Macrocyclic Ligands: Functional Host Guest Systems.

1
Macrocyclic Ligands Functional Host Guest
Systems.

• Post Graduate Lecture Course
• February 2000
• Lecture 5

2
Kinetic Stability of Macrocyclic Ligand Complexes

• Ligand dissociation
• T1/2 22 days in
• 6.1 M HCl
• 107x slower than linear complex
• Kinetic Macrocycle Effect

3
Factors Affecting Rate of Ligand Dissociation

• Ring Size
• eg for Ni2 complexes of N2O2 macrocycles
• 14 gt 15 gt 16 lt 17
• Metal Ion
• Donor atoms (ligand field strength)
• Ligand flexibility
• pH

4
Redox Behaviour of Macrocycle Complexes

• Macrocyclic ligands support metal ions in unusual
  oxidation states.
• Prearranged donor set
• strong ligand field
• slow ligand dissociation
• Protected metal ion environment

5
High Oxidation States

• Nickel(III) and copper(III) N4-macrocycles
• Cu(Ln)2 ? Cu(Ln)3
• E1/2/V T1/2/s
• L1 1.00 3
• L2 1.03 60
• L3 1.07 0.6
• L4 1.09 1.9

6
Redox properties

• Macrocycles tend to lower oxidation potentials,
  making CuIII, NiIII, PtIV, AgIII etc accessible.
• Also stabilise lower oxidation states.
• eg NiI, CoI, FeI.
• Ligand oxidation/reduction.
• Non-innocence.

7
Example

• Ni(L)2 catalysed epoxidation of alkenes
• Stabilisation of NiIV intermediate
• Kochi, J. Amer. Chem. Soc., 1987, 26, 908.

8
Stabilisation of Unusual Oxidation States

• Stabilisation of RhII, PdIII and AuII.
• M(L)2n ? M(L)2(n1)
• L trithiacyclononane (ttcn)
• Molecular Dewar

9
(No Transcript)
10
Ligand Oxidation/Reduction

• Two types
• Non innocent ligands
• eg porphyrins - donate or accept electrons into ?
  - system
• oxidative dehydrogenation/reductive hydrogenation
• Designed redox active ligands
• organic eg quinones
• organometallic eg ferrocenes

11
Example
12
Designed Redox Active Ligands

• Some examples shown below

13
Sensors

• To sense a substrate and report its presence
• Requires
• A Receptor Site (preferably substrate specific)
• A signalling unit
• should change some property in the presence of
  the substrate
• should preferably do so reversibly
• Communication between sites

14
Redox Sensing of Na Ions
15
Redox Switch Ion Transport
16
A Photochemical Switch

• Complexation of K switches on fluorescence

17
Ion Selective Electrodes

• Incorporation of L1 into an electrode enables
  electrochemical detection of Ag ions.
• Detection Limit 3 x 10-7M
• Selectivity (KM/KAg) gt10-3 in worst case (Hg2)
  and typically 10-5 - 10-6.
• General difficulty in finding
• suitably selective hosts.