Preparations of metal trichalcogenophosphonates from organophosphonate esters

Article abstract of DOI:10.1021/ic301504p

A new method for the preparation of metal trichalcogenophosphonates is presented wherein organophosphonate esters are first reduced with LiAlH ₄ and subsequently treated with an organometallic reagent and elemental sulfur or selenium to give th

Full text of DOI:10.1021/ic301504p

Article
pubs.acs.org/IC
Preparations of Metal Trichalcogenophosphonates from
Organophosphonate Esters
Robert P. Davies,* Laura Patel, and Andrew J. P. White
Department of Chemistry, Imperial College London, South Kensington, London, SW7 2AZ U.K.
S
* Supporting Information
ABSTRACT: A new method for the preparation of metal
trichalcogenophosphonates is presented wherein organophosph-
onate esters are first reduced with LiAlH4 and subsequently
treated with an organometallic reagent and elemental sulfur or
selenium to give the desired trichalcogenophosphonate complex.
n
Using this synthetic protocol with BuLi as the organometallic
reagent, the lithium trithiophosphonate complexes
[Li2(S3PCH2Ph)(THF)(TMEDA)]2 (1) and [Li4(S3PnPr)2(TMEDA)3]∞ (3), where THF = tetrahydrofuran and TMEDA =
N,N,N′,N′-tetramethylethylenediamine, have been prepared. In both cases, the formation of byproducts is also evident, including,
for 1, the tetrathiohypodiphosphonate complex [(PhCH2P(S2))2Li2(THF)4] (2), which has been structurally characterized.
n
n
Replacement of BuLi with Bu2Mg as the metallating agent led to much cleaner products and improved yields, with the new
trithio- and triselenoorganophosphonate complexes [Mg(S3PCH2Ph)(TMEDA)]2 (4) and [Mg(Se3PnPr)(TMEDA)]2 (5)
reported. All trichalcogenophosphonate complexes have been structurally characterized in the solid state: 1 adopts a dimer
structure in which the [PhCH2PS3]2− ligand exhibits a unique μ3-η2,η2,η2-coordination mode; 3 is polymeric comprising of
[Li4(S3PnPr)2(TMEDA)2] dimers linked via additional bridging bis(monodentate) TMEDA molecules; 4 and 5 both adopt
dimeric motifs with μ2-η2,η2 coordination of the magnesium centers.
reagents with E = S, R = 4-anisyl and E = Se, R = Ph,
respectively) in reactions with metal salts or complexes. This
approach has been reported by the groups of Woollins,3−6
Rothenberger,7,8 and others,9−14 and usually proceeds via the
asymmetric cleavage of dichalcogenodiphosphetane dichalco-
genides (Scheme 1a). However, the scope of the reaction has
INTRODUCTION
■
Despite their potential wide-ranging applications, studies on
metal trichalcogenophosphonates [RPX3]2− (X = S, Se) remain
sparse in the scientific literature, primarily because of the lack of
a reliable and generally applicable synthetic route to these
compounds. In stark contrast, salts and complexes of the
analogous oxygen-containing organophosphonates, [RPO3]2−,
have been subjected to extensive investigations. Thus, metal
organophosphonates have shown potential and realized
applications in many fields including catalysis, guest inter-
calation, sorbents, ion exchange, proton conduction, nonlinear
optics, photochemically active materials, and sensors.1 The
wide applicability of organophosphonate materials is, in part,
due to the ability of the organophosphonate ligands to act as
versatile building blocks in the design of layered structures and
cages.1 Analogous behavior in trichalcogenophosphonates
might lead to new materials with similarly useful or novel
properties. Additionally, in common with other metal organo-
phosphorus−chalcogen complexes, trichalcogenophosphonates
have potential applications as single-source precursors to metal
chalcogenide thin films and quantum dots, lubricant additives,
catalysts or chalcogen-exchange reagents in organic synthesis,
biological agents, and heavy-metal extractants.2 Hence, a better
understanding of the synthesis, structure, and properties of
trichalcogenophosphonates is highly desirable and is the focus
of this work.
Scheme 1. Representative Synthetic Routes to Metal
Trichalcogenophosphonates (E = S, Se)
been somewhat limited both by the availability of dichalcoge-
nodiphosphetane dichalcogenide starting materials and also by
competition from the associated symmetric cleavage reaction.
An alternative route to trithiophosphonates uses thiometal
reagents to induce symmetric cleavage of dithiodiphosphetane
disulfides (Scheme 1b).10,11 Other reported syntheses of
trichalcogenophosphonates include a gallium trithiophospho-
nate prepared by Cowley et al. via the reaction of P4S10 with
Preparations of metal trichalcogenophosphonates to date
have mainly employed dichalcogenodiphosphetane dichalcoge-
nides [{RP(E)(μ-E)}2] (such as Lawesson’s or Woollins’
Received: July 11, 2012
© XXXX American Chemical Society
A
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Inorganic Chemistry
Article
DMSO-d6) δ/ppm 7.41 (d, 3JHH = 6.8 Hz, 2H, o-C6H5), 7.09 (m, 2H,
tBu3Ga,15 platinum trithiophosphonate complexes formed from
the reaction of [Pt(SP(NPh)(Ph)(NPh))(L2)] (L2 = 2PPh3,
dppp) with phenyl isothiocyanate or carbon disulfide by
Kemmitt et al.,16 and a tungsten trithiophosphonate complex
isolated from the reaction of [PPh4]2[WS4] with PhPCl2 by
Garner and co-workers.17 Transmetalation,7,12,18 ion ex-
change,19−22 and silyl ester cleavage10,17,23,24 protocols have
also been used to introduce a metal center to an existing RPS3
group. Moreover, and particularly salient to this work, we have
previously reported on the reaction of primary phosphines with
an organometal deprotonating agent and elemental chalcogen
to give metal trithio- and triselenophosphonate complexes
(Scheme 1c).25−27
We now report on a significant extension of this final
synthetic route using organophosphonate esters, RP(O)-
(OR′′)2, as starting materials and thus taking advantage of
the widespread commercial availability of such esters with a
large variety of organo R groups. In a “one-pot” reaction,
organophosphonate esters are first reduced by lithium
aluminum hydride to the corresponding primary phosphine
followed by the reaction with an organometallic metallating
reagent and elemental chalcogen to give the desired metal
trichalcogenophosphonate. The employment of such a protocol
avoids any risk associated with the isolation and handling of
neat primary phosphines, which are often malodorous and
highly pyrophoric, and significantly simplifies the reaction
protocol. In addition to providing a new route to these
complexes, the coordination chemistry of trichalcogenophosph-
onates is further elaborated upon in this work.
2
m-C6H6), 7.03 (m, 1H, p-C6H6), 3.60 (m, 4H, THF), 3.34 (d, JHP
=
13.5 Hz, 2H, CH2), 2.28 (s, 4H, TMEDA), 2.12 (s, 12H, TMEDA),
1.75 (m, 4H, THF); 31P NMR (162 MHz, DMSO-d6) δ/ppm 83.9 (t,
7
2JPH = 13.5 Hz); Li NMR (156 MHz, DMSO-d6) δ/ppm −0.9. A
spectroscopically pure sample of 2 was obtained as colorless crystals by
the slow cooling to room temperature of a hot THF (8.0 mL) and
hexane (3.0 mL) solution of the original 1 + 2 mixture: mp 120 °C
(dec); 1H NMR (400 MHz, DMSO-d6) δ/ppm 7.36 (d, 3JHH = 7.3 Hz,
3
4H, o-C6H5), 7.18 (dd, JHH = 7.1 and 7.3 Hz, 4H, m-C6H5), 7.10 (t,
3JHH = 7.1 Hz, 2H, p-C6H5), 3.61 (m, 16H, THF), 3.52 (dd, 2JHP = 6.5
3
Hz, JHP = 3.8 Hz, 4H, CH2), 1.76 (m, 16H, THF); 31P{1H} NMR
(162 MHz, DMSO-d6) δ/ppm 82.0; 7Li NMR (156 MHz, DMSO-d6)
δ/ppm −1.0.
Preparation of [Li4(S3PnPr)2(TMEDA)3]∞ (3). nPrP(O)(OEt)2
(0.88 mL, 5 mmol) was added dropwise to a suspension of LiAlH4
(0.28 g, 7.4 mmol) in toluene (15 mL) at 0 °C. The mixture was
stirred for 16 h and then quenched with thoroughly degassed saturated
aqueous Na2SO4 (10 mL) at 0 °C. The white mixture was then filtered
and the organic layer separated. The remaining aqueous layer was
washed with toluene (2 × 3.0 mL), and the combined organic layers
were dried over MgSO4. NMR studies on the resultant solution show
n
complete conversion to PrPH2: 31P{1H} NMR (162 MHz, toluene/
C6D6) δ/ppm −139.9 (lit.31 −138). The phosphine solution was
cooled to −78 °C, and TMEDA (1.5 mL, 10 mmol) and nBuLi (2.5 M
in hexanes, 4.0 mL, 10 mmol) were added. This solution was then
transferred onto S8 (0.480 g, 1.9 mmol), stirred for 16 h at room
temperature, and filtered through Celite. Removal of the solvent under
vacuum and recrystallization from hexane/toluene (2 mL/4 mL) at
−35 °C for 4 days gave a batch of colorless block crystals of 3 along
with a small amount of red solid (see the Results and Discussion
section): yield 0.334 g, 20%; mp 108 °C; 1H NMR (400 MHz, C6D6)
δ/ppm 2.91 (m, 4H, CH2), 2.50 (m, 4H, CH2), 2.32 (s, 36H,
TMEDA), 2.01 (s, 12H, TMEDA), 1.23 (t, 3JHH = 7.4 Hz, 6H, CH3);
EXPERIMENTAL SECTION
■
7
31P{1H} NMR (162 MHz, C6D6) δ/ppm 84.8; Li NMR (156 MHz,
General Procedures. All experimental work was carried out under
an inert atmosphere of nitrogen using standard Schlenk double-
manifold techniques for the synthesis, and a glovebox for the storage,
of the reported complexes. Purification and drying of the solvents was
carried out following standard methods or using an Innovative
Technologies PureSolv Solvent Purification System with purification-
grade solvents. The reduction of organophosphonate esters using
LiAlH4 is based upon an adaptation of published procedures.28,29 1H,
31P, and 7Li NMR spectra were recorded on a Bruker DPX400
C6D6) δ/ppm 1.5.
Preparation of [Mg(S3PCH2Ph)(TMEDA)]2 (4). A solution of
PhCH2PH2 (2.5 mmol) in toluene (∼10 mL) was prepared from
PhCH2P(O)(OEt)2 and LiAlH4 using a procedure identical with that
n
described in the preparation of 1 above. Bu2Mg (1.0 M in heptane,
2.5 mL, 2.5 mmol) and TMEDA (0.60 mL, 4.0 mmol) were then
added to the phosphine solution. The resultant cloudy yellow solution
was transferred onto S8 (0.255 g, 0.94 mmol) and stirred for 16 h to
yield a batch of pale-yellow solid 4: yield 0.922 g, 52%; mp >250 °C;
1H NMR (400 MHz, DMSO-d6) δ/ppm 7.41 (d, 3JHH = 7.5 Hz, 2H, o-
spectrometer with internal standards. 77Se NMR spectra were recorded
on a JOEL EX270 Delta Upgrade spectrometer with an external
standard (Me2Se). Melting points were measured in capillaries sealed
under nitrogen, and microanalytical data were obtained from the
Science Technical Support Unit, London Metropolitan University.
Preparation of [Li2(S3PCH2Ph)(THF)(TMEDA)]2 (1) and
[(PhCH2P(S2))2Li2(THF)4] (2). PhCH2P(O)(OEt)2 (3.15 mL, 7.5
mmol) was added dropwise to a suspension of LiAlH4 (0.42 g, 11
mmol) in toluene (15 mL) at 0 °C. The mixture was stirred for 16 h
and then quenched with thoroughly degassed saturated aqueous
Na2SO4 (15 mL) at 0 °C. The white mixture was filtered and the
organic layer separated. The remaining aqueous layer was washed with
toluene (2 × 4.0 mL), and the combined organic layers were dried
over MgSO4. NMR studies on the resultant solution show complete
conversion to PhCH2PH2: 31P{1H} NMR (162 MHz, toluene/C6D6)
δ/ppm −122.5 (lit.30 −120.9). The phosphine solution was cooled to
0 °C, and nBuLi (2.5 M in hexanes, 6.0 mL, 15 mmol) and N,N,N′,N′-
tetramethylethylenediamine (TMEDA; 2.3 mL, 15 mmol) were added.
The solution was then transferred onto S8 (0.720 g, 2.8 mmol) and
stirred at room temperature for 16 h. Removal of the solvent under
vacuum and recrystallization from tetrahydrofuran (THF)/hexane (8
mL/3 mL) at −35 °C gave a crystalline precipitate (1.36 g) that was
shown by 31P NMR analysis to be an approximately equimolar mixture
of 1 (∼15% yield) and 2 (∼18% yield). Further concentration of the
filtrate and storage at −35 °C for 2 weeks gave a small (0.07 g) batch
C6H5), 7.08 (dd, 3JHH = 7.2 and 7.5 Hz, 2H, m-C6H5), 6.99 (t, 3JHH
=
7.2 Hz, 1H, p-C6H5), 3.31 (d, 2JHP = 13.9 Hz, 2H, CH2), 2.23 (s, 4H,
TMEDA), 2.12 (s, 12H, TMEDA); 31P NMR (162 MHz, DMSO-d6)
2
δ/ppm 84.1 (t, JPH = 12.0 Hz). Calcd for C26H46Mg2N4P2S6: C,
43.51; H, 6.46; N, 7.81. Found: C, 43.51; H, 6.51; N, 7.87.
Recrystallization from THF at 5 °C gave colorless crystals of 4 suitable
for X-ray crystallographic studies.
Preparation of [Mg(Se3PnPr)(TMEDA)]2 (5). A solution of
nPrPH2 (2.5 mmol) in toluene (∼10 mL) was prepared from
nPrP(O)(OEt)2 and LiAlH4 using a procedure identical with that
n
described in the preparation of 3 above. Bu2Mg (1.0 M in heptane,
2.5 mL, 2.5 mmol) and TMEDA (0.75 mL, 5.0 mmol) were then
added to the phosphine solution. The resultant cloudy yellow solution
was transferred onto Se (0.592 g, 7.5 mmol) at 0 °C and stirred for 16
1
h to give the white solid 5: yield 0.662 g, 37%; mp 221 °C (dec); H
NMR (400 MHz, DMSO-d6) δ/ppm 2.27 (s, 2H, TMEDA), 2.24−
2.20 (m, 2H, CH2), 2.11 (s, 12H, TMEDA), 1.84−1.72 (m, 2H, CH2),
3
0.90 (t, JHH = 7.4 Hz, 3H, CH3); 31P10 NMR (162 MHz, DMSO-d6)
1
δ/ppm −18.5 (s + d satellites, JPSe = −542 Hz); 77Se NMR (95.4
1
MHz, DMSO-d6) δ/ppm 353.6 (d, JSeP = −542 Hz). Calcd for
C18H46Mg2N4P2Se6: C, 23.94; H, 5.14; N, 6.21. Found: C, 23.87; H,
5.12; N, 6.54. Recrystallization from dichloromethane at 30 °C gave
crystals of 2 suitable for X-ray crystallographic studies.
1
of colorless block crystals of 1: mp >250 °C; H NMR (400 MHz,
B
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Inorganic Chemistry
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Table 1. Crystal Data, Data Collection, and Refinement Parameters for the Structures of 1−5
1
2
3
4
5
formula
C34H62Li4N4O2P2S6
C4H8O
C30H46Li2O4P2S4
C24H62Li4N6P2S6
C26H46Mg2N4P2S6
2(C4H8O)
861.80
C18H46Mg2N4P2Se6
2(CH2Cl2)
1072.76
solvent
fw
913.04
674.73
716.86
color, habit
temp/K
colorless, blocks
173
colorless, tablets
173
colorless, blocks
173
colorless, prisms
173
colorless, tablets
173
cryst syst
space group
a/Å
triclinic
triclinic
triclinic
monoclinic
P21/n (No. 14)
10.0755(3)
18.8928(4)
12.4849(4)
triclinic
P1 (No. 2)
̅
P1 (No. 2)
̅
P1 (No. 2)
̅
P1 (No. 2)
̅
8.8544(4)
10.7325(5)
14.0520(6)
94.743(4)
91.093(4)
110.391(4)
1245.76(10)
10.8127(8)
12.1431(6)
13.4353(7)
88.900(4)
84.026(5)
88.591(5)
1753.70(18)
10.5462(5)
10.7732(5)
10.9125(6)
117.717(5)
105.696(5)
95.638(4)
10.1998(3)
10.2810(4)
10.4612(4)
68.813(4)
76.831(3)
86.348(3)
995.75(6)
b/Å
c/Å
α/deg
β/deg
109.942(3)
γ/deg
V/Å3
1019.47(11)
2234.05(12)
a
b
a
a
a
Z
1
2
1
2
1
Dc/g cm−3
radiation used
μ/mm−1
1.217
Mo Kα
0.375
65
1.278
Mo Kα
0.394
66
1.168
Mo Kα
0.437
61
1.281
Cu Kα
4.046
145
1.789
Mo Kα
5.911
63
2θmax/deg
no. of unique reflns
measd (Rint)
obsd |Fo| > 4σ(|Fo|)
no. of variables
R1 (obsd), wR2 (all)
8173 (0.0188)
6262
15098 (c)
9568
5379 (0.0167)
4044
4402 (0.0259)
3774
5969 (0.0185)
4127
285
401
207
269
172
d
0.0388, 0.1062
0.0666, 0.1942
0.0345, 0.0827
0.0383, 0.1056
0.0250, 0.0488
a
b
c
The complex has crystallographic Ci symmetry. The structure contains two crystallographically independent Ci-symmetric complexes. Not
d
applicable because of twinning; see the Supporting Information for more details. R1 = ∑||Fo| − |Fc||/∑|Fo|; wR2 = {∑[w(Fo − Fc2)2]/
2
2
2
∑[w(Fo )2]}1/2; w−1 = σ2(Fo ) + (aP)2 + bP.
Crystallographic Studies. Crystals to be analyzed by X-ray
diffraction were taken directly from the mother liquor, covered with a
perfluorinated ether, and mounted on the top of a glass capillary under
a flow of cold gaseous nitrogen. Table 1 provides a summary of the
crystallographic data for the structures of 1−5. Data were collected
using Oxford Diffraction Xcalibur 3 (1−3 and 5) and Xcalibur PX
Ultra (4) diffractometers, and the structures were refined based on F2
using the SHELXTL and SHELX-97 program systems.32 The THF
molecules in 1 and 2 and the TMEDA molecules in 3 and 4 all exhibit
some disorder in the crystal structures; see the Supporting Information
for full details. The crystal structure data have been deposited with the
Cambridge Crystallographic Data Center under deposition numbers
CCDC 878883−878887. This material can be obtained free of charge
via www.ccdc.cam.ac.uk/data_request/cif, by emailing data_request@
ccdc.cam.ac.uk, or by contacting the Cambridge Crystallographic Data
Centre, 12, Union Road, Cambridge CB2 1EZ, U.K.
the handling and storage of neat primary phosphines, which are
often malodorous and pyrophoric.
Immediate reaction of the toluene solution of PhCH2PH2
n
3
with 2 equiv of BuLi and /8 equiv of elemental S8 in the
presence of TMEDA gave a mixture of products (Scheme 2),
Scheme 2. Synthesis of 1 and 2
RESULTS AND DISCUSSION
■
n
The diethyl phosphonate esters PrPO3Et2 and PhCH2PO3Et2
were used in these reactions because of their commercial
availability and lack of any additional functionalization. In both
cases, reduction with LiAlH4 in toluene (with rigorous
exclusion of oxygen) gave conversions in 95+% yield to the
n
corresponding primary phosphines (PhCH2PH2 or PrPH2).
After an aqueous workup using a deoxygenated sodium sulfate
solution under an atmosphere of nitrogen, the organic layer was
separated (again under nitrogen) and dried to give a toluene
solution of the phosphine. 31P NMR studies on this solution
showed clean formation of the primary phosphines in both
cases (δ −122.5 and −139.9 for PhCH2PH2 and nPrPH2
respectively), with no other phosphorus-containing species
present. No further purification of the phosphine solutions was
undertaken, thus avoiding any complications associated with
showing that the reaction had not proceeded cleanly to the
trithiophosphonate complex as desired. Two compounds were
identified in the product mixture, the targeted trithiophosph-
onate 1 and tetrathiohypodiphosphonate 2 (31P NMR in C6D6:
δ 83.9 and 82.0, respectively), each of which was subsequently
purified using differing crystallization conditions and charac-
C
dx.doi.org/10.1021/ic301504p | Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
terized using single-crystal X-ray diffraction (see the Exper-
imental Section).
X-ray structure elucidation reveals complex 1 to exist as a
centrosymmetric dimer in the solid state in which each
dianionic trithiophosphonate ligand bonds to three of the
four lithium centers present via μ3-η2,η2,η2 coordination (Figure
1 and Table 2). Although observed in triselenophosphonate
[PhCH2PS3]2− ligand and is additionally chelated by a TMEDA
molecule, resulting in distorted tetrahedral geometry at the
metal center.
Within complex 1, the P−S bonds are intermediate in length
between the expected P−S single (2.13 Å35) and double (1.88
Å35) bonds, suggesting delocalization of the double negative
charge over the PS3 group. Nonetheless, the phosphorus bond
to S3 is marginally shorter at 2.0153(5) Å than those to S1 and
S2, 2.0218(5) and 2.0213(4) Å, respectively. There is an
associated variation in the S−Li bond lengths, with S3−Li2
being slightly longer than S2−Li2 [2.462(2) vs 2.445(2) Å] and
S3−Li1A being longer than S1−Li1A [2.854(3) vs 2.646(2) Å].
The longer bonds to Li1 relative to Li2 are to be expected,
given the differing coordination numbers of these metal centers
(5 and 4, respectively).
X-ray structural studies on the other reaction product 2
reveal a monomeric tetrathiohypodiphosphonate complex
(Figure 2 and Table 3). This complex is most likely formed
Figure 1. (a) Molecular structure of 1, with hydrogen atoms and
disorder in the THF molecules omitted for clarity. (b) Central core
unit of 1. Thermal ellipsoids are displayed at the 40% probability level.
Table 2. Selected Bond Lengths (Å) and Angles (deg) for 1
P1−S1
P1−S2
P1−S3
P1−C14
Li1−O1
Li1−S1
Li1−S1A
2.0218(5)
2.0213(4)
2.0153(5)
1.8464(14)
2.001(2)
Li1−S2
Li1−S3A
Li2−S2
Li2−S3
Li2−N6
Li2−N9
2.697(3)
2.854(3)
2.445(2)
2.462(2)
2.093(3)
2.107(3)
Figure 2. Molecular structure of one (2-A) of the two similar but
independent Ci-symmetric complexes present in the crystals of 2.
Hydrogen atoms and disorder in the THF molecules are omitted for
clarity. Thermal ellipsoids are displayed at the 40% probability level.
2.529(3)
2.646(2)
through condensation of two PhCH2PS3Li2 species (present in
1) with the elimination of lithium sulfide. A similar mechanism
has been previously documented for the reaction of [Al2(S2P-
(H)C6H11)2(S3PC6H11)2] with nBuLi to give [(C6H11P-
(S2))2Li2(TMEDA)2].27
S2−P1−S1
S3−P1−S1
S3−P1−S2
112.34(2)
110.43(2)
111.30(2)
104.90(5)
108.14(5)
109.47(5)
97.92(8)
S1A−Li1−S3A
O1−Li1−S2
O1−Li1−S3A
O1−Li1−S1A
S2−Li1−S3A
S2−Li2−S3
S2−Li2−N6
S2−Li2−N9
S3−Li2−N6
S3−Li2−N9
N6−Li2−N9
74.04(7)
95.29(10)
92.24(10)
152.12(13)
166.05(10)
85.56(8)
C14−P1−S1
C14−P1−S2
C14−P1−S3
S1−Li1−S1A
S1−Li1−S2
S1−Li1−S3A
O1−Li1−S1
S1A−Li1−S2
The unit cell of 2 contains two independent complexes, 2-A
and 2-B, both of which adopt the same structure with just
minor variations in the bonding parameters (see Table 3).
Within each complex, the [PhCH2P(S)2P(S)2CH2Ph]2− ligand
coordinates the two metal centers in a doubly bidentate
manner, with each PS2 moiety chelating a lithium center to
create two four-membered PSLiS rings. Two THF molecules
also coordinate each lithium, resulting in a distorted tetrahedral
geometry at the metal centers. Chelation of the lithium atoms
by the PS2 units is symmetrical with P1−S1 and P1−S2, and
also Li1−S1 and Li1−S2, equal in length within experimental
error. Further analysis of the P−S bond lengths in 2 [range
1.9888(10)−1.9927(11) Å] indicates complete delocalization
of the single negative charge within the PS2 unit. Notably, the
formally higher bond order of the P−S bonds in 2 (11/2)
relative to those in 1 (11/3) is reflected in their shorter bond
lengths (mean 1.990 Å in 2 vs 2.019 Å in 1). The P−P bond is
rotated such that the benzyl groups are staggered with respect
113.20(11)
137.83(12)
118.35(11)
118.44(11)
86.66(10)
79.91(7)
108.55(9)
109.54(11)
94.10(8)
complexes,33 this coordination mode is, to the best of our
knowledge, previously undocumented for trithiophosphonate
ligands. The phenyl groups are oriented such that the C14−
C
ipso bond is anti to the P1−S1 bond. The central lithium atom,
Li1, is coordinated by both trithiophosphonate ligands and is
bridged to Li1A by S1 and S1A to give a SLiSLi four-membered
ring. Li1 adopts a distorted square-pyramidal-based geometry
(τ = 0.23),34 with S1 in the apical position and the outward-
facing basal coordination site occupied by a THF molecule.
The other lithium atom, Li2, is coordinated by just one
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a
Table 3. Selected Bond Lengths (Å) and Angles (deg) for the Two Independent Complexes 2-A and 2-B Present in 2
2-A
2-B
2-A
2-B
P1−S1
P1−S2
P1−C11
P1−P1A
1.9927(11)
1.9896(11)
1.842(3)
1.9906(11)
1.9888(10)
1.831(3)
Li1−S1
Li1−S2
Li1−O1
Li1−O6
2.499(7)
2.500(7)
1.907(7)
1.908(8)
2.500(6)
2.491(6)
1.929(6)
1.931(6)
2.2321(13)
2.2339(13)
S1−P1−S2
114.87(5)
112.06(12)
109.20(6)
111.19(11)
108.97(6)
99.39(10)
80.36(17)
114.69(5)
111.77(11)
109.26(5)
111.54(11)
109.08(5)
99.37(9)
P1−S2−Li1
S1−Li1−S2
S1−Li1−O1
S1−Li1−O6
S2−Li1−O1
S2−Li1−O6
O1−Li1−O6
80.41(16)
84.34(18)
120.0(3)
115.7(5)
119.1(4)
112.2(4)
105.0(4)
80.62(14)
84.35(16)
118.8(3)
114.2(3)
119.8(3)
113.2(3)
105.8(3)
S1−P1−C11
S1−P1−P1A
S2−P1−C11
S2−P1−P1A
C11−P1−P1A
P1−S1−Li1
80.35(13)
a
Phosphorus atoms labeled with an A are related to their parent atoms by a center of symmetry; full details of symmetry operations for 2-A and 2-B
are given in the Supporting Information.
to one another. The “end-on” coordination mode observed in 2
is very different from the “side-on” coordination observed in
tetrathiohypodiphosphonate [(C6H11P(S2))2Li2(TMEDA)2],27
in which two five-membered PPSLiS chelate rings are present,
both of which include the P−P bond. To our knowledge, the
only other example of similar “end-on” coordination as featured
in 2 is in the tetraselenohypodiphosphonate complex [(PhP-
(Se2))2Li2(TMEDA)2].36
the cage and the bridging TMEDA molecule. Within each
dimeric cage, the [nPrPS3]2− trithiophosphonate ligands are
staggered with respect to each other, with two ligand sulfur
atoms, S1 and S2, each bridging two lithium centers and the
remaining sulfur, S3, binding to a single lithium to give an
overall μ4-η2,η1,η1,η1-coordination mode. Both lithium environ-
ments within the structure are distorted tetrahedral: Li1 is
coordinated to two sulfur and two nitrogen atoms from a
chelating TMEDA molecule, while Li2 is coordinated by three
sulfur and one nitrogen atoms from a bridging TMEDA
molecule. All P−S bonds in 3 are of similar length and are
intermediate between those expected for single and double P−
S bonds, again suggesting charge delocalization (vide supra).
However, the P1−S3 bond involving the nonbridging sulfur
atom is, at 2.0113(6) Å, slightly shorter than the P1−S1 and
P1−S2 bonds, 2.0406(6) and 2.0391(6) Å, which both involve
bridging sulfur atoms. The difference in coordination modes of
the sulfur atoms is also reflected in the S−Li bond lengths, with
lithium bonds to S3 being slightly shorter than those to S1 and
S2, 2.405(3) Å vs 2.427(3)−2.486(3) Å. Similar but discrete
lithium trithiophosphonate dimer cages have previously been
reported for [Li4(S3PC6H11)2(THF)2(TMEDA)2] in which
THF molecules can be considered to formally replace the
bridging TMEDA molecules in 3.26
Attempts to prepare the trithiophosphonate 1 in higher
purity and yield by lowering the reaction temperature to −78
°C in order to minimize the occurrence of the condensation
reaction proved unsuccessful, with 31P NMR showing
tetrathiohypodiphosphonate 2 to still be present in the product
mixture in appreciable quantity. However, substitution of
n
PhCH2PO3Et2 with PrPO3Et2 did lead to an improvement in
yield and purity with the trithiophosphonate 3 obtained in 20%
yield as colorless crystals (Scheme 3). Nevertheless, even in this
Scheme 3. Synthesis of 3
A significant improvement to the reaction protocol was
n
n
further realized by the replacement of BuLi with Bu2Mg as
the organometallic agent. This change in the metallating agent
resulted in inhibition of the condensation reaction (with no
tetrathiohypodiphosphonate byproduct detected) and led to
increased yields of the desired metal trithiophosphonate
complex. In addition, using this protocol, it was also possible
to prepare the homologous triselenophosphonate complex.
Thus, magnesium trithiophosphonate and triselenophospho-
nate complexes 4 and 5, respectively, were prepared from
diethylphosphonate ester starting materials via a reaction
protocol involving reduction with LiAlH4 followed by a
reaction with dibutylmagnesium and either elemental sulfur
or selenium (Scheme 4).
case, a small quantity of insoluble red powder (<5% yield),
thought to be the corresponding tetrathiohypodiphosphonate
complex, was also formed in the reaction. Preparation of the
homologous lithium triselenophosphonate (nPrPSe3Li2) was
also attempted via substitution of the sulfur reagent in Scheme
3 with elemental selenium; however, the reaction yielded a
number of inseparable phosphorus- and selenium-containing
products as determined by 31P NMR, with the major product
(δ 55.9) thought to be the tetraselenohypodisphosphonate
[(nPrP(Se2))2Li2].
X-ray diffraction experiments reveal 3 to form a polymeric
chain in the solid state, comprising dimeric [Li2(S3PnPr)-
(TMEDA)]2 cages linked together by bridging bis-
(monodentate) TMEDA molecules (Figure 3 and Table 4).
Independent centers of symmetry are present at the centers of
n
Reduction of PrP(O)(OEt)2 followed by treatment with 1
equiv of nBu2Mg and 3/8 equiv of S8 in the presence of TMEDA
gave 4 as a crystalline solid in 51% overall yield (31P NMR in
DMSO-d6: δ 84.1). The yield of 4 from the phosphonate ester
starting material is therefore directly comparable to that
obtained for the only previously reported magnesium
trithiophosphonate complex [Mg(S3PC6H11)(THF)2]2 (52%),
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Figure 3. View of a section of the polymeric structure of 3. Hydrogen atoms and disorder in the bridging TMEDA molecule are omitted for clarity.
Thermal ellipsoids are displayed at the 40% probability level.
coordination. One chalcogen atom from each ligand
coordinates to both metal centers, while the remaining two
donors bind to a single magnesium only. The orientation of the
ligands in both complexes is staggered with respect to one
another, and in 4, the orientation of the C1−Cipso bond of the
benzyl groups is anti to the P1−S3 bond. Chelating TMEDA
molecules occupy the outward-facing coordination sites around
the magnesium centers. Inspection of the P−E (E = S, Se)
bond lengths in both complexes shows the bonds to the
bridging chalcogens, S2 or Se2, to be longer than those
involving nonbridging chalcogens [4, 2.0390(7) vs 2.0090(8)
and 2.0181(8) Å; 5, 2.1948(6) vs 2.1716(6) and 2.1720(6) Å].
Nevertheless, all P−E bonds are intermediate in length between
the expected P−E single (P−S, 2.13 Å; P−Se 2.24 Å35) and
double (PS, 1.88 Å35; PSe, 1.96 Å35) bond lengths,
suggesting delocalization of the double negative charge over the
PE3 group similar to 1 and 3. The Mg−E bond lengths also
vary depending on the chalcogen atom environment, with
magnesium distances to bridging chalcogens longer than those
to the other chalcogens [4, 2.6811(9) and 2.6856(9) vs
2.5780(9) and 2.5899(9) Å; 5, 2.8219(7) and 2.8383(7) vs
2.7297(7) and 2.7111(7) Å]. A comparable core structure,
exhibiting similar variation in the P−E and E−Mg bond
lengths, has also been observed in the THF-solvated
magnesium trithiophosphonate [Mg(S3PPh)(THF)2]2.26
Table 4. Selected Bond Lengths (Å) and Angles (deg) for 3
P1−S1
P1−S2
P1−S3
P1−C13
Li1−S1
Li1−S2A
2.0406(6)
2.0391(6)
2.0113(6)
1.8242(16)
2.444(3)
Li2−S1
Li2−S2
Li2−S3A
Li1−N1
Li1−N4
Li2−N9
2.486(3)
2.469(3)
2.405(3)
2.117(3)
2.123(3)
2.185(6)
2.427(3)
S2−P1−S1
S3−P1−S1
S3−P1−S2
C13−P1−S1
C13−P1−S2
C13−P1−S3
108.40(3)
115.40(3)
115.50(3)
105.03(6)
105.46(6)
106.01(6)
P1−S1−Li1
P1−S1−Li2
P1−S2−Li2
P1−S2−Li1A
P1−S3−Li2A
109.83(8)
81.83(7)
82.28(8)
110.52(8)
114.01(8)
Scheme 4. Synthesis of 4 and 5
31P and 77Se NMR spectroscopic studies on 5 in dimethyl
sulfoxide (DMSO)-d6 suggest that on the NMR time scale all of
the selenium atoms are identical: only one doublet is observed
in the 77Se NMR spectrum, and one set of 77Se satellites is
observed in the 31P NMR spectrum. A feasible explanation for
this is the breakup of the complex in DMSO to give an ion-
separated species comprised of a [nPrPSe3]2− anion and a
solvated magnesium cation. The 31P NMR signal for 5 is, at
−18.5 ppm, within the range reported for triselenophospho-
formed in the much more direct reaction of C6H11PH2 with
nBu2Mg and S8.26 Similarly, the reaction of a prepared toluene
solution of PrPH2 with 1 equiv of Bu2Mg and 3 equiv of
elemental gray selenium proceeded cleanly to give triseleno-
phosphonate 5 in 37% overall yield [31P NMR in DMSO-d6 δ
−18.5 (1JPSe = −542 Hz)].
Following recrystallization from either THF (4) or dichloro-
methane (5), both trichalcogenophosphonates complexes were
structurally characterized by single-crystal X-ray diffraction and
shown to exhibit similar molecular structures in the solid state,
forming dimeric aggregrates with comparable core bonding
motifs (Figure 4 and Table 5).
n
n
nates (13.44 to −29.47 ppm). The JPSe coupling constant
1
(−542 Hz) is, however, slightly larger than those previously
reported for similar complexes (−47725 to −5047,33 Hz) but is
still consistent with a P−Se bond order of 11/3 (which is
indicative of complete charge delocalization over the PSe3
moiety), being between values reported for P−Se single
(−405 Hz)37 and 11/2 bonds (−585 Hz).25 The 77Se NMR
Complexes 4 and 5 adopt Ci-symmetric dimeric structures,
with two ligands bridging the two metal centers via μ2-η2,η2
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Figure 4. Molecular structures of (a) 4 and (b) 5. Hydrogen atoms in 4 and 5 and disorder in the TMEDA molecule in 4 are omitted for clarity.
Thermal ellipsoids are displayed at the 40% probability level.
all four trichalcogenophosphonates in this paper have been
structurally characterized and discussed.
Table 5. Selected Bond Lengths (Å) and Angles (deg) for
Complexes 4 and 5
Trichalcogenophonate materials are promising candidates for
a range of future potential applications both as homologues to
metal organophosphosphonates and as phosphorus−chalcogen
ligand systems in their own right. The synthetic protocol
pioneered in this work will allow access to a wide range of new
trichalcogenophosphonate systems with differing organo
groups thanks to the commercial availability and easy
accessibility of the organophosphonate ester starting materials.
Work to prepare and exploit these materials is currently
ongoing in our group.
4 (E = S)
5 (E = Se)
4 (E = S)
5 (E = Se)
P1−E1
P1−E3
P1−E2
P1−C1
2.0090(8)
2.0181(8)
2.0390(7)
1.848(2)
2.1720(6) E1−Mg1
2.1948(6) E2−Mg1
2.1716(6) E2−Mg1A
2.5899(9)
2.6856(9)
2.6811(9)
2.5780(9)
2.7297(7)
2.8219(7)
2.8383(7)
2.7111(7)
1.821(2)
E3−Mg1A
E1−P1−
E3
114.13(4)
109.06(3)
109.34(3)
109.27(9)
109.14(9)
105.78(8)
113.48(3) P1−E1−
86.43(3)
83.32(3)
83.53(3)
86.67(3)
85.10(2)
82.47(2)
82.08(2)
85.57(2)
Mg1
E1−P1−
E2
109.84(2) P1−E2−
Mg1
E3−P1−
E2
109.04(2) P1−E2−
Mg1A
ASSOCIATED CONTENT
■
C1−P1−
E1
109.35(8) P1−E3−
Mg1A
S
* Supporting Information
C1−P1−
E2
109.71(8)
105.33(7)
X-ray crystallographic data in CIF format and X-ray crystal
structures of 1−5. This material is available free of charge via
the Internet at http://pubs.acs.org.
C1−P1−
E3
AUTHOR INFORMATION
1
■
spectrum shows a doublet signal at 353.6 ppm with JSeP
=
Corresponding Author
*E-mail: r.davies@imperial.ac.uk. Tel: +44 (0)207 5945754.
Notes
−542 Hz. This is consistent with chemical shift values reported
for similar [RPSe3]2− triselenophosphonate ligands: 166 (R =
C6H11)25 and 364 (R = Ph)7,33 ppm.
The authors declare no competing financial interest.
CONCLUDING REMARKS
■
This work has demonstrated a new synthetic route to metal
trichalcogenophosphonates from readily available organo-
phosphonate esters. An all-in-one protocol involving reduction
of the organophosphonate ester using LiAlH4 followed by an
aqueous workup under nitrogen and immediate reaction with a
metallating agent and elemental chalcogen gave the titled
compounds. The use of n-butyllithium as the metallating agent
yielded the lithium trithiophosphonate complexes 1 and 3,
although contamination from the tetrathiohypodiphosphonate
condensation product (2) was evident. Switching from n-
butyllithium to di-n-butylmagnesium as the metallating reagent
avoided the formation of condensation byproducts, giving the
trithiophosphonate 4 in moderate yield and also allowing access
to the triselenophosphonate homologue 5. Structures of metal
trichalcogenophosphonates are still rare in the literature; thus,
ACKNOWLEDGMENTS
■
We thank the EPSRC (Grant EP/E021077/1) for supporting
this work.
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