Synthesis and reactivity of a molybdenum(IV) η4-butadiene complex

Article abstract of DOI:10.1021/om010508q

Molybdenum olefin complexes of the type [Mo(NPh)(η²-olefin)(o-(Me₃SiN)₂ C₆H₄)] (olefin = propene (1a), isobutene (1b)) reacted with butadiene gas, affording [Mo(NPh)(η⁴-H₂ C=CHCH=CH₂)(o-(Me₃SiN)₂C₆ H₄)] (2), a molybdenum η⁴-butadiene complex. The crystal structure of 2 is reported. Treatment of 2 with 1 equiv of 2-butyne afforded [Mo(NPh)(η⁴-CH₂CH=C(CH₃) C(CH₃)=CHCH₂) (o-(Me₃SiN)₂ C₆H₄)] (3) by ¹H NMR spectroscopy. Two intermediates arising during the progress of the reaction were trapped at low temperature and characterized as the isomeric η³-allyl metallacycles [syn-Mo(NPh)(C(CH₃)=C(CH₃) CH₂-CHCHCH₂)(o-(Me₃SiN)₂ C₆H₄)] and [anti-Mo(NPh)(C(CH₃)=C (CH₃)CH₂CHCHCH₂)(o-(Me₃ SiN)₂C₆H₄)] (4a,b, respectively) by NMR spectroscopy. Complex 3 was made independently by reaction of 1b with 1,2-dimethyl-1,4-cyclohexadiene, and the crystal structure of 3 is reported. Treatment of 2 with 2 equiv of 2-butyne afforded 1,2-dimethyl-1,4-cyclohexadiene as an organic product with concomitant formation of [Mo(NPh)(η²-alkyne)(o-(Me₃SiN)₂ C₆H₄)] (alkyne = 2-butyne; 5) by ¹H NMR spectroscopy. Reaction of 1b with 2-butyne also affords 5, and the X-ray structure of 5 is reported.

Full text of DOI:10.1021/om010508q

4378
Organometallics 2001, 20, 4378-4383
Syn th esis a n d Rea ctivity of a Molybd en u m (IV)
η4-Bu ta d ien e Com p lex
Thomas M. Cameron, Ion Ghiviriga, Khalil A. Abboud, and J ames M. Boncella*
Department of Chemistry and Center for Catalysis, University of Florida,
Gainesville, Florida 32611-7200
Received J une 13, 2001
Molybdenum olefin complexes of the type [Mo(NPh)(η2-olefin)(o-(Me3SiN)2C6H4)] (olefin
) propene (1a ), isobutene (1b)) reacted with butadiene gas, affording [Mo(NPh)(η4-H2Cd
CHCHdCH2)(o-(Me3SiN)2C6H4)] (2), a molybdenum η4-butadiene complex. The crystal
structure of 2 is reported. Treatment of 2 with 1 equiv of 2-butyne afforded [Mo(NPh)(η4-
CH2CHdC(CH3)C(CH3)dCHCH2)(o-(Me3SiN)2C6H4)] (3) by 1H NMR spectroscopy. Two
intermediates arising during the progress of the reaction were trapped at low temperature
and characterized as the isomeric η3-allyl metallacycles [syn-Mo(NPh)(C(CH3)dC(CH3)CH2-
CHCHCH2)(o-(Me3SiN)2C6H4)] and [anti-Mo(NPh)(C(CH3)dC(CH3)CH2CHCHCH2)(o-(Me3-
SiN)2C6H4)] (4a ,b, respectively) by NMR spectroscopy. Complex 3 was made independently
by reaction of 1b with 1,2-dimethyl-1,4-cyclohexadiene, and the crystal structure of 3 is
reported. Treatment of 2 with 2 equiv of 2-butyne afforded 1,2-dimethyl-1,4-cyclohexadiene
as an organic product with concomitant formation of [Mo(NPh)(η2-alkyne)(o-(Me3SiN)2C6H4)]
1
(alkyne ) 2-butyne; 5) by H NMR spectroscopy. Reaction of 1b with 2-butyne also affords
5, and the X-ray structure of 5 is reported.
Sch em e 1
In tr od u ction
Low-valent zirconium(0) butadiene complexes with
bidentate phosphine ligands were among the first well-
characterized examples of group 4 metal butadiene
complexes.1 The well-known, higher valent, base-free
group 4 metallocene transition-metal butadiene com-
plexes were independently synthesized by both Erker2
and Nakamura3 (Scheme 1). Among the striking struc-
tural characteristics of these complexes is the ability of
the butadiene fragment to coordinate to the metallocene
core in cis and trans modes and the dynamic envelope
shift isomerization process associated with the cis
coordination mode. The structure and bonding of the cis-
butadiene complex varies between the π2 and the σ2,π
designation depending on the substitution of the buta-
diene ligand and the identity of the metal. To date there
are several examples of structurally characterized group
4 and 5 cis-4 and trans-butadiene5 metal complexes, the
cisoid conformation being the most common.
Sch em e 2
The characteristic reactivity of group 4 and 5 buta-
diene complexes involves reactions with unsaturated
substrates such as carbonyl compounds, nitriles, and
alkynes.6 This reactivity is typified by Cp2Zr(butadiene),
where C-C coupling of the unsaturated substrate with
a terminal butadiene carbon results in metallacyclic
complexes (C-E; Scheme 2). In some cases the inter-
conversion of these metallacyclic compounds can be
followed experimentally. The metallacycles are gener-
ally stable, and hydrolysis protocols are required to free
useful acyclic organic products from the metal center.
In sharp contrast, later transition metals (groups 9 and
(1) (a) Datta, S.; Wreford, S. S.; Beatty, R. P.; McNeese, T. J . J . Am.
Chem. Soc. 1979, 101, 1053. (b) Beatty, R. P.; Datta, S.; Wreford, S. S.
Inorg. Chem. 1979, 18, 3139.
(2) Erker, G.; Wicher, J .; Engel, K.; Rosenfeldt, F.; Dietrich, W.;
Kruger, C. J . Am. Chem. Soc. 1980, 102, 6346.
(3) (a) Yasuda, H.; Kajihara, Y.; Mashima, K.; Lee, K.; Nakamura,
A. Chem. Lett. 1981, 519. (b) Yasuda, H.; Kajihara, Y.; Mashima, K.;
Lee, K.; Nakamura, A. Organometallics 1982, 1, 388.
(4) For a Cambridge Structural Database compilation of group 3,
4, and 5 η4-s-cis transition-metal butadiene complexes see: Dahlmann,
M.; Erker, G.; Frohlich, R.; Meyer, O. Organometallics 1999, 18, 4459
(supporting information).
(5) For a compilation of examples see: (a) Wang, L.-S.; Fettinger,
J . C.; Poli, R. J . Am. Chem. Soc. 1997, 119, 4453. (b) Mashima, K.;
Fukumoto, H.; Tani, K.; Haga, M.; Nakamura, A. Organometallics
1998, 17, 410 and references therein. (c) Strauch, H. C.; Erker, G.;
Frohlich, R. Organometallics 1998, 17, 5746.
(6) Selected examples: see Dahlmann, M.; Erker, G.; Frohlich, R.;
Meyer, O. Organometallics 1999, 18, 4459.
10.1021/om010508q CCC: $20.00 © 2001 American Chemical Society
Publication on Web 09/12/2001

A Molybdenum(IV) η4-Butadiene Complex
Organometallics, Vol. 20, No. 21, 2001 4379
Ta ble 1. Cr ysta l Da ta a n d Da ta Collection a n d Str u ctu r e Refin em en t Deta ils for 2, 3, a n d 5
2
3
5
empirical formula
fw
C
22H33MoN3Si2‚CH2Cl2
C26H39MoN3Si2
545.72
C22H33MoN3Si2
491.63
576.56
space group
a (Å)
b (Å)
P21/n
P21/c
P1h
11.9736(6)
15.8844(8)
14.5066(7)
16.031(1)
10.0300(8)
16.906(1)
10.7483(5)
10.8358(6)
12.3049(6)
77.788(1)
64.377(1)
77.812(1)
1251.2(1)
1.305
c (Å)
R (deg)
â (deg)
γ (deg)
Vc (Å3)
95.324(1)
93.351(1)
2747.2(2)
1.394
4
2713.7(4)
1.336
4
Dc (Mg m-3
)
Z
2
µ(Mo KR) (mm-1
final R indices
)
0.775
0.590
0.632
R1 ) 0.0307, wR2 ) 0.0755 (5170)
R1 ) 0.0408, wR2 ) 0.0815
R1 ) 0.0356, wR2 ) 0.0907 (5487)
R1 ) 0.0409, wR2 ) 0.0947
R1 ) 0.0275, wR2 ) 0.0719 (5120)
R1 ) 0.0304, wR2 ) 0.0742
R indices (all data)
10) catalyze the intermolecular 4 + 2 cycloaddition of
nonactivated substrates,7 while related transition-metal-
catalyzed cycloisomerization reactions constitute a rap-
idly developing area of research.7a,8
Reports concerning the synthesis and characterization
of group 6 transition metal butadiene complexes are not
as common as those of the earlier groups.9 Furthermore,
reactivity of these group 6 butadiene complexes differs
from the well-explored reaction chemistry associated
with the group 4 and 5 butadiene complexes and the
catalytic cycloisomerization reactions of the later met-
als.9
denum(IV) cis-butadiene complex, [Mo(NPh)(η4-H2Cd
CHCHdCH2)(o-(Me3SiN)2C6H4)] (2), is discussed below.
While structurally similar to other molybdenum buta-
diene complexes, the reactivity of 2 departs from these,
showing similarities to both early- and late-transition-
metal reactivity.
Resu lts a n d Discu ssion
Syn th esis of th e High -Va len t, Ba se-F r ee, η4-
Bu t a d ien e Com p lex [Mo(NP h )(η4-H 2CdCH CH d
CH2)(o-(Me3SiN)2C6H4)] (2). Treatment of a pentane
solution of [Mo(NPh)(η2-propene)(o-(Me3SiN)2C6H4)] (1a )
or [Mo(NPh)(η2-isobutene)(o-(Me3SiN)2C6H4)] (1b) with
molecular butadiene affords the η4-cis-butadiene com-
plex [Mo(NPh)(η4-H2CdCHCHdCH2)(o-(Me3SiN)2C6H4)]
(2) in good yield (eq 1). Decomposition of 2 occurs within
The synthesis, structural characterization, and reac-
tivity studies of a monomeric, diamagnetic molyb-
(7) (a) Hilt, G.; du Mesnil, F.-X. Tetrahedron Lett. 2000, 41, 6757.
(b) Paik, S.-J .; Son, S. U.; Chung, Y. K. Org. Lett. 1999, 13, 2045. (c)
Matsuda, I.; Shibata, M.; Sato, S.; Izumi, Y. Tetrahedron Lett. 1987,
3361. (d) tom Dieck, H.; Diercks, R. Angew. Chem., Int. Ed. Engl. 1983,
22, 778. (e) Mach, K.; Antropiusova´, H.; Petrusova´, L.; Turecek, F.;
Hanus, V. J . Organomet. Chem. 1985, 289, 331. (f) Murakami, M.;
Ubukata, M.; Itami, K.; Ito, Y. Angew. Chem., Int. Ed. Engl. 1998, 37,
16. (g) Murakami, M.; Itami, K.; Ito, Y. J . Am. Chem. Soc. 1997, 119,
7163.
(8) (a) Wender, P. A.; J enkins, T. E. J . Am. Chem. Soc. 1989, 111,
6432. (b) J olly, R. S.; Luedtke, G.; Sheehan, D.; Livinghouse, T. J . Am.
Chem. Soc. 1990, 112, 4965. (c) Wender, P. A.; Smith, T. E. J . Org.
Chem. 1996, 61, 824. (d) McKinstry, L.; Livinghouse, T. Tetrahedron
1994, 50, 6145. (e) O’Mahony, D. J . R.; Belanger, D. B.; Livinghouse,
T. Synlett 1998, 443. (f) Gilbertson, S. R.; Hoge, G. S. Tetrahedron Lett.
1998, 39, 2075. (g) Gilbertson, S. R.; Hoge, G. S.; Genov, D. G. J . Org.
Chem. 1998, 63, 10077. (h) Kumar, K.; J olly, R. S. Tetrahedron Lett.
1998, 39, 3047. For a review of transition-metal-mediated cycloaddition
reactions see: Lautens, M.; Klute, W.; Tam, W. Chem. Rev. 1996, 96,
49.
4 h at room temperature, affording an intractable
mixture as determined by 1H NMR spectroscopy. If kept
at -30 °C in the solid state, 2 does not decompose
appreciably over an 8 month period, as noted by 1H
NMR spectroscopy.
(9) Homoleptic Mo(d6) butadiene and derivative complexes (synthe-
sis and structure): (a) Skell, P. S.; Van Dam, E. M.; Silvon, M. P. J .
Am. Chem. Soc. 1974, 96, 626. (b) Skell, P. S.; McGlinchy, M. J . Angew.
Chem., Int. Ed. Engl. 1975, 14, 195. (c) Gausing, W.; Wilke, G. Angew.
Chem., Int. Ed. Engl. 1981, 20, 186. (d) Bogdanovic, B.; Bonnemann,
H.; Goddard, R.; Startsev, A.; Wallis, J . M. J . Organomet. Chem. 1986,
299, 347. (e) Yun, S. S.; Kang, S. K.; Suh, I.-H.; Choi, Y. D.; Chang, I.
S. Organometallics 1991, 10, 2509. Nonhomoleptic systems (synthesis
and reactivity): (f) Le Grognec, E.; Poli, R.; Richard, P. Organometallics
2000, 19, 3842. (g) Poli, R.; Wang, L.-S. Polyhedron 1998, 17, 3689.
(h) Wang, L.-S.; Fettinger, J . C.; Poli, R.; Meunier-Prest, R. Organo-
metallics 1998, 17, 2692. (i) Galindo, A.; Gutierrez, E.; Monge, A.;
Paneque, M.; Pastor, A.; Perez, P. J .; Rogers, R. D.; Carmona, E. J .
Chem. Soc., Dalton Trans. 1995, 23, 3801. (j) Kingsbury, K. B.; Carter,
J . D.; Mc-Elwee-White, L.; Ostrander, R. L.; Rheingold, A. L. Orga-
nometallics 1994, 13, 1635. (k) Vong, W.-J .; Peng, S.-M.; Lin, S.-H.;
Lin, W.-J .; Liu, R.-S. J . Am. Chem. Soc. 1991, 113, 573. (l) Benyunes,
S. A.; Binelli, A.; Green, M.; Grimshire, M. J . J . Chem. Soc., Dalton
Trans. 1991, 895. (m) Hunter, A. D.; Legzdins, P.; Einstein, F. W. B.;
Willis, A. C.; Bursten, B. E.; Gatter, M. G. J . Am. Chem. Soc. 1986,
108, 3843. (n) Davidson, J . L.; Davidson, K.; Lindsell, W. E.; Murrall,
N. W.; Welch, A. J . J . Chem. Soc., Dalton Trans. 1986, 1677. (o)
Brookhart, M.; Cox, K.; Cloke, F. G. N.; Green, J . C.; Green, M. L. H.;
Hare, M. P.; Bashkin, J .; Derome, A.; Grebenik, P. D. J . Chem. Soc.,
Dalton Trans. 1985, 423. (p) Hunter, A. D.; Legzdins, P.; Nurse, C. R.;
Einstein, F. W. B.; Willis, A. C. J . Am. Chem. Soc. 1985, 107, 1791.
An X-ray crystallographic study was carried out on a
single crystal of 2 grown from a -30 °C solution of
pentane/methylene chloride. The crystal data and de-
tails of the structure refinement are summarized in
Table 1. The butadiene complex crystallizes in a mono-
clinic unit cell with one molecule of methylene chloride.
The molecular structure of 2, accompanied by selected
bond lengths and angles, is shown in Figure 1. The
butadiene fragment clearly adopts a cis arrangement
when bound to the metal center. The metal to terminal
butadiene carbon atom distances of 2.254(3) Å (Mo-
C(19)) and 2.257(2) Å (Mo-C(22)) and the Mo-C(20)
and Mo-C(21) bond lengths of 2.336(2) and 2.355(2) Å,
respectively, support a π2,η4-butadiene bonding motif for
2. Other molybdenum butadiene complexes adopt simi-
lar bonding modes.9 Within the butadiene fragment the
similar C(19)-C(20), C(20)-C(21), and C(21)-C(22)

4380 Organometallics, Vol. 20, No. 21, 2001
Cameron et al.
Sch em e 3
agreement with the formulation of 2 as a π2-butadiene
complex. In light of these results regarding 2, the formal
oxidation state at the metal center is best represented
by Mo(IV).
Rea ctivity of th e Molybd en u m Bu ta d ien e Com -
p lex w ith 2-Bu tyn e. The reactivity of 2 with 1 or 2
equiv of 2-butyne was explored. Proton NMR spectra of
these reaction mixtures revealed the formation of two
intermediates that disappear within 30 min at room
temperature, giving rise to the final products, as out-
lined in Scheme 3. Reaction of 2 with 1 equiv of 2-butyne
afforded the molybdenum 2,3-dimethyl-1,3-cyclohexa-
diene complex 3 by 1H NMR spectroscopy, while reaction
with 2 equiv of 2-butyne gave 1,2-dimethyl-1,4-cyclo-
hexadiene and a molybdenum η2-alkyne complex (5) by
1H NMR spectroscopy (this alkyne complex has been
synthesized independently, vide infra). Complex 3 was
prepared independently by treatment of 1b with excess
1,2-dimethyl-1,4-cyclohexadiene (Scheme 3). In addition
the 2,3-dimethyl-1,3-cyclohexadiene ligand in 3 is not
displaced by 2-butyne at room temperature by 1H NMR
spectroscopy.
The identity of 3 was confirmed by X-ray structural
analysis (Figure 2) and NMR spectroscopy. The crystal
data and details of the structure refinement are sum-
marized in Table 1. In 3 the metal is bound to the 2,3-
dimethyl-1,3-cyclohexadiene in an η4 mode reminiscent
of 2. The C-C distances for C(21)-C(20), C(20)-C(19),
and C(19)-C(24) in 3 (1.418(3), 1.418(3), and 1.405(3)
Å respectively), as well as the metal-carbon bond
lengths, are similar to the corresponding distances in
2. The Mo-N(1) distance of 1.7686(18) Å is consistent
with a molybdenum-nitrogen triple bond interaction.10
The Mo-N(2) and N(3) amide distances of 2.0561(18)
and 2.0492(17) Å, respectively, are within the range
expected for Mo-N single bonds.10(e),(f),(g) Complex 3 is
stable in solution at room temperature for weeks. We
attribute the difference in stabilities between 2 and 3
in part to steric crowding around the metal center in 3.
F igu r e 1. Thermal ellipsoid plot of 2 (50% probability
thermal ellipsoids). Selected bond lengths (Å) and angles
(deg): Mo-N(1), 1.7517(19); Mo-N(2), 2.0556(17); Mo-
N(3), 2.0536(18); Mo-C(19), 2.254(3); Mo-C(20), 2.336(2);
Mo-C(21), 2.355(2); Mo-C(22), 2.257(2); C(19)-C(20),
1.401(4); C(20)-C(21), 1.397(4); C(21)-C(22), 1.405(4);
C(19)-C(20)-C(21), 124.4(2); C(20)-C(21)-C(22), 123.3(2).
bond lengths of 1.401(4), 1.397(4), and 1.405(4) Å,
respectively, also support a π2,η4-bonding mode for 2.
The Mo-N(1) length of 1.7517(19) Å is typical of a
Mo-N triple-bond interaction and is comparable to the
Mo-N(1) lengths in similar complexes.10
It is common practice to use the 1J (C-H) coupling
constants to determine the relative degree of sp2-sp3
hybridization of the coordinated diene carbons in metal
butadiene complexes. By determination of the relative
degree of hybridization of the diene carbon atoms, the
structure, in terms of butadiene bonding, can be as-
signed a position somewhere between the two extremes
π2 and σ2,π. Using Newton’s semiempirical rule,11 it is
possible to calculate the percent s character of carbon
atoms in the dienes and, hence, the hybridization. The
value of n for the carbons at diene termini reaches 2.8-
2.9 (132-128 Hz) when the molecule adopts a σ2,π type
structure, while the value is in the range of 2.1-2.3
(165-154 Hz) in the case of a π2 complex.12 We have
assigned the observed triplet in the 13C NMR spectrum
of 2 at 75.0 ppm to the terminal butadiene carbons, and
the observed 158 Hz coupling constant is in good
(10) (a) Dyer, P. W.; Gibson, V. C.; Howard, J . A. K.; Whittle, B.;
Wilson, C. Polyhedron 1995, 14, 103. (b) Dyer, P. W.; Gibson, V. C.;
Clegg, W. J . Chem. Soc., Dalton Trans. 1995, 3313. (c) Dyer, P. W.;
Gibson, V. C.; Howard, J . A. K.; Whittle, B.; Wilson, C. J . Chem. Soc.,
Chem. Commun. 1992, 1666. (d) Bryson, N.; Youinou, M. T.; Osborn,
J . A. Organometallics 1991, 10, 3389. (e) Cameron, T. M.; Ortiz, C.
G.; Abboud, K. A.; Boncella, J . M.; Baker, R. T.; Scott, B. L. Chem.
Commun. 2000, 573. (f) Ortiz, C. G.; Abboud, K. A.; Cameron, T. M.;
Boncella, J . M. Chem. Commun. 2001, 247. (g) Cameron, T. M.; Ortiz,
C. G.; Ghiviriga, I.; Abboud, K. A.; Boncella, J . M. Organometallics
2001, 20, 2032.
When 2 is treated with 1 equiv of 2-butyne at -20
°C, the accumulation of two metal-containing species is
observed by 1H NMR spectroscopy over 24 h. These
species are the fleeting intermediates observed during
this reaction at room temperature and are stable at -20
°C for extended periods of time (over 2 days). An array
of two-dimensional NMR spectroscopic techniques was
(11) Newton, M. D.; Schulmann, T. M.; Manus, M. M. J . Am. Chem.
Soc. 1974, 96, 17.
(12) Yamamoto, H.; Yasuda, H.; Tatsumi, K.; Lee, K.; Nakamura,
A.; Chen, J .; Kai, Y.; Kasai, N. Organometallics 1989, 8, 105.

A Molybdenum(IV) η4-Butadiene Complex
Organometallics, Vol. 20, No. 21, 2001 4381
F igu r e 4. Structure of 4b, a molybdenum η3-allyl metal-
lacycle. Only proton resonances are assigned. The chelating
ligand has been omitted for clarity.
range couplings between the quaternary carbons at
177.5 and 154.7 ppm and the protons of both the
methylene and the methyl groups, which confirm the
metallacycle fragment. The relative sizes of the NOE’s
place Hb, Hd, and He on one side of the cycle and Ha,
Hc, and Hf on the other side. In 4a Ha, Hc, and Hf showed
NOE’s to the aromatic proton Hg. We state that Hg is
an ortho phenyl imido proton and not a phenylene
proton because in the DQCOSY spectrum they displayed
a phenyl coupling pattern (7.15 ppm (Hg)-6.98 ppm-
6.79 ppm). Therefore, Hc occupies a syn relationship
with respect to the phenyl imido group. This intermedi-
ate metallacyclic π-allyl system most likely arises from
C-C coupling at the conjugated diene terminus. The
minor species also consists of a metallacyclic π-allyl
system (4b) which is very similar to that in 4a (Figure
4). Metallacycle 4b differs from 4a in that in 4b the
central allyl proton Hc is anti with respect to the phenyl
imido group, as revealed by the NOE’s of Hg at 7.19 ppm
and Ha, Hd, and Hf. The details of the structural
elucidation of 4b are similar to those of 4a ; however,
the low concentration of 4b in solution prevented the
assignment of the 13C chemical shifts of the metallacycle
fragment from the GHMQC and GHMBC spectra. The
NOESY spectrum did not display any exchange peaks
between 4a and 4b at -20 °C.
F igu r e 2. Molecular structure of 3 (50% probability
thermal ellipsoids). Selected bond lengths (Å) and angles
(deg): Mo-N(1), 1.7686(18); Mo-N(2), 2.0561(18); Mo-
N(3), 2.0492(17); Mo-C(21), 2.258(2); Mo-C(20), 2.342(2);
Mo-C(19), 2.402(2); Mo-C(24), 2.274(2); C(21)-C(20),
1.418(3); C(20)-C(19), 1.418(3); C(24)-C(19), 1.405(3);
C(24)-C(23), 1.518(3); C(23)-C(22), 1.542(3); C(22)-C(21),
1.525(3).
F igu r e 3. Structure of intermediate 4a , a molybdenum
η3-allyl metallacycle. The proton and carbon resonances are
assigned to corresponding atoms (carbon resonances are
underlined). The chelating ligand has been omitted for
clarity.
There has been extensive mechanistic work by Erker
et al. and Nakamura et al. on the reactivity of group 4
metallocene derivatives of 1,3-butadienes with unsatur-
ated substrates. The pathway favored for such reactions
involves coupling of the unsaturated moiety with one
of the diene double bonds, producing a 2-vinyl metal-
lacyclopentane species (C in Scheme 2). This species can
then isomerize as shown in Scheme 2.
used to elucidate the structures of these intermediates.
The intermediates consist of two isomeric complexes
present in a 4:1 ratio. The major intermediate (4a ) has
been characterized as a syn η3-allyl metallacyclic sys-
tem, and the results of the structural elucidation study
are presented in Figure 3. The metallacycle Ha-C and
It thus seems reasonable to propose that 2 reacts with
2-butyne to give 4e, which does not accumulate to any
1
1
Hb-C J (C-H) values of ca. 160 Hz are consistent with
detectable levels by H NMR spectroscopy (Scheme 4).
an sp2 hybridization of the carbon resonance at 76.8
ppm. This, along with the lack of resolved coupling
between Ha and Hb, supports the existence of an η3-allyl
functionality. The chemical shifts of the remaining allyl
proton and carbon nuclei also support the allyl fragment
in 4a . The sequence of the protons which display
multiplets was seen in the DQCOSY spectrum. NOE’s
with the methylene protons at 2.59 and 2.67 ppm
identified the methyl at 1.58 ppm as the one close to
the methylene. The protonated carbons were assigned
to the corresponding protons on the basis of the GHMQC
spectrum. The GHMBC spectrum displayed the long-
Intermediate 4e rapidly rearranges, affording 4a and
4b, which can be observed at low temperature. At room
temperature 4a , 4b, or some combination of the two
most likely rearranges to form 4f, which rapidly gener-
ates 4g via reductive elimination. Isomerization of 4g
ultimately results in the formation of 3.13 The produc-
tion of 3 from treatment of the olefin complex 1b with
excess 1,2-dimethyl-1,4-cyclohexadiene gives evidence
supporting formation of 4g in the proposed mechanism.
The difference in product distribution when 2 equiv
of 2-butyne reacts with 2 can be rationalized on the
basis of the proposed mechanism in Scheme 4. Excess

4382 Organometallics, Vol. 20, No. 21, 2001
Cameron et al.
Sch em e 4
2-butyne competes for the metal center with the 1,2-
dimethyl-1,4-cyclohexadiene ligand in 4g, liberating free
1,2-dimethyl-1,4-cyclohexadiene and forming 5 as the
metal-containing product.
Syn th esis of th e Molybd en u m 2-Bu tyn e Com -
p lex [Mo(NP h )(η2-2-bu tyn e)(o-(Me3SiN)2C6H4)] (5).
Treatment of a pentane solution of 1b with 2-butyne
affords the η2-2-butyne complex [Mo(NPh)(η2-2-butyne)-
(o-(Me3SiN)2C6H4)] (5). This alkyne complex is not stable
in solution at room temperature and decomposes, af-
fording an intractable mixture by 1H NMR spectroscopy.
At -30 °C 5 is stable for a brief period of time and X-ray-
quality crystals can be grown overnight, allowing us to
perform a single-crystal X-ray diffraction study. The
crystal data and details of the structure refinement are
summarized in Table 3 and a thermal ellipsoid plot of
5 along with selected bond lengths and angles is
displayed in Figure 5. Complex 5 adopts a square-
pyramidal geometry in which the imido group occupies
the apical position. The metal-imido interaction (Mo-
N(1), 1.7511(17) Å) is consistent with a metal-nitrogen
triple-bond interaction,10 and the molybdenum amide
contacts are consistent with metal-nitrogen single-bond
interactions.10e-g The C(21)-C(20) bond length of 1.297(3)
Å and bond angles C(21)-C(20)-C(19) (137.3(2)°) and
C(20)-C(21)-C(22) (140.2(2)°) support a considerable
amount of sp2 character of carbons C(20) and C(21). The
metal-coordinated 2-butyne carbons of 5 resonate at
181.5 ppm in the 13C NMR spectrum. On the basis of
this chemical shift the 2-butyne ligand is best viewed
as donating more than 2e to the metal center.14 We have
been able to generate a variety of η2-alkyne complexes
of disubstituted alkynes that show similar trends in
bonding. We are currently in the process of investigating
the bonding in these compounds via DFT studies. These
results will be reported separately.
F igu r e 5. Molecular structure of 5 (50% probability
thermal ellipsoids). Selected bond lengths (Å) and angles
(deg): Mo-N(1), 1.7511(17); Mo-N(2), 2.0314(16); Mo-
N(3), 2.0014(16); Mo-C(20), 2.056(2); Mo-C(21), 2.052(2);
C(20)-C(21), 1.297(3); C(20)-C(21)-C(22), 140.2(2); C(21)-
C(20)-C(19), 137.3(2).
of other group 6 metal butadiene complexes. The
reactivity of our butadiene complex is comparable to the
reactivity of early-metal butadiene complexes. Our
system is unique in that unlike group 4 chemistry,
where the final products of butadiene substrate C-C
coupling are metallacyclic, our system is governed by a
reductive-elimination step, resulting in formation of a
metal 1,3-cyclohexadiene complex. Additional studies
into the reactivity of this butadiene complex with other
unsaturated organics are underway.
Exp er im en ta l Section
Gen er a l Meth od s. All reactions were conducted under a
dry argon atmosphere using standard Schlenk techniques, and
all compounds were handled in a nitrogen-filled drybox. All
solvents were distilled under nitrogen from sodium or sodium
benzophenone ketyl or passed over activated alumina, stored
over molecular sieves, and degassed prior to use. Complexes
1a and 1b were synthesized according to published procedure.10g
NMR spectra were obtained on a Varian Gemini 300, VXR
300, or Mercury 300 instrument with C6D6, C7D8, and CDCl3
as solvents, as noted, and referenced to residual solvent peaks.
A Varian Inova 500 equipped with an indirect detection probe
was used as indicated for the GHMQC,15 GHMBC,16 and
NOESY17 experiments.
Con clu sion
We have characterized a cis molybdenum butadiene
complex with structural characteristics similar to those
Syn th esis a n d Ch a r a cter iza tion of [Mo(NP h )(η4-H2Cd
CHCHdCH2)(o-(Me3SiN)2C6H4)] (2). In a typical procedure
butadiene gas (ca. 15 psi) was added to a degassed flask
containing a green solution of 1a (0.42 g, 0.88 mmol) in
(13) (a) Along these lines we have observed that 1b catalyzes the
isomerization of allylanisole to trans-â-methylstyrene. Although no
hydride species are detected in solution for these isomerization
reactions, we have isolated hydride complexes of a closely related
tungsten system13b and have spectroscopically observed a molybdenum
PMe3 stabilized dihydrogen complex of this system (manuscript in
preparation). (b) Boncella, J . M.; Wang, S.-Y. S.; VanderLende, D. D.
J . Organomet. Chem. 1999, 591, 8.
(14) (a) Templeton, J . L.; Ward, B. C. J . Am. Chem. Soc. 1980, 102,
3288. (b) Templeton, J . L.; Ward, B. C. J . Am. Chem. Soc. 1980, 102,
1532. (c) Tatsumi, K.; Hoffmann, R.; Templeton, J . L. Inorg. Chem.
1982, 21, 466.
(15) Bodenhausen, G.; Ruben, D. J . Chem. Phys. Lett. 1980, 69, 185.
(16) Bax, A.; Summers, M. F. J . Am. Chem. Soc. 1986, 108, 2093.
(17) (a) States, D. J .; Haberkorn, R. A.; Ruben, D. J . J . Magn. Reson.
1982, 48, 286. (b) Bodenhausen, G.; Kogler, H.; Ernest, R. R. J . Magn.
Reson. 1984, 58, 370. (c) Wider, G.; Macura, S.; Kumar, A.; Ernst, R.
R.; Wuthrich, K. J . Magn. Reson. 1984, 56, 207.

A Molybdenum(IV) η4-Butadiene Complex
Organometallics, Vol. 20, No. 21, 2001 4383
pentane. The mixture was stirred for 10 min and concentrated
in vacuo, affording 2 as a green solid in 90% yield. Complex 2
decomposes rapidly in solution at room temperature and is
best stored as a solid at -30 °C under an inert atmosphere.
1H NMR (C6D6; room temperature): δ 0.33 (18H, SiMe3), 0.33
(ov, 2 H, butadiene protons), 4.20 (mult, 2 H, butadiene
protons), 5.49 (mult, 2 H, butadiene protons), 6.7-7.0 (ov,
mult, 7 H), 7.23 (m, 2 H, aromatic protons). 13C{1H} NMR
(C6D6; room temperature): δ 4.7, 75.0, 114.7, 117.9, 119.0,
119.4, 127.3, 129.3, 152.8, 158.9. 1H NMR (C7D8; -30 °C;
assigned via GHMQC; 500 MHz): δ 0.13 (br, mult, 2 H,
terminal butadiene protons), 0.33 (br, 18 H, SiMe3), 4.21 (br,
m, 2 H, terminal butadiene protons), 5.53 (br, mult, 2 H,
internal butadiene protons), 6.69 (t, 8.0 Hz, para proton), 6.72
(d, 8.0 Hz, ortho protons), 6.85 (t, 8.0 Hz, meta proton), 6.94
(m, 2 H, o-(Me3SiN)2C6H4 protons), 7.25 (m, 2 H, o-(Me3-
SiN)2C6H4 protons). 13C NMR (C7D8; -30 °C; assigned via
GHMQC and J CH from the gated decoupled 13C NMR spec-
trum): δ 4.2 (q, 122 Hz, Si(CH3)3), 75.0 (br, t, 158 Hz,
butadiene terminal carbon), 114.4 (d, 164 Hz, butadiene
internal carbon), 117.3 (d, 147 Hz), 118.3 (d of d, 157 Hz, 8
Hz), 118.7 (d, 162 Hz), 126.9, 128.8, 152.3 (t, 7 Hz), 158.3 (t,
8 Hz). Because of the thermal instability of 2, elemental
analysis was not possible.
F or m a tion of 3 by Tr ea tm en t of 2 w ith 2-Bu tyn e. An
NMR tube containing 2 (136.3 mg, 27.7 mmol) in C6D6 was
charged with 2-butyne (21.7 µL, 27.7 mmol) at room temper-
ature. The reaction was complete after 30 min. Complex 3 is
observed in the 1H NMR spectrum of the reaction mixture.
Syn th esis a n d Ch a r a cter iza tion of In ter m ed ia tes 4a
a n d 4b. An NMR tube containing 2 (136.3 mgs, 27.7 mmol)
in cold C7D8 was charged with 2-butyne (21.7 µL, 27.7 mmol)
at room temperature. The mixture was frozen with liquid
nitrogen after 1 min and placed in a cooled (-20 °C) NMR
probe. All 2D spectra (GHMQC, GHMBC, NOESY, DQCOSY,
and TOCSY) were acquired at -20 °C on an Inova 500. For
selected 1H and 13C NMR data for 4a and 4b see Figures 4
and 5.
Syn th esis a n d Ch a r a cter iza tion of [Mo(NP h )(η2-CH3-
CCCH3)(o-(Me3SiN)2C6H4)] (5). To a stirred solution of 1b
(0.340 g, 6.880 mmol) in pentane was added 2-butyne (0.0373
g, 6.880 mmol). The reaction mixture was stirred for 10 min,
and 5 was isolated as a red solid upon concentration of the
reaction mixture under reduced pressure (yield 76%). Complex
5 is not stable in solution at room temperature and also
1
decomposes in solution at -30 °C overnight. H NMR (C6D6;
room temperature): δ 0.45 (18 H, SiMe3), 2.17 (6 H, CH3-
CCCH3), 6.9-7.2 (ov, mult, 9 H). 13C NMR (C7D7; -25 °C): δ
1.8, 17.8, 121.9, 123.3, 123.8, 124.1, 157.8, 181.5. Two reso-
nances are overlapping with solvent. Because of the thermal
instability of 5 elemental analysis was not possible.
F or m a tion of 5 a n d 1,2-Dim eth yl-1,4-cycloh exa d ien e
by Tr ea tm en t of 2 w ith 2 Equ iv of 2-Bu tyn e. An NMR
tube containing 2 (136.3 mg, 27.7 mmol) in C6D6 was charged
with 2-butyne (43.4 µL, 55.4 mmol) at room temperature. The
reaction is complete after 30 min. Complex 5 and 1,2-dimethyl-
1,4-cyclohexadiene are observed in the 1H NMR spectrum of
the reaction mixture.
Syn th esis a n d Ch a r a cter iza tion of [Mo(NP h )(η4- 2,3-
d im eth yl-1,3-cycloh exa d ien e)(o-(Me3SiN)2C6H4)] (3). 1,2-
Dimethyl-1,4-cyclohexadiene (0.145 g, 1.33 mmol) was added
to a stirred solution of 1b (0.660 g, 1.33 mmol) in pentane.
This mixture was stirred for 5 days and then concentrated
under reduced pressure, affording 3 in 76% yield. Analytically
pure 3 can be obtained by washing the solid with pentane at
room temperature. Anal. Calcd for C26H39MoN3Si2: C, 57.22;
1
H, 7.20; N, 7.70. Found: C, 57.08; H, 7.26; N, 7.52. H NMR
(C6D6; room temperature; 500 MHz; assigned by GHMQC,
GHMBC, and NOESY): δ 0.32 (18 H, SiMe3), 0.31 (ov, 2 H,
cyclohexadiene methylene protons), 1.45 (br, mult, 2 H,
cyclohexadiene methylene protons), 2.05 (Me), 4.59 (br, Cd
C(H)), 6.68 (ortho protons), 6.73 (para proton), 6.83 (m, 2 H,
o-(Me3SiN)2C6H4 protons), 6.93 (meta protons), 7.16 (m, 2 H,
o-(Me3SiN)2C6H4 protons). 13C NMR (C6D6; room temperature;
500 MHz; assigned by GHMQC and GHMBC): δ 3.6 (Si(CH3)3),
18.4 (CH2), 22.0 (Me), 87.4 (CdC(H)), 117.3 (o-(Me3SiN)2C6H4),
117.6 (ortho carbon), 118.2 (o-(Me3SiN)2C6H4), 126.0 (para
carbon), 126.2 (CdC(H)), 128.7 (meta carbon), 153.3 (ipso,
o-(Me3SiN)2C6H4), 158.7 (ipso, phenyl).
Ack n ow led gm en t. We thank the National Science
Foundation (Grant No. CHE 0094404) for funding of this
work. K.A.A. thanks the NSF and the University of
Florida for funding X-ray equipment purchases.
Su p p or tin g In for m a tion Ava ila ble: Tables of atomic
coordinates, anisotropic thermal parameters, and bond lengths
and angles for 2, 3, and 5. This material is available free of
charge via the Internet at http://pubs.acs.org.
OM010508Q