Complexation of Sc₃N@C₈₀ Endohedral Fullerene with Cyclic Zn-Bisporphyrins: Solid State and Solution Studies

We report the synthesis of two cyclic β-pyrrole unsubstituted meso-tetraphenyl bisporphyrins in which the porphyrin units are connected by two 2,3-hexadiynyl-1,6-dioxo or two hexyl-1,6-dioxo spacers, respectively. Both cyclic porphyrin dimers exist in solution as mixtures of two conformational isomers. In the solid state, the receptor with diynyl spacers forms a 1:1 complex with the icosahedral (Ih) isomer of the trimetallic nitride endohedral fullerene Sc3N@C80. In this complex the receptor adopts a scoop-shaped conformation having a dihedral angle of 87.25° between the two porphyrin planes. The hexyl spaced analogue, however, adopts a similar conformation upon encapsulation of one molecule of Sc3N@C80 in a self-assembled dimeric capsule. The capsular complexes pack in columns and render the fullerene units completely isolated. In toluene solution, 1H NMR experiments indicate that the endohedral fullerene Sc3N@C80 is exclusively bound by the expanded isomer of both dimers. UV−vis and fluorescence titration experiments confirmed the existence of strong π−π interactions between the fullerene Sc3N@C80 and the flexible bisporphyrin dimer with hexyl spacers. At micromolar concentration, the flexible receptor forms only a 1:1 complex with the endohedral fullerene with stability constant value of Ka = 2.6 ± 0.3 × 105 M−1.本工作报道了两种β-吡咯未取代的环状中位四苯基双卟啉的合成，两类卟啉单元分别通过两个2,3-己二炔基-1,6-二羰基或两个己基-1,6-二羰基间隔基连接。两种环状双卟啉二聚体在溶液中均以两种构象异构体的混合体系形式存在。在固态下，带有二炔基间隔基的受体可与三金属氮化物内嵌富勒烯（trimetallic nitride endohedral fullerene）Sc3N@C80的二十面体（Ih）异构体形成1:1配合物。该配合物中的受体采取勺形构象，两个卟啉平面之间的二面角为87.25°。而带有己基间隔基的类似物则在自组装二聚胶囊中封装一个Sc3N@C80分子时，采取相似构象。该胶囊状配合物呈柱形堆积，使富勒烯单元完全相互隔离。在甲苯溶液中，氢核磁共振（1H NMR）实验表明，Sc3N@C80仅与两种双卟啉二聚体的扩张型构象异构体结合。紫外-可见光谱（UV−vis）与荧光滴定实验证实，Sc3N@C80与带有己基间隔基的柔性双卟啉二聚体之间存在强π-π相互作用。在微摩尔浓度下，该柔性受体仅与内嵌富勒烯形成1:1配合物，其稳定常数Ka = (2.6 ± 0.3) × 10^5 M^−1。