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Original article
9 (
1_suppl
); S54-S63
doi:
10.1016/j.arabjc.2011.02.018

Manganese(II) chelates of bioinorganic and medicinal relevance: Synthesis, characterization, antibacterial activity and 3D-molecular modeling of some penta-coordinated manganese(II) chelates in O,N-donor coordination matrix of β-diketoenolates and picolinate

, , , ,
 Coordination and Bioinorganic Chemistry Laboratory, Department of P.G. Studies and Research in Chemistry, Rani Durgavati University, Jabalpur 482 001, Madhya Pradesh, India

⁎Corresponding author. Tel.: +91 761 2601303; fax: +91 761 2603752. rcmaurya1@gmail.com (R.C. Maurya)

Received: , Accepted: ,
© The Author(s) 2011
Disclaimer:
This article was originally published by Elsevier and was migrated to Scientific Scholar after the change of Publisher.

Abstract

Four new mixed-ligand complexes of manganese(II) of the composition [Mn(pa)(L)(H2O)], where paH = picolinic acid and LH = acetoacetanilide (aaH), o-acetoacetanisidide (o-aansH), o-acetoacetotoluidide (o-aatdH) or ethylacetoacetate (eacacH), have been synthesized by the interaction of MnCl2·4H2O with the said ligands in aqueous-ethanol medium. The complexes so obtained have been characterized on the basis of elemental analyses, molar conductance and magnetic measurements, thermogravimetric analyses, mass, EPR, infrared and electronic spectral studies. Synthesized complexes have shown significantly greater antibacterial activity against Escherichia coli and Vibrio cholera than ligands. The 3D-molecular modeling and analysis for bond lengths and bond angles have also been carried out for one of the representative compound [Mn(pa)(aa)(H2O)] (1) to substantiate the proposed structures.

Keywords

Manganese(II) chelates
O,N-donor ligands
Penta-coordinate
3D-Molecular modeling
Bioinorganic and medicinal relevance
1

1 Introduction

The half-filled 3d5 shell of the Mn(II) ion makes it behave as a spherically polarizable ion, with no crystal-field stabilization energy in the high-spin complexes (Blake and Cotton, 1964). Therefore, there are no electronic restraints, and the structures of Mn(II) complexes are those that minimize the sterical repulsion: octahedral and tetrahedral; although in biological systems there are also examples of five-coordination (Stallings et al., 1985) or even seven-coordination (Gill and Eisenberg, 2001; Johnson et al., 2001).

The great upsurge in the coordination chemistry of manganese(II) involving oxygen and nitrogen donor ligands comes from the increasing recognition of the biological role of this metal ion (Christou, 1989; Pecoraro et al., 1994; Riley, 1999; Que and True, 1990; Pecoraro, 1992). Among the first-row transition metal ions, the high-spin Mn(II) centers are quite desirable because they contain the highest possible number of unpaired electrons (Yokum and Pecoraro, 1999). Mn(II) is found in a variety of enzymes such as pyruvate carboxylase, oxaloacetate decarboxylase, superoxide dismutases and diamine oxidases, among others (Mukhopadhyay et al., 2004). Manganese is also well known to be required for oxygen evolution in the photosystem-II (Hurley et al., 1960; Tuormaa, 1996; Hughes, 1988). Moreover, it is involved in many biological processes including hydrogen peroxide decomposition or superoxide dismutation, and it appears to play an important role in several metabolic processes such as bone growth, glucose tolerance, reproduction and development of the inner ear (Vaiana et al., 2007).

Mitochondria play a central role in cell life and death and are known to be important in a wide range of diseases including the cancer, diabetes, cardiovascular disease, etc. (Armstrong, 2007). The unique structure and functional characteristics of mitochondria enable the selective targeting of drugs designed to modulate the function of this organelle for therapeutic gain (Gogvadze et al., 2009). Manganese(II) ions are the required co-factor for many ubiquitous enzymes. The mitochondria can accumulate Mn(II) ions through an ATP dependent Ca transporter (Frezza and Gottlieb, 2009). Transport mechanism of Mn(II) in vivo may make Mn(II)-based compounds possible to be tumor-targeted. Research has proven that Mn(II) ions were mainly up taken and transported by a divalent metal transporter and transferring receptor (Aschner et al., 2007), which were highly expressed in some tumor tissues (Calzolari et al., 2007; Sciot et al., 1990).

Two new Mn(II) complexes in the ONNN-donor coordination matrix, namely [(Etdpa)MnCl2] (1) and [(Adpa)Mn(Cl) (H2O)] (2) are reported (Chen et al., 2010) to be active against the four cancer cells (MCF-7, ECA-109, U251, HeLa). Especially, complex (2) inhibits the proliferation of cancer cells MCF-7 and U251 with IC50 values of 6.5 and 9.5 μM. The highly inhibiting activity complex (2) on the tumor cells in vitro may be due to the good mitochondrial membrane potentials induced permeation. The direct interaction between the complex (2) with mitochondria could be the main reason for its cytotoxic activity.

β-Diketones and related derivatives are considered to be a class of very important ligands in the growth of coordination chemistry. Their complexes have been thoroughly studied. Due to the presence of two oxygen donor atoms and facile keto-enol tautomerism, they easily coordinate with metal ions after deprotonation of the enolic hydrogen and give rise stable metal complexes with six membered chelate rings. Water molecules are usually associated with metal β-diketones when the coordination number of the metal ion is more than four (Maurya et al., 1991; Maurya, 2003).

Found in humans as a metabolite of the amino acid tryptophan and is, therefore, less toxic to mammals (Yasumatsu et al., 2007), picolinic acid (pyridine-2-carboxylic acid) forms a variety of metal complexes. Picolinic acid and its metal complexes have been the target of several studies in the last few years because of their broad spectrum of physiological effects (Yasui et al., 2002; Sakurai et al., 1995; Hepburn and Vincent, 2003; Chaudhary et al., 2005; Kanth and Mishra, 2005). In the search for developing compounds that can replace or mimic insulin, the bis(picolinato)oxovanadaium(IV) complex, [VO(pic)2] (having N2O2 coordination environment) has been found to be a good candidate to do so and to provide a long-term acting insulin-mimetic activity [Yasui et al., 2002; Sakurai et al., 1995]. The insulinomimetic effect of this compound results from its ability to normalize the blood glucose levels in several types of diabetic animals. Tris(picolinato)chromium(III), Cr(pic)3 is considered to be among the most popular nutritional supplements (Hepburn and Vincent, 2003) and the compound showed DNA cleavage in the presence and absence of H2O2 (Chaudhary et al., 2005). Also, iron, zinc and copper complexes of picolinic acid exhibited antibacterial and antifungal activities (Kanth and Mishra, 2005). Moreover, Fe(III) and Mn(II) picolinates were used to disproportionate the superoxide ion ( O 2 · - ), an ion which may cause damage to the animal cells, in a matter related to the metalloprotein superoxide dismutase (SOD) (Jain and Bhattacharyya, 2005; Yamaguchi et al., 1986). The picolinic acid is known to bind a variety of metal ions ranging from alkali metals, transition metals and lanthanides (Goher and Mautner, 1995; Mautner et al., 1997; Zhang et al., 2005; Heren et al., 2006a,b; Vargova et al., 2004; Chattopadhyay et al., 2003; Orjiekwe et al., 2005; Wu et al., 2005; Sendor et al., 2003; Fang et al., 1996). This may result from the versatility of the picolinate ion to act as a monodentate ligand via the oxygen atoms of the carboxylate group or through its pyridine nitrogen atom or to act as a bidentate chelating agent via both the nitrogen and oxygen (Mautner and Massoud, 2007).

Maurya et al. have recently reported some new monomeric (Maurya et al., 2003a,b,c) and dimeric (Maurya et al., 2008a,b) manganese(II) complexes with a variety of Schiff bases derived from o-vanillin and furyl methyl ketone, in tetrahedral and octahedral stereochemistries. In a recent communication, Maurya and Sutradhar (2008) have reported the synthesis and characterization of two mixed-ligand cyanonitrosyl Mn(I) and two mixed-ligand dinitrosyl Mn(0) complexes involving {MnNO}6 and {Mn(NO)2}7 electron configurations, respectively.

In view of the diverse and important applications of metal complexes having (N,O)- and (O,O)-donor coordination environments in general and manganese compounds in the (N,O)-donor coordination matrix in particular, a study was undertaken of the coordination chemistry of some mixed-ligand manganese(II) complexes involving picolinic acid (O,N-donor) and some β-diketoenolates (O,O-donor), such as, actoacetanilide (aaH), o-acetoacetanisidide (o-aansH), o-acetoacetotoluidide (o-aatdH) or ethylacetoacetate (eacacH).

2

2 Experimental

2.1

2.1 Materials

Manganese(II) chloride tetrahydrate (BDH, Chemicals, Bombay), 2-picolinic acid (Sisco Chem. Pvt. Ltd., Mumbai), acetoacetanilide, o-acetoacetanisidide, o-acetoacetotoluidide (Aldrich Chemical Co., USA), ethylacetoacetate and DMF (Tomas Baker Chemical Ltd., Mumbai), ethanol (Bengal Chemicals and Pharmaceuticals Ltd., Kolkata) were used as purchased. All other chemicals used were of analytical reagent (AR) grade.

2.2

2.2 Preparation of mixed-ligands complexes

To a solution of 0.001 M of picolinic acid (0.123 g) in 10 mL of ethanol was added 0.001 M of acetoacetanilide (0.177 g), o-actoacetanisidide (0.207 g), o-actoacetotoluidide (0.191 g) or ethyl acetoacetate (0.13 mL) in 10 mL of ethanol. The resulting mixture of the two appropriate ligands was added slowly with constant stirring to the solution of 0.001 M of MnCl2·4H2O (0.198 g) in 1 mL water and 10 mL of ethanol. After refluxing the mixture for ∼4 h, the pH of the solution was raised by drop wise addition of dilute aq. NH3 solution when a dark brown solid precipitated immediately. After heating the mixture over a hot plate at 50 °C for 15 min, the solid mass was suction filtered, repeatedly washed with 1:1 ethanol water mixture and dried in vacuo. The analytical data of the complexes are given in Table 1.

Table 1 Analytical data and some physical properties of the synthesized complexes.
S. No. Complexes (Empirical formula) (M.W.) Found (Calcd), (%) Color Decomp. temp. (°C) ΛM (Ω−1 cm2 mol−1) Yield (%) μeff (B.M.)
C H N Mn
1 [Mn(pa)(aa)(H2O)] 51.98 4.24 7.49 14.58 Leaf brown 244 12.4 60 5.86
(C16H16N2O5Mn) (370.93) (51.76) (4.31) (7.55) (14.81)
2 [Mn(pa)(o-aans)(H2O)] 50.68 4.62 6.65 13.49 Leaf brown 258 14.3 70 5.89
(C17H18N2O6Mn) (400.93) (50.88) (4.49) (6.98) (13.70)
3 [Mn(pa)(o-aatd)(H2O)] 52.89 4.80 7.45 14.10 Brown 265 13.6 64 5.92
(C17H18N2O5Mn) (384.93) (53.00) (4.68) (7.27) (14.27)
4 [Mn(pa)(eacac)(H2O)] 44.60 4.45 4.51 16.73 Leaf brown 240 13.0 68 5.85
(C12H15NO6Mn) (323.93) (44.45) (4.63) (4.32) (16.96)

3

3 Physical methods

Carbon, hydrogen and nitrogen were determined micro-analytically at C.D.R.I., Lucknow. Manganese was determined as ammonium manganese phosphate monohydrate, MnNH4PO4·H2O using the standard procedure (Vogel, 1979) after decomposing the complexes with concentrated nitric acid and then with concentrate sulphuric acid. From the weight of MnNH4PO4·H2O manganese content of the complex was calculated in each case.

Solid-state infrared spectra were recorded in KBr pellets using a Perkin–Elmer model 1620 FT-IR spectrophotometer and mass spectra of a compound was recorded at S.A.I.F., C.D.R.I., Lucknow. Magnetic measurements were performed by the vibrating sample magnetometer method at Indian Institute of Technology, Roorkee. Conductance Measurements were made at room temperature in dimethylformamide (DMF) solution using Toshniwal Conductivity Bridge and dip type cell with a smooth platinum electrode of a cell constant 1.02. The decomposition temperatures of the complexes were recorded using an electrically operated melting point apparatus (Kumar Industries, Bombay) of heating capacity up to 360 °C. The X-Band EPR spectra of compounds were recorded at LNT on ESP X-Band EPR spectrophotometer using a powdered sample at the microwave frequency of 9.1 GHz. Thermogravimetric analysis was done by heating the sample at the rate of 15 °C min−1 up to 1000 °C on a thermal analyzer at R.S.I.C., I.I.T., Mumbai. Electronic spectra of the complexes were recorded in DMF on a Systronics Double Beam UV–vis Spectrophotometer, model 2201.

3.1

3.1 3D-Molecular modeling studies

The 3D-molecular modeling of a representative compound was carried out on a CS Chem 3D Ultra Molecular Modeling and Analysis Programme (www.cambridgesoft.com). It is an interactive graphics programme that allows rapid structure building, geometry optimization with minimum energy and molecular display. It has the ability to handle transition metal compounds.

4

4 Results and discussion

The mixed-ligand manganese(II) complexes in the present investigation were prepared as given in Scheme 1. These complexes are found to be air stable. They are thermally stable and their decomposition temperatures are given in the Table 1. These are insoluble in the most of the common organic solvents but are fairly soluble in DMF. These complexes are characterized on the basis of their elemental analysis, molar conductance and magnetic measurements, thermogravimetric analysis, infrared spectra, electron spin resonance, mass and electronic spectral studies.

Synthesis of mixed-ligand Mn(II)complexes.
Scheme 1
Synthesis of mixed-ligand Mn(II)complexes.

4.1

4.1 Infrared spectral studies

The characteristic IR spectral bands and their plausible assignments of the synthesized complexes are given in Table 2. In the complexes (1)–(3), the two important modes on account of coordination of acetoacetanilide, o-acetoacetanisidide or o-acetoacetotoluidide (acetoacetylarylamides) anions are ν(C⚌O) (acetyl carbonyl) and ν(C⚌O) (amide carbonyl). These two modes have been observed at 1571–1578 and 1602–1604 cm−1, respectively, in the said complexes, which are in agreement with the reported results (Maurya and Singh, 2004). These three complexes exhibit ν(NH) mode at ∼3280 cm−1, which is almost similar to the uncoordinated acetoacetylarylamides. This suggests that the amide nitrogen of the acetoacetylarylamides is not taking part in the coordination. The characteristic absorption bands due to coordinated ethyl acetoacetate anion are ν(C⚌O) (acetyl) and ν(C⚌C) modes, and these have been observed at 1604 and 1571 cm−1, respectively, in the complex (4). This result is comparable to the data reported elsewhere (Maurya and Rajput, 2004) for such compounds.

Table 2 Some important IR spectral bands (cm−1) of the synthesized complexes.
S. No. Complexes νas(COO)(pa) (a) νs(COO)(pa) (b) Δ = (a) − (b) ν(C⚌O) (acetyl carbon.) ν(C⚌O) (Amide carbon.) ν(C⚌C) ν(C⚌N) (pa) PRB mode ν(OH) ν(Mn–O) ν(Mn–N)
1 [Mn(pa)(aa)(H2O)] 1643 1348 295 1578 1602 – a 1030 3418 545 457
2 [Mn(pa)(o-aans)(H2O)] 1643 1347 296 1571 1604 – a 1032 3448 546 459
3 [Mn(pa)(o-aatd)(H2O)] 1640 1348 293 1571 1604 – a 1032 3448 546 459
4 [Mn(pa)(eacac)(H2O)] 1644 1347 297 1604 – 1571 b 1032 3448 546 459

PRB = pyridine ring breathing mode.

a Merged with ν(C⚌O) amide.
b Merged with ν(C⚌O) acetyl.

Picolinic acid used as a co-ligand in all the complexes contains two potential donor sites: (i) the pyridine ring nitrogen and (ii) the carbonyl oxygen. Coordination of the pyridine ring nitrogen of the picolinic acid should display a strong band due to ν(C⚌N) at ∼1600 cm−1 in these complexes. But due to presence of ν(C⚌O) (amide/acetyl carbonyl) of the ligands acetoacetanilide/o-acetoacetanisidide/o-acetoacetotoluidide and ethyl acetoacetate in this range, it is difficult to say with certainty the coordination of ring nitrogen of picolinic acid to manganese. However, the presences of the pyridine ring breathing mode (Maurya et al., 1995) at ∼1030 cm−1 in these complexes suggests the coordination of the pyridine ring nitrogen to manganese in these complexes. The asymmetric and symmetric vibrations of the carboxylate group of picolinic acid appear at 1643–1644 and 1347–1348 cm−1 (Jain et al., 2004), respectively, showing a difference (Maurya et al., 2003a,b,c) of 293–297 cm−1. These observations suggest the monodentate coordination of the carboxylate group. The non-ligand bands occurring at 545–546 cm−1 (broad) and 457–459 cm−1 (sharp) have been assigned to ν(Mn–O) and ν(Mn–N), respectively (Maurya et al., 2003a,b,c).

In all the complexes the presence of coordinated/lattice water is indicated by the presence of a broad band in the region of 3418–3448 cm−1. The IR spectrum of compound [Mn(pa)(o-aatd)(H2O)] (3) is given in Fig. 1.

IR spectrum of [Mn(pa)(o-aatd)(H2O)] (3).
Figure 1
IR spectrum of [Mn(pa)(o-aatd)(H2O)] (3).

4.2

4.2 Conductance measurements

The molar conductance values of the synthesized complexes in 10−3 M DMF solution are in the range 12.6–16.3 ohm−1 cm2 mol−1 (Table 1). These results are indicative of the non-electrolytic nature of these complexes (Howader et al., 2008; Geary, 1971).

4.3

4.3 Magnetic measurements

The manganese(II) belongs to S = 5/2 system. Magnetically dilute manganese(II) complexes usually exhibit magnetic moments close to their spin only value of 5.92 B.M. because of the additional stability of the half filled d-shell. At room temperature, the observed values of the magnetic moment for the present complexes are in the range of 5.85–5.92 B.M. (Table 1), as expected for a simple of S = 5/2 paramagnet with a 6S ground state. These data suggest that the complexes under this investigation are mononuclear. The magnetic moment of both octahedral and tetrahedral complexes should be the same since a 6S ground state persists in both symmetries. Thus, the magnetic results give no specific information about the geometry of these compounds (Lal et al., 2007).

4.4

4.4 Electronic spectral studies

Electronic spectra of two complexes (1) and (4) were recorded in 10−3 M DMF solutions. The electronic spectral peaks observed in each of the complexes along with the molar extinction coefficients are given in the Table 3. The two high intensity peaks in the ultraviolet region in each of the complexes are due to intra ligand n–π∗/π–π∗ transitions, while the third peak near the visible region may be due to the ligand to metal charge transfer transitions (Bonomon et al., 1991).

Table 3 Electronic spectral data of the synthesized complexes.
Compound no. Complexes λmax (nm) ν (cm−1) ε (L mol−1 cm−1) Peak assignment
(2) [Mn(pa)(o-aans)(H2O)] 286 34,965 2287 Intra ligand transitions
355 28,169 2262
391 25,575 2285 Ligand → metal charge transfer transition
(4) [Mn(pa)(eacac)(H2O)] 272 36,764 2290 Intra ligand transitions
358 27,932 2278
385 25,974 2284 Ligand → metal charge transfer transition

4.5

4.5 EPR spectral studies

The X-band EPR spectrum of a representative compound namely [Mn(pa)(eacac)(H2O)] (4) (Fig. 2) at liquid nitrogen temperature (LNT) was recorded in the powder form at the microwave frequency 9.1 GHz without using DPPH as a marker. The EPR spectrum of a manganese(II) complex should exhibit six hyperfine lines (Mn, I = 5/2). However, this complex shows an isotropic spectrum with g = 2.01 at LNT. No other signal is observed in the 4000 G scan spectrum as could have been expected for a non cubic manganese(II) complex with an appreciable zero field splitting. Such a simple one line spectrum devoid of hyperfine structure has been interpreted as arising from a manganese(II) complex with a neighboring atom magnetic interaction (Yasumatsu et al., 2007; Thimmaiah et al., 1985).

EPR spectrum of [Mn(pa)(eacac)(H2O)] (4).
Figure 2
EPR spectrum of [Mn(pa)(eacac)(H2O)] (4).

4.6

4.6 Mass spectral studies

The FAB mass spectrum of a representative complex [Mn(pa)(eacac)(H2O)] (4) (Fig. 3) was recorded on a JEOL SX 102/DA-6000 mass spectrometer/data system using xenon/argon (6 kV, 10 mA) as the FAB gas in the m/z range of 89.98–355.54. The mass spectral peaks observed at 136, 137 and 154 m/z are matrix (m-nitrobenzyl alcohol) peaks. The appearance of a peak at 307 m/z is consistent with the molecular mass of the present compound excluding one water molecule present therein, which is supposed to be lost before ionization. The other spectral peaks observed at, 178, 260 and 289 m/z in the complex might be correlated possibly with the following types of ion associations: [ Mn ( pa ) ( eacac ) ] + = 307 [ Mn ( pa ) ( eacac ) ] + ( 307 ) - [ CH 3 ] + ( 15 ) - 3 [ H ] + = 289 [ pa ] + ( 123 ) + [ matrix ] + ( 137 ) = 260 [ Mn ( pa ) ] + ( 176 ) + 2 [ H ] + = 178 These results are consistent with the proposed molecular composition of the complex (4).

Mass spectrum of [Mn(pa)(eacac)(H2O)] (4).
Figure 3
Mass spectrum of [Mn(pa)(eacac)(H2O)] (4).

4.7

4.7 Thermogravimetric studies

The thermogravimetric analysis of two representative compounds, namely, [Mn(pa)(o-aans)(H2O)] (2) and [Mn(pa)(eacac)(H2O)] (4) were carried out in the temperature range of 20–1000 °C at the heating rate of 15 °C/min. The compound (2) shows a weight loss of 4.63% at 160 °C, which corresponds to the elimination of one molecule of coordinated (Maurya et al., 2003a,b,c) water per molecule of the complex (calculated weight loss = 4.48%). The second weight loss in this compound was noticed at 370 °C, which corresponds to the decomposition of o-aans ligand group from the complex (observed weight loss: 52%, calculated weight loss: 55.38%). The third weight loss was noticed at 720 °C which roughly corresponds to the removal of all the ligand moieties from the complex (observed weight loss: 78%, calculated weight loss: 86.22%) The final residue (observed, 22%) corresponds to MnO2 (calculated, 21.66%).

The thermogram of compound (4) shows the first weight loss of 5.12% at 126 °C against a calculated weight loss of 5.54%. This corresponds to the removal of one coordinated (Maurya et al., 2003a,b,c) water molecule from the complex. The second weight of 48% was noticed at 410 °C against a theoretical weight loss of 45.29%, for the removal of an eacac ligand group from the complex. This compound shows a third weight loss of 78% around 880 °C, which corresponds to removal of all the ligand moieties from the complex (calculated weight loss: 82.88%). The final residue (observed: 22%) roughly corresponds to MnO2 (calculated: 26.78). The TG curve of compound (4) is given in Fig. 4.

TG curve of [Mn(pa)(eacac)(H2O)] (4).
Figure 4
TG curve of [Mn(pa)(eacac)(H2O)] (4).

4.8

4.8 Antibacterial screening

One of the co-ligands picolinic acid and the two representative metal complexes, namely, [Mn(pa)(o-aans)(H2O)] (2) and [Mn(pa)(eacac)(H2O)] (4) have been screened for their antibacterial activity against Escherichia coli and Vibrio cholera at a concentration of 300 μg cm−3 in DMSO by the agar diffusion method (Ong and Martelli, 1994; Ollee et al., 1989) using streptomycin as the standard antibacterial agent. The results obtained are presented in Table 4. It is observed that all the metal complexes are more potent bactericides than the ligand picolinic acid. The enhancement in the activity of the complexes may depend upon the metal ions (Thimmaiah et al., 1985; Franklin and Snow, 1971), viz., size, charge distribution, shape and redox potential of the metal chelates and chelation. Moreover, steric and pharmacokinetic factors also play a decisive role in deciding the potency of an antimicrobial agent. Thus, antibacterial property of the metal complexes is an intricate blend of several contributions (Levingson et al., 1978; Murukan and Mohanan, 2006).

Table 4 Antibacterial study of the picolinic acid and metal Complexes.
S. No. Compound Compound zone of inhibition in (nm)
E. coli V. cholera
1 Piconilic acid 28 27
2 [Mn(pa)(o-aans)(H2O)] (2) 32 33
3 [Mn(pa)(eacac)(H2O)] (4) 31 35

4.9

4.9 3D-Molecular modeling and analysis

In view of the penta-coordination of the present complexes, and also taking into account the trigonal bipyramidal geometry (Cotton et al., 2004) of high-spin penta-coordinate complexes, the 3D-molecular modeling of compound, [Mn(pa)(aa)(H2O)] (1) as a representative, is based on its trigonal bipyramidal structure. The details of bond lengths and bond angles as per the 3D structure (Fig. 5) are given in Tables 5 and 6, respectively. For convenience of looking over the different bond lengths and bond angles, the various atoms of the compound in question are numbered in Arabic numerals. In all, 116 measurements of the bond lengths (43 in numbers), plus the bond angles (73 in numbers) are listed. Except a few cases, optimal values of both the bond lengths and the bond angles are given in the tables along with the actual ones. The actual bond lengths/bond angles given in Tables 5 and 6 are obtained as a result of energy optimization in CHEM 3D Ultra, while the optimal bond length/bond angle values are the standard bond lengths/bond angles established by the builder unit of the CHEM 3D. The missing of some values of standard bond lengths/bond angles may be due to the limitations of the software, which we had already noticed in the modeling of other systems (Maurya and Rajput, 2007; Maurya et al., 2008a,b, 2010). In most of the cases, the actual bond lengths and bond angles are close to the optimal values, and thus the proposed structures of the compound (1) as well as of the others are acceptable (Maurya and Rajput, 2007; Maurya et al., 2008a,b, 2010).

3D Structure of [Mn(pa)(aa)(H2O)] (1).
Figure 5
3D Structure of [Mn(pa)(aa)(H2O)] (1).
Table 5 Various Bond Lengths of Compound [Mn(pa)(aa)(H2O)] (1).
No Atoms Actual bond lengths Optimal bond lengths S. No. Atoms Actual bond lengths Optimal bond lengths
1 C(24)–H(40) 1.1 1.1 23 C(12)–C(13) 1.337 1.337
2 C(23)–H(39) 1.1 1.1 24 C(9)–O(11) 1.7648 1.338
3 C(23)–C(24) 1.337 1.42 25 C(9)–O(10) 1.208 1.208
4 C(22)–H(38) 1.1 1.1 26 C(8)–H(30) 1.1 1.1
5 C(22)–C(23) 1.337 1.42 27 C(7)–H(29) 1.1 1.1
6 C(21)–H(37) 1.1 1.1 28 C(7)–C(8) 1.337 1.503
7 C(21)–C(22) 1.337 1.42 29 C(6)–H(28) 1.1 1.1
8 C(20)–H(36) 1.1 1.1 30 C(6)–C(7) 1.3894 1.337
9 C(20)–C(21) 1.337 1.42 31 C(5)–H(27) 1.1 1.1
10 C(19)–C(24) 1.337 1.42 32 C(5)–C(6) 1.337 1.503
11 C(19)–C(20) 1.337 1.42 33 N(4)–C(5) 1.266 1.266
12 N(18)–H(35) 1.05 1.05 34 C(3)–C(9) 1.351 1.517
13 N(18)–C(19) 1.266 1.462 35 C(3)–C(8) 1.337 1.337
14 C(16)–H(34) 1.113 1.113 36 C(3)–N(4) 1.266 1.462
15 C(16)–H(33) 1.113 1.113 37 O(2)–H(26) 0.986 –
16 C(16)–H(32) 1.113 1.113 38 O(2)–H(25) 0.986 –
17 C(14)–C(16) 1.497 1.497 39 Mn(1)–O(17) 1.81 –
18 C(14)–O(15) 1.7944 1.355 40 O(15)–Mn(1) 1.81 –
19 C(13)–H(31) 1.1 1.1 41 N(4)–Mn(1) 1.846 –
20 C(13)–C(14) 1.337 1.503 42 Mn(1)–O(11) 1.81 –
21 C(12)–N(18) 1.266 1.462 43 Mn(1)–O(2) 1.81 –
22 C(12)–O(17) 1.355 1.355
Table 6 Various Bond Angles of Compound [Mn(pa)(aa)(H2O)] (1).
S. no Atoms Actual bond angles Optimal bond angles S. no. Atoms Actual bond angles Optimal bond angles
1 H(39)–C(23)–C(24) 120.0002 120 38 C(9)–O(11)–Mn(1) 86.767 –
2 H(39)–C(23)–C(22) 119.9999 120 39 H(26)–O(2)–H(25) 120.0004 –
3 C(24)–C(23)–C(22) 119.9999 – 40 H(26)–O(2)–Mn(1) 119.9996 –
4 H(38)–C(22)–C(23) 120.0001 120 41 H(25)–O(2)–Mn(1) 120 –
5 H(38)–C(22)–C(21) 119.9999 120 42 C(14)–O(15)–Mn(1) 76.7537 –
6 C(23)–C(22)–C(21) 120.0001 – 43 O(11)–C(9)–O(10) 116.3884 122
7 H(37)–C(21)–C(22) 120 120 44 O(11)–C(9)–C(3) 127.2226 124.3
8 H(37)–C(21)–C(20) 120 120 45 O(10)–C(9)–C(3) 116.389 123
9 C(22)–C(21)–C(20) 120 – 46 H(30)–C(8)–C(7) 119.9998 120
10 H(40)–C(24)–C(23) 119.9999 120 47 H(30)–C(8)–C(3) 120.0001 120
11 H(40)–C(24)–C(19) 120 120 48 C(7)–C(8)–C(3) 120.0001 –
12 C(23)–C(24)–C(19) 120 – 49 H(29)–C(7)–C(8) 121.3316 120
13 H(36)–C(20)–C(21) 120.0002 120 50 H(29)–C(7)–C(6) 121.3319 120
14 H(36)–C(20)–C(19) 119.9997 120 51 C(8)–C(7)–C(6) 117.3366 –
15 C(21)–C(20)–C(19) 120.0001 – 52 H(28)–C(6)–C(7) 120.673 120
16 C(24)–C(19)–C(20) 120 120 53 H(28)–C(6)–C(5) 120.6734 120
17 C(24)–C(19)–N(18) 119.9999 120 54 C(7)–C(6)–C(5) 118.6536 –
18 C(20)–C(19)–N(18) 120.0001 120 55 O(17)–Mn(1)–O(15) 136.7386 –
19 H(34)–C(16)–H(33) 109.5199 109 56 O(17)–Mn(1)–O(11) 89.9999 –
20 H(34)–C(16)–H(32) 109.4613 109 57 O(17)–Mn(1)–N(4) 89.9998 –
21 H(34)–C(16)–C(14) 109.4621 110 58 O(17)–Mn(1)–O(2) 180 –
22 H(33)–C(16)–H(32) 109.4419 109 59 O(15)–Mn(1)–O(11) 109.5001 –
23 H(33)–C(16)–C(14) 109.4419 110 60 O(15)–Mn(1)–N(4) 120.0003 –
24 H(32)–C(16)–C(14) 109.5003 110 61 O(15)–Mn(1)–O(2) 43.2613 –
25 C(16)–C(14)–O(15) 105.6499 120 62 O(11)–Mn(1)–N(4) 104.0001 –
26 C(16)–C(14)–C(13) 105.65 121.4 63 O(11)–Mn(1)–O(2) 90.0002 –
27 O(15)–C(14)–C(13) 148.7001 124.3 64 N(4)–Mn(1)–O(2) 90.0002 –
28 H(35)–N(18)–C(19) 120.0001 118 65 H(27)–C(5)–C(6) 119.9994 120
29 H(35)–N(18)–C(12) 119.9997 118 66 H(27)–C(5)–N(4) 120.0003 113.5
30 C(19)–N(18)–C(12) 120.0002 124 67 C(6)–C(5)–N(4) 120.0002 119
31 H(31)–C(13)–C(14) 119.9997 120 68 C(5)–N(4)–C(3) 124.0001 124
32 H(31)–C(13)–C(12) 120 120 69 C(5)–N(4)–Mn(1) 124.9986 –
33 C(14)–C(13)–C(12) 120.0003 – 70 C(3)–N(4)–Mn(1) 110.9991 –
34 N(18)–C(12)–O(17) 117.8498 – 71 C(9)–C(3)–C(8) 128.9981 117.6
35 N(18)–C(12)–C(13) 117.8503 120 72 C(9)–C(3)–N(4) 110.9995 120
36 O(17)–C(12)–C(13) 124.2999 124.3 73 C(8)–C(3)–N(4) 119.9999 120
37 C(12)–O(17)–Mn(1) 109.5 –

5

5 Conclusions

The satisfactory analytical data and all of the physico-chemical evidence presented above suggest that the complexes under this investigation may be formulated as, [Mn(pa)(L)(H2O)] where paH = 2-picolinic acid, LH = β-diketones: acetoacetanilide (aaH), o-acetoacetanisidide (o-aansH), o-acetoacetotoluidide (o-aatdH) or ethylacetoacetate (eacacH). Considering the monomeric penta-coordination in these complexes, trigonal bipyramidal structures (Cotton et al., 2004) have been proposed for them as shown in Fig. 6.

Proposed trigonal bipyramidal structure of complexes.
Figure 6
Proposed trigonal bipyramidal structure of complexes.

Acknowledgements

The authors are thankful to Prof. Ram Rajesh Mishra, Vice-Chancellor, Rani Durgavati University, Jabalpur, Madhya Pradesh, India for encouragement. Analytical facilities provided by the Central Drug Research Institute, Lucknow, India, and the Regional Sophisticated Instrumentation Centre, Indian Institute of Technology, Mumbai and Roorkee, India, are gratefully acknowledged.

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