A Luminescent Zirconium(IV) Complex as a Molecular Photosensitizer for Visible Light Photoredox Catalysis

Article abstract of DOI:10.1021/jacs.6b05934

Titanium and zirconium complexes carrying two 2,6-bis(pyrrolyl)pyridine ligands have been synthesized and characterized. The neutral complexes Ti(^MePDP)₂ and Zr(^MePDP)₂ (^MePDP = 2,6-bis(5-methyl-3-phenyl-1H-pyrrol-2-yl)pyridine) show intense ligand-to-metal charge-transfer bands in the visible region and undergo multiple reversible redox events under highly reducing conditions. Zr(^MePDP)₂ exhibits photoluminescent behavior and its excited state can be quenched by mild reductants to generate a powerful electron transfer reagent with a ground state potential of -2.16 V vs Fc^+/0. This reactivity was utilized to facilitate dehalogenation reactions, the reduction of electron-poor olefins, and the reductive coupling of benzyl bromide via photoredox catalysis. In these reactions, the earth-abundant metal complex Zr(^MePDP)₂ acts as a substitute for the precious metal photosensitizer [Ru(bpy)₃]²⁺.

Full text of DOI:10.1021/jacs.6b05934

Subscriber access provided by Northern Illinois University
Communication
A Luminescent Zirconium(IV) Complex as a Molecular
Photosensitizer for Visible Light Photoredox Catalysis.
Yu Zhang, Jeffrey L. Petersen, and Carsten Milsmann
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.6b05934 • Publication Date (Web): 19 Sep 2016
Downloaded from http://pubs.acs.org on September 19, 2016
Just Accepted
“Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted
online prior to technical editing, formatting for publication and author proofing. The American Chemical
Society provides “Just Accepted” as a free service to the research community to expedite the
dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts
appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been
fully peer reviewed, but should not be considered the official version of record. They are accessible to all
readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered
to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published
in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just
Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor
changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers
and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors
or consequences arising from the use of information contained in these “Just Accepted” manuscripts.
Journal of the American Chemical Society is published by the American Chemical
Society. 1155 Sixteenth Street N.W., Washington, DC 20036
Published by American Chemical Society. Copyright © American Chemical Society.
However, no copyright claim is made to original U.S. Government works, or works
produced by employees of any Commonwealth realm Crown government in the course
of their duties.

Page 1 of 6
Journal of the American Chemical Society
1
2
3
4
5
6
A Luminescent Zirconium(IV) Complex as a Molecular Photosensitiz-
er for Visible Light Photoredox Catalysis.
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Yu Zhang, Jeffrey L. Petersen, and Carsten Milsmann*
C. Eugene Bennett Department of Chemistry, West Virginia University, 100 Prospect Street, Morgantown, West
Virginia 26506, United States
Supporting Information Placeholder
0
11
clopentadienyl complexes of d metal ions. However, most
of these complexes require excitation with UV rather than
visible light. Luminescent LMCT states from visible light
excitation have been observed in group 5 and 6 complexes
ABSTRACT: Titanium and zirconium complexes carrying
two 2,6-bis(pyrrolyl)pyridine ligands have been synthesized
Me
and characterized. The neutral complexes Ti( PDP)2 and
Me
Me
Zr( PDP) ( PDP = 2,6-bis(5-methyl-3-phenyl-1H-pyrrol-2-
2
1
2
carrying a single oxo or imido ligand. Electron rich metal
complexes with luminescent LMCT states have been report-
yl)pyridine) show intense ligand-to-metal charge-transfer
bands in the visible region and undergo multiple reversible
13
Me
ed for Re and Tc using bidentate phosphine ligands. Our
redox events under highly reducing conditions. Zr( PDP)2
interest in this type of mechanism was sparked by the poten-
tial to develop molecular photosensitizers based on group 4
transition metals. The high earth-abundance and the result-
exhibits photoluminescent behavior and its excited state can
be quenched by mild reductants to generate a powerful elec-
tron transfer reagent with a ground state potential of -2.16 V
vs. Fc . This reactivity was utilized to facilitate dehalogena-
tion reactions, the reduction of electron-poor olefins, and the
reductive coupling of benzyl bromide via photoredox cataly-
nd
th
+
/0
ing low cost of titanium and zirconium as the 2 and 4
most-abundant transition metals in the earth’s crust, respec-
1
4
tively, makes these metals attractive candidates for large-
1
5
scale solar energy applications. Herein, we present a new
molecular photosensitizer based on the electron-deficient
early transition metal Zr(IV) and a π-donating pyridine di-
pyrrolide, PDP, ligand and demonstrate its utility in photo-
redox catalysis.
sis. In these reactions, the earth-abundant metal complex
Me
Zr( PDP) acts as a substitute for the precious metal photo-
2
2+
sensitizer [Ru(bpy)3]
.
Transition metal complexes using a pincer-type 2,6-
bis(pyrrolyl)pyridine ligand framework were only recently
Photoluminescent transition metal complexes have re-
ceived considerable attention due to their importance in
1
6
reported. This ligand architecture exhibits a number of at-
tractive features for the design of LMCT photosensitizers: a)
Pyrrolide ligands are π-donors due to the amide character of
the nitrogen atom and the 2,2’-connectivity between the pyr-
rolide and pyridine rings confers amide character to the pyr-
idine nitrogen via conjugation; b) the extended π-system
allows for facile charge delocalization and the ligand was
1
2
photovoltaic devices, solar fuel production, and photoredox
3
catalysis. A common design element of many popular, met-
al-based molecular photosensitizers is the combination of an
electron-rich metal center with strong π-acceptor ligands. In
these complexes, initial charge separation occurs via metal-
to-ligand charge-transfer (MLCT) transitions often followed
by inter-system crossing (ISC) leading to long-lived excited
1
6b
shown to be readily oxidized by up to two electrons; c) the
pincer-type backbone provides a rigid framework for the
synthesis of coordinatively saturated octahedral complexes
and its modular synthesis allows for straightforward tuning
of the steric and electronic properties of the ligand. For the
purpose of this study, we focused on 2,6-bis(5-methyl-3-
4
states. The light energy stored in the luminescent excited
state alters the redox-potentials of the photosensitizer and
enables facile single-electron transfer (SET) reactions. Prom-
inent examples for this type of photosensitizer are
II
2+
6
5
III
6
[Ru (bpy) ] (d , bpy = 2,2’-bipyridine), [Ir (ppy) ] (d , ppy
3
3
6
I
1+
10
Me
= 2,2’-phenylpyridine), and [Cu (dmp) ] (d , dmp = 2,9-
dimethyl-1,10-phenanthroline) as well as the more recently
2
phenyl-1H-pyrrol-2-yl)pyridine, H2 PDP, which was synthe-
7
sized from commercially available 2,6-pyridinedicarbaldhyde
and benzylideneacetone via a straightforward two step/one
6
reported complexes
W(C≡NAr)6
(d , Ar
=
2,6-
(d , terpy
,2';6',2"-terpyridine). A notable exception are cerium(III)
8
1+
8
1
7
dimethylphenyl) and [Pt(terpy)(C≡CR)]
=
pot protocol.
9
2
Me
Addition of two equivalents of n-BuLi to H2 PDP result-
amide and guanidinate complexes recently reported by
Schelter et al., in which the strong luminescence originates
Me
ed in clean deprotonation yielding Li2 PDP. Treatment of
Me
2
1
0
ZrCl
4
with two equivalents of Li
PDP at room temperature
from 5d → 4f transitions.
Me
provided Zr( PDP) in 69% yield (Scheme 1). A similar pro-
2
Ligand-to-metal charge transfer (LMCT) presents an alter-
native strategy for the design of photoactive transition metal
compounds. Among the most well-studied examples are cy-
tocol using TiCl (thf) as the metal precursor did not furnish
the corresponding titanium complex, but lead to the for-
mation of unidentified paramagnetic products. This reactivi-
4
2
ACS Paragon Plus Environment

Journal of the American Chemical Society
Page 2 of 6
Me Me
Scheme 1. Synthesis of Zr( PDP) , Ti( PDP) , and
2
2
Me
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
[
Li(thf) ][Ti( PDP) ].
4 2
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Me
Me
Figure 2. Absorption spectra of Ti( PDP) and Zr( PDP)2
2
Me
and emission spectrum of Zr( PDP) recorded in THF solu-
2
Me
tion at room temperature. Inset: THF solution of Zr( PDP)2
IV
ty is probably due to facile reduction of Ti by the lithium
salt of the ligand. In contrast, reaction of TiCl (thf) with two
under UV irradiation (365 nm).
3
3
Me
equivalents of Li2 PDP resulted in clean formation of para-
magnetic [Li(thf) ][Ti( PDP) ] in 62% yield, which was con-
verted to Ti( PDP) by oxidation with half an equivalent of
respectively.
Me
4
2
Despite their structural similarities, the optical properties
Me
Me
Me
2
of the neutral complexes Zr( PDP)2 and Ti( PDP)2 are
Me
I2 (92% yield). The molecular structures of the three com-
quite different. Solutions of Ti( PDP) in THF exhibit a dark
2
plexes were established by X-ray diffraction and the structure
brown color without any visible luminescence under ambient
light or upon irradiation with UV light at 365 or 254 nm. In
contrast, THF solutions of the zirconium analog show an
intense pink color and are photoluminescent. Electronic ab-
Me
of Zr( PDP) is shown in Figure 1. In all complexes, the co-
2
ordination environment around the central metal ion is best
described as distorted octahedral with two meridionally co-
Me
2-
ordinating tridentate PDP ligands. The geometric con-
straints enforced by the ligand framework result in reduced
average Npyrrol-M-Npyrrol angles of 140.00(9)°, 147.57(11)°, and
sorption spectra for both complexes as well as the emission
Me
spectrum of Zr( PDP) upon excitation at 528 nm are shown
2
Me
in Figure 2. The spectrum of Ti( PDP) exhibits two absorp-
2
Me
Me
149.40(9)° for the pincer ligands in Zr( PDP) , Ti( PDP)2,
2
tion bands above 400 nm with maxima at 777 nm (ε = 9669
Me
1-
-
1
-1
-1
-1
and [Ti( PDP) ] , respectively. The two tridentate ligands
2
M cm ) and 459 nm (ε = 22012 M cm ), which are tenta-
tively assigned as charge transfer bands based on their inten-
sities. Even stronger absorption bands were observed in the
exhibit nearly perfect perpendicular orientation in all com-
plexes as indicated by the angle between the N(1)-N(2)-N(3)
1
-
1
-1
and the N(4)-N(5)-N(6) planes. The H NMR spectroscopy
UV region at 386 nm (ε = 79819 M cm ), 330 nm (ε = 59659
Me
Me
-
1
-1
-1
-1
data for diamagnetic Zr( PDP) and Ti( PDP) as well as
2
Me
2
M cm ), and 255 nm (ε = 69944 M cm ). No emission
for paramagnetic [Li(thf) ][Ti( PDP) ] are in agreement
4
2
bands were detected upon excitation at wavelengths corre-
sponding to these absorption maxima. The spectrum ob-
with D2d symmetric structures in solution. No significant
Me
changes in the intra-ligand bond distances are observed be-
tained for Zr( PDP)2 shows similar features with blue-
Me
tween Ti( PDP) and the one-electron reduced complex ion
2
shifted absorption maxima. A single absorption band was
observed in the visible region with a maximum at 528 nm (ε
Me
1-
[
Ti( PDP) ] (Table S1) indicating metal-centered reduction
2 ,
-
1
-1
and
a +III oxidation state for the titanium ion in
=
27001 M cm ). Additional bands with maxima at 346 nm
Me
-
1
-1
-1
-1
[
Li(thf) ][Ti( PDP) ]. Consistent with this assignment, the
4 2
(ε = 40028 M cm ) and 300 nm (ε = 61582 M cm ) are lo-
cated in the UV part of the spectrum. A weaker absorption
band is visible as a shoulder around 395 nm. Excitation at
any of these wavelengths resulted in the detection of an
emission spectrum with a maximum at 594 nm. A lumines-
cence quantum yield, Φ, of 0.08 was determined via a com-
parative method using Rhodamine 6G in ethanol as the ref-
average Ti-Npyrrole bond length increases from 2.039(3) Å in
the neutral complex to 2.113(2) Å in the anionic compound.
The average Ti-Npyridine distance remains constant with
Me
Me
1-
2.122(3) Å and 2.129(2) Å in Ti( PDP) and [Ti( PDP) ] ,
2
2
18
erence. This value is similar to the one reported for
2+
[Ru(bpy)3] in acetonitrile (Φ = 0.09) or water (Φ = 0.06)
1
9
under oxygen-free conditions.
To establish the nature of the electronic transitions, time-
dependent density functional theory (TD-DFT) calculations
were performed at the B3LYP level of theory. Solvent effects
were included using the conductor-like screening model
(COSMO). The calculated absorption spectra are in good
agreement with the experimental data (Figure 3 and Figure
S41). The lowest energy band in both neutral complexes cor-
responds to a transition from an exclusively ligand centered
π orbital (b in D symmetry) to a degenerate set of orbitals
2
2d
Figure 1. Representation of the molecular structure of
(
e) with significant contributions from the metal (d , d )
xz yz
Me
Zr( PDP) with 50% probability ellipsoids. Hydrogen atoms
2
and the pyridine rings of the ligands (Figure 3). The metal
character of the acceptor orbitals was found to be 61% in the
were omitted for clarity.
ACS Paragon Plus Environment

Page 3 of 6
Journal of the American Chemical Society
Me
phenyl-2,3-dihydro-1H-7-methylbenzo-[d]imidazole,
BIH,
Me
1
2
3
4
5
6
7
8
9
was identified as a potential reductant for Zr( PDP) *. The
2
Me
redox potential for one-electron oxidation of BIH is slightly
more negative (-0.16 V vs. Fc in MeCN) than the one re-
ported for BIH (-0.10 V vs. Fc in MeCN), which is fre-
quently used as a terminal reductant in photoredox reduc-
tions of organic substrates and CO2. Addition of BIH to
+
/0
+
/0
21
22
23
Me
Me
Zr( PDP) in THF solution lead to a significant reduction of
2
luminescence intensity indicating quenching of the excited
state (Figure S13).
Encouraged by these results, the potential for photoredox
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Me
catalysis using Zr( PDP)2 as the photosensitizer was ex-
plored (Scheme 3). As a first proof of concept, the dehalo-
genation of ethyl bromodifluoroacetate was attempted. Pho-
tocatalytic dehalogenation reactions with organic hydride
sources such as 1-benzyl-1,4-dihydronicotinamide (BNAH) or
Hantzsch esters were among the earliest examples of photo-
redox reactions using the reductive quenching cycles of
Figure 3. TD-DFT (COSMO) predicted electronic absorption
Me
-1
spectrum of Zr( PDP) (red line, fwhm of 2000 cm ). Verti-
2
cal bars indicate the position of the predicted transitions.
The experimental spectrum is shown as a dotted line for
comparison. The orbital pictures represent the donor (b2)
and acceptor (e) orbitals for the calculated transitions in the
2+
24
[
Ru(bpy)3] or Ir(ppy)3. Irradiation of an equimolar mixture
Me
-1
of ethyl bromodifluoroacetate and BIH in benzene-d in
the presence of catalytic amounts of Zr( PDP) with green
visible region at 19980 cm (500.5 nm).
6
Me
2
Me
Me
case of Ti( PDP)2 and 34% for Zr( PDP)2 in agreement
LED light (λmax = 520 nm) resulted in clean conversion to
Me
with significant LMCT contributions.
ethyl difluoroacetate and precipitation of BIBr within 2.5 h.
To investigate the potential for outer sphere electron
transfer, cyclic voltammetry (CV) experiments were per-
No reaction was observed in the absence of either light or
Me
Zr( PDP)
. Additionally, no reaction was observed using
Ti( PDP) under irradiation with green or red LED light
2
2
Me
Me
Me
formed for Ti( PDP) and Zr( PDP) in THF using ferro-
2
2
+
/0
cene, Fc , as an internal standard. For both compounds an
irreversible oxidation event with a peak potential around 0.9
V was observed. This feature is readily assigned as an oxida-
tion of the ligand framework followed by rapid decomposi-
tion of the oxidized product as metal centered oxidation re-
(λmax = 630 nm). A second reductive transformation that has
been well-established for precious metal photosensitizers in
combination with BNAH is the reduction of electron-
2
5
Me
Me
deficient olefins. Employing the Zr( PDP) / BIH system
2
described herein, the reduction of diethyl maleate to diethyl
succinate proceeded readily upon irradiation with green light
for 8 h followed by aqueous work up. Again, no reduction
was observed in the absence of zirconium catalyst or in the
dark.
IV
IV
actions can be excluded for Ti and Zr complexes. More
interestingly, both compounds undergo multiple reductions
Me
at negative potentials. The CV of Ti( PDP) exhibits two
2
fully reversible redox waves at -1.23 V and -2.64 V followed by
a quasi-reversible redox event at -3.16 V (Figure S20). For
While the two reactions described above clearly establish
Me
Me
Zr( PDP) , reversible redox events were observed at -2.16 V
2
the photosensitizer properties of Zr( PDP) , excitation en-
2
Me
and -2.63 V with a quasi-reversible feature at -3.22 V (Figure
ergy transfer to BIH followed by hydride transfer cannot be
Me
1-
S22). Based on the previously isolated complex [Ti( PDP)2]
ruled out as a mechanistic alternative to the desired single-
electron transfer pathway. Therefore, the reductive coupling
and the strong dependence on the nature of the transition
metal ion, the first reduction event can be tentatively as-
signed as a predominantly metal centered reduction. The
more negative potential for Zr is in agreement with the gen-
erally more difficult reduction of second vs. first row transi-
tion metals. The similar potentials for the second and third
reduction events indicate primarily ligand centered reduc-
tions.
of benzyl bromide to bibenzyl was investigated as an exam-
7b,26
ple for a reduction without net hydride transfer.
Initial
Me
Me
experiments with Zr( PDP)2/ BIH and benzyl bromide in
benzene-d resulted in poor conversion and decomposition
6
of the zirconium catalyst. However, small amounts of biben-
Scheme 3. Reaction Conditions for Photoredox Ex-
periments.
Having established the electrochemical properties of the
ground state and the emission profile, the excited state po-
Me
Me
1-
tential for the redox-couple Zr( PDP) */[Zr( PDP)2] was
2
+
/0
estimated as -0.07 V vs. Fc using the Rehm-Weller formal-
ism (Scheme 2). Based on this potential, 1,3-dimethyl-2-
20
Scheme 2. Estimation of the Excited State Potential
Me
for Zr( PDP)2.
ACS Paragon Plus Environment

Journal of the American Chemical Society
Page 4 of 6
1
zyl were detected by H NMR spectroscopy. The potential
formation of HBr during turnover was identified as a likely
reason for the observed catalyst decomposition via protona-
tion of the pyrrolide arms of the ligand. In agreement with
this hypothesis, addition of pyridine or 2,6-lutidine resulted
in full conversion of benzyl bromide and an increased yield
of the desired bibenzyl product (40%). The presence of ben-
(2) For recent reviews see the Special Issue: Solar Fuels in: Chem.
Soc. Rev. 2013, 2205.
3) (a) Xuan, J.; Xiao, W.-J. Angew. Chemie - Int. Ed. 2012, 51, 6828.
b) Prier, C. K.; Rankic, D. A.; MacMillan, D. W. C. Chem. Rev. 2013,
13, 5322. (c) Yoon, T. P.; Ischay, M. A.; Du, J. Nat. Chem. 2010, 2, 527.
d) Zeitler, K. Angew. Chemie - Int. Ed. 2009, 48, 9785. (e) Tucker, J.
W.; Stephenson, C. R. J. J. Org. Chem. 2012, 77, 1617.
(4) (a) McCusker, J. K. Acc. Chem. Res. 2003, 36, 876. (b) Yeh, A. T.;
Shank, C. V; McCusker, J. K. Science 2000, 289, 935.
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
(
(
1
(
Me
zylic C-H bonds in BIH could result in the formation of
unintended by-products. To examine this, we utilized alter-
native quenchers (BIH and BIH) that lack benzylic protons.
(5) Kalyanasundaram, K. Coord. Chem. Rev. 1982, 46, 159.
Cl
(
6) (a) King, K. A.; Spellane, P. J.; Watts, R. J. J. Am. Chem. Soc.
1
985, 107, 1431. (b) Dixon, I. M.; Collin, J.-P.; Sauvage, J.-P.; Flamigni,
L.; Encinas, S.; Barigelletti, F. Chem. Soc. Rev. 2000, 29, 385.
(7) (a) McMillin, D. R.; Buckner, M. T.; Ahn, B. T. Inorg. Chem.
977, 16, 943. (b) Kern, J. M.; Sauvage, J. P. J. Chem. Soc. Chem.
While the number of by-products was decreased, slow con-
version was observed. This can be attributed to the less fa-
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Cl
vorable potentials of BIH and BIH for reduction of
1
Me
21
Zr( PDP) *, which is supported by Stern-Volmer quench-
2
Commun. 1987, 287, 546.
ing experiments showing
a
clear correlation between
(8) Sattler, W.; Ener, M. E.; Blakemore, J. D.; Rachford, A. A.;
Labeaume, P. J.; Thackeray, J. W.; Cameron, J. F.; Winkler, J. R.; Gray,
H. B. J. Am. Chem. Soc. 2013, 135, 10614.
(9) Yang, Q.-Z.; Wu, L.-Z.; Wu, Z.-X.; Zhang, L.-P.; Tung, C.-H.
Inorg. Chem. 2002, 41, 5653.
quenching efficiency and redox potential (Table 1).
Table 1. Stern-Volmer Constants and Redox Poten-
R
tials for BIH Derivatives Used in Photoredox Reac-
tions.
(
10) (a)Yin, H.; Carroll, P. J.; Manor, B. C.; Anna, J. M.; Schelter, E. J.
J. Am. Chem. Soc. 2016, 138, 5984. (b) Yin, H.; Carroll, P. J.; Anna, J.
M.; Schelter, E. J. J. Am. Chem. Soc. 2015, 137, 9234.
Me
Cl
BIH
BIH
BIH
(
11) (a) Kenney, J. W.; Boone, D. R.; Striplin, D. R.; Chen, Y.; Hamar,
+
/0
Eox / V vs. Fc
-0.16
-0.10
0.00
K. B. Organometallics 1993, 12, 3671. (b) Loukova, G. V.; Huhn, W.;
Vasiliev, V. P.; Smirnov, V. A. J. Phys. Chem. A 2007, 111, 4117. (c)
Loukova, G. V.; Smirnov, V. A. Chem. Phys. Lett. 2000, 329, 437. (d)
Loukova, G. V.; Starodubova, S. E.; Smirnov, V. A. J. Phys. Chem. A
2007, 111, 10928. (e) Loukova, G. V. Chem. Phys. Lett. 2002, 353, 244.
(f) Yam, V. W.-W.; Qi, G.-Z.; Cheung, K.-K. Organometallics 1998, 17,
5448. (g) Patrick, E. L.; Ray, C. J.; Meyer, G. D.; Ortiz, T. P.; Marshall,
J. A.; Brozik, J. A.; Summers, M. A.; Kenney, J. W. J. Am. Chem. Soc.
003, 125, 5461. (h) Bruce, M. R. M.; Kenter, A.; Tyler, D. R. J. Am.
Chem. Soc. 1984, 106, 639. (i) Loukova, G. V.; Vasiliev, V. P.; Milov, A.
A.; Smirnov, V. A.; Minkin, V. I. J. Photochem. Photobiol. A Chem.
016, 327, 6. (j) Pfennig, B. W.; Thompson, M. E.; Bocarsly, A. B.
Organometallics 1993, 12, 649.
(12) (a) Heinselman, K. S.; Hopkins, M. D. J. Am. Chem. Soc. 1995,
117, 12340. (b) Choing, S. N.; Francis, A. J.; Clendenning, G.;
Schuurman, M. S.; Sommer, R. D.; Tamblyn, I.; Weare, W. W.; Cuk,
T. J. Phys. Chem. C 2015, 119, 17029. (c) Tonks, I. A.; Durrell, A. C.;
Gray, H. B.; Bercaw, J. E. J. Am. Chem. Soc. 2012, 134, 7301.
(13) (a) Chatterjee, S.; Del Negro, A. S.; Smith, F. N.; Wang, Z.;
Hightower, S. E.; Sullivan, B. P.; Heineman, W. R.; Seliskar, C. J.;
Bryan, S. A. J. Phys. Chem. A 2013, 117, 12749. (b) Adams, J. J.;
Arulsamy, N.; Sullivan, B. P.; Roddick, D. M.; Neuberger, A.;
Schmehl, R. H. Inorg. Chem. 2015, 54, 11136.
-
1
KSV / L mol
47,900 ± 600 3,500 ± 100 no quench-
ing
In conclusion, we have developed a photoluminescent zir-
conium complex supported by 2,6-bis(pyrrolyl)pyridine lig-
ands that acts as an earth-abundant metal substitute for pre-
cious metal photosensitizers in reductive photoredox cataly-
2
sis using visible light. Experimental and computational stud-
Me
ies of Zr( PDP) and its titanium analog suggest that the
2
visible light absorption bands exhibit significant ligand-to-
metal charge-transfer character.
2
ASSOCIATED CONTENT
Supporting Information
Experimental and computational details, spectroscopic and
electrochemical characterization, and crystallographic data
(
CIF). This material is available free of charge via the Internet
at http://pubs.acs.org.
AUTHOR INFORMATION
(
14) Yaroshevsky, a. a. Geochemistry Int. 2006, 44, 48.
(15) Radivojevic, I.; Bazzan, G. J. Phys. Chem. C 2012, 116, 15867.
(16) (a) Searles, K.; Fortier, S.; Khusniyarov, M. M.; Carroll, P. J.;
Sutter, J.; Meyer, K.; Mindiola, D. J.; Caulton, K. G. Angew. Chemie -
Int. Ed. 2014, 53, 14139. (b) Komine, N.; Buell, R. W.; Chen, C.-H.;
Hui, A. K.; Pink, M.; Caulton, K. G. Inorg. Chem. 2014, 53, 1361.
Corresponding Author
camilsmann@mail.wvu.edu
Notes
(
17) (a) Nagata, T.; Tanaka, K. Bull. Chem. Soc. Jpn. 2002, 75, 2469.
The authors declare no competing financial interests.
(b) Jones, R. A.; Karatza, M.; Voro, T. N.; Civeir, P. U.; Franck, A.;
Ozturk, O.; Seaman, J. P.; Whitmore, A. P.; Williamson, D. J.
Tetrahedron 1996, 52, 8707.
ACKNOWLEDGMENT
(
18) (a) Brouwer, A. M. Pure Appl. Chem. 2011, 83, 2213. (b) Würth,
C.; Grabolle, M.; Pauli, J.; Spieles, M.; Resch-Genger, U. Nat. Protoc.
013, 8, 1535.
19) Suzuki, K.; Kobayashi, A.; Kaneko, S.; Takehira, K.; Yoshihara,
West Virginia University and the Don and Linda Brodie Re-
source Fund for Innovation are acknowledged for financial
support. This work used X-ray crystallography (CHE-1336071)
and NMR (CHE-1228336) equipment funded by the National
Science Foundation.
2
(
T.; Ishida, H.; Shiina, Y.; Oishi, S.; Tobita, S. Phys. Chem. Chem. Phys.
2009, 11, 9850.
(
(
20) Rehm, D.; Weller, A. Isr. J. Chem. 1970, 8, 259.
21) Zhu, X.-Q.; Zhang, M.-T.; Yu, A.; Wang, C.-H.; Cheng, J.-P. J.
REFERENCES
Am. Chem. Soc. 2008, 130, 2501.
(1) (a) Nazeeruddin, M. K.; Baranoff, E.; Grätzel, M. Sol. Energy 2011,
85, 1172. (b) Grätzel, M. J. Photochem. Photobiol. C Photochem. Rev.
(22) Hasegawa, E.; Tateyama, M.; Hoshi, T.; Ohta, T.; Tayama, E.;
Iwamoto, H.; Takizawa, S. Y.; Murata, S. Tetrahedron 2014, 70, 2776.
(23) Yamazaki, Y.; Takeda, H.; Ishitani, O. J. Photochem. Photobiol.
C Photochem. Rev. 2015, 25, 106.
2003, 4, 145. (c) Hagfeldt, A.; Boschloo, G.; Sun, L.; Kloo, L.;
Pettersson, H. Chem. Rev. 2010, 110, 6595.
ACS Paragon Plus Environment

Page 5 of 6
Journal of the American Chemical Society
(
24) (a) Fukuzumi, S.; Mochizuki, S.; Tanaka, T. J. Phys. Chem.
(26) Katsuhiko, H.; Fukuzumi, S.; Tanaka, T. J. Chem. Soc. Perkin
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
1990, 94, 722. (b) Nguyen, J. D.; D’Amato, E. M.; Narayaman, J. M. R.;
Stephenson, C. R. J. Nat. Chem. 2012, 4, 854.
Trans. II 1984, 1705.
(
25) Pac, C.; Ihama, M.; Yasuda, M.; Miyauchi, Y.; Sakurai, H. J. Am.
Chem. Soc. 1981, 103, 6495.
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
ACS Paragon Plus Environment

Journal of the American Chemical Society
Page 6 of 6
Insert Table of Contents artwork here
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
6
ACS Paragon Plus Environment