Coordinative flexibility in an acyclic bis(sulfonamide) ligand

Abstract

The new acyclic and potentially heptadentate dinucleating ligand, 2,6-bis(N,N-bis-(2-pyridylmethyl)-sulfonamido)-4-methylphenolato (bpsmp(-)) contains two tertiary sulfonamide groups. The sulfonamide donors permit a greater degree of control over the accessibility of mono versus dinuclear complexes compared to their closely related amine-containing counterparts, on account of their relatively weaker donor properties. A series of air-stable dinuclear complexes of Co(II), Mn(II) and Cu(II) containing two auxiliary acetate ligands have been prepared. The absence of acetate in reaction mixtures containing Co(II) and Mn(II) led to mononuclear complexes, with water ligands completing the coordination spheres of the metal ions, even in the presence of large excess of the metal ions. Thus, bridging acetate ligands appear to stabilise the dinuclear structures for the relatively labile Co(II) and Mn(II) ions. A mononuclear complex of V(IV)=O was isolated even in the presence of acetate, possibly because the oxyl groups on each V(IV) prevent formation of a bis-acetato-bridged complex. Reaction of one equivalent of CuCl(2) with bpsmpH led to isolation of two different mononuclear complexes, dependent on the identity of the solvent. The phenol group is coordinated in only one of these complexes. A dinuclear Cu(II) complex was isolated when two equivalents of the metal salt were used in the reaction.

Formation of a Metal-to-Nitrogen Bond of Normal Length by a Neutral Sufonamide Group within a Tridentate Ligand. A New Approach to Radiopharmaceutical Bioconjugation

“…The evidence for angles close to 109.5° (Table 2) clearly indicates that binding to Re I results in sp 2 -to-sp 3 re-hybridization of the tertiary sulfonamide nitrogen. This finding that all angles are close to the tetrahedral value is a common feature in the complexes of many metals in various oxidation states (Ni II , Co II , Mn II , Cu II , La III ) 26,27,32,33 with a bound sulfonamide. One useful approach to appreciating the sp 3 nature of the sulfonamide nitrogen is to determine the distance by which the N lies out of the plane defined by the S atom and the two C atoms attached to N. For complexes 1 , 2 , 4 , and 5 , this out-of-the-plane distance ranges from 0.486–0.515 Å.…”

“…This result and features of structures in the literature (see below) suggest that the angles of the sulfonamide group are quite pliable, a feature that can lead to ligands which exhibit coordinative flexibility. 27 …”

“…Likewise, for complexes with other metals and differing widely in the ligands and geometries, values of 0.38–0.51 Å are typically found. 26,27,32,33 In contrast, for complexes in which the sulfonamide nitrogen is not bound, the out-of-the-plane distances in reported structures typically range from 0.06–0.125 Å. 26,27 …”

“…27 In fact, only in such rare cases is the M–N(sulfonamide) bond length as short as a typical M–N(sp 3 ) distance. 27 In these rare tripodal tetradentate cases, the sulfonamide N and the two flanking N donor atoms linked to it coordinate meridionally, not facially. 27 In the only other case of meridional coordination, the M– N(sulfonamide) bond length is long (2.298 Å).…”

“…In the rare examples in which the M–N(sulfonamide) bond length reflects a strong bond (enforced by the tripodal nature of the ligand), the N–S distance is ∼1.73–1.76 Å. 27 For more common cases in which the M–N(sulfonamide) bond length is much longer than the typical value for the M–N distance of that metal in its oxidation state, the N–S distance is shorter, ∼1.7 Å or less. 27 For complexes in which the sulfonamide is not coordinated, the N–S distance is about 1.62–1.64 Å.…”