Potassium complexes containing bidentate pyrrole ligands: Synthesis, structures, and catalytic activity for the cyclotrimerization of isocyanates

Article abstract of DOI:10.1039/c9dt01246j

Bidentate pyrrolyl ligands, 2-(t-butyliminomethyl)pyrrole and 2-(t-butylaminomethyl)pyrrole, reacted with KH to give complexes [C₄H₃N(2-CHN^tBu)K(THF)]_n (1) and [C₄H₃N(2-CH₂NH

Full text of DOI:10.1039/c9dt01246j

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ISSN 1477-9226
PAPER
Joseph T. Hupp, Omar K. Farha et al.
Efficient extraction of sulfate from water using
framework
a Zr-metal–organic
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DOI: 10.1039/C9DT01246J
ARTICLE
Potassium Complexes Containing Bidentate Pyrrole Ligands:
Synthesis, Structures, and Catalytic Activity for the
Cyclotrimerization of Isocyanates
Received 00th January 20xx,
Accepted 00th January 20xx
Zhiqiang Guo, a Yuan Xu,b Xiaoqin Wu,a Xuehong Wei *a and Chanjuan Xi*c
DOI: 10.1039/x0xx00000x
The bidentate pyrrolyl ligand, 2-(t-butyliminomethyl)pyrrole and 2-(t-butylaminomethyl)pyrrole, reacted with KH to give
complexes [C4H3N(2-CH=NtBu)K(THF)]n (1) and [C4H3N(2-CH2NHtBu)K]n (2), respectively. Each has been characterized by
satisfactory C, H and N microanalysis, NMR spectroscopy, and single crystal X-ray structural analysis. The X-ray structure of
complex 1 (n ≥1) demonstrated that it exists as a 1D zig-zag coordination polymer in the solid state. Conversely, the structure
of complex 2 (n ≥1) showed that it is a 2D supramolecular network. They proved to be an effective class of catalysts for
cyclotrimerization of isocyanate in excellent yield under mild conditions.
state, let alone the characterization of their structural features.
Thus, full realization of the utilization potential of these
elements still requires substantial advances in understanding
Introduction
Bidentate 2-imino/aminopyrrole ligand precursors (Chart 1),
making possible the introduction of several kinds of exceptional
tunable steric and electronic features required for
compensating coordinative unsaturation of metal centers, have
attracted considerable attention in the areas of organometallic
and coordination chemistry in the recent year.1 The synthesis
and characterization of several classes of metal complexes
containing 2-imino/aminopyrrolyl ligands, such as Ca(II),2
Mg(II),3 Al(III),4 Ni(II),5 Cu(II),6 Zr(IV),7 Hf(IV),7a,8 Ti(IV),9 W(V),9a
Ta(V)10 and rare-earth metals,11 have been particularly studied,
being mainly used as polymerization catalysts and molecular
catalysts.
their basic coordination and organometallic chemistry.
Driven by the lack of structural information on 2-
imino/aminopyrrolyl potassium complexes, and in light of our
recent studies on alkali metal complexes incorporating 2-
aminopyrrolyl ligands,13 we decided to isolate and characterize
the potassium complexes with bidentate pyrrolyl ligand
[C4H3NH(2-CH=NtBu)] and [C4H3NH(2-CH2NHtBu)], respectively.
More remarkable, their structural characterizations have
uncovered a surprisingly diverse range of novel structures from
1D coordination polymer to 2D supramolecular network, and
these potassium complexes exhibited highly catalytic activities
for the cyclotrimerization of aryl isocyanate to the
corresponding 1,3,5-triaryl isocyanurate under mild conditions.
N
H
N
H
N
HN
R
R
Results and discussion
A
B
Synthesis and characterization of potassium complexes
Chart 1 2-Iminopyrrole (A) and 2-aminopyrrole (B) ligand precursors
Treatment of KH with one equiv. of the iminopyrrolyl ligand
[C4H3NH(2-CH=NtBu)] in THF solution resulted in the one-
dimensional chain potassium complex of the molecular formula
[C4H3N(2-CH=NtBu)K(THF)]n (1). Similarly, treatment of one
equiv. of KH with [C4H3NH(2-CH2NHtBu)] in THF resulted in the
corresponding two-dimensional plane potassium complex
[C4H3N(2-CH2NHtBu)K]n (2) with good yield (see Scheme 1). Each
of 1 and 2 were fully characterized by satisfactory C, H and N
microanalysis, 1H, 13CNMR spectra at ambient temperature, and
their solid-state structures were established using single-crystal
X-ray diffraction analysis. The 1H NMR spectra of 1 exhibits two
doublets for C(CH3)3 and CH=N at δ 1.25 and 8.06, respectively,
However, their alkali metal complexes are generally
prepared and employed in situ,12 they have rarely been isolated
from reaction solution and are poorly characterized in the solid
a. Scientific Instrument Center, Shanxi University, Taiyuan, 030006, P.R. China. E-
mail: xhwei@sxu.edu.cn
b. School of Chemistry and Chemical Engineering, Shanxi University, Taiyuan, 030006,
P.R. China.
c. MOE Key Laboratory of Bioorganic Phosphorus Chemistry & Chemical Biology,
Department of Chemistry, Tsinghua University, Beijing 100084, P.R. China. E-mail:
cjxi@tsinghua.edu.cn
Electronic Supplementary Information (ESI) available: All experimental details and
characterization data. CCDC 1566725-1566726. For ESI and crystallographic data in
CIF or other electronic format see DOI: 10.1039/x0xx00000x
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ARTICLE
Journal Name
and O atom of THF to give 1D coordination chains running along
View Article Online
tBu
K
tBu
K
N
N
+
the y axis. As a result, the center metal ioDnOKI: 1o0f.1c0o39m/Cp9leDxT0112w4a6Js
N
N
O
N
O
H
NtBu
hexa-coordinated by two O atoms from two THF molecular and
four N atoms from two ligand molecules. Therefore, the
geometry around the potassium ion in 1 can be best described
as a distorted octahedral. The K1–N1pyrrolyl and K1–N2imino bond
distances, 2.806(3) and 2.893(3) Å respectively, are much
shorter than the corresponding K–N distances observed in
iminopyrrolyl potassium complex [{2-(Ph3CN=CH)C4H3N}K(THF)
0.5]4 (2.971(3) and 3.005(3) Å respectively).14 The K1–O1 bond
distances of 2.820(3) Å are within the range of K–O bond
distances reported in the literature. The bite angle of 61.93(12)°
for N(1)-K(1)-N(2) was observed for the iminopyrrolyl chelated
to the potassium atom.
K
K
K
KH
THF
O
O
N
N
N
N
tBu
tBu
n
1
K
K
K
N
K
N
HN
N
NH
tBu
HN
H
NHtBu
tBu
tBu
tBu
tBu
H
tBu
KH
N
NH
K
N
THF/Toluene
K
N
N
N
K
K
2
In contrast to complex 1, the potassium complex 2
crystallizes in a mixed solvent of THF and toluene. It shows in
the orthorhombic space group Pccn with four molecules in the
unit cell. The asymmetric unit of potassium complex 2 contains
two aminopyrrolyl ligand and two potassium ions (Figure 2). It
is noted that the coordination spheres of the two potassium
ions are different. The ion K1 is coordinated by two nitrogens
from an aminopyrrolyl ligand in a bidentate fashion and π
interaction (η5-mode) from one pyrrole ring of the adjacent
aminopyrrolyl ligand while the second ion, K2, is coordinated by
K-π interaction (η2-mode) from pyrrole ring of aminopyrrolyl
ligand, and chelated by nitrogen atom of the adjacent pyrrole
ring in a κ1 fashion. In the grown structure, the potassium ion K1
is observed as a sandwich structure between two pyrrolyl ring
π-electron densities in a η5-fashion and further chelated by two
nitrogen atoms of one aminopyrrolyl ligand. The other
potassium ion K2 are surrounded by three aminopyrrolyl
Scheme 1 Synthetic routes to complex 1 and 2.
representing the manner of molecular structure 1 in a solution
maintain the solid ones at room temperature. The protons of
complex 2 were downfield shifted and exhibited similar spectral
1
feature of the corresponding lithium complex in the H NMR
spectrum13b. In addition, the pyrrole ring protons and other
protons showed resonance signals in each complex at expected
regions. The NMR spectra also supported the presence of the
carbon atom in each complex, which are consistent with the
structure of X-ray analysis.
X-ray single crystal structures of 1 and 2.
Complex 1 crystallizes in the orthorhombic space group Pnma
with two formula units in the unit cell. The asymmetric unit
contains one potassium ion K+, one pyrrolyl ligand L-, and one
solvent molecular THF (Figure 1). In the crystal structure of 1, it
presents
a one-dimensional coordination polymer, the
connection of potassium ion by bridging iminopyrrolyl ligand
(a)
(b)
(a)
(b)
(c)
(d)
(c)
Fig. 1 (a) Asymmetric unit of complex 1, with atom-numbering scheme and
displacement ellipsoids drawn at the 30% probability level; (b) and (c) fragment of
Fig. 2 (a) Asymmetric unit of complex 2, with atom-numbering scheme and displacement
ellipsoids drawn at the 30% probability level; (b) and (c) Section of the extended
framework structure showing K-C or K-N atom connectivity; (d) the resulting two-
dimensional layer packing. Hydrogen atoms are omitted for clarity.
polymeric coordination chain, running along the
y axis from a different
perspective, in the crystal structure of complex 1. Hydrogen atoms are omitted for
clarity.
2 | J. Name., 2012, 00, 1-3
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moieties, three pyrrolyl rings bind to potassium ion through its
central N-atom or ring C-atoms into μ3-(η1-η2-η4)-mode to form
a 2D supramolecular network structure formed. The K1–
View Article Online
DOI: 10.1039/C9DT01246J
Scheme 2 Substrate scope for cyclotrimerization of isocyanatesa
N1pyrrolyl, K1–N3pyrrolyl, K2–N1
distances, 2.747(3), 2.968(3), 2.898(3) and 2.765(4)
and K2–N3pyrrolyl bond
R
O
pyrrolyl
Å
R
N
O
O
Cat.
N
C O
R
respectively, are in good agreement with K–N distances
observed in complex 1. The K1–N2amino bond distance (2.969(3)
Å) fits well with the corresponding K–N distances observed in
iminopyrrolyl potassium complex [{2-(Ph3CN=CH)C4H3N}K(THF)
0.5]4 3.005(3) Å.14 The distances between the potassium ion K1
and the pyrrole ring atoms (C10, C11, C12, C13 and N3) were
found to be 3.124(4), 3.248(5), 3.125(4), 2.932(4) and 2.968(3)
Å, respectively. These distances are within the range of K-
pyrrolyl centroid distances found in the polymeric potassium
compound of the sandwiched type reported in the literature.13f
To the best of our knowledge, this is the first example of a μ3-
(η1-η2-η4)-mode developed by a pyrrole ring towards potassium
atoms.
Solvent, r.t.
N
N
R
Me
Me
O
O
N
O
O
N
O
O
N
O
O
O
N
N
Me
N
N
N
N
Me
Me
Me
2a
98%
2b
97%
2c
95%
OCH3
F
Me
O
O
N
O
O
N
O
N
O
O
O
Me
N
N
N
N
N
N
The cyclotrimerization of isocyanate catalysed by potassium
complexes
O
H3CO
OCH3
F
F
Me
2e
95%
2d
90%
2f
93%
Cl
NO2
Over the past few years, the industrial and commercial
application of isocyanurates have attracted much attention
because it could enhance the physical properties of
polyurethanes, copolymer resins and coating materials15, such
as increased thermal and chemical resistance, water-resistance,
O
N
O
O
N
O
O
O
N
O
O
N
N
N
N
N
N
O
Cl
Cl
O2N
NO2
transparency,
and
impact
resistance16.
And
the
2g
94%
2h
91%
2i
95%
cyclotrimerization of isocyanate has been studied thoroughly
and many types of catalysts have been developed to obtain the
aReaction conditions: isocyanate (30 mmol), room temperature, 3 h, complex 1 as
catalyst with 0.05% mol. bIsolated yield.
Table1. Optimization of cyclotrimerization of isocyanate catalyzed by potassium
complex a
isocyanurates in high yields. For example, metal-based catalytic
systems including inexpensive transition metals such as Zn17,
Sn18, Cu19, Ni19, Ti20, noble metal Pd21, rare earth metals22 and
so on. Organic and inorganic molecular catalysts including N-
heterocyclic carbenes,23 phosphines,24 amines,25 fluoride
anions,26 p-toluenesulfinate27and carbamate anions.28 Our
group also have reported utilizing various type pyrrolyl lithium
O
N
O
Cat.
N
C O
Solvent, r.t.
N
N
O
1a
2a
Catalyst
loading
(mol %)
1.0
complexes
as
catalysts
for
cyclotrimerization
of
Time
(h)
Yield
(%)b
Entry
Solvent
isocyanates.13b,c,g For further exploiting new reaction systems
based on alkali metal complexes with functionalized pyrrole
ligands, the above potassium complexes as catalysts for
cyclotrimerization of aryl isocyanats were studied, and the
results are listed in Table 1.
1
2
Et2O
THF
0.5
0.5
0.5
0.5
0.5
0.5
0.5
3
99
>99
97
1.0
1.0
3
PhMe
None
THF
4
1.0
98
The cyclotrimerization of phenyl isocyanate to 1,3,5-
triphenyl-1,3,5-triazinane-2,4,6-trione worked well in various
solvents at room temperature for 0.5 h in the presence of 1
mol% of the complex 1 (Table 1, entries 1-4), indicating the
solvent compatibility of the catalysts though the best result was
obtained when tetrahydrofuran was employed as solvent.
Pleasingly, the title complexes could promote this reaction
almost quantitatively at the 0.1 to 1.0 mol% loadings (Table 1,
entries 5-8). It is worth noting that the cyclotrimerization
reaction also can afford 95% isolated yield after 3h when the
loading of catalyst is decreased to 0.025% (Table 1, entry 10).
We also evaluated the catalytic behavior of complex 2 for
cyclotrimerization of isocyanates. it also exhibited good activity
5
0.5
>99
>99
96
6
0.25
0.1
THF
7
THF
8
0.1
THF
99
9
0.05
0.025
0.05
THF
3
98
10
11c
THF
3
95
THF
3
92
aReaction conditions: isocyanate (30 mmol), room temperature, complex 1 as
catalyst. bIsolated yield. cComplex 2 as catalyst.
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 3
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for catalytic reaction (Table 1, entry 11), giving the product CH=NtBu)] and [C4H3NH(2-CH2NHtBu)] were synthesized
View Article Online
yields over 90%.
Under the optimized reactions, the representative
according to the literature procedure.13a
DOI: 10.1039/C9DT01246J
Preparation of the Potassium complexes
Synthesis of [C4H3N(2-CH=NtBu)K(THF)]n (1).
isocyanates were also examined using above complex 1 as a
catalyst at 0.05 mol% loading (Scheme 2). The para-substituted
isocyanates with electron-donating groups exhibited higher
reactivity, and the corresponding isocyanurates were obtained
over 95% yields (2b, 2e). Comparatively, the para-substituted
isocyanates with electron-withdrawing groups are relatively
inert, giving the desired products in 91-94% yield because of
weak nuclephilicity (2f-2g). And the substrate with stronger
electron-withdrawing group such as 4-nitrophenyl isocyanate
also gave the corresponding product 2h. In addition, the
substituent groups at the meta or ortho positions of aromatic
ring, as well as 1-naphthyl isocyanate, were well tolerated, the
desired isocyanurates 2c-2d and 2i were isolated in moderate
to excellent yields (90-95%), indicating that the electronic
effects were not evident in this process.
General, this kind of potassium complex shows the higher
activities in the cyclotrimerization of isocyanates. Their catalytic
effects are superior to that of the corresponding lithium
complex [Li{C4H3N(2-CH2NHtBu)}2Li(THF)2] (TON=980)13b and
lanthanide complexes (TON=392)22. A proposed mechanism for
the cyclotrimerization of isocyanates catalyzed by this complex
would be similar to the reaction catalyzed by Lewis base
(Scheme 3).13b
A solution of [C4H3NH(2-CH=NtBu)] (0.450 g, 3.0 mmol) in THF
(15 mL) was added slowly to a suspension of KH (0.12 g, 3.0
mmol) in THF (15 mL) at -78 °C, the reaction was warmed to
room temperature and stirred overnight. The resulting solution
was filtered and concentrated, followed by overnight storage at
-10 °C afforded colorless, block-like X-ray quality crystals (0.478
g, 61%). Mp: 51 oC (dec). 1H NMR (d-THF): 8.06 (d, J = 3.9 Hz, 1H,
CH=NtBu), 6.86 (s, 1H, C4H3N), 6.33 (m, 1H, C4H3N), 6.02 (m, 1H,
C4H3N), 3.58(s, 4H, THF), 1.73(s, 4H, THF), 1.25 (d, J= 3.8 Hz, 9H,
NBut); 13C NMR (d-THF): 149.4 (CH=NtBu), 129.6 (C4H3N),
128.0(C4H3N), 114.5 (C4H3N), 107.7 (C4H3N), 67.1 (THF), 54.9
(C(CH3)3), 29.6(C(CH3)3), 25.2 (THF); Anal. Calcd for C13H22KN2O:
C, 59.73; H, 8. 48; N, 10.72. Found: C, 59.51; H, 8.13; N, 10.44.
Synthesis of [C4H3N(2-CH2NHtBu)K]n (2).
A solution of [C4H3NH(2-CH2NHtBu)] (0.457 g, 3.0 mmol) in THF
(15 mL) was added slowly to a suspension of KH (0.12 g, 3.0
mmol) in THF (15 mL) at -78 °C, the reaction was warmed to
room temperature and stirred overnight. The resulting solution
was filtered and concentrated to a small amount and
recrystallized to generate colorless, slice-like X-ray quality
crystals (0.296 g, 52%). Mp: 73 oC (dec). 1H NMR (C6D6+C5D5N):
8.92 (br, 1H, NH), 6.59(s, 2H, C4H3N), 6.36 (s, 2H, C4H3N), 6.19
(s, 2H, C4H3N), 3.58 (d, 2H, J = 7.5 Hz, CH2NBut), 0.92 (s, 18H,
CH2NBut); 13C NMR (C6D6+C5D5N): 131.8 (C4H3N), 116.4 (C4H3N),
108.1 (C4H3N), 105.2 (C4H3N), 49.7 (C(CH3)3), 40.0 (CH2NBut),
28.7 (C(CH3)3); Anal. Calcd for C18H30K2N4: C, 56.80; H, 7.94; N,
14.72. Found: C, 56.27; H, 7.71; N, 14.59.
K
O
R-N=C=O
R
N
L
K
L
R
N
2 R-N=C=O
O
R
O
R
R
N
K
N
N
L
O
O
O
N
N
R
R
Typical procedure for the catalytic reaction of isocyanate to
isocyanurate
O
Scheme 3 Proposed mechanism for the cyclotrimerization of isocyanates.
A 100 mL Schlenk flask was charged with potassium complexes
(0.05 mol%) and 20 mL of tetrahydrofuran was added, then
phenyl isocyanate (30 mmol) was added dropwise through a
syringe with stirring. The solution became cloudy gradually and
the white suspension was stirred at room temperature for 3 h
after the addition was completed. The resulting suspension was
concentrated under reduced pressure and washed with diethyl
ether (3 x 10 mL), gave the corresponding isocyanurate as a
white solid. All of the products were characterized by NMR
techniques.
Experimental
General
Unless otherwise noted, all syntheses and manipulations of air-
sensitive materials were performed under a purified nitrogen
atmosphere
using
standard
Schlenk
techniques.
Tetrahydrofuran and diethyl ether were distilled from sodium
benzophenone under nitrogen. Hexane and toluene were dried
using sodium potassium alloy and distilled under nitrogen prior
to use. KH (30% dispersion in mineral oil) was purchased from
Aldrich and washed with hexane (2x10 mL) in a Schlenk tube
before used. All chemicals were sublimed, recrystallized or
Conclusions
distilled before use. H NMR (600 MHz), 13C NMR (150.9 MHz)
1
In summary, we have presented the synthesis and
characterization of two potassium complexes incorporating 2-
imino/aminopyrrolyl ligand. According to X-ray analysis, the
synthesized complexes 1 and 2 are 1D zig-zag coordination
polymer and 2D supramolecular network in the solid state,
spectra of the compounds were recorded on a BRUKER AVANCE
III 600MHz instrument at 298 K. Elemental analyses were
performed on a Vario EL-III instrument. The ligand [C4H3NH(2-
4 | J. Name., 2012, 00, 1-3
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respectively. This work also represents rare example of the
potassium complex supported by pyrrolide species in various
coordination mode (η1-η5). Two potassium complexes exhibited
very good catalytic activities for the cyclotrimerization of aryl
isocyanate to 1,3,5-triaryl-1,3,5-triazinane-2,4,6-trione under
mild conditions.
Conflicts of interest
The authors declare no competing financial interest.
Acknowledgements
Financial supports from the National Natural Science
Foundation of China (No.91645120 and 21871163), the Coal-
based Key Scientific and Technological Project (No. MH2014-07)
and the Natural Science Foundation of Shanxi Province (No.
201801D121041) are gratefully acknowledged.
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27 F. M. Moghaddam, M. G. Dekamin, M. S. Khajavi and S. Jalili,
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DOI: 10.1039/C9DT01246J
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Graphic Abstract
for
Potassium Complexes Containing Bidentate Pyrrole Ligands: Synthesis, Structures,
and Catalytic Activity for the Cyclotrimerization of Isocyanates
Zhiqiang Guo,a Yuan Xu,b Xiaoqin Wu,a Xuehong Wei* a and Chanjuan Xi* c
aScientific Instrument Center, Shanxi University, Taiyuan, 030006, P.R. China
bSchool of Chemistry and Chemical Engineering, Shanxi University, Taiyuan, 030006,
P.R. China
cMOE Key Laboratory of Bioorganic Phosphorus Chemistry & Chemical Biology，
Department of Chemistry, Tsinghua University, Beijing 100084, P.R. China
E-mail: cjxi@tsinghua.edu.cn
KH
N
THF
tBu
N
H
KH
THF
tBu
N
H
HN
2-(t-Butyliminomethyl)pyrrole and 2-(t-butylaminomethyl)pyrrole react with KH
to give potassium complexes with intriguing 1D zig-zag coordination polymer and 2D
supramolecular network structures, respectively. They proved to be an effective class
of catalysts for cyclotrimerization of isocyanate in excellent yield under mild
conditions.