Synthesis, Spectroscopic Characterization and Theoretical Studies of Copper (II) 1, 3-Diphenylpropane-1, 3-Dione Complex and its 1, 10-Phenanthroline, 2, 2-Bipyridine Adducts

Abstract

Copper (II) 1, 3-diphenylpropane-1, 3-dione complex and its 1, 10-phenanthroline, 2, 2-bipyridine adducts were synthesized and accordingly characterized by elemental analysis, solubility, infrared, electronic spectroscopic methods. Infrared spectral of the complex and adducts revealed that, lower frequency shifts of varying magnitudes were observed in the carbonyl and aromatic stretching vibrations from their ligand values. Electronic spectral data indicated the geometries of the complex and adducts. The experimental vibration frequencies are compared with those obtained theoretically at semi-empirical level of calculations.

Headnote

Abstract: Copper (II) 1, 3-diphenylpropane-1, 3-dione complex and its 1, 10- phenanthroline (phen), 2, 2-bipyridine (bipy) adducts were synthesized and accordingly characterized by elemental analysis, solubility, infrared, electronic spectroscopic methods. Infrared spectral of the complex and adducts revealed that lower frequency shifts of varying magnitudes were observed in the carbonyl (C=O) and (C=C) aromatic stretching vibrations from their ligand values. Electronic spectral data indicated the geometries of the complex and adducts. The experimental vibration frequencies are compared with those obtained theoretically at semi-empirical (PM3) level of calculations.

Keywords: Copper (II) complex, adducts, 1, 10-phenantroline, spectroscopic, semi-empirical method.

Introduction

?-diketones are a class of high functional compounds with outstanding optical, electric and magnetic properties, and the negative ion may act as an excellent chelating agent [1, 2]. ?-diketone complexes of transition metals have been the subject of many different studies ranging from synthetic, kinetic, and structural topics to catalysis and many others [3-7]. The isolation of various substituted beta- diketones complexs have been reported [8, 9].

Complexes of copper with ?-diketones are of significant theoretical interest in addition to their practical implementation [10, 11]. In the case of dibenzoylmethane (dbm),previous studies mainly involved trivalent metal, Sc, Cr, Mn, Fe, Ga, Al, Ln, while for divalent metal ions, the structures are relatively limited [12]. In the case of divalent metal ions, the [M (?-diketonate)2] complexes are able to coordinate additional ligands forming octahedral metal complexes [13-15]. The nature of the chelating ligands can have a significant effect on the properties of the complexes.

This group had earlier reported the kinetic study of formation of copper (II) dibenzoylmethane complex [16]. In continuation of our studies on copper (II) dibenzoylmethane complex and its derivatives, we hereby presents the theoretical studies of the Copper (II) dibenzoylmethane complex and adducts with 1, 10-phenanthroline and 2, 2-bipyridine. The synthesis, spectroscopic data are included for clearity . The complex and its adducts are modeled based on the spectroscopic interpretations using Spartan program (Spartan 06) implemented on an Intel Pentium M 2.0 GHz computer. The optimization and frequency calculation of the Cu (II) complex and its adducts were performed using semi-empirical method (PM3), since PM3 has been successfully used either alone or with other theorertical methods for structural analysis of Cu(II) complexes [17-22]. The Mulliken charges and highest molecular orbitals (HOMO) of adducts and the metal complex was reported.

Experimental

All chemicals used were of analytical grade dibenzoylmethane (DBM) (Aldrich), Copper (II) acetate monohydrate, 1, 10-phenanthroline and 2, 2-bipyridine.

Preparation of the Complexes and Adducts

Preparation of Cu (DBM)2H2O

50ml Copper (II) acetate monohydrate (1.00g, 5mmoles) dissolved in 40% ethanol was added to 40ml dibenzoylmethane (2.24g 10mmoles) in absolute ethanol while stirring. The precipitated solid was filtered off, washed with absolute ethanol and dried over calcium chloride in a dessicator.

Preparation of Cu(DBM)2phen

The 1,10-phenanthroline adduct of Cu(DBM)2H2O was prepared by adding 1.42g (2.77mmoles) of Cu(DBM)2H2O bit by bit into a 25ml of chloroform containing 1g (5.5mmoles) of 1, 10- phenanthroline in a 250ml beaker while stirring continues for 1 hour. 10ml of chloroform was added when the adduct dries up before 1hour. The resultant mixture was filtered, washed with acetone, dried over calcium chloride in a desiccator (yield 42%).

Preparation of Cu(DBM)2bpy2

1.64g (3.2mmoles) of the Cu(DBM)2H2O was added gradually to 1.00g (6.4mmoles) of the organic base (2,2-bipyridine) in 25ml of chloroform and was stirred continuously for 1hour, filtered by suction, washed with acetone and dried in a desiccators over calcium chloride

Physical Measurements

The percentage Copper in the Cu(DBM)2H2O and its adducts were determined using complexometric titration using EDTA. Infrared spectra were recorded on a Perkin-Elmer Lambda 950 spectrometer, FTIR spectra (chloroform). Electronic spectra were recorded on a Heλios? UV-visible spectrophotometer V4.60.

Results and Discussion

Elemental Analyses

The result of the metal analysis on the digested Copper (II) dibenzoylmethane complex suggested that the complex synthesized was a bis (dibenzoylmethane) copper (II) complex [Cu (DBM)2H2O] with a probable shape of square pyramidal with the coordination number of five. The result of the metal analysis on the 1,10-phenanthroline adduct of Copper (II) bis (dibenzoylmethane) suggested that the adduct synthesized was of the formula Cu (DBM)2phen with probable shape of distorted octahedral due to the coordination number of six. The result of the metal analysis on the 2, 2-bipyridine adduct of Copper (II) bis (dibenzoylmethane) suggested that the adduct synthesized was of the formula Cu (DBM)2bpy2 with probable octahedral shape due to the coordination number of six. The metal analyses showed a fairly good agreement between the observed and calculated values (Table 1).

Solubility

The Copper (II) dibenzoylmethane complex, its 1, 10-phenanthroline and 2, 2-bipyridine adducts exhibits good solubility in chloroform. They possess shades of green (Table 2).

Infrared

The main stretching modes in the infrared spectra of the complex and adducts resulting from dibenzoylmethane, 1-10-phenantroline, and 2, 2-bipyridine are υC=O, υC-O, υC-N υC=C [13]. The absence of any absorption above 1700cm-1, as well as the broad band observed at 3411cm-1 due to hydrogen bonded O-H in the ligand is an indication that dbm is in the enol form. Insertion of Copper (II) shifted the broad band to 3021cm-1(υH2O). A slight shift to lower frequencies of υc-o absorption (Table 3) between 1543cm-1 and 1538cm-1 was observed on complexation and adducts formation. This indicates that the ?-diketonate ligands adopt a chelating coordination mode [13]. Theoretical calculation gave similar results between 1691cm-1 and 1620cm-1 .The υ(c=c) vibrations of the 1, 10-phenanthroline and 2,2-bipyridine adduct was observed at lower frequency relative to the parent ligand and complex which is consistent with the theoretical result. The coupled vibrations of Cu-O and Cu-N stretching modes appeared below 700cm-1 in the complex and adducts while the 457-489 cm-1 band have been assigned as pure υ(Cu-O) vibrations [23, 24]. Theoretical results show the Cu-O vibrations below 700cm-1 in the complex and adducts except for the Cu-N vibrations which are very close to 700cm-1.

Electronic Spectra

On complexation, hypsochromic shifts were observed in the π3-π4* tr ansition relative to the ligand The spectra of the Copper (II) complex and adducts have additional bands assigned to dl - 3dxy ring, π- π* and d-d transitions appearing around 50000-49000 cm-1, 45000-41000cm-1 and 19000-14000cm cm 45000-41000cm and 19000-14000cm-1 respectively [25, 26]. The d-d transition 23201cm-1 in the complex shows a square pyramidal geometry with coordination number of 5. A shift to higher frequency (complex to adducts) indicate a change from five to six coordinate; 23584cm-1 and 23640cm-1 for 1,10-phenanthroline and 2,2- bipyridine adducts respectively. These charge transfer transitions; 23584cm-1 for 1,10-phenanthroline adduct and 23640cm-1 for 2,2-bipyridine adduct show distorted octahedral geometry in 1,10- phenanthroline adduct and an octahedral geometry in the 2,2-bipyridine adduct having considered Jahn- Teller distortion. The d-d transition of the complex was accompanied by bathochromic shifts on adduct formation (Table 4) [23, 27, 28].

Computational Studies

Optimized Geometries

The complexes were modeled based on the data obtained from infrared spectra and elemental analysis of the complexes. Cu(DBM)2H2O complex modeled structure consists of five coordinated Copper (II) complex with four unit of carbonyl oxygen bonded to the metal ion from the ligands and the other one unit attaching to a water molecule, thus giving five coordination around the metal ion as shown in figure 1.

Cu(DBM)2phen adduct modeled structure consists of six coordinated copper (II) complex with four unit of carbonyl oxygen bonded to the metal ion from the ligands and the other two unit attaching to the two nitrogen atom of 1,10-phenanthroline, thus giving six coordination around the metal ion as shown in figure 1. However, modeled Cu(DBM)2bpy2 adduct consists of six coordinated copper (II) complex with one unit of carbonyl oxygen bonded to the metal ion from each of the ligands and the other four unit attaching to the four nitrogen atom of two 2,2-bipyridine molecule thus giving six coordination around the metal ion as shown in figure 1.

The geometric parameters calculated at PM3 level of the semi-empirical method for complex (A) and adducts (B, C) are listed in table 5. The calculated results of the bond distances of Cu-O2 (Cu-O3) for complex A, B, C are 1.90Å (1.906Å), 2.024Å (1.960Å) and 2.081Å (2.062Å) respectively. It is observed that there is an increase in the bond distance from complex A through complex B to complex C. This is consistent with the experimental infrared spectra which show lower frequency shifts indicating stronger Cu-O bonds in complex. The bond distances of Cu-N1 (Cu-N2) for complexes B and C are 1.930Å (1.928Å) and 1.947Å (1.950Å) respectively. Comparing the values of Cu-O in complex (A) and adducts (B, C), the Cu-O bonds in the Complex are stronger than that of adducts (B, C). This is due to the interactions of the ligand with the complex resulted to formation of Cu-N bonds in adducts which is consistent with experimental infrared spectra obtained [21].

The bond angle O3-Cu-O2 for complexes A, B, C are 94.81o, 83.23o and 178.59o respectively. The O3-Cu-N1 bond angle for complexes B and C are 90.25o and 92.60o respectively.

There is a decrease in the bond angle from complex A to complex B showing the presence of distortion in the octahedral geometry of complex B which is consistent with electronic spectra result. This agreed with the electronic spectra results in which distorted octahedral geometry was assigned to complex B and octahedral geometry to complex C.

The Frontier molecular orbitals for the complex (A), adducts (B, C) and ligand (DBM) computed at the PM3 level of semi-empirical method are shown in Figure 2. The HOMO orbitals are mainly localized on the phenyl group of the DBM while LUMO is principally delocalized on either the 1,10- phenanthroline for adduct (B) or 2,2-bipyridine for adduct (C). The HOMO-LUMO band gap of the complex and adducts suggested that they might have good stability.

The Mulliken atomic charges for copper, oxygen and nitrogen atoms are displayed in table 6. The Mulliken charge of copper atom in the complexes A, B, C are -0.154, - 0.240 and -0.267 respectively. The charge increases from A through B to C showing that more electrons are present in the d-orbital of the copper ion of adducts than that of the complex. This is due to electron donating ability of nitrogen atoms in the ligand which is consistent with the geometry obtained from the electronic spectra of complexes A, B, C. The higher Mulliken charges on copper in adduct (C) is due more π electrons system than in adduct (B). This is also evident in the Mulliken charges on O2(O3) in complex A and adducts (B and C) which are -0.114 (-0.082), -0.216 (-0.162) and -0.296 (-0.273) respectively. The values of Mulliken charges on nitrogen atoms are all positive, although the Mulliken charges on N1 and N2 are 0.490 and 0.485 in adduct B and 0.403 and 0.392 in adduct C respectively. The higher positive Mulliken charges on the nitrogen atoms of adduct B show that more of their electrons lone pairs have been donated to the copper central atom. This is in agreement with the experimental vibrational frequency bands of C=C aromatic which was observed at 1397 cm-1 and 1479 cm-1 in adduct B and C respectively.

Conclusion

Copper (II) complex of dibenzoylmethane and its 1,10-phenanthroline and 2,2-bipyridine adducts were synthesized. Their elemental analysis agreed with the expected theoretical values. A hypsochromic shift in the carbonyl vibrational band was observed on the addition of a nitrogenous base (1,10-phenanthroline and 2,2-bipyridine) to the parent complex A. Geometries were assigned to the complex and adducts using the electronic spectra results. These geometries were theoretically modeled using the semi empirical PM3 method. The calculated PM3 results were quantitatively agreed with the experimental results.

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