Oxidation of 1,3-diphosphacyclobutane-2,4-diyl with ammoniumyl antimonate and EPR study of the corresponding cation radical

Article abstract of DOI:10.1246/cl.2006.1136

A sterically protected biradical species, 1-tert-butyl-3-methyl-2,4-bis(2, 4,6-tri-tert-butylphenyl)-1,3-diphosphacyclobutane-2,4-diyl was allowed to react with tris(4-bromophenyl)-ammoniumyl hexachloroantimonate as an oxidant and the product was analyzed by EPR to show that the biradical was oxidized to give the corresponding radical cation species. The structure of the cation radical was also supported by ab initio calculations on a model compound. Copyright

Full text of DOI:10.1246/cl.2006.1136

1136
Chemistry Letters Vol.35, No.10 (2006)
Oxidation of 1,3-Diphosphacyclobutane-2,4-diyl with Ammoniumyl Antimonate
and EPR Study of the Corresponding Cation Radical
Masaaki Yoshifuji,ꢀ1 Anthony J. Arduengo, III,1 Tatyana A. Konovalova,1 Lowell D. Kispert,1
Manabu Kikuchi,2 and Shigekazu Itoꢀ2
1Department of Chemistry, The University of Alabama, Tuscaloosa, AL 35487-0336, U. S. A.
2Department of Chemistry, Graduate School of Science, Tohoku University, Aoba, Sendai 980-8578
(Received August 1, 2006; CL-060874; E-mail: myoshifuji@bama.ua.edu)
A sterically protected biradical species, 1-tert-butyl-3-
methyl-2,4-bis(2,4,6-tri-tert-butylphenyl)-1,3-diphosphacyclo-
butane-2,4-diyl was allowed to react with tris(4-bromophenyl)-
ammoniumyl hexachloroantimonate as an oxidant and the prod-
uct was analyzed by EPR to show that the biradical was oxidized
to give the corresponding radical cation species. The structure
of the cation radical was also supported by ab initio calculations
on a model compound.
Me
P
•
(4-BrC6H4)3N+ •SbCl6
–
3
Mes*
Mes*
SbCl6
+
(4-BrC6H4)3N
P
t-Bu
4
Scheme 2.
discussed in light of ab initio calculations on a model system.
Ledwith and co-workers have developed an efficient and
clean one-electron oxidizing reagent, tris(4-bromophenyl)-
ammoniumyl hexachloroantimonate,8 which prompted us to
carry out an oxidation reaction of 3 with this ammoniumyl
antimonate, since radical cation 4 is expected to be generated
without formation of EPR-active by-product.
A blue solution of tris(4-bromophenyl)ammoniumyl hexa-
chloroantimonate (slightly less than one equivalent) in dichloro-
methane was added to a dark blue dichloromethane solution of
compound 3 (ca. 1 mmol) in an EPR sample tube producing a
green solution. The reaction mixture containing the putative
cation radical 4 was sealed under nitrogen and analyzed by
EPR spectroscopy.9 A qualitative UV–vis spectrum (dichloro-
methane) of the adduct (cation radical) showed absorptions at
510, 625, 732, and 846 nm as well as an intensive absorption
due to (4-BrC6H4)3N at 310 nm.10,11
By use of steric protection technique, we have been success-
ful in isolation and characterization of various unusual phospho-
rus compounds.1 An extremely bulky substituent such as 2,4,6-
tri-tert-butylphenyl (here after abbreviated as Mesꢀ) enables us
to isolate compounds containing multiple bonds to phosphorus
(cf. diphosphenes2 and phosphaalkynes3,4).
In a seminal report Niecke and co-workers described an
isolable phosphorus-containing four-membered cyclic biradical,
2,4-dichloro-1,3-diphosphacyclobutane-2,4-diyl (1).5 Compound
1 is sterically stabilized by Mesꢀ at the phosphorus atoms and
electronically stabilized by P atoms in the four-membered ring
together with Cl at the radical carbon center. The chemical prop-
erties of 1 are those of a singlet ground state biradical. Bertrand
et al. reported the synthesis of a four-membered B2P2 ring sys-
tem 2,6 which is the first stable singlet biradical. We recently
reported the formation of 1,3-diphosphacyclobutane-2,4-diyl
(3) in the reaction of a bulky phosphaalkyne, 2-(2,4,6-tri-tert-bu-
tylphenyl)-1-phosphaethyne (MesꢀCꢁP) with tert-butyllithium
and iodomethane.4 This latter species is highly stabilized by
Mesꢀ groups at the carbon atoms, as can be recognized from
the X-ray analysis (Scheme 1).
Figure 1a shows the spectrum which is an overlap of the
reaction product and the starting material.12 Subtraction of this
signal from the EPR spectrum (a) gives us a four-line signal with
small outer peaks and the isotropic g ¼ 2:0025 ꢂ 0:0002 which
(a)
Indeed, compound 3 is extremely stable even at room tem-
perature.4,7 The parent peak in the mass spectrum (ESI-positive)
for 3 was pronounced (Found: m/z 648.4949; Calcd for
C43H70P2: m/z 648.4947). We have also investigated the electro-
chemistry (CV) of 3.4 The biradical 3 is easily and reversibly
oxidized; an observation that suggests a stable cation radical
might also be formed by chemical oxidation. We report here
chemical one-electron oxidation of 3 to produce a P-heterocyclic
radical cation (Scheme 2). The EPR-spectroscopic properties are
(b)
(c)
Me
P
Mes*
P
Me
P
i-Pr
i-Pr
P
P
Figure 1. (a) EPR spectrum of the reaction mixture of 3 with
(4-bromophenyl)ammoniumyl antimonate measured at room
temperature in dichloromethane (experimental). Parameters: mi-
crowave frequency 9.09096 GHz; modulation amplitude 1 G;
microwave power 2 mW; gain 4 ꢃ 103. (b) EPR spectrum of 4
obtained by subtraction of the starting material spectrum from
spectrum (a). (c) Simulated (b) spectrum, see text.
t-Bu
B
B
t-Bu Mes*
Mes*
H
Cl
Cl
C
C H
P
P
P
i-Pr
i-Pr
t-Bu
3
Mes*
1
t-Bu
5
2
Mes* = 2,4,6-t-Bu3C6H2
Scheme 1.
Copyright ꢀ 2006 The Chemical Society of Japan

Chemistry Letters Vol.35, No.10 (2006)
1137
0342921 and CHE-0079498), U. S. A., US Department of
Energy Grant (DE-FG02-86ER13465), and the Department of
Energy, Office of Energy Efficiency and Renewable Energy
under the Hydrogen Storage Grand Challenge (DE-PS36-
03GO93013). The authors thank Prof. Noboru Morita at Tohoku
University for his helpful suggestions and comments.
H
C
–0.59
1.847
1.722
t-Bu 1.868 P89.2
94.6
81.5
P
1.846 Me
+0.84
+0.70
C
1.072
–0.59
H
References and Notes
Figure 2. A visual map of electron spin density in the opti-
mized structure of 5 at the UHF/6-31Gꢀ level. The geometry
at the optimized structure is shown on the right: bond distances
in A and bond angles (italics) in degrees ( ). The sums of the an-
gles: ꢀ(C): 360.0ꢆ, ꢀ(P): 292.8ꢆ (PMe), 350.0ꢆ (Pt-Bu). Mulliken
atomic charges on the ring are also given.
1
2
3
Multiple Bonds and Low Coordination in Phosphorus Chemistry,
ed. by M. Regitz, O. J. Scherer, Georg Thieme Verlag, Stuttgart,
1990. b) K. B. Dillon, F. Mathey, J. F. Nixon, Phosphorus: The
Carbon Copy, Wiley, Chichester, 1998.
a) M. Yoshifuji, I. Shima, N. Inamoto, K. Hirotsu, T. Higuchi,
J. Am. Chem. Soc. 1981, 103, 4587; 1982, 104, 6167. b) M.
Yoshifuji, K. Shibayama, N. Inamoto, T. Matsushita, K.
Nishimoto, J. Am. Chem. Soc. 1983, 105, 2495.
ꢆ
˚
is characteristic of ꢀ-organic radicals (Figure 1b) and could be
attributed to the radical cation 4. The radical cation spectrum
(b) was simulated by using the XSophe computer simulation
software.13 The best fit was obtained with the Lorentzian line
width of 2.2 G, two different 31P(I ¼ 1=2) hyperfine coupling
constants (hfc) constants aP1 ¼ 39:5 G, aP2 ¼ 22:3 G and the
13C (I ¼ 1=2) hfc of 30 G (Figure 1c).14 The large 31P(aP1) hfc
corresponds to the phosphorus atom with Me and the smaller
31P(aP2) corresponds to that with t-Bu, according to an ab initio
a) G. Markl, H. Sejpka, Tetrahedron Lett. 1986, 27, 171. b) R.
¨
Appel, M. Immenkeppel, Z. Anorg. Allg. Chem. 1987, 553, 7.
c) M. Yoshifuji, T. Niitsu, N. Inamoto, Chem. Lett. 1988,
1733. d) M. Yoshifuji, H. Kawanami, Y. Kawai, K. Toyota, M.
Yasunami, T. Niitsu, N. Inamoto, Chem. Lett. 1992, 1053.
H. Sugiyama, S. Ito, M. Yoshifuji, Angew. Chem., Int. Ed. 2003,
42, 3802.
a) E. Niecke, A. Fuchs, F. Baumeister, M. Nieger, W. W.
Schoeller, Angew. Chem., Int. Ed. Engl. 1995, 34, 555. b) O.
Schmidt, A. Fuchs, D. Gudat, M. Nieger, W. Hoffbauer, E.
Niecke, W. W. Schoeller, Angew. Chem., Int. Ed. 1998, 37, 949.
D. Scheschkewitz, H. Amii, H. Gornitzka, W. W. Schoeller, D.
Bourissou, G. Bertrand, Science 2002, 295, 1880.
4
5
ꢄ
calculation using [t-BuP(CH)2PMe]þ (5) as a model for 4.15
Figure 2 shows the optimized structure of 5 with the total
energy of ꢅ954:3680984 au at the UHF/6-31Gꢀ level. The
two carbon atoms assume sp2 configurations while the two phos-
phorus centers are pyramidalized. The computation depicts that
most of the electron spin density is concentrated equally on the
two carbon atoms of the C2P2 ring, and there is much smaller
electron spin density observed on the phosphorus atom attached
to methyl (PMe), while the spin density does not seem to reside
on the other phosphorus attached to t-Bu (Pt-Bu). Accordingly,
the unambiguous assignment of aP1 and aP2 was possible based
on the model calculation. The experimental spectrum (Figure 1b)
was well-reproducible if the protons with hfc < 1:0 G were used
in the simulations. Most of the positive charges in 5 locate on
the phosphorus atoms (with the larger charge at Pt-Bu), which
supports the partially delocalized structure formula 4.
6
7
a) H. Sugiyama, S. Ito, M. Yoshifuji, Chem.—Eur. J. 2004, 10,
2700. b) M. Yoshifuji, H. Sugiyama, S. Ito, J. Organomet. Chem.
2005, 690, 2515.
8
9
a) F. A. Bell, A. Ledwith, D. C. Sherrington, J. Chem. Soc. (C)
1969, 2719. b) A. Ledwith, Acc. Chem. Res. 1972, 5, 133.
EPR measurements at X-band (9 GHz) were carried out with a
Varian E-12 EPR spectrometer, equipped with a rectangular cav-
ity. The magnetic field was measured with a Bruker EPR 035M
gaussmeter, and the microwave frequency was measured with a
model HP 5245L frequency counter for g value determination.
ꢄ
ꢅ
.
10 UV–vis spectrum for (4-BrC6H4)3Nþ SbCl6 was reported
(ꢁmax ¼ 700 nm) and the spectrum of the reaction mixture did
not seem to contain the ammoniumyl antimonite: W. Schmidt,
E. Steckhan, Chem. Ber. 1980, 113, 577.
11 An EI-MS (70 eV) spectrum of the adducts showed obvious
ion peaks corresponding to 3þ (m/z 648) and (4-BrC6H4)3Nþ
(m/z 479).
12 The three-line spectrum of the starting reagent was also observed
(not shown in Figure 1). It could be due to a trace of mono-radical
admixture in 3, the assignment of which is in progress. See: H. D.
Brauer, J. Stieger, J. S. Hyde, L. D. Kispert, G. R. Luckhust,
Molecular Physics 1969, 17, 457. The four-line signals due to 4
are reproducible and stayed alive at room temperature even after
2 months.
13 XSophe Computer Simulation Software Suite (Version 1.1.4),
1993–2004, developed in the Centre for Magnetic Resonance
and Department of Mathematics, The University of Queensland,
Brisbane, Queensland, Australia and Bruker Biospin, GmbH,
Rheinstetten, Germany.
Our results described herein constitute a chemical transfor-
mation of 3 to a novel heterocyclic radical ion through single-
electron transfer procedure. This chemical process mirrors nota-
ble feature of the redox properties of 1,3-diphosphacyclobutane-
2,4-diyls. Recently, Niecke and co-workers reported chemical
reduction and protonation of 1,3-diphosphacyclobutane-2,4-
diyls affording the intriguing heterocyclic anion and cation,
respectively, through removal or addition of the substituents
on the ring.16,17 Therefore, such reactivity of the biradicals,
1,3-diphosphacyclobutane-2,4-diyls, promises to develop heter-
ocyclic phosphorus chemistry by furnishing novel molecular
functionalities.
In conclusion, we were successful in generating a cation rad-
ical chemically by the reaction of 1,3-diphosphacyclobutane-
2,4-diyl with tris(4-bromophenyl)ammoniumyl hexachloro-
antimonate and characterized it by EPR measurement and ab
initio calculation.
14 S. Ito, M. Kikuchi, M. Yoshifuji, A. J. Arduengo, III, T. A.
Konovalova, L. D. Kispert, Angew. Chem., Int. Ed. 2006, 45,
4341.
15 Ab initio calculations were performed at the UHF/6-31Gꢀ
level using a SPARTAN’04 Program Package available from
Wavefunction Inc., Irvine, CA, 2004.
This work was supported in part by Grant-in-Aid for
Scientific Research (Nos. 13304049 and 14044012) from the
Ministry of Education, Culture, Sports, Science and Technology,
Japan, the National Science Foundation (CHE-0413521, CHE-
´
16 M. Sebastian, M. Nieger, D. Szieberth, L. Nyulaszi, E. Niecke,
Angew. Chem., Int. Ed. 2004, 43, 637.
17 M. Sebastian, A. Hoskin, M. Nieger, L. Nyulaszi, E. Niecke,
Angew. Chem., Int. Ed. 2005, 44, 1405.
´
Published on the web (Advance View) September 9, 2006; doi:10.1246/cl.2006.1136