Stoichiometric carbon-carbon bond formation mediated by well defined Nb(III) complexes Dedicated to Professor John E. Bercaw, for his deep and wide-ranging contributions to organometallic chemistry and catalysis.

Article abstract of DOI:10.1016/j.poly.2014.05.031

The discovery of a Nb-mediated double coupling of terminal alkynes and carbon monoxide is reported. The Nb(III) complex (BDI)Nb(N^tBu)(CO)₂ reacts with two equivalents of t-butylacetylene to form a metal-bound yne-one-ene moiety resulting from coupling of CO to two equivalents of the terminal alkyne. Additionally, Z-alkene and α,β-unsaturated imine formation from hydrogenolysis of an internal alkyne bound complex in the presence of an isocyanide ligand is described. ¹H NMR spectroscopy and synthesis of a bromide analog support the involvement of an alkenyl iminyl ligand intermediate.

Full text of DOI:10.1016/j.poly.2014.05.031

Accepted Manuscript
Stoichiometric carbon-carbon bond formation mediated by well defined Nb(III)
complexes
Thomas L. Gianetti, Robert G. Bergman, John Arnold
PII:
S0277-5387(14)00347-7
DOI:
http://dx.doi.org/10.1016/j.poly.2014.05.031
POLY 10745
Reference:
Polyhedron
To appear in:
Received Date:
Accepted Date:
15 April 2014
15 May 2014
Please cite this article as: T.L. Gianetti, R.G. Bergman, J. Arnold, Stoichiometric carbon-carbon bond formation
mediated by well defined Nb(III) complexes, Polyhedron (2014), doi: http://dx.doi.org/10.1016/j.poly.2014.05.031
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Stoichiometric carbon-carbon bond formation mediated by well defined Nb(III) complexes
Thomas L. Gianetti, Robert G. Bergman* and John Arnold*
Department of Chemistry, University of California, Berkeley, California 94720
Abstract. The discovery of a Nb-mediated double coupling of terminal alkynes and carbon
monoxide is reported. The Nb(III) complex (BDI)Nb(NtBu)(CO)2 reacts with two equivalents
Keywords:
Alkynes coupling
Niobium
Imido
C-C bond formation
of t-butylacetylene to form a metal-bound yne-one-ene moiety resulting from coupling of CO
to two equivalents of the terminal alkyne. Additionally, Z-alkene and α,β-unsaturated imine
formation from hydrogenolysis of an internal alkyne bound complex in the presence of an
1
isocyanide ligand is described. H NMR spectroscopy, and synthesis of a bromide analog
support the involvement of an alkenyl iminyl ligand intermediate.
1. Introduction
2. Experimental
The use of d2 early-transition metal complexes as 2.1 General methods
reducing agents is a powerful tool in synthetic organic
Unless otherwise noted, all reactions were performed
chemistry.1 The ability to form C–C bonds via the reductive
coupling of unsaturated organic molecules (alkenes, alkynes,
ketones and ketimines) provides alternative routes to dienes,
amino alcohols, diols, and bicyclic products. However, the
small number of coordinatively unsaturated d2 complexes
has limited the development of new coupling chemistry and
these transformations are mainly observed with d2
metallocenes of group IV.2 The most common reaction
reported is the coupling of two alkynes, via the formation of
a metallacyclopentadiene, to form a diene moiety that is
released via acidic work-up.3 Hydroformylation and
hydroiminylation of alkenes and alkynes involving coupling
with carbon monoxide or isocyanide to form an enone or
enamine products is also known.4 However, the coupling of
a carbon monoxide and two alkynes,5 as well as the
formation of an α,β-unsaturated imine from a metal-
mediated reaction of alkyne and isocyanide,6 remains rare.
using standard Schlenk line techniques or in an MBraun
inert atmosphere box under an atmosphere of purified
nitrogen (<1 ppm O2/H2O). Glassware, cannulae, and Celite
were stored in an oven at ca. 160 ºC. n-Pentane, n-hexane,
Et2O and toluene, were purified by passage through a
column of activated alumina, stored over 3 or 4 Å molecular
sieves, and degassed prior to use.9 Deuterated solvents
(C6D6, C7D8) were dried over sodium/benzophenone,
vacuum transferred to a storage flask containing activated
molecular sieves, and degassed by three freeze-pump-thaw
cycles before being stored in the dry box. PhC≡CMe and
tBuC≡CH were stored over 4 Å activated molecular sieves
and degassed by three freeze-pump-thaw cycles. 1,3,5-
trimethoxybenzene was sublimed under static vacuum.
(BDI)(CO)2Nb(NtBu),10a
(BDI)(Cl)2PyNb(NtBu),10b
t 8
t
≡CMe)(
(BDI)Nb(N Bu)(PhC
L) with (L = CO and CN Bu),
and Ph(MgBr)C=C(H)Me11 were prepared using literature
procedures. All other reagents were acquired from
commercial sources and used as received. NMR spectra
were recorded on Bruker AV-300, AVQ-400, AVB-400,
DRX-500, AV-500, and AV-600 spectrometers. Chemical
shifts were measured relative to residual solvent peaks,
which were assigned relative to an external TMS standard
set at 0.00 ppm. 1H and 13C NMR assignments were
In our effort to develop new group 5 catalysts,7 we
recently discovered the selective semi-hydrogenation of
alkynes under H2:CO atmosphere, catalyzed by a complex 1:
a dicarbonyl niobium(III) imido supported by a BDI ligand
(BDI = N,N-diisopropylphenyl-β-diketiminate, Scheme 1).8
Our mechanistic investigations revealed the involvement of
an η2-alkyne bound monocarbonyl intermediate, 2-CO,
which is rapidly formed via displacement of one equivalent
of carbon monoxide. This species, which was isolated and
characterized, then interacts with a molecule of H2 to release
the cis-alkene organic product and reform the dicarbonyl
niobium catalyst.6
1
1
routinely confirmed by H-1H (COSY, NOESY) or H-13C
(HSQC and HMBC) experiments. The uncorrected melting
points were determined on an Optmelt SRS using sealed
capillaries prepared under nitrogen. Elemental analyses were
determined at the College of Chemistry, University of
California, Berkeley. The X-ray structural determinations
were performed at CHEXRAY, University of California,
Berkeley on Bruker SMART 1000 or SMART APEX
diffractometers. GC/MS analyses were performed using a
Agilent 6890 N Network GC system coupled to a 5973
Network mass selective detector.
SCHEME 1
Ph
Me
CO
Ar
Ar
Hexanes
t
N
t
N
N Bu
N Bu
Nb
Nb
Me
Ph
N
CO
N
Ar
r.t., 15 mins
CO
Ar
1
2-CO
2.2 Synthesis and characterization
Here we report the reaction between the dicarbonyl
complex 1 and a terminal alkyne leading to an unusual
coupling product, as well as hydrogenolysis of the NCtBu
bound analog of 2-CO, resulting in both Z-alkene and α,β-
unsaturated imine formation.
2.2.1 (BDI)(tBuCCC(O)(C(H)=C(H)tBu)Nb(NtBu), 3
tBuC≡CH (48 µL, 3.90 mmol , 5 equiv.) was added to
a solution of 1 (0.51 g, 0.78 mmol, 1 equiv.) in hexane at

room temperature. The solution immediately changed color Hz), 5.51 (s, 1 H, HC(C(Me)NAr)2), 3.24 (sept, 1 H,
3
3
from yellow-green to orange. The solution was stirred for 15 CHMe2, JHH = 6.8 Hz), 3.15 (sept, 1 H, CHMe2, JHH = 6.8
3
min at room temperature, the volatile materials were Hz), 3.08 (sept, 1 H, CHMe2, JHH =6.8 Hz), 2.86 (sept, 1 H,
3
3
removed under vacuum and the orange residue was CHMe2, JHH = 6.8 Hz), 1.75 (d, 3 H, PhC=C(H)Me, JHH
=
extracted with hexanes (40 mL). Crystallization from 6.4 Hz), 1.71 (s, 3 H, HC(C(Me)NAr)2), 1.47 (s, 3 H,
hexane at -40 oC resulted in the formation of orange crystals HC(C(Me)NAr)2), 1.39 (m, 9 H, CHMe2), 1.23 (d, 3 H,
3
of 3 (68 %). X-ray quality crystals were obtained by CHMe2, JHH = 6.8 Hz), 1.20 (d, 6 H, CHMe2, 3JHH =7.2 Hz),
3
recrystallization from toluene at -20 oC. 1H NMR (400 MHz, 1.11 (d, 3 H, CHMe2, JHH = 6.8 Hz), 1.07 (d, 3 H, CHMe2,
3
C6D6, 293 K): δ(ppm) 7.36 (d, 1H, Ar, JHH = 7.2 Hz), 7.27 3JHH =6.8 Hz), 0.93 (s, 9 H, tBu). Chloride complex: 7.53 (d,
3
(m, 2H, Ar), 7.21 (m, 2H, Ar), 7.12 (m, 2H, Ar), 7.06 (d, 1H, 2 H, PhC=C(H)Me, JHH = 7.2 Hz), 6.62 (q, 1 H,
3
3
3
Ar, JHH = 7.0 Hz), 7.01 (d, 1H, Ar, JHH = 6.8 Hz), 6.55 (d, PhC=C(H)Me, JHH
=
6.4 Hz), 5.46 (s,
1
H,
1H, RCH=CHtBu, JHH = 16.4 Hz), 6.18 (d, 1H, HC(C(Me)NAr)2), 1.80 (d, 3 H, PhC=C(H)Me, JHH = 6.4
3
3
RCH=CHtBu, JHH
=
16.4 Hz), 5.28 (s,
1
H, Hz), 1.62 (s, 3 H, HC(C(Me)NAr)2), 1.49 (s, 3 H,
H, tBu). No further
3.83 (sept, 1 H, CHMe2, JHH = 7.0 Hz), 3.11 (sept, 1 H, characterization was performed due to the mixed
3
3
HC(C(Me)NAr)2), 4.50 (sept, 1 H, CHMe2, JHH = 7.0 Hz), HC(C(Me)NAr)2), 0.85 (s,
9
3
3
3
CHMe2, JHH = 6.9 Hz), 2.76 (sept, 1 H, CHMe2, JHH = 6.9 composition of this sample.
Hz), 1.77 (s, 3 H, HC(C(Me)NAr)2), 1.71 (s, 3 H,
3
(BDI)(PhC=C(H)Me)(Br)Nb(NtBu), 5-Br.
HC(C(Me)NAr)2), 1.65 (d, 3 H, CHMe2, JHH = 6.8 Hz), 1.53
(d, 3 H, CHMe2, JHH = 6.8 Hz), 1.40 (d, 3 H, CHMe2, 3JHH
=
3
MgBr2·Et2O was added to a solution of 5-X (X = Br:Cl,
in a 9:1 ratio, 0.220 g, 0.282 mmol, 1 equiv.) in ether (20
mL) and the mixture was stirred overnight. The volatile
material was removed under vacuum and the residue was
extracted with pentane (3 x 30 mL). The extracts were
concentrated to 50 mL to afford orange crystals after storage
at -40 ºC for two days. Yield: 164 mg, 75 %. X-ray quality
crystals were obtained by recrystallization from hexanes at -
20 C. The H NMR spectrum was completely identical to
that of the bromide complex described previously. 13C NMR
(100 MHz, C6D6, 293 K): 171.3 and 169.3 (C,
HC(C(Me)NAr)2), 143.1 (CH, Nb-C(Ph)=CHMe)), 145-140
(CH, Nb-C(Ph)=CHMe)), 141 (C, Nb-C(Ph)=CHMe)), 145-
140 (4 peaks, C, ArαN), 121.3 (C, Nb-C(Ph)=CHMe)), 127-
123 (6 peaks, CH, Ar), 105.5 (CH, HC(C(Me)NAr)2), 70.5
(C, Nb=NC(CH3)3), 30.5 (CH3, Nb=NC(CH3)3), 34.5 (CH3,
Nb-C(Ph)=CHMe)), 29.6-28.5 (4 peaks, CH, CHMe2), 25.3
and 25.0 (CH3, HC(C(Me)NAr)2), 27-25 (8 peaks, CH3,
CHMe2). Anal. Calcd for C42H59BrN3Nb: C, 64.78; H, 7.64;
N, 5.40; found: C, 65.13; H, 7.93; N, 5.55. Mp: 160-165 ºC
(decomp).
3
7.2 Hz), 1.37 (d, 3 H, CHMe2, JHH = 6.8 Hz), 1.27 (d, 3 H,
CHMe2, JHH =6.8 Hz), 1.25 (s, 9 H, Nb=NtBu), 1.15 (d, 6 H,
3
3
3
CHMe2, JHH =7.2 Hz), 1.11 (d, 3 H, CHMe2, JHH =6.8 Hz),
0.96 (s, 9 H, RCH=CHtBu), 0.88 (d, 3 H, CHMe2, JHH = 6.8
3
Hz), 0.81 (s, 9 H, BuCCC(O)R’). 13C NMR (100 MHz,
t
C6D6, 293 K): 171.3 and 169.3 (C, HC(C(Me)NAr)2), 155.3
t
t
(C, BuCCC(O)R’), 150.2 (C, BuCCC(O)R’), 147.5 (CH,
RCH=CHtBu), 145-140 (4 peaks, C, ArαN), 135.4 (C,
tBuCCC(O)R’), 125.3 (CH, RCH=CHtBu), 127-123 (6
peaks, CH, Ar), 103.5 (CH, HC(C(Me)NAr)2), 72.5 (C,
o
1
t
Nb=NC(CH3)3), 34.2 and 33.5 (C and CH3, BuCCC(O)R’),
32.5 (CH3, Nb=NC(CH3)3), 29.4 and 29.0 (CH3 and C,
RCH=CHtBu), 29.0-27.9 (4 peaks, CH, CHMe2), 26.3 and
26.0 (CH3, HC(C(Me)NAr)2), 26-24 (8 peaks, CH3, CHMe2).
To simplify the assignment R and R’ were used to described
t
the yne-one-ene moiety with R = BuCCC(O) and R’=
RCH=CHtBu. IR (nujol): 1522 cm-1. Anal. Calcd for
C50H71N3NbO: C, 71.38; H, 9.12; N, 5.43; found: C, 71.14;
H, 9.36; N, 5.65. Mp: 185-190 ºC (decomp).
2.2.2 (BDI)(PhC=C(H)Me)(X)Nb(NtBu), 5-X
(BDI)(PhC=C(H)Me)(X)Nb(NtBu) 5-X, (X=Br:Cl, in a 9:1
2.3 Stoichiometric hydrogenation of 2-CNtBu
ratio).
To a J-Young NMR tube with a Teflon cap containing
0.5 mL of C6D6 and trimethoxybenzene (0.011 M, 1 equiv.)
was added complex 2-CNtBu (51 mg, 0.078 mmol, 1 equiv),
and t-butyl isocyanide (44 µL, 0.390 mmol, 5 equiv.). The
solution was freeze-pump-thawed three times and then
refilled with H2 (1 atm, 2.5 mL, 0.102 mmol, 10 equiv). The
mixture was allowed to warm to room temperature. The
transformation was then monitored by 1H NMR
spectroscopy until completion (4 h). NMR yields were
calculated by integration of the vinyl protons of the cis-β-
To a stirred suspension of (BDI)pyCl2Nb(NtBu) (0.369
g, 0.490 mmol, 1 equiv.) in 20 mL Et2O cooled to -78 ºC
was
added
dropwise
the
trans-Grignard
Ph(MgBr)C=C(H)Me in Et2O (1.2 mL, 0.38 M, 0.45 mmol,
0.9 equiv.). The resulting mixture was stirred for 20 min at -
78 ºC. The solution was allowed to warm to room
temperature, resulting in a color change from deep red to
green. The mixture was stirred for 1 h at room temperature,
and then the volatile materials were removed under vacuum.
The residue was extracted with pentane (3 x 30 mL) and the
extracts were concentrated to 50 mL, at which time yellow
crystals started to form on the walls of the flask. The flask
was stored at -40 ºC for 2 days, resulting in isolation of
1
methylstyrene and N-(t-butyl)-2-phenylbut-2-en-1-imine. H
NMR
(500MHz,
C6D6,
293
K),
δ(ppm),
trimethoxylbenzene: 6.26 (s, 3 H, benzene ring); cis-β-
3
methylstyrene: 6.42 (dd, 1 H, PhC(H)=C(H)Me, JHHvinyl
=
1
3
yellow crystals of 5-X. Analysis by H NMR spectroscopy
11.5 Hz, JHHphenyl = 2 Hz); N-(t-butyl)-2-phenylbut-2-en-1-
imine: 5.95 (q, 1 H, MeCHC(Ph)NtBu, 3JHH =6.8 Hz). Once
the reaction was complete, the organic products were
analyzed by GC/MS further confirming the nature of the
α,β-unsaturated imine product. (m/z): P(2) = 201.
revealed a mixture of two complexes: 90 % of the bromide
and 10 % of the analogous chloride. Yield: 152 mg, 64 %.
1H NMR (400 MHz, C6D6, 293 K): δ(ppm) Bromide
complex: 7.58 (d, 2 H, PhC=C(H)Me, JHH = 7.2 Hz), 7.26-
7.20 (m, 5 H, Ar and PhC=C(H)Me), 7.11-7.00 (m, 5 H, Ar
3
3
and PhC=C(H)Me), 6.73 (q, 1 H, PhC=C(H)Me, JHH = 6.4
2

2.4 X-ray crystallography study
Table 1 Crystallographic parameters of 3 and 5-Br
A crystal of appropriate size was coated in Paratone-N
oil and mounted on a Kaptan® loop. The loop was
transferred to a diffractometer equipped with a CCD area
detector,12a centered in the beam, and cooled by a nitrogen
flow low-temperature apparatus that had been previously
calibrated by a thermocouple placed at the same position as
the crystal. Preliminary orientation matrices and cell
constants were determined by collection of 3x20 10 s frames,
followed by spot integration and least-squares refinement.
An arbitrary hemisphere of data was collected, and the raw
data were integrated using SAINT.12b Cell dimensions
reported were calculated from all reflections with I > 10 σ.
The data were corrected for Lorentz and polarization effects;
no correction for crystal decay was applied. Data were
analyzed for agreement and possible absorption using
XPREP.12c An empirical absorption correction based on
comparison of redundant and equivalent reflections was
applied using SADABS.12d Structures were solved by direct
methods with the aid of successive difference Fourier maps
and were refined on F2 using the SHELXTL 5.0 software
package. Thermal parameters for all non-hydrogen atoms
Compound
3.C7H8
5-Br
Formula
C50H70N3NbO C42H59BrN3Nb
Formula weight
Space Group
a (Å)
822.00
P21/c
778.74
P21/n
9.377(5)
22.177(5)
22.588(5)
90
13.268(2)
19.718(3)
15.667(3)
90
b (Å)
c (Å)
α (°)
β (°)
90.240(5)
90
98.086(2)
90
γ (°)
V (Å3)
4697(3)
4
4058.0(12)
4
Z
ρcalcd (g/cm3)
1.162
1.275
F000
µ (mm-1)
1760
1632
0.294
1.312
Tmin/Tmax
0.8940/0.9328 0.7267/0.8800
were refined anisotropically. For all structures, R1 = Σ(|Fo| -
No. rflns measured
No. indep. Rflns
Rint
55745
8591
0.0255
8591
581
23885
7386
0.0577
7386
438
2 2
|Fc|)/Σ(|Fo|); wR2 = [Σ{w(Fo2 - Fc2)2}/Σ{w(Fo ) }]1/2
.
3. Results and discussion
No. obs. (I > 2.00σ(I))
No. variables
R1, wR2
3.1 Reactivity with terminal alkyne
0.0287, 0.0716 0.0471, 0.1010
Reaction of 1 with 5 equivalents of t-butylacetylene led
to a fast color change from deep yellow green to orange.
Subsequent workup provided orange crystals of 3 in 87 %
yield. The absence of a terminal carbonyl band in the IR
spectrum suggests that a new species, different from 1 and
2-CO, was formed (1: νCO = 1988 and 1999 cm-1, and 2-
CO: νCO = 2031 cm-1). Instead, an intense band at 1522 cm-1
was observed, which suggests the presence of highly
R1 (all data)
GoF
Res. peak/hole (e–/Å3)
0.0363
1.046
0.0919
1.011
0.349/-0.285
0.555/-0.793
The structure of
3 was revealed using X-ray
crystallography (Figure 1), which shows the unprecedented
coupling of two alkynes and one carbon monoxide to form
an ene-one-yne moiety bound in a κ4-fashion. Complex 3
exhibits a pseudo square-based pyramidal geometry (τ =
0.42),13 in which both the BDI ligand and the ene-one-yne
moiety occupy the basal position, while the imido group is
in apical position. Both the metal-BDI nitrogen and metal-
imido distances are within the range of distances previously
1
reduced CO moiety. H NMR spectroscopic analysis reveals
the formation of a low-symmetry complex displaying three
singlets in the alkyl region integrating for 9 H each,
consistent with three inequivalent t-butyl groups (one from
the imido and two from the alkyne moieties). This
observation implies coupling of the alkynes, a reaction that
is well known in low-valent early transition metal
chemistry.3
reported for this system (Nb-NBDI = 2.22 Å and Nb-Nimido
=
1.757(4) Å). The bond distances of the moiety formed by
alkyne-carbonyl coupling as well as the distance between
this fragment and the metal center are presented in Figure 2,
left. The long C(1)-C(2) and C(3)-O(1) bond distances of
1.283(3) Å and 1.337(4) Å respectively, compared to 1.20 Å
for alkynes and 1.23 Å for ketones, along with the short
C(2)-C(3) of 1.386(2) Å, suggest a delocalization of the π-
system within the κ4-yne-one ligand along with formal
oxidation of the metal center to Nb(V). Additionally, the
Nb-C and Nb-O bond distances (Nb(1)-C(1) = 2.356(3) Å,
Nb(1)-C(2) = 2.272(3) Å, Nb(1)-C(3) = 2.308(3) Å and
Nb(1)-O(1) = 2.116(2) Å) are within the range of Nb(V)-
C(alkyl) and Nb(V)-C(alkoxide) bonds reported previously
for this system. 13C NMR spectroscopy reveals a significant
formal reduction of the κ4-moiety (Figure 2, right). Both C1
and C2 possess significantly downfield shifted 13C
resonances (C1 = 155 ppm and C2 = 150 ppm) compared to
Coupling of terminal alkynes can form two
metallacyclopentadiene regioisomers, the head to head
product (M-(RC=CH-CH=CR-)) or the head to tail isomer
(M-(HC=CR-CH=CR-)). The β-Hs of the first regioisomer
1
are often observed as singlets in H NMR spectroscopy,3c-e
while the α-H / β-H set of the second regioisomer exhibits
two doublets with a small coupling constant (4JH-H = 1 to 3
Hz),3f,g both within the aromatic region (6.5 to 7.5 ppm). In
the present case, two doublets, integrating for one proton
each, are observed. COSY and HSQC experiments reveal
that these two resonances are not attributable to
diasterotopic hydrogens and are coupled to each other,
which imply the formation of a metallacyclopentadiene in a
head to tail fashion. However, their downfield chemical
shifts (6.0 to 6.5 ppm) along with their large coupling
constants (3JH,H = 16.2 Hz) are inconsistent with the
formation of a metallacyclopentadiene.
3

those observed in free t-butyl acetylene (92 and 67 ppm), or believed to be the bis- or tris-tBuNC niobium analog of 1.
compared to signals for a free ynone such as but-3-yn-2-one The thermal instability of this complex hampered its
(82 and 78 ppm). Similarly C3 has a chemical shift of 135 isolation and characterization. NMR spectroscopy and
ppm, upfield compared to that of a free yneone molecule GC/MS analysis revealed the formation of the expected cis-
(185 ppm), and consistent with the low stretching frequency alkene (P1) along with N-(t-butyl)-2-phenylbut-2-en-1-
1
observed by IR spectroscopy (see above). Both the H and imine (P2). The formation of the second organic product is
13C chemical shifts of H1, H2, C4 and C5 are within believed to have occurred via an alkenyl iminyl intermediate
1
expected values for an enone moiety.
B (Scheme 3b). Monitoring of this transformation by H
NMR spectroscopy revealed the presence of an
organometallic intermediate that exhibits a quartet at 5.56
ppm (3JH,H = 6.5 Hz) integrating for 1 H relative to the BDI
ligand backbone proton (δ = 5.18 ppm). The similarity with
SCHEME 2
CO
t
N Bu
Ar
Ar
Hexanes
O
H
t
N
N Bu
CO
N
tBu
Nb
2
Nb
1
N
Ar
N
Ar
t
r.t., 15 mins
Bu
the H NMR chemical shift of the free organic product
(singlet at 7.95 ppm for Ha and quartet at 5.95 ppm for Hb
with JH,H = 6.8 Hz, Scheme 3b) suggests that the species
H
t
Bu
1
3
3
observed can be assigned to one of the two intermediates A
and B proposed in Scheme 3b or a product-bound species.
However, the complexity of the spectrum along with the
short lifetime of this compound hampered its complete
solution characterization.
Interestingly, during the reported catalytic semi-
hydrogenation of 1-phenylpropyne with 1 and under high
concentration of CO, no hydroformylation products were
observed. However, the DFT calculations we reported
suggested that an alkenyl formyl complex (the analog of the
proposed intermediate B) was energetically accessible; but
such a species was not experimentally observed. Thus, we
believe that the reactivity differences observed between 2-
CO (hydrogenation only) vs. 2-CNtBu (hydrogenation and
C-C coupling) toward H2, is governed by the relative rates
of the hydride transfer from Nb to the alkenyl moiety vs. the
CX ligand (X = O or NtBu).
SCHEME 3
Figure 1. ORTEP diagram of 3. Hydrogen atoms and isopropyl of
the arene groups have been omitted for clarity.
Ph
Me
Hexanes
5 equiv. CN Bu
CO
Ar
Ar
t
t
N
t
N
N Bu
N Bu
Nb
Nb
a.
Me
Ph
CO
N
Ar
t
N
Ar
r.t., 15 mins
CN Bu
1
2-CNtBu
O
6.18
H1
O
2.116
1.337
H
2.308
2.272
1.470
Me
Nb
125
C4
C3
Me
Nb
C3
135
C4
Ph
Ar
t
Ph
1.316
C5
Ph
Ar
Bu
Me
t
1.386
H
C5
147
H2
6.55
Ar
Bu
H
NtBu
C2
N
N
Ar
NtBu
1 atm H2
2.356
N
NtBu
N
Nb
C2
1.283
150
b.
Nb
Nb
H
C1
155
t
t
N
N
Ar
C
H
H
CN Bu xs. BuNC
C6D6
C1
t
Ar CN Bu
t
N Bu
B
2-CNtBu
t
A
Bu
t
Bu
Chemical shift in ppm
Bond distances in Å
Ph
Me
Hb
H
H
Figure 2. Selected bond distances (in Å, left) and chemical shifts
(in ppm, right) for the yne-one-ene moiety of 3.
N
Ph
Me
t
Ha
Bu
P1: 65 %
P2: 35 %
3.2 Reactivity of internal alkyne
In order to obtain further structural information on this
alkenyl intermediate, the halide analog was synthesized by
reaction between (BDI)(Cl)2PyNb(NtBu), 4, and one
equivalent of the E-alkenyl Grignard (Scheme 4). After
workup, yellow crystals of 5-X were isolated in moderate
yield (45%). In the first attempt, incomplete halide transfer
between Mg and Nb was observed leading to the isolation of
a halide mixture, 5-Br and 5-Cl, in a 9:1 ratio. Pure 5-Br
was formed by adding an excess of anhydrous MgBr2·Et2O
once the reaction reached room temperature.
While the mechanism of this unusual terminal alkyne
transformation is not yet understood, C-C bond formation
involving internal alkynes was also observed within this
system. We recently reported that addition of 1-
phenylpropyne to 1 in presence of 5 equivalents of CNtBu
led to the formation of the alkyne bound complex 2-CNtBu,
the isocyanide bound analog of 2-CO (Scheme 3a). We now
present the stoichiometric hydrogenation of 2-CNtBu with
t
an excess of BuNC, which led to a mixture of two organic
products in a 1.6 : 1 ratio (P1 and P2, Scheme 3b), along
with the formation of an organometallic species, which is
An ORTEP diagram derived from the single crystal X-
ray analysis of 5-Br along with selected bond distances and

angles are presented in Figure 3. This complex exhibits a imine product formed by coupling between the alkyne, H2
1
distorted square pyramidal geometry (τ = 0.28)13 with the
and the isocyanide. H NMR monitoring along with the
synthesis of an alkenyl niobium bromide complex suggests
that an alkenyl iminyl species is involved in the C-C
coupling. In our previous report, in which CO was used in
place of CNtBu, related coupling chemistry was not
observed, which shows the sensitivity of the system toward
modest changes in reaction conditions. We are currently
performing further synthetic and mechanistic studies in
order to understand the mechanism involved in these
transformations as well as their scope and catalytic potential.
alkenyl moiety in the apical position. The C(2)-C(3) bond
distance is within the values reported for other niobium
alkenyl complexes and consistent with a carbon-carbon
double bond. However, the Nb(1)-C(3) bond distance of
2.155(3) Å is shorter than other reported distances (between
2.25 and 2.40 Å).14 1H NMR spectroscopy showed that the
BDI proton backbone can be found at 5.51 ppm, and that the
alkenyl proton is coupled to the methyl group resulting in a
quartet (3JH,H = 6.4. Hz) at 6.73 ppm. Although the chemical
shifts are more downfield than those obtained for the
intermediate observed during hydrogenation of 2-CNtBu
(see above), these data support its assignment as an alkenyl
complex; however, they are insufficient to allow further
structural elucidation. Unfortunately, treatment with hydride
sources (e.g. super hydride, red-Al, NaH) in the presence or
absence of CNtBu resulted in either no conversion, or rapid
formation of an intractable mixture, respectively.
Acknowledgments
We thank the Air Force Office of Scientific Research
(grant number FA9550-11-1-0008) for financial support, Dr.
A. DiPasquale for experimental assistance, and, Dr Neil
Tomson, and B. Kriegel for helpful discussions.
Notes and References
Department of Chemistry, University of California,
USA.
SCHEME 4
Berkeley,
California,
94720,
E-mail:
arnold@berkeley.edu; rbergman@berkeley.edu
Me
Ph
† Cif files for complexes
997085).
3 and 5-Br (CCDC: 997084-
Cl
Ar
Ar
N
MgBr
Ph
t
Et2O
t
N
N Bu
N Bu
Nb
Nb
Py
4
+
N
Ar
X
[1]
[2]
F. Sato, H. Urabe, S. Okamoto, Chem. Rev. 100 (2000) 2835.
N
Ar
20 min, -78 o
1 h, r.t.
C
Cl
Me
(a) B. Demerseman, G. Bouquet, M. Bigorgne, J. Organomet. Chem.
107 (1976) C19. (b) B. Demerseman, G. Bouquet, M. Bigorgne, J.
Organomet. Chem. 132 (1977) 223. (c) G. Fachinetti, G. Fochi, C.
Floriani, J. Chem. Soc., Chem. Comm. (1976) 230. (d) K. I. Gell, J.
Schwartz, J. Am. Chem. Soc. 103 (1981) 2687. (e) J. L. Thomas, JK.
T. Brown, J. Organomet. Chem. 111 (1976) 297.
5-X
X = Br,Cl
[3]
For selected examples with early transition metals: (a) A. D. Miller, J.
L. McBee, T. D. Tilley, J. Am. Chem. Soc. 130 (2008) 4992. (b) C.
García-Yebra, F. Carrero, C. López-Mardomingo, M. Fajardo, A.
Rodríguez, A. Antiñolo, A. Otero, D. Lucas, Y. Mugnier,
Organometallics 18 (1999) 1287. (c) Y-W Chao, P. A. Wexler, D. E.
Wigley, Inorg. Chem. 28 (1989) 3860. (d) F. Guérin, D. H.
McConville, J. J. Vittal, Organometallics 16 (1997) 1491. (e) D. P.
Smith, J. R. Strickier, S. D. Gray, M. A. Bruck, R. S. Holmes, D. E.
Wigley, Organometallics 11 (1992) 1275. (f) D. P. Smith, J. R.
Strickier, S. D. Gray, M. A. Bruck, R. S. Holmes, D. E. Wigley,
Organometallics 11 (1992) 1275. (g) J. R. Strickler, P. A. Wexler, D.
E. Wigley, Organometallics 10 (1991) 118.
[4]
[5]
For selected examples with niobium complexes: (a) H. Wakgari, M.
D. Curtis, Organometallics 13 (1994) 2706. (b) M. Etienne, P. S.
White, J. L. Templeton, Organometallics 12 (1993) 4010.
Selected examples: (a) S. R. Allen, M. Green, N. C. Norman, K. E.
Paddick, A. G. Orpen, J. Chem. Soc., Dalton Trans. (1983) 1625. (b)
R. D. Adams, G. Chen, L. Chen, W. Wu, J. Yin, Organometallics 12
(1993) 3431.
[6]
[7]
Selected examples: (a) C. Cao, Y. Shi, A. L. Odom, J. Am. Chem.
Soc. 125 (2003) 2880. (b) C. J. Adams, K. M. Anderson, I. M.
Bartlett, N. G. Connelly, A. G. Orpen, T. J. Paget, Organometallics
21 (2002) 3454.
Figure 3 ORTEP diagram of 5-Br. Hydrogen atoms have been
omitted for clarity. Selected bond lengths (Å): C(1)-C(2): 1.497(6);
C(2)-C(3): 1.330(6); C(3)-C(4): 1.487(6), Nb(1)-C(3): 2.155(6),
Nb(1)-Br(1): 2.588(4). Selected angles (o): C(1)-C(2)-C(3):
130.1(3), C(2)-C(3)-C(4): 123.5(4), Nb(1)-C(3)-C(2): 102.0(3),
Nb(1)-C(3)-C(4): 133.9(3).
(a) N. C. Tomson, A. Yan, J. Arnold, R. G. Bergman, J. Am. Chem.
Soc. 130 (2008) 11262; (b) H. S. La Pierre, J. Arnold, F. D. Toste,
Angew. Chem. Int. Ed. 50 (2011) 3900; (d) T. L. Gianetti, H. S. La
Pierre, J. Arnold, Eur. J. Inorg. Chem. 22-23 (2013) 3771. (e) T. L.
Gianetti, G. Nocton, S. G. Minasian, N. C. Tomson, A. L. D.
Kilcoyne, S. A. Kozimor, D. K. Shuh, T. Tyliszczak, R. G. Bergman,
J. Arnold, J. Am. Chem. Soc. 135 (2013) 3224. (f) T. L. Gianetti, R.
G. Bergman, J. Arnold, J. Am. Chem. Soc. 135 (2013) 8145. (g) T. L.
Gianetti, R. G. Bergman, J. Arnold, Chem. Sci. 5 (2014) 2517.
T. L. Gianetti, N. C. Tomson, J. Arnold, R. G. Bergman, J. Am.
Chem. Soc. 133 (2011) 14904.
4. Conclusions
We have described the unusual coupling between two
terminal alkynes and one carbon monoxide molecule to
form an yne-one-ene bound moiety mediated by a well-
defined dicarbonyl niobium (III) complex. We have also
shown that hydrogenation of the alkyne adduct 2-CNtBu in
presence of isocyanide leads to the formation of the
expected Z-β-methylstyrene along with an α,β-unsaturated
[8]
[9] P. J. Alaimo, D. W. Peters, J. Arnold, R. G. Bergman, J. Chem. Educ.
78 (2001) 64.
[10] (a) N. C. Tomson, J. Arnold, R. G. Bergman, Organometallics 29
(2010) 5010. (b) N. C. Tomson, J. Arnold, R. G. Bergman,
Organometallics 29 (2010) 2926.
[11] (a) F. Sato, J. Organomet. Chem. 285 (1985) 53. (b) F. Sato, H.
Ishikawa, M. Sato, Tetrahedron Lett. 22 (1981) 85.

[12] (a) SMART: Area-Detector Software Package; Brucker Analytical
X-ray Systems, I. M., WI, 2001-2003. (b) SAINT: SAX Area-
detector Integration Program; Brucker Analytical X-ray Systems, I.
M., WI, 2003. (c) XPREP; Brucker Analytical X-ray Systems, I. M.,
WI, 2003. (d) SADABS: Brucker-Nonius Area Detector Scaling and
Absorption v. 2.05; Brucker Analytical X-ray Systems, I. M., WI,
2003.
[13] As determined using the continuous symmetry parameter τ = (α -
β)/60, where α and β are the largest and second-largest angles about
the metal center, respectively, See: A. W. Addison, T. N. Rao, J.
Reedijk, J. van Rijn, G. C. Verschoor, J. Chem. Soc., Dalton Trans.
(1984) 1349.
[14] (a) J. Amaudrut, J. Sala-Pala, J. E. Guerchais, R. Mercier, J. of
Organomet. Chem. 391 (1990) 61. (b) G. E. Herberich, H. Mayer,
Organometallics 9 (1990) 2655. (c) M. Etienne, R. Mathieu, B.
Donnadieu, J. Am. Chem. Soc. 119 (1997) 3218.
6

Entry for the Table of Contents
t
CO
Ar
N Bu
Ar
Hexanes
O
H
t
N
N
N Bu
CO
tBu
H
Nb
2
Nb
N
N
Ar
t
r.t., 15 mins
- CO
Bu
Ar
H
t
Bu
7