Catalytic alkyne metathesis and stoichiometric metal-alkylidyne formation from N=Mo(OR)3 complexes promoted by Lewis acids

Article abstract of DOI:10.1016/j.jorganchem.2011.08.001

Nitride complex NMo(OCMe₂CF₃)₃ is synthesized in 78% yield on a multigram scale. Although the irreversible but sluggish conversion of terminal molybdenum-nitride complexes NMo(OCMe(CF ₃)₂)₃

Full text of DOI:10.1016/j.jorganchem.2011.08.001

Journal of Organometallic Chemistry 708-709 (2012) 1e9
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Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Catalytic alkyne metathesis and stoichiometric metalealkylidyne formation
from N^Mo(OR)3 complexes promoted by Lewis acids
1
*
Andrea M. Geyer , Michael J. Holland, Robyn L. Gdula, Joel E. Goodman, Marc J.A. Johnson , Jeff W. Kampf
Department of Chemistry, University of Michigan, 930 North University Ave, Ann Arbor, MI 48109, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
Nitride complex N^Mo(OCMe2CF3)3 is synthesized in 78% yield on a multigram scale. Although the
irreversible but sluggish conversion of terminal molybdenumenitride complexes N^Mo(OCMe(CF3)2)3
(1) and N^Mo(OC(CF3)3)3(NCMe) (2) to their propylidyne analogs EtC^Mo(OR)3(DME) (OCMe(CF3)2 (3),
OC(CF3)3 (4)) via metathesis with 3-hexyne occurs, the analogous reaction with N^Mo(OR)3 (OR]
OCMe2CF3 (5), OCMe3 (6)) complexes does not occur under similar reaction conditions. However, the
kinetic barrier to alkylidyne formation from 5 and 6 with internal alkynes can be overcome through the
addition of simple Lewis acids, including MgBr2, MgI2 and BPh3 in specific instances. Although this
typically leads to accelerated decomposition of the alkylidyne complex so formed, the combination of
metalenitride complex plus exogenous Lewis acid frequently leads to alkyne metathesis of the test
substrates 1-phenyl-1-propyne and 1-phenyl-1-butyne under milder conditions than possible in the
absence of Lewis acid, in some cases at room temperature. The interaction of solvent, ancillary alkoxide
ligands, and Lewis acid is complex and was not predicted a priori. New benzylidyne complexes
4-MeOC6H4C^Mo(OC(CF3)3)3(MeOC6H4CN) (22%), 4-PheC6H4C^Mo(OC(CF3)3)3(4-PhC6H4CN) (46%)
were isolated in low yield via the nitride-to-alkylidyne route upon reaction with suitable diarylalkynes.
Several related alkylidyne complexes were formed but could not be separated cleanly from the alkyne
reagents used.
Received 1 May 2011
Received in revised form
30 July 2011
Accepted 1 August 2011
Dedicated to Professor Kenneth G. Caulton
on the occasion of his 70th birthday.
Keywords:
Alkyne
Metathesis
Molybdenum
Nitride
Alkylidyne
Lewis acid
Ó 2012 Published by Elsevier B.V.
1. Introduction
complex conversion. Unfortunately, conversion of 5 or 6 to
EtC^Mo(OR)3 complexes via metathesis with 3-hexyne has not
been achieved. The potential pre-catalysts 5 and 6 were treated
with 20 equiv of 1-phenyl-1-propyne at 95 ꢀC in order to probe for
any alkyne-metathesis activity. The formation of an equilibrium
mixture of diphenylacetylene, 1-phenyl-1-propyne, and 2-butyne
in the presence of 5 indicated that at least trace conversion to an
alkylidyne complex occurred (Scheme 2). However, the rate of
alkyne metathesis with 5 was slow, requiring 9 days to achieve
equilibrium, and no alkylidyne complexes were directly observed.
No successful metathesis was observed by 1H NMR spectroscopy
with 6 under these conditions.
At this point we sought to enhance the rate of alkylidyne
complex formation from a nitride precursor. The earliest homoge-
neous alkyne metathesis system consisted of Mo(CO)6, and phenol
[3]. Modifications of this system have been reported by Bunz, Grela,
and Mori [4,5]; the phenol exerts a substantial influence on catalyst
performance [6e8]. Although the active species in these reaction
mixtures has yet to be identified, it is frequently assumed to be
a Schrock-type alkylidyne complex [9e11].
The conversion of 1 to 3 via metathesis with 3-hexyne is one of
the most facile methods for preparing a molybdenumealkylidyne
complex in good yield (Scheme 1) [1]. The analogous metathesis of
2 with 3-hexyne to afford 4 can be effected at a lower temperature
(Scheme 1). However, decomposition of 4 during concentration of
the reaction mixture has hindered its isolation. The in situ
decomposition of 4 is enhanced when the reaction is conducted at
the 95 ꢀC originally reported. With a limited number of examples of
the nitride-to-alkylidyne moiety conversion at molybdenum [1,2],
the potential breadth of this methodology has yet to be fully
exploited.
The incorporation of more economical OCMe2CF3 and OCMe3
ligands would enhance the scope of this nitride-to-alkylidyne
* Corresponding author. Present address: Chemical Sciences and Engineering
Division, Argonne National Laboratory, 9700 South Cass Avenue, Argonne, IL 60439,
USA. Tel.: þ1 630 252 3418; fax: þ1 630 972 4567.
E-mail address: mjjohnson@anl.gov (M.J.A. Johnson).
Present address: Chemistry Department, University of Saint Francis, 2701
1
Inspired by this work and the potential role that a Lewis acid
such as phenol has on alkyne metathesis, we decided to investigate
Spring Street, Fort Wayne, IN 46808, USA.
0022-328X/$ e see front matter Ó 2012 Published by Elsevier B.V.
doi:10.1016/j.jorganchem.2011.08.001

2
A.M. Geyer et al. / Journal of Organometallic Chemistry 708-709 (2012) 1e9
using standard air-free techniques. 1H NMR spectra were recorded
Et
10 EtC
DME
CEt
C
at 499.909 MHz, 399.967 MHz on a Varian Inova 400 spectrometer
or 300.075 MHz on a Varian Inova 300 spectrometer and refer-
enced to the residual protons in toluene-d8 (2.09 ppm), CDCl3
(7.26 ppm), CD2Cl2 (5.33 ppm), and C6D6 (7.15 ppm). 19F NMR
spectra were recorded at 282.384 MHz on a Varian Inova 300
spectrometer or 376.303 MHz on a Varian Inova 400 spectrometer
and were referenced to an external standard of CFCl3 in CDCl3
(0.00 ppm). 13C NMR spectra were recorded at 100.596 MHz on
a Varian Inova 400 spectrometer and were referenced to naturally
abundant 13C nuclei in CD2Cl2 (54.00 ppm). GC/MS data were
collected on a Shimadzu GCMS-QP5000 with a Restek XTI-5 phase
column (30 m, 0.25 I.D., 0.25 D. F.). EI MS data were collected on
a VG (Micromass) 70-250-S Magnetic sector mass spectrometer.
N
OCMe(CF3)2
OCMe(CF3)2
(F3C)2MeCO Mo
Mo
OCMe(CF3)2
OCMe(CF3)2
toluene
95oC
(F3C)2MeCO
O
O
1
Isolated 3: 62%
Et
C
10 EtC
DME
CEt
N
OC(CF3)3
OC(CF3)3
OC(CF3)3
OC(CF3)3
(F3C)3CO
MeCN
Mo
toluene
80oC
(F3C)3CO Mo
O
2
O
Conversion 4: 100%
Scheme 1. Syntheses of 3 and 4 via metathesis of N^Mo(OR)3 complexes with 3-
hexyne.
the influence of simple Lewis acids on the conversion of
N^Mo(OR)3 into RC^Mo(OR)3 complexes [12]. There are three
potentially favorable interaction modes of the Lewis acids with our
nitride pre-catalysts (Fig. 1). First, the Lewis acid could bind directly
to the nitride moiety (A). This is observed when B(C6F5)3 is used [2],
and should affect the rate of nitride-to-alkylidyne conversion, but
not subsequent alkyne metathesis. Molybdenumealkylidyne
complexes are in general more difficult to synthesize than their
tungsten counterparts, but several efficient syntheses of benzyli-
dyne complexes of molybdenum and tungsten via low- and high-
oxidation-state precursors have been reported recently
[11,13e18]. Alternatively, coordination of the Lewis acid to the
alkoxide ligand could occur (B). Lastly, if an additional coordinating
ligand is present in the coordination sphere of a precatalyst such as
a molybdenum nitride complex, the Lewis acid could activate the
precursor by abstracting or trapping this ligand upon its dissocia-
tion, as noted in the reaction of recently reported N^Mo(O-
SiPh3)3(py) [18] with one equivalent of B(C6F5)3 (C) [2].
2.2. Materials and methods
All solvents used were dried and deoxygenated by the method
of Grubbs [23]. Bis(4-methoxyphenyl)acetylene [24,25], VCl3(thf)3
[26], N^Mo(OCMe3)3 (6) [27], N^Mo(OCMe(CF3)2)3 (1) [28], and
N^Mo(OC(CF3)3)3(NCMe) (2) [28], were prepared according to
literature procedures. Mesitylene, diphenylacetylene, and chlor-
otitanium tri-isopropoxide were obtained from Acros. 3-hexyne, 1-
phenyl-1-butyne, and 1-phenyl-1-propyne were obtained from GFS
Chemicals and dried over 4 Å molecular sieves for at least 24 h.
Magnesium bromide, magnesium iodide, and hydrochloric acid
(2.0 M) in Et2O were obtained from Aldrich. Zirconium (IV) chloride
and copper (II) chloride were obtained from Strem Chemicals, Inc.
4-bromobiphenyl was obtained from TCI Organic Chemicals. NMR
solvents were obtained from Cambridge Isotope Laboratories and
were dried over 4 Å molecular sieves for at least 24 h. All reagents
were used as received unless otherwise noted.
The first two Lewis acid binding modes are equivalent to
decreasing the pKa of the conjugate acid of the ancillary ligand.
Comparison of the metal centers of an alkylidyne and a nitride
complex with same ancillary ligand set reveals that a lesser positive
charge is present on the alkylidyne center [19]. Thus, alkylidyne
complex formation should become more favorable relative to the
nitride as the electron-donating capacity of the ancillary ligands
decreases (Fig. 2). Furthermore, theoretical calculations and
experimental data have demonstrated that altering the electron
donating-ability of the alkoxide ligands influences the rate of
alkyne metathesis dramatically [20e22]. These studies indicated
that introduction of a poor electron-donor ancillary ligand stabi-
lizes the transition state for metalacyclobutadiene formation. Thus,
enhanced rates of alkyne metathesis should occur if the Lewis acid
coordinates to the alkoxide of the alkylidyne complex due to the
resulting increased Lewis acidity of the metal center.
2.3. Nitride-to-alkylidyne complex syntheses
2.3.1. Synthesis of N^Mo(OCMe2CF3)3 (5)
MoCl4(NCMe)2 (3.0 g, 9.4 mmol) was slurried in 100 mL aceto-
nitrile. NaN3 (735 mg, 11.3 mmol, 1.20 equiv) was added to the
suspension. [Warning! Sodium azide is potentially explosive and is
used here in 20% excess. Care should be taken not to heat the crude
material, particularly during subsequent solvent removal, as some
NaN3 remains. This must be disposed of properly. As an alternative,
0.99 equiv NaN3 may be used, though the isolated yield of 5
decreased to 65% in one attempt under these conditions. The
mixture was capped with a needle-pierced septum and left to stir
for 2 h at 30 ꢀC. The acetonitrile was removed from the resulting
dark red mixture in vacuo. The resultant solid residue was triturated
with 10 mL of toluene before it was slurried in 100 mL toluene.
Solid LiOMe2CF3 (3.85 g, 28.7 mmol, 3.05 equiv) was added to the
toluene solution and the mixture was left to stir for 12 h at 30 ꢀC.
The mixture was then heated and filtered through celite using
excess toluene. The solvent was removed in vacuo and the resulting
residue was redissolved in ca. 18 mL boiling toluene and cooled
to ꢁ35 ꢀC overnight. Upon vacuum filtration of the resulting
mixture, 3.62 g (7.37 mmol, 78.4%) of pale brown crystals were
2. Experimental section
2.1. General procedures
All reactions were performed in an atmosphere of dinitrogen,
either in a nitrogen-filled MBRAUN Labmaster 130 glove box or by
recovered. 1H NMR (C6D6): 1.41 (s, OC(CH3)2CF3). 19F NMR (C6D6):
d
N
20 Ph
Me
Mo
10
5
OCMe2CF3
OCMe2CF3
Ph
Me + Me
Me +
5
Ph
Ph
tol, 95oC, 9d
F3CMe2CO
5
Scheme 2. Alkyne metathesis with 5.

A.M. Geyer et al. / Journal of Organometallic Chemistry 708-709 (2012) 1e9
3
LA
LA
N
N
N
N
RO
RO
RO
RO
R
Mo
Mo
Mo
N
Mo
OR
CMe
OR
O
OR
OR
RO
RO
N
LA
CMe
OR
LA
A
B
C
Fig. 1. Potential Lewis acid binding modes.
d
ꢁ82.99 (s). 13C{1H} NMR (C6D6):
d
126.18 (q, CF3, JCeF ¼ 284.59 Hz),
3-hexyne (522 mL, 4.59 mmol, 3 equiv) was added to the reaction
82.52 (q, CH3, JCeF ¼ 29.69 Hz), 22.91 (s, OC(CH3)2CF3). Anal. Calc for
N^MoO3C12H18F9: C, 29.34; H, 3.70; N, 2.85. Found: C, 28.75; H,
3.62; N, 3.03.
mixture via syringe. The mixture was sealed and heated to 95 ꢀC for
24 h. The reaction mixture was filtered through celite and the celite
was washed with pentane (40 mL). The volatiles were then removed
in vacuo. The reaction mixture was taken up in toluene/pentane
2.3.2. Synthesis of EtC^Mo(OCMe(CF3)2)3(NCEt) (7)
(16 mL) and DME (159 mL, 1.53 mmol, 1 equiv) was added. The
Complex 1 (10.0 mg, 0.0153 mmol) was dissolved in C6D6
(0.5 mL). Then 3-hexyne (17.4 mL, 0.153 mmol, 10 equiv) was added
mixture was then cooled in the freezer. Following repeated recrys-
tallizations PhC^Mo(OCMe(CF3)2)3(DME) (158 mg, 0.205 mmol,
20%) was isolated. Further isolation could not be achieved through
recrystallization. Characterization data agreed with the literature
[29].
to the solution via syringe. The solution was frozen and the over-
lying volatiles were removed in vacuo. The solution was then
heated to 95 ꢀC for 29 h. At this point the reaction mixture consisted
of 7 (80%) and a decomposition product (unidentified). The volatiles
were removed in vacuo from the reaction mixture. The resulting
residue was then reconstituted in C6D6. At this point insoluble
material was present in the reaction mixture along with increased
evidence of decomposition with 7 only accounting for 63% of the 19F
2.3.5. Synthesis of 4-PhC6H4C^Mo(OCMe(CF3)2)3(4-PhC6H4CN)
(10)
Complex 1 (5.0 mg, 0.0077 mmol) and bis(4-biphenyl)acetylene
(25.3 mg, 0.0766 mmol, 10 equiv) were slurried in toluene-d8
NMR spectrum. 1H NMR (400 MHz, C6D6):
d
2.44 (q, 2H, ^CH2CH3,
(0.5 mL). Then 3-hexyne (1.9 mL, 0.023 mmol, 3 equiv) was added
J ¼ 7.6 Hz), 1.55 (s, 9H, OC(CH3)2CF3), 1.10 (s br, 2H, N^CH2CH3),
via syringe to the reaction mixture. The reaction mixture was
heated to 95 ꢀC for 3 d. At this point the reaction mixture consisted
of 10 (51%) and 1 (39%). Further heating of the reaction mixture for
1 d only resulted in an additional 1% formation of 10 (52%).
0.56 (t, 3H, ^CH2CH3, J ¼ 7.6 Hz), 0.34 (t, 3H, N^CH2CH3, J ¼ 7.6 Hz)
1H NMR (400 MHz, CD2Cl2, ꢁ40 ꢀC):
d 3.15 (q br, 2H, ^CH2CH3,
J ¼ 7.5 Hz), 2.62 (q br, 2H, N^CH2CH3, J ¼ 7.5 Hz), 1.81 (s, 9H,
OC(CH3)2CF3), 1.32 (t br, 3H, N^CH2CH3, J ¼ 7.5 Hz), 1.02 (t br, 3H,
^CH2CH3, J ¼ 7.5 Hz). 19F NMR (300 MHz, C6D6):
EI/MS [m/z]þ: 730.0 (EtC^Mo(OCMe(CF3)2)3).
d
ꢁ77.67 (s, CF3).
2.3.6. Synthesis of 4-MeOC6H4C^Mo(OC(CF3)3)3(4-MeOC6H4CN)
Complex 2 (500.0 mg, 0.5840 mmol) and bis(4-methoxyphenyl)
acetylene (339.2 mg, 1.424 mmol, 2.438 equiv) were dissolved in
toluene (25 mL). The reaction mixture was heated to 60 ꢀC for 6 d.
The reaction volume was reduced by half and the mixture was
heated to 60 ꢀC for an additional 2 d. At this point, the reaction
mixture was 84% 4-MeOC6H4C^Mo(OC(CF3)3)3(MeOC6H4CN), 7% 2,
and 9% of a decomposition product. The volatiles were removed in
vacuo and the reaction mixture was extracted with pentane (30 mL)
and filtered. The resulting filtrate was extracted with pentane
(10 mL) and the volatiles were removed in vacuo. The resulting
material was dissolved in Et2O/pentane (5 mL) and cooled to ꢁ35 ꢀC.
A purple powder of 4-MeOC6H4C^Mo(OC(CF3)3)3(MeOC6H4CN)
was isolated via filtration (133.2 mg, 0.1075 mmol, 22%). 1H NMR
2.3.3. Synthesis of EtC^Mo(OC(CF3)3)3(NCEt) (8)
Complex 2 (100.0 mg, 0.117 mmol) was dissolved in toluene
(3 mL). 3-hexyne (26.5 mL, 0.234 mmol, 2 equiv) was added via
syringe to the solution. The solution was then heated to 75 ꢀC for
12 h. Upon removal of volatiles in vacuo a few orange crystals of 8
crystallized on the side of the reaction vial. 1H NMR (400 MHz,
C6D6):
d
2.79 (q, 2H, ^CH2CH3, J ¼ 7.6 Hz), 0.62 (t, 3H, ^CH2CH3,
J ¼ 7.6 Hz), 0.35 (t, 3H, N^CH2CH3, J ¼ 7.6 Hz). 1H NMR (400 MHz,
toluene-d8, ꢁ20 ꢀC): 2.83 (q br, 2H, ^CH2CH3, J ¼ 7.0 Hz), 1.08 (q br,
2H, N^CH2CH3, J ¼ 7.6 Hz), 0.67 (t, 3H, ^CHCH3, J ¼ 7.0 Hz), 0.39 (t,
3H, N^CH2CH3, J ¼ 7.4 Hz). 19F NMR (300 MHz, C6D6): ꢁ72.44 (s,
CF3). EI/MS [m/z]þ: 843.9 (EtC^Mo(OC(CF3)3)3).
(400 MHz, C6D6):
d
7.23 (d, 2H, ArH, J ¼ 9.0 Hz), 7.06 (d, 2H, ArH,
J ¼ 9.0 Hz), 6.35 (d, 2H, ArH, J ¼ 9.0 Hz), 6.15 (d, 2H, ArH, J ¼ 9.0 Hz),
2.3.4. Synthesis of PhC^Mo(OCMe(CF3)2)3(DME)
Complex 1 (1.00 g, 1.53 mmol) and diphenylacetylene (2.73 g,
15.3 mmol, 10 equiv) were dissolved in toluene (50 mL). Then
3.04 (s, 3H, OMe), 2.95 (s, 3H, OMe). 19F NMR (300 MHz,
C6D6): ꢁ72.48 (s, CF3). 13C{1H} NMR (400 MHz, CD2Cl2):
d 321.94 (s,
Mo^C), 165.95 (s, ArC), 162.20 (s, ArC), 137.52 (s, ArC), 135.54 (s,
ArC), 133.23 (s, ArC), 133.14 (s, ArC), 121.60 (q, OC(CF3)3,
JCeF ¼ 293.2 Hz), 116.15 (s, ArC), 113.22 (s, ArC), 101.83 (s, ArC) 99.38
(s, CN), 87.02 (m, OC(CF3)3), 56.44 (s, OMe), 55.86 (s, OMe).
Decreasing
Alkoxide
Donor
Strength
2.3.7. Synthesis of 4-PhC6H4C^Mo(OC(CF3)3)3(4-PhC6H4CN)
CR
OR
Energy
Complex 2 (5.0 mg, 0.0058 mmol) and bis(4-biphenyl)acetylene
(19.3 mg, 0.0584 mmol, 10 equiv) were slurried in C6D6 (0.5 mL).
The reaction mixture was frozen and the overlying volatiles were
removed in vacuo. The mixture was then heated to 95 ꢀC for 3 d.
At this point the reaction mixture consisted of 88% 4-
M
X
X
N
X
Introduction
of a Lewis
N
M
N
M
CR
M
R'C
X
Acid
X'
X'
R'C N LA
X'
X'
X
X'
X'
X
R'C CR
R'C CR LA
PheC6H4C^Mo(OC(CF3)3)3(4-PhC6H4CN). Scale-Up. Complex
(1.0 g, 1.17 mmol) and bis(4-biphenyl)acetylene (2.88 g, 8.76 mmol,
2
Fig. 2. Qualitative influence of alkoxide and Lewis acids on relative stabilities of nitride
and alkylidyne complexes.

4
A.M. Geyer et al. / Journal of Organometallic Chemistry 708-709 (2012) 1e9
Et
N
OCMe(CF3)2
OCMe(CF3)2
10
Et
Et
Mo
OCMe(CF3)2
OCMe(CF3)2
(F3C)2MeCO
EtCN
Mo
C6D6, 95oC
29 h
(F3C)2MeCO
1
7:
Conversion
80%
Et
OC(CF3)3
OC(CF3)3
N
2
OC(CF3)3
OC(CF3)3
Et
Et
(F3C)3CO
EtCN
Conversion
Mo
toluene, 75oC
12 h
(F3C)3CO
MeCN
Mo
8:
89%
2
Scheme 3. Formation of 7 and 8 via metathesis of N^Mo(OR)3 complexes with 3-hexyne.
7.5 equiv) were slurried in toluene (50 mL). The reaction mixture
was then heated at 95 ꢀC for 3 d. The mixture and the resulting
white precipitate were washed with toluene (40 mL) and then
pentane (10 mL). The volatiles were removed in vacuo from the
filtrate. The resulting material was extracted with toluene and
filtered. The resulting filtrate was reduced to dryness and dissolved
in 1:1 Et2O/hexane (10 mL) and cooled to ꢁ35 ꢀC. The resulting tan
powder was filtered and the filtrate was taken up in toluene/
hexane (7 mL) and cooled to ꢁ35 ꢀC. A dark red-brown powder of
4-PheC6H4C^Mo(OC(CF3)3)3(4-PhC6H4CN) formed (508.9 mg,
0.444 mmol, 38%). 19F NMR (400 MHz, C6D6): ꢁ72.3 (s, CF3). 1H
NMR (400 MHz, C6D6): 7.40 (d, J ¼ 8.2 Hz), 7.00e7.278 (m), 7.01 (d,
J ¼ 7.0 Hz). EI/MS [m/z]þ: 968.0, 4-PhC6H4C^Mo(OCMe(CF3)2)3.
transferred to a J. Young Tube. The reaction was monitored via 1H
NMR spectroscopy at room temperature.
2.5. Solvent studies for Lewis acid testing
2.5.1. General procedure with 1
Complex 1 (5.0 mg, 0.0077 mmol) was dissolved in an appro-
priate solvent (500
mL). Then 1-phenyl-1-propyne (18.9 mL,
0.153 mmol, 20 equiv) and an internal standard of mesitylene were
added via syringe. The reaction was monitored at room
temperature.
2.5.2. General procedure with 2
Complex 2 (5.0 mg, 0.0058 mmol) was dissolved in an appro-
priate solvent (500
mL). Then 1-phenyl-1-propyne (14.4 mL,
2.4. General protocol for Lewis acid testing
0.117 mmol, 20 equiv) and an internal standard of mesitylene
were added via syringe. The reaction was monitored at room
temperature.
2.4.1. General procedure with 5
Complex 5 was dissolved in CD2Cl2 (20.4 mM). 1-phenyl-1-
propyne (20 equiv) and an internal standard of mesitylene were
added to the solution via syringe. This solution was placed in a vial
containing the Lewis acid (2.0 equiv). The resulting slurry was
transferred to a J. Young Tube. The reaction mixture was frozen and
the overlying volatiles were removed in vacuo. The reaction was
monitored via 1H NMR spectroscopy at 40 ꢀC.
2.5.3. General procedure with 5
Complex 5 (10.0 mg, 0.0204 mmol) was dissolved in an appro-
priate solvent (1 mL). Then 1-phenyl-1-butyne (57.9
0.407 mmol, 20 equiv) and an internal standard of mesitylene were
added via syringe. This solution was transferred to a vial containing
mL,
2.4.2. General procedure with 6
Complex 6 was dissolved in C6D6 (30.4 mM). 1-phenyl-1-
propyne (20 equiv) and an internal standard of mesitylene were
added to the solution via syringe. This solution was placed in a vial
containing the Lewis acid (2.0 equiv). The resulting slurry was
transferred to a J. Young Tube. The mixture was frozen and the
overlying volatiles were removed in vacuo. The reaction was
monitored via 1H NMR spectroscopy at 80 ꢀC.
2.4.3. General procedure with 1
Complex 1 was dissolved in CDCl3 (1.0 mL). 1-phenyl-1-propyne
(20 equiv) and an internal standard of mesitylene were added to
the solution via syringe. This solution was placed in a vial con-
taining the Lewis acid (2.0 equiv). The resulting slurry was trans-
ferred to a J. Young Tube. The reaction was monitored via 1H NMR
spectroscopy at room temperature.
2.4.4. General procedure with 2
Complex 2 was dissolved in C6D6 (1.0 mL). 1-phenyl-1-propyne
(20 equiv) and an internal standard of mesitylene were added to
the solution via syringe. This solution was placed in a vial con-
taining the Lewis acid (2.0 equiv). The resulting slurry was
Fig. 3. . 50% Thermal ellipsoid plot of 8.

A.M. Geyer et al. / Journal of Organometallic Chemistry 708-709 (2012) 1e9
5
Table 1
a Bruker SMART APEX CCD-based X-ray diffractometer equipped
with a low temperature device and fine focus Mo-target X-ray tube
Selected Bond Lengths and Angles for 8.
(
l
¼ 0.71073 A) operated at 1500 W power (50 kV, 30 mA). The X-
Complex 8
ray intensities were measured at 85(2) K; the detector was placed
at a distance 5.055 cm from the crystal. A total of 3490 frames were
Bond distances (Å)
MoeC(13)
MoeO(1)
MoeO(2)
Bond angles (deg)
C(13)eMoeO(1)
C(13)eMoeO(2)
C(13)eMoeO(3)
C(13)eMoeN(1)
1.722(2)
1.9604(16)
1.9390(16)
MoeO(3)
MoeN(1)
1.9528(16)
2.182(2)
collected with a scan width of 0.5ꢀ in
u
and 0.45ꢀ in
4
with an
exposure time of 15 s/frame. The integration of the data yielded
a total of 90081 reflections to a maximum 2
value of 60.20ꢀ of
which 7850 were independent and 7227 were greater than 2 (I).
The final cell constants were based on the xyz centroids of 9987
reflections above 10 (I). Analysis of the data showed negligible
q
105.30(9)
105.16(9)
104.72(9)
91.30(11)
O(2)eMoeO(3)
O(1)eMoeN(1)
O(3)eMoeN(1)
O(1)eMoeO(2)
94.16(7)
81.47(8)
83.30(8)
92.10(7)
s
s
decay during data collection; the data were processed with
SADABS and corrected for absorption. The structure was solved
and refined with the Bruker SHELXTL software package, using the
space group P2(1)/n with Z ¼ 4 for the formula C18H10F27NO3Mo.
All non-hydrogen atoms were refined anisotropically with the
hydrogen atoms placed in idealized positions. The propionitrile
ligand is disordered over two positions. Full matrix least-squares
magnesium bromide (7.5 mg, 0.041 mmol, 2 equiv). The resulting
reaction mixture was then placed in a J. Young tube. The reaction
mixture was frozen and the overlying volatiles were removed in
vacuo. The reaction was then monitored at 60 ꢀC.
refinement based on F2 converged at R1
¼
0.0402 and
2.5.4. General procedure with 6
Complex 6 (5.0 mg, 0.015 mmol) was dissolved in an appropriate
solvent (500 mL). Then 1-phenyl-1-propyne (37.5 mL, 0.304 mmol,
wR2 ¼ 0.1040 [based on I > 2
s
(I)], R1 ¼ 0.0436 and wR2 ¼ 0.1069
for all data.
20 equiv) and an internal standard of mesitylene were added via
syringe. This solution was transferred to a vial containing magne-
sium bromide (5.6 mg, 0.030 mmol, 2 equiv). The resulting reaction
mixture was then placed in a J. Young tube. The reaction mixture
was frozen and the overlying volatiles were removed in vacuo. The
reaction was then monitored at 60 ꢀC.
3. Results and discussion
3.1. Nitride-to-alkylidyne complex conversion with OCMe(CF3)2 and
OC(CF3)3 ligands
Due to the continued decomposition of 4 during attempted
isolation in the presence of DME, the nitride-to-alkylidyne complex
conversion was investigated in the absence of DME. Spectroscopic
evidence for the formation of EtC^Mo(OR)3(NCEt) (OR]
OCMe(CF3)2 (7), OC(CF3)3 (8)) was found (Scheme 3). Upon removal
of the volatiles, a similar decomposition of the reaction mixtures
has prevented isolation of 7 and 8 in good yield. The identity of the
decomposition product(s) is under investigation.
Further reaction of the 7 and 8 upon concentration under
reduced pressure is not surprising, as 1H NMR spectroscopy indi-
cates that bound and unbound propionitrile exchange on the NMR
time scale in both 7 and 8. Upon cooling solutions of 7 and 8
to ꢁ40 ꢀC and ꢁ20 ꢀC respectively, distinct quartets that correspond
to bound propionitrile appear in the 1H NMR spectra (S1, S2, S3, and
S4). Isolation of a small sample of 8 in crystalline form was achieved
as the reaction mixture was concentrated under vacuum. A thermal
ellipsoid plot of 8 (Fig. 3) reveals an approximately square pyra-
midal geometry about molybdenum (Table 1) with propionitrile
coordinated trans to one alkoxide ligand. Structure refinement data
are collected in Table 2.
2.6. Alkyne dependence studies
Complex 5 was dissolved in CD2Cl2 (20.4 mM). 1-phenyl-1-
butyne (20 equiv) or 1-phenyl-1-propyne (20 equiv) and an
internal standard of mesitylene were added to the solution via
syringe. This solution was placed in a vial containing the Lewis acid
(2.0 equiv). The resulting slurry was transferred to a J. Young Tube.
The reaction mixture was frozen and the overlying volatiles were
removed in vacuo. The reaction was monitored via 1H NMR spec-
troscopy at 40 ꢀC.
2.7. X-ray crystallography
Orange needles of 8 were grown from a toluene solution at
25 ꢀC. A crystal of dimensions 0.32 ꢂ 0.14 ꢂ 0.11 mm mounted on
Table 2
Crystallographic Data for Complex 8.
Complex 8
Formula
FW
C18H10F27O3NMo
897.21
Crystal system
Space group
a (Å)
b (Å)
c (Å)
Monoclinic
P21/n
10.9445(9)
18.1387(15)
15.0623(12)
90
110.8650(10)
90
2794.1(4)
4
0.7103
108(2)
2.133
0.676
1736
0.0402
0.1040
1.062
Ph
RO
RO
Mo
NCPh
OR
toluene,
95oC, 2d
a
b
g
(deg)
(deg)
(deg)
9:
Conversion
80%
Et
Et
3
N
10 Ph
Ph
Mo
V (Å)
Z
OC(CF3)2Me
OC(CF3)2Me
Me(F3C)2CO
C6D6,
1
Radiation (Ka, Å)
T (K)
Ar
10
C
C
Et
95oC, 3d
Ph
Ar
Dcalcd (Mg mꢁ3
)
Ar=
mcalcd (mmꢁ1
)
RO
RO
Conversion
Scheme 4. Formation of 9 and 10 from 1.
Mo
NCAr
OR
F000
R1
wR2
GOF
10:
78%

6
A.M. Geyer et al. / Journal of Organometallic Chemistry 708-709 (2012) 1e9
Ar
N
Mo
2
OMe 80%, 1.5 h
22% Isolated Yield
Ar=
OC(CF3)3
OC(CF3)3
OC(CF3)3
OC(CF3)3
10 Ar
toluene, 95oC
Ar
(F3C)3CO
MeCN
(F3C)3CO
ArCN
Mo
Ph 88%, 4 d
38% Isolated Yield
Ar=
Scheme 5. Formation of ArC^Mo(OC(CF3)3)3(NCAr) complexes from 2.
3.2. Nitride-to-benzylidyne complex conversion with OCMe(CF3)2
and OC(CF3)3 ligands
Supporting Information). All reactions were monitored via 1H NMR
spectroscopy to within 5% equilibrium with their reaction quotients
(Q) being reported in Table 3. An equilibrium amount of alkyne
metathesis products corresponds to a Keq ¼ 0.25 with the aryl-
containing products consisting of 33% unsymmetrical alkyne and
66% symmetrical alkyne. Since no evidence of polymer formation
was observed this value is calculated assuming the absence of 3-
hexyne (2-butyne) polymerization, as highlighted in Fig. 4. The
corresponding solvent studies for 1 and 2 in the absence of a Lewis
acid are also reported in Table 3.
Methods for the preparation of ArC^Mo(OR)3 complexes from 1
and 2 were explored next (Scheme 4). Direct formation of
PhC^Mo(OCMe(CF3)2)3(NCPh) (9) via metathesis of
1 with
diphenylacetylene was not observed by 1H NMR spectroscopy, even
upon heating to 90 ꢀC for 2 days. In contrast, treatment of 1 with
3 equiv of 3-hexyne and 10 equiv of diphenylacetylene resulted in
80% conversion to 9. Difficulty in separating diphenylacetylene and
9 has prevented isolation of 9 to date. Addition of DME to the
reaction mixture allowed for isolation of PhC^Mo(OC-
Me(CF3)2)3(DME) in low yield (20%). Similarly, treatment of 1 with
10 equiv of the unsymmetrical alkyne 4-biphenyl-1-butyne resul-
ted in the formation of ArC^Mo(OCMe(CF3)2)3(NCAr) (10) (Ar ¼ p-
PhC6H4) (78%). Although bis(4-biphenyl)acetylene can be readily
separated from 10, separation from the remaining unsymmetrical
alkyne has not been achieved to date.
3.3.1. Solvent studies
In the absence of Lewis acids, the optimal solvents for alkyne
metathesis activity with 1 and 2 were C6D6 and CDCl3, respectively.
Despite the increased Lewis acidity of 2 relative to 1, more rapid
alkyne metathesis activity was observed with 1 than with 2. We
attribute this difference to the presence of a coordinating ligand in
2 hindering access to the active catalyst. Schrock demonstrated
Since the RC^Mo(OC(CF3)3)3 complexes were usually unstable
upon attempts to isolate them as solids, stable benzylidyne analogs
were sought. As shown in Scheme 5, direct conversion of 2 to
ArC^Mo(OR)3(NCAr) was achieved via metathesis with bis(4-
methoxyphenyl)acetylene (80%) or bis(4-biphenyl)acetylene
(88%). Slow metathesis was observed with bis(4-biphenyl)acety-
lene due to the poor solubility of this alkyne under the reaction
conditions. The increased Lewis acidity of 2 relative to 1 likely
accounts for the direct scission of ArC^CAr by 2, a reaction that is
not observed with 1.
a similar rate disparity in comparing Me3CC^Mo(OC(CF3)2
-
Me)3(DME) and Me3CC^Mo(OC(CF3)3)3(DME). More rapid alkyne
metathesis was observed with the former catalyst due to the
presence of a less strongly bound DME ligand [29] In order to avoid
overlapping resonances in the 1H NMR spectrum during the solvent
studies with 5, 1-phenyl-1-butyne was employed in place of 1-
phenyl-1-propyne. The preferred reaction medium with 5 was
CD2Cl2 due to the increased solubility of magnesium bromide under
the reaction conditions. Catalyst 6 displayed the highest conversion
to diphenylacetylene in CDCl3 prior to catalyst decomposition at
60 ꢀC. The rate of metathesis can be enhanced relative to the rate of
catalyst decomposition with 6 by increasing the reaction temper-
ature to 80 ꢀC, achieving an equilibrium mixture of alkyne
metathesis products.
3.3. Nitride-to-alkylidyne complex conversions assisted by Lewis
acids
Investigations of the influence of Lewis acids on metathesis
initially focused on the effect that 2 equiv of magnesium bromide
had on the alkyne metathesis activity of 5 and 6 with 20 equiv of
PheC^CeR (R ¼ Et, Me) in three different solvents, CDCl3, CD2Cl2,
and C6D6 (Table 1). Magnesium bromide was selected for the
studies because it promoted minimal catalyst decomposition under
the reaction conditions, unlike some other Lewis acids (see
3.3.2. Nitride-to-alkylidyne moiety conversions with 6 assisted by
Lewis acids
The presence of alkyne metathesis activity with
6 was
surprising, since the conversion to an alkylidyne complex is least
favorable thermodynamically and kinetically with 6 relative to the
other nitride complexes investigated. The activity of 6 with 2 equiv
of several different Lewis acids and 20 equiv of 1-phenyl-1-propyne
was examined in CDCl3 at 80 ꢀC. The reactions were monitored to
20% diphenylacetylene and 80% 1-phenyl-1-propyne (Q ¼ 0.07)
(Table 4). Although seven different Lewis acids assisted in alkyne
metathesis with 6, only magnesium bromide resulted in further
Table 3
Solvent effect on metathesis of PhC^CR catalyzed by nitride complexes.a
Catalyst
C6D6
(h)
CD2Cl2
(h)
CDCl3
(h)
R
Temp
(ꢀC)
Q
5/MgBr2
6/MgBr2
1
2
14b
43d
4
4
11
43f
8
Et
60
60
RTg
RT
0.20 ꢃ 0.01c
e
NRe
6
Me
Me
Me
catalyst
0.25 ꢃ 0.01h
2Ph
C
C
C
Et
Ph
C
C
Ph
Et
C
C
Et
9
13
7
0.24 ꢃ 0.02h
a
Ph
C
Ph Et
C
C
Et
NMR scale reactions with 5 mol% catalyst.
Toluene-d8.
31% Diphenylacetylene/69% 1-phenyl-1-butyne.
Keq
=
b
c
2
Ph
C
C
Et
Since:
d
e
f
Ph
C
C
Ph
=
Et
C
C
Et
2
2
15% Diphenylacetylene/85% 1-phenyl-1-propyne (Q ¼ 0.03).
Ph
Ph
C
C
C
C
Ph
Et
NR ¼ no reaction.
Keq
=
24% Diphenylacetylene/76% 1-phenyl-1-propyne (Q ¼ 0.11).
RT ¼ room temperature.
g
h
33% Diphenylacetylene/67% 1-phenyl-1-propyne.
Fig. 4. Equilibrium calculations.

A.M. Geyer et al. / Journal of Organometallic Chemistry 708-709 (2012) 1e9
7
Table 4
Table 5
Alkyne metathesis studies with 1-phenyl-1-propyne assisted by Lewis acids with
Alkyne effect on metathesis rate with 5 and PheC^CeR (R ¼ Et, Me) assisted by
complexes 5, 6, 1 and 2a: Time to reported Q-value.
Lewis acids.a
Entry
Lewis acid
Time (h)
5b
Lewis Acid
Et (Time, h)
Me (Time, h)
6c
23f
19
88
11
27
27
1d
2e
MgBr2
MgI2
CuCl2
CuBr2
BPh3
18
5.5
93
59
1
2
3
4
5
6
7
8
9
MgBr2
MgI2
93
59
NREg
NRE
Dec
6
NRE
NRE
NRE
6
NRE
NRE
Dec
NRE
NRE
NRE
5
44
99
8b
59b
NR
NR
TiCl(OiPr)3
ZrCl4
Dech
Dec
99
45 (polymer)
None
NRc
CuCl2
CuBr2
BPh3
HCl
None
Q
0.22 ꢃ 0.3d
0.07b
59
NRi
Dec
NR
Polymerj
NR
NR
a
NMR scale reactions with 5 mol% 5, 40 ꢀC, CD2Cl2.
NRE
9
b
c
20% Diphenylacetylene/80% 1-phenyl-1-propyne (Q ¼ 0.07 ꢃ 0.02).
8
NR ¼ no reaction.
d
Q
0.06 ꢃ 0.01
0.07 ꢃ 0.01
0.20 ꢃ 0.01
0.20 ꢃ 0.01
33% Diphenylacetylene/67% 1-phenyl-1-butyne.
a
NMR scale reactions with 5 mol% catalyst.
CD2Cl2 at 40 ꢀC.
b
c
d
e
f
C6D6 at 80 ꢀC.
80% 1-phenyl-1-propyne. No further alkyne metathesis was
observed upon additional heating. As observed with 5, an induction
period was required for catalyst conversion prior to alkyne
metathesis occurring. Additionally, no alkylidyne complexes were
observed spectroscopically.
CDCl3 at room temperature.
C6D6 at room temperature.
Q ¼ 0.21.
g
h
i
NRE ¼ no rate enhancement.
Dec. ¼ catalyst decomposition.
NR ¼ no reaction.
j
Poly(3-hexyne) present.
3.3.4. Enhancing the rate of alkyne metathesis with 1 and 2 through
Lewis acid assistance
Activity studies were completed with 1 and 2 in the presence of
2 equiv of Lewis acid and 20 equiv of 1-phenyl-1-propyne to
determine whether the rate of alkyne metathesis could be
enhanced with these pre-catalysts (Table 4). These reactions were
monitored to a composition of 31% diphenylacetylene and 69% 1-
phenyl-1-propyne. With each catalyst, the solvent that resulted in
the slowest rate of metathesis in the absence of a Lewis acid was
employed in order to observe the greatest impact on metathesis
rate. As observed with 5 and 6, 1 required an activation period prior
to observing alkyne metathesis under the reaction conditions
(CDCl3, room temperature). Only zirconium (IV) chloride and
hydrochloric acid were found to enhance the rate of alkyne
metathesis with 1. Unlike 1, triphenylborane was the only Lewis
acid that assisted in the rate of alkyne metathesis with 2 in C6D6 at
room temperature. No evidence of poly-3-heyxne formation was
noted. In this case, the lack of an extended catalyst induction period
is indicated in Fig. 5, with the rate of metathesis slowing as equi-
librium is approached.
conversion (Q ¼ 0.21) to alkyne metathesis products upon addi-
tional heating. Some alkyne metathesis was observed in the pres-
ence of triphenylborane; however, unlike with the other Lewis
acids, alkyne polymerization dominated as indicated by the
formation of an insoluble gelatinous material. Additional Lewis
acids were tested with no alkyne metathesis activity being
observed (See supplementary information). There is presently no
obvious trend in the Lewis acids that assist in alkyne metathesis
with 6. The length of time required to convert 6 to trace amounts of
alkylidyne complex in order for alkyne metathesis to occur varied
with each Lewis acid.
3.3.3. Nitride-to-alkylidyne moiety conversions with 5 assisted by
Lewis acids
Examination of the activity of 5 with 20 equiv of 1-phenyl-1-
propyne under its optimized reaction conditions (CD2Cl2, 40 ꢀC)
revealed that a subset of the Lewis acids that assisted the alkyne
metathesis activity of 6 also did so with 5 (Table 4). The reactions
were monitored to a composition of 20% diphenylacetylene and
3.3.5. Substrate influence on alkyne metathesis assisted by Lewis
acids with 5
In addition to Lewis acids influencing the rate of alkyne
metathesis, an unanticipated substrate-dependent rate difference
was observed when comparing PheC^CeMe and PheC^CeEt
with 5. The rates of alkyne metathesis when R ¼ Et were enhanced
relative to those when R ¼ Me (Table 5). Additionally, an equilib-
rium mixture of alkyne products was achieved when R ¼ Et under
the reaction conditions, since alkyne metathesis occurred at a more
rapid rate than catalyst decomposition. The difference in alkyne
metathesis rates might be attributed to a difference in 3-hexyne
and 2-butyne polymerization rates; however, metathesis with
toluene-d8
9 d, 85oC
toluene-d8
3 d, 80oC
C6D6
3 d, 80oC
+
7
EtC CEt
+
2 MgBr2
PhC CPh + 2 MgBr2
MgBr2
N
Mo
10
+
+
OR
OR
OR=OCMe3
RO
PhC CEt
+
Fig. 5. Conversion toward equilibrium: alkyne metathesis of 1-phenyl-1-propyne with
2 and BPh3.
Scheme 6. Isobutylene formation with 6.

8
A.M. Geyer et al. / Journal of Organometallic Chemistry 708-709 (2012) 1e9
OR=OCMe2CF3
N
Et
C
Mo
toluene-d8
1 d, 95oC
toluene-d8
1 d, 95oC
+ 10 EtC CEt + 2 MgBr2
decomp
Mo
OR
RO
OR
OR
RO
OR
12%
N
C6D6
1 d, 70oC
20
20
+
+
PhC CEt + 2 MgBr2
PhC CPh + 2 MgBr2
no reaction
Mo
OR
OR
RO
N
C6D6
1 d, 70oC
no reaction
Mo
OR
OR
RO
Scheme 7. Attempted formation of alkylidyne complexes from 5.
1-phenyl-1-propyne should then proceed faster than with 1-
phenyl-1-butyne. The decreased rate of alkyne polymerization
relative to alkyne metathesis as the substrate alkyl-chain (R group)
length increases has been noted previously in alkyne metathesis
systems [30]. Furthermore, no evidence of polymer formation was
noted in the current study. The origin of the rate differences asso-
ciated with alkyl-chain length upon catalysis with 5 and a Lewis
acid is still under investigation.
or benzylidyne analogs of 1 or 2 can be readily accessed via
metathesis if they are desired. Future studies with these systems
will focus on determining the mode of interaction of the Lewis acid
with the nitride complexes. Additionally, the influence of Lewis
acids on metathesis reactions with tungsten nitride complexes will
be pursued.
Acknowledgments
3.3.6. Attempted isolation of alkylidyne complexes from 5 to 6
Now that the ability to form an alkylidyne complex from 5 and 6
in the presence of a Lewis acid had been established, isolation of an
alkylidyne complex from the reaction mixture was desired. Since
magnesium bromide was found to readily assist in alkyne metath-
esis, several attempts at isolating alkylidyne complexes were
completed with this Lewis acid. Symmetrical alkyl and aryl-based
alkynes and unsymmetrical alkynes were examined at elevated
temperatures in aromatic solvents with both 5 and 6. As shown in
Scheme 6, the CeO bond scission product, isobutylene, was the only
readily identifiable product in the reaction mixtures with 6.
Unlike 6, for 5 1H and 19F NMR spectroscopies provided direct
evidence of alkylidyne complex formation (12%) upon heating with
10 equiv of 3-hexyne and 2 equiv of magnesium bromide at 95 ꢀC
for 1 day (Scheme 7). Attempts to drive the formation of the pro-
pylidyne complex only resulted in destruction of the alkylidyne
complex, which decomposes upon prolonged heating in the pres-
ence of magnesium bromide. Formation of alkylidyne complexes
via treatment of 5 with other alkyne substrates was unsuccessful.
This material is based upon work supported by the National
Science Foundation under Grant No. CHE-0449459. We also thank
the University of Michigan for financial support.
Appendix. Supplementary data
Supplementary data related to this article can be found online at
doi:10.1016/j.jorganchem.2011.08.001.
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