 addition compound (Me2N)3PSBr2. Selected bond lengths
and angles are in Table 4. Clearly, this material cannot be con-
sidered as representative of the only bulk product from the
reaction of (Me2N)3PS with Br2, but it is nevertheless isolated in
significant yield (ca. 30%) and provides an interesting insight
into the complex reaction of certain R3PS compounds with
dibromine. One possible other product is the free phosphine
(Me2N)3P, although this was not observed in the 31P-{1H}
NMR spectrum of the bulk material. It is possible to speculate
that during the reaction phosphorus–sulfur bond cleavage has
again occurred, but only for some of the (Me2N)3PS molecules.
The free elemental sulfur thus produced may then react with
dibromine to produce transient dications (e.g. SBr2) which then
react with other (Me2N)3PS moieties producing the dipositive
cation [{(Me2N)3PS}2S]2ϩ, the charge being balanced by tri-
bromide anions. Again, no evidence for a sulfur–bromine bond
is observed. No cation of the formula [(R3PS)2S]2ϩ has previ-
ously been crystallographically characterised; however, the
analogous tellurium containing cation, [(But3PTe)2Te]2ϩ, has
been described by Kuhn et al.17 This cation may be considered
as a tellurophosphorane Te2ϩ complex or as a phosphine stabil-
In order to gain further information concerning the reaction
of R3PS with dibromine, we also investigated the reaction of
(C6H11)3PS with dibromine in diethyl ether solution. Reaction
of dibromine with this triorganophosphine sulfide appeared to
proceed in a different way than the analogous reaction with
Ph3PS, described above. Tricyclohexylphosphine sulfide reacts
with dibromine over ca. 2 d to produce a yellow solid. Elem-
ental analysis of this material suggests the formation of a 1:1
adduct, (C6H11)3PSBr2, Table 1. Further evidence for this
adduct formation may be inferred from its IR spectrum, clearly
illustrating a band due to the phosphorus–sulfur stretch, thus
confirming that, in contrast to the reaction of Ph3PS with Br2,
which results in cleavage of the P–S bond to produce Ph3PBr2,
reaction of (C6H11)3PS does not result in cleavage of the P–S
bond. Moreover, ν(P–S) for (C6H11)3PSBr2, 573 cmϪ1, is shifted
downfield compared to ν(P–S) for the parent tricyclohexyl-
phosphine sulfide, 629 cmϪ1. Both we11–13 and other workers3,10
have previously noted both in the present and previous studies
that this is a good indication of adduct formation since ν(P–S)
shifts to lower frequency upon co-ordination of a halogen atom
to the sulfur donor, as expected. Final confirmation of the
formation of (C6H11)3PSBr2 should be provided by its 31P-{1H}
NMR spectrum, which would be expected to exhibit a single
peak, shifted from that observed from the parent tertiary phos-
phine selenide. Unfortunately, (C6H11)3PSBr2 is insoluble in
non-polar solvents. Dissolution of the material in polar sol-
vents such as CDCl3 results in cleavage of the P–S bond to
produce (C6H11)3PBr2 and, presumably, elemental sulfur, since a
single resonance at δ 103.9 is observed which is very close to the
reported value for tricyclohexylphosphine dibromide.16 This
behaviour mirrors the triphenylphosphine sulfide–dibromine
system, although in this case P–S bond cleavage occurs regard-
less of the relative permittivity of the solvent.
2228
J. Chem. Soc., Dalton Trans., 1999, 2225–2230
Cross, Wendy I.
