| Abstract
| - 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 synthesizedby the use of four newly synthesized tridentate ligand precursors. Several carbon and boron compoundswere characterized by X-ray crystallographic analysis, showing that compounds 27, 56−59 bearing anoxygen-donating anthracene skeleton had a trigonal bipyramidal (TBP) pentacoordinate structure withrelatively long apical distances (ca. 2.38−2.46 Å). Despite the relatively long apical distances, DFT calculationof carbon species 27 and boron species 56 and experimental accurate X-ray electron density distributionanalysis of 56 supported the existence of the apical hypervalent bond even though the nature of thehypervalent interaction between the central carbon (or boron) and the donating oxygen atom was relativelyweak and ionic. On the other hand, X-ray analysis of compounds 50−52 bearing a nitrogen-donatinganthracene skeleton showed unsymmetrical tetracoordinate carbon or boron atom with coordination byonly 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 59having two methoxy groups on the central boron atom, indicating that the B−O interaction became strongerdue to the electron-withdrawing nature of the fluorine atoms.
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