Chiral metal complexes Part 33. Coordination stereoselectivity in ternary cobalt(III) complexes of dipeptides and an optically active triamine

A number of complexes of the type [Co(R,R-benzet)(dipeptidato)]⁺, where R,R-benzet is N-benzyl-N′-(2-picolyl)-1R,2R-diaminocyclohexane and dipeptidato is the dianion of glygly, gly-S-val, gly-R-val, gly-S-leu, gly-R-leu, S-leugly, R-leugly, S-valgly, S-leu-S-ala and S-leu-S-phe, where gly = glycine, val = valine, leu = leucine, ala = alanine and phe = phenylalanine have been synthesized and characterized. The complexes were prepared by reacting the peptides with the facially coordinated precursor [OC-6-432-C]-Co(R,R-benzet)Cl₃ in water. All of the ternary complexes containing a peptide are coordinated meridionally, and are formed stereospecifically, except in one case (S-valgly). A combination of CD and NMR studies has been used to assign the stereochemistry of the complexes. The assignment was made possible by the low-temperature single-crystal X-ray structure determinations of two of the compounds isolated. Both of these complexes crystallize in the orthorhombic system with space group P2₁2₁2₁ and Z = 4; for C-[Co(R,R-benzet)(gly-R-val)]ClO₄·H₂O a = 12.520(7), b = 13.954(14), c = 16.594(8) Å and for C-[Co(R,R-benzet)(glygly)]ClO₄·2H₂O a = 9.998(2), b = 19.301(4), c = 13.919(4) Å. The structures were refined by full-matrix least-squares procedures to values for R of 0.029 and for R′ of 0.032 for 2014 reflections and for R of 0.042 and R′ of 0.047 for 2150 reflections, respectively. In both structures the complex cations have the configuration [OC-6-64-C-(R),(R),(R)].