“…The initial structures of the transition states were obtained by rotating the external ring relative to the ferrocenyl group and were fully optimized without restrictions. Though there has been some discussion in the literature regarding the multiplicity of cyclopentadienyl cations, the transition states must be singlets and were treated as such. The computed bond lengths in the transition state, 14 , of the neutral molecule were very similar to those in the ground state, and no major geometric reorganization occurred upon rotation.…”
Protonation of 3-ferrocenyl-2,4,5-triphenylcyclopentadienone with trifluoroacetic acid
results in the formation of the first hydroxyfulvalene complex. The dramatic deshielding
and decoalescence of the 1H and 13C NMR signals in the ferrocenyl region offer evidence for
the participation of the metal in the stabilization of the positive charge, which results in
restricted rotation of the ferrocenyl group. As determined by NMR spectroscopy, the
maximum barrier to rotation in the neutral species is ca. 9 kcal mol-1, whereas the minimum
barrier in the cation considerably exceeds 13 kcal mol-1. The difference in rotational barriers
for the neutral and protonated species is rationalized by using DFT calculations.
“…The initial structures of the transition states were obtained by rotating the external ring relative to the ferrocenyl group and were fully optimized without restrictions. Though there has been some discussion in the literature regarding the multiplicity of cyclopentadienyl cations, the transition states must be singlets and were treated as such. The computed bond lengths in the transition state, 14 , of the neutral molecule were very similar to those in the ground state, and no major geometric reorganization occurred upon rotation.…”
Protonation of 3-ferrocenyl-2,4,5-triphenylcyclopentadienone with trifluoroacetic acid
results in the formation of the first hydroxyfulvalene complex. The dramatic deshielding
and decoalescence of the 1H and 13C NMR signals in the ferrocenyl region offer evidence for
the participation of the metal in the stabilization of the positive charge, which results in
restricted rotation of the ferrocenyl group. As determined by NMR spectroscopy, the
maximum barrier to rotation in the neutral species is ca. 9 kcal mol-1, whereas the minimum
barrier in the cation considerably exceeds 13 kcal mol-1. The difference in rotational barriers
for the neutral and protonated species is rationalized by using DFT calculations.
“… 4 B3LYP/6-31G* and MP2/6-31G* (in parentheses) calculated geometries of singlet cyclopentadienyl ( 5 ), indenyl ( 27 ), and fluorenyl ( 28 ) cations. (Reprinted with permission from ref a. Copyright 1997 American Chemical Society.)…”
Section: Cyclopentadienyl and Homocyclopentadienyl
Cationsmentioning
“…By using EPR spectroscopy, it was shown that the planar triplet structure is generated for Cp + 19g. Depending upon the level of calculation, ab initio results show that there is a change in the relative energies of the lower energy structures such as nido ‐ 3 , planar triplet 4 , and vinylcyclopropenyl 5 20e–i. At the G2 level of theory, it is shown that the energy difference between the triplet D 5 h structure 4 or a vinylcyclopropenyl cation 5 is negligible, whereas nido ‐ 3 is high in energy 20j…”
Classical and non-classical isomers of both neutral and dianionic BC(2)P(2)H(3) species, which are isolobal to Cp(+) and Cp(-), are studied at both B3LYP/6-311++G(d,p) and G3B3 levels of theory. The global minimum structure given by B3LYP/6-311++G(d,p) for BC(2)P(2)H(3) is based on a vinylcyclopropenyl-type structure, whereas BC(2)P(2)H(3)(2-) has a planar aromatic cyclopentadienyl-ion-like structure. However, at the G3B3 level, there are three low-energy isomers for BC(2)P(2)H(3): 1) tricyclopentane, 2) nido and 3) vinylcyclopropenyl-type structures, all within 1.7 kcal mol(-1) of each other. On the contrary, for the dianionic species the cyclic planar structure is still the minimum. In comparison to the isolobal Cp(+) and H(n)C(n)P(5-n)(+) isomers, BC(2)P(2)H(3) shows a competition between pi-delocalised vinylcyclopropenyl- and cluster-type structures (nido and tricyclopentane). Substitution of H on C by tBu, and H on B by Ph, in BC(2)P(2)H(3) increases the energy difference between the low-lying isomers, giving the lowest energy structure as a tricyclopentane type. Similar substitution in BC(2)P(2)H(3)(2-) merely favours different positional isomers of the cyclic planar geometry, as observed in 1) isoelectronic neutral heterodiphospholes EtBu(2)C(2)P(2) (E = S, Se, Te), 2) monoanionic heterophospholyl rings EtBu(2)C(2)P(2) (E = P(-), As(-), Sb(-)) and 3) polyphospholyl rings anions tBu(5-n)C(n)P(5-n) (n = 0-5). The principal factors that affect the stability of three-, four-, and five-membered ring and acyclic geometrical and positional isomers of neutral and dianionic BC(2)P(2)H(3) isomers appear to be: 1) relative bond strengths, 2) availability of electrons for the empty 2p boron orbital and 3) steric effects of the tBu groups in the HBC(2)P(2)tBu(2) systems.
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