Solid state NMR of tin containing compounds

Abstract

A recent development in chemical research has been concerned with molecular assemblies, and all manner of structural aggregates, leading to the relatively new area of supramolecular chemistry. The systems under study are organometallic ion exchangers where the structural motif is [(Me(_3)Sn(^IV))(_4)M(^II)(CN)(_6)]=(_oo) (M=Fe, Ru, Os...). These 3D cyanides are not simply molecular crystals, and their chemistry and topology are more reminiscent of inorganic solids. By replacing the bridging unit (Me(_3)Sn(^IV)) with guests such as (nPr)(_4)N(^+) and (nBu)(_4)N(^+), new 3D structures can be engineered, leading to new compounds. Tin units show unchanged trigonal bypyramidal coordination upon variation of metal (M), guest size, hindrance by the ligands. Changes in the metal affect mainly the isotropic and anisotropic (^119)Sn shielding parameters. The interplay of the electronic nature of the metal(s) and the bonding capacity of the ligands is studied by (^59)Co NMR. Quadrupolar coupling constants and asymmetry parameters show how the coordinative bond is sensitive to spatial reorganisation. Shielding calculations for the [Me(_3)Sn(CN)(_2)]’ model and for different X-ray structures have been performed to reproduce trends in chemical shift changes. Relativity effects have been omitted from the computations. This approach had four major aims: (a) to establish the extent to which (^119)Sn isotropic chemical shifts can be computed by DPT methods with acceptable basis sets for model molecular fragments relevant to four selected compounds; (b) the computation of the U^Sn isotropic chemical shifts for different coordination geometries of the CN ligands; (c) the attempt to correlate between computed and observed isotropic chemical shifts for four selected compounds; (d) to ascertain whether such a correlation can be used to establish the assignment of three experimental (^119)Sn shifts in a predictive fashion.