Synthesis , Spectral Characterization , Magnetic Moment , Mass , X-Ray Diffraction , and Kinetic ( TGA ) Studies of Cr ( III ) Complex with Metformin : An Oral Antidiabetic Drug

The present work describes synthesis, spectral characterization, and X-ray diffraction studies of Cr(III) with “metformin,” an oral antidiabetic drug. The conductometric titration using monovariation method indicates that complex is ionic and of L 2 M type. Analytical data agree with the molecular formula of complex namely, [(C 4 H 11 N 5 ) 2 Cr(H 2 O) 2 ]2Cl. Geometry of complex assigned is octahedral, supported by IR, electronic spectra, magnetic moments, H-NMR and mass studies, and proposed structure (I) for complex. X-ray diffraction data also supports the complex formation and symmetry of complex and various parameters like particle size, porosity, volume of unit cell, and density have been calculated.The Freeman-Carroll and Sharp-Wentworthmethods have been used to calculate activation energy and thermal stability.The activation energy (E a ) calculated with the help of these methods are in agreement with each other. Thermodynamic parameters such as free energy change (ΔF) and entropy change are also determined on the basis of TG curves and by using data of the Freeman-Carroll method.


Introduction
Metal ions are required for many critical functions in humans.Scarcity of some metal ions can leads to disease [1].Well-known example are pernicious anemia resulting from iron deficiency; growth retardation arising from insufficient dietary of zinc and heart disease in infants due to copper deficiency.The ability to recognize and to understand at the molecular level and the diseases caused by inadequate metalion function constitutes an important aspect of medicinal bioinorganic chemistry.Understanding the biochemistry and molecular biology of natural detoxification mechanisms and designing and applying ion-specific chelating agents to treat metal overloads are two components of a second major aspect of the new science that is evolving at the interface of bioinorganic chemistry and medicine.
Diabetes is a deceptive disease and if not detected in early stage may cause even death.It is considered hereditary but actual genetic disorder is still a mystery.Several million people are suffering from this disease all over the world which is reported by Sadilot and Phatak [2], Bloomgarden [3], and Sanger and Thompson [4].Zinc-insulin was discovered as early as in 1921 and later it proved to be a very efficacious medicine in the treatment of diabetes mellitus.To avoid the daily pricks of hypodermic syringe, oral hypoglycemic agents were discovered, which has revolutionized the treatment of diabetes.It is worthwhile to mention here that the majority of the essential metallic elements of biological importance are transition metals, whose ability to form coordination complexes and a chelate is the characteristic aspects of their chemistry.
In recent years much attention is given to the use of sulphonylurea because of their high complexing nature with essential metals.Sulphonylurea are effective for noninsulin dependent diabetes mellitus, reported by Sadilot and Phatak [2], Bloomgarden [3], and Sanger and Thompson [4].These compounds are completely absorbed by oral administration.They are metabolized by liver and are excreted predominantly through urine.Complexation of sulphonylurea with lighter transition metals has been studied in detail by Yoshinaga and Yamamotto [5,6] and complexation of sulphonylurea and biguanidine by Iqbal et al. [7] and others [8].
Metformin is sold under several trade names, including Glucophage XR, Riomet, Fortamet, Glumetza, obimed, Gludormin, Diarber, Diaber, and Diaformin.Broad interest in metformin was not rekindled until the withdrawal of the other biguanides in the 1970s.Generic formulations are now available in several countries and metformin is believed to have become the most widely prescribed antidiabetic drug in the world; therefore looking the importance of metformin, we have studied metformin complex with chromium (see Scheme 1).

Ligand-Metal Ratio
(a) Pure "Metformin" 0.828 gram (0.005 M) was dissolved in 100 mL of ethanol and 0.66 gram (0.01 M) chromium chloride in 100 mL ethanol.10 mL metformin solution was diluted to 100 mL in a beaker.This was titrated conductometrically against chromium chloride solution taken in burette using fractions of 1 mL.Conductance was recorded after each addition with proper stirring at temperature 30 ± 1 ∘ C. Results were plotted in the form of a graph between corrected conductance and volume of metal salt.From the equivalent point in the graph, ratio between metal and ligand was noted to be 1 : 2. (b) Formation of 2 : 1 (L 2 M) ratio was also confirmed by Job's method [9] of continuous variation as modified by Turner and Anderson [10] (Table 1 and Figures 1(a) and 1(b)) using absorbance as index property; from these values the stability constant (log k) and free energy change (−Δ) were also calculated by using formula  = /(( where k = Stability constant, x = Conc. of complex, and Δ = − ln k, respectively.

Synthesis of Complex.
The chemicals used in this synthesis were all of analytical grade E-Merck.A weighed  = 1.5060Å).The samples were scanned in the range from 10.0084 to 79.9804 (2).Powder data were indexes using computer software (FPSUIT V 2.0).TGA spectra were recorded in IIT Bombay, India.
From stoichiometry and analytical data, the composition of the complex comes out to be [(C 4 H 10 N 5 ) 2 Cr(OH 2 ) 2 ]2Cl, which favors 2 : 1 (L 2 M) ratio.The tentative structure (I) assigned to complex, on the basis of analytical data, was further supported by XRD data.Mass spectra of [(C 4 H 10 N 5 ) 2 Cr(OH 2 ) 2 ]2Cl complex were recorded at C.D.R.I Lucknow, India, which provides information about the complex by examination of the fragmentation pattern and total mass of the complex.Proposed structures (I) for the isolated complex are also supported by mass spectral studies.

Electronic Spectral Studies.
In Cr(III) complex of "Metformin" the observed magnetic moment is 4.38 and bands observed at 380 cm −1 attributed to charge transfer while the values 350 cm −1 and 295 cm −1 are assignment to the 4 A 2g (F) → 4 T 2g (F), 4 A 2g (F) → 4 T 1g (F) transition, respectively, suggest a octahedral environment around "Metformin"-chromium complex.

Magnetic Moments Studies.
In the present study, the room temperature of magnetic susceptibility measurement was carried out on a vibration sample magnetometer (VSM) at the Indian Institute of Technology, Roorkee (India).The hysteresis curve given in Figure 2 shows the magnetic behavior of complex.The magnetic measurements of complex were carried out on Vibrating Sample Magnetometer (mode 155) which directly gives the  eff values without diamagnetic correction with the help of the following equation: where  is reading of magnetic moment on magnetometer,  is absolute temperature,  is molecular weight of the compound,  is weight of the sample, and  is magnetic field.Magnetic moment has been calculated and is equal to 4.38 B.M.; moreover hysteresis curve has been shown in Figure 2 that gives the information about magnetism; that is, "metformin"-chromium complex is diamagnetic.

1 H-NMR Studies
. By 1 H-NMR spectral data it was observed a signal of imide (NH) proton around (6.93) in the spectrum of the ligand and found less broad band in complex (Figure 4).This also confirms the deprotonation of imide -NH group through enolization (the appearance of >C=N stretching band observed in IR spectra).Assignment of "Metformin"-chromium complex, molecular formula The proposed structure for the isolated complex is also supported by Slichter [17], Akit [18], and others [19].
3.6.Mass Spectral Studies.Mass spectrum (Figure 5) of the compound is a plot which represents the intensities of the signals at various m/z values.It is highly characteristic of the compound that no two compounds can have similar mass spectra.It provides information regarding the molecular structure of organic and inorganic compounds [20].We have found various fragmentation peaks but in the present paper focused only on molecular ion peak of the "Metformin" complex of chromium and assignments are molecular formula of "Metformin"-chromium complex [ base peak ion 100% relative abundance.
The number of peaks in "metformin" is 13 while that of "Metformin"-chromium complex is 14 as shown in X-ray diffractogram (Figure 6), respectively, indicating that complex formed is a well-knit one; moreover all the reflections are new ones and the patterns are fairly strong.The X-ray pattern have been indexed by using computer software (FPSUIT 2.0V) and applying interactive trial and error method keeping in mind the characteristics of the various symmetry system, till a good fit was obtained between the observed and the calculated sin 2  value.The unit cell parameters were calculated (Table 2) from the indexed data and from  cell data and crystal lattice parameters of Cr(III) indicate complex attributed to monoclinic crystal system.Powder Xray diffraction patterns indicate that the synthesized complex are of monoclinic structure.The particle size is 8.301 microns which is calculated from X-ray line broadening using the Scherrer formula ℎ = /ℎcos , where  is the particle diameter in ångstroms,  is a coefficient and is equal to 0.89 here,  is the half-maximum line width, and  is the wavelength of X-rays and porosity is 0.0207 calculated by formula ( true −  obs × 100)/ true and volume of the unit cell is 13748 calculated by Volume( sin ) Å where a, b, and c are lattice parameters.Density = Weight/Volume is found as 0.03007 g/cm 3 for "metformin"-chromium complex.Space group is Pmmm and  = 90 ∘ ,  = 89.2∘ , and  = 90 ∘ .

Thermal Study of Metformin-Chromium Complex.
In the present investigation, the weight loss for complex was calculated within corresponding temperature ranges of 20-600 ∘ C. The complex is thermally decomposed in two decomposition steps within the temperature range of 50-600 ∘ C. The TGA/DTA curves for the complex are shown in Figure 7.
The thermoanalytical data of Freeman-Carroll and Sharp-Wentworth method are presented in Tables 4 and 5.In studying the decomposition kinetics methods mentioned in the literature were used in each case in which the least square plots were drawn.The first few points that did not fall on straight line were discarded.These types of deviations of points are reported in literature by several research workers [26].This is explained as due to the failure of obeying as first order kinetics always by the solids in their decomposition in the early stages.

Theoretical Consideration.
To provide further evidence regarding the degradation system of analyzed compounds we derived the TG curves by applying an analytical method proposed by Freeman-Carroll [27] and Sharp-Wentworth [28].[27].The straight line equation derived by Freeman and Carroll is in the form of

Freeman-Carroll Method
where / = rate of change of weight with time,   =   − ,   = Wt.loss at completion of reaction,  = Total wt.loss up to time "t, "   = Energy of activation, and  = Order of reaction.
The plot between the term Δ log(/)/Δ log   versus Δ(1/)/Δ log   gives a straight line (Figure 8) from which slope can be calculated; also we obtained energy of activation (  ) and intercept on -axis as order of reaction () and the change in entropy (ΔS) also.[28].Using the equation derived by Sharp and Wentworth,

Sharp-Wentworth Method
where / = Rate of change of fraction of weight with change in temperature and  = Linear heating rate /, by plotting the graph between Δ log(/)/(1 − ) versus 1/.We obtained the straight line (Figure 9) which gives energy of activation (  ), from its slope.
Table 3 is given for thermodynamic parameters of complexes, namely, activation energy (  ).Entropy (Δ) and Gibb's free energy change of decomposition (Δ ∘ ) are evaluated graphically by employing Freeman-Carroll and Sharp-Wentworth relation.The activation energies of decomposition are found to be in the range from 40 to 46 kJ Mol −1 .The high value of activation energies reflects the thermal stability of complex.The entropy of activation is found to have negative values in complex which indicate that decomposition reactions process with lower rate than the normal ones [26,[29][30][31][32][33].In TGA curve (Figure 7) the initial loss (8.30%) (also given in Tables 4 and 5) is due to two molecules of water of crystallization (70-90 ∘ C) in complex which is in agreement with theoretically calculated percentage of water molecule, that is, 8.70%.The 52% loss at temperature range 100-550 ∘ C  is due to loss of organic moiety and above 550 ∘ C there is no loss (45%) which is due the formation metal oxide.

Discussion
For supporting the proposed structure of chromium complex with "metformin" initially monovariation method was conducted which indicates 1 : 2 metal : ligand ratio which was further confirmed by Job's method of continuous variation as modified by Turner and Anderson (Scheme 2).Moreover stability constant and free energy change were also calculated from modified Job's method.From stoichiometry and analytical data, the composition of the complex comes out to be [(C 4 H 10 N 5 ) 2 Cr(OH 2 ) 2 ]2Cl, which favors 2 : 1 (L 2 M) ratio.The proposed structure (I) for chromium complex of "metformin" was further supported by spectroscopic methods like IR, 1 H-NMR, electronic spectra and mass spectral studies, and so forth.The biguanidine unit inside the coordination sphere has a positive charge while Cl − occupies the outer place of the coordination sphere.The IR spectral assignments show the M-N frequency and confirm the linkage of N 4 to the metal through the covalent bond.Moreover, the difference in the NMR peak of the drug (wide peak) and the complex (narrow peak) are further supporting the removal of one of the "H" from the N-H group through enolisation.Mass spectral results further support the formation of complex which observed a molecular ion peak.A detailed study of X-ray also supports the complex formation and the various parameters like particle size, porosity, volume of unit cell, density, and crystal system were evaluated and discussed.Thermal analysis has been studied by giving their relative thermal stability, applying the Sharp-Wentworth and Freeman-Carroll methods: energy of activation (  ) and kinetic parameter, namely, Δ, Δ, and order of reaction () were determined by applying Freeman-Carroll method.TGA curve gives strong support of presence of coordinate water molecule.

Conclusion
In the present paper we have syntheses complex of antidiabetic drug with chromium and have given its spectral characterization and kinetics parameters, which gives the

Table 2 :
Cell data and crystal parameter of metformin-chromium complex.Heat from 30.00 ∘ C to 630.00 ∘ C at 20.00 ∘ C/min

Table 3 :
Thermogravimetric data of metformin complex with corresponding to heating rate of 10 ∘ C/min.

Table 4 :
Thermogravimetric data of metformin-chromium complex by Sharp-Wentworth method.

Table 5 :
Thermogravimetric data of metformin-chromium complex by Freedman and Carroll method.