Cage Precursor Ligands: A Sexidentate Triamine Trioxime and Its Complexes With Cobalt(III)

Abstract

The ligand ' tamox ' (CH₃C{CH₂NHC(CH₃)₂C(CH₃)=NOH}₃) has been synthesized from 1,1,1-tris ( aminomethyl )ethane and 2-chloro-2-methyl-3-nitrosobutane. The ligand forms a deep yellow cobalt(III) complex which crystallizes readily from acidic solution as a monohydrated chloride diperchlorate salt. An X-ray structure determination on the monoclinic crystals, space group P2₁/c, a 10.254(4), b 22.30(1), c 15.375(8) Ǻ, β 117.16(4)°, Z 4, was refined to conventional R= 0.048 for 4573 'observed' reflections. The ligand is sexidentate with distinctly different bonding interactions of the two types of nitrogen-donor atoms ({Co- N_amine } 1.97, {Co- N_oxime } 1.92 Ǻ). The chloride anion interacts strongly with the complex cation through symmetrical hydrogen bonding to all three oxime hydroxyl protons. From neutral solution, the complex can be crystallized as an approximately trihydrated dichloride, the ligand having lost one proton ( pKa ≈ 3, I= 1.0, at 298 K). An X-ray structure determination for these crystals, monoclinic, P2₁/c, a 12.565(3), b 14.686(12), c 18.524(16) Ǻ, β 123.88(2)°, Z 4, R 0.054 for 3900 'observed' reflections, shows that neither chloride anion has any special mode of association with the cation. Attempts to 'cap' the [Co( tamox )]³⁺ complex by reactions with aqueous borate and phenyl boronate were unsuccessful. There is apparently a minor side reaction in the formation of the cobalt complex, a reaction which leads to the hydrolysis of one of the oxime groups of the ligand and the generation of a trans- dichlorocobalt (III) complex of this hydrolysed ligand in a quadridentate form. The nature of this green material has been established by an X-ray structure determination of its perchlorate salt, monoclinic, P2₁/c, a 14.38(2), b 18.20(1), c 14.71(2)Ǻ, β 118.80(7)°, Z 4, R 0.070 for 2120 'observed' reflections.