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• Soft Ligands for Soft Metal Ions Properties of alkali metal cations • Hard, nonpolarizable spheres • Little fixed preference for particular coordination geometries • Relatively high free energies of hydration • Affinity for highly charged, nonpolarizable (hard) bases. So, how about soft cations? (e.g. Ag+ and many lower oxidiation state transition metals) X-ray molecular structures of the Ag+ complexes of (a) crown-6 and (b) its sulfur analogue (nitrate salts). Products arising from the reactions of transition metal dications with crown-6, crown-5 and crown-4 in aqueous or alcohol solution. Nitrogen and Sulfur Analogues of Cryptands Azamacrocycles: Basicity Effects Tetramethyl cyclam binds metal ions less strongly than its secondary amine analogue because the complexation energy must over come the unfavorable steric repulsions between the methyl groups themselves and also between the methyl groups and the methylene groups of the macrocycle backbone. The five most stable conformers of cyclam (M = transition metal, X = coordinating anion, R H, Me, etc.). Phosphorus–Containing Macrocycles Template synthesis of the triphosphine macrocycle Ph3ane-P3 Mixed Cryptates A CO substrate is sandwiched between a redox-active transition metal and a Lewis acidic alkali metal cation by a cryptand with both hard and soft donor sites. Schiff Base Macrocycles The original Schiff base macrocycles Schiff base reactions (a) 1:1 cyclocondensation templated by small metal ions; (b) larger macrocycles are obtained with larger templates. Torands Torands (from ‘torus’ and ‘ligand’) are a series of extremely rigid ligands based on fused aromatic rings. Calixarenes The descriptive name ‘calixarene’ was coined by C. David Gutsche (Washington University, USA) because of the resemblance of the bowl-shaped conformation of the smaller calixarenes to a Greek vase called a calix crater. Hayes and Hunter stepwise synthesis of a calixarene. Calixarenes as enzyme mimics Conformational aspects of cation inclusion by calixarene A bis(calixarene) cage host for K+ complexation Metal “pumping” by a molecular syringe Siderophores Optimum microorganism growth requires an intracellular concentration in the region of 107 mol dm-3, a factor of about a hundred billion times more concentrated than the solubility-limited concentration of Fe(III). As a result, plants and bacteria require highly effective iron-complexing ligands in order to mobilize Fe3+ and deliver it to the cell. These naturally occurring ligands are termed siderophores (from the Greek meaning ‘iron bearer’), and may be regarded as essential microorganism growth promoters. Trigonal prismatic coordination of Fe3+ in [Fe(enterobactin)]3- Template synthesis of enterobactin Synthetic siderophores Fe(III) specific colorimetric sensing by a squarine-based siderophore Ionophores: natural products that bind metal ions valinomycin X-ray crystal structure of the K+ complex of valinomycin. The carrier (ionophore) encapsulates the alkali metal cation, stripping away most or all of the water molecules bound to it. The carrier then presents a lipophilic surface, moving across the membrane and releasing the ion back into aqueous solution at the other end. The channel is, primarily, an aqueous hole running through the membrane. Ions can traverse swiftly through the channel without losing their solvent sphere throughout most of their journey, as long as the gate (activated by potential changes or hormones) is open and the ion can pass through the selectivity filter (which discriminates between Na+ and K+).