Pentaarylfullerenes as noncoordinating cyclopentadienyl anions

Article abstract of DOI:10.1021/ic802034m

The first example of an early-transition-metal complex involving a pentaarylfullerene was prepared. Instead of half-sandwich complexes, solvent separated ion pairs were obtained in which the pentaarylfullerene moiety acts as noncoordinating cyclopentadien

Full text of DOI:10.1021/ic802034m

Inorg. Chem. 2009, 48, 8-9
Pentaarylfullerenes as Noncoordinating Cyclopentadienyl Anions
Marco W. Bouwkamp* and Auke Meetsma
Molecular Inorganic Chemistry Department, Stratingh Institute for Chemistry,
UniVersity of Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands
Received October 23, 2008
Scheme 1
The first example of an early-transition-metal complex involving a
pentaarylfullerene was prepared. Instead of half-sandwich com-
plexes, solvent separated ion pairs were obtained in which the
pentaarylfullerene moiety acts as noncoordinating cyclopentadienyl
anion.
About a decade ago, the group of Nakamura introduced a
new family of cyclopentadienyl ligands derived from
fullerenes.1 Since their initial report, a number of transition-
metal complexes with these pentaalkyl- and pentaaryl-
fullerene ligands have emerged, illustrating the versatility
of this type of ligand. Examples include sandwich2 and half-
sandwich complexes3 of group 6,3h 7,3g 8,2,3a,b,e,g 9,3d,f,g and
103c metals. To date, however, the chemistry of these ligands
is limited to the middle and right-hand sides of the periodic
table, and no early-transition-metal complexes are known.3h
Here we report our initial efforts to isolate their early-
transition-metal counterparts, which resulted in the synthesis
of cationic zirconium complexes with noncoordinating pen-
taarylfullerene anions.
1
the reaction by H NMR spectroscopy in THF-d8 revealed
that, during the reaction, 1 equiv of dimethylamine was
formed. Both the H and 13C NMR spectra of the product
1
revealed the correct number of peaks expected for a half-
sandwich complex of the type [(Ph5C60)Zr(NMe2)3]. For
example, the 1H NMR spectrum shows resonances at δ 7.8,
7.0, and 2.87 ppm in a 10:15:18 ratio.
On the other hand, a close comparison of the NMR
spectroscopic data of the fullerene moiety with that of the
corresponding lithium reagent [Li(THF)4][Ph5C60] (3a)5
shows that the two are virtually identical. This suggests the
formation of an ion pair of the type [Zr(NMe2)3(THF)n]-
[Ph5C60] (2a; Scheme 1), in which there is no direct contact
between the zirconium tris(dimethylamide) cation and the
pentaphenylfullerene anion. Compound 3a was prepared
separately by the addition of 1 equiv of LiNMe2 to ligand
1a in THF (Scheme 1). To further confirm the nature of ion
pair 2a, a zirconium tris(dimethylamide) cation with a known
weakly coordinating anion, [BPh4]-, was prepared as a
reference by treatment of [Zr(NMe2)4] with 1 equiv of
[PhNMe2H][BPh4] in a THF solution. The compound was
obtained as its bis- or tris(THF) adduct, [Zr(NMe2)3(THF)n]-
[BPh4] (n ) 2, 4; n ) 3, 4′), depending on the workup
procedure (vide infra). The NMR spectroscopic data for the
[Zr(NMe2)3(THF)n] cation of 4 and 4′ are virtually identical
with those of compound 2a, as expected for a solvent-
separated ion pair.
The addition of tetrahydrofuran (THF) to an equimolar
mixture of the pentaarylfullerene ligand precursor Ph5C60H
(1a) and zirconium amide [Zr(NMe2)4]4 resulted in gas
evolution and the disappearance of the poorly soluble ligand
precursor, affording a reddish-brown solution. Monitoring
*
To whom correspondence should be addressed. E-mail:
m.w.bouwkamp@rug.nl.
(1) (a) Sawamura, M.; Iikura, H.; Nakamura, E. J. Am. Chem. Soc. 1996,
118, 12850. (b) Nakamura, E.; Sawamura, M. Pure Appl. Chem. 2001,
73, 355. (c) Nakamura, E. Pure Appl. Chem. 2003, 75, 427. (d)
Nakamura, E. J. Organomet. Chem. 2004, 689, 4630.
(2) (a) Sawamura, M.; Kuninobu, Y.; Toganoh, M.; Matsuo, Y.; Yamanaka,
M.; Nakamura, E. J. Am. Chem. Soc. 2002, 124, 9354. (b) Matsuo, Y.;
Kuninobu, Y.; Ito, S.; Nakamura, E. Chem. Lett. 2004, 33, 68. (c)
Herber, R. H.; Nowik, I.; Matsuo, Y.; Toganoh, M.; Kuninobu, Y.;
Nakamura, E. Inorg. Chem. 2005, 44, 5629.
(3) (a) Matsuo, Y.; Nakamura, E. Organometallics 2003, 22, 2554. (b)
Tonagoh, M.; Matsuo, Y.; Nakamura, E. J. Am. Chem. Soc. 2003, 125,
13974. (c) Kuninobu, Y.; Matsuo, Y.; Toganoh, M.; Sawamura, M.;
Nakamura, E. Organometallics 2004, 23, 3259. (d) Matsuo, Y.; Iwashita,
A.; Nakamura, E. Organometallics 2005, 24, 89. (e) Matsuo, Y.; Mitani,
Y.; Zhong, Y.-W.; Nakamura, E. Organometallics 2006, 25, 2826. (f)
Sawamura, M.; Kuninobu, Y.; Nakamura, E. J. Am. Chem. Soc. 2000,
122, 12407. (g) Matsuo, Y.; Kuninobu, Y.; Muramatsu, A.; Sawamura,
M.; Nakamura, E. Organometallics 2008, 27, 3403. (h) Matsuo, Y.;
Iwashita, A.; Nakamura, E. Organometallics 2008, 27, 4611.
(4) Chandra, G.; Lappert, M. F. J. Chem. Soc. A 1968, n/a, 1940.
Recrystallization of compound 2a from chlorobenzene/
cyclohexane eventually resulted in single crystals that were
(5) The stoichiometries of lithium salts 3a and 3b were determined by
elemental analysis. For 3b, this was confirmed by X-ray analysis.
8 Inorganic Chemistry, Vol. 48, No. 1, 2009
10.1021/ic802034m CCC: $40.75  2009 American Chemical Society
Published on Web 12/02/2008

COMMUNICATION
Table 1. Selected Bond Distances (Å) for Compounds 4 and 4′
4
4′
Zr1-N11
Zr1-N12
Zr1-N13
Zr1-O11
Zr1-O12
2.001(4)
2.042(3)
2.056(2)
2.053(2)
2.057(2)
2.3588(18)
2.3652(18)
2.261(2)
With the hope of obtaining better crystals of an ion pair
of the type 2a, a similar compound was prepared with
different ligand substituents and thus with different solubili-
ties. Ligand (4-n-PrC6H4)5C60H (1b)7 was prepared in the
same way as 1a, by treatment of an excess (15 equiv) of an
in situ generated arylcopper reagent to C60.8 Like in the case
of 1a, treatment of ligand 1b with [Zr(NMe2)4] resulted in a
Figure 1. Ball-and-stick representation of ion pair 2a. The hydrogen atoms
were omitted for clarity.
1
brownish-red reaction mixture, and the H and 13C NMR
spectroscopic data are consistent with the formation of ion
pair [Zr(NMe2)3(THF)2][(4-n-PrC6H4)5C60] (2b). Again, the
spectroscopic data for ion pair 2b are nearly identical with
those of the corresponding ions in compounds 3b7 and 4.
Unfortunately, by using this ligand, no single crystals suitable
for X-ray analysis were obtained.
Other attempts to prepare half-sandwich zirconium com-
plexes using these types of pentaarylfullerene ligands include
salt metathesis using ZrCl4 and lithium reagent 3b in THF
and the aforementioned amine elimination reaction in a
weakly coordinating solvent, such as toluene. In the case of
the first, ligand precursor 1b was obtained at room temper-
ature in THF-d8, suggesting the initial formation of ion pair
[ZrCl3(THF)n][(4-n-PrC6H4)5C60], which, in turn, abstracts
a proton from the reaction mixture. Accordingly, treatment
of compound 2b with Me3SiCl in THF-d8 resulted in the
precipitation of 1b and concurrent formation of Me3SiNMe2.
When ligand 1b was treated with a solution of [Zr(NMe2)4]
in toluene-d8, no reaction was observed by 1H NMR
spectroscopy, even at elevated temperatures (80 °C).
In summary, attempts to prepare half-sandwich complexes
of zirconium bearing pentaarylfullerene ligands resulted in
solvent-separated ion pairs involving a zirconium tris(dim-
ethylamide) cation and a noncoordinating fullerene-derived
cyclopentadienyl anion. This is most likely the result of the
reduced nucleophilicity of the pentaarylfullerene anion
compared to that of more traditional cyclopentadienyl
anions.9
Figure 2. ORTEP representations of 4 (left) and 4′ (right) showing 50%
probability levels. The hydrogen atoms and counterions were omitted for
clarity.
suitable for X-ray analysis. The scattering power of the crystals
was very weak, and refinement was complicated by disorder
in the zirconium tris(dimethylamide) cation. Yet, the data were
of sufficient quality to reveal the nature of the ion pair. A ball-
and-stick representation of its structure is depicted in Figure 1.
The cation of 2a is best described as a five-coordinate zirconium
tris(dimethylamide) species with trigonal-bipyramidal geometry.
The amide ligands are bound in the equatorial plane, and the
apical positions are occupied by two molecules of THF. No
apparent interaction between the cation and the pentaphenyl-
fullerene anion was found. As yet, there is one other example
of an early-transition-metal cation with a fullerene anion. In
that case, the ion pair is obtained by reduction of C60 using a
titanium(III) compound.6
Because the cation of 2a was disordered, the structure of
compound 4 was determined as well, in order to obtain a
better description of the cation. Single crystals suitable for
an X-ray diffraction study were obtained by recrystallization
of 4 from chlorobenzene/cyclohexane. Initially the compound
was recrystallized from THF/cyclohexane, but this resulted
in single crystals of the six-coordinate tris(THF) adduct 4′.
An ORTEP representation of both cations is depicted in
Figure 2 (see Table 1 for pertinent bond distances). As
expected, the metal-ligand bond distances in 4′ [average
Zr-N ) 2.055(3) Å; average Zr-O ) 2.357(3) Å] are
slightly longer compared to those in 4 [average Zr-N )
2.022(5) Å; average Zr-O ) 2.261(2) Å], a result of the
higher coordination number in 4′.
Acknowledgment. This investigation was supported by
The Netherlands Organization for Scientific Research (NWO).
Prof. B. Hessen and Prof. J. C. Hummelen are acknowledged
for use of the facilities.
Supporting Information Available: Experimental details and
crystallographic data for compounds 1b, 2a, 3b, 4, and 4′. This
materialisavailablefreeofchargeviatheInternetathttp://pubs.acs.org.
IC802034M
(7) Ligand 1b and compound 3b were characterized by single-crystal X-ray
analysis.
(8) Sawamura, M.; Iikura, H.; Ohama, T.; Hackler, U. E.; Nakamura, E. J.
Organomet. Chem. 2000, 599, 32.
(9) Iikura, H.; Mori, S.; Sawamura, N.; Nakamura, E. J. Org. Chem. 1997,
62, 7912.
(6) Barroso, S.; Cui, J.; Dias, A. R.; Duarte, M. T.; Ferreira, H.; Henriques,
R. T.; Oliveira, M. C.; Ascenso, J. R.; Martins, A. M. Inorg. Chem.
2006, 45, 3532.
Inorganic Chemistry, Vol. 48, No. 1, 2009 9