Synthesis, structure, and reactions of a (η3-α- silabenzyl)molybdenum complex: A synthetic equivalent of a coordinatively unsaturated silyl complex

Article abstract of DOI:10.1021/om400225j

The (η³-α-silabenzyl)molybdenum complex Cp*Mo(CO)₂{η³(Si,C,C)-Si(p-Tol)₃} (1; Cp* = η⁵-C₅Me₅, p-Tol = p-C ₆H₄Me) was synthesized in high yield by removal of 4-(dimethylamino)pyridine (DMAP) from the (arylsilyl)(DMAP)molybdenum complex Cp*Mo(CO)₂(DMAP){Si(p-Tol)₃} (2) with BPh ₃. The precursor, complex 2, was readily prepared by reaction of the (DMAP)(methyl)molybdenum complex Cp*Mo(CO)₂(DMAP)Me (3) with tri-p-tolylsilane (HSi(p-Tol)₃) through methane elimination. Study on the reactivity of 1 toward DMAP and nitrile revealed that complex 1 serves as a synthetic equivalent of the 16-electron silyl complex Cp*Mo(CO) ₂{Si(p-Tol)₃}. Thus, complex 1 reacted with DMAP quantitatively at room temperature to reproduce arylsilyl complex 2 through dissociation of the arene carbon atoms coordinated to molybdenum. Complex 1 also reacted with acetonitrile at room temperature to give the N-silyliminoacyl complex Cp*Mo(CO)₂{η²(C,N)-C(Me)-NSi(p-Tol) ₃} (4) exclusively via cleavage of the Mo-C(arene) bonds followed by insertion of nitrile into the Mo-Si bond.

Full text of DOI:10.1021/om400225j

Article
pubs.acs.org/Organometallics
Synthesis, Structure, and Reactions of a (η3‑α-
silabenzyl)molybdenum Complex: A Synthetic Equivalent of a
Coordinatively Unsaturated Silyl Complex
Takashi Komuro, Yuto Kanno, and Hiromi Tobita*
Department of Chemistry, Graduate School of Science, Tohoku University, Sendai 980-8578, Japan
S
* Supporting Information
ABSTRACT: The (η3-α-silabenzyl)molybdenum complex Cp*Mo-
(CO)2{η3(Si,C,C)-Si(p-Tol)3} (1; Cp* = η5-C5Me5, p-Tol = p-C6H4Me)
was synthesized in high yield by removal of 4-(dimethylamino)pyridine
(DMAP) from the (arylsilyl)(DMAP)molybdenum complex Cp*Mo-
(CO)2(DMAP){Si(p-Tol)3} (2) with BPh3. The precursor, complex 2,
was readily prepared by reaction of the (DMAP)(methyl)molybdenum
complex Cp*Mo(CO)2(DMAP)Me (3) with tri-p-tolylsilane (HSi(p-Tol)3)
through methane elimination. Study on the reactivity of 1 toward DMAP
and nitrile revealed that complex 1 serves as a synthetic equivalent of the 16-electron silyl complex Cp*Mo(CO)2{Si(p-Tol)3}.
Thus, complex 1 reacted with DMAP quantitatively at room temperature to reproduce arylsilyl complex 2 through dissociation of
the arene carbon atoms coordinated to molybdenum. Complex 1 also reacted with acetonitrile at room temperature to give the
N-silyliminoacyl complex Cp*Mo(CO)2{η2(C,N)-C(Me)NSi(p-Tol)3} (4) exclusively via cleavage of the Mo−C(arene)
bonds followed by insertion of nitrile into the Mo−Si bond.
toward nitrogen-containing molecules revealed that complexes
A serve as synthetic equivalents for both base-free silylene
complexes and coordinatively unsaturated silyl complexes
(Scheme 1).2 Thus, reactions of A with DMAP afforded
INTRODUCTION
■
Reactive transition-metal complexes with a metal−silicon bond
such as silylene complexes and coordinatively unsaturated silyl
complexes play important roles in the synthesis of organo-
silicon compounds.1 These complexes are presumed to be key
intermediates in metal-mediated transformations of organo-
silanes such as hydrosilylation of unsaturated organic molecules,
dehydrogenative silylation of organic molecules, dehydrogen-
ative coupling of hydrosilanes, scrambling of the substituents of
silanes, etc.1 Therefore, in order to elucidate detailed reaction
mechanisms for these transformations of organosilanes, basic
studies on the synthesis and reactions of reactive metal−silicon
complexes are indispensable.
Scheme 1. Reactions of (η3-α-silabenzyl)tungsten Complexes
A with DMAP and Acetonitrile
We have recently succeeded in the synthesis of a new type of
reactive metal−silicon complex, i.e., the (η3-α-silabenzyl)-
tungsten complexes Cp*W(CO)2{η3(Si,C,C)-Si(p-Tol)2R}
(A; R = Me, p-Tol), which adopt an unusual η3 coordination
of one silicon and two aromatic carbon atoms to the tungsten
center.2 These complexes have been demonstrated to be key
intermediates in the interconversion between DMAP-stabilized
(aryl)(silylene)tungsten complexes Cp*W(CO)2(p-Tol){
Si(p-Tol)R·DMAP} (B) and (arylsilyl)(DMAP)tungsten com-
plexes Cp*W(CO)2(DMAP){Si(p-Tol)2R} (C) through 1,2-
migration of an aryl group (eq 1).2,3 A study on the reactivity
Received: March 16, 2013
Published: April 23, 2013
© 2013 American Chemical Society
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(aryl)(silylene·DMAP)tungsten complexes B via cleavage of the
coordinated Si−C(aryl) bond, whereas A (R = p-Tol) reacted
with acetonitrile to give the η2-N-silyliminoacyl complex
Cp*W(CO)2{η2(C,N)-C(Me)NSi(p-Tol)3} (D) through
dissociation of the coordinated aryl group followed by insertion
of nitrile into the W−Si bond.
In this work, we synthesized the molybdenum analogue of
η3-α-silabenzyl complex A (R = p-Tol), i.e., Cp*Mo-
(CO)2{η3(Si,C,C)-Si(p-Tol)3} (1). Although the covalent
radius of molybdenum is about the same as that of tungsten
absorption bands due to the symmetric and antisymmetric CO
vibrations at 1905 and 1807 cm−1, respectively, where the
intensity of the former absorption is slightly stronger than that
of the latter absorption. These observations indicate that the
CO ligands in 3 were located at mutually cis positions on the
metal center. This geometry is the same as that of the tungsten
analogue E.3b
Synthesis of (arylsilyl)(DMAP)molybdenum Complex
2. (DMAP)(methyl)molybdenum complex 3 reacted with tri-p-
tolylsilane (HSi(p-Tol)3) in toluene at room temperature to
give the (DMAP)(silyl)molybdenum complex Cp*Mo-
(CO)2(DMAP){Si(p-Tol)3} (2) as yellow crystals in 69%
isolated yield (eq 3). A possible mechanism for the formation
according to the recent reports by Pyykko et al. (single-bond
̈
covalent radii, Mo = 1.38 Å, W = 1.37 Å; double-bond covalent
radii, Mo = 1.21 Å, W = 1.20 Å),4 molybdenum uses much
smaller d orbitals (4d orbitals) for the construction of
coordinate bonds with ligands in comparison with those of
tungsten (5d orbitals). Therefore, it is expected that the
structure and reactivity of molybdenum complex 1 are fairly
different from those of the tungsten analogue A (R = p-Tol)
because of the significant change of orbital interaction between
the metal center and the η3-α-silabenzyl ligand.5 This
speculation turned out to be consistent with the results of
this work: in contrast with the reactions of tungsten complexes
A depicted in Scheme 1,2 we found that both reactions of
molybdenum complex 1 with DMAP and acetonitrile favor
initial formation of silyl complexes through cleavage of the
Mo−C(arene) bonds. Thus, 1 predominantly serves as a
synthetic equivalent of the 16-electron silyl complex Cp*Mo-
(CO)2{Si(p-Tol)3}.
of 2 involves (1) dissociation of DMAP in 3 followed by Si−H
oxidative addition of HSi(p-Tol)3, (2) reductive elimination of
methane from the resulting (hydrido)(methyl)(silyl)molybde-
num complex F, and (3) coordination of DMAP to the metal
center (eq 3).
RESULTS AND DISCUSSION
■
Synthesis and Characterization of (DMAP)(methyl)
molybdenum Complex 3. The (DMAP)(methyl)molybde-
num complex Cp*Mo(CO)2(DMAP)Me (3), which is the
precursor of (DMAP)(silyl) complex 2 in this research (vide
infra), was synthesized by a method analogous to that for the
synthesis of the tungsten analogue Cp*W(CO)2(DMAP)Me
(E).3b Thus, 3 was obtained in 62% yield by treatment of the
acetonitirile(methyl)molybdenum complex Cp*Mo-
(CO)2(NCMe)Me,5a prepared as the main product by
photoirradiation of Cp*Mo(CO)3Me in acetonitrile, with
DMAP in toluene at room temperature (eq 2).
This reaction of molybdenum complex 3 sharply contrasts
with that of the tungsten analogue E with HSi(p-Tol)3, which
leads to the (aryl)(DMAP·silylene)tungsten complex Cp*W-
(CO)2(p-Tol){Si(p-Tol)2·DMAP} (B; R = p-Tol) through
1,2-migration of a p-tolyl group from the silicon atom to the
metal center.3b Silylene complex B (R = p-Tol) is thermally
unstable and is converted into the (arylsilyl)(DMAP)tungsten
complex Cp*W(CO)2(DMAP){Si(p-Tol)3} (C; R = p-Tol) on
heating at 40 °C in toluene through 1,2-aryl migration from
tungsten to silicon.3b Therefore, silylene complex B (R = p-
Tol) and silyl complex C (R = p-Tol) can be regarded as the
kinetically controlled product and the thermodynamically
controlled product, respectively, of the reaction of E with
1
HSi(p-Tol)3. On the other hand, in the H NMR monitored
reaction of molybdenum complex 3 with HSi(p-Tol)3 (1:2
molar ratio) in C6D6 at room temperature, we did not observe
any signals assignable to the (aryl)(silylene)molybdenum
complex Cp*Mo(CO)2(p-Tol){Si(p-Tol)2·DMAP} (G) in
the reaction mixture but observed weak signals of η3-α-
silabenzyl complex 1 (vide infra) as a transient species. This
difference in reactivity on a change of the central metal implies
that the stability of the hypothetical aryl(silylene)molybdenum
complex G is considerably lower than that of the tungsten
analogue B (R = p-Tol). This is attributable to less effective π-
back-donation from the metal center to the silylene silicon
atom in molybdenum complex G caused by the smaller orbital
overlap between a 4d orbital of molybdenum and the vacant 3p
orbital of the silylene silicon atom in comparison with that in
the tungsten analogue B (R = p-Tol).
Complex 3 was thermally unstable at room temperature in
C6D6 solution (t1/2 = ca. 76 h at 25 °C) but was characterized
1
by NMR and IR spectroscopy. The H NMR spectrum of 3
shows a signal assignable to the methyl ligand at 0.54 ppm. This
chemical shift is similar to that of the tungsten analogue E (0.52
ppm).3b Two signals attributable to the carbonyl ligands were
observed at 254.2 and 269.7 ppm in the 13C{1H} NMR
spectrum of 3. The IR spectrum of 3 in C6D6 shows two
Characterization of (arylsilyl)(DMAP)molybdenum
Complex 2. Single-crystal X-ray analysis revealed that 2
adopts a four-legged piano-stool geometry composed of η5-
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C5Me5, silyl, DMAP, and two carbonyl ligands (Figure 1 and
Table 1). The silyl ligand and the DMAP ligand are located at
molybdenum complexes with a SiR3 (R3 = Me3, Et3, MePh2)
ligand (27.0−35.2 ppm).8,9 The 29Si signal is also shifted more
downfield than that of the tungsten analogue C (R = p-Tol)
(24.3 ppm)3b by 10.4 ppm. The latter trend is similar to that
observed for the molybdenum and tungsten complexes of
Cp2M(H)(SiMe3) (Cp = η5-C5H5, M = Mo (27.0 ppm) and W
(0.5 ppm)).8
The IR spectrum of 2 exhibits two νCO bands at 1892
(symmetric) and 1805 cm−1 (antisymmetric). The intensity of
the latter band is stronger than that of the former, which is
consistent with the mutual trans arrangement of the two
carbonyl ligands in 2. Also, these absorption bands are shifted
to higher wavenumbers in comparison with each of the
corresponding bands of the tungsten analogue C (R = p-Tol)
(1874 and 1786 cm−1, respectively),3b which implies the weaker
back-donation from the metal center to the CO ligands in 2 in
comparison with that in C (R = p-Tol).
Synthesis of (η3-α-silabenzyl)molybdenum Complex
1. Reaction of 2 with BPh3, an effective Lewis acid for removal
of a pyridine molecule,10 in toluene at room temperature led to
formation of the η3-α-silabenzyl complex Cp*Mo-
(CO)2{η3(Si,C,C)-Si(p-Tol)3} (1) and the adduct DMAP·BPh3
(eq 4). Complex 1 was isolated in 83% yield as reddish purple
Figure 1. ORTEP drawing of Cp*Mo(CO)2(DMAP){Si(p-Tol)3}
(2). Thermal ellipsoids are drawn at the 50% probability level, and all
hydrogen atoms are omitted for clarity.
Table 1. Selected Bond Distances (Å) and Angles (deg) in
Cp*Mo(CO)2(DMAP){Si(p-Tol)3} (2)
Mo−Si
Mo−C1
2.6218(10)
1.965(4)
Mo−N1
Mo−C2
2.255(3)
1.933(4)
Si−Mo−N1
Si−Mo−C2
N1−Mo−C2
133.20(8)
70.68(10)
83.83(13)
Si−Mo−C1
N1−Mo−C1
C1−Mo−C2
70.03(10)
79.65(13)
104.23(15)
crystals. This reaction is considered to proceed through initial
dissociation of DMAP, which was readily trapped with BPh3 to
form the adduct DMAP·BPh3, followed by π coordination of
one of the p-tolyl groups on silicon to the coordinatively
unsaturated molybdenum center.
In contrast with the reaction in eq 4, the formation of
tungsten η3-α-silabenzyl complexes A by the reaction of arylsilyl
complexes C with BPh3 requires photoirradiation.2 This is
possibly because the coordination of DMAP to tungsten in C is
stronger than that of molybdenum complex 2.
mutually trans positions. This molecular structure is similar to
that of the tungsten analogue Cp*W(CO)2(DMAP){Si(p-
Tol)3} (C, R = p-Tol).3b
The Mo−Si bond distance (2.6218(10) Å) is relatively long
in the range of normal Mo−Si single-bond distances
(2.480(2)−2.6815(13) Å).6 This elongation is attributable to
the steric repulsion between the Cp*Mo(CO)2(DMAP)
fragment and the Si(p-Tol)3 ligand in 2: some interatomic
distances between methyl carbon atoms in the C5Me5 ligand
and aromatic carbon atoms in two p-tolyl groups, i.e.,
C10···C37 (3.493(6) Å), C11···C37 (3.604(7) Å), C17···C36
(3.621(7) Å), and C18···C36 (3.595(7) Å), are shorter than the
sum (3.7 Å) of the van der Waals radius of the methyl group
(2.0 Å) and the half-thickness of the plane of an aromatic ring
(1.7 Å).7
Crystal Structure of (η3-α-silabenzyl)molybdenum
Complex 1. The molecular structure of 1, bearing a η3-α-
silabenzyl ligand, was unequivocally determined by a single-
crystal X-ray analysis (Figure 2 and Table 2). Similarly to the
tungsten analogue A (R = p-Tol),2 the η3-α-silabenzyl ligand in
1 takes an exo geometry. The Mo−Si distance (2.5172(8) Å) is
typical of single bonds and is comparable with those for (η3-
silapropargyl)molybdenum complexes Cp*Mo-
(CO)2{η3(Si,C,C)-Ph2SiCCR} (R = t-Bu, 2.5249(10) Å; R =
i-Pr, 2.5221(9) Å) with a related η3(Si,C,C)-coordinated
organosilicon moiety.5a The Mo−C3 and Mo−C4 distances
in 1 (2.452(3) and 2.646(3) Å, respectively) are longer than
the corresponding Mo−Cβ and Mo−Cγ distances for the (η3-
benzyl)molybdenum complex CpMo(CO)2{η3(C,C,C)-CH2(p-
Tol)} (Mo−Cβ = 2.364(5) Å, Mo−Cγ = 2.480(6) Å),11 but
similarly long distances have been reported for a molybdenum
complex with a η3-PPh3 ligand, [CpMo(CO)2{η3(P,C,C)-
PPh3}](BAr′4) (Mo−Cβ = 2.566(9) Å, Mo−Cγ = 2.645(9)
Å; Ar′ = 3,5-bis(trifluoromethyl)phenyl).12 The elongation of
the Mo−C distances for 1 is attributable to the steric repulsion
between the η3-α-silabenzyl ligand and the Cp*Mo(CO)2
moiety, especially that between one methyl group (C30) of
the Cp* ligand and ring carbons of the silabenzyl ligand. Thus,
1
The H NMR spectrum of 2 in C6D6 at room temperature
shows two sets of signals assignable to p-tolyl groups on silicon
with a 1:2 intensity ratio, indicating the existence of hindered
rotation around the Mo−Si bond on the NMR time scale. This
hindered rotation in 2 is attributable to the steric repulsion
between the Cp*Mo(CO)2(DMAP) fragment and the Si(p-
Tol)3 ligand (vide supra). A similar hindered rotation around a
Mo−Si bond has been reported by Berry et al. for the
molybdenum complexes bearing a bulky Si(t-Bu)2H ligand: (η5-
C5H4R)2Mo(H){Si(t-Bu)2H} (R = H, Me).8 In the 13C{1H}
NMR spectrum of 2, two carbonyl ligands show only one signal
at 249.1 ppm because of the mutual trans arrangement of them.
The 29Si{1H} NMR spectrum of 2 shows a signal at 34.7 ppm,
which is shifted more downfield than that of HSi(p-Tol)3
(−17.9 ppm) and is comparable with those of typical (silyl)
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Si−C single-bond distances (1.86−1.91 Å)13 and coordinated
SiC double-bond distances in η2-silene complexes (1.78(2)−
1.810(6) Å).14 The sum of the bond angles around the silicon
atom (340.6(2)°) is similar to that of the tungsten analogue A
(R = p-Tol) (341.8(3)°)2 and lies midway between the
tetrahedral (329°) and trigonal (360°) valence angles, implying
some contribution of sp2 hybridization at the silicon atom.
The Mo−Si and Mo−C3 bond distances (2.5172(8) and
2.452(3) Å, respectively) are comparable to the corresponding
distances for the tungsten analogue A (R = p-Tol) (2.5257(12)
and 2.416(4) Å, respectively), whereas the Mo−C4 bond
distance (2.646(3) Å) is longer than the corresponding distance
for A (R = p-Tol; 2.564(4) Å) by ca. 0.08 Å.2 This implies that
the coordination of the aromatic carbon atom at the γ position
in 1 is weaker than that for tungsten complex A (R = p-Tol).
This bond weakening is attributable to a smaller contribution of
the (aryl)(η2-silene)molybdenum canonical form I (Scheme 2)
for 1 in comparison with the corresponding canonical form for
A (R = p-Tol). In accord with the aforementioned observation,
it is expected that the η2-coordinated arene moiety in
molybdenum complex 1 is more labile than that in tungsten
complexes A. This is consistent with the result that reactions of
1 with DMAP and nitrile exclusively proceed through
dissociation of the η2-arene moiety (vide infra).
Figure 2. ORTEP drawing of Cp*Mo(CO)2{η3(Si,C,C)-Si(p-Tol)3}
(1). Thermal ellipsoids are drawn at the 50% probability level, and all
hydrogen atoms are omitted for clarity.
Table 2. Selected Bond Distances (Å) and Angles (deg) in
Cp*Mo(CO)2{η3(Si,C,C)-Si(p-Tol)3} (1)
Mo−Si
Mo−C2
Mo−C4
Si−C10
C3−C4
C4−C5
C6−C7
Si−Mo−C1
Si−Mo−C3
C1−Mo−C2
Mo−Si−C3
Mo−Si−C17
C3−Si−C17
2.5172(8)
1.944(3)
2.646(3)
1.868(3)
1.424(4)
1.417(4)
1.422(4)
64.53(9)
43.68(7)
83.62(14)
66.28(9)
125.44(10)
113.69(13)
Mo−C1
Mo−C3
Si−C3
Si−C17
C3−C8
1.924(3)
2.452(3)
1.850(3)
1.873(3)
1.435(4)
1.373(4)
1.359(4)
109.11(10)
64.70(7)
32.16(9)
119.06(10)
119.00(14)
107.87(13)
Spectroscopic Data for (η3-α-silabenzyl)molybdenum
Complex 1. The NMR spectroscopic analysis for 1 also
demonstrates a molecular structure bearing the η3-α-silabenzyl
ligand. The 1H NMR spectrum of 1 at room temperature shows
two sets of two doublet signals assignable to ortho and meta
aromatic protons of the p-tolyl groups with a 1:2 intensity ratio
at 7.02 (ortho), 6.52 ppm (meta) and 7.84 (ortho), 7.04 ppm
(meta).15 The first two signals are shifted upfield in comparison
with the last two signals and with those of HSi(p-Tol)3 (7.61
and 7.03 ppm). This upfield shift is attributable to the η2
coordination of the aryl ring to the metal. A similar upfield shift
C5−C6
C7−C8
Si−Mo−C2
Si−Mo−C4
C3−Mo−C4
Mo−Si−C10
C3−Si−C10
C10−Si−C17
1
of H NMR signals of aromatic protons was also observed in
the tungsten analogue A (R = p-Tol): 6.73 (ortho), 6.44 (meta)
ppm for the protons of the coordinated aromatic ring and 7.80
(ortho), 7.03 (meta) ppm for the protons of the uncoordinated
aromatic rings.2
the C6···C30, C7···C30, and C8···C30 interatomic distances
(3.540(4), 3.339(4), and 3.585(5) Å) are much shorter than
the sum of the van der Waals radii of the methyl group and the
aromatic ring plane.7 The C−C bonds in the aryl ring other
than the coordinated C3−C4 bond alternate, i.e., C3−C8 =
1.435(4) Å, C4−C5 = 1.417(4) Å, C5−C6 = 1.373(4) Å, C6−
C7 = 1.422(4) Å, and C7−C8 = 1.359(4) Å, indicating the
existence of bond localization in the aromatic ring. Similar bond
alternations have been observed for the tungsten analogue A (R
= p-Tol)2 and for the (η3-benzyl)molybdenum complex
CpMo(CO)2{η3(C,C,C)-CH2(p-Tol)}.11
The structure of complex 1 can be drawn as a resonance of
two canonical forms, i.e., (η2-arene)(silyl)molybdenum form H
and (aryl)(η2-silene)molybdenum form I (Scheme 2), on the
basis of the following structural features. The Si−C3 bond
distance (1.850(3) Å) is slightly shorter than the other two Si−
C bonds (1.868(3) and 1.873(3) Å) and lies between typical
The 13C{1H} NMR spectrum of 1 at room temperature
exhibits a signal at 79.6 ppm, which is assignable to the silicon-
bound ipso carbon in the coordinated p-tolyl group of the
silabenzyl ligand. This chemical shift is similar to that of the
corresponding 13C signals for tungsten complexes A (70.2 (R =
Me) and 69.6 ppm (R = p-Tol)).2 In the same 13C NMR
spectrum of 1, a single broad signal attributable to the ortho
carbon atoms in the coordinated p-tolyl group was observed at
124.7 ppm, whereas in the low-temperature 13C NMR
spectrum of 1 in toluene-d8 at −90 °C (vide infra) the ortho
carbon signal appears as two separated broad resonances at
101.6 and 140.7 ppm. The former signal is shifted considerably
upfield in comparison with the latter signal and also with the
signal assignable to the ortho carbon atoms of the
uncoordinated p-tolyl groups for 1 (136.4 ppm at −90 °C),
indicating the coordination of an ortho carbon atom in the
silabenzyl ligand to the metal center. Similar upfield-shifted
resonances of coordinated ipso and ortho carbons have been
reported for the η3-PPh3 complex [CpMo(CO)2{η3(P,C,C)-
PPh3}](BAr′4) (81.5 (ipso) and 90.0 ppm (ortho)).12 There-
fore, the aforementioned 13C NMR data for 1 support the η2
coordination of a C−C unsaturated bond in the p-tolyl group of
the η3-α-silabenzyl ligand.
Scheme 2. Two Possible Canonical Forms of (η3-α-
silabenzyl)molybdenum Complex 1
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The 29Si{1H} NMR spectrum of 1 at room temperature
exhibits a signal at 15.8 ppm, which is shifted downfield in
comparison with those of tungsten complexes A (8.8 (R = Me)
and 9.5 ppm (R = p-Tol)).2 This trend is similar to that
observed for the 29Si NMR data of silyl complex 2 (vide supra).
The chemical shift of the 29Si signal of 1 is between that of silyl
complex 2 (34.7 ppm) and those of typical η2-silene complexes
(from −20.77 to 6.14 ppm),14,16 indicating some contribution
of the (aryl)(η2-silene)molybdenum canonical form I in 1
(Scheme 2). Similar 29Si NMR chemical shifts have also been
reported for related (η3-1-silaallyl)tungsten complexes Cp*W-
(CO)2{η3(Si,C,C)-Me2SiCHCR2} (13.8 (R = H) and 8.8 ppm
(R = Me)).17
The IR spectrum of 1 shows νCO,sym and νCO,asym bands at
1873 and 1807 cm−1, respectively, with nearly the same
intensity. This supports the mutual cis arrangement of the two
carbonyl ligands in 1. The wavenumbers of these bands are
almost identical with the corresponding wavenumbers for the
tungsten analogue A (R = p-Tol) (1873 and 1805 cm−1,
respectively),2 indicating that the degree of π back-donation
from the metal to the CO ligands in 1 is comparable with that
for A (R = p-Tol). This is in contrast with the previously
described trend of CO stretching frequencies between silyl
complexes 2 and C (R = p-Tol), where the degree of the
corresponding π back-donation of 2 is weaker than that for
tungsten complex C (R = p-Tol).
These NMR spectroscopic observations indicate that the
dynamic behavior of 1 entails exchange between each of two
ortho positions and two meta positions in the η3-α-silabenzyl
ligand and also exchange between two uncoordinated p-tolyl
groups on silicon and between two CO ligands. Although these
data are insufficient to clarify the mechanism of the dynamic
behavior of 1, we suggest that a possible mechanism of the
dynamic behavior involves ring slippage of the coordinated p-
tolyl group on the Cp*Mo(CO)2 moiety without rotation
around the coordinated Si−C bond (see Scheme S1 in the
Supporting Information). The same mechanism has been
proposed for tungsten complexes A.2
Reactions of (η3-α-silabenzyl)molybdenum Complex 1
with DMAP and Acetonitrile. To compare the reactivity of
molybdenum complex 1 with that of tungsten complexes A
(Scheme 1),2 we examined reactions of 1 with nitrogen-
containing organic molecules (DMAP and acetonitrile),
revealing that 1 can serve as a synthetic equivalent of the 16-
electron silyl complex Cp*Mo(CO)2{Si(p-Tol)3}. Thus, treat-
ment of complex 1 with DMAP in toluene at room temperature
produced silyl complex 2 in 99% yield through dissociation of
the coordinated aromatic carbons of the silabenzyl ligand in 1
(Scheme 3). This result indicates that the reaction in eq 4 is
Scheme 3. Reactions of (η3-α-silabenzyl)molybdenum
Complex 1 with DMAP and Acetonitrile
Dynamic Behavior of (η3-α-silabenzyl)molybdenum
Complex 1 in Solution. An NMR spectroscopic analysis of 1
in solution demonstrates the existence of some dynamic
behaviors. As mentioned above, in the 1H NMR spectrum of 1
in C6D6 at room temperature, each set of two ortho and two
meta protons of the η3-α-silabenzyl ligand and also two
uncoordinated p-tolyl groups appears equivalently on the NMR
time scale. A similar spectroscopic feature was observed for the
tungsten analogue A (R = p-Tol).2 The 13C{1H} NMR
spectrum of 1 in C6D6 at room temperature also shows only
one set of signals assignable to the ortho and meta carbons in
the silabenzyl ligand at 124.7 and 125.6 ppm, respectively,
where the former signal is broadened (Δν1/2 = 8 Hz).15 In the
same spectrum, a single 13C signal assignable to CO ligands of 1
was observed at 234.6 ppm, indicating that the two carbonyl
ligands also exchange quickly on the NMR time scale.
Furthermore, low-temperature 1H and 13C{1H} NMR
spectroscopic measurements for 1 in toluene-d8 revealed that
the dynamic behavior was very fast (see Figures S1−S5 in the
Supporting Information). When a toluene-d8 solution of 1 was
cooled, the 1H NMR spectra (700 MHz) showed broadening of
the four ArH signals of the coordinated and uncoordinated p-
tolyl substituents and the methyl signal of the latter
substituents. Especially, the ortho ArH signal of the silabenzyl
ligand (6.93 ppm at room temperature) broadened most
significantly and finally disappeared at −70 °C. The 13C{1H}
NMR spectrum (176 MHz) of 1 in toluene-d8 at room
temperature exhibited only one signal for the two CO ligands at
234.4 ppm and a broad signal assignable to the ortho carbon of
the η3-α-silabenzyl ligand at 124.2 ppm. At −30 °C, the CO
signal broadened, and the ortho carbon signal apparently
disappeared because of significant broadening. Finally, the
13C{1H} NMR spectrum of 1 at −90 °C showed two sets of
two separated broad resonances attributable to the ortho
carbons and CO signals at 101.6, 140.7 ppm and at 229.4, 239.9
ppm, respectively.
reversible. In contrast, the reaction of (η3-α-silabenzyl)tungsten
complexes A with DMAP gave DMAP-stabilized (aryl)-
(silylene)tungsten complexes B as the kinetically controlled
products almost quantitatively (Scheme 1).2 As has been
previously described, this difference of reactivity on a change of
the metal center is attributable to the lower stability of
(aryl)(silylene)molybdenum complex G in comparison with
that of the tungsten analogue B (R = p-Tol).
Complex 1 smoothly reacted with excess acetonitrile (2
equiv) in toluene at room temperature to give the η2-N-
silyliminoacyl complex Cp*Mo(CO)2{η2(C,N)-C(Me)NSi-
(p-Tol)3} (4) in 86% yield (Scheme 3). This reaction pattern is
the same as that of the tungsten analogue A (R = p-Tol), which
reacts with acetonitrile to give the corresponding η2-N-
silyliminoacyl complex D (Scheme 1).2
The X-ray crystal structure analysis of 4 (Figure 3 and Table
3) revealed that 4 adopts a pseudo three-legged piano-stool
geometry composed of η5-C5Me5, η2-N-silyliminoacyl, and two
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Scheme 4. Two Possible Canonical Forms of (η2-N-
silyliminoacyl)molybdenum Complex 4
in complex D, possibly due to the stronger π back-donation
from the tungsten center to the iminoacyl carbon atom. The
29Si{1H} NMR spectrum of 4 shows a signal at −15.9 ppm, a
chemical shift is close to that of the tungsten analogue D
(−17.9 ppm),19 indicating that the Si(p-Tol)3 group is bound
to the iminoacyl nitrogen.
A plausible formation mechanism of 4 involves (1)
substitution of the coordinated aromatic carbons in 1 with
acetonitrile to generate (arylsilyl)(nitrile)molybdenum complex
L and (2) insertion of the coordinated CN bond of nitrile
into the Mo−Si bond in L (Scheme 3). The related migratory
insertion of nitrile into a metal−silyl bond to produce an η2-N-
silyliminoacyl complex has been reported by Bergman,
Brookhart, et al. for rhodium complexes21 and by Nakazawa
et al. for an iron complex.22
Figure 3. ORTEP drawing of Cp*Mo(CO)2{η2(C,N)-C(Me)
NSi(p-Tol)3} (4). Thermal ellipsoids are drawn at the 50% probability
level, and all hydrogen atoms are omitted for clarity.
Table 3. Selected Bond Distances (Å) and Angles (deg) in
Cp*Mo(CO)2{η2(C,N)-C(Me)NSi(p-Tol)3} (4)
Mo−N
Mo−C2
Si−N
N−Mo−C1
N−Mo−C3
C1−Mo−C3
Mo−N−Si
Mo−C3−N
N−C3−C4
2.2242(13)
1.9660(19)
1.7556(14)
110.69(6)
33.62(6)
Mo−C1
Mo−C3
N−C3
N−Mo−C2
C1−Mo−C2
C2−Mo−C3
Mo−N−C3
Si−N−C3
1.9373(19)
2.0906(16)
1.254(2)
CONCLUSION
■
The (η3-α-silabenzyl)molybdenum complex Cp*Mo-
(CO)2{η3(Si,C,C)-Si(p-Tol)3} (1) was synthesized by the
DMAP-removal reaction from (arylsilyl)(DMAP)molybdenum
complex Cp*Mo(CO)2(DMAP){Si(p-Tol)3} (2) with BPh3.
The η3(Si,C,C) coordination of the silabenzyl ligand in 1 was
confirmed by an X-ray crystallographic analysis, where the
coordination of the aromatic carbon at the γ position of the
silabenzyl ligand is considerably weaker than that of the
tungsten analogue A (R = p-Tol).2 The reactions of 1 with
DMAP and acetonitrile exclusively proceeded through
dissociation of the η2-coordinated aromatic carbons in the
silabenzyl ligand. The result of the reaction of 1 with DMAP
that reproduces 2 is very different from that of (η3-α-silabenzyl)
tungsten complexes A, which gives DMAP-stabilized (aryl)-
(silylene)tungsten complexes Cp*W(CO)2(p-Tol){Si(p-
Tol)R·DMAP} (B) as the kinetically controlled products.2,3
These results indicate that η3-α-silabenzyl complex 1 solely
serves as a synthetic equivalent of the 16-electron triarylsilyl
complex Cp*Mo(CO)2{Si(p-Tol)3}. The molybdenum ana-
logue of complex B (R = p-Tol), i.e., the hypothetical silylene
complex G, appears to be less stable than B possibly because
the d orbitals of molybdenum (4d orbitals) that are used mainly
for bonding with silicon are smaller in size than those of
tungsten (5d orbitals), which causes less effective π back-
donation from the metal center to the silylene silicon in G.
Further studies on the reactivity of 1 toward other organic
molecules and on its application to catalytic reactions are in
progress.
93.58(6)
77.35(8)
88.69(7)
114.07(7)
67.34(9)
152.00(8)
79.05(10)
130.59(16)
140.10(12)
carbonyl ligands. The Mo−C3, Mo−N, and N−C3 distances
(2.0906(16), 2.2242(13), and 1.254(2) Å, respectively) of the
Mo−N−C three-membered ring in 4 are similar to the
corresponding distances of the (N-silyliminoacyl)molybdenum
complex Cp*Mo[κ4Si,H,N,C-xantsil(H){NC(t-Bu)}](CO)
(Mo−C = 2.073(3) Å, Mo−N = 2.236(2) Å, and N−C =
1.262(4) Å).18
The 13C{1H} NMR spectrum of 4 shows a signal assignable
to the iminoacyl carbon at 236.4 ppm, and this chemical shift is
almost identical with that for the tungsten analogue D (235.2
ppm).19 However, the signal is shifted considerably downfield
in comparison with those (189.7−199.2 ppm) of iminoacyl
carbons in the (N-alkyliminoacyl)molybdenum complexes
Cp′Mo(CO)2{η2(C,N)-C(R′)NR″} (Cp′ = η5-C5H5, η5-
C5H4Me, η5-C5Me5; R′ = Me, Et; R″ = Me, Et, i-Pr, t-Bu).20
The IR spectrum of 4 shows a CN stretching absorption
band at 1647 cm−1. This band is shifted to lower wavenumbers
in comparison with those of the (N-alkyliminoacyl)-
molybdenum complexes Cp′Mo(CO)2{η2(C,N)-C(R′)
NR″} (1693−1732 cm−1).20 These results indicate the
existence of a significant contribution of the (amido)(carbene)-
molybdenum canonical form K in addition to the (alkyl)-
(imino)molybdenum canonical form J in the bonding mode of
4 illustrated in Scheme 4. This trend is characteristic of N-
silyliminoacyl complexes.19 On the other hand, the CN
stretching absorption band of 4 is shifted slightly to a higher
wavenumber region in comparison with that of the tungsten
analogue D (1620 cm−1).19 This is attributable to a stronger
contribution of the (amido)(carbene)tungsten canonical form
EXPERIMENTAL SECTION
General Procedures. All manipulations were carried out under
dry argon or nitrogen in a glovebox or using a high-vacuum line and
standard Schlenk techniques.
■
Materials. Benzene-d6, toluene, toluene-d8, hexane, and acetonitrile
were dried over CaH2, vacuum-transferred, and stored under argon
over 3 Å (for acetonitrile) or 4 Å (for other solvents) molecular sieves
in a glovebox. BPh3 (Strem Chemicals) was used as received.
Cp*Mo(CO)3Me was prepared according to the literature method.23
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Physical Measurements. 1H, 13C{1H}, and 29Si{1H} NMR
spectra were recorded on a Bruker AVANCE-300, Bruker AVANCE
III 400, JEOL JNM-ECA 700, or JEOL JNM-ECA 600 Fourier
transform spectrometer. Chemical shifts are reported in parts per
pyH). 13C{1H} NMR (75.5 MHz, C6D6): δ 10.9 (C5Me5), 21.37, 21.44
(C6H4Me), 38.0 (NMe2), 103.1 (C5Me5), 107.9 (pyC), 135.6, 136.1,
136.8, 137.2, 138.5, 141.3, 144.2 (ArC), 153.7, 157.6 (pyC), 249.1
(CO). One aromatic carbon signal could not be observed due to the
overlap with the signal of benzene-d6. 29Si{1H} NMR (59.6 MHz,
C6D6): δ 34.7. IR (KBr pellet, cm−1): 1892 (w, νCO,sym), 1805 (s,
νCO,asym). HRMS (FD): m/z calcd for [C40H46N2O2SiMo]+ 712.2377,
found 712.2379. Anal. Calcd for C40H46N2O2SiMo: C, 67.59; H, 6.52;
N, 3.94. Found: C, 67.61; H, 6.56; N, 3.98.
million. Coupling constants (J) and line widths at half-height (Δν1/2
)
are given in Hz. 29Si{1H} NMR measurements were performed using
the DEPT pulse sequence. The residual proton (C6D5H, 7.15 ppm;
C6D5CD2H, 2.09 ppm) and the carbon resonances (C6D6, 128.0 ppm;
C6D5CD3, 20.4 ppm) of deuterated solvents were used as internal
1
references for H and 13C resonances, respectively. Aromatic protons
Synthesis of Cp*Mo(CO)2{η3(Si,C,C)-Si(p-Tol)3} (1). Cp*Mo-
(CO)2(DMAP){Si(p-Tol)3} (2; 150 mg, 0.211 mmol) and BPh3 (46
mg, 0.19 mmol) were dissolved in toluene (5 mL), and the solution
was stirred at room temperature for 18 h. The dark red reaction
mixture was evaporated to dryness under vacuum. To the residue was
added hexane (5 mL), and then the mixture was stirred at room
temperature for 8 h. DMAP·BPh3 was precipitated as a colorless solid.
The mixture was filtered to remove DMAP·BPh3, and the reddish
purple filtrate was concentrated under vacuum. When the concen-
trated solution was cooled to −35 °C, reddish purple crystals were
precipitated. Removing the mother liquor, washing the crystals with
hexane, and drying under vacuum afforded reddish purple crystals of 1
in 83% yield based on BPh3 (93 mg, 0.16 mmol). Mp: 160 °C dec. 1H
NMR (400 MHz, C6D6): δ 1.52 (s, 15H, Cp*), 1.98 (s, 3H,
C6H4Me(silabenzyl)), 2.06 (s, 6H, C6H4Me), 6.52 (d, 3J = 7.8 Hz, 2H,
meta-ArH(silabenzyl)), 7.02 (d, 3J = 7.8 Hz, 2H, ortho-ArH-
and carbons are abbreviated as ArH and ArC, respectively. Pyridyl
protons and carbons are abbreviated as pyH and pyC, respectively.
The 1H signals of pyridyl protons of DMAP in Cp*Mo-
(CO)2(DMAP){Si(p-Tol)3} (2) and Cp*Mo(CO)2(DMAP)Me (3)
appeared as AA′XX′ multiplets, but coupling constants of the signals
could not be determined because the peak tops of each signal were
poorly resolved. 29Si{1H} NMR chemical shifts were referenced to
SiMe4 (0 ppm) as an external standard. The NMR data were collected
at room temperature unless indicated otherwise. Infrared spectra were
measured on a C6D6 solution placed between KBr plates in a liquid
cell or on a KBr pellet using a Horiba FT-730 or Horiba FT-720
spectrometer. The UV−visible spectrum of Cp*Mo(CO)2{η3(Si,C,C)-
Si(p-Tol)3} (1) was acquired on a Shimadzu MultiSpec-1500
spectrometer at room temperature. High-resolution mass spectra
(HRMS) and mass spectra were recorded on a Hitachi M-2500S
spectrometer operating in the electron impact (EI) mode or on a
JEOL JMS-T100GCV spectrometer operating in the field desorption
(FD) mode. Measurements of some NMR and mass spectra and
elemental analyses were performed at the Research and Analytical
Center for Giant Molecules, Tohoku University.
3
3
(silabenzyl)), 7.04 (d, J = 8.2 Hz, 4H, meta-ArH), 7.84 (d, J = 8.2
Hz, 4H, ortho-ArH).15 1H NMR (700 MHz, 203 K, toluene-d8): δ 1.51
(s, 15H, Cp*), 1.99 (s, 3H, C6H4Me(silabenzyl)), 2.05 (br s, Δν1/2
=
9.6 Hz, 6H, C6H4Me), 6.41 (br, Δν1/2 = 19 Hz, 2H, meta-
3
ArH(silabenzyl)), 6.98 (br, 4H, meta-ArH), 7.85 (br d, J = 7.0 Hz,
Synthesis of Cp*Mo(CO)2(DMAP)Me (3). The title compound
was synthesized by modifying a literature method for the synthesis of
the tungsten analogue Cp*W(CO)2(DMAP)Me (E).3b An acetonitrile
(7 mL) solution of Cp*Mo(CO)3Me (86 mg, 0.26 mmol) in a Pyrex
tube (20 mm o.d.) with a Teflon vacuum valve was irradiated for 30
min with a 450 W medium-pressure Hg lamp immersed in a water
bath (ca. 5 °C). During the photoreaction, the mixture was degassed
after 10 and 20 min of irradiation to remove generated carbon
monoxide by a conventional freeze−pump−thaw cycle on a vacuum
line. The reaction mixture containing Cp*Mo(CO)2(NCMe)Me5a as
the main product was transferred into a Schlenk tube and evaporated
to dryness under reduced pressure. The residue was dissolved in
toluene (2 mL), and the solution was added to a solid of DMAP (35
mg, 0.29 mmol). The mixture was stirred at room temperature for 70
min. The reaction mixture was concentrated to three-fourths volume,
and then hexane (2 mL) was slowly layered on it. After crystals were
precipitated, the mother liquor was removed by a syringe, and the
crystals were washed with hexane (1 mL × 3). Purple crystals of 3 (68
4H, ortho-ArH). The ortho-ArH(silabenzyl) signal is too broad to be
observed, indicating that the signal is nearly decoalesced. The line
width at half-height of the broad signal at 6.98 ppm could not be
determined due to partial overlap with a signal of the residual protons
of toluene-d8. 13C{1H} NMR (75.5 MHz, C6D6): δ 10.0 (C5Me5), 21.4
(C6H4Me), 21.8 (C6H4Me(silabenzyl)), 79.6 (ipso-ArC(silabenzyl)),
100.7 (C5Me5), 124.7 (br, Δν1/2 = 8 Hz, ortho-ArC(silabenzyl)), 125.6
(meta-ArC(silabenzyl)), 129.1 (meta-ArC), 130.9 (ipso-ArC), 136.6
(ortho-ArC), 139.7 (para-ArC), 142.1 (para-ArC(silabenzyl)), 234.6
(CO).15 13C{1H} NMR (176 MHz, 183 K, toluene-d8) δ 9.7 (C5Me5),
21.3 (br, Δν1/2 = 11 Hz, C6H4Me), 21.7 (C6H4Me(silabenzyl)), 77.2
(ipso-ArC(silabenzyl)), 100.1 (C5Me5), 101.6 (br, Δν1/2 = ca. 190 Hz,
ortho-ArC(silabenzyl)), 129.5 (ipso-ArC), 136.4 (br, Δν1/2 = 41 Hz,
ortho-ArC), 139.7 (para-ArC), 140.7 (br, ortho-ArC(silabenzyl)),
141.3 (para-ArC(silabenzyl)), 229.4 (br, Δν1/2 = ca. 190 Hz, CO),
239.9 (br, Δν1/2 = ca. 180 Hz, CO). Other aromatic carbon signals
could not be observed due to the overlap with signals of toluene-d8.
The line width at half-height of the broad signal at 140.7 ppm could
not be determined because of partial overlap with the aromatic carbon
signals at 139.7 and 141.3 ppm. 29Si{1H} NMR (59.6 MHz, C6D6): δ
15.8. IR (KBr pellet, cm−1): 1873 (s, νCO,sym), 1807 (s, νCO,asym). UV−
vis (λmax, nm (ε, M−1 cm−1), hexane): 250 sh (25000), 329 (7200),
377 sh (4800). EI-MS (70 eV): m/z 590 (M+, 33), 534 (M+ − 2 CO,
100). HRMS (EI): m/z calcd for [C33H36O2SiMo]+ 590.1533, found
590.1539. Anal. Calcd for C33H36O2SiMo: C, 67.33; H, 6.16. Found:
C, 67.36; H, 6.22.
1
mg, 0.16 mmol) were obtained in 62% yield. Mp: 133 °C dec. H
NMR (300 MHz, C6D6): δ 0.54 (s, 3H, MoMe), 1.74 (s, 15H, Cp*),
1.97 (s, 6H, NMe2), 5.50−5.56 (m, 2H, pyH), 7.88−7.94 (m, 2H,
pyH). 13C{1H} NMR (75.5 MHz, C6D6): δ 5.4 (MoMe), 10.6
(C5Me5), 38.0 (NMe2), 102.9 (C5Me5), 107.2, 153.4, 154.5 (pyC),
254.2, 269.7 (CO). IR (C6D6, cm−1): 1905 (s, νCO,sym), 1807 (m,
νCO,asym). HRMS (FD): m/z calcd for [C20H28N2O2Mo]+ 426.1199,
found 426.1197. Anal. Calcd for C20H28N2O2Mo: C, 56.60; H, 6.65;
N, 6.60. Found: C, 56.39; H, 6.53; N, 6.37.
Reaction of (η3-α-silabenzyl)molybdenum Complex 1 with
DMAP. Complex 1 (30 mg, 0.051 mmol) and DMAP (9 mg, 0.07
mmol) were dissolved in toluene (3 mL), and the solution was stirred
at room temperature for 50 min. The orange reaction mixture was
evaporated under vacuum. The residue was washed with hexane (1 mL
× 3) and dried under vacuum to give a yellow powder of
Cp*Mo(CO)2(DMAP){Si(p-Tol)3} (2) in 99% yield (36 mg, 0.051
mmol). The product was identified as 2 by comparison of its 1H NMR
and IR spectra with those of a sample synthesized from 3 in the
previous section. The purity of the obtained 2 was confirmed by
elemental analysis. Anal. Calcd for C40H46N2O2SiMo: C, 67.59; H,
6.52; N: 3.94. Found: C, 67.97; H, 6.60; N: 4.16.
Synthesis of Cp*Mo(CO)2(DMAP){Si(p-Tol)3} (2). Cp*Mo-
(CO)2(DMAP)Me (3; 34 mg, 0.080 mmol) and HSi(p-Tol)3 (48
mg, 0.16 mmol) were dissolved in toluene (4 mL), and the solution
was stirred for 90 min at room temperature. After concentration of the
reaction mixture, hexane (3 mL) was slowly layered on the
concentrated solution. On standing at room temperature, yellow
crystals precipitated. Removing the mother liquor, washing the crystals
with hexane (1 mL × 2), and drying under vacuum afforded yellow
1
crystals of 2 in 69% yield (39 mg, 0.055 mmol). Mp: 136 °C dec. H
NMR (300 MHz, C6D6): δ 1.64 (s, 15H, Cp*), 1.97 (s, 6H, NMe2),
2.02 (s, 3H, C6H4Me), 2.23 (s, 6H, C6H4Me × 2), 5.44−5.51 (m, 2H,
3
3
Reaction of (η3-α-silabenzyl)molybdenum Complex 1 with
Acetonitrile. To a solid sample of complex 1 (27 mg, 0.046 mmol)
pyH), 7.04 (d, JHH = 7.8 Hz, 2H, ArH), 7.21 (d, JHH = 7.8 Hz, 4H,
3
ArH), 8.16 (d, JHH = 7.8 Hz, 4H, ArH), 8.19−8.27 (m, 4H, ArH +
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was added acetonitrile (5 μL, d = 0.782 g mL−1, 0.09 mmol), and the
mixture was dissolved in toluene (3 mL). The solution was stirred at
room temperature for 70 min. The resulting red solution was
evaporated under vacuum. The residue (red oil) was washed with
hexane (1 mL × 3) and dried under vacuum to give a red powder of
Cp*Mo(CO)2{η2(C,N)-C(Me)NSi(p-Tol)3} (4) in 86% yield (25
We are grateful to Mr. Shinichiro Yoshida for his help with the
variable-temperature NMR spectroscopic measurements of 1,
to Mr. Hiroyuki Momma for MS measurements, and to Ms.
Satomi Ahiko for elemental analyses in the Research and
Analytical Center for Giant Molecules, Tohoku University. This
work was also performed under the Cooperative Research
Program of “Network Joint Research Center for Materials and
Devices”. We acknowledge Prof. Hideo Nagashima (Kyusyu
University) for his warm support in this program. We are also
1
mg, 0.040 mmol). Mp: 147 °C dec. H NMR (300 MHz, C6D6): δ
1.74 (s, 15H, Cp*), 2.06 (s, 9H, C6H4Me), 2.50 (s, 3H, NCMe),
3
3
7.05 (d, JHH = 7.8 Hz, 6H, ArH), 7.63 (d, JHH = 7.8 Hz, 6H, ArH).
13C{1H} NMR (75.5 MHz, C6D6): δ 11.1 (C5Me5), 21.4 (C6H4Me),
25.1 (NCMe), 105.2 (C5Me5), 129.3, 130.0, 136.2, 140.6 (ArC),
236.4 (NCMe). The CO signals could not be observed probably
because of dynamic behavior involving inversion of the chiral metal
center in 4, which led to exchange between the CO ligands.19b
29Si{1H} NMR (59.6 MHz, C6D6): δ −15.9. IR (KBr pellet, cm−1):
1917 (s, νCO,sym), 1817 (s, νCO,asym), 1647 (w, νCN). EI-MS (70 eV):
m/z 631 (M+, 6), 534 (M+ − 2 CO − MeCN, 100). Anal. Calcd for
C35H39NO2SiMo: C, 66.76; H, 6.24; N, 2.22. Found: C, 67.14; H,
6.40; N, 2.39.
1
grateful to Ms. Keiko Ideta for her help with the H−13C
1
HMQC and H−13C HMBC NMR spectroscopic measure-
ments of 1 in the Evaluation Center of Materials Properties and
Function, Institute for Material Chemistry and Engineering,
Kyusyu University.
REFERENCES
■
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S., Rappoport, Z., Eds.; Wiley: New York, 1989; Chapter 24. (b) Eisen,
M. S. In The Chemistry of Organic Silicon Compounds; Rappoport, Z.,
Apeloig, Y., Eds.; Wiley: New York, 1998; Vol. 2, Chapter 35.
(c) Corey, J. Y. Chem. Rev. 2011, 111, 863. (d) Corey, J. Y.; Braddock-
Wilking, J. Chem. Rev. 1999, 99, 175. (e) Braunstein, P.; Knorr, M. J.
Organomet. Chem. 1995, 500, 21. (f) Sharma, H. K.; Pannell, K. H.
Chem. Rev. 1995, 95, 1351. (g) Ogino, H. Chem. Rec. 2002, 2, 291.
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X-ray Crystal Structure Determination. Selected crystallo-
graphic data for 1, 2, and 4 are listed in Table S1 (see the Supporting
Information). X-ray quality single crystals were obtained from hexane
at −35 °C (for 1 and 4·0.5(hexane) as reddish purple platelike crystals
and red platelike crystals, respectively) and from THF at −35 °C (for
2·1.5THF as yellow prismatic crystals). In the case of 4·0.5hexane, a
large single crystal was cut to a suitable size (0.35 × 0.35 × 0.33 mm3).
Intensity data for the analysis were collected on a Rigaku RAXIS-
RAPID imaging plate diffractometer with graphite-monochromated
Mo Kα radiation (λ = 0.71069 Å) under a cold nitrogen stream (T =
150 K (for 1 and 4) or 223 K (for 2)). Numerical absorption
corrections were applied to the data. The structures were solved by the
Patterson method using the DIRDIF-99 or DIRDIF-2008 program24
and refined by full matrix least-squares techniques on all F2 data with
SHELXL-97.25 Except for the carbon and oxygen atoms of the THF
molecules, the solvent of crystallization, in 2·1.5THF, anisotropic
refinements were applied to all non-hydrogen atoms. The carbon and
oxygen atoms of these THF molecules were isotropically refined, and
no hydrogen atoms on the carbon atoms were included. In the case of
2·1.5THF, a THF molecule was disordered over two sites related to
each other by an inversion center. The hydrogen atom on the
coordinated ortho aromatic carbon C4 in 1 was found on the difference
Fourier map and was refined isotropically. Other hydrogen atoms were
placed at calculated positions. All calculations were carried out using
Yadokari-XG 2009.26 CCDC reference numbers: 932045 (1), 932046
(2), and 932047 (4). Crystallographic data are available in the
Supporting Information as a CIF file.
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ASSOCIATED CONTENT
* Supporting Information
■
S
A scheme illustrating a possible mechanism of the dynamic
behavior of 1 (Scheme S1), figures depicting variable-
temperature 1H and 13C{1H} NMR spectra of 1 (Figures
S1−S5), a table giving selected X-ray crystallographic data for 1,
2, and 4 (Table S1), and a CIF file giving X-ray crystallographic
data for 1, 2, and 4. This material is available free of charge via
the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*E-mail for H.T.: tobita@m.tohoku.ac.jp.
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by Grants-in-Aid for Scientific
Research (Nos. 22350024 and 23750053) from the Ministry of
Education, Culture, Sports, Science and Technology of Japan.
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Organometallics
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1
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