Tetravalent and pentavalent uranium acetylide complexes prepared by oxidative functionalization with CuC≡CPh

Article abstract of DOI:10.1021/om800466m

Oxidation of (C₅Me₅)₂U(NPh ₂)(THF) and (C₅Me₅)₂U(=N-2,6- ⁱPr₂-C₆H₃)(THF) with CuC≡CPh yields the corresponding U^IV and U^V acetylide complexes (C₅Me₅)₂U(NPh₂)(C≡CPh) and (C₅Me₅)₂U(=N-2,6-ⁱPr ₂-C₆H₃)(C≡CPh), respectively. The complexes were characterized using a combination of ¹H NMR, X-ray crystallography, UV-visible-near-IR spectroscopy, and cyclic voltammetry.

Full text of DOI:10.1021/om800466m

Organometallics 2008, 27, 3335–3337
3335
Tetravalent and Pentavalent Uranium Acetylide Complexes
Prepared by Oxidative Functionalization with CuCt CPh
Christopher R. Graves, Brian L. Scott, David E. Morris,* and Jaqueline L. Kiplinger*
Los Alamos National Laboratory, Los Alamos, New Mexico 87545
ReceiVed May 22, 2008
crystals of 2 suitable for X-ray diffraction were not obtained,
1H NMR, elemental analysis, mass spectrometry, cyclic volta-
mmetry, and UV-visible-near-IR absorption spectroscopy data
are consistent with the formulation of 2 as (C5Me5)2U(dN-2,6-
iPr2-C6H3)(Ct CPh). To the best of our knowledge, complex 2
represents the first example of a pentavalent uranium complex
with an anionic carbon ligand other than a carbocyclic (C5R5,
C7H7, C8H8) ligand.6
Summary: Oxidationof(C5Me5)2U(NPh2)(THF)and(C5Me5)2U(dN-
2,6-iPr2-C6H3)(THF) with CuCt CPh yields the corresponding
U
IV and UV acetylide complexes (C5Me5)2U(NPh2)(Ct CPh) and
(C5Me5)2U(dN-2,6-iPr2-C6H3)(Ct CPh), respectiVely. The com-
1
plexes were characterized using a combination of H NMR,
X-ray crystallography, UV-Visible-near-IR spectroscopy, and
cyclic Voltammetry.
Despite the key role that the metal alkynyl functionality plays
in various homogeneous catalytic processes promoted by
organoactinide complexes,1 only a few examples of actinide
complexes exhibiting a terminal acetylide (AnCt CR) unit have
been isolated and structurally characterized.2,3 This absence is
further underscored when contrasted with the vast number of
known transition-metal acetylide complexes.4 We recently
demonstrated that copper(I) salts are ideal reagents for the
preparation of functionalized UV imido complexes from UIV
imido precursors.5 Given the success of this reaction manifold,
we envisioned that organometallic copper acetylide reagents
could also be employed in a similar fashion to generate
uranium-carbon bonds. Herein, we report a simple procedure
for the synthesis of stable tetravalent and pentavalent uranium
terminal acetylide complexes using copper(I) phenylacetylide
(CuCt CPh).
1
The H NMR spectrum of 2 has diagnostic features compa-
rable to those recorded for the UV imido halides (C5Me5)2U(dN-
2,6-iPr2-C6H3)(X) (X ) F, Cl, Br, I):5b a broad signal (∆ν1/2
)
80 Hz) corresponding to the C5Me5 ligand protons and inequiva-
lent iPr groups. At δ 4.11 ppm, the C5Me5 signal for 2 intersects
the respective C5Me5 resonances for the halide complexes,7
indicating that the acetylide ligand is commensurate in donating
ability with the fluoride ligand.5b This observation is in accord
with electrochemical findings discussed below.
As depicted in eq 1, reaction of (C5Me5)2U(dN-2,6-iPr2-
C6H3)(THF) (1) and CuCt CPh at 75 °C for 12 h produced the
UV acetylide complex (C5Me5)2U(dN-2,6-iPr2-C6H3)(Ct CPh)
(2) as a dark brown solid in good isolated yield. Although single
The generality of this Cu-based oxidative functionalization
extends beyond the UIV imido system, as demonstrated by the
synthesis of the UIV acetylide (C5Me5)2U(NPh2)(Ct CPh) (4)
from the UIII amide complex (C5Me5)2U(NPh2)(THF) (3) (eq
2). This is the first application of any Cu(I)-oxidation protocol
to the UIV/UIII redox couple. While elevated temperatures were
needed for the synthesis of 2 from 1, the analogous oxidation
of 3 to 4 proceeded smoothly at ambient temperature under
otherwise identical reaction conditions in excellent yield.
* To whom correspondence should be addressed. E-mail: kiplinger@
lanl.gov (J.L.K.).
(1) Terminal acetylides have been proposed to be key intermediates in
various catalytic processes utilizing actinide organometallic complexes, such
as hydrosilylation, alkyne dimerization/oligomerization, and hydroamination.
For a detailed account, see: Burns, C. J.; Eisen, M. S. Homogeneous and
Heterogeneous Catalytic Processes Promoted by Organoactinides. In The
Chemistry of the Actinide and Transactinide Elements, 3rd ed.; Morss, L. R.,
Edelstein, N. M., Fuger, J., Eds.; Springer: Dordrecht, The Netherlands,
2006; pp 2911-3012, and references therein.
(2) (a) Atwood, J. L.; Hains, C. F., Jr.; Tsutsui, M.; Gebala, A. E.
J. Chem. Soc., Chem. Commun. 1973, 452–453. (b) Atwood, J. L.; Tsutsui,
M.; Ely, N.; Gebala, A. E. J. Coord. Chem. 1976, 5, 209–215. (c) Boaretto,
R.; Roussel, P.; Kingsley, A. J.; Munslow, I. J.; Sanders, C. J.; Alcock,
N. W.; Scott, P. Chem. Commun. 1999, 1701–1702.
(3) A handful of lanthanide acetylide complexes have been prepared
and structurally characterized: (a) Boncella, J. M.; Tilley, T. D.; Andersen,
R. A J. Chem. Soc., Chem. Commun. 1984, 710–712. (b) Evans, W. J.;
Ulibarri, T A.; Chamberlain, L. R.; Ziller, J. W., Jr. Organometallics 1990,
9, 2124–2130. (c) Evans, W. J.; Keyer, R. A.; Ziller, J. W. Organometallics
1993, 12, 2618–2633. (d) Tazelaar, C. G. J.; Bambirra, S.; van Leusen, D.;
Meetsma, A.; Hessen, B.; Teuben, J. H. Organometallics 2004, 23, 936–
939.
(4) Manna, J.; John, K. D.; Hopkins, M. D. AdV. Organomet. Chem.
1995, 38, 79–154.
(5) (a) Graves, C. R.; Scott, B. L.; Morris, D. E.; Kiplinger, J. L. J. Am.
Chem. Soc. 2007, 129, 11914–11915. (b) 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.
Single crystals of complex 4 suitable for X-ray diffraction
were obtained from the slow evaporation of a concentrated
pentane solution at ambient temperature. As shown in Figure
1, 4 features a bent-metallocene framework with the amide and
acetylide ligands contained within the metallocene wedge. At
2.409(4) Å, the U-Cacetylide bond distance observed in 4 falls
within the range of bond distances observed for the three other
structurally characterized UIV acetylide complexes. For example,
the (C5H5)3U(Ct CR) system has U-Cacetylide bond lengths of
10.1021/om800466m CCC: $40.75
2008 American Chemical Society
Publication on Web 07/02/2008

3336 Organometallics, Vol. 27, No. 14, 2008
Communications
Figure 1. Molecular structure of 4 with thermal ellipsoids projected
at the 50% probability level. Hydrogen atoms have been omitted
for clarity. Selected bond distances (Å) and angles (deg): U(1)-N(1),
2.298(4);U(1)-C(33),2.409(4);C(33)-C(34),1.197(6);N(1)-U(1)-
C(33), 112.38(15); U(1)-C(33)-C(34), 176.9(4).
Figure2.Cyclicvoltammogramsfor2and4in∼0.1M[nBu4N][B(1,3-
(CF3)2-C6H3)4]/THF at a Pt working electrode at 200 mV/s. Half-
wave potentials (in parentheses) were determined from peaks in
the square-wave voltammograms (not shown).
2.33-2.36 Å (R ) H)2b and 2.33(2) Å (R ) Ph),2a while the
U-Cacetylide bond distance in the strained uranium metallacycle
UV complex 2. In contrast, the lesser electron donation from
the amide ligand in 4 permits the observation of the UIV/III redox
couple, but the UVI/V couple is not accessible.
t
U[N(CH2CH2NSiMe2 Bu)3](Ct C-4-Me-C6H4) is 2.468(7) Å.2c
At 176.9(4)° and 1.197(6) Å, the U-Ct C angle and Ct C bond
distance in 4 are consistent with the parameters reported for
the (C5H5)3U(Ct CR) systems2a,b and analogous transition-metal
complexes.4
The IR spectra of 2 and 4 were obtained, marking the first
report of Ct C stretches for an actinide acetylide system. Both
complexes display similar spectra in the high-energy region with
bands at 2056 cm-1 (for 2) and 2062 cm-1 (for 48) that can be
assigned to the Ct C stretch by analogy to transition-metal
acetylide complexes.4
Cyclic voltammetric data for 2 and 4 (Figure 2) are consistent
with data reported previously for structurally related pentavalent5
and tetravalent9,10 uranium metallocene complexes, but the
comparison here vividly illustrates the impact of the different
ligand sets on the redox energetics for these two oxidation states.
The much greater electron-donating ability of the imide ligand
in 2 stabilizes the UVI oxidation state, enabling the observation
of the UVI/V redox process while concomitantly destabilizing
the UV/IV couple, shifting it to quite negative potential relative
to that found in most UIV systems, including the acetylide
complex 4. No further metal-based couples are resolved for the
The separation between the adjacent metal-based redox
couples (∼1.5 V for 2 and ∼2.1 V for 4) is the same as that
foundforawiderangeofUV andUIV complexes,respectively.5,9,10
On an absolute potential basis, both redox processes for 2 occur
at values slightly more negative (∼0.1-0.2 V) than those for
(C5Me5)2U(dN-2,6-iPr2-C6H3)(X) (X ) Cl, Br, I),5b indicating
that the acetylide ligand is a somewhat stronger electron donor
than these halides. The potentials for the metal-based couples
in 4 are comparable to those found in UIV hydrazonato
complexes (C5Me5)2U[η2-(N,N′)-CH3-N-NdCPh2](X) (X )
OSO2CF3, Cl) and ∼0.25 V more positive than for UIV ketimide
complexes (e.g., (C5Me5)2U[-NdC(Ar)2]2).9,11
The electronic absorption spectral data for 2 and 4 also
provide an informative contrast in electronic structure (Figure
3). In both systems the open-shell 5f electronic configurations
give rise to highly characteristic near-infrared intraconfiguration
transitions with narrow line widths and relatively low extinction
coefficients (ε) commonly associated with f-f transitions. Both
complexes also possess broader, more intense spectral bands
in the visible and UV region that derive from metal-ligand
charge-transfer and ligand-localized excited states. The spectral
properties of 2 are very similar to those of other UV imido
complexes, and the assignments are described in detail else-
where.5 The much simpler f-f spectral region (compared to
that for 4) is a consequence of the 5f1 electronic configuration
that gives rise to only two term levels (2F5/2 ground-state
(6) For a selection of pentavalent organouranium complexes with
carbocyclic aromatic ligands see: (a) Boisson, C.; Berthet, J.-C.; Lance,
M.; Viger, J.; Nierlich, M.; Ephritikhine, M. J. Chem. Soc., Dalton Trans.
1996, 947–953. (b) Gourier, D.; Caurant, D.; Arliguie, T.; Ephritikhine,
M. J. Am. Chem. Soc. 1998, 120, 6084–6092. (c) Boaretto, R.; Roussel, P.;
Alcock, N. W.; Kingsley, A. J.; Munslow, I. J.; Sanders, C. J.; Scott, P. J.
Organomet. Chem. 1999, 591, 174–184. (d) Arliguie, T.; Rourmigue, M.;
Ephritikhine, M. Organometallics 2000, 19, 109–111. (e) 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: Dordrecht, The Netherlands,
2006; pp 2799-2910, and references therein.
2
manifold and F7/2 excited-state manifold) from spin-orbit
coupling. The spectral data for 4 are most like those found for
UIV hydrazonato complexes having a 5f2 valence electronic
configuration.9
(7) The chemical shifts for the C5Me5 protons in the 1H NMR spectrum
for (C5Me5)2U(dN-2,6-iPr2-C6H3)(X) in C6D6 are as follows: δ 3.94 ppm
(X ) F), 4.92 ppm (X ) Cl), 5.31 ppm (X ) Br), and 5.78 ppm (X ) I).
(8) A second stretch was observed in the IR spectrum of complex 4 at
The most interesting aspect of the spectroscopic data for 2
and 4 is the oscillator strength of the f-f bands. The f-f
transitions are parity-forbidden in centrosymmetric systems, and
although bent-metallocene complexes of uranium having the
generic formulation (C5Me5)2U(L)2 have much lower sym-
2189 cm-1. Although we cannot rule out this band as an additional Ct
C
stretching mode, it’s usually high energy makes this assignment unlikely.
(9) Morris, D. E.; Da Re, R. E.; Jantunen, K. C.; Castro-Rodriguez, I.;
Kiplinger, J. L. Organometallics 2004, 23, 5142–5153.
(10) 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.
(11) Kiplinger, J. L.; John, K. D.; Morris, D. E.; Scott, B. L.; Burns,
C. J. Organometallics 2002, 21, 4306–4308.

Communications
Organometallics, Vol. 27, No. 14, 2008 3337
pronounced in the spectrum of 2, for which the lowest energy
bands assigned to metal imide ligand charge-transfer transitions5
tail into the near-IR region, and for which the most intense f-f
band at ∼6100 cm-1 has ε ) 650 M-1 cm-1. The enhanced
intensities in these f-f bands have been attributed to mixing of
these higher lying charge-transfer excited states into the ligand-
field states,5,9,10 and these new data for 2 and 4 add to the
growing body of evidence of the importance of energetic
proximities of these higher lying states to enhance the mixing
into the f-f states.5,9,11
In conclusion, using CuCt CPh we have shown that Cu-based
oxidative functionalization is applicable to both UIII and UIV
oxidation states through the synthesis of the respective UIV
amide and UV imide acetylide complexes. The isolation of the
UV acetylide complex reported in this work clearly demonstrates
that pentavalent uranium can support anionic carbon ligands
other than carbocycles and, therefore, may serve as a useful
platform for the generation of carbon-based functional groups
on uranium.
Acknowledgment. For financial support of this work, we
acknowledge the LANL (Director’s PD Fellowship to
C.R.G.), the LANL G. T. Seaborg Institute (PD Fellowship
to C.R.G.), the LANL LDRD program, and the Division of
Chemical Sciences, Office of Basic Energy Sciences, Heavy
Element Chemistry program. We also thank Dr. Eric J.
Schelter (LANL) for assistance with collection of the IR data
and Dr. Kevin D. John (LANL) for useful discussions
regarding the IR of these complexes.
Figure 3. UV-visible (top) and near-IR (bottom) spectral data for
2 and 4 in toluene solution at room temperature. The near-IR spectra
have been baseline-corrected to remove the contribution from the
higher energy tails of the visible bands.
metries, the f-f transitions are typically still quite weak. For
example, in (C5Me5)2UCl2, typical ε values for the f-f bands
are 20-50 M-1cm-1.9 In the f-f spectra of 2 and 4 we find
substantially greater extinction coefficients that correlate with
the intensities and energetic proximity of the visible charge-
transfer and ligand-based electronic transitions. This is more
Supporting Information Available: Text, tables, figures, and
CIF files giving full experimental and characterization details and
crystallographic data for 2-4. This material is available free of
charge via the Internet at http://pubs.acs.org.
OM800466M