Time-Resolved Spectroscopic Study of N,N-Di(4-bromo)nitrenium Ions in Selected Solutions

Abstract

Nitrenium ions are important reactive intermediates in chemistry and biology. In this work, femtosecond and nanosecond transient absorption (fs-TA and ns-TA) along with nanosecond time-resolved resonance Raman (ns-TR3) experiments were employed to examine the photochemical pathways of N-(4,4′-dibromodiphenylamino)-2,4,6-trimethylpyridinium BF4− (salt (DN) from just absorption of a photon of light to the production of the important N,N-di(4-bromophenyl)nitrenium ion 2. In acetonitrile (MeCN), the formation of halogenated diarylnitrenium ion 2 was observed within 4 ps, showing the vibrational spectra with strong intensity. The nucleophilic adduct reaction of ion 2 with H2O was also examined in aqueous solutions. The direct detection of the unique ortho adduct intermediate 3 shows that there is an efficient and exclusive reaction pathway for 2 with H2O. The results shown in this paper give new characterization of 2, which can be used to design time-resolved spectroscopy investigations of covalent addition reactions of nitrenium ions with other molecules in future studies.

Time-Resolved Spectroscopic Study of N,N–Di(4–bromo)nitrenium Ions in Acidic Aqueous Solution

“…The singlet ground state properties of the DN was also demonstrated by the nanosecond time-resolved resonance Raman (ns-TR 3 ) spectroscopy data, in which the direct fingerprint information of DN was obtained. Similar reaction mechanisms to those found in Falvey and coworkers’ previous study were also concluded in the near-neutral aqueous solution, as found by a vibrational spectroscopic study [15].…”

“…Additionally, Thomas and coworkers reported that the N,N –di(4–bromophenyl)nitrenium ion ( DN ) formed after photoexcitation of the N –(4,4′–dibromodiphenylamino)–2,4,6–trimethylpyridinium BF 4 - salt ( 1 ) showed more stability and a much longer lifetime than the unsubstituted N,N –diphenylnitrenium ion, the lifetime of which was 124 µs in neat MeCN solution and 6.57 µs in an aqueous solution [13]. Furthermore, our group has determined that the generation of DN both in MeCN and near neutral aqueous solution was around 4 ps [15]. The singlet ground state properties of the DN was also demonstrated by the nanosecond time-resolved resonance Raman (ns-TR 3 ) spectroscopy data, in which the direct fingerprint information of DN was obtained.…”

“…Unlike the 2-fluorenyl nitrenium ion [9,16,17,18,19,20], the lack of the hydrogen on the positive charge nitrogen on DN might show poor acidity for the difficulty in dissociation of a phenyl ring. According to a previous study by Falvey and coworkers [13], the DN in an acidic solution should be able to abstract an electron, followed with an abstraction of a proton to form the radical cation (intermediate 3 ) as shown in Scheme 1 [15]. However, McClelland and coworkers [21] also mentioned that nitrenium ions were like weak bases in acidic aqueous solution and could accept a proton to form the aniline dication species.…”

Photochemical α-Cleavage Reaction of 3’,5’-Dimethoxybenzoin: A Combined Time-Resolved Spectroscopy and Computational Chemistry Study

“…The fs-TA and ns-TA experiments here utilized the same experimental apparatus and methods that have been detailed in previous work [ 25 , 26 , 27 ].…”

Generation of N,N-Di(4-bromophenyl)nitrenium Ion Under Acidic Conditions: Search for a Nitrenium Dication

“…Earlier studies 10,12 demonstrated that N,N-di (4-bromophenyl) for >500 μs in water and does not react at an observable rate with any added nucleophiles. These experiments support the original assignment of the 720 nm signal to the radical cation, rather than the nitrenium dication.…”