Ligand exchange experiment of SiO2–Mo-500 with alcohol
=
After sublimation of the [(N )Mo(OtBu) ] onto a silica pellet
=
3
as described above (pretreated at 500 ꢀC under vacuum at
10À5 mmHg for 12 h), CD3OD was introduced into the reac-
tor; after 5 minutes, the reactor was placed under vacuum at
70 ꢀC for 1 h to remove the excess CD3OD, then CH3OH
was introduced, the reactor was placed under vacuum after 5
minutes to remove the excess CH3OH at 70 ꢀC for 1 h. This
procedure was repeated twice and the ligand exchange reaction
was followed by in-situ FT-IR spectroscopy.
=
Grafting of [(N )Mo(OtBu) ] onto support by liquid–solid
=
3
reaction
Preparation of Mo–Si-500. Grafting was typically carried
=
out by stirring a mixture of [(N )Mo(OtBu) ] (164 mmg, 0.5
=
3
mmol) and SiO2-500 (1.00 g) (pretreated at 500 ꢀC under
vacuum at ꢁ10À5 mmHg for 12 h) in pentane (10 mL) at
20 ꢀC for 3 h. After filtration, the solid was washed three times
with pentane and dried under vacuum to yield 1.1 g of a white
powder. Elemental analysis: C, 1.09%wt; N, 0.28%wt and Mo,
1.74%wt. Solid state 1H NMR d 0.84 ppm. CP/MAS 13C
NMR d 29.4 ppm. IR: 2976, 2945, 2902, 2871, 1471, 1451,
Fig. 1 In situ IR spectroscopy before and after sublimation of
=
[(N )Mo(OtBu) ] onto an SiO -700 pellet; inset figure is the gas phase
=
evolved during the sublimation.
3
2
1362 cmÀ1
.
agreement with a grafting via the displacement of the tert-
butoxy ligands for siloxyls from the silica surface, which is clo-
sely related to the alcoholysis reaction reported by Chisholm
et al.11
Usually, the structure of the product obtained from the reac-
tion of the organometallic compound and triphenylsilanol was
a reliable reference to determine the structure of the surface
complex. In our previous study of the reaction between
Preparation of Mo–Si-700. SiO2-700 (pretreated at 700 ꢀC
under vacuum at ꢁ10À5 mmHg for 12 h) was used as support,
the preparation procedure is similar to that described above.
Elemental analysis: C, 2.13%wt; N, 0.30%wt and Mo,
1.90%wt. Solid state 1H NMR d 0.74 ppm. CP/MAS 13C
NMR d 27.4 ppm. IR: 2976, 2945, 2902, 2871, 1471, 1451,
1362 cmÀ1
.
=
[(N )Mo(CH tBu) ] and silica, we found that protonation of
=
nitride occurred to give an imido surface complex [( SiO)-
2
3
=
Preparation of Mo–MCM-41. A mixture of [(N )Mo-
=
=
=
(OtBu)3] (492 mmg, 1.5 mmol) and MCM-41-500 (1.00 g) (pre-
treated at 500 ꢀC under vacuum at ꢁ10À5 mmHg for 12 h) in
pentane (15 mL) was stirred at 20 ꢀC for 3 h. After filtration,
the solid was washed three times with pentane and dried in
vacuo to yield 1.4 g of a white powder. Elemental analysis:
C, 4.18%wt; N, 1.26%wt and Mo, 6.69%wt. Solid state 1H
NMR d 1.45. CP/MAS 13C NMR d 29.6. IR: 2977, 2943,
=
Mo( NH)(CH2tBu)3]. Using triphenylsilanol in place of silica
=
provided the same reactivity and yielded [Ph3SiO–Mo( N-
H)(CH2tBu)3].16 The reaction of (N )Mo(OtBu) with tri-
=
=
3
=
phenylsilanol gave a mixture of [(Ph SiO) –Mo( N)(OtBu) ]
=
3
x
3Àx
(observed by liquid H and 13C NMR) as in the case of the
alcoholysis reaction.11 This result is consistent with the FT-
IR data, which support the reaction pathway between
1
2902, 2871, 1471, 1451, 1361 cmÀ1
.
=
[(N )Mo(CH tBu) ] and siloxyls of the silica surface. The dif-
2
=
ferent reactivity of the two analogous molybdenum complexes
with OH groups is probably related to the difference in Mo–N
3
Epoxidation of olefins using various surface Mo-complexes
˚
bond strengths, which is 1.66 and 1.72 A for [(N )Mo(OtBu) ]
=
=
3
The Mo-containing solid (18 mmol of Mo) was introduced into
a two-necked 50 mL glass reactor fitted with a reflux condenser
under argon. After addition of the substrate (50 mmol) and
CH2Cl2 (5 mL), the reaction mixture was heated to reflux
and TBHP (17.5 mmol) was added via a syringe. The reaction
was monitored at certain time intervals by GC analysis using
dodecane as an internal standard. The amount of remaining
TBHP was measured by iodometric titration at the end of
the reaction.
=
and [(N )Mo(CH tBu) ], respectively.
2
=
3
The surface complex shows a free ligand exchange property
as exhibited by the reaction with CD3OD and CH3OH. The
FT-IR shows the disappearance of the n(C–H) vibration asso-
ciated with the OtBu groups for n(C–D) vibrations corre-
sponding to CD3O–M when the surface complex is reacted
with CD3OD (Fig. 2). When it was further reacted with
CH3OH, the n(C–D) vibration associated with CD3O–M was
replaced by the n(C–H) vibration of CH3OH. The same pro-
cess could be repeated several times. This further corroborates
the proposed reaction pathway.
Liquid–solid reaction provides the same material according
to IR spectroscopy. The supported surface complexes prepared
by liquid–solid reaction were characterized by elemental analy-
sis (Table 1). The results show that the pretreatment tempera-
ture of the support has an important effect on the amount of
the molybdenum species grafted. With increasing pretreatment
temperature, the amount of grafted molybdenum species
decreased, this is mainly due to the decrease in the amount
of reactive hydroxyl groups when the support was pretreated
at higher temperature. Elemental analysis of the supported sur-
face complexes gave C/N/Mo ratios of 5/1/1 and 9/1/1 for
the surface complexes obtained with SiO2-500 and SiO2-700,
respectively. Theoretical values in elemental analysis for a
Results and discussion
Preparation and characterization of the surface complexes
=
The reaction of [(N )Mo(OtBu) ] with SiO partially dehy-
=
3
2
droxylated at either 500 ꢀC or 700 ꢀC (respectively named
Mo–Si-500 and Mo–Si-700) was performed using both gas–
solid and liquid–solid reactions.15 When [(N )Mo(OtBu) ] is
=
=
3
sublimed at 85 ꢀC onto a silica disc, the intensity of the
À1
=
=
n( Si–OH) band at 3745 cm decreases, while bands in the
range of 3000–2860 cmÀ1 and 1500–1350 cmÀ1, attributed to
n(C–H) and d(C–H) respectively, appear (Fig. 1). The evolu-
tion of 2,2-dimethylpropanol in the gas phase was detected
by IR spectroscopy (Fig. 1 inset). These observations are in
320
New J. Chem., 2003, 27, 319–323