title: Surprising acid/base and ion-sequestration chemistry of Sn9(4-): HSn9(3-), Ni@HSn9(3-), and the Sn9(3-) ion revisited.
abstract: K(4)Sn(9) dissolves in ethylenediamine (en) to give equilibrium mixtures of the diamagnetic HSn(9)(3-) ion along with K(x)Sn(9)((4-x)-) ion pairs, where x = 0, 1, 2, 3. The HSn(9)(3-) cluster is formed from the deprotonation of the en solvent and is the conjugate acid of Sn(9)(4-). DFT studies show that the structure is quite similar to the known isoelectronic RSn(9)(3-) ions (e.g., R = i-Pr). The hydrogen atom of HSn(9)(3-) (δ = 6.18 ppm) rapidly migrates among all nine Sn atoms in an intramolecular fashion; the Sn(9) core is also highly dynamic on the NMR time scale. The HSn(9)(3-) cluster reacts with Ni(cod)(2) to give the Ni@HSn(9)(3-) ion containing a hydridic hydrogen (δ = -28.3 ppm) that also scrambles across the Sn(9) cluster. The Sn(9)(4-) ion competes effectively with 2,2,2-crypt for binding K(+) in en solutions, and the pK(a) of HSn(9)(3-) is similar to that of en (i.e., Sn(9)(4-) is a very strong Brønsted base with a pK(b) comparable to that of the NH(2)CH(2)CH(2)NH(-) anion). Competition studies show that the HSn(9)(3-) ⇄ Sn(9)(4-) + H(+) equilibrium is fully reversible. The HSn(9)(3-) anion is present in significant concentrations in en solutions containing 2,2,2-crypt, yet it has gone undetected for over 30 years.
authors.affiliation: Department of Chemistry and Biochemistry, University of Maryland, College Park, Maryland 20742, USA.
authors.forename: F Sanem
authors.initials: FS
authors.lastname: Kocak
authors.forename: Domonique O
authors.initials: DO
authors.lastname: Downing
authors.forename: Peter
authors.initials: P
authors.lastname: Zavalij
authors.forename: Yiu-Fai
authors.initials: YF
authors.lastname: Lam
authors.forename: Andrei N
authors.initials: AN
authors.lastname: Vedernikov
authors.forename: Bryan
authors.initials: B
authors.lastname: Eichhorn