Reactions of quadruply chelated silyl- and germyl-molybdenum hydrido complexes with carboxylic acids and carbon dioxide: A first example of carbon dioxide fixation utilizing the trans effect of a silyl ligand

Article abstract of DOI:10.1039/b107837m

The reaction of molybdenum complexes [MoH₃{E(Ph)[Ph₂PCH₂CH₂P(Ph)C ₆H₄-o]₂}] (E = Si or Ge) with carboxylic acids gave carboxylato complexes where the carboxylato ligand coordinat

Full text of DOI:10.1039/b107837m

Reactions of quadruply chelated silyl– and germyl–molybdenum
hydrido complexes with carboxylic acids and carbon dioxide: a first
example of carbon dioxide fixation utilizing the trans effect of a silyl
ligand
Makoto Minato, Da-Yang Zhou, Ken-ichiro Sumiura, Ryo Hirabayashi, Yoshitaka Yamaguchi and
Takashi Ito*
Department of Materials Chemistry, Graduate School of Engineering, Yokohama National University,
79-5 Tokiwadai, Hodogaya-ku, Yokohama, Japan 240-8501. E-mail: t-ito@ynu.ac.jp
Received (in Cambridge, UK) 30th August 2001, Accepted 7th November 2001
First published as an Advance Article on the web 4th December 2001
The reaction of molybdenum complexes [Mo-
H3{E(Ph)[Ph2PCH2CH2P(Ph)C6H4-o]2}] (E = Si or Ge)
with carboxylic acids gave carboxylato complexes where the
carboxylato ligand coordinates to the metal in a unidentate
mode; the formato complex was found to be an effective
catalyst for carbon dioxide fixation.
to an A2K2X spin system. The 31P{H} NMR spectra of
complexes 4 contain two resonances at around d 87 (br d, JPP
122 Hz) and 58 (br d, JPP 122 Hz) in a 1+1 ratio. The presence
of only two resonances indicates that the molecule 4 has an
effective mirror plane which contains the Mo–Si bond. The IR
spectra of complexes 4 showed n(Mo–H) at 1816–1819 cm21
and two n(OCO) stretching vibrations (asym and sym) at
1609–1618 cm21 and 1326–1360 cm21, respectively. The fairly
large values of Dn [n(OCO)asym2n(OCO)sym] ranging from
248 to 272 cm21 suggest that the carboxylato ligands in the
complexes coordinate to the metal in a unidentate fashion as
shown in Scheme 1.7 The molybdenum–germyl complexes 5
have very similar IR and NMR spectra to 4, implying that 4 and
5 are isostructural.
Since the first preparation of the transition-metal silyl deriva-
5
tive, CpFe(CO)2SiMe3 (Cp = h -C5H5), by Wilkinson and
coworkers in 1956,1 numerous relevant examples have been
reported.2 We previously reported the unexpected formation of
a
novel molybdenum–silyl complex, [MoH3{Si(Ph)-
[Ph2PCH2CH2P(Ph)C6H4-o]2}] 1a, which is obtained by the
thermal reaction of [MoH4(dppe)2] 2, (dppe
=
Ph2PCH2CH2PPh2) with PhSiH3 and possesses an unusual
quadruply chelated ligand.3 Recently this methodology has
been extended to the reaction of 2 with PhGeH3, which is a
higher homologue of PhSiH3, and have obtained the same type
of complex 1b having a Mo–Ge bond.4 Complexes in which two
metal atoms of very different character are held in close
proximity seem to offer fascinating opportunities to achieve
novel types of reaction. We have studied the reactivity of these
complexes and a series of new complexes with this quinqui-
dentate ligand has been synthesized from 1. For example, 1a
reacted with a dialkyl malonate or gaseous dioxygen to afford
The basic features of the structure of these complexes
deduced from the spectra were fully confirmed by single crystal
X-ray diffraction on 4b. A view of the molecule is shown in Fig.
1 with selected bond distances and angles.8 As expected from
the IR and NMR results, this complex contains a quinquidentate
P2SiP2 ligand and a unidentate acetato ligand occupying the
axial site. The structure can be compared with that of the above-
mentioned
peroxo
complex,
[MoH{Si(Ph)[Ph2-
2
PCH2CH2P(Ph)C6H4-o]2}(h -O2)] 3.5 The Mo–Si distance of
1
2
an h -O-enolato type complex or a peroxo type h -O2 complex
3, respectively.5,6 Herein we report the synthesis and some
reactions of dihydrido(carboxylato)molybdenum complexes.
Treatment of 1 with two equiv. of carboxylic acids such as
formic acid, acetic acid or benzoic acid in THF at room
temperature readily led to the formation of monocarboxylato
molybdenum complexes (4, E = Si; 5, E = Ge) in good yields
(70–90%, Scheme 1).† In these reactions, the evolution of
hydrogen gas was qualitatively confirmed by GLC analysis. In
the 1H NMR spectra of the complexes 4, hydrido signals
appeared at around d 27.9 as a triplet of triplets corresponding
Fig. 1 Thermal ellipsoid plot (30% probability level) for 4b. Selected bond
lengths (Å) and angles (°): Mo1–Si1 2.515(2), Mo1–P1 2.485(2), Mo1–P2
2.507(2), Mo1–P3 2.496(2), Mo1–P4 2.466(2), Si1–C47 1.887(7), Mo1–O1
2.174(5); P1–Mo1–P2 80.31(6), P2–Mo1–P3 96.32(6), Si1–Mo1–O1
143.7(1).
Scheme 1
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Chem. Commun., 2001, 2654–2655
This journal is © The Royal Society of Chemistry 2001
DOI: 10.1039/b107837m

2.515(2) Å in 4b is slightly shorter than that of 2.554(5) Å in 3.
On the other hand, the mean Mo–P distance of 2.489 Å in 4b is
a little longer than that found in 3 (2.462 Å).
The parent complex 2 is known to react with carboxylic acids
or allyl carboxylates to give cationic and neutral carboxylato
complexes, respectively, where the carboxylato ligand coor-
dinates to the metal in a bidentate mode.9,10 The unusual
unidentate coordination of acetate of 4b may be ascribed to the
stability of the P2SiP2 coordination.
using Group 6 metals are rare.15 To our knowledge, complex 1a
is the first molybdenum compound which can catalyze the
transformation of carbon dioxide into dialkylformamide ef-
fectively.
This work was supported by a Grant-in-Aid for Scientific
Research (B) No. 10450340 and a Grant-in-Aid for Scientific
Research on Priority Areas (No. 11120217) from the Ministry
of Education, Science, Sports and Culture of Japan.
A toluene solution of complex 1a was allowed to react under
CO2 pressure (17 atm) at room temperature and 4a was isolated
in 82% yield from the reaction mixture (Scheme 2). Inter-
estingly, 4a reverted to 1a quantitatively with accompanying
evolution of one mol of CO2, when warmed in benzene. The
reversible insertion reaction of CO2 into the Mo–H bond under
mild conditions is an outstanding feature exhibited by complex
1a since the parent complex 2 does not react with CO2 at room
temperature although irradiation of a benzene solution of 2
under CO2 atmosphere yielded a chelated formato-O,O’
complex.11 We thought that the high reactivity of 1a is due to
the strong trans-influence of the Si fragment since it has been
pointed out that the R3Si group has a strong s-donor ability and
may labilize the ligand in the position trans to it.12 Yet, to date
there have been few reports concerning the influence of silyl
ligands on the reactivity of transition-metal complexes.13
Subsequently, we explored the catalytic activity of 1a
towards carbon dioxide fixation [eqn. (1)]. A toluene solution of
Notes and references
† Typical procedure for the preparation of 4b: to a solution of 1a (0.268 g,
26.8 mmol) in dry THF (20 mL) was added acetic acid (0.03 mL) under
argon. The resultant solution was stirred for 3 h at ambient temperature.
After removal of the solvent under reduced pressure, the residue was
washed with diethyl ether and hexane. Recrystallization from THF–hexane
gave 4b (0.255g, 90%). Complex 4b: 1H NMR (benzene-d6, 25 °C, 270
MHz): d 27.96 (tt, J 43.3, 17.2 Hz, 2H, Mo–H), 1.23 (s, 3H, CH3CO)
2.0–3.0 (br m, 8H, PCH2); 31P{1H} NMR (benzene-d6, 109.4 MHz): d 87.7
(br d, J 122 Hz), 59.1 (br d, J 122 Hz); Satisfactory elemental analysis data
are so far not available, possibly due to the presence of silicon in the
molecule.
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(1)
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complex 1a was allowed to react under CO2/H2 (25/35 atm)
pressure in the presence of dimethylamine at 110 °C affording
115 equivalents of N,N-dimethylformamide with respect to the
complex. In this process, formato complex 4a was also found to
be an efficient catalyst. Presumably, the first step in the catalytic
cycle using 1a may be the formation of 4a. Neither 2 nor 1b
catalyzed this process suggesting both the Mo–Si linkage and
the P2SiP2 girdle in 1a play an important role.
7 K. Nakamoto, Infrared and Raman Spectra of Inorganic and Coordina-
tion Compounds, Wiley, NY, 3rd edn, 1978, p. 232.
8 Crystal data for 4b: yellow prism; 0.30 3 0.10 3 0.10 mm;
C60H56O2P4MoSi·C4H8O·H2O, M = 1147.14; monoclinic, space group
P21/c(no. 14); a = 23.858(1), b = 12.1098(3), c = 19.5279(7) Å, b =
90.027(1)°, V = 5641.9(3) Å3, Z = 4; Dc = 1.350 g cm23; Rigaku
RAXIS-II imaging plate area detector; 298 K; Mo-Ka radiation (l =
0.71069 Å); m(Mo-Ka) = 4.15 cm21; R = 0.051. Rw = 0.080 for 5376
unique reflections [I > 5.00s(I)]. CCDC reference number 172503. See
http://www.rsc.org/suppdata/cc/b1/b107837m/ for crystallographic data
in CIF or other electronic format.
Catalytic carbon dioxide fixation has attracted considerable
interest. Most of the reported studies deal with iridium,
palladium and ruthenium complexes14 and reports of catalysis
9 T. Ito, A. Takahashi and S. Tamura, Bull. Chem. Soc. Jpn., 1986, 59,
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Science Books, Mill Valley, CA, 1987, p. 243.
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H. Ogino, Organometallics, 1999, 18, 2058.
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15 D. J. Darensbourg and C. Ovalles, J. Am. Chem. Soc., 1987, 109,
3330.
Scheme 2
Chem. Commun., 2001, 2654–2655
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