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

A facile synthesis of the tetrapodal pentaamine ligand 2,2'-dimethyl-2,2'-i minodimethylenebis(1,3-propanediamine), ditame, has been achieved and some unusual effects of its topology and preference for square pyramidal co-ordi nation in cobalt(III) complexes explored. Potential influences of the ditam e structure on substitution chemistry in [Co(ditame)X](n+) systems are defi ned by crystal structure analyses for [Co(ditame)Cl][ZnCl4] and [Co(ditame) (NH3)]Cl[ZnCl4]. Proton exchange, nitrogen inversion and chloride anation r eactivity, and substitution stereochemistry studies have been carried out o n the [Co(ditame)Cl](2+) and [Co(ditame)(OD2)](3+) complexes by using C-13 NMR spectroscopy. The base hydrolysis rate constant for [Co(ditame)Cl](2+) (68 dm(3) mol(-1) s(-1) at 25 degrees C, I 1.0 mol dm(-3)) is 250 fold grea ter than that for the analogous [Co(NH3)(5)Cl](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 processe s 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(di tame)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) substi tutions. One is the very small amount of [Co(ditame)(N-3)](2+) (1.1 +/- 0.3 %) formed in competition with [Co(ditame)(OH)](2+) during base hydrolysis i n aqueous 1 mol dm(-3) NaN3, which indicates an unusually short lifetime fo r the proposed intermediate. Also, there were no species detected that aris e from partial dissociation of the amine, neither in the base hydrolysis no r in the aquation and anation experiments, even at temperatures > 50 degree s C and in the presence of strong acids. There are important consequences f or substitution chemistry in other [M(pentaamine)X] (n+) systems where rear rangement of the MN5(amine) fragment is restricted. The quantitatively simp le substitution processes also make these reagents valuable as protective g roups in synthetic applications such as peptide cleavage and synthesis.