Helical _∞¹[Pb₂O] Chains in Polymorphs of Pb₂O(C₆H₅COO)₂

Three new lead oxide benzoates have been prepared solvothermally. In two polymorphs, 1 and 2, of Pb2O­(C6H5­COO)2, distorted Pb4O tetrahedra share edges to form helical ∞1[Pb2O] chains. Unprecedented among Pb4O-based structures, these motifs may be viewed as double helices of lead atoms surrounding the central oxygen atoms. Both right- and left-handed helices are included in each structure. Formation of the polymorphs was controlled through variation in synthetic conditions. Polymorph 1 crystallizes in the achiral, polar space group Fdd2. Polymorph 2, in P21, is a rare example of a kryptoracemate in which crystallographically independent chains of both chiralities occur in the asymmetric unit. This second polymorph is both chiral and polar, and the pseudosymmetric relationship between the helices has been examined. In Pb4O­(C6H5­COO)6 (3), discrete Pb4O tetrahedra are bridged by benzoate ligands to form a one-dimensional hybrid structure. The compounds have been further characterized via IR spectroscopy, elemental analysis, and thermogravimetric analysis. Measurement of the second-harmonic generating activity of 1 and 2 corroborates the crystallographic findings of noncentrosymmetry.本研究通过溶剂热法制备了三种新型氧化苯甲酸铅类化合物。在Pb₂O(C₆H₅COO)₂的两种多晶型物1和2中，畸变的Pb₄O四面体通过共边连接形成螺旋状一维∞¹[Pb₂O]链。在已报道的Pb₄O基结构中，此类结构基元尚属首例，可视为围绕中心氧原子的铅原子双螺旋结构。每种结构中均同时包含右旋与左旋螺旋链。通过调控合成条件可实现两种多晶型物的定向制备。多晶型物1结晶于非手性极性空间群Fdd2。多晶型物2结晶于空间群P2₁，属于罕见的隐消旋体（kryptoracemate），其不对称单元中同时存在两种手性的晶体学独立螺旋链。该多晶型物2同时具有手性与极性，研究人员对其螺旋链间的假对称关系进行了分析。在Pb₄O(C₆H₅COO)₆（化合物3）中，离散的Pb₄O四面体通过苯甲酸根配体桥联，形成一维杂化结构。采用红外光谱、元素分析及热重分析对上述化合物进行了全面表征。对化合物1和2的二次谐波发生活性进行测试，结果证实了晶体学分析得出的非中心对称结论。