Nickel(II) Complexes of 1,5-Dialkyl-2,4-pentanediones

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Abstract
Bis(heptane-3,5-dionato)nickel(II) monohydrate [Ni(DNPM)2·H2O], bis(nonane-4,6-dionato)nickel(II) monohydrate [Ni(DNBM)2·H2O], bis(2,6-dimethylheptane-3,5-dionato)nickel(II) monohydrate [Ni(DIBM)2·H2O], bis(2,8-dimethylnonane-4,6-dionato)nickel(II) monohydrate [Ni(DIVM)2·H2O] and bis(tridecane-6,8-dionato)nickel(II) monohydrate [Ni(DNCM)2·H2O] were prepared, and a dimeric octahedral structure was proposed for each of them by means of spectroscopic studies. These dimeric chelates, (NiL2·H2O)2 were stable in solid state, however they disproportionated in non-coordinating solvents to afford dimeric monohydrates, Ni2L4·H2O, and monomeric dihydrates, NiL2·2H2O. DTA studies show that these monohydrates dehydrate in two steps and that the temperature of dehydration decreases in the order of the ligands, DNPM, DNBM, DIBM, DIVM, and DNCM. The dehydrated chelates are green in the neighborhood of room temperature while they changed into red at an elevated temperature. This thermochromism indicates that the chelates are in the trimeric octahedral form at low temperature and in monomeric tetra-coordinated square planar form at high temperature. On the other hand a nickel(II) chelate of 2,2,6,6-tetramethylheptane-3,5-dione (DPVM) was obtained as a pentacoordinated monodioxane adduct from aqueous dioxane. The coordinated dioxane was readily released to afford a monomeric tetra-coordinate square planar, red colored complex, Ni(DPVM)2.