Trapping unstable terminal M-O multiple bonds of monocyclopentadienyl niobium and tantalum complexes with Lewis acids

Article abstract of DOI:10.1021/ic1016609

Hydrolysis of [NbCp′Cl₄] (Cp′ = η⁵- C₅H₄SiMe₃) with the water adduct H ₂O·B(C₆F₅)₃afforded the oxo-borane compound [NbCp′Cl₂{O·B(C₆F <

Full text of DOI:10.1021/ic1016609

10642 Inorg. Chem. 2010, 49, 10642–10648
DOI: 10.1021/ic1016609
Trapping Unstable Terminal M-O Multiple Bonds of Monocyclopentadienyl
Niobium and Tantalum Complexes with Lewis Acids
,†,‡
§
,†
§
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ꢀ
~
Javier Sanchez-Nieves,* Luis M. Frutos, Pascual Royo,* Obis Castano, Eberhardt Herdtweck, and
Marta E. G. Mosquera†
†
§
‡
´
ꢀ
´
ꢀ
Departamento de Quımica Inorganica, CIBER-BBN, Departamento de Quımica Inorganica, and
´
´
ꢀ
ꢀ
Departamento de Quımica Fısica, Universidad de Alcala, Campus Universitario, E-28871 Alcala de Henares,
||
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Madrid, Spain, and Anorganisch-Chemisches Institut der Technischen Universitat Munchen,
Lichtenbergstrasse 4, D-85747 Garching bei Mu€nchen, Germany
Received August 17, 2010
Hydrolysis of [NbCp0Cl4] (Cp0 = η5-C5H4SiMe3) with the water adduct H2O B(C6F5)3 afforded the oxo-borane
3
compound [NbCp0Cl2{O B(C6F5)3}] (2a). This compound reacted with [MgBz2(THF)2] giving [NbCp0Bz2{O
3
3
B(C6F5)3}] (2b), whereas [NbCp0Me2{O B(C6F5)3}] (2c) was obtained from the reaction of [NbCp0Me4] with
3
H2O B(C6F5)3. Addition of Al(C6F5)3 to solutions containing the oxo-borane compounds [MCpRX2{O B(C6F5)3}]
3
3
(M = Ta, CpR = η5-C5Me5 (Cp*), X = Cl 1a, Bz 1b, Me 1c; M = Nb, CpR = Cp0, X = Cl 2a) afforded the oxo-alane complexes
[MCpRX2{O Al(C6F5)3}] (M = Ta, CpR = Cp*, X = Cl 3a, Bz 3b, Me 3c; M = Nb, CpR = Cp0, X = Cl 4a), releasing
3
B(C6F5)3. Compound 3a was also obtained by addition of Al(C6F5)3 to the dinuclear μ-oxo compound [TaCp*Cl2-
(μ-O)]2, meanwhile addition of the water adduct H2O Al(C6F5)3 to [TaCp*Me4] gave complex 3c. The structure of 2a
and 3a was obtained by X-ray diffraction studies. Density functional theory (DFT) calculations were carried out to
further understand these types of oxo compounds.
3
Introduction
characterized for vanadium.7-9 Analogous unstable diiodide
and dibromide tantalum compounds are postulated to be
mononuclear,10 but no X-ray structure could be determined,
although the corresponding dichloride compound was for-
mulated as a binuclear complex with μ2-O bridges.11,12
Related mononuclear 16- and 18-electron imide1,2,13-22
and alkylidene1,2,23-30 half-sandwich complexes have been
described for vanadium, niobium, and tantalum.
Metal complexes in high oxidation states are stabilized via
formation of terminal multiple bonds with ligands such as
alkylidene (CR2), imide (NR), and oxo (O).1,2 The similar-
ities between these three ligands and the cyclopentadienyl
ligand (C5R5) has allowed relationships between the frag-
ments [M(C5R5)2] (M= group 4), [M(C5R5)(L)] (M= group
5, L=CR2, NR) and [M(L)2] (M= group 6, L=CR2, NR or
O) to be established.3-6 It is noteworthy that while for
compounds of group 6 metals, terminal oxo ligands are
widely studied,1,2 in group 5 metals, half-sandwich 16 elec-
tron compounds of the type [M(C5R5)(O)R2] (M= group 5)
with a terminal oxo ligand have only been unequivocally
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*To whom correspondence should be addressed. E-mail: pascual.royo@
uah.es (P.R.), javier.sancheznieves@uah.es (J.S.-N.). Fax: 00 34 91 885 4683.
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2010 American Chemical Society
pubs.acs.org/IC
Published on Web 10/15/2010

Article
The very unstable oxo halide compounds [M(C5Me5)-
Inorganic Chemistry, Vol. 49, No. 22, 2010 10643
in insertion reactions44 of CO and CNR than related mono-
cyclopentadienyl imido complexes [TaCp*R2(NR)],34,35,45,46
although the presence of the oxo ligand helped to stabilize the
new acyl derivatives.
(O)X2]n (n = 1, X = Br, I; n = 2, X = Cl) were obtained
by exposure of carbonyl complexes to air in halogenated
solvents,10 oxygen abstraction from CO2 by [TaCp*Cl2-
(PMe)3]11 and oxo-imido exchange.12 Any attempt to intro-
duce the terminal oxo ligand via hydrolysis reactions of
monocyclopentadienyl compounds [M(C5R5)X4] (M = Nb,
Ta) gave polynuclear oxo-bridged complexes.15,31-33 Con-
versely, in previous work by our group, we successfully
isolated stable 18 electron monocyclopentadienyl tantalum
compounds [TaCp*Cl(O){η2-C(Me)NR}] by intramolecular
exchange between imide and acyl groups.34,35 For niobium,
the transfer of an oxo group from oxythiolate compounds
yields terminal oxo complexes.36,37 These are to our knowl-
edge the only examples of fully characterized (X-ray diffrac-
tion studies) terminal oxo monocyclopentadienyl com-
pounds reported for tantalum and niobium, while terminal
oxo complexes are easily achieved for dicyclopentadienyl
compounds of both tantalum and niobium.1,38 Very few
other examples of non cyclopentadienyl terminal oxo tanta-
lum complexes have been described, while for niobium it is a
more common ligand.1
Terminal oxo-borane adducts have been previously
synthesized for Ti,47 V,47,48 Mo,49-52 W,52 Re,51-53 and
U54 compounds starting from the stable terminal oxo com-
plexes in reactions with the Lewis acid B(C6F5)3, also by
trapping a non-detected oxo Ti derivative,55 meanwhile Al43
and Ta39,56 complexes have been obtained by hydrolysis of
M-alkyl bonds with H2O B(C6F5)3 and Zr57 complexes by
3
hydrolysis with [NEt3H][HOB(C6F5)3]. However, similar
oxo-adduct compounds with the related Lewis acid Al-
(C6F5)3 are unknown.
Continuing with our aim to synthesize group 5 mono-
nuclear oxo complexes, we have explored the reactivity
of monocyclopentadienyl niobium compounds [NbCp0X4]
(CpR = C5H4SiMe3, X=Cl, Me) with the water adduct
H2O B(C6F5)3, analogous reactions of these complexes and
related monocyclopentadienyl tantalum derivatives with the
3
alane adduct H2O Al(C6F5)3, and finally borane-alane
3
exchange processes.
We have reported that hydrolysis of Ta-Cl and Ta-C
Results and Discussion
bonds with the water adduct H2O B(C6F5)3 was an efficient
3
method to generate mononuclear oxo compounds of the type
Hydrolysis with H2O E(C6F5)3 (E = B, Al). The same
method reported to obtain the oxo-borane tantalum
complexes [TaCp*R2{O B(C6F5)3}] (R = Cl 1a, CH2Ph
3
[TaCp*R2{O B(C6F5)3}] (R=Cl 1a, CH2Ph 1b, Me 1c),39
3
stabilized by the presence of the strong Lewis acid
3
40-42
1b, Me 1c)39 has been used to isolate the related niobium
B(C6F5)3
which avoids molecular aggregation via
adduct formation.43 These complexes showed similar behavior
derivatives. Thus, a hydrolysis reaction of [NbCp0Cl4]
(Cp0 = η5-C5H4SiMe3) with 1 equiv of H2O B(C6F5)3
afforded the green mononuclear oxo-borane complex
3
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3
heating in toluene at 90 °C for 3 h (Scheme 1). Similar
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3
[NbCp0Me2{O B(C6F5)3}] (2c), whereas the correspond-
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3
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Sanchez-Nieves et al.
10644 Inorganic Chemistry, Vol. 49, No. 22, 2010
Scheme 1. Synthesis of Oxo-Borane Compoundsa
Scheme 2. Synthesis of Oxo-Alane Compoundsa
a (i) H2O Al(C6F5)3, toluene, -78°C; ii) Al(C6F5)3, toluene.
3
Scheme 3. Synthesis of Mononuclear Oxo Adducts from a Dinuclear
Compounda
a (i) H2O B(C6F5)3, toluene, reflux; (ii) H2O B(C6F5)3, toluene,
3
3
-78°C; [MgBz2(THF)2], toluene, -78°C, only for 2b.
in toluene at low temperature. In contrast, we were unable
to detect the formation of the corresponding dimethyl
derivative using different alkylating agents. Compounds
2 are soluble in halogenated and aromatic solvents but
insoluble in alkanes. Compound 2a is thermally stable
and air stable in solution for a few hours, whereas the
alkyl derivatives 2b and 2c were thermally unstable decom-
posing in solution within a few hours.
a (i) E(C6F5)3 (E= B, Al), toluene.
Finally, the mononuclear oxo-alane derivative 3a was
also achieved upon addition of Al(C6F5)3 to the dinuclear
μ-oxo compound [TaCp*Cl2(μ-O)]2, in an analogous way
to its reaction with B(C6F5)3 (Scheme 3).
Analogous hydrolytic reactions of both Nb and Ta
compounds were studied using the adduct H2O Al-
3
(C6F5)3. Addition of this water adduct to the tantalum
complexes [TaCp*(CHPh)Bz2] or [TaCp*Me4] at low
temperature allowed us to isolate the oxo-alane com-
NMR Characterization. The 11B- and 19F-NMR spec-
troscopy of complexes 2 indicated that the boron atom
was tetracoordinated, by means of a broad peak in
the 11B-NMR spectra around δ 0 and by the shift
to higher field of the resonances corresponding to the
meta-fluorine nuclei of the C6F5 groups with regard to
free B(C6F5)3.39,43,44,47-53,56,57 A comparison for com-
pounds 1 and 2 of the 11B NMR chemical shifts and of the
δ(m-F) - δ(p-F) differences (Δδ) in the 19F-NMR spectra
showed a shifting to higher field of the 11B resonances and
a decrease of the Δδ values when the chloro ligands were
replaced by alkyl substituents, indicating a slightly stron-
ger interaction of the oxygen atom with the borane as
consequence of the lower electronegative and higher
inductive effect of the alkyl group.
plexes [TaCp*R2{O Al(C6F5)3}] (R=CH2Ph 3b, Me
3
3c) in low yield. Unfortunately, any attempt to obtain the
corresponding chloro derivatives [MCpRCl2{O Al(C6-
F5)3}] (M = Ta, Nb) or the alkyl niobium derivatives
3
[NbCp0R2{O Al(C6F5)3}] (R = B, Al) by hydrolysis
3
reactions of [MCpRCl4] and [NbCp0(CHPh)Bz2] or
[NbCp0Me4], respectively, failed; a mixture of unidenti-
fied compounds being obtained.
Exchange from M-O-B to M-O-Al. The higher
oxophilicity of Al(C6F5)3 led us to try borane-alane
exchange reactions. Thus, the oxo-borane compounds
[MCpRX2{O B(C6F5)3}] (M = Ta, CpR = η5-C5Me5
3
(Cp*), X = Cl 1a, Bz 1b, Me 1c; M = Nb, CpR = Cp0,
X = Cl 2a, Bz 2b) reacted with one equiv of Al(C6F5)3
affording the corresponding oxo-alane derivatives
The 1H and 13C NMR spectra of compounds 3-4 were
very close to those of the related compounds 1-2, with no
significant change in their chemical shifts. More informa-
tion can be obtained from 19F NMR spectroscopy of the
borane-alane exchange reaction, which clearly showed
resonances corresponding to free B(C6F5)3, whereas the
resonances corresponding to the Al(C6F5)3 moiety were
as expected for this type of tetracoordinated aluminum
derivative.
X-ray Characterization. The X-ray structure of com-
pound 2a is depicted in Figure 1, and selected bond
distances and angles are collected in Table 1. The molecular
structure of compound 2a corresponds to a three-legged
piano stool disposition with the cyclopentadienyl ring at
[MCpRX2{O Al(C6F5)3}] (M = Ta, CpR = Cp*, X =
3
Cl 3a, Bz 3b, Me 3c; M = Nb, CpR = Cp0, X = Cl 4a, Bz
4b) (Scheme 2). This process rendered the more thermo-
dynamically stable compound, as has been inferred
from theoretical calculations (see below). The B(C6F5)3
released in these reactions did not abstract any alkyl
group from the metal center. However, all of these
compounds were thermally unstable even in the solid
state, contrasting with the inertness shown by the
corresponding oxo-borane derivatives, probably as
consequence of the higher oxophilicity of the alumi-
num atom.

Article
Inorganic Chemistry, Vol. 49, No. 22, 2010 10645
Figure 2. X-ray structure of compound 3a.
respect to the boron atom environment, it presents a
distorted tetrahedral geometry with three closer O-B-C
angles about 104° and three more open C-B-C angles
about 113°, as consequence of the steric demand of the
C6F5 groups.
Figure 1. X-ray structure of compound 2a.
Table 1. Selected Bond Distances (A) and Angles (deg) for Compounds 1a,39 2a,
and 3a
The X-ray structure of compound 3a is depicted in
Figure 2, and selected bond distances and angles are
collected in Table 1. The disposition of the substituents
about the Ta atom is as expected, as in compounds 1a and
2a. The Ta-O bond length of 1.788(3) A is very similar to
that in compound 1a (1.784(2) A). This value is longer
than those reported for mononuclear tantalum complexes
containing terminal free Ta-O multiple bonds, ranging
from 1.730 to 1.760 A,34,60 but shorter than the related
1a
2a
3a
M-O
1.784(2)
1.546(4)
2.283(1)
2.284(1)
169.6(2)
104.88(4)
104.49(8)
101.30(8)
1.765(1)
1.558(3)
2.2829(7)
2.2840(7)
177.0(1)
106.77(2)
102.12(5)
103.14(5)
1.788(3)
1.807(4)
2.278(1)
2.279(1)
165.0(2)
105.30(4)
105.01(1)
103.3(1)
O-E
M-Cl(1)
M-Cl(2)
M-O-E
Cl(1)-M-Cl(2)
Cl(1)-M-O
Cl(2)-M-O
oxo-borane adduct [TaCp*{(κ3-OCH2CH2)2O}{O
3
the top. The distribution of the ligands about the Nb atom
is similar to that found in the tantalum oxo-borane com-
pound 1a39 and analogues half-sandwich imido complexes,3,58
with three short M-C bonds and two long M-C-cyclo-
pentadienyl bonds and a slightly distorted cyclopenta-
dienyl ring. The SiMe3 group of the cyclopentadienyl ring
points away from the B(C6F5)3 ligand, as in related imido
compounds.
B(C6F5)3}] (1.829(2) A),56 which is coordinatively satu-
rated, and also shorter than bridging Ta-O-Ta
(1.82-2.10 A). The O-Al bond distance of 1.807(4) A
is shorter than that found in the water adduct H2O Al-
3
(C6F5)3 (1.857(3) A),61 where interactions of both oxygen
lone pairs with the aluminum were proposed from the
oxygen sum of angles (360°), although this compound
also presented intermolecular H-F interactions. How-
ever, the O-Al bond distance in 3a is similar to the one in
The Nb-O bond length of 1.765(1) A is in the high
region of the range found for terminal Nb-O bonds
t
the adduct [WBr(CH2 Bu)3(O) AlBr3] (1.794(3) A), con-
3
(1.678-1.783 A), similar to the one in the oxo-alane
taining a third period transition metal, but longer than
those in the niobium and titanium adduct complexes
[NbCl4(O) AlCl3]2 (1.772 A) and [TiLCl2(O) AlCl3]
2-
complex [NbCl4(O) AlCl3]2
(1.768 A), longer than
3
the values reported for monocyclopentadienyl oxo com-
pounds [Nb2CpR2(μ-η1:η2-edt)2(edt)(O)]36 (mean 1.735
A) and [NbCpR(O){edt2O}]37 (1.734(3) and 1.735(3) A)
(CpR = C5H4Me, edt = SC2H4), and dicyclopentadienyl
compounds [NbCpR2X(O)] (1.718-1.752 A), but clearly
shorter than the bond length found in the oxo-borane
2-
3
3
(1.758 A), respectively. Conversely, all these values are
longer than the O-Al bond distances in bridging
M-O-Al containing complexes (1.710-1.727 A). The
Ta-O-Al angle value of 165.0(2) is slightly smaller than
the corresponding angle in 1a (169.6(2)°) but clearly
smaller than the Nb-O-B angle in 2a. This difference
may be attributed to the steric demand of the Cp* ligand
in 1a and 3a when compared with the Cp0 ligand in 2a.
With respect to the geometry around the aluminum atom,
the pseudotetrahedral disposition of the substituents is
slightly different to the disposition around the boron
atom in 2a but analogous to the environment about the
boron atom in 1a, observing for 1a and 3a one O-X-C
and one C-X-C angle (X = B, Al) closer to 109° than in
2a, whereas the remaining O-X-C and C-X-C angle
adduct [NbCp2 Cl(O BF3)]59 (1.830(4) A). The O-B bond
0
3
distance of 1.558(3) A is within the range observed
for neutral compounds of the type M-O B(C6F5)3
3
(1.496-1.591 A), although clearly longer than the O-B
0
bond distance in [NbCp2 Cl(O BF3)] (1.486(3) A), this
3
difference is attributed to softer Lewis acidity of B(C6F5)3
and also to a stronger interaction of the oxygen atom with
the metal center in compound 2a. Finally, the M-O-B
angle is close to linearity (177.0(1)°), indicative of
π delocalization through the Nb-O-B system. With
ꢀ
ꢀ
(58) Galakhov, M.; Gomez, M.; Jimenez, G.; Pellinghelli, M. A.; Royo,
(60) Parkin, G.; Asselt, A. v.; Leahy, D. J.; Whinnery, L.; Hua, N. G.;
Quan, R. W.; Henling, L. M.; Schaefer, W. P.; Santarsiero, B. D.; Bercaw,
J. E. Inorg. Chem. 1992, 31, 82.
P.; Tiripicchio, A. Organometallics 1994, 13, 1564.
(59) Thiyagarajan, B.; Kerr, M. E.; Bollinger, J. C.; Young, V. G.; Bruno,
J. W. Organometallics 1997, 16, 1331.
(61) Chakraborty, D.; Chen, E. Y. X. Organometallics 2003, 22, 207.

ꢀ
Sanchez-Nieves et al.
10646 Inorganic Chemistry, Vol. 49, No. 22, 2010
Figure 3. Optimized structures for compounds A and 2a. Different bond distances and atom (O, Nb) and molecular fragment (η5-C5H4SiMe3 and
B(C6F5)3) charges are displayed.
values are similar to those in 2a. This situation is also
probably influenced by the different steric requirements
of each type of monocyclopentadienyl ligands.
In contrast, the [B(C6F5)3] moiety has an electronic with-
drawing effect when complex 2a is formed, provoking an
electronic deficit in the Nb center which is compensated for
by theotherligands; thiseffect is clearly seenintheshortening
of the bond distances with both Cl and Cp0 ligands. The
Nb-Cl bond distances are reduced from 2.40 A in A to 2.37 A
in 2a. Also, the average Nb-C distance in the Cp0 moiety
is slightly shortened from 2.53 in A to 2.49 A in 2a. Not only
are the bond distances shortened, but the bond orders are
increased because of the decrease of Nb-O bond order
in 2a. In effect, the Nb-Cl bond order is 1.17 in A and
1.28 in 2a, while the total Nb-Cp bond order is 1.51 in A and
1.70 in 2a.
The formation of complex 4a (Figure 4) occurs in a similar
way to 2a. The main difference is the electronic withdrawing
effect of the [Al(C6F5)3] fragment in 4a, which is lower than
that of the corresponding [B(C6F5)3] moiety in compound 2a.
This is reflected in the charge of the [Al(C6F5)3] moiety in 4a
(-0.12 e) compared with that of the [B(C6F5)3] moiety in 2a
(-0.25 e). Because of this slight difference, the Al-O bond
order is lower (0.25) than the B-O bond order (0.51). This
effect has only a slight effect on the Nb-O bond, the distance
and the bond order remain almost unaffected compared with
complex 2a, with only a small increase in the ionic contribu-
tion of the Nb-O character because of the increase of the
negative charge in oxygen from -0.62e in 2a to -0.76e in 4a.
Finally, the Nb-Cp0 and Nb-Cl bonds are shortened,
increasing the bond order of the corresponding bonds, as
was observed for 2a.
Theoretical Calculations
0
Optimizations of 2a, 4a, [NbCp2 Cl(O)] (A), B(C6F5)3 (B)
and Al(C6F5)3 (C) were carried out to determine the mini-
mum energy structures of these species.62 The data obtained
for 2a are in quantitative agreement with those obtained by
X-ray diffraction (see Supporting Information for details).
The formation of 2a as well as 4a implies a stabilization of
19.3 and 38.1 kcal/mol, respectively, with respect to isolated
compounds A and B or C, with formation of complex 4a
being more energetically favorable. In the following discus-
sion, we analyze the electronic structure of these complexes.
The formation of complex 2a (Figure 3) provokes
a reorganization of the electronic structure compared with
complex A. The Nb-O bond is enlarged from 1.74 A to 1.79 A
and, moreover, the bond order is reduced from 2.1 to 1.5.
Also, the charge separation between both atoms increases
because of the slight increase of the positive charge in the Nb
atom (from þ0.70 in A to þ0.73 in 2a), and the increase of the
negative charge on the oxygen atom (from -0.47 in A to
-0.62 in 2a). The charge separation is explained in terms of
total charge of B(C6F5)3, which is raised from 0 (B) to -0.25e
in 2a. Therefore the Nb-O covalent character is lowered and
the ionic character increased with a net loss of bond order of
about 0.6.
Conclusions
(62) Frisch, J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb,
M. A.; Cheeseman, J. R.; J. A. Montgomery, J.; Vreven, T.; Kudin, K. N.;
Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.;
Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.; Nakatsuji,
H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida,
M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li, X.;
Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Bakken, V.; Adamo, C.;
Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.;
Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.;
Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich,
S.; Daniels, A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.;
Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.;
Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz,
P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.;
Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson,
B.; Chen, W.; Wong, M. W.; Gonzalez, C.; Pople, J. A. Gaussian 03, Revision
E.01; Gaussian, Inc.: Wallingford, CT, 2004.
Mononuclear monocyclopentadienyl oxo niobium and
tantalum complexes [MCpRX2{O E(C6F5)3}] can be ob-
3
tained by hydrolysis of M-Cl and M-C bonds with the
water adducts H2O E(C6F5)3 (E = B, Al). The higher
3
oxophilicity of Al(C6F5)3 with respect to B(C6F5)3 produces
the displacement of the coordinated borane ligand in com-
plexes [MCpRX2{O B(C6F5)3}] and the formation of the
3
corresponding alane derivatives [MCpRX2{O Al(C6F5)3}].
3
The presence of the Lewis acid coordinated to the oxygen
atom is responsible of the complex’s mononuclearity.
X-ray diffraction studies showed that the M-O bonds are
longer than in complexes of the type [MCpR2(O)] and
[MCpR(O)], with terminal non-coordinated oxo ligands, for

Article
Inorganic Chemistry, Vol. 49, No. 22, 2010 10647
(m-C6F5). C26H13BCl2F15NbOSi (829.07): calcd C 37.67, H
1.58; found C 37.83, H 1.69.
[NbCp0(CH2Ph)2{O B(C6F5)3}] (2b). A solution of [Mg-
(CH2Ph)2(THF)2] (0.26 g, 0.72 mmol) in toluene (15 mL) was
3
slowly added to a solution of [NbCp0Cl2{O B(C6F5)3}] (2a)
3
(0.60 g, 0.72 mmol) in toluene (15 mL) at -78 °C. When the
addition was complete, the solution was allowed to reach
ambient temperature and stirred for 2 h. The solution was then
filtered and the volatiles removed under vacuum leaving
a yellow oil that was washed with cold hexane (10 mL) to yield
2b (0.10 g., yield 28%). Data for 2b: 1H NMR (δ, CDCl3): 0.26
(s, 9 H, SiMe3), 1.37 (d, 2J(H-H) = 10.0 Hz, 2 H, Nb-CH2Ph),
3.00 (d, 2J(H-H) = 10.0 Hz, 2 H, Nb-CH2Ph), 5.58 (m, 2 H,
C5H4), 5.83 (m, 2 H, C5H4), 6.54 (d, 3J(H-H) = 7.5 Hz, 4 H,
o-C6H5), 7.24 (m, 4 H, m-C6H5), 7.43 (t, 3J(H-H) = 6.3 Hz, 2 H,
p-C6H5); 11B NMR (δ, CDCl3): -0.35; 13C NMR (δ, CDCl3):
0.0 (SiMe3), 59.8 (Nb-CH2Ph), 114.7 (C5H4), 118.4 (C5H4),
126.7 (i-C5H4), 130.5 (C6H5), 131.1 (C6H5), 132.1 (C6H5),
137.3 (C6F5), 141.1 (C6F5), 147.6 (C6F5); 19F NMR (δ, CDCl3):
-131.3 (o-C6F5), -157.0 (p-C6F5), -163.1 (m-C6F5). C40H27-
BF15NbOSi (940.42): calcd C 51.10, H 2.89; found C 50.73,
H 2.99.
Figure 4. Optimized structures for 4a complex. Bond distances and
atom (O, Nb) and molecular fragment (η5-C5H4SiMe3 and Al(C6F5)3)
charges are displayed.
[NbCp0Me2{O B(C6F5)3}] (2c). A Teflon-valved NMR tube
which bond orders of two and three respectively are pro-
posed, to conform with the 18 electron rule. The oxygen atom
environments indicate that the bond order in these oxo-
adduct complexes could be higher than two, comparing the
3
was charged with a solution of [NbCp0Me4] (0.012 g, 0.041
mmol) in C6D6, and a solution of H2O B(C6F5)3 (0.022 g, 0.041
3
mmol) in C6D6 was added; a bubble of a gas was immediately
observed. NMR spectroscopy showed formation of 2c as the
main reaction product. Data for 2c: 1H NMR (δ, C6D6): -0.20
(s, 9 H, SiMe3), 0.90 (s, 6 H, NbMe2), 5.28 (m, 2 H, C5H4), 5.66
(m, 2 H, C5H4); 11B NMR (δ, C6D6): -0.58; 13C NMR
(δ, C6D6): -1.0 (SiMe3), 48.9 (NbMe), 117.5 (i-C5H4), 119.5
(C5H4), 128.4 (C5H4), 137.1 (C6F5), 141.4 (C6F5), 147.6
(C6F5); 19F NMR (δ, C6D6): -132.7 (o-C6F5), -156.4 (p-
C6F5), -162.9 (m-C6F5).
M-O-E angle in these compounds with that found in
0
[NbCp2 Cl(O BF3)] (140.07°), where the free electron pair
3
on the oxygen atom forces this situation. Also, the linear
M-O-E indicates π delocalization through this system.
Experimental Section
All manipulations were carried out under an argon atmo-
sphere, and solvents were distilled from appropriate drying
agents. NMR spectra were recorded at 300.13 (1H), 282.2
(19F), 75.47 (13C), and 128.4 (11B) at room temperature on a
Varian Unity 300 (1H, 13C, 19F) or Bruker Advance 400 (11B),
and chemical shifts (δ) are given in parts per million (ppm).
1H and 13C resonances were measured relative to solvent
peaks considering TMS = 0 ppm, and 19F and 11B were
[TaCp*Cl2{O Al(C6F5)3}] (3a). A solution of [TaCp*Cl2-
3
{O B(C6F5)3}] (1a) (0.30 g, 0.33 mmol) and Al(C6F5)3 (0.20 g,
3
0.33 mmol) in toluene (5 mL) was stirred for 1 h when the
volatiles were removed under vacuum and the yellow solid
remaining was washed with hexane (2 ꢀ 5 mL) yielding 3a
(0.20 g, 68%).
Data for 3a: 1H NMR (δ, CDCl3): 2.44 (s, 15 H, C5Me5); 13
C
NMR (δ, CDCl3): 12.01 (C5Me5), 129.0 (C5Me5), 136.5 (C6F5),
141.7 (C6F5), 149.5 (C6F5);19F NMR (δ, CDCl3): -121.6 (o-
C6F5), -153.0 ( p-C6F5), -161.1 (m-C6F5). C28H15AlCl2F15OTa
(931.23): calcd C 36.11, H 1.62; found C 35.89, H 1.69.
measured relative to external CFCl3 and BF3 OEt2, respec-
3
tively. Elemental analyses were performed on a Perkin-
Elmer 240C. The compounds [TaCp*Cl4],63 [NbCp0Cl4],31
[NbCp0Me4],64 [TaCp*Me4],63 [TaCp*(CHPh)(CH2Ph)2],23
[TaCp*R2{O B(C6F5)3}] (R=Cl 1a, CH2Ph 1b, Me 1c),39
[TaCp*(CH2Ph)2{O Al(C6F5)3}] (3b). Method (A): A solu-
tion of [TaCp*Bz2{O B(C6F5)3}] (1b) (0.30 g, 0.29 mmol) and
3
3
66
H2O B(C6F5)3,65 (0.5 toluene) Al(C6F5)3 and H2O Al-
3
3
3
3
(C6F5)361 were prepared by literature methods.
[NbCp0Cl2{O B(C6F5)3}] (2a). A solution of [H2O B(C6F5)3]
Al(C6F5)3 (0.17 g, 0.29 mmol) in toluene (5 mL) was stirred for
1 h when the solution was filtered, volatiles were removed under
vacuum, and the yellow solid remaining was washed with
hexane (2 ꢀ 5 mL) yielding 3b (0.12 g, 40%).
3
3
(0.70 g, 1.33 mmol) in toluene (15 mL) was added slowly to a
suspension of [NbCp0Cl4] (0.50 g, 1.33 mmol) in toluene (10 mL)
at ambient temperature. When the addition was complete, the
solution was refluxed for 3 h when the solution was filtered and
the volatiles removed under vacuum. The green solid was
washed with hexane (2 ꢀ 20 mL) yielding 2a (0.80 g, 73%).
Data for 2a: 1H NMR (δ, CDCl3): 0.38 (s, 9 H, SiMe3), 6.67 (m, 2
Method (B): A solution of [H2O Al(C6F5)3] (0.140 g, 0.25
3
mmol) in toluene (5 mL) was added slowly to a solution of
[TaCp*(CHPh)(CH2Ph)2] (0.150 g, 0.25 mmol) in toluene
(5 mL) at -78 °C. When the addition was complete, the solution
was warmed to room temperature and stirred for an additional
1 h when the volatiles were partially removed under vacuum to
about 5 mL. The solution was then filtered and volatiles
removed under vacuum, and the yellow remaining solid was
washed with hexane (2 ꢀ 5 mL) yielding 3b (0.060 g, 23%).
Data for 3b: 1H NMR (δ, C6D6): 1.32 (s, 15 H, C5Me5), 1.73
(d, 2J(H-H) = 12.9 Hz, 2 H, Ta-CH2Ph), 2.70 (d, 2J(H-H) =
12.9 Hz, 2 H, Ta-CH2Ph), 6.72 (m, 4 H, o-C6H5), 7.03 (m, 4 H,
m-C6H5), 7.12 (m, 2 H, p-C6H5); 13C NMR (δ, C6D6): 9.3
(C5Me5), 68.8 (Ta- CH2Ph), 122.8 (C5Me5), 123.4 (C6H5),
127.0 (C6H5), 128.2 (C6H5), 135.1 (m, C6F5), 139.0 (m, C6F5),
149.7 (m, C6F5); 19F NMR (δ, C6D6): -122.1 (o-C6F5), -150.3
(p-C6F5), -159.9 (m-C6F5). C42H29AlF15OTa (1042.58): calcd
C 48.38, H 2.80; found C 48.19, H 2.69.
H, C5H4), 7.14 (m, 2 H, C5H4); 11B NMR (δ, CDCl3): 4.68; 13
C
NMR (δ, CDCl3): -1.0 (SiMe3), 117.5 (i-C5H4), 119.5 (C5H4),
128.4 (C5H4), 137.1 (C6F5), 141.4 (C6F5), 147.6 (C6F5); 19F
NMR (δ, CDCl3): -132.3 (o-C6F5), -155.3 (p-C6F5), -162.4
(63) Sanner, R. D.; Carter, S. T.; Bruton, W. J. J. Organomet. Chem. 1982,
240, 157.
ꢀ
(64) Sanchez-Nieves, J.; Tabernero, V.; Camejo, C.; Royo, P. J. Organo-
met. Chem. 2010, 695, 2469.
(65) Doerrer, L. H.; Green, M. L. H. J. Chem. Soc., Dalton Trans. 1999,
4325.
(66) Feng, S. G.; Roof, G. R.; Chen, E. Y. X. Organometallics 2002, 21,
832.

ꢀ
Sanchez-Nieves et al.
10648 Inorganic Chemistry, Vol. 49, No. 22, 2010
[TaCp*Me2{O Al(C6F5)3}] (3c). Method (A): A solution
of [TaCp*Me2{O B(C6F5)3}] (1c) (0.120 g, 0.14 mmol) and
Table 2. Crystallographic Data of Compounds 2a and 3a
3
3
2a
3a
Al(C6F5)3 (0.080 g, 0.14 mmol) in toluene (5 mL) was stirred
for 1 h when the solution was filtered, volatiles were removed
under vacuum and the yellow solid remaining was washed with
hexane (2 ꢀ 5 mL) yielding 3c (0.070 g, 52%).
formula
Mw
T (K)
C26H13BCl2F15NbOSi C28H15AlCl2F15TaO
829.07
173
931.23
150
λ (A)
0.71073
triclinic
P1
0.71073
triclinic
P1
Method (B): A solution of [H2O Al(C6F5)3] (0.140 g, 0.25
3
crystal system
space group
Z
mmol) in toluene (5 mL) was added slowly to a solution of
[TaCp*Me4] (0.096 g, 0.25 mmol) in toluene (5 mL) at -78 °C.
When the addition was finished the solution was warmed
to room temperature and stirred for an additional 1 h when
the volatiles were partially removed under vacuum to about
5 mL. Then, the solution was filtered, volatiles removed under
vacuum, and the yellow solid remaining was washed with
hexane (2 ꢀ 5 mL) yielding 3c (0.070 g, 30%).
2
2
crystal size, mm3
a (A)
b (A)
0.10 ꢀ 0.13 ꢀ 0.18
10.9783(1)
11.7477(1)
12.4249(1)
88.2373(5)
86.5981(5)
70.7876(3)
1510.42(2)
1.823
0.49 ꢀ 0.40 ꢀ 0.38
10.7086(10)
11.2122(10)
14.0319(14)
90.945(9)
110.889(7)
104.017(8)
1517.3(2)
2.038
c (A)
R (deg)
β (deg)
γ (deg)
V (A3)
Data for 3c: 1H NMR (δ, CDCl3): 0.72 (s, 6 H, Ta-Me), 2.08
(s, 15 H, C5Me5); 13C NMR{1H} (δ, CDCl3): 11.0 (C5Me5), 62.7
(Ta-Me), 123.0 (C5Me5), 136.9 (m, C6F5), 141.0 (m, C6F5), 149.9
(m, C6F5); 19F NMR (δ, CDCl3): -122.1 (o-C6F5), -153.7 (p-
C6F5), -161.3 (m-C6F5). C30H21AlF15OTa (890.39): calcd C
40.47, H 2.38; found C 40.39, H 2.29.
D(Mg/m3)
μ(mm-1
F (000)
)
0.730
812
3.944
892
θ range for data
collection (deg)
reflections collected
1.64-25.38
3.05-27.50
[NbCp0Cl2{O Al(C6F5)3}] (4a). A solution of [NbCp0Cl2{O -
32416
12838
3
3
independent reflections 5534 [R(int) = 0.036] 6811 [R(int) = 0.072]
B(C6F5)3}] (2a) (0.100 g, 0.120 mmol) and 0.5(toluene) Al-
3
R1, wR2 [I > 2σ(I)]
R1, wR2 [all data]
largest diff. peak and
0.0251, 0.0604
0.0251, 0.0626
0.24-0.47
0.0322, 0.0724
1.57-1.92
1.066
(C6F5)3 (0.070 g, 0.120 mmol) in toluene (5 mL) was stirred
for 1 h. The solution was filtered, the volatiles were removed
under vacuum, and the brown oil was washed with hexane (2 ꢀ 3
mL), obtaining 4a as a yellowish solid (0.025 g, 25%).
hole (e A-3
)
GOF
1.052
Data for 4a: 1H NMR (δ, C6D6): -0.13 (s, 9 H, SiMe3), 5.94
(m, 2 H, C5H4), 6.05 (m, 2 H, C5H4); 13C NMR (δ, C6D6): -1.56
(SiMe3), 121.9 (C5H4), 125.6 (i-C5H4), 129.3 (C5H4), 137.3
(C6F5), 142.2 (C6F5), 148.2 (C6F5);19F NMR (δ, C6D6): -121.8
(o-C6F5), -151.5 (p-C6F5), -160.3 (m-C6F5). C26H13AlCl2F15N-
bOSi (845.24): calcd C 36.95, H 1.55; found C 36.55, H 1.39.
X-ray Diffraction Studies. Crystal data and details of the
structure determination are presented in Table 2. Suitable single
crystals of 2a for the X-ray diffraction study were grown from
toluene/hexane. A clear orange crystal was stored under per-
fluorinated ether, transferred in a Lindemann capillary, fixed,
and sealed. Preliminary examination and data collection were
carried out on an area detecting system (NONIUS, MACH3,
κ-CCD) at the window of a rotating anode (NONIUS, FR591)
and graphite-monochromated Mo-KR radiation (λ = 0.71073 A).
The unit cell parameters were obtained by full-matrix least-
squares refinement of 5517 reflections. Data collection was
performed at 173 K (OXFORD CRYOSYSTEMS) within a
θ-range of 1.64°< θ < 25.38°. The data were collected with nine
sets in rotation scan modus with Δj/Δω = 1.0°. A total number
of 32416 intensities were integrated. Raw data were corrected
for Lorentz, polarization, and, arising from the scaling proce-
displacement parameters. All hydrogen atoms were found in
the final difference Fourier maps and refined with individual
isotropic displacement parameters. Full-matrix least-squares
refinements with 476 parameters were carried out by minimizing
P
2
2 2
w(Fo - Fc ) with the SHELXL-97 weighting scheme and
stopped at shift/err <0.001. The final residual electron density
maps showed no remarkable features.67-72
A suitable single crystal of 3a for the X-ray diffraction study
was selected (Table 2), covered with perfluorinated ether and
mounted on a Bruker-Nonius Kappa CCD single crystal dif-
fractometer. Data collection was performed at 150(2) K. The
structure was solved, using the WINGX package,73 by direct
methods (SHELXS-97) and refined by using full-matrix least-
squares against F2 (SHELXL-97).69 All non-hydrogen atoms
were anisotropically refined, and hydrogen atoms were geome-
trically placed and left riding on their parent atoms.
Acknowledgment. We gratefully acknowledge Ministerio de
ꢀ
Educacion y Ciencia (projects MAT2007-60997 and CTQ2009-
07120) and DGUI-Comunidad de Madrid (programme S-0505/
PPQ-0328, COMAL-CM) (Spain) for financial support. L.M.F.
ꢀ
acknowledges Spanish MEC for a “Ramon y Cajal” contract.
dure, latent decay, and absorption effects. After merging (Rint
=
0.036) a sum of 5534 (all data) and 5075 [I>2σ(I)], respectively,
remained and all data were used. The structure was solved
by a combination of direct methods and difference Fourier syn-
theses. All non-hydrogen atoms were refined with anisotropic
Supporting Information Available: Tables and figures giving
computational details. Crystallographic data for compounds
2a and 3a are available in CIF format. This material is available
free of charge via the Internet at http://pubs.acs.org. Crystal-
lographic data (excluding structure factors) for the structures
reported in this paper have been deposited with the Cambridge
Crystallographic Data Centre as supplementary publication
no. CCDC-785359 (2a) and CCDC-794327 (3a). Copies of the
data can be obtained free of charge on application to CCDC, 12
Union Road, Cambridge CB2 1EZ, U.K. (fax: (þ44)1223-336-
033; e-mail: deposit@ccdc.cam.ac.uk).
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