Lewis-base-stabilized dichlorosilylene: A two-electron σ-donor ligand

Article abstract of DOI:10.1021/ic9023162

The first structurally described cobalt(I) Lewis-base-stabilized silylene complex [Co(CO)₃{SiCl₂(IPr)}₂] ⁺[CoCl₃(THF)^- [1; IPr = 1,3-bis(2,6- diisopropylphenyl)imidazol-2-ylidene] was prepared by applying the two-electron σ-donor ligand SiCl₂(IPr) through coordination with Co ₂(CO)₈. The bonding situation between ligand SiCl _2- (IPr) and the cobalt(I) metal center in [Co(CO) ₃{SiCl₂(IPr)}₂]⁺ of 1 was investigated by ¹H NMR and IR spectroscopy, single-crystal X-ray structural analysis, and density functional theoretical calculations.

Full text of DOI:10.1021/ic9023162

Inorg. Chem. 2010, 49, 775–777 775
DOI: 10.1021/ic9023162
Lewis-Base-Stabilized Dichlorosilylene: A Two-Electron σ-Donor Ligand
€
Jianfeng Li, Sebastian Merkel, Julian Henn, Kathrin Meindl, Alexander Doring, Herbert W. Roesky,*
Rajendra S. Ghadwal, and Dietmar Stalke
€
€
€
Institut fu€r Anorganische Chemie, Universitat Gottingen, Tammannstrasse 4, 37077 Gottingen, Germany
Received November 23, 2009
The first structurally described cobalt(I) Lewis-base-stabilized
silylene complex [Co(CO)3{SiCl2(IPr)}2]þ[CoCl3(THF)]- [1; IPr =
1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene] was prepared by
applying the two-electron σ-donor ligand SiCl2(IPr) through coordi-
nation with Co2(CO)8. The bonding situation between ligand SiCl2-
(IPr) and the cobalt(I) metal center in [Co(CO)3{SiCl2(IPr)}2]þ of 1
was investigated by H NMR and IR spectroscopy, single-crystal
X-ray structural analysis, and density functional theoretical calcula-
tions.
transition metals has become an active area of research,5,6 even
though, except stable N-heterocyclic silylenes, other types of
silylene ligands were still extremely scarce until now.6
Dihalosilylenes as high-temperature molecules have at-
tracted academic and industrial interest for decades.7 How-
ever, because of the extreme instability and rapid poly-
merization of dihalosilylenes at ambient temperature,8,9 as
far as we know, the study of stable dihalosilylenes serving as
free ligands that coordinate to metals has not been reported
in the literature.10 Recently, we synthesized the first Lewis-
base-stabilized dichlorosilylene SiCl2(IPr) [IPr = 1,3-bis(2,6-
diisopropylphenyl)imidazol-2-ylidene]11 and investigated the
1
An important part of the art of organometallic chemistry is
to select suitable ligands to influence the electronic and steric
properties of the complexes to favor the desired reactivity.
N-Heterocyclic carbenes (NHCs) as representatives of strong
two-electron σ-donor ligands have been successfully used in
a number of catalytic transformations.1 Apparently, small
changes in the ligand properties can entirely change the
chemistry. Silylenes, as heavier analogues of carbenes, are
divalent neutral silicon species that nominally have a singlet
ground state (1A1), with the lone pair of electrons as the highest
occupied molecular orbital and an empty p orbital as the
lowest unoccupied molecular orbital.2 Consequently, silylenes
are capable of behaving as σ-donor/π-acceptor ligands resem-
bling triorganophosphines (PR3). The important break-
through is the synthesis and structure of Ni(CO)2(NHSi)2
[NHSi = (tBuNCHdCHNtBu)Si] published in 1994,3 which
is derived from the coordination of two free NHSi ligands to a
nickel center instead of other indirect preparative methods.4
Since then, the incorporation of stable silylenes as ligands for
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(b) Feldman, J. D.; Mitchell, G. P.; Nolte, J.-O.; Tilley, T. D. J. Am. Chem. Soc.
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A.; West, R.; Fogg, D. E. Organometallics 2002, 21, 534–540. (j) Avent, A. G.;
Gehrhus, B.; Hitchcock, P. B.; Lappert, M. F.; Maciejewski, H. J. Organomet. Chem.
2003, 686, 321–331. (k) Kong, L.; Zhang, J.; Song, H.; Cui, C. Dalton Trans. 2009,
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Jana, A. Inorg. Chem. 2009, 48, 5058–5060.
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26, 1-35 and references cited therein.
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H.; Bruderreck, H.; Morcher, B. Z. Anorg. Allg. Chem. 1967, 352, 122–137.
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Chem. Soc. 1965, 87, 2824–2828. (b) Hengge, E.; Kovar, D. Angew. Chem.
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*To whom correspondence should be addressed. E-mail: hroesky@
gwdg.de.
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Chem. Rev. 2009, 253, 687–703.
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3683–3690.
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1994, 1, 33–34.
(10) For iron and chromium dichlorosilylene complexes derived from an
indirect strategy of salt elimination, see: Leis, C.; Wilkinson, D. L.;
(4) For the first examples of transition-metal silylene complexes derived
from an indirect salt elimination method and a triflate abstraction method,
see: (a) Straus, D. A.; Tilley, T. D.; Rheingold, A. L.; Geib, S. J. J. Am. Chem.
Soc. 1987, 109, 5872–5873. (b) Zybill, C.; M€uller, G. Angew. Chem. 1987, 99,
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€
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(11) (a) Ghadwal, R. S.; Roesky, H. W.; Merkel, S.; Henn, J.; Stalke, D.
Angew. Chem. 2009, 121, 5793–5796. Angew. Chem., Int. Ed. 2009, 48, 5683-
5686. At the same time, Filippou et al. reported a similar compound, SiBr2(IPr): (b)
Filippou, A. C.; Chernov, O.; Schnakenburg, G. Angew. Chem. 2009, 121, 5797–
5800. Angew. Chem., Int. Ed. 2009, 48, 5687-5690. (c) Ghadwal, R. S.; Roesky, H.
W.; Merkel, S.; Stalke, D. Chem. Eur. J. 2009, DOI: 10.1002/chem.200902930.
r
2009 American Chemical Society
Published on Web 12/22/2009
pubs.acs.org/IC

776 Inorganic Chemistry, Vol. 49, No. 3, 2010
Li et al.
Scheme 1. Synthesis of Cobalt Carbonyl Lewis-Base-Stabilized
Dichlorosilylene Complex 1
reactivity of SiCl2(IPr) toward Lewis acid11c B(C6F5)3, from
which we are inspired to explore the coordination behavior of
SiCl2(IPr) to transition metals. Herein we report on the
synthesis and characterization of the first cobalt carbonyl
Lewis-base-stabilized dichlorosilylene complex.
Figure 1. Anisotropic displacement parameters of 1, depicted at the
50% probability level. Hydrogen atoms and the counterion [CoCl3-
(THF)]- have been omitted for clarity. Selected bond lengths (A) and
˚
The reaction of 2 equiv of SiCl2(IPr) with Co2(CO)8 in
toluene at room temperature afforded air- and moisture-
sensitive blue crystals of [Co(CO)3{SiCl2(IPr)}2]þ[CoCl3-
(THF)]- (1, where THF=tetrahydrofuran; Scheme 1). The
ratio of the starting materials is different in comparison to that
in Scheme 1, due to the problem of separating the products.
The relatively high yield (74%) of 1 demonstrates that SiCl2-
(IPr) functions as a coordinate ligand as well as a chlorinating
and oxidizing agent. In contrast to the anion formed only by
disproportionation in [Co(CO)3(L)2]þ[Co(CO)4]- (L = PR3
and NHC),12,13 the anion [CoCl3(THF)]- in 1 is exclusively
generated even if an excess of Co2(CO)8 was used. Compound
1 is soluble in THF but insoluble in benzene and toluene. 1 has
been characterized by NMR, IR and UV-vis spectroscopy,
elemental analysis, and single-crystal X-ray analysis.
angles (deg): Co1-Si1 2.2278(13), Co1-Si2 2.2276(12), Co1-C55
1.767(4), Co1-C56 1.773(5), Co1-C57 1.773(4), Si1-Cl1 2.0635(16),
Si1-Cl2 2.0846(15), Si1-C1 1.944(4), Si2-Cl3 2.0696(15), Si2-Cl4
2.0778(15), Si2-C28 1.952(4); Si1-Co1-Si2 176.78(5), Co1-Si1-Cl1
109.64(6), Co1-Si1-Cl2 115.54(6), Co1-Si2-Cl3 110.58(6), Co1-Si2-
Cl4 115.13(6), Cl1-Si1-Cl2 104.09(7), Cl3-Si2-Cl4 103.11(6).
as a strong σ donor accompanied by weak π-accepting proper-
˚
ties. The average Co-CO bond distance of 1.771(5) A in 1 is
similar to those reported for [Co(CO)3(PPh3)2]þ[PF6]-
˚
[1.778(10)
A
(av)] and [Co(CO)3(IMes)2]þ[Co(CO)4)]-
[IMes=1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene;
13,18
˚
1.790(6) A (av)].
The geometries of the two silicon atoms in 1 can be
described as distorted tetrahedral. Compared with the calcu-
lated LPsi-Si-Cl (LPsi = lone-pair density) bond angle in
SiCl2(IPr) [122.65° (av)], the Co-Si-Cl bond angle in 1
[112.72(6)° (av)] is smaller.11 The Cl-Si-Cl bond angle of
103.60(7)° (av) in 1 is comparable to that in SiCl2(IPr)
[97.25(6)°]. The mean Si-Cl and Si-C bond lengths in 1
The molecular structure of 1 is shown in Figure 1. The
formal oxidation states of cobalt in the cationic and anionic
moieties are 1þ and 2þ, respectively. The cation adopts a
slightly distorted trigonal-bipyramidal geometry, with three
carbonyl ligands occupying the equatorial plane and two
SiCl2(IPr) ligands arranged in the apical positions. [Co(CO)3-
{SiCl2(IPr)}2]þ has essentially C2 symmetry. The cobalt
atom in the anion is pseudotetrahedrally coordinate. The
Si1-Co1-Si2 moiety is close to linear [176.78(5)°]. Notably,
to the best of our knowledge, no crystal structure of the
cobalt(I) silylene complex has been described in the litera-
ture.14 The Co-Si bond lengths of 2.2278(13) and 2.2276(12)
˚
[2.0739(16) and 1.948(4) A] are significantly shorter than
˚
those observed in SiCl2(IPr) [2.1664(16) A (av) and 1.985(4)
˚
A], which is probably caused by an increased Lewis acidity of
the silicon center due to σ-electron donation from Lewis-
base-stabilized silylene to the cobalt atom.
Compound 1 displays abnormal resonances in the 1H
NMR spectrum at room temperature, which might be caused
by the d7 cobalt(II) tetrahedral anion.19 These resonances
shifted by different extents from þ50 to -90 °C and became
broader at lower temperature. The 29Si{1H} NMR spectrum
of 1 exhibited a singlet at 44.11 ppm. The characteristic
downfield shift Δδ of 25.05 ppm from the free SiCl2(IPr)
ligand (19.06 ppm) demonstrates coordination of SiCl2(IPr)
to the cobalt center,5l,6b,11 which reveals the ambiphilicity of
Lewis-base-stabilized silylene in 1. Consistent with ArHSi-
Co(CO)Cp [Ar = 2-(Me2NCH2)C6H4, δ = 95.00 ppm;
Ar = 8-(Me2NCH2)C10H6, δ=85.99 ppm], 1 features quite
˚
A in 1 are slightly shorter than those of covalently bonded
˚
Co-Si units [range of 2.254(4) A for Cl3SiCo(CO)4 to
15,16
˚
˚
and longer by ca. 0.04 A
2.381(7) A for H3SiCo(CO)4]
than that observed in the seven-coordinate cobalt silylene
17
˚
complex [2.1848(8) A]. This suggests that the Lewis-base-
stabilized dichlorosilylene ligand SiCl2(IPr) can be considered
(12) (a) Hieber, W.; Freyer, W. Chem. Ber. 1958, 91, 1230–1234. (b) Vohler,
O. Chem. Ber. 1958, 91, 1235–1238. (c) Sacco, A.; Freni, M. J. Inorg. Nucl.
Chem. 1958, 8, 566–568. (d) Hieber, W.; Freyer, W. Chem. Ber. 1960, 93,
462–467. (e) Attali, S.; Poilblanc, R. Inorg. Chim. Acta 1972, 6, 475–479.
(13) van Rensburg, H.; Tooze, R. P.; Foster, D. F.; Otto, S. Inorg. Chem.
2007, 46, 1963–1965.
(14) For cobalt(I) silylene complexes derived from an indirect photoche-
mical method without structural proof, see: Corriu, R. J. P.; Chauhan,
B. P. S.; Lanneau, G. F. Organometallics 1995, 14, 1646–1656.
(15) Robinson, W. T.; Ibers, J. A. Inorg. Chem. 1967, 6, 1208–1213.
(16) Robiette, A. G.; Sheldrick, G. M.; Simpson, R. N. F.; Aylett, B. J.;
Campbell, J. A. J. Organomet. Chem. 1968, 14, 279–283.
(18) MacNeill, J. H.; Chiverton, A. C.; Fortier, S.; Baird, M. C.; Hynes,
R. C.; Williams, A. J.; Preston, K. F.; Ziegler, T. J. Am. Chem. Soc. 1991,
113, 9834–9842.
(19) 1H NMR of 1 (500 MHz, THF-d8, 50 °C): δ -4.82, -0.63, 0.40, 0.73,
1.15, 2.04, 5.85, 6.59, 6.87, 7.31, 7.53, 9.69. 1H NMR (500 MHz, THF-d8,
25 °C): δ -6.28, -1.04, 0.37, 0.78, 1.17, 2.08, 6.11, 6.52, 6.78, 7.35, 7.57, 9.95.
1H NMR (500 MHz, THF-d8, 0 °C): δ -7.49, -1.41, 0.36, 0.84, 1.19, 2.13,
6.45, 6.68, 7.40, 7.60, 10.36. 1H NMR (500 MHz, THF-d8, -50 °C): δ -8.91,
-1.98, 0.39, 1.29, 2.41, 6.25, 7.55, 12.93. 1H NMR (500 MHz, THF-d8,
-90 °C): δ 0.86, 1.41, 2.60, 5.82, 7.77.
(17) Ingleson, M.; Fan, H.; Pink, M.; Tomaszewski, J.; Caulton, K. G.
J. Am. Chem. Soc. 2006, 128, 1804–1805.

Communication
Inorganic Chemistry, Vol. 49, No. 3, 2010 777
Table 1. Comparison of Carbonyl IR Bands in Various Cations of
In order to explore the electronic structure and bonding
properties of the cationic moiety in complex 1, density
functional theoretical calculations were performed for the
cation singlet, in which the isopropyl groups were substituted
by methyl groups, with exchange-correlation functional
B25P8626 and the 6-31G* basis set employing polarization
functions for non-hydrogen atoms with Gaussian03.27 After
geometry optimization starting from the crystal coordinates,
the calculated structural parameters compare well with
experimental values (see the Supporting Information).
Natural bond orbital28 analysis assigns a single weak
covalent Co-Si bond with occupation 1.82226 from one of
the two silicon atoms, and the other silicon atom is described
in terms of a strongly donating silicon lone pair (occupation
1.05441; no covalent bond here). The donation is into the
same empty predominantly s-type orbital [s (88.95%) p0.00
(0.09%) d0.12 (10.95%) f0.00 (0.01%)] on cobalt into which
also the carbonyl carbon atoms donate their lone pairs.
Additionally, for the same silicon atom, there is another
donation into the antibonding covalent Co-Si bond. Both
donation modes lead to a distinct stabilization energy (see the
Supporting Information). The Co-Si bonds, however, are
chemically equivalent, so the complete characteristics of the
Co-Si bond are given by an average of two (or even more)
contributing natural Lewis structures. This leads to a strong
σ-donating silicon lone pair.
[Co(CO)3(L)2]þX-
L
X-
νh(CO) (cm-1 a,b
)
SiCl2(IPr)
PnBu3
PPh3
P(p-MeC6H4)3
IMes13
[CoCl3(THF)]-
[Co(CO)4]-
[Co(CO)4]-
[PF6]-
2052 (vw), 1994 (s), 1969 (s)
2066 (vw), 1997 (s), 1983 (s)
2071 (vw), 2014 (s), 2001 (s)
2073 (vw), 2009 (sh), 2001 (s)c
1981 (mw)c,d
20
20
20
[Co(CO)4]-
a Nujol mull, unless indicated otherwise. b v, very; m, medium;
w, weak; s, strong; sh, shoulder. c THF. d E0 band.
different 29Si{1H} NMR resonances from those of covalently
bonded metal silyl complexes (below 150 ppm).14
The IR spectrum of 1 recorded as a Nujol mull shows three
bands attributed to carbonyl groups on cobalt(I) and a weak
A1 band centered at 2052 cm-1 along with two strong bands
0
at 1994 and 1969 cm-1 for the E0 mode. The well-split of the
three distinct bands is characteristic for C2 symmetry of the
cationic moiety [Co(CO)3{SiCl2(IPr)}2]þ.20,21 In a compari-
son of analogous complexes containing [Co(CO)3(PR3)2]þ,
the coordinate CO in 1 as a “reporting group” features lower
stretching frequencies (Table 1). According to Tolman’s
method, this indicates higher electron density on the cobalt
(I) center in [Co(CO)3{SiCl2(IPr)}2]+ of 1.22 Moreover, we
can deduce that the Lewis-base-stabilized dichlorosilylene
SiCl2(IPr) is either a stronger two-electron σ donor or a
weaker π acceptor than PR3.5g,23 We can state that the
electron-donating to electron-accepting ratio for SiCl2(IPr)
is distinctly larger than that for PR3. For the same reason,
SiCl2(IPr) and IMes possess the similar ratios of electron
donation to electron acceptance.
In summary, we use the two-electron σ-donor ligand
SiCl2(IPr) for synthesis of the first structural characterized
cobalt(I) Lewis-base-stabilized silylene complex 1 through
coordination with Co2(CO)8. The Lewis-base-stabilized di-
chlorosilylene SiCl2(IPr) can be considered as a strong σ
donor accompanied by a weak π-accepting ability.
The THF solution of 1 exhibits two characteristic absorp-
tions at 587 and 693 nm, which are ascribed to the d-d
transition from the ground state 4A2 to the 4T1(P) state of the
pseudotetrahedral cobalt(II) anionic moiety.24 Though the
expected band should be around 600 nm, the spin-orbit
coupling causes a splitting of this band into two parts. Thus,
the cationic moiety [Co(CO)3{SiCl2(IPr)}2]þ of 1 shows
essentially no absorption beyond λ>400 nm, consistent with
that observed for [Co(CO)3(PnBu3)2]þ[ClO4]-.20
Acknowledgment. Dedicated to Professor Michael
Veith on the occasion of his 65th birthday. Support of
the Deutsche Forschungsgemeinschaft is highly acknow-
ledged.
Supporting Information Available: Experimental preparation,
computational details, and a CIF file for compound 1. This
material is available free of charge via the Internet at http://
pubs.acs.org.
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Advanced Inorganic Chemistry, 6th ed.; John Wiley & Sons: New York, 1999;
pp 817-822.
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