Structural Determination of Ruthenium Complexes Containing Bi-Dentate Pyrrole-Ketone Ligands

A series of ruthenium compounds containing a pyrrole-ketone bidentate ligand, 2-(2′-methoxybenzoyl)pyrrole (1), have been synthesized and characterized. Reacting 1 with [(η6-cymene)RuCl2]2 and RuHCl(CO)(PPh3)3 generated Ru(η6-cymene)[C4H3N-2-(CO-C6H4-2-OMe)]Cl (2) and {RuCl(CO)(PPh3)2[C4H3N-2-(COC6H4-2-OMe)]} (3), respectively, in moderate yields. Successively reacting 2 with sodium cyanate and sodium azide gave {Ru(η6-cymene)[C4H3N-2-(CO-C6H4-2-OMe)]X} (4, X=OCN; 5, X=N3) with the elimination of sodium chloride. Compounds 2–5 were all characterized by 1H and 13C-NMR spectra and their structures were also determined by X-ray single crystallography.

Here we report the synthesis and structural characterization of a series of ruthenium compounds containing the multidentate pyrrole-ketone ligand.The successive reacting of these ruthenium complexes with sodium salts of NaOCN and NaN 3 is also reported.These compounds were structurally determined in order to understand their steric geometries for further applications.

Molecular Geometries of Compounds 2-5
Single-crystals of Compounds 2-5 for X-ray diffraction analysis were obtained from either methanol or THF saturated solution at −20 °C.The summary of the X-ray crystal data and selected bond lengths and angles are shown in Tables 1 and 2, respectively.The molecular geometries of 2-5 are depicted in Figures 1-4.The molecular geometry of 2 could be described as a three-legged piano stool with the nitrogen atom of the pyrrole, the oxygen atom of the carbonyl atoms, and the chloride atom forming the three legs and the cymene ring acting as the stool plane.The bond length of the ruthenium atom to the center of the cymene ring is 1.6598(1) Å, relatively close to the bond lengths of published ruthenium-cymene compounds [41,42].It is interesting to note that the methine proton (H8) of the cymene ring was close to the oxygen atom (O2) of the methoxy group of ligand 1, with a bond length of 2.511 Å, which is assumed to be a weak hydrogen bonding between the H8 and O2 atoms [43].Successively replacing the chloride anion of 2 with sodium salts of NaX in methanol resulted in new ruthenium compounds {Ru(η 6 -cymene)[C 4 H 3 N-2-(CO-C 6 H 4 -2-OMe)]X} (4, X=OCN; 5, X=N 3 ).Compounds 4 and 5 were both characterized by 1 H and 13 C-NMR spectra and showed patterns similar to those of Compound 2, indicating that the electronic properties around the ruthenium atom was not affected by the coordinated mono-anionic ligands.

Molecular Geometries of Compounds 2-5
Single-crystals of Compounds 2-5 for X-ray diffraction analysis were obtained from either methanol or THF saturated solution at −20 • C. The summary of the X-ray crystal data and selected bond lengths and angles are shown in Tables 1 and 2, respectively.The molecular geometries of 2-5 are depicted in Figures 1-4.The molecular geometry of 2 could be described as a three-legged piano stool with the nitrogen atom of the pyrrole, the oxygen atom of the carbonyl atoms, and the chloride atom forming the three legs and the cymene ring acting as the stool plane.The bond length of the ruthenium atom to the center of the cymene ring is 1.6598(1) Å, relatively close to the bond lengths of published ruthenium-cymene compounds [41,42].It is interesting to note that the methine proton (H8) of the cymene ring was close to the oxygen atom (O2) of the methoxy group of ligand 1, with a bond length of 2.511 Å, which is assumed to be a weak hydrogen bonding between the H8 and O2 atoms [43].The molecular geometry of 4 can also be described as a three-legged piano stool geometry, as shown in Figure 3.The bond length from the ruthenium atom to the center of the cymene ring is at ca. 1.667 Å.The ambi-dentate NCO ligand was bound to the ruthenium atom at the N-end.Similar ruthenium cymene-NCO structures have been reported in the literature [49,50].The bond lengths of ruthenium to the cyanate-N atom as well as N=C and C=O are all consistent with the results in the literature.
The pale orange crystals of 5 were obtained from a saturated methanol solution at −20 °C.The molecular structure of 5 is shown in Figure 4 and its geometry is similar to that of 2. It can also be described as a three-legged piano stool geometry with the azide and pyrrolyl nitrogen atoms and carboxyl oxygen atom taking the three leg positions and the cymene acting as the planar surface.The bond length of O(2)-H(8) was ca.2.868 Å, indicating a very weak or no hydrogen bonding effect.Ru(cymene)azido compounds have been reported in the literature [51][52][53][54].For 5, the bond lengths of Ru(1)-N(1), N(1)-N(2), and N(2)-N(3) were 2.109(2) Å, 1.204(3) Å, and 1.157(3) Å, respectively and these data are consistent that reported in the literature.

General Consideration
All reactions were performed under a nitrogen atmosphere using standard Schlenk techniques or in a glove box.Toluene was dried by refluxing over sodium benzophenone ketyl.CH2Cl2 was dried over P2O5.All solvents were distilled and stored in solvent reservoirs that contained 4-Å molecular sieves and were purged with nitrogen.The 1 H and 13 C-NMR spectra were recorded using a Bruker Avance 300 spectrometer and the chemical shifts were recorded in ppm relative to the residual protons of CDCl3 (δ = 7.24, 77.0 ppm).Elemental analyses were performed using a Heraeus CHN-OS The molecular geometry of 4 can also be described as a three-legged piano stool geometry, as shown in Figure 3.The bond length from the ruthenium atom to the center of the cymene ring is at ca. 1.667 Å.The ambi-dentate NCO ligand was bound to the ruthenium atom at the N-end.Similar ruthenium cymene-NCO structures have been reported in the literature [49,50].The bond lengths of ruthenium to the cyanate-N atom as well as N=C and C=O are all consistent with the results in the literature.
The pale orange crystals of 5 were obtained from a saturated methanol solution at −20 • C. The molecular structure of 5 is shown in Figure 4 and its geometry is similar to that of 2. It can also be described as a three-legged piano stool geometry with the azide and pyrrolyl nitrogen atoms and carboxyl oxygen atom taking the three leg positions and the cymene acting as the planar surface.The bond length of O(2)-H(8) was ca.2.868 Å, indicating a very weak or no hydrogen bonding effect.Ru(cymene)azido compounds have been reported in the literature [51][52][53][54].For 5, the bond lengths of Ru(1)-N(1), N(1)-N(2), and N(2)-N(3) were 2.109(2) Å, 1.204(3) Å, and 1.157(3) Å, respectively and these data are consistent that reported in the literature.

General Consideration
All reactions were performed under a nitrogen atmosphere using standard Schlenk techniques or in a glove box.Toluene was dried by refluxing over sodium benzophenone ketyl.CH 2 Cl 2 was dried over P 2 O 5 .All solvents were distilled and stored in solvent reservoirs that contained 4-Å molecular sieves and were purged with nitrogen.The 1 H and 13 C-NMR spectra were recorded using a Bruker Avance 300 spectrometer and the chemical shifts were recorded in ppm relative to the residual protons of CDCl 3 (δ = 7.24, 77.0 ppm).Elemental analyses were performed using a Heraeus CHN-OS Rapid Elemental Analyzer at the Instrument Center of the NCHU.Ligands [33][34][35] 1 and [Ru(η 6 -p-cymene)Cl 2 ] 2 were prepared using a modified procedure [19].
A methanol solution (10 mL) of 1 (0.20 g, 0.99 mmol) and [Ru(η 6 -p-cymene)Cl 2 ] 2 (0.304 g, 0.50 mmol) in a 50 mL of flask was stirred for 1 h at room temperature.Sodium bicarbonate then was added into the solution and stirred for 3 h.The volatiles were removed under vacuum and the solid was extracted with methylene chloride and then solvent was removed again to give crude product of 2. The product was recrystallized from a methanol solution to generate 0.149 g of red brown crystals in a 32.0% yield.A toluene solution (10 mL) of 1 (0.08 g, 0.40 mmol) and RuHCl(CO)(PPh 3 ) 3 (0.378 g, 0.40 mmol) in a 50 mL of flask was refluxed for 24 h under nitrogen.The volatiles were removed under vacuum and residue was washed with heptane to remove excess of PPh 3 .The resulting solid was recrystallized from a saturated THF solution at −20 • C to give 0.24 g of yellow crystals in a 66.9% yield. 1  A 25 mL Schlenk flask charged with 2 (0.117 g, 0.25 mmol), excess sodium cyanate and methanol (10 mL) was stirred at room temperature for overnight.Methanol was removed under vacuum and residue was extracted with methylene chloride (10 mL × 3).The methylene chloride was removed and residue was recrystallized from a methanol solution at −20 A 25 mL Schlenk flask charged with 2 (0.047 g, 0.10 mmol), excess sodium azide, and methanol (10 mL) was stirred at room temperature for overnight.Methanol was removed under vacuum and residue was extracted with methylene chloride (10 mL × 3).The methylene chloride was removed and residue was recrystallized from a methanol solution at −20

X-ray Crystallography
Suitable crystals of Compounds 2-5 were attached to a fine glass fiber and mounted in goniostat for data collection.Data collections were performed at 150 K under liquid nitrogen vapor for all compounds.Data were collected on a Bruker SMART CCD diffractometer with graphite monochromated Mo-Kα radiation.No significant crystal decay was found.Data were corrected for absorption empirically by means of ψ scans.All non-hydrogen atoms were refined with anisotropic displacement parameters.For all the structures, the hydrogen atom positions were calculated and they were constrained to idealized geometries and treated as rigid where the H atom displacement parameter was calculated from the equivalent isotropic displacement parameter of the bound atom.An absorption correction was performed with the program SADABS [55] and the structures of both complexes were determined by direct methods procedures in SHELXS [56] and refined by full-matrix least-squares methods, on F 2 's, in SHELXL [57].All the relevant crystallographic data and structure refinement parameters are summarized in Table 1.

Conclusions
We have successfully employed a bidentate pyrrole-ketone ligand with ruthenium compounds to form a series of Compounds 2-5 and their structures were determined by X-ray single crystallography.A preliminary test of hydrogen transfer reactions of acetophenone and isopropyl alcohol using these ruthenium compounds showed low conversion.We are currently using Compounds 2-5 to investigate the catalytic activities of hydrogen transfer reactions toward varieties of ketones and alcohols and the hydroaminations of inter-and intra-molecular alkenes and alkynes.

2Figure 1 .
Figure 1.The molecular geometry of Compound 2. The thermal ellipsoids were drawn at 50% probability and all the hydrogen atoms except the methine proton on the cymene ring were omitted for clarity.The H8-O2 hydrogen bonding is shown.

Figure 1 .
Figure 1.The molecular geometry of Compound 2. The thermal ellipsoids were drawn at 50% probability and all the hydrogen atoms except the methine proton on the cymene ring were omitted for clarity.The H8-O2 hydrogen bonding is shown.

Figure 2 .
Figure 2. The molecular geometry of Compound 3. The thermal ellipsoids were drawn at 50% probability and all the hydrogen atoms were omitted for clarity.

Figure 3 .
Figure 3.The molecular geometry of Compound 4. The thermal ellipsoids were drawn at 50% probability and all the hydrogen atoms were omitted for clarity.

Figure 2 . 11 Figure 2 .
Figure 2. The molecular geometry of Compound 3. The thermal ellipsoids were drawn at 50% probability and all the hydrogen atoms were omitted for clarity.

Figure 3 .
Figure 3.The molecular geometry of Compound 4. The thermal ellipsoids were drawn at 50% probability and all the hydrogen atoms were omitted for clarity.Figure 3. The molecular geometry of Compound 4. The thermal ellipsoids were drawn at 50% probability and all the hydrogen atoms were omitted for clarity.

Figure 3 .
Figure 3.The molecular geometry of Compound 4. The thermal ellipsoids were drawn at 50% probability and all the hydrogen atoms were omitted for clarity.Figure 3. The molecular geometry of Compound 4. The thermal ellipsoids were drawn at 50% probability and all the hydrogen atoms were omitted for clarity.

Figure 4 .
Figure 4.The molecular geometry of Compound 5.The thermal ellipsoids were drawn at 50% probability and all the hydrogen atoms except the methine proton on the cymene ring were omitted for clarity.The H8-O2 hydrogen bonding is shown.

Figure 4 .
Figure 4.The molecular geometry of Compound 5.The thermal ellipsoids were drawn at 50% probability and all the hydrogen atoms except the methine proton on the cymene ring were omitted for clarity.The H8-O2 hydrogen bonding is shown.

Table 1 .
The summary of X-ray crystal data for Compounds 2-5.