The interactions between Ni II cations and bidentate Schiff base ligands, N-alkyl-5-X-salicylaldimine HL (X = H; R = Et, nPr, tBu; X = Cl, OMe, NO 2 ; R = nPr), and bis(N-n-alkyl-5-X-salicylaldiminato)nickel(II) complexes [NiL 2 ] (X = H; R = Et, nPr, nBu; X = Cl, OMe, Me; R = nPr) were investigated by UV/Vis spectrophotometry in acetonitrile. The kinetics for the formation of Ni II /HL 1:1 adducts was studied by stoppedflow techniques, which showed that the process followed a two-consecutive mechanism. Experimental evidence and theoretical calculations indicated the formation of an unid-
Strontium ion catalyses the reactions of the crown ether derivatives 1 and 2 with methoxide ion in methanol at 40 "C to yield the product of substitution 3 and elimination 4, respectively.Metal-ion promoted catalysis in supramolecular systems is of current interest in developing 'artificial enzymes'. 1Catalysis in such systems depends, among other things, (solvent, cation, etc.) on the reaction type and the position of coordination sites which hold the metal ion in close proximity to the reaction centre. Previous studies of sN2 reactions have regarded, in particular, the formation of benzo-crown ethers via intramolecular Williamson reaction (template effect) ,192 and methyl-transfer reactions to a-toluenethiolate from substrates in which a crown ether ring is part of the leaving group structure . 3 In searching for systems in which rate-enhancing metal-ion effects could arise owing to suitably placed donor atoms, we found that compound 1 4 reacts with MeO-in MeOH to yield exclusively the substitution product 3,4 whereas compound with a different leaving group 2,t yields, under the same conditions, exclusively the elimination product 4. $ These two systems appeared to us ideally suited for the purpose of comparing rate-enhancing metal-ion effects in sN2 and E2 reactions. This is the first investigation of this type on an elimination reaction, and on a substitution reaction in which a crown ether is part of the alkyl structure.Rate measurements were carried out in MeOH at 40.0 "C on very dilute substrate solutions (cu.rnol dm-3) in the presence of excess Me4NOMe (0.100 rnol dm-3) and of varying amounts of alkali (Na, K) and alkaline-earth (Sr, Ba) metal bromides. In all cases ionic strength was kept constant by adjusting total salt concentration to 0.2 rnol dm-3 with Et4NBr. Reactions were followed spectrophotometrically by monitoring the appearance of tosylate ion in the reaction 1 -+ 3 (h 234 nm), and iodide ion in the reaction 2 -+ 4 (h 238 nm).Preliminary rate measurements in the absence of metal salts and with varying amounts of Me4NOMe in the range ca. 5 x to 0.2 rnol dm-3 allowed the evaluation of the secondorder rate constants for both the reactions 1 -+ 3 (ko 2.6 x dm3 s-1 mol-l) and 2 -+ 4 (ko 1.5 x dm3 s-* mol-l).
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