Structures of Reactive Nitrenium Ions:  Time-Resolved Infrared Laser Flash Photolysis and Computational Studies of Substituted N-Methyl-N-arylnitrenium Ions

Abstract

A series of para-substituted N-methyl-N-phenylnitrenium ions (N-(4-biphenylyl)-N-methylnitrenium ion, N-(4-chlorophenyl)-N-methylnitrenium ion, N-(4-methoxyphenyl)-N-methylnitrenium ion, and N-(4-methylphenyl)-N-methylnitrenium ion) were generated through photolysis of the appropriately substituted 1-aminopyridinium salt. Laser flash photolysis using UV−vis detection as well as photoproduct analysis verified that the expected nitrenium ions were formed cleanly and rapidly following photolysis. Laser flash photolysis with time-resolved infrared detection allowed for structural characterization of the nitrenium ions through observation of a symmetrical aromatic CC stretch in the region 1580−1628 cm-1. The specific frequencies reflect the degree of quinoidal character present in each phenylnitrenium ion (i.e., the degree to which the nitrenium ion resembles a 4-iminocyclohexa-2,5-dienyl cation). The 4-methoxy derivative shows the highest frequency CC stretch, indicating that this strongly π-electron-donating substituent imparts more quinoidal character, and the 4-chloro derivative shows the lowest frequency CC stretch, suggesting that it possesses the least quinoidal character. Quantum calculations using density functional theory (BPW91/cc-pVDZ) were carried out on the same nitrenium ions. The theoretically derived IR frequencies showed excellent quantitative agreement with the experiment. The computed structures show significant bond length alternation in the phenyl rings, shortened C−N bond lengths, and substantial positive charge delocalization into the phenyl rings. All of these effects are more pronounced with increasing π-donating character of the ring substituent. Arylnitrenium ions are well described as 4-iminocyclohexa-2,5-dienyl cations.

Time-Resolved Resonance Raman Spectroscopy and Density Functional Study of 2-Fluorenylnitrene and Its Dehydroazepine Products

“…[66] These short CÀN bonds and the alternation of the C À C bond lengths in the phenyl rings observed for the 2-fluorenylnitrene species indicates both the singlet and triplet states have significant iminocyclohexadienyl character, although to a varying extent. This behavior is similar to that found for several arylnitrenium ions [48,56,60] including the 2-fluorenylnitrenium ion. [60] Symmetric aromatic CC stretch vibrational modes in the 1568 ± 1628 cm À1 region were previously found to correlate with the degree of iminocyclohexadienyl character of several arylnitrenium ions.…”

“…Several studies have used a comparison of vibrational frequencies computed with density functional to experimental frequencies observed in either time-resolved Raman or IR spectra to convincingly assign several different arylnitrenium ions to their singlet ground states, [48,56,60] and we expect a similar approach may be useful for arylnitrene species. We have performed UBPW91/cc-PVDZ calculations to find the optimized geometry and predicted vibrational frequencies for the singlet and triplet 2-fluorenylnitrene species.…”

“…[60] Symmetric aromatic CC stretch vibrational modes in the 1568 ± 1628 cm À1 region were previously found to correlate with the degree of iminocyclohexadienyl character of several arylnitrenium ions. [48,56,60] Comparison of similar modes in 2-fluorenylnitrene indicates this kind of correlation for arylnitrene species is not as clear and appears to be complicated by the nature of the nitrene moiety. For example, the singlet and triplet 2-fluorenylnitrenes have very similar vibrational frequencies of the CC stretch mode (1575, 1579, and 1604 cm À1 for the singlet state and 1577, 1578, and 1605 cm À1 for the triplet state) although the aromatic C À C and CÀN bonds are noticeably different in the optimized geometries shown in Figure 3.…”

“…This is probably partly caused by the complex and by the delocalized nature of the vibrational normal modes that are not clearly associated with mainly one or the other carbon atom rings as was the case for the singlet 2-fluorenylnitrenium ion and some other singlet arylnitrenium ions. [56,60] It is very interesting to compare the optimized geometry of the singlet 2-fluorenylnitrene species to the singlet 2-fluorenylnitrenium ion species (see Table 2 of Ref. [60]).…”

The 2-carbazolylnitrenium ion and its conjugate acid and base forms

“…8 This approach allows for the their generation in solvents such as acetonitrile and water, and has provided direct kinetic information about the reactivity of nitrenium ions with the solvent and with added nucleophiles. The recent use of IR 9 and Raman 10 detection systems is also providing detailed structural information. An approach that our group has used is outlined in Scheme 1.…”