Syntheses and Structures of Hypervalent Pentacoordinate Carbon and Boron Compounds Bearing an Anthracene Skeleton − Elucidation of Hypervalent Interaction Based on X-ray Analysis and DFT Calculation

Pentacoordinate and tetracoordinate carbon and boron compounds (27, 38, 50−52, 56−61)
bearing an anthracene skeleton with two oxygen or nitrogen atoms at the 1,8-positions were synthesized
by the use of four newly synthesized tridentate ligand precursors. Several carbon and boron compounds
were characterized by X-ray crystallographic analysis, showing that compounds 27, 56−59 bearing an
oxygen-donating anthracene skeleton had a trigonal bipyramidal (TBP) pentacoordinate structure with
relatively long apical distances (ca. 2.38−2.46 Å). Despite the relatively long apical distances, DFT calculation
of carbon species 27 and boron species 56 and experimental accurate X-ray electron density distribution
analysis of 56 supported the existence of the apical hypervalent bond even though the nature of the
hypervalent interaction between the central carbon (or boron) and the donating oxygen atom was relatively
weak and ionic. On the other hand, X-ray analysis of compounds 50−52 bearing a nitrogen-donating
anthracene skeleton showed unsymmetrical tetracoordinate carbon or boron atom with coordination by
only one of the two nitrogen-donating groups. It is interesting to note that, with an oxygen-donating skeleton,
the compound 61 having two chlorine atoms on the central boron atom showed a tetracoordinate structure,
although the corresponding compound 60 with two fluorine atoms showed a pentacoordinate structure.
The B−O distances (av 2.29 Å) in 60 were relatively short in comparison with those (av 2.44 Å) in 59
having two methoxy groups on the central boron atom, indicating that the B−O interaction became stronger
due to the electron-withdrawing nature of the fluorine atoms.

五配位与四配位碳、硼化合物（编号27、38、50−52、56−61）的骨架为1,8位连有两个氧或氮原子的蒽环，通过四种新合成的三齿配体前驱体制备得到。研究人员采用X射线晶体衍射表征对部分碳、硼化合物进行了结构表征，结果显示：带有氧配位蒽环骨架的化合物27、56−59呈现三角双锥（trigonal bipyramidal, TBP）五配位结构，其轴向键长相对较长（约2.38−2.46 Å）。尽管轴向键长较长，但针对碳化合物27与硼化合物56的密度泛函理论（DFT）计算，以及对化合物56的高精度X射线电子密度分布分析，均证实了轴向超价键的存在；尽管中心碳（或硼）与配位氧原子之间的超价相互作用相对较弱且呈离子性。另一方面，带有氮配位蒽环骨架的化合物50−52的X射线分析结果表明，其中心碳或硼原子为不对称四配位，仅与两个氮配位基团中的一个形成配位作用。值得注意的是，在氧配位骨架体系中，中心硼原子连有两个氯原子的化合物61呈现四配位结构，而对应中心硼原子连有两个氟原子的化合物60则为五配位结构。与中心硼连有两个甲氧基的化合物59相比，化合物60中的B-O键长（平均2.29 Å）更短（化合物59的B-O键长平均为2.44 Å），这表明氟原子的吸电子特性增强了B-O相互作用。