Tetracoordination Binding with Solvent or Anion

In the same study as claims for SnH, Birchall and Manivannan also reported that the Sn chemical shift for a purported SnMe ion was 5 322. This material was prepared by the reaction of trimethyltin hydride with fluorosulfonic acid. At the time (1985), this value could be considered unusually deshielded, so that this species clearly had at least some stannylium ion character. No further structural data, however, were available. 

Shortly thereafter (1994), Kira, Sakurai, and coworkers reported a very similar reaction of tributhyltin hydride with trityl tetrakis[3,5-di(trifluoromethyl)phenyl]borate in CD2Cl2. The resulting species, which they called a trivalent tin cation, had a chemical shift of 5 356 at —20 °C. The material decomposed above this temperature and reacted with ethyl ether at —70 °C to give a more solvated species [ Bu3Sn(Et20)+ ] with a chemical shift of 5 165 at —20 °C. 

The difficulty in interpreting these results, at the time, arose from the lack of reliable points of comparison for the expected Sn chemical shifts of stannylium ions. There was no doubt that values higher than 300 were deshielded to an unprecedented extent, but were these shifts sufficient to demonstrate tricoordination or trivalency Computation in the early 1990s could not provide a reliable answer. Arshadi et al bypassed the calculational problem in 1996 by publishing a remarkable empirical correlation between structurally analogous silicon and tin compounds ( Si chemical shift vs Sn chemical shift). Since reliable calculations were available for the Si chemical shifts of trialkylsilylium ions, this plot could provide at least an indication of the expected Sn chemical shifts of trialkylstannylium ions, which proved to be 5 ca. 1700. The species observed by Birchall, Lambert, and Sakurai thus were very far from the expected chemical shift and hence from the ideal tricoordinate geometry of the stannylium ion. Since the values are deshielded to some extent, pentacoordination could be ruled out. The best description of the structures observed by all these authors, therefore, is the bond-stretched, solvent-coordinated stannyl cation 8. Lambert and Kuhlmann observed high conductivity, so that the neutral anion-coordinated variant 9 could be eliminated. Such structures (8) also apply to those reported in 1992 by Edlund et al as the tetrahedral part of the equilibrium with pentacoordinate species. 

The most deshielded material within this category (8) was reported in 2002 by Piers and coworkers. They used a novel, highly fluorinated borate anion, the solvent benzene, and hydride abstraction with trityl to produce the species Bu3Sn(benzene)+ with a Sn chemical shift of 5 434. By this time, authors were referring to such species as solvated stannyl cations. 

This family of stannyl cations (8) has a flattened tetrahedral structure and somewhat deshielded Sn chemical shifts (values up to S 434). The bond between tin and the fourth coordination site is lengthened, and tin clearly has considerable positive charge density. Although one may speak of stannylium ion character, these materials, nonetheless, are not free, tricoordinate stannylium ions. 

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