Generation of powerful tungsten reductants by visible light excitation

Article abstract of DOI:10.1021/ja4047119

The homoleptic arylisocyanide tungsten complexes, W(CNXy)₆ and W(CNIph)₆ (Xy = 2,6-dimethylphenyl, Iph = 2,6-diisopropylphenyl), display intense metal to ligand charge transfer (MLCT) absorptions in the visible region (400-550 nm). MLCT emission (λ_max ≈ 580 nm) in tetrahydrofuran (THF) solution at rt is observed for W(CNXy)₆ and W(CNIph)₆ with lifetimes of 17 and 73 ns, respectively. Diffusion-controlled energy transfer from electronically excited W(CNIph) ₆ (*W) to the lowest energy triplet excited state of anthracene (anth) is the dominant quenching pathway in THF solution. Introduction of tetrabutylammonium hexafluorophosphate, [Buⁿ₄N][PF ₆], to the THF solution promotes formation of electron transfer (ET) quenching products, [W(CNIph)₆]⁺ and [anth] ^?-. ET from*W to benzophenone and cobalticenium also is observed in [Buⁿ₄N][PF₆]/THF solutions. The estimated reduction potential for the [W(CNIph)₆]⁺/ *W couple is -2.8 V vs Cp₂Fe^+/0, establishing W(CNIph)₆ as one of the most powerful photoreductants that has been generated with visible light.

Full text of DOI:10.1021/ja4047119

Communication
pubs.acs.org/JACS
Generation of Powerful Tungsten Reductants by Visible Light
Excitation
†
†
†
,‡
§
Wesley Sattler, Maraia E. Ener, James D. Blakemore, Aaron A. Rachford,* Paul J. LaBeaume,
§
§
†
,†
James W. Thackeray, James F. Cameron, Jay R. Winkler, and Harry B. Gray*
†
Beckman Institute, California Institute of Technology, Pasadena, California 91125, United States
‡
The Dow Chemical Company, Analytical Sciences, 1897 Building, Midland, Michigan 48667, United States
§
The Dow Chemical Company, Dow Electronic Materials, 455 Forest Street, Marlborough, Massachusetts 01752, United States
S Supporting Information
*
10
substitution for other ligands, (e.g., acetate, acetonitrile)
where excess isocyanide can also act as a reducing agent; and
11
ABSTRACT: The homoleptic arylisocyanide tungsten
complexes, W(CNXy)6 and W(CNIph)6 (Xy = 2,6-
dimethylphenyl, Iph = 2,6-diisopropylphenyl), display
intense metal to ligand charge transfer (MLCT)
absorptions in the visible region (400−550 nm). MLCT
(iii) reduction of metal halides by metallic reducing agents (e.g.,
4
,12
Na(Hg), Mg) in the presence of free isocyanide.
We
obtained red, crystalline W(CNAr) by Na(Hg) reduction of
6
13
WCl in the presence of arylisocyanide in THF (Scheme 1).
6
emission (λ
≈ 580 nm) in tetrahydrofuran (THF)
max
solution at rt is observed for W(CNXy) and W(CNIph)
6
6
Scheme 1
with lifetimes of 17 and 73 ns, respectively. Diffusion-
controlled energy transfer from electronically excited
W(CNIph) (*W) to the lowest energy triplet excited
6
state of anthracene (anth) is the dominant quenching
pathway in THF solution. Introduction of tetrabutylam-
n
monium hexafluorophosphate, [Bu N][PF ], to the THF
4
6
solution promotes formation of electron transfer (ET)
quenching products, [W(CNIph)6]+ and [anth] . ET
from *W to benzophenone and cobalticenium also is
•−
n
observed in [Bu N][PF ]/THF solutions. The estimated
4
6
+
While several homoleptic W(CNAr) complexes are
reduction potential for the [W(CNIph) ] /*W couple is
6
6
4
,5,12
13
+
/0
known,
inspection of the Cambridge Structural Database
−
2.8 V vs Cp Fe , establishing W(CNIph) as one of the
2
6
suggests that only W(CNXy) has been structurally charac-
most powerful photoreductants that has been generated
with visible light.
6
14,15
16
terized.
We determined the structure of W(CNIph)6 by
single-crystal X-ray diffraction (Figure 1) from large red crystals
he development of photosensitizers that can serve as
T
powerful photoreductants is of great interest, as such
reagents are used in many organic transformations and as key
1
−3
components in devices for the production of solar fuels.
Very promising photosensitizers are the hexakis phenyliso-
4
,5
cyanide complexes of the group six transition metals, which
absorb strongly in the visible region and emit with lifetimes (τ)
5
ranging from ps to ns, according to Cr < Mo < W. One
problem is that ligand loss competes with excited state redox
decay pathways, most especially in Cr(0) and Mo(0)
complexes. Less prone to photosubstitution is the tungsten
6
complex containing a bulkier ligand, IphNC (IphNC = 2,6-
diisopropylphenyl isocyanide), whose photosubstitution quan-
tum yield (ϕ = 0.0003) is much lower than that (ϕ = 0.011) for
5
W(CNPh) in pyridine solution. We have now extended this
6
Figure 1. Molecular structure of W(CNIph) (H-atoms omitted for
clarity).
work on W(0) photosensitizers to include exploration of the
6
rich photoredox chemistry of W(CNAr) (Ar = Xy and Iph, Xy
6
=
2,6-dimethylphenyl) complexes.
Methods for preparing M(0) isocyanide complexes include:
7
8
Received: May 10, 2013
Published: July 15, 2013
9
(
i) substitution of isocyanides in metal carbonyls; (ii)
©
2013 American Chemical Society
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dx.doi.org/10.1021/ja4047119 | J. Am. Chem. Soc. 2013, 135, 10614−10617

Journal of the American Chemical Society
Communication
of W(CNIph) obtained by slow evaporation from benzene
6
(
̅
triclinic, space group P1, with two benzene molecules per unit
cell). The average W−C and WC−N bond lengths are 2.062
and 1.176 Å, respectively, with an average WC−N−Iph angle of
1
64.4°.
The solid-state IR spectrum of W(CNIph) features a single,
6
−1
−1
broad CN stretch at 1944 cm , which is 10 cm higher than
the corresponding CN stretching frequency for W(CNXy)6.
Both W(CNXy) and W(CNIph) absorb strongly between
6
6
400 and 550 nm (Figure 2). Interestingly, the lowest energy
Figure 3. Cyclic voltammograms of W(CNIph) (ca. 1 mM) in THF
6
n
with 0.1 M [Bu N][PF ] supporting electrolyte: scan rate 100 mV/s.
4
6
By performing a controlled-potential electrolysis at −0.15 V
in a spectroelectrochemical cell, we have obtained a rough
+
estimate for the absorption spectrum of [W(CNIph) ] (SI,
6
Figure S9), which also can be formed by ET from *W to an
acceptor. However, the spectrum obtained by the controlled-
potential electrolysis is slightly different from that obtained by
+
photochemical generation of [W(CNIph) ] ; we believe that
6
decomposition of [W(CNIph)6]+ could occur during the
electrolysis, as this happens on a much longer time scale
(many minutes) compared to the photochemistry (<10 ms, vide
infra).
Figure 2. Absorption and emission spectra of W(CNXy) (red) and
6
W(CNIph) (blue) in THF solutions.
6
These electrochemical and photophysical data suggest that
*
W is an extremely powerful reductant, with an estimated
4
−1
absorption maximum of W(CNIph) (ε = 9.09 × 10 M
18
6
465
potential of −2.80(4) V (Scheme 2). For comparison, data
for inorganic photosensitizers that have been used for
reductions are set out in Table 1.
−1
cm ) is substantially more intense than that of W(CNXy)
ε455 = 6.04 × 10 M cm ), a finding that could be related to
differences in the orientation of the π-systems on the isocyanide
6
4
−1
−1
(
ligands. Specifically, the trans XyNC ligands of W(CNXy) are
approximately orthogonal to each other, while the trans
6
Scheme 2
14
IphNC ligands of W(CNIph) are roughly coplanar. Both
6
complexes are luminescent, emitting yellow-to-red light with
maxima at ca. 580 nm (Figure 2). The energy of the E
00
−1
transition is estimated to be 18 350 cm above the ground
state, as judged by the 77 K emission spectrum (Supporting
Information (SI), Figure S3). The lifetime of electronically
excited W(CNXy) , *W(CNXy) , is ca. 17 ns in THF solution
6
6
at rt, whereas that of *W(CNIph) (*W) is ca. 73 ns under the
6
same conditions (SI, Figure S4). In accord with previous
literature reports, we suggest that the isopropyl groups of the
IphNC ligand provide greater steric protection of the metal
center than the methyl groups of the XyNC ligand, disfavoring
excited state decay that would occur by collisions of the metal
We have studied the reactions of *W with anthracene (anth),
+
benzophenone, and cobalticenium (Cp Co ). Following
2
excitation of W(CNIph) , transient absorption (TA) spectros-
6
copy was used to characterize the reaction products. For
anth0 in glyme, E1/2 = −2.47 V,
/−1
38,19
so we estimate that the
5
center with solvent molecules.
driving force (−ΔG°) for its reduction by *W is 0.3 eV.
As W(CNIph) is relatively robust, we investigated its
Quenching of *W (λ
= 488 nm) by anth is diffusion
6
excitation
1
1,17
10
−1 −1
electrochemistry
and photochemistry in more depth;
controlled (Stern−Volmer analysis, k = 1.1 × 10 M s , SI,
q
importantly, we have found that this W(0) complex is among
the most powerful photoreductants generated with visible light.
Figure S10). However, TA spectroscopy clearly demonstrates
that the photoproducts are primarily due to excitation energy
2
0,11
Cyclic voltammograms of W(CNIph) at a basal-plane graphite
transfer (EET) and not ET (Figure 4, top and inset).
This
6
2
electrode (electrode area = 0.09 cm ) in a 0.1 M THF solution
outcome is not unexpected, as the lowest triplet excited state of
n
3
−1
of [Bu N][PF ] exhibit reversible waves at E = −0.53(2) V
anth, anth, is 14,850 cm above its ground state (ΔG°
≈
6
4
6
1/2
EET
21
(
all potentials are reported relative to ferricenium/ferrocene,
−0.5 eV). TA spectra recorded after excitation of W(CNIph)
in the presence of anth (>10 mM) exhibit large absorbance
increases at 400 and 424 nm (Figure 4), consistent with
production of anth.
with a peak around 460−470 nm, and increased ΔOD peaking
at 390 nm) is attributable to the production of [W(CNIph) ] .
+
/0
Cp Fe ) assigned to the W(+1/0) couple (Figure 3, top).
2
This couple exhibits a scan rate dependence consistent with a
diffusional process (SI, Figures S7 and S8). Additional
3
22,23
A small TA signal (negative ΔOD
oxidation events (Ep,a = 0.42(3) V and 0.72(3) V, Ep,a
=
+
anodic peak potential) are only partially reversible at a scan rate
of 100 mV/s (reductions occur at Ep,c = 0.45(3) and −1.93(3)
V, Ep,c = cathodic peak potential) (Figure 3, bottom). No sign
of the W(0/−1) couple was observed at E > −2.5 V.
6
3
The decay kinetics of anth, monitored at ca. 424 nm, are
dominated by a second-order reaction, consistent with a prior
24
report.
1
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Journal of the American Chemical Society
Communication
Table 1. Selected Inorganic Photosensitizers
+
a
b
+
a
sensitizer (D)
E°(D /D), (V)
E00 (eV)
2.6
τ
E°(D /*D) (V)
solvent
reference
c
d
[
(PNP)Cu]2
W(CNIph)6
Ir(μ-Pz)(COD)]
−0.55
−0.53
−0.15
+0.37
+0.39
10.2 μs
73 ns
−3.2
−2.8
CH Cl2
27
2
2.3
THF
this work
28
e
2
[
2.06
250 ns
1.9 μs
2.7 ns
1200 μs
85 ns
−2.21
−2.10
−1.66
−1.20
−1.51
−1.23
−0.97
CH CN
3
f
fac-Ir(ppy)3
2.50
CH CN
29, 30
31, 32
3
g
ZnTPP
2.05 (singlet)
CH CN
3
1
.59 (triplet)
+
h
i
[
Cu(dmp) ]
+0.53
2.04
2.12
CH Cl2
33, 34
35, 36
37
2
2
+
j
[
Ru(bpy) ]2
+0.89
+1.41
855 ns
211 ns
CH CN
3
3
+
k
fac-[Re(CO) (bpy)(py)]
2.38
+/0
CH CN
3
3
a
Data reported at rt; redox potentials are reported vs Cp Fe . Literature values vs the saturated calomel electrode (SCE) are corrected according to
ref 38 (i.e., in CH CN, Cp Fe is +0.4 V vs SCE; CH Cl , Cp Fe is +0.46 V vs SCE). Data pertaining to the emitting state. PNP = bis(2-
2
+
/0
+/0
b
c
−
3
2
2
2
2
d
(
diisobutylphosphino)phenyl)amide. Electrochemistry was performed in CH Cl , while the photophysics were studied in THF and cyclohexane.
2 2
f g h
e
2
μ-Pz = bridging pyrazolyl, COD = 1,5-cylooctadiene. ppy = (2-pyridinyl-κN)phenyl-κ C. TPP = tetraphenylporphyrin. dmp = 2,9-dimethyl-1,10-
i
n
+
phenanthroline. A potential of +0.64 V vs Ag/AgNO in 0.1 M [Bu N][PF ]/CH Cl was measured for the (D /D) redox couple. The Ag/AgNO
redox couple was found to be +0.35 V vs SCE using ferrocene as an internal standard (see ref 34). bpy = 2,2′-bipyridine. py = pyridine.
3
4
6
2
2
3
j
k
Figure 4. Transient difference spectra for quenching of *W by
anthracene (ca. 35 mM) in THF without (top) and with (bottom) 0.1
Figure 5. Transient difference spectra (points taken from single-
wavelength data) for quenching of *W by benzophenone (ca. 50 mM)
n
M [Bu N][PF ] at selected time delays after laser excitation (black =
4
6
n
2
0 μs, blue = 100 μs, and red = 300 μs). Inset: Transient difference
in THF with 0.1 M [Bu N][PF ] at selected time delays after laser
4
6
n
spectra at 50 ns in the absence of [Bu N][PF ]. λ = 488 nm.
excitation. Inset: Single-wavelength TA (460 nm) for the decay of
4
6
excitation
+
The spectra from ca. 478 to 493 nm are not shown due to the use of a
notch filter to protect the detector from scattered laser light.
[W(CNIph) ] . λ
= 488 nm.
6
excitation
We suggest that the lack of observable ET reactivity between
20
+
*
W and anth is due to limited cage escape. Addition of
We also investigated the ET quenching of *W with Cp Co
2
+/0
38,41
n
25
electrolyte ([Bu N][PF ], 0.1 M in THF) promotes ET
(Cp Co in THF, E1/2 = −1.35 V).
As expected, ET is
diffusion-controlled (SI, Figure S13) and produces [W-
(CNIph) ] and cobaltocene (Cp Co), as detected by TA
spectroscopy. The second-order back ET reaction also is
s , SI, Figures S17−
S19). It is of special interest that W(CNIph) can generate
Cp Co efficiently upon visible light excitation, as the latter has
been used as a homogeneous reducing agent for the reduction
4
6
2
from *W to anth, as observed by TA spectroscopy (Figure 4,
2
6
n
+
bottom). The lifetime of *W in the presence of [Bu N][PF ]
4
6
6
2
is unaffected, and Stern−Volmer analysis confirms that
10
10
−1 −1
quenching by anth is still diffusion-controlled (1.0 × 10
diffusion-controlled (ca. 2 × 10
M
−
1
−1
M
s , SI, Figure S11). These experiments illustrate that
6
the ratio of EET to ET quenching products can be tuned by
2
n
4
variations in the concentration of [Bu N][PF ] in THF.
6
38
42
43
ET from *W to benzophenone in THF (E1/2 = −2.30 V)
should be the favored reaction channel (ΔG°ET ≈ −0.5 eV),
since EET is energetically unfavorable (ΔG° ≈ +0.7 eV,
of protons to dihydrogen and dinitrogen to ammonia.
Interest in photosensitizers continues to be driven by the
desire to carry out challenging inorganic and organic reactions
by exposure to visible light. Our work confirms that
arylisocyanide metal complexes are among the most powerful
photoreductants to date. The lowest energy excited states of
these molecules can potentially be active in a very large range of
ET reaction cycles.
EET
1
39
−
lowest excited state energy of ∼24 000 cm ). Stern−Volmer
analysis of *W (λ = 488 nm) quenching by
excitation
n
4
benzophenone in THF solution (with and without [Bu N]-
[
PF ]) confirms that the quenching reaction is diffusion-
6
1
0
−1 −1
controlled (k = 1.0 × 10 M s , SI, Figure S12). TA spectra
q
obtained following excitation are consistent with the
production of [W(CNIph)6]+ and [benzophenone] , indi-
•−
ASSOCIATED CONTENT
40
■
cated by positive TA between ca. 600 and 800 nm (Figure 5).
The ET reaction between [W(CNIph) ] and [benzophe-
none]
+
*
S
Supporting Information
6
•
−
proceeds on the millisecond time scale with
Experimental details and crystallographic data in CIF format.
This material is available free of charge via the Internet at
http://pubs.acs.org.
concentration dependent kinetics indicative of a second-order
process (Figure 5, inset; SI, Figures S14−S16).
1
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Journal of the American Chemical Society
Communication
(
19) Dessy, R. E.; King, R. B.; Waldrop, M. J. Am. Chem. Soc. 1966,
AUTHOR INFORMATION
Corresponding Author
hbgray@caltech.edu; raaaron@dow.com
Notes
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8
(
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The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
■
(
Dedicated to the memory of Nicholas J. Turro. The authors
thank Jeffrey J. Warren, James R. McKone, and Oliver S.
Shafaat for helpful discussions. Our work is supported by the
National Science Foundation Center for Chemical Innovation
in Solar Fuels (CHE-0802907); CCI postdoctoral fellowship to
W.S.; and the Dow Chemical Company through the university
partnership program (Agreement No. 227027AH).
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