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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)
[18]crown-6 and (b) its sulfur
analogue (nitrate salts).
Products arising from the reactions of transition metal dications with
[18]crown-6, [15]crown-5 and [12]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 Ph3[12]ane-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 calix[4]arene.
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+).