Softening the Donor-Set : From [ Cu ( PˆP ) ( NˆN ) ] [ PF 6 ] to [ Cu ( PˆP ) ( NˆS ) ] [ PF 6 ]

We report the synthesis and characterization of [Cu(PˆP)(NˆS)][PF6] complexes with PˆP = bis(2-(diphenylphosphino)phenyl) ether (POP) or 4,5-bis(diphenylphosphino)-9,9dimethylxanthene (xantphos) and NˆS = 2-(iso-propylthio)pyridine (iPrSpy) or 2-(tert-butylthio)pyridine (tBuSpy). The single crystal structures of [Cu(POP)(iPrSPy)][PF6] and [Cu(POP)(tBuSPy)][PF6] have been determined and confirm a distorted tetrahedral copper(I) centre and chelating PˆP and NˆS ligands in each complex. Variable temperature (VT) 1H and 31P{1H} NMR spectroscopy reveals dynamic behavior with motion of the POP backbone in [Cu(POP)(iPrSPy)][PF6] and [Cu(POP)(tBuSPy)][PF6] frozen out at 238 K. VT NMR spectroscopic data including EXSY peaks in the ROESY spectrum of [Cu(xantphos)(tBuSPy)][PF6] at 198 K reveal that two conformers exist in an approximate ratio of 5:1. Replacing bpy by the NˆS ligands shifts the Cu+/Cu2+ oxidation to a higher potential. The copper(I) compounds are weak emitters in the solid state with PLQY values of <2%. These values are similar to those for [Cu(POP)(bpy)][PF6] and [Cu(xantphos)(bpy)][PF6] in the solid state.

Scheme 1. Structures of POP, xantphos and the N^S ligands with ring and atom numbering for NMR spectroscopic assignments.The phenyl rings in the PPh2 groups of POP and xantphos are labelled D. (The ring labels are chosen to allow comparison with our previous work, for example, see References [13,14]).
[Cu2(dppdtbpf)2(μ-NCS)2] (dppdtbpf = 1-diphenylphosphino-1′-di-tert-butylphosphinoferrocene) exhibits a broad emission with λ em max ≈ 500 nm assigned to a metal-centered transition [24].However, detailed studies of the emission behavior of [Cu(PR3)2(N^S)] + complexes are, to the best of our knowledge, absent from the literature.We were therefore motivated to investigate a series of compounds that combined simple N^S chelates with the POP and xantphos ligands.

Preliminary Theoretical Investigation
Before embarking on a synthetic study, we examined the ground state electronic structure of the model [Cu(POP)(MeSPy)] + cation shown in Figure 1.After geometry optimization, DFT calculations revealed a similar partitioning of orbital character in the HOMO and LUMO, as has been shown for [Cu(N^N)(P^P] + (see Introduction).The LUMO of [Cu(POP)(MeSPy)] + is localized on the N^S domain and largely on the pyridine ring (Figure 1a), while the HOMO displays dominant copper character with smaller contributions from the ligands (Figure 1b).These results suggested that the LUMO energy of [Cu(P^P)(N^S)] + complexes may be modified by structural modification of the N^S domain.Most investigations of copper(I) complexes for applications in LECs continue to focus on [Cu(PˆP)(NˆN)] + compounds, and we were interested to note that relatively little attention has been paid to copper(I) complexes containing PR 3 or bis(phosphane) ligands in combination with hard-soft chelating NˆS donors.A number of [Cu(PPh 3 ) 2 (NˆS)] and related compounds, in which H(NˆS) is a thiosemicarbazone, have been described [16][17][18][19][20], and there are several reports of thiocyanato-bridged dicopper(I) compounds, for example [Cu 2 (PPh 3 ) 2 (2-Mepy) 2 (µ-NCS) 2 ] [21], which present a distorted tetahedral CuP 2 NS coordination sphere.Copper(I) complexes incorporating heterocyclic thioamide and phosphane ligands have also been described [22,23], and are shown to exhibit broad emission bands in the range λ em max = 490-495 nm (λ exc = 270-291 nm) [21].[Cu 2 (dppdtbpf) 2 (µ-NCS) 2 ] (dppdtbpf = 1-diphenylphosphino-1 -di-tert-butylphosphinoferrocene) exhibits a broad emission with λ em max ≈ 500 nm assigned to a metal-centered transition [24].However, detailed studies of the emission behavior of [Cu(PR 3 ) 2 (NˆS)] + complexes are, to the best of our knowledge, absent from the literature.We were therefore motivated to investigate a series of compounds that combined simple NˆS chelates with the POP and xantphos ligands.

Preliminary Theoretical Investigation
Before embarking on a synthetic study, we examined the ground state electronic structure of the model [Cu(POP)(MeSPy)] + cation shown in Figure 1.After geometry optimization, DFT calculations revealed a similar partitioning of orbital character in the HOMO and LUMO, as has been shown for [Cu(NˆN)(PˆP] + (see Introduction).The LUMO of [Cu(POP)(MeSPy)] + is localized on the NˆS domain and largely on the pyridine ring (Figure 1a), while the HOMO displays dominant copper character with smaller contributions from the ligands (Figure 1b).These results suggested that the LUMO energy of [Cu(PˆP)(NˆS)] + complexes may be modified by structural modification of the NˆS domain.

Synthesis of Ligands and Copper(I) Complexes
For an initial investigation, we chose the N^S ligands iPrSPy and tBuSPy shown in Scheme 1.They were prepared by following literature procedures and the 1 H and 13 C{ 1 H} NMR spectra (Figures S1 and S2) were in agreement with those reported [25,26]

Structural Characterizations
Colourless single crystals of [Cu(POP)(iPrSPy)][PF6] and [Cu(POP)(tBuSPy)][PF6] were grown by diffusion of Et2O into acetone solutions of the complexes.Both compounds crystallize in the monoclinic P21/c space group.Figures 2 and S3 depict the cations in the two compounds.In [Cu(POP)(iPrSPy)][PF6] (Figure S3), the phenyl ring with C40 was disordered and was modelled over two sites with 65% and 35% occupancies.A disorder involving the iPrSPy ligand was modelled over two equal-occupancy sites; the S atom was common to both ligand positions.The crystal structures confirm the κ 2 N,S-binding modes of iPrSPy and tBuSPy and the distorted tetrahedral geometry of copper(I).Selected bond parameters are given in the captions to Figures 2 and S3 and are unexceptional.The unit cell dimensions for the two compounds are comparable (see Materials and Methods Section) and the similarity between the ligand conformations in the [Cu(POP)(iPrSPy)] + and [Cu(POP)(tBuSPy)] + cations is seen in Figure 3.Each cation is chiral and both enantiomers are present in the unit cell.No intracation π-stacking of aromatic rings is observed in either [Cu(POP)(iPrSPy)] + or [Cu(POP)(tBuSPy)] + .This contrasts with the numerous examples of structurally characterized salts of [Cu(POP)(N^N)] + and [Cu(xantphos)(N^N)] + complexes that exhibit intracation π-stacking [27].It is noteworthy that the PM3 optimized geometry of the model [Cu(POP)(MeSPy)] + cation, shown in Figure 1, also exhibited no π-stacked pairs of aromatic rings.

Synthesis of Ligands and Copper(I) Complexes
For an initial investigation, we chose the NˆS ligands iPrSPy and tBuSPy shown in Scheme 1.They were prepared by following literature procedures and the 1 H and 13 C{ 1 H} NMR spectra (Figures S1 and S2) were in agreement with those reported [25,26]

Structural Characterizations
Colourless single crystals of [Cu(POP)(iPrSPy)][PF 6 ] and [Cu(POP)(tBuSPy)][PF 6 ] were grown by diffusion of Et 2 O into acetone solutions of the complexes.Both compounds crystallize in the monoclinic P2 1 /c space group.Figure 2 and Figure S3 depict the cations in the two compounds.In [Cu(POP)(iPrSPy)][PF 6 ] (Figure S3), the phenyl ring with C40 was disordered and was modelled over two sites with 65% and 35% occupancies.A disorder involving the iPrSPy ligand was modelled over two equal-occupancy sites; the S atom was common to both ligand positions.The crystal structures confirm the κ 2 N,S-binding modes of iPrSPy and tBuSPy and the distorted tetrahedral geometry of copper(I).Selected bond parameters are given in the captions to Figure 2 and Figure S3 and are unexceptional.The unit cell dimensions for the two compounds are comparable (see Materials and Methods Section) and the similarity between the ligand conformations in the [Cu(POP)(iPrSPy)] + and [Cu(POP)(tBuSPy)] + cations is seen in Figure 3.Each cation is chiral and both enantiomers are present in the unit cell.No intracation π-stacking of aromatic rings is observed in either [Cu(POP)(iPrSPy)] + or [Cu(POP)(tBuSPy)] + .This contrasts with the numerous examples of structurally characterized salts of [Cu(POP)(NˆN)] + and [Cu(xantphos)(NˆN)] + complexes that exhibit intracation π-stacking [27].It is noteworthy that the PM3 optimized geometry of the model [Cu(POP)(MeSPy)] + cation, shown in Figure 1, also exhibited no π-stacked pairs of aromatic rings.

NMR Spectroscopic Characterization and Solution Dynamics
The solution NMR spectra of the compounds were recorded in acetone-d6.The complexes are kinetically stable in this solvent with respect to ligand redistribution, at least for the period of data collection.Atom labelling for the NMR assignments is given in Schemes 1 and 2. The solution 31

NMR Spectroscopic Characterization and Solution Dynamics
The solution NMR spectra of the compounds were recorded in acetone-d6.The complexes are kinetically stable in this solvent with respect to ligand redistribution, at least for the period of data collection.Atom labelling for the NMR assignments is given in Schemes 1 and 2. The solution 31

NMR Spectroscopic Characterization and Solution Dynamics
The solution NMR spectra of the compounds were recorded in acetone-d 6 .The complexes are kinetically stable in this solvent with respect to ligand redistribution, at least for the period of data collection.Atom labelling for the NMR assignments is given in Schemes 1 and 2. The solution 31  In addition, the coordinated sulfur atom (which is a stereogenic center) can undergo inversion.As in the crystallographically determined structure of the cations shown in Figure 3, the two P atoms in these [Cu(PˆP)(NˆS)] + complexes would be inequivalent if inversion at sulfur were frozen out.Since this was never observed in the low temperature 31 P{ 1 H} NMR spectra (see below), we conclude that inversion at the coordinated-S atom is a low energy process in all four complexes.

Figure S11
. There is no corresponding NOESY correlation between H A6 and H D '2 , indicating that the D rings point up towards to N^S ligand while the D' rings are directed away (see Figure 3 and Scheme 2).For a molecular weight of around 700, NOESY peaks in a ROESY spectrum are expected to have a close to zero intensity [28].In summary, dynamic processes in [Cu(POP)(iPrSPy)][PF6] and [Cu(POP)(tBuSPy)][PF6] involve motion of the POP backbone that is frozen out at 238 K, while inversion at the coordinated sulfur atom is still rapid on the NMR timescale at 238 K.        S12).This integral ratio is the same as that observed for signals arising from two species present in the 1 H NMR spectrum of [Cu(xantphos)(tBuSPy)][PF6] at 198 K (Figure 6 for the aromatic region and Figure S13 for the alkyl region).COSY, ROESY, HMQC, and HMBC spectra recorded at 198 K were used to assign the major peaks in the spectrum, and EXSY peaks in the ROESY spectrum at 198 K were used to correlate the major and minor species.The EXSY peaks for [Cu(xantphos)(tBuSPy)][PF6] are instructive.Figure 7 shows the aromatic region and reveals the very different environments of proton H A6 of the pyridine ring (Scheme 2) in the two species.The EXSY peaks shown in Figure 8 verify the exchange between the tert-butyl groups of the two species, as well as the exchange between the inner and outer pointing methyl groups of the xantphos CMe2 units in the two species.This can only occur if the xanthene unit undergoes the inversion, as shown in Scheme 3, for one pair of possible conformers.In summary for [Cu(xantphos)(tBuSPy)][PF6], two species are present in solution at 198 K, assigned to conformers which interconvert at higher temperatures through inversion of the xanthene "bowl"; fast inversion at the coordinated sulfur atom occurs on the NMR timescale at 198 K.  S12).This integral ratio is the same as that observed for signals arising from two species present in the 1 H NMR spectrum of [Cu(xantphos)(tBuSPy)][PF 6 ] at 198 K (Figure 6 for the aromatic region and Figure S13 for the alkyl region).COSY, ROESY, HMQC, and HMBC spectra recorded at 198 K were used to assign the major peaks in the spectrum, and EXSY peaks in the ROESY spectrum at 198 K were used to correlate the major and minor species.The EXSY peaks for [Cu(xantphos)(tBuSPy)][PF 6 ] are instructive.Figure 7 shows the aromatic region and reveals the very different environments of proton H A6 of the pyridine ring (Scheme 2) in the two species.The EXSY peaks shown in Figure 8 verify the exchange between the tert-butyl groups of the two species, as well as the exchange between the inner and outer pointing methyl groups of the xantphos CMe 2 units in the two species.This can only occur if the xanthene unit undergoes the inversion, as shown in Scheme 3, for one pair of possible conformers.In summary for [Cu(xantphos)(tBuSPy)][PF 6 ], two species are present in solution at 198 K, assigned to conformers which interconvert at higher temperatures through inversion of the xanthene "bowl"; fast inversion at the coordinated sulfur atom occurs on the NMR timescale at 198 K.

Electrochemistry
Cyclic voltammetry was used to investigate the electrochemistry of the copper(I) complexes.Each undergoes a quasi-reversible or irreversible oxidation, which is assigned to a Cu + /Cu 2+ process (Table 1 and Figures S14 and S15).For the benchmark-compounds [Cu(POP)(bpy)][PF6] and [Cu(xantphos)(bpy)][PF6], the Cu + /Cu 2+ process occurs at +0.72 and +0.76 V, respectively [13].Thus, replacing bpy by the N^S ligands shifts the oxidation to a higher potential.This observation results from an interplay of two opposing effects.A change from bpy to the less π-accepting N^S ligands should make Cu + easier to oxidize, whereas the soft S-donor favours the copper(I) oxidation state.If the forward CV scan is taken past about +1.1 V, a second (irreversible) oxidation process, assigned to phosphane oxidation, is observed.No reduction processes were observed in the cyclic voltammograms within the solvent accessible window.

Electrochemistry
Cyclic voltammetry was used to investigate the electrochemistry of the copper(I) complexes.Each undergoes a quasi-reversible or irreversible oxidation, which is assigned to a Cu + /Cu 2+ process (Table 1 and Figures S14 and S15).For the benchmark-compounds [Cu(POP)(bpy)][PF6] and [Cu(xantphos)(bpy)][PF6], the Cu + /Cu 2+ process occurs at +0.72 and +0.76 V, respectively [13].Thus, replacing bpy by the N^S ligands shifts the oxidation to a higher potential.This observation results from an interplay of two opposing effects.A change from bpy to the less π-accepting N^S ligands should make Cu + easier to oxidize, whereas the soft S-donor favours the copper(I) oxidation state.If the forward CV scan is taken past about +1.1 V, a second (irreversible) oxidation process, assigned to phosphane oxidation, is observed.No reduction processes were observed in the cyclic voltammograms within the solvent accessible window.

Electrochemistry
Cyclic voltammetry was used to investigate the electrochemistry of the copper(I) complexes.Each undergoes a quasi-reversible or irreversible oxidation, which is assigned to a Cu + /Cu 2+ process (Table 1 and Figures S14 and S15).For the benchmark-compounds [Cu(POP)(bpy)][PF 6 ] and [Cu(xantphos)(bpy)][PF 6 ], the Cu + /Cu 2+ process occurs at +0.72 and +0.76 V, respectively [13].Thus, replacing bpy by the NˆS ligands shifts the oxidation to a higher potential.This observation results from an interplay of two opposing effects.A change from bpy to the less π-accepting NˆS ligands should make Cu + easier to oxidize, whereas the soft S-donor favours the copper(I) oxidation state.If the forward CV scan is taken past about +1.1 V, a second (irreversible) oxidation process, assigned to phosphane oxidation, is observed.No reduction processes were observed in the cyclic voltammograms within the solvent accessible window.S16; the absorption spectra of POP and xantphos have previously been reported [31].The compounds are colourless, and the intense high-energy absorption bands (Table 2) are assigned to ligand-based, spin-allowed π*←π and π*←n transitions.The compounds are non-emissive in solution, and solid samples excited at 280 nm gave emission maxima between 470 and 490 nm (Table 2 and Figure S16; the absorption spectra of POP and xantphos have previously been reported [31].The compounds are colourless, and the intense high-energy absorption bands (Table 2) are assigned to ligand-based, spin-allowed π*←π and π*←n transitions.The compounds are non-emissive in solution, and solid samples excited at 280 nm gave emission maxima between 470 and 490 nm (Table 2 and Figure

General
1 H, 13 C{ 1 H} and 31 P{ 1 H} NMR spectra were recorded on a Bruker Avance 500 spectrometer (Bruker BioSpin AG, Fällanden, Switzerland); spectra for the ligands were recorded at 295 K, spectra for the complexes were recorded at 238 K unless otherwise stated. 1 H and 13 C NMR chemical shifts were referenced to the residual solvent peaks with respect to δ(TMS) = 0 ppm and 31 P NMR chemical shifts and with respect to δ(85% aqueous H 3 PO 4 ) = 0 ppm.Solution absorption and emission spectra were measured using an Agilent 8453 spectrophotometer (Agilent Technologies Inc., Santa Clara, CA, USA) and a Shimadzu RF-5301PC spectrofluorometer (Shimadzu Schweiz GmbH, Roemerstr., Switzerland), respectively.A Shimadzu LCMS-2020 instrument or a Bruker esquire 3000plus instrument was used to record electrospray ionization (ESI) mass spectra.Quantum yields (CH 2 Cl 2 solution and powder) were measured using a Hamamatsu absolute photoluminescence (PL) quantum yield spectrometer C11347 Quantaurus-QY.Powder emission spectra were measured with a Hamamatsu Compact Fluorescence lifetime Spectrometer C11367 Quantaurus-Tau, using an LED light source with λ exc = 280.Electrochemical measurements were carried out using a CH Instruments 900B potentiostat with [ n Bu 4 N][PF 6 ] (0.1 M) as the supporting electrolyte and at a scan rate of 0.1 V s −1 .The working electrode was glassy carbon, the reference electrode was a leakless Ag + /AgCl (eDAQ ET069-1) and the counter-electrode platinum wire.Final potentials were internally referenced with respect to the Fc/Fc + couple (E 1/2 = 1.126V with respect to Ag/AgCl at 298.15 K [32]).

Crystallography
Data were collected on a Bruker Kappa Apex2 diffractometer (Bruker Biospin AG, Fällanden, Switzerland) with data reduction, solution and refinement using the programs APEX [34] and CRYSTALS [35].

Density Functional Theory (DFT) Calculations
Ground state DFT calculations were carried out using Spartan 16 (v.2.0.10)[38] at the B3LYP level with a 6-31G* basis set in vacuum.Although the choice of atomic orbital basis set (6-311++G** basis set on all atoms, 6-311++G** on Cu and 6-31G* basis set on C, H and N, or 6-31G* basis set on all atoms) greatly influences the calculated absorption spectra of related copper(I) dyes, the MO characteristics are little affected [39], and therefore a 6-31G* basis set on all atoms was selected to optimize computer time.Initial geometry energy optimization was carried out at a semi-empirical (PM3) level.

Conclusions
We , two conformers are present in acetone solution and at 198 K, signal integrals in both the 31 P{ 1 H} and 1 H NMR spectra show that the ratio of these species is ~5:1.Exchange between the species has been confirmed through the observation of EXSY peaks in the ROESY spectrum.In all four compounds, inversion at the coordinated stereogenic sulfur atom is rapid on the NMR timescale over the temperature ranges studied.Replacing bpy by the NˆS ligands shifts the Cu + /Cu 2+ oxidation to higher potential.All the copper compounds are weak emitters in the solid state, but PLQY values of <2% are of the same order of magnitude as for

Scheme 1 .
Scheme 1. Structures of POP, xantphos and the NˆS ligands with ring and atom numbering for NMR spectroscopic assignments.The phenyl rings in the PPh 2 groups of POP and xantphos are labelled D. (The ring labels are chosen to allow comparison with our previous work, for example, see References [13,14]).

Figure 1 .
Figure 1.Characters of the (a) LUMO and (b) HOMO of the model [Cu(POP)(N^S)] + compound shown, and calculated at a DFT level (6-31G* basis set in vacuum).
. The heteroleptic [Cu(P^P)(N^S)][PF6] complexes were obtained by addition of the N^S ligand to a CH2Cl2 solution containing a 1:1 mixture of [Cu(MeCN)4][PF6] and the P^P ligand.The [Cu(P^P)(N^S)][PF6] compounds were isolated as colourless solids in 41% to 67% yield.The base peak in the positive mode electrospray mass spectrum of each compound arose from the [M-PF6] + ion (see Materials and Methods Section).
P{ 1 H} NMR spectrum of each [Cu(P^P)(N^S)][PF6] compound at 298 K (Figure S4) showed a broadened singlet arising from POP or xantphos (δ −12.6 ppm for each POP-containing compound, and δ −13.1 and −14.4 ppm for [Cu(xantphos)(iPrSPy)][PF6] and [Cu(xantphos)(tBuSPy)][PF6], respectively) in addition to a septet from the [PF6] − ion.At 298 K, the solution 1 H NMR spectra (Figures S5-S8) contained sharp signals for the N^S ligands, with broadened signals from POP or xantphos being indicative of dynamic behavior.In addition, the coordinated sulfur atom (which is a stereogenic center) can undergo inversion.As in the crystallographically determined structure of the cations shown in Figure 3, the two P atoms in these [Cu(P^P)(N^S)] + complexes would be inequivalent if inversion at sulfur were frozen out.Since this was never observed in the low temperature 31 P{ 1 H} NMR spectra (see below), we conclude that inversion at the coordinated-S atom is a low energy process in all four complexes.Over the temperature range 298-238 K, the 1 H NMR spectra of [Cu(POP)(iPrSPy)][PF6] and [Cu(POP)(tBuSPy)][PF6] remain essentially unchanged in the alkyl region as shown for
P{ 1 H} NMR spectrum of each [Cu(P^P)(N^S)][PF6] compound at 298 K (Figure S4) showed a broadened singlet arising from POP or xantphos (δ −12.6 ppm for each POP-containing compound, and δ −13.1 and −14.4 ppm for [Cu(xantphos)(iPrSPy)][PF6] and [Cu(xantphos)(tBuSPy)][PF6], respectively) in addition to a septet from the [PF6] − ion.At 298 K, the solution 1 H NMR spectra (Figures S5-S8) contained sharp signals for the N^S ligands, with broadened signals from POP or xantphos being indicative of dynamic behavior.In addition, the coordinated sulfur atom (which is a stereogenic center) can undergo inversion.As in the crystallographically determined structure of the cations shown in Figure 3, the two P atoms in these [Cu(P^P)(N^S)] + complexes would be inequivalent if inversion at sulfur were frozen out.Since this was never observed in the low temperature 31 P{ 1 H} NMR spectra (see below), we conclude that inversion at the coordinated-S atom is a low energy process in all four complexes.Over the temperature range 298-238 K, the 1 H NMR spectra of [Cu(POP)(iPrSPy)][PF6] and [Cu(POP)(tBuSPy)][PF6] remain essentially unchanged in the alkyl region as shown for

Scheme 2 .
Scheme 2. Inequivalence of the phenyl rings in each PPh 2 group and of the xantphos methyl groups in [Cu(RSPy)(xantphos)] + .A similar inequivalence occurs in [Cu(RSPy)(POP)] + .Over the temperature range 298-238 K, the 1 H NMR spectra of [Cu(POP)(iPrSPy)][PF 6 ] and [Cu(POP)(tBuSPy)][PF 6 ] remain essentially unchanged in the alkyl region as shown for [Cu(POP)(tBuSPy)][PF 6 ] in Figure S9.Figure 4 and Figure S10 show the aromatic regions of the variable temperature spectra of [Cu(POP)(iPrSPy)][PF 6 ] and [Cu(POP)(tBuSPy)][PF 6 ], respectively, and reveal that signals assigned to the protons of the phenyl rings, which are broad at 298 K, are partly resolved into two sets (labelled D and D') at 238 K. Exchange (EXSY) cross-peaks are observed in the ROESY spectrum at 238 K between signals for the pairs of protons H D2 /H D'2 and H D3 /H D'3 , as shown for [Cu(POP)(tBuSPy)][PF 6 ] in Figure 5.A low intensity NOESY cross-peak is observed in the ROESY spectrum between protons H D2 and H A6 , and this is displayed for [Cu(POP)(tBuSPy)][PF 6 ] in Figure S11.There is no corresponding NOESY correlation between H A6 and H D'2 , indicating that the D rings point up towards to NˆS ligand while the D' rings are directed away (see Figure 3 and Scheme 2).For a molecular weight of around 700, NOESY peaks in a ROESY spectrum are expected to have a close to zero intensity [28].In summary, dynamic processes in [Cu(POP)(iPrSPy)][PF 6 ] and [Cu(POP)(tBuSPy)][PF 6 ] involve motion of the POP backbone that is frozen out at 238 K, while inversion at the coordinated sulfur atom is still rapid on the NMR timescale at 238 K.

Figure 5 .
Figure 5. EXSY peaks in the ROESY spectrum of [Cu(POP)(tBuSPy)][PF 6 ] (500 MHz, acetone-d 6 , 238 K).The xantphos ligand has less flexibility than POP, because of the insertion of the CMe 2 bridge across the back of the ligand (Scheme 1).However, we have previously detailed dynamic processes involving the inversion of the xanthene unit in coordinated xantphos, which leads to the interconversion of, for example, conformers of [Cu(xantphos)(Phbpy)] + [29] and of [Cu(xantphos)(1-Pyrbpy)] + [30], (Phbpy = 6-phenyl-2,2 -bipyridine, 1-Pyrbpy = 6-(1-pyrenyl)-2,2 -bipyridine).For [Cu(xantphos)(tBuSPy)][PF 6 ], the broad 31 P{ 1 H} NMR signal at 298 K from the xantphos ligand (δ −14.4 ppm) is replaced at 198 K by two signals at δ −16.8 and −10.4 ppm with relative integrals of 5:1 (FigureS12).This integral ratio is the same as that observed for signals arising from two species present in the 1 H NMR spectrum of [Cu(xantphos)(tBuSPy)][PF 6 ] at 198 K (Figure6for the aromatic region and FigureS13for the alkyl region).COSY, ROESY, HMQC, and HMBC spectra recorded at 198 K were used to assign the major peaks in the spectrum, and EXSY peaks in the ROESY spectrum at 198 K were used to correlate the major and minor species.The EXSY peaks for [Cu(xantphos)(tBuSPy)][PF 6 ] are instructive.Figure7shows the aromatic region and reveals the very different environments of proton H A6 of the pyridine ring (Scheme 2) in the two species.The EXSY peaks shown in Figure8verify the exchange between the tert-butyl groups of the two species, as well as the exchange between the inner and outer pointing methyl groups of the xantphos CMe 2 units in the two species.This can only occur if the xanthene unit undergoes the inversion, as shown in Scheme 3, for one pair of possible conformers.In summary for [Cu(xantphos)(tBuSPy)][PF 6 ], two species are present in solution at 198 K, assigned to conformers which interconvert at higher temperatures through inversion of the xanthene "bowl"; fast inversion at the coordinated sulfur atom occurs on the NMR timescale at 198 K.

Figure 6 .
Figure 6.Aromatic region in the variable temperature 1 H NMR spectra of [Cu(xantphos)(tBuSPy)][PF6] (500 MHz, acetone-d6).At 198 K, the major species is represented by black labels, and minor species by red labels.See Materials and Methods Section for full assignment of major species.

Figure 6 .
Figure 6.Aromatic region in the variable temperature 1 H NMR spectra of [Cu(xantphos)(tBuSPy)][PF 6 ] (500 MHz, acetone-d 6 ).At 198 K, the major species is represented by black labels, and minor species by red labels.See Materials and Methods Section for full assignment of major species.

Figure 6 .
Figure 6.Aromatic region in the variable temperature 1 H NMR spectra of [Cu(xantphos)(tBuSPy)][PF6] (500 MHz, acetone-d6).At 198 K, the major species is represented by black labels, and minor species by red labels.See Materials and Methods Section for full assignment of major species.

Figure 7 .
Figure 7. EXSY (red) and NOESY (blue) cross-peaks in the aromatic region of the ROESY spectrum of [Cu(xantphos)(tBuSPy)][PF 6 ] (500 MHz, acetone-d 6 , 198 K).Major species represented by black labels, minor by red labels.See Materials and Methods Section for full assignment of major species.
Figure S16: Solution absorption spectra of iPrSPy and tBuSPy.