Oxygen extrusion from amidate ligands to generate terminal TaO units under reducing conditions. How to successfully use amidate ligands in dinitrogen coordination chemistry

Article abstract of DOI:10.1039/c1dt11595b

A series of mixed Cp* amidate tantalum complexes Cp*Ta(RNC(O) R′)X₃ (where R = Me₂C₆H₃, ⁱPr, R′ = ^tBu, Ph, X = Cl, Me) have been prepared via salt metathesis and their fundamental reactivities under reducing conditions have been explored. Reaction of the tantalum chloro precursors with potassium graphite under N₂ or Ar leads to the stereoselective formation of the terminal tantalum oxo species, Cp*TaO(η²-RNCR′)Cl. This represents the formal extrusion of oxygen from the amidate ligand to the reduced tantalum center and is accompanied by the formation of the iminoacyl fragment bound to Ta(v). Amidate dinitrogen complexes, [Cp*TaCl(RNC(O) ^tBu)]₂(μ-N₂) (where R = Me₂C ₆H₃, ⁱPr) were synthesized via salt metathesis from the known [Cp*TaCl₂]₂(μ-N₂) precursor, establishing that amidate ligands can support dinitrogen complexes, but not the reduction process often necessary for their synthesis.

Full text of DOI:10.1039/c1dt11595b

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PAPER
Oxygen extrusion from amidate ligands to generate terminal Ta O units
under reducing conditions. How to successfully use amidate ligands in
dinitrogen coordination chemistry†
Patricia Horrillo-Martinez, Brian O. Patrick, Laurel L. Schafer* and Michael D. Fryzuk*
Received 24th August 2011, Accepted 24th October 2011
DOI: 10.1039/c1dt11595b
A series of mixed Cp* amidate tantalum complexes Cp*Ta(RNC(O)R¢)X3 (where R = Me2C6H3, iPr,
t
R¢ = Bu, Ph, X = Cl, Me) have been prepared via salt metathesis and their fundamental reactivities
under reducing conditions have been explored. Reaction of the tantalum chloro precursors with
potassium graphite under N2 or Ar leads to the stereoselective formation of the terminal tantalum oxo
2
species, Cp*Ta O(h -RN CR¢)Cl. This represents the formal extrusion of oxygen from the amidate
ligand to the reduced tantalum center and is accompanied by the formation of the iminoacyl fragment
bound to Ta(V). Amidate dinitrogen complexes, [Cp*TaCl(RNC(O)tBu)]2(m-N2) (where R = Me2C6H3,
iPr) were synthesized via salt metathesis from the known [Cp*TaCl2]2(m-N2) precursor, establishing that
amidate ligands can support dinitrogen complexes, but not the reduction process often necessary for
their synthesis.
The amidate ligand (RNC(O)R¢) is isoelectronic to the amidi-
nate donor set and differs only by replacement of the oxygen atom
of the former with a monosubstituted nitrogen donor. Given the
aforementioned success of the combination of Cp* and amidinate
ligands to stabilize dinitrogen complexes, we were interested in
examining the effect of modifying the Cp*M(amidinate) system to
the Cp*M(amidate) fragment (Chart 1). The amidate N,O chelat-
ing monoanionic ligands are easily accessible from commercially
available acid chlorides and primary amines to give a wide range
of white crystalline amide proligands.
Introduction
Our interest in the preparation of dinitrogen complexes of the
early metal complexes has focused mainly on reduction of high
oxidation state metal complexes in the presence of N2.1–8 While
this method is very common, it uses quite harsh reagents that can
lead to side reactions such as ancillary ligand rearrangements,9,10
metal-metal bond formation,11 and solvent activation.12 In order to
avoid these kinds of harsh conditions, efforts have made to examine
the use of dihydrogen to generate reduced hydride derivatives that
can activate dinitrogen mildly and directly. However, so far this
latter approach is still in its infancy and has been difficult to apply
broadly to generate activated dinitrogen complexes.13,14
A number of recent reports15–17 have described the isolation of
a series of dinitrogen complexes with a common ligand set, that
of pentamethylcyclopentadienyl (Cp*) and substituted-amidinates
i
(amidinates = (RN)2CR¢, where R = Pr, R¢ = Me). Under reducing
conditions, the Cp*M(amidinate) fragment has been shown to
stabilize dinuclear derivatives that incorporate N2 in both the side-
on and end-on modes for M = Zr, Hf, Ta, Mo and W. Such a range
of metal complexes with coordinated dinitrogen is unique and
has allowed some interesting conclusions to be made about the
importance of formal d-electron count on the extent of dinitrogen
activation.17
Chart 1
While amidate ligands have been successfully utilized for the
synthesis of group 418,19 and, more recently, group 520 complexes for
applications in catalysis, to our knowledge, early transition metal
amidate derivatives have not been subjected to reducing conditions
to examine their ability to support dinitrogen coordination. Herein
we wish to report the reduction of amidate-containing tantalum
complexes, which results in the extrusion of oxygen from the
amidate moiety. While this unanticipated process did not allow
the synthesis of dinitrogen complexes directly via reduction under
N2, we were able to prepare dinitrogen complexes of tantalum via
Department of Chemistry, The University of British Columbia, 2036 Main
Mall, Vancouver, BC, V6T 1Z1, Canada. E-mail: fryzuk@chem.ubc.ca;
Fax: +1 604-822-2847; Tel: +1 604-822-2897
† Electronic supplementary information (ESI) available. CCDC reference
numbers 841449 (3a), 841450 (3b), 841451 (3c), 841452 (4a), 841453 (4b),
841454 (7b) and 841455 (Cp*TaMe4). For ESI and crystallographic data
in CIF or other electronic format see DOI: 10.1039/c1dt11595b
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Fig. 1 ORTEP representation of the solid-state molecular structure of 3a–c (ellipsoids at 50% probability, hydrogens omitted for clarity) with selected
◦
˚
bond lengths (A), and angles ( ): 3a: Ta1–Cl1A, 2.449(19); Ta1–Cl2A, 2.418(2); Ta1–Cl3A, 2.394(2); Ta1–N1A, 2.274(7); Ta1–O1A, 2.025(6); N1A–C11A,
1.296(10); O1A–C11A, 1.318(10); N1A–C16A, 1.462(9); O1A–Ta–N1A, 60.1(2); O1A–C11A–N1A, 111.7(7); 3b: Ta1–Cl1, 2.399(2); Ta1–Cl2, 2.365(2);
Ta1–Cl3, 2.445(2); Ta1–N1, 2.308(7); Ta1–O1, 2.061(6); N1–C1, 1.305(11); O1–C1, 1.320(10); N1–C7, 1.434(11); O1–Ta1–N1, 59.6(3); O1–C1–N1,
112.5(8); 3c: Ta1–Cl1, 2.415(1); Ta1–Cl2, 2.369(1); Ta1–Cl3, 2.410(1); Ta1–N1, 2.315(3); Ta1–O1, 2.053(2); N1–C1, 1.295(5); O1–C1, 1.324(4); N1–C8,
1.444(4); O1–Ta1–N1, 59.47(10); O1–C1–N1, 112.5(3); O1–C1–C2–C3, 33.9.
an indirect method to show that amidate ligands can support N2
coordination.
The solid state structures of complexes 3a–c (Fig. 1) have been
determined by single-crystal X-ray diffraction and they closely
resemble the previously characterized Ta-amidinate complex
Cp*Ta[MeC(NiPr)2]Cl3.21 In all of these derivatives, the amidate
NCO face lies in a molecular plane that bisects the molecule.
The structures can be viewed as distorted octahedral with the
N of the amidate oriented trans to the Cp*. The amidates are
bound unsymmetrically with Ta–N distances substantially longer
Results and discussion
Synthesis and characterization of Cp* tantalum amidate precursors
In analogy to the Cp* amidinate chemistry, we prepared
Cp*Ta(amidate)Cl3 from Cp*TaCl4 via salt metathesis as shown
in Scheme 1. Complexes 3a–c are yellow crystalline solids that
can be purified (up to 56% yield) and structurally characterized.
˚
˚
than Ta–O distances (av. 2.299 A vs. av. 2.046 A). The N–C
distances in the amidate core are shorter than the N–R (R =
˚
˚
alkyl, aryl) distances (av. 1.299 A vs. av. 1.447 A). The O–C
˚
1
According to H and 13C NMR spectra, the structures of 3a–c
distances of 1.321 A av. in the amidate core compare favourably
to literature values.20,22 This indicates that the resulting amidate
bonding motif is best described as an imine-alkoxide mode of
bonding. Crystals of 3a were obtained from saturated cyclohexane
at 3 ◦C, while 3b and 3c were obtained from toluene solutions at
-40 ◦C. Complex 3a crystallizes in the monoclinic space group
P21/n as a nonmerohedral twin (ratio of the two twin components
approximately 60 : 40), while 3b crystallizes with two independent
molecules in the unit cell. Complex 3c crystallizes in a triclinic
system. The Ta–Cl bond distance trans to the oxygen in 3b and
show the NCO plane to be a symmetry plane perpendicular to the
cyclopentadienyl ring, making the N-isopropyl methyl groups and
those on the N-phenyl ring equivalent. Indeed, a single doublet
corresponding to N-isopropyl methyl groups of 3a is observed at
1.82 ppm by 1H NMR spectroscopy. The 13C NMR spectra display
the characteristic peak near 180 ppm for k2-amidate binding
mode.20 At room temperature, 3a–c are stable in the solid state;
however, solutions of these complexes are very moisture sensitive
as evidenced by the formation of the free amide proligand and
insoluble ill-defined ‘Ta’ species, even with attempted rigorous
exclusion of water.
˚
3c is shorter than the two cis-Cl’s (2.365(2) vs. av. 2.422 A and
˚
2.369(1) vs. av. 2.412 A, respectively). Notably, changing the steric
bulk and/or electronic properties of the amidate results in little
or no effect on the fundamental coordination geometry of the
resulting solid-state structure. However, the Ta–N bond length
increases slightly as the steric bulk of the N-substituent is increased
˚
(2.274(7) in 3a vs. 2.308(7) A in 3b).
Reduction of Cp*Ta(amidate)Cl3
Following the procedure reported for the reduction of
Cp*Ta(amidinate)Cl3,15–17 the amidate derivatives 3a–c were re-
duced with KC8 under 4 atm of nitrogen in tetrahydrofuran (THF)
at low temperatures (Scheme 2). After filtration over Celite and
crystallization in diethyl ether, white crystals were obtained that
Scheme 1
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Fig. 2 ORTEP representation of the solid-state molecular structure of 4a (left) and 4b (right) (ellipsoids at 50% probability, hydrogen atoms omitted
˚
for clarity) with selected bond lengths (A): 4a: Ta1–Cl1, 2.420(1); Ta1–O1, 1.738(2); Ta1–N1, 2.113(2); C1–N1, 1.268(3); N1–C6, 1.475(3); 4b: Ta1–Cl1,
2.406(1); Ta1–O1, 1.750(1); Ta1–N1, 2.119(2); C1–N1, 1.270(2); N1–C6, 1.435(2).
display solution NMR data consistent with reduced symmetry
from that of the starting materials. For example, reduction of
3a generates a species that shows diastereotopic Me’s for the
N-isopropyl group; reduction of 3b and 3c results in complexes
that also display inequivalent o-methyl groups of the N-aryl
unit at room temperature. Suitable crystals for X-ray analysis
were obtained, which revealed that the amidate complexes have
been transformed to iminoacyl derivatives containing a terminal
tantalum oxo unit (Fig. 3). 13C NMR spectroscopy revealed a
diagnostic peak for the N C resonance around 235 ppm, in
2
good agreement with related h -iminoacyl complexes23,24 (Scheme
2). The IR spectrum of 4a–c showed strong bands at 1620 and
900 cm-1, which are consistent with the N C and the terminal
Ta O vibrations, respectively.25 The same complexes could be
obtained by the use of permethylcobaltocene, Cp2*Co, as the
reducing agent (-1.94 V vs. Fc/Fc+ in CH2Cl2); interestingly, no
reduction was observed with the milder reducing agent Cp2Co
(-1.33 V vs. Fc/Fc+ in CH2Cl2).
Fig. 3 ORTEP representation of the solid-state molecular structure of
7b (ellipsoids at 50% probability, hydroge◦n atoms omitted for clarity)
˚
with selected bond lengths (A), and angles ( ): Ta1–N1, 1.802(3); N1–N2,
1.292(4); Ta1–Cl1, 2.421(1); Ta1–O1, 2.158(3); Ta1–N3, 2.218(3); C21–N3,
1.296(5); N3–C26, 1.431(5); O1–C21, 1.299(5); C21–C22, 1.524(6);
Ta1–N1–N2, 168.5(3); O1–Ta1–N3, 58.63(11).
The formation of the oxo imino-acyl derivatives 4a–c via
reduction of the amidate precursors is a stereoselective process,
in which only one diastereomer is observed, and the carbon
and the oxygen of the amidate carbonyl are cis disposed in
the product. We propose that reduction of the Ta(V) amidate
complexes 3a–c produces a Ta(III) amidate, which subsequently
undergoes oxygen extrusion to generate the observed Ta(V) oxo
species 4a–c. Presumably, it is the high oxophilicity of tantalum
that promotes this outcome.
Scheme 2
Crystallographic data for 4a and 4b show a distorted square
Such isolable terminal oxo species have been targeted syntheti-
cally, as group 5 species often result in the isolation of bridging oxo
compounds.23,24,26–30 While the synthesis of terminal oxo complexes
is easily accomplished for bis(cyclopentadienyl) compounds of
both tantalum and niobium,31 few other examples of terminal
oxo tantalum complexes supported by non-cyclopentadienyl or
mono-cyclopentadienyl ligands are known.27,28 The attempted
introduction of terminal oxo ligands via hydrolysis reactions of
monocyclopentadienyl compounds M(C5R5)X4 (M = Nb, Ta) gives
pyramidal geometry with the Cp* ligand situated in the apical
2
position (Fig. 2). The iminoacyl moiety is bound in the h -
coordination mode, consistent with literature precedent for other
early metal derivatives. Complex 4b crystallized together with a
molecule of amide in the unit cell. The Ta O bond distance
˚
in 4a and 4b (1.738(2) and 1.750(1) A, respectively) is in good
agreement with the previously reported terminal oxo tantalum
2
23
˚
complex Cp*Ta O(h -2,6-Me2C6H3N CMe)Cl (1.731(7) A).
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polynuclear oxo-bridged complexes.32 However, in the presence of
a strong Lewis acid such as E(C6F5)3 (E = B, Al),33–36 molecular
aggregation is suppressed via adduct formation and complexes
of the type Cp*MR2( O∑E(C6F5)3) (M = Ta, Nb; R = Cl,
CH2Ph, Me; E = B, Al) can be isolated.24,37 Furthermore, the only
Hydrogenation at normal pressures (1–4 atm) did not result in
the formation of hydride complexes; only starting materials were
recovered. We did not investigate higher pressures to force the
hydrogenolysis process. We also attempted the reaction of KBEt3H
with the amidate trichloro complex 3a, however, we were unable
to isolate or characterize any of the species that were produced.
2
monocyclopentadienyl h -iminoacyl oxo complexes reported are
formed either by insertion of isocyanides into Ta–C bonds or by
reacting the tantalum imido species, Cp*Ta NAr(Me)Cl (where
Ar = 2,6-Me2C6H3) with CO.23,25,38,39 In both cases, the formation
of chelated N,O complexes have been proposed as intermediates en
route to the observed and characterized Ta oxo species. Thus, this
approach starting from discrete amidate complexes, is consistent
with these previous literature suggestions.24,37
With the observation that amidate ligands are transformed
under the reducing conditions normally required for the formation
of N2 complexes, a fundamental question regarding the suitability
of amidates as a supporting auxiliary ligand arises. To address
this concern, other milder synthetic routes into amidate ligated N2
complexes were pursued.
Amidate supported dinitrogen complexes
Given the outcome of the attempt to form amidate-stabilized
dinitrogen complexes by reduction, and the inability to ac-
cess azophilic hydride complexes, we revised our strategy and
examined a preformed dinitrogen complex that already in-
cludes the Cp*TaCl2 fragment, namely the dinuclear derivative,
[Cp*TaCl2]2(m-N2) (6).43 Reduction of Cp*TaCl4 with Na/Hg
under N2, or dehydrochlorination of the hydrazine adduct
[(C5Me5)TaCl4]2(m-N2H4), gives the dinuclear dinitrogen complex
6. Due to the success of the aforementioned salt metathesis
reaction (Scheme 1), two equivalents of the amidate sodium salts
2a,b were reacted with 6 to yield the corresponding dinuclear dini-
trogen complexes 7a,b (Scheme 4); spectroscopic and analytical
data of the resultant complexes are consistent with the proposed
structure. For example, the mass spectrum of 7a and 7b both
show a molecular ion peak at 1014 and 1138 (m/z), respectively,
confirming the formation of dinuclear species with two amidate
ligands present, along with two Cp* units and one N2 ligand,
Attempts to hydrogenate Cp*Ta(amidate)Me3 complexes
As already mentioned, a milder but not general method for
the preparation of dinitrogen complexes is the reaction of N2
with certain hydride complexes.14 For example, hydrogenation of
the tantalum(V) trimethyl complex [NPN]TaMe3 (where [NPN]
is [PhP(CH2SiMe2NPh)2]) generates the dinuclear tetrahydride
([NPN]Ta)2(m-H)4, which reacts spontaneously with N2 to gen-
erate the side-on, end-on dinitrogen complex ([NPN]Ta)2(m-
1
presumably bridging. In addition, the H NMR spectrum of 7a
shows a downfield septet corresponding to the CH of the N-
isopropyl group at 4.48 ppm that integrates for 2H with respect to
the 30H of the Cp*, thus suggesting that the complex has the
required stoichiometry of Cp* : amidate being 1 : 1. A notable
feature in this spectrum is the two sets of signals for the Me’s
of the N-isopropyl groups at 1.39 and 1.19 ppm, which indicates
diastereotopicity in the molecule due to the chiral Ta centres. In
7b, these are two singlets for the o-methyl groups of the N-aryl
unit at 2.68 and 2.04 ppm, which can be attributed to restricted
rotation of this group.
2
1
h :h -N2)(m-H)2.40 In analogy to the formation of [NPN]TaMe3,
we examined the preparation of the corresponding derivative,
Cp*Ta(amidate)Me3. As one of the most common methods of
introducing an amidate ligand is via protonolysis,41 we first
attempted the direct reaction of the amide proligand with
Cp*TaMe4; unfortunately, only intractable mixtures resulted. We
were able to prepare the desired organometallic amidate complexes
via metathesis methods. The reaction of the known monochloro
metal-alkyl complex, Cp*TaMe3Cl,42 with the amidate salt 2a
yielded the desired trimethyl complex 5a in 60% yield (Scheme 3).
The 1H NMR spectrum exhibits a singlet at 1.86 ppm corre-
sponding to the Cp* ligand and two singlets at 0.30 and 0.02
ppm in a 2 : 1 ratio, respectively, attributed to the two cis and one
trans Ta–Me groups. As for 3a–c, according to 1H and 13C NMR
spectra, the ligand coordination mode in 5a has a NCO plane of
symmetry perpendicular to the cyclopentadienyl ring, making the
N-isopropyl methyl groups equivalent. Alternatively, complex 5a
can be obtained by the reaction of 3a with 3 equivalents of MeLi
or MeMgCl at low temperature. However, this reaction yielded
a mixture of the desired 5a and the tetramethyl Cp* derivative
Cp*TaMe4 in a 1 : 2 ratio, indicating that the amidate ligands are
labile and are prone to displacement.
Although the high solubility of 7a in nonpolar solvents pre-
vented us from obtaining crystalline material suitable for solid
state analysis, crystals of 7b suitable for X-ray diffraction could
be obtained from a solution of bis(trimethylsilyl)ether and a few
drops of toluene at -40 ◦C (Fig. 3). Complex 7b crystallizes in
¯
the triclinic space group P1 with two molecules of 7b and three
molecules of toluene present in the unit cell. The N2 ligand remains
1
1
bounded in a m-h :h fashion, as in the dinitrogen precursor 6.
˚
The Ta1–N1 distance, 1.802(3) A, is consistent with a Ta-imido-
like linkage, and the N1–N2 distance, 1.292(4) A◦, is similar to
that observed in 6 and other reported m-h :h -N2 complexes.44–47
The Ta1–N1–N2 angle of 168.5(3)◦ is comparable to that of 6
(166.3(4)◦). As in the Cp*Ta-amidate chloro complexes 3a–c, the
amidate ligand is bound with the nitrogen trans to the Cp* ligand.
The Ta–O1 bond length slightly increases compared to 3b (2.158(3)
1
1
˚
vs. 2.061(6) A), while the Ta1–N3 bond length is now significantly
˚
shorter (2.218(3) vs. 2.308(7) A). The particular stereoisomer in
the solid state corresponds to the meso diastereomer.
These results illustrate that amidate ligands can indeed support
isolable dinitrogen complexes, however, they are incompatible with
strong reducing metals, as oxygen extrusion from the amidate
moiety occurs.
Scheme 3
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Scheme 4
according to literature procedures.51 Cp*TaCl4 (1),52 Cp*TaMe4,52
Conclusions
TaMe3Cl2,53 Cp*TaMe3Cl,42 KC8 and [Cp*TaCl2]2(m-N2) (6)43
54
were prepared according to literature procedures. 1H and 13C
spectra were recorded with a Bruker Avance 300 or Avance 400
spectrometer. Mass spectra were recorded with a Kratos MS-50
spectrometer using an electron-impact (70 eV) source. Samples for
IR spectroscopy were prepared as KBr pellets and recorded with
a Nicolet 4700 FT-IR spectrophotometer in the region 4000–400
cm-1. The samples for the IR measurements were prepared and
handled in a N2-filled glovebox. Elemental analyses were recorded
with a Carlo Erba Elemental Analyzer EA 1108.
In summary, we have described the facile preparation of a new fam-
ily of Ta(V) amidate complexes of the formula Cp*Ta(amidate)R3
(R = Me, Cl) via salt metathesis. These complexes are structurally
analogous to previously characterized Ta-amidinate complexes,
although in the case of complexes 3a–c, the unsymmetrical
amidate unit coordinates with the amido nitrogen trans to the
Cp* moiety.
Upon reduction of 3a–c under N2, instead of isolating dinuclear
dinitrogen complexes analogous to related amidinate derivatives,
we observed the stereoselective formation of a series of terminal
2
tantalum oxo h -iminoacyl species 4a–c. While imino acyl species
Cp*Ta(iPrNC(O)tBu)Cl3 (3a)
can be accessed via insertion, the oxygen extrusion process
described here represents a new method to generate terminal
Ta O units; however, it also reveals a limitation in using amidates
as ancillary ligands as they are non-innocent under reducing con-
ditions. This approach for the preparation of terminal tantalum
oxo complexes is notable considering that such species have been
previously postulated and attempts to introduce terminal oxo
ligands in mono-cyclopentadienyl group 5 metal complexes of
tantalum and niobium have been particularly elusive.48–50
Finally, we have synthesized the first amidate dinitrogen com-
plexes 7a,b by incorporating the amide ligand in the last step of the
synthesis, thus avoiding ligand degradation under strong reducing
conditions. The preparation of these complexes begs the question
of whether amidate ligands can be used under alternative reducing
conditions that avoid the use of strong reducing metals.
A THF (40 mL) solution of N-isopropyl(tert-butyl)amide (1.432
g, 10.00 mmol) was added dropwise to a suspension of sodium
bis(trimethylsilyl)amide (1.833 g, 10.00 mmol) in THF (40 mL)
at r.t.. The suspension was stirred for 1 h and the solvent,
together with the bis(trimethylsilyl)amine formed, was removed
in vacuo. The white solid was dissolved again in THF (80 mL)
and was added via cannula to a solution of Cp*TaCl4 (4.580
g, 10.00 mmol) in 80 mL of THF at -30 ◦C. After stirring at
r.t. overnight, the THF was removed in vacuo, and the resulting
yellow solid was extracted with cyclohexane and filtered through
Celite. Concentration and cooling in the fridge yielded 2.48 g
1
of yellow crystalline analytically pure material (44% yield). H
NMR (benzene-d6): d 4.68 (sept, 1H, NCH(CH3)2), 2.27 (s, 15H,
3
C5(CH3)5), 1.82 (d, JHH = 6.8 Hz, 6H, NCH(CH3)2), 1.03 (s,
9H, C(CH3)3). 13C NMR (benzene-d6): d 181.1 (NC(C(CH3)3)O),
130.6 (C5(CH3)5), 52.1 (NCH(CH3)2), 39.4 (NC(C(CH3)3)O), 26.7
NC(C(CH3)3)O), 21.4 (NCH(CH3)2), 12.4 (C5(CH3)5). MS(EI),
m/z = 563 ([M]+, 62%), 565 ([M+2]+, 62%), 567 ([M+4]+, 22%), 527
([M]+-Cl, 28%), 529 ([M+2]+-Cl, 20%), 421 ([M]+- (iPrNC(O)tBu),
34%), 423 ([M+2]+- (iPrNC(O)tBu), 34%), 126 ([iPrNC(O)tBu]+-
CH3, 100%). Anal. calcd for C18H31NCl3OTa (564.75): C 38.28, H
5.53, N 2.48. Found: C 38.17, H 5.49, N 2.50.
Experimental
General considerations
All operations were carried out with standard Schlenk and
glove-box inert-gas techniques under dry nitrogen. Anhydrous
diethyl ether, tetrahydrofuran, hexanes, pentanes and toluene
were purchased from Aldrich, sparged with dinitrogen, and
passed through columns containing activated alumina and Ridox
catalyst before use. Anhydrous diethyl ether and tetrahydrofuran
were stored over a sodium mirror. Nitrogen gas was dried and
deoxygenated by passage through a column containing activated
molecular sieves and CuO. Deuterated benzene was degassed by
Cp*Ta((2,6-Me2C6H3)NC(O)tBu)Cl3 (3b)
In a similar procedure as for complex 3a, a THF (60 mL)
solution of N-2¢,6¢-dimethylphenyl(tert-butyl)amide (2.10 g,
10.23 mmol) was added dropwise to a suspension of sodium
bis(trimethylsilyl)amide (1.88 g, 10.23 mmol) in THF (40 mL)
at r.t.. The suspension was stirred for 1 h and the solvent, together
with the bis(trimethylsilyl)amine formed, was removed in vacuo.
The white solid was dissolved again in THF (80 mL) and was added
via cannula to a solution of Cp*TaCl4 (4.685 g, 10.23 mmol) in
˚
several freeze-pump-thaw cycles and dried with activated 4-A
molecular sieves. Isopropyl amine, 2,6-dimethylaniline, pivaloyl
chloride, benzoyl chloride and pyridin-2-ol were purchased from
Aldrich and used as received. TaCl5 (Strem), Cp*2Co (Aldrich) and
Cp2Co (Aldrich) were used as received. Amides 2a–c were prepared
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80 mL of THF at -30 ◦C. After stirring at r.t. overnight, the THF
was removed in vacuo, and the resulting orange solid was extracted
with toluene and filtered through Celite. The toluene was removed
in vacuo and the yellow-orange solid washed with hot hexanes to
give 3.612 g (56% yield) of a yellow-orange powder of 3b. Single
crystals suitable for X-ray diffraction were obtained by cooling a
saturated toluene solution of 3b to -40 ◦C. 1H NMR (benzene-d6):
d 6.96 (m, 2H, ArH), 6.84 (m, 1H, ArH), 2.80 (s, 6H, Ar(CH3)),
2.25 (s, 15H, C5(CH3)5), 0.96 (s, 9H, C(CH3)3). 13C NMR (benzene-
d6): d 184.2 (NC(C(CH3)3)O), 140.9, 134.4, 128.7, 126.0 (Ar),
130.6 (C5(CH3)5), 41.7 (NC(C(CH3)3)O), 26.5 NC(C(CH3)3)O),
23.0 (Ar(CH3)), 12.5 (C5(CH3)5). MS(EI), m/z = 625 ([M]+, 18%),
627 ([M+2]+, 18%), 421 ([M]+-((2,6-Me2C6H3)NC(O)tBu), 94%),
423 ([M+2]+-((2,6-Me2C6H3)NC(O)tBu), 90%). Anal. calcd for
C23H33NCl3OTa (626.82): C 44.07, H 5.31, N 2.23. Found: C 44.10,
H 5.36, N 2.30.
15H, C5(CH3)5), 1.35 (d, JHH = 6.8 Hz, 3H, NCH(CH3)), 1.14
3
(s, 9H, C(CH3)3) 1.07 (d, JHH = 6.8 Hz, 3H, NCH(CH3)). 13C
3
NMR (benzene-d6): d 242.24 (NC(C(CH3)3)), 117.3 (C5(CH3)5,
55.4 (NCH(CH3)2), 40.22 (NC(C(CH3)3)), 28.8 (NCH(CH3)2),
22.3 NC(C(CH3)3)), 11.7 (C5(CH3)5). MS(EI), m/z = 493 ([M]+,
i
12%), 450 ([M]+-iPr, 100%), 394 ([M]+- Pr-tBu, 40%), 367 ([M]+-
(iPrN CtBu), 50%). Anal. calcd for C18H31NClOTa (493.16): C
43.78, H 6.33, N 2.84. Found: C 44.34, H 6.38, N 2.94. IR (KBr,
n, cm-1) = 1624 (vs) [C N], 898 (vs) [Ta O].25
Cp*Ta O((2,6-Me2C6H3)N CtBu)Cl (4b)
Method A: Complex 3b (626 mg, 1.00 mmol) and freshly prepared
KC8 (338 mg, 2.5 mmol) were placed in a glass tube with a teflon
valve and then THF (~ 20 mL) was added via vacuum transfer
at -196 ◦C. Dinitrogen (1 atm) was added at this temperature
and then the glass tube was sealed and warmed to -78 ◦C. After
stirring for 5 h, the reaction mixture was allowed to warm to r.t.
and stirred overnight. The black suspension was filtered through
Celite and the solvent was removed in vacuo. The crude deep
green material was then extracted with diethyl ether via cannula
filtration (red-orange solution). Concentration and cooling at
-30 ◦C yielded a white crystalline material, which was used for
single X-ray crystallography. Method B: Complex 3b (313 mg,
Cp*Ta((2,6-Me2C6H3)NC(O)Ph)Cl3 (3c)
In a similar procedure as for complex 3a, a THF (40 mL) solution
of N-2¢,6¢-dimethylphenyl(phenyl)amide (660 mg, 2.93 mmol) was
added dropwise to a suspension of sodium bis(trimethylsilyl)amide
(537 mg, 2.93 mmol) in THF (40 mL) at r.t.. The suspen-
sion was stirred for 1 h and the solvent, together with the
bis(trimethylsilyl)amine formed, was removed in vacuo. The white
solid was dissolved again in THF (40 mL) and was added via
cannula to a solution of Cp*TaCl4 (1.342 g, 2.93 mmol) in
60 mL of THF at r.t.. After stirring at r.t. overnight, the THF
was removed in vacuo, and the resulting yellow-orange solid
was extracted with diethyl ether and filtered through Celite.
The diethyl ether was removed in vacuo and the yellow powder
washed with hexanes to give 860 mg (45% yield) of 3c. Single
crystals suitable for X-ray diffraction were obtained by cooling a
0.5 mmol) and Cp* Co (411.7 mg, 1.25 mmol) were placed in a
2
glass tube with a teflon valve and then THF (40 mL) was added
via cannula at -196 ◦C. Dinitrogen (1 atm) was added at this
temperature and then the glass tube was sealed and warmed to
r.t. overnight. The solvent was removed in vacuo and the crude
brown material was extracted with pentane via cannula filtration.
The solvent was removed under vacuo and hexane was added.
Concentration and cooling at -40 ◦C yielded 53 mg of a white
powder (19% yield). 1H NMR (benzene-d6): d 6.83 (m, 3H,
ArH), 2.30 (s, 3H, Ar(CH3)), 2.06 (s, 15H, C5(CH3)5), 1.65 (s,
3H, Ar(CH3)), 1.06 (s, 9H, C(CH3)3). 13C NMR (benzene-d6):
d 243.0 (NC(C(CH3)3)), 142.8, 131.1, 128.9, 127.4, 126.5 (Ar),
117.6 (C5(CH3)5, 42.3 (NC(C(CH3)3)), 27.5 NC(C(CH3)3)), 19.0,
18.8 (Ar(CH3)) 11.8 (C5(CH3)5). MS(EI), m/z = 557 ([M+2]+,
14%), 555 ([M]+, 34%), 500 ([(M+2)-tBu]+, 34%), 498 ([M-tBu]+,
100%), 367 ([M-((2,6-Me2C6H3)N CtBu)]+, 14%). Anal. calcd for
C23H33NClOTa (555.92): C, 49.69, H 5.98, N 2.52. Found: C 49.48,
H 6.07, N 2.55. IR (KBr, n, cm-1) = 1622 (vs) [C N], 908 (vs)
[Ta O].25
◦
1
saturated toluene solution of 3c to -40 C. H NMR (benzene-
d6): d 7.52 (m, 2H, ArH), 6.97 (m, 2H, ArH), 6.87 (m, 2H,
ArH), 6.77 (m, 2H, ArH), 2.75 (s, 6H, Ar(CH3)), 2.33 (s, 15H,
C5(CH3)5). 13C NMR (benzene-d6): d 174.3 (NC(C(CH3)3)O),
141.4, 134.6, 133.0, 132.4 (Ar), 131.0 (C5(CH3)5), 129.6,
128.58, 128.56, 127.6, 126.5 (Ar), 22.6 (Ar(CH3)), 12.6
(C5(CH3)5). MS(EI), m/z = 645 ([M]+, 24%), 647 ([M+2]+, 24%),
421 ([M]+-((2,6-Me2C6H3)NC(O)Ph), 100%), 423 ([M+2]+-((2,6-
Me2C6H3)NC(O)Ph), 94%), 208 ([(2,6-Me2C6H3)NC(O)Ph]+-
CH3, 100%). Anal. calcd for C25H29NCl3OTa (646.81): C 46.42,
H 4.52, N 2.17. Found: C 46.48, H 5.59, N 2.56.
Cp*Ta O((2,6-Me2C6H3)N CPh)Cl (4c)
Cp*Ta O(iPrN CtBu)Cl (4a)
Complex 3c (515 mg, 0.8 mmol) and freshly prepared KC8 (270
mg, 2.0 mmol) were placed in a glass tube with a teflon valve
and then THF (40 mL) was added via cannula at -196 ◦C.
Dinitrogen (1 atm) was added at this temperature and then the
glass tube was sealed and warmed to r.t. overnight. The solvent
was removed in vacuo and the crude material was extracted with
diethyl ether via cannula filtration. Concentration and cooling
at -30 C yielded 119 mg of a white powder (26% yield). H
NMR (benzene-d6): d 7.38 (m, 2H, ArH), 6.92 (m, 4H, ArH),
6.86 (m, 1H, ArH), 6.66 (m, 1H, ArH), 2.04 (s, 15H, C5(CH3)5),
2.02 (s, 3H, Ar(CH3)), 1.75 (s, 3H, Ar(CH3)). 13C NMR (benzene-
d6): d 233.3 (NCPh), 141.9, 133.6, 132.5, 130.4, 129.8, 129.7,
129.3, 129.1, 126.9 (Ar) 117.4 (C5(CH3)5, 18.7, 18.2 (Ar(CH3)),
Complex 3a (595 mg, 1.00 mmol) and freshly prepared KC8 (338
mg, 2.5 mmol) were placed in a glass tube with a teflon valve and
then THF (~ 20 mL) was added via vacuum transfer at -196 ◦C.
Dinitrogen (1 atm) was added at this temperature and then the
glass tube was sealed and warmed to -78 ◦C. After stirring for
5 h, the reaction mixture was allowed to warm to r.t. and stirred
overnight. The black suspension was filtered through Celite and
the solvent was removed in vacuo. The crude deep green material
was then extracted with diethyl ether via cannula filtration (red-
orange solution). Concentration and cooling at -30 ◦C yielded
112 mg of white crystalline analytically pure material (23% yield).
1H NMR (benzene-d6): d 4.39 (sept, 1H, NCH(CH3)2), 1.99 (s,
◦
1
1614 | Dalton Trans., 2012, 41, 1609–1616
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11.40 (C5(CH3)5). MS(EI), m/z = 575 ([M]+, 56%), 470 ([M-2,6-
Me2C6H3]+, 10%), 208 ([(2,6-Me2C6H3)N CPh]+, 100%). Anal.
calcd for C25H29NClOTa (575.90): C 52.14, H 5.08, N 2.43. Found:
C 52.72, H 5.13, N 2.41. IR (KBr, n, cm-1) = 1621 (s) [C N], 902
(vs) [Ta O].25
41.2 (NC(C(CH3)3)O), 27.6 NC(C(CH3)3)O), 20.4 (Ar(CH3)), 19.2
(Ar(CH3)), 11.1 (C5(CH3)5). MS(EI), m/z = 1138 ([M]+, 32%).
Acknowledgements
NSERC is gratefully acknowledged for support of this work
through the Strategic Grants program in the form of an SRO
grant to LLS and MDF.
Cp*Ta(iPrNC(O)tBu)Me3 (5a)
A THF (40 mL) solution of the sodium salt of N-isopropyl(tert-
butyl)amide (0.196 g, 1.18 mmol) was added via cannula to a
solution of Cp*TaMe3Cl (0.470 g, 1.18 mmol) in 40 mL of THF
at -30 ◦C. The reaction mixture was allowed to slowly warm
to r.t. and stirred overnight. The THF was removed in vacuo,
and the resulting yellow solid was extracted in toluene. After
removal of the toluene, 0.360 g (60% yield) of a yellow powder was
obtained. 1H NMR (benzene-d6): d 4.68 (sept, 1H, NCH(CH3)2),
1.86 (s, 15H, C5(CH3)5), 1.71 (d, 3JHH = 6.4 Hz, 6H, NCH(CH3)2),
1.08 (s, 9H, C(CH3)3), 0.29 (s, 6H, Ta-CH3), 0.02 (s, 3H, Ta-
CH3). 13C NMR (benzene-d6): d 176.6 (NC(C(CH3)3)O), 120.4
(C5(CH3)5), 51.0 (Ta-CH3), 50.6 (Ta-CH3), 45.2 (NCH(CH3)2),
39.5 (NC(C(CH3)3)O), 27.6 NC(C(CH3)3)O), 23.3 (NCH(CH3)2),
11.1 (C5(CH3)5).
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1
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