QUINQUEDENTATE COORDINATION OF YL)AMINO]-2-[(2''-AMINOETHYL)AMINOMETHYL]PROPIONIC ACID (LH) - CRYSTAL-STRUCTURE OF TRANS-[CO(L)CL][CLO4].H2O AND MOLECULAR MECHANICS EXAMINATION OF ISOMERS

The anion (1c) of the branched polyamino acid l)amino]-2-[(2''-aminoet hyl)-aminomethyl]propionic acid (1b) binds as a quinquedentate ligand to cobalt(iii). Reaction with cobalt(II) chloride, followed by oxidati on and acidification, yields exclusively one isomer of [Co(1c)Cl]+. Th e complex was crystallized readily as a perchlorate monohydrate salt, in the monoclinic space group P2(1)/n, a 7.921(2), b 17.357(4), c 11.8 52(4) angstrom, beta 107.25(2)-degrees, Z = 4; a single-crystal X-ray structure determination was refined to R 0.033 for 2099 'observed' ref lections. 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) angstrom] is at the short end of the range of cobalt-carboxyla te bonds, and Co-N distances [average 1.94(4) angstrom (primary amines ), average 1.95(0) angstrom (secondary amines)] are slightly shorter t han average for Co-N bonds. The Co-Cl distance is 2.255(1) angstrom. M olecular 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 acros s the range of isomers and conformers in the latter more strained syst em.