Diastereomeric separations and crystal structures of rhodium(III) and iridium(III) complexes containing adenosine and related nucleosides

Six C3 cyclic trinuclear complexes [{M(Cp*)(L)}3]3+ (M = RhIII and IrIII; Cp* = η5–C5Me5) containing adenosine nucleosides [L: adenosine (Hado), 2′-deoxyadenosine (Hdeoado) and 5′-acetyl-2′,3′-isopropylideneadenosine (Haipado)] were prepared and characterized by UV/Vis and circular dichroism (CD) spectra, NMR spectroscopy, electrospray ionization mass spectroscopy and X-ray crystal structure analysis. The isolations of one and/or two diastereomers were successfully carried out for the four systems by second-order asymmetric transformation and/or fractional crystallization. Interestingly, a striking kinetic difference was found between the present RhIII– and IrIII–ado systems. The crystal structure of CCC-[{Rh(Cp*)(ado)}3](CF3SO3)3·2.5H2O·CH3OH revealed that the ado ligand adopts a μ-1κN1:2κ2N6,N7 bridging mode and the three purine rings forming a triangle dome-like cavity can include one methanol molecule into its cavity. The absolute configurations were assigned to these complexes based on the crystal structural result and CD spectral arguments. The RhIII–N-methyl adenosine (HNMeado) system exceptionally gave a di-μ-hydroxy anti-dinuclear structure because of the weak coordination ability of the N(6) donor.