Aryloxide anions can form outer sphere complexes with metals as electropositive as uranium

Article abstract of DOI:10.1039/b912222b

A uranium complex containing an outer sphere aryloxide anion is formed by the proteolytic cleavage of the methyl group in the mono-methyl uranium metallocene, (C₅Me₅)₂[ⁱPrNC (Me)NⁱPr]UMe, by 2,6-di-te

Full text of DOI:10.1039/b912222b

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Aryloxide anions can form outer sphere complexes with metals
as electropositive as uraniumw
William J. Evans,* Justin R. Walensky and Joseph W. Ziller
Received (in Cambridge, UK) 22nd June 2009, Accepted 20th October 2009
First published as an Advance Article on the web 5th November 2009
DOI: 10.1039/b912222b
A uranium complex containing an outer sphere aryloxide anion
is formed by the proteolytic cleavage of the methyl group in the mono-
methyl uranium metallocene, (C5Me5)2[iPrNC(Me)NiPr]UMe,
by 2,6-di-tert-butyl-4-methyl phenol.
The coordination chemistry of uranium is of considerable
importance due to the use of uranium as a nuclear fuel1 and
the control of its speciation in nuclear waste streams.2 Recent
studies have shown that the secondary coordination sphere of
uranium, as well as other actinides, plays a critical role in
overall speciation.3 The coordination chemistry of uranium
with oxygen-donor ligands is of particular interest since
Fig. 1 Thermal ellipsoid plot of the cation {(C5Me5)2[iPrNC(Me)-
NiPr-k2N,N0]U}1+
in 2, shown at the 50% probability level.
actinides are oxophilic and oxygen-donor ligands are
prevalent in waste streams and in the environment.
,
Hydrogen atoms have been omitted for clarity. U(1)–Cnt(C5Me5) =
2.433, 2.455 A, U(1)–N(1) = 2.331(1) A, U(1)–N(2) = 2.344(1) A,
U(1)–C(21) = 2.749(2) A, Cnt(C5Me5)–U(1)–Cnt(C5Me5) = 137.71,
Cnt(C5Me5)–U(1)–N(1) = 105.91, 109.01, Cnt(C5Me5)–U(1)–N(2) =
109.61, 108.81, N(1)–U(1)–N(2) = 57.86(7)1.
Recent studies of uranium metallocenes in sterically crowded
coordination environments have generated metallocene amidi-
nate complexes like (C5Me5)2[iPrNC(Me)NiPr-k2N,N0]UMe,
1, that contain a single hydrocarbyl ligand.4 Mono-methyl 1 is
unusual compared to its dimethyl metallocene precursor,
(C5Me5)2UMe2, in that the methyl group in 1 is much less
reactive. For example, in contrast to (C5Me5)2UMe2, the
U–Me bond in 1 does not react with HCRCPh or HNPh2
or H2, even at 80 psi.5 In efforts to find reagents that would
cleave the methyl group, aryl alcohols were studied and found
to form a product that contained an outer sphere aryloxide
anion. This was unexpected with a metal as electropositive as
uranium.6
Complex 1 does not react with 2,6-di-tert-butyl-4-methyl
phenol, ArOH, in toluene at room temperature, but
Fig. 2 Thermal ellipsoid plot of the hydrogen bonding between the
outer sphere aryloxide and aryl alcohol in 2 is shown at the 50%
probability level. O(1)–C(29) = 1.370(3) A, O(2)–C(44) = 1.302(3) A,
O(1)–H(1) = 0.762 A, O(2)ꢀ ꢀ ꢀH(1) = 1.810 A.
upon heating to 85 1C for
1
h, the maroon
complex {(C5Me5)2[iPrNC(Me)NiPr]U}1+{MetBu2C6H2Oꢀ ꢀ ꢀ
t
HOC6H2 Bu2–Me}1ꢁ, 2, is formed according to eqn (1).
1
The presence of methane was identified by H NMR spectro-
crystallographically characterized in the past. However, these
involve either a transition metal that coordinates the ligand via
the phenyl ring, e.g. (C8H12)Rh(Z6-ArO)(HOAr)7 or
scopy in a reaction in a sealed NMR tube. The reaction
of 1 with one equiv. of the aryl alcohol gave lower yields
of 2.z
The structure of 2 was determined by X-ray diffraction and
the data revealed a metallocene amidinate uranium cation
complex, Fig. 1, with an outer sphere aryloxide anion
hydrogen bonded to an aryl alcohol as shown in Fig. 2. To
our knowledge, this is the first example of an outer sphere
aryloxide metal complex. Aryloxide anions that do not
coordinate to a metal via the oxygen-donor atom have been
(1)
Department of Chemistry, University of California, Irvine,
CA 92697-2025, USA. E-mail: wevans@uci.edu;
Fax: +1-949-824-2210
w CCDC 735106. For crystallographic data in CIF or other electronic
format see DOI: 10.1039/b912222b
ꢂc
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complexes
that
contain
no
metal,
e.g.
8
protected. Hence, when there is sufficient steric bulk around
uranium from other ligands, it is possible to have anionic
oxygen-donor atom ligands that exist as outer sphere ions.
This steric argument is supported by the reaction of 1 with a
smaller aryl alcohol. In contrast to eqn (1), phenol reacts
immediately at room temperature with 1 to form an orange
[(Ph3PMe)1+(OC6H3Ph2-2,6)1ꢁ]2,8 [(Ph3PEt)1+(OAr)1ꢁ
]
,
N
and the [C3(NMes)2H]1+ imidazolium salt of [(OAr)]1ꢁ 9
.
Hydrogen bonding with aryloxide ligands is observed in the
latter three cases, but only in the rhodium complex does this
involve interaction with an aryl alcohol as in 2.
1
The aryloxide anion was characterized in several ways. The
hydrogen of the alcohol hydrogen bonded to the aryloxide
anion was located and refined in the crystallographic analysis.
solution. The H NMR spectrum and the unit cell of orange
crystals grown from this solution were identical to that of the
previously reported (C5Me5)2U(OPh)2, 3, eqn (2).18 Whether
this reaction is done with one equiv. of phenol or at ꢁ35 1C,
only 3 was observed. Resonances for methane and the
protonated amidinate ligand were observed by 1H NMR
spectroscopy.
1
Aryloxide resonances at 7.05, 2.24, and 1.38 ppm in the H
NMR spectrum are nearly identical to those of the protonated
ligand, but no resonance for the alcohol proton is observed.
The fact that these resonances are not shifted by the para-
magnetic uranium center is consistent with the solid state
structure that shows the aryloxide as an outer sphere ligand.
The infrared spectrum of 2 is also consistent with the
presence of an aryl alcohol–aryloxide unit. A characteristic
hydrogen bonded OH absorption is seen at 3650 cmꢁ1. This is
shifted from the normal OH stretching frequency of 3630 cmꢁ1
in the free aryl alcohol.
(2)
In conclusion, the protonation of the methyl group in 1 with
a large aryl alcohol leads to the formation of an outer sphere
aryloxide that is hydrogen bonded to its protonated counter-
part. With appropriate steric manipulation, in this case using
the {(C5Me5)2[iPrNC(Me)NiPr-k2N,N0]}3ꢁ ligand set, outer
sphere aryloxide complexes will result even in the presence
The 1.370(3)
A C(29)–O(1) bond distance in the
{ArOꢀ ꢀ ꢀHOAr}1ꢁ anion in 2 is indicative of a C–O single
bond10 and is close to the analogs of 1.383(3) A in ArOH,11
1.381(3) A in 2,6-di-tert-butyl-4-phenyl phenol,12 and 1.387(1) A
in 2,6-di-tert-butyl phenol.13 The C(29)–O(1) bond in 2 also
compares well to the 1.382(6) A C–O bond distance for the
aryl alcohol that is hydrogen bonded to an aryloxide in
(C8H12)Rh(Z6-ArO)(HOAr)7 and the 1.379(4) A C–O distance
in the 2,6-di-tert-butyl phenol in MoO2(OC6H3-2,6-tBu2)2ꢀ
HO-2,6-tBu2C6H3.14 The 1.302(3) A C(44)–O(2) length in 2
is shorter than C(29)–O(1) and the 1.345(6) A length in the
lithiated salt [Li(OAr)(OEt2)]2,15 but it is very close to the
1.306(2) A length in [C3(NMes)2H]1+[(OAr)]1ꢁ.9 The hydrogen
bond length of 1.810(3) A in 2 is shorter than the 1.935(5) and
1.998(3) A distances in [(Ph3PMe)1+(OC6H3Ph2-2,6)1ꢁ]2 and
of large, highly charged, electropositive metal ions like U4+
.
We gratefully acknowledge the Chemical Sciences,
Geosciences, and Biosciences Division of the Office of Basic
Energy Sciences of the Department of Energy for support of
this research. We thank Michael K. Takase for assistance with
X-ray crystallography.
Notes and references
z Synthesis of2. In a glovebox, 2,6-di-tert-butyl-4-methyl phenol
(150 mg, 0.681 mmol) was added to a stirred solution of 1 (223 mg,
0.335 mmol) in toluene (8 mL). When the solution was heated to 85 1C,
the color turned from yellow-brown to red. After 1 h, the solvent was
removed to generate 2 as a maroon powder (175 mg, 60%). Crystals
suitable for X-ray crystallography were grown from a saturated
solution of pentane–toluene at ꢁ35 1C. 1H NMR (C6D6, 298 K): d
7.05 (s, 2H, C6H2), 4.14 (s, 6H, CHMe2), 2.24 (s, 3H, Me), 1.38
(s, 18H, CMe3), ꢁ1.65 (s, 30H, C5Me5), ꢁ8.14 (s, 6H, CHMe2). 13C
NMR (C6D6, 298 K): d 155.3 (C5H2), 136.9 (C5H2), 130.8 (C5H2),
126.3 (C5H2), 37.2 (C5H2), 31.0 (tBu), 25.8 (Me). IR (KBr): 3650m,
3279w, 2965s, 2903s, 2858s, 2722m, 1637m, 1556m, 1440s, 1414s,
1378s, 1311s, 1194s, 1118m, 1017m, 805m cmꢁ1. Anal. calcd for
C58H94N2O2U: C, 63.95; H, 8.70; N, 2.57%. Found: C, 64.14; H,
8.48; N, 2.39%. When the reaction was done in a sealed J-Young tube,
the presence of methane was observed in the 1H NMR spectrum
(d 0.14 ppm). Crystallographic data for 2: C58H94N2O2Uꢀ2.5C7H8,
M = 1319.72, monoclinic, space group P21/n, a = 13.8613(5) A, b =
21.4698(7) A, c = 23.4715(8) A, b = 101.001, V = 6856.7(4) A3, Z =
4, T = 103(2) K, m = 2.412 mmꢁ1, 90 785 reflections measured on a
Bruker CCD diffractometer, 19 175 unique (Rint = 0.0303) which were
used in all calculations. The final R1 was 0.0280 (I > 2.0s(I)) and wR2
(all data, 0.72 A) was 0.0687. The APEX220 program package was
used to determine the unit-cell parameters and for data collection
(20 seconds per frame scan time for a sphere of diffraction data). The
raw frame data were processed using SAINT21 and SADABS22 to
yield the reflection data file. Subsequent calculations were carried out
using the SHELXTL23 program. The diffraction symmetry was 2/m
and the systematic absences were consistent with the orthorhombic
space group P21/n that was later determined to be correct. The
structure was solved by direct methods and refined on F2 by
full-matrix least-squares techniques. Synthesis of3. In a glovebox,
[(Ph3PEt)1+(OAr)1ꢁ N, respectively,8 which involve multiple
]
hydrogen bonds to the oxygen atoms.
The structure of the {(C5Me5)2[iPrNC(Me)NiPr-k2N,N0]U}1+
cation is similar to that of 1 except that the (C5Me5 ring
centroid)–U–(C5Me5 ring centroid) angle has opened up to
137.71 in 2 compared to 130.91 in 1. A similar situation is
found with [(C5Me5)2ThMe][B(C6F5)4]16 and (C5Me5)2ThMe2
17
which have 140.11 and 133.91 angles, respectively. Since the
cation in 2 has one less ligand than 1, shorter U-ligand
distances are expected. Indeed, the 2.433 and 2.455 A
U–(C5Me5 ring centroid) bond distances in 2 are shorter than
the 2.527 and 2.538 A values in 1. Similarly, the U–N bond
distances of 2.331(1) and 2.344(1) A in 2 are shorter than the
2.461(2) and 2.453(2) A lengths in 1. However, the shortening
is more than the 0.05 A difference in the ionic radii of 8- and
9-coordinate U4+ 18
.
The isolation of a structure for 2 with an outer sphere
aryloxide anion can be explained using steric arguments. The
{(C5Me5)2[iPrNC(Me)NiPr-k2N,N0]}3ꢁ metallocene amidinate
ligand set apparently does not allow another ligand as large as
(OAr)1ꢁ to coordinate to the U4+ center. This is consistent
with the limited reactivity of the methyl group in 1: although a
group as small as methyl can coordinate to U4+ along with
two (C5Me5)1ꢁ ligands and an amidinate, it is sterically
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The H NMR spectrum and unit cell confirmed the product to be the
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¨
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