Biometals DOI 10.1007/s10534-016-9914-8
Interactions fulvate-metal (Zn2+, Cu2+ and Fe2+): theoretical investigation of thermodynamic, structural and spectroscopic properties Alexandre C. Bertoli . Jerusa S. Garcia . Marcello G. Trevisan . Teodorico C. Ramalho . Matheus P. Freitas
Received: 2 September 2015 / Accepted: 3 February 2016 Ó Springer Science+Business Media New York 2016
Abstract The use of theoretical calculation to determine structural properties of fulvate-metal complex (zinc, copper and iron) is here related. The species were proposed in the ratio 1:1 and 2:1 for which the molecular structure was obtained through the semi-empirical method PM6. The calculation of thermodynamic sta0 bility (DHðaq:Þ ) predicted that the zinc complex were more exo-energetic. Metallic ions were coordinated to the phtalate groups of the model-structure of fulvic acid Suwannee River and the calculations of vibrational frequencies suggested that hydrogen bonds may help on the stability of the complex formation. Keywords Metallic complex Fulvic Acid Theoretical Calculation PM6
Introduction Humic substances, the main constituents of organic matter, are important components of soil that influence its chemical, physical and biological properties. They are responsible for improving the soil fertility, for the A. C. Bertoli (&) J. S. Garcia M. G. Trevisan Instituto de Quı´mica, Universidade Federal de Alfenas, UNIFAL, Alfenas, MG 37130-000, Brazil e-mail: [email protected] T. C. Ramalho M. P. Freitas Departamento de Quı´mica, Universidade Federal de Lavras, Lavras, MG 37200-000, Brazil
bioavailability of chemical elements, as well as transport and degradation of natural xenobiotics and other organic compounds (Piccolo 1996). Moreover, they present dark color and polymerics formed by the association of heterogeneous macromolecules containing high molecular weight polyfunctional groups (de Moraes et al. 2004). Since it is a complex matrix that varies in composition depending on the origin (animal, vegetal and microbiological), the structure of these substances is not completely elucidated yet. The order of ionization of humic substances is mainly guided by the amount of phenolic and carboxylic ionizable groups. Humic substances may be divided in three main fractions: humic acids, fulvic acids and humin (Silva et al. 2002; Piccolo 2002). The chemical structure of humin is similar to the humic acid, however it is strongly complexed by clays and hydrated oxides, and it cannot be extracted by any diluted base or acid. Humic and fulvic acids contains similar amounts of aliphatic and aromatic carbons. Furthermore, fulvic acids have lower molecular weight and their structure is rich on carboxylic groups (Saparpakorn et al. 2007). Due to the structural characteristics, fulvic acids may interact with several ions of transition metals which are essential for the functioning of biological systems. The list comprehends from vanadium (V) to zinc (Zn) in the first row series of periodic table, besides the molybdenum (Mo) (Clemens 2006). When complexed with metallic ions, fulvic acids are considered natural chelates. They are organic
123
Biometals
substances that ease the plant nutrition, mainly regarding micronutrients. In the absence of chelates on the soil, elements like Zn, Cu and Fe become little available, thus causing deficiencies in the plant and inhibiting the root growth and other several undesirable conditions. Many available minerals in soil are presented as positive-charge ions and hydrophilic character, while pores or openings in the roots and leaves have a hydrophobic character (Curie and Briat 2003). However, the addition or natural presence of chelate substances lead to the encapsulation of metallic cations and the total positive charge is altered for neutral or negative, what allows the element to migrate throughout the vegetal pores (Alva and Obreza 1998). The coordination of fulvic acids with metals allows a great variety of arrays, as well as the creation of several possibilities of supramolecular matrices formation by means of intermolecular interactions. An approach for this problem is the use of computational chemistry that may help on proposing the formed structures. However, calculations involving coordination compounds may become difficult since several geometries are possible for the complex and many times the elevated number of atoms in the system may restrict the calculation (Hoops et al. 1991). The use of semi-empirical methods is interesting alternative to DFT calculations (for big systems) because of a good result/computational-time ratio. Indeed, nowadays the semi-empirical methods may be used in systems with hundreds or even thousands of atoms without problems (Gonc¸alves and Livotto 1999). The semi-empirical method PM6 is parameterized for most of the transition metals, and is a good option to conduct calculations with molecules of biological interest (Stewart 2007). Then, the molecular modelling and optimization of possible structures of formed complex can serve as model-structures for reactions between fulvic acid and metals. Thus, understanding interactions between metallic ions and fulvic acid is particularly important in the rationalization of the of bioavailability and transport processes of nutrients for plants. In this sense, the objective of this paper is to investigate the structural and thermodynamic effects of coordinating the metallic ions Zn2?, Cu2? and Fe2? with fulvic acid. In this study we chose to use the fulvic acid model Suwannee River (Fig. 1), which is based on experimental evidences (spectroscopic and electrochemical properties) (Averett et al. 1994;
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Fig. 1 Fulvic acid model Suwannee River
Nantsis and Carper 1998) and was already theoretically studied regarding metallic complex (de Castro Ramalho et al. 2007).
Methodology Computational details We used the software Gaussian 09 W (Frisch et al. 2010), for modeling and optimization of possible structures of complex formed from the relationship fulvate-metal. The previsions of molecular structure and vibrational frequencies of the free fulvate and complex fulvate:Zn2?, fulvate:Cu2? and fulvate:Fe2? were made through the semi-empirical method PM6 (Stewart 2007), which is parameterized for most of the transition metals. All calculations were performed considering molecules free in the vacuum, as well as in solution implicitly considering the solvent water by means of the polarizable continuous model (PCM) (Tomasi et al. 2005). Thermodynamic studies The thermodynamic study aimed to promote a theoretical discussion about the complex in order to obtain the parameters that determine its chemical properties. Thereunto the values of absolute energy (DH0) of the complex were obtained in the different relations fulvate-metal using the thermodynamic cycle of the Fig. 2. O DH(aq.) of one complex in the
Biometals Fig. 2 Thermodynamic cycle
M 2 +(gas)
ΔH(solv.)M 2 +
M 2 +(aq.)
+
Fulvate(gas) +
ΔH(solv.)Fulvate
+
ðDHðsolv:Þ M 2þ þ DHðsolv:Þ Fulvate þ DHðvap:Þ H2 OÞ ð1Þ 0 The calculation of relative energy (DDHðaq:Þ ) was performed to identify the most stable complex in relation to the same metal and the same stoichiometry. 0 The DDHðaq:Þ was determined by the difference of
energy variation between the complex of highest energy (DH02 ) and the complex of lowest energy (DH01 ) according to the Eq. 2. 0 ¼ DH20 DH10 DDHðaqÞ
ΔH(vap.)H 2O
Fulvate(aq.) +
thermodynamic cycle was calculated by the Eq. 1 (Ramalho et al. 2004). n DHðaq:Þ ¼ DHðgÞ þ DHðsolv:Þ MðFulvateÞðH2 OÞx
ð2Þ
Results and Discussion Structures and thermodynamic stability A theoretical study of complex formation was performed in order to understand the different ways of interaction and stability between fulvic acid and the metals Zn2?, Cu2? and Fe2?. Formerly the species formed between fulvic acid and metals may show a great number of conformations due to the presence of several coordination sites for complexation. For the calculations, despite of several possibilities of conformers we chose the group phtalate as binding site once previous studies suggested it is the preferred place for complexation (de Castro Ramalho et al. 2007). Therefore, we considered the phtalate present on the completely deprotonated structure of fulvate (Fulv2-) coordinated to the metallic cation. Different conditions fulvate:metal (Figs. 3, 4) were considered: 1:1 (one molecule of fulvate for one metallic cation) and 2:1 (two molecules of fulvate for one metallic
xH 2O(gas) ⎯⎯→ [M ( Fulvate)(H 2O)x ] n −(gas)
Δ H(g)
ΔH(solv.) [M ( Fulvate)(H 2O)x ] n −
xH 2O(aq.) ⎯⎯→ [M ( Fulvate)(H 2O)x ] n −(aq.)
Δ H(aq.)
cation). For better representation of geometries, water molecules were incorporated in the sphere of internal coordination of the complex. The method of calculation used for optimizations was the semi-empirical (PM6) that uses some parameters obtained from experimental data (Stewart 2007). Figures 3 and 4 present, respectively, the most stable complex of fulvate:zinc, fulvate:copper and fulvate:iron in the different stoichiometric relations after optimization. Energetic results for complex in the different studied conditions 1:1 and 2:1 are presented on Table 1. The energy calculated for the reactions of complex formation were given in DH0. According to the results reported in Table 1, the order of thermodynamic stability in both relations was Fe [ Cu [ Zn. The complex [Fe(Fulv)(H2O)4] is 185.42 and 290.60 kcal mol-1 more stable than Cu2? and Zn2 complexes ?, respectively. The energy difference is attenuated to 119.29 kcal mol-1 for the Cu2?complex and 140.60 kcal mol-1 for Zn2? in the relation 2:1. The order of stability of complexes are in accordance with the ligand field stabilization energy (CFSE), since the complexes formed were octahedral Fe2? and tetrahedral Cu2? and Zn2?. In the octahedron the ligands exert maximum influence on the e.g. orbital and very low on the orbital t2g. Tetrahedral complexes in the opposite occurs. This means that the energy released in the unfolding of the d orbitals in an octahedral compound is higher, so the complex is more stable (Huheey et al. 1997). Formerly the stability of complex formed by ions M2? also may be related to the series of Irving-Williams (Irving and Williams 1953), which presents the relative stabilities, reflects the combination of electrostatic effects and the CFSE. The increase of stability is related to the ionic radii, however for the ions Fe2? (d6) and Cu2? (d9) there is a sharp increase in the Kf value with strong field ligands. These ions experienced an additional stabilization proportional to the CFSE (Shriver and Atkins 2008). In relation to Zn2?, the metallic ions
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Biometals
Fig. 3 Most stable complexation forms in the gas phase obtained by optimization in PM6: a [Zn(Fulv)(H2O)2], b [Cu(Fulv)(H2O)2] and c [Fe(Fulv)(H2O)4]
present configuration 3d10, closed layer, it does not have stabilization energy in the ligand field (Bock et al. 2006) and generally its complex are very labile, which may explain the small energy difference between the complexes [Zn(Fulv)2]2- and [Cu(Fulv)2]2-. By increasing the number of ligands in the system, several possibilities (stereoisomers) involving metals can be formed. It should be kept in mind that it is delicate to compare the stoichiometric conditions reported, since they are systems with different numbers of atoms. However, the energy values found for the 2:1 stoichiometry suggest greater stability. A justification for this energy variation in the complexes can be attributed to classical intramolecular interactions—repulsion effects between bulky groups and
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electrostatic interactions. Another explanation can be related to the electron pair repulsion theory (VSEPR), in which regions of high electron concentrations repel each other and, due to this repulsion, they are organized so that they are distant from each other (Bertoli et al. 2015a, b). Table 2 presents the bond length between atoms of oxygen of phthalate groups and water with the studied metals in the formation of complex. Except for the complexes Fe2?, the geometries of metallic complexes were similar. This regularity probably is a consequence of the electrons distribution that corresponds to the preferred approach direction of electrophile (metal) and nucleophiles (ligands) together with the non-bonding interactions (Ramalho et al. 2004).
Biometals
Fig. 4 Most stable complexation forms in the gas phase obtained by optimization in PM6: a [Zn(Fulv)2]2-, b [Cu(Fulv)2]2- and c [Fe(Fulv)2(H2O)2]2-
The bond length Zn–O ranged from 1.93 to 1.96. These values are similar to the bond distance Zn–O of ˚ ), theoretically the complex Zn-FA (1.90 and 1.96 A studied by DFT calculations (DFT-B3LYP/6-31G**) related by de Castro Ramalho et al. (2007). The length of Zn–O bonds obtained through the technique of X-ray diffraction was related to values of other works and it is similar to those found in the present study. Chen et al. studied complex formed between Zn and phthalate groups: [Zn(phth)(bpy)(H2O)]2.H2O, [Zn(phth)(bpy)(H2O)]2 and {[Zn(phth)(bpy)(H2O)]{[Zn(phth)(bpy)].H2O}n, which presented bond
˚ . The authors had observed distances of 1.95 and 2.48 A that heating the reaction mixture reduced the coordination number of the octahedron distorted to tetrahedral (Chen et al. 2013). Zn complex may adopt square planar, tetrahedral or octahedral geometry, however some Zn complex tend to present a distorted octahedral structure. This fact is due to environments with a greater number of ligands in which bonds are farther from the atom of zinc, thus approaching from a square planar geometry and then evolving to a more stable tetrahedral configuration (Ramalho and Figueroa-Villar 2002).
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Biometals 0 0 Table 1 Enthalpy values (kcal mol-1) for the reactions of complex formation in solution (DHðaqueousÞ and DDHðaqueousÞ ) between fulvate (Fulv) and the metal cations Zn2?, Cu2? and Fe2?
Models
0 DHðaq:Þ
0 DDHðaq:Þ
Models
0 DHðaq:Þ
0 DDHðaq:Þ
[Zn(Fulv)(H2O)2]
-1114.40
290.60
[Zn(Fulv)2]2-
-1221.75
140.60
[Cu(Fulv)(H2O)2]
-1219.58
185.42
[Cu(Fulv)2]2-
-1243.06
119.29
[Fe(Fulv)(H2O)4]
-1405.00
0.00
[Fe(Fulv)2(H2O)2]2-
-1362.35
0.00
Structures optimized in the semi-empirical method (PM6) ˚ ) (Zn–O, Cu–O and Fe–O) between the atoms of oxygen of phthalates groups and of water with Table 2 Relevant bond lengths (A the metals ˚) Bond lengths (A [Zn(Fulv)(H2O)2]
[Cu(Fulv)(H2O)2]
Atoms
˚) (A
Zn61–O19 Zn61–O20
[Fe(Fulv)(H2O)4]
Atoms
˚) (A
Atoms
˚) (A
1.94
Cu55–O19
1.99
Fe61–O19
1.97
1.93
Cu55–O20
1.99
Fe61–O20
1.96
Zn61–O55
1.96
Cu55–O56
2.06
Fe61–O55
1.86
Zn61–O57
1.95
Cu55–O58
2.05
Fe61–O62
2.05
Fe61–O64
2.04
˚) Bond lengths (A [Zn(Fulv)2]2-
[Cu(Fulv)2]2-
[Fe(Fulv)2(H2O)2]2-
Atoms
˚) (A
Atoms
˚) (A
Atoms
˚) (A
Zn109–O19
1.95
Cu109–O19
2.02
Fe109–O19
1.88
Zn109–O20
1.93
Cu109–O20
1.99
Fe109–O20
1.85
Zn109–O73
1.95
Cu109–O73
1.99
Fe109–O73
1.74
Zn109–O74
1.93
Cu109–O74
2.01
Fe109–O74
1.79
Fe109–O110
2.20
Fe109–O112
2.11
Structures optimized in the semi-empirical method (PM6)
The Cu–O bond distances ranged, from 1.99 to ˚ , are similar in other complexes of Cu2? 2.06 A phtalate obtained experimentally by X-ray diffraction: {[Cu(2,20 -bipyridine)(l-phthalate)H2O]3.5H2˚ ), according to what was stated O}n (1.93 and 1.95 A by Zhang et al. and {[Cu(Pht)(Im)2]1.5H2O}n (from ˚ ) described by Baca et al. Suksrichavalit 1.96 to 1.99 A and coworkers reported the coordination of Cu2? with nicotinic acid and the phtalate group with bond ˚ calculated at the DFTdistances of 1.83 to 1.87 A B3LYP/LANL2DZ level (Zhang et al. 2000; Baca et al. 2006; Suksrichavalit et al. 2008). The distortions in the complex [Cu(Fulv)2]2typical of transitions d–d are in accordance with the
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effect Jahn–Teller due to the configuration d9 of Cu2? (Menelaou et al. 2012). However once neither the orbitals t2g in an octahedral complex nor any orbital d in a tetrahedral complex point directly to the ligands, the effect is very small to induce a measurable distortion on the structure (Shriver and Atkins 2008). In relation to the Fe2? complex, the Fe–O bond ˚ . These bond length ranged from 1.74 to 2.20 A distances are next to the values found by studies conducted by van Schaik et al., who evaluated the interaction of Fe with fulvic acid (standard IHSS1S102F) by means of extended X-ray absorption fine structure (EXAFS) spectroscopy. The bond length Fe– ˚ , highlighting that the O ranged from 1.98 to 2.10 A
Biometals
Vibrational assignments The frequencies and intensities of vibration were calculated using the semi-empirical method PM6 and the vibrational spectra for the complex are shown on Figs. 5, 6 and 7, respectively. Generally, the calculated number of waves is larger than the experimental number due to factors like neglect of anharmonicity, electronic correlation and deficiencies on the basis set (Go¨kce and Bahc¸eli 2012). The correlation of theoretical data with experimental values obtained by other authors (Matczak 2010; Rageh et al. 2015; Bertoli et al. 2015a, b), shows that the calculated vibrational modes (Table 3) by the semi-empirical method PM6 offers a good quantitative performance on predicting the vibrational frequencies. A detailed description of the main vibrational assignments calculated for the complex is shown on Table 3. Vibrational modes C–H The typical bands of chemical groups useful to the identification of the molecular structure frequently involve coupled vibrations. According to the vibrational assignments presented on Table 3, bands in the region from 790.88 to 1012.35 cm-1 may be
IR Intensity
[Zn(Fulv)(H2O)2]
2-
[Zn(Fulv)2]
3000
2500
2000
1500
1000
500
-1
Wavenumber (cm ) Fig. 5 IR spectra for the complex [Zn(Fulv)(H2O)2] and [Zn(Fulv)2]2-
IR Intensity
ligands present on fulvic acid may be acetate, phenolate, phthalate, salicylate, and catecholate (van Schaik ¨ hrstro¨m and Michaud-Soret, evaluated et al. 2008). O the geometry of the compounds Fe(catecholate)2-, Fe(4-methylcatecholate)2-, [Fe(oxalate)3]3- and Fe(oxalate)2- at the DFT level and observed bond ˚ (O ¨ hrstro¨m and distances between 1.86 and 2.04 A Michaud-Soret 1999). Distortions were also expected on the geometry of Fe2? complex, however if the ligands connected to the metal were strong field, situations of low-spin are created in the orbitals d preventing distortions (Bertoli et al. 2015a, b). Therefore, the groups C-O complexed to the metal are strong field ligands, what favors the electron pairing (Shriver and Atkins 2008). The complex [Fe(Fulv)(H2O)4]- adopted the pentagonal planar geometry, what may be caused by several anionic ligands around the metal, the negative charge that must be stabilized by steric protection and strong p-donors ligands to destabilize the sterically preferred octahedral geometry (Holland 2011).
[Cu(Fulv)(H2O)2]
2-
[Cu(Fulv)2] 3000
2500
2000
1500
1000
500
-1
Wavenumber (cm ) Fig. 6 IR spectra for the complex [Cu(Fulv)(H2O)2] and [Cu(Fulv)2]2-
attributed to the angular deformation of C–H (Balachandran et al. 2014), in the modes, in the plane (drocking (HCH)) and out of the plane in the ring of 6 members [dwagging Ring (CH)]. Low-intensity stretches came from the H–C–H bonds of aliphatic chain were observed in the region between 2667.44 and 2674.83 cm-1. C–C stretches Angular deformations C–C in the twisting mode [dtwist (C–C]) were present in the region from 1138.84 to 1157.12 cm-1, while symmetric stretches C=C of the
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IR Intensity
Biometals
[Fe(Fulv)(H2O)4]
2-
[Fe(Fulv)2(H2O)2] 3000
2500
2000
1500
1000
500
Wavenumber (cm-1 ) Fig. 7 IR spectra for the complex [Fe(Fulv)(H2O)4] and [Fe(Fulv)2(H2O)2]2-
aromatic rings (msymRing (C=C)) were observed between 1410.39 and 1638.53 cm-1. For vibrations of the aromatic nuclei, bands in the region of 1600 cm-1 are intense when the phenyl group is conjugated to unsaturations or even linked to atoms with pairs of free electrons. C=C stretches of ring, but with lower intensity, were also found in the region from 1048.77 to 1195.70 cm-1 coupled to vibrations C–OH and the bond vibrations of intramolecular hydrogen. For complex formed around the central metallic cation, it makes physical sense to find vibrational couplings that describe simple bonds such as C–C and C–O (Silverstein et al. 2006). The assignments to vibrations C–C are close to the results found by Go¨kce and Bahc¸eli, who studied metallic complex of Co2?, Cu2? and Zn2? through experimental and theoretical methods with B3LYP/ LANL2DZ (Go¨kce and Bahc¸eli 2013). C–O, C=O and COO- stretches The carboxylic acids present as characteristic a C–O stretch in the region from 1200 to 1300 cm-1 (Balachandran et al. 2014). The frequencies for this group are concentrated in the region from 1236.87 to 1305.83 cm-1 and correspond to the C-O stretches of carboxylate groups linked to the aromatic rings [mRing (C–O)]. The bands related to this stretch may be greatly overlapped in the range between 1150 and 1300 cm-1 such as confirmed by Moreira et al., who observed this vibration in 1278 cm-1 using the method DFT. Stretches of C–OH groups of carboxyl
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linked to the rings and phenyl groups [mRing (C–OH)] were observed in the region from 1122.14 to 1137.13 cm-1 (Moreira et al. 2012). Carboxylic acids, ketones and lactones show an intense band in the region from 1540 to 1870 cm-1 that originates in the axial deformation of the double bond carbon–oxygen. Inside this region the position of the deformation band of C=O is very sensible to several factors, like physical state, hydrogen bond, the electronic effect per substituent, ring strain, etc. (Silverstein et al. 2006; Balachandran et al. 2012). For the highlighted complex, bands between 1587.50 and 1795.45 cm-1 and 1819.16 and 1866.06 cm-1 were attributed to the stretches m(C=O) of ketones and mRing (C=O) of lactones, respectively. The most intense bands of C=O related to the symmetric [msymRing(COO-)] and asymetric stretches [masymRing(COO-)] of the phthalate groups were present in the region from 1297.23 to 1478.48 cm-1 and from 1587.50 to 1816.54 cm-1, respectively. The theoretical frequencies were similar to the experimental assignments, however the values were overestimated mainly for asymmetric stretches of the phthalate group [masymRing(COO-)]. Matczak, in works with organotins using the methods B3LYP, PM6 and B3PW91 previewed that, in general, the theoretical approaches tend to overestimate the frequencies of vibration (Matczak 2010). Hydrogen bonds In most of the chemical environments, the hydroxyl group does not exist separately; the vibration may, generally, be coupled, thus resulting in extensive hydrogen bonds. These groups may be associated within the same molecule (intramolecular hydrogen bond) or among neighboring molecules (intermolecular hydrogen bond). In compounds like carboxylic acids, which exhibit extremely strong hydrogen bonds, one typical characteristic is the presence of this bond in lower frequencies (Balachandran et al. 2014; Silverstein et al. 2006). Through the theoretical calculations conducted in the present work, it was possible to the observe vibrations related to intramolecular hydrogen bonds in the regions from 790.88 to 1195.70 cm-1, between the phthalate coordinated to the metals and the phenyl group [dwagging (OHO)). These bonds are typical of dimeric carboxylic acids from the angular deformation
2670.42
m (HCH) 2667.44
2539.95
2389.35
1863.15 1824.10
1820.33
1786.63
1297.23
1305.83
2668.52
2542.84
2422.11
1858.45 1817.98
1819.50
1726.86
1795.45
1638.53
1364.73
1236.87
1138.84
906.30
[Fe(Fulv)(H2O)4]
1412.93
m(HCH)
m(OH)
2674.83
2564.77
1825.70
1771.46 1805.35
m(C=O) ? masymRing(COO-) msym(COOH) mRing (C=O)
1601.28
masymRing (C=C)
1469.75
msymRing (C=C) msymRing (COO-)
1137.13
1048.77
1012.35
940.10
898.19
[Zn(Fulv)2]2-
Complex 2:1
mRing (C–OH) ?msymRing (C=C)
dwagging (OHO) ?msymRing (C=C)
dwagging (OHO) ?dwagging Ring (CH)
dwagging (OHO) ?dwagging Ring (CH)
drocking (HCH) ?dwagging Ring (CH)
Approximate Assignments
2673.00
2561.56
1829.08
1697.92 1825.73
1625.49
1359.95
1526.28
1122.14
1195.70
1003.77
903.18
790.88
[Cu(Fulv)2]2-
2674.41
2560.49
1819.16
1587.50 1804.34
1603.40
1465.42
1410.39
1132.32
1123.12
979.97
944.40
890.67
[Fe(Fulv)2(H2O)2]2-
dtwist twisting deformation, drocking rocking deformation, dwagging wagging deformation, dsciss scissors deformation, msym symmetric stretching, masym asymmetric stretching
2151.70
2525.22
m (OHO)
1866.06 1912.31
m (OH)
1837.46
msym (COOH)
mRing (C=O) mRing (C=O)
1806.80
1717.45
1816.54
1633.88
msym Ring (C=C)
m (C=O)
1478.48
msym Ring (COO-)
masymRing (COO-)
1423.07
1238.98
drocking Ring (C–H) ?mRing (C–O)
1157.12
1144.02
dtwist (C–C) ? drocking (O– H)
910.58
905.93
[Zn(Fulv)(H2O)2] [Cu(Fulv)(H2O)2]
Complex 1:1
drocking (HCH) ? dwagging Ring (CH)
Approximate Assignments
Table 3 Main calculated frequencies (cm-1) and vibrational assignments of the complex by the semi-empirical (PM6) method
Biometals
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Biometals
of the plane in the group O–H and generally they appear between 900 and 1000 cm-1 (Silverstein et al. 2006). Vibrations were also observed between 2151.70 and 2564.77 cm-1 in dimers of carboxylic acids that absorbs an axial deformation of intense and wide O–H bond in the region from 2500 to 3000 cm-1. These O–HO interactions were greatly important to the system to elucidate the relation structure–property (Grabowski 2011).
Conclusions Fulvic acids extracted from organic materials can be enriched with macro and micronutrients and be marketed as ‘‘organic’’ fertilizers or ‘‘natural’’, and according to the manufacturers are released to the plants according to their needs. This is a relatively new market that has economic potential. However, the industry remains poorly regulated, primarily due to the lack of scientific data on nutrient release mechanisms and structures formed since it can be formed supramolecular aggregates. Therefore, studies of interactions (such as complexation) between the metallic species of macro and micronutrients with fulvic acids are important in improving agricultural productivity and reducing environmental impacts (Botero et al. 2010). 0 The DHðaq:Þ results suggest that the zinc complex may be preferably formed and the order of stability was Fe [ Cu [ Zn for the ratio 1:1 and Fe [ Zn [ Cu in 2:1. Furthermore, vibrational frequency calculations showed that hydrogen bonds are greatly important on predicting the structure and stability of complex. For the stoichiometry 1:1 the structures had similar geometries except for the compound [Fe(Fulv)(H2O)4]. The most stable structure with 2:1 of zinc and copper complex formed a distorted tetrahedral, while the complex [Fe(Fulv)2(H2O)2]2- was octahedral. The use of semi-empirical method PM6, which is parameterized for transition metals and applies some parameters obtained from experimental data, may be useful on understanding the interactions between metallic ions and a physiologically relevant ligand, like the fulvic acid. Acknowledgments To Coordenac¸a˜o de Aperfeic¸oamento de Pessoal de Nı´vel Superior (CAPES) by the scholarship; to Instituto de Quı´mica of Universidade Federal de Alfenas (UNIFAL); to the Laborato´rio de Ana´lise e Caracterizac¸a˜o de
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Fa´rmacos (LACFar-UNIFAL); to Fundac¸a˜o de Amparo a` Pesquisa do Estado de Minas Gerais (FAPEMIG).
References Alva AK, Obreza TA (1998) By-product iron-humate increases tree growth and fruit production of orange and grapefruit. Hort Sci 33:71–74 Averett RC, Leenheer JA, McKnight DM, Thorn KA (1994) Humic substances in the Suwannee River, Georgia; interactions, properties, and proposed structures (USGS Numbered Series No. 2373), Water Supply Paper. U.S. G.P.O.; U.S. Geological Survey, Map Distribution Baca SG, Reetz MT, Goddard R, Filippova IG, Simonov YA, Gdaniec M, Gerbeleu N (2006) Coordination polymers constructed from o-phthalic acid and diamines: syntheses and crystal structures of the phthalate-imidazole complex {[Cu(Pht)(Im)2]1.5H2O}n and [Co(Pht)(Im)2]n and their application in oxidation catalysis. Polyhedron 25:1215–1222. doi:10.1016/j.poly.2005.09.003 Balachandran V, Rajeswari S, Lalitha S (2012) DFT computations, vibrational spectra, monomer, dimer, NBO and NMR analyses of antifungal agent: 3,5-dibromosalicylic acid. J Mol Struct 1007:63–73. doi:10.1016/j.molstruc.2011.10.014 Balachandran V, Lalitha S, Rajeswari S, Rastogi VK (2014) Theoretical investigations on the molecular structure, vibrational spectra, thermodynamics, HOMO–LUMO, NBO analyses and paramagnetic susceptibility properties of p-(p-hydroxyphenoxy)benzoic acid. Spectrochim Acta A 121:575–585. doi:10.1016/j.saa.2013.11.014 Bertoli AC, Carvalho R, Freitas MP, Ramalho TC, Mancini DT, Oliveira MC, de Varennes A, Dias A (2015a) Structural determination of Cu and Fe–Citrate complex: theoretical investigation and analysis by ESI-MS. J Inorg Biochem 144:31–37. doi:10.1016/j.jinorgbio.2014.12.008 Bertoli AC, Carvalho R, Freitas MP, Ramalho TC, Mancini DT, Oliveira MC, de Varennes A, Dias A (2015b) Theoretical spectroscopic studies and identification of metal-citrate (Cd and Pb) complex by ESI-MS in aqueous solution. Spectrochim Acta A 137:271–280. doi:10.1016/j.saa.2014. 08.053 Bock CW, Markham GD, Katz AK, Glusker JP (2006) The arrangement of first- and second-shell water molecules around metal ions: effects of charge and size. Theor. Chem. Acc. 115:100–112. doi:10.1007/s00214-005-0056-2 Botero WG, Oliveira LC, Rocha JC, Rosa AH, dos Santos A (2010) Peat humic substances enriched with nutrients for agricultural applications: competition between nutrients and non-essential metals present in tropical soils. J Hazard Mater 177:307–311. doi:10.1016/j.jhazmat.2009.12.033 Chen Q-L, Chen H-B, Zhou Z-H (2013) Solid and solution evidences on the temperature effect of N-chelated zinc phthalates. J Mol Struct 1035:198–202. doi:10.1016/j. molstruc.2012.09.054 Clemens S (2006) Toxic metal accumulation, responses to exposure and mechanisms of tolerance in plants. Biochim Facets Environ Nucl Toxicol 88:1707–1719. doi:10.1016/j. biochi.2006.07.003
Biometals Curie C, Briat JF (2003) Iron transport and signaling in plants. Annu Rev Plant Biol 54:183–206. doi:10.1146/annurev. arplant.54.031902.135018 de Castro Ramalho T, da Cunha EFF, de Alencastro RB, Espı´nola A (2007) Differential complexation between Zn2? and Cd2? with fulvic acid: a computational chemistry study. Water Air Soil Pollut 183:467–472. doi:10.1007/s11270007-9364-6 de Moraes SL, Landgraf MD, Rezende MOO (2004) Caracterizac¸a˜o de a´cidos hu´micos de diferentes origens por eletroforese capilar de zona. Eclet Quim 29:47–52. doi:10. 1590/S0100-46702004000200007 Frisch MJ, Trucks GW, Schlegel HB, Scuseria GE, Robb MA, Cheeseman JR (2010) Gaussian 09, Revision B.01. Gaussian, Inc, Wallingford Go¨kce H, Bahc¸eli S (2012) A study of molecular structure and vibrational spectra of copper(II) halide complex of 2-(20 thienyl)pyridine. Spectrochim Acta A 96:139–147. doi:10. 1016/j.saa.2012.05.005 Go¨kce H, Bahc¸eli S (2013) FT-IR, Micro-Raman and UV–vis spectroscopic and quantum chemical investigations of free 2,20 -dithiodipyridine and its metal (Co, Cu and Zn) halide complex. Spectrochim Acta A 114:61–73. doi:10.1016/j. saa.2013.04.112 Gonc¸alves PFB, Livotto PR (1999) Optimization of molecular cavities in the PCM of neutral molecules using charge dependent atomic radii: applications to the semi-empirical AM1 and MNDO/PM3 methods. Chem Phys Lett 304:438–444. doi:10.1016/S0009-2614(99)00341-3 Grabowski SJ (2011) What is the covalency of hydrogen bonding? Chem Rev 111:2597–2625. doi:10.1021/cr800346f Holland PL (2011) Coordination chemistry: all square for highspin iron(II). Nat. Chem. 3:507–508. doi:10.1038/nchem. 1078 Hoops SC, Anderson KW, Merz KM (1991) Force field design for metalloproteins. J Am Chem Soc 113:8262–8270. doi:10.1021/ja00022a010 Huheey JE, Keiter EA, Keiter RL (1997) Inorganic chemistry: principles of structure and reactivity, 4th edn. Harper Collins, New York Irving H, Williams RJP (1953) The stability of transition-metal complex. J Chem Soc Resume 637:3192–3210. doi:10. 1039/JR9530003192 Matczak P (2010) Quantum-chemical calculations of spectroscopic, electric and thermochemical properties of some n-butyltin compounds. J. Mol Struc. Theochem 950:83–92. doi:10.1016/j.theochem.2010.03.030 Menelaou M, Lalioti N, Psycharis V, Raptopoulou CP, Terzis A, Mateescu C, Salifoglou A (2012) Coordination polymeric materials in binary and ternary Cu(II)–tetracarboxylato– bipy systems: structure–reactivity correlation in Cu(II)–(O, N) 1D–3D lattice assemblies. Polyhedron 40:134–144. doi:10.1016/j.poly.2012.04.012 Moreira MA, Cormanich RA, de Rezende FMP, Silla JM, Tormena CF, Rittner R, Ramalho TC, Freitas MP (2012) Theoretical and infrared studies on the conformations of monofluorophenols. J Mol Struct 1009:11–15. doi:10. 1016/j.molstruc.2011.05.012 Nantsis EA, Carper WR (1998) Molecular structure of divalent metal ion–fulvic acid complex. J Mol Struct Theochem 423:203–212. doi:10.1016/S0166-1280(97)00125-5
¨ hrstro¨m L, Michaud-Soret I (1999) Fe-catecholate and FeO oxalate vibrations and isotopic substitution shifts from DFT quantum chemistry. J Phys Chem A 103:256–264. doi:10.1021/jp981508f Piccolo A (1996) Humus and soil conservation. In: Piccolo A (ed) Humic substances in terrestrial ecosystems. Elsevier, Amsterdam, pp 225–264 Piccolo A (2002) The supramolecular structure of humic substances: a novel understanding of humus chemistry and implications in soil science. Adv Agron 75:57–134. doi:10. 1016/S0065-2113(02)75003-7 Rageh HM, Ibrahim SA, Selim MA, Alsoghier HM (2015) Spectroscopic and semiempirical investigation of the structural features of hetarylazo-5-isoxazolones tautomerism. J Saudi Chem Soc. doi:10.1016/j.jscs.2015.01.007 Ramalho TC, Figueroa-Villar JD (2002) Thermodynamic evaluation of complex of zinc and cadmium that mimetize metallic centers in transcription factors. J Mol Struct Theochem 580:217–223. doi:10.1016/S0166-1280(01)00618-2 Ramalho TC, da Cunha EFF, Bicca de Alencastro R (2004) A density functional study on the complexation of ethambutol with divalent cations. J Mol Struct Theochem 676:149–153. doi:10.1016/j.theochem.2004.02.006 Saparpakorn P, Kim JH, Hannongbua S (2007) Investigation on the binding of polycyclic aromatic hydrocarbons with soil organic matter: a theoretical approach. Mol Basel Switz 12:703–715. doi:10.3390/12040703 Shriver DF, Atkins PW (2008) Quı´mica Inorgaˆnica, 4th edn. Bookman, Porto Alegre Silva WTL, Gautier CT, Rezende MOO, El Murr N (2002) Electrochemical behavior of Cu (II) on carbon paste electrode modified by humic acid, cyclic voltametry study. Electroanal. 14:71–77. doi:10.1002/15214109(200201)14: 1\71:AID-ELAN71[3.0.CO;2-B Silverstein RM, Webster FX, Kiemle DJ (2006) Spectrometric identification of organic compounds. Livros Te´cnicos e Cientı´ficos, Rio de Janeiro Stewart JJP (2007) Optimization of parameters for semiempirical methods V: modification of NDDO approximations and application to 70 elements. J Mol Model 13:1173–1213. doi:10.1007/s00894-007-0233-4 Suksrichavalit T, Prachayasittikul S, Piacham T, Isarankura-NaAyudhya C, Nantasenamat C, Prachayasittikul V (2008) Copper complex of nicotinic-aromatic carboxylic acids as superoxide dismutase mimetics. Molecules 13:3040–3056. doi:10.3390/molecules13123040 Tomasi J, Mennucci B, Cammi R (2005) Quantum mechanical continuum solvation models. Chem Rev 105:2999–3093. doi:10.1021/cr9904009 Van Schaik JWJ, Persson I, Kleja DB, Gustafsson JP (2008) EXAFS study on the reactions between iron and fulvic acid in acid aqueous solutions. Environ Sci Technol 42:2367–2373. doi:10.1021/es072092z Zhang Y, Li J, Su Q, Wang Q, Wu X (2000) Synthesis, structure and spectroscopic properties of an o-phthalate-bridged copper(II) chain complex. J Mol Struct 516:231–236. doi:10.1016/S0022-2860(99)00203-3
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Interactions fulvate-metal (Zn2+, Cu2+ and Fe2+): theoretical investigation of thermodynamic, structural and spectroscopic properties Alexandre C. Bertoli . Jerusa S. Garcia . Marcello G. Trevisan . Teodorico C. Ramalho . Matheus P. Freitas
Received: 2 September 2015 / Accepted: 3 February 2016 Ó Springer Science+Business Media New York 2016
Abstract The use of theoretical calculation to determine structural properties of fulvate-metal complex (zinc, copper and iron) is here related. The species were proposed in the ratio 1:1 and 2:1 for which the molecular structure was obtained through the semi-empirical method PM6. The calculation of thermodynamic sta0 bility (DHðaq:Þ ) predicted that the zinc complex were more exo-energetic. Metallic ions were coordinated to the phtalate groups of the model-structure of fulvic acid Suwannee River and the calculations of vibrational frequencies suggested that hydrogen bonds may help on the stability of the complex formation. Keywords Metallic complex Fulvic Acid Theoretical Calculation PM6
Introduction Humic substances, the main constituents of organic matter, are important components of soil that influence its chemical, physical and biological properties. They are responsible for improving the soil fertility, for the A. C. Bertoli (&) J. S. Garcia M. G. Trevisan Instituto de Quı´mica, Universidade Federal de Alfenas, UNIFAL, Alfenas, MG 37130-000, Brazil e-mail: [email protected] T. C. Ramalho M. P. Freitas Departamento de Quı´mica, Universidade Federal de Lavras, Lavras, MG 37200-000, Brazil
bioavailability of chemical elements, as well as transport and degradation of natural xenobiotics and other organic compounds (Piccolo 1996). Moreover, they present dark color and polymerics formed by the association of heterogeneous macromolecules containing high molecular weight polyfunctional groups (de Moraes et al. 2004). Since it is a complex matrix that varies in composition depending on the origin (animal, vegetal and microbiological), the structure of these substances is not completely elucidated yet. The order of ionization of humic substances is mainly guided by the amount of phenolic and carboxylic ionizable groups. Humic substances may be divided in three main fractions: humic acids, fulvic acids and humin (Silva et al. 2002; Piccolo 2002). The chemical structure of humin is similar to the humic acid, however it is strongly complexed by clays and hydrated oxides, and it cannot be extracted by any diluted base or acid. Humic and fulvic acids contains similar amounts of aliphatic and aromatic carbons. Furthermore, fulvic acids have lower molecular weight and their structure is rich on carboxylic groups (Saparpakorn et al. 2007). Due to the structural characteristics, fulvic acids may interact with several ions of transition metals which are essential for the functioning of biological systems. The list comprehends from vanadium (V) to zinc (Zn) in the first row series of periodic table, besides the molybdenum (Mo) (Clemens 2006). When complexed with metallic ions, fulvic acids are considered natural chelates. They are organic
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Biometals
substances that ease the plant nutrition, mainly regarding micronutrients. In the absence of chelates on the soil, elements like Zn, Cu and Fe become little available, thus causing deficiencies in the plant and inhibiting the root growth and other several undesirable conditions. Many available minerals in soil are presented as positive-charge ions and hydrophilic character, while pores or openings in the roots and leaves have a hydrophobic character (Curie and Briat 2003). However, the addition or natural presence of chelate substances lead to the encapsulation of metallic cations and the total positive charge is altered for neutral or negative, what allows the element to migrate throughout the vegetal pores (Alva and Obreza 1998). The coordination of fulvic acids with metals allows a great variety of arrays, as well as the creation of several possibilities of supramolecular matrices formation by means of intermolecular interactions. An approach for this problem is the use of computational chemistry that may help on proposing the formed structures. However, calculations involving coordination compounds may become difficult since several geometries are possible for the complex and many times the elevated number of atoms in the system may restrict the calculation (Hoops et al. 1991). The use of semi-empirical methods is interesting alternative to DFT calculations (for big systems) because of a good result/computational-time ratio. Indeed, nowadays the semi-empirical methods may be used in systems with hundreds or even thousands of atoms without problems (Gonc¸alves and Livotto 1999). The semi-empirical method PM6 is parameterized for most of the transition metals, and is a good option to conduct calculations with molecules of biological interest (Stewart 2007). Then, the molecular modelling and optimization of possible structures of formed complex can serve as model-structures for reactions between fulvic acid and metals. Thus, understanding interactions between metallic ions and fulvic acid is particularly important in the rationalization of the of bioavailability and transport processes of nutrients for plants. In this sense, the objective of this paper is to investigate the structural and thermodynamic effects of coordinating the metallic ions Zn2?, Cu2? and Fe2? with fulvic acid. In this study we chose to use the fulvic acid model Suwannee River (Fig. 1), which is based on experimental evidences (spectroscopic and electrochemical properties) (Averett et al. 1994;
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Fig. 1 Fulvic acid model Suwannee River
Nantsis and Carper 1998) and was already theoretically studied regarding metallic complex (de Castro Ramalho et al. 2007).
Methodology Computational details We used the software Gaussian 09 W (Frisch et al. 2010), for modeling and optimization of possible structures of complex formed from the relationship fulvate-metal. The previsions of molecular structure and vibrational frequencies of the free fulvate and complex fulvate:Zn2?, fulvate:Cu2? and fulvate:Fe2? were made through the semi-empirical method PM6 (Stewart 2007), which is parameterized for most of the transition metals. All calculations were performed considering molecules free in the vacuum, as well as in solution implicitly considering the solvent water by means of the polarizable continuous model (PCM) (Tomasi et al. 2005). Thermodynamic studies The thermodynamic study aimed to promote a theoretical discussion about the complex in order to obtain the parameters that determine its chemical properties. Thereunto the values of absolute energy (DH0) of the complex were obtained in the different relations fulvate-metal using the thermodynamic cycle of the Fig. 2. O DH(aq.) of one complex in the
Biometals Fig. 2 Thermodynamic cycle
M 2 +(gas)
ΔH(solv.)M 2 +
M 2 +(aq.)
+
Fulvate(gas) +
ΔH(solv.)Fulvate
+
ðDHðsolv:Þ M 2þ þ DHðsolv:Þ Fulvate þ DHðvap:Þ H2 OÞ ð1Þ 0 The calculation of relative energy (DDHðaq:Þ ) was performed to identify the most stable complex in relation to the same metal and the same stoichiometry. 0 The DDHðaq:Þ was determined by the difference of
energy variation between the complex of highest energy (DH02 ) and the complex of lowest energy (DH01 ) according to the Eq. 2. 0 ¼ DH20 DH10 DDHðaqÞ
ΔH(vap.)H 2O
Fulvate(aq.) +
thermodynamic cycle was calculated by the Eq. 1 (Ramalho et al. 2004). n DHðaq:Þ ¼ DHðgÞ þ DHðsolv:Þ MðFulvateÞðH2 OÞx
ð2Þ
Results and Discussion Structures and thermodynamic stability A theoretical study of complex formation was performed in order to understand the different ways of interaction and stability between fulvic acid and the metals Zn2?, Cu2? and Fe2?. Formerly the species formed between fulvic acid and metals may show a great number of conformations due to the presence of several coordination sites for complexation. For the calculations, despite of several possibilities of conformers we chose the group phtalate as binding site once previous studies suggested it is the preferred place for complexation (de Castro Ramalho et al. 2007). Therefore, we considered the phtalate present on the completely deprotonated structure of fulvate (Fulv2-) coordinated to the metallic cation. Different conditions fulvate:metal (Figs. 3, 4) were considered: 1:1 (one molecule of fulvate for one metallic cation) and 2:1 (two molecules of fulvate for one metallic
xH 2O(gas) ⎯⎯→ [M ( Fulvate)(H 2O)x ] n −(gas)
Δ H(g)
ΔH(solv.) [M ( Fulvate)(H 2O)x ] n −
xH 2O(aq.) ⎯⎯→ [M ( Fulvate)(H 2O)x ] n −(aq.)
Δ H(aq.)
cation). For better representation of geometries, water molecules were incorporated in the sphere of internal coordination of the complex. The method of calculation used for optimizations was the semi-empirical (PM6) that uses some parameters obtained from experimental data (Stewart 2007). Figures 3 and 4 present, respectively, the most stable complex of fulvate:zinc, fulvate:copper and fulvate:iron in the different stoichiometric relations after optimization. Energetic results for complex in the different studied conditions 1:1 and 2:1 are presented on Table 1. The energy calculated for the reactions of complex formation were given in DH0. According to the results reported in Table 1, the order of thermodynamic stability in both relations was Fe [ Cu [ Zn. The complex [Fe(Fulv)(H2O)4] is 185.42 and 290.60 kcal mol-1 more stable than Cu2? and Zn2 complexes ?, respectively. The energy difference is attenuated to 119.29 kcal mol-1 for the Cu2?complex and 140.60 kcal mol-1 for Zn2? in the relation 2:1. The order of stability of complexes are in accordance with the ligand field stabilization energy (CFSE), since the complexes formed were octahedral Fe2? and tetrahedral Cu2? and Zn2?. In the octahedron the ligands exert maximum influence on the e.g. orbital and very low on the orbital t2g. Tetrahedral complexes in the opposite occurs. This means that the energy released in the unfolding of the d orbitals in an octahedral compound is higher, so the complex is more stable (Huheey et al. 1997). Formerly the stability of complex formed by ions M2? also may be related to the series of Irving-Williams (Irving and Williams 1953), which presents the relative stabilities, reflects the combination of electrostatic effects and the CFSE. The increase of stability is related to the ionic radii, however for the ions Fe2? (d6) and Cu2? (d9) there is a sharp increase in the Kf value with strong field ligands. These ions experienced an additional stabilization proportional to the CFSE (Shriver and Atkins 2008). In relation to Zn2?, the metallic ions
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Biometals
Fig. 3 Most stable complexation forms in the gas phase obtained by optimization in PM6: a [Zn(Fulv)(H2O)2], b [Cu(Fulv)(H2O)2] and c [Fe(Fulv)(H2O)4]
present configuration 3d10, closed layer, it does not have stabilization energy in the ligand field (Bock et al. 2006) and generally its complex are very labile, which may explain the small energy difference between the complexes [Zn(Fulv)2]2- and [Cu(Fulv)2]2-. By increasing the number of ligands in the system, several possibilities (stereoisomers) involving metals can be formed. It should be kept in mind that it is delicate to compare the stoichiometric conditions reported, since they are systems with different numbers of atoms. However, the energy values found for the 2:1 stoichiometry suggest greater stability. A justification for this energy variation in the complexes can be attributed to classical intramolecular interactions—repulsion effects between bulky groups and
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electrostatic interactions. Another explanation can be related to the electron pair repulsion theory (VSEPR), in which regions of high electron concentrations repel each other and, due to this repulsion, they are organized so that they are distant from each other (Bertoli et al. 2015a, b). Table 2 presents the bond length between atoms of oxygen of phthalate groups and water with the studied metals in the formation of complex. Except for the complexes Fe2?, the geometries of metallic complexes were similar. This regularity probably is a consequence of the electrons distribution that corresponds to the preferred approach direction of electrophile (metal) and nucleophiles (ligands) together with the non-bonding interactions (Ramalho et al. 2004).
Biometals
Fig. 4 Most stable complexation forms in the gas phase obtained by optimization in PM6: a [Zn(Fulv)2]2-, b [Cu(Fulv)2]2- and c [Fe(Fulv)2(H2O)2]2-
The bond length Zn–O ranged from 1.93 to 1.96. These values are similar to the bond distance Zn–O of ˚ ), theoretically the complex Zn-FA (1.90 and 1.96 A studied by DFT calculations (DFT-B3LYP/6-31G**) related by de Castro Ramalho et al. (2007). The length of Zn–O bonds obtained through the technique of X-ray diffraction was related to values of other works and it is similar to those found in the present study. Chen et al. studied complex formed between Zn and phthalate groups: [Zn(phth)(bpy)(H2O)]2.H2O, [Zn(phth)(bpy)(H2O)]2 and {[Zn(phth)(bpy)(H2O)]{[Zn(phth)(bpy)].H2O}n, which presented bond
˚ . The authors had observed distances of 1.95 and 2.48 A that heating the reaction mixture reduced the coordination number of the octahedron distorted to tetrahedral (Chen et al. 2013). Zn complex may adopt square planar, tetrahedral or octahedral geometry, however some Zn complex tend to present a distorted octahedral structure. This fact is due to environments with a greater number of ligands in which bonds are farther from the atom of zinc, thus approaching from a square planar geometry and then evolving to a more stable tetrahedral configuration (Ramalho and Figueroa-Villar 2002).
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Biometals 0 0 Table 1 Enthalpy values (kcal mol-1) for the reactions of complex formation in solution (DHðaqueousÞ and DDHðaqueousÞ ) between fulvate (Fulv) and the metal cations Zn2?, Cu2? and Fe2?
Models
0 DHðaq:Þ
0 DDHðaq:Þ
Models
0 DHðaq:Þ
0 DDHðaq:Þ
[Zn(Fulv)(H2O)2]
-1114.40
290.60
[Zn(Fulv)2]2-
-1221.75
140.60
[Cu(Fulv)(H2O)2]
-1219.58
185.42
[Cu(Fulv)2]2-
-1243.06
119.29
[Fe(Fulv)(H2O)4]
-1405.00
0.00
[Fe(Fulv)2(H2O)2]2-
-1362.35
0.00
Structures optimized in the semi-empirical method (PM6) ˚ ) (Zn–O, Cu–O and Fe–O) between the atoms of oxygen of phthalates groups and of water with Table 2 Relevant bond lengths (A the metals ˚) Bond lengths (A [Zn(Fulv)(H2O)2]
[Cu(Fulv)(H2O)2]
Atoms
˚) (A
Zn61–O19 Zn61–O20
[Fe(Fulv)(H2O)4]
Atoms
˚) (A
Atoms
˚) (A
1.94
Cu55–O19
1.99
Fe61–O19
1.97
1.93
Cu55–O20
1.99
Fe61–O20
1.96
Zn61–O55
1.96
Cu55–O56
2.06
Fe61–O55
1.86
Zn61–O57
1.95
Cu55–O58
2.05
Fe61–O62
2.05
Fe61–O64
2.04
˚) Bond lengths (A [Zn(Fulv)2]2-
[Cu(Fulv)2]2-
[Fe(Fulv)2(H2O)2]2-
Atoms
˚) (A
Atoms
˚) (A
Atoms
˚) (A
Zn109–O19
1.95
Cu109–O19
2.02
Fe109–O19
1.88
Zn109–O20
1.93
Cu109–O20
1.99
Fe109–O20
1.85
Zn109–O73
1.95
Cu109–O73
1.99
Fe109–O73
1.74
Zn109–O74
1.93
Cu109–O74
2.01
Fe109–O74
1.79
Fe109–O110
2.20
Fe109–O112
2.11
Structures optimized in the semi-empirical method (PM6)
The Cu–O bond distances ranged, from 1.99 to ˚ , are similar in other complexes of Cu2? 2.06 A phtalate obtained experimentally by X-ray diffraction: {[Cu(2,20 -bipyridine)(l-phthalate)H2O]3.5H2˚ ), according to what was stated O}n (1.93 and 1.95 A by Zhang et al. and {[Cu(Pht)(Im)2]1.5H2O}n (from ˚ ) described by Baca et al. Suksrichavalit 1.96 to 1.99 A and coworkers reported the coordination of Cu2? with nicotinic acid and the phtalate group with bond ˚ calculated at the DFTdistances of 1.83 to 1.87 A B3LYP/LANL2DZ level (Zhang et al. 2000; Baca et al. 2006; Suksrichavalit et al. 2008). The distortions in the complex [Cu(Fulv)2]2typical of transitions d–d are in accordance with the
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effect Jahn–Teller due to the configuration d9 of Cu2? (Menelaou et al. 2012). However once neither the orbitals t2g in an octahedral complex nor any orbital d in a tetrahedral complex point directly to the ligands, the effect is very small to induce a measurable distortion on the structure (Shriver and Atkins 2008). In relation to the Fe2? complex, the Fe–O bond ˚ . These bond length ranged from 1.74 to 2.20 A distances are next to the values found by studies conducted by van Schaik et al., who evaluated the interaction of Fe with fulvic acid (standard IHSS1S102F) by means of extended X-ray absorption fine structure (EXAFS) spectroscopy. The bond length Fe– ˚ , highlighting that the O ranged from 1.98 to 2.10 A
Biometals
Vibrational assignments The frequencies and intensities of vibration were calculated using the semi-empirical method PM6 and the vibrational spectra for the complex are shown on Figs. 5, 6 and 7, respectively. Generally, the calculated number of waves is larger than the experimental number due to factors like neglect of anharmonicity, electronic correlation and deficiencies on the basis set (Go¨kce and Bahc¸eli 2012). The correlation of theoretical data with experimental values obtained by other authors (Matczak 2010; Rageh et al. 2015; Bertoli et al. 2015a, b), shows that the calculated vibrational modes (Table 3) by the semi-empirical method PM6 offers a good quantitative performance on predicting the vibrational frequencies. A detailed description of the main vibrational assignments calculated for the complex is shown on Table 3. Vibrational modes C–H The typical bands of chemical groups useful to the identification of the molecular structure frequently involve coupled vibrations. According to the vibrational assignments presented on Table 3, bands in the region from 790.88 to 1012.35 cm-1 may be
IR Intensity
[Zn(Fulv)(H2O)2]
2-
[Zn(Fulv)2]
3000
2500
2000
1500
1000
500
-1
Wavenumber (cm ) Fig. 5 IR spectra for the complex [Zn(Fulv)(H2O)2] and [Zn(Fulv)2]2-
IR Intensity
ligands present on fulvic acid may be acetate, phenolate, phthalate, salicylate, and catecholate (van Schaik ¨ hrstro¨m and Michaud-Soret, evaluated et al. 2008). O the geometry of the compounds Fe(catecholate)2-, Fe(4-methylcatecholate)2-, [Fe(oxalate)3]3- and Fe(oxalate)2- at the DFT level and observed bond ˚ (O ¨ hrstro¨m and distances between 1.86 and 2.04 A Michaud-Soret 1999). Distortions were also expected on the geometry of Fe2? complex, however if the ligands connected to the metal were strong field, situations of low-spin are created in the orbitals d preventing distortions (Bertoli et al. 2015a, b). Therefore, the groups C-O complexed to the metal are strong field ligands, what favors the electron pairing (Shriver and Atkins 2008). The complex [Fe(Fulv)(H2O)4]- adopted the pentagonal planar geometry, what may be caused by several anionic ligands around the metal, the negative charge that must be stabilized by steric protection and strong p-donors ligands to destabilize the sterically preferred octahedral geometry (Holland 2011).
[Cu(Fulv)(H2O)2]
2-
[Cu(Fulv)2] 3000
2500
2000
1500
1000
500
-1
Wavenumber (cm ) Fig. 6 IR spectra for the complex [Cu(Fulv)(H2O)2] and [Cu(Fulv)2]2-
attributed to the angular deformation of C–H (Balachandran et al. 2014), in the modes, in the plane (drocking (HCH)) and out of the plane in the ring of 6 members [dwagging Ring (CH)]. Low-intensity stretches came from the H–C–H bonds of aliphatic chain were observed in the region between 2667.44 and 2674.83 cm-1. C–C stretches Angular deformations C–C in the twisting mode [dtwist (C–C]) were present in the region from 1138.84 to 1157.12 cm-1, while symmetric stretches C=C of the
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IR Intensity
Biometals
[Fe(Fulv)(H2O)4]
2-
[Fe(Fulv)2(H2O)2] 3000
2500
2000
1500
1000
500
Wavenumber (cm-1 ) Fig. 7 IR spectra for the complex [Fe(Fulv)(H2O)4] and [Fe(Fulv)2(H2O)2]2-
aromatic rings (msymRing (C=C)) were observed between 1410.39 and 1638.53 cm-1. For vibrations of the aromatic nuclei, bands in the region of 1600 cm-1 are intense when the phenyl group is conjugated to unsaturations or even linked to atoms with pairs of free electrons. C=C stretches of ring, but with lower intensity, were also found in the region from 1048.77 to 1195.70 cm-1 coupled to vibrations C–OH and the bond vibrations of intramolecular hydrogen. For complex formed around the central metallic cation, it makes physical sense to find vibrational couplings that describe simple bonds such as C–C and C–O (Silverstein et al. 2006). The assignments to vibrations C–C are close to the results found by Go¨kce and Bahc¸eli, who studied metallic complex of Co2?, Cu2? and Zn2? through experimental and theoretical methods with B3LYP/ LANL2DZ (Go¨kce and Bahc¸eli 2013). C–O, C=O and COO- stretches The carboxylic acids present as characteristic a C–O stretch in the region from 1200 to 1300 cm-1 (Balachandran et al. 2014). The frequencies for this group are concentrated in the region from 1236.87 to 1305.83 cm-1 and correspond to the C-O stretches of carboxylate groups linked to the aromatic rings [mRing (C–O)]. The bands related to this stretch may be greatly overlapped in the range between 1150 and 1300 cm-1 such as confirmed by Moreira et al., who observed this vibration in 1278 cm-1 using the method DFT. Stretches of C–OH groups of carboxyl
123
linked to the rings and phenyl groups [mRing (C–OH)] were observed in the region from 1122.14 to 1137.13 cm-1 (Moreira et al. 2012). Carboxylic acids, ketones and lactones show an intense band in the region from 1540 to 1870 cm-1 that originates in the axial deformation of the double bond carbon–oxygen. Inside this region the position of the deformation band of C=O is very sensible to several factors, like physical state, hydrogen bond, the electronic effect per substituent, ring strain, etc. (Silverstein et al. 2006; Balachandran et al. 2012). For the highlighted complex, bands between 1587.50 and 1795.45 cm-1 and 1819.16 and 1866.06 cm-1 were attributed to the stretches m(C=O) of ketones and mRing (C=O) of lactones, respectively. The most intense bands of C=O related to the symmetric [msymRing(COO-)] and asymetric stretches [masymRing(COO-)] of the phthalate groups were present in the region from 1297.23 to 1478.48 cm-1 and from 1587.50 to 1816.54 cm-1, respectively. The theoretical frequencies were similar to the experimental assignments, however the values were overestimated mainly for asymmetric stretches of the phthalate group [masymRing(COO-)]. Matczak, in works with organotins using the methods B3LYP, PM6 and B3PW91 previewed that, in general, the theoretical approaches tend to overestimate the frequencies of vibration (Matczak 2010). Hydrogen bonds In most of the chemical environments, the hydroxyl group does not exist separately; the vibration may, generally, be coupled, thus resulting in extensive hydrogen bonds. These groups may be associated within the same molecule (intramolecular hydrogen bond) or among neighboring molecules (intermolecular hydrogen bond). In compounds like carboxylic acids, which exhibit extremely strong hydrogen bonds, one typical characteristic is the presence of this bond in lower frequencies (Balachandran et al. 2014; Silverstein et al. 2006). Through the theoretical calculations conducted in the present work, it was possible to the observe vibrations related to intramolecular hydrogen bonds in the regions from 790.88 to 1195.70 cm-1, between the phthalate coordinated to the metals and the phenyl group [dwagging (OHO)). These bonds are typical of dimeric carboxylic acids from the angular deformation
2670.42
m (HCH) 2667.44
2539.95
2389.35
1863.15 1824.10
1820.33
1786.63
1297.23
1305.83
2668.52
2542.84
2422.11
1858.45 1817.98
1819.50
1726.86
1795.45
1638.53
1364.73
1236.87
1138.84
906.30
[Fe(Fulv)(H2O)4]
1412.93
m(HCH)
m(OH)
2674.83
2564.77
1825.70
1771.46 1805.35
m(C=O) ? masymRing(COO-) msym(COOH) mRing (C=O)
1601.28
masymRing (C=C)
1469.75
msymRing (C=C) msymRing (COO-)
1137.13
1048.77
1012.35
940.10
898.19
[Zn(Fulv)2]2-
Complex 2:1
mRing (C–OH) ?msymRing (C=C)
dwagging (OHO) ?msymRing (C=C)
dwagging (OHO) ?dwagging Ring (CH)
dwagging (OHO) ?dwagging Ring (CH)
drocking (HCH) ?dwagging Ring (CH)
Approximate Assignments
2673.00
2561.56
1829.08
1697.92 1825.73
1625.49
1359.95
1526.28
1122.14
1195.70
1003.77
903.18
790.88
[Cu(Fulv)2]2-
2674.41
2560.49
1819.16
1587.50 1804.34
1603.40
1465.42
1410.39
1132.32
1123.12
979.97
944.40
890.67
[Fe(Fulv)2(H2O)2]2-
dtwist twisting deformation, drocking rocking deformation, dwagging wagging deformation, dsciss scissors deformation, msym symmetric stretching, masym asymmetric stretching
2151.70
2525.22
m (OHO)
1866.06 1912.31
m (OH)
1837.46
msym (COOH)
mRing (C=O) mRing (C=O)
1806.80
1717.45
1816.54
1633.88
msym Ring (C=C)
m (C=O)
1478.48
msym Ring (COO-)
masymRing (COO-)
1423.07
1238.98
drocking Ring (C–H) ?mRing (C–O)
1157.12
1144.02
dtwist (C–C) ? drocking (O– H)
910.58
905.93
[Zn(Fulv)(H2O)2] [Cu(Fulv)(H2O)2]
Complex 1:1
drocking (HCH) ? dwagging Ring (CH)
Approximate Assignments
Table 3 Main calculated frequencies (cm-1) and vibrational assignments of the complex by the semi-empirical (PM6) method
Biometals
123
Biometals
of the plane in the group O–H and generally they appear between 900 and 1000 cm-1 (Silverstein et al. 2006). Vibrations were also observed between 2151.70 and 2564.77 cm-1 in dimers of carboxylic acids that absorbs an axial deformation of intense and wide O–H bond in the region from 2500 to 3000 cm-1. These O–HO interactions were greatly important to the system to elucidate the relation structure–property (Grabowski 2011).
Conclusions Fulvic acids extracted from organic materials can be enriched with macro and micronutrients and be marketed as ‘‘organic’’ fertilizers or ‘‘natural’’, and according to the manufacturers are released to the plants according to their needs. This is a relatively new market that has economic potential. However, the industry remains poorly regulated, primarily due to the lack of scientific data on nutrient release mechanisms and structures formed since it can be formed supramolecular aggregates. Therefore, studies of interactions (such as complexation) between the metallic species of macro and micronutrients with fulvic acids are important in improving agricultural productivity and reducing environmental impacts (Botero et al. 2010). 0 The DHðaq:Þ results suggest that the zinc complex may be preferably formed and the order of stability was Fe [ Cu [ Zn for the ratio 1:1 and Fe [ Zn [ Cu in 2:1. Furthermore, vibrational frequency calculations showed that hydrogen bonds are greatly important on predicting the structure and stability of complex. For the stoichiometry 1:1 the structures had similar geometries except for the compound [Fe(Fulv)(H2O)4]. The most stable structure with 2:1 of zinc and copper complex formed a distorted tetrahedral, while the complex [Fe(Fulv)2(H2O)2]2- was octahedral. The use of semi-empirical method PM6, which is parameterized for transition metals and applies some parameters obtained from experimental data, may be useful on understanding the interactions between metallic ions and a physiologically relevant ligand, like the fulvic acid. Acknowledgments To Coordenac¸a˜o de Aperfeic¸oamento de Pessoal de Nı´vel Superior (CAPES) by the scholarship; to Instituto de Quı´mica of Universidade Federal de Alfenas (UNIFAL); to the Laborato´rio de Ana´lise e Caracterizac¸a˜o de
123
Fa´rmacos (LACFar-UNIFAL); to Fundac¸a˜o de Amparo a` Pesquisa do Estado de Minas Gerais (FAPEMIG).
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