Some coordination chemistry of the bidentate nitrogen-donor ligand 2-(2-aminophenyl)pyridine

Complexes of the chelating bidentate nitrogen-donor ligand 2-(2-aminophenyl)pyridine (L) are described. [PdL₂] [PF₆]₂ (1) forms two types of crystal (1a,b) of which (1a) was crystallographically characterized [space group P2₁/c; a 10.974(6), b 14.757(4), c 8.223(2) Å; β 104.66(5)°; V 1288.3(8) ų; Z 2]. It has a square-planar donor set about the metal, with the ligands mutually trans and with a twist of 35° between the pyridyl and phenyl rings. ¹H n.m.r. spectroscopy shows that pure (1a) and (1b) equilibrate in solution; it is likely that (1b) is the cis form of the complex. Electrochemical measurements on [Ru(bipy)₂L] [PF₆]₂ (2) show that the Ru^II/Ru^III couple is cathodically shifted by 0.34 V with respect to [Ru(bipy)₃]²⁺, indicating that replacing a pyridyl donor by an amine results in a significant increase of electron density on the metal centre: this effect is much larger with a primary amine ligand than with tertiary amines. The e.p.r. spectrum of [CuL₂] [PF₆]₂ (3) shows that a small extent of association into a dinuclear complex occurs, a reaction which is solvent-dependent: addition of pyridine breaks up the dimer by coordination to the axial sites of the monomer, whose spectrum is characteristic of a tetragonal geometry (i.e. a planar array of two ligands L, and axial solvent molecules). Attempted recrystallization of other complexes of L from acetone afforded [C₁₄H₁₅N₂ [PF₆] (4), in which the free ligand L has condensed with acetone and then undergone an intramolecular cyclization by attack of the pyridyl nitrogen atom on the electrophilic carbon atom of the imine bond; this compound was also crystallographically characterized [space group P2₁/n; a 8.258(2), b 10.036(4), c 17.976(7) Å β 100.12(3)°; V 1466.8(10) ų; Z 4].