A "push-pull" phosphasilene and phosphagermene and their anion-radicals

Article abstract of DOI:10.1021/om900310u

Phosphasilene la andphosphagermene lb, featuring unprecedented substitution patterns (donors on Si or Ge and acceptor on P), were synthesized utilizing a one-step synthetic approach: direct coupling of dilithiosilane/dilithiogermane and dichlorophosphine. The structural features of both 1a and 1b based on their NMR and X-ray crystal data are discussed, as well as their one-electron reduction forming persistent phosphasilene and phosphagermene anion-radicals.

Full text of DOI:10.1021/om900310u

4262 Organometallics 2009, 28, 4262–4265
DOI: 10.1021/om900310u
A “Push-Pull” Phosphasilene and Phosphagermene and Their
Anion-Radicals
Vladimir Ya. Lee,† Manami Kawai,† Akira Sekiguchi,*,† Henri Ranaivonjatovo,‡ and
Jean Escudie
ꢀ‡
†Department of Chemistry, Graduate School of Pure and Applied Sciences, University of Tsukuba, Tsukuba,
‡
Ibaraki 305-8571, Japan, and Universite de Toulouse, UPS, LHFA, 118 Route de Narbonne, 31062,
ꢀ
Toulouse, France, and CNRS, LHFA UMR 5069, 31062, Toulouse, France
Received April 23, 2009
Summary: Phosphasilene 1a and phosphagermene 1b, featur-
ing unprecedented substitution patterns (donors on Si or Ge
and acceptor on P), were synthesized utilizing a one-step
synthetic approach: direct coupling of dilithiosilane/dilithio-
germane and dichlorophosphine. The structural features of
both 1a and 1b based on their NMR and X-ray crystal data are
discussed, as well as their one-electron reduction forming
persistent phosphasilene and phosphagermene anion-radicals.
type >E14dE15- have been reported, including phosphasi-
lenes >SidP-, phosphagermenes >GedP-, phosphastan-
nenes >SndP-, and arsasilenes >SidAs-.3 The double
bond between silicon/germanium and phosphorus is polar-
ized, >Eδ+dPδ-- (E = Si or Ge), because of the difference
in electronegativity between Si (or Ge) and P atoms: 1.90 (or
2.01) vs 2.19 (Pauling scale). Such bond polarization results
in the enhanced reactivity of both >SidP- and >GedP-.
Using substituents with opposing electronic effects on
the >EdP- bond (E = Si or Ge), namely, electron-donat-
ing groups (D) on silicon/germanium and electron-accepting
groups (A) on phosphorus (“push-pull” substitution
pattern), one can expect that such double-bond polarization
would be decreased or even reversed as in D2Eδ-dPδ+-A.
However, this interesting synthetic challenge, directed to-
ward a changed electron distribution in the group 14-15
element double bonds, has not yet been achieved experimen-
tally.2
Multiple bonding between the main group elements
heavier than the elements of the second row is one of the
main themes in contemporary heteroatomic and organome-
tallic chemistry. Being well established in the case of homo-
nuclear derivatives of the types -E13dE13-, >E14dE14<,
or -E15dE15- (E13, E14, or E15 = group 13, 14, or 15
elements),1 the chemistry of heteronuclear combinations,
particularly those of different group elements of the types
-E13dE15-, -E13dE14<, and >E14dE15-, is much less
developed.1 Of the heavy doubly bonded derivatives of the
main group elements, those of group 14 and group 15
elements are particularly interesting, since they contain two
reactive sites in one molecule: an E14dE15 π-bond and lone
electron pair on the E15 element.2 Several compounds of the
In this communication, we report the synthesis of phos-
phasilene and phosphagermene derivatives with such a sub-
stitution pattern (electron-releasing silyl substituents on Si or
Ge and an electron-withdrawing aryl group on P). The one-
step procedure used takes advantage of the high and selective
reactivity of (tBu2MeSi)2ELi2 (E = Si, Ge) reagents. The
structural features of these phosphasilene and phosphager-
mene compounds, based on their structures as determined by
X-ray crystallography and their NMR and UV spectroscopic
data, are discussed, and their reduction with potassium
graphite to form persistent anion-radical species is described.
Previously reported phosphasilenes and phosphagermenes
are typically prepared employing a two-step procedure:
initial coupling of phosphorus and silicon/germanium
units to form a >E(X)-P(H)- (E = Si, Ge) bond followed
by reductive dehydrohalogenation to form a >EdP- (E =
Si or Ge) bond.2,3a-3k We applied a one-step coupling of a
dihalophosphine witha dilithiosilane/dilithiogermane to effect
the direct formation of the >EdP- (E = Si or Ge) double
*To whom correspondence should be addressed. Phone: +81-29-853-
4314. Fax: +81-29-853-4314 E-mail: sekiguch@chem.tsukuba.ac.jp.
(1) Selected reviews on the >E14dE14< and -E15dE15- doubly
ꢀ
bonded derivatives: (a) Escudie, J.; Couret, C.; Ranaivonjatovo, H.;
ꢀ
Satge, J. Coord. Chem. Rev. 1994, 130, 427. (b) Okazaki, R.; West, R. Adv.
ꢀ
Organomet. Chem. 1996, 39, 231. (c) Escudie, J.; Ranaivonjatovo, H. Adv.
Organomet. Chem. 1999, 44, 113. (d) Power, P. P. Chem. Rev. 1999, 99,
3463. (e) Weidenbruch, M. In The Chemistry of Organic Silicon Com-
pounds; Rappoport, Z., Apeloig, Y., Eds.; Wiley: Chichester, 2001, Vol. 3,
Chapter 5. (f) Kira, M.; Iwamoto, T. Adv. Organomet. Chem. 2006, 54, 73.
(g) Sasamori, T.; Tokitoh, N. Dalton Trans. 2008, 1395. (h) Ottosson, H.;
€
Eklof, A. M. Coord. Chem. Rev. 2008, 252, 1287.
(2) Driess, M. Coord. Chem. Rev. 1995, 145, 1.
(3) Phosphasilenes: (a) Smith, C. N.; Bickelhaupt, F. Organometal-
lics 1987, 6, 1156. (b) Niecke, E.; Klein, E.; Nieger, M. Angew. Chem., Int.
Ed. Engl. 1989, 28, 751. (c) Driess, M. Angew. Chem., Int. Ed. Engl. 1991,
30, 1022. (d) Bender, H. R. G.; Niecke, E.; Nieger, M. J. Am. Chem. Soc.
1993, 115, 3314. (e) Driess, M.; Rell, S.; Pritzkow, H. J. Chem. Soc., Chem.
Commun. 1995, 253. (f) Driess, M.; Pritzkow, H.; Rell, S.; Winkler, U.
Organometallics 1996, 15, 1845. (g) Driess, M.; Block, S.; Brym, M.;
Gamer, M. T. Angew. Chem., Int. Ed. 2006, 45, 2293. (h) Yao, S.; Block, S.;
Brym, M.; Driess, M. Chem. Commun. 2007, 3844. Phosphagermenes:
4
bond. Thus, reaction of Mes*PCl2 (Mes* = 2,4,6-tri-tert-
5
butylphenyl) with (tBu2MeSi)2SiLi2 (or (tBu2MeSi)2GeLi2)6
in THF produced the phosphasilene (tBu2MeSi)2SidPMes*
(1a) and phosphagermene (tBu2MeSi)2GedPMes* (1b),
ꢀ
ꢀ
(i) Escudie, J.; Couret, C.; Satge, J.; Andrianarison, M.; Andriamizaka, J.-D.
€
ꢀ
J. Am. Chem. Soc. 1985, 107, 3378. (j) Drager, M.; Escudie, J.; Couret, C.;
Ranaivonjatovo, H.; Satge, J. Organometallics 1988, 7, 1010.
(k) Ranaivonjatovo, H.; Escudie, J.; Couret, C.; Satge, J.; Drager, M. New
J. Chem. 1989, 13, 389. Phosphastannenes: (l) Couret, C.; Escudie, J.; Satge,
J.; Raharinirina, A.; Andriamizaka, J. D. J. Am. Chem. Soc. 1985, 107, 8280.
ꢀ
ꢀ
ꢀ
€
(4) Yoshifuji, M.; Shima, I.; Inamoto, N.; Hirotsu, K.; Higuchi, T. J.
Am. Chem. Soc. 1981, 103, 4587.
(5) Ichinohe, M.; Arai, Y.; Sekiguchi, A.; Takagi, N.; Nagase, S.
Organometallics 2001, 20, 4141.
(6) Sekiguchi, A.; Izumi, R.; Ihara, S.; Ichinohe, M.; Lee, V. Ya.
Angew. Chem., Int. Ed. 2002, 41, 1598.
ꢀ
ꢀ
ꢀ
ꢀ
(m) Ranaivonjatovo, H.; Escudie, J.; Couret, C.; Satge, J. J. Chem. Soc.,
Chem. Commun. 1992, 1047. Arsasilenes: (n) Driess, M.; Pritzkow, H.
Angew. Chem., Int. Ed. Engl. 1992, 31, 316. (o) See ref 3e. (p) See ref 3f
.
r
pubs.acs.org/Organometallics
Published on Web 07/07/2009
2009 American Chemical Society

Communication
Organometallics, Vol. 28, No. 15, 2009 4263
Scheme 1. One-Step Synthesis of Phosphasilene 1a and Phos-
phagermene 1b
It is determined by the Ramsey formula10 and is inversely
proportional to the energy difference between the interacting
occupied and vacant molecular orbitals, which are coupled
by the external magnetic field.9 In the case of phosphasilenes,
the most relevant molecular orbitals under consideration are
nP (occupied) and π*SidP (vacant) orbitals. In the case of
silyl-substituted phosphasilenes, appreciable nP-π*SidP or-
bital mixing can be promoted by the remarkable lowering of
the π* energy level, resulting in the overall decrease of the n-
π* energy gap. Given the fact that, according to our compu-
tations on the model (Me3Si)2SidPMes (Mes = 2,4,6-
trimethylphenyl) compound 1a0, the n and π* orbitals re-
present the HOMO and LUMO, respectively, such a para-
magnetic contribution should be responsible not only for the
extreme deshielding of the doubly bonded Si and P atoms in
1a but also for the notable red shift in its UV spectrum.11
Indeed, the longest wavelength absorption of 1a was found
to be markedly red-shifted compared with those of phos-
phasilenes featuring aryl groups on the doubly bonded Si
atom: 461 nm vs 331-343 nm.3f In accord with such ob-
servations, our computations on a series of differently sub-
stituted phosphasilenes (from σ-accepting alkyl/aryl to σ-
donating silyl groups on the doubly bonded silicon atom)
revealed the general tendencies of significant deshielding of
doubly bonded atoms taking place on decreasing the n-π*
energy gap upon the successive introduction of silyl substit-
uents on the sp2-Si center (ΔE(n-π*) in eV, 29Si NMR in
ppm, 31P NMR in ppm): 4.33, 186.0, 85.4 [tBu2SidPMes];
3.97, 205.2, 189.6 [(E)-tBu(Me3Si)SidPMes]; 3.73, 236.1,
322.6 [(Me3Si)2SidPMes].11
isolated as orange crystals in 34% and 40% yield, respectively
(Scheme 1).7
In the 1H NMR spectra of both 1a and 1b, the resonances
t
of the methyl groups of the two Bu2MeSi substituents
were observed in distinctly different regions: -0.40 and
0.50 ppm (Δδ = 0.90 ppm) for 1a, -0.30 and 0.55 ppm
(Δδ = 0.85 ppm) for 1b. Such a difference can be explained
by the nonequivalent spatial arrangement of these methyl
groups, one of which is directed toward the shielding area of
the aromatic ring of the Mes* substituent, which results in
the remarkable shift of its resonance to higher magnetic field.
The 29Si and 31P NMR spectra of 1a and 1b are particularly
informative in elucidating the structures in solution. Thus, in
the 29Si NMR spectrum of 1a, three signals were observed,
each split into a doublet by coupling to the phosphorus
nucleus: 13.5 (d, 2J(29Si-31P) = 25.2 Hz), 19.3 (d, 2J-
The crystal structures of 1a and 1b are shown in Figures 1
and 2.7 The most important structural feature of both 1a and
˚
1b is the length of the double bond: 2.1114(7) A (Si1dP1 in
˚
1a) and 2.1748(14) A (Ge1dP1 in 1b). Although these two
1
(29Si-31P) = 6.9 Hz), and 201.2 (d, J(29Si-31P) = 171.3
Hz) ppm. The latter resonance due to the central Si atom,
observed in the diagnostic low-field region, exhibits a very
large 1J(29Si-31P) coupling constant.8 This clearly testifies to
its sp2-hybridization, confirming the presence of a >SidP-
double bond. In agreement, the resonance of the P atom was
found in extremely low-field region at 389.3 ppm, being by
far more deshielded than the P atoms of other phosphasi-
lenes reported to date (from -33.0 to 292.5 ppm).3a-3h
Expectedly, the doubly bonded phosphorus atom in 1b is
more deshielded, being observed at 416.3 ppm. Such an
extreme low-field shift of the doubly bonded phosphorus
resonance can be attributed in part to the electron-with-
drawing effect of the aromatic substituent. However, one
should recognize the σ-donating effect of the silyl substitu-
ents bonded to the central Si atom as the other, even more
important, contributor to the overall deshielding of the
doubly bonded atoms. Such a phenomenon, well known in
the chemistry of disilenes >SidSi<, resulting from a para-
magnetic contribution,9 can also be seen in the great de-
shielding of the central Si atom in 1a (201.2 ppm). This
markedly exceeds the range of most other known phospha-
silenes.3a-3h The paramagnetic contribution, responsible for
the overall low-field isotropic chemical shift, stems from the
paramagnetic currents induced by the applied magnetic field.
values are greater than those of other reported phosphasi-
1,3b,3c,3f
˚
lenes (2.062(1)-2.094(5) A)
and phosphagermenes
3j
˚
(2.138(3)-2.144(3) A) because of the appreciable steric
congestion around the double bonds in 1a,b, they are smaller
˚
than the sum of the covalent radii of Si and P (2.27 A) or Ge
and P (2.32 A)12 by ca. 6-7%, suggesting >EdP- (E = Si
˚
or Ge) double-bond character.13 The >EdP - (E = Si or
Ge) double bonds exhibited no kind of distortion, being
neither pyramidalized (both Si1 and Ge1 centers in 1a and 1b
are perfectly planar, with the sum of the bond angles around
them being 360°) nor twisted (torsional angles C19-P1-
Si1-Si2 and C19-P1-Ge1-Si1 in 1a and 1b are 178.8° and
178.9°, respectively).7 One should note that the overall
geometry of both 1a and 1b is mainly dictated by the degree
t
of steric interactions. Thus, Bu2MeSi substituents facing
bulky Mes* groups are bent away from them because of the
steric repulsions: 128.05(3)° (P1-Si1-Si3) vs 104.83(3)°
(P1-Si1-Si2) in 1a; 127.50(6)° vs 104.88(5)° in 1b.
In the context of structural discussion, it is interesting to
mention that the tendencies seen in both 1a and 1b are
reminiscent of those of the well-studied silenes >SidC<.
Thus, quite similar to the case of 1a and 1b, Apeloig’s silene
(7) For the experimental procedures and spectral and X-ray crystal
data of 1a and 1b, see the Supporting Information.
(11) All computations (geometry optimizations, GIAO NMR che-
mical shifts) were performed with the GAUSSIAN 98 program package
at the DFT B3LYP/6-31G(d) level.
1
(8) JP-Si coupling constants in the previously reported phosphasi-
lenes lie in the range 130-224 Hz (see refs 3a-3h).
(9) West, R.; Cavalieri, J. D.; Buffy, J. J.; Fry, C.; Zilm, K. W.;
€
Duchamp, J. C.; Kira, M.; Iwamoto, T.; Muller, T.; Apeloig, Y. J. Am.
Chem. Soc. 1997, 119, 4972.
(10) Ramsey, N. F. Phys. Rev. 1950, 78, 699.
(12) Emsley, J. The Elements, 3rd ed.; Oxford University Press Inc.:
New York, 1998.
(13) Calculated values of the >SidP- and >GedP- bond lengths
in the model (Me3Si)2SidPMes (1a0) and (Me3Si)2GedPMes (1b0) are
˚
2.113 and 2.163 A, respectively.

4264 Organometallics, Vol. 28, No. 15, 2009
Lee et al.
Scheme 2. One-Electron Reduction of 1a and 1b with KC8 in
THF to Form the Potassium Salts of the Corresponding Anion-
Radicals
The electrochemical reduction of 1a,b under cyclic vol-
tammetry conditions (vs Ag/Ag+, THF, rt, 0.1 M [nBu4N]-
ClO4) revealed reversible one-electron reduction waves with
the reduction potentials E1/2(red) = -1.78 V (for 1a) and
E1/2(red) = -1.75 V (for 1b). As expected, these values are
just in between those of (tBu2MeSi)2SidSi(SiMetBu2)2
(-1.47 V)17 and Mes*PdPMes* (-1.93 V).18 Given the
reversibility of the electrochemical reduction, one can sug-
gest the stability (and hence accessibility) of the resulting
phosphasilene/phosphagermene anion-radical species. We
have therefore undertaken the chemical reduction of 1a
and 1b with an equimolar amount of potassium graphite.
In both cases, one-electron reduction resulted in immediate
change of the orange color of the starting 1a and 1b to the
dark green color of their anion-radicals [1a]-• and [1b]-•
(Scheme 2).7
Figure 1. ORTEP drawing of 1a (30% thermal ellipsoids).
Hydrogen atoms are omitted for clarity. Selected bond lengths
˚
(A): Si1-P1 = 2.1114(7), Si1-Si2 = 2.3855(8), Si1-Si3 =
2.4100(8), P1-C19 = 1.863(2). Selected bond angles (deg): P1-
Si1-Si2 = 104.83(3), P1-Si1-Si3 = 128.05(3), Si2-Si1-Si3 =
127.12(3), Si1-P1-C19 = 110.66(7).
Both anion-radicals were persistent at room temperature,
which allowed their characterization by EPR spectroscopy.
Thus, in THF solution the EPR spectrum of the phosphasi-
lene anion-radical [1a]-• showed a doublet resulting from
coupling to the phosphorus nucleus (a(31P) = 5.4 mT) with a
g-factor of 2.0083, accompanied by a pair of silicon satellites
(a(29Si) = 5.0 mT) (Figure 3).
Although the value of the hyperfine coupling constant
(hfcc) a(29Si) of [1a]-• is close to those of silyl-substituted silyl
radicals (5.6-6.4 mT),19 the value of a(31P) hfcc is remark-
ably lower than that of phosphorus-centered radicals (9.2-
10.8 mT),20 being very close to those of delocalized dipho-
sphene anion-radicals (4.1-5.5 mT).21 Likewise, the EPR
spectrum of the phosphagermene anion-radical [1b]-• con-
tained a doublet resonance with the very characteristic set of
10 satellite signals (73Ge coupling) with the following EPR
parameters: g-factor = 2.0161, a(31P) = 5.6 mT, a(73Ge) =
2.5 mT. The EPR spectrum pattern of [1a]-• dramatically
changed upon changing the solvent from polar THF to
Figure 2. ORTEP drawing of 1b (30% thermal ellipsoids).
Hydrogen atoms are omitted for clarity. Selected bond lengths
(A): Ge1-P1 = 2.1748(14), Ge1-Si1 = 2.4297(14), Ge1-Si2 =
˚
2.4406(16), P1-C19 = 1.854(5). Selected bond angles (deg): P1-
Ge1-Si1 = 104.88(5), P1-Ge1-Si2 = 127.50(6), Si1-Ge1-
Si2 = 127.59(6), Ge1-P1-C19 = 109.90(16).
Me3Si(tBuMe2Si)SidAd (Ad = adamantylidene)14 with the
substitution pattern Si2SidCC2, which favors relaxation
of the natural bond polarity >Siδ+dCδ-< due to the
inductive effect of the substituents, featured a relatively long
(17) Inoue, S.; Ichinohe, M.; Sekiguchi, A. J. Am. Chem. Soc. 2008,
130, 6078.
(18) Bard, A. J.; Cowley, A. H.; Kilduff, J. E.; Leland, J. K.; Norman,
N. C.; Pakulski, M.; Heath, G. A. J. Chem. Soc., Dalton Trans. 1987,
249.
(19) Latest review: Lee, V. Ya.; Sekiguchi, A. In Reviews of Reactive
Intermediates Chemistry; Moss, R. A., Platz, M. S., Jones, M., Jr., Eds.;
Wiley: Hoboken, 2007; Chapter 2.
15
˚
>SidC< bond (1.741(2) A).
In marked contrast,
Wiberg’s silene Me2SidC(SiMetBu2)SiMe316 with the oppo-
site substitution pattern C2SidCSi2, enhancing the intrinsic
>Siδ+dCδ-< bond polarity by the substituent effect,
manifested a remarkably short >SidC< bond (1.702-
(5) A). Likewise, silyl substitution at the Si atom of the
>SidC< bond resulted in the notable red shifts in their UV
spectra,1h similar to the situation observed in 1a and 1b.
15
˚
(20) Cetinkaya, B.; Hudson, A.; Lappert, M. F.; Goldwhite, H. J.
Chem. Soc., Chem. Commun. 1982, 609.
(21) (a) See ref 18. (b) See ref 20. (c) Culcasi, M.; Gronchi, G.;
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Escudie, J.; Couret, C.; Pujol, L.; Tordo, P. J. Am. Chem. Soc. 1986, 108,
3130. (d) Binder, H.; Riegel, B.; Heckmann, G.; Moscherosch, M.; Kaim, W.;
€
von Schnering, H.-G.; Honle, W.; Flad, H.-J.; Savin, A. Inorg. Chem. 1996,
(14) Apeloig, Y.; Bendikov, M.; Yuzefovich, M.; Nakash, M.; Bravo-
€
12228.
(15) Apeloig, Y.; Karni, M. J. Am. Chem. Soc. 1984, 106, 6676.
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(16) Wiberg, N.; Wagner, G.; Muller, G. Angew. Chem., Int. Ed. Engl.
35, 2119. (e) Shah, S.; Burdette, S. C.; Swavey, S.; Urbach, F. L.;
Protasiewicz, J. D. Organometallics 1997, 16, 3395. (f) Dutan, C.; Shah,
S.; Smith, R. C.; Choua, S.; Berclaz, T.; Geoffroy, M.; Protasiewicz, J. D.
Inorg. Chem. 2003, 42, 6241. (g) Geier, J.; Harmer, J.; Gr€utzmacher, H.
Angew. Chem., Int. Ed. 2004, 43, 4093. (h) Nagahora, N.; Sasamori, T.;
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1985, 24, 229.

Communication
Organometallics, Vol. 28, No. 15, 2009 4265
THF f benzene solvent exchange can be explained in terms
of a more delocalized structure of the anion-radical in THF
and a more localized (unpaired electron on Si and negative
charge on P) structure in benzene. In nonpolar benzene the
anionic phosphorus center can be stabilized by the bonding
interaction to the potassium counterion (contact ion pair),
thus favoring unpaired electron-negative charge localiza-
tion and separation, whereas in polar THF such cation-
anion interaction is, apparently, broken down because
of solvation effects, thus leading to a more delocalized
structure.
Acknowledgment. This work was financially supported
by the JSPS-CNRS Joint Research Project and Grants-
in-Aid for Scientific Research program (Nos. 19105001,
20038006, 21550033) from the Ministry of Education,
Science, Sports, and Culture of Japan.
Figure 3. EPR spectrum of the potassium salt of phosphasilene
anion-radical [1a]-• [K+] in THF.
3
nonpolar benzene.22 The value of a(31P) hfcc of the central
doublet (g=2.0076) was significantly decreased to 1.5 mT,
whereas that of a(29Si) hfcc was increased to 6.5 mT. Overall,
such a decrease in the a(31P) hfcc and simultaneous increase
in the a(29Si) hfcc in the EPR spectrum of [1a]-• upon the
Supporting Information Available: The experimental proce-
dures and spectral data for compounds 1a, 1b, and their anion-
radicals, CV charts of 1a and 1b, tables of crystallographic data
including atomic positional and thermal parameters for 1a and
1b (PDF/CIF), and computational details (tables of atomic
coordinates for optimized geometries of the model compounds
1a0 and 1b0 and values of their total energies). This material is
available free of charge via the Internet at http://pubs.acs.org.
(22) Such a change is reversible, and by replacing benzene with THF
again, we were able to recover the original structure exhibiting the EPR
spectrum shown in Figure 3.