Isolation of a uranium(iii) benzophenone ketyl radical that displays redox-active ligand behaviour

Article abstract of DOI:10.1039/c4dt01636j

The first uranium(iii) charge separated ketyl radical complex, Tp₂U(OC·Ph₂), has been isolated and characterized by infrared, ¹H NMR, and electronic absorption spectroscopies, along with X-ray crystallography. Tp₂U(OC·Ph₂) is a potent two-electron reductant towards N₃Mes (Mes = 2,4,6-trimethylphenyl) and (2,2,6,6-tetramethyl-piperidin-1-yl)oxyl (TEMPO), with reducing equivalents derived from the metal centre and the redox-active benzophenone. This journal is

Full text of DOI:10.1039/c4dt01636j

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Isolation of a uranium(III) benzophenone ketyl
radical that displays redox-active ligand
behaviour†
Cite this: Dalton Trans., 2014, 43,
17885
Received 3rd June 2014,
Accepted 11th July 2014
Ellen M. Matson, John J. Kiernicki, Nickolas H. Anderson, Phillip E. Fanwick and
Suzanne C. Bart*
DOI: 10.1039/c4dt01636j
www.rsc.org/dalton
The first uranium(III) charge separated ketyl radical complex, ligands. For instance, C–C bond formation occurs between
Tp*2U(OC·Ph2), has been isolated and characterized by infrared, a proposed furanaldehyde ketyl radical and the dianionic
1H NMR, and electronic absorption spectroscopies, along with
[ ]
MesPDIMe 2− ligand in tetravalent CpRUI(MesPDIMe), forming a
X-ray crystallography. Tp*2U(OC·Ph2) is a potent two-electron tetradentate chelate.10 We have also demonstrated that
reductant towards N3Mes (Mes 2,4,6-trimethylphenyl) and exposure of trivalent Cp*2U(2,2′-bpy), which contains a radical
=
(2,2,6,6-tetramethyl-piperidin-1-yl)oxyl (TEMPO), with reducing bipyridine ligand, to both aldehydes and ketones results in
equivalents derived from the metal centre and the redox-active reductive C–C heterocoupling of the carbonyl-carbon and 2,2′-
benzophenone.
bipyridine to form the uranium(IV) family, Cp*2U[(NC5H4)-
NC4H3CH-C(R′R)O] (R = R′ = Ph; R = R′ = Me; R = H, R′ = tolyl;
R = H, R′ = furanyl).13 The coupled products are proposed to
form via a ketyl radical intermediate that is generated upon
carbonyl coordination.
Alkali metals,1–3 transition metals,4,5 and lanthanides6,7 have
been studied extensively for their ability to reduce the π-bond
in carbonylated substrates. Single electron transfer from these
metals results in the formation of charge separated radical
ketyl complexes, which are highly reactive and thus typically
invoked as intermediates in C–C coupling reactions. The oxo-
philicity and highly reducing nature of low-valent uranium
renders this metal a prime candidate for such chemistry.
Uranium mediated pinacolate coupling, in which two ketyl
radicals are formed and subsequently coupled, has also been
proposed to occur through radical intermediates.8–10 For
instance, treating trivalent Cp3U with benzophenone results in
dimeric Cp3U(OC(Ph)Ph-CPh2O)UCp3,11 proposed to form via
a uranium(IV) ketyl radical intermediate, Cp3U(OC·Ph2), pro-
duced by electron transfer from U(III) to the ketone followed by
coupling with the para-carbon radical. By analogy, [((AdArO)3-
tacn)U(OC(Ph)Ph-CPh2O)U((AdArO)3tacn)] has been formed in
low yields from addition of benzophenone to trivalent [((AdArO)3-
tacn)U], proposed to occur by generation of the uranium(IV)
charge separated ketyl species, [((AdArO)3tacn)U(OC·Ph2)].12
Recently, work from our lab has demonstrated reductive
heterocoupling of ketyl radical intermediates with redox-active
Isolation of uranium ketyl radical species has proven more
challenging but has been demonstrated with the formation of
[((t-BuArO)3tacn)U(OC·tBuPh2)], which was recently fully charac-
terized by Meyer and co-workers.12 Using the para-tert-butyl
substituted benzophenone prevents the C–C coupling pre-
viously observed in the adamantyl ligand derivative. This
species was found to be quite reactive, readily abstracting an
H atom from THF solvent or other H-donor substrates such as
1,4-cyclohexadiene. Interestingly, the isolation of the corres-
ponding uranium(III) ketyl radical intermediate has remained
elusive. We reasoned that making use of the sterically bulky
hydrotris-(3,5-dimethylpyrazolyl)borate (Tp*) ligand framework
could make isolation of such a reactive species possible.
Herein, we report the characterization of a rare uranium(III)
charge separated ketyl complex, Tp*2U(OC·Ph2). Unlike pre-
vious examples, this complex resists H-atom abstraction and
C–C coupling. Reactivity studies show the ketyl radical is truly
redox-active, using electrons derived both from the metal
centre and the ketyl for reduction of organic substrates.
An initial attempt at formation of the trivalent uranium
ketyl species was made by reduction of Tp*2UI (1-UI) with KC8
in the presence of an equivalent of benzophenone, which
resulted in a bright purple reaction mixture (eqn (1)). Analysis
of an aliquot of this solution by vibrational spectroscopy did
not show a carbonyl stretch, suggesting reduction of the CvO
H.C. Brown Laboratory, Department of Chemistry, Purdue University, West Lafayette,
IN 47906, USA. E-mail: sbart@purdue.edu
†Electronic supplementary information (ESI) available: Full experimental pro-
cedures, spectroscopic and crystallographic data. CCDC 1003448. For ESI and
crystallographic data in CIF or other electronic format see DOI: 10.1039/
c4dt01636j
1
multiple bond occurred (Fig. S2†). Analysis by H NMR spec-
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troscopy revealed thirteen paramagnetically broadened reson-
ances ranging from −227 to 93 ppm, suggestive of a C2 sym-
metric complex in solution (Fig. S1†).
Table 1 Experimental structural parameters of 2
Bond
Distance (Å)
Bond
Angle (°)
162.7(6)
124.8(7)
118.8(8)
116.4(7)
124.4(8)
120.1(8)
U1–N11
U1–N21
U1–N31
U1–N61
U1–N71
U1–N81
U1–O4
2.551(6)
2.586(7)
2.703(6)
2.602(6)
2.594(6)
2.634(7)
2.240(5)
1.339(10)
U1–O4–C4
C41–C4–C51
O4–C4–C41
O4–C4–C51
C42–C41–C46
C52–C51–C56
O4–C4
significantly longer than that of the uranium(III) aryloxide
Tp*2U(ODIPP) (DIPP = 2,6-diisopropylphenyl) (2.159(10) Å)
reported by Marques and co-workers.17 This observation is
consistent with data for [((tBuArO)3tacn)UIV](OC·tBuPh2), which
has an elongated U–O bond distance (2.178 Å) as compared to
its corresponding uranium(IV) methoxide, [((tBuArO)3tacn)-
UIV(OCHtBuPh2)] (2.077(3) Å).12
Further evidence for the formation of the ketyl species was
obtained by examination of intraligand distances of the newly
formed ligand. The O4–C4 distance in complex 2 (1.339(10) Å)
is significantly longer than that reported for free benzophe-
none (1.23(1) Å),18 but consistent with that of the alkali metal-
ketyl complex, [(HMPA)2(Ph2CO)Na]2 (1.299(10) Å)3 and
[((tBuArO)3tacn)UIV](OC·tBuPh2) (1.334(6) Å).12 The C–O distance
in 2 is shorter than that reported for the benzophenone
dianion, [Na2(Ph2CO)(tetraglyme)]∞ (1.401(4) Å),19 supporting
the formulation of 2 as a uranium(III) species. Furthermore,
the bond angles around the ketyl carbon sum to 360°
(118.8(8)°, 116.4(7)°, 124.8(7)°), suggesting retention of sp2
hybridization at C4. Similarly, sp2 hybridization at the ipso
carbons (C46–C41–C42 = 124.4(8)°; C56–C51–C52 = 120.1(8)°)
suggests a single reduction of benzophenone, whereas a sig-
nificant decrease in this angle has been observed for the
benzophenone dianion.19
ð1Þ
To aid in the spectral assignment, the analogous species
was synthesized with benzophenone-d10 and characterized by
1H NMR spectroscopy. For the latter, the absence of the
furthest two upfield (−227.59 ppm, 2H; −178.86 ppm, 4H) and
downfield (93.86 ppm, 4H) resonances confirmed these as
protons arising from coordinated benzophenone. Protons
proximal to ligand radicals on uranium have been previously
observed to have far-ranging chemical shifts, as is the case
with Tp*2U(2,2′-bpy) (1-U(bpy)) (−254.80, −134.80 ppm),14
Cp*2U(2,2′-bpy) (−93.91, −81.40 ppm),15 and CpPU(O2C2Ph4)-
(
MesPDIMe) (−310.96, 177.43 ppm).10 Thus, the purple product
was tentatively assigned as the charge-separated ketyl radical
species, Tp*2U(OC·Ph2) (2).
Crystals of 2‡ suitable for X-ray crystallographic analysis
were obtained from a concentrated solution of n-pentane and
toluene (20 : 1) at −35 °C. Refinement of the data set revealed a
seven coordinate uranium species, Tp*2U(OC·Ph2), in a dis-
torted pentagonal bipyramidal geometry (Fig. 1, Table 1). The
U–N distances for the Tp* ligands range from 2.551(6) to
2.703(6) Å and are consistent with previously reported bis(Tp*)
uranium complexes.16 The U1–O4 distance of 2.240(5) Å is
ˉ
Complex 2 crystallizes in the same unit cell (P1) as the iso-
structural samarium(III) ketyl radical complex, Tp*2Sm-
(OC·Ph2), synthesized by Takats and co-workers.20 Like the
uranium examples, the Sm–O distance of 2.201(3) Å is longer
than that of reported Sm(III)–OR alkoxide complexes.21,22 The
O–C distance in 2 is similar to that of Tp*2Sm(OC·Ph2)
(1.322(6) Å), making 2 the first isolated and crystallographically
characterized uranium(III) charge separated ketyl complex.
The electronic absorption spectrum for 2 was collected
from 400 to 2100 nm in THF at ambient temperature (Fig. 2).
Complex 2 exhibits an intense, color-producing absorption in
the visible region at 556 nm (ε = 863 M−1 cm−1). This absorp-
tion is representative of the π–π* transition of a benzophenone
radical bound to a uranium centre, as previously observed in
the electronic absorption spectrum for [((tBuArO)3tacn)UIV]-
(OC·tBuPh2) (562 nm) and the fleeting [((AdArO)3tacn)-
UIV(OC·Ph2)] (550 nm).12 This transition is blue-shifted from
the π–π* of sodium benzophenone ketyl (624 nm),23 a trend
previously observed for both transition metal5 and lanthanide7
complexes. The near-infrared region is comprised of broad,
Fig. 1 Molecular structure of 2 displayed at 30% probability ellipsoids.
Co-crystallized solvent molecules and selected hydrogen atoms have
been removed for clarity.
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Next, the reducing ability of 2 was tested. Addition of
N3Mes to a solution of 2 resulted in an immediate colour
change to pink with concomitant loss of dinitrogen as indi-
cated by effervescence of the reaction mixture (eqn (2)). Inspec-
tion of the crude product mixture by infrared spectroscopy
revealed the formation of a carbonyl stretch at 1660 cm−1
(Fig. S7†) suggesting oxidation of the C–O single bond in 2.
1H NMR spectroscopy of the crude reaction revealed the pre-
viously characterized uranium(IV) imido species, Tp*2UNMes
(3-UNMes),26 as the sole product in high purity as well as an
equivalent of uncoordinated, neutral benzophenone (Fig. S6†).
Interestingly, 3-UNMes was previously found to react with alde-
hydes to form the corresponding aldimine and Tp*2UO
(3-UO);27 however, in the presence of the sterically more
demanding benzophenone, 3-UNMes is stable.
Fig. 2 Electronic absorption spectrum of 2 recorded in tetrahydrofuran
from 400–2100 nm. Solvent overtones from 1670–1760 nm have been
removed for clarity.
ill-defined f–f transitions of moderate intensity (ε
25–150 M−1 cm−1) characteristic of a uranium(III) centre.24
=
ð2Þ
The reactivity of benzophenone with Tp*2U(2,2′-bpy)
(1-U(bpy)) was investigated to determine if reductive hetero-
coupling of the carbonyl substrate would occur by analogy to
our previous observations with Cp*2U(2,2′-bpy).13 Exposure of
a brown THF solution of 1-U(bpy) to one equivalent of benzo-
phenone resulted in a gradual colour change to purple
(eqn (1)). Following workup, inspection of the product by
1H NMR spectroscopy surprisingly revealed the formation of 2
as well as an equivalent of uncoordinated 2,2′-bipyridine.
While both trivalent uranium bipyridine complexes, 1-U(bpy)
and Cp*2U(2,2′-bpy), reduce the C–O π-bond, formation of a
new C–C bond and oxidation to uranium(IV) is only observed
in the case of Cp*2U(2,2′-bpy), whereas the trivalency of
1-U(bpy) is maintained in formation of 2. We attribute this
difference in reactivity to the increased steric bulk of the ancil-
lary Tp* ligand framework, which cannot accommodate a
bulky tridentate heterocoupled ligand.
To determine whether this redox-active nature of benzophe-
none was general, complex 2 was exposed to an equivalent of
TEMPO (TEMPO
=
(2,2,6,6-tetramethyl-piperidin-1-yl)oxyl)
resulting in an immediate colour change to light pink. Inspec-
tion of the crude sample by infrared spectroscopy revealed the
same carbonyl stretch observed in the reaction with N3Mes,
suggesting reformation of uncoordinated benzophenone
(Fig. S5†). 1H NMR spectroscopy displayed a single product,
3-UO,27 as well as two organic products, benzophenone and
2,2,6,6-tetramethylpiperidine (TMP) (formed by H-atom
abstraction from solvent) (Fig. S4†). To our knowledge, this is
the first homogeneous system in which benzophenone acts as
a redox-active ligand by first accepting an electron in the for-
mation of 2 and then ejecting this electron in the formation of
3-UNMes and 3-UO.
In order to determine how trivalent 2 compares to pre-
viously observed uranium ketyl radicals, reactivity studies were
undertaken. Exposure of 2 to an additional equivalent of
benzophenone resulted in decomposition to an unidentified
complex commonly observed with this ligand system,
suggesting formation of a pinacolate ligand is sterically in-
In summary, the first trivalent charge separated radical
ketyl complex of uranium, Tp*2U(OC·Ph2) (2), has been syn-
thesized. X-ray crystallography was used to establish structural
parameters which were found to be consistent with a reduced
benzophenone ligand, while electronic absorption spec-
troscopy confirmed the U(III) oxidation state. Complex 2 dis-
plays reactivity in sharp contrast to literature precedent by
resisting both H-atom abstraction and C–C coupling. Alter-
nately, 2 can perform the two electron reduction of organic oxi-
dants by using reducing equivalents derived from the metal
centre and the ketyl ligand, demonstrating the redox-active be-
haviour of the latter.
accessible with the bis(Tp*) framework. This was corroborated
25
by treating Tp*2UCl2
with one equivalent of 1,1,2,2-tetra-
phenyl-1,2-ethanediol in the presence of KCH2Ph, which
resulted in a complex mixture of uranium containing products.
Complex 2 also did not show similar reactivity to [((t-BuArO)3tacn)-
U(OC·tBuPh2)],12 as addition of 1,4-cyclohexadiene to a THF
solution of 2 yielded no reaction with neither consumption of
cyclohexadiene nor C–C coupling apparent.
We gratefully acknowledge funding by the National Science
Foundation (CAREER grant to SCB, CHE-1149875).
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Notes and references
‡Crystal data for 2: C43H54B2N12OU, 0.5(C7H7), Mr = 1060.199; triclinic; space
ˉ
group P1; a = 11.0395(8), b = 13.5231(9), c = 17.6638(18) Å; α = 72.590(5), β =
86.210(5), γ = 67.426(4)°; V = 2320.04 Å3; Z = 2; ρ = 1.518 g cm−3; T = 150(1) K;
R1 = 0.068; wR2 = 0.179.
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17888 | Dalton Trans., 2014, 43, 17885–17888
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