1892 Organometallics, Vol. 28, No. 6, 2009
Cruz et al.
in combination with activators such as MAO.12 Marks et al.
also reported highly active heterogeneous olefin polymerization
catalysts, which are similar in nature to [Cp*2ThR][A], but are
formed by reaction of thorium and/or uranium polyalkyl
complexes with dehydroxylated γ-alumina or MgCl2.13 Other
actinide alkyl cations are [(Cp*2ThMe)2(µ-Me)][B(C6F5)4],4,8,14
which exists in equilibrium with [Cp*2ThMe2] and [Cp*2-
ThMe][B(C6F5)4] in solution, and the Lewis base-stabilized
cations [Cp*2ThMe(L)x][A] (L ) THF or NR3; x ) 1-3),6
[Cp*2UMe(THF)][MeBPh3],15,16 and [LU(CH2Ph)(OEt2)][BPh4]
{L ) Fe(C5H4NSitBuMe2)2}17 (Figure 1). Several cationic
actinide aryl, alkynyl, and borohydride complexes have also
been reported.18,19
As with the lanthanide metals and group 3 metals Y and
Lu,3,20 many of the challenges in the chemistry of thorium and
uranium stem from the large radii and electropositivity of these
elements,21 which leads to a propensity for donor solvent
coordination, dinuclear complex formation, ligand redistribution
reactions, and often kinetically facile decomposition.22 However,
actinide complexes are unique in their potential for significant
(12) Campbell, R. E., Jr. (The Dow Chemical Co.) U.S. Patent 4,665,046,
1987.
(13) (a) Marks, T. J. Acc. Chem. Res. 1992, 25, 57. (b) Finch, W. C.;
Gillespie, R. D.; Hedden, D.; Marks, T. J. J. Am. Chem. Soc. 1990, 112,
6221. (c) Hedden, D.; Marks, T. J. J. Am. Chem. Soc. 1988, 110, 1647. (d)
Toscano, P. J.; Marks, T. J. J. Am. Chem. Soc. 1985, 107, 653. (e) He,
M.-Y.; Xiong, G.; Toscano, P. J.; Burwell, R. L., Jr.; Marks, T. J. J. Am.
Chem. Soc. 1985, 107, 641.
Figure 1. Literature examples of cationic thorium and uranium alkyl
complexes.
covalency and f-orbital involvement in bonding,23 and in actinide
organometallic chemistry, unlike lanthanide and group 3
chemistry, the tetravalent state is also dominant.24 These
properties not only present challenges for the design of new
actinide polymerization catalysts, especially those based on
noncyclopentadienyl ligands, but also offer the tantalizing
potential for polymerization behavior25 that is significantly
different from that of both early transition metal and lanthanide
catalysts.
(14) Reaction of [Cp*2ThMe][B(C6F4SitBuMe2)4] or [Cp*2ThMe]-
[B(C6F4SiiPr3)4] with [Cp*2ThMe2] did not result in the formation of 1H
NMR detectable binuclear species. However, exchange of the methyl group
of the cation with those of the neutral dimethyl complex was observed,
suggesting that dinuclear cations analogous to [(Cp*2ThMe)2(µ-
Me)][B(C6F5)4] are accessible as part of an equilibrium that lies well to the
side of the starting materials: ref 4.
(15) Evans, W. J.; Kozimor, S. A.; Ziller, J. W. Organometallics 2005,
24, 3407.
(16) Base-free [Cp*2UMe][MeBPh3] has also been detected in solution.
However, at temperatures above 238 K, it exists in equilibrium with
[Cp*2UMe2] and free BPh3: ref 15.
A major focus of our research program is the development
of noncyclopentadienyl organoactinide chemistry and, in par-
ticular, exploration of ligand features necessary for the synthesis
of (a) thermally robust thorium(IV) dialkyl precursors, (b)
isolable actinide(IV) alkyl cations, and (c) active ethylene and
R-olefin polymerization catalysts. To this end, our research has
focused on the tridentate, dianionic supporting ligands 2,6-
bis(2,6-diisopropylanilidomethyl)pyridine (BDPP)26 and 4,5-
(17) Monreal, M. J.; Diaconescu, P. L. Organometallics 2008, 27, 1702.
(18) The cationic borohydride complex [(COT)U(BH4)(THF)2][BPh4]
was prepared from [(COT)U(BH4)2(THF)] by reaction with [NEt3H][BPh4],
and the adducts [(COT)U(BH4)L3][BPh4] (L ) HMPA and OPPh3) were
prepared in situ by reaction with 3 equiv of the appropriate Lewis base.
The complex [(η5:η1-C5Me4-pyridyl-o)2U(BH4)][BPh4] has also been
reported: (a) Cendrowski-Guillaume, S. M.; Lance, M.; Nierlich, M.;
Ephritikhine, M. Organometallics 2000, 19, 3257. (b) Moisan, L.; Le
Borgne, T.; Villiers, C.; Thuery, P.; Ephritikhine, M. C. R. Chimie 2007,
10, 883.
(19) The cationic aryl and alkynyl complexes [Cp*2Th(κ2-C6H4CH2NMe2-
t
t
ortho)][BPh4] and [(Et2N)2U(C2 Bu)(HC2 Bu)][BPh4] have been reported: (a)
ref 6b. (b) Dash, A. K.; Wang, J. X.; Berthet, J.-C.; Ephritikhine, M.; Eisen,
M. S. J. Organomet. Chem. 2000, 604, 83.
(23) Kaltsoyannis, N.; Hay, P. J.; Li, J.; Blaudeau, J.-P.; Bursten, B. E.
Theoretical Studies of the Electronic Structure of Compounds of the Actinide
Elements. In The Chemistry of the Transactinide Elements, 3rd ed.; Morss,
L. R., Edelstein, N. M., Fuger, J., Eds.; Springer: Dordrecht, The
Netherlands, 2006; Vol. 3, p 1893.
(20) (a) Piers, W. E.; Emslie, D. J. H. Coord. Chem. ReV. 2002, 233,
131. (b) Edelmann, F. T.; Freckmann, D. M. M.; Schumann, H. Chem.
ReV. 2002, 102, 1851. (c) Marques, N.; Sella, A.; Takats, J. Chem. ReV.
2002, 102, 2137.
(21) (a) Shannon, R. D. Acta Crystallogr. 1976, A32, 751. (b) Cordero,
B.; Go´mez, V.; Platero-Prats, A. E.; Reve´s, M.; Echeverr´ıa, J.; Cremades,
E.; Barraga´n, F.; Alvarez, S. Dalton Trans. 2008, 2832.
(22) (a) Marks, T. J.; Day, V. W. Actinide Hydrocarbyl and Hydride
Chemistry. In Fundamental and Technological Aspects of Organo-f-Element
Chemistry, Nato Science Series C; Marks, T. J., Fragala`, I. L., Eds.; D.
Reidel Publishing Co.: Dordrecht, The Netherlands, 1985; Vol. 155, p 115.
(b) Castro-Rodr´ıguez, I.; Meyer, K. Chem. Commun. 2006, 1353. (c)
Ephritikhine, M. Dalton Trans. 2006, 2501. (d) Barnea, E.; Eisen, M. S.
Coord. Chem. ReV. 2006, 250, 855. (e) Burns, C. J.; Eisen, M. S.
Organoactinide Chemistry: Synthesis and Characterization. In The Chemistry
of the Transactinide Elements, 3rd ed.; Morss, L. R., Edelstein, N. M., Fuger,
J., Eds.; Springer: Dordrecht, The Netherlands, 2006; Vol. 5, p 2799. (f)
Burns, C. J.; Eisen, M. S. Homogeneous and Heterogeneous Catalytic
Processes Promoted by Organoactinides. In The Chemistry of the Trans-
actinide Elements, 3rd ed.; Morss, L. R., Edelstein, N. M., Fuger, J., Eds.;
Springer: Dordrecht, The Netherlands, 2006; Vol. 5, p 2911.
(24) Cerium also has access to the tetravalent oxidation state. However,
cerium(IV) alkyl complexes have not yet been prepared, and even [Ce(η8-
C8R8)2] and [Ce(pentalene)2] are best described as complexes of cerium(III),
rather than cerium(IV). However, [Ce{OCMe2CH2(NHC)}4] (NHC )
NCHCHNiPrC) has recently been prepared, and the oxidation state in
[Cp3CeX] and [Cp2CeX2] (X ) OR or halide) may be closer to cerium(IV)
than to cerium(III):(a) Noh, W.; Girolami, G. S. Polyhedron 2007, 26, 3865.
(b) Casely, I. J.; Liddle, S. T.; Blake, A. J.; Wilson, C.; Arnold, P. L. Chem.
Commun. 2007, 5037. (c) Evans, W. J.; Deming, T. J.; Ziller, J. W.
Organometallics 1989, 8, 1581. (d) Edelstein, N. M.; Allen, P. G.; Bucher,
J. J.; Shuh, D. K.; Sofield, C. D.; Kaltsoyannis, N.; Maunder, G. H.; Russo,
M. R.; Sella, A. J. Am. Chem. Soc. 1996, 118, 13115. (e) Ashley, A.; Balazs,
G.; Cowley, A.; Green, J.; Booth, C. H.; O’Hare, D. Chem. Commun. 2007,
1515.
(25) Substrate specificity, co-polymerization opportunities, degree of
branching, polymerization by high activity single component catalysts,
etc.
(26) Gue´rin, F.; McConville, D. H.; Vittal, J. J. Organometallics 1996,
15, 5586.