Aromatic Character of Tria- and Pentafulvene and Their Exocyclic Si, Ge, and Sn Derivatives. An ab initio Study

Abstract

The structures and dipole moments have been calculated for both methylenecyclopropene (triafulvene) and pentafulvene and their exocyclic Si, Ge, and Sn analogues 1a − d and 2a − d, respectively. Ab initio calculations employing the HF, DFT, and MP2 methods, each using split valence plus polarization and triple split valence with two sets of polarization functions basis sets, have been performed. The results of these six levels of theory on each of the eight molecules were compared, and the aromatic character of these systems was examined. The structures 1a − d exhibited pronounced bond alternation in the ring suggesting the existence of only weak cyclopropenyl cation-like resonance contribution. However, very large dipole moments were predicted in the direction that would be expected from an aromaticity contribution. Since Si, Ge, and Sn are less electronegative than C, the large dipole moments in the direction of the exocyclic heteroatom were surprising. The most striking finding was that, while 1a is planar, 4-sila-, 4-germa-, and 4-stannatriafulvene were nonplanar with trans-bent structures in which the exocyclic double bond is bent slightly out of the cyclopropene ring plane and out of the HXH plane (X = Si, Ge, Sn) by large angles. Thus, the Si, Ge, and Sn atoms exhibit pyramidal geometries. The dipole moment of pentafulvene, 2a, (oriented toward the ring) was substantially smaller than that of its Si, Ge, and Sn analogues, 2b − d, The large dipoles of 2b − d were due to greater charge separation resulting from the lower electronegativities of Si, Ge, and Sn versus that of C. The cyclopentadienyl rings exhibited strong bond alternation indicating only a modest cyclopentadienyl anion-like contribution in 2b − d.

Formation of Phosphaalkyne Trimers: A Mechanistic Study

“…Because the weights of each form strongly depend on the substituents or on the heteroatom, the aromaticity of pentafulvenes is an interesting question . The parent compound fulvene is an isomer of benzene, with a very small degree of aromaticity. Accordingly, its small dipole moment (0.42 D) suggests a small charge separation. , It was pointed out that electron-donor substituents on its exocyclic methylene group increase the aromaticity, , in agreement with the positively charged exo atom in the aromatic resonance structure.…”

Aromaticity of substituted fulvene derivatives: substituent-dependent ring currents

“…14 Combinations of structural, electric and magnetic properties have been used to probe aromaticity of some exotic fulvene systems, in which the CH 2 group is replaced successively by SiH 2 , GeH 2 , SnH 2 , SiF 2 , GeF 2 , and SnF 2 groups. 15,16 One unambiguous criterion of aromatic character in cyclic systems, which underlies all other magnetic indices, is the existence of an induced ring current in the presence of an external magnetic field. The previous considerations suggest substantial substituent-induced changes in the current-density map of fulvene, even to the extent of the growing-in of a full ring current with increase in electron-donating power of the substituent.…”

Methylenecyclopropene: local vision of the first 1B2 excited state

“…But the first synthesis of the molecule was done more than 30 years after, in 1984, and the expected strong dipole moment was finally measured [4][5] [6]. Substitution of the exocyclic position by more (or less) electronegative substituents give rise to very interesting effects [7] [8]. The MCP molecule is also particularly interesting because of the inversion of the dipole moment upon excitation to the first singlet ( 1 B 2 ) [9].…”