Synthesis and characterization of trianionic pincer-type complexes of tantalum(V) including solid (X-ray) and solution (NMR) state assignment of an intraligand N-H - F hydrogen bonding interaction the Veige research group dedicates this manuscript to our colleague and friend, Professor George Christou on the occasion of his 60th birthday.

Article abstract of DOI:10.1016/j.poly.2013.07.004

Tanatalum(V) complexes ligated with a trianionic ONO^3- pincer-type ligand were synthesized and characterized. Using the ONO trianionic pincer ligand precursor 2,2′-(azanediylbis(3-methyl-6,1-phenylene))bis(1, 1,1,3,3,3-hexafluoropropan-2-ol) [CF₃-ONO]H₃ (1) and various sources of Ta(V), the following pincer complexes were synthesized: (I) anionic trichloride complex {[CF₃-ONO]TaCl₃}{HNEt ₃}·(HNEt₃Cl)_x (2), (II) diamido-amine complex [CF₃-ONO]Ta(NMe₂)₂(HNMe₂) (3-NHMe₂) and its amine free version [CF₃-ONO]Ta(NMe ₂)₂ (3), (III) the neutral trichloride [CF ₃-ONHO]TaCl₃ (4), (IV) the neutral dichloride [CF ₃-ONO]TaCl₂(THF) (5), and (V) the dibenzyl complex [CF₃-ONO]TaBn₂ (6). Characterization methods include the complete assignment (where possible) for ¹H, ¹³C, ¹⁹F, and ¹⁵N NMR spectra for complexes 2-6, and single crystal X-ray diffraction characterization for complexes 2, 3, and 4. Interesting aspects regarding the interplay of the ligand in its dianionic form versus its trianionic form, and an unusual Ha?F hydrogen bonding interaction within complex 4 were discovered from these synthetic pursuits.

Full text of DOI:10.1016/j.poly.2013.07.004

Polyhedron 64 (2013) 377–387
Contents lists available at SciVerse ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
Synthesis and characterization of trianionic pincer-type complexes
of tantalum(V) including solid (X-ray) and solution (NMR)
state assignment of an intraligand N–H—F hydrogen bonding interaction
⇑
Sudarsan VenkatRamani, Matias E. Pascualini, Ion Ghiviriga, Khalil A. Abboud, Adam S. Veige
University of Florida, Center for Catalysis, P.O. Box 117200, Gainesville, FL 32611, United States
a r t i c l e i n f o
a b s t r a c t
Tanatalum(V) complexes ligated with a trianionic ONO3ꢀ pincer-type ligand were synthesized and
characterized. Using the ONO trianionic pincer ligand precursor 2,20-(azanediylbis(3-methyl-6,1-pheny-
lene))bis(1,1,1,3,3,3-hexafluoropropan-2-ol) [CF3-ONO]H3 (1) and various sources of Ta(V), the following
pincer complexes were synthesized: (I) anionic trichloride complex {[CF3-ONO]TaCl3}{HNEt3}ꢁ(HNEt3Cl)x
(2), (II) diamido-amine complex [CF3-ONO]Ta(NMe2)2(HNMe2) (3-NHMe2) and its amine free version
[CF3-ONO]Ta(NMe2)2 (3), (III) the neutral trichloride [CF3-ONHO]TaCl3 (4), (IV) the neutral dichloride
[CF3-ONO]TaCl2(THF) (5), and (V) the dibenzyl complex [CF3-ONO]TaBn2 (6). Characterization methods
include the complete assignment (where possible) for 1H, 13C, 19F, and 15N NMR spectra for complexes
2–6, and single crystal X-ray diffraction characterization for complexes 2, 3, and 4. Interesting aspects
regarding the interplay of the ligand in its dianionic form versus its trianionic form, and an unusual
Hꢁ ꢁ ꢁF hydrogen bonding interaction within complex 4 were discovered from these synthetic pursuits.
Published by Elsevier Ltd.
Article history:
Received 1 May 2013
Accepted 3 July 2013
Available online 11 July 2013
The Veige research group dedicates this
manuscript to our colleague and friend,
Professor George Christou on the occasion of
his 60th birthday.
Keywords:
Pincer
Trianionic
Tantalum
Enamine
Inorganic
Pincer-type
N–H–F hydrogen bonding
1. Introduction
metal–carbon multiple bonds by employing an ONO3- pincer li-
gand. Constraining the lone-pair on the central N-atom of the pin-
One of the most important developments in the field of homo-
geneous transition metal catalysis is the creation of tailored ancil-
lary ligands [1]. Critical to the expansion of catalyst development is
the ability to fine-tune catalyst properties via ligand modification.
Monoanionic pincer ligands [2–7] are particularly well-suited for
this task because they offer multiple sites for functionalization
and a structural framework that is relatively easy to modify [8].
New growth in pincer ligand chemistry is being fuelled, in part,
by the use of novel trianionic pincer [9–20], and pincer-type ligands
[21–24]. Recent application of trianionic pincer ligands in catalysis
is good evidence that growth in this area should mirror the success
of traditional pincer ligands [25–31].
cer to be collinear with M@C or M„C bonds creates an
a a
‘‘inorganic enamine’’ bonding combination [32]. This concept was
demonstrated within tungsten-alkylidene and tungsten-alkylidyne
complexes featuring the ONO3- pincer ligand 2,20-(azanediylbis-
(3-methyl-6,1-phenylene))bis(1,1,1,3,3,3-hexafluoropropan-2-ol)
[CF3-ONO]H3 (1; Fig. 1) [32].
In an effort to further understand the coordination chemistry of
this unique ligand, ONO3ꢀ trianionic pincer ligand complexes of
tantalum were synthesized. Specifically, this work involves the
synthesis and characterization of the anionic trichloride complex
{[CF3-ONO]TaCl3}{HNEt3}ꢁ(HNEt3Cl)x (2), the diamido-amine
complex [CF3-ONO]Ta(NMe2)2(HNMe2) (3-NHMe2), the amine free
species [CF3-ONO]Ta(NMe2)2 (3), the neutral trichloride
[CF3-ONHO]TaCl3 (4), the neutral dichloride [CF3-ONO]TaCl2(THF)
(5), and the dibenzyl complex [CF3-ONO]TaBn2 (6). These synthetic
endeavors establish conditions that provide access to two different
bonding modes for ONO3ꢀ ligand: its dianionic and trianionic form.
In addition, an unusual Hꢁ ꢁ ꢁF hydrogen bonding interaction within
complex 4 was also observed.
Being relatively new, some of the fundamental principles of
trianionic pincer ligand coordination chemistry are still emerging.
For example, it is possible to accentuate the nucleophilicity of
⇑
Corresponding author. Tel.: +1 352 392 9844; fax: +1 352 392 3255.
E-mail address: veige@chem.ufl.edu (A.S. Veige).
0277-5387/$ - see front matter Published by Elsevier Ltd.
http://dx.doi.org/10.1016/j.poly.2013.07.004

378
S. VenkatRamani et al. / Polyhedron 64 (2013) 377–387
and used as received. Hexafluoroacetone ((CF3)2CO) used in the
CF3
CF3
CF3
CF3
OH HO
synthesis of [CF3-ONO]H3 (1) [32] was provided by Synquest Labs
Inc. TaBn5 [33] was prepared according to reported procedures.
Elemental analyses were performed at Complete Analysis Labora-
tory Inc., Parsippany, New Jersey.
H
N
2.1.1. Synthesis of {[CF3-ONO]TaCl3}{HNEt3}ꢁ(HNEt3Cl)x (2)
[CF3-ONO]H3 (1)
In a nitrogen filled glove box a glass vial was charged with [CF3-
ONO]H3 (1) (0.149 g, 0.281 mmol) and 2 mL of CH2Cl2, and then
cooled to ꢀ35 °C. In another vial, a suspension of TaCl5 (0.106 g,
0.296 mmol) in CH2Cl2 was cooled to ꢀ35 °C. To the stirring sus-
pension of TaCl5 the solution of 1 was added drop-wise. The reac-
tion mixture color gradually changed from pale red to deep brown.
After achieving the deep brown color the solution was stirred an
additional 2 h. At the end of 2 h, NEt3 (0.029 g, 0.286 mmol) was
added drop-wise to the deep brown solution resulting in a color
change to deep red and the solution was stirred for an additional
30 min. The volatiles were removed in vacuo and the resulting
sticky residue was dissolved in a minimum amount of benzene
and filtered. The filtrate was slowly added drop-wise to cold pen-
tane (ꢀ35 °C) to afford compound 2 as a fine deep red microcrys-
Fig. 1. Trianionic pincer-type proligand [CF3-ONO]H3 (1).
2. Experimental
2.1. Materials and methods
Unless specified otherwise, all manipulations were performed
under an inert atmosphere using standard Schlenk or glovebox
techniques. Glassware was pre-dried in an oven before us. Pentane,
toluene, tetrahydrofuran (THF) and diethyl ether (Et2O) were dried
using a GlassContours drying column. Benzene-d6 (Cambridge
Isotopes) was dried over sodium-benzophenone-ketyl and distilled
or vacuum transferred and stored over 4 Å molecular sieves.
Chloroform-d1 (Cambridge Isotopes) was dried over anhydrous
CaCl2; vacuum transferred, passed over a plug of basic alumina,
and stored over 4 Å molecular sieves. Toluene-d8 (Cambridge
Isotopes) was dried over CaH2; vacuum transferred and stored over
4 Å molecular sieves. Resublimed TaCl5 was purchased from Strem
Chemicals Inc. and used as received. Benzylmagnesium chloride
(1 M in Et2O, and 2 M in THF) was received from Sigma–Aldrich
talline powder. Single crystals were obtained by layering
a
concentrated benzene solution of 2 with pentane and allowing
the solution to rest overnight at ambient temperature. Yield:
0.125 g (49%). 1H NMR (300.15 MHz, C6D6, 25 °C): d = 9.56 (bs,
1H, HN(CH2CH3)3), 7.71 (s, 2H, Ar–H), 6.77 (dd, 2H, Ar–H,
3J = 8.8 Hz, 4J = 1.3 Hz),6.74 (d, 2H, Ar–H, 3J = 8.5 Hz), 2.43 (m,
11H, HN(CH2CH3)3), 2.01 (s, 6H, –CH3),0.81 (t, 16H, HN(CH2CH3)3,
3J = 6.7 Hz) ppm. 13C NMR (from 1H–13C gHMBC) (500 MHz, C6D6,
10
8
6
7
CF3
9
1
CF3
5
2
O
3
4
N
14 13
15
O
11
CF3
12
18
CF3
16 17
19
20
CF3
CF3
CF3
CF3
CF3
CF3
22
22
O
Ta
O
NHMe2
O
Ta
21
O
Ta
O
Cl
N
H
21
NMe2
N
Cl
N
Me2N
N
NMe2
21
NMe2
21
Cl O
CF3
CF3
21
CF3
CF3
CF3
CF3
2
3-HNMe2
3
CF3
CF3
CF3
CF3
CF3
CF3
25
21
22
O
Ta
O
Ta
Cl
Cl
O
24
23
N
O
24 23
HN
Cl
N
Ta
O
22
Cl O
CF3
CF3
Cl O
CF3
CF3
21
CF3
CF3
6
4
5
Fig. 2. Labeling scheme for 1H and 13C NMR resonances of compounds 2–6.

S. VenkatRamani et al. / Polyhedron 64 (2013) 377–387
379
4
25 °C): d (ppm) = 148.5 (Ar C), 132.6 (Ar C), 131.1 (Ar C), 126.2 (Ar
C), 125.0 (Ar C), 124.6 (Ar C), 124.1 (CF3), 122.6 (CF3), 83.5 (C(CF3)2),
and 20.2 (CH3). 19FNMR (282.39 MHz, C6D6, 25 °C):d = ꢀ70.1 (q, 3F,
4JFF = 9.1 Hz), ꢀ71.6 (q, 3F, JFF = 9.2 Hz) and ꢀ73.1 (dq, 3F,
4JFF = 10.4 Hz, JHꢁ ꢁ ꢁF = 7 Hz) ppm. Anal. Calc. for C20H13Cl3F12NO2Ta:
C, 29.49; H, 1.61; N, 1.72. Found: C, 29.46; H, 1.78; N, 1.64%.
4
4JFF = 9.6 Hz), and ꢀ73.8 (q, 3F, JFF = 9.6 Hz) ppm.
2.1.5. Synthesis of [CF3-ONO]TaCl2(THF) (5)
2.1.2. Synthesis of [CF3-ONO]Ta(NMe2)2(HNMe2) (3-NHMe2)
A 100 mL flask was charged with 4 (0.212 g, 0.260 mmol) as a
brown powder; dissolution in THF (20 mL) produced an immediate
color change to deep red. The solution was stirred for 3.5 h. The
volatiles were removed in vacuo and the product was dried under
high vacuum overnight to afford a fine red powder. The product
may additionally be purified by washing with pentane. It is to be
noted however that prolonged exposure to pentane causes decom-
position. Yield: 0.144 g (65%). 1H NMR (300.15 MHz, C6D6, 25 °C):
d = 7.63 (s, 2H, Ar–H), 6.69 (dd, 2H, Ar–H, 3J = 8.3 Hz, 4J = 1.6 Hz),
6.55 (d, 2H, Ar–H, 3J = 8.6 Hz), 3.82 (br, 4H, THF) 1.96 (s, 6H, –
CH3), 1.10 (br, 4H, THF) ppm. 13C NMR (from 1H–13C gHMBC)
(500 MHz, C6D6, 25 °C): d (ppm) = 146.6 (Ar C), 134.5 (Ar C),
131.6 (Ar C), 126.3 (Ar C), 124.5 (Ar C), 124.1 (Ar C), 123.4 (CF3),
122.1 (CF3), 84.2 (C(CF3)2), and 20.0 (CH3). 19F NMR (282.39 MHz,
In a nitrogen-filled glove box a vial was charged with Ta(NMe2)5
(100 mg, 0.250 mmol) and Et2O. The solution was stirred vigor-
ously and a 1 mL Et2O solution of [CF3-ONO]H3 (1) (130 mg,
0.246 mmol) was added drop-wise, producing an instantaneous
color change from orange to yellow. Stirring the solution for
20 min results in the deposition of crystals of 3-NHMe2 on the
walls of the vial. Placing the vial in a ꢀ35 °C freezer completes
the precipitation. The supernatant is then decanted. Note: it is dif-
ficult to isolate 3-NHMe2 from residual solvent since evaporation
leads to concomitant loss of the HNMe2 ligand. 1H NMR (C6D6,
300 MHz, 25 °C) d (ppm): 7.58 (s, 2H), 6.95 (d, 2H, 3J = 9 Hz), 6.79
(d, 2H, 3J = 9 Hz), 3.27 (s, 12H), 2.04 (bs, 3H), 2.03 (s, 6H), 0.89
(bs, 1H). 13C NMR (from 1H–13C gHMBC) (500 MHz, toluene-d8,
25 °C): d (ppm) = 150.9 (Ar C), 131.7 (Ar C), 129.2 (Ar C), 127.1
(Ar C), 126.4 (Ar C), 124.1 (CF3), 123.6 (Ar C), 123.3 (CF3), 83.6
(C(CF3)2), 44.8 (N(CH3)2), 39.0 (HN(CH3)2), and 20.2 (CH3). 19F
NMR (282.39 MHz, toluene-d8, 25 °C): d (ppm) = ꢀ70.6 (dq, 3F,
4
4
C6D6, 25 °C):d = ꢀ70.7 (q, 6F, JFF = 9.7 Hz), ꢀ74.3 (q, 6F, JFF = 9.6 -
Hz) ppm. Anal. Calc. for C24H20Cl2F12NO3Ta: C, 33.90; H, 2.37; N,
1.65. Found: C, 33.93; H, 2.46; N, 1.61%.
4
4JFF = 9.9 Hz, JHꢁ ꢁ ꢁF = 1.2 Hz), and ꢀ75.1 (q, 3F, JFF = 9.9 Hz).
2.1.6. Synthesis of [CF3-ONO]TaBn2 (6)
Method A:
A 100 mL flask was charged with 5 (0.045 g,
2.1.3. Synthesis of [CF3-ONO]Ta(NMe2)2 (3)
0.053 mmol) and 2 mL ether and cooled to ꢀ35 °C. BnMgCl (2 M
in THF) (0.053 mL) was added drop wise to a stirred solution of
5. After approximately 3 h the color had gradually changed from
red to orange with copious salt precipitation. The salt was removed
via filtration and the clear orange filtrate was evaporated in vacuo.
The orange solid obtained was extracted with pentanes to afford a
yellow solution and complex 5 was obtained as a yellow solid upon
solvent removal. Yield: 0.015 g (32%).
Method B: TaBn5 (0.090 g, 0.141 mmol) and [CF3-ONO]H3 (1)
(0.060 g, 0.113 mmol) were mixed as solids. 1.5 mL C6D6 was
added and the solution was stirred for 16 h at 50 °C. The reaction
was monitored at regular intervals by NMR for completion. Upon
completion, the resulting red–orange solution was filtered and
the volatiles were removed in vacuo. The crude product was
triturated with pentane several times and filtered. Extracting
with pentane and solvent removal afforded 6 as a yellow solid.
Yield: 0.074 g (83%). 1H NMR (300.15 MHz, C6D6, 25 °C): d = 7.41
(s, 2H, Ar–H), 6.8 (m, 10H, (C6H5CH2)2ꢀ), 6.62 (dd, 2H, Ar–H,
3J = 8.6 Hz, 4J = 1.8 Hz), 6.53 (d, 2H, Ar–H, 3J = 8.5 Hz), 2.29 (s, 4H,
In a nitrogen-filled glove box a vial was charged with Ta(NMe2)5
(100 mg, 0.249 mmol) and 1 mL of C6D6. The solution was stirred
vigorously and [CF3-ONO]H3 (1) (130 mg, 0.246 mmol) in 1 mL of
C6D6 was added drop-wise, producing an instantaneous color
change from orange to yellow. After stirring the resulting solution
for 1 h, all the volatiles were removed under vacuum and the sam-
ple was dried under vacuum for 2 h. Yield: (140 mg, 71%). 1H NMR
(C6D6, 300 MHz, 25 °C) d (ppm): 7.60 (s, 2H), 6.86 (d, 2H, 3J = 9 Hz),
6.74 (d, 2H, 3J = 9 Hz), 3.11 (s, 12H), 1.98 (s, 6H). 13C NMR (from
1H–13C gHMBC) (500 MHz, C6D6, 25 °C): d (ppm) = 149.3 (Ar C),
131.9 (Ar C), 130.7 (Ar C), 126.8 (Ar C), 126.7 (Ar C), 124.5 (Ar C),
123.9 (CF3), 123.2 (CF3), 83.2 (C(CF3)2), 43.9 (N(CH3)2), and 20.2
(CH3). 19F NMR (282.39 MHz, benzene-d6, 25 °C): d (ppm) = ꢀ71.5
(dq, 3F, 4JFF = 9.9 Hz, JHꢁ ꢁ ꢁF = 1.2 Hz), and ꢀ76.0 (q, 3F, 4JFF = 9.9 Hz).
Anal. Calc. for C24H24F12N3O2Ta: C, 36.24; H, 3.04; N, 5.28. Found: C,
36.13; H, 2.99; N, 5.26%.
2.1.4. Synthesis of [CF3-ONHO]TaCl3 (4)
In a nitrogen filled glove box, a 100 mL round bottom flask was
charged with [CF3-ONO]H3 (1) (0.120 g, 0.227 mmol) and TaCl5
(0.089 g, 0.248 mmol). Toluene (10 mL) was added to the solid
mixture to afford a red solution. The flask was fitted with a reflux
condenser and a Y-adapter and immediately taken out of the box
and connected to a Schlenk line. The solution changed from red
to brown upon refluxing the solution for 1.5 h and then all volatiles
were removed in vacuo to obtain 4 as a microcrystalline powder.
Yield: 0.169 g (95%). Note: the product could be purified further
by washing with pentane. Single crystals were obtained by cooling
a concentrated ether solution to ꢀ35 °C. 1H NMR (300.15 MHz,
C6D6, 25 °C): d = 7.61 (q, 1H, Npincer–H, JHꢁ ꢁ ꢁF = 7 Hz), 7.59 (s, 1H,
Ar-H), 7.43 (s, 1H,Ar-H), 6.54 (dd, 1H, Ar–H, 3J = 8.3 Hz,
4J = 1.7 Hz), 6.44 (dd, 1H, Ar–H, 3J = 8.3 Hz, 4J = 1.7 Hz), 6.28 (d,
1H, Ar–H, 3J = 8.2 Hz), 6.04 (d, 1H, Ar–H, 3J = 8.3 Hz), 1.76 (s, 3H,
–CH3), 1.69 (s, 3H, –CH3) ppm. 13C NMR (from 1H–13C gHMBC)
(500 MHz, C6D6, 25 °C): d (ppm) = 146.6 (Ar C), 142.3 (Ar C),
140.7 (Ar C), 137.3 (Ar C), 133.0 (Ar C), 132.9 (Ar C), 131.9 (Ar C),
128.6 (Ar C), 128.5 (Ar C), 127.6 (Ar C), 123.9 (Ar C), 123.6 (CF3),
122.9 (CF3), 122.9 (CF3), 121.9 (CF3), 121.8 (Ar C), 86.4 (C(CF3)2),
84.2 (C(CF3)2), 20.3 (CH3) and 20.0 (CH3). 19F NMR (282.39 MHz,
(C6H5CH2)2ꢀ), 1.92 (s, 6H, –CH3) ppm. 13C NMR (from 1H–13
C
gHMBC) (500 MHz, C6D6, 25 °C): d (ppm) = 147.8 (Ar C), 132.7
(Ar CBn), 132.0 (Ar C), 131.8 (Ar C),131.5(Ar CBn), 129.3 (Ar CBn),
127.5 (Ar CBn), 126.3 (Ar C), 126.0 (Ar C), 123.4 (Ar C), 123.3
(CF3), 122.3 (CF3), 83.7 (C(CF3)2), 71.2 (CH2Ph) and 20.2 (CH3).
4
19F NMR (282.39 MHz, C6D6, 25 °C):d = ꢀ70.7 (q, 6F, JFF = 9.9 Hz),
4
ꢀ75.2 (q, 6F, JFF = 9.9 Hz) ppm. Anal. Calc. for C34H26F12NO2Ta: C,
45.91; H, 2.95; N, 1.57. Found: C, 45.96; H, 2.86; N, 1.51%.
2.2. NMR spectroscopy characterization and method
NMR spectra were obtained on a Varian Mercury spectrometer
operating at 300 MHz for 1H, or a Varian Inova spectrometer oper-
ating at 500 MHz for 1H. The chemical shifts are reported in d
(ppm) and were referenced to the lock signal on the TMS scale
for 1H and 13C NMR spectra, on the CFCl3 scale for 19F NMR spectra
and neat NH3 scale for 15N NMR spectra. Compounds 2–6 were
characterized by 1H, 13C, 19F and 15N NMR. The chemical shifts
are presented in Table 1 and an atom labelling scheme is provided
in Fig. 2. The assignments were made primarily based on the
cross-peaks observed in the 1H–13C gHMBC spectra. The chemical
4
C6D6, 25 °C): d = ꢀ68.9 (q, 3F, JFF = 10.4 Hz), ꢀ70.2 (q, 3F,
shifts of the fluorinated carbons were measured in the 19F–13
C

380
S. VenkatRamani et al. / Polyhedron 64 (2013) 377–387
Table 1
couplings H10–C5, H10–C7, H5–C7. H4 was identified by its cou-
1H, 13C{1H}, 19F, 15N NMR chemical shift data.
pling with H5, or C6. A similar method was applied to C1-symmet-
ric complexes that have unique atoms in position 11–20. One
coupling of F8 or F9 with C1 was sufficient to identify these fluo-
rine atoms, since the pairs F8–F9 and F18–F19 are revealed by
selective decoupling in the 19F spectra. For complexes featuring
NMR active nuclei such as coordinated THF, and Et3NH+, the proton
signals for positions 21–27 were assigned based on their intensity
and multiplicity. The carbons in these positions were assigned
based on their one-bond and long-range couplings to protons.
d
Compd.
1
2
3-HNMe2
3
4
5
6
H4
H5
H7
6.69
6.63
7.47
1.83
6.69
6.63
7.47
1.83
–
6.74
6.77
7.71
2.01
6.74
6.11
7.71
2.01
2.43
0.81
–
6.92
6.76
7.52
2.05
6.92
6.76
7.52
2.05
3.29
2.05
–
6.82
6.71
7.57
1.95
6.82
6.71
7.57
1.95
3.07
–
6.28
6.54
7.59
1.76
6.04
6.44
7.43
1.69
–
–
–
–
–
84.2
127.6
146.6
132.9
133.0
140.7
128.6
122.9
121.9
20.3
86.4
123.9
142.3
121.8
131.9
137.3
128.5
122.9
123.6
20.0
–
6.55
6.69
7.63
1.96
6.55
6.69
7.63
1.96
3.82
1.10
1.10
3.82
–
84.2
124.1
146.6
124.5
131.6
134.5
126.3
123.4
122.1
20.0
84.2
124.1
146.6
124.5
131.6
134.5
126.3
123.4
122.1
20.0
a
6.53
6.62
7.41
1.92
6.53
6.62
7.41
1.92
2.29
–
H10
H14
H15
H17
H20
H21
H22
H23
H24
H25
C1
C2
C3
C4
C5
C6
C7
C8
C9
C10
C11
C12
C13
C14
C15
C16
C17
C18
C19
C20
C21
C22
C23
C24
C25
HNEt3
NMe2
NHMe2
Npincer
F8
–
–
2.3. X-ray crystallography experimental details
–
–
–
6.89
6.85
6.85
83.7
123.4
147.8
126.0
132.0
131.8
126.3
123.3
122.3
20.2
83.7
123.4
147.8
126.0
132.0
131.8
126.3
123.3
122.3
20.2
71.2
131.5
132.7
129.3
127.5
–
–
–
–
–
–
–
X-ray Intensity data were collected at 100 K on a Bruker SMART
80.6
120.9
143.0
126.1
132.1
134.4
128.3
123.2
123.3
20.2
80.6
120.9
143.0
126.1
132.1
134.4
128.3
123.2
123.3
20.2
–
83.5
124.6
148.5
125.0
131.1
132.6
126.2
124.1
122.6
20.2
83.5
124.6
148.5
125.0
131.1
132.6
126.2
124.1
122.6
20.2
46.0
8.1
83.6
123.6
150.9
126.4
131.7
129.2
127.1
124.1
123.3
20.2
83.6
123.6
150.9
126.4
131.7
129.2
127.1
124.1
123.3
20.2
44.8
39.0
–
83.2
124.5
149.3
126.7
131.9
130.7
126.8
123.9
123.2
20.2
83.2
124.5
149.3
126.7
131.9
130.7
126.8
123.9
123.2
20.2
43.9
–
(DUO for [CF3-ONHO]TaCl3) diffractometer using Mo K
a radiation
(k = 0.71073 Å) and an APEXII CCD area detector. Raw data frames
were read by program SAINT [34] and integrated using 3D profiling
algorithms. The resulting data were reduced to produce hkl reflec-
tions and their intensities and estimated standard deviations. The
data were corrected for Lorentz and polarization effects and
numerical absorption corrections were applied based on indexed
and measured faces. The structure was solved and refined in
SHELXTL6.1, using full-matrix least-squares refinement. The non-
H atoms were refined with anisotropic thermal parameters (except
in disordered partitions) and the H atoms (unless specified) were
calculated in idealized positions and refined riding on their parent
atoms. The refinement was carried out by minimizing the wR2
function using F2 rather than F values. R1 is calculated to provide
a reference to the conventional R value but its function is not
minimized. Solid state structure drawings were created using the
OLEX2 software package [35].
–
–
–
–
–
–
–
–
a
a
a
–
–
–
–
–
–
–
–
3. Results and discussion
–
–
–
–
–
–
–
–
–
57.9
–
–
–
3.1. Synthesis and characterization of {[CF3-ONO]TaCl3}{HNEt3}ꢁ
(HNEt3Cl)x (2)
197.2
a
192.2
–
66.2
233
167.4
180.2
78.4
253
205
ꢀ74.16 ꢀ70.1 ꢀ70.6b
ꢀ75.69 ꢀ73.8 ꢀ75.1c
ꢀ74.16 ꢀ70.1 ꢀ70.6b
ꢀ75.69 ꢀ73.8 ꢀ75.1c
ꢀ71.5b ꢀ70.2 ꢀ70.7 ꢀ70.7
F9
F18
F19
ꢀ76c
ꢀ71.6 ꢀ74.3 ꢀ75.2
ꢀ71.5b ꢀ68.9 ꢀ70.7 ꢀ70.7
ꢀ76c
ꢀ73.1 ꢀ74.3 ꢀ75.2
a
not observable, HNEt3 appears at 9.56 ppm (1H, C6D6), HNMe2 appears at
1.92 ppm. (1H NMR, C6D6), [ONHO]TaCl3 appears at 7.61 Hz (q, JHꢁ ꢁ ꢁF = 7 Hz, 1H, C6D6).
b
quartet (JFF = 9.9 Hz).
doublet of quartets (JFF = 9.9 Hz; JHꢁ ꢁ ꢁF = 1.2 Hz).
measured in toluene-d8.
c
d
ð1Þ
gHSQC spectra, and their assignment to positions 8 and 9 versus 18
and 19 was made based on the long-range coupling observed in the
19F–13C gHMBC spectra of the fluorine atoms to the quaternary car-
Attempts to metalate [CF3-ONO]H3 (1) with TaCl5 employing
lithium transmetalation from n-BuLi (2 or 3 equiv) resulted in
intractable mixtures, thus a milder route was sought. Exposing
TaCl5 to proligand 1 alone in toluene resulted in a color change
from pale red to maroon, but again the reaction mixture appeared
to have multiple products as determined by 1H NMR spectroscopy.
One explanation for the complicated mixture is the fact that HCl is
one of the products from this reaction. To remove the HCl, the
reaction was conducted in the presence of triethylamine (NEt3).
Treating TaCl5 with 1 in CH2Cl2, followed by the addition of a stoi-
chiometric amount of Et3N results in the formation of the trichlo-
ride pincerate complex {[CF3-ONO]TaCl3}{HNEt3}ꢁ(HNEt3Cl)x (2) in
49% yield as a red microcrystalline powder Eq. (1). Upon addition
of the NEt3, vapors of HNEt3Cl appear above the solution indicating
that HCl must evolve prior to addition. Red single crystals precip-
itate from a concentrated benzene solution of 2 layered with
bon two bonds away. Cross-peaks with H4 and H14 reveal the 15
N
chemical shifts in the 1H–15N gHMBC spectra. No stereochemical
assignments were made for H7 and H17 or C8 and C9 since they
are interchangeable. Some assignments were made based on rela-
tive shielding. For example, C1 and C2 were assigned as the most
shielded of the pairs between C1 and C11, and C2 and C12. For
the pairs of fluorine atoms F8 and F18, and F9 and F19, F8 and F9
were assigned as the most deshielded. For compound 4 in specific,
H7 and H17 are interchangeable, as are C8 and C9. C1 was arbi-
trarily taken as the most shielded between C1 and C11, and F8
was taken as the most deshielded between F8 and F9.
In a typical assignment procedure, H7 displays cross-peaks with
the C10 carbon (ꢂ20 ppm), the C1 carbon (80–85 ppm), the C3
carbon (145–160 ppm), and the C5 carbon (130–140 ppm). H10,
H5 and C7 are then identified by one-bond correlations, or by the

S. VenkatRamani et al. / Polyhedron 64 (2013) 377–387
381
pentane. Fig. 3 depicts the results of a single crystal X-ray diffrac-
tion experiment performed on 2 and Table 2 lists crystallographic
refinement data. In the solid state, complex 2 is pseudo-C2-sym-
metric with a Ta(V) ion in a slightly distorted octahedral geometry
with meridional coordination by the ONO3- pincer ligand and chlo-
ride ions. Fig. 3 (right) depicts a truncated view of the Ta(V) core
and highlights the modest deviation from octahedral geometry.
The oxygen atoms create an angle of 164.70(7)° across the Ta(V)
ion. Another distortion, though minimal, occurs across the trans-
chlorides, Cl2 and Cl3, which create an angle of 169.81(2)°. Signif-
icantly elongated due to the trans influence of the amido N-atom of
the pincer, the Ta1–Cl1 bond (2.4508(6) Å) is 0.0233(6) and
0.0938(7) Å longer than Ta1–Cl2 (2.4275(6) Å) and Ta1–Cl3
(2.3570(7) Å), respectively. Perhaps the most interesting solid state
Map and refined freely) points towards the anionic complex,
presumably to minimize charge separation. In fact, it is clear that
a hydrogen bonding interaction occurs. The distance between
H2–Cl1 and H2–Cl2, is 2.52(3) and 2.73(3) Å, respectively. The
H2–Cl2 interaction leads to
a 0.706(7) Å elongation in the
Ta1–Cl2 bond (2.42756(6) Å) relative to the Ta1–Cl3 bond
(2.3570(7) Å).
Consistent with the solid state structure, complex 2 exhibits
C2-symmetry in solution. Most notably, in the 1H NMR spectrum
the ONO3ꢀ methyl protons appear as a singlet at 2.01 ppm. Also, in
the 19F NMR spectrum two quartet resonances appear at ꢀ70.10
and ꢀ73.78 ppm (JFF = 9.6 Hz), for the CF3 groups indicating C2-sym-
metry. The solid state crystal structure does not contain residual
HNEt3Cl; however, the solution phase characterization of
+
feature is the orientation of the HNEt3+ counter cation. The HNEt3
recrystallized
2 indicates the presence of HNEt3Cl, which
orients such that the proton (located within the Difference Fourier
co-crystallizes and is thus not separable. The 1H NMR spectrum of
Fig. 3. Left: Solid state structure of {[CF3-ONO]TaCl3}{HNEt3}ꢁ(HNEt3Cl)x (2), with hydrogen atoms removed for clarity. Also removed is a bromine atom that occurs with 2.5%
occupancy on C14. The brominated impurity occurs from a small amount of over-bromination during the synthesis of 1[32]. Right: A truncated thermal ellipsoid plot of the
Ta(V) core. Selected bond distances [Å]: Ta1–O1 1.8986(17), Ta1–O2 1.9125(17), Ta1–Cl1 2.4508(6), Ta1–Cl2 2.4275(6), Ta1–Cl3 2.3570(7), Ta1–N1 2.0222(19), N2–H2
0.80(3), Cl2–H2 2.74(3), Cl1–H2 2.52(3). Bond angles [°]: O1–Ta1–O2 164.70(7), Cl1–Ta1–N1 175.75(6), Cl2–Ta1–Cl3 169.81(2), C13–N1–Ta1 121.56(15), C3–N1–C13
113.87(17), and C3–N1–Ta1 124.56 (15).
Table 2
X-ray crystallographic structure parameters and refinement data.
2
3-NHMe2
4
Empirical formula
Formula weight
Crystal system
Space group
Dimensions (mm)
a (Å)
C
27H27.98Br0.03Cl3F12N2O2Ta
C26H32F12N4O2Ta
841.51
monoclinic
C2/c
C20H13Cl3F12NO2Ta
814.61
monoclinic
P2(1)/n
917.78
triclinic
P1
ꢀ
0.23 ꢃ 0.07 ꢃ 0.03
0.123 ꢃ 0.118 ꢃ 0.079
0.17 ꢃ 0.05 ꢃ 0.03
9.0697(2)
9.5512(3)
9.0662(6)
b (Å)
c (Å)
11.7937(2)
15.66676(3)
94.393(1)
16.0480(4)
19.8285(6)
90
22.3688(15)
12.8803(8)
90
a
(°)
b (°)
93.049(1)
99.823(2)
106.3170(10)
c
(°)
107.822(1)
1585.59(5)
2
3.850
894
90
90
2506.9(3)
4
4.815
1552
2.158
V (Å3)
2994.71(15)
4
3.777
1652
Z (Å)
Absorption coefficient (mmꢀ1
F (000)
)
Dcalc (g/cm3)
1.922
1.866
c
(Mo K
a
) (Å)
0.71073
0.71073
0.71073
T (K)
100(2)
100(2)
100(2)
h (°)
1.82–27.50
100.0%
2.08–27.50
100.0%
1.82–27.50
100.0%
Completeness to hmax
Reflections collected
25433
20921
47657
Independent reflections (Rint
Data/restraints/parameter
)
7278 (0.0440)
7278/0/427
R1 = 0.0212, wR2 = 0.0464 [6632]
R1 = 0.0253, wR2 = 0.0475
1.273/ꢀ0.640
1.008
3447 (0.0295)
3447/0/211
R1 = 0.0208, wR2 = 0.0426 [3253]
R1 = 0.0231, wR2 = 0.0431
0.442/ꢀ0.533
1.159
5761 (0.0383)
5761/0/351
R1 = 0.0236, wR2 = 0.0559 [4840]
R1 = 0.0316, wR2 = 0.0580
1.086/ꢀ0.836
1.043
Final R1 indices [I > 2
r(I)]
R indices (all data)
Largest difference peak/hole (e Åꢀ3
)
Goodness of fit (GOF) on F2
P
P
P
R1
=
||Fo – |Fc||/ |Fo|. wR2 = (
R
(w(Fo2 – Fc2)2)/ (w(Fo2)2))1/2. GOF = (
R
w(Fo2 – Fc2)2/(n – p))1/2. w = 1/[
r
2(Fo2)+(m⁄p)2 + n⁄p], p = [max(Fo2,0) + 2⁄Fc2]/3, m & n are constants.

382
S. VenkatRamani et al. / Polyhedron 64 (2013) 377–387
2 reveals broad multiplets attributable to the ethyl groups of the
counter cation HNEt3 at 2.43 ppm and 0.81 ppm. Integration of
Ta-NHMe2 bond is longer (2.281(5) Å). Further, the sum of angles
around the two disordered N-atoms, N3 (334.3646(15)°) and N30
(356.6821(17))° is different and this serves as compelling evidence
that N3 is the NHMe2 and N30 is the NMe2 ligand. Another short
N—F distance of 2.90893(6) Å suggests that there may be an
N–H—F hydrogen bonding interaction. However, the solution
phase NMR data does not provide convincing data that the
interaction remains in solution.
+
these resonances indicates the presence of HNEt3Cl as an impurity.
However, this amount is not consistent for each synthesis of 2.
Attempts to separate the two salts via fractional crystallization were
not successful. Data provided in the supporting information comes
from a sample that contains 0.73 equivalents of HNEt3Cl. In addition,
attemptsto alkylatecomplex 2 with Grignard reagentswere notsuc-
cessful. The complication most likely arises from the presence of
HNEt3Cl, thus an alternative Ta(V) synthon was explored.
Table 1 lists the absolute assignment of all 1H, 13C, 19F, and 15N
NMR resonances. The 1H NMR spectrum of the complex, generated
within a sealable NMR tube (toluene-d8) to prevent loss of HNMe2,
provides support for the solid state structure, though in solution
the complex exhibits pseudo-C2 symmetry instead of C1 symmetry
observed in the solid state. In the 19F NMR spectrum of 3-HNMe2, a
3.2. Synthesis and characterization of [CF3-ONO]Ta(NMe2)2(HNMe2)
(3-NHMe2) and [CF3-ONO]Ta(NMe2)2 (3)
quartet at ꢀ75.1 (JFF = 9.9 Hz) and
a doublet of quartets at
ꢀ70.6 ppm (JFF = 9.9 Hz and JHꢁ ꢁ ꢁF = 1.2 Hz) are consistent with the
pseudo-C2 symmetry. The 1.2 Hz coupling arises from the intramo-
lecular C–Hꢁ ꢁ ꢁF interaction between F9 to the C7–H proton of the
pincer aromatic ring and the symmetry-related F19 to the C17–H
(see Fig. 1 and Table 1 for labeling).
In the 1H NMR spectrum, a singlet integrating to twelve protons
for the dimethylamido N–Me’s resonates at 3.29 ppm. A broad res-
onance corresponding to the methyl groups on the pincer ligand
and the NHMe2 in solution, integrating to 21 protons, overlap at
2.05 ppm. 3-NHMe2 exchanges its amine ligand rapidly with free
HNMe2. Evidence for the exchange within 3-NHMe2 comes from
a variable-temperature NMR experiment. Cooling a solution of 3-
HNMe2 in toluene-d8 to ꢀ85 °C affords a spectrum with resonances
attributable to a C1 symmetric compound. The methyl resonances
from the pincer ligand appear at 2.03 ppm and 1.99 ppm. Methyl
resonances attributable to the dimethylamido ligand trans to the
pincer ligand resonate at 3.31 ppm (coincidentally equivalent),
and cis to the pincer ligand at 3.32 and 3.14 ppm. The dimethyl-
amine ligand and free amine resolve into distinct resonances (free;
2.19 ppm, and bound; 1.62 and 1.53 ppm) as determined by cross-
peaks in a gHMBC spectrum. Evidence that distinguishes which
amine resonances correspond to the coordinated amine comes
from NOE data. The methyl protons associated with the coordi-
nated amine that resonate at 1.62 and 1.53 exhibit nOes with the
adjacent methyl protons of the dimethylamido ligand (Fig. 5). A
1H–1H ROESY experiment conducted at ꢀ85 °C in toluene-d8 pro-
vides additional support for the amine ligand exchange. The free
amine proton (HNMe2) that resonates at ꢀ0.02 ppm shows an ex-
change peak with the bound amine proton at 1.92 ppm (Fig. 5a).
The corresponding methyl groups for the free amine, which resonate
at 2.19 ppm, also show an exchange peak inthe ROESY spectrum with
the bound amine methyl protons at 1.62 and 1.53 ppm (Fig. 5b). Com-
plex 3-NHMe2 is not isolable as a bulk solid since evaporation of all
volatiles to dryness results in the loss of NHMe2 from the Ta(V) core
to yield analytically pure [CF3-ONO]Ta(NMe2)2 (3) in quantitative
yield (Eq. (2b)). Table 1 lists the absolute assignment of all 1H, 13C,
19F, and 15N NMR resonances for 3. The 1H NMR spectrum of 3 does
notdiffersignificantlyfromthat of3-NHMe2. The two dimethylamido
methyl protons and the two pincer ligand methyl protons resonate at
3.07 and 1.95 ppm, respectively. Again, the 19F NMR reveals a quartet
at ꢀ76.0 (JFF = 9.9 Hz) and a doublet of quartets at ꢀ71.5 ppm
(JFF = 9.9 Hz and JHꢁꢁ ꢁF = 1.2 Hz). The 1.2 Hz coupling arises from a C–
Hꢁ ꢁ ꢁF interaction between F9 to the C7–H proton of the pincer aro-
matic ring, and the equivalent interaction between F19 to the C17–
H (see Fig. 1 and Table 1 for labeling).
ð2aÞ
ð2bÞ
Considering transmetalation employing n-BuLi did not work,
and the use of mild base NEt3 leads to co-crystallization of insepa-
rable HNEt3Cl, a Ta(V) reagent featuring an internal base was
sought. Treating [CF3-ONO]H3 (1) with Ta(NMe2)5 at ambient tem-
perature in Et2O results in the immediate color change from orange
to yellow that signals the formation of the amido-amine complex
[CF3-ONO]Ta(NMe2)2(HNMe2) (3-NHMe2), according to Eq. (2a).
Evidence for the composition of 3-NHMe2 comes from an X-ray dif-
fraction experiment performed on a single crystal that precipitates
on cooling the Et2O reaction mixture to ꢀ35 °C. This method is re-
quired since evaporating the solution to dryness results in the loss
of NHMe2 from the Ta(V) coordination sphere vide infra.
Fig. 4 depicts the solid state structure of 3-NHMe2 and Table 2
lists crystallographic refinement data. The distorted octahedral, C2-
symmetric complex resides on a 2-fold rotation axis, thus the
asymmetric unit comprises half the molecule (Fig. 4, left). The
dimethylamido group (N2) distorts away from the 2-fold rotation
axis and disorders over two positions that refine with 0.5 occupa-
tion factors (Fig. 4 (bottom) depicts the two partitions).
A second disorder exists at N3 and comprises a mixture of NMe2
and NHMe2 ligands that refine with 50% occupancy. Related, the
Ta(V) metal center also disorders over two positions. Electron den-
sity associated with a proton was identified from the Difference
Fourier Map for the amino N3 proton, but it did not refine well.
A proton was included in the final refinement, but was riding on
its parent atom. As expected, a short bond occurs between the
Ta-NMe2 disordered partition of 1.861(6) Å, whereas the
For further functionalization of the Ta(V) core it is helpful to
have a Ta–Cl bond. An example of –NMe2 exchange for Clꢀ on
Ta(V) using Me3SiCl was reported by Chisholm et al. by converting
Ta(NMe2)5 to Ta(NMe2)2Cl3(HNMe2). Unfortunately, treating 3
with Me3SiCl (2 equivalents) results in a mixture of compounds,
and as a consequence this route was abandoned.

S. VenkatRamani et al. / Polyhedron 64 (2013) 377–387
383
Fig. 4. Top: Asymmetric unit of [CF3-ONO]Ta(NMe2)2(HNMe2) (3-NHMe2) with hydrogen atoms (except H3) removed for clarity. Left: Disordered partition of Ta1 with
ellipsoids drawn at the 50% probability level. Hydrogen atoms (except H3) are removed for clarity. Right: Disordered partition of Ta10 with ellipsoids drawn at the 50%
probability level. Hydrogen atoms (except H3A) are removed for clarity. Selected bond distances [Å]: Ta1–O1 1.9373(15), Ta10–O1 1.995(5), Ta1–N1 2.119(5), Ta10–N1
2.120(3), Ta1–N2 1.959(4), Ta10–N2 1.965(4), Ta1–N3 2.281(5), Ta10–N30 1.861(6), N3–F6 2.90803(6). Bond angles [°]: O1–Ta1–O1 163.03(10), O1–Ta10–O1 158.10 (16), C13–
N3–C14 102.0(5), C13–N3–Ta1 120.8(4), C14–N3–Ta1 111.5(4), C130–N30–C140 107.3(5), C130–N30–Ta10 127.2(4), C140–N30–Ta10 122.1(4).
Fig. 5. 1H–1H ROESY experiment of 3-HNMe2 at ꢀ85 °C in toluene-d8. Black peaks denote through-space (NOE) interaction. Red peaks denote chemical exchange. (A)
exchange peaks between proton on free amine (HNMe2) and proton on bound amine (Ta-NHMe2). (B) Presence of exchange peaks between the methyl groups on the bound
amine (Ta-NHMe2) and the free amine (HNMe2).

384
S. VenkatRamani et al. / Polyhedron 64 (2013) 377–387
3.3. Synthesis and characterization of [CF3-ONHO]TaCl3 (4)
methyl protons are not equivalent and resonate as singlets at
1.69 and 1.76 ppm, and each proton on the aromatic rings is un-
ique. Supporting the assignment of a proton on N1 is a resonance
at 7.61 ppm. A 1H–15N gHMBC correlation experiment reveals
cross-peaks to the proton directly attached to the N-atom. In com-
plex 4 the N-atom resonates at 78.4 ppm indicating it is proton-
ated. Additional support comes from the fact the N-atom in
protonated proligand 1 appears at 66.2 ppm, whereas in complex
2, which contains a deprotonated N-atom, the resonance appears
well downfield at 233 ppm.
ð3Þ
Perhaps the most important feature of complex 4 is the crystal-
lographically observed N–H—F hydrogen bonding interaction. To
discount the influence of packing forces within the lattice as the
cause, solution phase characterization included 19F NMR spectro-
scopic interrogation. Three quartet resonances appear in the 19F
NMR spectrum at ꢀ68.9 (JFF = 10.4 Hz), ꢀ70.2 (JFF = 9.1 Hz), and
ꢀ71.6 (JFF = 9.2 Hz). However, the fourth CF3 group resonates as a
doublet of quartets -73.1 (JFF = 10.4 Hz, JHF = 7.0 Hz) as a conse-
Instead of adding an external base or incorporating an internal
one, an alternative strategy involves refluxing the two reagents un-
der an active flow of argon to promote the release of HCl. Thus,
refluxing ligand 1 with TaCl5 in toluene for 1.5 h under argon,
results in the formation of the neutral trichloride complex
[CF3-ONHO]TaCl3 (4) in 95% yield as a red-brown microcrystalline
powder Eq. (3). Alternatively, combining the reagents in dichloro-
methane (CH2Cl2) at 25 °C, placing the system under static vacuum
for 3 h, removing all volatiles, re-dissolving the material in CH2Cl2,
and stirring for an additional 5 h also provides complex 4. This
latter method has the advantage of being prepared in a vial within
the convenient confines of a glovebox.
quence of the F-atoms coupling with the N–H proton. A 19F–13
C
gHSQC experiments permits assignment of the carbon associated
with the interacting CF3 group, to a resonance at 123.6 ppm. The
important point is that in both solution and the solid state, the
N–H—F hydrogen bonding interaction only occurs with one out
of the four possible CF3 groups. Complex 4 was cooled down to
ꢀ85 °C (in toluene-d8) to observe if the N–Hꢁ ꢁ ꢁF interaction could
be limited to just one F-atom in a –CF3 group, similar to what is ob-
served crystallographically. However, this only causes an overall
broadening of the resonances in both the 1H and the 19F NMR
spectra.
Assignment of the ONHO2ꢀ ligand in its dianionic form comes
from an X-ray diffraction experiment performed on single crystals
that deposit from a concentrated Et2O solution of 4 at ꢀ35 °C. Fig. 6
depicts the results of a single crystal X-ray diffraction experiment
performed on 4 and Table 2 lists crystallographic refinement data.
Note: complex 4 contains a large disordered region that extends
over most of the atoms. The disorder was modeled in two parts:
atoms Cl1, Cl2, F7–12, N1, C3–6, C10, C13–20 were included in
one part, and the atoms Cl10, Cl20, F70-120, N10, C30-60, C100, C130-
200 were included in the other (see Supporting information). The
disordered amine protons on the pincer backbone were obtained
from a Difference Fourier Map and were refined riding on their
respective N atoms.
3.4. Synthesis and characterization of [CF3-ONO]TaCl2(THF) (5)
In contrast to 2 and 3, complex 4 contains a Ta(V) ion in a highly
distorted octahedral environment. The truncated depiction of the
Ta(V) core in Fig. 6 provides visual evidence for the distortions
and the metric parameters concur. For example, the N–Ta1–Cl1 an-
gle of 162.9(3) and the O1–Ta1–O2 angle of 155.79(9)° deviate sig-
nificantly from 180°. There is an obvious reason for the severe
distortion: though not located within the Difference Fourier Map,
the presence of a proton on N1 is evident. The sum of angles
around N1 of 349.7(10)° is much less than 360°, indicating pyrami-
dal geometry; and a long Ta1–N1 bond of 2.327(4) Å contrasts
ð4Þ
Converting the dianionic pincer within complex 4 into its tri-
anionic form is remarkably easy. Simply dissolving 4 in CH2Cl2
and adding a small amount (typically 1 ml) of THF results in
the loss of HCl and coordination of one solvent molecule (THF)
to give the dichloride complex [CF3-ONO]TaCl2(THF) (5). Solvent
removal and washing with pentane provides reasonably pure
material in 65% yield as a microcrystalline red powder Eq. (4).
Complex 5 is not stable indefinitely in the solid state and in solu-
tion decomposition occurs over several hours. Consequently, at-
tempts to grow single crystals of 5 were unsuccessful. Solution
phase characterization via NMR techniques provides adequate
data for the unequivocal assignment of complex 5; however,
some minor impurities develop during the timeframe of the
NMR experiments, including free 1. Table 1 provides the absolute
1H, 13C, 15N, and 19F NMR assignments for complex 5. The data is
consistent with the assignment of complex 5 as a C2-symmetric
complex. For example, in the 1H NMR spectrum (C6D6) a singlet
appears at 1.96 ppm for the aryl methyl protons of the ONO
strikingly with the
p-bonding-capable amido N-atom in 2 featur-
ing a much shorter Ta1–N1 bond of 2.0222(19) Å. Additional sup-
port for a proton comes from a hydrogen bonding interaction
between the H1 and F6 and the corresponding disordered pairing,
H10 and F70 (not drawn). The distance between N1 and F6 is
2.927(6) Å and for N10 and F70 the distance is 2.929(10) Å. This
value compares well with other purported N–Hꢁ ꢁ ꢁF hydrogen
bonding interactions derived from crystallographic data where
the N–F separation was found to fall in the range of 2.7 Å [36]
and 3.0 Å [37]. For comparison to an example in which an N–H
bond is absent, the closest N–F distances in complex
0.031(7) Å longer, and the second closest is 0.311(8) Å.
2 is
NMR spectroscopic data supports the solid state assignment of a
neutral tantalum trichloride complex featuring the pincer ligand in
its dianionic form. Present in the 1H NMR spectrum (C6D6) are hall-
mark resonances for a C1-symmetric complex. The pincer aryl

S. VenkatRamani et al. / Polyhedron 64 (2013) 377–387
385
Fig. 6. Left: Solid state structure of [CF3-ONHO]TaCl3 (4), with hydrogen atoms (except H1A), disordered N10, and disordered –C(CF3)2 groups removed for clarity. Right: A
truncated thermal ellipsoid plot (50%) of the Ta(V) core. Selected bond distances [Å]: Ta1–O1 1.8822), Ta1–O2 1.880(2), Ta1–Cl1 2.370(9), Ta1–Cl2 2.311(5), Ta1–Cl3
2.3391(9), Ta1–N1 2.327(4), N1–F6 2.927(6), N10–F70 2.929(10). Bond angles [°]: O1–Ta1–O2 155.79(9), Cl1–Ta1–N1 162.9(3), Cl2–Ta1–Cl3 170.00(9), C13–N1–Ta1 121.7(3),
C3–N1–C13 113.8(4), C3–N1–Ta1 114.2(3).
the reaction (Eq. (6)). Characterization methods employed for com-
plex 6 include 1H, 13C, 19F, 15N NMR spectroscopy and combustion
analysis. The N-atom correlates with the ligand aryl o-proton three
bonds away in a 1H–15N gHMBC experiment and appears at
205 ppm, which is consistent with the ONO3- ligand in its trianion-
ic form. A 19F NMR spectrum (C6D6) of 6 reveals two quartets for
the CF3 groups at ꢀ70.65 and ꢀ75.23 ppm (JFF = 9.9 Hz) and is clear
indication of a C2-symmetric complex. Consistent with this assign-
ment, the 1H NMR spectrum of 6 exhibits a singlet for the aryl-
methyl protons on the pincer ligand at 1.92 ppm. The most striking
feature is a singlet at 2.29 ppm attributable to the benzyl methy-
lene protons that correlate in a gHMBC experiment with the benzyl
ð5Þ
(–CH2C6H5) carbon at 71.2 ppm. Within a C2 symmetric structure
these protons should be diastereotopic. However, at room temper-
ature the resonances appear as a singlet. A variable temperature 1H
NMR study reveals that the chemicals shifts for the benzyl methy-
lene protons are temperature dependent (Fig. 7). Upon cooling to
ꢀ55 °C (1H NMR, 500 MHz, toluene-d8) the resonances do indeed
separate into an AB pattern. At ꢀ85 °C the resonances broaden,
presumably due to restricted rotation. More pronounced tempera-
ture dependent chemical shifts are observed for the pincer back-
bone protons H4 and H5 (also H14 and H15, see Fig. 1 for
labeling). The resonances appear as an AB pattern at room temper-
ð6Þ
ature, they cross and appear as a singlet at ꢀ55 °C, and then sepa-
rate again as an AB pattern at ꢀ85 °C. These data are consistent
ligand. Also, the 19F NMR spectrum exhibits only two quartets for
with the C2-symmetric structure proposed for complex 6.
Some of the earliest alkylidene complexes were derived from
the –CF3 groups at ꢀ70.7 and ꢀ74.3 ppm (JFF = 9.6 Hz). In complex
Ta-alkyls. Subjecting complex 6 to thermolysis conditions (toluene,
110 °C) did not result in the elimination of toluene. Instead, com-
plex 6 exhibits remarkable stability. In contrast, TaBn5 readily
undergoes decomposition when heated to 40 °C; Specifically,
TaBn5 when refluxed in C6D6 does not produce an alkylidene and
instead releases toluene (2.5 mol per Ta) and bibenzyl (0.1 mol
per Ta) [38,39]. In some examples of Ta-alkylidene synthesis, donor
ligands such as phosphines have been shown to coordinate and
prevent dimerization or decomposition [40]. Again, heating 6 at
80 °C for two days in C6D6, in the presence of PCy3, PPh3 or PMe3
does not lead to the formation of an alkylidene. The complex shows
no signs of decomposition either.
5 the N-atom resonates at 253 ppm; this is well downfield from
the N-atom in 4, and is clear evidence of the ligand in its trianion-
ic form.
3.5. Synthesis and characterization of [CF3-ONO]TaBn2 (6)
Treating 5 with two equivalents of BnMgCl (2 M in THF) in Et2O
at ꢀ35 °C provides the dibenzyl complex [CF3-ONO]TaBn2 (6) in
32% yield as a microcrystalline powder (Eq. 5). Alternatively, heat-
ing a benzene solution of TaBn5 with one equivalent of 1 also pro-
duces 6. Purification involves simply removing solvent and
triturating with pentane to remove residual toluene formed in

386
S. VenkatRamani et al. / Polyhedron 64 (2013) 377–387
NONAME04
ONAME04
1.0
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
a
b
-25 °C
-55 °C
-0.1
-0.2
-0.3
-85 °C
6.95
6.90
6.85
6.80
6.75
6.70
6.65
6.60
6.55
6.50
6.45
2.40
2.35
2.30
2.25
2.20
2.15
2.10
2.05
2.00
1.95
1.90
1.85
1.80
1.75
Chemical Shift (ppm)
Chemical Shift (ppm)
Fig. 7. Variable temperature NMR of [CF3-ONO]TaBn2 (6) (1H, 500 MHz, toluene-d8). Region a shows the change in the benzyl methylene resonance (ꢂ 2.30 ppm) in the
temperature range ꢀ85 to ꢀ25 °C. Region b shows the change in the pincer aryl resonance (H4/H14 and H5/H15 pairs) in the same temperature range.
Cambridge Crystallographic Data Centre, 12 Union Road, Cam-
bridge CB2 1EZ, UK; fax: (+44) 1223 336 033; or e-mail: depos-
it@ccdc.cam.ac.uk. Supplementary data associated with this
article can be found, in the online version, at http://dx.doi.org/
10.1016/j.poly.2013.07.004.
4. Conclusions
The synthetic chemistry explored in this study provides several
noteworthy points for discussion. The synthesis of the pincerate
complex {[CF3-ONO]TaCl3}{HNEt3}ꢁ(HNEt3Cl)x (2), the amido-
amine complex [CF3-ONO]Ta(NMe2)2(HNMe2) (3-HNMe2) and the
neutral dichloride complex [CF3-ONO]TaCl2(THF) (5) indicate the
ligand does not provide enough steric protection and, given the
opportunity, the tantalum ion will coordinate small ligands to sat-
isfy its electronic requirements. However, if the tantalum is pro-
References
[1] J.F. Hartwig, Organotransition Metal Chemistry: From Bonding to Catalysis,
University Science Books, Sausalito, 2010.
[2] C. Crocker, H.D. Empsall, R.J. Errington, E.M. Hyde, W.S. Mcdonald, R. Markham,
M.C. Norton, B.L. Shaw, B. Weeks, J. Chem. Soc., Dalton Trans. (1982) 1217.
[3] C. Crocker, R.J. Errington, R. Markham, C.J. Moulton, B.L. Shaw, J. Chem. Soc.,
Dalton Trans. (1982) 387.
[4] R.J. Errington, W.S. Mcdonald, B.L. Shaw, J. Chem. Soc., Dalton Trans. (1982)
1829.
[5] C. Crocker, R.J. Errington, R. Markham, C.J. Moulton, K.J. Odell, B.L. Shaw, J. Am.
Chem. Soc. 102 (1980) 4373.
[6] C. Crocker, R.J. Errington, W.S. Mcdonald, K.J. Odell, B.L. Shaw, R.J. Goodfellow,
J. Chem. Soc., Chem. Commun. (1979) 498.
[7] C.J. Moulton, B.L. Shaw, J. Chem. Soc., Dalton Trans. (1976) 1020.
[8] D. Morales-Morales, C.M. Jensen (Eds.), The Chemistry of Pincer Compounds,
Elsevier, Amsterdam, 2007.
[9] A. Sattler, G. Parkin, J. Am. Chem. Soc. 134 (2012) 2355.
[10] I. Korobkov, S. Gorelsky, S. Gambarotta, J. Am. Chem. Soc. 131 (2009) 10406.
[11] T. Watanabe, T. Matsuo, H. Kawaguchi, Inorg. Chem. 45 (2006) 6580.
[12] M.E. O’Reilly, T.J. Del Castillo, K.A. Abboud, A.S. Veige, Dalton Trans. 41 (2012)
2237.
vided with strongly
p-donating ligands, such as –NMe2, then low
coordinate species are possible. This is observed in the case in
which HNMe2 is released from 3-HNMe2 to provide 3. The synthe-
sis of complex [CF3-ONHO]TaCl3 (4) with the ligand in the dianion-
ic form reveals an important diagnostic feature. From experience,
during metalation studies of 1 with other transition metal ions, it
is not immediately obvious from 1H NMR data that deprotonation
of the N–H bond has occurred. Often the N-H proton is not visible
via 1H NMR spectroscopy. However, the observation of a N–H—F
hydrogen bonding interaction and, in particular, the distinct 19F
NMR resonance featuring a 7 Hz coupling to the N–H proton, will
greatly facilitate structural assignment in future syntheses.
Another important observation is that the trianionic proligand
[CF3-ONO]H3 (1) reacts with TaCl5 directly, even at room tempera-
ture. The alcohol protons, by virtue of the electron withdrawing
CF3 groups, are acidic, which facilitates protonation of the M–Cl
bonds. This demonstrated concept of simple addition of the proli-
gand to the M-halide salts will open access to numerous new tran-
sition metal complexes for further study.
[13] M.T. Jan, S. Sarkar, S. Kuppuswamy, I. Ghiviriga, K.A. Abboud, A.S. Veige, J.
Organomet. Chem. 696 (2011) 4079.
[14] S. Sarkar, K.P. McGowan, J.A. Culver, A.R. Carlson, J. Koller, A.J. Peloquin, M.K.
Veige, K.A. Abboud, A.S. Veige, Inorg. Chem. 49 (2010) 5143.
[15] S. Kuppuswamy, I. Ghiviriga, K.A. Abboud, A.S. Veige, Organometallics 29
(2010) 6711.
[16] S. Sarkar, A.R. Carlson, M.K. Veige, J.M. Falkowski, K.A. Abboud, A.S. Veige, J.
Am. Chem. Soc. 130 (2008) 1116.
[17] S. Sarkar, K.A. Abboud, A.S. Veige, J. Am. Chem. Soc. 130 (2008) 16128.
[18] J. Koller, S. Sarkar, K.A. Abboud, A.S. Veige, Organometallics 26 (2007) 5438.
[19] T. Agapie, J.E. Bercaw, Organometallics 26 (2007) 2957.
[20] T. Agapie, M.W. Day, J.E. Bercaw, Organometallics 27 (2008) 6123.
[21] A.I. Nguyen, K.J. Blackmore, S.M. Carter, R.A. Zarkesh, A.F. Heyduk, J. Am. Chem.
Soc. 131 (2009) 3307.
[22] R.A. Zarkesh, J.W. Ziller, A.F. Heyduk, Angew. Chem., Int. Ed. 47 (2008) 4715.
[23] R.R. Schrock, J. Lee, L.-C. Liang, W.M. Davis, Inorg. Chim. Acta 270 (1998) 353.
[24] D.W. Shaffer, G. Szigethy, J.W. Ziller, A.F. Heyduk, Inorg. Chem. 52 (2013) 2110.
[25] S. Sarkar, K.P. McGowan, S. Kuppuswamy, I. Ghiviriga, K.A. Abboud, A.S. Veige,
J. Am. Chem. Soc. 134 (2012) 4509.
Acknowledgements
The authors thank the National Science Foundation in the form
of a CAREER award to ASV (CHE-0748408) and UF for financial
support. ASV is the recipient of an Alfred P. Sloan Fellowship
(2010-2012). K.A.A. thanks the NSF (CHE-0821346) and UF for
funding the purchase of X-ray equipment.
[26] K.P. McGowan, A.S. Veige, J. Organomet. Chem. 711 (2012) 10.
[27] M.E. O’Reilly, T.J. Del Castillo, J.M. Falkowski, V. Ramachandran, M. Pati, M.C.
Correia, K.A. Abboud, N.S. Dalal, D.E. Richardson, A.S. Veige, J. Am. Chem. Soc.
133 (2011) 13661.
Appendix A. Supplementary data
[28] K.P. McGowan, K.A. Abboud, A.S. Veige, Organometallics 30 (2011) 4949.
[29] M. O’Reilly, J.M. Falkowski, V. Ramachandran, M. Pati, K.A. Abboud, N.S. Dalal,
T.G. Gray, A.S. Veige, Inorg. Chem. 48 (2009) 10901.
CCDC 932766–932768 contains the supplementary crystallo-
graphic data for 2–4. These data can be obtained free of charge
via http://www.ccdc.cam.ac.uk/conts/retrieving.html, or from the
[30] S.R. Golisz, J.E. Bercaw, Macromolecules 42 (2009) 8751.

S. VenkatRamani et al. / Polyhedron 64 (2013) 377–387
387
[31] T. Agapie, L.M. Henling, A.G. DiPasquale, A.L. Rheingold, J.E. Bercaw,
Organometallics 27 (2008) 6245.
[36] S.J. Lancaster, A. Rodriguez, A. Lara-Sanchez, M.D. Hannant, D.A. Walker, D.H.
Hughes, M. Bochmann, Organometallics 21 (2002) 451.
[32] M.E. O’Reilly, I. Ghiviriga, K.A. Abboud, A.S. Veige, J. Am. Chem. Soc. 134 (2012)
11185.
[37] M. Taddei, F. Costantino, V. Manuali, R. Vivani, Inorg. Chem. 50 (2011) 10835.
[38] R.R. Schrock, J. Organomet. Chem. 122 (1976) 209.
[33] S. Groysman, I. Goldberg, M. Kol, Z. Goldschmidt, Organometallics 22 (2003)
3793.
[34] SHELXTL6.1, BRUKER-AXS, Madison, WI, 2008.
[35] O.V. Dolomanov, L.J. Bourhis, R.J. Gildea, J.A.K. Howard, H. Puschmann, J. Appl.
Crystallogr. 42 (2009) 339.
[39] V. Malatesta, K.U. Ingold, R.R. Schrock, J. Organomet. Chem. 152 (1978) C53.
[40] G.A. Rupprecht, L.W. Messerle, J.D. Fellmann, R.R. Schrock, J. Am. Chem. Soc.
102 (1980) 6236.