Synthesis of Camphor-Derived Bis(pyrazolylpyridine) Rhodium(III) Complexes: Structure-Reactivity Relationships and Biological Activity.

Two novel rhodium(III) complexes, namely, [Rh ^III (X)Cl ₃ ] (X = 2 2,6-bis((4 S,7 R)-7,8,8-trimethyl-4,5,6,7-tetrahydro-1 H-4,7-methanoindazol-3-yl)pyridine or 2,6-bis((4 S,7 R)-1,7,8,8-tetramethyl-4,5,6,7-tetrahydro-1 H-4,7-methanoindazol-3-yl)pyridine), were synthesized from camphor derivatives of a bis(pyrazolylpyridine), tridentate nitrogen-donor chelate system, giving [Rh ^III (H ₂ L*)Cl ₃ ] (1a) and [Rh ^III (Me ₂ L*)Cl ₃ ] (1b). A rhodium(III) terpyridine (terpy) ligand complex, [Rh ^III (terpy)Cl ₃ ] (1c), was also synthesized. By single-crystal X-ray analysis, 1b crystallizes in an orthorhombic P2 ₁ 2 ₁ 2 ₁ system, with two molecules in the asymmetric unit. Tridentate coordination by the N,N,N-donor localizes the central nitrogen atom close to the rhodium(III) center. Compounds 1a and 1b were reactive toward l-methionine (l-Met), guanosine-5'-monophosphate (5'-GMP), and glutathione (GSH), with an order of reactivity of 5'-GMP > GSH > l-Met. The order of reactivity of the Rh ^III complexes was: 1b> 1a > 1c. The Rh ^III complexes showed affinity for calf thymus DNA and bovine serum albumin by UV-vis and emission spectral studies. Furthermore, 1b showed significant in vitro cytotoxicity against human epithelial colorectal carcinoma cells. Since the Rh ^III complexes have similar coordination modes, stability differences were evaluated by density functional theory (DFT) calculations (B3LYP(CPCM)/LANL2DZp). With (H ₂ L*) and (terpy) as model ligands, DFT calculations suggest that both tridentate ligand systems have similar stability. In addition, molecular docking suggests that all test compounds have affinity for the minor groove of DNA, while 1b and 1c have potential for DNA intercalation.