Synthesis,characterisation and activity in catalytic epoxidation of olefins of new dinuclear nickel(ll) complexes

Abstract

Two new dinuclear nickel(ll) compounds of formula [Ni^ll₂(μ-L¹)₂](CIO₄)₂·MeCN (1·MeCN) and [Ni^II₂(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 E_1/2 = 0.566 V and 0.603 V (vs Fc⁺/Fc), respectively, attributed to Ni^IIINi^II/Ni^II₂ redox couples. Additional reversible Ni^III₂/Ni^IIINi^II redox responses were observed at relatively higher potential near E_1/2 = 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 Ni₂^III 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⊥ > g_II. 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.