EFFECT OF NONCOMPLEXING ANIONS ON COPPER(II) MACROCYCLIC TETRATHIAETHER COMPLEXES - EVALUATION OF DISSOCIATION KINETICS IN AQUEOUS-SOLUTION USING MERCURY(II) ION AS SCAVENGER

As a follow-up to earlier work which indicated that copper(II) macrocyclic tetrathiaether complexes are stabilized by the presence of anions normally considered to be noncomplexing, the dissociation kinetics of Cu(II) complexes with [12]aneS4, [13]aneS4, and [14]aneS4 have been investigated in aqueous solution at 25-degrees-C as a function of perchlorate ion concentration using aquamercury(II) ion as the scavenging species. For the Cu(II)([14]aneS4) complex, additional kinetic studies have been conducted in the presence of tetrafluoroborate, trifluoromethanesulfonate, and nitrate ions. For all systems, the kinetic data are interpreted to represent a combination of the direct Cu(II)L complex dissociation pathway plus a parallel pathway in which the Hg(II) actively displaces the Cu(II) from the ligand, presumably by forming coordinate bonds to the ligand before Cu(II)L is fully dissociated. Both the direct Cu(II)L dissociation rate and the Hg(II) displacement rate decrease markedly upon increasing the concentration of the perchlorate, tetrafluoroborate, and trifluoromethanesulfonate ions but show no dependence upon nitrate ion. For the studies involving variable perchlorate ion concentration, two specific dissociation rate constants (k(CuL) and k(CuLX)) are resolved representing the dissociation of CuL2+ and the anion adduct CuLX+ (where X- represents ClO4-); in addition, two specific mercury(II) displacement rate constants (k(Hg)CuL' and k(Hg)CuLX', both expressed as first-order rate constants after correction for outer-sphere complex formation) are resolved for these same two Cu(II) complex species. The resultant values at 25-degrees-C for all four specific rate constants (s-1), listed in the order k(CuL), k(CuLX), k(Hg)CuL', and k(Hg)CuLX' are as follows: Cu(II)([12]aneS4), 8 +/- 2, 1.3 +/- 0.4, (2.6 +/- 0.3) x 10(3), (9 +/- 6) x 10(1); Cu(II)([13]aneS4), 95 +/- 11, 9 +/- 2, (1.0 +/- 0.1) x 10(4), (4 +/- 1) x 10(3); Cu(II)([14]aneS4), 18 +/- 2, 1.2 +/- 0.3, (1.4 +/- 0.3) x 10(3), (5.3 +/- 0.6) x 10(2). The mechanistic implications of the observed kinetic effects for these "noncomplexing" anions are discussed.