Carbon-carbon bond forming reactions with tantalum diamidophosphine complexes that incorporate alkyne ligands

Article abstract of DOI:10.1021/om500775c

The incorporation of o-phenylene-linked diamidophosphine ligands onto the readily available alkyne complexes Ta(alkyne)Cl₃(DME) (where alkyne = hex-3-yne or 1,2-bis(trimethylsilylacetylene); DME = 1,2-dimethoxyethane) results in the formation o

Full text of DOI:10.1021/om500775c

Article
pubs.acs.org/Organometallics
Carbon−Carbon Bond Forming Reactions with Tantalum
Diamidophosphine Complexes That Incorporate Alkyne Ligands
Kyle D. J. Parker and Michael D. Fryzuk*
Department of Chemistry, The University of British Columbia, 2036 Main Mall, Vancouver, BC, Canada V6T 1Z1
S
* Supporting Information
ABSTRACT: The incorporation of o-phenylene-linked dia-
midophosphine ligands onto the readily available alkyne
complexes Ta(alkyne)Cl3(DME) (where alkyne = hex-3-yne
or 1,2-bis(trimethylsilylacetylene); DME = 1,2-dimethoxy-
ethane) results in the formation of a versatile set of starting
materials of the general formula [PhNPN*]Ta(alkyne)Cl
(where [PhNPN*] = PhP(2-(N-mesityl)-5-Me-C6H3)2).
Upon reaction with KBEt3H, the synthesis of the correspond-
ing hydride complexes [PhNPN*]Ta(alkyne)H can be achieved; these complexes feature extremely downfield (δ ∼21 ppm)
1
doublet resonances (2JHP = ∼35 Hz) in the respective H NMR spectra that are assigned to the newly formed Ta−H moieties.
Subsequent reaction of these Ta hydrides with 2,6-dimethylphenylisocyanide and phenylacetylene results in the insertion of these
species into the Ta−H bond and the formation of the corresponding iminoformyl and phenylvinyl complexes, respectively. While
the former intermediate cannot be detected, the latter was characterized by NMR spectroscopy. Both of these processes result in
the further transformation to generate C−C coupled products by a reductive elimination sequence with the coordinated alkyne;
in the case of the iminoformyl, an azadiene results, whereas with the phenylvinyl derivative a butadienyl fragment is generated.
Single-crystal X-ray diffraction and a suite of NMR spectroscopic techniques were used to characterize these species. A discussion
of the bonding of the products in the context of the process by which they form is presented. The rate of formation of the
butadienyl moiety from the phenylvinyl intermediate results in the activation parameters of ΔH⧧ = 22.2 0.3 kcal/mol and ΔS⧧
= −8.7 0.2 cal/(mol)(K).
aminoalkylation, respectively.26−32 In addition, there are a
INTRODUCTION
■
variety of examples of Nb and Ta complexes serving as hydride
Alkynes are well known in the organometallic literature as being
or alkyl transfer agents for ketones,33−38 imines,33,37−41 and
versatile ligands for transition metal complexes. They offer a
alkyne substrates.42−46 These systems also provide further
number of different bonding modes and electron counts1−6 and
examples of metal-mediated C−C bond formation, as the
are important participants in a variety of carbon−carbon bond
resulting iminoacyl or vinyl moieties are also known to couple
forming reactions. Historically, late metal (Pt, Pd, Ni, Co)
with coordinated alkyne units, resulting in the formation of
alkyne complexes dominated in terms of their use as reagents in
more complex metallacyclic products featuring 1,3-butadienyl44
or 1-aza-1,3-butadienyl34,35,47 (AD) organic fragments.
organic synthesis, mediating a variety of transformations
ranging from cyclotrimerization of alkynes to the preparation
In this work, we report the synthesis of a series of tantalum
alkyne complexes that incorporate an ortho-phenylene-bridged
diamidophosphine ancillary ligand. We also present the
of pyridines and cyclopentadienones from alkynes and
isonitriles, olefins, or carbon monoxide.7−9 With respect to
early transition metals, a number of group 5 (Nb and Ta) metal
synthesis of monohydride derivatives of these complexes and
alkyne complexes have also been shown to participate in
their reactivity with 2,6-dimethylphenyl isocyanide and phenyl-
cyclotrimerization reactions,10−17 as well as mediate coupling
reactions between alkynes and a variety of small-molecule
substrates.16,18,19 Related work by Rosenthal and co-workers
has focused on a series of group 4 (Zr and Hf) alkyne
complexes that react with ketones, olefins, carbodiimides, and
acetylene.
RESULTS AND DISCUSSION
■
Synthesis of [PhNPN*]Ta Complexes Featuring an
Activated Alkyne Unit. Tantalum complexes of [PhNPN*]
other cumulenes, resulting in metallacycle ring expansion via
(where [PhNPN*] = PhP(2-(N-mesityl)-5-Me-C6H3)2) can be
prepared via the salt metathesis reaction between [PhNPN*]-
K2(THF)x (1) and Ta(alkyne)Cl3(DME) (where DME = 1,2-
dimethoxyethane), as shown in eq 1.[PhNPN*]Ta(hex-3-
the formation of new C−C bonds.20−25
The formation of C−C bonds is an area of particular utility
and interest for synthetic chemists. Aside from the seminal
advances in the realm of Pd-catalyzed cross-coupling,
complexes of group 4 and 5 metals are well known to catalyze
the coupling of amines and unsaturated hydrocarbons to afford
new C−N and C−C bonds, via hydroamination and hydro-
Received: July 30, 2014
Published: October 13, 2014
© 2014 American Chemical Society
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yne)Cl, 2, is obtained as a dark yellow solid in good yield. In
1
C6D6, the H NMR spectrum of 2 is consistent with a Cs
symmetric complex; there are resonances due to four distinct
aryl methyl groups, along with the expected aryl resonances. At
room temperature, the methyl and methylene groups of the
hexyne unit appear as two broad singlets at δ 2.95 (methylene)
and 1.03 (methyl), which integrate to the expected four and six
protons, respectively; the broadness of these resonances
suggests that the position of the hexyne unit is fluxional, likely
as a result of slow rotation of the hexyne unit about the Ta−
hexyne centroid. In the variable-temperature 1H NMR
experiment, the methyl and methylene protons of the hexyne
unit resolve into two pairs of triplets and quartets (3JHH = 7 Hz
in both cases) at 243 K and coalesce into a single triplet and
Figure 1. ORTEP drawing of the solid-state molecular structure of 2
(ellipsoids at 50% probability). All hydrogen atoms have been omitted
for clarity. Selected bond lengths (Å) and angles (deg): Ta01−N01
2.060(2), Ta01−N02 2.055(2), Ta01−P01 2.5728(6), Ta01−Cl01
2.4112(6), Ta01−C41 2.126(3), Ta01−C42 2.075(3), C41−C42
1.295(4), N01−Ta01−N02 131.46(8), P01−Ta01−Cl01 153.63(2),
N01−Ta01−P01 74.41(6), N02−Ta01−P01 73.98(6), N01−Ta01−
Cl01 94.98(6), N02−Ta01−Cl01 97.34(6).
distances observed in the starting trichloride complex19
(2.046(9) and 2.102(7) Å); the Ta01−C41 bond is slightly
longer than the Ta01−C42 bond, likely due to steric crowding
from the phosphine Ph group. The C41−C42 bond length is
1.295(4) Å, which is noticeably shorter than the analogous
bond length found in the starting trichloride19 (1.39(1) Å) and
more indicative of a bond order intermediate between 2 and 3.
Alkyne ligands are well known to be variable electron donors,
in part mediated by the electrophilicity of the metal center.4−6
Consequently, the interaction between the metal center and the
3-hexyne unit in complex 2 can be viewed as a dative bond
between Ta(III) and a neutral (two- or four-electron-donating)
alkyne ligand. Alternatively, the interaction can be viewed as a
metallacyclopropene-type structure, with the donation of four
electrons via two formal covalent bonds between Ta(V) and an
“alkenediyl” dianion. Resonance structures that depict these
two extremes are depicted in Scheme 1.
quartet at 358 K. From these data, the ΔG⧧ was determined
rot
to be 16.4 0.3 kcal/mol; the full details of these VT-NMR
experiments can be found in the Supporting Information.
The 13C{1H} NMR spectrum features all the expected
resonances for the [PhNPN*] ligand; however, the carbon
resonances of the hexyne unit are more complicated.
Resonances for the methyl and methylene carbons appear as
very weak singlets at δ 14.1 and 29.2, respectively; no
resonances attributable to the metal-bound carbons are
observable in a room-temperature 13C{1H} spectrum, likely
due to signal broadening of these weak quaternary carbon
resonances caused by the rotation of the hexyne unit. Indeed,
the 13C{1H} NMR spectrum collected at 243 K contains two
weak downfield resonances at δ 182.1 and 200.6 assigned to the
metal-bound hexyne carbons. In addition, these carbon atoms
can be detected indirectly using a 1H−13C HMBC NMR
experiment, via their long-range coupling to the hexyne methyl
and methylene protons; in C6D6 at room temperature, this
method correlates these metal-bound hexyne carbons to a
single 13C NMR resonance at δ 204.4.
Scheme 1
Slow evaporation of a concentrated toluene/pentane solution
of 2 afforded bright yellow single crystals suitable for X-ray
crystallography; the ORTEP representation of the solid-state
molecular structure of 2 is shown in Figure 1. The [PhNPN*]
ligand coordinates facially to Ta, resulting in significantly
distorted trigonal bipyramidal geometry about the metal center;
the quasi-apical positions are occupied by the Cl and P atoms,
and the equatorial plane consists of N01, N02, and the centroid
of the C41−C42 bond. The P01−Ta01−N01, P01−Ta01−
N02, and P01−Ta01−Cl01 bond angles are significantly more
acute than the expected value of 90° or 180°, respectively, due
to structural demands of the arene bridge of the ligand.48 The
angle between the plane defined by P01, Ta01, and Cl01 (the
σv symmetry plane of the molecule) and the plane defined by
C41, C42, and Ta01 is 2.8°; this results in inequivalent alkyne
carbon atoms in the solid state, which agrees with what is
observed in the 13C{1H} NMR spectrum at 243 K.
Despite these two possible formalisms, complex 2 and its
various congeners will be referred to as metal−alkyne
complexes; this nomenclature is chosen for simplicity and
brevity, despite the fact that the spectroscopic data point
toward a more metallacyclopropene-type structure.
[PhNPN*]Ta(BTA)Cl (3, where BTA is bis(trimethylsilyl-
acetylene)) is prepared via the reaction of eq 1 and
Ta(BTA)Cl3(DME) and is isolated as a light orange powder
in good yield. The 1H NMR spectrum of 3 is similar to that of 2
with four aryl methyl resonances; the aryl region of the
spectrum is consistent with Cs symmetry. Two distinct
trimethylsilyl groups appear in the expected region (δ 0.16
and 0.08), suggesting that in solution the entire alkyne unit lies
in the σv plane of symmetry. The metal-bound carbons of the
alkyne unit are attributed to two singlets at δ 225.3 and 205.4 in
the 13C{1H} NMR spectrum.
The bond lengths between the Ta center and the hexyne unit
are 2.075(3) and 2.126(3) Å, which are slightly shorter than a
typical Ta−C single bond (2.20−2.25 Å),49,50 but similar to the
The ORTEP representation of the solid-state molecular
structure of 3 is shown in Figure 2. As with 2, the [PhNPN*]
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phosphorus-31 nucleus of the [PhNPN*] ligand, suggesting that
in solution these hydrides are trans to the phosphine. This
assertion is borne out by the solid-state structural data
discussed below. The remainder of 1H NMR spectra for
complexes 4 and 5 is consistent with Cs symmetry in solution.
As with 3, the trimethylsilyl groups in 5 give rise to two distinct
singlets, which indicates that the alkyne unit lies in the σv plane
of symmetry (a hypothesis supported by the inequivalent
metal-bound alkyne carbon atoms mentioned above). Whereas
in the case of 2 the ethyl arms of the 3-hexyne unit displayed
considerable fluxionality and consequently result in two broad
singlets in the room-temperature 1H NMR spectrum, in
complex 4 these methyl and methylene protons give rise to
two pairs of well-resolved triplets and quartets (3JHH = 7 Hz in
both cases), respectively. This observation, along with the
presence of two inequivalent alkyne quaternary carbons in the
13C{1H} spectrum, implies that the two ethyl arms are
inequivalent in solution and that the hexyne unit lies along
the σv plane of symmetry, similar to the examples discussed
above.
Figure 2. ORTEP drawing of the solid-state molecular structure of 3
(ellipsoids at 50% probability). All hydrogen atoms have been omitted
for clarity. Selected bond lengths (Å) and angles (deg): Ta01−N01
2.058(2), Ta01−N02 2.042(2), Ta01−P01 2.619(6), Ta01−Cl01
2.405(6), Ta01−C39 2.168(2), Ta01−C40 2.095(2), C39−C40
1.326(3), N01−Ta01−N02 132.86(8), P01−Ta01−Cl01 145.59(2),
N01−Ta01−P01 73.62(6), N02−Ta01−P01 73.99(6), N01−Ta01−
Cl01 91.96(6), N02−Ta01−Cl01 95.91(6).
The ORTEP representation of the solid-state molecular
structure of 4 is shown in Figure 3. The structure of 4
ligand coordinates facially to Ta, resulting in significantly
distorted trigonal bipyramidal geometry about the metal center;
the apical positions are occupied by the Cl and P atoms, and
the equatorial plane consists of N01, N02, and the centroid of
the C39−C40 bond. Compounds 2 and 3 have similar solid-
state structures; in general, they exhibit small but unremarkable
differences with respect to bond lengths and angles. These
structural similarities include the coordinated alkyne; the
Ta01−C39, Ta01−C40, and C39−C40 bond lengths are
comparable to those found in 2. In addition, the plane defined
by C39, C40, and Ta01 deviates from the P01−Ta01−Cl01
plane (the molecular plane of symmetry, σv) by only 5.2°. This
is consistent with the observation of two distinct SiMe3 groups
in solution.
Figure 3. ORTEP drawing of the solid-state molecular structure of 4
(ellipsoids at 50% probability). All hydrogen atoms (except for H99,
which was located from the difference map and refined isotropically)
have been omitted for clarity. Selected bond lengths (Å) and angles
(deg): Ta01−N01 2.041(2), Ta01−N02 2.037(2), Ta01−P01
2.647(1), Ta01−H99 1.82(3), Ta01−C41 2.109(3), Ta01−C42
2.084(3), C41−C42 1.300(5), N01−Ta01−N02 130.27(10), P01−
Ta01−H99 148.8(11), N01−Ta01−P01 74.84(7), N02−Ta01−P01
73.52(7), N02−Ta01−H99 93.20(11), N01−Ta01−H99 94.60(11).
Synthesis of [PhNPN*]Ta Alkyne Monohydride Com-
plexes. Both [PhNPN*]Ta(hex-3-yne)H (4) and [PhNPN*]-
Ta(BTA)H (5) can be prepared from their corresponding
chloride complexes via a salt metathesis reaction with freshly
prepared KBEt3H, as shown in eq 2.
resembles that of 2, with only small differences in bond angles
and lengths. Again, the geometry at Ta is that of a significantly
distorted trigonal bipyramid; the P atom of [PhNPN*] and the
hydride ligand occupy the apical positions, with N01, N02, and
the centroid of the C41−C42 bond constituting the equatorial
plane. In the solid state the hexyne unit lies in the σv plane of
symmetry, leading to two inequivalent ethyl arms, which is
consistent with the solution NMR data. The bond lengths
related to the coordinated hexyne unit (Ta01−C41, Ta01−
C42, and C41−C42) are all very similar to those found in 2 and
reflect an analogous metal−ligand interaction.
The 13C{1H} NMR spectra of both 4 and 5 are similar to
their chloride precursors (2 and 3); all of the [PhNPN*]
resonances are present in their expected regions and are
indicative of Cs symmetric complexes. In contrast to 2, the
quaternary alkyne carbons for 4 are directly observable in the
expected region (at δ 205 and 184); the quaternary alkyne
carbons for 5 also appear as expected at δ 220 and 193.
Reactions of 4 and 5 with 2,6-Dimethylphenyl
Isocyanide. The addition of 1 equiv of 2,6-dimethylphenyl
isocyanide to a toluene solution of 4 or 5 leads within 5 min to
the full consumption of the starting hydride complex and the
quantitative formation of a new species, 6 or 7 (Scheme 2).
1
A noteworthy feature of the H NMR spectrum for both 4
1
and 5 is the extremely downfield chemical shift of the hydride
ligand;51−53 in both cases these doublet resonances appear at δ
∼21 ppm and exhibit strong coupling (2JHP ≈ 35 Hz) to the
Upon workup, the H NMR spectrum of 6 or 7 in C6D6
features aryl [PhNPN*] ligand resonances suggestive of a C1
symmetric complex, in addition to a new singlet (6 = δ 4.85; 7
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The near planarity (dihedral angle ≅1°) of the N03−C3−
C2−C1 fragment is a defining feature of this structural motif.
Also, the AD fragment is bent back toward the tantalum in a
supine-type mode, which is well established for AD-type ligands
coordinated to the early transition metal complexes.54−56 Two
possible resonance forms are shown in Scheme 3 for the AD
Scheme 2
Scheme 3
= δ 5.24) for a proton that a 13C−1H HSQC experiment
indicates to be carbon-bound (6 = δ 93.9; 7 = δ 107.6). As well,
there are 10 distinct singlets attributable to aryl methyl groups
(eight from the [PhNPN*] ligand and two from the isocyanide
moiety) in the expected region (δ 2.3−1.6). An obvious
structure consistent with this data is that of an iminoformyl
complex generated from insertion of the isocyanide into the
Ta−H bond (square brackets, Scheme 2).
However, an X-ray diffraction study instead revealed the
formation of a five-membered tantallacyclic product. Bright red
single crystals of 7 were obtained from a concentrated hexanes
solution cooled to −30 °C, and the ORTEP representation
(Figure 4) shows a structure in which the putative iminoformyl
moiety has coupled with the coordinated alkyne to generate an
1-aza-1,3-butadiene ligand. Three moleculesrepresenting
both enantiomeric forms of 7are present in the unit cell,
although for clarity only one is depicted.
metallacycle. The solid-state structural data are more consistent
with the amido−alkylidene resonance structure: in particular,
the short Ta−C1 bond length of 1.976(3) Å is indicative of a
metal−carbene interaction. Furthermore, the Ta−N3 distance
of 2.022(2) Å is similar to the Ta−amido bond lengths of the
diamidophosphine [PhNPN*] ancillary ligand (cf. Ta−N01:
2.077(2), Ta−N02: 2.103(3) Å).
The 13C{1H} NMR spectra of 6 and 7 are typical of C1
symmetric complexes featuring a 1-aza-1,3-butadiene moiety.
The 13C{1H} NMR spectrum of 6 features two pairs of singlets
that correspond to the inequivalent methylene and methyl
carbons of the former hexyne moiety, whereas in the case of 7,
there are two distinct singlets that correspond to inequivalent
trimethylsilyl groups. Resonances for the carbon atoms of the 1-
aza-1,3-butadiene fragment are in excellent agreement with
other similar complexes:47 C1 (6: δ 232, 7: δ 242), C2 (6: δ
111, 7: δ 111), and C3 (6: δ 94, 7: δ 107). The extremely
downfield chemical shift for C1 is typical for a metal-bound
alkylidene and is consistent with similar metal−AD bonding
models.47,54,55,57
The synthesis of an 1-aza-1,3-butadienyl moiety via the
reaction of an organic isocyanide and an early transition metal
complex is not without precedent. There are numerous
examples in the literature of isocyanide insertion into Ta−C
bonds to produce an η2-iminoacyl33−38,47,58−60 functionality,
and there are several examples of the coupling of these
iminoacyl units with coordinated alkynes,34,35,47 or indeed
other iminoacyls,58−65 to form 1-aza-1,3-butadiene or 1,4-
diazabutadiene ligands, respectively.
However, there are few examples of stable, well-characterized
iminoformyl complexes46 generated from isocyanide insertion
into early metal−hydride bonds. The majority of reports feature
products that are either incompletely characterized (usually due
to rapid decomposition),66−68 contain a μ-η2:η2 RNCH unit
bridging between two metal centers,69−71 or have a strongly
coordinated phosphine adduct at the iminoformyl C atom.72,73
In this context, it appears that the highly electrophilic
iminoformyl moiety strongly favors further reactivity, which
frustrates isolation and characterization of a mononuclear,
adduct-free example. Complexes 6 and 7 appear to be the first
Figure 4. ORTEP diagram of the solid-state molecular structure of 7
(left) and truncated ORTEP diagram of the core of 7 (right) with
ellipsoids drawn at 50% probability. All hydrogen atoms (except H3)
and the silyl methyl groups at Si01 and Si02 have been omitted for
clarity; H3 was located from the difference map and refined
isotropically. Selected bond lengths (Å) and angles (deg): Ta−N03:
2.022(2), Ta−C3: 2.418(3), Ta−C2: 2.445(3), Ta−C1: 1.976(3),
N03−C3: 1.412(4), C3−C2: 1.401(4), C2 −C1: 1.463(4), Ta−P01:
2.595(1) Ta−N01: 2.077(2), Ta−N02: 2.103(3), Ta−N03−C3:
87.7(2), Ta−C1−C2: 89.3(2), N03−Ta−C1: 89.59(10), N01−Ta−
N02: 124.46(9), P01−Ta−N03: 168.97, N03−C3−C2: 121.9(3),
H3−C3−C2: 118.4(20), C3−C2−C1: 117.2(2), N03−C3−C2−C1
(dihedral): 1.4.
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reported examples of a 1-aza-1,3-butadiene moiety generated
from the coupling of an iminoformyl and a coordinated alkyne.
This type of C−C bond formation is not limited to the
highly reactive iminoformyl fragment. As shown in the next
section, a similar process also occurs with the more stable
phenylvinyl fragment generated by addition of phenylacetylene.
Reactions of 4 and 5 with Phenylacetylene. The
addition of 1 equiv of phenylacetylene to a benzene solution of
4 at room temperature results in an immediate brown to dark
red color change. After 5 min the starting Ta hydride complex
is rapidly converted to complex 8 via 1,2-insertion of
phenylacetylene into the Ta−H bond to generate a Ta
spectra of 10 are extremely complicated, and full structural
elucidation was possible only after obtaining a solid-state
molecular structure from a single-crystal X-ray analysis; ORTEP
representations for complex 10 are shown in Figure 5.
alkyne−phenylvinyl complex, as shown in eq 3.In C6D6, the
1H NMR spectrum features two doublets, at δ 8.62 and 6.08,
that correspond to the newly formed phenylvinylic protons; the
large coupling constant shared by these protons (3JHH = 18 Hz)
is indicative of trans-vicinal, rather than geminal, stereo-
chemistry, which is reflected in the depiction of the alkene
moiety in eq 3.46,72,73 This assignment is buttressed by the
results of a 1H−13C HSQC experiment, which indicates that the
phenylvinylic protons are located on separate carbon atoms.
Synthesis of the isotopologue of 8 with PhCCD, and the
Figure 5. ORTEP diagram of the solid-state molecular structure of 10
(left) and truncated ORTEP diagram of the core of 10 (right) with
ellipsoids drawn at 50% probability.
All hydrogen atoms (except H3 and H4) and the mesityl
group at N02 (except for Cipso) have been omitted for clarity;
H3 and H4 were located from the difference map and refined
isotropically. Selected bond lengths (Å) and angles (deg): Ta−
N01: 2.050(8), Ta−N02: 2.125(8), Ta−P01: 2.581(3), Ta−
C1: 2.021(10), Ta−C2: 2.391(11), Ta−C3: 2.295(11), Ta−
C4: 2.221(12), C2−C1: 1.470(15), C3−C2: 1.437(15), C4−
C3: 1.450(16), N01−Ta−N02: 108.8(3), N01−Ta−P01:
73.1(2), N02−Ta−P01: 75.3(2), C3−C2−C1: 118.8(10),
C4−C3−C2: 123.0(10), C1−C2−C3−C4 (dihedral): 23.67
Complex 10 features a five-membered tantallacycle that
arises from the coupling of one of the coordinated hexyne
carbons to the Cα of the phenylvinyl moiety in 8. There is
evidence of bond delocalization around the four-carbon chain,
with the C1−C2, C2−C3, and C3−C4 distances all equal
(within experimental error). The exact nature of the bonding
between the newly formed four-carbon chain and the tantalum
center is complicated; two limiting resonance forms that best
describe this bonding motif are shown in Scheme 4. Other
workers have synthesized niobium complexes that feature
similar metallacyclic structures, and therein the C4R5 fragment
is described as a monoanionic η3-butadienyl44 or η4-butadienyl45
moiety, depending on the length of the various metal−carbon
bonds; in the case of complex 10, a structure of this type would
imply a formally Ta(III) metal center. However, the short Ta−
C1 (2.021(10) Å) and Ta−C4 (2.221(12) Å) distances are
typical of formal double and single Ta−C bonds, respectively;74
this, in concert with the solution-state NMR data (vide infra),
provides compelling evidence for a trianionic alkyl−alkylidene
ligand coordinated to Ta(V).
1
subsequent absence of the H resonance at δ 8.62, allows for
the unambiguous identification of these phenylvinylic carbons:
δ 204.3 (Cα), 142.1 (Cβ). The methyl and methylene groups on
the hexyne moiety appear as one quartet and one triplet (JHH
=
7.5 Hz) integrating to four and six protons, respectively, rather
than the two pairs of triplets and quartets seen in complex 4.
These data indicate that the hexyne unit is oriented
perpendicular to the σv plane of the molecule, in contrast to
the coplanar orientation seen in complexes 2−5. However, as
the static geometry depicted in eq 3 would result in
diastereotopic methylene protons, the NMR data also imply
fast exchange of the two halves of the hexyne ligand, via
rotation about the Ta−alkyne bond.
Complex 5 also reacts with phenylacetylene to generate 9, a
1
Ta alkyne−phenylvinyl complex similar to 8 (eq 3). The H
NMR spectrum of 9 in C6D6 contains two doublets at δ 8.92
and 5.83 (3JHH = 18 Hz) that correspond well with the data for
the phenylvinylic protons in 8; the rest of the 1H NMR
spectrum of 9 agrees with the proposed Cs symmetric structure,
including a single resonance for the two TMS groups, implying
that the BTA moiety is arranged perpendicular to the σv plane.
Further characterization of 9 is hampered by its thermal
instability and rapid (∼8 h) structural rearrangement to
complex 11, as will be discussed in more detail below.
Structural Rearrangement of Complexes 8 and 9. A
benzene solution of 8 left to sit at room temperature cleanly
converts to a second complex, 10, over the course of
1
approximately 3 days (eq 4). The H and 13C{1H} NMR
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δ 251.4) and lends credence to the Ta(V) alkyl−alkylidene
structural motif.
Scheme 4
Kinetic Study of the Rearrangement of Complex 8.
Although the initial reaction between 4 and phenylacetylene
that forms 8 occurs too rapidly to easily monitor by NMR
spectroscopy, the subsequent rearrangement of 8 to 10
proceeds slowly enough to permit a study of its kinetic
parameters. Based on the linearity of a ln[8] versus time plot,
the rearrangement was determined to be first order in 8; details
regarding this determination, as well as a discussion of NMR
spectrum processing, integral choice, error analysis, and a
sample plot of ln[8] versus time (at 318 K) used in the
determination of kobs can be found in the Supporting
Information. From the Eyring plot of ln(kobs/T) versus 1/T,
the best fit gave the activation parameters ΔH⧧ = 22.2 0.3
kcal/mol and ΔS⧧ = −8.7
0.2 cal/(mol)(K). The slightly
It is possible to envision the actual C−C bond forming event
that gives rise to complex 10 in terms of the reductive
elimination of the phenylvinyl unit and one end of the
alkenediyl moiety in complex 8; such a mechanism would
necessarily lead to the Ta(III)-butadienyl structure shown in
Scheme 4. Nevertheless, as the Ta(V) formalism better matches
the spectroscopic and crystallographic data, it is the preferred
bonding description for 10; it is likely that the [PhNPN*]
Ta(III) moiety is strongly reducing and can formally add a pair
of electrons to the butadienyl unit, which results in the
oxidation to Ta(V), as shown in Scheme 5.
negative ΔS⧧ value is consistent with an ordered transition state
for the aforementioned intramolecular rearrangement.
CONCLUSIONS
■
The results of this study illustrate the utility of tantalum alkyne
complexes for stoichiometric C−C bond forming reactions.
The two reaction sequences featured in this work both involve
a migratory insertion step of a Ta(V) hydride species with an
aryl isocyanide and phenylacetylene. Although the iminoformyl
intermediate could not be detected, in the case of the terminal
acetylene, the phenylvinyl species could be characterized via
solution-state NMR spectroscopy; depending on the alkyne
substituents, the rate of the transformation to the butadienyl
species could also be monitored by NMR spectroscopy. In the
case of the hexyne derivative 8, the sequence of reductive
elimination followed by internal reduction of the organic
fragment by the electron-rich Ta(III) species leads to the
observed product, which is best described as an alkyl−
alkylidene species. This matches nicely the analogous reaction
of the putative iminoformyl species with the alkyne unit, which
generates an amido−alkylidene description of the final product.
As both imino−formyl alkyne and the phenylvinyl−alkyne
complexes undergo C−C bond forming reactions via a formal
reductive elimination, it is plausible that these processes are
facilitated by the phosphine donor in the NPN ligand set, which
stabilizes the lower oxidation state, at least transiently.
Scheme 5
As was mentioned above, the NMR spectra of 10 are quite
complicated as a result of the chiral center that is generated at
the terminal carbon of the butadienyl unit; the unsymmetrical
nature of 10 is evident in the 1H and 13C{1H} NMR spectra, as
there are twice the number of ligand resonances than are
present in the spectra of the starting phenylvinyl complex 8.
Similar complexity is observed for 11, although in this case, the
lack of diastereopic methylene groups of the ethyl substituents
makes the spectrum less crowded in the 0−5 ppm region. On
the basis of the results of a battery of NMR experiments
(1H−1H COSY, 13C-APT, 1H−13C HSQC, and 1H−13C
HMBC) it was possible to assign all of these proton
resonances; while the full analysis can be found in the
Supporting Information, the diagnostic peaks for coupled
products in the 1H NMR spectra are the two resonances for the
unique, trans-disposed protons of the “dienyl” unit: in 10, they
EXPERIMENTAL SECTION
■
General Procedures. Unless otherwise noted, all experiments
were conducted by means of standard Schlenk line techniques or in a
glovebox (Innovative Technology) equipped with a freezer (−35 °C),
under an atmosphere of dry oxygen-free dinitrogen, using oven-dried
(200 °C) glassware cooled under dynamic vacuum. Anhydrous
hexanes, toluene, diethyl ether, and tetrahydrofuran were purchased
from Aldrich, sparged with dinitrogen, and dried further by passage
through towers containing activated alumina and molecular sieves.
Pentane was refluxed over sodium benzophenone ketal, distilled under
positive argon pressure, and degassed via several freeze−pump−thaw
cycles. THF-d8 and C6D6 were stirred over sodium benzophenone
ketal, vacuum transferred, and freeze−pump−thaw degassed; toluene-
d8 and pyridine-d5 were stirred over activated molecular sieves and
freeze−pump−thaw degassed. KBEt3H (1.0 M in THF) was
purchased from Aldrich, evaporated to dryness, and used as a solid.
Phenylacetylene was purchased from Aldrich, distilled, degassed, and
stored over molecular sieves; phenylacetylene-d1 was prepared by
treating dry phenylacetylene with 1 equiv of nBuLi and quenching with
an excess of DCl (35% w/w in D2O). Benzyl potassium,75
[PhNPN*]H2,48 Ta(hexyne)Cl3(DME),19 and Ta(BTA)Cl3(DME)76
were prepared according to literature methods. NMR spectra were
3
2
appear at δ 0.61 (dd, JHH = 8 Hz, JHP = 3 Hz) and 4.33 (d,
3JHH = 8 Hz); in 11, these resonances are at δ 1.51 (d, 3JHH = 9
3
2
Hz) and 4.95 (dd, JHH = 9 Hz, JHP = 2 Hz). As mentioned
above, the extremely downfield chemical shift for C1 (cf.
Scheme 4) is typical for a carbene-type carbon (10: δ 245.3; 11:
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3
recorded on a Bruker AV-400 MHz or AV-300 MHz spectrometer.
Except where noted, all spectra were recorded at room temperature.
1H NMR spectra were referenced to residual proton signals in C6D6 (δ
7.16), toluene-d8 (δ 2.09), or THF-d8 (δ 1.73). 31P{1H} NMR spectra
were referenced to an external sample of P(OMe)3 (δ 141.0 with
respect to 85% H3PO4 at δ 0.0). 13C{1H} NMR spectra were
referenced to the solvent resonances of C6D6 (δ 128.06), toluene-d8 (δ
20.9), or THF-d8 (δ 25.31). Some coupling constants are not assigned
bond connectivity because of the inability to make unambiguous
assignments; therefore, the actual number of bonds between the two
nuclei is not specified. This is especially evident for the phosphorus-31
coupling to aryl protons in the ligand. Elemental analyses were
performed using a FISONS 1108 elemental analyzer by Mr. David
Wong or Mr. Derek Smith at the Department of Chemistry, University
of British Columbia. Electron ionization−mass spectrometry (EI-MS)
analyses were performed using a Kratos MS-50 spectrometer (70 eV
source) by Mr. Marshall Lapawa at the Department of Chemistry,
University of British Columbia.
2H, JHH = 7 Hz, hexyne CH2), 2.05 (s, 6H), 1.99 (s, 6H), 1.73 (s,
6H) (ArCH3), 1.27 (t, 3H, JHH = 7 Hz), 0.79 (t, 3H, JHH = 7 Hz)
1
(hexyne CH3). H NMR (toluene-d8, 400 MHz, 343 K): δ 7.57 (bd,
JHP = 7.8 Hz, 2H), 7.44 (m, 2H), 7.1−6.9 (overlapping signals,
approximately 3 aromatic protons and residual toluene-d8 protons),
6.87 (s, 2H), 6.77 (bd, JHH = 8.7 Hz, 2H), 6.63 (s, 2H), 5.98 (dd, JHP
=
5 Hz, JHH = 8.7 Hz, 2H) (ArH), 2.90 (bs, 4H, hexyne CH2), 2.51 (s,
6H), 2.05 (s, 6H), 1.99 (s, 6H), 1.68 (s, 6H) (ArCH3), 0.98 (bs, 6H,
hexyne CH3). 31P{1H} NMR (C6D6, 120 MHz): δ 32.4 (s). 13C{1H}
NMR (C6D6, 75 MHz): δ 163.10 (d, JCP = 23 Hz), 139.25, 138.48,
136.9 (d, JCP = 32 Hz), 136.1 (d, JCP = 10 Hz), 135.27, 134.19, 132.81
(d, JCP = 9 Hz), 131.15, 130.42, 129.9, 129.6, 129.2, 125.7, 121.8 (d,
JCP = 32 Hz), 115.5 (d, JCP = 8 Hz) (ArC), 29.2 (hexyne CH2), 21.12,
20.28, 20.19, 18.66 (ArCH3), 14.1 (hexyne CH3). 13C{1H} NMR
(toluene-d8, 100 MHz, 243 K): δ 200.6, 182.1 (d, JCP = 2 Hz) (hexyne
EtCCEt), 163.10 (d, JCP = 32 Hz), 139.05, 137.4, 135.8 (d, JCP = 10
Hz), 135.27, 134.6 (d, JCP = 32 Hz), 134.19, 132.81 (d, JCP = 9 Hz),
131.15, 130.42, 129.9, 129.6, 129.2, 125.6, 121.8 (d, JCP = 32 Hz),
115.3 (d, JCP = 8 Hz) (ArC), 30.6, 26.7 (hexyne CH2), 21.05, 20.17,
20.165, 18.6 (ArCH3), 15.7, 13.7 (hexyne CH3). Anal. Calcd for
C44H49Cl1N2P1Ta1: C, 61.94; H, 5.79; N, 3.28. Found: C, 61.89; H,
5.87; N, 3.10.
[PhNPN*]K2(THF)0.5 (1). At room temperature, 30 mL of THF was
added to [PhNPN*]H2 (1.00 g, 1.80 mmol) to give a clear, pale yellow
solution. Solid benzyl potassium (468 mg, 3.60 mmol) was added to
the solution, and the mixture was left to stir for 30 min. The resulting
bright yellow solution was evaporated to dryness and then triturated
with 30 mL of hexanes to generate a bright yellow solid. This solid was
collected on a sintered-glass frit, washed with hexanes (3 × 30 mL),
and dried in vacuo to yield 1.10 g (1.74 mmol, 97%). Samples for
NMR spectroscopy were prepared in toluene-d8, with a drop of
pyridine-d5 for additional solubility.
1
For 3: H NMR (C6D6, 300 MHz): δ 7.62 (bd, JHP = 8 Hz, 2H),
7.55 (m, 2H), 7.05 (m, 3H), 6.93 (s, 2H), 6.80 (bd, JHH = 8 Hz, 2H),
6.69 (s, 2H), 6.05 (dd, JHP = 5 Hz, JHH = 9 Hz, 2H) (ArH), 2.48 (s,
6H), 2.12 (s, 6H), 1.98 (s, 6H), 1.73 (s, 6H) (ArCH3), 0.16, (s, 9H)
0.08 (s, 9H) (Si(CH3)3). 31P{1H} NMR (C6D6, 120 MHz): δ 29.7 (s).
13C{1H} NMR (C6D6, 75 MHz): δ 225.3, 205.4 (d, JCP = 9 Hz)
(TMSCCTMS), 161.9 (d, JCP = 30 Hz), 139.5 (d, JCP = 4 Hz),
138.8, 137.3 (d, JCP = 39 Hz), 136.4, 135.2, 134.9 (d, JCP = 2 Hz),
134.2 (d, JCP = 5 Hz), 134.1 (d, JCP = 4 Hz), 131.3, 130.4 (d, JCP = 2
Hz), 130.0 (d, JCP = 5 Hz), 129.7, 128.8 (d, JCP = 10 Hz), 122.6 (d, JCP
= 41 Hz), 115.2 (d, JCP = 10 Hz) (ArC), 21.0, 20.5, 20.3, 20.1
(ArCH3), 2.8, 2.0 (SiCH3). Anal. Calcd for C46H57ClN2PSi2Ta: C,
58.68; H, 6.10; N, 2.98. Found: C, 58.87; H, 6.37; N, 3.30.
1H NMR (toluene-d8, 300 MHz): δ 8.06 (bs, 2H), 7.2−6.9
(overlapping resonances, 9H plus residual toluene-d8), 6.66 (d, JHH = 8
Hz, 2H), 6.07 (dd, JHP = 6 Hz, JHH = 6 Hz, 2H) (ArH), 3.55 (THF,
2H), 2.36 (s, 6H), 2.12 (bs, 12H), 2.04 (s, 6H) (ArCH3), 1.48 (THF,
2H). 31P{1H} NMR (toluene-d8, 120 MHz): δ −25.7 (s). 13C{1H}
NMR (toluene-d8, 75 MHz): δ 159.3 (d, JCP = 21 Hz), 152.79, 142.65
(d, JCP = 12 Hz), 136.11, 134.8 (d, JCP = 17 Hz) 131.92, 131.37,
130.52, 130.38, 129.56, 128.28, 126.20, 127.05, 117.68, 115.58, 111.50
(ArC), 67.7, 25.82 (THF), 21.05, 20.59, 20.12, 19.86 (ArCH3).
Elemental analysis of 1 was hampered by its pronounced air sensitivity.
Despite several attempts, results that were significantly low in carbon
were found. The data for one representative attempt are reported:
Anal. Calcd for C80H86K4N4O2P2: C, 71.82; H, 6.48; N, 4.19. Found:
C, 63.68; H, 6.47; N, 5.40.
[PhNPN*]Ta(RCCR)H (R = Et (4); R = SiMe3 (5)). At room
temperature, solid KHBEt3 (164 mg, 1.22 mmol) was added at once to
a stirring toluene solution (20 mL) of 2 (1.03 g, 1.20 mmol) or 3 (1.14
g, 1.21 mmol). The resulting (4: dark brown, 5: dark red) solution was
stirred for 3 h, during which the formation of a light-colored
precipitate was observed. This suspension was filtered through a pad of
silica on a sintered glass frit, and the filtrate was evaporated to dryness
in vacuo to afford a dark brown residue. This residue was triturated
with 20 mL of pentane and cooled to −35 °C, whereupon a (4: tawny
brown, 5: red) precipitate formed. This solid was collected on a frit,
washed with cold pentane (2 × 5 mL), and dried in vacuo. (Yields = 4:
589 mg, 61%, 5 = 493 g, 45%.)
[PhNPN*]Ta(RCCR)Cl (R = Et (2); R = SiMe3 (3)). A 200 mL
Kontes-seal glass reactor was charged with a magnetic stir bar, 1 (3.57
g, 5.65 mmol), Ta(RCCR)Cl3(DME) (2 = 2.60 g, 5.66 mmol; 3 =
2.75 g, 5.99 mmol)), and 60 mL of THF. The resulting solution was
stirred at 54 °C (2: 36 h 3: 6 h), during which the formation of a light-
colored precipitate was observed. This suspension was filtered through
a pad of silica on a sintered glass frit, and the filtrate was evaporated to
dryness in vacuo to afford a dark yellow (2) or orange (3) powder.
This powder was triturated with 30 mL of pentane and cooled to −35
°C, whereupon a precipitate formed. This solid (2: dark yellow, 3:
dark orange) was collected on a frit, washed with cold pentane (2 × 10
mL), and dried in vacuo. (Yields = 2: 3.47 g, 72%; 3 = 3.78 g, 65%.)
For 2: 1H NMR (C6D6, 300 MHz, 298 K): δ 7.62 (bd, JHP = 7.8 Hz,
2H), 7.52 (m, 2H), 7.05 (m, 3H), 6.93 (s, 2H), 6.84 (bd, JHH = 8.7 Hz,
2H), 6.70 (s, 2H), 6.09 (dd, JHP = 5 Hz, JHH = 8.7 Hz, 2H) (ArH),
2.95 (bs, 4H, hexyne CH2), 2.58 (s, 6H), 2.07 (s, 6H), 2.01 (s, 6H),
1.76 (s, 6H) (ArCH3), 1.03 (bs, 6H, hexyne CH3). 1H NMR (toluene-
d8, 400 MHz, 298 K): δ 7.57 (bd, JHP = 7.8 Hz, 2H), 7.44 (m, 2H),
7.1−6.9 (overlapping signals, approximately 3 aromatic protons and
residual toluene-d8 protons), 6.87 (s, 2H), 6.77 (bd, JHH = 8.7 Hz,
2H), 6.63 (s, 2H), 5.98 (dd, JHP = 5 Hz, JHH = 8.7 Hz, 2H) (ArH),
2.90 (bs, 4H, hexyne CH2), 2.51 (s, 6H), 2.05 (s, 6H), 1.99 (s, 6H),
1.68 (s, 6H) (ArCH3), 0.98 (bs, 6H, hexyne CH3). 1H NMR (toluene-
d8, 400 MHz, 243 K): δ 7.60 (bd, JHP = 7.8 Hz, 2H), 7.47 (m, 2H),
7.1−6.9 (overlapping signals, approximately 3 aromatic protons and
residual toluene-d8 protons), 6.87 (s, 2H), 6.75 (bd, JHH = 8.7 Hz,
2H), 6.64 (s, 2H), 6.05 (dd, JHP = 5 Hz, JHH = 8.7 Hz, 2H) (ArH),
3.49 (q, 2H, 3JHH = 7 Hz, hexyne CH2), 2.59 (s, 6H, ArCH3), 2.33 (q,
For 4: H NMR (C6D6, 400 MHz): δ 21.6 (d, 2JHP = 34.8 Hz, 1H,
TaH), 7.71 (d, JHP = 7 Hz, 2H), 7.62 (dd, JHP = 9 Hz, JHH = 6 Hz, 2H),
7.12 (m, 3H), 6.93 (s, 2H), 6.86 (d, JHH = 8 Hz, 2H), 6.67 (s, 2H),
6.00 (dd, JHP = 6 Hz, JHH = 8 Hz, 2H) (ArH), 3.10 (q, JHH = 7 Hz,
1
3
2H), 2.78 (q, JHH = 7 Hz, 2H) (hexyne CH2), 2.68 (s, 6H), 2.10 (s,
6H), 2.01 (s, 6H), 1.79 (s, 6H) (ArCH3) 1.16 (t, JHH = 7 Hz, 3H),
0.77 (t, JHH = 7 Hz, 3H) (hexyne CH3). 31P{1H} NMR (C6D6, 160
3
MHz): δ 20.1 (s). 13C{1H} NMR (C6D6, 75 MHz): δ 205.3 (d, JCP = 4
Hz), 184.2 (d, JCP = 11 Hz) (EtCCEt), 162.80 (d, JCP = 34 Hz),
141.8, 137.0, 136.3 (JCP = 35 Hz), 135.2, 134.8, 133.4 (d, JCP = 6 Hz),
132.2 (d, JCP = 12 Hz), 129.9 (d, JCP = 3 Hz), 129.69, 129.67, 129.65,
129.60, 129.2 (d, JCP = 9 Hz), 121.81 (d, JCP = 37 Hz), 115.8 (d, JCP
=
11 Hz) (ArC), 30.54 (d, JCP = 4.9 Hz), 30.40 (JCP = 2.8 Hz) (hexyne
CH2), 21.43, 20.81, 20.34, 18.72 (ArCH3), 15.75, 14.21 (hexyne CH3).
EI-MS (m/z): 817 (100, [M − H]+), 735 (40, [Ta{PhNPN*}]+), 541
(20, [{PhNPN*} − Me]+). Multiple attempts to obtain acceptable
elemental analyses failed; a representative set is shown. Anal. Calcd for
C44H50N2PTa: C, 64.54; H, 6.15; N, 3.42. Found: C, 62.71; H, 5.83;
N, 3.98.
For 5: H NMR (C6D6, 400 MHz): δ 20.6 (d, JHP = 35 Hz, 1H,
TaH), 7.72 (d, JHP = 7 Hz, 2H), 7.62 (dd, JHP = 8 Hz, JHH = 7 Hz, 2H),
7.02 (m, 3H), 6.91 (s, 2H), 6.82 (bd, JHH = 8 Hz, 2H), 6.61 (s, 2H),
5.92 (dd, JHP = 5 Hz, JHH = 8 Hz, 2H) (ArH), 2.67 (s, 6H), 2.12 (s,
1
2
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6H), 1.98 (s, 6H), 1.64 (s, 6H) (ArCH3), 0.13, (s, 9H), −0.1 (s, 9H)
(Si(CH3)3). 31P{1H} NMR (C6D6, 120 MHz): δ 16.3 (s). 13C{1H}
NMR (C6D6, 101 MHz): δ 220.3, 193.2 (d, JCP = 5 Hz) (TMSC
CTMS), 161.9 (d, JCP = 32 Hz), 141.8, 137.6, 136.5, 135.7, 134.9 (d,
JCP = 2 Hz), 134.7 (d, JCP = 3 Hz), 133.3 (d, JCP = 12 Hz), 133.2,
131.8, 130.0 (d, JCP = 5 Hz), 129.9 (d, JCP = 5 Hz), 129.6 (d, JCP = 9
Hz), 128.8 (d, JCP = 9 Hz), 123.2 (d, JCP = 35 Hz), 115.3 (d, JCP = 10
Hz) (ArC), 21.1, 20.9, 20.4, 19.5 (ArCH3), 2.3, 0.4 (SiCH3). Multiple
attempts to obtain acceptable elemental analyses failed; a representa-
tive set is shown. Anal. Calcd for C46H58N2PSi2Ta: C, 60.91; H, 6.45;
N, 3.09. Found: C, 57.10; H, 7.46; N, 2.83
[PhNPN*]TaC(R)C(R)C(H)N(xylyl) (R = Et (6); R = SiMe3 (7)). To
a mixture of 4 (94 mg, 0.11 mmol) or 5 (104 mg, 0.11 mmol) and 2,6-
dimethylphenyl isocyanide (15 mg, 0.11 mmol) was added 10 mL of
toluene. This solution was stirred for 15 min, after which the volatiles
were removed in vacuo. The resulting residue (6: dark brown, 7: dark
red) was triturated with ∼10 mL of cold pentane and filtered to yield
solid 6 (94 mg, 90%) or 7 (102 mg, 86%).
For 6: 1H NMR (C6D6, 300 MHz): δ 7.38 (dd, JHH = 8 Hz, JHP = 7
Hz, 1H), 7.2−6.6 (15H plus residual C6D6 protons), 6.04 (dd, JHH = 8
Hz, JHP = 5 Hz, 1H), 5.91 (dd, JHH = 8 Hz, JHP = 5 Hz, 1H) (ArH),
4.88 (s, 1H, “H3”), 3.38 (dt, 2JHH = 7 Hz, 3JHH = 7 Hz, 1H, “H4a/b”),
2.61 (s, 3H), 2.36 (s, 3H) (ArCH3), 2.29 (m, 1H, “H4a/b”), 2.27 (s,
3H), 2.22 (s, 3H), 2.20 (s, 6H), 1.97 (s, 3H), 1.94 (s, 6H), 1.87 (s,
3H), 1.86 (s, 3H) (ArCH3), 1.60 (m, 1H, “H5a/b”), 1.09 (t, JHH = 7
Hz, “Me2”), 0.77 (m, 1H, “H5a/b”) 0.716 (t, JHH = 7 Hz, “Me1”).
31P{1H} NMR (C6D6, 120 MHz): δ 16.1 (s). 13C{1H} (C6D6, 75
MHz): δ 231.6 (d, JCP = 30 Hz, C1), 163.7 (d, JCP = 30 Hz), 163.2 (d,
JCP = 25 Hz), 151.8, 147.1 (d, JCP = 7 Hz), 146.8 (d, JCP = 3 Hz),
137.2, 136.4, 135.6, 135.4, 134.6, 134.4, 134.2, 133.9, 133.7, 133.6,
133.3, 133.2, 133.0, 132.2, 131.8, 130.2 (d, JCP = 15 Hz), 129.8 (d, JCP
= 5 Hz), 129.6, 129.3, 129.2, 128.7, 128.6, 128.4, 127.0 (d, JCP = 5 Hz),
Figure 6. Schematic representation of the core of complexes 6, 7, 10,
and 11. These depictions are only meant to indicate connectivity and
1
serve as an aid for H and 13C{1H} NMR spectral assignments; they
do not accurately reflect the bonding in the tantallacycles.
164.1 (d, JCP = 25 Hz), 143.3, 142.0 (TaCHCHPh), 139.3, 137.9 (d,
JCP = 5 Hz), 137.1 (d, JCP = 28 Hz), 135.1, 134.7, 132.8 (d, JCP = 9
Hz), 131.1, 130.1, 129.8, 129.5 (d, JCP = 4 Hz), 129.1 (d, JCP = 7 Hz),
128.5, 126.1, 125.6, 121.1 (d, JCP = 28 Hz), 115.7 (d, JCP = 8 Hz)
(ArC), 29.2 (bs, hexyne CH2), 21.1, 20.3, 19.6, 18.7 (ArCH3), 14.6
(bs, hexyne CH3).
For 9: Due to rapid thermal decomposition (to 11), this complex
1
1
was characterized by H and 31P{1H} NMR spectroscopy only. H
NMR (C6D6, 300 MHz): δ 8.92 (dd, JHH = 18 Hz, JHP = 3 Hz, 1H,
TaCHCHPh), 7.65 (d, JHH = 8 Hz, 2H), 7.5−6.7 (overlapping
signals, 16H plus residual C6D6 protons) (ArH), 6.03 (dd, JHH = 8 Hz,
JHP = 7 Hz, 2H, ArH), 5.83 (d, JHH = 18 Hz, 1H, TaCHCHPh),
2.32 (s, 6H), 2.09 (s, 6H), 1.97 (s, 6H), 1.78 (s, 6H) (ArCH3), 0.10 (s,
18H, Si(CH3)3). 31P{1H} NMR (C6D6, 160 MHz): δ 23.9 (s).
[PhNPN*]Ta(EtCCEt)(CDCHPh) (d1-8). A sample of d1-8 was
prepared using PhCCD and 2, in a manner identical to that for 8;
the reaction was scaled down by a factor of 10 and performed in a
sealed J-Young NMR tube.
125.2, 120.5 (d, JCP = 35 Hz), 116.8 (d, JCP = 10 Hz), 116.1 (d, JCP
=
10 Hz), 113.8 (d, JCP = 36 Hz) (ArC), 111.3 (C2), 93.9 (C3), 29.0
(C5), 21.5, 21.1, 20.8, 20.52 (ArCH3), 20.50 (C6), 20.15, 20.12, 20.10,
20.08, 19.7, 19.0, (ArCH3), 17.7 (Me1), 15.2 (Me2). Anal. Calcd for
C53H59N3PTa: C, 67.01; H, 6.26; N, 4.42. Found: C, 67.20; H, 6.24;
N, 4.09.
For 7: 1H NMR (C6D6, 300 MHz): δ 8.22 (dd, JHH = 8 Hz, JHP = 7
Hz, 2H), 7.44 (d, JHP = 7 Hz, 1H), 6.90 (d, JHH = 7 Hz, 1H), 6.75 (m,
4H), 6.63 (d, JHH = 7 Hz, 1H), 6.55 (bs, 2H), 6.31 (dd, JHH = 8 Hz,
JHP = 5 Hz, 1H), 6.18 (bs, 1H), 6.03 (dd, JHH = 8 Hz, JHP = 5 Hz, 1H)
(ArH), 5.24 (s, 1H, “H3”), 2.20 (s, 6H), 2.09 (s, 3H), 2.06 (s, 3H),
2.04 (s, 3H), 2.02 (s, 3H), 1.94 (s, 6H), 1.64 (s, 3H), 1.59 (s, 3H)
(ArCH3), 0.49 (s, 9H), 0.11 (s, 9H) (SiCH3). 31P{1H} NMR (C6D6,
120 MHz): δ 28.8 (s). 13C{1H} (C6D6, 75 MHz): δ 242.6 (C1), 162.0
(d, JCP = 30 Hz), 158.1 (d, JCP = 25 Hz), 148.7 (d, JCP = 7 Hz), 147.5,
146.8 (d, JCP = 7 Hz), 136.4, 135.9, 135.8, 134.8, 134.7, 134.6, 134.55,
1H NMR (C6D6, 400 MHz): δ 7.65 (d, JHH = 8 Hz, 2H), 7.52 (m,
2H), 7.21 (s, 2H), 7.10 (m, 5H), 6.85 (m, 5H), 6.74 (s, 2H) (ArH),
6.20 (bs, 1H, TaCDCHPh), 6.03 (dd, JHH = 8 Hz, JHP = 7 Hz, 2H,
ArH), 2.99 (q, 3JHH = 7.5 Hz, 4H, hexyne CH2), 2.34 (s, 6H), 2.13 (s,
3
6H), 2.04 (s, 6H), 1.78 (s, 6H) (ArCH3), 0.99 (t, JHH = 7.5 Hz, 6H,
hexyne CH3). 31P{1H} NMR (C6D6, 160 MHz): δ 26.7 (s).
[PhNPN*]TaC(R)C(R)C(H)C(H)Ph (R = Et (10); R = SiMe3 (11)). A
50 mL Kontes-sealed glass reactor was charged with a magnetic stir
bar, 2 (300 mg, 0.37 mmol) or 3 (332 mg, 0.37 mmol),
phenylacetylene (40 μL, 37 mg, 37 mmol), and 20 mL of toluene.
The resulting solution was stirred at 54 °C (2: 6 h; 3: 2 h), during
which a brown to red-brown color change was observed. After heating,
the volatiles were removed in vacuo; the resulting red-brown residue
was triturated with ∼20 mL of cold pentane and filtered to yield solid
10 (231 mg, 68%) or 11 (227 mg, 61%).
134.3, 134.2, 133.9, 133.2, 133.1, 132.3 (d, JCP = 5 Hz), 131.0 (d, JCP
=
3 Hz), 130.8, 130.4, 130.3, 130.2, 130.0 (d, JCP = 36 Hz), 129.3, 129.2,
129.0, 128.95, 128.9, 128.6, 128.5, 123. 5, 120.4 (d, JCP = 45 Hz), 120.2
(d, JCP = 7 Hz), 116.7 (d, JCP = 10 Hz), 111.72 (d, JCP = 2 Hz, C2),
111.6 (d, JCP = 36 Hz) (ArC), 107.6 (C3), 22.3, 22.1 (d, JCP = 2 Hz),
21.7, 21.2, 21.2, 20.9, 20.6, 20.5, 20.4, 19.2 (ArCH3), 5.22, 1.14
(Si(CH3)3). Multiple attempts to obtain acceptable elemental analyses
failed; a representative set is shown. Anal. Calcd for C55H67N3PSi2Ta:
C, 63.63; H, 6.50; N, 4.05. Found: C, 65.01; H, 7.09; N, 4.26.
[PhNPN*]Ta(RCCR)(CHCHPh) (R = Et (8); R = SiMe3 (9)).
Phenylacetylene (5 μL, 4.1 mg, 43 umol) was added to a C6D6
solution (∼0.5 mL) of 4 (33 mg, 42 umol) or 5 (38 mg, 42 umol),
which led to an immediate bright red color change. By NMR, the
reaction is quantitative and complete within 5 min.
For 10: 1H NMR (C6D6, 400 MHz): δ 7.83 (m, 2H), 7.60 (d, JHP
=
9 Hz, 1H), 7.45 (d, JHP = 9 Hz, 1H), 7.10 (m, 3H), 6.96 (m, 4H), 6.81
(d, JHH = 8 Hz, 1H), 6.79 (s, 1H), 6.72 (d, JHH = 8 Hz, 1H), 6.69 (s,
1H), 6.57 (s, 1H), 6.21 (dd, JHH = 8 Hz, JHP = 5 Hz, 1H), 5.70 (dd,
JHH = 8 Hz, JHP = 5 Hz, 1H), 5.59 (d, JHH = 6.5 Hz, 2H) (ArH), 4.33
(d, JHH = 8 Hz, 1H, “H2”), 3.48 (m, 1H), 2.89 (m, 2H), 2.49 (m, 1H)
(“H5a/b, H6a/b”) 2.31, 2.15, 2.09, 2.02, 1.94, 1.90, 1.86, 1.62, (s, 3H)
For 8: 1H NMR (C6D6, 400 MHz): δ 8.69 (dd, JHH = 18 Hz, JHP = 3
Hz, 1H, TaCHCHPh), 7.65 (d, JHH = 8 Hz, 2H), 7.52 (m, 2H),
7.21 (s, 2H), 7.10 (m, 5H), 6.85 (m, 5H), 6.74 (s, 2H) (ArH), 6.21 (d,
JHH = 18 Hz, 1H, TaCHCHPh), 6.03 (dd, JHH = 8 Hz, JHP = 7 Hz,
2H, ArH), 2.99 (q, JHH = 7.5 Hz, 4H, hexyne CH2), 2.34 (s, 6H), 2.13
(s, 6H), 2.04 (s, 6H), 1.78 (s, 6H) (ArCH3), 0.99 (t, JHH = 7.5 Hz, 6H,
hexyne CH3). 31P{1H} NMR (C6D6, 160 MHz): δ 26.4 (s). 13C{1H}
NMR (C6D6, 75 MHz): δ 204.3 (d, JCP = 18 Hz, TaCHCHPh),
3
(ArCH3), 1.34 (t, JHH = 7.5 Hz, 3H, “Me2”), 0.61 (dd, JHH = 8 Hz,
2JHP = 3 Hz, 1H, “H1”), 0.45 (t, JHH = 7.5 Hz, 3H, “Me1”). 31P{1H}
NMR (C6D6, 120 MHz): δ 28.5 (s). 13C{1H} NMR (C6D6, 75 MHz):
δ 245.3 (d, JCP = 11 Hz, C1), 168.4 (d, JCP = 32 Hz), 160.4 (d, JCP
=
28 Hz), 151.2 (d, JCP = 5 Hz), 147.6, 141.7 (d, JCP = 31 Hz), 137.9,
135.9, 135.6, 134.6, 134.5, 134.4, 133.9, 133.7 (C2), 133.6, 133.5 (d,
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JCP = 14 Hz), 133.3, 132.5 (d, JCP = 14 Hz), 131.3 (d, JCP = 25 Hz),
130.9 (d, JCP = 6 Hz), 130.1 (d, JCP = 4 Hz), 129.9 (d, JCP = 3 Hz)
129.6, 129.4, 129.2, 129.1, 129.0, 126.1, 126.0, 125.1 (d, JCP = 42 Hz),
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10 Hz) (ArC), 93.6 (C3), 93.1 (d, JCP = 15 Hz, C4), 31.5 (d, JCP = 6
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Hz, JHP = 2 Hz, 1H), 7.23 (dt, JHH = 9 Hz, JHP = 2 Hz, 2H), 7.15−6.95
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ASSOCIATED CONTENT
■
S
* Supporting Information
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Corresponding Author
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Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
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M.D.F. thanks NSERC of Canada for a Discovery Grant, and
K.D.J.P. thanks NSERC for Postgraduate Scholarships.
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