Syntheses and X-ray Structures of Mixed-Ligand Salicylaldehyde Complexes of Mn(III), Fe(III), and Cu(II) Ions: Reactivity of the Mn(III) Complex toward Primary Monoamines and Catalytic Epoxidation of Olefins by the Cu(II) Complex

The Schiff base ( L ), synthesized from 2-(dimethylamino)ethylamine and salicylaldehyde acts as a tridentate ligand. This ligand, when stirred with 1 equiv of KOH in methanol, undergoes partial hydrolysis of the imine bond. This solution readily takes up Mn(II)/Mn(III) acetate or Fe(III) chloride/perchlorate to form mixed-ligand Mn(III) or Fe(III) complexes, respectively. The neutral dark brown complex, [Mn(L - H )(NCS){ o -(CHO)C 6 H 4 O - }] ( 1 ), crystallizes in the presence of thiocyanate in the orthorhombic space group Pbca with a = 15.271(8), b = 19.522(7), c = 13.213(7) Å; Z = 8; R = 0.060; and R w = 0.062. The coordination geometry around Mn(III) ion is distorted octahedral with donation from one L - H, one salicylaldehyde and, one thiocyanate ligand. With Fe(III), the dark red complex isolated in the solid state is found to be a neutral μ-oxo Fe(III) dimer with the formula [{ o -(CHO)C 6 H 4 O - }(L - H )Fe] 2 O ( 3 ). The structure of 3 has been solved and successfully refined in the monoclinic space group C 2/ c with a = 18.558(7), b = 11.231(5), c = 16.943(6) Å; β = 95.81(3)°; Z = 4; R = 0.052; and R w = 0.055. Each of the Fe(III) ions is hexadentate with donation from one L - H and one salicylaldehyde besides the bridged O atom. The Fe(III)−O−Fe(III) angle is found to be 166.05(4)°, which is well within the normal range observed for monobridged Fe(III)−O−Fe(III) complexes. Due to structural trans effects, the coordination geometry around each metal center is distorted from the ideal octahedral geometry. Cu(II) makes the neutral complex, [Cu(L - H ){ o -(CHO)C 6 H 4 O - }] ( 2 ) when L, salicylaldehyde, and a Cu(II) salt are allowed to react in equimolar quantities in the presence of excess of KOH. It crystallizes in the monoclinic space group C 2/ c with a = 18.077(4), b = 11.514(2), c = 16.716(4) Å; β = 93.66(2)°; Z = 8; R = 0.057; and R w = 0.061. The coordination geometry around Cu(II) is square pyramidal where, out of the four equatorial donors, three are provided by the Schiff base L and the fourth one by the phenolate O of the salicylaldehyde group. The Cu(II) ion is 0.170(5) Å above the equatorial plane and is bonded axially to the O atom of the carbonyl group of the salicylaldehyde. The bound salicylaldehyde in the Mn(III) complex 1 readily reacts with the reagents 2-aminophenol, 2-aminothiophenol, or 2-aminoethanol to form mononuclear, neutral Mn(III) complexes 4 − 6, respectively, with the general formula,[Mn(L - H )(L‘ - 2H )]. The Schiff base L ‘ is formed by condensation of the bound salicylaldehyde in 1 with the amino group of the added reagent. Complex 4 is low-spin (μ eff /μ B = 3.01; S = 1) at 300 K, which is quite rare. It also exhibits a pseudoreversible Mn(IV)/Mn(III) couple with E 1/ 2 = 0.54 V (vs SCE) in DMF. All the other Mn complexes are high-spin (μ eff /μ B range, 4.89−4.94; S = 2) at 300 K. The μ eff /μ B number for 2 is 1.93 and for 3 is 1.84. Complex 2 shows catalytic activity in the oxidation of olefins to epoxides in the presence of 2-methylpropanal and molecular oxygen.