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

The 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 99.5degrees in B, 94.0degrees in C, and 128.2degrees 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 I 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(tineda)(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 P-31 NMR spectroscopy for ligand C. (C) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003).