Silver Aggregation Caused by Stanna-<i>closo</i>-dodecaborate Coordination: Syntheses, Solid-State Structures and Theoretical Studies

Stanna-closo-dodecaborate [SnB11H11]2- reacts as a nucleophile with various silver electrophiles ([Ag(PMe3)]+, [Ag(PEt3)]+, [Ag(PPh3)]+, and Ag+) to form silver−tin bonds. Aggregation of two, three, or four units of [{Ag(SnB11H11)(PR3)}n]n- (PPh3, n = 2; PEt3, n = 3; PMe3, n = 4) was found, depending on the size of the coordinating phosphine. The structures of the silver−tin clusters in the solid state were determined by single-crystal X-ray diffraction. In these phosphine silver coordination compounds, the tin ligand exhibits μ2- and μ3-coordination with the silver atoms. From the reaction with silver nitrate, an octaanionic stanna-closo-dodecaborate coordination compound, [Et4N]8[Ag4(SnB11H11)6], was isolated. In this cluster, arranged as butterfly, the stannaborate shows various coordination modes at four silver atoms. In the reported silver−tin complexes, the silver−silver interatomic distances are in a range of 2.6326(10)−3.1424(6) Å. Silver−tin distances were found between 2.6416(5) and 3.1460(6) Å. Analysis of the molecular orbitals calculated by means of density functional theory shows that the LUMO of the core compound without [SnB11H11]2- units is always a totally symmetric combination of (mainly) s-orbitals of Ag atoms. This core is filled with electrons of the HOMOs of the [SnB11H11]2- units and is leading, in this way, to a stable compound.笼状十二硼锡酸盐(Stanna-closo-dodecaborate)阴离子[SnB11H11]²⁻可作为亲核试剂，与多种银基亲电试剂（[Ag(PMe3)]⁺、[Ag(PEt3)]⁺、[Ag(PPh3)]⁺及Ag⁺）发生反应，生成银-锡键。 根据配位膦配体的空间体积差异，可得到聚集数分别为2、3、4的[{Ag(SnB11H11)(PR3)}ₙ]ⁿ⁻簇合物（其中PPh3对应n=2，PEt3对应n=3，PMe3对应n=4）。 通过单晶X射线衍射解析了固态下银-锡簇合物的晶体结构。在这类膦配位银化合物中，锡配体与银原子间存在μ₂-及μ₃-两种桥联配位模式。 由硝酸银参与的反应体系中，可分离得到八阴离子型笼状十二硼锡酸盐配位簇合物的四乙基铵盐[Et4N]8[Ag4(SnB11H11)6]。该簇合物呈蝶形排布，其中的锡硼酸盐配体与4个银原子展现出多种配位模式。 在已报道的银-锡配合物中，银-银原子间距离介于2.6326(10)~3.1424(6) Å之间，银-锡键长则处于2.6416(5)~3.1460(6) Å范围内。 通过密度泛函理论(DFT)计算得到的分子轨道分析显示：不含[SnB11H11]²⁻单元的核心化合物的最低未占据分子轨道(LUMO)，始终为以银原子s轨道为主的全对称组合；该核心可被[SnB11H11]²⁻单元的最高占据分子轨道(HOMO)贡献的电子填充，从而形成稳定的配位化合物。