Tri-chlorido, 2-methylallyl and 2-butenyl tert-butylimido niobium and tantalum complexes: Synthesis, multinuclear NMR spectroscopy and reactivity

Article abstract of DOI:10.1039/c0dt00878h

Pseudooctahedral complexes [MCl₃(NtBu)L₂] (M = Nb, L = py 1, tmeda 3; M = Ta, L = py 2, tmeda 4) have been studied by spectroscopic methods. By a VT ¹H NMR experiment a mutual exchange process between the py_ax and py_free in the complexes 1-2 was observed, whereas ¹³C and ¹⁵N NMR studies showed in the complexes 3-4 a tmeda ligand with an axial/equatorial coordination mode. The reaction of 2 with 3 equiv of Grignard reagent produces the methathesis products [TaR ₃(NtBu)] (R = CH₂CMeCH₂5, CH ₂CHCHCH₃6) in which 2-methylallyl and 2-butenyl groups appear with a η³- and σ-coordination mode, respectively. When, toluene solutions of the compounds 5-6 were treated with 2 equiv of 2,6-dimethylphenylisocyanide the imido bisiminoacyl compounds [TaR(NtBu){C(R)NAr-κ¹C}₂] (Ar = 2,6-Me ₂C₆H₃; R = CH₂CMeCH₂7, CH₂CHCHCH₃8) can be isolated, via an imido iminoacyl intermediate [TaR₂(NtBu){C(R)NAr-κ¹C}] (Ar = 2,6-Me₂C₆H₃; R = CH₂CMeCH ₂9) as we have observed in the treatment of 5 with 1 equiv of isocyanide; however, the analogous reaction between 5 and COPh₂ leads to the formation of the trisalkoxo imido compound [Ta(OCPh₂R) ₃(NtBu)] (R = CH₂CMeCH₂10). All new complexes were studied by IR and multinuclear NMR spectroscopy.

Full text of DOI:10.1039/c0dt00878h

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PAPER
Tri-chlorido, 2-methylallyl and 2-butenyl tert-butylimido niobium and
tantalum complexes: Synthesis, multinuclear NMR spectroscopy and
reactivity†
Miguel Galajov,b Carlos Garc´ıaa and Manuel Go´mez*a
Received 20th July 2010, Accepted 19th October 2010
DOI: 10.1039/c0dt00878h
1
2
1
2
Pseudooctahedral complexes [MCl3(NtBu)L2] (M = Nb, L = py 1, tmeda 3; M = Ta, L = py 2, tmeda
4) have been studied by spectroscopic methods. By a VT 1H NMR experiment a mutual exchange
process between the pyax and pyfree in the complexes 1–2 was observed, whereas 13C and 15N NMR
studies showed in the complexes 3–4 a tmeda ligand with an axial/equatorial coordination mode. The
reaction of 2 with 3 equiv of Grignard reagent produces the methathesis products [TaR3(NtBu)] (R =
3
CH2CMeCH2 5, CH2CHCHCH3 6) in which 2-methylallyl and 2-butenyl groups appear with a h - and
s-coordination mode, respectively. When, toluene solutions of the compounds 5–6 were treated with
2 equiv of 2,6-dimethylphenylisocyanide the imido bisiminoacyl compounds [TaR(NtBu){C(R)NAr-
1
k C}2] (Ar = 2,6-Me2C6H3; R = CH2CMeCH2 7, CH2CHCHCH3 8) can be isolated, via an imido
1
iminoacyl intermediate [TaR2(NtBu){C(R)NAr-k C}] (Ar = 2,6-Me2C6H3; R = CH2CMeCH2 9) as we
have observed in the treatment of 5 with 1 equiv of isocyanide; however, the analogous reaction
between 5 and COPh2 leads to the formation of the trisalkoxo imido compound [Ta(OCPh2R)3(NtBu)]
(R = CH2CMeCH2 10). All new complexes were studied by IR and multinuclear NMR spectroscopy.
In this paper, some trichlorido tert-butylimido niobium and tan-
Introduction
talum complexes were studied by multinuclear NMR spectroscopy
and the nature of 15N chemical shifts analyzed. In addition, by
alkylation reactions 2-methylallyl and 2-butenyl tert-butylimido
derivatives were prepared and their reactivity investigated with
unsaturated organic substrates as isocyanides and ketones.
High-valent early transition metal complexes containing strong
p-donor organoimido substituents1 are being intensively studied
in relation to their potential applications in catalytic and stoi-
chiometric processes. A lot of examples show that over the last
few years we have witnessed an extensive development of imido
group 5 nonmetallocene chemistry.2
Protonated lithium amides3 together with other synthetic
strategies4 have been extensively used to generate the imido ligand
and in this context, neutral niobium and tantalum complexes have
been synthesized5 and their functionalities6 were used as both
ancillary and reactive sites. Although as ancillary ligands they have
been electronically compared with cyclopentadienide,6a,7 a series
of important stoichiometric transformations such as, protonations
of metal amides,6e additions of hydrogen to metal–alkyl groups,6j,6k
insertions of unsaturated molecules (CO, ArNC) into metal–alkyl
bonds,6b–6d,6f–6h,6l imido-to-oxo ligand exchange and the activation
of benzene C–H bonds6h have been observed.
Results and discussion
Although the first metal-imido complexes of the type
[MX3(NR)L2] (M = Nb, Ta; R = alkyl, aryl; X = halide; L =
neutral s-donor ligand)8 were synthesized twenty years ago,
their structural study is incomplete and only some molecular
structure,8c,9 reactivity8b and luminescent properties9 have been
previously reported. Pseudooctahedral trichlorido imido com-
pounds [MCl3(NtBu)py2] (M = Nb 1, Ta 2) were prepared in
better yields by a modification of the reported method8c (see
Experimental) when toluene solutions containing 1 equivalent
of the pentachlorides MCl5 were treated with 2 equivalents of
NH(tBu)(SiMe3) in the presence of an excess of pyridine. In addi-
tion, reactions of 1–2 with stoichiometric amounts of N,N,N¢,N¢-
tetramethylethylenediamine (tmeda) give solutions from which the
adducts [MCl3(NtBu)(tmeda)] (M = Nb 3, Ta 4) can be isolated in
good yield as yellow and white microcrystalline solids, respectively
(Scheme 1). 1–4 are soluble in most organic solvents, including
saturated hydrocarbons. They are extremely air- and moisture-
sensitive, and rigorously dried solvents and handling under an
aDepartamento de Qu´ımica Inorga´nica, Universidad de Alcala´ de Henares,
Campus Universitario, E-28871, Alcala´ de Henares, Spain
bCentro de Espectroscopia de Resonancia Magne´tica Nuclear, CAI en
Qu´ımica, Universidad de Alcala´ de Henares, Campus Universitario, E-28871,
Alcala´ de Henares, Spain. E-mail: manuel.gomez@uah.es; Fax: + 34 91 885
46 83; Tel: + 34 91 885 47 64
† This paper is dedicated to the memory of Dr Amelio Va´zquez de Miguel
who died April 20, 2010 and whom we remember with affection.
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Scheme 2 Exchange process between pyax and free py*.
When a C7D8 solution of 1–2 and pyridine in an 1 : 1.5 molar
ratio was studied by 1H VT NMR spectroscopy between 223 and
343 K (Fig. 1), the proposed mutual exchange process6c,11 between
the axial pyridine (pyax) and free pyridine (py*) was confirmed and
free Gibbs energy values were obtained in the collapse point (1,
DG‡313 K = 15.3 kcal mol-1; 2, DG‡317 K = 15.4 kcal mol-1).
1
The 13C{ H} NMR spectra of both complexes 1–2 (see Experi-
mental) are in agreement with the observed behaviour by 1H NMR
spectroscopy and although it hardly shows any differences between
the pyax and pyeq carbon atom resonances, the quaternary carbon
resonances of the tBu moiety are different: d 72.6 (1), 67.0 (2).
Moreover, in the gHMBCd2_N15 spectra we observe the signals
for the imido nitrogen atom at d 64.2 (1) and 30.8 (2), and the
corresponding signals for the equatorial pyridine ligand at d 112.6
(1) and 113.1 (2), whereas the cross-peaks for the axial pyridine
ligand were not detected in both complexes probably due to the
spin exchange process.
Scheme 1 Preparation of complexes 1–4.
argon atmosphere were found to be imperative for successful
preparations.
Compounds 1–4 were characterized by analytical and spec-
troscopic methods, and the data are in agreement with a mer,
cis-pseudooctahedral structure. The IR spectra of all complexes
show the characteristic absorption corresponding to the M N-
The NMR spectra of complexes 3–4 show two carbon (13C)
and two nitrogen (15N) resonances for tmeda in agreement with
a Cs symmetry and with the presence of a bidentate ligand
coordinated to the axial/equatorial positions of a metal centre
-1
stretching vibration5a,6g,10 at n ª 1358 cm . At room temperature,
˜
1
the H NMR spectrum of the complexes 1–2 shows two signals
set with different width, probably due to an intermolecular
exchange process between the py coordinated in only one po-
sition and a small amount of free undetected py. A noesy 1d
experiment with selective excitation of tBu resonance showed
that the well resolved multiplets corresponding to the pyeq and
therefore, only the pyax participate in the spin exchange process
(see Scheme 2).
1
in a pseudooctahedral environment. Moreover, in the H NMR
spectra we have observed very well resolved multiplets for the –
CH2-CH2– moiety and there is no inter- and/or intramolecular
exchange process in the NMR time scale.
The 15N resonances of Ta NtBu moiety (see Scheme 3)
are remarkably more shielding (Dd ª -34) with respect to the
Fig. 1 VT 1H NMR spectra of the mixture 2 + 1.5 py* in C7D8.
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Scheme 3 NMR data assignment for complexes 1–4.
analogous niobium complexes (Dd2-1 = 30.8–64.2 = -33.4; Dd4-3
=
20.0–54.7 = -34.7). We propose that the coincident Dd (15N) value
is basically due to the change of the diamagnetic component of the
magnetic shielding constant in agreement with Lamb’s formalism
for a M N- direct bond. On the other hand, the axial position
in the complexes 3 and 4 is characterized by more shielding
resonances (Dd = dax - deq) in the 15N (NMe2) [Dd = -15.2 (3), -16.7
(4)] and the 13C (NMe2) [Dd = -4.7 (3), -5.5 (4)] spectra due to the
well known trans effect of the M N- triple bond.6g Moreover, the
pyridine by tmeda substitution also causes a notable shielding of
the 15N imido resonance (Dd3-1 = 54.7–64.2 = -9.5; Dd4-2 = 20.0–
30.8 = –10.8) probably due to the major donor character of the
axial nitrogen amino atom in the trans position with respect to the
imido group in comparison with the pyridine ligand implied in
a coordination–decoordination process. In addition, the carbon
chemical shift corresponding to the quaternary carbon atom of
the tBu moiety in niobium complexes (1,3) is larger (Dd1-2 = 5.6;
Dd3-4 = 2.8) in agreement with the major electronic density of the
imido nitrogen atom in the tantalum compounds (2,4).
Scheme 4 Preparation of complexes 5–6.
room temperature by treatment of diethyl ether solutions of 2 with
a stoichiometric amount of the corresponding Grignard reagent.
They are soluble in most organic solvents, including saturated
hydrocarbons and in addition, are extremely air- and moisture-
sensitive, and rigorously dried solvents and handling under an
argon atmosphere were found to be imperative for successful
preparations.
Compounds 5–6 were characterized by spectroscopic methods
and the data are in agreement with the proposed structures.
Trialkylated imido tantalum complexes [TaR3(NtBu)] (R =
CH2CMeCH2 5, CH2CHCHCH3 6) were obtained (Scheme 4) at
The IR spectra show the characteristic absorption for the
-1
Ta N- stretching vibration5a,6g,10 at n ª 1351 cm . The different
˜
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coordination mode of the 2-methylallyl and 2-butenyl groups to
the pseudotetrahedral metal centre in the complexes 5–6 was
deduced by NMR spectroscopic studies. Complex 5 shows in
1H NMR, we proposed that the spectral behaviour of 6 in a C6D6
solution can be described by a typical s1–s3 isomerization process
of the 2-butenyl group coordinated to the metal center in which
the population of s1 ground state is very much larger than s3, as
shown in Scheme 5.
1
its H NMR spectra one singlet at d 0.85 (9H) for the t-butyl
group, two narrow resonances at d 3.11 (12H) and 1.70 (9H)
3
for the three equivalent h -coordinated 2-methylallyl groups.12
1
The 13C{ H} NMR spectrum (see Experimental) is in agreement
with the proposed structure showing the -CH2- resonance at
d 73.4 and in addition, in the 15N NMR spectrum the imido
nitrogen signal appears at d 13.2. Although the Grignard reagent
used MgCl(CH2CMeCH2) is seemingly described in the Aldrich
catalogue as a magnesium compound with a s-coordinated
3
methylallyl group, we propose a h -coordination mode for this
ligand as we deduced after studying a C6D6 solution of the reagent
by NMR spectroscopy.13 In contrast, the 1H NMR spectrum of 6
shows a narrow singlet at d 0.97 (9H) for the t-butyl substituent,
a resolved doublet at d 2.34 (6H) and a doublet of triplets at d 5.1
Scheme 5 Isomerization process of complex 6.
Unfortunately, we do not observe this process for complex 6
by a H VT NMR experiment. However, an important spectral
3
(3H) (3Jd = 15, Jt = 10.6 Hz) for -CH2–CH = resonances, also a
1
broad signal (Dn = 18 Hz) at d 1.84 (9H) and another very broad
(Dn = 40 Hz) at d 3.8 (ª 3H) for the CH3–CH = moiety. In the
variation was detected at 173–253 K in a CD2Cl2 solution of
starting Grignard reagent MgCl(CH2CHCHCH3) in the presence
of tetrahydrofuran. The VT 1H NMR spectra show a typical two
spin exchange process corresponding to very different populations.
The more significant experimental data comprise the detection
at 183 K of a new signal for the methyl group (1 : 6 ratio) in
accordance with the tocsy 1d and ASAPHMQC spectra (1H,
d 1.47; 13C, d 20). We suggest that this resonance is due to
the MgC(CH3)CHCH2 moiety in the s3-isomeric structure (see
Scheme 5).
1
13C{ H} spectrum all signals for the 2-butenyl group appear as
slightly wide resonances at d 125.6 (Dn ª 5 Hz) for –CH2-CH
,
97.5 (Dn = 12.5 Hz) for = CH–CH3, 44.4 (Dn = 14.3 Hz) for –CH2–
CH , 17.8 (Dn = 5.4 Hz) for CH3–CH , whereas the resonance
at d 32.2 corresponding to -CMe3 is characterized by a natural
1
line width (Dn = 1.2 Hz). The 13C{ H} spectrum of 6 exhibits
significant chemical shift differences with respect to the Grignard
reagent MgCl(CH2CHCHCH3).14 So, while the resonances for a-
CH2 and CH3 groups are deshielding Dd = dTa - dMg = +26.9 and
Dd = +2.8, the resonances corresponding to carbons b- and g-
CH = CH- have a shielding of Dd = -15.3 and -3.7, respectively.
Furthermore, the chemical shift of the methylene carbon atom
d 44.4 (6) has really the same value as that in the methylallyl
complex d 42.4 (5) and, in addition, the imido nitrogen resonance
in 6 was observed at d 6.7 [Dd = d(6) - d(5) = -8.2]. Assuming
the anomaly a large value of vicinal SSCC between –CH2–CH =
protons (3J = 10.4 Hz) and observation of some broad signals in
Complexes
5 and 6 react with xilylisocyanide in an
1 : 1 or 1 : 2 molar ratio (Scheme 6) at room temperature
in toluene to give, in good yields, the bisiminoacyl com-
1
pounds [TaR(NtBu){C(R)NAr-k C}2] (Ar = 2,6-Me2C6H3; R =
CH2CMeCH2 7, CH2CHCHCH3 8), as result of a double iso-
cyanide insertion reaction into the two different Ta–C bonds
of the starting trialkylated derivatives. However, 5 reacts with
1 equivalent of isocyanide leading to a mixture in which the
main product is a di(2-methylallyl) imido iminoacyl derivative
Scheme 6 Preparation of complexes 7–9.
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1
[TaR2(NtBu){C(R)NAr-k C}] (Ar
=
2,6-Me2C6H3;
R
=
CH2CMeCH2 9) together with small amounts of the bisiminoacyl
complex 7 and starting material 5.
This result is in contrast with to that observed for
[TaCp*Cl4-xMex] (x = 2–4)6b,6c which in the presence of 1 or
2 equivalents of isocyanide leads to the formation of azatantalacy-
2
clopropane [TaCp*XY(CMe2NAr-k C,N)] (x = 2, 4; X = Y = Cl,
Me; x = 3, X = Cl, Y = Me; Ar = 2,6-Me2C6H3) or chloride/methyl
imido alkenylamido [TaCp*X(NAr){NAr(CMeCMe2)-k N}] (x =
Scheme 7 Insertion of Ph2CO in complex 5.
1
corresponding to the phenyl ring, and also four signals at d 1.47
(9H), 3.2 (6H), 4.84 (3H) and 4.91 (3H) for the 2-methylallyl group
in accordance with a pseudotetrahedral tantalum(V) complex with
3, 4; X = Cl, Me; Ar = 2,6-Me2C6H3) species, respectively. In the
present case, the first step consists in the insertion of the first co-
ordinated isocyanide molecule with migration of an 2-methylallyl
or 2-butenyl group to give the corresponding iminoacyl complex
9, as we have detected by NMR spectroscopy and subsequently,
the coordination of a second isocyanide to the vacant position is
followed by the migration of the second group to the electrophilic
isocyanide carbon atom leading to bisiminoacyl complexes 7–
8. The remaining 2-methylallyl or 2-butenyl group does not
migrate, probably because it would lead to a much less favourable
unsaturated electron deficient compound and in both cases (R =
CH2CMeCH2, CH2CHCHCH3), we do not detect the formation
of azatantalacyclopropane or imido alkenylamido systems.
The IR spectra of the imido iminoacyl complexes 7–9 show
1
a C3v symmetry. The 13C{ H} spectrum displays all expected
signals (see Experimental, d 65.2, 34.1 CMe3; 50.6, 141.6, 24.9,
116.6 CH2CMeCH2; 148.1, 129.0, 127.4, 126.9 Ci, Co, Cm, Cp
C6H5; 88.1 OCPh2R) whereas the 15N imido resonance is detected
at d -30.2 and is very shielding [Dd = 43.4] with respect to
the precursor tris(methylallyl) complex 5. We propose that the
observed difference in the 15N chemical shift is due to the major
back donation (pp–dp hyperconjugation) of oxygen free electron
pairs to the imido nitrogen atom, which increases their electronic
density.
the characteristic absorption due to the Ta N-imido stretching
Conclusions
-1
vibration5a,6g,10 at n ª 1352 cm , but the corresponding absorption
˜
bands due to the C C6d and C(R) = N-6g moieties cannot
NMR spectroscopic studies in [MCl3(NtBu)(tmeda)] (M = Nb, Ta)
complexes show a mutual exchange process between axial pyridine
and free pyridine and confirm a pseudooctahedral geometry
for [MCl3(NtBu)(tmeda)] (M = Nb, Ta) with a bidentate ligand
coordinated in an axial/equatorial fashion. Alkylation of the
trichlorido imido tantalum compounds with Grignard reagents
˜
be unambiguously assigned because both appear around n ª
1600 cm-1 together with other more intense absorption bands
corresponding to the 2,6-Me2C6H3 substituent. However, the
NMR data (see Experimental) of the complexes 7–8 are in
agreement with the structural proposal indicated in the Scheme 6,
as pseudotetrahedral complexes with a Cs symmetry. The 15N
imide resonance appears at d +5.1 [Dd = d(7) - d(5) = -8.1]
and d +3.8 [Dd = d(8) - d(6) = -2.9]. The iminoacyl moiety
shows carbon-13 signals6g,15 located at d ª 254 for the complexes
7–8 and the 15N resonance at d -91 for 7, whereas this signal
for 8 is not detected. The methylene proton resonances of the
migrated allyl groups are diastereotopics16 in both complexes [dav.
3
produces trialkylated imido derivatives [TaR3(NtBu)] (R = h -
CH2CMeCH2, s-CH2CHCHCH3), which in the presence of 2,6-
Me2C6H3NC and COPh2 leads to 2-methylallyl/2-butenyl imido
bisiminoacyl and trialkoxo imido compounds, respectively, by
conventional insertion processes.
Experimental
2
2
3.28, 4H, JH–H = 15.7 Hz (7); dav. 3.34, 4H, JH–H = 16.2 Hz
(8)] due to the prochiral character of the metal centre, while
the carbon resonances of CH2 appear at d 45 (7) and 41.5 (8).
In complex 7 the allyl group bonded to the tantalum atom
appears as one singlet at d 2.05 and a broad resonance (Dn =
25 Hz) at d 3.3 for CH3 and CH2 moieties, respectively, in
All operations were carried out under a dry argon atmosphere
using standard Schlenk-tube and cannula techniques or in a con-
ventional argon-filled glove-box. Solvents were refluxed over an
appropriate drying agent and distilled and degassed prior to use:
C6D6 and hexane (Na/K alloy), diethyl ether (Na/benzophenone)
and toluene (Na). Starting materials [MCl3(NtBu)py2] (M = Nb,
Ta)8c were prepared by a modification of the method described
previously. Reagent grade MCl5 (M = Nb, Ta), SiClMe3, tBuNH2,
py, tmeda, 2,6-Me2C6H3NC and MgClR (R = CH2CMeCH2,
CH2CHCHCH3; 0.5 M in diethyl ether, Aldrich) were purchased
from commercial sources and were used without further purifica-
tion.
3
accordance with a h -coordinated methylallyl group but with a
slower s1–s3 isomerization process with respect to the starting
3
tris-h -methylallyl complex 5. The formation of the monoinsertion
compound 9 can be observed when the reaction is followed by
NMR spectroscopy. The observation of two inequivalent methyl
groups for the 2,6-Me2C6H3 ring of the iminoacyl ligand is
consistent with the restricted rotation of the aryl group around
the N–Ci(aryl) bond.6g
Samples for IR spectroscopy were prepared as KBr pellets and
recorded on a Perkin-Elmer Spectrum 2000 spectrophotometer
Contrary to the isocyanide insertion, diphenyl ketone reacts
with 5 (Scheme 7) in a molar ratio 3 : 1 at room temperature to give
in good yield the trialkoxo imido complex [Ta(OCPh2R)3(NtBu)]
(R = CH2CMeCH2 10).
(4000–400 cm-1). The 1H and 13C{ H} NMR spectra were
1
recorded on MercuryPlus-300 and UnityPlus-300 spectrometers. The
inverse detection (ASAPHMQ, gc2hsqcsc13 and gHMBC_d215),
selective excitation and VT experiments were realized in a three
channel VNMRS-500 spectrometer. The 1H and 13C chemical
shifts were referenced to TMS by using the solvent signals and
1
The H NMR spectrum shows one singlet at d 0.99 (9H) for
the tBu moiety, three characteristic multiplets at d 7.46 (12H),
7.12 (12H), and 7.04 (6H) for o-, m- and p- proton resonances
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15N chemical shifts to external CH3NO2. Microanalyses (C, H, N)
were performed in a LECO CHNS 932 microanalyzer.
Me2Neq–C2H4–NaxMe2), 1.85(AA¢BB¢, 4H, Me2Neq–CH2-CH2-
1
NaxMe2), 1.38(s, 9H, NCMe3). 13C{ H} NMR (C6D6): d =
67.7(NCMe3), 59.3(Me2Neq-CH2-CH2–NaxMe2), 56.7(Me2Neq-
CH2-CH2–NaxMe2), 55.1(Me2Neq–C2H4–NaxMe2), 49.7(Me2Neq–
C2H4–NaxMe2), 32.0(NCMe3). 15N gHMBC NMR (C6D6): d =
20.0(NCMe3), -329.0(Me2Neq-C2H4–NaxMe2), -345.7(Me2Neq–
C2H4-NaxMe2). Anal. Calcd. for C10H25N3Ta: C, 25.31; H, 5.26;
N, 8.85. Found: C, 25.27; H, 5.26; N, 8.78%.
Synthesis of [MCl3(NtBu)py2] (M = Nb 1, Ta 2)
To a toluene (40 mL) solution of tBuNH2 (2.60 mL, 25 mmol) was
added SiClMe3 (1.50 mL, 12 mmol), which immediately produced
a white precipitate of [NH3(tBu)]Cl. This mixture was stirred at
room temperature for 15 min, the suspension was filtered through
Celite and the filtrate was added to a toluene (M = Nb 30 mL,
Ta 100 mL) suspension of MCl5 (5.50 mmol; M = Nb 1.45 g, Ta
2.20 g). The reaction mixture was stirred for 1 h and a large excess
of pyridine (1.60 mL, 21 mmol) was added. The resulting yellow
suspension was stirred for 12 h and filtered, and the filtrate was
pumped to dryness giving 1–2 as yellow microcrystalline solids.
Synthesis of [Ta(CHCMeCH2)3(NtBu)] (5)
Under rigorously anhydrous conditions, a 0.5 M solution of
MgCl(CH2CMeCH2) in diethyl ether (4.94 mL, 2.47 mmol) was
added at room temperature to a solution of 2 (0.43 g, 0.82 mmol) in
diethyl ether (15 mL) and the reaction mixture was stirred for 12 h.
The resulting suspension was decanted and the magnesium salt
filtered off. The filtrate was evaporated to dryness and the residue
extracted with hexane (3 ¥ 10 mL). The solution was concentrated
to ca. 5 mL and cooled to -40 ◦C overnight to give 5 as a dark red
microcrystalline solid.
˜
1. Yield 2.10 g (89%). IR (KBr): n = 2972(vs), 1606(vs),
1444(vs), 1359(s), 1247(vs), 1066(s), 758(vs), 697(vs), 425(w) cm-1.
1H NMR (C6D6): d = 9.20(br, 2H, Ho, pyax), 8.83(2H, Ho, pyeq),
3
4
6.81(1H, Hp, pyax), 6.64(1H, JH–H = 7.7 Hz, JH–H = 1.6 Hz,
Hp, pyeq), 6.50(2H, Hm, pyax), 6.27(m, 2H, Hm, pyeq), 1.48(s, 9H,
˜
Yield 0.19 g (48%). IR (KBr): n = 2946(s), 1606(m), 1445(m),
1
NtBu).13C{ H} NMR (C6D6): d = 152.6(Co, pyeq), 151.7(Co, pyax),
1
1376(m), 1354(vs), 1263(vs), 1032(w), 884(m), 697(w) cm-1. H
138.6(Cm, pyax), 138.1(Cm, pyeq) 123.7(Cp, pyeq), 124.1(Cp, pyax),
72.0(NCMe3), 30.7(NCMe3). 15N gHMBC NMR (C6D6): d =
65.2(NtBu), -108(Npyeq), not detected(Npyax).
NMR (C6D6): d = 3.11(br, 12H, CH2CMeCH2), 1.69(s, 9H,
1
CH2CMeCH2), 0.84(s, 9H, NCMe3). 13C{ H} NMR (C6D6):
d = 146.3(CH2CMeCH2), 67.6(NCMe3), 42.4(CH2CMeCH2),
32.2(NCMe3), 25.5(CH2CMeCH2). 15N gHMBC NMR (C6D6):
d = 14.9(NCMe3). Anal. Calcd. for C16H30NTa: C, 46.07; H, 7.19;
N, 3.35. Found: C, 45.86; H, 7.21; N, 3.26%.
˜
2. Yield 2.32 g (82%). IR (KBr): n = 2971(vs), 1609(vs),
1444(vs), 1357(s), 1281(vs), 1067(s), 758(vs), 697(vs), 427(w) cm-1.
1H NMR (C6D6): d = 9.17(br, 2H, Ho, pyax), 8.90(m, 2H, 3JH–H
=
4
5.2 Hz, JH–H = 1.6 Hz, Hp, pyax), 6.80(m, 1H, Hp, pyax), 6.61(m,
1H, Hp, pyeq), 6.47(br, 2H, Hm, pyax), 6.23(m, 2H, Hm, pyeq), 1.53(s,
Synthesis of [Ta(CH2CHCHCH3)3(NtBu)] (6)
1
9H, NtBu).13C{ H} NMR (C6D6): d = 153.2(Co, pyax), 152.0(Co,
A sample of 2 (0.43 g, 0.82 mmol) was dissolved in 15 mL of diethyl
pyeq), 139.1(Cm, pyax), 138.1(Cm, pyeq), 124.0(Cp, pyax), 123.8(Cp,
pyeq), 66.6(NCMe3), 32.2(NCMe3). 15N gHMBC NMR (C6D6):
d = 30.8(NtBu), -113.2(Npyeq), not detected (Npyax).
◦
ether in a Schlenk tube and at -78 C was treated with a 0.5 M
solution of MgCl(CH2CHCHCH3) in diethyl ether (4.94 mL,
4.94 mmol), and the mixture was stirred for 1 h. It was warmed
to room temperature for 12 h, the solvent was removed in vacuo,
and the residue was extracted into hexane (3 ¥ 10 mL). The filtrate
was concentrated to ca. 10 mL and cooled to -40 ◦C to give 6 as
a brown microcrystalline solid.
Synthesis of [MCl3(NtBu)(Me2N-CH2-CH2-NMe2-j2N,N)] (M =
Nb 3, Ta 4)
A toluene (25 mL) solution of [MCl3(NtBu)py2] (M = Nb, 0.16 g,
0.45 mmol; Ta, 0.20 g, 0.38 mmol) was treated at room temperature
with tmeda (M = Nb, 0.10 mL, 0.67 mmol; Ta, 0.11 mL, 0.58 mmol)
and the mixture was stirred for 12 h. The resulting suspension was
filtered, the solution was concentrated to ca. 5 mL and cooled to
-40 ◦C overnight to afford 3 and 4 as yellow and white air-sensitive
microcrystalline solids, respectively.
˜
Yield 0.18 g (46%). IR (KBr): n = 2961(s), 1605(m), 1445(vs),
1
1376(m), 1353(m), 1273(vs), 1022(w), 838(w), 757(w) cm-1. H
3
3
NMR (C6D6): d = 5.08(dt, 3H, JH–H = 14.69 Hz, JH–H
=
10.47 Hz, CH2–CH = CH-CH3), 3.82(br, 3H, CH2-CH CH-
3
CH3), 2.34(d, 6H, JH–H = 10.47 Hz, CH2-CH CH-CH3),
1
1.62(br, 9H, CH2-CH CH–CH3), 0.96(s, 9H, NCMe3). 13C{ H}
NMR (C6D6): d = 125.38(CH2-CH CH-CH3), 97.42(CH2-
CH = CH-CH3), 65.73(NCMe3), 44.26(CH2-CH CH-CH3),
31.94(NCMe3), 17.63(CH2-CH CH-CH3). 15N gHMBC NMR
(C6D6): d = 6.7(NCMe3). Anal. Calcd. for C16H30NTa: C, 46.07;
H, 7.19; N, 3.35. Found: C, 45.88; H, 7.23; N, 3.29%.
˜
3. Yield 0.16 g (91%). IR (KBr): n = 2974(vs), 1474(s),
1402(m), 1358(m), 1243(s), 1010(m), 951(m), 797(s) cm-1. 1H
NMR (C6D6): d = 2.47(s, 6H, Me2Neq–C2H4–NaxMe2), 2.26(s, 6H,
Me2Neq–C2H4–NaxMe2), 1.54(2H), 1.59(2H, AA¢BB¢, Me2Neq–
1
CH2-CH2-NaxMe2), 1.29(s, 9H, NCMe3). 13C{ H} NMR (C6D6):
d = 70.5(NCMe3), 58.6(Me2Neq-CH2-CH2–NaxMe2), 56.4(Me2Neq-
CH2-CH2–NaxMe2), 53.8(Me2Neq–C2H4–NaxMe2), 49.1(Me2Neq–
C2H4–NaxMe2), 29.5(NCMe3). 15N gHMBC NMR (C6D6): d =
54.7(NCMe3), -331.9(Me2Neq-C2H4–NaxMe2), -347.1(Me2Neq–
C2H4-NaxMe2). Anal. Calcd. for C10H25N3Nb: C, 31.08; H, 6.47;
N, 10.87. Found: C, 30.87; H, 6.44; N, 10.40%.
Synthesis of [TaR(NtBu){C(R¢)NAr-j1C}2] (R =
g3-CH2CMeCH2, R¢ = CH2CMeCH2, Ar = 2,6-Me2C6H3 7)
A solution of 5 (0.18 g, 0.41 mmol) in toluene (20 mL) was
treated with 2,6-Me2C6H3NC (0.12 g, 0.82 mmol) under rigorously
anhydrous conditions. The mixture was stirred for 12 h and then
filtered. The filtrate was concentrated to ca. 5 mL and cooling
overnight at -40 ◦C produced 7 as a garnet microcrystalline
solid.
˜
4. Yield 0.16 g (87%). IR (KBr): n = 2969(vs), 1475(s),
1356(m), 1280(s), 1008(m), 950(m), 799(s) cm-1.1H NMR
(C6D6): d = 2.49(s, 6H, Me2Neq–C2H4–NaxMe2), 2.39(s, 6H,
418 | Dalton Trans., 2011, 40, 413–420
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˜
Yield 0.24
1467(vs), 1353(s), 1260(vs), 1091(m), 764(w) cm-1. 1H NMR
(C6D6): 6.93[m, 6H, TaC(CH2CMeCH2)NC6H3Me2],
4.18[av, m, 4H, TaC(CH2CMeCH2)NAr], 3.30(br, 4H,
g
(80%). IR (KBr):
n
=
2963(s), 1590(s),
Formation of 9 was confirmed by NMR spectroscopy, however,
repeated attempts to obtain 9 as a unitary product by preparative
scale were unsuccessful.
d
=
1H NMR (C6D6): d = 6.90[m, 3H, TaC(CH2CMeCH2)-
NC6H3Me2], 4.60[br, 2H, TaC(CH2CMeCH2)NAr], 3.20[br, 2H,
TaC(CH2CMeCH2)NAr], 3.18[br, 8H, Ta(CH2CMeCH2)2], 2.04[s,
6H, Ta(CH2CMeCH2)2], 1.99[s, 3H, TaC(CH2CMeCH2)NAr],
1.74[s, 6H, TaC(CH2CMeCH2)NC6H3Me2], 1.11(s, 9H, NCMe3).
2
Ta–CH2CMeCH2), 3.34, 3.22[AB, 4H, JH–H
TaC(CH2CMeCH2)NAr], 2.14(s, 6H), 1.73[s, 6H, TaC(CH2CMe-
CH2)NC6H3Me2], 2.05(s, 3H, Ta-CH2CMeCH2), 1.92[t,
= 14.55 Hz,
4
6H, JH–H = 1.27 Hz, TaC(CH2CMeCH2)NAr], 1.32(s, 9H,
NCMe3). 13C{ H} NMR (C6D6): d = 254.4[TaC(CH2CMeCH2)-
13C{ H} NMR (C6D6): d = 252.6[TaC(CH2CMeCH2)NAr],
1
1
NAr], 156(TaCH2CMeCH2), 141[TaC(CH2CMeCH2)NAr],
153.8(Ci), 141(Cp), 129.2, 128.1(Cm), 122, 121(Co)[TaC(CH2CMe-
CH2)NC6H3Me2], 114[TaC(CH2CMeCH2)NAr], 66.5(NCMe3),
45.7[TaC(CH2CMeCH2)NAr], 34.3(NCMe3), 26.9(TaCH2CMe-
CH2), 23.7[TaC(CH2CMeCH2)NAr], 18.7, 18.3[TaC(CH2CMe-
CH2)NC6H3Me2], not observed(TaCH2CMeCH2). 15N gHMBC
NMR (C6D6): d = 5.8(NCMe3), -90.6[TaC(CH2CMeCH2)NAr].
Anal. Calcd. for C34H48N3Ta: C, 60.11; H, 7.06; N, 6.18. Found:
C, 59.88; H, 6.78; N, 6.24%.
150.9[Ta(CH2CMeCH2)2], 142, 128.2, 124.4[several phenyl,
TaC(CH2CMeCH2)NC6H3Me2], 141.8[TaC(CH2CMeCH2)NAr],
114.8[TaC(CH2CMeCH2)NAr], 77.05[Ta(CH2CMeCH2)2], 68-
(NCMe3),
46.7[TaC(CH2CMeCH2)NAr],
32.8(NCMe3),
26.3[Ta(CH2CMeCH2)2], 23.5[TaC(CH2CMeCH2)NAr], 19.6-
[TaC(CH2CMeCH2)NC6H3Me2], not observed[TaC(CH2CMe-
CH2)NAr]. 15N gHMBC NMR (C6D6): d = 9.0(NCMe3),
-96.7[TaC(CH2CMeCH2)NAr].
Synthesis of [Ta(OCPh2R)3(NtBu)] (R = CH2CMeCH2 10)
Synthesis of [TaR(NtBu){C(R)NAr-j1C}2] (R = CH2CHCHCH3;
Ar = 2,6-Me2C6H3 8)
At room temperature, a 0.87 mM solution of COPh2 (72 mL,
0.06 mmol) in C6D6 was added to a C6D6 (0.75 mL) solution
of 5 (0.01 g, 0.02 mmol) placed in a NMR valved tube. The color
of the mixture quickly changed from dark red to orange and the
A 100 mL Schlenk flash was charged with a toluene (30 mL)
solution of 6 (0.18 g, 0.42 mmol) and treated with 2,6-Me2C6H3NC
(0.11 g, 0.84 mmol) under rigorously anhydrous conditions. After
the mixture had been stirred at room temperature for 12 h, the
volume of the solution was concentrated to ca. 5 mL under reduced
pressure. The resulting solution was cooled to -40 ◦C overnight to
afford a brown microcrystalline solid of 8.
1
reaction was checked by H NMR spectroscopy until no further
change was observed. After 1 h the spectrum showed the presence
of 10 in quantitative yield.
1H NMR (C6D6):
d = 7.46[12H, Ho, TaOC(C6H5)2R],
7.12[12H, Hm, TaOC(C6H5)2R], 7.04[6H, Hp, TaOC(C6H5)2R],
4.91(3H), 4.84[AB, 3H, TaOCPh2(CH2CMeCH2)], 3.20[s, 6H,
˜
Yield 0.22 g (77%). IR (KBr): n = 2915(s), 1612(vs), 1458(vs),
1
1373(s), 1351(m), 1266(s), 964(s), 728(s) cm-1. H NMR (C6D6):
TaOCPh2(CH2CMeCH2)], 1.47[s, 9H, TaOCPh2(CH2CMe-
1
CH2)], 0.99(s, 9H, NCMe3). 13C{ H} NMR (C6D6): d
=
d = 6.90[m, 6H, TaC(CH2CHCHCH3)NC6H3Me2], 5.95(br, 2H,
TaCH2CHCHCH3), 5.85[m, 2H, TaC(CH2CHCHCH3)NAr],
148.1(Ci), 129.0(Co), 127.4(Cm), 126.9(Cp, TaOC(C6H5)2R],
141.6[TaOCPh2(CH2CMeCH2)],
116.6[TaOCPh2(CH2CMe-
5.52[m,
2H,
TaC(CH2CHCHCH3)NAr],
4.60(br,
1H,
CH2)], 88.1[TaOCPh2(CH2CMeCH2)], 65.2(NCMe3), 50.6[TaO-
CPh2(CH2CMeCH2)], 34.1(NCMe3), 24.9[TaOCPh2(CH2CMe-
CH2)]. 15N gHMBC NMR (C6D6): d = -30.3(NCMe3).
TaCH2CHCHCH3),
3.30(m, 2H, TaCH2CHCHCH3),
3.30[m, AB, 4H, TaC(CH2CHCHCH3)NAr], 2.13(s, 6H),
1.72[s, 6H, TaC(CH2CHCHCH3)NC6H3Me2], 1.60[m, 6H,
TaC(CH2CHCHCH3)NAr], 1.58(m, 3H, TaCH2CHCHCH3),
1
1.29(s, 9H, NCMe3). 13C{ H} NMR (C6D6): d = 254.1[TaC(CH2-
Acknowledgements
CHCHCH3)NAr], not observed(Ci), 142.5(Cp), 130, 129(Cm),
128.1, 128(Co)[TaC(CH2CMeCH2)NC6H3Me2], 127.8[TaC(CH2-
We thank the Ministerio de Ciencia e Innovacio´n (project
CTQ 2006-04540/BQU), the Comunidad de Madrid (project S-
0505/PPQ/0328-02) and the Universidad de Alcala´ (project UAH
GC2010-002) for their financial support of this research. C.G. is
grateful to Universidad de Alcala´ for a fellowship.
CHCHCH3)NAr],
125.6[TaC(CH2CHCHCH3)NAr],
66.1-
(NCMe3), 40.8[Ta(CH2CHCHCH3)], 40.5[TaC(CH2CHCHCH3)-
NAr], 34.2(NCMe3), 19.5[TaC(CH2CHCHCH3)NAr], 18.6(Ta-
CH2CHCHCH3), 16.2, 16 [TaC(CH2CHCHCH3)NC6H3Me2], not
observed (TaCH2CHCHCH3), not observed(TaCH2CHCHCH3).
15N gHMBC NMR (C6D6): d = 3.85(NCMe3), -91 [TaC(CH2-
CMeCH2)NAr]. Anal. Calcd. for C34H48N3Ta: C, 60.11; H, 7.06;
N, 6.18. Found: C, 60.08; H, 6.96; N, 6.16%.
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13 NMR data for MgCl(h -CH2CMeCH2): 1H NMR (C6D6): = 2.98(s, 4H,
1
CH2CMeCH2), 2.23(s, 3H, CH2CMeCH2). 13C{ H} NMR (C6D6): d =
156.4(CH2CMeCH2), 58.4(CH2CMeCH2), 27.4(CH2CMeCH2).
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1
CH2CHCHCH3), 1.44(s, 2H, CH2CHCHCH3). 13C{ H} NMR
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d
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17.6(CH2CHCHCH3), 15.0(CH2CHCHCH3).
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420 | Dalton Trans., 2011, 40, 413–420
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©