Pyridine-2,6-Dicarboxylic Acid Esters (pydicR 2 ) as O,N,O-Pincer Ligands in Cu II Complexes

: The pyridine-2,6-carboxylic esters pydicR 2 with R = Me or Ph form the unprecedented mononuclear Cu II complexes [Cu(pydicR 2 )Cl 3 ] − in one-pot reactions starting from pyridine-2,6-carboxychloride pydicCl 2 , Cu II chloride, and NEt 3 in MeOH or PhOH solution under non-aqueous conditions. The triethylammonium salts (HNEt 3 )[Cu(pydicR 2 )Cl 3 ] were isolated. The methyl derivative could be crystallized to allow a XRD structure determination. Both structures were optimized using DFT calculations in various surroundings ranging from gas phase and the non-coordinating solvent CH 2 Cl 2 to the weakly coordinating acetone and well-coordinating solvents acetonitrile (MeCN) or dimethylformamide (DMF), while detailed calculation showed the charge distribution, dipole moments, and HOMO–LUMO gap energies changing upon solvation. According to these calculations, the ion pairs and the anionic Cu II complexes were stable, which shows only Cu–Cl bond elongation and weakening of the charge transfer between the anionic complex and the cation as solvents become polar. Synthesis attempts in the presence of water yielded the Cu II complexes [Cu(pydic)(OH 2 ) 2 ] n and [Cu(OH 2 ) 6 ][{Cu(pydic)} 2 ( µ -Cl) 2 ], which results from pydicCl 2 hydrolysis. Alternatively, the new pydic(IPh) 2 (IPh = 2-iodo-phenyl) ester ligand was synthesized and reacted with anhydrous CuCl 2 , which yields the new binuclear complex [{Cu(pydic(IPh) 2 )Cl} 2 ( µ -Cl) 2 ]. EPR spectroscopy of the solid compounds reveals typical axial spectra in line with the observed and DFT calculated geometries. Cyclic voltammetry and UV–vis absorption spectroscopy in solution are in line with un-dissociated complex species [Cu(pydicR 2 )Cl 3 ] − .

In this contribution, we will report on our investigations about the coordination of Cu II toward the pyridine-2-6-dicarboxylic ester pydicR2 ligands with R = Me, Ph, and 2-iodo-phenyl (IPh).We report their synthesis and compare their molecular structures in the solid using single crystal XRD with the structures under various environments (gas phase, solution) using quantum chemical calculations based on the density functional theory (DFT).In these calculations, we were probing for their structural variability in terms of isomers, ion pair stabilization, or ligand replacement by potentially coordinating solvents such as MeCN and DMF.Detailed DFT calculations also revealed the influence of increasing solvent polarity on the dipole moments and charge distribution in the ion pairs.Since Cu II (d 9 , S = ½) provides very useful spectroscopic (d-d bands, EPR) and electrochemical (Cu II /Cu I couple) patterns to study geometries and the strength of M-ligand bonds, we added EPR spectroscopy in the solid and solution to this study, together with UV-vis absorption, cyclic voltammetry, and UV-vis spectroelectrochemistry of selected compounds in the solution.
In this contribution, we will report on our investigations about the coordination of Cu II toward the pyridine-2-6-dicarboxylic ester pydicR 2 ligands with R = Me, Ph, and 2-iodo-phenyl (IPh).We report their synthesis and compare their molecular structures in the solid using single crystal XRD with the structures under various environments (gas phase, solution) using quantum chemical calculations based on the density functional theory (DFT).In these calculations, we were probing for their structural variability in terms of isomers, ion pair stabilization, or ligand replacement by potentially coordinating solvents such as MeCN and DMF.Detailed DFT calculations also revealed the influence of increasing solvent polarity on the dipole moments and charge distribution in the ion pairs.Since Cu II (d 9 , S = 1 2 ) provides very useful spectroscopic (d-d bands, EPR) and electrochemical (Cu II /Cu I couple) patterns to study geometries and the strength of M-ligand bonds, we added EPR spectroscopy in the solid and solution to this study, together with UV-vis absorption, cyclic voltammetry, and UV-vis spectroelectrochemistry of selected compounds in the solution.

Synthesis and Molecular Structures of pydicR 2 Ligands and their Cu II Complexes
Two different reaction strategies were used to synthesize Cu II complexes (Scheme 2).The first route has been previously used, e.g., for the formation of pydic ester complexes [{Cu(pydicR 2 )Cl} 2 (µ-Cl) 2 ] (R = Me, Et, i Pr) [17,22] and consists of two subsequent steps, the synthesis of the pydicR 2 ligand, and its isolation followed by the complex formation reaction (1).This method allowed the synthesis of the new ligand pydic(IPh)2 (IPh = 2-iodo-phenyl) in 82% yield (spectroscopic characterization, see Experimental Section).Single crystals suitable for XRD were obtained from acetone solutions by slow evaporation and the structure was solved in the orthorhombic space group Pbca (further details, full data, and Figure S1 in the Supplementary Materials).Reacting pydic(IPh)2 with anhydrous CuCl2 gave a new complex as a brown powder in 67% yield with a ligand:Cu ratio of 1:1 from elemental analysis.Similar reactions using pydicMe2, pydicEt2, or pydic i Pr2 have been reported, which yielded binuclear complexes [{Cu(pydicR2)Cl}2(µ-Cl)2] [17,22,23].We assume a binuclear structure for [{Cu(pydic(IPh)2)Cl}2(µ-Cl)2].DFT-calculated optimized geometries confirmed this with the binuclear structure being more stable by about 75 kJ/mol compared with the mononuclear [Cu(pydic(IPh)2)Cl2] (Figure 1).Importantly, both the mononuclear and the binuclear complex show the CC isomeric structure (Scheme 1) with the two carbonyl O atoms binding to Cu II in the two markedly elongated axial positions of a distorted trigonal pyramidal (mononuclear) or pseudo-octahedral (binuclear) coordination.Comparison of the calculated coordination modes CC, CA, and AA shows that the CC isomer is energetically favored.In addition to this, we had the idea to set up a one-pot reaction using pyridine-2,6-dicarbonyl dichloride (pydicCl2), a Cu II source, and the corresponding alcohol (methanol or phenol) to form the ligand in the presence of the coordinating metal ion and a base (2).The base was quite similar to what has been reported recently for the formation of lanthanide coordinated IRMOF-3-PY through a post-synthetic modification [5].This method allowed the synthesis of the new ligand pydic(IPh) 2 (IPh = 2-iodo-phenyl) in 82% yield (spectroscopic characterization, see Experimental Section).Single crystals suitable for XRD were obtained from acetone solutions by slow evaporation and the structure was solved in the orthorhombic space group Pbca (further details, full data, and Figure S1 in the Supplementary Materials).Reacting pydic(IPh) 2 with anhydrous CuCl 2 gave a new complex as a brown powder in 67% yield with a ligand:Cu ratio of 1:1 from elemental analysis.Similar reactions using pydicMe 2 , pydicEt 2 , or pydic i Pr 2 have been reported, which yielded binuclear complexes [{Cu(pydicR 2 )Cl} 2 (µ-Cl) 2 ] [17,22,23].We assume a binuclear structure for [{Cu(pydic(IPh) 2 )Cl} 2 (µ-Cl) 2 ].DFT-calculated optimized geometries confirmed this with the binuclear structure being more stable by about 75 kJ/mol compared with the mononuclear [Cu(pydic(IPh) 2 )Cl 2 ] (Figure 1).Importantly, both the mononuclear and the binuclear complex show the CC isomeric structure (Scheme 1) with the two carbonyl O atoms binding to Cu II in the two markedly elongated axial positions of a distorted trigonal pyramidal (mononuclear) or pseudo-octahedral (binuclear) coordination.Comparison of the calculated coordination modes CC, CA, and AA shows that the CC isomer is energetically favored.This method allowed the synthesis of the new ligand pydic(IPh)2 (IPh = 2-iodo-phenyl) in 82% yield (spectroscopic characterization, see Experimental Section).Single crystals suitable for XRD were obtained from acetone solutions by slow evaporation and the structure was solved in the orthorhombic space group Pbca (further details, full data, and Figure S1 in the Supplementary Materials).Reacting pydic(IPh)2 with anhydrous CuCl2 gave a new complex as a brown powder in 67% yield with a ligand:Cu ratio of 1:1 from elemental analysis.Similar reactions using pydicMe2, pydicEt2, or pydic i Pr2 have been reported, which yielded binuclear complexes [{Cu(pydicR2)Cl}2(µ-Cl)2] [17,22,23].We assume a binuclear structure for [{Cu(pydic(IPh)2)Cl}2(µ-Cl)2].DFT-calculated optimized geometries confirmed this with the binuclear structure being more stable by about 75 kJ/mol compared with the mononuclear [Cu(pydic(IPh)2)Cl2] (Figure 1).Importantly, both the mononuclear and the binuclear complex show the CC isomeric structure (Scheme 1) with the two carbonyl O atoms binding to Cu II in the two markedly elongated axial positions of a distorted trigonal pyramidal (mononuclear) or pseudo-octahedral (binuclear) coordination.Comparison of the calculated coordination modes CC, CA, and AA shows that the CC isomer is energetically favored.In addition to this, we had the idea to set up a one-pot reaction using pyridine-2,6-dicarbonyl dichloride (pydicCl2), a Cu II source, and the corresponding alcohol (methanol or phenol) to form the ligand in the presence of the coordinating metal ion and a base (2).The base was quite similar to what has been reported recently for the formation of lanthanide coordinated IRMOF-3-PY through a post-synthetic modification [5].In addition to this, we had the idea to set up a one-pot reaction using pyridine-2,6-dicarbonyl dichloride (pydicCl 2 ), a Cu II source, and the corresponding alcohol (methanol or phenol) to form the ligand in the presence of the coordinating metal ion and a base (2).The base was quite similar to what has been reported recently for the formation of lanthanide coordinated IRMOF-3-PY through a post-synthetic modification [5].

DFT Calculations on the Solvation of (HNEt3)[Cu(pydicR2)Cl3] (R = Me or Ph)
To achieve insight into the electronic structure of the compounds, density functional theory (DFT) calculations in the gas phase and various solvent environments are performed (B3LYP/6-31+G(d,p) using Gaussian).The Cu atom was treated using LANL2DZ relativistic pseudo potentials.The final optimized geometry of the compounds in the gas phase is depicted in  In CH2Cl2, acetone, DMF, and MeCN solution, the geometry of the complexes remains rather intact.Only the Cu-Cl3 bond is markedly elongated with a slight tendency of longer bonds for more polar solvents along this series.Even more pronounced is the increasing distance between the ions due to the shielding of electrostatic interactions in these solutions (see Tables S3-S7 in the

DFT Calculations on the Solvation of (HNEt 3 )[Cu(pydicR 2 )Cl 3 ] (R = Me or Ph)
To achieve insight into the electronic structure of the compounds, density functional theory (DFT) calculations in the gas phase and various solvent environments are performed (B3LYP/6-31+G(d,p) using Gaussian).The Cu atom was treated using LANL2DZ relativistic pseudo potentials.The final optimized geometry of the compounds in the gas phase is depicted in

DFT Calculations on the Solvation of (HNEt3)[Cu(pydicR2)Cl3] (R = Me or Ph)
To achieve insight into the electronic structure of the compounds, density functional theory (DFT) calculations in the gas phase and various solvent environments are performed (B3LYP/6-31+G(d,p) using Gaussian).The Cu atom was treated using LANL2DZ relativistic pseudo potentials.The final optimized geometry of the compounds in the gas phase is depicted in  In CH2Cl2, acetone, DMF, and MeCN solution, the geometry of the complexes remains rather intact.Only the Cu-Cl3 bond is markedly elongated with a slight tendency of longer bonds for more polar solvents along this series.Even more pronounced is the increasing distance between the ions due to the shielding of electrostatic interactions in these solutions (see Tables S3-S7 in the In CH 2 Cl 2 , acetone, DMF, and MeCN solution, the geometry of the complexes remains rather intact.Only the Cu-Cl3 bond is markedly elongated with a slight tendency of longer bonds for more polar solvents along this series.Even more pronounced is the increasing distance between the ions due to the shielding of electrostatic interactions in these solutions (see Tables S3-S7 in the Supplementary Materials).The computed dipole moments of the complexes are significantly increased in solution compared with the gas phase (Table 1) and the electric permittivity of the solvents clearly has a very pronounced effect on the dipole moments, even in CH 2 Cl 2 .In order to investigate the solubility and thermodynamic stability of (HNEt 3 )[Cu(pydicR 2 )Cl 3 ] in the surrounding medium, the solvation energies are calculated.The solvation energy of the system is determined by calculating the free energy differences in the solvent and the gas phase (Table 1).The solvation energies vary from about −110.88 to −135.12 kJ/mol and increase together with the dipole moment of the complex when increasing the electric permittivity of the solvents.Negative values of solvation energies denote that solvation of these complexes is spontaneous.To get an idea about the kinetic stability and chemical reactivity of these complexes, we also calculated the difference between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) (E LUMO −E HOMO ) for the complexes in each solvent (Table 1).The spatial distributions of HOMO, SOMO, and LUMO of the complexes are supplied in the Supplementary Materials (Figure S8).In general, molecules with a small energy gap are more polarizable [43] and will have higher chemical activity and lower kinetic stability [44].Overall, the HOMO−LUMO gap decreases as the electric permittivity of the solvents declines along the series DMF (ε = 37.22) > MeCN (ε = 35.69)>> acetone (ε = 20.49)>> CH 2 Cl 2 (ε = 8.93).However, the absolute values are quite similar for all four solvents, which means that a pronounced activation by the polar solvents DMF or MeCN compared with acetone or CH 2 Cl 2 seems not to be the case.The electron density of (HNEt 3 )[Cu(pydicMe 2 )Cl 3 ] and (HNEt 3 )[Cu(pydicPh 2 )Cl 3 ] in the gas phase can be visualized from the three-dimensional electrostatic potential map (EPM) surfaces (Figure 5).The distribution of negative and positive charges within the EPM surfaces are depicted by red and blue colors, respectively.As a result of change in electron densities in EPM surfaces upon solvation, the charges of the anions and cations both decrease along the series.DMF > MeCN > acetone > CH 2 Cl 2 >> gas phase ( Supplementary Materials).The computed dipole moments of the complexes are significantly increased in solution compared with the gas phase (Table 1) and the electric permittivity of the solvents clearly has a very pronounced effect on the dipole moments, even in CH2Cl2.In order to investigate the solubility and thermodynamic stability of (HNEt3)[Cu(pydicR2)Cl3] in the surrounding medium, the solvation energies are calculated.The solvation energy of the system is determined by calculating the free energy differences in the solvent and the gas phase (Table 1).The solvation energies vary from about −110.88 to −135.12 kJ/mol and increase together with the dipole moment of the complex when increasing the electric permittivity of the solvents.Negative values of solvation energies denote that solvation of these complexes is spontaneous.To get an idea about the kinetic stability and chemical reactivity of these complexes, we also calculated the difference between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) (ELUMO−EHOMO) for the complexes in each solvent (Table 1).The spatial distributions of HOMO, SOMO, and LUMO of the complexes are supplied in the Supplementary Materials (Figure S8).In general, molecules with a small energy gap are more polarizable [43] and will have higher chemical activity and lower kinetic stability [44].Overall, the HOMO−LUMO gap decreases as the electric permittivity of the solvents declines along the series DMF (ε = 37.22) > MeCN (ε = 35.69)>> acetone (ε = 20.49)>> CH2Cl2 (ε = 8.93).However, the absolute values are quite similar for all four solvents, which means that a pronounced activation by the polar solvents DMF or MeCN compared with acetone or CH2Cl2 seems not to be the case.
The electron density of (HNEt3)[Cu(pydicMe2)Cl3] and (HNEt3)[Cu(pydicPh2)Cl3] in the gas phase can be visualized from the three-dimensional electrostatic potential map (EPM) surfaces (Figure 5).The distribution of negative and positive charges within the EPM surfaces are depicted by red and blue colors, respectively.As a result of change in electron densities in EPM surfaces upon solvation, the charges of the anions and cations both decrease along the series.DMF > MeCN > acetone > CH2Cl2 >> gas phase (Table 1, Figures S9 and S10 in the Supplementary Materials).
. Table 1.DFT-calculated dipole moments and solvation energies of (HNEt3)[Cu(pydicMe2)Cl3] and (HNEt 3 )[Cu(pydicPh 2 )Cl 3 ] in the gas phase and solvent media.To better understand the inclusion of the solvent effect on the DFT calculations and also the nature of interactions in the (HNEt 3 )[Cu(pydicR 2 )Cl 3 ] compounds, natural bond orbital (NBO) analyses were carried out as implemented in the Gaussian program package.In the NBO analysis, the donor and acceptor interaction energies can be estimated through the second-order perturbation theory, which is described by the equation below.

Gas
where q i is the donor orbital occupancy, ε j and ε i are diagonal elements, and F(i,j) is the off-diagonal NBO Fock matrix element.The most important donor-acceptor interactions with high second-order perturbation energies E (2) are provided in Table 2 (data for the pydicPh derivative in Table S8 in the Supplementary Materials).As the interaction between (HNEt 3 ) + and [Cu(pydicR 2 )Cl 3 ] − as the electron acceptor and electron donor increases, the calculated E (2) value become larger, as expected.Furthermore, the increase in donating tendency from electron donors to electron acceptors is consistent with the enhancement of conjugation of the whole system from R = Me to R = Ph.The results suggest strong orbital interactions between the antibonding molecular orbitals of the Cu atom (LP*) and the antibonding orbitals of the N2-H22 bond (BD*) in the gas phase, which leads to a stabilization of 18.91 kJ/mol for (HNEt 3 )[Cu(pydicMe 2 )Cl 3 ] and 21.30 kJ/mol for (HNEt 3 )[Cu(pydicPh 2 )Cl 3 ], respectively.In solution with an effect of the solvents, this donor-acceptor behavior between (HNEt 3 ) + and [Cu(pydicMe 2 )Cl 3 ] − leads to a reduction of the intermolecular charge transfer with lower values of E (2) (1.66).The table also reveals that the E (2) in the LP*(8) Cu1 and the BD*(1) N2-H22 and also the LP(3) Cl2 and BD*(1) N2-H22 are decreasing with an increasing dielectric constant of the solvent.The NBO analysis results illustrate that, in the gas phase, LP(4) Cl2 and LP(4) Cl1 participate as donors and the LP*(7) Cu1 acts as an acceptor with the strongest intramolecular charge transfer interactions (Table 2).In solvent environments, the E (2) in the LP(4) Cl2 and LP*(7) Cu1 and also the E (2) in the LP(4) Cl1 and LP*(7) Cu1 are enlarged with increasing dielectric constants of the solvent.However, this trend for the E (2) in the LP(4) Cl3 and LP*(7) Cu1 are completely reversed.The trend of charge transfer from LP(1) O1, LP(1) O2, and LP(1) N1 as donors and LP* Cu1 as an acceptor are increased with a growing dielectric constant of the solvents.

Molecular Dynamics (MD) Simulations of Bulk Solutions
In addition, we also performed molecular dynamic simulations using the GROMACS 4.5.5 package and utilizing the GROMOS force field 53A6.Six ion pairs solvated in 1200 solvent molecules were simulated for each compound.As an example, Figure 6A  The first peak of the radial distribution function is located at around 1.84 Å, which is an indicator of a strong hydrogen bond.Combined radial/angular distribution function (CDF) is a powerful tool for defining hydrogen bond criteria, since it offers much more information than g(r) [45].The combined radial/angular distribution function (see Figure 6D) indicates that this preferred interaction occurs for N2•••H22•••Cl2 angles (α) between 160° and 180°.
All of these calculated data points show a marked difference between the gas phase and solvent surrounding these ion pairs and emphasizing the importance of the proper inclusion of solvents in any DFT calculations.According to our calculations, the electronic structure of the studied solutes is influenced in a different manner by the non-coordinating CH2Cl2, the weakly coordinating acetone, and MeCN and DMF, which are considered suitable ligands for Cu II .With increasing polarity along the series gas phase < CH2Cl2 < acetone < MeCN < DMF solutions, the Cl3-Cu bond is markedly elongated, the distance between the cations and anions are increased, and the dipole moments as well as the extent of charge localization of the cation and anion increases.However, the differences The first peak of the radial distribution function is located at around 1.84 Å, which is an indicator of a strong hydrogen bond.Combined radial/angular distribution function (CDF) is a powerful tool for defining hydrogen bond criteria, since it offers much more information than g(r) [45].The combined radial/angular distribution function (see Figure 6D) indicates that this preferred interaction occurs for N2•••H22•••Cl2 angles (α) between 160 • and 180 • .
All of these calculated data points show a marked difference between the gas phase and solvent surrounding these ion pairs and emphasizing the importance of the proper inclusion of solvents in any DFT calculations.According to our calculations, the electronic structure of the studied solutes is influenced in a different manner by the non-coordinating CH 2 Cl 2 , the weakly coordinating acetone, and MeCN and DMF, which are considered suitable ligands for Cu II .With increasing polarity along the series gas phase < CH 2 Cl 2 < acetone < MeCN < DMF solutions, the Cl3-Cu bond is markedly elongated, the distance between the cations and anions are increased, and the dipole moments as well as the extent of charge localization of the cation and anion increases.However, the differences between CH 2 Cl 2 and MeCN or DMF solutions are not very pronounced.Although more detailed kinetic calculations would be necessary to make a statement on the dissociation of the complexes in MeCN or DMF solution, we assume from the present calculations that the complexes remain stable in solution, which is important in view of the following spectroscopy and electrochemical measurements in the solution.

EPR Spectroscopy of Solids and Solutions
The X-band EPR spectrum of [{Cu(pydic(IPh) 2 )Cl} 2 (µ-Cl) 2 ] in the solid shows a very intense signal for the forbidden ∆M S = ±2 transition and a rhombic ∆M S = ±1 signal (Figure 7), which is completely in line with the binuclear character of the complex and a triplet ground state.In acetone solution, the spectrum is essentially retained and, thus, the binuclear character remains unchanged.A similar spectrum with very similar g values has been reported for [{Cu(pydicEt 2 )Cl} 2 (µ-Cl) 2 ] [23], while the derivative [{Cu(pydic i Pr 2 )Cl} 2 (µ-Cl) 2 ] exhibits a markedly different averaged g value (Table 3) and no ∆M S = ±2 transition is observed.The difference has been attributed to the different geometries for the central Cu(µ-Cl) 2 Cu unit, which is planar for the pydicEt 2 complex but bent for the iso-propyl derivative [23].Thus, our results are fully in line with the DFT-calculated structure of [{Cu(pydic(IPh) 2 )Cl} 2 (µ-Cl) 2 ], which shows a planar Cu(µ-Cl) 2 Cu core (Figure 1).Although the averaged g value is quite similar for the related binuclear [46], the g anisotropy ∆g is markedly increased, while the ∆M S = ±2 is far less intense.This reflects the different "ligand setting" in this complex containing an ONO pincer ligand based on pyridine-2,6-dimethanol.As deduced from EXAFS data, the O, N, O atoms of the pydotH 2 ligand and the bridging chlorido ligand form a square plane of the tightly bound ligands.From this bridging Cl − , an elongated Cu-Cl bond connects to the second Cu II , while the terminal chloride ligand has a short Cu-Cl bond [46].Thus, while in the binuclear complexes [{Cu(pydicR 2 )Cl} 2 (µ-Cl) 2 ] with R = Me, Et, i Pr and IPh, the tight binding ligands are N, Cl, and 2× µ-Cl with the two square planes fused by the bridging µ-Cl atoms.In the pydotH 2 complex, the two square planes O,N,O,µ-Cl and the central Cu(µ-Cl) 2 Cu core are perpendicular to each other.The EPR spectra fully reflect this difference.completely in line with the binuclear character of the complex and a triplet ground state.In acetone solution, the spectrum is essentially retained and, thus, the binuclear character remains unchanged.A similar spectrum with very similar g values has been reported for [{Cu(pydicEt2)Cl}2(µ-Cl)2] [23], while the derivative [{Cu(pydic i Pr2)Cl}2(µ-Cl)2] exhibits a markedly different averaged g value (Table 3) and no ΔMS = ±2 transition is observed.The difference has been attributed to the different geometries for the central Cu(µ-Cl)2Cu unit, which is planar for the pydicEt2 complex but bent for the iso-propyl derivative [23].Thus, our results are fully in line with the DFT-calculated structure of [{Cu(pydic(IPh)2)Cl}2(µ-Cl)2], which shows a planar Cu(µ-Cl)2Cu core (Figure 1).Although the averaged g value is quite similar for the related binuclear Cu II complex[{Cu(pydotH2)Cl}2(µ-Cl)2] (pydotH2 = 2,6-bis(1-hydroxy-1-o-tolyl-ethyl-η 2 -O,O')pyridine) [46], the g anisotropy Δg is markedly increased, while the ΔMS = ±2 is far less intense.This reflects the different "ligand setting" in this complex containing an ONO pincer ligand based on pyridine-2,6-dimethanol.As deduced from EXAFS data, the O, N, O atoms of the pydotH2 ligand and the bridging chlorido ligand form a square plane of the tightly bound ligands.From this bridging Cl − , an elongated Cu-Cl bond connects to the second Cu II , while the terminal chloride ligand has a short Cu-Cl bond [46].Thus, while in the binuclear complexes [{Cu(pydicR2)Cl}2(µ-Cl)2] with R = Me, Et, i Pr and IPh, the tight binding ligands are N, Cl, and 2× µ-Cl with the two square planes fused by the bridging µ-Cl atoms.In the pydotH2 complex, the two square planes O,N,O,µ-Cl and the central Cu(µ-Cl)2Cu core are perpendicular to each other.The EPR spectra fully reflect this difference.The mononuclearcompounds (HNEt3)[Cu(pydicMe2)Cl3] and (HNEt3)[Cu(pydicPh2)Cl3] reveal EPR spectra of axial symmetry with g|| > g⊥ in the solid (Figure 8), which is typical for elongated octahedral or square pyramidal Cu II complexes [23,[47][48][49][50][51][52].The Cu hyperfine structure (coupling to the 63 Cu (69.17%) and 65 Cu (30.83%) nuclei with I = 3/2 [53]) was not observed in line with observations for similar mononuclear complexes [46,52].A rhombic EPR spectrum was reported for the complex [Cu(pydotH2)(DMF)Cl2] [46], which has an axially elongated distorted pseudo-octahedral geometry with the O,N,O ligand located in the square plane of tight Cu-ligand bonds.Yet, in (HNEt3)[Cu(pydicMe2)Cl3] and (HNEt3)[Cu(pydicPh2)Cl3], the N,Cl3 coordination represents the short = strong bonds and the Ocarbonyl functions are in the elongated positions.Nevertheless, the averaged g values and g anisotropy (Δg) are very similar.Far smaller averaged g values were reported for the square pyramid or trigonal bipyramidal complexes listed in Table 3.Thus, the observed spectra represent the overall geometry around Cu II than the ligand strength of Cl − , DMF, and O atoms in the O, N, O ligands.The mononuclearcompounds (HNEt 3 )[Cu(pydicMe 2 )Cl 3 ] and (HNEt 3 )[Cu(pydicPh 2 )Cl 3 ] reveal EPR spectra of axial symmetry with g || > g ⊥ in the solid (Figure 8), which is typical for elongated octahedral or square pyramidal Cu II complexes [23,[47][48][49][50][51][52].The Cu hyperfine structure (coupling to the 63 Cu (69.17%) and 65 Cu (30.83%) nuclei with I = 3/2 [53]) was not observed in line with observations for similar mononuclear complexes [46,52].A rhombic EPR spectrum was reported for the complex [Cu(pydotH 2 )(DMF)Cl 2 ] [46], which has an axially elongated distorted pseudo-octahedral geometry with the O,N,O ligand located in the square plane of tight Cu-ligand bonds.Yet, in (HNEt 3 )[Cu(pydicMe 2 )Cl 3 ] and (HNEt 3 )[Cu(pydicPh 2 )Cl 3 ], the N,Cl 3 coordination represents the short = strong bonds and the O carbonyl functions are in the elongated positions.Nevertheless, the averaged g values and g anisotropy (∆g) are very similar.Far smaller averaged g values were reported for the square pyramid or trigonal bipyramidal complexes listed in Table 3.Thus, the observed spectra represent the overall geometry around Cu II than the ligand strength of Cl − , DMF, and O atoms in the O, N, O ligands.[52].c From Reference [46].sqp = square pyramidal, tbp = trigonal bipyramidal, aeo = axially elongated pseudo-octahedral.d From Reference [23].
Samples dissolved in acetone or MeCN (at ambient temperatures) exhibit axial signal symmetry even though the averaged g values and the g anisotropy (Δg) are markely decreased.We conclude that the ion pairs and the anionic complexes in (HNEt3)[Cu(pydicR2)Cl3] are basically retained in acetone or MeCN solution with minor alterations in complex geometry.This is completely in line with our DFT calculations showing changes in the geometry and charge transfer of the ion pairs upon solvation and likely shows no dissociation.
Samples dissolved in acetone or MeCN (at ambient temperatures) exhibit axial signal symmetry even though the averaged g values and the g anisotropy (∆g) are markely decreased.We conclude that the ion pairs and the anionic complexes in (HNEt 3 )[Cu(pydicR 2 )Cl 3 ] are basically retained in acetone or MeCN solution with minor alterations in complex geometry.This is completely in line with our DFT calculations showing changes in the geometry and charge transfer of the ion pairs upon solvation and likely shows no dissociation.

Electrochemical Measurements
The electrochemical properties of the new complexes and the new ligand pydic(IPh) 2 were studied by cyclic voltammetry in MeCN/ n Bu 4 NPF 6 solution.The complex [{Cu(pydic(IPh) 2 )Cl} 2 (µ-Cl) 2 ] shows a reversible redox wave around 0 V vs. ferrocene/ferrocenium, which can be assigned to the Cu II /Cu I couple (Figure 9).Additionally, an irreversible oxidation wave at 0.73 V and an irreversible reduction wave at −1.33 V were observed.They both occurred for the uncoordinated ligand at similar potentials (Table 5) and were, thus, assigned to ligand-centered processes.Assuming two electrons for the reversible Cu II /Cu I wave of the binuclear complex, the oxidation wave is markedly larger and more than two electrons might be involved in this process.The redox features of the mononuclear compounds (HNEt 3 )[Cu(pydicMe 2 )Cl 3 ] and (HNEt 3 )[Cu(pydicPh 2 )Cl 3 ] are very similar, with a reversible Cu II /Cu I wave at around 0.1 V, irreversible oxidations at around 1 V, and reductions at around −1.3 V (Figure S4 in the Supplementary Materials).For the phenyl derivative [Cu(pydicPh 2 )Cl 3 ] − , the second reduction wave is partially reversible, while the methyl derivative shows a broad irreversible wave.The bulky phenyl groups seem to stabilize the reduced complex, which is in line with the idea that the reduction is ligand-centered.

Electrochemical Measurements
The electrochemical properties of the new complexes and the new ligand pydic(IPh)2 were studied by cyclic voltammetry in MeCN/ n Bu4NPF6 solution.
The complex [{Cu(pydic(IPh)2)Cl}2(µ-Cl)2] shows a reversible redox wave around 0 V vs. ferrocene/ferrocenium, which can be assigned to the Cu II /Cu I couple (Figure 9).Additionally, an irreversible oxidation wave at 0.73 V and an irreversible reduction wave at −1.33 V were observed.They both occurred for the uncoordinated ligand at similar potentials (Table 5) and were, thus, assigned to ligand-centered processes.Assuming two electrons for the reversible Cu II /Cu I wave of the binuclear complex, the oxidation wave is markedly larger and more than two electrons might be involved in this process.The redox features of the mononuclear compounds (HNEt3)[Cu(pydicMe2)Cl3] and (HNEt3)[Cu(pydicPh2)Cl3] are very similar, with a reversible Cu II /Cu I wave at around 0.1 V, irreversible oxidations at around 1 V, and reductions at around −1.3 V (Figure S4 in the Supplementary Materials).For the phenyl derivative [Cu(pydicPh2)Cl3] − , the second reduction wave is partially reversible, while the methyl derivative shows a broad irreversible wave.The bulky phenyl groups seem to stabilize the reduced complex, which is in line with the idea that the reduction is ligand-centered.Spectro-electrochemical (SEC), in situ UV-vis absorption spectra during cathodic or anodic electrolysis measurements of (HNEt 3 )[Cu(pydicPh 2 )Cl 3 ] and [{Cu(pydic(IPh) 2 )Cl} 2 (µ-Cl) 2 ] confirm the full reversibility of the Cu II /Cu I waves.However, no conclusive spectra were obtained for the reduced species at around −1.3 or the oxidized complexes at around +1 V (for figures, see Supplementary Materials).Importantly, the reduction wave at −1.3 V for (HNEt 3 )[Cu(pydicPh 2 )Cl 3 ], which appears reversible in the relatively fast cyclic voltammetry experiment with a timescale of a few seconds turns out to be completely irreversible in the much slower SEC experiments, which are run on a minute timescale.

Methods and Instrumentation
NMR spectra were recorded on a Bruker Avance II 300 MHz spectrometer (Bruker, Rheinhausen, Germany), using a triple resonance 1 H, n BB inverse probe head.The unambiguous assignment of the 1 H and 13 C resonances was obtained from 1 H NOESY, 1 H COSY, gradient selected 1 H, 13 C HSQC, and HMBC experiments.All 2D NMR experiments were performed using standard pulse sequences from the Bruker pulse program library.Chemical shifts were relative to TMS.UV-vis absorption spectra were measured on a Shimadzu UV-3600 photo spectrometer (Shimadzu Europe, Duisburg, Germany).Elemental analyses were carried out using a HEKAtech CHNS EuroEA 3000 Analyzer (Hekatech, Wegberg, Germany).EPR spectra were recorded in the X-band on a Bruker System ELEXSYS 500E equipped with a Bruker Variable Temperature Unit ER 4131VT (500 to 100 K) (Bruker, Rheinhausen, Germany).The g values were calibrated using a dpph sample.Electrochemical experiments were carried out in 0.1 M n Bu 4 NPF 6 solutions using a three-electrode configuration (glassy carbon working electrode, Pt counter electrode, Ag/AgCl pseudo reference), an Autolab PGSTAT30 potentiostat (Metrohm, Filderstadt, Germany), and a function generator.Experiments were run at a scan rate of 100 mV/s at ambient temperature.The ferrocene/ferrocenium couple (FeCp 2 /FeCp 2 + ) served as the internal reference.UV-vis spectro-electrochemical measurements were performed with an optical transparent thin-layer electrochemical (OTTLE) cell [55,56].
(HNEt 3 )[Cu(pydicR 2 )Cl 3 ] (R = Me or Ph) structures were optimized using DFT calculations at the B3LYP/6-31+G(d,p) level in different environments (gas phase, various solvents) using the Gaussian 09 program [70].The Cu atom was treated using LANL2DZ relativistic pseudo potentials.The solvent effect was introduced by applying the polarized continuum model (PCM) embedded in the Gaussian 09 package.In the natural bond, the orbital (NBO) analysis and the donor and acceptor interaction energies can be estimated through the second-order perturbation theory, described using the equation below.
where q i is the donor orbital occupancy, ε j and ε i are diagonal elements, and F(i,j) is the off-diagonal NBO Fock matrix element.Molecular dynamics (MD) simulations on (HNEt 3 )[Cu(pydicR 2 )Cl 3 ] (R = Me or Ph) were carried out using the GROMACS package [71] and utilizing the GROMOS force field 53A6 [72].Six-ion pairs solvated in 1200 solvent molecules were simulated for each compound.The equations of motion have been solved by the leapfrog integrator with a time step of 2 fs.After initial energy minimizations and preliminary simulations adjustments in NVT and NPT ensembles, MD simulations were extended for an additional time of 10 ns collecting statistical data under the NPT condition at T = 298 K with periodic boundary conditions in all directions.The temperature was maintained at 298 K using the Berendsen thermostat [73] and the pressure was kept at 1 bar by coupling semi-isotopically to a barostat with a coupling constant of 1 ps.Both the electrostatic and van der Waals interactions used a short-range cutoff of 1.2 nm and the long-range electrostatics were evaluated using the particle mesh Ewald (PME) method [74].

General
Water-free reactions were carried out under inert gas conditions and performed using Schlenk techniques.Solvents were dried using a MBRAUN MB SPS-800 solvent purification system.Pyridine-2,6-dicarbonyl dichloride [51] was synthesized according to a literature procedure.

Complexes from pydicCl 2 Hydrolysis
In a typical reaction, 100 mg (0.5 mmol) pyridine-2,6-dicarbonyl dichloride were dissolved in 15 mL moist MeOH.85 mg (0.5 mmol) CuCl 2 •2H 2 O and 1 mL (excess) NEt 3 were added and the whole mixture was stirred at an ambient temperature for 16-24 h.The resulting green solutions were slowly evaporated and individual crystals were collected.Thus, 33

Table 1 ,
Figures S9 and S10 in the Supplementary Materials).

Table 3 .
X-band EPR data of the reported and comparable Cu II complexes a .

Table 3 .
X-band EPR data of the reported and comparable Cu II complexes a .

Table 4 .
Absorption maxima for Cu II complexes a .

Table 4 .
Absorption maxima for Cu II complexes a .