Synthesis, Characterization and Semi-empirical study of Di-nuclear Mixed-Ligands Complexes of Ru(II)

INTRODUCTION

The chemistry of polynuclear transitions metal complexes has been of immense current interest, due to the fascinating and versatile properties exhibited by them [1]. Transition metal complexes containing polypyridyl- type ligands exhibit attractive electrochemical and photophysical properties [2 – 8]. The ligand 2, 2’, 6’,2’’-terpyridine has been used widely in the preparation of molecular systems with diverse chemical properties [9]. In the ligand, the position of the nitrogen atom of each pyridine ring allows tridentate and meridian coordination to Ru, Os and Ir [9]. The synthesis of the ligand can be applied to the preparation of derivatives containing an additional functional group which can be used as a bridge to connect two or more different metal centers  to produce symmetrical and unsymmetrical metal complexes [9].In addition, much work has been devoted to the bridged bi- and poly-nuclear ruthenium complexes particularly in their mixed valence states [10]. The short-bridging ligands such as pyrazine, tert-butylmalonitrile, dinitrogen, cyanogen or 4,4’-dithiodipyridine provide often a strong enough coupling between ruthenium – amine centers to allow complete electronic delocalization in the mixed – valence (MV) state [10]. Polynuclear complexes of ruthenium(II)are valuable sensitizers for the conversion of light into chemical or electrical energy as they are efficient MLCT chromophores in which the absorption maximum can be tuned to almost any wavelength of the visible spectrum[10]. In order to ensure a potentially good charge separation from the excited state in a donor-chromophore-acceptor triad, it is good to aim at linear, rigid assembly [10].

Herein, we report the synthesis and characterization of the Ru(phen)₂Cl₂.2H₂O and Ru(bipy)₂Cl₂.2H₂O with the condensate of diacetyldihydrazone with 2-acetylpyridine and the semi-empirical study of the complexes investigated.

EXPERIMENTAL

MATERIALS AND METHOD

Reagents

Reagent grade RuCl₃.3H₂O, 1,10-phenanthroline, LiCl, DMF, diethyl ether,  2,2’-bipyridyl, diacetyl, hydrazine hydrate, glacial acetic acid, 2-acetylpyridine, NaClO₄ were purchased from the British Drug House Chemicals Ltd (BDH)and Aldrich Chemicals Co., and were used without further purification.

Preparation of the DDACP

0.456g(1mmole) diacetydihydrazone[11] was transferred into 100mL round-bottom flask, 20mL methanol and 0.90mL2-acetylpyridine added. The mixture stirred with calcium chloride guard-tube fixed to the flask. The bright–yellow solution obtained was stirred for 4hrs, after which the solution was left overnight. The yellow compounds obtained was filtered by suction and dried in the air (0.620g, 48%).

 

Fig. 1: The structure of the DDACP Click here to View figure

 

Reaction of Ru(phen)­₂Cl­₂.2H₂O [12] with DDACP

0.114g(0.2mmole) Cis-Ru(phen)₂Cl₂.2H₂O [12] was transferred to a 100mL two-neck round bottom flask, 1:1 MeOH  – H₂O mixture(20mL) added and degassed . To this 0.064g (0.2mmole) DDACP was added and the mixture refluxed under N₂ gasfor 6hrs. It was then cooled to room temperature and filtered. To the filterate, 0.246g (2mmole) NaClO₄solution in 5mL distilled water was added dropwisely with constant stirring. The red precipitate obtained immediately was stirred for 30mins before it was filtered by suction, washed with 15mL diethyl ether and then dried in the vacuo. Yield: 0.07g(20%) [Ru(phen)₂]₂L(ClO₄)₄.4H₂O Anal. Cal. C₆₆H₆₀N₁₄O₂₀Cl₄Ru­₂(M.M: 1712.718): C, 46.24; H, 3.53; N, 11.45. Found: C, 46.73; H, 3.37; N, 11.40.FTIR: ν/cm^-1: 1601 (C=N); 3063(C-H aromatic);625s, 1090vs (ClO₄).UV-Vis: cm^-122,756; 38,487; 45,274.ESI-MS (CH₃CN) m/z: 391.02 (28% {Ru(phen)₂L}²⁺), 310.14 (20% {Ru₂(phen)₄L}⁴⁺),  324.51 (25% {Ru₂(phen)₄L}⁴⁺ .3H₂O), 560.99 (20% {Ru₂(phen)₄(ClO₄)₂}²⁺).¹H NMR: δ/ppm:2.17(3H, CH₃-C-C-CH₃), 3.13(3H, CH₃-C=N), 7.20 – 8.91 (40H, aromatic).Ʌ_M/mho cm²mol^-1562(CH₃CN) (1:4 electrolyte).

Reaction of Ru(bipy)­₂Cl­₂.2H₂O [13] with DDACP

0.064 g (0.2mmole) DDACP was added to 0.104g(0.2mmole) cis-Ru(bipy)₂Cl₂.2H₂O[13]dissolved in degassed 10mL MeOH – H₂O mixture(1:1) added and degassed for 30mins. To this 0.064 g (0.2mmole) DDACP was added and the mixture refluxed for 6hrs under N₂ gas. It was then cooled to room temperature and filtered. 0. 246g (2mmole) NaClO₄solution distilled water (5mL) was added dropwisely with constant stirring. The red precipitate obtained immediately was stirred for 30mins before it was filtered by suction, washed with 15mL diethyl ether and dried in the vacuo.  Yield: 0.062g, (20%) [Ru(bipy)₂]₂L(ClO₄)₄.4H₂O Anal. Cal. C₅₈H₅₆N₁₄O₁₈Cl₄Ru­₂ (M.M: 800.28): C, 44.03; H, 3.57; N, 12.41. Found: C, 43.98; H, 3.67; N, 12.40.FTIR: ν/cm^-1: 1600 (C=N); 3070 (C-H aromatic);2972 (C-H aliphatic); 625s, 1090vs (ClO₄).UV-Vis: cm^-122,437; 23,915sh; 35,452; 39,828sh; 41,787. ESI-MS (CH₃CN) m/z: 286.84 (25% {Ru₂(bipy)₄L}⁴⁺),288.82 (28% {Ru(bipy)L}²⁺),  366.91 (32% {Ru(bipy)₂L}²⁺), 445.82 (18% {Ru(bipy)₃L}²⁺), 674.27{Ru₂(bipy)₄L(ClO₄)₂}²⁺.¹H NMR: δ/ppm:2.17(3H, CH₃-C-C-CH₃), 2.62 (CH₃-C=N), 7.49 – 8.84 (40H, aromatic).Ʌ_M/mho cm²mol^-1575 (CH₃CN) (1:4 electrolyte).

Physical measurements

Microanalyses were performed by a Perkin-Elmer 2400II CHNS analyzer. UV/VIS spectra were recorded on a Perkin-Elmer Lambda 950 spectrophotometer, FTIR spectra(KBr) on a Shimadzu FTIR-8400S spectrometer and ESI mass spectra on a Waters Qtof Micro YA263 Spectrometer. Molar conductances were measured by a Syntronics (India) conductivity meter (model 306) in acetonitrile and 300MHz NMR spectra on a Bruker DPX300 Spectrometer in deuterated dimethylsulphoxide (DMSO-d₆).

Results and Discussion

The complexes were obtained by refluxing the Ru(phen)₂Cl₂.2H₂O and Ru(bipy)₂Cl₂H₂O with DDACP in MeOH-H₂O mixture under N₂. The red precipitates obtained were found to have low yields probably due to the formation of dinuclear Ru(II) complexes in which the L acts as a bridge between two [Ru(phen)₂]²⁺. The microanalyses data for these complexes gave satisfactory results. The colours of the compounds are consistent with those of similar systems [9,14].

The infrared spectra of the complexes revealed the -C=N stretching frequencies at 1601cm^-1 and 1600cm^-1 respectively for the 1,10-phenanthroline and 2,2’-bipyridine complexes[9]. The aromatic –C-H of both complexes appeared at 3063cm^-1 and 3070cm^-1 respectively for [Ru(phen)₂]₂L(ClO₄)₄. 4H₂O and [Ru(bipy)₂]₂L(ClO₄)₄.2H₂O. The strong bandsat ca 625cm^-1 and 1090cm^-1observed in both complexescorrespond to the -ClO₄stretching frequencies [15].

The single spin-allowed d-d transition from the ground term⁵T₂g to ⁵Eg [16] was at 22,756cm^-1and 22,437cm^-1 with a shoulder 23,915cm^-1 respectively for  [Ru(phen)₂]₂L(ClO₄)₄. 4H₂O and [Ru(bipy)₂]₂L(ClO₄)₄.2H₂O. The ligand transitions in the complexes are observed between the range 35,452cm^-1– 45,274cm^-1 in the two complexes.

The fragmentation patterns observed in the complexes is also an indication that DDACP acts as a bridge between two of the starting Ru(II) compounds.

In the [Ru(phen)₂]₂L(ClO₄)₄. 4H₂O complex,   peaks at m/z = 391.02, 310.14, 324.51and 560.99 correspond to {Ru(phen)₂L}²⁺, {Ru₂(phen)₄L}⁴⁺,   {Ru₂(phen)₄L}⁴⁺ .3H₂O, {Ru₂(phen)₄(ClO₄)₂}²⁺ respectively. The peaks at m/z = 286.84 and m/z = 288.82were assigned to {Ru₂(bipy)₄L}⁴⁺) and{Ru(bipy)L}²⁺) respectively in  [Ru(bipy)₂]₂L(ClO₄)₄. 2H₂O while the other peaks in the complex at m/z = 366.91, m/z = 445.82 and m/z = 674.27 stand for  {Ru(bipy)₂L}²⁺, {Ru(bipy)₃L}²⁺ and {Ru₂(bipy)₄L(ClO₄)₂}²⁺ respectively. The fragmentation patterns observed in these complexes have molecular weights which were consistent with the expected values.

The ¹H NMR of the complexes showed the three methyl protons of (CH₃-C-C-CH₃) at 2.17ppm  for both of them and while the three methyl protons of CH₃-C=N- at 3.13ppm and 2.62ppm respectively. The aromatic protons were observed in the complexes between 7.20ppm and 8.91ppm. The number of the aromatic protons in each case indicated that the 1,10-phenanthroline and 2,2’-bipyridine rings in the starting Ru(II) compounds are present in the mixed –ligand complexes coupled with the aromatic ring of the introduced ligand, L. The molar conductance of 562mho cm²mol^-1 was obtained for the [Ru(phen)₂]₂L(ClO₄)₄. 4H₂O complex whilethat of [Ru(bipy)₂]₂L(ClO₄)₄.2H₂O is at 575mho cm²mol^-1. These values showed that the complexes are   dimeric.

 

Fig. 2: The proposed structure of the complexes Click here to View figure

Computational methods

Quantum chemical methods (Semi-empirical, PM3) was used for the optimization of the complex geometries. Heat of formation, binding energy for the two mixed-ligand binuclear Ru^II complexes were also calculated using PM3 as implemented in Spartan 06 computational software package. It has been reported that geometrical parameters from PM3 calculation for transition metal complexes are more accurate than that of ab initio methods [17], however, HF/3-21G* was used to calculate the stabilization energy for the two binuclear Ru(II) complexes.The average Ru1-N (Ru2-N) bond distance are 2.061 (2.068) and 2.061Å (2.063Å) for [Ru₂(bipy)₄L]⁴⁺ and [Ru₂(phen)₄L]⁴⁺ respectively (Table 1).

 

Table 1.  Ru-N bond distances for the two di-nuclear Ru(II) complexes  Click here to View table

Figure 3: Optimized Ru(II) complexes at PM3; (a) = [Ru2(bipy)4L]4+ and (b) = [Ru2(phen)4L]4+: Click here to View figure

Figure 4: The HOMO map at PM3; (a) = [Ru2(bipy)4L]4+ and (b) = [Ru2(phen)4L]4+ Click here to View figure

Figure 5: The LUMO map at PM3; (a) = [Ru2(bipy)4L]4+ and (b) = [Ru2(phen)4L]4+ Click here to View figure

 

Optimized geometries, HOMO and LUMO of [Ru₂(bipy)₄L]⁴⁺ and [Ru₂(phen)₄L]⁴⁺complex ions are shown in Figures 3, 4 and 5 respectively. The highest occupied molecular orbital (HOMO) representing π-electrons of the complexes arelocalized on one unit of 1,10 phenanthroline and 2,2’-bipyridine ligands for [Ru₂(phen)₄L]⁴⁺ and [Ru₂(bipy)₄L]⁴⁺ respectively (Figure 4). The lowest unoccupied molecular orbital (LUMO)is mainly on (1-pyridin-2-yl-ethylidene) -hydrazine subunit of DDACP for the two complexes (Figure 5).The HOMO and LUMOenergies calculated are -17.80 and -9.80 eV for[Ru₂(bipy)₄L]⁴⁺ and -17.15 and -9.59 eV for [Ru₂(phen)₄L]⁴⁺respectively. The E_HOMO-E_LUMOenergy presenting π – π* main transitions in the complexes are 8.00 and 7.56 eV for [Ru₂(bipy)₄L]⁴⁺ and [Ru₂(phen)₄L]⁴⁺ respectively. The dipole moment (D.M) and polar surface area (PSA) are some important parameters to be considered in solute-solvent interactions which have overall effect on the reactivity, therefore D.M and PSA calculated at Semi-empirical method show that [Ru₂(phen)₄L]⁴⁺ may exhibit more complex-solvent interactions. The Mulliken charges on nitrogen atoms involved in the coordination are all positive and the average charge on these nitrogen atoms are 0.541/0.471e for [Ru₂(bipy)₄L]⁴⁺ and 0.457/0.474e for [Ru₂(phen)₄L]⁴⁺as compared to -0.032/-0.273e and -0.041/-0.223e on N7/N7′ for [Ru₂(bipy)₄L]⁴⁺and [Ru₂(phen)₄L]⁴⁺respectively. These show that electrons are transferred from ligands to the Ru(II) ion during coordination (Table 2).

 

Table 2: Frontier molecular energies, heat of formation, Mulliken charges, binding and stabilization energies Click here to View table

 

To evaluate the effect of DDACP on dinuclear Ru(II) complexes, binding energy (BE)  and stabilization energy were calculated from the energy  involved in the dissociation processes as shown in equations 1 and 2.

Binding energy (B.E) = [Ru₂X₄L]⁴⁺  – 2E[RuX₂]²⁺  – EL                         (1)

Stabilization energy (S.E) = E[Ru₂X₄L]⁴⁺  – 2ERu²⁺ – 4EX – EL            (2)

where X = 1,10-phenanthroline or 2,2’-bipyridine and L = DDACP and E = energy of each species.

The calculated binding energy and stabilization energy as presented in Table 2 were carried out only for the ground states of the complex ions. The BE calculated for [Ru₂(bipy)₄L]⁴⁺ are-270.85  and -279.01 kJ/mol for PM3 and HF/3-21G(d) calculations respectively. These are calculated to be -330.16 and -342.94 kJ/mol for [Ru₂(phen)₄L]⁴⁺ at PM3 and HF/3-21G(d) levels. The stabilization energies calculated at HF/3-21G(d) level are -726.82  and -785.01 kJ/mol for [Ru₂(bipy)₄L]⁴⁺ and[Ru₂(phen)₄L]⁴⁺ respectively. Comparison of the binding energies and stabilization energies suggest that [Ru₂(phen)₄L]⁴⁺is more favoured thermodynamically. The higher binding energy and lower stabilization energy in the [Ru₂(phen)₄L]⁴⁺ may be attributed to the availability of more  π-electrons on phenanthroline moiety of the compound.

Conclusion

The formation of the bridged mixed – ligand dimeric complexes was supported by the microanalyses, fragmentation pattern in the mass spectra, and the conductivity measurements. The ¹HNMR and infrared spectra in conjunction with other analytical data all lends credence to the formation of the compounds.

Furthermore, the quantum chemical methods (Semi-empirical, PM3) used for the optimization of the complex geometries have the HOMO map localized on 1,10- phenanthroline and 2,2’-bipyridine and LUMO is mainly on (1-pyridin-2-yl-ethylidene)-hydrazine subunit of DDACP for the two complexes. The binding and stabilization energiescalculated for the two mixed-ligand dinuclear Ru^IIcomplexes revealed that [Ru₂(phen)₄L]⁴⁺ion is more stable thermodynamically.

ACKNOWLEGEMENT

F.A.O.Adekunle is grateful to Professor Dipankar Datta of Indian Association for the Cultivation of Science (IACS) Calcutta India for a laboratory space and Third World Academy of Science(TWAS), Trieste, Italy and Indian Association for the Cultivation of Science for a Post-Doctoral Fellowship.

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