Modular, Self-Assembling Metallaphotocatalyst for Cross-Couplings Using the Full Visible-Light Spectrum

Using the Full Visible-Light Spectrum

Research Article

Modular, Self-Assembling Metallaphotocatalyst for Cross-Couplings

Susanne Reischauer, Volker Strauss, and Bartholomaus Pieber*

Cite This: ACS Catal. 2020, 10, 13269 −13274

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sı Supporting Information

ABSTRACT: The combination of nickel and photocatalysis has

unlocked a variety of cross-couplings. These protocols rely on a few

photocatalysts that can only convert a small portion of visible light

(

<500 nm) into chemical energy. The high-energy photons that

excite the photocatalyst can result in unwanted side reactions. Dyes

that absorb a much broader spectrum of light are not applicable

because of their short-lived singlet excited states. Here, we describe

a self-assembling catalyst system that overcomes this limitation.

Immobilization of a nickel catalyst on dye-sensitized titanium

dioxide results in a material that catalyzes carbon−heteroatom and carbon−carbon bond formations. The modular approach of dye-

sensitized metallaphotocatalysts accesses the entire visible light spectrum and allows tackling selectivity issues resulting from low

wavelengths strategically. The concept overcomes current limitations of metallaphotocatalysis by unlocking the potential of dyes that

were previously unsuitable.

KEYWORDS: metallaphotocatalysis, dual catalysis, nickel catalysis, photoredox catalysis, dye-sensitized semiconductor

he combination of photo and nickel catalysis (metal-

laphotocatalysis) has emerged as a powerful strategy for

We wondered whether immobilization of a suitable nickel

complex on dye-sensitized TiO₂accesses a bifunctional

material that serves as the metallaphotocatalyst (Figure 1C).

We hypothesized that such a system overcomes limitations

related to short excited state lifetimes and diﬀusion-controlled

energy or single-electron transfer (SET) events because of the

close spatial proximity between the PC and the nickel catalyst.

The proposed, modular design of dye-sensitized metal-

laphotocatalysts (DSMPs) allows selecting dyes/wavelengths

and nickel complexes depending on the respective application.

Our investigations started with the O-arylation of carboxylic

carbon−carbon and carbon−heteroatom cross-couplings (Fig-

−3

ure 1A).

Key to the success is redox or photosensitization

events between a nickel catalyst and a photocatalyst (PC).

Applicable PCs are iridium and ruthenium polypyridyl

complexes, or carbazolyl dicyanobenzenes, with tailored

redox potentials or triplet energies, and long-lived excited

triplet states (Figure 1B). These PCs are limited to short

excitation wavelengths. Current approaches toward photo-

catalysis with low photon energies require complex catalytic

cocktails that enable photon upconversion, osmium com-

acids that was reported using the PC Ir(ppy) (ppy = 2-

plexes as PCs, or multiphoton excitation processes.

phenylpyridine) and a nickel bipyridine complex. The cross-

2,13

The use of abundant dyes that absorb broadly across the

visible-light spectrum is highly desirable. The redox potentials

and excited state energies of many commodity chemicals, such

as ﬂuorescein, rose bengal, or coumarins are in theory suitable

coupling proceeds via an energy transfer mechanism,

is feasible using semiconductors that absorb blue light,

and

4−16

but does not work using simple organic dyes. We ﬁrst tested if

anchoring of a nickel complex on TiO P25 results (i) in an

for metallaphotocatalysis, but only their singlet exited states

active, heterogeneous metallaphotocatalyst, and (ii) improves

the reaction yield because of spatial proximity of the PC and

the nickel catalyst using near-UV light. A ligand equipped with

carboxylic acid groups (dcbpy = 2,2′-bipyridine-4,4′-dicarbox-

ylic acid) indeed gave a higher yield of the C−O cross-

are available for photocatalysis because of slow intersystem

crossing. The short excited state lifetime of S1 renders a

diﬀusion-limited interaction with a nickel catalyst in a

homogeneous solution unlikely (Figure 1B). These dyes are,

however, able to sensitize metal oxide (MO) semiconductors,

such as TiO , in dye-sensitized solar cells, or dye-sensitized

Received: September 9, 2020

Revised: October 20, 2020

photocatalysts (DSPs) for light-driven H production. The

carboxylic acid groups of the dyes bind to the surface hydroxyl

groups of TiO . This facilitates electron injection into the

conduction band of the semiconductor upon photoexcitation.

This results in a charge-separated species that is sustained for

several microseconds.

XXXX American Chemical Society

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ACS Catal. 2020, 10, 13269−13274

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Figure 1. Working hypothesis toward a modular, heterogeneous metallaphotocatalyst. Cross-coupling reactions using homogeneous

metallaphotocatalysis (A). Onset of absorption of PCs. The suitability for homogeneous metallaphotocatalysis depends on the excited state

lifetime (B). DSMPs are proposed to overcome excited state lifetime limitations (C).

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ACS Catal. 2020, 10, 13269−13274

Research Article

Figure 2. Self-assembling, modular metallaphotocatalysts enable cross-couplings using the entire visible-light spectrum. Anchoring of nickel

complexes on TiO P25 improves yield because of spatial proximity (A). UV−vis spectra and nickel/dye loadings of two representative DSMPs

(

B). Optimized conditions and control experiments for the O-arylation of carboxylic acids using DSMPs (C). C−S, C−N, and C−C cross-coupling

catalyzed by DSMPs using blue, green, or red light (D). Yields were determined by H NMR using 1,3,5-trimethoxybenzene as the internal

standard if not stated otherwise. Isolated yields. DSMP = dye-sensitized metallaphotocatalyst, n.d. = not detected, dcbpy = 2,2′-bipyridine-4,4′-

dicarboxylic acid, mcbpy = 4′-methyl-2,2′-bipyridine-4-carboxylic acid, glyme = 1,2-dimethoxyethane, and NaFluo = ﬂuorescein sodium.

coupling product 1 than a ligand that lacks functionalities

capable of binding to the semiconductor’s surface (dtbbpy =

preparation of the DSMPs was carried out to characterize the

bifunctional materials (Figure 2B). The UV−vis spectra of the

materials conﬁrmed immobilization of the dyes on the MO,

and inductively coupled plasma−optical emission spectrometry

corroborated anchoring of the nickel complex.

The in situ DSMP approach resulted in a highly selective

formation of 1 using blue (440 nm), green (525 nm), or red

(666 nm) light (Figure 2C). The best catalytic activities were

obtained when the amount of dye (1.25 mol %) and NiCl₂·

,4′-ditert-butyl-2,2′-dipyridyl) (Figure 2A). Next, dyes that

contain a suitable anchoring group were studied as sensitizers

using green light (525 nm). Fluorescein sodium (NaFluo)

showed the best results (see Supporting Information). The C−

O coupling was also feasible at higher wavelengths (666 nm)

using the ruthenium dye N3 that has an excited state lifetime

of 20 ns. The DSMPs self-assemble in situ. An ex situ

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ACS Catal. 2020, 10, 13269−13274

Supporting Information ).

Research Article

Figure 3. Mechanistic investigations. Recycling experiments showed that catalytic inactive materials still contain nickel complexes and dye

molecules (A). DSMPs might work via an “on-particle” and/or “through-particle” mechanism (B). Experiments using insulating MO and diﬀusion-

controlled metallaphotocatalysis (C). Spectrophotometric titrations show static quenching for DSMPs, whereas solely dynamic quenching was

observed in other systems (D). MO = metal oxide, dcbpy = 2,2′-bipyridine-4,4′-dicarboxylic acid, dtbbpy = 4,4′-ditert-butyl-2,2′-bipyridine, and

Fluo = ﬂuorescein.

2−16

dcbpy (10 mol %) exceeded the loading that was determined

using ex situ DSMP preparation. This is rationalized by a

dynamic equilibrium between immobilized and unbound

laphotocatalyzed C−O arylation of carboxylic acids,

the

substrate scope is limited to electron-poor aryl halides (see

NiCl ·dcbpy as well as dye molecules, which also contribute

to productive catalysis in the course of the bulk experiment.

The DSMP approach also enabled metallaphotocatalyzed

C−S, C−N, and C−C bond formations that proceed via

17 19

Reducing the amount of dye (0.1 mol %) or NiCl ·dcbpy (1

SET processes (Figure 2D). Because the C−S and C−C

mol %) still resulted in almost quantitative yield of the desired

ester. When the dye and the nickel complex are used in such

low amounts, no product formation was observed. We assume

couplings proceed via a single-electron oxidation of the

substrate and a single-electron reduction of a nickel complex,

we assume that the aﬃnity of the substrates to the

semiconductors’ surface has a positive eﬀect on the catalytic

activity.

that high loadings of either NiCl ·dcbpy or the dye are

suﬃcient for the formation of a monolayer on TiO and the

resulting close spatial proximity of Fluo and the nickel complex

is responsible for the catalytic activity. It has to be noted that

also the substrates and the base are likely to bind to the surface

The modular design principle and the self-assembling

strategy facilitated a straightforward optimization of dyes,

nickel salts, and ligands resulting in selective cross-couplings

using blue (440 nm), green (525 nm), and red (666 nm) light.

With regard to the C−C coupling, 68% of the desired product

(4) were obtained using blue light, which is similar to the

of TiO , which could contribute to the performance of the

catalytic system. No product formation was observed in the

absence of TiO P25, the dye, dcbpy, nickel salt, or light. In

agreement with previously reported protocols for metal-

homogeneous metallaphotocatalysis system. Reactions at

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ACS Catal. 2020, 10, 13269−13274

https://pubs.acs.org/doi/10.1021/acscatal.0c03950.

Research Article

Figure 4. C−O arylation of (E)-cinnamic acid with 4-iodobenzotriﬂuoride with diﬀerent metallaphotocatalyst systems. Yields were determined by

H NMR using 1,3,5-trimethoxybenzene as the internal standard if not stated otherwise. Isolated yield in parentheses.

higher wavelengths, not accessible with the original, homoge-

neous approach, resulted in a higher selectivity.

of “on-particle” and “through-particle” processes is responsible

for the high catalytic activity of DSMPs with TiO P25.

The dynamic equilibrium between immobilized and un-

bound molecules is responsible for leaching of nickel and

ﬂuorescein during catalyst recycling studies and caused a

gradual decrease of the yield of 1 (Figure 3A). Addition of

either NaFluo or the nickel salt restored the catalytic activity,

which is in agreement with the experiments using low amounts

of either the dye or the nickel complex (Figure 2C). This

indicated that the amount of immobilized dye molecules and

nickel complexes has to be above a certain limit to observe

catalytic activity. We, therefore, questioned if only the close

proximity of the dye molecules and the nickel complex is

In addition, we sought to compare the selectivity of DSMPs

with homogenous metallaphotocatalysis systems in the C−O

arylation of (E)-cinnamic acid with 4-iodobenzotriﬂuoride

(

Figure 4). The resulting coupling product (E-5) is prone to

subsequent photocatalytic isomerization or cycloadditions that

20,21

could lead to low selectivities.

We indeed observed

signiﬁcant amounts of the undesired Z-isomer (Z-5) when

Ir(ppy ) was used as PC using 440 nm irradiation. Other Ir

complexes gave even worse results (see Supporting Informa-

tion ). Control experiments showed that E-5 is also slowly

converted to Z -5 at 440 nm in the absence of a PC (see

Supporting Information). The selectivity was, however, not

responsible for productive catalysis, and TiO P25 only acts as

the support. In other words, an “on-particle” rather than a

improved using Ir(ppy ) and 525 nm irradiation, because the

“through-particle” mechanism could be responsible for over-

triplet energy of the PC is not wavelength-dependent. The E−

Z isomerization was also observed when the reaction was

catalyzed by a DSMP at 440 nm, but was totally suppressed by

switching to higher wavelengths, resulting in the selective

formation of E-5.

coming the short excited state lifetime (Figure 3B). To test this

hypothesis, we substituted TiO P25 with the insulating MO

SiO and Al O that only enable binding of the dye and the

nickel complex and observed product formation for all cross-

couplings, but with a signiﬁcantly lower eﬃciency compared to

The DSMP concept overcomes the constraint of long

excited triplet state lifetimes of PCs for metallaphotocatalysis

and unlocks the potential of many dyes that were previously

unsuitable because of their short-lived excited S1 states. Many

cross-couplings can be carried out using the entire visible light

spectrum and selectivity issues can be tackled strategically. The

simplicity and modularity of DSMPs suggest that the present

approach will complement the existing methods.

TiO P25 (Figure 3C). A second set of experiments was

carried out using a nickel complex that is not able to bind to

the surface of MO. Here, only experiments with the

semiconductor TiO₂P25 gave productive catalysis. This

conﬁrms that dye sensitization leads to a charge-separated

species that is suﬃciently long-lived to turn over a

homogeneous nickel catalyst in a semiheterogeneous,

diﬀusion-controlled reaction. Spectrophotometric titrations

unveiled the electronic communication between the excited

dye and the immobilized nickel complex “through” a

semiconductor. Static ﬂuorescence quenching of ﬂuorescein-

ASSOCIATED CONTENT

■

sı Supporting Information

sensitized TiO P25 was observed with a nickel complex that

binds to the semiconductor’s surface (NiCl ·dcbpy). NiCl ·

The Supporting Information is available free of charge at

dtbbpy showed solely dynamic quenching (Figure 3D).

Titration experiments with ﬂuorescein bound to SiO instead

Experimental procedures, characterization of products,

of TiO P25 displayed dynamic quenching behavior in the case

and copies of the H/ C/ F NMR spectra of the

prepared compounds (PDF )

of both nickel complexes, and signiﬁcantly lower quenching

rates. Taking all results together, we assume that a combination

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ACS Catal. 2020, 10, 13269−13274

ACS Catalysis

AUTHOR INFORMATION

Research Article

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■

Corresponding Author

Bartholoma

Potsdam, Germany; orcid.org/0000-0001-8689-388X;

us Pieber − Department of Biomolecular Systems,

Sensitized Solar Cells. Chem. Rev. 2010, 110, 6595−6663.

Max Planck Institute of Colloids and Interfaces, 14476

Email: bartholomaeus.pieber@mpikg.mpg.de

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Nanocrystalline Systems. Chem. Rev. 1995, 95, 49−68.

12) Welin, E. R.; Le, C.; Arias-Rotondo, D. M.; McCusker, J. K.;

Authors

Susanne Reischauer − Department of Biomolecular Systems,

Max Planck Institute of Colloids and Interfaces, 14476

Potsdam, Germany; Department of Chemistry and

(

MacMillan, D. W. C. Photosensitized, energy transfer-mediated

organometallic catalysis through electronically excited nickel(II).

Science 2017, 355, 380−385.

(13) Tian, L.; Till, N. A.; Kudisch, B.; MacMillan, D. W. C.; Scholes,

G. D. Transient Absorption Spectroscopy Offers Mechanistic Insights

for an Iridium/Nickel-Catalyzed C −O Coupling. J. Am. Chem. Soc.

Biochemistry, Freie Universitat Berlin, 14195 Berlin, Germany

Volker Strauss − Department of Colloid Chemistry, Max Planck

Institute of Colloids and Interfaces, 14476 Potsdam, Germany;

orcid.org/0000-0003-2619-6841

020, 142, 4555−4559.

Complete contact information is available at:

https://pubs.acs.org/10.1021/acscatal.0c03950

14) Zhu, X.; Lin, Y.; San Martin, J.; Sun, Y.; Zhu, D.; Yan, Y. Lead

halide perovskites for photocatalytic organic synthesis. Nat. Commun.

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Notes

(15) Pieber, B.; Malik, J. A.; Cavedon, C.; Gisbertz, S.; Savateev, A.;

Cruz, D.; Heil, T.; Zhang, G.; Seeberger, P. H. Semi-heterogeneous

Dual Nickel/Photocatalysis using Carbon Nitrides: Esterification of

Carboxylic Acids with Aryl Halides. Angew. Chem., Int. Ed. 2019, 58,

An early version of this manuscript was previously deposited

on ChemRxiv: Reischauer, S., Strauss, V., and Pieber, B. A

Modular, Self-Assembling Metallaphotocatalyst for Cross-

Couplings Using the Full Visible-Light Spectrum. https://

doi.org/10.26434/chemrxiv.12444908.v1 (2020).

The authors declare no competing ﬁnancial interest.

16) Malik, J. A.; Madani, A.; Pieber, B.; Seeberger, P. H. Evidence

575−9580.

for Photocatalyst Involvement in Oxidative Additions of Nickel-

Catalyzed Carboxylate O-Arylations. J. Am. Chem. Soc. 2020, 142,

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ACKNOWLEDGMENTS

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Johannes, J. W. Photoredox Mediated Nickel Catalyzed Cross-

Coupling of Thiols With Aryl and Heteroaryl Iodides via Thiyl

Radicals. J. Am. Chem. Soc. 2016, 138, 1760−1763.

■

We gratefully acknowledge the Max-Planck Society for

generous ﬁnancial support. S.R. and B.P. thank the German

Chemical Industry Fund (Fonds der Chemischen Industrie,

FCI) for funding through a Liebig Fellowship. B.P. thanks the

Deutsche Forschungsgemeinschaft (DFG, German Research

Foundation) under Germany’s Excellence StrategyEXC

(

18) Corcoran, E. B.; Pirnot, M. T.; Lin, S.; Dreher, S. D.; DiRocco,

D. A.; Davies, I. W.; Buchwald, S. L.; MacMillan, D. W. C. Aryl

amination using ligand-free Ni(II) salts and photoredox catalysis.

Science 2016, 353, 279−283.

(19) Remeur, C.; Kelly, C. B.; Patel, N. R.; Molander, G. A.

008390540038UniSysCat for ﬁnancial support. We

Aminomethylation of Aryl Halides Using α -Silylamines Enabled by

Ni/Photoredox Dual Catalysis. ACS Catal. 2017, 7, 6065−6069.

thank our colleagues Prof. Peter H. Seeberger, Dr. Jamal

Malik, Dr. Martina Delbianco, Dr. Kerry Gilmore, Dr. Lukas

(

20) Lei, T.; Zhou, C.; Huang, M.-Y.; Zhao, L.-M.; Yang, B.; Ye, C.;

Zeininger, Cristian Cavedon, Silvia Furstenberg, Jessica Brandt,

Xiao, H.; Meng, Q.-Y.; Ramamurthy, V.; Tung, C.-H.; Wu, L.-Z.

General and Efficient Intermolecular [2+2] Photodimerization of

Chalcones and Cinnamic Acid Derivatives in Solution through

Visible-Light Catalysis. Angew. Chem., Int. Ed. 2017, 56, 15407−

and Katharina ten Brummelhuis (all MPIKG) for scientiﬁc,

technical, and analytical support.

(

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