Substitution Reactions and Equilibrium Studies of Some Labile Metal Complexes

Abstract

The kinetics of the formation reactions of E-(1,3-bis-(2′-pyridyl)-1,2-diazaprop-2-ene), abbrev. paphy, and 1-(2′-pyridyl)3-(6″-methy1-2″-pyridyl)1,2-diazaprop-2-ene, abbrev. 6-Me-paphy, with Ni2+ and Ni(paphy)2+, Ni(phen)2+, Ni(bipy)2+ and Ni(terpy)2+ are reported for I =0.10 mol dm-3(NaNO3),25.0°C. The formation rates for the reactions Ni2+ + paphy and Ni2+ + 6-Me-paphy are : kf=2.2 x 103 and 1.4 x 103 dm3, mol-1 s-1 respectively. The lower rate measured for 6-Me-paphy compared with paphy is attributed to steric hindrance to coordination of the 6-methyl pyridyl group. Rate constants are also reported for the following reactions ( kf values, units dm3 mol-1 s-1 ):Ni(paphy)2+ + paphy 1.2 x 104; + 6-Me-paphy 6.0 x 103; + terpy 8.1 x 104; + phen 1 2 x 103; + bipy 2.6 x 104; + pada 1.17 x 105; Ni(phen)2+ + paphy 1.5 x 104; + 6-Me-paphy 6.8 x 103; Ni(terpy)2+ + paphy 3.5 x 104; Ni(bipy)2+ + paphy 1.6 x 104. The rates for formation of ternary complexes with paphy result in larger outer sphere association constants than are theoretically calculated for such systems. Stacking interactions between the incoming aromatic ligand and a bound aromatic ligand are proposed for the larger KOS values. The effect of subtle changes in the ligands on the stacking interactions are reported. The aqueous solution equilibria of the five-coordinate complexes formed by the triazamacrocycles 2,4,4-trimethyl-1,5,9-triazacyclododec-1-ene, abbrev. L1, with Ni (II) and 2,2,4-trimethyl-1,5,9-triazacyclododecane, abbrev. L2, with Ni(II), Cu(II) and Zn(II) are also reported. Equilibrium constants were determined potentiometrically and solution equilibria examined with UV-visible spectrophotometric techniques, I = O.10 mol dm-3(NaNO3), 25.0°c.Acid dissociation constants of L2 are 12.3, 7.34 and 2.51. The formation constants (log K values) for the reaction M2+ + L2 ≠ M(L2)2+ are 11.58 (Cu) and 7.68 (Zn); the very slow formation rate of the nickel complex prevented a potentiometric determination of log K(NiL). The nickel and Copper complexes are assigned from their UV-visible spectra five-coordinate square pyramidal geometries in solution. The formation constants are compared with analogous six-coordinate systems and the effect of methyl substituents on the ligands discussed. Bromide ions accelerated the formation of the copper and zinc complexes and bromide complexes were observed in solution for the copper system. Hydrolysis of the aquo complexes occurs above pH 7, pKa (H2O) values for the reaction ML(H2O) 2 2+ ≠ ML(H2O)OH+ + H+ are (L = L1) 9.85 (Ni), (L = L2) 9.81 (Ni), 8.48 (Cu) and 9.6 (Zn)- Di-µ-hydroxo dimers are shown to be formed in aqueous solution with the nickel and copper complexes. The formation constants (log K values) for the reaction 2ML(OH)+ ≠ M2L2(OH)2 2+ determined potentiometrically are (L = L1) 2.78 (Ni) and (L = L2) 2.44 (Ni), 2.44 (Ni),. 2.00 (Cu). The hydrolysis constants are compared with those of other five-coordinate systems and six-coordinate analogues of L1 and L2. Above pH 10 Cu(L2) (OH)2, was also formed, pKa(H2O) 12.2. Absorption maxima and molar absorptivities of the nickel and copper complexes formed in aqueous solution are reported.