Novel biscapped and monocapped tris(dioxime) Mn(II) complexes: x-ray crystal structure of the first cationic tris(dioxime) Mn(II) complex [Mn(CDOH)3BPh]OH (CDOH2= 1,2-cyclohexanedione dioxime).

Abstract

This report describes the synthesis and characterization of a series of novel biscapped and monocapped tris(dioxime) Mn(II) complexes [Mn(dioxime)3(BR)2] and [Mn(dioxime)3BR]+ (dioxime = cyclohexanedione dioxime (CDOH2) and 1,2-dimethylglyoxyl dioxime (DMGH(2)); R = Me, n-Bu, and Ph). All tris(dioxime) Mn(II) complexes have been characterized by elemental analysis, IR, UV/vis, cyclic voltammetry, ESI-MS, and, in the cases of [Mn(CDOH)3BPh]OH.CHCl3 and [Mn(CDO)(CDOH)2(BBu(OC2H5))2], X-ray crystallography. It was found that biscapped Mn(II) complexes [Mn(dioxime)3(BR)2] are not stable in the presence of water and readily hydrolyze to form monocapped cationic complexes [M(dioxime)3BR]+. This instability is most likely caused by mismatch between the size of Mn(II) and the coordination cavity of the biscapped tris(dioxime) ligands. In contrast, monocapped cationic complexes [M(dioxime)3BR]+ are very stable in aqueous solution even in the presence of PDTA (1,2-diaminopropane-N,N,N',N'-tetraacetic acid) because of the kinetic inertness imposed by the monocapped tris(dioxime) chelators that are able to completely "wrap" Mn(II) into their N6 coordination cavity. [Mn(CDO)3BPh]OH has a distorted trigonal prismatic coordination geometry, with the Mn(II) being bonded by six imine-N donors. The hydroxyl groups from three dioxime chelating arms form very strong intramolecular hydrogen bonds with the hydroxide counterion so that the structure of [Mn(CDOH)3BPh]OH can be considered as being the clathrochelate with the hydroxide counterion as a "cap".