Rational Syntheses, Structure, and Properties of the First Bismuth(II) Carboxylate

Bismuth(II) trifluoroacetate (1), the first inorganic salt of bismuth in oxidation state +2, has been obtained in its
pure, unstabilized form. Several synthetic routes suggested for the isolation of the new compound include (i) mild
oxidation of elemental bismuth with some metal trifluoroacetates, e.g., AgI and HgII; (ii) mild reduction of bismuth(III) trifluoroacetate with metals, such as Zn; (iii) comproportionation reaction between Bi and Bi(O2CCF3)3. The last
approach gives the title compound 1 in quantitative yield as a sole product. Bismuth(II) trifluoroacetate has been
characterized by NMR, IR, and UV−vis spectroscopy as well as by single-crystal X-ray diffraction. Crystallographic
study reveals the dinuclear paddle-wheel structure for diamagnetic molecules Bi2(O2CCF3)4. The Bi−Bi bond distances
in dimetal units of 1 are averaged to 2.9462(3) Å, and there are no axial intermolecular contacts between these
units in the solid state. The compound is volatile and exists in vapor phase up to 220 °C when it disproportionates
back to Bi0 and BiIII species, i.e., by the reverse of the synthetic route iii. In contrast, the solution chemistry is quite
limited:  the bismuth(II) trifluoroacetate is decomposed by the majority of common solvents, but it can be stabilized
by aromatic systems. The dibismuth unit has been shown to be preserved in the latter solvents and can be crystallized
out in a form of π-adducts with arenes. Two such adducts, Bi2(O2CCF3)4·(C6H5Me) (2) and Bi2(O2CCF3)4·(1,4-C6H4Me2)2 (3), have been isolated as single crystals and characterized by X-ray diffraction techniques. In the
structures of both 2 and 3, the bismuth(II) centers exhibit weak η6-coordination to aromatic rings.

三氟乙酸铋(II)（Bismuth(II) trifluoroacetate，1）是首例+2氧化态铋的无机盐，现已以纯态、未稳定化的形式成功分离。针对该新型化合物的分离，已有多条合成路径被报道：(i) 利用部分三氟乙酸金属盐（如碘化银、二价汞盐）对单质铋进行温和氧化；(ii) 以金属锌为还原剂，对三氟乙酸铋(III)进行温和还原；(iii) 单质铋与三氟乙酸铋(III)之间的归中反应。其中第三种方法可定量得到目标化合物1，且为唯一反应产物。三氟乙酸铋(II)已通过核磁共振波谱（NMR）、红外光谱（IR）、紫外-可见光谱（UV−vis）以及单晶X射线衍射（single-crystal X-ray diffraction）技术完成表征。晶体学研究表明，抗磁性分子Bi₂(O₂CCF₃)₄具有双核桨轮状结构。该双核金属单元内的Bi−Bi键平均键长为2.9462(3) Å，固态下这些双核单元之间不存在轴向分子间接触。该化合物具有挥发性，在220 ℃以下的气相中可稳定存在；当温度升至220 ℃时，其会发生歧化反应，重新分解为单质铋（Bi⁰）与三价铋（BiIII）物种，即对应合成路径(iii)的逆反应。与之相对，其溶液化学性质较为有限：三氟乙酸铋(II)可被绝大多数常见溶剂分解，但可通过芳香体系实现稳定化。在这类芳香溶剂中，二铋单元得以保留，且可通过与芳烃形成π加合物的形式结晶析出。目前已分离得到两种此类加合物：Bi₂(O₂CCF₃)₄·(C₆H₅Me)（2）与Bi₂(O₂CCF₃)₄·(1,4-C₆H₄Me₂)₂（3），并通过X射线衍射技术完成表征。在2和3的晶体结构中，铋(II)中心与芳香环存在弱η⁶配位作用。