Calix[4]pyrrole Hydridosilicate: The Elusive Planar Tetracoordinate Silicon Imparts Striking Stability to Its Anionic Silicon Hydride

Article abstract of DOI:10.1021/jacs.8b11137

Anionic hydridosilicates are highly reactive and strong hydride donors. In contrast, calix[4]pyrrole hydridosilicate is an entirely water-stable, anionic silicon hydride, which does not show hydridic reactivity. However, it still acts as an electron donor and enables the detection of a single electron transfer process in the reduction chemistry with hydridosilicates. Most important, these unusual properties are imparted by the unique planar structure of its elusive parent neutral silane-substantiating the effect of planar tetracoordinate silicon for the first time.

Full text of DOI:10.1021/jacs.8b11137

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Calix[4]pyrrole hydridosilicate: the elusive planar tetracoordinate
silicon imparts striking stability to its anionic silicon hydride.
Fabian Ebner, and Lutz Greb
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b11137 • Publication Date (Web): 30 Nov 2018
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Journal of the American Chemical Society
Calix[4]pyrrole hydridosilicate: the elusive planar tetracoordinate
silicon imparts striking stability to its anionic silicon hydride.
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Fabian Ebner, Lutz Greb*
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Anorganisch-Chemisches Institut, Ruprecht-Karls-Universität Heidelberg, Im Neuenheimer Feld 275, 69120 Heidelberg
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Supporting Information Placeholder
ABSTRACT: Anionic hydridosilicates are highly reactive and
strong hydride donors. In contrast, calix[4]pyrrole hydridosilicate
is an entirely water-stable, anionic silicon hydride, which does not
show hydridic reactivity. However, it still acts as an electron donor
and enables the detection of a single electron transfer process in the
reduction chemistry with hydridosilicates. Most important, these
unusual properties are imparted by the unique planar structure of
its elusive parent neutral silane – substantiating the effect of planar
Scheme 1: Synthesis of different salts of [1-H-][M+].
tetracoordinate silicon for the first time.
The methyl-calix[4]pyrrole macrocyclic ligand, easily prepared in
a one-step procedure,44 was fully deprotonated with benzyl potas-
sium in toluene at 120 °C (Scheme 1). Changing the solvent to ac-
Anionic hypercoordinate silicon hydrides (hydridosilicates) are
etonitrile and addition of HSiCl3 at room temperature gave the po-
highly reactive species and powerful reducing agents.1 The arche-
tassium salt of methyl-calix[4]pyrrole hydridosilicate [1-][K+] in
-
2-
50% isolated yield at mmol scale.45 The respective lithium and
typal SiH5 or SiH6 anions have only been observed under gas
phase or high-pressure conditions, respectively.2-4 Recently, hy-
dridosilicates were demonstrated to even react with Csp2-F bonds in
hydrodefluorination reactions.5 Already neutral silanes turn into ef-
fective reductants upon transient coordination of neutral or anionic
donors.2-3, 6-14 Besides being potent hydride donors, hydridosili-
cates are also involved in fundamentally different reactivity modes,
like acting as strong bases,3, 15-17 electron or hydrogen atom do-
nors,16-20 making valuable dehydrogenative coupling processes fea-
sible.21-24 A reason for their high reactivity is the low hydride ion
affinity of neutral silanes, which is indeed among the lowest known
for neutral molecules, comparable with H2O or benzene.2-3, 25-26 By
consequence, the isolation of hydridosilicates needs special precau-
tions, like the stabilization of the Si-H bonds by agostic interactions
with transition metals.27-32 However, also a small number of non-
transition metal salts of [HSiR4]- or [H2SiR3]- (R = Ar, OR, F) have
been observed spectroscopically.1, 16-17, 20, 33-39 The first structural
tetraphenyl phosphonium salts [1-H-][Li+] and [1-H-][PPh4 ] were
+
obtained by subsequent salt metathesis with LiCl or PPh4Cl.
Multinuclear NMR spectroscopy revealed noticeable spectro-
scopic features of [1-H-], being similar for the various cations. 1H-
NMR in thf-d8 showed two doublets for the pyrrole hydrogen atoms
and four different signals for the methyl groups, consistent with a
C2v symmetry of [1-H-] in solution. The Si bound hydrogen reso-
nated at δ = 6.60 ppm with a coupling constant 1JSi-H = 270 Hz. The
29Si chemical shift of -140 ppm rather lies in the range of hexacoor-
dinate silicon.46 In comparison to other hydridosilicates, the Si-H
1H-NMR signal is shifted downfield and the 29Si-NMR signal high-
field, indicating an unusually deshielded hydrogen and a shielded
silicon atom.16, 37, 47 The experimental values agree ideally with the
computed NMR data of [1-H-] (ZORA-SO-PBE0(COSMO,
CH2Cl2)/TZ2P), refuting any solvent coordination responsible for
the observed strong shift.
-
characterization of a hydridosilicate, H2SiPh3 , was achieved only
IR spectroscopy for salts [1-H-][M+] showed distinct stretching
in 2001,40 followed by some related derivatives.41-43 A general re-
quirement were strict anhydrous conditions to prevent immediate
hydrolysis of the activated and unstable Si-H bonds.
modes of the Si-H bonds at 2176 cm-1 [K+] or 2243 cm-1 [PPh4 ].
+
Such values are much higher compared to other hydridosilicates
(1550 – 1970 cm-1),4, 40, 43 but rather match with those of Si-H bonds
in neutral silanes or aminosilanes (2100 – 2200 cm-1).41, 48-49 The
IR-spectra thus indicate a remarkably strong Si-H bond in [1-H-].
We herewith present the synthesis and characterization of an en-
tirely water stable, anionic hydridosilicate [1-H-] (Figure 1). The
hydridic reactivity of [1-H-] is suppressed, but it still undergoes sin-
gle electron transfer, supporting such process as feasible reactivity
mode of hydridosilicates. Most important, these features are caused
by the unique planar conformation of its (so far elusive) parent neu-
tral silane 1.
Single crystals of [1-H-][K+] were obtained from dichloromethane
and of [1-H-][Li+] from acetonitrile. [1-H-][K+] exists as a coordi-
nation polymeric structure in the solid state, wherein the cation
bridges the anionic units via K…π-pyrrole interactions. In
[1-H-][Li+], the lithium cation is coordinated by four acetonitrile
molecules, whereas the anion [1-H-] resides freely (Figure 2). In all
previously reported hydridosilicates, the silicon was coordinated in
a trigonal-bipyramidal fashion, with the hydrides being in the api-
cal position.40-41, 43 In [1-H-], the coordination sphere around silicon
is almost square-pyramidal, wherein the basal plane is generated by
four nitrogen atoms (Si-Navg = 1.86 Å, N-Si-N = 157°). The Si-H
bond is rather short (1.36 Å) in comparison to the previously re-
ported structures (1.50 – 1.60 Å).40-41, 43
Figure 1: Highly reactive anionic hydridosilicates compared with the water stable sil-
icon hydride of this work.
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Planar 1 has even a higher HIA than B(C6F5)3 (512 kJ mol-1), with-
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out bearing any electron withdrawing groups. NBO analysis of the
Si-H bond in [1-H-] reveals a sp1.26 hybrid orbital at silicon. The
high s-orbital character is in line with the strong bonding as well as
with the large Si-H NMR coupling constant. Energy decomposition
analysis (BP86-D3/TZ2P) further rationalized the much stronger
hydride binding energy in [1-H-], in comparison to [2-H-]. The pre-
organization in 1 causes a strongly diminished deformation energy
(96 kJ mol-1) upon distortion to the structure in [1-H-], in compari-
son to 2 (281 kJ mol-1). Moreover, the contribution of orbital inter-
action in the Si-H bond of [1-H-] (48 %) are more pronounced as in
[2-H-] (45 %), in agreement with the very low LUMO energy in 1.
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The spectroscopic and theoretical results consistently identify
the unusually strong silicon-hydride bond as responsible for the sta-
bility of [1-H-]. The weak steric demand of the methyl groups in
the ligand refutes a strong kinetic effect, although the blocked po-
sition trans to the Si-H bond hinders associative processes, and
could cause high reaction barriers. It is known for almost 40 years,
that the planarization of silicon should significantly lower the
LUMO energy of neutral silanes.50-51 However, this effect has
never been verified experimentally until now.
In contrast to the resistance of [1-H-] towards heterolytic Si-H
bond cleavage (hydride donation), single electron transfer (SET)
does readily occur. Upon mixing of [1-H-] with the electron accep-
tor p-chloranil, two independent short-lived radical species were
detected by EPR spectroscopy, tentatively assigned as a neutral
radical [1-H●] and the p-chloranil radical anion. Upon prolonged
reaction time, the exhaustive reduction to p-perchlorohydroqui-
none was verified. Accordingly, reaction of [1-H-] with AgSbF6 led
to the formation of only one short-lived radical and elemental sil-
ver. The observation of radical intermediates during the reduction
with a hydridosilicate is an important finding. Even though penta-
coordinate hydridosilicates have been considered as electron trans-
fer agents, respective intermediates were never detected.16-19 The
metastability of the radical species again relies on the effect im-
parted by the underlying planar silane 1. Evidently, not only the
HIA but also the electron affinity of 1 is larger as that of common
neutral silanes – rendering the intermediate as metastable and ena-
bling its observation.
Figure 2: Molecular structure of [1-H-][Li+(CH3CN)4] obtained by X-ray diffraction.
Only one of the two independent fragments in the unit cell is shown. Selected hydrogen
atoms are omitted for clarity. Ellipsoids 50 % probability, selected bond lengths [Å]
and angles [°]: Si-Navg = 1.860(3) Å, Si-H1 = 1.36(3) Å, N1-Si-N3 = 158.5(2), N2-Si-
N4 = 157.4(2).
The calix[4]pyrrole ligand adopts a ruffled conformation, with
two opposite methyl groups being identical, in agreement with the
1
C2v symmetry in solution as observed by H-NMR spectroscopy.
The methyl groups block the coordination side trans to the Si-H
bond.
The water and air stability of the [1-H-] salts were tested by ex-
posing solutions to air at room temperature. In contrast to the im-
mediate hydrolysis of all previously reported hydridosilicates, the
lithium and potassium salts are hydrolyzed only over the range of
hours. The tetraphenylphosphonium salt of [1-H-] remained en-
tirely intact under such conditions and even in a 1:1 mixture of
CD3CN/D2O, no hydrolysis was detected after several weeks (see
SI). Moreover, [1-H-] does neither react with p-methyl benzalde-
hyde, benzyl chloride, benzyl bromide nor with the TEMPO radi-
cal. These findings are in strong contrast to the pronounced water
sensibility, the strong hydride donor capabilities and the weak Si-
H bond energy of all previously described hydridosilicates.
But how can the stability and lack of reactivity be rationalized?
To answer this question, the neutral silane 1 that results from hy-
dride abstraction of [1-H-] was analyzed by quantum-theoretical
tools, and compared with the related but structurally unrestricted
Si(pyrrole)4 (2) (Figure 3). For the neutral silane 1, irrespective of
the used method (wavefunction or various density functionals) and
starting geometry, a planar conformation at silicon is obtained (e.g.
N-Si-N = 178.3° with B3LYP-D3(BJ)/def2-TZVPP). The neutral
silane 2 has a common tetrahedral geometry. The LUMO energy in
1 (-2.3 eV) is reduced dramatically in comparison to 2 (-0.1 eV),
and reveals almost pure pz-type character at silicon (Figure 3). The
computed (DLPNO-CCSD(T)/cc-pVQZ, isodesmic) hydride ion
affinity (HIA) of 1 is exceptionally high (517 kJ mol-1, cf. for 2
HIA = 343 kJ mol-1), as a consequence of the geometrical strain.
The present contribution provides the first experimental proof
for the effect of a planar tetracoordinate silicon, expressed by the
taming of the reactive class of hydridosilicates. It furthermore sup-
ports SET steps in the reduction chemistry with hydridosilicates.
Finally, it introduces the calix[4]pyrrole ligand as new, four fold
anionic platform for the emerging field of geometrically con-
strained main-group element species.52-54
ASSOCIATED CONTENT
Supporting Information
The Supporting Information, containing all experimental and com-
putational details, is available free of charge on the ACS Publica-
tions website.
AUTHOR INFORMATION
Corresponding Author
Dr. Lutz Greb, greb@uni-heidelberg.de
Notes
The authors declare no conflict of interest.
Figure 3: LUMO energies and HIA of planar 1 and Si(pyrrole)4 2 with computed struc-
ture of 1, including the isodensity plot of the Kohn-Sham LUMO.
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ACKNOWLEDGMENT
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fruitful discussions, the FCI and DFG for financial support and the
BWFor/BWUniCluster for computational resources. F.E. is grate-
ful to the Foundation of German Business (sdw) for a fellowship.
A
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