Chlorometal(III) complexes of a new chiral quinquedentate polyamino acid based on (SS)-trans-cyclohexane-1,2-diamine

Abstract

The potentially quinquedentate ligand (SS,SS)-3-(2-aminocyclohexylamino)-2-(2-aminocyclohexylaminomethyl)propionic acid (HL¹), prepared via a metal-directed condensation reaction, readily formed a (carboxylatotetraamine)chlorocobalt(III) complex cation as well as the rhodium(III) and chromium(III) analogues. The compound SS,SS-[CoL¹(Cl)][ClO₄]·0.75HClO₄·2H₂O crystallizes in the orthorhombic space group P2₁2₁2₁, a= 7.825(2), b= 16.566(6), c= 19.737(11)Å, and the crystal structure determined by X-ray diffraction methods refined to a residual of 0.066 for 1475 independent observed reflections. The co-ordinated chloride has a trans disposition relative to the carboxylate group of the ligand, the two pairs of nitrogen donors from the cyclohexane-1,2-diamine residues being coplanar with the cobalt ion. The NHCH₂CH(CO₂^–)CH₂NH core of the ligand necessarily occupies an octahedral face. Metal–nitrogen distances [Co–N 1.920(11)–1.982(11)Å] are slightly longer than in the 3-(2-aminoethylamino)-2-(2-aminoethylaminomethyl)propionic acid analogue although the metal–oxygen distance [1.916(10)Å] is comparable to that in the analogue, as is the Co–Cl distance [2.223(4)Å]. Comparisons of structures and kinetics of base hydrolysis with related systems are drawn.