Activation of a Si=Si bond by η1-coordination to a transition metal

Article abstract of DOI:10.1021/ja052593p

Reaction of the disilenide 1 with Cp₂ZrCl₂ yields 2, the first transition metal complex with an η¹-σ-bonded disilene moiety. The compound 2 exhibits a strongly red-shifted UV-vis absorption, resulting in an intensely green color. The Si=Si bond in 2 is considerably activated as shown by the rearrangement to a silyl-substituted zirconocene 3. X-ray diffraction studies on 2 reveal that the Si=Si bond is significantly longer than that in the precursor 1. Conversely, the Zr-Si distance is considerably smaller than the corresponding distance in 3, suggesting a certain degree of electron donation toward the 16e^- zirconium center. Copyright

Full text of DOI:10.1021/ja052593p

Published on Web 07/01/2005
Activation of a SidSi Bond by η1-Coordination to a Transition Metal
Thi-loan Nguyen and David Scheschkewitz*
Institut fu¨r Anorganische Chemie, UniVersita¨t Wu¨rzburg, Am Hubland, D-97074 Wu¨rzburg, Germany
Received April 21, 2005; E-mail: scheschkewitz@mail.uni-wuerzburg.de
Since the ground breaking discovery of a stable disilene by West,
Michl, and Fink in 19811, low-valent silicon compounds have re-
ceived considerable attention.2 In the meantime, numerous stable
disilenes have been prepared, taking advantage of either kinetic
stabilization by bulky substituents or electronic stabilization within
the coordination sphere of transition metals. To date, examples of
η2-disilene complexes cover a wide range of transition metals from
groups 4, 6, 8, and 10.3 Conversely, no η1-disilenide complexes,
3rd row analogues of the extensively studied η1-vinyl complexes,4
have yet been reported. The disilenides recently obtained by our-
selves and others should be suitable precursors for such com-
pounds.5
Here, we report the reaction of the lithium salt of disilenide 15a
with Cp2ZrCl2, yielding the η1-disilenide zirconium complex 2
almost quantitatively. Compared to that of the anionic precursor,
the SidSi bond of 2 appears to be far more reactive, as shown by
its facile isomerization to the silyl complex 3 (Scheme 1).6
Scheme 1
Figure 1. Structure of 2 in the solid state. H atoms and disordered hexanes
are omitted for clarity. Ellipsoids at 50%. Selected bond lengths (Å): Si1-
Si2 2.2144(7), Si1-Zr1 2.7611(6), Zr1-Cl1 2.4512(6), Si1-C1 1.925(2),
Si2-C16 1.907(2), Si2-C31 1.884(2). Selected bond angles (°): Cl1-
Zr1-Si1 99.912(18), Si2-Si1-Zr1 132.07(3), Zr1-Si1-C1 119.91(6),
C1-Si1-Si2 107.70(6), Si1-Si2-C16 134.45(7), Si1-Si2-C31 116.64(7),
C16-Si2-C31 108.42(9).
ligand. For instance, the Zr-Si distance in Cp2Zr(SiPh3)Cl (4) was
determined to 2.813(2) Å.10 Nonetheless, the Zr1-Cl1 bond in 2
(2.4512(6) Å) is somewhat longer than that in 4 (2.430(3) Å). The
Si1dSi2 bond of 2 is considerably twisted, as indicated by the angle
of 20.24° between the planes defined by Zr1, Si1, C1 and C31,
Si2, C16, respectively. Both silicon atoms are, however, only
slightly pyramidalized (sum of angles: Si1 359.68, Si2 359.51). A
strong twisting of a SidSi bond was also observed in the blue
disilene [(tBu2MeSi)2Si]2 and its diradical nature discussed.11
To our knowledge, the facile rearrangement of 2 at room
temperature to the zirconocene silyl complex 3 is without precedent
in disilene chemistry.12 On the other hand, the insertion of carbene
analogues into C-H bonds is well-known, for example, for the
germylene GeAr2 (Ar ) 2,4,6-tri-tert-butylphenyl).13 The actual
mechanism of the isomerization of green 2 to red 3, which formally
requires the 1,2-addition of a methyl C-H bond to the SidSi bond,
remains obscure. While an excess of 1 in the synthesis of 2 sped
up the latter’s rearrangement considerably (complete conversion
after 1 h), a slight surplus of Cp2ZrCl2 stabilized 2 in benzene
solution for at least 24 h at room temperature. Therefore, either a
base-catalyzed mechanism or a radical pathway initiated by electron
transfer from excess disilenide 1 can be envisaged.
1
Compound 2 was characterized by means of H, 13C, and 29Si
NMR, and UV-vis spectroscopy. It shows an absorption band at
λmax ) 715 nm, which is even more bathochromically shifted than
the highest wavelength signal of the recently reported disilyne
featuring a SitSi bond (λmax ) 690 nm).7 Given its high extinction
coefficient (ꢀ = 7000 L‚mol-1‚cm-1), the unusual red-shift of the
absorption of 2 could be due to a ligand-to-metal π f d charge-
transfer transition, as depicted by the resonance structures in Scheme
1.
A contribution by the upper resonance form might also explain
the low-field 29Si NMR resonances at 116.8 and 152.5 ppm.
Compared to the signals of its precursor 1,5a the latter is shifted
more than 50 ppm to lower field. It is unambiguously assigned to
the Si atom bonded to zirconium on the basis of 2D-29Si-1H
experiments. On these grounds, a certain silylene character of the
disilenide ligand in 2 can be considered. The 29Si NMR shifts of
cationic tungsten silylene complexes, for instance, have been
observed between 277 and 314 ppm.8
Further indications that charge transfer from the disilenide ligand
to Zr might contribute to satisfy the electron demand of the formal
16e- center are provided by X-ray crystallography (Figure 1).9
While the Si1-Si2 bond in 2 is significantly longer than that in
the precursor (2, 2.2144(7); 1, 2.1920(6) Å), the Zr1-Si1 distance
of 2.7611(6) Å is relatively short considering the bulkiness of the
As expected, 3 is formed as a diastereomeric mixture. However,
presumably due to its favorable arrangement of the bulky substit-
uents, the l,l-diastereomer14 predominates and was crystallized from
hexane in 57% yield.6
9
10174
J. AM. CHEM. SOC. 2005, 127, 10174-10175
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Figure 2. Structure of l,l-3 in the solid state. H atoms, except for H2, are
omitted for clarity. Ellipsoids at 50%. Selected bond lengths (Å): Si1-Si2
2.3827(6), Si2-H2 1.327(19), Si1-Zr1 2.8214(4), Si1-C23 1.9161(15),
Zr1-Cl1 2.4181(5), Si1-C1 1.9527(15), Si2-C16 1.8937(16), Si2-C31
1.9189(15). Selected bond angles (°): Cl1-Zr1-Si1 101.461(17),
Si2-Si1-Zr1 119.194(18), Zr1-Si1-C1 118.32(5), C1-Si1-Si2 109.71-
(5), C1-Si1-C23 103.45(7), Si2-Si1-C23 89.11(5), Zr1-Si1-C23
112.12(5).
199.
(5) (a) Scheschkewitz, D. Angew. Chem., Int. Ed. 2004, 43, 2965. (b) Ichinohe,
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(6) For experimental details, see Supporting Information. Compound 2: air-
sensitive green solid; 1H NMR (C6D6, 500 MHz) δ 7.11, 7.07, 6.97 (s,
2H each, ArH), 5.91 (s, 10H, CpH), 4.23, 4.10, 3.55, 2.79, 2.73, 2.64
(hept., 9H, iPrCH), 1.46, 1.39, 1.33, 1.20, 1.14, 1.08 (d, 54H, iPrCH3);
13C NMR (C6D6, 125 MHz) δ 155.14, 154.89, 154.73, 151.46, 150.42,
148.96, 142.24, 140.34, 138.14 (ArC), 122.64, 122.28, 122.02 (ArCH),
111.08 (CpC), 38.30, 37.23, 36.17, 34.74, 34.68, 34.33 (iPrCH), 26.43,
24.86, 24.13, 23.86 (iPrCH3); 29Si NMR (C6D6, 99 MHz) δ 152.5 (ZrSi),
116.8 (SiAr2); UV-vis (hexane) λmax/nm (ꢀ) 416 (3000), 715 (7000).
Compound 3: air-sensitive red solid; mp 154 °C dec; 1H NMR (C6D6,
500 MHz) δ 7.20, 7.11, 7.05, 6.93, 6.91 (s, 6H, ArH), 5.914, 5.910 (s,
10H, CpH), 5.74 (d, 1H, 4J ) 2.44 Hz, SiH), 3.83, 3.66, 3.60, 3.35, 2.78,
2.73, 2.62 (hept., 9H, iPrCH), 2.31 (pseudo-dt, 1H, 2J ) 15.37 Hz, SiCH2),
The 29Si NMR shifts of l,l-3 at δ ) -46.1 and -8.4 ppm are
indicative of the absence of a SidSi bond. The formal addition of
a methyl C-H bond of one of the o-isopropyl moieties was verified
by 2D-29Si-1H experiments. The UV-vis absorption of l,l-3 at
490 nm is in accordance with the deep-red color usually observed
for zirconocene(chloro)silylcomplexes.15
X-ray crystallography proved the stereochemistry of l,l-3 (Figure
2).9 The achiral point group P1h implies that both enantiomers are
present as symmetry equivalents in the lattice. While Si1-Zr1 is
significantly longer than that in 2, Zr1-Cl1 is noticeably shorter.
These changes could be attributed to a significantly weaker
interaction between Zr1 and the σ-bonded silyl ligand in 3.
However, it has been shown that the Zr-Si and Zr-Cl distances
in complexes such as 3 can depend on the steric congestion of the
silyl group.15
In conclusion, we have shown that the SidSi bond in the first
η1-disilenide transition metal complex 2 is considerably activated
by the σ-coordination to zirconium. The enhanced reactivity leads
to an unprecedented addition to a methyl C-H bond of the
substituents, affording the silyl zirconocene complex 3. Investiga-
tions of the reactivity of 2 toward nucleo- and electrophiles are
currently underway.
2
2.18 (dd, dt, 1H, J ) 15.49 Hz, SiCH2), 1.64, 1.59, 1.51, 1.48 (d, 15H,
iPrCH3), 1.44, 1.36 (br, 9H, iPrCH3), 1.25, 1.20, 1.19, 1.17, 1.10, 1.09,
0.90, 0.81 (d, 27H, iPrCH3); 13C NMR (C6D6, 125 MHz) δ 156.63, 156.30,
155.45, 155.23, 154.60, 152.16, 150.39, 148.85, 148.24, 137.32, 134.85,
130.35 (ArC), 126.83, 122.94, 122.20, 121.64, 121.48, 120.67 (ArCH),
111.62, 111.48 (CpC), 38.88, 36.00, 35.10, 34.67, 34.57, 34.31, 34.12,
33.45, 33.06 (iPrCH), 31.06 (iPrCH3), 30.67 (SiCH2), 28.64, 27.39, 27.27,
26.36, 26.05, 25.48, 25.40, 24.84, 24.54, 24.30, 24.13, 24.05, 24.03, 24.00,
23.91 (iPrCH3); 29Si NMR (C6D6, 99 MHz) δ -8.4 (SiCH2), -46.1 (SiH);
UV-vis (hexane) λmax/nm (ꢀ) 490 (1400). Anal. Calcd for C55H79ClSi2-
Zr: C, 71.56; H, 8.63. Found: C, 71.42; H, 8.26.
(7) Sekiguchi, A.; Kinjo, R.; Ichinohe, M. Science 2004, 305, 1755.
(8) Mork, B. V.; Tilley, D. T. J. Am. Chem. Soc. 2004, 126, 4375.
(9) Crystal data for 2 (173 K):
C55H79ClSi2Zr‚(1.5 C6H14); FW 1009.20;
triclinic; P1h (No. 2); a ) 10.5012(10) Å; b ) 13.4970(13) Å; c )
22.406(2) Å; R ) 74.515(2)°; â ) 81.705(2)°; γ ) 87.295(2)°; V )
3028.4(5) Å3; Dcalc ) 1.107 g/cm3; Z ) 2; R1 ) 0.0404 (I > 2σ(I)); wR2
) 0.1176 (all data); S ) 1.058. Crystal data for 3 (173 K): C55H79ClSi2-
Zr; FW 923.03; triclinic; P1h (No. 2); a ) 10.7783(7) Å; b ) 13.5790(9)
Å; c ) 19.7407(13) Å; R ) 87.6510(10)°; â ) 78.4380(10)°; γ ) 71.8970-
(10)°; V ) 2689.8(3) Å3; Dcalc ) 1.140 g/cm3; Z ) 2; R1 ) 0.0361 (I >
2σ(I)); wR2 ) 0.0988 (all data); S ) 1.043.
(10) Muir, K. W. J. Chem. Soc. A 1971, 2663.
(11) Sekiguchi, A.; Inoue, S.; Ichinohe, M.; Arai, Y. J. Am. Chem. Soc. 2004,
126, 9626.
(12) For (Mes2Si)2, a similar rearrangement has been observed upon melting
at 180 °C: Fink, M. J.; DeYoung, D. J.; West, R.; Michl, J. J. Am. Chem.
Soc. 1983, 105, 1070.
(13) (a) Lange, L.; Meyer, B.; du Mont, W.-W. J. Organomet. Chem. 1987,
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Organometallics 1996, 15, 741.
(14) The like (RR, SS)-unlike (RS, SR) descriptors are used for the configuration
of the three stereogenic centers in 3: Seebach, D.; Prelog, V. Angew.
Chem., Int. Ed. Engl. 1982, 21, 654.
(15) Kayser, C.; Frank, D.; Baumgartner, J.; Marschner, C. J. Organomet.
Chem. 2003, 667, 149 and references therein.
Acknowledgment. Funding of this work by the Deutsche
Forschungsgemeinschaft (DFG SCHE 906/3-1) is gratefully ac-
knowledged. The author thanks Prof. H. Braunschweig for continu-
ous support, and Dr. R. Bertermann for NMR spectroscopy.
Supporting Information Available: Details of the synthesis and
structure determinations of 2 and 3. The X-ray crystallographic data
JA052593P
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J. AM. CHEM. SOC. VOL. 127, NO. 29, 2005 10175