1

Synthesis and Electronic Structure of a Mononuclear Copper(II) Complex

Supported by Tris(2-hydroxyliminopropyl)amine

Alexander Roseborough,a Kraig A. Wheeler,b Małgorzata Hołyńska,c Sebastian A. Stoiana,* a Department of Chemistry, University of Idaho, 875 Perimeter Dr, Moscow, ID 83844, USA

b Department of Chemistry, Whitworth University, 300 W Hawthorne Road, Spokane WA 99251, USA c Fachbereich Chemie and Wissenschaftliches Zentrum für Materialwissenschaften(WZMW), Philipps-Universität Marburg Hans-Meerwein-Straße, 35043 Marburg, Germany * Corresponding author: sstoian@uidaho.edu Keywords: copper(II) complexes; EPR; DFT; CASSCF; coordination chemistry, electrochemistry, hydroxylimine

Manuscript Click here to view linked References

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Abstract

The blue-green, mononuclear copper complex [CuL(NO3)]NO3, (1), was synthesized by reacting copper nitrate with L = tris(2-hydroxyliminopropyl)amine in methanol. The structure of 1 was determined by single-crystal X-ray diffraction, which revealed a copper(II) ion that exhibits a pseudo-octahedral coordination geometry. While one of the nitrate anions is coordinated in a η2 fashion the other is non-coordinated and is involved in intra- and inter-molecular hydrogen bonds. The nitrate-promoted intermolecular hydrogen bonding is supported by the protonated ligand and leads to the formation of supramolecular zig-zagging chains. The electronic structure of 1 was investigated using Electron Paramagnetic Resonance (EPR) measurements performed at cryogenic temperatures on both neat powders and frozen solutions. In the solid state we observe a quasi-axial S = 1/2 signal described by gz = 2.256, gy = 2.086, and gx = 2.048. These values were rationalized using coupled-perturbed (CP) Density Functional Theory (DFT) and CASSCF/NEVPT2 calculations. The computational investigation of 1 suggests an (x2-y2)-type ground state and a g -tensor dominated by the (xy) → (x2-y2) transition. EPR investigation of frozen solution samples revealed the presence of two similar spectral components indicating that upon dissolution the nitrate anion is displaced from the coordination sphere and 1 speciates into six- and five-coordinated complex-cations. The cyclic voltammograms of 1 revealed a quasi-reversible Cu(I)/Cu(II) redox couple, E1/2 = +0.262 V vs. SHE, and that 1 functions as a precursor for catalytically-active, electrodeposited Cu(0)/Cu2O film.

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Introduction

An essential transition metal in many biological systems, copper is also a low-cost component sought after for a variety of valuable applications, including catalysts and magnetic materials.1 Therefore, the investigation of copper-containing species continues to play a significant role in advancing our understanding of copper-mediated processes in both chemical and biological systems. Recently, copper compounds have been reported to function as photoredox catalysts that can be activated by visible light.2 While some of these species effectuate cross-coupling reactions, during the oxo-azidation of vinylarenes the copper(II) ions mediate the concomitant transfer of electrons and oxygen atoms.3 Therefore, certain aspects of this chemistry emulate the biological activity of copper enzymes, in particular the oxygen-activating oxidases.4 In the field of magnetic materials, copper(II)-supported Kagomé lattices play a prominent role in the quest of using spin-frustration to develop new classes of quantum magnets.5,6 Central to these efforts is the synthesis and characterization of new copper compounds.7 Their spectroscopic and theoretical investigation permits the derivation of detailed structure-property and -activity correlations which, in turn, allow us to rationalize experimental observation and advance the development of new copper-containing materials and their applications.

Aiming at coordination compounds that are catalytically active and exhibit interesting magnetic properties we have investigated the reaction of Cu2+ ions with the tripodal ligand tris(2-hydroxyliminopropyl)amine, L (Scheme 1). Although the tripodal amine has been known for some time,8 its exploration as a ligand in coordination chemistry is much more recent. In an interesting report, Goldcamp et al have shown that L not only coordinates to Ni(II) but also its protonation level has a dramatic influence on its reactivity.9,10 Fully protonated L leads to the formation of a mononuclear complex, partial deprotonation to a Ni2 dimer, and complete deprotonation to a putative [Ni(Ox3)]- species which binds dioxygen reversibly. The latter species can perform oxygen atom transfer to substrates such as PPh3. A surprising feature of L is that it can undergo, under certain conditions, an intramolecular cyclization to yield an adamantane-like molecule, that is, tris-hydroxylaminatotetraazaadamantane.11 While this reaction can be accomplished under metal-free conditions through acid catalysis, it can also be mediated by Mn(II) ions, with the concomitant formation of Mn(IV) complexes supported by the deprotonated, cyclized ligand.12, 13 Additionally, L-supported CuII

2 , ZnII, and CoIII4LnIII

3 complexes have been reported.14,15,16

In this manuscript we report the synthesis, crystal structure, and electrochemical behavior of [CuL(NO3)]NO3, 1 (Figure 1). The electronic structure of this complex was investigated using X-band EPR spectroscopy and computational methods including DFT and CASSCF/NEVPT2 calculations. The previously reported reaction of L with copper (II) was carried out in the presence of a lanthanide, which resulted in the formation of a dimer, [Cu2L2](ClO4)4. However, the synthesis of 1 was performed under lanthanide-free conditions yielding a mononuclear copper(II) complex, 1, instead of a dimer. The crystallographic investigation of 1 revealed a Cu(II) site with a distorted geometry intermediate between square-pyramid and octahedral. The second-coordination sphere of the metal ion is modulated by the hydrogen-bond forming properties of the hydroxylimino groups. The non-coordinated nitrate anion is involved in three hydrogen bonds, two intra- and one inter-molecular. Our combined theoretical and spectroscopic investigation of 1 uncovered that the single unpaired electron of the Cu2+ ion is accommodated in an antibonding eg

*, (x2-y2)-like orbital. This orbital is aligned with the shortest four Cu-L bonds, which also define the base of the square-pyramidal polyhedron of coordination. The properties of 1 are dominated by the (xy) → (x2-y2) transition that has an energy of ~16 500 cm-1 which leads to a g tensor such that gz >> gx gy. The electrochemical investigation of 1 revealed a surprisingly exciting behavior. Thus, limited-range cyclic voltammograms of 1 reveal a quasi-reversible CuI/CuII redox couple. In contrast, extending the range of the cathodic potential sweeps leads to the electrodeposition of a catalytically active film.17 This observation suggests that 1 is similar to several other Cu(II) complexes with N-donor ligands int that it can function as a precursor for the formation of electroactive copper/copper oxide films.18

Scheme 1. Chemical structure of L =

tris(2-hydroxyliminopropyl)amine.

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Materials and Methods Synthesis. Copper(II) nitrate hemi(pentahydrate) (Alfa Aesar) and methanol (Macron) were obtained from commercial sources and used as received. The tris(2-hydroxyliminopropyl)amine ligand was synthesized from the reaction of hydroxylamine (Acros Organics) with chloroacetone (96% stabilized, Acros Organics) and ammonium hydroxide based on the method reported by Goldcamp et al.9

X-Ray Crystallography. The X-ray intensity data for 1 were measured at 173 K on a Bruker Photon II D8 Venture diffractometer equipped with both IμS-Cu and IμS-Mo microfocus X-ray sources. The Mo K ( = 0.71073 Å) source was used for this crystallographic assessment. A single crystal of 1 was suspended in Paratone® N oil (Hampton Research) and mounted on a cryoloop, which was cooled using a stream of cold N2 gas. Intensities were collected using ø and ω scans and were corrected for Lorentz polarization and absorption effects. The X-SEED software platform,19 equipped with SHELXS and SHELXL modules on a PC computer,20 was used for all structure solution and refinement calculations and molecular graphics. The structure was solved by direct methods and refined by an anisotropic full-matrix least-squares procedure for all non-hydrogen atoms. Crystal data, data collection, and structure refinement details are presented in Table 1.

Electron Paramagnetic Resonance. Low-temperature X-band EPR spectra were recorded on a Bruker EMX 6/1-HX spectrometer equipped with a liquid-helium and liquid-nitrogen cooled Oxford ESR 900 cryostat. This instrument was equipped with a high-sensitivity, ER4119HS-W1, cylindrical resonator and a ER041X microwave bridge. The concentration of solution samples was checked through spin quantitations obtained using a CuII-EDTA standard. Temperatures were determined using a calibrated Cernox© sensor. The magnetic field modulation was 100 kHz at an amplitude of 1.0 mT. All spectra presented in this manuscript were collected under non-saturating conditions.

The EPR spectra were analyzed in the framework of a typical S = 1/2 spin-Hamiltonian (eq. 1). While the first term of this equation describes the electronic Zeeman interaction, the second term accounts for the hyperfine splitting and the last term for the nuclear Zeeman effect. Spectral simulations were obtained using the Spin Count program developed by Prof. Michael Hendrich at Carnegie Mellon University.21

H = 𝜇B ∙ ∙ + ∙ ∙ − 𝜇n𝑔n ∙ (1)

Electrochemistry. Cyclic voltammetry (CV) of 1 was performed at room temperature for a 2.4 mM DMSO solution, using a PalmSens4 potentiostat. The measurements were carried out at the sweep rate of 100 mV·s–1, with 0.1 M (Bu4N)PF6 electrolyte solution, a glassy carbon working electrode, a Pt-wire counter electrode, and a Pine Research sol-gel Ag/AgCl reference electrode. Potentials are reported against the standard hydrogen electrode (SHE) and were

Table 1. Data collection and crystal structure

refinement parameters for 1.

Formula CuO9N6C9H18 (1)

CCDC number 1957762

T, K 173(2)

Molar mass, g/mol 417.83

Space group Pbca

a, Å 10.2348(10)

b, Å 14.3612(12)

c, Å 21.7444(16)

V, Å3 3196.1(5)

Z 8

Crystal color blue-green

dcalc, g cm–3 1.720

, mm–1 1.428

, Å 0.71073

2max, deg 26.402

Total reflections 20197

Rint 0.0362

Unique reflections 3265

Parameters refined 238

Restraints used 0

R1, wR2 [I > 2(I)]a 0.0251/0.0619

R1, wR2 (all data) 0.0318/0.0659

Goodness of fitb 1.032

Diff. peak/hole, e Å–

3

0.337/-0.534

a R1 = ||Fo|–|Fc||/|Fo|; wR2 = [[w(Fo2–

Fc2)2]/[w(Fo2)2]]½ b Goodness-of-fit = [[w(Fo2–Fc2)2]/(Nobs–Nparams)]½,

based on all data

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determined using an internal Fc+/Fc (Fc = ferrocene) standard;22 Fc was added to the solution upon completion of the CV experiment.

Theoretical Calculations. The electronic structure of 1 was interrogated through electronic structure calculations performed using the Gaussian’09 and ORCA4 quantum chemical software packages.23,24 Predicted g tensors were using coupled-perturbed (CP) Density Functional Theory and state-averaged CASSCF/NEVPT2 methods as implemented in ORCA 4. The predicted g and A tensors of [LCu(NO3)]1+ and [LCu(NO3)](NO3) were determined using structural models derived from the experimental structure and for [LCu(NO3)]1+ also using geometry optimized structures obtained using Gaussian 09 at the B3LYP/6-311G level of theory. While our DFT calculations employed the hybrid TPSSh, B3LYP and the pure BP86 functionals in conjunctions with Pople’s 6-311G basis set, the complete active space self-consistent filed/n-electron valence state perturbation theory (CASSCF/NEVPT2) calculations employed the def2-TZVPP basis set and the def2/JK the auxiliary basis set. The RI-JK approximation was also used. The guess orbitals used for the CASSCF calculations were obtained from a single-point restricted open-shell (ROKS) DFT (BP86) calculation. The subsequent NEVPT2 calculations used as initial guesses the converged CASSCF states. The desired active spaces were prepared by swapping the energies of the appropriate orbitals using the rotate option of the scf ORCA keyword. All calculations were completed using either tight or standard convergence criteria. The ground state character of the electronic configurations obtained using DFT were assessed using time-dependent (TD) DFT calculations, i.e, all single-electron excitations were found to be positive. The character of the TD states was established following a procedure that was previously described.25 Charge and distributions of spin densities were estimated using Mulliken population analysis, natural populations of the valence shells obtained Natural Bond Orbitals analysis (NBO 3.0) as implemented in Gaussian’09. Finally, all our calculations used unsymmetrized, unabridged computational models were derived from the experimental structure of 1. Results and Discussion Synthesis. Tris(2-hydroxyliminopropyl)amine (46 mg; 0.2 mmol) was added to a light-blue solution of CuII(NO3)2·2.5H2O (48 mg; 0.2 mmol) in 10 mL methanol. The resulting dark-green solution was stirred for 15 minutes, filtered and left for slow evaporation. Blue-green crystals of 1 with a 61% yield were obtained after several days. Interestingly, two seemingly different type of crystals were formed during the crystallization process. Thus, after 2-3 days green-tinted blue hexagonal platelets, which adhered to the sidewalls of the crystallization flasks were first observed. Subsequently, prismatic, blue-green, needle-like crystals formed at the bottom of the flask. Despite of the different crystal morphologies the subsequent complete crystallographic investigations of these crystals revealed that they have identical compositions and diffraction patterns. Crystal Structure. Complex 1 crystallizes in the orthorhombic space group Pbca. Considering the tripodal character of the tris-(hydroxylimine)amine ligand, we anticipated a trigonal-bipyramid coordination geometry. However, the crystallographic investigation of 1 revealed a copper(II) ion with a coordination intermediate between octahedral and square pyramidal. Albeit the metal ion is formally six-coordinated, one of the bonds of the η2-bound nitrate ion is 0.61 Å longer than the other. The shorter

Figure 1. The asymmetric unit of 1. Except the hydrogen atoms of oxime groups all other Hs are omitted. Selected interatomic distances (in Å): Cu–O4 1.966(1), Cu-O5 2.679(1), Cu–N1 2.016(2), Cu-N4 2.037(1), Cu-N2 1.978(1), Cu-N3 2.170(1), O7-H 1.92(2), O8-H 1.99(2). Selected bond angles (in deg): N1-Cu-N3 100.95(6), N3-Cu-N2 103.55(6), N3-Cu-O5 166.10(5), N4-Cu-N3 79.25(6).

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bond has a length of ~1.966(1) Å, value which is typical of Cu-O bonds found in copper oxides. The other bond is unusually long indicating perhaps a semi-coordinative bond analogous to the Cu-X bonds of Cu(en)2X2 and Cu(NH3)4X2 complexes.26

The first coordination sphere of the copper(II) ion is completed by the tris-(hydroxylimine)amine ligand. The position in trans of the short Cu-O bond is occupied by the tertiary amine group. Interestingly, the Cu-N bond found in trans of the long Cu-O bond of the coordinated nitrate is 0.1 Å longer than the other two analogous bonds. This leads to a geometry for which the four shortest Cu-L bonds, ranging from 1.966(1) to 2.037(1) Å, are found in the same approximate plane and the two longest bonds are parallel to an axis orthogonal to this plane. Therefore, the geometry of 1 is perhaps best described as square-pyramid and is reminiscent of a Jahn-Teller active copper(II) ion which undergoes a tetragonal, z-out distortion along an axis parallel to the two long Cu-N/O bonds.28

Albeit the second nitrate anion is non-coordinating, it is tightly bound to the complex cation through two intramolecular hydrogen bonds.29 These supramolecular interactions are mediated by the protonated hydroxylimine groups and have O…H bond lengths of 1.92(2)-1.99(2) Å. Interestingly, the remaining oxygen atom of the non-coordinating nitrate anion forms a strong, intermolecular hydrogen bond with a neighboring complex cation. This intermolecular bond length is even shorter that the other two such that the O…H distance is about 1.87(2) Å. These three hydrogen bonds lead to a supramolecular, zig-zagging chain of complex cations that propagates along the a-axis of the unit cell Electrochemistry. Cyclic-voltammograms of 1 done using a cathodic sweep with a low switching potential show a quasi-reversible redox event, centered at E1/2 = +0.262 V, which is assigned to a CuI/CuII couple, see Figure 2a. While for a reversible redox process, we anticipate a peak-to-peak separation of 0.059 V and an anodic to cathodic current ratio of one, for 1 we observe a peak-to-peak separation of 0.179 V and current ratio of 0.74. Albeit this process is quasi-reversible, cycling of the oxidation and reduction events did not lead to a decline in the intensities of the electrochemical response. This observation suggests that, for this redox couple, 1 undergoes a slow ligand rearrangement to accommodate the preferred coordination geometry of the metal ion for that particular oxidation state.30

Extending the cathodic range of the potential sweep engenders, for the first CV cycle, the development of two additional reduction peaks at -0.292 V and -0.440 V, see Figure 2b. Subsequent cycling induces these peaks to coalesce, leading to a single irreversible redox wave centered at -0.330 V, see Figure 2c. Comparison with literature values suggests that these events can be assigned to the reduction of cuprous ions to Cu(0).31 The reverse anodic scan leads to the apparition of a very large oxidation wave centered at +0.305 V and a shoulder at +0.469 V which, most likely, are generated by the electrocatalytic oxidation of the solvent. The observed electrochemical behavior is typically associated with the electrodeposition on the working electrode of a catalytically active film or species. Interestingly, the electrochemical behavior of 1 is essentially identical to that observed for [CuL1(CF3SO3)](CF3SO3) where L1 = 1,1-bis(6-(1H-imidazol-2-yl)pyridine-2-yl)ethan-1-ol.17 For the latter complex, it has been demonstrated that, upon reduction, the Cu(I) complex becomes unstable which, in turn, allows for the deposition of an electroactive Cu(0)/Cu2O film. Considering the similitude of the first coordination spheres and of their electrochemical behavior we conclude that, just like [CuL1(CF3SO3)](CF3SO3), 1 functions as a precursor for the electrodeposition of Cu(0)/Cu2O.

Figure 2. Cyclic voltammograms of 1 recorded for a 2.4 mM

DMSO solution using 0.1 M (Bu4N)PF6 as supporting

electrolyte. Potential values were determined using a Fc+/0

standard (E1/2 = 0.676 V vs SHE).

-1.8 -1.2 -0.6 0.0 0.6 1.2Potential vs. SHE [ V ]

10 A

a

b

c

CuII/ICuI/0

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X-band EPR spectroscopy. To investigate the electronic structure of 1, a neat powder sample consisting of about 1 mg ground polycrystalline powder dispersed in mineral oil was studied at temperatures ranging from 4 to 150 K. Below 20 K, we observe a signal which saturates easily even for a microwave power as low as 8 µW. Above this value, up to the highest temperature investigated, other than the expected Curie-type change in the intensity of the signal, the observed spectra are essentially temperature-independent.32 Inspection of Figure 3, which shows a representative spectrum recorded at 25 K, reveals a nearly axial S = 1/2 signal with g∥ > g⊥. Spectral simulations indicate that gz = 2.256(2), gx = 2.086(1), and gy = 2.048(1). The relative broadness of the observed resonances could be reproduced by considering an intrinsic linewidth of 10 G and a g strain such that 𝜎(g𝑧) ≫ 𝜎(g𝑥) ≈ 𝜎(g𝑦), see Table 2. Albeit the considerably greater distribution of the low-field resonance, gz, might be indicative of heterogenous broadening, it is also consistent with the presence of unresolved hyperfine splitting. Thus, we anticipate a pattern of hyperfine splitting constants analogous to that of the g tensor, such that A∥ ≫ A⊥. Although A∥ is expected to be quite large, possibly as high as 160 G (600 MHz), the lack of resolution is, most likely, due to strong intermolecular spin-spin prevalent in the solid state. Including hyperfine splitting instead of g-strain in our simulations did not improve the quality of our theoretical spectra and did not allow us to infer the values of the hyperfine splitting constants.

To elucidate the hyperfine splitting pattern of 1 we investigated a series of ~2.5 mM solution samples prepared using solvents with differing coordinating and glass-forming properties. By exploring dilute solutions, we attempted to examine magnetically dilute samples and thus eliminate the effect of intermolecular spin-spin interactions. Regardless, of the solvent used we did not detect a signal at room temperature for liquid solutions. In contrast, we observed strong signals for frozen solutions at cryogenic temperatures, i.e., T < 150 K. For some solvents such as acetonitrile and pure water we observed just one broad, featureless resonance with an apparent linewidth of 75 - 90 G centered at giso = 2.11 - 2.12, spectra not shown. Some Cu2+-containing materials exhibit temperature-dependent signals with dynamic spectral shapes that change from rhombic or axial at low temperatures to isotropic at high temperatures. It has been proposed that this behavior originates from a dynamic Jahn-Teller effect.33 However, for our samples the observed spectral shapes were essentially temperature-independent and thus, we inferred that they originate from heterogeneous polycrystalline materials formed by the extrusion of the solute during freezing.

Table 2. Parameters used to simulate the EPR spectra of 1.

Sample

Comp. gz gx gy Az Ax Ay LW

[G]

%

σ(gz) σ(gx) σ(gy) σ(Az)

[MHz]

σ(Ax)

[MHz]

σ(Ay)

[MHz]

neat

powder

2.256 2.086 2.048 10 100

0.017 0.004 0.004

DMSO

solution

a

(blue)

2.262 2.098 2.061 517 38 26 10 85

0.027 0.063 0.016 0 1 17

b

(green)

2.405 2.071 2.000 400 33 197 10 15

0.015 0.004 0.010 0 0 0

water/

glycerol

solution

a

(blue)

2.219 2.157 2.076 337 159 10 12 45

0.012 0.012 0.003 5 5 5

b

(green)

2.412 2.037 2.000 408 60 232 12 55

0.015 0.001 0.007 7 8 6

Figure 3. Shown in red is the X-band EPR spectrum

recorded at 25 K and 9.378 GHz for a neat powder of 1.

The black curve is a simulation is obtained using the

parameters listed in Table 2.

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Interestingly, adding glycerol to the water solution or using a weakly coordinating solvent such as DMSO leads to frozen solutions samples that exhibit a rich hyperfine structure, see Figure 5. The observed hyperfine splitting patterns originate from the interaction of the S = 1/2 electronic spin with the I = 3/2 nuclear spin of 63Cu (69.1%) and 63Cu (30.9%) nuclei. Since for these isotopes their nuclear g values differ by only 7% we could determine only an average value of the respective hyperfine coupling constants. Inspection of Figure 4 shows that, in both cases, we observe at least two distinct spectral components. Focusing on the low-field part of the spectra it becomes readily apparent that one component exhibits a low-field g-value, gz ~ 2.4, which is considerably higher than the analogous value observed for the solid gz ~ 2.22. While for the water/glycerol solution this component is dominant (55%), for the DMSO solution it accounts for only small fraction (15%) of the Cu(II)-containing species present in the sample. Interestingly, the pattern of g and A values observed for the minor component of the water/glycerol solution values is reversed from that observed for the solid and for all other components of the solution samples, i.e., A⊥ ≫ A∥ and g⊥ ≫ g∥. This switch is indicative of change in the orbital ground state of the copper(II) and most likely it results from the reorganization of the first coordination sphere of the copper ions. In contrast, the major component of the DMSO solution (85%) exhibits a g tensor that is essentially identical to that observed for the solid state. Consequently, this observation suggests that, in this solvent, the first coordination sphere of 1 is essentially conserved. Finally, the simulation of the individual spectral components including their sum, together with the experimental spectra, are presented in Figure 4. The theoretical spectra were generated using the parameters listed in Table 2 which were obtained after performing an extensive series of fits and simulations. Electronic structure of 1. Based on their EPR spectra mononuclear copper(II) complexes with a quasi-octahedral coordination geometries fit into three broad categories: compounds with axial or nearly axial g-tensors such that (i) g∥ > g⊥, (ii) g⊥ >g∥, and complexes with rhombic spectra for which (iii) g𝑧 > g𝑥 > g𝑦 .34,35 Cu2+ ions have a d9 electronic configuration

Table 3. Theoretical g and A tensors.

Cmplx. Method Struct. gz gx gy Az Ax Ay

[CuL(NO3)]+

B3LYP x-ray 2.180 2.082 2.043 -509 118 -24

opt. 2.181 2.062 2.058 -559 41 14

BP86 x-ray 2.116 2.062 2.026 -411 118 -36

opt. 2.120 2.062 2.029 -430 114 -35

TPSSh x-ray 2.138 2.067 2.034 -482 143 -11

opt. 2.140 2.067 2.036 -494 137 -6

[CuL(NO3)]NO3

B3LYP x-ray 2.187 2.086 2.047 -505 123 -15

BP86 x-ray 2.120 2.060 2.032 -338 -107 -11

TPSSh x-ray 2.145 2.071 2.039 -451 144 2.42

CASSCF(9,5) x-ray 2.598 2.171 2.084

not available NEVPT2(9,5) x-ray 2.430 2.131 2.067 CASSCF(13,7) x-ray 2.599 2.171 2.084 NEVPT2(13,7) x-ray 2.429 2.131 2.067

[CuL(NO3)]NO3 experimental 2.25(1) 2.08(1) 2.04(2) 517(4) 38(5) 26(20)

Figure 4. X-band EPR spectra of water/glycerol (A) and DMSO (B)

solutions of 1. While A was recorded at 100 K and 9.383 GHz, B

was recorded at 50 K and 9.387 GHz. The black curves overlaid on

the experimental spectra are simulations obtained considering two

spectral components. The individual components are drawn below

each spectrum the major component being shown in blue (a) and

the minor component in green (b). The parameters used to obtain

these simulations are listed in Table 2.

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9

which, for an octahedral geometry, leads to a doubly degenerate 2Eg orbital ground state and a 2Tg excited triplet state. The orbital degeneracy is, however, lifted due to either a metal site symmetry lower than Oh or through a Jahn-Teller distortion. Regardless, we anticipate singlet orbital ground states corresponding to the single unpaired electron of Cu(II) being accommodated either into a (z2) or a (x2-y2) orbital. Albeit these two states are not mixed by spin-orbit coupling (SOC) they do mix with the higher 2Tg - derived excited orbital states.36 Consequently, for a (x2-y2) ground state the non-zero matrix elements, ⟨𝑥2 − 𝑦2| 𝑧|𝑥𝑦⟩ = −2𝑖 and ⟨𝑥2 − 𝑦2| 𝑥|𝑦𝑧⟩ = ⟨𝑥2 − 𝑦2| 𝑦|𝑥𝑧⟩ = 𝑖, lead to a type (i), g∥ >g⊥ EPR signal such as that observed for 1. Analogously, for a (z2) ground state the ⟨𝑧2| 𝑥|𝑦𝑧⟩ = −⟨𝑧2| 𝑦|𝑥𝑧⟩ = 𝑖√3 non-zero matrix elements yield a type (ii), g⊥ > g∥~ge EPR signal. Finally, the spectra of type (iii) are a diagnostic of an orbital ground state for which the unpaired electron is accommodated in an orbital obtained from a linear combination of (x2-y2)/(z2) orbitals.

The simple crystal-field theory-derived arguments presented above strongly suggest that the quasi-axial EPR signal of 1 is indicative of a (x2-y2) ground state. However, in order to validate this conclusion and to obtain a more quantitative understanding of the g and A tensors we have performed a series of DFT and CASSCF calculations. At the DFT level, the g-tensor and the orbital part of the A tensor were obtained using the coupled-perturbed SOC formalism developed by Neese et al.37 The g-tensor was also calculated using the CASSCF and CASSCFF/NEVPT2 methods, vide infra. For our DFT calculations we compared the predictions obtained using a pure functional (BP86) and two hybrid functionals (B3LYP and TPSSh), in conjunctions with Pople’s 6-311G basis set. All three functionals have been extensively used to evaluate the SOC of transition metal complexes. Inspection of Table 3 shows that, in our case, B3LYP works best in predicting the g tensor of 1. Interestingly, these calculations suggest that the second coordination sphere of the copper(II) ion does not influence the magnetic properties of 1. Thus, the predicted g tensors obtained for the neutral [CuL(NO3)](NO3) complex, which also includes the hydrogen-bound nitrate anion, are virtually indistinguishable from those obtained for the [CuL(NO3)]+ complex cation. Additionally, the results obtained for geometry-optimized structures are quite close to those obtained for the experimental structure. Since our geometry optimizations leave most of the metric parameters essentially unchanged, the latter observation suggests that unlike the zero-field splitting tensor of S > 1/2 systems, the g tensor of S = 1/2 states of complexes such as 1 is more robust to minute structural changes. Analysis of the DFT-predicted ground-state electronic configurations reveals that, as expected, the unpaired electron of 1 occupies an (x2-y2)-like orbital. This orbital and, also, the two smallest components of the predicted g tensor are aligned with the four shortest Cu-ligand bonds. Analysis of the time-dependent (TD) DFT result reveals that the lowest d-d transition can be assigned to the

Figure 5. Crystal-field splitting diagram derived from the TD DFT

calculation performed at the B3LYP/6-311G level of theory. The axes

shown in black indicate the frame of the predicted g-tensor obtained.

Apart from the teal-colored hydroxyl-imine hydrogen atoms all other Hs

are omitted.

16

12

8

4

0

Tran

sitio

n En

ergi

es [1

03 c

m-1

]

𝑧2 𝑥2 − 𝑦2

𝑥𝑧 𝑥2 − 𝑦2

𝑥𝑦 𝑥2 − 𝑦2

𝑦𝑧 𝑥2 − 𝑦2

Ground State𝑥𝑧 2 𝑦𝑧 2 𝑥𝑦 2 𝑧2 2 𝑥2 − 𝑦2

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65

10

promotion of a spin-down, beta electron from a z2-like orbital to the LUMO beta, (x2-y2)-like orbital, see Figure 5. For this transition DFT, at the B3LYP/6-311G level of theory, predicts an energy of ~9000 cm-1. However, largest contribution to the g tensor is given by the (𝑥𝑦) (𝑥2 − 𝑦2) transition which has a predicted energy of ~16 500 cm-1. This contribution leads to an increase to the g-tensor component aligned with the z direction which is roughly parallel with the two longest bonds.

The predicted g tensor of 1 was also evaluated using state-averaged CASSCF performed on a structural model of [LCu(NO3)](NO3) derived from the experimental structure. These calculations were augmented by including the dynamic correlation calculated using the NEVPT2 method. The active space of the CASSCF/NEVPT2(9,5) calculations spanned the canonical d orbitals represented by the antibonding eg

*- and non-bonding t2g-like orbitals. The active space of the CASSCF/NEVPT(13,7) calculations was expanded to include the ligand-based eg-like bonding orbitals. Inspection of Table 3 reveals that expanding the active space leaves the predicted g tensor unchanged. However, the individual tensor components, in particular gz, are considerably larger than those predicted by DFT or the experimental values. While the CASSCF-predicted pattern of excited states predicted is essentially the same with that inferred from TD DFT, the energies of the excited states are considerably lower. In some cases, these energies are 60% lower than those of the analogous DFT-derived one-electron excitations and thus responsible for the larger predicted g-values. Conclusion A new copper(II) complex, [CuL(NO3)](NO3), (1), was synthesized using a tripodal ligand which incorporates three hydroxylimine groups. The electronic structure of this complex was investigated using X-band EPR. The experimental g-tensor was best reproduced using CP DFT at the B3LYP/6-311G level of theory. These calculations also reveal that the unpaired electron of the copper(II) is accommodated in a x2-y2 orbital which is found along the four shortest Cu-ligand bonds. Electrochemical studies of 1 have revealed a quasi-reversible Cu(I)/Cu(II) redox couple and that 1 function as a precursor for electroactive films. Acknowledgments S.A.S. and A.R. acknowledge support from the University of Idaho. This work used the Extreme Science and Engineering Discovery Environment (XSEDE), which is supported by National Science Foundation grant number ACI-1548562 (project CHE180015 to S.A.S.). WU’s Single Crystal Diffractometer was acquired using funds provided by the NSF-MRI grant CHE-1827313 (to K.A.W.). Appendix A. Supplementary data Supplementary data associated with this article can be found in the online version. References (1) Carter, B. Boom, Bust, Boom: Story about Copper, the Metal That Runs the World; First Schaffner Press:

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Graphical Abstract:

gz

gy

gx

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65