Two new dinuclear nickel(ll) compounds of formula [Nill2(μ-L1)2](CIO4)2·MeCN (1·MeCN) and [NiII2(μ-L2)2](CIO4)2 (2) where HL1 and HL2 stand for 3-(2-(dimethylamino)ethylimino)butan-2-one oxime and 1-(2-(dimethylamino)ethyl-imino)-1-phenylpropan-2-one oxime respectively, have been synthesised. Single crystal X-ray analyses of the complexes reveal that the nickel(ll) ions are in square-planar N3O environments and form six-membered (NiNO)2 metallacycles. Cyclic voltammetric measurements of 1·MeCN and 2 in MeCN solution show quasirreversible one-electron oxidations at E1/2 = 0.566 V and 0.603 V ( vs Fc+/Fc), respectively, attributed to NiIIINiII/NiII2 redox couples. Additional reversible NiIII2/NiIIINiII redox responses were observed at relatively higher potential near E1/2 = 0.832 V and 0.850 V ( vs Fc+/Fc), respectively, for 1·MeCN and 2. Complexes 1·MeCN and 2 display intense charge-transfer bands at ∼390 and ∼345 nm in the visible region. Chemical oxidation of complex 1·MeCN by sodium hexachloroiridate(IV) hexahydrate generates red Ni2III species with characteristic new bands at ∼520 and 427 nm in the visible region as well as the characteristic EPR signals at 77 K with g⊥ > gII. Similar phenomena were observed for complex 2 upon chemical oxidation. The dinickel(ll) complexes are catalytically active for epoxidation of olefins using iodosylbenzene as the terminal oxidant.
Study on catalytic epoxidation of olefins in ketone-hydrogen peroxide system
