Reactions of quadruply chelated silyl- and germyl-molybdenum hydrido complexes with isocyanides: Observation of the trans-influence on the silyl ligand

Article abstract of DOI:10.1039/b209624m

The reactions of a series of silyl- and germyl-molybdenum hydrido complexes, [MoH₃{E(Ar)[Ph₂PCH₂CH₂P(Ph)-C₆ H₆-o]₂}] (E = Si, Ge; Ar = Ph, C₆F₅, 4-Me₂NC₆H₄), with various isocyanides result in the liberation of hydrogen and formation of the molybdenum isocyanide complexes, [MoH(CNR){E(Ar)[Ph₂PCH₂CH₂P(Ph)C₆H ₄-o]₂}] (R = t-Bu, cyclo-C₆H₁₁, PhCH₂, Ph, 2,6-Me₂C₆H₃), in which the isocyanide ligand ligates at the site opposite to the E atom. The new complexes show in their solid-state IR spectra an intense band assignable to C=N stretching, which is found to be considerably lower than that of the corresponding free isocyanide (more than 100 cm^-1), demonstrating the existence of π-back-bonding from the electron-rich metal centre to the isocyanide ligand. Both X-ray crystallographic analyses and spectroscopic evidence confirm the strong trans-influence of the Si group.

Full text of DOI:10.1039/b209624m

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Reactions of quadruply chelated silyl- and germyl-molybdenum
hydrido complexes with isocyanides: observation of the
trans-influence on the silyl ligand
Makoto Minato,* Jun-ya Nishiuchi, Masaki Kakeya, Takaomi Matsumoto,
Yoshitaka Yamaguchi and Takashi Ito*
Department of Materials Chemistry, Graduate School of Engineering,
Yokohama National University, 79-5 Tokiwadai, Hodogaya-ku, Yokohama 240-8501, Japan
Received 1st October 2002, Accepted 9th December 2002
First published as an Advance Article on the web 23rd December 2002
The reactions of a series of silyl- and germyl-molybdenum hydrido complexes, [MoH3{E(Ar)[Ph2PCH2CH2P(Ph)-
C6H4–o]2}] (E = Si, Ge; Ar = Ph, C6F5, 4-Me2NC6H4), with various isocyanides result in the liberation of hydrogen
and formation of the molybdenum isocyanide complexes, [MoH(CNR){E(Ar)[Ph2PCH2CH2P(Ph)C6H4–o]2}]
(R = t-Bu, cyclo-C6H11, PhCH2, Ph, 2,6-Me2C6H3), in which the isocyanide ligand ligates at the site opposite to the
E atom. The new complexes show in their solid-state IR spectra an intense band assignable to C᎐N stretching,
᎐
which is found to be considerably lower than that of the corresponding free isocyanide (more than 100 cmϪ1),
demonstrating the existence of π-back-bonding from the electron-rich metal centre to the isocyanide ligand. Both
X-ray crystallographic analyses and spectroscopic evidence confirm the strong trans-influence of the Si group.
Introduction
We have previously shown that the novel silyl- and germyl-
molybdenum complexes [MoH3{E(Ph)[Ph2PCH2CH2P(Ph)-
C6H4–o]2}] (E = Si (1a) and Ge (2a)) are readily prepared by
the thermal reaction of the molybdenum hydrido complex
[MoH4(dppe)2] (dppe = Ph2PCH2CH2PPh2) with PhEH3.1,2
These complexes possess an unusual quadruply chelated ligand
comprised of a P–P–E–P–P framework and some interesting
reactivity features of these species have been observed.3 For
example, 1a reacted with carbon dioxide to give a formato
complex at room temperature indicating that CO2 can insert
into the Mo–H bond of the complex 1a, although the parent
complex, [MoH4(dppe)2], does not react with CO2 under the
same conditions at all. Furthermore, 1a has exhibited a unique
characteristic for catalytic carbon dioxide fixation and has
represented the first molybdenum compound which can
catalyze hydrogenation of CO2 effectively.4 We thought that the
high reactivity of 1a was due to the strong trans-effect of the Si
fragment since it has been pointed out that the R3Si group is a
Scheme 1
strong σ-donor and may labilize the ligand in the position trans
to good yields (61–82%, Scheme 1 and Table 1). The resultant
complexes were obtained as yellow or orange powders. They
were found to be soluble in benzene, toluene, or THF, but not
soluble in hexane and could be purified by recrystallization
from toluene–hexane.
to it.5
It is well known that spectral features of a coordinated
isocyanide ligand reflect the property of the molecule under
consideration and often afford reliable evidence for compound
identity.6 We have therefore studied isocyanide complexes
derived from 1 or 2 in order to obtain more detailed inform-
ation on the trans-influencing ability imparted by the Si and the
Ge fragment. Isocyanides form complexes with transition
metals and resultant isocyanide complexes are, in some
respects, similar to metal carbonyls. However, isocyanide
ligands are more basic and much weaker π-acceptors than
carbon monoxide because of the competition for the carbon–
nitrogen multiple bond antibonding orbitals between the nitro-
gen lone electron pair and the metal lone electron pairs.6,7
Infrared spectra
In their solid-state IR spectra, the complexes show the intense
band assignable to C᎐N stretching; these frequencies are listed
᎐
in Table 2. The present values are found to be considerably
lower than those of the corresponding free isocyanides (more
than 100 cmϪ1), demonstrating the existence of π-back-bonding
from the electron-rich metal centre to the isocyanide ligand.8
C᎐N Stretching frequencies can be regarded as an indicator
᎐
of relative amounts of charge on a given metal centre and
variations in ν(CN) are usually ascribed to differences in
π-back-bonding.9 In a series of the silylmolybdenum complexes
studied in this work, the electron density on the molybdenum
atom would be expected to increase in the order: C6F5 com-
pounds (3f–h) < Ph compounds (3a,b,e) < 4-Me2NC6H4 com-
pounds (3i–k). The observed spectroscopic trend for ν(CN) is
partially in line with this order. The increased donor properties
of 4-Me2NC6H4 compared to phenyl result in a 18 cmϪ1 shift to
lower frequency for 3k (1961 cmϪ1) compared with 3e (1979
Results and discussion
Syntheses of the isocyanide complexes
Treatment of 1 or 2 with one equivalent of isocyanide such as
tert-butyl isocyanide, cyclohexyl isocyanide, benzyl isocyanide,
phenyl isocyanide, or 2,6-dimethylphenyl isocyanide in toluene
at ambient temperature readily led to the formation of molyb-
denum isocyanide complexes (3, E = Si; 4, E = Ge) in moderate
T h i s j o u r n a l i s © T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 3
D a l t o n T r a n s . , 2 0 0 3 , 4 8 3 – 4 8 7
483

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Table 1 Preparation of isocyanide complexes
Compound
Complex/mmol
Isocyanide (R/mmol)
Toluene (mL)
Yield (%)
3a
3b
3c
3d
3e
3f
3g
3h
3i
1a/0.088
1a/0.14
1a/0.16
1a/0.11
1a/0.10
1b/0.092
1b/0.092
1b/0.092
1c/0.096
1c/0.096
1c/0.096
2a/0.11
t-Bu/0.088
cyclo-C6H11/0.20
PhCH2/0.24
Ph/0.16
2,6-Me2C6H3/0.30
t-Bu/0.74
cyclo-C6H11/0.14
2,6-Me2C6H3/0.28
t-Bu/0.77
cyclo-C6H11/0.48
2,6-Me2C6H3/0.29
t-Bu/0.11
10
10
15
10
10
10
10
10
10
10
10
10
72
65
68
61
82
67
65
69
74
80
75
72
3j
3k
4a
Table 2 Infrared spectra (CN stretching region, cmϪ1
)
state of the metal centre can be tuned by the selection of sub-
stituents on the silicon atom. The value of ν(CN) for 3a(2025
cmϪ1) is slightly lower than that of 4a (2031 cmϪ1); this may be
interpreted in terms of a difference in the π-interaction between
the central molybdenum atom and the two elements (Si and
Ge), increasing in the order Si < Ge.12
Compound
Ar
E
R
ν(CN)a
3a
3b
3c
3d
3e
3f
3g
3h
3i
Ph
Ph
Ph
Ph
Ph
C6F5
C6F5
C6F5
4-Me2NC6H4
4-Me2NC6H4
4-Me2NC6H4
Ph
Si
Si
Si
Si
Si
Si
Si
Si
Si
Si
Si
Ge
t-Bu
cyclo-C6H11
PhCH2
2025
2038
2033
1969
1979
2039
2053
1979
2023
2038
1961
2031
Ph
2,6-Me2C6H3
t-Bu
cyclo-C6H11
2,6-Me2C6H3
t-Bu
cyclo-C6H11
2,6-Me2C6H3
t-Bu
NMR spectra
1
In the H NMR spectra of 3, which are collected in Table 3, a
hydrido signal appears at around δ Ϫ7 as a triplet of triplets
(2JHP = ca. 28 and 4 Hz) corresponding to an A2K2X spin
system. In the case of 3e, the hydrido ligand is observed at
δ Ϫ6.90 as an apparent broad triplet (2JHP = 23.7 Hz). The
hydride region of 4a displays a triplet of triplets (δ Ϫ6.80, 2JHP
= 6.2 and 42.1 Hz), which is at a lower field than that of 3a by
about 0.7 ppm. This would appear to suggest that the Si ligand
imparts somewhat more electron density on the metal centre
than does the Ge ligand. In the series of silylmolybdenum
complexes studied in this work, the hydrido resonances for the
C6F5 compounds (3f–h) are shifted to higher field by about 0.2
ppm and those for the 4-Me2NC6H4 compounds (3i–k) are
shifted to lower field by about 0.1 ppm versus the corresponding
resonances for the phenyl compounds (3a,b,e). Taking into
account the electronic effect of the substituents, the present
results are unexpected and somewhat puzzling at first glance.
However, let us assume that the hydride and the Si ligand are
positioned pseudo-trans to the isocyanide ligand, and the Mo
H σ-donation is influenced by σ-donor effect of the
coordinated isocyanide ligand. σ-Donor ability of the Si ligand
increases in the order: C6F5 compound < phenyl compound <
4-Me2NC6H4 compound, suggesting that the relative σ-donor
influence of the coordinated isocyanide toward the hydride
decreases in this order.
3j
3k
4a
a KBr disk.
cmϪ1). In addition, replacement of the phenyl group by the
most electron-withdrawing C6F5 leads to a 15 cmϪ1 shift to
higher frequency: 2053 cmϪ1 for 3g compared with 2038 cmϪ1
for 3b and similar higher shift is observed in the pair 3f (2039
cmϪ1) and 3a (2025 cmϪ1). However, it is quite unexpected that
the ν(CN) frequencies in 3e and 3h are identical in spite of the
large difference in the electronic effect of the substituents, as are
the ν(CN) frequencies in 3b and 3j. Likewise, the ν(CN)
frequencies in 3a are nearly equal to those in 3i. Consequently,
in these complexes, the influence of the substituent in the aro-
matic ring on the ν(CN) frequencies can be completely ignored.
An intriguing feature of the present results is as follows: the
complexes containing a phenyl and a C6F5 group on the silicon
atom behave similarly with aryl isocyanide but behave differ-
ently with the aliphatic isocyanides, on the other hand the com-
plexes containing a phenyl and 4-Me2NC6H4 group behave
similarly with aliphatic isocyanides but behave differently in the
presence of an aryl isocyanide. Coordinated aryl isocyanides
are generally capable of delocalizing excess charge density on a
metal centre from M–L back-donation into the aromatic ring
system, while such delocalization cannot be expected for
coordinated aliphatic isocyanides, and aliphatic isocyanides
impart somewhat more electron density on a metal centre
than do aryl isocyanides.10 The C6F5 group in the aliphatic iso-
cyanide complexes (3f,g) may be responsible for withdrawing
the extra electron density through the Si–Mo–C linkage,
although in 3h its electron-withdrawing capability would not be
enough to attract π-density from the coordinated aryl iso-
cyanide. On the other hand, the N,N-dimethylamino substitu-
ent acts as a potent electron-donating group, rendering the
silicon a good donor and a poor π-acceptor. This effect would
play a significantly greater role in the aryl isocyanide complex
(3k) since the π-acceptor strength of aliphatic isocyanides is
much less than that of aryl ones.11 The observed spectroscopic
trend for ν(CN) is indicative of a temperate electronic com-
munication between a functional group on the silicon atom and
a coordinated isocyanide. These results show that the electronic
All of the 31P{1H} NMR spectra for 3 show a pair of
doublets at around δ 105 and 80 with coupling constant 2JPP
=
ca. 75 Hz in a 1 : 1 ratio due to the two inequivalent equatorial
phosphines (see Table 3), indicating that the molecule 3 has an
effective mirror plane which contains the Mo–Si bond.
Crystal structures
The solid-state structures of 3a, 3b, and 3e were determined by
X-ray crystallography (Figs. 1–3). Crystallographic data are
listed in Table 4, and selected bond distances and angles are
given in Table 5. Complex 3a crystallizes as a C7H8 solvate.
The molecular structures of all three complexes can be con-
sidered to consist of a seven-coordinate molybdenum centre.
Although fairly weak the reflection intensity prevents us from
defining the hydride ligand precisely, it is postulated as being at
a similar site to the Si ligand (as described in the NMR section).
In these structures, the quadruply chelated P–P–Si–P–P frame-
work is found to be kept intact and the isocyanide ligand binds
to the Mo centre in an end-on fashion. The Mo1–Si1 bond
length is 2.519(3) Å in 3a, 2.5188(9) Å in 3b, and 2.5593(9) Å in
D a l t o n T r a n s . , 2 0 0 3 , 4 8 3 – 4 8 7
484

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Fig. 1 Molecular structure and atom labeling for 3a.
Fig. 2 Molecular structure and atom labeling for 3b.
3e and the mean Mo–P distances (2.4465 Å for 3a, 2.4391 Å for
3b, and 2.4502 Å for 3e) are nearly identical, indicating that
geometries around the molybdenum and silicon atom in 3a, 3b
and 3e are similar. It is noteworthy that the Mo1–Si1 bond
distances are closely correlated with the C1–N1 bond distances.
Namely, the Mo1–Si1 bonds are elongated in the order 3b < 3a
< 3e, and accordingly the C1–N1 bond distances increase in the
order 3b (1.136(4) Å) < 3a (1.15(2) Å) < 3e (1.171(4) Å). The Si
ligand is trans to the isocyanide and competes with the iso-
cyanide ligand for π-density. The C1–N1 bond length reflects a
degree of π-back-bonding from the metal into the isocyanide π*
frontier orbitals. Thus, we can anticipate that the extent of the
π-interaction between the central molybdenum atom and the
silicon atom decreases as the C1–N1 bond distance increases.
The Mo1–C1 bonds are elongated in the order 3e (2.072(3) Å) <
3b (2.091(3) Å) < 3a (2.10(1) Å) and hence 3e has a significantly
shortened Mo–C bond length. This indicates that 2,6-di-
methylphenyl isocyanide is the strongest π-acceptor of the three
ligand types, consistent with frontier molecular orbital studies
which show that aryl isocyanides should be better π-acceptors
D a l t o n T r a n s . , 2 0 0 3 , 4 8 3 – 4 8 7
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Table 4 Crystallographic data for compounds 3a, 3b and 3e
3aؒC7H8
3b
3e
Formula
M
Crystal system
Space group
a/Å
b/Å
c/Å
α/Њ
β/Њ
γ/Њ
V/Å
C70H69MoNP4Si
C65H63MoNP4Si
1106.14
C67H61MoNP4Si
1128.15
1172.24
Triclinic
Triclinic
Triclinic
¯
¯
¯
P1
P1
P1
14.42(2)
17.611(5)
13.275(4)
90.50(2)
109.72(5)
76.78(7)
3081(4)
2
1.263
0.377
0.1316 (0.3214)
3.384
23
12.473(3)
20.304(5)
12.078(3)
98.56(2)
112.41(2)
78.80(2)
2764(1)
2
1.317
0.412
0.0562 (0.1563)
1.495
Ϫ55
14.298(6)
17.097(9)
12.788(4)
104.04(4)
113.30(3)
90.16(5)
2768(2)
2
1.353
0.417
0.0398 (0.1273)
1.023
23
Z
ρc/g cmϪ3
µ/cmϪ1
R/wR
GOF
T /ЊC
Table 5 Selected bond distances (Å) and angles (Њ) for 3a, 3b and 3e
cyanide ligand in 3a and 3b exhibits greater deviation from
linearity than in 3e. In general, bent C–N–C linkages are found
in aliphatic isocyanides, bound to an electron-rich metal centre,
while significant C–N–C bending is rare in aryl isocyanide
ligands, due in large part to their ability to delocalize charge
away from the nitrogen atom on the isocyanide into the
aromatic ring.13
3a
3b
3e
Mo(1)–P(1)
Mo(1)–P(2)
Mo(1)–P(3)
Mo(1)–P(4)
Mo(1)–Si(1)
Mo(1)–C(1)
Si(1)–C
2.414(3)
2.480(3)
2.454(3)
2.438(3)
2.519(3)
2.10(1)
1.93(1)
1.89(1)
1.912(8)
1.15(2)
1.45(2)
2.3964(8)
2.4568(9)
2.4715(8)
2.4315(8)
2.5188(9)
2.091(3)
1.911(4)
1.904(3)
1.918(3)
1.136(4)
1.429(5)
2.4207(9)
2.4978(9)
2.4612(9)
2.4210(9)
2.5593(9)
2.072(3)
1.917(3)
1.915(3)
1.899(3)
1.171(4)
1.396(4)
The structure of 3e can be compared and contrasted with
that of the related molybdenum isocyanide complex, trans-
[Mo(PhNC)(CO)(dppe)2] (5), which was synthesized and
characterized by Hidai’s group.14 The Mo–C1 bond distance
(2.072(3) Å) is nearly identical to that found in 5 (2.072(4) Å)
and is longer than that of the known Mo–C bond lengths
in other related molybdenum isocyanide complexes, trans-
[Mo(PhNC)(L)(dppe)2] (L = N2, PhNC, p-MeOC6H4CN), by
about 0.04–0.1 Å.14 The long bond in 3e trans to the Si atom is
thus entirely consistent with the purported strong trans
influence of the silyl ligand. The C–N–C bond angle (177.7(4)Њ)
agrees well with that found in 5 (177.6(3)Њ). Of great interest is
the fact that the C1–N1 bond length of 1.171(4) Å in 3e is
considerably longer than that found in 5 (1.099(4) Å), indicat-
ing that there is more electron density on the molybdenum atom
of 3e available for back-donation. These observations seem to
reflect a difference in the trans-influence of the silyl and the
carbonyl ligand, although both are known to exhibit a strong
trans-influence.5a As noted above, the trans-influence of the silyl
ligand can be mainly interpreted in terms of its σ-donor ability.
On the other hand, the carbonyl ligand behaves as a strong π
acid causing reduction of dπ electron density in the metal, and
hence the M(dπ)–CN(pπ*) interaction in 5 would be weaker
than that in 3e. This also explains significant changes in IR
C(1)–N(1)
N(1)–C(2)
P(1)–Mo(1)–P(2)
P(2)–Mo(1)–P(3)
P(3)–Mo(1)–P(4)
P(1)–Mo(1)–P(4)
C(1)–N(1)–C(2)
Si(1)–Mo(1)–C(1)
80.72(9)
97.73(9)
78.97(10)
103.81(10)
169.8(8)
149.9(3)
80.48(3)
99.78(3)
78.97(3)
102.33(3)
172.3(5)
147.20(9)
79.00(3)
102.15(3)
80.67(3)
98.25(3)
177.7(4)
154.76(9)
C᎐N stretching frequencies for these complexes. Thus, the value
᎐
of ν(CN) for 3e (1979 cmϪ1) is considerably lower than that of
5 (2004 cmϪ1(av.)) and the Si ligand plays an important role in
enabling the isocyanide function to accept electron density from
the metal into the π* antibonding orbital.
Synthesis of the carbonyl complex
To learn more about the properties of the Mo–Si fragment, we
have examined the reaction of 1a with carbon monoxide
(Scheme 2). A pale yellow THF solution of 1a turned green on
exposure to 1 atm of carbon monoxide with stirring for 3 h at
ambient temperature. The carbonyl complex 6 was obtained as
a green powder in a good yield (91%). Coordination of the
carbonyl ligand at the molybdenum centre was proved on the
basis of the solid-state IR spectrum of 6 (KBr), which shows an
intense absorption at 1885 cmϪ1 for the CO stretch. The value is
much lower than that found in free carbon monoxide (2143
cmϪ1), consistent with the view that the Mo–Si fragment
participates in back-donation and can be an excellent σ-donor
Fig. 3 Molecular structure and atom labeling for 3e.
than aliphatic isocyanides, as noted above.11 The Si1–Mo1–C1
linkages of 3a, 3b, and 3e (149.9(3), 147.20(9) and 154.76(9)Њ,
respectively) are bent, while the C1–N1–C2 angles [169.8(8),
172.3(5) and 177.7(4)Њ, respectively] are close to 180Њ. The iso-
D a l t o n T r a n s . , 2 0 0 3 , 4 8 3 – 4 8 7
486

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X-Ray crystallographic study of 3a, 3b and 3e
Crystals of complexes 3a, 3b and 3e suitable for X-ray crystal-
lography were grown in toluene–hexane at room temperature,
and crystals thus obtained were mounted on a glass fiber.
Measurement were made on a Rigaku AFC7R (for 3b and 3e)
or a Rigaku RAXIS-IV (for 3a) diffractometer by using Mo Kα
radiation (λ = 0.71069 Å) for data collection. The unit-cell
parameters were determined by least-squares fitting of 25
reflections for 3b and 3e with ranges 28.3Њ < 2θ < 30.0Њ and
20.4Њ < 2 θ < 23.9Њ, respectively. The parameters used during the
collection of diffraction data are given in Table 4. The structure
was solved and refined by using Fourier techniques. The non-
hydrogen atoms were refined anisotropically. Hydrogen atoms
were included but not refined. In compound 3a, the solvent has
been refined isotropically and with no hydrogens. As all the
residuals in 3a are high, there are large residual density peaks,
all indicating that the data is poor for refinement.
Scheme 2
in accord with the results obtained for isocyanides. The 1H
NMR spectrum of 6 displays a hydrido resonance at δ Ϫ6.40 as
2
an apparent broad triplet JHP = 23.3 Hz. The 31P{1H} NMR
spectrum for 6 shows a pair of doublets at δ 110 and 81 with
coupling constants 2JPP = 58 and 61 Hz in a 1 : 1 ratio.
Experimental
CCDC reference numbers 194603–194605.
See http://www.rsc.org/suppdata/dt/b2/b209624m/ for crystal-
lographic data in CIF or other electronic format.
General
Unless otherwise noted, all manipulations were conducted
using standard Schlenk techniques under purified argon or
nitrogen. Commercially available reagent grade chemicals were
used as such without any further purification. All solvents
were dried by standard methods and were stored under argon.
Complexes 1, 21,2 and phenyl isocyanide15 were prepared as
previously described. All NMR spectra were recorded on a
JEOL-JNM-270 spectrometer. 31P{1H} NMR peak positions
were referenced to external PPh3. Microanalyses were per-
formed but none of the products analyzed properly; carbon
percentage values were found to be approximately 2–4% below
their calculated values, and reproducibility was poor on sam-
Acknowledgements
This work was partially 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.
References
1
1 D.-Y. Zhou, M. Minato, T. Ito and M. Yamasaki, Chem. Lett., 1997,
1017.
2 M. Minato, R. Hirabayashi, T. Matsumoto, Y. Yamaguchi and
T. Ito, Chem. Lett., 2001, 960.
ples which were clean as determined by H NMR. In silicon
compounds, carbon contents are occasionally too low due to
formation of silicon carbide.16
3 (a) T. Ito, Bull. Chem. Soc. Jpn., 1999, 72, 2365; (b) D.-Y. Zhou,
L.-B. Zhang, M. Minato, T. Ito and K. Osakada, Chem. Lett., 1998,
187; (c) T. Ito, K. Shimada, T. Ono, M. Minato and Y. Yamaguchi,
J. Organomet. Chem., 2000, 611, 308.
4 M. Minato, D.-Y. Zhou, K. Sumiura, R. Hirabayashi, Y. Yamaguchi
and T. Ito, Chem. Commun., 2001, 2654.
5 (a) J. P. Collman, L. S. Hegedus, J. R. Norton and R. G. Finke,
Principles and Applications of Organotransition Metal Chemistry,
University Science Books, Mill Valley, CA, 1987, p.243; (b)
M. Okazaki, S. Ohshitanai, H. Tobita and H. Ogino, Chem. Lett.,
2001, 952; (c) M. J. Auburn, R. D. Holmes-Smith, S. R. Stobart,
P. K. Bakshi and T. S. Cameron, Organometallics, 1996, 15, 3032;
(d ) R. D. Brost, G. C. Bruce, F. L. Joslin and S. R. Stobart,
Organometallics, 1997, 16, 5669; (e) T. Koizumi, K. Osakada and
T. Yamamoto, Organometallics, 1997, 16, 6014.
6 R. B. King and M. S. Saran, Inorg. Chem., 1974, 13, 74.
7 M. Minelli and W. J. Maley, Inorg. Chem., 1989, 28, 2954.
8 L. Malatesta and F. Bonati, Isocyanide Complexes of Metals, Wiley,
New York, 1969.
Synthesis of the isocyanide complexes
A typical procedure is as follows. A mixture of 1a (88 mg, 0.088
mmol) and tert-butyl isocyanide (10 µL, 0.088 mmol) was
dissolved in toluene (10 mL) at room temperature. The mixture
was stirred for 12 h at room temperature. Slow addition of
hexane (about 2 mL) to the solution caused the separation of
a precipitate, which was removed by filtration. The resultant
filtrate was concentrated to dryness leaving a yellow powder.
The crude product was washed with hexane and dried under
reduced pressure. The complex 3a thus obtained could be
recrystallized from toluene–hexane to give yellow crystals (95
mg, 72% yield). This procedure is also applicable to the
synthesis of the other isocyanide complexes. Experiments listed
in Table 1 were carried out under essentially the same
conditions.
9 J. Chatt, C. M. Elson, A. J. L. Pombeiro, R. L. Richards and
G. H. D. Royston, J. Chem. Soc., Dalton Trans., 1978, 165.
10 N. L. Wagner, F. E. Laib and D. W. Bennett, J. Am. Chem. Soc.,
2000, 122, 10856.
11 M. P. Guy, J. T. Guy and D. W. Bennett, Organometallics, 1986, 5,
1696.
12 M. A. Esteruelas, F. J. Lahoz, E. Oñate, L.A. Oro and L. Rodriguez,
Organometallics, 1996, 15, 3670.
13 C. Hu, W. C. Hodgeman and D. W. Bennett, Inorg. Chem., 1996, 35,
1621.
14 H. Seino, C. Arita, D. Nonokawa, G. Nakamura, Y. Harada,
Y. Mizobe and M. Hidai, Organometallics, 1999, 18, 4165.
15 W. P. Weber, G. W. Gokel and I. K. Ugi, Angew. Chem., Int. Ed.
Engl., 1972, 11, 530.
16 M. Söldner, M. Sandor, A. Schier and H. Schmidbaur, Chem. Ber.,
1997, 130, 1671.
Synthesis of the carbonyl complex
A pale yellow solution of 1a (120 mg, 0.120 mmol) in THF (15
mL) turned green on exposure to 1 atm of carbon monoxide,
which was generated by conc. H2SO4 and HCOOH,17 with
stirring for 3 h at ambient temperature. Slow addition of hexane
to the solution caused the separation of a precipitate, which was
removed by filtration. The resultant filtrate was concentrated to
dryness leaving a green powder. The crude product was washed
with hexane and dried under reduced pressure to give 6 (112
mg, 91% yield). IR (KBr) ν(C᎐O): 1885 cmϪ1, ν(Mo–H): 1715
᎐
cmϪ1. 1H NMR (270 MHz at 20 ЊC, in C6D6): δ Ϫ6.40 (br t, 1H,
J = 23.2 Hz, Mo–H ), 1.8–3.6 (m, 8H, C2H4), 6.5–8.0 (m, 43H,
aromatic). 31P{1H} NMR (109.25 MHz at 20 ЊC, in C6D6):
δ 81.4 (d, J = 61.0 Hz), 110.3 (d, J = 58.0 Hz).
17 W. L. Gilliland and A. A. Blanchard, in Inorganic Synthesis,
ed. W. C. Fernelius, McGraw-Hill, New York, 1946, vol. II, p. 81.
D a l t o n T r a n s . , 2 0 0 3 , 4 8 3 – 4 8 7
487