Observation of the Hammick Intermediate:  Reduction of the Pyridine-2-ylid Ion in the Gas Phase

Abstract

Azacyclohexatriene-2-ylidene (1), the 2-isomer of pyridine (2), has been generated by one-electron reduction of the corresponding radical cation in neutralization−reionization mass spectrometric experiments. The experimental finding that this molecule is stable on the microsecond time scale is in agreement with results of quantum chemical calculations that indicate both 1 and its radical cation, 1•+ , correspond to minima on the C5H5N and C5H5N•+ potential energy surfaces. The calculations predict that 1 is less stable than pyridine, 2, by 50 and 49 kcal/mol (MP2/6-31G** and CASSCF-MP2/6-31G**, respectively) or 47 kcal/mol (B3LYP/6-31G**), whereas the radical cations 1•+ and 2•+ are much closer in energy. The ylid ion 1•+ is predicted to be 6 and 7 kcal/mol lower in energy than 2•+ at the MP2 and CASSCF-MP2/6-31G** levels, respectively, and 1 kcal/mol higher according to the hybrid density functional theory. Calculations also suggest that facile isomerization of the ions is prohibited by an energy barrier, amounting to 62 and 57 kcal/mol at MP2/6-31G** and B3LYP/6-31G**, respectively, relative to 1•+ , which is even larger than the 38 kcal/mol obtained at both levels of theory required for the neutral transformation. Despite the substantial impediments, isomerization of excited species is possible since the lowest dissociation channels lie even higher in energy but the experimental observations confirm that neither the ions or neutrals undergo particularly facile isomerization. Using known thermochemical data a value for ΔH f (1•+ ) = 237 ± 5 kcal/mol was obtained from the measured appearance energy, 10.14 eV, of the C5H5N•+ ion generated from methyl picolinate, which is completely consistent with the theoretical predictions of 237−242 kcal/mol derived from the calculated energy differences between the various species and the known heat of formation of 2.

Abnormal Carbenes as Ligands in Transition Metal Chemistry: Curiosities with Exciting Perspectives

“…Hitherto, free pyridylidenes have been characterized only in the gas phase by mass spectrometry. [34] A versatile methodology to install transition metal centers on pyridinium salts comprises oxidative addition. Following pioneering work by Raubenheimer and coworkers, [35] we have introduced potentially chelating donor groups to the pyridine skeleton.…”

Pyridine-Derived N-Heterocyclic Carbenes: An Experimental and Theoretical Evaluation of the Bonding in and Reactivity of Selected Normal and Abnormal Complexes of Nickel(II) and Palladium(II)

“…28 Significant barriers for intramolecular interconversion allowed for detecting the co-existence of isomers I and II under high vacuum conditions. 26,27 In the condensed phase, intermolecular interconversion is more facile and pyridylidenes have thus far only been observed in complexes, i.e. stabilized by a metal center.…”

Activation of Methane by the Pyridine Radical Cation and its Substituted Forms in the Gas Phase

“…A distinction between ionized pyridine (B) from its α-distonic form (C) (see Scheme 1) has been identified by Schwarz and co-workers [51] 4 . CH 4 and CD 4 were admitted directly to the ion trap via a precision leak valve at a pressure of 3×10 -6 mbar differentiation between the two ions was obtained through an associative ion molecule reaction with dimethyl disulfide as a neutral reagent [50].…”