Preparation, crystal structures and properties of half-sandwich ruthenium complexes containing salicylbenzoxazole ligands

Abstract Three half-sandwich ruthenium complexes [Ru(p-cymene)LCl] containing salicylbenzoxazole ligands [LH = 2-(5-methyl-benzoxazol-2-yl)-4-methyl-phenol (2a), LH = 2-(5-methyl-benzoxazol-2-yl)-4-chloro-phenol (2b), and LH = 2-(5-methyl-benzoxazol-2-yl)-4-bromo-phenol (2c)] were synthesized and characterized. All half-sandwich ruthenium complexes were fully characterized by 1H and 13C NMR spectra, MS, elemental analyses, and UV–vis as well as cyclic voltammetry (CV). The molecular structures of 2a, 2b, and 2c were confirmed by single-crystal X-ray diffraction. Single-crystal X-ray structures show that the synthesized ruthenium complexes are three-legged piano-stools with a six-membered metallocycle formed by coordination of the bidentate salicylbenzoxazole ligands to the metal centers. Data from CV and UV–vis absorption of the ruthenium complexes indicated that by changing the substituent on the para position of (donating or withdraw group) the salicylbenzoxazole ligands, minor changes in redox and electronic properties of the ruthenium complexes were observed.


Introduction
The coordination chemistry and reactivity of half-sandwich ruthenium complexes have been extensively studied due to the fact that these complexes have a wide range of potential applications in metallomacrocycles, host-guest, anticancer drugs, biological activities, electrochemical sensors, and catalysts in the past decades [1][2][3][4][5][6][7]. Various metal-organic frameworks including metallomacrocycles and cages have been obtained using Ru(p-cymene) as a building block [8][9][10]. Half-sandwich ruthenium complexes show high catalytic activities toward organic transformation, such as C-C bond formation via direct C-H bond activation [11,12], addition of arylboronic acid [13,14], acceptorless dehydrogenative coupling of alcohols [15], transfer hydrogenations of ketones [16] and water oxidation [17,18]. Ruthenium complexes as anti-cancer drugs have entered clinical trials [19]. Thus, synthesis of half-sandwich ruthenium complexes and studying their chemical properties would be desirable.

Materials and measurements
All manipulations were carried out under pure nitrogen using standard Schlenk techniques. All solvents were purified and degassed by standard procedures. The bidentate monoanionic [N,o] ligands were synthesized according to procedures described [20]. 1 H and 13 C NMR spectra were recorded on 300 or 500 MHz NMR spectrometers at room temperature. Chemical shifts (δ) are in ppm relative to internal TMS and are referenced to residual 1 H and 13 C solvent resonances. iR spectra were recorded on a Nicolet AVATAR-360iR spectrometer. elemental analyses were performed on an elementar iii vario ei Analyzer. Mass spectrometry was performed on a Bruker BifLeX iii MALDi-Tof-MS instrument. A CHi400c computerace instrument was employed to obtain cyclic voltammograms in acetonitrile solutions of the ruthenium complexes (2 mM) under nitrogen at room temperature (298 K) using 0.20 M tetrabutylammonium perchlorate (TBAP) as supporting electrolyte. A glassy carbon working electrode, a platinum auxiliary electrode, and a Ag/AgCl reference electrode were used to obtain cyclic voltammograms. All E 1/2 values estimated from cyclic voltammetry (CV) were calculated as the average of the oxidative and reductive peak potentials (E pa + E pc )/2 at a scan rate of 100 mV s −1 .

Synthesis of 2-(5-methyl-benzoxazol-2-yl)-4-methyl-phenol (1a)
A solution of 5-methylsalicylaldehyde (1.36 g, 10 mmol), 2-amino-4-methylphenol (1.23 g, 10 mmol), and 3-5 drops of acetic acid in 50 mL of methanol was refluxed with stirring for 3 h after bulk orange precipitate formation under nitrogen. When the resulting mixture was cooled to room temperature, Phi(oAc) 2 (3.54 g, 11 mmol) was added and the mixture immediately turned black. After the mixture was refluxed again for 1 h in open air, the solvent was evaporated to dryness and the black crude product was chromatographed on silica gel with petroleum ether to give white powder (1.63 g, 68%). Anal. Calcd for C 15

Synthesis of half-sandwich ruthenium complex 2c
Complex 2c was prepared by the same procedure as described above for 2a using [Ru(p-cymene)(μ-Cl)Cl] 2

X-ray structure determination
Diffraction data of 2a, 2b, and 2c were collected on a Bruker AXS SMART APeX diffractometer equipped with a CCD area detector using Mo Kα radiation (λ = 0.71073 Å). All the data were collected at 298 K and the structures were solved by direct methods and subsequently refined on F 2 using full-matrix least squares techniques (SHeLXL) [22]. SADABS [23] absorption corrections were applied to the data, all non-hydrogen atoms were refined anisotropically, and hydrogens were located at calculated positions. All calculations were performed using the Bruker Smart program. A summary of the crystallographic data and selected experimental information is given in Table 1 with selected bond distances and angles given in Table 2.

Synthesis of ligands and half-sandwich ruthenium complexes
As outlined in Scheme 1, the salicylbenzoxazoles (1a-1c) were readily accessible from salicylaldehyde in two steps [20], condensation of the corresponding salicylaldehyde with o-aminophenol and then oxidation with Phi(oAc) 2 in refluxing CH 3 CN. The desired products were separated through column chromatography on silica gel as white powders in moderate to good yields. A general synthetic route for half-sandwich ruthenium complexes is shown in Scheme 2. The complexes 2a-2c were obtained by the treatment of two equiv of the salicylbenzoxazole ligands with [Ru(p-cymene)(μ-Cl)Cl] 2 in the presence of K 2 Co 3 in CH 3 CN under reflux for 3 h, affording orange crystals of 2a-2c in yields of 52-58%. These complexes are stable toward air and moisture in the solid state. These ruthenium complexes were characterized by iR and NMR spectroscopy and MS. These half-sandwich ruthenium complexes have similar NMR spectra, so we use 2c for an example. The 1 H NMR spectrum of 2c show signals at δ 1.11, 1.20, and 2.75 ppm, which can be assigned to the isopropyl groups, which exhibited signals at δ 5.35-5.55 ppm for the arene of p-cymene ring, respectively. Mass spectra also provided further evidence for the formation of the half-sandwich ruthenium complexes, with peaks at m/z = 474.1007 (for 2a), 494.2528 (for 2b), and 537.9956 (for 2c), corresponding to the loss of the Cl anion [M-Cl] + .
Crystals of 2a, 2b, and 2c suitable for X-ray crystallographic diffraction were obtained by slow diffusion of diethyl ether into a concentrated solution of the ruthenium complexes in dichloromethane or methanol solution. The crystallographic data for 2a, 2b, and 2c are summarized in Table 1, and selected bond lengths and angles are given in Table 2. The molecular structures of 2a-2c are shown in figures 1-3.
As shown in figures 1-3, the half-sandwich ruthenium complexes have remarkably similar structures. each Ru is surrounded by one chloride, one nitrogen, and one oxygen from the ligand and one of the p-cymene rings. All of the ruthenium centers have six-coordinate geometry, assuming that the p-cymene ring is a three-coordinate ligand. Ruthenium of 2a, 2b, and 2c has a three-legged piano-stool conformation with a six-membered metallocycle formed by coordination of the bidentate salicylbenzoxazole ligands to the metal centers. The Ru-N distances (2.121(9) Å in 2a, 2

The CV and UV-vis absorbance of 2a-2c
The electrochemical properties of 2a-2c have been studied by CV in degassed CH 3 CN solution under nitrogen using 0.2 M TBAP as supporting electrolyte. A glassy carbon working electrode, a platinum auxiliary electrode, and a Ag/AgCl reference electrode were used to obtain cyclic voltammograms. The data for 2a-2c are collected in Table 3, and the cyclic voltammograms of the complexes are shown in figure 4. Complexes 2a-2c displayed a one-electron oxidation process Ru(ii)/Ru(iii) with quasi-reversible half-potentials and similar to those of other half-sandwich ruthenium complexes [26]. The quasi-reversible reduction couples (e 1/2 ) for 2a-2c were −0.782, −0.796, and −0.783 V, respectively. on modifying     the para substituent on the ligand from -Me to -Br on the ligands, the oxidation potential increases from −0.660 V (2a) to −0.691 V (2c). This slight variation is attributed to the electron-rich (-Me) and electron-deficient (Cl − and Br − ) substituents on the salicylbenzoxazole ligands. it is apparent from the cyclic voltammograms that the e 1/2 values are sensitive to substituents on the salicylbenzoxazole ligands.
The absorption spectra of 2a-2c were measured at room temperature in CH 3 CN. The UV/Vis absorption data are summarized in Table 3, and the corresponding electronic absorption spectra of ruthenium complexes are depicted in figure 5. All ruthenium complexes show intense absorptions at 299 and 400 nm with molar extinction coefficients (ε) in the order of 10 4 dm 3 mol −1 cm −1 , which can be assigned to charge transfer between salicylbenzoxazole ligands and ruthenium (MLCT). The salicylbenzoxazole compounds have a strong absorption between 220 and 280 nm due to the π-π* transition. When complexed to ruthenium, this absorption band was slightly red shifted for 2a-2c, as observed in ruthenium complexes with [3] radialene ligands [27].

Conclusion
We have synthesized and characterized three half-sandwich salicylbenzoxazole ruthenium complexes. A combination of spectroscopic studies and X-ray crystallographic confirmed the molecular structures of 2a-2c. The redox properties and electronic absorptions of 2a-2c showed that the electron-rich or electron-deficient substituents on the salicylbenzoxazole ligands have minor influences to the redox and spectroscopic properties of the ruthenium complexes.

Disclosure statement
No potential conflict of interest was reported by the authors.

Funding
We acknowledge financial support from the National Nature Science foundation of China [grant number 21102004], and the Project-sponsored by SRf for RoCS and Special and excellent Research fund of Anhui Normal University.