Utility of a redox-active pyridine(diimine) chelate in facilitating two electron oxidative addition chemistry at uranium

Article abstract of DOI:10.1039/c4cc03355h

Exposure of the uranium(iv) complex, Cp^PU(^MesPDI ^Me) (1) (^MesPDI^Me = 2,6-((Mes)NCMe) ₂-C₅H₃N; Mes = 2,4,6-trimethylphenyl; Cp ^P = 1-(7,7-dimethylbenzyl)cyclopentadienyl), which contains a [ ^MesPDI^Me]^3- chelate, to I₂, Cl ₂, PhSeCl, and PhEEPh (E = S, Se, Te) results in oxidative addition to form the uranium(iv) family, Cp^PU(XX′)( ^MesPDI^Me) (X = X′ = I, Cl, EPh; X = SePh, X′ = Cl). Spectroscopic and structural studies support products with [ ^MesPDI^Me]^1-, indicating the reducing equivalents derive from this redox-active chelate. This journal is the Partner Organisations 2014.

Full text of DOI:10.1039/c4cc03355h

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Utility of a redox-active pyridine(diimine) chelate
in facilitating two electron oxidative addition
chemistry at uranium†
Cite this: Chem. Commun., 2014,
0, 8189
5
Received 5th May 2014,
Accepted 5th June 2014
John J. Kiernicki, Phillip E. Fanwick and Suzanne C. Bart*
DOI: 10.1039/c4cc03355h
www.rsc.org/chemcomm
P
Mes
Me
Mes
Me
P
Exposure of the uranium(IV) complex, Cp U( PDI ) (1) ( PDI
,6-((Mes)NQCMe)
–C N; Mes
-(7,7-dimethylbenzyl)cyclopentadienyl), which contains a [ PDI
chelate, to I , Cl
=
=
Compound 2-Cl was isolated as a brown solid in moderate yields
(40–60%), and showed a paramagnetic H NMR spectrum with
2
1
2
2
5
H
3
=
2,4,6-trimethylphenyl; Cp
Mes
Me 3ꢀ
1
]
twelve broadened resonances ranging from ꢀ46.67 to 80.20 ppm,
2
2
, PhSeCl, and PhEEPh (E = S, Se, Te) results in oxidative reminiscent of that for 2-I
2
. The most intense singlet (ꢀ2.84 ppm)
P
0
Mes
Me
0
3
addition to form the uranium(IV) family, Cp U(XX )( PDI ) (X = X = I, was assigned as the ortho-mesityl –CH (12H) protons, suggesting a
0
Cl, EPh; X = SePh, X = Cl). Spectroscopic and structural studies support
C2v symmetric compound in solution.
Mes
Me 1ꢀ
products with [ PDI
] , indicating the reducing equivalents derive
from this redox-active chelate.
1
,2
0
3–5
6
6,7
Redox-active a-diimines, 2,2 -bipyridine, Z -arenes, and
iminoquinones
8
,9
all promote multi-electron chemistry at
uranium centres, opening up additional reactivity pathways
not typically observed for the actinides. One such example is
oxidative addition, which until recently, has required multi-
1
0–12
nuclear systems to accomplish this two electron reaction.
(1)
Analysis of single crystals of 2-Cl suitable for X-ray diffraction
Recently, we have described the synthesis of the cyclopentadienyl
uranium(IV) species bearing a triply reduced pyridine(diimine)
chelate, Cp*U( PDI )(THF), which uses ligand electrons to facili- revealed the expected trans-dichloro-uranium pyridine(diimine)
tate the oxidative addition of azobenzene. Both Cp*U( PDI )- complex bound by an Z -Cp (U–Ct = 2.510 Å) ligand (Fig. 1, left).
THF) and its analogue, Cp U( PDI ) (1), are active for C–C The structural parameters for the newly installed trans-chloride
2
Mes
Me
13
Mes
Me
5
P
P
Mes
Me
(
coupling of carbonylated substrates mediated by ligand reducing ligands (157.60(4)1; 2.7382(12) and 2.6246(13) Å) are similar
14
equivalents, serving to maintain the U(IV) oxidation state. Thus, we to other uranium(IV) trans-chloride complexes including
set out to determine if 1 could also perform oxidative addition [(Me Si) N] UCl (DME) (155.9(1)1, 2.640(3) and 2.630(3) Å)
reactions in analogy to the Cp* derivative, given that its Cp ligand is reported by Templeton and co-workers.
3
2
2
2
P
15
less electron donating.
Previous work from our laboratory has established that
Studies commenced with exploring dihalide activation by com- uranium–nitrogen distances for the pyridine(diimine) chelate are a
13,14
pound 1. Addition of one equivalent of either I or PhICl to the dark reliable metric to determine the extent of ligand reduction.
The
) and two longer U–Nimine distances (U–N1 = 2.497(4); U–N3 = 2.440(4) Å)
), respectively (eqn (1)). Formation of 2-I are consistent with dative interactions, whereas the shorter
2
2
Me
P
Mes
brown solution resulted in isolation of Cp UI
2
(
PDI ) (2-I
2
P
Mes
Me
Cp UCl
2
(
PDI ) (2-Cl
2
2
proceeded in near quantitative yield, and was confirmed by com- U–Npyr bond (U–N2 = 2.402(4) Å) suggests an anionic interaction
1
14
parison of the H NMR spectrum to that previously published.
that would occur with a reduced ligand. These distances compare
favourably to 2-I (U–N1 = 2.522(10); U–N2 = 2.368(10); U–N3 =
2.484(9) Å), which is established to bear a [ PDI anion by
variable temperature magnetic susceptibility studies.
The intraligand bond distances further support the formula-
2
Mes
Me 1ꢀ
14
H.C. Brown Laboratory, Department of Chemistry, Purdue University,
]
West Lafayette, IN 47907, USA. E-mail: sbart@purdue.edu; Fax: +1-765-494-0239;
Tel: +1-765-494-5451
†
Electronic supplementary information (ESI) available: Experimental details,
1
2
tion of 2-Cl as containing a monoanionic ligand. As compared
supporting figures, H NMR data, and crystallographic details. CCDC 1000462–
000465. For ESI and crystallographic data in CIF or other electronic format see
Mes
Me
to the free
PDI ligand, the elongated C–Nimine distances
1
DOI: 10.1039/c4cc03355h
(N1–C2 = 1.320(6); N3–C8 = 1.324(6) Å) and contracted adjacent
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Fig. 1 Molecular structures of (left to right) 2-Cl
2
, 3, 4-SPh, and 4-SePh displayed at 30% probability level. Hydrogen atoms, solvent molecules, selected
phenyls (top), and mesityls (bottom) have been omitted for clarity.
C
imine–Cpyridine distances (C2–C3 = 1.447(7); C7–C8 = 1.438(7) Å)
With the successful oxidative addition of halide substrates, the
16
are indicators of pyridine(diimine) reduction. These distances are bis(chalcogenides), PhEEPh (E = S, Se, Te), were tested under
in agreement with those reported for Cp U(O
P
Mes
Me
2
C
2
Ph
4
)( PDI
)
analogous conditions to 3. Adding one equivalent of either PhSSPh,
(
N1–C2 = 1.305(6); N3–C8 = 1.321(6); C2–C3 = 1.440(7); C7–C8 = PhSeSePh, or PhTeTePh to a toluene solution of 1 afforded
14
OMe
Me
P
Mes
Me
P
Mes
Me
1
.435(7) Å)
Mn(III)][PF ], ( PDI = 2,6-bis[1-(4-methoxyphenylimino)ethyl]- Cp U(TePh)
pyridine) (average C–N = 1.321(1); average Cimine–Cpyridine Complexes 4-SPh, 4-SePh, and 4-TePh were investigated by
and the manganese(III) complex, [( PDI
)
2
-
Cp U(SPh)
2
(
2
(
PDI ) (4-SPh), Cp U(SePh)
PDI ) (4-TePh), respectively, in high yields (eqn (2)).
2
(
PDI ) (4-SePh), and
OMe
Me
P
Mes
Me
6
=
imine
17
1
1
.442(1) Å), both of which are established to have monoanionic
H NMR spectroscopy, and showed analogous paramagnetically
shifted and broadened spectra with C2v symmetry for each (Fig. 2).
Mes
Me 1ꢀ
pyridine(diimine) ligands, suggesting a [ PDI
]
ligand in 2-Cl
2
.
In order to test the generality of the oxidative addition reaction, All spectra have an intense resonance for the equivalent ortho-methyl
one equivalent of phenylselenyl chloride was added to a toluene protons of the imine-aryl substituent in the range of 2 to ꢀ9 ppm.
P
Mes
Me
solution of 1, forming Cp U(SePh)(Cl)( PDI ) (3) in modest Additionally, in each case, the para-pyridine proton has the most
1
yields (eqn (1)). Characterization by H NMR spectroscopy upfield shift, in the range of ꢀ40 to ꢀ85 ppm. For 4-SPh and 4-SePh,
s
revealed a C symmetric complex with seventeen paramagnetically the furthest downfield resonance, 44.56 and 29.28 ppm, respectively,
broadened resonances ranging from ꢀ77.54 to 45.12 ppm. The
furthest upfield (1H) and downfield (6H) resonances are assigned
as the p-pyridine proton and the equivalent imine methyl protons,
respectively, suggesting radical character delocalized throughout
Mes
Me
18–20
the [ PDI ] ligand plane.
No disproportionation of 3
, in contrast to that
occurred as evidenced by the absence of 2-Cl
noted in dimeric high-valent systems.
2
12
The absolute configuration of 3 was confirmed by an X-ray
diffraction analysis of single crystals. Refinement revealed a
5
P
pyridine(diimine) uranium complex bound by an Z -Cp (U–Ct =
.509 Å) in which the PhSeCl substrate has been added across the
metal centre (Fig. 1, second from left). The U–Cl distance (U–Cl1 =
.621(2) Å) is similar to 2-Cl , as well as the indenyl uranium(IV)
complex reported by Goffart and co-workers, (Z -C
2
2
2
5
9
H
7
)UCl
U–Clavg = 2.609(2) Å), while the uranium–selenide distance (U–Se =
.8729(11) Å) is slightly longer than other uranium(IV) complexes,
3 2
(THF)
2
1
(
2
2
2
2
such as Cp* U(SePh) (U–Se1 = 2.8011(7) Å) and Tp* U(Z -Se )
2
2
2
2
2
3
(
1
2
U–Se: 2.8147(5), 2.7745(5) Å). Pseudo-octahedral 3 (Se1–U–Cl1 =
60.43(6)1) has two long U–N bonds (U–N1 = 2.485(6); U–N3 =
.491(7) Å) assigned as dative interactions, as well as a short U–Npyr
distance (U–N2 = 2.367(7) Å) on the order of an anionic bond,
1
6 6
Fig. 2 H NMR spectra (C D , 23 1C) of 4-SPh (top), 4-SePh (middle), and
crossover experiment (bottom; major product is 4-SPh/SePh). Portions of
the plot from ꢀ65 to ꢀ35 ppm and 15 to 25 ppm have been removed to
increase clarity. The furthest upfield resonances are inset.
Mes
Me 1ꢀ
indicating a uranium(IV) ion with a [ PDI
]
ligand.
8190 | Chem. Commun., 2014, 50, 8189--8192
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ꢀ
1
ꢀ1
is assigned as the imine-methyl protons, whereas in 4-TePh, this All complexes display similar strong (e = 400–1100 M cm ),
resonance is shifted to 3.56 ppm. The strong shifting of resonances broad absorptions throughout the near infrared region arising
for protons in the plane of the pyridine(diimine) ligand is indicative of from intramolecular electron transfer (IET) between the pyridine
radical delocalization throughout this area in solution at 23 1C, causing radical and the uranium(IV) metal centre, a phenomenon previously
1
18,20
14
29
the observed symmetry in the H NMR spectrum.
resonances shifting depending on the identity of the arylchalcogenide ligand radical. Complex 2-Cl
have also been previously observed in complexes of the type electron withdrawing chloride ligands. Each complex also con-
Trends of NMR observed in both actinides and transition metals containing a
2
exhibits a red-shifted IET due to the
30
24
22
25
Cp*
2
U(EPh)
2
(E = S, Se, Te ) where both the Cp* and the –EPh tains three distinct transitions in the visible region (440–460 nm,
550 nm, 620–650 nm) comparable to the previously reported
resonances shift downfield as the atomic number of E increases.
Mes
Me 1ꢀ
14
uranium(IV) [ PDI
]
complexes.
The synthesis of 2-I , 2-Cl , 3, and the 4-EPh family highlights
2
2
Mes
Me
the role of the redox-active
PDI
in oxidative addition
reactions by compound 1. In each case, the substrate is formally
reduced by two electrons that are derived solely from the
Mes
Me 3ꢀ
Mes
Me 3ꢀ
[
[
PDI
PDI
]
trianion in 1. Thus, [ PDI
]
is oxidized to
Mes
Me 1ꢀ
]
in the process. Making use of the stored reducing
equivalents in the pyridine(diimine) chelate allows activation to
occur at a mononuclear uranium species, whereas previously
(2)
Complexes 4-EPh were independently synthesized via salt such chemistry has been observed at dinuclear uranium species.
metathesis of MEPh (M = K, E = S, Se; M = Na, E = Te) with 2-I2 For instance, the two electron reduction of PhSSPh has been
(
eqn (2)) in greater (4-SPh) or similar (4-SePh, 4-TePh) yields to accomplished by Cummins and co-workers, who reported the
the oxidative addition reaction. inverted sandwich dimer, (m-C )[U(N[R]Ar) (R = –C(CH
Absolute configuration of 4-SPh was confirmed by analysis of Ar = 3,5-C Me ), reacts with two equivalents of diphenyldi-
single, X-ray quality crystals obtained by layering a diethyl ether sulphide to yield the dimeric thiolate-bridged uranium(IV) com-
solution with n-pentane (ꢀ35 1C). Refinement of the data revealed a plex, [U(m-SPh)(SPh)(N[R]Ar) and toluene. In this case, the
pyridine(diimine) uranium complex bearing trans-phenylsulphide reduced toluene ring, along with the uranium centre, is the
H
7 8
]
2 2
3 3
) ,
H
6 3
2
2 2
]
5
P
6
(
2
S1–U–S2 = 163.821) substituents, capped by an Z -Cp (U–Ct = source of the reducing equivalents. Diphenyldisulphide reduc-
.520 Å) ligand (Fig. 1, second from right). The uranium–sulphide tion was also observed by Evans and co-workers with the
distances (U–S1 = 2.7440(16); U–S2 = 2.7404(16) Å) are similar to the uranium(IV) ‘‘tuck-in tuck-over’’ Cp*U[m-C Me (CH ) ](m-H) UCp*
5
3
2 2
2
2
4 3
tetra(phenylthiolate) complex, U(SPh) (py) (U–S: 2.717–2.764 Å), complex. Activation proceeds by C–H reductive elimination of the
2
6
31
reported by Neu and co-workers. The newly formed thiolate cyclometallated Cp* ring to yield Cp*
ligands, U–S1–C27 and U–S2–C33, possess bond angles of 113.8(2) With the ability of 1 to perform two-electron reduction of
and 112.5(2)1, respectively, similar to the trans-phenylthiolate diphenyldichalcogenides established, the mechanism for addi-
2 2
U(SPh) .
t
27
complex (Ph
3 2 2 2
PO) U(N Bu) (SPh) (U–S–Cipso = 110.6(2)1). As in the tion to uranium was explored. While trans addition products
case of 2-I , 2-Cl , and 3, the pyridine(diimine) ligand displays two might immediately suggest an S 2 or radical mechanism,
2
2
N
long U–Nimine distances (U–N1 = 2.526(5); U–N3 = 2.480(5) Å) and concerted oxidative addition followed by an isomerisation
one shorter U–N distance (U–N2 = 2.375(5) Å), supporting a could also be operative. Thus, a crossover experiment was
pyr
Mes
Me 1ꢀ
14
[
PDI
]
anion.
conducted by stirring two equivalents of 1 with a pre-mixed
Similarly, refinement of the data obtained from single, X-ray equimolar toluene solution of PhSSPh and PhSeSePh (eqn (3)).
1
quality crystals of 4-SePh (Fig. 1, right) revealed a complex isostruc- Analysis of the reaction mixture by H NMR spectroscopy
tural to 4-SPh with trans-phenylselenolate (Se1–U–Se2 = 166.564(16)1) revealed two minor products, 4-SPh (22%), 4-SePh (25%), as
P
Mes
Me
substituents. Metrical parameters of the uranium–nitrogen distances well as the major crossover product Cp U(SPh)(SePh)( PDI
suggest similar bonding and degree of ligand reduction to that in (4-SPh/SePh) (50%) (Fig. 2, bottom). Combining independently
-SPh. The U–Se distances (U–Se1 = 2.8694(4), U–Se2 = 2.9032(5) Å) synthesized samples of 4-SPh and 4-SePh did not scramble the
)
4
are longer than those reported for the cis-uranium(IV) complex –EPh ligands even at elevated temperatures. Additionally,
2
2
2
23
Cp*
2
U(SePh)
2
(2.800, 2.801 Å) or Tp*
2
U(Z -Se
2
), but similar to scrambling prior to oxidative addition was also dismissed by
1
compound 3, and the trans-uranium(VI)-diselenolate, (Ph
3
PO)
2
H NMR spectroscopic analysis of an equimolar solution of
t
27
U(N Bu)
decrease in the angle of the newly formed phenylselenolate ligands
U–Se1–C27 = 109.43(12)1; U–Se2–C33 = 107.90(12)1) compared to
2
(SePh)
2
(2.885 Å), reported by Boncella and coworkers.
A
(
that of 4-SPh reflects the trend of decreasing degree of
s-hybridization as the atomic number of the chalcogenide increases,
28
a
Cp*
trend previously established for the Cp* Sm(EPh)(THF),
2
22,25
t
27
2
U(EPh)
2
,
and (Ph
3
PO)
2
U(N Bu)
2
(EPh)
2
families.
Further confirmation of ligand and metal oxidation states
was gained using electronic absorption spectroscopy (Fig. S1, ESI†).
(3)
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PhSSPh and PhSeSePh. While the solution phase bond
7 W. J. Evans, C. A. Traina and J. W. Ziller, J. Am. Chem. Soc., 2009, 131,
ꢀ1
17473–17481.
strengths of PhSSPh and PhSeSePh (46 and 41 kcal mol
,
8
E. M. Matson, S. R. Opperwall, P. E. Fanwick and S. C. Bart, Inorg.
Chem., 2013, 52, 7295–7304.
3
2
respectively) are slightly different, the control experiments
confirm the observed scrambling to form 4-SPh/SePh was
unequivocally due to oxidative addition. Thus, oxidative addi-
tion of PhEEPh substrates by 1 occurs via a radical mechanism,
9 E. M. Matson, S. M. Franke, N. H. Anderson, T. D. Cook,
P. E. Fanwick and S. C. Bart, Organometallics, 2014, 33, 1964–1971.
0 R. G. Finke, Y. Hirose and G. Gaughan, J. Chem. Soc., Chem.
Commun., 1981, 232–234.
1
Mes
Me 3ꢀ
and is facilitated by reducing equivalents from [ PDI
The major product, 4-SPh/SePh, displays C symmetry in the
H NMR spectrum, showing twenty paramagnetically broa-
]
.
11 R. G. Finke, D. A. Schiraldi and Y. Hirose, J. Am. Chem. Soc., 1981,
03, 1875–1876.
1
s
1
2 L. P. Spencer, P. Yang, B. L. Scott, E. R. Batista and J. M. Boncella,
1
Inorg. Chem., 2009, 48, 11615–11623.
dened resonances with five resonances integrating to 6 protons 13 D. P. Cladis, J. J. Kiernicki, P. E. Fanwick and S. C. Bart, Chem.
Commun., 2013, 49, 4169–4171.
each. Most interesting are those resonances furthest downfield
1
4 J. J. Kiernicki, B. S. Newell, E. M. Matson, N. H. Anderson,
P. E. Fanwick, M. P. Shores and S. C. Bart, Inorg. Chem., 2014, 53,
3730–3741.
5 L. G. McCullough, H. W. Turner, R. A. Andersen, A. Zalkin and
D. H. Templeton, Inorg. Chem., 1981, 20, 2869–2871.
6 Q. Knijnenburg, S. Gambarotta and P. H. M. Budzelaar, Dalton
Trans., 2006, 5442–5448.
(
36.70 ppm, 6H, imine-CH ) and upfield (ꢀ74.99, 1H, para-pyr).
3
Compound 4-SPh/SePh has resonances that lie at the average of
the chemical shifts of the corresponding resonances of 4-SPh
1
(
44.56, ꢀ83.83 ppm) and 4-SePh (29.28, ꢀ68.30 ppm).
1
In summary, tetravalent 1, which contains a reduced
Mes
Me 3ꢀ
17 B. De Bruin, E. Bill, E. Bothe, T. Weyherm u¨ ller and K. Wieghardt,
[
PDI
]
trianion, undergoes oxidative addition of diha-
Inorg. Chem., 2000, 39, 2936–2947.
lides, phenylselenyl chloride and diphenyldichalcogenides. In
1
8 S. C. Bart, K. Chlopek, E. Bill, M. W. Bouwkamp, E. Lobkovsky,
F. Neese, K. Wieghardt and P. J. Chirik, J. Am. Chem. Soc., 2006, 128,
13901–13912.
all cases, oxidative addition occurs to afford the trans products,
Mes
Me
with subsequent
PDI
oxidation to the monoanion. A
1
9 S. C. Bart, E. Lobkovsky and P. J. Chirik, J. Am. Chem. Soc., 2004, 126,
crossover experiment confirms the radical mechanism for the
13794–13807.
PhEEPh family of substrates. Using reducing equivalents 20 R. P. Yu, J. M. Darmon, C. Milsmann, G. W. Margulieux,
S. C. E. Stieber, S. DeBeer and P. J. Chirik, J. Am. Chem. Soc., 2013,
derived from the ligand facilitates this two-electron chemistry
135, 13168–13184.
at a single uranium centre. Further studies are focused on
expanding the limits of the small molecule activation by this
highly reduced uranium(IV) complex.
We acknowledge the Division of Chemical Sciences, Geo-
sciences, and Biosciences, Office of Basic Energy Sciences of the
U.S. Department of Energy through Grant DE-AC02-12ER16328.
SCB is a Cottrell Scholar funded by the Research Corporation.
2
2
2
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2
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