Rotational energy barrier in PtII–N7(purine) bonds by one- and two-dimensional nuclear magnetic resonance spectroscopy

Abstract

Rotational energy barriers about Pt^II-N⁷ purine bonds for cis-[Pt(NH₃)₂L₂][L = 1-methyladenosine, adenosine 5′-mono-, -di- or -tri-phosphate, or guanosine 5′-monophosphate) were evaluated from one-and two-dimensional phosphorus-31 exchange spectroscopy. Two diastereomers in a head-to-tail conformation exist for all the aminopurine complexes in the temperature range 25–60 °C due to slow rotation about Pt–N⁷ bonds. The rotational activation energies for all the 6-aminopurine complexes lie in the range 46–95 kJ mol^–1 while the 6-oxopurine complex exhibits the lowest energy barrier, 25 kJ mol^–1. The two exocyclic amines of the aminopurine complexes in a head-to-tail configuration are positioned above and below the platinum square plane. The increased rotational energy barrier in the AMP compared to that of GMP complexes is attributed to the absence of hydrogen bonding with amines, a direct interaction of the lone-pair electrons of the 6-NH₂ group with the σ- bonding orbitals of the platinum atom, and a decrease in the crystal-field stabilization energy due to the interaction with the filled d_z² orbital. The rotational energy barrier can be taken as the ground-state energy difference between the head-to-tail and head-to-head configurations since the latter is encountered near to the highest potential-energy surface during the conversion of one head-to-tail rotamer into the other. An energy difference >45 kJ mol^–1 may be estimated between the bis(GMP) complex and the AMP analogue in a head-to-head configuration.