Solvent effects on host-guest complexation | SpringerLink

2 downloads 0 Views 516KB Size Report
Apr 4, 1994 - Solvent effects on host-guest complexation. Authors; Authors and affiliations. J. N. Spencer; J. E. Mihalick; I. M. Paul; W. J. Nicholson; T. J.
Journal of Solution Chemistry, Vol. 23, No. 6, 1994

Solvent Effects on Host-Guest Complexation J. N. Spencer, 1,2 J. E. Mihalick, 1,3 I. M. Paul, 1 W. J. Nicholson, 1 T. J. Nicholson, 1 X. Ke, 1 Q. He, 1 F. J. Carter, 1 S. E. Daniels, l L. J. Fenton, 1 J. L. Ealy, 1 S. Puppala, 1 and C. H. Yoder 1 Received April 4, 1994 Complex formation between 15-crown-5 and malononitrile was studied in twelve solvents by calorimetry. Thermodynamic parameters for the crown ether adduct were determined and used in a LFER analysis to ascertain the solvent effects on the complexation process. Enthalpy of solution data show that malononitrile is solvated by electron pair donation by the solvents and the crown ether is solvated by donating electron pairs to the solvents. The complex is more solvated than the monomers. KEYWORDS: Solvation; crown ether; thermodynamic parameters; malononitrile.

1. I n t r o d u c t i o n

Perhaps the first correlation of equilibrium constants for hostguest complexes with solvent properties was reported by Schneider et al.~ 1) using mixed solvents generally containing water and methanol. They found a linear correlation with solvophobicity parameters and log K. Chapman and Still(2) determined equilibrium constants for the complex formation between the five-membered lactam, 2-pyrrolidinone, and imidazole in organic solvents. They found that binding was favored by solvents which weakly solvated the uncomplexed components or strongly solvated the complex. 1Department of Chemistry, Franklin and Marshall College, Lancaster, PA 17604. 2To whom correspondence should be addressed. 3Current address: Department of Chemistry, University of Wisconsin - Oshkosh, Oshkosh, WI 54901. 711 0095-9782/94/0600-0711507.0040 9 Plenum Publishing Corporation

712

Spencer et al.

In a recent study of the effect of solvent on host-guest complexation, Smithrud and Diederich determined the equilibrium constants for inclusion complexes formed by a macrobicyclic cyclophane host (polar) and a pyrene guest (apolar) in water and 17 organic solvents.(3) The correlation between solvent polarity and the free energy of host-guest binding supported the conclusion that "upon complexation these solvent cages around the ... host and guest molecules break down and the solvent molecules are released into the bulk which represents a less ordered state." This is one of the first suggestions that destruction of a solvent cage may be as important in organic as in aqueous media. None of these studies include the enthalpy and entropy contributions from which solvent effects can be best understood. The present work was undertaken to determine the enthalpy and entropy components of solvent effects on the host-guest complex of 15crown-5 and malononitrile.

2. Experimental Two models of calorimeters were used in this study, a Tronac Model 450 isoperibol calorimeter and a Parr solution calorimeter Model 1451 thermistor bridge. The instruments were enclosed in glove bags purged with argon to maintain a dry atmosphere. The calorimeters were calibrated by reaction of tris(hydroxymethyl)aminoethane with HC1 and by enthalpy of solution measurements on known systems such as KC1 and pyridine in H20. The procedures for data collection have been described34) Reagents were purchased in the highest purities available. Malononitrile was sublimed under reduced pressure. Concentrations of malononitrile ranged from 0.03 to 0.05M, while crown ether concentrations varied from 0.01 to 0.1M. The standard state is 1M for all systems. The data analysis has been previously described35) The resulting equilibrium constants and enthalpies for the complexation reactions, as well as enthalpies of solution at infinite dilution for both host and guest, are given in Table I. The reported uncertainties are based on a least-squares fitting of the data.

Solvent Effects on Host-Guest Complexation

713

Table I. Thermodynamic Data For Complexation of 15-Crown-5 ond Malononitrile Solvent

K

Ethyl Acetate 2.2+0.3 1-Propanol 3.5_+0.1 N,N-Dimethylacetamide 0.7_+0.1 Benzene 12.0• Toluene 17.5-+1.5 Dioxane 1.5_+0.3 Cyclohexanone 1.6-+0.3 Formamide Methanol 0.65_+0.1 THF 1.6_+0.1 1,2-Dichloroelhane 3.8-+0.6 Acetonitrile 1.0-+0.2

-Aft"

-A~S b

Asol~Hm