Exceptional Structural Compliance of the B12F122– Superweak Anion

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.

本研究报道了一系列由十二氟合硼阴离子(B12F122–)与全同配体银(I)阳离子Ag(L)n+形成的盐的单晶X射线衍射结构、热重分析及/或傅里叶变换红外光谱(FTIR)，其中配体L分别为：二氯甲烷(CH2Cl2)，配位数n=2；甲苯(PhCH3)，n=3；乙腈(CH3CN)，n=2~4；一氧化碳(CO)，n=1或2。十二氟合硼阴离子(B12F122–，后文简称Y2–)的超弱阴离子特性，可通过如下实验证实：微晶态Ag2(Y)与1 atm一氧化碳快速反应，生成非经典银(I)羰基化合物，该产物的FTIR光谱中出现2198 cm–1处的ν(CO)特征峰，其推定分子式为[Ag(CO)n]2[Y]。与之形成对比的是，微晶态Ag2(B12Cl12)即使在24 atm一氧化碳氛围中反应32小时，也未出现ν(CO)特征峰，因此未生成Ag(CO)+物种。当Ag2(Y)在高于1 atm的一氧化碳压力下反应时，光谱中会出现2190 cm–1处的新ν(CO)特征峰，该峰对应双羰基银(I)阳离子Ag(CO)2+的特征。Ag2(CH3CN)8(Y)与Ag2(CH3CN)5(Y)均会在25 ℃下快速失去配位乙腈配体，转化为Ag2(CH3CN)4(Y)；该产物仅在加热至100 ℃以上时，才能脱除所有溶剂配体得到无溶剂Ag2(Y)。同理，Ag2(PhCH3)6(Y)在25 ℃下快速失去配位甲苯配体，转化为Ag2(PhCH3)2(Y)，该产物需加热至150 ℃以上才能得到Ag2(Y)；而Ag2(CH2Cl2)4(Y)会在25~100 ℃间快速失去4个配位二氯甲烷配体中的3个，仅在加热至200 ℃以上时才能完全脱除溶剂得到Ag2(Y)。无溶剂态Ag2(Y)在加热至380 ℃以上前均可保持稳定。无孔晶体在25 ℃下快速挥发脱除配位配体的过程，需要晶格同时发生快速结构重排，这是本研究报道的Y2–阴离子结构柔性的三种表现形式之一。第二种表现形式（更具定量性）为：当配位数n分别为8、5、4时，Ag2(CH3CN)n(Y)中银(I)离子的键价和几乎保持恒定，为1.20±0.03；这是因为Y2–阴离子可通过形成适量的弱Ag–F(B)键，精准补偿因脱除乙腈配体所损失的配位作用。第三种（也是最特殊的）表现形式为：伴随Ag2(CH3CN)8(Y) → Ag2(CH3CN)5(Y) → Ag2(CH3CN)4(Y)这一概念性转化过程的理想化结构重排，可描述为：原本夹载Ag(CH3CN)4+配合物的Y2–阴离子密排层，会解离为交错排布的扁平带状结构——由相互连接的(Y2–)3三角形单元构成，该带状结构从四个方向包围Ag2(CH3CN)52+配合物；随后阴离子层会重新形成密排结构，再次夹载Ag(CH3CN)2+配合物。Ag2(CH3CN)5(Y)中由相互连接的(Y2–)3三角形构成的阴离子晶格，可能是该结构类型的首例报道。