Multidentate Phenanthroline Ligands Containing Additional Donor Moieties and Their Resulting Cu(I) and Ru(II) Photosensitizers: A Comparative Study

Martin Rentschler, Marie-Ann Schmid, Wolfgang Frey, Stefanie Tschierlei,* and Michael Karnahl*

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Multidentate Phenanthroline Ligands Containing Additional Donor

Moieties and Their Resulting Cu(I) and Ru(II) Photosensitizers: A

Comparative Study

†

Cite This: https://dx.doi.org/10.1021/acs.inorgchem.9b03687

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sı Supporting Information

ABSTRACT: To bind or not to bind: Driven by the motivation to

increase the (photo)stability of traditional Cu(I) photosensitizers,

multidentate diimine ligands, which contain two additional donor sites,

were designed. To this end, a systematic series of four 1,10-

phenanthroline ligands with either OR or SR (R = Pr or Ph) donor

groups at the 2 and 9 positions and their resulting hetero- and

homoleptic Cu(I) complexes were prepared. In addition, the related

Ru(II) complexes were also synthesized to study the eﬀect of another

metal center. In the following, a combination of NMR spectroscopy

and X-ray analysis was used to evaluate the impact of the additional

donor moieties on the coordination behavior. Most remarkably, for the

homoleptic bis(diimine)copper(I) complexes, a pentacoordinated

copper center, corresponding to a (4 + 1)-fold coordination mode,

was found in the solid state. This additional binding is the ﬁrst

indication that the extra donor might also occupy a free coordination site in the excited-state complex, modifying the nature of the

excited states and their respective deactivation processes. Therefore, the electrochemical and photophysical properties of all novel

complexes (in total 13) were studied in detail to assess the potential of these photosensitizers for future applications within solar

energy conversion schemes. Finally, the photostabilities and a potential degradation mechanism were analyzed for representative

samples.

,26−28

INTRODUCTION

stability.

Unfortunately, the lifetime of the triplet excited

■

state is usually only on the order of a few nanoseconds, and the

quantum yields are often much lower than those of the

competing Ru(II) photosensitizers.

Photosensitizers based on transition-metal complexes have

received intense attention owing to their advantageous

photophysical and electrochemical properties.

,22,29

−5

Conse-

In comparison to their homoleptic counterparts, heteroleptic

diiminediphosphinecopper(I) complexes with the general

quently, these compounds are already successfully used for a

variety of photonic applications such as dye-sensitized solar

formula [(P^P)Cu(N^N)]⁺possess increased emission

8,9

cells (DSSCs) or organic light-emitting diodes (OLEDs)

23,24,30,31

0−15

quantum yields and excited-state lifetimes.

compounds can also be tuned in a wider range because of the

presence of two diﬀerent types of ligands. However, as a major

drawback, these heteroleptic Cu(I) complexes suﬀer from a

reduced photostability.

in solution, under reaction conditions, a dissociation of the

diphosphine ligand occurs.

As a common feature, all Cu(I) complexes possess a

These

and for photoredox catalysis.

In this respect, particularly

ruthenium(II) polypyridine complexes have been studied in

detail for decades because of their high chemical stability,

strong absorption of visible light, and tunable redox

28,32−34

Several studies evidenced that

,5,16−18

behavior.

However, ruthenium is a rare and expensive

noble metal (only 0.001 ppm in the Earth’s crust), which

complicates the broad application of Ru(II) photosensitizers in

solar-energy conversion schemes.

For this reason, luminescent Cu(I) complexes have become

more and more interesting as a promising alternative.

28,32−35

2,23

(

distorted) tetrahedral ground-state structure.

Upon light

19−24

Special Issue: Light-Controlled Reactivity of Metal

Complexes

Nevertheless, a basic distinction must be made between homo-

and heteroleptic Cu(I) compounds, which bear diﬀerent pros

,22,23,25

and cons.

Homoleptic bis(diimine)copper(I) com-

Received: December 19, 2019

plexes of the type [Cu(N^N) ] , where N^N indicates a

chelating diimine ligand, are commonly characterized by a

decent absorption in the visible range and a high

XXXX American Chemical Society

Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry

(see Figure 2 ) were prepared. For comparison, also the related

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excitation, a metal-to-ligand charge-transfer (MLCT) tran-

oxidation reaction compared to the corresponding phos-

41,42

sition from the 3d orbital of Cu(I) to the π* orbital of the

phines.

23,29,36,37

diimine ligand takes place.

This causes a formal

After the successful synthesis of the multidentate phenan-

throline ligands L2−L5, the corresponding hetero- and

homoleptic Cu(I) complexes Cu2−Cu5 and Cu2′−Cu5′

oxidation from Cu(I) to Cu(II) and a single-electron reduction

of one diimine ligand. As a result, a signiﬁcant change in the

coordination geometry and a ﬂattening toward a more square-

planar structure occurs, corresponding to the 3d conﬁguration

3,29,36−38

of Cu(II).

In this ﬂattened geometry, the complex is

more open for a nucleophilic attack by solvent molecules or

counterions, leading to a pentacoordinated excited-state

,29,37

complex (exciplex).

This exciplex is then eﬃciently

deactivated via nonemissive deactivation pathways, causing

short excited-state lifetimes and restricting the applicability.

It is known that sterically demanding and rigid ligands can

suppress ﬂattening distortion upon photoexcitation and shield

20,23,24,30

the copper center against nucleophilic attack.

As a

possible alternative, multidentate ligands could be used that

occupy the free coordination site in the exciplex with an

35,39,40

additional donor from the beginning (see Figure 1).

Figure 2. Overview and numbering of the diﬀerent Cu(I) and Ru(II)

complexes that were studied.

Ru(II) photosensitizers Ru2−Ru5 with the general formula

[

(tbbpy) Ru(L)] (with tbbpy = 4,4′-di-tert-butyl-2,2′-bipyr-

idine) were made. The three reference complexes Cu1, Cu1′,

and Ru1 with the 2,9-dimethyl-1,10-phenanthroline ligand L1

were used to systematically evaluate the electronic and steric

eﬀects of the additional donor moiety. It should be noted that,

although there are already a large variety of ruthenium(II)

phenanthroline complexes, the reference Ru1 was not yet

presented. All in all, crystals suitable for X-ray analysis were

obtained for 8 of the 13 new complexes (in total 15

complexes), which allowed for a comprehensive elucidation

of the coordination behavior.

Figure 1. General concept of this study investigating the impact of an

additional donor moiety on the coordination behavior of the

corresponding homo- and heteroleptic Cu(I) complexes.

Thereby, a nucleophilic attack might be prevented in the

excited state. The understanding of this issue and of the

coordination behavior of multidentate diimine ligands is the

main goal of the present study.

For this purpose, four phenanthroline ligands, which contain

two additional donor moieties in the 2 and 9 positions, were

designed (see Scheme 1). As suitable donor functions, OR and

In addition, the photophysical (i.e. absorption and emission

behavior, excited-state lifetimes, and emission quantum yields)

and electrochemical properties were studied in detail for all

complexes. As a result, structure−property relationships have

been identiﬁed, providing valuable information for future

studies. Interestingly, the O Pr group has been proven as the

best substituent in terms of the resulting photophysical and

electrochemical properties. This can be understood by a

proposed decomposition mechanism of the other ligands

under reductive conditions.

Scheme 1. Depiction of the Two-Step Synthesis Procedure

of the Ligands L2−L5

RESULTS AND DISCUSSION

■

Synthesis and Structural Characterization. Synthesis

of the Phenanthroline-Derived Ligands. Starting from the

commercially available 2,9-dimethyl-1,10-phenanthroline li-

gand L1, 2,9-dibromomethyl-1,10-phenanthroline was pre-

Reaction conditions: (1) (i) NBS, MeCN, reﬂux, 18 h; (ii)

HPO(OEt) , PrNEt , THF, 0 °C to rt; (2) NaX, solvent, reﬂux,

pared in two steps following a known literature procedure.

−24 h.

The obtained dibrominated phenanthroline derivative serves

then as a universal intermediate that can be further

functionalized by nucleophilic substitution of the two bromine

atoms. Applying the sodium salts of either (thio)isopropyl

alcohol or (thio)phenol as nucleophiles results in the

phenanthroline ligands L2−L5 containing two (thio)ether

functionalities each (see Scheme 1) with 48−96% yield. Only

SR groups (with R = Pr or Ph) were chosen because of their

good availability and the fact that substituents in this position

are known to improve the photophysical properties of the

0,23,24,30

resulting Cu(I) complexes.

chalcogen elements only carry one additional substituent R

compared to phosphines PR . As a consequence, the OR and

Furthermore, these

L4 (S Pr, 43%) had to be additionally recrystallized from n-

SR donor groups are less sterically demanding, which may

facilitate a 5-fold coordination (see Figure 1). Moreover, the

sulfur moiety is less prone to undergoing an unwanted

hexane to increase purity. The ligand L3 (OPh) was already

presented in the literature but prepared by a diﬀerent

procedure.

Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry

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Table 1. Selected Crystallographic Bond Lengths (pm) and Angles (deg) of the Hetero- and Homoleptic Cu(I) Complexes

Cu1, Cu2, Cu4, and Cu1′−Cu5′

Cu1(space

Cu2(space

group P1

Cu4(space

group P1

Cu1′(space

Cu2′ (space

group C2/c)

Cu3′(space

group P1

Cu4′(space

group P1

Cu5′(space

group P1

)

group P2 /c)

)

Cu−N1

Cu−N2

Cu−P1

Cu−P2

Cu−X1

Cu−X2

208.4(3)

211.2(3)

226.26(11)

228.63(13)

208.4(4)

210.6(4)

224.94(13)

231.27(13)

470.3(4)

210.7(3)

210.2(3)

225.34(11)

230.48(11)

506.1(1)

Cu−N1

Cu−N2

Cu−N3

Cu−N4

Cu−X1

Cu−X2

Cu−X3

Cu−X4

202.8(2)

204.2(2)

205.3(2)

207.0(2)

202.06(14)

204.80(14)

204.81(14)

352.31(13)

462.43(13)

352.31(13)

462.43(13)

203.6(3)

204.2(2)

204.1(3)

203.5(2)

328.9(2)

350.3(2)

457.3(2)

472.7(2)

205.3(3)

202.0(3)

206.9(3)

201.0(3)

389.7(1)

420.2(2)

496.8(2)

486.3(2)

204(1)

200.0(9)

205(1)

376.3(4)

392.3(4)

404.0(5)

394.6(5)

447.4(4)

483.2(1)

N1−Cu−N2

P1−Cu−P2

N1−Cu−P1

N1−Cu−P2

N2−Cu−P1

N2−Cu−P2

80.53(13)

112.93(4)

108.77(9)

120.33(9)

118.59(9)

80.97(15)

120.43(5)

119.73(11)

102.33(11)

126.99(11)

97.86(10)

79.92(13)

116.59(4)

121.55(10)

103.1(1)

127.1(1)

101.3(1)

N1−Cu−N2

N3−Cu−N4

N1−Cu−N3

N1−Cu−N4

N2−Cu−N3

N2−Cu−N4

81.78(8)

81.95(8)

110.75(8)

126.67(8)

128.42(8)

132.11(8)

82.53(6)

134.40(8)

121.75(5)

118.50(8)

82.4(1)

82.2(1)

133.8(1)

123.4(1)

117.1(1)

123.6(1)

81.8(1)

82.4(1)

117.5(1)

121.6(1)

121.2(1)

136.5(1)

82.4(5)

83.0(4)

128.4(5)

124.4(5)

124.9(5)

119.0(5)

P−P−N−N

76.9

85.9

87.8

N−N−N−N

79.4

86.9

82.3

86.3

88.7

The values in parentheses represents the experimental estimated standard deviation of the measurement. Cu1 and Cu1′ were already published

earlier but are given here for comparison. Reference 45. Reference 49.

7,48

Synthesis of the Cu(I) and Ru(II) Complexes. After the

successful synthesis of multidentate phenanthroline ligands

L2−L5, which contain two additional chalcogen donor

moieties, we were interested in their coordination behavior.

Therefore, these ligands were coordinated to diﬀerent

potentially photoactive metal centers (see Figure 2). The

heteroleptic Cu(I) complexes Cu1−Cu5 of the type [(xant)-

applied.

However, all Ru(II) complexes were prepared

by reﬂuxing a mixture of [(tbbpy) RuCl ] and the respective

phenanthroline derivatives L1−L5 in ethanol and water.

Further synthetic details are given in the Supporting

Information (SI). Complexes Ru1−Ru5 were obtained as

orange-to-red solids in 38−86% yield.

After their synthesis, all ligands and complexes were fully

characterized by H, C, and P NMR spectroscopy, high-

resolution mass spectrometry (HRMS), and elemental analysis

Cu(L)]PF (with xant = xantphos) as well as their homoleptic

counterparts Cu1′−Cu5′ [Cu(L) ]PF and the heteroleptic

Ru(II) complexes Ru1−Ru5 with the general formula

(

see the SI ). The expected elemental composition and purity

[

(tbbpy) Ru(L)](PF ) (with tbbpy = 4,4′-di-tert-butyl-2,2′-

6 2

were conﬁrmed by the obtained HRMS and elemental analysis.

Furthermore, the NMR spectra are in accordance with the

proposed structures (see the SI , Chapter 4). Interestingly, the

heteroleptic Cu(I) complexes Cu2−Cu5 with the additional

donor moieties at the phenanthroline ligand showed no

signiﬁcant deviation in the P NMR signal (δ = −13.2 to

13.9 ppm, measured in acetonitrile-d ) compared to complex

Cu1 (δ = −13.4 ppm) without additional donors. This

indicates that there is no variation in the coordination behavior

of the copper center because a change in the type or geometry

bipyridine) were prepared to evaluate the potential of these

phenanthroline-derived ligands.

The heteroleptic Cu(I) complexes Cu1−Cu5 (see Figure 2)

were synthesized following a procedure that was established for

21,30,45

these types of complexes previously.

First, the xantphos

ligand was coordinated to the copper center by reﬂuxing a

−

mixture of [Cu(MeCN) ]PF₆and xantphos in dichloro-

methane for 16 h under inert conditions. Then, the respective

phenanthroline ligands L1−L5, dissolved in dichloromethane,

was added dropwise at 0 °C. The solution was again reﬂuxed

for 4 h, whereby a signiﬁcant yellow coloring occurred. After

cooling to room temperature, the desired complexes were

precipitated by the addition of n-hexane. Complexes Cu1−

Cu5 were received in 37−86% yield.

of coordination would most likely also cause a shift in the

NMR signal. This nonaltered coordination behavior has been

further conﬁrmed by crystal structure analysis and is discussed

below.

In the H NMR spectra of the homoleptic Cu(I) complexes

The homoleptic Cu(I) complexes Cu1′−Cu5′ (see Figure

Cu2′−Cu5′, only one set of signals is present. Therefore, the

interaction between one of the additional donor atoms and the

copper center that is observed in the crystal structure of these

complexes is weak and undergoes a fast exchange at higher

rates than the NMR time scale. Even temperature-dependent

H NMR measurements of Cu2′ in a methanol-d solution,

performed in the range from 303 to 183 K, resulted in no

) were prepared by following a previously reported

5,46

procedure.

For this purpose, the copper precursor

[

Cu(MeCN) ]PF and the respective phenanthroline ligands

L1−L5 were dissolved in dichloromethane and heated to reﬂux

for 3 h. After cooling to room temperature, the desired

complexes were precipitated by the addition of n-hexane.

Complexes Cu1′−Cu5′ were isolated in 43−87% yield. It

should be noted that, for complex Cu3′, a crystal structure

already existed, but no further synthetic details and structural,

obvious shift of the H NMR signals and no occurrence of

additional signals because they would be expected for

nonequivalent protons. Only a broadening of the ligand signals

and a shift of the solvent signal are observed at lower

temperatures (see Figure S4.18).

photophysical, or electrochemical data were provided.

For the synthesis of the heteroleptic Ru(II) complexes

Ru1−Ru5 (see Figure 2), two diﬀerent strategies were

Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry

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In the H NMR spectra of the Ru(II) complexes Ru2−Ru5,

a splitting of the signals belonging to the benzylic protons is

present. Furthermore, for complexes Ru2 and Ru4, also a

splitting of the signals belonging to the isopropylic methyl

groups in the respective H NMR spectra is observed (see

Figures S4.14 and S4.16). This suggests a stereogenic center at

the neighboring donor atoms, which could be formed by the

coordination of both additional donor atoms to the Ru(II)

center. The subsequent crystal structure analysis of Ru3 (OPh)

showed that the diastereotopic environment is created by the

coordination behavior of the ligand L3. L3 coordinates in a

deﬁned way, where the additional donor atoms point away

from the ruthenium center.

Figure 3. Solid-state structures (ORTEP representation) of the

heteroleptic Cu(I) complexes Cu2 (left) and Cu4 (right). Ellipsoids

are drawn at 50% probability. Hydrogen atoms, PF₆counteranions,

Single crystals suitable for X-ray analysis could be obtained

for 8 of the 13 novel complexes (see Tables 1 and 2) by

−

and solvent molecules are omitted for clarity.

Table 2. Selected Crystallographic Bond Lengths (pm) and

Angles (deg) of the Ru(II) Complexes Ru1 and Ru3

copper center, diﬀers clearly from the 90° of an ideal

tetrahedron (see Table 1). However, because of the additional

bulkiness of the OR and SR substituents at the 2 and 9

positions, Cu2 and Cu4 appear less distorted than the

reference complex Cu1 with methyl groups only. A

comparison of further structural parameters of Cu2, Cu4,

and Cu1 (see Table 1) does not show major diﬀerences with

respect to the basic coordination geometry (e.g., N1−Cu−N2

Ru1 (space group

Ru3 (space

P2 /c)

group P1

)

group P1)

Ru−N1

Ru−N2

Ru−N3

Ru−N4

Ru−N5

Ru−N6

Ru−X1

Ru−X2

210.4(5)

210.0(4)

206.8(5)

206.3(5)

203.8(4)

205.9(5)

206(1)

213(1)

214(1)

203(1)

207(1)

204(1)

208.0(9)

483(1)

490(1)

214.6(9)

204(1)

206(1)

207(1)

212.0(9)

485(1)

486(1)

80.53(13)° for Cu1 and 80.97(15)° for Cu2; Cu−N1 =

08.4(3) pm for Cu1 and 208.4(4) pm for Cu2). This

indicates a small impact of the substituents at 1,10-

phenanthroline on the general structure around the Cu(I)

center. Most importantly, in the solid-state structures of the

N1−Ru−N2

N3−Ru−N4

N5−Ru−N6

N1−Ru−N3

N1−Ru−N4

N1−Ru−N5

N1−Ru−N6

N2−Ru−N3

N2−Ru−N4

N2−Ru−N5

N2−Ru−N6

79.4(2)

171.3(2)

79.3(2)

92.9(2)

93.2(2)

101.4(2)

171.5(2)

88.3(2)

98.8(2)

177.3(2)

100.3(2)

80.1(4)

79.0(5)

78.6(5)

95.2(4)

173.4(5)

87.3(5)

102.1(5)

88.9(4)

102.5(5)

94.5(5)

172.6(5)

78.9(5)

78.2(4)

78.0(5)

97.4(5)

175.4(5)

88.6(5)

102.5(5)

92.4(5)

102.4(5)

93.1(5)

170.9(5)

heteroleptic Cu(I) complexes Cu2 (O Pr) and Cu4 (S Pr), the

additional donor functions point away from the copper center

(

see Figure 3), resulting in Cu−X distances of ≥4.5 Å. This

clearly shows that there is no interaction between the Cu(I)

center and additional donor atoms in the heteroleptic

complexes. Consequently, no change in the P NMR chemical

shift was observed when the additional donor functions were

introduced.

Crystal Structure Analysis of the Homoleptic Copper

Complexes Cu2′−Cu5′. All homoleptic Cu(I) complexes

Cu2′−Cu5′ could be examined by X-ray crystallography (see

Figure 4), and they show again the typical distorted tetrahedral

The values in parentheses represent the experimental estimated

standard deviation of the measurement. Ru3 crystallized in a lattice

with two twinned units of the respective complex. The two units are

referred as Ru3 and Ru3 .

,22,49

structure.

In addition, the reference complex Cu1′

exhibits the smallest angle between the two ligand planes

(

79.4°) compared to the substituted derivatives Cu2′−Cu5′

layering a dichloromethane solution of the respective complex

with ethanol and n-hexane. As a result, structural data were

received for the heteroleptic Cu(I) complexes Cu2 and Cu4,

the homoleptic Cu(I) complexes Cu2′−Cu5′, and the Ru(II)

complexes Ru1 and Ru3 and are discussed below. Interest-

ingly, almost all Cu(I) compounds crystallize in a triclinic

crystal system with the space group P1 (see Table 1).

Crystal Structure Analysis of the Heteroleptic Copper

Complexes. Both Cu(I) complexes Cu2 and Cu4 (see Figure

(82.3−88.7°; see Table 1). Hence, Cu1′ features the largest

distortion from the tetrahedral coordination sphere, whereas

the additional substituents in Cu2′−Cu5′ reduce the extent of

the ﬂattening. The diﬀerent types of substituents (O Pr, OPh,

S Pr, and SPh) have almost no inﬂuence on the Cu−N

distances (e.g., Cu−N1 = 203.6(3) pm for Cu3′, 205.3(3) pm

for Cu4′, and 204(1) pm for Cu5′) and the bite or tetrahedral

angles, which are in the same range for all complexes (see

Table 1). Special attention was then paid to the Cu−X

) possess a distorted tetrahedral ground-state structure, which

distances (XO Pr, OPh, S Pr, or SPh). As a main diﬀerence

to the heteroleptic Cu(I) complexes, all homoleptic com-

pounds Cu2′−Cu5′ possess one Cu−X distance, which is

signiﬁcantly shortened. For instance, in Cu3′, the Cu−O1

distance is 328.9(2) pm, whereas Cu−O3 is 457.3(2) pm. This

indicates a weak interaction between the copper center and

one of the additional donor atoms. It should be noted that the

crystal structure of complex Cu2′ has a higher symmetry than

24,30,45

is commonly observed for this class of compounds.

This

is mainly caused by the bulky and rigid xantphos ligand and the

large diﬀerence in the bite angles of the diimine and

diphosphine [e.g., for Cu2, N1−Cu−N2 = 80.97(15)° and

P1−Cu−P2 = 120.43(5)°]. With 76.9° (Cu1), 85.9° (Cu2),

and 87.8° (Cu4), the angle between the two ligand planes,

which are spanned through the chelating heteroatoms and

Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry

Ru1 −Ru5 (see Figure 6 ).

Forum Article

50−52

compounds.

Deviations from an ideal octahedron are

caused by the bite angles of the three diimine ligands that are

all smaller than 90°. The Ru−N bond lengths are between

03(1) and 214.6(9) pm. The N−Ru−N bite angles at the

phenanthroline ligand are between 78.9(5)° and 80.1(4)°. The

N−Ru−N angles, with one of the nitrogen atoms being part of

the phenanthroline ligand, are between 87.3(5)° and

02.5(5)° for neighboring nitrogen atoms and between

70.9(5) and 177.3(2) for opposing nitrogen atoms.

Crystal structure analysis shows that there is no coordination

of the additional donor atoms to the ruthenium center,

although it has been indicated by the H NMR spectra. The

additional donor atoms do all point away from the ruthenium

center (see Figure 5). As evidenced by the NMR data, the

benzylic protons do have distinct diastereotopic environments,

but they are not caused by the coordination of an additional

donor to the ruthenium center. To minimize steric repulsion,

the additional donor atoms point away from the metal center

and are in plane with the phenanthroline backbone. This leads

to one of the benzylic hydrogen atoms being above the

phenanthroline plane, while the other one is below. The helical

arrangement of the tbbpy ligands of the Ru(tbbpy) fragment

causes the two benzylic protons to have nonequivalent

chemical surroundings (see Figure S4.15).

Photophysical Studies. All complexes exhibit pronounced

absorption bands in the UV region at 280 nm [for the Cu(I)

Figure 4. Solid-state structures (ORTEP representation) of the

homoleptic Cu(I) complexes Cu2′ (top left), Cu3′ (top right), Cu4′

(

bottom left), and Cu5′ (bottom right). Ellipsoids are drawn at 50%

−

probability. Hydrogen atoms, PF counteranions, and solvent

complexes] and 300 nm [for the Ru(II) complexes], which can

molecules are omitted for clarity.

1,23

be assigned to ligand-centered (LC) transitions.

The

absorption bands attributed to the MLCT transitions are

generally broader and appear between 350 and 400 nm for

Cu1−Cu5 or between 400 and 500 nm for Cu1′−Cu5′ and

the other homoleptic Cu(I) complexes, which leads to two

shortened Cu−X distances instead of only one. For complexes

Cu2′, Cu4′, and Cu5′, it was further observed that the

shortened Cu−X distance is accompanied by an elongation of

23,29

the respective H C−X bond (see Table S3) because this is the

longest H C−X distance within the complex [e.g., for Cu2′,

42.3(3) pm compared to a distance of 141.5(2) pm, and for

Cu4′, 182.4(6) pm compared to distances of 180.1(6)−

82.0(6) pm].

Crystal Structure Analysis of the Heteroleptic Ruthenium

Complexes. Crystals suitable for X-ray analysis were obtained

for the Ru(II) complexes Ru1 and Ru3. It should be noted

that complex Ru3 crystallized in a lattice with two twinned

units, which are referred to as Ru3 and Ru3 (see Table 2).

Structural analysis of Ru1 and Ru3 (see Figure 5) revealed a

conventional octahedral coordination environment around the

ruthenium center, which is common for these Ru(II)

Figure 6. UV/vis absorption spectra (attenuation coeﬃcient) of Cu2

(dashed), Cu2′ (dotted), and Ru2 (solid) in acetonitrile. Cu2, Cu2′,

and Ru2 were selected as characteristic representatives of their

substance class (for the other UV/vis spectra, see the SI, Chapter 6).

The inset enlarges the MLCT region.

The maxima of the MLCT absorption of the heteroleptic

Cu(I) complexes Cu2−Cu5 are localized around 380 nm with

a slight bathochromic shift for the thioether-functionalized

complexes Cu4 (S Pr, 384 nm) and Cu5 (SPh, 382 nm; see

Table 3). The intensities of these MLCT bands are lower

compared to that of the reference Cu1; e.g., the attenuation

coeﬃcient of Cu4 is halved. Related heteroleptic Cu(I)

complexes containing 6-alkylthio- or 6-phenylthio-substituted

,2′-bipyridine ligands show similar behavior. The same

trend is observed for the homoleptic counterparts Cu2′−Cu5′.

Again the attenuation coeﬃcients are somewhat decreased

compared to the reference complex Cu1′ with methyl

substituents only. The absorption bands of the thioether-

Figure 5. Solid-state structures (ORTEP representation) of the

Ru(II) complexes Ru1 and Ru3. Ellipsoids are drawn at 50%

probability. Hydrogen atoms, PF₆⁻counteranions, and solvent

molecules are omitted for clarity.

Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry

Forum Article

Table 3. Summary of the Photophysical and Electrochemical Properties of the Diﬀerent Complexes (in Acetonitrile) and

Ligands (in Dichloromethane for Solubility Reasons)

−1

compound

λ_m_a_x_,_a_b_s, nm (ε, ×10 M cm⁻¹

)

λ_m_a_x_,_e_m, nm (τ, ns)

Φ, %

E₁_/₂_,_r_e_d, V

E₁_/₂_,_o_x, V

Cu1

Cu2

Cu3

Cu4

Cu5

Cu1′

Cu2′

Cu3′

Cu4′

Cu5′

Ru1

Ru2

Ru3

Ru4

Ru5

379 (3.12)

378 (2.39)

380 (1.94)

384 (1.43)

382 (1.94)

551 (64 )

−2.10

−2.06

−1.98

0.40

540 (15)

539 (10)

535 (14)

535 (13)

0.23

0.33, 0.95

1.09

−2.46, −2.06

0.96, 1.22

−1.84

−2.11

0.13, 0.27, 0.92

455 (7.95)

447 (8.1)

0.29

0.12

0.26

−2.26, −2.04

449 (6.77)

459 (5.28)

459 (7.35)

452 (12.86)

448 (14.75)

440 (15.53)

449 (14.58)

445 (13.59)

273 (58.48)

277 (29.1)

274 (43.49)

277 (20.63)

276 (21.1)

−2.38, −2.01

−2.28, −2.05

0.17, 1.04

−2.35, −1.87

0.19

0.80

0.84

0.97

0.90

0.87

611 (753)

613 (836)

612 (934)

613 (1029)

612 (888)

366

365

295

365

0.09

0.15

0.27

0.40

0.09

−2.26, −2.02, −1.80

−2.24, −2.00, −1.78

−2.16, −1.92, −1.73

−2.40, −2.12, −1.84

−2.40, −2.09, −1.87

Taken from ref 23. Taken from ref 55. Irreversible. Taken from ref 28.

functionalized Cu4′ and Cu5′, both located at 459 nm, are

slightly bathochromically shifted compared to Cu1′ (λ

max

55 nm). In contrast, complexes Cu2′ and Cu3′ with ether

−1

substituents are ∼340 cm hypsochromically shifted (see

Table 3). As a major diﬀerence, all Ru(II) complexes with

additional donor atoms (Ru2−Ru5) absorb stronger than the

reference complex Ru1, and the respective MLCT bands are all

slightly blue-shifted compared to Ru1. This means that the

introduction of additional donor moieties has contrasting

eﬀects on the absorption behavior of the Cu(I) and Ru(II)

complexes. The additional substituents at the 2 and 9 positions

in the phenanthroline seem to lower the absorbance of the

Cu(I) complexes but increase the attenuation coeﬃcient of the

MLCT bands of the Ru(II) complexes.

Figure 7. Normalized emission spectra of Cu1 (black, dashed), Cu2

(red, dashed), Cu3 (green, dashed), Cu4 (blue, dashed), and Cu5

(magenta, dashed) and Ru1 (black, solid), Ru2 (red, solid), Ru3

(green, solid), Ru4 (blue, solid), and Ru5 (magenta, solid) in

The emission behavior was examined in an acetonitrile

solution under inert conditions. Under these conditions, all

homoleptic Cu(I) complexes Cu1′−Cu5′ are not emissive. In

contrast, the heteroleptic Cu(I) complexes Cu1−Cu5 emit

weakly in an acetonitrile solution with emission maxima

between 518 and 551 nm (see Figure 7 and Table 3 ) and

between 508 and 533 nm in the solid state (Figure S6.7 and

Table S5 ). In solution, the emission of the functionalized

acetonitrile under inert conditions. All complexes are excited at their

respective MLCT wavelength.

lifetime is, with ∼1 μs, much longer compared to those of

Cu1−Cu5 but similar to those of structurally related Ru(II)

complexes containing diﬀerent phenanthroline derivatives

(

Table 3).

Electrochemical Studies. The electrochemical properties

−

Cu(I) complexes Cu2−Cu5 is shifted by ∼623 cm to higher

energies compared to Cu1 (see Figure 7). In the solid state, a

similar trend is observed, except for Cu5 (Figure S6.7 ).

of all complexes were studied by cyclic voltammetry, while

Cu2 and Cu2′ (both O Pr) were chosen as representatives of

Complex Cu2 (O Pr) has the strongest emission with an

their substance class (for all other cyclic voltammograms, see

the SI , Chapter 8). The cyclic voltammograms of the Cu(I)

complexes Cu2 and Cu2′ possess a reversible reduction event

emission quantum yield of 0.23%. Furthermore, the emission

lifetime of these complexes is short with τ ∼ 15 ns in solution

(

see Table 3). The emission behavior of the Ru(II) complexes

at about −2.05 V versus ferrocene/ferricenium (Fc/Fc ) in

Ru1−Ru5 is clearly diﬀerent compared to the Cu(I)

complexes. Ru1−Ru5 emit in an acetonitrile solution with

emission maxima at around 612 nm. Interestingly, no impact of

the diﬀerent substituents is obvious (see Figure 7), and the

acetonitrile (see Figure 8, bottom). This reduction is merely

shifted by 50 mV to higher potentials compared to the

reference Cu1, indicating only a weak inﬂuence of the O Pr

substituents. In the case of the homoleptic complex Cu2′, a

results are comparable to those of [(bpy) Ru(L1)] (with bpy

second reduction at −2.26 V occurs.

2,2′-bipyridine). The emission quantum yields of Ru2−

The thioether (SR, in Cu4, Cu5, Cu4′, and Cu5′) and ether

phenyl (OPh, in Cu3 and Cu3′) functionalities lead to an

irreversible reduction event. It seems that these ligands are not

Ru5 are quite low (0.09−0.40%) and on the same order of

magnitude as the copper complex Cu2. Instead, the emission

Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry

Forum Article

observed. At the same time, the continuous buildup of a new

band at around 450 nm (Figures S9.1 and S9.2) is a strong hint

for the formation of homoleptic counterparts, which is

characteristic for these types of heteroleptic Cu(I) com-

28,32−34

plexes.

In contrast, upon use of a 360 nm long pass

ﬁlter (to prevent UV light) and application of a reduced light

intensity, complexes Cu1 and Cu2 are stable over 2 h (Figures

S9.5 and S9.6). Solely, Cu3 still decomposes to a small extent

(

Figure S9.7). Most importantly, the homoleptic complexes

Cu1 ′−Cu3 ′ are fully stable under these conditions (Figures

S9.8 −S9.10).

As is evident from electrochemistry, Cu2 (O Pr) is the only

heteroleptic Cu(I) complex with additional donor moieties

that showed a reversible reduction. All other copper complexes

of this type possess an irreversible behavior under reductive

conditions. Therefore, the possible decomposition mechanism

and products were investigated in detail to derive new design

principles for the future. According to Scheme 2 we assumed

Figure 8. Cyclic voltammograms in an acetonitrile solution with 0.1

Scheme 2. Proposed Decomposition Mechanism under

Reductive Conditions

M Bu NPF as the supporting electrolyte referenced versus a Fc/Fc

−

couple with a scan rate of 100 mV s (further scan rates are depicted

in the SI, Chapter 8). The arrows illustrate the initial scan direction.

Top: Ru(II) complexes Ru1 (black, solid), Ru2 (red, solid), Ru3

(

green, solid), Ru4 (blue, solid), and Ru5 (magenta, solid). Bottom:

Cu(I) complexes Cu2 (red, dotted) and Cu2′ (red, dashed).

able to stabilize the negative charge and that a separation of the

respective thioether or ether unit occurs (Figures S8.1 and

S8.2 ). For all homoleptic copper complexes Cu1′−Cu5′, the

oxidation at around +0.20 V versus Fc/Fc in acetonitrile

that, under reductive conditions (either electro- or photo-

chemical), an electron is transferred to the additional donor

functionality (X = OR or SR), leading to cleavage of X . This

Figures S8.3 and S8.5) is quasi-reversible, except for Cu3′

(

OPh), and can be attributed to oxidation of the copper center

−

from Cu(I) to Cu(II). Moreover, the mild oxidation potentials

hypothesis was then conﬁrmed by gas chromatography, where

the respective decomposition products were found after

protonation of the liberated anions with acetic acid. For

instance, in the presence of visible light and triethylamine,

acting as a reductive quenching agent, phenol (HOPh) was

detected as a decomposition product for complex Cu3 (with

OPh). Moreover, the same result was obtained using bulk

of ∼0.20 V are rather unusual for such homoleptic

,22

bis(diimine)copper(I) complexes.

This observation could

be explained by a pentacoordinated copper center, which is

further supported by the stronger peak separation at higher

56−58

scan rates (Figures S8.4 −S8.7 ).

The redox behavior of the ruthenium complexes Ru1 and

Ru2 is fully reversible (Figure 8). They possess one reversible

oxidation and three reversible reductions. The last two

reduction events of these Ru(II) complexes can be assigned

electrolysis (−1.98 V vs Fc/Fc ; see the SI, Chapter 10) to

create reductive conditions. No decomposition was observed

to the reduction of the two tbbpy ligands and the ﬁrst

for Cu2 containing the O Pr substituents (Table S6). Hence,

reduction of the phenanthroline ligand. The introduction of

the OPh, S Pr, or SPh moieties are not able to stabilize an

the additional O Pr donor moiety in Ru2 causes only a slight

electron at the phenanthroline ligand. The results of the

decomposition experiments suggest that the additional donor

shift of 20−40 mV compared to Ru1 for the ﬁrst reduction as

well as the oxidation (Table 3). Furthermore, the oxidations of

all Ru(II) complexes are reversible, except of Ru3. In this case

the OPh substituents lead to a more pronounced shift of 170

mV. Further, the ﬁrst reduction processes of Ru3-Ru5 are

considerably irreversible and unaﬀected by the scan rate

−

function X is cleaved as the respective anion X . This explains

the signiﬁcantly increased stability of Cu2 and Cu2′ (both

O Pr) compared to the other complexes. The isopropoxide

anion is much less stable than the other leaving groups, as is

evident from the pK values of the corresponding protonated

9−61

Figure S8.12 −S8.14), which is in line with the results

compounds, and is therefore the worst leaving group.

obtained for the related Cu(I) complexes.

CONCLUSIONS

Photostability and Decomposition Studies. The

■

irreversible electrochemical behavior of the Cu(I) complexes

A new concept to stabilize traditional Cu(I)-based photo-

sensitizers was tested. For this purpose, a systematic series of

four 1,10-phenanthroline ligands with two additional group 16

donor functionalities (with X = OR or SR) were developed.

For the ﬁrst time, these ligands were then coordinated to either

the Cu(I) or Ru(II) centers for comparison. In total, 15

complexes, of which 13 are not yet presented in the literature,

were prepared. Furthermore, 8 crystal structures could be

obtained. X-ray analysis revealed that all homoleptic Cu(I)

containing OPh, S Pr, or SPh substituents gave a ﬁrst

indication that these compounds possess only a limited

stability. Therefore, the photostabilities of representative

samples were examined by irradiation of an acetonitrile

solution with a xenon lamp (150 W without ﬁlter) and

continuous monitoring of the absorption spectra (see the SI,

Chapter 9). As a result, stepwise decomposition of the

heteroleptic Cu(I) complexes Cu1 and Cu2 could be

Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry

Complete contact information is available at:

Forum Article

complexes Cu2′−Cu5′ possess one Cu−X distance, which is

signiﬁcantly shortened. Therefore, a pentacoordinated copper

center, corresponding to a (4 + 1)-fold coordination mode,

seems possible.

Authors

Martin Rentschler − Institute of Organic Chemistry, University

of Stuttgart, 70569 Stuttgart, Germany

Marie-Ann Schmid − Institute of Inorganic Chemistry I, Ulm

University, 89081 Ulm, Germany

Wolfgang Frey − Institute of Organic Chemistry, University of

Stuttgart, 70569 Stuttgart, Germany

The introduction of additional substituents (i.e., O Pr, OPh,

S Pr, or SPh) to the phenanthroline had only a slight impact on

the absorption maxima. They lowered the absorbance of the

Cu(I) complexes but increased the attenuation coeﬃcients of

the related Ru(II) complexes. The homoleptic Cu(I)

complexes Cu1′−Cu5′ do not show emission in solution,

while the heteroleptic Cu(I) complexes Cu1−Cu5 exhibit a

weak emission with excited-state lifetimes of about 15 ns.

Further, the Ru(II) complexes Ru2−Ru5 are clearly emissive

with emission lifetimes of ∼1 μs.

https://pubs.acs.org/10.1021/acs.inorgchem.9b03687

Author Contributions

†

These authors contributed equally to this work.

Notes

The Cu(I) complexes containing thioether (S Pr or SPh) or

The authors declare no competing ﬁnancial interest.

ether aryl (OPh) substituents are not stable under reductive

conditions. Only complexes Cu2 and Cu2′ with O Pr have a

ACKNOWLEDGMENTS

■

reversible reduction at about −2.05 V versus Fc/Fc . All

This work was supported by the Baden-Wu

rttemberg

Ru(II) complexes show one reversible oxidation and two

reversible reduction events. The limited electrochemical

stability of the Cu(I) compounds is in line with a reduced

photostability compared to that of the reference complex with

no additional donor atoms. Furthermore, under reductive

conditions (either by bulk electrolysis or irradiation in the

presence of an electron donor like triethylamine), stepwise

decomposition of the phenanthroline ligand (i.e., cleavage of

the OR or SR substituents) takes place.

Foundation (BW-Stiftung, Germany) and by the Deutsche

Forschungs-Gemeinschaft (DFG) within the Priority Program

SPP 2102 “Light-controlled reactivity of metal complexes” (TS

30/4-1 and KA 4671/2-1) and the DFG project TS 330/3-1.

Special thanks goes to Jannik Bruckmann (Ulm) for providing

the Ru(II) precursor and to Fiona Schneider (Stuttgart) for

preliminary photostability measurements.

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■

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