Unusual Example of Chelate Ring Opening upon Coordination of the 9-Ethylguanine Nucleobase to [Pt(di-(6-methyl-2-picolyl)amine)Cl]Cl

Anticancer-active monofunctional Pt(II) complexes have bulky carrier ligands and bind to G residues in DNA, causing structural distortions. To gain fundamental chemical information on such monofunctional adducts, we assessed the 9-ethylguanine (9-EtG) adducts formed by [Pt(N(H)6,6′-Me2dpa)Cl]Cl (N(H)6,6′-Me2dpa = di-(6-methyl-2-picolyl)amine). 9-EtG added to [Pt(N(H)6,6′-Me2dpa)Cl]Cl to form not only the expected [Pt(N(H)6,6′-Me2dpa)(9-EtG)]2+ monoadduct having syn and anti conformers but also a [Pt(N(H)6,6′-Me2dpa)(9-EtG)2]2+ bisadduct consisting of ΛHT and ΔHT conformers (HT = head-to-tail). For both adducts, the two conformers exist as a dynamic equilibrium mixture. Concomitant with formation of the bisadduct, the binding mode of the N(H)6,6′-Me2dpa ligand converts from tridentate to bidentate. A Pt(II)-bound 6-methyl-2-picolyl chain and the secondary amine constitute the bidentate chelate ring. The other 6-methyl-2-picolyl chain is dangling. The secondary nitrogen is an asymmetric center, and each con...