Stability trends and tautomerization of chalcocyclopentadienes. The role of aromaticity

Abstract

International audienceThe stability trends and tautomerization processes within the family of chalcocyclopentadienes (CpXH, X = O, S, Se, Te) have been investigated through the use of high-level G3B3 and G2 ab initio, as well as B3LYP density functional theory calculations. All methods predict the 5-substituted derivative to be the least stable tautomer, with the only exception of the Te derivative. For X = O, Se the dominant forms are the 2- and the 1-substituted derivatives, respectively, whereas for S, an equal distribution of the two forms is predicted. The enhanced stability of the 5-substituted compound on going from O to Te is due to a parallel increase of the aromaticity of the system, through a charge donation from the C-X bond toward the five membered ring. Independently of the nature of the chalcogen heteroatom, the global minimum of the PES is the asymmetric keto structure 2-cyclopenten-1-one. Its enhanced stability with respect to the symmetric keto form 3-cyclopenten-1-one is due to a more favorable conjugation between the C[double bond, length as m-dash]X and the C[double bond, length as m-dash]C π-systems. This interaction steadily increases on going from O to Te. The barriers connecting the keto and the enolic forms are rather high, as those connecting the enolic forms among each other. Hence, an important conclusion is that, in the gas-phase, if the 5-substituted derivative is formed, it should be observable no matter the nature of the chalcogen heteroatom

Aromaticity from the Viewpoint of Molecular Geometry: Application to Planar Systems

“…For example, the tautomerization of chalcocyclopentadienes (CpXH, X = O, S, Se, Te; see Figure 22) was investigated by means of several indices including magnetic (NICS), structural (HOMA), and electronic (FLU) ones. 312 These compounds can exist in a form of three isomers: 1-, 2-, or 5-substituted. Two rotamers are possible for 1-and 2derivatives, leading to the keto−enol tautomeric equilibrium.…”

On the Origin of the Enhanced Acidity of Chalcocyclopentadienes (Cyclopentadiene Chalcogenols) in the Gas Phase

“…The activation barriers connecting the different anions (see Figure 5) follow a similar trend, O < S Se, to that observed for the corresponding neutral compounds. [18] Also, as has been found for the neutral systems, the barriers are high enough to assume that, on deprotonation of the O-Iso1 and S-Iso1 compounds only the O-C5 and S-C5 anions should be observed. Similarly, only the 5Se and the 5Te anions should be detected in the gas phase after deprotonation of Se-Iso1 and Te-Iso1 derivatives.…”

“…found to depend significantly on the nature of the heteroatom. [18] For the oxygen-containing compounds only two isomers, namely Iso2 and Iso3 should be expected in the gas phase at room temperature, and the latter is the dominant one. For sulfur, a more or less equal distribution of the Iso2 and Iso3 isomers should be expected, whereas for Se, the Iso2 form becomes slightly dominant with respect to Iso3, and a small proportion (3 %) of Iso1 should also be expected.…”

“…Interestingly, for Te Iso1 becomes the dominant isomer in the gas phase at room temperature, with a much lower proportion of isomers Iso2 and Iso3. [18] Although the main goal of our study is to investigate whether these compounds are stronger or weaker acids than cyclopentadiene, other questions regarding their intrinsic acidity are open. The first important issue is to establish the most acidic center for the different isomers, and whether its nature depends on the relative position of the XH substituent in the five-membered ring.…”

Influence of Ring Methylation in Group 15 Tetramethylcyclopentadienyl Complexes, M(C5Me4H)nI3−n (M = As, Sb)

“…The similarities of the structure of 1 , 2 , and Sb(C 5 H 5 ) 3 suggested that a computational investigation of the structure of As(C 5 H 5 ) 3 would be of interest. Calculations have been reported for the cyclopentadienyl arsines As[(C 5 (i-Pr) 4 H]Cl 2 (semiempirical methods), As(C 5 H 5 )H 2 (HF, MP2, DFT, , and coupled cluster methods), and As(C 5 H 5 )Cl 2 (DFT and coupled cluster methods); we are not aware of previous calculations on the tris(cyclopentadienyl)arsines themselves.…”