High-spin cation radicals of methylenephosphoranes

Article abstract of DOI:10.1021/ja964185e

Novel di(cation radical)s and tri(cation radical)s are prepared by oxidation of the corresponding 1,3-phenylenebis[[(4-tert- butylphenyl)methylene]triphenylphosphorane] and 1,3,5-benzenetriyltris[[(4- tert-butylphenyl)methylene]triphenylphosphorane] precu

Full text of DOI:10.1021/ja964185e

5398
J. Am. Chem. Soc. 1997, 119, 5398-5403
High-Spin Cation Radicals of Methylenephosphoranes
Martijn M. Wienk and Rene´ A. J. Janssen*
Contribution from the Laboratory of Organic Chemistry, EindhoVen UniVersity of Technology,
P.O. Box 513, 5600 MB EindhoVen, The Netherlands
ReceiVed December 4, 1996X
Abstract: Novel di(cation radical)s and tri(cation radical)s are prepared by oxidation of the corresponding 1,3-
phenylenebis[[(4-tert-butylphenyl)methylene]triphenylphosphorane] and 1,3,5-benzenetriyltris[[(4-tert-butylphenyl)-
methylene]triphenylphosphorane] precursors. The oligo(cation radical)s are investigated in frozen solutions using
ESR spectroscopy. The di(cation radical) has a triplet state as evidenced from a ∆Ms ) (2 ESR transition exhibiting
hyperfine coupling to two identical phosphorus nuclei and is characterized by zero-field splitting parameters D )
350 MHz and E ) 0 MHz. The corresponding tri(cation radical) possesses a quartet state with D ) 262 MHz and
E ) 0 MHz and exhibits a ∆Ms ) (3 transition. Temperature dependent studies (4-100 K) reveal that the ESR
intensities follow Curie’s law, consistent with high-spin ground states. The stability of these oligo(cation radical)s
is assessed via cyclic voltammetry at room temperature in THF solution.
Introduction
coupling, 1,3-phenylene is known to be a fairly robust ferro-
magnetic coupling unit, even for significantly twisted geom-
etries.6,7
High-spin molecules in which organic radicals are coupled
via ferromagnetic coupling units attract considerable attention
as potential building blocks for future organic ferromagnetic
materials.1-3 This interest is motivated by the fact that
intramolecular coupling of unpaired electron spins is usually
associated with much stronger exchange interactions than the
spin alignment that can be obtained via intermolecular interac-
tions.
Compared to the well-known triarylmethyl radicals, substi-
tuted diphenylmethylene (Ph2C˙ X) radicals may provide a higher
spin density at the methylene carbon atom directly linked to
the 1,3-phenylene ring, which may be beneficial for a stronger
exchange interaction. As an example, ketyl radicals (Ph2C˙ O-),
which are isoelectronic with nitroxide radicals (R2N˙ O), have
recently been considered for the preparation of high-spin
molecules.8 While appealing for their stability, ketyl radicals
and nitroxide radicals exhibit an appreciable delocalization of
the spin density onto the oxygen nucleus, which is estimated to
be on the order of 50% for nitroxide radicals.9 Diphenylmeth-
ylenephosphorane cation radicals (Ph2C˙ PPh3+) can be expected
to give a higher spin density at the methylene carbon atom,
because almost no direct delocalization of electron spin occurs
onto the phosphorus nucleus.10,11
In designing novel high-spin molecules, a variety of intramo-
lecular ferromagnetic coupling units has been investigated.
Several studies have shown that 1,3-connected and 1,3,5-
connected benzenes act as strong and versatile ferromagnetic
couplers in one and two dimensions, respectively. The meta
substitution pattern of these units provides a non-Ke´kule
resonance structure in which the topological symmetry and in-
phase periodicity of spin polarization favor intramolecular
ferromagnetic coupling of the electron spins. Using 1,3-
phenylene and 1,3,5-benzenetriyl, high-spin ground state mol-
ecules have been obtained up to S ) 9 for a nonacarbene and
S ) 5 for a deca(triarylmethyl radical).4,5
An important aspect of high-spin molecules is the nature of
the radical center, since it determines the chemical stability and
spin density at the coupling unit. For effective spin alignment,
the spin density at the coupling unit must be high. In contrast,
however, chemical stability usually increases with the extension
of the delocalization of the unpaired electron. This inherent
dichotomy has resulted in an extensive search for novel organic
radicals that can be incorporated in high-spin systems. As a
result various oligoradicals have been described, often containing
carbon and nitrogen centered electron spins.1-3 Although the
dihedral angle between the spin-carrying unit and the ferro-
magnetic coupling unit is likewise important for effective
Here we describe the formation and characterization using
ESR spectroscopy of two novel high-spin molecules (1a2•2+ and
1b3•3+, Scheme 1) containing two and three methylenephos-
phorane cation radicals as spin centers, linked via central 1,3-
phenylene and 1,3,5-benzenetriyl coupling units, respectively.
The cation radicals are generated in situ via electrochemical or
chemical oxidation reactions. Recently, similar procedures have
been employed to generate high-spin phenylenediamine and
1,3,5-benzenetriyltriamine oligo(cation radicals).12,13
(6) Veciana, J.; Rovira, C.; Ventosa, N.; Crespo, N. I.; Palacio, F. J.
Am. Chem. Soc. 1993, 115, 57.
(7) Silverman, S. K.; Dougherty, D. A. J. Phys. Chem. 1993, 97, 13273.
(8) (a) Baumgarten, M. Mol. Cryst. Liq. Cryst. 1995, 272, 109. (b) Jang,
S.-H.; Mitchell, C.; Jackson, J. E.; Kahr, B. Mol. Cryst. Liq. Cryst. 1995,
272, 147.
(9) Forrester, A. R.; Hay, J. M.; Thomson, R. H. Organic Chemistry of
Stable Free Radicals; Academic Press: London, 1968.
(10) (a) Lucken, E. A. C.; Mazeline, C. J. Chem. Soc. A. 1966, 1074.
(b) Lucken, E. A. C.; Mazeline, C. J. Chem. Soc. A. 1967, 439.
(11) (a) Begum, A.; Lyons, A. R.; Symons, M. C. R. J. Chem. Soc. A,
1971, 2388. (b) Lyons, A. R.; Neilson, G. W.; Symons, M. C. R. J. Chem.
Soc., Faraday Trans. 2 1992, 68, 807.
(12) (a) Stickley, K. R.; Blackstock, S. C. J. Am. Chem. Soc. 1994, 116,
11576. (b) Stickley, K. R.; Selby, T. D.; Blackstock, S. C. J. Org. Chem.
1997, 62, 448.
X Abstract published in AdVance ACS Abstracts, May 15, 1997.
(1) Iwamura, H. AdV. Phys. Org. Chem. 1990, 26, 179.
(2) Rajca, A. Chem. ReV. 1994, 94 871.
(3) Miller, J. S.; Epstein, A. J. Angew. Chem., Int. Ed. Engl. 1994, 33,
385.
(4) (a) Nakamura, N.; Inoue, K.; Iwamura, H. Angew. Chem., Int. Ed.
Engl. 1993, 32, 872. (b) Matsuda, K.; Nakamura, N.; Inoue, K.; Koga, N.;
Iwamura, H. Bull. Chem. Soc. Jpn. 1996, 69, 1483.
(5) Racja, A.; Utamapanya, S.; Thayumanavan, S. J. Am. Chem. Soc.
1992, 114, 1884.
(13) (a) Wienk, M. M.; Janssen, R. A. J. Chem. Commun. 1996, 267.
(b) Wienk, M. M.; Janssen, R. A. J. J. Am. Chem. Soc. 1996, 118, 10626.
(c) Wienk, M. M.; Janssen, R. A. J. J. Am. Chem. Soc. 1997, 119, in press.
S0002-7863(96)04185-6 CCC: $14.00 © 1997 American Chemical Society

High-Spin Cation Radicals of Methylenephosphoranes
J. Am. Chem. Soc., Vol. 119, No. 23, 1997 5399
Scheme 1a
a (a) tert-butylbenzene, AlCl3, CH2Cl2. (b) LiAlH4, THF. (c) PBr3, toluene. (d) triphenylphosphine, toluene. (e) NaNH2, THF. (f) AgBF4, THF.
Scheme 2
Results and Discussion
at 0.18 V vs SCE which is associated with the formation of a
diphenylmethylenephosphorane cation radical (Figure 1). This
cation radical has a limited stability, and in a second scan an
additional oxidation wave is observed at -0.08 V vs SCE. This
secondary product is identified using ESR (Vide infra) as 4,4′-
biphenylenebis(methylenetriphenylphosphorane) formed by
coupling of the cation radical with a second molecule of 6 via
the para positions (Scheme 2).
Synthesis. The synthesis of 1a and 1b is outlined in Scheme
1. Isophthaloyl chloride or 1,3,5-benzenetricarbonyl trichloride
are reacted with tert-butylbenzene in a Friedel-Crafts acylation
to afford 1,3-phenylene diketone (2a) and 1,3,5-benzenetriyl
triketone (2b), respectively. Reduction to the corresponding
diol (3a) and triol (3b) is accomplished with lithium aluminum
hydride in THF, and subsequent conversion to the corresponding
bromides 4a and 4b is carried out using phosphorus tribromide
in toluene. Reaction of 4a and 4b with triphenylphosphine
provides the bis(phosphonium bromide) 5a and tris(phospho-
nium bromide) 5b. Deprotonation of these phosphonium salts
with potassium methoxide or sodium amide in THF affords the
methylenephosphoranes 1a and 1b. These precursors are stable
at -20 °C but slowly decompose at room temperature.
Cyclic Voltammetry. Cyclic voltammetry in THF solution
containing 0.1 M tetrabutylammonium hexafluorophosphate
(TBAH) was used to assess the stability of the different redox
states of the methylenephosphoranes. The cyclic voltammo-
gram of the parent (diphenylmethylene)triphenylphosphorane
(6, Scheme 2) displays a quasi-reversible one-electron oxidation
Cyclic voltammetry of 1a at ambient temperature reveals two
quasi-reversible one-electron oxidation waves at -0.14 and 0.23
V vs SCE (Figure 1). The quasi-reversible wave indicates that
the doubly oxidized state of bis(methylenephosphorane) 1a is
moderately stable at room temperature. In this case, however,
coupling reactions as observed for 6 are effectively suppressed
by the p-tert-butyl substituents on terminal phenyl rings. In a
similar experiment, one-electron oxidations of the tris(methyl-
enephosphorane) 1b occurred at -0.29, -0.01, and 0.27 V vs
SCE (Figure 1).
The electrochemical experiments demonstrate that the first
oxidation potential decreases by nearly 0.5 V when the number
of methylenephosphoranes linked to the central benzene unit

5400 J. Am. Chem. Soc., Vol. 119, No. 23, 1997
Wienk and Janssen
Figure 2. ESR spectra of (a) cation radical 6•+ obtained via iodine
oxidation of 6 in THF. (b) The adduct cation radical 6-6•+, prepared
via electrochemical oxidation of 6 in THF/TBAH (0.1 M). Hyperfine
parameters are described in the text.
Figure 1. Cyclic voltammograms of methylenephosphoranes 6, 1a,
and 1b recorded in THF/TBAH (0.1 M) at 295 K, scan rate 100 mV/s,
potential vs SCE calibrated against Fc/Fc+ (0.47 V).
increases from one to three. Clearly, this decrease results from
the increasing repulsive Coulombic interaction of the negative
charges at the methylene carbon atoms. The effect of conjuga-
tion on the oxidation potential is likely to be small as a result
of the nonresonant meta substitution pattern at the central ring.
The final oxidation step for 6, 1a, and 1b occurs at almost the
same potential (0.18-0.27 V vs SCE). Similar effects have
been described for perchlorotriarylmethyl anions.6
ESR Spectroscopy. Chemical oxidation of 6 at room
temperature in THF using iodine or AgBF4 produces the cation
radical 6•+ which exhibits a well-resolved ESR spectrum due
to hyperfine coupling with a phosphorus nucleus (A(P) ) 74.5
MHz) and the protons of the diphenylmethylene moiety (A(o-
H) ) 7.60 MHz, A(m-H) ) 3.28 MHz, and A(p-H) ) 8.32 MHz)
(Figure 2).14 Under these conditions the cation radical is
moderately stable. When prepared via in situ electrochemical
oxidation, the ESR spectrum initially indicates the formation
of 6•+, which under these conditions, however, rapidly couples
oxidatively with a second molecule of 6 at the para positions
to the adduct cation radical 6-6•+. The ESR spectrum of 6-6•+
(Figure 2) is a triplet of pentuplets resulting from hyperfine
coupling with two identical 31P nuclei (A(P) ) 32.82 MHz) and
Figure 3. ∆Ms ) (1 ESR spectrum of 1a recorded in THF at 120 K
after oxidation with AgBF4, showing the transitions assigned to 1a•+
-
([) and 1a2•2+(O). The simulation is a 2:5 superposition of the spectra
of mono(cation radical) 1a•+ and a di(cation radical) 1a2•2+ calculated
using the zero-field and hyperfine coupling parameters described in
the text. The inset shows the ∆Ms ) (2 ESR spectrum of 1a2•2+
.
MHz) and three different sets of two equivalent 1H nuclei (A(H)
) 11.5, 3.75, and 2.75 MHz) indicate appreciable spin delo-
calization over both the terminal and central benzene rings.
Further oxidation of 1a•+ produces 1a2•2+. The anisotropic ESR
spectrum recorded in frozen THF at 120 K reveals a superposi-
tion of the spectra of the mono(cation radical) and the triplet
di(cation radical) (Figure 3). The two strong central transitions
are attributed to the mono(cation radical) 1a•+, possessing an
essentially isotropic 31P hyperfine coupling. The broad lines
in the lateral regions of the spectrum are the ∆Ms ) (1
transitions of the di(cation radical) 1a2•2+, exhibiting a zero-
field splitting characteristic of a triplet state. Weak shoulders
on the xy components of the ESR spectrum of 1a2•2+ are
tentatively attributed to partially resolved 31P hyperfine coupling.
Further evidence for the formation of di(cation radical) 1a2•2+
in a triplet state is obtained from the ESR spectrum recorded in
the g ) 4 region, where the formally forbidden ∆Ms ) (2
transition of 1a2•2+ is observed (Figure 3). The ∆Ms ) (2
ESR spectrum exhibits a well-resolved 1:2:1 three-line pattern
resulting from hyperfine interaction of the S ) 1 spin with two
1
four identical H nuclei (A(H) ) 4.96 MHz, at 3-, 3′-, 5-, and
5′-positions), identical to the ESR spectrum obtained via
oxidation of 4,4′-biphenylenebis[phenylmethylenetriphenyl-
phosphorane] (6-6).15 When electrochemical oxidation is
carried out at 240 K, the coupling reaction is suppressed.
To prevent side reactions, chemical oxidation of the bis- and
trismethylenephosphoranes 1a and 1b was carried out at 195
K. At that temperature chemical oxidation of 1a with AgBF4
in THF initially produces the corresponding mono(cation radical)
1a•+, which is characterized by a partially resolved ESR spec-
trum. Hyperfine interactions with one 31P nucleus (A(P) ) 72.5
(14) Buck, H. M.; Huizer, A. H.; Oldenburg, S. J.; Schipper, P. Recl.
TraV. Chim. 1970, 89, 1085.
(15) Kooistra, C.; van Dijk, J. M. F.; van Lier, P. M.; Buck, H. M. Recl.
TraV. Chim. 1973, 92, 961.

High-Spin Cation Radicals of Methylenephosphoranes
J. Am. Chem. Soc., Vol. 119, No. 23, 1997 5401
equivalent 31P nuclei, which exhibit half the hyperfine coupling
constant observed for mono(cation radical) 1a•+. The identical
hyperfine coupling to two 31P nuclei directly relates the spectrum
to the proposed structure of 1a2•2+. Apart from the intensity,
the ESR spectrum does not change in the temperature range of
4 to 130 K.
The ESR spectrum of mono(cation radical) 1a•+ can be
simulated assuming an isotropic 31P hyperfine coupling constant
of A(P) ) 65 MHz and a Gaussian line width of 48 MHz.16
The hyperfine coupling constant of 1a•+ was used as starting
point to obtain a spectral simulation for 1a2•2+. We found that
the remaining transitions in the ESR spectrum attributed to
1a2•2+ are reproduced satisfactorily by taking D ) 350 MHz
and E ) 0 MHz for the zero-field splitting parameters, an
isotropic hyperfine constant of A(P) ) 32.5 MHz (i.e., half the
coupling constant of 1a•+), and using a Gaussian line width of
85 MHz (Figure 3).17 The increased line width of 1a2•2+ as
compared to 1a•+ explains the loss of hyperfine structure in
the ∆Ms ) (1 triplet spectrum. Adding the simulated spectra
of 1a•+ and 1a2•2+ in a 2:5 ratio provides a good agreement
with the experimental spectrum (Figure 3) and demonstrates
the fairly efficient production of di(cation radical)s.
Figure 4. Temperature dependence of ESR signal intensity of the ∆Ms
) (1 (9) and ∆Ms ) (2 (0) transitions of di(cation radical) 1a2•2+
.
Solid lines are least-square fits to Curie’s law.
The zero-field splittings and hyperfine couplings obtained
from the spectral simulation can be used to assess the electronic
structure of 1a•+ and 1a2•2+ in some detail. The isotropic
hyperfine coupling A(P) ) 65 MHz of 1a•+ suggests that the
spin density at the methylene carbon is about 57%, when
compared to A(P) ) 114 MHz of the methylenetriphenylphos-
phorane cation radical (Ph3PCH2•+), for which unit spin density
on carbon can be assumed.10b The zero-field splitting of D )
350 MHz for 1a2•2+ is significantly larger than the value of
237 MHz reported for Schlenk’s biradical. This indicates an
increased spin density on the methylene carbon for a diphenyl-
methylenephosphorane cation radical as compared to a triaryl-
methyl radical.18 The D value of 350 MHz for 1a2•2+
corresponds within the point-dipole approximation to a distance
between the radical centers of about 6.1 Å. Standard bond
lengths suggest a distance of only 4.85 Å between the two
methylene carbon nuclei. The difference can be interpreted as
an indication for some localization of spin density in the terminal
4-tert-butylphenyl rings which may result from a twisting of
the methylene carbons out of the plane of conjugation with the
central 1,3-phenylene unit as consequence of steric hindrance.
Figure 5. ∆Ms ) (1 ESR spectrum of 1b recorded in THF at 120 K
after oxidation with AgBF4, showing the transitions assigned to 1b•+
([), 1b2•2+(O), and 1b3•3+(#). The simulation is a 1:6:14 superposition
-
of the spectra of mono(cation radical) 1b•+, di(cation radical) 1b2•2+
,
and tri(cation radical) 1b3•3+ calculated using the zero-field and
hyperfine coupling parameters described in the text.
concluded that in either case the triplet state corresponds to a
low-energy state.19
The ESR spectrum of 1b recorded at 150 K after oxidation
with AgBF4 in THF at 195 K reveals the presence of the
corresponding mono(cation radical), di(cation radical), and tri-
(cation radical) (Figure 5). In the central region of the ESR
spectrum, the strong partially-resolved 31P doublet of 1b•+ is
observed, accompanied on each side by the transitions of triplet-
state 1b2•2+ showing some poorly resolved hyperfine structure.
The broad lines in the outer parts are attributed to the ∆Ms )
(1 transitions of 1b3•3+ in a quartet state. The appearance of
the ESR spectrum does not change between 4 and 130 K, apart
from intensity. The presence of high-spin states can further be
inferred from the ESR spectra at lower fields. In the g ) 4
region (Figure 6) the 1:2:1 three-line pattern of the ∆Ms ) (2
transition of 1b2•2+ is observed, resulting from a triplet state
with a hyperfine coupling to two identical 31P nuclei with a
coupling constant of about 33 MHz. In the outer regions of
the half-field spectrum a weak pattern can be discerned which
is attributed to the ∆Ms ) (2 signal of 1b3•3+ in a quartet state.
The ∆Ms ) (2 spectrum of a quartet state is expected to exhibit
Variable temperature experiments in the range from T ) 3.8
to 100 K on the ∆Ms ) (1 and ∆Ms ) (2 ESR transitions of
1a2•2+ demonstrate that the signal intensity follows Curie’s law
(I ) C/T), indicating that the population of the triplet state is
independent of temperature (Figure 4). Hence, the triplet state
is separated from the corresponding singlet state by either a
very large or a very small energy gap. Although it is not
possible to resolve this ambiguity using ESR, it can be
(16) The apparent decrease of A(P) in the spectrum recorded in frozen
THF matrix at 120 K as compared to the isotropic coupling of 72.5 MHz
in THF solution at 195 K, is readily explained by a small anisotropy of the
ESR spectrum, which is dominated by the perpendicular couplings. In fact,
an equally adequate simulation for 1a•+ can be obtained using an axially
symmetric hyperfine tensor with principal values A(P) ) 89.5, 64, and 64
MHz, consistent with an isotropic coupling of A(P) ) 72.5 MHz. However,
since this anisotropy is not resolved in the spectrum, we favor a description
using only one parameter.
(17) Simulations show that the weak shoulders in the experimental ESR
spectrum of 1a2•2+ can be due to unresolved hyperfine coupling, an E * 0
value, or a combination thereof. The limited resolution does not allow
discrimination between these possibilities.
(18) Kothe, G.; Denkel, K.-H.; Su¨mmermann, W. Angew. Chem. 1970,
82, 935.
(19) Berson, J. A. In The Chemistry of Quinoid Compounds; Patai, S.,
Rappaport, Z. Eds.; Wiley: 1988; Vol II, Chapter 10.

5402 J. Am. Chem. Soc., Vol. 119, No. 23, 1997
Wienk and Janssen
Figure 6. ∆Ms ) (2 ESR spectrum of di(cation radical) 1b2•2+ (O)
and tri(cation radical) 1b3•3+ (#). The signal marked with an asterisk is
a background signal. The inset shows the ∆Ms ) (3 ESR spectrum of
Figure 7. Temperature dependence of ESR signal intensity of the ∆Ms
) (1 (9) transition of tri(cation radical) 1b3•3+. Solid line is a least-
square fit to Curie’s law.
1b3•3+
.
Conclusion
four lines with a spacing of D/2, D, and D/2 respectively.20,21
Experimentally, the two inner lines appear as clear shoulders
on the ∆Ms ) (2 transition of 1b2•2+, while the outer two lines
could not be identified with certainty. The spectral intensity
of the ∆Ms ) (2 transition of 1b3•3+ is considerably reduced
compared to the ∆Ms ) (2 transition of 1b2•2+ as a result of
the increased spectral width. Direct spectral evidence of a
quartet state for 1b3•3+ is obtained in the g ) 6 region, where
the ∆Ms ) (3 transition is observed. This leaves no doubt on
the formation of a quartet state. In literature only few organic
polyradicals have been reported that exhibit a ∆Ms ) (3
transition in the ESR spectrum.8a,21-23
Novel oligo(cation radical)s 1a2•2+ and 1b3•3+ have been
prepared by chemical oxidation of the corresponding oligo-
(methylenephosphoranes). These oligo(cation radical)s have
been fully characterized using ESR spectroscopy and are found
to possess a triplet and quartet ground state, respectively, as a
consequence of the meta substitution pattern at the central
benzene ring.
Experimental Section
General Methods. Commercial grade reagents were used without
further purification. Solvents were purified, dried, and degassed
following standard procedures. NMR spectra were recorded on a
Bruker AM-400 spectrometer, and chemical shifts are relative to TMS
for 1H and 13C-NMR spectra and relative to aqueous 85% H3PO4
solution (external standard) for 31P NMR spectra. Cyclic voltammo-
grams were recorded with 0.1 M tetrabutylammonium hexafluorophos-
phate as supporting electrolyte using a Potentioscan Wenking POS73
potentiostat. The working electrode was a platinum disc (0.2 cm2),
the counter electrode was a platinum plate (0.5 cm2), and a saturated
calomel electrode was used as reference electrode, calibrated against a
Fc/Fc+ couple. ESR spectra were recorded on a Bruker ER 200D
spectrometer, operating with an X-band standard or TMH cavity,
interfaced to a Bruker Aspect 3000 data system. Temperature was
controlled by a Bruker ER4111 variable-temperature unit between 100
and 300 K or by an Oxford 3120 temperature controller combined with
an ESR900 continuous flow cryostat in the range 4-100 K. Saturation
of the ESR signal during variable temperature experiments was avoided
by using low microwave powers, i.e., 200 nW for the ∆Ms ) (1
transition and 1 mW for the ∆Ms ) (2 transition, which is well within
the range where signal intensity is proportional to the square root of
the microwave power at 4 K.
The spectral simulation of the g ) 2 region of the mixture
of the three oxidation states 1b•+, 1b2•2+, and 1b3•3+ is shown
in Figure 5. For this simulation the hyperfine and zero-field
parameters of 1b•+ (S ) 1/2, A(P) ) 65 MHz, Gaussian line
width of 45 MHz) and 1b2•2+ (S ) 1, D ) 350, E ) 0, A(P) )
32.5 MHz, Gaussian line width of 32 MHz) are identical to
.
those of 1a•+ and 1a2•2+ In contrast to 1a2•2+, the A(P)
hyperfine coupling of 1b2•2+ is partially resolved due to a
reduced line width of the triplet spectrum. For 1b3•3+ we found
that the experimental spectrum can be simulated by taking S )
3/2, D ) 262, E ) 0 MHz and a Gaussian line width of 98
MHz (no 31P hyperfine coupling was used in this case). By
adding the spectra of 1b•+, 1b2•2+, and 1b3•3+ in a 1:6:14 ratio,
an acceptable correspondence was obtained with the experi-
mental spectrum. Although the point-dipole approximation for
a triradical is not straightforward, the decreased zero-field
splitting for 1b3•3+ (D ) 262 MHz) as compared to 1a2•2+ and
1b2•2+ (D ) 350 MHz) suggests a rise in average distance
between the unpaired electrons from 6.1 to 6.7 Å.
1,3-Phenylenebis[(4-tert-butylphenyl)methanone] 2a. Isophthaloyl
chloride (10.15 g, 0.05 mol) in CH2Cl2 (20 mL) was added slowly to
a suspension of AlCl3 (16.0 g, 0.12 mol) in CH2Cl2 (50 mL) with stirring
and cooling in an ice bath. Then tert-butylbenzene (16.1 g, 0.12 mol)
was added keeping the temperature at 20 °C. After stirring for 18 h at
room temperature the mixture was poured onto ice. The organic phase
was separated, and the aqueous phase was extracted with CH2Cl2. The
combined organic fractions were washed with dilute NaOH solution
and water and dried over MgSO4. Evaporation of the solvent and
recrystallization from ethanol afforded pure 2 (14.6 g, 74%) as a white
The signals at g ) 4 and g ) 6 of 1b3•3+ were too weak to
be measured accurately at temperatures above 4 K, but the ∆Ms
) (1 transitions follow Curie’s law, when changing the
temperature in the range from T ) 3.8 to 100 K (Figure 7). We
conclude that the quartet state of 1b3•3+ is the ground state,
although an exact degeneracy with the corresponding doublet
state cannot be excluded from this experiment.19
(20) Brickmann, J.; Kothe, G. J. Chem. Phys. 1973, 59, 2807.
(21) (a) Rajca, A.; Utamapanya, S. J. Am. Chem. Soc. 1993, 115, 2396.
(b) Rajca, A.; Rajca, S.; Desai, S. R. J. Am. Chem. Soc. 1995, 117, 806.
(22) Weissman, S. I.; Kothe, G. J. Am. Chem. Soc. 1975, 97, 2537.
(23) Mu¨ller, U.; Baumgarten, M. J. Am. Chem. Soc. 1995, 117, 5840.
(24) Different signals arise from diastereoisomeric RS and RR + SS
isomers.
1
crystalline material: mp 153 °C; H NMR (CDCl3) δ 1.36 (18H, s,
CH3), 7.51 (4H, d, J ) 8.6 Hz, H-2′), 7.62 (1H, t, J ) 7.7 Hz, H-5),
7.78 (4H, d, J ) 8.6 Hz, H-3′), 8.02 (2H, dd, J ) 7.7 Hz and 1.8 Hz,
H-4), 8.17 (1H, t, J ) 1.8 Hz, H-2); 13C NMR (CDCl3): δ 31.10 (CH3),
35.16 (CCH3), 125.44 (C-3′), 128.41 (C-5), 130.17 (C-2′), 131.13 (C-
2), 133.21 (C-4), 134.26 (C-1), 138.01 (C-1′), 156.67 (C-4′), 195.63
(CO).
(25) One signal arises from RRR + SSS isomers, the other two from the
RRS and SSR isomers which contain two inequivalent phosphorus nuclei.

High-Spin Cation Radicals of Methylenephosphoranes
J. Am. Chem. Soc., Vol. 119, No. 23, 1997 5403
1,3,5-Benzenetriyltris[(4-tert-butylphenyl)methanone] 2b. From
1,3,5-benzenetricarbonyl trichloride (2.65 g, 10 mmol) using same
procedure as for 2a. Yield (3.52 g, 63%) as white crystals: mp 181
1,3-Phenylenebis{[(4-tert-butylphenyl)methyl]triphenylphos-
honium bromide} 5a. A solution of 4a (1.06 g, 2 mmol) and
triphenylphosphine (1.31 g, 5 mmol) was heated under reflux for 24 h.
After cooling, the solvent was evaporated. Column chromatography
(SiO2, CHCl3/MeOH 95:5-9:1) afforded pure 5a (0.90g, 43%) as a
white solid. 31P NMR (CDCl3) δ 23.14, 23.18.24 ES-MS m/z (M -
Br-) calcd 971.4, obsd 971.3.
1,3,5-Benzenetriyltris{[(4-tert-butylphenyl)methyl]triphenyl-
phoshonium bromide} 5b was obtained from 4b (0.75 g, 1 mmol)
using the same procedure as described for 5a. Yield (0.60 g, 41%) as
a white solid: 31P NMR(CDCl3) δ 23.34, 23.78, 24,08.25 ES-MS m/z
(M - Br-) calcd 1457.5, obsd 1457.5.
1,3-Phenylenebis[[(4-tert-butylphenyl)methylene]triphenyl-
phosphorane] 1a. NaNH2 (4.0 mg, 0.1 mmol) was added to a
suspension of 5a (21 mg, 0.02 mmol) in THF (2 mL) at room
temperature. The reaction mixture quickly turned deep red. After
stirring at room temperature for 1 h, the conversion was complete as
evidenced by NMR spectroscopy, using a small aliquot of the reaction
mixture: 31P NMR (THF) δ 6.39; 1H NMR (THF) δ 6.25 (2H, d, J )
8.0 Hz, H-4), 6.31 (4H, d, J ) 8.6 Hz, H-2′), 6.36 (1H, t, J ) 8.0 Hz,
H-5), 6.61 (4H, d, J ) 8.6 Hz, H-3′), 6.73 (1H, s, H-2), 7.23-7.30 (12
H, m, H-3′′), 7.33-7.39 (6H, m, H-4′′), 7.41-7.48 (12H, m, H-3′′).
1,3,5-Benzenetriyltris[[(4-tert-butylphenyl)methylene]triphe-
nylphosphorane] 1b. From 5b (0.75 g, 1 mmol) using the same
procedure as described for 1a: 31P NMR (THF) δ 5.80; 1H NMR (THF)
δ 6.06 (6H, d, J ) 8.3 Hz, H-2′), 6.33 (3H, s, H-2), 6.54 (6H, d, J )
8.3 Hz, H-3′), 7.21-7.26 (18 H, m, H-3′′), 7.29-7.45 (27H, m, H-2′′
+ H-4′′).
1
°C; H NMR (CDCl3) δ 1.36 (27H, s, CH3), 7.53 (6H, d, J ) 8.4 Hz,
H-2′), 7.81 (6H, d, J ) 8.4 Hz, H-3′), 8.39 (3H, s, H-2); 13C NMR
(CDCl3): δ 31.03 (CH3), 35.14 (CCH3), 125.56 (C-3′), 130.16 (C-2′),
133.80 (C-1 + C-2), 138.36 (C-1′), 157.03 (C-4′), 194.67 (CO).
r,r′-Bis(4-tert-butylphenyl)-1,3-benzenedimethanol 3a. Com-
pound 2a (9.96 g, 25 mmol) in THF (50 mL) was added slowly to a
suspension of LiAlH4 (1.9 g, 50 mmol) in THF (50 mL) with stirring
and cooling in an ice bath. After the addition was complete, the ice
bath was removed, and the mixture was stirred at room temperature
for 30 min. Then water was added carefully to quench the excess of
LiAlH4. The mixture was concentrated and extracted with diethyl ether.
The collected organic fractions were washed with water and dried with
MgSO4, and the solvent was evaporated affording pure 3a (9.73 g, 97%)
as a white solid: 1H NMR (CDCl3) δ 1.29 (18H, s, CH3), 2.15 (2H, b
s, OH), 5.77 (2H, 2 s, CHOH), 7.24-7.34 (11H, m, H-4, H-5, H-2′,
H-3′), 7.47, 7.49 (1H, 2 s, H-2); 13C NMR (CDCl3): δ 31.32 (CH3),
34.48 (CCH3), 76.00 (COH), 124.59, 124.62 (C-2), 125.39 (C-3′),
125.55, 125.65 (C-4), 126.30, 126.35 (C-2′), 128.53 (C-5), 140.76 (C-
1′), 144.02, 144.08 (C-1), 150.51 (C-4′).
r,r′,r′′-Tris(4-tert-butylphenyl)-1,3,5-benzenetrimethanol 3b was
obtained from 2b (2.24 g, 4 mmol) using the same procedure as for
1
3a. Yield (2.16 g, 96%) as a white solid: H NMR (CDCl3) δ 1.29
(27H, s, CH3), 3.2 (3H, 2 b s, OH) 5.67 (3H, m, CHOH), 7.24-7.34
(15H, m, H-2, H-2′, H-3′); 13C NMR (CDCl3): δ 31.27 (CH3), 34.39
(CCH3), 75.76, 75.80 (COH), 123.83 (C-2), 125.22 (C-3′), 126.34,
126.36 (C-2′), 140.63 (C-1′), 144.21, 144.27 (C-1), 150.22 (C-4′).
1,3-Bis[bromo(4-tert-butylphenyl)methyl]benzene 4a. PBr3 (5.41
g, 20 mmol) in toluene (40 mL) was added slowly to a stirred
suspension of 3a (4.02 g, 10 mmol) in toluene (20 mL) at room
temperature. The solution became clear and was stirred for 1 h. The
mixture was poured onto ice and extracted two times with toluene.
The combined organic layers were washed with water and dried over
MgSO4, and the solvent was evaporated affording pure 4a (4.91 g, 93%)
as a light yellow solid: 1H NMR (CDCl3) δ 1.30 (18H, s, CH3), 6.25
(2H, 2 s, CHBr), 7.2-7.4 (11H, m, H-4, H-5, H-2′, H-3′), 7.62 (1H, s,
H-2); 13C NMR (CDCl3): δ 31.23 (CH3), 34.57 (CCH3), 54.96 (CBr),
125.54 (C-3′), 128.09 (C-2′), 128.15 (C-4), 128.58, 128.70 (C-2, C-5),
137.74, 137.81 (C-1′), 141.27, 141.39 (C-1), 151.22 (C-4′).
Oxidation of Bis(methylenephosphorane) 1a. A freshly prepared
solution of phosphorane 1a (0.02 mmol) in THF (2 mL) was cooled to
-78 °C in a Schlenk vessel, and a solution of AgBF4 (0.5 mL, 0.08 M
in THF) was added slowly by syringe. After stirring at -78 °C for 30
min, the mixture was transferred as quickly as possible into a precooled
ESR tube equipped with a ground-glass joint. After sealing, the sample
was cooled to 77 K using liquid nitrogen.
Oxidation of tris(methylenephosphorane) 1b was performed using
from freshly prepared 1b (0.02 mmol) in THF (2 mL) and AgBF4 (0.75
mL, 0.08 M in THF), following the same procedure as described for
the oxidation of 1a.
Acknowledgment. We thank Professor E. W. Meijer for
valuable discussions and Philips Research for an unrestricted
research grant. This work has been supported by the Nether-
lands Foundation for Chemical Research (SON) with financial
aid from the Netherlands Organization for Scientific Research
(NWO).
1,3,5-Tris[bromo(4-tert-butylphenyl)methyl]benzene 4b was ob-
tained from 3b (1.69 g, 3 mmol) using the same procedure as described
for 4a. Yield (2.05 g, 91%) as a light brown solid: 1H NMR (CDCl3)
δ 1.29 (27H, s, CH3), 6.22 (3H, m, CHBr), 7.2-7.4 (15H, m, H-2′,
H-3′), 7.55 (3H, m, H-2); 13C NMR (CDCl3): δ 31.22 (CH3), 34.58
(CCH3), 54.45 (CBr), 125.60 (C-3′), 129.09 (C-2′), 128.35 (C-2),
137.43, 137.53, 137.63 (C-1′), 141.46, 141.59, 141.75 (C-1), 151.35
(C-4′).
JA964185E