Structural Analysis of Powdered Manganese(II) of 1,10-Phenanthroline (phen) as Ligand and Trifluoroacetate (TFA) as Counter Anion

Introduction

Synthesis and characterisation of complexes which were composed of transition metals,  phenanthroline (phen) as ligand, and various counter ions, NO₃^–, BF₄^–, ClO₄^–, PF₆^– have been reported long time ago^1-2. However, the cell parameters of a single crystal of tris (phenanthroline) manganese(II) cation with any counter anion is, in fact, not easy to confirmed, meaning that the preparation of single crystals seem unsuccessful. The chemistry of supramolecular tris (phenanthroline) manganese(II) cation has been reported to associate with I₃^– and I₅^– ions in the single crystals of [Mn(phen)₃](I₃)₂³ and [Mn(phen)₃]I₈⁴. Quite recently, the single crystal of simple perchlorate anion of [Mn(phen)₃]²⁺ was isolated, but it was also known together with carbonic acid⁵.

Trifluoroacetic acid, H-TFA, is known a strong acid. The corresponding pKa is about 0.23 compared to a pKa of 4.76 for the acetic acid and thus being about 100,000 times stronger⁶. It is associated with the strongest electronegativity of fluorine atoms which leads to the electron-withdrawing character of the trifluoromethyl grouap. This results in weakening the oxygen-hydrogen bond and therefore stabilising the conjugate base of anion. Thus, it might be considered in the same group of strong acids with pKa less than one, such as hydroiodic, hydrobromic, hydrochloric, perchloric, chloric, nitric and (mono)sulfuric acids⁷.

For these reasons, the TFA would be possible to be a counter anionic role in cationic complex synthesis. However, the 3d cationic transition complex of TFA has not been much observed, and it seems the only cationic mercuracycle was reported⁸. In fact the monodentate coordinated TFA to gold (I)⁹ and to cobalt(II)¹⁰ in a polymer complexes were observed. Complex of diisopropylammonium trifluoroacetate has also been synthesized and found to be monodentate coordinated TFA with hydrogen bonding of N–H···O¹¹. With transition metals, Ru, Os, and Ir, hydrogen bonding of TFA were produced¹². With the monodentate ligand of pyridine (py), some molecular complexes of M(Py)(CF₃CO₂) where M=Co(II), Ni(II), and Cu(II), were isolated, indicating the coordinated TFA as a ligand rather than counter anion¹³

The previous works clearly involved monodentate organic ligands, and it is readily understood that the TFA might prefer to coordinate as anionic ligand also. Thus, by involving relatively strong coordinated bidentate ligand such as 1,10-phenanthroline, the tendency of TFA to be mono/bi-dentate anionic ligand might be reduced and an cationic complex should result.

Thus the synthesis of manganese(II) complex of phenantroline and TFA anion should be useful in understanding the ionic-molecular characteristic of the complex.  The powder-XRD of the complex may be refined following Rietica method and this should lead to the structural lattice parameters, being comparable to some known single crystal structures as for example described by Polyanskaya et al¹⁴ and Wang¹⁵, and it is the main idea of this work. In the last two complexes  the corresponding magnetic moment and electronic spectral data have not been reported yet.

Experimental

Materials

The main chemicals, nickel nitrate hexahydrate (Ni(NO₃)₂.6H₂O), 1,10-phenanthroline (C₁₂H₈N₂), sodium trifluoroacetate (CF₃COONa), calcium nitrate (Ca(NO₃)₂), nickel sulfate (NiSO₄), calcium chloride (CaCl₂), ammonium nitrate (NH₄NO₃), aluminium nitrate (Al(NO₃)₃.6H₂O), and iron(III) chloride (FeCl₃) were purchased from Sigma-Aldrich. All the reagents were used without further purification.

Preparation of Tris(phenantroline)manganese(II) Trifluoroacetate

The mixture of about 5 mL solution of 0.32 mol phenanthroline in water with three drops of ethanol to clearly dissolve and about 10 mL of an aqueous solution of 0.1 mol Mn(NO₃)₂ was warmed while well stiring to obtain an homogeneous solution. A saturated aqueous solution of CF₃COONa (0.4 mol in 5 mL) which was stoichiometrically in excess was added to the mixture. It was then concentrated on heating, and the powdered complex which was precipitated on scratching while cooling was filtered, washed with a minimum of cold water, and then let the precipitate dry in exposure.

Physical Characterization

Magnetism. The magnetic moment of the complex was calculated through the measurement of the susceptibility for powdered samples which were obtained on Magnetic Susceptibility Balance of Auto Sherwood Scientific 240V-AC model. This was calibrated with CuSO₄.5H₂O. The corrected diamagnetism using Pascal’s constant¹⁶ was applied to the molar magnetic susceptibility data on the calculation of  effective magnetic moment (μ_eff) following  the formula, μ_eff = 2.828√ (χ_M.T) BM.

IR and UV-Vis Spectra. The IR spectrum of the sample which was set on the cell with potassium bromide pellets was recorded on a FTIR Spectrophotometer ABB MB3000 at 500 – 4000 cm^-1. UV-Vis spectrum of the powdered complex was recorded on a spectrophotometer of  Pharmaspec UV 1700. The powders were spread on a thin glass adhered with ethanol. It was then put in the cell holder and the spectrum was recorded at 200-1000 nm. The spectrophotometer UV-Vis Shimadzu 2400 PC Series was applied for recording the spectrum of complex in solution.

Ionic Property and Metal Content. The ionic character of the complex was based on the data of equivalent electrical conductance recorded using a conductometer Lutron CD-4301 calibrated with 1.0 M potassium chloride at 25⁰. The data obtained were compared to those of known ionic solutions, NH₄NO₃, CaCl₂, Ca(NO₃)₂, NiSO₄, MnSO₄, FeCl₃, and Al(NO₃)₃, which were also recorded on the same instrument. The metal content of the complex was etimated as observed by Atomic Absorption Spectrophotometer model of PinAAcle 900T Perkin Elmer.

TGA-DTA (Thermogravimetric Analysis and Differential Thermal Analysis). Thermal decomposition of the complex, [Mn(phen)₃](CF₃COO)₂.1.35H₂O , was performed was performed up to 600°C under nitrogen, to confirm the lost of particular materials contained in the compound. Thus, the loss of hydrated molecule of water and decomposition of other materials were performed on Diamond (Perkin Elmer Instruments), and simultaneous TGA-DTA graphs were obtained by a model NETZSCH STA 409C/CO thermal analyzer with in the rate  of 10°C/mins.

X-Ray Powder Diffraction. The X-ray diffractogram of the powdered complex was collected using a Rigaku Miniflex 600 40 kW 15 mA Benchtop Diffractometer, with CuK_α: λ=1.5406 Å. The powdered complex was spread on the glass plate which was then set on the cell holder. The data of reflection were collected in a scan mode with interval of 0.04 steps per 4 seconds within 2 hours at 2-90 degree of 2θ. The X-Ray diffractogram then was refined according to Rietica program of Le Bail method at the range 5-60 degree of 2θ which was run within 75 cycles.

SEM-EDX (Scanning Electron Microscopy with Energy Dispersive X-Ray). The SEM images of the complex were recorded in JEOL JED-2300 model to confirm the crystalinity as well as the content of the main elements

Results and Discussion

Conductance, AAS, TGA-DTA and Chemical Formula

Direct interactions of manganese(II) nitrate and  the phenanthroline in solution resulted in a light yellow-greenish solution of cationic complex which precipitated when the addition of excessive TFA salt. The equivalent electrical conductance of this complex was estimated on the basis of known simple compounds, and the results of measurement are listed in Table 1. It was found that the conductance value for the complex is in the range of those for ionic compounds of three ions per molecule, and hence the empirical formula of [Mn(phen)_n](CF₃COO)₂.xH₂O is then proposed for this complex, indicating a typical uncoordinated anion of TFA.

Table 1: Equivalent electric conductance of the complex and some known salts in aqueous solutions

Compounds	Equivalent conductance	Amount ratio of	Number of ions per molecule
	(Λc) Ω-1cm2mol-1	cation/anion
NH4 NO3	160.429	1:01	2
NiSO4	118.355	1:01	2
MnSO4	119.612	1:01	2
Ca(NO3)2	379.355	1:02	3
Al(NO3)3	519.095	1:03	4
FeCl3	476.975	1:03	4
[Mn(phen)n](CF3COO)2.xH2O	209.125	1:02	3

 

The metal content observed in the atomic absorption spectral data (6.9% in mass) followed by TGA data should estimate the coordination number (n) and the number of hydrates (x) of the empirical formula. As shown in Fig. 1, the mass loss of 2.856% in the first stage at 80-90⁰C is believed due to the loss of 1.35H₂O (ca. 2.873%). While the loss of mass in the second-third-forth stages are complicated to identify, the residual mass of 18.624% at 300-500⁰C seems to be due to α-Mn₂O₃ (ca. 18.664%)¹⁷, leading to Mn content of 6.96% (ca. 6.50%). Theoretically, the mass loss of TFA and phen  are 25.444 and 60.857 %, respectively, while total loss in the second-forth stages (150-400⁰C) is only about 78.52 %. The only thing we can say is that during the loss of TFA, the oxygen atom might react with metal to form final product of Mn₂O₃

Figure 1: The TGA-DTA of [Mn(phen)3](CF3COO)2.1.35H2O at 30-500°C Click here to View figure

 

Thus in summary, the percentage of each entity in the powdered complex as shown in Table 2 might confirm the formula proposed, [Mn(phen)₃](CF₃COO)₂.1.35H₂O, as expected also from stoichiometric preparation.

Table 2: Entity Found in [Mn(phen)₃](CF₃COO)₂.1.35H₂O

Type	Mn	1.35H2O	2(CF3COO)	3(phen)	Residue Mn2O3
Calculated	6.5	2.873	25.444	60.857	18.664
Found	6.9	2.856	24.932	51.731+1.857	18.624
Method	AAS	TGA	TGA	TGA	TGA

Magnetic Moment

The effective magnetic moments of the complex calculated from molar magnetic susceptibility data which were collected on measurement for the three separated preparation were found to be 5.81, 5.89, and 5.92 BM at 291K, being normal paramagnet. These reflect to the spin-only value (μ_s) which corresponds to five unpaired electrons in the electron configuration of Mn(II).  It is normal values in an octahedral environment as observed by some literatues to be 5.89-6.16 BM for various different ligands1^18-21. Much lower in magnetism, however, have been reported to the molecular complex of [Mn(Val)₂(phen)]²², the magnetic moment being only 5.12 BM

Electronic Spectrum

The UV-Vis spectrum of the powder complex as depicted in Figure 2, demonstrates clearly three absorption bands (ν₃, ν₄, and ν₅) at high range of energy, 28000-40000 cm^-1. Another lower shoulder at 24000-26000 cm^-1 (ν₂)and another bump at 17000 cm^-1 (ν₁) might establish the possible electronic transitions following Tanabe-Sugano diagram for MnF₂ and comparably observed by other literatures^{18-19, 23}

Figure 2: UV-Vis spectrum of  [Mn(phen)3](CF3COO)2.1.35H2O Click here to View figure

 

Thus the fully high-spin state of the complex as indicated by the magnetic moment values leads to the ground state of ⁶A_1g, and since other sextet excited states are not possible there should be no spin-allowed transitions, and thus the spin-forbidden transitions to quartet states govern the electronic absorption bands. Therefore, transitions of ⁶A_{1g →} ⁴T_1g(⁴G), ⁶A_{1g →} ⁴T_2g(⁴G), ⁶A_1g→ ⁴E_g,⁴A_1g(⁴G), ⁶A_1g→ ⁴T_2g(⁴D), and ⁶A_1g→ ⁴E_g(⁴D) are then proposed to occure at 17000 cm^-1 (ν₁), 24000-25000 cm^-1 (ν₂), 28000 cm^-1 (ν₃), 35500 cm^-1 (ν₄), and at 39500 cm^-1 (ν₅), respectively

Infrared Spectrum

IR spectrum of this complex was collected at the range of 500-4000 cm^-1(Fig. 3). By overlaying the corresponding spectrum of TFA anion, the main purpose to assign the typical vibration bands of phenanthroline (red) might come straight forward. The broad band at about 3418 and 3433 cm^-1 might be due to -OH stretching of H₂O molecules as indicated by the formula proposed, and also observed by Shad et al²⁴ at about 3441 cm^-1, though Kumar et al²⁵ assigned C-C aromatic at 3430 cm^-1. Another band at 3059 and 2341-2361 cm^-1 should be due to stretching vibration of C-H bonds of phenanthroline which were reported by Chen et al²⁶ at 3064 cm^-1, and Tosonian et al²⁷ at 3051-3068 cm^-1.

The typical ring modes of phenanthroline (ν_C-C and ν_C-N) in the range of 1700-600 cm^-1, might be strongly observed at 1620, 1589, 1415, 1304, and 775 cm^-1, being comparably observed by Ramalakshmi et al for the corresponding iodide³. Other stretching modes are due to TFA, ν_C-F, ν_C-C and ν_C-O, being observed at 1670, 1443, 1203, 1134, 845, 802, 725, and 602 cm^-1.

Figure 3: IR spectrum of  [Mn(phen)3](CF3COO)2.1.35H2O Click here to View figure

 

X-Ray Powder Diffaction and Structural Refinement

Diffractogram of the powdered [Mn(phen)₃](CF₃COO)₂.1.35H₂O was recorded and then analysed. As shown in Fig. 4, the black signs (+) are the experimental data, the red full line represents the result of refinement due to Rietica-Le Bail program performed at the range of 5-60 degree of 2θ within 75 cycles, the blue bar-lines are the expected lines of the symmetry and space group model, while the green line reflects the intensity difference between the experimental data (black) and the result of refinement (red). Clearly, the red full line does pass through almost all of the black experimental signs, as also demonstrated by almost flat green line. This suggests that the refinement of the model fitting to the experimental data is almost perfect. The detailed cell parameters of the structure are recorded in Table 3 together with similar data for some known single crystals of the same cations. Therefore,  this complex synthesized in this work might be considered as triclinic symmetry of PĪ.

Figure 4. XRD profile of [Mn(phen)₃](CF₃COO)₂.1.35H₂O (a-black), the refinement on triclinic symmetry of PĪ (b-red) with it’s posision of 2 θ (c-blue), and the intensity difference between the black and the red lines (d– green).

Figure 4: XRD profile of [Mn(phen)3](CF3COO)2.1.35H2O (a-black), the refinement on triclinic symmetry of PĪ (b-red) with it’s posision of 2 θ (c-blue), and the intensity difference between the black and the red lines (d– green). Click here to View figure

 

Table 3: Detailed cell parameters of [Mn(phen)₃] X, where X= (TFA)₂.1.35H₂O (a)*, (CF₃SO₃)₂.6.5H₂O (b)*²⁸, (B₆H₇)₂ (c)¹⁴, Cl₂ (d)¹⁵, (I₃)₂ (e)³,(ClO₄)₂(H₂CO₃)₂ (f)⁵, and (Rdtp)₂(g) ²⁹.(*this work due to Le Bail method of Rietica program)

X	(a)*	(b)*28	(c)14	(d)15	(e)3	(f)5	(g)29
Symmetry	Triclinic	Triclinic	Triclinic	Tetragonal	Hexagonal	Triclinic	Monoclinic
Space Group	P Ī	P Ī	P Ī	I41/a (no. 88)	R3 (no. 148)	PĪ	C2
a (Å)	13.6422	12.6338	10.3131	35.922(1)	16.456(3)	12.6685(10)	16.55(1)
b (Å)	18.2792	17.0627	13.4839	11.8977(8)	16.456(3)	12.9049(2)	17.50(1)
c (Å)	23.8741	22.481	15.1132		25.864(4)	13.2145(10)	17.46(1)
α (°)	114.4245	106.8768	97.696			86.341(10)
β (°)	94.8337	110.9014	108.324			74.337(10)	93.0(1)
γ (°)	99.7977	99.56265	102.211			73.701(10)
V (Å)	5261.6714	4130.417	1903.92	15352.4(12)	6066(2)	1996.39(4)	4680
Z	1	1	2	16	6	4	4
Figure of merit:
Rp	1.31	3.67
Rwp	2.39	7.34
Rexp	0.56	3.61
Bragg R-Factor	0.04	0.09
GOF	18	4.133	n.a.	n.a.	n.a.	n.a.	n.a.

Sem-Edx

The SEM photographs of [Mn(phen)₃](TFA)₂·1.35H₂O on various magnification reflects the high crystalinity of the complex which is also clearly demontrated by the corresponding diffractogram which shows sharp peaks. Fig. 5(d) might indicate triclinic crystal system as resolved by the Rietica program on the diffractogram.

Figure 5: SEM photographs of [Mn(phen)3](CF3COO)2.1.35H2O at magnification of 100x (a), 1000x (b), 3000x (c) and 10,000x (d) Click here to View figure

 

The corresponding energy dispersive X-ray (EDX) analysis resulted from the selected surface as shown in Fig. 6(a) strongly demonstrates the existence of all elements (Fig. 6(b)) except the hydrogen atom, and therefore, the percentage ratio of the number of atoms, being Mn: 1(2%), C:25.7(51.43%), N:12.7(25.56%), O: 6.6(13.34%), and F: 3.8(7.67%) clearly does not reflect the accurate quantitative stoichiometry.

Figure 6: The image of selected surface of [Mn(phen)3](CF3COO)2.1.35H2O and its EDX analysis result showing the content of elements Click here to View figure

 

Conclusion

The complex of [Mn(phen)₃](CF₃COO)₂.1.35H₂O has been successfully prepared and found to be normal paramagnet of high spin Mn(II). The electronic spectral property of the powdered complex might be resolved into five bands associated with the spin-forbidden transitions of the sexted ground state (⁶A_1g) to quartet states of ⁴T_1g(⁴G), ⁴T_2g(⁴G), ⁴E_g,⁴A_1g(⁴G), ⁴T_2g(⁴D), and ⁴E_g(⁴D). The IR spectral properties signify the mode of vibrations characteristic for phenanthroline and TFA groups. The SEM images show high crystalinity of the powder which is also reflected by its sharp peaks of the diffractogram. Following Rietica method of Le Bail program the diffractogram was analysed to have triclinic symmetry of PĪ space group.

Acknowlegment

Authors should thank to the Faculty of Mathematics and Natural Science (Yogyakarta State University) because of the funding, DIPA-2017 for this work.

References

1. Ito T.; Tanaka N.; Hanazaki I.; Nagakura S. Bulletin of the Chemical Society of Japan. 1969, 42, 702-709.
   CrossRef
2.  Lee C. S.; Gorton E. M.; Neumann H. M.; Hunt Jr. H. R. Inorg. Chem. 1966, 5, 1397-1399.
   CrossRef
3.  Ramalakshmi D.; Reddy K. R.; Padmavathy D.; Rajasekharan M. V.; Arulsamy N.; Hodgson D. J. Inorganica Chimica Acta. 1999,  284, 158-166.
   CrossRef
4. Horn C.; Scudder M.; Dance I. Cryst. Eng. Comm. 2001, 1, 1-8.
   CrossRef
5. Kani I.; Atlier Ö.; Güven K. J. Chem. Sci. 2016, 128, 523-536.
   CrossRef
6. Trifluoroacetic Acid. Available at: http://www.commonorganicchemistry.com/Common_Reagents/Trifluoroacetic_Acid/Trifluoroacetic_Acid.htm
7. Table of Acids with Ka and pKa Values (Compiled from Appendix 5 Chem 1A, B, C Lab Manual and Zumdahl 6th Ed.The pKa values for organic acids can be found in Appendix II of Bruice 5th Ed.). Available at:   http://clas.sa.ucsb.edu/staff/Resource%20folder/Chem109ABC/Acid,%20Base%20Strength/Table%20of%20Acids%20w%20Kas%20and%20pKas.pdf
8. Zheng Z.; Knobler C. B.; Curtis C. E.; Hawthorne M. F. Inorg. Chem. 1995, 34, 432-435.
   CrossRef
9. Tunyogi T.; Deák A. Acta Cryst. 2010, C66, m133-m136.
10. Peedikakkal A. M. P.; Song Y. M.; Xiong R. G.; Gao S.; Vittal J. J. Eur. J. Inorg. Chem. 2010, 3856-3865.
    CrossRef
11. Reiß G. J.; Meyer M. K. Z. Naturforsch. 2010, 65b, 479-484.
    CrossRef
12. Robinson S. D.; Sahajpal A. J. Chem. Soc. Dalton Trans. 1997, 3349-3351.
    CrossRef
13.  Agambar C. A.; Orrell K. G. J. Chem. Soc. (A), 1969, 897-904.
    CrossRef
14. Polyanskaya T. M.; Drozdova M. K.; Volkov V. V.; Myakishev K. G. Journal of Structural Chemistry. 2009, 50, 368-372.
    CrossRef
15. Wang X., Z. Kristallogr. 2013, NCS 228, 5-6.
16. Figgis B. N. and Lewis, J., Modern Coordination Chemistry, Edited by Lewis, J., and Wilkins, R. G., Interscience: New York, 1960: 400.
17. Pattanayak J.; Rao V. S.; Maiti H. S.; Thermochimica Acta. 1989, 153, 193-204.
    CrossRef
18. Singh B. K.; Mishra P.; Prakash A.; Narendar Bhojak N. Arabian Journal of Chemistry. 2012 (2017), 10, S472-S483 (to be published). https://doi.org/10.1016/j.arabjc.2012.10.007
    CrossRef
19. Verma R. International Journal of Pharmaceutical Sciences and Research. 2017, 8(3): 1504-1513.
20. Devereux M.; McCann M.; Leon V.; Kelly R.; O’Shea D.; McKee V. Polyhedron. 2003, 7, 3187-3194.
    CrossRef
21. Devereux M.; McCann M.; Leon V.; Geraghty M.; McKee V.; Wikaira J. Metal Based Drugs. 2001, 7, 275-288.
    CrossRef
22. Fayad N. K. ; Al-Noor T. H.; Mahmood A. A.; Malih I. K. Chemistry and Materials Research. 2013, 3, 66-73.
23. Lever A. B. P., Inorganic Electronic Spectroscopy, Second Edition, Elsevier: New York, 1968: p. 293.
24. Shad H. A.; Thebo K. H.; Ibupoto Z. H.; Malik M. A.; O’Brien P.; Raftery J. Journal of Coordination Chemistry. 2011, 64, 2353-2360.
    CrossRef
25. Kumar S. P.; Suresh R.; Giribabu K.; Manigandan R.; Munusamy S.; Muthamizh S.; Narayanan V. International Journal of ChemTech Research. 2014, 6, 3280-3283.
26. Chen H.; Xu X.-Y.; Gao J.; Yang X. J.; Lu L. D.; Wang X. Acta Phys. –Chim. Sin. 2006, 22, 856-859.
27. Tosonian S.; Ruiz C. J.; Rios A.; Frias E.; Eichler J. F. Open Journal of Inorganic Chemistry. 2013, 3, 7-13.
    CrossRef
28. Sugiyarto K. H.; Saputra H. W.; Permanasari L.; Kusumawardani C. AIP Conference Proceedings. 2017, 1847, (040006)1- 7.

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