Accessing Two-Coordinate ZnII Organocations by NHC Coordination: Synthesis, Structure, and Use as π-Lewis Acids in Alkene, Alkyne, and CO2 Hydrosilylation

Article abstract of DOI:10.1002/chem.201704382

Discrete two-coordinate Zn^II organocations of the type (NHC)Zn?R⁺ are reported, thanks to NHC stabilization. In preliminary reactivity studies, such entities, which are direct cationic analogues of long-known ZnR₂ species,

Full text of DOI:10.1002/chem.201704382

A Journal of
Accepted Article
Title: Accessing two-coordinate Zn(II) Organocations via NHC
coordination: synthesis, structure and use as π-Lewis acids in
alkene, alkyne, and CO2 hydrosilylation
Authors: Samuel Dagorne, David Specklin, Frédéric Hild, Christophe
Fliedel, Christophe Gourlaouen, and Luis Veiros
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To be cited as: Chem. Eur. J. 10.1002/chem.201704382
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Accessing two-coordinate Zn(II) Organocations via NHC
coordination: synthesis, structure and use as -Lewis acids in
alkene, alkyne, and CO2 hydrosilylation
David Specklin,[a] Frédéric Hild,[a] Christophe Fliedel,[a] Christophe Gourlaouen,[a] Luis. F. Veiros[b] and
Samuel Dagorne*[a]
Dedication ((optional))
Abstract: Discrete two-coordinate Zn(II) organocations are first
reported, presently of the type (NHC)Zn–R+, thanks to NHC
stabilization. In preliminary reactivity studies, such entities, which are
direct cationic analogues of long-known ZnR2 species, act as
effective and tunable π-Lewis acid catalysts in alkene, alkyne and
CO2 hydrosilylation.
Structurally characterized and isolable two-coordinate
Zn(II) organocations of the type [(L)Zn–R]+ (L monodentate) are
unknown thus far. To date, (DMF)Zn–alkyl+ cations have been
observed in the gas phase and a two-coordinate phenalenyl-
based Zn(II) cation was proposed to exist in solution.[7] In
principle, in addition to their fundamental interest, such
electrophilic entities may represent a better reactivity/stability
balance when compared to, on the one hand, more robust but
less Lewis acidic three-/four-coordinate cationic analogues and,
on the other hand, elusive mono-coordinate [RZn]+ cations.
Stable two-coordinate [(L)Zn–R]+ cations (L monodentate)
are expected to require a strong σ-donating L ligand (for a robust
L–Zn–R+ array) providing a significant steric protection of the
Lewis acidic Zn(II) center, ligand properties for which N-
heterocyclic carbenes (NHCs) are so priviledged and widely
used.[8] As part of our studies on NHC-stabilized Zn(II)
organocations,[5d,9] we herein report on the synthesis and
structure of two-coordinate Zn(II) di-organyl cations (NHC)Zn–R+
(R = alkyl, aryl), as stable and robust entities thanks to NHC
stabilization. Such entities, which are the first direct cationic
analogues of long-known ZnR2 species, are shown to act as
effective and tunable π-Lewis acid catalysts in alkene, alkyne
and CO2 hydrosilylation.
Neutral diorganozinc species, first prepared by Frankland in
1849, have long been established as useful reagents in organic
synthesis and catalysis, most notably for the fonctionalization of
unsaturated substrates.[1] Structurally, such species are typically
monomeric, display a linear C-Zn-C array and bears a two-
coordinate sp-hybridized Zn(II) center with two low-lying vacant
π orbitals available for coordination. Since Zn(II) is only
moderately Lewis acidic (HSAB scale), Zn(II) organocations are
of particular interest for enhanced Lewis acidity at Zn(II) to
improve
substrate
activation
in
Lewis-acid
stoichiometric/catalytic processes. In this area, a number of
structurally characterized three- and four-coordinate Zn(II)
supported by various N-/O-/P-/C-based supporting ligands have
been reported over the past twenty years and successfully
exploited in polymerization, alkene/alkyne hydroamination and
CO2 functionalisation catalysis.[2-5] Such ligand-stabilized Zn(II)
organocations are usually stable but typically display moderate
catalytic performances. For more Lewis acidic Zn-based
organocations, challenging studies were recently performed by
the groups of Wehmschulte and Krossing to access thus-far-
unknown “free” mono-coordinate cation [EtZn]+ via direct
ionization of ZnEt2 in the presence of a weakly coordinating
anion, such as perfluorinated aluminate anions of the type
[Al(OR)4]- or the carborane anion [CHB11Cl11]-.[4a,6] Ion-like
species such as EtZn(η3-C6H6)(CHB11Cl11) and EtZn[Al(OR)4]
display weak anion-Zn(II) bonding interactions leading, for
instance, to a three-coordinate Zn(II) center in EtZn(Al(OR)4.
Discrete [EtZn(arene)2]+ Zn(II) cations, with arene π-coordination
to the Zn(II) center, were also recently characterized.[6]
Two-coordinate cations (NHC)Zn–R+ may be prepared via
alkyde (R-) abstraction reaction from the corresponding neutral
dialkyl precursors with an electrophile such as Ph3C+, an
approach successfully used by Bochmann to access three-/four-
coordinate Zn(II)–alkyl cations.[2c-e] Thus, the reaction of adducts
(IDipp)ZnR2 (R = Me, Et) with 1 equiv [Ph3C][B(C6F5)4] (RT, 30
min, PhBr) led to the high yield formation and isolation of the
corresponding two-coordinate cations [(IDipp)Zn–R]+ (as
-
B(C6F5)4 salts; R = Me, [1][B(C6F5)4]; R = Et, [2][B(C6F5)4];
Scheme 1), as deduced from solution and solid state data.
[a]
[b]
Dr. D. Specklin, Dr. F. Hild, Dr. Fliedel, Dr. C. Gourlaouen, Dr. S.
Dagorne
Institut de Chimie de Strasbourg, CNRS-Université de Strasbourg
(UMR 7177), 1, rue Blaise Pascal, 67000 Strasbourg, France.
E-mail: dagorne@unistra.fr
Prof. L. F. Veiros
Centro de Quımica Estrutural, Instituto Superior Técnico,
́
Universidade de Lisboa, Av. Rovisco Pais No. 1, 1049-001 Lisboa,
Portugal
Supporting information for this article is given via a link at the end of
the document.((Please delete this text if not appropriate))
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Scheme 1. Synthesis of Zn(II) cations 1+-3+ and 6+
.
insertion reactivity (into the Zn–Me bond), unlike the typical
behavior of ZnR2 towards carbonyl substrates.
Cations 1+ and 2+ may be further derivatized for enhanced
Lewis acidity at Zn(II). Thus, 1+ cleanly reacts with 1 equiv
B(C6F5)3 (RT, PhF) via a Me/C6F5 exchange reaction to
+
quantitatively afford two-coordinate cation (IDipp)Zn–C6F5 (6+),
isolated as [6][B(C6F5)4] (Scheme 1).[13] From CH2Cl2, it
+
-
crystallizes as discrete (IDipp)Zn–C6F5 and B(C6F5)4 ions as
determined from XRD studies (Figure 2a). Two-coordinate
cation 6+ is structurally similar to 1+ and 2+, albeit with a shorter
Zn-CNHC bond distance [1.938(3) vs. 1.961(3) Å in 2+], indicative
of a more electrophilic Zn(II) center. DOSY 1H and 19F NMR data
for [6][B(C6F5)4] agree with separated ions in solution (CD2Cl2,
RT; Figures S17-20). From PhBr, cation 6+ crystallizes as adduct
(IDipp)Zn–C6F5+.PhBr (Figure 2b) with a side-on position of PhBr
and a Zn...Br distance [2.6909(5) Å] well below the sum of VdW
radii of Zn and Br (3.24 Å). π-interactions between the PhBr and
Zn-C6F5+ aromatic rings may also stabilize such an arrangement
in the solid state. Similar M...BrPh interactions were observed
with the strong Lewis acid Al(C6F5)3 and may account for the
excellent thermal stability of cation 6+ in PhBr.[14]
Figure 1. Molecular structure of cation 2+ (H atoms are omitted for clarity).
Selected bond distances (Å) and angles (°): Zn-Et = 1.909(5), Zn-NHC =
1.961(3), C-Zn-C = 173.0(3).
The molecular structures of salts [1-2][B(C6F5)4] were
determined through single-crystal X-ray diffraction studies (XRD)
and both display similar features (Table S1; Figures 1, S20-21).
Taking the example of [2][B(C6F5)4], it crystallizes from PhBr as
-
discrete two-coordinate (IDipp)Zn–Et+ cations and B(C6F5)4
anions with no cation/anion short contacts (Figure S22). Cation
2+ contains a central sp-hybridized two-coordinate Zn(II) center,
with a nearly linear CNHC-Zn-CEt array [173.0(3)°]. The Zn–CEt
bond distance in 2+ [1.909(5) Å] is significant shorter than that in
higher coordinate Zn(II)–alkyl cations [1.964(6) and 1.964(3) Å in
+
Et–Zn(η3-toluene)(κ2-CHB11Cl11)]
and
Et–Zn(OEt2)3 ,
respectively,[4a,10] and in ZnEt2 [1.948(5) Å],[11] reflecting an
enhanced electrophilicity at Zn(II). The NMR data for [1-
2][B(C6F5)4] (CD2Cl2, RT) are consistent with weakly interacting
anions and cations in solution. The 1H and 19F DOSY NMR data
for salt [1][B(C6F5)4] suggests the presence of contact ion pairs
(RT, CD2Cl2).[12] In line with their ionic nature, cations 1+ and 2+
are only sparingly soluble in benzene and toluene while they are
soluble and stable (for days) in CD2Cl2 and C6D5Br (RT, under
N2). Steric protection at Zn(II) by the NHC ligand may matter for
stability. Thus, the less sterically bulky cation (IMes)Zn–Me+ (3+,
Scheme 1), generated by ionization of (IMes)ZnMe2 with
[Ph3C][B(C6F5)4], is unstable at room temperature and
decomposes within a few hours to unknown species, precluding
its isolation and further use. Expectedly, cation 1+ reacts fast and
irreversibly with Lewis bases (L) such as THF and benzaldehyde
to form the corresponding (IDipp)Zn(Me)(L)+ adducts (4+ and 5+,
respectively). The Zn-PhCHO cation 5+ is however stable for
hours in C6D5Br (90 °C, 12h) or THF (70 °C, 12h) with no C=O
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Figure 2. Molecular structures of cation 6+ (a) and 6+.PhBr (b). Selected bond
distances (Å) and angles (°): for 6+; Zn-C6F5 = 1.919(3), Zn-NHC = 1.938(3),
C-Zn-C = 175.4(1). For 6+.PhBr; Zn...Br = 2.6909(5), C-Zn-C = 155.8(1).
Figure S27). This contrasts with the absence of any catalytic
activity when using B(C6F5)3 as 1-octyne hydrosilylation catalyst
under identical conditions (< 5% conv. to 9, 24h, CD2Cl2). A
Lewis-acid-mediated reaction is likely operating based on
experimental data, with a hydrosilylation proceeding an anti-1,2-
addition of Si–H to the C-C triple bond.[20]
To gain further insight on cations 1+, 2+ and 6+ as -Lewis
acids, their Lewis acidity was experimentally estimated with the
Gutmann-Beckett method, where the 31P NMR chemical shift of
Et3P=O (acting as a Lewis base) is used to probe Lewis
acidity.[15] According to these measurements, the Lewis acidity of
6+ is comparable to that of B(C6F5)3 and higher than that of 1+
and Zn(C6F5)2 [Δδ31P (CD2Cl2) = 26.4, 24.7, 17.2 and 14.5 ppm
for B(C6F5)3, 6+, Zn(C6F5)2 and 1+, respectively], albeit within the
limit of validity of such a method.[16] Since best suited for HSAB
hard Lewis acid/base interactions, the Fluoride Ion Affinity (FIA),
which also correlates to Lewis acidity (the higher the FIA, the
stronger the Lewis acid), was also calculated for cations 1+ and
6+ using a known methodology (see Supporting Information).[17]
FIA calculations agree with 6+ and 1+ being more Lewis acidic
than B(C6F5)3 [FIA = 156, 146, 106 and 86 kcal/mol for 6+, 1+,
B(C6F5)3 and Zn(C6F5)2, respectively]. In any case, the latter
estimations establish 6+ and, to a lesser extent, 1+ as potent π-
Lewis acids. Furthermore, as DFT-estimated (BLYP/def2-SVP),
it is interesting to note that the LUMOs of 1+ and 6+ both exhibit
a significant -bonding interaction between the Ccarbene and Zn(II),
which suggests that the NHC -Lewis acid character may further
enhance the Lewis acidity of these cations (for 1+ and 6+, see
Figures 3 and S35, respectively).
Scheme 2. Alkene and alkyne hydrosilylation catalysis by cation 6+
There are a number of metal-based catalysts able to
efficiently hydrosilylate CO2 to the formate level (HCO2SiR3).
However, effective and selective systems for CO2 reduction to
aldehyde-,
methanol-equivalent
and
methane
(i.e.
R3SiOCH2OSiR3, CH3OSiR3 and CH4) remain challenging.[21,22]
As recently shown, strong group 13 Lewis acids, such as ion
pair Et2Al(CHB11Cl11) and mixed Al(C6F5)3/B(C6F5)3 catalytic
systems, may convert CO2 to CH4.[5a,23] Satisfyingly, Zn(II) cation
1+ (5% mol.) catalyzes CO2 hydrosilylation in the presence of
HSiEt3 to quantitatively and selectively yield methanol-equivalent
MeOSiEt3 (1.5 bar of CO2, C6D5Br, 90 °C, 12h), along with
Et3SiOSiEt3 as a side-product (Scheme 3, Figures S28-S29).
Instead, when using more Lewis acidic 6+ (5% mol.), CO2 is
completely converted to CH4 within 6h under identical reaction
conditions (Scheme 3, Figures S30-S31). As a comparison,
+
benchmark Lewis acids Zn(C6F5)2, B(C6F5)3 and EtZn(OEt2)3
are inactive in CO2 hydrosilylation under such conditions. In line
with the robustness of 6+, decreasing catalyst loading by ten
(0.5% mol. in 6+) led to a 95% conv. of the CO2/HSiEt3 mixture to
CH4 within 23h (C6D5Br, 90 °C; Figure S32).
Figure 3. Front (left) and side (right) view of the DFT-computed (BLYP/def2-
SVP) LUMO of cation 6+ (-4.756 eV).
Cations 1+, 2+ and 6+ were evaluated in alkene, alkyne and
CO2 hydrosilylation catalysis and their performance compared to
B(C6F5)3.[18] Thus, in the presence of HSiEt3, cation 6+ (5% mol.)
catalyzes 1-hexene hydrosilylation at room temperature (CD2Cl2,
quantitative conv. to 7 within 18h, Scheme 2) while less Lewis
acidic cations 1+ and 2+ are inactive under these conditions
(Figures S25). Hydrosilylation of methylcyclohexene mediated
by 6+ (5% mol., CD2Cl2, RT, 14h) only led to hydrosilylated
product 8, thus consistent with a selective anti-1,2-addition of
Si–H to the C=C bond (Figures S26). Cation 6+ retains its
integrity during catalysis with no apparent catalyst/substrates
Scheme 3. CO2 hydrosilylation catalyzed by cations 1+ and 6+.
1
According to 1H NMR control experiments, both 2+ and 6+ are
unreactive under CO2 alone (1.5 bar, C6D5Br, 90 °C, 24h), and,
likewise, do not react with HSiEt3 on its own (5 equiv, C6D5Br,
90 °C, 24h). Also, while CO2 hydrosilylation is neglectable at
room temperature (< 5% conv. to CH4 after 24h), 6+ reacts at
interactions, as monitored by H NMR (CD2Cl2, RT). Altogether,
these data are consistent a Lewis-acid-catalyzed process, as
observed with strong group 13 Lewis acids E(C6F5)3 (E = B,
Al).[19] Also, cation 6+ is highly active and selective in 1-octyne
hydrosilylation to afford the Z-selective product 9 (5% mol. of 6+,
quantitative conv. to 9 within 15 min, RT, CD2Cl2, Scheme 2,
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room temperature with silyl formate HCO2SiEt3 (7 equiv) in the
presence of HSiEt3 (21 equiv) to quantitatively afford a mixture
of MeOSiEt3 and CH4 (15 and 85% conv., respectively; 24h,
C6D5Br), with catalyst 6+ retaining its integrity (Figure S33).
Under identical conditions, the stoichiometric reaction of 6+ with
a 1/1 HCO2SiEt3/HSiEt3 mixture leads to the fast formation of a
1/5 CH2(OSiEt3)2/MeOSiEt3 mixture (15 min, 50% conv. of
In summary, we first reported on discrete two-coordinate
Zn(II) organocations, presently of the type (NHC)Zn–R+, thanks
to NHC stabilization. Structurally analogous to ZnR2 compounds,
preliminary reactivity studies show such cations behave as
effective and selective π-Lewis acid catalysts for a number of
organic transformations of current interest, outperforming
classical Lewis acids such as B(C6F5)3 in some instances. These
HSiEt3) along with unreacted HCO2SiEt3 (Figure S34). Altogether, cations benefit from several attractive features which should
these observations suggest
a
Lewis-acid-mediated CO2
promote their further use and development, including: i) stability,
ii) possible tunability of the Lewis acidity (via the nature of the
NHC and Zn–R+ moieties) and iii) straightforward synthesis from
commonly used reagents. With Zn being a cheap and low toxic
metal source, this may lead to new developments in
fundamental and applied reactivity.
reduction catalysis with the initial CO2 functionalisation (i.e.
formation of HCO2SiEt3) being rate-limiting, followed by faster
sequential hydrosilylation of HCO2SiEt3 to CH2(OSiEt3)2,
MeOSiEt3 and finally CH4 (Scheme 4).
Acknowledgements ((optional))
The authors thank the CNRS, the University of Strasbourg and
the University of Lisbon for financial support. The high
computing center of the University of Strasbourg is
acknowledged for computing time. The laboratory of Prof. J.
Moran (ISIS, University of Strasbourg) is thanked for GC-MS
analysis.
Scheme 4. Stepwise hydrosilylation of CO2 to methanol-equivalent and CH4
catalyzed by cations 1+and 6+
Keywords: Zinc • organocation • carbene • Lewis acid •
hydrosilylation
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(kcal/mol) reaction of CO2 with HSiEt3 mediated by VI+ (model of 6+)
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-
of 1+ and B(C6F5)4 ions in close vicinity as in the solid state (see
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10.1002/chem.201704382
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Discrete two-coordinate Zn(II)
organocations, presently of the
type are first
(NHC)Zn–R+,
structurally characterized thanks to
NHC stabilization. Such entities,
Author(s), Corresponding Author(s)*
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Title
which
are
direct
cationic
analogues of long-known ZnR2
species, act as effective and
tunable π-Lewis acid as effective
and tunable π-Lewis acid catalysts
in alkene, alkyne and CO2
hydrosilylation.
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