Homoleptic and heteroleptic copper(I) complexes bearing diimine-diphosphine ligands

Chapter 8

Homoleptic and heteroleptic copper(I) complexes bearing diimine-diphosphine ligands Ganesan Mani and Vasudevan Subramaniyan Department of Chemistry, Indian Institute of Technology—Kharagpur, Kharagpur, India

Abbreviations BArF4 bpmtzH bcp binap bphen bpy BF42 CuCl CuPS C6H4Cl CH3NO2 CH3COCH3 CH3 CF3 CH3CN ClO42 dbp dfdppe dipp DMF dmp dpep dpp dppf dppb dppe dppm

tetrakis(3,5-bis(trifluoromethyl)phenyl)borate 5-tbutyl-3-(20 -pyrimidinyl)-1H-1,2,4-triazole 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline 2,20 -bis(diphenylphosphino)-1,10 -binaphthyl 4,7-diphenyl-1,10-phenanthroline 2,20 -bipyridine tetrafluoroborate anion copper(I) chloride copper photosensitizer 4-chlorophenyl nitromethane acetone methyl trifluoromethyl acetonitrile perchlorate anion 2,9-dibutyl-1,10-phenanthroline 1,2-bis[bis(pentafluorophenyl)phosphino]ethane 2,9-diisopropyl-1,10-phenanthroline dimethylformamide 2,9-dimethyl-1,10-phenanthroline 2,9-bis(2-phenylethyl)-1,10-phenanthroline 2,9-diphenyl-1,10-phenanthroline 1,10 -bis(diphenylphosphino)ferrocene 1,4-bis(diphenylphosphino)butane 1,2-bis(diphenylphosphino)ethane 1,10 -bis(diphenylphosphino)methane

Copper(I) Chemistry of Phosphines, Functionalized Phosphines and Phosphorus Heterocycles. DOI: https://doi.org/10.1016/B978-0-12-815052-8.00008-7 © 2019 Elsevier Inc. All rights reserved.

237

238

Phosphines, Functionalized Phosphines, and Phosphorus Heterocycles

dppp DPEphos DBFphos DPPMB dsbdpp dtbp Et3N EtOH fiqro fpho fpmtzH fpyro HI HOMO iBu iPr Im-CF3 Im-COOEt IndN ISC KSV LECs LITR-XAS LUMO M37 Me MeOH MLCT NaBF4 NCDP NO22 nBu nHex OLEDs pftpb PF62 Ph phen PNNP PPh2(C6H4OMe-p) PPh(C6H4OMe-p)2 P(C6H4OMe-p)3 PSs

1,3-bis(diphenylphosphino)propane bis[2-(diphenylphosphino)phenyl]ether 4,6-bis(diphenylphosphino)dibenzofuran bis[(diphenylphosphino)methyl]diphenylborate 2,9-di-sec-butyl-4,7-diphenyl-1,10-phenanthroline 2,9-di-tert-butyl-1,10-phenanthroline triethylamine ethanol 2-[3,5-bis(trifluoromethyl)-1H-pyrrol-2-yl]quinoline 2-[3,5-bis(trifluoromethyl)-1H-pyrrol-2-yl]-6-phenylpyridine 5-trifluoromethyl-3-(20 -pyrimidinyl)-1H-1,2,4-triazole 2-[3,5-bis(trifluoromethyl)-1H-pyrrol-2-yl]pyridine hydriodic acid highest occupied molecular orbital iso-butyl iso-propyl 2-(4-pyridinyl)-N-[3-(trifluoromethyl)phenyl]-4-[[3(trifluoromethyl)phenyl]imino]-4H-imidazol-5-amine 4-[[5-[[4-(ethoxycarbonyl)phenyl]amino]-2-phenyl-4H-imidazol-4ylidene]amino]ethyl benzoate N-(1H-indol-2-ylmethylene)benzenamine intersystem crossing SternVolmer constant light-emitting electrochemical cells light-initiated time-resolved X-ray absorption spectroscopy lowest unoccupied molecular orbital 8,11,14,17,20,23,26-heptaoxa-4(2,9)-phenanthrolina-1,7(1,4) dibenzenacyclo-hexacosaphane methyl methanol metal-to-ligand charge-transfer sodium tetrafluoroborate 1,2-bis(diphenylphosphino)carborane (nido-carboranediphosphine) nitrate anion n-butyl n-hexyl organic light-emitting diodes tetrakis(pentfluorophenyl)borate hexafluorophosphate anion phenyl phenanthroline 1,9-bis(diphenylphosphinomethyl)diphenyldipyrrolylmethane diphenyl-(4-methoxyphenyl)phosphine bis-(4-methoxyphenyl)phenylphosphine tris-(4-methoxyphenyl)phosphine photosensitizers

Homoleptic and heteroleptic copper(I) complexes Chapter | 8 PPh3 PyN PyrTet ppy pz2BH2 pz2BPh2 pz4B qbm sBu SR tBu tfpb THF TON WRC Xantphos 6dmbpy

239

triphenylphosphine N-(1H-pyrrol-2-ylmethylene)benzenamine 2-(2H-tetrazol-5-yl)pyridine 2-phenylpyridine bis(pyrazol-1-yl)borohydrate diphenylbis(1H-pyrazolato)borate tetrakis(1H-pyrazolato)borate 2-(1H-benzimidazol-2-yl)quinoline sec-butyl sacrificial reductant tert-butyl tetrakis[3,5-bis(trifluoromethyl)phenyl]borate tetrahydrofuran turnover number water reduction catalyst 4,5-bis(diphenylphosphino)-9,9-dimethylxanthene 6,60 -dimethyl-2,20 -bipyridine

8.1 Introduction Copper(I) complexes have attracted attention in the past and continue to be an active area of research because of their diverse structural, photophysical, and catalysis properties [1]. Copper(I) has a symmetric electronic charge distribution with d10 configuration. This low valent metal atom is a soft Lewis acid, electron-rich, and stabilized by ligands having π-acceptor property. Hence, a variety of phosphine ligands have been developed to support twoto five-coordinate [2] and multinuclear framework copper(I) complexes such as rhombohedra, cubane-like [3], stairstep [4], octahedral [5], butterfly [6], hourglass [7], and other shapes [8]. Among these, four-coordinate copper(I) centers are very common and the tetrahedral geometry is preferred because of the minimized interligand repulsions. Copper(I) complexes are diamagnetic and often colorless. However, some copper(I) complexes are colored because of the anion or charge-transfer bands [9]. In particular, metal-toligand charge-transfer (MLCT) occurs in copper(I) complexes containing 2,20 -bipyridine (bpy) or 1,10-phenanthroline (phen) and phosphine ligands to give an excellent luminescence character. Their MLCT excited states are relatively long-lived and give intense photoluminescence peaks with high quantum yields. These impressive photophysical properties have been found to be associated with homoleptic copper(I) complexes containing only chelating diimine ligands. In addition, it has been found that their luminescent properties are highly tunable with the chemical nature, size, and position of substituents on imine ligands. In the course of study, a dramatic improvement in photophysical property was found with heteroleptic copper(I) complexes containing both diimine and chelating wide bite-angle diphosphine ligands. This system offered a highly useful option of tuning the absorption and

240

Phosphines, Functionalized Phosphines, and Phosphorus Heterocycles

emission maxima with an improvement in lifetime by merely changing the nature of substituent on both ligands. The copper metal is relatively more Earth-abundant and cheaper. Usually photoactive copper(I) complexes are relatively easy to synthesize. Hence, improving the photophysical property of copper(I) complexes has been a major area of research in the past three decades [10]. Eventually, copper(I) complexes have become an alternative to the widely studied and expensive Ir(III) and Ru(II) complexes and have been employed in applications such as solar energy conversion [11], organic light-emitting diodes (OLEDs) [12], light-emitting electrochemical cells (LECs) [13], photosensitizers (PSs) in hydrogen evolution reactions [14], sensors for oxygen [15], and photoredox catalysts in various organic reactions [16].

8.2 Photophysical property of copper(I) complexes In 1978, McMillin et al. reported the first heteroleptic copper(I)phosphine complex and studied the photophysical properties [17]. While the homoleptic complex [Cu(dmp)2]BF4 exhibits a very weak luminescence, the heteroleptic complex, [Cu(dmp)(PPh3)2]X (X 5 ClO4, BF4, PF6, and BPh4) synthesized by the substitution of one of the imine ligands by two monodendate phosphine ligands possesses an intense luminescence in methanol with the lifetime of 330 ns and quantum yield of 0.0014 [18,19]. Interestingly, the introduction of chelating bisphosphine ligand showed further improvement in the photoluminescence character of this type of copper(I) complexes [20]. The lifetimes of [Cu(dmp)(DPEphos)]BF4 in methanol and dichloromethane are 2.4 and 14.3 μs, respectively. It was further improved to 5.4 μs in methanol or 16.1 μs in dichloromethane by using a bulky diimine ligand, 2,9-di-nbutyl-1,10-phenanthroline (dbp), for example [Cu(dbp)(DPEphos)]BF4. Furthermore, the quantum yield of [Cu(dmp)(DPEphos)]BF4 is about 100 times greater (0.15) than that of the heteroleptic complex [Cu(phen) (DPEphos)]BF4 (0.0018) containing the sterically less hindered diimine ligand, phen (see Table 8.1). The effect of phosphine ligands and substituents on the periphery of phenantharoline rings is explained using the structural distortion taking place upon photoirradiation. When a simple homoleptic complex [Cu(N͡ N)2]PF6 absorbs light, two kinds of absorption occur. One is in the UV region which is due to the diimine π-electrons and is designated as ligand-centered ππ transition and the other is in the visible region due to the transition of metalcentered d-electron to the singlet excited state, designated as 1MLCT state. The formation of the MLCT state is accompanied by a change in the formal oxidation of the copper atom from 1 I (d10) to 1 II (d9) state. This transient copper(II) species has a tendency to form a square planar geometry, that is, the flattened structure via pseudo-JahnTeller distortion, as shown in Fig. 8.1. The resulting flattened structure lies lower in energy, as compared

Homoleptic and heteroleptic copper(I) complexes Chapter | 8

241

TABLE 8.1 The absorption and emission data of a few heteroleptic copper (I) complexes relative to homoleptic complexes. Complex

λabs (nm)

ε (M21/cm)

λem (nm)

[Cu(phen)2]PF6

458

Very weakly emissive

τ (μs)

Φ

References

[Cu(phen) (PPh3)2]BF4

a

370

3900

680a

0.22

0.0007

[19]

[Cu(phen) (DPEphos)]BF4

391b

3000

700b

0.19

0.0018

[21]

[Cu(dmp)2]PF6

454c

8430

710d





[17]

[Cu(dmp)(PPh3)2] BF4

a

365

2500

560

0.33

0.0014

[18]

[Cu(dmp)(dppe)] PF6

400b

3200

630b

1.33

0.010

[21]

[Cu(dmp) (DPEphos)]BF4

383b

3100

565b

14.3

0.15

[21]

[Cu(dmp)(binap)] PF6

393b

5200

613b

0.6

0.001

[22]

[Cu(dbp) (DPEphos)]BF4

378b

2900

560b

16.1

0.16

[21]

[Cu(dmp)(PPh3)2] BF4

364c

2600

532c

0.70

0.0036

[23]

[Cu(dmp) (PPh2(C6H4OMep))]ClO4

366c

2600

538c

1.40

0.0073

[23]

[Cu(dmp)(PPh (C6H4OMe-p)2)] ClO4

368c

2900

543c

2.70

0.0162

[23]

[Cu(dmp)(P (C6H4OMe-p)3)] ClO4

373c

2700

546c

5.5

0.0309

[23]

a

In MeOH. In CH2Cl2. c In EtOH. d In EtOH at 77K. b

to the pseudo tetrahedral 1MLCT state and hence, the flattening decreases the energy gap. According to the energy gap law, the increase in the lifetime is approximately an exponential function of energy of the excited state. Therefore, the flattened structure tends to return back to the ground state by

242

Phosphines, Functionalized Phosphines, and Phosphorus Heterocycles

FIGURE 8.1 Schematic energy level diagram for homoleptic bis(imine) copper(I) complexes upon photoirradiation and excitation into MLCT absorption bands. fl and ph (radiative decays) are shown as dashed lines and the solid lines refer to nonradiative decays. FC, FranckCondon; fl, fluorescence; IC, internal conversion; ISC, intersystem crossing; MLCT, metal-to-ligand charge-transfer; ph, phosphorescence; PJT, pseudo-JahnTeller distortion.

the nonradiative decay pathway. This means if a complex follows this type of energy profile, it becomes a weak or nonluminescence material, for example, [Cu(phen)2]PF6. Apart from this, quenching of the excited state can also happen via the formation of another complex at the excited state, called exciplex formation. The transient Lewis acidic copper(II) atom undergoes attack by Lewis bases which are often the donor solvents such as MeOH, dimethylformamide, CH3CN, or tetrahydrofuran (THF) used for experiment and also by counterions, forming a pentacoordinated copper(II) species which prefers a flattened geometry and is a poor luminescent material or does not exhibit fluorescence because of the drastically decreased lifetime and the energy gap. The exciplex formation quenching mechanism has been supported by light-initiated time-resolved X-ray absorption spectroscopy [2426]. This pumpprobe method provides information about the transiently formed copper(II) state and its structure following photoexcitation via an ultrafast laser source. For example, this technique has successfully been applied to study [Cu(dmp)2]PF6 complex and clearly confirmed the formation of the pentacoordinated copper(II) atom in both toluene and acetonitrile solvents [27].

Homoleptic and heteroleptic copper(I) complexes Chapter | 8

243

This unfavorable flattening mechanism of quenching is controlled by creating a strong steric congestion at the copper atom coordination sphere. This has initially been carried out using phen ligand containing alkyl groups at 2,9-positions for homoleptic complexes and then with heteroleptic complexes containing two monodendate phosphine ligands. Later, suppression of quenching has been done with the bidentate wide bite-angle phosphine ligand DPEphos, which supports the tetrahedral geometry at the copper(I) ion and suppresses ligand dissociation and solvent attack considerably [21]. In addition, it was also controlled by adjusting the electronic properties of coordinated ligands. For example, as given in Table 8.1, complex [Cu(dmp) (DPEphos)]BF4 or [Cu(dbp)(DPEphos)]BF4 possesses a relatively high quantum yield and long lifetime. Heteroleptic copper(I) complexes display both absorption and emission maxima at shorter wavelengths relative to their corresponding homoleptic copper(I) complexes, as shown in Table 8.1. This blue shift is due to an increased gap between highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels caused by π-acceptor ability of diphosphine ligands from the electronrich copper(I) atom which brings down the HOMO level relative to the HOMO level of homoleptic complex. The emission property of heteroleptic complexes is not only affected by steric hindrance, but also by the electronic nature of the group attached to both phosphine and imine ligands. This is demonstrated via the absorption and emission maxima of a series of heteroleptic complexes [Cu(dmp)L2] ClO4 (L 5 Ph3P, PPh2(C6H4OMe-p), PPh(C6H4OMe-p)2, and P(C6H4OMep)3). These complexes show a slight shift toward longer wavelengths as the electron-donating power of the phosphine ligand increases in the order, Ph3P , PPh2(C6H4OMe-p) , PPh(C6H4OMe-p)2 , P(C6H4OMe-p)3 (see Table 8.1 for values) [23]. Following this pioneering work, several sterically and electronic different diimines and phosphine ligands coordinated heteroleptic complexes and their photophysical properties have been studied. A few diphosphine and diimine ligands are given in Figs. 8.2 and 8.3.

8.3 Mononuclear heteroleptic copper(I) complexes with phenanthroline ligands The majority of heteroleptic copper(I) complexes are mononuclear, and have been commonly synthesized by successive addition of phosphines followed by diimine ligands. However, the thermodynamic stability of heteroleptic complexes depends on the nature, size, and positions of the substituents present on both ligands. In several cases, homoleptic complexes are more stable and formed almost exclusively. To understand the factors determining the stability of complexes, Nicot, Armaroli, Nierengarten, et al. have recently carried out a systematic study. A series of diphosphine and diimine ligands have been employed to demonstrate the stability of heteroleptic versus

244

Phosphines, Functionalized Phosphines, and Phosphorus Heterocycles

Mononuclear heteroleptic copper(I) complexes with phenanthroline ligands Ph 2 P

PPh 2

PPh 2

PPh 2

PPh 2

PPh 2

PPh 2

O

O

dppm

dppe

PPh 2

dppp

dppb

PPh 2

PPh 2

PPh 2

DPEphos

2

Xantphos

C C PPh2

PPh

PPh2 PPh2

B

Fe PPh2

PPh 2

dppf

= BH

PPh 2

DPPMB[28]

Ph

NCDP[29]

Ph

NH

HN

O PPh2

Ph2 P PNNP

PPh 2

PPh2 DBFphos

FIGURE 8.2 A few diphosphine ligands used in the synthesis of heteroleptic copper(I) complexes containing phenanthroline derivatives.

homoleptic copper(I) complexes based on steric hindrance and bite-angles. Fig. 8.4 and Table 8.2 give specific details about the stability of heteroleptic complexes in solution [31]. The X-ray structures of heteroleptic complexes containing dmp or dpep showed the existence of steric conflicts between the 2,9-alkyl groups of the phenanthroline moeity and Ph2P groups, leading to the formation of less stable heteroleptic complexes in solution. Ligands like dppf and DPEphos exhibit wide bite-angles as compared to dppe, dppp, and dppb ligands. It has been found that dppf or DPEphos does not form a homoleptic complex of the type [Cu(DPEphos)2]BF4 containing two chelate rings because of steric hindrance between the PPh2 groups and the small size of the copper(I) atom.

245

Homoleptic and heteroleptic copper(I) complexes Chapter | 8

Ph

Bu n

CH3

N

Ph

CH3

N

N

N

N

N

N

N N

N

phen

dmp

Ph

Bu n Ph

CH3

CH3

dbp

bcp

dsdpp O

Ph

Bu t Ph

O

(CH2 )2 Ph

O

N

N

N

N

N O

N

N

N

N

N

O O

Bu t Ph

(CH2 )2 Ph

Ph

dtbp

bphen

dpp

O

dpep

m37[30]

FIGURE 8.3 A few phenanthroline derivatives used in the synthesis of heteroleptic copper(I) phosphine complexes.

PPh 2

1. [Cu(CH 3 CN) 4 ]BF4 CH 2Cl2 / CH 3CN

PPh 2

2. phen or bphen

R P Cu

N

P

PP PP

N R

dppe, dppp, dppb, dppf or DPEphos

BF 4 H [Cu(phen )(PP)]BF4 Ph [Cu(bphen)(PP)]BF 4

R R R R

P

Cu

N

P

R

N

N

Cu R

N N

N R

P +

P Cu

P

P

BF4

R

BF4

BF4 PP R

dppe, dppp, dppb, dppf, DPEphos or binap CH 3 or (CH2 )2 Ph

FIGURE 8.4 Stable and unstable heteroleptic copper(I) complexes and their dynamic ligand exchange behaviors in solution.

246

Phosphines, Functionalized Phosphines, and Phosphorus Heterocycles

TABLE 8.2 For [Cu(NN)(PP)]BF4-type complex, the stability of heteroleptic versus homoleptic complexes with different combinations of NN and PP ligands. [Cu(NN)(PP)]BF4

In solution

NN 5 phen or bphen and PP 5 dppm, dppe, dppp, dppb, dppf, or DPEphos

Stable

NN 5 dmp or dpep and PP 5 dppe, dppp, dppb, dppf, or DPEphos

Dynamic equilibrium with homoleptic complex

NN 5 dmp, dpep, or dpp and PP 5 dppf or DPEphos

Almost stable; with a small amount of homoleptic complex in equilibrium

Indeed, in the X-ray structure of [Cu(DPEphos)2]BF4, the copper atom is three-coordinate, with one chelated and one monodentate ligands, which avoids the otherwise steric hindrances [32]. Further, in the structure of complex [Cu2(dppf)3](BF4)2, dppf adopts both the bridging as well as chelating bonding modes [31].

8.4 Heteroleptic copper(I) complexes with non-phenanthroline ligands Several neutral heteroleptic copper(I) complexes have been synthesized using anionic non-phenanthroline ligand systems and a few of those ligands are given in Fig. 8.5. In line with phenanthroline heteroleptic complexes, these complexes are also sensitive to the electronic nature and positions of the groups attached to the diimine as well as diphosphine ligands, and accordingly show changes in lifetime, and absorption and emission maxima. The λmax of [Cu(bpy)(dppe)] PF6 is 601 nm, whereas that of [Cu(bpy)(dfppe)]PF6 is 578 nm (dfdppe 5 1,2bis[bis(pentafluorophenyl)phosphino]ethane) [33]. This blue shift is due to the presence of electron-withdrawing groups (C6F5) attached to the phosphorus atom, which increase the HOMOLUMO gap. In addition, a very long lifetime was measured for [Cu(6mbpy)(dfppe)]PF6 (6dmbpy 5 6,60 -dimethyl-2,20 -bipyridine) because of the presence of methyl groups on the bipyridine moiety which restrict the flattening as observed for phenanthroline heteroleptic complexes. On the other hand, having electron-withdrawing groups on diimine ligand showed a red-shift of emission maximum, because of the decrease in the HOMOLUMO gap. For example, as shown in Fig. 8.6, complexes 1 [Cu (PPh3)2(bpmtzH)](ClO4) (bpmtzH 5 5-tbutyl-3-(20 -pyrimidinyl)-1H-1,2,4-triazole) and 3 [Cu(PPh3)2(fpmtzH)](ClO4) (fpmtzH 5 5-trifluoromethyl-3-(20 -pyrimidinyl)-1H-1,2,4-triazole) show emission maxima at 529 and 605 nm, respectively, and the red shift for 3 is attributed to the presence of CF3 group [34].

Homoleptic and heteroleptic copper(I) complexes Chapter | 8

247

Heteroleptic copper(I) complexes with non-phenanthroline ligands

N

N N

H

N

N

B

H

Ph

N N

N N B

Ph

N N N N B N N N N

N N

pz4 B

N

N Ph

Ph

PyN[37]

IndN

N N PyrTet[35] pz2BH2[36] pz2Bph2

N

N

F 3C Ph N F3C

N

N F3C

N CF 3

CF 3

fiqro[38]

fpyro

N EtO 2 C

F3 C

N

N

N

N

N

N

N

N

F3C

CF 3

fphro

N N

qbm[39]

Im-CF3 [40]

N

N

N

CO 2 Et

Im-COOEt

FIGURE 8.5 A few non-phenanthroline anionic diimine ligands used for synthesizing heteroleptic copper(I) complexes.

8.5 Binuclear heteroleptic copper(I) complexes In pursuit of developing better luminescent materials, several binuclear heteroleptic copper(I) complexes containing diimine ligands have been studied. A few of those complexes 5 [41], 6 [42], 7 [43], and 8 [44] are shown in Fig. 8.7. While complexes 58 are fluorescent, complex 9 [45] is not emissive in toluene or dichloromethane solution. Complex 9 in dichloromethane exhibits an absorption band (λmax) at 439 nm, falling in the region characteristic for a MLCT transition. The reaction between the dipyrrolyldiphosphine ligand (PNNP) and CuCl in the presence of one equivalent of 1,10-phenanthroline monohydrate and NaBF4 yielded complex 9. Its X-ray structure showed ππ interactions between the two phenanthroline ligands (Fig. 8.7).

248

Phosphines, Functionalized Phosphines, and Phosphorus Heterocycles

N

N

Ph 3P

PPh 3 Cu

N N PPh 3 HN But

N

(ClO 4 )2

2

N

Ph 3P

PPh 3 Cu

N N PPh 3 HN CF 3

N

N

Cu

(ClO 4 ) Ph 3 P

PPh 3 Cu

N N PPh 3 HN CF 3

(ClO 4 )2

4

3

1.0

Normalized intensity (a.u.)

PPh 3 Cu

N N PPh 3 HN Bu t

Ph 3 P

1

N

N

Cu

(ClO 4 )

3 4

1 0.5

2

0 450

650 550 Wavelength (nm)

750

FIGURE 8.6 Neutral heteroleptic copper(I) complexes containing non-phenanthroline ligands and their emission spectra in CH2Cl2 solution.

8.6 Application of heteroleptic copper(I)phosphine complexes The emissive heteroleptic copper(I) complexes have found several applications, mainly as sensitizers. One of the prime applications is to promote hydrogen production from water. Water is a cheap, sustainable, and benign energy source of hydrogen gas which can be an alternative and efficient fuel for transportation, and can relieve mankind from its fossil fuel dependence. The production of hydrogen from water involves multielectron transfer reactions and has been studied with noble metals such as ruthenium, iridium,

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249

2+ Ph 3 P

2+

PPh 3 Cu

N Ph3 P

Cu

Ph 3P

(BF 4 )–2

N N

N

N

N

Cu

N

N

(PF 6 )–2

PPh3 N

PPh3

N Cu

5

6

PPh 3

Ph 3 P

λ em = 550 nm Φ = 0.17 at 77 K in CH2 Cl2 matrix Ph

dppb

Ph

P

N N

7

P

P

Cu P

dppm

P

dmp

Cu

N

N

N

Cu

N

P

Cu

(NO 3 )–2 N

2+

2+

P

(PF 6) –2

N

dppb P

8

dppm

λem = 670 nm in CH2 Cl2 Φ = 0.013, τ = 1.1 ms

Ph

Ph

2+

HN

NH Ph 2P

(BF 4) –2 PPh2

Cl Cu

Cu N

N N

λem = 590 nm with life time of, τ = 10.8 ms in CH2Cl2

N

9

FIGURE 8.7 A few binuclear heteroleptic copper(I) complexes.

rhenium, and platinum [46]. Alternatively, the light-absorbing property of copper(I) heteroleptic complexes has also been used together with a waterreduction catalyst, [Fe3(CO)12], and sacrificial electron donor, triethylamine. In this system, the emissive property of copper(I) complex plays a crucial role and, hence, several emissive heteroleptic copper(I) complexes have been developed and studied for their activities. Beller et al. have reported an efficient system to date using heteroleptic copper(I) complexes containing bulkier and more rigid diphosphine ligands, DBFphos and xantphos, in combination with phenanthroline ligands [47]. As shown in Fig. 8.8, for a given diphosphine ligand, the steric bulkiness of substituents at the 2 and 9 positions of the phenanthroline ligand was changed and their activities were investigated in the presence of [Fe3(CO)12] in water/ THF/Et3N medium with Xe-light irradiation. It was found that the activity increases with an increase in the steric bulk of the phenanthroline substitutent, that is, Me , nBu , sBu with DPEphos and xantphos ligands. This is in line with the increase in the lifetime of complex which follows the same order. Hence, there is a correlation between lifetime and activity. The higher the lifetime, the higher is the activity. For example, [Cu(dsbdpp)(xantphos)] PF6 exhibits the highest turnover number of 1330, which is attributed to its

250

Phosphines, Functionalized Phosphines, and Phosphorus Heterocycles

(A) O PPh 2 R O

Cu PPh 2

R

P

Ph N N

[PF 6 ] Ph

S

O

Cu

R N N R

Ph Ph

[PF 6]

P O

R = Me, nBu, s Bu, iBu, iPr and nHex

(C)

(B)

Cathode Conductive layers

hv

[Cu(N^N)(P^P)]+ emitting layer Conductive layers ITO glass

FIGURE 8.8 (A) Heteroleptic copper(I) complexes used for production of hydrogen from water; (B) plausible mechanism for hydrogen production; (C) typical OLED and LED device set up. LED, light emitting diode; CuPS, Copper photosensitizer; OLED, organic light-emitting diode; SR, sacrificial reductant; WRC, water reduction catalyst.

excellent lifetime of 54.1 μs in THF and quantum yield of 0.75, and a faster electron-transfer rate. It is to be noted that this system represents a noblemetal-free system and is relatively inexpensive and better than the wellknown PSs, such as [Ru(bipy)3]Cl2 and [Ir(bipy)(ppy)2]PF6. The plausible mechanism is given in Fig. 8.8B. The emissive heteroleptic copper(I) complexes are also used as emitter materials for constructing industrially important OLEDs and LECs. As the emission is sensitive and tunable with substituents on both diphosphine and diimine, they became suitable materials for covering the whole visible region. A typical OLED device set up is shown in Fig. 8.8C. Wang et al. have demonstrated the OLED application using a series of heteroleptic copper(I) complexes containing monodentate, bidentate phosphine and phenanthroline ligands. It has been shown that complexes containing DBFphos ligand are stable up to 320 C, whereas those containing PPh3 decompose around 200 C. In addition, the sterically hindered [Cu(dbp)(DPEphos)]BF4 complex exhibits a high current efficiency of 11.0 cd/A at 1.0 mA/cm2 with 23 wt.% of complex and a maximum brightness of up to 1663 cd/m2 [48]. The same group has also reported LEC application of heteroleptic complexes containing DBFphos and different phenanthroline ligands with different counter ions. A current efficiency of 56 cd/A has been reported [49].

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The photoexcited heteroleptic copper(I) complex is a good oxidant as well as reductant as compared to its ground state complex. By suppressing the oxidation property, the reduction ability resulting from copper(I) to copper(II) species formation is used for dehydrogenative CC coupling reactions. Collins et al. have reported the synthesis of [5]helicene and π-expanded helicenes using [Cu(xantphos)(dmp)]BF4 [50]. As shown in Fig. 8.9, the catalyst system consists of the in situ generated copper(I) complex as sensitizer, molecular iodine as oxidant, and propylene oxide as a hydriodic acid trap. The yields of [5]helicene are much greater than those obtained with ruthenium and iridium sensitizers which is attributed to the greater lifetime and quantum yields of complex [Cu(xantphos)(dmp)]BF4. Mann et al. have reported oxygen-sensing applications of heteroleptic copper(I)phosphine complexes such as [Cu(DPEphos)(dmp)]tfpb, [Cu (Xantphos)(dmp)]tfpb, [Cu(Xantphos)(dipp)]tfpb, and [Cu(Xantphos)(dipp)] pftpb and of homoleptic complex [Cu(dipp)2]tfpb [51]. Among these, [Cu (Xantphos)(dmp)]tfpb was found to possess an excellent quantum yield [0.66 (5)], the oxygen sensitivity parameter SternVolmer constant (KSV) of 5.65 (8) atm21, and higher photochemical stability. This excellent property is due to the enhanced excited state lifetime (B30 μs) of [Cu(Xantphos)(dmp)]tfpb as compared to other heteroleptic and homoleptic complexes [Cu(dipp)2] tfpb. The enhanced lifetime is due to the restriction caused by the rigid

[Cu(MeCN)4]BF4 (25 mol%) Xantphos (25 mol%) dmp (25 mol%) I2 (1 Eq) Visible light, 5 d THF:propylene oxide (56:1) 42% yield [5]Helicene

t-Bu t-Bu

MeO

t-Bu

t-Bu

MeO OMe 45%

41%

46%

FIGURE 8.9 Synthesis of [5]helicene and π-expanded helicenes using [Cu(xantphos)(dmp)] BF4 as a sensitizer.

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Xantphos, whereas in [Cu(dppe)(phen)]PF6 and DPEphos heteroleptic complexes, significant distortions are present in the coordination sphere, making them have a relatively shorter lifetime. It is to be noted that KSV of [Ru(4,7Me2Phen)3](tfpb) is 4.76 atm21, lower than that of this copper complex. [Cu(DPEphos)(dmp)]BF4 is used in mediating the photoredox synthesis of carbazoles from triarylamines in the presence of oxidant I2, as shown in the following reaction. Ph

N Ph

Ph

[Cu(Xantphos)(dmp)]BF 4 (in situ)

Ph N

I2 , THF/propylene oxide, visible light

The product is formed in 56% yield, higher than that obtained from the analogous reaction with [Ru(bpy)3]PF6 (27%) [52]. Chen et al. have reported the catalysis of cross-dehydrogenative coupling between tetrahydroisoquinoline, nitroalkanes, and acetone or indoles in the presence of heteroleptic copper(I) complexes and oxygen under visible light irradiation (Table 8.3). Better activities were found with sterically hindered complexes [Cu(dmp) (NCDP)] and [Cu(bcp)(NCDP)]; the less bulky heteroleptic complexes yielded lower yields [29].

TABLE 8.3 The cross-dehydrogenative coupling reactions and products catalyzed by heteroleptic copper(I) complexes. Tetrahydroisoquinoline

Nitroalkane

Cross-dehydrogenative product

CH3NO2 N

N

NO2

N R

R 5 Ph and C6H4Cl

N

N R1

R

R1 5 H and CH3

N R1

CH3COCH3 N

N R

R 5 Ph, C6H4(Me) and C6H4Cl

O

R

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8.7 Conclusions Diiminediphosphine heteroleptic copper(I) complexes have witnessed significant progress, mainly as an efficient photoactive material. Heteroleptic copper (I) complexes have been found to possess long excited lifetimes and high quantum yields, qualifying them to be alternatives to the conventionally used Ru(II), Ir(III), and other precious metal complexes. In addition, copper metal is Earthabundant, relatively inexpensive, and photoactive complexes can readily be synthesized from commercially available starting materials in a single step. Although poisonous phosphine ligands are being used, phosphine and phenanthroline ligands offer a wide range of possibilities for tuning the key properties such as absorption, emission, lifetime, quantum yield, and redox properties. These properties are sensitive to the size, nature, and position of substituents on both the diphosphine and diimine ligands. Eventually, it has been found that bulky and rigid ligands around the copper atom are necessary to control unwanted excited state flattening and exciplex quenching. Further, neutral heteroleptic copper(I) complexes containing a monoionic diimine ligand showed promising long excited-state lifetimes and high quantum yields. These complexes have become better photoactive materials for a wide range of applications such as OLED, LEC, solar energy conversion, hydrogen production from water, and catalysis of organic reactions. However, some of the heteroleptic complexes have a lower stability, narrow range of visible light absorption, and lower extinction coefficient as compared to other photoactive transition metal complexes. This limitation and the encouraging structurefunction relationship offer spacious room for doing further research in this field using novel diimine and diphosphine ligands with specific steric and electronic characters, warranting more potential specific applications to be realized in the future.

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