Hypervalent sulfur-functionalized diphosphagermylene and diphosphastannylene compounds

Article abstract of DOI:10.1021/om2008327

The reaction between either GeCl₂(1,4-dioxane) or SnCl ₂ and 2 equiv of the lithium phosphide [{(Me₃Si) ₂CH}P(C₆H₄-2-SMe)]Li(tmeda) gives the corresponding diphosphatetrylenes [{(Me₃Si)₂CH}P(C ₆H₄-2-SMe)]₂E [E = Ge (10), Sn (11)] in good yields. Both 10 and 11 crystallize as discrete monomers in which the Ge and Sn atoms are coordinated by both P and S atoms. Although 10 and 11 crystallize as racemic mixtures of the RR and SS diastereomers, variable-temperature NMR experiments suggest that, in solution, these compounds are in dynamic equilibrium with small amounts of the corresponding RS and SR diastereomers. DFT calculations reveal that the lowest-energy minima for both 10 and 11 possess rac stereochemistry; two higher-energy minima were located for each of 10 and 11, both of which have meso stereochemistry. The two calculated meso diastereomers differ in the location of the sulfur and phosphorus substituents within the pseudo-trigonalbipyramidal structures. Both 10 and 11 decompose on exposure to light, generating the diphosphine {(Me₃Si) ₂CH}(C₆H₄-2-SMe)P-P(C₆H ₄-2-SMe){CH(SiMe₃)₂} (14) as the major product.

Full text of DOI:10.1021/om2008327

Article
pubs.acs.org/Organometallics
Hypervalent Sulfur-Functionalized Diphosphagermylene and
Diphosphastannylene Compounds
Keith Izod,* Ewan R. Clark, William Clegg, and Ross W. Harrington
Main Group Chemistry Laboratories, School of Chemistry, Bedson Building, Newcastle University, Newcastle upon Tyne, NE1 7RU,
U.K.
S
* Supporting Information
ABSTRACT: The reaction between either GeCl2(1,4-dioxane) or
SnCl2 and 2 equiv of the lithium phosphide [{(Me3Si)2CH}P(C6H4-
2-SMe)]Li(tmeda) gives the corresponding diphosphatetrylenes
[{(Me3Si)2CH}P(C6H4-2-SMe)]2E [E = Ge (10), Sn (11)] in good
yields. Both 10 and 11 crystallize as discrete monomers in which the
Ge and Sn atoms are coordinated by both P and S atoms. Although
10 and 11 crystallize as racemic mixtures of the RR and SS
diastereomers, variable-temperature NMR experiments suggest that,
in solution, these compounds are in dynamic equilibrium with small
amounts of the corresponding RS and SR diastereomers. DFT
calculations reveal that the lowest-energy minima for both 10 and 11
possess rac stereochemistry; two higher-energy minima were located
for each of 10 and 11, both of which have meso stereochemistry. The
two calculated meso diastereomers differ in the location of the sulfur and phosphorus substituents within the pseudo-trigonal-
bipyramidal structures. Both 10 and 11 decompose on exposure to light, generating the diphosphine {(Me3Si)2CH}(C6H4-2-
SMe)P-P(C6H4-2-SMe){CH(SiMe3)2} (14) as the major product.
iPr3C6H2],5 and dimeric species, such as {(tBu2P)2Pb}2,6
INTRODUCTION
Over the last two decades, the chemistry of diaminocarbenes
■
{((Me3Si)2P)2Pb}2,7 and {(iPr2P)2Ge}2;8 several ate complexes
and mixed group 2/group 14 phosphide and phosphinidene
(R2N)2C and their cyclic analogues (N-heterocyclic carbenes,
complexes have also been reported.11−13 In addition, Bertrand
NHCs) has seen a rapid and extraordinary expansion, due
and co-workers have shown that, under very specific conditions,
principally to the unique properties of these compounds and
stable P-heterocyclic carbenes are accessible, in which the
their widespread application as ligands for catalytically active
transition-metal complexes.1 The stability of diaminocarbenes is
phosphorus atoms adjacent to the carbene center approach
planarity.14
largely due to efficient pπ−pπ overlap of the nitrogen lone pairs
with the vacant p orbital at carbon, which mitigates the electron
deficiency of this center and which results in these compounds
having good σ-donor, but poor π-acceptor, ligand character-
We have recently reported a series of intramolecularly base-
stabilized mono- and diphosphatetrylenes (1−8, Chart 1).15−18
The structures and dynamic behavior of these compounds are
significantly affected by the nature of the group 14 center and
istics.
the size of the potential chelate ring. In the solid state, the
diphosphagermylenes 5 and 7 crystallize as monomers in which
one phosphide ligand chelates the germanium center, while the
other acts as a terminal P-donor ligand; in both structures, the
second amino substituent is oriented away from the germanium
center.15,17 A similar situation prevails for the tin analogue 6,
which has one chelating and one terminal phosphide ligand;16
however, 8 (the tin analogue of 7) crystallizes with a somewhat
different structure in which the second amino substituent forms
a relatively close contact with the tin center (Sn−P, 3.306(3)
Å).17
While the first stable diaminocarbenes were reported in the
early 1990s, the heavier group 14 analogues of these
compounds, the diaminotetrylenes, (R2N)2E (E = Ge, Sn,
Pb), have been known for some time.2 This is principally due to
the increasing stability of the +2 oxidation state with increasing
atomic number, and the ready availability of Ge(II), Sn(II), and
Pb(II) precursors. However, despite this, there are surprisingly
few examples of stable diphosphacarbenes, (R2P)2C, and
diphosphatetrylenes, (R2P)2E. This may be attributed to the
large barrier to inversion at phosphorus, which inhibits
planarization at the phosphorus center, a key requirement for
effective pπ−pπ overlap of the phosphorus lone pair with the
vacant p orbital at the group 14 element center.3,4 Nonetheless,
a small number of diphosphatetrylenes and related species have
been reported,5−10 including both monomeric species, such as
[{(Tripp)2FSi}(iPr3Si)P]2E [E = Ge, Sn, Pb; Tripp = 2,4,6-
In solution, the diphosphagermylenes 5 and 7 are subject to
dynamic equilibria that (i) interconvert the chelating and
Received: September 5, 2011
Published: December 7, 2011
© 2011 American Chemical Society
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imperative as this prevents isomerization of the lithium salt 9 to
the corresponding thiolate [{(Me3Si)2CH}P(Me)(C6H4-2-S)]-
Li(tmeda) (see below).19 Compounds 10 and 11 are highly
soluble, even in hydrocarbon solvents, but were crystallized as
orange blocks suitable for X-ray crystallography from cold
hexamethyldisiloxane containing a few drops of diethyl ether.
Both 10 and 11 crystallize under these conditions as diethyl
ether solvates, although the two compounds differ in the
proportion of solvent incorporated: thus, 10 crystallizes as the
hemisolvate [{(Me3Si)2CH}P(C6H4-2-SMe)]2Ge·1/2Et2O
(10a), whereas 11 crystallizes as the monosolvate
[{(Me3Si)2CH}P(C6H4-2-SMe)]2Sn·Et2O (11a). For 11a, the
solvent of crystallization is only weakly held and is rapidly lost
under vacuum; 1H NMR spectra and elemental analyses of 11a
reveal substoichiometric quantities of diethyl ether to be
present after exposure of the crystalline material to vacuum for
a few minutes. The solvent of crystallization in 10a appears to
be more tightly held and is not lost on exposure to vacuum for
short periods.
Chart 1
We have previously reported the straightforward synthesis of
the heteroleptic compounds 1−4 from the reaction between
either GeCl2(1,4-dioxane) or SnCl2 and the corresponding
lithium or potassium phosphide.15−17 Somewhat surprisingly,
attempts to prepare the corresponding heteroleptic, thioether-
functionalized compounds [{(Me3Si)2CH}P(C6H4-2-SMe)]-
ECl (E = Ge, Sn) were unsuccessful. The reaction between 9
and 1 equiv of GeCl2(1,4-dioxane) yields only the homoleptic
compound 10, as judged by H and 31P{1H} NMR spectros-
1
terminal phosphide ligands and (ii) interconvert diastereomers
via inversion at the phosphorus and/or germanium centers.15,17
For 5 and 7, both of these processes may be observed by
variable-temperature NMR spectroscopy. For 6, epimerization
appears to be rapid on the NMR time scale and only
interconversion of the chelating and terminal phosphide ligands
is observed.16 For 8, a different situation prevails: at high
temperatures, both epimerization and chelating-terminal ligand
exchange are rapid, whereas at low temperatures, a single
pseudo-trigonal-bipyramidal species is observed in which both
amino groups are bound to the tin center (8a).17
We now report the synthesis of a new diphosphagermylene
and diphosphastannylene and show that incorporation of a soft
sulfur-donor functionality into the phosphide ligand favors the
formation of four-coordinate pseudo-trigonal-bipyramidal
structures for these species.
copy, and half an equivalent of the known adduct
GeCl2(tmeda) (the identity of this latter compound was
confirmed by X-ray crystallography).20 The corresponding
reaction between 9 and 1 equiv of SnCl2 similarly yields 11 as
the only phosphorus-containing product, which was crystallized
from n-hexane as the solvate [{(Me3Si)2CH}P(C6H4-2-
SMe)]2Sn·1/2C6H14 (11b), as confirmed by X-ray crystallog-
raphy (see the Supporting Information). In an attempt to rule
out the intermediacy of tmeda in the formation of 10 and 11,
we prepared the 12-crown-4 adduct of 9, [{(Me3Si)2CH}P-
(C6H4-2-SMe)]Li(12-crown-4) (9a) as a potential starting
material; however, the reaction between 9a and 1 equiv of
SnCl2 again yields the homoleptic compound 11. A small
number of colorless crystals, which were isolated from this
reaction, were shown by X-ray crystallography to be the
trichlorostannate complex [Li(12-crown-4)(THF)][SnCl3]
(12) (see the Supporting Information). The isolation of 10−
12 from these reactions suggests that ligand exchange is rapid
under the conditions employed and that there is a strong
preference for the formation of the homoleptic diphosphate-
trylenes. This is in marked contrast to the behavior of related
amino-functionalized ligands, where the heteroleptic species 1−
4 are readily isolated. The isolation of the homoleptic
compounds in these reactions may be attributed to the softer
nature and reduced steric demands of the SMe groups in 10
and 11, compared with the hard, more sterically demanding
NMe2 groups in 1−8.
RESULTS AND DISCUSSION
■
Synthesis. The reaction between 2 equiv of the lithium
phosphide [{(Me3Si)2CH}P(C6H4-2-SMe)]Li(tmeda) (9)19
and either GeCl2(1,4-dioxane) or SnCl2 in THF, in the absence
of light, gives the corresponding diphosphatetrylenes
[{(Me3Si)2CH}P(C6H4-2-SMe)]2E [E = Ge (10), Sn (11)]
as orange, air-sensitive solids in 64 and 90% yield, respectively
(Scheme 1). The exclusion of light during these reactions is
Scheme 1. R = CH(SiMe3)2
Solid-State Structures. The structures of 10a and 11a are
shown in Figure 1, along with selected bond lengths and angles.
Both 10a and 11a crystallize as discrete molecular species in
which the group 14 centers are four-coordinate, bound by the P
and S atoms of two chelating phosphide ligands [bite angles:
10a, 76.11(5)° and 75.41(5)°; 11a, 72.47(5)° and 72.97(5)°].
This affords a pseudo-trigonal-bipyramidal geometry at the
group 14 element center in each case, with the sterically
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2.393(2) and 2.386(2) Å].21 Similarly, there are no previous
reports of crystallographically characterized compounds in
which a Sn(II) center is bound by a thioether. The Sn−S
distances of 2.9055(17) and 2.9404(17) Å are rather long; for
example, the Sn−S distances in trimeric (DippS)Sn(μ-
DippS)2Sn(μ-DippS)2Sn(DippS) range from 2.471(5) to
2.838(4) Å,22 whereas the corresponding distances in
{(Me3Si)3C}Sn(μ-S-t-Bu)2Li(THF)2 are 2.568(1) and
2.574(1) Å [Dipp = 2,6-Pri2C6H3].21
Solution-State Behavior. Our previous variable-temper-
ature and multielement NMR spectroscopic studies have shown
that the amino-functionalized diphosphagermylene 7 is subject
to dynamic equilibria in solution.17 The low-temperature
31P{1H} NMR spectrum of 7 exhibits signals due to three of
the four possible diastereomers, whereas at room temperature
and above, a single set of ligand signals is observed in both the
1
31P{1H} and the H NMR spectra due to rapid exchange
between diastereomers and between the chelating and terminal
phosphide ligands. The solution behavior of 8 is somewhat
different: while at room temperature, 8 appears to be subject to
similar dynamic equilibria to those observed for 7, inter-
conversion between diastereomers appears to be more rapid
than exchange between the chelating and terminal phosphide
ligands, and exchange between diastereomers may not be
1
frozen out. At −80 °C, the 31P{1H}, H, and 119Sn NMR
spectra of 8 are consistent with the predominance of a single
pseudo-trigonal-bipyramidal species (8a).17
The 31P{1H} NMR spectra of 10 and 11 exhibit somewhat
different characteristics to those of both 7 and 8. At room
temperature, the 31P{1H} NMR spectrum of 10 in d8-toluene
exhibits a single, slightly broadened singlet at −26.6 ppm. As
the temperature is reduced, this signal broadens significantly
and moves to higher field, until, at −30 °C, the spectrum
consists of a broad singlet at −28.0 ppm. Below this
temperature, this signal sharpens again, until, at −60 °C, the
spectrum consists of a single, reasonably sharp singlet at −28.3
ppm.
Figure 1. Molecular structures of (a) 10a and (b) 11a with 40%
probability ellipsoids; H atoms and solvent of crystallization omitted
for clarity. Selected bond lengths (Å) and angles (deg). For 10a: Ge−
P(1) 2.4302(16), Ge−P(2) 2.4327(14), Ge−S(1) 2.7793(15), Ge−
S(2) 2.8220(15), P(1)−Ge−S(1) 76.11(5), P(2)−Ge−S(2) 75.41(5),
S(1)−Ge−S(2) 148.26(4), P(1)−Ge−P(2) 100.63(5). For 11a: Sn−
P(1) 2.6080(17), Sn−P(2) 2.6087(17), Sn−S(1) 2.9404(17), Sn−
S(2) 2.9055(17), P(1)−Sn−S(1) 72.47(5), P(2)−Sn−S(2) 72.97(5),
S(1)−Sn−S(2) 140.66(5), P(1)−Sn−P(2) 100.71(5).
1
At −60 °C, the H NMR spectrum of 10 in d8-toluene is
entirely consistent with the structure observed crystallo-
graphically: a single set of ligand signals is observed, in which
the diastereotopic SiMe3 groups give rise to distinct singlets at
−0.09 and −0.36 ppm. As the temperature is increased, the
SiMe3 signals coalesce to give a single peak, such that, at room
temperature, these protons give rise to a single, relatively sharp
singlet at 0.09 ppm. Similarly, the well-resolved set of aromatic
signals observed at −60 °C broadens with increasing
temperature, before sharpening to a new set of signals at
room temperature. Perhaps most notably, at −60 °C, the
Si2CHP group gives rise to a somewhat broad singlet at 1.93
ppm (A), which broadens substantially and moves upfield as
the temperature is increased, until, at −30 °C, this group gives
rise to a broad singlet at 1.76 ppm (Figure 2). Above this
temperature, this signal continues to move upfield, but now
sharpens, until, at room temperature, the Si2CHP group gives
rise to a relatively sharp singlet at 1.48 ppm. The line shape of
the signal due to the SMe protons (B) does not change
significantly as the temperature is increased from −60 °C, but
this signal shifts from 2.04 ppm at this temperature to 2.18 ppm
at room temperature.
hindered phosphorus atoms in the equatorial positions, as
expected. Because of the constraints associated with the
formation of the two chelate rings and the larger size of tin
compared with that of germanium, the S(1)−Sn−S(2) angle in
11a [140.66(5)°] is smaller than the S(1)−Ge−S(2) angle in
10a [148.26(4)°], and both are significantly distorted away
from the ideal 180°. Compounds 10a and 11a are chiral at both
of the phosphorus centers, and both compounds crystallize as a
racemic mixture of the PRPR and PSPS diastereomers.
The Ge−P distances of 2.4302(16) and 2.4327(14) Å in 10a
are similar to the few previously reported Ge(II)−P distances;
for example, the Ge−P distances in dimeric [(i-Pr2P)2Ge]2 are
2.4261(11), 2.4217(11), and 2.3981(11) Å,8 whereas the Ge−P
distance in [CH{(CMe)(2,6-i-Pr2C6H3N)}2]Ge{P(SiMe3)2} is
2.3912(8) Å.9 Similarly, the Sn−P distances in 11a [2.6080(17)
and 2.6087(17) Å] are comparable to those in 6 [2.5906(9)
and 2.6407(8) Å]16 and 8 [2.5995(10) and 2.6110(11) Å].17
Few Ge(II)−S distances have been reported, and none between
a Ge(II) center and a thioether; however, the Ge−S distances
[2.7793(15) and 2.8220(15) Å] are, as expected, somewhat
longer than the few reported Ge−S distances in Ge(II)
thiolates, such as {(Me3Si)3C}Ge(μ-SBu)2Li(THF)2 [Ge−S,
The variable-temperature NMR spectra of 11 exhibit similar
characteristics to those of 10. The room-temperature 31P{1H}
NMR spectrum of 11 in d8-toluene consists of a sharp singlet at
−34.7 ppm, exhibiting satellites due to coupling to 117/119Sn
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The low-temperature NMR spectra of both 10 and 11 are
consistent with the presence of a single, symmetrical species in
each case, with a structure corresponding to that observed
crystallographically. At higher temperatures, both compounds
appear to be subject to dynamic equilibria between the
dominant rac diastereomer observed at low temperatures and
the corresponding meso diastereomer, the latter of which is
present only in very low concentrations; thus, at room
1
temperature, a time-averaged H NMR spectrum is observed
in which the diastereotopic SiMe3 groups give rise to a single
peak.
The unusually low-field shifts of the Si2CHP methine
1
protons in the low-temperature H NMR spectra of both 10
and 11 may be attributed to the proximity of these protons to
the lone pairs at the sulfur and germanium/tin centers in the
rac diastereomer. In contrast, DFT calculations suggest that in
neither of the two meso diastereomer conformations located,
are both of the methine protons in such a position (see below).
The unusual temperature-dependent behavior of the chemical
shift of the Si2CHP protons in 10 and 11 may, therefore, be
attributed to a dynamic equilibrium between the rac and meso
diastereomers in each case. At low temperatures, the rac
diastereomer predominates and a low-field Si2CHP shift is
observed. However, as the temperature is raised, the proportion
of the meso diastereomer increases, and so a time-averaged
signal is observed for these protons, which moves to higher
field as the proportion of meso diastereomer increases.
DFT Calculations. To gain further insight into the relative
stabilities of the two diastereomers of 10 and 11, we have
undertaken a DFT study of these compounds. The ground-
state geometries of the complete molecules were optimized
using the hybrid B3LYP functional,23 with a Lanl2dz effective
core potential basis set on tin24 and a 6-31G(d,p) basis set on
all other atoms;25 the location of minima was confirmed by the
absence of imaginary vibrational frequencies.
For each compound, three distinct minima were located
(Figure 3). The lowest-energy minima located in each case (101
and 111) correlate extremely well with the structures obtained
by X-ray crystallography; details of selected bond lengths and
angles for all minima located are given in Table 1. In both 101
and 111, the germanium and tin atoms adopt a pseudo-trigonal-
bipyramidal structure in which the sterically less hindered, more
electronegative sulfur substituents lie in the axial positions. The
calculated Ge−P and Sn−P distances [2.4325 and 2.6320 Å,
respectively] in both 101 and 111 are slightly longer than those
observed crystallographically [Ge−P, 2.4302(16) and
2.4327(14) Å; Sn−P, 2.6080(17) and 2.6087(17) Å], while
there is a more significant difference between the calculated
Ge−S and Sn−S distances [2.8593 and 3.0504 Å, respectively]
compared with the corresponding distances in 10a [2.7793(15)
and 2.8220(15) Å] and 11a [2.9055(17) and 2.9404(17) Å].
There is a good correspondence between the calculated and
crystallographically determined angles about the Ge and Sn
centers. That the lowest-energy diastereomer for both 10 and
11 has a rac configuration is consistent with both the crystal
1
Figure 2. Expansion of the variable-temperature H NMR spectra of
10 in d8-toluene, showing the CHP region [* free phosphine
{(Me3Si)2CH}PH(C6H4-2-SMe) due to a small amount of hydrolysis
during sample preparation].
(JSnP = 1027 and 1072 Hz). This signal shifts to slightly higher
field as the temperature is reduced, with only a marginal change
to its line width; the 31P{1H} NMR spectrum of 11 at −70 °C
consists of a sharp singlet at −38.2 ppm, exhibiting poorly
resolved 117/119Sn satellites (JSnP = 1062 Hz). The variable-
1
temperature H NMR spectra of 11 follow a similar pattern to
those of 10. The diastereotopic SiMe3 groups give rise to two
sharp singlets at −0.04 and 0.39 ppm at −60 °C, which coalesce
to a single broad singlet at 0.06 ppm at room temperature. The
signal due to the Si2CHP group shifts from approximately 2.07
ppm at −60 °C (this signal is obscured by that due to the SMe
protons at this temperature) to 1.86 ppm at room temperature,
without any significant change to its line shape. At the same
time, the signal due to the SMe protons shifts from 2.07 ppm at
−60 °C to 2.17 ppm at room temperature. We were unable to
locate a 119Sn signal at room temperature, probably as a
consequence of rapid dynamic exchange (see below); however,
1
structures of 10a and 11a and the variable-temperature H and
31P{1H} NMR spectra of these compounds, which indicate that
rac-10 and rac-11 predominate at low temperatures.
Two distinct minima were located for the meso diastereomers
of both 10 and 11. Minima 102 and 112 possess pseudo-
trigonal-bipyramidal geometries at the germanium and tin
centers, respectively, in which the sulfur substituents occupy the
axial positions, as expected. The bond lengths and angles in 102
at −60 °C, a binomial triplet is observed at 657 ppm (JSnP
=
1072 Hz).
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Figure 3. Calculated minimum-energy geometries for 10 and 11 with H atoms omitted for clarity; energies (kJ mol−1) relative to the ground states in
square brackets [B3LYP/6-31G(d,p), Lanl2dz(Sn)].
Table 1. Selected Bond Lengths (Å) and Angles (deg) for the Calculated Minimum-Energy Geometries 101, 102, 103, and 111,
112, 113, and for Crystallographically Characterized 10a and 11a
E = Ge
E = Sn
10a
101
102
103
11a
111
112
113
E−P(1)
E−P(2)
E−S(1)
E−S(2)
S(1)−E−S(2)
P(1)−E−P(2)
P(1)−E−S(1)
P(2)−E−S(2)
2.4302(16)
2.4327(14)
2.7793(15)
2.8220(15)
148.26(4)
100.63(5)
76.11(5)
2.4325
2.4325
2.8593
2.8593
154.20
102.13
77.09
2.4147
2.4675
2.7487
2.7504
168.54
111.77
84.73
2.3773
2.4542
3.2632
2.5706
84.03
2.6080(17)
2.6087(17)
2.9055(17)
2.9404(17)
140.66(5)
100.71(5)
72.47(5)
2.6320
2.6320
3.0504
3.0503
142.35
101.31
71.21
2.6856
2.6821
2.9525
2.9573
160.83
113.76
78.47
2.5710
2.6697
3.3839
2.8293
77.70
100.52
79.78
98.58
74.38
75.41(5)
77.09
84.09
85.13
72.97(5)
71.21
78.53
78.04
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and 112 are similar to those in 101 and 111, although the E−S
distances are somewhat shorter and the P−E−S, S−E−S, and
P−E−P angles are somewhat larger in 102 and 112 than in the
ground-state geometries. Somewhat unexpectedly, we also
located a second minimum-energy geometry for the meso
diastereomers. The germanium and tin atoms in 103 and 113
again adopt a pseudo-trigonal-bipyramidal geometry; however,
in each case, one of the sulfur substituents lies in an equatorial
position, while one of the sterically bulky phosphorus
substituents lies in an axial position. As expected, the axial
Ge−P and Sn−P distances [2.4542 and 2.6697 Å, respectively]
in 103 and 113 are significantly longer than the equatorial Ge−
P and Sn−P distances [2.3773 and 2.5710 Å, respectively],
although these distances are not significantly longer than the
equatorial Ge−P and Sn−P distances in 102 and 112. More
strikingly, while the equatorial Ge−S and Sn−S distances
[2.5706 and 2.8293 Å, respectively] are significantly shorter
than the axial Ge−S and Sn−S distances in 102 and 112, the
axial Ge−S and Sn−S distances [3.2632 and 3.3839 Å,
respectively] are extremely long, suggesting only very weak
E···S interactions in these cases. In this regard, the structure of
113 closely resembles the X-ray crystal structure of the amino-
substituted analogue 8, in which there is one strong and one
weak Sn−N contact [Sn−N(1), 2.408(3) Å; Sn···N(2),
3.306(3) Å].
Perhaps surprisingly, given the above, the relative energies of
103 and 113 [22.7 and 47.5 kJ mol−1, respectively] are
substantially lower than those of 102 and 112 [87.1 and 71.3 kJ
mol−1, respectively], despite the positioning of one of the bulky
phosphorus substituents in an equatorial site in the former
geometries. This is likely a reflection of the ability of the bulky
CH(SiMe3)2 group to orientate itself in such a way as to
minimize steric interactions and of the somewhat similar
electronegativities of S and P (Pauling electronegativities of
2.19 and 2.55, respectively).
10 on photolysis may, at least partly, be attributed to the
presence of two adjacent phosphide ligands in the latter,
compared to a single ligand in the former compound. Thus,
intramolecular migration of a methyl group within a ligand is
favored for 9, whereas for 10, alternative decomposition
pathways are available. Consistent with this, a major
component of the photolysis products of 10 is the diphosphine
{(Me3Si)2CH}(C6H4-2-SMe)P-P(C6H4-2-SMe){CH(SiMe3)2}
(14), formed via a reductive elimination process, presumably
generating elemental germanium as a side product; compound
14 gives rise to the signal at −18.7 ppm in the 31P{1H} NMR
spectrum of the photolysis products of 10 (see below for
further details of the characterization of this compound). In
addition, it is possible for migration of the S-methyl group to
occur either from the sulfur atom to the phosphorus atom
within one ligand or from the sulfur atom of one ligand to the
phosphorus atom of an adjacent ligand; in either case, this will
potentially lead to the formation of more than one
diastereomer and thus a rather complex 31P{1H} NMR
spectrum.
Exposure of solutions of the diphosphastannylene 11 in
toluene to ambient light for 1 day leads to the deposition of
elemental tin and the formation of the diphosphine 14 as the
sole phosphorus-containing product, suggesting that reductive
elimination is more rapid than ligand isomerization for this
compound. The 31P{1H} NMR spectrum of 14 formed under
these conditions exhibits a dominant peak at −18.2 ppm along
with a much lower intensity signal at −24.8 ppm; the ratio of
these peaks is approximately 15:1, suggesting that 14 is formed
predominantly as a single diastereomer.
The identity of 14 was confirmed by X-ray crystallography.
Although both our DFT studies and our variable-temperature
NMR data suggest that the rac diastereomer of 11
predominates and so that the dominant product of reductive
elimination of 11 should be rac-14, the crystal of 14 studied by
crystallography was found to be of the meso diastereomer. The
molecular structure of meso-14 is shown in Figure 4, along with
selected bond lengths and angles.
Compound meso-14 crystallizes in an approximately
antiperiplanar conformation with a dihedral angle of 179°
between the nominal lone-pair directions. The P−P distance of
2.2278(12) Å is similar to the corresponding distance in the
related diphosphine {(Me3Si)2CH}(C6H4-2-CH2NMe2)P-P-
(C6H4-2-CH2NMe2){CH(SiMe3)2} [2.2304(6) Å]26 and in
other diphosphine compounds.27 The two ends of the
diphosphine adopt significantly different conformations, such
that the mean plane of the aromatic ring bound to P(2) lies
almost in the same plane as the P(1)−P(2) axis, whereas the
aromatic ring bound to P(2) lies nearly perpendicular to this
axis.
Photolysis of 10 and 11. We previously reported that the
lithium phosphide 9 is subject to a photolytically induced
isomerization to the corresponding thiolate [{(Me3Si)2CH}P-
(Me)(C6H4-2-S)]Li(tmeda) (13) (Scheme 2).
Scheme 2
We were interested to observe whether a similar phosphide-
thiolate isomerization would occur when the phosphorus center
was coordinated to a softer germanium(II) or tin(II) center
rather than a hard lithium cation. In the absence of air and light,
compounds 10 and 11 appear to be stable for long periods;
however, exposure of solutions of 10 in toluene to ambient
light for 3 days leads to slow isomerization. 31P{1H} NMR
spectra of the crude photolyzed solutions indicate the presence
of several species; significant intensity peaks were observed at
−18.7, −25.7, −39.5, and −57.0 ppm. The presence of multiple
species suggests that the photolysis of 10 is not straightforward;
in contrast, the photolysis of the lithium complex 9 proceeds
cleanly to give the thiolate 13 as the sole phosphorus-
containing product. The difference in behavior between 9 and
CONCLUSIONS
■
A small change in the nature of the peripheral donor
functionality in the ligands [{(Me3Si)2CH}P(C6H4-2-X)]−
(from X = NMe2 to SMe) has significant consequences for
the structure and dynamic behavior of the corresponding
diphosphatetrylenes [{(Me3Si)2CH}P(C6H4-2-X)]2E [X =
NMe2, E = Ge (7), Sn (8); X = SMe, E = Ge (10), Sn
(11)]. In both 7 and 8, the tetrel atom adopts a trigonal-
pyramidal configuration, bound by the two phosphorus atoms
and one of the nitrogen atoms of the ligands, although 8
exhibits a weak Sn···N contact to the second nitrogen atom in
the molecule. In contrast, in compounds 10 and 11, the better
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dx.doi.org/10.1021/om2008327 | Organometallics 2012, 31, 246−255

Organometallics
Article
absence of light, over 40 min. The reaction was allowed to attain room
temperature and was stirred overnight, and then solvent was removed
in vacuo to give a sticky yellow solid, which was extracted into light
petroleum (3 × 15 mL) and filtered. The solvent was removed in
vacuo from the filtrate to give a sticky yellow solid. Single crystals of
the diethyl ether solvate [{(Me3Si)2CH}P(C6H4-2-SMe)]2Ge·1/
2Et2O (10a) suitable for X-ray crystallography were grown from
cold (−20 °C) hexamethyldisiloxane containing a few drops of diethyl
ether. Yield: 1.03 g, 63.7%. Anal. Calcd for C28H52GeP2S2Si4·1/
1
2C4H10O: C, 48.9; H, 7.8. Found: C, 49.0; H, 7.8. H NMR (d8-
toluene, 293 K): δ 0.09 (s, 36H, SiMe3), 1.05 (t, J = 7.1 Hz, 3H, Et2O),
1.93 (s br, 2H, CHP), 2.18 (s, 6H, SMe), 3.21 (q, J = 7.1 Hz, 2H,
Et2O), 6.61−6.88 (m, 8H, ArH). 13C{1H} NMR (C6D6, 293 K): δ
2.67 (SiMe3), 8.57 (d, JPC = 57.8 Hz, CHP), 15.80 (Et2O), 18.13 (d,
JPC = 12.6 Hz, SMe), 67.57 (Et2O), 124.48 (d, JPC = 10.1 Hz, Ar),
127.17, 125.36 (Ar), 136.00 (d, JCP = 31.4 Hz, Ar), 144.71, 145.44 (d,
JPC = 45.3 Hz, Ar). 31P{1H} NMR (d8-toluene, 293 K): δ −26.6 (br s).
[{(Me3Si)2CH}P(C6H4-2-SMe)]2Sn (11). A solution of
[{(Me3Si)2CH}P(C6H4-2-SMe)]Li(tmeda) (0.83 g, 1.90 mmol) in
THF (10 mL) was added, dropwise, over half an hour, to a cold (−78
°C), stirred solution of SnCl2 (0.18 g, 0.95 mmol) in THF (15 mL) in
the absence of light. The reaction was allowed to attain room
temperature and was stirred overnight, and then solvent was removed
in vacuo. The resulting orange solid was extracted into light petroleum
(3 × 15 mL) and filtered, and the solvent was removed in vacuo from
the filtrate to give 11 as a sticky orange solid. Single crystals of the
diethyl ether solvate [{(Me3Si)2CH}P(C6H4-2-SMe)]2Sn·Et2O (11a)
suitable for X-ray crystallography were obtained from cold (−30 °C)
hexamethyldisiloxane containing a few drops of diethyl ether. Yield:
0.64 g, 90.3%. Anal. Calcd for C28H52P2S2Si4Sn: C, 45.1; H, 7.0%.
Figure 4. Molecular structure of meso-14, with 40% probability
ellipsoids and with H atoms omitted for clarity. Selected bond lengths
(Å) and angles (deg): P(1)−P(2) 2.2278(12), P(1)−C(1) 1.882(3),
P(1)−C(15) 1.853(3), P(2)−C(8) 1.871(3), P(2)−C(22) 1.848(3),
C(1)−P(1)−C(15) 100.55(15), C(1)−P(1)−P(2) 109.21(11),
C(15)−P(1)−P(2) 94.54(11), C(8)−P(2)−C(22) 106.85(15),
C(8)−P(2)−P(1) 97.33(11), C(22)−P(2)−P(1) 100.95(11).
match between the soft sulfur donor and the relatively soft
Ge(II) and Sn(II) centers leads to stronger E−S interactions,
resulting in pseudo-trigonal-bipyramidal geometry at the tetrel
centers. DFT calculations indicate that the rac diastereomers of
10 and 11, as observed in the solid state, are significantly more
stable than the corresponding meso diastereomers. However,
variable-temperature 1H and 31P{1H} NMR spectroscopies
suggest that, in solution, while rac-10 and rac-11 predominate
at low temperatures, these species are in dynamic equilibrium
with the corresponding meso diastereomers at higher temper-
atures.
1
Found: C, 45.7; H, 7.1%. H NMR (d8-toluene, 293 K): δ 0.06 (s,
36H, SiMe3), 1.05 (t, J = 7.1 Hz, 2H, Et2O), 1.86 (s, 2H, CHP), 2.17
(s, 6H, SMe), 3.20 (q, J = 7.1 Hz, 3H, Et2O), 6.57−6.77 (m, 8H,
ArH). 13C{1H} NMR (d8-toluene, 293 K): δ 2.37 (SiMe3), 6.61 (d, JPC
= 61.4 Hz, CHP), 15.26 (Et2O), 16.92 (d, JPC = 13.4 Hz, SMe), 65.60
(Et2O), 124.37, 127.11 (Ar), 138.33 (d, JPC = 42.2 Hz, Ar), 144.72 (d,
JPC = 49.9 Hz, Ar), 146.49 (Ar). 31P{1H} NMR (d8-toluene, 293 K): δ
−34.7 (s, JSnP = 1072 and 1027 Hz). 119Sn NMR (d8-toluene, 237 K):
δ 657 (t, JSnP = 1072 Hz).
Attempted Synthesis of [{(Me3Si)2CH}P(C6H4-2-SMe)]GeCl. A
solution of [{(Me3Si)2CH}P(C6H4-2-SMe)]Li(tmeda) (0.485 g, 1.11
mmol) in THF (10 mL) was added dropwise to a cold (−78 °C),
stirred solution of GeCl2·dioxane (0.257 g, 1.11 mmol) in THF (10
mL) over 20 min in the absence of light. The stirred reaction mixture
was allowed to attain room temperature overnight, yielding a yellow
solution. The solvent was removed in vacuo to give a sticky yellow
solid. The product was extracted into light petroleum (2 × 10 mL) and
filtered, and solvent was removed in vacuo from the filtrate to give a
EXPERIMENTAL SECTION
■
All manipulations were carried out using standard Schlenk techniques
under an atmosphere of dry nitrogen. Diethyl ether, THF, light
petroleum (bp 40−60 °C), n-hexane, and toluene were dried prior to
use by distillation under nitrogen from sodium, potassium, or sodium/
potassium alloy; hexamethyldisiloxane was purified by distillation
under nitrogen from CaH2. THF and hexamethyldisiloxane were
stored over activated 4A molecular sieves; all other solvents were
stored over a potassium film. Deuterated toluene was distilled from
potassium and deoxygenated by three freeze−pump−thaw cycles and
was stored over activated 4A molecular sieves. [{(Me3Si)2CH}P-
(C6H4-2-SMe)]Li(tmeda) was prepared by a previously published
procedure;19 n-butyllithium was purchased from Aldrich as a 2.5 M
solution in hexanes. All other compounds were used as supplied by the
manufacturer.
1
yellow solid, which was shown by H and 31P{1H} NMR spectros-
copies to be the homoleptic compound 10a.
This reaction was repeated under similar conditions. However, in
this instance, the sticky yellow solid was extracted into diethyl ether,
rather than light petroleum, and the ether extract was cooled to −20
°C, leading to the deposition of a small number of colorless crystals,
which were shown by X-ray crystallography to be the known adduct
GeCl2·TMEDA.
1H and 13C{1H} NMR spectra were recorded on a JEOL ECS500
spectrometer operating at 500.16 and 125.65 MHz, respectively, or a
Bruker Avance300 spectrometer operating at 300.15 and 75.47 MHz,
respectively; chemical shifts are quoted in parts per million relative to
tetramethylsilane. 31P{1H} and 119Sn NMR spectra were recorded on a
JEOL ECS500 spectrometer operating at 202.35 and 186.51 MHz,
respectively; chemical shifts are quoted in parts per million relative to
external 85% H3PO4 and Me4Sn, respectively. Elemental analyses were
obtained by the Elemental Analysis Service of London Metropolitan
University.
Attempted Synthesis of [{(Me3Si)2CH}P(C6H4-2-SMe)]SnCl.
Route 1. To a stirred, cold (−78 °C) solution of SnCl2 (0.278 g,
1.47 mmol) in THF (15 mL) was added a solution of [{(Me3Si)2CH}-
P(C6H4-2-SMe)]Li(tmeda) (0.640 g, 1.47 mmol) in THF (10 mL),
and the reaction mixture was allowed to attain room temperature
overnight. The solvent was removed in vacuo, the product was
extracted into diethyl ether (3 × 15 mL) and filtered, and solvent was
removed in vacuo from the filtrate to give a yellow solid, which was
1
shown by H and 31P{1H} NMR spectroscopies to be the homoleptic
compound 11. An alternative batch of single crystals of 11 suitable for
X-ray crystallography was grown from cold (−20 °C) n-hexane and
found to be the n-hexane solvate [{(Me3Si)2CH}P(C6H4-2-
SMe)]2Sn·1/2C6H14 (11b).
[{(Me3Si)2CH}P(C6H4-2-SMe)]2Ge (10). A solution of
[{(Me3Si)2CH}P(C6H4-2-SMe)]Li(tmeda) (2.02 g, 4.63 mmol) in
THF (20 mL) was added dropwise to a cold (−78 °C), stirred
solution of GeCl2·dioxane (0.54 g, 2.31 mmol) in THF (15 mL) in the
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Article
Table 2. Crystallographic Data for 10a, 11a, 11b, 12, and 14
compound
10a
11a
11b
12
14
+
−
formula
C28H52GeP2S2Si4·0.5C4H10O
736.8
C28H52P2S2Si4Sn·C4H10O
819.9
C28H52P2S2Si4Sn·0.5C6H14
788.9
C12H24LiO5 SnCl3
480.3
C28H52P2S2Si4
627.1
Mw
cryst size (mm)
0.12 × 0.10 × 0.10
0.20 × 0.10 × 0.10
0.24 × 0.10 × 0.10
0.40 × 0.20 × 0.01 0.40 × 0.30 ×
0.30
cryst syst
space group
a (Å)
monoclinic
C2/c
monoclinic
P21/c
monoclinic
P21/c
monoclinic
P21/n
monoclinic
P21/c
30.494(2)
11.1789(7)
22.9459(14)
91.760(5)
7818.3(9)
8
16.9798(15)
11.0771(10)
23.0424(18)
99.466(8)
4275.0(6)
4
10.4746(2)
21.3062(4)
18.4807(3)
102.3116(19)
4029.56(13)
4
9.6864(4)
8.3921(3)
23.1898(12)
94.973(4)
1877.98(14)
4
19.6717(7)
11.3658(4)
15.8141(5)
91.836(3)
3534.0(2)
4
b (Å)
c (Å)
β (deg)
V (Å3)
Z
μ (mm−1
)
1.115
0.905
0.956
1.802
0.394
trans coeff range
reflns measd
unique reflns
Rint
0.878−0.897
18342
0.840−0.915
25684
0.803−0.911
38038
0.533−0.840
10803
0.858−0.891
17814
6872
8243
10038
4509
5060
0.121
0.058
0.032
0.021
0.026
reflns with F2 > 2σ
3160
5104
7460
3578
4561
parameters
371
395
377
199
339
a
R (on F, F2 > 2σ)
0.061
0.060
0.023
0.023
0.048
a
Rw (on F2, all data)
0.068
0.169
0.050
0.056
0.120
a
goodness of fit
0.955
0.997
0.902
1.020
1.061
max, min electron density
0.72, −0.71
3.25, −0.95
0.37, −0.43
0.52, −0.50
1.58, −0.34
(e Å−3
)
a
2
2
2
Conventional R = ∑||Fo| − |Fc||/∑|Fo|; Rw = [∑w(Fo − Fc2)2/∑w(Fo )2]1/2; S = [∑w(Fo − Fc2)2/(no. data − no. params)]1/2 for all data.
Route 2. In the absence of light, [{(Me3Si)2CH}P(C6H4-2-
Crystal Structure Determinations of 10a, 11a, 11b, 12, and
14. Measurements were made at 150 K on an Oxford Diffraction
(now Agilent Technologies) Gemini A Ultra diffractometer using
graphite-monochromated Mo Kα radiation (λ = 0.71073 Å) and ω
scans. Cell parameters were refined from the observed positions of all
strong reflections. Intensities were corrected semiempirically for
absorption, based on symmetry-equivalent and repeated reflections.
The structures were solved by direct methods and refined on F2 values
for all unique data. Table 2 gives further details. All non-hydrogen
atoms were refined anisotropically, and H atoms were constrained with
a riding model. U(H) was set at 1.2 (1.5 for methyl groups) times Ueq
for the parent atom. Programs were Oxford Diffraction CrysAlisPro
and SHELXTL for structure solution, refinement, and molecular
graphics.28 Although disorder is likely for the ether molecule lying on a
2-fold rotation axis in 10a, this could not be resolved satisfactorily, and
the geometry was restrained on the basis of well-ordered molecules
found in a search of the Cambridge Structural Database.29 The highest
difference density peaks found at the end of refinement of 11a lie close
to the Sn atom and indicate imperfect absorption corrections.
DFT Calculations. Geometry optimizations were performed on
the gas-phase molecules with the Gaussian 03 suite of programs
(revision D.01)30 on a 224-core Silicon Graphics Altix 4700, with 1.6
GHz Montecito Itanium2 processors, via the EPSRC National Service
for Computational Chemistry Software (http://www.nsccs.ac.uk).
Ground-state optimizations were carried out using the B3LYP
hybrid functional24 with an Lanl2dz effective core potential basis set
for Sn,25 and a 6-31G(d,p) all-electron basis set on all other atoms26
(default parameters were used throughout). Minima were confirmed
by the absence of imaginary vibrational frequencies.
SMe)]Li(tmeda) (1.20 g, 2.75 mmol) was dissolved in hot light
petroleum (50 mL), and the solution was cooled to room temperature.
One equivalent of 12-crown-4 (0.44 mL, 0.48 g, 2.75 mmol) was
added to this solution, with immediate formation of a dark red solid,
and this mixture was stirred for 1 h. The supernatant was decanted and
the sticky solid washed with light petroleum (20 mL). The resulting
red solid was dissolved in THF (20 mL) and filtered to remove a pale,
insoluble precipitate and the solvent once more removed to give crude
[{(Me3Si)2CH}P(C6H4-2-SMe)]Li(12-crown-4) (1.04 g, 2.09 mmol).
The crown ether complex was dissolved in THF (25 mL) and added
dropwise to a cold (−78 °C), stirred solution of SnCl2 (0.397 g, 2.09
mmol) in THF (25 mL). The reaction was stirred overnight and
allowed to attain room temperature, giving a very pale yellow solution.
Solvent was removed in vacuo to give a pale paste that was extracted
into diethyl ether (2 × 20 mL); the combined extracts were filtered
and solvent was removed in vacuo from the filtrate to give a sticky,
cream solid. The solid was dissolved in diethyl ether (10 mL), filtered,
and stored at −20 °C, whereupon a small number of colorless single
crystals deposited, which were shown by X-ray crystallography to be
the trichlorostannate complex [Li(12-crown-4)(THF)][SnCl3] (12).
Photolysis of 11 and Isolation of [{(Me3Si)2CH}(C6H4-2-
SMe)P]2 (14). Compound 11 (0.14 g, 0.19 mmol) was dissolved in
toluene (20 mL) under nitrogen in a standard borosilicate glass
Schlenk tube and exposed to direct sunlight. After 16 h of continuous
exposure, the solvent was removed in vacuo, giving a thick, greasy solid
that was extracted with hot light petroleum (2 × 20 mL). The
combined extracts were filtered to remove the dark solids, and solvent
was removed in vacuo from the filtrate to afford 14 as a waxy yellow
solid. The product was recrystallized from cold (−20 °C) n-hexane as
colorless blocks suitable for X-ray crystallography. Yield: 0.17 g, 89.5%.
1H NMR (CDCl3, 293 K): δ −0.06 (s, 18H, SiMe3), 0.00 (s, 18H,
ASSOCIATED CONTENT
■
S
* Supporting Information
SiMe3), 2.46 (d, JPH = 12.4 Hz, 2H, CHP), 2.48 (s, 6H, SMe), 7.19−
7.13 (m, 4H, ArH), 7.35 (m, 2H, ArH), 8.08 (m, 2H, ArH). 13C{1H}
NMR (CDCl3, 293 K): δ 2.58 (t, JPC = 5.3 Hz, SiMe3,), 2.74 (s,
SiMe3), 9.65 (t, JPC = 23.0 Hz, CHP), 16.06 (t, JPC = 7.2 Hz, SMe),
For 10a, 11a, 11b, 12, and 14, details of structure
determination, atomic coordinates, bond lengths and angles,
and displacement parameters in CIF format; for 11b and 12,
structural diagrams and selected bond lengths and angles. For
10 and 11, details of DFT calculations, final atomic coordinates,
124.00, 129.69 (Ar), 136.66 (t, JPC = 7.7 Hz, ArH), 147.08 (t, JPC
=
17.3 Hz, ArH). 31P{1H} NMR (CDCl3, 293 K): δ −18.2.
253
dx.doi.org/10.1021/om2008327 | Organometallics 2012, 31, 246−255

Organometallics
Article
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AUTHOR INFORMATION
■
Corresponding Author
*E-mail: k.j.izod@ncl.ac.uk.
(s) Goller, A.; Clark, T. Chem. Commun. 1997, 1033. (t) Izod, K.;
̈
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ACKNOWLEDGMENTS
■
The authors are grateful to the EPSRC for support and
acknowledge the use of the EPSRC UK National Service for
Computational Chemistry Software (NSCCS) at Imperial
College London in carrying out this work.
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