Structural and Electrochemical Comparison of NiII in an N2S2 Plane with Two, One and No Axial Ligands: Isolation and Characterization of a Five‐Coordinate Nickel(II) Complex

Abstract

AbstractAddition of two equivalents of benzyl bromide to [N,N′‐bis(2‐mercaptoethyl)‐1,5‐diazacyclooctane]nickel(II) (Ni‐1) produced the green dibenzyl dithioether complex [Ni‐1‐Bzl2Br2] structurally characterized by X‐ray crystallography as a six‐coordinate octahedral complex in a monoclinic crystal system and the P21/c space group. Benzylation of the methyl thioether/thiolate [(Ni‐1‐Me)+I– yielded the mixed benzyl/methyl dithioether [(Ni‐1‐BzlMe)Br]I, structurally characterized by X‐ray crystallography as a five‐coordinate square pyramidal nickel(II) complex with a Ni–Br distance of 2.451 Å. The complex crystallizes in the orthorhombic Cmca space group. The latter is the first nickel(II) pentacoordinate complex isolated for the N,N′‐bis(2‐mercaptoethyl)‐1,5‐diazacyclooctane (daco) ligand. Structural comparison of these two complexes with the square‐planar dimethyl compound [Ni‐1‐Me2]I2 found an expansion of the N2S2 plane upon addition of axial ligands. The five‐coordinate nickel complex showed a displacement of the Ni ion from the N2S2 plane by 0.43 Å toward the axial bromide ligand, a common observation of pentacoordinate complexes of this ligand with metals such as Zn and Fe. Electrochemical studies as well as the X‐ray crystal structures for complex [Ni‐1‐Bzl2Br2] and [(Ni‐1‐BzlMe)Br]I are presented and discussed in detail.