Synthesis of tantalum fluoride complexes supported by bis(trimethylsilyl)benzamidinate ligands. X-ray structures of [PhC(NSiMe3)2]2TaF3 and [PhC(NSiMe3)2]2TaFPh2

Article abstract of DOI:10.1016/S0022-328X(00)00376-4

The reaction of [PhC(NSiMe₃)₂]Li(TMEDA) with TaF₅ yields complex 1, [PhC(NSiMe₃)₂]₂TaF₃, in 40% isolated yield. The solid-state structure of 1 shows a pentagonal, bipyramidal coo

Full text of DOI:10.1016/S0022-328X(00)00376-4

www.elsevier.nl/locate/jorganchem
Journal of Organometallic Chemistry 607 (2000) 227–232
Synthesis of tantalum fluoride complexes supported by
bis(trimethylsilyl)benzamidinate ligands. X-ray structures of
[PhC(NSiMe3)2]2TaF3 and [PhC(NSiMe3)2]2TaFPh2
Sarah M. Mullins, John R. Hagadorn, Robert G. Bergman *1, John Arnold *2
Department of Chemistry, Uni6ersity of California and Chemical Sciences Di6ision, Lawrence Berkeley National Laboratory, Berkeley,
CA 94720-1460, USA
Received 17 April 2000; received in revised form 15 June 2000
Dedicated to Professor Martin Bennett on the occasion of his retirement.
Abstract
The reaction of [PhC(NSiMe3)2]Li(TMEDA) with TaF5 yields complex 1, [PhC(NSiMe3)2]2TaF3, in 40% isolated yield. The
solid-state structure of 1 shows a pentagonal, bipyramidal coordination geometry. Variable temperature 19F-NMR experiments
reveal one singlet at an ambient temperature; decoalescence to two singlets (1:2 integration) is observed at −20°C. Complex 1
reacts with Me2Mg to produce [PhC(NSiMe3)2]2TaMe3 (2). The reaction of 1 with Ph2Mg yields [PhC(NSiMe3)2]2TaF2Ph (3) and
[PhC(NSiMe3)2]2TaFPh2 (4). Characterization of 4 by X-ray crystallography shows a ligand arrangement similar to that of 1.
© 2000 Elsevier Science S.A. All rights reserved.
Keywords: Tantalum; Amidinate complexes; Metal fluoride complexes
1. Introduction
organometallic complex with an alkali metal (CsF,
NaF) [3,7] or main group (AsF3, Me3SnF) [8,9] fluoride
transfer agent. Clearly, these routes add the complica-
tion of an additional step to first introduce the ancillary
ligand. These reactions also often proceed in low yield
and may produce solvated complexes.
Transition metal fluoride complexes catalyze a wide
variety of transformations, including olefin polymeriza-
tion [1], reduction of perfluorocarbons [2] and hydrosi-
lylation of amines [3]. In addition, transition metal
fluoride compounds have been studied for their poten-
tial role in CꢀF bond activation reactions [4]. The
ability of fluoride ligands to act as strong p-donors
toward metals sets their reactivity apart from that of
other halide derivatives [5,6]. In contrast to their chlo-
ride or bromide counterparts, transition metal fluorides
are often not suitable synthons for the introduction of
organometallic ligands. Consequently, the properties of
many potential fluoride derivatives remain unexplored.
The synthesis of fluoride complexes with cyclopentadi-
enyl ligands is most often achieved through the
metathesis of the bromide or chloride ligands of the
A limited number of organotantalum fluoride com-
plexes have been reported. Most are half-sandwich
complexes [10,11] which, along with a few alkyl deriva-
tives [9,12], have been prepared by halide metathesis
routes. The synthesis of the monomeric metallocene
complex, Cp*TaF3 (Cp*ꢁh5-C5Me5) by reaction of
2
pyridinium·HFx with Cp*2 TaH3 was recently reported
[13]. To the best of our knowledge, there have been no
reports of organometallic derivatives prepared directly
from TaF5.
Nitrogen-based ancillary ligands have received atten-
tion for their ability to support metal centers in a range
of catalytic reactions [14–16]. We have studied use of
the bis(trimethylsilyl)benzamidinate ligand in early
transition metal complexes [17–19]. This ligand imparts
considerable steric shielding (similar to Cp* [20]) and
1 *Corresponding author. Fax: +1-510-6427714; e-mail: bergman@
cchem.berkeley.edu
2 *Corresponding author. Fax: +1-510-6435482; e-mail: arnold@
socrates.berkeley.edu
0022-328X/00/$ - see front matter © 2000 Elsevier Science S.A. All rights reserved.
PII: S0022-328X(00)00376-4

228
S.M. Mullins et al. / Journal of Organometallic Chemistry 607 (2000) 227–232
Scheme 1. Synthesis of alkyl and aryl derivatives from 1.
excellent solubility properties to its metal complexes.
Additionally, the amidinate is a four-electron donor
[20], which should provide a more electrophilic metal
center than in analogous metallocene complexes.
Herein we report the synthesis of [PhC(NSiMe3)2]2TaF3
directly from tantalum pentafluoride, as well as substi-
tution of the remaining TaꢀF bonds with common
reagents to yield alkyl and aryl derivatives.
142°, and the inner fluoride, F3, is separated by 75–83°
from the outer fluorides. This arc geometry is observed
in the related compound, [PhC(NSiMe3)2]2TaMe3 [19].
The steric bulk of the nitrogen substituents affects
the amount of perturbation from an idealized pentago-
nal bipyramidal geometry, as well as the geometrical
arrangement of the amidinate ligands. If the nitrogen
i
substituents are small (e.g. Pr, Cy), all four nitrogen
atoms may be equatorial, making it possible for the
three remaining substituents to arrange in a T-forma-
tion [25,26]. If the steric bulk of the nitrogen sub-
stituents (e.g. SiMe3) or the other tantalum substituents
are large, then one nitrogen must be located in the axial
2. Results and discussion
2.1. Synthesis and characterization of
[PhC(NSiMe3)2]2TaF3 (1)
Complex 1 was prepared by reaction of TaF5 with
two equivalents of Li[PhC(NSiMe3)2](TMEDA) in tolu-
ene at room temperature (Scheme 1). The product is
very soluble in hydrocarbon solvents, but pale yellow
crystals may be isolated from hexanes in 40% yield. An
X-ray diffraction study was undertaken; an ORTEP dia-
gram of 1 is shown in Fig. 1. Selected bond lengths and
angles are shown in Table 1.
Inspection of the structural parameters suggests that
of the three common idealized geometries for seven-co-
ordinate molecules (capped octahedron, pentagonal
bipyramid, capped trigonal prism) [21,22], the environ-
ment of the complex is best described as a distorted
pentagonal bipyramid. The X-ray structures of other
(amidinate)2TaX3 (XꢁMe, Cl) complexes also show a
distorted pentagonal bipyramidal geometry [19,23,24].
Three of the amidinate nitrogens (N1, N3, N4) and two
of the fluorides (F1, F2) of 1 lie in the pseudo pentago-
nal plane and the remaining nitrogen and fluoride
atoms (N2 and F3) occupy the axial positions. The
mean distance of equatorial atoms from the least-
Fig. 1. ORTEP diagram of 1 drawn with 50% probability thermal
ellipsoids. Hydrogen atoms omitted for clarity.
Table 1
,
Selected bond distances (A) and angles (°) for 1
TaꢀF1
TaꢀF2
TaꢀF3
TaꢀN1
TaꢀN2
TaꢀN3
TaꢀN4
1.924(2)
1.926(2)
1.893(2)
2.181(3)
2.213(3)
2.192(3)
2.124(3)
F1ꢀTaꢀN4
F2ꢀTaꢀN1
N1ꢀTaꢀF1
F2ꢀTaꢀN3
N4ꢀTaꢀN3
N1ꢀTaꢀN2
F3ꢀTaꢀN2
77.9(1)
78.4(1)
75.8(1)
79.4(1)
61.3(1)
61.0(1)
171.8(1)
,
squares plane is 0.38 A. The three fluorides bound to
the tantalum in 1 are arranged in an arc; the FꢀTaꢀF
angle between the two outer fluorides, F1 and F2 is

S.M. Mullins et al. / Journal of Organometallic Chemistry 607 (2000) 227–232
229
1
observed in the H-NMR spectrum at room tempera-
ture. Decoalescence of this peak was not observed upon
cooling to −138°C. A variety of pentagonal bipyrami-
dal and capped trigonal prismatic configurations are
plausible solution structures of 1. Each of these, yields
two distinct fluoride environments and allows for equi-
libration of all four SiMe3 groups either by symmetry
alone or a combination of symmetry and rotation
about the twofold axis within the benzamidinate ligand
(‘ring flipping’). The facile configurational interconver-
sion is precedented in seven coordinate complexes
[22,31,32]; additionally, ring flipping has been observed
to occur in amidinate complexes [33–35].
The trifluoride (1) is stable in solution at room
1
temperature for several days, as monitored by H- and
19F-NMR spectroscopy in C6D6, but it decomposes to a
mixture of unidentified products at temperatures above
75°C. In solution, 1 does not react with dry O2; as a
solid or in solution it decomposes slowly in air.
Fig. 2. ORTEP diagram of 4 drawn with 50% probability thermal
ellipsoids. Hydrogen atoms omitted for clarity.
Table 2
,
Selected bond distances (A) and angles (°) for 4
2.2. Synthesis and characterization of aryl and alkyl
deri6ati6es
TaꢀF1
TaꢀC27
TaꢀC33
TaꢀN1
TaꢀN2
TaꢀN3
TaꢀN4
1.886(3)
2.281(5)
2.270(5)
2.212(3)
2.140(4)
2.232(4)
2.164(4)
N4ꢀTaꢀC27
N1ꢀTaꢀC27
N1ꢀTaꢀC33
C33ꢀTaꢀN3
N4ꢀTaꢀN3
N1ꢀTaꢀN2
F1ꢀTaꢀN2
80.0(1)
74.4(1)
72.5(1)
79.8(1)
61.6(1)
61.8(1)
165.3(1)
To explore the lability of the fluoride ligands, we
examined the reaction of 1 with alkylating agents.
Complex 1 reacts with an excess of Me2Mg to form the
known trimethyl compound, [PhC(NSiMe3)2]2TaMe3
(2) [19]. Complex 1 also undergoes salt metathesis
reactions with diphenylmagnesium to form tantalum
aryl derivatives (Scheme 1). The reaction of 1 with 1.5
equivalents of Ph2Mg in diethyl ether or THF for
several days yields a mixture of [PhC(NSiMe3)2]2-
TaF2Ph (3) and [PhC(NSiMe3)2]2TaFPh2 (4). Monitor-
position, leaving the equatorial plane significantly dis-
torted [19,23] and resulting in the arc formation de-
scribed above. The steric bulk from the pair of
bis(trimethylsilyl)benzamidinate ligands also inhibits
the formation of a dimeric structure. The monopen-
tamethylcyclopentadienyl complex, Cp*TaF4, is dimeric
in the solid state while the mixed ligand complex,
Cp*[(p-MeO)PhC(NSiMe3)2]TaF3 [27] and [PhC(NSi-
Me3)2]2TaF3 are both monomeric. This is consistent
with steric similarity of [PhC(NSiMe3)2] to Cp*.
1
ing the reaction by H-NMR spectroscopy reveals that
3 forms quantitatively after 4 h, while introduction of
the second phenyl group is substantially slower. Forma-
tion of 4 is still incomplete after 2 d, when decomposi-
1
tion is observed in the H-NMR spectrum. Colorless
Identical CꢀN bond lengths indicate delocalization
crystals of 3 can be isolated, however, from hexanes in
40% yield when the reaction of 1 with 1.5 equivalents
Ph2Mg in diethyl ether is stopped after 4 h.
within the NCN amidinate core. The TaꢀFax and
,
TaꢀFeq bond lengths are 1.893(2) and 1.925(2) A (aver-
age), respectively. The TaꢀF bond lengths of 1 are
longer than the terminal TaꢀF bonds in [TaF5]4 (1.77
The reaction of 1 with 1.5 equivalents Ph2Mg in
diethyl ether for 2 d affords pale purple bis(aryl) com-
plex 4 in 20% yield after crystallization from hexanes.
An ORTEP diagram of 4 is shown in Fig. 2 and selected
bond lengths and angles are shown in Table 2. Several
geometric features of 4 are similar to those revealed in
the X-ray structure of 1 (Table 3). The ligands are
coordinated to the tantalum atom in a distorted pentag-
onal bipyramidal configuration in which the two phenyl
groups (C27 and C33), both occupy equatorial posi-
tions, along with three of the amidinate nitrogen atoms
(N1, N3, N4). The mean deviation of the equatorial
,
A) [28]. The elongation is also observed in
,
{[Cp*TaF4]2}{AsF3} (1.90
length) [10].
A average TaꢀF bond
Variable temperature 19F-NMR spectroscopic studies
on 1 suggest fluxional behavior of the fluoride ligands
on the NMR timescale. At 25°C, a single resonance
(sharp singlet) is observed which upon cooling to
−50°C decoalesces into two resonances (singlets, ratio
2:1). No FꢀF coupling was observed, probably due to
line broadening from the quadrupolar 181Ta nucleus
(I=7/2) [29]. As observed in other reported group
III–V complexes with two bis(trimethylsilyl)benzamidi-
nate ligands [17,19,20,30], a single trimethylsilyl peak is
,
atoms from the plane is 0.30 A. The two ipso phenyl
carbons (C27 and C33) and the fluoride are arranged in
an arc, as the F ligands are in 1. For the carbon atoms

230
S.M. Mullins et al. / Journal of Organometallic Chemistry 607 (2000) 227–232
3. Conclusion
C27 and C33, which are in the outer positions, the
CꢀTaꢀC angle is 139.4°. The fluoride lies in the inside
position and is separated 82–83° from C27 and C33.
In this report, we have shown that the nitrogen-based
bis(trimethylsilyl)benzamidinate ligand is an excellent
reagent for synthesizing derivatives directly from tanta-
lum pentafluoride. This ligand provides easy access to
complexes which are analogous to those used in a wide
variety of useful reactions. Both the fluoride and amidi-
nate ligands of 1 exhibit dynamic behavior on the NMR
timescale. The metal–fluoride bonds of 1 are labile, as
demonstrated by their straightforward displacement in
the conversion of 1 to alkyl and aryl complexes. No ten-
dency for dimerization of the products was observed,
which we attribute to the steric bulk of the bis-
(trimethylsilyl)benzamidinate ligand. Future studies will
determine how the reactivity of these amidinate tanta-
lum fluoride complexes compares with the reactivity of
other tantalum fluoride derivatives.
,
TaꢀF bond length of 1.886(3) A is comparable to the
TaꢀFax bond length in 1. The two TaꢀC bond lengths
for the two phenyl groups are essentially equivalent, at
2.281(5) and 2.270(5) A. The room temperature 1H-
,
NMR spectrum shows only one set of resonances for
the metal-bound phenyl groups, suggesting an averaged
symmetrical configuration of the phenyl groups in
solution.
Table 3
Crystallographic data and refinement details for 1 and 4 a
Compound 1
Compound 4
Empirical formula
Formula weight
Temperature (°C)
Crystal system
Space group
C26H46N4F3Si4Ta C38H56N4Si4Ta
4. Experimental
764.96
881.18
−101
−93
4.1. General considerations
Monoclinic
P21/a (c14)
16.9883(2)
11.0893(2)
18.3154(1)
94.306(1)
3440.67(6)
4
Monoclinic
C2/c (c15)
22.2291(13)
12.5074(7)
30.4723(18)
102.952(1)
8257(1)
Standard Schlenk-line and glove box techniques were
used throughout [36]. Hexanes and tetrahydrofuran
(THF) were distilled from Na–benzophenone under ni-
trogen. Toluene was distilled from Na under nitrogen.
C6D6 was vacuum transferred from Na–benzophenone.
Ph2Mg was prepared according to the literature proce-
dure [37]. 1H- and 13C{1H}-NMR spectra were recorded
at ambient temperatures, unless otherwise noted. Chem-
ical shifts (l) are given relative to the residual proton in
the deuterated solvent at 7.15 for C6D6. 19F chemical
shifts are given relative to an external CFCl3 standard in
the given solvent at 0.00 ppm. IR samples were prepared
as mineral oil mulls and taken between KBr plates. Ele-
mental analysis data were obtained by the microanalysis
Facility of the College of Chemistry, University of Cali-
fornia, Berkeley, CA. Single crystal X-ray determina-
tions were performed at the College of Chemistry X-ray
diffraction facility, University of California, Berkeley,
CA.
,
a (A)
,
b (A)
,
c (A)
B(°)
3
,
V (A )
Z
8
Dcalc (g cm−3
)
1.477
1.418
Diffractometer
Radiation
Monochromator
Detector
Siemens SMART
Mo–Ka
Graphite
CCD area
detector
ꢀ, 0.3
Siemens SMART
Mo–Ka
Graphite
CCD area
detector
ꢀ, 0.3
Scan type (°)
Frame collection time (s)
Reflections measured
2q range (°)
10
20
Hemisphere
3–46.5
Hemisphere
3–46.5
v (mm−1
)
33.67
2.81
Tmin, Tmax
0.334, 0.567
0.40×0.31×0.10
14 173
0.658, 0.863
0.13×0.11×0.07
17 288
Crystal dimensions (mm)
Reflections measured
Unique reflections
Observations (I\3|)
Variables
5208
6222
4102
4666
343
433
Data/parameter
Refinement method
11.96
10.78
4.2. [PhC(NSiMe3)2]2TaF3
Full-matrix
least-squares on
F2
Full-matrix
least-squares on
F2
Toluene (125 ml) was added to a 500-ml round-bot-
tomed flask loaded with TaF5 (2.50 g, 9.05 mmol),
forming a slurry. A solution of Li[PhC(NSiMe3)2]-
(TMEDA) (7.00 g, 18.1 mmol) in toluene (100 ml) was
added dropwise to yield a yellow solution. After stirring
the mixture at room temperature overnight, the volatile
materials were removed under reduced pressure, afford-
ing a pale yellow solid. One extraction with hexanes
(225 ml) followed by filtration through a Celite pad on a
fritted disk gave a clear yellow solution. Concentration
Rint (%)
R (%)
Rw (%)
Goodness-of-fit
Largest difference peak
and hole (e A
Hydrogen atoms
2.5
2.1
2.7
2.7
2.74
2.38
3.01
1.192
0.54 and −0.71
1.71 and −0.80
−3
,
)
Idealized
positions
Idealized
positions
a R=[SꢀꢀFoꢀ−ꢀFcꢀꢀ]/SꢀFoꢀ, Rw={[Sw(ꢀFoꢀ−ꢀFcꢀ)2]/SwꢀF2o}1/2, Good-
ness-of-fit={[Sw(ꢀFoꢀ−ꢀFcꢀ)2]/(no−nv)}1/2
.

S.M. Mullins et al. / Journal of Organometallic Chemistry 607 (2000) 227–232
231
of the solution to 80 ml followed by cooling to −30°C
o-Ar), 7.45 (t, 4H, J=7 Hz, m-Ar), 7.35–7.25 (m, 6H,
Ar), 6.97–6.93 (m, 6H, Ar), −0.07 (s, 36H, SiMe3).
13C{1H}-NMR (C6D6, 75.5 MHz): l 179.1 (NCN),
140.3 (Ar), 140.2 (Ar), 140.1 (Ar), 130.1 (Ar), 128.5
(Ar), 126.0 (Ar), 3.5 (SiMe3). The remaining carbon
resonances for the phenyl groups were not located.
19F-NMR (C6D6, 376.5 MHz): l 131.1 (s, br). IR
(cm−1): 1601 (w), 1525 (s), 1484 (s), 1463 (s, br), 1444
(s, br), 1429 (s, br), 1400 (s, br), 1375 (s, br), 1249 (s),
1056 (w), 1008 (m), 987 (s), 925 (w), 842 (w), 835 (s, br),
784 (s), 773 (s), 756 (s), 736 (s), 711 (s), 702 (s), 580 (s),
553 (s), 509 (m). Anal. Calc. for C38H56N4FSi4Ta: C
51.80 H 6.41, N 6.36. Found: C 51.76, H 6.38, N 6.45.
afforded pale yellow crystals. Repeating this procedure
1
gave a second crop, total 2.77 g, 40% yield. H-NMR
(C6D6, 400 MHz): l 7.25 (d, 4H, J=8 Hz, o-Ar),
6.95–6.89 (m, 6H, m, p-Ar), 0.23 (s, 36H, SiMe3).
13C{1H}-NMR (C6D6, 75.5 MHz): l 184.6 (NCN),
139.3 (Ar), 129.9 (Ar), 128.1 (Ar), 127.0 (Ar), 2.0
(SiMe3). 19F-NMR (C6D6, 376.5 MHz): l 67.5. IR
(cm−1): 1459 (s, br), 1446 (s, br), 1423 (s), 1407 (s),
1247 (s), 1083 (w), 1000 (s), 918 (w), 848 (s, br), 833 (s),
786 (s), 765 (s), 721 (s), 703 (s), 694 (m), 599 (m), 549
(s), 516 (m). Anal. Calc. for C26H46N4F3Si4Ta: C 40.82,
H 6.06, N 7.32. Found: C 40.83, H 6.09, N 7.33.
4.3. [PhC(NSiMe3)2]2TaF2Ph
5. Supplementary material
THF (60 ml) was added to a 100-ml round-bottomed
flask loaded with [PhC(NSiMe3)2]2TaF3 (0.465 g, 0.608
mmol) and Ph2Mg (0.108 g, 0.608 mmol) forming a
clear yellow solution. After the reaction mixture was
stirred at room temperature for 4 h, the volatile materi-
als were removed under reduced pressure to afford a
red–brown solid. One extraction with hexanes (50 ml)
followed by filtration through a Celite pad on a fritted
disk gave a clear red solution. Concentration of the
filtrate in vacuo to 4 ml followed by cooling to −30°C
afforded colorless crystals (0.200 g, 40% yield). 1H-
NMR (C6D6, 400 MHz): l 8.83 (d, 2H, J=7 Hz,
o-Ar), 7.49 (t, 2H, J=7 Hz, m-Ar), 7.33–7.26 (m, 5H,
Ar), 6.97–6.93 (m, 6H, Ar), 0.11 (s, 36H, SiMe3).
13C{1H}-NMR (C6D6, 75.5 MHz): l 182.8 (NCN),
140.7 (Ar), 139.2 (Ar), 139.1 (Ar), 130.2 (Ar), 128.5
(Ar), 127.4 (Ar), 2.6 (SiMe3). The remaining carbon
resonances for the phenyl group were not located.
19F-NMR (C6D6, 376.5 MHz): l 108.5 (s, br). IR
(cm−1): 1601(w), 1452 (s, br), 1434 (s, br), 1413 (s, br),
1402 (s, br), 1395 (s, br), 1275 (s), 1064 (w), 1008 (m),
979 (s), 925 (w), 840 (s, br), 796 (s), 771 (s), 757 (s), 746
(s), 700 (s), 553 (s), 512 (m). Anal. Calc. for
C32H51N4F2Si4Ta: C 46.70, H 6.25, N 6.81. Found: C
46.89, H 6.27, N 6.86.
Crystallographic data for the structural analysis have
been deposited with the Cambridge Crystallographic
Data Center, CCDC no. 141165 for 1 and CCDC no.
141166 for 4. Copies of this information may be ob-
tained free of charge from The Director, CCDC, 12
Union Road, Cambridge CB2 1EZ, UK (Fax: +44-
1223-336033; email: deposit@ccdc.cam.ac.uk or www:
http://www.ccdc.cam.ac.uk).
Acknowledgements
We thank the Petroleum Research Fund, adminis-
tered by the ACS (to J.A.), and the NSF (grant no.
CHE-9633374 to R.G.B.) for funding.
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4.4. [PhC(NSiMe3)2]2TaFPh2
Et2O (60 ml) was added to a 100-ml round-bottomed
flask loaded with [PhC(NSiMe3)2]2TaF3 (0.251 g, 0.328
mmol) and Ph2Mg (0.097 g, 0.54 mmol) forming a clear
orange solution. After the reaction mixture was stirred
at room temperature for 2 days, the volatile materials
were removed under reduced pressure to afford a red–
brown solid. One extraction with hexanes (50 ml) fol-
lowed by filtration through a Celite pad on a fritted
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filtrate in vacuo to 4 ml followed by cooling to −30°C
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