Hydrogen-bridged cations are a type of charged species in which a hydrogen atom is simultaneously bonded to two atoms through partial sigma bonds. While best observable in the presence of superacids at room temperature, spectroscopic evidence has suggested that hydrogen-bridged cations exist in ordinary solvents. These ions have been the subject of debate as they constitute a type of charged species of uncertain electronic structure.
Theory Two models provide an explanation of their structure, the classical and the non-classical view. The classical view (Figure 1.a) involves a fast-equilibrating system in which a hydrogen atom rapidly shifts between two adjacent carbon atoms. In this model, fast equilibrium results from a low energy barrier between the two conformations of the molecule, and each conformer has a localized positive charge. The potential energy diagram of this model is characterized by a double-well with two energy minima. The non-classical view (Figure 1.b) involves the delocalization of two electrons over three atoms (1 hydrogen and 2 carbon atoms). The model is characterized by a 3-membered ring where a hydrogen atom is located between two other atoms with partial sigma bonds. The potential energy diagram of this model is characterized by a single energy minimum, where the structure of the cation corresponds to the transition state between the two conformers of the classical view.
Figure 1 - Potential Energy Surface (PES) of (a) classical and (b) non-classical view of a hydrogen-bridged cation.
History For several decades after they were first proposed around 1950, the existence and importance of non-classical ions in organic chemistry was bitterly controversial. As hydrogen bridges discussed here are 3 center (3 atom) - 2 electron bonds, the investigations over the possibilities of such systems laid an important framework from which to understand this bonding. Many of these studies centered around the 2-norbornyl cation. Observations made by Saul Winstein and others suggested that highly delocalized and symmetric intermediates were present in the reactions of various substituted norbornyl cations, evidence for non-classical ions. H. C. Brown, the most outspoken opponent of non-classical ions, believed that such non-classical bonding was invoked far too widely and saw no reason to deviate from the classical idea of rapidly equilibrating, discrete carbocations. In 1973, G. Olah was able to directly observe the 2-norbornyl cation by low-temperature NMR and confirm the presence of a non-classical 2-norbornyl cation, allowing the field to reach some conclusions about the possibilities of 3 center - 2 electron bonds involving carbon. This verification of delocalized sigma bonding in the 2-norbornyl system was a critical foundation for understanding hydrogen bridges between carbon atoms, another form of delocalized sigma bonding.
Scheme 1 - The classical and non-classical models for bonding in the 2-norbornyl cation are shown above. The non-classical model (a) shows one three-center two-electron bond with a delocalized positive charge. The classical model, (b) describes a rapid equilibration between three distinct carbocations rather than delocalization.
Structure Two different types of C-H-C bonding are recognized. The first is an "open" type, entailing linear geometry and negligible bonding between the terminal carbon atoms, while the second is the "closed" type, with triangular geometry allowing bonding interaction between terminal carbons. The relationship between these two types of 3-center 2-electron bonding can be shown through a molecular orbital diagram. Because of the additional overlap between the orbitals, the bonding orbital for the "closed" type is pushed lower in energy relative to the "open" type. The presence of two electrons in this system implies that the closed geometry will be energetically favorable, which has been seen in studies of metal-H-metal systems.
Figure 2 - A molecular orbital diagram for open and closed hydrogen bridged cations with carbon is shown above. The open and closed structures show different orbital overlap which leads to different bonding energy.
Closed C-H-C Bonds In closed 3 center 2 electron bonds, the atoms are arranged in a triangular shape to increase orbital overlap as shown above. Because there are only two electrons in the system, this overlap causes a net reduction in energy relative to the open, linear bond. While the closed C-H-C bond has not been isolated or studied, it is well established that hydrogen-bridged metals prefer the closed triangular bonding pattern. One example of closed C-H-C bonding is seen in the detection of "protonated ethene" through mass spectrometry, C2C+7, with the bridging hydrogen sitting atop the π-bond of ethene. These closed bridges are likely short-lived intermediates, as there is no steric hindrance to prevent further reaction.
Open C-H-C bonds While supposedly less favored energetically, the steric properties of some molecules promote the formation of open C-H-C bridges, as shown below in Scheme 2. In 1978 T.S. Sorenson obtained NMR evidence for hydrido-bridged carbocations with 3-center 2-electron bonds through using the 1,6-dimethyl-1-cyclodecyl cation. The steric restriction of the ten-membered ring allowed the formation of a bridging hydrogen species. Expanding upon this approach, in 1992 McMurry developed the in-bicyclo[4,4,4]-1-tetradecyl cation, where the additional ring vastly improved the stability of the molecule by maintain a more rigid structure around the hydrogen bridge.
Synthesis While they are reactive intermediates, hydrogen bridged cations can be stabilized sterically. Hydrogen bridged cations are generally formed by producing a carbocation through the addition of a proton to an alcohol or alkene. Instead of continuing the reaction through the nucleophile addition of the conjugate base to the carbocation, a neighboring C-H bond can interact with the carbocation to form the hydrogen bridge. In Sorensen's 1978 synthesis and observation of the 1,6-dimethyl-1-cyclododecyl anion, the hydroxyl group of 1,6-dimethyl-1-cyclodecanol was removed using fluorosulfonic acid. This allowed the formation of the 1,6 hydrogen bridge. With McMurry's in-bicyclo[4.4.4]-tetradecyl cation, the alkene across from the methyne hydrogen was protonated by trifluoroacetic acid (TFA) to allow a bridge to the tertiary carbocation bridgehead.
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