Quinquedentate Coordination of 3-[(2'-Amino-ethyl)amino]-2-[(2″-aminoethyl)aminomethyl]propionic Acid (LH). Crystal Structure of trans-[Co(L)Cl][ClO₄].H₂O And Molecular Mechanics Examination of Isomers

Abstract

The anion (1c) of the branched polyamino acid 3-[(2′-aminoethyl)amino]-2-[(2″-aminoethyl)- aminomethyl ] propionic acid (1b) binds as a quinquedentate ligand to cobalt(III). Reaction with cobalt(II) chloride, followed by oxidation and acidification, yields exclusively one isomer of [Co(1c) Cl ]⁺. The complex was crystallized readily as a perchlorate monohydrate salt, in the monoclinic space group P 2₁/n, a 7.921(2), b 17.357(4), c 11.852(4) Ǻ, β 107.25(2)°, Z = 4; a single-crystal X-ray structure determination was refined to R 0.033 for 2099 `observed' reflections. The carboxylate and two adjacent secondary amines occupy an octahedral face, with the chloro ligand trans to the carboxylate , and the secondary amines adopt R,S stereochemistries. The Co-O distance [1.901(2) Ǻ] is at the short end of the range of cobalt- carboxylate bonds, and Co-N distances [average 1.94₄ Ǻ (primary amines), average 1.95₀ Ǻ (secondary amines)] are slightly shorter than average for Co-N bonds. The Co- Cl distance is 2.255(1) Ǻ. Molecular mechanics examination of all possible isomers and conformers of [Co(1c) Cl ]⁺ and of the related pentaamine 1,5,9-triamino-5-methyl-3,7-diazanonane (2) indicate a greater variation in strain energy across the range of isomers and conformers in the latter more strained system.