Challenging the metallocene dominance in actinide chemistry with a soft PNP pincer ligand: New uranium structures and reactivity patterns

Full text of DOI:10.1002/anie.200806115

Angewandte
Chemie
DOI: 10.1002/anie.200806115
Actinide Chemistry
Challenging the Metallocene Dominance in Actinide Chemistry with a
Soft PNP Pincer Ligand: New Uranium Structures and Reactivity
Patterns**
Thibault Cantat, Christopher R. Graves, Brian L. Scott, and Jaqueline L. Kiplinger*
Cyclopentadienyl-based ligand sets, in particular the bis(pen-
tamethylcyclopentadienyl) ligand, are enormously successful
frameworks in actinide chemistry and no alternative has
proved as fruitful to date.[1] However, the bis(C5Me5) frame-
work suffers from inconvenient drawbacks such as the chronic
release of the (C5Me5)2 dimer upon oxidation of
“[(C5Me5)2U]” complexes.[2] Inspired by the ability of ligands
that combine both hard and soft coordination environments
to stabilize highly reactive functional groups (such as
alkylidenes, imides, and phosphinidenes) on transition
metals and lanthanides,[3] we have extended this strategy to
the actinides by using the monoanionic bis[2-(diisopropyl-
phosphino)-4-methylphenyl]amido (PNP) ligand. Compared
to the C5Me5 ligand, which only displays h5-coordination to
uranium,[4] this soft PNP pincer ligand adopts a variety of
coordination modes that range from k2-(P,N) to k3-(P,N,P’)
and provides more steric crowding and greater electronic
density at the metal center.[5] By capitalizing on this versa-
tility, we now demonstrate by using [(PNP)2U(I)] (1) and
[(C5Me5)2U(I)(thf)] (2) that the PNP ligand not only pro-
motes unprecedented reactivity patterns for low-valent ura-
nium but also supports new structures for the actinide series
that are not available with the C5Me5 ligand framework.
Trivalent [(C5Me5)2U(I)(thf)] (2) in the presence of Na/Hg
amalgam or KC8, a classic uranium(II) synthetic equivalent, is
known to provide up to four reducing equivalents.[6] As such,
initial studies with 1 were targeted toward establishing the
capacity of the bis(PNP) platform to support low-valent
uranium chemistry [Eq. (1)]. Treatment of a cold (À358C)
solution of 1 and KC8 in toluene with hexachloroethane
resulted in an immediate color change from deep green to
bright red and formation of the known uranium(IV)
[(PNP)2UCl2] complex[5]; following workup the yield of the
isolated product was 96%. These observations clearly dem-
onstrate that the PNP ligand can indeed support low-valent
uranium with the 1/KC8 system able to function as a new
uranium(II) synthon.
The ability of the bis(PNP) platform to promote new
uranium chemistry was validated by comparing the reactions
of 1/KC8 and 2/KC8 with diphenyldiazomethane. Reaction of
=
1/KC8 with Ph2C N2 afforded the hydrazonido complex 4,
which was isolated in 98% yield [Eq. (2)]. To the best of our
knowledge, complex 4 represents the first example of an
2
2À
= À =
actinide hydrazonido [h -(N,N’) N N CR2] complex and
the first example of such a ligand stabilized by a single metal
ion.[7] Figure 1 shows the molecular structure of complex 4,
2
2À
= À =
which reveals that the [h -(N,N’) N N CPh2] ligand is
stabilized by both a short U N double bond (U(1)–N(4) =
=
2.097(5) ꢀ) and an additional U–N dative interaction (U(1)–
=
N(3) = 2.370(5) ꢀ). The U N linkage is substantially shorter
than a uranium(IV)–amide bond (ca. 2.3 ꢀ) and compares
well with the few structurally characterized UIV complexes
that contain an imido functional group (1.95 ꢀ).[8]
[*] Dr. T. Cantat, Dr. C. R. Graves, Dr. B. L. Scott, Dr. J. L. Kiplinger
Los Alamos National Laboratory
2
2À
= À =
The hydrazonido [h -(N,N’) N N CPh2]
ligand is
formed by the two-electron reduction of diphenyldiazome-
Mail Stop J514, Los Alamos, NM 87545 (USA)
Fax: (+1)505-667-9905
E-mail: Kiplinger@lanl.gov
À
=
thane. The N N bond of the reduced Ph2C N2 unit (N(3)–
N(4) 1.359(7) ꢀ) is substantially elongated compared to that
in uncomplexed diazoalkanes (1.12–1.13 ꢀ)[9] and is slightly
[**] We acknowledge LANL (Director’s PD Fellowships to T.C. and
C.R.G.), G.T. Seaborg Institute for Transactinium Science (PD
Fellowship to C.R.G.), and the Heavy Element Chemistry Program,
Division of Chemical Sciences, Office of Basic Energy Sciences for
financial support of this work. We thank Anthony Mancinco for the
design of the inside cover art.
2
À
=
longer than that reported for the [h -(N,N’)-N-N CPh2]C
ligand in
[(tBuArO)3(tacn)U{h2-(N,N’)-N-N-CPh2}]
(1.338(5) ꢀ; tacn = 1,4,7-tris(3,5-di-tert-butyl-2-hydroxyben-
zyl)-1,4,7-triazacyclononane).[10] In fact, the N(3)–N(4) bond
distance in 4 is longer than those observed for the imido
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.200806115.
= À =
=
ligands in [(C5Me5)2U( N N CPh2)( N-2,4,6-tBu3-C6H2)]
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[11]
and other [(C5Me5)2UVI]
=
=
N CPh2)( N-2,4,6-tBu3-C6H2)]
[14]
=
bis(imido) complexes (average U N = 1.98 ꢀ).
The mechanisms of these two transformations were
probed to determine the origin of the difference in reactivity
between the [(PNP)2U(I)] (1) and [(C5Me5)2U(I)(thf)] (2)
systems. Compound 2 is known to be a two-electron reductant
and the active species in the reductive cleavage of azobenzene
=
(to give [(C5Me5)2U( NPh)2]) and the reductive coupling of
2
=
diphenylacetylene (to give [(C5Me5)2U{k -(C,C’)-CPh CPh-
CPh CPh}]. In the absence of KC8 or Na/Hg amalgam,
[6]
=
=
complex 2 reacts with one equivalent of Ph2C N2 to produce
0.5 equivalents of 5 and 0.5 equivalents of [(C5Me5)2U(I)2] (6)
[Eq. (4)].
Figure 1. Molecular structure of complexes 4 (a) and 5 (b) with
thermal ellipsoids projected at the 50% probability level. Hydrogen
atoms and methyl groups on the PNP ligands are omitted for clarity.
Selected bond distances [ꢀ] and angles [8] for 4: U(1)–N(3) 2.375(5),
U(1)–N(4) 2.097(5), N(3)–N(4) 1.359(7), N(3)–C(53) 1.302(8). For 5:
U(1)–N(1) 1.999(4), U(1)–N(3) 2.018(4), N(1)–N(2) 1.315(6), N(3)–
N(4) 1.324(5), N(2)–C(21) 1.314(6), N(4)–C(34) 1.305(6); N(3)-U(1)-
N(1) 103.26(16).
=
Similarly, in the absence of KC8, addition of Ph2C N2 to
one equivalent of 1 yielded a 50:50 mixture of 4/(PNP)2U(I)2
[15]
(1.308(8) ꢀ)[11] and in 5 (1.315(6), 1.324(5) ꢀ, see below). This
(7) to leave 0.5 equivalents of Ph2C N2 unreacted [Eq. (5)].
=
=
bond lengthening is accompanied by a shortening of the C N
bond from 1.32–1.33 ꢀ in uncomplexed diazoalkanes[9] to
1.302(8) ꢀ for C(53)–N(3) in 4. The same shortening is
= À =
=
observed in [(C5Me5)2U( N N CPh2)( N-2,4,6-tBu3-C6H2)]
(1.310(10) ꢀ)[11] and in 5 (1.314(6), 1.305(6) ꢀ), but not for
the radical anion ligand in [(tBuArO)3(tacn)U{h2-(N,N’)-N-
The combination of these observations suggests that the
active species are the uranium(III) complexes 1 and 2, which
can accomplish the two-electron reduction of the substrate.
As such, the role of the exogenous reductant (KC8) is merely
to regenerate the active uranium(III) species by reducing 6 to
2 and 7 to 1. The difference in the observed reactivity is likely
to arise from the different redox behavior of the uranium(IV)
complexes obtained by reaction with the diphenyldiazo-
2
=
N-CPh2}] (1.333(6) ꢀ), as expected for a [h -(N,N’)-N-N
CPh2]·À radical anion compared to a [h -(N,N’) N N CPh2]
2
2À
= À =
dianion.[10] Finally, the coordination sphere of the uraniu-
m(IV) center[12] in 4 is completed by two PNP ligands in k3-
(P,N,P’) and k2-(P,N) coordination modes.[13] This is the first
example of such a coordination environment for the bis(PNP)
framework and illustrates the flexibility of this ligand set to
accommodate various electronic and steric environments
which are needed to enable new reactivity patterns for
uranium.
= À =
methane oxidant (that is, [(C5Me5)2U N N CPh2] versus
2
= À =
(PNP)2U[h -(N,N’) N N CPh2] (4)).
To determine whether the reduction chemistry with 1 was
limited to the formation of uranium(IV) complexes, its
reaction with stronger oxidants was explored. A dramatic
distinction between PNP and C5Me5 as supporting ligands is
seen in the reactivity of 1 and 2 with pyridine-N-oxide.
Addition of one equivalent of pyridine-N-oxide to a solution
of 1 in toluene led to the complete consumption of pyridine-
N-oxide and formation of pyridine, 0.5 equivalents of 7, and
0.5 equivalents of the new uranium(VI) complex 8, which
showed that two equivalents of the oxidant had reacted. In
fact, reaction of 1/KC8 with two equivalents of pyridine-N-
oxide smoothly produced complex 8 as the only uranium
complex, which was isolated in 85% yield [Eq. (6)]. This
Indeed, different chemistry is observed in the reaction of
=
2/KC8 and Ph2C N2, which gives the bis(imido) uranium(VI)
complex 5 in 94% yield of isolated product; two equivalents
of the oxidant are needed for the completion of the reaction
[Eq. (3)]. The molecular structure of 5 features two imido
ligands obtained by the formal two electron reduction of each
diphenyldiazomethane (Figure 1). As noted for 4, this reduc-
À
tion results in a significant lengthening of the N N bonds and
=
=
a shortening of the C N bonds of the diazoalkane. The U N
À
double bonds are characterized by short U N bond distances
(U(1)–N(1) = 1.999(4), U(1)–N(3) = 2.018(4) ꢀ), which are
= À
in agreement with those found in the related [(C5Me5)2U( N
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transformation decisively illustrates the ability of 1 to act as a
four-electron reductant and to support uranium(VI).
Single-crystal X-ray diffraction confirmed that 8 consists
of a uranyl moiety supported by two PNP ligands in k3-
(P,N,P’) and k2-(P,N) coordination modes (Figure 2), which
exciting alternative for promoting reaction chemistry and
structural motifs at uranium that are simply not available for
the C5Me5 ligand set. As evidenced by the formation of the
uranyl complex 8, the PNP ligand is able to support UVI and
efforts are now directed towards exploiting systems such as 4
as entries to actinide metallanitrene and nitride chemistry.[19]
Received: December 15, 2008
Published online: February 16, 2009
Keywords: actinides · hydrazonido compounds · metallocenes ·
.
N,P ligands · uranium
[1] a) T. J. Marks, A. Streitwieser in The Chemistry of the Actinide
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Chapman and Hall, New York, 1986, pp. 1547 – 1587; b) C. J.
Burns, M. S. Eisen in The Chemistry of the Actinide and
Transactinide Elements, Vol. 5 (Eds.: L. R. Morss, N. M. Edel-
stein, J. Fuger), Springer, Dordrecht, 2006, pp. 2799 – 2910.
[2] For key references on the decomposition of low-valent uranium–
bis(C5Me5) complexes in oxidation reactions, see: a) P. B. Duval,
C. J. Burns, D. L. Clark, D. E. Morris, B. L. Scott, J. D. Thomp-
son, E. L. Werkema, L. Jia, R. A. Andersen, Angew. Chem. 2001,
113, 3461 – 3465; Angew. Chem. Int. Ed. 2001, 40, 3357 – 3361;
b) J. Maynadie, J.-C. Berthet, P. Thuery, M. Ephritikhine, Chem.
Commun. 2007, 486 – 488.
Figure 2. Molecular structure of complex 8 with thermal ellipsoids
projected at the 50% probability level. Hydrogen atoms and methyl
groups are omitted for clarity. Selected bond distances [ꢀ] and angles
[8]: U(1)–P(1) 3.066(3), U(1)–P(3) 3.017(3), U(1)–P(4) 2.984(3), U(1)–
O(1) 1.791(6), U(1)–O(2) 1.808(6), U(1)–N(1) 2.405(7), U(1)–
N(2) 2.402(8); O(1)-U(1)-O(2) 177.0(3).
[3] a) O. V. Ozerov in The Chemistry of Pincer Compounds (Eds.:
D. Morales-Morales, C. Jensen), Elsevier Science, Amsterdam,
2007, p. 287; b) J. D. Masuda, K. C. Jantunen, O. V. Ozerov,
K. J. T. Noonan, D. P. Gates, B. L. Scott, J. L. Kiplinger, J. Am.
Chem. Soc. 2008, 130, 2408 – 2409; c) J. Scott, F. Basuli, A. R.
Fout, J. C. Huffman, D. J. Mindiola, Angew. Chem. 2008, 120,
8630 – 8633; Angew. Chem. Int. Ed. 2008, 47, 8502 – 8505.
À
makes it the first uranyl phosphine complex. The U P bond
lengths in 8 (U(1)–P(1) = 3.066(3), U(1)–P(3) = 3.017(3),
U(1)–P(4) = 2.984(3) ꢀ) compare well with those reported
=
for
[U(I)2( NtBu)2(thf)(PMe3)2]
(U–P = 3.075(3),
3.042(2) ꢀ).[16] The uranium–oxo bond lengths are short,
with U(1)–O(1) = 1.791(6) ꢀ and U(1)–O(2) = 1.808(6) ꢀ,
which is typical for uranyl complexes.[17] It is noteworthy
that the formation of 8 represents only the second rational
synthesis of a uranyl fragment from a low-valent uranium
precursor; the other reported example relies on the oxidation
[4] Actinide–bis(C5Me5) complexes adopt
a bent metallocene
structural motif, with the exception of a few recent examples
of uranium linear metallocene complexes, see: a) A. Maynadiꢁ,
J. C. Berthet, P. Thuery, M. Ephritikhine, J. Am. Chem. Soc.
2006, 128, 1082 – 1083; b) J. Maynadiꢁ, N. Barros, J.-C. Berthet, P.
Thuery, L. Maron, M. Ephritikhine, Angew. Chem. 2007, 119,
2056 – 2058; Angew. Chem. Int. Ed. 2007, 46, 2010 – 2012.
[5] T. Cantat, B. L. Scott, O. V. Ozerov, D. E. Morris, J. L. Kiplinger,
Inorg. Chem. 2009, 48, 2114 – 2127.
[6] a) B. P. Warner, B. L. Scott, C. J. Burns, Angew. Chem. 1998, 110,
1005 – 1007; Angew. Chem. Int. Ed. 1998, 37, 959 – 960; b) J. M.
Manriquez, P. J. Fagan, T. J. Marks, S. H. Vollmer, C. S. Day,
V. W. Day, J. Am. Chem. Soc. 1979, 101, 5075 – 5078; c) P. J.
Fagan, J. M. Manriquez, T. J. Marks, C. S. Day, S. H. Vollmer,
of
[U(I)3(thf)3]
using
pyridine-N-oxide
to
form
[UO2(I)2(py)3].[18]
Finally, for purposes of comparison, the same reaction
using 2/KC8 instead of 1 was found to produce the dimer
(C5Me5)2 as the only C5Me5-containing compound [Eq. (7)].
V. W. Day, Organometallics 1982, 1, 170 – 180.
2
[7] Hydrazonido [h -(N,N’)-N-N CR2]2À complexes are only known
=
for the transition metals, in these complexes the ligand is always
bridging. For representative examples, see: a) L. K. Bell, W. A.
Herrmann, G. W. Kriechbaum, H. Pfisterer, M. L. Ziegler, J.
Organomet. Chem. 1982, 240, 381 – 393; b) S. Gambarotta, C.
Floriani, A. Chiesivilla, C. Guastini, J. Am. Chem. Soc. 1983, 105,
7295 – 7301; c) D. Nucciarone, N. J. Taylor, A. J. Carty, Organo-
metallics 1986, 5, 2565 – 2567; d) W. A. Herrmann, B. Menjon, E.
Herdtweck, Organometallics 1991, 10, 2134 – 2141; e) V. Taber-
nero, T. Cuenca, E. Herdtweck, J. Organomet. Chem. 2002, 663,
173 – 182; f) R. J. Wright, M. Brynda, J. C. Fettinger, A. R.
Betzer, P. P. Power, J. Am. Chem. Soc. 2006, 128, 12498 – 12509.
[8] a) D. S. J. Arney, C. J. Burns, J. Am. Chem. Soc. 1995, 117, 9448 –
9460; b) C. R. Graves, P. Yang, S. A. Kozimor, A. E. Vaughn,
D. L. Clark, S. D. Conradson, E. J. Schelter, B. L. Scott, J. D.
Thompson, P. J. Hay, D. E. Morris, J. L. Kiplinger, J. Am. Chem.
Soc. 2008, 130, 5272 – 5285.
This finding implies the generation of an unstable hexavalent
À
=
(C5Me5)2U( O)2 complex that is reduced by the C5Me5
ligand to produce (C5Me5)2 and uranium oxides.[2a,8] In
contrast, the stability of 8 reinforces the hypothesis that the
PNP ligand, as a better donor and a more flexible ligand than
C5Me5, can support structures that are simply not accessible
with the bis(C5Me5) ligand framework.
In conclusion, the work presented here demonstrates that
the PNP ligand is capable of supporting not only low-valent
uranium but also high-valent species and represents an
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Communications
=
10069; b) [(C5Me5)2U( NAd)2] (Ad = 1-adamantyl), U–N =
[9] S. Patai, The Chemistry of Diazonium and Diazo Groups, Parts 1
=
=
and 2, Wiley, New York, 1978.
1.94(2), 1.96(2) ꢀ: reference [6a]; c) [(C5Me5)2U( O)( N-2,6-
[10] O. P. Lam, P. L. Feng, F. W. Heinemann, J. M. OꢂConnor, K.
Meyer, J. Am. Chem. Soc. 2008, 130, 2806 – 2816.
[11] J. L. Kiplinger, D. E. Morris, B. L. Scott, C. J. Burns, Chem.
Commun. 2002, 30 – 31.
iPr2C6H3)], U–N = 1.988(4) ꢀ: D. S. J. Arney, C. J. Burns, J. Am.
= À =
Chem. Soc. 1993, 115, 9840 – 9841; d) [(C5Me5)2U( N N
=
CPh2)( N-2,4,6-tBu3-C6H2)], U–N = 2.031(6), 1.987(5) ꢀ: refer-
ence [11].
[12] The solution magnetic moment of 4 was determined in C6D6
(2.99 mB) by using Evansꢂ method and confirmed the UIV
oxidation state of the metal center. M. L. Naklicki, C. A.
White, L. L. Plante, C. E. B. Evans, R. J. Crutchley, Inorg.
Chem. 1998, 37, 1880 – 1885.
[15] Complex 7 was independently synthesized (isolated in 100%
yield) by oxidation of 1 with iodine. See the Supporting
Information for further details.
[16] T. W. Hayton, J. M. Boncella, B. L. Scott, E. R. Batista, P. J. Hay,
J. Am. Chem. Soc. 2006, 128, 10549 – 10559.
[17] J. J. Katz, L. R. Morss, G. T. Seaborg, Summary and Comparative
Aspects of the Actinide Elements, Chapman & Hall, London,
1986.
[13] Retention of the k3-(P,N,P’) and k2-(P,N) coordination modes for
the two PNP ligands in 4 in solution was demonstrated by using
1H and 31P NMR spectroscopy.
[14] A Cambridge Structural Database search identified four bis-
[18] L. Natrajan, F. Burdet, J. Pecaut, M. Mazzanti, J. Am. Chem. Soc.
2006, 128, 7152 – 7153.
=
(C5Me5)–uranium(VI) imido structures with an average U N
bond length of 1.98 ꢀ (range 0.09 ꢀ). For specific references,
[19] D. J. Mindiola, Angew. Chem. 2008, 120, 1580 – 1583; Angew.
Chem. Int. Ed. 2008, 47, 1557 – 1559.
=
see: a) [(C5Me5)2U( NPh)2], U–N = 1.952(7) ꢀ: D. S. J. Arney,
C. J. Burns, D. C. Smith, J. Am. Chem. Soc. 1992, 114, 10068 –
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