The Direct Synthesis of a Silene-Organometallic Complex

Article abstract of DOI:10.1002/anie.200353048

σ or π? The direct synthesis and the molecular and electronic structures of a novel η²-silene platinum complex, 1, is reported (see scheme). The nature of bonding in 1 was studied by using geometrical and electronic structure criteria, and the results suggested that 1 is best described as an hybrid between a π complex and a σ-cyclopropane structure with a more dominant π-complex character than in a related disilene complex.

Full text of DOI:10.1002/anie.200353048

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made in this field,[1] including the synthesis of several
transition-metal complexes of silenes and disilenes such as
compounds 1–4.[2–6] However, while numerous olefin–orga-
nometallic complexes were synthesized directly from the
corresponding olefins, all known silene complexes were
synthesized by indirect processes.For example, 1,[3] 2a,[4]
2b,[5] 3,[6] 4a and 4b[7a,b] were all obtained by cyclization
reactions.Only 4c was synthesized by a direct reaction of a
disilene with an organometallic reagent, but its molecular
structure was not determined by X-ray crystallography
(Mes = C6H2Me3).[7b]
We report herein the synthesis and the molecular and
electronic structure of a novel platinum h2-silene complex, 5,
the first silene complex synthesized by a direct reaction of a
silene (6) and an organometallic reagent Pt(PCy3)2.
Silene–Organometallic Complexes
The Direct Synthesis of a Silene–Organometallic
Complex**
Dmitry Bravo-Zhivotovskii,* Hanan Peleg-Vasserman,
Monica Kosa, Gregory Molev, Mark Botoshanskii, and
Yitzhak Apeloig*
Dedicated to Professor Henry F. Schaefer III
on the occasion of his 60th birthday
Our initial attempts to obtain a silene–platinum complex
by a reaction of Pt(PCy3)2[8] and the stable silenes 7[9] and 8[10]
failed, thus leading to a complex mixture of products.
Molecular models reveal that the double bond in 7 and 8 is
so effectively protected by the very bulky R1 and R2
substituents that the desired complex cannot be formed.A
less heavily substituted silene was required and we chose 6,
which is obtained as a short lived intermediate.[11] Reaction of
(Me3Si)3SiLi·3THF with 2-adamantanone in toluene at
À708C yields the isolable addition adduct 9.[12] On warming
of the reaction mixture (or warming of 9 directly) to À308C
the transient silene 6 was formed and its formation was
monitored by 29Si NMR (the NMR chemical shifts of 6 are
very similar to those of the stable 7[9]).[12] The warming of this
reaction mixture to room temperature yielded dimer 10, thus
supporting the presence of silene 6.Warming of the toluene
reaction mixture from À308C to room tempretature in the
presence of Pt(PCy3)2 led to the formation of 5 (31%).A
The first stable compounds with doubly bonded silicon atoms
were reported in 1981 and since then vast progress has been
[*] D. Bravo-Zhivotovskii, H. Peleg-Vasserman, M. Kosa, G. Molev,
M. Botoshanskii, Y. Apeloig
Department of Chemistry and
the Lise Meitner-Minerva Center for Computational Quantum
Chemistry
Technion-Israel Institute of Technology
Haifa 32000 (Israel)
Fax: (+972)4823-3735
E-mail: chrbrzh@tx.technion.ac.il
chrapel@tx.technion.ac.il
[**] This research was supported by the Israel Science Foundation
administrated by the Israel Academy of Sciences and Humanities.
We are grateful to Dr. Miriam Karni and Dr. Thomas Müller for
helpful discussions. D.B.-Z. is grateful to the Ministry of Immigrant
Absorption, State of Israel, for a Kamea scholarship.
Angew. Chem. Int. Ed. 2004, 43, 745 –745
DOI: 10.1002/anie.200353048
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745

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higher yield (40%) of the desired silene–platinum complex 5
feature of the structure of 5 is the near colinearity of the C25-
Pt1-P1 atoms (bond angle 177.78).
(together with 10, 37%) was obtained when Pt(PCy3)2 was
added to a toluene solution of 9 at À708C and the solution
was warmed to room temperature [Eq.(1)]. These experi-
The NMR spectrum is consistent with the molecular
structure of 5.The NMR signals of 29Si1 (d = 6.8 ppm) and of
13C25 (d = 137.8 ppm) of 5 are shifted upfield, as expected,
compared to silenes (e.g., d(29Si) = 51.7 ppm, d(13C) =
196.8 ppm in 7).The 31P signal in 5 of d = 21.5 ppm is similar
to that in Pt(PMe3)3.
Is 5 better described as a p-complex (A) or as a s-bonded
cyclopropane-type compound (B)?
ments support strongly the conclusion that 5 is obtained by a
direct reaction between the transient silene 6 and Pt(PCy3)2 as
shown in Equation (1).Compound 5 was obtained as pale
pink crystals and its molecular structure was determined by
X-ray crystallography.[13]
One criterion to answer this question is the structure of 5,
À
most importantly the Si C bond length and the degree of
bending around the complexed Si C bond.[14] Structural data
=
for comparison with model compounds is given in Scheme 1.
The molecular structure of 5 (Figure 1) reveals
the surprising presence of a tricoordinated Pt
atom.Thus, the reaction of silene 6 with Pt(PCy3)2
involves a ligand substitution reaction at the
dicoordinated Pt center at which one of the PCy3
ligands is displaced to allow coordination with the
sterically bulky silene.To our best knowledge this
ligand exchange reaction is unprecedented in
dicoordinated Pt compounds.Another unusual
À
Scheme 1. A comparison of Si C bond lengths (Š) in several organosilicon com-
pounds.
=
The Si C bond of silene 7 (a close model for 6) is elongated
(Dl) by 5.6%, from 1.741 Š in 7 to 1.838 Š in 5.This bond
elongation is similar to that found in 1, 2a, and 3, compared to
=
the corresponding Si E bonds (Dl = + 5.8%, + 6.4% and
À
+ 5.4%, respectively). However, the l(Si C) bond in 5 is
significantly shorter than in related singly bonded com-
pounds; e.g., 1.928 Š in 11a and 11b; [10b] thus Dl in 11a and
11b is almost twice as large as in 5 (Scheme 1).The closest
available comparison is between 5 and 12,[15] as both have a
À
three-membered ring; l(Si C) in 5 is 0.08 Š shorter than in
12.[16] The bending angles in 5 at Si1 and C25 of 160.48 and
154.28, respectively, are larger than those of analogous angles
in 3 (150.78[6]).The sum of the angles around Si1 and C25 are
353.88 and 353.18 respectively.In conclusion, the geometrical
parameters indicate that 5 is best described as a hybrid
between p complex (A) and s-bonded compound (B),as 5 has
a higher p-complex character than the disilene complex 3.[6]
The difference Fourier (DF) electronic map of 5 was
studied experimentally at À708C and a cut through the main
Figure 1. ORTEP diagram of the h2-silene–platinum complex 5. Hydro-
gen atoms are omitted for clarity. Selected bond lengths [Š] and angles
[8]: Si1-C25 1.838(12), Pt-Si1 2.298(3), Pt-C25 2.161(11), Pt-P 2.268(3),
Si1-Si2 2.344(5), P-C7 1.861(11), C25-Pt-Si1 48.6(3), Pt-Si1-C25
61.8(3), Pt-C25-Si1 69.6(4), P-Pt-C25 177.7(3), P-Pt-Si 131.24(11), Pt-
Si1-Si2 116.21(15), C25-Si1-Si2 119.5(4), C25-Si1-Si3 122.1(4), Si2-Si1-
Si3 112.22(18), P-Pt-Si1-C25 177.0(4), C25-Pt-Si1-Si2 111.0(4), P-Pt-Si1-
Si2 72.0(2).
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molecular plane is shown in Figure 2.Three main regions
of accumulation of nonnuclear electron density are
observed: (a) between Pt and the Si1 and C25 atoms,
with significant density overlap at the center of the PtSiC
À
ring and being polarized towards the Pt C internuclear
region (b) between the Si1 and C25 atoms—a maximum
which is shifted outwards of the three-membered ring and
À
towards the C atom, and (c) along the Pt P bond,
À
corresponding to a Pt P s bond.
Natural bond orbital (NBO)[17] calculations[18–20] for the
model complex 13 reveal that on complexation 0.15
electrons are transferred from the organometallic frag-
ment to the silene.In the complex, the p orbital of the
=
silene ((H3Si)2Si C(CH3)2), is depopulated to 1.64 elec-
currently studying the chemistry of 5 and using similar
strategies to prepare other silene and disilene organometallic
complexes.
Experimental Section
Standard Schlenk techniques were used for the syntheses and sample
manipulations.
A toluene solution of (Me3Si)3SiLi·3THF (0.123 g, 0.26 mmol;
10 mL) was added to a toluene solution of 2-adamantanone (0.038 g,
0.25 mmol; 10 mL), which was kept at À708C under vacuum.After
the reaction mixture had been stirred for 30 min, a toluene solution of
[8]
Pt(PCy3)2 (0.2 g, 0.26 mmol; 20 mL) was added and the mixture
allowed to reach room temperature and maintained at this temper-
ature overnight.The solvent was removed by evaporation and the
crude product was crystallized from hexane (0.083 g, 0.1 mmol; 40%
yield), of pale pink crystals suitable for X-ray crystallization of 5 were
obtained. 1H NMR (C6D6): d = 0.174 (18H s, SiMe3), 1.2–2.4 ppm
(14H m, Ad, 33H m, PCy3); 13C NMR (C6D6): d = 137.8 (1C
PtC(Ad)Si), 4.1 (6C SiMe3), 44.1, 43.0, 41.0, 35.9, 34.0, 32.7, 32.1,
31.9, 31.5, 29.1, 28.0, 26.9, 25.6 ppm (9C Ad, 18C PCy3); 29Si NMR
(C6D6): d = 6.8 (1Si PtSiAd), À14.67 ppm (2Si SiMe3); 31P NMR
(C6D6): d = 21.5 (1P t 1JPt,P = 1475 Hz, PCy3); MS(CI) m/z 756.3
(MÀ2Me).
Figure 2. Difference Fourier electronic mapof 5 at À708C, showing a
cut through the main molecular plane.
=
trons while the p*(C Si) orbital is populated by 0.42
À
electrons.The s* (Pt P) antibonding orbital, which is located
mainly on Pt, is populated with 0.31 electrons. According to
the calculated second-order perturbation interaction ener-
gies[18b] the strongest orbital interaction in complex 13 (and
Received: October 9, 2003 [Z53048]
Published Online: January 8, 2004
=
thus in 5) is interaction 14a, between the polarized p(Si C)
Keywords: bond theory · platinum · Si ligands · transition metals
.
À
orbital and the s*(Pt P) orbital.This interaction is also
evident in the observed (Si C), (Pt Si) and (Pt C) electron-
density peaks.The strong polarization of p(Si C) towards C
and the good acceptor ability of phosphorous dictates the
near linearity of the P-Pt-C angle.The energy difference
À
À
À
=
[1] T.Müller, W.Ziche, N.Auner, The Chemistry of Organic Silicon
Compounds, Vol. 2 (Eds.: Z. Rappoport, Y. Apeloig), Wiley,
Chichester, 1998, chap.16.
[2] T.D. Tilley, The Silicon-Heteroatom Bond (Eds.: S. Patai, Z.
Rappoport) 1991, chap.10, p.330.
À À
between optimized 13 (a P-Pt-Si = 129.68, P Pt C = 1808)
and its constrained isomer, in which a P-Pt-Si = 1808 is
15.9 kcalmolÀ1.The second strongest interaction is 14b
=
between the filled Pt(dxy) orbital and the empty p*(Si C)
[3] a) B.K.Campion, R.H.Heyn, T.D.Tilley,
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J.
orbital, which is polarized towards the Si atom.In general, the
NBO calculations produce the classical Dewar–Chatt–Dun-
canson model.[21]
In summary, the silene—platinum complex 5 is best
described as an hybrid between a p complex (A) and a s-
cyclopropane structure (B), which has a more pronounced p-
complex character than the disilene complex 3.We are
[4] T.S.Koloski, P.J.Carroll, D.H.Berry,
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Communications
À
À
[7] a) E.K.Pham, R.West,
J. Am. Chem. Soc. 1989, 111, 7667 –
shorter than l(Ru C) and l(W C) in 1 and 2a of 2.250 Š and
À
7668; b) E.K. Pham, R. West, Organometallics 1990, 9, 1517.
[8] R.Yoshida, S.Otsuka, Inorg. Synth. 1979, 19, 105.
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Zhivotovskii, D.Bläser, R.Boese, J. Am. Chem. Soc. 1996, 118,
12228 – 12229.
2.329 Š, respectively. l(Pt Si) in 5 of 2.298 Š is at the shorter end
of reported l(Pt Si) distances (2.255–2.444 Š[16b]), but it is
À
significantly shorter than in 3 (24. 3 Š).a) G.Chandra, P.Y.Lo,
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[10] a) M.Bendikov, MSc thesis, Technion-Israel Institute of Tech-
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[17] A.E. Reed, L.A. Curtiss, F. Weinhold,
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[11] D.Bravo-Zhivotovskii, V.Braude, A.Stanger, M.Kapon, Y.
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[12] D.Bravo-Zhivotovskii, G.Korogodsky, Y.Apeloig, J. Organo-
met. Chem. 2003, 686, 58.
[18] The fully optimized geometry of 13 is similar to that of 5. All
calculations were performed with the Gaussian 98 program[18a]
with implementation of NBO version 5.0;[18b] a) Gaussian98
(RevisionA.11.3), 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,
CY. .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, 2002.; b) NBO 5.0. E. D. Glendening, J. K.
Badenhoop, A.E.Reed, J.E.Carpenter, J.A.Bohmann, C.M.
Morales, F.Weinhold, Theoretical Chemistry Institute, Univer-
sity of Wisconsin, Madison, WI, 2001; http://www.chem.wis-
c.edu/ ~ nbo5.
[13] X-ray structure data: Nonius Kappa CCD diffractometer; phi-
scans; MoKa radiation (l = 0.7107); T= 220 K; semiempirical
absorption correction; the structure were solved by direct
methods with SHELXS-97 and refined against F2 with
SHELXL-97.The hydrogen atoms were generated with ideal-
ized geometry.Crystal structure data: Pt1 P1 Si3 C34 H65; Mr =
784.18; crystal size 0.14 ” 0.125 ” 0.038 mmmonoclinic space
group P21/n, a = 13.455(3) Š, b = 19.568(4) Š, c = 14.818(3) Š,
b = 106.15(3), V= 3747(1) Š3, Z = 4, 1calcd = 1.39, m(MoKa) =
3.904 mmÀ1, F(000) = 1616.0, 2qmax = 50.04, 24189 reflections
measured, 6587 unique reflections (Rint = 0.098), final R1 =
0.0623 for 3782 reflections [I > 2s(I)], Rw = 0.1142(all data),
residual maximum electron density 1.714 eÀ Š3.CCDC-220373
contains the supplementary crystallographic data for this paper.
These data can be obtained free of charge via www.ccdc.cam.a-
c.uk/conts/retrieving.html (or from the Cambridge Crystallo-
graphic Data Centre, 12 Union Road, Cambridge CB21EZ, UK;
fax: (+ 44)1223-336-033; or deposit@ccdc.cam.ac.uk).
[19] All calculation used the B3LYP/6-31G(d) level of theory for all
atoms, except for Pt for which B3LYP/SDD was used.
[20] For previous theoretical studies on Pt–silene complexes see:
a) T.R. Cundari, M.S. Gordon, THEOCHEM 1994, 47; b) S.
Sakaki, M.Ieki, Inorg. Chem. 1991, 30, 4218.
[14] J.Uddin, S.Dapprich, G.Frenking, B.F.Yates,
Organometallics
1999, 18, 457 – 465.
[15] M.J.M. Vlaar, A.W. Ehlers, F.J.J. de Kanter, M. Schakler,
AL. .Spek, M.Lutz, N.Sigal, Y.Apeloig, K.Lammertsma,
Angew. Chem. 2000, 112, 4296 – 4299; Angew. Chem. Int. Ed.
2000, 39, 4127 – 4129.
[21] a) M.J.S.Dewar, Bull. Soc. Chim. Fr. 1951, 18, C79; b) J.Chatt,
L.A.Duncanson, J. Chem. Soc. 1953, 2929.
À
[16] l(Pt C) in 5 of 2.161 Š is within the range found in a bisolefin
platinum monophosphane complex (2.154–2.185 Š),[16a] but it is
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