X-ray Diffraction and 2D Gradient-Assisted ¹H−¹¹⁹Sn HMQC NMR Studies of Structures Obtained from Nucleophilic Substitutions on Dimethyltin(IV) Salicylaldoximates

The reactivity of the trinuclear tin cluster [(Me₂Sn)₂(Me₂SnO)(OCH₃)(HONZO)(ONZO)] (3a; HONZOH = o-HONCH-C₆H₄-OH, salicylaldoxime) toward proton-donating nucleophiles is investigated with the purpose of studying the influence of the nature and acidity of the entering nucleophile on the reactivity of 3a and the stability of its cluster network. Reactions of 3a with benzaldoxime, HONCH-C₆H₅, and phenols HO-C₆H_5-nX_n (pK_a > 8) preserve the cluster network and give rise to smooth substitution of the μ₂-OCH₃ moiety for, respectively, a μ₂-bridging benzaldoximate, generating [(Me₂Sn)₂(Me₂SnO)(ONCHC₆H₅)(HONZO)(ONZO)] (4), and μ₂-bridging phenolates, generating [(Me₂Sn)₂(Me₂SnO)(OC₆H_5-nX_n)(HONZO)(ONZO)] (n = 1, X = 4-Me (5a), 4-Br (5b), 3-Cl (5c), 3-NO₂ (5d); n = 2, 3,5-Me₂ (5e)). More acidic or sterically hindered ortho-disubstituted phenols lead to decomposition of the trinuclear cluster or to mixtures from which no pure trinuclear cluster can be isolated. Reactions of 3a with carboxylic acids, acetic and p-toluic acids, and acetylacetone lead to clean decomposition of the trinuclear cluster, yielding the respective bis[dicarboxylatotetramethyldistannoxanes], 6a and 6b, and dimethyltin bis(acetylacetonate), Me₂Sn(acac)₂, 7. Reaction of 3a with ethylene glycol generates a mixture of trinuclear clusters, from which 1,3-dioxa-2,2-dimethyl-2-stannacyclopentane (8a) precipitates. Crystal structure determinations by X-ray diffraction for 4, 5a, and 5b reveal essentially the same trinuclear clusters as in 3a and related compounds, with a seven-coordinate pentagonal-bipyramidal tin atom linked to two five-coordinate trigonal-bipyramidal tin atoms via a network of oxygen atoms. All new trinuclear tin clusters have been characterized in solution by gradient-assisted 2D NMR.