NHC-coordinated silagermenylidene functionalized in allylic position and its behaviour as a ligand

Article abstract of DOI:10.1039/c4dt00094c

Vinylidenes are common in transition metal chemistry with catalytic applications in alkene and alkyne metathesis. We report here the isolation of a heavier analogue of vinylidene, an α-chlorosilyl functionalized silagermenylidene stabilized by an N-heterocyclic carbene (NHC). Silagermenylidene (Tip₂Cl)Si-(Tip)SiGe·NHCiPr ₂Me₂ (4-E/Z; Tip = 2,4,6-iPr₃C ₆H₂; NHCiPr₂Me₂ = 1,3-diisopropyl-4,5-dimethylimidazol-2-ylidene) is available as an E/Z-equilibrium mixture from Tip₂SiSi(Tip)Li and NHCiPr ₂Me₂·GeCl₂. Reaction of 4-E/Z with Fe₂(CO)₉ affords a silagermenylidene Fe(CO)₄ complex, which slowly isomerizes to its E-isomer at 25°C. A rearranged Fe(CO)₃ complex with an allylic SiGeSi ligand is obtained as a side product at 65°C.

Full text of DOI:10.1039/c4dt00094c

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NHC-coordinated silagermenylidene
functionalized in allylic position and its
behaviour as a ligand†
Cite this: Dalton Trans., 2014, 43,
5175
Anukul Jana, Moumita Majumdar, Volker Huch, Michael Zimmer and
David Scheschkewitz*
Vinylidenes are common in transition metal chemistry with catalytic applications in alkene and alkyne
metathesis. We report here the isolation of a heavier analogue of vinylidene, an α-chlorosilyl function-
alized silagermenylidene stabilized by an N-heterocyclic carbene (NHC). Silagermenylidene (Tip2Cl)Si-
(Tip)SivGe·NHCiPr Me (4-E/Z; Tip = 2,4,6-iPr3C6H2; NHCiPr Me = 1,3-diisopropyl-4,5-dimethylimidazol-
2
2
2
2
2-ylidene) is available as an E/Z-equilibrium mixture from Tip2SivSi(Tip)Li and NHCiPr Me ·GeCl2. Reaction
of 4-E/Z with Fe2(CO)9 affords a silagermenylidene Fe(CO)4 complex, which slowly isomerizes to its
E-isomer at 25 °C. A rearranged Fe(CO)3 complex with an allylic SiGeSi ligand is obtained as a side
product at 65 °C.
2
2
Received 10th January 2014,
Accepted 14th January 2014
DOI: 10.1039/c4dt00094c
www.rsc.org/dalton
Introduction
The chemistry of low-coordinate germanium has received con-
siderable attention in recent years.1 Important bonding motifs
experimentally realized include two-coordinate germylenes2
and digermynes,3 as well as three-coordinate digermenes,4
silagermenes,5 and germachalcogenones6 on the other hand.
Since Robinson et al. reported the NHC-stabilized disilicon(0)
species Ia,7 the use of strong donors for the isolation of highly
reactive low-valent species by raising the coordination number
has drastically increased.8 In germanium chemistry, germy-
lene-type compounds (e.g. dihalogermylenes,9 digermanium(0)
Ib10), and inherently polar/polarizable multiple bonds (e.g. ger-
machalcogenones,11 digermynes12) are prominent examples
that are stabilized by base-coordination under retention of
remarkable reactivity. Very recently, we reported on a N-hetero-
Scheme 1 Chemical structures of Ia, Ib, II, III, and IV (Dip
=
2,6-iPr2C6H3, R = Tip = 2,4,6-iPr3C6H2, Mes = 2,4,6-Me3C6H2).
cyclic carbene stabilized silagermenylidene, Tip2SivGe·NHCiPr Me
2
2
II (NHCiPr Me = 1,3-diisopropyl-4,5-dimethylimidazol-2-ylidene,
Scheme 1).13 With the SivGe bond, the lone pair of electrons
and the coordination site of the NHC, compound II offers
various potential sites for further manipulation. Initially, we
demonstrated the clean [2 + 2] cycloaddition of an alkyne to
the SivGe bond.13 In view of the prominent role of carbon-
2
2
based vinylidene complexes in catalysis,14 an open question
remains the coordination behavior of isolable heavier vinyli-
denes towards transition metals.15 In the case of heavier ana-
logues of carbenes, transition metal coordination compounds
are known.16
Our recent isolation of a stable NHCiPr Me -stabilized aryl
(disilenyl)silylene III17 encouraged us to target the corres-
ponding disilenyl-substituted chlorogermylene 3. We thus
reacted disilenide 118 and NHC-coordinated germanium(II)
2
2
Krupp-Chair of General and Inorganic Chemistry, Saarland University,
66125 Saarbrücken, Germany. E-mail: scheschkewitz@mx.uni-saarland.de
†Electronic supplementary information (ESI) available: NMR and UV/vis spectra
chloride, NHCiPr Me ·GeCl2 29c (Scheme 2). Monosubstituted
NHC-coordinated chlorogermylenes IV (Scheme 1) have been
prepared via similar approaches.19
2
2
of all new compounds, X-ray crystallographic data (CIF) for 4-E, 5-E, and 6, and
computational details. CCDC 953520–953522. For ESI and crystallographic data
in CIF or other electronic format see DOI: 10.1039/c4dt00094c
This journal is © The Royal Society of Chemistry 2014
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Scheme 2 Synthesis of 4 (R = Tip = 2,4,6-iPr3C6H2).
Results and discussion
Surprisingly, instead of the targeted 3 the 1 : 1 reaction of 118
and 29c in toluene at −78 °C affords the NHC-coordinated sila-
germenylidene 4 in 62% yield (mp. 126–128 °C) (Scheme 2)
with an additional peripheral Si–Cl functionality (see the
Experimental section). The reaction plausibly proceeds
through the NHC-stabilized chloro(disilenyl)germylene 3 as a
transient followed by subsequent 1,3-migration of chlorine
from germanium to the β-silicon. In solution, the 29Si reson-
Fig. 1 Structure of 4-E in the solid state (thermal ellipsoids at 30%).
Hydrogen atoms are omitted. Selected bond lengths [Å]: Si1–Ge1 =
2.2757(10), Ge1–C46
= 2.061(4), Si1–Si2 = 2.3776(13), Si2–Cl1 =
2.1179(13).
ances of 4 at 162.5 and 7.3 ppm served as the first indication it is almost identical with that of the bulkily substituted sila-
germene (tBu3Si)2SivGeMes2 (2.2769(8) Å; Mes
=
2,4,6-
of the formation of a silagermenylidene due to the close simi-
larity to the low-field resonance of II (158.9 ppm).13 The red
Me3C6H2).5c As in II, the NHC coordinates to germanium in a
color of 4 is due to the longest wavelength absorption in the near-orthogonal manner with respect to the Si1–Ge1 bond
UV/vis spectrum at λmax = 451 nm (Table 1, ε = 9220 L mol−1
vector (C46–Ge1–Si1 101.90(10)°). The Ge1–C46 distance in 4-E
(2.061(4) Å) is between that of the simple silagermenylidene II
cm−1), which almost matches with that of compound II (λmax
=
455 nm). In contrast to II, however, the second absorption of 4 (2.0474(18) Å) and the GeCl2 precursor 2 (2.106(3) Å).9c
appears as a shoulder (4: λmax = 389 nm, II: λmax = 365 nm). To
gain more information about the origins of the UV/vis absorp-
Interestingly, in solution, 4-E slowly converts to a new com-
pound with 29Si NMR resonances at 134.0 and −0.2 ppm,
tions, we performed TD-DFT calculations of the silagermenyli- which we assign to stereoisomer 4-Z (Scheme 3). Equilibrium
dene II on the basis of the experimentally determined
molecular structure in the solid state. Solvent effects were
is reached after approximately 4 h in benzene-d6 at an E/Z ratio
of 0.85 : 0.15, essentially unaffected by temperature (+70 to
20
2
approximated using the Tomasi’s polarized continuum model −60 °C) or the presence of excess NHCiPr Me
(PCM) at the B3LYP/6-31G(d,p) level of theory.† The calculated
.
2
The calculated 29Si shifts [GIAO/B3LYP/6-31G(d,p) for H, C,
N, 6-311+G(2d,p) for Si, Ge, Cl] of the truncated model systems
lowest-energy excitation of II at 439 nm is predominantly
associated with the π–π* transition (HOMO → LUMO), in very for both isomers 4Dip-E and 4Dip-Z (R = Dip = 2,6-iPr2C6H3
good agreement with the experimental value of 455 nm. The
instead of Tip) are 159.5, 1.7 and 90.4, −9.8 ppm, respectively.†
second experimental absorption band at λmax = 365 nm is due
Although the experimental trend is reproduced, the absolute
to various excitations, but does contain a significant com- agreement of the calculated and the experimental values is
ponent originating from the n–π* transition (HOMO−1 →
better for the major isomer 4-E. The deviations presumably
arise from the neglect of dispersive forces that should
LUMO) as suspected in our previous communication.13
Crystals of 4 suitable for X-ray diffraction analysis were affect the sterically unfavorable isomer 4-Z considerably more
than 4-E.17
obtained from pentane at 25 °C. The structure in the solid
state (Fig. 1) confirmed the constitution of 4 as the sterically
Mills et al. had obtained the first structurally characterized
most favorable E-stereoisomer. The Ge1–Si1 bond length is by transition metal complexes of diphenylvinylidene from diphenyl-
2.2757(10) Å slightly longer than in II (2.2521(5) Å),13 whereas
ketene and Fe(CO)5.21 The reaction of 4-E/Z with Fe2(CO)9 in
Table 1 29Si{1H}a, 13C{1H} NMR, UV/vis data of 4-E, 4-Z, 5-Z, 5-E, and 6
4-Eb
4-Zb
5-Zc
5-Ec
6c
δ
δ
δ
δ
29Si (SiTip)
29Si (SiTip2)
13C (NCN)
162.5 (159.5)
7.3 (1.7)
178.4 (168.4)
—
134.0 (90.4)
−0.2 (−9.8)
178.3(165.1)
100.7
3.0
165.8
217.6
512; 427
98.1
113.7 (138.3)
91.5 (110.7)
—
−3.9
167.0
216.8
368d
13C Fe(CO)4
219.0, 216.0
λmax [nm]
451, 389
503
a Calculated values in parentheses. b In d6-benzene. c In d8-toluene. d In THF.
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Fig. 2 Structure of 5-E in the solid state (thermal ellipsoids at 30%).
Hydrogen atoms are omitted. Selected bond lengths [Å]: Si1–Ge1 =
2.2480(10), Ge1–C16 = 2.020(3), Ge1–Fe1 = 2.3780(6), Fe1–C59 = 1.772(4),
C59–O3 = 1.162(5), Si1–Si2 = 2.3767(12), Si2–Cl1 = 2.1118(11).
Scheme 3 Equilibrium of 4-E to 4-Z and synthesis of transition metal
complexes 5-Z, 5-E and 6 (a: THF, Fe2(CO)9; R = Tip = 2,4,6-iPr3C6H2;
NHCiPr Me = 1,3-diisopropyl-4,5-dimethylimidazol-2-ylidene).
2
2
THF at room temperature initially affords only the Z-stereo- geometry (Si1: 30.27 (1)°, Ge1: 21.07 (8)°). Another noteworthy
isomer of the silagermenylidene complex 5, which corresponds feature of 5-E is the twisting angle of 17.09(1)° between the
to the relative orientation of the chlorosilyl group and the planes of C1–Si–Si2 and Fe1–Ge1–C16. The bond length of
NHCiPr Me ligand in 4-E (Scheme 3) (see the Experimental Ge1–C16 is by 2.020(3) Å significantly shorter than in the free
section). The iron germenylidene complex 5-Z was isolated as a ligand (4-E: 2.061(4) Å). The Ge1–Fe1 bond length (2.3780(6) Å)
red-brown solid (mp. 140–142 °C) in 63% yield. In the case of is slightly longer than in germylene-coordinated iron(0)tetra-
germylenes, similar complexes have been reported.16b In the carbonyl complexes (LGeOH Å 2.330(1) Å,23b LGeF 2.3262(7)
13C NMR of 5-Z, the two downfield resonances at 216.8 and Å,23d L = CH{(CMe)2(2,6-iPr2C6H3N)2}). The C16–Ge1–Si1 bond
2
2
167.0 ppm were assigned to the carbonyl ligands at the Fe- angle is by 115.59(10)° much wider than that in silagermenyli-
22
center and coordinated NHCiPr Me , respectively. The 29Si denes (II,13 C31–Ge–Si 98.90(5)° and 4-E, C(46)–Ge(1)–Si(1)
NMR exhibits signals at 100.7 and 3.0 ppm; the formal sp2-Si 101.90(10)°). These structural parameters are reminiscent of
is substantially upfield shifted compared to that of the free the η1-vinyl coordination mode in Tip2SivSiTip-(Cl)ZrCp2.24
ligand 4-E (Table 1). As shown by the new 29Si signals at 98.1 In the light of the current discussion on the use of arrows in
and −3.9 ppm in a 1 : 1 ratio, 5-Z slowly – but in this case ir- the context of donor–acceptor interactions25 it should be
reversibly – rearranges to the stereoisomer, 5-E (mp. 158–160 °C) noted that obviously the formulation of 5-Z as zwitterionic
in solution. This is in contrast to the steric preferences of the complex 5′-Z is equally valid.
2
2
NHCiPr Me ligand in 4-E/Z, which, however, can readily be
When the isomerization process of 5-Z was carried out at
2
2
explained by the comparatively larger Fe(CO)4 moiety 65 °C, an additional product 6 is formed in 14% yield
(Scheme 3) (see the Experimental section). In the IR, the most (Scheme 3) along with the major product 5-E (56%) (see the
intense carbonyl stretching bands of 5-Z and 5-E are observed Experimental section). Notably, 6 cannot be obtained by
at 2018, 2010, 1923, 1899 cm−1 and 2011, 2008, 1917, heating an isolated sample of 5-E. Spatial proximity between
1897 cm−1, respectively. Apparently, silagermenylidenes 4-E/Z the Fe(CO)4 and SiTip2Cl moieties seems to be required for the
are somewhat stronger σ-donors compared to, for instance, the isomerization under loss of one CO ligand. By fractional crys-
N-heterocyclic carbene, NHCDip (NHCDipFe(CO)4:23a ν = 2035, tallization, we isolated 6 as yellow blocks (mp. 197–199 °C). In
1947, 1928, 1919 cm−1) (NHCDip = C{N(Ar)CH}2, Ar = 2,6-di- 29Si NMR, both resonances of silicon appear at 113.7 (SiTip)
isopropylphenyl) and intramolecularly base-coordinated germy- and 91.5 (SiTip2) ppm, which hints at the absence of saturated
lene LGeOH23b (CO stretching frequencies of LGe(OH)Fe(CO)4: silicon atoms, such as in the chlorosilyl side chain of 5-Z/E.
ν = 2039, 1956, 1942 cm−1) (L = CH{(CMe)2(2,6-iPr2C6H3N)2}).
An X-ray diffraction study on single crystals of 6 revealed a
Incidentally, the carbonyl stretching frequencies of 5-Z and 5-E bicyclo[1.1.0]butane-like butterfly structure with the Fe1 and
are very similar to those of the carbon-based vinylidene Ge1 in the bridgehead positions (Fig. 3). Apparently, a chlorine
complex H(CHO)CvCvFe(CO)2(P(OMe)3)2 (ν
=
2015, migration from the SiTip2 moiety to the SiTip moiety took
2007 cm−1).23c
place during conversion from 5-Z to 6. The 29Si NMR shifts of
The iron complex 5-E crystallizes from concentrated 6 are close to those of Ogino’s alkoxy- and amido-bridged bis
pentane solution (Fig. 2). The structural model confirmed that (silylene)iron complexes 7 (Scheme 4).26 In the present case,
the SivGe bond is retained upon coordination (Ge1–Si1 however, the bridging unit is the NHCiPr Me -stabilized germy-
2.2480(10) Å). Both formally sp2-hybridized heavier atoms, Si lene moiety so that an analogous electronic description
and Ge, are pyramidalized (∑ of angles: Ge1 354.74°; Si1 (6′, Scheme 4) probably contributes less than a zwitterionic
2
2
351.25°). The SivGe bond adopts
a strongly trans-bent resonance form of allylic type (6″).
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the peaks of residual protons of the deuterated solvent (1H) or
the deuterated solvent itself (13C). 29Si{1H} NMR spectra were
referenced to external SiMe4. UV/vis spectra were acquired
using a Perkin-Elmer Lambda 35 spectrometer using quartz
cells with a path length of 0.1 cm. IR spectra were recorded
using a Varian 2000 FT-IR FTS 2000 spectrometer. Melting
points were determined under argon in closed NMR tubes and
are uncorrected. Elemental analyses were performed using a
Leco CHN-900 analyzer.
Synthesis of 4-E: A precooled (–78 °C) solution of 1 (2.70 g,
3.17 mmol, in 30 mL of toluene) was transferred by cannula to
a suspension of 2 (1.02 g, 3.17 mmol, in 15 mL of toluene) at
−78 °C. The reaction mixture was allowed to warm slowly to
room temperature and stirred overnight. All the volatiles were
removed under vacuum and the solid residue dissolved in
30 mL of hexane. After filtration, the solution was concen-
trated to about 10 mL and kept overnight at room temperature.
The red precipitate was separated from the supernatant solu-
tion, washed with 5 mL of pentane and dried under vacuum to
yield 1.90 g (62%) of 4-E. Single crystals suitable for X-ray dif-
fraction were obtained from a saturated pentane solution after
keeping for 2 days at room temperature. Mp: 126–128 °C.
1H NMR (300 MHz, benzene-d6, TMS): δ 7.08 (s, 4H, TipH),
7.03 (s, 2H, TipH), 5.38 (2H, sept, NiPr–CH), 4.47–3.96 (m and
br, altogether 6H, o-iPr–CH), 2.83–2.65 (m, 3H, p-iPr–CH), 1.52
(s, 6H, CH3CvC), 1.33–1.11 (br and m, altogether 48H,
Fig. 3 Structure of 6 in the solid state (thermal ellipsoids at 30%).
Hydrogen atoms are omitted. Selected bond lengths [Å]: Ge1–C46 =
2.053(2), Ge1–Si1 = 2.3870(7), Ge1–Si2 = 2.3249(7), Fe1–Si1 = 2.3520(7),
Fe1–Si2 = 2.3166(8), Ge1–Fe1 = 2.6875(4).
i
iPr–CH3), 1.01 (d, 6H, Pr–CH3), 0.85 (d, 12 H, NiPr–CH3) ppm.
Scheme 4 Chemical structures of 7 and canonical forms of 6 (R = Tip =
2,4,6-iPr3C6H2).
13C NMR (75.4 MHz, benzene-d6, TMS): δ 178.45 (NCN),
154.41, 153.96, 149.51, 149.17, 142.48, 138.44 (TipCquart),
126.43 (NCCN), 122.51, 121.64 (TipCH), 53.92 (NiPr–CH),
35.74, 34.79, 34.53, 34.42 (iPr–CH), 25.64, 25.25, 24.95, 24.29,
24.09, 24.05 (iPr–CH3), 20.69 (N iPr–CH3), 10.05 (CH3CvC) ppm.
29Si NMR (59.5 MHz, benzene-d6, TMS): δ 162.50 (SiTip), 7.3
(SiTip2) ppm. UV/vis (hexane): λmax(ε) = 451 nm (9220 L mol−1
cm−1), 389 nm (sh). Anal. Calcd for C56H89ClGeN2Si2 (954.58):
C, 70.46; H, 9.40; N, 2.93. Found: C, 70.47; H, 9.47; N, 2.93.
Crystallographic data: C56H89ClGeN2Si2, Mr = 954.51, mono-
clinic, space group P2(1)/c, a = 20.3816(11), b = 10.9027(6), c =
This assertion finds support in the pertinent structural fea-
tures of 6. The averaged distance between Fe1 and Si1/Si2 in 6
is 2.3343(7) Å, shorter than that in the tetracarbonyl iron com-
plexes of a Z-1,2-dichlorodisilene (2.4358(6) Å, average dis-
tance),27 but longer than the Si–Fe distances in silylene–iron
complexes ((CO)4FevSi(Me)2·HMPA,28a 2.280(1) and 2.294(1) Å
for two crystallographic independent molecules; (CO)4Fev
Si(StBu)2·HMPA28b (2.278(1) Å) (HMPA = (NMe2)3PO/hexamethyl-
phosphoric triamide). The average distance between silicon
and germanium in 6 is 2.3560(7) Å, considerably longer than
that of the reported 2-germadisilaallene (2.2370(7) Å, average
distance).29 The mechanism for formation of 6 remains obscure.
However, the intramolecular activation of a silicon–silicon bond
in oligosilyl iron complexes has been reported30 and recently
Marschner et al. demonstrated the Lewis acids catalyzed
shuttling of germanium atoms into branched polysilanes.31
25.5288(13) Å, β = 90.840(3)°, V = 5672.3(5) Å3; Z = 4, ρc
=
1.118 g cm−3, T = 133(2) K, λ = 0.71073 Å, 48 361 reflections,
14 017 independent (Rint = 0.1264), R1 = 0.0619 (I > 2σ(I))
and wR2(all data) = 0.1332, GooF = 0.970, max/min residual
electron density: 0.827/−0.923 e Å−3
.
Equilibrium between 4-E and 4-Z and NMR data of 4-Z: In
solution 4-E isomerizes to 4-Z reaching equilibrium after about
4 h. The ratio of the two isomers was approximately 0.84 : 0.16
(4-E/4-Z). 4-Z: 1H NMR (300 MHz, benzene-d6, TMS): δ 7.24
(br, 2H, TipH), 7.21 (br, 1H, TipH), 6.97 (br, 1H, TipH), 6.77
(br, 1H, TipH), 5.67 (2H, sept, NiPr–CH), 4.75–4.54 (m and br,
altogether 2H, o-iPr–CH), 3.76–3.64 (m, 2H, o-iPr–CH), 1.80
Experimental section
General remarks
All experiments were carried out under a protective atmos- (d, 3H, iPr–CH3), 1.76–1.69 (m, 6H, iPr–CH3), 1.61 (s, 6H,
phere of argon applying standard Schlenk techniques or in a CH3CvC), 0.61 (d, 3H, iPr–CH3), 0.47 (d, 3H, iPr–CH3), 0.40 (d,
i
i
glove box. All the solvents were refluxed over sodium/benzo- 3H, Pr–CH3), 0.30 (d, 3H, Pr–CH3) ppm. 13C NMR (75.4 MHz,
phenone, distilled and stored under argon. Benzene-d6, [D6]benzene, TMS): δ 178.33 (NCN) ppm. 29Si NMR (59.5 MHz,
toluene-d8, and THF-d8 were dried and distilled over potassium [D6]benzene, TMS): δ 134.02 (SiTip), −0.20 (SiTip2) ppm
under argon. H and 13C{1H} NMR spectra were referenced to (minor isomer).
1
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Synthesis of 5-Z: Dry and degassed THF (30 mL) was added (59.5 MHz, toluene-d8, TMS): δ 98.14 (SiTip), −3.89 (SiTip2)
Schlenk flask containing compound 4-E (1.90 g, ppm. UV/vis (hexane): λmax(ε) = 512 nm (7050 Lmol−1 cm−1),
to
a
1.99 mmol) and Fe2(CO)9 (0.80 g, 2.19 mmol) at room tempera- 427 nm (6670) nm (L mol−1 cm−1). IR (KBr, cm−1): ν = 2011 (s),
ture. The color of the reaction mixture changed immediately to 2008 (s), 1917 (s) 1897 (s). Anal. Calcd for C60H89ClFeGeN2-
deep red. The reaction mixture was stirred for another 4 h and O4Si2 (1122.47): C, 64.20; H, 7.99; N, 2.50. Found: C, 64.15;
all the volatiles were removed under vacuum. The solid residue H, 7.90; N, 2.26. Crystallographic data: C60H89ClFeGeN2-
ˉ
was extracted with 80 mL of hexane and the resulting solution O4Si2·0.25C5H12, Mr = 1140.44, triclinic, space group P1, a =
was filtered to remove insoluble impurities. The hexane was 13.2910(4), b = 19.9842(5), c = 24.8636(7) Å, α = 89.1430(10), β =
distilled off under vacuum. After addition of 20 mL of 95.311(2), γ = 76.122(2)°, V = 6407.6(3) Å3; Z = 4, ρc = 1.182 g cm−3
,
pentane, 1.40 g (63%) of 5-Z were isolated as a dark-red solid. T = 123(2) K, λ = 0.71073 Å, 105 034 reflections, 28 114 inde-
1
Mp: 140–142 °C. H NMR (300 MHz, toluene-d8, TMS): δ 7.26, pendent (Rint = 0.0436), R1 = 0.0619 (I > 2σ(I)) and wR2 (all
7.18, 6.86, 6.81, 6.65 (d, each having 1H, TipH), another Tip-H data) = 0.1744, GooF = 1.427, max/min residual electron
signal is masked by residual proton signals of toluene-d8, 5.83 density: 2.142/−0.789 e Å−3
.
i
i
(sept, 1H, Pr–CH), 5.09 (sept, 1H, Pr–CH), 5.00 (sept., 1H,
Synthesis of 6: A solution of 5-Z (0.50 g, 0.44 mmol) in
iPr–CH), 4.55 (sept, 1H, iPr–CH), 4.07 (sept, 1H, iPr–CH), toluene (30 mL) was stirred in a sealed Schlenk flask overnight
i
i
3.80–3.62 (m, 2H, Pr–CH), 2.78 (sept, 1H, Pr–CH), 2.70–2.53 at 65 °C. All the volatiles were removed under vacuum and the
i
i
i
(m, 2H, Pr–CH), 2.48 (sept, 1H, Pr–CH), 1.73 (d, 3H, Pr–CH3), solid residue extracted with 40 mL of hexane. The solution was
1.67–1.51 (s and m, altogether 15H, Pr–CH3 and CH3CvC), concentrated to about 20 mL and after keeping at room temp-
i
i
1.45–1.39 (s and m, altogether 6H, Pr–CH3 and CH3CvC), erature for two days afforded yellow blocks of 6 (0.075 g, 14%).
i
i
1.34–1.25 (m, 9H, Pr–CH3), 1.22 (d, 6H, Pr–CH3), 1.17–1.16 Keeping the mother liquor at −20 °C for a week afforded
i
i
i
(m, 9H, Pr–CH3), 1.09–1.03 (m, 6H, Pr–CH3), 0.48 (d, 3H, Pr– brown-red crystals of 5-E (0.28 g, 56%). 6: Mp: 197–199 °C.
i
i
CH3), 0.43 (d, 3H, Pr–CH3), 0.37 (d, 3H, Pr–CH3), 0.20–0.11 1H NMR (300 MHz, toluene-d8, TMS): δ 7.23 (d, 1H, TipH),
i
(m, 9H, Pr–CH3) ppm. 13C NMR (75.4 MHz, toluene-d8, TMS): 7.13 (1H, TipH, masked by toluene-d8), 7.02 (sept, 1H, NiPr–
δ 217.56 (CO), 165.81 (NCN) ppm. (We were unable to assign CH), 7.01 (1H, TipH, masked by toluene-d8), 6.98–6.95 (m, 2H,
other signals correctly, because they overlap with its isomer TipH), 6.87 (d, 1H, TipH), 6.41 (sept, 1H, NiPr–CH), 5.25 (sept,
i
i
i
5-E.) 29Si NMR (59.5 MHz, toluene-d8, TMS): δ 100.67 (SiTip), 1H, Pr–CH), 4.86 (sept, 1H, Pr–CH), 4.35 (sept, 1H, Pr–CH),
i
i
2.98 (SiTip2) ppm. Mp.: 140–142 °C. UV/vis (hexane): λmax(ε) = 4.22 (sept, 1H, Pr–CH), 3.61 (sept, 1H, Pr–CH), 3.44 (sept, 1H,
503 nm (8260 Lmol−1 cm−1). IR (KBr, cm−1): ν = 2018 (s), iPr–CH), 2.79 (sept, 1H, Pr–CH), 2.77 (sept, 1H, Pr–CH), 2.65
i
i
i
i
2010 (s), 1923 (s), 1899 (s). Anal. Calcd for C60H89ClFeGeN2- (sept, 1H, Pr–CH), 1.87–1.78 (m, 6H, Pr–CH3), 1.68 (s, 3H,
O4Si2 (1122.47): C, 64.20; H, 7.99; N, 2.50. Found: C, 64.10; CH3CvC), 1.66 (s, 3H, CH3CvC), 1.56–1.37 (m, altogether
i
i
H, 7.74; N, 2.63.
18H, Pr–CH3), 1.31–1.04 (m, altogether 30H, Pr–CH3), 0.72 (d,
i
i
i
Synthesis of 5-E: A solution of 5-Z (1.00 g, 0.89 mmol) in 3H, Pr–CH3), 0.48 (d, 3H, Pr–CH3), 0.45 (d, 3H, Pr–CH3), 0.19
i
1
toluene (60 mL) was stirred for five days at room temperature. (d, 3H, Pr–CH3) ppm. H NMR (300 MHz, thf-d8, TMS): δ 7.03
All volatiles were removed under vacuum and the solid residue (d, 1H, TipH), 6.96 (d, 1H, TipH), 6.95 (sept, 1H, NiPr–CH),
extracted with 70 mL of hexane. The solution was concentrated 6.80 (d, 1H, TipH), 6.76 (d, 1H, TipH), 6.71 (d, 1H, TipH), 6.67
to about 20 mL and after keeping at −20 °C for a week (d, 1H, TipH), 6.19 (sept, 1H, NiPr–CH), 4.82 (sept, 1H, Pr–
i
afforded brown-red crystals of 5-E (0.77 g, 76%). Mp: CH), 4.61 (sept, 1H, Pr–CH), 4.15 (sept, 1H, iPr–CH), 3.82
i
1
i
i
158–160 °C. H NMR (300 MHz, toluene-d8, TMS): δ 7.27–7.23 (sept, 1H, Pr–CH), 3.34–3.18 (m, 2H, Pr–CH), 2.87–2.68 (m,
i
i
(m, 2H, TipH), 7.21 (d, 1H, TipH), 7.05 (1H, TipH, masked by 2H, Pr–CH), 2.63 (sept, 1H, Pr–CH), 2.41 (s, 3H, CH3CvC),
i
i
toluene-d8), 6.96 (d, 1H, TipH), 6.73 (d, 1H, TipH), 5.72 (sept, 2.35 (s, 3H, CH3CvC), 1.85 (d, 3H, Pr–CH3), 1.62 (d, 3H, Pr–
1H, NiPr–CH), 5.51 (sept, 1H, NiPr–CH), 4.62 (sept, 1H, CH3), 1.58–1.49 (m, altogether 9H, Pr–CH3), 1.44 (d, 3H, Pr–
i
i
iPr–CH), 4.54 (sept, 1H, Pr–CH), 4.30 (sept, 1H, Pr–CH), 3.67 CH3), 1.24 (d, 3H, iPr–CH3), 1.20–1.11 (m, altogether 21H,
i
i
(sept, 1H, Pr–CH), 3.57 (sept, 1H, Pr–CH), 2.88–2.62 (m, 4H, iPr–CH3), 1.08–1.03 (m, altogether 9H, Pr–CH3), 0.88 (d, 3H,
i
i
i
iPr–CH), 2.00 (d, 3H, Pr–CH3), 1.79 (d, 3H, Pr–CH3), 1.68–1.46 iPr–CH3), 0.45 (d, 3H, iPr–CH3), 0.18–0.14 (m, altogether
i
i
(s and m, altogether 30H, Pr–CH3 and CH3CvC), 1.27–1.21 6H, iPr–CH3), −0.20 (d, 3H, iPr–CH3) ppm. 13C NMR (75.4 MHz,
i
i
i
(m, 12H, Pr–CH3), 1.19–1.15 (m, 12H, Pr–CH3), 0.56 (d, 3H, thf-d8, TMS): δ 218.99, 215.98 (CO), 157.19, 156.89, 155.09,
iPr–CH3), 0.43 (d, 3H, Pr–CH3), 0.34 (d, 3H, Pr–CH3), 0.24 (d, 154.75, 154.08, 152.28, 150.90, 150.71, 149.48, 143.26, 142.18,
i
i
3H, Pr–CH3) ppm. 13C NMR (75.4 MHz, toluene-d8, TMS): 136.52 ((TipCquart), 129.52, 129.26 (NCCN), 123.68, 122.72,
i
δ 216.85 (CO), 166.99 (NCN), 156.72, 155.76, 155.69, 154.05, 122.66, 122.06, 121.88, 121.43 (TipCH), 56.24, 54.08 (NiPr–CH),
153.23, 152.37, 151.32, 150.41, 137.67, 133.07, 130.55, 127.35, 38.20, 36.65, 36.46, 35.29, 35.39, 34.98, 34.83, 34.16, 30.24
127.00 (TipCquart and NCCN), 124.07, 123.37, 122.53, 122.40, (iPr–CH), 28.21, 26.76, 26.28, 25.97, 25.65, 25.48, 24.91, 24.38,
122.12, 120.96 (TipCH), 55.53, 54.84 (NiPr–CH), 38.48, 38.11, 24.36, 24.28, 24.18, 23.99, 23.90, 23.84, 23.40, 22.88, 22.23,
37.20, 35.24, 34.81, 34.62, 34.47, 33.97, 31.04 (iPr–CH), 32.00, 21.98 (iPr–CH3), 11.21, 10.73 (CH3CvC) ppm (we did not
30.66, 27.80, 27.30, 26.00, 25.36, 25.16, 24.83, 24.30, 24.16, observe the carbenic carbon resonance). 29Si NMR (59.5 MHz,
24.02, 23.97, 23.92, 23.87, 23.05, 22.50, 22.43, 22.40, 21.87, toluene-d8, TMS):
δ 113.66 (SiTip), 91.46 (SiTip2) ppm.
20.83, 14.29 (iPr–CH3) 10.17, 9.92 (CH3CvC) ppm. 29Si NMR 29Si NMR (59.5 MHz, [D8]THF, TMS): δ 111.36 (SiTip), 88.27
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(SiTip2) ppm. UV/vis (THF): λmax(ε) = 368 nm (6820 Lmol−1
cm−1). IR (KBr, cm−1): ν = 1982 (s), 1920 (s), 1916 (s). Anal.
Calcd for C59H89ClFeGeN2O3Si2 (1094.46): C, 64.75; H, 8.20;
N, 2.56. Found: C, 65.35; H, 8.08; N, 2.38. Crystallographic
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A. J. Thorne, J. Chem. Soc., Chem. Commun., 1984, 480–482;
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data: C59H89ClFeGeN2O3Si2, Mr
= 1094.39, orthorhombic,
space group Pbca, a = 19.6288(5), b = 24.6607(7), c = 24.6974(7)
Å, V = 11 955.0(6) Å3; Z = 8, ρc = 1.216 g cm−3, T = 132(2) K,
λ = 0.71073 Å, 104 743 reflections, 14 312 independent (Rint
=
0.0577), R1 = 0.0442 (I > 2σ(I)) and wR2 (all data) = 0.1131, GooF =
1.022, max/min residual electron density: 1.184/−0.536 e Å−3
.
Conclusions
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We have shown an efficient method for the synthesis of side
chain-functionalized silagermenylidene stabilized by coordi-
nation of an N-heterocyclic carbene. Its suitability as a ligand
for transition metal complexes was demonstrated by coordi-
nation to the Fe(CO)4 fragment. Moreover, the resulting
silagermenylidene iron complex thermally rearranges to an
apparently more stable complex of unprecedented allylic struc-
ture, which is undoubtedly a consequence of the ease of
migration of the residual chlorine functionality.
Acknowledgements
Support for this study was provided by the EPSRC (EP/
H048804/1), the Alfried Krupp von Bohlen und Halbach Foun-
dation. We thank Dr Carsten Präsang for assistance with the
IR measurement.
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