A silicon–carbonyl complex stable at room temperature

Article abstract of DOI:10.1038/s41557-020-0456-x

Main-group-element compounds with energetically high-lying donor and low-lying acceptor orbitals are able to mimic chemical bonding motifs and reactivity patterns known in transition metal chemistry, including small-molecule activation and catalytic reactions. Monovalent group 13 compounds and divalent group 14 compounds, particularly silylenes, have been shown to be excellent candidates for this purpose. However, one of the most common reactions of transition metal complexes, the direct reaction with carbon monoxide and formation of room-temperature isolable carbonyl complexes, is virtually unknown in main-group-element chemistry. Here, we show the synthesis, single-crystal X-ray structure, and density functional theory computations of a room-temperature-stable silylene carbonyl complex [L(Br)Ga]₂Si:–CO (L = HC[C(Me)N(2,6-ⁱPr₂-C₆H₃)]₂), which was obtained by direct carbonylation of the electron-rich silylene intermediate [L(Br)Ga]₂Si:. Furthermore, [L(Br)Ga]₂Si:–CO reacts with H₂ and PBr₃ with bond activation, whereas the reaction with cyclohexyl isocyanide proceeds with CO substitution. [Figure not available: see fulltext.]

Full text of DOI:10.1038/s41557-020-0456-x

Articles
https://doi.org/10.1038/s41557-020-0456-x
A silicon–carbonyl complex stable at room
temperature
Chelladurai Ganesamoorthy1, Juliane Schoening1, Christoph Wölper1, Lijuan Song2,3,
1
ꢀ✉
Peter R. Schreinerꢀ ꢀ2,3 and Stephan Schulzꢀ ꢀ
ꢀ✉
Main-group-element compounds with energetically high-lying donor and low-lying acceptor orbitals are able to mimic chemi-
cal bonding motifs and reactivity patterns known in transition metal chemistry, including small-molecule activation and cata-
lytic reactions. Monovalent group 13 compounds and divalent group 14 compounds, particularly silylenes, have been shown
to be excellent candidates for this purpose. However, one of the most common reactions of transition metal complexes, the
direct reaction with carbon monoxide and formation of room-temperature isolable carbonyl complexes, is virtually unknown in
main-group-element chemistry. Here, we show the synthesis, single-crystal X-ray structure, and density functional theory com-
putations of a room-temperature-stable silylene carbonyl complex [L(Br)Ga]2Si:–CO (Lꢀ=ꢀHC[C(Me)N(2,6-iPr2-C6H3)]2), which
was obtained by direct carbonylation of the electron-rich silylene intermediate [L(Br)Ga]2Si:. Furthermore, [L(Br)Ga]2Si:–CO
reacts with H2 and PBr3 with bond activation, whereas the reaction with cyclohexyl isocyanide proceeds with CO substitution.
helastdecadehaswitnessedremarkableanalogiesinthechem- react with CO to give non-covalent adducts R2Si:–CO I (Fig. 1)
ical bonding of transition-metal and main-group-element that are only stable under cryogenic matrix isolation conditions
T
compounds1,2. Heavy main-group-metal compounds with a in solid argon; they readily dissociate with elimination of CO upon
non-bonding (donor) electron pair and an energetically low-lying warming27–34. Uncomplexed silaketenes R2SiCO II (R=H, Me)
vacant (acceptor) orbital and main-group-element compounds with are transition structures for the degenerate interconversion of I30.
π-bonding contributions are able to mimic the σ-donor/π-acceptor Two bis-silylenes react with CO with selective deoxygenative
concept that is typical for bonding in transition metal complexes, homocoupling to disilylketenes III35. The importance of the
resulting in transition metal-like chemical reactivity including the Si···Si distance in the bis-silylene compound on CO binding and
activation of small molecules, stabilization of highly reactive spe- activation has been shown, and mechanistic studies have proved
cies3–5 and catalytic reactions6. However, one typical reaction of that CO acts as a Lewis acid in the initial step of the reaction.
transition metal complexes, the formation of room-temperature In contrast, the boryl-substituted acyclic silylene {Me3Si(Dipp)
isolable carbonyl complexes by reaction with carbon monox- N}{(HCNDipp)2B}Si: (Dipp=2,6-diisopropylphenyl) reacts with
ide (CO), is almost unknown for non-d-block elements, most CO with reductive C–C coupling and formation of IV36, whereas
probably due to the absence of suitable π back-bonding orbitals. oxidative addition occurred with H2 and C–H bonds at ambient
Carbonyl complexes of s-block elements are limited to homolep- temperature37. Furthermore, the reaction of an acyclic germylene
tic alkaline earth complexes M(CO)8 isolated in neon matrices at with CO was reported, which yielded compound V38,39. We are not
4K (refs. 7,8), whereas simple CO adducts of Lewis-acidic boranes, aware of any group 14 element carbonyl complex that is stable at
that is, BF3, BH3 (ref. ), and polyboranes10, as well as the dicar- ambient conditions.
9
bonyl compound TpB(CO)2 (Tp=2,6-[2,4,6-iPr3-C6H2]2-C6H3)11,
are the only room-temperature-stable p-block carbonyl com- Results
plexes. Phosphinophosphaketenes behave as (phosphino) Group 13 diyls LM (M=Al, Ga; L=HC[C(Me)N(2,6-iPr2-C6H3)]2)
phosphinidene-carbonyl adducts12, and the OCP− monoanion serve as two-electron reducing agents40, and recent studies have
undergoes reductive decarbonylation reactions with early tran- further demonstrated their capability in oxidative addition reac-
sition metal complexes13, so both are regarded as ‘masked’ car- tions. The L(X)Ga ligand shows superior properties for stabilizing
bonyl complexes3,14. In contrast, reactions of multiply bonded unusual bonding situations of group 15 element compounds41–43
,
main-group-element compounds15–19 and frustrated Lewis pairs resulting from the electropositive nature of the L(X)Ga metal frag-
(FLPs)20–22 with CO proceed with CO activation rather than with ment. We thus became interested in reactions of LGa with SiBr4, and
the formation of CO complexes.
we here demonstrate the generation of a silylene intermediate that is
So far, homoleptic carbonyl complexes of heavier group 14 ele- very reactive towards small molecules, including H2, CO and CO2.
ments have only been reported for matrix isolation studies23, includ-
ing SiCO, Si(CO)224 and larger silicon cluster carbonyls25. Reactions Synthesis, characterization and reactivity studies. Reaction of LGa
of carbenes R2C: (R=H, alkyl, aryl and so on) and silylenes R2Si: and SiBr4 yields L(Br)GaSiBr3 (1), which reacts with one additional
with CO have also been investigated. Singlet carbenes react with CO equivalent of LGa to give [L(Br)Ga]2SiBr2 (2) (Fig. 2). Compound 2
with formation of room-temperature-stable ketenes R2CCO26, while was isolated in low yield due to its thermal instability and reactivity
silylene H2Si: and diorganosilylenes R2Si: (R=alkyl, Cp*=C5Me5) towards additional amounts of LGa (Supplementary Figs. 36 and 37).
1Institute for Inorganic Chemistry and Center for Nanointegration Duisburg-Essen (Cenide), University of Duisburg–Essen, Essen, Germany. 2Institute
of Organic Chemistry, Justus Liebig University, Giessen, Germany. 3Center for Materials Research (LaMa), Justus Liebig University, Giessen, Germany.
✉
e-mail: prs@uni-giessen.de; stephan.schulz@uni-due.de
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tBu
Ph
tBu
Ar
tBu
Ph
tBu
N
Ar
B
Ar
N
O
Ar
Si
N
C
Si
N
N
C
O
O
C
O
O
R
R
C
N
O
SiMe3
Ar
Si
Si
C
iPr
Si
C
O
Fe
C
O
O
C
C
O
O O C
Si
R
R
Me3Si
O
N
C
Ge
Ar'
tBu
Si
Si
N
C
Ar
N
tBu
B
Ar'
tBu
Ph
N
Ar
IV
tBu
Ph
I
II
III
V
Fig. 1 | A selection of Si and Ge compounds formed in reactions of silylenes and germylene with CO. Silylene carbonyl adducts (i) were found under
cryogenic conditions, whereas silaketenes (ii) are transient structures. Stable singlet silylenes and germylenes react with CO, with formation of
compounds iii–V (Rꢀ=ꢀalkyl chain; Phꢀ=ꢀphenyl; tBuꢀ=ꢀtert-butyl; iPrꢀ=ꢀisopropyl; Arꢀ=ꢀ2,6-iPr2-C6H3; Ar′ꢀ=ꢀ2,4,6-Me3-C6H2).
Ar
Ga
Ar
NMR spectra of 1 and 2 show the characteristic resonances of the
1
L(Br)Ga
L(Br)Ga
Br
Br
N
N
L(Br)Ga
Br
Br
Br
β-diketiminate ligand. The H NMR spectra show two septets and
LGa
LGa
SiBr4
Si
LGa
Si
four doublets for the methine and methyl groups of the isopropyl
substituents, respectively, and single resonances for the γ-CH and
two methyl groups of the C3N2Ga rings. The 13C{1H} NMR spectra of
1 and 2 show all symmetry-related signals, including the characteris-
tic resonances due to the γ-CH backbone carbon atom.
C6H6
60 °C
C6H6
25 °C
1
2
Ar
N
L(Br)Ga
L(Br)Ga
L(Br)Ga
Br
Br
LGa
Compound 2 was heated in C6D6 to 60°C for 42h (Supplementary
Fig. 36) to force the formation of [L(Br)Ga](Br)Si: by intramo-
lecular elimination of LGaBr2, as observed for stibines [L(X)
Ga]2SbX (X=Cl, Br)43, but only LGaBr2 and elemental Si formed.
To trap the potentially formed silylene intermediate [L(Br)Ga](Br)
Si:, 1equiv. of 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene
Si
L(Br)Ga Si Ga
Si
C6H6, 60 °C
–LGaBr2
C6H6, 60 °C
Br
L(Br)Ga
N
2
Ar
3
CO
Cy
N
DippNHC; NHC=N-heterocyclic carbene) was added, but LGaBr2
O
(
and traces of DippNHC–SiBr2 and LGa again formed rather than the
NHC-stabilized silylene [L(Br)Ga](Br)Si–DippNHC. We therefore
reacted 2 with different reductants to form the Ga-coordinated
silylene [L(Br)Ga]2Si:. The reaction with 1equiv. of LGa in benzene
at 60°C yielded LGaBr2 and compound 3 (Supplementary Fig. 37)
rather than silylene [L(Br)Ga]2Si: as shown by in situ NMR spec-
troscopy. In addition, reduction reactions with KC8 and [LMg]2
(Supplementary Figs. 40 and 41) in C6D6 at 60°C also gave 3, but in
C
L(Br)Ga
L(Br)Ga
L(Br)Ga
L(Br)Ga
H
C
L(Br)Ga
L(Br)Ga
H2
CyNC
Si
Si
Si
C6H6, 60 °C
C6H6, 25 °C
–CO
H
4
5
6
Fig. 2 | Synthesis of compounds 1–6 and exemplary reactions of 5.
Reactions of 2ꢀequiv. of gallanediyl LGa with SiBr4 proceed stepwise
with insertion of LGa into two Si–Br bonds and formation of 1 and
2. Compound 2 reacts under an argon atmosphere with 1ꢀequiv. of
LGa leading to elimination of LGaBr2 and formation of the silylene
1
lower yields. The H NMR spectrum of compound 3 shows seven
septets, eleven doublets and one multiplet for the magnetically
inequivalent isopropyl substituents. In addition, four singlets for
the γ-CH, the SiCH and ArNCCH3 methyl protons bound to the Si
atom are observed. The 13C{1H} NMR spectrum of 3 displays three
resonances for the ArNCCH3 and two resonances for the γ-CH and
SiCH carbon atoms, while the 29Si{1H} NMR spectrum shows a sin-
glet at −33.1ppm.
intermediate [L(Br)Ga]2Si:, which immediately undergoes intramolecular
C–C bond activation to give 3. In contrast, analogous reactions under a
H2 and CO atmosphere proceed with formation of silane 4 and the stable
silylene carbonyl complex 5, which reacts with cyclohexyl isocyanide
with CO substitution and formation of 6 (CyNCꢀ=ꢀcyclohexyl isocyanide;
Arꢀ=ꢀ2,6-iPr2-C6H3).
The formation of 3 can be rationalized with the intermediacy
of a reactive silylene, [L(Br)Ga]2Si:, that activates the C–C bond of
the ‘L’ ligand (3). A comparable intramolecular cyclopropanation (Supplementary Fig. 43), yielding [L(Br)Ga]2SiH2 (4) and [L(Br)
reaction has been reported for an alkali-metal-substituted silylene44. Ga]2Si:–CO (5) (Fig. 2). Silane 4 and carbonyl complex 5 formed
To circumvent the C–C bond activation reaction and to isolate the via the silylene intermediate [L(Br)Ga]2Si: as was proven in control
silylene [L(Br)Ga]2Si:, 2 was reacted with LGa in benzene at ambi- reactions of 2 with H2 and CO in the absence of LGa (Supplementary
ent temperature, but the reaction proceeded slowly over a period of Figs. 38 and 39), which yielded LGaBr2 and elemental Si rather than
one month, again with formation of 3 and LGaBr2. The attempted 4 and 5. Compound 5 also formed in 30% yield in the reaction of
stabilization of [L(Br)Ga]2Si: by coordination of strong Lewis bases [L(Br)Ga]2SiBr2 (2) with 1equiv. of LGa in C6D6 at 60°C under a CO2
such as 4-dimethylaminopyridine (DMAP), EtNHC (EtNHC=1, atmosphere (Supplementary Fig. 43) by selective reduction of CO2
3-diethylimidazol-2-ylidene), DippNHC and coordinating solvents to CO, as was previously observed in reactions of a boryl-substituted
(THF, Et2O) failed and only 3 formed, as was also observed in the acyclic silylene and a disilyne bisphosphine adduct37,45. As-formed
reaction of carbene-stabilized DippNHC–SiBr4 with 3equiv. of LGa CO then reacts with the silylene intermediate to 5 in 30% yield.
(Supplementary Fig. 44). These findings indicate that silylene [L(Br)
Compounds 4 and 5 can be stored under an argon atmosphere at
Ga]2Si: is highly reactive and, once formed, immediately undergoes room temperature for weeks. The 1H NMR and 13C{1H} spectra of 4
C–C bond activation.
and 5 are very similar and each show the expected resonances due to
The high reactivity of silylene [L(Br)Ga]2Si: renders this com- the β-diketiminate ligand as well as singlets at 2.12ppm (1H NMR,
pound very promising for small-molecule activation reactions, as 4) and 206.5ppm (13C{1H} NMR, 5) for the SiH2 and SiCO groups.
was confirmed in reactions of 2 with 1equiv. of LGa in benzene The 29Si{1H} NMR spectra of 4 and 5 display singlets at −130.6 and
at 60°C under H2 (Supplementary Fig. 42) and CO atmosphere −256.5ppm, respectively. The presence of the SiH2 and SiCO groups
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C9
C54
C10
C11
C8
C7
C16
C9
C14
C49
C55
C13
C6
C17
C6
C8
C50
C12
C53
C47
C10
C15
C34
C48
C51
C7
C3
C57
C4
C52
C11
N1
C56
C45
N4
C14
C12
C32
C13
C1
N1
C16
C1
C15
C58
C44
C17
Ga1
Br1
Ga2
C2
C2
C4
Br1
N2
Si1
Ga1
Br2
Si1
C46
N2
C3
N3
C40
C37
C25
C20
C31
C35
C24
C30
O1
C30
C26
C39
C26
C27
C24
C33
C5
C5
C19
C21
C29
C42
C43
C23
C36
C38
C25
C19
C20
C23
C22
C41
C22
C29
C21
3
5
C29
C62
C21
C20
C22
C29
C22
C28
C58
C63
C57
C28
C27
C61
C23
C20
C56
C52
C23
C18
N2
C27
C50
C49
C60
C64
C65
C5
C18
C19
C19
C5
N5
C47
Br1
C26
N2
C34
C24
C3
C26
C48
Ga2
C53
C59
C25
H1
C3
C24
Br2
Ga1
C32
C31
C30
N4
C54
C25
C2
C2
Br1
N1
Si1
Ga1
Si1
C55
N1
N3
C1
C1
C46
C44
C12
C12
C13
C14
C17
C14
C4
C15
C4
C33
C45
C15
C11
C13
C43
C7
C36
C11
C10
C17
C39
C8
C9
C16
C8
C42
C37
C10
C38
C9
4
6
Fig. 3 | molecular structures of compounds 3–6 as derived from single-crystal X-ray crystallography. Thermal ellipsoids in the structures represent the
30% probability level. For clarity, hydrogen atoms have been removed except in 4, where Si–H are displayed as spheres of arbitrary radii.
in 4 and 5 was also confirmed by infrared spectroscopy, showing (Mes=2,4,6-Me3-C6H2; tbt=2,4,6-[CH(SiMe3)2]3–C6H2) show
stretching vibrations at νSi–H 2,112cm−1 and νSi-CO 1,945cm−1 (solid) either one (R=Tip =2,4,6-iPr3-C6H2, λmax =596nm) or two absorp-
and 1,943cm−1 (solution in mesitylene), respectively. The carbonyl tion bands (R=tbt, λmax =397, 632nm; Mes*=2,4,6-tBu3-C6H2,
stretching vibration in 5 is shifted to lower wavenumbers compared
λ .
max =390, 671nm)46,47
to Cp*2Si–CO (2,065cm−1)33 and H2Si–CO (2,043cm−1)30, but agrees
with values reported for Me2Si–CO (1,962cm−1) and Me(Ph)Si–CO Solid-state structures. The molecular structures of 1–6 were
(1,988cm−1)32. The UV–vis spectrum of 5 shows a sharp absorption determined by single-crystal X-ray diffraction analyses. Crystals
band at 358nm and a very broad absorption band at 444nm.
of 1 (Supplementary Fig. 51), 2 (Supplementary Fig. 52), 3, and 5
Compound 5 is stable in toluene solution up to 80°C but reacts were obtained from benzene solution at room temperature,
to LGa, LGaBr2 and 3 on irradiation with UV light or on heating whereas crystals of 4 and 6 formed in solutions in n-hexane (Fig. 3).
at 100°C (Supplementary Figs. 44 and 45). Compound 5 serves as Compounds 1, 4, 5, and 6 crystallize in the monoclinic space
masked silylene, as was shown in CO elimination reactions with groups P21/n (1), C2/c (4, 5) and P21/c (6), while 2 and 3 crystallize
H2 and PBr3, yielding [L(Br)Ga]2SiH2 (4) and [L(Br)Ga]2SiBr2 (2), in the triclinic space group P-1. Selected bond lengths and angles
respectively. However, the reaction of 5 with (coe)Cr(CO)5 are provided in Table 1.
(coe=cyclooctene) gave LGaBr2 and LGaCr(CO)5 (Supplementary
The molecular structure of 1 is similar to that of LGa(Cl)SiCl3
Fig. 49) rather than the silylene complex [L(Br)Ga]2SiCr(CO)5, and (ref. 48). The Ga and Si atoms in 1 to 4 each adopt a distorted tet-
5 failed to react with N-heterocyclic carbenes (DippNHC, EtNHC), rahedral geometry, whereas the Si atoms in 5 and 6 are triply coor-
whereas its reaction with sterically less hindered cyclohexyl iso- dinated. The six-membered GaN2C3 rings show typical boat-type
cyanide CyNC (Cy=cyclohexyl) yielded [L(Br)Ga]2Si:‒CNCy (6). conformations41,43. The Ga–Si bond lengths in 1, 3, and 4 are slightly
1
The H and 13C{1H} NMR spectra of 6 are comparable to those shorter than in 2, 5, and 6, respectively. The Ga–Si–Ga bond angles
of 5, whereas the 29Si{1H] NMR spectrum of 6 (‒175.4ppm) in 2, 4, and 6 are almost identical, whereas those in 3 and 5 deviate
shows a large downfield shift (5, ‒256.5ppm). The UV–vis spec- substantially. The Si–C–O (5) and Si–C–N (6) units are almost lin-
trum of 6 shows a single absorption band at λmax =357nm, ear and the Ga–Si–C bond angles in 5 and 6 indicate a large p-orbital
while other isocyanide silylene complexes Mes(tbt)Si:‒CNR contribution in the bonding electron pair, as is expected for a singlet
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Table 1 | Selected bond lengths (Å) and angles (°) of compounds 1–6
1
2
3
4
5
6
Ga–Si
2.3992(14)
2.4247(5), 2.4344(5) 2.3919(7), 2.3628(7) 2.3788(7)
2.4203(5)
1.865(6)
1.136(7)
2.3777(3)
122.73(4)
172.7(6)
92.97(15)
2.4107(5), 2.4058(5)
1.8419(19)
Si–C
–
–
–
1.864(2), 1.939(2)
–
–
–
C–O/N
Ga–Br
–
1.173(2), 1.377(6)
2.4418(4), 2.3962(4)
131.59(2)
2.3595(11)
2.3737(3), 2.3640(3) 2.3893(4), 2.3081(4) 2.3757(6)
Ga–Si–Ga
Si–C–O/N
Ga–Si–C
–
–
–
129.83(2)
153.97(3)
–
128.33(6)
–
–
–
–
172.24(17)
112.74(7), 87.05(7),
112.954(7), 76.81(7)
98.65(6), 91.68(6)
Si–Ga–N
117.53(6), 119.69(6) 117.29(4), 122.70(4), 120.30(6), 122.31(5), 121.10(8), 110.51(9)
115.31(4), 123.50(4) 94.74(6), 72.53(5)
118.39(6), 111.58(6) 120.88(4), 111.24(4),
137.65(4), 113.43(4)
Complete lists of bond lengths and angles are provided in Supplementary Tables 2–7.
spectra, all of which are reproduced well. In particular, the change
in absorption frequency of free CO (2,143cm−1, calc. 2,147cm−1,
using a scaling factor of 0.967)49 versus the CO stretch in 5
(1,945cm−1 (solid), 1,943 (solution in mesitylene), calc. 1,964cm−1)
of 198cm−1 (solid) and 200cm−1 (mesitylene, calc. 183cm−1) pro-
vides confidence regarding the applicability of the chosen DFT
level (Supplementary Fig. 57). The computed isocyanide stretch-
ing (2,115cm−1) agrees with the experimental one (6, 2,093cm−1,
Supplementary Fig. 59), and the key 13C{1H} and 29Si{1H} chemical
shifts of 5 and 6 (Supplementary Table 8) agree within 8ppm (C) and
5ppm (Si). The computed 29Si{1H} chemical shift for silylene [L(Br)
Ga]2Si: (758.8ppm) is similar to that of H2Si: and the large positive
value indicates a small highest occupied molecular orbital–lowest
unoccupied molecular orbital (HOMO–LUMO) gap, as was com-
puted for (H3Si)2Si: (29Si{1H} 1,223ppm)51. Time-dependent DFT
(TDDFT) computation on 5 revealed that the strong absorption
at 313nm (expt. 358nm) corresponds to the nSi→π*CO transition
(nSi: electron lone pair, sp2 orbital) and also involves contributions
from the LGa fragment. The HOMO→LUMO transition appeared
at 416nm (expt. 444nm) and also involves the Si lone pair to CO
and LGa π* transition. The absorption of 6 at 308 and 310nm (expt.
357nm) mainly involves the nSi→π*LGa transition and πLGa→π*LGa
transition of the LGa ligand.
Table 2 | experimental and computed structural data
(B3LyP-D3(BJ)/6-311G(d,p), def2-tZVP (Ga, Br)) for R2Si:–CO
complexes
Species
h2Si:–CO
me2Si:–CO
[L(Br)Ga]2Si:–CO (5)
expt.
Comp.
1.890c
1.148c
1.512c
99.2c
89.4c
167.4c
1.1
expt. Comp. expt.
Comp.
1.811
1.151
rSi–C (Å)
rC=O (Å)
rSi–R (Å)
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
1.886c 1.865(6)
1.155c
1.136(7)
1.926c 2.4203(5)
106.3c 122.73(4)
107.5c 92.97(15)
160.6c 172.7(6)
2.439
113.9
90.1
∠
R–Si–R
∠
R–Si–CO
∠
171.0
1.3
Si–C–O
WBISi–C
WBIC=O
νC=O
1.2
–
2.2
2.2
–
2.0
2,043a 2,067b
1,962 2,006b 1,945
1,964b
∆EST
–
−35.7
–
−26.8
–
−34.2
aMean value of observed bands. bScaled with a factor of 0.967 (as suggested by the NIST
Computational Chemistry Comparison and Benchmark Database). cFrom ref. 30, computed at
B3LYP/6-31G(d,p). WBI, Wiberg bond index; Expt., experimental; Comp., computed. ‘Free’ CO for
comparison: rC=Oꢀ=ꢀ1.128ꢀÅ (ref. 49), WBIC=Oꢀ=ꢀ2.3, νC=Oꢀ=ꢀ2,147ꢀcm−1
.
The formation of non-covalent H2C/Si:–CO adducts is always
thermodynamically favourable, but different for C versus Si (Fig. 4):
while ketene is highly favoured for carbon (−83.1 versus
silylene. The Si–C bond lengths of 1.865(6)Å (5) and 1.8419(19)Å −77.4kcalmol−1), silaketene is considerably less stable than the
(6) point to Si–C single bonds, whereas the short C–O (1.136(7)Å, 5) H2Si:–CO adduct (−10.7 versus −27.0kcalmol−1) owing to the
and C–N bonds (1.173(2)Å, 6) indicate slightly weakened C–O well-documented lower propensity of heavier group 14 elements
49
and C–N triple-bond character (compare to CO, 1.128Å, in ref.
,
for multiple bonding. Even though the exothermicity of the CO
and 1,4-diisocyanocyclohexane, 1.141(3)Å, Cambridge Structural association reaction is 10kcalmol−1 higher for 5 (−37.1kcalmol−1)
Database, ref. code YUNMOJ). The Si–C and C–O bond lengths of versus the parent system H2Si:–CO (−27.0kcalmol−1), the higher
5 are slightly shorter than the calculated values reported for H2Si:– stability of the CO adduct relative to the silaketene-like structure
CO (Si–C, 1.890Å; C–O, 1.148Å)30 and Me2Si:–CO (Si–C, 1.886Å; (−6.9kcalmol−1) is reduced by about the same amount. All of
C–O, 1.155Å)32, respectively, whereas the Si–C–O bond angle of 5 this is likely to be related to the wide Ga–Si–Ga angle (113.9° ver-
is larger than those of H2Si:–CO (167.4°) and Me2Si:–CO (160.6°) sus 99.2° for the parent H2Si:–CO adduct) of the L(Br)Ga ligand
(Table 2). The Ga–Si–C bond angle in 5 agrees well with the H–Si–C that results in energetic promotion of the sp2 lone pair (nSi), which
bond angle of H2Si:–CO (89.4°), whereas the CMe–Si–C bond angle is now more readily available for back-bonding the CO moiety
of Me2Si:–CO (107.5°) is substantially wider.
(nSi→π*CO, Fig. 4b); this also leads to a very substantial reduction
of the singlet–triplet energy separation (∆EST) in [L(Br)Ga]2Si:
Quantum chemical calculations. For
a
better understand- (∆EST =−1.4kcalmol−1, Supplementary Table 9) as compared to
ing of the electronic structures, we computed the key geom- the parent H2Si: (∆EST =−20.0kcalmol−1)52, which agrees with the
etries utilizing density functional theory (DFT) approaches computed 29Si{1H} NMR resonance of [L(Br)Ga]2Si:. Comparing
(B3LYP-D3(BJ)/6-311G(d,p), def2-TZVP, see Supplementary this analysis with the parent H2Si:–CO, we find that the Wiberg
Information for details)50.
bond index (WBI) of the Si–C bond increases substantially from
We first anchored our computational level with experiments 1.1 in H2Si:–CO to 1.3 in 5, in line with the shorter Si–C bond
through comparisons of the computed infrared, NMR, and UV–vis (1.890Å versus 1.811Å) and the larger interaction energy between
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O
C
a
b
H
H
H
H
H
C
CO
C
C
O
C
+
or
(1)
(i)
H
p
Electron
donation
1A1
–83.1 (–87.4 expt.)
sp2
HR (298 K)
–77.4
LBrGa
LBrGa
Enlarged
lone pair
Si
O
Electron
donation
C
H
H
H
H
H
Si
CO
Si
C
O
Si
+
or
(2)
H
(ii)
1A1
O
HR (298 K)
–10.7
–27.0
p*
(–21.0 to –29.0 expt.)
C
O
C
Back
bonding
sp2
LBrGa
LBrGa
LBrGa
LBrGa
Si
Si
LBrGa
LBrGa
Si
CO
Si
C
O
+
or
LBrGa
(3)
LBrGa
5
1A1
HR (298 K)
–30.2
–37.1
Fig. 4 | thermochemical comparisons. a, For equations (1) to (3), the enthalpy change ∆HR(298ꢀK) was computed at B3LYP-D3(BJ)/6-311G(d,p),
def2-TZVP (Ga and Br) in kcalꢀmol−1, including the zero point vibrational energy (ZPVE) of the association of parent closed-shell singlet methylene,
silylene, and [L(Br)Ga]2Si: with CO to give ketene and silaketene as well as the corresponding adducts. b, Key changes in orbital coefficients upon
electron–donor substitution through the ligand in the substituted silylene (i) and the back-bonding in the CO complex (ii).
Si and CO in 5 (Table 1). According to natural bond orbital (NBO) for a long time to react with CO, with formation of the carbonyl
analyses, the donation of electron density in 5 from the silicon lone adduct, the specific L(Br)Ga substituents in the silylene inter-
pair (nSi) to the antibonding π*CO orbital (second-order interaction mediate [L(Br)Ga]2Si: are necessary to stabilize the compound,
E(2) =15.2kcalmol−1) is comparable with the nSi→π*CO donation in both kinetically (that is, against dimerization) and thermody-
H2Si:–CO (E(2) =12.4kcalmol−1).
namically (back-bonding and dispersion, which amounts to
~2.6kcalmol−1 stabilization; Supplementary Fig. 64). An NBO
analysis shows a strong donation from nSi to the empty p orbital
Discussion
The formation of compounds 3 to 5 support the intermediacy of of Ga (E(2) =28–32kcalmol−1). The introduction of electropositive
silylene [L(Br)Ga]2Si: formed by reaction of compound 2 with LGa. metal-based ligands allows us to tailor the electronic properties of
[L(Br)Ga]2Si: exhibits the lowest HOMO–LUMO energy separa- silylenes R2Si:, and reduction of ∆EST and the HOMO–LUMO energy
tion (2.7eV versus 3.3eV for H2Si:, Supplementary Fig. 61) and gap in the reaction intermediate [L(Br)Ga]2Si: results in a dramatic
smallest ∆EST (−1.4kcalmol−1 versus −20.0kcalmol−1 for H2Si:) change in reactivity compared to known stable silylenes. Computed
reported so far. Because of the very much reduced ∆EST, [L(Br) reaction energies (Supplementary Fig. 65) agree with experi-
Ga]2Si: is a highly reactive species towards C–C bond activation to mental findings on the reactivity of silylene [L(Br)Ga]2Si: and 5.
yield compound 3 and a hydrogenation reaction to give silane 4, as Reactions of [L(Br)Ga]2Si: with H2 and CO as well as C–C bond
previously reported for compounds whose ∆EST is reduced due to activation leading to 3 are exergonic, whereas base coordination to
stereoelectronic factors53. Others have shown that {B(NDippCH)2} the silylene is strongly (MeNHC=1,3-dimethylimidazol-2-ylidene)
{N(SiMe3)Dipp}Si: undergoes oxidative addition reactions with or slightly (CyNC) endergonic.
H2 and alkyl C–H bonds due to the low ∆EST of −29.7kcalmol−1
The structural and spectroscopic findings in 5, in particular the
(−24.8kcalmol−1)37. {B(NDippCH)2}2Si: shows an even smaller shift of the C=O absorption band in the infrared spectrum, are
∆EST (−0.1kcalmol−1), which agrees with the computed ∆EST of best explained by strong nSi→π*CO electron donation. CO serves
[L(Br)Ga]2Si: (−1.4kcalmol−1)37.
as a mild Lewis acid in 5, as was recently postulated for interme-
However, silylene [L(Br)Ga]2Si: is not only capable of undergo- diates in reactions of CO with bis-silylene Xant(LSi:)2 (Xant=9,
ing C–C and H–H bond activation reactions under mild reaction 9-dimethyl-xanthene-4,5-diyl, L=PhC(NtBu)2)35, in which the
conditions, but also shows strong electron-donating properties, initial step was associated with an interaction of two silylenes
which originate from the electropositive L(Br)Ga substituents and with two π*CO orbitals and CO serving as a four-electron accep-
the wide Ga–Si–Ga bond angle. As a consequence, [L(Br)Ga]2Si: tor. The computed interaction energy (4.3kcalmol−1), WBIC=O
forms a room-temperature-stable heavy p-block element carbonyl (1.2) and C–O bond length (1.130Å) of the optimized structure of
complex 5 in reactions with CO. Although H2Si: has been known the bis-silylene carbonyl complex support this interpretation
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(‘free’ CO: WBIC=O =2.3; C–O, 1.326Å)35. The WBIC=O (1.3) and
C–O bond length (1.151Å) computed for 5 indicate slightly less
electron donation (nSi→π*CO), which is likely because only one
Si donor contributes to this bonding in 5. This is supported by
a natural charge analysis of 5 (q(Ga)=+1.24e, q(Si)=−0.29e,
q(Br)=−0.47e, q(C)=+0.26e, q(O)=−0.42e), giving values for the
carbonyl group that are less negative than the bis-silylene interme-
diates (q(C)=−0.80e, q(O)=−0.81e). We are not aware of other
room-temperature-stable silylene–carbonyl complexes or other
isolable metal carbonyl complexes in which CO mainly acts as a
Lewis acid. Compound 5 furthermore acts as a ‘masked’ silylene
and is expected to substantially enlarge the reactivity spectrum of
stable silylenes due to its unusual electronic properties.
17. Arrowsmith, M., Böhnke, J., Braunschweig, H. & Celik, M. A. Reactivity of a
dihydrodiborene with CO: coordination, insertion, cleavage and spontaneous
formation of a cyclic alkyne. Angew. Chem. Int. Ed. 56, 14287–14292 (2017).
18. Zhang, H., Cao, Z., Wu, W. & Mo, Y. Te transition-metal-like behavior of
B2(NHC)2 in the activation of CO: HOMO–LUMO swap without
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methods
Manipulations were carried out under a dry, oxygen-free argon atmosphere,
with reagents dissolved in benzene, and combined or isolated using cannula and
glovebox techniques. Compounds 1 and 2 were synthesized by reactions of SiBr4
with 1 and 2equiv. of LGa, while 3–5 were obtained from reactions of 2 with LGa
in an Ar (3), H2 (4) and CO2 or CO atmosphere (5) in benzene at 60°C, and 6
was formed through the reaction of 5 with cyclohexyl isocyanide. Compounds
1–6 were characterized by elemental analysis, multinuclear NMR (1H, 13C{1H},
29Si{1H}), infrared and UV–vis spectroscopy (5 and 6). Details are provided in the
Supplementary Information.
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with CO: experimental and theoretical characterization of Si CO (n =
n
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Online content
Any Nature Research reporting summaries, source data, extended data,
supplementary information, acknowledgements, peer review information; details
of author contributions and competing interests; and statements of data and code
availability are available at https://doi.org/10.1038/s41557-020-0456-x.
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Received: 2 October 2019; Accepted: 10 March 2020;
Published: xx xx xxxx
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Data availability
Author contributions
All data generated or analysed during this study are included in this Article
(and its Supplementary Information). The structures of 1–6 in the solid state were
determined by single-crystal X-ray diffraction and the crystallographic data
have been deposited with the Cambridge Crystallographic Data Centre under nos.
CCDC 1943186 (1), 1943187 (2), 1943188 (3), 1943189 (4), 1943190 (5) and
1967024 (6). Copies of the data can be obtained free of charge on application
to CCDC.
C.G., S.S. and P.R.S. conceived the experiments. S.S. and P.R.S. supervised the study. C.G.
and J.S. performed the synthetic studies, C.W. the single-crystal X-ray diffraction studies,
and L.S. the computational experiments. C.G., S.S. and P.R.S. wrote the manuscript, with
input from all authors. All authors analysed the results and commented on the manuscript.
Competing interests
The authors declare no competing interests.
Acknowledgements
Additional information
This work was supported by the University of Duisburg–Essen and the
Alexander-von-Humboldt Foundation (a scholarship to L.S.). We thank the Deutsche
Forschungsgemeinschaft for partial support within the Priority Program SPP 1807
(control of London dispersion interactions in molecular chemistry) and T. Benter
and H. Kersten for mass spectroscopy measurements.
Supplementary information is available for this paper at https://doi.org/10.1038/
s41557-020-0456-x.
Correspondence and requests for materials should be addressed to P.R.S. or S.S.
Reprints and permissions information is available at www.nature.com/reprints.
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