Tuning the electronic and steric parameters of a redox-active tris(amido) ligand

Article abstract of DOI:10.1021/ic401496w

A family of tantalum compounds was prepared to probe the electronic effects engendered by the addition of electron-donating or electron-withdrawing groups to the 4/4′ positions of the redox-active ligand derived from bis(2-isopropylamino-4-X-phenyl)amine

Full text of DOI:10.1021/ic401496w

Article
pubs.acs.org/IC
Tuning the Electronic and Steric Parameters of a Redox-Active
Tris(amido) Ligand
Rui F. Munha, Ryan A. Zarkesh, and Alan F. Heyduk*
́
Department of Chemistry, University of California, Irvine, California 92697-2025, United States
S
* Supporting Information
ABSTRACT: A family of tantalum compounds was prepared to probe
the electronic effects engendered by the addition of electron-donating or
electron-withdrawing groups to the 4/4′ positions of the redox-active
ligand derived from bis(2-isopropylamino-4-X-phenyl)amine
t
[
X,iPr(NNNcat)H3, X = F, H, Me, Bu]). A general synthetic procedure
for the X,iPr(NNNcat)H3 ligand family was developed starting from the 4/
4′ disubstituted diphenylamine derivative. A second ligand modification,
incorporation of aromatic substituents at the flanking nitrogen moieties,
was achieved via palladium-catalyzed cross-coupling to afford bis(2-3,5-
dimethylphenylamino-4-methoxy-phenyl)amine OMe,DMP(NNNcat)H3
(DMP = 3,5-C6H3Me2), allowing a comparative study to the less
sterically hindered isopropyl derivative. Treatment of the triamines with
1 equiv of TaMe3Cl2 generated the corresponding dichloro complexes
X,R(NNNcat)TaCl2(L) (L = empty or Et2O) in high yields. These neutral
dichloride derivatives reacted with [NBnEt3][Cl] to produce the anionic trichloride derivatives [NBnEt3][X,R(NNNcat)TaCl3],
whereas the neutral dichloride derivatives reacted with chlorine atom donors to produce the neutral trichloride derivatives
X,R(NNNsq)TaCl3, containing the one-electron-oxidized form of the redox-active ligand. Aryl azides reacted with the
X,R(NNNcat)TaCl2(L) derivatives, resulting in nitrene transfer to tantalum and two-electron oxidation of the ligand platform to
give X,R(NNNq)TaCl2(NR′) (R = iPr; X = OMe, F, H, Me; R′ = p-C6H4tBu, p-C6H4CF3; and R = 3,5-C6H3Me2; X = OMe; R′
= p-C6H4CH3). Electrochemistry, UV−vis−NIR, IR, and EPR spectroscopies along with X-ray diffraction methods were used to
characterize and compare complexes with different redox-active ligand derivatives in each oxidation state. This study
demonstrates that while the ligand redox potentials can be adjusted over a 270 mV range through substitutions at the 4/4′ ring
positions, the coordination chemistry and reactivity patterns at the bound tantalum center remain unchanged, suggesting that
such ligand modifications can be used to tune the redox potentials of a complex for a particular substrate of interest.
Two related redox-active pincer ligands, (ONO) and
X,R(NNN) of Chart 1, engender different multielectron
reactivity when coordinated to tantalum. Both of these ligands
are stable in three oxidation states when metal-bound: the
trianionic catecholate forms, the dianionic semiquinonate
forms, and the monoanionic quinonate forms.1,6 These ligands
afford isostructural and isoelectronic coordination complexes
when bound to tantalum; however, the tantalum complexes
display different reactivity due to redox potentials that vary by
approximately 500 mV.18 Whereas OMe,iPr(NNNcat)TaCl2
reacted with phenyl azide to extrude N2 and give the robust
terminal imido complex OMe,iPr(NNNq)TaCl2(NPh),19 the
reaction of (ONOcat)TaCl2 with phenyl azide resulted in
nitrene coupling to afford azobenzene by a bimolecular four-
electron reductive elimination.20 Experimental evidence
suggested that this latter reaction proceeds through an
analogous terminal imido intermediate, but that the more
strongly oxidizing (ONOq)− ligand drives the nitrene coupling
INTRODUCTION
■
Interest in noninnocent or redox-active ligands has increased
owing to the recognition that these ligand platforms can be
exploited to engender multielectron reactivity in coordination
complexes.1−14 Toward this end, one key feature of any redox-
active ligand platform is the reduction potential(s) at which the
ligand oxidation state changes in a coordination complex. While
the coordinated metal ion modulates the ligand reduction
potential(s), an appealing aspect of redox-active ligands is that
these reduction potential(s) also may be tailored through
systematic modifications to the ligand platform itself.
Comparisons of ortho-quinone, ortho-iminoquinone, and
ortho-diiminoquinone complexes of ruthenium(II) revealed
reduction potentials in isostructural and isoelectronic com-
plexes that varied by over 0.8 V.15 More recently, Chirik and
co-workers examined the addition of electron-donating and
electron-withdrawing groups to the redox-active pyridinedii-
mine (PDI) ligand.16,17 In this Article, we report a detailed
examination of the impact of electron-donating and electron-
withdrawing groups on a topologically similar redox-active
tris(amido) ligand platform.
Received: June 13, 2013
Published: September 6, 2013
© 2013 American Chemical Society
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Article
complex, yet afford afford different coordination isomers of the
metal coordination sphere.
Chart 1
EXPERIMENTAL SECTION
■
General Considerations. The complexes described below are air
and moisture sensitive, necessitating that manipulations be carried out
under an inert atmosphere of argon or nitrogen gas using standard
Schlenk, vacuum-line, and glovebox techniques. Ambient temperature
in the glovebox was 27 °C. Hydrocarbon solvents were sparged with
nitrogen and then deoxygenated and dried by passage through Q5 and
activated alumina columns, respectively. Ethereal and halogenated
solvents were sparged with nitrogen and then dried by passage through
two activated alumina columns. Bis(4-methylphenyl)amine was
purchased from TCI; isoamyl nitrite and N-chlorosuccinimide were
purchased from Alfa Aesar and used without further purification. The
compounds bis(4-tert-butylphenyl)amine,22 bis(4-fluorophenyl)-
amine,23 H,iPr(NNNcat)H3 (2c),24 TaMe3Cl2,25 and OMe,iPr(NNNcat)-
TaCl2 (3a)19 were prepared according to previously reported synthetic
methodologies. tert-Butylisocyanide was purchased from Aldrich, dried
with CaH2, and distilled prior to use. [nBu4N][PF6] and [NBnEt3][Cl]
were recrystallized three times from hot methanol and dried under
high vacuum.
reaction.6 While the stronger oxidizing nature of the (ONOq)−
ligand compared to the OMe,iPr(NNNq)− ligand is consistent
with the prior results from Lever’s studies on bidentate redox-
active ligands,15,21 we postulated that a secondary factor may be
the inclusion of electron-donating methoxy groups into the
(NNN) ligand backbone. As shown in Chart 2, these methoxy
groups are in resonance with the redox-active π-system of the
ligand and could further stabilize the quinonate form of the
(NNN) ligand.
Spectroscopic Methods. NMR spectra were collected on a
Bruker Avance 600, 500, or 400 MHz spectrometer in dry, degassed
1
deuterated solvents. H NMR spectra were referenced to TMS using
the residual proteo impurities of the solvent; 13C NMR spectra were
referenced to TMS using the natural-abundance 13C impurities 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
of 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 or a Perkin-Elmer Lambda
900 UV−vis spectrophotometer equipped with a NIR detector, both
using 1 cm quartz cuvettes. APCI-MS data was collected on a Waters
LCT Premier mass spectrometer. EPR spectra were collected on a
Bruker EMX X-band spectrometer equipped with an ER041XG
microwave bridge. Spectra for EPR samples were collected using the
following spectrometer settings: attenuation = 20 dB, microwave
power = 2.017 mW, frequency = 9.79 GHz; sweep width = 300 G,
modulation amplitude = 9.02 G, gain = 2.00 × 103, conversion time =
10.24 ms, time constant = 81.92 ms, and resolution = 2048 points. The
EPR spectra were modeled using MATLAB, assuming an isotropic
signal and hyperfine coupling to three equivalent nitrogen atoms.
Elemental analyses were collected on a Perkin-Elmer 2400 Series II
CHNS/O analyzer.
Chart 2
Electrochemical Methods. Electrochemical experiments were
performed on a Gamry Series G 300 potentiostat/galvanostat/ZRA
(Gamry Instruments, Warminster, PA) using a 1.5 mm diameter
platinum disk working electrode, a platinum wire auxiliary electrode,
and a silver wire reference electrode. Electrochemical experiments
were performed at ambient temperature (27 °C) under a dinitrogen
atomosphere in a MeCN solution containing 0.1 M [nBu4N][PF6] as
the supporting electrolyte and 1 mM of the desired analyte. All
potentials were referenced to [Cp2Fe]+/0, using [Cp2Co][PF6] as an
internal standard (−1.336 V vs [Cp2Fe]+/0). Under our conditions the
typical solvent window is from a reductive limit of −2.00 V to an
oxidative limit of +2.00 V.
X-ray Diffraction Methods. X-ray diffraction data for 3b−3f, 6f,
7a, and 7d were collected on crystals mounted on glass fibers with
Paratone oil using a Bruker CCD platform diffractometer equipped
with a CCD detector. Measurements were carried out at 88 K using
Mo Kα (λ = 0.710 73 Å) radiation, which was wavelength selected
with a single-crystal graphite monochromator. The SMART26 program
package was used to determine the unit-cell parameters and for data
collection. A full sphere of data was collected for each crystal structure.
The raw frame data were processed using SAINT27 and SADABS28 to
yield the reflection data files. Subsequent calculations were carried out
using the SHELXTL29 program suite. Structures were solved by direct
To assess the importance of ligand substituents on the
electronic properties of the X,R(NNN) ligand platform, new
ligand derivatives have been prepared containing different
functional groups in the ligand backbone (X) and in the ligand
arms (R). To determine the electronic influence of backbone
substitution, five different X,iPr(NNNcat)H3 ligand derivatives
were prepared (X = F, H, Me, tBu, and OMe), while the
importance of the amido arm substituent was evaluated by
comparing two derivatives, OMe,R(NNNcat)H3 (R = iPr; 3,5-
C6H3Me2). Isostructural tantalum complexes were prepared
containing these redox-active ligands in all three ligand
oxidation states of Chart 1. The metal complexes were
characterized by NMR, EPR, UV−vis−NIR, and IR spectros-
copies as appropriate, as well as by cyclic voltammetry and X-
ray crystallography. Notably, changes to the ligand backbone
functional groups (X) change the ligand-based redox potentials
over a range of 270 mV, providing a logical approach for tuning
the redox properties of a coordination complex during ligand
synthesis. Conversely, changes to the amido substituent
minimally affect the electronic properties of the coordination
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methods and refined on F2 by full-matrix least-squares techniques to
convergence. Analytical scattering factors30 for neutral atoms were
used throughout the analyses. Hydrogen atoms were generated at
calculated positions and their positions refined using the standard
riding models. ORTEP31 diagrams were generated using ORTEP-3 for
Windows, and all thermal ellipsoids are drawn at the 50% probability
level. Diffraction collection and refinement data are available in the
Supporting Information.
General Preparation of Bis(4-X-2-nitrophenyl)amine. This
preparation was adapted from a previous report.19 Here is outlined a
typical example. A round-bottom flask equipped with a stir bar was
charged with a solution of bis(4-fluorophenyl)amine (5.00 g, 24 mmol,
1 equiv) in 10 mL of concentrated nitric acid and 45 mL of glacial
acetic acid. The solution was stirred for 10 min in an ice−water bath
before isoamyl nitrite (8.75 g, 75 mmol, 3 equiv) was slowly dripped
into solution over the course of 2 min. After stirring for additional 5
min, an orange precipitate was collected and washed with diethyl
ether.
ppm: 6.69 (m, 4H, aryl-H), 6.59 (d, J = 1.5 Hz, 2H, aryl-H), 3.08 (br,
5H, N−H), 1.28 (s, 18H, aryl-tBu). 13C{1H} NMR (128 Hz, C6D6) δ/
ppm: 145.9 (aryl-C), 138.5 (aryl-C), 129.4 (aryl-C), 120.3 (aryl-C),
116.5 (aryl-C), 113.6 (aryl-C), 34.3 (aryl-C(CH3)3), 31.7 (aryl-
C(CH3)3). MS (ESI+) m/z: 311.2 (M+), 312.2 (MH+).
General Preparation of Bis(2-isopropylamino-4-X-phenyl)-
amine (X,iPr(NNNcat)H3). The new ligand precursors were all prepared
by a common procedure which is outlined for a typical example. A
Schlenk flask was charged with bis(2-amino-4-methyl-phenyl)amine
(0.969 g, 4.8 mmol, 1 equiv) and a stir bar. Under an inert atmosphere,
60 mL of degassed MeOH was cannula transferred to the flask.
Degassed acetone (0.78 mL, 10.6 mmol, 2.2 equiv) and degassed
concentrated HCl (0.87 mL, 10.5 mmol, 2.2 equiv) were transferred
via microsyringe to the reaction flask resulting in the reaction mixture
turning green. The green solution stirred in an ice−water bath for 20
min before a solid addition tube filled with NaCNBH3 (1.223 g, 19.5
mmol, 4 equiv) was attached to the Schlenk flask. The solid addition
tube was slowly upended as the green solution stirred. After stirring for
12 h the yellow solution was dried under vacuum, and the yellow-red
residue was treated with water. The organic layer was extracted with
CH2Cl2 (3 × 20 mL). The combined organic layers were dried over
MgSO4, and the solvent was removed under vacuum to a yellow-
brown oil.
Bis(4-fluoro-2-nitrophenyl)amine. This material was prepared in
1
71% yield (5.04 g). H NMR (400 MHz, CDCl3) δ/ppm: 10.68 (s,
1H, N−H), 7.91 (dd, J = 2.8 Hz, 2H, aryl-H), 7.47 (m, 2H, aryl-H),
7.32 (m, 2H, aryl-H). 13C{1H} NMR (77 Hz, CDCl3) δ/ppm: 133.8
(aryl-C), 123.1 (d, J = 92 Hz, aryl-C), 122.9 (aryl-C), 121.3 (d, J = 30
Hz, aryl-C), 113.3 (d, J = 100 Hz, aryl-C), 113.1 (aryl-C). 19F{1H}
NMR δ/ppm: −117.9. MS (ESI+) m/z: 295.3 (M+), 296.1 (MH+).
Bis(4-tert-butyl-2-nitrophenyl)amine. After stirring for 10 min, the
solution was treated with 50 mL of water, and the acidic solution was
neutralized with NaHCO3 to pH 7. The organic layer was extracted
with dichloromethane (3 × 60 mL). The extracts were collected and
dried under vacuum. The resulting red-orange residue was chromato-
graphed on silica gel with a 10:1 hexanes/ethyl acetate (v/v %)
elutent, and the red solution was collected and dried to give bis(4-tert-
butyl-2-nitrophenyl)amine (2.66 g, 40%). 1H NMR (500 MHz,
CDCl3) δ/ppm: 10.85 (s, 1H, N−H), 7.91 (d, J = 2 Hz, 2H, aryl-H),
7.53 (dd, J = 2 Hz, 2H, aryl-H), 7.51 (s, 2H, aryl-H), 1.35 (s, 18H,
tBu). 13C{1H} NMR (77 Hz, CDCl3) δ/ppm: 145.3 (aryl-C), 138.0
(aryl-C), 135.1 (aryl-C), 132.3 (aryl-C), 122.9 (aryl-C), 119.7 (aryl-
C), 34.5 (C(CH3)3), 31.0 (C(CCH3)3. MS (ESI+) m/z: 371.3 (M+),
372.2 (MH+).
General Preparation of Bis(2-amino-4-X-phenyl)amine. The
new compounds were all prepared by a common procedure which is
outlined for a typical example. A 250 mL Schlenk flask equipped with a
stir bar was filled with solid bis(4-methyl-2nitro-phenyl)amine (1.24 g,
4.3 mmol, 1 equiv), Zn powder (100 mesh, 3.712 g, 56.8 mmol, 14
equiv), and NH4Cl (2.78 g, 52 mmol, 13 equiv) before the contents
were exposed to vacuum. Backfilling the Schlenk flask with inert gas,
60 mL of THF was cannula transferred into the flask before a reflux
condenser was attached to the Schlenk flask. The solution stirred at
gentle reflux for 16 h before the pale yellow solution was cooled and
filtered under N2. The yellow solution was dried under vacuum to a
yellow-brown oil.
Bis(2-isopropylamino-4-fluorophenyl)amine, F,iPr(NNNcat)H3 (2b).
1
This material was prepared in 71% yield (0.56 g). H NMR (600
MHz, C6D6) δ/ppm: 6.35 (overlapping, 6H, aryl-H), 3.77 (s, 1H, N−
3
H), 3.62 (s, 2H, N−H), 3.09 (sept, JHH = 7.8 Hz, 2H, CH(CH3)2),
3
0.80 (d, JHH = 7.8 Hz, 12H, CH(CH3)3) . 13C{1H} NMR (128 Hz,
C6D6) δ/ppm: 146.3 (aryl-C), 136.8 (aryl-C), 130.9 (aryl-C), 120.8
(aryl-C), 115.5 (aryl-C), 109.9 (aryl-C), 34.5 (aryl-C(CH3)3), 31.7
(aryl-C(CH3)3), 31.7 (CH(CH3)2). 19F{1H} NMR δ/ppm: −118.0.
MS (ESI+) m/z: 319.2 (M+), 320.2 (MH+).
Bis(2-isopropylamino-4-methyl-phenyl)amine, Me,iPr(NNNcat)H3
1
(2d). This material was prepared in 71% yield (1.06 g). H NMR
(500 MHz, C6D6) δ/ppm: 6.73 (d, 3JHH = 8 Hz, 2H, aryl-H), 6.63 (s,
3
2H, aryl-H), 6.53 (d, JHH = 8 Hz, 2H, aryl-H), 4.86 (br, 3H, N−H),
3.43 (sept, 3JHH = 6 Hz, 2H, CH(CH3)2), 2.22 (s, 6H, aryl-CH3), 0.98
3
(d, JHH = 6.5 Hz, 12H, CH(CH3)3). 13C{1H} NMR (128 Hz, C6D6)
δ/ppm: 133.0 (aryl-C), 130.2 (aryl-C), 123.1 (aryl-C), 121.4 (aryl-C),
119.7 (aryl-C), 114.8 (aryl-C), 45.2 (CH(CH3)2), 22.6 (aryl-CH3),
21.5 (CH(CH3)3). MS (ESI+) m/z: 311.2 (M+), 312.2 (MH+).
Bis(2-isopropylamino-4-tert-butylphenyl)amine, tBu,iPr(NNNcat)H3
1
(2e). This material was prepared in 96% yield (0.36 g). H NMR
(500 MHz, C6D6) δ/ppm: 6.99 (s, 2H, aryl-H), 6.63 (d, 3JHH = 10 Hz,
2H, aryl-H), 6.53 (dd, 3JHH = 7.5 Hz, 4JHH = 2.5 Hz, 2H, aryl-H), 5.28
3
(br, 2H, N−H), 4.70 (br, 1H, N−H), 3.52 (sept, JHH = 8 Hz, 2H,
3
CH(CH3)2), 1.32 (s, 18H, aryl-tBu), 1.02 (d, JHH = 8 Hz, 12H,
CH(CH3)3) . 13C{1H} NMR (128 Hz, C6D6) δ/ppm: 146.3 (aryl-C),
136.8 (aryl-C), 130.9 (aryl-C), 120.8 (aryl-C), 115.5 (aryl-C), 109.9
(aryl-C), 34.5 (aryl-C(CH3)3), 31.7 (aryl-C(CH3)3), 31.7 (CH-
(CH3)2). MS (ESI+) m/z: 395.4 (M+), 396.5 (MH+).
Bis(2-3,5-dimethylphneyl-amino-4-methoxyphenyl)amine,
OMe,DMP(NNNcat)H3 (2f). A Schlenk flask was charged with Pd2(dba)3
(0.120 g, 0.13 mmol, 0.06 equiv), racemic-BINAP (0.240 g, 0.39
mmol, 0.18 equiv,) and 5 mL of THF. The solution was stirred for 2 h
at room temperature and transferred to a Schlenk flask containing
OMe(NNNcat)H5 (0.60 g, 2.31 mmol, 1.2 equiv), 3,5-methyl-
bromobenzene (0.76 g, 4.10 mmol, 2 equiv), and NaOtBu (0.55 g,
5.74 mmol, 2.8 equiv). The mixture was refluxed for 48 h, taken to
dryness, and the resulting slurry was extracted with diethyl ether. The
Bis(2-amino-4-fluorophenyl)amine (1b). A slight modification of
the procedure was used: Zn powder (14.04 g, 215 mmol, 30 equiv)
1
and NH4Cl (8.78 g, 164 mmol, 20 equiv). Yield 98% (1.65 g). H
NMR (400 MHz, C6D6) δ/ppm: 6.31 (td, JHF = 11 Hz, JHH = 3.5 Hz,
4
2H, aryl-H), 6.27 (d, JHH = 5.5 Hz, 2H, aryl-H), 6.12 (dd, JHH = 3.5
Hz, 3JHH = 9 Hz, 2H, aryl-H), 2.99 (br, 5H, N−H). 13C{1H} NMR (77
Hz, CDCl3) δ/ppm: 161.4 (aryl-C), 140.7 (aryl-C), 123.2 (aryl-C),
121.8 (d, J = 48 Hz, aryl-C), 104.9 (d, J = 111 Hz, aryl-C), 102.6 (d, J
= 129 Hz, aryl-C), 113.1 (aryl-C). 19F{1H} NMR δ/ppm: −120.3. MS
(ESI+) m/z: 235.1 (M+), 236.1 (MH+).
1
mixture was filtered and the solution taken to dryness. The H NMR
spectrum indicated the formation of OMe,DMP(NNNcat)H3 with 95%
purity. Pentane wash of the solids gave 0.59 g of the parent compound
in 62% yield. 1H NMR (400 MHz; C6D6; 298 K) δ/ppm: 7.06 (d, 4JHH
= 2.8 Hz, 2H, aryl-H), 6.94 (d, 3JHH4 = 8.7 Hz, 2H, aryl-H), 6.54 (s, 4H,
Bis(2-amino-4-methylphenyl)amine (1d). This material was
prepared in 90% yield (1.27 g). 1H NMR (500 MHz, C6D6) δ/
ppm: 6.62 (d, J = 8 Hz, 2H, aryl-H), 6.51 (d, J = 8 Hz, 2H, aryl-H),
6.26 (s, 2H, aryl-H), 3.08 (br, 5H, N−H), 2.15 (s, 6H, aryl-CH3).
13C{1H} NMR (128 Hz, C6D6) δ/ppm: 138.8 (aryl-C), 132.5 (aryl-C),
129.4 (aryl-C), 120.7 (aryl-C), 120.3 (aryl-C), 117.3 (aryl-C), 21.1
(aryl-CH3). MS (ESI+) m/z: 227.3 (M+), 228.2 (MH+).
3
aryl-H), 6.50 (dd, JHH = 8.7 Hz, JHH = 2.8 Hz, 2H, aryl-H), 6.48 (s,
2H, aryl-H), 5.49 (s, 2H, N−H), 5.09 (s, 1H, N−H), 3.34 (s, 6H, O−
CH3), 2.08 (s, 12H, aryl-CH3). 13C{1H} NMR (128 MHz; C6D6; 298
K) δ/ppm: 156.4 (ipso-C), 144.1 (ipso-C), 138.9 (ipso-C), 136.5
(ipso-C), 130.2 (ipso-C), 123.1 (aryl-C), 121.5 (aryl-C), 116.0 (aryl-
Bis(2-amino-4-tert-butylphenyl)amine (1e). This material was
prepared in 95% yield (1.50 g). 1H NMR (500 MHz, C6D6) δ/
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C), 108.1 (aryl-C), 106.6 (aryl-C), 55.1 (OCH3), 21.4 (aryl-CH3). MS
(ESI+) m/z: 466.1 (M+).
General Preparation of X,R(NNNcat)TaCl2(CNtBu). The new
compounds were all prepared by a common procedure which is
outlined for a typical example. A 20 mL scintillation vial equipped with
a stir bar was filled with solid tBu,iPr(NNNcat)TaCl2 (0.05 g, 0.08 mmol,
1 equiv) and dissolved with 5 mL of toluene. As the red solution
stirred, tert-butylisocyanide (0.01 g, 0.12 mmol, 1.5 equiv) was slowly
added to the solution via microsyringe causing an immediate color
change to dark red. After stirring for 3 h the volatiles were removed
under reduced pressure leaving the product as a dark red solid that was
washed with pentane.
General Preparation of X,R(NNNcat)TaCl2. The new compounds
were all prepared by a common procedure which is outlined for a
typical example. A 50 mL round-bottom flask equipped with a stir bar
was charged with TaMe3Cl2 (0.471 g, 1.59 mmol, 1 equiv) dissolved in
30 mL of diethyl ether and frozen in a cold well. A 0.3 M solution of
Me,iPr(NNNcat)H3 in toluene (5.28 mL, 1.59 mmol, 1 equiv) was
diluted with 10 mL of diethyl ether and chilled to −35 °C. The yellow
TaMe3Cl2 solution was removed from the liquid nitrogen filled cold
well, and just as the solution thawed the ligand solution was slowly
added to the flask with vigorous stirring. The reaction was allowed to
warm to room temperature and continued to stir overnight. The dark
red solution was concentrated down 5 mL before the solution was
diluted with 15 mL of pentane to give a brick red precipitate. The solid
was isolated by filtration and dried under vacuum. X-ray quality
crystals of 3b, 3c, 3d, 3e, 3f·Et2O were obtained from chilled (−35
°C), saturated diethyl ether/pentane solutions.
F,iPr(NNNcat)TaCl2(CNtBu) (4b). This material was prepared in 89%
yield (0.11 g). Anal. Calcd for C18H20N3F2Cl2Ta: C, 42.41; H, 4.49; N,
1
8.60. Found: C, 41.63; H, 4.50; N, 7.47. H NMR (400 MHz; C6D6)
δ/ppm: 6.92 (m, 2H, aryl-H), 6.38−6.27 (overlapping, 4H total, aryl-
H), 5.43 (m, 2H, CH(CH3)2), 1.28 (br, 12H, CH(CH3)2), 0.54 (s,
9H, (CN(C(CH3)3). 13C{1H} NMR (128 MHz; C6D6, 298 K) δ/
ppm: 149.5 (CN(C(CH3)3), 140.8 (ipso-C), 114.7 (d, JCF = 24 Hz,
aryl-C), 106.9 (d, JCF = 23 Hz, aryl-C), 100.2 (d, JCF = 10 Hz, aryl-C),
50.2 (CH(CH3)2), 28.9 (CN(C(CH3)3), 20.3 (CN(C(CH3)3), 16.5
F,iPr(NNNcat)TaCl2 (3b). This material was prepared in 87% yield
(0.32 g). Anal. Calcd for C18H20N3F2Cl2Ta: C, 38.05; H, 3.55; N, 7.40.
(CH(CH3)2). 19F {1H} NMR δ/ppm: −121.9. IR (KBr) νCN(CNtBu)
/
cm−1: 2203.
1
Found: C, 37.80; H, 3.50; N, 7.30. H NMR (500 MHz, C6D6) δ/
H,iPr(NNNcat)TaCl2(CNtBu) (4c). This material was prepared in 85%
ppm: 7.02 (td, JHF = 5.5 Hz, JHH = 2.5 Hz, 2H, aryl-H), 6.39
(overlapping, 4H, aryl-H), 4.40 (sept, 3JHH = 6.5 Hz, 2H, CH(CH3)2),
1.18 (d, 3J = 5 Hz, 12H, CH(CH3)2). 13C{1H} NMR (128 Hz, C6D6)
yield (0.05 g). Anal. Calcd for C18H22N3Cl2Ta: C, 44.89; H, 5.08; N,
1
9.10. Found: C, 45.13; H, 5.54; N, 8.69. H NMR (400 MHz; C6D6)
δ/ppm: 7.39 (d, 3JHH = 8, 2H, aryl-H), 6.62 (td, 3JHH = 8 Hz, 4JHH = 1
δ/ppm: 160.1 (ipso-C), 146.6 (aryl-C), 143.8 (aryl-C), 114.1 (d, JCF
=
3
3
Hz, 2H, aryl-H), 6.56 (t, JHH = 8 Hz, 2H, aryl-H), 6.46 (d, JHH = 8
Hz, 2H, aryl-H), 5.63 (m, 2H, CH(CH3)2), 1.44 (d, 3JHH = 7 Hz, 12H,
CH(CH3)2), 0.52 (s, 9H, (CN(C(CH3)3). 13C NMR {1H} (128 MHz;
C6D6) δ/ppm: 148.9 (ipso-C), 144.9 (CN), 128.3 (ipso-C), 122.5
(aryl-C), 121.3 (aryl-C), 116.0 (aryl-C), 112.6 (aryl-C), 49.9
(CH(CH3)2), 28.9 (CN(C(CH3)3), 20.9 (CN(C(CH3)3), 16.9
(CH(CH3)2). UV−vis (toluene) λmax/nm (ε/M−1 cm−1): 310 (11
000). IR (KBr) νCN(CNtBu)/cm−1: 2203.
24 Hz, aryl-C), 106.5 (aryl-C), 107.3 (d, JCF = 24 Hz, aryl-C), 99.9 (d,
JCF = 24 Hz, aryl-C), 48.1 (CH(CH3)2), 17.0 (CH(CH3)3). 19F {1H}
NMR δ/ppm: −122.6. UV−vis (toluene) λmax/nm (ε/M−1 cm−1): 308
(17 300).
H,iPr(NNNcat)TaCl2 (3c). This material was prepared in 87% yield
(0.65 g). Anal. Calcd for C18H22N3Cl2Ta: C, 40.62; H, 4.17; N, 7.89.
1
Found: C, 40.87; H, 4.20; N, 8.05. H NMR (500 MHz, C6D6) δ/
ppm: 7.52 (br, 2H, aryl-H), 6.70 (br, 4H, aryl-H), 6.52 (br, 2H, aryl-
H), 4.62 (br, 2H, CH(CH3)2), 1.35 (s, 12H, CH(CH3)2). 13C{1H}
NMR (128 Hz, C6D6) δ/ppm: 129.1 (ipso-C), 128.9 (ipso-C), 128.6
(aryl-C), 121.9 (aryl-C), 115.3 (aryl-C), 111.9 (aryl-C), 48.1
(CH(CH3)2), 17.4 (CH(CH3)3). UV−vis (toluene) λmax/nm (ε/
M−1 cm−1): 293 (29 100).
Me,iPr(NNNcat)TaCl2(CNtBu) (4d). Yield is quantitative (0.04 g). Anal.
Calcd for C25H35N4Cl2Ta: C, 46.67; H, 5.48; N, 8.71. Found: C,
1
46.63; H, 5.70; N, 8.88. H NMR (600 MHz; C6D6) δ/ppm: 7.33 (d,
3
3JHH = 8 Hz, 2H, aryl-H), 6.45 (d, JHH = 8 Hz, 2H, aryl-H), 6.43 (s,
2H, aryl-H), 5.64 (m, 2H, CH(CH3)2), 2.23 (s, 6H, aryl-CH3), 1.49
3
Me,iPr(NNNcat)TaCl2 (3d). This material was prepared in 72% yield
(0.17 g). Anal. Calcd for C20H26N3Cl2Ta: C, 42.87; H, 4.68; N, 7.50.
(d, JHH = 6 Hz, 12H, CH(CH3)2), 0.57 (s, 9H, (CN(C(CH3)3).
13C{1H} NMR (128 MHz; C6D6) δ/ppm: 149.1 (ipso-C), 142.9
(CN), 131.7 (ipso-C), 128.6 (aryl-C), 122.1 (aryl-C), 115.6 (aryl-C),
113.2 (aryl-C), 49.8 (CH(CH3)2), 29.0 (CN(C(CH3)3), 20.9
(overlapping, aryl-CH3 and CN(C(CH3)3), 17.0 (CH(CH3)2). IR
(KBr) νCN(CNtBu)/cm−1: 2198.
1
Found: C, 42.50; H, 4.60; N, 7.50. H NMR (600 MHz, C6D6) δ/
ppm: 7.47 (d, 3JHH = 7.8 Hz, 2H, aryl-H), 6.53 (d, 3JHH = 6.5 Hz, 2H,
3
aryl-H), 6.48 (s, 2H, aryl-H), 4.62 (sept, JHH = 7.8 Hz, 2H,
3
CH(CH3)2), 2.25 (s, 6H, aryl-CH3), 1.40 (d, JHH = 6.5 Hz, 12H,
tBu,iPr(NNNcat)TaCl2(CNtBu) (4e). Yield is quantitative (0.06 g). Anal.
Calcd for C31H47N4Cl2Ta: C, 51.17; H, 6.51; N, 7.70. Found: C,
CH(CH3)3). 13C{1H} NMR (128 Hz, C6D6) δ/ppm: 146.1 (ipso-C),
130.7 (ipso-C), 129.0 (ipso-C), 122.1 (aryl-C), 114.4 (aryl-C), 112.1
(aryl-C), 47.6 (CH(CH3)2), 20.7 (aryl-CH3), 17.1 (CH(CH3)3). UV−
vis (toluene) λmax/nm (ε/M−1 cm−1): 300 (19 500).
1
51.71; H, 6.69; N, 7.70. H NMR (400 MHz; C6D6) δ/ppm: 7.40 (d,
3JHH = 9 Hz, 2H, aryl-H), 6.73−6.66 (overlapping, 4H, aryl-H), 5.65
(m, 2H, CH(CH3)2), 1.52 (d, 3JHH = 6 Hz, 12H, CH(CH3)2), 1.24 (s,
18H, C(CH3)3), 0.51 (s, 9H, (CN(C(CH3)3). 13C{1H} NMR (128
MHz; C6D6) δ/ppm: 148.9 (ipso-C), 145.1(CN), 142.9 (ipso-C),
118.3 (aryl-C), 115.2 (aryl-C), 109.9 (aryl-C), 49.8 (CH(CH3)2), 34.0
(C(CH3)3), 31.9 (C(CH3)3), 28.9 (CN(C(CH3)3), 21.2 (CN(C-
(CH3)3), 17.2 (CH(CH3)2). IR (KBr) νCN(CNtBu)/cm−1: 2203.
OMe,DMP(NNNcat)TaCl2(CNtBu) (4f). Yield is quantitative (0.06 g).
Anal. Calcd for C35H39N4O2Cl2Ta: C, 52.58; H, 4.92; N, 7.01. Found:
tBu,iPr(NNNcat)TaCl2 (3e). This material was prepared in 72% yield
(0.35 g). Anal. Calcd for C26H38N3Cl2Ta: C, 48.46; H, 5.94; N, 6.52.
1
Found: C, 48.10; H, 5.80; N, 6.45. H NMR (500 MHz, C6D6) δ/
3
ppm: 7.55 (d, JHH = 8 Hz, 2H, aryl-H), 6.79 (overlapping, 4H, aryl-
H), 4.67 (sept, 3JHH = 6 Hz, 2H, CH(CH3)2), 1.45 (d, 3JHH = 5.5 Hz,
12H, CH(CH3)3), 1.31 (s, 18H, aryl-C(CH3)3). 13C{1H} NMR (128
Hz, C6D6) δ/ppm: 146.3 (ipso-C), 145.8 (ipso-C), 144.4 (ipso-C),
118.7 (aryl-C), 114.4 (aryl-C), 109.1 (aryl-C), 47.9 (CH(CH3)2), 34.2
(C(CH3)3), 31.8 (C(CH3)3), 17.4 (CH(CH3)3). UV−vis (toluene)
λmax/nm (ε/M−1 cm−1): 335 (18 300).
1
C, 53.08; H, 4.95; N, 6.61. H NMR (500 MHz; C6D6) δ/ppm: 7.53
3
(s, 2H, Ph-H), 7.23 (d, JHH = 9 Hz, 2H, Ph-H), 6.89 (s, 2H, Ph-H),
OMe,DMP(NNNcat)TaCl2(OEt2) (3f·Et2O). This material was prepared in
76% yield (0.47 g). Anal. Calcd for C34H40N3O3Cl2Ta: C, 51.66; H,
5.10; N, 5.32. Found: C, 52.05; H, 4.99; N, 5.19. 1H NMR (500 MHz,
C6D6) δ/ppm: 7.36 (br, 2H, aryl-H), 7.12 (s, 4H, aryl-H), 6.60 (m,
2H, aryl-H), 6.37 (br, 2H, Ph-H), 5.95 (br, 2H, Ph-H), 3.45 (m, 4H,
OCH2CH3), 3.29 (br, 6H, OCH3), 2.00 (s, 12H, Ph−CH3), 1.04 (m,
4H, OCH2CH3). 13C{1H} NMR (128 Hz, C6D6) δ/ppm: 157.1 (ipso-
OCH3), 152.6 (ipso-C), 142.3 (ipso-C), 139.5 (ipso-C), 129.8 (ipso-
C), 126.5 (aryl-C), 119.4 (aryl-C), 114.4 (aryl-C), 107.6 (aryl-C), 99.5
(aryl-C), 66.7 (OCH2CH3), 55.0 (aryl-OCH3), 20.8 (aryl-CH3), 14.4
(OCH2CH3). UV−vis (toluene) λmax/nm (ε/M−1 cm−1): 309 (29
200), 1050 (500).
6.63 (s, 2H, Ph-H), 6.29 (d, 3JHH = 9 Hz, 2H, Ph-H), 5.87 (m, 2H, Ph-
H), 3.23 (s, 6H, O−CH3), 2.11 (s, 6H, Ph−CH3), 2.00 (s, 6H, Ph−
CH3), 0.80 (s, 9H, C(CH3)3). 13C{1H} NMR (118 Hz, C6D6) δ/ppm:
158.1 (ipso-C), 154.6 (ipso-C), 144.4 (CN), 140.2 (ipso-C), 138.7
(ipso-C), 135.1 (ipso-C), 129.9 (aryl-C), 126.8 (aryl-C), 125.3 (aryl-
C), 115.6 (aryl-C), 108.3 (aryl-C), 99.0 (aryl-C), 55.1 (aryl-OCH3),
29.1 (CN(C(CH3)3), 21.3 (aryl-CH3). IR (KBr) νCN(CNtBu)/cm−1:
2212.
General Preparation of [NEt3Bn][X,R(NNNcat)TaCl3]. The new
compounds were all prepared by a common procedure which is
outlined for a typical example. A 20 mL scintillation vial equipped with
a stir bar was filled with OMe,iPr(NNNcat)TaCl2 (0.11 g, 0.18 mmol, 1
11247
dx.doi.org/10.1021/ic401496w | Inorg. Chem. 2013, 52, 11244−11255

Inorganic Chemistry
Article
equiv) and dissolved in 16 mL of THF. Next, finely ground
[NBnEt3][Cl] (0.04 g, 0.18 mmol, 1 equiv) was added to the
vigorously stirred reaction mixture. After stirring for 12 h the solution
was filtered. The solvent was removed from the filtrate under reduced
pressure by coevaporation with a 6:1 (v/v) mixture of pentane/THF
to afford the product as a fine red solid.
7.5 Hz, 9H, NCH2CH3), 1.27 (s, 18H, aryl-tBu). 13C{1H} NMR (118
Hz, CD3CN) δ/ppm: 148.8 (ipso-C), 144.8 (ipso-C), 141.4 (ipso-C),
133.4 (ipso-C), 131.6 (ipso-C), 130.3 (aryl-C), 128.2 (aryl-C), 117.6
(aryl-C), 114.2 (aryl-C), 110.0 (aryl-C), 61.1 (NCH2Ph), 53.5
(NCH2CH3), 49.6 (CH(CH3)2), 34.4 (C(CH3)3), 32.0 (C(CH3)3),
18.8 (CH(CH3)3), 8.2 (NCH2CH3). UV−vis (THF) λmax/nm (ε/M−1
cm−1): 313 (17 400).
[NEt3Bn][OMe,iPr(NNNcat)TaCl3] (5a). This material is prepared in
99% yield (0.15 g). Anal. Calcd for C33H48N4O2Cl3Ta: C, 48.33; H,
5.90; N, 6.83. Found: C, 48.26; H, 6.02; N, 7.23. 1H NMR (500 MHz,
[NEt3Bn][OMe,DMP(NNNcat)TaCl3] (5f). This material was prepared in
92% yield (0.15 g). Anal. Calcd for C43H52N4O2Cl3Ta(C4H8O)1.5: C,
3
1
CD3CN) δ/ppm: 7.50 (overlapping, 5H, NBn-H), 6.89 (d, JHH = 9
55.92; H, 6.13; N, 5.32. Found: C, 56.27; H, 5.87; N, 5.60. H NMR
3
4
Hz, 2H, aryl-H), 6.10 (dd, JHH = 6 Hz, JHH = 2.5 Hz, 2H, aryl-H),
(500 MHz, CD3CN) δ/ppm: 7.50 (overlapping, 5H, NCH2Ph-H),
6.96 (d, 3JHH = 9 Hz, 2H, aryl-H), 6.90 (s, 4H, o-Ph-H), 6.87 (s, 2H, p-
Ph-H), 6.14 (dd, 3JHH = 9 Hz, 4JHH = 3 Hz, 2H, aryl-H), 5.14 (d, 4JHH
= 3 Hz, 2H, aryl-H), 4.30 (s, 2H, NCH2Ph), 3.56 (s, 6H, O−CH3),
6.05 (d, 3JHH = 7.5 Hz, 2H, aryl-H), 5.32 (br, 2H, CH(CH3)2), 4.30 (s,
2H, NCH2Ph), 3.70 (s, 6H, OCH3), 3.12 (q, JHH = 7.0 Hz, 6H,
3
3
3
NCH2CH3), 1.39 (d, JHH = 7 Hz, 12H, CH(CH3)3), 1.31 (t, JHH
=
7.0 Hz, 9H, NCH2CH3). 13C{1H} NMR (118 Hz, CD3CN) δ/ppm:
156.2 (ipso-C), 149.8 (ipso-C), 133.4 (ipso-C), 131.6 (ipso-C), 130.2
(aryl-C), 129.9 (aryl-C), 128.1 (aryl-C), 114.1 (aryl-C), 105.3 (aryl-
C), 99.6 (aryl-C), 61.2 (N−CH2Ph), 56.0 (OCH3), 53.4 (N−
CH2CH3), 49.9 (CH(CH3)2), 18.7 (CH(CH3)3), 8.1 (N−CH2CH3).
UV−vis (THF) λmax/nm (ε/M−1 cm−1): 315 (18 300).
3.11 (q, JHH = 8 Hz, 6H, NCH2CH3), 2.25 (s, 12H, aryl-CH3), 1.32
3
3
(t, JHH = 8 Hz, 9H, NCH2CH3). 13C{1H} NMR (118 Hz, CD3CN)
δ/ppm: 157.2 (ipso-C), 154.3 (ipso-C), 145.9 (ipso-C), 139.4 (ipso-
C), 134.9 (ipso-C), 133.4 (aryl-C), 131.6 (aryl-C), 130.2 (aryl-C),
129.2 (aryl-C), 128.1 (ipso-C), 126.8 (aryl-C), 114.6 (aryl-C), 106.6
(aryl-C), 98.5 (aryl-C), 61.0 (NCH2Ph), 55.8 (aryl-OCH3), 53.4
(NCH2CH3), 21.3 (aryl-CH3), 8.2 (NCH2CH3).
[NEt3Bn][F,iPr(NNNcat)TaCl3] (5b). This material is prepared in 99%
yield (0.11 g). Anal. Calcd for C31H42N3F2Cl3Ta: C, 46.78; H, 5.32; N,
7.04. Found: C, 46.40; H, 5.32; N, 7.00. 1H NMR (500 MHz,
General Preparation of X,R(NNNsq)TaCl3. The new compounds
were all prepared by a common procedure which is outlined for a
typical example. Method A follows: A 20 mL scintillation vial equipped
with a stir bar was filled with solid H,iPr(NNNcat)TaCl2 (0.08 g, 0.15
mmol, 1 equiv) that was then dissolved in 10 mL of toluene. As the
solution stirred, N-chlorosuccinimide (0.02 g, 0.16 mmol, 1.05 equiv)
was added as a solid. After stirring for 12 h the solvent volume was
reduced in vacuo (3 mL), and the brown solution was cooled (−35°)
to induce precipitation of H,iPr(NNNsq)TaCl3. The product was
collected by filtration and dried under vacuum. Method B follows: A
20 mL scintillation vial equipped with a stir bar was filled with solid
Me,iPr(NNNcat)TaCl2 (0.07 g, 0.13 mmol, 1 equiv) that was then
dissolved in THF (5 mL). The resulting red solution was chilled to
−35 °C and then treated with a cold solution of PhICl2 (0.02 g, 0.07
mmol, 0.5 equiv) in diethyl ether (10 mL). The reaction mixture was
stored overnight at −35 °C. Complex Me,iPr(NNNsq)TaCl3 precipitated
as a dark green solid that was collected by filtration and dried under
vacuum.
3
CD3CN) δ/ppm: 7.50 (overlapping, 5H, NBn-H), 6.93 (dd, JHH = 6
Hz, 4JHH = 3 Hz, 2H, aryl-H), 6.55 (td, 3JHF = 9 Hz, 4JHH = 2.5 Hz, 2H,
3
Ph-H), 630−6.21 (overlapping, 4H, aryl-H), 5.33 (sept, JHH = 6 Hz,
3
2H CH(CH3)2), 4.31 (s, 2H, NCH2Ph), 3.12 (q, JHH = 7 Hz, 6H,
NCH2CH3), 1.39 (d, 3JHH = 6.5 Hz, 12H, CH(CH3)3), 1.31 (t, 3JHH
=
7 Hz, 9H, NCH2CH3). 13C{1H} NMR (118 Hz, CD3CN) δ/ppm:
149.6 (ipso-C), 139.4 (ipso-C), 133.4 (ipso-C), 131.6 (ipso-C), 130.2
(aryl-C), 128.1 (aryl-C), 113.7 (d, J = 40 Hz, aryl-C), 105.6 (d, J = 90
Hz, aryl-C), 104.3 (d, J = 90 Hz, aryl-C), 99.8 (d, J = 103 Hz, aryl-C),
61.1 (NCH2Ph), 53.4 (NCH2CH3), 50.4 (CH(CH3)2), 18.3 (CH-
(CH3)3), 8.1 (NCH2CH3). 19F{1H} NMR δ/ppm: −125.1. UV−vis
(THF) λmax/nm (ε/M−1 cm−1): 310 (16 700).
[NEt3Bn][H,iPr(NNNcat)TaCl3] (5c). This material was prepared in
99% yield (0.11 g). Anal. Calcd for C31H44N4Cl3Ta: C, 48.99; H, 5.84;
1
N, 7.37. Found: C, 48.80; H, 5.97; N, 7.29. H NMR (500 MHz,
3
CD3CN) δ/ppm: 7.50 (overlapping, 5H, NBn-H), 7.11 (d, JHH = 9
Hz, 1H, aryl-H), 6.55 (m, 4H, aryl-H), 6.46 (d, 3JHH = 9 Hz, 2H, aryl-
H), 5.42 (sept, 3JHH = 8 Hz, 2H, CH(CH3)2), 4.30 (s, 2H, NCH2Ph),
OMe,iPr(NNNsq)TaCl3 (6a). UV−vis (toluene) λmax/nm (ε/M−1 cm−1):
333 (27 000), 588 (4100), 1118 (3800). EPR (toluene, 298 K): g =
1.958; Aiso = 33 G.
3
3
3.12 (q, JHH = 9 Hz, 6H, NCH2CH3), 1.41 (d, JHH = 8.5 Hz, 12H,
CH(CH3)3), 1.33 (t, JHH = 9 Hz, 9H, NCH2CH3). 13C{1H} NMR
3
F,iPr(NNNsq)TaCl3 (6b). This material is produced in a yield of 62%
(0.07 g). Anal. Calcd for C18H22N3Cl3Ta: C, 35.81; H, 3.34; N, 6.96.
Found: C, 36.38; H, 3.53; N, 6.64. UV−vis (toluene) λmax/nm (ε/M−1
cm−1): 326 (19 200), 553 (1900), 1151 (1900). EPR (toluene, 298 K):
g = 1.964; Aiso = 30 G.
(118 Hz, CD3CN) δ/ppm: 149.0 (ipso-C), 143.4 (ipso-C), 133.5
(ipso-C), 131.6 (ipso-C), 130.3 (aryl-C), 128.1 (aryl-C), 122.1 (aryl-
C), 120.7 (aryl-C), 115.0 (aryl-C), 112.9 (aryl-C), 61.2(NCH2Ph),
53.5 (NCH2CH3), 50.0 (CH(CH3)2), 18.7 (CH(CH3)3), 8.2
(NCH2CH3). UV−vis (THF) λmax/nm (ε/M−1 cm−1): 310 (18 500).
[NEt3Bn][Me,iPr(NNNcat)TaCl3] (5d). This material was prepared in
99% yield (0.12 g). Anal. Calcd for C33H48N4Cl3Ta: C, 50.29; H, 6.14;
H,iPr(NNNsq)TaCl3 (6c). This material is produced in a yield of 53%
(0.05 g). Anal. Calcd for C18H20N3Cl3Ta: C, 38.08; H, 3.91; N, 7.40.
Found: C, 38.52 ; H, 4.26; N, 7.05. UV−vis (toluene) λmax/nm (ε/
M−1 cm−1): 326 (23 700), 567 (1700), 1265 (2200). EPR (toluene,
298 K): g = 1.964 ; Aiso = 30 G.
1
N, 7.11. Found: C, 49.90; H, 6.12; N, 6.92. H NMR (500 MHz,
3
CD3CN) δ/ppm: 7.50 (overlapping, 5H, NBn-H), 6.94 (d, JHH = 8
3
Me,iPr(NNNsq)TaCl3 (6d). This material is produced in a yield of 43%
(0.03 g). Anal. Calcd for C20H26N3Cl3Ta: C, 40.32; H, 4.40; N, 7.05.
Found: C, 40.79; H, 4.80; N, 6.65. UV−vis (toluene) λmax/nm (ε/M−1
cm−1): 328 (25 300), 580 (2400), 1228 (2800). EPR (toluene, 298 K):
g = 1.962; Aiso = 31 G.
Hz, 2H, aryl-H), 6.36 (d, JHH = 8 Hz, 2H, aryl-H), 6.30 (s, 2H, aryl-
H), 5.38 (br, 2H CH(CH3)2), 4.29 (s, 2H, NCH2Ph), 3.11 (q, 3JHH
=
3
7 Hz, 6H, NCH2CH3), 2.32 (s, 6H, aryl-CH3), 1.40 (d, JHH = 7 Hz,
3
12H, CH(CH3)3), 1.31 (t, JHH = 7.0 Hz, 9H, NCH2CH3). 13C{1H}
NMR (118 Hz, CD3CN) δ/ppm: 149.0 (ipso-C), 141.2 (ipso-C),
133.4 (ipso-C), 131.6 (ipso-C), 131.4 (aryl-C), 130.2 (aryl-C), 128.1
(aryl-C), 121.1 (aryl-C), 114.4 (aryl-C), 113.4 (aryl-C), 61.0
(NCH2Ph), 53.4 (NCH2CH3), 49.7 (CH(CH3)2), 20.8 (aryl-CH3),
18.6 (CH(CH3)3), 8.1 (NCH2CH3). UV−vis (THF) λmax/nm (ε/M−1
cm−1): 313 (14600).
tBu,iPr(NNNsq)TaCl3 (6e). This material is produced in a yield of 40%
(0.04 g). Anal. Calcd for C26H38N3Cl3Ta: C, 45.93; H, 5.63; N, 6.18.
Found: C, 46.19; H, 5.82; N, 6.49. UV−vis (toluene) λmax/nm (ε/M−1
cm−1): 330 (25 000), 576 (2100), 1237 (2100). EPR (toluene, 298 K):
g = 1.962; Aiso = 31 G.
[NEt3Bn][tBu,iPr(NNNcat)TaCl3] (5e). This material was prepared in
99% yield (0.09 g). Anal. Calcd for C39H60N4Cl3Ta: C, 53.70; H, 6.93;
OMe,DMP(NNNsq)TaCl3 (6f). This material is produced in a yield of
53% (0.08 g). Anal. Calcd for C26H38N3Cl3Ta(C7H8)0.25: C, 49.21; H,
4.16; N, 5.42. Found: C, 49.50; H, 4.35; N, 5.38. UV−vis (toluene)
λmax/nm (ε/M−1 cm−1): 335 (29 000), 586 (4100), 1035 (4500). EPR
(toluene, 298 K): g = 1.960; Aiso = 29 G.
1
N, 6.42. Found: C, 53.50; H, 6.90; N, 6.40. H NMR (500 MHz,
CD3CN) δ/ppm: 7.50 (overlapping, 5H, NBn-H), 7.00 (d, 3JHH = 8.5
3
4
Hz, 2H, aryl-H), 6.60 (dd, JHH = 6.5 Hz, JHH = 2 Hz, 2H, aryl-H),
4
3
General Procedure for the Preparation of X,R(NNNq)-
6.46 (d, JHH = 2 Hz, 2H, aryl-H), 5.42 (sept, JHH = 6.5 Hz, 2H
3
t
CH(CH3)2), 4.33 (s, 2H, NCH2Ph), 3.12 (q, JHH = 7.5 Hz, 6H,
TaCl2(NPh-p-R′) (R′ = Bu, CF3, CH3). The new compounds were
NCH2CH3), 1.42 (d, 3JHH = 6.5 Hz, 12H, CH(CH3)3), 1.31 (t, 3JHH
=
all prepared by a common procedure, which is outlined for a typical
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example. A 20 mL scintillation vial was charged with a toluene solution
of F,iPr(NNNcat)TaCl2 (0.05 g, 0.08 mmol, 1 equiv). tBu-p-PhN3 (0.02
g, 0.10 mmol, 1.2 equiv) was added dropwise resulting in a color
change from dark red to brownish green. The solution was stirred
overnight after which time the volatiles were removed and the
resulting tacky solid was coevaporated and washed with pentane to
give a greenish solid. X-ray quality crystals of 7a and 7b were obtained
from chilled (−35 °C), saturated toluene solutions.
OMe,DMP(NNNq)TaCl2(NPh-p-CH3) (9f). Anal. Calcd for
C37H37N4Cl2O2Ta: C, 54.09; H, 4.54; N, 6.82. Found: C, 54.35; H,
1
4.51; N, 6.69. Yield 79% (0.14 g). H NMR (400 MHz; CDCl3) δ/
3
ppm: 8.05 (d, JHH = 9 Hz, 2H, aryl-H), 7.13 (s, 4H, aryl-H), 6.97 (s,
2H, aryl-H), 6.80 (m, 2H, aryl-H), 6.73 (m, 2H, aryl-H), 6.05 (m, 2H,
aryl-H), 5.50 (d, 3JHH = 9 Hz, 2H, aryl-H), 3.70 (s, 6H, O−CH3), 2.35
(s, 12H, aryl-CH3), 2.32 (s, 3H, aryl-CH3). 13C{1H} NMR (128 MHz;
CDCl3; 298 K) δ/ppm: 166.5 (ipso-C), 166.3 (ipso-C), 152.7 (ipso-
C), 149.3 (ipso-C), 139.2 (ipso-C), 137.3 (ipso-C), 132.8 (ipso-C),
128.2 (aryl-C), 127.1 (aryl-C), 126.6 (aryl-C), 125.4 (aryl-C), 122.6
(aryl-C), 121.9 (aryl-C), 97.6 (aryl-C), 56.1 (aryl-OCH3), 21.4 (aryl-
CH3), 20.7 (aryl-CH3). UV−vis (toluene) λmax/nm (ε/M−1 cm−1):
310 (29 700), 364 (19 200), 467 (12 700), 566 (5100), 773 (16 600).
UV−vis (THF) λmax/nm (ε/M−1 cm−1): 309 (26 700), 364 (17 200),
466 (11 400), 566 (4600), 774 (15 000).
F,iPr(NNNq)TaCl2(NPh-p-tBu) (7b). This material was prepared in
1
92% yield (0.06 g). H NMR (400 MHz; CDCl3) δ/ppm: 7.98 (dd,
3
3
2H, JHH = 9.0 Hz, JHF3 = 6.0 Hz, aryl-H), 7.39 (d, 2H, JHH = 8.6 Hz,
aryl-H), 7.23 (d, 2H, JHH = 8.6 Hz, aryl-H), 7.01−6.98 (overlapping,
4H total, aryl-H), 4.78 (m, 2H, CH(CH3)2), 1.71 (d, 12H, JHH = 6.5
3
Hz, CH(CH3)2), 1.34 (s, 9H, C(CH3)3). 13C NMR (78 MHz; CDCl3)
δ/ppm: 166.9 (ipso-C), 164.6 (d, JCF = 12 Hz, ipso-C), 153.3 (ipso-
C), 147.3 (ipso-C), 139.1 (ipso-C), 127.5 (aryl-C), 126.0 (d, JCF = 12
Hz, aryl-C), 124.9 (aryl-C), 118.7 (d, JCF = 30 Hz, aryl-C), 102.1 (d,
JCF = 25 Hz, aryl-C), 56.7 (CH(CH3)2), 34.5 (C(CH3)3), 31.8
(C(CH3)3), 24.1 (CH(CH3)2). 19F NMR (376.5 MHz; CDCl3) δ/
ppm: −96.9 (br). UV−vis (THF) λmax/nm (ε/M−1 cm−1): 299 (21
000), 369 (10 300), 533 (2500), 848 (8400).
RESULTS AND DISCUSSION
■
Synthesis of X,R(NNNcat)H3 Ligands. Previously, we
described a three-step synthesis for the preparation of
OMe,iPr(NNNcat)H3 (2a, Scheme 1).19 An Ullmann coupling of
H,iPr(NNNq)TaCl2(NPh-p-tBu) (7c). Anal. Calcd for C28H35N4Cl2Ta:
C, 49.50; H, 5.19; N, 8.25. Found: C, 49.12; H, 5.34; N, 8.11. 85%
yield (0.04 g). 1H NMR (400 MHz; CDCl3) δ/ppm: 8.10 (d, 2H, 3JHH
Scheme 1
3
= 9.0 Hz, aryl-H), 7.51 (d, 2H, JHH = 9 Hz, aryl-H), 7.39
3
(overlapping, 4H total, aryl-H), 7.25 (d, 2H, JHH = 8 Hz, aryl-H),
7.17 (t, 2H, 3JHH = 7.5 Hz, aryl-H), 5.06 (br, 2H, CH(CH3)2), 1.75 (d,
12H, 3JHH = 6 Hz, CH(CH3)2), 1.35 (s, 9H, C(CH3)3). 13C NMR (78
MHz; CDCl3) δ/ppm: 163.0 (ipso-C), 153.6 (ipso-C), 146.8 (ipso-C),
142.5 (ipso-C), 135.2 (aryl-C), 127.7 (aryl-C), 127.4 (aryl-C), 124.9
(aryl-C), 124.5 (aryl-C),118.3 (aryl-C), 56.2 (CH(CH3)2), 34.4
(C(CH3)3), 31.8 (C(CH3)3), 24.4 (CH(CH3)2). UV−vis (THF)
λmax/nm (ε/M−1 cm−1): 304 (25 000), 374 (10 500), 565 (1900), 884
(8500).
Me,iPr(NNNq)TaCl2(NPh-p-tBu) (7d). Anal. Calcd for C30H39N4Cl2Ta:
C, 50.93; H, 5.56; N, 7.92. Found: C, 50.52; H, 5.27; N, 7.74. Yield
83% (0.06 g). 1H NMR (400 MHz; CDCl3) δ/ppm: 7.98 (d, 2H, 3JHH
= 9 Hz, aryl-H), 7.37 (d, 2H, 3JHH = 8 Hz, aryl-H), 7.25 (d, 2H, 3JHH
=
8 Hz, aryl-H), 7.15 (br, 2H, aryl-H), 6.98 (d, 2H, 3JHH = 9 Hz, aryl-H),
3
5.02 (br, 2H, CH(CH3)2), 2.42 (s, 6H, aryl-CH3), 1.74 (d, 12H, JHH
= 6 Hz, CH(CH3)2), 1.35 (s, 9H, C(CH3)3). 13C NMR (78 MHz;
CDCl3) δ/ppm: 163.0 (ipso-C), 153.8 (ipso-C), 147.0 (ipso-C), 146.6
(ipso-C), 140.8 (ipso-C), 130.0 (aryl-C), 127.4 (aryl-C), 124.8 (aryl-
C), 123.9 (aryl-C), 116.8 (aryl-C), 55.6 (CH(CH3)2), 34.4 (C-
(CH3)3), 31.8 (C(CH3)3), 24.4 (CH(CH3)2), 23.8 (aryl-CH3). UV−
vis (THF) λmax/nm (ε/M−1 cm−1): 306 (22 800), 379 (10 900), 553
(2200), 857 (9200).
OMe,iPr(NNNq)TaCl2(NPh-p-CF3) (8a). Anal. Calcd for
C27H30N4Cl2F3O2Ta: C, 43.16; H, 4.02; N, 7.46. Found: C, 43.41;
H, 3.88; N, 7.06. 81% yield (0.06 g). 1H NMR (400 MHz; CDCl3) δ/
ppm: 7.95 (d, 2H, 3JHH = 9 Hz, aryl-H), 7.62 (d, 2H, 3JHH = 8 Hz, aryl-
H), 7.39 (d, 2H, 3JHH = 8 Hz, aryl-H), 6.86 (dd, 3JHH = 9 Hz, 4JHH = 2
Hz, 2H total, aryl-H), 6.53 (d,4JHH = 2 Hz, 2H, aryl-H) 4.86 (m, 2H,
3
CH(CH3)2), 3.97 (s, 6H, OCH3), 1.70 (d, 12H, JHH = 6 Hz,
CH(CH3)2). 13C NMR (78 MHz; CDCl3) δ/ppm: 166.5 (ipso-C),
165.0 (ipso-C), 158.9 (ipso-C), 137.8 (aryl-C), 127.8 (aryl-C), 125.6
(aryl-C), 125.4 (aryl-C), 121.7 (aryl-C), 95.3 (aryl-C), 56.2 (OCH3),
55.4 (CH(CH3)2), 24.2 (CH(CH3)2). 19F NMR (376.5 MHz; CDCl3)
δ/ppm: −61.8 (br). UV−vis (THF) λmax/nm (ε/M−1 cm−1): 286 (25
300), 376 (12 300), 464 (9700), 528 (3800), 788 (11 000).
2-nitroanisidine and 3-iodo-4-nitroanisole with CuI gave the
bis(2-nitro-4-methoxyphenyl)amine. Because similar starting
materials are not commercially available for the other 4,4′-
substituted ligand derivatives, a different approach was used to
synthesize the ligand backbone of the general formula
X,iPr(NNNcat)H3 (X = F, 2b; H, 2c; Me, 2d; tBu, 2e). In each
case, the appropriate diarylamine (A, Scheme 1) was reacted
with isoamyl nitrite to effect selective nitration ortho to the
central nitrogen atom and form the corresponding bis(2-nitro-
4-X-phenyl)amine derivative (B, Scheme 1). The dinitro
species were reduced with zinc metal and ammonium chloride
to give the triammine derivatives 1a−e. Reductive amination of
F,iPr(NNNq)TaCl2(NPh-p-CF3) (8b). This material was prepared in
1
84% yield (0.04 g). H NMR (400 MHz; CDCl3) δ/ppm: 8.04 (m,
3
2H, aryl-H), 7.65 (dd, 2H, JHH = 9 Hz, JHF = 6 Hz, aryl-H), 7.38 (d,
3
2H, JHH = 8 Hz, aryl-H), 7.05−6.85 (overlapping, 4H total, Ph-H),
4.81 (m, 2H, CH(CH3)2), 1.68 (d, 12H, 3JHH = 6 Hz, CH(CH3)2). 19
F
NMR (376.5 MHz; CDCl3) δ/ppm: −61.9 (s, CF3), −96.0 (s, aryl-F).
13C NMR spectrum was not acquired due to the low solubility of the
product. UV−vis (THF) λmax/nm (ε/M−1 cm−1): 302 (20 800), 370
(9200), 528 (2100), 847 (7100).
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1a−e using acetone and sodium cyanoborohydride produced
the isopropyl-substituted ligand precursors 2a−e in 71−96%
yield (Scheme 1). In order to test a different steric group on the
(NNN) ligand platform, the related ligand OMe,DMP(NNNcat)H3
(DMP = 3,5-dimethylphenyl; 2f) was prepared from 1a and
3,5-dimethylbromobenzene via palladium-catalyzed cross-cou-
pling. With 8 mol % loading of Pd2(dba)3, 2f was obtained in
multigram quantities and in 60−70% yields.32
Structural data for 3b−e obtained by single-crystal X-ray
diffraction revealed five-coordinate complexes in the solid state,
consistent with the previously reported solid-state molecular
structure for 3a.19 Figure 1a depicts an ORTEP of 3d as a
Synthesis of X,R(NNNcat)3− Complexes of Tantalum.
Metalation of ligands 2b−f proceed smoothly using trimethyl-
tantalum dichloride as originally reported for 2a (Scheme 2).19
Scheme 2
Figure 1. ORTEP diagrams showing thermal ellipsoids drawn at 50%
probability for (a) Me,iPr(NNNcat)TaCl2 (3d) and (b)
OMe,DMP(NNNcat)TaCl2(OEt2) (3f·Et2O). Hydrogen atoms and
cocrystallized solvent molecules have been omitted for clarity.
Treatment of the ligand precursors 2a−f with cold diethyl ether
solutions of TaCl2Me3 afforded the corresponding dichlor-
otantalum complexes X,iPr(NNNcat)TaCl2 (X = OMe, 3a; F, 3b;
H, 3c; Me, 3d; tBu, 3e) and OMe,DMP(NNNcat)TaCl2(OEt2) (3f·
Et2O) upon loss of 3 equiv of methane. Depending on reaction
conditions, the complexes can be isolated as the diethyl ether
adducts, with derivative 3f showing the greatest affinity for
weakly coordinating solvents.
typical example of the structure type. Selected metrical
parameters for 3a (X = OMe), 3b (X = F), and 3d (X =
Me) are listed in Table 1 (ORTEP diagrams and crystallo-
graphic data for 3b−f are available as Supporting Information).
Tantalum derivatives 3a−e exhibit trigonal bipyramidal
geometry around the tantalum center with the (NNNcat)3−
ligand coordinated in a meridional fashion. The chloride ligands
share the equatorial plane with the central nitrogen atom of the
(NNNcat)3− ligand. The metal−ligand bond lengths for 3a−e
are within the expected ranges for tantalum(V) complexes, with
minimal deviations in bond lengths between individual
derivatives;34−36 for example, the Ta−Cl bond lengths only
vary between approximately 2.31 and 2.33 Å in 3a−e. The
carbon−nitrogen and carbon−carbon distances within the
(NNN) ligand backbone are consistent with the fully reduced,
catecholate form of the ligand.
NMR spectroscopic data for 3a−e are consistent with
1
diamagnetic C2v-symmetric tantalum(V) complexes. The H
and 13C NMR spectra showed single resonances attributed to
the methyl and methine moieties of the isopropyl groups; in
addition, only one set of signals assigned to the backbone aryl
resonances was observed.19,33 The H NMR spectrum of 3f·
1
Et2O shows significant broadening of the resonances for the
ligand backbone and for the coordinated ether molecule,
suggesting an equilibrium between the six-coordinate ether
adduct and a five-coordinate species analogous to that observed
for 3a−e.
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The electrochemistry of anions [5a−f]− was probed by cyclic
voltammetry. In MeCN solvent using [nBu4N][PF6] as the
electrolyte, [5a−f]− have two partially reversible (ipa ≥ ipc) one-
electron oxidations. Figure 2 shows the cyclic voltammograms
Table 1. Selected Bond Lengths (Å) and Angles (deg) for
X,iPr(NNNcat)TaCl2 (X = OMe, 3a; X = F, 3b; X = Me, 3d)
a
3a
3b
3d
Bond Distances/Å
2.3348(7)
Ta(1)−Cl(1)
Ta(1)−Cl(2)
Ta(1)−N(1)
Ta(1)−N(2)
N(1)−C(1)
N(2)−C(2)
C(1)−C(2)
C(2)−C(3)
C(3)−C(4)
C(4)−C(5)
C(5)−C(6)
C(6)−C(1)
2.3351(5)
2.3276(5)
2.0805(15)
1.9620(16)
1.424(2)
1.387(2)
1.411(3)
1.392(3)
1.377(3)
1.371(3)
1.391(3)
1.391(3)
2.3267(6)
2.082(3)
1.963(2)
1.423(3)
1.383(3)
1.414(4)
1.385(4)
1.391(4)
1.390(4)
1.401(4)
1.383(4)
2.077(3)
1.995(2)
1.406(3)
1.388(3)
1.423(3)
1.393(3)
1.391(3)
1.416(4)
1.393(4)
1.388(3)
Bond Angles/deg
117.90(2)
Cl(1)−Ta(1)−Cl(2)
120.72(3)
113.28(3)
a
Data taken from ref 19.
In contrast to 3a−e, derivative 3f crystallized as the six-
coordinate diethyl ether adduct 3f·Et2O. Figure 1b shows the
molecular structure of 3f·Et2O, which is best described as a
distorted octahedron containing a meridional (NNN) ligand
and two cis chloride ligands (selected metrical parameters
shown in Table 3). In general, the six-coordinate environment
of 3f·Et2O results in longer metal−ligand bonds than observed
in 3a−e. The Ta−Cl bonds are 2.38 Å (trans to N) and 2.39 Å
(trans to OEt2); similarly, the Ta−N bond lengths in 3f·Et2O
are elongated by 0.03 Å compared to 3a−e. The Cs symmetry
of 3f·Et2O in the solid state supports the hypothesis that the
symmetric NMR spectra for 3f derive from reversible
dissociation of the coordinated ether molecule on the NMR
time scale.
Figure 2. Cyclic voltammetry (MeCN, 0.1 M [Bu4N][PF6] at 200 mV
s−1) of 1 mM solutions of (from top to bottom) [F,iPr(NNNcat)TaCl3]−
(5b), [H,iPr(NNNcat)TaCl3]− (5c), [tBu,iPr(NNNcat)TaCl3]− (5e),
[
[
Me,iPr(NNNcat)TaCl3]− (5d), [OMe,iPr(NNNcat)TaCl3]− (5a), and
OMe,DMP(NNNcat)TaCl3]− (5f). The asterisk (*) denotes the
potentials of the bulk tantalum complex. Potentials are referenced to
[Cp2Fe]+/0
.
The dichlorotantalum complexes X,R(NNNcat)TaCl2 (3a−f)
reacted cleanly with tBuNC to generate dark red, six-coordinate
adducts of the general formula X,R(NNNcat)TaCl2(CNtBu)
(4a−f) as shown in Scheme 2. The 1H and 13C NMR spectra of
4a−f indicate static structures on the NMR time scale. For
complexes 4a−e, the NMR data are consistent with C2v-
symmetric structures, indicating the presence of trans chloride
ligands with the isocyanide ligand bound trans to the central
nitrogen of the (NNN) ligand. In contrast, the NMR data for 4f
indicates a Cs-symmetric complex, requiring that the isocyanide
binds trans to a chloride ligand, consistent with the solid-state
structure of 3f·Et2O. The solid-state IR spectra of 4a−f
confirmed the coordination of the isocyanide ligand to an
electron-poor tantalum(V) center since the complexes
displayed C≡N stretch frequencies of νC≡N ∼ 2203 cm−1,
which is higher in frequency than the free isonitrile molecule
(2125 cm−1).19,37−39
Table 2. Electrochemical Properties of
[NBnEt3][X,R(NNNcat)TaCl3] (5a−f)
E°2 (V vs [Cp2Fe]+/0
)
X
R
compd
[(NNN)TaCl3]0/−
[(NNN)TaCl3]+/0
OMe
F
iPr
iPr
5a
5b
5c
5d
5e
5f
−0.17
0.10
0.23
0.50
0.47
0.36
0.42
0.23
H
iPr
0.02
Me
tBu
OMe
iPr
−0.07
−0.02
−0.13
iPr
DMP
for [5a−f]−, and Table 2 summarizes the reduction potentials
with respect to an internal [Cp2Fe]+/0 couple. Both redox
potentials for [5a−e]− show a linear free energy relationship
with the nature of the functional group in the ligand backbone.
That is, derivative [5b]− with an electron-withdrawing fluoride
substituent is oxidized at less positive potentials than derivative
[5a]− with the electron-donating methoxy substituent.
Comparison of [5a]− and [5f]− shows that the nitrogen
substituent has a small effect on the ligand redox properties for
the first oxidation and no measurable effect on the second
oxidation.
One- and two-electron oxidized tantalum complexes are
described here. Tantalum complexes of the one-electron
oxidized X,R(NNNsq)2− ligand were prepared by the addition
of Cl• to 3a−f. The addition of 1 equiv of N-chlorosuccinimide
or 1/2 equiv of PhICl2 to 3a−f resulted in a rapid color change
In order to generate tantalum complexes stable for
electrochemical analysis, chloride adducts of 3a−f were
prepared. Red THF solutions of 3a−f were treated with
[NBnEt3][Cl] to afford [X,R(NNNcat)TaCl3]− (R = iPr, X =
OMe [5a]−; X = F [5b]−; X = H [5c]−; X = Me [5d]−; X = tBu
[5e]− and R = DMP, X = OMe [5f]−) as dark red, [NBnEt3]+
1
salts that were insoluble in aromatic solvents. The H and 13C
NMR spectra of 5a−f in CD3CN displayed resonances
consistent with C2v-symmetric, pseudo-octahedral species,
consistent with a meridional (NNN) ligand and three
meridional chloride ligands.
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from red to dark green signaling the formation of X,R(NNNsq)-
Supporting Information for the remaining EPR spectra). Also
diagnostic for the semiquinonate oxidation state of the (NNN)
ligand are the UV−vis−NIR spectra of 6a−f that show ligand-
based transitions in both the visible (λmax = 550−590 nm) and
near-IR (λmax = 1120−1270 nm) portions of the spectrum
(Figure 4).6,19
TaCl3 (R = iPr, X = OMe, 6a; X = F, 6b; X = H, 6c; X = Me,
t
6d; X = Bu, 6e; and R = DMP, X = OMe, 6f), which were
isolated as dark green microcrystalline solids in modest yields
(Scheme 3).
Scheme 3
Figure 4. UV−vis−NIR spectra of X,R(NNNsq)TaCl3 (6a−f) collected
at 298 K in toluene: OMe,iPr(NNNsq)TaCl3 (6a), F,iPr(NNNsq)TaCl3
(6b), H,iPr(NNNsq)TaCl3 (6c), Me,iPr(NNNsq)TaCl3 (6d),
tBu,iPr(NNNsq)TaCl3 (6e), OMe,DMP(NNNsq)TaCl3 (6f).
1
Complexes 6a−f are paramagnetic, S = /2 complexes with
Single crystals of OMe,DMP(NNNsq)TaCl3 (6f) were obtained
and provided the first X-ray structural data for an (NNN)
ligand in the semiquinonate oxidation state. An ORTEP of 6f is
depicted in Figure 5, and selected bond lengths and angles for
the unpaired electron localized primarily on the redox-active
(NNN) ligand. The room-temperature EPR spectra of 6a−f in
toluene all display isotropic signals between g values of 1.958
and 1.964 with an eight-line pattern indicating hyperfine
coupling to the I = 7/2 tantalum center. The isotropic hyperfine
coupling constants (aTa) for 6a−f all fall in the range 29−33 G,
which is significantly smaller than the coupling constants
observed for well-define tantalum(IV) complexes.19,40
A
representative EPR spectrum for 6d is shown in Figure 3,
along with the corresponding simulated spectrum (see
Figure 5. ORTEP diagram for OMe,DMP(NNNsq)TaCl3 (6f). Thermal
ellipsoids are drawn at 50% probability. Hydrogen atoms and
cocrystallized solvent molecules have been omitted for clarity.
6f are displayed alongside those for the reduced congener
OMe,DMP(NNNcat)TaCl2(OEt2) (3f·Et2O) in Table 3. As
expected, 6f displays a distorted octahedron geometry around
the tantalum center, with a meridional arrangement of chloride
ligands. As expected, the ligand N−C and C−C bond distances
in the structure of 6f fall between those observed for tantalum
complexes of the fully reduced catecholate and fully oxidized
quinonate ligands. For example, the semiquinonate oxidation
state of the ligand in 6f is evident from the N−C bond lengths,
Figure 3. EPR spectrum of Me,iPr(NNNsq)TaCl3 (6d) collected at 298
K in toluene (expt) and the corresponding simulated spectrum (sim).
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Table 3. Selected Bond Lengths (Å) and Angles (deg) for
OMe,DMP(NNNsq)TaCl3 (6f) and
OMe,DMP(NNNcat)TaCl2(OEt2) (3f·Et2O)
6f
3f·Et2O
Bond Distances/Å
2.3678(10)
2.3941(11)
2.3846(10)
2.215(3)
Ta(1)−Cl(1)
Ta(1)−Cl(2)
Ta(1)−Z(3)
Ta(1)−N(1)
Ta(1)−N(2)
Ta(1)−N(3)
N(1)−C(1)
N(2)−C(2)
N(1)−C(16)
N(3)−C(17)
C(1)−C(2)
C(16)−C(17)
O(1)−C(4)
O(2)−C(19)
2.3763(9)
2.3907(9)
a
b
2.222(2)
2.110(3)
2.007(3)
1.996(3)
1.413(4)
1.389(4)
1.415(4)
1.393(4)
1.402(5)
1.397(5)
1.376(4)
1.376(4)
2.008(3)
2.021(3)
1.387(5)
1.377(5)
1.367(5)
1.375(5)
1.419(5)
1.418(5)
1.363(5)
Figure 6. UV−vis−NIR spectra of OMe,iPr(NNNq)TaCl2(NPh4‑tBu
)
(7a), OMe,iPr(NNNq)TaCl2(NPh4‑CF3
)
(8a), F,iPr(NNNq)-
1.354(5)
TaCl2(NPh4‑tBu) (7b), F,iPr(NNNq)TaCl2(NPh4‑CF3) (8b), and
OMe,DMP(NNNq)TaCl2(NPh4‑Me) (9f) collected at 298 K in THF.
Bond Angles/deg
a
b
Cl(2)−Ta(1)−Z(3)
171.83(3)
169.09(3)
a
b
Z = Cl. Z = OEt2.
other hand, comparing the spectrum of 7a and 9f suggests that
the steric substituent, R, on the (NNN) ligand has a negligible
impact on the energy of the absorption bands. Similarly, the
imido substituent has no effect on the energy or intensity of the
absorption features (compare 7a to 8a or 7b to 8b). Taken
together, these results suggest that the frontier electronic
structure of 7−9 is dominated by the diaryl backbone of the
(NNN) ligand.
which are contracted by 0.03 Å relative to the N−C bonds of
catecholate derivatives 3, but elongated by 0.03 Å relative to the
N−C bonds of quinonate derivatives 7 (vide infra).
Nitrene transfer to complexes 3a−f was used to effect two-
electron oxidation of the redox-active (NNN) ligand. As
summarized in Scheme 3, the reaction of complexes 3a−f with
various aryl azides resulted in the liberation of N2 and
formation of the tantalum imido complexes X,R(NNNq)-
TaCl2(N-p-C6H4R′). Thus, 3b−d were treated with N3(p-
Single crystals suitable for X-ray diffraction studies were
t
obtained for OMe,iPr(NNNq)TaCl2(N-p-C6H4 Bu) (7a) and
for Me,iPr(NNNq)TaCl2(N-p-C6H4 Bu) (7d). Figure 7 shows
t
C6H4 Bu) giving X,iPr(NNNq)TaCl2(N-p-C6H4 Bu) (X = F,
7b; H, 7c; Me, 7d) in high yields. Tantalum imido complexes
bearing an electron-withdrawing group on the arylimido ligand
were synthesized by treating 3a and 3b with N3(p-C6H4CF3) to
generate X,iPr(NNNq)TaCl2(N-p-C6H4CF3) (X = OMe, 8a; F,
8b). To determine if sterics would influence the nitrene transfer
reaction, 3f·Et2O was treated with N3(p-C6H4CH3) to give
OMe,R(NNNq)TaCl2(N-p-C6H4CH3) (9f). The compounds
were isolated as microcrystalline brown to green solids that
were sparingly soluble in aromatic solvents. NMR spectra in
CDCl3 of 7−9 are consistent with diamagnetic C2v-symmetric
tantalum(V) complexes. The symmetry of 9f was assigned on
t
t
1
the basis of the H NMR spectra, which showed only one
resonance corresponding to the ortho protons of the DMP
substituent, indicating that the imido ligand is trans to the
central nitrogen of the (NNN) scaffold, as observed in the
other imido complexes.
Tantalum complexes 7−9 all have rich absorbance spectra
with features throughout the UV−vis−NIR regions. Figure 6
shows the electronic absorption spectra of 7a, 7b, 8a, 8b, and 9f
to illustrate the impact that changes to the redox-active (NNN)
ligand and to the terminal imido ligand have on the electronic
properties of the complexes. Sharp transitions between 300 and
500 nm are complimented by a broad and intense absorption in
the red portion of the spectrum.6,19 The electronic substituent,
X, on the (NNN) ligand has a measurable effect on the low-
energy transition as can be seen by comparing 7a to 7b or 8a to
8b in Figure 6. The more electron-withdrawing flouride of 7b
and 8b resulted in a 60 nm red shift of the band compared to
the electron-donating methoxy substituent of 7a and 8a. On the
t
Figure 7. ORTEP diagram for Me,iPr(NNNq)TaCl2(N-p-C6H4 Bu)
(7d). Thermal ellipsoids are drawn at 50% probability. Hydrogen
atoms and cocrystallized solvent molecules have been omitted for
clarity.
an ORTEP diagram for 7d, which is directly analogous to the
structure for 7a; Table 4 presents selected bond metrics for 7d
alongside those for 7a. In short, the structures for 7a and 7d are
indistinguishable with bond length differences of less than 0.01
Å.
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Inorganic Chemistry
Article
minimal impact on the coordination chemistry at the tantalum
center. It can be concluded that the 270 mV potential range set
forth by the electrochemistry of complexes 6a−e does not
change appreciably the nature of the metal−ligand interaction
between the tantalum center and the (NNN) ligand: across five
ligand derivatives, and in each ligand oxidation state, the redox-
active frontier orbital of the tantalum complexes is localized
primarily on the ligand. For other, less electropositive metals,
this conclusion may not hold, as more extensive mixing15
between metal d orbitals and ligand-based orbitals could lead to
changes in the interactions between the metal center and the
ancillary ligands.
Table 4. Selected Bond Lengths (Å) and Angles (deg) for
t
X,iPr(NNNq)TaCl2(N-p-C6H4 Bu) (X = OMe, 7a; X = Me,
7d)
X = OMe (7a)
X = Me (7d)
Bond Distances/Å
Ta(1)−N(4)
Ta(1)−Cl(1)
Ta(1)−Cl(2)
Ta(1)−N(1)
Ta(1)−N(2)
Ta(1)−N(3)
N(1)−C(1)
N(2)−C(2)
N(1)−C(11)
N(3)−C(12)
C(1)−C(2)
C(11)−C(12)
X−C(4)
1.793(2)
2.4393(8)
2.4248(8)
2.272(2)
1.790(2)
2.4345(7)
2.4355(7)
2.281(2)
2.128(2)
2.125(2)
1.353(3)
1.348(3)
1.341(3)
1.345(3)
1.460(4)
1.461(4)
1.503(4)
1.503(4)
2.119(2)
2.132(3)
A sixth ligand derivative, OMe,DMP(NNNcat)H3 (2f), contain-
ing the methoxy group in the backbone and 3,5-dimethylphenyl
steric groups, was used to probe the impact of different sterics
surrounding the metal center. Despite the fact that aryl groups
can significantly modulate π-interactions in amido-type ligands,
changing the steric substituent on the (NNN) ligand from
isopropyl to 3,5-dimethylphenyl had little effect on the ligand
redox potentials as evidenced by the nearly identical cyclic
voltammograms for 5a and 5f, respectively. Similarly,
comparisons of the UV−vis−NIR absorption spectra of 6a
and 6f or 7a and 9f, revealed only minor shifts in the absorption
bands resulting from inclusion of the DMP groups. The DMP
groups did impact coordination geometry though, as evidenced
in the NMR and structural data for 3f·Et2O and 4f. In these
complexes the data suggests that the DMP groups facilitate
formation of a Cs symmetric structure with cis chloride ligands
as opposed to the C2v symmetric structure with trans chloride
ligands that is observed for all isopropyl derivatives 3a−e and
4a−e. While it is not clear why the DMP-substituted (NNN)
ligand affords a different coordination isomer for tantalum
complexes 3f·Et2O and 4f, it provides another synthetic
strategy for controlling substrate binding in small molecule
activation reactions.
1.352(4)
1.347(4)
1.342(4)
1.338(4)
1.460(4)
1.465(4)
1.357(4)
X−C(14)
1.350(4)
Bond Angles/deg
171.4(2)
Ta(1)−N(4)−C(21)
167.1(2)
CONCLUDING REMARKS
■
A family of tantalum complexes containing derivatives of the
redox-active (NNN) ligand have been prepared and charac-
terized by a battery of analytical techniques with the goal of
benchmarking the impact of different electronic and steric
substituents on the ligand. Five ligand derivatives with different
electronic substituents incorporated into the ligand backbone
were studied, including OMe, tBu, Me, H, and F. On the basis
of electrochemical data, collected on [X,iPr(NNNcat)TaCl3]−
derivatives 5a−e, these groups rank from the most electron
donating, OMe, to the least donating, F, affording a 270 mV
range for the reduction potentials of the tantalum complexes.
The electronically different (NNN) ligands also manifest
differences in the UV−vis−NIR spectra of complexes
containing the oxidized X,R(NNNsq)2− or X,R(NNNq)− ligands.
In these cases, the lowest-energy absorption band is sensitive to
the electronic group of the (NNN) ligand with electron-
releasing methoxy groups pushing the absorption to higher
energy and the electron-withdrawing fluoro groups pushing the
absorption to lower energy. This effect was small, encompass-
ing differences in λmax of only 30 to 60 nm; moreover, the data
suggests that these transitions are π → π* transitions localized
entirely on the backbone of the (NNN) ligand with little to no
participation from the tantalum center or from the ancillary
ligands.
ASSOCIATED CONTENT
* Supporting Information
■
S
X-ray diffraction data (in CIF format) for 3b−f, 6f, 7a, and 7d.
ORTEP diagrams of the molecular structures, and EPR, UV−
vis−NIR and NMR spectra. This material is available free of
charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*E-mail: aheyduk@uci.edu.
■
Notes
The authors declare no competing financial interest.
Despite the readily measurable differences in redox potentials
for the electronic derivatives of the (NNN) ligand platform,
these substituents did not manifest significant changes to the
structural or spectroscopic properties of the various tantalum
complexes studied. Isonitrile stretching frequencies in 4a−e
were identical, suggesting that all X,R(NNNcat)3− ligand
derivatives act as similar electron donors to the tantalum(V)
ACKNOWLEDGMENTS
■
This work was supported by the NSF (CHE-1152543). The
authors thank Dr. Joseph W. Ziller for assistance with analysis
of X-ray diffraction data. A.F.H. is a Camille Dreyfus Teacher-
Scholar.
1
center. Similarly, EPR spectra of the S = /2 complexes 6a−e
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7d showed differences of less than 0.01 Å in the metal−ligand
bond lengths (including the TaNR bond) suggesting that the
different electronic substituents in X,R(NNNq)− ligands have
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