Chromium(III) complexes of the hexadentate ligand N,N,N ',N '-tetrakis(2-pyridylmethyl)ethane-1,2-diamine. Synthesis, structure and reactivity

Chromium(III) complexes of N,N,N',N'-tetrakis(2-pyridylmethyl)ethan-1,2-dia mine, tpen, have been synthesised by air oxidation of chromium(II) acetate and the ligand in a methanol-water mixture. Addition of perchlorate precipi tates [Cr(tpen)(OCOCH3)](ClO4)(2). H2O, which has been used as the starting material for the synthesis of a number of complexes including [Cr(tpen)](C lO4)(3), [Cr(tpen)(OH)](ClO4)(2) and cis-[Cr(tpen)(OH)(2)]ClO4. 3H(2)O. The se compounds have all been characterised by single-crystal structure determ inations. The four complexes are all monomeric and contain six-coordinate c hromium(III) cations. In [Cr(tpen)](3+) all six nitrogens of the potentiall y hexadentate tpen ligand are coordinated to chromium, in [Cr(tpen)(OCOCH3) ](2+) and [Cr(tpen)(OH)](2+) five nitrogen atoms are coordinated and in cis -[Cr(tpen)(OH)(2)](+) only four nitrogen atoms are coordinated. The remaini ng coordination sites in the last three cations are occupied by oxygen atom s of the acetate and the hydroxide ligands. In neutral solution the divalen t base [Cr(tpen)(OH)](2+) is relatively stable, but acidification leads to a fast equilibration reaction between [Cr(tpen)(OH2)](3+) and [Cr(tpen)](3), which has been characterized by stopped-flow measurements. In basic aque ous solution [Cr(tpen)(OH)](2+) reacts slowly to give cis-[Cr(tpen)(OH)(2)] (+). Acidification of cis-[Cr(tpen)(OH)(2)](+) solutions leads to the same hexaamine/pentaamine equilibrium mixture but by a considerably slower proce ss. Part of this significant reactivity difference may be attributed to the conformation of the coordinated part of the ligand, which is similar in th e hexaamine and pentaamine complexes, and markedly different in the tetraam ine complex.