tantalum to silicon and a siloxy transfer from silicon to tantalum were
observed [40].
3.1.4 Reactivity of Group VI (W) Alkyls and Alkylidene on Oxide
Surfaces
For the exploration of better catalyst for C–H bond activation especially, in the case
of alkane metathesis, the focus was shifted from group V to group VI metal
catalysts as these metals are well known for olefin metathesis as well as for C–H
bond activation. They also were found to make low-temperature hydrogenolysis of
alkanes and polymerization of olefin [19, 43]. Similar to group V metal alkyls,
group VI metal alkyls can undergo reaction with dehydroxylated silica. Similar to
Scheme 7, [W(C
t
Bu)(CH 2
t Bu) 3 ] (19) [Mo(C
t
Bu)(CH 2
t
Bu) 3 ] (20) was employed
for grafting of group VI metal alkyl on SiO 2-700 . In general, a pentane solution of an
excess of 19 or 20 was added to SiO 2-700 at room temperature to obtain [(SiO–)W
(C
t
Bu)(CH 2
t
Bu) 2 ] [44] (21) or [(SiO–)Mo(C
t
Bu)(CH 2
t
Bu) 2 ] (22)
(Scheme 10). The IR spectrum shows a decrease of the ν(Si–O–H) band at
3,747 cm
À1 with the formation of two new series of bands at 3,000–2,700 and
1,500–1,300 cm
À1 assigned to ν (CH) and δ (CH) vibrations.
However, in the case of [Ta(¼C
t
Bu)(CH 2
t Bu) 2 ], the reaction proceeds with the
addition of Si–OH bond onto the Ta¼C bond followed by α-H abstraction
(Scheme 7), but in the case of [W(C
t
Bu)(CH 2
t Bu) 3 ] or [Mo(C
t
Bu)(CH 2
t
Bu) 3 ],
the addition of Si–OH on WC or MoC was not observed when grafted on SiO 2700 (Scheme 10). It forms a monopodal carbyne species on SiO 2-700 . Gas quantification and elemental analysis confirmed that there are nearly three neopentyl
groups per tungsten atom which again corroborates the monopodal structure of
the above grafted complex. Furthermore, formation of a carbyne ligand on silica
surface was confirmed by
1 H,
13 C, and HETCOR solid-state NMR with the observation of characteristic peak at 318 ppm for the C (carbyne) . The supported complex
21 was found to be very active in olefin metathesis but exhibited no activity in
alkane metathesis. The corresponding hydride [(SiO–)W(C
t
Bu)(CH 2
t
Bu) 2 ] was
prepared by treating 21 with the excess of hydrogen at 150
C and found to be much
less active in alkane metathesis. To generate more electrophilic metal center for
better catalytic activity, alumina partially dehydroxylated at 500
C (PDA) was
chosen as solid support. Under similar condition, 19 was grafted on Al 2 O 3-500 at
room temperature (Scheme 11).
The supported organometallic complexes were characterized by IR, solid-state
NMR, EXAFS, and elemental analysis. All these data taken together confirm that it
is a mixture of monopodal carbyne (major) (Scheme 10) along with a minor amount
of cationic tungsten with the migration of neopentyl group from tungsten to nearby
electrophilic aluminum center. The corresponding polyhydride was prepared from
23 by the treatment of excess of hydrogen at 150
C (Scheme 12). IR spectra showed
partial consumption of the Al–OH with simultaneously formation of Al–H bond,
New Concept of C–H and C–C Bond Activation via Surface Organometallic. . .
165
observed [40].
3.1.4 Reactivity of Group VI (W) Alkyls and Alkylidene on Oxide
Surfaces
For the exploration of better catalyst for C–H bond activation especially, in the case
of alkane metathesis, the focus was shifted from group V to group VI metal
catalysts as these metals are well known for olefin metathesis as well as for C–H
bond activation. They also were found to make low-temperature hydrogenolysis of
alkanes and polymerization of olefin [19, 43]. Similar to group V metal alkyls,
group VI metal alkyls can undergo reaction with dehydroxylated silica. Similar to
Scheme 7, [W(C
t
Bu)(CH 2
t Bu) 3 ] (19) [Mo(C
t
Bu)(CH 2
t
Bu) 3 ] (20) was employed
for grafting of group VI metal alkyl on SiO 2-700 . In general, a pentane solution of an
excess of 19 or 20 was added to SiO 2-700 at room temperature to obtain [(SiO–)W
(C
t
Bu)(CH 2
t
Bu) 2 ] [44] (21) or [(SiO–)Mo(C
t
Bu)(CH 2
t
Bu) 2 ] (22)
(Scheme 10). The IR spectrum shows a decrease of the ν(Si–O–H) band at
3,747 cm
À1 with the formation of two new series of bands at 3,000–2,700 and
1,500–1,300 cm
À1 assigned to ν (CH) and δ (CH) vibrations.
However, in the case of [Ta(¼C
t
Bu)(CH 2
t Bu) 2 ], the reaction proceeds with the
addition of Si–OH bond onto the Ta¼C bond followed by α-H abstraction
(Scheme 7), but in the case of [W(C
t
Bu)(CH 2
t Bu) 3 ] or [Mo(C
t
Bu)(CH 2
t
Bu) 3 ],
the addition of Si–OH on WC or MoC was not observed when grafted on SiO 2700 (Scheme 10). It forms a monopodal carbyne species on SiO 2-700 . Gas quantification and elemental analysis confirmed that there are nearly three neopentyl
groups per tungsten atom which again corroborates the monopodal structure of
the above grafted complex. Furthermore, formation of a carbyne ligand on silica
surface was confirmed by
1 H,
13 C, and HETCOR solid-state NMR with the observation of characteristic peak at 318 ppm for the C (carbyne) . The supported complex
21 was found to be very active in olefin metathesis but exhibited no activity in
alkane metathesis. The corresponding hydride [(SiO–)W(C
t
Bu)(CH 2
t
Bu) 2 ] was
prepared by treating 21 with the excess of hydrogen at 150
C and found to be much
less active in alkane metathesis. To generate more electrophilic metal center for
better catalytic activity, alumina partially dehydroxylated at 500
C (PDA) was
chosen as solid support. Under similar condition, 19 was grafted on Al 2 O 3-500 at
room temperature (Scheme 11).
The supported organometallic complexes were characterized by IR, solid-state
NMR, EXAFS, and elemental analysis. All these data taken together confirm that it
is a mixture of monopodal carbyne (major) (Scheme 10) along with a minor amount
of cationic tungsten with the migration of neopentyl group from tungsten to nearby
electrophilic aluminum center. The corresponding polyhydride was prepared from
23 by the treatment of excess of hydrogen at 150
C (Scheme 12). IR spectra showed
partial consumption of the Al–OH with simultaneously formation of Al–H bond,
New Concept of C–H and C–C Bond Activation via Surface Organometallic. . .
165