The EPR Spectrum of Triplet Mesitylphosphinidene: Reassignment and New Assignment

Article abstract of DOI:10.1002/anie.201703629

Low-temperature UV-photolysis of mesitylphosphiranes under highly anaerobic conditions leads to the formation of the triplet mesitylphosphinidene (MesP). The recorded X-band EPR spectrum of triplet MesP and the derived zero-field splitting parameter D=4.116 cm^?1 differ significantly from those reported previously for this intermediate. New magnetic parameters of mesitylphosphinidene are discussed along with the results of DFT calculations.

Full text of DOI:10.1002/anie.201703629

A Journal of the Gesellschaft Deutscher Chemiker
Angewandte
Chemie
International Edition
www.angewandte.org
Accepted Article
Title: The EPR spectrum of triplet mesitylphosphinidene: reassignment
and new assignment
Authors: Alexander V. Akimov, Yulia S. Ganushevich, Denis V.
Korchagin, Vasili A. Miluykov, and Eugenii Ya Misochko
This manuscript has been accepted after peer review and appears as an
Accepted Article online prior to editing, proofing, and formal publication
of the final Version of Record (VoR). This work is currently citable by
using the Digital Object Identifier (DOI) given below. The VoR will be
published online in Early View as soon as possible and may be different
to this Accepted Article as a result of editing. Readers should obtain
the VoR from the journal website shown below when it is published
to ensure accuracy of information. The authors are responsible for the
content of this Accepted Article.
To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201703629
Angew. Chem. 10.1002/ange.201703629
Link to VoR: http://dx.doi.org/10.1002/anie.201703629
http://dx.doi.org/10.1002/ange.201703629

10.1002/anie.201703629
Angewandte Chemie International Edition
COMMUNICATION
The EPR spectrum of triplet mesitylphosphinidene: reassignment
a new assignment
Alexander V. Akimov,[a] Yulia S. Ganushevich,*[b] Denis V. Korchagin,[a, c] Vasili A. Miluykov, [b] and
Eugenii Ya. Misochko*[a]
Abstract: Low temperature UV-photolysis of mesitylphosphiranes
under strong anaerobic conditions leads to the formation of the triplet
mesitylphosphinidene (MesP). The recorded X-band EPR spectrum
of triplet MesP and the derived zero-field splitting parameter
D = 4.116 cm-1 differ significantly from those reported previously for
phosphinidene (i.e. mesitylphosphinidene), was reported by Li et
al.12 The recorded broad structureless EPR line at a magnetic
field of 1,150 T was assigned to this intermediate, and its ZFS
parameter of D = 3.521 cm-1 was estimated from the recorded
spectrum. 15 Afterwards, this D value and the recorded EPR
spectrum were used as an experimental test for quantum
chemical calculations4 and cited in various papers and reviews16
this
intermediate.
New
magnetic
parameters
of
mesitylphosphinidene are discussed along with the results of DFT
calculations.
as
an
important
magnetic
parameter
of
triplet
mesitylphosphinidene. It was also used as direct experimental
confirmation of the triplet ground state for this intermediate.
Among the great variety of organic compounds, there is a
relatively small, but very important from practical and theoretical
perspectives class of compounds that contain carbon, nitrogen,
and phosphorus atoms in their extraordinarily low valence state.
These are carbenes, nitrenes and phosphinidenes. Nitrenes and
phosphinidenes are characterized by very strong (~> 10
kcal/mol) ferromagnetic exchange interactions between unpaired
electrons and, as a result, mainly have high-spin ground states.
Thus, nitrenes and phosphinidenes are model systems for
investigations of magnetism in organic high-spin molecules. The
scalar parameter D of the zero-field splitting (ZFS) tensor is an
important magnetic parameter of such molecules. Nitrenes Ar-
(N)x, where X = 1, 2 and 3, have been used to study magnetic
anisotropy in organic molecules for cases in which anisotropic
spin-spin interactions are dominant.1, 2, 3 In contrast to the widely
studied high-spin nitrenes, the spin-orbit contribution to magnetic
anisotropy is expected to be dominant in triplet phosphinidenes
because the spin-orbit coupling constant of the phosphorus
atom is approximately 16 times higher than that of the nitrogen
Scheme 1. Photochemical generation of mesitylphosphinidene (2).
Our interest in the EPR spectra of organic phosphinidenes was
motivated by the study of the “heavy atom” effect on magnetic
anisotropy as a continuation of our previous studies.17, 18, 19 Our
first EPR measurements in photolyzed frozen solutions with
precursors 1a and 1b showed that the spectrum of photolytic
product
2 (Scheme 1) is significantly different from that
previously reported in [12]. In the present work, we focused our
attention on the assignment of the spectrum and magnetic
parameters of the generated triplet intermediate.
4 ,
5
atom.
Therefore, organic phosphinidenes are attractive
objects for studying “heavy atom” effects in magnetic anisotropy.
In contrast to the thoroughly studied carbenes and nitrenes, it is
a paradoxical situation. Although triplet phosphinidenes have
been postulated as short-lived intermediates in numerous
organophosphorus reaction systems,6, 7, 8, 9, 10 few reports of the
direct spectroscopic observations have been made to fully
establish their formation.11, 12, 13, 14 Several attempts have been
made to observe triplet phenylphosphinidene directly using X-
band EPR spectroscopy.12 - 14 In fact, the only example in the
literature, that involves an EPR observation of triplet
The X-band cw EPR spectra of mesitylphosphinidene 2 were
recorded after UV-irradiation of arylphosphiranes 1 in glassy
methylcyclohexane (MCH) at 5 K. In a typical procedure, the
arylphosphirane 1 was dissolved in freshly distilled MCH in the
ratio of 1:1000, placed in a 4 mm o.d. quartz EPR tube, and
sealed under vacuum. The sample tube was placed in the cavity
of an EPR spectrometer equipped with a helium gas-flow
thermostat. Then, the sample was cooled to 5 K to form glass
and photolyzed at 60 K using a low-pressure mercury lamp (at λ
= 254 nm).20 EPR spectra were recorded using a standard 9
GHz spectrometer (Bruker Elexsys 500, the modulation
frequency 100 KHz) at a sufficiently low microwave power to
avoid the saturation effects.
Photolysis of 1a and 1b lead to the fully identical EPR spectra,
demonstrating a strong EPR line at a magnetic field of 1.225 T
with well resolved hyperfine structure on the 31P atom, see Fig. 1.
The recorded EPR line is assigned to the allowed EPR transition
for (xy) canonical orientation of triplet species with a very large D
(where the ZFS parameter D exceeds a quantum of microwave
radiation h (h  0.33 cm-1 for X-band spectroscopy)). The
[a]
[b]
[c]
A. V. Akimov, D. V. Korchagin, E. Ya. Misochko
Institute of Problems of Chemical Physics of RAS
Chernogolovka, Moscow Region, Russia
E-mail: misochko@icp.ac.ru
Yu. S. Ganushevich, V.A.. Miluykov
A.E. Arbuzov Institute of Organic and Physical Chemistry of RAS
Kazan, Russia
E-mail: yu.ganushevich@gmail.com
D. V. Korchagin
Peoples’ Friendship University of Russia (RUDN University)
6 Miklukho-Maklaya St.,117198 Moscow, Russia
Supporting information for this article is given via a link at the end of
the document.
This article is protected by copyright. All rights reserved.

10.1002/anie.201703629
Angewandte Chemie International Edition
COMMUNICATION
parameter D derived from this spectrum equals 4.116 cm-1, see
Fig. 1. The obtained identical EPR spectra of the photolytic
products of 1a and 1b with resolved hyperfine structure belong
simulation was performed using EasySpin program package,25
which was operated with an exact numerical matrix
diagonalization of the spin Hamiltonian
^
^
to the same product, namely, to mesitylphosphinidene
(Scheme 1).
2
Η  gBS  S D S  S A I
for randomly oriented molecules with a total spin of S = 1,
nuclear spin I = ½ for atom 31P, isotropic value of g = 2.0023,
and assuming similar orientations of the principal axes for the
fine structure tensor D and the hyperfine structure tensor A.
Since only an EPR line for the perpendicular (xy) canonical
orientation was detected in the recorded spectrum, only two
perpendicular components of the hyperfine structure tensor A
were derived from this spectrum: Ax = 269 MHz and Ay = 356
MHz.
Figure 1. The EPR spectra recorded after photolysis of arylphosphirane 1b (a)
and 1a (b) in glassy MCH. Microwave frequency 9.47135 GHz. Spectra were
recorded at 5 K. The simulated spectrum for a triplet molecule with the ZFS
parameters D = 4.116 см-1, E = 0.004 см-1 and the hyperfine components on
31P atom Ax = 269 MHz and Ay = 356 MHz (c).
Initially, the recorded EPR spectra seemed surprising, since
these spectra were significantly different from those previously
assigned12 to mesitylphosphinidene 2, which was generated
under similar experimental conditions. Our hypothesis on the
nature of the spectrum, which was reported in the work [12]
(parameter D = 3.521 cm-1), was very close to that for triplet
Figure 2. The EPR spectra recorded after 50 min photolysis of
arylphosphirane 1a in glassy MCH in the sample possessing slow air leak from
the upper tip of the quartz tube (a) and after the subsequent aging of the
sample for 60 min in dark at 5 K (b). The simulated EPR spectrum with
DO2 = 3.572 cm-1 (data from Ref. [22]) for triplet oxygen in solid nitrogen (c).
oxygen in solid nitrogen (DO2 = 3.572 cm-1).21, 22 23 To verify this
,
assumption, we performed measurements on the samples
possessing a slow air leak from the upper tip of the quartz tube,
see the Supporting Information for details. In these samples,
there was slow air freezing over the top of the glassy solution of
precursor 1a in MCH. Figure 2 shows appearance of the well-
The obtained new value of D = 4.116 cm-1 for triplet
mesitylphosphinidene
2 is close to that of the simplest
phosphinidene PH, (DPH = 4.4 cm-1). 26 DFT calculations for
parameter D and hyperfine tensor A were performed to compare
the theoretical and experimental values. The simulation results
are shown in Tables 1 and 2 and Supporting Information. These
calculations suggest the following preliminary conclusions:
1. The contribution of the spin-orbit coupling (SOC) to parameter
D is one order of magnitude greater than the contribution of the
spin-spin (SS) interaction. The main SOC contribution is the
SOMO → SOMO spin flip interaction between excited singlet
and ground triplet states (Table S3).
resolved spectrum of
2
after 50 min of photolysis.
Simultaneously, a new EPR line at 1.17 T appeared in the
spectrum. Then, the intensity of this line continued to grow upon
the subsequent sample aging in the dark at 5 K. The line shape,
position (at magnetic field at 1.17 T), and parameter D = 3.521
cm-1, that corresponded to this line, were very similar to those
recorded in the previous work [12]. A similar EPR line at 1.17 T
was registered in experiments, where an empty quartz tube with
a slow air leak was placed in the cavity of the EPR spectrometer
for a long time at 5 K, see Fig. S11 in the Supporting Information.
These measurements definitely confirm that the previously
reported EPR spectrum12 belongs to triplet oxygen, whereas the
spectrum in Figure 1 and that the derived from the spectrum
parameter D = 4.116 cm-1 correspond to the triplet photolytic
product, specially, triplet mesitylphosphinidene 2.24
2. As proposed in this work, the PBE/Ahlrichs-DZ level of theory
is universal and describes the experimental results for all known
triplet phosphinidenes (Table 1), as well as for high-spin
nitrenes17, 18, 19
.
3. The calculated components (Ax and Ay) of hyperfine structure
tensor are very similar to those obtained from the
experimental spectrum, see Table 2.
A
The magnetic parameters of 2 were determined by comparing
the experimental and simulated spectra, see Fig. 1. The spectral
This article is protected by copyright. All rights reserved.

10.1002/anie.201703629
Angewandte Chemie International Edition
COMMUNICATION
Experimental Section
Table 1. Calculated (PBE/Ahlrichs-DZ) and experimental ZFS parameters
D (cm-1) for triplet phosphinidenes R -P.
Phosphiranes 1a and 1b were synthesized according to a previously
27
28
published literature procedure via reactions of mesitylphosphine and
29
ethylene glycol ditosylate or (±)-2,3-butanediol ditosylate12, respectively.
R-P
R=
Mes
H
DFT calculation
DSO
Expt.
D
Taking into account the high reactivity of 1, all manipulations (synthesis,
purification, and sample preparation) were performed under an
atmosphere of purified nitrogen using standard high-vacuum Schlenk
techniques. The synthesized compounds 1a and 1b were fully
characterized via 1H, 13C, and 31P NMR and MS, elemental analysis, see
the Supporting Information in details.
DSS
DSS+SO
4.209
4.365
0.319
0.414
3.890
4.116a)
4.4b)
3.951
F
0.418
5.295
5.713
5.9b)
Acknowledgements
[a] this work; [b] data from Ref. [26]
The authors thank Dirk Grote, Wolfram Sander and Shigeru
Sasaki for fruitful discussions. This study was supported by
FASO Russia (project 0089-2014-0014) and Russian Academy
of Sciences (program OX-01). The synthetic work was
sponsored by Russian Foundation for Basic Research (RFBR,
grant 16-33-00840 mol_a). D. V. K. thanks Russian Foundation
for Basic Research (RFBR, grant 16-33-00353 mol_a) and the
Ministry of Education and Science of the Russian Federation
(the Agreement number 02.a03.21.0008) for support of quantum
chemical calculations.
Table 2. The hyperfine components (MHz) on 31P atom in triplet
mesitylphosphinidene 2.
Axx
269
242
Ayy
356
339
Azz
-
Aiso
-
Experimental
Calculated
-163
139
(B3LYP/EPRIII)
Keywords: Reactive intermediates • Phosphinidenes • High-
spin molecules • EPR spectroscopy • Quantum chemistry
In summary, the experimental data obtained in the present
work demonstrated that photolysis of arylphosphiranes 1a and
1b in glassy MCH leads to a fully identical EPR spectrum, which
is assigned to the same product of triplet mesitylphosphinidene.
2. The DFT calculations for parameter D and hyperfine coupling
constants on phosphorous atom 31P in mesitylphosphinidene 2
gave parameters very close to those obtained from the recorded
spectra. The experimental EPR spectrum of triplet
1
2
D. Kraskoff, P. Bednarek, L. George, K. Waich, C. Wentrup,
J. Org. Chem. 2006, 71, 4049.
K. Sugisaki, K. Toyota, K. Sato, D.Shiomi, M. Kitagawa, T. Takui in
EPR of Free Radicals in Solids I, Progress in Theoretical Chemistry
and Physics (Eds.: A. Lund and M. Shiotani), Springer,
Science+Business Media: Dordrecht, 2013, 24, p. 363.
D. Grote, W. Sander in Nitrene and Nitrenium Ions (Eds.: D. E. Falvey
and A. D. Gudmundsdottir), John Wiley & Sons, Inc.: Hoboken, New
Jersey, 2013, p.317.
mesitylphosphinidene
2 and zero-field splitting parameter
D = 4.116 cm-1 from this spectrum differ significantly from those
of this intermediate that were previously reported in literature.
Additional experiments confirmed that the previously reported
EPR spectrum belongs to triplet oxygen in frozen air. Therefore,
we consider the results of our work as the first experimental
observation of the EPR spectrum of 2 and registration of its
magnetic parameters. This study demonstrates that highly
reactive intermediates, such as organic triplet phosphinidenes,
can be effectively stabilized in low-temperature solids, if
anaerobic conditions are met at all stages of the experiment.
The detected doublet hyperfine structure in the EPR spectra
provides reliable identification of these molecules in the complex
3
4
Z. Havlas, M. Kyvala, J. Michl, Collect. Czech. Chem. Commun. 2003,
68, 2335.
5
6
F. Neese, J. Chem. Phys. 2007, 127, 164112.
K. B. Dillon, F. Mathey, J. F Nixon, in Phosphorus: The Carbon Copy:
From Organophosphorus to Phospha-organic Chemistry; John Wiley:
Chichester, England, 1998.
7
8
K. Lammertsma, Top Curr. Chem. 2003, 229, 95.
A. W. Ehlers, E. J Bearends; K. Lammertsma, J. Amer. Chem. Soc.
2002, 124, 2831.
9
Z Benko, R. Streubel, L. Nyulaszi, Dalton Trans. 2006, 4321-4327.
A. A. Zagidullin, Y. S Ganushevich, A. G. Kafiyatullina, V. A. Miluykov,
O. G. Sinyashin, Russ. Chem. Bull. 2015, 64, 1986.
J. Glatthaar, G. Maier, Angew. Chem. 2004, 116, 1314-1317; Angew.
Chem. Int. Ed. 2004, 43, 1294.
10
chemical processes. This observation provides
new
opportunities for exploring the structure and chemical properties
of these types of phosphinidenes under low temperature
chemistry and matrix isolation conditions. In addition, these
molecules may serve as a convenient model for the quantum
chemical study of the magnetic anisotropy created by the spin-
orbit interaction that is important for the molecular magnetism
theory.
11
12
X. Li,
S. I Weissman,
T.-S. Lin,
P. P. Gaspar,
A. H. Cowley,
A. I. Smirnov, J. Amer. Chem. Soc. 1994, 116, 7899.
13
14
K. Tsuji, S. Sasaki, M. Yoshifuji, Heteroat. Chem. 1998, 9, 607.
G. Bucher, M. L. G. Borst, A. W. Ehlers, K. Lammertsma, S. Ceola,
M. Huber, D. Grote, W. Sander, Angew. Chem. 2005, 117, 3353.
Two further attempts in Ref’s 13 and 14 to obtain X-band EPR
spectrum of mesitylphosphinidene in the reported region were
unsuccessful.
15
This article is protected by copyright. All rights reserved.

10.1002/anie.201703629
Angewandte Chemie International Edition
COMMUNICATION
16
17
See, for examples Ref’s [7 - 10]. According to the collected data in Web
of Sci., this paper (Ref. 12) obtained 79 references.
23
24
Note that Tsuji et al [13] assigned the recorded weak EPR signal in the
reported region to triplet oxygen, see note 21 in Ref. [13].
E. Ya. Misochko, A. V. Akimov,
A. A. Masitov, D. V. Korchagin,
Note, that very weak broad doublet EPR line at 1260 mT is clearly seen
in the spectrum recorded in Ref. [12] (see Fig. 1 in Ref. [12]). However,
this weak pattern in the spectrum was not discussed in Ref. [12].
I. K. Yakushchenko, S. V. Chapyshev, J. Chem. Phys. 2012, 137,
06438.
18
19
20
E. Ya. Misochko,
S. V. Chapyshev, J. Phys. Chem. A 2015, 119, 2413-2419.
A. V. Akimov, A. A. Masitov, D. V. Korchagin, S. V. Chapyshev,
A. A. Masitov,
A. V. Akimov,
D. V. Korchagin,
25 S. Stoll, A. Schweiger, J. Magn. Reson. 2006, 178, 42-55.
26 K. P. Huber, G. Herzberg, in Constants of diatomic molecules. Molecular
spectra and molecular structure, Van Nostrand Reinhold: NY, 1979.
E. Ya. Misochko, A. Savitsky, J. Chem. Phys. 2015, 143, 084313.
Photolysis at lower temperatures occurred less efficiently, perhaps, due
to the “cage effect”.
27
28
X. Li, K. D. Robinson, P. Gaspar, J. Org. Chem. 1996, 61, 7702-7710
J. D. Masuda, K. C. Jantunen, O. V. Ozerov, K. J. T. Noonan,
D. P. Gates, B. L. Scott, J. L. Kiplinger, J. Am. Chem. Soc. 2008, 130,
21
22
H. Kon, J. Amer. Chem. Soc. 1973, 95, 1045-1049.
L. A. Pardi, J. Krzystek, J. Tesler, L.-C. Brunel, J. Magn. Reson. 2000,
146, 375-378.
2408–2409.
29 E. M. D. Keegstra, J. W. Zwikker, M. R. Roest, L. W. Jenneskens, J. Org.
Chem. 1992, 57, 6678-6680.
This article is protected by copyright. All rights reserved.

10.1002/anie.201703629
Angewandte Chemie International Edition
COMMUNICATION
Layout 1:
COMMUNICATION
EPR spectra of triplet aromatic
carbenes, nitrenes, and
phosphinidenes – feel the
difference: The recorded EPR
spectrum of triplet
Alexander V. Akimov, Yulia S.
Ganushevich,* Denis V. Korchagin,
Vasili A. Miluykov, and Eugenii Ya.
Misochko*
Page No. – Page No.
mesitylphosphinidene and zero-field
splitting parameter D from this
spectrum differ significantly from those
of this intermediate that were
previously reported in literature. The
presence of the doublet hyperfine
structure in the EPR spectra provides
reliable identification of these
molecules in the complex chemical
processes.
The EPR spectrum of triplet
mesitylphosphinidene: reassignment
and new assignment
This article is protected by copyright. All rights reserved.