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Exceptional Structural Compliance of the B12F122– Superweak Anion

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journal contribution
posted on 23.03.2017 by Dmitry V. Peryshkov, Steven H. Strauss
The single-crystal X-ray structures, thermogravimetric analyses, and/or FTIR spectra of a series of salts of the B12F122– anion and homoleptic Ag­(L)n+ cations are reported (L = CH2Cl2, n = 2; L = PhCH3, n = 3; L = CH3CN; n = 2–4; L = CO, n = 1, 2). The superweak-anion nature of B12F122– (Y2–) was demonstrated by the rapid reaction of microcrystalline Ag2(Y) with 1 atm of CO to form a nonclassical silver­(I) carbonyl compound with an FTIR ν­(CO) band at 2198 cm–1 (and with the proposed formula [Ag­(CO)n]2[Y]). In contrast, microcrystalline Ag2(B12Cl12) did not exhibit ν­(CO) bands and therefore did not form Ag­(CO)+ species, even after 32 h under 24 atm of CO. When Ag2(Y) was treated with carbon monoxide pressures higher than 1 atm, a new ν­(CO) band at 2190 cm–1 appeared, which is characteristic of a Ag­(CO)2+ dicarbonyl cation. Both Ag2(CH3CN)8(Y) and Ag2(CH3CN)5(Y) rapidly lost coordinated CH3CN at 25 °C to form Ag2(CH3CN)4(Y), which formed solvent-free Ag2(Y) only after heating above 100 °C. Similarly, Ag2(PhCH3)6(Y) rapidly lost coordinated PhCH3 at 25 °C to form Ag2(PhCH3)2(Y), which formed Ag2(Y) after heating above 150 °C, and Ag2(CH2Cl2)4(Y) rapidly lost three of the four coordinated CH2Cl2 ligands between 25 and 100 °C and formed Ag2(Y) when it was heated above 200 °C. Solvent-free Ag2(Y) was stable until it was heated above 380 °C. The rapid evaporative loss of coordinated ligands at 25 °C from nonporous crystalline solids requires equally rapid structural reorganization of the lattice and is one of three manifestations of the structural compliance of the Y2– anion reported in this work. The second, more quantitative, manifestation is that Ag+ bond-valence sums for Ag2(CH3CN)n(Y) are virtually constant, 1.20 ± 0.03, for n = 8, 5, 4, because the Y2– anion precisely compensated for the lost CH3CN ligands by readily forming the necessary number of weak Ag–F­(B) bonds. The third, and most exceptional, manifestation is that the idealized structural reorganization accompanying the conceptual transformations Ag2(CH3CN)8(Y) → Ag2(CH3CN)5(Y) → Ag2(CH3CN)4(Y) involve close-packed layers of Y2– anions that sandwich the Ag­(CH3CN)4+ complexes splitting into staggered flat ribbons of interconnected (Y2–)3 triangles that surround the Ag2(CH3CN)52+ complexes on four sides, conceptually re-forming close-packed layers of anions that sandwich the Ag­(CH3CN)2+ complexes. The interconnected (Y2–)3 triangle lattice of anions in Ag2(CH3CN)5(Y) may be the first example of this structure type.