Synthesis and Reactivity of [PCCP]-Coordinated Group 5 Alkyl and Alkylidene Complexes Featuring a Metallacyclopropene Backbone

Article abstract of DOI:10.1021/acs.organomet.9b00577

The R₂P-functionalized tolanes 1a (R = Ph) and 1b (R = ⁱPr) were employed for the synthesis of [PCCP]-coordinated group 5 complexes. In the case of vanadium, the d¹-configured dialkyl species [PCCP]V(CH₂SiMe₃)₂ (3a and 3b) were prepared and found to be resistant toward oxidation, i.e., the corresponding alkylidene complexes were not observed upon treatment with various oxidants. For niobium and tantalum, however, the high-valent d⁰-configured alkyl-alkylidene complexes [PCCP]M(=CHSiMe₃)(CH₂SiMe₃) (M = Nb: 6b, M = Ta: 7b) were obtained for the ⁱPr₂P-substituted ligand. Both these alkylidenes, i.e., 6b and 7b, were employed for ring-opening metathesis polymerizations (ROMP) of strained olefins. Although the ??²-alkyne motifs in 6b and 7b are best described as metallacyclopropenes, the unsaturated ligand backbone was neither degraded nor copolymerized over the course of these polymerizations. Upon hydrogenolysis of 7b, however, the metallacyclopropene backbone was partially hydrogenated, and the dinuclear (μ-H)₃-bridged tantalum(V) complex 8 featuring a metallacyclopropane substructure was formed. An intermediate in this reaction was shown to catalytically hydrogenate one or both arenes in naphthalene, while the isolated dimeric species 8 was found to be inactive under identical reaction conditions.

Full text of DOI:10.1021/acs.organomet.9b00577

Article
pubs.acs.org/Organometallics
Cite This: Organometallics XXXX, XXX, XXX−XXX
Synthesis and Reactivity of [PCCP]-Coordinated Group 5 Alkyl and
Alkylidene Complexes Featuring a Metallacyclopropene Backbone
Patrick Federmann, Tim Richter, Hubert Wadepohl, and Joachim Ballmann*
̈
Anorganisch-Chemisches Institut, Universitat Heidelberg, Im Neuenheimer Feld 276, D-69120 Heidelberg, Germany
S
* Supporting Information
ABSTRACT: The R2P-functionalized tolanes 1a (R = Ph)
and 1b (R = iPr) were employed for the synthesis of [PCCP]-
coordinated group 5 complexes. In the case of vanadium, the
d1-configured dialkyl species [PCCP]V(CH2SiMe3)2 (3a and
3b) were prepared and found to be resistant toward oxidation,
i.e., the corresponding alkylidene complexes were not
observed upon treatment with various oxidants. For niobium
and tantalum, however, the high-valent d0-configured alkyl−
alkylidene complexes [PCCP]M(=CHSiMe3)(CH2SiMe3)
i
(M = Nb: 6b, M = Ta: 7b) were obtained for the Pr2P-
substituted ligand. Both these alkylidenes, i.e., 6b and 7b, were
employed for ring-opening metathesis polymerizations
(ROMP) of strained olefins. Although the η2-alkyne motifs in 6b and 7b are best described as metallacyclopropenes, the
unsaturated ligand backbone was neither degraded nor copolymerized over the course of these polymerizations. Upon
hydrogenolysis of 7b, however, the metallacyclopropene backbone was partially hydrogenated, and the dinuclear (μ-H)3-
bridged tantalum(V) complex 8 featuring a metallacyclopropane substructure was formed. An intermediate in this reaction was
shown to catalytically hydrogenate one or both arenes in naphthalene, while the isolated dimeric species 8 was found to be
inactive under identical reaction conditions.
Chart 1. Different Descriptions for η2-Alkyne Complexes
That Are Commonly Used To Qualitatively Indicate
Internal alkynes are particularly versatile ligands that are
Whether Weak or Strong Metal-Alkyne Interactions Are
known to coordinate to d- and even to f-block metals in an η2-
Present in a Given Alkyne Complex
INTRODUCTION
■
fashion.1 To qualitatively describe the bonding situation in
these complexes, the Dewar−Chatt−Duncanson model, which
was originally developed for η2-alkene complexes,2 may be
employed; i.e., it was shown that the concept is equally
applicable to η2-alkyne derivatives.3 In comparison to alkene
complexes, which are well-described by a σ-bonding and a π-
backbonding interaction, a second set of perpendicular π and
vs 4e−-donor, cf., Chart 1).4 Hard and soft metals, i.e., metal
π* orbitals needs to be taken into account in the case of
coordinated alkynes.4 With this set of additional orbitals, a
supplementary dative π-bond may be formed, while δ-
backbonding is commonly considered a rather weak inter-
action.5 As σ-donation and π-backdonation normally go hand
in hand, a partial rehybridization of the acetylenic carbon
atoms is induced, which ultimately leads to the formation of a
metallacyclopropene.6 Depending on the degree of rehybrid-
ization, the four different notations depicted in Chart 1 are
commonly used for describing the bonding situation in η2-
alkyne complexes.7
While the activation and subsequent conversion of alkynes is
frequently exploited in metal-catalyzed or metal-mediated
organic transformations,8 only a very few alkyne-based chelate
ligands have been reported so far, although a certain structural
and electronic flexibility has to be expected for such chelates
due to the variable donor properties of their alkyne units (2e−-
ions in different oxidation states, may thus be stabilized equally
well, which in general is considered advantageousnot only in
catalysis. Despite these encouraging expectations, only a few
alkyne-based chelates are known at present (see Chart 2). In
this context, it is even more surprising that all these alkyne-
ligands (cf., structures A,9 B,10 C,11 and D12) were only
employed for the synthesis of late transition metal complexes.
In A (investigated by Gleiter and co-workers9), B
(investigated by Tor and co-workers10), and C (investigated
11
́
by Latos-Grazynski and co-workers ), only weak interactions
̇
between the alkyne moieties and the metal centers were found
according to the respective CC bond lengths and the
deviations of the alkyne from linearity. For B and C, this
Received: August 22, 2019
© XXXX American Chemical Society
A
DOI: 10.1021/acs.organomet.9b00577
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Article
However, the difference in thermal stability and sensitivity
toward air is merely due to the differently substituted
phosphines and thus not reflected in overall molecular
structures of 1a and 1b; i.e., the geometry of the alkyne
fragments is fairly similar in both ligands as established by
single crystal X-ray diffraction (see Figure 1). As expected,
Chart 2. Multidentate Ligand Scaffolds Featuring an Alkyne-
Backbone and Their Hitherto Known Late Transition Metal
Complexes
Figure 1. ORTEP plots of the molecular structures of 1a and 1b (only
one of the two independent half-molecules present in the asymmetric
unit is shown for 1b, displacement ellipsoids drawn at 50%
probability, hydrogen atoms omitted). Selected bond lengths (Å)
and angles (deg) for 1a: C6−C7 1.436(2), C7−C7′ 1.196(3), C6−
C7−C7′ 177.0(2). Selected bond lengths (Å) and angles (deg) for
1b: C2−C7 1.4319(17), C7−C7′ 1.199(2), C2−C7−C7′
174.65(16).
observation is ascribed to the rigidity of the employed ligand
systems together with the low tendency of the chosen metals
to undergo 2e−-oxidations (not only in B and C, but also in
A).5c In contrast, comparatively strong alkyne-metal inter-
actions were found by Ohe and co-workers upon coordination
of a Ph2P-substituted diphosphinotolane to rhodium and
iridium (cf., complexes D in Chart 2).12 Although even
stronger interactions are to be expected for diphosphinotolane-
coordinated early transition metals, complexes of these type
have not been reported yet. For that reason, we set out to
explore the coordination chemistry of 2,2′-diphosphinotolanes
with the group 5 metals in a curiosity driven approach. We
have found that [PCCP]-coordinate vanadium alkyl and
niobium or tantalum alkylidene complexes are readily
accessible and that the coordinated alkynes within these
complexes are best described as metallacyclopropenes. These
metallacyclopropene units were shown to be surprisingly
robust, even in the presence of strained olefins under ROMP
conditions. Under H2 pressure, however, metallacyclopropanes
were formed. These results are discussed in the following.
linear arylCCaryl arrangements were found with the
lengths of the CC triple bonds well within the usual range14
(cf., caption of Figure 1). In both cases, the aryl rings are
mutually coplanar with the phosphines pointing to opposite
directions.
With both ligand derivatives in hand, we first set out to
explore their coordination chemistry with vanadium. To our
surprise, no reaction was observed with VCl3(thf)3 and
15
VI3(thf)3. Upon addition of solid VI2(thf)4 to solutions of
1a (in 1,2-dichloroethane) or 1b (in toluene), however, slow
conversions were noticed as the metal precursor gradually
dissolved in both cases. Red-brown solutions were obtained
after a few hours at room temperature. From these solutions,
the products 2 were isolated as orange (2a) and orange-brown
(2b) powders in 41% and 88% yield, respectively (see Scheme
i
2). For the more electron-rich Pr-substituted diphosphine 1b,
RESULTS AND DISCUSSION
■
this reaction may be carried out in thf as well, while no
conversion is observed when 1a and VI2(thf)4 are stirred in thf.
Following a previously reported procedure, the required
i
ligands 1a (R = Ph) and 1b (R = Pr) were prepared starting
from 2,2′-dibromotolane via bromine−lithium exchange and
subsequent treatment of the dilithiated intermediate with the
respective chloro phosphines ClPR2 (see Scheme 1).13
Scheme 2. Synthesis of the [PCCP]-Coordinated Vanadium
Pincer16 Complexes 2 and 3 Starting from 1
Scheme 1. Synthesis of 1
Both compounds were obtained in gram quantities as off-
white solids with δ(31P) = −8.9 ppm (1a) and 4.9 ppm (1b).
In solution, both ligand derivatives slowly cyclize to the
corresponding P-substituted diphosphadibenzo[a,e]-
pentalenes,13 which is accompanied by a color change from
colorless to intensively green; i.e., heating these ligands in
solution needs to be avoided. In the solid state, the Ph2P-
substituted derivative was found to be stable indefinitely at
room temperature even in air, while the oxygen-sensitive iPr2P-
substituted ligand was kept below 0 °C to prevent cyclization.
B
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Therefore, it is not surprisingly that isolated samples of 2a
degrade in thf, while solutions of 2b in thf are stable at room
temperature. To elucidate whether this difference is reflected
by the molecular structures of the products, single crystals of
2a and 2b were grown from dichloroethane/pentane and
subjected to X-ray diffraction. In each case, both acetylenic
backbone carbons, both phosphines and the two iodides are
coordinated to the central vanadium ion. In contrast to 2b, an
additional thf ligand was found within the coordination sphere
of 2a (see Figure 2, see SI for an ORTEP plot of 2a).
(see SI for an ORTEP plot). Upon reaction of 2b with
Mg(CH2SiMe3)2, however, the desired vanadium(IV) dialkyl
3b was formed cleanly and isolated as a pentane soluble dark-
green homogeneously microcrystalline powder (see SI for
UV−vis spectra). Single crystals of 3b were grown from a
concentrated pentane solution and subjected to X-ray
diffraction as well. Considering the metallacyclopropene in
3b as a single contact, a distorted trigonal bipyramidal
structure (τ5 = 0.68)17 with the phosphines occupying the
axial positions was found in the crystalline state. The
delocalized [PCCP]-backbone of the ligand adapts to the
coordination environment of the central vanadium ion and is
curved in a concave manner with respect to the apical position
(concave refers to a view along the C31−V vector, see Figure
2). A similar curvature was also uncovered for the five-
coordinate complex 2b, which exhibits an overall analogous
structure (see SI for an ORTEP plot). In solution, a magnetic
moment of μeff = 1.49 μB was determined (Evans method) and
an octet was observed in the EPR spectrum of 3b (see Figure
3), which is in line with a monomeric d1-configured
Figure 2. ORTEP plots of the molecular structures of 2a and 3b
(displacement ellipsoids drawn at 50% probability, hydrogen atoms
omitted, see SI for ORTEP plots of the molecular structures of 2b and
3a). Selected structural parameters are summarized in Table 1.
The aryl−CC angles within the ligand backbone and the
C13−C14 distances (approximately 1.29 0.01 Å) are in line
with a CC double bond, i.e., with a metallacyclopropene
substructure (cf., Table 1).1f Nevertheless, considering the CC
Table 1. Structural Parameters of Compounds 2 and 3
a
V−CCC (Å)
V−P (Å)
V−X (Å)
Ar−CC (deg)
2a 1.998(4),
2.5308(12),
2.5547(12)
2.5407(12),
2.5363(12)
2.6282(7),
2.6600(7)
2.7475(8),
2.7854(8)
2.6089(7),
2.7085(7)
2.093(2),
2.096(2)
154.5(4),
153.9(4)
151.4(4),
154.0(4)
150.1(2),
150.3(2)
Figure 3. X-band EPR spectra (293 K) of 3a (left) and 3b (right)
1.998(4)
recorded in toluene (3a) and pentane (3b), respectively (3a: giso
=
2b 1.985(4),
1.97, |Aiso|(51V) = 184 MHz, |Aiso|(31P) = 69 MHz; 3b: giso = 1.98, |
Aiso|(51V) = 177 MHz, |Aiso|(31P) = 54 MHz).
2.001(4)
3a 2.007(2),
2.012(2)
vanadium(IV) complex. That the phosphines are coordinated
not only in the solid state, but also in solution is also evident
from the EPR spectra, as the central lines of the octets exhibit a
superimposed triplet pattern due to hyperfine coupling to the
two 31P nuclei (I = 1/2) that are present in each complex (cf.,
caption of Figure 3).
To explore the reactivity of 3b, hydrogenation reactions
were attempted, but no productive conversions were detected.
As 3b was inert toward simple donor ligands (e.g., CO, py) as
well, attempts were made to generate the corresponding
vanadium(V) cations by means of selective oxidation. Inspired
by Mindiola’s work on oxidatively induced α-eliminations,19 3b
was reacted with various oxidants (inter alia AgPF6, AgOTf,
thianthrenyl radical cation20), but no reaction was observed
unless exceedingly reactive oxidants (NOPF6, XeF2) were used,
which led to decomposition.21
3b 2.0046(17),
2.6385(5),
2.6463(5)
2.1384(17),
2.0926(17)
150.74(17),
150.93(17)
2.0048(17)
a
X = I for 2a and 2b; X = CH2SiMe3 for 3a and 3b.
bond as single contacts may be useful as well, namely, for
describing the coordination polyhedra around the central
metals. In this picture, an octahedral structure emerges for 2a
(see Figure 2) and a distorted square pyramid (τ5 = 0.28)17
with one of the iodides occupying the axial position is found
for 2b (see SI for an ORTEP plot).
In dichloromethane, 2a and 2b exhibit 51V-coupled (I = 7/
2) octet-signals in the EPR spectra (see SI), which is in line
with the presence of (formally) d1-configured vanadium(IV)
complexes and a formally dianionic pincer16 ligand. With the
structures of 2a and 2b ascertained in the solid state and in
18
solution, alkylation reactions with Mg(CH2SiMe3)2 were
In view of the inert character of complexes 3 toward
oxidation, the synthesis of niobium and tantalum complexes
was considered due to the tendency of these elements to more
readily attain their pentavalent oxidation state. Upon reaction
of NbCl3(dme)22 or (in situ generated) TaCl3(dme)23 with 1a
or 1b, the desired [PCCP]pincer16 complexes formed indeed,
but unidentified insoluble species were produced over the
course of these reactions as well. Hence, the alkyne-
explored. In the case of 2a, this reaction led to mixtures of
products containing the desired dialkyl species together with
certain amounts of decoordinated ligand (according by 31P
NMR spectroscopy) and unidentified vanadium(IV) com-
plexes (according to EPR spectroscopy). However, small
amounts of the desired dialkyl complex 3a were obtained after
repeated recrystallization and the structure of the product was
ascertained unambiguously by single crystal X-ray diffraction
C
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coordinated niobium and tantalum precursors (η2-Me3SiC
CSiMe3)MCl3(dme) (M = Nb, Ta)24,23 were used as synthons
for the trivalent metal halides. Upon reaction of these
complexes with 1a and 1b, Me3SiCCSiMe3 was liberated
and the [PCCP]MCl3 (M = Nb, Ta) target complexes 4a, 4b,
5a, and 5b were generated in high yields (see Scheme 3). All
The structural parameters of the metallacyclopropene
moieties within the trichlorides 4a, 4b, 5a, and 5b are
surprisingly unaffected by the central metal ion and the
phosphine substitution pattern, i.e., the CCC−CCC and M−CCC
bond lengths with the metallacyclopropene lie in the narrow
range of 1.29 0.02 Å and 2.11 0.02 Å, respectively, while
the Ar−C = C angles (153 2°) vary by two degrees only (see
Table 2). Nevertheless, slightly shorter M−P distances are
noticed for the Ph2P-substituted species 4a and 5a (2.65
0.03 Å) in comparison to the iPr2P-derivatives 4b and 5b (2.68
0.02 Å). As a consequence of the relative trans-influences,
significantly weakened M−Cl bonds were found for the
chlorides trans to the metallacyclopropene. Thus, the halides
perpendicular to plane of the ligand are closer to the metal ion
by approximately 0.15 Å in each case (cf., Table 2).
Scheme 3. Synthesis of the [PCCP]-Coordinated Niobium
and Tantalum Pincer16 Complexes 4, 5, 6, and 7 Starting
from 1
Starting from the [PCCP]-coordinated niobium and
tantalum trichlorides, alkylating reactions were carried out
i
using 1.5 equiv of Mg(CH2SiMe3)2. In the case of the Pr2P-
substituted species 4b and 5b, these reactions proceeded
without difficulties and led to the alkyl−alkylidene complexes
6b and 7b. The corresponding trialkyls were observed as
intermediates with δ(31P) = 61.6 and 64.5 ppm for niobium
and tantalum, respectively, while the 31P NMR resonances of
the products 6b and 7b were detected at δ(31P) = 65.2 and
67.4 ppm, respectively. Over the course of the reactions the
NMR signals corresponding to the former intermediates
vanished and the formation of SiMe4 was monitored in the
1
respective H NMR spectra. After 24 h at room temperature,
this process was complete and the alkylidene species exhibiting
1
characteristic H NMR signals at 11.00 (6b) and 8.47 ppm
(7b) were isolated in 77% (6b) and 58% (7b) yield. After
recrystallization, microcrystalline yellow-orange powders were
obtained, which were found to be remarkably stable even in
refluxing benzene. In contrast, the synthesis of the Ph2P-
substituted alkylidenes 6a and 7a turned out to be exceedingly
more challenging given that several unidentified species were
generated upon alkylation with Mg(CH2SiMe3)2 or
LiCH2SiMe3, even at low temperatures. Although these side-
reaction interfered with the isolation of analytically pure
four halide derivatives were isolated as greenish-yellow
diamagnetic powders, indicating that these complexes are
best described as d0-configured niobium(V) and tantalum(V)
species with a formally dianionic metallacyclopropene motif
present in each case. In the 31P NMR spectra, only one
resonance was observed for each complex and the correspond-
ing proton NMR spectra indicated that C2v-symmetric species
were present in solution. Crystallographic analyses of
complexes 4 and 5 confirmed the anticipated octahedral
coordination environments around the central metals,
provided that the metallacyclopropenes are seen as single
contacts. Interestingly, the backbones of the ligands are
1
samples of 6a and 7a (cf., Experimental Section), H and 31P
NMR spectroscopy unambiguously confirmed that both
products were generated, which was further corroborated for
7a by means of single crystal X-ray diffraction (see SI for an
i
ORTEP plot, cf., Table 2). For the analytically pure Pr2P-
substituted derivatives 6b and 7b, single crystals were grown
readily from concentrated pentane solutions at −40 °C and
subjected to X-ray diffraction as well (see Table 2).
i
perfectly planar for the Pr2P-substituted derivatives 4b and
5b, while slightly curved π-systems were noticed for the Ph2P-
substituted derivatives 4a and 5a (see Figure 4 for ORTEP
plots of 4a and 5b, see SI for ORTEP plots of 4b and 5a).
According to the distances between the acetylenic carbons
(approximately 1.306 0.01 Å) and the Ar−CC angles in 6b,
7a, and 7b, the metal-bound alkyne units are once again best
described as a metallacyclopropenes. The coordination
polyhedra around the central metal ions may either be
regarded as distorted square pyramids with the alkylidenes
occupying each apical position or as distorted trigonal
bipyramids with the P−M−P axis deviating from linearity by
approximately 8°/15° (two independent molecules in 7a with
τ5 = 0.63/0.56) and 20° (6b, 7b with each τ5 = 0.40),
respectively, i.e., the actual coordination polyhedra are best
seen as intermediates between both idealized forms. In the
solid state, the three crystallographically characterized alkyl−
alkylidene complexes are CS-symmetric (6b, 7b) or nearly CS-
symmetric (7a) with the alkylidene C−H vectors pointing
toward the open fronts in each case (6b and 7b: see Figure 5,
Figure 4. ORTEP plots of the molecular structures of 4a and 5b
(displacement ellipsoids drawn at 50% probability, hydrogen atoms
omitted, see SI for ORTEP plots of the molecular structures of 4b and
5a). Selected structural parameters are summarized in Table 2.
D
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Table 2. Structural Parameters of Compounds 4−7
a
M−CCC (Å)
M−P (Å)
M−X (Å)
Ar−CC (deg)
4a
4b
2.1143(15), 2.1152(15)
2.1118(18), 2.1068(18),
2.106(2), 2.1088(19)
2.6782(4), 2.6260(4)
2.6815(5), 2.6813(5),
2.6945(6), 2.6875(6)
2.5373(4), 2.3641(4), 2.3575(4)
2.5210(5), 2.3923(5), 2.3562(5), 2.5323(5),
2.3786(5), 2.3622(5)
152.85(16), 154.01(15)
152.70(19), 152.78(19),
152.56(19), 153.2(2)
b
5a
5b
6b
2.1084(14), 2.1073(14)
2.0981(17), 2.0981(17)
2.1208(14), 2.1208(14)
2.104(3), 2.117(3), 2.108(3),
2.111(3)
2.6682(4), 2.6214(4)
2.6912(4), 2.6755(4)
2.6718(3), 2.6717(3)
2.6357(9), 2.6367(9),
2.6158(9), 2.6282(8)
2.5250(4), 2.3588(4), 2.3538(4)
2.5050(4), 2.3776(4), 2.3531(4)
2.3369(19), 1.971(2)
152.55(14), 153.52(14)
152.38(16), 151.91(17)
150.89(8), 150.89(8)
150.6(3), 152.0(3), 150.4(3),
149.8(3)
b
7a
2.244(4), 1.975(4), 2.234(4), 1.966(4)
7b
2.114(2), 2.114(2)
2.6609(6), 2.6609(6)
2.292(4), 1.965(4)
150.77(14), 150.77(14)
a
b
X = Cl for 4a, 4b, 5a, and 5b; X = CH2SiMe3 or CHSiMe3 for 6b, 7a, and 7b. Two independent molecules are present in the asymmetric unit.
Scheme 4. ROMP of NBE Derivatives Catalyzed by 6b and
7b
Figure 5. ORTEP plots of the molecular structures of 6b and 7b
(displacement ellipsoids drawn at 50% probability, most hydrogen
atoms omitted, see SI for an ORTEP plot of the molecular structure
of 7a). Selected structural parameters: d(C17−H17) = 0.84(4)Å
(6b), 0.76(5)Å (7b); d(M−H17) = 2.29(4)Å (6b, M = Nb),
2.24(5)Å (7b, M = Ta); ∠M−C17−H17 = 102(3)° (6b, M = Nb),
101(4)° (7b, M = Ta). Other selected structural parameters are
summarized in Table 2.
In the case of NBE and NBD, the tantalum precatalyst 7b
was found to be more active in comparison to the niobium
analogue; i.e., reaction times of 24 h were sufficient for 7b,
while 120 h were required for achieving reasonable conversion
with 6b (see Table 3). The cis/trans-ratio in the polymeric
Table 3. Niobium- and Tantalum-Catalyzed ROMP
Reactions with Strained Olefins (1 mol % Catalyst, 85 °C in
Benzene)
7a: see SI). In 6b and 7b, the M−C17−H17 angles (M = Nb,
Ta) of approximately 101° and the M−C17−Si2 angles of
approximately 140° indicate that α-agostic interactions do not
play a significant role,25 which is also the case for 7a (see SI for
detailed structural parameters). The chemical shifts of the
conv.
(%)
cis/
cat.
monomer
NBE
NBD
2-MOM-5-NBE
NBE
NBD
t (h)
trans
Mw (PDI)
6b
6b
6b
7b
7b
7b
120
120
120
24
24
24
85
80
35
94
99
13
0.35
0.29
3.15 × 105 (2.60)
1
a
alkylidene protons in the respective H NMR spectra (vide
n.d.
b
1.82 × 105 (1.60)
9.34 × 104 (1.25)
supra) are in line with this interpretation, as significantly high-
field shifted resonances are commonly detected for niobium
and tantalum alkylidenes that are stabilized by α-agostic
interactions.26 The 1JC−H coupling constants determined for 6b
(104 Hz) and 7b (108 Hz) are less diagnostic, but within the
typical range for trimethylsilyl substituted niobium and
tantalum alkylidenes.27
n.d.
1.29
0.83
a
n.d.
a
2-MOM-5-NBE
n.d.
5.54 × 104 (1.20)
a
Not determined due to the polymers’ insolubility in all common
b
solvents. Not determined due to overlapping 1H NMR signals of the
cis and trans isomer (note that 2-MOM-5-NBE was used as a mixture
of the endo and exo isomer, which further complicates the analysis).
As sufficiently pure batches of the Ph2P-substituted
alkylidenes 6a and 7a were not obtained (vide supra),
reactivity studies were carried out only for the iPr2P-substituted
derivatives 6b and 7b. In view of a recently reported
relationship between the 13C NMR shifts of the alkylidene
resonances and the reactivities of the corresponding
alkylidenes,28 complexes 6b and 7b with δalkylidene(13C{1H})
= 285.7 and 272.8 ppm, respectively, were considered
promising candidates for ring-opening olefin metathesis
polymerization (ROMP) reactions. The question that arose
in this context, is whether the metallacyclopropene backbone
of the ligand was stable under ROMP conditions or whether
the ligand itself would act as a substrate and thus lead to
degraded catalysts only. Employing norbornene (NBE),
norbornadiene (NBD), and 2-methoxymethyl-5-norbornene
(2-MOM-5-NBE, mixture of the endo and the exo isomer) and
1 mol % of the catalyst (6b or 7b) indeed led to the formation
of the corresponding polymers, although heating to 85 °C was
required to reach significant conversions (see Scheme 4).
29
1
materials was determined by H NMR spectroscopy, but no
practically useful selectivities were found. For 2-methoxymeth-
yl-5-norbornene (2-MOM-5-NBE) low conversions were
observed with both precatalysts, presumably due to inhibition
via coordination of the methoxy groups to the active species.
For NBE and 2-MOM-5-NBE, the averaged molecular weights
(determined by GPC analysis) of the polymers were found to
be higher by an order of magnitude for the niobium catalyst in
comparison to the tantalum analogue, which is in line with the
relative reaction times and the PDI values for both catalyst (see
Table 3).
In comparison to state-of-the-art group 5 ROMP catalysts,
neither 6b nor 7b seems to be suited for an application as
nearly perfectly regioselective catalysts with high activities at
room temperature have been reported already.30 Nevertheless,
our results indicate that metallacyclopropene subunits within
E
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Article
the catalyst are tolerated in principle, i.e., that the latter
substructure is not prone to side-reactions per se and thus
possibly suited as a design element in other ligands as well. To
substantiate this concept, NBE and 1 mol % of 6b and 7b,
respectively, were loaded into NMR tubes and the polymer-
ization reactions monitored by 1H and 31P{1H} NMR
spectroscopy. In both cases, no signs of degraded catalysts
were detectable in the respective NMR spectra.31
additional bridging hydrides. These data are in line with a
trihydrido-bridged ditantalum complex with the tantalum(V)
cores coordinated by two partially hydrogenated capping
ligands (one on each Ta) and three terminal hydrides in total.
Upon deuteration starting form 7b and D2 or from 8 and D2,
the 10-fold deuterated species 8-d10 was obtained with all the
six hydrides and the four olefinic hydrogens exchanged for
2
deuterium (see SI for details). In the H{1H} NMR spectrum
To further probe the stabilities of 6b and 7b, hydrogenation
reactions were studied next, but no reactions were observed at
room temperature, not even under 10 bar H2-pressure. Upon
heating these pressurized solutions in C6H12 (or C6D12) to 65
°C, however, slow reactions set in, in both cases. For the
niobium derivative 6b, this reaction reached completeness after
3d and led to a black unidentified precipitate (insoluble in all
common organic solvents, supposedly finely divided niobium
metal) and a nearly colorless solution. Only one singlet at δ =
−8.0 ppm was detected upon inspection of the supernatant
solution by 31P{1H} NMR spectroscopy. This species was
unambiguously identified as the partially hydrogenated trans-
of 8-d10, all the expected deuterium signals were found
supporting the signal assignment discussed above.
To unambiguously ascertain the structure of the product
(compound 8, see Scheme 5), single crystals suitable for X-ray
Scheme 5. Synthesis of 8 via Hydrogenolysis of 7b Together
with the Catalytic Hydrogenation of Naphthalene Using 7b
as a Precatalysis (See Inset)
i
alkene ligand Pr2PC6H4CHCHC6H4PiPr2 (1b-
H2, see SI for an ORTEP plot of the corresponding molecular
structure).32 Employing identical reaction conditions, the
tantalum derivative 7b was hydrogenated as well and the
31P{1H} NMR signal corresponding to the latter alkene was
detected once again, but only in amounts of approximately
15% (by integration of the 31P NMR signals). For the main
reaction product (approximately 85% according to 31P NMR
spectroscopy), four phosphorus resonances in a 1:1:1:1 ratio
were observed at 89.1, 80.1, 79.1, and 67.0 ppm, indicative of
an asymmetrically coordinated dinuclear species. In the
diffraction were grown from a concentrated pentane/cyclo-
hexane mixture (see Figure 7). As expected on the basis of the
1
corresponding H NMR spectrum, typical signals for overall
four metallacyclopropane protons were found at 4.16, 4.71,
and 4.98 ppm in addition to three chemically different 31P-
coupled hydride resonances at −3.60, 4.98, and 13.19 ppm,
which were identified as three terminal hydride (see Figure 6).
Figure 7. ORTEP plot of the molecular structure of 8 (displacement
ellipsoids drawn at 50% probability, carbon-bound hydrogen atoms
omitted). Selected bond lengths (Å) and angles (deg): Ta1···Ta2
2.9364(2), Ta1−C7 2.275(4), Ta1−C8 2.290(4), C7−C8 1.459(6),
Ta1−P2 2.5121(10), Ta1−P1 2.5223(10), Ta1−H91 1.97(4), Ta1−
H92 1.69(5), Ta1−H93 1.84(5), Ta1−H94 1.77(6), Ta1−H96 1.84
(5), Ta2−C34 2.209(4), Ta2−C33 2.258(4), C33−C34 1.470(6),
Ta2−P42.5302(10), Ta2−P3 2.5761(10), Ta2−H91 1.68(4), Ta2−
H92 1.83(5), Ta2−H93 1.84(5), Ta2−H95 1.80(5), C7−Ta1−C8
37.29(14), P2−Ta1−P1 124.77(3), H91−Ta1−H93 60(2), H92−
Ta1−H93 65(2), C34−Ta2−C33 38.42(15), P3−Ta2−P4
133.04(3), H91−Ta2−H93 66(2), H92−Ta2−H93 63(2), Ta2−
Ta1−H94 100.3(18), Ta1−Ta2−H95 109.4(17), Ta2−Ta1−H96
170.4(15).
1
1
Figure 6. Selected regions of the H{31P} NMR (top) and H NMR
(bottom) spectra (600 MHz, C6D6, 295 K) of 8 showing the three
terminal hydrides. The H NMR signal of the three μ-hydrides (not
shown) is buried below one of the Pr resonances at 0.98 ppm.
1
i
NMR data, a dinuclear (μ-H)3-bridged structure with overall
three terminal hydrides was found with one cis-configured
partially hydrogenated ligand bound to each tantalum atom.
Interestingly, the P3−Ta2−P4 angle (approximately 133°) was
found to be widened significantly in comparison to the P1−
Ta1−P2 angle (approximately 125°), although Ta1 is
coordinated by two hydrides, while only one hydride is
bound to Ta2. This observation is in line with the DFT-
optimized structure of 8 (see SI) and seems to be related to
The resonances at −3.60 and 4.98 ppm appear as mutually
coupled signals in the 1H{31P} NMR spectrum, indicating that
these two hydrides are bound to one tantalum atom, while the
hydride at 13.19 ppm is bound to the second tantalum. In the
COSY NMR spectrum, each of the three terminal hydrides
exhibits a cross-peak to a broad signal at 0.98 ppm (partially
i
buried below the Pr-resonances), which corresponds to three
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Article
i
the overall spatial arrangement of the Pr groups. Due to this
arrangement, which minimizes through-space repulsions
between both ligands in 8, only one tantalum center is suitably
arranged for hosting a second hydride (see SI for space-filling
models). In the crystallographically determined structure, both
capping ligands are nearly perpendicular with respect to each
other (dihedral angle CCcentroid···Ta···Ta···CCcentroid = 92°),
which also agrees well with the DFT model.
CONCLUSIONS
■
In summary, it was shown that both derivatives of 1 (R = Ph,
iPr) are suitable for the synthesis of group 5 complexes,
although the Ph2P-substituted ligand 1a seemed to be inferior
i
in comparison to the Pr2P-derivative 1b, given that some of
the Ph2P-substituted complexes were not easily obtained in a
pure form (cf., Scheme 2 and Scheme 3). Regardless of the
ligand’s substitution pattern, however, the coordinated alkynes
are best described as metallacyclopropenes in all the examples
reported herein. Interestingly, only d1-configured complexes
were obtained for vanadium, while d0-configured alkylidene
complexes were readily prepared for niobium and tantalum, at
least in the case of ligand 1b. Despite their unsaturated ligand
backbone, the former alkylidenes (i.e., 6b and 7b) were found
to be surprisingly robust ROMP catalysts. Moreover, it was
shown that the tantalum derivative 7b may be employed as
hydrogenation catalyst, although the reactive species formed in
situ ultimately was converted to a catalytically inactive dimeric
hydride, namely, complex 8. Future efforts will be directed
toward the isolation of related monomeric hydrido-tantalum
hydrogenation catalysts bearing sterically more encumbered
2,2′-diphosphinotolanes.
In this context, it seems noteworthy that the coordination
chemistry of R2P-substituted cis- and trans-stilbene ligands has
been explored by the groups of Bennett,33 Iluc,34 Carmona,35
and Barrata.36 In their studies on group 9, 10, and 11
complexes, the diphosphino-alkene ligands were either
prepared via traditional routes, via templated C−C-coupling
or via dehydrogenation reactions; i.e., the templated hydro-
genation route reported herein may very well represent an
additional approach to species of this type. In contrast to the
former alkene-coordinated late metal complexes, however, a
formulation as metallacyclopropane with formally pentavalent
tantalum ions seems more appropriate in the case of 8 (see
caption of Figure 7 for selected metric parameters). The Ta(V)
oxidation state of 8 is reflected not only by the compounds
diamagnetism, but also by the Ta···Ta separation of
approximately 2.94 Å, which is lengthened significantly in
comparison to related dinuclear hydride-bridged Ta(IV)
species.37
Interestingly, all of the (μ-H)x-bridged dinuclear tantalum
complexes known in literature are formed via unknown
mechanisms, although it is known that fairly reactive species
are involved, which often coexist with cyclometalated
intermediates.37 In contrast, 8 is thermally stable at temper-
atures up to 110 °C and actually the first asymmetric (μ-H)3-
bridged ditantalum hydride. Due to this unique structure, the
hydrogenation of 7b was examined in more detail. Although no
intermediates were observed by 31P NMR spectroscopy at 65
°C,38 an interesting observation was made, when the reaction
between 7b and H2 was monitored in benzene-d6 or toluene-
d8. In both these solvents, varying amounts (up to 15%) of the
corresponding partially hydrogenated solvents (i.e., C6D6H6
and C7D8H6) were detected by proton NMR spectroscopy.
When samples of 8 in benzene-d6 or toluene-d8 were
pressurized with H2, no reaction was observed, not even at
80 °C (benzene-d6) or 110 °C (toluene-d8). Taken together,
these findings indicate that a reactive intermediate in the
reaction mixture may be employed as a homogeneous arene
hydrogenation catalysts, while the final product of the reaction
is unreactive toward arenes. This was further corroborated by
employing a less volatile substrate (naphthalene) and catalytic
amounts of 7b (1 mol %). After heating this mixture to 65 °C
(in hexanes) under H2 (10 bar) for 3d, 34% of the employed
naphthalene was consumed and the in situ generated catalyst
was converted to 8. The expected products, i.e., tetraline
(C10H12) and decaline (C10H18), were produced in a 2:3 ratio
EXPERIMENTAL SECTION
■
General Remarks. All experiments were carried out under an
atmosphere of oxygen-free and dry argon (Argon 5.0 passed over
P4O10) either in a glovebox (MBraun) or by using standard Schlenk
techniques. Glassware was heated to 130 °C overnight and evacuated
while still hot prior to use. Pentane, hexane, diethyl ether, thf, toluene,
and dichloromethane were purified using an MBraun Solvent
Purification System. All other solvents were dried by established
methods and distilled prior to use. Deuterated solvents were dried
over calcium hydride (CD2Cl2) or over sodium/benzophenone
(C6D6, thf-d8, and toluene-d8) and distilled before use. NMR spectra
were recorded on a Bruker Avance II 400 MHz or on a Bruker Avance
600 III spectrometer Carbon and phosphorus NMR spectra were
1
collected with H broadband or composite pulse decoupling unless
stated otherwise. Chemical shifts δ are given in parts per million
(ppm) relative to residual proton signals of the lock solvent. Coupling
constants J are given in Hertz (Hz). Elemental analyses were
measured at the Department of Inorganic Chemistry (Heidelberg
University) using an Elementar vario MICRO Cube. UV−vis spectra
were collected on a Cary 5000 UV−vis−NIR Spectrophotometer.
EPR spectra were recorded on a Magnettech MiniScope MS400
spectrometer at room temperature and fitted with EasySpin.39 Mass
spectrometry was conducted at the Department of Organic Chemistry
(Heidelberg University) using a JEOL AccuTOF by liquid injection
field desorption ionization (LIFDI).40 GPC measurements were
carried out at room temperature on a Jasco PU-2050 GPC unit
equipped with PSS-SDV columns (8.0 mm × 30.0 mm, 5 μm
particles, 102-, 103- and 105-Å pore size) and a Jasco RI-2031 RI-
detector. Compounds 1,13 VI2(thf)4,15 VI3(thf)3,41 NbCl3(dme),22
MCl3(TMSCCTMS)(dme) (M = Nb, Ta),23 and Mg(CH2SiMe3)2
18
were prepared according to the literature. NBE, NBD, and 2-
methoxymethyl-5-NBE were dried over sodium and distilled prior to
use.
Ph[PCCP]VI2(thf) (2a). Finely powdered 1a (195 mg, 357 μmol,
1.05 equiv) was added to a stirred solution of VI2(thf)4 (200 mg, 340
μmol, 1.00 equiv) in dichloroethane (40 mL) at room temperature.
After stirring was continued for 3.5 h, the volume of the solvent was
condensed to 10 mL, and the concentrated solution was stored at −40
°C overnight. The resulting microcrystalline precipitate was filtered
off and dried in vacuo to afford the product as an orange solid (128
1
according to H NMR spectroscopy (see Scheme 5). In view
of these promising results, we currently focus on the
preparation of sterically more demanding ligands (Ad2P- and
tBu2P-derivatives of 1) and their tantalum complexes, which is
thought to hamper the formation of dinuclear tantalum species
and possibly allow for the detection of the catalytically active
species.
α
mg, 139 μmol, 41%). EPR (ν = 9.442969 GHz, CH2Cl2, 22 °C): giso
= 1.98, |Aiso|α(51V) = 260 MHz, |Aiso|α(31P) = 51 MHz and gisoβ = 1.96,
|Aiso|β (51V) = 210 MHz, |Aiso|β(31P) = 56 MHz with α and β
representing two molecules in solution; i.e., it is assumed that the
G
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Article
coordinated thf is labile in solution; μeff (Evans method, CD2Cl2) =
1.61 μB; UV−vis (dichloroethane, 22 °C) λ (in nm) [ε (in L·mol−1·
cm−1)] = 410 [2528]; MS (LIFDI, dichloroethane) m/z (in g/mol)
calcd. for C42H36I2OP2V: 922.9770, found 923.06 (20%) [M]+;
Elemental analysis calcd. for C42H36I2OP2V: C 54.63, H 3.93. Despite
numerous attempts, the values were consistently low (e.g., found: C
53.63, H 3.89), which may be related to incomplete combustion or
carbide formation.
analysis calcd. for C38H28Cl3NbP2: C 61.19, H 3.78. Despite
numerous attempts, the values were consistently low (e.g., found: C
59.65, H 4.14), which may be related to incomplete combustion or
carbide formation.
iPr[PCCP]NbCl3 (4b). Following the procedure for 4a, solid 1b
(246 mg, 598 μmol, 1.10 equiv) was added to NbCl3(TMSCCTMS)-
(dme) (250 mg, 544 μmol, 1.00 equiv) in a mixture of toluene (10
mL) and CH2Cl2 (5 mL). After workup, 4b was obtained as bright
green solid (313 mg, 513 μmol, 94%). 1H NMR (600 MHz, CD2Cl2,
22 °C) δ (in ppm) = 8.20 (d, JH−H = 7.5 Hz, 2H, ArH), 7.90 (d, JH−H
iPr[PCCP]VI2 (2b). Akin to the synthesis of 2a, 1b (366 mg, 891
μmol, 1.05 equiv) was added to a stirred solution of VI2(THF)4 (500
mg, 849 μmol, 1.00 equiv) in toluene (50 mL) at room temperature.
After 2.5 h, the solvent was evaporated in vacuo. The remaining solid
was washed with pentane (10 mL) to afford the product as an orange-
brown solid (532 mg, 744 μmol, 88%). EPR (ν = 9.409001 GHz,
CH2Cl2, 22 °C): giso = 1.98, |Aiso|(51V) = 197 MHz, |Aiso|(31P) = 59
MHz; μeff (Evans method, CD2Cl2) = 1.68 μB; UV−vis (dichloro-
ethane, 22 °C) λ (in nm) [ε (in L·mol−1·cm−1)] = 485 [1320], 549
[640], 679 [370]; MS (LIFDI, dichloroethane) m/z (in g/mol) calcd.
for C26H36I2P2V: 714.9821, found 714.98 (70%) [M]+; Elemental
analysis calcd. for C26H36I2P2V·C4H8O: C 45.76, H 5.64. Despite
numerous attempts, the values were consistently low (e.g., found: C
45.24, H 5.19), which may be related to incomplete combustion or
carbide formation.
= 6.9 Hz, 2H, ArH), 7.81 (t, JH−H = 7.3 Hz, 2H, ArH), 7.65 (t, JH−H
=
7.2 Hz, 2H, ArH), 2.91−2.80 (m, 4H, iPrCH), 1.61−1.53 (m, 12H,
iPrCH3), 1.29−1.21 (m, 12H, iPrCH3); 13C{1H} NMR (151 MHz,
CD2Cl2, 22 °C) δ (in ppm) = 185.2 (t, J = 5.0 Hz, alkyneC), 147.0 (t, J
= 17.7 Hz, ArC), 142.1 (t, J = 12.3 Hz, ArC), 133.8 (s, ArCH), 132.8−
132.7 (m, ArCH), 131.3 (t, J = 2.4 Hz, ArCH), 28.5 (t, J = 6.7 Hz,
iPrCH), 20.0 (s, iPrCH3), 19.8 (s, iPrCH3); 31P{1H} NMR (243 MHz,
CD2Cl2, 22 °C) δ (in ppm) = 52.3 (s); MS (LIFDI, dichloroethane)
m/z (in g/mol) calcd. for C26H36Cl3NbP2: 608.0422, found 608.00
(100%) [M]+; Elemental analysis calcd. for C26H36Cl3NbP2: C 51.21,
H 5.95. Despite numerous attempts, the values were consistently low
(e.g., found: C 50.85, H 5.96), which may be related to incomplete
combustion or carbide formation.
Ph[PCCP]V(CH2SiMe3)2 (3a). To a solution of Ph[PCCP]VI2(thf)
2a (200 mg, 235 μmol, 1.00 equiv) in toluene (10 mL) was added
Mg(CH2SiMe3)2 (46.7 mg, 235 μmol, 1.00 equiv) and the resulting
mixture was stirred for 2.5 h at room temperature. The reaction
mixture was filtered through Celite and the filtrate was evaporated
under reduced pressure. The residual green-gray powder was
recrystallized twice from Et2O/pentane to afford the product (50
mg, 65 μmol, 27%) as dark green crystals. EPR (ν = 9.412462 GHz,
toluene, 22 °C): giso = 1.97, |Aiso| (51V) = 184 MHz, |Aiso| (31P) = 69
MHz; μeff (Evans method, CD2Cl2) = 1.90 μB; UV−vis (toluene, 22
°C) λ (in nm) [ε (in L·mol−1·cm−1)] = 440 [3395], 476 [2531], 637
[1096]; Elemental analysis calcd. for C46H50P2Si2V: C 71.57, H 6.53.
Despite numerous attempts, the values were consistently low (e.g.,
found: C 71.21, H 5.81), which may be related to incomplete
combustion or carbide formation.
Ph[PCCP]TaCl3 (5a). Finely powdered 1a (200 mg, 366 μmol, 1.10
equiv) was added to a stirred solution of TaCl3(TMSCCTMS)(dme)
(182 mg, 333 μmol, 1.00 equiv) in a mixture of toluene (10 mL) and
CH2Cl2 (5 mL). After stirring for 2.5 h at room temperature, the
mixture was concentrated to approximately half of its volume. Storage
of the resulting mixture at −40 °C overnight resulted in the formation
of a precipitate, which was collected by filtration. The solid was placed
on the sintered glass frit and washed repeatedly with pentane (10 mL)
and dried in vacuo to afford the title compound (244 mg, 293 μmol,
1
88%) as a green powder. H NMR (600 MHz, CD2Cl2, 22 °C) δ (in
ppm) = 8.35 (d, JH−H = 7.6 Hz, 2H, ArH), 7.99 (t, JH−H = 7.4 Hz, 2H,
ArH), 7.89−7.83 (m, 2H, ArH), 7.78−7.73 (m, 8H, ArH), 7.68 (t, JH−H
= 7.4 Hz, 2H, ArH), 7.43−7.37 (m, 12H, ArH); 13C{1H} NMR (151
MHz, CD2Cl2, 22 °C) δ (in ppm) = 190.7 (t, J = 3.9 Hz, alkyneC),
147.5 (t, J = 26.4 Hz, ArC), 146.2 (t, J = 13.4 Hz, ArC), 136.3 (s, ArC),
134.4 (t, J = 4.1 Hz, ArC), 134.3 (s, ArC), 134.0 (t, J = 5.6 Hz, ArC),
132.5 (t, J = 18.4 Hz, ArC), 131.9 (t, J = 3.0 Hz, ArC), 131.2 (s, ArC),
129.0 (t J = 4.8 Hz, ArC); 31P{1H} NMR (243 MHz, C2D2Cl4, 22 °C)
δ (in ppm) = 41.2 (s); MS (LIFDI, dichloroethane) m/z (in g/mol)
calcd. for C38H28Cl3P2Ta: 832.0212, found 831.95 (100%) [M]+;
Elemental analysis calcd. for C38H28Cl3P2Ta: C 54.73, H 3.38. Found:
C 55.02, H 3.78.
iPr[PCCP]V(CH2SiMe3)2 (3b). Solid Mg(CH2SiMe3)2 (83.4 mg,
419 μmol, 1.00 equiv) was added to a stirred solution of 2b (300 mg,
419 μmol, 1.00 equiv) in benzene (10 mL) at room temperature and
stirring was continued for 3 h. The solvent was evaporated under
reduced pressure and the residual solid was extracted with pentane.
The pentane extracts were filtered over Celite, and the filtrate
evaporated in vacuo to afford the product as a dark green solid (99.0
iPr[PCCP]TaCl3 (5b). Following the procedure for 5a, 1b (200 mg,
mg, 156 μmol, 37%). EPR (ν = 9.415962 GHz, pentane, 22 °C): giso
=
1.98, |Aiso|(51V) = 177 MHz, |Aiso|(31P) = 54 MHz; μeff (Evans
method, CD2Cl2) = 1.49 μB; UV−vis (toluene, 22 °C): (λ (in nm) [ε
(in L·mol−1·cm−1)] = 440 [2573], 475 [1751], 626 [879]; Elemental
analysis calcd. for C34H58P2Si2V: C 64.22, H 9.19. Found: C 63.92, H
9.29.
487 μmol, 1.10 equiv) was added to
a solution of
TaCl3(TMSCCTMS)(dme) (243 mg, 443 μmol, 1.00 equiv) in a
mixture of toluene (10 mL) and CH2Cl2 (5 mL). After workup, the
product was obtained as a microcrystalline green powder (241 mg,
345 μmol, 78%). 1H NMR (600 MHz, CD2Cl2, 22 °C) δ (in ppm) =
8.16 (d, JH−H = 7.5 Hz, 2H, ArH), 7.84−8.81 (m, 4H, ArH), 7.59 (t,
JH−H = 7.2 Hz, 2H, ArH), 2.91−2.88 (m, 4H, iPrCH), 1.59−1.53 (m,
12H, iPrCH3), 1.27−1.21 (m, 12H, iPrCH3); 13C{1H}v NMR (151
MHz, CD2Cl2, 22 °C) δ (in ppm) = 193.6 (t, J = 4.4 Hz, alkyneC),
148.6 (dd, J = 20.1 Hz, J = 21.4 Hz, ArC), 146.2 (t, J = 11.1 Hz, ArC),
134.2 (t, J = 3.5 Hz, ArC), 134.2 (s, ArC), 133.0 (t, J = 1.3 Hz, ArC),
130.9 (t, J = 2.6 Hz, ArC), 28.6 (t, J = 7.4 Hz, iPrCH), 20.0 (t, J = 1.3
Hz, iPrCH3), 19.8 (s, iPrCH3); 31P{1H} NMR (243 MHz, CD2Cl2, 22
°C) δ (in ppm) = 51.2 (s); MS (LIFDI, dichloroethane) m/z (in g/
mol) calcd. for C26H36Cl3P2Ta: 696.0838, found 696.08 (25%) [M]+;
Elemental analysis calcd. for C26H36Cl3P2Ta: C 44.75, H 5.20. Found:
C 44.65, H 5.40.
Ph[PCCP]NbCl3 (4a). Finely powdered 1a (150 mg, 274 μmol, 1.10
equiv) was added to a stirred solution of NbCl3(TMSCCTMS)(dme)
(115 mg, 250 μmol, 1.00 equiv) in a mixture of toluene (7.0 mL) and
CH2Cl2 (3.5 mL). The reaction mixture was stirred at room
temperature for 2 h. The solvent was concentrated in vacuo to
approximately half of its volume and then stored at −40 °C overnight.
The resulting precipitate was filtered off, washed with pentane (10
mL), and dried in vacuo to afford the title compound as a bright green
solid (166 mg, 223 μmol, 89%). 1H NMR (600 MHz, CD2Cl2, 22 °C)
δ (in ppm) = 8.40 (d, JH−H = 7.6 Hz, 2H, ArH), 7.96 (t, JH−H = 7.4 Hz,
2H, ArH), 7.94−7.89 (m, 2H, ArH), 7.79−7−74 (m, 10H, ArH), 7.43−
7.38 (m, 12H, ArH); 13C{1H} NMR (151 MHz, CD2Cl2, 22 °C) δ (in
ppm) = 182.4 (t, J = 4.3 Hz, alkyneC), 145.9 (dd, J = 23.3, 22.6 Hz,
ArC), 142.0 (t, J = 14.5 Hz, ArC), 136.0 (s, ArC), 134.0 (t, J = 5.7 Hz,
ArC), 133.1 (t, J = 4.7 Hz, ArC), 132.6 (s, ArC), 132.5 (s, ArC), 132.4
(s, ArC), 132.2 (t, J = 2.8 Hz, ArC), 131.2 (s, ArC), 129.0 (t, J = 4.9 Hz,
ArC); 31P{1H} NMR (243 MHz, C2D2Cl4, 22 °C) δ (in ppm) = 40.1
(s); MS (LIFDI, dichloroethane) m/z (in g/mol) calcd. for
C38H28Cl3NbP2: 743.9796, found 743.96 (80%) [M]+; Elemental
Ph[PCCP]Nb(CHSiMe3)(CH2SiMe3) (6a). To a solution of 4a
(30.0 mg, 40.2 μmol, 1.00 equiv) in toluene-d8 (0.60 mL) was added
LiCH2SiMe3 (11.1 mg, 119 μmol, 2.95 equiv). After storing the
reaction mixture for 17 h at room temperature, the solvent was
removed in vacuo. The residue was extracted with Et2O and the
mixture filtered through a syringe filter. Removing the solvent in
vacuo afforded a brown solid, which could not be isolated in an
H
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analytically pure form. 1H NMR (600 MHz, C6D6, 22 °C) δ (in ppm)
= 11.89 (s, 1H, NbCHSi), 8.23 (dd, JH−P = 9.7 Hz, JH−H = 7.3 Hz,
4H, ArH), 8.17 (d, JH−H = 7.5 Hz, 2H, ArH), 7.54−7.56 (m, 2H, ArH),
7.34 (t, JH−H = 7.5 Hz, 2H, ArH), 7.29 (s, 1H, ArH) 7.24 (t, JH−H = 7.5
Hz, 4H, ArH), 7.08−7.12 (m, 4H, ArH), 6.95−6.98 (m, 3H, ArH),
6.77−6.81 (m, 6H, ArH), 0.40 (s, 9H, TMSCH3), 0.30 (s, 2H,
NbCH2Si), −0.30 (s, 9H, TMSCH3); 31P{1H} NMR (243 MHz, C6D6,
22 °C) δ (in ppm) = 53.7 (s); MS (LIFDI, toluene) m/z (in g/mol)
calcd. for C46H49NbP2Si2: 812.1912, found 635.30 (14%) [M −
(NbCHSiMe3)]+.
150.9−150.4 (m, ArC), 148.7 (t, J = 10.8 Hz, ArC), 132.2 (s, ArC),
131.1 (s, ArC), 129.0 (t, J = 2.9 Hz, ArC), 126.8 (t, J = 2.5 Hz, ArC),
37.6 (s, TaCH2Si), 24.3 (t, J = 5.0 Hz, iPrCH), 21.8 (t, J = 4.0 Hz,
iPrCH3), 3.9 (s, TMSCH3); 31P{1H} NMR (243 MHz, C6D6, 22 °C) δ
(in ppm) = 67.4 (s); MS (LIFDI, toluene) m/z (in g/mol) calcd. for
C34H57P2Si2Ta: 764.2925, found 765.50 (17%) [M + H]+; Elemental
analysis calcd. for C34H57P2Si2Ta: C 53.39, H 7.51. Found: C 53.03,
H 7.40.
iPr[PCHCHP]Ta(H)2(μ-H)3Ta(H)iPr[PCHCHP] (8). A solution of 7b
(20.0 mg, 26.1 μmol, 1.00 equiv) in hexane (0.4 mL) in a high-
pressure NMR tube was pressurized with dihydrogen (10 bar).
Sealing the tube and heating at 65 °C for 3 d leaded to full conversion
of 7b to 8. Removing the solvent in vacuo and recrystallizing the
residue from pentane afforded the title compound (10.0 mg, 8.38
iPr[PCCP]Nb(CHSiMe3)(CH2SiMe3) (6b). A suspension of 4b
(150 mg, 246 μmol, 1.00 equiv) and Mg(CH2SiMe3)2 (97.8 mg, 492
μmol, 2.00 equiv) in toluene (30 mL) was stirred at room
temperature for 16 h. All volatiles were evaporated under reduced
pressure and the solid residue was extracted with pentane (30 mL).
The pentane extracts were filtered through Celite and the filtrate
evaporated under reduced pressure to afford the product as a
1
μmol, 64%) as a brown solid. H NMR (600 MHz, C6D6, 22 °C) δ
(in ppm) = 13.19 (t, J = 45.5 Hz, 1H, terminalH), 8.29 (d, J = 7.7 Hz,
1H, ArH), 7.96 (d, J = 5.6 Hz, 1H, ArH), 7.54 (d, J = 7.6 Hz, 1H, ArH),
7.19 (d, J = 7.4 Hz, 1H, ArH), 7.09 (t, J = 7.2 Hz, 1H, ArH), 7.05−6.83
(m, 6H, ArH), 6.81−6.69 (m, 5H, ArH), 5.09−4.88 (m, 2H, terminalH
1
greenish-yellow powder (128 mg, 189 μmol, 77%). H NMR (600
MHz, C6D6, 22 °C) δ (in ppm) = 11.0 (s, 1H, NbCHSi), 8.09 (d,
JH−H = 7.4 Hz, 2H, ArH), 7.37 (t, JH−H = 7.2 Hz, 4H, ArH), 7.10 (t,
metal-
and metallacyclopropaneH overlapping), 4.72 (d, J = 14.3 Hz, 1H,
H−H = 7.3 Hz, 2H, ArH), 2.58−2.45 (m, 4H, iPrCH), 1.57 (dd, JH−P
=
lacyclopropaneH), 4.16 (s, 2H, metallacyclopropaneH), 3.17−3.05 (m, 1H,
iPrCH), 2.71−2.26 (m, 7H, iPrCH), 1.65−1.40 (m, 24H, iPrCH3),
1.38−1.11 (m, 10H, iPrCH3), 1.04−0.92 (m, 8H, bridgeH and iPrCH3
overlapping), 0.81 (dd, J = 14.3, 6.8 Hz, 3H, iPrCH3), 0.73 (dd, J =
12.8, 6.8 Hz, 3H, iPrCH3), 0.08 (dd, J = 15.0, 6.8 Hz, 3H, iPrCH3),
−3.59 (ddd, J = 60.2, 34.0, 5.0 Hz, 2H, terminalH); 13C{1H} NMR (151
MHz, C6D6, 22 °C) δ (in ppm) = 138.6 (d, J = 8.8 Hz, ArC), 136.9 (d,
J = 10.3 Hz, ArC), 134.9 (d, J = 9.1 H, ArC), 131.9 (d, J = 8.9 Hz, ArC),
131.5 (s, ArC), 131.3 (s, ArC), 131.2 (s, ArC), 130.9 (s, ArC), 127.4 (d,
J = 13.9 Hz, ArC), 126.5 (s, ArC), 126.0 (d, J = 4.3 Hz, ArC), 123.4 (d,
J = 4.1 Hz, ArC), 122.8 (d, J = 5.2 Hz, ArC), 65.4 (m, metallacyclopropaneC),
65.0 (m, metallacyclopropaneC), 57.8 (t, J = 5.8 Hz, metallacyclopropaneC), 56.5
(m, metallacyclopropaneC), 33.0 (s, iPrCH), 31.0 (d, J = 6.3 Hz, iPrCH), 30.1
(d, J = 14.9 Hz, iPrCH), 28.3 (d, J = 14.2 Hz, iPrCH), 24.3 (d, J = 25.9
Hz, iPrCH), 23.4 (d, J = 21.1 Hz, iPrCH), 21.4 (d, J = 3.5 Hz, iPrCH),
21.1 (s, iPrCH), 20.5 (d, J = 22.7 Hz, iPrCH), 20.2 (d, J = 1.9 Hz,
iPrCH3), 19.7 (m, iPrCH3), 19.2 (d, J = 9.5 Hz, iPrCH3), 19.0 (d, J =
5.3 Hz, iPrCH3), 18.7 (d, J = 4.5 Hz, iPrCH3), 18.1 (d, J = 6.0 Hz,
iPrCH3), 17.6 (d, J = 5.2 Hz, iPrCH3); 31P{1H} NMR (243 MHz,
C6D6, 22 °C) δ (in ppm) = 89.1 (d, J = 37.3 Hz), 80.1 (d, J = 17.2
Hz), 79.1 (d, J = 37.4 Hz), 67.0 (d, J = 17.1 Hz); MS (LIFDI,
toluene) m/z (in g/mol) calcd. for C52H82P4Ta2: 1192.43269, found
1190.19 (100%) [M − H2]+, Elemental analysis calcd. for
C52H82P4Ta2·C5H12: C 54.11, H 7.49. Found: C 53.83, H 7.13.
General Procedure for Catalytic ROMP Reactions. To a
solution of 390 μmol of the substrate (norbornene, norbornadiene, or
2-methoxymethyl-5-norbornene) in benzene (2 mL) was added 3.90
μmol (1 mol %) of the catalyst (6b or 7b) in a Schlenk tube, and the
reaction mixture was heated at 85 °C. After stirring for 24 h (7b) and
120 h (6b), respectively, the reaction mixture was cooled to room
temperature and an aliquot was analyzed by NMR spectroscopy
(integrated against a defined amount of ferrocene) to determine the
conversion and cis/trans ratio of the respective polymers (cf., Table
3). 1H NMR spectral assignments were consistent with literature
reports.24 The reaction mixture was then poured into methanol (50
mL). The precipitate was filtered off, washed with methanol, and
dried in vacuo to afford the polymers as off-white solids. The
molecular weight distributions and the PDIs of the polymers were
determined by GPC using polystyrene as a standard.
J
15.1, JH−H = 6.9 Hz, 6H, iPrCH3), 1.11 (dd, JH−P = 15.6, JH−H = 6.9
Hz, 6H, iPrCH3), 0.99−0.93 (m, 6H, iPrCH3), 0.73 (s, 9H, TMSCH3),
0.19 (t, JH−P = 12.4 Hz, 2H, NbCH2Si), 0.07 (dd, JH−P = 14.6, JH−H
=
7.0 Hz, 6H, iPrCH3), −0.43 (s, 9H, TMSCH3); 13C{1H} NMR (151
MHz, C6D6, 22 °C) δ (in ppm) = 285.7 (bs, NbCHSi), 188.8 (t, J =
9.2 Hz, alkyneC), 149.0 (s, ArC), 148.9 (d, J = 3.8 Hz, ArC), 148.7 (s,
ArC), 146.1 (s, ArC), 146.0 (s, ArC), 145.9 (s, ArC), 131.5 (s, ArC),
130.8 (s, ArC), 128.5 (t, J = 3.3 Hz, ArC), 128.4 (s, ArC), 126.9 (t, J =
2.2 Hz, ArC), 24.2 (t, J = 4.0 Hz, iPrCH), 24.0 (bs, NbCH2Si), 21.8 (t,
J = 3.9 Hz, iPrCH), 19.8 (t, J = 4.6 Hz, iPrCH3), 19.7 (t, J = 5.1 Hz,
iPrCH3), 19.1 (t, J = 2.8 Hz, iPrCH3), 16.7 (t, J = 2.9 Hz, iPrCH3), 6.3
(s, TMSCH3), 2.9 (s, TMSCH3); 31P{1H} NMR (243 MHz, C6D6, 22
°C) δ (in ppm) = 65.2 (s);); MS (LIFDI, toluene) m/z (in g/mol)
calcd. for C34H57NbP2Si2: 676.2538, found 589.19 (8%) [M −
(CH2SiMe3)]+; Elemental Analysis calcd. for C34H57NbP2Si2: C
60.33, H 8.49. Despite numerous attempts, the values were
consistently low (e.g., found: C 58.62, H 8.43), which may be related
to incomplete combustion or carbide formation.
Ph[PCCP]Ta(CHSiMe3)(CH2SiMe3) (7a). Following the procedure
for 6a, 5a (30.0 mg, 36.0 μmol, 1.00 equiv) was dissolved in toluene-
d8 (0.60 mL) and LiCH2SiMe3 (9.90 mg, 106 μmol, 2.95 equiv) was
added. After workup, an orange solid was isolated, which could not be
obtained in an analytically pure form. 1H NMR (600 MHz, C6D6, 22
°C) δ (in ppm) = 9.34 (s, 1H, TaCHSi), 8.29−8.32 (m, 4H, ArH),
8.14 (d, JH−H = 7.5 Hz, 2H, ArH), 7.52−7.54 (m, 2H, ArH), 7.36 (t,
J
H−H = 7.4 Hz, 2H, ArH), 7.25 (t, JH−H = 7.5 Hz, 4H, ArH), 7.16−7.18
(m, 2H, ArH), 7.10 (m, 4H, ArH), 6.94 (t, JH−H = 7.4 Hz, 2H, ArH),
6.76−6.80 (m, 6H, ArH), 0.36 (s, 9H, TMSCH3), −0.27 (s, 9H,
TMSCH3), −0.49 (t, JH−P = 15.5 Hz, 2H, TaCH2Si); 31P{1H} NMR
(243 MHz, C6D6, 22 °C) δ (in ppm) = 56.1 (s); MS (LIFDI,
toluene) m/z (in g/mol) calcd. for C46H49P2Si2Ta 900.2328, found
900.21 (28%) [M]+.
iPr[PCCP]Ta(CHSiMe3)(CH2SiMe3) (7b). A suspension of 5b (200
mg, 287 μmol, 1.00 equiv) and Mg(CH2SiMe3)2 (114 mg, 573 μmol,
2.00 equiv) in toluene (30 mL) was stirred at room temperature for
24 h. The resulting mixture was evaporated to dryness and the
remaining solids were extracted with pentane (30 mL). The pentane
extracts were filtered through Celite and the filtrate evaporated under
reduced pressure to afford the title compound as an orange
Procedure for the Hydrogenation of Naphthalene. A high-
pressure NMR tube was charged with naphthalene (42 mg, 327 μmol)
and 7b (2.50 mg, 3.27 μmol, 1 mol %). Hexane (0.4 mL) was added
and the NMR tube was pressurized with H2 (10 bar). The sample was
kept at 65 °C for 3 d and then allowed to cool to room temperature.
The solvent was removed in vacuo, and the reaction was quenched by
1
microcrystalline powder (127 mg, 160 μmol, 58%). H NMR (600
MHz, toluene-d8, 22 °C) δ (in ppm) = 8.47 (s, 1H, TaCHSi), 8.00 (d,
JH−H = 7.5 Hz, 2H, ArH), 7.36 (t, JH−H = 7.4 Hz, 2H, ArH), 7.32 (d,
J
H−H = 6.3 Hz, 2H, ArH), 7.09−7.05 (m, 2H, ArH), 2.61−2.52 (m, 2H,
iPrCH), 2.51−2.42 (m, 2H, iPrCH), 1.58−1.50 (m, 6H, iPrCH3), 1.13−
1.05 (m, 6H, iPrCH3), 1.00−0.94 (m, 6H, iPrCH3), 0.63 (s, 9H,
TMSCH3), 0.10−0.02 (m, 6H, iPrCH3), −0.15 (t, JH−P = 14.0 Hz, 2H,
TaCH2Si), −0.50 (s, 9H, TMSCH3); 13C{1H} NMR (101 MHz, C6D6,
22 °C) δ (in ppm) = 272.8 (bs, TaCHSi), 202.3−202.1 (m, alkyneC),
1
addition of CDCl3 (0.4 mL). According to H NMR spectroscopy,
approximately one-third of the initially present naphthalene was
converted to afford a 2:3 mixture of tetraline (C10H12) and decaline
(C10H18).
I
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ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.organo-
met.9b00577.
■
S
Selected NMR, EPR, and UV−vis spectra, computa-
tional details, additional ORTEP plots, and crystallo-
graphic data (PDF)
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Accession Codes
CCDC 1947155−1947169 contain the supplementary crys-
tallographic data for this paper. These data can be obtained
free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by
emailing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
́
́
(c) Carbo, J. J.; Crochet, P.; Esteruelas, M. A.; Jean, Y.; Lledos, A.;
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Lopez, A. M.; Onate, E. Two-and Four-Electron Alkyne Ligands in
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AUTHOR INFORMATION
Corresponding Author
*E-mail: joachim.ballmann@uni-heidelberg.de.
ORCID
Patrick Federmann: 0000-0002-0278-0181
Joachim Ballmann: 0000-0001-6431-4197
Notes
■
(5) (a) Hartley, F. R. Metal-Olefin and -Acetylene Bonding in
Complexes. Angew. Chem., Int. Ed. Engl. 1972, 11, 596−606.
(b) Albright, T. A.; Hoffmann, R.; Thibeault, J. C.; Thorn, D. L.
Ethylene complexes. Bonding, rotational barriers, and conformational
preferences. J. Am. Chem. Soc. 1979, 101, 3801−3812. (c) Hertwig, R.
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
Funding from the DFG (BA 4859/2) is gratefully acknowl-
edged. We thank M. Bojanowski and D. Gutruf for collecting
GPC and CV data, respectively. We thank Prof. M. Enders for
help with some NMR experiments and H. Wagner for
preparing an additional batch of 7b during the revision
process. The authors acknowledge support by the state of
̈
̌
́
H.; Koch, W.; Schroder, D.; Schwarz, H.; Hrusak, J.; Schwerdtfeger, P.
A Comparative Computational Study of Cationic Coinage Metal-
Ethylene Complexes (C2H4)M+ (M = Cu, Ag, and Au). J. Phys. Chem.
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Schwartz, D. J.; Burns, C. J.; Andersen, R. A. Bonding of H2, N2,
Ethylene, and Acetylene to Bivalent Lanthanide Metallocenes: Trends
from DFT Calculations on Cp2M and Cp*2 M (M = Sm, Eu, Yb) and
Experiments with Cp*2Yb. Organometallics 2003, 22, 5447−5453.
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carbenoid species with two double bonds within the three-membered
metallacycle.
Baden-Wurttemberg through bwHPC and the German
̈
Research Foundation (DFG) through grant INST 40/467-1
FUGG (JUSTUS cluster). J. B. thanks Prof. L. H. Gade for
generous support over the past years and his enduring interest
in our work.
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