Heteroleptic [Cu(P^P)(N^N)][PF 6 ] Complexes: Effects of Isomer Switching from 2,2 ′ -biquinoline to 1,1 ′ -biisoquinoline

: The preparation and characterization of [Cu(POP)(biq)][PF 6 ] and [Cu(xantphos)(biq)][PF 6 ] are reported (biq = 1,1'-biisoquinoline, POP = bis(2-(diphenylphosphanyl)phenyl)ether, and xantphos = (9,9-dimethyl-9 H -xanthene-4,5-diyl)bis(diphenylphosphane). The single crystal structure of [Cu(POP)(biq)][PF 6 ] 0.5Et 2 O was determined and compared to that in three salts of [Cu(POP)(bq)] + in which bq = 2,2'-biquinoline. The P–C–P angle is 114.456(19) o in [Cu(POP)(biq)] + compared to a range of 118.29(3)–119.60(3) o [Cu(POP)(bq)] + . There is a change from an intra-POP PPh 2 -phenyl/(C 6 H 4 ) 2 O-arene π -stacking in [Cu(POP)(biq)] + to a π -stacking contact between the POP and bq ligands in [Cu(POP)(bq)] + . In solution and at ambient temperatures, the [Cu(POP)(biq)][PF 6 ] + and [Cu(xantphos)(biq)] + cations undergo several concurrent dynamic processes, as evidenced in their multinuclear NMR spectra. The photophysical and electrochemical behaviors of the heteroleptic copper (I) complexes were investigated, and the effects of changing from bq to biq are described. Short Cu···O distances within the [Cu(POP)(biq)] + and [Cu(xantphos)(biq)] + cations may contribute to their very low photoluminescent quantum yields.


General
1 H, 13 C{ 1 H}, and 31 P{ 1 H} NMR spectra were recorded at 298 K on a Bruker Avance III-500 or Bruker Avance-III 600 NMR spectrometer (Bruker BioSpin AG, Fällanden, Switzerland). 1 H and 13 C NMR chemical shifts were referenced to residual solvent peaks with respect to δ(TMS) = 0 ppm and 31 P NMR chemical shifts with respect to δ(85% aqueous H3PO4) = 0 ppm. A Shimadzu LCMS-2020 instrument was used to record electrospray (ESI) mass spectra (Shimadzu Schweiz GmbH, 4153 Reinach, Switzerland). Solution absorption and solution emission spectra were measured using a Shimadzu UV2600 spectrometer and a Shimadzu RF-5301PC spectrofluorometer, respectively. A Hamamatsu absolute photoluminescence quantum yield spectrometer C11347 Quantaurus-QY (Hamamatsu Photonics France, 4500 Solothurn) was used to measure PLQYs, and powder emission spectra were measured using a Hamamatsu Compact Fluorescence lifetime Spectrometer C11367 Quantaurus-Tau with an LED light source (λexc = 365 nm) (Hamamatsu, 4500 Solothurn, Switzerland). Electrochemical measurements were conducted using a CH Instruments 900B potentiostat (CH Instruments, Austin, United States) and a VersaSTAT 3F (AMETEK Princeton Applied Research, Oak Ridge, United States) with [ n Bu4N][PF6] (0.1 M) as the supporting electrolyte and a scan rate of 0.1 V s −1 ; the solvent used was CH2Cl2. The working electrode was glassy carbon, the reference electrode was a leakless Ag + /AgCl electrode (eDAQ ET069-1), and the counter electrode was a platinum wire. Final potentials were internally referenced with respect to the Fc/Fc + couple.

[Cu(POP)(biq)][PF6]
[Cu(MeCN)4][PF6] (130 mg, 0.350 mmol) and POP (207 mg, 0.385 mmol) were dissolved in CH2Cl2 (30 mL) and stirred for 1 h at around 22 °C. Then, biq (89.7 mg, 0.350 mmol) was added and the reaction mixture was stirred for 1 h. After filtration, the solvent was removed from the filtrate under reduced pressure. The crude product was dissolved in the minimum amount of CH2Cl2. Then, Et2O (3 × 10 mL) was added to precipitate the product. The solid was separated by centrifugation (9000 rpm, 4 min) and was dried under reduced pressure.

Crystallography
Single crystal data were collected on a STOE StadiVari diffractometer equipped with a Pilatus300K detector and with a Metaljet D2 source (GaKα radiation) and by solving the structure using Superflip [30,31] and Olex2 [32]. The structure was refined using ShelXL v. 2014/7 [33]. The program CSD Mercury 2020.1 [34] was used for the structure analysis and structural figures. The Et2O solvent molecule was disordered over a symmetry element and was modelled over two positions each with an occupancy of 0.5. [

Preparation and Characterization of [Cu(POP)(biq)][PF6] and [Cu(xantphos)(biq)][PF6]
The  (17) Å is typical, Cu1-N1 is noticeably lengthened (2.1434(16) Å), and this is possibly associated with a weak C-H···π interaction [35,36], involving the C1H1 unit and the arene ring containing C25 (C1H1···ring-plane = 2.78 Å and C1H1···centroid = 3.35 Å). The P-C bond lengths lie in the range 1.817(2) to 1.8315 (19) Å. The Cu1···O1 separation is 3.054(1) Å, and this is towards the shorter end of the range of values observed for [Cu(POP)(N^N)] + complexes; we return to this point in Section 3.4 [37]. The torsion angle N1-C5-C6-N2 is -38.5(2)° and the angle between the least-squares planes of the rings containing N1 and N2 is 43.7°. The twist in the biq ligand leads to each Cu-N bond vector being noncoincidental with the direction of the lone pair of the sp 2 hybridized N atom [38], as shown in Figure S3 in the Supplementary Materials. The angle subtended by the Cu-N vector and the plane of the corresponding NC5 ring is 30.8° for N1 and 22.1° for N2. Figure 1b illustrates that the POP ligand adopts a conformation that leads to a π-stacking interaction between a phenyl ring of one PPh2 group and an arene ring of the (C6H4)2O unit. Within the π-stacking contact, the angle between the planes of the rings containing C31 and C38 is 20.8° and the inter-centroid distance is 3.80 Å. This π-stacking interaction is a common motif within the structurally characterized [Cu(POP)(N^N)] + cations found in the Cambridge Structural Database (CSD version 5.4.1 searched using Conquest version 2020.2.0 [39,40]). The phenyl ring containing C19 (Figure 1a) is oriented such that the C20-H20 unit is directed towards the CuN2C2-chelate ring with a CH···centroid distance of 2.56 Å. This adds to the steric protection around the copper center.
1,1'-Biisoquinoline (biq) is distinct from its isomer 2,2'-biquinoline (bq), not only in possessing axial chirality, as discussed in the Introduction, but also in its steric requirements within the coordination sphere of a metal to which it is bound. We were therefore interested in comparing the structures of the [Cu(POP) ( [41] have previously been reported, and important structural data are given in Table 1 Table 1 confirm that the bq ligand is closer to being planar than the sterically hindered biq. The N-C-C-N torsion angles (defined in Figure 2) in the hexafluoridophosphate and triflate salts are similar (14.0 and 12.9°) but are larger than the 4.8° observed in the tetrafluoridoborate salt. Compare these torsion angles to the value of -38.

5(2)° in [Cu(POP)(biq)][PF6].
Although not discussed in the original report [15], Figure 3 illustrates that the near planarity of the 2,2'-biquinoline ligand is concomitant with face-to-face π-stacking with one phenyl ring of a PPh2 unit of POP (centroid···centroid = 3.56 Å, angle between ring planes = 12.2 o ). This is reminiscent of the π-stacking interactions in [Cu(POP)(pyq)][PF6], where pyq = 2-(pyridin-2-yl)quinoline [37]. Figure 2a,b illustrate the evolution of the π-stacking interaction in the [Cu(POP)(bq)] + cations in the hexafluoridophosphate and triflate salts, with the lack of an efficient interaction corresponding with the larger torsion angle of the bq ligand. The switch from PPh2-phenyl/(C6H4)2O-arene π-stacking in [Cu(POP)(biq)] + (i.e., an interaction within the POP domain) to an inter-ligand (but still intramolecular) πstacking in [Cu(POP)(bq)] + is associated with the extended conjugation in the N^N ligand.   [15] This work a The torsion angle is defined in Figure 2.  Table 1 is defined in the diagram at the top. X-ray-quality single crystals of [Cu(xantphos)(biq)][PF6] could not be obtained, and therefore, we calculated the energy minimized structure of the [Cu(xantphos)(biq)] + cation to gain insight into the relationship between the N^N and P^P ligands within the copper coordination sphere. The structure was first minimized at a molecular mechanics (MM2) level, and this geometry was used as the input for a density functional theory (DFT) level energy minimization (B3LYP 6-31G*) [42]. The calculated structure of [Cu(xantphos)(biq)] + is shown in Figure 4, and Cartesian coordinates are presented in Table S1. Within the distorted tetrahedral environment of the copper atom, the P-Cu-P bond angle is 116.0°. The P-Cu-N bond angles of 111.9, 112.5, 114.8, and 117.2 o lead to the biq ligand being skewed slightly towards the xanthene bowl (on the left of Figure 4a). This is related to the hosting of the C3-H3 unit of the biq ligand within the bowl-like cavity of the xanthene unit (Figure 4b). This is relevant to the NMR spectroscopic discussion to follow. A short Cu···O distance of 3.08 Å is found in the calculated structure, and this is similar to the crystallographically determined Cu···O distance

The solution NMR spectra of [Cu(POP)(biq)][PF6] and [Cu(xantphos)(biq)][PF6]
were recorded in acetone-d6 solutions at 298 K, but for [Cu(xantphos)(biq)][PF6], the signals were broad ( Figure S3a in the Supplementary Materials). In order to carry out NMR spectroscopic measurements at higher temperatures, the solvent was changed to C2D2Cl4. Even at 298 K, this led to sharp signals ( Figure S3b), presumably due to the differences in viscosities and dielectric constants of the solvents. Warming the sample to 328 K did not lead to any significant changes in the 1 H NMR spectrum. Figure 5 shows the 1 (Figure 1). However, we note that the change in solvent from acetone-d6 to C2D2Cl4 also causes a shift to lower frequency for the signal for H A3 from δ 8.13 ppm to δ 7.93 ppm ( Figure S3). The NMR spectra of the complexes are consistent with significant dynamic behavior in solution at ambient temperatures. We previously discussed the solution dynamics of related bpy-containing [Cu(POP)(N^N)] + and [Cu(xantphos)(N^N)] + complexes in detail; see, for example, [14,43]. In [Cu(POP)(biq)] + and [Cu(xantphos)(biq)] + , several dynamic processes may be considered: (i) rotation about the P-Cphenyl bonds which is fast on the NMR timescale at 298 K; (ii) atropisomerization of the biq ligand (Scheme 4a); and (iii) a conformational change of the POP (Scheme 4b) or xantphos backbone (Scheme 4c). Atropisomerization of the coordinated biq ligand is expected to occur rapidly on the NMR timescale at 298 K [38,44]. Although Figure 4; Figure 5 show that the two isoquinoline units of the biq ligand are in different environments in the solid-state, the conformational changes in the POP or xantphos ligand shown in Scheme 4b,c render them equivalent on the NMR timescale. Figure 1 illustrates two phenyl environments (rings C and C'), corresponding to rings facing towards or away from the biq ligand.  (Figure 6). This is evidence for exchange between the pairs of phenyl rings in chemically different environments. However, the dynamic process is slow enough on the NMR timescale for separate resonances for phenyl rings C and C' to be observed.

Photophysical and Electrochemical Properties
The  Table 2. Absorptions below 380 nm are assigned to ligand-based π*←π transitions, while the broad bands in the visible region arise from MLCT from copper to π* orbitals on the diimine ligand. The spectra for the 2,2'-biquinoline-containing compounds are in agreement with those previously published [15,17,18]. Figure 7 illustrates that incorporation of the bq ligand leads to a shift in the MLCT absorption to lower energies than that observed for [Cu(POP)(biq)][PF6] and [Cu(xantphos)(biq)][PF6]. This is rationalized in terms of the ring fusion in bq (Scheme 1) being adjacent to the N atoms but remote from the N atoms in biq (Scheme 2). An expansion of the MLCT regions is displayed in Figure S9.    Table 3. Figure 8 illustrates the cyclic voltammogram (CV) of [Cu(POP)(biq)][PF6]; that of the xantphoscontaining compound is similar. Both compounds undergo a reversible oxidation process, which is ascribed to the Cu + /Cu 2+ couple. For both complexes, if the forward scan is extended above +1.2 V, another oxidation wave is observed at ca. +1.3 V. This process arises from the oxidation of the phosphane ligand. A reversible reduction process ( Figure  8) is observed for each compound and is assigned to a biq-centered process. The copper (I) oxidations (Table 3) occur at lower potentials than in analogous complexes containing 2,2'-biquinoline (bq), consistent with the greater steric demands of the bq ligand compared to biq, which impede the flattening of the coordination sphere that accompanies copper (I) oxidation. Each of [Cu(POP)(bq)][PF6] and [Cu(xantphos)(bq)][PF6] is reported to undergo a quasi-reversible oxidation at +0.90 V and +0.95 V, respectively, in CH2Cl2 (Fc/Fc + reference) [17,18].   Table 3 for additional details.

Conclusions
We