N-heterocyclic silylene stabilized monocordinated copper(i)-arene cationic complexes and their application in click chemistry

Article abstract of DOI:10.1039/c9cc09115g

Herein for the first time we report monocoordinated cationic Cu(i) complexes with unsymmetrical arenes (toluene and m-xylene) [LCu(η³-C₇H₈)]⁺[SbF₆]^- and [{LCu(η²-Me₂C<

Full text of DOI:10.1039/c9cc09115g

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N-heterocyclic silylene stabilized monocordinated
copper(I)–arene cationic complexes and their
application in click chemistry†
Cite this: Chem. Commun., 2020,
56, 273
Received 22nd November 2019,
Accepted 28th November 2019
b
b
Nasrina Parvin,a Jabed Hossain,a Anjana George,
Pattiyil Parameswaran
*
a
and Shabana Khan
*
DOI: 10.1039/c9cc09115g
rsc.li/chemcomm
Herein for the first time we report monocoordinated cationic Cu(I)
complexes with unsymmetrical arenes (toluene and m-xylene)
[LCu(g3-C7H8)]+[SbF6]À and [{LCu(g2-Me2C6H4)}]+[SbF6]À [L
=
{PhC(NtBu)2SiN(SiMe3)2}], [IPr (1,3-bis(2,6-diisopropylphenyl)imidazol-
2-ylidene)], their reactivity and catalytic applications in CuAAC reactions
(12 examples). The bonding analysis was performed in both silylene
and carbene complexes using the EDA-NOCV method at the BP86/
TZ2P level of theory.
Chart 1 The selected examples of monocoordinated Cu(I)–arene complexes.
unsymmetrical arenes. To fill this void, we used [{PhC(NtBu)2}-
Si{N(SiMe3)2}]2Cu2Br2 (1)14 as
a precursor and synthesized
Coinage metal p-complexes are important due to the catalytic
functionalisation of p-substrates. Although there are a range of
examples on coinage metal(I)–arene complexes,1–3 the examples of
structurally authenticated monocoordinated Cu(I)–arene cationic
complexes are rare (A, B1 and B2)4,5 (Chart 1). Cu–arene complexes
are considered to be present as intermediates in several reactions
and they have shown their applications in a wide variety of catalytic
reactions.6–12 Owing to their highly reactive nature it is very challen-
ging to isolate them in the monocoordinated form. In a pioneering
work, Hayton and co-workers reported the first Cu(I)–arene com-
plexes (A) where Cu(I) is bound to the hexamethylbenzene ring in a
Z6-fashion.4 Very recently, our group has reported the first copper
cation [{PhC(NtBu)2SiN(SiMe3)2}Cu(Z6-C6H6)]+[SbF6]À (B1) bound to
the benzene ring in an unsupported Z6 mode5 using amidinato
silylene, [PhC(NtBu)2SiN(SiMe3)2]13 as a ligand. The synthetic
methodology also led to a copper cation (B2) bound to the hexa-
methylbenzene ring in Z6 mode.5 In light of these recent synthetic
advances concerning Cu(I) arene complexes, a thorough study on
other common arenes, especially unsymmetrical arenes such as
toluene and xylene, is anticipated as no structural evidence exists for
N-heterocyclic carbene (NHC)/silylene (NHSi) Cu(I) cations with such
[{PhC(NtBu)2SiN(SiMe3)2}Cu(Z3-C7H8)]+[SbF6]À (2) and [{PhC(NtBu)2-
SiN(SiMe3)2}Cu(Z2-Me2C6H4)]+[SbF6]À (3) complexes. It is noteworthy
to mention that this is the first report where a monocoordinated
Cu(I) cation is bound to free toluene and m-xylene. For an explicit
comparison, we carried out the same reaction with NHC (IPr) in
place of silylene by using IPrÁCuBr15 with toluene and m-xylene
which resulted in [IPrÁCu(Z3-C7H8)]+[SbF6]À (5) and [IPrÁCu(Z2-
Me2C6H4)]+[SbF6]À (6), respectively. Further we explored their reac-
tivity with strong donor ligands i.e., MeCN [PhC(NtBu)2SiN(SiMe3)2]
and IPr. We also explored 2 and 5 as catalysts in copper-catalyzed
azide–alkyne cycloaddition (CuAAC) reactions. Our results are
reported herein.
Treatment of 1 with AgSbF6 in an arene (toluene/m-xylene)-
CH2Cl2 mixture at room temperature for 10 hours afforded 2
and 3, respectively (Scheme 1). The coordination of the arene ring
1
to the Cu center is accompanied by the H NMR spectrum of 2
displaying a peak for three methyl protons of toluene at 2.51 ppm
which is slightly upfield shifted when compared to that in the free
toluene (2.36 ppm). The six protons of the methyl group of m-xylene
a Department of Chemistry, Indian Institute of Science Education and Research
Pune, Dr Homi Bhabha Road, Pashan, Pune-411008, India.
E-mail: shabana@iiserpune.ac.in
b Department of Chemistry, National Institute of Technology Calicut,
NIT Campus P. O., Calicut – 673601, India. E-mail: param@nitc.ac.in
† Electronic supplementary information (ESI) available. CCDC 1896663 (2), 1896664
(3), 1896665 (5), 1886948 (6), 1896666 (8) and 1896670 (9). For ESI and crystallographic
data in CIF or other electronic format see DOI: 10.1039/c9cc09115g
Scheme 1 Syntheses of complexes 2 and 3.
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Scheme 2 Syntheses of complexes 5 and 6.
Fig. 1 The molecular structures of 2 (left) and 3 (right) (ellipsoids are
shown at the probability level of 30%). Hydrogen atoms and the SbF6 anion
are omitted for clarity. Selected bond lengths (Å): (2) Si1–Cu1 2.234(2),
Si1–N3 1.704(6), Cu1–C14 2.230(1), Cu1–C22 2.330(1), Cu1–C23 2.426(1),
Cu1–C15 2.64(1), Cu1–C24 2.697(2), and Cu1–C27 2.828(2). (3) Si1–Cu1
2.244(8), Si1–N3 1.721(2), Cu1–C18 2.161(5), Cu1–C17 2.195(5), Cu1–
C2 2.573(4), Cu1–C6 2.678(3), Cu1–C9 2.827(4), and Cu1–C29 2.957(3).
were observed at 2.28 ppm as a singlet (for free m-xylene at
2.40 ppm) in the 1H NMR spectrum of 3. The 29Si NMR spectra of
2 and 3 display resonances at 2.28 ppm and 2.80 ppm, respectively,
which correspond to the central Si(II) atom and are slightly upfield
shifted in comparison to that of 1 (5.72 ppm).14
2 and 3 crystallize in orthorhombic P212121 and monoclinic
Pn space groups, respectively.16 The molecular structures of 2
and 3 (Fig. 1) unveil the Z3 and Z2 binding mode of the toluene
Fig. 2 The molecular structures of 5 (left) and 6 (right) (ellipsoids are
shown at the probability level of 30%). Hydrogen atoms are omitted for
clarity. Selected bond lengths (Å): (5) C2–Cu1 1.882(1), Cu1–C17 2.064(2),
Cu1–C20 2.314(2), Cu1–C26 2.316(1), Cu1–C13 2.746(1), Cu1–C21 2.827(9),
and Cu1–C15 3.001(1).
and m-xylene rings, respectively (as per the bond distances of previously reported CIPr–Cu bond length of [IPrÁCu(Z3-C6H6)]+-
Cu–Carene bonds). The assignment of hapticity for the complexes [SbF6]À (1.890(3) Å) and [IPrÁCu(Z3-Me6C6)]+[SbF6]À (1.886(5) Å),
having low hapticities (Z1–Z3) has always been a tough job. There- respectively.5 The Cu–Carene bond distances and the hapticity calcu-
fore, we adopted the method developed by Alvarez and co-workers to lations of 5 and 6 indicate the Z3 and Z2 binding mode of toluene
calculate the hapticity (Table 1).17 The central Si(II) atom is four and m-xylene to the Cu centre, respectively (Table 1). We attempted
coordinate in both the cases and adopts a distorted tetrahedral to isolate such coordination complexes with bigger arene systems
geometry with the Si - Cu bond distances of 2.234(2) Å (2) and (naphthalene, biphenyl, styrene, etc.) but unfortunately it did not
2.244(8) Å (3), which are slightly longer than those in 1 [2.222(2) Å].14 work, and instead led to the formation of 7 most of the time.
Furthermore, we treated the IPr - CuBr (4) adduct15 with AgSbF6
It is well known that s-donor ligands bind more strongly than
in the presence of toluene and m-xylene, respectively, which resulted p-donor ligands. Therefore, displacement reactions of arene rings
in the corresponding [IPrÁCu(Z3-C7H8)]+[SbF6]À (5) and [IPrÁCu(Z2- with s-donor ligands such as MeCN, NHC and NHSi with 2 and 5
1
Me2C6H4)]+[SbF6]À (6) complexes (Scheme 2). The H NMR spectra were performed. Upon reaction of 2 with MeCN, the formation of
of 5 and 6 further supported the coordination of the respective a dimeric copper complex 7 along with Cu(CH3CN)4SbF6 salt was
arenes to the copper centre. 5 and 6 crystallize in the monoclinic noted (Scheme 3). 7 was spectroscopically characterized but the
Cc and orthorhombic Pbca space groups, respectively (Fig. 2).16 single crystal X-ray data are not sufficiently good to discuss the
The Cu–CIPr distance of 5 is 1.882(1) Å and matched well with the structural parameters. An analogous reaction of 5 afforded a
Table 1 The EDA-NOCV results (BP86/TZ2P) in 2 and 5. (a) For the interaction of the L1L2Si fragment with Cu(Tol)+ and (b) the L1L2SiCu+ fragment with
Tol in complex 2. (c) The NHC (IPr) fragment with Cu(Tol)+ and (d) the NHCCu+ fragment with Tol in complex 5. L1 = N(SiMe3)2, L2 = (Ph)C(Nt-Bu)2, Tol =
C6H5(CH3). Energies are in kcal molÀ1
Parameter
(a)
(b)
(c)
(d)
DEint
DEPauli
DEelstat
DEorb
DETol-Cu
DECu-Tol
DESilylene/NHC-Cu
DECu-Silylene/NHC
À94.8
À27.2
À94.6
À37.2
102.2
76.1
109.5
69.9
a
À127.3 (64.6%)
À69.8 (35.4%)
—
À58.4 (56.4%)
À45.0 (43.6%)
À22.5 (50.0%)
À8.7 (19.3%)
—
À141.1 (69.1%)
À63.0 (30.9%)
—
À57.4 (53.6%)
À49.7 (46.4%)
À24.7 (49.7%)
À11.5 (23.1%)
—
a
b
b
—
—
b
b
À44.8 (64.2%)
À4.8 (6.9%)
À20.2
À29.0 (46.0%)
À9.7 (15.4%)
À24.3
—
—
b,c
DErest
À13.8
À13.5
d
DEprdep
7.9
3.5
3.2
4.5
ÀDe
À86.9
À23.7
À91.4
À32.7
a
b
Values in parentheses give the percentage contribution to the total attractive interactions, DEorb + DEelstat
.
Values in parentheses give the
c
d
percentage contribution to the orbital interactions, DEorb
dissociation energy, respectively.
.
DErest = DEorb À (DEM-L + DEL-M). DEprep and De represent the preparatory and
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Scheme 3 Reaction of 2 with acetonitrile and IPr.
Fig. 3 Plots of deformation densities (BP86/TZ2P) for (a) donation from
the Si(II)- sp-hybrid orbital on Cu; (b) backdonation from the d-orbital of
Cu - Si–N p**-MO of silylene in 2; (c) donation from the NHC - sp-
hybrid orbital on Cu and (d) back donation from the d-orbital of Cu - p**-
MO of NHC in 5. The direction of charge flow is from red to blue. The
isosurface value for the plot is 0.0003. The associated energy (DE) is given
in kcal molÀ1
.
Scheme 4 Reaction of 5 with MeCN and 2. The molecular structures of 8
and 9 (ellipsoids are shown at the probability level of 30%). Hydrogen
atoms and the SbF6 anion are omitted for clarity. Selected bond lengths (Å)
and bond angles (deg): (8) C1–Cu1 1.939(4), Cu1–Si1 2.271(1); and
of the vacant sp-hybrid orbital (80.4% s and 14.9% p) accepts
C1–Cu1–Si1 178.6(1); (9) C1–Cu1 1.926(5), C1–N2 1.361(4), Cu1–N4 1.976(4); the lone pair from the NHC to form the IPr - Cu bond. The
N4–Cu1–N4 111.11(19), N4–Cu1–C1 124.38(10), C4–C3–N4 179.7(5), and
nature of bonding between silylene/NHC and the Cu-centre as
well as the arene ring and the Cu-centre was further analysed by
C3–N4–Cu1 168.6(3).
the EDA-NOCV method and the results of the toluene com-
tricoordinate copper cation 9 where two acetonitrile molecules plexes 2 and 5 are discussed here (see the ESI† for 3 and 6).
were coordinated to the Cu center. The Cu–CIPr bond distance is
The analysis of Si(II) - Cu in 2 by the EDA-NOCV method
elongated to 1.926(5) Å (Scheme 4). The arene displacement (Table 1) indicates that the electrostatic interaction (DEelstat
=
reaction of 2 with IPr or 5 with NHSi afforded an identical 64.6%) is higher than the covalent interaction (s-type lone pair
complex, 8, where a dicoordinate copper cation is bound to one on silylene to the vacant sp-hybrid orbital on Cu accounts to
silylene and one carbene ligand. This is a rare example of a mixed 64.2% of the total covalent interaction) (Fig. 3a, À44.8 kcal molÀ1).
tetrelylene coinage metal cation. Note that, both the Cu–CIPr The back donation from the filled d-orbitals on Cu to the
[1.939(4) Å] and Cu–Si [2.271(1) Å] bond lengths have been Si–N s*-molecular orbital (MO) of silylene contributes to
increased compared to their parent bond lengths.
6.9% of the total orbital interaction (Fig. 3b, À4.8 kcal molÀ1).
We have carried out quantum mechanical calculations at the Similar to 2, the IPr - Cu bond in 5 (Table 1) also has
M06/def2-TZVPP//BP86/def2-SVP level of theory17 to explore the
bonding interaction between the silylene/IPr and Cu-centre as (DEelstat = 69.1%) (see Table S5 in the ESI† for 3 and 6).
well as the arene ring in complexes 2, 3, 5 and 6.18 The NBO
To explore the catalytic activity of our newly synthesized
a significant contribution from electrostatic interaction
charge analysis (Table S4, ESI†) of the complexes 2 and 3 shows complexes, 2 and 5, we have studied the CuAAC reaction, which
that the N(SiMe3)2 (À0.53 e) and (Ph)C(Nt-Bu)2 (À0.35 to has been widely used for the synthesis of 1,2,3-triazoles.21 Unlike
À0.36 e) ligands possess negative group charges. The positive NHC–Cu(I) complexes, which have ample literature precedence
charge is mainly located on the Si(II)-centre (1.43 e), Cu in CuAAC catalysis,22 there is only one report of the CuAAC
(0.27–0.29 e) and the arene ligand (0.16–0.189 e). Thus, a high reaction with NHSi–Cu(I) multinuclear complexes by Stalke
positive charge on Si and a low positive charge on the Cu-centre et al.;23 however, the methodology is limited to only benzyl azide
indicate a strong Si(II) - Cu donor acceptor character, which is (5 examples). Moreover, there is no report of a monocoordinated
commonly observed in silylene complexes.19
Si(II)–Cu complex for CuAAC reactions. Therefore, we have used 2
The NBO analyses of complexes 2 and 3 suggest that five d- as a catalyst for different azides and alkynes under ambient
orbitals of Cu+ (1S, 3d104s0) are occupied, and one of the vacant conditions (Scheme 5), which afforded the desired triazoles in
sp-hybrid orbital (79.6% s and 18.4% p) accepts the lone pair good to excellent yields. A higher yield is observed for aromatic
from silylene to form the Si(II) - Cu bond. The NBO charge alkynes (I, II, V, VI, IX, and X) than for the aliphatic ones
analysis (Table S4, ESI†) indicates significant differences in (Scheme 5). Aromatic azides bearing electron donating groups
charge distribution on the carbene complexes 5 and 6 com- afford better yields (IX and X) than those with electron with-
pared to that on 2 and 3. The Cu-centre in 5 (0.51–0.51 e) has a drawing ones (XI and XII). Replacement of benzyl azide with the
more positive charge than that in 2. The group charge on the naphthylmethyl moiety has little effect on the yield except in the
NHC ligand is positive (0.33 e and 0.32 e) which indicates the case of VII. It is to be noted here that the exploitation of
IPr - Cu donor–acceptor bond in the complexes 5 and 6.20 naphthylmethyl azide in homogeneous click chemistry is not
The NBO analysis further confirms that the five d-orbitals of known, and the formation of triazole with the naphthylmethyl
Cu+ (1S, 3d104s0) are occupied in the complexes 5 and 6 and one group is only accessed in a heterogeneous manner.24 The drop
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Notes and references
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´
´
´
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´
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Conflicts of interest
There are no conflicts to declare.
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