Di-n-butyltin Methyl- and Phenylphosphonates

Abstract

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 hydrogenophosphonate ligands. The tin atoms are formally five-coordinate, but exhibit also two additional remote contacts, d(Sn−O) ≈ 3.14 Å, which results in a “5+2” type coordination. This crystalline compound and the three other amorphous compounds, Bu2Sn(O3PMe) (2), Bu2Sn(HO3PPh)2 (3), and Bu2Sn(O3PPh) (4), have been characterized by solid state 31P and 119Sn MAS NMR. Compound 1 exhibits a very well resolved 31P MAS NMR spectrum in which three different 2J(31P−119/117Sn)iso scalar couplings can be measured. 31P and 119Sn NMR, 31P−19Sn HMQC spectroscopy, and various other 2D NMR techniques at variable temperatures were used to unravel 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 apical and one equatorial oxygen atom. Compound 1, in solution, displays a similar local geometry at tin and the same dimeric unit as in the crystalline state. In contrast with 2 and 4, however, compounds 1 and 3 display an extremely high degree of stereochemical fluxionality based on fast exchange of the bridging and terminal hydrogenophosphonate ligands.