How much steric crowding is possible in tris(η5-pentamethylcyclopentadienyl) complexes? Synthesis and structure of (C5Me5)3UCl and (C5Me5)3UF1 [13]

Full text of DOI:10.1021/ja002486p

J. Am. Chem. Soc. 2000, 122, 12019-12020
12019
the sequence of the multielectron reduction of (C5Me5)3U, we
sought a system in which (C5Me5)3U would reduce a substrate
stepwise, the intermediate(s) could be identified, and it could be
determined which of the two half reactions, eq 2 or 3, occurred
first. Phenyl halides, which had previously been useful in
organouranium chemistry,9,10 proved suitable for this purpose.
(C5Me5)3U reacts instantly at room temperature with one equiv
of PhCl to form a dark red complex, 1, as the primary product.
Upon addition of another equiv of PhCl, 1 is transformed over
several days to (C5Me5)2UCl2, 2. The latter complex can be made
in good yield using 2 equiv of PhCl as shown in eq 4 in which
(C5Me5)3U is functioning as a two-electron reductant.
How Much Steric Crowding Is Possible in
Tris(η5-pentamethylcyclopentadienyl) Complexes?
Synthesis and Structure of (C5Me5)3UCl and
(C5Me5)3UF1
William J. Evans,* Gregory W. Nyce,
Matthew A. Johnston, and Joseph W. Ziller
Department of Chemistry
UniVersity of California, IrVine
IrVine, California 92697-2025
ReceiVed July 10, 2000
During several decades of investigation of organometallic
pentamethylcyclopentadienyl chemistry, no examples of tris-
(ligand) complexes, (C5Me5)3M, were reported.2 Such complexes
were assumed to be too sterically crowded to exist since the C5-
Me5 cone angle was estimated to be much greater than 120°.3
The discovery of (C5Me5)3Sm4 not only showed that this class of
complexes could exist, but it also revealed new opportunities in
organometallic reaction chemistry.5-7 Most surprising was the fact
that this trivalent complex could accomplish one-electron reduc-
tion chemistry.5c This has led to the development of “sterically
induced reduction” chemistry in which sterically crowded com-
plexes of redox inactive metals act as reductants.6,7
Although three new synthetic routes in addition to the original
synthesis of (C5Me5)3Sm have been discovered,5a,8 only two other
crystallographically characterized (C5Me5)3M complexes have
been reported in the literature, (C5Me5)3Nd8a and (C5Me5)3U.5a It
is clear that formation and isolation of (C5Me5)3M complexes is
not easy and requires conditions where more sterically favorable
options are not accessible. Although it is expected that metals
larger than Sm(III) should form these complexes (e.g., La(III)-
Pr(III)), it is uncertain if complexes of smaller metals would be
isolable. We now report that the reaction chemistry of (C5Me5)3U
has led to the isolation of significantly more crowded (C5Me5)3M
systems in which a fourth ligand is present.
(C5Me5)3U + 2PhCl
8 (C5Me5)2UCl2 (4)
-
1/2(C5Me5)2
- Ph-Ph
As is typical in reactions of organic halides with f element
reductants,9 other metal-containing complexes are produced in
this reaction and, in this case, (C5Me5)2UCl(Ph) was also observed.
If the first equiv of PhCl was reduced by sterically induced
reduction according to eq 3, complex 1 would be the known
compound, [(C5Me5)2UCl]3.10 On the other hand, if the first
electron transfer was a result of a U(III)/U(IV) redox process, eq
2, the composition of the product would be (C5Me5)3UCl. Such
a product would be most surprising, since it would be much more
crowded than (C5Me5)3U, due to the extra ligand, and since U(IV)
is 0.135 Å smaller than U(III).11
1
The H NMR spectrum, the solubility in arene solvents, and
the red color of 1 were not consistent with the formation of [(C5-
Me5)2UCl]3.10 Since the NMR spectra were not definitive, an
X-ray diffraction study12 was conducted which established that 1
was in fact (η5-C5Me5)3UCl (Figure 1), eq 5.13
Previous studies of (C5Me5)3U showed that the sterically
induced reduction chemistry of this crowded molecule could be
coupled with a U(III)/U(IV) reduction to make this a multielectron
reductant.7 Hence, (C5Me5)3U reacts with 1,3,5,7-C8H8, as a three-
electron reductant, eq 1. One electron arises from U(III), eq 2,
and two result from two C5Me5-/C5Me5 half reactions, eq 3,
This is the most crowded (C5Me5)3M complex isolated to date,
and like (C5Me5)3Sm, it was not expected to be isolable.14,15 Once
(6) (a) Evans, W. J.; Nyce, G. W.; Clark, R. D.; Doedens, R. J.; Ziller, J.
W. Angew. Chem., Int. Ed. 1999, 38, 1801-1803. (b) Evans, W. J. Coord.
Chem. ReV. 2000, 206-207, 263-283.
2(C5Me5)3U + 3C8H8
8
- 2(C5Me5)2
(7) Evans, W. J.; Nyce, G. W.; Ziller, J. W. Angew. Chem., Int. Ed. 2000,
39, 240-242.
[(C5Me5)(C8H8)U]2(µ-C8H8) (1)
(8) (a) Evans, W. J.; Seibel, C. A.; Ziller, J. W. J. Am. Chem. Soc. 1998,
120, 6745-6752. (b) Evans, W. J.; Forrestal, K. J.; Leman, J. T.; Ziller, J.
W. Organometallics 1996, 15, 527-531.
U3+ f U4+ + e-
(C5Me5)- f 1/2(C5Me5)2 + e-
(2)
(3)
(9) (a) Finke, R. G.; Hirose, Y.; Gaughan, G. J. Chem. Soc., Chem.
Commun. 1981, 232-234. (b) Finke R. G.; Schiraldi, D. A.; Hirose, Y. J.
Am. Chem. Soc. 1981, 103, 1875-1876.
(10) Fagan, P. J.; Manriquez, J. M.; Marks, T. J.; Day, C. S.; Vollmer, S.
H.; Day, V. W. Organometallics 1982, 1, 170-180.
presumably via sterically induced reduction. To gain insight into
(11) Shannon, R. D. Acta Crystallogr. 1976, A32, 751-767.
(12) 1 crystallizes from toluene in the hexagonal space group P63/m with
a ) 9.9903(3) Å, b ) 9.9903(3) Å, c ) 15.3902(6) Å, V ) 1330.25(8) Å3,
Dcalc ) 1.696 mg/m3 for Z ) 2. At convergence, wR2 ) 0.1247 and GOF )
1.398 for 54 parameters refined against 1137 unique reflections.
(13) In a THF-free glovebox, PhCl (0.18 mL, 0.18 mmol) was added via
a microsyringe to a dark brown solution of (C5Me5)3U (1.16 g, 0.18 mmol) in
50 mL of toluene. The solution immediately became dark red. The reaction
was stirred for 1 h after which the solvent was removed under reduced pressure.
The solid was washed with hexamethyldisiloxane to yield (C5Me5)3UCl as a
rose colored powder (850 mg, 70%): 1H NMR (295 K , C6D6) C5Me5, 12.1
(s, 45H, ∆υ1/2 ) 15 Hz). IR 2964 s, 2910 s, 2856 s, 1490 w, 1440 m, 1378
(1) Reported in part at Contemporary Inorganic Chemistry II, College
Station, Texas, March, 2000.
(2) (a) ComprehensiVe Organometallic Chemistry; Wilkinson, G., Stone,
F. G. A., Abel, E. W., Eds.; Pergamon Press: New York, 1982. (b) Schumann,
H.; Esser, L.; Meese-Marktscheffel, J. A. Chem. ReV. 1995, 95, 866-985.
(3) (a) Tolman, C. A. Chem. ReV. 1977, 77, 313-348. (b) White, D.;
Taverner, B. C.; Leach, P. G. L.; Coville, N. J. J. Comput. Chem. 1993, 14,
1042-1049. (c) Stahl, L.; Ernst, R. D. J. Am. Chem. Soc. 1987, 109, 5673-
5680. (d) Lubben, T. V.; Wolczanski, P. T. J. Am. Chem. Soc. 1987, 109,
424-435.
(4) Evans, W. J.; Gonzales, S. G.; Ziller, J. W. J. Am Chem. Soc. 1991,
113, 7423-7424.
m, 1262 w, 1123 w, 1065 s, 1023 s, 1004 s, 984 s, 950 s, 803 m, 675 s cm-1
.
(5) (a) Evans, W. J.; Forrestal, K. J.; Ziller, J. W. Angew. Chem., Int. Ed.
Eng. 1997, 36, 774-776. (b) Evans, W. J.; Forrestal, K. J.; Ansari, M. A.;
Ziller, J. W. J. Am. Chem. Soc. 1998, 120, 2180-2181. (c) Evans, W. J.;
Forrestal, K. J.; Ziller, J. W. J. Am. Chem. Soc. 1998, 120, 9273-9282. (d)
Evans, W. J.; Cano, D. A.; Greci, M. A.; Ziller, J. W. Organometallics 1999,
18, 1381-1388.
Magnetic susceptibility: øm ) 2.2 × 10-3, µeff ) 2.3 µB. Anal. Calcd for C30H45-
UCl: C, 53.05; H, 6.68. Found: C, 52.90; H 6.97. (C5Me5)2UCl(Ph)14 is a
byproduct in this reaction which can be separated from the product by re-
crystallization.
(14) Fagan, T. J.; Manriquez, J. M.; Maatta, E. A.; Seyam, A. M.; Marks,
T. J. J. Am. Chem. Soc. 1981, 103, 6650-6667.
10.1021/ja002486p CCC: $19.00 © 2000 American Chemical Society
Published on Web 11/11/2000

12020 J. Am. Chem. Soc., Vol. 122, No. 48, 2000
Communications to the Editor
literature: 2.43(2) Å versus 2.073 and 2.086 Å in {[1,3-(Me3-
Si)2C5H3]2UF2}2 and [1,3-(Me3C)2C5H3]2UF2, respectively.20
Although reactions of (C5Me5)3U with PhBr and PhI have not
yet yielded crystallographically characterizable (C5Me5)3UX
analogues, the reactions provide further information on sterically
induced reduction. (C5Me5)3U reacts with 1 equiv of PhBr to make
a red intermediate which analyses for (C5Me5)3UBr21 and reacts
further with an additional equiv of PhBr to make (C5Me5)2UBr2,
in direct analogy with the (C5Me5)3U/PhCl system, eq 4. However,
the red intermediate can be thermally transformed within 2 min
at 60 °C to (C5Me5)2 and a pale green powder with properties
consistent with [(C5Me5)2UBr]n.22 This suggests that (C5Me5)3-
UBr can undergo sterically induced reduction chemistry without
an external substrate at 60 °C, eq 10.
∆
2(C5Me5)3UX
8 2(C5Me5)2UX
(10)
- (C5Me5)2
Figure 1. Structure of (C5Me5)3UCl with thermal ellipsoids drawn to
50% probability; (ring centroid)-U-(ring centroid), 120°, (ring cen-
troid)-U-Cl, 90°.
The (C5Me5)3U/PhI reaction product decomposes in the same
manner as that of the bromide analogue, but in just 3 h at room
temperature. By comparison, (C5Me5)3UCl shows no sign of
decomposition at 60 °C over a period of 3 days.
The isolation of (C5Me5)3UCl shows that significantly more
steric crowding is possible in tris(pentamethylcyclopentadienyl)
complexes than has previously been observed. The (C5Me5)3U/
PhCl reaction demonstrates that in this combination of a traditional
redox couple (U(III)/U(IV) with sterically induced reduction,
U(III) does reduction first. The (C5Me5)3U/PhX reactions show
that the balance between U(III) redox chemistry and sterically
induced reduction can be manipulated by slight changes in the
components of the complex.
the existence of (C5Me5)3UCl was established, several alternative
syntheses were examined and found to be successful as shown
in eq 6-8.
(C5Me5)3U + (C5Me5)2UCl2 f (C5Me5)3UCl +
1/3[(C5Me5)2UCl]3 (6)
2[(C5Me5)2UCl]3 + 3(C5Me5)2Pb
2(C5Me5)3U + PbCl2
8 6(C5Me5)3UCl (7)
- Pb0
8 2(C5Me5)3UCl
(8)
- Pb0
Acknowledgment. We thank the National Science Foundation for
support of this research.
1 crystallizes in the same P63/m space group as (C5Me5)3U,5a
and both have similar unit cell constants. A molecular mirror plane
bisects the three symmetry-equivalent C5Me5 rings, and the
chloride ligand is disordered on either side.16 The U-C(C5Me5)
distances (2.780(6)-2.899(9) Å range; 2.833(9) Å average) are
equivalent within experimental error to those of (C5Me5)3U (2.857-
(4) Å)5a and (C5Me4H)3UCl (2.791(12) Å).17 In fact, the positions
and orientations of the rings around U in 1 are indistinguishable,
within experimental error, from those of the (C5Me5)3M com-
plexes which have been reported to date: M ) Sm,4 U,5a Nd.8a
Thus, the chloride ligand in 1 does not appear to perturb the
U-C(C5Me5) parameters, but instead an exceptionally long U-Cl
bond of 2.90(1) Å is found relative to the U-Cl bond lengths in
(C5Me4H)3UCl17 (2.637 Å) and in (C4Me4P)3UCl15a (2.67(1) Å).
Once (C5Me5)3UCl was isolated, it seemed clear that the
fluoride analogue should be isolable.18 (C5Me5)3UF, 2, can be
readily made from HgF2, eq 9,
Supporting Information Available: Tables of crystal data, positional
parameters, bond distances and angles, and thermal parameters; listing
of observed and calculated structure factor amplitudes (PDF). X-ray
crystallographic files in CIF format. This material is available free of
charge via the Internet at http://pubs.acs.org.
JA002486P
(18) In a THF-free glovebox, (C5Me5)3U (98 mg, 0.152 mmol) and HgF2
(18 mg, 0.075 mmol) were combined in toluene (10 mL) and stirred for 12 h.
The mixture was centrifuged to remove Hg. The solvent was removed
under reduced pressure to afford a red solid (90 mg, 90%): 1H NMR (298 K,
C6D6) C5Me5, 7.25 (s, 45H, ∆υ1/2 ) 17 Hz). IR 2961 s, 2910 s, 2856 s, 1440
m, 1378 m, 1262 m, 1100 m, 1065 m, 1023 m, 803 w, 702 w, 675 w. Magnetic
susceptibility: øm ) 2.4 × 10-3, µeff ) 2.4 µB. Anal. Calcd. for (C5Me5)3UF:
C, 54.37; H, 6.84. Found: C, 54.34; H, 6.37.
(19) 2 crystallizes from toluene in the hexagonal space group P63/m with
a ) 9.9804(5) Å, b ) 9.9804(5) Å, c ) 15.4529(11) Å, V ) 1333.02(13) Å3,
Dcalc ) 1.651 mg/m3 for Z ) 2. At convergence, wR2 ) 0.1025 and GOF )
1.176 for 55 parameters refined against 1146 unique reflections.
(20) Lukens, W. W.; Beshouri, S. M.; Blosch, L. L.; Stuart, A. L.; Andersen,
R. A. Organometallics 1999, 18, 1235-1246.
(21) (C5Me5)3UBr was synthesized following the procedure in footnote 13.
(C5Me5)3UBr was isolated as light rose powder (73 mg, 65%): 1H NMR (295
K, C6D6) C5Me5, 13.3 (s, 45H, ∆υ1/2 ) 9 Hz). IR 2961 s, 2910 s, 2856 s,
1490 w, 1440 m, 1378 m, 1262 m, 1204 w, 1185 w, 1089 s, 1061 s, 1019 s,
2(C5Me5)3U + HgF2
8 2(C5Me5)3UF
(9)
- Hg0
803 s, 698 w, 679 w cm -1. Magnetic susceptibility: øm ) 2.2 × 10-3, µeff
)
and has been completely characterized by X-ray crystallography.19
As in 1, the U-X bond in 2 is much longer than those in the
2.3 µB. Anal. Calcd. for (C5Me5)3UBr: C, 49.80; H, 6.27; Br, 11.04. Found:
C, 49.42; H, 6.51; Br, 10.78.
(22) The green powder reacts cleanly with PhBr to make (C5Me5)2UBr2.
Addition of THF to the green powder forms (C5Me5)2UBr(THF): 1H NMR
(C6D6, 25 °C) δ -2.8 (s, 30 H, C5Me5, ∆ν1/2) 120 Hz), -16.9 (br s, 4H,
THF), -51.84 (br s, 4H, THF). These shifts are between those of (C5Me5)2-
UCl(THF) (Fagan, P. J.; Manriquez, J. M.; Marks, T. J., Day; C. S.; Vollmer,
S. H.; Day, V. W. Organometallics 1982, 1, 170-180) and (C5Me5)2UI(THF)
(Avens, L. R.; Burns, C. J.; Butcher, R. J.; Clark, D. L.; Gordon, J. C.; Schake,
A. R.; Scott, B. L.; Watkin, J. G.; Zwick, B. D. Organometallics 2000, 19,
451-457).
(15) (a) Gradoz, P.; Boisson, C.; Baudry, D.; Lance, M.; Nerlich, M.;
Vigner, J.; Ephritikhine, M. J. Chem. Soc., Chem. Commun. 1992, 1720-
1721. (b) Tilley, T. D.; Andersen, R. A. Inorg. Chem. 1981, 20, 3267-3270.
(16) The disorder is similar to that found in the P63/m structure of (C5H5)3-
ZrH. Edelbach, B. L.; Fazlur Rahman, A. K.; Lachicotte, R. J.; Jones, W. D.
Organometallics 1999, 18, 3170-3177.
(17) Cloke, F. G. N.; Hawkes, S. A.; Hitchcock, P. B.; Scott, P.
Organometallics 1994, 13, 2895-2897.