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

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 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 A). 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 A) in 60 were relatively short in comparison with those (av 2.44 A) 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.

Highly Electron‐Deficient and Air‐Stable Conjugated Thienylboranes

“…The corresponding values for FBDT2 proved to be similar with average values of 0.015 e a 0 À3 and 0.050 e a 0 À5 . The relatively lower electron density for FBDT and FBDT2 in comparison to that reported by Yamaguchi for related B···Cl [17] and by Yamashita for B···O contacts [18] is likely a result of the smaller atomic radii and higher electronegativity of fluorine.…”

Poly(anthrylenebutadiynylene)s: Precursor‐Based Synthesis and Band‐Gap Tuning

“…It remains to be determined whether the dramatic influence of the alkoxy substituents is a simple substituent effect or if the through-space interaction between the alkyne p system and the nonbonding electrons of the oxygen atoms plays a role in the low band gap of 4 d. The fact that peri-substituted anthracenes have served as a scaffold for the study of electronic interactions between adjacent p electron systems would appear to support the latter proposal. [16] To the extent that the peri substituents may also play a role in the steric stabilization of electron-rich 4 d and its derived radical cation (as demonstrated by cyclic voltammetry), the enforced proximity of p systems to a conjugated backbone could prove to be a general strategy for the design of stable, low-band-gap materials. …”

Mono- and Polynuclear Boron Dications

“…NHO-BH 3 complexes can be transformed into the corresponding hydride-bridged diboron dication [23] 2+ with one equivalent of Tf 2 NH (Figure 4). 57 The 2+ are also identified with NBO analysis and can be readily dissociated upon addition of neutral ligands to yield the tetracoordinated boronium cation.…”