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)LCl 2 complexes prepared with the new N-donor ligands N(SO 2 R)Me n dpa (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(SO 2 R)3,3',5,5'-Me 4 dpa ligands are coordinated in a bidentate fashion in Pt( N(SO 2 R)3,3',5,5'-Me 4 dpa)Cl 2 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(SO 2 R)6,6'-Me 2 dpa ligands coordinate in a monodentate fashion to form the binuclear complexes [ trans-Pt(DMSO)Cl 2 ] 2 ( N(SO 2 R)6,6'-Me 2 dpa). The monodentate instead of bidentate N(SO 2 R)6,6'-Me 2 dpa coordination is attributed to 6,6'-Me steric bulk. These binuclear complexes are indefinitely stable in DMF- d 7 , but in DMSO- d 6 he N(SO 2 R)6,6'-Me 2 dpa ligands dissociate completely. In DMSO- d 6 , the bidentate ligands in Pt( N(SO 2 R)3,3',5,5'-Me 4 dpa)Cl 2 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-PtLCl 2 complexes, the Pt( N(SO 2 R)3,3',5,5'-Me 4 dpa)Cl 2 complexes undergo monosolvolysis in DMSO- d 6 o form the [Pt( N(SO 2 R)3,3',5,5'-Me 4 dpa)(DMSO- d 6 )Cl] + cations. However, unlike typical cis-PtLCl 2 complexes, the Pt( N(SO 2 R)3,3',5,5'-Me 4 dpa)Cl 2 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(SO 2 R)3,3',5,5'-Me 4 dpa)Cl 2 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(SO 2 R)3,3',5,5'-Me 4 dpa)Cl 2 complexes arise from steric consequences imposed by the -CH 2 -N(SO 2 R)-CH 2 - chain in the eight-membered chelate ring.