A Cobalt(III) Chiroporphyrin and Its Amine Adducts. A Potential Chiral NMR Shift Reagent for Amines

The chlorocobalt(III) complex of alphabetaalphabeta-tetramethylchiroporphyrin, CoCl(TMCP), has been prepared as a potential enantioselective host or chiral NMR shift reagent for optically active amines. The X-ray structure of CoCl(OHCH(2)CH(3))(TMCP) shows the six-coordinate cobalt(III) ion at the center of a strongly ruffled porphyrin. The 2-fold-disordered ethanol ligand interacts with the chiroporphyrin host by two C-H.O hydrogen bonds to the carbonyl groups of two ester substituents. Primary amines bind to this diamagnetic cobalt(III) center to form cationic 2:1 complexes in which the (1)H NMR resonances of the axial ligands are shifted upfield of tetramethylsilane by the porphyrin ring current. Coordinated enantiopure 2-alkylamines exhibit NMR signals for the protons of the amine group which are characteristic of their (R or S) absolute configuration. The bis-complexes of the same amines in racemic form exist as three different species, (R,R), (S,S), and (R,S), in 1:1:2 relative ratios. Negligible enantioselection by the chiral host suggests kinetic control of bis(amine) complex formation on cobalt(III). The X-ray structure of the bis((S)-2-butylamine) complex [Co((S)-NH(2)CH(CH(3))CH(2)CH(3))(2)(TMCP)][CoCl(4)](0.5) shows a 2-fold-disordered amine on one face of the porphyrin only. The unique amine on the other face is held within the porphyrin groove by a network of weak interactions including N-H.O and C-H.O hydrogen bonds. With its ability to induce good resolution of axial ligand (1)H NMR resonances and slow dissociation kinetics of its bis-adducts, CoCl(TMCP) may be useful as a chiral NMR shift reagent for conformational studies of chiral amines and as an analytical reagent for the determination of their enantiomer composition.