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Macrocyclic Ligands: Functional Host Guest Systems.

Macrocyclic Ligands: Functional Host Guest Systems. Post Graduate Lecture Course February 2000 Lecture 4. Metal Complexes of Macrocyclic Ligands. M n+ + L  [M(L)] n+ [M(L`)] n+ + L  [M(L)] n+ + L` L = macrocyclic Ligand, L` = acyclic analogue

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Macrocyclic Ligands: Functional Host Guest Systems.

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  1. Macrocyclic Ligands: Functional Host Guest Systems. Post Graduate Lecture Course February 2000 Lecture 4

  2. Metal Complexes of Macrocyclic Ligands Mn+ + L  [M(L)]n+ [M(L`)]n+ + L  [M(L)]n+ + L` L = macrocyclic Ligand, L` = acyclic analogue Both equilibria lie far to the right hand side. 1st is a kinetic effect - slow ligand dissociation 2nd manifestation of macrocyclic effect

  3. The Macrocyclic Effect • Stability constant for macrocycle is ca 104 times that of linear analogue. Cabbiness & Margerum, J. Am. Chem. Soc., 1970, 92, 2151.

  4. Analogous to chelate effect? • Expect some increase in stability as number of chelate rings increases • In this case stabilisation is at least an order of magnitude greater than expected Term MACROCYCLIC EFFECT coined. Results in….. • Enhanced thermodynamic stability • Kinetic inertness • Metal ion selectivity

  5. Thermodynamic Data • Generally measure stability constants: Mn+ + L  M(L)n+ K = [M(L)n+] [Mn+][L] G = -RTlnK = H - TS

  6. Origin of the macrocyclic effect: enthalpy vs entropy Early data contradictory - used temperature dependence of K to obtain H. Resolved by calorimetric determination of H for linear and macrocyclic ligands.

  7. Consider….. [Ni(Loc)]2+ + Lmac [Ni(Lmac)]2+ + Loc Loc/Lmac G H TS L1/L2 -2.43 5.1 7.4 L3/L4 -21.05 5.3 26.4 L5/L6 -15.69 3.5 19.2 L7/L8 -33.67 -20.5 13.2

  8. There are many possible contributors to both enthalpy and entropy. • Consider the Born-Haber cycle…. • Each process (1-5) has G, H, TS contributions.

  9. Stability: Key Factors • Ligand Pre-organisation • Relief of dipole-dipole interactions • Differential Ligand Solvation • Intrinsic Basicity

  10. Size Selectivity • Example: Ion(r+,pm) log K -G -HTS Na+(95) 4.38 25.1 31.4 -6.3 K+(133) 6.06 34.7 56.1 -21.3 Rb+(148) 5.32 30.6 50.7 -20.1 Cs+(169) 4.79 27.2 47.3 -20.1

  11. Factors in Metal ion Selectivity • Cavity size • Ligand flexibility • Number of Donors • Identity of donors • Size of chelate rings formed • Relative positioning of donor atoms

  12. Selectivity best for alkali metal ions with O-donor ligands • Selectivity often poorer for transition metals • Selectivity improved with rigid ligands such as cryptands • Can tailor donors to suit the metal ion

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