Oxygenation of copper(I) complexes containing fluorine tagged tripodal tetradentate chelates: significant ligand electronic effects

Abstract Copper-dioxygen (O2) interactions are of great importance in biological and chemical transformations involving reversible dioxygen binding, activation, or reduction. In this report, we describe O2-reactions with the mononuclear copper(I) complexes containing two new analogues of the known nitrogen-containing tetradentate tripodal chelate, tris[(2-pyridyl)methyl]amine (TMPA). In both derivatives, one electron-rich and one electron-deficient, fluorine atoms are attached to the ligand framework, allowing for the use of 19F-NMR spectroscopy to probe the oxygenation process. Variations of ligand electronic properties are manifested in the electrochemical behavior of copper complexes and their reactivities toward O2. Our NMR spectroscopic studies, along with variable-temperature electronic absorption measurements, revealed that the copper(I) complexes reversibly react with O2 to form the corresponding 1:1 copper-O2 (i.e., end-on superoxo) intermediates which can further react reversibly with second equivalents of copper(I) complexes to form the related dinuclear 2:1 copper-O2 (i.e., trans-peroxo) adducts. However, considerable differences exist in detail at various temperatures, depending on the chelate. All three end-on superoxo and trans-peroxo species described here possess similar spectroscopic features, although small but significant shifts in the energy of their signature bands were observed, suggesting that the variation in the chelates directly affects the electronic properties of the copper-O2 cores. Graphical Abstract


Introduction
The dioxygen (O 2 ) chemistry of synthetic copper(I) complexes and the oxidative properties of the resulting copper-O 2 adducts are of importance due to their potential relevance to copper-containing proteins vital for aerobic life as well as their applications in chemical catalysis [1][2][3][4][5]. In nature, copper-dioxygen interactions are essential for facilitating an array of biological functions in many proteins including the dioxygen-carrier hemocyanin, monooxygenases where O 2 is activated such as in tyrosinase, dopamine b-hydroxylase, and phenylalanine hydroxylase, or oxidases where O 2 is reduced to H 2 O or H 2 O 2 including in laccase, galactose oxidase, ascorbate oxidase, amine oxidase, and the heme-copper binuclear active site of cytochrome c oxidase [2,[6][7][8].
Modifications of the TMPA framework to influence the steric and electronic properties of the donor atoms have shown that the presence of electron-donating substituents leads to energetic stabilization of the superoxo intermediate and chelates with large steric demands and a negative charge can prevent the dimerization and formation of the corresponding peroxo species in solution [10,13,18,19]. In this report, we describe and compare the oxygenation of the cuprous complexes of two new TMPA-derivatives, one electron-rich and one electron-deficient, with fluorine atoms attached to the chelate backbones which also provide the ability to probe the dioxygen reactivity of these systems through 19 F-NMR spectroscopy ( Figure 2). The dioxygen reactivities of the copper(I) complexes were investigated using 1 H-and 19 F-NMR spectroscopies along with variable-temperature UV-vis measurements. For comparison, the corresponding copper(II)-chloro complexes were also synthesized and characterized using a series of spectroscopic methods as well as cyclic voltammetry, mass spectrometry, and X-ray crystallography. The properties of copper(I) and copper(II)-chloro complexes are compared and contrasted, particularly with respect to the influence of the pyridyl substituents present in the new ligand scaffolds. Some mechanistic insights regarding oxygenation reactions are also presented.

General methods
All chemicals were of commercially available grade and used without purification, unless noted otherwise. Acetonitrile (MeCN), dichloromethane (DCM), tetrahydrofuran  (THF), and 2-methyltetrahydrofuran (MeTHF) were purchased from Sigma-Aldrich. Methanol (MeOH) and diethyl ether were purchased from Fisher Chemical. Deuterated solvents (CDCl 3 , CD 3 CN, and THF-d 8 ) were purchased from Cambridge Isotope Laboratories. Commercial ACS grade solvents were used for chromatography and extractions. All solvents were purified by an Innovative Technologies or Inert PureSolv Micro solvent purification system prior to use for the reactions and characterizations. Solvents were then deoxygenated by bubbling with argon for 1 h followed by storage over 3 or 5 Å molecular sieves for at least 72 h prior to use. Deionized water was purified by a PURELAB flex 1 Analytical Ultrapure Water System (ELGA) to obtain nanopure water with a specific resistance of 18.2 MX cm at room temperature. Air-and moisture-sensitive compounds were prepared and handled under nitrogen in a Vacuum Atmospheres OMNI-Lab inert atmosphere (<0.5 ppm of O 2 and H 2 O) glovebox, or under a dry, oxygen-free argon atmosphere using standard Schlenk techniques. Ultrahigh purity grade oxygen gas was purchased from Airgas and passed through a drying column containing Drierite desiccant and 3 Å activated molecular sieves prior to use. For Nuclear Magnetic Resonance (NMR) experiments, dry O 2 gas was transferred and stored in a capped 50 mL Schlenk flask, then slowly bubbled into the metal complex solutions via a three-way long syringe needle.

Single-crystal X-ray diffraction
Suitable X-ray quality single crystals of [(F 2 tmpa)Cu II (Cl)][B(C 6 F 5 ) 4 ] were obtained by recrystallization in hot MeOH/water. All reflection intensities were measured at 100(2) K using a Gemini R diffractometer (equipped with an Atlas detector) with Mo-Ka radiation (k ¼ 0.71073 Å) under the program CrysAlisPro (Version CrysAlisPro 1.171.38.43f, Rigaku OD, 2015). The same program (but a different version viz. CrysAlisPro 1.171.40.53, Rigaku OD, 2019) was used to refine the cell dimensions and for data reduction. The temperature of the data collection was controlled using the system Cryojet (manufactured by Oxford Instruments, Abingdon, UK). The structure was solved with the program SHELXT-2018/2 and was refined on F 2 by full-matrix least-squares using the SHELXL-2018/3 program package [21]. Numerical absorption correction based on Gaussian integration was applied using a multifaceted crystal model by CrysAlisPro. Non-hydrogen atoms were refined anisotropically. In the refinement, hydrogen atoms were treated as riding atoms using SHELXL default parameters. The SUMP command in SHELXL was used to fix the occupancy factors of the fluorine atoms on pyridyl ring to two.
The Cambridge Crystallographic Data Center (CCDC no. 2163521) contains the supplementary crystallographic data for this article. The data can be obtained free of charge via https://www.ccdc.cam.ac.uk/data_request/cif.

Electrochemical studies
Electrochemical data were collected under nitrogen in the glovebox using a threeelectrode cell. A leak-free Ag/AgCl reference electrode (Innovative Instruments, Inc.), a 3.0-mm glassy carbon working electrode, and a graphite carbon rod counter electrode were used for the measurements. The electrodes were cleaned using nanopure water and acetone prior to each experiment. The supporting electrolyte solution was 100 mM of [(nBu) 4 Table S1). All potentials were further confirmed with the ferricenium/ferrocene couple (E 1/2 ¼ 0.450 V vs. Ag/AgCl under identical conditions) as an internal reference [22].  4 ] in MeTHF (2 mL) was prepared inside the glovebox. Samples for UV-vis measurements (450 lM concentration) were prepared by diluting 180 mL of this stock solution with MeTHF to 1 mL, then transferred to a 4-mm modified Schlenk cuvette equipped with a septum. UV-vis spectra were recorded upon the bubbling of dry O 2 gas directly into the solution at room temperature.

Low-temperature UV-vis absorption studies
The general procedure for the dioxygen reactivities of the [(L)Cu I ][B(C 6 F 5 ) 4 ] complexes (L: F 2 TMPA, TMPA, or MeTFE-TMPA) is described below, with L being MeTFE-TMPA as a representative case. A stock solution of 6.9 mg (0.006 mmol) of [(MeTFEtmpa)Cu I ][B(C 6 F 5 ) 4 ] in MeTHF (2 mL) was prepared inside the glovebox. Samples for UVvis measurements (450 lM concentration) were prepared by diluting 180 mL of this stock solution with MeTHF to 1 mL, then transferred to a 4-mm modified Schlenk cuvette equipped with a septum. UV-vis spectra were recorded upon the bubbling of dry O 2 gas directly into the solution at -80 or -110 C. The solution was monitored at -80 or -110 C for 30 min after O 2 bubbling, then allowed to warm to room temperature. The solution was further monitored for 1 h 30 min, with spectra recorded at -80 or -110 C. , 0.028 mmol) was dissolved in THF-d 8 (800 mL) and transferred to an NMR tube. The sample was sealed with a rubber septum and transferred outside, and its spectrum was recorded at room temperature. For oxygenation, 4 mL of dry O 2 gas was slowly bubbled into the sample solution using a Hamilton gastight syringe equipped with a three-way valve. After O 2 bubbling, the first spectrum was recorded within 2 min of mixing, and the reaction was monitored over 6 h.

Results and discussion
3.1. Structural and physical properties of copper complexes

New ligands
To study the electronic effects in modulating the dioxygen reactivity of the cuprous complexes, we prepared two new TMPA-based ligands, with either electron-withdrawing or -donating groups on the pyridyl arms. Inclusion of fluorine groups on the backbone of the chelates also provided an additional tool for probing the oxygenation reactions. In the electron-deficient F 2 TMPA chelate, fluorine atoms occupy the 3 0 -position of the two substituted pyridyl rings whereas in the electron-rich MeTFE-TMPA derivative, a methyl group along with a trifluoroethoxy (TFE) pendant are attached to the 3 0 and 4 0 -positions of the substituted pyridyl ring, respectively. From the coordination chemistry perspective, the ligands were designed such that in both TMPA derivatives, two of the three pyridyl rings serve as weaker r-donors as compared to the third pyridyl arm (i.e., the two fluorine-substituted pyridine rings in F 2 TMPA relative to its unsubstituted arm, or the two unsubstituted pyridine rings in MeTFE-TMPA compared to the 3-methyl-4-(2,2,2-trifluoroethoxy)pyridine arm). The ligand F 2 TMPA was synthesized through reductive amination while MeTFE-TMPA was prepared via a nucleophilic substitution reaction. Further details are presented in the Experimental section and supporting material Scheme S1. Both tetradentate ligands contain groups with fluorine atoms which can be probed using 19 F-NMR spectroscopy. Both ligands were characterized through 1 H-and 19 F-NMR, FT-IR, and ESI-MS measurements. It is important to note that previously, synthetic difficulties were described as the reason for not including any O 2 -reactivity studies of the TMPA-based cuprous complexes with electron-withdrawing groups [24].

Copper(I) complexes
Cuprous complexes were synthesized with tetrakis(pentafluorophenyl)borate, [B(C 6 F 5 ) 4 ] -, as a counter anion for enhanced solubility, by mixing one equivalent of each ligand and [Cu I (MeCN) 4 ][B(C 6 F 5 ) 4 ] in THF in the glovebox (see Experimental section). Previous reports have shown that copper(I) complexes of TMPA-like ligands effect reductive dehalogenation reactions with a wide range of organohalide substrates [25][26][27][28]. Even a reactivity study of a TMPA-based chelate possessing a pendant R-Cl moiety (i.e., internal substrate) revealed the involvement of a copper-alkyl intermediate [29]. However, regarding the MeTFE-TMPA ligand, the presence of the trifluoroethoxy arm on the 4 0 -pyridyl position during the preparation of the copper(I) complex led to no observable reductive defluorination reaction, likely due to the stronger carbon-fluorine bonds as compared to those of other halides.
Both  (MeCN)] þ possesses one coordinated acetonitrile molecule per tetradentate ligand-copper core, preferring five-coordinate geometry while the bound MeCN ligand can be removed by multiple recrystallization from a weakly coordinating solvent such as THF or diethyl ether [25,28]. The 1 H-NMR and IR spectra of the two new isolated copper(I) complexes with F 2 TMPA and MeTFE-TMPA revealed that both systems adopt a four-coordinate formulation [(L)Cu I ] þ with no MeCN incorporated as a fifth ligand. This is not surprising as these complexes were prepared and isolated in THF.

Copper(II) complexes
The copper(II)-chloro complexes of the two new chelates, F 2 TMPA and MeTFE-TMPA, were prepared following a similar procedure to the one previously reported for the parent complex, [(tmpa)Cu II (Cl)][B(C 6 F 5 ) 4 ] [20]. The 19 F-NMR spectra of the copper(II)chloro complexes, with F 2 TMPA or MeTFE-TMPA, confirmed the association of one equivalent of [B(C 6 F 5 ) 4 ]with one ligand-copper moiety in each system. The complexation of copper(II)-chloro complexes was further confirmed via 1 H-NMR, FT-IR, mass spectrometry, and cyclic voltammetry measurements. The reduction potentials of the copper(II)-chloro complexes are significantly lower (>340 mV) than those of the cuprous complexes (vide infra).
In contrast to the four-coordinate structures of [(L)Cu I ][B(C 6 F 5 ) 4 ] (L: F 2 TMPA, TMPA, and MeTFE-TMPA) but common in copper(I) ion chemistry, the copper centers in the copper(II)-chloro counterparts are five-coordinate. In general, a complex with a fivecoordinate cupric center in a square pyramidal (SP) environment exhibits a broad band in the visible region (590-780 nm) along with a low-energy spin-forbidden shoulder at k > 800 nm, whereas in a trigonal bipyramidal (TBP) geometry, the complex displays a main d-d transition at k > 800 nm with a high-energy shoulder in the visible region [30]. The electronic absorption spectra of the parent as well as the two new copper(II)-chloro complexes exhibit one main d-d transition band centered at 964 nm and a shoulder at 734 nm, revealing that the TBP geometry is dominant is all three cupric systems in solution (supporting material Figure S15).
The intense ligand-to-metal charge transfer (LMCT) bands from the bound chloride ligand to the cupric center in the complexes bearing the F 2 TMPA, TMPA, and MeTFE-TMPA chelates appear at 310, 305, and 302 nm, respectively, supporting the more stabilized d z2 orbitals in the former, [(F 2 tmpa)Cu II (Cl)][B(C 6 F 5 ) 4 ].

Crystal structure of [(F 2 tmpa)Cu II (Cl)][B(C 6 F 5 ) 4 ]
The complex crystallizes in a triclinic crystal system with P-1 space group (see Experimental section, supporting material Figure S29 and Table S2). The molecular structure of the complex is shown in Figure 3 and relevant bond distances and angles are given in the figure captions. The cupric center is five-coordinate with three pyridyl nitrogens (N1, N3, and N4) and one tertiary alkyl amino nitrogen (N2) along with a chloride.
The Addison-Reedijk geometry analysis allows for the semiquantitative estimation of the prevalent geometry in crystalline phase [31], further supporting that the cupric center adjusts to a distorted TBP (s 5 ¼ 0.78) coordination environment in F 2 TMPA while the ligation in the parent TMPA analogue is reported to occur within a nearly perfect TBP geometry (s 5 ¼ 0.95) [20]. The prominent TBP organization of the ligands around the cupric center observed in the crystalline phase agrees well with those speculated from their d-d absorption patterns for the solution phase (vide supra).

Electrochemistry
The electrochemical behavior of the copper complexes was studied by cyclic voltammetry (CV) under an inert atmosphere in MeCN. The data are given in Table 1. All complexes display a single one-electron transfer process showing quasireversible behavior with peak-to-peak separation values, DE p , no more than 112 mV (Table 1) and anodic/cathodic peak current ratios (i pa /i pc ) between 0.96 and 0.79.
Our Randles-Sevcik analysis of the peak current vs. the square root of the scan rate confirmed that in all cases, the species involved in the redox reactions were freely diffusing through the solution. The diffusion coefficients (D) of the cupric and cuprous forms of the [(L)Cu] nþ species are between 1.39 and 3.65 Â 10 À6 cm 2 s À1 (supporting material Table S1). It is worth noting that the presence of the chloride ligand systematically decreases the reduction potential of the copper center by about 350 mV in all three chelate environments (Figure 4). The reversibility of the voltammograms of the copper(II)-chloro complexes, even at slower scan rates (e.g., 25 mVÁs À1 ), and the absence of any additional irreversible anodic peaks associated with the free copper  chelates also confirm that the axial chloride remains bound to the copper center during the reduction event.
The data shown for copper complexes reveal that, with substituted pyridyl ligands bearing electron-donating or -withdrawing groups, the E1 =2 value for the Cu II/I redox couple becomes more negative or more positive compared to that for the complex with the parent TMPA ligand, respectively. In other words, MeTFE-TMPA scaffold results in a more thermodynamically stable copper(II) complex while F 2 TMPA forms a more thermodynamically stable copper(I) species as compared to the parent system. The variation in electrochemical behavior of copper(I) species with pyridyl ligands correlates to the O 2 -reactivity differences observed in these systems (vide infra).

Oxygenation reactions of copper(I) complexes
The interaction of cuprous complexes, [(L)Cu I ][B(C 6 F 5 ) 4 ] (L: F 2 TMPA, TMPA, and MeTFE-TMPA), with dioxygen in MeTHF follows the same basic mechanism which has been reported for other TMPA-like systems as described in the Introduction (Figure 1). The oxygenation reactions were monitored by variable temperature electronic absorption as well as 1 H-and 19 F-NMR spectroscopies.

UV-vis absorption spectroscopy
Bubbling O 2 into the solution of each copper(I) complex at room temperature resulted in the signature, intense color change from light yellow to purple known for formation of binuclear trans-peroxo-dicopper(II) complexes ( Figure 5) [13,14,32]. The absorption spectra of the purple trans-peroxo-dicopper(II) intermediates with the F 2 TMPA, TMPA, and MeTFE-TMPA chelates in MeTHF solution showed intense absorption bands at 526, 525, and 523 nm along with shoulders at 617, 616, and 614 nm, respectively. These two characteristic spectral features in the visible region are respectively ascribed to p r Ã ! d and p v Ã ! d charge transfer (CT) transitions from the trans-peroxo ligand to the two copper centers and are consistent with the presence of highly covalent Cu-O bonds [7,33,34]. Close analysis of the previously reported X-ray structure of the parent trans-peroxo species, [(tmpa)Cu II -(O 2 )-Cu II (tmpa)] 2þ , reveals that the copper centers display an almost ideal TBP geometry (s 5 ¼ 0.89), consistent with the structure of its copper(II)-chloro analogue, [(tmpa)Cu II (Cl)] þ (s 5 ¼ 0.95). Assuming the same relationship exists between the two new trans-peroxo-dicopper(II) complexes and the corresponding copper(II)-chloro counterparts, the s 5 value and supporting electronic absorption results would suggest that the cupric site of the corresponding trans-peroxo species, though to different degrees, adjust to a distorted TBP coordination environment, thus, supporting a d z2 ground state. The systematic hypsochromic (i.e., blue) shift of both CT bands in moving from the electron-deficient system bearing F 2 TMPA to the electron-rich analogue with MeTFE-TMPA strongly suggests that the higher degree of electron donation from the chelate destabilizes d z2 , hence, shifting the LMCT bands from the bridging peroxide p Ã -orbitals to the cupric center to higher energies (i.e., hypsochromic shift).
Interestingly (1), some cuprous species were still present in the solution. The observed trend follows the redox potential pattern of the cuprous complexes. Thus, the thermodynamic stability of the trans-peroxo species bearing MeTFE-TMPA is much higher than that of the TMPA or F 2 TMPA analogues manifesting a considerable ligand electronic effect. This finding is in agreement with our NMR studies at room temperature, confirming that the trans-peroxo species of MeTFE-TMPA is significantly more stable than its more electron-deficient analogues (vide infra).
The association of two cuprous complexes with O 2 to form a trans-peroxo bridged dicopper assembly is an entropically unfavorable process, but the effect of this entropic cost can be alleviated by lowering the temperature. Lower temperatures also increase the lifetime of the initially formed copper-dioxygen intermediates not only by reducing the entropic costs of formation, but also by attenuating subsequent reactions. Therefore, further investigation of the oxygenation reactions of the copper(I) complexes were performed at lower reaction temperatures. At -80 C, the copper(I) complexes of TMPA and F 2 TMPA instantly reacted with dioxygen to form the corresponding trans-peroxo dicopper (i.e., 2:1 copper-dioxygen) intermediates with no sign of residual cuprous complex ( Figure 6). The resulting trans-peroxo species were stable at -80 C and only decomposed upon briefly warming to room temperature. While no end-on superoxo (i.e., 1:1 copper-dioxygen) intermediates were detected within the measurement time Further lowering the temperature to -110 C led to remarkable differences in the oxygenation reactivities compared to those at -80 C as is evident in Figures 6 and 7. The addition of O 2 to all three copper(I) complexes at -110 C in MeTHF instantly led to full formation of the corresponding end-on superoxo intermediates. In the case of MeTFE-TMPA, a clean and stable end-on superoxo species was formed with an intense absorption band at 420 nm, two weaker features at 519 and 587 nm, and a more prominent band at 759 nm. The former has been tentatively assigned to a p r Ã ! d CT transition, in which the in-plane p r Ã orbital of the superoxide ion overlaps in r fashion, with the d z2 orbital of the cupric center [13]. The oxygenation of the other two copper(I) complexes, 1 and 2, however, resulted in the formation of a mixture of the corresponding 1:1 and 2:1 copper-O 2 adducts, with the end-on superoxo intermediate initially being the major species and readily converting to the corresponding trans-peroxo dicopper complex.
Comparisons of copper(I)-dioxygen reactivities at different temperatures point to varying degrees of change in the rate of initial reversible O 2 binding and the subsequent formation of the bridged trans-peroxo complex. At -80 C, much slower transformation of the end-on superoxo into the trans-peroxo species with the MeTFE-TMPA chelate as compared to the other two systems suggests that the concentration of the copper(I) complex, 3, must be minute due to a very large equilibrium constant for formation of the end-on superoxo complex at that temperature. These results are indicative of a clear ligand electronic effect, precluding formation of the corresponding trans-peroxo at the lowest temperature, -110 C. At higher temperatures, the rate of formation of the trans-peroxo complex bearing MeTFE-TMPA strongly increases as is particularly noticeable in comparing Figures 5-7. This is again likely due to the shift in the equilibrium between 3 and the corresponding end-on superoxo species, making 3 available for the formation of the peroxo complex.  We also note that a similar correlation between the electronic effects of the three chelates and the energy of the absorption features in both end-on superoxo and trans-peroxo dicopper species were observed. As listed in Table 2, all CT bands shift to lower energies as the chelates become more electron-deficient and stabilize the d z2 orbitals.

Nuclear magnetic resonance spectroscopy
To further confirm the formation of trans-peroxo-dicopper(II) species and subsequent decomposition as observed in the UV-vis studies (vide supra), the dioxygen reactivity of the cuprous complexes was studied through 1 H-and 19 F-NMR spectroscopies at room temperature. Karlin and co-workers have previously shown that the two cupric centers in the parent trans-peroxo-dicopper(II) assembly were strongly antiferromagnetically coupled (i.e., singlet ground state) [25], therefore, well resolved ligand resonances in both 1 H-and 19 F-NMR spectra were expected.
The , reflecting the expected deshielding that was associated with the removal of electron density from the copper center. A similar trend was also observed in the 1 H-NMR spectra for oxygenation of the parent system with the TMPA chelate, in THF-d 8 (supporting material Figures S30 and S31) [23], consistent with the reactivity pattern previously reported in CD 2 Cl 2 [25].
Due to decomposition of the trans-peroxo-dicopper(II) intermediates to paramagnetic monomeric copper(II) species, different and growing amounts of these final decomposition products were present throughout the oxygenation reactions of all the cuprous systems. In general, 1 H-NMR signals of mononuclear copper(II) complexes are either not observed or very broad due to the relatively long electronic relaxation time (i.e., s s % 10 À9 s for Cu 2þ ), which leads to line broadening [35,36]. For instance, the initial spectrum for the dioxygen-exposed solution of [(F 2 tmpa)Cu I ][B(C 6 F 5 ) 4 ] showed additional broad peaks at d ¼ 10.66 and 9.54 ppm along with a series of paramagnetic signals in the downfield region (d ¼ 16.42-43.56 ppm) which all corresponded to the monomeric [(F 2 tmpa)Cu II (X)] nþ species [35]. Decomposition of the trans-peroxodicopper(II) intermediates to the final monomeric cupric products were completed within a few hours at room temperature. The dioxygen reactivity of 1 was also monitored through 19 F-NMR spectroscopy ( Figure  9). The 19  poses over time, as observed from the gradual decrease in these signal intensities. The final decomposition product was again a paramagnetic monomer copper(II) species identified by signals at d ¼ 13.80, 13.66, 12.52, 10.58, and 9.05 ppm along with resonances between 46.59 and 22.07 ppm in the paramagnetic region [35]. It is important to point out the presence of a more complicated 1 H-NMR pattern observed for the MeTFE-TMPA oxygenation reaction as compared to those observed for the TMPA or F 2 TMPA analogue is possibly due to the lower symmetry of the cupric center or more significant separation of chemical shifts. ( Figures 8 and 9), also support the findings concluded from the UV-vis experiments at room temperature ( Figure 5), viz., that the trans-peroxo-dicopper(II) species bearing the more electron-rich MeTFE-TMPA chelate is more stable as compared to its more electron-deficient counterparts bearing either TMPA or F 2 TMPA.

Conclusion
Tagging the two new, remotely substituted, TMPA-based chelates with fluorine and utilizing 19 F-NMR spectroscopy provided a useful means for probing the oxygenation reactions. This is particularly beneficial for antiferromagnetically coupled species such as the trans-peroxo dicopper(II) intermediate in which the sharpness and lack of extensive broadening of the NMR resonances result in their facile detection.
Our spectroscopic and electrochemical studies of the effects of variation in ligand electronic properties on the redox behavior and dioxygen reactivity of the corresponding copper(I) complexes support that ligand electron-donating ability can significantly affect the oxygenation reactions. The relative rates of formation of the 1:1 (i.e., end-on superoxo) and subsequent generation of the 2:1 (i.e., trans-peroxo) copper-O 2 intermediates are governed by temperature and ligand electronic effects. For both classes of the copper-dioxygen adducts, the main electronic transitions shift to lower energies as the electron-donating ability of the chelate diminishes. This change in the electronic character of the copper-O 2 core can be explained by stabilization of d z2 orbitals which is consistent with the increase in the reduction potentials.