Rates and activation parameters for the stepwise formation of mono complexes of the hexakis(dimethyl sulphoxide)aluminium(III) ion with uni-, bi-, and ter-dentate nitrogen-donor ligands in nitromethane solution

Abstract

Rates and activation parameters for the formation of mono complexes of the [Al(dmso)₆]³⁺ ion (dmso = dimethyi suiphoxide) with pyridine, 2,2′-bipyridine (bipy), 2.2′ : 6′,2″-terpyridine (terpy). 1, 10-phenanthroiine (phen), and t-butyiamine have been determined in nitromethane solution by following the release of the co-ordinated dmso molecules using stopped-flow Fourier-transform ¹H n.m.r. spectroscopy. Activation parameters for the soivent-exchange process (using [²H₆]dmso) are also reported from data covering a wide temperature range (263–338 K). The reactions with bipy and terpy proceed in two and three distinct steps respectively, and activation parameters are reported for the first bond formations and for the subsequent slower cheiate-ring closures. The reaction with pyridine is significantly retarded by the addition of a ten-fold excess of the non-ligating substrate 2,6-dibromo-pyridine, the rate constant decreasing from 167 to 55.6 dm³ mol^–1 s^–1 This rate retardation is in line with the observation of unusually large outer-sphere associations (K₀ in the range 2 × 10²–75 × 10² dm³ mol^–1) during the first bond formations, the large values of K₀ accounting for the unexpectedly rapid rates of these reactions.