Dirhodium(II,II) complexes: molecular characteristics that affect in vitro activity.

In the series Rh2(O2CR)4 (R=CH3, 1; R=CF3, 2), [Rh2(O2CR)2(phen)2]2+ (R=CH3, 3; R=CF3, 4), and [Rh2(O2CR)2(dppz)2]2+ (R=CH3, 5; R=CF3, 6), 2, 4, and 6 are twice as cytotoxic as 1, 3, and 5, respectively. The substitution reactions of 2 with 9-ethylguanine at various temperatures take place at faster rates than those of 1, and the activation energy Ea(1)=69+/-4 kJ/mol is twice Ea(2)=35+/-2 kJ/mol. The higher cytotoxicities of [Rh2(micro-O2CCH3)2(eta1-O2CCH3)L(MeOH)]+ (L=dppz, 7; L=dppn, 8) relative to [Rh2(micro-O2CCH3)2(bpy)L]2+ (L=dppz, 10; L=dppn, 11) are attributed to the labile equatorial groups in 7 and 8 not present in 10 and 11. The toxicities of complexes 1-8 are not related to their charge or the ease by which they transverse the cellular membrane but to the lability of the ligands on the dirhodium core.