Synthesis of a "half"-parent phosphasilene R2Si=PH and its metalation to the corresponding P-zinciophosphasilene [R2Si=PM]

Article abstract of DOI:10.1002/anie.200504145

(Chemical Equation Presented) Remarkably stable - even with a hydrogen atom at the low-coordinate phosphorus center! A straightforward synthesis provides the thermally robust phosphasilenes (E)/(Z)-1, which have a terminal PH group. These compounds can be

Full text of DOI:10.1002/anie.200504145

Angewandte
Chemie
=
Si P Species
DOI: 10.1002/anie.200504145
Synthesis of a “Half”-Parent Phosphasilene
=
R2Si PH and Its Metalation to the Corresponding
=
P-Zinciophosphasilene [R2Si PM]**
Matthias Driess,* Stefan Block, Markus Brym, and
Michael T. Gamer
Dedicated to Professor Hansgeorg Schnöckel
on the occasion of his 65th birthday
Scheme 1. The ferrio-substituted phosphasilene D and the conversion
of X-metalated silenes C into A and B.
The isolation of compounds with low-coordinate silicon
atoms,[1–3] in other words, silicon homologues of olefins, and
more recently the synthesis of the first inert disilyne, an
alkyne analogue with a silicon–silicon triple bond,[4] are major
breakthroughs in contemporary silicon chemistry.Unsatu-
phosphasilene derivative is hitherto unknown.We report
here the synthesis of the first isolable phosphasilene 1 with a
PH group and its P-metalation with Me2Zn in the presence of
N,N,N’,N’-tetramethylethylenediamine (tmeda) to afford the
crystalline P-zinciophosphasilene 2 in 87% yield (Scheme 2).
=
rated silicon compounds with Si X bonds (X = elements from
Groups 14,[1] 15,[2] or 16 [3]) show an intriguing reactivity
toward both electrophiles and nucleophiles, and are valuable
building blocks for the synthesis of electronically unusual
silicon compounds by means of addition reactions.
Another challenge is to use functional groups containing
low-coordinate silicon atoms for the synthesis of novel
conjugated heteroatom systems A and B by homo- and
=
heterocoupling reactions of metal-substituted Si X building
blocks C (X = elements from Groups 14 or 15; Scheme 1).
However, compounds of type C suitable as reagents for a
=
nucleophilic transfer of intact Si X moieties are still very
=
rare.While lithium disilenylide derivatives of the type R Si
2
SiR(Li), that is, disila analogues of vinyllithium, have been
reported very recently,[5] related metal-substituted heterosi-
=
=
lenes of the type R2Si [X]M and (M)RSi [X]R are currently
Scheme 2. Synthesis of 1 and 2 via the difluorosilane 3 and the
(fluorosilyl)phosphanes 4 and 5, respectively. dme=dimethoxyethane.
unknown, except for the P-ferrio-substituted phosphasilene
5
=
R2Si P[Fe(CO)2(h -C5H5)]
D
(R = 2,4,6-iPr3C6H2;
Scheme 1).[6] Lithium disilenylides have been shown to
undergo homocoupling to give the corresponding tetrasila-
buta-1,3-dienes.[7]
Phosphasilene 1 is easily accessible by a modified
procedure analogous to the preparation of P-silyl-substituted
phosphasilenes,[2f,g,i,k] starting from the bulky substituted
=
P-metalated phosphasilenes of the type R2Si PM should
be easily accessible through metalation of “half”-parent
difluorosilane (tBu3Si)RSiF2 3 (R = 2,4,6-iPr3C6H2) and
=
phosphasilenes of the type R2Si PH; however, such a
[LiPH2(dme)] in the molar ratio of 1:2 (Scheme 2).The first
step of this reaction leads solely to the corresponding lithium
(fluorosilyl)phosphanide
4
(characterized by 31P and
[*] Prof. Dr. M. Driess, Dipl.-Chem. S. Block, Dr. M. Brym
Institut für Chemie
29Si NMR spectroscopy, see the Supporting Information)
along with PH3 and LiF.Unexpectedly and in contrast to
other lithium (fluorosilyl)phosphanides,[2l,8] solutions of 4 in
THF already undergo LiF elimination above 308C, affording
a mixture of the Z and E isomers of 1 in moderate yield
(31P NMR) besides several side-products, which could not be
separated from 1.However, compound 1 is surprisingly
simple to prepare in quantitative yield by a modified
procedure using LiF-free samples of 4 in hexane solutions
at ambient temperature.This can be achieved by P-lithiation
of purified (fluorosilyl)phosphane 5 with BuLi in hexane
solution, and 5 is in turn easily accessible by mild protonation
of 4 with NMe3HCl (Scheme 2).Compound 5 can be isolated
Metallorganik und Anorganische Materialien
Technische Universität Berlin
Strasse des 17. Juni 135, Sekr. C2, 10623 Berlin (Germany)
Fax: (+49)30-314-22168
E-mail: matthias.driess@tu-berlin.de
Dr. M. T. Gamer
Institute of Chemistry
Freie Universität Berlin
Fabeckstrasse 34–36, 14195 Berlin (Germany)
[**] M={Zn(tmeda)Me}; tmeda=N,N,N’,N’-tetramethylethylene-
diamine.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or fromthe author.
Angew. Chem. Int. Ed. 2006, 45, 2293 –2296
ꢀ 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
2293

Communications
in 91% yield in the form of colorless crystals, which have been
characterized by NMR spectroscopy and elemental analysis
(see the Supporting Information).
P-silyl-substituted phosphasilenes.[2k,i] This indicates a shallow
[2n,o]
=
potential of the Si P bond on the potential energy surface.
ꢀ
The Si1 Si2 bond length of 239.8(5) pm is comparable to the
Phosphasilene 1 was isolated in the form of greenish
yellow crystals in 88% yield, and its composition was
confirmed by EIMS (m/z 462) and C,H analysis.The product
consists of a 1:1.5 mixture of the Z and E isomers as proven by
its 1H, 31P, and 29Si NMR spectra.Thus, the 31P NMR spectrum
shows two doublet signals with 29Si satellites at d = 123.1
(1J(P,H) = 123, 1J(31P, 29Si) = 157 Hz) and 134.2 ppm (1J(P,H) =
131, 1J(31P, 29Si) = 130 Hz).Accordingly, the 1H NMR spec-
trum of the E/Z mixture exhibits two doublets for the protons
of the PH groups at d = 5.14 (1J(31P, 1H) = 131 Hz) and
5.20 ppm (1J(31P, 1H) = 123 Hz), respectively.Based on selec-
values of other tBu3Si-substituted silicon compounds.[11]
Although the substituents at the phosphorus atom are not
very bulky, compound 1 survives heating in boiling toluene
=
solution for several days.Since the Si P bond appears to be
sufficiently protected, we decided to use 1 as the starting
material for the formation of an isolable, nucleophilic P-
metal-subsituted phosphasilene.While lithiation of 1 with
BuLi in nonpolar or polar donor solvents (ether, tmeda) at
ꢀ788C did not lead to the desired P-lithiated phosphasilene as
indicated by the 31P NMR spectrum of the reaction mixture
(no low-field resonances), its metalation with Me2Zn in the
presence of tmeda proceeded cleanly at ꢀ788C and furnished
solely the desired P-zinciophosphasilene 2 (low-field d(31P)
and d(29Si) signals).Alternatively, 2 was also accessible in high
yield by a one-pot procedure, starting from 5 (see the
Supporting Information).
1
tive NOE H NMR experiments, one can assign the NMR
resonances at d = 5.14 (1H) and 123 ppm (31P) to the E isomer
(tBu3Si,PH), while the signals at d = 5.20 (1H) and 134.2 ppm
(31P) belong to the Z form.
1
Interestingly, the J(P,H) coupling constants are approx-
imately 80 Hz smaller than the characteristic values observed
for secondary phosphanes R2PH (R = alkyl, aryl, silyl) and
significantly smaller than those of phosphaalkenes with a PH
group (1J(31P, 1H) = 140–208 Hz).[9] This indicates that the
P-zinciophosphasilene 2 was isolated in the form of
orange crystals in 87% yield, and its composition and
constitution was proven by EIMS (m/z 658), C,H analysis,
and NMR spectroscopy.Because of the dominantly covalent
ꢀ
ꢀ
phosphorus atom in 1 prefers more 3p character in the P H
character of the Zn P bond, 2 does not dissociate in solutions
bond.As expected, the 31P nucleus in 1 is less shielded than
in aromatic hydrocarbons and in ethers, as proven by
cryoscopic measurements.Not only that, the steric demand
of the {ZnMe(tmeda)} group at the phosphorus atom favors
the E configuration as the lowest energy configuration; this
was proven by variable-temperature (VT) 1H, 31P and
29Si NMR spectroscopy.Thus, solutions of 2 in toluene exhibit
in the temperature range from ꢀ80 to 808C only one singlet
resonance signal in the 31P NMR spectrum at unusual low
field (d = 227 ppm).The extremely low-field resonance of the
31P nucleus, which is roughly 100 ppm lower than that
observed for (Z)- and (E)-1 represents the least shielded
chemical shift hitherto reported for a phosphasilene.[2j,k]
In contrast to the deshielding of the 31P nucleus, the low-
coordinate 29Si atom in 2 (d = 203 ppm) is more shielded than
that in (Z)/(E)-1 (Dd = 42 ppm).This can be explained by
stabilization of the n orbital at phosphorus through n(P)!
=
those in P-silylated phosphasilenes of the type R2Si PSiR3
(d = ꢀ33 to 28 ppm)[2k,i] but almost identical with the values
=
reported for P-organo-substituted Si P compounds (d = 65–
136 ppm).[2k,i,o] The 29Si resonance for the low-coordinate Si
atom in 1 shows for each isomer a doublet at characteristically
low field with a typical scalar 29Si-31P coupling constant.The
signal at d = 248.9 ppm (1J(29Si,31P) = 130 Hz) can be assigned
to the Z isomer and that at d = 249.8 ppm (1J(29Si,31P) =
157 Hz), to the E form.
A single-crystal X-ray diffraction analysis[10] confirms that
the low-coordinate silicon atom adopts the trigonal-planar
coordination geometry (Figure 1).The Si1 P1 distance of
209.4(5) pm is about 7% shorter than a Si P single bond but
only marginally longer (ca.3 pm) than the respective value in
ꢀ
ꢀ
ꢀ
s*(Si Si) hyperconjugation which, apparently, is more effi-
cient in 2 than in 1 as a result of the higher negative partial
charge at the phosphorus atom (Figure 2).[12] Similar shielding
effects have been observed for related alkali-metal-substi-
=
tuted disilenylides (“disilavinyls”) of the type [M(R3Si)Si
Si(SiR3)2] (M = Li, Na, K),[5b,7b] which show extremely low-
field signals for the metal-substituted silicon atoms.Accord-
Figure 1. X-ray crystal structure of 1 (ellipsoids drawn at the 50%
probability level; hydrogen atoms omitted for clarity). Selected distan-
ces [pm] and angles [8]: Si1–P1 209.4(5), Si1–Si2 239.8(5), Si1–C1
188.0(12); Si2-Si1-P1 121.5(2), C1-Si1-Si2 120.3(4), C1-Si1-P1 118.2(4);
sumof the bond angles at Si1: 359.9 8.
ꢀ
Figure 2. Proposed negative hyperconjugation n(P)!s*(Si Si) in 2
ꢀ
ꢀ
(left) and the analogous hyperconjugation s(Si M)!s*(Si Si) in
isoelectronic silyl-substituted alkali-metal disilenylide compounds
(right).[5b,7b]
2294 www.angewandte.org
ꢀ 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2006, 45, 2293 –2296

Angewandte
Chemie
ingly, the resonance signal of the other low-coordinate 29Si
=
g) N.Tokitoh, Acc. Chem. Res. 2004, 37, 86.Si C compounds:
h) A.G.Brook, S.C.Nyburg, F.Abdesaken, B.Gutekunst, G.
Gutekunst, R.K.Kallury, Y.C.Poon, Y-.M.Chang, W.Wong-
Ng, J. Am. Chem. Soc. 1982, 104, 5667; i) N.Wiberg, G.Wagner,
G.Müller, 1985, 97, 220; Angew. Chem. Int. Ed. Engl. 1985, 24,
ꢀ
ꢀ
type is shifted to higher field as a result of s(Si M)!s*(Si
Si) negative hyperconjugation (Figure 2).
The E configuration of 2 was confirmed by X-ray
diffraction analysis.[10] This also proved that the Si1 atom
229; j) N.Wiberg, G.Wagner,
k) Reviews: A.G.Brook, K.M.Baines, Adv. Organomet. Chem.
1986, 25, 1; l) A.G. Brook, M.A. Brook, Adv. Organomet.
Chem. Ber. 1986, 119, 1467;
ꢀ
has trigonal-planar geometry and that the Si1 P1 distance of
206.4(1) pm is significantly shorter than that in 1 (Figure 3).
Chem. 1996, 39, 71; m) “Silicon-Carbon and Silicon-Nitrogen
Multiply Bonded Compounds”: T.Müller, W.Ziche, N.Auner in
The Chemistry of Organic Silicon Compounds, Vol. 2 (Eds.: Z.
Rappoport, Y.Apeloig), Wiley, Chichester, 1998, chap.16,
=
=
p.857. Si Ge and Si Sn compounds: n) K.M. Baines, J.A.
Cooke, Organometallics 1991, 10, 3419; o) K.M. Baines, J.A.
Cooke, Organometallics 1992, 11, 3487; p) V.Y. Lee, M.
Ichinohe, A.Sekiguchi, N.Takagi, S.Nagase,
J. Am. Chem.
Soc. 2000, 122, 9034; q) A.Sekiguchi, R.Izumi, V.Y.Lee, M.
Ichinohe, J. Am. Chem. Soc. 2002, 124, 14822.
=
[2] Si N compounds: a) N.Wiberg, K.Schurz, G.Reber, G.Müller,
J. Chem. Soc. Chem. Commun. 1986, 591; b) M.Hesse, U.
Klingebiel, Angew. Chem. 1986, 98, 638; Angew. Chem. Int. Ed.
Engl. 1986, 25, 649; Review: c) I.Hemme, U.Klingebiel, Adv.
=
P
Organomet. Chem. 1996, 39, 159; see also reference [1m].Si
=
and Si As compounds: d) C.N.Smit, F.Bickelhaupt, Organo-
metallics 1987, 6, 1156; e) Y.van den Winkel, HM. M. .Bas-
tiaans, F.Bickelhaupt, J. Organomet. Chem. 1991, 405, 183; f) M.
Driess, H.Pritzkow, Angew. Chem. 1992, 104, 350; Angew.
Chem. Int. Ed. Engl. 1992, 31, 316; g) M.Driess, S.Rell, H.
Pritzkow, J. Chem. Soc. Chem. Commun. 1995, 253; h) H.R.G.
Bender, E.Niecke, M.Nieger, J. Am. Chem. Soc. 1993, 115,
Figure 3. X-ray crystal structure of 2 (ellipsoids drawn at the 50%
probability level; hydrogen atoms omitted for clarity). Selected distan-
ces [pm] and angles [8]: Si1–P1 206.4(1), Si1–Si2 240.2(1), Si1–C2
190.3(4), P1–Zn1 234.9(1), C1–Zn1 199.5(4); Si2-Si1-P1 118.10(6), Si2-
Si1-C2 120.2(1), C2-Si1-P1 121.7(1), Si1-P1-Zn1 103.20(5); sumof the
bond angles at Si1: 3608.
3314; i) M.Driess, S.Rell, H.Pritzkow, R.Janoschek,
Angew.
Chem. 1997, 109, 1384; Angew. Chem. Int. Ed. Engl. 1997, 36,
1326; j) D.Lange, E.Klein, H.Bender, E.Niecke, M.Nieger, R.
Pietschnig, W.W. Schoeller, H. Ranaivonjatovo, Organometallics
1998, 17, 2425; k) M.Driess, S.Rell, K.Merz, Z. Anorg. Allg.
Chem. 1999, 625, 1119; Reviews: l) M.Driess, Adv. Organomet.
Chem. 1996, 39, 193; m) M.Driess, Coord. Chem. Rev. 1995, 145,
=
The shortening of the Si P bond, the slight elongation of the
ꢀ
Si1 Si2 bond, and the smaller P1-Si1-Si2 angle are consistent
=
with the presence of hyperconjugative interactions as
1; n) see also: reference [1m], p.1051; Theoretical aspects of Si
=
=
N, Si P and Si As compounds: o) K.J.Dykema, T.N.Truong,
M.S.Gordon, J. Am. Chem. Soc. 1985, 107, 4535; p) M.Driess,
R.Janoschek, J. Mol. Struct. 1994, 323, 129.
depicted in Figure 2.As expected, the Zn1 atom has
ꢀ
tetrahedral coordination geometry and the value of the P1
Zn1 distance is typical for that of zincphosphanides.[13]
In summary, we have prepared the first phosphasilene
derivatives with terminal hydrogen and zinc substituents at
phosphorus by an unexpectedly simple procedure.These
compounds represent a novel type of thermally robust
building blocks in the chemistry of unsaturated silicon–
phosphorus compounds.Investigations on the use of 1 and 2
for the synthesis of novel conjugated, electron-rich p systems
of types A and B (see Scheme 1) by homo- or hetero-cross-
coupling reactions are currently underway.
[3] a) H.Suzuki, N.Tokitoh, S.Nagase, R.Okazaki,
J. Am. Chem.
Soc. 1994, 116, 11578; b) N.Tokitoh, H.Suzuki, T.Matsumoto,
Y.Matsuhashi, R.Okazaki, M.Goto, J. Am. Chem. Soc. 1991,
113, 7047; c) H.Suzuki, N.Tokitoh, R.Okazaki, S.Nagase, M.
Goto, J. Am. Chem. Soc. 1998, 120, 11096; d) Review: “Chemis-
try of Compounds with Silicon–Sulfur, Silicon–Selenium and
Silicon–Tellurium Bonds”: D.A.Armitage in The Chemistry of
Organic Silicon Compounds, Vol. 2 (Eds.: Z. Rappoport, Y.
Apeloig), Wiley, Chichester, 1998, chap.31, p.1869.
[4] a) N.Wiberg, W.Niedermayer, G.Fischer, H.Nöth, M.Suter,
Eur. J. Inorg. Chem. 2002, 1066; b) N.Wiberg, S.K.Vasisht, G.
Fischer, P.Mayer, Z. Anorg. Allg. Chem. 2004, 630, 1823; c) A.
Sekiguchi, R.Kinjo, M.Ichinohe, Science 2004, 305, 1755.
[5] a) D.Scheschkewitz, Angew. Chem. 2004, 116, 3025; Angew.
Chem. Int. Ed. 2004, 43, 2965; b) S.Inoue, M.Ichinohe, A.
Sekiguchi, Chem. Lett. 2005, 34, 1564.
Received: November 21, 2005
Published online: March 3, 2006
Keywords: metalation · phosphanes · phosphasilenes · zinc
.
[6] M.Driess, H.Pritzkow, U.Winkler, J. Organomet. Chem. 1997,
529, 313.
[7] a) M.Weidenbruch, S.Willms, W.Saak, G.Henkel,
Angew.
=
[1] Si Si compounds: a) R.West, M.J.Fink, J.Michl, Science 1981,
Chem. 1997, 109, 2612; Angew. Chem. Int. Ed. Engl. 1997, 36,
2503; b) M.Ichinohe, K.Sanuki, S.Inoue, A.Sekiguchi,
Organometallics 2004, 23, 3088.
214, 1343; b) S.Matsumoto, S.Tsutsui, E.Kwon, K.Sakamoto,
Angew. Chem. 2004, 116, 4710; Angew. Chem. Int. Ed. 2004, 43,
4610.Reviews: c) R.West, Angew. Chem. 1987, 99, 1231; Angew.
Chem. Int. Ed. Engl. 1987, 26, 1201; d) R.Okazaki, R.West, Adv.
Organomet. Chem. 1995, 39, 231; e) R.West, Polyhedron 2002,
21, 467; f) A.Sekiguchi, V.Y.Lee, Chem. Rev. 2003, 103, 1429;
[8] a) M.Driess, U.Winkler, W.Imhof, L.Zsolnai, G.Huttner,
Chem. Ber. 1994, 127, 1031; b) S.Ionkin, W.J.Marshall,
Organometallics 2003, 22, 4136.
Angew. Chem. Int. Ed. 2006, 45, 2293 –2296
ꢀ 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
2295

Communications
[9] “Survey of 31P NMR Data”: K.Karaghiosoff in Multiple Bonds
and Low Coordination in Phosphorus Chemistry (Eds.: O.
Scherer, M.Regitz), Thieme, Stuttgart, 1990, p.467.
¯
[10] 1: Triclinic, space group P1, a = 8.488(2), b = 9.388(2), c =
18.944(4) Š, a = 78.627(17), b = 84.363(18), g = 81.652(18)8,
V= 1460.4(6) Š3, Z = 2, 1calcd = 1.053 mgmꢀ3
,
m(MoKa) =
0.19 mmꢀ1, 11516 collected reflections, 5122 crystallographically
independent reflections [Rint = 0.0359], 5121 reflections with I >
2s(I), qmax = 25.008, 271 refined parameters, R(Fo) = 0.0459 (I >
2
2s(I), wR(Fo ) = 0.1179 (all data). 2: Monoclinic, space group
P21/c, a = 14.6883(4), b = 12.5931(3), c = 24.4109(7) Š, b =
101.7130(10)8, V= 4421.3(2) Š3, Z = 4, 1calcd = 1.076 mgmꢀ3, m-
(MoKa) = 0.67 mmꢀ1, 26671 collected reflections, 7772 crystallo-
graphically independent reflections [Rint = 0.0448], 5757 reflec-
tions with I > 2s(I), qmax = 25.008,419 refined parameters,
2
R(Fo) = 0.0580 (I > 2s(I)), wR(Fo ) = 0.1476 (all data). Each of
the crystals were mounted on a glass capillary in perfluorinated
oil and measured in a cold N2 flow.The data of 1 were collected
on a STOE IPDS 2T diffractometer with an area detector at
173 K (MoKa radiation, l = 0.71073 Š) and those of 2 on a
Bruker-AXS SMART CCD diffractometer at 173(2) K (MoKa
radiation, l = 0.71073 Š). The structures were solved by direct
methods and were refined on F2 with the SHELX-97[14] software
package.The positions of the H atoms were calculated and
considered isotropically according to a riding model.Absorption
corrections were conducted using the SADABS program.[15]
CCDC 289280 (1) and CCDC 289281 (2) contains the supple-
mentary crystallographic data for this paper.These data can be
obtained free of charge from The Cambridge Crystallographic
Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
[11] N.Wiberg, W.Niedermayer, H.Nöth, J.Knizek, W.Ponikwar, K.
Polborn, Z. Naturforsch. B 2000, 55, 389, and references therein.
[12] T.Albright, J.K.Burdett, M-.H.Whangbo,
Orbital Interactions
in Chemistry, Wiley, Chichester, 1985, p.178.
[13] S.C.Goel, M.Y.Chiang, W.E.Buhro,
J. Am. Chem. Soc. 1990,
112, 5636.
[14] G.M. Sheldrick, SHELX-97 Program for Crystal Structure
Determination, Universitꢀt Göttingen (Germany), 1997.
[15] G.M. Sheldrick, SADABS Program for Empirical Absorption
Correction of Area Detector Data, Universitꢀt Göttingen (Ger-
many), 1996.
2296 www.angewandte.org
ꢀ 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2006, 45, 2293 –2296