Luminescent Ce(III) Complexes as Stoichiometric and Catalytic Photoreductants for Halogen Atom Abstraction Reactions

Article abstract of DOI:10.1021/jacs.5b05411

Luminescent Ce(III) complexes, Ce[N(SiMe₃)₂]₃ (1) and [(Me₃Si)₂NC(RN)₂]Ce[N(SiMe₃)₂]₂ (R = ⁱPr, 1-ⁱPr; R = Cy, 1-Cy), with C_3v and C_2v solution symmetries display absorptive 4f → 5d electronic transitions in the visible region. Emission bands are observed at 553, 518, and 523 nm for 1, 1-ⁱPr, and 1-Cy with lifetimes of 24, 67, and 61 ns, respectively. Time-dependent density functional theory (TD-DFT) studies on 1 and 1-ⁱPr revealed the ²A₁ excited states corresponded to singly occupied 5d_z² orbitals. The strongly reducing metalloradical character of 1, 1-ⁱPr, and 1-Cy in their ²A₁ excited states afforded photochemical halogen atom abstraction reactions from sp³ and sp² C-X (X = Cl, Br, I) bonds for the first time with a lanthanide cation. The dehalogenation reactions could be turned over with catalytic amounts of photosensitizers by coupling salt metathesis and reduction to the photopromoted atom abstraction reactions.

Full text of DOI:10.1021/jacs.5b05411

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Luminescent Ce(III) Complexes as Stoichiometric and Catalytic Pho-
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Haolin Yin, Patrick J. Carroll, Jessica M. Anna* and Eric J. Schelter*
P. Roy and Diana T. Vagelos Laboratories, Department of Chemistry, University of Pennsylvania, 231 South 34 Street, Philꢀ
adelphia, Pennsylvania 19104, United States
Supporting Information Placeholder
Ce3+ cations originates from 5d→4f transitions,9 distinct from
ABSTRACT: Luminescent Ce(III) complexes, Ce[N(SiMe3)2]3
most other Ln3+ cations where 4f→4f transitions underlie optical
properties. The interꢀconfigurational transition shows greater
emission intensity but shorter lifetimes compared to other lanthaꢀ
nide cations; the 5d→4f transition is allowed both by parity and
spin selection rules.10
(1) and [(Me3Si)2NC(RN)2]Ce[N(SiMe3)2]2 (R = Pr, 1ꢀiPr; R =
Cy, 1ꢀCy), with C3v and C2v solution symmetries display absorpꢀ
tive 4f→5d electronic transitions in the visible region. Emission
bands are observed at 553, 518 and 523 nm for 1, 1ꢀiPr and 1ꢀCy
with lifetimes of 24, 67 and 61 ns, respectively. Timeꢀdependent
density functional theory (TDꢀDFT) studies on 1 and 1ꢀiPr reꢀ
vealed the 2A1 excited states corresponded to singly occupied 5dz2
orbitals. The strongly reducing metalloradical character of 1, 1ꢀiPr
and 1ꢀCy in their 2A1 excited states afforded photochemical haloꢀ
gen atom abstraction reactions from sp3 and sp2 C‒X (X = Cl, Br,
I) bonds for the first time with a lanthanide cation. The dehaloꢀ
genation reactions could be turned over with catalytic amounts of
photosensitizers by coupling salt metathesis and reduction to the
photoꢀpromoted atom abstraction reactions.
i
Scheme 1. Synthesis of 1ꢀiPr and 1ꢀCy from 1
R
N
RN C NR
Me3Si
Me3Si
N(SiMe3)2
N(SiMe3)2
excess
Ce
Ce[N(SiMe3)2]3
1
N
N
R
R = iPr 1-iPr
Cy 1-Cy
0.46
67 ns
τ =
ΦPL
ΦPL
=
=
ΦPL
= 0.03 τ = 24 ns
0.54 τ = 61 ns
We noted that the common Ce(III) protonolysis reagent,
Ce[N(SiMe3)2]3 (1), emitted yellow light weakly under UV irradiꢀ
ation (365 nm). However, treatment of 1 with excess carꢀ
bodiimides, R−N=C=N−R (R = iPr, Cy) afforded isolation of their
monoꢀinsertion products, 1ꢀiPr and 1ꢀCy (Scheme 1), which were
found to be bright green emitters. Xꢀray studies confirmed the
yellow compounds 1ꢀiPr and 1ꢀCy as the monoꢀguanidinate inserꢀ
tion products (Figure S3‒S4). No further insertion was observed
for 1ꢀiPr or 1ꢀCy with excess carbodiimide to 80 °C.
In photoredox chemistry the energy of visible light, especially
of high energy blue light (440 nm, 2.8 eV), is absorbed by photoꢀ
sensitizers and converted into redox power for reactivity. Current
molecular photosensitizers including RuII(bpy)2+ (bpy = 2,2'ꢀ
bipyridine),1 facꢀIrIV(ppy)+ (ppy = 2,2'ꢀphenylpyridine),2 tungꢀ
sten(0) isocyanides,3 Cu(I) complexes4 and organic dyes,5 accomꢀ
plish photoredox reactions exclusively through outer sphere elecꢀ
tron transfer processes. Photoreductants activate substrates under
mild conditions and have drawn interests in catalysis.6 For examꢀ
ple, in catalytic oxidative quenching cycles,6d electrons are transꢀ
ferred from excitedꢀstate photosensitizers to acceptors to effect
chemical changes. Antiꢀbonding orbitals of C‒X (X = Cl, Br, I)
bonds are one such target, leading to reductive C‒X bond cleavꢀ
age. In such cases, the scope of accessible substrates containing
C‒X bonds depends solely on the excited state reduction potential
of the photosensitizer.7
In contrast to outer sphere processes, we hypothesized that
favourable M‒X bond formation enthalpies could be coupled with
metal redox power in an inner sphere electron transfer process,
providing additional driving force for the activation of substrates.
An inner sphere photosensitizer would require the association of
C‒X bonds to a metal cation in its excited state. As such, highly
electrophilic fꢀblock cations are excellent candidates for such
reactivity. Herein, we report the use of luminescent Ce(III) comꢀ
plexes as inner sphere photoreductants for C‒X bond activation.
Cerium(III) is a redox active cation.8 With its single 4f electron,
Ce(III) complexes have simple, wellꢀdefined electronic structures
Figure 1. UVꢀvis electronic absorption spectra (solid lines) and normalꢀ
ized emission spectra (dashed lines) of 1 (red), 1ꢀiPr (green), 1ꢀCy (blue)
recorded in toluene. Pictures of C6H6 solution containing 1(left), 1ꢀ
iPr(middle) and 1ꢀCy(right) in Pyrex NMR tubes (1.0 mM) under UV
irradiation (365 nm) (inset).
2
featuring F5/2 ground states. In fact, reported luminescence of
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Figure 2. Experimental (black solid lines) and TDꢀDFT predicted (red dashed lines) absorption spectra of 1 (left) and 1ꢀiPr (right). The predicted spectra
were rendered with FWHM 3000 cmꢀ1. Oscillator strengths for the electronic transitions are shown as red vertical lines. Stick representations of the crystal
structures of 1 and 1ꢀiPr are shown to the left of the spectra. Natural transition orbitals (NTOs) are shown for the donor/acceptor orbitals of each transition.
Electronic absorption spectra of 1ꢀiPr, 1ꢀCy resembled the
spectrum of 1, displaying two bands with ε ~ 300‒400 Mꢀ1 cmꢀ1 in
the visible range attributed to 4f→5d transitions (Figure 1). The
emission energies of 1ꢀiPr and 1ꢀCy were blue shifted by ca. 30
nm compared to 1, resulting in green luminescence. The guanidiꢀ
nate complexes 1ꢀiPr and 1ꢀCy exhibited relatively high photoluꢀ
minescence quantum yields: ΦPL = 0.46 and 0.54 for 1ꢀiPr and 1ꢀ
Cy, respectively, compared to 1: ΦPL = 0.03. Consistent with the
observation of low quantum yield is the short measured lifetime
for 1 (24 ns) compared to 1‒iPr (67 ns) and 1‒Cy (61 ns). No
variance of the lifetime was observed across the emission profiles
for all complexes, suggesting the emissions originated from a
single excited electronic state.10c All emission spectra were deꢀ
convoluted into pairs of overlapping Gaussian bands (Figure
S48‒S50), consistent with the transitions from their emissive
states to the F ground manifold. The F ground manifold is split
by spinꢀorbital coupling into the J = 5/2 ground state and J = 7/2
excited states.9b Excitation spectra collected at the emission maxꢀ
ima for 1, 1ꢀiPr, 1ꢀCy showed intense, overlapping bands with
their lowest energy 4f→5d absorption bands at ca. 420 nm (Figꢀ
ure S45‒S47), supporting the associated excited state as the longꢀ
lived emissive state.
from ground states of primarily 4f character to 5dz2 orbitalꢀbased
excited states: A1 in both the C3v and C2v point groups. The reꢀ
2
sults are readily rationalized as the 5dz2 orbitals are essentially
nonꢀbonding in both cases and therefore of the lowest in energy
within the 5dꢀmanifold.12 The acceptor orbitals of the transition at
341 nm were found to be a degenerate set of 5dxz and 5dyz orbitals
for 1; such degeneracy is lost for the lower symmetry complex, 1ꢀ
2
iPr. The A1 states were therefore assigned as the longꢀlived exꢀ
cited states for all three complexes. Additionally, a ligandꢀtoꢀ
metal charge transfer (LMCT) absorption tail located below 320
nm for 1ꢀiPr was attributed by the TDꢀDFT result to a guanidinate
nonꢀbonding πn orbital to 4fꢀorbital CT transition (Figure S78).
To evaluate the reduction potential of the Ce(III) upon photoꢀ
2
excitation, the excited state potentials (E*1/2) for the A1 states
were approximated using the ground state potentials (E1/2) and
emission band energies following the RehnꢀWeller formalism:13
E*1/2 = E1/2 ‒ E0,0
2
2
Cyclic voltammetry experiments on 1ꢀiPr and 1ꢀCy in CH2Cl2
with 0.1 M [nPr4N][BArF ] as supporting electrolyte revealed
4
quasiꢀreversible CeIV/III couples with E1/2 = + 0.03 V and + 0.13 V
versus Cp2Fe+/0, respectively (Figure S54‒S55). The electrochemꢀ
ical data of 1 in THF (E1/2 = + 0.35 V)14 was applied for its estiꢀ
mate of the excited state reduction potential because no feature
In an effort to identify the frontier orbitals involved in the elecꢀ
tronic transitions of 1, 1ꢀiPr and 1ꢀCy, timeꢀdependent density
functional theory (TDꢀDFT) calculations were performed for
complexes 1 and 1ꢀiPr. The predicted vertical excitations and
intensities were in reasonably good agreement with the experiꢀ
mental data, indicating two metal based 4f→5d transitions (Figure
2). Natural transition orbitals (NTOs)11 analyses revealed the lowꢀ
est energy transitions at ca. 420 nm for 1 and 1ꢀiPr were both
Table 1. Estimation of reduction potential for Ce(III) 2A1 excited states
E1/2a /eV
+ 0.35b
+ 0.03
E0,0 /eV
+ 2.24
+ 2.39
+ 2.37
E1/2* /eV
‒ 1.89
‒ 2.36
1
1ꢀiPr
1ꢀCy
+ 0.13
‒ 2.24
aCyclic voltammetry in 0.1 M [nPr4N][BArF ]/CH2Cl2. bIn THF.
4
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Scheme 2. Photoꢀinduced oxidation of 1, 1‒iPr and 1‒Cy with PhCH2Cl
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was observed for 1 in CH2Cl2. The E1/2* values were estimated to
In all cases, the Ce(IV)‒Cl products, 2, 2ꢀiPr and 2ꢀCy were
stronger absorbers than their Ce(III) congeners. As such, the
Ce(IV) products in the reaction mixture inhibited the absorption
of light by Ce(III) species and resulted in incomplete reactions. To
avoid accumulation of Ce(IV) species we found NaN(SiMe3)2
could effectively reduce the Ce(IV)‒Cl products to Ce(III) species
with precipitation of NaCl and formation of aminyl radical:
•N(SiMe3)2. However, under catalytic conditions with stoichioꢀ
metric NaN(SiMe3)2, only a low yield (22% in Et2O) of
PhCH2CH2Ph was achieved, due to sideꢀreactions of the aminyl
radical with the starting material, PhCH2Cl. Byproducts, including
PhCH2N(SiMe3)2 and PhCH2CHClPh, were identified by 1H
NMR spectroscopy and GCꢀMS (See Figure S22, S30‒34). To
address these problems, Zn or Ce metal powders were introduced
to the reaction mixtures as additives to quench the aminyl radical
and afforded bibenzyl products in reasonable yields; control exꢀ
periments showed slow background reactions of the metal powꢀ
ders with PhCH2Cl (Scheme 3 and Table S1). Further controls
demonstrated the metal powders reacted slowly with 2; direction
reaction of NaN(SiMe3)2 with 2 proceeded instantaneously.
be ‒1.89, ‒2.36 and ‒2.24 V versus Cp2Fe+/0 for 1, 1ꢀiPr and 1ꢀ
Cy, respectively (Table 1), providing opportunities for accessing
photoꢀreduction chemistry with Ce(III) complexes.
Based on the spectroscopic and electrochemical data, we hyꢀ
2
pothesized that combining the reducing power of the A1 excited
state and Ce(III)‒X bond enthalpy would allow us to activate
challenging C‒X bonds photoꢀchemically through an inner sphere
atom abstraction pathway. We choose PhCH2Cl as a first target
for activation since the cathodic reduction wave of PhCH2Cl was
reported at Epc = ‒2.66 V versus Cp2Fe+/0 in DMF,15 lower than
the ‒1.89 V estimated reduction potential for 1*. Thus, an outer
sphere electron transfer process would not be expected to occur
between 1* and PhCH2Cl on thermodynamic grounds. Combinaꢀ
tions of 1 and PhCH2Cl showed no reaction in the absence of
light. Irradiation of a C6D6 solution containing 1 with excess of
PhCH2Cl in a photoreactor equipped with 420 nm narrow band
lamps led to a color change of the solution from yellow to dark
red/purple within 5 min. The reaction also proceeded with comꢀ
mercially available compact fluorescent lamps (CFLs) as light
sources. An 1H NMR experiment showed the formation of
CeIVCl[N(SiMe3)2]3 (2), concomitant with the generation of
PhCH2CH2Ph (Scheme 2) through radical homoꢀcoupling.
The dehalogenation of PhCH2Cl proceeded much slower with
1ꢀiPr and 1ꢀCy despite their longer lifetimes than 1. This differꢀ
ence was attributed to steric congestion from the guanidinate ligꢀ
ands which disfavored coordination at the Ce(III) cation.
Similarly, photoꢀreactions of 1ꢀiPr (or 1ꢀCy) with excess
PhCH2Cl led to color changes from yellow to black (Scheme 2)
1
Scheme 3. PhCH2Cl coupling reactions with Ceꢀmetal as external
reductant
and the production of PhCH2CH2Ph was confirmed by H NMR
1
spectroscopy. Also noted in the solution H NMR spectra were
sets of resonances for the Ce(IV)‒Cl products, 2ꢀiPr (or 2ꢀCy). To
confirm the assessments of the oxidation products, 2ꢀiPr and 2ꢀ
Cy, were independently prepared from reactions of Ph3C‒Cl with
1ꢀiPr and 1ꢀCy. Xꢀray crystallography studies confirmed their
identities as the Ce(IV)‒Cl products (Figure S5‒6). The black
complexes 2ꢀiPr and 2ꢀCy are the first examples of κ2ꢀguanidinate
moieties coordinated to Ce(IV) cations; related cerium(III/IV)
amidates and formamidinates have been reported.16 The black
appearances of 2ꢀiPr and 2ꢀCy were evident in their UVꢀvis specꢀ
tra, where both compounds were found to absorb evenly in the
visible range, 400‒800 nm (3.1‒1.5 eV, Figure S42). TDꢀDFT
calculations performed on 2ꢀiPr afforded assignment of bands in
the visible region as LMCT (Figure S89). The computed
HOMO→LUMO excitation at 725 nm (1.71 eV) corresponded to
a guanidinate nonꢀbonding π orbital (πn) to a 4fꢀorbital.
Entry
[CeIII]
Solvent
Et2O
Et2O
Et2O
Et2O
Yielda/%
1
2
3
4
5
6
7
8
9
1
68
17
10
5
45
23
44
12
19
1ꢀiPr
1ꢀCy
ꢀ
1
1
1
1
1
C6H6
TMS2O
CPME
nꢀpentane
1,4ꢀdioxane
a. Determined by 1H NMR integration against internal standard CH2Br2.
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We also reasoned that the aminyl radical could be harnessed
Corresponding Authors
1
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for hydrogen atom abstraction reactions. This objective was
achieved using more challenging substrates containing C(sp2)‒X
bonds. Irradiation of a 20 mol% solution of 1 with 4ꢀFꢀC6H4X (X
= I, Br) and NaN(SiMe3)2 in benzene at room temperature for 6 d
afforded 1ꢀ(4ꢀfluorophenyl)benzene in 68% and 43% isolated
yields for X = I and Br, respectively (Scheme 4), while no reacꢀ
tion was observed for X = Cl (Figure S27). The identity of the
biphenyl product was also confirmed by 19F NMR spectroscopy as
well as high resolution mass spectroscopy (HRMS). The origin of
the phenyl group was confirmed with the use of deuterobenzene
as the reaction solvent; the molecular mass corresponding to 1ꢀ(4ꢀ
fluorophenyl)ꢀ2,3,4,5,6ꢀdeuterobenzene was detected by HRMS
(see Figure S29). Direct C(sp2)‒H and C(sp2)‒X coupling in the
presence of catalytic 1,10ꢀphenanthroline derivatives at elevated
temperatures was reported recently by Shi and Hayashi,17 and
were proposed to proceed through a single electron transfer (SET)
mechanism. In our case, NMRꢀscale reactions of 1 with excess 4ꢀ
*jmanna@sas.upenn.edu, *schelter@sas.upenn.edu
ACKNOWLEDGMENT
We gratefully acknowledge the University of Pennsylvania and
the National Science Foundation (CHEꢀ1362854) for financial
support. This work used the Extreme Science and Engineering
Discovery Environment (XSEDE), which is supported by U.S.
NSF grant number OCIꢀ1053575. We thank the E. J. Petersson
and S. J. Park groups at UPenn for use of their fluorometers.
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in
C6D6
revealed
the
formation
of
CeIVBr[N(SiMe3)2]3.18 Therefore, the catalytic generation of the
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2, 1044; c) Li, H.; Sun, C.ꢀL.; Yu, M.; Yu, D.ꢀG.; Li, B.ꢀJ.; Shi, Z.ꢀJ.
Chem. –Eur. J. 2011, 17, 3593; d) Sun, C.ꢀL.; Gu, Y.ꢀF.; Huang, W.ꢀP.;
Shi, Z.ꢀJ. Chem. Commun. 2011, 47, 9813.
Scheme 4. Catalytic arylations of benzene with ArX (X = I, Br)
Through combined spectroscopic and computational studies,
we demonstrated luminescent Ce(III) complexes 1, 1ꢀiPr and 1ꢀ
Cy possess singly occupied 5dz2 orbitals in their long lived excitꢀ
ed states. The metalloradical nature of the excited states allowed
electrophilic cerium(III) complexes to act as photosensitizers that
activated challenging substrates through inner sphere processes,
taking advantage of the enthalpy gain in the formation of
Ce(IV)‒X bonds. A drawback in the current system is the relativeꢀ
ly low absorptivity of Ce(III) complexes, which limits the catalytꢀ
ic turnover rates. Development of new sensitized cerium(III) phoꢀ
toꢀredox catalysts, expanded reactivity studies and physicochemiꢀ
cal studies on cerium(III) luminescence characteristics are curꢀ
rently underway in our laboratory.
ASSOCIATED CONTENT
Supporting Information
Crystallographic data (CIF), electrochemical data, electronic abꢀ
sorption data, excitation and emission data, computational details
and optimization data for catalysis. This material is available free
of charge via the Internet at http://pubs.acs.org.
(18) Williams, U. J.; Carroll, P. J.; Schelter, E. J. Inorg. Chem. 2014, 53,
6338.
AUTHOR INFORMATION
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