Synthesis,crystal structure,characterization and corrosion inhibition studies of a strontium coordination polymer with tartaric acid

Abstract

The reaction of anhydrous strontium chloride with the proton transfer compound 1-H-Imidazolium Hydrogen- meso-tartrate in water resulted in the formation of polymeric tartrate complex of strontium which is characterised by SXRD, IR, UV-Visible and TG/DTG analyses. The single crystal X-ray diffraction analysis reveals that each strontium atom is eight coordinate with distorted dodecahedral arrangement. The two metal fragments are linked through central four-membered Sr₂O₂ ring and there is no imidazole moiety in the complex. The compound displays a rare crystallographic feature where a shared hydrogen atom is located between two carboxylic groups. This unusual arrangement of hydrogen atom with an occupancy factor 0.5, located in between two carboxylate groups balances the total charge of the structure. FT- IR study reveals the bidentate coordination mode of the ligand. The thermal behaviour of the compound was studied using TG/DTG analysis. Corrosion inhibition property of the title compound on mild steel in 1.0M HCl has been investigated using weight loss method. The formation of protective film on the mild steel surface is confirmed by Scanning Electron Microscopy images. The inhibition efficiency increases with increase in concentration of inhibitor.

Keywords

Crystal growth coordination polymer corrosion inhibition studies SEM

1 Introduction

Research in coordination compounds and polymers have witnessed a significant growth in the last decades [1]. The remarkable structural advantage of inorganic- metal containing nodes and multitopic -organic linkers results in highly-porous and crystalline structures which find extensive applications in gas storage [2] catalysis [3] sensors [4] drug delivery [5] and energy applications [6]. Coordination polymers can be designed utilizing coordinate covalent bonds between metal and ligands or by the reaction of proton transfer ion pairs formed from acids and bases with metal ions. The proton transfer ion pairs can enhance the intermolecular interactions between the cationic and anionic fragments providing large stabilization energy. Some of the complexes of proton transfer ion pairs contain both anionic and cationic fragments while others have only one of the species as ligand [7, 8]. Tartaric acid form compounds which exhibits interesting electrical, optical and NLO properties [9 –12] and coordination polymers or MOF structures [13 –15]. Tartaric acid compounds have been investigated as efficient corrosion inhibitors for various metals in acid media [16]. Compounds of N-heterocyclic base, imidazole finds extensive applications in medicinal [17] and NLO properties [18]. Since there are no reports on the interaction of proton transfer ion pair of imidazole and tartaric with metals, this prompted us to explore the ability of the compound to form metal based frameworks. The proton transfer compound is prepared by reacting equivalent amounts of imidazole with meso-tartaric acid. Structural analysis carried out by single crystal XRD reveals that it crystallises in monoclinic system. The cell parameters agree well with the reported values [19]. The reaction of anhydrous SrCl₂ with this proton transfer ion pair resulted in the formation of polymeric tartrate complex of strontium with the formula [Sr₉ C₃₆ H₆₃ O₆₆] Cl₃ abbreviated as SrTA where the chloride ions are present as counter ions only. The component imidazole is not incorporated since its role was only in deprotonation. The flexible multi-functional coordination sites of tartaric acid give a high likelihood for the generation of coordination polymers with high dimensions. Moreover, based on the soft and hard acid and base theory, the functional groups carboxylate and imidazole ring binds to metal atom in different way. The “harder” strontium atom coordinates more preferably to oxygen atom of tartarate moiety than to “softer” nitrogen atom of imidazole [20]. The growth of strontium tartrate crystal by different methods is reported in literature [21, 22]. It crystallises in monoclinic system with space group P2_1 . But the single crystal obtained in the present work is a 3D strontium tartrate polymer which belongs to hexagonal system with P 61 2 2 space group. The presence of imidazole increases the pH of the reaction medium. It is reported that increase in pH results in higher connectivity of ligands and leads to higher dimensional structure [23, 24]. The compound is further characterised by IR, UV-Visible and TG/DTG analyses. The anticorrosive property of the title compound is examined on the corrosion of mild steel in hydrochloric acid medium.

2 Experimental

2.1 Reagents

AR imidazole (Sigma-Aldrich), AR meso-tartaric acid (Sigma-Aldrich) and AR anhydrous strontium chloride (Sigma-Aldrich) and ethanol were purchased and used as such.

2.2 Synthesis

Synthesis of imidazolium hydrogen meso- tartrate (IMTA) and polymeric strontium tartarate complex

Mesotartaric acid (0.750 g, 5 mmol) and imidazole (0.34 g, 5 mmol) were dissolved in ethanol and stirred for 45 minutes at room temperature. The white precipitate obtained was dissolved in water and kept for slow evaporation in a test tube. Good quality transparent crystals of IMTA appeared at the sides of the test tube after one week. Elemental analysis; Found (calculated) % Carbon 38.43 (38.54), hydrogen 4.62 (4.60), nitrogen 12.68 (12.84).

IMTA (0.436 g, 2 mmol) and anhydrous strontium chloride (0.316 g, 2 mmol) were dissolved in water and stirred for 45 minutes at room temperature. Slow evaporation of the resulting solution produced white crystals of the SrTA complex suitable for X-ray diffraction study. The scheme of the reaction is given below (Scheme 1).

Scheme 1

Synthesis of strontium complex of proton transfer compound (SrTA).

A control experiment is also performed by dissolving equimolar amounts (5 mmol each) imidazole (0.34 g), meso tartaric acid (0.750 g) and strontium chloride (0.7925 g) in water and stirred well. The solution is filtered and kept for slow evaporation. Single crystals suitable for SXRD study was obtained after 5 weeks.

2.3 Characterization

The elemental analysis is carried out using Perkin Elmer 2400 Series II CHNS/O analyser. FT-IR spectrum was recorded from potassium bromide pellets on Thermoscientific Nicolet iS50 spectrometer in the range 400–4000 cm⁻¹. The UV-Vis spectrum was recorded using a Varian Cary 5000 UV-Vis-NIR spectrometer in the range 200–1000 nm. Thermal analysis (TG/DTG) was carried out using PerkinElmer STA 8000 with a heating rate of 10°C/ minute in nitrogen atmosphere. Single crystal X-ray diffraction study was carried out using Bruker Kappa Apex II diffractometer at room temperature with graphite monochromated MoKα radiation (λ = 0.71073Å). The structure was solved by SIR92 and refined by full-matrix least squares on F² using SHELXL-97 [25] computer program. Molecular graphics are done using Diamond version 3.2 [26] and IUCr Mercury 3.8 [27] software version.

2.4 Corrosion inhibition study

Accurately weighed mild steel sample plates of dimensions 1.5 cm × 2 cm × 0.2 cm were polished and cleaned with distilled water and acetone. The test solutions were prepared from AR HCl and distilled water kept at 30°C. The solutions of different concentrations of sample were taken in 100 ml beakers and the plates were suspended in to the solution. After a period of 48 hours, the plates were removed, washed with water, dried and weighed. The weight loss, corrosion rate and inhibition efficiency were calculated.

3 Results and discussion

3.1 Single crystal X-ray diffraction study

Single crystal X-ray diffraction analysis of IMTA shows that the compound belongs to monoclinic system with space group P2_1 . The asymmetric unit contains one protonated imidazole unit and one deprotonated tartaric acid unit. The lattice parameters a = 7.57Å, b = 6.96Å, c = 9.00 Å and α = 90°, β = 101 . 49°, γ = 90° agree well with the reported values [19]. The Sr (II) complex of the proton transfer compound SrTA crystallises in hexagonal system (space group P 6₁ 2 2) with lattice parameters a = 10.1040(6) Å, b = 10.1040(6) Å, c = 41.746(2) Å and α = 90°, β = 90°, γ = 120°. The crystal data, structure refinement parameters, bond lengths and bond angles are given in Tables 1 and 2. The asymmetric unit (Fig. 1) contains one and half Sr(II) cations, one and half tartrate ligand, half chloride ion and one water molecule. The half of the tartaric acid ligand is completed by equivalent atoms generated by inversion symmetry. Even though both strontium atoms are eight coordinated with distorted dodecahedral arrangement, their chemical environments are different. Sr1 is coordinated to seven tartrate oxygens and one oxygen from water molecule whereas Sr2 is coordinated to six tartrate oxygens and two oxygen atoms from water molecule. The two Sr atoms are linked by central four membered Sr₂O₂ ring. The coordination environments are depicted in Fig. 2. The two tartrate units are crystallographically independent in which one is classed as μ₄, k⁵ (C₁-C₂-C₃-C₄) which binds to four different strontium atoms through five oxygen atoms whereas the other (-C₅-C₆-) as μ₄, k⁴ which binds four strontium atoms using four oxygen atoms (Fig. 3). Each half of the tartrate possesses 1, 2 chelation involving a carboxylate oxygen and the ortho-hydroxy group. The Sr-O _carboxylate distances are shorter than Sr-O hydroxyl distances. While most of the Sr-O distances lies in the range of 2.546 Å–2.589Å, the oxygen atoms coordinated to the alkoxy group of the ligand are slightly elongated in the range 2.649Å–2.703Å. It is interesting to observe that the elongated alkoxy Sr-O lies opposite to water ligand and thus experience a trans effect [28]. The oxygen O₆ is bonded to another O₆ by a unique H_6A leading to half occupancy of H_6A along the O₆ . . . . O₆ bond with distance 2.746Å (Fig. 4). The incorporation of this hydrogen atom justified the overall charge balance of the compound. This type of unusual crystallographic feature of shared hydrogen is reported in crystal structures [29]. The chloride ion is not coordinated to the metal but acts as acceptor of two hydrogen bonds O3-H3A....Cl (d _D - H=0.84Å, d _H …A=2.35Å, d _D …A=3.184Å) and O7-H7A....Cl (d _D - H=0.67Å, d _H …A=2.37Å, d _D …A=3.032Å). Hydrogen bonding which forms infinite network throughout the crystal is an interesting feature of the of the complex. The collection of all hydrogen bonds including those of coordinated water molecules forms the 3D architecture of the complex. Table 3 lists the various hydrogen bonds. The packing diagram and of the molecule with hydrogen bonds are depicted in Fig. 5.

Fig. 1

Asymmetric unit of SrTA.

Fig. 2

Coordination environment of strontium.

Fig. 3

Coordination environment of two tartaric acid units.

Fig. 4

Shared hydrogen atom H6A between two O6 atoms.

Fig. 5

Packing diagram of molecule.

Table 1

Crystal data and structure refinement for SrTA

Identification code	Shelx
Empirical formula	C36 H63 Cl3 O66 Sr9
Formula weight	2446.79
Temperature	296(2) K
Wavelength	0.71073 Å
Crystal system	Hexagonal
Space group	P 6₁ 2 2
Unit cell dimensions	a=10.1040(6) Å	α = 90°.
	b = 10.1040(6) Å	β = 90°.
	c = 41.746(2) Å	γ = 120°.
Volume	3690.9(5) Å³
Z	2
Density (calculated)	2.202 Mg/m³
Absorption coefficient	6.688 mm⁻¹
F(000)	2400
Crystal size	0.100 × 0.100 × 0.050 mm³
Theta range for data collection	2.328 to 25 . 262°.
Index ranges	–12< =h< =12, –12< =k< =12, –49< =l< =50
Reflections collected	60334
Independent reflections	2229 [R(int)=0.1416]
Completeness to theta = 25.242°	99.9%
Absorption correction	Semi-empirical from equivalents
Max. and min. transmission	0.7452 and 0.4495
Refinement method	Full-matrix least-squares on F²
Data / restraints / parameters	2229 / 6 / 195
Goodness-of-fit on F²	1.062
Final R indices [I > 2sigma(I)]	R1 = 0.0405, wR2 = 0.0631
R indices (all data)	R1 = 0.0587, wR2 = 0.0679
Absolute structure parameter	0.017(18)
Extinction coefficient	n/a
Largest diff. peak and hole	0.428 and –0.651 e.Å⁻³

Table 2

Bond lengths [Å] and angles [°] for SrTA

Sr(1)-O(1)	2.546(5)
Sr(1)-O(5)#1	2.578(5)
Sr(1)-O(8)	2.583(6)
Sr(1)-O(6)#2	2.587(5)
Sr(1)-O(5)#3	2.589(4)
Sr(1)-O(10)	2.637(6)
Sr(1)-O(7)	2.649(7)
Sr(1)-O(4)#3	2.703(5)
Sr(1)-O(3)#3	3.263(7)
Sr(1)-C(4)#3	3.404(7)
Sr(1)-Sr(2)	4.0155(11)
Sr(1)-Sr(1)#4	4.0165(15)
Sr(1)-H(4A)#3	1.98(6)
Sr(2)-O(11)	2.502(8)
Sr(2)-O(11)#3	2.502(9)
Sr(2)-O(8)#3	2.559(5)
Sr(2)-O(8)	2.559(5)
Sr(2)-O(1)#3	2.568(6)
Sr(2)-O(1)	2.568(6)
Sr(2)-O(3)	2.650(5)
Sr(2)-O(3)#3	2.650(5)
Sr(2)-H(11B)	2.66(13)
C(1)-O(2)	1.246(10)
C(1)-O(1)	1.247(10)
C(1)-C(2)	1.542(10)
C(2)-O(3)	1.431(9)
C(2)-C(3)	1.515(10)
C(2)-H(2)	0.9800
C(3)-O(4)	1.412(7)
C(3)-C(4)	1.527(10)
C(3)-H(3)	0.9800
C(4)-O(5)	1.236(8)
C(4)-O(6)	1.272(7)
C(5)-O(9)	1.243(9)
C(5)-O(8)	1.252(9)
C(5)-C(6)	1.529(10)
C(6)-O(7)	1.408(9)
C(6)-C(6)#4	1.521(12)
C(6)-H(6)	0.9800
O(3)-H(3A)	0.84(7)
O(4)-H(4A)	0.8200
O(6)-H(6A)	0.8200
O(7)-H(7A)	0.67(7)
O(10)-H(10B)	0.85(3)
O(10)-H(10A)	0.85(3)
O(11)-H(11A)	0.85(3)
O(11)-H(11B)	0.86(3)
O(1)-Sr(1)-O(5)#1	157.32(17)
O(1)-Sr(1)-O(8)	69.04(19)
O(5)#1-Sr(1)-O(8)	127.17(18)
O(1)-Sr(1)-O(6)#2	84.43(18)
O(5)#1-Sr(1)-O(6)#2	78.25(16)
O(8)-Sr(1)-O(6)#2	96.75(17)
O(1)-Sr(1)-O(5)#3	128.26(16)
O(5)#1-Sr(1)-O(5)#3	67.22(18)
O(8)-Sr(1)-O(5)#3	105.01(17)
O(6)#2-Sr(1)-O(5)#3	145.44(18)
O(1)-Sr(1)-O(10)	83.7(2)
O(5)#1-Sr(1)-O(10)	77.03(19)
O(8)-Sr(1)-O(10)	151.35(19)
O(6)#2-Sr(1)-O(10)	71.32(18)
O(5)#3-Sr(1)-O(10)	98.53(18)
O(1)-Sr(1)-O(7)	119.8(2)
O(5)#1-Sr(1)-O(7)	68.8(2)
O(8)-Sr(1)-O(7)	59.7(2)
O(6)#2-Sr(1)-O(7)	73.0(2)
O(5)#3-Sr(1)-O(7)	95.1(2)
O(10)-Sr(1)-O(7)	134.54(19)
O(1)-Sr(1)-O(4)#3	71.91(17)
O(5)#1-Sr(1)-O(4)#3	111.81(16)
O(8)-Sr(1)-O(4)#3	106.97(16)
O(6)#2-Sr(1)-O(4)#3	136.98(17)
O(5)#3-Sr(1)-O(4)#3	60.71(15)
O(10)-Sr(1)-O(4)#3	70.77(16)
O(7)-Sr(1)-O(4)#3	149.99(18)
O(1)-Sr(1)-O(3)#3	71.94(15)
O(5)#1-Sr(1)-O(3)#3	129.12(14)
O(8)-Sr(1)-O(3)#3	56.45(15)
O(6)#2-Sr(1)-O(3)#3	148.96(16)
O(5)#3-Sr(1)-O(3)#3	64.16(15)
O(10)-Sr(1)-O(3)#3	123.77(17)
O(7)-Sr(1)-O(3)#3	101.20(17)
O(4)#3-Sr(1)-O(3)#3	53.80(15)
O(1)-Sr(1)-C(4)#3	110.23(17)
O(5)#1-Sr(1)-C(4)#3	84.26(16)
O(8)-Sr(1)-C(4)#3	99.89(18)
O(6)#2-Sr(1)-C(4)#3	160.92(18)
O(5)#3-Sr(1)-C(4)#3	18.02(16)
O(10)-Sr(1)-C(4)#3	97.49(19)
O(7)-Sr(1)-C(4)#3	107.7(2)
O(4)#3-Sr(1)-C(4)#3	44.91(16)
O(3)#3-Sr(1)-C(4)#3	50.12(16)
O(1)-Sr(1)-Sr(2)	38.45(14)
O(5)#1-Sr(1)-Sr(2)	163.41(12)
O(8)-Sr(1)-Sr(2)	38.43(11)
O(6)#2-Sr(1)-Sr(2)	108.46(13)
O(5)#3-Sr(1)-Sr(2)	105.25(11)
O(10)-Sr(1)-Sr(2)	119.33(14)
O(7)-Sr(1)-Sr(2)	98.11(16)
O(4)#3-Sr(1)-Sr(2)	74.12(11)
O(3)#3-Sr(1)-Sr(2)	41.09(9)
C(4)#3-Sr(1)-Sr(2)	90.44(12)
O(1)-Sr(1)-Sr(1)#4	163.13(12)
O(5)#1-Sr(1)-Sr(1)#4	39.09(10)
O(8)-Sr(1)-Sr(1)#4	101.38(12)
O(6)#2-Sr(1)-Sr(1)#4	111.03(14)
O(5)#3-Sr(1)-Sr(1)#4	38.88(11)
O(10)-Sr(1)-Sr(1)#4	107.19(14)
O(7)-Sr(1)-Sr(1)#4	61.65(18)
O(4)#3-Sr(1)-Sr(1)#4	99.01(10)
O(3)#3-Sr(1)-Sr(1)#4	91.21(10)
C(4)#3-Sr(1)-Sr(1)#4	56.45(12)
Sr(2)-Sr(1)-Sr(1)#4	126.13(3)
O(1)-Sr(1)-H(4A)#3	64.6(7)
O(5)#1-Sr(1)-H(4A)#3	121.1(12)
O(8)-Sr(1)-H(4A)#3	97.8(16)
O(6)#2-Sr(1)-H(4A)#3	137.9(8)
O(5)#3-Sr(1)-H(4A)#3	65.6(3)
O(10)-Sr(1)-H(4A)#3	77.2(15)
O(7)-Sr(1)-H(4A)#3	146.7(14)
O(4)#3-Sr(1)-H(4A)#3	9.7(15)
O(3)#3-Sr(1)-H(4A)#3	46.6(15)
C(4)#3-Sr(1)-H(4A)#3	48.45(18)
Sr(2)-Sr(1)-H(4A)#3	64.4(15)
Sr(1)#4-Sr(1)-H(4A)#3	104.5(3)
O(11)-Sr(2)-O(11)#3	83.2(7)
O(11)-Sr(2)-O(8)#3	88.5(3)
O(11)#3-Sr(2)-O(8)#3	79.2(3)
O(11)-Sr(2)-O(8)	79.2(3)
O(11)#3-Sr(2)-O(8)	88.5(3)
O(8)#3-Sr(2)-O(8)	163.6(3)
O(11)-Sr(2)-O(1)#3	156.8(3)
O(11)#3-Sr(2)-O(1)#3	97.7(3)
O(8)#3-Sr(2)-O(1)#3	69.09(18)
O(8)-Sr(2)-O(1)#3	123.93(17)
O(11)-Sr(2)-O(1)	97.7(3)
O(11)#3-Sr(2)-O(1)	156.8(3)
O(8)#3-Sr(2)-O(1)	123.93(17)
O(8)-Sr(2)-O(1)	69.09(18)
O(1)#3-Sr(2)-O(1)	90.4(3)
O(11)-Sr(2)-O(3)	82.3(4)
O(11)#3-Sr(2)-O(3)	142.3(4)
O(8)#3-Sr(2)-O(3)	65.78(18)
O(8)-Sr(2)-O(3)	122.38(19)
O(1)#3-Sr(2)-O(3)	83.11(19)
O(1)-Sr(2)-O(3)	60.12(17)
O(11)-Sr(2)-O(3)#3	142.3(4)
O(11)#3-Sr(2)-O(3)#3	82.3(4)
O(8)#3-Sr(2)-O(3)#3	122.38(19)
O(8)-Sr(2)-O(3)#3	65.78(18)
O(1)#3-Sr(2)-O(3)#3	60.12(17)
O(1)-Sr(2)-O(3)#3	83.11(18)
O(3)-Sr(2)-O(3)#3	127.9(3)
O(11)-Sr(2)-Sr(1)#3	119.3(3)
O(11)#3-Sr(2)-Sr(1)#3	105.6(2)
O(8)#3-Sr(2)-Sr(1)#3	38.88(14)
O(8)-Sr(2)-Sr(1)#3	157.43(14)
O(1)#3-Sr(2)-Sr(1)#3	38.07(12)
O(1)-Sr(2)-Sr(1)#3	94.18(14)
O(3)-Sr(2)-Sr(1)#3	54.04(14)
O(3)#3-Sr(2)-Sr(1)#3	98.17(14)
O(11)-Sr(2)-Sr(1)	105.6(2)
O(11)#3-Sr(2)-Sr(1)	119.3(3)
O(8)#3-Sr(2)-Sr(1)	157.43(14)
O(8)-Sr(2)-Sr(1)	38.87(13)
O(1)#3-Sr(2)-Sr(1)	94.18(14)
O(1)-Sr(2)-Sr(1)	38.07(12)
O(3)-Sr(2)-Sr(1)	98.18(13)
O(3)#3-Sr(2)-Sr(1)	54.04(14)
Sr(1)#3-Sr(2)-Sr(1)	119.06(4)
O(11)-Sr(2)-H(11B)	18.8(11)
O(11)#3-Sr(2)-H(11B)	64.8(14)
O(8)#3-Sr(2)-H(11B)	81(5)
O(8)-Sr(2)-H(11B)	83(5)
O(1)#3-Sr(2)-H(11B)	148(4)
O(1)-Sr(2)-H(11B)	116.1(14)
O(3)-Sr(2)-H(11B)	95(4)
O(3)#3-Sr(2)-H(11B)	136(4)
Sr(1)#3-Sr(2)-H(11B)	118(5)
Sr(1)-Sr(2)-H(11B)	117(4)
O(2)-C(1)-O(1)	127.7(8)
O(2)-C(1)-C(2)	114.4(8)
O(1)-C(1)-C(2)	118.0(8)
O(3)-C(2)-C(3)	107.8(6)
O(3)-C(2)-C(1)	109.3(7)
C(3)-C(2)-C(1)	110.2(7)
O(3)-C(2)-H(2)	109.8
C(3)-C(2)-H(2)	109.8
C(1)-C(2)-H(2)	109.8
O(4)-C(3)-C(2)	108.6(6)
O(4)-C(3)-C(4)	110.8(5)
C(2)-C(3)-C(4)	108.3(6)
O(4)-C(3)-H(3)	109.7
C(2)-C(3)-H(3)	109.7
C(4)-C(3)-H(3)	109.7
O(5)-C(4)-O(6)	124.2(7)
O(5)-C(4)-C(3)	119.9(6)
O(6)-C(4)-C(3)	115.6(6)
O(5)-C(4)-Sr(1)#3	40.4(3)
O(6)-C(4)-Sr(1)#3	157.4(6)
C(3)-C(4)-Sr(1)#3	81.0(4)
O(9)-C(5)-O(8)	125.6(8)
O(9)-C(5)-C(6)	117.2(8)
O(8)-C(5)-C(6)	117.2(8)
O(7)-C(6)-C(6)#4	107.9(5)
O(7)-C(6)-C(5)	109.6(6)
C(6)#4-C(6)-C(5)	111.7(7)
O(7)-C(6)-H(6)	109.2
C(6)#4-C(6)-H(6)	109.2
C(5)-C(6)-H(6)	109.2
C(1)-O(1)-Sr(1)	126.7(5)
C(1)-O(1)-Sr(2)	127.6(5)
Sr(1)-O(1)-Sr(2)	103.5(2)
C(2)-O(3)-Sr(2)	122.8(4)
C(2)-O(3)-Sr(1)#3	111.0(4)
Sr(2)-O(3)-Sr(1)#3	84.87(16)
C(2)-O(3)-H(3A)	109(5)
Sr(2)-O(3)-H(3A)	109(5)
Sr(1)#3-O(3)-H(3A)	119(5)
C(3)-O(4)-Sr(1)#3	113.1(4)
C(3)-O(4)-H(4A)	109.5
Sr(1)#3-O(4)-H(4A)	24.2
C(4)-O(5)-Sr(1)#5	134.1(4)
C(4)-O(5)-Sr(1)#3	121.6(4)
Sr(1)#5-O(5)-Sr(1)#3	102.04(17)
C(4)-O(6)-Sr(1)#6	160.0(5)
C(4)-O(6)-H(6A)	109.5
Sr(1)#6-O(6)-H(6A)	86.4
C(6)-O(7)-Sr(1)	120.3(5)
C(6)-O(7)-H(7A)	107(7)
Sr(1)-O(7)-H(7A)	127(7)
C(5)-O(8)-Sr(2)	129.5(6)
C(5)-O(8)-Sr(1)	126.9(5)
Sr(2)-O(8)-Sr(1)	102.69(19)
Sr(1)-O(10)-H(10B)	123(10)
Sr(1)-O(10)-H(10A)	111(10)
H(10B)-O(10)-H(10A)	109(5)
Sr(2)-O(11)-H(11A)	159(10)
Sr(2)-O(11)-H(11B)	91(9)
H(11A)-O(11)-H(11B)	109(5)

Symmetry transformations used to generate equivalent atoms: #1 x,y+1,z #2 y,x+1,-z+4/3 #3 x,x-y+1,-z+7/6. #4 x,x-y+2,-z+7/6 #5 x,y-1,z #6 y-1,x,-z+4/3.

Torsion angles [°] for SrTA

O(2)-C(1)-C(2)-O(3)	174.0(7)
O(1)-C(1)-C(2)-O(3)	–5.5(10)
O(2)-C(1)-C(2)-C(3)	55.7(9)
O(1)-C(1)-C(2)-C(3)	–123.8(8)
O(3)-C(2)-C(3)-O(4)	–58.9(7)
C(1)-C(2)-C(3)-O(4)	60.4(8)
O(3)-C(2)-C(3)-C(4)	61.6(7)
C(1)-C(2)-C(3)-C(4)	-1-79.2(6)
O(4)-C(3)-C(4)-O(5)	14.9(10)
C(2)-C(3)-C(4)-O(5)	–104.1(8)
O(4)-C(3)-C(4)-O(6)	–170.2(7)
C(2)-C(3)-C(4)-O(6)	70.7(8)
O(4)-C(3)-C(4)-Sr(1)#3	26.0(5)
C(2)-C(3)-C(4)-Sr(1)#3	–93.1(5)
O(9)-C(5)-C(6)-O(7)	–165.6(7)
O(8)-C(5)-C(6)-O(7)	16.4(10)
O(9)-C(5)-C(6)-C(6)#4	75.0(7)
O(8)-C(5)-C(6)-C(6)#4	–103.0(6)
O(2)-C(1)-O(1)-Sr(1)	–2.5(13)
C(2)-C(1)-O(1)-Sr(1)	176.8(5)
O(2)-C(1)-O(1)-Sr(2)	–162.9(7)
C(2)-C(1)-O(1)-Sr(2)	16.5(10)
C(3)-C(2)-O(3)-Sr(2)	113.3(5)
C(1)-C(2)-O(3)-Sr(2)	–6.5(8)
C(3)-C(2)-O(3)-Sr(1)#3	15.6(6)
C(1)-C(2)-O(3)-Sr(1)#3	–104.2(6)
C(2)-C(3)-O(4)-Sr(1)#3	82.6(5)
C(4)-C(3)-O(4)-Sr(1)#3	–36.3(7)
O(6)-C(4)-O(5)-Sr(1)#5	2.2(13)
C(3)-C(4)-O(5)-Sr(1)#5	176.5(5)
Sr(1)#3-C(4)-O(5)-Sr(1)#5	159.5(9)
O(6)-C(4)-O(5)-Sr(1)#3	–157.4(6)
C(3)-C(4)-O(5)-Sr(1)#3	17.0(10)
O(5)-C(4)-O(6)-Sr(1)#6	–164.7(10)
C(3)-C(4)-O(6)-Sr(1)#6	20.7(19)
Sr(1)#3-C(4)-O(6)-Sr(1)#6	154.9(8)
C(6)#4-C(6)-O(7)-Sr(1)	93.4(7)
C(5)-C(6)-O(7)-Sr(1)	–28.3(8)
O(9)-C(5)-O(8)-Sr(2)	–7.3(12)
C(6)-C(5)-O(8)-Sr(2)	170.5(4)
O(9)-C(5)-O(8)-Sr(1)	–174.5(6)
C(6)-C(5)-O(8)-Sr(1)	3.3(10)

Symmetry transformations used to generate equivalent atoms: #1 x,y+1,z #2 y,x+1,-z+4/3 #3 x,x-y+1,-z+7/6. #4 x,x-y+2,-z+7/6 #5 x,y-1,z #6 y-1,x,-z+4/3.

Table 3

Hydrogen bonds for SrTA [Å and ∘]

D-H...A	d(D-H)	d(H...A)	d(D...A)	(DHA)°
O(4)-H(4A)...O(1)#3	0.82	2.47	3.085(7)	132.8
O(6)-H(6A)...O(6)#7	0.82	2.12	2.746(11)	133.2
O(6)-H(6A)...O(10)#6	0.82	2.54	3.046(8)	121.6
O(11)-H(11A)...O(2)#2	0.85(3)	2.07(11)	2.721(12)	134(13)
O(10)-H(10B)...O(9)#8	0.85(3)	2.26(7)	3.038(9)	153(14)
O(10)-H(10A)...O(2)	0.85(3)	2.12(12)	2.713(9)	126(13)
O(3)-H(3A)...Cl(1)#5	0.84(7)	2.35(7)	3.184(6)	172(7)
O(7)-H(7A)...Cl(1)	0.67(7)	2.37(7)	3.032(7)	170(9)

Symmetry transformations used to generate equivalent atoms: #1 x,y+1,z #2 y,x+1,-z+4/3 #3 x,x-y+1,-z+7/6. #4 x,x-y+2,-z+7/6 #5 x,y-1,z #6 y-1,x,-z+4/3. #7 y,x,-z+4/3 #8 x-1,x-y+1,-z+7/6.

The cell parameters for the crystal obtained from control experiment a = 9.462(6) Å, b = 9.462 Å, c = 10.926(8) Å and α = 90°, β = 90°, γ = 90° matches well with the reported values of the compound strontium tartrate tetrahydrate Sr(C₄H₄O₆)(H₂O)₂](H₂O)₂ [30]. As the single crystals resulted from two experiments are different, it can be concluded that the imidazole moiety in the proton transfer compound played a crucial role in the formation of the coordination polymer SrTA.

3.2 Infrared spectroscopy

The IR spectrum of the proton transfer compound IMTA (Fig. 6) contains a broad band between 3460–3370 cm⁻¹ is due to O-H stretching frequency of tartrate moiety [31]. The sharp peak at 3149 cm⁻¹ is assigned to the N-H vibrational frequency of imidazole moiety. The signal due to aromatic C-H vibration of imidazole is found at 2965 cm⁻¹ [32]. These two characteristic peaks confirm the presence of imidazole in the crystal lattice. The aliphatic C-H stretching vibration of tartrate is observed as two sharp bands at 2965 and 2883 cm⁻¹. The frequency at 1716 cm⁻¹ correspond to ν c = o of non-ionized carboxylic acid group of tartrate [33]. The asymmetric and symmetric stretching frequencies of COO- are observed at 1638 and 1435 cm⁻¹ The frequency at 1543 cm⁻¹ correspond to ν c = c of imidazole. The bending modes of C-OH and C-CH are assigned at 1381 cm⁻¹ and 1231 cm⁻¹. The CHOH stretching mode is observed at 1104 cm⁻¹ [34]. The C-C stretching modes of tartrate are observed at 907 and 861 cm⁻¹ respectively. The IR spectrum of the complex SrTA (Fig. 7) contains a broad band in the region 3430–2990 cm^–¹ centered at 3215 cm^–¹ due to the overlapping stretching vibrations of free hydroxyl of tartaric acid and water. The antisymmetric carboxylate stretching is observed at 1582 cm^–¹ and symmetric stretching at 1407 cm^–¹. The separation of 175 cm^–¹ indicates bidentate coordination mode of carboxylate [35]. The sharp band at 1367 cm^–¹ is attributed to O-H in plane bending vibrations. The – C—H in-plane and out of plane vibrations are observed at 1308 and 1257 cm^–¹ respectively. The bands at 1112 cm^–¹ and 1052 cm^–¹ are attributed to C-O stretching and C—C stretching vibrations respectively. The band at 843 cm^–¹ corresponds to the scissoring band of carboxylate group.

Fig. 6

FT-IR spectrum of IMTA.

Fig. 7

FT-IR spectrum of SrTA.

3.3 UV-Visible spectroscopy

The UV-Visible absorption spectrum (Fig. 8) of the complex in the wavelength range 200–1000 nm shows that it is transparent in the visible region 400–800 nm which suggests the suitability of the material in optoelectronic applications. The spectrum shows characteristic absorption band at 263 nm attributed to π ⟶π* transition. The band at 343 nm is due to n⟶π* transition.

Fig. 8

UV-Vis spectrum of SrTA.

3.4 TG/DTG analysis

Thermal analysis of the sample using TG/DTG technique (Fig. 9) indicates that the material is stable up to 53°C. The first stage of decomposition takes place in the temperature range of 53°C–70°C (DTG max = 60°C) due to the elimination of chloride ion as HCl. The calculated and observed mass losses are 4.47% and 3.5% respectively. The next decomposition takes place in the temperature range 72°C–110°C (DTG max = 105°C) due to the loss of coordinated water molecules. The third decomposition takes place in the temperature range of 259°C–347°C (DTG max = 276°C) corresponds to the conversion of anhydrous strontium tartrate to strontium oxalate with the calculated and observed mass losses. The final stage decomposition of strontium oxalate to strontium carbonate takes place in the temperature range of 404°C–484°C (DTG max = 450°C) with observed and calculated mass losses 10.26% and 9.8% respectively.

Fig. 9

TG-DTG of SrTA.

3.5 Corrosion inhibition study

Mild steel is widely used in industries due to low cost and good mechanical strength. Corrosion is the major economic and safety concern in industries. The simplest and relatively cheap method to prevent corrosion is the use of inhibitors [36]. Organic compounds having nitrogen, sulphur, oxygen or polar functional group possess corrosion inhibition efficiency because the hetero atom acts as centre for adsorption on the metal surface [37]. Literature survey reveals that tartaric acid compounds have efficient corrosion inhibition activity [38, 39]. So the corrosion inhibition property of the title compound on mild steel in 1.0M HCl has been investigated using weight loss method. On the basis of weight loss measurement, the corrosion rate, inhibition efficiency and surface coverage of the title compound for various concentrations of inhibitor is given in Table 4. The corrosion rate and inhibition efficiency are calculated as follows $Corrosion rate, V (mils per year) = \frac{W}{St} \times 100$ $Inhibition efficiency (%) = \frac{V_{0} - V}{V_{0}} \times 100$ where W is the weight loss of the mild steel, S is the total surface area of the specimen, t-immersion time, V₀- corrosion rate without inhibitor and V- corrosion rate with inhibitor.

Table 4
Corrosion inhibition study

Concentration Weight loss Corrosion rate Inhibition Surface

of SrTA (g/L) (mg) (mg cm⁻² h⁻¹) efficiency (IE) % coverage (Θ)

0 156 3.2500 0 0

0.2 70.17 1.4620 55 0.5500

0.4 64.35 1.3406 58.75 0.5875

0.6 55.06 1.1472 64.70 0.6470

0.8 45.04 0.9385 71.12 0.7112

1.0 36.70 0.7647 76.47 0.7647

1.2 28.90 0.6022 81.47 0.8147

Concentration	Weight loss	Corrosion rate	Inhibition	Surface
0	156	3.2500	0	0
0.2	70.17	1.4620	55	0.5500
0.4	64.35	1.3406	58.75	0.5875
0.6	55.06	1.1472	64.70	0.6470
0.8	45.04	0.9385	71.12	0.7112
1.0	36.70	0.7647	76.47	0.7647
1.2	28.90	0.6022	81.47	0.8147

From the table it is clear that, with the gradually increased concentration of inhibitor, the weight loss and corrosion rate (V) diminished significantly. The highest inhibitory efficiency of 81.47% is attained with a sample concentration of 1.2 g/L.

3.6 Adsorption isotherm

In order to get better insight into the mechanism of corrosion, adsorption isotherm experiments were performed. The corrosion inhibitor functions by forming thin layer (s) or protective film(s) on the metallic surface thereby reducing its availability.

The surface coverage (Θ) data obtained from weight loss experiment was fitted in various adsorption isotherm models and the suitable isotherm chosen was Langmuir isotherm based on correlation coefficient (R²). The langmuir isotherm C/Θ=C+1/K_ads where C is the concentration of inhibitor and K_ads is the equilibrium constant of adsorption. Figure 10 shows the linear relationship of C/Θ versus C with correlation nearly equal to 1. The slope obtained is greater than unity which suggests that each inhibitor unit occupies more than one adsorption site and there are interactions between adsorbed species on the metal surface [40]. The value of K_ads obtained from the reciprocal of intercept is 8.599. The free energy change of adsorption ΔG_ads as per the equation $Δ G_{ads} = - 2.303 RT log (55.5 K) is - 15.385 kJ / mol$

Fig. 10

Adsorption isotherm of SrTA.

where R is the gas constant, T is the absolute temperature and 55.5 is the molar concentration of water. Adsorption of the inhibitor on the steel surface is spontaneous as inferred from the negative value of ΔG_ads. The value less than –20 kJ/mol suggests that the adsorption is physisorption where only electrostatic interactions between inhibitor molecules and metal surface exist [41].

3.7 Surface analysis

The surface analysis was carried out using Scanning Electron Microscopy images for the mild steel surface immersed in 1M HCl in the absence and presence of inhibitor. The SEM images are given in Fig. 11. A significant surface damage was observed for mild steel immersed in 1M HCl without inhibitor indicating that the surface is corroded whereas the mild steel surface in presence of inhibitor is protected.

Fig. 11

SEM images of the sample without inhibitor and presence of inhibitor.

4 Conclusions

A polymeric tartrate complex of strontium is prepared by the reaction of 1-H-imidazolium Hydrogen- meso-tartrate and anhydrous strontium chloride. Single crystal X-ray diffraction analysis revealed that there is no imidazole moiety in the complex. The asymmetric unit consist of one and half Sr(II) cations, one and half tartrate ligand, half chloride ion and one water molecule. Two kinds of Sr(II) in different coordination environments are linked by two oxygen atoms to form four-membered Sr₂O₂ ring. The tartrate ligand also exhibited two different binding modes. The bidentate coordination mode of the ligand is revealed in FT-IR spectroscopy. The thermal behaviour of the compound was studied using TG/DTG analysis. Corrosion inhibition property of the title compound on mild steel in 1.0 M HCl has been investigated using weight loss method. The formation of protective film on the mild steel surface is confirmed by Scanning Electron Microscopy images. The inhibition efficiency increases with increase in concentration of inhibitor.

Footnotes

Acknowledgments

The authors are grateful to the authorities of Sophisticated Analytical Instrumental Facilities (SAIF), IIT MADRAS, Cochin University of Science and Technology, CLIF University of Kerala for providing the instrumental facilities.

Appendix A: Supplementary material

CCDC 2283395 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via .

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