Carbon radical generation by d0 tantalum complexes with α-diimine ligands through ligand-centered redox processes

Article abstract of DOI:10.1021/ja204665s

High-valent tantalum complexes having redox-active α-diimine ligands, (α-diimine)TaCl_n (n = 3, 4), are prepared by the reaction of TaCl₅, α-diimine ligands, and an organosilicon-based reductant, 1-methyl-3,6-bis-(trimethylsilyl)-1,4-cyclohexadiene. Reductive cleavage of the C-Cl bond of polyhaloalkanes is accomplished by trichlorotantalum complexes having dianionic α-diimine ligands via electron transfer from the dianionic ligands, whereas oxidative decomposition of tetraphenylborate is observed using tetrachlorotantalum complexes with monoanionic α-diimine ligands through electron transfer to the monoanionic ligands. Chemically oxidized or reduced complexes of (α-diimine)TaCl₄ are isolated as ligand-centered redox products, [Cp₂Co][(α-diimine)TaCl ₄] and [(α-diimine)TaCl₄][WCl₆], where the α-diimine ligand coordinates to the metal center as a dianionic or neutral ligand, respectively. On the basis of EPR measurements of (α-diimine)TaCl₄ complexes (which are key intermediates for reductive cleavage of C-Cl bond and oxidative decomposition of tetraphenylborate), two redox isomers-a tantalum-centered radical and ligand-localized radical-are present in solution.

Full text of DOI:10.1021/ja204665s

ARTICLE
pubs.acs.org/JACS
Carbon Radical Generation by d0 Tantalum Complexes with r-Diimine
Ligands through Ligand-Centered Redox Processes
Hayato Tsurugi,† Teruhiko Saito,† Hiromasa Tanahashi,† John Arnold,‡ and Kazushi Mashima*,†
†Department of Chemistry, Graduate School of Engineering Science, Osaka University, Toyonaka, Osaka 560-8531, Japan
‡Department of Chemistry, University of California, Berkeley, California 94720, United States
S Supporting Information
b
ABSTRACT: High-valent tantalum complexes having redox-active α-diimine
ligands, (α-diimine)TaCln (n = 3, 4), are prepared by the reaction of TaCl5, α-
diimine ligands, and an organosilicon-based reductant, 1-methyl-3,6-bis-
(trimethylsilyl)-1,4-cyclohexadiene. Reductive cleavage of the CÀCl bond of
polyhaloalkanes is accomplished by trichlorotantalum complexes having dia-
nionic α-diimine ligands via electron transfer from the dianionic ligands,
whereas oxidative decomposition of tetraphenylborate is observed using tetrachlorotantalum complexes with monoanionic α-
diimine ligands through electron transfer to the monoanionic ligands. Chemically oxidized or reduced complexes of (α-
diimine)TaCl4 are isolated as ligand-centered redox products, [Cp2Co][(α-diimine)TaCl4] and [(α-diimine)TaCl4][WCl6],
where the α-diimine ligand coordinates to the metal center as a dianionic or neutral ligand, respectively. On the basis of EPR
measurements of (α-diimine)TaCl4 complexes (which are key intermediates for reductive cleavage of CÀCl bond and oxidative
decomposition of tetraphenylborate), two redox isomers a tantalum-centered radical and ligand-localized radical are present in
;
;
solution.
Chart 1. Three Electronic Structures of α-Diimine Ligands
’ INTRODUCTION
Transition metal complexes may mediate bond formation and
bond cleavage by utilizing redox processes. One-electron redox
processes at a metal center are considered to be radical reactions,
whereas oxidative addition and reductive elimination reactions
are events accompanied by two electron redox at the metal.1 In
addition, ligand-centered redox processes have attracted much
interest as alternatives to metal-centered redox cycles because of
increased flexibility on designing catalysts.2À6 Recent progress
for utilizing redox-active metal and ligand combination, such as
bis(imino)pyridine iron(II)3 and bis(aminophenolate) cobalt-
(II)4 complexes, enable us to use first-row transition metals as
catalysts for carbonÀcarbon bond-forming reactions. In some
instances, it has been shown that ligand-localized π-radical species
are involved as key intermediates. Furthermore, several research
groups have focused on electrophilic d0 early transition metal
complexes supported by redox-active ligands, and oxidative
addition of halogens and dioxygen and reductive elimination
from dialkyl species are possible through formation of open-
shell ligand π-radical species as intermediates.6d,k Because of the
stability of high oxidation states of early transition metals, it is
possible to study purely ligand-based redox events and the
character of open-shell ligand π-radicals using d0-metal complexes.6
The 1,4-diaza-1,3-butadiene (α-diimine) class of ligands are
important in this regard because of (i) their flexibility on tuning
the coordination modes suitable to transition metals and (ii)
their one-electron or two-electron reductions, producing π-
radical monoanions and ene-diamide dianions, respectively
(Chart 1).5h,k,l,o,7,8 Based on the redox reactivity of the α-diimine
ligands, Abu-Omar et al. reported a multielectron transfer
process from the α-diimine ligands to dioxygen on the central
metal leading to η2-peroxo formation for (α-diimine)2Zr
complexes.6b In addition, (α-diimine)MClx complexes were an-
ticipated to be an interesting platform to investigate ligand-based
valence changes in d0 early transition metal complexes. However,
the preparation of (α-diimine)MClx via simple salt metathesis
reactions has been problematic, with intractable mixtures ham-
pering the isolation of desired products.9 These results impeded
precise evaluation of the role of α-diimine ligands during the
redox reaction.
We recently reported that 1-methyl-3,6-bis(trimethylsilyl)-
1,4-cyclohexadiene (MBTCD) acted as a unique reducing re-
agent for early transition metal halides, such as NbCl5 and TaCl5, to
give low valent species without any salt contamination.10 In situ
generated low-valent species reacted smoothly with redox-active
Received: May 21, 2011
Published: October 07, 2011
r
2011 American Chemical Society
18673
dx.doi.org/10.1021/ja204665s J. Am. Chem. Soc. 2011, 133, 18673–18683
|

Journal of the American Chemical Society
ARTICLE
α-diimine ligands to form (α-diimine)TaClx complexes. Herein
we report the reductive cleavage of the carbonÀhalogen bond of
polyhaloalkanes through electron transfer from the dianionic α-
diimine ligands to polyhaloalkanes. The generated radicals add
to the α-diimine ligand backbone or styrene to form new CÀC
bonds. Oxidative decomposition of tetraphenylborate proceeds
via transfer of one electron from the borate anion to the mono-
anionic α-diimine ligand. To the best of our knowledge, this is
the first example of controlled radical generation from poly-
haloalkanes and organoborate via redox reactions using d0 metal
complexes with redox active ligands. Furthermore, tantalum
complexes having open-shell ligand π-radicals are fully char-
acterized by EPR measurements and X-ray diffraction studies.
MHz, C6D5Br, 308 K) δ 14.2 (NC(CH3), 23.9 (CH(CH3)2), 24.3
(CH(CH3)2), 28.1 (CH(CH3)2), 110.0 (CH3CdCCH3), 123.7 (Ar),
127.7 (Ar), 142.0 (Ar), 145.0 (Ar). Anal. Calcd for C28H40Cl3N2Ta: C,
48.60; H, 5.83; N, 4.05. Found: C, 48.68; H, 6.04; N, 3.75.
Synthesis of [{(r-Diimine)TaCl}2(μ-Cl)3][TaCl6] (3c). A to-
luene (20 mL) solution of 1-methyl-3,6-bis(trimethylsilyl)-1,4-cyclohexad-
iene (0.17 mL, 0.72 mmol) and 1c (0.36 g, 0.72 mmol) was added to
TaCl5 (0.38 g, 1.1 mmol) in toluene (50 mL) at room temperature. The
color of the solution changed to brown. The reaction mixture was stirred
for 16 h, and then all volatiles were removed under reduced pressure.
The solid was washed with hexane (3 Â 10 mL) and then dried under
vacuum to give 3c as a brown powder in 84% yield (0.63 g, 0.30 mmol),
mp 287À289 °C (dec). 1H NMR (400 MHz, C6D5Br, 308 K) δ 0.90 (d,
J = 6.8 Hz, 12H, CH(CH3)2), 0.94 (d, J = 6.8 Hz, 12H, CH(CH3)2), 1.49
(d, J = 6.8 Hz, 12H, CH(CH3)2), 1.52 (d, J = 6.8 Hz, 12H, CH(CH3)2),
2.46 (m, J = 6.9 Hz, 4H, CH(CH3)2), 4.08 (m, J = 6.6 Hz, 4H,
CH(CH3)2), 7.16 (d, J = 7.8 Hz, 4H, 3-H of acenaphthylene), 7.52 (t,
J = 4.3 Hz, 4H, p-NAr), 7.54 (t, J = 7.8 Hz, 4H, 4-H of acenaphthylene),
7.73 (d, J = 4.3 Hz, 8H, m-NAr), 7.83 (d, J = 7.8 Hz, 4H, 5-H of
acenaphthylene). 13C NMR (100 MHz, C6D5Br, 308 K) δ 23.4
(CH(CH3)2), 23.9 (CH(CH3)2), 24.2 (CH(CH3)2), 25.8 (CH-
(CH3)2), 28.1 (CH(CH3)2), 28.5 (CH(CH3)2), 109.1 (NÀCd
CÀN), 124.85 (p-NAr), 124.89 (m-NAr), 126.9 (acenaphthylene),
127.7 (acenaphthylene), 128.1 (acenaphthylene), 130.0 (m-NAr),
130.4 (acenaphthylene), 140.0 (ipso-NAr), 141.8 (acenaphthylene),
144.3 (o-NAr), 144.6 (o-NAr). Anal. Calcd for C74H84Cl15N4Ta3-
(CH2Cl2)2: C, 42.24; H, 4.02; N, 2.66; Found: C, 41.88; H, 4.00; N,
2.67. UVÀvis (CH2Cl2) λmax/nm (ε/MÀ1 cmÀ1): 331 (2.25 Â 104).
Synthesis of (Enamido-imino)TaCl4 (4a). Carbon tetrachloride
(31.7 μL, 0.32 mmol) was added to a solution of complex 2a (226 mg,
0.32 mmol) in toluene (50 mL) at room temperature. The reaction
mixture was stirred for 4 h at 60 °C, and then all volatiles were removed
under reduced pressure to give yellow solid. The solid was washed with
hexane (3 Â 10 mL) and dried to give 4a as a yellow powder in 89% yield
’ EXPERIMENTAL SECTION
General. All manipulations involving air- and moisture-sensitive
organometallic compounds were carried out under argon using standard
Schlenk techniques or in an argon-filled glovebox. 1-Methyl-3,6-bis-
(trimethylsilyl)-1,4-cyclohexadiene (MBTCD)11 and α-diimine ligands
(1aÀ1c)12 were prepared according to literature procedures. Cp2Co,
and WCl6 were purchased and used as received. Anhydrous hexane,
toluene, THF, and dichloromethane were purchased from Kanto
Chemical and further purified by passage through activated alumina
under positive argon pressure as described by Grubbs et al.13 Benzene-
d6, toluene-d8, and bromobenzene-d5 were distilled from CaH2 and
degassed before use. 1H NMR (300 MHz, 400 MHz) and 13C NMR (75
MHz, 100 MHz) spectra were measured on Varian UNITY INOVA-300
1
and Bruker AVANCEIII-400 spectrometers. Assignments for H and
1
13C NMR peaks for some of the complexes were aided by 2D 1HÀ H
13
13
COSY, 2D 1HÀ C HMQC, and 2D 1HÀ C HMBC spectra. The EPR
spectrum was recorded at 298 K on a Bruker EMX-10/12 spectrometer
in toluene, CH2Cl2, and THF. Cyclic voltammograms were recorded in
a glovebox at room temperature in CH2Cl2 solution containing 0.1 M
[nBu4N][PF6] or 0.1 M [nBu4N][BF4] as the supporting electrolyte.
GCÀMS measurement was carried out using a DB-1 capillary column
(0.25 mm  30 m) on a Shimadzu GCMS-QP2010Plus. IR spectra were
recorded on a JASCO FT/IR-230 spectrometer. All melting points were
measured in sealed tubes under argon atmosphere. Elemental analyses
were recorded by using Perkin-Elmer 2400 at the Faculty of Engineering
Science, Osaka University.
1
(199 mg, 0.28 mmol), mp 199À203 °C (dec). H NMR (400 MHz,
C6D5Br, 308 K) δ 1.46 (d, J = 6.6 Hz, 6H, CH(CH3)2), 1.55 (d, J = 6.6
Hz, 6H, CH(CH3)2), 1.89 (d, J = 6.6 Hz, 6H, CH(CH3)2), 1.94 (d, J =
6.6 Hz, 6H, CH(CH3)2), 2.34 (s, 3H, NC(CH3)C), 3.71 (sept, J = 6.6
Hz, 2H, CH(CH3)2), 3.13 (sept, J = 6.6 Hz, 2H, CH(CH3)2), 4.62 (br d,
1H, NCdCH2), 5.33 (br d, 1H, NCdCH2), 7.5À7.8 (m, 6H, aromatic
protons). 13C NMR (100 MHz, C6D5Br, 308 K) δ 19.1 (NdCCH3),
24.5 (CH(CH3)2), 24.5 (CH(CH3)2), 24.6 (CH(CH3)2), 26.2
(CH(CH3)2), 28.1(CH(CH3)2), 28.5 (CH(CH3)2), 114.9 (NCd
CH2), 124.2 (m-Ar), 125.2 (m-Ar), 127.7 (Ar), 128.3 (Ar), 129.5
(Ar), 140.2 (o-Ar), 144.3 (Ar), 145.2 (o-Ar), 153.1 (NCdCH2), 182.0
(CdN). Anal. Calcd for C28H39Cl4N2Ta(CH2Cl2): C, 42.93; H, 5.09;
N, 3.45. Found: C, 43.30; H, 5.09; N, 3.50. UVÀvis (toluene) λmax/nm
(ε/MÀ1 cmÀ1): 341 (5.52 Â 103).
Synthesis of (r-Diimine)TaCl3 (2b). A solution of 1b (2.9 g, 7.8
mmol) in toluene (10 mL) was added to a solution of TaCl5 (2.8 g, 7.8
mmol) in toluene (50 mL) at room temperature. The color of the
solution changed to deep red. After the mixture was stirred for 10 min,
1-methyl-3,6-bis(trimethylsilyl)-1,4-cyclohexadiene (2.0 mL, 8.4 mmol)
was added. The reaction mixture was stirred for 12 h, and then all
volatiles were removed under reduced pressure to give brown solid. The
solid was washed with hexane (3 Â 10 mL). The remaining solid was
dried to give yellow powder of 2b in 92% yield (4.8 g, 7.2 mmol), mp
210À214 °C (dec). 1H NMR (400 MHz, C6D6, 308 K) δ 1.13 (d, J = 6.8
Hz, 12H, CH(CH3)2), 1.31 (d, J = 6.8 Hz, 12H, CH(CH3)2), 2.99 (sept,
J = 6.8 Hz, 4H, CH(CH3)), 5.67 (s, 2H, HCdCH), 7.10À7.15 (m, 6H,
aromatic protons). 13C NMR (100 MHz, C6D6, 308 K) δ 24.4
(CH(CH3)2), 24.7 (CH(CH3)2), 28.8 (CH(CH3)2), 104.8 (HCdCH),
124.3 (m-Ar), 129.0 (p-Ar), 144.3 (o-Ar), 145.9 (ipso-Ar). Anal. Calcd
for C26H36Cl3N2Ta: C, 47.04; H, 5.47; N, 4.22. Found: C, 46.76; H, 5.69;
N, 4.14. UVÀvis (toluene) λmax/nm (ε/MÀ1 cmÀ1): 403 (1.33 Â 103).
Complex 2a was prepared in similar manner as 2b. An orange powder
was obtained in 86% yield, mp 170À171 °C (dec). 1H NMR (400 MHz,
C6D5Br, 308 K) δ 1.46 (d, J = 6.8 Hz, 12H, CH(CH3)2), 1.61 (d, J = 6.8
Hz, 12H, CH(CH3)2), 2.45 (s, 4H, NC(CH3)), 3.24 (sept, J = 6.8 Hz,
4H, CH(CH3)2), 7.35À7.5 (m, 6H, aromatic protons). 13C NMR (100
Synthesis of (Cl3C-Amido-imino)TaCl4 (5b). Carbon tetra-
chloride (47.5 μL, 0.49 mmol) was added to a solution of complex 2b
(325 mg, 0.49 mmol) in toluene (5 mL) at room temperature. This
reaction mixture was stirred for 4 h at 60 °C, and then all volatiles were
removed under reduced pressure to give yellow solid. The solid was
washed with hexane (3 Â 10 mL) and dried to give 5b as a yellow
powder in 83% yield (332 mg, 0.41 mmol), mp 146À147 °C (dec). 1H
NMR (400 MHz, C6D6, 308 K) δ 0.99 (d, J = 6.8 Hz, 3H, CH(CH3)2),
1.01 (d, J = 6.8 Hz, 3H, CH(CH3)2), 1.20 (d, J = 6.8 Hz, 3H,
CH(CH3)2), 1.37 (d, J = 6.8 Hz, 6H, CH(CH3)2), 1.44 (d, J = 6.8
Hz, 3H, CH(CH3)2), 1.47 (d, J = 6.8 Hz, 3H, CH(CH3)2), 1.67 (d, J =
6.8 Hz, 3H, CH(CH3)2), 3.47 (sept, J = 6.8 Hz, 1H, CH(CH3)2), 3.60
(sept, J = 6.8 Hz, 1H, CH(CH3)2), 3.76 (sept, J = 6.8 Hz, 1H,
CH(CH3)2), 3.94 (sept, J = 6.8 Hz, 1H, CH(CH3)2), 7.04 (s, 1H,
CCl3CH), 7.0À7.1 (m, 6H, aromatic protons), 8.98 (s, 1H, NdCH).
13C NMR (100 MHz, C6D6, 308 K) δ 23.3 (CH(CH3)2), 24.2
18674
dx.doi.org/10.1021/ja204665s |J. Am. Chem. Soc. 2011, 133, 18673–18683

Journal of the American Chemical Society
ARTICLE
(CH(CH3)2), 25.4 (CH(CH3)2), 25.6 (CH(CH3)2), 25.8 (CH-
(CH3)2), 25.9 (CH(CH3)2), 26.1 (CH(CH3)2), 27.5 (CH(CH3)2),
28.7 (CH(CH3)2), 28.9 (CH(CH3)2), 29.4 (CH(CH3)2), 30.2 (CH-
(CH3)2), 89.6 (NCH(CCl3)CH), 101.5 (CCl3), 124.7 (Ar), 124.9 (Ar),
126.3 (Ar), 128.1 (Ar), 129.5 (Ar), 131.1 (Ar), 141.1 (Ar), 142.3 (Ar),
142.4 (Ar), 145.7 (Ar), 147.9 (Ar), 151.0 (Ar), 180.8 (CdN). Anal.
Calcd for C27H36Cl7N2Ta (C6H5CH3)0.25: C, 41.07; H, 4.56; N, 3.33.
Found: C, 41.12; H, 4.68; N, 3.39.
color of the solution changed to deep blue. The reaction mixture was
stirred for 14 h, and then volatiles were removed under reduced pressure
to give a blue solid. The solid was washed with hexane (3 Â 10 mL) and
then dried to give 8b as a blue powder in 77% yield (424 mg, 0.61 mmol),
mp 184À186 °C (dec). EPR (toluene): g = 2.0031 (Aiso = 7.0 G), g =
1.9332 (Aiso = 58 G). Anal. Calcd for C26H36Cl4N2Ta; C, 44.65; H, 5.19;
N, 4.01. Found: C, 44.86; H, 5.21; N, 3.98. λmax/nm (ε/MÀ1 cmÀ1):
403 (1.33 Â 103), 593 (2.09 Â 103).
Complex (Cl2HC-amido-imino)TaCl4 (6b) was prepared in a similar
manner as 5b. A yellow powder was obtained in 62% yield, mp 132À133 °C
(α-Diimine)TaCl4 (8a) was prepared in a similar manner as 8b. A
deep blue powder was obtained in 35% yield, mp 191À192 °C (dec).
EPR (toluene): g = 1.9313 (Aiso = 65 G). Anal. Calcd for C28H40-
Cl4N2Ta; C, 46.23; H, 5.54; N, 3.85. Found: C, 45.98; H, 5.42; N, 3.77.
λmax/nm (ε/MÀ1 cmÀ1): 624 (1.01 Â 103).
1
(dec). H NMR (400 MHz, C6D6, 308 K) δ 0.92 (d, J = 6.8 Hz, 3H,
CH(CH3)2), 1.05 (d, J = 6.8 Hz, 3H, CH(CH3)2), 1.10 (d, J = 6.8 Hz, 3H,
CH(CH3)2), 1.18 (d, J = 6.8 Hz, 3H, CH(CH3)2), 1.37 (d, J = 6.8 Hz, 3H,
CH(CH3)2), 1.44 (d, J = 6.8 Hz, 3H, CH(CH3)2), 1.48 (d, J = 6.8 Hz, 3H,
CH(CH3)2), 1.55 (d, J = 6.8 Hz, 3H, CH(CH3)2), 3.41 (sept, J = 6.8 Hz,
1H, CH(CH3)2), 3.72 (sept, J= 6.8 Hz, 1H, CH(CH3)2), 3.77 (sept, J=6.8
Hz, 1H, CH(CH3)2), 4.34 (sept, J = 6.8 Hz, 1H, CH(CH3)2), 5.52 (d, J =
1.8 Hz, 1H, Cl2CHÀ), 6.76 (br, 1H, Cl2CHCH), 7.0À7.15 (m, 6H,
aromatic protons), 8.72 (br, 1H, NdCH). 13C NMR (100 MHz, C6D6,
308 K) δ 23.4 (CH(CH3)2), 24.2 (CH(CH3)2), 25.2 (CH(CH3)2), 25.6
(CH(CH3)2), 25.9 (CH(CH3)2), 25.9 (CH(CH3)2), 26.4 (CH(CH3)2),
26.9 (CH(CH3)2), 28.5 (CH(CH3)2), 28.8 (CH(CH3)2), 29.1 (CH-
(CH3)2), 29.3 (CH(CH3)2), 71.6 (Cl2CHCH), 86.4 (Cl2CHÀ), 124.6
(Ar), 125.0 (Ar), 126.6 (Ar), 127.3 (Ar), 129.4 (Ar), 130.4 (Ar), 141.2
(Ar), 142.3 (Ar), 145.3 (Ar), 145.7 (Ar), 148.11 (Ar), 148.12 (Ar), 180.0
(CdN). Anal. Calcd for C27H37Cl6N2Ta: C, 41.40; H, 4.76; N, 3.58.
Found: C, 41.58; H, 5.21; N, 3.48.
Polymerization of Styrene by Complex 2a. Styrene (0.51 mL,
4.9 mmol) and carbon tetrachloride (4.9 μL, 0.049 mmol) were added to
a solution of complex 2a (34 mg, 0.049 mmol) in toluene (5 mL) at
room temperature. The reaction mixture was stirred for 18 h at 60 °C.
The mixture was added to excess of MeOH, yielding a white precipitate
of poly(styrene) (0.12 g, 26% yield), which was filtered and dried (Mn =
1.9 Â 103, Mn/Mw = 1.3).
(α-Diimine)TaCl4 (8c) was prepared in a similar manner as 8b. A
deep green powder was obtained in 89% yield, mp 213À215 °C (dec).
EPR (toluene): g = 2.0026 (Aiso = 6.1 G), g = 1.9152 (Aiso = 60 G). Anal.
Calcd for C36H40Cl4N2Ta(C6H5CH3); C, 56.41; H, 5.28; N, 3.06.
Found: C, 56.53; H, 5.09; N, 3.42. λmax/nm (ε/MÀ1 cmÀ1): 336 (1.65
 104), 686 (3.35  103).
Reduction of Complex 8b Leading to [Cp2Co][(α-Diimine)
TaCl4] (9b). A solution of Cp2Co (37.8 mg, 0.20 mmol) in toluene (5 mL)
was added to solution of complex 8b (139 mg, 0.20 mmol) in toluene (5 mL)
at room temperature. The color of the mixture turned to green. After 3 h of
stirring, the volatiles were removed under reduced pressure to give a green
solid that was washed with hexane (3 Â 10 mL). The remaining solid was
dried to give 9b as a green powder in 92% yield (163 mg, 0.18 mmol), mp
218À220 °C (dec). 1H NMR (400 MHz, CDCl3, 308 K) δ 1.16 (d, J = 6.5
Hz, 12H, CH(CH3)2), 1.39 (d, J = 6.5 Hz, 12H, CH(CH3)2), 4.29 (sept, J =
6.5 Hz, 4H, CH(CH3)2), 4.84 (br s, 2H, CHdCH), 5.51 (br s, 10H,
Cp2Co), 7.09 (br d, 2H, p-Ar), 7.19 (br t, 4H, o-Ar). 13C NMR (400 MHz,
CDCl3, 308K) δ23.8 (CH(CH3)2), 26.7(CH(CH3)2), 27.9(CH(CH3)2),
85.1 (Cp2Co), 122.6 (HCdCH), 123.7 (m-Ar), 126.6 (p-Ar), 146.8 (Ar),
150.2 (Ar). Anal. Calcd for C36H46Cl4CoN2Ta; C, 48.67; H, 5.22; N, 3.15.
Found: C, 49.18; H, 5.58; N, 2.99. λmax/nm (ε/MÀ1 cmÀ1):395(5.51Â 103).
Oxidation of Complex 8b Leading to [(α-Diimine)TaCl4]-
[WCl6] (10b). A solution of WCl6 (86.6 mg, 0.22 mmol) in toluene
(5 mL) was added to solution of complex 8b (154 mg, 0.22 mmol) in
toluene (5 mL) at room temperature. The color of the mixture turned to red.
After stirring for 3 h, the volatiles were removed under reduced pressure to
give reddish-brown solid. The solid was washed with hexane (3 Â 10 mL),
and dried vacuum to give 10b as reddish-brown powder in 45% yield (108
mg, 0.10 mmol), mp 134À136 °C (dec). 1H NMR (400 MHz, CDCl3,
308 K) δ 1.15 (d, J = 6.5 Hz, 12H, CH(CH3)2), 1.23 (d, J = 6.5 Hz, 12H,
CH(CH3)2), 3.02 (sept, J = 6.5 Hz, 4H, CH(CH3)2), 7.26 (m, 6H,
aromaticprotons), 9.53(brs, 2H, NdCH). 13CNMR(400 MHz, CDCl3,
308 K) δ23.6(CH(CH3)2), 28.0(CH(CH3)2), 30.5 (CH(CH3)2), 125.9
(Ar), 131.6 (p-Ar), 142.4 (Ar), 148.7 (Ar), 181.7 (NdC). Anal. Calcd for
C26H36Cl10N2TaW(C6H5CH3)0.5; C, 31.03; H, 3.53; N, 2.45. Found: C,
31.42; H, 3.78; N, 2.51. λmax/nm (ε/MÀ1 cmÀ1): 344 (1.69 Â 104).
Oxidation of Complex 9b Leading to (α-Diimine)TaCl4
(8b). A solution of WCl6 (30.2 mg, 0.08 mmol) in toluene (5 mL) was
added to a solution of complex 9b (68.0 mg, 0.08 mmol) in toluene (5 mL) at
À78 °C. The reaction mixture was allowed to slowly warm to ambient
temperature and was stirred for 3 h. The precipitate was filtered, and volatiles
were removed under reduced pressure to give 8b as a deep blue powder in
60% yield (33.6 mg, 0.05 mmol). The complex 8b was characterized by EPR
spectroscopy (see Figure S18a in Supporting Information).
Synthesis of (Cl3CCH2CH(Ph)-Amido-imino)TaCl4 (7b).
Styrene (0.33 mL, 2.9 mmol, 5 equiv) and carbon tetrachloride
(0.054 mL, 0.57 mmol) were added to a solution of complex 2b (0.38 g,
0.57 mmol) in toluene (8 mL) at room temperature. The reaction
mixture was stirred for 20 h at 60 °C, and then volatiles were removed
under reduced pressure to give yellow solid. The solid was washed with
hexane (3 Â 10 mL), and the solvent was evaporated to give 7b as a
yellow powder in 54% yield (0.28 g, 0.31 mmol), mp 157À159 °C (dec).
1H NMR (400 MHz, C6D6, 308 K) δ 0.79 (d, J = 6.8 Hz, 3H,
CH(CH3)2), 1.23 (d, J = 6.8 Hz, 3H, CH(CH3)2), 1.30 (d, J = 6.8
Hz, 6H, CH(CH3)2), 1.42 (d, J = 6.8 Hz, 3H, CH(CH3)2), 1.49 (d, J =
6.8 Hz, 3H, CH(CH3)2), 1.51 (d, J = 6.8 Hz, 3H, CH(CH3)2), 1.58 (d,
J = 6.8 Hz, 3H, CH(CH3)2), 3.12 (d, J = 14.8 Hz, 1H, PhCHCH2CCl3),
3.34 (sept, J = 6.8 Hz, 1H, CH(CH3)2), 3.42 (dd, J = 14.8 Hz, J = 10.3
Hz, 1H, PhCHCH2CCl3), 3.47 (m, J = 6.8 Hz, 2H, CH(CH3)2), 3.79
(m, J = 6.8 Hz, 1H, CH(CH3)2), 4.63 (d, J = 10.3 Hz, 1H, PhCH), 5.37
(d, J = 22.7 Hz, 1H, NCH2), 5.39 (d, J = 22.7 Hz, 1H, NCH2), 6.88À7.16
(m, 11H, aromatic protons). 13C NMR (100 MHz, C6D6, 308 K) δ 22.0
(CH(CH3)2), 23.6 (CH(CH3)2), 24.6 (CH(CH3)2), 25.1 (CH-
(CH3)2), 27.3 (CH(CH3)2), 27.7 (CH(CH3)2), 28.0 (CH(CH3)2),
28.6 (CH(CH3)2), 28.7 (CH(CH3)2), 29.2 (CH(CH3)2), 29.2
(CH(CH3)2), 29.7 (CH(CH3)2), 45.8 (PhCHCH2CCl3), 51.2 (PhCH-
CH2CCl3), 74.4 (ArNCH2), 96.5 (CCl3), 125.4 (Ar), 125.7 (Ar), 126.3
(Ar), 126.3 (Ar), 128.8 (Ar), 129.1 (Ar), 129.2 (Ar), 129.6 (Ar), 130.0
(Ar), 134.5 (Ar), 141.0 (Ar), 141.6 (Ar), 143.3 (Ar), 144.4 (Ar), 146.9 (Ar),
148.9 (Ar), 194 (CdN). Anal. Calcd for C35H44Cl7N2Ta: C, 45.60; H, 4.81;
N, 3.04; Found: C, 45.37; H, 5.05; N, 3.33.
Reduction of Complex 10b Leading to (α-Diimine)TaCl4
(8b). A solution of Cp2Co (19.4 mg, 0.10 mmol) in toluene (5 mL) was
added to a solution of complex 10b (112 mg, 0.10 mmol) in toluene
(5 mL) at À78 °C. The reaction mixture was allowed to slowly warm to
ambient temperature and was stirred for 3 h. The precipitate was filtered,
and volatiles were removed under reduced pressure to give 8b as a deep
blue powder in 54% yield (36.3 mg, 0.05 mmol). The complex 8b was
Synthesis of (r-Diimine)TaCl4 (8b). A solution of 1b (298 mg,
0.79 mmol) in toluene (10 mL) was added to a suspension of (TaCl4)n
(256 mg, 0.79 mmol) in toluene (10 mL) at room temperature. The
18675
dx.doi.org/10.1021/ja204665s |J. Am. Chem. Soc. 2011, 133, 18673–18683

Journal of the American Chemical Society
ARTICLE
characterized by EPR spectroscopy (see Figure S18b in Supporting
Information).
Scheme 1. Preparation of Tantalum Complexes with Dia-
nionic r-Diimine Ligands Using MBTCD as a Reductant
Decomposition of NaBPh4 by (r-Diimine)TaCl4 (8b).
NaBPh4 (45.0 mg, 0.13 mmol) was added to a solution of 8b (92.7
mg, 0.13 mmol) in CH2Cl2 (5 mL) at room temerature. The reaction
mixture was stirred for 24 h, over which time the color of the mixture
gradually changed to yellow. Naphthalene (18.0 mg, 0.14 mmol) was then
added to the reaction mixture as an internal standard for GC measure-
ment. The solution was quenched with water and the organic layer was
analyzed by GC and GCÀMS. Biphenyl was produced in 52% yield (as
determined by GC analysis). Formation of 8b was confirmed by the 1H
NMR spectrum (see Figure S2 and S3 in Supporting Information). A
similar reaction was observed for the mixture of 8a and NaBPh4.
Reaction of Complex 8a with AIBN Leading to 4a. Azobi-
sisobutyronitrile (26 mg, 0.16 mmol) was added to a solution of complex
8a (230 mg, 0.31 mmol) in toluene (5 mL) at room temperature. The
reaction mixture was stirred for 20 h at 60 °C, and then all volatiles were
removed under reduced pressure to give a red powder. The powder was
washed with hexane (3 Â 10 mL), and the remaining solid was dried to
give yellow powder of 4a in 72% yield (160 mg, 0.22 mmol). The 1H
NMR spectrum was consistent with that of 4a.
X-ray Crystallographic Analysis. All crystals were handled
similarly. The crystals were mounted on the CryoLoop (Hampton
Research Corp.) with a layer of light mineral oil and placed in a nitrogen
stream at 113(1) K. Measurements were made on Rigaku R-AXIS
RAPID imaging plate area detector or Rigaku AFC7R/Mercury CCD
detector with graphite-monochromated Mo Kα (0.71075 Å) radiation.
Crystal data and structure refinement parameters were listed below
(Table S1 in Supporting Information).
The structures of complexes 2b, 3c, 7b, 8b, 8c, and 9b were solved by
direct methods (SHELXS-97).14 The structure of complex 8a was solved
by direct methods (SIR92).15 The structures were refined on F2 by full-
matrix least-squares method, using SHELXL-97.14 Non-hydrogen atoms
were anisotropically refined. Hydrogen atoms were included in the
refinement on calculated positions riding on their carrier atoms. The
2
2
function minimized was [Σw(Fo À Fc2)2] (w = 1/[σ2 (Fo ) + (aP)2 +
Figure 1. Molecular structure of 2b with 30% thermal ellipsoids. All
hydrogen atoms are omitted for clarity. Selected bond distances (Å) and
angles (deg): TaÀN1, 1.968(6); TaÀCl1, 2.3778(18); TaÀCl2,
2.281(3); TaÀC1, 2.428(7); N1ÀC1, 1.376(9); C1ÀC1*, 1.406(13).
Dihedral angle between N1ÀTaÀN1* and N1ÀC1ÀC1*ÀN1* planes =
123.0°.
2
2
bP]), where P = (Max(Fo ,0) + 2Fc2)/3 with σ2(Fo ) from counting
statistics. The function R1 and wR2 were (Σ||Fo| À |Fc||)/Σ|Fo| and
2
4
[Σw(Fo À Fc2)2/Σ(wFo )]1/2, respectively. For the complex 3c, large
solvent accessible voids in the lattice were involved in the crystal packing,
but we could not find suitable solvent molecules due to the disordered
density. The molecular structure of 3c was shown in the Supporting
Information. The ORTEP-3 program was used to draw the molecule.16
In contrast, the reaction of in situ generated “TaCl3” with an
acenaphthene-based α-diimine ligand 1c gave a less soluble
tantalum complex 3c. The preliminary X-ray diffraction study
of 3c revealed the formation of the chloride-bridged dimeric
complex, [{(α-diimine)TaCl}2(μ-Cl)3][TaCl6].19 The 13C NMR
spectrum of 3c displayed a resonance for the N-bound acena-
phthylene carbon at δ 109.7, indicating the two electron reduc-
tion of 1c.
’ RESULTS AND DISCUSSION
Preparation of Tantalum Complexes with Dianionic r-
Diimine Ligands. Reduction of TaCl5 by MBTCD in the
presence of α-diimine ligands 1a and 1b resulted in the formation
of tantalum complexes 2a and 2b (Scheme 1). In the NMR
spectra of 2a and 2b, resonances assignable to alkene carbons
were observed at δC 110.0 for 2a and 104.8 for 2b, and the signal
for an imine proton for 1b was shifted to higher field (δH 5.67),
indicating the reduction of neutral α-diimine ligands 1a and 1b
to dianionic ene-diamide ligands. An X-ray diffraction study of
2b reveals a dianionic ene-diamido coordination mode to
the tantalum atom (Figure 1).17 The C1ÀN1 bond length
(1.376(9) Å), the C1ÀC1* bond length (1.406(13) Å), and the
fold angle between the N1ÀC1ÀC1*ÀN1 and N1ÀTaÀ
N1* planes (123.0°) are typical for dianionic ligation to early
transition metal centers.17,18 Thus, during the complexation
reaction, the α-diimine ligands were reduced by in situ generated
“TaCl3”.
Reductive Cleavage of CÀCl Bonds by (r-Diimine)TaCl3.
The complexes 2a and 2b showed unique reactivity toward alkyl
halides: reactions of CCl4 and CHCl3 with 2a afforded the same
TaCl4 complex 4a in good yields (Scheme 2a). By monitoring
1
the reaction of 2a with polyhaloalkanes using H NMR spec-
troscopy, we found generation of dehalogenated products,
CHCl3 and CH2Cl2, respectively, in the reaction mixtures. In
the 1H NMR spectrum of 4a, two broad doublet resonances were
observed at δ 4.62 and 5.33 due to the vinylidene moiety of the
ligand backbone, indicating an in situ generated radical from
polyhaloalkanes abstracted the hydrogen atom of the ligand
backbone. Similarly, complex 2b also activated a carbonÀchlor-
ide bond of CCl4 or CHCl3 to form a TaCl4 fragment; however,
18676
dx.doi.org/10.1021/ja204665s |J. Am. Chem. Soc. 2011, 133, 18673–18683

Journal of the American Chemical Society
ARTICLE
Scheme 2. Reductive Cleavage of CÀCl Bond of
Scheme 3. Radical Addition Reaction of Trichloromethyl
Radical to Styrene
Polyhaloalkanes
the organic residual fragments attached to the ligand backbone,
to give the corresponding amido-imino complexes 5b and 6b
(Scheme 2b).20 The 13C NMR spectra of 5b and 6b displayed
singlet resonances corresponding to trichloromethyl and dichlor-
omethyl groups at δ 101.5 for 5b and 86.4 for 6b. In both
reactions in Scheme 2, the carbonÀchlorine bond was activated
by a high-valent d0 metal complex. We assume that the initial step
was one electron transfer from the dianionic ene-diamide ligand
in 2a or 2b to the polyhaloalkane to generate tantalum tetra-
chloride complexes of π-radical monoanionic α-diimine ligands
and organic radicals (vide infra). If the generated organic radicals
attacked the metal center of 2a or 2b (which were still present in
the reaction mixture), new organometallic species were pro-
duced via formation of new tantalumÀcarbon bonds. Reversible
metalÀcarbon bond formation and homolytic cleavage processes
are known to feature in organometallic mediated radical polymer-
ization reactions.21 However, attacking the ligand backbone is
preferable to form complexes 4a, 5b, and 6b.
Another reaction pathway between the carbon radical and
metal halides is a halogen abstraction reaction from the metal
center to generate alkyl halides. Such CÀX bond formation is an
important step for atom transfer radical addition and polymer-
ization reactions.22 We thus examined a radical polymerization
reaction of styrene by the in situ generated radical from the
reaction of 2a or 2b with polyhaloalkanes. Treatment of catalytic
amounts of 2a and CCl4 in the presence of excess styrene
resulted in the formation of poly(styrene) (26% yield, Mn =
1.9 Â 103, Mw/Mn = 1.3) and unidentified tantalum species,
indicating the generated trichloromethyl radical initiated radical
polymerization of styrene (Scheme 3a). When PhCH2Br was used
as the initiator for the styrene polymerization reaction, poly(styrene)
was obtained in 53% yield (Mn = 1.6 Â 103, Mw/Mn = 1.4). The
slightly increased yield of the polymer is probably due to the low
concentration of the carbon radical from 2a/PhCH2Br.23 In the 1H
NMR spectra of the poly(styrene), resonances assignable to the
terminal proton adjacent to the halogen atom were not observed,24
suggesting the free-radical polymerization character and the difficulty
of the abstraction of halides bound to the high-valent early transition
metal center by carbon radicals.25 In contrast, the reaction of 2b, CCl4,
and styrene (5 equiv) gave a new tantalum complex 7b, which was
Figure 2. Molecular structure of 7b with 30% thermal ellipsoids. All
hydrogen atoms are omitted for clarity. Selected bond distances (Å) and
angles (deg): TaÀN1, 2.395(6); TaÀN2, 1.971(6); TaÀCl1, 2.323(3);
TaÀCl2, 2.360(2); TaÀCl3, 2.392(3); TaÀCl4, 2.304(2); N1ÀC1,
1.292(10); N2ÀC2, 1.451(10); C1ÀC2, 1.504(10); C1ÀC3, 1.535-
(11); C3ÀC4, 1.546(11); N1ÀTaÀN2, 72.2(2); N1ÀTaÀCl2,
80.91(14); N1ÀTaÀCl4, 169.06(18); N2ÀTaÀCl2, 153.06(16);
N2ÀTaÀCl4, 100.27(16).
Scheme 4. Synthesis of (r-Diimine)TaCl4 Complexes
1
characterized using H and 13C NMR spectra, aided by 2D NMR
measurements (Scheme 3b). The 1H NMR spectrum of 7b displayed
two doublet resonances assignable to the amidomethylene group at δ
5.37 and 6.39 (J = 22.7 Hz). The resonance for the imine carbon was
observed at δ 194.0, whereas the signal due to the imine proton was
Formation of poly(styrene) or incorporation of several styrene
molecules into the ligand backbone was not observed for the reaction
of 2b and CCl4 in the presence of excess styrene, indicating that the
radical coupling reaction of benzyl radical with the π-radical ligand was
faster than the radical addition to another styrene monomer.27 We
monitored the reaction of 2b with CCl4 in the presence of styrene by
1H NMR spectroscopy. When excess styrene (100 equiv) was added
1
not detected in the H NMR spectrum, suggesting intramolecular
hydrogen transfer in the ligand backbone.26 It was noteworthy that one
ABX type signal appeared at δ 3.12, 3.42, and 4.63 (J = 9.8 and 14.8
Hz) assignable to a 1-phenyl-3,3,3-trichloropropyl group, which was
generated by the addition of trichloromethyl radical to styrene.
18677
dx.doi.org/10.1021/ja204665s |J. Am. Chem. Soc. 2011, 133, 18673–18683

Journal of the American Chemical Society
ARTICLE
Figure 3. UVÀvis spectra of tantalum complexes in toluene at 25 °C. (a) 2a (- - - -) and 2b ( ). (b) 8a (- - - -), 8b ( ), and 8c (À À).
;
;
3
Figure 4. Molecular structures of (α-diimine)TaCl4 ((a) 8a, (b) 8b, and (c) 8c) with 30% thermal ellipsoids. All hydrogen atoms are omitted for clarity.
Table 1. Bond Distances (Å) and Angles (deg) of 8aÀ8c
8a
8b
8c
TaÀN1
TaÀN2
2.195(8)
2.148(7)
2.326(3)
2.364(2)
2.379(4)
2.276(2)
1.326(10)
1.342(10)
1.434(11)
71.8(3)
2.154(3)
2.164(3)
2.179(3)
TaÀCl1
2.3199(15)
2.3176(11)
2.3521(10)
2.3106(10)
2.3153(10)
2.3554(11)
1.332(4)
TaÀCl2
TaÀCl3
TaÀCl4
N1ÀC1
1.333(4)
N2ÀC2
1.333(4)
C1ÀC2
1.425(7)
74.44(15)a
94.44(8)
90.56(8)
1.442(4)
N1ÀTaÀN2
N1ÀTaÀCl1
N1ÀTaÀCl2
a N1ÀTaÀN1*
74.79(10)
89.83(8)
93.9(2)
Figure 5. EPR spectra in toluene at room temperature for (a) 8a, (b)
8b, and (c) 8c.
87.4(2)
91.51(8)
tantalumÀnitrogen dative bonds, while N2 bound to the metal
center as an amido nitrogen atom as evident from the short
TaÀN2 bond length (1.971(6) Å).17,28 The C2ÀN2 interaction
is longer than that of C1ÀN1, indicating intramolecular hydro-
gen transfer to form a saturated CH2 moiety in the ligand
backbone.26 The trichloromethyl group is bound to the β-position
of the imine group due to the addition of trichloromethyl radical to
the α-position of styrene.
Preparation and Structure of (r-Diimine)TaCl4. In the
reactions described in Schemes 2 and 3, tetrachlorotantalum
complexes were regarded as key intermediates. We thus examined
the preparation of (α-diimine)TaCl4 complexes. Reaction of TaCl5
and the reaction mixture was heated at 60 °C, complex 7b was formed
as the main product together with the formation of 5b (5b/7b = 13/
87). In the case where an equimolar amount of styrene was added to
the reaction mixture, complex 5b was the main product (5b/7b =63/
37), suggesting that the product ratio (5b/7b) was dependent on the
amount of styrene and the intramolecular radical coupling to form 5b
was favorable compared to intermolecular reaction.
The molecular structure of 7b was confirmed by X-ray
diffraction (Figure 2). The tantalum center is pseudo-octahedral
with four chloride and one amido-imino ligands. The TaÀN1
distance (2.395(6) Å) is in the range typically observed for
18678
dx.doi.org/10.1021/ja204665s |J. Am. Chem. Soc. 2011, 133, 18673–18683

Journal of the American Chemical Society
ARTICLE
Scheme 5. Chemical Oxidation and Reduction
Reactions of 8b
Figure 6. (a) Simulated spectrum of 8b for ligand-centered radical
species (g = 2.0031, aN = 6.80 G, aH = 6.31 G). (b) Simulated spectrum
of 8b for tantalum-centered radical species (g = 1.9332, aTa = 57.6 G).,
(c) Addition spectrum of simulated spectra for ligand- and tantalum-
centered radical species.
Chart 2. Coordination Mode of α-Diimine Ligands for
(α-Diimine)TaCl4
Figure 7. UVÀvis spectra of 8b ( ), 9b (À À) and 10b (- - - -) in
;
3
CH2Cl2 at 25 °C.
with a 0.5 equiv of MBTCD resulted in the formation of (TaCl4)n
species, which were treated with the α-diimine ligands 1aÀ1c to
yield deep blue to green powders 8aÀ8c (Scheme 4). Figure 3
shows the UVÀvis spectra of 2a and 2b and 8aÀ8c in toluene. The
absorption maxima in the visible region at 624, 593, and 686 nm
(ε > 103 MÀ1 cmÀ1) for 8aÀ8c are characteristic of the presence of
a ligand π radical anion;5h,8b the corresponding absorption band
was not observed for the UVÀvis spectra of 2a and 2b.
(C5R5)Ta(C7H7) complexes (Aiso = 61À113 G).33 Thus, the
relatively lower value of the coupling constant for 8a might be
ascribed to the two resonance structures of tantalum(IV)-loca-
lized and TaN2C2 metallacycle-delocalized unpaired electron of
8a;6h no superhyperfine coupling due to the nitrogen atoms of
the ligand backbone was resolved. In contrast, two resonances
were observed for 8b: an eight-line pattern, g = 1.9332, Aiso = 58
G and a nine-line pattern, g = 2.0031, Aiso = 7.0 G (Figure 5b).
The nine-line splitting resonance is simulated by taking into
account hyperfine coupling with two virtually identical nitrogen
atoms (aN = 6.80 G) and four equivalent hydrogen atoms of the
NdCH and p-hydrogen atom of the N-aryl group (aH = 6.31 G)
(Figure 6a). The eight-line patterned signal is simulated as a
tantalum-centered radical (aTa = 57.6 G) (Figure 6b). Accord-
ingly, the EPR spectrum of 8b is consistent with sets of two
signals: one corresponds to two resonance structures of
tantalum(IV)-localized and TaN2C2 metallacycle-delocalized
radical, and the other is an organic radical purely localized on
the α-diimine ligands.20,34 The strength of the signal for the π-
radical monoanionic ligand was controllable by extending the π-
aromatic backbone of α-diimine ligands. Analogous to the EPR
spectrum of 8b, two signals were observed for 8c (Figure 5c,
eight-line pattern, g = 1.9152, Aiso = 60 G; seven-line pattern, g =
2.0026, Aiso = 6.1 G).35 Based on the EPR spectra of 8aÀ8c, the
coordination modes of the α-diimine ligands of 8aÀ8c are
shown as neutral or π-radical α-diimine ligands, as shown in
Chart 2.36 Therefore, formation of 5b and 6b is consistent with
coupling of the carbon radical and the ligand-localized π-radical.
On the other hand, an unpaired electron in 4a mainly localized
1
The H NMR spectra of 8aÀ8c displayed only broad reso-
nances; the molecular structures were therefore elucidated using
X-ray diffraction. ORTEP drawings of the structures are shown in
Figure 4, and the selected geometries are summarized in Table 1.
In contrast to the structure of 2b, the five-membered metalla-
cycles formed by the α-diimine ligands and tantalum atom are in
the same plane. The TaÀN distances range from 2.148(7) to
2.195(8) Å and are slightly longer than other tantalumÀnitrogen
σ-bonds.28 The CÀN and CÀC bond lengths of the α-diimine
ligands deviate from those of the free ligands29 and dianionic
enediamido ligands. The elongated CÀN (ca. 1.33 Å) and
shortened CÀC (ca. 1.43 Å) bonds suggest the partial reduc-
tion of the α-diimine ligand to form a ligand-centered π-
radical.5d,h,i,k,l,o,8b,30
EPR Spectra of (r-Diimine)TaCl4. The EPR spectra of 8aÀ8c
clearly reflected the coordination mode of the α-diimine ligands
to the tantalum atom. The EPR spectrum of 8a displayed an
eight-line pattern due to the isotropic tantalum nuclear spin (g =
1.9313, Aiso = 65 G) (Figure 5a). The isotropic hyperfine
coupling constant is lower than that of the purely localized tanta-
lum-centered radical complexes, TaCl4(PEt3)2 (Aiso = 211 G)31
and Ta(η-C6H6)2 (Aiso = 138 G),32 and is correlated to the
electron density of the metal center, as observed for
18679
dx.doi.org/10.1021/ja204665s |J. Am. Chem. Soc. 2011, 133, 18673–18683

Journal of the American Chemical Society
ARTICLE
Scheme 6. Reduction of Complexes 8a and 8b by NaBPh4
Figure 8. Molecular structure of [Cp2Co][(α-diimine)TaCl4] (9b)
with 30% thermal ellipsoids. All hydrogen atoms are omitted for clarity.
Scheme 7. Reaction of 8a with AIBN-Derived Carbon
Radical
Table 2. Bond Distances (Å) and Angles (deg) of 9b
TaÀN1
2.068(7)
2.405(2)
2.4079(19)
1.391(10)
1.356(11)
75.4(3)
TaÀN2
TaÀCl2
TaÀCl4
N2ÀC2
2.075(7)
2.402(2)
2.371(2)
1.390(10)
TaÀCl1
TaÀCl3
N1ÀC1
C1ÀC2
N1ÀTaÀN2
N1ÀTaÀCl1
N1ÀTaÀCl2
97.25(19)
89.30(19)
(Figure 8). The bond distances and angles are summarized in Table 2.
The tantalum center again is pseudo-octahedral. The TaÀN bonds
are shorter than those of 8aÀ8c, with values in the range observed for
related tantalumÀnitrogen σ-bonds.28 Due to the electron-rich
character of the tantalum atom of 9b, the TaÀN bonds are elongated
compared with 2b. In contrast to the structure of 2b, the TaN2C2
metallacycle is almost planar. It is noteworthy that structural features
of the α-diimine ligand are typical of early transition metal complexes,
with a longÀshortÀlong bonding sequence, N1ÀC1(1.391(10) Å),
C1ÀC2 (1.356(11) Å), and N2ÀC2 (1.390(10) Å), reflecting a
dianionic σ2-enediamido canonical form.17
Reversibility of the redox processes of α-diimine ligands was
also shown by the reaction of 8a or 8b with NaBPh4. In this
reaction, the α-diimine ligands acted as an electron acceptor
from the organoborate anion. Reaction of 8a or 8b with
NaBPh4 in CH2Cl2 resulted in the formation of 2a or 2b and
biphenyl. During these reactions, the α-diimine ligands in 8a
and 8b were reduced to form a dianionic ene-diamide ligand
(Scheme 6).35 In the presence of oxidants, organoborate anions
were decomposed via single electron transfer to form organo-
borate radical,39,40 and thus the electron transfer reaction was
assumed to proceed after the formation of ionic species, [(α-
diimine)TaCl3][BPh4].
Next, we examined the reaction of 8a with AIBN as a source of
a carbon radical. In this case the generated carbon radical
abstracts a chloride atom bound to the tantalum center, and
similar to the reaction in Scheme 6, one-electron reduction of 8a
proceeds to regenerate a dianionic α-diimine ligand. However,
the AIBN-derived carbon radical abstracted a hydrogen atom
from the ligand backbone of 8a to give 4a (Scheme 7).41 As
mentioned in the mechanism for styrene polymerization, abstrac-
tion of a halide ligand from early transition metal complexes is
difficult compared to H-abstraction or radical recombination
reaction.
on the tantalum atom, and thus the carbon radical abstracted one
hydrogen atom from the ligand backbone in Scheme 2a.
Redox Reactivity of (r-Diimine)TaCl4. The redox properties
of (α-diimine)TaCl4 complexes were investigated using cyclic
voltammetry.35 In all cases, two reversible one-electron
redox processes, [8]/[8]À and [8]/[8]+, were observed. The
one-electron reduction potential was almost the same among
complexes 8aÀ8c, whereas the difference of the oxidation
potential was ca. 0.2 V (Eox1/2 = À0.01 V for 8a and 0.21 V for
8c vs [Cp2Fe]+/0). On the basis of the reversibility of the redox
processes, we examined the chemical oxidation and reduction of
8c using Cp2Co as a reductant and WCl6 as an oxidant.
Reduction of 8b with Cp2Co resulted in the formation of
ligand-centered reduced species 9b as a green powder in 97%
yield. The ligand-centered oxidized species 10b was formed as a
red-brown powder in 68% yield by oxidation of 8b using WCl6
(Scheme 5). The ligand-centered redox products 9b and 10b
were characterized by 1H and 13C NMR spectra. The resonances
due to the hydrogen atoms of the ligand backbone were observed
at δ 4.84 for 9b and δ 9.53 for 10b, clearly indicating the ligand-
based redox reaction to produce dianionic enediamido and
neutral α-diimine ligands. UVÀvis spectra of complexes 8b,
9b, and 10b are shown in Figure 7. In contrast to 8b having a π-
radical monoanionic ligand, an intense absorption band in the
visible region was not observed for 9b.37 The complex 10b has
broad absorbance in the visible region, probably due to the
presence of W(V) anion in the reaction mixture. These chemical
redox processes are reversible, and oxidation of 9b by 1 equiv of
WCl6 or reduction of 10b by 1 equiv of Cp2Co at low temperature
resulted in the regeneration of 8b.35,38
Single crystals of 9b were obtained from saturated chloroform
solution; the molecular structure was clarified by X-ray diffraction
18680
dx.doi.org/10.1021/ja204665s |J. Am. Chem. Soc. 2011, 133, 18673–18683

Journal of the American Chemical Society
ARTICLE
’ CONCLUSION
E.; Chirik, P. J. Organometallics 2008, 27, 1470. (i) Sylvester, K. T.;
Chirik, P. J. J. Am. Chem. Soc. 2009, 131, 8772. (j) Tondreau, A. M.;
Milsmann, C.; Patrick, A. D.; Hoyt, H. M.; Lobkovsky, E.; Wieghardt, K.;
Chirik, P. J. J. Am. Chem. Soc. 2010, 132, 15046.
(4) (a) Dzik, W. I.; van der Vlugt, J. I.; Reek, J. N. H.; de Bruin, B.
Angew. Chem., Int. Ed. 2011, 50, 3356. (b) Smith, A. L.; Hardcastle, K. I.;
Soper, J. D. J. Am. Chem. Soc. 2010, 132, 14358.
Our results show that (α-diimine)TaCl3 complexes, involving
dianionic α-diimine ligands, induced generation of carbon
radicals by reductive cleavage of the CÀX bond of polyhaloalk-
anes. During the reaction, the dianionic ligand is oxidized,
releasing one electron to the polyhaloalkane. The radical gener-
ated in this process either abstracted a hydrogen atom from the
ligand or added to the monoanionic ligand backbone or styrene.
Accordingly, these ligand-centered redox processes provide
interesting protocols for activating polyhaloalkanes to generate
synthetically useful carbon-centered radicals. Recombination of a
carbon radical and a halide ligand is difficult due to the strength of
the early transition metalÀhalide bond, but by tuning the
electronic property of the supporting ligand to weaken the
metalÀhalogen bond, ligand-based reduction of alkyl halides
provides a new strategy for creating early transition metal
catalyzed atom transfer radical coupling catalysts.
(5) (a) Shaffer, D. W.; Ryken, S. A.; Zarkesh, R. A.; Heyduk, A. F.
Inorg. Chem. 2011, 50, 13. (b) Lu, C. C.; Weyherm€uller, T.; Bill, E.;
Wieghardt, K. Inorg. Chem. 2009, 48, 6055. (c) van Gastel, M.; Lu, C. C.;
Wieghardt, K.; Lubitz, W. Inorg. Chem. 2009, 48, 2626. (d) Khusniyarov,
M. M.; Weyherm€uller, T.; Bill, E.; Wieghardt, K. J. Am. Chem. Soc. 2009,
131, 1208. (e) Spikes, G. H.; Milsmann, C.; Bill, E.; Weyherm€uller, T.;
Wieghardt, K. Inorg. Chem. 2008, 47, 11745. (f) Roy, N.; Sproules, S.;
Bill, E.; Weyherm€uller, T.; Wieghardt, K. Inorg. Chem. 2008, 47, 10911.
(g) Spikes, G. H.; Sproules, S.; Bill, E.; Weyherm€uller, T.; Wieghardt, K.
Inorg. Chem. 2008, 47, 10935. (h) Ghosh, M.; Sproules, S.; Weyherm€uller,
T.; Wieghardt, K. Inorg. Chem. 2008, 47, 5963. (i) Muresan, N.; Lu,
C. C.; Ghosh, M.; Peters, J. C.; Abe, M.; Henling, L. M.; Weyherm€uller,
T.; Bill, E.; Wieghardt, K. Inorg. Chem. 2008, 47, 4579. (j) Lu, C. C.; Bill,
E.; Weyherm€uller, T.; Bothe, E.; Wieghardt, K. J. Am. Chem. Soc. 2008,
130, 3181. (k) Ghosh, M.; Weyherm€uller, T.; Wieghardt, K. Dalton
Trans. 2008, 5149. (l) Muresan, N.; Weyherm€uller, T.; Wieghardt, K.
Dalton Trans. 2007, 4390. (m) Lu, C. C.; Bill, E.; Weyherm€uller, T.;
Bothe, E.; Wieghardt, K. Inorg. Chem. 2007, 46, 7880. (n) Kapre, R. R.;
Bothe, E.; Weyherm€uller, T.; George, S. D.; Muresan, N.; Wieghardt, K.
Inorg. Chem. 2007, 46, 7827. (o)Muresan, N.; Chiopek, K.; Weyherm€uller,
T.; Neese, F.; Wieghardt, K. Inorg. Chem. 2007, 46, 5327. (p) Chlopek,
K.; Bothe, E.; Neese, F.; Weyherm€uller, T.; Wieghardt, K. Inorg. Chem.
2006, 45, 6298. (q) Chlopek, K.; Bill, E.; Weyherm€uller, T.; Wieghardt,
K. Inorg. Chem. 2005, 44, 7087. (r) Kokatam, S.; Weyherm€uller, T.;
Bothe, E.; Chaudhuri, P.; Wieghardt, K. Inorg. Chem. 2005, 44, 3709. (s)
Bianchard, S.; Bill, E.; Weyherm€uller, T.; Wieghardt, K. Inorg. Chem. 2004,
43, 2324. (t) Chun, H.; Bill, E.; Weyherm€uller, T.; Wieghardt, K. Inorg. Chem.
2002, 41, 5091. (u)Chun, H.;Chaudhuri, P.; Weyherm€uller, T.; Wieghardt,
K. Inorg. Chem. 2002, 41, 790. (v) Chun, H.; Verani, C. N.; Chaudhuri,
P.; Bothe, E.; Bill, E.; Weyherm€uller, T.; Wieghardt, K. Inorg. Chem.
2001, 40, 4157. (w) Chaudhuri, P.; Verani, C. N.; Bill, E.; Bothe, E.;
Weyherm€uller, T.; Wieghardt, K. J. Am. Chem. Soc. 2001, 123, 2213.
(6) (a) Brand, H.; Arnold, J. Angew. Chem., Int. Ed. 1994, 33, 95.
(b) Stanciu, C.; Jones, M. E.; Fanwick, P. E.; Abu-Omar, M. M. J. Am.
Chem. Soc. 2007, 129, 12400. (c) Blackmore, K. J.; Ziller, J. W.; Heyduk,
A. F. Inorg. Chem. 2005, 44, 5559. (d) Haneline, M. R.; Heyduk, A. F.
J. Am. Chem. Soc. 2006, 128, 8410. (e) 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. (f) Blackmore, K. J.; Lal, N.; Ziller,
J. W.; Heyduk, A. F. J. Am. Chem. Soc. 2008, 130, 2728. (g) Blackmore,
K. J.; Sly, M. B.; Haneline, M. R.; Ziller, J. W.; Heyduk, A. F. Inorg. Chem.
2008, 47, 10522. (h) Zarkesh, R. A.; Ziller, J. W.; Heyduk, A. F. Angew.
Chem., Int. Ed. 2008, 47, 4715. (i) Nguyen, A. I.; Blackmore, K. J.; Carter,
S. M.; Zarkesh, R. A.; Heyduk, A. F. J. Am. Chem. Soc. 2009, 131, 3307.
(j) Zarkesh, R. A.; Heyduk, A. F. Organometallics 2009, 28, 6629.
(k) Szigethy, G.; Heyduk, A. F. Inorg. Chem. 2011, 50, 125. (l) Nguyen,
A. I.; Zarkesh, R. A.; Lacy, D. C.; Thorson, M. K.; Heyduk, A. F. Chem.
Sci. 2011, 2, 166.
’ ASSOCIATED CONTENT
S
Supporting Information. Experimental details, selected
b
EPR spectra, cyclic voltammograms, and CIF file of data for
complexes 2b, 3c, 7b, 8a, 8b, 8c, and 9b. This material is available
free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION
Corresponding Author
mashima@chem.es.osaka-u.ac.jp
’ ACKNOWLEDGMENT
T.S. expresses his special thanks for the financial support
provided by the JSPS Research Fellowships for Young Scientists.
H.T. acknowledged financial support by a Grant-in-Aid for
Mitsubishi Chemical Corporation Fund. This work was sup-
ported by the Core Research for Evolutional Science and
Technology (CREST) program of the Japan Science and Tech-
nology Agency (JST), Japan. J.A. thanks the US-NSF for funding
(0848931). This paper is dedicated to Professor Christian
Bruneau on the occassion of his 60th birthday.
’ REFERENCES
(1) (a) Curran, D. P. Comprehensive Organic Synthesis; Pergamon:
New York, 1992; p 715. (b) Crabtree, R. H. The Organometallic
Chemistry of the Transition Metals; Wiley Interscience: New York, 2005.
(2) (a) Kaim, W. Coord. Chem. Rev. 1987, 76, 187. (b) Jazdzewski,
B. A.; Tolman, W. B. Coord. Chem. Rev. 2000, 200À202, 633.
(c) Pierpont, C. G. Coord. Chem. Rev. 2001, 216À217, 99. (d) Butin,
K. P.; Beloglazkina, E. K.; Zyk, N. V. Russ. Chem. Rev. 2005, 74, 531.
(e) Zanello, P.; Corsini, M. Coord. Chem. Rev. 2006, 250, 2000. (f) Ray,
K.; Petrenko, T.; Wieghardt, K.; Neese, F. Dalton Trans. 2007, 1552.
(g) Kaim, W.; Schwederski, B. Coord. Chem. Rev. 2010, 254, 1580.
(3) (a) Chirik, P. J.; Wieghardt, K. Science 2010, 327, 794. (b) Bart,
S. C.; Lobkovsky, E.; Chirik, P. J. J. Am. Chem. Soc. 2004, 126, 13794.
(c) Bart, S. C.; Hawrelak, E. J.; Lobkovsky, E.; Chirik, P. J. Organome-
tallics 2005, 24, 5518. (d) Bouwkamp, M. W.; Bowman, A. C.;
Lobkovsky, E.; Chirik, P. J. J. Am. Chem. Soc. 2006, 128, 13340. (e)
Bart, S. C.; Chlopek, K.; Bill, E.; Bouwkamp, M. W.; Lobkovsky, E.;
Neese, F.; Wieghardt, K.; Chirik, P. J. J. Am. Chem. Soc. 2006, 128, 13901.
(f) Trovitch, R. J.; Lobkovsky, E.; Chirik, P. J. Inorg. Chem. 2006,
45, 7252. (g) Bart, S. C.; Bowman, A. C.; Lobkovsky, E.; Chirik, P. J.
J. Am. Chem. Soc. 2007, 129, 7212. (h) Trovitch, R. J.; Lobkovsky, E.; Bill,
(7) (a) van Koten, G.; Vrieze, K. Adv. Organomet. Chem. 1982,
21, 151. (b) Vrieze, K. J. Organomet. Chem. 1986, 300, 307. (c) Mealli,
C.; Ienco, A.; Phillips, A. D.; Galindo, A. Eur. J. Inorg. Chem. 2007, 2556.
(8) (a) Bart, S. C.; Hawrelak, E. J.; Lobkovsky, E.; Chirik, P. J.
Organometallics 2005, 24, 5518. (b) Kreisel, K. A.; Yap, G. P. A.;
Theopold, K. H. Inorg. Chem. 2008, 47, 5293.
(9) (a) Richter, B.; Scholz, J.; Sieler, J.; Thiele, K.-H. Angew. Chem.,
Int. Ed. 1995, 34, 2649. (b) Spaniel, T.; Gorls, H.; Scholz, J. Angew.
Chem., Int. Ed. 1998, 37, 1862. (c) Daff, P. J.; Etienne, M.; Donnadieu, B.;
Knottenbelt, S. Z.; McGrady, J. E. J. Am. Chem. Soc. 2002, 124, 3818.
(d) Scholz, J.; Gorls, H. Polyhedron 2002, 21, 305.
(10) Arteaga-M€uller, R.; Tsurugi, H.; Saito, T.; Yanagawa, M.; Oda,
S.; Mashima, K. J. Am. Chem. Soc. 2009, 131, 5370.
18681
dx.doi.org/10.1021/ja204665s |J. Am. Chem. Soc. 2011, 133, 18673–18683

Journal of the American Chemical Society
ARTICLE
(11) Laguerre, M.; Dunogues, J.; Calas, R.; Duffaut, N. J. Organomet.
Chem. 1976, 112, 49.
(12) (a) Dieck, H.; Svoboda, M.; Greiser, T. Z. Naturforsch. B 1981,
36, 823. (b) van Asselt, R.; Elsevier, C. J.; Smeets, W. J. J.; Spek, A. L.;
Benedix, R. Recl. Trav. Chim. Pays-Bas 1994, 113, 88.
(13) Pangborn, A. B.; Giardello, M. A.; Grubbs, R. H.; Rosen, R. K.;
Timmers, F. J. Organometallics 1996, 15, 1518.
(14) Sheldrich, G. M. Acta Crystallogr. 2008, A64, 112–122.
(15) Altomare, A.; Burla, M. C.; Camalli, M.; Cascarano, G. L.;
Giacovazzo, C.; Guagliardi, A.; Polidori, G. J. Appl. Crystallogr. 1994,
27, 435.
(24) (a) Kotani, Y.; Kamigaito, M.; Sawamoto, M. Macromolecules
1999, 32, 2420. (b) Stoffelbach, F.; Haddleton, D. M.; Poli, R. Eur.
Polym. J. 2003, 39, 2099.
(25) (a) Asandei, A. D.; Moran, I. W. J. Am. Chem. Soc. 2004,
126, 15932. (b) Asandei, A. D.; Chen, Y. Macromolecules 2006, 39, 7549.
(26) (a) Mashima, K.; Ohnishi, R.; Yamagata, T.; Tsurugi, H. Chem.
Lett. 2007, 36, 1420. (b) De Waele, P.; Jazdzewski, B. A.; Klosin, J.;
Murray, R. E.; Theriault, C. N.; Vosejpka, P. C.; Petersen, J. L.
Organometallics 2007, 26, 3896. (c) Tsurugi, H.; Ohnishi, R.; Kaneko,
H.; Panda, T. K.; Mashima, K. Organometallics 2009, 28, 680.
(27) For the comparison of the reactivity of trichloromethyl- and
benzyl-radicals, we examined the reaction of 2b with benzyl bromide in
the presence of excess styrene. Incorporation of styrene was not
observed when benzyl bromide was used, and the benzyl radical attacked
the ligand to form (PhCH2-amido-imino)TaCl4, indicating that recom-
bination of benzylic radical and ligand-based radical was much faster
than that of trichloromethyl-based and ligand-based radical due to
the stability of trichloromethyl radical compared with benzylic
radical toward radical recombination (see Scheme S1 in Supporting
Information).
(16) Farrugia, L. J. J. Appl. Crystallogr. 1999, 32, 837.
(17) (a) Kawaguchi, H.; Yamamoto, Y.; Asaoka, K.; Tatsumi, K.
Organometallics 1998, 17, 4380. (b) Mashima, K.; Matsuo, Y.; Tani, K.
Organometallics 1999, 18, 1471. (c) Sanchez-Nieves, J.; Royo, P.;
Pellinghelli, M. A.; Tiripicchio, A. Organometallics 2000, 19, 3161.
(d) Nakamura, A.; Mashima, K. J. Organomet. Chem. 2001, 621, 224.
(e) Tsurugi, H.; Ohno, T.; Yamagata, T.; Mashima, K. Organometallics
2006, 25, 3179. (f) Tsurugi, H.; Ohno, T.; Kanayama, T.; Arteaga-
M€uller, R.; Mashima, K. Organometallics 2009, 28, 1950.
(18) Discussion of the coordination mode of folded ene-diamido
chelates in d0 metal complexes: (a) Galindo, A.; Ienco, A.; Mealli, C. New
J. Chem. 2000, 24, 73. (b) Galindo, A.; Gꢀomez, M.; del Río, D.; Sꢀanchez,
F. Eur. J. Inorg. Chem. 2002, 1326. (c) del Río, D.; Galindo, A.
J. Organomet. Chem. 2002, 655, 16. (d) Galindo, A.; del Río, D.; Mealli,
C.; Ienco, A.; Bo, C. J. Organomet. Chem. 2004, 689, 2847. (e) Wang,
S.-Y. S.; Abboud, K. A.;Boncella, J. M. J. Am. Chem. Soc. 1997, 119, 11990.
(f) Cameron, T. M.; Abboud, K. A.; Boncella, J. M. Chem. Commun.
2001, 1224. (g) Cameron, T. M.; Ortiz, C. G.; Ghiviriga, I.; Abboud,
K. A.; Boncella, J. M. Organometallics 2001, 20, 2032. (h) Cameron,
T. M.; Ghiviriga, I.; Abboud, E. A.; Boncella, J. M. Organometallics 2001,
20, 4378. (i) Ison, E. A.; Cameron, T. M.; Abboud, K. A.; Boncella, J. M.
Organometallics 2004, 23, 4070. (j) Hayton, T. W.; Boncella, J. M.; Scott,
B. L.; Abboud, K. A.; Mills, R. C. Inorg. Chem. 2005, 44, 9506.
(19) See the Supporting Information for the molecular structure of
the cationic part of 3c.
(28) (a) Chao, Y.-W.; Wexler, P. A.; Wigley, D. E. Inorg. Chem. 1989,
28, 3860. (b) Suh, S.; Hoffman, D. M. Inorg. Chem. 1996, 35, 5015.
(c) Guꢀerin, F.; McConville, D. H.; Vittal, J. J.; Yap, G. A. P. Organome-
tallics 1998, 17, 1290. (d) Araujo, J. P.; Wicht, D. K.; Bonitatebus, P. J.;
Schrock, R. R. Organometallics 2001, 20, 5682. (e) Oshiki, T.; Tanaka,
K.; Yamada, J.; Ishiyama, T.; Kataoka, Y.; Mashima, K.; Tani, K.; Takai,
K. Organometallics 2003, 22, 464. (f) Oshiki, T.; Yamada, A.; Kawai, K.;
Arimitsu, H.; Takai, K. Organometallics 2007, 26, 173.
(29) (a) Cope-Eatough, E. K.; Mair, F. S.; Pritchard, R. G.; Warren,
J. E.; Woods, R. J. Polyhedron 2003, 22, 1447. (b) Laine, T. V.; Klinga,
M.; Maaninen, A.; Aitola, E.; Leskela, M. Acta Chem. Scand. 1999,
53, 968. (c) El-Ayaan, U.; Paulovicova, A.; Fukuda, Y. J. Mol. Struct. 2003,
645, 205.
(30) (a) Recknagel, A.; Noltemeyer, M.; Edelmann, F. T. J. Orga-
nomet. Chem. 1991, 410, 53. (b) Bochkarev, M. N.; Trifonov, A. A.;
Cloke, F. G. N.; Dalby, C. I.; Matsunaga, P. T.; Andersen, R. A.;
Schumann, H.; Loebel, J.; Hemling, H. J. Organomet. Chem. 1995,
486, 177. (c) Scholz, A.; Thiele, K.-H.; Scholz, J.; Weimann, R.
J. Organomet. Chem. 1995, 501, 195. (d) Trifonov, A. A.; Kirillov,
E. N.; Bochkarev, M. N.; Schumann, H.; Muehle, S. Russ. Chem. Bull.
1999, 48, 382. (e) Trifonov, A. A.; Kurskii, Y. A.; Bochkarev, M. N.;
Muehle, S.; Dechert, S.; Schumann, H. Russ. Chem. Bull. 2003, 52, 601.
(f) Trifonov, A. A.; Fedorova, E. A.; Ikorskii, V. N.; Dechert, S.;
Schumann, H.; Bochkarev, M. N. Eur. J. Inorg. Chem. 2005, 2812.
(g) Moore, J. A.; Cowley, A. H.; Gordon, J. C. Organometallics 2006,
25, 5207. (h) Walter, M. D.; Berg, D. J.; Andersen, R. A. Organometallics
2007, 26, 2296. (i) Cui, P.; Chen, Y.; Wang, G.; Li, G.; Xia, W.
Organometallics 2008, 27, 4013. (j) Panda, T. K.; Kaneko, H.; Pal, K.;
Tsurugi, H.; Mashima, K. Organometallics 2010, 29, 2610.
(20) Kaupp, M.; Stoll, H.; Preuss, H.; Kaim, W.; Stahl, T.; van Koten,
G.; Wissing, E.; Smeets, W. J. J.; Spek, A. L. J. Am. Chem. Soc. 1991,
113, 5606.
(21) (a) Poli, R. Angew. Chem., Int. Ed. 2006, 45, 5058. (b) Debuigne,
A.; Poli, R.; Jꢀer^ome, C.; Jꢀer^ome, R.; Detrembleur, C. Prog. Polym. Sci.
2009, 34, 211. (c) Wayland, B. B.; Poszmik, G.; Mukerjee, S. L.; Fryd, M.
J. Am. Chem. Soc. 1994, 116, 7943. (d) Grognec, E. L.; Claverie, J.; Poli,
R. J. Am. Chem. Soc. 2001, 123, 9513. (e) Stoffelbach, F.; Poli, R.;
Richard, P. J. Organomet. Chem. 2002, 663, 269. (f) Shaver, M. P.; Allan,
L. E. N.; Gibson, V. C. Organometallics 2007, 26, 4725. (g) Allan,
L. E. N.; Shaver, M. P.; White, A. J. P.; Gibson, V. C. Inorg. Chem. 2007,
46, 8963. (h) MacLeod, K. C.; Conway, J. L.; Tang, L.; Smith, J. J.;
Corcoran, L. D.; Ballem, K. H. D.; Patrick, B. O.; Smith, K. M.
Organometallics 2009, 28, 6798. (i) MacLeod, K. C.; Conway, J. L.;
Patrick, B. O.; Smith, K. M. J. Am. Chem. Soc. 2010, 132, 17325. (j)
Champouret, Y.; MacLeod, K. C.; Baisch, U.; Patrick, B. O.; Smith,
K. M.; Poli, R. Organometallics 2010, 29, 167. (k) Champouret, Y.;
MacLeod, K. C.; Smith, K. M.; Patrick, B. O.; Poli, R. Organometallics
2010, 29, 3125. (l) Shaver, M. P.; Hanhan, M. E.; Jones, M. R. Chem.
Commun. 2010, 46, 2127. (m) Zhou, W.; Tang, L.; Patrick, B. O.; Smith,
K. M. Organometallics 2011, 30, 603. (n) Allan, L. E. N.; Cross, E. D.;
Francis-Pranger, T. W.; Hanhan, M. E.; Jones, M. R.; Pearson, J. K.;
Perry, M. R.; Storr, T.; Shaver, M. P. Macromolecules 2011, 44, 4072.
(22) (a) Tsarevsky, N. V.; Matyajaszewski, K. Chem. Rev. 2007,
107, 2270. (b) Pintauer, T.; Matyajaszewski, K. Chem. Soc. Rev. 2008,
37, 1087. (c) Ouchi, M.; Terashima, T.; Sawamoto, M. Chem. Rev. 2009,
109, 4963. (d) Satoh, K.; Kamigaito, M. Chem. Rev. 2009, 109, 5120. (e)
Rosen, B. M.; Percec, V. Chem. Rev. 2009, 109, 5069 and references therein.
(23) We monitored the reaction of 2a/CCl4 and 2a/PhCH2Br by
UVÀvis spectra, and the activation of Cl-CCl3 was much faster than that
of Br-CH2Ph. Changes of the UVÀvis spectra for 2a/PhCH2Br are
shown in Figure S9 in Supporting Information.
(31) Labauze, G.; Samuel, E.; Livage, J. Inorg. Chem. 1980, 19, 1384.
(32) Cloke, F. G. N.; Dix, A. N.; Green, J. C.; Perutz, R. N.; Seddon,
E. A. Organometallics 1983, 2, 1150.
(33) Noh, W.; Girolami, G. S. Inorg. Chem. 2008, 47, 535.
(34) (a) Richter, S.; Daul, C.; Aelewsky, A. V. Inorg. Chem. 1976,
15, 943. (b) Gardiner, M. G.; Hanson, G. R.; Henderson, M. J.; Lee,
F. C.; Raston, C. L. Inorg. Chem. 1994, 33, 2456. (c) Rijnberg, E.;
Boersma, J.; Jastrzebski, J. T. B. H.; Lakin, M. T.; Spek, A. L.; van Koten,
G. Organometallics 1997, 16, 3158. (d) Rijnberg, E.; Richter, B.; Thiele,
K.-H.; Boersma, J.; Veldman, N.; Spek, A. L.; van Koten, G. Inorg. Chem.
1998, 37, 56.
(35) See Supporting Information.
(36) In the EPR spectra of 8b and 8c in THF, the resonances for
tantalum-centered radical became larger than those observed in toluene
or CH2Cl2 (Figures S15 and S16 in Supporting Information). The
equilibrium between metal and ligand-centered radical character was
probably affected by the coordinating solvent. In addition, the increase
of the intensity for signals of the ligand-centered radical was observed
with increasing the temperature (Figure S17 in Supporting
18682
dx.doi.org/10.1021/ja204665s |J. Am. Chem. Soc. 2011, 133, 18673–18683

Journal of the American Chemical Society
ARTICLE
Information), suggesting the equilibrium of the two redox isomers was
affected by the solvent and temperature.
(37) Absorption band at 400 nm corresponds to cobaltocene cation:
Warratz, R.; Peters, G.; Studt, F.; R€omer, R.-H.; Tuczek, F. Inorg. Chem.
2006, 45, 2531.
(38) Consumption of 8b was monitored by the EPR measurement
to check the disappearance of both resonances during the reduction:
controlled amount of Cp2Co (0.75 equiv) was added to the toluene
solution of 8b, and both resonances were weakened (Figure S19 in
Supporting Information). By the addition of 1 equiv of Cp2Co, the
reaction solution was EPR silent and the complex 9b was detected in the
1H NMR spectrum.
(39) (a) Ue, M.; Takeda, M.; Takehara, M.; Mori, S. J. Electrochem.
Soc. 1997, 144, 2684. (b) Shundrin, L. A.; Bardin, V. V.; Frohn, H.-J. Z.
Anorg. Allg. Chem. 2004, 630, 1253. (c) Sorin, G.; Mallorquin, R. M.;
Contie, Y.; Baralle, A.; Malacria, M.; Goddard, J.-P.; Fensterbank, L.
Angew. Chem., Int. Ed. 2010, 49, 8721.
(40) When NaBF4 was used instead of NaBPh4, reduction of the
ligand was not observed.
(41) In the case of the reaction of 8b with AIBN at 60 °C for 60 h, the
AIBN-derived radical attacked to the ligand backbone to form a new
tantalum complex as the main product together with unidentified
tantalum species (Scheme S2 in Supporting Information). Reaction
details are shown in Supporting Information.
18683
dx.doi.org/10.1021/ja204665s |J. Am. Chem. Soc. 2011, 133, 18673–18683