Bis-tropolonato derivatives of cobalt(III) complexes of bidentate aliphatic nitrogen mustards as potential hypoxia-selective cytotoxins.

A series of cobalt(III) complexes, [Co(trop)2(L)]+, where trop is the tropolonate anion and L is a bidentate amine or nitrogen mustard, have been prepared as potential hypoxia-selective cytotoxins (L = BEE, N,N'-diethylethylenediamine; DEE, N,N-diethylethylenediamine; BCE, N,N'-bis(2-chloroethyl)ethylenediamine; DCE, N,N-bis(2-chloroethyl)ethylenediamine). The 1H NMR and 13C{1H} NMR spectra of the complexes were assigned on the basis of chemical shift considerations, 2D NMR studies (including 13C-1H and 1H-1H COSY correlation experiments), and comparison to the related, known acetylacetonato (acac) complexes [Co(acac)2(L)]+. An x-ray crystal structure determination of the analogue [Co(trop)2(BEE)]ClO4 showed it to be the delta SS/lambda RR enantiomeric pair of diastereomers. The tropolonato complexes have significantly higher reduction potentials than the corresponding acac complexes, suggesting more facile cellular reduction. In agreement with this, the mustard complexes have IC50 values in cells little different to those of the free mustards even under aerobic conditions, and do not show hypoxic selectivity in a clonogenic assay under conditions where the corresponding 3-methylacac complex of DCE showed significant selectivity.