Cycloruthenates(III) with CNO pincer-like ligands: Regioselective metallation of N-(4-R-benzoyl)-N′-(2-naphthylidene)hydrazines

Abstract

Synthesis and characterization of a series of cyclometallated ruthenium(III) complexes with the Schiff bases N-(4-R-benzoyl)-N′-(2-naphthylidene)hydrazines (H2Ln, where n = 1–5 for R = H, Cl, Me, OMe and NO2, respectively) are described. All the complexes having the general molecular formula trans-[Ru(Ln)(PPh3)2Cl] (1–5) were characterized by elemental analysis, magnetic susceptibility, mass spectrometric, various spectroscopic (IR, UV-Vis and EPR) and electrochemical measurements. Molecular structures of trans-[Ru(L1)(PPh3)2Cl] (1) and trans-[Ru(L2)(PPh3)2Cl] (2) were confirmed by X-ray crystallography. In each of these two complexes, the metal center is in a distorted octahedral CNOClP2 coordination environment. The tridentate amidate-O, azomethine-N and the 2-naphthyl-C3 coordinating (L1/2)2– and the Cl– occupy the four equatorial positions and the P-atoms of the two triphenylphosphine molecules satisfy the two axial positions of the metal center. Electronic spectra of 1–5 in dichloromethane exhibit a ligand field transition within 1322–1160 nm and two ligand-to-metal and three intraligand transitions in the ranges 500–406 and 332–250 nm, respectively. The room temperature (300 K) magnetic moments of 1–5 are in the range of 1.86–1.99 µB. All five complexes display rhombic EPR spectra in frozen (77 K) dichloromethane-toluene (1:1) solution. A quasi-reversible reduction response in the range of –0.60–0.68 V and two irreversible oxidation responses around 0.9 and 1.4 V (vs. Ag/AgCl) were observed in the cyclic voltammograms of 1–5 in dichloromethane. Theoretical calculations were performed to get an insight of the regioselective ruthenation at the 3-position over the 1-position of the 2-naphthyl group of (Ln)2– in these complexes.