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Cyclic Bis(phosphanyl)carbenium Ion by Protonation of a 1 3-Diphosphacyclobutane-2 4-diyl.

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Stable Carbenium Ions
Cyclic Bis(phosphanyl)carbenium Ion by
Protonation of a 1,3-Diphosphacyclobutane-2,4diyl**
to give dianion 5.[5] We report here on a further means of
reducing the electron density of 1 by the transformation of a
bis(phosphanyl)methyl fragment into a bis(phosphanyl)carbenium ion.
In the phosphanyl-substituted carbenium ions 6 both the
phosphorus and the carbon center have trigonal-planar
Manuel Sebastian, Aaron Hoskin, Martin Nieger,
Lszl Nyulszi,* and Edgar Niecke*
Dedicated to Professor Martin Jansen
on the occasion of his 60th birthday
The 1,3-diphosphacyclobutane-2,4-diyl 1 is an electron-rich
diradicaloid heterocycle.[1] To reduce electron density, it
undergoes valence isomerizations forming an intermediate
phosphanylcarbene (2, R’ = R’’ = Cl, R = Mes* (2,4,6tBu3C6H2)[1] or Tmp (2,2,6,6-Tetramethylpiperidyl)[2]) or bicyclobutane (3, R’ = SiMe3, R’’ = H, R = Mes* [3]). Another way
to reduce the electron density is the excitation of one
electron. The excited species undergoes subsequent P aryl
bond cleavage forming the radical species 4.[4] Alternatively,
reduction of 1 leads to the formation of an intermediate
radical anion, which can also undergo P aryl bond cleavage
[*] Dr. M. Sebastian, Dr. A. Hoskin, Dr. M. Nieger, Prof. Dr. E. Niecke
Anorganisch-Chemisches Institut der Universitt
Gerhard-Domagk-Strasse 1, 53121 Bonn (Deutschland)
Fax: (+ 49) 228-735-327
Prof. Dr. L. Nyulszi
Department of Inorganic Chemistry
Technical University of Budapest
Gellrt tr 4, 1521 Budapest (Ungarn)
Fax: (+ 36) 1463-3642
coordination,[6] since the stabilization gained from the interaction of the carbenium ion with the good p-electrondonating phosphorus[7] overcomes the energy required for
planarization at the phosphorus center. Moiety 6 can be
regarded as a highly polar alkylidene, which is a valuable
building block in organophosphorus chemistry.[8]
1,3-Bis(phosphanyl)carbenium ions 7 formally correspond to allylic anions. The stabilization of the carbenium
ion, however, is insufficient to compensate for the inversion
barrier of two phosphorus atoms. Ab initio calculations
indicate that the parent 7 is nonplanar and without conjugative stabilization.[9] Hitherto, two derivatives of 7 have been
reported based on NMR evidence; however, this assignment
is not completely certain owing to downfield resonances in the
P NMR spectra.[10]
Isomers of 7 such as phosphonium-substituted phosphalkenes 8,[6b, 11] the cyclic isomers bis(phosphonium)methanide 9
(R = NiPr2),[12] and the diphosphiranium salt 10 (R = Mes*)[9]
are known. The incorporation of the parent P-C-P unit into a
small ring system like 1 is beneficial since it prevents the
possible formation of the P P bond to give the threemembered ring 9. The desired bis(phosphanyl)carbenium ion
is favored, even if this would be the thermodynamically less
stable species.[13]
Herein we describe the synthesis and characterization of
the bis(phosphanyl)carbenium ion 11 [Eq. (1)]. When 1,3diphosphacyclobutane-2,4-diyl 1 a was protonated with trifluoromethanesulfonic acid, the color of the solution changed
from red to yellow and the salt 11-Tfl was produced, which
[**] This work was supported by the Deutsche Forschungsgemeinschaft,
the Fonds der Chemischen Industrie, and the Alexander von
Humboldt Stiftung (fellowship to L.N.). Generous allocation of
computer time from NIIF/Hungary and support from OTKA T
034675 is also gratefully acknowledged.
Angew. Chem. Int. Ed. 2005, 44, 1405 –1408
DOI: 10.1002/anie.200462032
2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
was isolated as a crystalline solid. The sole signal in the
P NMR spectrum of 11-Tfl (d(31P) = 0.1 ppm) has a coupling with two protons (t, 2JPH = 14.0 Hz), and in the 1H NMR
spectrum the resonance at d(1H) = 5.3 also shows a triplet
structure with a coupling of 2JPH = 14.0 Hz. This indicates that
the two phosphorus atoms in 11-Tfl are equivalent and
connected through a bridging CH2 group. Thus, protonation
took place at the C(H) carbon atom in 1 a. The 13C NMR
signals of the ring carbon atoms of 11-Tfl d(13C) = 75.8 (t,
JPC = 18.6 Hz) and 123.3 ppm (t, 1JPC = 12.3 Hz) indicate that
the symmetry of the ring system is retained. The considerably
downfield shift (Dd = 10 ppm) of the silyl group in the 29Si
NMR spectrum on going from 1 a to 11-Tfl (d(29Si) = 3.6, t,
JPSi = 1.5 Hz) indicates the electron deficiency of 11. The
ionic structure of 11-Tfl is responsible for its poor solubility
in nonpolar solvents.
Crystals of 11-Tfl suitable for X-ray structure analysis
were obtained by recrystallization from toluene/CH2Cl2
(10:1) at 30 8C (Figure 1).[14] The four-membered P2C2 ring
surmise that the electrophilic elimination of the silyl group
in 11 should be facile.[16]
Density functional calculations[17] carried out on 11’[18] at
the B3LYP/6-31 + G* level of theory provided a structure
Figure 2. LUMO of the carbenium ion 11’.
Figure 1. Molecular structure of 11-Tfl with selected bond lengths
[pm], angles [8], and bond angle sums [8]. (Counterion and hydrogen
atoms, apart from those on the P2C2 ring, have been omitted for
clarity.) P1–C1 173.5(3), P1–C2 185.5(3), P2–C1 172.3(3), P2–C2
183.8(3), C1–Si1 189.3(3); C1-P1-C2 90.0(1), C1-P2-C2 90.9(1), P1-C1P2 93.2(1), P1-C2-P2 85.8(1); (C1) 360, (P1) 336, (P2) 341.
similar to that obtained by X-ray diffraction. The carbocationic character of 11’ is clearly evident in the LUMO
presented in Figure 2.[19] A carbenium ion should have a
low-energy empty orbital at the tricoordinate carbon atom. In
the case of 11’ the LUMO is indeed centered at the
tricoordinate carbon atom but with a significant contribution
from the lone-pair orbitals of the neighboring nonplanar
phosphorus atoms. The aryl substituents on the two phosphorus atoms occupy a trans position, consequently the two
“s”-type lone pairs are at opposite ends of the four-membered
ring. The interaction of the two lone pairs is substantial,
resulting in a bonding (Fb(P)) and an antibonding (Fa(P))
combination (Figure 3). (In the case of the saturated 1,3diphosphacyclobutane ring, the splitting of these two Kohn–
exhibits a planar geometry. The sums of the bond angles at the
phosphorus atoms ((P1) 3368, (P2) 3418) as well as at the
tricoordinated endocyclic carbon atom ((C1) 3608) do not
differ considerably from the corresponding
sums of 1 a.[3] This holds also for the bond
lengths within the bis(phosphanyl)carbenium fragment (P1 C1 173.5(3) pm, P2 C1
172.3(3) pm). The protonated carbon atom
C2 exhibits a distorted tetrahedral geometry. The P C2 bonds are significantly longer
(P1 C2 185.5(3) pm, P2 C2 183.8(3) pm)
and correspond to typical P C single bonds
(1 = 183 pm).[15] These structural motifs
indicate that the moderate pyramidalization
of the phosphorus atom enables the distinct
delocalization of the positive charge, resulting in an allyl-like system [Eq. (1)]. The Si
C bond in 11-Tfl (189.3(3) pm) is significantly longer than the Si Cring bonds in 1 a
(183.6 pm)[3] and cation 9 (R = NiPr2,
186.3 pm).[12] Taking the lowfield 29Si NMR
Figure 3. Stabilizing interaction between the phosphorus lone-pair orbitals and the empty
shift (see above) into consideration, we
p orbital of the carbenium ion 11’.
2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2005, 44, 1405 –1408
Sham (KS) MOs is 1.1 eV, indicating a strong through-space
interaction between the two nP MOs). The antibonding
combination Fa(P) is of suitable symmetry to overlap with
the empty p orbital of the carbenium ion. The stabilization
resulting from the interaction can be estimated from the
isodesmic reaction (2). The calculated stabilization energy is
137.7 kcal mol 1 at the B3LYP/3-21G(*) level and 132.5 kcal mol 1 at the B3LYP/6-31 + G*//B3LYP/3-21G(*) level, showing the effectiveness of this interaction.
Experimental Section
11-Tfl : To a solution of 1 a (325 mg, 0.5 mmol) in toluene (15 mL) at
room temperature was added trifluoromethanesulfonic acid (75 mg,
0.5 mmol). The reaction mixture was stirred for one hour, before a
precipitate was removed from the light yellow solution by filtration.
The residue was washed with toluene (5 mL). The bright yellow solid
was then dissolved in a mixture of toluene (10 mL) and CH2Cl2
(1 mL), and the solution was stored at 30 8C. Yellow crystals of
11-Tfl formed over a period of two days. Yield 348 mg (87 %); m.p.
185 8C; NMR (CD2Cl2, 80 8C): 31P{1H} NMR: d = 29.7 ppm (t, 2JPH =
14.0 Hz); 1H NMR: d = 0.6 (s, Si(CH3)3, 9 H), 1.2 (s, p-tBu, Mes*,
18 H), 1.3 (s, o-tBu, Mes*, 18 H), 1.7 (s, o-tBu, Mes*, 18 H), 5.7 (t,
JPH = 14.0 Hz, PCH2P), 7.4 (s, m-CH, Mes*, 2 H), 7.5 ppm (s, m-CH,
Mes*, 2 H); 13C{1H} NMR: d = 2.4 (t, 3JPC = 3.9 Hz, Si(CH3)3), 30.2
(s, p-CCH3, Mes*), 30.9 (s, o-CCH3, Mes*), 32.6 (s, o-CCH3, Mes*),
34.2 (s, o-CCH3, Mes*), 38.4 (s, o-CCH3, Mes*), 38.5 (s, p-CCH3,
Mes*), 75.8 (t, 1JPC = 18.6 Hz, PC(H2)P), 122.8 (pseudo t, (3JPC +
JPC) = 11.8 Hz, Cmeta, Mes*), 123.3 (t, 1JPC = 12.3 Hz, PC(Si)P), 128.3
(pseudo t, (1JPC + 3JPC) = 58.5 Hz, Cipso, Mes*), 149.6 (s, Cpara, Mes*),
157.0 (s, Cortho, Mes*), 158.3 (s, Cpara, Mes*), 160.1 ppm (pseudo t,
(2JPC + 4JPC) = 15.2 Hz, Cortho, Mes*); 29Si NMR: d = 3.6 (t, 2JPSi =
1.5 Hz).
Received: September 17, 2004
Published online: January 26, 2005
Keywords: ab initio calculations · carbenium ions ·
carbocations · diradicals · phosphorus heterocycles
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Sebastian, O. Schmidt, M. Nieger, L. Nyulszi, E. Niecke,
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[5] M. Sebastian, M. Nieger, D. Szieberth, L. Nyulszi, E. Niecke,
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Int. Ed. Engl. 1996, 35, 2242 – 2244; b) [tBu2PC(H)P(NiPr2)2]+:
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According to ab initio calculations,[9] 9 is more stable than 8, if
the substituents at the phosphorus atoms are amino groups,
while in case of the parent compound the bis(phosphanyl)
carbenium ion 8 is more stable than 9.
X-Ray structure analysis of 11-Tfl : C41H69P2Si + CF3SO3 +
0.5 CF3SO3H: yellow crystals, crystal dimensions 0.7 0.6 0.5 mm3, Mr = 876.10 g mol 1; monoclinic, space group P21/c
(No. 14), a = 13.7571(2), b = 19.6740(3), c = 18.1056(4) , b =
104.554(1)8, V = 4743.16(14) 3, Z = 4, m(MoKa) = 0.239 mm 1,
T = 123(2) K, F(000) = 1876. 21 636 reflections up to 2Vmax. = 508
were measured on a Nonius KappaCCD diffractometer with
MoKa radiation, 8377 of which were independent and used for all
calculations. The structure was solved by direct methods and
refined to F2 anisotropically; the H atoms were refined with a
riding model. wR2(F2) = 0.1692, R(F) = 0.0568 for 501 parameters and 89 restraints. CCDC-246722 (11-Tfl ) contains the
supplementary crystallographic data for this paper. These data
can be obtained free of charge via
retrieving.html (or from the Cambridge Crystallographic Data
Centre, 12, Union Road, Cambridge CB2 1EZ, UK; fax:
(+ 44) 1223-336-033; or
Mean value 3 standard deviation as a result of a CCSD
enquiry for P C single-bond lengths in cyclic systems.
The cleavage of the silyl group from the ring is observed during
different reactions of 11-Tfl .
Gaussian 98 (Revision A.5), M. J. Frisch, G. W. Trucks, H. B.
Schlegel, G. E. Scuseria, M. A. Robb, J. R. Cheeseman, V. G.
Zakrzewski, J. A. Montgomery, R. E. Stratmann, J. C. Burant, S.
Dapprich, J. M. Millam, A. D. Daniels, K. N. Kudin, M. C. Strain,
O. Farkas, J. Tomasi, V. Barone, M. Cossi, R. Cammi, B.
Mennucci, C. Pomelli, C. Adamo, S. Clifford, J. Ochterski, G. A.
Petersson, P. Y. Ayala, Q. Cui, K. Morokuma, D. K. Malick,
A. D. Rabuck, K. Raghavachari, J. B. Foresman, J. Cioslowski, J.
V. Ortiz, B. B. Stefanov, G. Liu, A. Liashenko, P. Piskorz, I.
Komaromi, R. Gomperts, R. L. Martin, D. J. Fox, T. Keith, M. A.
Al-Laham, C. Y. Peng, A. Nanayakkara, C. Gonzalez, M.
Challacombe, P. M. W. Gill, B. G. Johnson, W. Chen, M. W.
Wong, J. L. Andres, M. Head-Gordon, E. S. Replogle, J. A.
Pople, Gaussian, Inc., Pittsburgh, PA, 1998. Unless otherwise
2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
stated the geometries were optimized first at the B3LYP/321G(*) level of theory. At the optimized structures, second
derivatives were calculated to show whether a minimum
(positive eigenvalues only) had been obtained. Further optimization was performed at the B3LYP/6-31 + G* level of theory.
[18] To decrease the computational effort, both para-tert-butyl
groups of the 2,4,6-tri-tert-butylphenyl substituents were
replaced by hydrogen atoms. This simplification maintains the
flattening effect of the substituents on the tricoordinate phosphorus atoms.
[19] The Kohn–Sham orbitals shown in Figure 2 are essentially
identical to the canonical Hartree–Fock molecular orbitals.
2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2005, 44, 1405 –1408
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diyl, carbenium, ion, cyclic, protonation, phosphanyl, diphosphacyclobutane, bis
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