EFFECT OF LIGAND CONSTRAINTS UPON THE STABILITIES AND POTENTIALS OF MACROCYCLIC POLYTHIAETHER COMPLEXES - COPPER(II) AND COPPER(I) COMPLEXES WITH CYCLOHEXYL AND PHENYL DERIVATIVES OF [14]ANES(4) IN WATER, 80-PERCENT METHANOL, AND ACETONITRILE

Synthetic procedures are described for the preparation of all 11 of the possible derivatives of the 14-membered macrocyclic tetrathiaether [14]aneS(4) (1,4,8,11-tetrathiacyclotetradecane), in which one or both of the ethylene bridges have been replaced by 1,2-benzene and/or cis- or trans-1,2-cyclohexane. The visible spectra of the copper(II) complexes formed with the derivatized ligands have been recorded in aqueous solution, in 80% methanol, and in acetonitrile; and the Cu(II/I)L potentials have also been determined in these three solvents. Limited solubility of the uncomplexed ligands has hindered the measurement of the Cu(II)L or Cu(I)L stability constants in aqueous solution. However, it has proved possible to measure the Cu(II)L stability constants in 80% methanol with all ligands except the dibenzo derivative using a spectrophotometric approach; and the Cu(I)L stability constants have been measured by pulsed square-wave cyclic voltammetry in acetonitrile. The stability constants for the Cu(I)L complexes in 80% methanol and the Cu(II)L complexes in acetonitrile, though too large to be measured directly by these approaches, were calculated indirectly for each ligand system by means of the Nernst equation. The trends in the stability constant values in these two solvent matrices are relatively uniform with the Cu(II)L stability constants for the cyclohexyl and phenyl derivatives increasing by about 10(7)-fold and 10(8)-fold, respectively, on going from 80% methanol to acetonitrile while the Cu(I)L stability constants decrease by about 10(8)-fold. These solvent dependencies are primarily attributed to the differing abilities of the two solvents to coordinate with the two oxidation states of copper. For the Cu(II) complexes with the derivatized [14]aneS(4) ligands, the following trends are noted: (i) incorporation of a single cyclohexyl group increases the stability constant by 1.5 orders of magnitude as a result of a more favorable enthalpy; (ii) incorporation of a second trans-cyclohexyl group increases the stability constant by more than an additional 1.5 orders of magnitude for a total increase of more than 10(3); (iii) for the cis,cis-dicyclohexyl derivatives, the syn isomer increases the overall stability by 4 orders of magnitude, but the anti isomer yields only a 10-fold increase; (iv) incorporation of a single benzene lowers the stability by about 3 orders of magnitude, while two benzenes lower the stability to the point where measurements could not be obtained. In the Cu(I) complexes, incorporation of either a cyclohexane or benzene increases the stability constant by about 10-fold. The effect of introducing a second cyclohexane ring results in stability constant increases of about 1- to 10-fold with no pattern apparent.