Two new supramolecular metal diphosphonates: Synthesis, characterization, crystal structure and inhibiting effects on metallic corrosion

Article abstract of DOI:10.1016/j.jssc.2016.07.027

Two new divalent metal(II) aminodiphosphonates with layered structure, namely, Cu(H₃L¹)₂·2H₂O (1), [H4L¹=methyl-N(CH₂PO₃H₂)₂] and Cd₂(H₂L²)₄(2), [H₄L²=n-propyl-N(CH₂PO₃H₂)₂] were synthesized and characterized. The Cu(II) ions in complex 1 are octahedrally coordinated by four oxygen atoms from two chelating ligands and two phosphonate oxygen atoms from two neighboring Cu(H₃L¹)₂ units. The Cu(H₃L¹)₂ units are interconnected by bridging phosphonate groups, forming a 2-D metal phosphonate layer. The structure of complex 2 contains two unique Cd(II) ions octahedrally-coordinated by six phosphonate oxygen atoms from four H₂L² diphosphonate anions. Corrosion inhibition performances of 1 and 2 were also compared with each other in order to study the effect of combinations of externally added Cd/H₄L² and Cu/H₄L¹ (1:1 ratio) on corrosion rates of carbon steel. It was found that at pH 3.0, Cd/H₄L² or Cu/H₄L¹ combinations do not have noticeable corrosion inhibition efficiency for carbon steel. In contrast, at pH 7.0, higher corrosion inhibition efficiency was achieved for Cd/H₄L². Physical characterizations such as scanning electron microscopy (SEM), Energy-dispersive X-ray spectroscopy (EDS) and Fourier transform infrared spectroscopy (FTIR) were applied to study the corrosion specimens and film material.

Full text of DOI:10.1016/j.jssc.2016.07.027

Author’s Accepted Manuscript
Two new supramolecular metal diphosphonates:
Synthesis, characterization, crystal structure and
inhibiting effects on metallic corrosion
Khodayar Gholivand, Rouhollah Yaghoubi, Alireza
Farrokhi, Shahram Khoddami
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http://dx.doi.org/10.1016/j.jssc.2016.07.027
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Received date: 2 May 2016
Revised date: 25 July 2016
Accepted date: 26 July 2016
Cite this article as: Khodayar Gholivand, Rouhollah Yaghoubi, Alireza Farrokhi
and Shahram Khoddami, Two new supramolecular metal diphosphonates
Synthesis, characterization, crystal structure and inhibiting effects on metallic
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Two new supramolecular metal diphosphonates: Synthesis, characterization, crystal structure
and inhibiting effects on metallic corrosion
Khodayar Gholivanda,*, Rouhollah Yaghoubia, Alireza Farrokhib, Shahram Khoddamic
aDepartment of Chemistry, Faculty of Science, Tarbiat Modares University, Tehran, Iran
bDepartment of Chemistry, Faculty of Science, University of Birjand, Birjand, Iran
cDepartment of Material Engineering, Tarbiat Modares University, Tehran, Iran
* Corresponding author. Tel.: +98 2182883443; Fax: +98 2182883455.
E-mail address: gholi_kh@modares.ac.ir (K. Gholivand).
1

Abstract
Two new divalent metal(II) aminodiphosphonates with layered structure, namely,
Cu(H3L1)2.2H2O (1), [H4L1= methyl-N(CH2PO3H2)2] and Cd2(H2L2)4(2), [H4L2= n-propyl-
N(CH2PO3H2)2] were synthesized and characterized. The Cu(II) ions in complex 1 are
octahedrally coordinated by four oxygen atoms from two chelating ligands and two
phosphonate oxygen atoms from two neighboring Cu(H3L1)2 units. The Cu(H3L1)2 units are
interconnected by bridging phosphonate groups, forming a 2-D metal phosphonate layer. The
structure of complex 2 contains two unique Cd(II) ions octahedrally-coordinated by six
phosphonate oxygen atoms from four H2L2 diphosphonate anions. Corrosion inhibition
performances of 1 and 2 were also compared with each other in order to study the effect of
combinations of externally added Cd/H4L2 and Cu/H4L1 (1:1 ratio) on corrosion rates of
carbon steel. It was found that at pH 3.0, Cd/H4L2 or Cu/H4L1 combinations do not have
noticeable corrosion inhibition efficiency for carbon steel. In contrast, at pH 7.0, higher
corrosion inhibition efficiency was achieved for Cd/H4L2. Physical characterizations such as
scanning electron microscopy (SEM), Energy-dispersive X-ray spectroscopy (EDS) and
Fourier transform infrared spectroscopy (FTIR) were applied to study the corrosion
specimens and film material.
Keywords: Metal phosphonates; Crystal structures; Corrosion; Inhibition ; Layered materials
2

1. Introduction
The chemistry of metal phosphonates is of a great deal of importance because of their
potential applications in catalysis, ion exchange, proton conductivity, intercalation chemistry,
photochemistry, and materials chemistry [1]. The structures of metal phosphonates are
mainly affected by factors such as the extent of deprotonation for the diphosphonate ligand,
the nature of metal ion, the nature of substitution in the ligand [2-11]. Studies of metal
complexes with aminodiphosphonic acids, RN(CH2PO3H2)2, have shown that the R
substituent on the amine group plays an important role on the metal phosphonates structure
[12–14]. So far, few works have been conducted on metal phosphonates based on alkyl-
substituted aminodiphosphonates. For the purpose of understanding the effect of substituent
R group on metal phosphonates structure with alkyl-substituted aminodiphosphonic acids,
two alkyl derivatives of RN(CH2PO3H2)2 were synthesized, namely, methyl-N(CH2PO3H2)2
(H4L1) and n-propyl-N(CH2PO3H2)2 (H4L2). Methyliminobis(methylenephosphonic acid)
(H4L1) has complexes Mn(H3L1)2.H2O, Cd(H3L1)2.H2O, Pb(H2L1), Cu3(H2O)2(HL1)2 .2H2O,
Zn3(HL1)2 and Zn(H2L1)(H2O) as reported in literature [15-18]. Among mentioned
complexes, the first four complexes exhibit layered structures while Zn3(HL1)2 form a 3-D
network and Zn(H2L1)(H2O) adapt a double chain structure. The complexes of Zn(H2L2) ,
Mn2[(HL2)(H2O)F].H2O and Cd2L2 have been synthesized under hydrothermal conditions for
H4L2 ligand [19,20]. The structural and coordination properties of copper(II) and cadmium(II)
complexes with H4L1 and H4L2 ligands, under mild reaction conditions, have not been
reported yet. The above mentioned reactions leads to preparation of two new metal
aminodiphosphonates with slightly condensed structure, namely, Cu(H3L1)2.2H2O (1), and
Cd2(H2L2)4(2).
Generally, phosphonate additives have been widely applied as inhibitors against corrosion
[21, 22]. Phosphonates, blended with certain metal cations and polymers, reduce the optimal
3

inhibitor concentration needed for inhibition regarding their synergistic effects of dissolved
M2+ and polyphosphonates [23]. Synergism plays an important role in the inhibition process
serving as the basis to develop new corrosion inhibitor formulations. Despite the significant
body of literature, the molecular identity of the thin protective metal phosphonate films has
not been fully understood.
Herein, we study the synthesis, crystal structure, and physical characterization of 1 and 2 and
their application as inhibitors for the carbon steel corrosion.
2. Experimental
2.1. Materials and Physical techniques
All chemicals were of reagent grade quality and obtained from commercial sources without
further purification. IR spectra of these compounds were recorded with a Shimadzu model
IR-460 spectrometer using KBr pellets in the range 4000-400 cm–1. The elemental analyses
1
13
31
were performed with a Heraeus CHN-O-RAPID elemental analyzer. H, C, and P NMR
spectra of the ligand were recorded in D2O with a Bruker (Avance DRS) 500 spectrometer.
For the 31P solution NMR spectra in D2O 85 % H3PO4 was applied as standard reference.
TGA data were recorded with a Perkin–Elmer Pyris Diamond TG/DTA thermal analysis
system in an oxygen atmosphere with a heating rate of 10 K.min–1. Melting points were
obtained with an Electrothermal instrument. The samples were characterized by a scanning
electron microscope (SEM) Philips XL30 and S-4160. Single crystal of 1 was mounted on a
Bruker Smart CCD-1000 using Mo-Kα radiation (λ= 0.71069 Å) and a graphite
monochromator at 120(2) K. All data sets were corrected for Lorentz and polarization factors
as well as for absorption using the SADABS program or a multiscan method [24]. The
structure was solved by direct methods and refined by full-matrix least-squares fitting on F2
by SHELXTL version 5.10 [25]. All non-hydrogen atoms were refined by anisotropic thermal
4

parameters. The hydrogen atoms were located at the geometrically calculated positions and
refined with isotropical thermal parameters. Single crystal of 2 was mounted on a Xcalibur,
Atlas, Gemini ultra using Cu-Kα radiation (λ= 1.54180 Å) at 120(2) K. The structure was
solved by direct methods using SHELXS-97 and refined by full-matrix least-squares fitting
on F2 by SHELXL-97 [26]. Some of the data collection and refinement parameters are
summarized in table 1. Selected bond lengths and angles are given in the table 2 of
Supplementary Material.
2.2. Syntheses
2.2.1. Synthesis of H4L1 and H4L2
The aminodiphosphonic acids, H4L1 and H4L2, were prepared by a Mannich type reaction
according to procedures previously described [27]. For preparation of the H4L2 , n-propyl
amine (Merck, 83 mmol) was mixed with hydrochloric acid (Merck, 30 cm3), deionized water
(30 cm3), and phosphorous acid (Fluka, 249 mmol). The mixture was allowed to reflux at 120
°C for 1 h, then paraformaldehyde (Aldrich, 332mmol) was added in small portions over a
period of 1 h, and the mixture was then refluxed for an additional 1 h. Removal of solvents
afforded 18.15 g of a white powder of n-propyliminobis(methylenephosphonic acid) (yield 93
%). Its purity was confirmed by NMR measurements and elemental analysis. 31P NMR shows
only one single peak at 8.14 ppm. 1H NMR: 0.78 ppm (CH3,t, 3H), 1.61 ppm (CH2, m, 2H),
3.28 ppm (CH2, m, 2H), 3.40 ppm (-CH2-, d, 4H, JH-P = 12.5 Hz). Anal. Calcd. for
C5H15NO6P2:C, 24.30; H, 6.12; N, 5.67. Found: C, 24.25; H, 6.06; N, 5.61 .
2.2.2. Syntheses of Cd-H4L2 and Cu-H4L1 at pH 3.0
2.2.2.1 Synthesis of Cu(H3L1)2.2H2O (1)
5

A quantity of hydrated copper chloride (4.39mmol) is dissolved in deionized water
(25mL) and H4L1 (4.39mmol) is added to it dropwise under vigorous stirring. Solution pH is
adjusted to 3.0 with 1.0 M NaOH. The clear, slightly yellow solutions are stored at ambient
temperature. . Large octahedral blocks crystallize after 7 days. (yield: ca. 62% based on Cu).
m.p. 250 °C. Anal. Calcd. for C6H24CuN2O14P4(%): C, 13.45; H, 4.52; N, 5.23. Found: C,
13.40; H, 4.48; N, 5.29. IR (KBr, cm–1): ν = 3417 (m), 3023 (m), 1658 (m), 1473 (m), 1211
(s), , 1115 (s), 1004 (s), 721 (m), 585 (m).
2.2.2.2 Synthesis of Cd2(H2L2)4(2)
A quantity of hydrated Cadmium chloride (4.39mmol) is dissolved in deionized water
(25mL) and H4L2 (4.39mmol) is added to it dropwise under vigorous stirring. Solution pH is
adjusted to 3.0 with 1.0M NaOH. The clear, slightly yellow solutions are stored at ambient
temperature. Large octahedral blocks crystallize after 4 days. (yield: ca. 71 % based on Cd).
m.p. 293 °C. Anal. Calcd. for C20H52Cd2N4O24P8(%): C, 19.91; H, 4.31; N, 4.64. Found: C,
19.95; H, 4.26; N, 4.67. IR (KBr, cm–1): ν = 3421 (m), 3053 (m), 2358 (w), 1598(s), 1423
(m), 1131 (s), 993 (s), 784 (m), 581 (m).
2.2.3. Syntheses of Cd-H4L2 and Cu-H4L1 at pH 7.0
The same procedures as given above were followed except that the solution pH was adjusted
to 7.0 with 1.0 M NaOH and immediate precipitation of solids is observed. Yield is 70% for
Cu-H4L1 IR (KBr, cm–1): ν = 3435 (s), 3092 (w), 1676 (m), 1641(m), 1433 (w), 1291 (m),
1125 (s), 1071 (s),
1000 (s), 945 (s), 811 (m), 624 (m), 550 (m), 478 (s)
for Cd-H4L2 (yield: 80 %) IR (KBr, cm–1): ν = 3450 (w), 3053 (m), 2400 (m), 1641(m), 1459
(m), 1115 (s), 1024 (s), 928 (s), 764 (m), 576 (s), 454 (m).
6

These products are amorphous based on powder XRD measurements (Fig. S1).These are not
the same compounds as those synthesized at pH 3.0 and have the different molecular
structures.
2.3 Corrosion Inhibition Protocol
Corrosion gravimetric experiments were accomplished in order to study the nature of the
anticorrosion film on steel surface. For mass loss measurement, corrosion test were carried
out using rectangular coupons measuring 1.5×6×0.2 cm of carbon steel C1010. All
experimental procedures such as preparation and cleaning were carried out based on ASTM
G31 nom [28].
Each specimen immersed in blank (no inhibitor) or test solution (450 mL).All experiments
performed at pH 3 and 7 for H4L1 and H4L2 at 3 different concentrations (0.5, 1 and 5mM).
Synergism effect of Cd+ H4L2 and Cu+ H4L1 with 1:1 molar ratio at 1mM concentration was
investigatedat pH 3 and 7.
Corrosion process was monitored by visual inspection for five days. Then, the specimens are
bring out from solution and protective film which is formed at pH=7 and in presences of
(metal+H4L) materials were collected and subjected to FT-IR. Moreover, surface analysis
was investigated through SEM and EDS.
3. Results and discussion
3.1 Crystal Structures of Metal Phosphonates
Reaction of methyliminobis(methylenephosphonic acid) (H4L1) with copper(II) chloride
yields a layered metal phosphonate, Cu(H3L)2.2H2O, 1, which has a M/L ratio of 1:2. The
asymmetric unit of complex 1 contains one Cu(II) ion, two ligands, and two water molecules
(Figure 1). Cu atom lies on center of symmetry and is octahedrally coordinated by four
oxygen atoms from two chelating ligands and two phosphonate oxygen atoms from two
7

neighboring Cu(H3L)2 units. Cu1 exhibits ‘‘4+‏ 2’’ Jahn–Teller distorted geometry through
coordination to six phosphonate oxygen atoms (2O(1), 2O(4), and 2O(6)) from four different
ligands. The basal plane of the polyhedron is defined by the atoms 2O(1), and 2O(4). The Cu-
O bond distances involving these oxygens are in the range of 1.951-1.952 Å. The axial Cu1–
O6 distances are 2.566 Å. Three trans angles are exactly 180° and the cis angles are in the
range of 87.18(9)–92.82(9)°. Each ligand acts as a tridentate ligand by chelating with one
copper atom and bridging with another Cu(II) ion, thus forming an eight-membered chelate
ring. This ring is composed of Cu1, O1, P1, C1, N1, C2, P2, O4 atoms (Figure 1). The
nitrogen atom and both phosphonate groups of the ligand are 1H-protonated, thus each ligand
carries a negative charge of one, and two ligands are needed to balance the two positive
charges of a Cu(II) ion. The lattice water molecules (O1W) are not coordinated to the Cu(II)
ions, but form strong hydrogen bonds with phosphonate oxygen atoms (O1W…O2 2.830 Å,
O1W…O3 2.674 Å, and O1W…O5 2.528 Å) (Table 2). The Cu(H3L)2 units are
interconnected by bridging phosphonate groups, forming a 2D metal phosphonate layer along
the (200) plane (Figure 2(a)). The bridging phosphonate groups (P2, O4, O5, O6) form 16-
membered rings (4Cu1, 4P2, 4O4, 4O6) with copper atoms separation of 6.76 Å (Figure
2(b)). The unidentate-coordinating phosphonate groups (P1, O1, O2, O3) are attached above
and below the plane. The methyl groups are oriented toward the cavity created by the 16-
membered rings as shown in figure 2(a). Hydrogen bonds are formed between phosphonate
oxygen atoms within a layer (O2…O6 2.570 Å), as well as between nitrogen atoms and
phosphonate oxygen atoms within the layer (N1-H1...O1 2.783 Å (166°), N1- H1…O4 3.067
Å (122°)) (Table 2).
These metal phosphonate layers are connected only by lattice water molecules via hydrogen
bonds discussed above and form a 3-D supramolecular hydrogen-bonded network (Figure 3).
8

Upon reaction with copper(II) chloride under weak acidic conditions at room temperature,
one more proton on another phosphonate group is deprotonated; thus a complex with 1:2
(M/L) is formed. The ligand adopts a tridentate chelate and a bridging coordinate mode to
form a layer structure. The uncoordinated phosphonate oxygens are available for intra- and
interlayer hydrogen bonding.
Compound 2 crystallizes in the Triclinic space group Pī. The asymmetric unit of 2 with
z=1 contains two unique Cd(II) ions and four different partially deprotonated H2L2 anions
(Fig. 4). As shown in fig. 4, each two Cd (II) ions are octahedrally-coordinated by six
phosphonate oxygen atoms from four diphosphonate anions. The Cd–O distances range from
2.237(5) to 2.339(4) Å (Table 2) which are comparable to those reported for other Cd(II)
aminodiphosphonates [19,20]. The trans angles are exactly 180° and the cis angles are in
83.20(2)–96.80(2)° range. The H2L2 anion chelates with a Cd (II) ion bidentately and also
bridges with other Cd(II) ion, thus forming an eight-membered chelate ring (Cd–O–P–C–N–
C–P–O). Three phosphonate oxygen atoms (O1, O2 and O4) are involved in metal
coordination and atoms (O3, O5 and O6) remain non-coordinated.
The interconnection of the Cd(II) ions via bridging diphosphonate ligands leads to a 2D metal
phosphonate layer along the (001) plane (Figure 5). Within the 2D metal phosphonate layer,
there are 16-member atomic rings composed of four Cd(2Cd1, 2Cd2) and four phosphonate
anions. The propyl groups of the ligands are orientated toward the cavity created by the 16-
membered rings as shown in figure 5. Within these layers, hydrogen bonds are formed
between phosphonate oxygen atoms (O9…O1 2.642Å, O6…O8 2.721Å, O12…O3 2.492Å),
as well as between nitrogen and phosphonate oxygen atoms within the layer (N1-H1...O10
2.832Å, N2- H2…O4 2.721Å) (Table 2).
9

Between two adjacent layers, hydrogen bonds are formed via non-coordinated phosphonate
oxygen atoms (O5…O11 2.512Å), and form a 3-D supramolecular hydrogen-bonded network
(Figure 6). The propyl groups of the phosphonate ligands occupy the interlayer space.
3.2 IR spectra and Thermogravimetric analyses
IR spectra for the two compounds 1 and 2 were recorded between 4000 and 400 cm–1 (Fig.
S2). The IR spectrum of 1 shows an intense and broad band in the O-H stretching vibration
region at 3500-3200 cm–1, which is consistent with the presence of the lattice water and the
hydrogen bonds. The bonds at 2956 cm–1 for 1 and 2978 cm–1 for 2 are due to the CH2
stretching vibrations. The band at 2760 cm–1 for 2 is likely due to ν(P-OH), which is
characteristic of hydrogen phosphonate [29]. The bending vibration of the tetrahedral O3PC
groups for 1 is found at 573 cm–1. The compounds also show typical C-H stretching
vibrations and bending vibrations of low intensities of CH2 attached to the nitrogen atoms,
near 2700 and 1440 cm–1, respectively. This complexes show a series of strong bands in the
1200-900 cm–1 region, which are attributed to the stretching vibrations of the tetrahedral C–
PO3 groups [30]. Additional medium and sharp bands at low energy (between 800 and 400
cm–1) are found, probably due to bending vibrations of the tetrahedral O3PC groups and M–O
(M=Cu and Cd) stretching vibrations.
A TGA diagram of complex 1 (Fig. S3) shows three major weight losses. The first step is the
loss of two lattice water molecules, started at 120 °C. The observed weight loss at 190 °C
(7.27%) is in agreement with the calculated value of 6.72%. The second weight loss of 5.54%
from 233 to 290 °C corresponds to the pyrolysis of the methyl group of the ligand
(calculated: 6.60 %). The combustion of the remaining organic parts of the ligand starts at
300 °C. The final product is mainly Cu(PO3)2, a metal metaphosphate, and the calculated
weight loss of 28.96% is in agreement with the observed one (29.59%). The TGA of 2 shows
10

that it is stable up to 300°C. Above this temperature, it gradually loses weight until 800°C.
The total weight loss (65.8%) corresponds to burning of the organic ligands in good
agreement with the calculated value (66.7%). The final product is assumed to be mixture of
Cd(PO3)2 and CdO.
3.3 Corrosion Studies
Phosphonates are mostly known for their antiscaling and antifouling properties [31].
However, some literatures are introduced synergistic effect of phosphonates with metal ions
as carbon steel corrosion inhibitor. at first, corrosion experiments were conducted to
investigate corrosion inhibition effect of H4L2 and H4L1 on carbon steel specimens at pH 3
and 7. This experiment was carried out in order to determine the optimum concentration of
both inhibitors at two pH conditions (Table 3).
Figure 7 demonstrates the concentration effect of H4L2 on corrosion inhibition of carbon steel
at pH 7. Based on mass loss measurements, corrosion rate for Blank specimen (no inhibitor)
was 0.564 mm/y, whereas for specimen containing 1mM H4L2 was 0.300 mm/y which means
46.8% corrosion inhibition efficiency for H4L2 at this concentration. Decreasing
concentration of H4L2 to 0.5 mM decreases the performance (0.461mm/y, 18.3% inhibition
efficiency). It should be noted that an increase in H4L2 to 5mM adversely affect inhibition
efficiency and causes metal dissolution (0.620 mm/y). Table 3 shows corrosion rates for H4L2
and H4L1 at pH 3 and 7, it is obvious that pH has a profound role in carbon steel corrosion. A
decrease of 4 pH units causes about three-fold increase in the corrosion rate (from 0.564 to
1.832 mm/y for Blank specimen). Based on these results, 1 Mm H4L was chosen as the
optimum concentration and synergistic effect of Cd2+-H4L2 and Cu2+-H4L1 in 1:1 molar ratio
was investigated through mass loss measurement (Table 3 and Figure 8).
11

Initially, corrosion experiments were performed at pH 3.0 (under identical conditions used to
prepare crystalline M-H4L at pH 3). Based on mass loss measurement, the corrosion rate for
the Blank sample (no inhibitor) was 1.832 mm/y, whereas it was the 1.982 and 2.187 mm/y
for Cd-H4L2 and Cu-H4L1, respectively (Table 3). Corrosion rates in the presence of
inhibitors are greater than those for blank sample (Figure 8, left). It can be explained by
several statements. First, metal phosphonates (Cd-H4L2 and Cu-H4L1) are too soluble to
deposit onto the metal surface; therefore they do not form a protective film layer on carbon
steel surface. Second, H4L2 and H4L1 react preferentially with Fe-oxide layer which exists on
carbon steel surface as it immerses in the solution, before they interact with soluble Cd2+ or
Cu2+. Another presumption is that Cd-H4L2 and Cu-H4L1 compounds that may form in
solution never reach the steel surface as they undergo bulk precipitation. Since no precipitate
forms in the bulk in our corrosion experiment at pH 3.0, this assumption is not acceptable.
Due to the lack of performance of Cd-H4L2 and Cu-H4L1 as corrosion inhibitors at pH 3.0, no
further experiments were performed at this pH.
Synergistic combination of metal ions and H4L materials at pH 7.0 were investigated through
mass loss measurements (Figure 8, right). To further characterization of the corrosion, the
specimens and film materials were studied by SEM, EDS and FT-IR techniques. Although
diffraction between the Blank and metal-H4L specimens was evident within the first hours,
the corrosion experiment was left to proceed over a 5 days period.
A combination of Cd2+ and H4L2 in a 1:1 molar ratio offers excellent corrosion protection for
carbon steel. Based on mass loss measurement, the corrosion rate for the blank specimen was
0.564 mm/y, whereas for the Cd2+-H4L2, it was 0.005 mm/y which means 99.1% inhibition
efficiency (Figure 8, specimen 4, 5). It can be observed from SEM micrograph that carbon
steel surface was strongly damaged in the absence of the inhibitor (Figure 9-A). However,
figure 9-B shows carbon steel surface and protective film are adsorbed on specimen surface
12

which is responsible for inhibition of corrosion with Cd2+-H4L2. The corresponding EDS
spectra shows that the inhibiting film is a material containing Cd (Cd from externally added
salt) and P (from added H4L2) in an approximately 1:1 ratio. Fe was also present originating
from the carbon steel surface. Further study of the protective film was carried out using FT-
IR spectroscopy. Fig. S4 shows FT-IR spectra of the protective film (from a corrosion
specimen with Cd2+-H4L2 at pH 7) and Cd2+-H4L2 that synthesized at pH 7, a similar FT-IR
spectrum was obtained. There is an excellent agreement between two spectra.
Figure 8, specimen 6 shows synergism effect of Cu-H4L1 at pH 7. Based on mass loss
measurement, the corrosion rate for this specimen was 0.631 mm/y, whereas for blank
specimen was 0.564 mm/y. Therefore, Cu-H4L1 essentially enhances the dissolution of bare
metal. None of assumptions that mentioned at pH 3 are acceptable here; Since, SEM image
and corresponding EDS spectra prove that protective film was covered carbon steel surface at
pH 7 (Figure 9-C). Another assumption would be that metal phosphonate film (Cu-H4L1) was
nonuniform and porous in microscopic nature. This would lead to localize attack and metal
pitting. Metal pits are obvious in SEM image (Figure 9-C) which are shown by black arrows
in (Figure 8). Fig. S5 shows FT-IR spectra of the protective film (from a corrosion specimen
with Cu-H4L1 at pH 7) and Cu-H4L1 that was synthesized at pH 7. The FT-IR spectra in Fig.
S5 indicates that the molecular structure of the filming material is not the same as the
synthesized compound at pH 7.
Comparison of the FT-IR spectra in Figures (S4, S5) and those (for 1 and 2) in Figure S2
indicates that the molecular structure of the filming material is not the same as the structurally
characterized materials 1 and 2. Several attempts have devoted in order to synthesize Cd-
H4L2 and Cu-H4L1 crystalline materials at pH 7 , instead, powder products could be obtained.
As shown by XRD analyze, these compounds are amorphous (Figure S1).
13

4. Conclusions
Two new Cu(II) and Cd(II) compounds were synthesized under mild reaction conditions at
room temperature with two different layered structures. The results indicated that the mild
reaction conditions result in a lower condensed structure. The Cu(II) ions in complex 1 are
octahedrally coordinated by four oxygen atoms from two chelating ligands and two
phosphonate oxygen atoms from two neighboring Cu(H3L1)2 units, forming a 2-D metal
phosphonate layer. The structure of complex 2 contains two unique Cd(II) ions which are
octahedrally-coordinated by six phosphonate oxygen atoms from four H2L2 diphosphonate
anions. The corrosion inhibition performances of the two synthesized complexes were
compared with each other. It was found that at pH 3.0, the combinations of Cd/H4L2 or
Cu/H4L1 (1:1 ratio) are ineffective corrosion inhibitors for carbon steel while a significant
corrosion inhibition efficiency was achieved at pH 7.0 for Cd/H4L2.
Supporting information available
TGA diagrams of compounds 1 and 2, XRD powder patterns and FT-IR spectra (in KBr
pellets) of corrosion films and compounds 1 and 2.
Acknowledgement
Financial support of this work by Tarbiat Modares University is gratefully acknowledged.
14

Appendix A. Supplementary material
Crystallographic data for the structure of 1 and 2 reported in this paper have been deposited
with the Cambridge Crystallographic Data center, CCDC Number 763103 for 1 and 1061932
for 2. Copies of this information may be obtained free of charge from The Director,
CCDC,12 Union Road, Cambridge, CB21EZ, UK (Fax:+44-1223-336-033;e-mail:
deposit@ccdc.cam.ac.uk).
References
[1] A. Clearfield, K. Demadis, Metal Phosphonate Chemistry: From Synthesis to
Applications, Eds, RSC Publishing, Cambridge, UK. (2012).
[2] (a) A. Clearfield, A.K. Cheetham, G. Ferey, T. Loiseau, Angew. Chem. Int. Ed., 38
(1999) 3268; (b) A. Clearfield, J. Zhu, X. Bu, P. Feng, G.D. Stucky, J. Am. Chem. Soc.,
122 (2013) 11563.
[3] (a) J.G. Mao, Z.K. Wang, A. Clearfield, Inorg. Chem. 41 (2012) 3713; (b) J.G. Mao, A.
Clearfield, Inorg. Chem. 41 (2002) 2334; (c) J.G. Mao, Z.K. Wang, A. Clearfield, Inorg.
Chem. 41 (2002) 6106; (d) J.G. Mao, Z.K. Wang, A. Clearfield, J. Chem. Soc. Dalton
Trans. (2002) 4457.
[4] B. Zhang, A. Clearfield, J. Am. Chem. Soc. 119 (1997) 2751.
[5] Y.D. Chang, J. Salta, J. Zubieta, Angew. Chem. Int. Ed. 33 (1994)325.
[6] O.R. Evans, H.L. Ngo, W.B. Lin, J. Am. Chem. Soc. 123 (2014)10395.
[7] (a) N. Stock, S.A. Frey, G.D. Stucky, A.K. Cheetham, J. Chem. Soc., Dalton Trans.
(2000) 4292; (b) N. Stock, T. Bein, J. Solid State Chem.167 (2002) 330.
[8] M.A. Petruska, B.C. Watson, M.W. Meisel, D.R. Talham, Chem.Mater. 14 (2002) 2011.
15

[9] E.K. Brechin, R.A. Coxall, A. Parkin, S. Parsons, P.A. Tasker, R.E.P. Winpenny, Angew.
Chem. Int. Ed. 40 (2001) 2700.
[10] A. Cabeza, X. Ouyang, C.V.K. Sharma, M.A.G. Aranda, S. Bruque, A. Clearfield, Inorg.
Chem. 41 (2012) 2325.
[11] D.M. Poojary, B. Zhang, A. Clearfield, J. Am. Chem. Soc. 119 (1997)12250.
[12] (a) Z.M. Sun, B.P. Yang, Y.Q. Sun, J.G.Mao, A. Clearfield, J. Solid State
Chem. 176 (2003) 62; (b) Z.M. Sun, J.G. Mao, Y.Q. Sun, H.Y. Zeng, A. Clearfield, New
J. Chem. 27 (2003) 1326.
[13] B.I. Makaranets, T.N. Polynova, N.D. Mitrofanova, M.A. Porai-Koshits, J. Struct.
Chem. 32 (1991) 94.
[14] (a) S.M Ying, J.G Mao Mao, J. Mol. Struct. 748 (2005) 63; (b) S.M Ying, J.G Mao Mao,
Eur. J. Inorg. Chem. (2004) 1270.
[15] J.-G. Mao, Z.-K. Wang, A. Clearfield. Inorg. Chem. 41(2002)2334.
[16] J.-G. Mao, Z.-K. Wang, A. Clearfield. Dalton Trans. (2002)4457.
[17] J.-G. Mao, Z. Wang, A. Clearfield. Inorg. Chem. 41(2002)6106.
[18] K. Gholivand, R. Yaghoubi, S. Farshadian, Z. Hosseini, J. Coord. Chem. 64(2011)4201.
[19] L. Meng, Z. Sun, D.Dong, H.Chen, J. Coord. Chem. 60(2007)2075.
[20] L. Meng, Z. Sun, D.Dong, H.Chen, J. Coord. Chem. 61(2008)2478.
[21] E. Barouda, K. D. Demadis, S.Freeman, F. Jones, Cryst. Growth Des. 7(2007), 321.
[22] K. D. Demadis, In Solid State Chemistry Research Trends; Buckley, R. W., Ed.; Nova
Science Publishers: New York. (2007) 109.
[23] N.Palaniswamy, R. Natarajan, B. V. Appa Rao, Anti-Corros. Methods Mater. 46 (1999)
23.
[24] Sheldrick, G. M. SADABS v.2.01, Bruker/Siemens Area Detector Absorption
Correction Program, Bruker AXS, Madison, WI, USA (1998).
16

[25] Sheldrick G. M. SHELXTL v. 5.10, Structure Determination Software Suite, Bruker
AXS, Madison, WI, USA (1998).
[26] Sheldrick G. M. Programs SHELXS97 (crystal structure solution) and
SHELXL97(crystal structure refinement),University of Gottingen, Germany (1998).
[27] K. Moedritzer, R.R. Irani, J. Org. Chem. 31(1966)1603.
[28] ASTM Standard G31, Standard Practice for Laboratory Immersion Corrosion Testing of
Metals, ASTM International, West Conshohocken, (2004)0302.
[29] E. Matczak-Jon, B. Kurzak, A. Kamecka, W. Sawka-Dobrowolska, P. Kafarski, J.
Chem. Soc., Dalton Trans. (1999)3627.
[30] L.C. Thomas. Interpretation of the Infrared Spectra of Organophosphorus Compounds,
Hyden & Son Ltd., London (1974).
[31] K. D. Demadis, S. D. Katarachia, Phosphorus Sulfur Silicon Relat.Elem. 179 (2004)
627.
17

Table legends
Table 1. Crystal data and structure refinement for complex 1 and 2.
Table 2. Selected bond lengths (Å), angles (°) and hydrogen bonds for compound 1 and 2.
Table 3. Corrosion rates and inhibition efficiency of carbon steel in presence and absence of
H4L or M2+-H4L combination obtained from weight loss measurements.
Figure captions
Fig. 1 ORTEP representation of the asymmetric unit of 1. Thermal ellipsoids are drawn at
50% probability.
Fig. 2 (a) Ball and stick representation of a copper(II) phosphonate layer along the [200]
plane. (b) One 16-membered ring of (-Cu-O-P-O-)4 of 1. The dotted lines indicate hydrogen
bonds.
Fig. 3 View of 3-D crystal structure of complex 1 along the b axis. The dotted lines indicate
hydrogen bonds.
Fig. 4 ORTEP representation of the asymmetric unit of 2. Thermal ellipsoids are drawn at
50% probability.
Fig. 5 The 2D metal phosphonate layer along the (001) plane of complex 2.
Fig. 6 View of 3-D crystal structure of complex 2 along the a axis. The dotted lines indicate
hydrogen bonds.
Fig. 7 H4L2 concentration effect on corrosion inhibition of carbon steel at pH=7. The
specimen corresponding to 5mM H4L2 has higher corrosion rate than the specimen which
immersed in Blank solution (no inhibitor). This indicates metal dissolution at this
concentration.
Fig. 8 Effect of metal-H4L films on carbon steel corrosion behavior at pH 3 and 7. Specimen
5 show synergistic inhibition effect of Cd and H4L2 compared to Blank (no inhibitor)
18

specimens 4. Specimen 3 and 6 have higher corrosion rate than Blank specimens which
indicate metal dissolution.
Fig. 9 SEM micrograph and corresponding EDS spectra of carbon steel surface after
immersion in a solution with pH 7 for 5 days. A) Blank (no inhibitor), B) Cd-H4L2, C) Cu-
H4L1.
Supporting information
Fig. S1 XRD powder patterns of a) Cu(H4L1) As-synthesized at pH 7 and b) Cd(H4L2) As-
synthesized at pH 7.
Fig. S2 FT-IR (in KBr pellets) of compounds a) 1 and b) 2.
Fig. S3 TGA diagrams of compounds 1 and 2.
Fig. S4 a) FT-IR (in KBr pellets) of Cd(H4L2) corrosion film at pH 7 b) FT-IR of Cd(H4L2)
As-synthesized at pH 7.
Fig. S5 a) FT-IR (in KBr pellets) of Cu(H4L1) corrosion film at pH 7 b) FT-IR of Cu(H4L1)
As-synthesized at pH 7 .
19

Table 1. Crystal data and structure refinement for complex 1 and 2.
Compound
1
2
Molecular formula
Formula weight
Crystal color, habit
Dimensions, mm
Crystal system
a, Å
C6H24N2O14P4Cu
535.69
Light blue
0.2 × 0.20 × 0.05
Monoclinic
10.150(1)
C20H52Cd2N4O24P8
1205.24
Colorless
0.242×0.13 ×0.034
Triclinic
9.5742(4)
b, Å
8.745(6)
9.7009(4)
c, Å
α, deg
10.313(8)
90
12.3625(6)
69.400(4)
β, deg
102.661(3)
90
893.2(8)
P 21/c
2, 1.992
73.961(4)
89.811(4)
1027.31(8)
Pī
1, 1.942
γ, deg
V, Å3
space group
Z, Calculated density (mg.m-3)
F(000)
550
604.0
θ range, deg
Absorption coefficient, mm-1
T, K
3.09 - 29.00
1.656
120(2)
3.98- 67.03
12.056
120(2)
λ, Å
0.71073
1.54180
Reflections collected
Independent reflections
Final R indices
R indices (all data)
GOF on F2
9565
3644
2371 [R(int) = 0.0451]
R1 = 0.0470, wR2 = 0.0900
R1 = 0.0630, wR2 = 0.0955
1.009
2999 [R(int) = 0.0624]
R1 = 0.0467, wR2 = 0.1224
R1 = 0.0589, wR2 = 0.1305
1.065
peak, hole /eÅ-3
1.404 , -0.586
2.085, -1.278
20

Table 2. Selected bond lengths (Å), angles (°) and hydrogen bonds for compound 1 and 2.
complex 1
Cu(1)-O(1)
Cu(1)-O(1)#1
Cu(1)-O(4)
Cu(1)-O(4)#1
Cu(1)-O(6)
1.952(2)
1.952(2)
1.951(2)
1.951(2)
2.566(2)
2.566(2)
P(1)-O(2)
P(1)-O(1)
P(1)-O(3)
P(2)-O(5)
P(2)-O(4)
P(2)-O(6)
1.577(2)
1.527(2)
1.474(2)
1.503(2)
1.563(2)
1.514(2)
O(4)-Cu(1)-O(4)#1
O(4)-Cu(1)-O(1)#1
180.0
87.18(9)
O(4)#1-Cu(1)-O(1)#1
O(4)-Cu(1)-O(1)
92.82(9)
92.82(9)
87.18(9)
180.0
O(4)#1-Cu(1)-O(1)
O(1)#1-Cu(1)-O(1)
Cu(1)-O(6)#1
Hydrogen Bond of 1
O(2)-H(2O)...O(6)#5
O(5)-H(5O)...O(1W)#1
2.570(3)
2.528(3)
3.067(4)
172
171
122
N(1)-H(1N)...O(1)#3
O(1W)-H(1W)...O(3)#4
O(1W)-H(2W)...O(2)#6
2.783(4)
2.674(3)
2.830(3)
166
165
158
N(1)-H(1N)...O(4)#2
complex 2
Cd(1)-O(1)
2.288(4)
2.285(5)
2.244(5)
2.237(5)
2.240(3)
2.339(4)
P(1)-O(1)
P(1)-O(2)
P(1)-O(3)
P(2)-O(4)
P(2)-O(5)
P(2)-O(6)
1.521(4)
1.492(5)
1.547(5)
1.511(4)
1.483(5)
1.568(5)
O(1)-Cd(1)-O(7)
O(1)-Cd(1)-O(10)
88.9(1)
83.2(2)
Cd(1)-O(7)
Cd(1)-O(10)
O(10)-Cd(1)-O(1)
O(2)-Cd(2)-O(4)
O(2)-Cd(2)-O(8)
O(4)-Cd(2)-O(8)
96.8(2)
91.7(2)
96.2(2)
88.5(2)
Cd(2)-O(2)
Cd(2)-O(4)
Cd(2)-O(8)
Hydrogen Bond of 2
O(9)-H(9O)...O(1)
N(2)-H(2N)...O(4)#3
O(6)-H(6O)...O(8)#1
2.642(5)
2.721(9)
2.721(5)
165(8)
151(9)
164(8)
O(3)-H(3O)...O(12)
O(11)-H(11O)...O(5)#2
N(1)-H(1N)...O(10)
2.492(5)
2.512(5)
2.832(8)
164(9)
167(7)
171(7)
Symmetry transformations used to generate equivalent atoms For 1 : #1 -x+1,-y+1,-z+1; #2 -x+1,y-1/2,-z+3/2;
#3 x,-y+1/2,z+1/2; #4 -x+2,-y+1,-z+1; #5 -x+1,y+1/2,-z+3/2 ; #6 x,y,z For 2 : #1 –x, –y + 2, –z; #2 –x + 1, –y
+ 2, –z – 1; #3 x, y + 1, z.
Table 3- Corrosion rates a and inhibition efficiencyb of carbon steel in presence and absence
of H4L or M2+-H4L combination obtained from weight loss measurements
H4L or metal concentration Corrosion rate (mm/y)
+ H4L material (mM)
pH=3.0
1.832
1.702
1.522
1.910
1.982
1.676
1.502
2.037
2.187
pH=7.0
0.564
0.461
0.300
Blank
H4L2
0.5
1
7.1% inhibition
16.9% inhibition
18.3% inhibition
46.8% inhibition
Metaldissolution
99.1% inhibition
14.5% inhibition
37.4% inhibition
metal dissolution
metal dissolution
5
metal dissolution 0.620
metal dissolution 0.005
Cd+ H4L2
H4L1
1c
0.5
1
8.5% inhibition
18% inhibition
0.482
0.353
5
metal dissolution 0.590
metal dissolution 0.631
Cu+ H4L1
1c
aThe corrosion rate is calculated from the equation CR= [(8.76×104)(mass loss)]/[(area)(time)(metal density)].
Units: CR in mm/y, mass loss in g, area in cm2, time in h, metal density= 7.86 g/cm3.
b Inhibition efficiency is defined as [(CRBlank-CRinhibited)/CRBlank]×100
c Concentration refers to [M2+] and [H4L] that are added in 1:1 molar ratio
21

Fig. 1
(a)
(b)
Figure 2.
22

Fig. 3
Fig. 4
23

Fig. 5
24

Fig. 6
25

Fig. 7
‏
‏
26

Fig. 8
27

A)
B)
C)
Fig. 9
28

Graphical abstract
Two new metal phosphonates have been synthesized and characterized by single-crystal X-ray diffraction
and thermogravimetric analysis. corrosion inhibition performances 1 and 2 are also compared.
29