Multiple-bond metathesis mediated by sterically pressured uranium complexes

Article abstract of DOI:10.1002/anie.200501667

(Figure Presented) Completing a circle of reactions: Reaction of a U ^V-imido complex (2) with a π acid (e.g., CH₃NC) affords a U^IV-heterocumulene complex (3), which reacts with an alkyl halide to yield the free carbodiimide and corresponding U^IV-halide complex (4). Reduction of complex 4 recovers 1, thereby closing a cycle in which a nitrogen atom is transferred from a U^V-imido complex to a π acid by multiple-bond metathesis.

Full text of DOI:10.1002/anie.200501667

Angewandte
Chemie
Nitrogen-Transfer Chemistry
possible applications for nitrogen-and group-transfer chemis-
try.
DOI: 10.1002/anie.200501667
Multiple-Bond Metathesis Mediated by Sterically
Pressured Uranium Complexes**
Ingrid Castro-Rodríguez, Hidetaka Nakai, and
Karsten Meyer*
In recent years, sterically encumbering ligands have been used
increasingly to synthesize coordinatively unsaturated, highly
reactive metal complexes.[1] The steric constraints imposed by
spectator ligands and chelators often translate into molecular
and electronic structural changes as they directly impact the
coordination mode and metal–ligand orbital interactions; the
latter being particularly important in atom-transfer chemistry.
Relative to complexes with less customized ligands, unsatu-
rated metal ions with molecularly engineered ligand environ-
ments often show altered and increased reactivity as a result
of the increased steric pressure. Sterically tailored, highly
reactive transition-metal complexes achieve a wide variety of
small-molecule activation and atom-transfer chemistry very
successfully.[2–6] The strategy of applying steric pressure,
however, is not always sufficient to achieve certain desired
reactivities. The most recent class of transition-metal-based
nitrogen-transfer reagents (aziridination catalysts), for exam-
ple, takes advantage of high-valent transition-metal nitrido
complexes that readily transfer the imidoacyl entity to olefins
upon activation with trifluoroacetic acid anhydride
We recently reported the reaction of the trivalent
precursor complex [((tBuArO)3tacn)U] (1, (tBuArOH)3tacn =
1,4,7-tris(3,5-di-tert-butyl-2-hydroxybenzyl)-1,4,7-triazacyclo-
nonane)[13] with trimethylsilyl azide (Me3SiN3), which
yielded the azido and imido uranium complexes
[((tBuArO)3tacn)U(L)] (L = N3ꢁ (2) and Me3SiN2ꢁ (3)).[14] We
found that formation of the pentavalent imido complex is
accompanied by evolution of N2, whereas the tetravalent
azido complex is formed by radical elimination of Me3SiC and
subsequent formation of Me6Si2.[14] However, complexes 2
and 3 did not exhibit the desired nitrogen-atom nucleophi-
licity and resulting imido-group transfer chemistry. We now
report a highly reactive imido complex, which can be
synthesized by employing the trivalent [((AdArO)3tacn)U]
(4)[15] with the sterically demanding adamantane (Ad)-
functionalized hexadentate (AdArO)3tacn3ꢁ chelator.
Complex 4 reacts with Me3SiN3 to form the highly reactive
Uv imido complex [((AdArO)3tacn)U(NSiMe3)] (5; Scheme 1,
4!5). In contrast to 2 and 3, this complex is highly reactive
toward p acids, such as CO and methyl isocyanide, to yield the
Uiv isocyanate [((AdArO)3tacn)U(NCO)] (6) and cyanamide
[((AdArO)3tacn)U(NCNCH3)] (7) complexes (Scheme 1, 5!
7).
(TFAA).[7–9] The M N activation with TFAA is indispensable
ꢀ
due to the highly covalent dp–pp interaction, thus resulting in
very strong metal–nitrido triple bonds.[10–12] In uranium
chemistry, however, the valence f orbitals do not participate
in bonding to the same extent as metal d orbitals. As a result,
ꢀ
the U N formal triple bond in the uranium imido and the
elusive nitrido species is considerably more ionic in nature
and can be described as U(d+)–N(dꢁ). Accordingly, we
expected sterically pressured uranium imido and/or nitrido
complexes with custom-designed chelators to exhibit
increased nucleophilicity toward organic substrates. We
therefore aimed to synthesize high-valent uranium imido
and nitrido complexes of varying steric demand to explore
The linear N ligand of complex 7 can in turn be trans-
ferred onto organic substrates to yield functionalized carbo-
diimides (Scheme 1, 7!10a). This observed nitrogen-transfer
chemistry in successive one-electron steps appears to be
generally applicable to various p-acceptor ligands and, to the
best of our knowledge, is unprecedented for metal–imido
complexes.
[*] Dr. I. Castro-Rodríguez,Dr. H. Nakai,Prof. Dr. K. Meyer
Friedrich-Alexander-University of Erlangen-Nuremberg
Institute of Inorganic Chemistry
The synthesis of complex 5 was achieved by treatment of a
red-brown solution of 4 in benzene with one equivalent of
Me3SiN3, which results in the formation of a deep-green
solution and N2 evolution. After filtration, the volume of the
reaction mixture was reduced by applying a vacuum, thus
causing the green microcrystalline solid 5 to precipitate
( ꢂ 65% yield).
Egerlandstrasse 1,91058 Erlangen (Germany)
Fax: (+49)9131-852-7367
E-mail: kmeyer@chemie.uni-erlangen.de
[**] This work was supported by the US Department of Energy (US DE-
FG02-04ER15537) and,in parts,through the UCSD/LANL CARE
program. We thank NIH for a fellowship to I.C.-R. (3T32DK07233-
2651) and Prof. Arnold Rheingold,Dr. Peter Gantzel,and Dr. Lev
Zakharov for assistance with the crystallographic details. K.M. is an
Alfred P. Sloan fellow.
Block-shaped, deep-green single crystals of 5 were
obtained from diffusion of diethyl ether into CH2Cl2 at
ꢁ408C and studied by X-ray diffraction. An ORTEP plot of
the molecular structure of [((AdArO)3tacn)U(NSiMe3)] in
crystals of 5·2CH2Cl2 is shown in Figure 1 (top).
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
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ꢁ
ꢁ
The average U O(ArO) and U N(tacn) lengths, as well
as the displacement of the U ion from an idealized plane of
the three aryl oxide oxygen atoms, were determined to be
2.211(2), 2.696(2), and ꢁ0.187 Š and thus are similar to those
found in 3 (2.213(4), 2.692(4), and ꢁ0.151 Š). A comparison
ꢁ ꢁ
of the parameters within the U N SiMe3 imido group,
however, reveals remarkable differences between complexes
3 and 5. Similar to transition-metal imido complexes, high-
valent Uv and Uvi imido species typically exhibit short, formal
ꢀ
U N(imido) triple bonds with bond lengths that range from
1.85 to 2.01 Š and a(U-N-R) bond angles that vary from
slightly bent to linear (163.33–180.08).[14,16–23] Accordingly, the
ꢁ
structural parameters of imido complex 3 (d(U N(imido)):
1.989(5) Š and a(U-N-R): 173.7(3)8) are similar to those
reported for metal imido complexes.[14] In contrast, the U
N(imido) bond length found in 5 is the longest ever reported
ꢁ
for metal imido complexes and deviates significantly from
ꢁ
linearity (d(U N(imido)): 2.1219(18) Š and a(U-N-R):
162.55(12)8). These unusual structural features of 5 are
likely due to the steric pressure brought about by the
sterically encumbering adamantane groups which form a
narrow cylindrical cavity and prevent the Me3SiN2ꢁ ligand
from optimal binding. Accordingly, the imido nitrogen
Scheme 1. Synthesis of complexes and nitrogen-transfer chemistry in
successive one-electron steps.
ꢁ
p orbitals cannot participate in efficient M L p bonding,
thus resulting in the observed unusual structural parameters
and increased reactivity of 5. A space-filling model of 5
(Figure 2) illustrates that the sterically encumbering ligand
=
still provides access for small molecules to the functional U
ꢁ
N TMS (TMS = trimethylsilyl) moiety through the cleft
perpendicular to the threefold axis. The resulting increased
reactivity is evidenced by the unusual multiple-bond meta-
thesis chemistry of this complex with p acids.
Figure 2. Top (left) and side view (right) of a space-filling representa-
tion of complex 5.
The exposure of deep-green solutions of 5 in benzene to
CO (1 atm) leads to a gradual discoloration of the solution
with concomitant formation of Me6Si2. A colorless micro-
crystalline precipitate, which exhibits an intense absorption
band at 2185 cmꢁ1 in the IR vibrational spectrum, is formed
after several minutes of stirring. Upon exposure of 5 to 13C-
labeled 13CO, this band shifts to 2122 cmꢁ1, which is in
agreement with the formation of a Uiv isocyanate complex
[((AdArO)3tacn)U(h1-N12/13CO)] (6). A control experiment
was carried out to confirm that the steric pressure induced by
the bulky adamantyl groups weakens the Uv imido bond and
Figure 1. Molecular structures of [((AdArO)3tacn)U(NSiMe3)] (5,top),
[((AdArO)3tacn)U(NCO)] (6,middle),and [(( AdArO)3tacn)U(NCNCH3)]
(7,bottom) in crystals of 5·2CH2Cl2, 6·2CHCl3,and 7·4C6H6,with the
thermal ellipsoids at the 50% probability level. Hydrogen atoms and
co-crystallized solvent molecules are omitted for clarity.
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increases the complex reactivity. In a competition experiment
of a 1:1 mixture of 3 and 5 with CO, only 5 reacted to yield the
isocyanate complex 6, whereas 3 remained unreactive toward
CO.
coordinate
UIII
complexes
(e.g.,
ꢁ0.45 Š
for
[((tBuArO)3tacn)U(NCCH3)][14]). The out-of-plane shifts of 6
and 7 fall between the values found for the analogous UV and
UIII complexes and therefore appear to indicate a formal UIV
oxidation state for 6 and 7. This oxidation state is also evident
from the UV/Vis/NIR electronic absorption spectra and
SQUID magnetization data. Although magnetic moments
mB of UIII (f3) and UIV (f2) complexes at room temperature are
generally very similar (mB ꢂ 3 B.M.) and often do not allow for
an unambiguous assignment of the formal oxidation state, the
temperature dependence of mB in the range 4–300 K, however,
clearly supports the + IV oxidation state of the central U ion
in 6 and 7 (see the Supporting Information).[26] This assign-
ment is additionally supported by electronic absorption
spectra recorded at 190–2100 nm (see the Supporting Infor-
mation). Colorless complexes 6 and 7 display very similar
spectra with various sharp, low-intensity bands (e = 5–
80mꢁ1 cmꢁ1), which are characteristic for spectra of UIV
complexes with an f2 electron configuration (3H4 ground
level).[27]
In an attempt to evaluate the versatility and general
applicability of this type of imido reactivity, we studied the
reaction of 5 with other p acids, such as methyl isocyanide. We
found that 5 cleanly reacts with CH3NC to form the Uiv
carbodiimide complex [((AdArO)3tacn)U(h1-NCNMe)] (7)
and Me6Si2 in 45% yield of the isolated and purified product.
Similar to 6, the IR spectrum of 7 exhibits one strong
vibrational band centered at 2101 cmꢁ1, which can be assigned
to the h1-coordinate carbodiimide ligand in 7. Elemental
analysis, 1H NMR spectroscopy, and electronic absorption
spectroscopy suggested that complexes 6 and 7 are isoelec-
tronic and isostructural to the previously reported Uiv
heterocumulene complexes [((AdArO)3tacn)U(h1-N3)] (8)
and [((AdArO)3tacn)U(h1-OCO)] (9).[24]
X-ray diffraction studies on single crystals of 6 and 7
confirmed the formation of nearly linear axial h1-bound
isocyanate and carbodiimide ligands in these complexes
(Figure 1). A comparison of selected bond lengths and
angles in 6 and 7 with the corresponding parameters found
in 5 is given in Table 1.
Interestingly, the isocyanate and carbodiimide ligands of 6
and 7 are reactive and can be transferred to organic
molecules. For example, carbodiimide species 7 quantitatively
reacts with CH2Cl2 and CH3I to yield the free carbodiimides
CH3NCNCH2Cl and CH3NCNCH3
Table 1: Selected bond lengths (Š) and angles (8) for 5, 6, 7,and 10a.
as well as the corresponding halide
complexes [((AdArO)3tacn)U(X)]
5
6
7
10a
(X = Cl (10a; see the Supporting
Information); I (10b; Scheme 1,
U–O(av)
U–N(av)
2.2105(17)
2.696(2)
2.146(4)
2.676(5)
2.164(2)
2.683(3)
2.183(5)/2.154(5)
2.642(6)/2.664(7)
7!10a)) in near-quantitative
yield. Reduction of 10a,b with
Na/Hg recovers the trivalent start-
ing material and is thereby closing
a cyclic reaction pathway. In this
cycle, a nitrogen atom is first trans-
ferred from the UV imido complex
U(oop shift)
U–Cl(1)
U–N(4)
ꢁ0.187
ꢁ0.301
ꢁ0.318
ꢁ0.283/ꢁ0.285
2.761(3)/2.875(5)
2.1219(18)
1.623(2)
2.389(6)
2.327(3)
1.184(4)
N(4)–Si(1)
N(4)–C(70)
C(70)–O(4)
C(70)–N(5)
N(5)–C(71)
U-N(4)-Si(1)
U-N(4)-C(70)
N(4)-C(70)-O(4)
N(4)-C(70)-N(5)
N(5)-C(70)-C(71)
1.156(10)
1.195(10)
1.231(4)
1.426(5)
ꢀ
ꢀ
ꢀ
to a p acid by U N/C O and C N
multiple-bond metathesis and in a
subsequent step to haloalkanes,
such as CH2Cl2 or CH3I
(Scheme 1).
162.55(12)
171.2(6)
178.2(9)
161.9(3)
174.3(4)
123.2(4)
av=average,oop =out of plane.
There have been previous
reports of a nitrene-group transfer
ꢁ
The U N4 bond lengths and a(U-N4-C70) angles were
determined to be 2.389(6) Š and 171.2(6)8 in 6 and 2.327(3) Š
and 161.9(3)8 in 7 and thus are similar to the corresponding
from metal imido complexes to p-accepting ligands, such as
CO for example, thus yielding various organic isocyanates
and the corresponding reduced metal carbonyl species[28–31]
and the archetypal Bergman [(Cp*)Ir(N(CO)tBu)(CO)]
(Cp* = pentamethylcyclopentadienyl) complex with an
h2:C,N bound CO fragment.[32] Chirik and co-workers
ꢁ
parameters determined for 8 and 9 (8: d(U Na): 2.384(10) Š
and a(U-Na-Nb): 176.9(8)8; 9: d(U-OCO ): 2.351(3) Š and
a(U-O-C): 171.1(2)8). The inner N-C-O and N-C-NMe
angles of 178.2(9)8 and 174.3(4)8, respectively, are also close
to linear.
A structural feature that is particularly interesting is the
shift of the U ion out of the tris(aryloxide) plane. These out-
of-plane shifts (Uoop) are ꢁ0.301 and ꢁ0.318 Š for 6 and 7 and
are generally very sensitive to the formal oxidation state of
the U center.[25] For example, whereas the Uoop shifts in
pentavalent 3 and 5 are ꢁ0.151 and ꢁ0.187 Š, complexes 6–9
exhibit out-of-plane shifts in the range ꢁ0.291 to ꢁ0.318 Š,
which in turn differ significantly from values found in seven-
2
recently reported the reaction of [(Cp’’)2Ti(NSiMe3)] (Cp’’ =
5
ꢁ
h -C5H3-1,3-(SiMe3)2) with CO to yield Me3Si NCO and the
two-electron-reduced [(Cp’’)2Ti(CO)2] complex.[31] The dis-
ꢀ
covery of metal imido complexes M NR that react with p-
accepting ligands L to form one-electron-reduced nucleo-
1
ꢁ
philic h -M NL species, as reported herein, represents a new
type of metal–imido transformation.
In conclusion, the sterically congested complexes
[((AdArO)3tacn)U(NSiMe3)]
and
[((AdArO)3tacn)U-
ꢁ
ꢁ
(NCNMe)] have reactive U N(imido) and U N(heterocu-
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2391

Communications
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philes. Due to the steric constraints of the bulky adamantane
derivatized aryl oxide chelator, the N(imido) and N(hetero-
cumulene) ligands have reduced p-bonding ability that results
ꢀ
in increased reactivity. We established that the U N(imido)
group acts as a nucleophile toward p-accepting ligands and
the trimethylsilyl radical as a leaving group. This reaction is
followed by nucleophilic attack of the newly formed hetero-
cumulene ligand on alkyl halides. Both reactions result in new
IV
ꢁ
N C bonds with the substrates. The U halide complex can
be reduced to recover the UIII starting material, thereby
closing a cycle, in which a nitrogen atom is transferred from a
UV imido complex to a p acid by C N/C O multiple-bond
ꢀ
ꢀ
metathesis in successive one-electron steps. The driving force
ꢀ
ꢀ
ꢀ
ꢀ
for the initial U NR/C O and U NR/C NR metathesis
reaction and one-electron reduction likely is the formation of
the stable UIV species 6 and 7, respectively. For complexes of
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Keywords: actinides · multiple-bond metathesis ·
.
nitrogen transfer · N,O ligands · uranium
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