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Cross-Coupling Reactions Hot Paper

Intraligand Charge Transfer Enables Visible-Light-Mediated Nickel-

Catalyzed Cross-Coupling Reactions**

Cristian Cavedon+, Sebastian Gisbertz+, Susanne Reischauer, Sarah Vogl, Eric Sperlich,

John H. Burke, Rachel F. Wallick, Stefanie Schrottke, Wei-Hsin Hsu, Lucia Anghileri,

Yannik Pfeifer, Noah Richter, Christian Teutloff, Henrike Müller-Werkmeister,

Dario Cambié, Peter H. Seeberger, Josh Vura-Weis, Renske M. van der Veen,*

Arne Thomas,* and Bartholomäus Pieber*

Abstract: We demonstrate that several visible-light-mediated carbonheteroatom cross-coupling reactions can be carried

out using a photoactive NiII precatalyst that forms in situ from a nickel salt and a bipyridine ligand decorated with two

carbazole groups (Ni(Czbpy)Cl2). The activation of this precatalyst towards cross-coupling reactions follows a hitherto

undisclosed mechanism that is different from previously reported light-responsive nickel complexes that undergo metal-

to-ligand charge transfer. Theoretical and spectroscopic investigations revealed that irradiation of Ni(Czbpy)Cl2with

visible light causes an initial intraligand charge transfer event that triggers productive catalysis. Ligand polymerization

affords a porous, recyclable organic polymer for heterogeneous nickel catalysis of cross-coupling reactions. The

heterogeneous catalyst shows stable performance in a packed-bed flow reactor during a week of continuous operation.

Introduction

Transition-metal-catalyzed cross-coupling reactions are key

to the synthesis of fine chemicals.[1,2] Nickel catalysts are an

abundant alternative to palladium complexes, particularly

by combining nickel- and photocatalysis in a dual catalytic

approach (metallaphotocatalysis).[3–8] This catalytic strategy

enables the use of bench-stable, commercially available NiII

salts, and bipyridine ligands that form the nickel catalyst in

situ. Photocatalysts for metallaphotocatalyzed cross-cou-

pling reactions range from ruthenium and iridium polypyr-

idyl complexes and organic dyes to heterogeneous semi-

conductors, including polymers.[6] The initial mechanistic

hypothesis for dual photo/nickel-catalyzed

carbonheteroatom cross-coupling reactions suggested that

energy or electron transfer between the photocatalyst and a

thermodynamically stable NiII intermediate triggers reduc-

tive elimination of the desired product.[6,9] Recent evidence

suggests that these reactions proceed through NiI/NiIII cycles

(Figure 1).[10–12] These findings imply that the role of the

photocatalyst (PC) is in the generation of a catalytically

active NiIspecies through a single-electron reduction of the

NiII precatalyst and NiII resting states (Figure 1a). This is

supported by a protocol that uses sub-stoichiometric

[*] C. Cavedon,+S. Gisbertz,+S. Reischauer, W.-H. Hsu, L. Anghileri,

N. Richter, D. Cambié, P. H. Seeberger, B. Pieber

Department of Biomolecular Systems,

Max-Planck-Institute of Colloids and Interfaces

Am Mühlenberg 1, 14476 Potsdam (Germany)

E-mail: [email protected]

C. Cavedon,+S. Gisbertz,+S. Reischauer, L. Anghileri,

P. H. Seeberger

Department of Chemistry and Biochemistry,

Freie Universität Berlin

Arnimallee 22, 14195 Berlin (Germany)

S. Vogl, A. Thomas

Department of Chemistry, Functional Materials,

Technische Universität Berlin

Hardenbergstraße 40, 10623 Berlin (Germany)

E-mail: [email protected]

E. Sperlich, Y. Pfeifer, H. Müller-Werkmeister

Institute of Chemistry, University of Potsdam

Karl-Liebknecht-Strasse 24–25, 14476 Potsdam (Germany)

J. H. Burke, R. F. Wallick, J. Vura-Weis, R. M. van der Veen

Department of Chemistry, University of Illinois Urbana-Champaign

Urbana, Illinois 61801 (USA)

S. Schrottke, C. Teutloff

Department of Physics, Freie Universität Berlin

Arnimallee 22, 14195 Berlin (Germany)

R. M. van der Veen

Helmholtz Zentrum Berlin für Materialien und Energie GmbH

Hahn-Meitner-Platz 1, 14109 Berlin (Germany)

E-mail: [email protected]

[+] These authors contributed equally to this work.

[**]A previous version of this manuscript has been deposited on a

preprint server (https://doi.org/10.26434/chemrxiv-2021-kt2wr).

published by Wiley-VCH GmbH. This is an open access article under

the terms of the Creative Commons Attribution Non-Commercial

NoDerivs License, which permits use and distribution in any med-

ium, provided the original work is properly cited, the use is non-

commercial and no modifications or adaptations are made.

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How to cite: Angew. Chem. Int. Ed. 2022, 61, e202211433

International Edition: doi.org/10.1002/anie.202211433

German Edition: doi.org/10.1002/ange.202211433

amounts of zinc in place of a photocatalyst and light for

CN and CO cross-couplings.[13]

Doyle and co-workers showed that these key NiIspecies

can be accessed without photocatalysts or additives by

illuminating NiII(dtbbpy) aryl halide complexes (dtbbpy=

4,4’-di-tert-butyl-2,2’-bipyridyl), which are synthesized from

Ni(COD)2(COD=1,5-cyclooctadiene) or nickel phosphine

complexes (Figure 1b).[14,15] Here, excitation generates a

metal-to-ligand charge transfer (MLCT) state that either

decays to a triplet metal-centered d-d state,[14] or a ligand-to-

metal charge transfer (LMCT) state.[16,17] The weakened

bonds in these latter states were reported to result in

homolysis of the NiII-aryl bond to generate the key NiI

catalyst, thereby initiating cross-coupling catalysis.[14,15] The

scope of this photocatalyst-free strategy was studied for

CO and CN cross-coupling reactions using ultraviolet

(UV, 390 nm) irradiation.[18,19]

Inspired by Doyle’s elegant strategy, we wondered

whether modification of the bipyridine ligand could access a

catalytically active NiIspecies through intraligand charge

transfer (ILCT; Figure 1c). We hypothesized that a ligand-

centered excited state would decay into a labile metal-

centered d-d state undergoing bond homolysis to initiate

cross-coupling catalysis. Here, we demonstrate that this can

be achieved by decorating a bipyridine ligand with two

carbazole units. The resulting complex enables visible-light-

mediated CO, CS, and CN cross-coupling of nucleo-

philes with aryl halides. Heterogeneous catalysis was

realized by polymerizing the ligand. The resulting conju-

gated microporous polymer coordinates nickel and serves as

a recyclable heterogeneous catalyst for light-mediated

carbonheteroatom cross-coupling reactions. The robust

catalyst showed stable performance during one week of

operation in continuous flow mode using a packed-bed

reactor.

Results and Discussion

Our underlying strategy was to equip the bipyridine ligand

with a structural motif that allows intraligand charge transfer

upon light excitation in the visible. Inspired by organic

donor–acceptor fluorophores, such as 1,2,3,5-tetrakis-

(carbazol-9-yl)-4,6-dicyanobenzene (4CzIPN),[20,21] we de-

cided to decorate a bipyridine motif with two carbazole units

as electron donor moieties via a copper-catalyzed Ullmann

coupling (Figure 2a).[22] The resulting 5,5’-dicarbazolyl-2,2’-

bipyridyl (Czbpy) forms the desired Ni(Czbpy)Cl2complex

upon mixing with NiCl2·glyme. A comparison of the electro-

chemical properties of this complex with Ni(dtbbpy)Cl2,

which is commonly used in metallaphotocatalytic cross-

coupling reactions, shows that Ni(Czbpy)Cl2is easier to

reduce (Figure 2b). Furthermore, the cyclic voltammogram

of Ni(Czbpy)Cl2shows two additional reduction waves that

are tentatively assigned as ligand reductions.

The UV/Visible spectrum of the unbound Czbpy ligand

shows a broad and strong absorption band in the UV region

(λmax =343 nm; Figure 2c). Complexation with NiCl2·glyme

gave a red-shift of this band with an absorption onset in the

visible region of the electromagnetic spectrum (onset

~450 nm, λmax =386 nm). The structure of this absorption

band, its extinction coefficient, and the fluorescence lifetime

are similar for the complex and the unbound ligand,

indicating that they share the same origin (see the Support-

ing Information for details). To determine the nature of the

electronic transitions, time-dependent density functional

theory (TD-DFT) calculations were performed (see the

Supporting Information for details). The transition that

makes up the 386 nm band in the experimental spectrum of

the complex is of predominantly ILCT character, involving

the transfer of electron density from the carbazole groups to

the bipyridine moiety (the highest occupied molecular

Figure 1. Strategies for visible-light-mediated nickel-catalyzed carbonheteroatom cross-couplings. a) Dual photo/nickel-catalysis uses in situ-

formed nickel precatalyst and an exogenous photocatalyst. b) NiII(dtbbpy) aryl halide complexes can be activated via MLCT. c) In situ-formed nickel

precatalysts can be activated via ILCT (this work).

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orbital is of Cz π character and the lowest unoccupied

molecular orbital is of bpy π* character; Figure 2d, Fig-

ure S37, and Table S2). A similar transition was assigned to

the 343 nm band of the unbound Czbpy ligand (Table S3).

The red-shift of the ILCT band upon complexation is due to

a slight delocalization of electron density involving Ni and

Cl atoms.

We conducted femtosecond-resolved optical transient

absorption (OTA) experiments to elucidate the excited-state

relaxation pathway for this complex (see the Supporting

Information for details). Upon excitation into the ILCT

band at 415 nm, the complex undergoes a fast (~ ~1.3 ps)

intersystem crossing from a singlet to a triplet ILCT state,

convolved with a partial back relaxation to the ground state

on the same time scale (Figure 2e). Within ~14 ps, the

triplet ILCT decays into an optically dark excited state

manifold. This assignment is supported by OTA experi-

ments and TD-DFT on the bare ligand. Figure 2f compares

the transient spectra of the Ni(Czbpy)Cl2complex and the

Czbpy ligand at 10 ps after photoexcitation. The transient

spectra of the complex and ligand are similar, except for an

energy shift as expected from the ground-state spectra

(Figure 2c). Furthermore, we simulated the transient spec-

trum of the ligand using TD-DFT; the difference between

Figure 2. Synthesis and characterization of Czbpy and Ni(Czbpy)Cl2. a) The ligand was synthesized via an Ullmann CN coupling and forms the

desired complex upon treatment with NiCl2·glyme. b) Cyclic voltammetry studies of Ni(Czbpy)Cl2and Ni(dtbbpy)Cl2. c) Experimental and calculated

UV/Visible spectra of Czbpy and Ni(Czbpy)Cl2. The band indicated by a vertical line is assigned to an intraligand charge transfer (ILCT) transition.

The theoretical spectra have been shifted by 0.25 eV. d) Natural transition orbitals (NTOs) of Ni(Czbpy)Cl2. e) A simplified excited state relaxation

diagram of Ni(Czbpy)Cl2. The states are separately labeled for the metal center and Czbpy ligand in parenthesis, respectively. f) Optical transient

absorption data of Ni(Czbpy)Cl2and Czbpy at 10 ps after photoexcitation at 415 nm (arrow). For comparison, the inverted static absorption spectra

are shown as well. The solid black curve is the simulated transient spectrum from TD-DFT calculated by subtracting the ground state spectrum

from the spectrum of the lowest-triplet T1 state. The spectra have been arbitrarily scaled to approximately match the amplitude of the transient

czpbpy spectrum (blue). g) Spin trapping experiment of Ni(Czbpy)Cl2in DMAc in the presence of phenyl N-t-butylnitrone (PBN) with and without

illumination. Radical species are formed upon irradiation.

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the TD-DFT spectrum of the lowest ligand-centered triplet

state (with ILCT character) and of the ground state matches

the transient spectra strikingly well. These findings corrobo-

rate the finding that the ~14 ps excited state of Ni-

(Czbpy)Cl2corresponds to a triplet intraligand excited state

with charge-transfer character.

While the ground state adopts a tetrahedral geometry

and is of triplet character, the lowest lying excited d-d state

is square-planar and of singlet 1A1g character (Figure S34).

Several triplet d-d 3T1states are calculated to lie between

the ILCT and singlet 1A1g states (Figure S38). We propose

that the triplet ILCT state decays into this metal-centered d-

d state manifold, while a partial back relaxation to the

ground state on the same time scale may also occur. The

excited d-d states are optically dark, meaning that they

cannot be detected in the UV/Visible part of the spectrum.

Indeed, the ground-state spectrum is dominated by ligand-

centered absorption bands that are not expected to be

different for the metal-centered states. The 1(d-d) and 3(d-d)

states have anti-bonding character along the nickel halide

bonds (Figure S39), signifying their propensity for NiCl

bond homolysis and the formation of catalytically active NiI

species. A similar pathway has been suggested in other

catalytic nickel complexes.[14,23] We note, however, that in

similar Ni(bipyridine) aryl halide complexes, repulsive

ligand-to-metal charge-transfer (LMCT) excited state poten-

tial energy surfaces were invoked in Ni-aryl bond

homolysis.[16,17] While the involvement of such surfaces

cannot be excluded here, the LMCT states are expected to

lie much higher in energy in Ni dichloride complexes

compared to Ni heteroaromatic complexes. A crossing from

the relatively low-lying ILCT states onto such LMCT

surfaces is thus unlikely in the present case.

Electron paramagnetic resonance (EPR) spin-trapping

experiments clearly showed the formation of a spin adduct

when a mixture of Ni(Czbpy)Cl2and phenyl N-tert-butylni-

trone (PBN) is irradiated with 440 nm LEDs at room

temperature (Figure 2g, see the Supporting Information for

details). The spectrum resulting from illumination of Ni-

(Czbpy)Cl2and PBN in DMAc is characteristic for a spin

adduct of an O- or C-centered radical rather than a chlorine

radical. Chlorine radicals are difficult to identify using spin-

trap methods, because trapping products are typically

unstable.[24] More specifically, the resulting NiIcomplex

potentially re-abstracts chlorine from the spin adduct to

regenerate the NiII complex.[25] However, we assume that the

chlorine radical reacts with DMAc to generate an O- or C-

centered radical species that is responsible for the EPR

signal in the trapping experiment using 440 nm irradiation.

Building on these favorable properties (strong absorp-

tion band in the visible and the formation of labile excited

states and radical species), we investigated if in situ-formed

Ni(Czbpy)Cl2serves as a suitable nickel precatalyst for

carbonheteroatom cross-couplings that mitigates the neces-

sity of an exogenous photocatalyst for its activation (Fig-

ure 3).[26] The coupling of aryl halides with sodium sulfinate

salts was originally reported using combinations of a nickel

catalyst with iridium[27] or ruthenium[28,29] polypyridyl com-

plexes as photocatalysts. Irradiating a mixture of 4-iodoben-

zotrifluoride, sodium 4-methylbenzenesulfinate,

NiCl2·glyme, and Czbpy in DMSO with 440 nm LEDs

afforded sulfone 1in excellent yield after 22 h (Figure 3a,

entry 1). No conversion was detected when the reaction was

carried out in the dark (entry 2). Sodium sulfinates and aryl

halides can assemble in electron-donor acceptor (EDA)

complexes and afford sulfones upon UV light irradiation.[30]

Accordingly, even in absence of NiCl2·glyme small amounts

of 1were formed due to partial emission in the UV region

of the light source (entry 3). When 2,2’-bipyridine (bpy), 9H-

carbazole or a combination of bpy and 9H-carbazole were

used, the desired product was also formed, although with

low selectivity (entries 4–6).

The CO arylation of 4-iodobenzotrifluoride with N-

Boc-proline under optimized conditions resulted in 88% of

the desired product (2) (Figure 3b, entry 7). No product

formation was observed in the dark, without NiCl2·glyme, or

when 9H-carbazole was used as a ligand (entries 8–10). Only

small amounts (<10%) of 2were formed using bpy, or bpy

together with 9H-carbazole (entries 9, 11). This transforma-

tion was used to probe if the proposed NiI/NiIII cycle could

perpetuate if light irradiation is interrupted. We indeed

observed a significant increase in product formation when

the reaction was subjected to photon-free conditions after

6 h irradiation, supporting the overall mechanistic proposal

(see the Supporting Information for details).

The light-mediated, nickel-catalyzed N-arylation of 4-

methylbenzenesulfonamide afforded 3in 75% yield (Fig-

ure 3c, entry 13). Light and the nickel salt were crucial for

product formation (entries 13, 14). Partial consumption of

the starting material in the absence of a nickel salt (entry 15)

might be a result of a photocatalytic activation of aryl

iodides.[31] Product formation was not detected using bpy

(entry 17), but significant amounts of 3were obtained in the

presence of 9H-carbazole (entry 16) or a combination of

9H-carbazole and bpy (entries 16 and 18). This might result

from formation of a Ni-carbazole complex that induces

cross-coupling reactions through MLCT.[32,33]

Czbpy was not suitable as a ligand for coupling of aryl

halides with amines or thiols, and only low amounts of the

respective coupling product was obtained when an alcohol

was used as a nucleophile (Tables S27–S29).[6] Attempts to

form carboncarbon bonds through coupling of aryl halides

with trifluoroborates[34] or α-silylamines[35] did not meet with

success or suffered from low selectivity (Tables S30–S32).

The scope and limitations of photocatalyst-free, visible-

light-mediated carbonheteroatom cross-coupling reactions

were explored next (Table 1). Aromatic sulfinate salts were

coupled with 4-iodobenzotrifluoride (Table 1a, 1,4–7).

Optimized reaction conditions did not result in the desired

product using sodium methane sulfinate. This substrate was

earlier reported as a successful coupling partner when

Ni(bpy)Cl2was used in combination with tris(2,2’-bipyridyl)-

dichlororuthenium(II) hexahydrate as an exogenous

photocatalyst.[29] With regard to the aryl iodide, the reaction

affords the corresponding sulfones in the presence of

electron-withdrawing groups such as trifluoromethyl (1),

nitrile (8), ketone (9–11), amide (12), boron pinacolate ester

(13), and methyl ester (14). Para- (9) and ortho- (11)

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substitution showed similar reactivity, whereas meta-substi-

tution (10) required longer reaction time. Electron-rich aryl

iodides, such as 4-iodotoluene (15) and 4-iodoanisole (16),

were suitable substrates and the presence of an unprotected

amine group (17) was tolerated. Coupling of 2-iodothio-

phene (18) and 4-iodopyridine (19) showed that heteroaryl

iodides are suitable substrates. Aryl iodides reacted signifi-

cantly faster than the corresponding bromide (14). This is in

contrast to dual nickel/photocatalytic protocols, where

iodides and bromides exhibit similar reactivity.[27,28] Aryl

chlorides undergo nickel/photocatalytic reactions when the

more electron-donating ligand 4,4’-dimethoxy-bpy is used in

combination with a exogenous photocatalyst,[27] but Czbpy

was not suitable. These observations were applied for

selective couplings of aryl iodides that contain a chloride

(20) or a bromide substituent (21). Moreover, diarylated

product 22 was synthesized from 1,4-diiodobenzene.

Good to excellent isolated yields were obtained for the

CO arylation of 4-iodobenzotrifluoride with aliphatic and

aromatic carboxylic acids (Table 1b, 2,23–26). A range of

aryl iodides containing electron-withdrawing groups af-

forded the corresponding products (2,27–35). Longer

reaction times were required for the coupling of meta-

substituted aryl iodides (27,29,31), compared to their para-

substituted analogues (28,30,32). The coupling of ortho-

substituted aryl iodides was not possible in case of 2-

iodoacetophenone and 2-iodobenzonitrile, but 36 was suc-

cessfully synthesized from methyl 3-methyl-4-iodobenzoate.

A heteroaryl iodide was also susceptible to the optimized

reaction conditions (38). High electron densities on the aryl

iodide decreased their reactivity towards the CO coupling,

as showcased for the series 4-iodobenzene (38), 4-iodoto-

luene (39), and 4-iodoanisole (40). Aryl iodides work best in

the reaction (32, 90% from Ar-I). The reaction is rather

slow using the corresponding bromide (46% NMR yield

from Ar-Br), and a chloride afforded only traces of the

desired product. As a result, 1-chloro-4-iodobenzene (41)

coupled selectively on the iodo-position, but 1-bromo-4-

iodobenzene reacted unselectively. These results are in

agreement with previous reports on the dual nickel/photo-

catalytic cross-coupling.[13,31,36–38]

Aromatic and aliphatic sulfonamides (3,42–46) gave

selective CN cross-couplings with 4-iodobenzotrifluoride

(Table 1c). In contrast to the CS and CO coupling,

reactivity is not affected by the substitution pattern of the

aryl iodide (47–49,54) or by the presence of either electron-

withdrawing or -donating functional groups (3,47–53,56–

58). Heteroaryl halides are problematic substrates in dual

Figure 3. Optimized reaction conditions and control experiments for light-mediated carbonheteroatom cross-coupling reactions catalyzed by

Ni(Czbpy)Cl2. a) CS arylation of sodium p-toluensulfinate with 4-iodobenzotrifluoride. b) CO arylation of N-Boc-proline with 4-iodobenzotri-

fluoride. c) CN arylation of p-toluensulfonamide with 4-iodobenzotrifluoride. [a] NMR yields determined by 1H NMR spectroscopy using 1,3,5-

trimethoxybenzene as an internal standard. n.d.=not detected.

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Table 1: Visible-light-mediated nickel-catalyzed CS, CO and CN cross-couplings.[a]

[a] Reaction conditions: aryl halide (300 μmol), nucleophile (panel a, sodium sulfinate, 600 μmol; panel b, carboxylic acid, 450 μmol; panel c,

sulfonamide, 450 μmol), NiCl2·glyme (15 μmol), Czbpy (15 μmol), base (panel b, N-tert-butylisopropylamine, 900 μmol; panel c, 1,8-diazabicyclo-

[5.4.0]undec-7-ene, 450 μmol), solvent (panel a, DMAc, 6 mL; panel b, DMSO, 3 or 6 mL; panel c, DMSO, 6 mL), 440 nm LED (2 lamps at full

power) at room temperature. Isolated yields are reported. NMR yields are in parentheses and were calculated via 1H NMR spectroscopy using

1,3,5-trimethoxybenzene or maleic acid as an internal standard. n.d.=not detected. Bpin=boronic acid pinacolate ester. BIPA=N-tert-

butylisopropylamine. DBU=1,8-diazabicyclo[5.4.0]undec-7-ene.

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nickel/photocatalytic sulfonamidation protocols and require

a ligand-free approach at elevated temperature.[39] Under

our optimized conditions 3-iodopyridine gave 55 in good

yield, but no product was observed for 2-iodothipohene.

Previously, aryl bromides were coupled with sulfonamides

using combinations of nickel and iridium catalysts.[39] Aryl

bromides are suitable substrates but iodide reactivity is

superior (52, 84% from Ar-I, 31% NMR yield from Ar-Br

within 24 h). Aryl chlorides are not reactive and 59 was

obtained with good selectivity from 1-chloro-4-iodobenzene.

Having shown that Czbpy serves as a versatile ligand for

visible-light-mediated cross-coupling reactions via homoge-

neous nickel catalysis, we aimed to extend this approach to

develop a heterogeneous, recyclable catalyst.[40] Defined

porous materials are ideal candidates for immobilization of

metal catalysts, as they enable optimal access to the catalytic

sites. The microporous organic polymer network poly-Czbpy

was synthesized from Czbpy via oxidative polymerization

and exhibits a Brunauer–Emmett–Teller surface area (SBET)

of 853 m2g1(Figure 4a, see the Supporting Information for

details).[22] The chemical structure of poly-Czbpy was

confirmed by 13C CPMAS NMR spectroscopy showing

signals between 130 and 152 ppm (Figure S4), which verify

the existence of bipyridine moieties within the structure.

Additionally, at 137 ppm a signal corresponding to carbons

in vicinity to carbazolyl nitrogen CAr-N was detected.

Ni@poly-Czbpy was prepared by refluxing a suspension of

poly-Czbpy and NiCl2in methanol. Investigation of the

porosity showed a decreased BET surface of 470 m2g1due

to the immobilization of NiII. Presence of nickel shifts the

absorption of the material up to 650 nm, while the metal-

free ligand framework poly-Czbpy absorbs until 550 nm

(Figure 4b). Characterization of Ni@poly-Czbpy by X-ray

photoelectron spectroscopy (XPS) confirmed successful

immobilization of NiII species on the polymeric material

(Figure 4c). The N 1s core spectrum contains three signals

for nitrogen: i) an intense peak at 400.4 eV corresponding to

polymerized carbazole moieties, ii) a signal at 399.7 eV

which is assigned to NNi coordination of the NiII-complex

and iii) a low-intensity peak at 400.2 eV derived from

bipyridine nitrogen species that are not coordinated to

nickel. The Ni 2p spectrum shows a doublet and its

corresponding satellites. Peaks located at 855.6 eV and

873.3 eV are assigned to Ni 2p3/2 and Ni 2p1/2 signals for NiII

species, respectively. ICP-OES analysis indicated the pres-

ence of 3.7% w/w of nickel, corresponding to an occupation

of 40% of bipyridine functionalities. Scanning electron

microscopy (SEM) images of Ni@poly-Czbpy show the

morphology of the amorphous polymeric particles (Fig-

ure S5). The images depict a homogeneous distribution of

nickel, nitrogen, and chlorine within the material.

After confirming that poly-Czbpy is suitable to coordi-

nate and immobilize nickel atoms, its use in the coupling

reactions was studied (Figure 4d). The desired CS, CO,

and CN coupling products were obtained by irradiation at

440 nm of mixtures of NiCl2·glyme (5 mol%) and poly-

Czbpy (5 mol%), but the selectivity was initially lower than

using homogeneous conditions. According to ICP-OES

analysis, 40% of the pyridine sites in poly-Czbpy coordinate

to nickel. Therefore, equimolar amounts of nickel and poly-

Czbpy lead to an excess of unligated nickel in solution that

presumably has a detrimental effect on the selectivity. This

was confirmed during a series of experiments using a lower

nickel salt/macroligand ratio (2.5 mol% of NiCl2·glyme,

5 mol% poly-Czbpy) that improved selectivity for all trans-

formations significantly.

Next, we studied whether the heterogeneous catalytic

system based on poly-Czbpy can be recycled. The heteroge-

neous material was recovered after the CS coupling

reaction by centrifugation and was reused for the same

reaction (Figure 4e). Initial results confirmed that poly-

Czbpy can be recycled ten times without significant loss in

reactivity using 5 mol% NiCl2·glyme for each reaction cycle

(orange bars). The addition of the nickel salt at each

reaction cycle was not necessary (green bars) and the

selectivity of the reaction improved upon washing and

reusing the material without addition of fresh nickel salt (1st

cycle: 78% yield, 2nd cycle: 90% yield). This supported our

earlier observation that an excess of unligated nickel results

in lower selectivity. A final recycling experiment was carried

out by adding 2.5 mol% of NiCl2·glyme for the first reaction

cycle (Figure 4e, blue bars) and resulted in excellent yields

for the CS coupling reaction without significant loss in

activity during ten recycling experiments. The Ni 2p XPS

spectra of the recycled catalyst confirmed that the NiII

species remained intact within the polymer (Figure S53).

The signals for the doublet were detected at 856.6 eV (Ni

2p3/2) and 874.6 eV (Ni 2p1/2), respectively. Furthermore, N

1s XPS core-level spectra show that, by single addition of

NiII precursor, predominantly the pyridinic nitrogen signal at

399.2 eV was detected due to a relatively low amount of NiII

coordinated to bipyridine, while addition of NiII after each

cycle result mainly in NiN coordination signals at 399.7 eV

(Figure S54).

The promising results of the recycling studies prompted

us to study the long-term stability of the heterogeneous

catalyst in a continuous flow packed-bed reactor.[41] Here,

the heterogeneous catalyst remains located in a specific part

of the reactor through which the reaction mixture is

pumped. The desired chemical reaction and separation of

the catalyst from the solution take place simultaneously,

enabling straightforward studies on the robustness of the

polymeric ligand and metal leaching.[42] We carried out a

continuous long-run CS cross-coupling experiment over

seven days using a mixture of poly-Czbpy, silica and glass

beads in an irradiated packed-bed reactor (Figure 4f, see the

Supporting Information for details). To our delight, a stable

catalytic activity was observed after reaching steady-state

conditions and a total nickel leaching of only ~15% was

determined.

Conclusion

Installing two carbazole units on a bipyridine motif enables

the formation of a homogeneous NiII complex that absorbs

at up to 450 nm and enables visible-light-mediated CS,

CO, and CN cross-coupling reactions without the addition

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of an exogenous photocatalyst. Studies using time-depend-

ent density functional theory calculations, femtosecond-

resolved optical transient absorption, and electron para-

magnetic resonance spectroscopy revealed the formation of

Figure 4. Polymerization of Czbpy enables heterogeneous visible-light-mediated metallaphotocatalyzed cross-coupling reactions. a) Preparation of

the porous organic polymer poly-Czbpy and complexation with NiCl2. b) Characterization of poly-Czbpy and Ni@poly-Czbpy by UV/Visible

spectroscopy. c) XPS analysis of Ni@Czbpy: N 1s and Ni 2p core-level spectra. d) Visible-light-mediated carbonheteroatom cross-coupling

reactions using Ni@poly-Czbpy as a heterogeneous metallaphotocatalyst. e) Catalyst recycling study (orange: 5 mol% of NiCl2·glyme added at

each reaction cycle; green: 5 mol% of NiCl2·glyme added only for the first cycle; blue: 2.5 mol% of NiCl2·glyme added only for the first cycle).

f) Catalyst lifetime study using a continuous long-run CS cross-coupling experiment over seven days flow in a packed-bed reactor.

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a paramagnetic species upon irradiation of this complex with

visible light, which occurs via a hitherto undisclosed initial

intraligand charge transfer mechanism. This scenario holds

significant potential for nickel catalysis and light-mediated

(transition) metal catalysis in general, because it accesses

light-responsive complexes solely through ligand excitation.

Future work will focus on systematic variations of the Czbpy

ligand scaffold to tune its electronic structure and correlate

with photocatalytic reactivity, as well as multireference/

multiconfigurational electronic structure calculations to

investigate the potential role of higher-lying repulsive states

and estimate the thermal barrier for Ni-chloride bond

homolysis.[16,17]

In addition, the ligand scaffold can be easily converted

into a porous organic polymer by a straightforward oxidative

polymerization. The resulting macroligand immobilizes nick-

el for heterogeneous, visible-light-mediated nickel catalysis.

The heterogeneous material recovered after the reaction can

be reused, maintaining a high activity over ten reaction

cycles. The robustness and reusability of the heterogeneous

catalyst was further demonstrated during a flow experiment

using a packed-bed reactor that showed stable conversion

over seven days of continuous operation.

Acknowledgements

C.C., S.G., S.R., W.-H. H., N.R., L.A., D.C. P.H.S, and B.P.

gratefully acknowledge the Max-Planck Society for generous

financial support. S.G. and B.P. thank the International Max

Planck Research School on Multiscale Bio-Systems for

funding. B.P. acknowledges financial support by a Liebig

Fellowship of the German Chemical Industry Fund (Fonds

der Chemischen Industrie, FCI). H.M.-W., A.T,. and B.P.

thank the Deutsche Forschungsgemeinschaft (DFG, Ger-

man Research Foundation) under Germany’s Excellence

Strategy—EXC 2008-390540038—UniSysCat. S.V. and A.T.

thank the Deutsche Forschungsgemeinschaft (DFG, Ger-

man Research Foundation) for financial support (TH

1463/15-1). L.A. and B.P. thank the Deutsche Forschungs-

gemeinschaft (DFG, German Research Foundation) for

financial support (BP 1635/2-19). R.M.V. acknowledges

funding from the David and Lucile Packard Foundation.

This material is based upon work supported by the U.S.

Department of Energy, Office of Science, Office of Basic

Energy Sciences under Award Number DE-SC0018904 to

J.V.W. J.H.B. was supported by the Robert C. and Caro-

lyn J. Springborn Endowment for Student Support Program

and the National Science Foundation Graduate Research

Fellowship Program. We thank Prof. R. Bittl and Dr. John J.

Molloy for scientific support and fruitful discussions. Open

Access funding enabled and organized by Projekt DEAL.

Conflict of Interest

The authors declare no conflict of interest.

Data Availability Statement

All experimental procedures and analytical data are avail-

able in the Supporting Information. All data is available

from the authors on reasonable request.

Keywords: Flow Chemistry ·Heterogeneous Catalysis ·

Homogeneous Catalysis ·Photocatalysis ·Nickel Catalysis

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Manuscript received: August 3, 2022

Accepted manuscript online: September 26, 2022

Version of record online: October 21, 2022

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