(η5-Pentamethylcyclopentadienyl)iridium(III) complexes of purine nucleobases and nucleotides: a comparison with (η6-arene)ruthenium(II) and (η5-pentamethylcyclopentadienyl)rhodium(III) species

Treatment of [(η5-C5Me5)Ir(H2O)3](CF3SO3)2 with adenine (AH) and 9-ethyladenine (9-EtA) affords the respective tetra- and trinuclear complexes [{(η5-C5Me5)Ir(A)}4](CF3SO3)4 (1) and [{(η5-C5Me5)Ir(9-EtAH−1)}3](CF3SO3)3 (2), whose structures were established by X-ray analysis. The former compound exhibits a μ-1κN9:2κ2N6,N7 coordination mode for the bridging adeninate ligands, the latter a μ-1κN1:2κ2N6,N7 binding pattern, associated with a wide IrN7C8 angle of 149° and substantial respectively upfield (δ 7.65) and downfield (δ 8.82) shifts for the purine ring protons H2 and H8. Exclusive formation of analogous diastereomeric trimers is likewise observed in the pH* range 3.5–9.0 for equimolar equilibrium systems of [(η5-C5Me5)Ir(H2O)3]2+ with 5′-AMP2− and 5′-ATP4−. Reaction of this organometallic fragment with guanine (GH) and hypoxanthine (HxH) also provides tetramers [{(η5-C5Me5)Ir(B)(H2O)}4](CF3SO3)43 (B=G) and 4 (B=Hx) in which, however, O6 only participates in outer-sphere coordination through O6⋯H–O interactions to the water ligand. The μ-N7,N9 coordination mode in 3 and the analogous complex [{(η6-C6H6)Ru(Hx)(H2O)}4](CF3SO3)4 (5) was confirmed by X-ray analysis. Two cyclic (presumably trimeric) oligomers with respectively μ-1κN9:2κ2O6,N7 and μ-1κN1:2κ2N7,O(H2O) binding patterns are present at an approximately 1:1 ratio in weakly acid equimolar equilibrium systems of [(η5-C5Me5)Ir(H2O)3]2+ with 5′-IMP2− and 5′-ITP4−. Phosphate coordination is absent for this fragment in the presence of purine nucleoside 5′-triphosphates, in striking contrast to (η6-C6H6)Ru(II) and (η5-C5Me5)Rh(III), whose pH-dependent reaction behaviour is reported for comparison purposes.