Edge Article
Chemical Science
To determine if EuIII remains complexed aer the oxidation Energy (DOE), Office of Science, Basic Energy Sciences (BES),
of EuII, luminescence intensities were compared of solutions under Award # DE-SC0012628 for nancial support. We thank
containing EuCl3, EuCl3 in the presence of 1, and EuII1 that was Jennifer Stockdill for helpful conversations and use of her gas
opened to air to oxidize (Fig. S17†). The spectra were normalized chromatograph, and we thank Jeremy Kodanko for use of his
to the 5D0
/
7F1 transition at 591 nm that is insensitive to spectrophotometer. The authors thank Duke Debrah and Lin
ligand environment, and the emission intensities of the spectra Fan for help with experimental setup for lifetime
were compared at the 5D0 / 7F2 transition (610–630 nm) that is measurements.
hypersensitive to ligand environment.26 The change in spectral
5
7
prole of the D0 / F2 transitions indicates that there is an
interaction between EuIII and 1, but the exact nature of this
interaction is ambiguous.
Notes and references
Based on the data presented here, we propose that the
photocatalytic reductive coupling of benzyl chloride using EuII1
proceeds through the catalytic cycle shown in Scheme 1. From
luminescence experiments, EuII1 is excited by blue light into an
excited state (EuII1*). Two molecules of EuII1* reduce two
molecules of substrate through a collisional electron transfer
based on Stern–Volmer analyses, followed by reductive coupling
of substrate molecules. The electron transfer also generates
EuIII that interacts with 1 to some extent. Zn0 reduces EuIII to
EuII either as the complex or the uncomplexed ion. Spectro-
scopic evidence (Fig. S17†) supports the presence of interac-
tions between EuIII and 1, but this evidence is not conclusive
with respect to the nature of speciation of the trivalent ion.
Regardless of the extent of encapsulation of EuIII by 1, reduction
by Zn0 regenerates EuII1, evidenced by spectroscopy and the
crystal structure in Fig. 4, restarting the catalytic cycle.
1 (a) A. G. Amador and T. P. Yoon, Angew. Chem., Int. Ed., 2016,
55, 2304; (b) M. H. Shaw, J. Twilton and D. W. C. MacMillan,
J. Org. Chem., 2016, 81, 6898; (c) Y. Slutskyy and
L. E. Overman, Org. Lett., 2016, 18, 2564; (d) A. Singh,
C. J. Fennell and J. D. Weaver, Chem. Sci., 2016, 7, 6796; (e)
J. J. Douglas, M. J. Sevrin and C. R. J. Stephenson, Org.
Process Res. Dev., 2016, 20, 1134; (f) J. A. Terrett,
J. D. Cuthbertson, V. W. Shurtleff and D. W. C. MacMillan,
Nature, 2015, 524, 330; (g) C. C. Nawrat, C. R. Jamison,
Y. Slutskyy, D. W. C. MacMillan and L. E. Overman, J. Am.
Chem. Soc., 2015, 135, 11270; (h) A. Arora, K. A. Teegardin
and J. D. Weaver, Org. Lett., 2015, 17, 3722; (i) J. C. Tellis,
D. N. Primer and G. A. Molander, Science, 2014, 345, 433;
(j) K. Singh, S. J. Staig and J. D. Weaver, J. Am. Chem. Soc.,
2014, 136, 5275; (k) G. Bergonzini, C. S. Schindler,
C.-J. Wallentin, E. N. Jacobsen and C. R. J. Stephenson,
Chem. Sci., 2014, 5, 112; (l) Y. Xi, H. Yi and A. Lei, Org.
Biomol. Chem., 2013, 11, 2387; (m) J. M. R. Narayanam and
C. R. J. Stephenson, Chem. Soc. Rev., 2011, 40, 102; (n)
T. P. Yoon, M. A. Ischay and J. Du, Nat. Chem., 2010, 2,
527; (o) E. R. Welin, C. Le, D. M. Arias-Rotondo,
J. K. McCusker and D. W. C. MacMillan, Science, 2017, 355,
380.
Conclusions
We have described the rst report of photoredox catalysis based
on europium. Exposure of EuII1 to visible light forms an excited
state with a calculated electrochemical potential that rivals SmI2
in the presence of hexamethylphosphoramide, has a long
luminescence lifetime, is tolerant of protic solvents and some
H2O, and can be assembled in situ starting from air-stable and
relatively inexpensive EuCl3$6H2O. We expect that the mecha-
nistic insight provided here will open the door for the study of
visible-light-promoted photoredox catalysis using EuII1 in
reactions that require large negative electrochemical potentials
between ꢁ0.9 and approximately ꢁ3 V vs. Ag/AgCl, including
challenging systems like unactivated halides such as aryl
bromides. Furthermore, studies from our laboratory have
shown that ligand modications to EuII1 can inuence its
spectroscopic properties,25a and these modications are likely
to impact excited-state redox properties. Studies exploring
ligand modications and the scope of reactivity of EuII1 are
underway in our laboratory.
2 (a) W. Sattler, L. M. Henling, J. R. Winkler and H. B. Gray, J.
Am. Chem. Soc., 2015, 137, 1198; (b) S. B. Harkins and
J. C. Peters, J. Am. Chem. Soc., 2005, 127, 2030; (c) H. Huo,
K. Harms and E. Meggers, J. Am. Chem. Soc., 2016, 138,
¨
6936; (d) L. A. Buldt, X. Guo, A. Prescimone and
O. S. Wenger, Angew. Chem., Int. Ed., 2016, 55, 11247; (e)
D. Li, C.-M. Che, H.-L. Kwong and V. W.-W. Yam, J. Chem.
Soc., Dalton Trans., 1992, 23, 3325; (f) S. E. Creutz,
K. J. Lotito, G. C. Fu and J. C. Peters, Science, 2012, 338,
647; (g) W. Sattler, M. E. Ener, J. D. Blakemore,
A. A. Rachford, P. J. LaBeaume, J. W. Thackeray,
J. F. Cameron, J. R. Winkler and H. B. Gray, J. Am. Chem.
Soc., 2013, 135, 10614; (h) J.-M. Kern and J.-P. Sauvage, J.
Chem. Soc., Chem. Commun., 1987, 8, 546.
3 (a) H. Yin, P. J. Carroll, J. M. Anna and E. J. Schelter, J. Am.
Chem. Soc., 2015, 137, 9234; (b) H. Yin, P. J. Carroll,
B. C. Manor, J. M. Anna and E. J. Schelter, J. Am. Chem.
Soc., 2016, 138, 5984.
Conflicts of interest
There are no conicts to declare.
4 H. Yin, Y. Jin, J. E. Hertzog, K. C. Mullane, P. J. Carroll,
B. C. Manor, J. M. Anna and E. J. Schelter, J. Am. Chem.
Soc., 2016, 138, 16266.
Acknowledgements
This research was supported by the National Science Founda-
tion (CHE-1564755). W. L. thanks the U.S. Department of
5 (a) K. Suzuki, F. Tang, Y. Kikukawa, K. Yamaguchi and
N. Mizuno, Angew. Chem., Int. Ed., 2014, 53, 5356; (b)
This journal is © The Royal Society of Chemistry 2017
Chem. Sci.