Catalytic Cyclization and Dimerization of 1.5‐Hexadiene by Transition Metals: The Reactivity of NiII and PdII Complexes containing Bi‐ or Tridendate Ligands and of Bis(alkyne)nickel(0) Compounds

The novel nickel(II)‐ and palladium(II) complexes (box)Ni(acac)₂ ( 1 ), (box)Ni(mes)(Br) ( 2 ), (box)Pd(mes)₂ ( 3 ), (PEMA)NiBr]Br ( 4 a ) and (PEMA)PdCl]Cl ( 4 b ) were synthesized and investigated by IR‐, MS‐ and (except the paramagnetic compound 1 ) by NMR spectroscopy (box: 4,4,4′,4′‐tetramethyl‐2,2′‐bisoxazoline, PEMA: N‐diphenylphosphinoethyl‐N‐(N′,N′‐dimethylaminoethyl)‐methylamine). According to the X‐ray analyses the metal atoms in 1 are in a distorted‐octahedral surrounding, in the organometallic compounds 2 and 3 they have square‐planar coordination, in the ionic complex 4 a is one halogen anion uncoordinated. According to the X‐ray analyses the analogous complexes (DEPA)NiCl]Cl ( 5 a ) and (DEPA)PdCl]Cl ( 5 b ) (DEPA: N,N‐bis(diphenylphosphinoethyl)‐amin)) display the same structure in the solid state as found in 4 a . The ni compounds catalyze at ambient temperature in the presence of Et₃Al the selective cyclization of 1.5‐hexadiene to form methylene‐cyclopentane (MeCP) in high yields. The combination of the Ni compounds with methylalumoxanes (MAO) leads to the catalytic formation of isomeric dimers containing one cyclopentyl ring as main products. The binuclear Ni(0) complex (alkyne)Ni]₂ ( 6 ) (alkyne: bis(t‐butylalkynyl)‐bis‐phenylsilan) is more reactive in both catalytic reactions.