Unique Approach to Copper(I) Silylene Chalcogenone Complexes

Article abstract of DOI:10.1021/acs.inorgchem.6b02833

Silylene-S-thione [PhC(NtBu)₂Si(S)N(SiMe₃)₂] (2) and silylene-Se-selone [PhC(NtBu)₂Si(Se)N(SiMe₃)₂] (3) compounds were prepared from the silylene [PhC(NtBu)₂SiN(SiMe₃

Full text of DOI:10.1021/acs.inorgchem.6b02833

Article
pubs.acs.org/IC
Unique Approach to Copper(I) Silylene Chalcogenone Complexes
Nasrina Parvin,† Shiv Pal,† Shabana Khan,*,† Shubhajit Das,‡ Swapan K. Pati,*,§
and Herbert W. Roesky*,∥
†Department of Chemistry, Indian Institute of Science Education and Research, Dr. HomiBhabha Road, Pashan, Pune 411 008, India
‡New Chemistry Unit, Jawaharlal Nehru Centre for Advanced Scientific Research, Jakkur, Bangalore, 560 064, India
§Theoretical Sciences Unit, Jawaharlal Nehru Centre for Advanced Scientific Research, Jakkur, Bangalore, 560 064, India
∥Institute of Inorganic Chemistry, University of Gottingen, Tammannnstrasse 4, Gottingen 37077, Germany
̈
̈
S
* Supporting Information
ABSTRACT: Silylene-S-thione [PhC(NtBu)2Si(S)N(SiMe3)2] (2) and silylene-Se-
selone [PhC(NtBu)2Si(Se)N(SiMe3)2] (3) compounds were prepared from the
silylene [PhC(NtBu)2SiN(SiMe3)2] (1) with 1 equiv of 1/8 S8 and 1 equiv of Se
powder, respectively, in high yields. Furthermore, compounds 2 and 3 reacted with
CuCl and CuBr and yielded [{PhC(NtBu)2}Si(S→CuX)N(SiMe3)2] (X = Cl (4),
Br (5)) and [{PhC(NtBu)2}Si(Se→CuX)N(SiMe3)2] (X = Cl (6), Br (7)),
respectively. Complexes 4−7 can also be obtained from the direct reaction of sulfur
and selenium with the corresponding silylene copper complexes [{PhC(NtBu)2}Si-
{N(SiMe3)2}]2Cu2X2 (X = Cl (8), Br (9)). The latter route avoids the preparation of the highly reactive silylene chalcogenones.
For comparison purposes the silylene PhC(NtBu)2SiN(SiMe3)2 in 2 and 3 was replaced by NHC (1,3-bis(2,6-
bis(diphenylmethyl)-4-methylphenyl)imidazol-2-ylidene) (10). The resulting products NHCS (thione 11) and NHCSe
(selenone 12) react with CuBr and lead to the expected complexes (NHCS→CuBr) (13) and (NHCSe→CuBr) (14).
However, unlike silylene complexes, 13 and 14 cannot be prepared by reacting NHC-CuBr (15) with chalcogens.
In 2007, we synthesized and characterized a silicon thioester
with the Si(S)−S skeleton by using the benzamidinato
chlorosilylene [PhC(NtBu)2SiCl].10 Furthermore, this silylene
was utilized to prepare the functionalized silylene [PhC-
(NtBu)2SiN(SiMe3)2]11 (1) in high yield. However, 1 has yet
to be explored for preparing Si chalcogenones. The first aim of
our investigation focused on the reactivity of 1 toward
chalcogens (E = S, Se). When we observed that compound 1
can smoothly afford the formation of silylene-S-thione (2) and
silylene-Se-selone (3), we investigated the donating abilities of
the Si chalcogenones toward transition metals. The reaction of
CuCl and CuBr resulted in the monomeric Lewis acid−base
adducts 4−7, respectively. It must be noted here that this is the
first report on sterically encumbered silicon-based silathione
and silaselenone donor ligands for the synthesis of low-
coordinate transition-metal complexes. Moreover, the present
study pointed to a distinctly superior donating ability of silylene
chalcogenones over silylenes, as the latter were found to form
dimeric complexes with transition metals from analogous
reactions.12
INTRODUCTION
■
Ketones are one of the most important functional groups in
organic chemistry. However, the heavier double-bonded
compounds between silicon and chalcogen are highly reactive
due to poor pπ−pπ overlap between chalcogens and silicon
atoms.1 In 1989, Corriu et al. reported the first stable diaryl-Si-
thiones and diaryl-Si-selones Ia-Ic (Chart 1).2 Subsequently,
Tokitoh, Okazaki, and their co-workers synthesized and
characterized a series of kinetically stable heavier diaryl Si
chalcogenones IIa−c using sterically hindered aromatic
groups.3 Since then numerous examples of compounds
containing a Si chalcogen double bond have been reported,
thanks to extensive work from the groups of Kira (III), West
(IV), Driess (V and VI), and others.4 With these compounds in
hand, one can envisage using these Si chalcogenones as ligands
in transition-metal chemistry. It has also been known for some
time that aldehydes and ketones can serve as ligands in
transition-metal complexes.5 In fact, the frequent use of acetone
often led to serendipitous acetone complexes such as [(η5-
C5H5)Re(NO)(PPh3)(ηl-(CH3)2CO)]+PF6 and [Fe-
−
(C19H25N5)(C3H6O)(H2O)2](BF4)2·2C3H6O.6 There are sev-
eral reports where SiO double-bonded compounds were
isolated as adducts with transition metals;7 however, other Si
E (E = S, Se, Te) bonded compounds were not explored for
such complexation. The only available report on a SiS→LA
type (LA = Lewis acid) linkage is observed in the monomeric
silicon disulfide (SiS2) VII8 stabilized by the Lewis acid GaCl3
(Chart 2). The same ligand system was further used to isolate
the germanium analogue of VII.9
RESULT AND DISCUSSION
■
Our efforts began by exploring the reactivity of 1 with
chalcogens (E = S, Se). Treatment of 1 in toluene with 1/8
equiv of S8 at room temperature for 12 h afforded a pale yellow
solution of 2 (66% isolated yield) (Scheme 1). Similarly, 3 was
Received: November 26, 2016
© XXXX American Chemical Society
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Chart 1. Reported Compounds with a Double Bond Involving Chalcogens and Silicon Atoms
Chart 2. Lewis Acid−Base Adduct with Si Bis-Thione
CuX)N(SiMe3)2] (X = Cl (4), Br (5)), [{PhC(NtBu)2}Si(
Se→CuX)N(SiMe3)2] (X = Cl (6), Br (7)) by using Si
chalcogenones 2 and 3, respectively. Alternatively, the reactions
of silylene copper complexes [{PhC(NtBu)2}Si{N-
(SiMe3)2}]2Cu2X2 (X = Cl (8), Br (9))12 with chalcogens
also resulted in the formation of products 4−7.
The 1H NMR spectra of 2 and 3 each display a sharp singlet
at δ 1.30 and 1.34 ppm for 18 tBu protons, which are shifted
downfield in comparison to those of 1 (δ 1.23 ppm). The 29Si
NMR spectra of both 2 and 3 display three resonances (2, δ
−16.82, 1.69, 6.26 ppm; 3, δ −20.22, 2.78, 8.40 ppm). The
resonances at δ −16.82 and −20.22 ppm for 2 and 3
respectively correspond to the central silicon atom, which are
shifted upfield in comparison with that of 1 (δ −8.07 ppm).
The other two resonances for both 2 and 3 are assigned to the
two SiMe3 groups. A sharp resonance is exhibited at δ −458.42
ppm for 3 in the 77Se NMR spectrum. In the HRMS spectrum,
synthesized in 70% yield by reacting 1 with 1 equiv of selenium
powder (Scheme 1). The reaction mixture turned from yellow
to colorless. Products 2 and 3 were isolated as colorless
crystalline solids with good solubility in toluene, THF, and
dichloromethane. After the isolation of 2 and 3, we explored
the donor properties of the Si chalcogenones. We were able to
prepare the Lewis acid−base adducts [{PhC(NtBu)2}Si(S→
Scheme 1. Reactions of 1 with Chalcogens (S, Se) and Copper Halides (CuCl, CuBr)
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the most abundant ion was observed at m/z 452.2405 for 2 and
m/z 500.1852 for 3 with the highest relative intensity (100%).
The ORTEP diagrams of 2 and 3 are shown in Figures 1 and
2 with their structural parameters. 2 crystallizes in the
distance in 3 is 2.1369(9) Å, which is longer than those of III
and V (2.096(1) and 2.117(1) Å) but shorter than those of IV
and VI (2.153(1) and 2.146(1) Å). The average Si−S−N bond
angle is 116.50°, which matches with an average Si−Se−N
bond angle (116.52°).
The reactions of 2 and 3 with 1 equiv of CuCl in toluene
afforded colorless crystalline solids of 4 and 6 with
approximately 60−61% yield (Scheme 1). The coordination
of the Si chalcogens to CuX is observed by a shift in the
respective 29Si NMR spectrum, while the resonance for the
central Si atom is exhibited at δ −14.96 and −19.79 ppm for 4
and 6, respectively. These resonances are shifted downfield in
comparison with those of their parent compounds 2 and 3. The
77Se NMR spectrum of 6 shows a sharp singlet at δ −466.68
ppm, which is shifted upfield in comparison to 3 (δ −458.42
ppm).
Compounds 4 and 6 were further characterized by single-
crystal X-ray diffraction analysis shown in Figures 3 and 4, and
Figure 1. Molecular structure of 2 with anisotropic displacement
parameters depicted at the 50% probability level. Hydrogen atoms are
not shown for clarity. Selected bond lengths (Å) and bond angles
(deg): Si1−S1 1.987(2), Si1−N3 1.716(4), Si1−N2 1.816(4), Si1−N1
1.834(4); N3−Si1−S1 119.04(17), N2−Si1−S1 117.13(14), N1−
Si1−S1 113.32(16).
Figure 3. Molecular structure of 4 with anisotropic displacement
parameters depicted at the 50% probability level. Hydrogen atoms are
not shown for clarity. Selected bond lengths (Å) and bond angles
(deg): Si1−S1 2.039(2), Cu1−S1 2.1260(18), Cu1−Cl1 2.092(2),
Si1−N3 1.692(5), Si1−N2 1.819(5), Si1−N1 1.804(5); Cl1−Cu1−S1
175.00(9), Si1−S1−Cu1 99.64(9), N3−Si1−S1 115.13(19), N2−
Si1−S1 115.83(18), N1−Si1−S1 112.66(18).
Figure 2. Molecular structure of 3 with anisotropic displacement
parameters depicted at the 50% probability level. Hydrogen atoms are
not shown for clarity. Selected bond lengths (Å) and bond angles
(deg): Se1−Si1 2.1369(9), N3−Si1 1.715(2), N2−Si1 1.815(2), N1−
Si1 1.827(2); N3−Si1−Se1 121.86(8), N2−Si1−Se1 113.23(8), N1−
Si1−Se1 114.48(8).
monoclinic space group P21/c, whereas 3 crystallizes in the
triclinic space group P1.13 Both crystals were obtained in
̅
toluene at −30 °C in a freezer. The silicon center in both 2 and
3 is four-coordinate, and each exhibits a distorted-tetrahedral
geometry. The two sites are occupied by the N atoms from the
amidinato ligand: one site is occupied by the N atom of the
N(TMS)2 moiety, and the other site is connected with the
chalcogens (S, 2; Se, 3). The SiS bond length in 2 is
1.987(2) Å, which is in good agreement with the previously
reported SiS double bond in [{PhC(NtBu)2}Si(S)StBu]
(1.984(8) Å).10 The SiS bond length in 2 is slightly longer
than those in IIa (1.948(4) and 1.952(4) Å, as two nonidentical
molecules of IIa were present in the unit cell)3a,b and IIIa
(1.957(2) Å),4a respectively, but shorter than that of the base
stabilized Si thione Ia2 (2.013(3) Å). Similarly, the SiSe bond
Figure 4. Molecular structure of 6 with anisotropic displacement
parameters depicted at the 50% probability level. Hydrogen atoms are
not shown for clarity. Selected bond lengths (Å) and bond angles
(deg): Si1−Se1 2.1903(8), Cu1−Se1 2.2412(5), Cu1−Cl1 2.0953(9),
Si1−N3 1.694(2), Si1−N2 1.821(2), Si1−N1 1.808(2); Cl1−Cu1−
Se1 174.15(4), Si1−Se1−Cu1 98.96(2), N3−Si1−Se1 115.14(8),
N2−Si1−Se1 115.22(8), N1−Si1−Se1 113.20(8).
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selected bond parameters are given in the legends of the figures.
Both complexes crystallize in the orthorhombic space group
Pbca. The silicon center is tetracoordinate for compounds 4
and 6. In both cases, the chalcogens are dicoordinate: one site is
connected with the silicon center, and the other site is
coordinated with the copper center of the CuCl moiety. In 4,
the SiS bond length is 2.039(2) Å, which is longer than that
of 2 due to coordination with the Lewis acid CuCl and also
closely matches with that of the previously reported VII
(2.106(2) Å),8 where S donates its lone pair of electrons to the
Lewis acid of GaCl3. The S→Cu bond length is 2.1260(18) Å.
S−Cu−Cl exhibits an angle of 175.00(9)°, while the Si−S−Cu
angle is 99.64(9)°. Similarly, in the case of 6, the SiSe bond
length is 2.1903(8) Å, which is slightly longer than that of the
precursor. The Se→Cu bond length is 2.2412(5) Å. Here, as in
the previous case the Se, Cu, and Cl are arranged in the same
plane with a bond angle of 174.15(4)°.
Figure 6. Molecular structure of 7 with anisotropic displacement
parameters depicted at the 50% probability level. Hydrogen atoms are
not shown for clarity. Selected bond lengths (Å) and bond angles
(deg): Se1−Si1 2.1901(18), Se1−Cu1 2.2460(11), Cu1−Br1
2.2194(11), Si1−N3 1.696(5), Si1−N2 1.811(5), Si1−N1 1.817(5);
Se1−Cu1−Br1 173.22(5), Si1−Se1−Cu1 99.55(6), N3−Si1−Se1
114.93(19), N2−Si1−Se1 113.32(18), N1−Si1−Se1 115.38(18).
Similarly, compounds 5 and 7 were obtained in good yield
(60−62%) from the reaction of 2 and 3 with 1 equiv of CuBr in
1
toluene at room temperature (Scheme 1). The H and 29Si
NMR spectra of 5 and 7 reflect the coordination of S and Se
centers via one of their lone pair of electrons to the CuBr
moiety. The resonances for the silicon center were observed at
δ −17.19 ppm (5) and δ −19.93 ppm (7). The slight upfield
shift in the resonance of 5 is unexpected, as there should be an
electron density shift from Si to S upon coordination with the
Lewis acid (CuX). However, this unusual behavior can be
attributed to the domination of a Si−S σ-bonding MO, which
was previously reported for the SiS→GaCl3 compound VII.8
The solid-state structures of 5 and 7 are the bromide
analogues of 4 and 6 (Figures 5 and 6). 5 crystallizes in the
the Se1−Cu1−Br1 angle of 173.22(5)° in 7 indicate that Cu,
Cl, and E (E = S, Se) are arranged in the same plane.
Compounds 4−7 can also be prepared by the insertion of E
(E = S, Se) into the Si→Cu bond of the complexes
[PhC(NtBu)2SiN(SiMe3)2CuCl]2 (8) and [PhC(NtBu)2SiN-
(SiMe3)2CuBr]2 (9), respectively. Complexes 8 and 9 can be
prepared by a modified literature procedure.12 The reactions of
complexes 8 and 9 with chalcogens (E = S, Se) resulted in the
Lewis acid−base adducts 4−7, respectively. This is another
useful route to synthesize Lewis acid−base adducts from the
SiII→CuX (X = Cl, Br) coordination complexes. DFT
calculations reveal that the Si→CuBr bond (WBI = 0.68) is
much weaker in comparison to the SiS bond (WBI = 1.55).
Therefore, the formation of a stronger Si−S (Si−Se) bond is at
the expense of a weaker Si−Cu bond that renders the
thermodynamic driving force for the aforementioned process.
For comparison reasons we further investigated the reactivity
of an N-heterocyclic carbene instead of silylene with chalcogens
to explore their coordination behavior (see the Supporting
Information for the experimental details and crystal structures).
For this purpose, we prepared the previously reported sterically
demanding N-heterocyclic carbene 1,3-bis(2,6-bis-
(diphenylmethyl)-4-methylphenyl)imidazol-2-ylidene (10).14
Treating 10 with sulfur and selenium afforded the thione 11
and the previously reported selenone 12,15 respectively.
Subsequently, these chalcogenones act as a Lewis base to
form adducts 13 and 14 (Scheme 2). We have also prepared
the carbene CuBr complex 15 and subsequently attempted to
react the latter with chalcogens (E = S, Se). However, unlike 8
and 9, no reaction between 15 and chalcogens was observed.
Therefore, this synthetic protocol did not lead to the formation
of 13 and 14, which was explained by density functional theory
(DFT) calculations. The theoretical study was performed using
isodesmic reactions (see the Supporting Information for
details), and a comparison of the solvent-corrected Gibbs free
energies of these isodesmic reactions shows that carbene-S→
CuBr formation is 51.6 kcal/mol more endergonic than
silylene-S→CuBr formation. Compounds 11 and 13−15 were
characterized by state of the art spectroscopic tools and single-
crystal X-ray diffraction analysis. Although X-ray diffraction on
Figure 5. Molecular structure of 5 with anisotropic displacement
parameters depicted at the 50% probability level. Hydrogen atoms are
not shown for clarity. Selected bond lengths (Å) and bond angles
(deg): Si1−S1 2.0491(10), S1−Cu1 2.1304(8), Cu1−Br1 2.2316(4),
Si1−N3 1.702(2), Si1−N2 1.814(2), Si1−N1 1.816(2); S1−Cu1−Br1
175.79(3), Si1−S1−Cu1 105.79(4), N3−Si1−S1 115.00(8), N2−
Si1−S1 112.80(8), N1−Si1−S1 114.64(8).
monoclinic space group P21/c, and 7 crystallizes in the
orthorhombic space group Pbca.13 For all of the complexes,
the silicon center is tetracoordinate. Furthermore, each of the
dicoordinate chalcogen atoms is bound to the Lewis acid CuBr.
Compounds 5 and 7 show SiE (E = S, Se) bond lengths of
2.0491(10) and 2.1902(18) Å, respectively. The E→Cu (E = S,
Se) bond lengths are 2.1304(8) and 2.2460(11) Å for 5 and 7,
respectively. The S1−Cu1−Br1 angle of 175.79(3)° in 5 and
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a
shows the σ-bonding interaction between Si and S. NBO
analysis suggests that the Si−S σ-bonding orbital is polarized
toward the S atom (64% of NBO density on S). The calculated
NPA charges on Si and S atoms are 1.829 and −0.802,
consistent with a polarized Si−S bond. Furthermore, at the
second-order perturbation level, a donor−acceptor interaction
is found between the lone pairs of S atom and the empty p
orbitals of Si (interaction energy sums up to 105.8 kcal/mol),
which is in support of the back-bonding picture.
Scheme 2. Reaction of 10 with Chalcogens and CuBr
The bonding situation in 3m is found to be similar to that in
2m with a calculated WBI of 1.59. MO analysis shows that both
σ- and π-bonding interactions are present in the SiSe bond
(Figure 8). The SiSe σ-bonding orbital is polarized toward
a
Ar*= 4-Me-C6H2-2,6-(CHPh2)2.
a single crystal unambiguously established the connectivity of
11 and 13−15, we refrain from a discussion of their bonding
parameters because of the low quality of the data (see Figures
S1−S3 in the Supporting Information for the crystal
structures).
In order to obtain a deeper insight into the electronic
structure of the silicon−chalcogen double bonds, we have
carried out density functional theory (DFT) calculations on
model compounds (see Computational Details in the
Supporting Information) [PhC(NMe)2Si(S)N(SiMe3)2]
(2m) and [PhC(NMe)2Si(Se)N(SiMe3)2] (3m). The
calculated structural parameters are fairly consistent with the
crystallographic data, and the geometry optimized structure of
2m is shown in Figure 7. The SiS bond length in 2m is found
Figure 8. Molecular geometry optimized structure and relevant
molecular orbitals of 3m.
the Se atom (60.5% of NBO density on Se). However, as
expected from the relatively smaller electronegativity of the Se
in comparison to S, the SiSe bond is slightly less polarized
than the SiS bond; the NPA charges on Si and Se atoms are
1.747 and −0.709 (Δq = 2.456 vs 2.631 in 2m). Furthermore,
the Se→Si back-donation is confirmed by NBO analysis, which
suggests a donor−acceptor interaction between the lone pairs
of Se and the empty p orbitals of Si.
The coordinate bonded 1:1 complexes between Si
chalcogenones and CuX (X = Cl, Br) have also been subjected
to computational analysis. Geometry optimized structures of
these complexes (4m−7m) are depicted in Figure S4 in the
Supporting Information. Our results indicate that in all the
complexes, silicon−chalcogen double bonds are elongated in
comparison to those in the Si chalcogenones. For example, the
SiS bond in 5m is elongated to 2.064 Å (computed 1.997 Å
in 2m), while the WBI is reduced to 1.22 (1.55 in 2m). This
can be explained by the electron donation from S to Cu. In fact,
NBO analysis shows that a significant donor−acceptor
interaction is found between a lone pair of the S atom and
an empty s orbital in Cu (at the second-order perturbation level
the interaction energy amounts to 104.9 kcal/mol).
Apart from the bonding analysis, we also posed a few
questions which we tried to answer from DFT calculations. For
example, silylene copper halide complexes 8 and 9 are dimeric,
while silachalcogenone copper halides complexes 4−7 are
monomeric. The free energy of dimerization of SiS→CuBr
(5m) is endergonic by 7.4 kcal/mol, while dimerization of Si→
CuBr is almost thermoneutral in toluene. It is also interesting to
note that sulfur and selenium insert into the Si−Cu bond but
Figure 7. Molecular geometry optimized structure and relevant
molecular orbitals of 2m.
to be 1.997 Å, which is close to the experimentally measured
value of 1.987 Å in 2. The Wiberg bond index (WBI) for the
SiS bond is found to be 1.55, which indicates that the SiS
bond has a multiple bonding character in addition to a
contribution from the charge-separated canonical forms (vide
infra; NPA calculations). An inspection of the molecular
orbitals (MOs) confirms the bonding picture. MO analysis
shows both σ- and π-bonding interactions between the Si and
the S atoms. As illustrated in Figure 7, the HOMO is a lone pair
on the S atom and HOMO-1 is a π-type bonding orbital
(concentrated on the Si−S bond), while HOMO-4 clearly
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not into the C−Cu bond. To rationalize this observation of
why 13 and 14 cannot be afforded from 15, we have formulated
a couple of isodesmic reactions through which we aim to
examine the propensity of the formation of (silylene/carbene)-
S→CuBr complexes from (silylene/carbene)-CuBr complexes.
For construction of the isodesmic equations, H2Si: and H2C:
are used as reference molecules and the equations are
DEDICATION
■
Dedicated to Professor Ionel Haiduc on the occasion of his
80th birthday.
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In summary, we have done a systematic study of the
synthesis and characterization of Si chalcogenones 2 and 3 and
their transition-metal-based Lewis acid−base adducts 4−7 of
composition SiE→LA (E = S, Se) with electron-deficient
CuX (X = Cl, Br). In addition to the SiS-based complexes
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adducts 13 and 14 from sterically hindered carbene 10 to
compare them with their silicon analogues. On comparison, we
observed that Si chalcogen based Lewis acid−base adducts 4−7
can also be obtained from the reaction of silylene copper
complexes 8 and 9 with chalcogens; however, this route does
not work for the carbene copper complex 15, as the latter does
not react with chalcogens to form 13 and 14. The dichotomy
was rationalized by theoretical calculations, which reveal that
carbene-S→CuBr formation is 51.6 kcal/mol more endergonic
than silylene-S→CuBr formation.
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ASSOCIATED CONTENT
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S
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.6b02833.
Experimental, crystal, and theoretical data (PDF)
Crystal data (CIF)
Crystal data (CIF)
Crystal data (CIF)
Crystal data (CIF)
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AUTHOR INFORMATION
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Corresponding Authors
*E-mail for S.K.: shabana@iiserpune.ac.in.
*E-mail for S.K.P.: pati@jncasr.ac.in.
*E-mail for H.W.R.: hroesky@gwdg.de.
ORCID
Shabana Khan: 0000-0002-6844-3954
Notes
The authors declare no competing financial interest.
́
2509−2513. (d) Fernandez, J. M.; Emerson, K.; Lamen, R. D.;
Gladysz, J. A. A General Method for the Selective Binding and
Activation of one Aldehyde Enantioface. Synthesis and Reactivity of
Rhenium Aldehyde Complexes [(η5-C5H5)Re(NO)(PPh3)(η2-
ACKNOWLEDGMENTS
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RCHO)]+ PF6 . J. Am. Chem. Soc. 1986, 108, 8268−8270.
S.K. thanks the SERB (India), DST-FIST, and IISER Pune for
financial support. N.P. and S.P. thank the UGC for providing
fellowships. H.W.R. thanks the Deutsche Forschungsgemein-
schaft for financial support.
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