A stable dialkylphosphinyl radical

Article abstract of DOI:10.1021/ja205001m

Dialkylphosphinyl radical 1 was synthesized as thermally stable yellow crystals and found to be monomeric both in solution and in the solid state. EPR spectrum showed that the spin density of 1 is mainly localized on the 3p orbital of the dicoordinated phosphorus atom. A distinct absorption band due to the electronic transition from nonbonding electron pair orbital to singly occupied 3p orbital on the phosphorus atom of 1 was observed at 445 nm in solution. Phosphinyl radical 1 underwent facile reaction with carbon tetrachloride, hydrogen abstraction, and a unique reaction with a persistent radical, galvinoxyl, giving a cyclic phosphaalkene and a silylether.

Full text of DOI:10.1021/ja205001m

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A Stable Dialkylphosphinyl Radical
Shintaro Ishida,* Fumiya Hirakawa, and Takeaki Iwamoto*
Department of Chemistry, Graduate School of Science, Tohoku University, Aoba-ku, Sendai 980-8578, Japan
S Supporting Information
b
and Power et al. exists as a monomeric form in the gas phase and in
solution but it dimerizes to give the corresponding diphosphane
Dis2PPDis2 upon crystallization.6 We have extensively explored the
chemistry of isolable dialkyl-substituted divalent species of group-14
elements (silylene, germylene, and stannylene) utilizing a helmet-
shaped bidentate alkyl ligand, 1,1,4,4-tetrakis(trimethylsilyl)butane-
ABSTRACT: Dialkylphosphinyl radical 1 was synthesized
as thermally stable yellow crystals and found to be mono-
meric both in solution and in the solid state. EPR spectrum
showed that the spin density of 1 is mainly localized on the
3p orbital of the dicoordinated phosphorus atom. A distinct
absorption band due to the electronic transition from
nonbonding electron pair orbital to singly occupied 3p
orbital on the phosphorus atom of 1 was observed at
445 nm in solution. Phosphinyl radical 1 underwent facile
reaction with carbon tetrachloride, hydrogen abstraction,
and a unique reaction with a persistent radical, galvinoxyl,
giving a cyclic phosphaalkene and a silylether.
1,4-diyl (RH ).7 The successful synthesis of these species prompted
2
us to examine the synthesis of dialkylphosphinyl radical utilizing the
RH ligand. We report herein synthesis, structure, and reactions
2
of the first stable dialkylphosphinyl radical, 2,2,5,5-tetrakis-
(trimethylsilyl)-1-phosphacyclopentane-1-yl (RH P , 1), which is
2
3
monomeric both in solution and in the solid state.
Phosphinyl radical 1 was synthesized as air-sensitive yellow
crystals in 57% yield on a multigram scale by the reduction of the
corresponding dialkylchlorophosphine 28 with potassium gra-
phite in hexane at room temperature for 19 days (eq 1). The 1H
NMR spectrum of the yellow crystals shows no signals except for
very weak signals due to a trace amount of hydrophosphine
RH2PH (3) and hexane used as solvent for crystallization.9
Radical 1 is highly air sensitive but thermally very stable; it can
survive at room temperature for several months in an inert
atmosphere both in 3-methylpentane (3-MP) solution and in the
solid state.
hosphinyl radicals are fascinating reactive intermediates in
P
phosphorus chemistry because they have a bifunctional
dicoordinated phosphorus center with an unpaired electron
and a lone pair.1 Although several persistent phosphinyl radicals
stabilized by kinetic and resonance stabilization have been
reported,2 stable phosphinyl radicals that do not dimerize in
the solid state1b are still quite rare. Very recently, stable phos-
phinyl radicals and related phosphinyl radical ions were synthe-
sized by Cummins et al.3 and Bertrand et al.4,5 (Chart 1). These
radicals have at least one nitrogen substituent that can affect the
electronic properties of the phosphorus center.
Chart 1. Stable Dicoordinated Phosphorus Centered
Radicalsa
X-ray analysis showed that 1 exists as a monomeric form in the
solid state as shown in Figure 1. The shortest distance between
the dicoordinated phosphorus centers of 6.9414(5) Å is far
longer than the sum of van der Waals radii of two phosphorus
atoms (3.6 Å)10a indicating that 1 is monomeric in the solid state.
The PÀC bond length of 1 [1.869(1) Å] is within the range of
standard PÀC single bond lengths (1.84À1.87 Å)10b but slightly
shorter than those of hydrophosphine 3 [1.886(2) and 1.885(2) Å].
The C1ÀPÀC4 angle of 1 of 96.16(6)ꢀ is comparable to that of 3
[96.28(7)ꢀ].
EPR spectrum of 1 in 3-MP at 298 K showed a doublet signal
(g-factor = 2.0086) due to a coupling with one 31P nucleus
(Figure 2a). The hyperfine coupling constant (hfc) due to 31P
nucleus, a(31P) of 9.07 mT, is in the range of typical hfc’s of the
pertinent phosphinyl radicals (6.5À9.6 mT)1À6 and close to
aNp = neopentyl, Ar = 3,5-dimethylphenyl, Dip = 2,6-diisopropylphenyl.
those of structurally similar Dis2P (9.63 mT)6 and related
3
Neutral dialkylphosphinyl radicals, which are least perturbed
electronically by substituents and should be suitable for investi-
gation of intrinsic properties of phosphinyl radicals such as
electronic structures and reactivity, have not been isolated as a
monomeric form in the solid state. Bis(trimethylsilyl)methyl
11a
transient dialkylphosphinyl radicals (9.57 mT for Me2P
3
and 9.67 mT for i-Pr2P 11b). On the basis of the intensity relative
3
Received: May 31, 2011
Published: July 20, 2011
(Dis)-substituted phosphinyl radical, Dis2P reported by Lappert
3
r
2011 American Chemical Society
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Figure 1. (a) Molecular structure of 1. Thermal ellipsoids are drawn at
the 30% probability level. Hydrogen atoms are omitted for clarity.
Selected bond lengths (Å) and angles (deg): PÀC1 1.869(1); PÀC4
1.869(1); C1ÀC2 1.562(2); C2ÀC3 1.538(2); C3ÀC4 1.562(2);
C1ÀSi1 1.913(1); C1ÀSi2 1.916(1); C4ÀSi3 1.914(1); C4ÀSi4
1.916(1); C1ÀPÀC4 96.16(6). (b) A packing diagram of 1. The
shortest distance between the dicoordinated phosphorus atoms shown
as a blue line is 6.9414(5) Å.
Figure 3. Highest occupied molecular orbital (HOMO, left) and singly
occupied molecular orbital (SOMO, right) of 1opt calculated at the
UHF/6-31G(d)//UB3LYP/6-31G(d) level.
interactions of 1.15 The spin density on the phosphorus atom of
1opt is slightly smaller than that of parent 1-phosphacyclopen-
tane-1-yl, 1H (1.00), suggesting that radical 1 is thermodynami-
cally stabilized to some extent due to the 3p(P)Àσ*(Me3SiÀC)
interactions. This is supported by the isodesmic reaction (eq 2)
showing that radical 1opt is more stable than 1H by 6.8 kcal molÀ1
at the (U)B3LYP/6-31G(d) level.
to those of the major doublet signal, the doublet satellite signals
with the hfc of 1.73 mT are assigned to signals due to the four
trimethylsilyl 29Si nuclei.12
EPR spectrum of 1 in 3-MP glass matrix displayed typical axial
symmetry (Figure 2b). The g-factors and hfc tensors were
determined as follows: g// = 2.0013, a//(31P) = 27.9 mT, g^ =
2.0084, and a^(31P) = 0.49 mT. Accordingly, the isotropic and
anisotropic coupling constants are aiso = 9.63 mT and aaniso
=
9.14 mT, respectively. On the basis of the comparison with hfc’s
of atomic phosphorus,13 70% and 2% spin are localized on the
3p(P) and 3s(P) orbitals in 1, respectively. The spin localization
of 1 is significantly higher than those of the hitherto known
radicals B4 [57% and 2% for 3p(P) and 3s(P)] and C5 [62% and
2% for 3p(P) and 3s(P)].
UVÀvis spectrum of 1 in 3-MP showed two distinct absorp-
tion bands at 445 nm (ε 157, band I) and 304 nm (ε 1790, band II)
(Figure 4). Band I is the origin of the yellow color of 1. Judging from
TD-DFT calculations of 1opt, band I is assignable to n(P) f 3p(P)
transition (HOMO-SOMO transition).14 To the best of our knowl-
edge, this is the first observation and characterization of electronic
transitions of phosphinyl radical by UVÀvis spectroscopy. Band II is
assigned to overlapped transitions of σ(CÀP) f 3p(P) and
σ(CÀSiMe3) f 3p(P).16
Compound 1 shows high reactivity as a phosphinyl radical
toward various reagents even though the phosphorus center of 1
is highly protected by the helmet-shaped RH2 ligand (Chart 2).
Figure 2. EPR spectra of 1 in (a) 3-methylpentane (3-MP) solution at
298 K and (b) 3-MP frozen solution at 112 K.
Theoretical calculations were carried out to obtain further
insight into the electronic structure of 1.14 The optimized
structure (1opt) calculated at the UB3LYP/6-31G(d) level well
reproduced the structure of 1 determined by X-ray analysis. The
SOMO of 1opt calculated at the UHF/6-31G(d) level was almost
pure 3p orbital on the dicoordinated phosphorus atom with small
contribution of σ*(Me3SiÀC) orbitals, while the HOMO of 1opt
was a nonbonding orbital on the phosphorus atom (Figure 3).
The Mulliken spin densities on phosphorus atom and each
silicon atom of 1opt calculated at the UM05-2X/def2-SVP level
are 0.90 and 0.02À0.03, respectively, which are consistent with
the considerably high spin localization on the phosphorus
atom and the existence of small 3p(P)Àσ*(Me3SiÀC) orbital
Figure 4. UVÀvis absorption spectrum of 1 in 3-MP at room tem-
perature. Superimposed vertical blue bars indicate band positions and
oscillator strengths of 1opt calculated at the TD-UB3LYP/6-311+
G(d,p)//UB3LYP/6-31G(d) level.
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Chart 2. Reaction Products
Tordo, P. Top. Curr. Chem. 2005, 250, 43. (d) Armstrong, A.; Chivers,
T.; Boere, R. T. ACS Symp. Ser. 2006, 917, 66.(e) Konu, J.; Chivers, T. In
Stable Radicals, Fundamentals and Applied Aspects of Odd-Electron
Compounds; Hicks, R. G., Ed.; Wiley: Chichester, U.K., 2010; p 395.
(f) Martin, D.; Soleilhavoup, M.; Bertrand, G. Chem. Sci. 2011, 2, 389.
(2) For recent examples of related persistent and stable phosphorus
radicals, see: (a) Bezombes, J. P.; Borisenko, K. B.; Hitchcock, P. B.;
Lappert, M. F.; Nycz, J. E.; Rankin, D. W. H.; Robertson, H. E. Dalton
Trans. 2004, 1980. (b) Dumitrescu, A.; Rudzevich, V. L.; Romanenko,
V. D.; Mari, A.; Schoeller, W. W.; Bourissou, D.; Bertrand, G. Inorg.
Chem. 2004, 43, 6546. (c) Ito, S.; Kikuchi, M.; Yoshifuji, M.; Arduengo,
A. J., III; Konovalova, T. A.; Kispert, L. D. Angew. Chem., Int. Ed. 2006,
45, 4341. (d) Sasamori, T.; Mieda, E.; Nagahora, N.; Sato, K.; Shiomi,
D.; Takui, Y.; Hosoi, Y.; Furukawa, Y.; Takagi, N.; Nagase, S.; Tokitoh,
N. J. Am. Chem. Soc. 2006, 128, 12582. (e) Scheer, M.; Kuntz, C.;
Stubenhofer, M.; Linseis, M.; Winter, R. F.; Sierka, M. Angew. Chem., Int.
Ed. 2009, 48, 2600. (f) Back, O.; Donnadieu, B.; Parameswaran, P.;
Frenking, G.; Bertrand, G. Nat. Chem. 2010, 2, 369. (g) Kinjo, R.;
Donnadieu, B.; Bertrand, G. Angew. Chem., Int. Ed. 2010, 49, 5930. See,
also refs 3À6.
The reaction of 1 with carbon tetrachloride in hexane completed
within a minute to afford chlorophosphine 2 in 49% yield.17
Hydrogen abstraction of 1 from cyclohexa-1,4-diene in the dark
condition also occurred within 3 days to give the corresponding
hydrophosphine 3 and benzene in 99 and 49% yields, respec-
tively. This is the first example of hydrogen abstraction of
phosphinyl radical from a hydrocarbon. The reaction of 1 with
a persistent oxygen-centered radical, galvinoxyl, afforded inter-
esting products, phosphaalkene 4 and trimethylsilyl ether 5
instead of a simple radical recombination product.
Possible mechanism for formation of 4 and 5 can involve the
radical coupling between phosphinyl radical center of 1 and
terminal oxygen atom of galvinoxyl followed by elimination of
silyl ether 5 to give phosphaalkene 4.18 In the latter process,
severe steric repulsion between geminal trimethylsilyl groups
would be responsible for the facile elimination of silyl ether 5
giving phosphaalkene 4. Consistent with this explanation, the
theoretical calculations for the model reactions (eq 3) at the
B3LYP/6-31G(d) level show that the reaction of 6a (R = H)
giving 7a and 8 is slightly exothermic (ΔE = À0.2 kcal molÀ1),
while that of more sterically congested 6b (R = SiMe3) giving 7b
and 8 is considerably exothermic (ΔE = À10.0 kcal molÀ1).
(3) Agarwal, P.; Piro, N. A.; Meyer, K.; Muller, P.; Cummins, C. C.
Angew. Chem., Int. Ed. 2007, 46, 3111.
(4) Back, O.; Celik, M. A.; Frenking, G.; Melaimi, M.; Donnadieu,
B.; Bertrand, G. J. Am. Chem. Soc. 2010, 132, 10262.
(5) Back, O.; Donnadieu, B.; Hopffgarten, M. v.; Klein, S.; Tonner,
R.; Frenking, G.; Bertrand, G. Chem. Sci. 2011, 2, 869.
(6) (a) Gynane, M. J. S.; Hudson, A.; Lappert, M. F.; Power, P. P.;
Goldwhite, H. J. Chem. Soc., Chem. Commun. 1976, 623. (b) Gynane,
M. J. S.; Hudson, A.; Lappert, M. F.; Power, P. P.; Goldwhite, H. J. Chem.
Soc., Dalton Trans. 1980, 2428. (c) Hinchley, S. L.; Morrison, C. A.;
Rankin, D. W. H.; Macdonald, C. L. B.; Wiacek, R. J.; Cowley, A. H.;
Lappert, M. F.; Gundersen, G.; Clyburnee, J. A. C.; Power, P. P. Chem.
Commun. 2000, 2045. (d) Hinchley, S. L.; Morrison, C. A.; Rankin,
D. W. H.; Macdonald, C. L. B.; Wiacek, R. J.; Voigt, A.; Cowley, A. H.;
Lappert, M. F.; Gundersen, G.; Clyburnee, J. A. C.; Power, P. P. J. Am.
Chem. Soc. 2001, 123, 9045.
(7) (a) Kira, M.; Yauchibara, R.; Hirano, R.; Kabuto, C.; Sakurai, H.
J. Am. Chem. Soc. 1991, 113, 7785. (b) Kira, M.; Ishida, S.; Iwamoto, T.;
Ichinohe, M.; Kabuto, C.; Ignatovich, L.; Sakurai, H. Chem. Lett. 1999, 263.
(c) Kira, M.; Ishida, S.; Iwamoto, T.; Kabuto, C. J. Am. Chem. Soc. 1999,
121, 9722. For reviews, see:(d) Kira, M.; Ishida, S.; Iwamoto, T. Chem. Rec.
2004, 4, 243. (e) Kira, M.; Iwamoto, T.; Ishida, S. Bull. Chem. Soc. Jpn. 2007,
80, 258. (f) Kira, M. Chem. Commun. 2010, 46, 2893.
(8) Molecular structures of chlorophosphine 2 and hydrophosphine
3 were determined by single crystal X-ray diffraction study. For details,
see Supporting Information.
(9) Purity of 1 was estimated to be >98% on the basis of NMR and
EPR spectroscopies.
(10) (a) Bondi, A. J. Phys. Chem. 1964, 68, 441.(b) Allen, F. H.;
Kennard, O.; Watson, D. G.; Brammer, L.; Orpen, A. G.; Taylor, R.
J. Chem. Soc., Perkin Trans. 2 1987, 12, S1.
Further investigations of the reactivity of 1 are in progress.
’ ASSOCIATED CONTENT
S
Supporting Information. Experimental details and full
b
characterizations for 1À5, details of theoretical calculations, and
X-ray crystallographic data of 1À3 in CIF format. This material is
available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION
(11) (a) Roberts, B. P.; Singh, K. J. Organomet. Chem. 1978, 159, 31.
(b) Fullam, B. W.; Mishra, S. P.; Symons, M. C. R. J. Chem. Soc., Dalton
Trans. 1974, 20, 2145.
Corresponding Author
sishida@m.tohoku.ac.jp; iwamoto@m.tohoku.ac.jp
(12) The a(29Si) value of 1 is similar to those of the isoelectronic
anion radical of 2,2,5,5-tetrakis(trimethylsilyl)-1-silacyclopentane-1,1-
diyl (1.28À1.70 mT). Ishida, S.; Iwamoto, T.; Kira, M. J. Am. Chem. Soc.
2003, 125, 3212.
’ ACKNOWLEDGMENT
(13) Morton, J. R.; Preston, K. F. J. Magn. Reson. 1978, 30, 577.
(14) TD-DFT calculations were carried out at the UB3LYP/6-311
+G(2df,p)//UB3LYP/6-31G(d) level. The shapes of KohnÀSham
orbitals were quite similar to those of the corresponding molecular
orbitals calculated at the UHF/6-31G(d) level. Details including full
citations are in Supporting Information.
This work was supported by Asahi Glass Foundation (T.I.)
and KAKENHI (S.I., No. 21750035). We thank Tatsuya
Yokoyama for preliminary investigations and reviewers for valu-
able suggestions.
’ REFERENCES
(15) Spin densities estimated by theoretical calculations are usually
higher than those by powder pattern EPR spectrum.4,5
(1) For recent reviews of persistent and stable radicals of the heavier
main group elements, see:(a) Geoffroy, M. Recent Res. Dev. Phys. Chem.
1998, 2, 311. (b) Power, P. P. Chem. Rev. 2003, 103, 789. (c) Marque, S.;
(16) TD-DFT calculations showed that the position of HOMO-
SOMO transition (band I) of 1H was the same as that of 1opt (437 nm),
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while no transition band corresponding to band II was predicted for 1H,
suggesting that Me3SiÀC orbitals are responsible for the band II of 1.
(17) Chlorophosphine 2 did not react with CCl4. Although the facile
reaction of 1 with CCl4 giving chlorophosphine 2 in nonpolar hexane
suggested chlorine abstraction of 1 from CCl4, we failed identifying
the fate of the CCl3 moiety, and hence, heterolytic pathway may not
be ruled out.
(18) This phosphaalkene formation proceeded in neutral condition
is in contrast to phospha-Peterson reaction in basic conditions. For an
example, see: Yam, M.; Chong, J. H.; Tsang, C.-W.; Patrick, B. O.; Lam,
A. E.; Gates, D. P. Inorg. Chem. 2006, 45, 5225.
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