^{1}

^{1}

^{*}

^{1}

^{1}

The molecular structure, the Natural Bond orbital (NBO) and the Time Dependent-DFT of both isomers
*cis* or
*γ*-Cl and
*trans* or
*δ*-Cl of RuCl
*2*(L)
_{2}, where L stands respectively for 2-phenylazopyridine (Azpy), 2,4-dimethyl-6-[phenylazo]pyridine (Dazpy), 2-[(3,5-dimethylphenyl)azopyridine] (Mazpy) and 2-pyridylazonaphtol (Nazpy) were calculated with DFT method at B3LYP/LANL2DZ level. The prediction of the frontier orbitals (Highest Occupied Molecular Orbital or HOMO and Lowest Unoccupied Molecular Orbital or LUMO) shows that the most active complexes suitable for electronic reactions are admitted to be the trans isomers. Moreover,
*δ*-RuCl
*2* (Azpy)
*2* is discovered to react more actively as photo-sensitizer since its energy gap is the minimum. Besides, electronic structures of all complexes through NBO calculation indicate that Ru-N bonds are made of delocalization of occupancies from lone pair orbital of N atoms to the ruthenium. Moreover, Ru was assumed to have almost the same charge regardless the structure of the azopyridine ligands in the complex indicating that the ligands provide only a steric effect that is responsible for the ruthenium’s selectivity. Concerning the transition state, NBO analysis also highlights that the transition LP(Ru) π*(N1-N2) does correspond to t
_{2g }π*(L). This transition is assumed to correspond to Metal to Ligand Charge Transfer (MLCT) that is responsible for the photo-sensitiveness of the metallic complex. Besides, TDDFT calculation of complexes showed that
*δ*-RuCl
_{2}(Nazpy)
_{2} displays the largest band during the absorption. For that reason, it is admitted to be the best photosensitizer due to a large system of conjugation provided by Nazpy ligand.

Since azopyridine complexes of ruthenium have been of real interest for their capability not merely to limit the metal degree of oxidation to II or III rendering it more selective but also to throw off cancer disease [_{3}・3H_{2}O with any azopyridine ligand, only both isomers cis and trans are obtained as displayed by

We assumed therefore that by this method of synthesis, the most producible complex is up today γ-Cl. Moreover, literature explains that azopyridine complexes of ruthenium can be used as sensitizer better than bipyridine complexes of ruthenium [_{2}(Azpy)_{2}, RuCl_{2}(Dazpy)_{2}, RuCl_{2}(Mazpy)_{2} and RuCl_{2}(Nazpy)_{2} whose_{ }ligands are displayed by

The regarding isomers are all admitted to be C_{2}-symmetrical except δ-RuCl_{2}(Nazpy)_{2} that is C_{i}-symmetrical. Anyhow, they all present a C_{2} axis that makes both azopyridine ligands identical within each complex [

All geometry optimizations were performed with DFT method using Becke’s hybrid three parameters exchange functional of non local correlation functional of Lee Yang and Parr (B3LYP). They were carried out using an effective core Lanl2dz basis set owing to the relativist effect of core electrons from ruthenium atom [_{2} [

where q_{i} stands for the orbital occupancy, ε_{i} and ε_{j} are diagonal elements (orbital energies) and F_{ij} is the off-diagonal NBO Fock matrix element. The Natural Localized Molecular Orbital NLMO that displays the delocalization from a Lewis orbital to a non Lewis orbital was also predicted at B3LYP/Lanl2dz level. It thus provides additional evidence of the intermolecular delocalization effects [_{i} is expressed as a linear combination of the parent Lewis-type NBO σ_{i} (with coefficient

Clearly, NLMO completes the NBO summary and the perturbation theory energy. Regarding electronic prediction, it was carried out using TDDFT method with combined basis set. Thus Ru was lonely calculated with ECP Lanl2dz while the remaining atoms were performed with polarized split valence 6-31G(d) basis set.

The frontier molecular orbitals comprising the highest occupied molecular orbital (HOMO), the lowest unoccupied molecular orbital (LUMO) with the energy gap between HOMO and LUMO (ΔE) of the complexes are calculated and displayed in

where I_{p} = −E_{HOMO} is ionization potential (Kcal・mol^{−1}), E_{A} = −E_{LUMO} is electronic affinity (Kcal・mol^{−1}).

The hardness emphasizes the reactivity of the complex. Therefore, the larger the gap between HOMO and LUMO is, the harder the molecule is and the worse its reactivity is. In contrary, the chemical softness that is defined as the inverse of hardness highlights the capacity of an atom or group of atoms to receive electrons. Therefore, the soft molecule needs small energy to liberate an electron from HOMO since the gap ΔE is narrow. Besides, the chemical potential was calculated to account for the capability for

RuCl_{2}(Azpy)_{2} | RuCl_{2}(Dazpy)_{2} | RuCl_{2}(Mazpy)_{2} | RuCl_{2}(Nazpy)_{2} | RuCl_{3}・3H_{2}O | |||||
---|---|---|---|---|---|---|---|---|---|

γ-Cl | δ-Cl | γ-Cl | δ-Cl | γ-Cl | δ-Cl | γ-Cl | δ-Cl | ||

HOMO | −123.6 | −121.0 | −121.6 | −118.6 | −126.6 | −122.3 | −128.0 | −130.5 | −153.2 |

LUMO | −77.2 | −79.5 | −70.9 | −74.7 | −75.6 | −78.4 | −82.2 | −86.0 | −95.7 |

ΔE^{a} | 46.4 | 41.5 | 50.7 | 43.9 | 51.0 | 43.9 | 45.8 | 44.5 | 57.5 |

η | −23.2 | −20.75 | −25.3 | −21.9 | −25.5 | −21.9 | −22.9 | −22.2 | −28.7 |

μ | −100.4 | −100.25 | −96.2 | −96.6 | −101.1 | −100.3 | −105.1 | −108.2 | −124.4 |

^{a}DE = LUMO-HOMO.

electron to leave from the molecule through Equation (3) as:

In _{2}(Azpy)_{2} is the most reactive complex with 41.5 Kcal・mol^{−1}. In consequence, it is assumed to be the soft molecule though it displays the low value of hardness [_{3}・3H_{2}O presents the highest energy gap with 57.5 Kcal・mol^{−1}. Therefore, it is admitted to be the hardest molecule.

The natural bond orbital was performed on complexes using the pseudo-potential LANL2DZ basis set whose particularity is to freeze the core electrons [_{3}・3H_{2}O and also on ruthenium atom.

According to NBO analysis, _{2}(L)_{2} complexes display almost a constant value of 43.44 electrons while RuCl_{3}・3H_{2}O displays 43.1 electrons. Consequently, we can admit that these results tend to highlight the electron donor’s strength of azopyridine ligands to Ru. In reality, the nominal

γ-RuCl_{2}(Azpy)_{2} | [core] 5s(0.28) 4d(7.12) 5p(0.01) 5d(0.03) 6p(0.02) |
---|---|

δ-RuCl_{2}(Azpy)_{2} | [core] 5s(0.27) 4d(7.14) 5p(0.01) 5d(0.03) 6p(0.02) |

γ-RuCl_{2}(Dazpy)_{2} | [core] 5s(0.28) 4d(7.12) 5p(0.01) 5d(0.03) 6p(0.02) |

δ-RuCl_{2}(Dazpy)_{2} | [core] 5s(0.27) 4d(7.12) 5p(0.1) 5d(0.03) 6p(0.02) |

γ-RuCl_{2}(Mazpy)_{2} | [core] 5s(0.27) 4d(7.10) 5p(0.01) 5d(0.03) 6p(0.02) |

δ-RuCl_{2}(Mazpy)_{2} | [core] 5s(0.27) 4d(7.13) 5p(0.01) 5d(0.03) 6p(0.02) |

γ-RuCl_{2}(Nazpy)_{2} | [core] 5s(0.27) 4d(7.14) 5p(0.1) 5d(0.03) 6p(0.02) |

δ-RuCl_{2}(Nazpy)_{2} | [core] 5s(0.27) 4d(7.16) 5p(0.01) 5d(0.03) 6p(0.02) |

Ru atom | [core] 5s(0.49) 4d(7.51) |

RuCl_{3}・3H_{2}O | [core] 5s(0.28) 4d(6.77) 5p(0.02) 5d(0.03) 6p(0.01) |

Core | Valence | Rydberg | Total | |
---|---|---|---|---|

γ-RuCl_{2}(Azpy)_{2} | 35.98 | 7.40 | 0.06 | 43.44 |

δ-RuCl_{2}(Azpy)_{2} | 35.98 | 7.40 | 0.06 | 43.44 |

γ-RuCl_{2}(Dazpy)_{2} | 35.98 | 7.40 | 0.06 | 43.44 |

δ-RuCl_{2}(Dazpy)_{2} | 35.98 | 7.39 | 0.06 | 43.43 |

γ-RuCl_{2}(Mazpy)_{2} | 35.98 | 7.37 | 0.06 | 43.41 |

δ-RuCl_{2}(Mazpy)_{2} | 35.98 | 7.40 | 0.06 | 43.44 |

γ-RuCl_{2}(Nazpy)_{2} | 35.98 | 7.41 | 0.06 | 43.45 |

δ-RuCl_{2}(Nazpy)_{2} | 35.98 | 7.42 | 0.06 | 43.46 |

Ru atom | 36.00 | 8.00 | 0.00 | 44.00 |

RuCl_{3}・3H_{2}O | 36.00 | 7.04 | 0.06 | 43.10 |

charge of Ru in RuCl_{2}(L)_{2} is +2. So, the natural atomic charge that corresponds to the difference between the nuclear charge of Ru (44) and its total electron population in _{3}・3H_{2}O, the nominal charge of Ru is +3 due to the presence of three chloride atoms while the natural charge is +0.9. This discrepancy of charge is attributed to the presence of water molecules that are known to be strong electrons donors.

_{2}(Azpy)_{2}, δ-RuCl_{2}(Dazpy)_{2} and δ-RuCl_{2}(Mazpy)_{2} are C_{2}-symmetrical with different chloride atomes, δ-RuCl_{2 }(Nazpy)_{2} presents a C_{i} symmetry indicating an inversion center [_{1} and N_{py}. Therefore, since the azopyridine ligands are known to be bidentate structures [_{1} should be delocalized either on N_{py} or on N_{2} that leads to the formation of Ru-N_{py} and Ru-N_{2} bonds. This process will give then rise to a five-ring stable shape of complex [_{1}, N_{2} and N_{py} atoms through NLMO delocalization of N_{1} electrons within the four azopyridine ligands.

Through _{1}) in the same natural atomic orbitals NAOs. Principally, LP(N_{1}) delocalizes greatly onto the C_{1}-N_{py} antibond. This fact confirms the identical electronic behavior of the azopyridine ligands.

Atoms | RuCl_{2}(Azpy)_{2} | RuCl_{2}(Dazpy)_{2} | RuCl_{2}(Mazpy)_{2} | RuCl_{2}(Nazpy)_{2} | ||||
---|---|---|---|---|---|---|---|---|

γ-Cl | δ-Cl | γ-Cl | δ-Cl | γ-Cl | δ-Cl | γ-Cl | δ-Cl | |

Ru | 0.55 | 0.55 | 0.56 | 0.57 | 0.59 | 0.55 | 0.55 | 0.53 |

N_{1} | −0.25 | −0.22 | −0.24 | −0.22 | −0.23 | −0.20 | −0.22 | −0.20 |

N_{2} | −0.16 | −0.16 | −0.15 | −0.18 | −0.17 | −0.16 | −0.19 | −0.18 |

N_{py} | −0.48 | −0.46 | −0.52 | −0.50 | −0.47 | −0.46 | −0.47 | −0.47 |

Cl_{1} | −0.52 | −0.54 | −0.53 | −0.54 | −0.51 | −0.53 | −0.51 | −0.51 |

Cl_{2} | −0.52 | −0.52 | −0.53 | −0.53 | −0.51 | −0.53 | −0.51 | −0.51 |

Atoms | Azpy | Dazpy | Mazpy | Nazpy |
---|---|---|---|---|

N_{py} | −0.48 | −0.51 | −0.49 | −0.49 |

N_{1} | −0.26 | −0.26 | −0.26 | −0.28 |

N_{2} | −0.18 | −0.18 | −0.19 | −0.17 |

Azpy | σ = 97.79%σ(N_{1}) + 0.80%sp^{5}^{.}^{26}(C_{1}) + 0.21%sp^{1}^{.}^{32}(N_{2}) + 0.35%sp^{0.97}(N_{py}) |
---|---|

Dazpy | σ = 97.74%σ(N_{1}) + 0.86%sp^{5}^{.}^{62}(C_{1}) + 0.19%sp^{1}^{.}^{56}(N_{2}) + 0.38%sp^{1.13}(N_{py}) |

Mazpy | σ = 97.19%σ(N_{1}) + 0.98%sp^{7}^{.}^{22}(C_{1}) + 0.25%sp^{1}^{.}^{67}(N_{2}) + 0.45%sp^{1.91}(N_{py}) |

Nazpy | σ = 96.97%σ(N_{1}) + 1.064%sp^{8}^{.}^{02}(C_{1}) + 0.23%sp^{2}^{.}^{08}(N_{2}) + 0.49%sp^{2.08}(N_{py}) |

All azopyridine complexes studied herein are admitted to show a C_{2} axis [_{2}(L)_{2} comprising electron delocalization. L stands for 2-phenylazopyridine (Azpy), 2-phenylazo-4,6-dime- thylpyridine (Dazpy), 2,6-dimethylphenylazo-2-pyridine (Mazpy) and 2-pyridylazo- naphtol (Nazpy). In these aforementioned Tables, only the AOs that occupancies are far lower than the ideal occupancy (2.0e) and the MOs involved in electronic transition are presented. It is assumed that the low occupancies of an AO disclose the orbital to be involved in MO formation [

With azopyridine ligands and according to the reactive RuCl_{3}・3H_{2}O, ruthenium is involved in an octahedral structure with six single bonds. Regarding RuCl_{2}(L)_{2}, Cl atoms that are responsible for the nominal charge +II of Ru are involved in an ionic

Donor Lewis-type^{a} NBO (Ru-N) | Occupancy | Hybrid^{b} | AO(c)^{c} | AO(%)^{d} |
---|---|---|---|---|

γ-RuCl_{2}(Azpy)_{2} | ||||

LP(N_{1}) | 1.93 | sp^{1.5} | 0.63(2s) + 0.62(2p_{y}) − 0.46(2p_{z}) | s(40.03%)p(59.97%) |

LP(N_{2}) | 1.66 | sp^{1.68} | 0.61(2s) + 0.63(2p_{y}) + 0.48(2p_{z}) | s(37.34%)p(62.66%) |

LP(N_{py}) | 1.68 | sp^{2.75} | 0.52(2s) + 0.72(2p_{y}) − 0.46(2p_{z}) | s(26.69%)p(73.31%) |

LP(Ru) | 1.59 | d^{99.99} | 0.37(4dz²) − 0.92(4d_{xy}) − 0.10(4d_{x² − y²}) | s(0.03%)d(99.97%) |

LP*(Ru) | 0.84 | pd^{99.99} | 0.99(4d_{yz}) + 0.15(4d_{xz}) | p(0.02%)d(99.98%) |

δ-RuCl_{2}(Azpy)_{2} | ||||

LP(N_{py}) | 1.68 | sp^{2.64} | 0.52(2s) − 0.77(2p_{x}) − 0.25(2p_{y}) + 0.25(2p_{z}) | s(27.48%)p(72.52%) |

LP(N_{2}) | 1.68 | sp^{1.65} | 0.61(2s) + 0.50(2p_{x}) − 0.61(2p_{y}) | s(37.78%)p(62.22%) |

LP(N_{1}) | 1.94 | sp^{1.52} | 0.63(3s) + 0.50(2p_{x}) − 0.59(2p_{y}) | s(39.66%)p(60.34%) |

LP(Ru) | 1.58 | d^{100} | 0.88(4d_{xz}) − 0.47(4d_{yz}) | p(0.01%)d(99.99%) |

LP*(Ru) | 0.80 | d^{100} | 0.18(4d_{z²}) − 0.88(4d_{xy}) − 0.43(4d_{x² − y²}) | s(0.39%)d(99.61%) |

γ-RuCl_{2}(Dazpy)_{2} | ||||

LP(N_{py}) | 1.69 | sp^{2.73} | 0.52(2s) + 0.70(2p_{y}) − 0.21(2p_{y}) − 0.44(2p_{z}) | s(26.82%)p(73.18%) |

LP(N_{2}) | 1.65 | sp^{1.69} | 0.60(2s) + 0.60(2px) + 0.18(2p_{y}) + 0.49(2p_{z}) | s(37.17%)p(62.83%) |

LP(N_{1}) | 1.93 | sp^{1.52} | 0.63(2s) + 0.59(2px) + 0.20(2p_{y}) − 0.47(2p_{z}) | s(39.75%)p(60.25%) |

LP(Ru) | 1.61 | d^{100} | 0.83(4d_{xy}) − 0.23(4d_{x² − y²}) + 0.5(4d_{z²}) | s(0.03%)d(99.97%) |

LP*(Ru) | 0.92 | d^{100} | 0.90(4d_{x² − y²}) − 0.42(4d_{z²}) | s(0.31%)d(99.69%) |

δ-RuCl_{2}(Dazpy)_{2} | ||||

LP(N_{py}) | 1.68 | sp^{2.67} | 0.52(2s) − 0.84(2p_{x}) − 0.11(2p_{y}) | s(27.28%)p(72.72%) |

LP(N_{2}) | 1.68 | sp^{1.62} | 0.62(2s) + 0.22(2p_{x}) + 0.69(2p_{y}) + 0.30(2p_{z}) | s(38.14%)p(61.86%) |

LP(N_{1}) | 1.93 | sp^{1,53} | 0.63(2s) + 0.22(2p_{x}) + 0.70(2p_{y}) − 0.27(2p_{z}) | s(39.47%)p(60.53%) |

LP(Ru) | 1.56 | d^{100} | 0.89(4d_{yz}) − 0.46(4d_{xz}) | p(0.02%)d(99.98%) |

LP*(Ru) | 0.79 | d^{100} | 0.23(4d_{z²}) + 0.26(4d_{xy}) − 0.94(4d_{x² − y²}) | s(0.26%)p(0.01%)d(99.73%) |

γ-RuCl_{2}(Mazpy)_{2} | ||||

LP(N_{py}) | 1.67 | sp^{2.77} | 0.51(2s) + 0.72(2p_{y}) − 0.46(2p_{z}) | s(26.54%)p(73.46%) |

LP(N_{2}) | 1.67 | sp^{1.56} | 0.63(2s) − 0.11(2px) + 0.63(2p_{y}) + 0.44(2p_{z}) | s(39.12%)p(60.88%) |

LP(N_{1}) | 1.93 | sp^{1.48} | 0.63(2s) + 0.63(2p_{y}) − 0.44(2p_{z}) | s(40.36%)p(59.64%) |

LP(Ru) | 1.64 | d^{100} | − 0.95(4dxy) − 0.20(4d_{x² − y²}) + 0.22(4d_{z²}) | d(100%) |

LP*(Ru) | 0.84 | d^{99.99} | 0.99(4d_{yz}) + 0.13(4d_{xz}) | p(0.01%)p(99.99%) |

δ-RuCl_{2}(Mazpy)_{2} | ||||

LP(N_{py}) | 1.67 | sp^{2.73} | 0.52(2s) + 0.25(2p_{x}) − 0.81(2p_{y}) | s(26.81%)p(73.19%) |

LP(N_{2}) | 1.68 | sp^{1.55} | 0.63(2s) − 0.77(2p_{y}) | s(39.21%)p(60.79%) |

LP(N_{1}) | 1.93 | sp^{1.53} | 0.63(2s) − 0.77(2p_{x}) | s(40.06%)p(59.94%) |

LP(Ru) | 1.60 | d^{100} | 0.92(4d_{xz}) − 0.40(4d_{yz}) | d(100%) |

LP*(Ru) | 0.82 | d^{99.99} | 0.17(4d_{xy}) + 0.98(4d_{x² − y²}) | s(0.27)d(99.73%) |

γ-RuCl_{2}(Nazpy)_{2} | ||||

LP(N_{py}) | 1.66 | sp^{2.78} | 0.51(2s) + 0.68(2p_{x}) − 0.21(2p_{y}) − 0.47(2p_{z}) | s(26.44%)p(73.56%) |

LP(N_{2}) | 1.68 | sp^{1.71} | 0.61(2s) + 0.64(2px) + 0.47(2p_{z}) | s(36.85%)p(63.15%) |

LP(N_{1}) | 1.93 | sp^{1.47} | 0.64(2s) + 0.63(2px) − 0.44(2p_{z}) | s(40.42%)p(59.58%) |
---|---|---|---|---|

LP(Ru) | 1.62 | d^{100} | 0.71(4d_{xy}) + 0.62(4d_{x² − y²}) + 0.33(4d_{z²}) | d(100%) |

LP*(Ru) | 0.90 | d^{99.99} | 0.70(4d_{xy}) − 0.57(4d_{x² − y²}) − 0.42(4d_{z²}) | s(0.31%)p(99.69%) |

δ-RuCl_{2}(Nazpy)_{2} | ||||

LP(N_{py}) | 1.68 | sp^{2.72} | 0.52(2s) − 0.21(2p_{x}) + 0.82(2p_{z}) | s(26.88%)p(73.12%) |

LP(N_{2}) | 1.68 | sp^{1.67} | 0.61(2s) + 0.11(2p_{x}) − 0.71(2p_{y}) − 0.33(2p_{z}) | s(37.41%)p(62.59%) |

LP(N_{1}) | 1.94 | sp^{1.51} | 0.63(2s) − 0.15(2p_{x}) + 0.68(2p_{y}) + 0.33(2p_{z}) | s(39.90%)p(60.10%) |

LP(Ru) | 1.61 | d^{100} | 0.46(4d_{xy}) + 0.53(4d_{xz}) + 0.20(4d_{yz}) − 0.63(4d_{x² − y²}) + 0.26(4d_{z²}) | d(100%) |

LP*(Ru) | 0.81 | d^{99.99} | 0.22(4dyz) − 0.26(4d_{xy}) + 0.52(4d_{yz}) − 0.16(4d_{x² − y²}) − 0.76(4d_{z²}) | s(0.34%)d(99.66%) |

^{a}LP represents Lone Pair Orbital; ^{b}Hybrid concerns the first atom of the bond; ^{c}Linear combination of NAOs of the atom concerned in the NBO hybrid; ^{d}Percentage contribution of each NAO in the NBO hybrid.

Donor?acceptor^{a} | E_{2} | E(j)-E(i) | F(i,j) | Donor ®acceptor^{a} | E_{2} | E(j)-E(i) | F(i,j) |
---|---|---|---|---|---|---|---|

γ-RuCl_{2}(Azpy)_{2} | δ-RuCl_{2}(Azpy)_{2} | ||||||

(N_{1}-N_{2})?π*(C_{1}-N_{py}) | 16.23 | 0.33 | 0.074 | π(N_{1}-N_{2})?π*(C_{1}-N_{py}) | 12.40 | 0.39 | 0.066 |

LP(N_{2})?LP*(Ru) | 80.2 | 0.28 | 0.155 | LP(N_{2})?LP*(Ru) | 63.49 | 0.30 | 0.141 |

LP(N_{py})?LP*(Ru) | 76.5 | 0.22 | 0.135 | LP(N_{py})?LP*(Ru) | 75.21 | 0.24 | 0.137 |

LP(Ru)?π*(N_{1}-N_{2}) | 12.8 | 0.14 | 0.038 | LP(Ru)?π*(N_{1}-N_{2}) | 14.07 | 0.14 | 0.04 |

γ-RuCl_{2}(Dazpy)_{2} | δ-RuCl_{2}(Dazpy)_{2} | ||||||

(N_{1}-N_{2}) ?π*(C_{1}-N_{py}) | 15.70 | 0.034 | 0.074 | π(N_{1}-N_{2})?π*(C_{1}-N_{py}) | 12.25 | 0.39 | 0.066 |

LP(N_{2})?LP*(Ru) | 82.43 | 0.28 | 0.157 | LP(N_{2})?LP*(Ru) | 53.91 | 0.30 | 0.13 |

LP(N_{py})?LP*(Ru) | 65.83 | 0.21 | 0.123 | LP(N_{py})?LP*(Ru) | 73.16 | 0.24 | 0.134 |

LP(Ru)?π*(N_{1}-N_{2}) | 12.77 | 0.14 | 0.038 | LP(Ru)?π*(N_{1}-N_{2}) | 18.28 | 0.14 | 0.045 |

γ-RuCl_{2}(Mazpy)_{2} | δ-RuCl_{2}(Mazpy)_{2} | ||||||

(N_{1}-N_{2})?π*(C_{1}-N_{py}) | 15.52 | 0.33 | 0.073 | π(N_{1}-N_{2})?π*(C_{1}-N_{py}) | 13.73 | 0.33 | 0.069 |

LP(N_{2})?LP*(Ru) | 75.9 | 0.29 | 0.152 | LP(N_{2})?LP*(Ru) | 74.76 | 0.29 | 0.153 |

LP(N_{py})?LP*(Ru) | 72.47 | 0.23 | 0.132 | LP(N_{py})?LP*(Ru) | 71.16 | 0.23 | 0.133 |

LP(Ru)?π*(N_{1}-N_{2}) | 10.10 | 0.15 | 0.035 | LP(Ru)?π*(N_{1}-N_{2}) | 13.27 | 0.14 | 0.039 |

γ-RuCl_{2}(Nazpy)_{2} | δ-RuCl_{2}(Nazpy)_{2} | ||||||

(N_{1}-N_{2})?π*(C_{1}-N_{py}) | 15.74 | 0.33 | 0.073 | π(N_{1}-N_{2})?π*(C_{1}-N_{py}) | 11.87 | 0.39 | 0.065 |

LP(N_{2})?LP*(Ru) | 72.59 | 0.28 | 0.147 | LP(N_{2})?LP*(Ru) | 54.05 | 0.30 | 0.130 |

LP(N_{py})?LP*(Ru) | 79.80 | 0.23 | 0.136 | LP(N_{py})?LP*(Ru) | 83.44 | 0.24 | 0.144 |

LP(Ru)?π*(N_{1}-N_{2}) | 9.72 | 0.15 | 0.034 | LP(Ru)?π*(N_{1}-N_{2}) | 13.15 | 0.14 | 0.040 |

^{a}Stared label (*) indicates anti-bonding, LP (A) is a valence lone pair orbital on atom A.

bonding. Therefore, σ(Ru-Cl) is a strong bond. Whereas Ru-N bonds, they are actually formed of electron transfer from lone pair LP(N) to Ru atom. _{py}) and LP(N_{2}) have almost identical populations indicating their equal ability to form Ru-N bond and confirming therefore the bidentate state of the ligands. These interactions are highlighted through

Besides, both transitions LP(N_{2})?LP*(Ru) and LP(N_{py})?LP*(Ru) show that only N_{py} and N_{2} are involved in bonding with the same LP*(Ru) orbital.

Moreover, _{1}) with 1.94e as occupancy. This carries out its non involvement in Ru-N bondings. However, it delocalizes its electrons in the C_{1}-N_{py} bonding as confirmed by

Whereas LP(Ru)?p*(N_{1}-N_{2}), it indicates the electron delocalization regarding the metal to ligand charge transfer (MLCT) transition_{xz} and d_{yz} as indicated in

TD-DFT is performed to understand the electronic absorption and find out the ability for the complex to behave as sensitizer [

_{max}) calculated for each complex, their excited energy, the frontier orbital’s composition and the main transitions regarding the

Complexes | Composition of frontier orbitals | ΔE(eV) | λ_{max} (nm) | f | Main transition | |
---|---|---|---|---|---|---|

HOMO | LUMO | |||||

γ-RuCl_{2}(Azpy)_{2} | Ru (55%) | Azpy (86%) | 1.78 | 697.7 | 0.052 | H®L+1 (48%) |

2.15 | 577.8 | 0.066 | H-2®L (65%) | |||

2.83 | 438.2 | 0.153 | H-3®L (62%) | |||

δ-RuCl_{2}(Azpy)_{2} | Ru (61%) | Azpy (93%) | 1.61 | 768.7 | 0.061 | H-1®L (70%) |

γ-RuCl_{2}(Dazpy)_{2} | Ru (55%) | Dazpy (87%) | 1.85 | 671.0 | 0.053 | H®L+1 (49%) |

2.30 | 537.9 | 0.109 | H-2®L (48%) | |||

2.89 | 429.1 | 0.111 | H-3®L (48%) | |||

δ-RuCl_{2}(Dazpy)_{2} | Ru (61%) | Dazpy (94%) | 1.68 | 738.4 | 0.079 | H-2®L (69%) |

γ-RuCl_{2}(Mazpy)_{2} | Ru (54%) | Mazpy (83%) | 2.00 | 620.4 | 0.053 | H-2®L (43%) |

2.12 | 583.9 | 0.068 | H-3®L (6%) | |||

δ-RuCl_{2}(Mazpy)_{2} | Ru (59%) | Mazpy (92%) | 1.62 | 763.2 | 0.053 | H-1®L (70%) |

γ-RuCl_{2}(Nazpy)_{2} | Nazpy (97%) | Nazpy (90%) | 1.68 | 737.8 | 0.104 | H®L (69%) |

2.30 | 538.2 | 0.050 | H-6®L (48%) | |||

δ-RuCl_{2}(Nazpy)_{2} | Ru (67%) | Mazpy (95%) | 1.62 | 748.1 | 0.106 | H-1®L (68%) |

2.17 | 572.0 | 0.055 | H-4®L (56%) | |||

2.41 | 515.3 | 0.061 | H-2®L (69%) | |||

RuCl_{3}・3H_{2}O | Ru (60%) | Ru (70%) | 2.00 | 516.8 | 0.061 | H-2®L (98%) |

visible region. Through this table we can see that γ-RuCl_{2}(Nazpy)_{2} presents the highest wavelength with the important extinction coefficient (λ_{max} = 748.1 nm and f = 0.106). We can assume that it should be the most sensitive complex. Moreover, all except one of them show that from the HOMO orbital until HOMO-4, molecular orbitals MOs are made principally of Ru orbital. Therefore, the regarding transitions are assumed to be metal to ligand charge transfer (MLCT) types. However, with γ-RuCl_{2}(Nazpy)_{2}, although maximum wavelength_{ }and extinction coefficient are also slightly important, HOMO is mainly made of ligand Nazpy orbital indicating that this transition is a ligand to ligand charge transfer (LLCT) type, which is not suitable for photochemical caracterisation since azopyridine ligands are reportedly insulator [_{2}(Nazpy)_{2} is not sufficiently active as sensitizer. Besides, regarding LUMO and LUMO + 1 orbitals, they are exclusively made of Ligand orbitals in all complexes. Whereas the reactive RuCl_{3}・3H_{2}O, its HOMO and LUMO are both made of Ru orbitals and the maximum wavelength and oscillation strength f regarding transitions are low (λ_{max} = 515.3 nm and f = 0.061). Since ΔE = 57.5 kcal・mol^{−1}, we can see here the importance of ligands that improve the sensitivity of the ruthenium. Regarding excitation energy, we observe that δ-RuCl_{2}(Azpy)_{2 }presents the lowest value confirming its softness. Whereas the δ-RuCl_{2}(Nazpy)_{2} it displays many metallic orbitals involved in the transition such as H-1, H-2 and H-4 with a large band of absorption. This strength can be attributed to the larger conjugate system that the ligand Nazpy provides [

Four azopyridine complexes of ruthenium were predicted in this paper by NBO and TD calculations with DFT method. In order to recover the relativistic effect due to ruthenium atom, the pseudo-potential Lanl2dz basis set was used to perform calculation. Frontier molecular orbital energies calculation show first and foremost that δ- RuCl_{2}(Azpy)_{2} is the most sensitive and soft complex expected to be used as sensitizer in photochemistry. Besides, the calculation shows that Ru atom in all complexes displays almost the same charge comprised between +0.53 and +0.59 that is significantly different from the nominal charge +2. This decrease in charge shows that azopyridine ligands are strong electrons donors. Nevertheless, the constant charge of ruthenium highlights that azopyridine ligands electronically behave similarly and the difference between them must be a steric effect for selective reactions. Furthermore, a natural bond orbital NBO analysis performed at B3LYP/Lanl2dz indicates that Ru-N bondings are made of delocalization of occupancies from Lone Pair atomic orbital of N_{2} and N_{py} to Ru. Moreover, as N_{1} does not link to ruthenium, it is assumed to delocalize its occupancies either in N_{2} or in N_{py}. This fact confirms the bidentate structure of azopyridine ligands. In addition, NBO shows that the transition regarding LP(Ru)?π*(N_{1}-N_{2}) corresponds to _{2}(Azpy)_{2}. However, δ-RuCl_{2}(Nazpy)_{2} is admitted to be the most sensitive with a large band of absorption and an involvement of many molecular orbitals in electron transfer. On behalf of that investigation, the coming work will consist on applying δ-RuCl_{2}(Nazpy)_{2} as photo-sensitizer over a well known active semi-conductor compound as TiO_{2} anatase through dye-sensitized solar cell (DSSC) device.

Nobel, N.K., Bamba, K., Patrice, O.W. and Ziao, N. (2017) NBO Population Analysis and Electronic Calculation of Four Azopyridine Ruthenium Complexes by DFT Method. Computational Chemistry, 5, 51-64. http://dx.doi.org/10.4236/cc.2017.51005