Facile Ligand Oxidation and Ring Nitration in Ruthenium Complexes Derived from a Ligand with Dicarboxamide-N and Phosphine-P Donors

The reaction of the tetradentate dicarboxamide ligand 1,2-bis-N-[2′(diphenylphosphanyl)benzoyl]diaminobenzene (dppbH2) with RuCl3 in DMF or ethanol results in metal-mediated ligand oxidation and formation of the diamagnetic Ru(II) complex [(dppQ)Ru(Cl)2] (1) with N2P2 chromophore. The o-phenylenedicarboxamide portion of the dppb2− ligand is oxidized to a o-benzoquinonediimine (bqdi) moiety in [(dppQ)Ru(Cl)2]. Presence of oxygen accelerates the ligand oxidation process. Unlike other tetradentate dicarboxamide ligands with pyridine-N, phenolato-O, or thiolato-S donors, dppb2− provides stability to the +2 oxidation state of ruthenium and facilitates oxidation of the coordinated ligand frame. Results of spectroscopic and redox studies strongly support the +2 oxidation state of Ru in 1. Exposure of 1 to NO(g) does not lead to formation of any metal nitrosyl; instead, the bqdi ring is nitrated to afford [(NO2dppQ)Ru(Cl)2] (2).