Solvent-free mechanochemical synthesis of dithiophosphonic acids and corresponding nickel(II) complexes

Article abstract of DOI:10.1080/10426507.2017.1358717

We report a green chemistry route for dithiophosphonic acids of the type [HS₂P(OR)(4-MeOC₆H₄)] [R = H, (1); Me (2); Et (3); ⁱPr (4)]. The different dithiophosphonic acids formed through the stoichiometric addition of water or alcohols to Lawesson’s Reagent (molar ratio 2:1), followed by an intimate grinding of themixture (mechanochemistry). The products formed without the use of solvent or external heat in less than 5 minutes. The acids are formed with 100% atomeconomy, and because they form in essentially quantitative yield, are also formed with >98% atom efficiency and an E-factor = 0, because no waste is produced. Of importance is that this methodology is different from conventional methods in forming dithiophosphonic acids where the use of organic solvents, added heat, long reaction times and lower yields are commonplace. We further demonstrate that nickel(II) complexes can form directly from the in situ generated acids. Thus, the reaction between 1–4 and NiCl2 · 6 H2O (molar ratio 2:1) lead to complexes of the type [Ni{S₂P(OR) (4-MeOC₆H₄)}₂] [R = H, (5);Me (6); Et (7); ⁱPr (8)] with no use of organic solvent. All compounds were characterized or verified by a combination of ¹H, ³¹P NMR, elemental analysis (solids), and FT-IR.

Full text of DOI:10.1080/10426507.2017.1358717

Phosphorus, Sulfur, and Silicon and the Related Elements
ISSN: 1042-6507 (Print) 1563-5325 (Online) Journal homepage: http://www.tandfonline.com/loi/gpss20
Solvent-free mechanochemical synthesis of
dithiophosphonic acids and corresponding
nickel(II) complexes
Tomilola J. Ajayi, Michael N. Pillay & Werner E. Van zyl
To cite this article: Tomilola J. Ajayi, Michael N. Pillay & Werner E. Van zyl (2017): Solvent-free
mechanochemical synthesis of dithiophosphonic acids and corresponding nickel(II) complexes,
Phosphorus, Sulfur, and Silicon and the Related Elements, DOI: 10.1080/10426507.2017.1358717
To link to this article: http://dx.doi.org/10.1080/10426507.2017.1358717
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Dithiophosphonic acids and nickel(II) complexes
SOLVENT-FREE MECHANOCHEMICAL SYNTHESIS OF DITHIOPHOSPHONIC ACIDS AND
CORRESPONDING NICKEL(II) COMPLEXES
TOMILOLA J. AJAYI, MICHAEL N. PILLAY, AND WERNER E. VAN ZYL*
School of Chemistry and Physics, University of KwaZulu-Natal, Westville Campus, Private Bag X54001,
Durban 4000, South Africa.
Author to whom correspondence should be addressed: Tel. +27 (0) 31 260 3188; Fax +27 (0) 31 260 3091.
Email: vanzylw@ukzn.ac.za
Abstract
We report a green chemistry route for dithiophosphonic acids of the type [HS2P(OR)(4-
i
MeOC6H4)] [R = H, (1); Me (2); Et (3); Pr (4)]. The different dithiophosphonic acids formed
through the stoichiometric addition of water or alcohols to Lawesson’s Reagent (molar ratio
2:1), followed by an intimate grinding of the mixture (mechanochemistry). The products formed
without the use of solvent or external heat in less than 5 minutes. The acids are formed with
100% atom economy, and because they form in essentially quantitative yield, are also formed
with > 98% atom efficiency and an E-factor = 0, because no waste is produced. Of importance is
that this methodology is different from conventional methods in forming dithiophosphonic acids
where the use of organic solvents, added heat, long reaction times and lower yields are
commonplace. We further demonstrate that nickel(II) complexes can form directly from the in-
situ generated acids. Thus, the reaction between 1-4 and NiCl2 ‧ 6 H2O (molar ratio 2:1) lead to
complexes of the type [Ni{S2P(OR)(4-MeOC6H4)}2] [R = H, (5); Me (6); Et (7); iPr (8)] with no
1
use of organic solvent. All compounds were characterized or verified by a combination of H,
31P NMR, elemental analysis (solids), and FT-IR.
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Keywords
Green chemistry; dithiophosphonic acid; solvent-free; nickel(II) complexes; atom economy;
hydrated
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INTRODUCTION
Dithio-organophosphorus compounds are used in diverse and important areas of the chemical
and allied industries. Applications range from anti-oxidant additives in the oil- and petroleum
industry1 to green lubricants2, agricultural pesticide derivatives3 and metal ore extraction
reagents in the mining industry4. While there is an initiative to become more aware of solvents
from nature5, conventional synthetic methods still greatly rely on inherently wasteful organic
solvents, which are toxic to the environment. Solvent-free methods are therefore considered
more beneficial.6
The phosphorus-1,1-dithiolate class of compounds is multifaceted and includes, amongst others,
the dithiophosphates A, dithiophosphinates B, and dithiophosphonates C, shown in Figure 1.
The dithiophosphonato ligand, C, which is the focus of the present study, may be described as a
hybrid of A and B.7 The literature reveals that [HS2PR(OR’)] type acids are less common as
compared to the better established [HS2PR2] and especially [HS2P(OR)2] counterparts.8 A reason
for this may be the unavailability of key starting materials, but this problem was overcome with
the development of dimers of the type [RP(µ-S)S]2 (R = aryl). Such dimers have traditionally
been used by organic chemists to convert ketones into thiones.9
The preparation and characterization of the phenyl substituted dimer [PhP(µ-S)S]2 was first
described in 1962 but the procedure is not in line with green chemistry procedures and requires
the use of wasteful excess and toxic H2S gas introduced sub-surface at elevated temperature (>
210°C) with the release of copious amounts of corrosive HCl as by-product.10,11 A significant
improvement came in 1978 when Lawesson and co-workers reported12 the reaction between the
electron-rich aromatic anisole and P4S10 which leads directly to the formation of the dimer 2,4-
bis(4-methoxyphenyl)-1,3-dithiadiphosphetane-2,4-disulfide, [(4-MeOC6H4)P(µ-S)S]2, Scheme
1. Anisole serves as both the solvent and reactant and this dimer has become known as
Lawesson’s Reagent (LR); it is commercially available and its general chemistry has been
comprehensively reviewed.13
Although the conversion of ketones into thiones still occupy the largest part of LR’s utility,
alcoholysis of dithiadiphosphetane dimers, including LR, yields the corresponding
dithiophosphonic acid.14 A wide variety of alcohols are now known to be utilized in this manner,
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including unsaturated (allyl), sterically demanding (adamantyl), and quasi-strained (cyclopentyl)
alcohol, as well as silanols and trialkyl silanols.15
As part of our ongoing studies on phosphorus-1,1-dichalcogenates of the late-transition metals
(Groups 10-12),16 we report i) an entirely green and solvent-free synthesis of dithiophosphonic
acids with 100% atom economy and ii) the direct formation of the corresponding nickel(II)
complexes using the unmodified acid ligands.
RESULTS AND DISCUSSION
Preparation of dithiophosphonic acids
The conventional synthetic methodology used to form dithiophosphonic acids, HS2PR(OR’),
typically starts with the anisyl substituted Lawesson’s Reagent (LR), although the phenetyl17
and even ferrocenyl18 versions of the dimer have also been successfully utilized. LR is placed in
a Schlenk tube and heated to ~80°C using an oil bath followed by the slow addition of the
appropriate alcohol. A small amount of inert solvent such as toluene is added to give the
magnetic stirrer bar mobility. The addition of solvent immediately leads to a dilution effect,
which in turn results in more time needed for the reaction to proceed to completion. Afterward,
the acid needs to be separated from the solvent, which requires further heating for distillation,
and loss of yield is inevitable. The acid is generally very viscous and difficult to manipulate but
can be precipitated as an ammonium salt by bubbling ammonia through the solution in an ice
bath at 0°C which turns it into a free-flowing powder upon drying.
By contrast, the green synthetic methodology described in the present study circumvents many of
these drawbacks. LR is placed in a Schlenk tube and is covered under a blanket of nitrogen to
avoid hydrolysis, vide infra. At room temperature, 2 molar equivalents of mono-hydroxy
alcohols (i.e. simple alcohols such as MeOH, EtOH, etc.) are dripped directly onto LR; higher
alcohols such as diols (ethylene glycol), pentaerythritol, etc. would require different molar ratios.
Mechanochemical synthesis in general terms refers to reactions induced by the input of
mechanical energy, usually grinding.19-20 Once the alcohol is added, the mixture is gently
grinded, Scheme 2. This process appears very discouraging at first because it seems that no
reaction is taking place. For this reason previous attempts of a mechanochemical synthesis for
this system have been prematurely abandoned. But the reaction dynamics changes quite
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dramatically in less than 5 minutes when the mixture turns viscous, which was subsequently
identified to be generated by the acid product. This highlights another advantage of the green
method, namely the reaction both starts and finishes at room temperature, giving no opportunity
for any alcohol to evaporate through extensive heating during the reaction. By contrast,
conventional methods require much longer (> 30 min.) reaction times.
Theories to comprehensively explain the reaction taking place at the solid-liquid interface during
solvent-free mechanochemical processes are lacking. Following from work by Paul and Curtin,21
Bala and Coville reviewed different scenarios for reactions in the melt22 (a sub-class of solvent-
free reactions) most commonly observed for solid-solid and solid-gas reactions, which can be
described as binary systems with a minimum eutectic temperature. In this study, the focus is on a
solid-liquid interface system which appears to be a rare example for a reaction, for which both
start and finish are at room temperature. We focused on alcohols in liquid form as reagents,
which turned out to be very efficient for this particular transformation; we also attempted
materials with alcohol functionalities in the solid state, such as cholesterol etc., but they were
less efficient and require further optimization.
Preparation of nickel(II) dithiophosphonate complexes
Once we were able to routinely form dithiophosphonic acids 1-4, we investigated their use as
ligands to form metal complexes. Here the goal was not to produce new metal complexes
through conventional solvent-based methods, but instead to show a new green chemistry and
sustainable synthetic methodology. We started from the hexahydrate NiCl2 ‧ 6 H2O (distinct
green colour) and obtained the corresponding Ni(II) dithiophosphonates, which have a distinctly
purple colour, allowing to detect product formation by simple colorimetric methods. The metal
most frequently reported to form complexes with the dithiophosphonato ligand7, is Ni(II); yet to
date these complexes were all formed by traditional methods. Our methodology differs from the
conventional methods in a number of ways: i) we found no need to first convert the ligands to
their respective ammonium or sodium salts, but used the ligands in the acid form directly; ii) this
had the advantage that no filtration step (to remove NH4Cl and/or NaCl) was later needed; iii)
the reaction proceeded without solvent and at room temperature, and iv) there was no need to
first isolate the ligand before using it for complex formation, since the ligand formed in > 98%
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yield in the first step, without unwanted side-products, and complex formation was immediate
by adding the Ni(II) precursor. The yields for the acids were all exceptionally high, which we
attribute to i) the 100% concentration of the reactants in intimate contact with each other (no
solvents, no dilution effects) and ii) the oxophilic nature of the P atom which would always
prefer P-O bonding to P-S bonding, the former providing a continuous driving force for the
reaction to reach completion. The yields for the nickel(II) complexes were slightly lower but still
much higher than could be achieved by synthesis through conventional methodology, which we
ascribe to the absence of salt formation as a byproduct, which makes extraction and filtration
steps unnecessary. The reaction is shown in Scheme 3.
Again, as is the case for the acids, we propose a mechanochemical synthesis for this system,
which had been overlooked due to the preconceived notion that the reaction always requires a
solvent. For example in 1997, Verani and coworkers reported23 a number of [Ni{S2P(OR)(4-
i
MeOC6H4)}2] (R = Me, Et, Pr) complexes similar to those in the present study. A typical
synthesis started in a favorable green chemistry manner by adding equivalent amounts of
nickel(II) chloride and LR, but then turned to a conventional route by suspending the solids in a
huge excess (100 mL) of the appropriate alcohol, ROH. The stirred reaction mixture was
refluxed for 1--3 h, then filtered and the solvent was removed under vacuum. In the present
study, we hope to demonstrate, that the excess alcohol is not required, thus making unnecessary
additional reflux, filtration, and solvent and waste removal steps.
In another study, that used a large excess of alcohol, Ni(II) complexes similar to 6 and 7 were
formed by mixing and refluxing two ROH solutions, namely LR and ROH (R = Me or Et, 0.02
M) and NiCl2 and ROH (0.02 M) in a closed vial and single crystals were grown from the
solution. These complexes were analysed solely by X-ray and IR studies, and it is thus not clear
whether they formed from acids or by another mechanism, and whether the bulk material formed
is the same as the single crystals.24
We are aware of only two examples where Ni(II) had previously been used in a solvent-free
system for any ligand, the one was obtained by mechanochemical and the other by
thermochemical means. The use of dimethylglyoxime (H2dmg) as a chelating agent for the
gravimetric analysis of Ni(II) was discovered by Tschugaeff in 1905.25 Yet the
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mechanochemical preparation of [Ni(Hdmg)2] and [Ni(H2dmg)2]2+ from nickel halides and
H2dmg was only reported a century later.26 Thermochemical work by Fabrizzi and coworkers
focused27 on a Ni(II) bis(diamine) complex having a pink-red nitro isomer and a blue coloured
nitrito isomer. The facile reversible thermally induced nitro / nitrito interconversion for the
design of thermochromic devices was studied.
Characterisation
We report the first series of characterized dithiophosphonic acids formed by green chemistry
methods. Due to the viscous nature of the acids, we did not attempt to measure the boiling
points. The melting points for the Ni(II) complexes were determined and the melting points of
complexes 7 and 8 corresponded within 1°C to those from identical complexes previously
reported through more cumbersome means.23 Together with satisfactory elemental analysis (see
Table 1) we were satisfied with the bulk purity. The determined melting point of complex 6
differed substantially from the literature value,23 but we did establish bulk purity based on
elemental analysis and NMR results and concluded that after work-up the bulk powder contained
no solvated molecules in the structure. We prepared and characterized complex 5, and came to
the same conclusion. However, when 5 was formed in situ and dissolved in water for about 1
week (no further work-up), we discovered through X-ray analysis that it formed the hydrated
ammonium salt of the Ni(II) complex, namely (NH4)2[Ni{S2P(O)(4-C6H4-OMe)}2], 9. There are
related neutral complexes reported28 but with THF solvates, and not with H2O, and with much
1
higher melting points. In the H NMR spectra the S-H signals for the acids 1-4 are the only
peculiarity to point out. There are other reports where the S-H proton for acids of the related
dithiophosphates are not observed or recorded.29 We observed the NMR signal of the S-H proton
for 1 at δ = 1.76 and for 2 at δ = 3.41 whilst no S-H signal was observed for 3 and 4. Note the
distinct upfield shift for 1 compared to 2; a similar effect was observed for the corresponding
complexes 5 and 6 in the 31P NMR spectra, see below. We nevertheless substituted H2O for D2O
(1) and MeOH for CD3OD (2) and noted, that the previously observed S-H protons have now
both disappeared. The 31P NMR spectra of all the acids show singlet peaks in the range 74 to 89
ppm, but this requires a cautionary note. Initially, we made no attempt to keep the reaction in an
inert environment because we reasoned the product forms rapidly and there is not sufficient time
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for side-products to form. But the 31P NMR spectrum for each acid we obtained, under those
circumstances, showed two singlets ca. > 10 ppm apart, with the upfield signal of lower intensity
but with growing intensity over time, and with further signals also appearing upfield over time.
What we observed with regards to by-product formation was akin to a reported series of elegant
hydrolysis studies performed by Burn et al.30-32 more than 2 decades ago. They followed
mechanisticaly and kinetically by 31P NMR the hydrolysis of zinc(II) dithiophosphate
[Zn{S2P(OEt)2}2] in 10-fold water excess at 85°C and could observe the replacement of S atoms
by O atoms, as well as C-O bond cleavage, giving rise to further signals at higher field, right to
the final hydrolysis product, H3PO4, at 0 ppm. It is clear from those studies that the ultimate fate
of possibly all phosphorus-1,1-dithiolate acids, salts, and complexes, in the presence of moisture
(hydrolysis) and oxygen (oxidation), is phosphoric acid, but this can be well avoided by
31
excluding both H2O and O2 from the system. The P NMR spectrum of the Ni(II) complexes
showed singlets for both [Ni{S2P(OH)(4-MeOC6H4)}2] 5 at 58.1 ppm and for [Ni{S2P(OCH3)(4-
MeOC6H4)}2] 6 at 104.4 ppm; a remarkable difference of 46 ppm between seemingly closely
related complexes. The chemical shift observed for 5 is similar to that reported for the complex
[Ni{S2P(OH)(4-MeOC6H4)}2(THF)2] by Rothenberger and coworkers, but in their case they
found two resonance peaks at 42.3 and 56.9 ppm, which they assign to partial decomposition.28
We found only one peak, however. It should also be pointed out that the heavily solvated
31
tetranuclear Ni(II) complex [Ni2{AnP(O)S2}2(THF)2(H2O)2]2 displays a P NMR resonance at
56.1 ppm.33
31
Complex 5 was obtained in high yield and displayed a singlet P NMR peak closely related to
similar compounds. The starting complex NiCl2 ‧ 6 H2O (0.50 g) leads to the release of 12.62
mmol of water (ca. 0.23 mL) upon reaction with the acid ligand. This is a sufficient volume to
already wet all generated powder of the product and only requires a few additional drops of
water to partly solubilise the complex. Amongst all Ni(II) complexes 5-8 formed, complex 5 was
by far the most soluble in water. As a result, we attempted to grow crystals of 5 from a water
solution. X-ray quality single crystals indeed formed from the aqueous solution by slow
evaporation of the solvent, but instead of 5, X-ray diffraction revealed the isolated crystals to be
complex 9. At first glance it appears as though the ligand had undergone a reduction in going
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from a negative to double negative charge, as shown in Figure 2A. But closer inspection
revealed that a double deprotonation occurred, followed by the coordination of the ligand
resulting in the best binding mode to the Ni(II) center (i.e. NiS4 vs. NiO2S2), shown in Figure 2B,
while the proposed formation of complex 9 is shown in Scheme 4.
This is the first example of a decidedly ‘green structure’ for any Ni(II) dithiophosphonate
complex and contains only water in the crystal, all other reported solvated structures contain
organic molecules, especially THF. The structure is shown in Figure 3. The molecular structure
is unsurprising, showing a 4-coordinate, slightly distorted square-planar geometry around the
Ni(II) center. The bond lengths and angles are comparable with related structures7 and the P =
O groups are in a trans orientation with respect to each other, generated by the Ni atom that acts
as a crystallographic inversion center. Details of the X-ray data are shown in Table 2.
Although this paper describes solvent-free methods to form acids and corresponding complexes,
knowledge about the solubility of compounds is often useful. To this end, a series of qualitative
solubility tests were performed on all the compounds, 1-8. A variety of solvents were tested over
a wide range of dielectric constants, including protic/aprotic and polar/non-polar solvents.
Noteworthy is for example how the acids are soluble in protic solvents such as water and
ethanol, but the corresponding complexes are insoluble in those solvents. Results are shown in
Table 3.
EXPERIMENTAL
General
Mechanochemical grinding was carried out in a standard Schlenk tube (~2 cm diameter) using a
ball head glass rod under a blanket of nitrogen. Lawesson’s reagent was either purchased from
Aldrich or prepared according to literature procedures.15 All other reagents were purchased from
Aldrich and used as received. 1H and 31P NMR spectra were recorded with a Bruker Avance 400
MHz spectrometer. Proton NMR chemical shifts (δH) are given in ppm and referenced internally
31
to the residual proton impurity in the deuterated solvent. P NMR chemical shifts are reported
relative to an 85% H3PO4 in D2O as external standard. All NMR measurements were performed
at 298 K. The yields were determined by performing the reaction in a dry pre-weighed Schlenk
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tube, weighing the sample after drying. Infrared spectra were recorded in the range 4000--380
cm1 with a Perkin-Elmer Spectrum 100 FT-IR spectrometer.
Synthesis
Hydroxy-4-methoxyphenyldithiophosphonic acid (1)
Lawesson’s Reagent (1.00 g, 2.48 mmol) was placed in a clean and dry Schlenk tube, and
distilled water (0.10 mL, 4.90 mmol) was added. The reactants were ground together for 15 min.
This derivative took the longest to complete the reaction compared to alcohols, that took < 5
min. A viscous liquid formed. (1.06 g, 98.6%). 31P NMR (CDCl3): δ = 74.1, 1H NMR (CDCl3): δ
4
3
= 1.76 (s, 1H, SH), 3.71 (s, 3H, ArOMe), 6.83 (dd, JPH = 2.6 Hz, JHH = 9.0 Hz, 2H, m-ArH),
3
3
7.51 (s, 1H, OH), 7.78 (dd, JPH = 13.8 Hz, JHH = 9.0 Hz, 2H, o-ArH). Selected FT-IR (4000--
350 cm1): 2945 (m), 2305 (b), 1595 (s), 1501 (s), 1256 (s), 1120 (s), 1023(s), 892(b), 803 (w),
612 (w), 526 (m).
(Methoxy)-4-methoxyphenyldithiophosphonic acid (2)
To Lawesson’s Reagent (1.00 g, 2.48 mmol) in a clean and dried Schlenk tube, methanol (0.2
mL, 4.9 mmol) was added. The reactants were ground together for about 2 min. A viscous liquid
resulted. (1.14 g, 98.5%). 31P NMR (CDCl3): δ = 89.5. 1H NMR (CDCl3): δ = 3.41 (s, 1H, SH),
4
3
3.81 (d, 2JHH = 8.7 Hz, 3H, CH3), 3.85 (s, 3H, ArOMe), 6.92 (dd, JPH = 3.5 Hz, JHH = 9.0 Hz,
3
3
2H, m-ArH), 7.88 (dd, JPH = 14.4 Hz, JHH = 9.0 Hz, 2H, o-ArH). Selected FT-IR (4000-350
cm1): 2941 (m), 2433 (b), 1593 (s), 1498 (s), 1256 (s), 1114 (s), 827 (w), 679 (s), 530 (m).
(Ethoxy)-4-methoxyphenyldithiophosphonic acid (3)
To Lawesson’s Reagent (1.00 g, 2.48 mmol) in a clean and dried Schlenk tube, ethanol (0.30
mL, 4.90 mmol) was added. The reactants were ground together for 3 min until a viscous liquid
formed. (1.21 g, 98.9%). 31P NMR (CDCl3): δ = 86.4. 1H NMR (CDCl3): δ = 1.36 (t, 3JHH = 7.1
Hz, 3H, CH3), 3.78 (s, 3H, ArOMe), 4.25 (m, 3JHH = 7.2 Hz, 2H, CH2), 6.92 (dd, 4JPH = 3.3 Hz,
3JHH = 9.0 Hz, 2H, m-ArH), 7.88 (dd, 3JPH = 14. 7 Hz, 3JHH = 9.0 Hz, 2H, o-ArH). Selected FT-
IR (4000-350 cm1): 2941 (m), 2433 (b), 1593 (s), 1498 (s), 1256 (s), 1114 (s), 827 (w), 679 (s),
530 (m).
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Bis (Isopropoxy)-4-methoxyphenyldithiophosphonic acid (4)
To Lawesson’s reagent (1.00 g, 2.48 mmol) in a clean and dried Schlenk tube, isopropanol (0.4
mL, 4.9 mmol) was added. The reactants were ground together for 3 min until viscous liquid
formed. (1.27 g, 98.7%). 31P NMR (CDCl3): δ = 84.3. 1H NMR (CDCl3): δ = 1.40 (d, 3JHH = 6.0
Hz, 6H, CH3), 3.82 (s, 3H, ArOMe), 5.08 (sept, 3JHH = 6.4 Hz, 1H, CH), 6.95 (d, 3JHH = 9.0 Hz,
2H, m-ArH), 7.90 (dd, 3JPH = 14.6 Hz, 3JHH = 9.0 Hz, 2H, o-ArH,). Selected FT-IR (4000--350
cm1): 2977 (m), 2434 (b), 1595 (s), 1499 (s), 1255 (s), 1111 (s), 961 (m), 828 (m), 617 (s), 530
(m).
[Ni{S2P(OH)(4-C6H4-OMe)}2] (5)
Non-green route: Ammonia gas was bubbled into compound 1 (1.00 g, 4.54 mmol) and NiCl2 ∙
6 H2O (0.45 g, 1.92 mmol) was added and ground together for 5 min. The immediate formation
of a purple complex resulted which was subsequently used for analysis and following work-up,
gave the same result as that found for the ‘green route’. Note: In the absence of further work-up,
X-ray quality crystals of 9 were grown by slow evaporation of an aqueous solution. Because of
+
the NH4 counterion, the same structure (9) can thus not be obtained by following the green
route below.
Green route: Solid NiCl2 ∙ 6 H2O (0.45 g, 1.92 mmol) was added to acid 1 (1.0 g, 4.54 mmol)
and ground together for 5 min. The complex was dried under vacuum to give a purple solid
(1.09 g, 96.3%). M.p.: 125°C. Anal. Calcd. for C14H16O4P2S4Ni: C, 33.81; H, 3.24. Found: C,
31
32.91; H, 3.55%. P NMR (DMSO): δ = 58.1. 1H-NMR (DMSO): δ = 3.80 (s, 3H, ArOMe),
6.95 (d, J = 8.0 Hz, 1H, m-ArH), 8.25 (d, J = 19.3 Hz, 1H, o-ArH). Selected FT-IR (4000--350
cm1): 3109 (m), 2559 (b), 1593 (s), 1499 (s), 1260 (s), 1113 (s), 1028 (s), 937 (s), 800 (w), 768
(s), 619 (s), 546 (s).
[Ni{S2P(OMe)(4-C6H4-OMe)}2] (6)
A mixture of NiCl2 ∙ 6 H2O (0.50 g, 2.13 mmol) and viscous acid 2 (1.00 g, 4.30 mmol) were
ground together and an immediate purple complex resulted. The complex precipitated upon
addition of water and was then filtered under vacuum to give a dry purple solid (1.08 g, 96.8%).
Anal. Calcd. for C16H20O4P2S4Ni: C, 36.59; H, 3.84. Found: C, 36.65; H, 3.77%. 31P NMR
1
(CDCl3): δ = 104.4. H NMR (CDCl3): δ = 3.87 (s, 3H, ArOMe), 3.99 (d, J = 14.8 Hz, 3H,
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CH3), 7.00 (d, J = 5.9 Hz, 2H, m-ArH), 7.99 (d, J = 5.4 Hz, 2H, o-ArH). Selected FT-IR (4000-
350 cm1): 2968 (m), 2292 (b), 1593 (s), 1501 (s), 1259 (s), 1116 (s), 962 (b), 827 (w), 616 (s),
549 (s).
[Ni{S2P(OEt)(4-C6H4-OMe)}2] (7)
A mixture of NiCl2 ∙ 6 H2O (0.48 g, 2.02 mmol) and viscous acid 3 (1.00 g, 4.00 mmol) was
ground and an immediate purple complex resulted. Workup was performed as for 6 above. Dry
purple solid (1.09 g, 97.8%). Anal. Calcd. for C18H24O4P2S4Ni: C, 39.08; H, 4.37. Found: C,
39.12; H, 4.29%. 31P NMR (CDCl3): δ = 101.9. 1H NMR (CDCl3): δ = 1.35 (t, J = 7.1 Hz, 3H,
CH3), 3.80 (s, 3H, ArOMe), 4.29 (d, J = 17.1 Hz, 2H, CH2), 6.97 (d, J = 8.7 Hz, 2H, m-ArH),
7.91 (dd, J = 8.8 Hz, 14.6 Hz, 1H, o-ArH). Selected FT-IR (4000-350 cm1): 2971 (m), 2288
(b), 1593 (s), 1498 (s), 1257 (s), 1112 (s), 1020 (s), 800 (m), 616 (s), 551 (s).
[Ni{S2P(OiPr)(4-C6H4-OMe)}2] (8)
A mixture of NiCl2 ∙ 6 H2O (0.45 g, 1.92 mmol) and viscous acid 4 (1.00 g, 3.8 mmol) were
ground together and an immediate purple complex resulted. Workup was performed as for 6
above. Dry purple solid (1.07 g, 96.7%). Anal. Calcd. for C20H28O4P2S4Ni: C, 41.32; H, 4.85.
Found: C, 41.39; H, 4.82%. 31P NMR (CDCl3): δ = 97.8. 1H NMR (CDCl3): δ = 1.41 (d, J = 6.2
Hz, 6H, (CH3)2), 3.81(s, 3H, ArOMe), 5.22 (d, J = 1.2 Hz, 1H, CH), 7.00 (d, J = 5.8 Hz, 2H, m-
ArH), 8.01 (d, J = 5.2 Hz, 1H, o-ArH). Selected FT-IR (4000--350 cm1): 2967(m), 2274 (b),
1593 (s), 1501 (s), 1260 (s), 1116 (s), 1023(s), 959(m), 800 (w), 616 (s), 550 (m).
Crystallography
A crystal of complex 9, (NH4)2[Ni{S2P(O)(4-C6H4-OMe)}2], with approximate dimensions
0.106 × 0.256 × 0.432 mm, was mounted on a glass fibre with epoxy resin. All intensity and
geometric data were collected on a Bruker APEXII CCD diffractometer equipped with a
graphite monochromated Mo-Kα radiation (λ = 0.71073 Å). APEXII software was used for
preliminary determination of the unit cell.35 Determination of integrated intensities and unit cell
refinement were performed using SAINT.36 The SADABS program was used to apply an
empirical absorption correction.37 The integration of the data yielded a total of 21960 reflections
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to a maximum θ angle of 28.51° (0.74 Å resolution). XPREP determined the space group to be
C 1 2/c 1, with Z = 4 for the formula unit, C14H14NiO4P2S4 ∙ 2 NH4 ∙ 2 H2O.38 The structure was
solved with XS39 and subsequent structure refinements were performed with XL.40 A complete
listing of crystallographic data and parameters are reported in Table 2.
CONCLUSIONS
The first mechanochemical synthesis procedure is described for the formation of
dithiophosphonate acid derivatives by using green chemistry methods with 100% atom economy
and essentially 100% atom efficiency. The procedure circumvented tedious consecutive steps
such as heating, reflux, filtration and solvent removal. In a proof-of-concept experiment, the
acids were then used in situ to form Ni(II) dithiophosphonate complexes in high yield. A
comparative analysis of the solubilities for all the compounds were also performed. The present
study focuses on common alcohols in the liquid phase under ambient conditions. We are in the
process of investigating the more challenging mechanochemical reaction where precursor
alcohols are in the solid phase.
ACKNOWLEDGEMENT
We thank the University of KwaZulu-Natal for financial support and Prof MD Bala (UKZN) for
helpful discussions.
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Table 1 Yield, melting points, 31P NMR and elemental analyses data for 1-8
Entry
Yield
(%)
m.p.
(°C)
31P
NMR
(ppm)
Elem.
anal.
(%)
H
C
1a
2
98
98
99
98
96
96
97
97
visc.
liquid
visc.
liquid
visc.
liquid
visc.
liquid
125
74.1
89.5
N/O
N/O
N/O
N/O
86.4
3
(102.9)h
84.3 /
84.4c
4
5b
6
58.1
(56.1)e
3.55
32.91
(N/A)d
(3.24)
(33.82)
36.65
160
104.4
3.77
(184)f
130
(131)f
(105.7)g (3.84)
101.9 4.29
(102.2)g (4.37)
(36.59)
39.12
7
(39.08)
41.39
167
97.8
4.82
8
(168)f
(99.1)g
(4.85)
(41.32)
N/O: not obtained. a: all four acids (1-4) are viscous colourless liquids; b: all four Ni(II)
complexes (5-8) are purple solids; c: in d6-DMSO; d: N/A = not available; m.p. for related THF
solvated [Ni{S2P(OH)(4-MeOC6H4)}2(THF)2] and [Ni{S2P(OH)(4-MeOC6H4)}2(THF)4] is 195
(dec.) and 250°C (dec.), respectively, ref. 28; e: [Ni2{AnP(O)S2}2(THF)2(H2O)2]2 δ = 56.1 ppm
(An = anisyl), ref. 33; f: ref. 23; g: ref. 34; h: as ammonium salt, ref. 15.
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Table 2. Crystal data and structure refinement of 9.
C14H14NiO4P2S4‧2NH4‧
2H2O
Empirical formula
Formula weight
Temperature
567.28
100(2) K
Wavelength
0.71073 Å
Crystal system
Space group
Monoclinic
C2/c (no. 15)
a = 23.7087(15) Å
b = 7.4340(4) Å
c = 13.4003(7) Å
Unit cell dimensions
= 90°.
= 105.507(4)°.
= 90°.
3
Volume
2275.8(2) Å
4
Z
3
Density (calculated)
Absorption coefficient
F(000)
1.6555 g/cm
-1
1.394 mm
1176.00
Crystal size
Theta range for data
collection
0.559 × 0.322 × 0.106 mm
1.78 to 28.51°.
Index ranges
-31 ≤ h ≤ 31, -9 ≤ k ≤ 9, -17
≤ l ≤ 17
Reflections collected
Independent reflections
Completeness to theta =
28.5138°
21960
2865 (Rint = 0.0290)
99.34%
Absorption correction
Semi-empirical from
equivalents
Max. and min.
transmission
0.866 and 0.509
Refinement method
Full-matrix least-squares on
2
F
Data / restraints /
parameters
2865 / 0 / 147
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2
Goodness-of-fit on F
Final R indices
1.0318
R1 = 0.0473, wR2 = 0.1262
[I>2sigma(I)]
R indices (all data)
R1 = 0.0492, wR2 = 0.1282
Largest diff. peak and
hole
-3
1.8737 and -0.5040 e ∙ Å
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Table 3. Solubility data for compounds 1-8
Entry
Acetone CH3CN
EtOH
Hexane
Ether
CH2Cl2
Water
THF
I
S
S
I
VS
VS
VS
VS
PS
PS
PS
PS
I
I
I
I
I
I
I
I
PS
PS
PS
PS
PS
PS
PS
PS
PS
PS
PS
PS
S
VS
S
S
1
2
3
4
5
6
7
8
I
S
I
VS
S
S
I
S
I
S
PS
PS
PS
PS
S
VS
VS
VS
VS
I
VS
VS
VS
I
I
I
I
I
Key to table: I = Insoluble (the material was quantitatively recovered after filtration); PS = Partly
Soluble (10% of the material dissolved); S = Soluble (80% of the material dissolved); VS = Very
Soluble (a clear solution emerged immediately). The solubility data is based on the following
experimental criteria: 0.02 g of the compound was dissolved in 1.0 mL of the appropriate solvent
and was hard shaken for 5-10 seconds at 23°C
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Figure 1 Different types of the most common phosphor-1,1,-dithiolates.
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Figure 2. Complex 9 formed through a double deprotonation of the ligand (A), followed by
coordination resulting in the best binding mode (B).
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Figure 3. ORTEP representation of the molecular structure of Ni(II) complex 9. Symmetry code
for the symmetry generated atoms: -x, 2-y, -z. Selected bond lengths (Å) and angles (°): Ni-S(1)
2.214(1), Ni-S(2) 2.219(1), S(1)-P(1) 2.044(1), S(2)-P(1) 2.042(1), P(1)-O(1) 1.510(2), P(1)-
C(1) 1.801(3), S(1)-Ni-S(2) 88.9(1), Ni-S(2)-P(1) 84.9(1), S(1)-P(1)-S(2) 98.9(1), S(1)-P(1)-
O(1) 115.6(1), S(1)-P(1)-C(1) 107.6(1), A complete listing can be found in the Supplementary
information.
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Scheme 1 Synthesis of the dimer [PhP(µ-S)S]2 (above) and of Lawesson’s Reagent (LR)
(below).
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Scheme 2 Comparison between the “traditional” and “green” method for the formation of
dithiophosphonic acids. Note the differences in terms of solvent, temperature and time.
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Scheme 3 Synthesis of the Ni(II) complexes 5-8.
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Scheme 4. Proposed formation of complex 9 from aqueous solution.
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