5-Methylquinoxaline as a versatile mono-, bi- and tridentate ligand in Palladium(II) chemistry. Crystal structures of trans-[Pd(OAc)2(N1-C8H5N2Me-5)2] and [Pd(OAc)(C,N4-CH2C8H5N2-5)(PPh3)]

To study the potential coordination versatility of 5-methylquinoxaline, the preparation of complexes which contain it as a mono-, bi- or tridentate ligand was carried out. On reaction with [Pd(OAc)2]3 it coordinates to the metal through its less sterically blocked nitrogen atom (N(1), distal to the Me group) to give trans-[Pd(OAc)2(N1-C8H5N2Me-5)2] (1). Reflux of 1, or a mixture of [Pd(OAc)2]3 and 5-methylquinoxaline, in glacial acetic acid gave the cyclometallated acetate-bridged dimer [Pd(C,N4-CH2C8H5N2-5)(μ-OAc)]2 (2). Complex 2 reacted with: (i) LiCl to give [Pd(C,N4-CH2C8H5N2-5)(μ-Cl)]2 (3); (ii) PPh3 to give [Pd(C,N4-CH2C8H5N2-5)(OAc)(PPh3)] (4); (iii) PR3 and LiCl to give [Pd(C,N4-CH2C8H5N2-5)Cl(PR3)] [R=Ph (5), Me (6), Et (7)]; (iv) Tl(acac) to give [Pd(C,N4-CH2C8H5N2-5)(O,O-acac)] (8); and (v) 2,2′-bipyridine (bpy)–NaClO4·H2O to give [Pd(C,N4-CH2C8H5N2-5)(bpy)]ClO4 (9). Complex 5 reacted with [PdCl2(NCPh)2] in a 2:1 molar ratio to form the C,N4,N1-coordinated tripalladium derivative [PdCl2{PdCl(C,N4,N1-CH2C8H5N2-5)(PPh3)}2] (10). By contrast, when this reaction was carried out with an excess or equimolecular amount of PdCl2(NCPh)2] compound 3 was obtained together with [PdCl(μ-Cl)(PPh3)]2. We present evidence for formation of novel multimetallic species of the type [Pd(C,N4,N1-CH2C8H5N2-5)(PR3)]nXn by reaction of 5 or 6 with TlOTf (OTf=O3SCF3), AgClO4 or TlClO4. The crystal structures of 1 and 4·2CHCl3 were determined by X-ray diffraction studies. They conclusively prove that Pd is coordinated to N(1) in 1 and to N(4) in 4.