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Surprising Acid/Base and Ion-Sequestration Chemistry of Sn94–: HSn93–, Ni@HSn93–, and the Sn93– Ion Revisited

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posted on 20.02.2016, 19:38 by F. Sanem Kocak, Domonique O. Downing, Peter Zavalij, Yiu-Fai Lam, Andrei N. Vedernikov, Bryan Eichhorn
K4Sn9 dissolves in ethylenediamine (en) to give equilibrium mixtures of the diamagnetic HSn93– ion along with KxSn9(4–x)– ion pairs, where x = 0, 1, 2, 3. The HSn93– cluster is formed from the deprotonation of the en solvent and is the conjugate acid of Sn94–. DFT studies show that the structure is quite similar to the known isoelectronic RSn93– ions (e.g., R = i-Pr). The hydrogen atom of HSn93– (δ = 6.18 ppm) rapidly migrates among all nine Sn atoms in an intramolecular fashion; the Sn9 core is also highly dynamic on the NMR time scale. The HSn93– cluster reacts with Ni­(cod)2 to give the Ni@HSn93– ion containing a hydridic hydrogen (δ = −28.3 ppm) that also scrambles across the Sn9 cluster. The Sn94– ion competes effectively with 2,2,2-crypt for binding K+ in en solutions, and the pKa of HSn93– is similar to that of en (i.e., Sn94– is a very strong Brønsted base with a pKb comparable to that of the NH2CH2CH2NH anion). Competition studies show that the HSn93– ⇄ Sn94– + H+ equilibrium is fully reversible. The HSn93– anion is present in significant concentrations in en solutions containing 2,2,2-crypt, yet it has gone undetected for over 30 years.