Protonation of Tris(iminocatecholato) Complexes of Gallium(III) and Titanium(IV)

The coordination behaviour of the chiral tren-type tripodal ligand 3-H6 with gallium(III) and titanium(IV) ions was investigated and it was found that only mononuclear complexes are formed. In the presence of Na+, one of the cations acts as a template and is bound inside the “cap” of [Na(3)M]n– (M = Ga, n = 2; M = Ti, n = 1). If no such cation is present, three protons are bound to the iminocatechol units in order to prevent repulsion between lone pairs at nitrogen and oxygen. To minimize the charge repulsion between the protonated imines of [H3(3)M]n+ (M = Ga, n = 0; M = Ti, n = 1), an enaminone/chinomethine mesomeric structure becomes important. The adoption of this unusual mesomeric form by the iminocatecholates was supported by computational methods. In further investigations, we found, that the addition of protons (H2SO4) to the dinuclear (catecholimine)titanium helicate K4[(4)3Ti2] leads to protonation of the imine part of the iminocatechol. An excess of acid results in the hydrolysis of the complexes as well as of the imine units. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)