Photochemical C-C Bond Formation in Luminescent Zirconium Complexes with CNN Pincer Ligands

Article abstract of DOI:10.1021/acs.organomet.8b00388

The complexes Zr(^HCNN)₂ and Zr(^MeCNN)₂ were prepared via reaction of the ligand precursors 2-phenyl-6-(5-methyl-3-phenyl-1H-pyrrol-2-yl)pyridine, H₂^HCNN, and 2-(3,5-dimethyl-phenyl)-6-(5-methyl-3-phenyl-1H-pyrrol-2-yl)pyridine, H₂^MeCNN, with tetrabenzyl zirconium. Both complexes are photoluminescent upon excitation with visible light and exhibit remarkably long emission lifetimes of tens to hundreds of microseconds in solution at room temperature. The nature of the emissive state was investigated using density functional theory, which allowed the assignment as a predominantly intraligand triplet state with significant ligand-to-metal charge transfer contributions (³IL/³LMCT). Electrochemical studies revealed two fully reversible one-electron reductions for each complex and an additional reversible oxidative EC process for the more sterically protected complex Zr(^MeCNN)₂. On the basis of the optical and electrochemical properties, the utility of the two zirconium complexes as photosensitizers for photoredox catalytic transformations was investigated. While Zr(^MeCNN)₂ readily promotes the photochemical homocoupling of benzyl bromide in the presence of a sacrificial benzimidazolium hydride reductant, Zr(^HCNN)₂ undergoes an intramolecular photochemical reaction with formation of a new carbon-carbon bond between the phenyl units of the two ^HCNN ligands.

Full text of DOI:10.1021/acs.organomet.8b00388

Article
pubs.acs.org/Organometallics
Cite This: Organometallics XXXX, XXX, XXX−XXX
Photochemical C−C Bond Formation in Luminescent Zirconium
Complexes with CNN Pincer Ligands
Yu Zhang, Jeffrey L. Petersen, and Carsten Milsmann*
C. Eugene Bennett Department of Chemistry, West Virginia University, Morgantown, West Virginia 26506, United States
S
* Supporting Information
ABSTRACT: The complexes Zr(HCNN)2 and Zr(MeCNN)2 were prepared via reaction of the ligand precursors 2-phenyl-6-(5-
H
methyl-3-phenyl-1H-pyrrol-2-yl)pyridine, H2 CNN, and 2-(3,5-dimethyl-phenyl)-6-(5-methyl-3-phenyl-1H-pyrrol-2-yl)-
pyridine, H2MeCNN, with tetrabenzyl zirconium. Both complexes are photoluminescent upon excitation with visible light
and exhibit remarkably long emission lifetimes of tens to hundreds of microseconds in solution at room temperature. The
nature of the emissive state was investigated using density functional theory, which allowed the assignment as a predominantly
intraligand triplet state with significant ligand-to-metal charge transfer contributions (3IL/3LMCT). Electrochemical studies
revealed two fully reversible one-electron reductions for each complex and an additional reversible oxidative EC process for the
more sterically protected complex Zr(MeCNN)2. On the basis of the optical and electrochemical properties, the utility of the two
zirconium complexes as photosensitizers for photoredox catalytic transformations was investigated. While Zr(MeCNN)2 readily
promotes the photochemical homocoupling of benzyl bromide in the presence of a sacrificial benzimidazolium hydride
reductant, Zr(HCNN)2 undergoes an intramolecular photochemical reaction with formation of a new carbon−carbon bond
H
between the phenyl units of the two CNN ligands.
While the well-established MLCT pathway for photo-
induced charge separation has been successfully adapted for
low-valent isocyanide complexes of group 6 and 7
elements,14−19 the generally electron-deficient nature of early
transition metals requires the study of alternative excitation
mechanism. Recent reports of long-lived d−d excited states in
luminescent CrIII (d3) complexes20−22 and the successful use of
emissive f−d states in lanthanide complexes for photo-
catalysis23−27 highlight the potential of unconventional
approaches. Inspired by reports of ligand-to-metal charge
transfer (LMCT) as a design principle for the generation of
luminescent metallocene complexes of group 3 and 4
elements,28−35 we recently reported the synthesis of a
luminescent zirconium complex with strongly π-donating
pyridine dipyrrolate (PDP) pincer ligands.36 The complex
Zr(MePDP)2 exhibits a remarkably long excited state lifetime of
325 μs and is a competent photosensitizer for reductive
photoredox catalytic transformations. Detailed studies estab-
INTRODUCTION
■
Transition metal photosensitizers play an important role in the
development of new technologies for solar energy con-
version1−3 and in the discovery of novel photochemical
reactions.4−9 For several decades, the field of photoactive
metal complexes has primarily been the domain of electron-
rich late transition metals and particularly precious metals. In
combination with strong π acceptor ligands, often derived from
polypyridine or phenylpyridine architectures, the central d6, d8,
or d10 metal ions exhibit strong metal-to-ligand charge transfer
(MLCT) transitions that feature high extinction coefficients
and readily undergo intersystem crossing resulting in long-lived
triplet excited states.10−12 Motivated by a long-standing
interest to develop photoactive metal complexes based on
earth-abundant elements,13 early transition metals have
become the focus of more recent research efforts. In addition
to their higher abundance in the Earth’s crust and the
associated lower costs, early transition metals often have the
added benefit of decreased toxicity compared to their late
transition metal counterparts, which could be particularly
attractive for photochemical applications in the production of
pharmaceuticals.
Special Issue: Organometallic Complexes of Electropositive Ele-
ments for Selective Synthesis
Received: June 6, 2018
© XXXX American Chemical Society
A
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lished photoinduced single electron transfer (SET) as the
mechanism for photocatalysis.37
°C resulted in clean formation of Zr(HCNN)2, which was
isolated in excellent yield. The synthesis of Zr(MeCNN)2
employing H2MeCNN required more forcing conditions of
130 °C in a pressure tube and provided only a modest yield of
the desired homoleptic zirconium complex after workup. The
more challenging C−H activation step for H2MeCNN is
consistent with the increased steric profile imposed by the
additional methyl groups in the ortho-position of the newly
formed Zr−C bond. The 1H and 13C NMR spectra of
Zr(HCNN)2 and Zr(MeCNN)2 are consistent with C2
symmetric molecules with equivalent pincer ligands in
solution. Most prominently, both complexes exhibit a
significantly downfield shifted resonance for the ipso-carbon
of the Zr-Ph moiety, which can be found at 191.5 and 191.9
ppm for Zr(HCNN)2 and Zr(MeCNN)2, respectively. Both
compounds are moisture-sensitive in the solid state and in
solution and react with water to yield the ligand precursors and
ZrO2. The complexes are stable under an atmosphere of dry
oxygen in the absence of light but react with O2 upon
irradiation with visible light in solution furnishing dark red
solids that have so far eluded characterization due to their
insolubility in organic solvents.
Here we describe the synthesis and characterization of two
photoluminescent zirconium complexes with 2-phenyl-6-
(pyrrol-2-yl)pyridine (CNN) ligands. These ligands can be
viewed as a hybrid between the strongly π-accepting 2-
phenylpyridine (ppy) ligands most prominently employed in
the design of luminescent iridium complexes11 and the π-
donating PDP ligands featured in our previous studies
(Scheme 1). The physical properties of the new complexes
Scheme 1. Relationship between Previously Studied
Pyridine Dipyrrolate and 2-Phenylpyridine Ligands and the
CNN Ligand Framework Used in this Study
were established via a combined experimental and computa-
tional approach and the photochemical reactivity was
investigated. Minor changes to the substitution pattern of the
ligand backbone resulted in a dramatic shift in the primary
reactivity of the complexes from photoinduced outersphere
electron transfer to photochemical intramolecular C−C bond
formation.
Single crystals of Zr(HCNN)2·Et2O suitable for crystallo-
graphic analysis by X-ray diffraction were obtained by cooling a
saturated solution of the complex in diethyl ether to −35 °C.
The compound crystallizes in the monoclinic space group C2/c
and a representation of the molecular structure of Zr(HCNN)2
is shown in Figure 1. Important bond lengths and angles are
RESULTS AND DISCUSSION
■
Synthesis and Characterization of the Complexes
Zr(HCNN)2 and Zr(MeCNN)2. The two ligand precursors 2-
phenyl-6-(5-methyl-3-phenyl-1H-pyrrol-2-yl)pyridine,
H
H2 CNN, and 2-(3,5-dimethyl-phenyl)-6-(5-methyl-3-phenyl-
1H-pyrrol-2-yl)pyridine, H2MeCNN, were prepared from
commercially available starting materials via a straightforward
three-step protocol outlined in Scheme 2. The direct reaction
H
of H2 CNN with 0.5 equiv of ZrBn4 in benzene solution at 80
a
Scheme 2. Synthesis of Zr(HCNN)2 and Zr(MeCNN)2
Figure 1. Solid-state molecular structure of Zr(HCNN)2 at 30%
probability ellipsoids. Hydrogen atoms were omitted for clarity.
summarized in Table 1. The coordination environment around
the central zirconium ion is best described as a distorted
octahedron. The most significant deviation from idealized
octahedral geometry is imposed by the small bite angles of the
pincer ligand with Npy−Zr−CPh and Npy−Zr−Npyrrole angles
being slightly below 70°. A second, smaller distortion is
apparent from the Npy−Zr−Npy angle of 170.50(6)° for the
trans-coordinating pyridine units. The two planes defined by
the donor atoms of the pincers are nearly perpendicular with
an angle of 85.66°, and both ligand backbones exhibit only
minor deviations from planarity. While the molecule does not
lie on a crystallographic C2 axis, the differences between the
two ligands are generally within experimental error and the
solid-state structure is consistent with the C2 symmetry in
solution observed by NMR spectroscopy.
a
Reagents and conditions: (a) Pd(PPh3)4 (3 mol %), Na2CO3 (3
equiv), toluene/methanol/water (2:1:1), reflux; (b) (1) 3-benzyl-5-
(2-hydroxyethyl)-4-methyl-thiazolium chloride (50 mol %), NaOtBu
(50 mol %), EtOH, reflux; (2) NH4OAc, EtOH, reflux; (c) C6H6, 80
°C (R = H) or 130 °C (R = Me).
B
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is related to these irreversible oxidations and is absent in CVs
with turnaround potentials at 0 V. The peak separation
between the two oxidation events is solvent-dependent with
slightly larger peak separation and potentials at 0.46 and 0.68 V
in THF solution. In both solvents, the reversibility of the
reduction events is negatively affected if the CV scans are
conducted with an initial sweep direction toward positive
potential which highlights the irreversibility of the oxidation
processes.
Table 1. Selected Bond Lengths (Å) and Angles (deg) for
Zr(HCNN)2
Zr(HCNN)2
Zr(1)−N(1)
Zr(1)−N(2)
Zr(1)−C(15)
Zr(1)−N(3)
Zr(1)−N(4)
Zr(1)−C(37)
2.179(2)
2.330(2)
2.256(2)
2.169(2)
2.345(2)
2.268(2)
On first glance, the redox properties of Zr(MeCNN)2 seem
nearly identical to those of Zr(HCNN)2. Two reversible
reductions can be detected at −2.26 and −2.63 V indicating
that the introduction of electron-donating methyl substituents
has only a limited effect on the reduction potentials of the
complex compared to the unsubstituted system. In contrast, a
significant change is observed upon oxidation of the complex.
A single, irreversible oxidation with an anodic peak maximum
at 0.51 V is observed for Zr(MeCNN)2. While the oxidation
potential is comparable to those for the two events observed
for Zr(HCNN)2, this redox process has no negative influence
on the two reduction events when scans are run toward
positive potentials first. This behavior is consistent with a
reversible electrochemical/chemical (EC) process and is
reflected in the well-behaved reductive feature with a cathodic
peak potential of −1.06 V. While the nature of the reversible
chemical process following oxidation of Zr(MeCNN)2 is
unknown at this point, it is likely that the steric protection
introduced by the methyl substituents confers increased
stability to the product compared to Zr(HCNN)2 and enables
reversible (electro)chemistry.
N(1)−Zr(1)−N(2)
N(1)−Zr(1)−C(15)
N(2)−Zr(1)−C(15)
69.70(6)
138.02(7)
69.50(7)
N(3)−Zr(1)−N(4)
N(3)−Zr(1)−C(37)
N(4)−Zr(1)−C(37)
68.60(6)
137.56(7)
69.85(7)
N(2)−Zr(1)−N(4)
170.50(6)
Electrochemistry. The electrochemical properties of
Zr(HCNN)2 and Zr(MeCNN)2 were studied via cyclic
voltammetry (CV). All CV experiments were conducted with
[N(n-Bu)4]PF6 as the supporting electrolyte (0.1 M) and all
potentials were referenced against the Fc+/Fc couple using
ferrocene (Fc) as an internal standard. The CVs for the two
zirconium complexes measured in 1,2-difluorobenzene sol-
ution are shown in Figure 2. Zr(HCNN)2 undergoes two fully
reversible redox events at −2.27 and −2.64 V, which were
assigned to one-electron reductions of the complex. Two
irreversible redox processes are observed with anodic peak
potentials of 0.41 and 0.53 V and are likely due to ligand-
centered oxidation events. An additional feature around −1 V
Optical Properties. The optical properties of Zr(HCNN)2,
Zr(MeCNN)2, and their corresponding ligand precursors
R
H2 CNN (R = H, Me) were studied in benzene solution at
room temperature. The electronic absorption spectra of all four
compounds are shown in Figure 3. Both ligand precursors
exhibit a single absorption band in the UV region of the
spectrum with a peak maximum at 341 nm (ε = 19 300 M−1
cm−1) and 340 nm (ε = 20 200 M−1 cm−1) for H2 CNN and
H
H2MeCNN, respectively. Deprotonation of the pyrrole ring by
addition of 1 equiv of n-BuLi resulted only in a small red-shift
of the absorption maximum for both ligand precursors (Figure
S15; LiHHCNN: 360 nm; LiHMeCNN: 361 nm). In contrast,
both Zr(HCNN)2 and Zr(MeCNN)2 possess a single strong
absorption band in the visible region of the spectrum with
maxima at 470 nm (ε = 14 300 M−1 cm−1) and 471 nm (ε =
12 400 M−1 cm−1), respectively. An additional absorption band
is observed at 325 nm (ε = 25 600 M−1 cm−1) for Zr(HCNN)2.
A similar feature for Zr(MeCNN)2 is split into two overlapping
signals with maxima at 340 nm (ε = 22 800 M−1 cm−1) and
316 nm (ε = 22,400 M−1 cm−1). Analogous to our previous
studies for the closely related complex Zr(MePDP)2,36,37 the
occurrence of a new absorption band with high extinction
coefficient in the visible region illustrates significant impact of
the central zirconium ion on the optical properties of the
compounds likely due to ligand-to-metal charge transfer
contributions.
The steady-state emission spectra obtained upon excitation
at the lowest energy absorption maximum for each compound
mirror the trends observed for the absorption spectra (Figure
3). The emission profiles for the free ligand precursors are
almost indistinguishable with emission maxima at 412 and 410
Figure 2. Cyclic voltammograms of Zr(HCNN)2 (black, top) and
Zr(MeCNN)2 (red, bottom) in 1,2-difluorobenzene (glassy carbon
working electrode, 0.1 M [n-Bu4N][PF6], scan rate 200 mV/s, 295
K).
H
nm for H2 CNN and H2MeCNN, respectively. Similar to the
absorption maxima, deprotonation of the pyrrole rings resulted
C
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(MeCNN)2 can be readily fit by a single exponential decay with
a lifetime of τ = 412 μs, the data for Zr(HCNN)2 is more
complicated and can only be fit satisfactorily using a
biexponential approach. This behavior is consistent with two
separate decay pathways, in which the majority of the
molecules in the excited state decay to the ground state via a
faster process with a lifetime of τ1 = 77 μs, while a smaller
portion can access a slower process with τ2 = 296 μs. This
significant change in excited state lifetime is also reflected in
the emission quantum yields of Φ = 0.03 and 0.18 obtained via
comparative method for Zr(HCNN)2 and Zr(MeCNN)2,
respectively. With experimental values of Φ and τ in hand,
the rate constants for radiative decay were determined via kr =
Φ/τ as 390 s−1 and 437 s−1 for Zr(HCNN)2 and its methyl-
substituted analog, respectively. The similarity between the
obtained kr values suggests that the methyl substitutents in
Zr(MeCNN)2 exert little influence on the rate of phosphor-
escent emission but are essential for minimizing nonradiative
decay pathways by lowering knr (τ = 1/(kr + knr)). The limited
influence of these substituents on the electronic properties
established via steady-state absorption and emission spectros-
copy implies that this outcome is primarily due to steric effect.
To further clarify the role of the central zirconium ion in
Zr(RCNN)2, time-resolved emission data for both ligand
precursors and their corresponding monolithium salts were
collected (Figure S15). All four species exhibit relatively short
luminescence lifetimes in the low nanosecond range
(H2 CNN: τ = 9 ns; H2MeCNN: τ = 7 ns; LiHHCNN: τ =
H
4 ns; LiHMeCNN: τ = 5 ns). We note that these values are
close to the detection limit of the available instrumentation
(∼2 ns). Nevertheless, the recorded values clearly establish
that the zirconium ion has a pronounced effect on the nature
and properties of the emissive state. On the basis of the
dramatic increase in excited state lifetime by several orders of
magnitude, it is likely that the zirconium center plays a key-role
in intersystem crossing to the triplet manifold.
Figure 3. Optical properties of Zr(HCNN)2 (top, black), Zr-
(
MeCNN)2 (bottom, red), and the corresponding ligand precursors
R
H2 CNN (blue) recorded in benzene solution at room temperature.
Solid lines represent the UV−vis absorption profiles and dotted lines
show the emission spectra obtained upon excitation at the lowest
absorption maximum (λex = 470 nm for Zr(HCNN)2 and Zr-
Density Functional Theory. To gain further insight into
the frontier molecular orbitals and the resulting optical
transitions, ground state and time-dependent density func-
tional theory, (TD-)DFT, calculations were performed for
Zr(HCNN)2. All computations were conducted at the B3LYP
level using the crystallographically determined geometry of the
untruncated complex as the starting point for geometry
optimization. The geometric parameters after optimization
shown in Table 2 are in excellent agreement with the
experimental data and reproduce the approximately C2
symmetric structure observed in the solid state. The largest
deviation between corresponding bond distances between the
two ligands is observed for the Zr−Npyrrole bonds (2.2 pm). For
all remaining bonds, the deviation is less than 1 pm. As is
typically observed for DFT calculations,38 the computed
zirconium-ligand bond lengths are slightly overestimated by
4−5 pm, while all intraligand bond distances are within 1−2
pm of the experimental values. The optimized structure
reproduces the slight deviation from 180° for the Npy−Zr−Npy
angle of the trans-coordinating pyridine rings providing a value
of 168.4° (exp. 170.5°). The angle between the two planes
defined by the three donor atoms of each pincer ligand is also
well-reproduced at 87.1° (exp. 85.7°). The electronic
absorption spectrum obtained via TD-DFT calculations
based on the optimized geometry is shown in Figure 4. The
conductor-like screening model (COSMO) was used to
R
(
MeCNN)2; λex = 341 nm for H2 CNN). The insets show time-
resolved emission data detected at 565 nm after excitation at 456 nm.
in a red-shift of the emission maxima (LiHHCNN: 523 nm;
LiHMeCNN: 521 nm). The emission profiles for the zirconium
species are significantly red-shifted compared to the ligand
precursors but are nearly identical between the two complexes
with maxima at 562 and 565 nm for Zr(HCNN)2 and
Zr(MeCNN)2, respectively. It is again notable that the
introduction of electron-donating substituents to the phenyl
moieties has a negligible effect on the absorption and emission
profiles. Combined with the observations from the electro-
chemical data this suggests that the phenyl units should have
limited contributions to the frontier molecular orbitals
involved in the optical transitions as well as outersphere
electron transfer.
While the absorption and emission profiles are mostly
unperturbed by the addition of methyl substituents, the excited
state lifetimes reveal dramatic differences between Zr(HCNN)2
and Zr(MeCNN)2. As shown in the insets in Figure 3, both
complexes exhibit long-lived emissive states consistent with
phosphorescence. The absence of fast decay pathways upon
termination of the excitation pulse is consistent with efficient
intersystem crossing to a long-lived triplet state in both
complexes. While the time-resolved emission data for Zr-
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Table 2. Selected Bond Distances for the S0 and T1
Geometries of Zr(HCNN)2 Obtained via DFT Calculations
a
a
singlet (RKS)
triplet (UKS3)
ligand 1
ligand 2
ligand 1
ligand 2
Figure 4. Electronic absorption spectrum of Zr(CNN)2 in benzene
obtained via TD-DFT calculations (red line, fwhm of 2500 cm−1).
The stick plot indicates the positions and relative intensities of
individual transitions. The major contributions to each numbered
state are listed in Table 3. The experimental spectrum is shown as a
dotted black line for comparison.
Zr(1)−N(1)
Zr(1)−N(2)
Zr(1)−C(15)
2.208
2.376
2.287
2.230
2.366
2.290
2.229
2.380
2.299
2.266
2.290
2.286
N(1)−C(1)
C(1)−C(2)
C(2)−C(3)
C(3)−C(4)
N(1)−C(4)
C(4)−C(5)
1.369
1.384
1.416
1.400
1.390
1.439
1.369
1.387
1.411
1.397
1.392
1.434
1.367
1.384
1.416
1.401
1.390
1.440
1.386
1.403
1.387
1.479
1.371
1.416
Table 3. Vertical Electronic Excitation Energies and Main
Excitations Contributing to the Absorption Bands of
Zr(HCNN)2 Obtained via TD-DFT Calculations
TD-
DFT
state
energy/cm−1
(λ/nm)
excitations
ab
character
(% LMCT)
N(2)−C(5)
C(5)−C(6)
C(6)−C(7)
C(7)−C(8)
C(8)−C(9)
N(2)−C(9)
1.359
1.404
1.380
1.393
1.390
1.354
1.358
1.405
1.379
1.395
1.388
1.354
1.360
1.404
1.380
1.392
1.391
1.355
1.397
1.401
1.383
1.405
1.389
1.365
,
fosc
(weight)
1
18820.3 (531.3)
0.031
181 → 182 (0.98) 1IL/1LMCT
(26%)
2
20089.5 (497.8)
0.004
0.062
180 → 182 (0.80) 1IL/1LMCT
(26%)
181 → 183 (0.19)
3
20268.4 (493.4)
181 → 183 (0.77) 1IL/1LMCT
(20%)
180 → 182 (0.18)
C(9)−C(10)
C(10)−C(11)
C(11)−C(12)
C(12)−C(13)
C(13)−C(14)
C(14)−C(15)
C(10)−C(15)
1.475
1.398
1.389
1.391
1.393
1.399
1.410
1.474
1.397
1.389
1.391
1.394
1.399
1.410
1.476
1.399
1.388
1.391
1.393
1.401
1.411
1.469
1.399
1.388
1.392
1.393
1.400
1.412
4
5
6
21286.9 (469.8)
22811.1 (438.4)
23829.7 (419.6)
0.050
0.058
0.054
180 → 183 (0.94) 1IL/1LMCT
(20%)
181 → 184 (0.86) 1IL/1LMCT
(30%)
180 → 184 (0.86) 1IL/1LMCT
(30%)
b
a
Only excitations with a weight larger than 0.1 are shown. HOMO
181, LUMO 182.
a
No symmetry constraints were applied during geometry optimiza-
tion.
20% for the LUMO+1 to 30% for the LUMO+2. On the basis
of this analysis, the transitions giving rise to the absorption
band in the visible region are best described as an intraligand
(IL) transition with significant ligand-to-metal charge transfer
(LMCT) transitions character of 20−30%. A closer inspection
of the frontier MOs also provides an explanation for the
limited influence of the substituents on the phenyl rings on the
visible absorption profile of Zr(HCNN)2 compared to
Zr(MeCNN)2: Neither the donor nor the acceptor orbitals
show significant contributions from the π-system of the phenyl
rings. The maximum contribution can be found for the LUMO
and amounts to less than 10% phenyl character for this orbital.
The UV region exhibits a more complex pattern of
transitions with a multitude of states giving rise to a single
broad absorption feature consistent with the experimental data.
simulate a dielectric medium corresponding to the solvent
benzene used during experimental data acquisition.39
The calculated spectrum is in good agreement with the
experimental data. Six TD-DFT states contribute to the
absorption band observed in the visible region of the spectrum.
The main single-electron excitations contributing to each state
are listed in Table 3. A molecular orbital diagram depicting the
frontier molecular orbitals (HOMO−1 to LUMO+2) involved
in these excitations is provided in Figure 5. Consistent with a
central zirconium ion in the +IV oxidation state (d0) the
doubly occupied donor orbitals (HOMO and HOMO−1)
exhibit no contributions from the metal d orbitals and are
exclusively ligand centered. The three main acceptor orbitals
contain varying amounts of zirconium character ranging from
E
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Figure 6. Spin density distribution for the lowest energy triplet state
of Zr(HCNN)2 obtained via Mulliken population analysis.
Figure 5. Frontier molecular orbital diagram showing the donor and
acceptor orbitals contributing to the TD-DFT excitations computed
in the visible region of the electronic absorption spectrum. The gray
numbers to the right correspond to the MO number with 181 being
the HOMO and 182 being the LUMO.
Zr(MePDP)2.37 On the basis of these results, we propose that
emission from the triplet to the singlet state occurs from a
symmetric structure closer to the equilibrium geometry of the
singlet ground state in which spin−orbit coupling is maximized
due to contributions from a degenerate set of metal d orbitals.
According to the computed asymmetric structure of the triplet
state, such a transition would likely have to occur from a
vibrationally excited state of the triplet state, which could
provide an explanation for the long phosphorescence lifetime
observed experimentally. More detailed spectroscopic studies
supporting this hypothesis are currently underway in our
laboratory.
In an attempt to gain more insight into the ground state
redox properties of Zr(RCNN)2 DFT calculations were also
performed for the one-electron reduced and oxidized species
[Zr(HCNN)2]1− and [Zr(HCNN)2]1+, respectively. The spin
density distributions obtained via Mulliken population analysis
for both complexes assuming doublet ground states (S = 1/2)
are shown in Figure 7. On the basis of these data, the
additional electron in the one-electron reduced complex
occupies an orbital that is highly delocalized over both ligand
and metal. Hence, the electronic structure of the anion is best
described by the two resonance structures [ZrIV(HCNN2−)-
(HCNN1−•)]1− and [ZrIII(HCNN2−)2]1− with a majority
contribution of 65% from the first resonance form. The high
degree of delocalization is consistent with the nature of the
LUMO in the neutral complex Zr(HCNN)2. Consequently, a
clear assignment as ligand- versus metal-centered reduction is
not possible. In contrast, the SOMO of the one-electron
oxidized species is completely localized on the pincer ligands
with major contributions from the pyrrole rings analogous to
the HOMO of the neutral compound. The pure organic radical
character of the SOMO in [Zr(HCNN)2]1+ explains the
increased reactivity of this species reflected in the irreversibility
of the one-electron oxidation of Zr(HCNN)2
A more detailed analysis of the most intense transitions in this
area is provided in the Supporting Information.
The electronic structure of the emissive triplet state was also
explored by DFT calculations. The optimized geometry
obtained for the lowest energy triplet state shows a substantial
distortion from the close to C2 symmetric structure computed
for the singlet ground state (Table 2). The most significant
changes are a shortening of the Zr−Npy bond length by almost
10 pm and distortions within the pyridine pyrrole moiety for
one of the two pincer ligands. The structural parameters for the
second pincer are very similar to those found in the singlet
ground state. Among the intraligand distances, the most
characteristic difference between the two ligands is found for
the C(3)−C(4) bond length (Δ(C(3)−C(4)) = 7.8 pm) in
the pyrrole ring and the N(2)−C(5) bond length of the
pyridine ring (Δ(N(2)−C(5)) = 3.7 pm). Both bonds are
elongated compared to the second pincer ligand of the triplet
state and the corresponding bonds in the singlet ground state.
Altogether, these structural changes are consistent with a
triplet excited state that contains a majority contribution from
an intraligand charge transfer (3ILCT) process, in which an
electron is transferred from the predominantly pyrrole-
centered HOMO of the singlet ground state (C(3)−C(4)
bonding) to the mixed pyridine- and zirconium-based LUMO
(N(2)−C(5) antibonding, Zr(1)−N(2) bonding). The partial
metal character of one of the two resulting SOMOs in the
triplet excited state is reflected in the spin density distribution
(20% Zr) shown in Figure 6.
To computationally approximate the emissive triplet-to-
singlet transition responsible for the observed phosphores-
cence of Zr(HCNN)2 TD-DFT calculations including spin-
forbidden transitions were performed. Initial calculations were
conducted based on the structure obtained for the triplet
excited state assuming emission from the triplet following the
Franck−Condon principle. To our surprise, the lowest triplet-
to-singlet transition was predicted to occur at 723 nm,
significantly red-shifted with respect to the experimental
emission maximum at 562 nm. However, calculations using
the same approach based on the more symmetric singlet
geometry provided a more accurate value of 568 nm for the
lowest triplet-to-singlet transition. A similar situation was
observed in our previous study for the closely related complex
Photoredox Properties of Zr(HCNN)2 and Zr(MeCNN)2.
By combining the information obtained from the electro-
chemical and steady-state emission measurements, the excited
state redox potentials of the two zirconium complexes were
estimated. The emission maxima were used as first-order
approximations for the energy difference, E0, between the
triplet excited state and the ground state. With E0 values of
2.21 and 2.19 eV and ground state redox potentials, E1/2([Zr]/
[Zr]1−), of −2.27 and −2.26 V, the excited state potentials,
E
1/2([Zr]*/[Zr]1−), were calculated as −0.06 and −0.07 V for
Zr(HCNN)2 and Zr(MeCNN)2, respectively, via E1/2([Zr]*/
[Zr]1−) = E1/2([Zr]/[Zr]1−) + E0.40 These potentials are on
F
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Figure 7. Spin density distributions for [Zr(HCNN)2]1+ (left) and [Zr(HCNN)2]1− (right) obtained via Mulliken population analysis.
par with the excited state redox potential of Zr(MePDP)2,
which can act as a photosensitizer for reductive photoredox
reactions.36 Notably, the more negative ground state redox
potentials of Zr(RCNN)2 compared to Zr(MePDP)2 are offset
by the blue-shifted emission profiles, which indicate increased
energy storage in the excited state.
conversion was detected for the methyl derivative in the same
time. Furthermore, the methyl substituted complex Zr-
(MeCNN)2 reacts only sluggishly in the presence of 10 equiv
of benzyl bromide even upon prolonged irradiation with blue
1
light. Analysis of the reaction mixture by H NMR spectros-
copy after 4 days of irradiation revealed a complex mixture of
products in addition to unreacted starting material (Figure
S28). In contrast, clean conversion to a new diamagnetic
species was observed under similar conditions for Zr(HCNN)2
within 4 h of irradiation on a small scale in an NMR tube
(Figure S27). An equimolar amount of bibenzyl was detected
as the only byproduct of the reaction. Preparative scale
reactions required longer irradiation times of up to 24 h but
allowed isolation of the new zirconium species in 64% yield.
The most remarkable feature of this complex immediately
detectable by 13C NMR spectroscopy is the absence of a
downfield shifted signal indicative of a Zr−Ph bond, which
suggests photochemical Zr−C bond cleavage.
The identity of the zirconium complex Zr(DPDP)Br2
(H4DPDP = 2,2′-bis(6-(5-methyl-3-phenyl-1H-pyrrol-2-yl)-
pyridin-2-yl)biphenyl) was established by X-ray diffraction
and a representation of the molecular structure is shown in
Figure 8. The structure confirms the absence of a Zr−C bond
and reveals C−C bond formation between the two ipso-C
atoms of the Zr(HCNN)2 starting material. It is noteworthy
that the solid-state molecular structure of Zr(DPDP)Br2
exhibits C1 symmetry, which is inconsistent with the data
obtained via NMR spectroscopy in solution that suggests
With the excited state potentials in hand, Zr(HCNN)2 and
Zr(MeCNN)2 were tested as photosensitizers in the reductive
homocoupling of benzyl bromide to bibenzyl. This well-
studied reaction has been employed by several research groups
as a benchmark system for reductive photoredox trans-
formations using various precious metal and earth-abundant
photosensitizers.14,23,41 Using our previously reported con-
ditions37 with 1,3-dimethyl-2-phenyl-2,3-dihydro-1H-7-
methoxybenzo[d]imidazole, MeOBIH, as the sacrificial reduc-
tant and triethylamine as the base the desired coupling product
was obtained in 40% yield after irradiation with blue light
(LED, λmax = 462 nm) for 20 h in the presence of 5 mol %
Zr(MeCNN)2 (Scheme 3). The outcome of the reaction
Scheme 3. Photoredox Catalytic Homocoupling of Benzyl
Bromide with Zr(MeCNN)2 as Photosensitizer
changed dramatically when Zr(HCNN)2 was used as the
photosensitizer under otherwise identical conditions. While
only a small amount of bibenzyl (10% yield) was obtained, 1H
NMR spectroscopy indicated complete decomposition of
Zr(HCNN)2. Further experiments showed that catalyst
decomposition occurred rapidly within only 2 h of irradiation
and without consumption of sacrificial reductant but providing
a consistent amount of bibenzyl (10%).
Inspired by these observations, direct photoreactions
between benzyl bromide and Zr(RCNN)2 were investigated.
Initial control experiments conducted in C6D6 solution
established that both complexes are photostable upon
irradiation with blue light (72 h) and that neither zirconium
complex reacts directly with benzyl bromide at room
temperature in the absence of light. Some reactivity between
benzyl bromide and either complex was observed upon
prolonged heating to 120 °C, but proceeded with low
selectivity yielding several unidentified products. Complete
decomposition of the metal complex upon heating in the dark
was observed after 2 days for Zr(HCNN)2, while only 20%
Figure 8. Solid-state molecular structure of Zr(DPDP)Br2 at 30%
probability ellipsoids. Hydrogen atoms were omitted for clarity.
G
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higher symmetry with equivalent pyrrole and pyridine
heterocycles. The appearance of higher symmetry in solution
is likely due to a fast dynamic process that renders the
heterocycles equivalent on the NMR time scale. While the
mechanism of the overall transformation (Scheme 4) is not
process upon visible light excitation that is not available to the
more sterically protected complex. However, the long-time
photostability of Zr(HCNN)2 established by NMR spectros-
copy after prolonged irradiation in the absence of oxidants
indicates that this process must be fully reversible. More
detailed studies supporting this hypothesis are currently
underway in our laboratory.
Scheme 4. Photochemical Reaction of Zr(HCNN)2 with
Benzyl Bromide
CONCLUDING REMARKS
■
The luminescent early transition metal complexes Zr(HCNN)2
and Zr(MeCNN)2 were successfully synthesized and charac-
terized and their electrochemical and optical properties were
established. The complexes exhibit rich electron-transfer
chemistry with two reversible single-electron reductions at
chemically accessible potentials. The introduction of methyl
groups adjacent to the zirconium−carbon bond of the phenyl
ring enables an additional reversible EC process in Zr-
(MeCNN)2. Both compounds can access long-lived excited
states upon excitation with visible light resulting in
phosphorescent emission with lifetimes in the microsecond
range and luminescence quantum yields of 3% for Zr(HCNN)2
and 18% for Zr(MeCNN)2 in benzene solution at room
temperature. Density functional theoretical calculations
allowed the assignment of the absorption bands in the visible
region as predominantly IL transitions with significant LMCT
character of 20−30%. Computational studies of the lowest
clear at this point and likely proceeds via single-electron steps,
Zr(DPDP)Br2 can be viewed as the product of a formal C−C
reductive elimination with concomitant oxidation by two
equivalents of benzyl bromide. Formal aryl−aryl reductive
eliminations in zirconium complexes have been reported
previously for diaryl zirconocene derivatives42−45 and a
diphenyl zirconium complex with redox-active ligands.46
However, these examples require either activation by UV
light or occur under thermal conditions upon treatment with
strong single electron oxidants. To the best of our knowledge,
the system presented herein represents the first example for
photochemical aryl−aryl bond formation at zirconium using
visible light.
On the basis of the observed photochemical reactivity of
Zr(HCNN)2, dibenzyl disulfide was employed as an alternative
mild oxidant to promote oxidative C−C bond formation
between the two HCNN ligands. Upon irradiation with blue
light, clean conversion to Zr(DPDP)(SBn)2 was observed and
the complex was isolated in 75% yield (Scheme 5). Control
experiments in the absence of light confirmed the photo-
chemical nature of this transformation as no reaction was
observed under dark conditions.
3
energy triplet state are consistent with a mixed IL/3LMCT
state and revealed a significant distortion from the C2
symmetric structure of the singlet ground state that likely
contributes to the remarkably long emission lifetimes.
The absence of methyl substituents adjacent to the Zr−C
bond in Zr(HCNN)2 compared to Zr(MeCNN)2 has a
pronounced effect on the photochemical reactivity of the
complexes. While the predominant mode of reactivity for
Zr(MeCNN)2 is photoinduced outersphere electron transfer
that enables photoredox catalysis, Zr(HCNN)2 undergoes C−
C reductive elimination upon photoexcitation in the presence
of mild oxidants resulting in the formation of Zr(DPDP)X2 (X
= Br or SBn) complexes. A clear difference in the photophysics
between the two complexes is also apparent from time-resolved
emission studies that show a simple single-exponential decay
for Zr(MeCNN)2 and a more complex biexponential behavior
for Zr(HCNN)2.
Scheme 5. Photochemical Reaction of Zr(HCNN)2 with
Dibenzyl Disulfide
EXPERIMENTAL SECTION
■
Materials. All air- and moisture-sensitive manipulations were
carried out using standard Schlenk line and cannula techniques or in
an MBraun inert atmosphere drybox containing an atmosphere of
purified nitrogen. Solvents for air- and moisture-sensitive manipu-
lations were dried and deoxygenated using a Glass Contour Solvent
Purification System and stored over 4 Å molecular sieves. Tetrabenzyl
zirconium,47 2-phenyl-6-pyridincarboxaldehyde,48−50 and 6-(3,5-
dimethylphenyl)-2-pyridinecarboxyaldehyde51 were prepared follow-
ing literature procedures. All solids were dried under high vacuum; all
liquids were dried over CaH2 and vacuum transferred into oven-dried
glassware in order to bring into the glovebox. Deuterated benzene
(C6D6) for NMR spectroscopy was distilled from sodium metal.
Physical Measurements. Electronic absorption spectra were
recorded using a Shimadzu UV-1800 spectrophotometer in gas-tight
quartz cuvettes with a 10 mm path length fitted with J. Young valves.
Extinction coefficients are reported with three significant figures.
Emission spectra were obtained in 10 mm path length gas-tight quartz
cuvettes with J. Young valves using a Shimadzu RF-5301 PC
spectrofluorophotometer. Time-resolved emission data were collected
As a final control experiment the reaction between both
Zr(RCNN)2 complexes and iodine was explored to address
whether the C−C bond formation could be induced in the
absence of light by sufficiently strong oxidants. Both
compounds react readily but unselectively with I2 providing a
complex mixture of products and highlighting the remarkable
selectivity of the photoreactivity of Zr(HCNN)2 with mild
oxidants.
The combination of the photochemical experiments with the
data from time-resolved emission spectroscopy (vide supra)
provides an interesting starting point for mechanistic
considerations. The biexponential decay profile and reduced
lifetime for Zr(HCNN)2 compared to Zr(MeCNN)2 is
consistent with an additional intramolecular photochemical
H
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using a Horiba Jobin Yvon Fluorolog-3 Spectrofluorometer equipped
with a single-photon-counting module in multichannel scaler mode
and a 456 nm spectraLED pulsed excitation light source. Emission
lifetimes were determined using the provided decay analysis software
package, DAS v6.1. 1H and 13C {1H} NMR spectra were recorded on
an Agilent 400 MHz spectrometer, JEOL 400 MHz YH spectrometer
or a Varian INOVA 600 MHz spectrometer. All chemical shifts are
the resulting residue was diluted with H2O. The product was
extracted with ethyl acetate three times, and the combined organic
layers were washed with saturated NaHCO3 solution, brine, and dried
over Na2SO4. Removal of the ethyl acetate and drying under high
vacuum yielded the desired product as a light brown solid (Yield: 3.02
g, 73%). Mp 148−150 °C. 1H NMR (400 MHz, C6D6; δ, ppm): 9.58
(br, 1H), 7.79 (s, 2H), 7.63 (dm, J = 6.8 Hz, 2H), 7.39 (d, J = 8.0 Hz,
1H), 7.26 (tm, J = 7.6 Hz, 1H), 7.16 (tt, J = 8.0, 1.2 Hz, 1H), 7.07 (d,
J = 7.6 Hz, 1H), 6.92 (t, J = 8.0 Hz, 1H), 6.88 (s, 1H), 6.06 (d, J = 3.2
Hz, 1H), 2.22 (s, 6H), 1.77 (s, 3H). 13C NMR (101 MHz, C6D6; δ,
ppm): 157.60, 151.52, 140.50, 138.70, 138.16, 136.84, 130.84, 129.91,
129.44, 128.89, 126.76, 126.23, 126.11, 125.54, 118.23, 117.48,
111.48, 21.49, 12.48. HRMS (ESI) calcd for C24H23N2+ [M + H]+ m/
z 339.18558. Found 339.18514.
1
reported relative to SiMe4 using H (residual) chemical shifts of the
solvent as a secondary standard. To obtain NMR spectra free of
solvent molecules present in the crystal lattice, which could overlap
with signals from the complex in question, the compounds were first
dissolved in a small amount of deuterated solvent, which was then
removed under vacuum to coevaporate the cocrystallized solvent
molecules. Fresh deuterated solvent was then added to the residue to
obtain a (close to) solvent-free NMR spectrum. High-resolution mass
spectra were obtained on a Thermo Finnigan Linear Trapping
Quadrupole mass spectrometer. Elemental analyses were performed at
Robertson Microlit Laboratories, Inc., in Ledgewood, NJ. Cyclic
voltammetry measurements were conducted under nitrogen atmos-
phere inside an MBraun drybox using a Gamry Interface 1000
electrochemical workstation in a single compartment cell using 1 mM
sample solutions in THF or 1,2-difluorobenzene with 0.1 M
tetrabutylammonium hexafluorophosphate as supporting electrolyte.
A three electrode setup was employed with a glassy carbon electrode
as working electrode, a platinum sheet as the counter electrode and a
silver wire as a quasi-reference electrode. Ferrocene was added as an
internal standard after completion of the measurements and all
potentials are referenced versus the Fc+/Fc couple.
Preparation of Zr(HCNN)2. In the glovebox, a solution of
tetrabenzyl zirconium (587 mg, 1.29 mmol, 0.50 equiv) in 3 mL of
benzene was added to a 50 mL thick-walled glass vessel containing a
solution of H2CNN (800 mg, 2.58 mmol, 1.00 equiv) in 5 mL of
benzene. The thick-walled vessel was sealed with a PTFE screw cap
and heated to 80 °C overnight, resulting in a brown solution. After
cooling to room temperature, the reaction vessel was brought back
into the glovebox and the reaction mixture was concentrated in
vacuum until a large amount of orange solid precipitated. The solid
was collected by filtration, washed three times with Et2O, and dried in
vacuum. The product was collected as a light orange solid (Yield: 856
1
mg, 94%). H NMR (400 MHz, C6D6; δ, ppm): 7.73 (d, J = 6.4 Hz,
2H), 7.64 (d, J = 6.8 Hz, 4H), 7.43 (d, J = 8.4 Hz, 2H), 7.34−7.22
(m, 6H), 7.19 (d, J = 7.2 Hz, 2H), 6.91 (t, J = 7.6 Hz, 2H), 6.86−6.77
(m, 4H), 6.73 (d, J = 7.6 Hz, 2H), 5.93 (s, 2H), 2.00 (s, 6H). 13C
NMR (101 MHz, C6D6; δ, ppm): 191.45, 164.37, 157.09, 145.99,
141.94, 141.50, 138.09, 135.31, 134.13, 131.39, 129.97, 129.81,
129.09, 128.81, 127.13, 122.65, 116.31, 114.04, 112.10, 14.33. Anal.
Calcd for C44H32N4Zr·Et2O: C, 73.71; H, 5.41; N, 7.16. Found: C,
73.70; H, 5.54; N, 7.13. Single crystals suitable for X-ray
crystallographic analysis were grown from a saturated solution of
Zr(HCNN)2 in Et2O at −35 °C.
Preparation of 2-Phenyl-6-(5-methyl-3-phenyl-1H-pyrrol-2-
yl)pyridine, H2HCNN. 2-Phenyl-6-pyridincarboxaldehyde (2.00 g,
10.92 mmol, 1.00 equiv), 4-phenyl-3-buten-2-one (1.76 g, 12.01
mmol, 1.10 equiv), and 3-benzyl-5-(2-hydroxyethyl)-4-methyl-thiazo-
lium chloride (1.47 g, 5.46 mmol, 0.5 equiv) were mixed in a 250 mL
Schlenk flask under an argon atmosphere. Then, 20 mL of absolute
ethanol were added. A solution of sodium tert-butoxide (525 mg, 5.46
mmol, 0.5 equiv) in ethanol (20 mL) was added via syringe and the
mixture was heated to 80 °C for 1 h during which the 1,4-diketone
intermediate precipitated. The reaction was cooled to room
temperature. The solid was collected via filtration and washed three
times with cold ethanol. Then, the 1,4-diketone intermediate was
mixed with ammonium acetate (8.41 g, 109.17 mmol, 10.00 equiv) in
10 mL of absolute ethanol and the reaction mixture heated to reflux.
After 12 h, the solvent was removed. The resulting residue was diluted
with H2O, and the product was extracted with ethyl acetate three
times. The combined organic layers were washed with saturated
NaHCO3 solution, brine, and dried over Na2SO4. Removal of ethyl
acetate and drying under high vacuum yielded the desired product as
a brown solid (Yield: 2.456 g, 72%). Mp 112−113 °C. 1H NMR (400
MHz, C6D6; δ, ppm): 9.30 (br, 1H), 8.06 (d, J = 7.2 Hz, 2H), 7.61 (d,
J = 7.2 Hz, 2H), 7.40−7.10 (m, 7H), 7.04 (d, J = 7.6 Hz, 1H), 6.89 (t,
J = 7.6 Hz, 1H), 6.08 (s, 1H), 1.82 (s, 3H). 13C NMR (101 MHz,
C6D6; δ, ppm): 156.89, 151.39, 140.41, 138.59, 136.93, 129.90,
129.34, 129.05, 128.89, 128.89, 127.41, 126.81, 126.31, 126.06,
Preparation of Zr(MeCNN)2. In the glovebox, a solution of
tetrabenzyl zirconium (202 mg, 443 mmol, 0.50 equiv) in 3 mL of
benzene was added to a 50 mL thick-walled glass vessel containing a
solution of H2(MeCNN) (300 mg, 886 mmol, 1.00 equiv) in 5 mL of
benzene. The thick-walled vessel was sealed with a PTFE screw cap
and heated to 130 °C for 36 h, resulting in a brown solution. After
cooling to room temperature, the reaction vessel was brought back
into the glovebox and the solvent was removed in vacuum. The crude
product was redissolved in 3 mL of Et2O. Then, pentane was added to
precipitate the solid product, which was collected by filtration. The
resulting solid was washed 10 times with Et2O and dried in vacuum.
The product was collected as an orange solid (Yield: 145 mg, 43%).
1H NMR (400 MHz, C6D6; δ, ppm): 7.62 (d, J = 7.2 Hz, 4H), 7.38
(dd, J = 7.2, 1.6 Hz, 2H), 7.28 (tm, J = 7.2 Hz, 4H), 7.20−7.16 (m,
4H), 6.85−6.77 (m, 4H), 6.45 (s, 2H), 5.81 (s, 2H), 2.42 (s, 6H),
2.08 (s, 6H), 2.00 (s, 6H). 13C NMR (101 MHz, C6D6; δ, ppm):
191.87, 164.62, 156.84, 146.33, 143.57, 141.66, 141.43, 138.35,
138.28, 136.03, 132.55, 130.96, 129.96, 128.78, 127.05, 120.75,
116.03, 114.50, 112.31, 23.02, 21.56, 14.69. Anal. Calcd for
C48H40N4Zr·Et2O: C, 74.51; H, 6.01; N, 6.68. Found: C, 74.37; H,
5.59; N, 6.97.
+
118.09, 117.10, 111.54, 12.60. HRMS (ESI) calcd for C22H19N2 [M
+ H]+ m/z 311.15428. Found 311.15409.
Preparation of 2-(3,5-Dimethyl-phenyl)-6-(5-methyl-3-phe-
nyl-1H-pyrrol-2-yl)pyridine, H2MeCNN. 6-(3,5-Dimethylphenyl)-2-
pyridinecarboxyaldehyde (2.60 g, 12.31 mmol, 1.00 equiv), 4-phenyl-
3-buten-2-one (1.89 g, 12.92 mmol, 1.05 equiv), and 3-benzyl-5-(2-
hydroxyethyl)-4-methyl-thiazolium chloride (1.68 g, 6.15 mmol, 0.5
equiv) were mixed in a 250 mL Schlenk flask under an argon
atmosphere. Then, 10 mL of absolute ethanol were added. A solution
of sodium tert-butoxide (591 mg, 6.15 mmol, 0.5 equiv) in ethanol
was added via syringe, and the mixture was heated to 70 °C for 1 h
during which the 1,4-diketone intermediate precipitated. The reaction
was cooled to room temperature. The solid was collected via filtration
and washed three times with cold ethanol. Then, the 1,4-diketone
intermediate was mixed with ammonium acetate (9.49 g, 123.07
mmol, 10.00 equiv) in 10 mL of absolute ethanol, and the reaction
mixture was heated to reflux. After 12 h, the solvent was removed, and
Preparation of Zr(DPDP)Br2. Benzyl bromide (169 mg, 989
mmol, 7.00 equiv) was added to a 50 mL thick-walled glass vessel
containing a solution of Zr(CNN)2 (100 mg, 141 mmol, 1.00 equiv)
in 10 mL of benzene. The thick-walled vessel was sealed with a PTFE
screw cap and irradiated with blue LED light (λmax = 462 nm) for 24
h, resulting in a dark red solution. The reaction vessel was brought
back into the glovebox, and the solvent was removed in vacuo. After
evaporation of benzene, the crude product was recrystallized from
THF and Et2O cooled to −35 °C. The analytically pure product was
1
collected as red crystals (Yield: 79 mg, 64%). H NMR (400 MHz,
C6D6; δ, ppm): 7.48 (d, J = 7.2 Hz, 4H), 7.22−7.10 (m, 10H), 6.90
(t, J = 7.2 Hz, 2H), 6.85 (t, J = 7.2 Hz, 2H), 6.71(d, J = 7.2 Hz, 2H),
6.59 (t, J = 8.4 Hz, 2H), 6.36 (d, J = 6.8 Hz, 2H), 6.02 (s, 2H), 2.34
I
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(s, 6H). 13C NMR (101 MHz, C6D6; δ, ppm): 158.21, 156.22, 143.47,
140.21, 140.12, 139.43, 137.89, 133.33, 130.82, 130.41, 129.91,
129.86, 128.96, 128.89, 127.47, 127.32, 120.62, 117.42, 115.40, 17.70.
Anal. Calcd for C44H32Br2N4Zr: C, 60.90; H, 3.72; N, 6.46. Found: C,
61.10; H, 4.04; N, 6.16. X-ray quality crystals of Zr(DPDP)2Br2 were
grown from THF and Et2O solution cooled to −35 °C.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: camilsmann@mail.wvu.edu.
ORCID
Yu Zhang: 0000-0001-8948-1434
Carsten Milsmann: 0000-0002-9249-5199
Notes
Preparation of Zr(DPDP)(SBn)2. Dibenzyl disulfide (88 mg, 356
mmol, 1.20 equiv) was added to a 50 mL thick-walled glass vessel
containing a solution of Zr(CNN)2 (210 mg, 297 mmol, 1.00 equiv)
in 10 mL of benzene. The thick-walled vessel was sealed with a PTFE
screw cap and irradiated with blue LED light (λmax = 462 nm) for 38
h, resulting in a bright orange solution. The reaction vessel was
brought back into the glovebox and the solvent was removed in
vacuum. After evaporation of benzene, the resulting solid was washed
multiple times with Et2O and gave analytically pure product as a light
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
West Virginia University, the Don and Linda Brodie Resource
Fund for Innovation, and the National Science Foundation
(CHE-1752738) are acknowledged for financial support. This
work used X-ray crystallography (CHE-1336071) and NMR
(CHE-1228336) equipment funded by the National Science
Foundation. The WVU High Performance Computing
facilities are funded by the National Science Foundation
EPSCoR Research Infrastructure Improvement Cooperative
Agreement #1003907, the state of West Virginia (WVEPSCoR
via the Higher Education Policy Commission), the WVU
Research Corporation, and faculty investments.
1
orange solid (Yield: 211 mg, 75%). H NMR (400 MHz, C6D6; δ,
ppm): 7.61 (d, J = 7.2 Hz, 4H), 7.32 (d, J = 7.2 Hz, 2H), 7.28 (d, J =
7.2 Hz, 4H), 7.23 (t, J = 7.6 Hz, 4H), 7.18−7.11 (m, 4H), 7.02 (t, J =
7.6 Hz, 4H), 6.92 (t, J = 7.6 Hz, 4H), 6.69 (td, J = 7.6, 1.2 Hz, 2H),
6.64 (dd, J = 8.4, 7.6 Hz, 2H), 6.45 (d, J = 7.6 Hz, 2H), 6.30 (d, J =
7.2 Hz, 2H), 6.21 (s, 2H), 4.34 (d, J = 14.0 Hz, 2H), 4.24 (d, J = 14.0
Hz, 2H), 2.40 (s, 6H). 13C NMR (101 MHz, C6D6; δ, ppm): 158.03,
154.99, 144.87, 143.72, 141.45, 139.41, 138.43, 138.32, 133.35,
129.82, 129.73, 129.61, 129.33, 129.17, 129.07, 128.82, 128.59,
127.49, 127.03, 126.22, 119.21, 115.97, 115.86, 40.45, 17.24. Anal.
Calcd for C58H46N4S2Zr: C, 72.99; H, 4.86; N, 5.87. Found: C, 72.32;
H, 4.98; N, 5.83. Elemental analysis results for this complex showed
consistently low values for carbon, which is likely due to the formation
of zirconium carbide species during combustion analysis even in the
presence of combustion aid. Hydrogen and nitrogen values were
reproducible within 0.2 of the calculated value.
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ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.organo-
met.8b00388.
■
S
Additional experimental procedures, spectroscopic and
electrochemical data, , and computational details
(DOCX)
Crystallographic data for Zr(HCNN)2·Et2O and Zr-
(DPDP)Br2·2Et2O (XYZ)
(13) Larsen, C. B.; Wenger, O. S. Photoredox Catalysis with Metal
Complexes Made from Earth-Abundant Elements. Chem. - Eur. J.
2018, 24, 2039−2058.
Accession Codes
CCDC 1847795−1847796 contain the supplementary crys-
tallographic data for this paper. These data can be obtained
free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by
emailing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
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Molybdenum(0) Isocyanide Analogue of Ru(2,2′-Bipyridine)32+: A
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