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Universityof Durham

Department of Chemistry

Dr. J. A. Gareth Williams



Metal complexes of macrocyclic ligands

Applications of macrocyclic metal complexes are numerous.  Our own interest lies primarily in:


1.  New tetra-azamacrocyclic ligands

The macrocycles cyclen and cyclam {[12-ane]N4 and [14-ane]N4} have been known for several decades, and their complexation chemistry with a large variety of metal ions has been studied thoroughly.  Such macrocyclic ligands often lead to complexes with enhanced thermodynamic and kinetic stability with respect to metal ion dissociation, compared to their open-chain analogues.


 

Metal ions with a preference for coordination numbers > 4 will require further ligands to be bound (apart from the four nitrogen atoms of the macrocycle), and these may be provided by the functionalisation of the macrocycle with additional pendent coordinating groups.  This leads to higher-dentate ligands whose properties and selectivity for certain metal ions over others may be quite different from those of the unsubstituted parent macrocycles.

During recent work in our group, cyclam-based macrocycles incorporating two additional coordinating groups (eg. pyridyl groups) have been prepared via di-N-alkylation.  Since cyclam has C2 symmetry, there are three different isomers for a di-N-substituted system, the 1,4-, 1,8- and 1,11-functionalised ligands.  We have been investigating the effect of this isomerism on the structures of the complexes formed with various transition metal ions (reference 1).  For example, 1,8- and 1,11-bis(pyridylmethyl) cyclam have been synthesised, and it turns out that the stuctures of the  complexes they form with copper(II) are completely different!


 

Below:  Molecular structures of the copper(II) complexes formed by the above ligands in the solid state (hydrogen atoms are omitted for clarity).  The structures were determined by X-ray crystallography in the laboratory of Professor J.A.K.Howard at Durham.

 

The nickel(II) complexes are different again.  For example, whilst the copper complex of the 1,8-ligand has pyridine groups occupying the axial positions with the four nitrogens of the macrocycle in the orthogonal plane, nickel forms a very different complex, in which the pyridine groups coordinate in mutually cis positions (reference 2).
 
 

        In some closely related work, we have been investigating the chemistry and complexation properties of macrocyclic dioxotetraamines.  These macrocycles contain two amino nitrogens and two amides, for example the three macroycles below, which we refer to (non-systematically!) as gem-, cis- and trans-dioxocyclam.


 

As with cylam and cyclen, we are able to alkylate the amino nitrogens with additional coordinating groups to obtain new hexadentate ligands.  They are able to bind to metals like copper(II) and nickel(II) with simultaneous dissociation of the two amide protons, such that metal binding is highly pH-sensitive and reversible (- a very useful property for metal-sensing applications).  We have crystallised the copper(II) complex of a functionalised trans system at neutral and basic pH, and found very different stuctures according to whether just one or both of the amides are deprotonated.  See reference 1 for more details!
 
 

Selected publications on macrocycle syntheses and complexation with d-block metals:

  1. A.E. Goeta, J.A.K. Howard, D. Maffeo, H. Puschmann, J.A.G. Williams and D.S. Yufit, Copper(II) complexes of the isomeric tetraazamacrocyclic ligands 1,11- and 1,8-bis(2-pyridylmethyl)-1,4,8,11-tetraazacyclotetradecane and of the 1,4,8,11-tetraazacyclotetradecane-5,12-dione analogue at neutral and basic pH, J. Chem. Soc., Dalton Trans., 2000, 1880.Available on-line here.
  2. A.S. Batsanov, A.E. Goeta, J.A.K. Howard, D. Maffeo, H. Puschmann and J.A.G. Williams, Nickel(II) complexes of the isomeric tetraazamacrocyclic ligands 1,11- and 1,8- bis(2-pyridylmethyl)-cyclam and of a structurally constrained N4,N8-methylene bridged analogue, Polyhedron, 2001, 20, 981-986.  Available on-line here.


2.  Luminescent macrocylic metal complexes for use as sensors

We are studying metal ions which have emissive metal-centred excited states (d-d or f-f), especially those where the natural lifetime of luminescence is long.  These include the first row transition metal ion chromium(III), and several of the lanthanide(III) ions, especially europium(III) and terbium(III), which emit in the visible region of the spectrum.  Although the natural (i.e. theoretical!) lifetimes of these ions are long (of the order of milliseconds), the aqua ions are quenched efficiently under ambient conditions by non-radiative processes.  Incorporation into a macrocycle can provide the rigidity and protection necessary to inhibit these competitive processes: macrocyclic complexes of these metal ions may emit sufficiently strongly to render them attrcative for use in sensors amenable to time-resolved detection procedures.


What is a luminescent sensor ?  Why bother with metal complexes ?

If you haven't already looked at our brief introduction to the concepts then click here!


Owing to the very low extinction coefficient of the lanthanides, direct metal excitation is inefficient and it is preferable to sensitise the excited state via a suitable chromophore covalently linked to the ligand.  This chromophore needs to act as an "antenna", absorbing light very strongly in a suitable region of the spectrum, and transferring the energy of the absorbed light to the excited state of the lanthanide ion.


 

Ultimately, the efficiency of light-emission (as measured by the quantum yield of luminescence flum) depends on the triplet yield of the chromophore (fT), the efficiency of energy transfer hET and the efficiency of metal centred luminescence hLn:

flum  =  fThEThLn

Currently, we are looking at ways to optimise the overall efficiency of the process.  hLn can be maximised by complexing the metal ion to an octadentate ligand which protects it efficiently from the deactivating effect of solvent water molecules.  The quantities fT and hET depend on the choice of chromophore.  We have found that aryl ketones are attractive from this point of view.  For the complex shown below which incorporates benzophenone as sensitiser, fT is unity, which leads to particularly impressive flum values of 0.1 (±0.01) and 0.27 (±0.03) for the europium and terbium complexes respectively (air-equilibrated aqueous solution, 293K).  All four macrocycle nitrogens bind to the metal ion, as do the three carboxylates and the amide oxygen, leading to outstanding thermodynamic and kinetic stability with respect to metal ion dissociation and excellent protection from solvent water molecules.


 
 

Selected publications on luminescent macrocyclic metal complexes:

  1. A. Beeby, L.M. Bushby, D. Maffeo and J.A.G. Williams, Intramolecular sensitisation of lanthanide(III) luminescence by acetophenone-containing ligands: the critical effect of para-substituents and solvent, J. Chem. Soc., Dalton Trans., 2002, 48-54.  Available on-line here.
  2. A. Beeby, L.M. Bushby, D. Maffeo and J.A.G. Williams, The efficient intramolecular sensitisation of terbium(III) and europium(III) by benzophenone-containing ligands, J. Chem. Soc., Perkin Trans. 2, 2000, 1281-1283.  Available on-line here.
  3. A. Beeby, S.W. Botchway, I.M. Clarkson, S Faulkner, A.W. Parker, D. Parker and J.A.G. Williams, Luminescence imaging microscopy and lifetime mapping using kinetically stable lanthanide(III) complexes,  J. Photochem. Photobiol. B, 2000, 57, 83-89.
  4. C.L. Maupin, D. Parker, J.A.G. Williams and J.P. Riehl, Circularly polarised luminescence from chiral octadentate complexes of Yb(III) in the near-infrared, J. Am. Chem. Soc., 1998, 120, 10563-10564. Available here for JACS subscribers.
  5. D. Parker, P.K. Senanayake and J.A.G. Williams, Luminescent sensors for pH, pO2, halide and hydroxide ions using phenanthridine as a photosensitiser in macrocyclic europium and terbium complexes, J. Chem. Soc., Perkin Trans. 2, 1998, 2129-2139. Available on-line here.
  6. D. Parker and J.A.G. Williams, Taking advantage of the pH and pO2 sensitivity of a luminescent macrocyclic terbium phenanthridyl complex, Chem. Commun., 1998, 245Available on-line here.
  7. D. Parker, K. Senanayake and J.A.G. Williams, Luminescent chemosensors for pH, halide and hydroxide ions based on kinetically stable, macrocyclic europium-phenanthridinium conjugates, Chem. Commun., 1997, 1777. Available on-line here.
  8. D. Parker and J.A.G. Williams, Getting excited about lanthanide complexation chemistry, J. Chem. Soc., Dalton Trans., 1996, 3613-3628.


This work has been funded by:



 


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