Reactivity of Ir–NSiN Complexes: Ir-Catalyzed Dehydrogenative Silylation of Carboxylic Acids

Article abstract of DOI:10.1002/cctc.201701488

This work describes the results from the studies on the potential of [Ir(μ-Cl)(cod)]₂ (cod=1,5-cyclooctadiene) as metallic precursor for the preparation of Ir(NSiN) complexes (NSiN=fac-bis-(pyridine-2-yloxy)methylsilyl). The reaction of [Ir(μ-Cl)(cod)]₂ with bis-(pyridine-2-yloxy)methylsilane enabled the synthesis of [Ir(H)(Cl)(NSiN)(η²-cod)] with an uncommon η²-coordination mode for the cod ligand. The application of Ir–NSiN species as catalysts precursors for the dehydrogenative silylation of carboxylic acids was also explored. The outcomes from these catalytic studies revealed a clear influence of the ancillary ligand on the catalytic activity of Ir–NSiN species. Thus, whereas [Ir(H)(CF₃SO₃)(NSiN)(coe)] shows poor catalytic activity, the related complex [Ir(H)(CF₃CO₂)(NSiN)(coe)] with a trifluoroacetate ligand was demonstrated to be a highly active catalyst precursor.

Full text of DOI:10.1002/cctc.201701488

Accepted Article
Title: Reactivity of Ir-NSiN Complexes: Ir-Catalyzed Dehydrogenative
Silylation of Carboxylic Acids
Authors: Luis A. Oro, Alejandro Julián, Karin Garces, Ralte Lalrempuia,
E. A. Jaseer, Pilar García-Orduña, Francisco Fernández-
Alvárez, and Fernando Lahoz
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10.1002/cctc.201701488
ChemCatChem
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Reactivity of Ir-NSiN Complexes: Ir-Catalyzed Dehydrogenative
Silylation of Carboxylic Acids
Alejandro Julián,[a] Karin Garcés,[a] Ralte Lalrempuia,[a][b] E. A. Jaseer,[c] Pilar García-Orduña,[a]
Francisco J. Fernández-Alvarez,*[a] Fernando J. Lahoz,[a] and Luis A. Oro*[a][c]
[13,14]
Abstract: This work describes the results from the studies on the
potential of [Ir(µ-Cl)(cod)]2 (cod = 1,5-cyclooctadiene) as metallic
precursor for the preparation of Ir(NSiN) complexes (NSiN = fac-bis-
(pyridine-2-yloxy)methylsilyl). It is noteworthy that the reaction of
[Ir(µ-Cl)(cod)]2 with bis-(pyridine-2-yloxy)methylsilane has allowed
the synthesis of [Ir(H)(Cl)(NSiN)(h2-cod)] with an uncommon η2-
coordination mode for the cod ligand. Moreover, the application of Ir-
NSiN species as catalysts precursors for the dehydrogenative
silylation of carboxylic acids has also been explored. The outcomes
from these catalytic studies revealed a clear influence of the ancillary
ligand on the catalytic activity of Ir-NSiN species. Thus, while
HSiMe(OSiMe3)2
has been observed. These outcomes
stimulated us to study the potential of Ir-NSiN species as
catalyst precursors for the reduction of carboxylic acids with
HSiMe(OSiMe3)2.
Results and Discussion
Synthesis and reactivity of Ir-NSiN complexes. Ir-NSiN
species (NSiN = fac-bis-(pyridine-2-yloxy)methylsilyl, or fac-bis-
(4-methylpyridine-2-yloxy)methylsilyl)[7] have been so far
[Ir(H)(CF3SO3)(NSiN)(coe)] shows
a poor catalytic activity, the
prepared using [Ir(µ-Cl)(coe)2]2 (coe
= cis-cyclooctene) as
related complex [Ir(H)(CF3CO2)(NSiN)(coe)] with a trifluoroacetate
ligand has demonstrated to be a highly active catalyst precursor.
metallic precursor. This work shows that [Ir(µ-Cl)(cod)]2 (cod =
1,5-cyclooctadiene) could also be used as metallic precursor for
the preparation of Ir-NSiN species.
Introduction
Catalytic hydrosilylation has proven to be an effective
methodology for the reduction of unsaturated organic molecules
including alkenes, alkynes, ketones, nitriles, imines, carboxylic
acids and carbonic acid derivatives.[1,2] Besides that, a number
of catalytic systems has been efficiently employed for the
selective reduction of CO2 with hydrosilanes to give the
corresponding silylformate, bis-silylacetal, methoxysilane or
methane.[3] It should be noted that the selective reaction of
carboxylic acids with hydrosilanes to afford silylesters,[4]
alcohols[5] or aldehydes[6] still represents a challenge.
Rh and Ir complexes with tridentate monoanionic NSiN ligands[7]
were found to be effective catalysts for CO2-hydrosilylation,[3,8]
dehydrogenative silylation of arenes[9] and acetophenone[10]
derivatives and for processes involving CO2-insertion into N-Si[11]
and P-Si[12] bonds. In this context, it should be mentioned that
the formation of the silylether CF3CH2OSiMe(OSiMe3)2 by
reduction
of
the
trifluoroacetate
ligand
during
the
Scheme 1. Preparation of Ir-NSiN complexes with 1,5-cyclooctadiene.
Ir(CF3CO2)(NSiN)-catalyzed hydrosilylation of CO2 with
The treatment of [Ir(µ-Cl)(cod)]2 with bis-(pyridine-2-
yloxy)methylsilane (1) in CH2Cl2 quantitatively leads to the
iridium(III) complex [Ir(H)(Cl)(NSiN)(h2-cod)] (2), which has been
isolated as a white solid in 87% yield (Scheme 1). The Ir-NSiN
species 2 has been fully characterized by elemental analysis,
mass spectrometry and NMR spectroscopy. In addition, the h2-
coordination mode of the cod ligand to the iridium atom in 2 has
been confirmed by X-ray diffraction studies (Figure 1).
[a]
A. Julián, Dr. K. Garcés, Dr. R. Lalrempuia, Dr. P. García-Orduña,
Dr. F. J. Fernández-Alvarez, Prof. F. J. Lahoz and Prof. L. A. Oro.
Departamento de Química Inorgánica
– Instituto de Síntesis
Química Catálisis Homogénea (ISQCH). Universidad de
y
Zaragoza. Facultad de Ciencias 50009, Zaragoza – Spain.
E-mail: paco@unizar.es, oro@unizar.es
Dr. R. Lalrempuia. University of Bergen, Department of Chemistry,
Allégaten 41, N-5007 Bergen, Norway.
Dr. E. A. Jaseer and Prof. L. A. Oro (visiting Prof.) Center of
Refining & Petrochemicals, King Fahd University of Petroleum &
Minerals, 31261 Dhahran - Saudi Arabia.
[b]
[c]
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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10.1002/cctc.201701488
ChemCatChem
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with two resonances which appear at d 130.2 and 130.1 ppm,
respectively, in the 13C{1H} NMR spectra of 2. It should be
mentioned that 13C{1H} NMR spectra of free 1,5-cyclooctadiene
show the olefinic carbon atoms around d 129 ppm. As it should
be expected, the carbon atoms of the coordinated olefinic
fragment appear shifted at higher field at d 68.7 and 65.9 ppm,
respectively. The 1H NMR spectra of species 2 in CD2Cl2 also
show a singlet resonance corresponding to the hydride ligand at
d -15.35 ppm, which is in the same range of that observed for
related species [Ir(H)(Cl)(NSiN)(coe)] (4)[8] with a cyclooctene
instead of the cyclooctadiene ligand. The 29Si{1H} NMR spectra
of 2 in CD2Cl2 exhibit a singlet at d 31.3 ppm, which compares
well with the corresponding resonances reported for 4.
Figure 1. Molecular structure of complex 2. Selected bond lengths (Å) and
angles (°) for compound 2. Ir-Si, 2.2356(12); Ir-Cl, 2.5229(10); Ir-Ct[a]
,
2.068(4); Ir-N1, 2.197(3); Ir-N2, 2.114(4); Ir-H, 1.53(7); Si-Ir-Cl, 170.51(4); Si-
Ir-Ct[a], 100.47(13); Si-Ir-N1, 79.93(9); Si-Ir-N2, 80.60(11); Si-Ir-H, 96(2); Cl-Ir-
Ct[a], 86.29(13); Cl-Ir-N1, 92.26(9); Cl-Ir-N2, 93.61(10); Cl-Ir-H, 90(2); Ct[a]-Ir-
N1, 101.58(15); Ct[a]-Ir-N2, 172.08(15); Ct[a]-Ir-H, 89(2); N1-Ir-N2; 86.34(13);
N1-Ir-H, 169(2); N2-Ir-H, 83(2). [a]Ct represents the centroid of the C12=C13
olefinic bond in compound 2.
Compound
2
reacts with one equivalent of silver
trifluoromethanesulfonate in CH2Cl2, in the absence of light, to
give a light-orange solid, which has been fully characterized by
means of elemental analysis, mass spectrometry and NMR
spectroscopy as the ionic iridium(III) species 3 (Scheme 1).
1H NMR studies of the reaction of 2 with PCy3 in C6D6 show the
resonances due to free cyclooctadiene together with those
corresponding to the complex [Ir(H)(Cl)(NSiN)(PCy3)] (5). It
should be mentioned that analogous behavior was observed
when using complex 4 instead of 2 (Scheme 2). In addition,
when the reactions of 2 (or 4) with one equivalent of PCy3 were
carried out at preparative level, the corresponding phosphane
derivative [Ir(H)(Cl)(NSiN)(PCy3)] (5) was obtained in high yield.
The reaction of complex 5 with AgCF3SO3 quantitatively gives
the triflate derivative [Ir(H)(CF3SO3)(NSiN)(PCy3)] (6) (Scheme
2). The triflate derivatives 3 or [Ir(H)(CF3SO3)(NSiN)(coe)] (7)[13]
also reacts with one equivalent of PCy3 to afford complex 6.
However, this methodology allows the formation of other minor
reaction side products, corresponding to the coordination of two
phosphane ligands to the iridium atom.
The NSiN ligand in 2 is facially coordinated to the iridium center.
Si-Ir-N cis angles (Si-Ir-N1, 79.93(9)° and Si-Ir-N2, 80.60(11)°)
are deviated from the ideal value of 90° due to the NSiN
chelating bonding through the formation of two Ir-Si-O-C-N
metallacycles, with ring puckering parameters (Ir-Si-O1-C2-N1:
q= 0.148(2) Å, φ= 29.6(15)°, Ir-Si-O2-C7-N2: q= 0.135(2) Å, φ= -
141.0(15))[15] typical of envelope E2 and 2E enantiomorphic
configurations. The pyridine-N atoms are located trans to the
hydride and to the ƞ2-cyclooctadiene ligands. The Ir-N1 bond
length (2.197(3) Å) is longer than Ir-N2 (2.114(4) Å), most likely
associated to the high structural trans effect of the hydride ligand
compared to that exerted by the olefin. The chloride ligand (Ir–Cl,
2.5229(10) Å) is coordinated trans to the Si atom. Structural
parameters describing the metal coordination sphere are close
to those reported in related Rh and Ir-(NSiN) metal
complexes.[8,10,13,14]
The coordination mode of the cod fragment to the metal in
compound 2 deserves special attention. The bond length found
between the metal atom and the olefin in compound 2 (2.068(4)
Å) is similar to those observed in related Ir(NSiN)(η2-coe)
complexes (in the range 2.062(12)-2.102(2) Å).[8,13,14] Although
the versatility of cyclooctadiene ligand to coordinate to iridium in
different forms has been well established,[16] it tends to chelate
to the metal in a η4 coordination mode. It should be mentioned
that the equilibrium between Ir(η2-cod) and Ir(η4-cod) has been
proposed as key step in some catalytic processes.[17] In addition,
to the best of our knowledge only three mononuclear complexes
containing an Ir(η2-cod) fragment have been crystallographically
characterized:
[(η5-Cp)Ir(η2-cod)(NHC)][18]
,
[(η5-Cp)Ir(η2-
cod)(biph)][19] and [Ir(η2-cod)(4-Ph)Tr(NP(iPr)2)(NHP(iPr)2)].[20]
The coordinated olefinic bond length in compound 2 (C12=C13:
1.411(6) Å) has been found to be longer than that of the
uncoordinated one (C16=C17:1.334(9) Å), a feature that has
been related to a π-back donation from the metal atom.[18,19]
Scheme 2. Synthesis of Ir-NSiN phosphane complexes.
1
The H and 13C{1H} NMR spectra of 2 in CD2Cl2 support the ƞ2-
coordination of the cod ligand in solution. Thus, the 1H NMR
spectra of 2 exhibit one multiplet resonance between d 5.67 and
5.51 ppm, assigned to the two Csp2-H protons of the free olefinic
moiety of the cyclooctadiene ligand, which show correlation in
the heteronuclear single quantum correlation (HSQC) spectra
The Ir-NSiN derivatives 5 and 6 have been fully characterized.
Moreover, the solid state structure of 5 has been determined by
X-ray diffraction studies (Figure 2).
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Scheme 3. Reduction of the ancillary trifluroacetate ligand in 8 to give
CF3CH2OSiMe(OSiMe3)2 during the 8-catalyzed hydrosilylation of CO2 with
HSiMe(OSiMe3)2.[14]
1H NMR studies of the reaction of ECOOH (E = H, CH3, Ph) with
one equivalent of HSiMe(OSiMe3)2 in C6D6 at 328 K in presence
of catalytic amounts of complexes 7 or 8 show a clear influence
of the ancillary ligands on the performance of the resulting
catalytic system (Table 1). Thus, while using complex 8 as
catalyst, it is possible to achieve the quantitative conversion of
the parent carboxylic acids into the corresponding silylester in
less than 4 hours, the related triflate derivatives 6 and 7 have
Figure 2. Molecular structure of complex 5. Selected bond lengths (Å) and
angles (°) for compound 5. Ir-Si, 2.2243(13); Ir-Cl, 2.5099(11); Ir-P,
2.2763(11); Ir-N1, 2.204(4); Ir-N2, 2.172(3); Ir-H, 1.641(10); Si-Ir-Cl, 166.07(4);
Si-Ir-P, 99.22(4); Si-Ir-N1, 80.14(10); Si-Ir-N2, 79.07(10); Si-Ir-H, 95.0(18); Cl-
Ir-P, 92.83(4); Cl-Ir-N1, 90.39(10); Cl-Ir-N2, 90.13(10); Cl-Ir-H, 93.8(18); P-Ir-
N1, 103.01(10); P-Ir-N2, 170.78(10); P-Ir-H, 80.7(18); N1-Ir-N2, 85.69(13); N1-
Ir-H, 174.2(18), N2-Ir-H, 90.4(18)
The iridium atom in 5 displays slightly distorted octahedral
coordination geometry (Figure 2) with similar structural features
than those found in its rhodium analogous.[10] The NSiN ligand is
facially coordinated to the iridium center, exhibiting Si-Ir-N cis
angles deviated from the ideal value of 90º due to the chelating
bonding of the ligand. Ring puckering parameters of the
iridacycles (Ir-Si-O1-C2-N1: q= 0.094(3) Å, φ= 11(3)°, Ir-Si-O2-
C7-N2: q= 0.210(3) Å, φ= -156.4(10)) characterize their twisted
1T2 and 2T1 (with an envelope 2E contribution) conformations.
The Ir-Si bond lenght (2.2243(13) Å) is shorter than that found in
complex 2, and indeed, it is in the lower limit of an iridium-silyl
mononuclear species (2.22-2.42 Å).[21] The nitrogen atoms are
located trans to the hydride and the phosphane ligands, and
although their different trans effect is reflected in the Ir-N bond
lengths, the disparity between Ir-N1 and Ir-N2 bond lengths in
complex 5 is less marked than that observed in compound 2.
The coordination sphere is completed by a chloride ligand.
Interestingly, the Ir-Cl bond length in 5 is shorter (2.5099(11) Å)
than that found for compound 2. This feature may be due to the
different intermolecular hydrogen bond patterns involving
chlorine atom in each structure (ꢀꢁꢁ(10) in compound 2 and C(7),
ꢀꢁꢁ(12) and ꢀꢁꢁ (14) in compound 5) (see SI).
shown
a poorer catalytic activity (Table 1). It should be
mentioned that the phosphane complex 6 is slightly more active
than 7 but less than 8 (Table 1).
Table 1. Reaction products from the reaction of carboxylic acids with one
equivalent of HSiMe(OSiMe3)2 in presence of catalytic amounts (5.0 mol%)
of Ir-NSiN complexes in C6D6.
Acid
catalyst
T (K)
328
328
328
328
328
328
328
Silyl ester(%)[a],[b]
10a (63.7)
10a (52.0)
10a (95.0)
10b (43.5)
10b (84.5)
10c (65.0)
10c (90.0)
HCOOH
HCOOH
HCOOH
CH3COOH
CH3COOH
PhCOOH
PhCOOH
6
7
8
7
8
7
8
Study of the Ir-NSiN catalyzed reaction of carboxylics acids
with
silanes.
The
formation
of
the
silylether
[a] % calculated by 1H NMR integration using hexamethylbenzene (0.07
mmol) as internal standard. [b] Data corresponding to the 1H NMR spectra
registered after 3.5 h of reaction.
CF3CH2OSiMe(OSiMe3)2 during the hydrosilylation of CO2 with
HSiMe(OSiMe3)2 using complex [Ir(H)(CF3CO2)(NSiN)(coe)] (8)
as catalyst precursor has been recently reported (Scheme 3).[14]
This evidence prompted us to study the potential of Ir-NSiN
species as catalysts for the reduction of carboxylic acids using
HSiMe(OSiMe3)2 as reducing agent.
The silylester species 10a-c were characterized by comparison
of their 1H, 13C and 29Si NMR spectra with the reported data
(Figures S1 to S6).[8,14,4e] In this context, it should be mentioned
that few catalytic systems effective for the selective
dehydrogenative silylation of carboxylic acids with silanes have
1
been reported so far.[4] It is worth noting that H NMR studies of
the 8-catalyzed reactions of formic, acetic or benzoic acid, with
3.0 equivalents of HSiMe(OSiMe3)2 in C6D6 at 328 K showed
that, after 24h, the corresponding silylester species 10 remains
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as the major reaction product and no evidence of its reduction
was observed.
1JC-F = 286.0 Hz) that are low field shifted in comparison with the
related resonance observed for trifluoroacetic acid (d 114.8 ppm,
1JC-F = 284.5 Hz) (Figure S8). These results confirm the
formation of at least two silylester species with the CF3CO2Si
moiety but different from 10d. This behavior could be explained
taking into account that complex 8 promoted the rearrangement
of HSiMe(OSiMe3)2 in absence of coordinating substrates to give
mixtures of HSi(OSiMe3)3, HSiMe2(OSiMe3) and other
hydrosiloxanes.[14] Indeed, when using HSiEt3 or HSiMe2Ph
instead of HSiMe(OSiMe3)2 under the same reaction conditions
the selective 8-catalyzed silylation of CF3COOH to give the
Kinetic studies of the reaction of ECOOH (E = H, CH3, Ph) with
HSiMe(OSiMe3)2 in presence of 8 (1.0 mol%) using dry-toluene
as solvent were carried out in a micro-reactor.[22] This allowed
measuring the hydrogen pressure generated during the process
(Figure 3). As a result from these studies a turnover frequency
number at 50% conversion (TOF1/2) value of 195, 2000 and 136
h-1 were found for the H2 generation from the dehydrogenative
Si-O coupling of HSiMe(OSiMe3)2 with formic, acetic and
benzoic acid, respectively. Therefore, the dehydrogenative
silylation is faster for acetic acid. However, in agreement with
the NMR experiments (Table 1) the reaction yield has found to
be lower for CH3COOH than for HCOOH and PhCOOH. These
data showed that the Ir-NSiN based catalytic system for
dehydrogenative silylation of carboxylic acids could be
considered as one of the most active so far reported.4
corresponding
silyl-ester
CF3COOSiEt3
(10e)
or
CF3COOSiMe2Ph (10f) was achieved in 1.5
h
(Table 2).
Compounds 10e and 10f were characterized by means of 1H,
29Si and 13C{1H} NMR spectroscopy (Figure S9 to S14). It should
be noted that the silylesters 10e and 10f were previously
obtained in low yield from the reaction of the corresponding
silane, HSiEt3 or HSiMe2Ph, with 3 equivalents of CF3COOH at
343 K for 10 h.[23]
Table 2. Reaction products from the reaction of CF3COOH with one
equivalent of HSiR3 in presence of catalytic amounts complex 8 (5.0 mol%)
in C6D6.
Silane
HSiMe(OSiMe3)2
HSiEt3
catalyst
T (K)
328
328
328
Silyl ester(%)[a],[b]
mixture of silylesters
10e (91.0)
8
8
8
HSiMe2Ph
10f (93.0)
[a] % calculated by 1H NMR integration using hexamethylbenzene (0.07
mmol) as internal standard. [b] Data corresponding to the 1H NMR spectra
registered after 1.5 h of reaction.
Figure 3. Hydrogen generation (% related to 1.0 mmol as 100%) versus time
(min) for the catalytic dehydrogenative Si-O coupling of acetic acid, formic acid
and benzoic acid (1.0 mmol) with HSiMe(OSiMe3)2 (1.01 mmol) using 8 (1.0
mol%) as catalyst in dry toluene (1.0 mL) at 328 K.
A mechanism proposed for the 8-dehydrogenative silylation of
the above mentioned carboxylic acids is shown in Scheme 4. It
has been proven that C6D6 solutions of 8 did not react with the
carboxylic acids in absence of silane. Therefore, it is reasonable
to assume that, analogously to that reported for the 8-catalyzed
CO2-hydrosilylation processes,[14] the first step of the reaction
could be the formation of species A, which is in equilibrium with
B (Scheme 4).[14] This is also supported by the experimentally
observed formation of cyclooctane. Intramolecular elimination of
molecular hydrogen from C could afford D. Subsequently, the
(ƞ2-Si-H)-coordination of a molecule of silane[14] to the free
1H and 29Si{1H} NMR studies of the 8-catalyzed reaction of
CF3COOH with HSiMe(OSiMe3)2 under the same conditions as
above described for others acids evidenced the formation of
molecular hydrogen together with a mixture of unidentified
species (Table 2). 29Si{1H} NMR spectra of the final reaction
product show that the intensity of the resonances assigned to
CF3COOSiMe(OSiMe3)2 (10d) (d 8.5 and -65.2 ppm)[4e] is very
low in comparison with the intensity of the resonances
corresponding to others not identified species (Figure S7). In
addition, no evidence of {SiMe(OSiMe3)2}2O nor residual
CF3COOH were observed in the 29Si{1H} and 13C{1H} NMR
spectra, respectively. The 13C{1H} NMR spectra of these reaction
samples show the presence of two quartet resonances centered
at d 156.8 ppm (2JC-F = 42.1 Hz) and d 155.6 ppm (2JC-F = 42.6
Hz), high field shifted with respect to the corresponding
vacancy in
D affords intermediate E (related to A). The
carboxylate ligand assisted Si-H bond activation leads to the
silyl-hydride intermediate F (related to B), which contains a
molecule of the corresponding silylester κ1-O-bonded to iridium.
Finally, the substitution of the silylester in F by a molecule of the
corresponding carboxylic acid regenerates the active species C.
This mechanistic proposal is based on Ir-NSiN intermediates
and/or transition states which have been recently reported.[14]
2
resonance for CF3COOH (d 160.8 ppm, JC-F = 43.8 Hz) (Figure
S8). These spectra showed another two quartet resonances due
1
CF3 groups (d 115.32 ppm, JC-F = 286.2 Hz and d 115.30 ppm,
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10.1002/cctc.201701488
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catalyst for the selective dehydrogenative silylation of ECOOH
(E = H, CH3 and Ph) to give the corresponding silylester.
Conversely, when the reaction with CF3COOH was studied
under the same reaction conditions the expected silylester was
obtained as minor product together with a mixture of not
identified compounds containing the CF3CO2Si moiety. This
behaviour could be due to 8-catalyzed rearrangement of
HSiMe(OSiMe3)2. Indeed, the reaction of CF3COOH with HSiEt3
or HSiMe2Ph in presence of catalytic amounts of 8 quantitatively
affords the corresponding silylester CF3COOSiEt3 or
CF3COOSiMe2Ph, respectively.
Experimental Section
General Considerations. All reactions and manipulations were carried
out under an argon atmosphere using Schlenk-type techniques. Organic
solvents were dried by standard procedures and distilled under argon
prior to use or obtained oxygen- and water-free from
a Solvent
Purification System (Innovative Technologies). NMR spectra were
obtained on a Bruker AV-300 and AV-400 spectrometer using TMS as
the internal reference. All chemical shifts (δ) are reported in ppm and
coupling constants (J) are reported in Hz to apparent peak
multiplications.1H-1H-COSY, 13C-APT, 1H/13C HSQC, 1H/13C HMBC and
1H/29Si HMBC sequences were used for help in the assignments of the
1H and 13C spectra. Compound 1 and the Ir-NSiN precursors 4,[8] 7[8] and
8[12] were prepared according to the reported method. RCO2H (R = H,
CH3, CF3 and Ph) and PCy3, were purchased from commercial sources
and used without further purifications.
Preparation of IrHCl(η2-cod)(NSiN) (2). A solution of [IrCl(cod)]2 (0.202
g, 0.30 mmol) in 7 mL of dichloromethane was treated with 1.0 equiv of
bis-(2-(oxy)pyridine)methylsilane 1 (0.140 g, 0.60 mmol) and stirred at
room temperature for 3 h. Then, the solvent was removed in vacuum.
The addition of hexane (3 mL) led to a white solid, which was separated
by decantation, washed with further portions of hexane (3 x 3 mL) and
dried in vacuum. Yield: 0.297 g (87 %). Anal. calcd. for C19H24N2IrClO2Si
(568.165): C, 40.17; H, 4.26; N, 4.93. Found: C, 39.45; H, 4.25; N, 5.85.
Mass spectrometry (ESI): m/z 533.1, [M-Cl]+ (100%).
Scheme 4. Mechanism proposal for the 8-catalyzed dehydrogenative silylation
of carboxylic acids. [Ir] = [Ir(fac-NSiN)]; E = H, CH3 or Ph).
Conclusions
The potential of [Ir(µ-Cl)(cod)]2 as metallic precursor for the
preparation of Ir-NSiN species and the reactivity of the resulting
compounds have been successfully studied. The complex
[Ir(H)(Cl)(NSiN)(h2-cod))] (2) reacts with silver triflate to afford
the ionic species [Ir(H)(NSiN)(h4-cod))][CF3SO3] (3). The
uncommon h2-cod coordination mode of the cod ligand to the
iridium atom in 2 has been confirmed by X-ray diffraction
methods. The reactivity of the Ir-NSiN(olefin) species with PCy3
evidenced that the olefin ligand is easily replaced by the
phosphane to give [Ir(H)(X)(NSiN)(PCy3)] (X = Cl, CF3SO3)
complexes.
1H NMR plus COSY and NOESY (400.13 MHz, CD2Cl2, 298 K): d 9.53
(ddd, JH-H = 6.1, 1.9, 0.6, 1H, H´1py), 9.25-9.22 (m, 1H, H1py), 7.71 (ddd,
JH-H = 8.3, 7.2, 2.0, 1H, H3py), 7.65 (ddd, JH-H = 8.4, 7.2, 1.9, 1H, H´3py),
7.01-6.95 (m, 2H, Hpy (H4 + H2)), 6.93 (ddd, JH-H = 8.3, 1.3, 0.6, 1H, H´4py),
6.79 (ddd, JH-H = 7.3, 6.1, 1.4, 1H, H2py), 5.67-5.51 (m, 2H, =CH3,4cod),
4.61-4.54 (m, 1H, =CH2cod), 3.83-3.75 (m, 1H, =CH1cod), 2.60-1.32 (m, 8H,
-CH2-cod), 0.69 (s, SiCH3, 3H), -15.35 (s, 1H, Ir-H). 13C-APT NMR plus
HMBC and HSQC (100.4 MHz, CD2Cl2, 298 K): d 167.6 and 165.8 (both
s, C´ipso-py and Cipso-py, respectively), 150.9, 148.1, 142.1 and 141.7 (all
On
the
other
hand,
the
trifluoroacetate
derivative
s, C´1py, C1py, C´3 and C3py, respectively), 130.2 and 130.1 (both s,
py
[Ir(H)(CF3CO2)(NSiN)(coe)] (8) has proven to be an efficient
=CH3,4cod), 118.3, 117.4, 112.7 and 112.0 (all s, C2py, C´2py, C4py, C´4
,
py
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10.1002/cctc.201701488
ChemCatChem
FULL PAPER
respectively), 68.7 and 65.9 (both s, =CH2 and =CH1cod, respectively),
Preparation of [Ir(H)(CF3SO3)(NSiN)(PCy3)] (6). A solution of 5 (0.250 g,
0.32 mmol) in 12 mL of THF was treated with 1.0 equiv of silver
trifluoromethanesulfonate (0.081 g, 0.32 mmol) in the absence of light.
After stirring the mixture for 1.5 h at room temperature, the suspension
was filtered through Celite and the filtrate was evaporated to dryness.
The addition of hexane (3 mL) caused the precipitation of a light orange
solid, which was separated by decantation, washed with further portions
of hexane (3 x 3 mL), and dried in vacuum. Yield: 0.268 g (84%). 1H
NMR (300 MHz, CD2Cl2): δ 8.62 (m, 2H, py), 7.63 (m, 2H, py ), 6.97 (m,
1H, py), 6.89 (m, 3H, py), 2.24 (brs, 3H, P-CH), 1.70 (m, 15H, Cy), 1.25
(m, 15H, Cy), 0.86 (s, 3H, SiMe), -19.17 (d, JP-H = 22.5 Hz, 1H, Ir-H). 13C-
cod
35.0, 32.7, 31.4 and 30.8 (all s, -CH2-cod), -1.0 (s, SiCH3). 29Si{1H} NMR
(59.6 MHz, CD2Cl2, 298 K): d 31.3 (s, SiCH3).
Preparation of [IrH(η4-cod)(NSiN)](CF3SO3) (3). A solution of 2 (0.179 g,
0.32 mmol) in 12 mL of THF was treated with 1.0 equiv of silver
trifluoromethanesulfonate (0.081 g, 0.32 mmol) in the absence of light.
After stirring the mixture for 1.5 h at room temperature, the suspension
was filtered through Celite and the filtrate was evaporated to dryness.
The addition of hexane (3 mL) caused the precipitation of a light orange
solid, which was separated by decantation, washed with further portions
of hexane (3 x 3 mL), and dried in vacuum. Yield: 0.176 g (82 %). Anal.
calcd. for C20H24N2SF3IrO5Si (681.778): C, 35.23; H, 3.55; N, 4.11; S,
4.70. Found: C, 34.78; H, 3.58; N, 3.99, S, 4.48. Mass spectrometry
(ESI): m/z 533.1, [M]+ (100 %).
APT NMR plus HSQC and HMBC (75 MHz, CD2Cl2): δ 165.2 (br, Cipso
,
2C, py), 147.4 (s, 2C, CH-py), 141.0, 140.9 and 118.1 (s, CH-py), 117.6
(d, 3JP-C = 2.5 Hz, CH-py), 112.8 and 112.3 (s, CH-py), 37.0 (d, 1J = 31.4
Hz, P-C), 30.1 (d, 4J = 2.5 Hz, Cy), 29.3 (br s, Cy), 27.9 (d, 3J = 11.0 Hz,
3
Cy), 27.7 (d, J = 10.8 Hz, Cy), 27.0 (s, Cy), 0.20 (s, SiMe). 1H-29Si-
HMBC NMR (C6D6): δ 15.3 (SiMe).
General procedure for the NMR scale reactions of equimolar
mixtures of HSiMe(OSiMe3)2 with carboxylic acids. A NMR tube was
charged with the precursor catalyst 7 or 8 (3.5 10-3 mmol) and C6D6 (0.5
mL). After which, HSiMe(OSiMe3)2 (22 µL, 0.07 mmol) and the
corresponding carboxylic acid (0.07 mmol) and hexamethylbenzene as
internal standard (0.07 mmol) were added to the homogeneous solution
and the resulting mixtures were heated at 333 K and monitored by 1H
NMR spectroscopy.
1
Selected data for 10b. H NMR (300 MHz, 298 K, C6D6): δ 1.70 (s, 3H,
CH3), 0.37 (s, 3H, SiMe), 0.20 (s, 18H, OSiMe3). 13C-APT NMR plus
HSQC and HMBC (75 MHz, C6D6): δ 169.7 (s, CO2), 22.3 (s, CH3), 1.6 (s,
OSiMe3), -2.6 (s, SiMe). 29Si{1H} plus 1H-29Si-HMBC NMR (60 MHz, 298
K, C6D6): δ 10.0 (s, OSiMe3), -58.2 (s, SiMe).
1H NMR plus COSY and NOESY (400.13 MHz, CD2Cl2, 298 K): 8.84
(ddd, JH-H = 6.0, 1.6, 0.8, 1H, H1py), 7.96 (ddd, JH-H = 6.1, 1.3, 0.4, 1H,
H´1py), 7.90 (ddd, JH-H = 8.4, 7.3, 1.8, 1H, H3py), 7.80 (ddd, JH-H = 8.5, 7.3,
1.8, 1H, H´3py), 7.19 (ddd, JH-H = 7.3, 6.0, 1.3, 1H, H2py), 7.10-7.08 (m, 1H,
H4py), 7.08-7.06 (m, 1H, H´4py), 7.00 (ddd, JH-H = 7.4, 6.1, 1.4, 1H, H´2py),
5.97-5.91 (m, 1H, =CH2cod), 4.71-4.64 (m, 1H, =CH4cod), 4.54-4.62 (m, 1H,
=CH1cod), 3.74-3.69 (m, 1H, =CH3cod), 3.00-1.84 (m, 8H, -CH2-cod), 0.79 (s,
SiCH3, 3H), -15.47 (s, 1H, Ir-H). 13C-APT NMR plus HMBC and HSQC
(100.4 MHz, CD2Cl2, 298 K): d 167.3 and 166.1 (both s, C´ipso-py and
Selected data for 10c. 1H NMR (300 MHz, 298 K, C6D6): δ 8.20 (m, 2H,
Ph), 7.06 (m, 3H, Ph), 0.48 (s, 3H, SiMe), 0.23 (s, 18H, OSiMe3). 13C-
APT NMR plus HSQC and HMBC (75 MHz, C6D6): δ 165.6 (s, CO2),
133.1 (s, CH, Ph), 131.7 (s, Cipso, Ph), 130.5 (s, CH, Ph) 128.5 (s, CH,
Ph), 1.7 (s, OSiMe3), -2.5 (s, SiMe). 29Si{1H} NMR (60 MHz, 298 K,
C6D6): δ 10.3 (s, OSiMe3), -57.0 (s, SiMe).
Cipso-py, respectively), 152.1, 149.1, 144.0 and 143.8 (all s, C1py, C´1
,
,
py
C´3 and C3py, respectively), 121.1, 120.3, 114.0 and 113.7 (all s, C2
py
py
C´2py, C4py, C´4py, respectively), 112.5, 105.8, 69.2 and 69.0 (all s,
=CH2cod, =CH1cod, =CH3cod, =CH4cod, respectively), 35.9, 35.1, 31.5 and
22.4 (all s, -CH2-cod), -1.5 (s, SiCH3). 29Si{1H} NMR (59.6 MHz, CD2Cl2,
298 K): d 51.5 (s, SiCH3). 19F NMR (282.4 MHz, CD2Cl2, 298 K): d -78.8
(s, CF3).
1
Selected data for 10e. H NMR (400 MHz, 298 K, C6D6): δ 0.56 (q, 6H,
3JH-H = 7.7, CH2), 0.78 (t, 9H, 3JH-H = 7.7, CH3). 13C{1H} NMR plus HSQC
(100 MHz, C6D6): δ 156.5 (q, JC-F = 42.0, CO2), 115.2 (q, JC-F = 286.5,
CF3), 5.8 (s, CH3), 4.0 (s, CH2). 29Si{1H} NMR (99.0 MHz, 298 K, C6D6): δ
34.5 (s). 19F{1H} NMR (282 MHz, 298 K, C6D6): δ -75.8 (s).
2
1
Preparation of [Ir(H)(Cl)(NSiN)(PCy3)] (5). Complex 2 (0.200 g, 0.35
mmol) and PCy3 (0.980 g, 0.35 mmol) were mixed inside the glovebox
and then 8.0 mL of toluene were added to give a yellow solution, which
was stirred at room temperature during 24h. The solvent was removed in
vacuum and the resulting residue was washed with hexane (2 x 3 mL.)
and dried in vacuum to obtain a white powder. Yield: 0.210 g (80%). Anal.
calcd. for C29H45ClIrN2O2PSi: C, 47.04; H, 6.13; N, 3.78. Found: C,
46.78; H, 6.20; N, 3.87. 1H NMR plus HSQC (300 MHz, 298 K, CDCl3): δ
9.64 (m, 1H, py), 9.40 (m, 1H, py), 7.54 (t, JH-H = 7.6 Hz, 1H, py), 7.48 (t,
JH-H = 7.6 Hz, 1H, py ), 6.91– 6.68 (m, 4H, py), 2.01–1.07 (m, 33H, Cy),
0.81(s, 3H, Me), –20.47 (d, JH-P = 23 Hz, Ir-H). 31P{1H} NMR (121 MHz,
298 K, C6D6) δ 15.0 (s, PCy3). 13C-APT plus HSQC and HMBC NMR (75
MHz, 298 K, CDCl3): δ 166.1, 166.0 (s, Cipso-py), 148.6, 148.1, 140.0,
139.8, 116.6, 116.5, 111.8 and 111.2, (s, CH-py), 36.8 and 35.0 (br s,
CH-PCy3), 29.3, 27.6, 26.7 (CH2-Cy). 29Si NMR (DEPT 45, 60 MHz, 298
K, C6D6): δ 41.3 (d, JSi-P = 14.9 Hz, SiMe). ESI+: 705.2 [M+-Cl, 100%].
Selected data for 10f. 1H NMR (400 MHz, 298 K, C6D6): δ 7.42 (m, 2H,
Ph), 7.12 (m, 3H, Ph), 0.30 (s, 6H, SiMe2). 13C{1H} NMR plus HSQC (100
MHz, C6D6): δ 156.7 (q, 2JC-F = 42.0, CO2), 114.9 (q, 1JC-F = 282.2, CF3),
128.5, 131.3 and 133.4 (s, CH, Ph), 133.1 (s, Cipso, Ph), -2.5 (s, SiMe2).
29Si{1H} NMR (99.0 MHz, 298 K, C6D6): δ 20.5 (s). 19F{1H} NMR (282
MHz, 298 K, C6D6): δ -75.9 (s).
General procedure for the measurement of H2 evolution from the 1-
catalyzed reactions of carboxylic acids with HSiMe(OSiMe3)2. The
reactions were carried out in a Man on the Moon X102 kit® micro-reactor
(www.manonthemoontech.com), with a total volume of 19.0 mL, placed
in a silicone bath at 333 K. In a typical procedure 1.0 mmol of
HSiMe(OSiMe3)2 (281 µL, 1.01 mmol) was added to a solution of the
catalyst 8 (0.01 mmol, 6.50 mg) in 1 mL of dry-toluene. The reactor was
closed and the pressure measurement started. Once the pressure was
stabilized, the corresponding ECOOH (E = H, 37.7µL; CH3, 57.2 µL; 1.0
mmol) was added with a syringe. In the case of PhCOOH, 1.0 mmol of
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10.1002/cctc.201701488
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HSiMe(OSiMe3)2 (281 µL, 1.01 mmol) was added to a solution of the
catalyst 8 (6.50 mg, 0.01 mmol) and PhCOOH (122 mg, 1.0 mmol) in 1
mL of dry-toluene. Hydrogen evolution was measured till inner pressure
in the micro-reactor remained constant. The difference in pressure was
used to calculate the amount of H2 produced during the reaction using
the Ideal Gas Law, P·V = n·R·T.
Keywords: Carboxylic acids reduction • Silylation • Ir-NSiN •
Homogeneous catalysis • Hydrogen generation
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Crystal Structure Determination of Complexes 2 and 5. Single crystal
X-ray diffraction data were collected at 100(2)
K with graphite-
monochromated MoKα radiation (λ 0.71073 Å) using narrow
=
ω
rotations (0.3°) on a Bruker APEX DUO (compound 2) or a Bruker
SMART APEX diffractometer (compound 5). SAINT+[24] and SADABS[25]
programs, integrated in APEX2 package, were used to integrate and
correct the absorption effect of the intensities. The structures were solved
by direct methods with SHELXS-2013[26] and refined by full-matrix least-
squares refinement in F2 with SHELXL-2014[27] included in Wingx
package.[28] Hydride ligand has been included in the model in observed
position and freely refined (compound 2) or refined with a restraint in Ir-H
bond length (compound 5).
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Crystal data for 2. C29H24ClIrN2O2Si M = 568.14; yellow prism, 0.070 x
0.082 x 0.082 mm3; monoclinic P21/c; a = 9.8151(7), b = 12.3333(8), c =
16.2337(11) Å, β = 90.1270(10)°; V = 1965.1(2) Å3; Z = 4; ρcalc = 1.920 g
cm–3; μ = 7.007 cm–1; min. and max. transmission factors 0.522 and
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0.672; 2θmax
reflections [Rint
=
59.056°; 30016 reflections collected; 5214 unique
0.0441]; number of data/restraints/parameters:
=
5214/0/256; final GOF 1.046; R1 = 0.0320 [4438 reflections, I > 2σ(I)],
wR2 = 0.0837 for all data; largest difference peak: 7.05 e Å–3. At the end
of the refinement five residual density peaks bigger than 1 e Å–3 have
been found. They are close to iridium atom and reveal bonding effects
between the metal and the ligands.
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Crystal data for 5. C29H45ClIrN2O2PSi M = 740.38; colorless prism,
0.060 x 0.160 x 0.175 mm3; monoclinic P21/c; a = 11.7418(8), b =
12.1443(8), c = 21.6800(14) Å, β = 104.6250(10)°; V = 2991.3(3) Å3; Z =
4; ρcalc = 1.644 g cm–3; μ = 4.675 cm–1; min. and max. transmission
factors 0.498 and 0.640; 2θmax = 56.214°; 34218 reflections collected;
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7094
unique
reflections
[Rint
=
0.0552];
number
of
data/restraints/parameters: 7094/3/394; final GOF 0.982; R1 = 0.0345
[5637 reflections, I > 2σ(I)], wR2 = 0.0844 for all data; largest difference
peak: 3.926 e Å–3. At the end of the refinement five residual density
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Crystallographic
Data
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via
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Financial support from MINECO/FEDER projects: CTQ2013-
42532-P and CTQ2015-67366-P and DGA/FSE group E07) is
gratefully acknowledged. Dr. P. García-Orduña acknowledges
CSIC, European Social Fund and Ministerio de Economía y
Competitividad of Spain for a PTA contract. Authors would like
to acknowledge the use of Servicio General de Apoyo a la
Investigación-SAI, Universidad de Zaragoza. The support of
KFUPM is also appreciated.
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Entry for the Table of Contents (Please choose one layout)
Layout 1:
FULL PAPER
Ir-NSiN species have proven to
be efficient catalysts for the
dehydrogenative silylation of
carboxylic acids
A. Julián, K. Garcés, R.
Lalrempuia, E. A. Jaseer, P.
García-Orduña, F. J. Fernández-
Alvarez,* F. J. Lahoz,[a] and L. A.
Oro*
Page No. – Page No.
Reactivity of Ir-NSiN
Complexes: Ir-Catalyzed
Dehydrogenative Silylation of
Carboxylic Acids
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