A tetrapodal pentaamine for stabilizing square pyramidal co-ordination modules: synthesis, structure and reactivity of cobalt(III) complexes of 2,2′-dimethyl-2,2′-iminodimethylenebis(1,3-propanediamine)

Abstract

A facile synthesis of the tetrapodal pentaamine ligand 2,2′-dimethyl-2,2′-iminodimethylenebis(1,3-propanediamine), ditame, has been achieved and some unusual effects of its topology and preference for square pyramidal co-ordination in cobalt(III) complexes explored. Potential influences of the ditame structure on substitution chemistry in [Co(ditame)X]ⁿ⁺ systems are defined by crystal structure analyses for [Co(ditame)Cl][ZnCl₄] and [Co(ditame)(NH₃)]Cl[ZnCl₄]. Proton exchange, nitrogen inversion and chloride anation reactivity, and substitution stereochemistry studies have been carried out on the [Co(ditame)Cl]²⁺ and [Co(ditame)(OD₂)]³⁺ complexes by using ¹³C NMR spectroscopy. The base hydrolysis rate constant for [Co(ditame)Cl]²⁺ (68 dm³ mol^–1 s^–1 at 25 °C, I 1.0 mol dm^–3) is 250 fold greater than that for the analogous [Co(NH₃)₅Cl]²⁺ ion. This difference is attributed to an enhanced trans influence and a bond-coupled co-operative mechanism that facilitate the Cl^– dissociation in the conjugate base of [Co(ditame)Cl]²⁺. The bond-coupled mechanism also aids dissociative processes for the relatively fast aquation and anation chemistry of [Co(ditame)Cl]²⁺ and [Co(ditame)(OH₂)]³⁺. Two results for the reactivity of the [Co(ditame)X]ⁿ⁺ (X = Cl^– or H₂O) ions are attributed to restricted rearrangement of the square pyramidal Co(ditame) fragment in the course of X^n – 3 substitutions. One is the very small amount of [Co(ditame)(N₃)]²⁺ (1.1 ± 0.3%) formed in competition with [Co(ditame)(OH)]²⁺ during base hydrolysis in aqueous 1 mol dm^–3 NaN₃, which indicates an unusually short lifetime for the proposed intermediate. Also, there were no species detected that arise from partial dissociation of the amine, neither in the base hydrolysis nor in the aquation and anation experiments, even at temperatures >50 °C and in the presence of strong acids. There are important consequences for substitution chemistry in other [M(pentaamine)X]ⁿ⁺ systems where rearrangement of the MN₅(amine) fragment is restricted. The quantitatively simple substitution processes also make these reagents valuable as protective groups in synthetic applications such as peptide cleavage and synthesis.