Di-n-butyltin methyl- and phenylphosphonates

The structure of [Bu2Sn(HO3PMe)(2)](2) (1), as determined by single-crystal X-ray diffraction, is based on a dimer containing bridging and terminal hy drogenophosphonate ligands. The tin atoms are formally five-coordinate, but exhibit also two additional remote contacts, d(Sn-O) approximate to 3.14 A ngstrom, which results in a "5+2" type coordination. This crystalline compo und and the three other amorphous compounds, Bu2Sn(O3PMe) (2), Bu2Sn(HO3PPh )(2) (3), and Bu-2-Sn(O3PPh) (4), have been characterized by solid state P- 31 and Sn-119 MAS NMR. Compound 1 exhibits a very well resolved P-31 MAS NM R spectrum in which three different (2)J-(P-31-Sn-119/117)(iso) scalar coup lings can be measured. P-31 and Sn-119 NMR, (31)p-Sn-19 HMQC spectroscopy, and various other 2D NMR techniques at variable temperatures were used to u nravel the basic structural unit of compounds 2 and 4 in solution, which is proposed to be based on a trigonal bipyramid of the type R2SnO3 with two a pical and one equatorial oxygen atom. Compound 1, in solution, displays a s imilar local geometry at tin and the same dimeric unit as in the crystallin e state. In contrast with 2 and 4, however, compounds 1 and 3 display an ex tremely high degree of stereochemical fluxionality based on fast exchange o f the bridging and terminal hydrogenophosphonate ligands.