Investigation of N-Heterocyclic Carbene-Supported Group 12 Triflates as Pre-catalysts for Hydrosilylation/Borylation

Article abstract of DOI:10.1002/chem.201603704

N-Heterocyclic carbene (NHC) complexes of Cd and Hg triflates (OTf) were prepared and their attempted conversion into rare cadmium and mercury hydrides was explored. In contrast to zinc, which forms stable [ZnH]⁺complexes with NHCs, the heavier Cd and Hg congeners could not be formed; the increased instability of Cd-H and Hg-H units was rationalized with the aid of computations. It was also discovered that the dimeric adduct [IPr?Cd(μ-OTf)₂]₂(IPr=[(HCNDipp)₂C:]; Dipp=2,6-iPr₂C₆H₃) is an active precatalyst for the hydrosilylation and hydroborylation of hindered aldehydes and ketones. The related zinc congener was inactive as a catalyst highlighting a distinct advantage of using heavy Group 12 metals to promote catalytic hydrosilylation/borylation.

Full text of DOI:10.1002/chem.201603704

DOI: 10.1002/chem.201603704
Full Paper
&
Inorganic Chemistry
Investigation of N-Heterocyclic Carbene-Supported Group 12
Triflates as Pre-catalysts for Hydrosilylation/Borylation
[
a]
Matthew M. D. Roy, Michael J. Ferguson, Robert McDonald, and Eric Rivard*
Abstract: N-Heterocyclic carbene (NHC) complexes of Cd
and Hg triflates (OTf) were prepared and their attempted
conversion into rare cadmium and mercury hydrides was ex-
also discovered that the dimeric adduct [IPr·Cd(m-OTf)2]2
(IPr=[(HCNDipp) C:]; Dipp=2,6-iPr C H ) is an active preca-
2
2
6
3
talyst for the hydrosilylation and hydroborylation of hin-
dered aldehydes and ketones. The related zinc congener
was inactive as a catalyst highlighting a distinct advantage
of using heavy Group 12 metals to promote catalytic hydro-
silylation/borylation.
+
plored. In contrast to zinc, which forms stable [ZnH] com-
plexes with NHCs, the heavier Cd and Hg congeners could
not be formed; the increased instability of Cd-H and Hg-H
units was rationalized with the aid of computations. It was
Introduction
drides, molecular species containing Cd–H or Hg–H moieties
[1d,5]
remain extremely rare in the literature (Scheme 1),
while
The study of inorganic hydrides has increased dramatically re-
cently due to the use of sterically encumbered anionic co-li-
gands or neutral donors to stabilize the more reactive mem-
their use in promoting catalysis has not been documented
thus far.
[
1]
bers of this important molecular class. Of particular interest
to this current study on Group 12 element hydrides, the
parent binary MH species (M=Zn, Cd and Hg) are reactive
2
solids that have been known for over sixty years. The thermal
stability of these metal hydrides decreases dramatically as one
descends the group, with decomposition noted for ZnH upon
2
heating to 908C (slow decomposition at room temperature),
while CdH and HgH decompose into their constituent ele-
2
2
[
2]
ments at ꢀ208C and ꢀ1258C, respectively.
In 2013 Okuda and co-workers reported the N-heterocyclic
carbene (NHC)-supported molecular zinc dihydride dimer
[3]
[
IPr·ZnH(m-H)] (IPr=[(HCNDipp) C:]; Dipp=2,6-iPr C H ). This
2 2 2 6 3
zinc dihydride complex exhibits low thermal stability as it
slowly decomposes in the solid state at room temperature,
and degradation was noted upon prolonged storage at
Scheme 1. Selected cadmium and mercury hydride complexes (Dipp=2,6-
iPr C H ; R=CH C(CH ) CH OH, CH , p-NO C H ; R ’=C H , H).
2 6 3 2 3 2 2 3 2 6 4 2 5
[
4]
ꢀ
358C. This result prompted our group to design the NHC-
complex [IPr·ZnH(OTf)·THF] which contains a formal cationic
+
zinc hydride unit [ZnH] along with a weakly associated triflate
Herein we report the synthesis of a series of new carbene-
ꢀ
ꢀ
[4,5]
(OTf =O SCF ) counterion.
Notably [IPr·ZnH(OTf)·THF] is
supported cadmium and mercury triflate compounds and the
attempted preparation of their corresponding hydrides; we
also provide guiding computational data on the stabilities of
3
3
catalytically active with respect to the hydrosilylation and hy-
[
4,6]
droborylation of benzophenone, and contrary to Okuda’s in-
+
itial NHC-ZnH adduct, our [ZnH] species is monomeric and
the homologous NHC adduct series [IPr·MH ] (M=Zn, Cd and
2
2
has considerable thermal stability (up to 2608C) in the solid
state. Despite the abovementioned successes with zinc hy-
Hg). The central tenet driving this study is the expected en-
hanced reactivity of Cd and Hg hydrides due to the increased
hydridic character and size of the Group 12 element in relation
to zinc hydrides, which should promote catalysis; in regard to
this concept, we now show that a Cd-containing species is far
superior as a hydrosilylation/borylation precatalyst than its zinc
congener, representing a promising new direction in catalysis.
[a] M. M. D. Roy, Dr. M. J. Ferguson, Dr. R. McDonald, Prof. Dr. E. Rivard
Department of Chemistry, University of Alberta
11227 Saskatchewan Drive, Edmonton, AB T6E 2G2 (Canada)
E-mail: erivard@ualberta.ca
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201603704.
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Results and Discussion
[
7]
In previous work we reported the attempted formation of
the heavy Group 12 dihydride complexes [IPr·MH ] (M=Cd
2
and Hg) from their corresponding halide precursors [IPr·CdI(m-
[
8]
[9]
I)]2 and [IPr·HgI2], using Li[HBEt ] (4 and 2 equiv, respective-
3
ly) as a hydride source. This reaction invariably led to the im-
mediate liberation of Cd and Hg metal and the recovery of
soluble [IPr·BEt ] and the dihydroaminal IPrH2 [IPrH =
3
2
[
10]
(
HCNDipp) CH ] as byproducts. Given the enhanced thermal
2 2
stability of [IPr·ZnH(OTf)·THF] relative to the parent dimer
IPr·ZnH(m-H)] , we looked to form a series of NHC-bound cad-
[
2
mium and mercury triflates as potential precursors to [IPr·M-
H(OTf)]x (M=Cd, Hg). Due to the apparent instability of
[
7]
[
IPr·MH ] complexes (M=Cd and Hg), our approach was to
2
first install triflate groups onto the metal followed by hydride
Figure 1. Molecular structure of 1a with thermal ellipsoids plotted at the
ꢀ
ꢀ
30% probability level. All hydrogen atoms, CH Cl and Et O solvate mole-
2
2
2
complex formation via OTf /H exchange.
cules have been omitted for clarity. Selected bond lengths [ꢁ] and angles [8]:
C(1A)-Cd(A) 2.191(4), Cd(A)-I(OA) 2.7923(4), Cd(AA)-I(OA) 2.8204(4), Cd(A)-
O(1A) 2.281(4), Cd(A)-O(3AA) 3.276(5); Cd(A)-I-Cd(AA) 74.709(12), I(OA)-
Cd(A)-I(OAA) 105.290(12).
The first stable cadmium triflato complex prepared in this
study [IPr·Cd(m-I)(OTf)] (1) was generated as a colorless solid in
2
90% yield by adding 2 equiv of AgOTf to the CdI2 adduct
[
IPr·CdI(m-I)]2 in CH Cl , followed by removal of the bright
2
2
yellow AgI precipitate by filtration (Scheme 2).
As shown in Figures 1 and 2, compound 1 crystallizes with
two different structural arrangements in the solid state: one
species features monodentate (terminal) Cd–OTf coordination
mode (1a, Figure 1), while in the second molecule in the asym-
(Figure 3) confirmed the presence of a cage-like structure for 2
derived from four symmetric OTf-bridges that each link two
formally 5-coordinate (square pyramidal) cadmium centers.
Presently, there are two related structures in the literature
which also exhibit triflate groups bridging two cadmium cen-
ꢀ
metric unit (1b) the OTf ligands adopt bridging motifs span-
[12]
ning two Cd centers (Figure 2). In each structure, the iodo li-
gands link two Cd atoms to yield centrosymmetric Cd I rhom-
ters in a similar fashion as in 1b and 2.
The bridging CdꢀO bond lengths in 2 [2.270(2)–2.299(2) ꢁ]
are notably shorter than in 1b [2.432(3) and 2.660(3) ꢁ] and
are almost the same value within experimental error as in the
mixed element cluster [(m-OTf)Cd{Zn (OiPr) }] where CdꢀO dis-
2
2
[
11]
boid cores. Despite the presence of two structurally distinct
molecules in the crystalline lattice of 1, the NMR spectra of
ꢀ
1
in CDCl reveal the presence of one carbene and one OTf
3
2
9
2
ꢀ
[12a]
environment; thus it appears that the OTf groups in 1 are
fluxional/mobile in solution.
tances of 2.234(5) and 2.256(4) ꢁ are found.
Consistent with
stronger average Cd–OTf interactions in [IPr·Cd(OTf)2]2 (2)
When four equivalents of AgOTf were added to [IPr·CdI(m-I)]2
the complete substitution of the iodine atoms for triflate
groups transpired to yield the centrosymmetric dimer
versus in the iodo-OTf complex [IPr·Cd(OTf)(m-I)] (1), a down-
2
19
1
field shifted F{ H} NMR resonance at ꢀ74.4 ppm (in CDCl ) is
3
found in 2, corresponding to the presence Cd-coordinated
ꢀ
[13]
[IPr·Cd(m-OTf)2]2 (2) in a 93% yield. X-ray crystallography
OTf groups in solution. The structure of 2 is notably differ-
Scheme 2. Preparation of the NHC-Cd triflate complexes 1–3.
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In an attempt to disrupt the persistent bridging OTf interac-
tions in 2, this complex was combined with an excess (6 equiv)
of the strong donor 4-dimethylaminopyridine (DMAP) in THF
1
(
Scheme 2). The crude H NMR spectrum revealed the com-
plete consumption of 2 and the presence of two distinct IPr
environments, along with small amounts of the imidazolium
salt [IPrH]OTf. Crystals were obtained by cooling a concentrated
CH Cl solution of the crude material to ꢀ358C. The resulting
2
2
crystallographic structure determination identified the pres-
ence of two discrete molecules in the lattice in a 1:1 ratio:
[
IPr·Cd(DMAP) ][OTf]2 (3a) and [IPr·Cd(DMAP) (OTf) ] (3b;
3 2 2
shown in Figure 4 and Figure 5, respectively). Although com-
pounds 3a and 3b both exhibit high solubility in CH Cl and
2
2
moderate solubility in THF, concentration of the crude reaction
mixture in THF results in the exclusive initial precipitation of
the dicationic cadmium salt 3a as a pure microcrystalline solid.
The remaining DMAP complex 3b could not be isolated in an
analytically pure form due to its similar solubility as the imida-
Figure 2. Molecular structure of 1b with thermal ellipsoids plotted at the
0% probability level. All hydrogen atoms, CH Cl and Et O solvate mole-
cules have been omitted for clarity. Selected bond lengths [ꢁ] and angles [8]:
C(1B)-Cd(B) 2.193(4), Cd(B)-O(1B) 2.432(3), Cd(BA)-O(2B) 2.660(3); Cd(B)-I(OB)-
Cd(BA) 71.273(12), I(OB)-Cd(B)-I(OBA) 108.727(12).
3
2
2
2
+
zolium [IPrH] salt byproduct. Even when the reaction of 2 is
carried out with a large excess of DMAP (10 equiv), the con-
comitant formation of 3b is always observed (ca. 5% by
1
H NMR spectroscopy).
Figure 3. Molecular structure of 2 with thermal ellipsoids plotted at the 30%
probability level. All hydrogen atoms, OTf-containing fluorine atoms and
Figure 4. Molecular structure of 3a with thermal ellipsoids plotted at the
CH
angles [8]: C(1)-Cd 2.189(2), Cd-O range 2.270(2)–2.299(2); C(1)-Cd-O range
09.79(9)-114.04(9), “cis” O-Cd-O range 79.87(10)-82.93(10), “trans” O-Cd-O
2
Cl
2
solvate have been omitted for clarity. Selected bond lengths [ꢁ] and
30% probability level. All hydrogen atoms and CH
omitted for clarity. Selected bond lengths [ꢁ] and angles [8]: C(6)-Cd(2)
2 2
Cl solvate have been
2.200(2), N-Cd(2) 2.242(2)–2.258(2); C(6)-Cd(2)-N 114.83(8)-125.81(8).
1
range 135.55(10)-136.15(10).
As mentioned previously, we were also interested in access-
ing stable NHC–Hg complexes to see if reactive HgꢀH bonding
environments could be obtained for subsequent catalysis. The
target carbene-bound mercury triflate precursors were pre-
ent from the 4-coordinate monomeric zinc bis(triflato)-carbene
[
14]
complex, [IMes·Zn(OTf) ·THF]
(IMes=[(HCNMes) C:]; Mes=
2
2
ꢀ
2
,4,6-Me C H ). When 2 is dissolved in THF and the solvent is
pared (Scheme 3) again by using AgOTf as a source of OTf
3
6
2
removed in vacuo, the remaining product is the THF-free
groups. When 1 equiv of AgOTf was combined with [IPr·HgI2]
adduct [IPr·Cd(m-OTf) ] (2) suggesting that any coordination of
in CH Cl , the expected iodo-triflato complex [IPr·HgI][OTf] (4)
2
2
2
2
THF to Cd in solution is both reversible and weak in nature.
In order to probe the nature of 2 in solution, a diffusion-or-
dered spectroscopy (DOSY) NMR experiment was carried out in
was obtained as a colorless solid in quantitative yield.
The molecular structure of [IPr·HgI][OTf] (4; Figure 6) shows
a slightly bent quasi-two coordinate geometry about mercury
[
D ]THF. The diffusion coefficient of 2 was estimated to be
.0ꢂ10 m s , corresponding to a hydrodynamic radius of
.9 ꢁ. This is in good agreement with the end-to-end distance
with a C -Hg-I angle of 166.46(7)8. The canting of the C-Hg-I
8
IPr
ꢀ
10
2
ꢀ2
8
angle away from a linear arrangement is due to the presence
of a pair of weak Hg–O interactions [2.679(2) and 2.880(2) ꢁ]
involving a proximal triflate group (Figure 6). The coordinative
CIPr–Hg distance is 2.075(3) ꢁ and is the same within experi-
5
of 2 in the solid state (14.3 ꢁ), implying the dimeric nature is
retained in solution.
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Figure 5. Molecular structure of 3b with thermal ellipsoids plotted at the
0% probability level. All hydrogen atoms and CH Cl solvate have been
omitted for clarity. Selected bond lengths [ꢁ] and angles [8]: C(1)-Cd(1)
.220(2), Cd(1)-O 2.449(7)–2.478(2), Cd(1)-N 2.218(2)–2.230(2); O(1)-Cd(1)-
Figure 6. Molecular structure of 4 with thermal ellipsoids plotted at the 30%
probability level. All hydrogen atoms have been omitted for clarity. Selected
bond lengths [ꢁ] and angles [8]: C(1)-Hg 2.075(3), Hg-I 2.5689(3), Hg-O(1)
3
2
2
2
2.679(2), Hg-O(2) 2.880(2); C(1)-Hg-I 166.46(7), C(1)-Hg-O(1) 99.14(9), C(1)-Hg-
O(4 A) 163.27(18), C(1)-Cd(1)-N 120.37(8)-123.94(8).
O(2) 93.57(9).
Scheme 3. Preparation of the NHC–mercury triflato complexes 4 and 5.
mental error as the corresponding distance in the monomeric
II
[9]
Hg halide adduct [IPr·HgI ].
2
The last member of the NHC-Group 12 element triflato
Figure 7. Molecular structure of 5·THF with thermal ellipsoids plotted at the
series to be discussed here, [IPr·Hg(OTf) ] (5), was synthesized
2
30% probability level. All hydrogen atoms have been omitted for clarity. Se-
by adding 2 equiv of AgOTf to [IPr·HgI ] in CH Cl . Crystals of
2
2
2
lected bond lengths [ꢁ] and angles [8]: C(1)-Hg 2.049(3), Hg-O(11) 2.435(2),
Hg-O(21) 2.109(2), Hg-O(1) 2.492(2); O(1)-Hg-O(21) 78.27(9), C(1)-Hg-O(21)
162.47(11), C(1)-Hg-O(1) 112.95(10), C(1)-Hg-O(11) 110.24(10).
the resulting compound were grown from a THF solution of 5
layered with hexanes and the corresponding X-ray structure
was found to contain THF within the coordination sphere of
[
15]
mercury [IPr·Hg(OTf) ·THF] (Figure 7). The Hg center in 5·THF
2
adopts a rare distorted cis-divacant octahedral geometry
wherein the ligating carbene carbon (C(1)), the THF oxygen
atom, and one OTf-based oxygen donor (O(21)) all lie in an
equatorial plane and form a T-shaped motif with Hg [sum of
2 equiv of K[sBu BH] in THF at room temperature, the immedi-
3
ate formation of a metallic precipitate was noted in each case;
1
H NMR analysis revealed the formation of free IPr, minor
amounts of IPrH and multiple unidentified products (with no
2
ꢀ
angles in this plane=353.69(17)8]; the remaining OTf group
forms a capping axial interaction with Hg.
discernable Cd–H resonances) for the Cd compounds 1–3,
while the Hg analogues 4 and 5 afforded only free IPr as a solu-
ble product. In order to get a sense of whether these reactive
hydrides could be observed at lower temperatures, THF solu-
tions of compounds 1–5 were first frozen in a cold well in a N2
filled glove-box, and were then layered with THF solutions of
We then explored the reactivity of the new iodo-triflato and
bis(triflato) carbene adducts 1–5 with the commonly used mild
[
4,16]
hydride source K[sBu BH]
to possibly yield [IPr·M-
3
H(OTf)(THF)] x (M=Cd and Hg) complexes. The known zinc
congener [IPr·ZnH(OTf)·THF] is an active hydrosilylation/boryla-
K[sBu BH] that were pre-cooled to ꢀ358C. Warming these reac-
3
[
4]
tion catalyst, and the increased polarity of the M–H linkages
M=Cd and Hg) could accelerate the rate of catalysis. However
when compounds 1–5 were combined with either one or
tion mixtures slowly to room temperature revealed the imme-
diate formation of metallic precipitates upon thawing of the
THF solutions containing the cadmium and mercury triflates;
(
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1
the resulting H NMR spectra were similar to those described
Table 1. Summary of the catalytic hydrosilylation and hydroborylation of
selected aldehydes and ketones (outlined in Scheme 4).
above. Undaunted by these unsuccessful attempts to generate
stable [IPr·MH(OTf)·THF] complexes (M=Cd and Hg), we decid-
ed to explore the catalytic activity of [IPr·Cd(OTf) ] (2) and
[
a]
Cat.
Cat.
mol%]
R/R’
Substrate
t
Yield TOF
2
2
[
b]
[c]
ꢀ1 [d]
[
[min] [%]
[h
]
[
IPr·Hg(OTf) ] (5) with respect to the hydrosilylation and hydro-
2
2
2
2
5
1
5
5
5
1
0.1
1
5
1
1
5
1
1
Ph/Ph
Ph/Ph
Ph/Ph
Ph/Ph
Ph/Ph
Ph/Ph
Ph/Ph
Ph/Ph
Ph/Ph
PhMeSiH
PhMeSiH
PhMeSiH
PhMeSiH
PhMeSiH
HBPin
HBPin
HBPin
HBPin
HBPin
2
60
7
13
14
10
N/A
N/A
526
borylation of benzophenone (Scheme 4) as the in situ forma-
tion of MꢀH bonds could transpire in the presence of silane or
boronic ester.
2
2
2
2
70
120
90
30
10
10
90
30
105
60
10
20
5
79
96
0
0
9
[Zn]
2
2
5
98 593
0
0
32
>99
89 534
>99 297
58 696
N/A
N/A
19
[Zn]
2
2
2
2
2
4-ClC
4-ClC
6
6
H
H
4
/4-ClC
/4-ClC
6
6
H
H
4
4
4
HBPin
HBPin
HBPin
HBPin
20
Mes/H
Mes/H
Cy/Cy
1
[
1
a] [Zn]=[IMes·Zn(OTf)
H NMR integration in C
2
·THF]. [b] [Cat.]=0.02m. [c] Yield determined by
6
D
6
. [d] Calculated per mol. of dimer 2.
+
Scheme 4. Catalytic hydrosilylation and hydroborylation of aldehydes and/or
ketones by using [IPr·Cd(OTf) ] (2) as a precatalyst.
2 2
where 37% of 2 had converted into an [IPrH] salt and a new
carbene containing product in an about 1:1 ratio, however no
Cd–H resonances were observed; the presence of H was also
2
1
noted in the H NMR spectrum. The formation of the known si-
[
18]
Following the successful ketone hydrosilylation and hydro-
borylation conditions used previously by our group with the
active catalyst [IPr·ZnH(OTf)·THF], a THF solution containing
lyltriflate PhMeSiH(OTf) was observed when 2 and PhMeSiH2
were combined at room temperature which can be taken as
ꢀ
ꢀ
substantial evidence of H /OTf exchange between 2 and
5
mol% of [IPr·Cd(OTf) ] (2) and benzophenone was combined
PhMeSiH to initially form a Cd–H intermediate. Compound 2
2
2
2
with phenylmethylsilane (PhMeSiH ) at room temperature.
and PhMeSiH were also combined in toluene in a 1:1 ratio,
2
2
Somewhat to our surprise given our failure at obtaining well-
defined Cd-H complexes, we found 2 to be an active precata-
lyst, with 96% conversion of the ketone to the corresponding
however no reaction was observed at room temperature pre-
sumably due to the insolubility of 2 in toluene.
In order to give more credence to the proposed hydride in-
sertion mechanism, we synthesized the cadmium alkoxide
[IPr·CdI(OiPr)] from [IPr·CdI(m-I)]2 and 2 equiv NaOiPr.
[IPr·CdI(OiPr)] represents a model of the first intermediate
formed in the proposed hydride insertion mechanism, and
[
4]
hydrosilylation product Ph CH-OSi(H)PhMe
after 120 min
2
ꢀ
1
1
(TOF=10 h ), as determined by H NMR spectroscopy. Using
a lower catalyst loading of 1 mol% resulted in the formation of
only a small amount of hydrosilylated product (7% conversion,
Table 1); in all cases the visible formation of Cd metal was
noted.
treatment of this species with PhMeSiH afforded the expected
2
silane PhMeSiH(OiPr) via OiPr/H exchange at cadmium; Cd
metal formation and unidentified soluble products were also
noted.
Although the mechanism of hydrosilylation by cationic NHC
[
4]
zinc hydrides is unknown at this time, it has been postulated
that the ketone inserts into the ZnꢀH bond, forming a zinc
The hydrosilylation of benzophenone was also attempted in
+
alkoxy intermediate [IPr·Zn(OCHR )] which in turn can react
the presence of 5 mol% of [IPr·Hg(OTf) ] (5) in THF. In this case,
2
2
+
with a silane to regenerate the [IPr·ZnH] catalyst and yield
the formation of mercury metal was noted immediately upon
the addition of phenylmethylsilane to the mixture of 5 and
benzophenone, with no sign of benzophenone conversion
after 90 min. When 5 was combined with 2 equiv of PhMeSiH2
in THF, the rapid decomposition to Hg metal occurred along
with the formation of several unidentified soluble products,
[
4,6]
the desired hydrosilylation product in the process.
An alter-
nate possible mechanism for hydrosilylation is a scenario
where the metal complex [M] is acting like a Lewis acid, lead-
ing to a transient [M]–H–SiR –OCR species, as is known for the
3
2
[
17]
B(C F ) -catalyzed hydrosilylation of ketones.
6
5 3
[18]
Our preliminary investigations (vide infra) suggest that a tran-
sient cadmium hydride may be forming from 2 during catalytic
hydrosilylation, which can then react rapidly with benzophe-
none to form a cadmium alkoxy intermediate. The reaction of
the known silyltriflate PhMeSiH(OTf) and free IPr.
The hydroborylation of benzophenone with HBpin was ex-
plored using the Cd complex 2 as a pre-catalyst. The resulting
catalytic hydroborylation reaction was found to be much more
rapid than hydrosilylation, with the reaction going to 98%
completion after 10 min in the presence of a 1 mol% loading
a stoichiometric amount of 2 and PhMeSiH was monitored by
2
1
variable temperature H NMR spectroscopy in [D ]THF. Begin-
8
ꢀ
1
ning at ꢀ808C, the temperature was slowly increased until re-
activity was observed. No reaction occurred until +108C,
of 2 (TOF=593 h ). For comparison, 2 is more active than
Jones’ novel low-coordinate tin(II) hydride hydroborylation cat-
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†
†
alyst [Ar N(SiiPr )SnH] (Ar =C H {CHPh } iPr-2,6,4; TOF=
ent as demonstrated by the short carbene carbon–zinc dis-
tance (2.190 ꢁ) and significant bending of the H-Zn-H angle
from linearity (141.88). The formation of the carbene adducts
of CdH and HgH were also evaluated computationally. Specif-
3
6
2
2 2
ꢀ
1 [19]
8
0 h ), and a recently reported diazaphospholene catalyst
ꢀ
1
[20]
prepared by Kinjo and co-workers (TOF=1.8 h at 908C).
The hydroborylation of benzophenone with 1 mol% of 2 was
attempted in toluene at room temperature, however only 5%
conversion was noted after 8 h, likely due to the insolubility of
the precatalyst. When 2 was combined with 4 equiv of HBpin,
the formation of a metallic precipitate was noted along with
several unidentified soluble products. Notably, the formation
of PinB(OTf) was also detected by NMR spectroscopy, and the
identity of this new species was confirmed by its independent
2
2
ically, [IPr·CdH ] formation was found to be favorable in the
2
gas phase, however less so than the zinc dihydride complex
ꢀ
1
ꢀ1
(D H8=ꢀ17.7 kcalmol , D G8=ꢀ10.0 kcalmol ). The carbene
r
r
carbon–cadmium distance was computed to be elongated rel-
ative to the CꢀZn bond in [IPr·ZnH ] (2.466 ꢁ), a more linear H-
2
Cd-H angle (152.78) was present in [IPr·CdH ], consistent with
2
a
weaker carbene–CdH2 interaction. The formation of
synthesis from HBPin and Me Si(OTf) (see the Experimental
[IPr·HgH ] was found to be the least favorable of the series
3
2
ꢀ1
ꢀ1
Section for full details).
(D H8=ꢀ9.4 kcalmol , D G8=ꢀ0.9 kcalmol ) and a corre-
r
r
Motivated by the high hydroborylation activity observed
with benzophenone, we investigated the ability of 2 to cata-
lyze the hydroborylation of various ketones and aldehydes.
Borylation of 4,4’-dichlorobenzophenone occurred more slowly
than benzophenone, however complete conversion to the
boryl ether (4-ClC H ) CHOBPin was observed after 60 min
spondingly long carbene carbon–mercury distance of 2.882 ꢁ
was found; furthermore the H-Hg-H angle in the carbene com-
plex was only slightly bent from a linear arrangement (172.78),
illustrating the very weak nature of the C –Hg donor–acceptor
IPr
interaction. The general trend of decreasing stability of
[IPr·MH ] down the group was further observed by second-
6
4 2
2
ꢀ
1
(
TOF=20 h ) with a 5 mol% loading of 2. Again, this repre-
order perturbation theory analysis of the C !M interaction
IPr
sents a more enhanced activity relative to Kinjo’s catalyst
(see the Supporting Information for full details). Of note, our
attempts to compute the Cd and Hg dimers [IPr·MH(m-H)]2
(M=Cd and Hg) failed due to IPr dissociation from the M H
ꢀ
1
[20]
(
TOF=1.7 h , T=908C).
High turnover frequencies were
also maintained with more sterically hindered substrates. For
example, the hydroborylation of mesitaldehyde (R/R’=Mes/H
in Scheme 4) occurred rapidly in the presence of 1 mol% 2
with 89% of the aldehyde being converted to product after
2
4
dimeric core during geometry optimization; in contrast, the Zn
congener [IPr·ZnH(m-H)] was previously shown to be an ener-
2
[4]
getic minimum on the potential energy surface. The noted
hydrosilylation and hydroborylation activity in the presence of
the Cd triflate complex 2 suggests the possible formation of
short-lived cadmium hydride species that participates in cataly-
sis. While in the case of the mercury analogue 5, if any hy-
drides were formed, these species decompose rapidly.
ꢀ
1
1
0 min (TOF=534 h ) and quantitative conversion observed
after 20 min. Dicyclohexyl ketone was also investigated as
a substrate, and it was found that 58% of the starting ketone
was converted to the borylated product Cy CHOBPin after just
2
ꢀ
1
5
min with a 1 mol% loading of 2 (TOF=696 h ). To our
knowledge, this represents the first example of the catalytic
hydroborylation of dicyclohexyl ketone. As was the case with
hydrosilylation, when 5 was examined as a hydroborylation
precatalyst, metal formation was observed immediately follow-
2
Table 2. Computational investigation of the formation of [IPr·MH ] ad-
ducts.
ing the addition of HBPin and benzophenone, with no sign of
Adduct
D
r
H
D
r
G
CꢀM
H-M-H
angle [8]
1
ꢀ
1
[a]
ꢀ1 [a]
the borylated product Ph CHOBPin by H NMR spectroscopy.
[kcalmol
]
[kcalmol
]
distance [ꢁ]
2
We also attempted to utilize the known zinc bistriflate ana-
[
IPr·ZnH
[IPr·CdH2]
2
[IPr·HgH ]
2
]
ꢀ23.7
ꢀ17.7
ꢀ9.4
ꢀ15.6
ꢀ10.0
ꢀ0.9
2.190
2.466
2.882
141.8
152.7
172.7
[
14]
logue, [IMes·Zn(OTf) ·THF]
as a precatalyst. In the case of
2
both hydrosilylation and hydroborylation no catalytic activity
was observed. This is likely due to the oxophilic nature of zinc
relative to the soft Lewis acids cadmium and mercury which in-
hibits the formation of a catalytically active zinc-hydride con-
[
a] Calculated for the gas-phase reaction IPr+MH
2
![IPr·MH
2
].
taining species from the mild hydride sources PhMeSiH and
2
Conclusions
ꢀ
ꢀ
HBPin (via H /OTf exchange).
The observation that the IPr-bound cadmium and mercury
triflates 1–5 did not form stable hydride complexes led us to
postulate that the carbene–metal interactions are not very
A series of IPr-supported cadmium and mercury triflates were
synthesized and used as precursors in the attempted formation
of cadmium and mercury hydrides. It was found that the
target metal hydride complexes [IPr·MH(OTf)·THF] (M=Cd or
Hg) could not be prepared, and rapid decomposition into free
carbene and Cd or Hg metal was noted. Computations reveal
the stability of IPr-bound Group 12 metal hydrides [IPr·MH2]
(M=Zn, Cd and Hg) decreases drastically down the group,
which is in agreement with our experimental observations.
strong in the target [IPr·MH ] and [IPr·MH(OTf)] complexes
2
(M=Cd and Hg). This prompted us to study the stability of the
IPr-bound Group 12 element dihydride complexes [IPr·MH2]
computationally at the M06-2X/cc-pVTZ level of theory.
The formation of [IPr·ZnH ] from the free carbene and linear
2
ZnH fragments was found to be significantly exothermic and
2
ꢀ
1
ꢀ1
exergonic (D H8=ꢀ23.7 kcalmol , D G8=ꢀ15.6 kcalmol ;
[IPr·Cd(OTf) ] (2) was found to be an active precatalyst for the
r
r
2 2
Table 2). In comparison with the heavier Group 12 element
congeners, a significant donor–acceptor interaction was pres-
hydrosilylation and hydroborylation of hindered aldehydes and
ketones. In contrast, [IMes·Zn(OTf) ·THF] is completely inactive
2
&
&
Chem. Eur. J. 2016, 22, 1 – 12
www.chemeurj.org
6
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!

Full Paper
as a hydrosilylation/borylation precatalyst; thus the larger size
and increased hydridic character of the putative CdꢀH bonds
formed during the use of 2 as a catalyst is in line with the in-
crease in catalytic activity expected for heavier Group 12 ele-
ment complexes. Future work will include the extension of the
presented concept of using heavy (low oxophilicity) elements
in catalysis to include more environmentally benign systems.
integration from the indexing of the crystal faces. Crystal structures
[28]
were solved using intrinsic phasing (SHELXT) and refined using
[29]
SHELXL-2014. The assignment of hydrogen atom positions were
2
3
based on the sp or sp hybridization geometries of their attached
carbon atoms and were given thermal parameters 20% greater
than those of their parent atoms.
Special refinement conditions: 3a/b·0.5(CH Cl )
2
2
Attempts to refine peaks of residual electron density as disordered
or partial-occupancy solvent dichloromethane chlorine or carbon
atoms were unsuccessful. The data were corrected for disordered
electron density through use of the SQUEEZE procedure as imple-
Experimental Section
General
[30]
All reactions were performed in an inert atmosphere glovebox (In-
novative Technology, Inc.) and were manipulated in the absence of
light until the products were isolated as solids. Solvents were dried
mented in PLATON. A total solvent-accessible void volume of
3
170 ꢁ with a total electron count of 49 (consistent with 1 molecule
of solvent dichloromethane) was found in the unit cell. The NꢀC
distance was restrained to be 1.450(2) ꢁ for the following pairs of
atoms: N6AꢀC48A, N6AꢀC49A, N6BꢀC48B, N6BꢀC49B. Additionally,
an anti-bumping restraint (2 ꢁ) was applied to H44···H48C.
[21]
using a Grubbs-type solvent purification system manufactured
by Innovative Technologies, Inc., degassed (freeze-pump-thaw
method), and stored under an atmosphere of nitrogen prior to
use. CdI , HgI , K[sBu BH] (1.0m solution in THF), dicyclohexyl
2
2
3
ketone, 4,4’-dichlorobenzophenone, trimethylsilyl triflate, sodium
isopropoxide, and PhMeSiH2 were purchased from Aldrich and
used as received. AgOTf and HBPin were purchased from Matrix
Scientific and used as received. Mesitaldehyde was purchased from
Computational methods
All calculations were carried out using the Gaussian 09, Rev. D.01,
[
31]
software package.
Input structures were optimized using the
[22]
[32]
[33]
Tokyo Chemical Industry and used as received. IPr,
B3LYP functional and 6–31G(d,p) basis set in the gas phase.
[
14]
[8]
[9]
[34]
[
IMes·Zn(OTf) ·THF],
[IPr·CdI(m-I)]2, and [IPr·HgI ] were pre-
The structures were then optimized using the M06–2X functional
2
2
1
11
1
13
1
[35]
pared according to literature procedures. H, B{ H}, C{ H} and
and cc-pVTZ basis set in the gas phase with “very tight” conver-
1
9
1
F{ H) NMR spectra were recorded on 400, 500, 600 or 700 MHz
Varian Inova instruments and were referenced externally to SiMe
gence criteria and an ultrafine integration grid. A 28 electron, fully
relativistic effective core potential (ECP28MDF)
for cadmium with the cc-pVTZ-PP basis set and a 60 electron,
fully relativistic effective core potential (ECP60MDF) was utilized
for mercury with the same cc-pVTZ-PP basis set. All optimized
structures were then confirmed to be local energy minima on the
potential energy surface by frequency analysis.
[36]
was employed
4
1
13
1
19
1
11
1
[37]
(
H, C{ H}), CFCl ( F{ H}) or BF ·Et O ( B{ H}). Elemental analyses
3 3 2
[
36]
were performed by the Analytical and Instrumentation Laboratory
at the University of Alberta. Melting points were measured in
sealed glass capillaries under nitrogen by using a MelTemp melting
point apparatus and are uncorrected. Diffusion-ordered spectrosco-
py (DOSY) experiments were performed on Varian 500 and
[37]
6
00 MHz instruments equipped with a Z-gradient broadband
Synthesis of [IPr·Cd(m-I)(OTf)] (1)
2
ꢀ1
probe capable of outputting 61.6 Gcm of gradient strength. All
measurements were carried out non-spinning and at a calibrated
temperature of 278C using the Oneshot45 pulse sequence.
To a vial charged with [IPr·Cd(I)(m-I)]
AgOTf (0.059 g, 0.21 mmol) was added 5 mL of CH
(0.174 g, 0.115 mmol) and
Cl which result-
2
[23,24]
For
2
2
all DOSY experiments a spectral window of 6 kHz was used with
a 3 s acquisition time and a 2 s S4 relaxation delay with 8 scans for
each gradient increment. Pulse widths and gradient strengths were
optimized for each sample. For both solvent systems a diffusion
delay of 50 ms and a diffusion gradient length of 2 ms was used.
ed in the immediate formation of a yellow precipitate. The result-
ing mixture was stirred for 2 h, filtered and the solid was discarded.
The solvent was removed from the colorless filtrate under vacuum
affording [IPr·Cd(m-I)(OTf)]
tals of [IPr·Cd(m-I)(OTf)] suitable for X-ray crystallographic analysis
were obtained by storing a solution of the product in CH Cl lay-
, 400.0 MHz):
as a colorless solid (0.140 g, 90%). Crys-
2
2
ꢀ1
Gradient strengths of 1.9 to 47.1 Gcm incremented in 20 steps
were used for the toluene solutions and gradient strengths of 1.9
2
2
1
ered with Et
O at ꢀ358C, overnight; H NMR (CDCl
2
3
ꢀ
1
3
to 37.6 Gcm also incremented in 20 steps were used for the THF
solutions. The spectra were Fourier transformed and baseline cor-
rected prior to discrete processing, fitting the data to a double ex-
d=7.61 (broad s, 4H, NCH), 7.49 (t, 4H, J(H,H)=7.6 Hz, p-ArH),
7.27 (d, 8H, J(H,H)=7.6 Hz, m-ArH), 2.33 (septet, 8H, J(H,H)=
3
3
3
7.2 Hz, CH(CH
)
3
), 1.16 (d, 24H, J(H,H)=8.0 Hz, CH(CH
)
2
), 1.14 ppm
, 125.7 MHz):
d=145.3 (NCH), 132.8 (ArC), 131.8 (ArC), 126.7 (ArC), 124.8 (ArC),
28.9 (CH(CH ), 25.5 (CH(CH ), 23.4 ppm (CH(CH ). NCN and OTf
resonances were not observed; F{ H} NMR (CDCl , 376.3 MHz):
d=ꢀ77.9 ppm; m.p. 2808C (decomp.); elemental analysis calcd (%)
: C 43.28, H 4.67, N 3.61, S 4.13. Found: C
3.54, H 4.68, N 3.60, S 3.92.
2
3
[25]
3
13
1
ponential fit and applying corrections for non-uniform gradients.
(d, 24H, J(H,H)=7.2 Hz, CH(CH ) ); C{ H} NMR (CDCl
3 2 3
The diffusion dimension was zero filled to 1024 data points and
the directly detected dimension was zero filled to 128K data
points.
)
)
3
)
3 2
3
2
2
19
1
3
[26]
for C56H72Cd F I N O S
2 6 2 4 6 2
4
X-ray crystallography
Crystals for X-ray diffraction studies were removed from a vial (in
a glovebox) and immediately coated with a thin layer of hydrocar-
bon oil (Paratone-N). A suitable crystal was then mounted on
a glass fiber and quickly placed in a low temperature stream of ni-
Synthesis of [IPr·Cd(m-OTf) ] (2)
2
2
To a vial charged with [IPr·Cd(I)(m-I)]2 (0.181 g, 0.120 mmol) and
AgOTf (0.127 g, 0.494 mmol) was added 5 mL of CH Cl , which re-
sulted in the immediate formation of a yellow precipitate. The re-
sulting mixture was stirred for 2 h, filtered and the solid was dis-
carded. The solvent was removed from the colorless filtrate under
[27]
trogen on the X-ray diffractometer. All data were collected using
a Bruker APEX II CCD detector/D8 or PLATFORM diffractometer
using MoKa or CuKa radiation, with the crystals cooled to ꢀ808C or
2
2
ꢀ
1008C. The data were corrected for absorption through Gaussian
Chem. Eur. J. 2016, 22, 1 – 12
www.chemeurj.org
7
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
&
&
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Full Paper
vacuum affording [IPr·Cd(OTf) ] as a colorless solid (0.178 g, 93%).
Synthesis of [IPr·HgI][OTf] (4)
To a vial charged with [IPr·HgI ] (0.105 g, 0.125 mmol) and AgOTf
2
2
Crystals of [IPr·Cd(OTf) ] suitable for X-ray crystallographic analysis
2
2
2
were obtained by storing a solution of the product in CH Cl lay-
2
2
1
(0.031 g, 0.12 mmol) was added 5 mL of CH Cl , which resulted in
2 2
ered with hexanes in a ꢀ358C freezer for two days; H NMR (CDCl ,
3
3
the immediate formation of a yellow precipitate. The resulting mix-
ture was stirred for 2 h, filtered and the solid was discarded. The
solvent was removed from the colorless filtrate under reduced
pressure, affording [IPr·HgI][OTf] as a white solid (0.103 g, quantita-
tive yield). Crystals of [IPr·HgI][OTf] suitable for X-ray crystallo-
graphic analysis were obtained by storing a concentrated solution
4
98.1 MHz): d=7.44 (t, 4H, J(H,H)=8.0 Hz, p-ArH), 7.25 (d, 8H,
J(H,H)=8.0 Hz, m-ArH), 7.21 (s, 4H, satellites: J(H,Cd)=9.7 Hz,
NCH), 2.42 (septet, 8H, J(H,H)=7.0 Hz, CH(CH ) ), 1.24 (d, 24H,
J(H,H)=7.0 Hz, CH(CH ) ), 1.08 ppm (d, 24H, J(H,H)=7.0 Hz,
3
4
3
3
2
3
3
3
2
1
3
1
CH(CH ) ); C{ H} NMR (CDCl , 125.7 MHz): d=175.3 (NCN), 145.1
3
2
3
(
(
ArC), 133.1 (ArC), 131.4 (NCH), 125.7 (ArC), 124.3 (ArC), 28.9
CH(CH ) ), 25.3 (CH(CH ) ), 22.6 ppm (CH(CH ) ). A resonance for
of the product in THF layered with hexanes in a ꢀ358C freezer for
3
2
3 2
3 2
1
19
1
three days; H NMR (CDCl
3
, 499.8 MHz): d=7.66 (s, 2H, satellites:
the OTf groups was not observed; F{ H} NMR (CDCl , 468.7 MHz):
3
4
3
J(H,Hg)=28.5 Hz, NCH), 7.56 (t, 2H, J(H,H)=7.5 Hz, p-ArH), 7.34
d, 4H, J(H,H)=7.5 Hz, m-ArH), 2.42 (septet, 4H, J(H,H)=6.5 Hz,
CH(CH ) ), 1.29 (d, 12H, J(H,H)=6.5 Hz, CH(CH ) ), 1.18 ppm (d,
2H, J(H,H)=6.5 Hz, CH(CH ) ); C{ H} NMR (CDCl , 125.7 MHz):
3 2 3
d ꢀ74.4 ppm; m.p.: stable >3008C; elemental analysis calcd (%)
3
3
(
for C H Cd F N O S : C 43.59, H 4.54, N 3.51, S 8.02. Found: C
5
8
72
2
12
4
12
4
3
3
2
3 2
4
3.87, H 4.69, N 3.45, S 7.77.
3
13
1
1
d=145.5 (NCH), 132.4 (ArC), 131.6 (ArC), 127.0 (ArC), 125.2 (ArC),
9.0 (CH(CH ) ), 25.3 (CH(CH ) ), 23.6 ppm (CH(CH ) . A NCN reso-
Synthesis of [IPr·Cd(DMAP) ][OTf] (3a)
3
2
2
3
2
3 2
3 2
nance was not observed. A resonance for the OTf group was not
A
1
[
solution of 4-dimethylaminopyridine (DMAP) (0.146 g,
.20 mmol) in 8 mL of THF was added to a 8 mL THF solution of
IPr·Cd(OTf)2]2 (0.330 g, 0.206 mmol). The reaction mixture was
19
1
observed; F{ H} NMR (CDCl , 376.7 MHz): d=ꢀ78.0 ppm; m.p.:
3
stable >2608C; elemental analysis calcd (%) for C H F HgIN O S:
28 36
3
2
3
C 38.87, H 4.19, N 3.24, S 3.71. Found: C 38.98, H 4.19, N 3.22, S
.64.
stirred, overnight, yielding a white precipitate. The mixture was
concentrated to 4 mL under vacuum, and the supernatant was
decanted. The resulting white precipitate was washed with 3ꢂ
3
5
mL portions of hexanes and dried under vacuum, affording
Synthesis of [IPr·Hg(OTf) ] (5)
2
[
IPr·Cd(DMAP) ][OTf] (3a) as a colorless solid (0.416 g, 90%). Data
3
2
1
3
for 3a: H NMR (CDCl , 498.1 MHz): d=7.61 (t, 2H, J(H,H)=7.9 Hz,
To a vial charged with [IPr·HgI
2
] (0.079 g, 0.094 mmol) and AgOTf
Cl , which resulted in
the immediate formation of a yellow precipitate. The resulting mix-
ture was stirred for 2 h and filtered. The solvent was removed from
3
p-ArH), 7.54 (broad s, 2H, NCH), 7.41 (broad s, 6H, DMAP-ArH), 7.36
(0.054 g, 0.21 mmol) was added 5 mL of CH
2
2
3
3
(
d, 4H, J(H,H)=7.9 Hzm-ArH), 6.41 (d, 6H, J(H,H)=7.0 Hz, DMAP-
3
ArH), 3.01 (s, 18H, DMAP N-CH ), 2.61 (septet, 4H, J(H,H)=6.8 Hz,
CH(CH ) ), 1.19 (d, 12H, J(H,H)=6.8 Hz, CH(CH ) ), 1.13 ppm (d,
1
3
3
the colorless filtrate under reduced pressure, yielding [IPr·Hg(OTf)
]
2
3
3
2
3 2
13
1
2H, J(H,H)=6.8 Hz, CH(CH ) ); C{ H} NMR (CDCl , 125.7 MHz):
3 2 3
as a colorless solid (0.079 g, 95%). Crystals of the THF adduct,
[15]
d=155.1 (NCH), 148.9 (ArC), 145.9 (ArC), 134.0 (ArC), 131.8 (ArC),
25.1 (ArC), 107.1 (ArC), 39.2 (DMAP-CH ), 29.0 (CH(CH ) ), 25.4
[IPr·Hg(OTf) ·THF], suitable for X-ray crystallographic analysis
2
1
were obtained by storing a concentrated solution of the product
3
3 2
1
(
CH(CH ) ), 23.0 ppm (CH(CH ) ). An NCN resonance was not ob-
in THF layered with hexanes in a ꢀ358C freezer, overnight; H NMR
3
2
3 2
19
1
served. A resonance for the OTf groups was not observed; F{ H}
(CDCl , 699.8 MHz): d=7.72 (broad s, 2H, NCH), 7.58 (t, 2H,
3
3
3
NMR (CDCl , 468.7 MHz): d=ꢀ77.9 ppm; m.p. 1878C (decomp.); el-
J(H,H)=8.0 Hz, p-ArH), 7.35 (d, 4H, J(H,H)=7.8 Hz, m-ArH), 2.35
3
3
3
emental analysis calcd (%) for C H CdF N O S : C 51.52, H 5.71, N
(sept, 4H, J(H,H)=7.0 Hz, CH(CH
)
3
2
), 1.24 (d, 12H, J(H,H)=6.9 Hz,
50
66
6
8
6 2
3
13
1
9
.61, S 5.50. Found: C 50.60, H 5.62, N 8.87, S 5.58.
CH(CH
3
)
2
), 1.18 ppm (d, 12H, J(H,H)=6.9 Hz, CH(CH
3
) ); C{ H}
2
NMR (CDCl , 176.0 MHz): d=188.6 (NCN), 145.3 (NCH), 132.1 (ArC),
The supernatant of the above synthesis (3a) was dried under
3
1
31.6 (ArC), 127.6 (ArC), 125.0 (ArC), 29.1 (CH(CH ) ), 25.1 (CH(CH ) ),
3 2 3 2
vacuum, affording a colorless solid (0.051 g). The solid was found
1
23.4 ppm (CH(CH ) ). A resonance for the OTf groups was not ob-
3 2
by H NMR spectroscopy to contain 3a (39%), 3b (39%) and [IPrH]
1
9
1
[7,22]
served; F{ H} NMR (CDCl , 376.1 MHz): d=ꢀ77.1 ppm; m.p.
[
OTf]
(22%).
3
1
908C (decomp.); elemental analyses were performed on three dif-
ferent crystalline samples. In all cases the CHNS values were sys-
tematically low; NMR spectra for 5 are given in the Supporting In-
formation.
NMR data for [IPr·Cd(DMAP) (OTf) ] (3b)
2
2
1
H NMR (CDCl , 498.1 MHz): d=7.54 (broad s, 4H, DMAP-ArH), 7.46
3
3
3
(
t, 2H, J(H,H)=8.0 Hz, p-ArH), 7.13 (d, 4H, J(H,H)=8.0 Hz, m-ArH),
3
6
2
7
.41 (d, 4H, J(H,H)=7.0 Hz, DMAP-ArH), 3.01 (s, 12H, DMAP-CH ),
3
Synthesis of [IPr·CdI(OiPr)] (6)
3
3
.25 (septet, 4H, J(H,H)=7.0 Hz, CH(CH ) ), 1.06 (d, 12H, J(H,H)=
3
2
3
.0 Hz, CH(CH ) ), 0.77 ppm (d, 12H, J(H,H)=7.0 Hz, CH(CH ) ). A
To a vial charged with [IPr·Cd(I)(m-I)]2 (0.160 g, 0.106 mmol) and
NaOiPr (0.018 g, 0.22 mmol) was added 10 mL of fluorobenzene,
immediately affording a white slurry. The reaction mixture was
stirred for 1.5 h and filtered. The solvent was removed from the
3
2
3 3
1
9
NCH resonance was not observed; a single F signal was observed
for the mixture; F{ H} NMR (CDCl , 468.7 MHz): d=ꢀ78.0 ppm.
When the above reaction was repeated and the crude product
1
9
1
3
colorless filtrate under reduced pressure, yielding a colorless solid
containing both 3a and 3b was crystallized from CH Cl (ꢀ358C;
2
2
1
(
0.135 g). The H NMR spectrum identified [IPr·CdI(OiPr)] as the
one week), colorless crystals were obtained which contained a 1:1
major product (84%) along with about 3% free IPr and another
mixture of 3a [IPr·Cd(DMAP) ][OTf] and 3b [IPr·Cd(DMAP) (OTf) ]
3
2
2
2
1
minor carbene containing product (ca. 13%); H NMR spectra of
in the lattice. Attempts to form 3b exclusively by reacting 4 equiv
DMAP with [IPr·Cd(OTf) ] were unsuccessful as 3a remained as the
[
IPr·CdI(OiPr)] at room temperature and +758C are given in the
Supporting Information.
Data for [IPr·CdI(OiPr)] (6): H NMR (C D , 399.9 MHz): d=7.18 (t,
2
2
1
major product (74% by H NMR spectroscopy), along with the for-
1
1
mation of 3b (14% by H NMR spectroscopy) and [IPrH][OTf] (12%
6
6
1
3
3
by H NMR spectroscopy). Attempts to separate 3b and [IPrH][OTf]
2H, J(H,H)=8.0 Hz, p-ArH), 7.06 (d, 4H, J(H,H)=8.0 Hz, m-ArH),
6.40 (s, 2H, satellites: J(H,Cd)=6.0 Hz), 2.99–2.66 (broad m, 5H,
CH(CH ) ), 1.57 (broad d, 12H, J(H,H)=ca. 8 Hz, CH(CH ) ), 1.01–
4
by crystallization from CH Cl /hexanes and THF/hexanes solvent
2
2
3
mixtures were unsuccessful.
3
2
3 2
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3
0
8
.96 (broad m, 3H, OCH(CH ) ), 0.93 (broad d, 12H, J(H,H)=ca.
was increased by 208C. This was repeated until a temperature of
08C was reached. The temperature was then increased in incre-
ments of 58C. No reaction was observed until a temperature of
3
2
Hz), 0.70–0.39 ppm (broad m, 3H, OCH(CH ) ).
3
2
+
+
108C was reached. After monitoring the reaction for 1 h at
108C, 37% of 2 had been consumed and converted to an [IPrH]
Catalytic hydrosilylation of benzophenone
+
To a vial charged with [IPr·Cd(OTf)2]2 (0.014 g, 0.0088 mmol,
salt and a new carbene containing product in a about 1:1 ratio,
4
5
mol%) and benzophenone (0.033 g, 0.18 mmol) was added 1 mL
of THF which formed a colorless solution. Phenylmethylsilane
24 mL, 0.18 mmol) was then immediately added to the reaction
however no Cd-H resonances or J(H,Cd) satellites were observed.
1
The presence of H was also confirmed by H NMR spectroscopy
2
(
and a metallic precipitate was observed in the NMR tube after
completion of the experiment.
mixture. The mixture was allowed to stir for 40 min at which time
a 0.2 mL aliquot was taken from the reaction mixture and the vola-
1
tiles were removed under vacuum. H NMR analysis in C D6 re-
6
Reaction of 2 with PhMeSiH in toluene
2
vealed that the reaction had proceeded to 42% completion
ꢀ
1
(
TON=8, TOF=13 h ). In a similar manner, 0.2 mL aliquots were
To a vial charged with [IPr·Cd(m-OTf)2]2 (2) (0.010 g, 0.013 mmol)
removed from the reaction mixture after 70 and 120 min, revealing
was added 3 mL of toluene forming an insoluble slurry. An aliquot
ꢀ1
7
9% completion (TON=16, TOF=14 h ) and 96% completion
of a freshly prepared 1.0m toluene solution of PhMeSiH (12.5 mL,
2
ꢀ
1
(
TON=19, TOF=10 h ), respectively. Data for Ph C(H)OSi(H)MePh
0.0125 mmol) was added to the slurry and the reaction was let stir
for 6 h after which time an aliquot was removed for NMR analysis.
2
[4]
are consistent with the literature.
1
19
1
H and F{ H} NMR revealed that no reaction had occurred.
Representative catalytic hydroborylation procedure
Reaction of 2 with 2 equiv of PhMeSiH2
To a vial charged with [IPr·Cd(OTf)2]2 (0.015 g, 0.0094 mmol,
5
mol%) and benzophenone (0.035 g, 0.19 mmol) was added 1 mL
of THF which afforded colorless solution. HBPin (28 mL,
.19 mmol) was then immediately added to the reaction mixture.
To a vial charged with [IPr·Cd(m-OTf)2]2 (2) (0.035 g, 0.022 mmol)
was added about 3 mL of THF, forming a colorless solution. PhMe-
a
0
SiH (12 mL, 0.087 mmol) was added to the solution. After 1.5 h, the
2
The reaction mixture was allowed to stir for 10 min at which time
formation of a metallic precipitate was observed and an aliquot
was removed and filtered for NMR analysis. The H NMR spectrum
1
a 0.2 mL aliquot was taken from the reaction mixture and the vola-
1
tiles were removed under vacuum. H NMR analysis in C D6 re-
revealed the partial consumption of 2 (68%) and the formation of
6
[
18]
vealed that the reaction had proceeded to completion (TON=21,
PhMeSiH(OTf)
along with several other unidentified soluble
ꢀ1
4
TOF=124 h ). In a similar manner, the reaction was conducted
products. No Cd-H resonances or J(H,Cd) satellites were observed.
The F{ H} spectrum revealed two peaks corresponding to 2 and
19
1
with a 1 mol% loading of [IPr·Cd(OTf) ] (0.008 g, 0.005 mmol), ben-
2
2
zophenone (0.092 g, 0.51 mmol), HBPin (73 mL, 0.51 mmol) in 2 mL
PhMeSiH(OTf).
of THF. The reaction was also conducted with a 0.1 mol% loading
of
Reaction of 2 with 2 equiv of HBpin
[
IPr·Cd(OTf)2]2 (0.006 g, 0.004 mmol), benzophenone (0.681 g,
.74 mmol), HBPin (542 mL, 3.74 mmol) and 5 mL of THF. Relevant
data are presented in Table 1. Data for Ph C(H)OBPin are consistent
3
To a vial charged with [IPr·Cd(m-OTf) ] (2; 0.028 g, 0.018 mmol) was
2
2
2
added 3 mL of THF, leading to a colorless solution. HBPin (10 mL,
0.070 mmol) was added to the solution and the formation of a met-
allic precipitate was observed after 5 min. After 1.5 h an aliquot
[38]
with the literature.
This general procedure was followed for the hydroborylation of
,4’-dichlorobenzophenone, mesitaldehyde and dicyclohexyl
1
4
was removed and filtered for NMR analysis. The H NMR spectrum
[20]
[38]
ketone. Data for (4-ClC H ) CHOBPin and MesCH OBPin were
revealed the complete consumption of 2 and the formation of Pin-
B(OTf) (see below for independent synthesis) along with several
other unidentified soluble products. No Cd-H resonances or
6
4 2
2
consistent with the literature.
4
11
1
19
J(H,Cd) satellites were observed. The B{ H} and F spectra were
NMR data for Cy C(H)OBPin
2
consistent with that of PinB(OTf).
1
H NMR (C D , 498.1 MHz): d=3.83 (m, 1H, OCH(Cy) ), 1.92–1.86
6
6
2
(
1
m, 2H, CyH), 1.76–1.67 (m, 4H, CyH), 1.66–1.51 (m, 6H, CyH), 1.32–
.11 (m, 10H, CyH), 1.10 ppm (s, 12H, BPin); C{ H} NMR (C D ,
6 6
Independent synthesis of PinB(OTf)
13
1
1
26.7 MHz): d=82.8 (C-CH in BPin), 82.1 (OCH), 39.6 (Cy-CH), 30.1
To a vial containing HBPin (100 mL, 0.689 mmol) and 1 mL of tolu-
3
(
2
2
Cy-CH ), 27.6 (Cy-CH ), 26.9 (Cy-CH ), 26.7 (Cy-CH ), 26.5 (Cy-CH ),
ene was added Me Si(OTf) (125 mL, 0.689 mmol). Bubbling was
3
2
2
2
2
2
11
1
4.6 ppm (C-CH3 in BPin); B{ H} NMR (C D , 159.8 MHz): d=
noted upon addition. After 1 h, an aliquot was removed from the
6
6
1
2.6 ppm.
reaction mixture revealing the formation of PinB(OTf); H NMR
1
1
1
(
C D , 400.0 MHz): d=1.01 ppm (s, 12H, BPin); B{ H} NMR (C D ,
6
6
6
6
19
1
1
ꢀ
a
28.3 MHz): d=21.9 ppm; F{ H} NMR (C D , 376.3 MHz): d=
78.0 ppm; C{ H} NMR (C D ): d=82.9 (C-CH ), 24.6 ppm (C-CH );
C{ H} resonance for the OTf group was not located.
6
6
Stoichiometric reaction of 2 with PhMeSiH2
A J-Young NMR tube was loaded with 2 (0.006 g, 0.004 mmol) in
13
1
6
6
3
3
13
1
4
00 mL of [D ]THF in a glovebox and frozen in a cold well. A 0.3m
8
solution of PhMeSiH in [D ]THF (25 mL, 0.008 mmol) was added to
2
8
Reaction of 5 with 2 equiv of PhMeSiH2
the J-Young NMR tube and the total volume was increased to
00 mL with [D ]THF and frozen in the cold well. The J-Young tube
7
To a vial charged with [IPr·Hg(OTf) ] (5; 0.056 g, 0.063 mmol) was
8
2
was removed from the glovebox and transported to the NMR in-
strument in a liquid nitrogen dewar. The NMR tube was quickly in-
added 3 mL of THF, forming a colorless solution. PhMeSiH (17 mL,
2
0.13 mmol) was added to the solution and the formation of a met-
allic precipitate was immediately observed. After 45 min, a 1 mL ali-
1
serted into the NMR instrument and let warm to ꢀ808C. A H NMR
1
spectrum was recorded every 20 min. After 1 h the temperature
quot was filtered through Celite for NMR analysis. The H NMR
Chem. Eur. J. 2016, 22, 1 – 12
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4
identified soluble products. No Hg-H resonances or J(H,Hg) satel-
19
1
lites were observed. The F{ H} spectrum revealed a single peak
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Reaction of 6 with PhMeSiH2
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Canada Foundation for Innovation, and the Faculty of Science
at the University of Alberta (Research Fellowship to E.R.; 2015–
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6796.
Published online on && &&, 0000
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FULL PAPER
&
Inorganic Chemistry
M. M. D. Roy, M. J. Ferguson,
R. McDonald, E. Rivard*
&& – &&
Investigation of N-Heterocyclic
Carbene-Supported Group 12 Triflates
as Pre-catalysts for Hydrosilylation/
Borylation
Cadmium is just right: N-Heterocyclic
carbene-supported cadmium and mer-
cury triflates were prepared and ex-
plored as pre-catalysts. Unlike zinc and
mercury, cadmium triflates were found
to be catalytically active hydrosilylation
and borylation pre-catalysts with the
necessary balance of thermal stability
and reactive CdꢀE bonds (see scheme).
&
&
Chem. Eur. J. 2016, 22, 1 – 12
www.chemeurj.org
12
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!