268
M.K. Barman et al. / Journal of Organometallic Chemistry 772-773 (2014) 265e270
main group organometallic compounds based catalysis of cyclo-
trimerization of aryl isocyanate is poorly documented, though
there are some reports [28].
Experimental section
General
The addition of complexes 1e3 (2 mol %) to neat arylisocyanates
and followed by stirring at room temperature for 1 h led to the
formation of cyclotrimerized products, i.e., triarylisocyanurates in
quantitative yields. Our catalysts show a high degree of selectivity
of only triaryl isocyanurate formation (93e96% yield). No other
isomeric products were observed. Moreover, catalysts 1e3 can be
recovered and confirmed by the 1H NMR without any decomposi-
tion products. The catalytic activity of guanidinate stabilized ger-
manium(II) and tin(II) amide complexes 1e3 along with data for
some related catalysts is summarized in Table 1.
From Table 1 (Entries 1e6), it is very clear that more bulky
guanidinate stabilized tin(II) amide catalyst 3 is showing slightly
better catalytic activity compare to 1 and 2. Further, we extended
these studies to test the catalytic activity of germanium bis(amide)
and tin bis(amide) compounds (Entry 7 & 8). These compounds
show less activity compare to catalysts 1e3 (Entries 1e6). This
suggests that basicity of the proligand attached to metal site and
Lewis acidity of the metal center play a role in the activity of these
complexes. And also, it might be a solubility effect. Homoleptic
metal catalysts may form polymeric insoluble structures in solu-
tion. However, the bulky guanidine ligand which is attached to
metal atom keeps the catalyst active in solution. Furthermore, it is
very important to note that bulky guanidinato stabilized tin(II)
amide complex which is better catalyst than other related amidi-
nato tin(II) amide complexes (Entries 9e13).
All manipulations of air and moisture sensitive materials were
performed with the rigorous barring of oxygen and moisture in
flamed Schlenk-type glassware either on a duel manifold Schlenk
line, or in a nitrogen-filled MBraun glovebox. NMR scale reactions
were conducted in Young valve NMR tubes and sealed in a glove-
box. NMR spectra were recorded on Bruker AV 400 MHz spec-
trometer for 1H NMR (13C{1H} NMR 100 MHz and 29Si {1H} NMR
80 MHz). IR Spectra were recorded in PerkineElmer FTeIR Spec-
trometer. Dry n-hexane and tetrahydrofuran(THF) solvents were
collected from MBraun Solvent Purification System and degassed
by freeze-pump-thaw cycles, prior to use. C6D6 was purchased from
SigmaeAldrich and dried over sodium before distillation under
nitrogen and storage over molecular sieves. L2H [16] and Ge
[N(SiMe3)2]2 [29] were prepared according to reported literature
procedures. SnCl2, GeCl2 (dioxane), KN(SiMe3)2, Sn[N(SiMe3)2]2
were purchased from SigmaeAldrich and used without further
purification.
Synthesis and characterization of compounds 1e3
Preparation of [{ArNC(NiPr2)NAr]GeN(SiMe3)2}; (Ar ¼ 2,6-
Me2eC6H3)] (1)
A solution of L1H (0.25 g, 0.711 mmol, 1.0 equiv) in THF (10 mL)
was added drop by drop to a stirred suspension of KN(SiMe3)2
(0.29 g,1.43 mmol, 2.01 equiv) in THF (5 mL) at 0 ꢀC and stirring was
continued for 12 h at room temperature. The resulting solution was
added drop by drop to a stirred suspension of GeCl2 (dioxane)
(0.165 g, 0.711 mmol, 1.0 equiv) in THF (5 mL) at 0 ꢀC under stirring
for another 24 h at room temperature. After removal of all the
volatiles, the residue was extracted with n-hexane (20 mL) and
concentrated to about 5 mL and finally stored in a ꢁ30 ꢀC freezer.
Colorless crystals of compound suitable for X-ray diffraction anal-
Conclusion
In conclusion, we have presented the synthesis and charac-
terization of three new metal complexes of bulky guanidinate
stabilized germanium(II) and tin(II) amides, which can be readily
prepared by two synthetic routes; i) deprotonation of free bulky
guanidine ligand with two equiv of KN(SiMe3)2 and followed by
metathesis reaction with one equiv of metal dihalide of germa-
nium or tin ii) deprotonation of ligand with metal bis(amide) of
germanium or tin. Furthermore, compounds 1 and 2 were
confirmed by single crystal X-ray structural analysis, and revealed
that both are monomeric in nature. Moreover, compounds 1e3
display excellent catalytic activity for the cyclotrimerization of
aryl isocyanurates.
ysis are obtained after one day. Yield: 0.73
g
(88%). M.
p. ¼ 120e122 ꢀC. 1H NMR (400 MHz, C6D6, 25 ꢀC):
d
¼ 0.26 (s, 18H,
NSi(CH3)3), 0.67 (d, J ¼ 8 Hz, 12H, CH(CH3)2), 2.58 (s, 12H, CH3), 3.90
(sept, J ¼ 8 Hz, 2H, CH(CH3)2), 6.87e6.93 (m, 4H, AreH), 7.00 (d,
J ¼ 8 Hz, 2H, AreH) ppm. 13C {1H} NMR (100 MHz, C6D6, 25 ꢀC):
d
¼ 5.6 (SieC), 20.7 (AreCH3), 21.0(AreCH3), 24.6 (iPreCH3), 50.7
(N-iPreCH), 125.6 (AreC), 129.6 (AreC), 129.6 (AreC), 136.0 (AreC),
136.3 (AreC), 144.3 (AreC), 165.0 (NCN) ppm. 29Si {1H} NMR
(80 MHz, C6D6, 25 ꢀC):
d
¼ ꢁ3.68 (NSi(CH3)3) ppm. IR (KBr)
n
(cmꢁ1): 2924(s), 2854(s), 1459(m), 1377(s), 932(w), 721(m).
Table 1
Data for the catalytic cyclotrimerization of arylisocyanatesa.
Preparation of [{ArNC(NiPr2)NAr}SnN(SiMe3)2}; (Ar ¼ 2,6-
Me2eC6H3)] (2)
Entry Substrate
Catalyst
Time Isolated
(min) yield (%)
A solution of L1H (0.5 g, 1.422 mmol, 1.0 equiv.) in THF (10 mL)
was added drop by drop to a stirred suspension of KN(SiMe3)2
(0.57 g, 2.85 mmol, 2.01 equiv) in THF (5 mL) at 0 ꢀC and stirring
was continued overnight at room temperature. The resulting so-
lution was added drop by drop to a stirred suspension of SnCl2
(0.269 g, 1.422 mmol, 1.0 equiv) in THF (5 mL) at 0 ꢀC and continued
the stirring for another 24 h at room temperature. After removal of
all the volatiles, the residue was extracted with n-hexane (20 mL)
and concentrated to about 5 mL and finally stored in a ꢁ30 ꢀC
freezer. Colorless crystals of compound suitable for X-ray diffrac-
tion analysis are formed after one day. Yield: 0.71 g (86%). M.
1
2
3
4
5
6
7
8
C6H5NCO
P-MeOC6H4NCO
C6H5NCO
P-MeOC6H4NCO
C6H5NCO
P-MeOC6H4NCO
P-MeOC6H4NCO Ge[N(SiMe3)2]2
P-MeOC6H4NCO Sn[N(SiMe3)2]2
C6H5NCO
C6H5NCO
C6H5NCO
1
1
2
2
3
3
60 93
60 94
30 95
30 96
20 97
20 96
1440 83
480 91
b
9
10
11
[Me3SiNC(tBu)NSiMe3]Sn[N(SiMe3)2]c 210 94
[Me3SiNC(tBu)NSiMe3]Ge[N(SiMe3)2]c
16 98
10 35 (52%
dimer)
12 95
60 68
c
Sn[Me3SiNC(Me)NSiMe3]2
c
12
13
C6H5NCO
C6H5NCO
[CyNC(Me)NCy]Sn[N(SiMe3)2]S4
[CyNC(tBu)NCy]Sn[N(SiMe3)2]S4
c
p. ¼ 125e127 ꢀC. 1H NMR (400 MHz, C6D6, 25 ꢀC):
d
¼ 0.20 (s, 18H,
NSi(CH3)3), 0.66 (d, J ¼ 8 Hz, 12H, CH(CH3)2), 2.50 (s, 6H, CH3), 2.60
(s, 6H, CH3), 3.84 (sept, J ¼ 8 Hz, 2H, CH(CH3)2), 6.85e6.88 (t, 2H,
AreH), 6.93 (d, J ¼ 8 Hz, 2H, AreH), 7.01 (d, J ¼ 8 Hz, 2H, AreH) ppm.
a
All reactions were carried out in neat aryl isocyanate (substrates) at room
temperature and catalysts 1e3 in 2 mol %.
b
5 mol % catalyst was used.
Data from Ref. [8].
13C {1H} NMR (100 MHz, C6D6, 25 ꢀC):
d
¼ 5.4 (SieC), 20.6 (AreCH3),
c