2,3‐Bis(diarylphosphanyl)‐1,4‐diazadienes: P,P Coordination of PdII, PtII and a Nickelacyclopentanone with Subsequent Formation of Quinoxalines by a Ring‐Closure Reaction at the Periphery

Abstract

AbstractThe solid‐state structures of several 2,3‐bis(phosphanyl)‐1,4‐diazadiene ligands of the general formula RN=C(PPh2)−C(PPh2)=NR (B: R = 4‐tolyl; C: R = 4‐tert‐butylphenyl; D: R = mesityl) were determined by single‐crystal X‐ray diffraction. The ligands B and C are nonplanar and lie between the (E) and the (Z) form (N=C−C=N is 99.5° in B, 94.0° in C, and 128.2° in D). In the presence of air, B is oxidized to its corresponding phosphane oxide. Reaction of D with elemental sulfur yields the corresponding phosphane sulfide. Furthermore, ligand C reacts with [PtCl2(cod)] and [PdCl2(CH3CN)2] to form mononuclear complexes (1 and 2, respectively) in which the two P atoms are bound to the metal center. X‐ray structural analysis showed that in both complexes the 1,4‐diazadiene functionality at the periphery had undergone a ring‐closure reaction with one of the aromatic N‐substituents thus resulting in a quinoxaline ring. The bulkier ligand D reacts with [PdCl2(CH3CN)2] to form the mononuclear complex 3. In the solid state, the two phosphorus atoms are coordinated to the metal center in a five‐membered chelate ring. In contrast to 1 or 2, a cyclohexadiene system is formed as result of a cycloaddition reaction of one aromatic N‐substituent with the 1,4‐diazadiene system. The nickel complex [Ni(tmeda)(C2H4COO)] reacts with D by displacement of tmeda to form a mononuclear complex 4a with P,P coordination. A subsequent ring‐closure reaction (cycloaddition) at the periphery does not occur in 4a. If the ligand is sterically less demanding (B or C), a cycloaddition between the 1,4‐diazadiene system and one aromatic substituent takes place in the nickel complex to form complexes of the type 5 containing a quinoxaline ring. This reaction could be monitored by 31P NMR spectroscopy for ligand C. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003)