Preparation of four-membered phosphonickelocycles. Unusual facile stabilization of five-co-ordinate complexes

Abstract

Three different types of organometallic compounds [[graphic omitted]R₂)-2}₂](R = Ph 1 or Et 1′), [[graphic omitted]R₂)-2}L](R = Ph 2 or Et 2′) and [NiCl{C₆Cl₄(PR₂)-2}L₂](R = Ph 3 or Et 3′) have been obtained from 1 equivalent of PR₂(C₆Cl₅)(R = Ph or Et), [Ni(cod)₂](cod =cis,cis-cycloocta-1,5-diene), and L = PMe₂Ph a, PEt₃b, P(CH₂Ph)₃c or PPh₃d. Complexes 2 evolve in solution, either to 1 and [NiCl₂L₂], or to 3 by breaking of the Ni–P bond of the four-membered ring by free phosphine. The selective preparation of compounds 1 or 3 can be achieved by performing the oxidative-addition reaction in the absence or with 2 equivalents of L respectively. When 1 equivalent of a diphosphine was used in the oxidative-addition reaction a mononuclear five-co-ordinate complex was obtained, [[graphic omitted]R₂)-2}-(L–L)]4[L–L = Ph₂P(CH₂)_nPPh₂, n= 2 or 3]. However, dppm (Ph₂PCH₂PPh₂) acts as a monodentate ligand to give the five-co-ordinate compound [[graphic omitted]Ph₂)-2}(dppm)₂]. Complexes 2, 2′ show preferentially a cis geometry, 1 is trans, and 3, 3′ have the L ligands in trans position. Insertion of CO or alkynes into the Ni–C bond was not observed. Compounds 1 and 1′in the presence of neutral ligands L = CO or PR₃(PR₃= PMe₂Ph a or PEt₃b) gave five-co-ordinate complexes [[graphic omitted]Ph₂)-2}₂L] without cleavage of the Ni–P bond of the ring. Stabilization of the four-membered ring is achieved when two bidentate ligands are present or in the five-co-ordinate compound [[graphic omitted]Ph₂)-2}(dppm)₂]. Two bidentate ligands are also needed to stabilize the formation of five-co-ordinate complexes. The molecular structures of complexes 1′, 3b, and [[graphic omitted]Ph₂)-2}₂(PEt₃)] were determined by single-crystal X-ray diffraction.