Facile rotation around a silicon-phosphorus double bond enabled through coordination to tungsten

Article abstract of DOI:10.1039/c5cc04247j

Unprecedented E/Z isomerisation of a SiP bond was observed by temperature dependent NMR spectroscopy. DFT calculations showed that the coordination of phosphasilene to tungsten lowered the rotational barrier from 19.1 to 14.2 kcal mol^-1. The th

Full text of DOI:10.1039/c5cc04247j

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Facile rotation around a silicon–phosphorus
double bond enabled through coordination to
Cite this:DOI: 10.1039/c5cc04247j
tungsten†
Nora C. Breit,a Tibor Szilvasib and Shigeyoshi Inoue*a
Received 22nd May 2015,
Accepted 9th June 2015
´
DOI: 10.1039/c5cc04247j
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Q
Unprecedented E/Z isomerisation of a Si P bond was observed by
temperature dependent NMR spectroscopy. DFT calculations showed
that the coordination of phosphasilene to tungsten lowered the rota-
tional barrier from 19.1 to 14.2 kcal molÀ1. The thermodynamically
more stable phosphinosilylene tungsten complex is formed at elevated
temperatures through substituent migration.
Chart 1 E/Z-Isomerisation of compounds containing a double bond.
substituents that can enhance the elemental polarization induced
by the different electronegativities of silicon and phosphorus.6 The
push–pull effect could also be used for decreasing the rotation
barrier of the SiQP double bond with the intention to observe
E/Z-isomerisation of phosphasilenes. In addition, phosphasilenes
feature an electron lone pair on phosphorus that can coordinate
to transition metals. This coordination behaviour on phosphorus
may lead to a lower rotation energy of the SiQP double bond.
However, surprisingly, little is known about its coordination to
transition metals.7
It is well known that a p bond is usually not free to rotate. For
instance, alkenes have very high activation energies (cis-2-butene =
62 kcal molÀ1) for rotation around the CQC bond, making them
inert against E/Z-isomerisation at room temperature.1 However, this
isomerisation energy can be decreased by introducing bulky
substituents (cf. stilbene = 43 kcal molÀ1, bifluorenylidene =
23 kcal molÀ1) or using the push–pull effect1c (cf. dimethyl[(dimethyl-
amino)methylene]malonate = 16 kcal molÀ1).
Their higher homologues, disilenes, display relatively low
rotation barriers of 26–28 kcal molÀ1 at 350 K for the SiQSi
bond, which enable E/Z-isomerisation.2 In the interesting case
of tetrakis(trialkylsilyl)-disilenes, rotation around the SiQSi
bond becomes possible at room temperature (activation barrier
15.3 kcal molÀ1 at 303 K), which is attributed to an effective s–p
conjugation in the transition state.3 It should be noted that
additional pathways for E/Z-isomerisation of disilenes through
dissociation–recombination reactions, disilene–silylsilylene inter-
conversions and through irradiation are also viable.4 Diphosphenes
with a PQP double bond can undergo photoisomerisation from
E- to Z-conformations.5 However, to the best of our knowledge no
E/Z-isomerisation was reported for phosphasilenes (Chart 1).
The SiQP bond can be strongly influenced by the introduction of
With this in mind, we utilized zwitterionic phosphasilene 16a
and engaged the electron lone pair on phosphorus in coordination
to tungsten (Scheme 1). By this we obtained an even weaker SiQP
bond in 2 and a low rotation barrier of only 14.2 kcal molÀ1 leading
to fast E/Z-isomerisation at room temperature.
The phosphasilene tungsten carbonyl complex 2 was synthesized
from 1 and W(CO)5Áthf in a very good yield of 92% (Scheme 1).
Compound 2 exists as two stable isomers labeled as (E)-2 and (Z)-2. It
should be noted that only (E)-2 is having the proper Si2–Si1–P1–W1
torsion angle of 175.131, while the isomer referred to as (Z)-2 actually
exhibits a Si2–Si1–P1–W1 torsion angle of 38.81. The rotational isomer
with a torsion angle of 01 is higher in energy (6.8 kcal molÀ1, see the
ESI,† Fig. S24). DFT calculations based on the B97-D/6-31G(d) level of
theory revealed that the rotation barrier for the silicon–phosphorus
a
¨
¨
Institut fur Chemie, Technische Universitat Berlin, Straße des 17. Juni 135,
Sekr. C2, D-10623 Berlin, Germany. E-mail: shigeyoshi.inoue@tu-berlin.de
b Department of Inorganic and Analytical Chemistry, Budapest University of
´
´
Technology and Economics, Szent Gellert ter 4, 1111 Budapest, Hungary
† Electronic supplementary information (ESI) available: Details of experimental
procedures and characterization methods, NMR spectra, IR spectra, the UV/Vis
spectrum, crystallographic and computational details, molecular orbitals and
mechanistic schemes. CCDC 1061059 (2) and 1061060 (3). For ESI and crystallo-
graphic data in CIF or other electronic format see DOI: 10.1039/c5cc04247j
Scheme 1 Synthesis and isomerism of complex 2.
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Fig. 2 Molecular structure of compound (E)-2. Thermal ellipsoids are drawn at
the 30% probability level. Hydrogen atoms are omitted for clarity. Selected bond
lengths (Å) and angles (1) in (E)-2: Si1–P1 2.158(2), Si1–Si2 2.369(2), P1–Si3
2.253(2), P1–W1 2.6265(18); Si1–P1–W1 118.82(8), Si3–P1–W1 115.55(8).
phosphorus to tungsten. The coordination to gold in the only
known phosphasilene complexes with a coordinative bond
showed little effect on the Si1–P1 bond lengths (2.0924(2) Å
and 2.0917(5) Å).7c The angles around phosphorus are nearly
1201 and the torsion angle Si2–Si1–P1–W1 amounts to 175.131
establishing the Z1-coordination of the SiQP bond to tungsten.
Fig. 1 The dynamic behavior of 2 observed by temperature dependent
NMR spectroscopy in toluene-d8.
double bond in 2 is 14.2 kcal molÀ1, which is feasible at room The P1–W1 bond length (2.6265(18) Å) is longer than in phospha-
temperature, but not at low temperature. Indeed, we observed by alkene tungsten complexes (2.51–2.54 Å)9 and closer to that of the
NMR spectroscopy that the signals belonging to one set at room (phosphoranylidenephosphine)penta-carbonyltungsten complex
temperature undergo coalescence and give rise to two sets of signals (2.603(1) Å).8b DFT calculations were utilized to understand the
at À60 1C (Fig. 1). The ratio between (E)-2 and (Z)-2 is 3 : 1 in favor of molecular structure of (Z)-2. Only minor differences were found
(E)-2, which was assigned based on lower thermodynamic stability of for the interatomic distances in (Z)-2 and (E)-2 (Si1–P1 2.159 Å
(Z)-2 (2.9 kcal molÀ1). The most prominent differences in the NMR and 2.152 Å and P1–W1 2.663 Å and 2.648 Å, respectively).
shifts of (E)-2 and (Z)-2 were observed for the phosphorus and the
UV/Vis spectroscopy revealed a red-shift from 1 to 2 (lmax =
silene silicon. The 31P{1H} NMR signals of 2 ((E)-2: d = À282.0 ppm, 333 nm and lmax = 354 nm, respectively), which was also observed
1JP–W = 123 Hz and (Z)-2: d = À304.3 ppm, 1JP–W = 128 Hz) are upfield for the phosphasilene gold complexes (8–10 nm).7c TD-DFT calcula-
shifted from that of 1 (d = À252.9 ppm).6a The low tungsten– tions based on the B3LYP/cc-pVTZ//B97-D/6-31G(d) level of theory
phosphorus coupling constants of 123 and 128 Hz are indicative of similarly predicted a red shift for (E)-2 (lmax = 383 nm, 3.24 eV,
the strong negative polarization on phosphorus and can be compared oscillator strength 0.050). The UV/Vis transition of (E)-2 occurs
to those of (phosphoranylidenephosphine)pentacarbonyltungsten mainly from the HOMO to the LUMO (weight 0.55), while the
complexes (102.5–109.9 Hz).8 The silene 29Si{1H} NMR signals of 2 LUMO+1 and LUMO+2 (weight 0.11 and weight 0.15, respectively)
((E)-2: d = 49.8 (1JSi–P = 106 Hz) ppm and (Z)-2: d = 45.9 (1JSi–P = 130 Hz) play a less significant part (see the ESI,† Fig. S25). The HOMO of
ppm) are downfield shifted from that of 1 (d = 40.5 (1JSi–P = 191.4 Hz) (E)-2 (À4.13 eV) is determined by the antibonding combination of
ppm).6a In addition the silene–phosphorus coupling constant is the p orbital of the phosphasilene and a filled d-orbital of tungsten.
significantly decreased in 2 compared to 1, which is all in agreement It is higher in energy than the HOMO of 1 (À5.94 eV) marked by
with the weakened and more polarized SiQP bond in 2. It is the silicon–phosphorus p-bonding orbital which can explain the
important to note that the NMR signals for the silene silicon and observed red-shift.6e,10 According to NBO analysis, the already very
phosphorus are broad at room temperature presumably due to the polarized double bond of 1 (silicon: +0.99 charge, 21.45% of the
rotation around this bond. Solid state 29Si{1H} and 31P{1H} NMR p-bond, phosphorus: À0.75 charge, 78.55%)6e gets even further
spectroscopy revealed that only (E)-2 is present in the solid state. polarized in 2 (silicon: +1.14 charge, phosphorus: À0.49 charge),
1
Variable temperature H and 31P{1H} NMR measurements of phos- to the extent of being dissected by NBO analysis into an empty
phasilene 1 did not reveal coalescence or two sets of signals at low orbital on silicon and a lone pair on phosphorus.11 This confirms
temperatures down to À60 1C, proving experimentally that a rotation the increasing importance of zwitterionic resonance structures, also
around the SiQP bond at r.t. is not applicable for 1.
supported by the amidinato ligand and its additional coordination
X-ray diffraction analysis of single crystals obtained from (Scheme 2), which explains the tendency of 2 for E/Z-isomerisation.
hexane elucidated the molecular structure of (E)-2 (Fig. 2). The Though compound 2 is stable at room temperature, at elevated
Si1–P1 bond length (2.158(2) Å) in (E)-2 is clearly longer than temperatures it undergoes a slow reaction to yield the phosphino-
that in 1 (2.095(3) Å),6a which confirms further the weakening silylene tungsten carbonyl complex 3 (Scheme 3).12 DFT calculations
of the silicon–phosphorus double bond by the coordination of showed that a stepwise mechanism in contrast to a concerted shift
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Scheme 2 Resonance structures of 2.
Scheme 3 Reactivity of 2 and independent synthesis of 3 from 4.
Fig. 3 Molecular structure of compound 3. Thermal ellipsoids are drawn
at the 30% probability level. Hydrogen atoms are omitted and only one of
the disordered fractions is depicted for clarity. Selected bond lengths (Å)
and angles (1) in 3, values in brackets refer to the other disordered fraction:
Si1–P1 2.249(9) {2.237(6)}, Si1–W1 2.562(3) {2.619(2)}, P1–Si2 2.228(13)
{2.260(10)}, P1–Si3 2.177(10) {2.375(9)}, W1–Si1–P1 122.0(3) {120.7(2)}.
of the TMS group and W(CO)5 moiety occurs for the transforma-
tion of 2 to 3 (see the ESI,† Fig. S26). The first step is the shift of
the TMS group, which is also rate determining (transition state:
19.7 kcal molÀ1) and thus responsible for the kinetic stability of 2 at
room temperature. The formation of 3 from 2 is thermodynamically
favoured by 10.1 kcal molÀ1 and can be explained by the stronger
donor strength of the silylene.13 In accordance with the calculated
values, no formation of 2 from 3 at elevated temperatures was
observed. This is contrasted by the equilibrium between 1 and
phosphinosilylene 4 (Scheme 3), which is determined by a smaller
thermodynamic difference (1.9 kcal molÀ1 in favour of 1) and a
higher activation barrier (32.1 kcal molÀ1).14
and the additional stabilization provided by the transition metal.
In fact, further reactivity studies are currently under way and
shall be reported in due course.
Notes and references
1 (a) E. L. Eliel and S. H. Wilen, Stereochemistry of Organic Compounds,
John Wiley & Sons, Ltd, New York, 1994, pp. 544–550; (b) I. Fleming,
Molecular Orbitals and Organic Chemical Reactions, John Wiley &
Sons, Ltd, 2010, p. 101; (c) Y. Shvo, E. C. Taylor and J. Bartulin,
Tetrahedron Lett., 1967, 3259.
2 Selected observations and calculations for thermodynamic E/Z-
isomerisation via rotation of SiQSi bonds: (a) M. J. Michalczyk,
R. West and J. Michl, Organometallics, 1985, 4, 826; (b) S. A. Batcheller,
T. Tsumuraya, O. Tempkin, W. M. Davis and S. Masamune, J. Am.
Chem. Soc., 1990, 112, 9394.
To date, merely four stable phosphinosilylenes have been
reported15 and their reactivity studies were limited to N2O, tBuCOCl
and Ni(COD)2.6d,14,16 Complex 3 was synthesized independently from
the parent phosphinosilylene 4 in a moderate to low yield of 30%
(Scheme 3).17 The silylene tungsten complex 3 was thus fully
characterized using NMR and IR spectroscopy, mass spectrometry,
elemental analysis and single crystal X-ray diffraction. As expected,
the 29Si{1H} NMR signal of the silylene–silicon in 3 is downfield
shifted from that of 4 and the 1JSi–P coupling constant has consider-
3 M. Kira, S. Ohya, T. Iwamoto, M. Ichinohe and C. Kabuto, Organo-
metallics, 2000, 19, 1817.
4 First example of thermal dissociation of a disilene into a silylene:
(a) N. Tokitoh, H. Suzuki and R. Okazaki, J. Am. Chem. Soc., 1993,
115, 10428. Example for a dyotropic rearrangement rather than a
disilene–silylsilylene interconversion: (b) H. B. Yokelson, J. Maxka,
D. A. Siegel and R. West, J. Am. Chem. Soc., 1986, 108, 4239. For a
review see: (c) M. Kira, Proc. Jpn. Acad., Ser. B, 2012, 88, 167.
5 Selected examples: (a) A.-M. Caminade, M. Verrier, C. Ades, N. Paillous
and M. Koenig, J. Chem. Soc., Chem. Commun., 1984, 875; (b) M. Yoshifuji,
T. Hashida, N. Inamoto, K. Hirotsu, T. Horiuchi, T. Higuchi, K. Ito and
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O. Altmeyer and M. Nieger, Angew. Chem., Int. Ed. Engl., 1991, 30, 1136.
1
ably decreased (d = 70.7 ppm, JSi–P = 134 Hz and d = 44.0 ppm,
1JSi–P = 194 Hz, respectively).6a The 31P{1H} NMR signal of 3 is
slightly downfield shifted from that of 4 (d = À199.4 ppm and d =
À211.0 ppm, respectively).6a The presence of the carbonyl group is
evident from 13C{1H} NMR spectroscopy (d = 202.0 ppm) and IR
spectroscopy (nCO = 2054, 1918, 1905 cmÀ1). The molecular structure
of compound 3 is shown in Fig. 3 and it revealed that the silicon–
phosphorus bond length in 3 (2.249(9) Å and 2.237(6) Å) is shorter
than that of 4 (2.2838(12) Å).6a The silicon–tungsten bond length in
3 (2.562(3) Å and 2.619(2) Å) is longer than those of (PhC{NtBu}2)-
Si(W{CO}5)Cl and (PhC{NtBu}2)Si(W{CO}5)F (2.5086(11) Å and
2.4990(8) Å, respectively)18 and similar to that of (PhC{NiPr}2)2-
Si(W{CO}5) (2.5803(9) Å).19
¨
6 Selected zwitterionic phosphasilenes: (a) S. Inoue, W. Wang, C. Prasang,
M. Asay, E. Irran and M. Driess, J. Am. Chem. Soc., 2011, 133, 2868;
(b) S. S. Sen, S. Khan, H. W. Roesky, D. Kratzert, K. Meindl, J. Henn,
D. Stalke, J.-P. Demers and A. Lange, Angew. Chem., Int. Ed., 2011,
´
50, 2322; (c) K. Hansen, T. Szilvasi, B. Blom, S. Inoue, J. Epping and
´
M. Driess, J. Am. Chem. Soc., 2013, 135, 11795; (d) K. Hansen, T. Szilvasi,
B. Blom, E. Irran and M. Driess, Chem. – Eur. J., 2014, 20, 1947;
´
(e) N. C. Breit, T. Szilvasi and S. Inoue, Chem. – Eur. J., 2014, 20, 9312.
7 One phosphasilene iron complex was observed using 29Si{1H} and 31P{1H}
NMR spectroscopy and mass spectrometry. (a) M. Driess, H. Pritzkow and
U. Winkler, J. Organomet. Chem., 1997, 529, 313. One phosphasilene zinc
complex with a mainly covalent bond that remains in (E)-configuration at
variable temperatures was reported. (b) M. Driess, S. Block, M. Brym and
M. T. Gamer, Angew. Chem., Int. Ed., 2006, 45, 2293. Few phosphasilene
gold complexes with a coordination of the lone pair on phosphorus were
described. (c) B. Li, T. Matsuo, T. Fukunaga, D. Hashizume, H. Fueno,
K. Tanaka and K. Tamao, Organometallics, 2011, 30, 3453.
In conclusion, the coordination of zwitterionic phosphasilene 1
to a tungsten carbonyl complex leads to a more polarized and weaker
silicon–phosphorus bond in 2. Interestingly, it also lowers the
rotation barrier of the SiQP bond from 19.1 kcal molÀ1 in 1 to
14.2 kcal molÀ1 in 2 to allow facile E/Z-isomerisation at room
temperature. In the presence of the transition metal, the phosphino-
silylene complex 3 is thermodynamically preferred to 2. Both
species, 2 and 3, are versatile building blocks for low-valent
silicon compounds due to their labile trimethylsilyl groups6a,e,14
8 (a) F. Mercier, F. Mathey, C. Afiong-Akpan and J. F. Nixon,
J. Organomet. Chem., 1988, 348, 361; (b) P. Le Floch, A. Marinetti,
L. Ricard and F. Mathey, J. Am. Chem. Soc., 1990, 112, 2407.
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9 L. Weber, M. Meyer, H.-G. Stammler and B. Neumann, Chem. – Eur.
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10 For a more detailed description of the MOs of (E)-2 and 1, please see
the ESI†.
11 It is important to note that the increased positive charge on silicon
represents the stronger polarization, since only silicon is in the
same environment in 1 and 2, while the charge on phosphorus is
significantly influenced by its coordination to tungsten.
H. W. Roesky, P. M. Gurubasavaraj, R. B. Oswald, M. T.
Gamer, P. G. Jones and S. Blaurock, J. Am. Chem. Soc., 2007,
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¨
12 A flame sealed NMR tube containing a solution of 2 in C6D6 was 16 R. Azhakar, K. Propper, B. Dittrich and H. W. Roesky, Organometallics,
heated to 70 1C, after a few hours the first signs of the formation of 3 2012, 31, 7586.
were observed, but complete consumption of 2 took 7 weeks at 17 3 is the main product in this reaction, but either W(CO)5Áthf or 3 is
70 1C.
13 Z. Benedek and T. Szilvasi, RSC Adv., 2015, 5, 5077.
catalyzing the formation of 1 from 4, so that 2 and sometimes 1 are
observed as side products.
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´
14 N. C. Breit, T. Szilvasi, T. Suzuki, D. Gallego and S. Inoue, J. Am. 18 R. Azhakar, R. S. Ghadwal, H. W. Roesky, H. Wolf and D. Stalke,
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G. Mu¨ller, Angew. Chem., Int. Ed. Engl., 1990, 29, 295; (b) C.-W. So,
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