Received 5th September 2018,Accepted 8th October 2018
DOI: 10.1039/c8dt03604g
rsc.li/dalton
Photoexcited state chemistry of metal–oxygencomplexes
Claudio Saracini, a Shunichi Fukuzumi, *a,b Yong-Min Lee *a andWonwoo Nam *a
Recent advances on the excited state chemistry of metal–oxygen synthetic complexes based on earth-
abundant metals such as copper, cobalt, and manganese are reviewed to show a much enhanced reactiv-
ity of the photoexcited states as compared with their relative ground states. Mononuclear copper(II)-
superoxide and dinuclear copper(II)-peroxo complexes underwent copper–oxygen bond cleavage, dioxy-
gen release, and copper(I)/dioxygen rebinding upon photoexcitation at low temperature. Photoirradiation
of the cobalt–oxygen compound [(TAML)CoIV(O)]2− (6) (TAML = tetraamidomacrocyclic ligand) at 5 °C
yielded a cobalt–oxygen excited state with 0.6(1) ns lifetime, showing a high reactivity in the bimolecular
electron-transfer oxidations of m-xylene and anisole. An extremely long-lived excited state was generated
upon photoexcitation of a manganese(IV)-oxo complex binding two Sc(OTf)3 molecules, which enabled
the hydroxylation of benzene.
1. Introduction
Light absorption by rare d6 metal complexes, such as thosebased on ruthenium(II), iridium(III), osmium(II), rhenium(I)1–6
and the tris(2,2′-bipyridine) [RuII(bpy)3]2+ complex,6,7 yields
long-lived high energy excited states that have been employedas efficient photosensitizers8–12 and photocatalysts.13–20 Therehas been great interest in replacing these expensive metalswith earth-abundant elements from the first row of thed-block.21–32
In particular, photoexcited states of earth-abundant high-valent metal-oxo species are expected to be very promisingsuperoxidants because of the already remarkable oxidationreactivity of their ground states.33–50 High-valent metal-oxointermediates have been, in fact, found to be powerful oxi-dants in some of the most challenging redox reactions in
Claudio Saracini
Claudio Saracini received hisPh.D. degree in Chemistry atThe Johns Hopkins University(Baltimore, Maryland, USA)under the supervision ofProfessor Kenneth D. Karlin in2014. After his post-doctoralwork at the Lawrence BerkeleyNational Laboratory (Berkeley,California, USA) under the direc-tion of Dr. Junko Yano, hebecame a Korean ResearchFellow at the Department ofChemistry and Nano Science of
Ewha Womans University in Korea in 2016. His research interestsinvolve photoredox dynamics and charge transfer chemistry oftransition metal ions in synthetic and in biological systems.
Shunichi Fukuzumi
Shunichi Fukuzumi earned abachelor’s degree and Ph.D.degree in applied chemistry atTokyo Institute of Technology in1973 and 1978, respectively.After working as a postdoctoralfellow (1978–1981) at IndianaUniversity in USA, he joined theDepartment of AppliedChemistry at Osaka University,as an Assistant Professor in 1981and was promoted to a FullProfessor in 1994. His researchinterests are artificial photosyn-
thesis and electron transfer chemistry. He is now DistinguishedProfessor of Ewha Womans University, Designated Professor ofMeijo University and Professor Emeritus of Osaka University.
aDepartment of Chemistry and Nano Science, Ewha Womans University, Seoul
nature, such as those catalysed by iron-oxo moieties incytochromes P450 and nonheme iron enzymes51–53 andthe manganese-oxo cluster in photosystem II.54–58 Excitedstates of high-valent rare metal-oxo complexes, such as trans-dioxo compounds derived from rhenium(V),59 osmium(VI),60
technetium(V),61 and the relatively more abundanttungsten(V),62 have been explored earlier. In contrast to suchrare metal-oxo complexes, little is known about photo-dynamics and bimolecular electron-transfer reactivity of earth-abundant metal-oxo excited states. In addition, rapid de-activation pathways of excited states of earth-abundant metaland metal-oxo complexes has often precluded the utilizationof their high oxidizing power.30,31
This Frontier article focuses on the emerging field of photo-excited chemistry of earth-abundant metal–oxygen speciessuch as those shown in Chart 1: the copper(II)-superoxide com-plexes [(TMG3tren)Cu
pyridyl-methyl)amine),64 and the high-valent metal-oxo com-plexes [(TAML)CoIV(O)]2− (6) (H4TAML = 3,4,8,9-tetrahydro-3,3,6,6,9,9-hexamethyl-1H-1,4,8,11-benzotetraazo-cyclotridecane-2,5,7,10-(6H,11H)tetrone)65 and [(Bn-TPEN)MnIV(O)]2+-(Sc(OTf)3)2 (7) (Bn-TPEN = N-benzyl-N,N′,N‘-tris(2-pyridyl-methyl)-1,2-diaminoethane).66 The copper/O2 complexesexhibited novel photodynamics with excited states and inter-mediates that could, potentially, be used in bimolecular reac-tivity with exogenous substrates, while the photoexcited statesof metal–oxygen species based on cobalt and manganeseshowed remarkably enhanced reactivity as compared withtheir ground states.
2. Photochemistry of mononuclearcopper(II)-superoxide complexes
The study of copper(I)/dioxygen binding at mononuclearcopper centres is relevant to the O2-activation occurring in thecopper enzymes, peptidylglycine α-hydroxylating monooxygen-ase67,68 and dopamine β-monooxygenase,67,68 with end-onbound mononuclear copper(II)-superoxide species suggested asthe substrate hydrogen atom-abstracting intermediates.42,69–72
Yong-Min Lee
Yong-Min Lee received his M.S.and Ph.D. degrees in InorganicChemistry at Pusan NationalUniversity, Republic of Korea,under the supervision ofProfessor Sung-Nak Choi in1999. After he worked in theMagnetic Resonance Centre(CERM) at University ofFlorence, Italy, as a Postdoctoralfellow and Researcher under thedirection of Professors IvanoBertini and Claudio Luchinatfrom 1999 to 2005, he joined the
Centre for Biomimetic Systems at Ewha Womans University, as aResearch Professor in 2006. He is currently a Special AppointmentProfessor at Ewha Womans University since 2009.
Wonwoo Nam
Wonwoo Nam received his BS(Honors) degree in Chemistry atCalifornia State University, LosAngeles (1985), and his Ph.D.degree in Inorganic Chemistry atUCLA (1990). After one year post-doctoral experience at UCLA, hebecame an Assistant Professor atHong Ik University in 1991. Hemoved to Ewha WomansUniversity in 1994, where he ispresently a DistinguishedProfessor of Ewha WomansUniversity. His current research
focuses on the dioxygen activation, water oxidation, and sensorsfor metal ions in bioinorganic chemistry.
Chart 1 Molecular structures of the metal–oxygen ground stateprecursors.
These types of copper(II)-superoxide moieties were laterfound to be photoactive with O2 being photo-ejected [eqn (1)]from both [(TMG3tren)Cu
II(O2)]+ (1) and [(PV-tmpa)CuII(O2)]
+
(2) (Chart 1) as shown in Scheme 1.63 The transient absorptionspectra observed upon photoexcitation of 1 and 2 (Fig. 1 forcomplex 1) were, in fact, the mirror image of the electronicabsorption spectra of 1 and 2 (not shown), consistent with thegeneration of [(TMG3tren)Cu
I]+ and O2 from 1 and of[(PV-tmpa)CuI]+ and O2 from 2, respectively [eqn (1)].63
Interestingly, the quantum yields for the generation of thecopper(I) and O2 products were wavelength-dependent (ϕ436 =0.29 and 0.11 for 1 and 2, and ϕ683 = 0.035 and 0.078 for 1 and2),63 in contrast to heme systems, such as myoglobin (ϕ = 0.3with either 488 nm (Soret band) or 580 (Q band) excitation).73
LCuðiiÞ-O2•� þ hν ! LCuðiÞ þ O2 ð1Þ
The photo-ejected O2 underwent rebinding with thecopper(I) formed in situ [eqn (2) and Fig. 1 for complex 1] withrate constants [e.g., kO2
from eqn (2)] and equilibrium con-stants [KO2
= kO2/k−O2
from eqn (3)] all determined from laserexperiments for complex 1.63 Activation parameters (ΔH‡ and
ΔS‡ for kO2and k−O2
) and thermodynamic parameters (ΔH°and ΔS° for KO2
) were also determined from laser experiments,using the Eyring [eqn (4)] and van’t Hoff [eqn (5)] equations,respectively.63
Given the already high reactivity of the mononuclearcopper(II)-superoxide ground states found in theenzymes,42,69–72 the high photoactivity observed for the excitedstates of the copper(II)-superoxide complexes 1 and 2 suggeststhat these species could be, indeed, suitable for the oxidationof exogenous substrates, which has yet to be explored.
3. Photochemistry of dinuclearcopper(II)-peroxo complexes
Single photon absorption reactions that drive two-electrontransfer mechanisms are quite rare, yet very important forphotocatalytic multi-electron transfer processes.74,75 Thephotodynamics of the dinuclear copper(II)-peroxo complexes,[(N5)CuII
2 (O2)]2+ (3) and [(N3)CuII
2 (O2)]2+ (4) (Chart 1),76–78
showed the first unambiguous example of a stepwise one-photon two-electron oxidation chemistry of a metal-bound per-oxide to O2 [eqn (6)].64
O22� þ hν ! O2 þ 2e� ð6Þ
Photoexcitation of the μ–η2:η2-(side-on) peroxo dicopper(II)complexes 3 and 4 with visible light at −80 °C in acetone led,in fact, to one-photon two-electron peroxide-to-dioxygen oxi-dation chemistry to release O2 with quantum yield 0.14, esti-mated for 3.64 The transient absorption spectra observed afternanosecond laser excitation of 3 and 4 (Fig. 2 for complex 3)were the mirror image of the electronic absorption spectra of 3and 4 (not shown), consistent with the generation of dicopper(I)and O2 in each case which, then, rebound in situ (Fig. 2 forcomplex 3). The results of the fast CuI
2/O2 rebinding kineticsexamined for 3 and 4 were remarkably similar with thosereported for the dicopper proteins hemocyanin andtyrosinase.64,76–78
Interestingly, no O2 was released when the trans-1,2 (end-on) peroxo dicopper(II) complex [(tmpa)2CuII
2 (O2)]2+ (5)
(Chart 1) (λmax, 525, 600 nm)79–81 was photoexcited under thesame conditions.64 Novel ultra-fast low-temperature transientabsorption apparatus revealed formation of previouslyunknown mixed-valent superoxide intermediates [CuII(O2
•−)CuI]2+ (λmax, 685–740 nm) upon photoexcitation of 3, 4, and 5,though [CuII(O2
•−)CuI]2+ underwent O2 release only for 3 and4.64 Such remarkable differences in electron-transfer pathwaysand O2 photochemistry64 are caused by the different ligand
Scheme 1 O2 photorelease from the copper(II)-superoxide complexes1 and 2. Reprinted with permission from ref. 63. Copyright 2014,American Chemical Society.
Fig. 1 Transient absorption difference spectra collected at the indi-cated delay times after 436 nm laser excitation (15 mJ per pulse, 8–10ns fwhm) of 1 in MeTHF at −55 °C showing the copper(I)/O2 rebindingkinetics. Overlaid in red on the experimental data is a simulated spec-trum [Abs([(TMG3tren)Cu
I]+) – Abs(1)]. Reprinted with permission fromref. 63. Copyright 2014, American Chemical Society.
architecture: (i) tridentate N5 and N3 (3 and 4) ligands vs.tetradentate tmpa (5) and (ii) 6-membered ring for 3 and 4 vs.5-membered ring for 5,85 which translates into differentligand–CuII/I redox potentials.77,82–84
The photogenerated [CuII(O2•−)CuI]2+ intermediates of 3, 4,
and 5 showed lifetimes of 55, 260, and 140 ps,64 respectively,suggesting that these reactive intermediates could, possibly, beemployed as oxidants in bimolecular reactions with exogenoussubstrates.
4. Excited state bimolecular oxidationreactivity of a cobalt–oxygen complex
The short lifetimes of the cobalt(I)-,86 cobalt(II)-,87–89 andcobalt(III)-based88,90–94 excited states reported so far precludedthe study of their bimolecular electron-transfer reactivity withexogenous substrates.86–94 Recently, a cobalt(III) excited state wasreported to selectively trifluoromethylate polycyclic aromatichydrocarbons through a mixed energy transfer/electron transfertwo-photon-induced mechanism.95 An interesting example of acobalt excited state performing bimolecular electron-transfer oxi-dation of m-xylene and anisole solely via a single-electron transfermechanism was also recently reported, from the mononuclearcobalt–oxygen complex [(TAML)CoIV(O)]2− (6) (see Chart 1).65
Photoexcitation of 6 (λexc = 393 nm) in an acetone/aceto-nitrile solvent mixture (acetone/MeCN v/v 1 : 1) at 5 °C resultedin generation of the excited state D2*, exhibiting a 1.4(4) pslifetime, and converting to the excited state D1* (λmax =580 nm, D2* → D1*; Fig. 3a). The excited state D1* showed a0.6(1) ns lifetime, remarkably long for a cobalt excited state,and converted back to 6 (D1* → 6; Fig. 3b).65 Interestingly, theexcited state dynamics observed was the same regardless theoxidant used to generate 6 from Li[(TAML)CoIII]·3(H2O) withone-electron oxidants, such as copper(II), [FeIII(bpy)3]
3+ (bpy =2,2′-bipyridine), and tris(4-bromophenyl)ammoniumyl radicalcation (TBPA•+).65
The 0.6(1) ns lifetime exhibited by D1* was long enough toallow probing of its one-electron transfer bimolecular reactiv-ity65 towards (Eox vs. SCE) benzene (2.35 V),96 toluene (2.20V),96 m-xylene (2.02 V),96 and anisole (1.67 V).97 When 6 wasirradiated with pulsed laser light in the presence of a largeconcentration of m-xylene (Eox = 2.02 V vs. SCE, 2.7 M), a newNIR absorption band (λmax = 1040 nm) appeared, consistentwith the generation of m-xylene π-dimer radical cation,98–100
which is known to form from m-xylene monomer radicalcation in the presence of a large excess of m-xylene,100 confirm-ing the bimolecular one-electron transfer occurred fromm-xylene to D1* (Scheme 2).98,99 A similar photoinduced reac-tion was observed when anisole was used as a substrate, withthe generation of anisole π-dimer radical cation, confirmed bydetection of its NIR band. In contrast, no reactivity wasobserved towards benzene and toluene (Scheme 2).65 Thedifferent reactivity of D1* depending on the Eox values of theelectron donors allowed the estimation of its one-electronreduction potential E*
red
� �as 2.1(1) V vs. SCE, which is remark-
ably more positive than that of the ground state (Ered = 0.86 Vvs. SCE).65
The light-absorption of 6 across the visible and NIR spectralregions,65 combined with the high reduction potential of its
Fig. 2 Transient absorption difference spectra collected at the indi-cated delay times after 532 nm laser excitation (10 mJ per pulse, 8–10ns fwhm) of 3 in acetone at −80 °C showing the dicopper(I)/O2 rebind-ing kinetics. Overlaid in red is a simulated spectrum [Abs([(N5)CuI
2-(CH3CN)]
2+) – Abs(3)], corresponding to the negative absorption of thespectrum of [(N5)CuII(O2
2–)CuII]2+ (3). Reprinted with permission fromref. 64. Copyright 2015, American Chemical Society.
Fig. 3 (a) Representative transient absorption difference spectralchanges due to decay of the lower-energy excited state D1* to theground state 6. (b) Representative time course monitored for the changeof absorbance difference at 580 nm due to the decay of D1*. The redline shows the fit to a first-order kinetics. Reprinted with permissionfrom ref. 65. Copyright 2018, American Chemical Society.
Scheme 2 Bimolecular electron-transfer reactivity of D1* with organicsubstrates. Reprinted with permission from ref. 65. Copyright 2018,American Chemical Society.
excited state D1* (E*red ¼ 2:1ð1ÞV vs. SCE), enabled challenging
bimolecular reactivity65 and suggests that earth-abundantcobalt complexes such as 6 or similar can be exploited for one-electron transfer chemistry with organic substrates in solar-driven oxidation reactions.
5. Benzene hydroxylation to phenolby the photoexcited state of amanganese(IV)-oxo-(Sc(OTf)3)2 complex
The long-lived photoexcited state of an earth-abundant metal-oxo species was reported recently for a MnIV-oxo complex thatbinds two Sc(OTf)3 molecules ([(Bn-TPEN)MnIV(O)]2+-(Sc(OTf)3)2 (7); see Chart 1) in a trifluoroethanol/acetonitrilesolvent mixture (TFE/MeCN v/v 1 : 1).66 XANES/EXAFS analysisand DFT calculations of 7 indicate that the second Sc3+ ion islocated at the secondary coordination sphere rather thandirect binding to the oxo moiety.100 Nanosecond laser exci-tation of 7 resulted in generation of the long-lived doubletexcited state [(Bn-TPEN)MnIV(O)-(Sc(OTf)3)2]
2+* (7*), whichshows an absorption band at λmax = 640 nm (Fig. 4a) and a6.4 µs lifetime.66 The doublet excited state 7* (2E) was gener-ated by rapid intersystem crossing from the quartet excitedstate (4E) (4E → 2E; rate constant: 2.9(3) × 1010 s−1) initiallyformed upon femtosecond laser excitation of 7 (7 + hν → 4E).66
The long lifetime of 7* is due to the spin-forbidden transitionfrom the doublet excited state to the quartet ground state(4B).101 Isoelectronic Cr(III) complexes were also reported toexhibit emission from a very long-lived excited state (2E).102–105
The one-electron reduction potential E*red
� �of 7* was esti-
mated as 2.1(1) V vs. SCE with a reorganization energy for theelectron transfer of λ = 0.64(4) eV, determined by analysing thedriving force dependence of the rate constant of electron trans-fer from a series of electron donors to 7* in light of the Marcustheory of electron transfer.66,106,107 Due to its high reductionpotential (E*
red ¼ 2:1ð1ÞV vs. SCE), the excited state 7* was ableto oxidize benzene through electron transfer, leading to for-mation of phenol as shown in Scheme 3.66 Photoexcitation of
7 in the presence of benzene resulted in electron transfer frombenzene to 7* to produce benzene monomer radical cationand [(Bn-TPEN)MnIII(O)]+-(Sc(OTf)3)2. The benzene monomerradical cation, in equilibrium with the π-dimer radicalcation,107 reacted with H2O to produce the OH adductradical,108 which, in turn, is oxidized by [(Bn-TPEN)MnIII(O)]+-(Sc(OTf)3)2 to yield phenol after removal of a proton(Scheme 3A). The resulting manganese(II) complex reactedrapidly with 7 to yield the [MnIII(O)MnIII]4+ species(Scheme 3B).66
6. Conclusions
As discussed above, photoexcitation of mononuclear copper(II)-superoxo species (complexes 1 and 2) results in release of O2,with the generation of copper(I) complexes, whereas photo-excitation of dinuclear copper(II)-peroxo species (complexes 3and 4) results in release of O2, occurring through a rare one-photon two-electron mechanism. Such reactive excited statesand intermediates of earth-abundant metal–oxygen speciescould be used in the bimolecular oxidation of exogenous sub-strates. In addition, the photodynamics of high-valent earth-abundant metal-oxo species was elucidated, with the excitedstates generated undergoing remarkable bimolecular electron-transfer oxidation reactivity with exogenous substrates. In par-ticular, the D1* excited state of [(TAML)CoIV(O)]2− (6) engagedin electron-transfer oxidation of m-xylene and anisole. The 2Eexcited state of [(Bn-TPEN)MnIV(O)]2+-(Sc(OTf)3)2 (7), thatshowed a remarkable lifetime of 6.4 μs because of the spin-for-bidden transition to the quartet ground state, was capable ofoxidising benzene to phenol. Such long-lived metal-oxo excitedstates have tremendous potential as superoxidants towardschallenging substrates, performing chemistry, which isimpossible to be carried out by the relative ground states. Thefurther elaborate design of ligands for metal complexes willlead to development of more reactive and longer-lived photo-
Fig. 4 (a) Transient absorption spectral changes observed upon nano-second laser excitation (λexc = 355 nm) of a solvent mixture of TFE/MeCN (v/v = 1 : 1) containing 7 (0.50 mM) and benzene (100 mM) at25 °C. Inset: Decay time profile of the absorbance at 640 nm due to the2E excited state of 7. (b) Plot of ket vs. concentration of benzene in TFE/MeCN (v/v = 1 : 1) at 25 °C. Reprinted with permission from ref. 66.Copyright 2018, American Chemical Society.
Scheme 3 Photoinduced oxidation of benzene to phenol by 7*.Reprinted with permission from ref. 66. Copyright 2018, AmericanChemical Society.
excited states of metal–oxygen complexes, which may act aseffective redox catalysts for photocatalytic oxidation ofsubstrates.
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
The authors gratefully acknowledge the contributions of theircollaborators and co-workers mentioned in the cited references,and financial supports from CRI (NRF-2012R1A3A2048842 toW. N.), GRL (NRF-2010-00353 to W. N.), Basic Science ResearchProgram (2017R1D1A1B03029982 to Y. M. L. and2017R1D1A1B03032615 to S. F.), and Korea Research FellowshipProgram (NRF-2015H1D3A1066507 to C. S.) through the NRF ofKorea, and JSPS KAKENHI (16H02268 to S. F.).
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