Metal-silicon triple bonds: The molybdenum silylidyne complex [Cp(CO) 2Mo = Si-R]

Article abstract of DOI:10.1002/anie.201000837

Carbene transfer from the silylidene complex [Cp(CO)₂Mo= Si(C₆H₃-2,6-Trip₂)(ImMe₄)] (Trip = 2,4,6-triisopropylphenyl, ImMe₄ = tetramethylimidazol-2-ylidene) to a triarylborane leads to the silylidyne complex [Cp(CO)₂Mo = Si (C₆H₃-2,6-Trip₂)], which contains the first Mo-Si triple bond (see picture). (Figure Presented)

Full text of DOI:10.1002/anie.201000837

Communications
DOI: 10.1002/anie.201000837
Silylidyne Complexes
Metal–Silicon Triple Bonds: The Molybdenum Silylidyne
Complex [Cp(CO)2Mo ꢀ Si-R]**
Alexander C. Filippou,* Oleg Chernov, Kai W. Stumpf, and Gregor Schnakenburg
Angewandte
Chemie
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Transition metal alkylidyne complexes are among the most
important organometallic compounds. The great utility of
these compounds in organometallic and organic chemistry
was highlighted by E. O. Fischer and R. R. Schrock in their
Nobel lectures.[1,2] Numerous stoichiometric and catalytic
transformations have been accomplished with these com-
plexes[3] since their discovery in 1973.[4] In comparison, silicon
analogues of the alkylidyne complexes are presently
unknown,[5,6] thereby emphasizing the marked difference
between silicon and carbon in forming multiple bonds. A
series of compounds featuring metal–germanium, metal–tin,
or metal–lead triple bonds have been obtained by reacting
carbonyl metalates[7] or dinitrogen and homoleptic phosphane
complexes of molybdenum and tungsten[8] with organo
Group 14 element(II) halides.[9] Extension of this efficient
process to silicon analogues was hampered to date by the
absence of any suitable organosilicon(II) halide precursors. In
fact, the isolation of transition metal silylidyne complexes is
one of the most challenging unresolved targets in organo-
silicon chemistry.
Scheme 1. Stepwise synthesis of the silylidyne complex 3.
Recent studies in our group have shown that N-hetero-
cyclic carbenes can be used to stabilize aryl silicon(II)
chlorides.[10] The synthetic potential of these compounds is
demonstrated herein by the isolation of the first complex
featuring a metal–silicon triple bond.
Heating a toluene solution of SiRCl(Im-Me4) (1: R =
C6H3-2,6-Trip2, Im-Me4 = tetramethylimidazol-2-ylidene)[10]
with Li[CpMo(CO)3][11] at 1008C was accompanied by a
color change from yellow over brown-green to brown.
Monitoring of the reaction progress by IR spectroscopy
revealed a rapid conversion of 1 into the silylidene complex 2,
which was isolated after recrystallization from a toluene/
hexane mixture as a dark-brown, air-sensitive solid in 51%
yield (Scheme 1).[12]
Complex 2 was fully characterized, and its molecular
structure was determined by single-crystal X-ray diffraction
(Figure 1).[13,14] The three-legged piano-stool complex fea-
Figure 1. DIAMOND plot of the molecular structure of the silylidene
complex 2. Thermal ellipsoids are set at 50% probability, and hydrogen
atoms are omitted for clarity. Selected bond lengths [ꢀ] and angles [8]
(values in square brackets are of the second independent molecule of
2 found in the asymmetric unit): Mo1–Si1 2.3474(6) [2.3430(6)], Mo1–
C49 1.917(2) [1.906(2)], Mo1–C50 1.936(2) [1.938(3)], Si1–C1 1.918(2)
[1.922(2)], Si1–C37 1.943(2) [1.945(2)]; Mo-Si1-C1 145.71(6)
[144.92(6)], Mo-Si1-C37 111.08(6) [111.58(6)], C1-Si-C37 99.64(8)
[101.12(8)], Si1-Mo1-C49 90.67(6) [91.36(7)], Si1-Mo1-C50 82.43(7)
[82.53(8)], C49-Mo1-C50 78.80(9) [80.2(1)].
À
tures a Mo Si double bond (2.345 ꢀ), which lies in the range
À
of Mo Si bonds reported for molybdenum arylsilylidene
complexes (d(Mo–Si) = 2.288(2)–2.3872(7) ꢀ).[5b,15,16] The
silylidene ligand has a trigonal planar coordinated silicon
center (sum of angles at Si = 357.08) and it adopts an upright
conformation, with the m-terphenyl group pointing towards
the cyclopentadienyl ring.[17] The angles at silicon differ
markedly; the Mo-Si-CAr angle is considerably widened to
145.38 owing to the large steric demand of the m-terphenyl
substituent; the CAr-Si-Ccarbene angle is lowered to 100.48,
[*] Prof. Dr. A. C. Filippou, O. Chernov, K. W. Stumpf,
Dr. G. Schnakenburg
which reflects the low tendency of silicon for hybridization.[18]
À
Institut fꢁr Anorganische Chemie, Universitꢂt Bonn
Gerhard-Domagk-Strasse 1, 53121 Bonn (Germany)
E-mail: filippou@uni-bonn.de
The Si Ccarbene bond of 2 (1.944 ꢀ) compares well with that of
1 (1.963(2) ꢀ)[10] and is only slightly longer than the Si CAr
À
bond of 2 (1.920 ꢀ) and 1 (1.937(2) ꢀ), thus indicating the
Homepage: http://www.filippou.chemie.uni-bonn.de
À
presence of a rather strong Ccarbene Si donor–acceptor inter-
[**] We thank the Deutsche Forschungsgemeinschaft for the generous
financial support of this work. We also thank C. Schmidt, K.
Prochnicki, and H. Spitz for recording the solution NMR spectra
and A. Martens for the elemental analyses. Cp=h5-C5H5; R=C6H3-
2,6-Trip2, Trip=2,4,6-triisopropylphenyl.
action.[19]
Further structural information is provided by the IR and
NMR spectra of 2. The IR spectrum of 2 in toluene has two
n(CO) absorption bands at considerably lower wavenumbers
(1859 and 1785 cmÀ1) than those of 3 (1937 and 1875 cmÀ1 in
toluene), which indicates that the silylidene ligand in 2 is a
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201000837.
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3297

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considerably weaker p-acceptor ligand than the silylidyne
1
ligand in 3. The H and 13C{1H} NMR spectra reveal a Cs
symmetric structure of the silylidene complex in solution, and
that rotation of the m-terphenyl substituent about the Si–CAr
bond is fast on the NMR timescale at room temperature. The
most distinctive signal in the 13C{1H} NMR spectrum of 2 is
that of the silicon-bonded Ccarbene atom (d = 165.3 ppm),
which appears considerably upfield to that of Im-Me4 (d =
212.7 ppm),[20] but at similar position to that of 1 (d =
166.7 ppm).[10] The 29Si{1H} NMR spectrum of 2 has a
characteristic downfield-shifted signal at d = 201.8 ppm,
which compares well with those of the molybdenum arylsi-
lylidene complexes [(h5-C5Me5)(dmpe)Mo(H){Si(R)Ar}]
(R = H, Cl, Me; Ar= Ph, Mes; d = 182–250 ppm).[5b,15a] All
the spectroscopic data and bonding parameters suggest that 2
is a new type of silylidene complex, which is best described by
the zwitterionic structure depicted in Scheme 1.
The bond dissociation enthalpy Do(0) and Gibbs free
Figure 2. DIAMOND plot of the molecular structure of the silylidyne
complex 3. Thermal ellipsoids are set at 50% probability, and hydrogen
atoms are omitted for clarity. Selected bond lengths [ꢀ] and angles [8]:
Mo–Si 2.2241(7), Mo–C37 1.968(3), Mo–C38 1.973(3), Si–C1 1.859(2);
Mo-Si-C1 173.49(8), Si-Mo-C37 90.56(7), Si-Mo-C38 89.63(8), C37-Mo-
C38 87.0(1).
o
À
dissociation energy DGD (298) required to cleave the Si
Ccarbene bond of 2 was calculated to be 62.1 and 1.2 kJmolÀ1,
respectively.[21,22] Both values are smaller than the respective
values
of
1
(Do(0) = 94.3 kJmolÀ1,
DGDo(298) =
28.1 kJmolÀ1),[10] which suggests that dissociation of the N-
heterocyclic carbene Im-Me4 from 2 might occur to some
extent at elevated temperatures and in the presence of a
suitable carbene trapping agent may lead to the silylidyne
complex 3. Indeed, reaction of 2 with one equivalent of the
triarylborane B(C6H4-4-Me)3 in refluxing o-xylene afforded
selectively the silylidyne complex 3 and the carbene–borane
adduct Im-Me4·B(C6H4-4-Me)3 (4-Me; Scheme 1).[12] Com-
plex 3 was easily separated from the adduct 4-Me upon
fractional crystallization from pentane and was isolated as a
brick-red, air-sensitive solid in 53% yield.
ligands. Further evidence for the stronger metal–carbonyl
À
back-bonding in [Cp(CO)2Mo ꢀ E C6H3-2,6-Trip2] (E = Si,
Ge) is provided by the downfield-shifted 13C NMR signal of
the carbonyl ligands (E = Si (3): d = 231.1 ppm, E = Ge: d =
231.4 ppm[7b]) than that of [Cp(CO)2Mo ꢀ C C6H3-2,6-Me2]
À
(d = 228.7 ppm).[25]
The thermochemical parameters for the carbene transfer
reaction of 2 with the borane B(C6H4-4-Me)3 to give 3 and 4-
Me were computed.[21,22] Formation of the silylidyne complex
3 is an exergonic process (DGR8(298) = À39.0 kJmolÀ1),
which is favored by both the reaction enthalpy
(DHR8(298) = À21.8 kJmolÀ1) and the reaction entropy
(DSR8(298) = 57.7 JmolÀ1 KÀ1). The negative reaction
enthalpy results from the higher bond dissociation enthalpy
Do(0) of the carbene–borane adduct 4-Me (88.4 kJmolÀ1)
than that of 2 (62.1 kJmolÀ1) and suggests that triarylboranes,
such as B(C6H4-4-Me)3, should be useful N-heterocyclic
carbene abstracting agents owing to the formation of a
The molecular structure of 3 was determined by single-
crystal X-ray diffraction (Figure 2).[13] The almost Cs-sym-
metric three-legged piano-stool complex[23] is isostructural
À
with the germanium analogue [Cp(CO)2Mo ꢀ Ge C6H3-2,6-
Trip2].[7b] It features an almost linearly coordinated silicon
À
center (Mo-Si-CAr = 173.49(8)8) and a very short Mo Si bond
À
(2.2241(7) ꢀ), which is 12 pm shorter than the Mo Si double
À
bond of 2. The Mo Si bond length of 3 compares well with the
calculated Mo Si bond lengths of the hypothetical silylidyne
À
ꢀ À
complexes [Cp(CO)2Mo Si R] (R = H: 2.213 ꢀ, R = Me:
5
2.229 ꢀ)[24] and the Mo Si bond length of [(h -C5Me5)-
rather strong B Ccarbene bond.
À
À
(dmpe)(H)MoSiMes][B(C6F5)4] (2.219(2) ꢀ).[5b]
The isolation of the silylidyne complex 3 shows the
potential of the carbene adduct 1 as a source for the
generation of unprecedented compounds featuring silicon
multiple bonds. Studies are currently in progress to explore
this potential and the chemistry of the silylidyne complex 3.
The IR and NMR spectra support the structure of
complex 3. The 29Si{1H} NMR spectrum of 3 has a distinctive
signal that is considerably downfield (at d = 320.1 ppm) to
that of 2 (d = 201.8 ppm). The IR spectrum of 3 in toluene has
two n(CO) bands (1937 and 1875 cmÀ1) that appear at almost
the same position as those of the germylidyne complex
[Cp(CO)2Mo ꢀ Ge-C6H3-2,6-Mes2] (1930 and 1875 cmÀ1 in
nujol),[7a] but at considerably lower wavenumbers than those
Received: February 10, 2010
Keywords: molybdenum · silicon · silylidene complexes ·
.
silylidyne complexes · triple bonds
ꢀ À
of the alkylidyne complex [Cp(CO)2Mo C C6H3-2,6-Me2]
(1992 and 1919 cmÀ1 in CH2Cl2).[25] This result indicates that
metal–carbonyl back-bonding is stronger in the complexes
À
[Cp(CO)2Mo ꢀ E R] (E = Si, Ge). It also suggests that
[1] E. O. Fischer, Nobel Lecture, December 11, 1973 (http://
nobelprize.org/chemistry/laureates/1973/fischer-lecture.pdf; see
also E. O. Fischer, Angew. Chem. 1974, 86, 651.
silylidyne and germylidyne ligands have a similar s-donor/p-
acceptor ratio, which is however larger than that of alkylidyne
3298
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[2] R. R. Schrock, Nobel Lecture, December 8, 2005 (http://nobel-
prize.org/chemistry/laureates/2005/schrock-lecture.pdf; see also
R. R. Schrock, Angew. Chem. 2006, 118, 3832; Angew. Chem. Int.
Ed. 2006, 45, 3748.
[3] Selected monographs and reviews on alkylidyne complexes:
a) E. O. Fischer, Adv. Organomet. Chem. 1976, 14, 1; b) F. G. A.
Stone, Angew. Chem. 1984, 96, 85; Angew. Chem. Int. Ed. Engl.
1984, 23, 89; c) R. R. Schrock, Acc. Chem. Res. 1986, 19, 342;
d) H. P. Kim, R. J. Angelici, Adv. Organomet. Chem. 1987, 27,
51; e) H. Fischer, P. Hofmann, F. R. Kreißl, R. R. Schrock, U.
Schubert, K. Weiss, Carbyne Complexes, VCH, Weinheim, 1988;
f) A. Mayr, H. Hoffmeister, Adv. Organomet. Chem. 1991, 32,
227; g) R. R. Schrock, Chem. Rev. 2002, 102, 145.
formed upon slow evaporation of a concentrated toluene
solution at room temperature. CCDC 760807 (2·toluene),
760808(3·n-pentane), and 764217 (4-H) contain the supplemen-
tary crystallographic data for this paper. These data can be
obtained free of charge from The Cambridge Crystallographic
Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
[14] Two independent molecules of 2 with very similar geometric
parameters were found in the asymmetric unit of 2·toluene. The
mean values xm of the individual bonding parameters of the two
molecules were used for the discussion of the structure. The
individual bonding parameters are given in the legend of
Figure 1 and the Supporting Information.
[15] a) B. V. Mork, T. Don Tilley, A. J. Schultz, J. A. Cowan, J. Am.
Chem. Soc. 2004, 126, 10428; b) M. Hirotsu, T. Nunokawa, K.
Ueno, Organometallics 2006, 25, 1554.
[4] E. O. Fischer, G. Kreis, C. G. Kreiter, J. Mꢁller, G. Huttner, H.
Lorenz, Angew. Chem. 1973, 85, 618; Angew. Chem. Int. Ed.
Engl. 1973, 12, 564.
[16] A few molybdenum diaminosilylidene complexes have also been
À
[5] Only a donor-stabilized silylidyne complex of ruthenium con-
taining a four-coordinate, singly-bonded silicon atom and a
molybdenum complex with considerable silylidyne character
have been isolated to date: a) S. D. Grumbine, R. K. Chadha,
T. D. Tilley, J. Am. Chem. Soc. 1992, 114, 1518; b) B. V. Mork,
T. D. Tilley, Angew. Chem. 2003, 115, 371; Angew. Chem. Int. Ed.
2003, 42, 357.
reported. Their Mo Si bonds are considerably longer than those
of the aryl or alkylsilylidene complexes owing to the weaker
metal–ligand p back-bonding, and range from 2.413 to 2.480 ꢀ:
a) S. H. A. Petri, D. Eikenberg, B. Neumann, H.-G. Stammler, P.
Jutzi, Organometallics 1999, 18, 2615; b) M. Haaf, T. A. Schme-
dake, R. West, Acc. Chem. Res. 2000, 33, 704; c) S. B. Clenden-
ning, B. Gehrhus, P. B. Hitchcock, D. F. Moser, J. F. Nixon, R.
West, J. Chem. Soc. Dalton Trans. 2002, 484.
À
[6] The silylidyne complex H3Mo ꢀ Si H has been recently sug-
gested to form in the reaction of laser-ablated molybdenum
atoms with SiH4 in excess argon during condensation at 4 K: X.
Wang, L. Andrews, J. Am. Chem. Soc. 2008, 130, 6766.
[7] a) R. S. Simons, P. P. Power, J. Am. Chem. Soc. 1996, 118, 11966;
b) L. Pu, B. Twamley, S. T. Haubrich, M. M. Olmstead, B. V.
Mork, R. S. Simons, P. P. Power, J. Am. Chem. Soc. 2000, 122,
650.
[17] The upright conformation of the silylidene ligand in 2 is best
indicated by the dihedral angle of 10.98 between the silylidene
ligand plane (least-square plane passing through the atoms Mo,
Si, CAr, and Ccarbene) and the plane defined by the atoms Si, Mo,
and Cg, where Cg denotes the center of gravity of the cyclo-
pentadienyl ring. Several isolobal metal–cyclopentadienyl car-
bene complexes also feature an upright conformation of the
carbene ligand: a) A. C. Filippou, D. Wꢃssner, G. Kociok-Kꢃhn,
I. Hinz, J. Organomet. Chem. 1997, 541, 333 and references
therein.
[8] a) A. C. Filippou, A. I. Philippopoulos, P. Portius, D. U. Neu-
mann, Angew. Chem. 2000, 112, 2881; Angew. Chem. Int. Ed.
2000, 39, 2778; b) A. C. Filippou, P. Portius, A. I. Philippopoulos,
Organometallics 2002, 21, 653; c) A. C. Filippou, P. Portius, A. I.
Philippopoulos, H. Rohde, Angew. Chem. 2003, 115, 461; Angew.
Chem. Int. Ed. 2003, 42, 445; d) A. C. Filippou, A. I. Philippo-
poulos, G. Schnakenburg, Organometallics 2003, 22, 3339;
e) A. C. Filippou, H. Rohde, G. Schnakenburg, Angew. Chem.
2004, 116, 2293; Angew. Chem. Int. Ed. 2004, 43, 2243; f) A. C.
Filippou, N. Weidemann, G. Schnakenburg, H. Rohde, A. I.
Philippopoulos, Angew. Chem. 2004, 116, 6674; Angew. Chem.
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Portius, G. Schnakenburg, Organometallics 2004, 23, 4503;
h) A. C. Filippou, G. Schnakenburg, A. I. Philippopoulos, N.
Weidemann, Angew. Chem. 2005, 117, 6133; Angew. Chem. Int.
Ed. 2005, 44, 5979; i) A. C. Filippou, N. Weidemann, A. I.
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[9] For a review covering the recent developments in the chemistry
of compounds containing triple bonds between transition metals
and the heavier elements of Groups 14 and 15, see: G. Balꢂzs,
L. J. Gregoriades, M. Scheer, Organometallics 2007, 26, 3058.
[10] A. C. Filippou, O. Chernov, B. Blom, K. W. Stumpf, G. Schna-
kenburg, Chem. Eur. J. 2010, 16, 2866.
[11] A. C. Filippou, J. G. Winter, G. Kociok-Kꢃhn, I. Hinz, J. Chem.
Soc. Dalton Trans. 1998, 2029.
[12] The Supporting Information contains the synthesis and analyt-
ical and spectroscopic data of the complexes 2 and 3 and the
carbene–borane adducts Im-Me4·B(C6H4-4-R)3 (R = H (4-H),
R = Me (4-Me)).
[18] Various metal tetrelylidene complexes of the general formula
=
[LnM E(Ar)X] (Ln = ligand sphere; M = Fe, Mo, W, Re, Pt; E =
Si, Ge, Sn; Ar= 2,6-Mes2C6H3, 2,6-Trip2C6H3; X = singly-bonded
substituent) featuring trigonal planar coordinate Group 14
element centers have been isolated in our group recently. In all
cases, the CAr-E-X angles were found by single-crystal X-ray
diffraction studies to be considerably smaller than 1208, ranging
from 858 to 1008. The small CAr-E-X angles indicate that the
Group 14 element centers use hybrid orbitals of high p character
for the bonding to the substituents Ar and X.
[19] The interplane angle of 74.08 between the imidazol-2-ylidene
least-square plane and the silylidene ligand plane (Ref. [17])
reveals that the Im-Me4 substituent is arranged almost orthog-
onally to the silicon coordination plane. In comparison, the m-
terphenyl substituent is slightly tilted to minimize the intra-
molecular repulsion between the peripheral Trip groups and the
Cp and Im-Me4 groups. The tilting is evidenced by the interplane
angle of 38.68 between the central aryl ring plane of the m-
terphenyl substituent and the silylidene ligand plane.
[20] N. Kuhn, T. Kratz, Synthesis 1993, 561.
[21] All quantum chemical calculations were carried out at the
B3LYP level of theory. The basis set TZVPP was used for the Mo
and Si atoms, and the basis set 6-31G* for the other atoms.
Details of the computational methods, tables of selected
calculated and experimental bonding parameters for 2, 3, 4-H,
and 4-Me, illustrations of the calculated structures of 2, 3, and 4-
Me, and their cartesian atomic coordinates are given in the
Supporting Information.
[22] Do(0) is the enthalpy of the dissociation of 2 or 4-Me at 0 K to
give the products 3 and Im-Me4 or B(C6H4-4-Me)3 and Im-Me4,
respectively, and includes the correction for the zero-point
vibrational energies. DGDo(298) is the Gibbs free energy of the
dissociation of 2 or 4-Me to give the products 3 and Im-Me4 or
[13] Brown crystals of 2·toluene were grown from an oversaturated
toluene solution at ambient temperature. Orange-red crystals of
3·n-pentane were obtained upon cooling a pentane solution from
room temperature to À608C, and colorless crystals of 4-H
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Communications
B(C6H4-4-Me)3) and Im-Me4 at 298 K, respectively. DGRo(298) is
[23] The silylidyne ligand adopts an upright conformation in which
the interplane angle between the central aryl ring plane of the m-
terphenyl substituent and the plane passing through the atoms
Si, Mo, and Cg (center of gravity of the Cp ring) is 6.48.
[24] a) N. Takagi, K. Yamazaki, S. Nagase, Bull. Korean Chem. Soc.
2003, 24, 832; b) K. K. Pandey, A. Lledꢄs, Inorg. Chem. 2009, 48,
2748.
the Gibbs free energy and DHR8(298) the enthalpy of the
reaction of 2 with B(C6H4-4-Me)3 to give 3 and 4-Me at 298 K.
DGRo(298) = DGDo(298)(2)ÀDGDo(298)(4-Me). DGDo(298)(2)
and DGDo(298)(4-Me) were calculated to be 1.2 and
40.2 kJmolÀ1
,
respectively. DHRo(298) = Do(298)(2)ÀDo(298)
(4-Me), where Do(298)(2) is the enthalpy of the dissociation of
2 to give 3 and Im-Me4 at 298 K, and Do(298)(4-Me) is the
enthalpy of the dissociation of 4-Me to give B(C6H4-4-Me)3 and
Im-Me4 at 298 K. Do(298)(2) and Do(298)(4-Me) were calcu-
lated to be 60.7 and 82.5 kJmolÀ1, respectively.
[25] S. J. Dossett, A. F. Hill, J. C. Jeffery, F. Marken, P. Sherwood,
F. G. A. Stone, J. Chem. Soc. Dalton Trans. 1988, 2453.
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