^{1}

^{1}

^{*}

^{1}

^{2}

Ground state geometries, spectral (IR and UV-Vis) properties, analysis of frontier molecular orbitals (FMOs), natural bond orbital (NBO) analysis and molecular electrostatic potential (MEP) surfaces of three transition metal complexes [Cu(AOYP)2(OH2)2] (A), [Ni(AOYP)2(OH2)2] (B) and [Zn-(AOYP)2(OH2)2] (C), have been studied theoretically by the Density Functional Theory (DFT) and Time-Dependent Density Functional Theory (TD-DFT) methods. AOYP is the oxadiazole ligand 2-(5-amino-[1,3,4]-oxadiazol-2-yl)phenol. The geometries of these complexes were initially optimized using two basis sets: LAN2DZ and a generic basis set, the latter of which was selected for subsequent analysis. The stability of the complexes arising from intramolecular interactions and electron delocalization was estimated by natural bond orbital (NBO) analysis. The NBO results showed significant charge transfer from lone pair orbitals on the AOYP donor atoms O19, O21, N15 and N36 to central metal ions in the complexes, as well as to the benzene and oxadiazole rings. The electronic spectrum of (A) showed bands at 752 and 550 nm mainly attributable to ligand-to-metal charge transfer (LMCT) transitions, and a band at 446 nm assigned to a d-d transition. The electronic spectrum of (B) consisted of bands at 540, 463 and 395 nm mainly due to d-d transitions. Calculated electronic bands for (C) occurred at 243, 238 and 235 nm, arising from intraligand charge transfer (ILCT) transitions within AOYP. A good agreement in terms of band positions was found between experimental and calculated absorption spectra of the complexes.

Oxadiazoles constitute a very important class of ligands in coordination chemistry due to their wide applications in the synthesis of a large variety of transition metal complexes with diverse biological activities such as anti-inflammatory, antifungal, antibacterial, antiviral, anti-HIV and anticancer activities [

In the search for novel antimicrobial agents based on a backbone of oxadiazoles, researchers have in the recent years synthesized, characterized and studied the antimicrobial properties of a variety of complexes with 1,3,4-oxdiazole ligands. In 2013, Wanale and co-workers synthesized and characterized complexes of 2-(5- amino-[1,3,4]-oxadiazol-2-yl)phenol with Cu(II), VO(IV), Ni(II), Zn(II) and Cd(II), but did not determine their geometrical parameters (bond lengths, bond angles and dihedral angles). These parameters can be conveniently determined theoretically, by quantum chemical calculations. To the best of our knowledge, a theoretical study of the geometries and properties of these complexes has not been reported in the literature. This inadequacy encouraged us to pursue theoretical studies on the work of Wanale and co-workers in order to determine stable geometries and shapes of [Cu(AOYP)_{2}(OH_{2})_{2}] (A), [Ni(AOYP)_{2}(OH_{2})_{2}] (B) and [Zn(AOYP)_{2}(OH_{2})_{2}] (C) as well as various microscopic properties. Shape is a fundamentally important molecular feature that often determines drug activity [

The purpose of the present study was to theoretically optimize the geometries of (A)-(C) and then determine their spectral (IR and UV-Vis) properties, charge delocalization patterns from NBO analysis, frontier molecular orbital compositions and MEP surfaces, via Density Functional Theory (DFT) and Time-Dependent Density Functional Theory (TD-DFT) calculations. The DFT method was chosen for this study because it is faster, less computationally intensive, takes electron correlation into account and has a precise accuracy in reproducing experimental data [

Theoretical calculations on (A)-(C) were carried out using Gaussian 09 rev. A.02 [

Careful selection of basis sets is very important for accurate prediction of the properties of transition metal complexes. To select an appropriate basis set for the current study, we optimized the geometries of (A)-(C) using LANL2DZ and a generic basis sets. The optimized molecular structures of (A), (B) and (C) in the gas phase, are shown in Figures 1-3 respectively. In _{2}O ligands, where significant discrepancies are

Geometrical parameter^{a} | (A) | (B) | (C) | |||
---|---|---|---|---|---|---|

Generic | LANL2DZ | Generic | LANL2DZ | Generic | LANL2DZ | |

Bond lengths (Å) | (A1) | (A2) | (B1) | (B2) | (C1) | (C2) |

N15-M20 | 2.015 | 2.007 | 1.900 | 1.901 | 2.152 | 2.151 |

N36-M20 | 2.017 | 2.018 | 1.900 | 1.900 | 2.163 | 2.110 |

O19-M20 | 1.967 | 1.988 | 1.859 | 1.867 | 2.085 | 2.002 |

O21-M20 | 1.957 | 1.949 | 1.859 | 1.867 | 2.077 | 2.077 |

O40-M20 | 3.079 | 5.149 | 5.095 | 5.063 | 2.326 | 2.043 |

O43-M20 | 2.631 | 2.363 | 5.107 | 5.062 | 2.294 | 3.549 |

Bond angles (˚) | (A1) | (A2) | (B1) | (B2) | (C1) | (C2) |

N15-M20-O19 | 88.84 | 87.64 | 91.25 | 91.17 | 83.12 | 83.94 |

N15-M20-O21 | 93.99 | 91.50 | 88.75 | 88.96 | 107.89 | 168.61 |

N36-M20-O19 | 90.75 | 93.30 | 88.78 | 88.63 | 91.03 | 102.84 |

N36-M20-O21 | 88.74 | 89.30 | 91.23 | 91.25 | 83.66 | 85.19 |

O40-M20-N15 | 73.90 | 45.80 | 49.00 | 46.87 | 79.23 | 81.76 |

O40-M20-O21 | 63.20 | 45.71 | 39.89 | 42.18 | 73.34 | 87.80 |

O43-M20-N15 | 103.33 | 106.19 | 131.15 | 132.61 | 101.15 | 123.74 |

O43-M20-O21 | 98.04 | 103.53 | 140.09 | 138.22 | 86.82 | 45.19 |

Dihedral angles (˚) | (A1) | (A2) | (B1) | (B2) | (C1) | (C2) |

O19-M20-N36-N35 | 24.46 | 5.52 | 1.87 | 2.62 | 35.85 | 89.27 |

O21-M20-N15-N14 | 14.11 | 2.20 | 0.79 | 1.65 | 30.20 | 50.82 |

O40-M20-O19-N36 | 98.24 | 163.69 | 174.72 | 176.55 | 91.53 | 173.68 |

O40-M20-O21-N15 | 69.72 | 1.42 | 4.33 | 3.41 | 72.44 | 23.52 |

O43-M20-N36-O19 | 71.55 | 72.42 | 0.00 | 4.74 | 72.83 | 137.13 |

O43-M20-N15-O21 | 99.24 | 104.55 | 179.18 | 175.30 | 90.24 | 12.70 |

^{a}M represents the central metal ion which is Cu(II) for (A), Ni(II) for (B) and Zn(II) for (C). For atomic numbering of (A), (B) and (C), refer to Figures 1-3 respectively.

observed. The values of bond angles and dihedral angles around the central metal ions calculated using the generic basis set are considered more acceptable because they are much closer to standard values in regular octahedral complexes (bond angles around the central metal in ideal octahedral complexes are 90˚ each). From the values of geometrical parameters around the central metal ions, the generic basis set was found to be more appropriate than the LANL2DZ basis set for calculations on the complexes and was then chosen for all calculations reported in this paper.

When the octahedral input geometry of (B) was fully optimized at both levels of theory, the output geometry was found to be square planar with the two H_{2}O ligands originally coordinated to the central Ni(II) ion, now linked to the NH_{2} groups of the AOYP ligands through hydrogen bonds as shown in _{2}O ligands (O40 and O43) are roughly in the same plane that contains the AOYP ligands and the central Ni(II) ion. Also, the nickel-oxygen distances Ni20-O40 and Ni20-O43 calculated at both levels of theory are around 5.1 Å which is unreasonable for a metal oxygen bond; therefore, the H_{2}O molecules are not coordinated to the central Ni(II) ion. The two H_{2}O ligands are lattice held and, hence, constitute water of crystallization. The square planar structure of the Ni(II) complex is expected because the Ni(II) ion has a d^{8} electronic structure and transition metal ions or atoms with this structure form stable square planar complexes with dsp^{2} hybridization.

Cu(II) and Zn(II) complexes optimized with the generic basis set, (A1) and (C1) respectively have distorted octahedral geometries around the central metal ions, while the same complexes optimized with LANL2DZ basis set, (A2) and (C2) respectively, have distorted square pyramidal geometries around the central metal ions as shown in Figures 1-3. In both cases, the water molecules in the complexes are involved in hydrogen bonding. (A1) is Jahn-Teller distorted because the bonds to the central Cu(II) ion along the z-axis (O40-M20 and O43-M20 with bond lengths 3.08 Å and 2.63 Å respectively) are much longer than the bonds along the x-axis and y-axis: 2.02 Å for M20-N15 and M20-N36 and 1.96 Å for M20-O19 and M20-O21. This corresponds to an extension along the z-axis and compression along the x- and y-axes.

Some calculated IR gas phase frequencies and their probable assignments for AOYP and its transition metal complexes are listed in

Vibrational^{a} assignment | IR frequencies of AOYP (cm^{−1}) | IR frequencies of the complexes (cm^{−1}) | ||||
---|---|---|---|---|---|---|

Calculated (scaled)^{b} | Experimental | Calculated (scaled)^{b} | Experimental | |||

(A) | (B) | (C) | ||||

3237 | 3382 | _ | _ | _ | _ | |

3428 | 3300 - 3200 | 3433 | 3278 | 3433 | 3300 - 3200 | |

1582 | 1597 | 1519 | 1520 | 1519 | 1575 - 1555 | |

1259 | 1255 | 1338 | 1350 | 1344 | 1235 | |

_ | _ | 3456 | 3399 | 3429 | 3500 - 3400 | |

_ | _ | 432 | 419 | 483 | 570 - 550 | |

_ | _ | 384 | 392 | 335 | 440 - 420 |

^{a} represents symmetric stretching vibrations of N-H and O-H bonds in the NH_{2} group and H_{2}O ligand respectively, Phe stands for Phenolic and Azo stands for Azomethine. ^{b}The theoretical wavenumbers were scaled by 0.9614.

mode vibrations of the complexes. This is because the DFT/B3LYP method tends to overestimate normal mode frequencies due to a combination of electron correlation effects and basis set deficiencies. Therefore, scaling factors have to be used to obtain a considerably better agreement with experimental data [

After scaling, we determined the correlation between experimental (FT-IR) and calculated wavenumbers of both AOYP and its transition metal complexes. The relation between these results is linear as described by the following correlation equations:

^{2} = 0.985); for the AOYP ligand

^{2} = 0.992); for M(II) complexes of AOYP

In the process of establishing the correlation equations, we used average wavenumbers for each of the normal modes, calculated from the wavenumbers of all the complexes and average values in a range for experimental wavenumbers. A comparison of the scaled wavenumbers calculated by the B3LYP/GEN method with experimental values reveals a very good agreement; with correlation coefficients of 0.985 and 0.992 for AOYP and its transition metal complexes respectively. The experimental IR frequency assigned to the phenolic O-H vibrational mode of AOYP is 3382 cm^{−1}, according to [_{2} (amino group) with SH (mercapto group), is 3219 cm^{−1}. The calculated frequency for the phenolic O-H stretching vibration is 3237 cm^{−1}, which is closer to the experimental value reported in [

In Equation (1), cal represents the calculated value and exp represents the experimental value. The vibrational frequency of the phenolic O-H disappeared in the spectra of all complexes. This indicates that the AOYP ligand coordinates to the central metal ion in each complex through the phenolic oxygen via deprotonation. The calculated frequency for the C=N stretching vibration in AOYP is 1582 cm^{−1}, which agrees very well with the experimental value since the relative error is only −0.94%. The calculated frequency undergoes a shift towards lower wavenumbers by 63 - 62 cm^{−1} in the complexes. This is indicative of the coordination of one of the azomethine nitrogen atoms in the oxadiazole ring to the central metal ion.

The calculated IR frequency for the phenolic C-O vibration is 1259 cm^{−1} in AOYP. This agrees very well with the experimental value. Contrary to experimental results, the theoretical band shifted to higher (instead of lower) wavenumbers by 91 - 79 cm^{−1} in the complexes. In reality, a shift to lower wavenumbers is expected because the phenolic C-O bond length reduces from 1.35 Å in AOYP to 1.30 Å in the complexes. Also, the phenolic oxygen atom is involved in intensive hydrogen bonding with the H_{2}O ligands, which is not the case in the AOYP ligand. The presence of calculated IR bands in the range 3456 - 3399 cm^{−1} in the spectra of the complexes due to stretching vibrations of O-H bonds in H_{2}O, indicate the presence of coordinated water or water of crystallization in the complexes. This is consistent with experimental results. Calculated IR bands due to the vibrations of O-H bonds in H_{2}O are completely absent in the spectrum of AOYP because it does not contain attached H_{2}O molecules.

The calculated IR spectra for AOYP and its transition metal complexes showed the persistence of two small bands in the region 3510 - 3259 cm^{−1} corresponding to the stretching vibrations of N-H bonds in the NH_{2} group. This suggests the non-coordination of the NH_{2} groups to the central metal ion. However, the IR frequency corresponding to the vibrations of N-H bonds in the NH_{2} group is shifted to lower wavenumbers by 150 cm^{−1} in (B). This can be attributed to the strong hydrogen bonds formed between this group and the non-coordinated water molecules, which act as water of crystallization.

The calculated spectra of the complexes showed the appearance of new bands due to ῦ (M-N) and ῦ (M-O) metal-ligand vibrations in the regions; 550 - 419 cm^{−1} and 419 - 335 cm^{−1} respectively. This further confirms the participation of the phenolic oxygen atom and one azomethine nitrogen atom in coordination to the central metal ion. The M-L vibrational frequencies of these complexes are in good agreement with the experimental values reported in [

Frontier molecular orbitals (FMOs) are the highest occupied molecular orbitals (HOMO) and the lowest-lying unoccupied molecular orbitals (LUMO) [

The LUMO of each complex consists of π anti-bonding MOs designated by π*(AOYP), located on each of the two AOYP ligands, AOYP(1) and AOYP(2). The total contribution from the two AOYP ligands to the LUMO of (A) is 38.59%, to the LUMO of (B) is 33.93% and to the LUMO and LUMO + 1 each of (C) is 98.50%. In

Complex | Molecular orbital | Energy ( ) | Molecular orbital composition (%) | Main bond type | ||||
---|---|---|---|---|---|---|---|---|

Index^{a} | Orbital^{b} | M(II)^{c} | AOYP(1) | AOYP(2) | O atoms in H_{2}O | |||

(A) | 111B | L | −3.203 | 61.35 | 19.11 | 19.48 | 0.06 | (Cu) + π*(AOYP) |

110B | H | −5.646 | 1.80 | 40.96 | 56.33 | 0.91 | π(AOYP) | |

109B | H − 1 | −5.807 | 1.37 | 55.91 | 40.92 | 1.80 | π(AOYP) | |

105B | H − 5 | −7.577 | 2.43 | 23.81 | 11.25 | 62.51 | (O) + π(AOYP) | |

102B | H − 8 | −7.918 | 15.76 | 12.93 | 24.15 | 47.16 | (Cu) + (O) + π(AOYP) | |

95B | H − 15 | −9.430 | 60.62 | 11.05 | 14.79 | 13.54 | (Cu) + (O) + π(AOYP) | |

(B) | 111 | L | −1.871 | 66.03 | 16.95 | 16.98 | 0.04 | (Ni) + π*(AOYP) |

110 | H | −5.374 | 11.59 | 44.15 | 44.16 | 0.10 | (Ni) + π(AOYP) | |

108 | H − 2 | −6.323 | 10.96 | 44.47 | 44.38 | 0.19 | (Ni) + π(AOYP) | |

106 | H − 4 | −6.846 | 89.70 | 5.12 | 5.13 | 0.05 | (Ni) + π(AOYP) | |

104 | H − 6 | −7.531 | 56.14 | 21.96 | 21.86 | 0.04 | (Ni) + π(AOYP) | |

101 | H − 9 | −7.675 | 45.68 | 26.92 | 26.50 | 0.90 | (Ni) + π(AOYP) | |

(C) | 109 | L + 1 | −1.195 | 0.97 | 14.94 | 83.55 | 0.54 | π*(AOYP) |

108 | L | −1.217 | 0.98 | 83.66 | 14.82 | 0.54 | π*(AOYP) | |

107 | H | −5.432 | 1.23 | 56.53 | 40.68 | 1.56 | π(AOYP) | |

106 | H − 1 | −5.481 | 1.43 | 40.31 | 55.73 | 2.53 | π(AOYP) |

^{a}B denotes a beta molecular orbital. ^{b}H represents HOMO and L represents LUMO. ^{c}M(II) represents the central metal ion which is Cu(II) for (A), Ni(II) for (B) and Zn(II) for (C).

addition, the LUMO of (A) and that of (B) have significant contributions from -orbitals on the central metal ions (61.35% from Cu^{2+} and 66.03% from Ni^{2+}), denoted by d(Cu) and d(Ni) respectively. The HOMO of each complex comprises of π bonding MOs located on the AOYP ligands, designated as π(AOYP). Besides the contribution from AOYP, HOMO-5 in (A) has a 62.51% contribution from p orbitals on oxygen atoms in the H_{2}O ligands denoted p(O), while HOMO-8 and HOMO-15 in the same complex each have contributions from d(Cu) and p(O). Apart from the contribution from AOYP, all HOMOs of (B) also have contributions from d(Ni), the largest of which is 89.70% for HOMO-4. The HOMOs of (C) are almost entirely dominated by the AOYP ligands.

In order to assign the already reported experimental electronic absorption bands of (A), (B) and (C) by [_{cal}) of the complexes, along with their oscillator strengths, assignments and transitions with significant coefficients of the wavefunction are listed in _{exp}) obtained from [^{*} and n → π^{*} transitions and are affected by the type of coordination.

The scaling factor 0.72 was used to correct the calculated singlet-singlet electronic excitation wavelengths

Complex | Singlet excited state | Electronic Transition | Transition coefficient (c) | Weight (%)^{a} of transition | Oscillator strength (f) | Assignment | Excitation energy (eV) | Unscaled λ_{cal} (nm) | Scaled^{b} λ_{cal} (nm) | λ_{exp} (nm) |
---|---|---|---|---|---|---|---|---|---|---|

(A) | S_{1} | H → L | 0.92422 | 85.42 | 0.0001 | LMCT/ILCT | 1.19 | 1044 | 752 | 620 |

S_{2} | H − 1 → L | 0.98385 | 96.80 | 0.0052 | LMCT/ILCT | 1.62 | 764 | 550 | ||

S_{3} | H − 15 → L | 0.58664 | 34.41 | d-d/MLCT/LMCT/LLCT/ILCT | 2.00 | 620 | 446 | |||

H − 8 → L | 0.47213 | 22.29 | 0.0005 | d-d/MLCT/LMCT/LLCT/ILCT | ||||||

H − 5 → L | −0.38320 | 14.68 | LMCT/LLCT/ILCT | |||||||

(B) | S_{1} | H → L | 0.57404 | 32.95 | 0.0000 | d-d/MLCT/LMCT/ILCT | 1.65 | 750 | 540 | 888 |

S_{2} | H − 4 → L | 0.70420 | 49.59 | 0.0000 | d-d/MLCT/LMCT/ILCT | 1.93 | 643 | 463 | 540 | |

S_{3} | H − 2 → L | 0.41462 | 17.19 | 0.0000 | d-d/MLCT/LMCT/ILCT | 2.26 | 548 | 395 | 371 | |

H − 9 → L | 0.38776 | 15.04 | d-d/MLCT/LMCT/ILCT | |||||||

H − 6 → L | 0.37403 | 13.99 | d-d/MLCT/LMCT/ILCT | |||||||

(C) | S_{1} | H → L | 0.59781 | 35.74 | 0.0422 | ILCT | 3.67 | 337 | 243 | - |

H − 1 → L+1 | 0.30839 | 9.51 | ILCT | |||||||

S_{2} | H → L+1 | 0.57763 | 33.37 | 0.3811 | ILCT | 3.75 | 331 | 238 | - | |

S_{3} | H − 1 → L | 0.58459 | 34.17 | 0.0311 | ILCT | 3.80 | 326 | 235 | - | |

H → L+1 | −0.33135 | 10.98 | ILCT |

^{a}Weight (%) of transition =^{b}The theoretical transition wavelengths were scaled with 0.72.

[^{3}A_{2g}(F) → ^{3}T_{2g}(F), ^{3}A_{2g}(F) → ^{3}T_{1g}(F) and ^{3}A_{2g}(F) → ^{3}T_{1g}(P) transitions respectively, that correspond to the three characteristic spin allowed transitions of octahedral complexes [_{2}O ligands that were initially directed along the z-axis are completely removed and solvent molecules occupy the vacant axial positions.

Each absorption band of (B) resulted from [d(Ni) + π(AOYP)] → [d(Ni) + π*(AOYP)] electronic transitions with a mixed character of d-d/MLCT/LMCT/ILCT. Experimental excitation energies were not reported for (C), given that it did not show any d-d transitions in the experimental spectrum. Theoretical electronic absorption bands at 243, 238 and 235 nm have been calculated for this complex and are found to result from ILCT transitions within the AOYP ligands. These bands occur principally due to π(AOYP) → π*(AOYP) electronic transitions. No d-d transitions were found in our calculations, confirming the experimental observation that d-d electronic transitions do not occur in Zn(II) complexes because the Zn(II) ion has a completely filled d sub-shell.

Comparing the experimental and calculated absorption spectra of the complexes, one can notice that both sets of data are in good agreement in terms of band positions, except for the first excited state (S_{1}) of (B) where a large discrepancy is observed between λ_{cal} (540 nm) and λ_{exp} (888 nm). The best agreement between experimental and theoretical absorption bands is observed for the S_{3} excited state of (B) with λ_{cal} (395 nm) and λ_{cal} (371 nm).

NBO analysis [^{2.25} on C3 (which is a mixture of 30.71% s and 69.24% p) and sp^{1.35} on C11 (which is a mixture of 42.46% s and 57.50% p). Similarly, the bonding LP(1)N15 in (C) consists of sp^{1.89} on N15 (which is a mixture of 34.62% s and 65.36% p). On the other hand, all anti-bonding NBOs are predominantly either p-type orbitals as is the case with the NBOs LP*(7) Zn20, LP*(8) Zn20 and LP*(9) Zn20 or s-type orbitals as in the NBOs LP*(6) Cu20 and LP*(6) Zn20.

The delocalization of ED between occupied Lewis-type and unoccupied non-Lewis NBOs correspond to stabilizing donor-acceptor interactions that contribute predominantly to the stabilization of the entire molecular system. The strength of these interactions can be estimated by the second order perturbation theory [^{(2)} associated with the delocalization from i → j was estimated using Equation (2). Values of this energy are proportional to the intensities of NBO interactions or to the extent of ICT within a molecular entity. The greater the electron donating tendency from donor to acceptor NBOs, the larger the E^{(2)} values and the more intense the interaction between the electron donors and the electron acceptors [

Parameters^{a} | Occupancies (e) | Energies (a.u) | Hybrid | AO(%)^{b} |
---|---|---|---|---|

(A) | ||||

σ(C3-C11) | 0.98572 | −0.71831 | sp^{2.25} (C3) | s(30.71) p(69.24) |

sp^{1.35} (C11) | s(42.46) p(57.50) | |||

σ(C11-N15) | 0.99368 | −0.91230 | sp^{1.98} (C11) | s(33.50) p(66.38) |

sp^{1.45} (N15) | s(40.87) p(59.06) | |||

σ(C23-C32) | 0.98568 | −0.71633 | sp^{2.26} (C23) | s(30.66) p(69.29) |

sp^{1.36} (C32) | s(42.37) p(57.59) | |||

σ(C32-N36) | 0.99363 | −0.91057 | sp^{1.99} (C32) | s(33.44) p(66.44) |

sp^{1.46} (N36) | s(40.69) p(59.24) | |||

LP(1) N15 | 0.85703 | −0.44384 | sp^{1.93} | s(34.18) p(65.81) |

LP(1) N36 | 0.85242 | −0.44142 | sp^{1.93} | s(34.13) p(65.86) |

LP(2) O19 | 0.87018 | −0.32116 | sp^{14.53} | s(06.44) p(93.51) |

LP(3) O19 | 0.81730 | −0.34860 | sp^{8.52} | s(10.49) p(89.47) |

LP(3) O21 | 0.82296 | −0.37646 | sp^{6.40} | s(13.52) p(86.45) |

LP*(6) Cu20 | 0.14855 | 0.13237 | sd^{0.01} | s(98.70) d(01.26) |

(B) | ||||

σ(C3-C11) | 1.97116 | −0.71880 | sp^{2.25} (C3) | s(30.76) p(69.19) |

sp^{1.34} (C11) | s(42.71) p(57.26) | |||

σ(C11-N15) | 1.98651 | −0.90473 | sp^{2.01} (C11) | s(33.16) p(66.72) |

sp^{1.54} (N15) | s(39.30) p(60.64) | |||

σ(N14-N15) | 1.97288 | −0.82783 | sp^{3.25} (N14) | s(23.43) p(76.22) |

sp^{2.48} (N15) | s(28.70) p(71.24) | |||

σ(C23-C32) | 1.97116 | −0.71878 | sp^{2.25} (C23) | s(30.76) p(69.19) |

sp^{1.34} (C32) | s(42.71) p(57.26) | |||

σ(C32-N36) | 1.98651 | −0.90477 | ^{2.01} (C32) | s(33.16) p(66.71) |

^{1.54} (N36) | s(39.30) p(60.64) | |||

σ(N35-N36) | 1.97287 | −0.82774 | sp^{3.25} (N35) | s(23.43) p(76.23) |

sp^{2.48} (N36) | s(28.70) p(71.24) | |||

(C) | ||||

σ(C3-C11) | 1.97203 | −0.70807 | sp^{2.26} (C3) | s(30.67) p(69.28) |

sp^{1.35} (C11) | s(42.57) p(57.39) | |||

σ(C2-C3) | 1.96964 | −0.67111 | sp^{1.81} (C2) | s(35.56) p(64.39) |

sp^{1.93} (C3) | s(34.10) p(65.87) | |||

σ(N14-N15) | 1.96953 | −0.79214 | sp^{3.41} (N14) | s(22.60) p(77.03) |

sp^{3.14} (N15) | s(24.14) p(75.78) | |||

LP(1) N15 | 1.85272 | −0.43562 | sp^{1.89} | s(34.62) p(65.36) |

LP(1) N36 | 1.85222 | −0.43133 | sp^{1.89} | s(34.58) p(65.41) |

LP(2) O19 | 1.86372 | −0.50083 | sp^{2.07} | s(32.59) p(67.40) |

LP*(6) Zn20 | 0.28506 | 0.04337 | s | s(99.84) |

LP*(7) Zn20 | 0.13581 | 0.23576 | p | p(99.96) |

LP*(8) Zn20 | 0.12722 | 0.16987 | p | p(99.96) |

LP*(9) Zn20 | 0.11758 | 0.18917 | p | p(99.97) |

^{a}LP(n)A generally represents a valence lone pair orbital (n) on atom A. For atomic numbering, refer to (A1), (B1) and (C1) in Figures 1-3 respectively. ^{b}Percentage contribution of atomic orbitals in NBO hybrid, given in their respective brackets.

In Equation (2), q is the donor orbital occupancy, ε_{i} and ε_{j} are diagonal elements (orbital energies) of donor and acceptor NBOs respectively and

(A) | (B) | (C) | |||
---|---|---|---|---|---|

Donor (i) → Acceptor (j) | E^{(2)} (kcal/mol) | Donor (i) → Acceptor (j) | E^{(2)} (kcal/mol) | Donor (i) → Acceptor (j) | E^{(2)} (kcal/mol) |

σ*(O12-C13) → σ*(N14-N15) | 12.18 | σ(O19-Ni20) → σ*(Ni20-O21) | 10.74 | π(C4-C5) → π*(C1-C6) | 21.43 |

π*(C11-N15) → π*(C3-C4) | 30.55 | σ(O19-Ni20) → σ*(Ni20-N36) | 150.10 | π(C4-C5) → LP (1) C3 | 37.28 |

π(C3-C4) → π*(C11-N15) | 17.89 | σ(O19-Ni20) → σ*(N15-Ni20) | 133.48 | π(C26-C27) → π*(C24-C25) | 15.19 |

π(C3-C4) → LP*(1)C2 | 27.71 | σ(Ni20-O21) → σ*(O19-Ni20) | 10.74 | π(C26-C27) → LP*(1)C22 | 48.95 |

π(C32-N36) → π*(C34-N35) | 52.21 | σ(Ni20-O21) → σ*(Ni20-N36) | 131.19 | π(C24-C25) → *(C26-C27) | 21.52 |

π(C32-N36) → π*(C23-C24) | 29.60 | σ(Ni20-O21) → σ*(N15-Ni20) | 147.69 | π(C24-C25) → LP(1)C23 | 37.96 |

π(C25-C26) → π*(C23-C24) | 13.72 | π(C4-C5) → LP (1) C3 | 38.48 | π(C1-C6) → LP*(1)C2 | 48.35 |

π(C23-C24) → π*(C32-N36) | 17.25 | π(C3-C4) → π*(C1-C6) | 21.22 | LP*(8) Zn20 → LP*(9) Zn20 | 19.05 |

π(C23-C24) → LP*(1)C22 | 27.64 | π(C32-N36) → LP(1)C23 | 10.25 | LP*(1) C22 → π*(C26-C27) | 50.08 |

π(C1-C6) → LP*(1)C2 | 25.61 | π(C26-C27) → π*(C24-C25) | 15.24 | LP*(1) C2 → π*(C1-C6) | 49.08 |

LP*(1) C22 → π*(C23-C24) | 30.79 | π(C26-C27) → LP*(1)C22 | 50.02 | LP(3) O21 → LP*(1) C22 | 113.46 |

LP*(1) C2 → π*(C3-C4) | 30.92 | π(C24-C25) → π*(C26-C27) | 21.22 | LP(3) O19 → LP*(1) C2 | 117.47 |

LP*(1) C2 → π*(C1-C6) | 24.48 | π(C24-C25) → LP(1)C23 | 38.48 | LP(2) O43 → LP*(9) Zn20 | 20.87 |

LP(3) O21 → LP*(7) Cu20 | 12.16 | π(C1-C6) → π*(C4-C5) | 15.24 | LP(2) O43 → LP*(6) Zn20 | 15.00 |

LP(3) O21 → LP*(6) Cu20 | 19.17 | π(C1-C6) → LP*(1)C2 | 50.01 | LP(2) O40 → LP*(9) Zn20 | 18.02 |

LP(3) O21 → LP*(5) Cu20 | 20.35 | π(C13-N14) → π*(C11-N15) | 10.02 | LP(2) O33 → π*(C34-N35) | 35.83 |

LP(3) O19 → LP*(7) Cu20 | 11.14 | π(C11-N15) → LP (1) C3 | 10.25 | LP(2) O33 → π*(C32-N36) | 34.31 |

LP(3) O19 → LP*(6) Cu20 | 14.68 | LP*(1) C2 → π*(C1-C6) | 48.39 | LP(2) O21 → LP*(8) Zn20 | 17.38 |

LP(3) O19 → LP*(5) Cu20 | 20.87 | LP*(1) C22 → π*(C26-C27) | 48.38 | LP(2) O21 → LP*(7) Zn20 | 17.72 |

LP(3) O19 → LP*(1) C2 | 10.18 | LP(2) O19 → LP*(1) C2 | 105.66 | LP(2) O21 → LP*(6) Zn20 | 25.55 |

LP(2) O33 → π*(C34-N35) | 17.60 | LP(2) O12 → π*(C13-N14) | 36.07 | LP(2) O12 → π*(C13-N14) | 37.01 |

LP(2) O33 → π*(C32-N36) | 17.80 | LP(2) O12 → π*(C11-N15) | 35.34 | LP(2) O12 → π*(C11-N15) | 33.79 |

LP(2) O21 → LP*(1) C22 | 39.73 | LP(2) O33 → π*(C34-N35) | 36.09 | LP(1) N36 → LP*(7) Zn20 | 18.69 |

LP(2) O19 → LP*(1) C2 | 27.70 | LP(2) O33 → π*(C32-N36) | 35.35 | LP(1) N36 → LP*(6) Zn20 | 29.31 |

LP(2) O12 → π*(C13-N14) | 18.17 | LP(1) N16 → π*(C13-N14) | 48.29 | LP(1) N16 → π*(C13-N14) | 47.36 |

LP(2) O12 → π*(C11-N15) | 17.47 | LP(1) N14 → σ*(O12-C13) | 10.59 | LP(1) N15 → LP*(7) Zn20 | 21.10 |

LP(1) O19 → LP*(7) Cu20 | 10.00 | LP(1) C3 → π*(C4-C5) | 64.04 | LP(1) N15 → LP*(6) Zn20 | 27.96 |

LP(1) N37 → π*(C34-N35) | 22.71 | LP(1) C3 → π*(C11-N15) | 138.81 | LP(1) C3 → π*(C4-C5) | 66.03 |

LP(1) N36 → LP*(8) Cu20 | 14.99 | LP(1) C3 → LP*(1) C2 | 1913.34 | LP(1) C3 → π*(C11-N15) | 120.93 |

LP(1) N36 → LP*(6) Cu20 | 25.14 | LP(1) N37 → π*(C34-N35) | 48.27 | LP(1) C3 → LP*(1) C2 | 1563.95 |

LP(1) N36 → LP*(5) Cu20 | 10.74 | LP(1) N35 → σ*(O33-C34) | 10.58 | LP(1) C23 → π*(C32-N36) | 115.47 |

LP(1) N16 → π*(C13-N14) | 24.43 | LP(1) C23 → π*(C32-N36) | 138.73 | LP (2) O19 → LP*(8) Zn20 | 15.93 |

LP(1) N15 → LP*(8) Cu20 | 13.57 | LP(1) C23 → π*(C24-C25) | 64.04 | LP (2) O19 → LP*(7) Zn20 | 16.46 |

LP(1) N15 → LP*(6) Cu20 | 24.14 | LP(1) C23 → LP*(1) C22 | 1912.27 | LP (2) O19 → LP*(6) Zn20 | 24.39 |

LP(1) C5 → π*(C1-C6) | 32.92 | LP (2) O21 → LP*(1) C22 | 104.60 | LP (1) N37 → π*(C34-N35) | 43.46 |

LP(1) C5 → π*(C3-C4) | 49.50 | LP (1) N35 → σ*(O43-H45) | 14.28 | LP (1) C23 → π*(C24-C25) | 66.20 |

LP(1) C27 → π*(C25-C26) | 41.35 | LP (1) N14 → σ*(O40-H41) | 14.36 | LP (1) C23→ LP*(1) C22 | 1521.47 |

LP*(1)C22 → π*(C23-C24) and π*(C11-N15) → π*(C3-C4), producing a total of 337.57 kcal/mol stabilization energy. The most intensive interactions in (B) are: σ(O19-Ni20) → σ*(Ni20-N36), σ*(N15-Ni20); σ(Ni20-O21) → σ*(Ni20-N36), σ*(N15-Ni20); LP(1)C3 → LP*(1)C2, π*(C11-N15); LP(1)C23 → LP*(1)C22; LP(2)O19 → LP*(1)C2 and LP(2)O21→ LP*(1)C22, leading to total stabilization energy of 4737.14 kcal/mol. The strongest interactions in (C) are: LP(1)C3 → π*(C11-N15), LP*(1)C2; LP(1)C23 → π*(C32-N36), LP*(1)C22; LP(3)O19 → LP*(1)C2 and LP(3)O21 → LP*(1)C22, resulting in total stabilization energy of 3552.75 kcal/mol. The intense NBO interactions in (A) result in total stabilization energy that is far smaller than each of those in (B) and (C), suggesting less electron delocalization and consequently minimal ICT in (A). The results of NBO analysis reflect a generally charge transfer from lone pair orbitals located on the donor atoms O19, O21, N15, N36 on AOYP to the central metal ions, the benzene and oxadiazole rings. NBO analysis provides the most accurate possible “natural Lewis structure”. In each of the complexes, donor-acceptor interactions result in a loss of occupancy from “filled” localized NBOs of this “natural Lewis structure” into “empty” Non-Lewis orbitals. These interactions are observed as an increase in ED in σ and π anti bonding orbitals C-C, C-O, C-N and Ni-N, which weakens the respective bonds and results in ICT stabilization of the complexes.

To investigate reactive sites for electrophilic and nucleophilic attack, the MEP surfaces for (A)-(C) were plotted by DFT calculations over optimized geometries at the B3LYP/GEN level of theory in water as solvent. Water was chosen instead of DMSO for these calculations since most biological reactions occur in water. A MEP surface is an electron density isosurface mapped with an electrostatic potential surface. The MEP surfaces for the complexes (shown in

The ground state geometries, spectral (IR and UV-Vis) properties, natural bond orbital (NBO) analysis,

electrostatic potentials and frontier molecular orbital analysis of three complexes of 2-(5-amino-[1,3,4]-oxadi- azol-2-yl)phenol with Cu(II), Ni(II) and Zn(II) were investigated by DFT and TD-DFT methods. The complexes were optimized in the gas phase and solution phase at two levels of theory (B3LYP/LANL2DZ and B3LYP/ GEN) for comparison, after which B3LYP/GEN was chosen for all remaining calculations. The calculated IR wavenumbers were compared with the experimental values and a good agreement was found. The results of NBO analysis reflect charge transfer from lone pair orbitals located on the donor atoms O19, O21, N15 and N36 on 2-(5-amino-[1,3,4]-oxadiazol-2-yl)phenol to the central metal ions, as well as to the benzene and oxadiazole rings. The electrostatic potential surface of each complex showed that the highest electron density is on the phenolic oxygen atoms, indicating that they are the most suitable atomic sites for attack by electrophiles. The maximum positive region was found to be localized on the N-H bonds of the NH_{2} groups, showing that they are possible sites for attack by nucleophiles. The theoretical electronic spectrum of (A) showed bands at 752 and 550 nm mainly attributable to ligand-to-metal charge transfer (LMCT) transitions and a band at 446 nm assigned to a d-d transition. The electronic spectrum of (B) consisted of three bands at 540, 463 and 395 nm mainly due to d-d transitions. Electronic absorption bands at 243, 238 and 235 nm were calculated for (C) and found to arise from intra-ligand charge transfer (ILCT) transitions within the 2-(5-amino-[1,3,4]-oxadiazol-2-yl)phenol ligand. A comparison of experimental and calculated absorption spectra of the complexes showed that both sets of data are in good agreement in terms of band positions.

The authors are thankful to the IIT Kanpur, India for the resources made available through a CV Raman International Fellowship award (Grant No. 101F102), offered to Julius Numbonui Ghogomu by the Ministry of External Affairs of India and FICCI (Federation of Indian Chambers of Commerce and Industry). They also wish to acknowledge the efforts of Dr. Matthew J. McGrath for proof reading this manuscript.

Nyiang KennetNkungli,Julius NumbonuiGhogomu,Ludovid NgouoNogheu,Shridhar RamachandraGadre, (2015) DFT and TD-DFT Study of Bis[2-(5-Amino-[1,3,4]-Oxadiazol-2-yl) Phenol](Diaqua)M(II) Complexes [M = Cu, Ni and Zn]: Electronic Structures, Properties and Analyses. Computational Chemistry,03,29-44. doi: 10.4236/cc.2015.33005