Reactivity of Tin(II) Guanidinate with 1,2- and 1,3-Diones: Oxidative Cycloaddition or Ligand Substitution?

A series of tin(IV) guanidinates were prepared by a (4+1) oxidative cycloaddition of four 1,2-diones (3,5-di-tert-butyl-o-benzoquinone, 3,4,5,6-tetrachloro-1,2-benzoquinone, 9,10-phenanthrenedione, 1,2-diphenylethanedione) or by an oxidative addition of a C–Br bond (from 2-bromo-1,3-diphenylpropane-1,3-dione followed by rearrangement) and a Cl–Cl bond (Cl2 generated from (dichloro-λ3-iodanyl)benzene) with {pTol-NC[N(SiMe3)2]N-pTol}2Sn (1). The reactivity of five pentane-1,3-diones and dimethyl malonate with compound 1 was assessed on the basis of the effect of 1,3-diones on the reaction mechanism in comparison with 1,2-diones. In contrast with oxidation reactions observed for compounds containing conjugated C═O bonds, the reactions of the tin(II) guanidinate with 1,3-diones revealed a high ability for ligand substitution. All the tin compounds prepared were characterized, and ligand substitution reactions were monitored using 1H, 13C, and 119Sn NMR spectroscopy. The molecular structures of one tin(II) and five tin(IV) guanidinato complexes investigated were determined by X-ray diffraction. All tin(IV) compounds display six- or seven-coordination. The UV–vis absorption spectra were recorded and simulated by TDDFT methods in order to get insight into the origin of the nontypical colors of the target tin(IV) diolato-guanidinates and their keto-functionalized precursors.