Thermolysis of half-sandwich vanadium(V) imido complexes to generate vanadium(III) imido species via a vanadium(IV) intermediate

Article abstract of DOI:10.1021/om100561y

Thermolysis of half-sandwich vanadium imido complexes Cp(RN)V(i-Pr ₂N)Me (R = p-Tol, 1a; R = t-Bu, 1b) results in a mixture of products, including vanadium(IV) dimers [CpVMe]₂(μ-RN)₂, i-Pr₂NH, i-PrNCMe₂, CH₄, and one or more paramagnetic species. In the presence of dmpe (dmpe = bis(dimethylphosphino) ethane), PMe₃, or PhSSPh, the initially formed paramagnetic species can be trapped, providing Cp(RN)VL₂ (R = p-Tol, L₂ = dmpe, 2a; R = t-Bu, L₂ = dmpe, 2b; R = p-Tol, L = PMe₃, 3a; R = t-Bu, L = PMe₃, 3b; R = t-Bu, L = SPh, 4b). This suggests that the initially formed species is [Cp(RN)V]. When generated in situ, [Cp(RN)V] is active in the [2+2+2] cyclotrimerization of PhCCH (R = p-Tol, t-Bu) and PhCCPh (R = t-Bu). Furthermore, the data presented indicate a mechanism in which both [CpVMe]₂(μ-RN)₂ and [Cp(RN)V] originate from a common intermediate, vanadium(IV) imido species [Cp(RN)VMe].

Full text of DOI:10.1021/om100561y

6230 Organometallics 2010, 29, 6230–6236
DOI: 10.1021/om100561y
Thermolysis of Half-Sandwich Vanadium(V) Imido Complexes to
Generate Vanadium(III) Imido Species via a Vanadium(IV) Intermediate
Aurora A. Batinas, Jeroen Dam, Auke Meetsma, Bart Hessen, and Marco W. Bouwkamp*
Stratingh Institute for Chemistry, University of Groningen, Nijenborgh 4, 9747 AG, Groningen,
The Netherlands
Received June 7, 2010
Thermolysis of half-sandwich vanadium imido complexes Cp(RN)V(i-Pr2N)Me (R = p-Tol, 1a; R =
t-Bu, 1b) results in a mixture of products, including vanadium(IV) dimers [CpVMe]2(μ-RN)2, i-Pr2NH,
i-PrNCMe2, CH4, and one or more paramagnetic species. In the presence of dmpe (dmpe = bis(dimethyl-
phosphino)ethane), PMe3, or PhSSPh, the initially formed paramagnetic species can be trapped, providing
Cp(RN)VL2 (R=p-Tol,L2 =dmpe,2a; R = t-Bu, L2 =dmpe,2b;R=p-Tol,L=PMe3, 3a; R = t-Bu,
L=PMe3, 3b; R = t-Bu, L = SPh, 4b). This suggests that the initially formed species is [Cp(RN)V]. When
generated in situ, [Cp(RN)V] is active in the [2þ2þ2] cyclotrimerization of PhCCH (R = p-Tol, t-Bu) and
PhCCPh (R = t-Bu). Furthermore, the data presented indicate a mechanism in which both [CpVMe]2-
(μ-RN)2 and [Cp(RN)V] originate from a common intermediate, vanadium(IV) imido species
[Cp(RN)VMe].
Introduction
imido complexes are generated by unusual rearrangement reac-
tions involving t-BuCN.4 Here we report the generation of
vanadium(III) imido complexes by thermolysis of Cp(RN)V-
(i-Pr2N)Me (R=p-Tol, 1a; R=t-Bu, 1b) and their application as
catalysts for the [2þ2þ2] cyclotrimerization of phenylacetylenes.
Group 4 and 5 imido complexes have gained much interest as
hydroamination catalysts.1 In the case of vanadium, the chem-
istry of half-sandwich vanadium(V) imido complexes has been
extensively studied.2 By contrast there are only a few examples of
imido complexes of vanadium(III). In general these complexes
are prepared by reduction of their vanadium(V) dichloride
precursors Cp(RN)VCl2 with magnesium in the presence of
phosphine or phosphite ligands or CO.3 Two notable examples,
however, generate half-sandwich vanadium(III) imido com-
plexes via an alternative route. In both cases, vanadium(III)
Results
Compound Cp(p-TolN)V(i-Pr2N)Me (1a)5 is moderately
stable at room temperature, though warming a solution of 1a
in toluene-d8 to 50 °C results in degradation to a mixture of
1
products. H NMR spectroscopy reveals the formation of a
mixture of [CpVMe]2(μ-p-TolN)2,6 diisopropylamine, N-isopro-
pylisopropylideneamine, methane, and one or more unidentified
paramagnetic species (resonances at δ 39.72, 28.55, 19.02, 16.29,
9.32, 5.38, -5.21, and -8.44 ppm). It is interesting to note that
the d0-isotopologue of i-Pr2NH was observed (see Discussion).
We propose that these paramagnetic species are in the V(III)
oxidation state and are EPR silent at RT and 77 K.7 On the basis
of the stoichiometry of the reaction, we tentatively identify this
species as [Cp(p-TolN)V] or at least to a compound of this
general formula. However, attempts to convert such species into
low-spin vanadium(III) dmpe adducts (dmpe = bis(dimethyl-
phosphino)ethane) did not provide the corresponding complex
*To whom correspondence should be addressed. E-mail: M.W.
Bouwkamp@rug.nl.
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Muller, T. E.; Beller, M. Chem. Rev. 1998, 98, 675. (b) Nobis, M.;
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Driessen-Holscher, B. Angew. Chem., Int. Ed. 2001, 40, 3983. (c) Pohlki,
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Devore, D. D.; Lichtenhan, J. D.; Takusagawa, F.; Maatta, E. A. J. Am.
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r
pubs.acs.org/Organometallics
Published on Web 11/16/2010
2010 American Chemical Society

Article
Organometallics, Vol. 29, No. 23, 2010 6231
Scheme 1
Scheme 2
Cp(p-TolN)V(dmpe) (vide infra). See Scheme 1 for an overview
of the constituents of the reaction mixture, including their
relative amounts.8
Although this coupling is small compared to most other
vanadium(IV) sandwich and half-sandwich complexes known
in the literature (61-80 G),11 it is larger than the hyperfine
coupling found in complexes of the type Cp*2VNAr (Ar = aryl;
5-10 G).12
Compound Cp(t-BuN)V(i-Pr2N)Me (1b), which was pre-
pared by addition of MeLi to the corresponding chloride
precursor Cp(t-BuN)V(i-Pr2N)Cl,9 also decomposes to gen-
erate a mixture of products. In this case thermolysis at 50 °C
In the case of the decomposition of 1b, addition of dmpe to
the initially obtained reaction mixture did result in the forma-
tion of the dmpe adduct of the expected vanadium(III) imido
species, Cp(t-BuN)V(dmpe) (2b, see below). Apart from the
spectral features for compound 2b, the 1H NMR spectrum of
the reaction mixture revealed that one of the paramagnetic
species had disappeared (the remaining paramagnetic species
shows resonances at δ 15.40, 12.11, 10.06, and -6.44 ppm). In
addition, an additional amount (22%) of N-isopropylisopro-
pylideneamine was observed, indicating that the vanadium(III)
imido species was initially supported by an imine ligand.
When the thermolysis of 1a and 1b was carried out in the
presence of a small excess (1.2 equiv) of dmpe, the corre-
sponding dmpe adducts, Cp(RN)V(dmpe) (R=p-Tol: 2a;
1
1
is complete within 6 h by H NMR spectroscopy. The H
NMR spectrum includes contributions from two diamag-
netic species, and a mixture of presumably more than one
paramagnetic species was observed (δ 91.45, 58.88, 38.30,
17.55, 15.40, 12.07, 10.06, 5.02, and -6.44 ppm), as well as
i-Pr2NH, i-PrNCMe2, and methane (Scheme 2).8 The dia-
magnetic species were tentatively assigned as the cis- and
trans-isomers of the vanadium dimer [CpVMe]2(μ-t-BuN)2.
Vroegop et al. previously reported the synthesis of the
corresponding titanium compound [CpTiCl]2(μ-t-BuN)2.10
This compound was initially obtained as the kinetic trans-
isomer, which undergoes thermal equilibration (4:1) in favor
of the cis-isomer.
1
R=t-Bu: 2b, Scheme 3), were trapped. H NMR spectros-
Addition of the oxidant PbCl2 to the reaction mixture results
in the formation of Cp(t-BuN)V(i-Pr2N)Cl9 and an additional
amount (20%) of N-isopropylisopropylideneamine, indicating
that one of the paramagnetic species is the vanadium(IV)
compound Cp(t-BuN)V(i-Pr2N)(i-PrNCMe2). This compound
could be identified by EPR spectroscopy, which shows, prior to
oxidation, a signal corresponding to a d1-vanadium compound
with a typical octet hyperfine structure with a(51V) = 18 G.
copy revealed that in both cases this is the major species
observed (90% and 94% conversion, respectively). In the
case of compound 2a, a small amount of the vanadium dimer
[CpVMe]2(μ-p-TolN)2 was observed also (6%), whereas
the only other half-sandwich vanadium species observed in
the case where 2b is the starting material. The formation of
the expected amounts of i-Pr2NH, i-PrNCMe2, and methane
were observed in both reactions.
(8) The amount of [CpVMe]2(μ-p-TolN)2, diisopropylamine, and N-
isopropylisopropylideneamine was determined using integration of
representative signals in the 1H NMR spectrum of the reaction mixture;
the amount of methane was quantified using a Toepler pump.
(9) Preuss, F.; Steidel, M.; Vogel, M.; Overhoff, G.; Hornung, G.;
Towae, W.; Frank, W.; Rein, G.; Muller-Becker, S. Z. Anorg. Allg.
Chem. 1997, 623, 1220.
(10) (a) Vroegop, C. T.; Teuben, J. H.; Van Bolhuis, F.; Van der
Linden, J. G. M. J. Chem. Soc., Chem. Commun. 1983, 550. (b) Jekel-
Vroegop, C. T.; Teuben, J. H. J. Organomet. Chem. 1985, 286, 309.
(11) (a) Foust, D.-F.; Rausch, M. D.; Samuel, E. J. Organomet.
Chem. 1980, 193, 209. (b) Mishra, S. P.; Singh, S. J. Mol. Struct. 1984,
112, 59. (c) Nieman, J.; Teuben, J. H. Organometallics 1986, 5, 1149. (d)
Choukroun, R.; Douziech, B.; Pan, C.; Dahan, F.; Cassoux, P. Organome-
tallics 1995, 14, 4471. (e) Ghosh, P.; Kotchevar, A. T.; DuMez, D. D.; Ghosh,
S.; Peiterson, J.; Uckun, F. M. Inorg. Chem. 1999, 38, 3730. (f ) Honzıcek, J.;
Palackova, H.; Cisarova, I.; Vinklarek, J. J. Organomet. Chem. 2006, 691,
202.
(12) Osborne, J. H.; Rheingold, A. L.; Trogler, W. C. J. Am. Chem.
Soc. 1985, 107, 7945.

6232 Organometallics, Vol. 29, No. 23, 2010
Batinas et al.
Figure 1. ORTEP representations at the 50% probability level of 2a and 2b.
Scheme 3
˚
Table 1. Selected Bond Distances (A) and Angles (deg) for
Compounds 2a and 2b
Compounds 2a and 2b were prepared on a preparative scale
by warming toluene solutions of 1a and 1b in the presence of
1 equiv of dmpe. The compounds were isolated in 55% and
45% yield, respectively, as purple and green crystals. Both
compounds were studied using single-crystal X-ray diffraction
(see Figure 1 for an ORTEP representation and Table 1 for
selected bond distances and angles). The structures of 2a and 2b
are very similar. Both compounds adopt a three-legged piano-
stool geometry. The V-N bond distances (1.7193(18) and
2a
2b
V-N
1.7193(18)
2.3596(7)
2.3522(7)
2.275
1.380(3)
172.13(16)
80.95(2)
94.42(6)
88.41(6)
1.6903(1)
2.3507(7)
2.3522(7)
2.277(9)
1.451(3)
172.15(16)
80.95(2)
96.56(6)
96.26(6)
V-P1
V-P2
Cpa-V
N-C
V-N-C
P1-V-P2
P1-V-N
P2-V-N
˚
1.6903(1) A, respectively) are characteristic for a V-N double
bond and are similar to those reported elsewhere.2e,f,j,3a,4,5,13
Furthermore, the V-N-C bond angle is, as expected, close to
linear (172.13(16)° and 172.15(16)°). The compounds were fur-
ther identified by a typical “horned” plateau in the 31P NMR
spectrum (2a: δ 87.41 ppm, Δνtop=2485 Hz; 2b: δ 87.34 ppm,
Δνtop =2696 Hz)14 and a single resonance in the 51V NMR
spectrum (2a: δ -4.9 ppm, JVP =341 Hz; 2b: δ -381 ppm,
Δν1/2=5190 Hz).
a Cp is defined as the centroid defined by the five carbon atoms of the
cyclopentadienyl ligands.
The vanadium(III) imido species [Cp(RN)V] was also
trapped with PMe3 or PhSSPh (Scheme 3). Thermolysis of
1a and 1b in the presence of PMe3 (2 equiv) affords the
corresponding bis-trimethylphosphine adducts Cp(RN)V-
(PMe3)2 (R = p-Tol: 3a; R = t-Bu: 3b) in a 82% and 86%
yield (determined by integration of the 1H NMR spectrum of
the reaction mixture using Cp2Fe as internal standard). In
addition to the formation of compound 3a, a small amount
of dimer [CpVMe]2(μ-p-TolN)2 was observed (5%) as well as
the same paramagnetic species that was observed after
thermolysis in the absence of any of the other trapping agent.
(13) (a) Preuss, F.; Wieland, T.; Perner, J.; Heckmann, G. Z. Nat-
urforsch. 1992, 47b, 1355. (b) Witte, P. T.; Meetsma, A.; Hessen, B.;
Budzelaar, P. H. M. J. Am. Chem. Soc. 1997, 119, 10561. (c) Preuss, F.;
Perner, J.; Frank, W. A.; Reiss, G. Z. Anorg. Allg. Chem. 1999, 625, 901. (d)
Billen, M.; Hornung, G.; Wolmershauser, G.; Preuss, F. Z. Naturforsch.
2003, 58b, 237. (e) Sato, Y.; Nakayama, Y.; Yasuda, H. J. Appl. Polym. Sci.
2005, 97, 1008.
(14) Billen, M.; Hornung, G.; Preuss, F. Z. Naturforsch. 2003, 58b, 975.

Article
Organometallics, Vol. 29, No. 23, 2010 6233
Scheme 4
Scheme 5
This indicates that PMe3 is less efficient in trapping a species
of the type [Cp(RN)V], relative to the chelating dmpe. Cp(t-
BuN)V(PMe3)2 was prepared previously by Preuss and co-
workers by reduction of the corresponding dichloride pre-
cursor Cp(t-BuN)VCl2 with Mg in the presence of 2 equiv of
PMe3.14,15 For compounds 3a and 3b resonances at 54.45
and 52.3 ppm are observed by 31P NMR spectroscopy and
Discussion
In the present study the trapping of vanadium imido
species using a series of trapping agents was examined. The
main finding is that the yield of the vanadium(III) imido
species increased significantly when dmpe was employed as
trapping agent. The more conventional route to species A
and B is depicted in Scheme 5. Compound A is generated by
β-hydrogen transfer involving one of the i-Pr methine pro-
tons and the methyl ligand to generate N-isopropylisopro-
pylideneamine and methane, whereas species B is the result
of a homolysis of the V-N bond of the diisopropylamide
ligand, followed by dimerization of the resulting vanadium-
(IV) species, [Cp(RN)VMe]. The mechanism shown in
Scheme 5, however, does not rationalize (i) the increased
yield of the vanadium(III) imido species A upon addition of
dmpe and (ii) that the generation of diisopropylamine during
this reaction is observed as its d0-isotopologue. In the case of
the mechanism described in Scheme 5, the d1-isotopologue is
expected where the hydrogen atom source is the solvent.
Therefore, an alternative mechanism can be proposed, in
which the formation of both species A and B and the effect of
the dmpe ligand and the absence of deuterium incorporation
in the diisopropylamine generated are rationalized. The mech-
anism is depicted in Scheme 6 and involves initial homolysis
of the vanadium-nitrogen bond of the diisopropylamide
ligand to generate a vanadium(IV) species [Cp(RN)VMe]
and the diisopropylamide radical. The latter disproportion-
ates to generate equimolar amounts of diisopropylamine and
triplets at δ 267.59 (JVP =399.66 Hz) and -111.39 (JVP
=
420.58 Hz) ppm by 51V NMR spectroscopy. In the case of the
thermolysis of 1b in the presence of PhSSPh, the formation
of vanadium(V) bisthiolate species Cp(t-BuN)V(SPh)2 (4b)
was observed,13d the result of an oxidative addition of the
disulfide reagent to the d2-vanadium center (Scheme 3).
The propensity of the [Cp(RN)V] species to undergo oxidative
addition reactions was utilized in the catalytic [2þ2þ2] cyclo-
trimerization of phenylacetylenes PhCCR (R = H, Ph). Heating
a PhCCH solution in benzene-d6 in the presence of 10 mol % of
compounds 1a or 1b to 80 °C for 28 and 8 h, respectively, resulted
in 88% and 90% conversion of the alkyne to form a mixture of
products. The main product observed is 1,2,4- and 1,3,5-triphe-
nylbenzene (1a: 65% yield, 1.5:1 ratio; 1b: 64% yield, 1.2:1 ratio),
with small amounts of dimers, tetramers, and higher oligomers
observed as well. In the case of compound 1a, the trimerization
of diphenylacetylene resulted in 80% conversion of the substrate
to yield 71% of hexaphenylbenzene (Scheme 4).
(15) Preuss, F.; Billen, M.; Tabellion, F.; Wolmershauser, G. Z. Anorg.
Allg. Chem. 2000, 626, 2446.

6234 Organometallics, Vol. 29, No. 23, 2010
Batinas et al.
Scheme 6
N-isopropylisopropylideneamine.16 The initially generated
vanadium(IV) species [Cp(RN)VMe] can either dimerize to
generate the vanadium dimer [CpVMe]2(μ-RN)2 or alterna-
tively react with diisopropylamine to generate the vanadium-
(IV) amide species, Cp(RN)V(i-Pr2N). The latter can undergo
a second homolysis reaction to generate vanadium(III) species
[Cp(RN)V] and an additional equivalent of the diisopropyl-
amide radical. In the presence of dmpe, the formation of the
vanadium dimer [CpVMe]2(μ-RN)2 is surpressed, increasing the
yield of dmpe adducts 2a and 2b. In addition, the hydrogen atom
source for the formation of diisopropylamine is another equiva-
lent of the diisopropylamide radical, which would anticipate
only the formation of the d0-isotopologue of i-Pr2NH, as is
observed. Finally, the formation of the vanadium(IV) species
Cp(t-BuN)V(i-Pr2N)(i-PrNCMe2) further supports the interme-
diacy of Cp(NR)V(N-i-Pr2) in the thermolysis reaction.
atmosphere using standard Schlenk line, vacuum line, and
glovebox techniques. Reagents were obtained from commercial
suppliers and used as received unless stated otherwise. Solvents
were passed, under a dinitrogen atmosphere, over a column
containing BASF R3-11 supported Cu-based scavenger and a
˚
˚
mixture of alumina and either 3 A mol sieves (toluene) or 4 A
mol sieves (pentane, toluene, Et2O) before use. Benzene-d6 was
dried on Na/K alloy and transferred in vacuo prior use. PhCCH
was dried over CaH2, and Cp(p-TolN)V(N-i-Pr2)Me was pre-
pared according to the literature procedures.5 NMR spectra
were recorded on Varian VXR-300 (300 MHz), Varian Mercury
Plus (400 MHz), or Unity (500 MHz) spectrometers. Chemical
shifts were determined relative to the residual solvent peaks and
referenced to TMS. 51V NMR chemical shifts are reported in
ppm relative to VOCl3, which is used as an external reference
(VOCl3, δ = 0 ppm). EPR spectra (X-band, 9.46 GHz) were
recorded on a Bruker ECS106 instrument. GC analyses were
performed on a HP 6890 instrument with a HP-1 dimethyl-
polysiloxane column (19095 Z-123). GC-MS spectra were recorded
at 70 eV using a HP 5973 mass-selective detector attached to a
HP 6890 GC as described above. Elemental analyses were
performed at the Microanalytical Department of the University
of Groningen or Kolbe Mikroanalytisches Laboratorium
Conclusions
A new route to half-sandwich vanadium(III) imido species
is reported. As a side-product, the vanadium dimer [CpVMe]2-
(μ-RN)2 is observed also. The observation of i-Pr2NH rather
than i-Pr2ND in the reaction mixture, as well as the increased
yield of the vanadium(III) imido species upon addition of dmpe,
suggests a mechanism in which both products originate from the
same intermediate. This intermediate is a vanadium(IV) imido
species [Cp(RN)VMe], the result of a homolysis of the vana-
dium-amido bond. The in situ-generated vanadium(III) imido
species can be trapped using phosphine ligands and PhSSPh or
used as a [2þ2þ2] cyclotrimerization catalysts for phenylacety-
lenes to generate substituted benzenes.
€
(Mulheim an der Ruhr, Germany).
Thermolysis of Cp(N-p-Tol)V(i-Pr2N)Me. In an NMR tube
equipped with a Teflon (Young) valve, Cp(p-TolN)V(i-Pr2N)Me
(0.055 g, 0.16 mmol) was dissolved in benzene-d6 (0.6 mL). The
tube was attached to the vacuum line and degassed using three
freeze-pump-thaw cycles, after which the tube was placed in an
oven at 50 °C for 3 days. The tube was attached to the vacuum line,
and the amount of methane formed in the reaction was determined
using a Toepler pump (0.78 equiv of CH4). The volatiles were
transferred to a second tube and analyzed by 1H NMR spectros-
copy using Cp2Fe as internal standard and GC-MS analysis.
This revealed formation of diisopropylamine (m/z=101) and
N-isopropylisopropylideneamine (m/z=99) in 9% and 75% yield,
respectively. i-Pr2NH: 1H NMR (400 MHz, benzene-d6, 25 °C): δ
2.79 (sept, JHH =6.2Hz, 2HCHMe2), 0.96 (d, JHH = 6.2 Hz, 12H,
CHMe2), 0.23 (br, 1H NH) ppm. i-PrNCMe2:1H NMR (400 MHz,
benzene-d6, 25°C): δ3.44 (sept, JHH =6.2 Hz, 1H CHMe2),1.80(s,
3H, CMe2), 1.36 (s, 3H CMe2), 1.15(d, JHH =6.2 Hz, 6H, CHMe2)
Experimental Section
General Considerations. All manipulations of air- and mois-
ture-sensitive compounds were performed under a nitrogen
(16) (a) Seetula, J.; Kalliorinne, K.; Koskikallio, J. J. Photochem.
Photobiol. 1998, 43A, 31. (b) Janovsky, I.; Knolle, W.; Naumov, S.;
Williams, F. Chem.—Eur. J. 2004, 10, 5524.

Article
Organometallics, Vol. 29, No. 23, 2010 6235
ppm. The residue was redissolved in benzene-d6. 1H NMR analysis
revealed the presence of both diamagnetic and paramagnetic spe-
cies. For the paramagnetic species: 1H NMR (400 MHz, benzene-
d6, 25 °C): δ 39.7 (br, Δν1/2=403 Hz), 28.6 (br, Δν1/2=340 Hz),
[CpVMe]2(μ-p-TolN)2 (6%). Spectral data for Cp(p-TolN)V-
(dmpe): 1H NMR (400 MHz, benzene-d6, 25 °C): δ 7.06 (d,
JHH = 8.0 Hz, 2H, p-TolN), 6.90 (d, JHH = 8.0 Hz, 2H, p-TolN),
4.99 (br, 5H, Cp), 2.13 (s, 3H, pTolN Me), 1.71-1.49 (m, 2H,
PCH2), 1.43-1.32 (m, 6H, PMe2), 1.17-0.97 (m, 2H, PCH2),
0.67-0.45 (m, 6H, PMe2). 13C{1H} NMR (100 MHz, benzene-d6,
25 °C): δ 128.96 (p-TolN CH), 125.34 (p-TolN CH), 92.58 (Cp
CH), 32.16 (PCH2), 21.15 (p-TolN Me), 17.51 (PMe2). Cq of
p-TolN not observed. 31P{1H} NMR (100 MHz, benzene-d6,
25 °C): δ 87.51 (plateau, Δνtop = 2449 Hz). 51V NMR (131
MHz, benzene-d6, 25 °C, VOCl3 external standard): δ -4.90 (t,
19.0 (br, Δν1/2=276 Hz), 16.3 (br, Δν1/2=177 Hz), 9.3 (br, Δν1/2
=
193 Hz), 5.4 (br, Δν1/2 =33 Hz), -5.2 (br, Δν1/2 =25 Hz), -8.4
(br, Δν1/2 =52 Hz) ppm. The diamagnetic species was identified
as [CpVMe]2(μ-p-TolN)2. EPR (benzene-d6, 298 K): g = 1.97;
a(51V)=18 G.
[CpVMe]2(μ-p-TolN)2. A solution of Cp(N-p-Tol)V(i-Pr2N)-
Me (0.40 g, 1.2 mmol) in toluene (3 mL) was warmed at 50 °C for
3 days. Slow cooling to room temperature afforded red-brown
crystals, which were separated. The compound was identified by
1H NMR spectroscopy as [CpVMe]2(μ-p-TolN)2 and were
obtained in 30% isolated yield.
JPV = 340.9 Hz).
Cp(p-TolN)V(dmpe). One equivalent of dmpe (0.12 g,
0.80 mmol) was added to a solution of Cp(p-TolN)V(i-Pr2N)Me
(0.27 g, 0.80 mmol) in toluene (20 mL). The reaction mixture was
warmed to 50 °C and stirred for 48 h. The solvent was removed
in vacuo, and the mixture was stripped twice with pentane
(5 mL). The solvent was removed in vacuo, and pentane (2 ꢀ
5 mL) was added and pumped off, to remove residual toluene.
Upon concentrating the solution to 10 mL and cooling to
-80 °C, 0.16 g (0.44 mmol, 55%) of reddish-purple block
crystals of the title compound were obtained. Anal. Calcd for
C18H28NP2V: C, 59.07; H, 8.09; N, 3.63. Found: C, 58.7; H,
7.69; N, 3.82.
Cp(t-BuN)V(i-Pr2N)Me. To a cold (-40 °C) solution of Cp(t-
BuN)V(i-Pr2N)Cl (7.16 g, 22.2 mmol) in diethyl ether (150 mL)
was added dropwise MeLi (14 mL, 1.6M, 22 mmol). The
mixture was warmed to -10 °C and stirred for 1 h. The color
of the solution changed from red to yellow-brown. The workup
was carried out at -10 °C. After removal of the solvent in vacuo,
the residual ether was stripped off by addition of 2 ꢀ 20 mL of
cold pentane and subsequent removal of the volatiles in vacuo.
Extraction of the reaction mixture with cold pentane (100 mL)
and removal of the solvent in vacuo at -10 °C yielded 5.65 g
(18.7 mmol, 84%) of the title compound as a brown oil, which
crystallizes at -30 °C. 1H NMR (400 MHz, benzene-d6, 25 °C): δ
5.66 (s, 5H, Cp), 4.40 (sept, JHH = 6.5 Hz, 1H, i-Pr CH), 3.19 (m,
1H, i-Pr CH), 1.65 (d, JHH = 6.4 Hz, 3H, i-Pr Me), 1.24 (s, 9H,
t-Bu Me), 1.33 (d, JHH = 6.5 Hz, 3H, i-Pr Me), 0.90 (d, JHH=6.5
Hz, 3H, i-Pr Me), 0.83 (d, JHH = 6.6 Hz, 3H, i-Pr Me), 0.41 (br,
3H, V-Me) ppm. 13C{1H} NMR (100 MHz, benzene-d6,
25 °C): δ 104.93 (Cp), 62.03 (i-Pr CH), 52.90 (i-Pr CH), 32.36
(i-Pr Me), 31.76 (t-Bu Me), 27.09 (i-Pr Me), 20.41 (i-Pr Me),
19.38 (i-Pr Me), Cq of t-Bu and V-Me not observed. 51V NMR
(131 MHz, benzene-d6, 25 °C, VOCl3 external standard): δ
-667.98 (t, JVN = 86.60 Hz).
Thermolysis of Cp(t-BuN)V(i-Pr2N)Me. In an NMR tube
equipped with a Teflon (Young) valve, Cp(t-BuN)V(i-Pr2N)Me
(0.050 g, 0.17 mmol) was dissolved in benzene-d6 (0.6 mL).
The tube was attached to the vacuum line and degassed using three
freeze-pump-thaw cycles, after which the tube was placed in
an oven at 50 °C for 6 h. The tube was attached to the vacuum
line, and the amount of methane formed in the reaction was
determined using a Toepler pump (0.48 equiv of CH4). The
volatiles were transferred to a second tube and analyzed by 1H
NMR spectroscopy using Cp2Fe as internal standard and GC-
MS analysis. This revealed formation of diisopropylamine and
N-isopropylisopropylideneamine in 25% and 6% yield, respec-
tively. The residue was redissolved in benzene-d6. 1H NMR
analysis revealed the presence of both diamagnetic and para-
magnetic species. For the paramagnetic species: 1H NMR (400
MHz, benzene-d6, 25 °C): δ 91.5 (br, Δν1/2 = 336 Hz), 58.9 (br,
Δν1/2 = 257 Hz), 38.3 (br, Δν1/2 = 96 Hz), 17.6 (br, Δν1/2 = 234
Hz), 15.4 (br, Δν1/2 = 390 Hz), 12.1 (br, Δν1/2 = 156 Hz), 10.1
Generation of Cp(t-BuN)V(dmpe). In an NMR tube equipped
with a Teflon (Young) valve, Cp(t-BuN)V(i-Pr2N)Me (0.060 g,
0.20 mmol) was dissolved in benzene-d6 (0.6 mL). To the
resulting solution was added dmpe (0.030 g, 0.20 mmol). The
tube was attached to the vacuum line and degassed using three
freeze-pump-thaw cycles, after which the tube was placed in
the oven at 50 °C for 6 h. Subsequently the tube was attached to
the vacuum line and the amount of methane formed during the
reaction was measured using a Toepler pump (0.94 equiv of
1
CH4). H NMR analysis confirmed the presence of N-isopro-
pylisopropylideneamine and the title compound (94%). 1H
NMR (400 MHz, benzene-d6, 25 °C): δ 4.87 (s, 5H Cp), 1.41-
1.28 (m, 8H, overlapping resonances for PMe2 and PCH2), 1.27
(s, 9H, t-BuN), 1.10 (m, 2H, PCH2), 0.68 (m, 6H, PMe2) ppm.
13C{1H} NMR (100 MHz, benzene-d6, 25 °C): δ 89.88 (Cp CH),
34.60 (t-BuN Me), 32.44 (PCH2), 24.55 (PMe2), 17.92 (PMe2),
Cq of t-Bu not observed. 31P{1H} NMR (100 MHz, benzene-d6,
25 °C): δ 87.34 (plateau, Δνtop= 2689 Hz). 51V NMR (131 MHz,
benzene-d6, 25 °C, VOCl3 external standard): δ -381 (Δν1/2
5190 Hz).
=
Cp(t-BuN)V(dmpe). Dmpe (0.116 g, 0.771 mmol) was added
at ambient temperature to a solution of Cp(t-BuN)V(i-Pr2N)Me
(0.233 g, 0.771 mmol) in toluene (20 mL). The red-brown
solution was warmed to 50 °C and stirred overnight. The solvent
was removed in vacuo, and pentane (2 ꢀ 5 mL) was added and
pumped off, to remove residual toluene. The residue was
extracted with 20 mL of pentane. The solution was concentrated
to 10 mL and cooled to -80 °C, affording 0.10 g (0.30 mmol,
45%) of green block-shaped crystals of the title compound.
Anal. Calcd for C15H30NP2V: C, 54.54; H, 9.44; N, 3.98. Found:
C, 54.10; H, 9.12; N, 4.21.
(br, Δν1/2 = 208 Hz), 5.0 (br, Δν1/2 = 345 Hz), -6.4 (br, Δν1/2
=
X-ray Analysis of Cp(RN)V(dmpe) (R = p-Tol, t-Bu). Suitable
crystals of 3.1, 3.5, and 3.6 were mounted on top of a glass fiber
in a drybox and transferred, using inert-atmosphere handling
techniques, into the cold nitrogen stream on a Bruker SMART
APEX CCD diffractometer. Intensity data were corrected for
Lorentz and polarization effects, scale variation, and decay and
absorption: a multiscan absorption correction was applied, based
on the intensities of symmetry-related reflections measured at
different angular settings (SADABS). The structures were solved
by Patterson methods, and extension of the model was accom-
plished by direct methods applied to difference structure factors
using the program DIRDIF. A subsequent difference Fourier
synthesis resulted in the location of all the hydrogen atoms, whose
coordinates and isotropic displacement parameters were refined.
All refinement and geometry calculations were performed with the
1375 Hz) ppm. For the diamagnetic species two major reso-
nances at δ 6.30 and 5.27 ppm are observed in a 1:4 ratio.
Generation of Cp(p-TolN)V(dmpe). In an NMR tube equipped
with a Teflon (Young) valve, Cp(p-TolN)V(i-Pr2N)Me (0.058 g,
0.172 mmol) and Cp2Fe (0.032 g, 0.172 mmol, internal standard)
were dissolved in benzene-d6 (0.6 mL). To the resulting solution
was added dmpe (0.026 g, 0.172 mmol). The tube was attached to
the vacuum line and degassed using three freeze-pump-thaw
cycles, after which the tube was placed in the oven at 50 °C for
3 days. Subsequently the tube was attached to the vacuum line, and
the amount of methane formed during the reaction was measured
1
using a Toepler pump (0.84 equiv of CH4). H NMR analysis
confirmed the presence of N-isopropylisopropylideneamine
(88%), diisopropylamine (6%), the title compound (88%), and

6236 Organometallics, Vol. 29, No. 23, 2010
Batinas et al.
Table 2. Crystallographic Data for 2a and 2b
2a
Cq), 34.54 (t-BuN Me), 25.02 (dd, JCP = 9.6, 4.8 Hz, PMe3).
31P{1H} NMR (100 MHz, benzene-d6, 25 °C): δ 52.30 (plateau,
Δνtop= 2914.98 Hz). 51V NMR (131 MHz, benzene-d6, 25 °C,
VOCl3 external standard): δ -111.39 (t, JVP = 420.58 Hz).
Generation of Cp(t-BuN)V(SPh)2. To a reddish-brown solu-
tion of Cp(t-BuN)V(i-Pr2N)Me (0.126 g, 0.420 mmol) in hexane
(1 mL) was added a 1 mL hexane solution of PhSSPh (0.091 g,
0.42 mmol). After 5 h at 50 °C the mixture turned dark green.
The solvent was subsequently pumped off, leaving the title
compound as a green oil. 1H NMR (400 MHz, benzene-d6,
25 °C): δ 7.93 (d, JHH = 7.7 Hz, 4H, SPh), 7.12 (d, JHH = 7.6 Hz,
4H, SPh), 6.98-6.80 (m, 2H, SPh), 5.61 (s, 5H, Cp), 1.02 (s, 9H,
t-BuN Me). 13C{1H} NMR (100 MHz, benzene-d6, 25 °C): δ
148.81 (SPh Cipso), 132.81 (SPh CH), 128.28 (SPh CH), 125.94
(SPh CH), 108.17 (Cp), 30.07 (t-BuN Me), Cq of t-BuN not
observed.
Catalytic Cyclotrimerization of Phenylacetylene Using Cp-
(RN)V(N-i-Pr2)Me (R = p-Tol, t-Bu). All cyclotrimerization
experiments were performed in NMR tubes equipped with a
Teflon (Young) valve. To a solution of the precatalysts Cp(p-
TolN)V(N-i-Pr2)Me or Cp(t-BuN)V(N-i-Pr2)Me (0.045 mmol)
in benzene-d6 (0.6 mL) was added 1 equiv of cyclohexane (0.045
mmol, 5 μL added via microsyringe), followed by 10 equiv of
PhCCH (0.046 g, 0.45 mmol). The tube was placed at 80 °C and
monitored by 1H NMR spectroscopy. After completion of the
reactions, the mixtures were analyzed by GC-MS and GC as
well, using cyclohexane as internal standard (Rf = 1.02).
Catalytic Cyclotrimerization of Diphenylacetylene Using Cp(t-
BuN)V(N-i-Pr2)Me. The cyclotrimerization experiment was
performed in an NMR tube equipped with a Teflon (Young) valve.
A solution of Cp(t-BuN)V(N-i-Pr2)Me (0.03 mmol, 0.010 g) in
benzene-d6 (0.6 mL) was made, and diphenylacetylene (0.30 mmol,
0.053 g) was added. The tube was warmed at 80 °C. After 3 h at this
temperature white crystals started to form in the tube, and after 24 h
the amount of crystals had increased considerably. The tube was
kept at this temperature for another 60 h, after which the solid part
was separated from the liquid by centrifuge. After washing with
pentane (2 ꢀ 1 mL), white crystals of hexamethylbenzene were
obtained in 71% yield. The product was identified by GC-MS
analysis (THF solvent).
2b
formula
fw
C
18H28NP2V
C15H30NP2V
337.30
monoclinic
P21/c
100(1)
15.1320(1)
8.9402(6)
13.7110(1)
102.480(1)
1811.0(2)
4
371.32
monoclinic
P21/c
100(1)
10.404(1)
10.939(1)
17.404(2)
95.317(2)
1972.2(3)
4
cryst syst
space group
temperature (K)
˚
a (A)
˚
b (A)
˚
c (A)
β (deg)
3
V (A )
˚
Z
F
F(000)
calc (g cm-3
)
1.250
784
6.62
2.71-29.25
0.0470
0.1092
1.042
1.237
720
7.14
2.74-26.73
0.0399
0.0950
1.021
3028
294
μ(Mo KR) (cm-1
θ range (deg)
R(F )
)
wR(F2)
GooF
obsd reflns for F0 g 4.0σ(F0)
params
3837
317
program packages SHELXL and PLATON. Crystal data and
details on data collection and refinement are presented in Table 2.
Generation of CpV(N-p-tolyl)(PMe3)2. In an NMR tube
equipped with a Teflon (Young) valve, Cp(p-TolN)V(i-Pr2N)Me
(0.03 g, 0.08 mmol) and Cp2Fe (0.015 g, 0.08 mmol) were
dissolved in benzene-d6 (0.5 mL). To the resulting solution
was added PMe3 (0.01 g, 0.02 mL, 0.16 mmol). This solution
was warmed to 50 °C for 48 h. After this time the color of the
mixture had changed from red-brown to brown-green. The
spectroscopic data confirmed the formation of CpV(N-p-tolyl)-
(PMe3)2 in 82% yield (relative to Cp2Fe internal standard). 1H
NMR (400 MHz, benzene-d6, 25 °C): δ 7.14 (d, JHH = 8.1 Hz,
2H, p-TolN CH), 6.94 (d, JHH = 7.7 Hz, 2H, p-TolN CH), 4.88
(s, 5H, Cp), 2.16 (s, 3H, p-TolN Me), 1.03 (m, 18H, PMe3).
13C{1H} NMR (100 MHz, benzene-d6, 25 °C): δ 161.52 (p-TolN
C
ipso), 129.06 (p-TolN CH), 128.72 (p-TolN Cipso), 125.56
(p-TolN CH), 94.49 (Cp), 23.29 (dd, JCP = 10.3, 5.9 Hz, PMe3),
21.22 (p-TolN Me). 31P{1H} NMR (100 MHz, benzene-d6,
25 °C): δ 54.45. 51V NMR (131 MHz, benzene-d6, 25 °C, VOCl3
external standard): δ 267.59 (t, JVP = 399.66 Hz).
Acknowledgment. This research was financially sup-
ported by The Netherlands Organization for Scientific
Research (NWO). M.W.B. acknowledges the NWO for a
VENI grant. We would like to thank Dr. W. R. Browne at
the University of Groningen for his assistance with the
EPR spectra.
Generation of CpV(N-t-Bu)(PMe3)2. In an NMR tube
equipped with a Teflon (Young) valve, Cp(t-BuN)V(i-Pr2N)Me
(0.040 g, 0.13 mmol) was dissolved in benzene-d6. To the
solution PMe3 (0.020 g, 0.26 mmol) was added. The tube was
warmed at 50 °C for 6 h, and the color changed to green-brown.
The volatiles were removed in vacuo, affording the title com-
pound as a brown-green oil (86% yield, Cp2Fe internal
1
standard). H NMR (400 MHz, benzene-d6, 25 °C): δ 4.81 (s,
Supporting Information Available: Crystallographic informa-
tion files of compounds 2a,b. This material is available free of
charge via the Internet at http://pubs.acs.org.
5H, Cp), 1.07 (s, 9H, t-BuN Me), 0.82 (s, 18H, PMe3). 13C{1H}
NMR (100 MHz, benzene-d6, 25 °C): δ 92.00 (Cp), 67.8 (t-BuN