Download Lewis Acid receptors

Lewis Acid receptors
X-ray crystal of 4.69
Comparison of “proton sponge” and
“hydride sponge”
Lewis Acid Receptors
OH-
log K11 = 6.3 (CHCl3)
Melaimi and Gabbai, JACS 2005, 127, 9680-9681
Sensing fluoride ion in water
Kim and Gabbai, JACS, 2009, 131, 3363–3369
Increasing hydrophobicity
R = Ph
R = Me, K = 840 M-1
R = Et, K = 2500 M-1
R = n-Pr, K = 4000 M-1
R = Ph, K = 10,500 M-1
Measured in H2O/MeOH (9/1 vol.; 9 mM
pyridine buffer, pH 4.6).
Anticrowns
A crown ether analogue made up from electron acceptor residues, as
opposed to Lewis bases in the conventional crown ethers, can be thought of
as an anticrown because of its opposite complexation behavior—affinity for
Lewis basic anions instead of Lewis acids such as alkali metal cations.
Chloride inclusion within
the carborane-based
mercura[12]crown-4
F-
Cl-
Metal Containing Receptors
Anion receptors based on metal centers are a well established class of anion
binding compound and can be classified into three broad categories:
• Those in which an inert or labile metal centre plays a structural role
• Those in which an inert or labile the metal is a key component of the anion
binding site
• Those in which an inert or labile metal acts as part of a redox, luminescent or
colorimetric reporter or sensing group
Use of an inert Ru(II) center to organize an anion chelate ligand
Labile coordination compounds are not true anion hosts in the conventional
sense. Instead, they fall into the category of self-assembly and are frequently
templated by anions, cations or both.
Self-assembly of a labile nitrate-binding Ag(I) complex and its evolution
in the presence of excess nitrate.
Organometallic Receptors
Cyclic voltammetric waves (a) in the
absence and (b) in the presence of Cl-.
Br-
Cyclic Voltammetry of Supramolecular Compounds
(a) Potential versus time profile in a cyclic voltammetric experiment. (b) The
resulting current versus potential trace (voltammogram) for a reversible redox
process. ipa = current for anodic peak, ipc = cathodic (assuming scan to positive
potential), E electrode potential, usually very close to the formal redox potential
for the redox process under study.
Schematic diagram of the experimental apparatus for
a cyclic voltammetric experiment
The working electrode is the electrode at which the electrochemical process
being investigated takes place. The reference electrode is the electrode with a
constant potential that is used as the reference standard against which the
potentials of the other electrodes in the cell are measured. The auxiliary
electrode serves as a sink for electrons so that current can be passed from the
external circuit through the cell.
Redox-switchable cation-binding cryptands
Y3+ and Eu3+ binding lead to
anodic shifts by 326 and 302
mV respectively, whereas
Na+ binding results in only a
70 mV anodic shift.
Venus flytrap anion sensor
Cl-
Simultaneous cation and anion receptors
Schematic illustrating the various states that ions are generally
found in solvents, i.e. (a) solvated ion pairs, (b) contact ion pairs
and (c) aggregated contacted pairs.
In real-world applications, non-competitive counter-ions are not generally
encountered and hence inter-ion competition can be significant. Also,
salts rarely, if ever, exist as separate ions unless the medium is highly
solvating. Therefore, the most effective ionic recognition strategy is to
design a receptor capable of explicitly recognizing both the anion and
cation of an ion pair (either as a contaction pair or in different regions of
the receptor) within the same molecular host.
Schematics of three different types of simultaneous receptors
Cascade receptor
Ditopic receptor
Zwitterion receptor
• Cascade receptors
When more than one metal ion (cation) coordinates to a particular ligand
(often a Schiff base or macrocyclic heteroalkane) in a well-defined geometry
and the anionic species then coordinates to the metal centre, this complex is
known as a casacade complex.
Dioxygen binding by haemocyanin
• Ditopic receptors
Ditopic ion-pair receptors involve the binding of ion pairs, either as contact or
separated ion pairs with a separate binding site for the cation and another site
for the anion.
Alkali-metal chloride transport (symport)
through a membrane
• Zwitterion receptors
Zwitterionic receptors differ from the two previous examples because the
charges are both on the same molecule, hence rendering the receptor’s
overall charge neutral.
Chiral discrimination of amino acid derivatives by the zwitterions
(+)-tubocurarine (2.113)
A zwitterion duplex involved in templation of a condensation reaction
between complementary components to give rudimentary self-replication.
• Cation and neutral simultaneous receptors
The X-ray structure and chemical diagram of the receptor 2.116,
binding Na+ and a single molecule of toluene within the cavity.