Diazoalkane reduction for the synthesis of uranium hydrazonido complexes

Article abstract of DOI:10.1002/ejic.201200606

The reactivity of the uranium(III) alkyl Tp*₂UCH ₂Ph (1) toward diazoalkanes is reported. Addition of 1 equiv. N ₂CPh₂ produces 0.5 equiv. bibenzyl, along with Tp*₂U(N₂CPh₂) (2). This species is dynamic in solution at room temperature and rapidly interconverts between the η¹- and η² isomers as determined by variable-temperature ¹H NMR spectroscopy. X-ray crystallographic analysis at low temperature shows exclusively the η² isomer, which features a short U-N multiple bond analogous to an imido species. The η¹ isomer reacts quantitatively with aldehydes and ketones through multiple bond metathesis to produce Tp*₂U(O) and the corresponding ketazine. Treatment of 1 with N₂CHSiMe₃ generates 0.5 equiv. bibenzyl and the η¹ isomer Tp*₂U(N₂CHSiMe₃) (3). This species is unstable over the course of hours, and there is no spectroscopic evidence for the η² isomer. Tp*₂U(η¹-N ₂CHSiMe₃) can be trapped by addition of phenylacetylene by a [2+2] cycloaddition to afford the uranium(IV) metallacycle Tp*₂U[(N-N = CHSiMe₃)CHCPh] (4). Crystallographic data for 4 are presented. Copyright

Full text of DOI:10.1002/ejic.201200606

Job/Unit: I20606
/KAP1
Date: 30-08-12 16:13:20
Pages: 9
FULL PAPER
DOI: 10.1002/ejic.201200606
Diazoalkane Reduction for the Synthesis of Uranium Hydrazonido Complexes
[a]
[a]
[a]
Ellen M. Matson, Phillip E. Fanwick, and Suzanne C. Bart*
Keywords: Uranium / Diazo compounds / Metallacycles / Tridentate ligands / Actinides
The reactivity of the uranium(III) alkyl Tp*
ward diazoalkanes is reported. Addition of 1 equiv. N
produces 0.5 equiv. bibenzyl, along with Tp* U(N CPh
This species is dynamic in solution at room temperature and
2
UCH
2
Ph (1) to-
CPh
) (2).
bond metathesis to produce Tp*
ketazine. Treatment of with
0.5 equiv. bibenzyl and the η isomer Tp*
2
U(O) and the corresponding
2
2
1
N
2
CHSiMe
3
generates
CHSiMe
1
2
2
2
2
U(N
2
3
)
(3). This species is unstable over the course of hours, and
1
2
2
rapidly interconverts between the η - and η isomers as de- there is no spectroscopic evidence for the
η
isomer.
) can be trapped by addition of phen-
ylacetylene by a [2+2] cycloaddition to afford the uranium-
(IV) metallacycle Tp* U[(N-N = CHSiMe )CHCPh] (4). Crys-
tallographic data for 4 are presented.
1
1
termined by variable-temperature H NMR spectroscopy. X-
ray crystallographic analysis at low temperature shows ex-
2 2 3
Tp* U(η -N CHSiMe
2
clusively the η isomer, which features a short U–N multiple
2
3
1
bond analogous to an imido species. The η isomer reacts
quantitatively with aldehydes and ketones through multiple
CH , PhCH , Ph; An =U, R = CH , PhCH
).[9] The pincer
3 2 3 2
Introduction
bis(ligand) derivative (PNP) UI {PNP = bis[2-(diisoprop-
2
Diazoalkanes are versatile reagents as they are a conve-
nient source of carbenes through loss of the dinitrogen frag-
ylphosphanyl)-4-methylphenyl]amido} in the presence of
KC and diphenyldiazomethane generates the first example
8
[
1–3]
ment
or they serve as models for studying metal-medi-
2
of an actinide hydrazonido complex, (PNP) U(η -
N CPh ). Recently, Liddle has reported the reactivity of
(tren
2
[
4–6]
ated dinitrogen reduction when dinitrogen is retained.
These molecules are better ligands than nonpolar N and
[10]
2
2
2
TMS
TMS
[
)UCl(THF)] [tren
= N(CH CH NSiMe ) ]
2 2 3 3
offer multiple bonding modes. Although electron-rich tran-
sition metals are typically sought after for diazoalkane re-
duction because of their ability to engage in π backbonding,
uranium is a highly reducing element as well, and thus a
logical choice for the activation of these molecules.
[11]
with the lithiated diazoalkane LiN CHSiMe . The prod-
2
3
uct is not a diazoalkane species, but rather the isocyanotri-
TMS
methylsilylamide dimer [(tren
)U{μ-N(SiMe )NC}] .
3 2
Diazoalkanes are structurally similar to organic azides
and access analogous resonance forms. Recent work from
our laboratory has demonstrated that organic azides react
with trivalent Tp* UCH Ph (1) to form the uranium(IV)
Early studies of uranium demonstrated the synthesis of
the diazoalkane adduct (NNЈ )U(η -N CHSiMe ) [NNЈ =
1
3
2
3
3
2
2
N(CH CH NSiMe tBu) ], although no crystallographic
2
2
2
3
imido species Tp* UNR [R = Ph, Ad, Mes (Mes = 2,4,6-
2
evidence was provided for its structure.[7] Following this
trimethylphenyl)], with extrusion of bibenzyl, rather than
IV
2
study, Meyer reported [{(tBuArO) tacn}U (η -NNCPh )]
[12]
3
2
insertion into the U–C bond.
We sought to determine
[
(tBuArO) tacn = trianion of 1,4,7-tris(3,5-di-tert-butyl-2-
3
whether analogous reactivity would occur for diazoalkanes
given the structural similarities or whether migratory inser-
hydroxybenzyl)-1,4,7-triazacyclononane], which has a sin-
gle electron delocalized over the C–N–N unit.[ Use of the
more sterically demanding adamantyl variant of the sup-
8]
[9]
tion would result as Kiplinger described. Herein we pres-
ent the results of a study focusing on the reactivity of both
diphenyldiazomethane (N CPh ) and trimethylsilyldiazo-
Ad
porting ligand produced the end-on analogue [{( ArO)3-
2
2
III
1
tacn}U (η -NNCPh )]. Migratory insertion of diphenyldi-
2
methane (N CHSiMe ) with Tp* UCH Ph. Characteriza-
2
3
2
2
azomethane was observed by Kiplinger for actinide alkyl
and aryl species to form the family of hydrazonato com-
pounds Cp* An[η -(N,NЈ)–R–N–N=CPh ]
1
tion by H NMR and IR spectroscopy as well as X-ray
crystallography is presented.
2
(Cp*
=
2
2 2
1,2,3,4,5-pentamethylcyclopentadienide; An = Th, R =
Results and Discussion
[
a] Department of Chemistry, Purdue University,
West Lafayette, IN 47906, USA
Initial studies explored the reactivity of diphenyldiazo-
Fax: +1-765-494-0239
E-mail: sbart@purdue.edu
methane with Tp*
UCH Ph (1). Treatment of a THF solu-
2 2
Homepage: www.chem.purdue.edu/people/faculty/
faculty.asp?itemID = 85
tion of 1 with 1 equiv. N CPh at room temperature re-
2
2
sulted in an immediate color change from dark green to
form a yellow–brown compound, 2 (Scheme 1). Analysis of
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejic.201200606.
Eur. J. Inorg. Chem. 0000, 0–0
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1

Job/Unit: I20606
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Date: 30-08-12 16:13:20
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E. M. Matson, P. E. Fanwick, S. C. Bart
FULL PAPER
Scheme 1.
1
2
by H NMR spectroscopy shows 14 broad paramagnetic
resonances ranging from –61.65 to 105.98 ppm, which
could not be assigned. Concomitant formation of 0.5 equiv.
bibenzyl was noted, as determined by integration against a
ferrocene internal standard. It was determined that the
same paramagnetic spectrum could be generated by using
3
equiv. of the diazoalkane with the uranium(III) species
Tp* U(2,2Ј-bpy), which has a monoanionic bipyridine li-
2
[
13]
gand.
In this case, along with 2 and 2,2Ј-bipyridine was
formed a small amount of the previously synthesized
[
14]
Tp*U(dmpz)3
(dmpz
=
3,5-dimethylpyrazolide).
Tp*U(dmpz) was originally discovered by Takats as a by-
3
2
product from heating (Tp*)(κ -Tp*)UCp, who confirmed
this structure by independent synthesis by salt metathesis
[
14]
of Tp*UCl (THF) with 3 equiv K(dmpz). In our hands,
3
IV
thermal decomposition of bis-Tp* U complexes with
uranium–nitrogen
multiple
bonds
also
produces
Figure 1. Molecular structure of 2 shown with 30% probability el-
Tp*U(dmpz) . To confirm that this product arises from the
3
lipsoids. Selected hydrogen atoms are omitted for clarity.
thermal instability of 2 and is not formed during conversion
from 1, the crude reaction mixture was heated to 80 °C in
toluene, which resulted in exclusive formation of
[
14]
Table 1. Structural parameters for compounds 2 and 4.
Tp*U(dmpz)3.
Analysis by IR spectroscopy of a KBr
pellet of 2 shows two B–H absorptions consistent with the Bond
2 [Å]
4 [Å]
–1
intact bis-Tp* ligand framework at 2523 and 2552 cm .
U1–N11
2.739(8)
2.535(7)
2.666(8)
2.596(8)
2.536(8)
2.666(9)
2.146(9)
2.347(8)
–
2.523(7)
2.577(6)
2.636(7)
2.676(6)
2.568(6)
2.566(6)
2.348(6)
–
2.372(8)
1.373(9)
1.380(11)
1.349(12)
Also isolated from the Tp* U(2,2Ј-bpy) reaction is tet- N1–N21
2
1
U1–N31
raphenylketazine, whose formation was confirmed by H
U1–N61
U1–N71
U1–N81
NMR spectroscopy and by a comparison of the unit cell of
an isolated crystal by X-ray crystallography.[
15]
To further identify the structure of the organometallic U1–N41
product 2, X-ray crystallography was employed. Single, U1–N42
U1–C45
brown crystals suitable for analysis were grown from a con-
N41–N42
centrated THF/pentane solution (1:4) at –35 °C. Refine-
N42–C40
1.354(11)
1.283(13)
–
ment reveals an eight-coordinate uranium center with a mo-
C44-C45
2
lecule of diphenyldiazomethane coordinated in a η fash-
2
2
ion, Tp* U(η -N CPh ) (2-η -N CPh ) (Figure 1, Table 1).
2
2
2
2
2
The molecular structure is a distorted dodecahedron de- tiple bonding. Uranium(IV) amide species, such as
[
19]
fined by two trapezia containing N11–N61–N81–N31 and Cp* U(CCPh)(NPh ) [2.298(4) Å] and (CGC)U(NMe2)2
2
2
5
[20]
N42–N71–N21–N41. The trapezia intersect at an angle of [CGC = Me –Si(η -Me C )(tBuN); 2.207(4) Å (NMe ) ]
2
4
5
2 2
77.4°. The U–N distances for the hydrotris(3,5-dimethylpyr- commonly show longer U–N bonds. The bonding in 2 is
azolyl)borate ligand are consistent with those previously more reminiscent of uranium imido species, in which U–N
[
21–27]
observed, and each ligand has one U–N distance that is distances range from 1.85 to 2.05 Å.
In 2, the imido
Compound 2 fea- moiety has an acute U–N–N angle of 80.8(5)° that prevents
tures a short U1–N41 (diazomethane Nα) bond length of Nα nitrogen lone pair bonding typically seen in species with
elongated relative to the others.[
12,13,16–18]
2
2
.146(7) Å, which is indicative of uranium–nitrogen mul- shorter distances, which creates a slightly longer U–N mul-
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Diazoalkane Reduction for the Synthesis of Uranium Hydrazonido Complexes
tiple bond. This angle is enforced by the uranium–Nβ- sights obtained from this analysis indicate that the low-tem-
(diazoalkane) bond, U1–N42, which has a distance of perature NMR spectroscopic data represents the side-on
2
.347(8) Å and shows dative uranium–nitrogen bonding. coordinated diazoalkane compound. Upon warming of the
2
The structure of 2 is analogous to Kiplinger’s (PNP) U(η - sample to 25 °C, the dynamic behavior is most likely a re-
N CPh ), which has a U–Nα bond of 2.097(5) Å, slightly sult of interconversion between the η -N CPh (2-η -
shorter than that in 2. The U–Nβ bond of (PNP) U(η - N CPh ) and the η -N CPh (2-η -N CPh ) species
2
[
10]
2
2
2
2
2
2
2
1
1
2
2
2
2
2
2
2
N CPh ) is 2.370(5) Å and is essentially the same as that for (Scheme 1). Thus, the spectrum measured at ambient tem-
2
2
1
2
within error. The N–N and C–N distances in compound 2 perature is representative of either 2-η -N CPh or the
are 1.354(11) and 1.283(13) Å, respectively, and are statistic- time-averaged structure of the solution mixture of 2-η -
ally the same as those found in (PNP) U(η -N CPh ) N CPh and 2-η -N CPh . The related family of linear ura-
2 2
2
2
1
2
2
2
2
2
2
2
[
1.359(7) and 1.302(8) Å]. Distances for the same bonds in nium(IV) imidos, Tp* UNR (R = Ph, Ad, Mes), displays
2
the uncoordinated diazoalkanes range from 1.130–1.145 Å sharp resonances different from those observed for 2 at am-
N–N) and 1.28–1.32 Å (C–N),[
28–30]
which indicates that bient temperature, which supports the notion that the ob-
(
the diazoalkane has been significantly reduced by inter- served spectrum for 2 is most consistent with the time-
1
2
action with the uranium center; a hydrazonido ligand, averaged mixture of 2-η -N CPh and 2-η -N CPh in solu-
2
2
2
2
2
–
[
N CPh ] , is formed.
tion.
Such dynamic behavior has previously been observed for
U(η -NNCPh )] differs from 2 in that its molecular struc- the titanium diazoalkane species CpЈЈ Ti(N CPh ) [CpЈЈ =
2
2
2
The η -N CPh ligand in tetravalent [{(tBuArO) tacn}-
2
2
[8]
3
2
2
2
2
2
5
[36]
ture has a U–Nα bond of 2.259(4) Å, longer than those η -C H -1,3-(SiMe )].
This species undergoes intercon-
5
3
3
2
1
2
observed for 2 or (PNP) U(η -N CPh ). This distance is version between the η and η isomers in solution at –20 °C.
2
2
2
more consistent with an amide linkage to the uranium cen- Similar to that of 2, this species has a broad and featureless
1
ter, while the U–Nβ distance of 2.582(4) Å shows dative
H NMR spectrum at ambient temperature that sharpens
coordination to uranium. Activation of the diazoalkane in to display resonances for both isomers as the sample is co-
2
[
{(tBuArO) tacn}U(η -NNCPh )] is noted by the respective oled. Further support for the isomeric forms of 2 lies in the
3 2
1
N–N and C–N distances of 1.338(5) and 1.333(6) Å. The previous synthesis of the end-on η -diazoalkane uranium
formation of [{(tBuArO) tacn}U(η -NNCPh )] results species, including Kiplinger’s Cp* U(η -N CPh )
from a one-electron reduction of diphenyldiazomethane by Burns’ Cp* U(η -N CPh )[N(2,4,6-tBu Ph)].
trivalent [{(tBuArO) tacn}U].
2
1
[10]
and
Both of
these species feature reduced η -N CPh hydrazonido li-
3
2
2
2
2 2
1
[37]
2
2
2
3
1
3
2
2
2
The η -hydrazonido uranium species can be contrasted gands with U–N bonds reminiscent of imido linkages,
with analogous examples involving transition metals. which indicates this isomeric form effectively supports the
2
[31]
Bergman’s Cp* Ti(η -N CHSiMe ) displays single bond uranium centers. Diazoalkane ligands have also been shown
2
2
3
character between the titanium atom and both Nα and Nβ to change hapticity on the uranium centers in the
Ad
in the molecular structure. In this species, the N–N bond is [{( ArO) tacn}U] system. Upon addition of N CPh to
3
2
2
1
elongated to 1.276(3) Å, which indicates that backbonding this molecule, the end-on η -N CPh uranium compound,
2
2
Ad
1
[8]
from the titanium to the N=N π* orbital occurs. The au- [{( ArO) tacn}U(η -NNCPh )], is isolated. Heating of
3
2
thors note that imide-like character may be present in solu- the latter causes a shift of the diazoalkane hapticity from
2
1
2
tion. The bonding in Ni(iPr Im) (η -N,NЈ-N CPh ) (iPr Im η to η , which facilitates C–H activation of the phenyl
2
2
2
2
2
[32]
=
1,3-diisopropylimidazol-2-ylidene)
diazofluorene) differs from the titanium example in that [{( ArO) tacn}U {η -3-phen(Ind)}].
and Ni(tBuNC)2- ring to form the uranium(IV) indazole product
[
33]
Ad
IV
2
[8]
(
3
the Ni–N distances are intermediate of single and double
Reactivity studies with aldehydes and ketones also pro-
bonds. The respective elongated N–N bonds of 1.266(3) and vide insight into the various isomeric forms of 2. Addition
2
1
.245(4) Å in Ni(iPr Im) (η -N,NЈ-N CPh ) and Ni- of 1 equiv. p-tolualdehyde to a THF solution of 2 at 25 °C
2 2 2 2
(
tBuNC) (diazofluorene) compare well with those in azo- caused an immediate color change from yellow–brown to
2
[
34,35]
benzene (1.244 Å),
bonding is prevalent in both molecules.
and, again, indicate that π back- bright pink, which indicates the formation of the previously
[
18]
reported base-free uranium(IV) oxo species Tp* U(O),
2
1
1
The H NMR spectrum of 2 at 25 °C is extremely broad as confirmed by H NMR spectroscopy [Equation (1)]. Si-
and shows resonances consistent with inequivalency multaneous formation of the ketazine Ph C=N-N=CH(p-
2
throughout the molecule, but it is not useful for structural tolyl)[
38,39]
was noted, supporting the reaction progresses
assignment. Thus 2 was analyzed by variable-temperature through multiple bond metathesis. The same reactivity was
1
H NMR spectroscopy to determine if dynamic behavior observed for benzophenone and acetone, and ketazine
on the NMR timescale is the origin for the broad lines at products were identified by comparison to literature
[
40,41]
room temperature (Figure S1). As the sample is cooled to data.
°C and eventually to –5 °C, the resonances become very been observed for the uranium(IV) imido species
Analogous reactivity with benzophenone has
5
5
[24]
sharp and shift slightly from their values at 25 °C. At –5 °C, CpЈ U{N(p-tolyl)} [CpЈ = η -1,2,4-(Me C) C H ]
to
2
3
3
5
2
it is clear that there is inequivalency in the molecule, which form CpЈ U(O) and the corresponding imine product
2
2
supports the formation of the η -N CPh species. The crys- Ph C[N(p-tolyl)]. Furthermore, treatment of Tp* UNR (R
2
2
2
2
tals obtained for 2 were grown at –35 °C, and the X-ray = Ph, Ad, Mes) with p-tolualdehyde also generates
[
12]
structured was determined at 150 K; thus the structural in- Tp* U(O) and H(p-tolyl)CNR.
In contrast, addition of
2
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E. M. Matson, P. E. Fanwick, S. C. Bart
FULL PAPER
2
1
aldehydes or ketones to a solution of 2-η -N CPh at visible by H NMR spectroscopy are the resonances for
2
2
–35 °C resulted in no reaction. However, warming of this 0.5 equiv. bibenzyl. Infrared spectroscopy on a solid sample
solution to room temperature started the reaction, which of recrystallized 3 shows two inequivalent B–H absorptions
–1
ultimately led to conversion of the products, presumably be- at 2529 and 2555 cm , similar to those observed for com-
1
cause of the accessibility of 2-η -N CPh at this tempera- plex 2 and as expected on the basis for other bis(Tp*) deriv-
2
2
2
[13,16–18]
1
ture. These reactivity studies support that 2-η -N CPh , atives.
The symmetric nature of the H NMR spec-
2
2
which predominates at low temperature, does not perform trum and loss of N=N functionality by IR spectroscopy
metathesis chemistry. This most likely is a result of the indicates that compound 3 is the linear hydrazonido prod-
1
1
dative coordination of Nβ, which prevents subsequent coor- uct Tp* U(η -N CHSiMe ) (3), in analogy to 2-η -
2
2
3
dination of the carbonylated substrate. However, once the N CPh . This species was found to be unstable upon stor-
2
2
1
solution is warmed, η -N CPh is present in solution, and age and forms Tp*U(dmpz) , which prevents full charac-
2
2
3
the metathesis chemistry can proceed at the open coordina- terization.
tion site. Attempts to freeze out the dynamic behavior and An experiment to trap unstable 3 was performed by re-
trap η -N CPh with Lewis bases such as pyridine, DMAP, peating the synthesis reaction, with subsequent addition of
1
2
2
and isonitriles were unsuccessful, as no reaction was ob- excess phenylacetylene prior to warming of the mixture to
served at room temperature.
room temperature. This procedure was accompanied by an
immediate color change from green to orange–brown
1
(
Scheme 1). Analysis of this product, 4, by H NMR spec-
troscopy reveals 25 paramagnetically shifted sharp reso-
nances ranging from –32.18 to 163.14 ppm; no symmetry in
the new product is seen. Notably, a large doublet present at
–
4.24 ppm corresponds to the protons of the –SiMe group.
3
Infrared spectroscopy shows loss of both the CϵC triple
bond of phenylacetylene and the N=N double bond of free
trimethylsilyldiazomethane. The two signature B–H
stretches of the bis-Tp* ligand framework are also present
–1
With an understanding of compound 2 established, we (2521, 2553 cm ).
turned our efforts to understanding the formation of Small orange crystals of 4 were grown from a concen-
equiv. ketazine in the reaction from Tp* U(2,2Ј-bpy). trated pentane/THF (6:1) solution and analyzed by X-ray
1
2
Ketazine extrusion is not unique to this system and has crystallography. Following refinement of the data, com-
been reported for transition metals. Recent work by Hey- pound 4 is assigned as the uranium(IV) metallacycle
cat
duk describes similar chemistry with [ONO ]TaCl (OEt ) Tp* U-N(N=CHSiMe )CH=CPh (4) (Figure 1, Table 1),
2
2
2
3
cat
{
[ONO ]H3
=
N,N-bis(3,5-di-tert-butyl-2-phenol)- which features an eight-coordinate uranium atom in a dis-
amine}. Addition of 30 equiv. N CPh to this Ta species torted square-antiprismatic geometry. One face of the anti-
[
42]
V
2
2
catalytically forms Ph CN=NCPh , dinitrogen, and the prism is made up of N61, N71, N81, and N41, while the
2
2
cat
2
new tantalum ketazine species [ONO ]TaCl (η - other is composed of N21, N31, C45, and N11. The re-
2
Ph CN=NCPh ). By performing the same reaction in neat sulting structure of complex 4 has a U1–N41 bond length
2
2
styrene 1,1,2-triphenylcyclopropane is generated, and this of 2.348(6) Å, indicative of a uranium(IV)–nitrogen(amide)
[
43,44]
is indicative of a transient tantalum alkylidene. Unlike the bond.
Additionally, 4 has a new uranium–carbon bond
Heyduk system, compound 2 does not form tetraphenylket- of 2.372(8) Å, which is at the short end of the range for U–
azine catalytically at any temperature. Furthermore, no cy- C σ bonds observed in uranium(IV) alkyl species (2.395–
[
19,45–62]
clopropanation chemistry was observed upon addition of 2.498 Å).
This is most likely due to increased s char-
N CPh to Tp* U(2,2Ј-bpy) in neat styrene, which indicates acter at the U–C44 bond as a result of multiple bonding at
2
2
2
that a transient uranium alkylidene is not responsible for the adjacent carbon–carbon bond (C44–C45). The former
ketazine formation in this system. CϵC triple bond has been reduced to a double bond upon
The reactivity of trimethylsilyldiazomethane with 1 was metallation, as supported by the lengthening of the bond
also explored to determine how the steric and electronic to 1.349(12) Å (from 1.203 Å in acetylene). A new N–C
variation in the diazoalkane changes the reactivity. Upon bond has been formed between the Nα nitrogen atom and
mixing 1 equiv. N CHSiMe with 1 in thawing THF, a the carbon atom of phenylacetylene. This bond has an
2
3
change from green to red–brown ensues immediately, and N41–C44 distance of 1.381(11) Å, which is intermediate of
workup and isolation affords a brown–orange oil (3). The a carbon–nitrogen single and double bond. Finally, the U–
1
H NMR spectrum of 3 displays equivalent resonances for N distances of the Tp* ligands are similar to those in 2.
1
the Tp*-CH groups pointing towards and away from the Complex 4 is derived from η coordination of the diazoal-
3
uranium center at –7.03 and 2.56 ppm, respectively. Ad- kane ligand with imido-type bonding through the terminal
ditionally, resonances for the B–H and pyrazole C–H atoms nitrogen atom in 3, followed by [2+2] cycloaddition of
of the two Tp* ligands are located at –21.78 and 17.00 ppm, phenylactylene (Figure 2). This serves to reduce the bond
respectively. The large peak for the protons of the SiMe3 order of the N–N bond as well, which is now elongated to
group is located downfield at δ = 35.98 ppm (9 H). Also 1.373(9) Å. The new cyclometallated species is stable up to
4
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Eur. J. Inorg. Chem. 0000, 0–0

Job/Unit: I20606
/KAP1
Date: 30-08-12 16:13:20
Pages: 9
Diazoalkane Reduction for the Synthesis of Uranium Hydrazonido Complexes
8
0 °C. Attempts to elaborate the organic fragment with CO, the diazoalkane ligand is dynamic and shifts between the
1
2
CO , H , amines, hydrazines, or other diazoalkanes have η and η isomers in solution at ambient temperature. This
been unsuccessful.
2
2
1
has a profound effect on the reactivity, as the η species
can undergo metathesis with carbonylated small molecules,
while the η2 isomer cannot. Use of trimethylsilyldiazo-
1
methane results in the fleeting η -N CHSiMe adduct,
2
3
which can be effectively trapped by phenylacetylene to
make the corresponding metallacycle.
Treatment of the uranium(III) alkyl species with diazoal-
kanes does not result in an alkylidene species by N ex-
2
trusion or by U–C insertion as previously observed.[
9,65]
In-
stead, the dinitrogen fragments are retained and reduced by
two electrons to create stable uranium hydrazonido species
containing U–N multiple bond linkages. This is in contrast
to transition-metal reactivity, where a more charge-delocal-
ized N fragment, as a result of superior π backbonding by
2
d-block metals, is common. Because the f-block elements
typically do not engage in such activity to a significant ex-
tent, more π bonding from the nitrogen to the uranium oc-
curs, which creates U–N multiple bonds.
Experimental Section
Figure 2. Molecular structure of 4 shown with 30% probability el-
lipsoids. Selected hydrogen atoms are omitted for clarity.
General Considerations: All air- and moisture-sensitive manipula-
tions were performed by using standard Schlenk techniques or in
an MBraun inert atmosphere drybox with an atmosphere of puri-
fied nitrogen. The MBraun drybox was equipped with a coldwell
designed for freezing samples in liquid nitrogen and with two
–35 °C freezers for cooling samples and for crystallization. Solvents
for sensitive manipulations were dried and deoxygenated by using
The cycloaddition chemistry observed for the formation
of 4 is rare for diazoalkanes. Bergman previously reported
2
[31]
analogous reactivity for Cp* Ti(η -N CHSiMe )
with
2
2
3
both acetylene and phenylacetylene to produce the corre-
[
66]
sponding metallacycles. Hillhouse observed similar metall- literature procedures with a Seca solvent purification system.
2
acyclic products from the reaction of Cp* Zr(η -PhCCPh) [D
6
]Benzene and [D
8
]toluene were purchased from Cambridge Iso-
2
[
63]
tope Laboratories, degassed by three freeze-pump-thaw cycles, and
dried with 4-Å molecular sieves and sodium. Elemental analyses
were performed by Midwest Microlab, LLC in Indianapolis, IN.
Trimethylsilyldiazomethane (2.0 m in hexanes) was purchased from
Sigma Aldrich and used as received. Phenylacetylene, p-tolual-
dehyde, and acetone were purchased from Sigma Aldrich, dried
with calcium hydride, and distilled or vacuum transferred before
use. Benzophenone was purchased from Sigma Aldrich, dried on a
Schlenk line overnight, recrystallized from anhydrous diethyl ether
and N CPh .
In both cases, no structural characteriza-
2
2
tion for the cyclic products was presented, which makes that
shown for 4 unique. The [2+2] cycloaddition chemistry is
[
64]
more common for transition-metal imido and hydrazido
species. For uranium, the reactivity observed for 4 parallels
that found by Andersen,[ who reported [2+2] cycload-
24]
dition of CpЈ U{N(p-tolyl)} with phenylacetylene, which
2
IV
resulted in the analogous U metallacycle. This observa-
[
13]
tion also supports the imido character proposed for 3. The at –35 °C, and dried under vacuum again. Tp* U(2,2Ј-bipy),
2
16]
[67]
U–C and U–N bond lengths observed for CpЈ U=N(p- Tp*
2
UCH
2
Ph,[ and N
2
CPh
2
were prepared according to lit-
2
erature procedures.
tolyl) are the same within error [2.270(6) and 2.202(6) Å,
respectively] and slightly shorter than those observed in 4.
The longer distances for 4 are most likely a result of steric
1H NMR spectra were recorded on a Varian Inova 300 spectrome-
ter operating at 299.992 MHz. All chemical shifts were reported
1
congestion imparted by the bulky bis-Tp* ligand frame- relative to the peak for SiMe by using H (residual) chemical shifts
4
work relative to that by the bis(Cp) framework.
of the solvent as a secondary standard. The spectra for paramag-
netic molecules were obtained by using an acquisition time of 0.5 s,
thus the peak widths reported have an error ofϮ2 Hz. For para-
magnetic molecules, the H NMR spectroscopic data are reported
with the chemical shift, followed by the peak width at half height in
1
Conclusions
The reactivity of the uranium(III) alkyl species Hz, the integration value, and where possible, the peak assignment.
Tp* UCH Ph with N CPh and N CHSiMe is presented. Solid-state infrared spectra were recorded by using a Perkin–Elmer
2
2
2
2
2
3
FTIR Spectrum RX I spectrometer. Samples were made by crush-
ing the solids, mixing with dry KBr, and pressing into a pellet.
In analogy to previous work with organic azides, the new
uranium species exhibit structural features for uranium–ni-
trogen linkages consistent with imido functionalities with
Single crystals for X-ray diffraction were coated with polybutene
concomitant formation of 0.5 equiv. bibenzyl, as deter- oil in a glovebox and quickly transferred to the goniometer head
1
mined by H NMR spectroscopy. In the case of N CPh , of a Rigaku Rapid II image plate diffractometer equipped with a
2
2
Eur. J. Inorg. Chem. 0000, 0–0
© 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
5

Job/Unit: I20606
/KAP1
Date: 30-08-12 16:13:20
Pages: 9
E. M. Matson, P. E. Fanwick, S. C. Bart
FULL PAPER
MicroMax002+ high intensity copper X-ray source with confocal
optics. Preliminary examination and data collection were per-
formed with Cu-K radiation (λ = 1.54184 Å). Cell constants for
α
General Procedure for Addition of Carbonylated Substrates
Synthesis of Tp* U(O): A 20-mL scintillation vial was charged with
2
2
(0.050 g, 0.049 mmol) and THF (approximately 3 mL). The solu-
data collection were obtained from least-squares refinement. The
space group was identified by using the program XPREP.[
68]
tion was cooled in the coldwell until the THF was frozen. In a
separate vial, 1 equiv. of a carbonylated organic (O=CRЈR) was
dissolved in diethyl ether [R = H, RЈ = p-tolyl (6 μL, 0.051 mmol);
R = RЈ = Ph, (0.009 g, 0.049 mmol); R = RЈ = Me (4 μL,
The
structures were solved by using the structure solution program
PATTY in DIRDIFF99.[
69]
Refinement was performed on a LI-
The data were collected at a tem-
NUX PC with SHELX-97.[
68]
0.052 mmol)]. This colorless solution was added to the thawing
perature of 150(1) K. CCDC-884974 and CCDC-884975 contain
the supplementary crystallographic data for this paper. These data
can be obtained free of charge from The Cambridge Crystallo-
graphic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
uranium solution while stirring. No initial color change was ob-
served; however, after the solution warmed to room temperature,
it changed to bright pink. The uranium-containing product was
1
isolated by removing the solvents in vacuo, and H NMR spec-
2
Synthesis of Tp*
2
U(η -N,N-N
2
CPh
Ph (1) (0.100 g, 0.108 mmol), THF
approximately 5 mL), and a stirrer bar, and was cooled in a liquid-
nitrogen coldwell for approximately 30 min. In a separate vial,
equiv. N CPh (0.020 g, 0.101 mmol) was dissolved in THF (ap-
proximately 2 mL), and the solution was also cooled in the cold-
well. After 30 min, the thawing solution of N CPh was added
dropwise to the thawing Tp* UCH Ph solution while stirring. The
2
) (2): A 20-mL scintillation vial
2
troscopy confirmed the formation of Tp* U(O) in quantitative
was charged with Tp*
2
UCH
2
[18]
[41]
yield. 1 equiv. of the corresponding ketazines, Ph
2
CNNCPh
2
,
(
[41]
[38,39]
Me
2
CNNCPh
2
,
2
and H(p-tolyl)CNNCPh ,
were identified
by comparison to literature data.
1
2
2
1
Synthesis of Tp*
2
U[η -(N)=N–N=CHSiMe
3
] (3): A 20-mL scintil-
UCH Ph (1) (0.100 g,
.108 mmol), THF (approximately 5 mL), and a stirrer bar, and
was cooled in a liquid-nitrogen coldwell for approximately 30 min.
In a separate vial, N CHSiMe solution (51 μL, 0.108 mmol) was
lation vial was charged with Tp*
2
2
2
2
0
2
2
mixture was warmed to room temperature while stirring, and the
volatiles were removed in vacuo. The brown–yellow oily residue
was recrystallized from a concentrated diethyl ether/pentane solu-
tion (1:1), which resulted in the isolation of a brown solid (0.103 g,
2
3
diluted with THF (approximately 3 mL) and cooled for 30 min in
the coldwell. After this time, the thawing pale yellow solution of
N
2
CHSiMe
solution of Tp*
served, but after stirring for 5 min, the solvents were removed in
3
was added dropwise to the vial containing a thawing
0
.100 mmol, 93%), assigned as 2. X-ray quality crystals were grown
from a concentrated mixture of THF and pentane (1:4) at –35 °C.
14U (1026.64): calcd. C 50.31, H 5.30, N 19.10; found
2
UCH Ph. No immediate color change was ob-
2
43 54 2
C H B N
1
vacuo to leave a crude brown–orange oil, assigned as Tp*
N)=N–N=CHSiMe ]. The yield was determined by integration of
the H NMR spectrum against a ferrocene internal standard,
2
U[η -
C 51.02, H 5.76, N 18.09. Elemental analysis results are reproduc-
ible over multiple independently synthesized samples; however, they
are outside of the +/– 0.4% range because of the instability of this
compound as mentioned in the text. Because of the broadness of
(
3
1
1
(
0.074 g, 0.079 mmol, 73%). H NMR (300 MHz, C
6 6
D , 25 °C): δ
1
= –21.78 (204, 2 H, B-H), –7.03 (8, 18 H, Tp*-CH ), 2.56 (60, 18
the resonances of the H NMR spectrum at 25 °C, chemical shifts
3
3
H, Tp*-CH ), 17.00 (11, 6 H, Tp*-CH), 29.98 (17, 1 H, C-H), 35.98
are reported without decimal places and peak widths at half height
–
1
1
[7, 9 H, –Si(CH ) ] ppm. IR: ν˜ = 2529, 2555 (B–H) cm . The
are reported to two significant figures. H NMR (300 MHz, C
5 °C): δ = –62 (660), –35 (740), –30 (180), –17 (770), –9 (280), –3
450), 2 (110), 8 (260), 12 (250), 27 (110), 32 (240), 38 (620), 85
430), 106 (700) ppm. 1H NMR (300 MHz, C
, –5 °C): δ =
67.98 (211, 3 H, Tp*-CH ), –66.56 (211, 3 H, Tp*-CH ), –38.27
197, 1 H, Tp*-CH), –33.95 (165, 2 H, Ph-CH), –19.99 (142, 3 H,
Tp*-CH ), –18.94 (129, 3 H, Tp*-CH ), –17.77 (117, 3 H, Tp*-
CH ), –13.03 (110, 3 H, Tp*-CH ), –11.11 (115, 3 H, Tp*-CH ),
8.00 (94, 1 H, Tp*-CH), –7.83 (172, 2 H, B-H), –6.18 (92, 1 H,
Tp*-CH), –4.16 (81, 1 H, Tp*-CH), –3.92 (67, 3 H, Tp*-CH ), 3.10
), 4.07 (8, 1 H, Tp*-CH), 8.83 (107, 3 H, Tp*-
), 11.76 (64, 1 H, Tp*-CH), 12.36 (115, 2 H, Ph-CH), 29.43
6
D
6
,
3 3
1
by-product in this reaction, bibenzyl, was observed by H NMR
2
spectroscopy as a singlet at δ = 2.78 ppm with multiplets in the
aromatic region ranging from 7.01–7.23 ppm. Isolation and charac-
terization of this species by elemental analysis was not possible
because of instability.
(
(
7
D
8
–
3
3
(
3
3
Synthesis of Tp*
tillation vial was charged with Tp*
.108 mmol) and THF (approximately 5 mL) and was cooled in the
coldwell until frozen. In a separate vial, 1 equiv. N CHSiMe
54 μL, 0.108 mmol) was dissolved in THF and cooled until frozen.
2
U–N(N=CHSiMe
3
)CH=CPh (4): A 20-mL scin-
3
3
3
2
UCH Ph (1) (0.100 g,
2
–
0
3
2
3
(14, 3 H, Tp*-CH
3
(
CH
3
The thawing diazoalkane solution was added dropwise to the green
uranium solution as it thawed, which resulted in a change to red–
brown. The vial was stored in the coldwell for approximately
(
115, 1 H, Ph-CH), 34.16 (126, 2 H, Ph-CH), 40.75 (174, 2 H, Ph-
CH), 41.87 (165, 1 H, Ph-CH), 93.05 (261, 3 H, Tp*-CH ), 114.38
305, 3 H, Tp*-CH ) ppm. IR: ν˜ = 2523, 2552 (B–H) cm . The
3
–
1
(
3
10 min to ensure complete reaction at low temperature. After this
1
by-product in this reaction, bibenzyl, was observed by H NMR
spectroscopy as a singlet at δ = 2.78 ppm with multiplets in the
aromatic region ranging from 7.01–7.23 ppm.
time, excess phenylacetylene was added to the solution, which re-
sulted in an orange–brown solution. After stirring for 5 min, the
volatiles were removed under reduced pressure. Product 4 was iso-
lated in moderate yields (0.072 g, 0.062 mmol, 57%) by recrystalli-
2
Alternative Synthesis of Tp*
2
U(η -N,N-N
2
CPh
2
) (2): A 20-mL scin-
tillation vial was charged with Tp*
.101 mmol), THF (approximately 5 mL) and a stirrer bar, and was
cooled in a liquid-nitrogen coldwell for approximately 30 min. In a
separate vial, 3 equiv. N CPh (0.059 g, 0.303 mmol) were dissolved
in THF (approximately 2 mL), and the solution was cooled in the
coldwell. After 30 min, the thawing solution of N CPh was added
dropwise to the thawing uranium solution while stirring. The mix-
ture was warmed to room temperature over 5 min, and the volatiles
were then removed by vacuum. The brown–yellow oily residue was
recrystallized from a concentrated diethyl ether solution, which re-
sulted in isolation of 2 (0.080 g, 0.078 mmol, 77%).
2
U(2,2Ј-bipy) (0.100 g, zation from a concentrated diethyl ether/pentane (1:2) solution at
0
–35 °C. Orange X-ray quality crystals were grown from a concen-
trated solution of diethyl ether/pentane (1:1) at –35 °C.
C H B N14SiU (1048.76): calcd. C 48.10, H 5.77, N 18.70; found
42 60 2
2
2
1
C 48.28, H 5.82, N 18.68. H NMR (300 MHz, C
–32.18 (23, 3 H, Tp*-CH ), –30.43 (23, 3 H, Tp*-CH
2 H, B-H), –20.21 (23, 1 H, Tp*-CH), –12.01 (17, 3 H, Tp*-CH
–11.75 (21, 3 H, Tp*-CH ), –8.45 (17, 3 H, Tp*-CH ), –6.37 (16,
3 H, Tp*-CH ), –5.75 (21, 3 H, Tp*-CH ), –4.24 (d, J = 11.4 Hz,
9 H, Si(CH ), –2.21 (20, 1 H, Tp*-CH), 0.91 (10, 1 H, Tp*-CH),
1.15 (16, 3 H, Tp*-CH ), 1.31 (7, 3 H, Tp*-CH ), 1.64 (6, 1 H,
6
D
6
, 25 °C): δ =
), –24.21 (82,
),
2
2
3
3
3
3
3
3
3
3 3
)
3
3
6
www.eurjic.org
© 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 0000, 0–0

Job/Unit: I20606
/KAP1
Date: 30-08-12 16:13:20
Pages: 9
Diazoalkane Reduction for the Synthesis of Uranium Hydrazonido Complexes
Tp*-CH), 2.38 (3, 1 H, Tp*-CH), 3.29 (16, 3 H, Tp*-CH
21, 3 H, Tp*-CH ), 11.27 (15, 3 H, Tp*-CH ), 11.79 (3, 1 H, Tp*-
CH), 16.56 (14, 1 H, Tp*-CH), 21.08 [15, 1 H, C(SiMe )H], 25.53
23, 1 H, phenyl-CH), 45.54 (27, 2 H, phenyl-CH), 63.96 (35, 2 H,
phenyl-CH), 163.14 (50, 1 H, olefin-H) ppm. IR: ν˜ = 2521, 2553
3
), 9.09
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2, 4836.
2
(
3
3
[21] T. W. Hayton, Dalton Trans. 2010, 39, 1145.
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[
23] C. R. Graves, P. Yang, S. A. Kozimor, A. E. Vaughn, D. L.
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(
B–H) cm .
Alternative Synthesis of Tp*
0-mL scintillation vial was charged with Tp*
0.100 g, 0.101 mmol), THF (approximately 2 mL), and 1.3 equiv.
phenylacetylene (0.020 g, 0.130 mmol). To an additional vial,
equiv. N CHSiMe (50 μL, 0.101 mmol) was added by microsy-
2
U–N(N=CHSiMe
3
)CH=CPh (4): A
U(2,2Ј-bipy)
2
(
2
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2
3
ringe and diluted with THF (2 mL). This pale yellow solution was
cooled for 30 min in the coldwell. The thawing diazomethane solu-
tion was added dropwise to the original brown–red uranium solu-
tion. An immediate color change to brown–orange was observed.
The volatiles were immediately removed, and the resulting residue
was washed with cold pentane. After drying, the resulting oil was
dissolved in diethyl ether and filtered, and the resulting filtrate
evaporated to dryness (0.100 g, 0.096 mmol, 89%).
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Date: 30-08-12 16:13:20
Pages: 9
Diazoalkane Reduction for the Synthesis of Uranium Hydrazonido Complexes
Uranium Hydrazonido Complexes
Uranium hydrazonido complexes were syn-
thesized by reduction of diazoalkanes by
E. M. Matson, P. E. Fanwick,
S. C. Bart* ........................................ 1–9
2 2
the uranium(III) alkyl Tp* UCH Ph with
extrusion of bibenzyl.
Diazoalkane Reduction for the Synthesis
of Uranium Hydrazonido Complexes
Keywords: Uranium / Diazo compounds /
Metallacycles / Tridentate ligands / Actin-
ides
Eur. J. Inorg. Chem. 0000, 0–0
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