New fac-[ReI(CO)3(Ltri)]Y complexes of linear N,N′,N′′-tridentate ligands (Ltri) with terminal imidazolyl or pyridyl ring (N and N′′) and central tertiary sulfonamide (N′) N-donors. Effects of size and substituents of the terminal N-donor rings and the rarely found direct sulfonamide N to metal bond

Abstract

fac-[Re(CO)3Ltri]Y (Y = ClO4- or PF6-) complexes were prepared with N,N',N''-donor tridentate ligands (Ltri), N(SO2R)1,1′-Me2dma and N(SO2R)Mendpa (R = Me or Tol). The two new N(SO2R)1,1′-Me2dma ligands are derivatives of the parent ligand, N(H)1,1′-Me2dma (bis(1-methyl-2-methylimidazolyl)amine), by having the central N within a tertiary sulfonamide group instead of a secondary amine group. In addition to having a tertiary sulfonamide group, the N(SO2R)Mendpa ligands differ from the parent ligand, N(H)dpa (2-picolylamine), by having Me groups at the 6,6′- (n = 2) or 3,3′,5,5′- (n = 4) positions of the pyridyl rings. Ltri is facially coordinated with a Re—N' (sulfonamide N) bond of normal length. Only the Ltri = N(SO2R)6,6′-Me2dpa complexes exhibit ligand dissociation over time in DMSO‑d6, a finding attributable to Re—N and Re—N'' bond weakening caused by the steric effect of the pyridyl 6,6′-Me groups projecting toward the coordination sphere, while the Ltri = N(SO2R)1,1′-Me2dma and N(SO2R)3,3′,5,5′-Me4dpa complexes are indefinitely robust in solution. In contrast to the similar behavior of fac-[Re(CO)3Ltri]Y complexes for Ltri = N(SO2R)1,1′-Me2dma and N(SO2R)3,3′,5,5′-Me4dpa, only the latter dissociates over time in DMSO‑d6 for [Pt(“Ltri”)Cl2] complexes (unpublished), where the sulfonamide N in “Ltri” is not bound, forming a rare 8-membered chelate ring. On the basis of NMR and X-ray crystallographic data, we attribute the high robustness of [Pt(N(SO2R)1,1'-Me2dma)Cl2] mainly to the presence of the terminal N imidazolyl donors. The smaller imidazolyl moieties for Ltri = N(SO2R)1,1'-Me2dma (relative to the pyridyl rings in the dpa ligands) make dma ligands a better platform for developing fac-[99mTc(CO)3Ltri]Y diagnostic agents and also most probably for developing Pt(II) anticancer drugs.