Reactivity of Diarylnitrenium Ions

Article abstract of DOI:10.1002/chem.202000930

Hydride abstraction from diarylamines with the trityl ion is explored in an attempt to generate a stable diarylnitrenium ion, Ar₂N⁺. Sequential H-atom abstraction reactions ensue. The first H-atom abstraction leads to intensely colored aminium radical cations, Ar₂NH^.+, some of which are quite stable. However, most undergo a second H-atom abstraction leading to ammonium ions, Ar₂NH₂⁺. In the absence of a ready source of H-atoms, a unique self-abstraction reaction occurs when Ar=Me₅C₆, leading to a novel iminium radical cation, Ar=N^.+Ar, which decays via a second self H-atom abstraction reaction to give a stable iminium ion, Ar=N⁺HAr. These products differ substantially from those derived via photochemically produced diarylnitrenium ions.

Full text of DOI:10.1002/chem.202000930

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Accepted Article
Title: Reactivity of diarylnitrenium ions
Authors: Agnes Kütt, Gunnar Jeschke, Lauri Toom, Jaak Nerut, and
Christopher A. Reed
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10.1002/chem.202000930
Chemistry - A European Journal
Reactivity of diarylnitrenium ions
Agnes Kütt,a,c* Gunnar Jeschke,b Lauri Toom,a Jaak Nerut,a Christopher A. Reedc*
a Institute of Chemistry, University of Tartu, Ravila 14A, 50411 Tartu, Estonia
b Department of Chemistry and Applied Biosciences, ETH Zurich, Vladimir-Prelog-Weg 2, CH-8093 Zurich, Switzerland
c Department of Chemistry, University of California, Riverside, CA 92521, USA
* agnes.kutt@ut.ee and chris.reed@ucr.edu
Nitrenium Conundrum: When exploring synthetic-scale generation of stable diarylnitrenium
ions, Ar2N+, we find products quite different to those derived from photochemical generation.
Keywords: Nitrenium ion, bulky substituents, aminium radical cation, iminium radical cation,
triplet
Hydride abstraction from diarylamines with trityl ion is explored in an attempt to generate
a stable diarylnitrenium ion, Ar2N+. Sequential H-atom abstraction reactions ensue. The
first H-atom abstraction leads to intensely colored aminium radical cations, Ar2NH∙+, some
of which are quite stable. However, most undergo a second H-atom abstraction leading to
+
ammonium ions, Ar2NH2 . In the absence of a ready source of H-atoms, a unique self-
abstraction reaction occurs when Ar = Me5C6, leading to a novel iminium radical cation,
Ar=N∙+Ar, which decays via a second self H-atom abstraction reaction to give a stable
iminium ion, Ar=N+HAr. These products differ substantially from those derived via
photochemically-produced diarylnitrenium ions.
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Introduction
Nitrenium ions, R2N+, isoelectronic with carbenes, are highly reactive intermediates of
dicoordinate positively-charged nitrogen having two free electrons.[1–4] While known as short-
lived species, the replacement of C-bonded with N-bonded substituents (R) can greatly stabilize
two-coordinate cations,[5–7] albeit at the loss of nitrenium ion character. There is a single report
of room temperature stability and isolation of a C-bonded nitrenium ion.[8] The substituents are
meso-porphyrins but in the absence of IR data, which should be diagnostic,[9] it is difficult to be
certain that this apparent nitrenium ion is not instead an amine with a porphyrin π-radical cation
substituent.
We are exploring the possible stabilization and potential isolation of nitrenium ions with bulky
aryl substituents. This strategy has been successful in the isolation of the first silylium ion,
Mes3Si+,[10] and the first borenium ion, Mes2B+.[11] (Mes = mesityl). We employ carborane anions
as counterions in these studies because of their legendary inertness and negligible
nucleophilicity.[12]
Mono-aryl nitrenium ions, ArNH+, have been intensively investigated because of their suspected
role in carcinogenesis[13,14] and their arguable role in the formation of conducting polyanilines.[15]
Di-aryl nitrenium ions have received much less attention. Diphenyl and di-p-halophenyl
nitrenium ions,[16–18] as well as carbazole analogues,[19,20] have been investigated computationally
and experimentally via laser flash photolysis and product trapping. Their lifetimes are short and,
based on transient spectra and product analysis, their electronic states are deduced to be
singlets. In general, singlets act as Lewis acids adding nucleophiles whereas triplets act as
diradicals, sequentially abstracting H-atoms. Herein, we communicate results from attempts to
generate bulky diarylnitrenium ions on a synthetic scale.
Results and Discussion
Synthesis. Because of the availability of diphenylchloroamine, Ph2NCl, we utilized halide
abstraction by silylium ion-like carboranes[21] in attempts to generate the diphenyl nitrenium ion
-
-
(Eq. 1). The counterion of choice is the undeca-chlorocarborane CHB11Cl11 , abbreviated {Cl11 },
and the solvent is weakly nucleophilic o-dichlorobenzene (ODCB).
Ph2NCl + Et3Si+ → Ph2N+ + Et3SiCl
(1)
For bulkier diarylamines, where the corresponding chloroamines are unavailable, hydride
abstraction using trityl ion is the preferred route (Eq. 2). The investigated aryl groups (Ar) include
mesityl (Mes), pentamethyl (C6Me5) and 2,6-di-isopropylphenyl (i-Pr2-C6H3).
Ar2NH + Ph3C+ → Ar2N+ + Ph3CH
(2)
1
Diphenyl. By following H NMR spectra, the product after carrying out reaction (1) is not the
-
diphenylnitrenium ion, rather it is the diphenylammonium ion, 3a. Colorless crystals of the {Cl11 }
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10.1002/chem.202000930
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salt were isolated and characterized by IR and X-ray (Supp Info). This product is consistent with
initial formation of a diradical, possibly a triplet diphenylnitrenium ion, followed by successive H-
atom abstraction reactions (Scheme 1). It confirms that phenyl substituents are unable to
stabilize a long-lived nitrenium ion 1a, or even its single H-atom abstraction product, the
presumed aminium radical cation, Ph2NH∙+, 2a. These synthetic scale results contrast with
conclusions drawn from photochemically-generated diphenylnitrenium ion where spectral data
and trapping reactivity are consistent with reactivity from a singlet state.[16,22] In the absence of
trapping agents, the major product from photo-generation was found to be a carbazole, arising
from internal cyclization.[16] In the presence of trapping nucleophiles, the products were those
arising from reactivity at o,p positions on the aromatic ring.[3,13,23] Such products are absent in
the present work. The present results, suggesting product origin from a triplet state
diphenylnitrenium ion, also contrast with calculations[18] that predict a singlet ground state with
ΔEST > -10 kcal/mol.
●
●
+ H
+ H
+
+
+
●
:
Ar2N
Ar2NH
Ar2NH2
1
2
3
(a) Ar = Ph (b) Ar = Mes (c) Ar = C6Me5 (d) Ar = i-Pr2-C6H3
Scheme 1. Sequential H-atom abstraction from a triplet state nitrenium cation forming an aminium radical cation and then an
ammonium cation.
Dimesityl. Carrying out reaction (2) with Ar = mesityl leads to a fairly stable deep-blue solution
-
consistent with the formation of an aminium radical cation [Mes2NH∙+]{Cl11 }, 2b, possibly via
single H-atom abstraction from solvent by a triplet nitrenium ion. An alternative formation
pathway involving electron transfer from the amine with formation of trityl radical is unlikely, if
not impossible on the synthetic timescale, because of an unfavorable redox potential difference
(554 mV, E°´(Ph₃C⁺/Ph₃C) << E°´(Mes2NH+/Mes2NH), Supp. Info.) and by the absence of
observable trityl radical coupling products, but only triphenylmethane (NMR and GC-MS, Supp.
Info). Aryl aminium radical cations, 2, are best known[24] in the form of “magic blue”, (p-BrC6-
H4)3N∙+, and their salts are notorious for giving oils rather than crystals. The present case is no
exception, even though we have used a carborane counterion known to aid crystallization.
Nevertheless, the deep blue color and the EPR and Mims electron nuclear double resonance
(ENDOR) spectra (Figure 1) are characteristic and agree with expectations for this structure.
Notably, 2b is quite stable and has a sufficiently long electron-spin relaxation time that the Mims
ENDOR spectrum can be measured in liquid solution at ambient temperature. Only very slowly
does it undergo a further H-atom abstraction reaction to eventually give an ammonium ion,
+
Mes2NH2 , 3b.
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Figure 1. X-band CW EPR experimental (black) and simulated (red) spectrum (a) and Mims ENDOR spectrum (b) of 2b at room
temperature in ODCB.
Bis-pentamethylphenyl. When reaction (2) was carried out with Ar = pentamethylphenyl
substituents, the initially orange solution (color from trityl ion) first became red. Then, over
several hours, it turned dark brown and a brown precipitate was obtained upon addition of
1
hexane. H NMR suggested formation of the iminium ion 5 where one phenyl ring has distinct
quinoidal character. This was confirmed by X-ray after recrystallization from ODCB (Figure 2).
Figure 2. Crystal structure of iminium salt 5. The C2-N distance is short (1.33Å), there are alternating single/double C-C bond
lengths in the C2-C7 phenyl ring and C5-C10 is a double bond (1.34Å). The N-C13 distance is long (1.45Å) and the C-C distances
in the C13-C18 phenyl ring are approximately equal (1.38-1.41Å). Between the cation and anion are hydrogen bonds: NH∙∙∙Cl
and CH∙∙∙centroid.
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We understand this reaction to be the result of self H-atom abstraction. The five methyl groups
on the phenyl rings apparently prevent the cationic N center from abstracting H-atoms from the
solvent and instead, H-atom abstraction occurs from its own most vulnerable site, the para
methyl group. As indicated in Scheme 2, the first H-atom abstraction would produce equal
amounts of aminium radical cation 2c and iminium radical cation 4. A second “self” H-atom
abstraction by 4 from the p-methyl group of 2c leads to a high yield of iminium ion 5 (X-ray) and
explains why there is no buildup of color from the required aminium ion intermediate 2c.
●
●
2
4
2c
●
+ H
●
5
Scheme 2. Self H-atom abstraction leading to iminium ion 5 (X-ray).
When the synthetic reaction is carried out in a better H-atom donor solvent, acetonitrile rather
than ODCB, a green-colored solution is obtained. This is ascribed to the aminium radical cation
2c. We attempted to find the byproducts of H∙ abstraction from solvents (by GC MS) but their
identities are swamped by the preponderance of solvent.
Bis(2,6-diisopropylphenyl). Carrying out reaction (2) with extremely bulky 2,6-diisopropyl
groups on the diphenylamine in o-difluorobenzene gave a light green-colored solution from
which colorless crystals were obtained with hexane. The resolution of a single crystal X-ray study
for this material was too low to obtain meaningful metrical data but it can be ascertained that
the cation and anion are well separated – too distant for N-H∙∙∙Cl hydrogen bonding expected of
an ammonium ion, 3d. This implied absence of N-H bonds gave tantalizing evidence for the
isolation of a nitrenium ion, 1d, especially since heating the crystal to 150°C gave rise to bright
green coloration that might be ascribable to radical cation formation, 2c. However, such
optimism was unwarranted. The IR spectrum, obtained on the single crystal used for X-ray,
shows clear evidence of N-H bonds with NH at 3174 cm-1, indicating the product is the
ammonium ion 3d. Thus, the unexpected separation of anion and cation and the absence of
N-H∙∙∙∙Cl hydrogen bonding must arise from crystal packing effects engendered by the bulky
isopropyl groups.
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We note that good old-fashioned IR spectroscopy provides an excellent way of distinguishing the
various species containing N-H bonds. Neutral amines have N-H stretches in the range 3400 –
3450 cm-1 while in aminium radical cations they appear at 3300 – 3400 cm-1, with distinctive
overtones 3560 – 3570 cm-1 and near 3640 cm-1. Ammonium cations that are H-bonded by
weakly basic anions,[25] show two bands in the range 3100 – 3200 cm-1. The iminium ion 5 has a
unique sharp N-H band at 3330 cm-1. IR spectra were not reported for the proposed stable bis-
porphyrin nitrenium ion[8] so it is difficult to be sure that N-H bonds are absent.
Figure 3. IR spectra (4000-2500 cm-1) of selected compounds.
Conclusions
We conclude that no matter how bulky and numerous are the substituents on the phenyl rings,
there is no evidence for a stable diarylnitrenium ion on the synthetic timescale. Our products
outcomes differ unexpectedly from those of photochemically-produced diarylnitrenium ions,
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raising interesting questions about the nature of synthetically-generated intermediate. We are
exploring these questions via calculation. One possibility includes the existence of an n,p open-
shell singlet nitrenium ion, for which there is suggestive evidence.[20]
In attempting to prepare stable diarylnitrenium ions we have expanded the scope of H-atom
abstraction chemistry to produce new amine radical cations whose tendency towards
subsequent H-atom abstraction is shown to vary with the nature of the o,p substituents. A new,
self H-atom abstraction reaction has been uncovered that ultimately leads to formation of a
stable iminium ion.
The prospects for isolating stable diarylnitrenium ions analogous to silylium and borenium ions
appear slim, making the one report of a stable nitrenium[8] all the more remarkable, or doubtful.
Acknowledgements
This study was supported by the Ministry of Education and Science of Estonia grant PUT182 and
PRG676, by the EU through the European Regional Development Fund (TK141 “Advanced
materials and high technology devices for energy recuperation systems”) and the US National
Science Foundation (CHE 1144838). Crystallographers Sergey Belyakov and Anatoly Mishnev
from Latvian Institute in Organic Synthesis are gratefully acknowledged.
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