Probing the chemistry, electronic structure and redox energetics in organometallic pentavalent uranium complexes

Article abstract of DOI:10.1021/ic8017375

A series of organometallic pentavalent uranium complexes of the general formula (C₅Me₅)₂U(=N-2,6-ⁱPr ₂-C₆H₃)(Y) (Y = monoanionic, non-halide ligand) have been prepared using a variety of routes. Utilizing the direct oxidation of (C₅Me₅)₂U(=N-2,6-ⁱPr ₂-C₆H₃)(THF) (2) with the appropriate copper(I) salt yielded the triflate (Y = OTf (OSO₂CF₃), 11), thiolate (Y = SPh, 12), and acetylide (Y = C≡CPh, 13) complexes, while a salt metathesis route between the U^V-imido (C₅Me ₅)₂U(=N-2,6-ⁱPr₂-C₆H ₃)(I) (10) and various alkali salts gave the diphenylamide (Y = NPh₂, 14), aryloxide (Y = OPh, 15), alkyl (Y = Me, 16), and aryl (Y = Ph, 17) complexes. Paired with 13, the isolation of 16 and 17 shows that U ^V can support the full range of carbon anions (sp, sp², and sp³), and these are, to the best of our knowledge, the first examples of pentavalent uranium complexes with anionic carbon moieties other than carbocyclic (C₅R₅, C₇H₇, C ₈H₈) ligands. Finally, both protonolysis and insertion pathways afforded the U^V-imido ketimide complex (C₅Me ₅)₂U(=N-2,6-ⁱPr₂-C₆H ₃)(N=CPh₂) (18). The complexes have been isolated in good yield and characterized using various combinations of ¹H NMR spectroscopy, elemental analysis, mass spectrometry, single crystal X-ray diffraction, cyclic voltammetry, UV-visible-NIR absorption spectroscopy, and magnetic susceptibility measurements. All (C₅Me₅) ₂U(=N-Ar)(X) (X = F, Cl, Br, I) and (C₅Me ₅)₂U(=N-Ar)(Y) complexes exhibit U^VI/U ^V and U^V/U^IV redox couples by voltammetry. The potential separation between these couples remains essentially constant at ～1.50 V, but both processes shift in tandem in potential by ～700 mV across the series of X/Y ligands. No significant differences between μ_eff values or temperature dependencies in the magnetic susceptibility were observed for these complexes regardless of the identity of the ancillary X/Y ligand. However, an excellent linear correlation was observed between the chemical shift values of C₅Me₅ ligand protons in the ¹H NMR spectra and the oxidation potentials of (C ₅Me₅)₂U(=N-2,6-ⁱPr ₂-C₆H₃)(X/Y), suggesting that there is a common origin, overall σ-/π-donation from the ancillary X/Y ligand to the metal, contributing to both observables. Combined, these data confer the following trend in increasing σ/π-donating ability of the X/Y ligand to the U^V metal center: OTf < I < Br < Cl < SPh < C≡CPh < F < [OPh ～ Me ～ Ph] ? NPh₂ < N=CPh₂. These (C₅Me₅)₂U(=N-2,6- 1Pr₂-C₆H₃)(X/Y) complexes also show distinct hallmarks of a covalent bonding interaction between the metal and the imide ligand that is modulated to varying degrees by the interaction between the X/Y ancillary ligand and the U^V metal center. These signatures of covalency include stabilization of multiple metal oxidations states [U ^VI, U^V, and U^IV] and enhanced intensities in the intraconfiguration (f-f) transitions. Of particular note in this regard is the more than 20-fold enhancement in the f-f intensities observed for Y = C?CPh and N=CPh₂, which is a clear reflection of the covalent metal-ligand bonding interactions sustained by the acetylide and ketimide ligands in these pentavalent systems.

Full text of DOI:10.1021/ic8017375

Inorg. Chem. 2008, 47, 11879-11891
Probing the Chemistry, Electronic Structure and Redox Energetics in
Organometallic Pentavalent Uranium Complexes
Christopher R. Graves, Anthony E. Vaughn, Eric J. Schelter, Brian L. Scott, Joe D. Thompson,
David E. Morris,* and Jaqueline L. Kiplinger*
Los Alamos National Laboratory, Los Alamos, New Mexico 87545
Received September 9, 2008
A series of organometallic pentavalent uranium complexes of the general formula (C5Me5)2U()N-2,6-iPr2-C6H3)(Y) (Y )
monoanionic, non-halide ligand) have been prepared using a variety of routes. Utilizing the direct oxidation of (C5Me5)2U()N-
2,6-iPr2-C6H3)(THF) (2) with the appropriate copper(I) salt yielded the triflate (Y ) OTf (OSO2CF3), 11), thiolate (Y )
SPh, 12), and acetylide (Y ) CtCPh, 13) complexes, while a salt metathesis route between the UV-imido (C5Me5)2U()N-
2,6-iPr2-C6H3)(I) (10) and various alkali salts gave the diphenylamide (Y ) NPh2, 14), aryloxide (Y ) OPh, 15), alkyl (Y
) Me, 16), and aryl (Y ) Ph, 17) complexes. Paired with 13, the isolation of 16 and 17 shows that UV can support the
full range of carbon anions (sp, sp2, and sp3), and these are, to the best of our knowledge, the first examples of pentavalent
uranium complexes with anionic carbon moieties other than carbocyclic (C5R5, C7H7, C8H8) ligands. Finally, both protonolysis
and insertion pathways afforded the UV-imido ketimide complex (C5Me5)2U()N-2,6-iPr2-C6H3)(N)CPh2) (18). The complexes
1
have been isolated in good yield and characterized using various combinations of H NMR spectroscopy, elemental
analysis, mass spectrometry, single crystal X-ray diffraction, cyclic voltammetry, UV-visible-NIR absorption spectroscopy,
and magnetic susceptibility measurements. All (C5Me5)2U()N-Ar)(X) (X ) F, Cl, Br, I) and (C5Me5)2U()N-Ar)(Y)
complexes exhibit UVI/UV and UV/UIV redox couples by voltammetry. The potential separation between these couples
remains essentially constant at ∼1.50 V, but both processes shift in tandem in potential by ∼700 mV across the series
of X/Y ligands. No significant differences between µeff values or temperature dependencies in the magnetic susceptibility
were observed for these complexes regardless of the identity of the ancillary X/Y ligand. However, an excellent linear
1
correlation was observed between the chemical shift values of C5Me5 ligand protons in the H NMR spectra and the
oxidation potentials of (C5Me5)2U()N-2,6-iPr2-C6H3)(X/Y), suggesting that there is a common origin, overall σ-/π-donation
from the ancillary X/Y ligand to the metal, contributing to both observables. Combined, these data confer the following
trend in increasing σ/π-donating ability of the X/Y ligand to the UV metal center: OTf < I < Br < Cl < SPh < CtCPh <
F < [OPh ∼ Me ∼ Ph] , NPh2 < N)CPh2. These (C5Me5)2U()N-2,6-iPr2-C6H3)(X/Y) complexes also show distinct
hallmarks of a covalent bonding interaction between the metal and the imide ligand that is modulated to varying degrees
by the interaction between the X/Y ancillary ligand and the UV metal center. These signatures of covalency include
stabilization of multiple metal oxidations states [UVI, UV, and UIV] and enhanced intensities in the intraconfiguration (f-f)
transitions. Of particular note in this regard is the more than 20-fold enhancement in the f-f intensities observed for Y )
CtCPh and N)CPh2, which is a clear reflection of the covalent metal-ligand bonding interactions sustained by the
acetylide and ketimide ligands in these pentavalent systems.
Introduction
considered to be ionic, the issue of covalency remains a
subject of debate in the area of actinide science,10-14 and
studies aimed at elucidating key bonding interactions with
5f-orbitals continue to garner attention. Towards this end,
our interests have focused on the role that metal oxidation
state plays in the structure, reactivity, and spectral properties
Complexes of the early actinides (Th-Pu) have gained
considerable prominence in organometallic chemistry as they
have been shown to undergo chemistries not observed with
their transition- or lanthanide metal counterparts.1-9 Further,
while bonding in f-element complexes has historically been
(1) For a recent review of the chemistry concerning UIII as a reducing
agent, see: (a) Evans, W. J.; Kozimor, S. A. Coord. Chem. ReV. 2006,
250, 911–935.
* To whom correspondence should be addressed. E-mail: kiplinger@
lanl.gov.
10.1021/ic8017375 CCC: $40.75  2008 American Chemical Society
Inorganic Chemistry, Vol. 47, No. 24, 2008 11879
Published on Web 11/18/2008

Graves et al.
of organouranium complexes. In recent years, tetravalent and
hexavalent complexes have dominated the landscape of
uranium chemistry,15,16 with the corresponding pentavalent
systems remaining rare. This is presumably a consequence
of their instability towards redistribution,17,18 and most of
our current knowledge of the physicochemical properties of
uranium in this oxidation state come from classical coordina-
tion complexes of the halides, (e.g., UX5 where X )
halide).19,20
uranium complexes with powerful two-electron oxidants such
as azides or N-oxides to give the corresponding UV-imido
or -oxo complexes, respectively.21c,25-28 While successful,
these routes utilize reagents that can be difficult to handle
and control,29 and are also synthetically limited in scope and
cannot be easily fine-tuned.
We recently reported30-32 a general and versatile Cu(I)-
based oxidation procedure that enables easy access to
pentavalent organometallic uranium complexes of the type
(C5Me5)2U()N-Ar)(X) (where X ) halide) from the cor-
responding UIV-imido systems ((C5Me5)2U()N-2,4,6-tBu3-
C6H2) (1);33 (C5Me5)2U()N-2,6-iPr2-C6H3) (2)25) (eq 1).
While the one-electron oxidations utilizing silver salts have
produced cationic uranium complexes, this Cu-based oxida-
tive functionalization results in neutral UV species which
feature a bent-metallocene framework with the imido and
halide ligands contained within the metallocene wedge.
Applicable for the synthesis of the full suite of halides (X
) F, Cl, Br, I), this protocol allowed for a systematic study
of the electronic structure and bonding in these pentavalent
uranium halidessa study which suggested that only minor
differences existed between the structures as the halide ligand
was varied. As such, our search has been broadened to
include the synthesis of non-halide UV-imido complexes,
(C5Me5)2U()N-Ar)(Y) (where Y ) non-halide monoanionic
From a heuristic perspective, the UV oxidation state with
its simple 5f1 valence electronic configuration remains a
highly attractive target. While more structurally elaborate
uranium(V) systems have been noted in the literature,
specifically those containing metal-nitrogen and -oxygen
bonds,13,21 systems containing U-carbon bonds are com-
paratively less common.15,22 Generally speaking, the syn-
thetic routes available for accessing pentavalent systems have
been limited, relying on either the one-electron oxidation of
UIV complexes with silver22a,23 or thallium24 salts to give
the corresponding UV species, or by reacting trivalent
(2) While transition metal complexes follow the 18-electron rule, no such
rule exists for the actinides. For pertinent examples, see: (a) Reynolds,
L. T.; Wilkinson, G. J. Inorg. Nucl. Chem. 1956, 2, 246–253. (b)
Fischer, E. O.; Hristidu, Y. Z. Naturforsch. 1962, 173, 275–276. (c)
Streitwieser, A., Jr.; Mueller-Westerhoff, U. J. Am. Chem. Soc. 1968,
90, 7364. (d) Avdeef, A.; Raymond, K. N.; Hodgson, K. O.; Zalkin,
A. Inorg. Chem. 1972, 11, 1083–1088. (e) Maier, R.; Kanellakopulos,
B.; Apostolidis, C.; Meyer, D.; Rebizant, J. J. Alloys Compd. 1993,
190, 269–271.
(3) Schelter, E. J.; Yang, P.; Scott, B. L.; Da Re, R. E.; Jantunen, K. C.;
Martin, R. L.; Hay, P. J.; Morris, D. E.; Kiplinger, J. L. J. Am. Chem.
Soc. 2007, 129, 5139–5152.
(4) Schelter, E. J.; Yang, P.; Scott, B. L.; Thompson, J. D.; Martin, R. L.;
Hay, P. J.; Morris, D. E.; Kiplinger, J. L. Inorg. Chem. 2007, 46,
7477–7488.
(21) For representative examples of UV-amide and -alkoxide/aryloxide
complexes, see: (a) Zalkin, A.; Brennan, J. G.; Andersen, R. A. Acta
Crystallogr. 1988, C44, 1553–1554. (b) Roussel, P.; Hitchcock, P. B.;
Tinker, N. D.; Scott, P. Inorg. Chem. 1997, 36, 5716–5721. (c) Castro-
Rodriguez, I.; Olsen, K.; Meyer, K. J. Am. Chem. Soc. 2003, 125,
4565–4571. (d) Salmon, P.; Thuery, P.; Ephritikhine, M. Polyhedron
2007, 26, 631–636. (e) Hayton, T. W.; Wu, G. J. Am. Chem. Soc.
2008, 130, 2005–2014.
(5) Schelter, E. J.; Morris, D. E.; Scott, B. L.; Kiplinger, J. L. Chem.
Commun. 2007, 1029–1031.
(22) For examples of organometallic pentavalent uranium complexes, see:
(a) Boisson, C.; Berthet, J. -C.; Lance, M.; Nierlich, M.; Vigner, J.;
Ephritikhine, M. J. Chem. Soc., Chem. Commun. 1995, 543–544. (b)
Gourier, D.; Caurant, D; Berthet, J. C.; Boisson, C.; Ephritikhine, M.
Inorg. Chem. 1997, 36, 5931–5936. (c) Ephritikhine, M.; Berthet, J. C.;
Boisson, C.; Lance, M.; Nierlich, M. J. Alloys Compd. 1998, 271-273,
144–149. (d) Arliguie, T.; Rourmigue, M.; Ephritikhine, M. Organo-
metallics 2000, 19, 109–111; also see reference 25.
(6) Kiplinger, J. L.; Pool, J. A.; Schelter, E. J.; Thompson, J. D.; Scott,
B. L.; Morris, D. E. Angew. Chem., Int. Ed. 2006, 45, 2036–2041.
(7) Jantunen, K. C.; Scott, B. L.; Kiplinger, J. L. J. Alloys Compd. 2007,
444-445, 363–368.
(8) Pool, J. A.; Scott, B. L.; Kiplinger, J. L. Chem. Commun. 2005, 2591–
2593.
(9) Pool, J. A.; Scott, B. L.; Kiplinger, J. L. J. Am. Chem. Soc. 2005,
127, 1338–1339.
(10) Burns, C. J.; Bursten, B. E. Comments Inorg. Chem. 1989, 9, 61–93.
(11) Diaconescu, P. L.; Arnold, P. L.; Baker, T. A.; Mindiola, D. J.;
Cummins, C. C. J. Am. Chem. Soc. 2000, 122, 6108–6109.
(12) Mazzanti, M.; Wietzke, R.; Pecaut, J.; Latour, J.-M.; Maldivi, P.;
Remy, M. Inorg. Chem. 2002, 41, 2389–2399.
(13) Meyer, K.; Mindiola, D. J.; Baker, T. A.; Davis, W. M.; Cummins,
C. C. Angew. Chem., Int. Ed. 2000, 39, 3063–3066.
(14) Raymond, K. N.; Eigenbrot, C. W., Jr. Acc. Chem. Res. 1980, 13,
276–283.
(15) Burns, C. J.; Eisen, M. S. Organoactinide Chemistry: Synthesis and
Characterization. In The Chemistry of the Actinide and Transactinide
Elements, 3rd ed.; Morss, L. R., Edelstein, N. M., Fuger, J., Eds.;
Springer: The Netherlands, 2006; Vol. 5, pp 2799-2910, and
references therein.
(23) Boisson, C.; Berthet, J.-C.; Lance, M.; Vigner, J.; Nierlich, M.;
Ephritikhine, M. J. Chem. Soc., Dalton Trans. 1996, 947–953.
(24) Berthet, J. C.; Ephritikhine, M. J. Chem. Soc., Chem. Commun. 1993,
1566–1567.
(25) Arney, D. S. J.; Burns, C. J. J. Am. Chem. Soc. 1993, 115, 9840–
9841.
(26) Brennan, J. G.; Andersen, R. A. J. Am. Chem. Soc. 1985, 107, 514–
516.
(27) Castro-Rodriguez, I.; Nakai, H.; Meyer, K. Angew. Chem., Int. Ed.
2006, 45, 2389–2392.
(28) Roussel, P.; Boaretto, R.; Kingsley, A. J.; Alcock, N. W.; Scott, P.
J. Chem. Soc., Dalton Trans. 2002, 1423–1428.
(29) Although silver salts are among the most widely utilized oxidants for
metal complexes, their behavior can be difficult to predict and control.
For an excellent review on chemical oxidants for organometallic
systems, see: (a) Connelly, N. G.; Geiger, W. E. Chem. ReV. 1996,
96, 877–910.
(30) This protocol has also been successfully extended to the oxidative
functionalization of UIII organometallics to their UIV equivalent, see:
Graves, C. R.; Schelter, E. J.; Cantat, T.; Scott, B. L.; Kiplinger, J. L.
Organometallics 2008, 27, 5371–5378.
(31) Graves, C. R.; Scott, B. L.; Morris, D. E.; Kiplinger, J. L. J. Am.
Chem. Soc. 2007, 129, 11914–11915.
(16) Ephritikhine, M. Dalton Trans. 2006, 2501–2516.
(17) For a review on the synthesis, stability and properties of pentavalent
uranium, see: (a) Selbin, J.; Ortego, J. D. Chem. ReV. 1969, 69, 657–
621.
(18) Bagnall, K. W. ComprehensiVe Coordination Chemistry; Wilkinson,
G., Gillard, R. D., Mc Cleverty, J. A., Eds.; Pergamon: Oxford, 1987;
Vol. 3, p 1129.
(19) For a detailed list of various UV halide complexes, see: Grenthe I.;
Drozdzynski, J.; Fujino, T.; Buck, E. C.; Albrecht-Schmitt, T. E.; Wolf,
S. F. Uranium. In The Chemistry of the Actinide and Transactinide
Elements, 3rd ed.; Morss, L. R., Edelstein, N. M., Fuger, J., Eds.;
Springer: The Netherlands, 2006; Vol. 1, pp 501-529.
(20) Ryan, J. L. J. Inorg. Nucl. Chem. 1971, 33, 153–177.
(32) Graves, C. R.; Yang, P.; Kozimor, S. A.; Vaughn, A. E.; Clark, D. L.;
Conradson, S. D.; Schelter, E. J.; Scott, B. L.; Thompson, J. D.; Hay,
P. J.; Morris, D. E.; Kiplinger, J. L. J. Am. Chem. Soc. 2008, 130,
5272–5285.
(33) Arney, D. S. J.; Burns, C. J. J. Am. Chem. Soc. 1995, 117, 9448–
9460.
11880 Inorganic Chemistry, Vol. 47, No. 24, 2008

Organometallic PentaWalent Uranium Imido Complexes
of selected geometric parameters can be found in Table 1.
Both 11 and 12 have nearly linear UdN-CAr angles
(168.3(5)° and 171.6(3)°, respectively) and short UdNimido
bond distances (1.9575(5) Å and 1.976(4) Å, respectively),
which compare well with the corresponding metrical pa-
rameters observed for the structurally characterized UV-imido
halides (3, 5, 8-10)32 and other high-valent uranium (UIV,
UV, UVI) imido compounds.35 To the best of our knowledge,
complex 11 is the first structurally characterized pentavalent
uranium species incorporating a triflate group. At 2.378(4)
Å, the U-OTf bond length observed in 11 is similar to those
reported for other structurally characterized uranium triflate
complexes. For example, (C5Me5)2U[η2-(N,N′)-CH3-N-
N)CPh2](OTf) has a U-OTf bond distance of 2.395(14)
Å,36 while the UIV bis(triflate) complexes (C5Me5)2-
U(OTf)2 · OH2 and (C5H5)2U(OTf)2(pyridine)2 have U-OTf
bond lengths of 2.36(1) Å and 2.40(1) Å37 and 2.395(4) Å
and 2.385(4) Å,38 respectively. For comparison with 12, one
other pentavalent organouranium complex with a U-S bond
has been reported: [Na(18-crown-6)(THF)][(η8-C8H8)U-
(C4H4S4)2],withU-Sbondlengthsintherange2.687(2)-2.700(2)
Å.39,40 Not surprisingly, the U-S distance of 2.7230(13) Å
found for neutral 12 is slightly longer than the range observed
for the UV cationic system, but it is in agreement with U-S
bond distances observed for complexes of uranium in other
oxidation states.41
ligand) to investigate larger variations in the bonding
environment around the uranium metal. Herein, we report
our progress in the synthesis of substituted UV-imido
complexes using various routes: (1) Direct oxidation of UIV-
imido complexes with copper(I) salts; (2) Salt metathesis
with UV-imido halides; (3) Protonolysis and insertion of an
UV-imido alkyl or aryl complex with H-N)CPh2 or
NtCPh, respectively, to form a UV-imido ketimide complex.
Further, we report and compare the crystallographic, elec-
trochemical, spectroscopic, and magnetic characterization of
the pentavalent uranium (C5Me5)2U()N-Ar)(Y) series (Y
) OTf, SPh, CtCPh, NPh2, OPh, Me, Ph, N)CPh2) to
further interrogate the molecular, electronic, and magnetic
structures of this new class of uranium complexes.
Results and Discussion
Synthesis and Structural Characterization. I. Direct
Oxidation. The direct oxidation of complex 2 with func-
tionalized copper(I) salts was examined as a primary route
for the synthesis of (C5Me5)2U()N-2,6-iPr2-C6H3)(Y) com-
plexes with extended ligand sets (Y * halide). A logical
extension of the halide chemistry, reaction of 2 with 1 equiv
of the (CuOTf) 2 ·toluene adduct (OTf ) OSO2CF3) afforded
the UV-imido triflate derivative, (C5Me5)2U()N-2,6-iPr2-
C6H3)(OTf) (11), in good isolated yield (eq 2). 31 Oxidations
using the copper halides were optimized to occur over 12 h
with an excess of the oxidant (5 equiv); however, use of
excess (CuOTf)2 · toluene or extended reaction times lead to
decompositionofthetriflateproduct.Conversely,(C5Me5)2U()N-
2,6-iPr2-C6H3)(SPh) (12)31 and (C5Me5)2U()N-2,6-iPr2-
C6H3)(CtCPh) (13)34 could only be prepared by reacting 2
with excess oxidant (CuSPh or CuCtCPh, 5 equiv) at 75
°C over 12 h. Following workup, complexes 11-13 were
reproducibly isolated as analytically pure solids and were
characterized by a combination of 1H NMR, electrochemistry,
UV-visible near-IR spectroscopy, elemental and mass spec-
trometric analyses, and magnetic susceptibility.
II. Salt Metathesis. An alternative route for the generation
of substituted UV-imido complexes is salt metathesis from
(35) For other examples of imido complexes featuring short U)N bond
distances (Å) and nearly linear U)N-C imido angles (deg), see: (a)
Brennan, J. G.; Andersen, R. A. J. Am. Chem. Soc. 1985, 107, 514–
516. (b) Stewart, J. L. Tris[bis(trimethylsilyl)amido]uranium: Com-
pounds with Tri-, Tetra-, and Pentavalent Uranium. Ph. D. Dissertation,
University of California, Berkeley, CA, 1988. (c) Arney, D. S. J.;
Burns, C. J.; Smith, D. C. J. Am. Chem. Soc. 1992, 114, 10068–10069.
(d) Roussel, P.; Boaretto, R.; Kingsley, A. J.; Alcock, N. W.; Scott,
P. J. Chem. Soc., Dalton Trans. 2002, 1423–1428. (e) Hayton, T. W.;
Boncella, J. M.; Scott, B. L.; Palmer, P. D.; Batista, E. R.; Hay, P. J.
Science 2005, 310, 1941–1943. (f) Zi, G.; Jia, L.; Werkema, E. L.;
Walter, M. D.; Gottfriedsen, J. P.; Andersen, R. A. Organometallics
2005, 24, 4251–4264. (g) Hayton, T. W.; Boncella, J. M.; Scott, B. L.;
Batista, E. R.; Hay, P. J. J. Am. Chem. Soc. 2006, 128, 10549–10559.
(h) Spencer, L. P.; Yang, P.; Scott, B. L.; Batista, E. R.; Boncella, J.
M J. Am. Chem. Soc. 2008, 130, 2930–2931; also see references
21a, c, 25, 31, 32, 44d .
(36) Kiplinger, J. L.; John, K. D.; Morris, D. E.; Scott, B. L.; Burns, C. J.
Organometallics 2002, 21, 4306–4308.
(37) Berthet, J. C.; Lance, M.; Nierlich, M.; Ephritikhine, M. Chem.
Commun. 1998, 1373–1374.
(38) Berthet, J. C.; Nierlich, M.; Ephritikhine, M. Eur. J. Inorg. Chem.
2002, 850–858.
(39) Arliguie, T.; Fourmigue, M.; Ephritikhine, M. Organometallics 2000,
19, 109–111.
The identities of compounds 11 and 12 were confirmed
by single-crystal X-ray crystallography.31 Like their halide
counterparts, these bent-metallocene complexes contain the
imido and Y ligands within the metallocene wedge. A list
(34) Graves, C. R.; Scott, B. L.; Morris, D. E.; Kiplinger, J. L. Organo-
metallics 2008, 27, 3335–3337.
(40) Belkhiri, L.; Arliguie, T.; Thuery, P.; Fourmigue, M.; Boucekkine,
A.; Ephritikhine, M. Organometallics 2006, 25, 2782–2795.
Inorganic Chemistry, Vol. 47, No. 24, 2008 11881

Graves et al.
Table 1. Selected Metrical Parameters for the (C5Me5)2U()N-2,6-iPr2-C6H3)(X/Y) Complexes 8-12, 14, 18
imido parameters
C5Me5(cent)-U C5Me5(cent)-U-
(Å)
C5Me5(cent)(deg) UdN (Å) N-C (Å) UdN-C (deg)
X/Y ligand parameters
N-U-X/Y (deg)
8 X ) Cl
9 X ) Br
10 X ) I
2.505, 2.453
2.523, 2.452
2.454, 2.459
134.87
137.98
134.07
1.963(4) 1.404(7)
1.969(7) 1.40(2)
1.974(7) 1.406(10)
169.6(4)
172.2(9)
170.7(6)
U-Cl (Å) 2.6209(15)
U-Br (Å) 2.789(3)
U-I (Å) 3.0385(7)
N-U-Cl 105.79(13)
N-U-Br 105.3(2)
N-U-I 106.6(2)
11 Y ) OTf
12 Y ) SPh
14 Y ) NPh2
2.435, 2.440
2.466, 2.457
2.511, 2.530
135.92
136.24
125.01
1.9575(5) 1.416(8)
1.976(4) 1.398(6)
1.984(4) 1.399(6)
168.3(5)
171.6(3)
174.0(3)
U-O (Å) 2.378
U-S (Å) 2.7230(13) U-S-C(deg) 131.08(17) N-U-S103.35(12)
U-N (Å) 2.322(4) N-U-N 93.73(15)
U-O-S (deg) 160.4(3) N-U-O 109.16(19)
18 Y ) N)CPh2 2.493, 2.484
138.32
2.012(4) 1.391(7)
174.6(4)
U-N (Å) 2.199(4) U-N-C (deg) 177.8(4) N-U-N 111.99(17)
Scheme 1. Salt Metathesis Route to Substituted UV-Imido
Organometallic Complexesa
Figure 1. Molecular structure of complex 14 with thermal ellipsoids
projected at the 50% probability level.
Both 14 and 15 were obtained in good isolated yields.
The molecular structure of 14 was confirmed by single-
crystal X-ray diffraction (Figure 1);43 the metrical parameters
of 14 agree well with those observed for complexes 11 and
12. Further, the U-Nimido distance and UdN-CAr angle of
1.984(4) Å and 174.0(3)° for 14 are in agreement with those
found for the other UV-imido complexes.35 At 2.322(4) Å,
the U(1)-N(1) bond in 14 compares well with the geometric
parameters obtained for other neutral ((Me3Si-N))U-
a
i: 1.2 equiv KNPh2, toluene, 25 °C, 12 h, 80%; ii: 1.2 equiv KOPh,
toluene, 75 °C, 12 h, 75%; iii: 1.5 equiv Me2Mg, 1,4-dioxane, toluene, 25
°C, 12 h, 70%; iV: 2.0 equiv Ph2Mg(THF)2, 1,4-dioxane, toluene, 25 °C,
12 h, 80%.
the imido iodide (C5Me5)2U()N-2,6-iPr2-C6H3)(I) (10)42
using alkali metal reagents. This pathway is particularly
attractive for experimental conditions where the copper(I)
reagent required for the direct oxidation pathway outlined
above is unavailable or unstable; use of salt metathesis
expands the breadth of the UV-imido chemistry and available
structural types. As illustrated in Scheme 1, (C5Me5)2U()N-
2,6-iPr2-C6H3)(NPh2) (14) was prepared by the reaction
between 10 and KNPh2. While this reaction occurred at room
temperature over 12 h, the corresponding synthesis for
(C5Me5)2U()N-2,6-iPr2-C6H3)(OPh) (15) required elevated
temperatures (75 °C) to occur in an equivalent timeframe.
[N(SiMe3)2]3, U-Namide
)
2.295(10) Å),21a anionic
([U(dbabh)6]-(Hdbabh)2,3:5,6-dibenzo-7-azabicyclo[2.2.1]hepta-
2,5-diene), U-Namide ) 2.230(11)-2.267(13) Å)13 and
cationic ([(C5Me5)U(NMe2)3(THF)]+, U-Namide ) 2.25(2)-
2.35(5) Å; [(C5Me5)2U(NEt2)2]+, U-Namide ) 2.162(9),
2.167(9) Å)22a uranium(V) complexes containing an
U-Namide linkage.44
Complex 10 also served as a useful platform for the
synthesis of both UV-imido alkyl and aryl compoundssreaction
between 10 and either Me2Mg or Ph2Mg(THF)2 afforded the
alkyl ((C5Me5)2U()N-2,6-iPr2-C6H3)(Me) (16)) and aryl
((C5Me5)2U()N-2,6-iPr2-C6H3)(Ph) (17)) complexes, respec-
tively, in good isolated yield (Scheme 1). Paired with 13,
the isolation of 16 and 17 shows that UV can support the
full range of carbon anions (sp, sp2, and sp3), and these are,
to the best of our knowledge, the first examples of pentava-
(41) U-S distances in the range of ∼2.6-2.8 Å have been reported for
tetravalent uranium complexes. For examples, see: (a) Clark, D. L.;
Miller, M. M.; Watkin, J. G. Inorg. Chem. 1993, 32, 772–774. (b)
Leverd, P. C.; Ephritikhine, M.; Lance, M.; Vigner, J.; Nierlich, M.
J. Organomet. Chem. 1996, 507, 229–237. (c) Lescop, C.; Arliguie,
T.; Lance, M.; Nierlich, M.; Ephritikhine, M. J. Organomet. Chem.
1999, 580, 137–144. (d) Diaconescu, P. L.; Arnold, P. L.; Baker, T. A.;
Mindiola, D. J.; Cummins, C. C. J. Am. Chem. Soc. 2000, 122, 6108–
6109. (e) Arliguie, T.; Lescop, C.; Ventelon, L.; Leverd, P. C.; Thuery,
P.; Nierlich, M.; Ephritikhine, M. Organometallics 2001, 20, 3698–
3703.
(42) Experiments performed on an NMR scale indicated that the corre-
sponding Cl (8), Br (9), and OTf (11) complexes also react cleanly
and can serve as starting materials using this salt metathesis route.
(43) While publication quality data for the structure of 15 were not obtained,
collected data showing connectivity consistent with the molecular
framework of (C5Me5)2U()N-2,6-iPr2-C6H3)(OPh) were obtained.
11882 Inorganic Chemistry, Vol. 47, No. 24, 2008

Organometallic PentaWalent Uranium Imido Complexes
Scheme 2. Synthesis of (C5Me5)2U()N-2,6-iPr2-C6H3)(N)CPh2) (18)
length (U(1)-N(2) ) 2.199(4) Å) and nearly linear U-NdC
bond angle (U(1)-N(2)-C(33) ) 178.2(5)°) found in 18
are comparable to those found for other structurally char-
Using Protonolysis Routes from UV-Imido Alkyl and Aryl Complexesa
acterized uranium ketimide complexes (U-Nketimide
)
2.179(6)-2.220(3) Å).4,36,45-48
Magnetic Susceptibility. The magnetic response of the
(C5Me5)2U()N-2,6-iPr2-C6H3)(Y) complexes 11-15 and 18
was measured between 2 and 350 K to evaluate changes in
the magnetic properties based on varying the auxiliary ligand
(Y). With µeff (µB/U) values of 2.65 (11), 2.48 (12), 2.22
(13), 2.27 (14), 2.38 (15), and 2.03 (18) (determined from
fitting the linear portion of the 1/ꢀ vs T data) the UV-imido
complexes exhibit magnetic susceptibilities similar to those
observed for their UV-imido halide counterparts32 and other
reported pentavalent uranium complexes.17,21c,49-53 No
significant differences in µeff values or temperature depend-
encies were observed for these complexes regardless of the
identity of Y. Plots of both the ꢀT product versus T and
constant temperature magnetization (M versus H) at 2 K are
provided as Supporting Information.
a
i: 1.2 equiv benzophenone imine, toluene, 75 °C, 12 h, 86%; ii: 1.2
equiv benzonitrile, toluene, 12 h, 91%.
1H NMR Spectroscopy. The 1H NMR spectra of the UV
complexes 11-13 and 15-1854 are similar to those found
for the UV-imido halidessthey exhibit a signal corresponding
to the C5Me5 ligand protons and an inequivalency of the ortho
iPr groups not seen for the UIV imido starting material.25 As
has been observed for other paramagnetic uranium systems,55
there is a dependence on the chemical shift for like protons
(i.e. the C5Me5 resonances) and the identity of the donating
ligand (X or Y) in the paramagnetic (C5Me5)2U()N-2,6-iPr2-
C6H3)(X/Y) framework (Table 2). While the exact reasons
(44) The U-Namide bond length for 14 also compares well to U-Namide
bond lengths for tri-, tetra- and hexavalent uranium amide complexes.
For examples, see: (a) U[N(SiMe3)2]3, U-Namide ) 2.320(4) Å Stewart,
J. L.; Andersen, R. A. Polyhedron 1998, 17, 953–958. (b)
(C5Me5)2U[N(SiMe3)2], U-Namide ) 2.352(2) Å Evans, W. J.; Nyce,
G. W.; Forrestal, K. J.; Ziller, J. W Organometallics 2002, 21, 1050–
1055. (c) (C5Me5)2U[NH(2,6-Me2-C6H3)]2, U-U-Namide ) 2.276(6)
Å Straub, T.; Frank, W.; Reis, G. J.; Eisen, M. S. J. Chem. Soc., Dalton
Trans. 1996, 2541–2546. (d) [N(SiMe3)2]3U()N-SiMe3)(F), U-Namide
) 2.217(17) -2.252(17) Å; [N(SiMe3)2]3U()N-C6H5)(F), U-Namide
) 2.206(7) -2.226(6) Å Burns, C. J.; Smith, W. H.; Huffman, J. C.;
Sattelberger, A. P. J. Am. Chem. Soc. 1990, 112, 3237–3238.
(45) Diaconescu, P. L.; Cummins, C. C. J. Am. Chem. Soc. 2002, 124,
7660–7661.
(46) Jantunen, K. C.; Burns, C. J.; Castro-Rodriguez, I.; Da Re, R. E.;
Golden, J. T.; Morris, D. E.; Scott, B. L.; Taw, F. L.; Kiplinger, J. L.
Organometallics 2004, 23, 4682–4692.
(47) Schelter, E. J.; Veauthier, J. M.; Thompson, J. D.; Scott, B. L.; John,
K. D.; Morris, D. E.; Kiplinger, J. L. J. Am. Chem. Soc. 2006, 128,
2198–2199.
(48) Silva, M.; Antunes, M. A.; Dias, M.; Domingos, A.; dos Santos, I. C.;
Marcalo, J.; Marques, N. Dalton Trans. 2005, 3353–3358.
(49) Edelstein, N. M.; Lander, G. H. Magnetic Properities. In The Chemistry
of the Actinide and Transactinide Elements, 3rd ed.; Morss, L. R.,
Edelstein, N. M., Fuger, J., Eds.; Springer: The Netherlands, 2006;
Vol. 4, pp 2241-2247.
(50) Arnold, P. L.; Patel, D.; Wilson, C.; Love, J. B. Nature 2008, 415,
315–318.
(51) Miyake, C.; Hirose, M.; Ohya-Nishiguchi, H. Inorg. Chim. Acta 1989,
165, 179–183.
Figure 2. Molecular structure of complex 18 with thermal ellipsoids
projected at the 50% probability level.
lent uranium complexes with anionic carbon moieties other
than carbocyclic (C5R5, C7H7, C8H8) ligands.15,22
III. Protonolysis and Insertion. (C5Me5)2U()N-2,6-iPr2-
C6H3)(Me) (16) and (C5Me5)2U()N-2,6-iPr2-C6H3)(Ph) (17)
served as useful starting materials for the synthesis of the
UV-imido ketimide complex (C5Me5)2U()N-2,6-iPr2-
C6H3)(N)CPh2) (18) (Scheme 2). Complex 16, prepared in
situ, was heated at 75 °C in the presence of excess
benzophenone imine to afford 18 in good isolated yield
(86%) through protonolysis and concomitant loss of methane.
Alternatively, insertion of benzonitrile into the U-Caryl bond
of in situ formed 17 gave the identical product 18 in 91%
yield. These pathways not only extend the scope of this UV-
imido chemistry but also confirm the existence of both 16
and 17, which were not structurally characterized.
Single crystals of 18 suitable for X-ray diffraction studies
were obtained by slow evaporation of a concentrated
hexamethyldisiloxane solution at room temperature. Complex
18 (Figure 2) represents the first uranium-ketimide complex
with the metal in a pentavalent oxidation state. As with
complexes 11-15, 18 has a nearly linear UdN-CAr angle
(C(21)-N(1)-U(1) ) 174.6(4)°) and a short UdNimido bond
distance (U(1)-N(1) ) 2.012(4) Å). The U-Nketimide bond
(52) Selbin, J.; Ahmad, N.; Pribble, M. J. J. Chem. Soc., Chem. Commun.
1969, 759–760.
(53) Selbin, J.; Ahmad, N.; Pribble, M. J. J. Inorg. Nucl. Chem. 1970, 32,
3249–3258.
1
(54) No resonances were observed in the H NMR spectrum of 14 over
the temperature range 0-100 °C.
Inorganic Chemistry, Vol. 47, No. 24, 2008 11883

Graves et al.
1
Table 2. H NMR Chemical Shift of the C5Me5 Protons in the
geometric parameters, this peak position implies that the
ketimide ligand is strongly donating to the uranium center,
consistent with previous studies that suggest that a bond order
(C5Me5)2U()N-2,6-iPr2-C6H3)(X/Y) Complexes at 25 °C in C6D6
∆ν1/2
∆ν1/2
δ (ppm) (Hz)
compound
δ (ppm) (Hz)
compound
>1 exists between the uranium and Nketimide.
4,61 Interestingly,
7 (X ) F)
3.94
4.92
5.31
5.78
5.98
109
84
72
125
104
12 (Y ) SPh)
4.57
83
80
71
63
117
52
both the Me (δ 3.30 ppm) and Ph (δ 3.00 ppm) ligands are
also strongly σ-donating to the uranium center in this system,
to an extent approximately equal to or slightly better than
the alkoxide derivative. Overall, given the relative positions
of the C5Me5 resonances for the UV-imido complexes, the
trend in increasing donating ability for the X/Y ligand is
OTf < I < Br < Cl < SPh < CtCPh < F < OPh ∼ Me <
Ph , N)CPh2. This is in good agreement with the
electrochemical data which suggest a similar trend in ease
of oxidation across the series (vide infra).
8 (X ) Cl)
9 (X ) Br)
10 (X ) I)
11 (Y ) OTf)
13 (Y ) CtCPh) 4.11
15 (Y ) OPh)
16 (Y ) Me)
17 (Y ) Ph)
3.33
3.30
3.00
18 (Y ) N)CPh2) 1.41
for this phenomena have been debated,56-58 it has been
proposed for the uranium systems that the trend tracks with
the amount of π-donation from the X/Y ligand to the metal
center.55a,c In effect, the better the π-donor, the more
electron-rich the uranium center, manifesting in a larger
shielding and an upfield shift of the auxiliary protons. In
this regard, the chemical shift of a set of analogous protons
for structurally related uranium complexes tracks with the
electron density at the metal center as a result of the change
in coordination environment in the complex. Therefore, 11
(L ) OTf), which has the most downfield shifted C5Me5
resonance at δ 5.98 ppm, has the most electron deficient
uranium center, presumably arising from poor donation from
the weakly coordinating OTf ligand. The C5Me5 shifts for
the halide series vary systematically with the identity and
overall donating capacity of the halide ligand: 10 (X ) I, δ
5.78 ppm) f 9 (X ) Br, δ 5.31 ppm) f 8 (X ) Cl, δ 4.92
ppm) f 7 (X ) F, δ 3.94 ppm), indicating that fluoride is
the strongest donor compared to the other halogens. The
chemical shift for the C5Me5 protons in 12 (δ 4.57 ppm)
intersects the halide series, indicating that the polarizable
SPh group is a better donor than the larger halides, although
fluoride is still a better π-donor.59 The C5Me5 resonance for
(C5Me5)2U()N-2,6-iPr2-C6H3)(CtCPh) (δ 4.11 ppm) is
slightly less than that of 7, which is not surprising given
that while the CtCPh fragment is a strong σ-donor, its ability
to π-donate is negligible.60 The C5Me5 resonance for
(C5Me5)2U()N-2,6-iPr2-C6H3)(OPh) (δ 3.33 ppm) is further
upfield, and the UV-imido ketimide complex 18 is shifted to
the largest upfield position: δ 1.41 ppm. Paired with the
Electrochemistry. Cyclic and square-wave voltammetric
data have been collected for (C5Me5)2U()N-2,6-iPr2-C6H3)(X/
Y) complexes 7-16 and 18 in ∼0.1 M [Bu4N][fluoro-
arylborate]/tetrahydrofuran solution ([fluoroarylborate]-
)
[B(C6F5)4]- or [B(3,5-(CF3)2-C6H3)4]-). The potential data
are summarized in Table 3, and typical cyclic voltammo-
grams are illustrated in Figure 3. Data for 7-1032 and 1334
have been previously reported, but are included here for
completeness. We were unable to obtain voltammetric data
for 17 because of very rapid decomposition of this complex
in the supporting electrolyte solution. While similar problems
were overcome previously for the fluoride complex (7)32
using low-temperature voltammetry to stabilize that complex
for data collection, no such efforts were made for 17.
The voltammetric data for the new UV-imido complexes
(11, 12, 14-16, 18) reported in this work are completely
consistent with that reported previously for the halides
(7-10) and the acetylide complex (13). In particular, all these
systems exhibit two chemically reversible one-electron redox
transformations; an oxidation wave attributable to the UVI/
UV process and a reduction wave attributable to the UV/UIV
process. As illustrated in Figure 3, the separations between
the anodic and cathodic peaks for these waves deviate, in
some cases substantially, from the nominal electrochemically
reversible value of 60 mV indicating that the heterogeneous
electron-transfer rates vary significantly across this series.62
This behavior was reported previously for the halide com-
plexes.32 Of greater interest is the variability in the half-
wave potentials for these metal-based redox transformations
across this series reflecting the role of the ancillary ligand
in perturbing the redox energetics in these systems (Table
3). This perturbation appears to derive from an interplay
between purely electrostatic effects (e.g., for the triflate
complex 11) and more intrinsic σ- and π-bonding interactions
that shift electron density at the metal center thereby
impacting the redox energetics. The overall effect across the
series is quite dramatic for a seemingly small structural
(55) This trend in chemical shift for like protons as a function of halide
has been seen for other paramagnetic uranium systems: (a) [(1,3-R2-
C5H3)2UX]2 (R ) SiMe3, CMe3; X ) F, Cl, Br, I): Lukens, W. W.,
Jr.; Beshouri, S. M.; Stuart, A. L.; Andersen, R. A. Organometallics
1999, 18, 1247–1252. (b) (C5Me5)3UX(X ) F, Cl, Br): Evans, W. J.;
Nyce, G. W.; Johnston, M. A.; Ziller, J. W. J. Am. Chem. Soc. 2000,
122, 12019–12020. (c) (1,3-R2-C5H3)2UX2 (R ) SiMe3, CMe3; X )
F, Cl, Br, I): Lukens, W. W., Jr.; Beshouri, S. M.; Blosch, L. L.; Stuart,
A. L.; Andersen, R. A. Organometallics 1999, 18, 1235–1246. (d)
The 1H NMR behavior of the (C5H5)3UX system has also been
extensively studied not only where X ) halide, but also for other donor
groups such as OR, NR2, SR, PR2, etc.: Fisher, R. D. NMR-
Spectroscopy of organo-f-element and pre-lanthanoid complexes. In
Fundamental and Technological Aspects of Organo-f-Element Chem-
istry; Marks, T. J., Fragala, I. L., Eds.; NATO ASI Series - Series C:
Mathematical and Physical Sciences. D. Reidel Publishing Company:
Dordrecht, 1985; Vol. 155, p 294.
(56) Dhingra, M. M.; Ganguli, P.; Mitra, S. Chem. Phys. Lett. 1974, 25,
579–581.
(57) Drago, R. S.; Wayland, B. B. Inorg. Chem. 1968, 7, 628–630.
(58) LaMar, G. N.; Fischer, R. H.; Horrocks, W. D., Jr. Inorg. Chem. 1967,
6, 1798–1803.
(59) Figgis, B. N.; Hitchman, M. A. Ligand Field Theory and Its
Applications; Wiley-VCH: New York, 2000.
(60) Elschenbroich, C.; Salzer, A. Organometallics. A Concise Introduction;
VCH: Weinheim, Germany, 1989.
(61) Kiplinger, J. L.; Morris, D. E.; Scott, B. L.; Burns, C. J. Organome-
tallics 2002, 21, 3073–3075.
(62) While positive-feedback IR compensation was employed in the
collection of the voltammetric data, there is still some contribution
from uncompensated solution resistance to the potential separation
between cathodic and anodic peaks. Thus, deviations in the peak
separation from 60 mV cannot be attributed solely to heterogeneous
electron-transfer kinetic effects.
11884 Inorganic Chemistry, Vol. 47, No. 24, 2008

Organometallic PentaWalent Uranium Imido Complexes
Table 3. Summary of Redox Potential Dataa for the (C5Me5)2U()N-2,6-iPr2-C6H3)(X/Y/L) Complexes 2, 7-16 and 18 in ∼0.1 M
[Bu4N][fluoroarylborate]b/THF Solution at Room Temperature
UV complexes
X/Y
electrolyte anionb
E1/2(UVI/UV) (V)
E1/2(UV/UIV) (V)
|∆E1/2| (V)
ref.
7c
8
9
10
11
12
13
14
15
16
18
F
Cl
Br
I
OTf
SPh
CtCPh
NPh2
OPh
Me
BArF
BArF
BArF
B(C6F5)4
BArF
BArF
BArF
BArF
BArF
BArF
BArF
-0.14
0.03
0.07
0.11
0.36
-1.81
-1.52
-1.44
-1.37
-1.21
-1.43
-1.64
-1.65
-1.75
-1.71
-1.84
1.67
1.55
1.51
1.48
1.57
1.43
1.54
1.35
1.53
1.58
1.50
1.52 ( 0.03
32
32
32
31, 32
31, this work
31, this work
34
this work
this work
this work
this work
0.00
-0.10
-0.30
-0.22
-0.13
-0.34
N)CPh2
|∆E1/2 (ave)
|
UIV precursor
L
electrolyte anionb
BArF
E1/2(UV/UIV) (V)
-0.86
E1/2(UIV/UIII) (V)
|∆E1/2| (V)
1.54
ref
28
2
THF
-2.40
a All E1/2 values are versus [(C5H5)2Fe]+/0 and were determined from the peak position in a square-wave voltammogram or from the average of the
cathodic and anodic peaks in a cyclic voltammogram. Electrolyte anion was either [B(3,5-(CF3)2-C6H3)4]- (BArF), or [B(C6F5)4]-. c Although scan-rate
b
dependent behavior was explored at ∼-50 °C, potential calibration data were collected for this complex at room temperature using a freshly prepared
solution.
Figure 4. Linear correlation between 1H NMR chemical shift of the C5Me5
protons and oxidation potential for (C5Me5)2U()N-2,6-iPr2-C6H3)(X/Y). The
correlation includes all ten available data points (R2 ) 0.93).
Finally, we note that the nearly constant potential separa-
tion between the UVI/UV and UV/UIV couples described
previously32 is retained across this entire series of eleven
complexes, with an average |∆E1/2| value of 1.52 ( 0.03 V.
This implies that the factors responsible for (de)stabilizing
the oxidation process are equally at play in shifting the
reduction wave in the same direction. For example, the σ/π-
donor ligands stabilize the UVI oxidation state while con-
comitantly destabilizing the more electron-rich UIV oxidation
state by an approximately equal amount.
The existence of three different uranium oxidation states
(UVI, UV, and UIV) within this relatively narrow potential
window is significant, and reminiscent of the behavior
observed previously for (C5Me5)2UIV(ketimide)2 complexes
for which the UV, UIV, and UIII states all lie within an ∼2.1
V range.4,63 This accessibility to three different oxidation
states in these organouranium complexes having partial
(ketimide) or full (imide) metal-ligand multiple bonding can
be contrasted with the behavior seen for many other
Figure 3. Cyclic voltammograms for the (C5Me5)2U()N-2,6-iPr2-C6H3)(X/
Y) complexes 7-16 and 18 in ∼0.1M [Bu4N][fluoroarylborate]/THF
solution at 200 mV/s scan rate at a Pt electrode.
perturbation to the otherwise constant UV-imido core (Table
1). If one considers the potential of the UVI/UV oxidation
wave, the process shifts by ∼0.7 V on going from the triflate
complex (11) to the ketimide complex (18). As noted above,
1
the chemical shift of C5Me5 ligand protons in the H NMR
spectra (Table 2) reflects the extent of electronic donation
from the ancillary X/Y ligand to the metal center. Interest-
ingly, there is an excellent linear correlation between these
chemical shift values and the oxidation potentials (Figure
4), suggesting that there is a common origin, overall electron
donation from the ancillary X/Y ligand to the metal,
contributing to both observables.
(63) Morris, D. E.; Re, R. E. D.; Jantunen, K. C.; Castro-Rodriguez, I.;
Kiplinger, J. L. Organometallics 2004, 23, 5142–5153.
Inorganic Chemistry, Vol. 47, No. 24, 2008 11885

Graves et al.
(C5Me5)2UL2 complexes containing only simple σ-donor
ligands (L) such as alkyl or halide. In this latter class of
complexes only a single metal-based redox couple (e.g., UIV/
UIII) is observed under standard non-aqueous voltammetric
conditions. The greater range of readily accessible oxidation
states in the former systems is attributable to the stabilizing
influence of the more covalent metal-ligand multiple bonding
environment. Similar effects have been noted by Meyer and
co-workers for Ru and Os polypyridyl complexes where the
higher-valent metal centers are stabilized and hence become
accessible through the formation of covalent metal oxo
(M)O) bonds.64
Electronic Spectroscopy. As noted previously,32,34 these
pentavalent uranium complexes provide an excellent op-
portunity to explore in detail the electronic/molecular
structure relationship. Their simple 5f1 valence electronic
configuration gives rise to relatively straightforward elec-
tronic spectra, and the essentially constant molecular struc-
tural framework provided by the (C5Me5)2U()N-2,6-iPr2-
C6H3) core enables a focus on perturbations introduced by
the ancillary ligand (X/Y) in the metallocene wedge. Ad-
ditionally, the imido ligand with its covalent multiple bonding
to the metal center introduces additional molecular electronic
states of both ligand-localized and charge-transfer character
that have the potential to interact with the ligand-field (f-f)
states in ways that provide additional diagnostics for the
electronic structure and electronic interactions in these
systems.
Figure5.UV-visible-NIRelectronicabsorptionspectraofthe(C5Me5)2U()N-
2,6-iPr2-C6H3)(X/Y) complexes 7-18 in toluene solution at room temper-
ature.
appear to be in the spectra for the acetylide (13) and ketimide
(18) as evidenced by the substantially larger transition
intensity in the higher energy region. Both of these ligands
possess low-energy non-bonding, π and/or π* orbitals that
should contribute to the spectral density of states in this
region.3,4,65,66 This becomes significant in the context of the
f-f transition intensities discussed below.
Consideration of the electronic spectra begins by examin-
ing the entire UV-visible-near IR region illustrated in Figure
5. For comparison purposes we have also included previously
published spectra for complexes 7-1032 and 13.34 There is
remarkable consistency in the spectral band shapes and
intensities, particularly for the spectra in the upper panel of
Figure 5. The variability is somewhat greater in the spectra
in the lower panel, although it should be noted that the
intensity data for the methyl (16) and phenyl (17) complexes
are less certain because these samples were prepared from
material that was of an oily consistency that made accurate
weighing problematic. The spectral assignments for the bands
in the region from 10000-33000 cm-1 have been described
in detail previously and derive principally from transitions
localized on the (C5Me5)2U()N-2,6-iPr2-C6H3) core.32 Given
the correlations among these spectra, it is likely that these
assignments remain valid for all these complexes. Specifi-
cally, the lowest energy broad bands at ∼10000 and 14000
cm-1 are assigned to the two different spin components of
the imido-to-metal charge-transfer transition: 4(πM)Nfnb5f)
The 5f1 valence electronic configuration in all these
complexes gives rise to a fairly simple f-orbital density of
states. Spin-orbit coupling leads to only two manifolds of
states: a 2F5/2 ground-state manifold and a 2F7/2 excited-state
manifold. These manifolds will be further split according to
the symmetry and strength of the crystal field imposed by
the ligand environment. In the low symmetry of these UV-
imido complexes (nominally Cs symmetry), all orbital
degeneracies should be removed leading to a total of seven
states and six possible transitions. (Note that each state will
still be a Kramer doublet as a consequence of the odd
electron count.) Preliminary ligand-field calculations incor-
porating spin-orbit coupling reported by us previously32 for
structurally simplified model surrogates of the fluoride (7)
and iodide (10) complexes (i.e. (C5Me5)2U()N-C6H5)(X)
(X ) F, I)) are in general good agreement with published
data and interpretations for 5f1 systems in higher symmetry
(e.g., pseudo-octahedral) environments such as the UV
hexahalides.20 Specifically, the calculations predict that two
transitions lie within ∼2800 cm-1 of the ground state and
would not be observable in our experiments. The four
2
and (πM)Nfnb5f), respectively (nb ) non-bonding). The
higher energy bands in the UV-visible region are at least in
part composed of πM)Nfπ*Ph transitions. These assignments
are all supported by density functional theory calculations.32
The strong spectral correlation across most of this series
suggests that the ancillary (X/Y) ligand does not contribute
significantly to electronic excited states within the region
probed in our studies. The exceptions to this observation
(65) Lichtenberger, D. L.; Renshaw, S. K.; Bullock, R. M. J. Am. Chem.
Soc. 1993, 115, 3276–3285.
(66) Wong, C.-Y.; Che, C.-M.; Chan, M. C. W.; Han, J.; Leung, K.-H.;
Phillips, D. L.; Wong, K.-Y.; Zhu, N. J. Am. Chem. Soc. 2005, 127,
13997–14007.
(64) Meyer, T. J.; Huynh, M. H. V. Inorg. Chem. 2003, 42, 8140–8160.
11886 Inorganic Chemistry, Vol. 47, No. 24, 2008

Organometallic PentaWalent Uranium Imido Complexes
Figure 7. NIR electronic absorption spectra of the (C5Me5)2U()N-2,6-
iPr2-C6H3)(Y) complexes 11-18 in toluene-d8 solution at room temperature.
Higher-energy tails from the visible spectra have been subtracted (see text).
calculations performed previously for the fluoride and iodide
model systems (C5Me5)2U()N-C6H5)(X) (X ) F, I).32
Examination of the 77 K data (Figure 6) in this f-f spectral
region for the four halide complexes provides some quantita-
tive support for the interpretation that this region is comprised
of two different sets of electronic transitions each of which
has vibronic structure (instead of a single electronic transition
with vibronic structure). First, there is a very weak feature
in the room temperature data for each of the halide complexes
at ∼5900 cm-1 that disappears in the 77 K spectra.32 This
indicates that this feature is a vibronic hot-band, and therefore
the most intense band in the 6100-6200 cm-1 region is an
electronic origin. (Note that there is no inversion symmetry
in these complexes, so dipole-selection rules associated with
orbital parity are relaxed.) Next, the series of weak bands in
the region just above this very intense feature (∼6360, 6520,
and 6590 cm-1) is nearly identical for the chloride, bromide,
and iodide complexes (with very minor shifts of all bands
to slightly higher energy along the series Cl < Br < I)
consistent with vibronic structure derived from vibrational
modes of nearly identical energy for all three complexes.
Since these features occur at nearly the same energy in all
three spectra, they likely derive from modes localized on
the (C5Me5)2U()N-2,6-iPr2-C6H3) core, and do not involve
the M-X interaction to an appreciable extent. The spectrum
of the fluoride complex also has several weak vibronic
features in this same region, as expected since it, too, has
this same metallocene imido framework. To higher energy,
however, there is a significant difference in the spectral data
for the fluoride complex versus those of the other halides,
with the next distinctive feature in the fluoride spectrum lying
some ∼500 cm-1 to higher energy than those in the spectra
of the other halides (∼7250 cm-1 vs ∼6700 cm-1). This
strongly suggests that this next series of bands derives from
a new electronic transition (and related vibronic sidebands
Figure 6. NIR electronic absorption spectra of the (C5Me5)2U()N-2,6-
iPr2-C6H3)(X) complexes 7-10 in toluene-d8 solution at room temperature
(red, offset for clarity) and 77 K (blue). Higher-energy tails from the visible
spectra have been subtracted (see text).
remaining states are predicted to lie 6000-10000 cm-1 above
the ground state, with the two lowest levels occurring at 6200
and 7100 cm-1.
A set of narrow, lower intensity bands in this low energy
spectral region is readily discernible in the data shown in
Figure 5. These bands were isolated for better interpretation
and comparison by fitting a Gaussian profile to the broad
band at 10000 cm-1 and subtracting this broad band from
the lower energy region. In addition, for several of the
complexes, data were collected at 77 K in a toluene-d8 glass
to enhance the spectral resolution. Typical data for these f-f
bands after this higher-energy background subtraction routine
are illustrated in Figures 6 and 7. The spectra clearly are
composed of many bands resulting from both pure electronic
transitions and vibronic transitions built on the electronic
transitions. At a qualitative level, the spectral data for all
twelve complexes appear to be composed of two sets of
bands. The first set occurs in the same energy range (with
the most prominent band at ∼6000-6200 cm-1) for all
complexes. The second set establishes greater distinction
among the complexes. For the fluoride (7), diphenylamide
(14), and phenoxide (15) the second set begins with the most
prominent band at ∼7300 cm-1, whereas for all other
complexes, this second set begins with the most prominent
band occurring at lower energy (∼6800-7000 cm-1). Note,
however, that these ranges are in good agreement for all
complexes with prediction from the ligand field/spin orbit
Inorganic Chemistry, Vol. 47, No. 24, 2008 11887

Graves et al.
to higher energy) that lies to higher energy in the fluoride
complex than in the other halides because of the greater
perturbative influence of the fluoride ion compared to that
of the other halides on the metal-based energy levels.
The rich vibronic structure seen at 77 K for the halide
complexes is less readily discernible in the room temperature
data for the other non-halide systems (Figure 7), but the gross
spectral features clearly fall into one of two categories; those
with the higher energy set of f-f bands in the same higher-
energy range as those of the fluoride complex (i.e., the
diphenylamide (14) and the phenoxide (15)) and those whose
spectra are more like those of the heavier halides. The crystal
structure data (Table 1) that span most of the twelve
complexes under consideration demonstrate that all com-
plexes have very similar structural parameters, with the only
significant difference being the identity of the ancillary ligand
(X/Y) in the metallocene wedge, and the metal-ligand bond
distance for this ligand. It is also expected that the spin-
orbit coupling constant for UV should remain essentially
constant across the entire series of complexes. Thus, the
spectral data provide an important opportunity to focus
specifically on the contribution of the ancillary ligand to the
splitting of the f-f states in these systems. In principal, these
data can be used to arrange the ancillary ligands according
to a spectrochemical or nephelauxetic series.67-69 However,
the low symmetry of these complexes necessitates more
parameters to describe the energy level splitting than can be
deduced from the energies of just the two states identified
in our data. We are pursuing additional density functional
theory based calculations to enable a more quantitative
assessment. It is clear that the complexes can be divided into
two distinct classes based on these spectral data, and the
grouping of the fluoride, diphenylamide, and phenoxide
systems into the same class as suggested above is most
consistent with the propensity of these ligands to interact
strongly with the metal center through both σ- and π-donation
that would be expected to lead to greater splitting of the 5f1
levels. However, the observation of slightly different cat-
egorizations of complexes based on these electronic spectral
symmetry is reduced from pseudo-octahedral in the hexa-
halides to ∼Cs symmetry in the imido complexes. This effect
alone likely cannot account for the observed more than 20-
fold difference in intensities. It is more likely that the f-f
transitions in the imido complexes gain intensity by coupling
to the higher lying charge-transfer states that give rise to
the spectral band shown in Figure 5. Such coupling should
scale according to the transition intensity in these higher lying
states and their energetic proximity to the f-f transitions.67
This same mechanism was proposed previously to account
for the unusually large f-f transition intensities in the near-
IR spectra of (C5Me5)2U(ketimide)2 complexes.4,63 In effect,
some of the charge-transfer excited state character is mixed
into the metal-localized f-f states inducing greater electric
dipole-allowed character to the f-f transitions. The mixing
is facilitated because there is common 5f orbital parentage
in the states. Since the charge-transfer excited states have
been shown to involve the UdN imido bond, the observation
of f-f intensity enhancement implicates f-orbital participation
in the UdN covalent bond. Hence, the intensity enhancement
by this mixing of charge-transfer character is diagnostic of
5f orbital involvement in the bonding. In support of this
proposed enhancement mechanism, we note in particular that
the two complexes possessing the most intense f-f bands,
the acetylide (13) and the ketimide (18), are also the
complexes exhibiting the greatest intensities in the UV-visible
transitions (Figure 5).
Conclusions
Given the increasing prominence that UV complexes are
gaining in the bonding and reactivity in actinide chemistry,
the synthetic protocols reported herein provide a valuable
entry into the synthesis of a variety of pentavalent uranium
organometallic complexes (C5Me5)2U()N-2,6-iPr2-C6H3)(X/
Y) (where X ) F, Cl, Br, I; Y ) OTf, SPh, NPh2, OPh, Me,
Ph, CtCPh, N)CPh2), which include the first examples of
pentavalent uranium complexes with anionic sp, sp2, and sp3
carbon moieties other than carbocyclic (C5R 5, C7H7, C8H8)
ligands. This is a strong complement to other reported routes
to UV organometallic species, and we have demonstrated that
a multitude of substitution patterns can be achieved utilizing
an array of reaction pathways sdirect oxidation of the UIV-
imido precursor, salt metathesis, protonolysis, or insertion.
That a wide range of substituents can be supported within
the wedge of these UV-imido complexes refutes prior
assertions that pentavalent organouranium complexes are
inherently unstable. The methods reported herein also avoid
the use of silver and thallium salts (RCRA-listed), thereby
greatly diminishing the generation of mixed radioactive
hazardous waste and potential exposure to highly toxic
thallium reagents.
1
data versus the trend discussed above based on H NMR
and redox potential data suggests that the splitting pattern
for the f-f states reflects more subtle influences than simple
σ/π electron-donation effects from the ancillary (X/Y)
ligands.
The final consideration regarding these f-f spectral data
concerns the intensities in the absorption bands. In classical
coordination compounds of pentavalent uranium in pseudo-
octahedral symmetry, such as the hexahalides, typical molar
absorptivities are in the range ∼5-20 M-1 cm-1.20 In
contrast, for the UV-imido complexes herein, the molar
absorptivities in the principal bands are in the range from
∼100 to ∼1500 M-1 cm-1. Clearly some of this discrepancy
can be attributed to the fact that there is a relaxation in the
electric dipole selection rules for these f-f transitions as the
The series of (C5Me5)2U()N-2,6-iPr2-C6H3)(X/Y) com-
plexes with electronically diverse X/Y ligands was examined
using a combination of structural analysis, magnetic suscep-
tibility measurements, electrochemistry, and spectroscopy,
which provides a coherent description of the electronic and
magnetic structure of these pentavalent systems. The elec-
trochemical analysis shows that the ligand framework can
(67) Lever, A. B. P. Inorganic Electronic Spectroscopy, 2nd ed.; Elsevier:
Amsterdam, The Netherlands, 1984.
(68) Jorgensen, C. K. Chem. Phys. Lett. 1982, 89, 455–458.
(69) Krupa, J. C. J. Solid State Chem. 2005, 178, 483–488.
11888 Inorganic Chemistry, Vol. 47, No. 24, 2008

Organometallic PentaWalent Uranium Imido Complexes
stabilize both the UIV and UVI oxidation states. An excellent
linear correlation was observed between the chemical shift
values of C5Me5 ligand protons in the 1H NMR spectra and
the oxidation potentials of the (C5Me5)2U()N-2,6-iPr2-
C6H3)(X/Y) compounds, suggesting that there is a common
origin, overall σ- and π-donation from the ancillary X/Y
ligand to the metal, contributing to both observables.
Combined, these data allow for the following assessment of
the overall donating ability of the X/Y ligand to the UV metal
center: OTf < I < Br < Cl < SPh < CtCPh < F < [OPh
∼ Me ∼ Ph] , NPh2 < N)CPh2.
or Bruker P4/CCD diffractometer. Structural solution and refinement
was achieved using the SHELXL97 program suite.70 Details
regarding data collection are provided in the CIF files.
Celite (Aldrich), alumina (Brockman I, Aldrich), and 4 Å
molecular sieves (Aldrich) were dried under dynamic vacuum at
250 °C for 48 h prior to use. All solvents (Aldrich) were purchased
anhydrous, dried over KH for 24 h, passed through a column of
activated alumina, and stored over activated 4 Å molecular sieves
prior to use. Benzene-d6 (Aldrich) was purified by passage through
activated alumina and storage over activated 4 Å molecular sieves
prior to use. (C5Me5)2U()N-2,4,6-tBu3-C6H2) (1),33 (C5Me5)2U()N-
2,6-iPr2-C6H3)(THF) (2),25 Me2Mg and Ph2Mg(THF)2,71,72
[Bu4N][B(C6F5)4]73 and [Bu4N][B(3,5-(CF3)2-C6H3)4]73 were pre-
pared according to literature methods, and 11 and 12 were prepared
as previously reported.31 KOPh and KNPh2 were both prepared by
refluxing a THF solution of HOPh and HNPh2, respectively, over
1 equiv of KH for 24 h.74
With the spin-orbit coupling constant for UV remaining
essentially constant across the entire series of complexes,
the absorption spectroscopic data provided an important
opportunity to focus on the contribution of the X/Y ancillary
ligand to the splitting of the f-f states in these systems.
Interestingly, the complexes can be divided into two distinct
classes based on the absorption spectroscopic data, with one
group including X/Y ) OTf, I, Br, Cl, SPh, CtCPh, Me,
Ph, and N)CPh2, which are strong σ-donors and weaker
π-donors in comparison to the other group containing the
X/Y ) F, NPh2, and OPh, which interact strongly with the
UV metal center by both σ- and π-donation and lead to a
larger splitting of the 5f1 levels versus the former group of
X/Y ligands.
Caution! Depleted uranium (primary isotope 238U) is a weak
R-emitter (4.197 MeV) with a half-life of 4.47 x 109 years;
manipulations and reactions should be carried out in monitored
fume hoods or in an inert atmosphere drybox in a radiation
laboratory equipped with R- and ꢁ-counting equipment.
Instrumentation and Sample Protocols. Electronic absorption
spectral data were obtained for toluene or toluene-d8 solutions of
complexes over the wavelength range 300-2500 nm on a Perkin-
Elmer Model Lambda 950 UV-visible-NIR spectrophotometer.
Room temperature data were collected in 1 cm and 1 mm path
length cuvettes loaded in the Vacuum Atmospheres drybox system
described above run versus the appropriate toluene solvent refer-
ence. Samples were typically run at multiple dilutions to optimize
absorbance in the UV-visible and near-infrared, respectively. Data
collected at 77 K were obtained from samples in toluene-d8
contained in medium-walled NMR tubes (503-PS from Wilmad/
Lab Glass). The samples were immersed in an optical dewar with
a quartz cold-finger (WG-850-Q-PTI) and run in a standard
transmission spectrometer configuration versus air after ensuring
the solutions glassed upon freezing. Pure toluene-d8 was run under
identical conditions to perform solvent corrections to the sample
spectra post-data collection. No effort was made to be quantitative
in determining extinction coefficients of the samples under these
conditions. Spectral resolution was typically 2 nm in the visible
region and 4-6 nm in the near-infrared in all cases.
Voltammetric data were obtained in the Vacuum Atmospheres
drybox system described above. In addition, data for complex 7
were obtained in a Schlenk-line electrochemical cell immersed in
a dry ice/2-propanol bath at ∼-50 °C. All data were collected using
a Perkin-Elmer Princeton Applied Research Corporation (PARC)
Model 263 potentiostat under computer control with PARC Model
270 software. All sample solutions were ∼1-2 mM in complex
with 0.1 M [Bu4N][B(3,5-(CF3)2-C6H3)4] or [Bu4N][B(C6F5)4]
supporting electrolyte in THF solvent.73 All data were collected
with the positive-feedback IR compensation feature of the software/
When considered in the broader context of bonding in
actinide chemistry, the (C5Me5)2U()N-2,6-iPr2-C6H3)(X/Y)
complexes also show distinct hallmarks of covalent metal-
imide bonding interactions that are modulated to varying
degrees by the X/Y ancillary ligand. The signatures of
covalency include stabilization of multiple metal oxidations
states [UIV, UV, and UVI] and enhanced intensities in the
intraconfiguration (f-f) transitions, such as the more than 20-
fold enhancement in the f-f intensities observed for Y )
CtCPh and N)CPh2, which are diagnostic indicators of
covalency in the metal-ligand bonding interactions sustained
by the acetylide and ketimide ligands in these pentavalent
systems.
Experimental Section
General Considerations. Reactions and manipulations were
performed in either a recirculating Vacuum Atmospheres NEXUS
model inert atmosphere (N2) drybox equipped with a 40CFM Dual
Purifier NI-Train or using standard Schlenk techniques. Glassware
was dried overnight at 150 °C before use. All NMR spectra were
obtained in C6D6 using a Bruker Avance 300 MHz spectrometer.
1
Chemical shifts for H NMR spectra were referenced to solvent
(70) (a) Bruker AXS, SAINT 7.06, Integration Software; Bruker Analytical
X-ray Systems: Madison, WI, 2003. (b) Sheldrick, G. M. SADABS
2.03, Program for Adsorption Correction; University of Go¨ttingen:
Go¨ttingen, Germany, 2001. (c) Sheldrick, G. M. SHELXTL 5.10,Struc-
ture Solution and Refinement Package; University of Go¨ttingen:
Go¨ttingen, Germany, 1997.
(71) Tobia, D.; Baranski, J.; Rickborn, B. J. Org. Chem. 1989, 54, 4253–
4256.
(72) Waggoner, K. M.; Power, P. P. Organometallics 1992, 11, 3209–
3214.
impurities. Melting points were determined with a Melt-Temp II
capillary melting point apparatus equipped with a Fluke 50S K/J
thermocouple using capillary tubes flame-sealed under nitrogen;
values are uncorrected. Mass spectrometric (MS) analyses were
obtained at the University of California, Berkeley Mass Spectrom-
etry Facility, using a VG ProSpec (EI) mass spectrometer. Elemental
analyses were performed at the University of California, Berkeley
Microanalytical Facility, on a Perkin-Elmer Series II 2400 CHNS
analyzer. X-ray data were collected using either a Bruker APEX2
(73) Barriere, F.; Geiger, W. E. J. Am. Chem. Soc. 2006, 128, 3980–3989.
(74) Barnhart, D. M.; Clark, D. L.; Grumbine, S. K.; Watkin, J. G. Inorg.
Chem. 1995, 34, 1695–1699.
Inorganic Chemistry, Vol. 47, No. 24, 2008 11889

Graves et al.
potentiostat activated to ensure minimal contribution to the volta-
mmetric waves from uncompensated solution resistance (typically
∼1 kΩ under the conditions employed). For experiments at ambient
temperature, solutions were contained in PARC Model K0264
microcells consisting of a ∼3 mm diameter Pt disk working
electrode, a Pt wire counter electrode, and a Ag wire quasi-reference
electrode. For the low temperature experiment, a Schlenk cell was
employed consisting of Pt wire working and counter electrodes
sandwiching a Ag wire quasi-reference electrode. Scan rates from
20-5000 mV/s were employed in the cyclic voltammetry scans to
assess the chemical and electrochemical reversibility of the observed
redox transformations. Half-wave potentials were determined from
the peak values in the square-wave voltammograms or from the
average of the cathodic and anodic peak potentials in the reversible
cyclic voltammograms. Potential calibrations were performed at
the end of each data collection cycle using the ferrocenium/ferrocene
couple as an internal standard. Electronic absorption and cyclic
voltammetric data were analyzed using Wavemetrics IGOR Pro
(Version 4.0) software on a Macintosh platform.
ethyldisiloxane solution at -30 °C. 1H NMR (C6D6, 298 K): δ 49.91
(b, 1H, CH(Me)2), 22.79 (1H, Ar-H), 16.43 (1H, Ar-H), 13.56 (1H,
OPh), 11.43 (6H, CH(Me)2), 10.21 (2H, OPh), 8.54 (1H, OPh),
3.33 (30H, (C5Me5)2), -6.19 (6H, CH(Me)2), -6.79 (1H, OPh),
-11.33 (1H, Ar-H), -20.09 (b, 1H, CH(Me)2). Anal. Calcd for
C38H52NOU (mol. wt. 776.85): C, 58.75; H, 6.75; N, 1.80. Found:
C, 58.74; H, 6.96; N, 2.04. Mp ) 197-199 °C. MS (EI, 70 eV):
m/z 776 (M+). UVI/V E1/2 ) -0.22 V, UV/IV E1/2 ) -1.75 V (vs.
[(C5H5)2Fe]+/0 in THF/0.1 M [Bu4N][B(3,5-(CF3)2-C6H3)4]).
Synthesis of (C5Me5)2U()N-2,6-iPr2-C6H3)(Me) (16). A 125
mL side-arm flask equipped with a stir bar was charged with
(C5Me5)2U()N-iPr2-C6H3)(I) (10) (0.50 g, 0.62 mmol) and toluene
(50 mL). Me2Mg (0.045 g, 0.84 mmol) was added to the stirring
dark brown solution followed by 1,4-dioxane (∼0.5 mL), and the
reaction was stirred at room temperature. After 12 h, the reaction
mixture was filtered through a Celite-padded coarse frit, and
volatiles were removed from the filtrate. The residue was extracted
into hexanes (50 mL) and filtered through a Celite-padded coarse
porosity frit. The filtrate was collected, and the volatiles were
removed under reduced pressure to give 16 as a brown solid (0.30
Magnetic susceptibility data were collected using a Quantum
Design Superconducting Quantum Interference Device (SQUID)
magnetometer at 5 T from 2-350 K. The samples were sealed in
a 5mm Wilmad 505-PS NMR tube along with a small amount of
quartz wool, which held the sample near the tube center. Contribu-
tions to the magnetization from quartz wool and the NMR tube
were measured independently and subtracted from the total
measured signal. Diamagnetic corrections were made with the use
of Pascal’s constants. Magnetic moments were determined from
fits of the 1/ꢀ versus T data in the high temperature linear regimes.
Synthesis of (C5Me5)2U()N-2,6-iPr2-C6H3)(NPh2) (14). A 125
mL side-arm flask equipped with a stir bar was charged with
(C5Me5)2U()N-iPr2-C6H3)(I) (10) (0.25 g, 0.31 mmol), toluene (50
mL). KNPh2 (0.077 g, 0.37 mmol) was added to the stirring dark
brown solution, and the reaction was stirred at room temperature.
After 12 h, the reaction mixture was filtered through a Celite-padded
coarse frit and volatiles were removed from the filtrate. The residue
was extracted into hexanes (50 mL) and filtered through a Celite-
padded coarse porosity frit. The filtrate was collected, and the
volatiles were removed under reduced pressure to give 14 as a
brown solid (0.21 g, 0.25 mmol, 80%). X-ray quality samples of
14 were obtained by recrystallization from a toluene/hexamethyl-
disiloxane (1/1) mixture at -30 °C. Anal. Calcd for C44H57N2U
(mol. wt. 851.97): C, 62.03; H, 6.74; N, 3.29. Found: C, 61.94; H,
6.37; N, 3.54. Mp ) 205-207 °C. MS (EI, 70 eV): m/z 851 (M+).
1
g, 0.43 mmol, 70%) suitable for further reaction. H NMR (C6D6,
298 K): δ 68.20 (b, 1H, CH(Me)2), 28.57 (1H, Ar-H), 19.67 (1H,
Ar-H), 15.20 (6H, CH(Me)2), 3.30 (30H, (C5Me5)2), -2.20 (3H,
U-Me), -7.30 (6H, CH(Me)2), -7.84 (1H, Ar-H), -16.06 (b, 1H,
CH(Me)2). Anal. Calcd for C33H50NU (mol. wt. 698.79): C, 56.72;
H, 7.21; N, 2.00. Found: C, 54.64; H, 7.07; N, 1.87. Despite
numerous attempts, the long-term room temperature instability of
VI/V
complex 16 precluded obtaining good elemental analyses. U
E
1/2
) -0.13 V, UV/IV E1/2 ) -1.71 V (vs. [(C5H5)2Fe]+/0 in THF/0.1
M [Bu4N][B(3,5-(CF3)2-C6H3)4]).
Synthesis of (C5Me5)2U()N-2,6-iPr2-C6H3)(Ph) (17). A 125 mL
side-armflaskequippedwithastirbarwaschargedwith(C5Me5)2U()N-
iPr2-C6H3)(I) (10) (0.50 g, 0.62 mmol), toluene (50 mL).
Ph2Mg(THF)2 (0.40 g, 1.24 mmol) was added to the stirring dark
brown solution followed by 1,4-dioxane (∼0.5 mL), and the reaction
was stirred at room temperature. After 12 h, the reaction mixture
was filtered through a Celite-padded coarse frit, and volatiles were
removed from the filtrate. The residue was extracted into hexanes
(50 mL) and filtered through a Celite-padded coarse porosity frit.
The filtrate was collected, and the volatiles were removed under
reduced pressure to give 17 as a brown solid (0.38 g, 0.50 mmol,
80%) suitable for further reactions. 1H NMR (C6D6, 298 K): δ 1H
NMR (C6D6, 298 K): δ 52.71 (b, 1H, CH(Me)2), 30.79 (1H, Ar-
H), 24.15 (1H, Ar-H), 14.65 (2H, Ar-H), 13.98 (6H, CH(Me)2),
8.03 (3H, Ar-H), 3.00 (30H, (C5Me5)2), -3.57 (6H, CH(Me)2),
-4.72 (1H, Ar-H), -15.47 (b, 1H, CH(Me)2). The room temper-
ature instability of 17 precluded elemental analysis.
VI/V
V/IV
U
E
) -0.30 V, U
E
) -1.65 V (vs. [(C5H5)2Fe]+/0 in
1/2
1/2
THF/0.1 M [Bu4N][B(3,5-(CF3)2-C6H3)4]). No resonances were
observed in the 1H NMR of 14 over the temperature range 0-100
°C.
Synthesis of (C5Me5)2U()N-2,6-iPr2-C6H3)(N)CPh2) (18).
Method A - Protonolysis. A 125 mL side-arm flask equipped with
a stir bar was charged with (C5Me5)2U()N-iPr2-C 6H3)(I) (10) (0.25
g, 0.31 mmol), toluene (50 mL). Me2Mg (0.023 g, 0.42 mmol) was
added followed by 1,4-dioxane (∼0.25 mL), and the reaction was
stirred for 12 h at room temperature. The resulting dark brown
solution was filtered though a Celite-padded coarse frit. The filtrate
was transferred to a 150 mL thick-walled Schlenk tube equipped
with Teflon valve and a stir bar followed by benzophenone imine
(0.067 g, 0.37 mmol) as a solution in toluene (5 mL). The reaction
vessel was sealed and removed from the drybox to a fumehood,
where it was placed in a 75 °C oil bath. After 12 h, the reaction
was removed from heat, cooled to room temperature, and brought
back into the drybox. The reaction was filtered though a Celite-
padded coarse frit, and the volatiles were removed from the filtrate.
The crude product was extracted into hexanes (50 mL), filtered
Synthesis of (C5Me5)2U()N-2,6-iPr2-C6H3)(OPh) (15). A 150
mL thick-walled Schlenk tube equipped with Teflon valve and a
stir bar was charged with (C5Me5)2U()N-2,6-iPr2-C6H3)(I) (10)
(0.25 g, 0.31 mmol), a magnetic stir bar, and toluene (50 mL). To
the dark brown solution was added KOPh (0.049 g, 0.37 mmol) as
an off-white powder. The reaction vessel was sealed and removed
from the drybox to a fumehood, where it was placed in a 75 °C oil
bath. After 12 h, the reaction was removed from heat, cooled to
room temperature, and brought back into the drybox. The reaction
was filtered though a Celite-padded coarse frit, and the volatiles
were removed from the filtrate. The crude product was extracted
into hexanes (50 mL), filtered through a Celite-padded coarse frit,
and the volatiles removed under reduced pressure to give 15 as a
brown solid (0.18 g, 0.23 mmol, 75%). X-ray quality samples of
15 were obtained by recrystallization from a concentrated hexam-
11890 Inorganic Chemistry, Vol. 47, No. 24, 2008

Organometallic PentaWalent Uranium Imido Complexes
through a Celite-padded coarse frit, and volatiles were removed
under reduced pressure to give 18 as a brown solid (0.023 g, 0.27
mmol, 86%).
Found: C, 63.58; H, 6.39; N, 3.57. Mp ) 216-218 °C. MS (EI,
70 eV): m/z 863 (M+). UVI/V E1/2 ) -0.34 V, UV/IV E1/2 ) -1.84
V (vs [(C5H5)2Fe]+/0 in THF/0.1 M [Bu4N][B(3,5-(CF3)2-C6H3)4]).
Method B - Nitrile Insertion. A 125 mL side-arm flask equipped
with a stir bar was charged with (C5Me5)2U()N-iPr2-C6H3)(I) (10)
(0.25 g, 0.31 mmol) and toluene (50 mL). Ph2Mg(THF)2 (0.20 g,
0.62 mmol) was added followed by 1,4-dioxane (∼0.25 mL), and
the reaction was stirred for 12 h at room temperature. The resulting
dark brown solution was filtered though a Celite-padded coarse frit.
Benzonitrile (0.038 g, 0.37 mmol) was added to the dark brown
filtrate as a solution in toluene (5 mL), and the resulting reaction
was stirred at room temperature. After 12 h, the reaction was filtered
though a Celite-padded coarse frit and the volatiles were removed
from the filtrate. The crude product was extracted into hexanes (50
mL), filtered through a Celite-padded coarse frit, and volatiles were
removed under reduced pressure to give 18 as a brown solid (0.025
g, 0.28 mmol, 91%).
Acknowledgment. For financial support of this work, we
acknowledge LANL (Director’s PD Fellowship to C.R.G.
and E.J.S.; Frederick Reines PD Fellowship to E.J.S.), the
LANL G. T. Seaborg Institute (PD Fellowships to C.R.G.
and E.J.S.; summer graduate student fellowship to A.E.V.),
the Division of Chemical Sciences, Office of Basic Energy
Sciences, Heavy Element Chemistry program, and the LANL
Laboratory Directed Research & Development program. This
work was carried out under the auspices of the National
Nuclear Security Administration of the US. Department of
Energy at Los Alamos National Laboratory under Contract
DE-AC5206NA25396.
X-ray quality samples of 18 were obtained by recrystallization
1
from a concentrated hexane solution at -30 °C. H NMR (C6D6,
Supporting Information Available: Magnetic susceptibility data
for 11-15 and 18 and crystallographic data for 14 and 18 (PDF,
CIF). This material is available free of charge via the Internet at
http://pubs.acs.org.
298 K): δ 46.50 (b, 1H, CH(Me)2), 28.79 (1H, Ar-H), 23.81 (1H,
Ar-H), 21.69 (1H, Ar-H), 11.40 (6H, CH(Me)2), 9.61 (3H, Ar-H),
7.92 (4H, Ar-H), 1.58 (30H, (C5Me5)2), -0.58 (2H, Ar-H), -2.38
(6H, CH(Me)2), -8.17 (b, 1H, CH(Me)2), -18.30 (1H, Ar-H). Anal.
Calcd for C45H57N2U (mol. wt. 863.98): C, 62.56; H, 6.65; N, 3.24.
IC8017375
Inorganic Chemistry, Vol. 47, No. 24, 2008 11891