Evidences for chelating complexes of lithium with phenylphosphinic and phenylphosphonic acids: A spectroscopic and DFT study

Article abstract of DOI:10.1080/10426507.2013.842999

Lithium complexes were prepared with phenylphosphinic and phenylphosphonic acids. The complexes were studied in the solid state using Fourier transform infrared spectroscopy spectroscopy and in solution (methanol) using ¹H, ¹³C, and ³¹P Nuclear magnetic resonance spectroscopy (NMR) spectroscopy; the most preferred structures of the complexes were determined by density functional theory (DFT) computational method. Although methanol has a strong solvation effect on lithium ions and ligands, which causes dissociation of the complexes, significant changes of the NMR spectra of the complexes (relative to those of the free ligands) were observed. The new spectroscopic results indicate the presence of the phenylphosphinic acid tautomer (I: C₆H₅PH(O)OH) rather than that of phenyl-phosphorous acid (II: C₆H₅P(OH)₂) in deuterated methanol showing PH/PD exchange. On the other hand, tautomer I predominates in the complex with lithium without showing PH/PD exchange. The DFT calculations predict that tautomer I is the preferred structure in the case of free ligand and lithium complex. The absence of a PH/PD exchange in the complex is due to the formation of a chelating complex, rather than of a simple salt between lithium ion and the two oxygen atoms of I, which prevent tautomerization of I into II. DFT calculations support the formation of lithium chelating complexes. The lithium ion was found to affect the spectroscopic properties of phenylphosphinic acid more dramatically than those of phenylphosphonic acid.

Full text of DOI:10.1080/10426507.2013.842999

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Phosphorus, Sulfur, and Silicon and the
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Evidences for Chelating Complexes of
Lithium with Phenylphosphinic and
Phenylphosphonic Acids: A Spectroscopic
and DFT Study
Mahmoud Sunjuk a , Bassam El-Eswed b , Jamal N. Dawoud a , Amneh
Shtaiwi a , Monther Khanfar c & Mohammad El-Khateeb d
a Department of Chemistry, Faculty of Science , Hashemite
University , Zarqa , 13115 , Jordan
Click for updates
b Zarka University College , Al-Balqa Applied University , Zarka ,
13110 , Jordan
c Chemistry Department , Faculty of Science, University of Jordan ,
Amman , Jordan
d Chemistry Department, College of Sciences and Arts at Al-Kamil,
King Abdul Aziz University, Saudi Arabia; Department of Applied
Chemical Sciences, Faculty of Science and Arts , Jordan University
of Science and Technology , Irbid , 22110 , Jordan
Accepted author version posted online: 30 Sep 2013.Published
online: 12 Mar 2014.
To cite this article: Mahmoud Sunjuk , Bassam El-Eswed , Jamal N. Dawoud , Amneh Shtaiwi ,
Monther Khanfar & Mohammad El-Khateeb (2014) Evidences for Chelating Complexes of Lithium with
Phenylphosphinic and Phenylphosphonic Acids: A Spectroscopic and DFT Study, Phosphorus, Sulfur, and
Silicon and the Related Elements, 189:4, 558-575, DOI: 10.1080/10426507.2013.842999
To link to this article: http://dx.doi.org/10.1080/10426507.2013.842999
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Phosphorus, Sulfur, and Silicon, 189:558–575, 2014
Copyright ꢀC Taylor & Francis Group, LLC
ISSN: 1042-6507 print / 1563-5325 online
DOI: 10.1080/10426507.2013.842999
EVIDENCES FOR CHELATING COMPLEXES OF LITHIUM
WITH PHENYLPHOSPHINIC AND PHENYLPHOSPHONIC
ACIDS: A SPECTROSCOPIC AND DFT STUDY
Mahmoud Sunjuk,1 Bassam El-Eswed,2 Jamal N. Dawoud,1
Amneh Shtaiwi,1 Monther Khanfar,3
and Mohammad El-Khateeb4
1Department of Chemistry, Faculty of Science, Hashemite University, Zarqa 13115,
Jordan
2Zarka University College, Al-Balqa Applied University, Zarka 13110, Jordan
3Chemistry Department, Faculty of Science, University of Jordan, Amman, Jordan
4Chemistry Department, College of Sciences and Arts at Al-Kamil, King Abdul Aziz
University, Saudi Arabia; Department of Applied Chemical Sciences, Faculty of
Science and Arts, Jordan University of Science and Technology, Irbid 22110,
Jordan
GRAPHICAL ABSTRACT
Abstract Lithium complexes were prepared with phenylphosphinic and phenylphosphonic
acids. The complexes were studied in the solid state using Fourier transform infrared spec-
troscopy spectroscopy and in solution (methanol) using 1H, 13C, and 31P Nuclear magnetic
resonance spectroscopy (NMR) spectroscopy; the most preferred structures of the complexes
were determined by density functional theory (DFT) computational method. Although methanol
has a strong solvation effect on lithium ions and ligands, which causes dissociation of the com-
plexes, significant changes of the NMR spectra of the complexes (relative to those of the free
ligands) were observed. The new spectroscopic results indicate the presence of the phenylphos-
phinic acid tautomer (I: C6H5PH( O)OH) rather than that of phenyl-phosphorous acid (II:
C6H5P(OH)2) in deuterated methanol showing PH/PD exchange. On the other hand, tautomer
I predominates in the complex with lithium without showing PH/PD exchange. The DFT
Received 20 January 2013; accepted 12 August 2013.
Financial support by the Hashemite University is gratefully acknowledged. The authors would like to thank
Basem Nasrallah for running the elemental analysis.
Address correspondence to Dr. Mahmoud Sunjuk, Department of Chemistry, Faculty of Science, Hashemite
University, P.O. Box 150459, Zarqa 13115, Jordan. E-mail: mahmoud.sunjuk@hu.edu.jo
558

PREPARATION OF LITHIUM COMPLEXES WITH PHENYLPHOSPHINIC
559
calculations predict that tautomer I is the preferred structure in the case of free ligand and
lithium complex. The absence of a PH/PD exchange in the complex is due to the formation of a
chelating complex, rather than of a simple salt between lithium ion and the two oxygen atoms
of I, which prevent tautomerization of I into II. DFT calculations support the formation of
lithium chelating complexes. The lithium ion was found to affect the spectroscopic properties
of phenylphosphinic acid more dramatically than those of phenylphosphonic acid.
Keywords Lithium; chelating complexes; phenylphosphonic acid; phenylphosphinic acid; 31
P
NMR; FTIR; DFT calculations
INTRODUCTION
Metal phosphonates are a class of hybrid inorganic–organic compounds in which the
metal coordinates with phosphonate functional group.1 Recently, metal organophospho-
nates have attracted great interest because of their potential applications as ion-exchangers,
sensors, or catalysts and due to their intriguing host intercalation chemistry.2
The first study of alkali metal organophosphonates was reported by Rao and
Vidyasagar3 in 2005. They reported the X-ray structures of one-dimensional alkali metal
phenyl-phosphonates, which were found to have the formula M(HO3PC6H5)(H2O3PC6H5),
where M Li, Na, K, Rb, and Cs.3 The X-ray diffraction study of the complex of lithium
with phenylphosphonic acid indicated that lithium is tetrahedrally coordinated by four phos-
phonate moieties forming one-dimensional chain of Li(HO3PC6H5)(H2O3PC6H5), with the
hydrogen bonding holding the chains together.3
However, the first study of alkaline earth metal organophosphonates dates back to
1979, when magnesium phenylphosphonate corresponding to the formula MgC6H5PO3
H2O was reported.2 Among other alkaline earth metal phenylphosphonates the structures
of the calcium, barium, and strontium salts were described. These compounds were found
to have various metal/phosphorus ratios depending on the acidity of the reaction medium.4,5
The powder X-ray patterns indicated that these compounds are layered.5 Studies on alkali
and alkaline earth metal phenylphosphinates are very rare in the literature. Phenylphosphi-
nate complex of lithium was employed as a soil disinfectant; however, there is no detailed
investigation of the structure of this compound.6
The phosphonate dianion RPO32− has a promising structure capable to form units for
the self-assembly of polynuclear complexes.7 Polynuclear transition metal phosphonates
have attracted attention because of their ion-exchange and adsorption ability and their proton
conductivity, which makes these compounds useful in photochemistry, electro-optics, and
catalysis. Several transition metal phenylphosphonates were prepared.1,8–12 Burkovskaya
et al.7 reported the X-ray structure of decanuclear phenylphosphonate bridged Co(II)
complexes. Furthermore, Wang et al.8 reported a single chain cobalt phenylphosphonate
complex [Co(phen)(4,4ꢁ-bpy)(C6H5PO3H)2]n 0.5 H2O (phen=1,10-phenanthroline, 4,4ꢁ-
bpy=4,4ꢁ-bipyridine). Supramolecular copper(II) phenyl-phosphonate complexes, e.g.,
[Cu(II)(phen)Cl][C6H5PO(OH)2)((OH)O2PC6H5)], were also reported.12 The structure of
trimethyltin(IV) derivatives of phenylphosphonic acid was found to consist of infinite
polymeric one- or two-dimensional networks depending on reaction conditions.13
The solid-state chemistry of phenylphosphonates constitutes a significant part of this
research area, and to the best of our knowledge there are no studies of metal phosphonates
in solutions. The aim of the present work is to investigate the nature of interaction between
lithium and phenylphosphinic (1a) and phenylphosphonic acid (2a) in their complexes in
the solid state using Fourier transform infrared spectroscopy (FTIR), and in solution using

560
M. SUNJUK ET AL.
1H, 31C, and 31P nuclear magnetic resonance (NMR) spectroscopy, as well as by density
functional theory (DFT) computational techniques.
RESULTS AND DISCUSSION
IR Absorption Spectra
The IR spectraof phenylphosphinic acidligand 1a andits lithiumcomplex 1 areshown
in Figure 1; the IR data are summarized in Table 1. The broad band at 2100–2800 cm−1 in
the spectrum of the ligand was assigned to ν(O H) of the hydrogen bonded hydroxy group.
This band disappeared upon complexation with lithium. A sharp strong band at 2345 cm−1
,
assigned to ν(P H), is observed in the spectrum of the complex. The P H vibration was
reported to cause a weak band at 2408 cm−1 in the case of 2,4,6-trimethyl-phenylphosphinic
acid.14 Furthermore, the band at 1676 and 1589 cm−1 in the spectrum of the ligand 1a,
assigned to δ(O H), disappeared upon complexation with lithium. All these observations
support the suggestion that the proton of the hydroxy group was replaced by Li+. The bands
at 1199, 1152, and 1102 cm−1 in the ligand, which were assigned to ν(O P O), were
shifted to lower frequency upon complexation with Li+. This is due to the decrease of bond
order of O P O bands in the complexes because of the delocalization of the electrons of
the double bond over two P O bonds (Scheme 1).
The infrared spectra of phenylphosphonic acid ligand 2a and its lithium complex 2
are shown in Figure 2; the IR data are summarized in Table 1. The IR characteristics of
complex 2 strictly differ from those of complex 1. The broad band at 3300–2200 cm−1
in the spectrum of ligand, which was assigned to ν(O H) (hydrogen bonded hydroxy
group), remained broad upon complexation with lithium. The band assigned to δ(O H) at
1593 cm−1 in the spectrum of ligand shifted to higher frequency upon complexation with
lithium. Furthermore, the bands assigned to ν(O P O) at 1221, 1145, 1080, 1016, and
940 cm−1 in the spectrum of ligand 2a were shifted to higher frequency upon complexation
with Li+ due to the strong hydrogen bonding of PO3H in complex 2, which is further
supported by the broad band at about 2800–2200 cm .
−1 2,4 The FTIR study of complex 2 is
consistent with the reported3 X-ray structure of complex (Li(HO3PC6H5)a˙(H2O3PC6H5))
(2), where hydrogen bonding plays an important role in connecting the chains of lithium
cations and phenylphosphonate anions together.
Scheme 1

PREPARATION OF LITHIUM COMPLEXES WITH PHENYLPHOSPHINIC
561
Figure 1 FTIR spectra of 1a and its complex with lithium 1. (Color figure available online).
NMR Spectra
The 1H, 13C, and 31P NMR spectra of complexes 1 and 2 were investigated and the
chemical shifts and coupling constants (JHH, JPC, and JPH) were analyzed and compared
with those of the corresponding ligands. Although methanol has a strong solvation effect
on lithium ions and ligands, which causes dissociation of complexes, significant changes
in the NMR spectra of the complexes (relative to those of free ligands) were observed. This

562
M. SUNJUK ET AL.
Table 1 IR spectral data of ligand 1a and its complex 1 and ligand 2a and its complex 2
Absorption bands (cm−1
)
Assignments∗
1a
1
2a
2
ν(C H)
ν(O H)
ν(P H)
δ(OH)
3078 (w) 3072 (w)
2800–2100 (br)
—
1676 (br), 1589 (s)
1440 (s)
3047, 3003 (w)
—
—
3052 (w)
2200–3200 (br)
—
1628 (br)
1437 (s)
2200–3300 (br)
—
2345 (s)
—
1433 (s)
1593 (m)
1439 (s)
ν(C C)
ν(O
P
O)
O)
1199 (s), 1152 (s),
1102 (s)
1175 (s), 1147 (s),
1084 (s)
1221(s), 1145 (s),
1080 (s), 1016 (s),
940 (s)
550 (s), 521 (m),
488 (m), 420 (m)
1285 (m), 1165 (s),
1059 (s), 1024 (m)
δ(O
P
533 (s), 463 (m),
422 (m)
553 (s), 501 (m),
464 (m), 422 (m)
589 (s), 543 (m),
512 (m), 461 (m)
∗Assignments were made according to references 2, 4, 15, 17, 18.
reflects the presence of strong interactions between lithium ions and phenylphosphinic and
phenylphosphonic acids.
1H NMR Spectra
1
1
The H NMR chemical shifts and the JPH coupling constant of ligand 1a and its
1
lithium complex 1 are presented in Table 2; the H NMR spectra are shown in Figure 3.
The chemical shifts of the ortho, meta, and para protons of ligand (1a) were assigned to
the peaks at 7.75–7.82, 7.52–7.57, and 7.61–7.64 ppm, respectively. The chemical shift
values follow the following order: ortho > para > meta, which is consistent with the
resonance effect of the electron-withdrawing phosphinyl group. All peaks of the aromatic
protons were shifted upfield upon complexation with Li+. This is due to expected decrease
of electron withdrawal strength of phosphinyl group on formation of Li+ salt.
Phosphinic acid (I, R PH( O)OH) exists in equilibrium with the tautomer (II,
1
R P(OH)2) (Scheme 2).14,15 The doublet at 7.51 ppm with a value of JPH = 563 Hz in
1
the H NMR spectrum of ligand 1a was assigned to the proton attached to phosphorus.
The intensity of this peak was very small compared with that of the peaks of phenyl
protons (≈2% of the expected value relative to the aromatic protons). If we take PH/PD
exchange into consideration, then tautomer I may exist in the case of free ligand 1a in
methanol. However, resemblance between the signal patterns of aromatic protons of the
experimental 1H NMR spectrum of 1a and that of calculated for tautomer II might indicate
Scheme 2 Tautomerization equilibrium in phosphinic acids.

PREPARATION OF LITHIUM COMPLEXES WITH PHENYLPHOSPHINIC
563
Figure 2 FTIR spectra of 2a and its complex with lithium 2. (Color figure available online).
the predominance of tautomer II (Figure 3). Orthaber et al.14 detected only tautomer I in
the case of 2,4,6-trimethylphenylphosphinic acid in d6-acetone with 1JPH = 562 Hz.
Upon complexation with Li+, the chemical shift of P H proton was not affected;
the coupling constant 1JPH decreased to 511 Hz, however. More important, the integrated
intensity of the signal increased to 27% of the expected value upon complexation with
Li+. Thus, tautomeric P H form predominates in the lithium complex 1. Resemblance
between the signal pattern of the aromatic protons of the experimental 1H NMR spectrum
of 1 and that calculated for the P H tautomer supported this finding (Figure 3). This is

564
M. SUNJUK ET AL.
Table 2 1H NMR chemical shifts (at 400.1 MHz, in ppm relative to TMS) and coupling constants (Hz) of ligands
1a and 2a and their complexes in CD3OD
1a tautomer 1a tautomer
I calculated II calculated
2a
Assignment
1a
1
2a
calculated
2
H-2 (m, 1H) o-H
H-3 (m, 1H) m-H
H-4 (m, 1H) p-H
P-H (d, 1H)
7.75–7.82 7.76–7.87 7.68–7.72 7.69–7.74 7.76–7.82 7.67–7.73 7.77–7.82
7.52–7.57 7.47–7.53 7.22–7.26 7.41–7.45 7.44–7.49 7.39–7.43 7.33–7.35
7.61–7.64
7.34–7.38
7.52–7.56 7.51–7.55
7.51 (1JPH 5.7 (1JPH
= 563 Hz) = 320 Hz)
7.51 (1JPH
= 511 Hz)
further supported by the observation found in the FTIR study described above, where a
strong peak due to P H stretching was observed in the case of the complex. The formation
of a chelate complex between lithium ion and the two oxygen atoms of ligand 1a prevents
transfer of the hydrogen atom of P H moiety to the oxygen atom of the carbonyl group
and thus prevents PH/PD exchange with the deuterated solvent methanol. This indicates
the formation of chelating complex rather than a simple salt between the lithium ion and
the anion of 1a.
The chemical shifts of different protons of ligand 2a and its complex 2 as well as
the 1JPH coupling constants are presented in Table 2; the corresponding 1H NMR spectra
Figure 3 Left: Experimental 1H NMR (at 400.1 MHz) spectra of 1a and its complex with lithium 1 in CD3OD.
Chemical shifts are given in ppm relative to TMS. Right: Calculated 1H NMR spectra for tautomer I and II in the
aromatic region. (Color figure available online).

PREPARATION OF LITHIUM COMPLEXES WITH PHENYLPHOSPHINIC
565
Figure 4 1H NMR spectra (at 400.1 MHz) of 2a and its complex with lithium 2 in CD3OD. Chemical shifts are
given in ppm relative to TMS.
are shown in Figure 4. The chemical shifts of the ortho, meta, and para protons of 2a
were assigned to the peaks at 7.76–7.82, 7.44–7.49, and 7.52–7.56 ppm, respectively. The
signals of the meta and para protons of ligand 2a are shifted upfield upon complexation
with Li+, while the signals of the ortho protons are not affected. This may reflect a weaker
interaction between cation and anion in 2a as compared with 1a.

566
M. SUNJUK ET AL.
1
Figure 5 Left: Experimental 13C{ H} NMR spectra of 1a and its complex with lithium 1 at 100.8 MHz in
CD3OD. Chemical shifts are given in ppm relative to TMS. Right: Calculated 13C NMR spectra for tautomers I
and II. (Color figure available online).
13C NMR Spectra
The 13C NMR spectra of ligand 1a and its complex with Li+ 1 are shown in Figure 5;
the values of the chemical shifts for different carbon atoms are compiled in Table 3. All
the signals of carbon atoms were observed as doublets due to coupling with the phos-
phorus atom. A doublet pattern observed for the 13C NMR signals of the aromatic carbon
atoms was also reported by Orthaber et al.14 for 2,4,6-trimethyl-phenylphosphinic acid with
1JPC = 138 Hz and 2JPC = 12 Hz. As it is evident from Table 3, the coupling constant JPC
1
Table 3 13C{ H}-NMR chemical shifts (at 100.8 MHz, ppm, relative to TMS) and coupling constants (Hz) of
ligands 1a and 2a and their complexes 1 and 2 in CD3OD
Carbon
atom
1a tautomer
I calculated
1a tautomer
II calculated
2a
1a
1
2a
calculated
2
∗
C-1
1JPC
C-2
2JPC
C-3
3JPC
C-4
4JPC
132.6
3
131.4
12
129.9
13
134.0
3
127.9
181
131.0
10
128.5
8
151.6
2
131.2
10
129.5
8
130.4
5
139.4
103
131.6
20
129.2
13
131.0
11
132.9
185
131.6
10
129.1
15
137.8
181
131.3
10
128.0
8
—
131.4
9
128.2
13
130.1
3
132.6
5
132.5
3
134.3
5
∗No peaks were detected.

PREPARATION OF LITHIUM COMPLEXES WITH PHENYLPHOSPHINIC
567
Table 4 31P NMR chemical shifts (at 161.3 MHz, ppm, relative to 85% phosphoric acid) and coupling constants
(Hz) of ligands 1a and 2a and their complexes 1 and 2 in CD3OD
2,4,6-trimethyl-
phenylphosphinic
acid in d6-acetone15
2,3,5,6-tetrafluoro-
phenylphosphinic
acid in d6-acetone15
1a
1
47.4 (tt)
24.2
2.2 (initial doublet
turns into triplet)
37.9 (dt)
47.8 (dt)
1JPD = 86 Hz
1JPH = 572 Hz
3JPH = 14 Hz
1JPD = 92 Hz
1JPH = 630 Hz
1JPH = 562 Hz
1JPH = 510 Hz
3JPH = 13 Hz
2a
2
39.4 (t)
3JPH = 14 Hz
35.3 (t)
3JPH = 11 Hz
decreases as we move from C2 to C4. The comparison of the 13C NMR spectrum of 1a
with those calculated for tautomers I and II does not confirm the existence of any of the
two tautomers exclusively (Figure 5).
Resemblance between the 13C NMR data of lithium phenylphosphinate 1 (139.4,
1JPC = 103 Hz; 131.6, 2JPC = 20 Hz; 129.2, 3JPC = 13 Hz; 131.0, 4JPC = 11 Hz) and those
calculated for tautomer I (127.9, 1JPC = 181 Hz; 131.0, 2JPC = 10 Hz; 128.5, 3JPC = 8 Hz;
132.6, 4JPC = 5 Hz) indicates that most probably tautomer I predominates in the complex
(Table 3 and Figure 5).
The 13C NMR spectra of ligand 2a and its complex 2 are shown in Figure 6. The
values of the chemical shifts for different carbon atoms are given in Table 3. The chemical
shifts of C2, C3, and C4 of ligand 2a are shifted upfield upon complexation with Li+
(Table 3).
31P NMR Spectra
The chemical shifts and selected P,H coupling constants for ligands 1a and 2a and
their lithium complexes 1 and 2 are presented in Table 4; the corresponding 31P NMR
spectra are shown in Figures 7 and 8.
The NMR signal of the phosphorus atom in ligand 1a undergoes significant upfield
shift upon complexation with Li+ (9.5 ppm). In the case of ligand 1a two signals are
observed: a doublet of triplets and a much more intense triplet of triplets. These two signals
indicate that phenylphosphinic acid undergoes PH/PD exchange with deuterated methanol.
The 1:1:1 triplet results from the coupling of phosphorus with deuterium atom (1JPD
=
1
86 Hz). This value is comparable with JPD = 92 Hz reported in the case of 2,3,5,6-
tetrafluoro phenylphosphinic acid.14 Each line of the 1:1:1 triplet is further split into a 1:2:1
triplet resulting from the coupling of phosphorus with the two aromatic ortho hydrogen
atoms (3JPH = 14 Hz). The less intense doublet of triplets is caused by the coupling of the
phosphorus atom with the directly attached hydrogen atom (1JPH = 572 Hz) and further
coupling with the aromatic ortho hydrogen atoms (3JPH = 14 Hz). Thus, the triplet of
triplets indicates significant PH/PD exchange in 1a, while the doublet of triplets results
from the molecules of 1a with unexchanged P H.

568
M. SUNJUK ET AL.
1
Figure 6 13C{ H} NMR spectra of 2a and its complex with lithium 2 at 100.8 MHz in CD3OD. Chemical shifts
are given in ppm relative to TMS.
On complexation of 1a with Li+, PH/PD exchange is suppressed due to preventing
tautomerization of I to II. Thus, the coupling of phosphorus with the directly attached
proton (1JPH = 510 Hz) and with both aromatic ortho hydrogen atoms (3JPH = 13 Hz) is
observed in the spectrum of complex 1 resulting in a doublet of triplets’ line pattern.
The NMR signal of phosphorus in ligand 2a is less strongly shifted (≈4 ppm) to
higher field upon complexation with Li+ than that of ligand 1a. The coupling constants of
ligand 2a are not affected significantly upon complexation with Li+.

PREPARATION OF LITHIUM COMPLEXES WITH PHENYLPHOSPHINIC
569
Figure 7 31P NMR spectra of 1a and its complex with lithium 1 at 161.3 MHz in CD3OD. Chemical shifts are
given in ppm relative to 85% phosphoric acid.
It is worth to mention that no P,Li coupling was observed in the 31P NMR spec-
tra of the studied complexes 1 and 2. The quartet line patterns reported by Reich
1
2
et al.18,19 with JPLi values of 45–48 Hz and JPLi values of 7.5–11.6 Hz were not de-
tected in the present study. This indicates that the lithium complexes investigated in this
study are in a fast dynamic exchange with the corresponding acid anions and lithium
cations.

570
M. SUNJUK ET AL.
Figure 8 31P NMR spectra of 2a and its complex with lithium 2 at 161.3 MHz in CD3OD. Chemical shifts are
given in ppm relative to 85% phosphoric acid.
DFT Calculations
It is worth to mention that all of the phosphorus complexes computationally studied
exist in the singlet ground state. Selected atom distances and bond angles computed at
B3LYP/6-31G(d) and B3LYP/6-31+G(d) levels of theory are presented in Table 5. The
results obtained on the basis of the B3LYP/6-31+G(d) method were found to agree very

PREPARATION OF LITHIUM COMPLEXES WITH PHENYLPHOSPHINIC
571
Table 5 Selected atom distances (Å) and bond angles (◦) for tautomers I and II of phenylphosphinic acid, and
possible structures of lithium complexes 1 and 2 determined by B3LYP/6-31+G(d) calculations∗
Structure
I
II
III
IV
V
Atom distance
C(1)-C(2)
C(4)-H(9)
C(3)-P(8)
P(8)-O(13)
P(8)-O(14)
P(8)-O(15)
P(8)-H(13)
P(8)-Li(16)
1.396 (1.403)
1.086 (1.086)
1.814 (1.812)
—
1.493 (1.499)
1.639 (1.637)
1.409 (1.410)
—
1.397 (1.395)
1.086 (1.085)
1.844 (1.842)
1.683 (1.678)
1.667 (1.663)
—
1.397 (1.935)
1.087 (1.086)
1.817 (1.814)
—
1.544 (1.542)
1.544 (1.542)
1.417 (1.418)
2.312 (2.303)
1.888 (1.884)
1.397 (1.395)
1.087 (1.086)
1.856 (1.851)
1.595 (1.592)
1.684 (1.675)
—
1.395 (1.394)
1.086 (1.086)
1.815 (1.813)
1.539 (1.538)
1.639 (1.637)
—
—
—
—
—
—
2.451 (2.386)
—
2.305 (2.298)
—
O(14)-Li(16)
Bond angle
—
C(1)-C(2)-H(7)
C(4)-C(3)-P(8)
C(4)-C(5)-C(6)
C(3)-P(8)-O(13)
C(3)-P(8)-O(14)
O(14)-Li(16)-O(15)
119.9 (120.0)
118.8 (118.5)
120.1 (120.0)
—
113.5 (113.3)
—
119.5 (119.7)
122.4 (121.7)
120.3 (120.2)
95.5 (95.6)
100.2 (99.7)
—
120.3 (120.5)
120.2 (120.1)
120.0 (120.0)
—
111.8 (111.3)
83.2 (83.5)
119.7 (119.9)
120.7 (120.3)
120.1 (120.1)
103.4 (103.9)
99.7 (99.6)
—
120.1 (120.0)
120.3 (119.7)
120.1 (120.0)
111.2 (110.5)
104.7 (105.2)
—
∗The data in parenthesis is obtained on the basis of B3LYP/6-31G(d).
well and to be consistent with those of the B3LYP/6-31G(d) method (Table 5). For the
lithium complex of phenylphosphinic acid 2 with the suggested structure V, DFT results
show that there are two types of P O bonds with bond distances of 1.54 Å and 1.64 Å and
a C P distance of 1.82 Å. These results agreed well with the reported X-ray data of the
structure of complex 2, in which the P O distances are observed in the range of 1.53–1.56
Å3, and in which the C P distance lies in the range of 1.78–1.84 Å; this was found
experimentally for similar aromatic phosphorus compounds.20 Note that the frequency
tests of the optimized geometry of all structures using B3LYP/6-31+G(d) yielded zero
imaginary frequencies and hence the optimized geometry of these phosphorus compounds
is true minima. For example, the frequency test analysis of structure V showed that the

572
M. SUNJUK ET AL.
Table 6 Electronic energies (Eh) for tautomers I and II of phenylphosphinic acid and possible structures of
lithium complexes 1 and 2 calculated at B3LYP/6-31+G(d) level of theory. The change in Gibbs energy (δ G)
for the tautomerization of phenyl-phosphinic acid I to phenylphosphorous acid II was also calculated at the
B3LYP/6-31+G(d) level of theory
Structure
Eh (Hartree)
δ G (kcal mol−1
)
C6H5H(P O)OH (I)∗
−724.586
δ GI→II = +5.67 (in gas phase)
δGI→II = +8.02 (in methanol)
C6H5-P(OH)2 (II)
−724.576
−731.607
−731.563
−806.865
C6H5-PH( O)OLi (III)
C6H5-P(OH)OLi (IV)
C6H5-P( O)(OH)OLi (V)
∗For compound numbers, see Table 5.
values were in the range 3170–3180 cm−1 for the C H stretching vibration and 1477 cm−1
for the aromatic C C stretching vibrations. The P O stretching vibration was found to be
within the range of 1000–1213 cm−1 and the O P O bending vibration was in the range
of 408–525 cm−1. All these values are in good agreement with those found in the infrared
spectrum of complex 2 (Table 1). Hence, the DFT method applied in this study is suitable
to examine the geometrical parameters of the phosphorus compounds investigated.
Our results indicate that the phenyl ring in all structures still retains its planarity since
all C C C, C C H, and C C P bond angles are very close to 120◦, and hence the π
electrons in the phenyl moiety are still delocalized in these configurations. The optimized
geometries of all proposed structures of lithium complexes showed that P( O)(O−) anion
moiety exists preferably in the delocalized form on three sites (O P O) that it interacts
strongly with Li+ ion forming chelating complex as suggested by the spectroscopic study.
The structures with Li+ ion bonded to a single oxygen atom were found to be unstable and
directly converted to the delocalized chelating complexes. Similar results were obtained
by Remko et al.,21 where bidentate (specific and not merely electrostatic) bonding of
Li+ and Na+ (oriented symmetrically) to the two oxygen atoms of carboxylates, sulfates,
phosphates, and phosphonates (Li+ . . . O distance of 1.858 Å) was found depending on
DFT calculations.
To decide which of the structures, III or IV, is the preferred one for complex 1,
the electronic energy of each possible structure was calculated at the B3LYP/6-31+G(d)
level of theory and is listed in Table 6. The results show that the electronic energies of the
two structures are relatively close to each other. However, structure III exhibits the largest
electronic energy, and hence is the most preferred one for complex 1, as also found in the
spectroscopic study.
The change in Gibbs free energy, δ G at 298 K, indicates that the tautomeriza-
tion of phenylphosphinic acid I to phosphorous acid II is a non-spontaneous process
(+5.67 kcal/mol in gas phase and 8.02 kcal/mol in methanol, Table 6) in gas phase as
well as in methanol. This means that structure I is the most favorable and stable tautomer.
Similar δ G298 value was reported for 2,4,6-trimethyl phenylphosphinic acid by Orthaber
et al.14 using DFT calculations, which was found to be +6.5 kcal/mol in the gas phase.
For a fluorinated derivative 2,3,5,6-tetrafluoro phenylphosphinic acid, a smaller value was
obtained (+1.9 kcal/mol).14

PREPARATION OF LITHIUM COMPLEXES WITH PHENYLPHOSPHINIC
573
CONCLUSIONS
The 1H, 13C, and 31P NMR spectra of the organophosphorus ligands 1a and 2a and
their lithium complexes indicate that phenylphosphinic acid tautomer C6H5PH( O)OH
predominates in deuterated methanol with PH/PD exchange. The phenylphosphinic acid
tautomer also predominates in the case of lithium complex in deuterated methanol but
without PH/PD exchange due to the formation of lithium chelating complexes. DFT cal-
culations support the predominance of phenylphosphinic acid tautomer in the case of the
free ligand and its lithium complex. Furthermore, DFT calculations show preference for
chelating lithium complexes with the ligands investigated. The bidentate (specific and not
merely electrostatic) bonding of Li+ with carboxylates, sulfates, phosphates, and phospho-
nates, which was suggested by Remko et al.21, was confirmed by the present paper based on
spectroscopic and DFT computational techniques. This type of bonding is very important
in biopolymers such as nucleotides and glycosaminoglycans.
EXPERIMENTAL
Preparation of Lithium Complex of Phenylphosphinic Acid
(Li(O2P(H)C6H5)) (1)
The complex Li(O2P(H)C6H5) (1) was prepared by mixing 50 mL of acetone (an-
alytical grade, Aldrich) solutions of 0.30 g (7.1 × 10−3 mol) of LiCl (analytical grade,
Aldrich) with 50 mL of a solution of 1.006 g (7.1 × 10−3 mol) of phenylphosphinic acid
HO2P(H)C6H5 (1a) (extra pure, Aldrich). The white precipitate was filtered off, washed,
and dried at room temperature; m.p. > 360◦C; elemental analysis: Calcd.: C, 48.67; H,
4.06; found: C, 48.59; H 4.04%.
Preparation of Lithium Complex of Phenylphosphonic Acid
Li(HO3PC6H5)·(H2O3PC6H5) (2)
The complex Li(HO3PC6H5)c˙(H2O3PC6H5) (2) was previously prepared by the re-
action of phenylphosphonic acid and Li2CO3 in water as a solvent.3 In the present paper the
complex Li(HO3PC6H5)c˙(H2O3PC6H5) (2) was prepared by mixing of 50 mL of acetone
solution of 0.30 g (7.1 × 10−3 mol) of LiCl with 50 mL of solution of 1.119 g (7.1 × 10−3
mol) of phenylphosphonic acid (H2O3PC6H5) (2a) (extra pure, Aldrich). The white pre-
cipitate was filtered off, washed, and dried at room temperature; m.p. > 360◦C; elemental
analysis: Calcd.: C, 44.70; H 4.03; found: C, 44.28; H, 4.08%. The structure of the complex
reported by Rao and Vidyasagar3 is in very good agreement with the obtained elemental
analysis of the complex prepared in this work.
1H, 13C, and 31P NMR spectra were obtained using a Bruker-Avance 400 MHz spec-
1
trometer operating at 400.1 MHz for H, at 100.8 MHz for 13C, and at 161.3 MHz for
1
31P. The chemical shifts are given in ppm relative to TMS in the case of H and 13C
and relative to 85% phosphoric acid in the case of 31P as external standards. All samples
were dissolved in deuterated methanol. IR spectra were recorded using a MAGNA-IR560
spectrometer (Nicolet Instrument Corporation). Elemental analysis was carried out us-
ing eurovector model E.A.3000 instrument employing thin sample tubes. Melting points

574
M. SUNJUK ET AL.
were measured with a SPM2 Stuart apparatus. All calculated NMR spectra were ob-
tained using ACD/ChemSketch (Advanced Chemistry Development Inc.; Ontario, Canada;
http://www.acdlabs.com).
DFT Calculations
The geometrical parameters of phenylphosphinic acid and that of lithium compounds
1 and 2 were fully optimized using the B3LYP method as implemented in the Gaussian 03
suite of programs.15 The density functional theory calculations (B3LYP) were performed
using two types of basis sets: 6-31G(d) and 6-31+G(d), where the first basis set has only
polarization function, and the second basis set has polarization and diffuse functions, which
are suitable and flexible to examine the geometrical structure of the organophosphorus
compounds investigated. To make sure that all of these structures represent a minimum, a
frequency test has been performed for each structure, where the electronic energy, including
the zero point correction, was calculated on the basis of the B3LYP/6-31+G(d) method.
These results were also used to determine the most favorable structures of the complexes.
In addition, the change in Gibbs free energy at 298 K, ꢀGo298, was calculated for the
tautomerization of phenylphosphinic acid I to phenylphosphonic acid II.
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