Reactivity of organometallic tantalum complexes containing a bis(phenoxy)amide (ONO)3- ligand with aryl azides and 1,2-diphenylhydrazine

Article abstract of DOI:10.1021/om2004332

The reactivity of tantalum dimethyl and diphenyl complexes (ONO ^cat)TaMe₂ (2) and (ONO^cat)TaPh₂ (3) with aryl azides and 1,2-diphenylhydrazine is reported. p-Tolyl azide inserts into a Ta-C bond of 2 or 3 to afford (ONO^cat)TaMe(η²- MeNNN-p-C₆H₄Me) (6) or (ONO^cat) TaPh(η²-PhNNN-p-C₆H₄Me) (7), respectively. In the presence of another 1 equiv of azide, another insertion occurs, affording the bis(triazenido) complexes (ONO^cat)Ta(η²-MeNNN-p- C₆H₄Me)₂ (4) and (ONO^cat) Ta(η²-PhNNN-p-C₆H₄Me)₂ (5). Complexes 2 and 3 also react with 1,2-diphenylhydrazine, resulting in the loss of 2 equiv of methane or benzene, respectively, along with ¹/ ₂ equiv of azobenzene. The tantalum-containing product was isolated as the pyridine adduct {(ONO^cat)Ta(μ²-NPh)(py)} ₂ (8), which implicates a tantalum(III) intermediate. The viability of this (ONO^cat)Ta^III intermediate was proven by KC ₈ reduction of (ONO^cat)TaCl₂ (1) in THF, resulting in activation of a C-O bond of THF and formation of {(ONO ^cat)Ta(μ-O(CH₂)₃CH₂)} ₂ (10), which was characterized by single-crystal X-ray diffraction studies.

Full text of DOI:10.1021/om2004332

ARTICLE
pubs.acs.org/Organometallics
Reactivity of Organometallic Tantalum Complexes Containing
a Bis(phenoxy)amide (ONO)3ꢀ Ligand with Aryl Azides and
1,2-Diphenylhydrazine
Ryan A. Zarkesh and Alan F. Heyduk*
Department of Chemistry, University of California, 1102 Natural Sciences 2, Irvine, California 92677, United States
S Supporting Information
b
ABSTRACT:
The reactivity of tantalum dimethyl and diphenyl complexes (ONOcat)TaMe2 (2) and (ONOcat)TaPh2 (3) with aryl azides and 1,2-
diphenylhydrazine is reported. p-Tolyl azide inserts into a TaꢀC bond of 2 or 3 to afford (ONOcat)TaMe(η2-MeNNN-p-C6H4Me)
(6) or (ONOcat)TaPh(η2-PhNNN-p-C6H4Me) (7), respectively. In the presence of another 1 equiv of azide, another insertion
occurs, affording the bis(triazenido) complexes (ONOcat)Ta(η2-MeNNN-p-C6H4Me)2 (4) and (ONOcat)Ta(η2-PhNNN-p-
C6H4Me)2 (5). Complexes 2 and 3 also react with 1,2-diphenylhydrazine, resulting in the loss of 2 equiv of methane or benzene,
1
respectively, along with
/ equiv of azobenzene. The tantalum-containing product was isolated as the pyridine adduct
2
{(ONOcat)Ta(μ2-NPh)(py)}2 (8), which implicates a tantalum(III) intermediate. The viability of this (ONOcat)TaIII intermediate
was proven by KC8 reduction of (ONOcat)TaCl2 (1) in THF, resulting in activation of a CꢀO bond of THF and formation of
{(ONOcat)Ta(μ-O(CH2)3CH2)}2 (10), which was characterized by single-crystal X-ray diffraction studies.
’ INTRODUCTION
react further to form a metal imide by 1,2-elimination of a second
RꢀH equivalent. In cases where the metalꢀimido complex is
low-coordinate, this second elimination is reversible and RꢀH
bond activation has also been observed.19,20 Hydrazine deriva-
tives also react with early-transition-metal organometallic com-
plexes to give metal imido complexes.21,22
The synthesis and reactivity of transition-metal complexes
with redox-active ancillary ligands has enjoyed a rebirth, thanks
to reports of surprising reactivity derived from the cooperation
between the metal and ligand valence orbitals. Late-transition-
metal complexes with ligands derived from catechol have poten-
tial application as cross-coupling catalysts,1 H+/H2 electrocatalysts,2,3
and alcohol and amine oxidation catalysts.4ꢀ6 Similarly, mid-
transition-metal complexes with redox-active ligands have shown
disparate reactivity patterns such as N2 activation7,8 and water
oxidation.9ꢀ12 Our research group has been focused on the
development of early-transition-metal complexes with redox-
active ligands in the primary coordination sphere. This strategy
has allowed us to demonstrate oxidative addition and group transfer
reactivity at zirconium(IV) and tantalum(V) metal centers,
despite formal d0 electron configurations at the metal.13ꢀ18 To
further develop the chemistry of early transition metals with
redox-active ligands, we targeted organometallic platforms as a
way to exploit the reactivity of metalꢀcarbon bonds alongside
the redox reactivity imparted by the redox-active ligand set.
Organometallic complexes of the early transition metals are
useful synthons for the preparation of metal alkoxo, oxo, amido,
and imido complexes. Protonolysis of an MꢀR (R = alkyl, aryl)
functionality with a primary amine can release RꢀH to give the
corresponding amide; with a second MꢀR group this amide can
Organoazides (RN3) offer two common routes for the in-
stallation ofnitrogen-donorligandsonearlytransitionmetals.23,24 If
the metal center can be oxidized by two electrons, nitrene transfer to
form a metal imido complex can occur.25 For early-transition-
metal complexes this route is uncommon, though nitrene transfer
reactions have been reported for low-valent vanadium.26,27 We
have shown that ligand-based reducing equivalents are also com-
petent for facilitating nitrene transfer from an organoazide to an
early-transition-metal center. Hence, the reaction of PhN3 with
(NNNcat)TaCl2 ((NNNcat)3ꢀ = bis(2-isopropylamido-4-methy-
oxyphenyl)amide) resulted in the formation of (NNNq)TaCl2-
(dNPh) with a two-electron oxidation of the redox-active
(NNNcat)3ꢀ ligand to the quinonate ((NNNq)ꢀ) oxidation
state.17 A second reaction pathway for organoazides with early-
transition-metal organometallic complexes is insertion into metalꢀ
hydride or metalꢀcarbon bonds.28ꢀ30 For example, Cp*2Hf(H)2
Received: May 23, 2011
Published: August 25, 2011
r
2011 American Chemical Society
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Organometallics
ARTICLE
reacts with phenyl azide to produce the triazenido complex
Cp*2Hf(H)(PhNNNH); however, upon heating, N2 is lost to
afford Cp*2Hf(H)(NHPh), the product of formal nitrene inser-
tion into the HfꢀH bond.31
Herein we report the synthesis and reactivity of organome-
tallic (ONOcat)TaR2 (R = Me (2), Ph (3); (ONOcat)3ꢀ = N,N-
bis(3,5-di-tert-butyl-2-phenoxide)amide) complexes with aryl
azides and 1,2-diphenylhydrazine. Insertion of aryl azide into
the TaꢀC bonds of 2 and 3 are observed to form the corre-
sponding mono-triazenido and bis-triazenido complexes. In all
cases, the triazenido ligands appear to be bidentate. The reaction
of 2 with 1,2-diphenylhydrazine initially affords the hydrazido
complex (ONOcat)Ta(Me)(η2-PhNN(H)Ph) (9) with the loss
of 1 equiv of methane. Heating solutions of 9 with pyridine
resulted in the loss of another 1 equiv of methane, 1/2 equiv of
PhNNPh, and the quantitative formation of {(ONOcat)Ta(μ2-
NPh)(py)}2 (8). Evidence suggests that the azobenzene is
formed by reductive elimination from a (ONOcat)Ta(PhNNPh)
species, implicating the generation of a reduced (ONOcat)TaIII
fragment under the reaction conditions. Independent generation
of the putative tantalum(III) fragment by KC8 reduction of
(ONOcat)TaCl2 (1) in THF supported this contention, as CꢀO
bond activation of THF was observed, resulting in the formation
of {(ONOcat)Ta(μ-O(CH2)3CH2)}2 (10).
Figure 1. ORTEP drawing of (ONOcat)TaMe2 (2). Thermal ellipsoids
are drawn at the 50% probability level. Hydrogen atoms have been
omitted for clarity.
Table 1. Selected Bond Distances (Å) and Angles (deg) for
(ONOcat)TaMe2 (2)
’ RESULTS AND DISCUSSION
Bond Distances/Å
Synthesis and Characterization of (ONOcat)TaR2 Com-
plexes. Tantalum dimethyl and diphenyl complexes of the
redox-active (ONOcat)3ꢀ ligand were accessed by metathesis
routes. The dichloride (ONOcat)TaCl2(OEt2) (1) was treated
with 2 equiv of MeLi in cold diethyl ether to afford
(ONOcat)TaMe2 (2) as a pale yellow powder in 90% yield after
filtration to remove LiCl and subsequent solvent removal. As
previously reported,14 (ONOcat)H3 can be doubly deprotonated
with nBuLi and then treated with TaCl2Me3 to afford 2 directly;
however, the overall yield for the single-step route is only 60%.
Dichloride 1 also provides access to the tantalum diphenyl
analogue (ONOcat)TaPh2 (3) as a red amorphous solid in 95% yield.
The NMR spectroscopic characterization of complexes 2 and
3 are consistent with five-coordinate, C2v-symmetric complexes
Ta(1)ꢀN(1)
Ta(1)ꢀO(1)
Ta(1)ꢀC(20)
Ta(1)ꢀC(21)
O(1)ꢀC(1)
N(1)ꢀC(2)
2.074(3)
1.890(2)
2.122(5)
2.120(5)
1.381(4)
1.406(3)
C(1)ꢀC(2)
C(2)ꢀC(3)
C(3)ꢀC(4)
C(4)ꢀC(5)
C(5)ꢀC(6)
C(1)ꢀC(6)
1.394(4)
1.388(4)
1.394(4)
1.393(5)
1.387(5)
1.384(4)
Bond Angles/deg .
C(20)ꢀTa(1)ꢀC(21) 110.4(2)
N(1)ꢀTa(1)ꢀC(20)
N(1)ꢀTa(1)ꢀC(21)
O(1)ꢀTa(1)ꢀO(1)0 149.44(13)
116.86(18) O(1)ꢀTa(1)ꢀN(1)
132.73(17)
75.47(7)
1
in solution. The H NMR spectrum of 2 reveals a symmetric
ligand. The molecular structure of 2 is shown as an ORTEP
drawing in Figure 1; selected bond distances and angles are
provided in Table 1. As expected, 2 is a five-coordinate, distorted-
trigonal-bipyramidal complex. While the TaꢀC bond distances
are similar to those in the structurally analogous complex
[(OCO)TaMe2]+ ((OCO)2ꢀ = bis(2-oxyphenyl)carbene),32
the CꢀTaꢀC bond angle in 2 is significantly smaller than
120° and the two NꢀTaꢀC bond angles are quite different,
consistent with a slight distortion toward square-pyramidal
geometry about the metal center. The OꢀTaꢀO angle is small,
consistent with the limited bite angle afforded by two fused five-
membered chelate rings of the (ONOcat)3ꢀ ligand. The CꢀC
bond distances within the backbone of the (ONOcat)3ꢀ ligand
are all 1.39(1) Å, consistent with an aromatic ring and the
reduced catecholate form of the ligand.34
Insertion of Organoazides into TaꢀC Bonds. In an attempt
to generate tantalum imido complexes in the presence of alkyl or
aryl ligands, complexes 2 and 3 were treated with organic
azides. Treatment of a benzene solution of 2 with 2 equiv of
p-MeC6H4N3 at 50 °C resulted in an immediate color change
from yellow to red. There was no evidence for the liberation of
(ONOcat)3ꢀ ligand with the aryl proton resonances appearing at
7.03 and 7.70 ppm and the tBu proton resonances appearing at
1.43 and 1.58 ppm. The two equivalent methyl ligands of 2 are
readily identified by their chemical shifts of 0.77 ppm in the 1H
NMR spectrum and 59.7 ppm in the 13C NMR.32 The (ONOcat)3ꢀ
1
ligand of 3 shows nearly identical signatures in the H NMR
spectrum, with aromatic resonances at 7.04 and 7.70 ppm and
tBu resonances at 1.44 and 1.58 ppm. The two equivalent phenyl
ligands of 3 are readily identified by their 1H NMR resonances,
3
which show the appropriate JHH coupling pattern, at 7.02
(para), 7.20 (meta), and 8.18 (ortho) ppm. As expected, the
ipso carbon of the phenyl ligands is readily observed at 202.9
ppm in the 13C NMR spectrum of 3.33
Confirmation of the coordination geometry of 2 was obtained
by single-crystal X-ray diffraction experiments. Crystals of 2
suitable for X-ray diffraction studies were grown from saturated
pentane solutions containing a drop of benzene. Complex 2
crystallizes in the orthorhombic space group Pnma with a mirror
plane containing the tantalum center, the carbon atoms of the
two methyl ligands, and the nitrogen atom of the (ONOcat)3ꢀ
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Scheme 1
Table 2. Selected Bond Distances (Å) and Angles (deg) for
(ONOcat)Ta(η2-MeNNN-p-C6H4Me)2 (4)
Bond Distances/Å
Ta(1)ꢀN(1)
Ta(1)ꢀO(1)
Ta(1)ꢀN(2)
Ta(1)ꢀN(4)
N(2)ꢀN(3)
N(3)ꢀN(4)
O(1)ꢀC(2)
N(1)ꢀC(1)
2.063(3)
1.9567(15)
2.1135(19)
2.288(2)
1.349(3)
1.276(3)
1.363(3)
1.412(2)
C(1)ꢀC(2)
C(2)ꢀC(3)
C(3)ꢀC(4)
C(4)ꢀC(5)
C(5)ꢀC(6)
C(1)ꢀC(6)
1.395(3)
1.402(3)
1.391(4)
1.402(3)
1.396(3)
1.395(3)
Bond Angles/deg
N(1)ꢀTa(1)ꢀN(2) 107.96(5)
N(1)ꢀTa(1)ꢀN(4) 143.64(6)
O(1)ꢀTa(1)ꢀO(1)’ 148.73(9)
O(1)ꢀTa(1)ꢀN(1)
N(2)ꢀTa(1)ꢀN(4)
74.37(5)
O(1)ꢀTa(1)ꢀN(2)
97.87(7)
93.11(8)
O(1)ꢀTa(1)ꢀN(4) 132.84(7)
N(2)ꢀTa(1)ꢀN(2)0 144.07(11)
Figure 2. ORTEP drawing of (ONOcat)Ta(η2-MeNNN-p-C6H4Me)2
(4). Thermal ellipsoids are drawn at the 50% probability level. Hydrogen
atoms have been omitted for clarity.
N(2)ꢀN(3)ꢀN(4)
105.62(19) N(4)ꢀTa(1)ꢀN(4)0
72.72(11)
pointed outward. The TaꢀN bond distances to the triazenido
ligands are very different at 2.29 Å for Ta(1)ꢀN(4) and 2.11 Å
for Ta(1)ꢀN(2). This difference is well outside of the error
associated with the measurement of the TaꢀN distances and
suggests that resonance structure A in eq 1 is the better π-bond
description of the triazenido ligand. The NꢀN bond distances
within the triazenido ligand further support this electronic
picture, with a short N(3)ꢀN(4) distance of 1.28 Å being
consistent with an NdN double bond and a long N(2)ꢀN(3)
distance of 1.35 Å being consistent with an NꢀN single bond.35ꢀ37
N2 during the course of the reaction. Solvent removal afforded
modest yields of the double azide insertion product (ONOcat)-
Ta(η2-MeNNN-p-C6H4Me)2 (4), which was characterized in
solution by NMR, IR, and UVꢀvis spectroscopy and in the solid
state by single-crystal X-ray diffraction. Similarly, the addition of
2 equiv of p-MeC6H4N3 to a benzene solution of 3 at 50 °C
resulted in the formation of the analogous bis(triazenido) complex
(ONOcat)Ta(η2-PhNNN-p-C6H4Me)2 (5), asshowninScheme1.
The nature of the double azide insertion product was identi-
fied by X-ray diffraction experiments carried out on 4. Crystals of
4 suitable for X-ray diffraction studies were obtained by cooling
saturated solutions of the complex in a mixture of toluene and
pentane to ꢀ35 °C. Complex 4 crystallized in the monoclinic
space group C2/c; the molecular structure is depicted as an
ORTEP drawing in Figure 2, and selected metrical parameters
are given in Table 2. Since 4 is C2-symmetric, the centrosym-
metric space group requires a racemic mixture of isomers in the
crystal (δδ and λλ isomers with regard to the triazenido ligands).
The tantalum center is seven-coordinate, with the (ONOcat)3ꢀ
ligand occupying three meridional coordination sites as observed
in the structure of 2; the two triazenido ligands of 4 occupy two
coordination sites each. The metrical parameters for the
(ONOcat)3ꢀ ligand fully support its assignment as the formally
trianionic catecholate oxidation state of the ligand. Slightly longer
TaꢀO bond distances in 4 are consistent with the increase in
coordination number from 5 in 2 to 7 in 4. The triazenido ligands
are oriented with the methyl groups together and the tolyl groups
Solution NMR spectra suggest analogous C2-symmetric mo-
lecular structures for 4 and 5, as indicated in Scheme 1. The 1H
NMR spectrum for 4 shows only one methyl resonance for the
triazenido ligand at 3.37 ppm and no resonances in the region
associated with a tantalum-bound methyl ligand. Similarly, the
resonance for the tantalum-bound methyl at 59.7 ppm in the 13C
NMR spectrum of 2 is replaced by a diagnostic NꢀCH3
resonance at 42.4 ppm in the 13C NMR of 4. The (ONOcat)3ꢀ
ligand is symmetric, and the tolyl groups of the triazenido ligands
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Scheme 2
1
are equivalent in 4 on the NMR time scale. Similarly, the H
NMR spectrum of 5 shows resonances for the (ONOcat)3ꢀ
ligand and for the tolyl groups of the triazenido ligand that are
nearly identical with those of 4. The ipso carbon resonance for
the tantalum-bound phenyl ligands of 3 disappears in the 13C
NMR spectrum of 5, and the phenyl protons are shifted upfield to
6.8, 7.0, and 7.5 ppm in the 1H NMR spectrum of 5.
Scheme 2. When the reaction between 2 and 1 equiv of aniline was
conducted at room temperature, ca. 50% conversion to [(ONOcat)-
Ta(μ2-NPh)(NH2Ph)]2 was observed with the remainder of 2
recovered. When aniline, pyridine, and 2 were combined in a single
flask and heated, the pyridine adduct (ONOcat)TaMe2(py) was
isolated without conversion to 8. The diphenyl complex 3 also
reacted with aniline and pyridine to form 8 in high yields.
The initial reaction between 1,2-diphenylhydrazine and 2
resulted in the loss of 1 equiv of methane and the formation of
a tantalum hydrazido complex formulated as (ONOcat)TaMe-
(η2-NPhNHPh) (9). As depicted in Scheme 2, addition of 1,2-
diphenylhydrazine to a chilled pentane solution of 2 resulted in
the immediate precipitation of 9 as a bright yellow powder in
good yield. The presence of an intact TaꢀCH3 group in 9 was
confirmed by single resonances at 0.98 and 44.2 ppm in the 1H
and 13C NMR spectra, respectively. A single NꢀH resonance
Treatment oftantalum dimethyl complex2 or tantalum diphenyl
complex 3 with 1 equiv of organoazide resulted in nearly quanti-
tative formation of the monotriazenido complex. In the case of 2,
the reaction with 1 equiv of p-MeC6H4N3 afforded (ONOcat)-
TaMe(η2-MeNNN-p-C6H4Me) (6) in nearly quantitative yield
after 12 h at room temperature. Similarly, diphenyl complex 3
reacted to completion upon addition of 1 equiv of p-MeC6H4N3
at 25 °C to afford (ONOcat)TaPh(η2-PhNNN-p-C6H4Me) (7).
Addition of 2 equiv of p-MeC6H4N3 to either 2 or 3 resulted only
in the formation of the monoinsertion products after 12 h at
25 °C; however, when the mixtures were heated to 50 °C, a
second insertion reaction occurred to afford the bis(triazenido)
complexes 4 and 5, respectively. Complexes 6 and 7 were readily
identified by their 1H and 13C NMR spectra. The (ONOcat)3ꢀ
1
was observed at 5.43 ppm in the H NMR spectrum, and the
corresponding vibration appeared at 3214 cmꢀ1 in the IR spec-
trum. Four resonances for the tert-butyl groups of the (ONOcat)3ꢀ
ligand indicated a C1-symmetric molecule, consistent with η2
hapticity of the hydrazido ligand. While 9 was stable in the solid
state, solutions of the complex slowly released methane, pre-
cluding the isolation of X-ray-quality crystals for an absolute
structural determination. Attempts to elucidate the orientation of
the hydrazido ligand by 2-D NMR techniques were inconclusive.
Given the modest stability of 9 to methane loss, we favor the
isomer of 9 shown in Scheme 2, with the remaining NꢀH located
trans to the TaꢀMe bond.
1
ligands in 6 and 7 are symmetric according to the H NMR
spectra, indicating approximate Cs symmetry on the NMR time
scale. The proton resonance for the tantalum-bound methyl
ligand of 6 is shifted downfield relative to 2 by 0.5 ppm (to 1.28
ppm). In the case of 7, the 13C NMR resonance for the ipso
carbon of the phenyl ligand is barely shifted relative to 3,
appearing at 202.4 ppm. Attempts to obtain quantitative inser-
tion rate data under pseudo-first-order conditions were unsuc-
cessful, as the initial insertion (to give 4 or 5) was too fast to
monitor by NMR spectroscopy and the second insertion (to give
6 or 7) was complicated by secondary reactions with excess
p-MeC6H4N3. Thermolysis experiments aimed at forming a tanta-
lum imide complex from 4, 5, 6, or 7 resulted in decomposition of
the triazenido complex without measurable release of N2 gas.
NꢀH Protonolysis Reactivity of (ONOcat)TaR2. As pre-
viously reported,14 2 reacts with aniline derivatives to afford
binuclear tantalum complexes with bridging arylimido ligands.
The bridging anilide complex [(ONOcat)Ta(μ2-NPh)(py)]2 (8)
was prepared by heating a 1:1 solution of 2 and aniline in benzene
at reflux for 8 h followed by addition of pyridine, as shown in
The loss of methane from solutions of 9 is accelerated by the
presence of stoichiometric quantities of ligands such as pyridine,
THF, and even phosphines. Yellow benzene solutions of 9
containing 1 equiv of pyridine slowly turned reddish purple
and afforded 8 as the final tantalum-containing product, along
1
with /2 equiv of azobenzene, as summarized in Scheme 2.
Solutions of diphenyl complex 3 and 1,2-diphenylhydrazine
release 2 equiv of benzene at ambient temperature (30 °C) with
no isolable intermediate analogous to 9; however, the addition of
pyridine to these reaction solutions afforded quantative yields of
8 and azobenzene, consistent with Scheme 2.
Generation and Reactivity of a (ONOcat)TaIII Fragment.
The formation of 8 and azobenzene as the terminal products
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from reactions of 2 or 3 with 1,2-diphenylhydrazine suggests the
intermediacy of a tantalum(III) fragment. Equations 2ꢀ5 sum-
marize the series of reactions needed to convert 2 equiv of 9 into
8 and azobenzene. In eq 2, elimination of methane from 9 would
form a species with the composition (ONOcat)Ta(N2Ph2), con-
sistent with the observed reactivity of 9 in solution. Three plausible
formulations for the (ONOcat)Ta(N2Ph2) species would be as a
TaVꢀ(diazenido)2ꢀ complex, a TaIVꢀ(diazenido)•ꢀ complex, ora
TaIIIꢀ(diazene) complex. Regardless of the electronic formulation
of this species, loss of the neutral diazene PhNdNPh would
afford a (ONOcat)TaIII fragment, as shown in eq 3. Such a
tantalum(III) species would be expected to be strongly reducing,
and its addition to an intact (ONOcat)Ta(N2Ph2) species (eq 4)
would assemble the bimetallic tantalum imido core of 8. Final
trapping of the bimetallic tantalum imido core by pyridine (eq 5)
would be consistent with the preparation of 8 from 2 and aniline
as presented above.
Scheme 3
Equations 3 and 4 represent the key redox reactions leading to
the formation of 8 and azobenzene and implicate a (ONOcat)TaIII
intermediate. To test the plausibility of such a tantalum(III)
species, KC8 reductions of 1 were examined in various solvents.
The reduction of 1 with 2 equiv of KC8 in THF resulted in the
formation of a yellow-orange solution. The tantalum-containing
product was a tantalum(V) complex containing a ring-opened
THF molecule, [(ONOcat)Ta{μ2-O(CH2)3CH2}]2 (10), ob-
tained in modest yields after recrystallization from pentane
(Scheme 3). The 1H NMR spectrum of 10 showed a symmetric
t
(ONOcat)3ꢀ ligand with aromatic and Bu resonances that are
shifted downfield slightly relative to those of 2. The ring-opened
THF molecule gave rise to four well-defined resonances in the 1H
NMR spectrum at 4.74 (triplet), 2.48 (quintet), 1.86 (multiplet),
and 1.25 ppm (triplet). The diagnostic 13C NMR resonance for
the metal-bound methylene carbon was observed at 74.3 ppm.38ꢀ40
For reactions carried out in THF, the presence of a stoichio-
metric amount or excess azobenzene had no effect on the
outcome of the reaction, and 10 was the only product observed.
A single-crystal X-ray structure of 10 revealed the dimeric
nature of the ring-opened THF product. Figure 3 shows 10 as an
ORTEP drawing, and Table 3 gives selected metrical parameters
for the primary coordination sphere of the tantalum center. The
complex crystallizes in the monoclinic space group P21/n with
two crystallographically unique, but chemically equivalent, mol-
ecules of 10 in the asymmetric unit. The bimetallic complex is
best described as an edge-sharing bioctahedron with the oxygen
atoms of the ring-opened THF molecules occupying the bridging
positions. The (ONOcat)3ꢀ ligands are planar and parallel to one
another in the dimer. All bond distances within the (ONOcat)3ꢀ
ligands are consistent with their assignment to the catecholate
oxidation state. The ring-opened THF molecules chelate to a
single tantalum center to form a six-membered ring that sits in a
boat conformation with a TaꢀC distance longer than that
observed in the structure of dimethyl complex 2. The bridging
oxygen atoms are nearly trigonal planar, having a sum of angles of
359°. Surprisingly, the TaꢀO distance within the six-membered
chelate ring is longer (Ta(1)ꢀO(3) = 2.13 Å) than the TaꢀO
Figure 3. ORTEP drawing of [(ONOcat)Ta{μ2-O(CH2)3CH2}]2
(10). Thermal ellipsoids are drawn at the 50% probability level. A
crystallographically distinct but chemically equivalent second molecule
of 10, pentane solvent molecules, and hydrogen atoms have been
omitted for clarity.
distance to the adjacent metal center (Ta(1)ꢀO(3)0 = 2.08 Å),
even though the latter TaꢀO bond is located opposite the
strongly trans influencing alkyl of the ring-opened THF
molecule.
When 1 was reduced with potassium graphite in the absence of
THF, the putative (ONOcat)TaIII fragment could be trapped
with azobenzene. To avoid the reaction of the (ONOcat)TaIII
fragment with THF, 1 was treated with KC8 in cold toluene
followed by 1/2 equiv of PhNdNPh. When it was warmed to room
temperature, the solution was reddish purple, consistent with the
formation of the bimetallic tantalum imido core. Addition of
pyridine to the solution afforded 8 in 77% isolated yield, indicating
that two (ONOcat)TaIII fragments are capable of the four-
electron reduction of dizobenzene.
’ CONCLUSIONS
The reaction of tantalum dimethyl and diphenyl complexes 2
and 3 with aryl azides to form triazenido complexes was unex-
pected but not unprecedented. The related tantalum complex
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Table 3. Selected Bond Distances (Å) and Angles (deg) for
out using standard vacuum-line, Schlenk-line, and glovebox techniques.
Solvents were sparged with argon before being deoxygenated and dried
by passage through Q5 and activated alumina columns, respectively. To
test for effective oxygen and water removal, aliquots of each solvent were
treated with a few drops of a purple solution of sodium benzophenone
ketyl radical in THF. 1,2-Diphenylhydrazine was purchased from Alfa
Aesar and sublimed to remove traces of azobenzene before use. Aryl
azides46,47 and (ONOcat)TaCl2(OEt2) (1)14 were synthesized by lit-
erature procedures.
Spectroscopic Measurements. NMR spectra were collected on
Bruker Avance 600 and 500 MHz spectrometers in dry, degassed C6D6
or CDCl3. 1H NMR spectra were referenced to tetramethylsilane
(TMS) using the residual protio impurities of the solvent; 13C NMR
spectra were referenced to TMS using the natural-abundance 13C impu-
rities of the solvent. All chemical shifts are reported using the standard δ
notation in parts per million; positive chemical shifts are to a higher
frequency from the given reference. Infrared spectra were recorded as
KBr pellets with a Perkin-Elmer Spectrum One FTIR spectrophotometer.
Electronic absorption spectra were recorded with a Perkin-Elmer Lambda
800 UV/vis spectrophotometer in 1 cm path length cells at 25 °C.
X-ray Data Collection and Reduction. X-ray diffraction data for
[(ONOcat)Ta{μ2-O(CH2)3CH2}]2 (10)
Bond Distances/Å
Ta(1)ꢀN(1)
Ta(1)ꢀO(1)
Ta(1)ꢀO(2)
Ta(1)ꢀO(3)
Ta(1)ꢀO(3)’
Ta(1)ꢀC(32)
2.064(6)
1.923(5)
1.938(5)
2.130(5)
2.083(5)
2.171(9)
Ta(2)ꢀN(2)
Ta(2)ꢀO(4)
Ta(2)ꢀO(5)
Ta(2)ꢀO(6)
Ta(2)ꢀO(6)’
Ta(2)ꢀC(64)
2.080(6)
1.937(6)
1.938(5)
2.143(5)
2.064(6)
2.155(9)
Bond Angles/deg
O(3)ꢀTa(1)ꢀO(3)’
O(3)ꢀTa(1)ꢀC(32)
Ta(1)ꢀO(3)ꢀTa(1)’
68.2(2)
O(6)ꢀTa(2)ꢀO(6)’
O(6)ꢀTa(2)ꢀC(64)
Ta(2)ꢀO(6)ꢀTa(2)’
68.1(3)
82.3(3)
82.3(3)
111.8(2)
111.9(3)
[NNNcat]TaCl2 reacts smoothly with aryl azides: the donation of
two electrons from the redox-active ligand allows reduction of
the azide and formation of a tantalumꢀimido bond. In the case of
2 and 3, the aryl azide must insert into the TaꢀC bond faster than
the two-electron oxidation that leads to N2 extrusion and formation
of the imido ligand. While azide insertions to give triazenido species
have been observed for hydride complexes of the early transition
metals,35 the analogous alkyl complexes have been shown to be
robust to azide insertion,28,29,36 a direct reflection of the much lower
migratory aptitude of methyl versus hydride ligands. In the case of 2
and 3, the migration must be relatively facile, as it quantitatively
beats out nitrene transfer to the tantalum center.
4 and 10 2.6C5H12 were collected for single crystals mounted on a glass
3
fiber and held in place with Paratone oil using a Bruker CCD platform
diffractometer equipped with a CCD detector. Measurements were
carried out at at 143 K using Mo KR (λ = 0.710 73 Å) radiation, which
was wavelength-selected with a single-crystal graphite monochromator.
X-ray diffraction data for 2 were collected on a Bruker D8 platform
diffractometer equipped with an APEX CCD detector at UC San Diego.
Measurements were carried out at 208 K using Mo KR (λ = 0.710 73 Å)
radiation, which was wavelength-selected with a single-crystal graphite
monochromator. The SMART program package was used to determine
unit-cell parameters and for data collection. The raw frame data were
processed using SAINT and SADABS to yield the reflection data file.
Subsequent calculations were carried out using the SHELXTL program.
Analytical scattering factors for neutral atoms were used throughout the
Another interesting feature of the (ONOcat)Ta platform is the
apparent accessibility of the tantalum(III) oxidation state. As
expected, 1,2-diphenylhydrazine reacts with 2 or 3 by sequential
protonolysis reactions to lose methane or benzene. The expected
product of these reactions was a (ONOcat)Ta(PhNNPh) com-
plex, but instead reductive elimination of azobenzene occurred to
liberate the putative (ONOcat)TaIII fragment. In the reaction with 1,2-
diphenylhydrazine, the reduced fragment can be trapped by a second
(ONOcat)Ta(PhNNPh) to break the NꢀN bond and form imido
dimer 8. The reactivity of the (ONOcat)TaIII fragment was further
illustrated by its ability to cleave a CꢀO bond of THF to afford 10, a
reaction that is observed for highly reduced complexes of the early
transition metals39,41,42 but is unreported for tantalum(III).43,44
Finally, two (ONOcat)TaIII fragments react with azobenzene
in the absence of THF to give 8, highlighting the ability of this
platform to mediate four-electron reactivity. Previous results
showed that the four-electron oxidation of 8 resulted in the
extrusion of azobenzene with the formation of (ONOcat)TaCl2-
(py).14 Taken together, these two reactions indicate that the
(ONO)Ta platform remains intact through an overall four-
electron series with minimal changes to the molecular geometry:
(ONOcat)TaIII, (ONOcat)TaV, (ONOq)TaV ((ONOq)ꢀ = 3,5-
di-tert-butyl-1,2-quinone-1-(2-hydroxy-3,5-di-tert-butyl-phenyl)-
imine). Such a multielectron sequence is rare in mononuclear
transition-metal complexes45 but in this case is enabled through
the cooperation between a redox-active ligand and a tantalum
metal center.
analyses. Hydrogen atoms in 2, 4, and 10 2.6C5H12 were generated in
3
calculated positions and refined using a riding model. ORTEP diagrams
were generated using ORTEP-3 for Windows.48 Diffraction data for 2, 4,
and 10 2.6C5H12 are shown in Table 4.
3
(ONOcat)TaMe2 (2). Compound 2 was prepared by an alternative
method to the previously published procedure.14 A 20 mL scintillation
vial equipped with a stir bar was charged with a red solution of 1 (0.419 g,
0.559 mmol, 1 equiv) in diethyl ether (18 mL). The solution was frozen
in a liquid-nitrogen cold well, and then immediately upon thawing it was
treated with a 1.37 M solution of MeLi in diethyl ether (0.839 mL,
1.15 mmol, 2 equiv). The mixture was then stirred as it was warmed to
ambient temperature (30 °C). After it was stirred for 3 h, the mixture was
filtered and the volatiles were removed under reduced pressure. The
resulting yellow-brown residue was treated with 3 mL of pentane and
chilled (ꢀ35 °C) so that 2 could be isolated by filtration as a yellow
powder. Yield: 0.278 g (78%). Characterization data for 2 prepared by
this method matched the data previously reported for the complex.14
(ONOcat)Ta(Ph)2 (3). A 20 mL scintillation vial equipped with a stir
bar was filled with a red solution of 1 (0.164 g, 0.20 mmol, 1 equiv) in
diethyl ether (15 mL) and frozen in a liquid-nitrogen-filled cold well. To
the recently thawed solution was added solid PhLi (0.037 g, 0.44 mmol,
2.2 equiv), and the mixture was then stirred as it was warmed to ambient
temperature. After it was stirred for 16 h, the dark red solution was
filtered to remove LiCl, and the filtrate was dried under reduced
pressure. The resulting dark red residue was titurated with pentane
(5 ꢁ 5 mL) to remove diethyl ether and extracted with cyclopentane
(10 mL). The solution was concentrated down (∼3 mL) and cooled
to ꢀ35 °C to induce 3 to precipitate as a dull red powder that was
isolated by filtration and dried under vacumm. Yield: 0.168 g (92%).
’ EXPERIMENTAL PROCEDURES
General Considerations. All compounds and reactions reported
below are both air and moisture sensitive. All manipulations were carried
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Table 4. Crystal Data Collection and Refinement Parameters
(ONOcat)TaMe2 (2)
(ONOcat)Ta(η2-(p-tolyl)NNNMe)2 (4)
[(ONOcat)Ta(O(CH2)3CH2)]2 (10 2.6C5H12
)
3
empirical formula
formula wt
cryst syst
space group
a, Å
C30H46NO2Ta
633.63
C44H60N7O2Ta
899.94
C77H127N2O6Ta2
1539.91
orthorhombic
Pnma
monoclinic
C2/c
monoclinic
P21/n
11.752(3)
26.728(8)
9.396(3)
90
19.9660(15)
17.1104(13)
12.9627(10)
90
18.4766(14)
17.3171(13)
23.2598(18)
9
b, Å
c, Å
R, deg
β, deg
90
101.5030(10)
90
94.7570(10)
90
γ, deg
90
V, Å3
2951.5(15)
4
4339.2(6)
4
7416.6(10)
4
Z
F(000)
1288
1848
3189
no. of rflns collected
no. of indep rflns (Rint
R1 (I > 2σI)a
wR2 (all data)a
17 455
16 377
81 444
)
3634 (0.0437)
0.0320
4743 (0.0218)
0.0224
17 181 (0.0557)
0.0654
0.0757
2
0.0572
0.1554
a R1 = ∑||Fo| ꢀ |Fc||/∑|Fo|; wR2 = [∑[w(Fo ꢀ Fc2)2]/∑[w(Fo ) ]]1/2; GOF = [∑w(|Fo| ꢀ |Fc|)2/(n ꢀ m)]1/2
.
2 2
Anal. CalcdforC40H50NO2Ta: C, 63.40; H, 6.65; N, 1.85. Found: C, 60.67; H,
6.90; N, 1.83. 1H NMR (600 MHz; C6D6;298K;δ/ppm):8.18(d,3JHH =6.6,
4H, aryl H), 7.70 (d, 4JHH = 1.8, 2H, aryl H), 7.20 (t, 3JHH = 7.8 Hz, 4H, aryl
H), 7.04 (d, 4JHH = 1.8 Hz, 2H, aryl H), 7.02 (t, 3JHH = 7.8 Hz, 2H, aryl H),
1.58 (s, 18H, tBu), 1.44 (s, 18H, tBu). 13C NMR (128 MHz; C6D6; 298 K; δ/
ppm): 202.9 (aryl C), 154.5 (aryl C), 148.3 (aryl C), 145.6 (aryl C), 136.9
(aryl C), 134.4 (aryl C), 131.7 (aryl C), 115.4 (aryl C), 110.7 (aryl C), 35.0
(C(CH3)3), 34.7 (C(CH3)3), 32.0 (C(CH3)3), 30.4 (C(CH3)3). UV/vis
(C6H6; λmax/nm (ε/Mꢀ1 cmꢀ1)): 295 (52 500).
C6D6; 298 K; δ/ppm): 156.6 (aryl C), 145.4 (aryl C), 143.3 (aryl C),
135.4 (aryl C), 130.2 (aryl C), 129.7 (aryl C), 129.0 (aryl C), 128.7 (aryl
C), 127.1 (aryl C), 119.1 (aryl C), 110.2 (aryl C), 34.8 (C(CH3)3), 34.2
(C(CH3)3, 31.8 (C(CH3)3), 29.3 (C(CH3)3). UV/vis (C6H6; λmax/nm
(ε/Mꢀ1 cmꢀ1)): 309 (29 521), 478 (16 243).
(ONOcat)TaMe(η2-MeNNN-p-C6H4Me) (6). A 20 mL scintilla-
tion vial equipped with a stirbar was charged with a yellow solution of 2
(0.133 g, 0.209 mmol, 1 equiv) in benzene (15 mL) and stirred as N3-
(p-C6H4Me) (0.028 g, 0.21 mmol, 1 equiv) was slowly dripped into the
solution at ambient temperature. After the mixture was stirred for 12 h,
the solvent was removed under reduced pressure and the resulting
orange residue triturated with pentane (3 ꢁ 5 mL) to give 6 as a dull
orange powder (yield 0.160 g, 99%). Anal. Calcd for C37H53N4O2Ta: C,
57.96; H, 6.97; N, 7.31. Found: C, 57.60; H, 6.90; N, 7.28. 1H NMR (600
MHz; C6D6; 298 K; δ/ppm): 7.81 (s, 2H, aryl H), 7.17 (d, 3JHH = 10.2
Hz, 2H, aryl H), 7.06 (s, 2H, aryl H), 6.62 (d, 3JHH = 10.2, 2H, aryl H),
3.18 (s, 3H, N-CH3), 1.88 (s, 3H, N(p-C6H4-CH3), 1.46 (s, 18H, tBu),
1.42 (s, 18H, tBu), 1.28 (s, 3H, Ta-CH3). 13C NMR (128 MHz; C6D6;
298 K; δ/ppm): 156.0 (aryl C), 145.8 (aryl C), 145.5 (aryl C), 144.3
(aryl C), 134.6 (aryl C), 134.0 (aryl C), 130.0, (aryl C), 116.7 (aryl C),
116.3 (aryl C), 110.8 (aryl C), 56.5 (Ta-CH3), 42.4 (N-CH3), 35.1
(C(CH3)3), 34.7 (C(CH3)3), 32.0 (C(CH3)3), 29.8 (C(CH3)3), 20.7
(Ar-CH3). UV/vis (C6H6; λmax/nm (ε/Mꢀ1 cmꢀ1)): 292 (18 800).
(ONOcat)TaPh(η2-PhNNN-p-C6H4Me) (7). A 20 mL scintilla-
tion vial equipped with a stirbar was charged with a dark red solution of 3
(0.130 g, 0.172 mmol, 1 equiv) in benzene (10 mL) and stirred as N3-
(p-tolyl) (0.023 g, 0.172 mmol, 1 equiv) wasslowly drippedintothe solution
at ambient temperature. The reaction mixture was then stirred for 12 h,
after which time the solvent was stripped under vacuum and the resulting
residue was suspended in 5 mL of pentane before cooling to ꢀ35 °C.
Compound 7 was isolated by filtration as a dark red powder and dried
under vacuum. Yield: 0.148 g (99%). Anal. Calcd for C47H57N4O2Ta: C,
63.36; H, 6.45; N, 6.29. Found: C, 62.56; H, 6.71; N, 5.36. 1H NMR (600
MHz; C6D6; 298 K; δ/ppm): 8.10 (d, 3JHH = 6.6 Hz, 2H, aryl H), 7.80
(d, 4JHH = 1.8 Hz, 2H, aryl H), 7.30 (d, 3JHH = 7.8 Hz, 2H, aryl H), 7.22
(d, 3JHH = 8.4 Hz, 2H, aryl H), 7.16 (d, 3JHH = 7.8 Hz, 2H, aryl H), 7.06
(d, 4JHH = 1.8 Hz, 2H, aryl H), 7.01 (t, 3JHH = 8.4 Hz, 2H, aryl H), 6.93
(t, 3JHH = 7.8 Hz, 1H, aryl H), 6.79 (t, 3JHH = 7.8 Hz, 1H, aryl H), 6.75
(d, 3JHH = 7.8 Hz, 2H, aryl H), 6.68, (d, 3JHH = 8.4 Hz, 1H, aryl H), 1.93
(ONOcat)Ta(η2-MeNNN-p-C6H4Me)2 (4). A 25 mL Straus tube
equipped with a stir bar was charged with a yellow solution of 2 (0.141 g,
0.22 mmol, 1 equiv) and N3(p-C6H4Me) (0.059 g, 0.44 mmol, 2 equiv)
in benzene (10 mL). The solution was stirred at 50 °C for 1 h as the
yellow-orange solution darkened to red. The solvent was removed under
vacuum, and the resulting residue was washed with pentane to give 4 as a
dull red powder (yield 0.119 g, 60%). X-ray-quality crystals of 4 were
grown from chilled (ꢀ35 °C) saturated pentane solutions. Anal. Calcd
for C44H60N7O2Ta: C, 58.72; H, 6.72; N, 10.89. Found: C, 57.42; H,
6.92; N, 10.80. 1H NMR (600 MHz; C6D6; 298 K; δ/ppm): 7.88 (s, 2H,
aryl H), 7.37 (d, 3JHH = 8.4 Hz, 2H, aryl H), 7.12 (s, 3JHH = 1.8 Hz, 2H,
aryl H), 6.78 (d, 3JHH = 8.4 Hz, 2H, aryl H), 3.37 (s, 6H, N-CH3), 1.95
t
t
(s, 6H, N(p-C6H4-CH3), 1.44 (s, 18H, Bu), 1.40 (s, 18H, Bu). 13C
NMR (128 MHz; C6D6; 298 K; δ/ppm): 156.4 (aryl C), 145.1 (aryl C),
143.9 (aryl C), 134.9 (aryl C), 130.5 (aryl C), 129.9 (aryl C), 119.1 (aryl
C), 118.3 (aryl C), 117.3 (aryl C), 110.4 (aryl C), 42.4 (N-CH3), 35.0
(C(CH3)3), 34.6 (C(CH3)3), 32.2 (C(CH3)3), 29.8 (C(CH3)3), 20.8
(Ar-CH3). UV/vis (C6H6; λmax/nm (ε/Mꢀ1 cmꢀ1)): 300 (24 000).
(ONOcat)Ta(η2-PhNNN-p-C6H4Me)2 (5). A 25 mL Straus tube
equipped with a stir bar was charged with a red solution of 3 (0.106 g,
0.14 mmol, 1 equiv) and N3(p-C6H4Me) (0.039 g, 0.28 mmol, 2 equiv)
in benzene (10 mL). The solution was stirred at 50 °C for 1 h. The
solvent was stripped under vacuum, and the resulting dark red residue
was suspended in 5 mL of pentane before it was chilled to ꢀ35 °C.
Compound 5 was isolated as a dull red-purple solid by filtration (yield
0.140 g, 97.7%). Anal. Calcd for C54H64N7O2Ta: C, 63.33; H, 6.30; N,
9.57. Found: C, 61.90; H, 6.00; N, 9.10. 1H NMR (600 MHz; C6D6; 298
K; δ/ppm): 7.94 (s, 2H, aryl H), 7.53 (br, 7H, aryl H), 7.14 (s, 2H, aryl
C), 6.98 (m, 4H, aryl H), 6.80 (m, 7H, aryl H), 1.92 (s, 6H, N(p-C6H4-
t
t
CH3), 1.42 (s, 18H, Bu), 1.26 (s, 18H, Bu). 13C NMR (128 MHz;
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(s, 3H, (p-C6H4-CH3), 1.40 (s, 18H, tBu), 1.38 (s, 18H, tBu). 13C NMR
(128 MHz; C6D6; 298 K; δ/ppm): 202.4 (aryl C), 156.6 (aryl C), 146.8
(aryl C),145.9 (aryl C), 145.2 (aryl C), 144.3 (aryl C), 135.9 (aryl C),
134.9 (aryl C), 134.1 (aryl C), 130.5 (aryl C), 129.8 (aryl C), 129.5
(aryl C), 129.2 (aryl C), 129.0 (aryl C), 125.8 (aryl C), 119.1 (aryl C),
118.5 (aryl C), 117.9 (aryl C), 111.1 (aryl C), 35.0 (C(CH3)3),
34.7 (C(CH3)3), 31.9 (C(CH3)3), 30.0 (C(CH3)3), 20.9 (N(p-C6H4-
CH3). UV/vis (C6H6; λmax/nm (ε/Mꢀ1 cmꢀ1)): 293 (24 200), 415 (3150).
[(ONOcat)Ta(μ-NPh)(py)]2 (8). Method A. A 50 mL round-
bottom flask equipped with a stirbar was charged with a toluene slurry
(10 mL) of KC8 (0.034 g, 0.25 mmol, 1.9 equiv) and frozen in a liquid-
nitrogen-filled cold well. As the slurry thawed, a recently thawed red
solution of 1 (0.100 g, 0.13 mmol, 1 equiv) in toluene (10 mL) was
added rapidly and the mixture was stirred for 10 min before an orange
solution of PhNNPh (0.012 g, 0.06 mmol, 0.5 equiv) in toluene (5 mL)
was slowly dripped in. The solution was stirred for 1 h as it was slowly
warmed to ambient temperature. The graphite was removed by filtration
to give a purple-red solution that was treated with pyridine (0.011 mL,
0.13 mmol, 1 equiv) and concentrated (∼2 mL) before 20 mL of
pentane was added and the solution was cooled to ꢀ35 °C, resulting in
the precipitation of 8 as a purple-brown solid that was isolated by
filtration and dried under vacuum. Yield: 0.077 g (77%).
(ONOcat)Ta(η2-O(CH2)3CH2) (10). A 50 mL round-bottom flask
equipped with a stirbar was charged with a slurry of KC8 (0.098 g, 0.73
mmol, 2.2 equiv) in THF (10 mL) and frozen in a liquid-nitrogen-filled
cold well. As the slurry thawed, a recently thawed red solution of 1
(0.248 g, 0.33 mmol, 1 equiv) in THF (20 mL) was rapidly added and
the mixture was stirred as it was warmed to ambient temperature for 1 h.
The graphite was removed by filtration to give an orange-red solution
that was dried under reduced pressure. The resulting orange residue was
dissolved in pentane and cooled to ꢀ35 °C to give 10 as a yellow-orange
solid that was isolated by filtration and dried under vacuum (yield 0.145
g, 65%). Anal. Calcd for C64H96N2O6Ta2: C, 56.88; H, 7.16; N, 2.07.
Found: C, 55.52; H, 7.35; N, 2.24. 1H NMR (600 MHz; C6D6; 298 K; δ/
ppm): 7.83 (s, 2H, aryl H), 7.11 (s, 2H, aryl H), 4.74 (t, 3JHH = 4.8 Hz,
2H, OCH2ꢀ), 2.48 (quin, 3JHH =5.9Hz, 2H, ꢀCH2ꢀ), 1.86(m,3JHH =6.6
5.4 Hz, 2H, TaꢀCH2ꢀ). 13C NMR (128 MHz; C6D6; 298 K; δ/ppm):
155.7 (aryl C), 145.1 (aryl C), 144.9 (aryl C), 134.9 (aryl C), 116.7 (aryl C),
110.7 (aryl C), 74.8 (O-CH2), 74.3 (Ta-CH2), 35.1 (C(CH3)3, 34.9
(C(CH3)3, 32.1 (C(CH3)3), 32.0 (CH2), 30.5 (CH2), 26.4 ((C(CH3)3).
UV/vis (C6H6; λmax/nm (ε/Mꢀ1 cmꢀ1)): 300 (15000), 410 (2300).
Protonolysis of (ONOcat)TaR2 (R = Me (2), Ph (3)) with 1,2-
Diphenylhydrazine. A 100 mL Schlenk flask equipped with a stirbar and
a reflux condenser was charged with 10 mL of an orange-red solution of
2 (0.113 g, 0.14 mmol, 1 equiv), 1,2-diphenylhydrazine (0.026 g, 0.14 mmol,
1 equiv), and pyridine (0.011 mL, 0.14 mmol, 1 equiv), and the mixture was
then refluxed and stirred for 6 h. The solution was cooled for 1 h before an
aliquot was removed from the solution and placed in a 1 mL volumetric flask
with 100 μL of an internal standard (biphenyl) and diluted to 1 mL with
benzene. The remaining red-purple solution was concentrated (∼2 mL) and
treated with 20 mL of pentane. The resulting dark purple-brown precipitate
was isolated by filtration and identified as 8 (yield 0.095 g, 87%).
A similar procedure was performed with 3 (0.060 g, 0.08 mmol,
1 equiv), 1,2-diphenylhydrazine (0.015 g, 0.08 mmol, 1 equiv), and
pyridine (0.007 mL, 0.08 mmol, 1 equiv) to give 8 (yield 0.055 g, 88%).
Decomposition of (ONOcat)TaMe(η2-PhNNHPh). A 100 mL
Straus tube equipped with a stirbar was charged with a bright yellow
solution of 9 (0.113 g, 0.141 mmol, 1 equiv) in benzene (10 mL), and
pyridine (0.011 mL, 0.14 mmol, 1 equiv) was slowly dripped into the
solution, resulting in a rapid color change to red. The solution was stirred
at 65 °C for 6 h, resulting in a color change of the solution to purple-red.
The solution was cooled for 1 h before an aliquot was removed from the
solution and placed in a 1 mL volumetric flask with 100 μL of an internal
standard (biphenyl) and diluted to 1 mL with benzene. The sample was
analyzed by GC/MS to determine the yield of aryl diazene. The remaining
red-purple solution was concentrated (∼2 mL) and treated with 20 mL
of pentane. The resulting dark purple-brown precipitate was isolated by
filtration and identified as 8 (yield 0.090 g, 83%).
Hz, 2H, ꢀCH2ꢀ), 1.60 (s, 18H, tBu), 1.46 (s, 18H, tBu), 1.25 (quin, 3JHH
=
Method B. A 50 mL Schlenk flask equipped with a stirbar and a reflux
condenser was charged with a yellow solution of 2 (0.315 g, 0.49 mmol,
1 equiv) and aniline (0.045 mL, 0.49 mmol, 1 equiv) in benzene (20 mL).
The solution was heated to reflux and stirred for 8 h. The purple-red
solution was cooled to room temperature for 1 h before pyridine
(0.040 mL, 0.49 mmol, 1 equiv) was added via syringe, resulting in
the development of a purple-brown color. After it was stirred for 12 h,
the mixture was concentrated down (∼4 mL) and 15 mL of pentane was
cannula-transferred into the flask, resulting in the precipitation of 8 as a
purple-brown solid that was isolated by filtration and dried under
vacuum. Yield: 0.210 g (65%). Anal. Calcd for C78H100N6O4Ta2: C,
60.54; H, 6.51; N, 5.43. Found: C, 60.34; H, 6.71; N, 5.40. 1H NMR (500
MHz; CDCl3; 298 K; δ/ppm): 8.25 (d, 3JHH = 5 Hz, 4H, aryl H), 7.37
(t, 3JHH = 7.5 Hz, 2H, aryl H), 6.95 (s, 4H, aryl H), 6.88 (t, 3JHH = 6.5 Hz,
4H, aryl H), 6.63 (s, 4H, aryl H), 6.55 (t, 3JHH = 7.5 Hz, 4H, aryl H), 6.26
(d, 3JHH = 7.5 Hz, 4H, aryl H), 6.24 (t, 3JHH = 7.5 Hz, 4H, aryl H), 1.55
(s, 36H, tBu), 1.22 (s, 36H, tBu). 13C NMR (77 MHz; CDCl3; 298 K; δ/
ppm): 157.0 (aryl C), 155.6 (aryl C), 147.7 (aryl C), 142.4 (aryl C),
142.3 (aryl C), 134.1 (aryl C), 127.0 (aryl C), 124.4 (aryl C), 123.7 (aryl
C), 114.8 (aryl C), 110.3 (aryl C), 34.7 (C(CH3)3), 34.5 (C(CH3)3),
31.7 (C(CH3)3), 30.6 (C(CH3)3). UV/vis (toluene; λmax/nm (ε/
M
ꢀ1 cmꢀ1)): 298 (46 000), 354 (20 600), 445 (7800).
(ONOcat)TaMe(η2-PhNNHPh) (9). A 20 mL scintillation vial was
charged with a cold (ꢀ35 °C) yellow solution of 2 (0.200 g, 0.32 mmol,
1 equiv) in pentane (15 mL) before it was treated with solid 1,2-
diphenylhydrazine (0.058 g, 0.32 mmol, 1 equiv), resulting in efferves-
cence. The solution was shaken for 5 min as a bright yellow precipitate
formed that was isolated by filtration. The precipitate was washed with
pentane and identified as 9 (yield 0.228 g, 90%). Anal. Calcd for
C41H54N3O2Ta: C, 61.41; H, 6.79; N, 5.24. Found: C, 60.81; H, 6.88;
N, 5.33. 1H NMR (600 MHz; C6D6; 298 K; δ/ppm): 7.96 (s, 2H, aryl
H), 7.11 (s, 2H, aryl H), 6.93 (br s, 4H, aryl H), 6.81 (br s, 4H, aryl H),
6.69 (br s, 2H, aryl H), 5.43 (s, 1H, NꢀH), 1.56 (s, 9H, tBu), 1.51 (s, 9H,
’ ASSOCIATED CONTENT
S
Supporting Information. CIF files giving X-ray crystal-
b
lographic data for 2, 4, and 10 2.6C5H12 as well as figures giving
3
1H NMR spectra for all new complexes. This material is available
free of charge via the Internet at http://pubs.acs.org.
t
t
tBu), 1.49 (s, 9H, Bu), 1.39 (s, 9H, Bu), 0.98 (s, 3H, Ta-CH3). 13C
NMR (128 MHz; C6D6; 298 K; δ/ppm): 155.7 (aryl C), 155.3 (aryl C),
148.6 (aryl C), 145.9 (aryl C), 145.5 (aryl C), 144.6 (aryl C), 144.5 (aryl
C), 143.4 (aryl C), 134.6 (aryl C), 134.3 (aryl C), 129.7 (aryl C), 129.4
(aryl C), 125.4 (aryl C), 122.6 (aryl C), 117.5(aryl C), 116.9 (aryl C),
115.5 (aryl C), 115.3 (aryl C), 110.6 (aryl C), 110.5 (aryl C), 44.2 (Ta-
CH3), 35.1 (C(CH3)), 34.8 (C(CH3)), 34.6 (C(CH3)), 32.2 (C(CH3)),
32.2 (C(CH3)), 30.2 (C(CH3)), 30.0 (C(CH3)). IR (KBr; νNH/cmꢀ1):
3214. UV/vis (C6H6; λmax/nm (ε/Mꢀ1 cmꢀ1)): 295 (15 000).
’ AUTHOR INFORMATION
Corresponding Author
*E-mail: aheyduk@uci.edu.
’ ACKNOWLEDGMENT
We thank Dr. Joe Ziller (UCI), Dr. Arnie Rheingold, and
Dr. Antonio DiPasquale (UCSD) for assistance with X-ray
4897
dx.doi.org/10.1021/om2004332 |Organometallics 2011, 30, 4890–4898

Organometallics
ARTICLE
crystallography. The NSF CAREER Program (No. CHE-
0645685) funded this work. A.F.H. is a Camille Dreyfus Tea-
cher-Scholar.
(33) Steffey, B. D.; Fanwick, P. E.; Rothwell, I. P. Polyhedron 1990,
9, 963–968.
(34) Bhattacharya, S.; Gupta, P.; Basuli, F.; Pierpont, C. G. Inorg.
Chem. 2002, 41, 5810–5816.
(35) Hillhouse, G. L.; Goeden, G. V.; Haymore, B. L. Inorg. Chem.
1982, 21, 2064–2071.
(36) Guzei, I. A.; Liable-Sands, L. M.; Rheingold, A. L.; Winter, C. H.
Polyhedron 1997, 16, 4017–4022.
’ REFERENCES
(1) Smith, A. L.; Hardcastle, K. I.; Soper, J. D. J. Am. Chem. Soc. 2010,
132, 14358–60.
(37) Barrett, A. G.; Crimmin, M. R.; Hill, M. S.; Hitchcock, P. B.;
Kociok-K€ohn, G.; Procopiou, P. A. Inorg. Chem. 2008, 47, 7366–76.
(38) Matsunaga, M. T.; Hillhouse, G. L.; Rheingold, A. L. J. Am.
Chem. Soc. 1993, 115, 2075–2077.
(2) Ringenberg, M. R.; Kokatam, S. L.; Heiden, Z. M.; Rauchfuss,
T. B. J. Am. Chem. Soc. 2008, 130, 788–789.
(3) Ringenberg, M. R.; Nilges, M. J.; Rauchfuss, T. B.; Wilson, S. R.
Organometallics 2010, 29, 1956–1965.
(39) Bradley, C. A.; Veiros, L. F.; Pun, D.; Lobkovsky, E.; Keresztes,
I.; Chirik, P. J. J. Am. Chem. Soc. 2006, 128, 16600–12.
(40) Beweries, T.; J€ager-Fiedler, U.; Bach, M. A.; Burlakov, V. V.;
Arndt, P.; Baumann, W.; Spannenberg, A.; Rosenthal, U. Organometallics
2007, 26, 3000–3004.
(41) Bradley, C. A.; Veiros, L. F.; Chirik, P. J. Organometallics 2007,
26, 3191–3200.
(42) Wong, E. W.; Walsby, C. J.; Storr, T.; Leznoff, D. B. Inorg. Chem.
2010, 49, 3343–50.
(43) Wallace, K. C.; Dewan, J. C.; Schrock, R. R. Organometallics
1986, 5, 2162–2164.
(44) Wallace, K. C.; Liu, A. H.; Dewan, J. C.; Schrock, R. R. J. Am.
Chem. Soc. 1988, 110, 4964–4977.
(45) Betley, T. A.; Peters, J. C. J. Am. Chem. Soc. 2004, 126,
6252–6254.
(46) Smith, P. A. S.; Brown, B. B. J. Am. Chem. Soc. 1951, 73,
2438–2441.
(47) Barral, K.; Moorhouse, A. D.; Moses, J. E. Org. Lett. 2007,
(4) Chaudhuri, P.; Hess, M.; Weyhermuller, T.; Wieghardt, K.
Angew. Chem., Int. Ed. 1999, 38, 1095–1098.
(5) Chaudhuri, P.; Hess, M.; M€uller, J.; Hildenbrand, K.; Bill, E.;
Weyherm€uller, T.; Wieghardt, K. J. Am. Chem. Soc. 1999, 121, 9599–9610.
(6) Paine, T. K.; Weyherm€uller, T.; Wieghardt, K.; Chaudhuri, P.
Dalton Trans. 2004, 2092–101.
(7) Bart, S. C.; Lobkovsky, E.; Chirik, P. J. J. Am. Chem. Soc. 2004,
126, 13794–13807.
(8) Bart, S. C.; Chzopek, K.; Bill, E.; Bouwkamp, M. W.; Lobkovsky,
E.; Neese, F.; Wieghardt, K.; Chirik, P. J. J. Am. Chem. Soc. 2006,
128, 13901–12.
(9) Muckerman, J. T.; Polyansky, D. E.; Wada, T.; Tanaka, K.; Fujita,
E. Inorg. Chem. 2008, 47, 1787–802.
(10) Tsai, M. K.; Rochford, J.; Polyansky, D. E.; Wada, T.; Tanaka,
K.; Fujita, E.; Muckerman, J. T. Inorg. Chem. 2009, 48, 4372–4383.
(11) Boyer, J. L.; Rochford, J.; Tsai, M. K.; Muckerman, J. T.; Fujita,
E. Coord. Chem. Rev. 2010, 254, 309–330.
(12) Wada, T.; Muckerman, J. T.; Fujita, E.; Tanaka, K. Dalton Trans.
2011, 40, 2225–33.
(13) Blackmore, K. J.; Ziller, J. W.; Heyduk, A. F. Inorg. Chem. 2005,
9, 1809–11.
(48) Farrugia, L. J. J. Appl. Crystallogr. 1997, 30, 565–565.
44, 5559–61.
(14) Zarkesh, R. A.; Ziller, J. W.; Heyduk, A. F. Angew. Chem., Int. Ed.
2008, 47, 4715–8.
(15) Blackmore, K. J.; Sly, M. B.; Haneline, M. R.; Ziller, J. W.;
Heyduk, A. F. Inorg. Chem. 2008, 47, 10522–32.
(16) Ketterer, N. A.; Fan, H.; Blackmore, K. J.; Yang, X.; Ziller, J. W.;
Baik, M. H.; Heyduk, A. F. J. Am. Chem. Soc. 2008, 130, 4364–74.
(17) Nguyen, A. I.; Blackmore, K. J.; Carter, S. M.; Zarkesh, R. A.;
Heyduk, A. F. J. Am. Chem. Soc. 2009, 131, 3307–3316.
(18) Nguyen, A. I.; Zarkesh, R. A.; Lacy, D. C.; Thorson, M. K.;
Heyduk, A. F. Chem. Sci. 2011, 2, 166–169.
(19) Walsh, P. J.; Hollander, F. J.; Bergman, R. G. J. Am. Chem. Soc.
1988, 110, 8729–8731.
(20) Cummins, C. C.; Baxter, S. M.; Wolczanski, P. T. J. Am. Chem.
Soc. 1988, 110, 8731–8733.
(21) Zambrano, C. H.; Fanwick, P. E.; Rothwell, I. P. Organometallics
1994, 13, 1174–1177.
(22) Arney, D. S. J.; Burns, C. J. J. Am. Chem. Soc. 1995, 117,
9448–9460.
(23) Cenini, S.; Gallo, E.; Caselli, A.; Ragaini, F.; Fantauzzi, S.;
Piangiolino, C. Coord. Chem. Rev. 2006, 250, 1234–1253.
(24) Nugent, W. A.; Haymore, B. L. Coord. Chem. Rev. 1980,
31, 123–175.
(25) Wigley, D. E. Prog. Inorg. Chem. 1994, 42, 239–482.
(26) Gambarotta, A. J. Organomet. Chem. 1984, 270, C49–C52.
(27) Osborne, J. H.; Rheingold, A. L.; Trogler, W. C. J. Am. Chem.
Soc. 1985, 107, 7945–7952.
(28) Chiu, K. W.; Wilkinson, M. G. Polyhedron 1984, 3, 79–85.
(29) Luker, T.; Whitby, R. J.; Webster, M. J. Organomet. Chem. 1995,
492, 53–57.
(30) Evans, W. J.; Walensky, J. R.; Ziller, J. W.; Rheingold, A. L.
Organometallics 2009, 28, 3350–3357.
(31) Hillhouse, G. L.; Bercaw, J. E. Organometallics 1982, 1,
1025–1029.
(32) Watanabe, T.; Matsuo, T.; Kawaguchi, H. Inorg. Chem. 2006,
45, 6580–2.
4898
dx.doi.org/10.1021/om2004332 |Organometallics 2011, 30, 4890–4898