Cooperativity and the Chelate Effect

Hancock, R. D., Chelate ring size and metal ion selection , J. Chem. Ed., 1992, 69, 615-621 Ercolani, G., Assessment of cooperativity in self-assembly , J. Am. Chem. Soc., 2003, 125, 16097-16103. 

The chelate effect is well known in coordination chemistry and relates to the observation that metal complexes of bidentate ligands (such as 1,2-diaminoethane, en) are significantly more stable than closely 

Strained ring Oplimum geometry Increasing flexibiiity-less emropically favourable for larger cations 

On statistical grounds it can be shown that Equation 1.25 holds true. Equation 1.25 implies that the binding constant for each added ligand is less than the previous one. In fact successive equilibrium 

From Equation 1.25 we can derive Equation 1.26. The quantity may be used as a measure 

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Lewis acidic hosts (Section V.A) illustrated important theoretical concepts such as the chelate effect and binding cooperativity, which have now been shown to exist for anion as well as cation binding. This work has also resulted in the crystallographic determination of eye-catching solid state receptor-anion complexes, while heteroelement NMR has allowed an accurate means of probing the solution phase structure of these complexes. Already, multinuclear tin systems are being built into functioning anion selective electrodes. 

We conclude by remarking that self-assembly ultimately rests on the chelate effect, possibly amplified by multivalency and cooperativity. No doubt, these will be the major themes in the field of supramolecular chemistry in the next years, influencing the approach of supramolecular chemists to nanotechnology. 

Jiang W, Nowosinski K, Low NL, et al. Chelate cooperativity and spacer length effects on the assembly thermodynamics and kinetics of divalent pseudorotaxanes. J Am Chem Soc. 2012 134 1860-1868. 

Another level of cooperativity can be observed for the chelate effect in terms of bonding of additional chelating ligands, as seen in the examples below. In coordination chemistry, a simple form of cooperativity can be observed in the successive complex formation constants (ATml,) for certain metal-ligand complexation reactions (9 and 10). 

We have extended application of the Hill formalism to poly-dentate ligands, polyamines competing with tetrahydrofuran for solvation of the sodium cation (118). The Hill plots obtained are no longer linear, denoting cooperativity. Entries of the first and of the second diamine molecules into the sodium coordination shell are independent and equiprobable steps Kj = K-j and K2 X. This permits also a direct determination of the chelate effect (118). 

Systematic variation in chirality at both the chelate backbone and the terminal groups revealed a remarkable effect on the enantioselectivity of the catalysts. Ligand (109) generates chiral cooperativity between the backbone and the terminal moieties in Pt-catalyzed hydroformylation. The highest ee (65%) for 2-phenylpropanal was found for the ligand R-bis(S)-(110) in combination with Pt. The chemoselectivities with all ligands described in association with Pt were rather low. The comparative... 

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