The pyridyl group in ligand design for selective metal ion complexation and sensing

Abstract

Factors in polypyridyl ligands that control their thermodynamic metal ion selectivity in aqueous solution, and their use in selective fluorescent sensing, are examined. Preorganization of polypyridyl ligands ranging from bidentate to tetradentate by bridging benzo groups, as are present in 1,10-phenanthroline (phen) compared to 2,2'-bipyridyl (bpy), is discussed. The role of solvation is considered in relation to the relative affinity of ligands containing pyridyl groups for divalent and trivalent metal ions in aqueous solution. The effects of steric clashes between H atoms on polypyridyl ligands in decreasing complex stability are evaluated, as well as the effect of chelate ring size on metal ion selectivity. Phen ligands with other donor groups present at the 2 and 9 positions, such as alcohols, amides, carboxylates, and oximes are discussed. The design of pyridyl-based ligands for the separation of Am(III) from lanthanide(III) ions is considered, as well as ligands for the removal of metal ions such as Cu(II) or Zn(II) in neurological diseases such as Alzheimer's. The design of pyridyl-based fluorescent sensors for selective sensing of metal ions is examined in terms of the role of spin-orbit coupling constants (ζ), paramagnetism, and steric effects in the development of selective fluorescent sensors that operate via chelation enhanced fluorescence (CHEF). It is concluded that for lighter metal ions with smaller ζ values such as Zn(II) and Ca(II), and to a lesser extent Cd(II), that the CHEF effect can be achieved with pyridyl-containing fluorophores that coordinate directly to the metal ion. The way in which steric effects can be used to decrease the CHEF effect in Zn(II) relative to Cd(II) to enable selective sensing of the latter is analyzed. For heavier metal ions such as Hg(II) and Pb(II), because of their large ζ values which quench fluorescence, it is concluded that the fluorophore should be tethered to the metal-binding part of the sensor, and prevented from binding to the metal ion by steric and electronic factors. How Hg(II) can quench the CHEF effect by π-contact with fluorophores such as the anthracenyl group, which at first sight might not seem able to bond with metal ions, is examined.

Rational design of a material for rapid colorimetric Fe2+ detection

“…Various derivatives of 2,2′:6′,2″-terpyridine (terpy) have been developed and implemented as such sensing receptors as summarized in seminal works by Constable [4] and Hancock [5]. Terpy based molecules were utilized to identify various metal cations [5][6][7] and cation-anion pairs [8,9]. To adjust polarity and therefore solubility, as well as to enhance sensing characteristics, terpy motifs have been combined into a variety of organic, organometallic, and supramolecular architectures.…”

(4′-Phenyl-2,2′:6′,2′′-terpyridine-κ3N,N′,N′′)bis(thiocyanato-κN)zinc(II) unknown solvate

“…Polydentate ligands containing N-donor heterocyclic rings are versatile building blocks in the construction of coordination compounds because of their ability to form stable complexes with transition metals and their various coordination modes (Hancock, 2013;Li et al, 2011;Bhaumik et al, 2011). One such ligand is 4 0 -phenyl-2,2 0 :6 0 ,2 00 -terpyridine (pypyt), first synthesized by Constable et al (1990) and used by them for the formation of nickel complexes.…”

Muconato‐bridged Manganese Coordination Polymer exhibiting rare Distorted‐trigonal Prismatic Coordination Arrangement

“…The use of 2,2Ј-bipyridine as ancillary ligand had become relevant in our earlier studies on coordination polymers. [8] Therefore, we have been using one of the most studied nitrogen donor ligand, [9] and just varying the alkyl-substituents on it, in order to verify the possible influence of the steric hindrance on the transition metal coordination spheres, dimensionality, and crystalline structures of coordination polymers. As it is well known, transition metal six-coordination continues to be ruled by octahedral arrangements.…”