Synthesis , Spectroscopic Properties and Structural Studies of Copper ( II ) Complexes of 2-Substituted-1 , 3-Diphenyl-1 , 3-Propanedione , Their 2 , 2 ′-Bipyridine and 1 , 10-Phenanthroline Adducts

The complexes of 2-substituted-1,3-diphenyl-1,3-propanedione with copper(II) ion and their 2,2′-bipyridine and 1,10-phenanthroline adducts were synthesised and characterised by microanalysis, conductance, magnetic and spectral measurements. The conductance data showed that [Cu(dbm)(phen)](dbm) and [Cu(Me-dbm)(phen)](Me-dbm) are 1:1 electrolytes. The infrared spectra revealed the different shifts of the carbonyl frequency. While the electronic solid reflectance spectra of the prepared complexes exhibited three peaks with varying λmax between 12,121-21,505 cm, the adducts displayed single bands in the visible region between 13,553-14,698 cm except for [Cu(dbm)(phen)](dbm) and [Cu(Me-dbm)(phen)](Me-dbm) with additional two peaks. These peaks have been assigned as d-d transitions. Using Density Functional Theory (DFT) and Semi-empirical PM3, the modeled compound showed a distorted five coordinate square pyramidal geometry.


Introduction
β-diketonates have been used as shift reagents for the structural determination of steroids and other complex molecules (Hinckley, 1969(Hinckley, , 1970)).Metal -diketonates have also been used in fuel additives; trace metal analysis by gas chromatography and other numerous extraction applications (Wenzel, 1985).We have recently reported the magnetic and spectral properties of nickel(II) complexes of 2-substituted-1,3-diphenyl-1,3-propanedione and their 2,2′-bipyridine and 1,10-phenanthroline adducts and were able to correlate the band shifts with the positive inductive effect of the substituted alkyl group on the system (Woods et al., 2009).These studies have been extended to the copper(II) derivatives of 2-substituted-1,3-diphenyl-1,3-propanedione [R-dbmH, where R = methyl (Me), ethyl (Et) and normal-butyl (n-Bu)] and their 2,2′-bipyridine (bipy) and 1,10-phenanthroline (phen) adducts to investigate the effects of these substituents on the properties of these compounds.Density functional theory is a widely used method for electronic structure calculations and provides useful predictions for molecular parameters (Perdew et al., 2005).In this paper, we present our report on synthesis, microanalysis, conductance, magnetic and spectral measurements of Copper(II) complexes of 2-substituted-1,3-diphenyl-1,3-propanedione, their 2,2′-bipyridine and 1,10-phenanthroline adducts and the DFT and PM3 calculations of Cu(Me-dbm) 2 bipy.

Computational Methods
Complete geometrical optimisation without symmetry constraint was performed using DFT (Density Functional Theory) with Becke's three parameter exchange functional, along with the Lee-Yang-Parr correlation functional and with LANL2DZ basis set.All calculations were done with Spartan '06 V112 (Shao et al., 2006).The vibrational wavenumbers were calculated using the PM3 method and a C1 point group.The positive value of all the calculated wavenumbers indicates the stability of the optimised geometry.

Results and Discussion
The analytical and the physical data are shown in Table 1.Different shades of green colour were observed for all the copper(II) compounds.The Microanalytical data, as depicted in Table 2, shows that the elemental analyses are consistent with the proposed stoichiometry.
The prepared 2-substituted-1,3-diphenyl-1,3-propanedionato copper(II) complexes have moments in the range 1.75-2.00B.M.An increase was observed on substituting the 2-position of Cu(dbm) 2 with alkyl groups except Cu(n-Bu-dbm) 2 with decreased moment.The observed magnetic moments of the synthesised 2,2′-bipyridine and 1,10-phenanthroline adducts of 2-substituted-1,3-diphenyl-1,3-propanedionato copper(II) complexes are in the range 1.95-2.10B.M.This shows that they are magnetically dilute compounds and that there is no intermolecular magnetic interaction.An increase in magnetic moments was observed in all the adducts as compared with the appropriate parent complexes.
The molar conductivities of these complexes were very low with  m values of 4.2-29.7 ohm -1 cm 2 mole -1 , which suggests that they are non-electrolytes.Similarly, the molar conductivities of the Cu(R-dbm) 2 adducts indicate that they are non-electrolytes except [Cu(dbm)(phen)](dbm) and [Cu(Me-dbm)(phen)](Me-dbm), which are 1:1 electrolytes with an outer sphere anion as reflected in the  m values of 82-84 ohm -1 cm 2 mole -1 .
The carbonyl and carbon-carbon double bonds have less double bond character and more single bond character in the conjugated ring, which accounted for the large frequency shifts usually observed.Studies have revealed that three factors determine the position of the perturbed carbonyl band in the spectrum of a given chelate: the masses of the groups attached at the ends of the ligand molecule to the carbonyl groups; interaction of the carbonyl with neighbouring π or d-orbitals and the relative electron density of the σ bond.Moreover, the comparison of the infrared spectra taken at low temperature with those at room temperature shows that there is a sharpening up of the bands in the low temperature spectrum owing to the decreased population of the higher vibrational states of the molecule (Fackler et al., 1968).
In the complexes studied, the frequencies of the asymmetric C=O and C=C stretching vibrations were lowered from their free ligand values (Table 3).The  as (C=O)   as (C=C) vibrations of the copper(II) complexes occurred as multiple bands in the 1514-1653 cm -1 region.Single band of  s (C-O) C-H were observed in Cu(dbm) 2 while Cu(Me-dbm) 2 , Cu(Et-dbm) 2 and Cu(n-Bu-dbm) 2 had double bands.Upon adduct formation, hypsochromic shifts of the  as (C=O)   as (C=C) in all the adducts relative to the parent complexes were observed except Cu(Me-dbm) 2 bipy, Cu(Me-dbm) 2 phen, and Cu(Et-dbm) 2 phen which had bathochromic shifts.The observed shifts can be used to predict the type of bonds in the adducts (Holtzclaw & Collman, 1957;Tanaka et al., 1969).
The hypsochromic shifts probably indicate stronger Cu-N and C-O bonds and weaker Cu-O bonds while the reverse is applicable for bathochromic shifts.The symmetric and asymmetric methyl bending vibrations of the adducts appeared with varying shifts as compared with the complexes.CH deformation bands of 2,2′-bipyridine were observed as strong bands in the 745-778 cm -1 region while the phenanthroline adducts bands were observed around 717-722 cm -1 and 843-856 cm -1 region.Coupled Cu-O and Cu-N stretching vibrational modes occurred in the range 420-696 cm -1 in the 2,2′-bipyridine and 1,10-phenanthroline adducts (Patel & Woods, 1990b).The solution spectra of the copper(II) complexes were studied in chloroform and methanol.The assignments of the bands have been made with the help of literature on similar compounds (Fackler et al., 1968;Patel & Woods, 1990b, 1990c).Hypsochromic shift of the π 3 -π* 4 band was observed in Cu(Et-dbm) 2 as compared with Cu(dbm) 2 in chloroform.Cu(Me-dbm) 2 had bathochromic shift while Cu(n-Bu-dbm) 2 had no shift.Coordinating solvents have been found to have a particular dramatic effect on the ligand field spectra of copper(II) compounds (Patel & Woods, 1990b).As a result of this, when there is a higher frequency shift in the ligand field spectra band of the copper compounds in coordinating solvents relative to non-coordinating (chloroform), it indicates a probable square pyramidal structure.A probable four coordinate square planar structure is observed when there is lower frequency shift in coordinating solvent relative to non-coordinating.For six-coordinate octahedral geometry, the band position remains unchanged in both coordinating and non-coordinating solvents.Thus, the various synthesised Cu(R-dbm) 2 have probable, four-coordinate, square planar geometry due to their lower frequency shifts in methanol relative to chloroform.Copper(II) complexes with square planar stereochemistry commonly exhibit a broad structured band between 13,000 to 20,000 cm -1 .They have absorption that shows little structure between 18,000 to 21,000 cm -1 and no electronic absorption below 10,000 cm -1 (Lever, 1986).In the synthesised complexes, the little structure absorption was observed between 18,051-18,087 cm -1 in chloroform, which also shows that the various synthesised Cu(R-dbm) 2 have probable four-coordinate, square planar geometry.In a study, the visible bands shifted to higher frequencies on replacement of hydrogen by alkyl groups and the magnitude of the shift was about the same in all the solvents.This was attributed to the inductive effect of the alkyl groups leading to a higher ligand field.It was also detected that lengthening of the alkyl side chain produced no further change in the formation constants (Graddon & Schulz, 1965).Transitions in Copper(II) of β-diketonates with absorption at energies higher than 24,000 cm -1 originate from charge transfer from β-diketones ion to the metal (Melnik et al., 1996;Gorbenko et al., 1997).In the prepared complexes, high-energy absorption was observed at 27,855 cm -1 in Cu(dbm) 2. .
The tentative assignment of the reflectance spectra of the ligands (R-dbmH), copper(II) complexes and their adducts in calcium carbonate are presented in Table 5.The visible spectra of all the complexes studied displayed three bands in this region with varying λ max between 12,121-21,505 cm -1 , which is consistent with square planar geometry for copper(II) complexes.The spectra of the Cu(R-dbm) 2 adducts displayed single bands in the visible region between 13,553-14,698 cm -1 .This is also consistent with square pyramidal geometry for copper(II) compounds (Odunola et al., 2003) except [Cu(dbm)(Phen)](dbm) and [Cu(Me-dbm)(Phen)](Me-dbm) with little structure absorption at 18,149-21,978 cm -1 which corresponded with square planar structures (Lever, 1986).In the synthesised complexes,  3 - 4 * transitions were observed in the 31,153-33,445 cm -1 region.Splitting of this band was not observed in any of the prepared complexes.Bands in the 40,000-44,843 cm -1 regions have been assigned as benzenoid/σ L -3d xy transitions (Johnson & Thornton, 1975).The ultraviolet region of the solid reflectance spectra of the adducts showed hypsochromic shift of the π 3 -π 4 * transition upon adduct formation.In the adducts, π 3 -π 4 * transition appeared as single bands at 32,154-35,971 cm -1 region except Cu(Et-dbm) 2 bipy which had an additional band.The electronic and its adducts

Geometry and Structural Data
Density Functional Theory at B3LYP/LANL2DZ was used for geometry optimisation and electronic structure determination (Karakas & Sayin, 2013;Malecki et al., 2010).The optimised geometry of the Cu(Me-dbm) 2 bipy is shown in Figures 3(a) and (b).Table 6 shows the selected calculated bond distances, angles, of the modeled compound and the experimental X-ray crystallographic data (Zheng et al., 1991) of a five-coordinated copper(II) complex of 2,2-bipyridine.The optimised bond distances at both semi-empirical PM3 and B3LYP levels are comparable with the corresponding values obtained from the X-ray diffraction as can be seen in Table 6 (  The molecular data from DFT calculation are summarized in Table 7.The energy gap between LUMO (lowest unoccupied molecular orbital) and HOMO (highest occupied molecular orbital) is 0.329 eV.The low energy gap indicates high reactivity (Fan et al., 2007;Herrag et al., 2010;Obot & Obi-Egbedi, 2010;Obot et al., 2012).The dipole moment is 28.(Sheela,Sampathkrishnan, Kumar, & Muthu, 2013).
high tendency to react with other charged compounds (Herrag et al., 2010).The modeled Copper(II) complex has 83 atoms, 234 normal modes of fundamental vibrations and possesses C1 point group.By using semi-empirical PM3 method, the vibrational frequencies of the modeled Copper(II) complex were calculated in the ground state.Calculated vibrational frequencies are scaled by a factor 0.974, that was recommended by Scott and Radom (Scott & Radom, 1996).Fourteen calculated vibrational frequencies are selected and reported in Table 8.Some of the calculated vibrational wavenumbers were found to agree quite well with the available experimental data.Discrepancy between calculated and experimental values could be due, in part, to anharmonicity and the tendency of the quantum chemical methods to overestimate the force constants at the exact equilibrium geometry (Sheela et al., 2013). s = symmetric stretching; γ = out of plane bending;  as = assymetric; Int = IR intensity.

Table 6 .
Selected calculated bond distances, angles, of the modeled compounds and the experimental X-ray crystallographic data

Table 8 .
Comparison of the observed and calculated vibrational wavenumber (cm -1 ) of Cu(Me-dbm) 2 bipy with PM3 method