Unprecedented silicon(ii)→calcium complexes with N-heterocyclic silylenes

Article abstract of DOI:10.1039/c4dt03148b

The first N-heterocyclic silylene (NHSi) complexes of any s-block element to date are reported for calcium: [(η⁵-C₅Me₅)₂Ca←:Si(O-C₆H₄-2-^tBu){(N^tBu)₂CPh}] (6

Full text of DOI:10.1039/c4dt03148b

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Unprecedented Silicon(II)ꢀCalcium Complexes with
NꢀHeterocyclic Silylenes
Cite this: DOI: 10.1039/x0xx00000x
Burgert Blom,a* Günter Klatt,b Daniel Gallego,a Gengwen Tana
and Matthias Driess a*
Received 00th October 2014,
Accepted 00th xx 2014
DOI: 10.1039/x0xx00000x
The first NꢀHeterocyclic Silylene (NHSi) complexes of any sꢀblock element to date are
reported for calcium: [(
C5Me5)2Ca←:Si(NtBuCH)2] (7). Complexes 6 and 7 are isolable in a facile way upon reactions
of the corresponding free NꢀHeterocyclic Silylenes (NHSis) with [(
5ꢀC5Me5)2Ca] (2).
η η
5ꢀC5Me5)2Ca←:Si(OꢀC6H4ꢀ2ꢀtBu){(NtBu)2CPh}] (6) and [( 5ꢀ
www.rsc.org/
η
Complexes 6 and 7 were fully characterised by spectroscopic means and single crystal Xꢀray
diffraction analysis of 6 is also reported. Analysis of the bonding situation by DFT methods
including a Bader Atoms in molecules (AIM) analysis is also reported. The bonding
interaction between the Si and Ca centres in 6 and 7 can best be viewed as σꢀdonorꢀacceptor
interactions, with a considerable ionic contribution in the bond. The reactivity towards the
oxygen containing substrates: THF and benzophenone is also discussed.
Introduction
NꢀHeterocyclic Silylene (NHSi) complexes have been reported
across the transition metals,1 and a plethora of studies have been
carried out to date, even highlighting their potential applications in
catalysis.2 These efforts have been heralded by the high abundance,
low toxicity and cost of silicon. Remarkably, no reports of NHSi
complexes for any sꢀblock elements exist, particularly for the
alkalineꢀearth metals, although two seminal reports for fꢀblock NHSi
complexes have been reported.3 This is somewhat surprising since
employing environmentally benign alkaline earth metals, such as
calcium, might enable access to inexpensive and environmentally
friendly NHSi complexes
for catalysis, stoichiometric
transformations, or as precursors for silicate glasses. Herein we
report the first group 2 (alkaline earth metal) calcium NHSi
complexes readily accessible in a quantitative fashion by simple
coordination reactions with the corresponding NHSis. Preliminary
reactivity studies are also reported in addition to the theoretical
calculations by DFT methods elucidating the bonding situation
between the Ca and Si centres.
Fig. 1 ORTEP representation of the molecular structure of dimer 3 in
the solid state, resulting from the reaction of the chlorosilylene 4
with [(η5ꢀC5Me5)2Ca] (2). Thermal ellipsoids are set at the 50 %
probability interval and H atoms are omitted for clarity.
In order to suppress this metathetical reaction, the NHꢀsilylene 1 was
functionalised with an aryloxy substituent affording the novel NHSi
:Si(OꢀC6H4ꢀ2ꢀtBu){(NtBu)2CPh} (5), since cleavage of the SiꢀO
bond in the latter is highly unlikely, due to the oxophilic nature of
silicon. Indeed this strategy enables facile and quantitative entry to
the first sꢀblock silylene complex: [(η5ꢀC5Me5)2Ca←:Si(OꢀC6H4ꢀ2ꢀ
tBu){(NtBu)2CPh}] (6) as a colourless solid, upon reaction of 5 with
2 in toluene at room temperature (Scheme 1). Strikingly, preferential
coordination to Ca occurs via the Si atom and not the O atom in
compound 6. This is likely due to the high steric demand of the
aryloxy substituent precluding O coordination.
Results and Discussion
The starting point in our study was to investigate the reaction of the
chlorosilylene of Roesky :Si(Cl)(PhC(NtBu)2)4 (1) with [(η5ꢀ
C5Me5)2Ca] (2), since earlier investigations showed that 1 is robust
in coordination to a range of transition metals.5 The reaction thereof
with 2, however, affords an inseparable mixture of the novel dimer
[(η5ꢀC5Me5)CaCl(thf)2]2 (3) (Fig. 1) and :Si(Cp*)(PhC(NtBu)2) (4),
when dissolved in thf, by salt metathesis as opposed to the desired
coordination compound [(η5ꢀC5Me5)2Ca←:Si(Cl){(NtBu)2CPh}]6
(See ESI†).
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DOI: 10.1039/C4DT03148B
Ca
N
N
toluene, RT
N
Si
Ca
+
Si
N
O
O
2
5
6:
colourless solid
100 % yield
Scheme 1 Synthetic entry to the first NHSi calcium complex by
coordination of 5 to calcium precursor 2.
Fig. 2 ORTEP representation of the molecular structure of
compound
% probability level and H atoms omitted for clarity. The
geometry optimised structure of (right) (BP86ꢀD3 functional
: d(CaꢀSi) = 3.2732(5) Å. : d(CalꢀSi)
6 in the solid state (left). Thermal ellipsoids set at 50
The same synthetic methodology can also be employed to prepare
the three coordinate NHSi complex [(η5ꢀC5Me5)2Ca←:Si(NtBuCH)2]
(7) by reaction of the West silylene :Si(NtBuCH)2 (8)7 with the
calcium precursor 2 in an analogous way. This affords compound 7
nearly quantitatively as a colourless solid (Scheme 2).
7
plus DZP basis set). For
3.090 Å (calcd.).
6
7
Both compounds feature rather long CaꢀSi bond lengths: in
compound , bearing a threeꢀcoordinate Si atom, this distance
is shorter by 18 pm compared to compound , indicating
7
6
stronger coordination in the former compound. The observed
difference in bond length might be due to sterics as compound
toluene, RT
N
N
N
N
Ca
Si
Si
Ca
+
6
bears a substantially more sterically congested NHSi than
compound which would impede coordination thereby
7
resulting in an elongated bond length. The bond lengths are
somewhat longer than those reported in the calcium silyl
complex Ca(thf)3(Si(SiMe3)3)2 at 3.042(9) and 3.086(9) Å
respectively.8a Moreover, in both compounds, this bond
distance is significantly larger than the sum of the single bond
covalent radii proposed by Pyykkö and Atsumi9 for Ca (1.71 Å)
and Si (1.16 Å) = 2.87 Å which led us to question whether it is
merely an electrostatic interaction, with no covalent character.
In order to address this point we performed DFT calculations to
elucidate the nature of the bonding interaction between Si and
Ca in both compounds (BP86ꢀD3 and the DZP basis).
7:
2
8
colourless solid
99 % yield
Scheme 2 Synthetic entry to the three coordinate NHSi calcium
complex 7.
Complexes
6 and 7 are remarkable examples of molecular
compounds featuring CaꢀSi bonds, which are exceptionally
rare.8 The most diagnostic spectroscopic feature for compounds
6
and
= 81 ppm, both in C6D6). Strikingly, these chemical shifts are
only marginally deshielded from the uncoordinated NHSis (
ꢀ22.8 and
= 78.3 ppm7 respectively) in both cases. This
7 6: δ = ꢀ13.7 ppm; 7: δ
is the 29Si NMR shift in solution (
In compound 6, the LUMO, E = ꢀ0.085 eV, is located mostly
on the phenyl residue while the HOMO, E = ꢀ0.146 eV, mostly
involves the Cp* ligands with some involvement of the Ca
δ
=
δ
observation is in contrast to dꢀblock NHSi complexes where a
dramatic deshielding effect is observed on coordination and
suggests only a very weak interaction between the Si and Ca
centres in both complexes.
centre (Fig. 3a). Similarly, in compound
7 the LUMO E = ꢀ
0.063 eV, is mostly Si(p) + 2 × N(p) + 2 × C(p) while the
HOMO, E = ꢀ0.151 eV, is also mostly located on the Cp*
ligands with some involvement of the Ca centre and (Fig. 3b).
Dissociation in solution can be ruled out since 29Si MASꢀsolid
state NMR spectroscopy revealed an identical chemical shift as
to the solution spectrum for compound
suitable for single crystal Xꢀray diffraction analysis were
obtained from a concentrated toluene solution with slow
cooling to ꢀ30 °C. In the case of compound a similar
7. Crystals of compound
6
7
procedure also consistently afforded crystals visually of high
quality, but the quality of the diffraction data was of low quality
(R1 = 0.1855 for I > 2
many other crystallisation procedures. Despite this, the Fig. 3a Plot of the boundary surfaces of the LUMO (left) (E = ꢀ
structural motif was unambiguously located in the structure 0.085 eV) and HOMO (right) (E = ꢀ0.146 eV) in compound
solution (see ESI ), and the metrical parameters are in good
σ(I)), despite several measurements and
6
.
†
agreement with the calculated structure (BP86ꢀD3 functional
plus DZP basis set). Fig. 2 depicts the solid state structure of
compound
6 and the geometry optimised structure of 7.
Fig. 3b Plot of the boundary surfaces of the LUMO (left) (E = ꢀ
0.063 eV) and HOMO (right) (E = ꢀ0.151 eV) in compound
7
.
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The most interesting MO in both compounds signifying the Siꢀ The labile coordination of the NHSi in compound
Ca interaction is in both cases the HOMOꢀ5 : E = ꢀ0.207 eV; by the reaction with THF where immediate NHSi elimination
: E = ꢀ0.230 eV, which clearly shows a donorꢀacceptor
7 was shown
6
results with concomitant formation of [(
2•2thf), observed in the 1H NMR spectra. This result is
certainly due to the highly oxophilic nature of the Ca centre.
We next attempted a cycloaddition reaction of benzophenone
η
5ꢀC5Me5)2Ca(thf)2]
7
interaction between Si and Ca in both compounds (Fig. 4).
Notably, in both cases some contribution arises from the Ca
centre in the MO. Moreover, the calculated Wibergꢀbond index
(
(WBI) of compound
6 = 0.47 while in compound 7, in accord
with
7
with the goal of forming the cycloaddition product
is eliminated with
5ꢀ
9 (see
with the decreased CaꢀSi bond length observed experimentally,
is slightly higher at 0.53. These indices are indicative of a
covalent bonding interaction between the Ca and Si centres in
both compounds.
Scheme 3). Even in this instance, the NHSi
8
concomitant formation of a dark purple ketone adduct [(
η
C5Me5)2Ca←:O=CPh2] (10) (Fig. 6).11 The isolation and
formation of 10 in this reaction dramatically highlights the
weak coordination of the NHSi to Ca, which is even eliminated
by a ketone coordination.12 Again, the oxophilic nature of the
Ca centre likely drives this process.
O
N
N
N
Ca
Si
Ca
Si
N
O
C6D6
Ph
Ph
9
7
O
Fig. 4 Plot of the boundary surfaces of the HOMOꢀ5 in
Occupancy is 1.91, with 86% Si spꢀ and 14% Ca spdꢀ
contribution. In (right) occupancy is 1.95, with 85% Si spꢀ
6 (left)
N
N
7
N
N
Ca
+
Si
O
Ca
Si
and 15% Ca spdꢀcontribution.
C6D6
In contrast, a Bader atoms in molecules (AIM) analysis10 of
10
8
7
compound
critical point (BCP) between the Ca and Si centres with
(r) = 0.0175 a.u and ∇2ρ(r) = 0.0657 a.u. at the bond critical
7 (See ESI† and Fig. 5) revealed a (3, ꢀ1) bond
Scheme 3 Reaction of compound
not afford the expected cycloaddition product (
formation of the ketone adduct 10 with concomitant elimination
of NHSi is observed.
7
with benzophenone does
9
), rather the
ρ
points (BCP). These values indicate a closedꢀshell (ionic)
interaction between the two centres. Moreover the ratio of the
8
eigenvalues |λ1|/λ3 = 0.127 (λ1 = ꢀ0.0115, λ2 = ꢀ0.0130, λ3 =
0.0902) is also indicative of a closeꢀshell interaction. Hence the
AIM analysis points to a substantial ionic character in the SiꢀCa
interaction; at odds with the positive WBI clearly indicating
covalency in the bond. These results collectively show that the
CaꢀSi bonding interaction is perhaps therefore best described as
a σꢀdonor acceptor interaction between Si and Ca which bears
considerable ionic character.
Fig. 6 ORTEP representation of the molecular structure of
ketone adduct 10 in the solid state. Thermal ellipsoids set at the
50 % probability level, H atoms omitted for clarity. Selected
bond lengths (Å): Ca1ꢀO1 2.293(2), O1ꢀC33 1.238(3).
Conclusion
The first examples of sꢀblock NꢀHeterocyclic Silylene
complexes (NHSi) have been reported for the group 2 alkalineꢀ
earth metal element calcium. These complexes are also the only
existing examples of any sꢀblock NHSi coordination compound
to date. The four or three coordinate NHSi complexes can be
prepared in a quantitative fashion upon reaction of the free
NHSis with the readily available precursor [(
affording [(
5ꢀC5Me5)2Ca←:Si(OꢀC6H4ꢀ2ꢀtBu){(NtBu)2CPh}]
) and [( ) respectively. Both
5ꢀC5Me5)2Ca←:Si(NtBuCH)2] (
compounds exhibit remarkably long CaꢀSi bond lengths, but on
the basis of DFT investigations these can be considered simple
donorꢀacceptor interactions from Si to Ca, with considerable
η
5ꢀC5Me5)2Ca]
η
(
6
η
7
Fig. 5 Contour map of the Laplacian in compound 7. Negative
values are indicated by a dotted line (where there is charge
concentration), and solid lines where the Laplacian is positive
(charge depletion).
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DOI: 10.1039/C4DT03148B
ionic character. The NHSi in compound
7 is very labile to temperature overnight. Removal of volatiles in vacuo, hexane
substitution, and is readily eliminated by THF and even extraction and crystallization at ꢀ30°C afforded 0.958 g of the
1
benzophenone, affording in the latter case the ketone adduct: desired product in high purity (65% yield). H NMR (400.13 MHz,
[(η
5ꢀC5Me5)2Ca←:O=CPh2] (10). This showcases the rather
C6D6, 298K, ppm): δ = 1.10 (s, 18H, 2 × NC(CH3)3), 1.67 (s, 9H, 1 ×
ArꢀC(CH3)3), 6.86ꢀ7.00 (m, 5H, Ph), 7.20ꢀ7.24 (m, 2H, C3ꢀH and C4ꢀ
low bond energy of the CaꢀSi bond. We are currently exploring
NHSis that bind more strongly to the Ca centre which might
enable novel Ca,Si mediated processes, without NHSi
elimination. We are also currently extending this synthetic
approach to other group 2 elements (Ba and Sr) and will report
these finding in due course.
3
4
H) 7.41 (dd, 1H, J(HꢀH) = 7.9 Hz, J(HꢀH) = 1.8 Hz, C2ꢀH) 7.73
(dd, 1H, J(HꢀH) = 8.2 Hz, J(HꢀH) = 1.4 Hz, C5ꢀH). 13C{1H} NMR
(100.61 MHz, C6D6, 298K, ppm): δ = 30.4 (s, 1 × ArꢀC(CH3)3), 31.6
(s, 2 × NC(CH3)3), 32.3 (s, 1 × ArꢀC(CH3)3), 53.1 (s, 2 × NC(CH3)3),
119.4 (s, 1 × C5, OAr), 120.4 (s, ArꢀC, Ph), 126.9 (s, 1 × C4, OAr),
127.1 (s, 1 × C3, OAr), 127.9 (ArꢀC crossed with solvent peaks,
located by HMQ/BC), 129.7 (s, ArꢀC, Ph), 129.8 (s, 1 × C5, OAr),
133.8 139.3 (both s, ArꢀC), 157.5 (s, 1 × OꢀC, OAr), 163.9 (s, 1 ×
NCN). 29Si{1H} NMR (79.49 MHz, C6D6, 298K, ppm): δ = ꢀ22.8.
3
4
Experimental Section
All experiments were carried out under dry oxygenꢀfree
nitrogen using standard Schlenk techniques. Solvents were dried by
standard methods and freshly distilled and degassed prior to use. The
NMR spectra were recorded on Bruker spectrometers (AV400 or
AV200) referenced to residual solvent signals as internal standards
(1H NMR: C6D6, 7.15; THFꢀd8 3.58 (left signal) ppm and 13C{H}
NMR: C6D6, 128.0; THFꢀd8 67.6 (left signal) ppm) or with an
external standard (SiMe4 for 29Si NMR). Concentrated solutions of
Reaction of 2 with chlorosilylene 1
A Schlenk tube was charged with [(η5ꢀC5Me5)2Ca] (2) (0.200 g,
0.644 mmol) and PhC(NtBu)2SiCl (1) (0.189 g, 0.644 mmol).
Freshly distilled toluene (40 mL) was recondensed onto this solid
mixture in a liquid nitrogen trap under static vacuum. The reaction
mixture was slowly allowed to warm to room temperature with
samples were sealed off in
a Youngꢀtype NMR tube for
stirring. On warming to room temperature
a bright yellow
measurements. Signals were unambiguously assigned by
a
suspension is formed. The reaction mixture was allowed to stir at
combination of H,H COSY, HSQC and HMBC correlation 2D
experiments. Melting points were recorded on a “Melting point
tester” device from BSGT company and are uncorrected. All the
samples are sealed off in capillary under vacuum and each sample
was measured in duplicate. High resolution ESI mass spectra were
recorded on an Orbitrap LTQ XL of Thermo Scientific mass
spectrometer and the raw data evaluated using the Xcalibur
computer program. For the single crystal Xꢀray structure analyses
the crystals were each mounted on a glass capillary in perfluorinated
oil and measured in a cold N2 flow. The data were collected on an
Agilent Technologies SuperNova (single source) at 150 K (CuKα
radiation, λ= 1.5418 Å) and refined on F2 with the SHELXꢀ971
software package. The positions of the H atoms were calculated and
considered isotropically according to a riding model. Solid state
29Si{1H} static and MAS cross polarization (CP) measurements were
carried out on a Bruker Avance II spectrometer at an external
magnetic field of 9.4 T ( i.e. a 29Si resonance frequency of 79.46
MHz) using a standard Bruker 4 mm doubleꢀresonance HꢀX MAS
probe. The CP spectra were recorded with a cross polarization time
1
room temperature for 2h. H NMR of the reaction solution at this
point revealed no benzene soluble products. The toluene supernatant
solution was separated from the yellow solid by cannula filtration,
and discarded. The remaining bright yellow solid was dried in vacuo
for 2 h affording 0.356 g of the product. This solid is insoluble in
tolꢀd8, C6D6, and affords decomposed products in acetonitrileꢀd3 and
CD2Cl2. It dissolves completely in thfꢀd8 and is shown to be a
mixture of the dimer [(η5ꢀC5Me5)CaCl(thfꢀd8)2]2 (3) and
1
:Si(Cp*)(PhC(NtBu)2) (4) on the basis of H NMR, 13C NMR, 29Si
NMR and Xꢀray diffraction analysis. Attempts at separating the two
components by fractional crystallisation methods failed, however
some crystals, suitable for XRD of [(η5ꢀC5Me5)CaCl(thf)2] were
obtained fortuitously upon dissolving a sample of the yellow solid
obtained from this reaction in THF (nonꢀdeuterated) and cooling to ꢀ
30 °C. 1H NMR (200 MHz, THFꢀd8, 298 K, ppm): δ = 1.00 (s, 9H, 2
× NC(CH3)3, :Si(Cp*)(PhC(NtBu)2)), 1.73 (s, 15H, 1 × C5(CH3)5, 0.5
[(η5ꢀC5Me5)CaCl(thfꢀd8)2]2), 1.87 (s, 6H,
:Si(Cp*)(PhC(NtBu)2)), 1.92 (s, 6H,
:Si(Cp*)(PhC(NtBu)2)), 1.98 (s, 3H,
2
×
×
×
η1ꢀCp*ꢀCH3,
η1ꢀCp*ꢀCH3,
η1ꢀCp*ꢀCH3,
2
1
1
of 2 ms and composite pulse H decoupling was applied during the
:Si(Cp*)(PhC(NtBu)2),
7.05
–
7.72
(m,
5H,
ArꢀH,
acquisition. The spectra were referenced externally to TMS
(tetramethylsilane) using TKS (tetrakis(trimethylsilyl)silane) as a
secondary reference. Benzophenone was purchased from Aldrich
and used as received. [(η5ꢀC5Me5)2Ca] (2) was prepared from
Ca{N(SiMe3)2}2 upon reaction with Cp*H and purified by
sublimation as a white crystalline solid as previously reported by
Tanner et al.13 The NHSi 14 and 87a was prepared by literature
procedures.
:Si(Cp*)(PhC(NtBu)2). No signals detected for coordinated thf in
[(η5ꢀC5Me5)CaCl(thfꢀd8)2]2 due to deuteration. 13C{1H} NMR (50
MHz, THFꢀd8, 298 K, ppm):
δ =
11.9 (s, η5ꢀC5(CH3)5,
:Si(Cp*)(PhC(NtBu)2), 13.6 (s, η5ꢀC5(CH3)5, [(η5ꢀC5Me5)CaCl(thfꢀ
d8)2]2), 32.3(s, NC(CH3)3, :Si(Cp*)(PhC(NtBu)2)), 53.9 (s,
NC(CH3)3, :Si(Cp*)(PhC(NtBu)2)), 111.9 (s, η5ꢀC5(CH3)5,
:Si(Cp*)(PhC(NtBu)2), 122.3 (s, η5ꢀC5(CH3)5, [(η5ꢀC5Me5)CaCl(thfꢀ
d8)2]2), 128.4, 128.9, 130.4, 130.7, 131.9, 135.3 (all s, ArꢀC,
:Si(Cp*)(PhC(NtBu)2), 160.8 (s, NCN, :Si(Cp*)(PhC(NtBu)2). (on
the time scale of the 13C measurement, the Cp* coordinated to Si is
fluxional and in the η5 coordination mode). 29Si{1H} NMR (79 MHz,
THFꢀd8, 298 K, ppm): δ = 52.4 ppm (s, :Si(Cp*)(PhC(NtBu)2).
Synthesis of N,N’ꢀdiꢀtertꢀbutyl(phenylamidinate)(2ꢀtertꢀbutylꢀ
phenolate)silylene (5)
The synthesis was carried out following the reported procedure for
the synthesis of Nꢀdonor stabilized silylenes PhC(NtBu)2SiX
reported by Roesky et al.14 with slight modifications. 2ꢀtertꢀ
butylphenol (0.558 g, 3.71 mmol) was dissolved in 20 mL of diethyl
ether and nꢀBuLi 1.6 M (2.3 mL, 3.7 mmol) was added via syringe at
ꢀ20 °C. After stirring for 2 h at room temperature, the reaction
Synthesis of [(η5ꢀC5Me5)2Ca←:Si(OꢀC6H4ꢀ2ꢀtBu){(NtBu)2CPh}]
(6)
A Schlenk tube was charged with [(η5ꢀC5Me5)2Ca] (2) (0.250 g,
0.805 mmol) and :Si(OꢀC6H4ꢀ2ꢀtBu){(NtBu)2CPh}] (5) (0.328 g,
0.805 mmol) in the gloveꢀbox. Toluene (40 mL) was recondensed
onto this solid mixture in a liquid nitrogen trap under static vacuum.
mixture was cooled down to ꢀ78 °C and
a solution of
PhC(NtBu)2SiCl (1) (1.066 g, 3.61 mmol) in 20 mL of toluene was
transferred via cannula. The reaction was slowly warmed up to room
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Schlenk tube was charged with [(η5ꢀC5Me5)2Ca] (2) (0.412 g, 1.326
mmol) and benzophenone (0.240 g, 1.326 mmol). The two solids
were mixed by stirring at room temperature and a colour change (in
the solid state) was immediately observed to bright purple. Toluene
(10 mL) was added to the purple solid and the formed purple
solution stirred at room temperature for 1 h. The solvent was
removed in vacuo affording the product as a purple solid. Yield:
0.650 g, 100 %). 1H NMR (400 MHz, C6D6, 298 K, ppm): δ = 2.07
(s, 30H, 2 × 2 × C5(CH3)5), 7.00 (t, 4H, 3J(H,H) = 7.8 Hz, 2 × C2,6ꢀH,
Ph), 7.11(t, 2H, 3J(H,H) = 7.8 Hz, 2 × C4ꢀH, Ph), 7.46 (d, 4H,
3J(H,H) = 7.8 Hz, 2 × C3,5ꢀH, Ph). 13C{1H} NMR (100.2 MHz, C6D6,
298 K, ppm): δ = 11.0 (s, 2 × C5(CH3)5), 112.5 (s, (s, 2 × C5(CH3)5),
129.0 (s, 2 × C2,6, Ph), 130.6 (s, 2 × C3,5, Ph), 134.4 (s, (s, 2 × C4,
Ph), 201.6 (s, 1 × C=O…Ca). ESIꢀMS(THF): 491.2613 (expt.)
491.2621 (calcd.) [MꢀHꢀ]+, 183.0800 (expt.) 183.0804 (calcd.)
[C13H10O + H]+ /[free benzophenone + H]+.
The reaction mixture was allowed to come to room temperature with
rapid stirring. Upon warming to room temperature a pale–yellow
solution is observed. After 1 h of stirring at room temperature a H
1
NMR spectrum showed completion of the reaction. The reaction
solution was concentrated to ca. 3 mL and cooled to ꢀ30 °C for 18 h.
A colourless precipitate was isolated by removal of the supernatant
liquid via syringe and dried in vacuo for 1 h (5 × 10ꢀ2 mbar). Yield:
0.570 g, 99%. m.p. 134 °C (turns to a pale yellow oil). 1H NMR (400
MHz, C6D6, 298 K, ppm): δ = 1.09 (s, 18H, 2 × NC(CH3)3), 1.66 (s,
9H, 1 × OꢀC6H4ꢀ2ꢀC(CH3)3), 2.00 (s, 30H, 2 × C5(CH3)5), 6.83 –
7.02 (m, 5H, 1 × {(NtBu)2CPhꢀH}), 7.18 – 7.24 (m, 2H, 1 × C3,4ꢀH,
3
OAr), 7.40 (d, 1H, J(H,H) = 7.3 Hz, 1 × C2ꢀH, OAr), 7.90 (d, 1H,
3J(H,H) = 7.9 Hz, 1 × C5ꢀH, OAr). 13C{1H} NMR (100.2 MHz,
C6D6, 298 K, ppm): δ = 10.7 (s, 2 × C5(CH3)5), 30.4 (s, 1 × C(CH3)3,
OAr), 31.6 (s, 2 × NC(CH3)3), 35.2 (s, 1 × C(CH3)3, OAr), 53.1 (s, 2
× NC(CH3)3), 114.0 (s, 2 × C5(CH3)5), 119.3 (s, 1 × C5, OAr), 120.6
(s, ArꢀC, Ph), 126.9 (s, 1 × C3 or 4, OAr), 127.2 (s, 1 × C2, OAr),
127.9 ArꢀC crossed with solvent peaks, found by HMB/QC), 129.5
(s, 1 × C3 or 4, OAr), 129.9 (s, ArꢀC, Ph), 133.4, 139.1 (all ArꢀC, Ph)
157.1 (s, 1 × OꢀC, OAr), 165.9 (s, 1 × NCN). 29Si{1H} NMR (79.5
MHz, C6D6, 298 K, ppm): δ = ꢀ13.7 ppm. ESIꢀMS (THF): [M + H]+
signal not detected; 409.2659 (expt.) 409.2670 (calcd.) [:Si(OꢀC6H4ꢀ
2ꢀtBu){(NtBu)2CPh} +H]+ .
Notes and references
a Technische Universität Berlin, Department of Chemistry: Metalorganics
and Inorganic Materials, Sekr. C2, Strasse des 17. Juni 135, 10623 Berlin
(Germany) Fax: (+49)30ꢀ314ꢀ29732 Eꢀmail: burgert.blom@tuꢀberlin.de;
matthias.driess@tuꢀberlin.de.
b
Synthesis of [(η5ꢀC5Me5)2Ca←:Si(NtBuCH)2] (7)
PhysikalischꢀChemisches Institut der Universität Heidelberg, Im
Neuenheimer Feld 229, Dꢀ69120 Heidelberg (Germany).
The authors wish to acknowledge the Unicat cluster of excellence for
funding and Dr. J.D. Epping for the Solidꢀstate NMR measurement. Dr.
A Schlenk tube was charged with [(η5ꢀC5Me5)2Ca] (2) (0.310 g, 1.00
mmol) and :Si(NtBuCH)2 (8) (0.196 g, 1.000 mmol). Freshly
distilled toluene (15 mL) was recondensed onto this solid mixture in
a liquid nitrogen trap under static vacuum. The reaction mixture was
allowed to come to room temperature upon which a colourless
solution had formed. The reaction was stirred at room temperature
for 40 minutes and the solvent removed in vacuo, affording a
colourless solid as product, which is spectroscopically pure and
requires no further purification. Crystals suitable for Xꢀray
diffraction analysis were grown from toluene, although despite
several measurements the data set is only of average quality. Yield:
0.507 g (100 %). 1H NMR (400 MHz, C6D6, 298 K, ppm): δ = 1.32
(s, 18H, 2 × NC(CH3)3), 2.12 (s, 30H, 2 × C5(CH3)5), 6.58 (s, 2H, 1
× CH=CH). 13C{1H} NMR (100.2 MHz, C6D6, 298 K, ppm): δ =
11.8 (s, 2 × C5(CH3)5), 32.1 (s, 2 × C(CH3)3), 54.4 (s, 2 × C(CH3)3),
113.6 (s, 2 × C5(CH3)5), 120.8 (s, 1 × CH=CH). 29Si{1H} NMR (79.5
MHz, C6D6, 298 K, ppm): δ = 81.0 ppm. 29Si{1H}ꢀMASꢀNMR: δ =
81.4 ppm ESIꢀMS (THF): 507.3449 (expt.) 507.3397 (calcd.)
[M+H]+.
S. Kohl is thanked for a generous gift of Cp* Ca. Dr. J. Li (TU Berlin) is
2
thanked for performing the AIM analysis of
Electronic Supplementary Information (ESI) available: [NMR and
ESIꢀMS spectra and the crystal structure of dimer and compound
along with crystallographic details of compounds
available. Details concerning the DFT calculations of
of Zꢀmatrices are also included.]. See DOI: 10.1039/c000000x/
‡ CCDC 1021899 ( ) CCDC 1021900 ( ) CCDC 1021901 (10) contain
7.
†
3
7,
3,
6
and
7
and 10 are
6
and
7
and tables
3
6
the supplementary 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.
1
(
a
b
) B. Blom, M. Stoelzel and M. Driess, Chem. Eur. J, 2013, 19, 40;
) B. Blom and M. Driess, Struct. Bond. 2014, 156, 85.
(
2
B. Blom, D. Gallego and M. Driess, Inorg. Chem. Front. 2014,
) X. Cai, B. Gehrhus, P. B. Hitchcock and M. F. Lappert, Can. J.
Chem. 2000, 78, 1484; ( ) W. J. Evans, J. M. Perotti, J. W. Ziller, D.
F. Moser and R. West, Organometallics 2003, 22, 1160.
4 ( ) C. –W. So, H. W. Roesky, J. Magull and R. B. Oswald, Angew.
Chem. 2006, 118, 4052; Angew. Chem. Int. Ed. 2006, 45, 3948; ( ) S.
1, 134.
3
(
a
b
a
Reaction of 7 with thf
b
S. Sen, H. W. Roesky, D. Stern, J. Henn and D. Stalke, D. J. Am.
Chem. Soc. 2010, 132, 1123.
A sample (30 mg) of 7 was dissolved in THF and the THF removed
in vacuo. The remaining colourless residue was reꢀdissolved in C6D6
which clearly showed clean formation of free :Si(NtBuCH)2 (8) and
[(η5ꢀC5Me5)2Ca(thf)2] (2.2thf) by NMR spectroscopy on comparison
to authentic samples of both compounds (See Fig S11).
5
See as selected examples: (
Roesky, H. Wolf, and D. Stalke, J. Am. Chem. Soc. 2012, 134, 2423
) B. Blom, S. Inoue, D. Gallego and Driess, M. Chem. Eur. J. 2012, 18,
a) R. Azhakar, R. S. Ghadwal, H. W.
(b
13355; (c) R. Azhakar, R. S. Ghadwal, H. W. Roesky, J. Hey and D.
Stalke, Chem. Asian. J. 2012, 7, 528; (d) B. Blom, S. Enthaler, S. Inoue, E.
Irran, and M. Driess, J. Am. Chem. Soc. 2013, 135, 6703; (e) G. Tan, B.
Blom, D. Gallego, and M. Driess, Organometallics 2014, 33, 363.
Recently similar reactivity has been reported with Zinc. see: S. Schäfer, R.
Köppe, M. T. Gamer and P. W. Roesky, Chem. Commun. 2014, 50, 11401.
(a) M. Denk, R. Lennon, R. Hayashi, R. West, A. V. Belaykov, H. P.
Verne, A. Haaland, M. Wagner and N. Metzler, J. Am. Chem. Soc. 1994,
Reaction of 7 with benzophenone affording 10 and free NHSi
A Schlenk tube was charged with (0.168 g, 0.330 mmol) 7 and
dissolved in C6D6 (3 mL). Benzophenone (0.60 g, 0.33 mmol) was
added to the solution of the 7 with stirring and an immediate colour
6
7
1
change to purple was observed. A H NMR sample at this stage
revealed selective formation of free :Si(NtBuCH)2 (8)7a with the
formation of a new product: [(η5ꢀC5Me5)2Ca…O=CPh2] (10). The
identity of 10 was confirmed by the synthesis of 10 directly from the
reaction of [(η5ꢀC5Me5)2Ca] (2) with Benzophenone as follows: A
This journal is © The Royal Society of Chemistry 2012
J. Name., 2012, 00, 1-3 | 5

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ARTICLE
DALTON TRANS
DOI: 10.1039/C4DT03148B
116, 2691; (b) M. Haaf, A. Schmiedl, T. A. Schmedake, D. R. Powell, A. 11 Compound 10 is surprisingly the first ketone adduct of Cp* Ca.
2
J. Millevolte, M. Denk, R. West, J. Am. Chem. Soc. 1998, 120, 12714.
Ketone adducts or ketyl complexes of calcium have been reported but
are somewhat rare. See: ( ) D. Cook, Can. J. Chem. 1963, 41, 515; (
D. Cook, Can. J. Chem. 1963, 41, 522; ( ) Z. Hou and Y. Wakatsuki,
Chem. Eur. J. 1997, , 1005; ( ) L. Orzechowski and S. Harder,
Organometallics 2007, 26, 2144; ( ) V. Vyas, P. Raizada and U.
8
Only very few reports exist of wellꢀdefined molecular compounds
a
b)
featuring CaꢀSi bonds. See as examples: (
RuhlandtꢀSenge, Organometallics 2004, 23, 2694; (
Schoendorff, B. M. Upton, A. Ellern, T. L. Windus, A. D. Sadow,
Organometallics 2013, 32, 1300; ( ) K. Takanashi, V. Y. Lee, T.
Yokoyama, A. Sekiguchi, J. Am. Chem. Soc. 2009, 131, 916; (
a
) W. Teng and K.
c
b
) K. Yan, G.
3
d
e
c
Sharma, Int. J. Electrochem. 2011, 798321.
d
) V. 12 The synthesis of ketone adduct 10 can also be achieved by the reaction
Leich, T. P. Spaniol, L. Maron, J. Okuda, Chem. Commun. 2014, 50
2311.
,
of benzophenone with 2, affording 10 quantitatively. This reaction can
remarkably even be carried out in the solid state without any solvent.
9
P. Pyykkö and M. Atsumi, Chem. Eur. J. 2009, 15, 186.
13 P. S. Tanner, D. J. Burkey, T. P. Hanusa, Polyhedron 1995, 14, 331.
10 See as examples: (
a
) R. F. W. Bader, Chem. Rev. 1991, 91, 893: (
b
) R. 14 R. Azhakar, R. S. Ghadwal, H. W. Roesky, H. Wolf, D. Stalke,
F. W. Bader, H. Essen, J. Chem. Phys. 1984, 80, 1943.
Organometallics 2012, 31, 4588.
6 | J. Name., 2012, 00, 1-3
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DOI: 10.1039/C4DT03148B
Abstract
The first NꢀHeterocyclic Silylene (NHSi) complexes of any sꢀblock element to date are reported for calcium:
[(
5ꢀC5Me5)2Ca←:Si(OꢀC6H4ꢀ2ꢀtBu){(NtBu)2CPh}] and [( 5ꢀC5Me5)2Ca←:Si(NtBuCH)2]. The synthesis, structure, bonding analysis
η
η
by DFT methods and the reactivity towards oxygen containing substrates is discussed.
This journal is © The Royal Society of Chemistry 2012
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