Synthesis, Structure, and Reactivity of Lewis Base Stabilized Plumbacyclopentadienylidenes

Abstract

Plumbacyclopentadienylidenes, in which the lead atoms have divalent states and are coordinated by THF, pyridine and N-heterocyclic carbene, were synthesized and characterized. The THF- and pyridine-stabilized compounds can be regarded as rare examples of hypervalent 10-X-4 species. The equilibrium between the THF adduct and the free plumbacyclopentadienylidene was evidenced by spectroscopic analysis and theoretical calculations. The THF adduct in benzene converted into a plumbylene dimer, where one of the lead centers is coordinated by THF and the other lead atom is coordinated by a divalent lead atom, the dimer gradually decomposing into spiroplumbole. The THF adduct unexpectedly reacted with trifluoroborane and trichlorogallane to afford fluoroborole and chlorogallole, which are the first examples of non-annulated fluoroborole and gallole, respectively.

Quantum‐chemical analyses of aromaticity, UV spectra, and NMR chemical shifts in plumbacyclopentadienylidenes stabilized by Lewis bases

“…The first absorption peak is observed at around 700 nm in plumbole (a), whereas it disappears in the ligand-coordinated plumboles. [8] This trend is reproduced by the present calculations. The disappearance of the lowest transition can be explained by the orbital diagram shown in Figure 4.…”

“…This proposes that plumbole 1 NHC 2 is dominant in solution, although plumbole 1 NHC is formed in the crystalline state. [8] In the 13 C-NMR chemical shifts, 13 C b is shifted upfield compared to 13 The experimental trend can be reproduced quantitatively by the ZORA method, whereas the nonrelativistic method gives the opposite result. The trend in the 13 C a and 13 C b chemical [a] Experimental values are observed in the crystalline state.…”

“…The C a AC b bond lengths of plumbole 1 THF 2 are 1.364 and 1.363 Å , respectively, whereas the experimental ones are 1.346 and 1.347 Å . [8] Conversely, the C b AC b bond length is 1.539 Å , whereas the experimental one is 1.517 Å . The calculated and experimental CAC bond lengths agree well and bond alternation occurs.…”

“…Recently, plumbacyclopentadienylidenes, stabilized by tetrahydrofuran (THF), pyridine, and N-heterocyclic carbene (NHC), were synthesized and characterized. [8] In these compounds, the total number of pelectrons is fractional, and therefore, the H€ uckel's (4n 1 2) pelectron rule seems to be partially satisfied.…”

“…Plumbole 1 THF 2 , plumbole 1 Py 2 , and plumbole 1 NHC can be recrystallized, and thus, their molecular structures are determined experimentally. [8] The other three compounds cannot be recrystallized; nevertheless, we perform all calculations of them because they may exist in solution.…”