Linear Bidentate Ligands (L) with Two Terminal Pyridyl N‑Donor Groups Forming Pt(II)LCl₂ Complexes with Rare Eight-Membered Chelate Rings

NMR and X-ray diffraction studies were conducted on Pt­(II)­LCl2 complexes prepared with the new N-donor ligands N(SO2R)­Mendpa (R = Me, Tol; n = 2, 4). These ligands differ from N(H)­dpa (di-2-picolylamine) in having the central N within a tertiary sulfonamide group instead of a secondary amine group and having Me groups at the 6,6′-positions (n = 2) or 3,3′,5,5′-positions (n = 4) of the pyridyl rings. The N(SO2R)­3,3′,5,5′-Me4dpa ligands are coordinated in a bidentate fashion in Pt­(N(SO2R)­3,3′,5,5′-Me4dpa)­Cl2 complexes, forming a rare eight-membered chelate ring. The sulfonamide N atom did not bind to Pt­(II), consistent with indications in the literature that tertiary sulfonamides are unlikely to anchor two meridionally coordinated five-membered chelate rings in solutions of coordinating solvents. The N(SO2R)­6,6′-Me2dpa ligands coordinate in a monodentate fashion to form the binuclear complexes [trans-Pt­(DMSO)­Cl2]2(N(SO2R)­6,6′-Me2dpa). The monodentate instead of bidentate N(SO2R)­6,6′-Me2dpa coordination is attributed to 6,6′-Me steric bulk. These binuclear complexes are indefinitely stable in DMF-d7, but in DMSO-d6 the N(SO2R)­6,6′-Me2dpa ligands dissociate completely. In DMSO-d6, the bidentate ligands in Pt­(N(SO2R)­3,3′,5,5′-Me4dpa)­Cl2 complexes also dissociate, but incompletely; these complexes provide rare examples of association–dissociation equilibria of N,N bidentate ligands in Pt­(II) chemistry. Like typical cis-PtLCl2 complexes, the Pt­(N(SO2R)­3,3′,5,5′-Me4dpa)­Cl2 complexes undergo monosolvolysis in DMSO-d6 to form the [Pt­(N(SO2R)­3,3′,5,5′-Me4dpa)­(DMSO-d6)­Cl]+ cations. However, unlike typical cis-PtLCl2 complexes, the Pt­(N(SO2R)­3,3′,5,5′-Me4dpa)­Cl2 complexes surprisingly do not react readily with the excellent N-donor bioligand guanosine. A comparison of the structural features of over 50 known relevant Pt­(II) complexes having smaller chelate rings with those of the very few relevant Pt­(II) complexes having eight-membered chelate rings indicates that the pyridyl rings in Pt­(N(SO2R)­3,3′,5,5′-Me4dpa)­Cl2 complexes are well positioned to form strong Pt–N bonds. Therefore, the dissociation of the bidentate ligand and the poor biomolecule reactivity of the Pt­(N(SO2R)­3,3′,5,5′-Me4dpa)­Cl2 complexes arise from steric consequences imposed by the −CH2–N­(SO2R)–CH2– chain in the eight-membered chelate ring