Vanadocene-mediated ionization of water in the aqua species [H 2O·B(C6F5)3]: Structural characterization of the hydride and hydroxide complexes [Cp2V(μ-H) B(C6F5)3] an

Article abstract of DOI:10.1021/om050965x

The reaction of Cp₂V with B(C₆F₅) ₃ in the presence of water leads to the hydride complex [Cp ₂V(μ-H)B(C₆F₅)₃] (1) and the hydroxide complex [Cp₂V(μ-OH)B(C<

Full text of DOI:10.1021/om050965x

Organometallics 2006, 25, 1551-1553
1551
Vanadocene-Mediated Ionization of Water in the Aqua Species
[H2O‚B(C6F5)3]: Structural Characterization of the Hydride and
Hydroxide Complexes [Cp2V(µ-H)B(C6F5)3] and
[Cp2V(µ-OH)B(C6F5)3]
Robert Choukroun,* Christian Lorber,† Laure Vendier, and Christine Lepetit
Laboratoire de Chimie de Coordination, CNRS UPR 8241, 205 route de Narbonne,
31077 Toulouse Cedex 04, France
ReceiVed NoVember 10, 2005
Summary: The reaction of Cp2V with B(C6F5)3 in the presence
of water leads to the hydride complex [Cp2V(µ-H)B(C6F5)3] (1)
and the hydroxide complex [Cp2V(µ-OH)B(C6F5)3] (2), both of
which haVe been characterized by X-ray structure determina-
tions. Both complexes are deriVed from the ionization of water
in the presence of B(C6F5)3.
Addition of Cp2V in toluene to a solution of hydrated borane9
gives a blue-purple solution and, after 1 day, yellow-orange and
blue crystals (Scheme 1). After hand separation, both compounds
Scheme 1
Various aspects of the chemistry and applications of B(C6F5)3
have been recently reviewed.1,2 Though B(C6F5)3 is well-known
as an activator for homogeneous group 4 transition-metal-
catalyzed olefin polymerization,3 it displays versatile chemistry
and unique reactivity. It is highly reactive with water, and its
aqua complex H2O‚B(C6F5)3 is known to behave as a strong
Brønsted acid.4 Indeed, adventitious water has given access to
the hydroxyborate complexes [M][O(H)B(C6F5)3] (M ) Ti, Zr).5
The identification of the hydroxyborate anions [HOB(C6F5)3]-,
[(F5C6)3B(µ-OH)B(C6F5)3]-, and [(F5C6)3B(Η3O2)B(C6F5)3]-,
which are derived from the reaction of B(C6F5)3 with water,
suggests the presence of an acidic-proton-containing counter-
cation.6 Several reactions reported recently in the literature
indirectly support the presence of such a cation; treatment of
[MCp2] (M ) Cr, Fe, Co) with H2O‚B(C6F5)3 resulted in metal
oxidation with H2 elimination.7 The hydride species [M(µ-H)B-
(C6F5)3] (M ) Ti, Zr, Nb) has been suggested to be the key
intermediate. In some cases the intermediate has been observed
by NMR spectroscopy,8 but it has never been isolated and
studied. Here we report the first isolation of this hydride species
in a vanadocene system.
were unambiguously characterized by an X-ray structure
analysis. Figures 1 and 2 give ORTEP views of the molecular
structures of 1 and 2, respectively.
* To whom correspondence should be addressed. E-mail:
choukroun@lcc-toulouse.fr.
† E-mail: lorber@lcc-toulouse.fr.
(1) Piers, W. E. AdV. Organomet. Chem. 2005, 52, 1-76.
(2) Erker, G. Dalton Trans. 2005, 1883-1890.
Figure 1. Ortep drawing of the molecular structure of 1 showing
50% probability ellipsoids and partial atom-labeling schemes.
Hydrogen atoms are omitted for clarity. Selected parameters (bond
lengths in Å and angles in deg): V1-H100 ) 1.87(2), B1-H1)
1.34(2); V1-H100-B1 ) 152.8, Cp1-V1-Cp2 ) 142.8 (Cp1
and Cp2 are the centroids of the C1-C5 and C6-C10 Cp ligands,
respectively).
(3) For a recent review on this subject see: Chen, E. Y-X.; Marks, T.
J. Chem. ReV. 2000, 100, 1391-1434.
(4) (a) Beringhelli, T.; Maggioni, D.; D’Alfonso, G. Organometallics
2001, 20, 4927-4938. (b) Bergquist, C.; Bridgewater, B. M.; Harlan, C.
J.; Norton, J. R.; Friesner, R. A.; Parkin, G. J. Am. Chem. Soc. 2000, 122,
10581-10590.
(5) (a) Spanneberg, A.; Burlakov, V. V.; Arndt, P.; Baumann, W.; Shur,
V. B.; Rosenthal, U. Z. Kristallogr. 2002, 217, 546-548. (b) Choukroun,
R.; Lorber, C.; Vendier, L. Eur. J. Inorg. Chem. 2004, 317-321.
(6) (a) In presence of the “metal base” [Ir(Cp)(C8H12)], a salt containing
the hydride ligand on iridium, [Ir(Cp)(C8H12)H][(F5C6)3B(µ-OH)B(C6F5)3],
was isolated.4b (b) Danopoulos, A. A.; Galsworthy, J. R.; Green, M. L. H.;
Cafferkey, S.; Doerrer, L. H.; Hursthouse, M. B. Chem. Commun. 1998,
2529-2530. (c) Di Saverio, A.; Focante, F.; Camurati, I.; Resconi, L.;
Beringhelli, T.; d’Alfonso, G.; Donghi, D.; Maggioni, D.; Mercandelli, P.;
Sironi, A. Inorg. Chem. 2005, 44, 5030-5041.
(8) (a) Harlan, C. J.; Hascall, T.; Fujita, E.; Norton, J. R. J. Am. Chem.
Soc. 1999, 121, 7274-7275. (b) Plecnik, C. E.; Liu, F.-C.; Liu, J.; Meyers,
E. A.; Shore, S. G. Organometallics 2001, 20, 3599-3606. (c) Yang, X.;
Stern, C. L.; Marks, T. J. Angew. Chem., Int. Ed. Engl. 1992, 31, 1375-
1377. (d) Liu, F. C.; Liu, J.: Meyers, E. A.; Shore, S. G. J. Am. Chem.
Soc. 2000, 122, 6106-6107. (e) Liu, S.; Liu, F.-C.; Renkes, G.; Shore, S.
G. Organometallics 2001, 20, 5717-5723.
(7) Doerrer, L. H.; Green, M. L. H. J. Chem. Soc., Dalton Trans. 1999,
4325-4329.
(9) Prepared by the addition 2 equiv of B(C6F5)3 with 1 equiv of H2O or
from isolated H2O‚B(C6F5)3) (1 equiv) with B(C6F5)3 (1 equiv).
10.1021/om050965x CCC: $33.50 © 2006 American Chemical Society
Publication on Web 02/24/2006

1552 Organometallics, Vol. 25, No. 7, 2006
Communications
Scheme 2
suggest a bridging hydride in 1 with weakened V-H or B-H
bonds. In a sense, 1 can be regarded as a vanadium hydride
stabilized by a borane before its abstraction by the borane,
leading to a salt containing the [HB(C6F5)3]- anion.8d,13
The blue crystals were structurally characterized as being the
expected [hydroxytris(pentafluorophenyl)borato]vanadium(III)
complex [Cp2V(µ-OH)B(C6F5)3] (2), in which the hydrogen
atom of the hydroxide group was also located by the X-ray
structure determination. The V-O (2.0875(14) Å) and O-B
(1.518(3) Å) distances are those expected for a bridging
hydroxide of a zwitterionic complex. In contrast to the case for
the analogous titanium complexes [Cp2Ti(µ-OH)B(C6F5)3] and
[rac-(ebthi)Ti(µ-OH)B(C6F5)3], in which a C-F‚‚‚Ti interaction
between the Ti center and an ortho fluorine atom of the borane
moiety occurs in the solid-state structure,5 no such interaction
is observed in the vanadium complex 2.
According to the stoichiometry of the reaction (Scheme 1)
compounds 1 and 2 should be obtained in equimolar amounts.
However, only a low yield of 1 is obtained. Varying the
experimental (solvent, concentration of reactants) or workup
conditions did not improve the yield of isolated 1 significantly,
and as a consequence this precluded its full characterization
and its reactivity study.13 Furthermore, meticulously dried
aprotic solvent added to a mixture of 1 and 2 immediately
emitted fumes and led to partial decomposition of 1 (leaving 2
unchanged, as monitored by IR). The very high sensitivity of 1
toward adventitious water may explain the difficulty we
encountered in its isolation and may also explain why such
hydride species have remained elusive in other metal systems
that have been studied. The magnetic moments measured for 1
and 2 are in agreement with those of vanadium(III) species (µeff
) 2.85 and 2.95 µB, respectively).13 The νOH infrared frequency
of 2 in KBr is observed at 3614 cm-1. The deuterated analogue
[Cp2V(µ-OD)B(C6F5)3]14 has a νOD value of 2666 cm-1. For 1
(which was always contaminated by a small amount of 2) the
medium-broad B-H stretching bands centered at 1904 cm-1
are too complex to be assigned but are compatible with a
monodentate borohydride species;15 the B-D stretching band
of the deuterated analogue is not observed, due to its superposi-
tion with other bands of the complex.
Figure 2. Ortep drawing of the molecular structure of 2 showing
50% probability ellipsoids and partial atom-labeling schemes.
Hydrogen atoms are omitted for clarity. Selected parameters (bond
lengths in Å and angles in deg): V1-O1 ) 2.0875(14), B1-O1
) 1.518(3), O1-Ho1 ) 0.75(2); V1-O1-B1 ) 142.57(13), Cp1-
V1-Cp2 ) 141.7 (Cp1 and Cp2 are the centroids of the C1-C5
and C6-C10 Cp ligands, respectively).
The yellow-orange crystals were determined to be the hydride
complex [Cp2V(µ-H)B(C6F5)3] (1). The data were of sufficiently
high quality that we were able to locate a hydrogen atom
between the vanadocene moiety and the borane Lewis acid. The
V-H distance in 1 (1.87(2) Å) is comparable to those in
complexes having an hydrogen atom bridging a vanadium center
and a second metal (V, Zn) or a boron atom of a η1- or η2-
borohydride ligand (V-H ) 1.69(4)-1.94(3) Å).10 The B-H
distance in 1 (1.34(2) Å) is longer than that found in the
aforementioned η1- and η2-BH4 complexes (1.12 (av) and 1.10
Å, respectively). This B-H bond is also longer than in the anion
[HB(C6F5)3]- (for example, for [Cp2V(CO)2][HB(C6F5)3] B-H
) 1.14(2) Å11 and for [Cp*2Zr][HB(C6F5)3] B-H ) 1.06(6)
Å8c). It is worth noting that the sums of the angles C-B-C
around the boron atom are nearly the same in 1 and in the
[HB(C6F5)3]- anion.
A comparative topological analysis was performed using the
electron localized function (ELF)12 in order to get more insight
into the V-H-B interaction in 1. The ELF picture of 1 was
compared to those of the free B-H bond in the [HB(C6F5)3]-
anion, free V-H in Cp2VH, and V-H-V in Cp4V2H2. As no
crystallographic structure was available for the last two com-
plexes, their structures were calculated at the B3PW91/6-31G*
level (see the Supporting Information). According to the ELF
analysis of 1, the H atom interacts with both V and B, as it is
involved in a three-center-two-electron bond (defined in the
ELF by the trisynaptic basin V(V,H,B)). The atomic contribution
of V to the population of the trisynaptic basin V(V,H,B) is close
to but slightly lower for 1 than for Cp4V2H2. Similarly, the
atomic contribution of the B atom to the population of V(V,H,B)
is slightly lower for 1 than for the [HB(C6F5)3]- anion. These
findings are in agreement with the structural parameters, which
The concomitant formation of a hydride species 1 and a
hydroxide species 2, starting from [H2O][B(C6F5)3], implies the
two-electron reduction of H2O by Cp2V. We suggest the reaction
to proceed via the overall pathway depicted in Scheme 2, which
involves the intermediacy of a V(IV) species ([Cp2VH]+[HOB-
(C6F5)3]-) that further evolves through disproportionation with
Cp2V into the vanadium(III) complexes 1 and 2.16
(13) The spectroscopic and magnetic studies were conducted at room
temperature on a pure crystalline sample obtained after separation by hand
of a mixture of large crystals of 1 and 2. Spectroscopic studies of 1 in
THF-d8 showed the formation of the vanadium(III) EPR-silent salt [Cp2V]+-
[HB(C6F5)3]-: 1H NMR (400 MHz) 145 (br, Cp), 4.28 ppm (br q, BH, JBH
) 80 Hz); 11B NMR -24.5 ppm (d, JBH ) 76 Hz); 19F NMR -46.2, -83.9,
-85.3 ppm. Complex 2 is EPR-silent. Spectroscopic studies of 1 in THF-
d8: 1H NMR (400 MHz) 134 ppm (br, Cp) (HO unobserved); 11B NMR
-3.3 ppm; 19F NMR unresolved. (note: 19F NMR (188.3 MHz), reference
CF3CO2H; 11B NMR (160.5 MHz), reference BF3‚Et2O).
(14) [Cp2V(µ-OD)B(C6F5)3] was obtained from the above reaction
scheme using D2O in place of H2O.
(15) (a) Marks, T. J.; Kolb, J. R. Chem. ReV. 1977, 77, 263-293. (b)
Marks, T. J.; Kennelly, W. J. J. Am. Chem. Soc. 1975, 97, 1439-1443.
(10) (a) Jonas, K.; Wiskamp, V.; Tsay, Y.-H.; Kru¨ger, C. J. Am. Chem.
Soc. 1983, 105, 5480-5481. (b) Bansemer, R. L.; Huffman, J. C.; Caulton,
K. J. J. Am. Chem. Soc. 1983, 105, 6163-6164. (c) Berno, P.; Gambarotta,
S. Angew. Chem., Int. Ed. Engl. 1995, 34, 822-824. (d) Gerlach, C. P.;
Arnold, J. J. Chem. Soc., Dalton Trans. 1997, 4795-4805. (e) Jensen, J.
A.; Girolami, G. S. Inorg. Chem. 1989, 28, 2107-2113.
(11) Choukroun, R.; Lorber, C.; Donnadieu, B. Organometallics 2004,
23, 1434-1437.
(12) Silvi, B.; Savin, A. Nature 1994, 371, 683-686.

Communications
Organometallics, Vol. 25, No. 7, 2006 1553
We have already reported the formation of a hydride species
(at that time wrongly formulated as the salt [Cp2V][HB(C6F5)3])
from the reaction of [Cp2VCO] with B(C6F5)3.11 We can now
see that redox and disproportionation reactions afford four
different complexes: 1, the structurally established, unprec-
edented, zwitterionic ring-borylated vanadium(III) complexes
CpCpBV and CpCpBV(CO)2 (CpB ) η5-C5H4B(C6F5)3), and the
ionic vanadium(III) species [Cp2V(CO)2][HB(C6F5)3].
we have demonstrated that its formation implies a two-electron
reduction of water (mediated, in our work, by Cp2V). Due to
the powerful water scavenging ability of B(C6F5)3, one may
not exclude the possibility that such hydride and hydroxide
species can be formed during catalytic olefin polymerization
processes involving metallocenes and B(C6F5)3 cocatalyst in the
presence of traces of water.1 We are currently working to answer
this point.
In conclusion, the existence of complex 1 is of considerable
importance and contributes to a better understanding of ion
pairs.17 It shows for the first time that such a hydride intermedi-
ate postulated for early-transition-metal complexes exists, and
Acknowledgment. We are grateful to the CNRS for financial
support.
Supporting Information Available: CIF files giving crystal
data for complexes 1 and 2 and tables giving computational details.
This material is available free of charge via the Internet at
http://pubs.acs.org.
(16) Disproportionation and redox reactions are often occurring in
vanadium chemistry; see for example: (a) Choukroun, R.; Lorber, C. Eur.
J. Inorg. Chem. 2005, 4683-4692. (b) Choukroun, R.; Moumboko, P.;
Chevalier, S.; Etienne, M.; Donnadieu, B. Angew. Chem., Int. Ed. 1998,
37, 3169-3172. (c) Berno, P.; Gambarotta, S.; Richeson, D. ComprehensiVe
Organometallic Chemistry II; Abel, E. W., Stone, F. G. A., Wilkinson, G.,
Eds.; Pergamon: New York, 1995; Vol. 4, pp 1-55.
OM050965X
(17) Luo, L.; Marks, T. J. Top. Catal. 1999, 7, 97-106.