Function of substituents in coordination behaviour, thermolysis and ligand crossover reactions of phosphine oxides

Article abstract of DOI:10.1039/c4ra13645d

Some selected aminophosphine oxides (AmPOs) of the type OP(NMe₂)₃, OPPh(NMe₂)₂, OP(NC₂H₄O)₃, OPPh(NC₂H₄O)₂ and their corresponding La(III) and Th(IV) complexes are synthesized and analyzed by FT-IR, ¹H-NMR, ³¹P{¹H}-NMR, elemental analysis and TGA data. The coordination behavior of AmPOs was compared with some of the known ligands that include trioctylphosphine oxide (TOPO), tributylphosphate (TBP) and diethylphosphite (DEP). Thermogravimetric analysis of these complexes showed a distinct decomposition trend either by a single step or multi-step elimination of ligand species, which are strongly dependent on the electronic and steric behaviour of substituents on the P=O group and the nature of the metal. Phosphine oxide based La(III) and Th(IV) complexes undergo three unique intermolecular ligand exchange reactions at room temperature: relative competition among phosphine oxides to form a strong complex by exchanging the weaker ligand and complete ligand transfer from La(III) to Th(IV) metal centers. Ligand crossover is well controlled by priority rules and the trend is TOPO > TBP > DEP > AmPO > Ph₃PO. This tendency closely agrees with the stability constants of metal complexes. On comparison, Th(IV) complexes showed slightly higher stability than La(III) analogues.

Full text of DOI:10.1039/c4ra13645d

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DOI: 10.1039/C4RA13645D
Function of Substituents on Coordination Behaviour, Thermolysis and Ligand
Crossover Reactions of Phosphine Oxides
1B.B. Pavankumar, 1E. Veerashekhar Goud, 1R. Selvakumar, 1S.K. Ashok Kumar, 1Akella
Sivaramakrishna*, and 1Kari Vijayakrishna
1Chemistry Division, School of Advances Sciences, VIT University, Vellore 632 014, Tamil Nadu, India
2C.V.S. Brahmananda Rao and 2K.N. Sabharwal
2Chemistry Group, Indira Gandhi Centre for Atomic Research, Kalpakkam 603 102, Tamil Nadu, India
3Prakash C. Jha
3School of Chemical Sciences, Central University of Gujarat, Sector-30, Gandhinagar, Gujarat, India
E-mail: asrkrishna@rediffmail.com
==========================================================================
Abstract
Some selected aminophosphine oxides (AmPOs) of the type OP(NMe2)3, OPPh(NMe2)2, OP(NC2H4O)3,
OPPh(NC2H4O)2 and their corresponding La(III) and Th(IV) complexes are synthesized and analyzed by
1
31
FTꢀIR, HꢀNMR, P{1H}ꢀNMR, elemental analysis and TGA data. The coordination behavior of AmPOs
was compared with some of the known ligands that include trioctylphosphine oxide (TOPO),
tributylphosphate (TBP) and diethylphosphite (DEP). Thermogravimetric analysis of these complexes
showed a distinct decomposition trend either by a single step or multiꢀstep elimination of ligand species,
which are strongly dependent on the electronic and steric behaviour of substituents on P=O group and
nature of metal. Phosphine oxide based La(III) and Th(IV) complexes undergo three unique intermolecular
ligand exchange reactions at room temperature: relative competition among phosphine oxides to form a
strong complex by exchanging the weaker ligand and complete ligand transfer from La(III) to Th(IV) metal
centers. Ligand crossover is well controlled by priority rules and the trend follows as TOPO > TBP > DEP
> AmPO > Ph3PO. This tendency closely agrees with the stability constants of metal complexes. On
comparison, Th(IV) complexes showed slightly higher stability than La(III) analogues.
Keywords: aminophosphine oxide; synthesis; comparative study; ligand crossover reaction; coordination
behavior; thermolysis
Introduction
Pentavalent phosphine oxides (R3P=O) are known as reagents or precursors and their association in metal
mediated catalytic reactions as dynamic ligands and in bioactivity studies for many years [1ꢀ3]. Apart from
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this, phosphine oxides with active Oꢀdonor functionality were also reported as active ligands in the
separation of actinides and lanthanides e.g. carbamoylmethylphosphine oxides (CMPOs) are being used in
the TRUEX (transuranium extraction) process [4] through the formation of a sixꢀmembered chelate ring
upon complexation [5]. In general, the extraction efficiency is dependent on the nature of substituents,
which can greatly influence the performance of the ligand. When the Nꢀsubstituents are phenyl groups, the
resulting less basic carbonyl oxygen improve the extraction selectivity [6]. The basicity of some of the
reported ligands with respect to extraction of uranium(VI) and thorium(IV) salts in nitric acid medium was
in the following order: TOPO (triꢀnꢀoctylphosphine oxide) > TEHPO (tris(2ꢀethylhexyl)phosphine oxide)
and HDPM (bis((diꢀnꢀhexylphosphinyl)methane) > EBDPM (bis[bis(2ꢀethylbutyl)phosphinyl]methane)
[7]. There is a pressing demand to design the ligands with the desired properties for the effective separation
of actinides and lanthanides from nuclear waste. In this perspective, it seemed a constructive exercise to
compare the nature of AmPO ligands with some selected and known phosphine oxides in order to
understand their coordination performance with some selected fꢀmetals.
As our recent focus is on the exploration of chemistry of aminophosphine oxides (AmPOs) which are
relatively new classes of ligands derived from very well known phosphine oxide family and offer
fascinating opportunities [8ꢀ11], we have recently reported the synthesis and coordination behavior of
AmPOs [12]. The present work describes the synthesis, thermolysis, coordination behavior and ligand
crossover reactions of some selected phosphine oxides as given in Chartꢀ1. We now present evidence for
complete intermolecular ligand crossover among different metal complexes, further extending the flexible
nature of these ligands to understand the degree of coordination strength.
(Chart-1 here)
2. Experimental Part
2.1. Instrumentation
1H, 13C, and 31P{1H}ꢀNMR spectra were recorded on a Bruker DMXꢀ400 spectrometer and all 1H chemical
shifts were reported relative to the residual proton resonance in deuterated solvents (all at 298 K, CDCl3).
Infrared spectra were recorded on a Shimadzu Affinity 1 FTꢀIR Spectrometer. Thermogravimetric analyses
were carried out on a Perkin Elmer TGAꢀ7 and Mettler Toledo SDTA851e from ambient to about 800 °C
under air (50 mL minꢀ1) and at a heating rate of 10.0°C minꢀ1. Approximately 5 mg of the samples was used
in the TG experiments. Heraeus CHNS rapid micro analyzer was used for elemental analysis.
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2.2. Materials and methods
All manipulations were carried out under nitrogen atmosphere using Schlenk line techniques, unless
otherwise stated. Commercially available solvents were distilled from Na metal/benzophenone ketyl before
use. The phosphorus reagents (Merck and Sigma–Aldrich) including POCl3, Ph2POCl, PhPOCl2, TOPO
(trioctylphosphine oxide), TBP (tributylphosphate) and DEP (diethoxyphosphate) were used as received
and morpholine (Merck) was purified by distillation over KOH [9]. Dimethylamine (40%) solution from
Loba (India) was used as such without any further purification. Hydrated metal salts (Loba and Aldrich),
[Th(NO3)4.5H2O)] and [La(NO3)3.7H2O)] were used as received. UV–vis absorption spectra were recorded
with a UNICAM UV4ꢀ100 type doubleꢀbeam spectrophotometer (ATI UNICAM, Cambridge, UK)
controlled by a software VISION 3.4. Quartz cuvettes with path length of 1 cm were used.
2.3. Experimental
2.3.1. Preparation of Aminophosphine Oxides (L1-L4)
Tris(morpholino)phosphine oxide (L1) and bis(morpholino)phenyl phosphine oxide (L1) were prepared by
the literature methods [12,13].
General procedure for the synthesis of bis(dimethylamino)phenylphosphine oxide (L3): To the 40%
aqueous solution of dimethylamine (13.33 g, 0.295 mol) was added dichlorophenylphosphine oxide (11.5
g, 0.059 mol) drop wise in ice cold condition for over 2h via dropping funnel using 50 mL of
dichloromethane as solvent under N2 atm under constant stirring. Then the reaction mixture was brought to
room temperature and was allowed to stir for 24h. As the products (AmPOs) are highly soluble in water, it
is very difficult to extract them from water to organic solvents. In order to overcome this problem and to
solubilize these AmPOs into the organic solvents like dichloromethane, the aqueous solution was treated
with equal volume of CH2Cl2 and 10 g of NaCl. The mixture was stirred for 30 min and the two layers
were separated. The organic layer was dried over anhydrous Na2SO4 (5 g) for 30 min. The product (L3)
obtained as a colourless crystalline solid (6.0 g) Yield 60%). m.p. 135ꢀ37 oC. 1H NMR (400 MHz, CDCl3):
δ 2.62ꢀ2.63 (s, 12H, ꢀNCH3), 7.45ꢀ 7.77 (m, 5H, Ph); 31P{1H}ꢀNMR (CDCl3): δ 29.98. Anal. Calc. for
C20H22Cl2N4O2Si: C, 56.59; H, 8.07; N, 13.20. Found: C, 56.83; H, 8.57; N, 13.28%.
Synthesis of tris(dimethylamino)phosphine oxide (L4): To the solution of dimethylamine in THF (15.09 g,
0.334 mol) was added phosphorous oxychloride (7.66 g, 0.055 mol) drop wise under ice cold temperature
for over 2h via dropping funnel using dicholoromethane (50 mL) as solvent under N2 atm. The workꢀup of
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the reaction mixture was carried out as mentioned above. The product (L4) was obtained as yellow
1
crystalline solid (3.5 g). Yield 70%. m.p. 122ꢀ24oC. HꢀNMR (400 MHz, CDCl3): δ 2.66ꢀ2.67 (s, 18H, ꢀ
NCH3); 31P{1H}ꢀNMR (CDCl3): δ 20.6. Anal. Calc. for C6H18N3OP: C, 40.21.45; H, 10.12; N, 23.45.
Found: C, 40.26; H, 10.16; N, 23.48%. The pure compounds were isolated and characterized by various
spectroscopic techniques (Tableꢀ1).
(Table-1 here)
2.3.2. General Procedure for the preparation of metal complexes: The following general procedure was
used for the preparation of [Th(NO3)4(L3)2] complex and the same procedure was followed for the other
metal complexes.
A finely powdered solid of Th(NO3)4.6H2O (0.56 g, 0.98 mmol) was added all at once to a stirred solution
of L3 (0.417g, 1.96 mmol) in solvent mixture of CHCl3 (15mL) and the mixture was stirred for 24h to get
1
the colorless crystalline solid. Yield: 0.82 g, 93%. IR (Nujol mull/cmꢀ1): 970.19 (s) [P=O]. HꢀNMR
31
(CDCl3, ppm): δ 7.42–7.79 (m, phenyl), 2.82 (m, CH2). P{1H}ꢀNMR (D2O): δ ꢀ1.16 (s). Then the solid
residue was washed with dichloromethane to obtain the pure product, Th(NO3)4(L3)2.
In a similar way, all the following complexes were prepared by taking the appropriate amounts as given
below: LaCl3(L1)3 [LaCl3.7H2O (0.5 g, 1.346 mmol) and L1 (1.196 g, 4.039 mmol)], LaCl3(L2)3
[LaCl3.7H2O (0.159 g, 0.43 mmol) and L2 (0.393 g, 1.29 mmol)], LaCl3(L3)3 [LaCl3.7H2O (0.5 g, 1.34
mmol) and L3 (0.853 g, 4.02 mmol)], LaCl3(L4)3 [LaCl3.7H2O (0.5 g, 1.34 mmol) and L4 (0.724 g, 4.02
mmol)], LaCl3(L5)3 [LaCl3.7H2O (0.082 g, 0.744 mmol) and L5 (0.262 g, 2.232 mmol)], LaCl3(L6)3
[La(NO3)3.7H2O (1.066 g, 2.84 mmol) and L6 (2.29 g, 8.61mmol)], LaCl3(L7)3 [La(NO3)3.7H2O ( 1.02g,
2.72 mmol) and L7 (2.24 g, 8.07 mmol)], LaCl3(L8)3 [La(NO3)3.7H2O (0.6133 g, 1.49 mmol) and L8 (0.554
g, 4.47 mmol)], Th(NO3)4(L1)2 [Th(NO3)4.5H2O (1.02 g, 1.78 mmol) and L1 (1.03 g, 3.51 mmol)],
Th(NO3)4(L2)2 [Th(NO3)4.5H2O (0.26 g, 0.458 mmol) and L2 (0.279 g, 0.916 mmol)], Th(NO3)4(L4)2
[Th(NO3)4.5H2O (0.56 g, 0.98 mmol) and L4 (0.35 g, 1.96 mmol)], Th(NO3)4(L5)2 [Th(NO3)4.5H2O (0.54
g,0.79 mmol) and L5 (0.62 g, 1.59 mmol)], Th(NO3)4(L6)2 [Th(NO3)4.5H2O (1.68 g, 2.96 mmol) and L6
(1.569 g, 5.92 mmol)], Th(NO3)4(L7)2 [Th(NO3)4.5H2O (0.51 g, 0.871 mmol) and L7 (0.588 g, 1.75
mmol)], Th(NO3)4(L8)2 [Th(NO3)4.5H2O (0.240 g, 0.876 mmol) and L8 (0.5 g, 1.75 mmol)]. The unreacted
ligands were collected from the worked up solvent mixtures of dichloromethane and petroleum ether (3:1)
and this indirectly supports the metalꢀligand stoichiometries. The metal complexes were dried in vacuuo
over anhydrous calcium chloride. The complete list of isolated metal complexes is given in Table 1.
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2.3.3. General procedure for ligand crossover reactions:
A series of experiments were carried out to investigate the priority of the ligands in complex formation
with Th(IV) and La(III) salts.
Th(NO3)4(L5)2 (21.24 mg, 0.0798 mmol) and L6 (50 mg, 0.0398 mmol) were taken in 5 mL of chloroform.
This reaction mixture was heated at different temperatures for different time intervals and monitored by 31P
NMR. The formation of mixed ligand complex was confirmed by 31P{1H}ꢀNMR. In the similar way, other
reactions were carried out by taking Th(NO3)4(L6)2 (12 mg, 0.012 mmol) complex, L5 (9.15 mg, 0.0237
mmol); LaCl3(L5)3 (24 mg, 0.017 mmol), L6 (40 mg, 0.153 mmol) and LaCl3(L5)3 (12 mg, 0.0115 mmol),
L6 (13.3 mg, 0.0345 mmol) [L5 = TOPO, L6 = TBP].
3. Results and Discussion
3.1. Synthesis and characterization of ligands (L1-L4):
The compounds L3 and L4 were prepared as given in the Eq.1. The 1HꢀNMR spectra of the free ligands L5–
L8 and their corresponding metal complexes were shown good agreement with the literature results [12ꢀ18].
The pure nature of all the samples prepared has been established by spectral and elemental analysis data,
especially by observing characteristic peaks in 31PꢀNMR spectra (Table 2). By comparing the 31P NMR
results of the free ligand (for example L3 shows δ 29.98) as well as the corresponding metal complexes of
[Th(NO3)4(L3)2] (δ ꢀ1.16 ppm), it is evident that the metal complexes showed significant chemical shift
with relative to the free ligands, showing evidence of bonding through the phosphoryl group.
(Table 2 here)
(Eq.1 here)
The 31P{1H}ꢀNMR spectra of L3 and L4 showed single peaks at 29.6 and 20.0 ppm respectively, indicating
1
the formation of a single compound. The chemical shifts in HꢀNMR pertaining to the aromatic protons in
the range of 7.2ꢀ8.4 ppm and two signals for NMe2 protons in the range of 2.59ꢀ2.62 ppm were obtained
for L3. In a similar way, the structural confirmation of L4 was carried out using IR and NMR spectral data.
All the metal complexes derived from various ligands L1 to L8 were characterized by IR and NMR spectral
analysis (Tableꢀ3).
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(Table-3 here)
3.2. Synthesis and characterization of metal complexes:
The metal complexes were synthesized using phosphine oxides, L1-L8 (chartꢀ1) with LaCl3 and Th(NO3)4
are shown in Tableꢀ2. The subsequent studies gave satisfactory yields of metal complexes with 1:2 ratio
with Th(NO3)4 salts and 1:3 ratio for LaCl3 salts, the reactions were carried out at room temperature for 12
h using CHCl3 as solvent. The solvent was concentrated in vaccuo resulting colourless products,
[LaCl3(L1)3], [LaCl3(L2)3], [LaCl3(L3)3], [LaCl3(L4)3], [LaCl3(L5)3], [LaCl3(L6)3], [LaCl3(L7)3],
[LaCl3(L8)3], [Th(NO3)4(L1)2], [Th(NO3)4(L2)2], [Th(NO3)4(L3)2], [Th(NO3)4(L4)2], [Th(NO3)4(L5)2],
[Th(NO3)4(L6)2], [Th(NO3)4(L7)2] and [Th(NO3)4(L8)2] which on further washed with chloroform to
remove unreacted phosphine oxide later dried in vaccuo and further characterized by spectroscopic and
analytical techniques. Most of these complexes were soluble in high polar solvents like water and DMSO.
All the analytical and spectroscopic data of the metal complexes of L1ꢀL8 are given in Tableꢀ1. Reaction of
thorium nitrate with tertiary phosphine oxides in solvents like dichloromethane or methanol has afforded
the corresponding complexes having coordination number 10. All lanthanum(III) complexes indicated the
coordination number as 6 (Chartꢀ2).
(Chart-2 here)
1HꢀNMR spectra of all the complexes derived from L1ꢀ L8 ligands showed significant downfield shifts for
their respective metal complexes. For example, the position of methyl protons of –NMe2 groups (L3) and
the complex [Th(NO3)4(L3)2] appear at δ 2.59 and δ 2.63 respectively. Additionally the IR spectra of the
metal complexes derived from the ligands L1ꢀL8 for a v(P=O) ranges from 1015 cmꢀ1 to 1138 cmꢀ1 as strong
bands, depending on the nature of substituents on the phosphorus atom (free ligands ranging from 1124–
1192 cmꢀ1 as strong bands), indicating coordination through the phosphoryl oxygen. The broad and weak
bands for LaꢀCl modes are assigned in the range of 221ꢀ243 cmꢀ1. The IR spectrum of v(P=O) stretch for
the free ligand of TOPO showing strong band at 1190 cmꢀ1 where as the v(P=O) stretch of the
[Th(NO3)4(L5)2] observed at 1090 cmꢀ1 this shows the coordination of phosphine oxide with metal centre.
The free ligand (DEP) shows a band in the range of 1052ꢀ1024 cmꢀ1 for P=O and the corresponding metal
complex showed in the range of 1052ꢀ1024 cmꢀ1.
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The trends related to phosphoryl stretching frequencies show that the infrared band shift of La(III) and
Th(IV) complexes follow the increasing trend : L4 > L1 > L3 > L8 > L7 > L5 > L3 > L2 and L5 = L7 > L4 >
L6 > L8 > L3 > L1 > L2 respectively with regard to the free ligand bands (Tableꢀ2). These results further
indicate that the coordination strength of ligands strongly depending on the nature of substituents attached
to the P=O group.
The absorption pattern of the complexes (205 to 300 nm range) depends on the nature of substituents on
phosphoryl group and the nature of metal ions. The electronic absorption of these ligands could be due to
n→π* transitions as per the literature reports [19]. The metal complexes coordinated with ligands
containing both electron withdrawing groups and electron donating groups respectively exhibited
bathochromic shifts compared to the free ligands of phosphine oxides and metal salt precursors in solution.
It may be speculated that this unusual trend could be due to the character of the metal complexes associated
with the nature of substituents on phosphoryl group.
3.3. Ligand exchange reactions
Phosphine oxide based La(III) and Th(IV) complexes undergo three unique intermolecular ligand exchange
reactions: relative competition among phosphine oxides to form a strong complex by exchanging the
weaker ligand and complete ligand transfer from La(III) to Th(IV) metal centers. Ligand crossover is well
controlled by priority rules. The evidence for complete intermolecular ligand crossover among different
metal complexes, further extending the flexible nature of these ligands to understand the degree of
coordination strength is obtained.
Three unique intermolecular exchange reactions are described below:
(i) Ligand crossover reactions among Th(IV) or La(III) complexes
The ligand crossover reactions were monitored in chloroform solutions by 31PꢀNMR spectral data. The data
showed that TOPO can easily replace the other ligands in both the complexes of Th(IV) and La(III). This
was confirmed by obtaining two new signals at δ 63.4 for the complex of TOPO and the released free
ligand(s) at their respective positions as given in the Tableꢀ2. Among the other ligands, aminophosphine
oxide ligands have not shown any significant effect in chloroform solution on the coordination. The
schematic representation of ligand crossover reactions is given in Eq.2. The priority of the ligand crossover
in chloroform solution is observed as follows: TOPO > TBP > DEP > TMPO > PPh3PO for Th(IV)
complexes.
(Eq.2 here)
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Perhaps the most striking feature of this exchange is its apparent irreversibility: within NMR detection
limits, the replacement of any ligand such as TBP, DEP, TMPO, Ph3PO and by TOPO is quantitative and
irreversible. This is demonstrated here by the example shown in Eq.2, in which addition of a large excess
of Ph3PO, TMPO and DMP did not result in reversal of the reaction and detection of even a trace of free
TOPO.
(ii) Complete ligand transfer from La(III) to Th(IV) metal centers.
When LaCl3(TOPO)3 was mixed with two equivalents of Th(NO3)4.5H2O in chloroform at room
temperature, 31PꢀNMR spectrum of this reaction mixture was flanked with new peaks at δ 65.82 and 71.13
(Eq.3) [20]. It clearly indicates that Th(NO3)4(TOPO)2 (δ 65.64) was formed. The stability of the
complexes follows the trend: Th(NO3)4(TOPO)2 > LaCl3(TOPO)3. After heating the reaction mixture at
70°C for 2h it showed peaks (31PꢀNMR spectrum) at δ 75.98, 71.79, 66.35, and 58.31. After prolonged
heating for 12 h and 24 h, the formation of new signals was observed due to the formation of mixed ligand
complex, Th(NO3)4(TOPO)(TBP).
(iii) Formation of mixed ligand complex
31
The PꢀNMR of LaCl3(TOPO)3 showed single peak at δ 54.72 and after mixing with thorium complex it
showed a signal at δ ꢀ3.10, 58.93, 66.40 and 67.64. It indicates that Th(NO3)4(TOPO)2 (δ 65.64),
Th(NO3)4(TOPO)(TBP) (δ 58.93) and LaCl3(TBP)3 (δ ꢀ0.30) were formed [21]. The reaction mixture was
31
heated at 70°C and the reaction was monitored at regular intervals using PꢀNMR as a tool. For instance
the 31PꢀNMR taken after 2h and 12 h gave signals at δ 68.19, 66.47, 59.04, ꢀ3.19 and δ 68.02, 66.40, 59.08,
ꢀ3.29 respectively with little shift in their values. When [Th(NO3)4(TOPO)2] reacts with TBP at ambient
31
temperature in CDCl3 solution, within minutes new signals in the P NMR spectrum indicate that ligand
TOPO has formally replaced TBP. On heating at 120°C, the mixture showed new signals in 31PꢀNMR
spectrum affording a mixed ligand complex as shown in Eq.4. The product was identified by its spectral
31
analogy with the independently characterized sample, and progress of the reaction was monitored by P
NMR spectral analysis a shown Fig.1.
(Eq.3 here)
(Eq.4 here)
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(Fig.1 here)
In this type of exchange, equimolar amounts of any two of the differently coordinated La(III) and Th(IV)
complexes [LaCl3(TOPO)3] and [Th(NO3)4(TBP)2] are mixed in chloroformꢀd solution at room
31
temperature, and complete crossover of ligand is observed (Eq.5). This is seen in the PꢀNMR spectra of
the mixture of [LaCl3(TBP)3] and [Th(NO3)4(TOPO)2], in which signals for all four species are well
resolved.
(Eq.5 here)
Studies on exchange of ligands in water were not possible for TOPO, TBP and Ph3PO ligands as they are
water insoluble. However, AmPOs were shown better affinity towards Th(IV) and La(III) than DEP.
3.4. Calculation of Stability Constants
The spectroscopic parameters (extinction coefficient in ultraviolet measurements and chemical shifts in the
NMR studies) are the fundamental properties of the pure molecular complex and they do not depend upon
the choice of a concentration scale. When complex (MꢀꢀꢀL) formation by the addition of ligand to the metal
salt in solution is followed spectroscopically, the absorbance of the complex (A) at the path length (l) is
measured at different concentrations of the components.
The BenesiꢀHildebrand analysis of K involves the measurement of the MꢀꢀꢀL absorbance as a function of
varied [M] when [L] >> [M]. A plot of x = 1/[M] vs y = Ao/(AoꢀA) gives a yꢀintercept = 1/ε and slope =
(1/Kfε) as defined by the BenesiꢀHildebrand equation [22]. Then the stability constant (K) was calculated
by dividing intercept with slope.
As shown in Tableꢀ4, TOPO complexes of Th(IV) and La(III) showed highest stability followed by TBP
complexes. The decreasing trends of stability constants follow the sequence TOPO > TBP > DEP > AmPO
≈ Ph3PO. This sequence is identical to the observed order for the 31PꢀNMR trends during the ligand
exchange reactions as well as thermal stability trends. It is significant to note that the substituent groups on
central phosphorus atom influence the ligand exchange behavior and thermal stability of the metal
complexes. For example, Fig.2 and Fig.3 depict the absorption patterns for Th(IV) and La(III) solutions in
the presence of DEP in DMSO respectively. Further the stability constants were calculated from the slope
as shown in Fig.4.
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(Fig.2 here), (Fig.3 here), (Fig.4 here)
(Fig. 5 here) (Table-4 here)
3.5. Thermogravimetric analysis
In the present study, the heating rate was suitably controlled at 10 °C minꢀ1, and the weight loss was
measured from the ambient temperature up to 700°C. The residues were found to be rare earth oxides, as
reported earlier. The weight loss for each chelate was calculated within the temperature range at which the
hydrated water molecules were expelled. The experimentally found and calculated weight losses are listed
in Tableꢀ5.
(Table-5 here)
(Fig. 6 here) & (Fig.7 here)
On comparison, all the thorium complexes clearly showed better thermal stability than the corresponding
lanthanum complexes. The minimum temperature ranges at which the decomposition initiated were found
to be from 100 to 380°C for thorium complexes where as that of lanthanum complexes exhibited 70 to 120
°C. The initial weight loss occurring in the temperature range of 70°C may be interpreted as loss of crystal
water molecules for the respective Th(IV) and La(III) complexes. The TG curves showed the stepꢀwise
decomposition of the ligands till a constant weight, in which the metal oxide residue is formed as final
product. The basicity of the ligands portraits the thermal stability of Th(IV) and La(III) complexes in solid
state as in the following order: TOPO > TBP > DEP > AmPO > PPh3PO (Th(IV) complexes) and TOPO >
TBP > DEP ≈ AmPO > PPh3PO (La(III) complexes). The thermogravimetric analysis of some TMPO
complexes of Th(IV) and La(III) complexes were reported earlier and the trends are in good agreement
with the corresponding metal complexes of DMPO.
4. Conclusion:
As phosphine oxides are key ligand systems due to the hard nature of the oxygen atom and are proficient
extracting agents in radioactive waste treatment plants, the current focus is on synthesis and coordination
1
behavior of some selected aminophosphine oxides. All the products were characterized by FTꢀIR, H
31
NMR, PꢀNMR, TGA and UV/Vis Spectroscopic techniques. The coordination behavior of AmPOs was
compared with some of the known ligands that include trioctylphosphine oxide (TOPO), tributylphosphate
(TBP) and diethoxyphosphate (DEP). Thermogravimetric analysis of these complexes showed a distinct
decomposition trends either single step or multiꢀstep elimination of ligand species, which are strongly
dependent on the electronic and steric behaviour of substituents on P=O group and nature of metal. The
10

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DOI: 10.1039/C4RA13645D
tendency in thermal stability of these complexes is almost similar to the trends in ligand crossover
reactions. The changes in chemical shifts can be attributed to variations in the extent and degree of
coordination of donor atoms towards metal(s). The present work describes the influence of various
substituents attached to the tetracoordinate phosphorous atom on the basicity of phosphoryl group and this
work is significant in designing the new ligands of the required properties. The priority of ligands explains
the influence of ligands for example the ability of TOPO ligand to exchange all other ligands in organic
solutions. Thermogravimetric analysis of the complexes showed the elimination of ligands in a step wise
manner in most of the complexes but the amino phosphine oxide indicates the removal of ammine group
from the ligands by breaking the PN bonds this further supports the flexibility of PꢀN bonds and they are
weaker than OꢀM bond dative bond. Changes in the substituents of ‘P=O’ groups can influence the thermal
and coordination behaviour of the corresponding La(III) and Th(IV) complexes. In summary, the results
reported in the present work will increase our knowledge of phosphorus chemistry and these findings may
be useful for tailoring new complexes for catalytic and medical applications.
Acknowledgments
Dr ASRK is highly thankful to DAEꢀBRNS (INDIA) (No. 2012/37C/6/ BRNS/623) for the financial
assistance. BBPK thanks VIT University for the fellowship. Dr. ASRK and his group members thank DSTꢀ
VITꢀFIST for NMR and SIFꢀVIT University for other instrumentation facilities.
References
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104ꢀ113.
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S. Huguet, P. Leglaye, J. Organomet. Chem. 566 (1998) 117ꢀ123; (c) N.R. Price, J. Chambers, The
Chemistry of Organophosphorus Compounds, F. R. Hartley, (Ed.), Wiley, Chichester, 1 (1990) 649;
(d) J. Ansell, M. Wills, Chem. Soc. Rev. 31 (2002) 259ꢀ268; (e) M.C.B. Dolinsky, W.O. Lin, M. L.
Dias, J. Mol. Catal. A: Chem. 258 (2006) 267ꢀ274.
12. E.V.S. Goud, B.B. Pavankumar, A. Paul, Y. Shruthi, A. Sivaramakrishna, K. Vijayakrishna, H.S.
Hadley, C.V.S.B. Rao, and K.N. Sabharwal, J. Coord. Chem., 66 (2013) 2647ꢀ2658.
13. a) J.T. Donoghue, E. Fernadez, D.A. Peters, Inorg. Chem. 8 (1968) 1191ꢀ1194.; b) Christian
Romming, Jon Songstad, Acta Chem. Scand. A 36 (1982) 665ꢀ671; c) J.T. Donoghue, Bull. Chem.
Soc. Jpn. 43 (1970) 932ꢀ934; d) J.T. Donoghue, E. Fernadez, Bull. Chem. Soc. Jpn. 43(1970) 271ꢀ273.
14. a) Y.A. ElꢀNadi, Hydrometallurgy, 119ꢀ120 (2012) 23ꢀ29; b) K.S.R. Murthy, R.J. Krupadam, Y.
Anjaneyulu, J. Chromatogr. Sci. 36 (1998) 595ꢀ599.
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15. a) K.M.A. Malik, J.W. Jeffery, Acta Cryst. (1973). B29, 2687; b) M.R. Yaftian, M.E. Eshraghi, L.
Hassanzadeh, Iran. J. Chem. & Chem. Eng. 22 (2003) 71ꢀ76; c) J.M.P.J. Verstegen, J. Inorg. Nucl.
Chem. 26 (1964) 25ꢀ35; d) N. Kumar, Dennis G. Tuck, Can. J. Chem. 62 (1984) 1701ꢀ1704.
16. a) E. Jorjani, M. Shahbazi, Arab. J. Chem. (2012), http://dx.doi.org/10.1016/j.arabjc.2012.04.002
b) B. Mokili, C. Poitrenaud, Sol. Extr. Ion Exch. 15(3), 455ꢀ481 (1997).
;
17. a) M.D. Reinoso Lopez, S. Gonzalez Garcia, Ars Pharmaceutica 27 (1986) 435ꢀ45; b) K.
Changkrueng, D. Chatsiriwech, J. Rare Earths, 29 (2011) 896ꢀ901; c) V. Vallet, I. Grenthe, Sol.
Extr. Ion Exch. 31(2013) 358ꢀ369.
18. a) M.J. Glazier, W. Levason, M.L. Matthews, P.L. Thornton, M. Webster, Inorg. Chim. Acta 357
(2004) 1083–1091; b) D.R. Cousins, F.A. Hart, J. Inorg. Nucl. Chem. 30 (1968) 3009ꢀ3015; c) B. C.
Smith, Marguerite A. Wassef, J. Chem. Soc. (A), (1968) 1817ꢀ1818.
19. G. Manca, M. Caporali, A. Ienco, M. Peruzzini, C. Mealli, J. Organomet. Chem. 760 (2014) 177ꢀ
185; b) H.P. Benschop, M. Halmann, J.C.S. Perkin II, 1972, 1095.
20. The 31PꢀNMR chemical shifts for TOPO (free ligand), LaCl3(TOPO)3 are δ 48.93 and δ 54.72
respectively.
21. 50 mg (0.0355 mmol) of [LaCl3(TOPO)3] was mixed with 36 mg (0.0355 mmol) of
[Th(NO3)4(TBP)2] and stirred the mixture for 30 min in chloroform under different temperatures.
22. M. Kadar, A. Biro, K. Toth, B. Vermes, P. Huszthy, Spectrochim. Acta Part A 62 (2005) 1032–1038;
b) I. D. Kuntz, Jr., F. P. Gasparro, M. D. Johnston, Jr., R. P. Taylor, J. Am. Chem. Soc. 90 (1968)
4778; c) H.A. Benesi, J.H. Hildebrand, J. Am. Chem. Soc. 1949, 71, 2703.
13

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Graphical Abstract

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Chart-1: Ligands employed in this study

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Table 1: Characterization of synthesized La(III) and Th(IV) metal complexes of phosphine oxides
Sl. Metal salt taken
No. mmol (amount)
Ligand taken
(amount in
Structural Formula of the % Yield, (wt of the
m.p.
(λmax),
nm
P=O stretch
in FTꢀIR,
cmꢀ1
[Ref]
formed complex
product)
mmol)
1.346 (0.5 g)
0.43 (0.159 g)
1.34 (0.5 g)
1.34 (0.5 g)
4.039 (1.196 g)
[LaCl3(L1)3]
[LaCl3(L2)3]
79.2 (1.21 g)
78.3 (0.391 g)
92.0 (1.09 g)
89.0 (0.94 g)
81.7 (0.85 g)
82.9 (2.48 g)
81.0 (2.36 g)
80.0 (0.781 g)
85.4 (1.598 g)
82.5 (0.412 g)
93.0 (0.82 g)
91.0 (0.75 g)
80.8 (0.80 g)
80.9 (2.42 g)
83.0 (0.75 g)
81.0 (0.54 g)
235 (dec)
240
216
1028
[12]
[12]
1.29 (0.393 g)
205 (dec)
1112
4.02 (0.853 g)
[LaCl3(L3)3]
>250
265
1105
4.02 (0.724 g)
[LaCl3(L4)3]
>250
ꢀꢀꢀꢀ
1016
La
0.744 (0.082 g) 2.232 (0.262 g)
[LaCl3(L5)3]
L
242
1060
[14,15]
[16,17]
[18]
2.87 (1.066 g)
2.69 (1.0 g)
8.61 (2.29 g)
8.07 (2.24 g)
4.47 (0.554 g)
3.5 (1.039 g)
0.916 (0.279 g)
1.96 (0.417 g)
1.96 (0.35 g)
1.59 (0.62 g)
5.92 (1.569 g)
1.75 (0.488 g)
1.75 (0.5 g)
[LaCl3(L6)3]
L
224, 300
255
1033
[LaCl3(L7)3]
>250
1052
1047
1.49 (0.6133 g)
1.75 (1.0 g)
[LaCl3(L8)3]
105
263
[Th(NO3)4(L1)2]
[Th(NO3)4(L2)2]
[Th(NO3)4(L3)2]
[Th(NO3)4(L4)2]
[Th(NO3)4(L5)2]
[Th(NO3)4(L6)2]
[Th(NO3)4(L7)2]
[Th(NO3)4(L8)2]
213ꢀ215 (dec)
258
1132,1111
1138
[12]
[12]
0.458 (0.26 g)
0.98 (0.56 g)
0.98 (0.56 g)
0.79 (0.45 g)
2.96 (1.68 g)
0.87 (0.5 g)
217ꢀ218 (dec)
248
>250
>250
L
230, 260
225, 295
244
1055
1031
Th
1024
[14,15]
[16,17]
[18]
L
232
1033
265
L
260
1024
0.876 (0.240 g)
274
1035

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Table 2: 31P{1H}ꢀNMR chemical shifts of all the ligands and their metal complexes
Ligands Used
TOPO
31P{1H}ꢀ NMR
for Free ligand
(ppm)
Expected structure of
complex
31P{1H}ꢀNMR
spectral data, ppm
Elemental Analysis of the metal
complexes obtained (calculated)
% C
% H
% N
48.5
Th(TOPO)2(NO3)4
70.0
54.72
46.12
8.31
4.52
(4.47)
ꢀ
(46.00) (8.20)
61.62 11.07
(61.54) (10.97)
41.84 2.96
(41.71) (2.92)
45.64
(45.62)
12.73
(12.71) (2.93)
21.91 5.11
(21.85) (5.04)
26.84 3.82
(26.56) (3.79)
40.92 5.87
(40.86) (5.83)
17.26 4.86
(17.19) (4.33)
27.68 6.99
(27.62) (6.95)
31.48*
(31.35) (3.95)
44.52 5.62
(44.48) (5.60)
24.12 4.23 11.41 (11.76)
(24.20) (4.03)
La(TOPO)3Cl3
TPPO
29.3
Th(TPPO)2(NO3)4
45.5
5.44
(5.40)
ꢀ
35.0
5.44
(5.36)
2.95
La(TPPO)3Cl3
Diethylphosphite
9.48, 5.21
Th(DEPO)2(NO3)4
4.92, 10.86
2.36, ꢀ1.64
7.42
(7.41)
ꢀ
La(DEPO)3Cl3
Th(BDMPO)2(NO3)4
La(BDMPO)3Cl3
Th(TDMPO)2(NO3)4
La(TDMPO)3Cl3
Th(DMPO)2(NO3)4
La(DMPO)3Cl3
Bis(dimethylamino)phenyl
phosphine oxide
29.6
20.0
24.6
20.5
ꢀ0.75
ꢀ1.1
35.1
22.2
ꢀ2.5
12.43
(12.39)
9.61
(9.53)
16.74
(16.71)
16.14
(16.10)
Tris(dimethylamino)phosphine
oxide
Dimorpholinephenylphosphine
oxide
29.0
28.71
23.0
2181
4.08 10.54 (10.45)
7.50 (7.41)
Trimorpholinephosphine oxide
Th(TMPO)2(NO3)4
La(TMPO)3Cl3
37.28
6.30 10.91 (10.86)
(37.24) (6.25)
Tributylphosphate
ꢀ1.21
ꢀ0.30
28.54
(28.46)
37.11
5.42
(5.37)
7.06
5.56
(5.53)
ꢀ
Th(TBP)2(NO3)4
La(TBP)3Cl3
(37.06)
(7.00)
*with one mole of CH2Cl2

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Table 3: 1Hꢀ and 13CꢀNMR data of all the ligands and their metal complexes
S.
Ligand
1H-NMR spectral data, ppm
13C-NMR spectral data,
No.
ppm
Free ligand (L)
[Th(NO3)4L2]
[LaCl3L3]
Free ligand (L)
13C NMR (400 MHz,
CDCl3)
1
TOPO
1.87ꢀ1.03 [m, 14H; 1.92ꢀ1.05 [m, 14H; 1.89ꢀ1.03 [m, 14H;
CH2] CH2] CH2]
31.78, 31.12, 29.04, 28.30,
27.66, 22.60, 21.69, 14.04
(CH2 and CH3)
2
3
4
TPPO
7.58ꢀ7.39 [m, 5H; 7.60ꢀ7.41 [m, 5H; Arꢀ 7.55ꢀ7.37 [m, 5H;
133.05, 132.11, 131.92,
128.56, 128.44 (Ph)
61.62 (OCH2), 16.15 (CH3)
ArꢀH]
H]
ArꢀH]
Diethyl Phosphite
1.31 [t, 6H; CH3],
1.76 [q, 4H; CH2]
1.32 [t, 6H; CH3],
1.77 [q, 4H; CH2]
1.30 [t, 6H; CH3],
1.75 [q, 4H; CH2]
Bis(dimethylamino)
phenylphosphine
oxide
7.74ꢀ7.42 [m, 5H; 7.83ꢀ7.39 [m, 5H; Arꢀ 7.62ꢀ7.74 [m, 5H;
132.13, 131.03, 129.51,
128.41, 128.28 (Ph), 36.33
(NꢀCH3)
ArꢀH], 2.62 [s,
12H; CH3]
H], 2.59 [s, 12H;
CH3]
ArꢀH], 2.51 [s,
12H; CH3]
5
6
Tris(dimethylamino) 2.67 [s, 18H; CH3]
phosphine oxide
2.59 [s, 18H; CH3]
2.61 [s, 18H; CH3]
35.24 (NꢀCH3)
Bis(morpholino)
phenylphosphine
oxide
7.68ꢀ7.47 [m, 5H; 7.70ꢀ7.51 [m, 5H; Arꢀ 7.66ꢀ7.45 [m, 5H;
ArꢀH], 3.58 [t, 8H; H], 3.59 [t, 8H; CH2], ArꢀH], 3.55 [t, 8H; 128.68 (Ph), 67.08 (OꢀCH2),
CH2], 3.02 [t, 8H;
CH2]
132.12, 131.87, 128.81,
3.04 [t, 8H; CH2]
CH2], 3.00 [t, 8H;
CH2]
44.45 (NꢀCH2)
7
8
Tris(morpholino)
phosphine oxide
3.62 [t, 12H; CH2], 3.66 [t, 12H; CH2],
3.63 [t, 4H; CH2], 67.29 (OꢀCH2), 45.17 (Nꢀ
3.08 [t, 4H; CH2] CH2)
3.06 [t, 12H; CH2]
3.11 [t, 12H; CH2]
TBP
3.90 [t, 6H; CH2],
1.52 [m, 6H; CH2],
3.94 [t, 6H; CH2],
1.56 [q, 6H; CH2],
3.91 [t, 6H; CH2],
1.53 [q, 6H; CH2],
0.81 [t, 6H; CH2]
67.23 (OCH2), 32.41 (CH2),
18.73 (CH2), 13.43 (CH3)
1.29 [m, 6H; CH2], 1.30[ m, 6H; CH2],
0.813 [t, 6H; CH2] 0.82 [t, 6H; CH2]

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Eq.1
a)
b)
Chart-2: Predicted general structures of [LaCl3(R3PO)3] and [Th(NO3)4(R3PO)2].
TOPO
[Th(NO3)4(L)2]
[Th(NO3)4(TOPO)2]
CHCl3, RT
L = Ph3PO, TBP, TMPO, DEP
Eq.2: Ligand crossover reactions among Th(IV) complexes

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Eq.3
Eq.4
Eq.5: Ligand crossover reactions between Th(IV) and La(III) metal centers

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(c)
(a)
(a)
(b)
Fig.1: 31P{1H}ꢀNMR spectrum of mixed ligand complex, [Th(NO3)4(TOPO)(TBP)];
(a) ꢀ [Th(NO3)4(TOPO)(TBP)], (b) – free TOPO, (c) free TBP

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Fig.2

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Fig.3
Fig.4

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Table-4: Stability constants of various metal complexes
Stability Constants
Stability Constants of
S.No.
Ligand
of La(III)
Complexes*
6.23
Th(IV) Complexes*
1.
2.
3.
4.
5.
6.
7.
8.
L1
L2
L3
L4
L5
L6
L7
L8
6.42
6.78
6.58
7.25
11.7
9.82
7.43
7.04
6.48
6.33
6.67
9.46
8.82
7.13
6.8
*calculated in DMSO solutions due to the solubility problems
associated with some ligands in aqueous solutions.

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14
12
10
8
L6
L1-L4
L5
Th(IV) Complexes
La(III) Complexes
Stability
6
Constant, K
L7
L8
4
2
0
Fig. 5
Various phosphine oxide ligands with different substituents

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Table-5: Thermogravimetric analysis data of Th(IV) and La(III) Complexes
Complex
Temperature Range,
%
Weight
Obtained
52%
loss Calculated weight loss
(%) for loss of fragment
52.59 for both TBP
ligands
ºC
220 ꢀ330
[Th(NO3)4.(TBP)2]
[Th(NO3)4.(DEPO)2]
[Th(NO3)4.(DMPO)2]
140ꢀ180; 240ꢀ330
100ꢀ320
18%;
30%
55%
18.26% (monoligand);
36.52% (for two ligands)
55.28 (for both the
ligands)
290ꢀ380
120ꢀ320
100ꢀ180;
61%
54%
51%
61.69% (both DMPO
ligands)
[Th(NO3)4.(TOPO)2]
[LaCl3.(TOPO)3]
[LaCl3.(TBP)3]
54.50% (for to free
ligands)
50.91% (free TOPO
ligands)
300ꢀ490
70ꢀ190;
27% and gradual
decomposition
26.12% (free DMPO
ligands)
[LaCl3.(DMPO)3]
[LaCl3.(DEPO)3]
410ꢀ525
70ꢀ145;
20% and gradual
decomposition
20.93% (free DEPO
ligands)
410ꢀ585

Page 27 of 27
RSC Advances
100
80
60
40
20
0
Th(TBP)2(NO3)4
Th(DMPO)2(NO3)4
Th(DEPO)2(NO3)4
Th(TOPO)2(NO3)4
0 100 200 300 400 500 600 700 800
Temperature, °C
Fig.6: TGA of thorium(IV) complexes
100
80
60
40
20
0
La(DEPO)3Cl3
La(TOPO)3Cl3
La(TBP)3Cl3
La(DMPO)3Cl3
0
200
400
600
800
Temperature, °C
Fig.7: TGA of lanthanum(III) complexes