Manganese(ii) complexes of tetradentate 4N ligands with diazepane backbones for catalytic olefin epoxidation: effect of nucleophilicity of peroxo complexes on reactivity

Abstract

A series of Mn(II) complexes of the types [Mn^II(L)](ClO₄)₂ 1–4 and [Mn^II(L)Cl₂] 1a–4a, where L = N,N′-bis(2-pyrid-2-ylmethyl)-1,4-diazepane (L1), N-(6-methylpyrid-2-ylmethyl)-N′-(pyrid-2-ylmethyl)-1,4-diazepane (L2), N-(1-methyl-1H-imidazol-2-ylmethyl)-N′-(pyrid-2-ylmethyl)-1,4-diazepane (L3) and N-(quinol-2-ylmethyl)-N′-(pyrid-2-ylmethyl)-1,4-diazepane (L4), has been isolated and characterized both in the solid state and in solution. In the X-ray crystal structure of 4, Mn(II) adopts a distorted octahedral coordination geometry. The peroxo adducts [Mn^III(O₂)(L)]⁺ have been generated by adding H₂O₂ (30%) and triethylamine to 1–4 in CH₃CN solution at −30 °C and have been characterized by UV-Vis and mass spectral and electrochemical techniques. The role of the nucleophilic nature of the peroxo adducts in the aldehyde deformylation reaction has been studied by using cyclohexanecarboxaldehyde (CCA) and para-substituted benzaldehydes as substrates. The rate constant (k₂) for the deformylation reaction of CCA follows the trend 3b > 1b > 2b > 4b, which illustrates the dependence of reaction rate upon the Lewis basicity of the ligand donor atoms, as reflected by the Mn^II/Mn^III redox potential of the complexes. Also, a linear correlation between the rate of deformylation catalysed by 3b and the substituent constant (σ) has been observed for the deformylation of various para-substituted benzaldehydes. The complexes catalyse the epoxidation of cis-cyclooctene, cyclohexene and styrene with H₂O₂ as the oxygen source in the presence of acetic acid with high conversion and selectivity, and the product yields are dependent upon the ligand stereoelectronic factors.