Pyramidanes: The Covalent Form of the Ionic Compounds

Abstract

Pyramidane and its derivatives are among the most desirable synthetic chemistry targets, whose appealing square-pyramidal design, fascinating nonclassical structure, and unusual bonding features have attracted the permanently growing interest of organic chemists for decades. Although they have been comprehensively approached on theoretical grounds, no member of the pyramidane family was experimentally realized until very recently, thus remaining one of the biggest synthetic challenges for experimental pursuits. In this paper, we report on a series of stable hybrid pyramidanes of group 14 elements, featuring germanium, tin, or lead at the apex of the square pyramid, capping the four-membered-ring base made of carbon, silicon, or germanium atoms. On the basis of the experimental results (X-ray diffraction and NMR and Mossbauer spectroscopy) and computational studies at the B3LYP/Def2TZVP level of theory (MO, NBO, NRT, and AIM), an extraordinarily high degree of ionicity of the pyramidal apex-to-base bonds was attributed to the overall structure of these nonclassical covalent compounds.

Stable Silapyramidanes

“…These NMR spectroscopic data compare favorably with the reported germa, stanna, and plumbapyramidanes, leaving little doubt about the identity of the product as the missing silapyramidane (Table ). , …”

“…While the heavier group 14 pyramidanes V had been prepared in a related manner from inorganic dichlorotetrylenes (e.g., GeCl 2 •dioxane, SnCl 2 , etc.) and cyclobutadiene dianions, 29,30 an analogous reaction with an N-heterocyclic carbene-complex of dichlorosilylene did not yield the corresponding silapyramidane. We also employed Roesky's amidinate-supported chlorosilylene PhC(NtBu) 2 SiCl instead but also observed the formation of neutral cyclobutadiene and the concomitant reductive dimerization of the silylene to the reported Si(I)− Si(I) dimer.…”

“…As in the case of the germa-, stanna-, and plumbapyramidanes, the molecular point group of silapyramidane 1 is not an ideal C 4v but a C 2 due to the sterically encumbered TMS groups. 29,30,32,33 The bond distances between the apex silicon atom and the carbon atoms of the base [2.027(1) to 2.037(2) Å] are significantly longer than single Si−C bonds (1.91 Å) 67 and indicate some extent of ionic character of the Si−C bond. The almost square shape of the four-membered ring [1.483(2)−1.490(2) Å] is reminiscent of cyclobutadiene metal complexes.…”

A Cationic Phosphapyramidane

“…Such exceptional shielding of the phosphorus nucleus in phosphapyramidane 2 + ⋅[B(C 6 F 5 ) 4 ] − requires a sophisticated explanation. It is therefore appropriate to recall a strikingly similar record shielding of the tin nucleus in the isostructural stannapyramidane ( 119 Sn NMR: δ =−2441.5 ppm), that was realized as an indication of the presence of an extraordinarily high s ‐character lone pair at the tin apex ,. Likewise, it is quite reasonable to hypothesize a presence of the nonbonding electron density at the phosphorus center in phosphapyramidane.…”

Borole/Borapyramidane Relationship

“…However, a variety of heteroatom analogues of elusive (C 5 H 4 ) pyramidane A , charged or neutral, were described in the literature; some of them were characterized by X-ray diffraction . Compounds B and C are typical examples. , Formal protonation of the neutral (C 5 H 4 ) pyramidane gives the (C 5 H 5 ) + cation AH + . On the (C 5 H 5 ) + surface, the pyramidane structure seems not to be the global minimum, but the possible observation of a dimethyl substituted derivative by NMR spectroscopy in a superacidic medium was reported .…”