Synthesis, structure, and reactivities of the five-coordinate molybdenum(0) mono(acetylene) complex [Mo(HC triple bond CH)(dppe)2]

Article abstract of DOI:10.1021/om0007623

The excess amounts of alkyne ligands were reacted with the five-coordinated molybdenum monoacetylene complexes. The synthesis and molecular structure of the complex were analyzed using the X-ray diffraction techniques. The reactivities of the molybdenum c

Full text of DOI:10.1021/om0007623

Organometallics 2001, 20, 13-15
13
Syn th esis, Str u ctu r e, a n d Rea ctivities of th e
F ive-Coor d in a te Molybd en u m (0) Mon o(a cetylen e)
Com p lex [Mo(HCtCH)(d p p e)2]1
Hiroshige Ishino, Shigeki Kuwata, Youichi Ishii,* and Masanobu Hidai*,†
Department of Chemistry and Biotechnology, Graduate School of Engineering,
The University of Tokyo, Hongo, Bunkyo-ku, Tokyo 113-8656, J apan
Received September 4, 2000
Summary: The reaction of trans-[Mo(N2)2(dppe)2] (1;
dppe ) Ph2PCH2CH2PPh2) with excess amounts of
HCtCLi‚en (en ) H2NCH2CH2NH2) afforded the five-
coordinate Mo(0) mono(acetylene) complex [Mo-
(HCtCH)(dppe)2] (2). Treatment of 2 with 2 mol of
[Cp2Fe][BF4] (Cp ) η5-C5H5) in THF gave the Mo(II)
acetylene complex trans-[MoF(HCtCH)(dppe)2][BF4],
while that in THF-MeCN followed by anion metathesis
with NaOTf (OTf ) OSO2CF3) led to the formation of
the metallacyclopropene complex [Mo(η3-CHCHPPh2-
CH2CH2PPh2-C,C′,P)(MeCN)2(dppe)][OTf]2.
also provides a unique coordination site to bind and
activate various substrates such as dmf (dmf ) N,N-
dimethylformamide),15 nitriles,16,17 imines,18,19 and iso-
cyanides20 upon dissociation of the dinitrogen ligands.
Now we have found that the five-coordinate Mo(0) (d6)
mono(acetylene) complex [Mo(HCtCH)(dppe)2] (2) can
be derived from complex 1. Here we describe its syn-
thesis and the structure of complex 2 as well as some
chemical properties.
When complex 1 was treated with excess amounts of
HCtCLi‚en (10 equiv; en ) H2NCH2CH2NH2) in THF
at 50 °C for 2 h, a very dark red solution was formed,
from which black crystals of complex 2 were obtained
in 32% yield after recrystallization (eq 1).21 At room
The chemical bonding and transformation of four-
electron-donating alkyne ligands at early-transition-
metal centers have been a subject of extensive research
activity.2 A wide variety of six-coordinate d4 metal (most
typically Mo(II) and W(II)) complexes with four-electron-
donating alkyne ligands have been synthesized and
found to exhibit intriguing reactivities.2,3 Four-coordi-
nate d6 complexes of the types ML(alkyne)3
ML2(alkyne)2
and
4-8
9-11
(L ) monodentate ligand) have also
been reported. In contrast, five-coordinate d6 mono-
(alkyne) complexes with the general formula ML4-
(alkyne) have been surprisingly scarce; only a few
examples, including the chromium complexes [Cr(CO)2-
{P(OMe)3}2(RCtCPh)] (R ) Ph, H),12 have been de-
scribed in the literature. Our extensive studies on low-
valent molybdenum and tungsten complexes have
demonstrated that the Mo(0) center in the complex
trans-[Mo(N2)2(dppe)2] (1; dppe ) Ph2PCH2CH2PPh2) is
not only effective for the activation of dinitrogen13,14 but
temperature, a longer reaction time (10 h) was neces-
sary to allow the reaction to go to completion (29%
yield). The 13C{1H} NMR signal for the acetylene
carbons in 2 appears as a triplet (J CP ) 15.9 Hz) at δ
171.0, which falls within the region reported for the sp
carbons of four-electron-donating alkyne ligands.3 The
31P{1H} NMR spectrum displays two apparent triplets
(J PP ) 24 Hz), indicating a trigonal-bipyramidal geom-
etry with the acetylene ligand occupying an equatorial
position. The resonance for the acetylenic hydrogens
appears as a doublet at δ 10.18 due to the coupling to
one phosphorus atom (3J PH ) 12.4 Hz) and splits into a
broad triplet (3J PH ) 6.0 Hz) upon warming at 90 °C.22
On the basis of these observations, the acetylene ligand
is deduced to take the orientation perpendicular to the
equatorial plane.
† Present address: Department of Materials Science and Technology,
Faculty of Industrial Science and Technology, Science University of
Tokyo, Noda, Chiba 278-8510, J apan. E-mail: hidai@mail.rs.noda.
sut.ac.jp.
(1) Preparation and Properties of Molybdenum and Tungsten Dini-
trogen Complexes. 71. Part 70: Ishino, H.; Nagano, T.; Kuwata, S.;
Yokobayashi, Y.; Ishii, Y.; Hidai, M.; Mizobe, Y. Organometallics, in
press.
(2) Baker, P. K. Adv. Organomet. Chem. 1996, 40, 45-115.
(3) Templeton, J . L. Adv. Organomet. Chem. 1989, 29, 1-100.
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1998, 569, 21-27.
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467, 233.
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(8) Tate, D. P.; Augl, J . M. J . Am. Chem. Soc. 1963, 85, 2174-2175.
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5012-5014.
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(14) Hidai, M.; Ishii, Y. Bull. Chem. Soc. J pn. 1996, 69, 819-831.
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10.1021/om0007623 CCC: $20.00 © 2001 American Chemical Society
Publication on Web 12/08/2000

14 Organometallics, Vol. 20, No. 1, 2001
Communications
cordance with the electron-rich nature of the molybde-
num center in 2.3 EHMO calculations for the model
complex [Mo(HCtCH)(PH3)4] revealed that the con-
former with the acetylene ligand perpendicular to the
equatorial plane is more energetically favored than that
with the acetylene ligand lying in the equatorial plane,
since the former conformation is best suited for both the
π-donation from the acetylene π orbital and the π-back-
donation from the metal to the acetylene π * orbital.12,24
||
The formation of complex 2 is of special interest. It
has been known that HCtCLi is in equilibrium with
acetylene and LiCtCLi, and addition of a diamine
ligand stabilizes the monolithiated species.25 Acetylene
generated in a low concentration is considered to be
required for the production of complex 2 from 1. In fact,
PhCtCLi as well as internal acetylenes such as bis-
(trimethylsilyl)acetylene and diphenylacetylene failed
to react with 1. In contrast, it has been reported that
the reaction of complex 1 with acetylene gas results in
the formation of polyacetylene as the only identified
product, while that with other 1-alkynes gives rise to
the formation of alkynyl complexes via oxidative addi-
tion of the C-H bonds or bis(alkyne) complexes which
are regarded as the precursors for the C-H activa-
tion.26,27 No molybdenum complex with a four-electron-
donating alkyne ligand was obtained in these reactions.
We have also found that the related coordinatively
unsaturated species [Mo(CO)(dppe)2], generated in situ
from [Mo(CO)(dmf)(dppe)2], readily undergoes oxidative
addition of 1-alkynes.28 Considering the high ability of
the electron-rich Mo(0) fragment {Mo(dppe)2} to activate
CH bonds, the formation of complex 2 from 1 as the
thermally stable product is quite exceptional.
In the cyclic voltammogram, complex 2 showed an
irreversible oxidation wave at -0.57 V (vs [Cp2Fe]/
[Cp2Fe]+; Cp ) η5-C5H5) in THF, but no reduction wave
was observed. Thus, we have investigated the reaction
of complex 2 with the oxidant [Cp2Fe][BF4]. The reaction
with 2 equiv of [Cp2Fe][BF4] in THF led to the oxidation
of the molybdenum center to give the Mo(II) acetylene
complex trans-[MoF(HCtCH)(dppe)2][BF4] (3) in 82%
yield,21 in which the acetylene ligand behaves as a four-
electron donor.29 Use of 1 equiv of [Cp2Fe][BF4] resulted
in the formation of a mixture of 2 and 3. The reaction
of 2 with excess amounts of HBF4‚OEt2 also gave 3 in
73% yield along with H2 gas (57% yield).
F igu r e 1. Molecular structure of 2‚C7H8. Thermal el-
lipsoids are shown at the 50% probability level. Selected
bond distances (Å) and angles (deg): Mo(1)-P(1), 2.443-
(2); Mo(1)-P(2), 2.406(1); Mo(1)-P(3), 2.411(2); Mo(1)-
P(4), 2.446(2); Mo(1)-C(1), 2.061(5); Mo(1)-C(2), 2.071(5);
C(1)-C(2), 1.265(7); P(1)-Mo(1)-P(4), 161.28(5); P(2)-Mo-
(1)-P(3), 103.25(5); C(1)-Mo(1)-C(2), 35.7(2); Mo(1)-
C(1)-C(2), 72.6(4); Mo(1)-C(2)-C(1), 71.7(3).
The molecular structure of 2‚C7H8 was unambigu-
ously determined by an X-ray diffraction study (Figure
1).23 In full agreement with the spectral data, complex
2 adopts a distorted-trigonal-bipyramidal structure with
the acetylene ligand at an equatorial position. The Mo-
(1)-C(1)-C(2) and Mo(1)-P(2)-P(3) planes are nearly
perpendicular to each other (dihedral angle 86.7°), and
the Mo(1)-C(1) (2.061(5) Å) and Mo(1)-C(2) (2.071(5)
Å) bond distances are on the high end of the range of
Mo-C(four-electron-donating alkyne) distances in ac-
(21) Preparation of 2: 1 (94.9 mg, 0.100 mmol) and HCtCLi‚en (92.1
mg, 1.00 mmol) were dissolved in THF (3 mL), and the mixture was
stirred at 50 °C for 2 h. During this period, the color of the solution
changed from orange to black. The resulting solution was filtered, and
layering the THF solution with hexane afforded a black solid of 2 (29.4
mg, 32% yield). 1H NMR (toluene-d8): δ 2.21-2.53 (m, 8H, CH2 of
dppe), 6.35-7.90 (m, 40H, Ph of dppe), 10.18 (d, 2H, J PH ) 12.4 Hz,
C2H2). 13C{1H} NMR (C4D8O): δ 171.0 (t, J CP ) 15.9 Hz, C2H2). 31P-
{1H} NMR (C4D8O): δ 95.8 (t, J PP ) 24 Hz), 99.1 (t, J PP ) 24 Hz).
Anal. Calcd for C54H50MoP4: C, 70.59; H, 5.48. Found: C, 70.38; H,
5.56. Data for the complex 3‚1.5CH2Cl2 are as follows. 1H NMR
(CDCl3): δ 2.77, 2.92 (br, 4H each, CH2 of dppe), 6.75-7.34 (m, 40H,
Ph of dppe), 11.49 (s, 2H, C2H2). 13C{1H} NMR (CDCl3): δ 225.5 (br,
C2H2). 31P{1H} NMR (CDCl3): δ 43.3 (d, J PF ) 34 Hz). Anal. Calcd for
C55.5H53BCl3F5MoP4: C, 57.86; H, 4.64. Found: C, 57.98; H, 4.47.
Preparation of 4: to a solution of 2 (139.4 mg, 0.152 mmol) in THF (3
mL) and MeCN (0.3 mL) was added [Cp2Fe][BF4] (82.9 mg, 0.304 mmol)
at room temperature, and the mixture was stirred for 3 h at room
temperature. During this period, the color of the solution changed from
black to dark green. Na[OTf] (261.5 mg, 1.52 mmol) was added to the
resultant solution, and the mixture was stirred for a further 2 h at
room temperature. Then the mixture was evaporated under vacuum,
and the residual green solid was extracted with CH2Cl2. Further
recrystallization from MeCN/ether deposited green crystals of 4 (161.9
mg, 82% yield). 1H NMR (CD2Cl2): δ 0.70 (s, 3H, MeCN), 2.26 (m, 1H,
CHP), 2.46 (s, 3H, MeCN), 1.71-3.73 (m, 8H, CH2 of dppe), 6.75-7.34
(m, 40H, Ph of dppe), 11.36 (br d, 1H, CH, J PH ) 29.2 Hz). 31P{1H}
NMR (CD2Cl2): δ 14.7 (s), 38.1 (m), 59.4 (m), 72.4 (m). IR (KBr, cm-1):
2313 (w, CN), 2282 (w, CN). Anal. Calcd for C60H56F6MoN2O6P4S2: C,
55.47; H, 4.34; N, 2.16. Found: C, 55.26; H, 4.36; N, 2.21.
(22) At room temperature, each acetylene proton is considered to
have observable coupling only to the axial P atom located roughly trans
to the acetylene CH, while in the high-temperature limit the two axial
P atoms exhibit coupling to the acetylene proton with the averaged J
values.
(23) Crystals of 2‚C7H8 suitable for X-ray diffraction study were
obtained by recrystallization from C7H8/hexane. Crystal data for 2‚
C7H8: C61H58MoP4, Mr ) 1010.96, triclinic, space group P1h, a ) 12.100-
(3) Å, b ) 13.665(3) Å, c ) 17.400(5) Å, R ) 72.38(2)°, â ) 75.45(2)°, γ
) 74.75(2)°, V ) 2599(1) Å3, Z ) 2, Dcalcd ) 1.292 g cm-3, F(000) )
1052.00, µ(Mo KR) ) 4.13 cm-1, R ) 0.046, Rw ) 0.038 for 5279
reflections with I > 3σ(I). The acetylenic hydrogen atoms (H(1), H(2))
were found in the final difference Fourier map.
(24) Wink, D. J . Organometallics 1991, 10, 442-447.
(25) Midland, M. M. J . Org. Chem. 1975, 40, 2250-2252.
(26) Hills, A.; Hughes, D. L.; Kashef, N.; Lemos, M. A. N. D. A.;
Pombeiro, A. J . L.; Richards, R. L. J . Chem. Soc., Dalton Trans. 1992,
1775-1782.
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104, 13-38.
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Soc. J pn. 1996, 69, 3305-3315.
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nomet. Chem. 1987, 333, C6-C10.

Communications
Organometallics, Vol. 20, No. 1, 2001 15
On the other hand, when the oxidation with [Cp2Fe]-
[BF4] was carried out in THF-MeCN, nucleophilic
attack of a phosphorus atom in dppe ligands on the
acetylenic carbon took place to give the novel metalla-
cyclopropene (η2-vinyl) complex [Mo(η3-CHCHPPh2CH2-
CH2PPh2-C,C′,P)(MeCN)2(dppe)][OTf]2 (4; OTf ) OSO2-
CF3) in 82% yield after anion metathesis with NaOTf
1
(eq 2).21 The H NMR spectrum of 4 exhibited a broad
doublet at δ 11.36 (J PH ) 29.2 Hz) as well as a multiplet
at δ 2.26 assignable to the metallacyclopropene moiety.
The molecular structure of 4 was established by X-ray
crystallography to confirm the formation of the η2-
vinyl-phosphine ligand (Figure 2).30 The cation has a
distorted-octahedral structure with the two MeCN
ligands in mutually cis positions. The Mo(1)-C(1)
(1.901(9) Å) and Mo(1)-C(2) (2.226(9) Å) distances
correspond to a ModC double bond and a Mo-C single
bond, respectively, and these parameters support the
metallacyclopropene structure.3 Although the formation
of metallacyclopropene ligands by direct nucleophilic
addition of phosphines and phophites to alkyne ligands
has been known for some group 6 alkyne complexes,31-33
the coupling reaction between a coordinated alkyne and
F igu r e 2. Molecular structure of the cationic part of 4.
Thermal ellipsoids are shown at the 50% probability level.
Selected bond distances (Å) and angles (deg): Mo(1)-P(2),
2.546(3); Mo(1)-P(3), 2.473(3); Mo(1)-P(4), 2.500(3); Mo-
(1)-N(1), 2.200(8); Mo(1)-N(2), 2.181(8); Mo(1)-C(1), 1.901-
(9); Mo(1)-C(2), 2.226(9); P(1)-C(2), 1.776(8); C(1)-C(2),
1.44(1); C(1)-Mo(1)-C(2), 39.8(3); Mo(1)-C(1)-C(2), 82.4-
(6); Mo(1)-C(2)-P(1), 123.3(4); Mo(1)-C(2)-C(1), 57.8(5);
P(1)-C(2)-C(1), 122.5(7).
a coordinated phosphine has not been described. Fur-
ther studies on the reactivities of 2 are now under way.
Ack n ow led gm en t. This work was supported by a
Grant-in-Aid for Specially Promoted Research (No.
09102004) from the Ministry of Education, Science,
Sports, and Culture of J apan.
(30) Crystal data for 4: C60H56F6MoN2O6P4S2, Mr ) 1299.06, mono-
clinic, space group P21/c, a ) 10.294(5) Å, b ) 23.357(7) Å, c ) 24.868-
(6) Å, â ) 95.82(4)°, V ) 5948(3) Å3, Z ) 4, Dcalcd ) 1.450 g cm-3, F(000)
) 2664.00, µ(Mo KR) ) 4.68 cm-1, R ) 0.067, Rw ) 0.068 for 6567
reflections with I > 3σ(I). The hydrogen atoms (H(1), H(2)) of the
metallacyclopropene ring were found in the final difference Fourier
map.
(31) Davidson, J . L.; Wilson, W. F. J . Organomet. Chem. 1983, 254,
C6-C10.
(32) Allen, S. R.; Beevor, R. G.; Green, M.; Norman, N. C.; Orpen,
A. G.; Williams, I. D. J . Chem. Soc., Dalton Trans. 1985, 435-450.
(33) Morrow, J . R.; Tonker, T. L.; Templeton, J . L. J . Am. Chem.
Soc. 1985, 107, 6956-6963.
Su p p or tin g In for m a tion Ava ila ble: Text giving full
experimental procedures and characterization data for the
reported complexes and tables giving details of data collection,
structure solution, and refinement, atomic coordinates, aniso-
tropic thermal parameters, and bond lengths and angles for
2‚C7H8 and 4. This material is available free of charge via the
Internet at http://pubs.acs.org.
OM0007623