Yttrium dialkyl supported by a silaamidinate ligand: Synthesis, structure and catalysis on cyclotrimerization of isocyanates

Article abstract of DOI:10.1039/c9cc06282c

A sterically demanding silaamidine (ArN = Si(L)NHAr) ligand was synthesized and employed for the preparation of a yttrium dialkyl complex, which catalytically enabled the cyclotrimerization of isocyanate with high activity and excellent functional group t

Full text of DOI:10.1039/c9cc06282c

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Yttrium dialkyl supported by a silaamidinate
ligand: synthesis, structure and catalysis on
cyclotrimerization of isocyanates†‡
Cite this: Chem. Commun., 2019,
55, 12324
Received 13th August 2019,
Accepted 18th September 2019
a
a
a
Deshuai Liu,
Chunming Cui
Dahai Zhou,
*
Hao Yang,
Jianfeng Li *a and
ab
DOI: 10.1039/c9cc06282c
rsc.li/chemcomm
A sterically demanding silaamidine (ArN = Si(L)NHAr) ligand was
synthesized and employed for the preparation of a yttrium dialkyl
complex, which catalytically enabled the cyclotrimerization of isocya-
nate with high activity and excellent functional group tolerance.
Rare-earth metal (RE) dialkyl complexes have attracted considerable
attention in a variety of chemical transformations such as hydro-
elementation, olefin polymerization and cyclic ester ring-opening
polymerization because of the highly reactive RE–C bond.1,2 It is
well-known that low-coordinate metal complexes feature unique
reactivity and catalytic transformations.3,4 However, low-coordinate
Chart 1 Amidinate vs. silaamidinate ligands.
rare-earth metal dialkyl complexes are still relatively undeveloped.5 multiple bonds.9 Herein, we report the successful and facile
Thus, significant efforts have been made on the development of synthesis of silaamidine and its preliminary application as a ligand
bulky ancillary ligands for the support of low-coordinate complexes. in rare-earth chemistry (Chart 1). The low-coordinate yttrium dialkyl
Silaimines (R2SiQNR0), compared to analogous imines complex supported by the silaamidinate ligand [PhC(NtBu)2Si-
(R2CQNR0), possess highly polarized silicon–nitrogen double (NAr)2]Y(CH2SiMe3)2 (4, Ar = 2,6-iPr2C6H3) exhibited high activity
bonds because of the electropositive silicon atom.6 The related and excellent functional group tolerance for the catalytic cyclotrimer-
silaamidinates (Chart 1) are isoelectronic with widely used amidinate ization of isocyanates, demonstrating the potential of this class of
ligands that have played important roles in the stabilization of ligand in coordination chemistry and catalysis.
transition metals and main group and rare-earth complexes.7,8 It is
Preparation of the target silaamidine 3 was achieved in high yield
envisioned that silaamidinate with sterically demanding substituents over two steps from chlorosilylene (1) as outlined in Scheme 1.10
could support highly Lewis acidic metal ions because of the two Treatment of 1 with 2,6-diisopropylphenyl azide resulted in chloro-
highly negative nitrogen donors. However, silaamidinates have been silaimine (2) in 80% yield.9 The salt metathesis reaction between 2
rarely studied as ligands because they are not easily accessible and the lithium salt of 2,6-diisopropylaniline afforded silaamidine 3
regarding the poor p-bonding ability of the silicon atom. Recent in 67% yield. 3 has been characterized by elemental analysis as well
advances in silylene and silicon multiple bonding chemistry have as 1H, 13C, and 29Si NMR and IR spectroscopic methods. The 1H and
shown that a silaimine moiety could be effectively stabilized with a 13C NMR spectra of 3 corroborate the expected ligand connectivity.
suitable Lewis base, leading to facile entries to varieties of silicon The 29Si NMR spectrum shows a resonance at ꢀ85.7 ppm, which is
consistent with the structure of silaimine.9b
The yttrium dialkyl [PhC(NtBu)2Si(NAr)2]Y(CH2SiMe3)2 4 was
a State Key Laboratory of Elemento-organic Chemistry and College of Chemistry,
obtained in 60% yield via an alkane elimination reaction of 3 with
Nankai University, Tianjin 300071, China. E-mail: lijf@nankai.edu.cn,
Y(CH2SiMe3)3(THF)2. The 1H and 13C NMR spectra disclose that 4 is
cmcui@nankai.edu.cn
b Collaborative Innovation Center of Chemical Science and Engineering (Tianjin),
solvent free with C2 symmetry in solution. This is a notable feature
considering that four-coordinate and solvent-free rare-earth metal
dialkyl complexes were relatively rare.1b The two methylene protons
of each –CH2SiMe3 group in 4 are magnetically inequivalent in the
1H NMR spectrum (C6D6, 23 1C), indicating the rotation of the Y–C
bonds prohibited by a sterically demanding silaamidinate ligand.
Tianjin 300072, China
† Dedicated to 100th anniversary of Nankai University.
‡ Electronic supplementary information (ESI) available: Experimental proce-
dures, crystallographic data and NMR spectra. CCDC 1906987. For ESI and
crystallographic data in CIF or other electronic format see DOI: 10.1039/
c9cc06282c
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Fig. 1 ORTEP representation of the X-ray structure of 4 with 30% ellipsoid
Scheme 1 Synthesis of ligand 3 and yttrium dialkyl complex 4.
probability. Hydrogen atoms are omitted for clarity.
angles determined for amidinate, guanidinate and diimido-
sulfonate yttrium dialkyls (ranging from 138.2 to 143.81 for Y–N–C
angles and from 57.5 to 61.51 for N–Y–N angles). The average
dihedral angle between the N–Y–N plane and the Ar rings is
43.41, which is quite smaller than those (79.7 and 73.31) in
[PhC(NAr)2]Y(CH2SiMe3)2(THF) and [PhS(NAr)2]Y(CH2SiMe3)2(THF)2,
respectively. In the silaamidinate ligand of 4, the Si1–N1 and Si1–N2
bond lengths (1.662(4) and 1.670(3) Å) are noticeably longer
than the silicon–nitrogen double bond length in 2 (1.545(2) Å)
but shorter than the silicon–nitrogen single bond.
Isocyanurates, cyclotrimeric products of isocyanate, are
commonly used as additives in polyurethane materials in order
to enhance their physical properties.11 Isocyanurates have also
attracted considerable attention in microporous materials,12
selective ion-bonding13 and drug delivery.14 Lewis bases,15 the
main group complexes16 and transition metal complexes17 have
been used as catalysts for isocyanate cyclotrimerization. However,
many existing cyclotrimerization methodologies faced issues such
as low activity, harsh reaction conditions, by-product formation
and difficulty in product separation. Rare-earth metal catalysts have
displayed mild reaction conditions and good selectivity in this
transformation, but poor functional group tolerance is a serious
constraint.18
The diagnostic methylene protons exhibit two doublets of doublet
(d ꢀ0.12 and ꢀ0.26 ppm), due to the characteristic coupling to 1H
and 89Y nuclei (dd, 2JHH = 11.3 Hz, 2JYH = 1.7 Hz; dd, 2JHH = 11.4 Hz,
2JYH = 2.3 Hz). In the 13C NMR spectrum, the methylene carbons
give rise to a doublet at 32.5 ppm (1JYC = 39.8 Hz). In comparison
with the 13C NMR resonances of methylene carbons in the yttrium
dialkyls with amidinate [PhC(NAr)2]Y(CH2SiMe3)2(THF) (d 39.5 ppm),
guanidinate [Me2NC(NAr)2]Y(CH2SiMe3)2(THF) (d 37.6 ppm) and
diimidosulfonate [PhS(NAr)2]Y(CH2SiMe3)2(THF)2 (d 36.0 ppm)
ligands, the equivalent resonance in 4 is upfield-shifted, suggesting
that the silaamidinate ligand is more electron-donating than these
ligands.8a,c,g The low 2JYH coupling constants in 4, compared to those
in [PhC(NAr)2]Y(CH2SiMe3)2(THF) (2JYH = 3.0 Hz) and [Me2NC(NAr)2]-
Y(CH2SiMe3)2(THF) (2JYH = 2.9 Hz), may be attributed to the
increased electronegativity of the yttrium ion in 4. Both the 1H
and 13C NMR spectra of 4 show two singlets (d 0.32 and 4.65 ppm,
respectively) for the methyl protons and carbons of the
–CH2SiMe3 groups. The 29Si NMR spectrum of 4 shows two
resonances (d ꢀ62.82 and ꢀ2.78 ppm), attributed to the silicon
atom in PhC(NtBu)2Si(NAr)2 and silicon atoms in ꢀCH2SiMe3
groups, respectively. The IR spectrum of 4 shows a stretching
vibration band of SiQN at 1421 cmꢀ1, which is red shifted
compared to that of the free ligand 3 (1493 cmꢀ1).
The catalytic cyclotrimerization of 3-methylphenyl isocyanate
using the yttrium dialkyl complex 4 as a catalyst has been
examined (Table 1). It was observed that the catalytic reaction
in THF displayed excellent activity and selectivity (entry 3, 98%
isolated yield with 0.25 mol% catalyst loading) in contrast to
those in toluene and Et2O at 23 1C after 12 hours (entries 1 and 2).
However, a decrease of the catalyst loading (0.1 mol%) and the
reaction time (6 hours) led to the lowering of the isolated yields
(71% in entry 4 and 76% in entry 5).
The molecular structure of 4 was determined by X-ray single-
crystal analysis (Fig. 1). In the solid state, 4 is essentially C2
symmetric. The structure of 4 indicates that the yttrium atom is
four-coordinate and solvent-free, being bonded to the silaamidi-
nate ligand in Z2 fashion and two –CH2SiMe3 groups with a
distorted tetrahedral geometry. In contrast, the known yttrium
dialkyl complexes with similar bulky amidinate, guanidinate,
diimidosulfonate and imidophosphonamide ligands are not
solvent-free with at least one coordinated THF molecule.8 The
Y–C bond lengths (2.449(5) and 2.426(5) Å) in 4 are elongated
relative to those in [PhC(NAr)2]Y(CH2SiMe3)2(THF) (2.374(4) and
2.384(4) Å) and [Me2NC(NAr)2]Y(CH2SiMe3)2(THF) (2.388(2)
and 2.407(2) Å). In contrast, the Y–N bond lengths (2.338(4)
and 2.339(3) Å) in 4 are only marginally shorter than those in
[PhC(NAr)2]Y(CH2SiMe3)2(THF) (2.339(3) and 2.369(2) Å) and
[Me2NC(NAr)2]Y(CH2SiMe3)2(THF) (2.345(2) and 2.338(2) Å).
The bulkiness of the silicon substituents and the large ionic radius
of silicon in 4 result in small Y–N–C angles (av. 131.3(2)1) but a
large N–Y–N angle (67.89(12)1) compared to the corresponding
Table 1 Optimization of cyclotrimerization of 3-methylphenyl isocyanate
catalysed by 4a
Entry
Solvent
Cat. (mol%)
Time (h)
Isolated yield (%)
1
2
3
4
5
Toluene
Et2O
THF
THF
THF
0.25
0.25
0.25
0.1
12
12
12
12
6
27
31
98
71
76
0.25
a
Reaction conditions: isocyanate (8 mmol), 5 mL solvent, 23 1C.
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Table 2 Catalysed cyclotrimerization of functional isocyanatesa
Scheme 2 Reaction of 4 with 3-methylphenyl isocyanates.
[PhC(NtBu)2Si(NAr)2]Y[OC(CH2SiMe3)N(3-MeC6H4)]2 (6) in 59%
yield (Scheme 2). The reactions of rare-earth alkyl complexes
with isocyanate have been previously investigated.19 The mole-
cular structure of 6 was confirmed by 1H, 13C and 29Si NMR, IR
and X-ray single-crystal analysis (Fig. S1 in ESI‡). Significant
downfield shifts were observed for –CH2SiMe3 group reso-
nances in the 1H NMR (d ꢀ0.12 and ꢀ0.26 ppm in 4 and
1.85 ppm in 6) and 29Si NMR spectra of 6 (d ꢀ2.78 ppm in 4 and
0.86 ppm in 6) compared to those of 4, owing to the insertion
reactions. In the IR spectrum of 6, a new strong band was
observed at 1521 cmꢀ1, which is assigned to the absorption of
the delocalized O–C–N moieties.
The double insertion product 6 was the major product even
when one equivalent of isocyanate was used. When the amount
of isocyanate was increased (more than two equivalents),
a cyclotrimerization product can be observed. The catalytic
behaviours of the insertion product 6 (97% isolated yield of
5a) were comparable to those of 4, indicating that 6 is the key
intermediate in the catalytic cycle. The proposed mechanism is
depicted in Fig. 2.20 The insertion of isocyanates into the two
Y–C bonds of 4 resulted in the formation of the intermediate 6,
which further underwent insertion reactions with the isocyanate to
give intermediates A and B. Finally, the intermediate B underwent
intramolecular cyclotrimerization to give the product 5 and
regenerated the intermediate 6.
In conclusion, a sterically demanding silaamidinate ligand
has been prepared and successfully employed for the synthesis
of well-defined rare-earth complexes for the first time. The
unique electronic structure of the ligand renders the low-
coordinate and solvent-free yttrium complex less Lewis acidic
and tolerant to some functional groups. Preliminary reactivity
studies demonstrated that this complex enabled the catalytic
cyclotrimerization of a series of isocyanates with a wide range
a
Reaction conditions: catalyst 4 (0.017 g, 0.02 mmol, 0.25 mol%)
b
isocyanate (8 mmol), 5 mL of THF, 23 1C. Isolated yield.
Under the optimized conditions, a range of functional isocya-
nates were examined with 0.25 mol% of 4 in THF at 23 1C. As
shown in Table 2, 4 is one of the catalysts with the highest activity
among the reported rare-earth metal catalysts for cyclotrimeriza-
tion of isocyanates.18 Surprisingly, 4 exhibited excellent functional
group tolerance, which is quite rare in the chemical transforma-
tion catalysed by rare-earth alkyl complexes. The substituted
aromatic isocyanates can be catalysed to the corresponding cyclo-
trimeric products (5c cyano-, 5d chloro-, 5e bromo-, 5f nitro-, 5g
trifluoromethyl- and 5h methoxy-) in excellent yields (88–99%). We
reasoned that the excellent functional group tolerance of 4 could
be attributed to the electron-donating feature of the silaamidinate
ligand, which may increase the electron density and reduce the
Lewis acidity of the yttrium ion. It was found that the steric effects
of the substrates are also pronounced. The catalytic cyclotrimer-
ization of 2-methylphenyl (5i) and 2-nitrophenyl (5j) isocyanates
gave relatively low yields, and the reaction of 2,6-dimethylphenyl
isocyanate cannot take place under the optimized conditions. The
activities of aromatic and aliphatic isocyanates were different due
to the electronic effects. Benzyl isocyanate afforded the cyclotri-
meric product 5k in 56% yield, whereas isopropyl and cyclohexyl
isocyanates were not able to undergo cyclotrimerization under the
catalytic conditions. We also carried out catalytic cyclotrimeriza-
tion of phenyl isothiocyanate with 4, but no cyclotrimeric product
was observed.
In order to gain insight into the mechanism of isocyanate
cyclotrimerization, the stoichiometric reaction of
4 with
3-methylphenyl isocyanate was investigated. The insertion of
isocyanates into the two Y–C bonds led to the formation of
Fig. 2 Proposed mechanism of isocyanate cyclotrimerization.
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of substrate scope. Further studies on silaamidinate rare-earth
metal complexes and their reactions are currently in progress.
We thank the National Natural Science Foundation of China
(21890722 and 21632006) for financial support.
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There are no conflicts to declare.
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