Silane-catalyzed reaction: Fac - mer isomerization of [Mo(CO) 3(phosphite)3]

Article abstract of DOI:10.1021/om7008827

Me₃SiX (X = Cl, Br, I) and related silane compounds catalyze fac - mer isomerization of [Mo(CO)₃(P(OR)₃}₃] at room temperature. A hypervalent silicon structure has been shown to play a crucial role in the catalytic reaction.

Full text of DOI:10.1021/om7008827


Copyright 2007
Volume 26, Number 26, December 17, 2007
American Chemical Society
Communications
Silane-Catalyzed Reaction: fac-mer Isomerization of
Mo(CO) (phosphite) ]
[
3
3
Kozo Fukumoto and Hiroshi Nakazawa*
Department of Chemistry, Graduate School of Science, Osaka City UniVersity, Sumiyoshi-ku, Osaka
5
58-8585, Japan
ReceiVed September 3, 2007
Summary: Me3SiX (X ) Cl, Br, I) and related silane compounds
and meridional (mer) forms. The fac isomer is considered more
stable electronically, as this isomer achieves stronger M to CO
back-donation, but the mer isomer is favored from a steric point
of view. The M(CO)3L3 complexes prepared to date from their
transition-metal precursors and L have exclusively fac geometry.
The mer isomers were obtained using isomerization of the fac
isomers. Bond et al. reported fac-mer isomerization via a one-
electron oxidation to produce the 17e cation species for
catalyze fac-mer isomerization of [Mo(CO)3{P(OR)3}3] at room
temperature. A hyperValent silicon structure has been shown
to play a crucial role in the catalytic reaction.
Because of their unique, efficient, and selective reactivity, in
addition to their ready availability and low toxicity, organosili-
con compounds have been widely used, especially for organic
1,2
syntheses. Extensive studies over the past three decades have
examined silicon compounds and have created many useful
reactions, most of which involve conversion of a silicon
compound into another silicon compound. In addition, several
silicon-based catalysts have been developed: R3SiOTf,
3
3
M(CO)3(η - Ph2PCH2CH2P(Ph)CH2CH2PPh2). We reported the
related isomerization for fac-[M(CO)3(bpy){P(NMeCH2)2
(
OMe)}] (bpy ) 2,2′-bipyridine, P(NMeCH2)2(OMe) )
P(NMeCH2CH2NMe)(OMe)). The reaction of the fac isomer
2
R3SiB(OTf)4, R3SiB(OTf)3Cl, R3SiN(SO2F)2, and R3SiN(Tf)2.
with BF3 · OEt2 gives the cationic phosphenium complex fac-
+
+
These catalysts can be described as containing R3Si and are
[
Mo(bpy)(CO)3{P(NMeCH2)2}] , which then isomerizes to the
considered Lewis acids. In contrast, no report describes silane
mer form. It does so because the phosphenium ligand shows
(
a neutral four-coordinate silicon compound) serving as a
strong π acidity, leading to an electron-deficient metal center
4
catalyst instead of a reagent. It has been speculated that a silane
is not a catalyst any more than an alkane is. This is the first
report of a silane-catalyzed reaction. Me3SiX (X ) Cl, Br, I)
and related silane compounds catalyze fac-mer isomerization
of [Mo(CO)3{P(OR)3}3] at room temperature.
Regarding M(CO)3L3 (M ) Cr, Mo, W; L ) phosphine,
phosphite), principally two geometrical isomers exist: facial (fac)
(
not complete one-electron oxidation, but partial oxidation).
Thermal fac-mer isomerization has been reported by Rousche
2
i
5
et al. for fac-[Mo(CO)3(η -dppe){P(O Pr)3}] and by Howell
6
et al. for fac-[M(CO)3{P(OR)3}3].
We examined the reaction of fac-[Mo(CO)3{P(OMe)3}3] (fac-
1) with 1 equiv of Me SiCl in CH Cl at room temperature.
3
2
2
3
1
1
The reaction was monitored by measuring the P{ H} NMR
*
To whom correspondence should be addressed. E-mail:
nakazawa@sci.osaka-cu.ac.jp.
1) Larson, G. L. In The Chemistry of Organic Silicon Compounds; Patai,
S., Rappoport, Z., Eds.; Wiley: London, 1989; p763.
2) Miura, K.; Hosomi, A. In Main Group Metals in Organic Synthesis;
(3) Bond, A. M.; Colton, R.; Feldberg, S. W.; Mahon, P. J.; Whyte, T.
Organometallics 1991, 10, 3320.
(4) Nakazawa, H.; Yamaguchi, Y.; Miyoshi, K. J. Organomet. Chem.
1994, 465, 193.
(5) Rousche, J.-C.; Dobson, G. R. Inorg. Chim. Acta 1978, 28, L139.
(6) Howell, J. A. S.; Yates, P. C.; Ashford, N. F.; Dixon, D. T.; Warren,
R. J. Chem. Soc., Dalton Trans. 1996, 20, 3959.
(
(
Yamamoto, H., Oshima, K., Eds.;Wiley-VCH: Weinheim, Germany, 2004;
p 409.
1
0.1021/om7008827 CCC: $37.00  2007 American Chemical Society
Publication on Web 11/10/2007

6
506 Organometallics, Vol. 26, No. 26, 2007
Table 1. Isomerization by Me3SiCl in CH2Cl2 at Room
Temperature
Scheme 1. Proposed fac-mer Isomerization Mechanism
Catalyzed by Me SiCl
3
entry
complex
amt of Me3SiCl (equiv)
time (h)
fac:mer
1
2
3
4
5
6
fac-1
fac-1
fac-1
fac-2
fac-3
fac-4
1.0
0.5
0.1
1.0
1.0
1.0
3
5
8
2
72
12
1:3.4
1:3.4
1:3.4
1:2.2
1:1.3
1:0.5
a
a
The isomerization is very slow, and the equilibrium is not attained
after 72 h. The final ratio obtained by using Me3SiOTf is 1:30
unpublished results).
(
spectrum. A triplet at 167.1 ppm (JPP ) 44.3 Hz) and a doublet
at 175.3 ppm (JPP ) 41.7 Hz) attributable to mer-1 appeared at
the expense of an intensity of a singlet at 164.8 ppm because
of fac-1. After 3 h, equilibrium between fac-1 and mer-1 was
7
established with the ratio of 1:3.4. Next, the reactions with
0
1
.5 and 0.1 equiv of Me3SiCl were examined. Table 1 (entries
–3) shows that the final equilibrium position was found not to
depend on the amount of Me3SiCl used, although it takes a
longer time to reach the equilibrium when the amount of
Me3SiCl is reduced. The equilibrium ratio resembles that
obtained in thermolysis by Howell et al. (1:3). These results
show that Me3SiCl acts as a catalyst for fac-mer isomerization.
In our experiments dry CH2Cl2 distilled from CaH2 was used.
However, there is a possibility that adventitious HCl from
Me3SiCl hydrolysis could be responsible for the observed
2
9
isomerization. Thus, the Si NMR of Me3SiCl in CH2Cl2 was
measured. No signal other than a singlet attributable to Me3SiCl
was observed, indicating no silicon compound derived from
Me3SiCl hydrolysis exists (less than 5%, if any). We observed
that the isomerization of fac-1 is completed within 8 h in the
presence of 10 mol % of Me3SiCl. If 10% of the Me3SiCl
produces adventitious HCl, then the amount of HCl would be
3
1
Figure 1. P NMR spectrum of a Me
temperature in which fac-1 is dissolved.
3
SiCl solution at room
+
9
31
(
OMe)}2{P(NMeCH2)2}] . In contrast, on the basis of
P
NMR monitoring, the reaction of fac-4 with Me3SiCl generated
no cationic phosphenium complex, which suggests that the
pathway of the isomerization promoted by Me3SiCl differs from
that involving a cationic phosphenium complex produced by
Me3SiOTf.
Scheme 1 presents a plausible mechanism for the fac-mer
isomerization promoted by Me3SiCl. The silicon atom in
Me3SiCl interacts with one oxygen in P(OR)3 ligands to form
a Si hypervalent structure (a). The interaction weakens the
coordination of P(OR)3 · Me3SiCl toward the central metal and
makes the ligand bulky, thereby decreasing the isomerization
energy barrier to give its mer isomer (b). The following
dissociation of the Me3SiCl gives the mer isomer, reproducing
free Me3SiCl.
The P and Si NMR measurements of a CH2Cl2 solution
containing fac-1 and Me3SiCl showed no signals assignable to
either a or b in Scheme 1. To obtain some evidence of a and b,
we dissolved fac-1 in Me3SiCl and measured the P and Si
NMR spectra of the solution. The P NMR is shown in Figure
. In addition to a singlet at 166.7 ppm attributable to fac-1
1
mol % based on fac-1 under the above conditions. The CH2Cl2
solution of fac-1 containing 1 mol % of HCl was prepared and
31
monitored by the P NMR measurement, and no isomerization
was observed after 10 h. These results clearly indicate that the
fac-mer isomerization is catalyzed by Me3SiCl but not by HCl.
The corresponding isomerization catalyzed by Me3SiCl was
observed for the triethyl phosphite complex fac-2, the triphenyl
phosphite complex fac-3, and the diamino-substituted phosphite
31
complex fac-4. In all cases, the P NMR spectra of the reaction
mixture suggested the formation of the corresponding mer
8
isomers. The isomerization of fac-3 to mer-3 was much slower
(
more than 72 h) than that of fac-1 to mer-1 (3 h), which might
be attributed to a steric cause. The reaction of fac-4 is
noteworthy. We found that the reaction of fac-4 with Me3SiOTf
abstracts one OMe as an anion from the phosphorus to give a
cationic phosphenium complex, fac-[Mo(CO)3{P(NMeCH2)2
31
29
3
1
29
3
1
(
7) In the absence of Me3SiCl no fac-mer isomerization occurred at
1
room temperature in CH2Cl2.
3
1
1
(
8) P{ H} NMR (161.7 MHz, CH2Cl2, 25 °C, reference 85% H3PO4;
δ, ppm): fac-1, 164.8 (s); mer-1, 167.1 (t, JPP ) 44.3 Hz, cis-PP), 175.3
d, JPP ) 41.7 Hz, trans-PP); fac-2, 145.4 (s); mer-2, 148.8 (t, JPP ) 52.1
Hz, cis-PP), 155.7 (d, JPP ) 52.1 Hz, trans-PP); fac-3, 146.1 (s); mer-3,
43.9 (d, JPP ) 46.9 Hz, cis-PP), 180.9 (t, JPP ) 46.9 Hz, trans-PP).
and a doublet at 176.2 ppm and a triplet at 168.8 with JPP )
(
(9) Nakazawa, H.; Miyoshi, Y.; Katayama, T.; Mizuta, T.; Miyoshi, K.;
Tsuchida, N.; Ono, A.; Takano, K. Organometallics 2006, 25, 5913.
1

Organometallics, Vol. 26, No. 26, 2007 6507
a
41.7 Hz attributable to mer-1, five signals were observed clearly.
Table 2. Isomerization of fac-1 Promoted by R3SiX
Among them, a triplet at 181.3 ppm and a doublet at 173.9
ppm with JPP ) 41.7 Hz can be assigned to a. The triplet is
assignable to P(OMe)3 · Me3SiCl because of the molecular
symmetry. The remaining three signals at 184.8 ppm (dd, JPP
entry
R3SiX
time (h)
fac:mer
1
2
3
4
5
Me3SiCl
Me3SiBr
Me3SiI
Ph3SiCl
(EtO)3SiCl
3
1:3.4
1:3.6
1:3.7
no reacn
1:3.4
<0.1
<0.1
25
)
41.7 and 229.3 Hz), 174.3 ppm (dd, JPP ) 44.3 and 229.3
Hz), and 166.2 ppm (t, JPP ) 44.3 Hz) can be reasonably
assigned respectively to P(OMe)3 · Me3SiCl and trans and cis
25
a
fac-1 was treated with 1 equiv of R3SiX in CH2Cl2 at room
temperature.
2
9
P(OMe)3 in P(OMe)3 · Me3SiCl in b. The Si NMR showed
two singlets at 6.62 and 6.53 ppm in addition to a strong singlet
at 29.8 ppm attributable to the solvent Me3SiCl. Coordination
of nitrogen or oxygen to silicon to form a pentacoordinate bond
has been shown to produce a strong shielding effect of the
silicon chemical shift. Kummer et al. reported the Si NMR
chemical shift at 11.1 ppm in CD2Cl2 for an Si hypervalent
compound with Cl and N in apical positions and two Me groups
SiCl4 promoted the isomerization, but some byproducts were
also formed. In contrast, SiMe4, with no chloride substituent,
did not promote isomerization at all. Therefore, among the
MenSiCl4-n (n ) 0–4) series, Me3SiCl is the best promoter,
which might stem from its superior hypervalent arrangement:
two electronegative substituents (Cl and O from phosphite) in
apical positions and three electron -releasing groups (three Me
groups) in equatorial positions. Table 2 shows the R3SiX
activity. Bromosilane and iodosilane (entries 2 and 3) show
activity better than that of chlorosilane. Ph3SiCl (entry 4) does
not promote isomerization, presumably because of the bulky
substituents. Slow isomerization was observed in the reaction
with (EtO)3SiCl (entry 5), which might be attributable to the
weak electron-releasing ability of an OEt group compared to
that of the Me group. A similar fac-mer ratio was obtained
for entries 1-3 and 5, meaning that the ratio is determined
thermodynamically and is unaffected by the added silane.
In conclusion, this paper describes, for the first time, a
halosilane-catalyzed reaction. The results showed that an
interaction of silane with one phosphite oxygen to form a
hypervalent silicon structure reduces the fac-mer isomerization
energy barrier of [Mo(CO)3(phosphite)3]. The hypervalent
silicon species was detectable spectroscopically. This finding
is believed to be a starting point for exploring the potential
catalytic activity of halosilanes.
1
0
29
1
1
13
and one CH2 group in equatorial positions. The C NMR of
the Me3SiCl solution containing fac-1 was also measured. Two
lumps consisting of several signals were observed at 213.9-214.1
and 218.0-218.2 ppm. These signals are attributable to terminal
carbonyl ligands, indicating that Me3SiCl does not have an
interaction with an oxygen atom in the terminal carbonyl ligand.
Therefore, our observations suggest the formation of hypervalent
Si compounds.
No fac-mer isomerization was confirmed in reactions
between Me3SiCl and fac-[Mo(CO)3(phosphine)3] (phosphine
n
)
PEt3, Bu 3) having no OR group on a coordinating phos-
phorus, suggesting that a phosphite oxygen plays a crucial role
in fac-mer isomerization.
To elucidate whether fac-mer isomerization involves a
phosphite dissociation process or not, a crossover experiment
was performed. Addition of Me3SiCl to a CH2Cl2 solution
containing both fac-1 and fac-2 yielded mer-1 and mer-2 but
formed neither phosphite exchange products such as fac- and
mer-[Mo(CO)3{P(OMe)3}2{P(OEt)3}] nor OR exchange prod-
ucts such as fac- and mer-[Mo(CO)3{P(OMe)3}2{P(OMe)2
Acknowledgment. This work was supported by a Grant-
in-Aid for Science Research on Priority Area (No. 18033044,
Chemistry of Coordination Space, and No. 19027047,
Synergistic Effect of Elements) from the Ministry of Educa-
tion, Culture, Sports, Science and Technology.
(OEt)}]. These results show intramolecular isomerization, which
is consistent with the mechanism shown in Scheme 1.
The results showed that Me3SiCl can promote fac-mer
isomerization for fac-[Mo(CO)3(phosphite)3]; the activity of
other silanes was examined for fac-1. Me2SiCl2, MeSiCl3, and
Supporting Information Available: Detailed experimental
procedures and the characterization of products. This material is
available free of charge via the Internet at http://pubs.acs.org.
(
10) Williams, E. A. In The Chemistry of Organic Silicon Compounds;
Patai, S., Rappoport, Z. , Eds.; Wiley: London, 1989; p 511.
11) Kummer, D.; Halim, S. H. A.; Kuhs, W.; Mattern, G. J. Organomet.
Chem. 1993, 446, 51.
(
OM7008827