Laser-Induced Dynamics of Peroxo-Dicopper(II) Complexes Vary with Ligand Architecture; One-Photon Two-Electron O₂-Ejection and Formation of Mixed-Valent Cu^ICu^II-Superoxide Intermediates

Abstract

The overall one-photon two-electron peroxide-to-dioxygen oxidation chemistry (O₂²⁻ + 1 hν → O₂ + 2e⁻) was achieved with copper complexes. Interestingly, light excitation of peroxo dinuclear copper(II) complexes with an μ-η²-η²–(side-on) peroxo ligation was found to release dioxygen while those with a trans-1,2 (end-on) geometry did not, even though spectroscopic studies revealed that both reactions proceeded through superoxo intermediates. More specifically, femtosecond laser excitation of acetone solutions of a trans-μ–1,2(end-on) peroxo dinuclear copper(II) complex ([(tmpa)₂Cu^II₂(O₂)]²⁺ (1); λ_max, 525 & 600 nm) and μ-η²-η²–(side-on) peroxo dinuclear copper(II) complexes ([(N5)Cu^II₂(O₂)]²⁺ (2) and [(N3)Cu^II₂(O₂)]²⁺ (3)) at −80 °C resulted in the rapid formation of an intermediate that was reasonably assigned to be a previously unknown mixed-valent superoxide species, [Cu^II(O₂^•−)Cu^I]²⁺ (λ_max, 685–740 nm). For 1, this intermediate underwent further fast intramolecular electron transfer from the O₂^•− moiety to the Cu^II ion to yield an ‘O₂-caged’ dicopper(I) adduct, Cu^I_2–O₂, with rate constant of (2.8 ± 0.4) × 10¹² s⁻¹ but without release of O₂. Instead, data consistent with barrierless stepwise back electron transfer to regenerate 1 were observed with the rate constant (1.8 ± 0.1) × 10¹⁰ s⁻¹. Femtosecond laser excitation of the side-on peroxide complexes 2 and 3 under the same conditions led to the appearance of transient species well formulated as [Cu^II(O₂^•−)Cu^I]²⁺ intermediates that underwent further intramolecular electron-transfer from the O₂^•− center to the Cu^II ion resulting in complete O₂ release to produce the corresponding dicopper(I) compounds with rate constants of (3.9 ± 0.3) × 10⁹ s⁻¹ and (7.3 ± 0.3) × 10⁹ s⁻¹ for 2 and 3, respectively. Such remarkable differences in reaction pathways for the peroxo dinuclear copper(II) complexes likely result from the ligand-derived stability of the Cu^I vs Cu^II species (such as the Cu^II₂–O₂ intermediates); N5 and N3 Cu^I complexes are far more stable than tmpa-Cu^I species, as known from measured ligand-Cu^II/I redox potentials. With the evolution of O₂ upon laser excitation of 2 and 3, we could also then measure the very fast overall Cu^I₂/O₂ re-binding kinetics using nanosecond laser spectroscopy. The results closely track those known for the dicopper proteins hemocyanin and tyrosinase, for which the synthetic dicopper(I) precursors [(N5)Cu^I₂]²⁺ and [(N3)Cu^I₂]²⁺ and their dioxygen adducts serve as models. The biological relevance of the present findings is discussed, including the potential impact on the solar water splitting process.

Introduction

Copper-dioxygen intermediates play key roles in chemically mediated oxidation reactions, and in copper proteins involved in dioxygen (O₂) processing for reversible O₂-binding, O₂-activation leading to substrate oxidations (e.g., in monooxygenases) or protein oxidase reduction of O₂ to produce H₂O₂ or water. Among copper-dioxygen adducts, several types of peroxo-dicopper(II) complexes have been well-characterized, with the type of coordination/structure dictated by the nature (denticity, donor atom type, etc.) of the ligand bound to the copper ion. Prominent among these are the trans-μ–1,2–(end-on) peroxo dicopper(II) (^TP) and μ–η²:η²-peroxo-dicopper species (^SP); either of these may isomerize to the bis(μ-oxo)dicopper(III) complexes (O) (Scheme 1).^1–10 All of these Cu^I/O₂ derived dicopper product species have been studied extensively with respect to their inherant or comparative spectroscopic signatures; their differential reactivity with substrates has also been widely investigated.^11–19

Scheme 1.

Further, these Cu₂-O₂ species are strong light absorbers, with prominent UV and/or visible region peroxo- or oxo-to-copper-ion charge transfer transitions. In fact, there are now a few cases known where laser-induced photolysis-excitation leads to photoejection of molecular oxygen, two being synthetic cupric-superoxide (^ES) coordination complexes,^18a and others include protein Cu₂(O₂) adducts.²⁰ For such cases, nanosecond laser flash photolysis can be used to follow the rebinding kinetics-thermodynamics of O₂ to the copper(I) species produced for systems that bind molecular oxygen faster than can be followed by solution spectroscopic stopped-flow kinetics methods.^14a,b,d–g

On the other hand, the identification, characterization or investigation of the dynamics of photoexcited states has been an elusive subject since (i) they are very short lived and (ii) because these solution phase Cu-dioxygen adducts (i.e. 1, 2, 3 or similar) are generally only stable at low-temperatures (< −80 °C), precluding the investigation of excited-state fast photodynamics using femtosecond laser flash photolysis, normally measured at room temperature.

In the present work, we adjusted instrumental conditions (see Experimental Section) in order to enable low temperature (−55 to −94 °C) solution femtosecond laser induced excited state dynamics studies. When the new apparatus is used to study well-known peroxo-dicopper(II) complexes, it has become possible to detect and characterize unprecedented and perhaps unexpected (vide infra) mixed-valent transient Cu^ICu^II-superoxide species. Further, monitoring of their subsequent fast (i.e., picosecond) reactions dynamics has also become achievable. The species studied and described here are the ^TP complex [(tmpa)Cu^II-(O₂)-Cu^II(tmpa)]²⁺ (1) {tmpa = tris-(2-pyridyl-methyl)amine}⁷ and the complexes with side-on O₂²⁻-binding, μ–η²:η²-peroxo dicopper(II) complexes [(N5)Cu^II₂-(O₂)]²⁺ (2) {N5 = −(CH₂)₅-linked bis[(2–(2–pyridyl)ethyl)amine]) (PY2) units} and [(N3)Cu^II₂-(O₂)]²⁺ (3) {N3 = (CH₂)₃-linked PY2 units} (Chart 1).^8c,d,e

Chart 1.

Interestingly, the nature of the excited state species formed was similar in all cases (1–3): a previously unobserved Cu^II-superoxide…Cu^I species, ([Cu^II(O₂^•−)Cu^I]²⁺). Subsequent novel transformations occurred, and UV-vis spectroscopic monitoring allowed for the determination of kinetics and activation parameters. Notably, the two varying classes of compounds, represented by end-on (1) and side-on (2 and 3) peroxo-dicopper(II) complexes, follow unique reaction pathways. The differential behavior can be explained on the basis of the nature of the ligands employed within these complexes (Chart 1), that influenced the thermodynamic stabilities of (i) the ligand-Cu^I (vs Cu^II) complexes, and (ii) peroxo-Cu₂^II structures. The present study opens an exciting research area on reactive excited states of metal-O₂ intermediates.

Results and Discussion

Photodynamics of Peroxo (^TP) Complex [(tmpa)Cu^II-(O₂)-Cu^II-(tmpa)]²⁺ (1)

The previously well characterized trans–μ– 1,2–(end-on) peroxo dinuclear copper(II) complex 1 was prepared by reaction of [(tmpa)Cu^I(CH₃CN)]BArF (BArF = B(C₆F₅)₄⁻) with O₂ at low temperature in acetone (Figure 1a, red spectrum) at −80 °C.⁷ The complex was brightly colored due to the presence of intense peroxo-to-copper charge transfer bands in the visible region. The transient absorption spectrum measured 1.9 ps after femtosecond laser excitation of 1 exhibited a positive absorption band at λ_max = 739 nm and negative absorption difference (bleaching) features at λ_max = 525 and 600 nm, Figure 1b. The bleach features were a mirror image of the steady state absorption spectrum of 1, consistent with the loss of the ground state species. Indeed, a simulation based on the assumption that light excitation of 1 generated a species that did not absorb visible light, accurately models the two bleach absorption bands at λ_max = 525 and 600 nm. However, such modeling did not explain the positive absorption band at λ_max = 739 nm. The observed absorption maximum and full-width-at-half-maximum were similar to those previously reported for Cu^II-superoxide complexes [(tmpa)Cu^II(O₂)]⁺,^14b [(TMG₃tren)Cu^II(O₂)]⁺¹⁸, [(PV-tmpa)-Cu^II(O₂)]⁺ (PV-tmpa = [bis-(pyrid-2-ylmethyl){[6-(pival-amido)pyrid-2-yl]methyl}amine]),¹⁹ as well as to those of several other known copper(II)-superoxide compounds.^21,22 Based on this prior data, the transient absorption spectra observed upon femto-second laser excitation of 1 (black line in Figure 1b) was assigned to the mixed-valent Cu^I-Cu^II-superoxide (O₂^•−) complex [(tmpa)Cu^II(O₂^•−)Cu^I(tmpa)]²⁺ produced by intramolecular electron transfer from the peroxo moiety to one of the two Cu^II centers in the Franck-Condon excited state 1* (Scheme 2).

Figure 1.

(a) Absorption spectrum of 1 (red line) obtained from oxygenation of [(tmpa)Cu^I(CH₃CN)]⁺ (black line) at −80 °C in acetone. (b) Transient difference absorption spectra of 1 in acetone at −80 °C measured at the indicated delay times after 490 nm femto-second laser excitation (2 mJ/pulse and fwhm = 130 fs). Overlaid in green is a simulated spectrum for 1, generated by subtraction of the red spectrum from the black spectrum shown in (a).

Scheme 2.

Very similar absorption features were observed after pulsed laser excitation of 2 and 3 under the same conditions. The absorption maxima for the positive absorption feature assigned to the mixed-valent intermediates were determined from Gaussian fits of the observed data (Figure S1). A linear relationship between the absorption change at 750 nm,²³ due to the Cu^I-Cu^II-superoxide (O ^•−) complex [(tmpa)Cu^II(O₂^•−)Cu^I(tmpa)]²⁺, and the laser fluence was observed over a range of 11–46 μJ/pulse consistent with a monophotonic process in the formation of the Cu^II-O₂^•−-Cu^I complex (Figure S2 in SI).

To confirm that the chemistry described here (vide supra) was truly due to the light-induced reaction of the peroxo complex[(tmpa)Cu^II-(O₂)-Cu^II−(tmpa)]²⁺ (1), control experiments were carried out. No photoactivity was observed after irradiation of either the mononuclear tmpa-copper(I) precursor complex alone, or the decomposition products of [(tmpa)Cu^II-(O₂)-Cu^II−(tmpa)]²⁺ (1) (formed by warming a cryogenically produced solution of 1 up to room temperature) and subsequent re-chilling.

Further Reactions of [(tmpa)Cu^II(O₂^•−)Cu^I(tmpa)]²⁺ (λ_max, 739 nm)

Single wavelength kinetics monitored at 600 and 750 nm are shown in Figure 2. The fastest decay, observed within the first 5 ps after laser excitation of 1, was the partial recovery of the bleach at 600 nm that occurs with a first-order rate constant k₁ = (2.8 ± 0.4) × 10¹² s⁻¹ (Figure 2a). This corresponds to back electron transfer from the Cu^I center to O₂^•− to regenerate 1 from the mixed-valent Cu^II-O₂^•−-Cu^I intermediate (k₁ in Scheme 2): Cu^II−O₂^•−-Cu^I → Cu^II-O₂²⁻-Cu^II (1).

Figure 2.

Time courses of femtosecond laser-induced reactions of 1 in acetone at −80 °C. Overlaid in red are fits to first-order kinetic models. (a) Fast partial recovery of the bleaching at 600 nm. (b) Decay of the absorbance difference observed at 750 nm due to the complex ([(tmpa)Cu^II(O₂^•−)Cu^I(tmpa)]²⁺). (c) The slower recovery of the bleaching observed at 600 nm to completely regenerate 1.

On the other hand, the decay of the absorption at 750 nm with rate constant k₂ = (4.0 ± 0.5) × 10¹¹ s⁻¹ at −80 °C, due to the disappearance of the Cu^II-O₂^•− moiety in the mixed-valent intermediate (Figure 2b), is significantly slower than the fast partial recovery of the bleaching at 600 nm. This indicates that a second process involving electron transfer occurs, that from the O₂^•− moiety to the Cu^II center, to produce what formally is to be considered as the Cu^I₂-O₂ complex ([(tmpa)Cu^I(O₂)-Cu^I(tmpa)]²⁺, a ‘caged’ O₂-species (Cu^II-O₂^•−-Cu^I → Cu^I-O₂-Cu^I; k₂ in Scheme 2). The second step of the recovery of the bleaching at 600 nm with rate constant k₃ = (1.8 ± 0.1) × 10¹⁰ s⁻¹ (Figure 2c) corresponds, instead, to the stepwise back electron transfer from two Cu^I centers to the O₂ moiety in the Cu^I₂-O₂ complex to regenerate 1. In this case, the rate-determining step is a, first, uphill back electron transfer from one Cu^I center to the O₂ moiety in the Cu^I₂-O₂ complex (Cu^I-O₂-Cu^I → Cu^II-O₂^•−-Cu^I) followed by the much faster (k₁ >> k₃; vide supra) second step back electron transfer from the other Cu^I center to the superoxide moiety in the Cu^II-O₂^•−-Cu^I complex to give back 1 (Cu^II-O₂^•−-Cu^I → Cu^II- O₂²⁻-Cu^II).

The temperature dependence of the rate constants k₁, k₂, and k₃ was examined in the temperature range from −94 to −55 °C and the rate constants determined are listed in Table S1, S2, and S3, respectively, of the Supporting Information (SI). An Eyring plot of k1 for the electron transfer from the Cu^I center to the O₂^•− moiety in the mixed-valent Cu^II-O₂^•−-Cu^I complex ([(tmpa)Cu^II (O₂^•−)Cu^I(tmpa)]²⁺) to regenerate 1 is shown in Figure 3, where k₁ is seen to be nearly temperature independent.

Figure 3.

Eyring plots of k₁ (electron transfer from the Cu^I center to the O₂^•− moiety in the Cu^II-O₂^•−-Cu^I complex to regenerate 1), k₂ (electron transfer from the O₂^•− moiety to the Cu^II center in [(tmpa)Cu^II(O₂^•−)Cu^I(tmpa)]²⁺ to produce the ‘in cage’ Cu^I₂-O₂ complex) and k₃ (stepwise back electron transfer reaction from the Cu^I centers to the O₂ moiety in the Cu^I₂-O₂ intermediate to regenerate the peroxo complex [(tmpa)Cu^II-(O₂)-Cu^II−(tmpa)]²⁺ (1)).

The ΔH^‡ and ΔS^‡ values obtained are listed in Table 1. For k₁, both values were virtually zero. Such a small activation entropy is typical for an electron-transfer process involving no bond breaking. Thus, electron transfer from the Cu^I center to the O₂^•− moiety in the Cu^II-O₂^•−-Cu^I complex is a barrierless adiabatic process. Because the activation entropy for k₁ is nearly zero (Table 1), it is likely that a cupric-superoxide Cu^II-O-O^•− end-on structure is maintained in the Cu^II-O₂^•−-Cu^I complex, similar to that found in the μ-1,2-peroxo complex 1 (see Chart 1 and Scheme 3).

Table 1.

Kinetic Parameters for Femtosecond Laser-Induced Reactions of 1 at −80 °C

	rate constant, s−1	ΔH‡,a	ΔS‡,b	ΔG‡,a
k1	(2.8 ± 0.4) × 1012	−0.8 ± 0.8	−4 ± 4	0.0 ± 0.8
k2	(4.0 ± 0.1) × 1011	−0.5 ± 0.4	−7 ± 5	0.9 ± 0.4
k3	(1.8 ± 0.1) × 1010	1.9 ± 0.4	−4 ± 5	2.7 ± 0.4

^aΔH^‡ and ΔG^‡(−80 °C), kcal mol⁻¹

^bΔS^‡, cal K⁻¹ mol⁻¹.

Scheme 3.

The Eyring plot related to k₂ for the electron transfer from the O₂^•− moiety to the Cu^II center to produce the Cu^I₂-O₂ complex is also shown in Figure 3, where k₂ was nearly temperature independent as was the case of k₁. Because no O₂ release was observed from this Cu^I₂-O₂ complex, we postulate that O₂ remains in a complex cage; further, the observed negative activation entropy suggests that the geometry of O₂ in the Cu^I₂-O₂ complex is more restricted in the transition state; this could indicate, we speculate, that the binding mode of dioxygen changed to a side-on configuration also in the ‘in-cage’ Cu^I(O₂)Cu^I complex, in contrast with that of the end-on geometry of 1 (Scheme 3).

The Eyring plot of the rate constant k₃ for the back electron transfer from the Cu^I center to the O₂ in the Cu^I₂-O₂ complex (Figure 3) afforded a positive activation enthalpy (ΔH^‡ = 1.9 ± 0.4 kcal mol⁻¹) and a nearly zero activation entropy (ΔS^‡ = −4 ± 5 cal K⁻¹ mol⁻¹). The positive activation enthalpy is consistent with the barrierless electron transfer found for the reverse process (k₂ in Scheme 2); because of that, the back electron transfer from one of the two Cu^I centers to the O₂ moiety (k₃) must be uphill, as shown in Scheme 2. The subsequent electron transfer from the other Cu^I center to the O₂^•− moiety to regenerate 1 (k₁) is barrierless (vide supra) and much faster than the rate-determining back electron transfer from the Cu^I center to the O₂ moiety (k₁ >> k₃). The ΔH^‡ value found here for k₃ (1.9 ± 0.4 kcal mol⁻¹) is similar to that previously determined for the intermolecular electron transfer from [(tmpa)Cu^I]⁺ to O₂ to produce the superoxide complex [(tmpa)Cu^IIO₂]⁺ (ΔH^‡ = 1.8 kcal mol⁻¹).^14b In the case of this intermolecular electron transfer reaction, however, the ΔS^‡ value (−10 cal K⁻¹ mol⁻¹)^14b was much more negative than the intramolecular electron-transfer reaction shown in Scheme 2 (virtually zero; vide supra). This is consistent with the fact that an intermolecular electron transfer should involve a negative activation entropy because it requires diffusion and the formation of a precursor complex prior to electron transfer, a requirement, instead, not necessary for the intramolecular electron transfer process observed here.

The lack of O₂ release from the Cu^I₂-O₂ complex may be attributed to the high activation free energy barrier that needs to be overcome (ΔG^‡ ~ 20 kcal mol⁻¹ at −80 °C) to produce 2 equiv of [(tmpa)Cu^I]⁺ and O₂, which are ~ 5 kcal mol⁻¹ uphill as compared to the Cu^I₂-O₂ adduct, according to data previously reported for this reaction in THF solvent.^7,14a,b

Photodynamics of Side-On Peroxo Dinuclear Copper(II) Complexes

The results on the end-on peroxo complex (1) described above are in sharp contrast to those observed for the side-on peroxo compound 2 ([(N5)Cu^II(O₂²⁻)Cu^II]²⁺),^8c,d,e where O₂ was instead released after laser excitation of the complex. The adduct 2 was produced by the reaction of [(N5)Cu^I₂(CH₃CN)]²⁺ with O₂ in acetone at −80 °C and its electronic spectrum exhibits absorption bands at 360 nm (21800 M⁻¹ cm⁻¹) and at 430 nm (5200 M⁻¹ cm⁻¹), which have been previously assigned as peroxide-to-copper(II) ligand-to-metal charge-transfer (LMCT) transitions.^8c,d,e

Femtosecond laser excitation of 2 in acetone at −94 °C resulted in observation of a transient absorption feature at λ_max = 709 nm (Figure 4) which was consistent with the commonly observed low-energy but strong absorption peaks known for Cu^II-superoxide complexes.^{14b,18,19,21,22} Thus, the transient absorption spectrum observed upon femtosecond laser excitation of 2 in Figure 4 can be also assigned to a mixed-valent Cu^II-O₂^•−-Cu^I complex ([(N5)Cu^II(O₂^•−)Cu^I]²⁺) produced by intramolecular electron transfer from the peroxo ligand to the Cu^II center via the Franck-Condon LMCT excited state 2* shown in Scheme 4.

Figure 4.

Transient absorption difference spectra of 2 in acetone at −94 °C measured at the indicated delay times after 436 nm femto-second laser excitation (2 mJ/pulse and fwhm = 130 fs). Overlaid in green is a simulated spectrum. The probe light is interrupted by the pump laser in the wavelength region around 540 nm.

Scheme 4.

The decay of the absorption due to the Cu^II-O₂^•−-Cu^I complex derived from 2 in acetone at −80 °C, monitored at 720 nm, obeyed first-order kinetics with the rate constant (k₄ in Scheme 4) (3.9 ± 0.3) ×10⁹ s⁻¹ which is much smaller than that derived for 1 [(4.0 ± 0.1) × 10¹¹ s⁻¹]. Representative kinetic data collected at −80 °C are shown in Figure 5.

Figure 5.

Decay time profile of the absorbance difference observed at 720 nm due to the Cu^II-O₂^•−-Cu^I complex derived from photoexcitation of 2 in acetone at −94 °C; Inset: Eyring plot of k₄ for the intramo-lecular electron transfer from the O₂^•− ligand to the Cu^II center in the Cu^II-O₂^•−-Cu^I photoproduct complex derived from 2 in acetone.

The temperature dependence for the rate constant k₄ was also examined in the temperature range −94/−55 °C and the related Eyring plot is shown as inset in Figure 5, giving ΔH^‡ = 5.4 ± 1.0 kcal mol⁻¹ and ΔS^‡ = −9 ± 5 cal K⁻¹ mol⁻¹. The Cu^II-O₂^•−-Cu^I adduct formed from 2 decays much slower than that derived from 1 (smaller decay constant: k₄ < k₂) and shows a significantly larger ΔH^‡ value. These results are consistent with a larger bond and solvent reorganization energy for the electron transfer from the superoxide anion to Cu^II in the Cu^II-O₂^•−-Cu^I photoproduct obtained from laser excitation of [(N5)Cu^II(O₂²⁻)Cu^II]²⁺ (2) with its side-on μ-η²-η²–peroxo binding compared to that of the decay of the Cu^II-O₂^•−-Cu^I photoproduct obtained from laser excitation of [(tmpa)Cu^II-(O₂²⁻)-Cu^II− (tmpa)]²⁺ (1) (Scheme 1) with its end-on μ-1,2-peroxo bridging mode.

Femtosecond laser excitation of an acetone solution of the analogue complex [(N3)Cu^II(O₂²⁻)Cu^II]²⁺ (3), supported by a binucleating chelating ligand having a shorter poly-methylene chain as compared to 2 (N3 vs N5), also results in the formation of a mixed-valent Cu^II-O₂^•−-Cu^I complex: [(N3)Cu^II(O₂^•−)Cu^I]²⁺. The latter exhibits a transient absorption signature at λ_max = 685 nm (Figure 6).

Figure 6.

Transient absorption difference spectra of 3 in acetone at −94 °C measured at the indicated delay times after 436 nm femto-second laser excitation (2 mJ/pulse and fwhm = 130 fs). Overlaid in green is a simulated spectrum for the conversion 3 → Cu^I + Cu^I + O₂ based on subtraction of the red spectrum from the black shown in Figure 8a. The probe light is interrupted by the pump laser in the wavelength region around 580 and 630 nm.

The observed blue-shift of the absorption band of [(N3)-Cu^II(O₂^•−)Cu^I]²⁺ as compared to that of [(N5)Cu^II(O₂^•−)Cu^I]²⁺ (685 nm, Figure 6, vs 709 nm, Figure 4) may be caused by a stronger electrostatic interaction between the Cu^I ion and the O₂^•− ligand in [(N3)Cu^II(O₂^•−)Cu^I]²⁺. This situation could arise because of the Cu^I ion’s closer spatial proximity as compared to what would be expected in [(N5)Cu^II(O₂^•−)…Cu^I]²⁺, due to the difference in poly-methylene chain length in the N3 vs N5 binucleating ligand (see Chart 1). Such differences do manifest themselves in other situations, such as for oxygenation reaction rates of dicopper(I) complexes with N3 vs N5 ligands.^8c,d

The decay of the absorbance observed at 685 nm obeyed first-order kinetics with the rate constant (k₄) of (7.3 ± 0.3) × 10⁹ s⁻¹ (Figure S3 of the SI), which was larger than that derived from 2 [(3.9 ± 0.1) × 10⁹ s⁻¹].

The energy diagram for the femtosecond laser-induced reactions of 2 is shown in Scheme 4. Femtosecond laser excitation into the lower energy LMCT band of 2 results in formation of the mixed-valent Cu^II-O₂^•−-Cu^I complex ([(N5)Cu^II(O₂^•−)Cu^I]²⁺) which decays via intramolecular electron transfer from the O₂^•− ligand to the Cu^II center to produce [(N5)Cu^I₂]²⁺ with release of molecular oxygen. The finding of O₂ release from the Cu^I₂-O₂ complex (Scheme 4) is in sharp contrast to the case of 1 (Scheme 2) where back electron transfer from the Cu^I center to the O₂ moiety to regenerate 1 occurs without release of O₂. The same energy diagram drawn for 2 can be applied to 3.

Photodynamics of Side-On Peroxo Dinuclear Copper(II) Complexes in the ms time domain (O₂ re-binding kinetics)

The generated Cu^I complexes, [(N5)Cu^I₂]²⁺ and [(N3)Cu^I₂]²⁺, produced by laser excitation of [(N5)Cu^II(O₂²⁻)Cu^II]²⁺ (2) and [(N3)Cu^II(O₂²⁻)Cu^II]²⁺ (3) reacted cleanly with O₂ to regenerate 2 and 3, respectively. The intermolecular reactions of the Cu^I complexes with O₂ were monitored by nanosecond laser flash photolysis at low temperature. This approach is superior to the previously reported “flash-and-trap” experiments that required the inclusion of CO gas and/or other various experimental procedures.^14a,b Here, we have added in dioxygen at various concentrations to examine [O₂]-dependencies, and to also study variable temperature kinetics, allowing determination of the second-order rate constants and the activation parameters for the reactions of the Cu^I complexes with O₂ (vide infra).

Nanosecond laser excitation of an acetone solution of 2 at −80 °C resulted in formation of [(N5)Cu^I₂]²⁺ and O₂ as shown in Figure 7b. The observed bleaching is fully consistent with that expected for the loss of O₂ from the Cu^I₂-O₂ complex, because subtraction of the absorption spectrum of 2 from that of [(N5)Cu^I₂]²⁺ afforded a simulated spectrum (red line in Figure 7b) which agrees well with the observed transient difference spectrum. The quantum yield for photoinduced release of O₂ determined at 30 ns after the laser pulse was estimated to be 0.14 ± 0.01 (see Experimental Section). The absorption change was found to be linear with the laser fluence over a 5–100 mJ cm⁻² range indicating that photoinduced release of O₂ is a monophotonic process (inset in Figure 7b). Thus, the laser-induced photoejection of dioxygen from complexes 2 and 3 (see below) are formally very rarely observed (and see further discussion below) one-photon two-electron processes,

$$\begin{array}{l}{\left[(\text{Nn}){\text{Cu}}^{\text{II}}({{\mathrm{O}}_2}^{2-}){\text{Cu}}^{\text{II}}\right]}^{2+}(\mathbf{2}\text{or}\mathbf{3})+1\text{photon}\\ \to {\left[(\text{Nn}){{\text{Cu}}^{\mathrm{I}}}_2\right]}^{2+}+{\mathrm{O}}_{2(\mathrm{g})}\end{array}$$

Figure 7.

(a) Absorption spectrum of 2 (red line) obtained from oxygenation of [(N5)Cu^I₂]²⁺ (black line) in acetone at −80 °C. (b) Transient absorption difference spectra collected at the indicated delay times after 532 nm laser excitation (10 mJ/pulse, 8–10 ns fwhm) of 2 in acetone at −80 °C. Overlaid in red on the experimental data is a simulated spectrum (Abs([(N5)Cu^I₂(CH₃CN)]²⁺)- Abs(2)), corresponding to the negative of the spectrum of [(N5)Cu^II(O₂²⁻)Cu^II]²⁺ (2). Inset: Difference in absorbance observed at 360 nm as a function of the applied laser energy for 2 in acetone at −80 °C.

Similar transient absorption spectra were observed for nanosecond laser flash photolysis measurements of an acetone solution of [(N3)Cu^I₂]²⁺ at −80 °C (Figure 8b). The rate of recovery of the bleaching observed at 365 nm obeyed first-order kinetics in the presence of excess O₂ and the observed pseudo-first-order rate constant increased linearly with O₂ concentration (Figure S4).

Figure 8.

(a) Absorption spectrum of 3 (red line) obtained from oxygenation of [(N3)Cu^I₂]²⁺ (black line) in acetone at −80 °C. (b) Transient absorption difference spectra collected at the indicated delay times after 532 nm laser excitation (10 mJ/pulse, 8–10 ns fwhm) of 3 in acetone at −80 °C. Overlaid in blue is a simulated spectrum based on subtraction of the red spectrum from the black in (a). The inset in (b) shows the magnitude of the absorption change as a function of the incident irradiance.

The second-order rate constants (k_O2) for the reactions of [(N5)Cu^I₂]²⁺ and [(N3)Cu^I₂]²⁺ with O₂ were determined at various temperatures as listed in Table 2 and S4 in Supporting Information. The k_O2 values determined for [(N5)Cu^I₂]²⁺ [(7.0 ± 1.9) × 10³ M⁻¹ s⁻¹] and [(N3)Cu^I₂]²⁺ [(3.8 ± 1.2) × 10³ M⁻¹ s⁻¹] in acetone at −90 °C are only slightly larger than the corresponding reported values [(4.1 ± 0.3) × 10³ M⁻¹ s⁻¹ and (1.1 ± 0.1) × 10³ M⁻¹ s⁻¹] in CH₂Cl₂ at −90 °C, as measured by low-temperature stopped-flow spectroscopy,^8c possibly due to the difference in solvation.

Table 2.

Rate Constants and Activation Parameters for the Reactions of [(N5)Cu^I₂]²⁺, [(N3)Cu^I₂]²⁺, Deoxy-Tyr, and Deoxy-Hc with O₂

Cu complex, T	kO2, M−1 s−1	ΔH‡,a	ΔS‡,b
[(N5)CuI2]2+(−90°C)	(7.0 ± 1.9) × 103	5 ± 1	−9 ± 5
[(N3)CuI2]2+(−90°C)	(3.8 ± 1.2) × 103	8 ± 1	3 ± 6
[(N5)CuI2]2+(21°C)	(3.2 ± 1.0) × 106
[(N3)CuI2]2+(21°C)	(2.8 ± 1.2) × 107
Tyrc	1.9 × 107	13 ± 3
Hcd	3.1–15.4 × 107
Hce	2.3 × 107

^aΔH^‡, kcal mol⁻¹

^bΔS^‡, cal K⁻¹ mol⁻¹

^cRef 20a

^dRef 24a

^eRef 24b.

Eyring plots of k_O2 for [(N3)Cu^I₂]²⁺ and [(N5)Cu^I₂]²⁺ are shown in Figures 9a and 9b, respectively, and activation parameters are listed in Table 2. The activation enthalpy found was comparable in the two cases and a more negative activation entropy for the reaction between [(N5)Cu^I₂]²⁺ and O₂ suggests, as previously observed,^8c a greater inner-sphere reorganization energy in this case, probably due to the longer poly-methylene chain present in [(N5)Cu^I₂]²⁺.

Figure 9.

Eyring plots of k_O2 for O₂ binding to the dicopper(I) complex [(N3)Cu^I₂]²⁺ which leads to 3 (a) or [(N5)Cu^I₂]²⁺ which gives 2 (b) in acetone.

The kinetics of O₂ coordination determined here also reveals noticeable similarities to that observed for both the copper-containing proteins hemocyanin (Hc; O₂-carrier in mollusks and arthropods) and tyrosinase (Tyr; ubiquitous o-phenol monooxygenase).^1,20,24,25 These possess similar dicopper active sites where each separated copper(I) ion binds three protein-derived histidine imidazole N-donor ligands and the oxygenated form consists of a Cu^II-(μ-η²:η²-O₂²⁻)-Cu^II moiety, the same peroxo binding mode found in [(N5)Cu^II₂(O₂)] ²⁺ (2) and [(N3)Cu^II₂(O₂)]²⁺ (3) (Chart 1). Interestingly, the rate constants, when extrapolated at 21° C, as well as the activation parameters determined here for the formation of 2 and 3 are essentially the same with those previously reported for Tyr^20a and Hc,^20b,24 (Table 2).

Summary and Conclusion

The present study on the photodynamics of the excited states of Cu^II-O₂ intermediates opens a new and exciting research area of reactive excited states of metal-O₂ intermediates. Here, in summary, we reported on the first unambiguous example of an, overall, one-photon two-electron oxidation, that being the oxidation of peroxide to molecular oxygen occurring when the trans-μ-1,2–(end-on) peroxide dicopper(II) compound [(tmpa)Cu^II−(O₂)−Cu^II−(tmpa)]²⁺ (1), or the μ-η²-η²-(side-on)-peroxide complexes [(N5)Cu^II(O₂²⁻)Cu^II]²⁺(2) and [(N3)Cu^II(O₂²⁻)Cu^II]²⁺(3) are illuminated with visible light. A stepwise electron transfer mechanism was elucidated employing a novel ultra-fast low-temperature transient absorption apparatus and new intermediates were observed and assigned as a mixed-valent Cu^II(O₂^•−)Cu^I species. While complex 1 converted to the ‘O₂-caged’ dicopper(I) complex [(tmpa)Cu^I(O₂)Cu^I−(tmpa)]²⁺ without O₂-release, femtosecond illumination of complexes 2 and 3 under the same conditions showed a remarkably different behavior such that the observed mixed-valent Cu^II-superoxide-Cu^I intermediates derived from these complexes further decay to two Cu^I ions, with complete release of O₂ via electron transfer from the superoxide ligand to the Cu^II moiety. Thus, the results imply that in the O₂-binding process, certainly for oxygenation of [(N5)Cu^I₂]²⁺ and [(N3)Cu^I₂]²⁺, a transient mixed valent species [(Nn)Cu^II(O₂^•−)Cu^I]²⁺ forms first (see Eq. below); it is however unobservable because the initial reaction of O₂ with one cuprous ion resides in a strongly left-lying equilibrium and the reaction with the second cuprous ion is fast (see also below).^8c The only manner to observe

$$\begin{array}{l}{\left[(\text{Nn}){{\text{Cu}}^{\mathrm{I}}}_2\right]}^{2+}+{{\mathrm{O}}_{2(\mathrm{g})<-----}}^{->}{\left[(\text{Nn}){\text{Cu}}^{\text{II}}({{\mathrm{O}}_2}^{•-}){\text{…Cu}}^{\mathrm{I}}\right]}^{2+}\\ -->{\left[(\text{Nn}){\text{Cu}}^{\text{II}}({{\mathrm{O}}_2}^{2-}){\text{Cu}}^{\text{II}}\right]}^{2+}\end{array}$$

the Cu^II(O₂^•−)Cu^I species is as described here, the use of laser-photolysis starting with the peroxo-dicopper(II) complexes.

The different reaction pathways observed for the end-on (1 in Scheme 2) and the side-on (2 in Scheme 4 and complex 3) peroxo complexes may be thought of and explained in terms of the known much lower ligand-Cu^II/I reduction potentials (as determined by prior cyclic voltammetric measurements) on isolated ligand-copper(I) complexes (ligands: tmpa, N3, N5 or PY2). For tmpa-Cu^I, E° = 0.05 V vs SCE in acetone,^14a,26,28 as compared with that for Cu^I with PY2 moieties as found in 2 or 3 (E° = +0.37 V vs SCE in acetone).^8d,27,28 Thus, back-electron transfer from the Cu^I ion to O₂ is favored in [(tmpa)Cu^IO₂Cu^I(tmpa)]²⁺ (Scheme 2) {and in mixed-valent [(tmpa)Cu^II-(O₂^•−)Cu^I(tmpa)]²⁺} in comparison to the possible O₂ release and production of two equiv [(tmpa)Cu^I]⁺. By contrast, O₂ release from [(N5)Cu^I₂O₂]²⁺ and then the N5 mixed-valent [(N5)Cu^II(O₂^•−)Cu^I]²⁺ species is favored in comparison to the much slower back electron transfer from Cu^I to O₂ in these intermediates (Scheme 4).

The disparate Cu^II/I reduction potentials in the two classes of compounds results from the presence of an additional N-donor atom bound to each Cu^I in the compounds derived from 1 compared to those derived from 2 and 3 (tetradentate tmpa vs tridentate N3 and N5), on one hand, and the different chelate ring size in the two cases (5-membered ring for 1 vs 6-membered ring for 2 and 3),^8b on the other hand, which confer different coordination environments, peroxo binding modes, and electronic properties to the resulting O₂-adducts. These variations translate into dramatic effects in electron transfer thermodynamics and kinetics, including the ability to photorelease molecular oxygen for the complexes 2 and 3 while not for 1.

It is interesting to note that in an earlier computational study from Metz and Solomon,²⁹ on O₂ binding to deoxy-Hc, the findings were that the process proceeds via a simultaneous double electron transfer between the two cuprous ions and O₂, such that as dioxygen interacts with a first copper ion, the two copper ions approach each other and then the redox reaction occurs.: Cu^I…Cu^I (~ 4.3 Å separation) + O₂ → Cu^II(O₂)Cu^II (~3.6 Å Cu-Cu distance). The intermediacy of a discrete Cu^II-O₂^•−-Cu^I initial adduct was ruled out. Thus, our synthetic analog compounds (i.e., models) behave differently than the dicopper center found in that very special protein environment. The results observed here may be ascribed to the larger Cu^I…Cu^I average distances likely found in the “floppy” [(N5)Cu^I₂]²⁺ and [(N3)Cu^I₂]²⁺ complexes. In the Cu^I₂/O₂ reactions, the cuprous ion in a Cu^II(O₂^•−)Cu^I species is likely to initially not be close to the superoxide moiety. Supporting this view are findings in the solid state, where Cu…Cu distances in non-bridged dicopper complexes with N3, N4 or N5 ligands can vary from ~ 6 to 10 Å.^8d,e And as explained above, the initial unfavorable interaction of O₂ with a single cuprous ion in a PY2 environment precludes the possibility of detecting a mixed-valent Cu^II(O₂^•−)Cu^I intermediate species in the “forward” [(Nn)Cu^I₂]²⁺+ O₂ direction.

Single photon absorption reactions that drive two-electron transfer reactions are relatively rare yet important for photocatalytic multi-electron transfer reactions. A highly relevant example is water oxidation to molecular oxygen that in addition to proton management requires four redox equivalents: two to form the O-O bond in peroxide; and an additional two to yield dioxygen gas.³⁰ The last two are particularly important as the release of peroxide or superoxide can give rise to unwanted radical chemistry. The peroxide-to-copper charge transfer absorption bands reported herein harvest sunlight across the entire visible region and their photoreactivity indicates that with the proper peroxide coordination environment, light excitation does indeed drive the two-electron transfer release of dioxygen. It is interesting to speculate whether such photo-reactivity may also be included in the realm of newly discovered copper water oxidation catalysts.³¹

Experimental Section

Materials

All materials purchased were of the highest purity available from Sigma-Aldrich Chemical or Tokyo Chemical Industry (TCI) and were used as received, unless specified otherwise. Tetrahydrofuran for the synthesis was distilled under an inert atmosphere from Na/benzophenone and degassed with argon prior to use. Pentane and acetone were freshly distilled from calcium hydride and calcium sulfate, respectively, under an inert atmosphere and degassed prior to use. The tmpa ligand,⁷ the N3 and N5 ligands,^8c and the [Cu¹(CH₃CN)₄](BArF) complex³² (BArF = B(C₆F₅)₄⁻) were synthesized according to literature procedures. [(N3)Cu^I₂(CH₃CN)](BArF)₂ and [(N5)Cu^I₂-(CH₃CN)](BArF)₂ were synthesized adding [Cu^I(CH₃CN)₄]₋(BArF) (410 mg, 0.452 mmol) to either N3 (114 mg, 0.230 mmol) or N5 (120 mg, 0.230 mmol) in dry, air-free tetrahydrofuran (THF) (15 mL) and the resulting solution was allowed to stir for 30 minutes. The isolation of the compounds was afforded by precipitation, under argon atmosphere, in dry, deoxygenated pentane (60 mL). The yellow powders obtained were made re-precipitate from THF/pentane (10 mL/60 mL) for three times, in both cases. Identity and purity of ligands and complexes were verified by elemental analysis and/or ¹H-NMR spectroscopy (see Figures S5–S8 in Supporting Information). Synthesis and manipulations of copper salts were performed according to standard Schlenk techniques or in a MBraun glovebox (with O₂ and H₂O levels below 1 ppm). UV-Vis spectra were recorded with a Cary 50 Bio spectrophotometer equipped with a liquid nitrogen chilled Unisoku USP-203-A cryostat. NMR spectroscopy was performed on Bruker 300 and 400 MHz instruments with spectra calibrated to either internal tetramethylsilane (TMS) standard or to residual protio solvent.

Determination of O₂ Solubility in Acetone

Both the solubility of O₂ in acetone at 25 °C (0.01134 mol/L) and the solubility at different temperatures were determined using data from temperature-dependent studies previously carried out.³³ The formula used for the temperature dependence of the molar fraction solubility of O₂ in acetone is given by eq 1,

$$\text{ln}\mathrm{\chi }=-24.3100+(649.40/T)+[2.6414(\text{ln}T)]$$ (1)

where χ is the molar fraction solubility in of O₂ in acetone and T is the absolute temperature.

Gas Mixing

Dioxygen (O₂; Air Gas East, grade 4.4) was dried by passing the gas through a short column of supported P4010 (Aquasorb, Mallinkrodt). Red rubber tubing (Fisher Scientific; inner diameter: 1/4 in.; thickness: 3/16 in.) was used to attach the gas cylinders fitted with appropriate regulators to two MKS Instruments Mass-Flo Controllers (MKS Type 1179A) regulated by an MKS Instruments Multi Channel Flow Ratio/Pressure Controller (MKS Type 647C). The gas mixtures, N₂/O₂, were determined by the set flow rates of the two gases. For example, a 10% O₂ mixture would be made by mixing O₂ at a rate of 10 standard cubic centimeters per minute (sccm) with N₂ at 90 sccm for a total flow of 100 sccm. By varying the ratio of O₂ and N₂ with the gas mixer, the concentration of the gases were determined by taking the percentage of the gas added and multiplying by the solubility of the corresponding gas in acetone.

Transient Absorption Measurements

The source for the pump and probe pulses for the femtosecond laser spectroscopy performed here were derived from the fundamental output of Integra-C (780 nm, 2 mJ/pulse and fwhm = 130 fs) at a repetition rate of 1 kHz. A total of 75% of the fundamental output of the laser was introduced into TOPAS, which has optical frequency mixers resulting in tunable range from 285 to 1660 nm, while the rest of the output was used for white light generation. Prior to generating the probe continuum, a variable neutral density filter was inserted in the path to generate stable continuum, and then the laser pulse was fed to a delay line that provides an experimental time window of 3.2 ns with a maximum step resolution of 7 fs. In our experiments, a wavelength at 490 nm of TOPAS output, which is the fourth harmonic of signal or idler pulses, was chosen as the pump beam. As this TOPAS output consists of not only desirable wavelength but also of unnecessary wavelengths, the latter was deviated using a wedge prism with wedge angle of 18°. The desirable beam was irradiated at the sample dell with a spot size of 1 mm diameter where it was merged with the white probe pulse in a close angle (< 10°). The probe beam after passing through the 2 mm sample cell put in a UNISOKU CoolSpeK (USP-203-BTT-OF) cryostat was focused on a fiber optic cable that was connected to a CCD spectrograph for recording the time-resolved spectra (450–800 nm). Typically, 2500 excitation pulses were averaged for 3 s to obtain the transient spectrum at a set delay time. Kinetic traces at appropriate wavelengths were assembled from the time-resolved spectral data.

Experimental information for the setup of the Nd:YAG flash-photolysis apparatus has been previously reported.³⁴ The apparatus used for nanosecond laser spectroscopy was equipped with liquid nitrogen chilled Unisoku USP-203-A cryostat. The samples, [(N5)Cu^II₂(O₂)](BArF)₂ and [(N3)Cu^II₂(O₂)](BArF)₂, were irradiated with λ_ex = 532 nm pulsed light (10 mJ/pulse) and data were collected at the monitored wavelengths from averages of 30 laser pulses. Samples (about 70 μM) were prepared under an inert atmosphere (drybox) in 1 cm quartz cuvettes with four polished windows made custom by Quark glass. The cuvettes were equipped with a 14/20 joint and Schlenk stopcock. Gas mixtures were added to sample solutions by direct bubbling through a 24-inch needle (19-gauge) for 5 s for 10 times with intervals of 10 s between each time. During data collection the gas flowed through the headspace of the sample solution into the cuvette.

Quantum Yield Determination

Quantum yields were determined in acetone solvent at −80 °C for the 532 nm excitation wavelength. Samples were prepared bubbling O_2(g) in solutions of [(N3)Cu^I₂(CH₃CN)](BArF)₂ in dried and distilled acetone at −80 °C. The absorbance of the samples at 532 nm (i.e., 0.14 at 532 nm) was monitored using a Cary 50 Bio spectrophotometer equipped with a liquid nitrogen chilled Unisoku USP-203-A cryostat. [Ru(bpy)₃]Cl₂ (bpy = 2,2′-bipyridine) in CH₃CN at room temperature was used as an actinometer and the solutions were prepared to ensure to match the optical density at 532 nm. Data collection for the change in absorbance (ΔA) at the correspondent λ_max values (365 nm) where the change in extinction coefficients (ΔΣ) are known was made. The quantum yields at the two excitation wavelengths were calculated using eq 2,

$$\mathrm{\Phi }=\left(\mathrm{\Delta }{\mathrm{A}}_{450\text{Cu}}/\mathrm{\Delta }{\mathrm{A}}_{450\text{Actin}}\right)\left(\mathrm{\Delta }{\mathit{\sum }}_{450\text{Actin}}/\mathrm{\Delta }{\mathit{\sum }}_{450\text{Cu}}\right)\left({n_{\mathrm{acetone}}}^2/{n_{\text{CH3CN}}}^2\right)$$ (2)

where “Actin” is [Ru(bpy)₃]Cl₂ in CH₃CN at room temperature. The values used were ΔΣ_450Actin = −10600 M⁻¹ cm⁻¹,³⁵ and ΔΣ_450Cu = −7264 M⁻¹ cm⁻¹, with the latter determined in this work. For the refractive index, the value 1.34163 for CH₃CN (n_CH3CN) at 298.15 K has been used,³⁶ whereas a temperature correction of 0.00045 per Kelvin has been added to the refractive index of acetone at 293.15 K (1.359)³⁷ to obtain the refractive index which has been used for acetone at 193.15 K in eq 2; (n_acetone): 1.359 + [0.00045(293.15–193.15)] = 1.404.

Eyring Plots

Samples were prepared for the laser experiments bubbling O₂ gas into acetone solutions of the correspondent Cu^I complex. For example, O₂ was added to an acetone solution of [(tmpa)Cu^I(CH₃CN)](BArF) (976 μM) at low temperature to generate 1. Laser measurements were performed in a temperature range from −94 °C to −55 °C and data were fitted with the Eyring equation (eq 3),

$$\text{ln}(\mathit{\text{kh}}/k_{\mathrm{B}}T)=-\mathrm{\Delta }{H}^{‡}/(\mathit{RT})+\mathrm{\Delta }{\mathrm{S}}^{‡}/R$$ (3)

where h is Planck constant, k_B is Boltzmann constant, and k is the rate constant. Temperature dependence studies have been performed in this work using 490 nm excitation wavelength.