Uncommon η3-(N,C,C′)-1-azaallyl bonding mode for the Nacnac ligand: A bis(β-diketiminato)uranium(III) iodide complex

Article abstract of DOI:10.1021/om049342u

The reaction between 2 equiv of K(Nacnac) (Nacnac^- = [Ar]NC(Me)CHC(Me)N[Ar], Ar = 2,6-ⁱPr₂C₆H ₃) and UI₃(THF)₄ in toluene affords (Nacnac)₂UI (4), which features a β-diketiminate ligand bound to uranium(III) in an unusual η³-(N,C,C′)-1-azaallyl mode and possesses close U...C_alkene contacts.

Full text of DOI:10.1021/om049342u

© Copyright 2004
Volume 23, Number 21, October 11, 2004
American Chemical Society
Communications
Un com m on η3-(N,C,C′)-1-Aza a llyl Bon d in g Mod e for th e
Na cn a c Liga n d : A Bis(â-d ik etim in a to)u r a n iu m (III)
Iod id e Com p lex
Robert J . Wright,†,‡ Philip P. Power,† Brian L. Scott,‡ and
J aqueline L. Kiplinger*,‡
Department of Chemistry, University of CaliforniasDavis, One Shields Avenue,
Davis, California 95616, and Chemistry Division, Los Alamos National Laboratory,
Los Alamos, New Mexico 87545
Received August 22, 2004
Summary: The reaction between 2 equiv of K(Nacnac)
(Nacnac- ) [Ar]NC(Me)CHC(Me)N[Ar], Ar ) 2,6-
iPr2C6H3) and UI3(THF)4 in toluene affords (Nacnac)2UI
(4), which features a â-diketiminate ligand bound to
uranium(III) in an unusual η3-(N,C,C′)-1-azaallyl mode
and possesses close U‚‚‚Calkene contacts.
of ThCl4 and UCl4 with Li(Nacnac) (Nacnac-
[SiMe3]NC(Ph)CHC(Ph)N[SiMe3]), respectively.
)
â-Diketiminato ligands have found increasing utility
as ancillary ligands for organometallic and nonaqueous
coordination chemistry of main-group, d-block, and
lanthanide metals.1 Sterically encumbered ligands of
this type have been shown to encourage low coordina-
tion numbers in transition-metal complexes and, there-
fore, have been used to prepare a variety of coordina-
tively and electronically unsaturated reactive compounds,
polymerization catalysts, and structural models for the
active sites in metalloproteins.1 In contrast to the large
number of transition-metal and lanthanide â-diketim-
inate complexes, only two â-diketiminato actinide de-
rivatives have been reported: the thorium dichloride
complex (Nacnac)2ThCl2 (1)2 and the interesting mixed-
valence [{UVI}2][UIII]2 salt 2,3 prepared from the reaction
Since the â-diketiminate ligand basically combines
the steric bulk and multihapticity of a cyclopentadienyl
ligand and the “hardness” of an amide or alkoxide
ligand, we rationalized that the appropriately tailored
“Nacnac” ligand may serve as an excellent scaffold for
* To whom correspondence should be addressed. Phone: 505-665-
9553. Fax: 505-667-9905. E-mail: kiplinger@lanl.gov.
† University of CaliforniasDavis.
‡ Los Alamos National Laboratory.
10.1021/om049342u CCC: $27.50 © 2004 American Chemical Society
Publication on Web 09/16/2004

4802 Organometallics, Vol. 23, No. 21, 2004
Communications
the stabilization of coordinatively unsaturated uranium-
(III) complexes. Herein we report a synthetic methodol-
ogy, which accesses the first neutral, monometallic
â-diketiminatouranium complex. This molecule features
a â-diketiminate ligand bound to uranium(III) in an
unusual η3-(N,C,C′)-1-azaallyl mode.
4
As illustrated in eq 1, reaction of UI3(THF)4 with 2
equiv of the sterically demanding K(Nacnac) (Nacnac-
) [Ar]NC(Me)CHC(Me)N[Ar], Ar ) 2,6-iPr2C6H3) (3)5
in toluene solution at ambient temperature affords the
bis(â-diketiminato)uranium(III) complex 4 as a dark
blue crystalline solid in 42% isolated yield.6 The 1H
F igu r e 1. Molecular structure of 4 with thermal ellipsoids
at the 30% probability level. Hydrogen atoms and iPr
groups on the â-diketiminate ligands have been removed
for clarity. Selected bond distances (Å) and angles (deg):
U(1)-N(1), 2.362(4); U(1)-N(2), 2.379(4); U(1)-N(3), 2.387-
(4); U(1)-C(30), 2.928(4); U(1)-C(31), 2.891(4); U(1)-I,
3.0665(8); N(1)-C(1), 1.349(7); N(2)-C(3), 1.341(7); C(1)-
C(2), 1.399(8); C(2)-C(3), 1.413(7); N(3)-C(30), 1.372(6);
C(30)-C(31), 1.385(7); C(31)-C(32), 1.452(7); C(32)-N(4),
1.297(7); N(1)-U(1)-N(2), 82.65(14); N(3)-U(1)-C(31),
51.52(13); N(3)-C(30)-C(31), 116.0(4); C(30)-U(1)-C(31),
27.53(13).
NMR data are in accord with the compound’s formula-
tion as a monomeric uranium(III) derivative with two
different â-diketiminate ligand environments.
The identity of complex 4 as a bis(â-diketiminato)-
uranium(III) iodide complex was confirmed by a single-
crystal X-ray diffraction study (Figure 1).7 The molec-
ular structure of 4 reveals a six-coordinate uranium
complex having C1 symmetry with the metal bound to
two â-diketiminate ligands and an iodide. One of the
â-diketiminate ligands is coordinated to the metal center
in a typical N,N′-chelating fashion; the uranium-
nitrogen distances (U(1)-N(1) ) 2.362(4) Å, U(1)-N(2)
) 2.379(4) Å) and bite angle of 82.65(14)° compare
favorably with those observed for the uranium â-diketim-
inate complex 2.3
Å) suggests that the ligand is symmetrically disposed
about the uranium metal center; additionally, the five
ligand atoms are virtually coplanar and the uranium
metal dips out (∼1.4 Å) of the plane defined by the
N(1)-C(1)-C(2)-C(3)-N(2) atoms of the ligand back-
bone, possibly due to unfavorable steric interactions
between the aryl isopropyl groups on the ligand and the
other substituents on the metal.8
The most striking aspect of the structure is the second
â-diketiminate ligand, which is bound to the uranium
metal center in an unusual η3-(N,C,C′) fashion.9 This
may be a result of unfavorable steric interactions at the
metal center; however, electronic factors cannot be ruled
out. The U(1)-N(3) bond distance of 2.387(4) Å is
comparable to the other U-N(diketiminate) bond dis-
tances. The N(3)-C(30) (1.372(6) Å) and C(30)-C(31)
(1.385(7) Å) fragments have bond lengths typical for
N-C(sp2) and CdC bonds.10 These metrical parameters
coupled with the acute C(31)-U(1)-N(3) bite angle of
51.52(13)° are consistent with the formulation of the
ligand as a 1-azaallyl functional group.2,11 To the best
of our knowledge, there are no structurally character-
ized examples of uranium 1-azaallyl complexes and only
one thorium 1-azaallyl derivative has been reported.2
Perusal of the uranium to ligand atom contacts
(bonding distances to N(1) and N(2) and nonbonding
distances to the three carbon atoms; U-Cav ) 3.101(5)
(1) Bourget-Merle, L.; Lappert, M. F.; Severn, J . R. Chem. Rev. 2002,
102, 3031-3065 and references therein.
(2) Hitchcock, P. B.; Hu, J .; Lappert, M. F.; Tian, S. J . Organomet.
Chem. 1997, 536-537, 473-480.
(3) Hitchcock, P. B.; Lappert, M. F.; Liu, D. S. J . Organomet. Chem.
1995, 488, 241-248.
(4) Avens, L. R.; Bott, S. G.; Clark, D. L.; Sattelberger, A. P.; Watkin,
J . G.; Zwick, B. D. Inorg. Chem. 1994, 33, 2248-2256.
(5) Clegg, W.; Cope, E. K.; Edwards, A. J .; Mair, F. S. Inorg. Chem.
1998, 37, 2317-2319.
(6) In an inert-atmosphere (nitrogen) drybox, a 125 mL Erlenmeyer
flask equipped with a stir bar was charged with K(Nacnac) (3;5 0.421
g, 0.917 mmol), UI3(THF)44 (0.411 g, 0.452 mmol), and toluene (20 mL).
The resultant dark blue reaction mixture was stirred at ambient
temperature for 16 h and filtered through a Celite-padded coarse frit.
The filtrate was collected, and the volatiles were removed under
reduced pressure to give crude 4 as a dark blue solid. Analytically pure
samples of 4 were obtained by recrystallization from a concentrated
hexanes solution at -30 °C (0.225 g, 0.187 mmol, 42%). Mp: 156.0-
156.6 °C. Anal. Calcd for C64H96IN4U (mol wt 1286.38): C, 59.76; H,
7.30; N, 4.36. Found: C, 59.68; H, 7.52; N, 4.14.
(8) Hayes, P. G.; Piers, W. E.; Lee, L. W. M.; Knight, L. K.; Parvez,
M.; Elsegood, M. R. J .; Clegg, W. Organometallics 2001, 20, 2533-
2544.
(9) There is one report of a bimetallic tris(â-diketiminato)barium
complex that features a delocalized η3-(N,C,C′)-1-azaallyl bonding
interaction between
ligand: Clegg, W.; Coles, S. J .; Cope, E. K.; Mair, F. S. Angew. Chem.,
Int. Ed. 1998, 37, 796-798.
a barium metal center and a â-diketiminate
(7) Crystal data for a dark blue crystal of 4:
C
64H96N4IU, Mr
)
(10) Allen, F. H.; Kennard, O.; Watson, D. G.; Brammer, L.; Orpen,
A. G.; Taylor, R. J . Chem. Soc., Perkin Trans. 2 1987, S1-S19.
(11) Cui, C.; Hao, H.; Noltemeyer, M.; Schmidt, H. G.; Roesky, H.
W. Polyhedron 2000, 19, 471-474 and references therein.
1286.38, triclinic, P1h, Z ) 2, a ) 12.225(3) Å, b ) 14.361(3) Å, c )
19.449(5) Å, R ) 71.585(4)°, â ) 75.007(5)°, γ ) 83.868(4)°, V ) 3128.3-
(13) Å3, T ) 203(2) K, R(I > 2σ) ) 0.0470, and GOF ) 1.323.

Communications
Organometallics, Vol. 23, No. 21, 2004 4803
The electrophilic nature of the low-coordinate ura-
nium(III) center is reflected in the structure of 4 by the
close U‚‚‚Calkene contacts of 2.928(4) and 2.891(4) Å. In
addition, the U-I distance of 3.0665(8) Å is considerably
shorter than those observed for other uranium(III)
iodide complexes (3.103(2)-3.267(10) Å).4,12 The
U‚‚‚Calkene interactions fall within the sum of the van
der Waals radii of the two atoms and suggest that
complex 4 is a rare example of a crystallographically
characterized uranium(III)-alkene π-complex.13 The
U‚‚‚Calkene distances are ca. 0.4 Å longer than the U-C
bond lengths (2.48(2) Å) reported for the only struc-
turally characterized uranium(III) alkyl complex, U[CH-
(SiMe3)2]3;14 however, they are well within the range of
the U‚‚‚C contacts reported for other monometallic
uranium(III)-arene π-complexes (∼2.85-2.97 Å).15
In summary, we have shown that voluminous â-di-
ketiminate ligands can support a low-coordinate ura-
nium(III) complex. In this case, a â-diketiminate ligand
is bound to the uranium metal center in a rare η3-
(N,C,C′)-1-azaallyl fashion. The close U‚‚‚Calkene contacts
observed are consistent with the formulation of this
complex as a uranium(III)-alkene π-complex. We are
currently exploring the chemistry of complex 4 and
developing the chemistry of other actinide â-diketim-
inate complexes.
Ack n ow led gm en t. For financial support of this
work, we acknowledge the LANL G.T. Seaborg Institute
for Transactinium Science (fellowship to R.J .W.), the
Division of Chemical Sciences, Office of Basic Energy
Sciences, U.S. Department of Energy, and the LANL
Laboratory Directed Research and Development Pro-
gram. R.J .W. thanks Drs. J aime A. Pool, Felicia L. Taw,
and J effery T. Golden for technical assistance.
Su p p or tin g In for m a tion Ava ila ble: Tables with bond
lengths, bond angles, atomic coordinates, and anisotropic
displacement parameters for the structure of 4; these data are
also available as a CIF file. This material is available free of
charge via the Internet at http://pubs.acs.org.
(12) For example, see the following: (a) [UI2{H(µ-H)B(3-tBu-5-Me-
pz)2}(THF)2], U-I ) 3.104(2), 3.131(2) Å: Maria, L.; Campello, M. P.;
Domingos, A.; Santos, I.; Andersen, R. J . Chem. Soc., Dalton Trans.
1999, 2015-2020. (b) (C5Me5)UI2(THF)3, U-I ) 3.161(1), 3.162(1) Å,
and (C5Me5)UI2(py)3, U-I ) 3.182(4) Å: 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. (c)
UI3(bipy)2(py), U-Iav ) 3.22(3) Å: Rivie`re, C.; Nierlich, M.; Ephri-
tikhine, M.; Madic, C. Inorg. Chem. 2001, 40, 4428-4435. (d) U(tpza)-
I3(MeCN), U-I ) 3.1632(10)-3.2637(10) Å: Mazzanti, M.; Wietzke,
R.; Pe´caut, J .; Latour, J . M.; Maldivi, P.; Remy, M. Inorg. Chem. 2002,
41, 2389-2399.
OM049342U
(15) For example: (a) Cesari, M.; Pedretti, U.; Zazzetta, A.; Lugli,
G.; Marconi, W. Inorg. Chim. Acta 1971, 5, 439-444. (b) Cotton, F. A.;
Schwotzer, W. Organometallics 1987, 6, 1275-1280. (c) Baudry, D.;
Bulot, E.; Charpin, P.; Ephritikhine, M.; Lance, M.; Nierlich, M.;
Vigner, J . J . Organomet. Chem. 1989, 371, 155-162. (d) Odom, A. L.;
Arnold, P. L.; Cummins, C. C. J . Am. Chem. Soc. 1998, 120, 5836-
5837. (e) Evans, W. J .; Nyce, G. W.; Forrestal, K. J .; Ziller, J . W.
Organometallics 2002, 21, 1050-1055.
(13) For a Th(IV) example, see: Smith, G. M.; Suzuki, H.; Sonnen-
berger, D. C.; Day, V. W.; Marks, T. J . Organometallics 1986, 5, 549-
561.
(14) Van Der Sluys, W. G.; Burns, C. J .; Sattelberger, A. P.
Organometallics 1989, 8, 855-857.