No abstract

No abstract

The hollow cavities of coordination cages can provide an environment for enzyme-like catalytic reactions of small-molecule guests . We report here a new example (catalysis of the Kemp elimination -reaction of benzisoxazole with hydroxide to form 2-cyanophenolate) in the cavity of a water-soluble M8L12 coordination cage, with two features of particular interest. Firstly, the rate enhancement is amongst the largest so far observed: at pD 8.3, kcat/kuncat is 2 x 10 5 , due to the accumulation of a high concentration of partially desolvated hydroxide ions around the bound guest arising from ion-pairing with the 16+ cage. Secondly, the catalysis is based on two orthogonal interactions: (i) hydrophobic binding of benzisoxazole in the cavity, and (ii) polar binding of hydroxide ions to sites on the cage surface, both of which were established by competition experiments. Hundreds of turnovers occur with no loss of activity due to expulsion of the hydrophilic, anionic product. 2Since the realisation that hollow molecular container molecules can accommodate guest molecules in their central cavities, 1-3 their ability to modify the reactivity of their bound guests has been of great interest. [4][5][6][7][8] Well known examples include Cram's stabilisation of highly-reactive cyclobutadiene; 4 Fujita's 'ship-in-a-bottle' synthesis of cyclic silanol oligomers; 5 Nitschke's stabilisation of P4 in a tetrahedral cage; 6 and the demonstration of unusual regioselectivity in a Diels-Alder reaction when the two reacting molecules are co-confined in a host cavity. 7 The ultimate expression of this behaviour is efficient catalysis of a reaction occurring in the cavity of a container molecule. 8 These synthetic systems have the potential to achieve the selectivity and catalytic rate enhancements displayed by biological systems. For example, artificial container molecules provide relatively rigid and hydrophobic central cavities that may mimic binding pockets in enzymes. These containers may be purely organic hydrogen-bonded assemblies (such as Rebek's 'softball' dimer) 9 or may be metal-ligand polyhedral coordination cages [such as Fujita's Pd6/tris(pyridyl)triazine cage]. 10 Coordination cages offer particular promise in this field because of the ease with which they can be formed by a self-assembly process from very simple component parts, using the predictable coordination geometries of metal ions to provide the three-dimensional ordering of the components which generates the necessary cavity. [1][2][3]8,11,12 In order for a container molecule to act as an efficient catalyst it needs to (i) recognise and bind the guest(s); (ii) accelerate the reaction by increasing the local concentration of reactants and/or stabilising the transition state; and (iii) expel the product to allow catalytic turnover. 8 Guest binding in cage cavities has been very well studied and is becoming a mature field, 1 to the extent that a modeling tool for quantitative prediction of guest binding has recently been reported by us. 13 For a reaction of t...

The concept of 'non-innocence' in coordination chemistry has been around for several decades but is still a topical subject because of its relevance to helping us understand the redox, magnetic and spectroscopic properties of metal complexes. This article describes recent work with two quite different classes of polynuclear complex, based on ruthenium-dioxolene or tris(pyrazolyl)borato-molybdenum fragments, which show non-innocent behaviour. In particular the consequences of non-innocent behaviour for (i) metal-metal electronic interactions in dinuclear complexes, and (ii) optical spectroscopic properties in electrochromic complexes, are highlighted.

Many naturally occurring systems show us how multi-component supramolecular assemblies can generate useful functional behaviour. In this article the problems and limitations associated with achieving such behaviour in artificial multi-component assemblies is discussed, together with two examples of functions in artificial supramolecular assemblies based on (i) host-guest chemistry in cavities of cages, and (ii) light-harvesting in multi-chromophore arrays. Important challenges for the future are summarised.