1H NMR studies of ligand and H/D exchange reactions of cis- and trans-([14]aneN4)(H2O)RhH2+ in aqueous solutions.

Abstract

Substitution and exchange reactions of cis- and trans-L(1)(H(2)O)RhH(2+) (L(1) = 1,4,8,11-tetraazacyclotetradecane = [14]aneN(4)) were studied in aqueous solutions by UV-vis and (1)H NMR spectroscopies. At pH 1 and 25 degrees C, the substitution of SCN(-) for the coordinated molecule of water is rapid and thermodynamically favorable. Spectrophotometric determinations yielded the equilibrium constants K = 1.49 x 10(3) M(-1) (cis) and 1.44 x 10(3) (trans). (1)H NMR studies in D(2)O revealed a rapid dynamic process, interpreted as the exchange between coordinated water and X(-) (X = Cl, Br, or I). On the other hand, no line broadening was observed for the strongly bound ligands CN(-) and SCN(-). The complex trans-L(1)(D(2)O)RhH(2+) undergoes a base-catalyzed H/D exchange of the hydride in D(2)O with a rate constant of (1.45 +/- 0.02) x 10(3) M(-1) s(-1). The exchange in the cis isomer is very slow under similar conditions. The complex cis-[L(1)ClRhH](ClO(4)) crystallizes in the centrosymmetric Ponemacr; space group, unit cell dimensions a = 8.9805(11) A, b = 9.1598(11) A, c = 10.4081(13) A, alpha = 81.091(2) degrees, beta = 81.978(2) degrees, gamma = 88.850(2) degrees. The rhodium atom resides in a slightly distorted octahedral environment consisting of the four N atoms of the cyclam, a stereochemically active hydrogen, and a chlorine atom.