Developing low-melting alkali salts is of interest for both battery electrolytes and inorganic ionic liquids. In this study, we report a series of asymmetric alkali-metal sulfonamide salts based upon the (3-methoxypropyl)((trifluoromethyl)sulfonyl)amide (MPSA) anion. This family of salts features an unusual melting point trend, where the melting point of the salts decreases as the cation increases in size from Li to K but then the melting point increases as the cation further increases in size from K to Cs. Analyses of single crystals reveal that the unusual higher melting points of RbMPSA and CsMPSA in comparison to KMPSA can be attributed to the greater cation–cation distances as well as the increased rigidity of anion–cation coordination due to an increase in cyclic structures in comparison to KMPSA. Exceptionally, KMPSA features a very low melting point of only 50.79 ± 0.31 °C. This low melting point can be attributed to a relatively high degree of disorder, an unusual uncoordinated ether moiety, and a very short K–K distance of only 3.4348(7) Å among other factors, which is supported by the low cohesive energy and small elastic moduli among the rest according to density functional theory (DFT) calculations. The low melting point of KMPSA makes it interesting for low-temperature ionic liquids.

中文翻译：

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碱不对称磺胺盐系列的异常熔融趋势：单晶分析和建模

开发低熔点碱金属盐对电池电解质和无机离子液体都很重要。在这项研究中，我们报告了一系列基于（3-甲氧基丙基）（（三氟甲基）磺酰基）酰胺（MPSA）阴离子的不对称碱金属磺酰胺盐。这一系列盐具有不寻常的熔点趋势，其中盐的熔点随着阳离子尺寸从 Li 增加到 K 而降低，但随后熔点增加随着阳离子尺寸从 K 进一步增加到 Cs。单晶分析表明，与 KMPSA 相比，RbMPSA 和 CsMPSA 异常高的熔点可归因于更大的阳离子 - 阳离子距离以及由于环状结构增加导致的阴离子 - 阳离子配位刚性增加KMPA。例外的是，KMPSA 的熔点非常低，仅为 50.79 ± 0.31 °C。这种低熔点可归因于相对较高的无序度、不寻常的未配位的醚部分以及非常短的 K-K 距离仅为 3.4348(7) Å 等因素，这是由低内聚能和小根据密度泛函理论 (DFT) 计算，其余弹性模量。KMPSA 的低熔点使其对低温离子液体很有吸引力。