Chimera Diimine Ligands in Emissive [Cu(P^P)(N^N)][PF₆] Complexes

Abstract

The syntheses and characterizations of the chelating ligand 6-chloro-6′-methyl-2,2′-bipyridine (6-Cl-6′-Mebpy) and of the copper(I) compounds [Cu(POP)(6-Cl-6′-Mebpy)][PF₆] and [Cu(xantphos)(6-Cl-6′-Mebpy)][PF₆] (POP = bis(2-(diphenylphosphanyl)phenyl)ether and xantphos = 4,5-bis(diphenylphosphanyl)-9,9-dimethyl-9H-xanthene) are described. The single crystal structures of both complexes were determined; the copper(I) ion is in a distorted tetrahedral environment and in [Cu(xantphos)(6-Cl-6′-Mebpy)][PF₆], the disorder of the 6-Cl-6′-Mebpy ligand indicates there is no preference of the ‘bowl’-like cavity of the xanthene unit to host either the methyl or chloro-substituent, consistent with comparable steric effects of the two groups. The electrochemical and photophysical properties of [Cu(POP)(6-Cl-6′-Mebpy)][PF₆] and [Cu(xantphos)(6-Cl-6′-Mebpy)][PF₆] were investigated and are compared with those of the related compounds containing 6,6′-dichloro-2,2′-bipyridine or 6,6′-dimethyl-2,2′-bipyridine ligands. Trends in properties of the [Cu(P^P)(N^N)]⁺ complexes were consistent with 6-Cl-6′-Mebpy behaving as a combination of the two parent ligands.

1. Introduction

Light-emitting diode (LED) and organic LED (OLED) solid-state lighting technologies are firmly embedded in our society [1]. Light-emitting electrochemical cells (LECs) represent a less well developed, but nonetheless promising, area of solid-state lighting [2,3]. Among the compounds targeted for emissive components in the active layers of LECs are heteroleptic [Cu(P^P)(N^N)]⁺ complexes in which P^P is a wide bite-angle bisphosphane [4] and N^N contains a diimine metal-binding domain. Investigations in this field were inspired by the work of McMillin and coworkers who showed that [Cu(P^P)(N^N)]⁺ complexes possess low-lying metal-to-ligand charge transfer (MLCT) excited states [5,6]. In the last decade, it has been shown that [Cu(P^P)(N^N)]⁺ complexes exhibit thermally activated delayed fluorescence (TADF) [7,8]. This phenomenon involves fast intersystem crossing from the lowest-lying singlet excited state (S₁) to the triplet excited state (T₁); because the latter is long-lived and shows a relatively slow phosphorescence, thermal repopulation of the singlet excited state via a reverse intersystem crossing occurs with a concomitant fluorescence emission from S₁. This process significantly increases the photoluminescent quantum yield (PLQY), making [Cu(P^P)(N^N)]⁺ complexes of particular interest for use in LECs and allows the harvesting of both triplet and singlet excited states which are generated in a 3:1 ratio [7,8].

The most frequently used wide bite-angle bisphosphanes in these compounds are 4,5-bis(diphenylphosphanyl)-9,9-dimethyl-9H-xanthene (xantphos, IUPAC PIN (9,9-dimethyl-9H-xanthene-4,5-diyl)bis(diphenylphosphane)) and bis(2-(diphenylphosphanyl)phenyl)ether (POP, IUPAC PIN oxydi(2,1-phenylene)] bis(diphenylphosphane)). The structures of POP and xantphos are shown in Scheme 1. The diimine ligand is typically a derivative of 2,2′-bipyridine (bpy) or 1,10-phenanthroline (phen). Despite the wide range of N^N ligands introduced into [Cu(P^P)(N^N)]⁺ complexes [9], simple ligands including 6-alkyl-2,2′-bipyridines, 6,6′-dialkyl-2,2′-bipyridines [10,11], 6-alkyloxy-2,2′-bipyridines [12], 6,6′-bis(alkyloxy)-2,2′-bipyridines [13], 6-thioalkyl-2,2′-bipyridines [12] and 6,6′-dihalo-2,2′-bipyridines [14] often lead to the highest PLQY values and the best electroluminescence (EL) performances in LECs.

Powdered samples of [Cu(POP)(6,6′-Me₂bpy)][PF₆] and [Cu(xantphos)(6,6′-Me₂bpy)][PF₆] (6,6′-Me₂bpy = 6,6′-dimethyl-2,2′-bipyridine, Scheme 1) exhibit PLQYs of 43.2% [10] and 37% [11], respectively. The introduction of the methyl groups is critical to improving PLQY values and performance in LECs [10,11,15]. A LEC with [Cu(POP)(6,6′-Me₂bpy)][PF₆] in the active layer reached a maximum luminance of 53 cd m⁻² and maximum efficacy of 5.2 cd A⁻¹ [10], values which are appreciably higher than for [Cu(POP)(bpy)][PF₆] (1.64 cd A⁻¹) [15]. However, the LEC turn-on times were in the order of minutes [10,11]. Changing the methyl substituents for chlorine substituents (Scheme 1) results in a decrease in PLQY values with powdered [Cu(POP)(6,6′-Cl₂bpy)][PF₆] and [Cu(xantphos)(6,6′-Cl₂bpy)][PF₆] exhibiting PLQYs of 15% and 17%, respectively. The performances of LECs with [Cu(POP)(6,6′-Cl₂bpy)][PF₆] and [Cu(xantphos)(6,6′-Cl₂bpy)][PF₆] in the active layers were promising with rapid LEC turn-on times of <5 s and 12 s, respectively, and maximum luminances of 121 and 259 cd m⁻², respectively, when the average current density was 100 A m⁻² [14]. These results suggested the idea of a diimine ligand that incorporated both chlorine and methyl substituents. Here, we report the synthesis and spectroscopic and structural properties of the copper(I) complexes [Cu(POP)(6-Cl-6′-Mebpy)][PF₆] and [Cu(xantphos)(6-Cl-6′-Mebpy)][PF₆]. Surprisingly, reports of the ligand 6-Cl-6′-Mebpy (Scheme 1) are confined to the patent literature [16] and to inclusion in a theoretical study of redox potentials, pK_a values of [H₂bpy]²⁺ and solubilities of a wide range of bpy derivatives [17].

2. Results and Discussion

2.1. Synthesis and Characterization of 6-Cl-6′-Mebpy

The preparation of 6-Cl-6′-Mebpy reported in the patent literature [16] involved the Grignard reaction of MeMgBr with 6,6′-Cl₂bpy. We favored a Negishi coupling between 2,6-dichloropyridine and 6-methyl-2-pyridinylzinc bromide under microwave conditions, which led to 6-Cl-6′-Mebpy in a 23.5% yield after purification. The base peak in the electrospray mass spectrum was observed at m/z 205.02 and was assigned to the [M + H]⁺ ion. The isotopomer distribution (Figure S1 in the supplementary materials) was as predicted. The ¹H and ¹³C{¹H} NMR spectra were assigned using COSY, NOESY, HMQC and HMBC methods. In the ¹H NMR spectrum, two sets of pyridine ring signals (Figure S2) and the presence of one methyl signal were consistent with the structure of 6-Cl-6′-Mebpy shown in Scheme 1. The HMQC and HMBC spectra are displayed in Figures S3 and S4.

2.2. Synthesis and Characterization of [Cu(POP)(6-Cl-6′-Mebpy)][PF₆] and [Cu(xantphos)(6-Cl-6′-Mebpy)][PF₆]

The syntheses of the POP and xantphos-containing [Cu(P^P)(bpy)][PF₆] compounds require two different strategies, for reasons we have previously discussed in detail [11,12]. Treatment of [Cu(MeCN)₄][PF₆] with POP in CH₂Cl₂ followed, after two hours, by the addition of 6-Cl-6′-Mebpy resulted in the formation of [Cu(POP)(6-Cl-6′-Mebpy)][PF₆]. In contrast, [Cu(xantphos)(6-Cl-6′-Mebpy)][PF₆] was prepared in a one-pot process by combining [Cu(MeCN)₄] [PF₆], xantphos and 6-Cl-6′-Mebpy. After precipitation from CH₂Cl₂ solutions by the addition of Et₂O, the complexes were isolated in 72.0% and 87.0% yields, respectively. The electrospray mass spectra of the two compounds are shown in Figures S5 and S6. The highest mass peak corresponded to the [M − PF₆]⁺ cation (m/z = 805.12 for [Cu(POP)(6-Cl-6′-Mebpy)]⁺ and m/z = 845.16 for [Cu(xantphos)(6-Cl-6′-Mebpy)]⁺). Loss of the 6-Cl-6′-Mebpy ligand from the complex cation led to the base peak in each mass spectrum (m/z = 601.09 and 641.10, respectively).

The solution NMR spectra of [Cu(POP)(6-Cl-6′-Mebpy)][PF₆] and [Cu(xantphos)(6-Cl-6′-Mebpy)][PF₆] were recorded in acetone-d₆ and a comparison of the aromatic regions of the ¹H NMR spectra is given in Figure 1. The atom labeling for the NMR spectroscopic assignments is given in Scheme 1. As in the ¹H and ¹³C{¹H} NMR spectra of related compounds [11,12,18,19], two sets of signals are observed for the phenyl rings of the PPh₂ groups. In Figure 1, these are labeled D and D’. In the NOESY spectrum of each complex, cross-peaks between signals for the methyl group of 6-Cl-6′-Mebpy and one set of protons-D2 were observed, defining D2′ as the ortho-protons on the phenyl rings closest to the N^N ligand. Combined use of NOESY and HMQC allows the ¹³C resonances for C^D2 and C^D2′ to be distinguished (Figure 2). The distinction between the sets of phenyl rings is more clearly recognizable in the structural discussion that follows later.

Single crystals of [Cu(POP)(6-Cl-6′-Mebpy)][PF₆] were grown by diffusion of n-hexane into an acetone solution of the compound. For [Cu(xantphos)(6-Cl-6′-Mebpy)][PF₆], diffusion of Et₂O into a CH₂Cl₂ solution of the complex was used to grow crystals. The POP- and xantphos-containing compounds crystallize in the monoclinic P2₁/c and triclinic P–1 space groups, respectively. The copper atom in each complex cation is in the expected distorted tetrahedral geometry with two chelating ligands (Figure 3). The structure of the [Cu(POP)(6-Cl-6′-Mebpy)]⁺ cation was severely disordered in both the N^N and POP domains. Within the POP ligand, half of the {(C₆H₄)₂O} unit and two phenyl rings of one PPh₂ group were also disordered, and each fragment was modeled with a 50% site occupancy. The Cl and Me substituents in the 6-Cl-6′-Mebpy ligand in both compounds were disordered and were modeled over two sites, each with a 50% occupancy. The disorders do not affect the {CuP₂ N₂}-coordination sphere and relevant bond lengths and angles are given in

Table 1. Bond lengths and angles in the copper(I) coordination sphere in the cations in [Cu(POP)(6-Cl-6′-Mebpy)][PF₆] and [Cu(xantphos)(6-Cl-6′-Mebpy)][PF₆].

Cation	Cu–N/Å	Cu–P/Å	N–Cu–N/o	P–Cu–P/o	P–Cu–N/o
[Cu(POP)(6-Cl-6′-Mebpy)]+	2.103(2), 2.131(3)	2.2497(11), 2.2774(13)	78.95(14)	116.51(5)	116.38(8), 112.11(8), 120.35(10), 106.79(11)
[Cu(xantphos)(6-Cl-6′-Mebpy)]+	2.104(3), 2.123(3)	2.2750(13), 2.2978(13)	78.52(13)	120.36(5)	117.08(11), 105.66(10), 114.44(10), 113.21(11)

. ORTEP representations of the complex cations and the disorders in the [Cu(POP)(6-Cl-6′-Mebpy)]⁺ cation are depicted in Figures S12–S14 in the supporting information. In the [Cu(POP)(6-Cl-6′-Mebpy)]⁺ cation, the bpy unit is twisted with an angle between the least squares planes through the two pyridine rings of 19.5°. The corresponding angle in the [Cu(xantphos)(6-Cl-6′-Mebpy)]⁺ cation is 5.7°.

In [Cu(xantphos)(N^N)]⁺ complexes which contain asymmetrical 6-substituted 2,2′-bipyridines or 2-(pyridin-2-yl)quinolines, the N^N ligand may favor a conformation in which the 6-substituent or quinoline unit lies over, or is remote from, the xanthene unit [11,12,18,19]. In some cases, NMR spectroscopy has provided evidence for mixtures of conformers in solution [11,18]. Among the 152 hits arising from a search of the Cambridge Structural Database (CSD, v. 5.4.1 [20]) for compounds containing a {Cu(POP/xantphos)(bpy/phen)} or closely related unit, only a few contain asymmetrical N^N ligands [10,11,12,14,18,21,22,23]. Within the context of these N^N ligands, 6-Cl-6′-Mebpy is atypical in being asymmetrical by virtue of having different substituents in the 6- and 6′-positions of the bpy domain. The disorder in [Cu(xantphos)(6-Cl-6′-Mebpy)][PF₆], modeled with equal occupancies of Cl and Me groups, indicates that there is no preference for the orientation of the 6-Cl-6′-Mebpy (Figure 4). This is consistent with the more general observation of similar steric effects for these two substituents [24,25].

2.3. Electrochemical and Photophysical Properties of [Cu(POP)(6-Cl-6′-Mebpy)][PF₆] and [Cu(xantphos)(6-Cl-6′-Mebpy)][PF₆]

The electrochemical behavior of the copper(I) compounds was investigated using cyclic voltammetry. For the xantphos-containing compound, a reversible oxidation process (Figure 5 and Figure S15) was assigned to the Cu⁺/Cu²⁺ redox process, but for [Cu(POP)(6-Cl-6′-Mebpy)][PF₆], the corresponding process was irreversible (Figure S16). Two ligand-centered reductive processes are observed for [Cu(POP)(6-Cl-6′-Mebpy)][PF₆] with values of E_pa = −2.03 and −2.21 V (Figure S16), but for the xantphos-containing complex, no reductive processes are observed within the solvent accessible window (Figure S15).

Table 2. Copper(I)/(II) oxidation potentials in [Cu(POP)(6-Cl-6′-Mebpy)][PF₆] and [Cu(xantphos)(6-Cl-6′-Mebpy)][PF₆] compared to those of related compounds. CH₂Cl₂ solutions (ca. 10⁻⁴ mol dm⁻³); values are referenced to internal Fc/Fc⁺ = 0.0 V; [ⁿBu₄N][PF₆] as supporting electrolyte and scan rate of 0.1 V s⁻¹.

Compound	E1/2 /V	Epc – Epa/mV	Epca/V	Reference
[Cu(POP)(6-Cl-6′-Mebpy)][PF6]			+0.98	This work
[Cu(xantphos)(6-Cl-6′-Mebpy)][PF6]	+0.91	106		This work
[Cu(POP)(6,6′-Cl2bpy)][PF6]	+0.98	170		[14]
[Cu(xantphos)(6,6′-Cl2bpy)][PF6]	+0.93	90		[14]
[Cu(POP)(6,6′-Me2bpy)][BF4]	+0.82 b	– c		[26]
[Cu(xantphos)(6,6′-Me2bpy)][PF6]	+0.90	150		[21]
[Cu(POP)(bpy)][PF6]	+0.72	110		[21]
[Cu(xantphos)(bpy)][PF6]	+0.76	110		[21]

^a The value is given for E_pc when the process is irreversible. ^b In our hands, the compound shows only partial reversibility with E_pc = +0.93 V [27]. ^c No E_pc – E_pa value reported.

compares values of the Cu⁺/Cu²⁺ oxidation potential of [Cu(POP)(6-Cl-6′-Mebpy)]⁺ and [Cu(xantphos)(6-Cl-6′-Mebpy)]⁺ with those of the analogous complexes containing bpy, 6,6′-Me₂bpy and 6,6′-Cl₂bpy. Oxidation from copper(I) to copper(II) is accompanied by a change from preferred tetrahedral to square-planar geometry. On going from bpy to 6,6′-Me₂bpy, the shift in E_1/2 to higher potentials can be rationalized in terms of steric effects of the methyl substituents which hinder the flattening of the coordination sphere. In this case, steric effects appear to dominate over electronic effects since the electron-donating methyl substituents would also stabilize copper(II). On going from bpy or 6,6′-Me₂bpy to 6,6′-Cl₂bpy, the shift in E_1/2 to higher potentials is consistent with a combination of both steric and electronic effects; the electron-withdrawing chlorine substituents stabilize the copper(I) oxidation state. The value of E_1/2 for [Cu(xantphos)(6-Cl-6′-Mebpy)][PF₆] is in accord with an interplay of the effects of both chlorine and methyl groups, and the value of E_pc for [Cu(POP)(6-Cl-6′-Mebpy)][PF₆] is also consistent with this trend.

Dichloromethane solutions of [Cu(POP)(6-Cl-6′-Mebpy)][PF₆] and [Cu(xantphos)(6-Cl-6′-Mebpy)][PF₆] exhibit absorption spectra comprising intense high-energy, ligand-centered absorptions and a broad, lower intensity band arising from metal-to-ligand charge transfer (MLCT). The spectra are shown in Figure 6a and data are presented in

Table 3. Absorption maxima for CH₂Cl₂ solutions of [Cu(POP)(6-Cl-6′-Mebpy)][PF₆] and [Cu(xantphos)(6-Cl-6′-Mebpy)][PF₆] (2.5 × 10⁻⁵ mol dm⁻³).

Compound	λmax/nm (εmax/dm3 mol−1 cm−1)
	π*←π	MLCT
[Cu(POP)(6-Cl-6′-Mebpy)][PF6]	294 (18,800), 322 sh (11,500)	400 (2,160)
[Cu(xantphos)(6-Cl-6′-Mebpy)][PF6]	283 (23,370), 307 sh (15,200), 322 sh (9,500)	400 (2,160)

. The trend in the MLCT maximum on changing the N^N ligand from bpy, 6,6′-Me₂bpy or 6,6′-Cl₂bpy to 6-Cl-6′-Mebpy reflects the changing electronic properties of the substituents. Figure 3b compares the MLCT absorptions of [Cu(xantphos)(6-Cl-6′-Mebpy)][PF₆], [Cu(xantphos)(bpy)][PF₆] [21], [Cu(xantphos)(6,6′-Me₂bpy)][PF₆] [11] and [Cu(xantphos)(6,6′-Cl₂bpy)][PF₆] [14]. The values of λ_max for the MLCT absorption are 400, 383, 379 and 420 nm, respectively. The LUMO of a [Cu(P^P)(N^N)]⁺ complex is localized on the N^N ligand, and introducing electron-withdrawing chloro groups into the bpy unit stabilizes the LUMO with respect to that in [Cu(xantphos)(bpy)][PF₆] [14] or [Cu(xantphos)(6,6′-Me₂bpy)][PF₆], leading to a red shift in the MLCT absorption. A smaller red shift is observed when 6,6′-Cl₂bpy is replaced by 6-Cl-6′-Mebpy (Table 3 and Figure 3b), consistent with the combined characters of the N^N ligand.

When excited into the MLCT band at 400 nm, [Cu(POP)(6-Cl-6′-Mebpy)][PF₆] and [Cu(xantphos)(6-Cl-6′-Mebpy)][PF₆] are orange or yellow emitters (

Table 4. Photophysical properties of [Cu(POP)(6-Cl-6′-Mebpy)][PF₆] and [Cu(xantphos)(6-Cl-6′-Mebpy)][PF₆] and related compounds.

Compound	Solution (CH2Cl2, De-Aerated, 1.0 × 10−5 mol dm−3)				Powder
	λexc/nm	λmaxem/nm	PLQY/%	τ/ns	λexc/nm	λmaxem/nm	PLQY/%	τ/μsa	τ(1)/ μs (A1)	τ(2)/ μs (A2)
[Cu(POP)(6-Cl-6′-Mebpy)][PF6]	400	593	5.5	1300	400	558	24	4.8	6.0 (0.66)	1.8 (0.25)
[Cu(xantphos)(6-Cl-6′-Mebpy)][PF6]	400	582	1.4	420	400	548	16	4.0	6.0 (0.49)	1.3 (0.37)

^a A biexponential fit to the lifetime delay was used because a single exponential gave a poor fit; τ is calculated from the equation ${{\displaystyle \sum }}^{}{A}_i{\tau }_i/{{\displaystyle \sum }}^{}{A}_i$ and A_i is the pre-exponential factor for the lifetime and values of τ(1), τ(2), A₁ and A₂ are also given.

), and the solution and solid-state emission spectra are illustrated in Figure 7. In solution, the emission maximum of [Cu(POP)(6-Cl-6′-Mebpy)][PF₆] is at a longer wavelength compared to [Cu(xantphos)(Cl-Mebpy)][PF₆] with a redshift of 11 nm. The emission maxima of powdered samples are blue-shifted with respect to solution values (Table 4 and Figure 7) and again, the POP-containing complex is red shifted (by 10 nm) with respect to that containing the xantphos ligand. The solution emission bands exhibit a low-energy shoulder, consistent with the somewhat structured emissions observed for related [Cu(POP)(N^N)][PF₆] and [Cu(xantphos)(N^N)][PF₆] compounds [10,11,14]. As expected, the PLQY values are significantly enhanced from solution to powdered samples (Table 4). The solid-state PLQY of 24% for [Cu(POP)(6-Cl-6′-Mebpy)][PF₆] lies between the 43.2% reported for [Cu(POP)(6,6′-Me₂bpy)][PF₆] [10] and the 14.8% observed for [Cu(POP)(6,6′-Cl₂bpy)][PF₆] [14]. The excited-state lifetime, τ, (which was determined using a biexponential fit [27] as detailed in Table 4) of 4.8 μs also lies between the values for [Cu(POP)(6,6′-Me₂bpy)][PF₆] (10.5 μs [10]) and [Cu(POP)(6,6′-Cl₂bpy)][PF₆] (2.7 μs [14]). Similar trends were seen for the xantphos-containing compounds with values of τ being 3.3, 4.0, and 11 μs for N^N = 6,6′-Cl₂bpy [14], 6-Cl-6′-Mebpy (Table 4) and 6,6′-Me₂bpy [10]. In terms of quantum yields, we observed that the PLQY of 16% for powdered [Cu(xantphos)(6-Cl-6′-Mebpy)][PF₆] compares with 37% reported for [Cu(xantphos)(6,6′-Me₂bpy)][PF₆] [11] and 17.1% observed for [Cu(xantphos)(6,6′-Cl₂bpy)][PF₆] [14].

3. Materials and Methods

3.1. General

¹H, ¹³C{¹H} and ³¹P{¹H} NMR spectra were recorded at 298 K on a Bruker Avance III-500 NMR spectrometer (Bruker BioSpin AG, Fällanden, Switzerland). ¹H and ¹³C NMR chemical shifts were referenced to residual solvent peaks with respect to δ(TMS) = 0 ppm and ³¹P NMR chemical shifts with respect to δ(85% aqueous H₃PO₄) = 0 ppm. Shimadzu LCMS-2020 (Shimadzu Schweiz GmbH, Reinach, Switzerland) and Bruker esquire 3000plus instruments (Bruker BioSpin AG, Fällanden, Switzerland) were used to record electrospray ionization (ESI) mass spectra with samples introduced. Solution absorption and emission spectra were recorded using an Agilent 8453 spectrophotometer (Agilent Technologies, Inc., Santa Clara, CA, USA) and Shimadzu RF-5301PC spectrofluorometer (Shimadzu Schweiz GmbH, Reinach, Switzerland), respectively. A Hamamatsu absolute photoluminescence quantum yield spectrometer C11347 Quantaurus-QY (Hamamatsu Photonics France, Solothurn, Switzerland) was used to measure PLQYs and emission lifetimes and powder emission spectra were measured using a Hamamatsu Compact Fluorescence lifetime Spectrometer C11367 Quantaurus-Tau with an LED light source (λ_exc = 365 nm). Microwave reactions were performed in a Biotage Initiator 8 microwave reactor (Biotage EU, 75103 Uppsala, Sweden).

Cyclic voltammograms were recorded using a CH Instruments 900B potentiostat (CH Instruments, Bee Cave, TX, USA) with [ⁿBu₄N][PF₆] (0.1 mol dm⁻³) as the supporting electrolyte and a scan rate of 0.1 V s⁻¹; the solvent was HPLC grade CH₂Cl₂ and solution concentrations were ca. 1 × 10⁻⁴ mol dm⁻³. All solutions were degassed with argon. The working electrode was glassy carbon, the reference electrode was an Ag wire, and the counter-electrode was a Pt wire. Final potentials were internally referenced with respect to the Fc/Fc⁺ couple.

POP and xantphos were bought from Acros (Fisher Scientific AG, Reinach, Switzerland) and Fluorochem (Chemie Brunschwig AG, Basel, Switzerland), respectively. (6-Methylpyridin-2-yl)zinc(II) bromide was purchased from Sigma-Aldrich (Sigma Aldrich Chemie GmbH, Steinheim, Germany). [Cu(MeCN)₄][PF₆] was prepared as described in the literature [28].

3.2. 6-Chloro-6′-methyl-2,2′-bipyridine (6-Cl-6′-Mebpy)

A microwave vial (20 mL) was evacuated and backfilled three times with N₂. Then [Pd(PPh₃)₄] (393 mg, 3.40 × 10⁻⁴ mmol, 4.6 mol %), 2,6-dichloropyridine (1.00 g, 6.76 mmol, 1.0 eq.) and 6-methyl-2-pyridinylzinc bromide (0.5 M in THF, 13.5 mL, 13.5 g, 6.76 mmol, 1.0 eq.) were added to the vial. The solution was degassed with N₂ for 20 min. The reaction was carried out in a microwave reactor for 2 h at 110 °C, during which time, a precipitate formed. Aqueous saturated NaHCO₃ (15 mL) and brine (15 mL) were added to the reaction mixture and this was then diluted with Milli-Q water to improve the separation of the organic and aqueous layers. The mixture was extracted with CH₂Cl₂ (3 × 20 mL). The combined organic layers were washed with H₂O (3 × 20 mL), dried over MgSO₄ and the solvent was removed under reduced pressure. After purification by column chromatography (silica; cyclohexane/EtOAc 20:1; Rf = 0.24), 6-Cl-6′-Mebpy was isolated as a colorless oil (0.326 g, 1.59 mmol, 23.5%). ¹H NMR (500 MHz, 298 K, CDCl₃) δ/ppm: 8.38 (d, J = 7.6 Hz, 1H, H^B3), 8.20 (d, J = 7.8 Hz, 1H, H^A3), 7.76 (m, 1H, H^B4), 7.70 (m, 1H, H^A4), 7.32 (m, 1H, H^B5), 7.19 (d, J = 7.6 Hz, 1H, H^A5), 2.63 (s, 3H, H^Me). ¹³C{¹H} NMR (126 MHz, 298 K, CDCl₃) δ/ppm 158.2 (C^A6), 157.3 (C^B2), 154.1 (C^A2), 150.9 (C^B6), 139.5 (C^B4), 137.4 (C^A4), 124.1 (C^B5), 124.0 (C^A5), 119.5 (C^B3), 118.6 (C^A3), 24.5 (C^Me). ESI-MS (CH₂Cl₂/MeOH, positive mode) m/z 205.02 [M + H]⁺ (base peak, calc. 205.05).

3.3. [Cu(POP)(6-Cl-6′-Mebpy)][PF₆]

[Cu(MeCN)₄][PF₆] (93.3 mg, 0.250 mmol, 1.0 eq.) and POP (162 mg, 0.300 mmol, 1.2 eq.) were dissolved in CH₂Cl₂ (30 mL) and the mixture was stirred for 2 h at room temperature. Then 6-Cl-6′-Mebpy (51.3 mg, 0.250 mmol, 1.0 eq.) was added to the mixture and this was stirred for 2 h. The mixture was filtered and the solvent from the filtrate was removed under vacuum to give a yellow solid. The product was purified by precipitation with Et₂O (4 times) from a CH₂Cl₂ solution of the compound and was finally isolated after sonication (5 min) and centrifugation (3000 rpm, 3 min). [Cu(POP)(6-Cl-6′-Mebpy)][PF₆] was obtained as a yellow crystalline solid (172 mg, 0.180 mmol, 72.0%). ¹H NMR (500 MHz, 298 K, acetone-d₆) δ/ppm 8.40 (d, J = 7.9 Hz, 1H, H^B3), 8.33 (d, J = 8.0 Hz, 1H, H^A3), 8.16 (m, 1H, H^B4), 8.09 (m, 1H, H^A4), 7.68 (d, J = 7.9 Hz, 1H, H^B5), 7.43 (d, J = 7.8 Hz, 1H, H^A5), 7.41–7.35 (overlapping m, 8H, H^C5+D2+D4), 7.33 (m, 2H, H^D4′), 7.23–7.29 (overlapping m, 6H, H^C4+D3), 7.22–7.15 (overlapping m, 6H, H^C3+D3′), 7.04–6.97 (overlapping m, 6H, H^C6+D2′), 2.07 (s, 3H, H^Me). ¹³C{¹H} NMR (126 MHz, 298 K, acetone-d₆) δ/ppm 160.6 (C^A6), 158.8 (t, J_CP = 6 Hz, C^C1), 154.9 (C^B2), 152.1 (C^A2), 151.7 (C^B6), 142.6 (C^B4), 140.1 (C^A4), 135.2 (t, J_CP = 8 Hz, C^D2), 134.4 (C^C3), 133.2 (C^C5), 133.0 (t, J_CP = 6 Hz, C^D2′), 132.6 (t, J_CP = 17 Hz, C^D1+D1′), 131.2 (C^D4), 130.5 (C^D4′), 129.6 (t, J_CP = 4 Hz, C^D3+D3′), 128.1 (C^A5), 127.4 (C^B5), 126.4 (t, J_CP = 15 Hz, C^C2), 126.1 (t, J_CP = 2 Hz, C^C4), 122.4 (C^B3), 121.5 (C^A3), 120.8 (t, J_CP = 2 Hz, C^C6), 25.9 (C^Me). ³¹P{¹H} NMR (202 MHz, 298 K, acetone-d₆) δ/ppm −13.0 (broad, POP), −144.3 (heptet, J_PF = 707 Hz, PF₆⁻). ESI-MS (CH₂Cl₂/MeOH, positive mode) m/z 805.12 [M − PF₆]⁺ (calc. 805.14), 601.09 [M − PF₆ − (6-Cl-6′-Mebpy)]⁺ (base peak, calc. 601.09). Found: C 58.56, H 4.04, N 2.85; C₄₇H₃₇ClCuF₆N₂OP₃ requires C 59.31, H 3.92, N 2.94.

3.4. [Cu(xantphos)(6-Cl-6′-Mebpy)][PF₆]

[Cu(MeCN)₄][PF₆] (94.5 mg, 0.253 mmol, 1.0 eq.), xantphos (146.7 mg, 0.253 mmol, 1.0 eq.) and 6-Cl-6′-Mebpy (52.1 mg, 0.250 mmol, 1.0 eq.) were dissolved in CH₂Cl₂ (30 mL) and the mixture was stirred for 4 h at room temperature. The mixture was filtered and the solvent from the filtrate was removed in vacuo yielding a yellow solid. The product was purified by precipitation with Et₂O (4 times) from a CH₂Cl₂ solution and was finally isolated after sonication (5 min) and centrifugation (3000 rpm, 3 min). [Cu(xantphos)(6-Cl-6′-Mebpy)][PF₆] was obtained as a yellow crystalline solid (219 mg, 0.220 mmol, 87.0%). ¹H NMR (500 MHz, 298 K, acetone-d₆) δ/ppm 8.31 (d, J = 7.8 Hz, 1H, H^B3), 8.23 (d, J = 7.9 Hz, 1H, H^A3), 8.12 (m, 1H, H^B4), 8.04 (m, 1H, H^A4), 7.82 (dd, J = 7.8, 1.4 Hz, 2H, C^C5), 7.60 (d, J = 7.6 Hz, 1H, H^B5), 7.46 (d, J = 7.8 Hz, 1H, H^A5), 7.41 (m, 4H, H^D4+D4′), 7.30 (m, 2H, H^C4), 7.28–7.20 (overlapping m, 12H, H^D2+D3+D3′), 7.13 (m, 4H, H^D2′), 6.97 (m, 2H, H^C3), 2.06 (s, 3H, H^bpy-Me), 1.79 (s, 3H, H^xantphos-Me), 1.69 (s, 3H, H^xantphos-Me). ¹³C{¹H} NMR (126 MHz, 298 K, acetone-d₆) δ/ppm 159.6 (C^A6), 155.9 (t, J_CP = 6 Hz, C^C1), 154.2 (C^B2), 151.7 (C^A2), 142.5 (C^B4), 140.1 (C^A4), 134.6 (t, J_CP = 2 Hz, C^C6), 134.2 (t, J_CP = 8 Hz, C^D2), 134.0 (t, J_CP = 8 Hz, C^D2′), 132.4 (m, C^D1+D1′), 131.1 (C^C3), 131.0 (C^D4/D4′), 130.95 (C^D4/D4′), 129.8 (t, J_CP = 5 Hz, C^D3/D3′), 129.5 (t, J_CP = 5 Hz, C^D3/D3′), 128.6 (C^C5), 127.5 (C^A5), 127.2 (C^B5), 126.2 (t, J_CP = 2 Hz, C^C4), 122.6 (t, J_CP = 13 Hz, C^C2), 122.2 (C^B3), 121.5 (C^A3), 36.8 (C^{xantphos-bridge}), 29.5 (C^xantphos-Me), 28.1 (C^xantphos-Me), 26.8 (C^bpy-Me). ³¹P{¹H} NMR (202 MHz, 298 K, acetone-d₆) δ/ppm −13.5 (broad, xantphos), −144.3 (heptet, J_PF = 707, PF₆⁻). ESI-MS (CH₂Cl₂/MeOH, positive mode) m/z 845.16 [M − PF₆]⁺ (calc. 845.17), 641.10 [M − PF₆ − (6-Cl-6′-Mebpy)]⁺ (base peak, calc. 641.12). Found: C 58.63, H 4.11, N 2.71; C₅₀H₄₁ClCuF₆N₂OP₃·0.5CH₂Cl₂ requires C 58.65, H 4.09, N 2.71.

3.5. Crystallography

Single crystal data were collected on a STOE StadiVari diffractometer (STOE & Cie GmbH, Darmstadt, Germany) equipped with a Pilatus300K detector and a Metaljet D2 source (Ga Kα radiation). The structure was solved using Superflip [29,30] and CRYSTALS [31]. Structure analysis including the ORTEP representations, used Mercury CSD v. 4.1.1 [32,33]. The cation in [Cu(POP)(6-Cl-6′-Mebpy)][PF₆] suffered from severe disorder and all the aromatic rings were refined as rigid bodies; one ring was refined isotropically as it was not possible to identify two distinct orientations, and the C atom of the methyl group was also refined isotropically. In the 6-Cl-6′-Mebpy ligand in [Cu(xantphos)(6-Cl-6′-Mebpy)][PF₆], the Cl/Me sites were disordered and were modeled over two sites with equal occupancies.

3.6. [Cu(POP)(6-Cl-6′-Mebpy)][PF₆]

C₄₇H₃₇ClCuF₆N₂OP₃, M_r = 951.73, yellow block, monoclinic, space group P2₁/c, a = 10.4786(3), b = 18.9864(4), c = 22.1096(5) Å, β = 100.077(2)^o, V = 4330.87(18) ų, D_c = 1.460 g cm⁻³, T = 130 K, Z = 4, μ(GaKα) = 4.150 mm⁻¹. Total 48,628 reflections, 8755 unique (R_int = 0.035). Refinement of 7234 reflections (539 parameters) with I > 2σ(I) converged at final R₁ = 0.1124 (R₁ all data = 0.1124), wR₂ = 0.0443 (wR₂ all data = 0.0443), gof = 1.0315. CCDC 1995549.

3.7. [Cu(xantphos)(6-Cl-6′-Mebpy)][PF₆]

C₅₀H₄₁ClCuF₆N₂OP₃, M_r = 991.80, yellow plate, triclinic, space group P−1, a = 11.4363(6), b = 14.0871(6), c = 14.8030(7) Å, α = 89.133(4), β = 68.457(4), γ = 88.730(4)^o, V = 2217.62(19) ų, D_c = 1.485 g cm⁻³, T = 130 K, Z = 2, μ(Ga Kα) = 4.068 mm⁻¹. Total 23,726 reflections, 8675 unique (R_int = 0.045). Refinement of 7234 reflections (539 parameters) with I > 2σ(I) converged at final R₁ = 0.1112 (R₁ all data = 0.1112), wR₂ = 0.0483 (wR₂ all data = 0.0483), gof = 0.9937. CCDC 1995548.

4. Conclusions

We have described the synthesis and characterization of the asymmetrical ligand 6-Cl-6′-Mebpy. This was incorporated into the copper(I) compounds [Cu(POP)(6-Cl-6′-Mebpy)][PF₆] and [Cu(xantphos)(6-Cl-6′-Mebpy)] [PF₆], both of which were structurally characterized. The copper(I) ion is in a distorted tetrahedral environment and in [Cu(xantphos)(6-Cl-6′-Mebpy)][PF₆], the disorder of the 6-Cl-6′-Mebpy ligand indicates that there is no preference for the ‘bowl’-like cavity of the xanthene unit to host either the methyl or chloro-substituent, consistent with similar steric effects of the two groups.

[Cu(xantphos)(6-Cl-6′-Mebpy)][PF₆] and [Cu(POP)(6-Cl-6′-Mebpy)][PF₆] undergo a reversible and irreversible copper(I) oxidation, respectively. A comparison with reported data [14,21,26] for [Cu(P^P)(N^N]⁺ complexes in which P^P = POP and xantphos and N^N = 6,6′-Cl₂bpy and 6,6′-Me₂bpy reveals that values for [Cu(P^P)(6-Cl-6′-Mebpy)]⁺ are consistent with there being an interplay of the effects of both chloro and methyl groups. The photophysical properties also support these combined effects. For example, the xantphos-containing compounds, values of λ_max for the MLCT absorption are 379, 400 and 420 nm, respectively, for N^N = 6,6′-Me₂bpy, 6-Cl-6′-Mebpy and 6,6′-Cl₂bpy. For the POP-containing complexes, the solid-state emission maxima appear at 535, 558 and 584 nm, respectively, for N^N = 6,6′-Me₂bpy, 6-Cl-6′-Mebpy and 6,6′-Cl₂bpy, with PLQY values of 43.2%, 24% and 14.8% and excited state lifetimes of 10.5, 4.8 and 2.7 μs, respectively.