Orthophosphate and Sulfate Utilization for C-E (E = P, S) Bond Formation via Trichlorosilyl Phosphide and Sulfide Anions

Article abstract of DOI:10.1021/jacs.9b01475

Reduction of phosphoric acid (H₃PO₄) or tetra-n-butylammonium bisulfate ([TBA][HSO₄]) with trichlorosilane leads to the formation of the bis(trichlorosilyl)phosphide ([P(SiCl₃)₂]^-, 1) and t

Full text of DOI:10.1021/jacs.9b01475

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Article
Orthophosphate and Sulfate Utilization for C-E (E = P, S) Bond
Formation via Trichlorosilyl Phosphide and Sulfide Anions
Michael B. Geeson, Pablo Ríos, Wesley J. Transue, and Christopher C. Cummins
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.9b01475 • Publication Date (Web): 22 Mar 2019
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Orthophosphate and Sulfate Utilization for C−E (E =
P, S) Bond Formation via Trichlorosilyl Phosphide and
Sulfide Anions
9
Michael B. Geeson, Pablo R´ıos, Wesley J. Transue, and Christopher C. Cummins∗
Department of Chemistry, Massachusetts Institute of Technology, Cambridge, MA 02139, USA
Received March 20, 2019; E-mail: ccummins@mit.edu
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Abstract: Reduction of phosphoric acid (H3PO4) or tetra-n-butylammonium bisulfate ([TBA][HSO4]) with trichlorosilane leads
to the formation of the bis(trichlorosilyl)phosphide ([P(SiCl3)2]−, 1) and trichlorosilylsulfide ([Cl3SiS]−, 2) anions, respectively.
Balanced equations for the formation of the TBA salts of anions 1 and 2 were formulated based on the identification of hexachloro-
disiloxane and hydrogen gas as byproducts arising from these reductive processes: i) [H2PO4]− + 10 HSiCl3 → 1 + 4 O(SiCl3)2
+ 6 H2 for P and ii) [HSO4]− + 9 HSiCl3 → 2 + 4 O(SiCl3)2 + 5 H2 for S. Hydrogen gas was identified by its subsequent
use to hydrogenate an alkene ((−)-terpinen-4-ol) using Crabtree’s catalyst ([(COD)Ir(py)(PCy3)][PF6], COD = 1,5-cyclooctadiene,
py = pyridine, Cy = cyclohexyl). Phosphide 1 was generated in situ by the reaction of phosphoric acid and trichlorosilane, and
used to convert an alkyl chloride (n-chlorooctane) to the corresponding primary phosphine, which was isolated in 41% yield. Anion
1 was also prepared from [TBA][H2PO4] and isolated in 62% yield on a gram scale. Treatment of [TBA]1 with an excess of
benzyl chloride leads to the formation of tetrabenzylphosphonium chloride, which was isolated in 61% yield. Sulfide 2 was used
as a thionation reagent, converting benzophenone to thiobenzophenone in 62% yield. It also converted benzyl bromide to benzyl
mercaptan in 55% yield. The TBA salt of trimetaphosphate ([TBA]3[P3O9].2H2O), also a precursor to anion 1, was found to react
with either trichlorosilane or silicon(IV) chloride to provide bis(trimetaphosphate)silicate, [TBA]2[Si(P3O9)2], characterized by NMR
spectroscopy, X-ray crystallography and elemental analysis. Trichlorosilane reduction of [TBA]2[Si(P3O9)2] also provided anion 1.
The electronic structures of 1 and 2 were investigated using a suite of theoretical theories; the computational studies suggest that
the trichlorosilyl ligand is a good π-acceptor and forms σ-bonds with a high degree of s character.
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A
thermal process
Phosphorus
chemicals
P4
Introduction
white phosphorus
White phosphorus has long been the critical intermediate
for the synthesis of nearly all phosphorus-containing non-
fertilizer chemicals.1–3 The production of white phospho-
rus, in what is known as the “thermal process”, requires
the energy-intensive reduction of phosphate rock and is con-
ducted in an electric arc furnace at 1500 ◦C.3 White phos-
phorus is a toxic and pyrophoric substance that has been
used as a chemical warfare agent and its transport has led
to high-profile catastrophes.4 The legacy thermal process
for the production of P4 has served a secondary role as a
method of purification; the phosphorus volatilized from the
electric arc furnace allows for it to be separated from chiefly
sulfur- and silicon-containing impurities.5 However, it strug-
gles to separate arsenic, which is toxic to humans,6 because
As replaces P in the P4 lattice.3,7
In contrast, phosphoric acid, H3PO4, is produced by the
“wet process”, which involves treating phosphate rock with
sulfuric acid.5 In this scheme, the industrial chemistry of
sulfur and phosphorus are intimately intertwined; sulfur is
a secondary product from the petrochemical industry, and
the vast majority of it is used to make sulfuric acid, allow-
ing the wet process to operate.8 The scale of the latter is
much larger than that of the thermal process, accounting
for the fate of 95% of the phosphate rock mined annually.3
Improvements in the methods for purifying phosphoric acid
have been such that, nowadays, the purity of H3PO4 pro-
duced by the wet process rivals that produced by the thermal
process, allowing wet process phosphoric acid to be used in
food-grade applications.9 The higher energy requirements
Ca5(PO4)3X
apatite
previous work
this work
[P(SiCl3)2]−
1
H3PO4
phosphoric acid
[TBA]3[P3O9].2H2O
TBA trimetaphosphate
wet process
Na3P3O9
sodium trimetaphosphate
B
MeCN, HSiCl3
[TBA][Cl3SiS]
[TBA][HSO4]
[TBA]2
Scheme 1. A: Synthetic routes to phosphorus-containing chemi-
cals from phosphate rock. TBA = tetra-n-butylammonium. Most
commonly, phosphate rock has the formula Ca5(PO4)3X, where X
is typically F, Cl or OH, collectively referred to as apatite. B: Ap-
plication of trichlorosilane reduction to the chemistry of sulfur by
preparation of [TBA]2 from [TBA][HSO4].
of the thermal process, in addition to the toxicity and py-
rophoric properties of P4, provide motivation for eliminating
P4 in favor of phosphoric acid as the key starting material for
the production of P-containing chemicals.10,11 Along these
lines, a microwave-assisted preparation of PCl3 from calcium
phosphate in a scheme that is thought to bypass elemental
P has also been reported.12
We recently reported the synthesis and characterization
of the bis(trichlorosilyl)phosphide anion 1, as its TBA
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Page 2 of 24
salt, by reduction of trimetaphosphate with trichlorosilane
to target the stable TBA salt of 1, crystalline phosphoric
1
2
3
4
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8
(Scheme 1A).10 [TBA]1 was shown to be a versatile reagent
for the preparation of several phosphorus-containing prod-
ucts previously only available downstream of white phospho-
rus, such as Li[PF6], RPH2 and PH3. We also demonstrated
that [TBA]1 does not need to be isolated and purified, and
can instead be generated in situ for the conversion of a pri-
mary alkyl chloride to a corresponding alkyl phosphine.
Trichlorosilane, the reducing agent used to prepare anion
1, is manufactured on an industrial scale for the produc-
tion of high-purity elemental silicon, which is in turn used
to produce photovoltaic (PV) cells.13 It is listed as a high
production volume (HPV)14 chemical, produced in the US
in the range of 1–5 billion pounds (2016).15 The majority of
trichlorosilane is produced by the reaction of hydrogen chlo-
ride gas and elemental silicon, the latter being derived from
a similarly high energy process to thermal P4 production.13
More recently however, sustainable and less energy-intensive
approaches for preparing trichlorosilane have been explored.
One example, already practiced industrially, is the reaction
of silicon(IV) chloride with hydrogen.16,17 Silicon(IV) chlo-
ride is a waste product of the PV industry,16 or can alterna-
tively be prepared in a redox-neutral process that involves
converting silicate-containing minerals to a tetraalkyl or-
thosilicate18 and subsequent chlorination to silicon(IV) chlo-
ride.19,20 A method for producing silicon(IV) chloride from
silica in a process that avoids elemental silicon entirely was
described and patented in 2012.12,21 Finally, new methods
for preparing silicon from silica using electrochemical tech-
niques are areas of active investigation.22 These recent de-
velopments in the field of sustainable silicon chemistry hold
promise for the use of trichlorosilane as a reducing agent for
other element oxides.
acid was heated with [TBA]Cl in neat trichlorosilane. Af-
ter heating at 110 ◦C for 86 h an aliquot was analyzed by
31P NMR spectroscopy; the characteristic signal for anion
1 was observed at −171.7 ppm. Despite several attempts,
we were unable to obtain [TBA]1 as a pure compound using
this preparative method due to the formation of byprod-
ucts, including [TBA]2[Si6Cl14],28–30 which was identified
by 29Si NMR spectroscopy (S1.3.2). Instead, we used phos-
phoric acid to generate anion 1 in situ. In the presence of
n-octylchloride, anion 1 undergoes a key carbon-phosphorus
bond forming reaction and, after workup, n-octylphosphine
could be isolated in 41% yield based on n-octylchloride as
the limiting reactant (Scheme 2).
Although unactivated alkyl halides can be reduced with
trichlorosilane in the presence of a halide catalyst to give
trichlorosilyl-functionalized alkanes, such reactions typically
require even more forcing conditions than we have employed
here for P–C bond formation.31 In support of a mecha-
nism in which a primary alkyl chloride undergoes nucle-
ophilic attack by anion 1, we have previously shown that
cleanly isolated [TBA]1 reacts with an alkyl chloride to
give P–C bond-containing products in the absence of ad-
ditional trichlorosilane.10 This method for the preparation
of primary alkyl phosphines is attractive because it con-
stitutes a one-step procedure from commercially available
starting materials (Scheme 1A) and gives a primary alkyl
phosphine in two steps from phosphate rock, the raw mate-
rial that is mined from the ground. n-Octylphosphine is a
useful precursor to stannylphosphines,32,33 as well as phos-
phacyclohexanes.34 Octylphosphine can also be oxidized to
octylphosphonic acid,35 which finds applications as a ligand
for nanoparticles36,37 and for heavy metal extraction pro-
cesses.38,39
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HSiCl3
[TBA]H2PO4
[TBA][P(SiCl3)2]
62%, 1 gram
Results and discussion
110 °C, 6 d
Scheme 3. Preparation of [TBA]1 starting from [TBA][H2PO4].
Reduction of orthophosphate with
trichlorosilane
Interested in having a reliable preparation of pure sam-
ples of [TBA]1 from a source of orthophosphate, we con-
sidered the possibility that tetra-n-butylammonium dihy-
drogenphosphate, [TBA][H2PO4],40 might be a more suit-
able phosphate-containing starting material. By avoiding
the introduction of additional chloride to the reaction, as
was the case for the initial H3PO4/[TBA]Cl conditions,
we sought to minimize the formation of byproducts such
as [TBA]2[Si6Cl14] which had impeded isolation of pure
[TBA]1. Gratifyingly, [TBA]1 could be isolated pure and
in a moderate yield (62%) when [TBA][H2PO4] was sub-
jected to the standard conditions for preparing anion 1
(Scheme 3). Importantly, we encountered no difficulties
in the typical workup and purification procedure. Over
the prolonged reaction times employed, decomposition of
TBA to give tri-n-butylammonium ([Bu3NH]+) was ob-
served (S1.6.1), although based on the moderate yield of
[TBA]1 from [TBA][H2PO4] (62%, Scheme 3) this process
appears to be slow. [TBA]1 melts with slight decomposi-
tion at 105–108 ◦C; analysis of a sample of [TBA]1 that was
maintained at 110 ◦C for 20 minutes partially redissolved in
DCM to show the presence of anion 1 by 31P NMR spec-
troscopy (S1.3.3). Nonetheless, future work will seek to de-
termine which cations can be paired with anion 1 to provide
salts with superior thermal stabilities.
Trimetaphosphate, the precursor to [TBA]1, is produced
by the dehydration of monosodium dihydrogenphosphate
at 300–500 ◦C,5 or more recently by a similarly high-
temperature dehydration of H3PO4 in the presence of
sodium chloride.23 We wondered whether it might be possi-
ble to obviate this dehydration step, and instead prepare
anion 1 more directly from a source of orthophosphate.
Despite the well-documented reactivity of water and alco-
hols with Si–Cl bonds to give silanols,24 our initial sys-
tem tolerated the two equivalents of water present in the
[TBA]3[P3O9]·2H2O starting material.25 We therefore con-
sidered sources of orthophosphate containing acidic pro-
tons as potentially viable substrates for the preparation of
[TBA]1.
1.5 H3PO4
1.5 [TBA]Cl
PH2
CH3(CH2)6CH2Cl
HSiCl3, 110 °C, 6 d
then Al2O3 or H2O
41%, 1.3 g
Scheme 2. Synthesis of n-octylphosphine from n-octylchloride and
phosphoric acid.
Crystalline phosphoric acid, typically obtained from an
aqueous 85% solution of H3PO4,26,27 accordingly was tested
for its ability to serve as a precursor to anion 1. In order
With a method in hand for preparing [TBA]1 directly
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Cl
CH2Ph
P
10 PhCH2Cl
1
2
3
4
5
[TBA][P(SiCl3)2]
61%
CH2Ph
MeCN, 70 °C
PhH2C
O
CH2Ph
O
P
SiCl3
Scheme 4. Synthesis of tetrabenzyl phosphonium chloride from
[TBA]1 and benzyl chloride.
*
O
O
P
c
a
O
O
a
O
O
O
b
b
P
O
6
P
a
P
O
Si
O
O
O
7
8
9
P
O
O
b
a
O
from a source of orthophosphate, we set out to further ex-
plore the reactivity of 1 with carbon-based electrophiles.
Heating [TBA]1 with an excess (10 equiv.) of benzyl chloride
led to the formation of tetrabenzylphosphonium, identified
by its 31P NMR spectrum (δ 24.5 ppm, displaying coupling
c
3
O
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2
to 8 equivalent protons to give a binomial nonet with JP–H
= 14 Hz), in line with previous literature reports.41 The
phosphonium salt was separated from any TBA salts using
water; while both the TBA and phosphonium salts were sol-
uble in boiling water, only the TBA salts were soluble at
23 ◦C. Accordingly, recrystallization of the crude product
from boiling water provided tetrabenzylphosphonium chlo-
ride in 61% yield (Scheme 4). Tetrabenzylphosphonium can
be converted to the corresponding Wittig reagent and sub-
sequently used to make carbon-carbon double bonds.42
-30
-35
31P (δ, ppm)
-40
Figure 1. 31P{ H} NMR spectrum of the reaction mixture ob-
tained when [TBA]3[P3O9].2H2O and SiCl4 are mixed for 30 min-
utes in DCM at 23 ◦C. The resonances correspond to intermediate
3 (inset) in the synthesis of [TBA]2[Si(P3O9)2]. The asterisk (*)
denotes [TBA]2[Si(P3O9)2].
1
O3
O2
P1
Intermediates in the HSiCl3 reduc-
tion of phosphate sources
With trimetaphosphate as the P source
O1
Si1
Reduced P-containing species other than anion 1 were
not observed upon trichlorosilane treatment of phos-
phate sources. However, a small but significant shift
(10 ppm) of the trimetaphosphate resonance in the 31P
NMR spectrum was observed when the initial reaction of
[TBA]3[P3O9].2H2O with trichlorosilane was monitored at
23 ◦C. Such changes in chemical shift are diagnostic of
trimetaphosphate coordinating as a ligand.43,44 Diluting
the reaction mixture in dichloromethane, thereby lowering
the concentration of trichlorosilane and slowing the rate of
reaction, led to the observation of an additional species (3,
Fig. 1), which by 31P NMR spectroscopy was indicative
of two trimetaphosphate units coordinated to two separate
silicon centers. The connectivity of intermediate 3 was as-
signed based on the integration ratios and chemical shifts
of the κ3 coordinated43 (Pa), ultraphosphate44 (Pb) and
branched45 (Pc) polyphosphate regions, as well as the cou-
Figure 2. Molecular structure of [TBA]2[Si(P3O9)2] with the el-
lipsoids at the 50% probability level and the two TBA cations
omitted for clarity. Color coding: phosphorus (orange), oxygen
(red) and silicon (tan). Selected bond distances (given as the av-
erage of chemically equivalent bonds) (A): Si1–O1: 1.7717(21),
O1–P1: 1.5254(23), P1–O2: 1.4448(25), P1–O3: 1.6121(34).
˚
Silicophosphate [TBA]2[Si(P3O9)2] forms quickly and
quantitatively (≥20 min) when a DCM solution of [TBA]1
is treated with a large excess of trichlorosilane (23 equiv.) at
room temperature. It was confirmed that the [Si(P3O9)]2−
2
pling pattern (AB2, JP–P = 27 Hz) of Pb and Pc. After
48 h, this intermediate had converted to one having a single
31P NMR resonance at δ 29.4 ppm, which is formulated as
[TBA]2[Si(P3O9)2].
anion can serve as
a P-source in the generation of
1, by subjecting an independently prepared sample of
[TBA]2[Si(P3O9)2] to the standard synthesis conditions
(neat HSiCl3, 110 ◦C).Although no other intermediates
were observed by 31P NMR spectroscopy, the reduction of
[TBA]2[Si(P3O9)2] by trichlorosilane might proceed through
mechanisms similar to those which have been proposed for
related organic phosphine oxide reductions.49 These have
been postulated to proceed through either attack of the
trichlorosilyl anion, [SiCl3]−,50 at the phosphorus center
or via a λ5 phosphorane intermediate.51 Disproportiona-
tion and redistribution reactions of chlorosilanes by either
a nucleophile or a base are well known,28–30 and so a po-
tentially large number of silicon-containing species could
be responsible for reduction of the phosphate sources we
have identified as precursors to [TBA]1. Given that we
This compound could be prepared independently by re-
action of SiCl4 with [TBA]3[P3O9].2H2O, allowing for its
isolation in 27% yield. An X-ray diffraction study con-
firmed the connectivity of [TBA]2[Si(P3O9)2], featuring a
six-coordinate central silicate moiety sandwiched between
two trimetaphosphate ligands (Fig. 2). Anion [Si(P3O9)2]2−
is a rare example of an entirely inorganic molecular sil-
icophosphate; the majority of structurally characterized
molecular silicophosphates contain organic substituents at
silicon.46 Like [TBA]2[Si(P3O9)2], silicophosphate materials
generally feature silicon in an all-oxygen six-coordinate en-
vironment.47 However, their syntheses generally proceed at
higher temperatures48 than we have found to be the case for
[TBA]2[Si(P3O9)2].
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do not observe additional intermediates by 31P NMR spec-
hydrogen phosphate and phosphoric acid, it follows that 2
1
2
3
4
5
6
7
8
troscopy, the first reduction event at phosphate is likely
rate-determining. Remarkably, anion 1 forms selectively
under the reaction conditions; analogs in which a Cl is
replaced by a hydrogen atom, a trichlorosilyl group, or a
trichlorosiloxy group (–OSiCl3) were not observed by 31P
NMR. Each of these three scenarios seems reasonable, given
the facile redistribution of H for Cl in chlorosilanes,52 the
ease at which chlorosilane oligomers are formed29,53 and the
existence of a related anion in which a phosphide features
two dichloroaryloxysilyl functional groups ([P(SiCl2OR)2]−
(R = 2,4,6-(t-Bu)3C6H2)).54
might be accessed by reduction of bisulfate with trichlorosi-
lane: a procedure that would avoid the use of liquefied
H2S, which is both extremely toxic and dangerous.58 Ac-
cordingly, [TBA]2 was prepared in moderate yield (43%) by
stirring [TBA][HSO4] in an acetonitrile/trichlorosilane mix-
ture at 23 ◦C for 20 h (Scheme 6). Conditions leading to
the formation of [TBA]2 are notably milder than the condi-
tions required for the corresponding preparation of [TBA]1
(S1.3.6). We also found that reduction of [TBA][HSO4] with
trichlorosilane leads to the formation of some elemental sul-
fur. A single crystal obtained from the crude reaction mix-
ture in the preparation of [TBA]2 was unambiguously iden-
tified as S8 by comparison of its unit cell with previous lit-
erature reports.59
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With phosphoric acid as the P source
Condensed phosphates are observed (31P NMR) to form un-
der the reduction conditions for the preparation of 1 from
phosphoric acid, demonstrating trichlorosilane must serve
the role of dehydrating agent in addition to its role as the
reducing agent. At the end of the reaction used to pre-
pare anion 1 in situ from H3PO4, a white precipitate was
seen to have formed that was insoluble in common organic
solvents. The material dissolved completely in water and
analysis of this aqueous solution by 31P NMR spectroscopy
revealed the presence of pyrophosphate and linear triphos-
phate (Scheme 5). Trichlorosilane is clearly capable of con-
densing orthophosphate into longer chain polyphosphates,
and this may play a role in the mechanism governing the
formation of [TBA]1. Presumably, condensed phosphates
formed under the dehydrative conditions employed are ca-
pable of acting as ligands towards silicate centers, in a
manner similar to the formation of [TBA]2[Si(P3O9)2] from
trimetaphosphate and trichlorosilane. The longer reaction
times required for orthophosphate to be converted to anion
1, versus trimetaphosphate, are in line with our previous ar-
guments that metaphosphates are kinetically more prone to
reductive processes.10
MeCN, HSiCl3
[TBA][Cl3SiS]
[TBA][HSO4]
43%
[TBA]2
23 ℃, 20 h
Scheme 6. Preparation of [TBA]2 from [TBA][HSO44].
S1
Cl1
Si1
Cl2
Cl3
Figure 3. Molecular structure of [TBA]2, with ellipsoids at the
50% probability level and the TBA cat◦ion omitted for clarity.
˚
Selected bond lengths (A) and angles ( ): Si1–S1: 1.9756(14),
Si1–Clav.: 2.073(7), S1–Si1–Clav.: 115.59(25), Cl–Si1–Clav.:
102.69(30).
[TBA]2 was characterized by multinuclear NMR, IR, and
Raman spectroscopies in addition to elemental analysis and
X-ray crystallography (Fig. 3). The IR and Raman data are
consistent with those provided in the single previous report
of anion 2, which had been isolated as the tetraethylam-
monium (TEA) salt.57 The previous report also described
the solid-state structure of [TEA][2] but the data were of
insufficient quality to permit a detailed discussion of the
bond metrics of the anion. Fortunately, our crystallographic
study of [TBA]2 permitted unambiguous assignment of the
sulfur and chlorine atoms of anion 2. The assignment of
the sulfur and chlorine atoms is also supported by the dif-
ferent Si–S and Si–Cl lengths, which match well with those
we calculated at the ωB97X-D3/ma-Def2-QZVPP level of
O
O
O
P
P
P
triphosphate
O
O
O
O
[TBA][Cl] (1 equiv.)
HSiCl3, 110 °C, 6 d
O
O
O
H3PO4
[1] +
O
P
O
P
pyrophosphate
O
O
O
O
O
Scheme 5. Under the conditions used to prepare anion 1 in situ,
pyrophosphate and linear triphosphate were observed by 31P NMR
spectroscopy. Line drawings of the polyphosphate products are
shown in their fully deprotonated states for clarity.
Sulfate reduction by trichlorosi-
lane
theory.60,61 Anion 2 displays a Si–S bond of 1.9756(14) A,
˚
significantly shorter than the sum of the covalent single bond
62
radii, 2.19 A. This bond length contraction is indicative of
˚
The ability of trichlorosilane to effect the dehydration and
complete deoxygenation of phosphate suggests a likely strat-
egy for the reduction of other p-block oxoanions to give
products containing the E–SiCl3 group. Such compounds
(E = C, Si, Ge) have received interest in recent years due
to their potential applications in deposition processes55 and
chemical synthesis.56 A survey of the literature returned the
trichlorosilylsulfide anion ([Cl3SiS]−, 2), which was prepared
previously from tetraethylammonium hydrogen sulfide upon
treatment with silicon tetrachloride in liquid hydrogen sul-
fide.57 Given the above results for generation of 1 from di-
multiple bonding between silicon and sulfur, a phenomenon
we have analyzed using a suite of theoretical methods (see
Electronic Structure and Bonding section).
Carbon-sulfur bond-forming reac-
tions
With [TBA]2 in hand, we set out to test the value of
this reagent for the preparation of organosulfur compounds
(Scheme 7). Although the conjugate acid of 2, Cl3SiSH, has
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1
2
3
4
5
6
7
8
9
10
11
MP2 π MOs
π NBOs
Figure 4. Comparison of the π interaction between silicon and sulfur in anion 2. The MP2 π MOs resemble p-orbital lone pairs on
sulfur, whereas the corresponding π NBOs suggest the presence of polar π bonds. Green: chlorine, tan: silicon, orange: sulfur.
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S
O
at 2.037, while indicating the presence of multiple resonance
structures contributing greater than 10% to the overall elec-
tron density. The leading resonance structure, at ca. 22%, is
the one corresponding to the NBOs and having a triple bond
between sulfur and silicon together with three Si–Cl single
bonds. The π bonds for this resonance structure are highly
polar with 6% Si dp character and 94% S p character; thus,
these π bonds are close to the NBO bonding/nonbonding
cutoff. The S–Si σ bond is much more covalent with 35%
Si sp1.52 character and 64% S sp3.1 character. The short
nature of the Si–S bond is likely due to the large amount
of s character used by silicon to form the σ interaction.
This is in line with Bent’s rule considerations that a cen-
tral atom (here, Si) will accumulate s-orbital character in
forming bonds to less electronegative elements. Accord-
5 [TBA][Cl3SiS]
62%
55%
CHCl3, 80 °C, 96 h
Ph
Ph
Ph
SH
Ph
Ph
Ph
Br
1.1 [TBA][Cl3SiS]
DCM, 40 °C, 18h
then H2O
Scheme 7. Use of [TBA]2 as a thionation reagent to give ben-
zyl mercaptan and thiobenzophenone from benzyl bromide and ben-
zophenone respectively.
been known for half a century,63 the use of this species in
chemical synthesis seems to be largely unexplored. It seems
reasonable to expect that formation of Si–X bonds (X =
O, Cl, Br) from the reaction of anion 2 with C–X bond-
containing substrates would provide a thermodynamic driv-
ing force for such reactions. Such is the case for an organic
analog of 2, [S–SiMe3]−, recently prepared by desilylation
of S(SiMe3)2 by ionic liquids64 and thought to be an in-
termediate for thionation reactions employing S(SiMe3)2.65
Treatment of benzyl bromide with [TBA]2 gave rise to a ma-
jor new species in solution, identified as benzyl trichlorosi-
lylsulfide by its NMR data (S1.9.1). A hydrolytic workup
and purification by vacuum transfer allowed for the isola-
tion of benzyl mercaptan in 55% yield. We also found that
[TBA]2 showed comparable reactivity to other thionating
compounds, such as Lawesson’s reagent, which can be used
for the conversion of carbonyl to thiocarbonyl functional
groups.66 Accordingly, thiobenzophenone could be prepared
from benzophenone using [TBA]2 (5 equiv.) in chloroform.
An excess of [TBA]2 was required in order to achieve a
reasonable reaction time. Thiobenzophenone was extracted
from the TBA-containing byproducts using hexanes and iso-
lated in 62% yield.
ingly, silicon uses majority p-orbital character, sp2.5d1.4
,
in forming the polar (19% Si, 81% Cl) Si–Cl bonds. In
terms of their appearance (Fig. 4) the MP2 π molecular
orbitals (left) look like p-orbital lone pairs polarized in the
direction of silicon, while the corresponding S–Si π NBOs
(right) do a better job of suggesting the presence of polar
π bonds. Finally, analysis of the generalized valance bond
(GVB) wavefunction (Fig. 5) using the VB2000 program70
as implemented in GAMESS,67 treating the Si–S bond at
the CASVB(6,6)/6-31++G** level of theory, gave a Si–S
bond order of 1.84.
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
1, 35%
3, 12.5%
5, 12%
2, 16%
4, 12.5%
6, 12%
S
S
S
Si
S
S
S
Si
Cl
Cl
Cl
Cl
Si
Si
Cl
Cl
Cl
Cl
Si
Si
Cl
Figure 5. Summary of the generalized valance bond (GVB)
wavefunction for anion treating the Si–S bond at the
Cl
Electronic structure and bonding
Electronic structure calculations on sulfide
MP2/aug-cc-pVTZ level of theory (C3v-optimized metri-
cal parameters (A, ◦): Si–S, 1.984; Si–Cl, 2.103; S–Si–Cl,
˚
116.8) are indicative of multiple bonding between sulfur and
silicon. Analysis of this wavefunction using GAMESS67
gave a Mayer bond order of 1.784 for the S–Si linkage.68
Submitting the same wavefunction for natural bond orbital
(NBO) analysis69 provided via the natural resonance theory
(NRT) routine an even larger estimate of this bond order,
2
2 at the
CASVB(6,6)/6-31++G** level of theory. Analysis of the GVB
wavefunction gives a Si–S bond order of 1.84.
Analyzed similarly, there is some evidence of multiple
bond character between phosphorus and silicon in phosphide
1; the anion was geometry optimized with C2 point group
symmetry (key metrical parameters (A, ◦): P–Si, 2.145; Si–
˚
P–Si, 95.4). Analysis of the MP2/aug-cc-pVTZ wavefunc-
tion using GAMESS67 gives a P–Si Mayer bond order of
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Page 6 of 24
Table 1. Comparison of the experimental and predicted values for
the JP–Si coupling constants and the % s-orbital character in the
1.318. In this case, NBO analysis indicates the presence of
1
1
2
3
4
5
6
7
8
two lone pairs at the phosphorus atom, the first of which
points along the C2 rotational axis of symmetry and accu-
mulates two-thirds s-orbital character (sp0.5 hybridization).
The second P lone pair is contained in a pure p-orbital for
which the PSi2 plane is a nodal plane. The P–Si σ bonds
are formed from overlap of sp5 and sp1.44 hybrids at P and
Si, respectively, such that once again, silicon reserves much
of its p-orbital percentage for bonding to the chlorine sub-
stituents, with s-orbital character being strong in silicon’s
contributions to the P–Si σ bonds. In anion 1, the σ bonds
are quite covalent (55% P, 45% Si). From inspection of the
NBO second order perturbation theory analysis of the Fock
matrix, the strongest delocalizations come from the π lone
pair into the Si–Cl σ∗ orbitals. The NRT analysis gives a
natural bond order of 1.340 in this case, close to the value
obtained from the GAMESS analysis of the MP2 wavefunc-
tion. Once again, due to the importance of multiple ionic
resonance structures in which P–Si π bonds are formed at
the expense of Si–Cl σ bonds, the leading resonance struc-
ture has a weight of only 22%.
P–Si bonds for a series of compounds
1JP–Si / Hz
% s charactera
Species
Exp.b Pred.c σ(P–Si)d Sie
Me3SiPPh2·BH3
F3SiPH2
HP(SiH3)2
−4475
1576
3577
5278
7379
7772
−42
22
20
54
85
76
115
159
20
21
17
21
21
22
23
26
8
15
9
15
15
14
18
20
Cl3SiPH2
9
(i-Pr)P(SiCl3)2
[P(SiH3)2]−
10
11
12
13
14
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(i-Pr)P(NPh2)(SiCl3) 12580
[P(SiCl3)2]−, 1
15810
a
Determined by NBO analysis
Experimental value determined by 31P NMR spectroscopy
b
c Predicted value determined by evaluation of the calculated
1JP–Si value in Eqn. 1
Sum of the P and S s-orbital character in the P–Si bond
S s-orbital character in the P–Si bond
d
e
Balancing the equations for the
formation of anions 1 and 2
1
Calculation of JP–Si coupling con-
An important question we wished to address in these two re-
ductive processes was the fate of the hydrogen atoms, origi-
nating from trichlorosilane and the protic substrates H3PO4
and [TBA][HSO4]. H2 or HCl would be the most likely
gaseous hydrogen-containing byproducts with the former
considered the more likely candidate given that P–Si and
S–Si bonds of 1 or 2, respectively, would be cleaved by HCl.
Under the assumption of H2 as the gaseous byproduct, we
carried out a two-compartment experiment where the hydro-
gen generated from the reduction of bisulfate could be used
to hydrogenate an alkene. We selected Crabtree’s catalyst
for the hydrogenation of (−)-terpinen-4-ol (Scheme 8) due to
the high efficiency of this reaction at mild pressures and tem-
peratures.81 Using a two-flask setup, we were able to confirm
the formation of the hydrogenated product, trans-4-methyl-
1-isopropylcyclohexan-1-ol, by 1H NMR spectroscopy.
stants
Theoretical analysis of the electronic structure of anions 1
and 2 predicted the s character of the Si–E (E= P, S) bonds
to be high, principally due to the electronegative chloride
substituents on silicon. In the case of anion 1, we sought
to experimentally validate this analysis by interpretation of
1
the JP–Si coupling constant, as measured by NMR spec-
1
troscopy. The magnitude of the JP–X coupling constant
is dominated by the Fermi-contact interaction, proportional
to the bond s character.71 Comparison to a series of re-
1
lated JP–Si values was approached with quantum chemical
calculations (Table 1). These provided both the signs and
magnitudes of the scalar couplings, as these can be diffi-
cult to determine in the presence of second-order coupling
patterns.72 The geometry of each species was optimized at
a low level of theory (B3LYP/6-31G(d,p)) then the total
nuclear spin-spin coupling constants (J) were calculated at
the PBE/aug-cc-pVQZ level of theory.73,74 Linear regres-
sion between experimental and calculated scalar coupling
constants73 provided an excellent correlation (R2 = 0.98,
Eqn. 1):
MeCN
[TBA][HSO4] + HSiCl3
H2 + [TBA]2
cat. [(COD)Ir(py)(PCy3)][PF6]
DCM, 0 °C
1
1JP–Si(calc.) = 0.77 × JP–Si(exp.) − 38.57
(1)
HO
i-Pr
HO
i-Pr
The species included in our study contained a range of
different substituents on both phosphorus and silicon, as
well as two-, three-, and four-coordinate phosphorus com-
pounds (Table 1). Anion 1 exhibits the largest positive
1JP–Si value among all the species studied, consistent with
a large amount of s character in its P–Si bonds, imparted
by the trichlorosilyl groups. Trichlorosilyl groups consis-
tently enhance P–Si scalar coupling, giving rise to four of
the five largest constants in Table 1. The anionic charge of
1 may also increase 1JP–Si as suggested by the large value for
[P(SiH3)2]−. In order to determine the amount of s char-
acter of the phosphorus-silicon bonds, natural bond order
(NBO) calculations were carried out. The s character of
the σ(P–Si) bonds, and in particular the s character arising
from silicon, correlated moderately well with the experimen-
Scheme 8. Identification of the hydrogen produced from the reduc-
tion of [TBA][HSO4] to [TBA]2 using trichlorosilane; the hydrogen
generated from the reduction was used to hydrogenate (−)-terpinen-
4-ol using Crabtree’s catalyst.
Control experiments showed that this method was un-
suitable for the quantification of hydrogen (S1.11.3), so in-
stead we turned to a simple gas collection experiment.82
The evolved hydrogen gas was collected by the displace-
ment of water (S1.13), and this method indicated produc-
tion of 4.45 equivalents of hydrogen gas per mole of added
[TBA][HSO4], compared to the theoretical maximum of 5.0
(Eqn. 4). An analogous study to quantify the H2 produced
by the formation of [TBA]1 from H3PO4 gave 5.0 equiva-
lents of H2, representing 71% of the theoretical maximum of
7 equivalents (Eqn. 2). Although not experimentally veri-
1
tal JP–Si constants, (R2 = 0.54 and 0.76, respectively).
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fied, the corresponding balanced equation leading to the for-
mation of [TBA]1 from [TBA][H2PO4] is shown in Eqn. 3.
Supplementary Information
1
2
3
4
5
6
7
8
Experimental details, characterization data, X-ray crystal-
lographic information, computational details, and tables of
Cartesian coordinates are provided in the Supporting Infor-
mation document. This material is available free of charge
via the Internet at http://pubs.acs.org. Crystallographic
data are available from the Cambridge Structural Database
under refcodes 1841573 and 1888218.
[TBA][Cl] + H3PO4 + 11 HSiCl3 →
[TBA][1] + 4 O(SiCl3)2 + 7 H2 + SiCl4 (2)
[TBA][H2PO4] + 10 HSiCl3 →
9
[TBA][1] + 4 O(SiCl3)2 + 6 H2 (3)
References
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[TBA][HSO4] + 9 HSiCl3 →
[TBA][2] + 4 O(SiCl3)2 + 5 H2 (4)
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Acknowledgements
The authors acknowledge financial and logistical support re-
ceived through the UMRP, a partnership between UM6P
and OCP Group in Morocco and MIT. Driss Dhiba (OCP)
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of Perchlorinated Cyclohexasilanes from Si2Cl6: A Mechanistic
Scenario. Chem. - Eur. J. 2014, 20, 9234–9239.
(30) Choi, S.-B.; Kim, B.-K.; Boudjouk, P.; Grier, D. G. Amine-
Promoted Disproportionation and Redistribution of Trichlorosi-
lane: Formation of Tetradecachlorocyclohexasilane Dianion. J.
Am. Chem. Soc. 2001, 123, 8117–8118.
(31) (a) Benkeser, R. A. Chemistry of trichlorosilane-tertiary amine
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H.; Han, J. S.; Lee, M. E.; Yoo, B. R.; Jung, I. N. Phosphonium
Chloride Induced Dichlorosilylene Transfer from Trichlorosi-
lane. Organometallics 2003, 22, 2551–2553; (c) Kang, S.-H.;
Han, J. S.; Yoo, B. R.; Lee, M. E.; Jung, I. N. Phosphonium
Chloride-Catalyzed Dehydrochlorinative Coupling Reactions
of Alkyl Halides with Hydridochlorosilanes. Organometallics
2003, 22, 529–534.
(32) Greulich, T. W.; Suzuki, N.; Daniliuc, C. G.; Fukazawa, A.;
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ing two electron-donating and two electron-accepting moieties:
a rigid and small donor-acceptor-donor ladder system. Chem.
Commun. 2016, 52, 2374–2377.
(33) Li, Y.; Chakrabarty, S.; Mu¨ck-Lichtenfeld, C.; Studer, A. Ortho-
Trialkylstannyl Arylphosphanes by C–P and C–Sn Bond Forma-
tion in Arynes. Angew. Chem., Int. Ed. 2016, 55, 802–806.
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Papp, R.; Knoll, K.; Voss, H.; Roeper, M. Phosphacyclohex-
anes and the use thereof in the hydroformylation of olefins.
2004; CIB: B01J31/24; C07C45/50; C07F15/00; C07F15/04;
C07F15/06; C07F9/6568; B01J31/28; (IPC1-7): B01J31/00.
(35) Buckler, S.; Epstein, M. The preparation and reactions of pri-
mary phosphine oxides. Tetrahedron 1962, 18, 1221–1230.
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thesis of Soluble and Processable Organic-Capped Anatase TiO2
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Perchlorinated Oligosilanes: A Fresh Look at an Old Reaction.
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with this compound, it can be accessed in the Cambridge Struc-
tural Database as a private communication under the refcode
CCDC 183389.
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Tris(trichlorosilyl)tetrelide Anions and a Comparative Study of
Their Donor Qualities. Chem. - Eur. J. 2019, 25, 2740–2744.
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Exhaustively Trichlorosilylated C1 and C2 Building Blocks: Be-
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mins, C. C. Facile synthesis of mononuclear early transition-
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]
)
4−
and cyclo-trimetaphosphate ([P3O9]3−). Dalton Trans. 2014,
43, 1509–1518.
(44) Jiang, Y.; Chakarawet, K.; Kohout, A. L.; Nava, M.;
Marino, N.; Cummins, C. C. Dihydrogen Tetrametaphosphate,
[P4O12H2]2−
:
Synthesis, Solubilization in Organic Media,
2−
Preparation of its Anhydride [P4O11
]
and Acidic Methyl Es-
ter, and Conversion to Tetrametaphosphate Metal Complexes
via Protonolysis. J. Am. Chem. Soc. 2014, 136, 11894–11897.
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Page 9 of 24
Journal of the American Chemical Society
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1
2
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4
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8
Organic (Trimethylsilyl)chalcogenolate Salts Cat[TMS-E] (E
= S, Se, Te): the Methylcarbonate Anion as a Desilylating
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-a Fully Chlorinated N-Silyl Titanium Amide. Z. Naturforsch.,
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Montgomery, J. A. General atomic and molecular electronic
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Gordon, M. S.; Schmidt, M. W. In “Advances in electronic
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plications of Computational Chemistry: the first forty years;
Dykstra, C., Frenking, G., Kim, K., Scuseria, G., Eds.; Elsevier:
Amsterdam, Netherlands, 2005; pp 1167–1189.
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ing, E. D.; Badenhoop, J. K.; Reed, A. E.; Carpenter, J. E.;
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methylamino)ethan-N,N’]lithium-disilylphosphanid–Synthese
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pling Constants. J. Chem. Theory Comput. 2008, 4, 448–456.
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Gruppe in Silylphosphinen. Z. Anorg. Allg. Chem. 1974, 407,
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tions at Silicon-Silicon Bonds, V. Selective Trichlorosilylation
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(85) Olaru, M.; Hesse, M. F.; Rychagova, E.; Ketkov, S.; Mebs, S.;
ACS Paragon Plus Environment
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Journal of the American Chemical Society
Page 10 of 24
Graphical TOC Entry
1
2
3
4
5
14
Cl
H
Si
Cl
Cl
6
7
8
9
[P(SiCl3)2]−
[SSiCl3]−
15
O
16
O
O
O
C−X
C−X
P
S
O
O
O
O
P
C
S
C
10
11
12
13
14
15
16
17
18
19
20
21
22
23
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40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
phosphoric acid synthesis:
[Ca5(PO4)3F] + 5 H2SO4 3 H3PO4 + 5 CaSO4 + HF
ACS Paragon Plus Environment
10

A
thermal process
Phosphorus
chemicals
P4
Page 1J1ouorfn2wa4hlitoefpthhosephAormuserican Chemical Society
Ca5(PO4)3X
previous work
this work
apatite
[P(SiCl3)2]−
1
1
2
3
4
H3PO4
phosphoric acid
[TBA]3[P3O9].2H2O
TBA trimetaphosphate
wet process
5
6
Na3P3O9
7
8
9
10
sodium trimetaphosphate
ACS Paragon Plus Environment
B
MeCN, HSiCl3
[TBA][HSO4]
[TBA][Cl3SiS]
[TBA]2

1.5 H3PO4
ACS Paragon Plus Environment
1.5 [TBA]Cl
PH2
Journal of the American Chemical SPoagcieet1y2 of 24
CH3(CH2)6CH2Cl
HSiCl3, 110 °C, 6 d
then Al2O3 or H2O
41%, 1.3 g

HSiCl3
[TBA]H2PO4
[TBA][P(SiCl3)2]
Page 1J3ouorfn2a4l of11t0h°eC,A6md erican Chemical S6o2c%ie, 1tygram

Cl
CH2Ph
ACS Paragon Plus Environment
10 PhCH Cl
Journal of the American Chemical SPoagcieet1y4 of 24
2
P
[TBA][P(SiCl3)2]
61%
CH2Ph
CH2Ph
MeCN, 70 °C
PhH2C

Page 1J5ouorfn2a4l of the American Chemical Society
O
O
P
SiCl3
1 *
2a
3
4
5
O
O
P
c
a
O
O
O
O
O
b
b
P
O
P
a
P
O
Si
O
O
O
P
O
O
b
a
O
c
3
O
6
7
8
ACS Paragon Plus Environment
9
10
11
-30
-35
-40
31P (δ, ppm)

O3
Journal of the American Chemical SPoagcieet1y6 of 24
O2
P1
1
2
3
4
5
6
7
8
9
10
O1
Si1
ACS Paragon Plus Environment

O
P
O
P
O
P
Page 1J7ouorfn2a4l of the American Chemical Society
triphosphate
O
O
O
O
[TBA][Cl] (1 equiv.)
O
ACS Paragon Plus EnviroOnmenOt
[1] +
H3PO4
O
P
O
P
HSiCl3, 110 °C, 6 d
1
2
pyrophosphate
O
O
O
O
O

MeCN, HSiCl3
[TBA][Cl3SiS]
[TBA][HSO4]
43%
[TBA]2
Journal of the23A℃m, 2e0rhican Chemical SPoagcieet1y8 of 24

S1
Page 1J9ouorfn2a4l of the American Chemical Society
Cl1
1
Si1
2
3
Cl2
ACS Paragon Plus Environment
4
5
6
7
Cl3

S
O
5 [TBA][Cl3SiS]
Journal of the American Chemical SPoagcieet2y06o2%f 24
CHCl3, 80 °C, 96 h
Ph
Ph
Ph
Ph
Ph
ACS Paragon Plus Environment
1
2
3
4
1.1 [TBA][Cl3SiS]
55%
Br
Ph
SH
DCM, 40 °C, 18h
then H2O

Page 21 of 24
Journal of the American Chemical Society
ACS Paragon Plus Environment
1
2
3
4
5
6
7
MP2 π MOs
π NBOs

Cl
Cl
Cl
1, 35%Journal of the American Chemical SPoagcieet2y2 ofC2l4
2, 16%
S
Si
S
S
S
Si
Cl
Cl
Cl
Cl
Cl
Cl
1
4, 12.5%
23, 12.5%
S
Si
Si
3
Cl
Cl
Cl
ACS PaCrlagon Plus Environment
4
Cl
Cl
55, 12%
6, 12%
S
Si
Si
6
7
Cl
Cl

MeCN
[TBA][HSO4] + HSiCl3
Page 2J3ouorfn2a4l of the American ChemiHca2 l+S[oTBciAe]2ty
cat. [(COD)Ir(py)(PCy3)][PF6]
1
2
ACS Paragon Plus Environment
DCM, 0 °C
3
4
HO
i-Pr
HO
i-Pr

Journal of the American Chemical Society
Page 24 of 24
1
2
3
4
14
H
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
Si
Cl
Cl
Cl
[P(SiCl3)2]−
[SSiCl3]−
15
O
16
O
O
O
C−X
C−X
P
S
O
O
O
O
C
P C
S
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
phosphoric acid synthesis:
ACS Paragon Plus Environment
[Ca5(PO4)3F] + 5 H2SO4 3 H3PO4 + 5 CaSO4 + HF