Effects of metal ions on physicochemical properties and redox reactivity of phenolates and phenoxyl radicals: mechanistic insight into hydrogen atom abstraction by phenoxyl radical-metal complexes

Coordination Variations within Binuclear Copper Dioxygen-Derived (Hydro)Peroxo and Superoxo Species; Influences upon Thermodynamic and Electronic Properties

Copper ion is a versatile and ubiquitous facilitator of redox chemical and biochemical processes. These include the binding of molecular oxygen to copper(I) complexes where it undergoes stepwise reduction-protonation. A detailed understanding of thermodynamic relationships between such reduced/protonated states is key to elucidate the fundamentals of the chemical/biochemical processes involved. The dicopper(I) complex [CuI2(BPMPO-)]1+ {BPMPOH = 2,6-bis{[(bis(2-pyridylmethyl)amino]methyl}-4-methylphenol)} undergoes cryogenic dioxygen addition; further manipulations in 2-methyltetrahydrofuran generate dicopper(II) peroxo [CuII2(BPMPO-)(O22-)]1+, hydroperoxo [CuII2(BPMPO-)(-OOH)]2+, and superoxo [CuII2(BPMPO-)(O2•-)]2+ species, characterized by UV-vis, resonance Raman and electron paramagnetic resonance (EPR) spectroscopies, and cold spray ionization mass spectrometry. An unexpected EPR spectrum for [CuII2(BPMPO-)(O2•-)]2+ is explained by the analysis of its exchange-coupled three-spin frustrated system and DFT calculations. A redox equilibrium, [CuII2(BPMPO-)(O22-)]1+ ⇄ [CuII2(BPMPO-)(O2•-)]2+, is established utilizing Me8Fc+/Cr(η6-C6H6)2, allowing for [CuII2(BPMPO-)(O2•-)]2+/[CuII2(BPMPO-)(O22-)]1+ reduction potential calculation, E°' = -0.44 ± 0.01 V vs Fc+/0, also confirmed by cryoelectrochemical measurements (E°' = -0.40 ± 0.01 V). 2,6-Lutidinium triflate addition to [CuII2(BPMPO-)(O22-)]1+ produces [CuII2(BPMPO-)(-OOH)]2+; using a phosphazene base, an acid-base equilibrium was achieved, pKa = 22.3 ± 0.7 for [CuII2(BPMPO-)(-OOH)]2+. The BDFEOO-H = 80.3 ± 1.2 kcal/mol, as calculated for [CuII2(BPMPO-)(-OOH)]2+; this is further substantiated by H atom abstraction from O-H substrates by [CuII2(BPMPO-)(O2•-)]2+ forming [CuII2(BPMPO-)(-OOH)]2+. In comparison to known analogues, the thermodynamic and spectroscopic properties of [CuII2(BPMPO-)] O2-derived adducts can be accounted for based on chelate ring size variations built into the BPMPO- framework and the resulting enhanced CuII-ion Lewis acidity.

Phenolate-bonded bis(μ-oxido)-bis-copper(III) intermediates: hydroxylation and dehalogenation reactivities

Exogenous phenolate ortho-hydroxylation by copper oxidants formed from dioxygen is generally thought to occur through one of two limiting mechanisms defined by the structure of the active oxidant: an electrophilic μ-η²:η²-peroxo-bis-copper(II) species as found in the oxygenated form of the binuclear copper enzyme tyrosinase (oxyTyr), or an isomeric bis(μ-oxido)-bis-copper(III) species (O) with ligated phenolate(s) as evidenced by most synthetic systems. The characterization of the latter is limited due to their limited thermal stability. This study expands the scope of an O species with ligated phenolate(s) using N,N'-di-tert-butyl-1,3-propanediamine (DBPD), a flexible secondary diamine ligand. Oxygenation of the [(DBPD)Cu(I)]¹⁺ complex at low temperatures (e.g., 153 K) forms a spectroscopically and structurally faithful model to oxyTyr, a side-on peroxide intermediate, which reacts with added phenolates to form a bis(μ-oxido)-bis-copper(III) species with ligated phenolates, designated as an A species. The proposed stoichiometry of A is best understood as possessing 2 rather than 1 bonded phenolate. Thermal decomposition of A results in regiospecific phenolate ortho-hydroxylation with the ortho-substituent as either a C-H or C-X (Cl, Br) group, though the halogen displacement is significantly slower. DFT and experimental studies support an electrophilic attack of an oxide ligand into the π-system of a ligated phenolate. This study supports a hydroxylation mechanism in which O-O bond cleavage of the initially formed peroxide by phenolate ligation, which precedes phenolate aromatic hydroxylation.

Impact of Group 10 Metals on the Solvent-Induced Disproportionation of o-Semiquinonato Complexes

The oxidation of [M^II (3,5-DTBCat)(DTBbpy)] (M=Ni ([Ni]), Pd ([Pd]), and Pt ([Pt]); 3,5-DTBCat=3,5-di-tert-butylcatecholato; DTBbpy=4,4'-di-tert-butyl-2,2'-bipyridine) afforded the dimeric {[Ni^II (3,5-DTBSQ)(DTBbpy)](PF₆ )}₂ ({[Ni](PF₆ )}₂ ; 3,5-DTBSQ=3,5-di-tert-butylsemiquinonato) and monomeric semiquinonato (SQ) complexes [M^II (3,5-DTBSQ)(DTBbpy)](PF₆ ) (M=Pd ([Pd](PF₆ )) and Pt ([Pt](PF₆ ))). The negative solvatochromic properties of the SQ complexes allowed us to estimate the relative order of their dipole moments: [Pd](PF₆ )>[Pt](PF₆ )>{[Ni](PF₆ )}₂ . The complexes [Pd](PF₆ ) and [Pt](PF₆ ) adopt monomeric structures and are stable in CH₂ Cl₂ and toluene, whereas they gradually disproportionate at room temperature to [M] and 3,5-di-tert-butylbenzoquinone (3,5-DTBBQ) in polar solvents such as THF, MeOH, EtOH, DMF, or DMSO. The results of spectroscopic studies suggested that the oxidized nickel complex adopts a monomeric structure ([Ni](PF₆ )) in CH₂ Cl₂ , but a dimeric structure ({[Ni](PF₆ )}₂ ) in the other investigated solvents. In polar solvents, {[Ni](PF₆ )}₂ may disproportionate to [Ni] and 3,5-DTBBQ at 323 K, thereby demonstrating a significant solvent- and metal-dependence in temperature. The relative activities of {[Ni](PF₆ )}₂ and [M](PF₆ ) toward disproportionation are related to the electrochemically estimated K_dis values in CH₂ Cl₂ and DMF. The present work demonstrates that solvent polarity and the dipole moments of the SQ complexes promote disproportionation, which can be controlled by a judicious choice of the metal ion, solvent, and temperature.

Tethered cationic alkaline earth – olefin complexes

The utilisation of a specifically tailored aminofluoroalcohol possessing both a methoxy and an olefin dangling side arms enables the preparation of the first examples of discrete calcium- and strontium-olefin cationic complexes.

Radical pathways and O2 participation in benzyl alcohol oxidation, and catechol and o-aminophenol oxidase activity studies with novel zinc complexes: an experimental and theoretical investigation

Diphenoxo bridged dinuclear zinc complexes were found to be capable of oxidation of benzyl alcohol, catechol and o-aminophenol.

Alkali aminoether-phenolate complexes: synthesis, structural characterization and evidence for an activated monomer ROP mechanism

Several monometallic {LO(i)}M complexes of lithium (M = Li; i = 1 (1), 2 (2), 3 (3)) or potassium (M = K, i = 3 (4)) and the heteroleptic bimetallic lithium complex {LO(3)}Li·LiN(SiMe2H)2 (5), all supported by monoanionic aminoether-phenolate {LO(i)}(-) (i = 1-3) ligands, have been synthesized and structurally characterized. A large range of coordination motifs is represented in the solid state, depending on the chelating ability of the ligand, the size of the metal and the number of metallic centres found in the complex. Pulse-gradient spin-echo NMR showed that 1-4 are monomeric in solution, irrespective of their (mono- or di)nuclearity in the solid-state. VT (7)Li and DOSY NMR measurements conducted for 5 indicated that the two Li atoms in the complex do not exchange positions even at 80 °C. Upon addition of 1-10 equiv. of BnOH, the electron-rich and sterically congested {LO(3)}Li complex (3) promotes the controlled living and immortal ring-opening polymerisation of L-lactide. The combination of polymer end-group analyses and stoichiometric model reactions unambiguously provided evidence that ROP reactions catalyzed by these two-component {LO(i)}Li/BnOH catalyst systems operate according to an activated monomer mechanism, and not via the coordination-insertion scenario frequently assumed for similar alkali phenolate-alcohol systems.

Spin crossover in polymeric and heterometallic FeII species containing polytopic dipyridylamino-substituted-triazine ligands

We report the synthesis and characterisation of the new polytopic ligands, ddta (N,N-di(pyridin-2-yl)-4,6-di(1,4,7,10-tetraoxa-13-azacyclopentadecan-13-yl)-1,3,5-triazin-2-amine) and tptd (N(2),N(2),N(4),N(4)-tetra(pyridin-2-yl)-6-(1,4,7,10-tetraoxa-13-azacyclopentadecan-13-yl)-1,3,5-triazine-2,4-diamine). Each contains N-donor dipyridylamino binding sites as well as separate and distinct mono-aza-15-crown-5 binding sites. The ligand ddta has been used to synthesise the polymeric heterometallic SCO compound trans-[Fe(II)(NCS)(2)(ddta)(2)Na(2)](ClO(4))(2)·4CH(3)CH(2)CH(2)OH, 1, and tptd has been used to synthesise the polymeric SCO compound trans-[Fe(II)(NCS)(2)(tptd)]·CH(3)OH, 2, and the dinuclear compound cis-[(Fe(II))(2)(NCS)(4)(tptd)(2)], 3. Magnetic susceptibility measurements show that 1 and a desolvated sample of 2 each undergo a gradual, one-step spin transition with T(½) values of ~240 K and ~110 K, respectively. The paucity of inter-chain intermolecular interactions, as well as the flexible, covalent bridges between Fe(II) spin crossover sites, are likely to contribute to the gradual nature of the spin transition observed in each case. Variable temperature powder X-ray diffraction studies on 1 show the anisotropic behaviour of the unit cell parameters, where c and the b-c plane are most affected by structural changes occurring as the temperature is lowered.

Scandium ion-enhanced oxidative dimerization and N-demethylation of N,N-dimethylanilines by a non-heme iron(IV)-oxo complex

Oxidative dimerization of N,N-dimethylaniline (DMA) occurs with a nonheme iron(IV)-oxo complex, [Fe(IV)(O)(N4Py)](2+) (N4Py = N,N-bis(2-pyridylmethyl)-N-bis(2-pyridyl)methylamine), to yield the corresponding dimer, tetramethylbenzidine (TMB), in acetonitrile. The rate of the oxidative dimerization of DMA by [Fe(IV)(O)(N4Py)](2+) is markedly enhanced by the presence of scandium triflate, Sc(OTf)(3) (OTf = CF(3)SO(3)(-)), when TMB is further oxidized to the radical cation (TMB(•+)). In contrast, we have observed the oxidative N-demethylation with para-substituted DMA substrates, since the position of the C-C bond formation to yield the dimer is blocked. The rate of the oxidative N-demethylation of para-substituted DMA by [Fe(IV)(O)(N4Py)](2+) is also markedly enhanced by the presence of Sc(OTf)(3). In the case of para-substituted DMA derivatives with electron-donating substituents, radical cations of DMA derivatives are initially formed by Sc(3+) ion-coupled electron transfer from DMA derivatives to [Fe(IV)(O)(N4Py)](2+), giving demethylated products. Binding of Sc(3+) to [Fe(IV)(O)(N4Py)](2+) enhances the Sc(3+) ion-coupled electron transfer from DMA derivatives to [Fe(IV)(O)(N4Py)](2+), whereas binding of Sc(3+) to DMA derivatives retards the electron-transfer reaction. The complicated kinetics of the Sc(3+) ion-coupled electron transfer from DMA derivatives to [Fe(IV)(O)(N4Py)](2+) are analyzed by competition between binding of Sc(3+) to DMA derivatives and to [Fe(IV)(O)(N4Py)](2+). The binding constants of Sc(3+) to DMA derivatives increase with the increase of the electron-donating ability of the para-substituent. The rate constants of Sc(3+) ion-coupled electron transfer from DMA derivatives to [Fe(IV)(O)(N4Py)](2+), which are estimated from the binding constants of Sc(3+) to DMA derivatives, agree well with those predicted from the driving force dependence of the rate constants of Sc(3+) ion-coupled electron transfer from one-electron reductants to [Fe(IV)(O)(N4Py)](2+). Thus, oxidative dimerization of DMA and N-demethylation of para-substituted DMA derivatives proceed via Sc(3+) ion-coupled electron transfer from DMA derivatives to [Fe(IV)(O)(N4Py)](2+).

Discrete, solvent-free alkaline-earth metal cations: metal···fluorine interactions and ROP catalytic activity

Efficient protocols for the syntheses of well-defined, solvent-free cations of the large alkaline-earth (Ae) metals (Ca, Sr, Ba) and their smaller Zn and Mg analogues have been designed. The reaction of 2,4-di-tert-butyl-6-(morpholinomethyl)phenol ({LO(1)}H), 2-{[bis(2-methoxyethyl)amino]methyl}-4,6-di-tert-butylphenol ({LO(2)}H), 2-[(1,4,7,10-tetraoxa-13-azacyclopentadecan-13-yl)methyl]-4,6-di-tert-butylphenol ({LO(3)}H), and 2-[(1,4,7,10-tetraoxa-13-azacyclo-pentadecan-13-yl)methyl]-1,1,1,3,3,3-hexafluoropropan-2-ol ({RO(3)}H) with [H(OEt(2))(2)](+)[H(2)N{B(C(6)F(5))(3)}(2)](-) readily afforded the doubly acidic pro-ligands [{LO(1)}HH](+)[X](-) (1), [{LO(2)}HH](+)[X](-) (2), [{LO(3)}HH](+)[X](-) (3), and [{RO(3)}HH](+)[X](-) (4) ([X](-) = [H(2)N{B(C(6)F(5))(3)}(2)](-)). The addition of 2 to Ca[N(SiMe(3))(2)](2)(THF)(2) and Sr[N(SiMe(3))(2)](2)(THF)(2) yielded [{LO(2)}Ca(THF)(0.5)](+)[X](-) (5) and [{LO(2)}Sr(THF)](+)[X](-) (6), respectively. Alternatively, 5 could also be prepared upon treatment of {LO(2)}CaN(SiMe(3))(2) (7) with [H(OEt(2))(2)](+)[X](-). Complexes [{LO(3)}M](+)[X](-) (M = Zn, 8; Mg, 9; Ca, 10; Sr, 11; Ba, 12) and [{RO(3)}M](+)[X](-) (M = Zn, 13; Mg, 14; Ca, 15; Sr, 16; Ba, 17) were synthesized in high yields (70-90%) by reaction of 3 or 4 with the neutral precursors M[N(SiMe(3))(2)](2)(THF)(x) (M = Zn, Mg, x = 0; M = Ca, Sr, Ba, x = 2). All compounds were fully characterized by spectroscopic methods, and the solid-sate structures of compounds 1, 3, 7, 8, 13, 14, {15}(4)·3CD(2)Cl(2), {16}(4)·3CD(2)Cl(2), and {{17}(4)·EtOH}·3CD(2)Cl(2) were determined by X-ray diffraction crystallography. Whereas the complexes are monomeric in the case of Zn and Mg, they form bimetallic cations in the case of Ca, Sr and Ba; there is no contact between the metal and the weakly coordinating anion. In all metal complexes, the multidentate ligand is κ(6)-coordinated to the metal. Strong intramolecular M···F secondary interactions between the metal and F atoms from the ancillary ligands are observed in the structures of {15}(4)·3CD(2)Cl(2), {16}(4)·3CD(2)Cl(2), and {{17}(4)·EtOH}·3CD(2)Cl(2). VT (19)F{(1)H} NMR provided no direct evidence that these interactions are maintained in solution; nevertheless, significant Ae···F energies of stabilization of 25-26 (Ca, Ba) and 40 kcal·mol(-1) (Sr) were calculated by NBO analysis on DFT-optimized structures. The identity and integrity of the cationic complexes are preserved in solution in the presence of an excess of alcohol (BnOH, (i)PrOH) or L-lactide (L-LA). Efficient binary catalytic systems for the immortal ring-opening polymerization of L-LA (up to 3,000 equiv) are produced upon addition of an excess (5-50 equiv) of external protic nucleophilic agents (BnOH, (i)PrOH) to 8-12 or 13-17. PLLAs with M(n) up to 35,000 g·mol(-1) were produced in a very controlled fashion (M(w)/M(n) ≈ 1.10-1.20) and without epimerization. In each series of catalysts, the following order of catalytic activity was established: Mg ≪ Zn < Ca < Sr ≈ Ba; also, Ae complexes supported by the aryloxide ligand are more active than their parents supported by the fluorinated alkoxide ancillary, possibly owing to the presence of Ae···F interactions in the latter case. The rate law -d[L-LA]/dt = k(p)·[L-LA](1.0)·[16](1.0)·[BnOH](1.0) was established by NMR kinetic investigations, with the corresponding activation parameters ΔH(++) = 14.8(5) kcal·mol(-1) and ΔS(++) = -7.6(2.0) cal·K(-1)·mol(-1). DFT calculations indicated that the observed order of catalytic activity matches an increase of the L-LA coordination energy onto the cationic metal centers with parallel decrease of the positive metal charge.

Transition metal complexes coordinated by an NAD(P)H model compound and their enhanced hydride-donating abilities in the presence of a base

The ruthenium(II) and rhenium(I) complexes containing an NAD(P)H model compound, 1-benzyl-1,4-dihydronicotinamide (BNAH), as ligand, [Ru(tpy)(bpy)(BNAH)]2+ (1 a) and [Re(bpy)(CO)3(BNAH)]+ (1 b), were quantitatively produced by the reaction of the corresponding metal hydrido complexes with BNA(+) (1-benzylnicotinamidium cation). In the presence of base with pK(a) = 8.9, 1 a and 1 b have much greater reducing power than "free" BNAH. The oxidation potentials of 1 a in the absence and the presence of triethylamine were 0.55 V and -0.04 V, respectively, versus Ag/AgNO(3), whereas that of "free" BNAH was 0.30 V. Spectroscopic results clearly showed that the base extracts a proton from the carbamoyl group on 1 a and 1 b to give the deprotonated BNAH coordinating to the transition-metal complexes [Ru(tpy)(bpy)(BNAH-H+)]+ (3 a) and [Re(bpy)(CO)3(BNAH-H+)] (3 b); this deprotonation underlies the enhancement in reducing ability. The deprotonated forms 3 a and 3 b can efficiently reduce other NAD(P) models to give the corresponding 1,4-dihydro form, resulting in the deprotonated BNA+ being coordinated to the metal complexes [Ru(tpy)(bpy)(BNA(+)-H+)]2+ (2 a) and [Re(bpy)(CO)3(BNA+-H+)]+ (2 b); "free" BNAH and the protonated adducts 1 a and 1 b cannot act in this way. X-ray crystallography was performed on the PF6- salt of 2 a, and showed that the deprotonated nitrogen atom on the carbamoyl group coordinates to the ruthenium(II) metal center with a bond length of 2.086(3) Angstroms. Infrared spectral data suggested that the deprotonated carbamoyl group on the reduced forms 3 a and 3 b is converted to the imido group, and that the oxygen atom coordinates to the metal center.

Catalytic alcohol oxidation by an unsymmetrical 5-coordinate copper complex: electronic structure and mechanism

Density functional theory reveals the detailed mechanism of alcohol oxidation by a model copper complex, Cu(II)L, L = cis-1-(3',5'-dimethoxy-benzylideneamino)-3,5-[2-hydroxy-(3',5'-di-tert-butyl)benzylideneimino]cyclohexane. Despite the obvious structural and functional parallels between the title compound and the enzyme galactose oxidase, the details of the catalytic pathway are fundamentally different. In the enzyme, coordination of the substrate produces an active form containing a Cu(II) centre and a tyrosyl radical, the latter being responsible for the abstraction of hydrogen from the substrate. In the model system, in marked contrast, the active form contains a Cu(II) centre, but the ligand radical character is localised on the substrate (alcoholate) oxygen, rather than the phenolate ligand. The result is a significantly higher barrier to hydrogen-atom abstraction compared to the enzyme itself. The origin of these significant differences is traced to the rigid nature of the pentadentate ligand, which resists changes in coordination number during the catalytic cycle.

Vibrational spectroscopy to study the properties of redox-active tyrosines in photosystem II and other proteins

Tyrosine radicals play catalytic roles in essential metalloenzymes. Their properties--midpoint potential, stability...--or environment varies considerably from one enzyme to the other. To understand the origin of these properties, the redox tyrosines are studied by a number of spectroscopic techniques, including Fourier transform infrared (FTIR) and resonance Raman (RR) spectroscopy. An increasing number of vibrational data are reported for the (modified-) redox active tyrosines in ribonucleotide reductases, photosystem II, heme catalase and peroxidases, galactose and glyoxal oxidases, and cytochrome oxidase. The spectral markers for the tyrosinyl radicals have been recorded on models of (substituted) phenoxyl radicals, free or coordinated to metals. We review these vibrational data and present the correlations existing between the vibrational modes of the radicals and their properties and interactions formed with their environment: we present that the nu7a(C-O) mode of the radical, observed both by RR and FTIR spectroscopy at 1480-1515 cm(-1), is a sensitive marker of the hydrogen bonding status of (substituted)-phenoxyl and Tyr*, while the nu8a(C-C) mode may probe coordination of the Tyr* to a metal. For photosystem II, the information obtained by light-induced FTIR difference spectroscopy for the two redox tyrosines TyrD and TyrZ and their hydrogen bonding partners is discussed in comparison with those obtained by other spectroscopic methods.

Light-induced geometric isomerization of 1,2-diphenylcyclopropanes included within Y zeolites: role of cation-guest binding

Through a systematic study of several diphenylcyclopropane derivatives, we have inferred that the cations present within a zeolite control the excited-state chemistry of these systems. In the parent 1,2-diphenylcylopropane, the cation binds to the two phenyl rings in a sandwich-type arrangement, and such a mode of binding prevents cis-to-trans isomerization. Once an ester or amide group is introduced into the system (derivatives of 2beta,3beta-diphenylcyclopropane-1alpha-carboxylic acid), the cation binds to the carbonyl group present in these chromophores and such a binding has no influence on the cis-trans isomerization process. Cation-reactant structures computed at density functional theory level have been very valuable in rationalizing the observed photochemical behavior of diphenylcyclopropane derivatives included in zeolites. While the parent system, 1,2-diphenylcylopropane, has been extensively investigated in the context of chiral induction in solution, owing to its failure to isomerize from cis to trans, the same could not be investigated in zeolites. However, esters of 2beta,3beta-diphenylcyclopropane-1alpha-carboxylic acid could be studied within zeolites in the context of chiral induction. Chiral induction as high 20% ee and 55% de has been obtained with selected systems. These numbers, although low, are much higher than what has been obtained in solution with the same system or with the parent system by other investigators (maximum approximately 10% ee).

Significant enhancement of electron transfer reduction of NAD(+) analogues by complexation with scandium ion and the detection of the radical intermediate-scandium ion complex

4-Acetyl-N,N-diisopropyl-1-benzylnicotinamidinium ion (ABNA(+)) and 1-benzyl-4-phenylnicotinamidinium ion (PhBNA(+)) were newly synthesized as NAD(+) analogues to examine the electron-transfer reactivity and the effects of metal ions on the reactivity in comparison with those of 1-benzylnicotinamidinium ion (BNA(+)) and 1-methyl-4-phenylpyridinium ion (MPP(+)) which has no amide or acetyl group. A remarkable positive shift in the one-electron reduction potential of ABNA(+) was observed in the presence of Sc(3+) which forms a 1:1 complex with ABNA(+) through both acetyl and amide groups, whereas no such shift in the presence of Sc(3+) was observed for the one-electron reduction of MPP(+) which has no acetyl or amide group. Similar but less positive shifts in the one-electron reduction potentials were observed in the presence of Sc(3+) for the one-electron reduction of BNA(+) and PhBNA(+) both of which have only one amide group. The rate of electron-transfer reduction of ABNA(+) is enhanced significantly by the complexation with Sc(3+) to produce stable ABNA(*)-Sc(3+) complex which has been successfully detected by ESR. The electron self-exchange rates of the MPP(*)/MPP(+) system have been determined from the ESR line width variation and are compared with those of the ABNA(*)/ABNA(+) system.

Catalytic control of redox reactivities of coenzyme analogs by metal ions

Redox coenzymes and analogs have their own redox reactivities for both thermal and photochemical redox reactions. The redox activities of coenzymes can be tuned by using metal ions that can bind the redox coenzymes and analogs. Quantitative measure to determine the Lewis acidity of a variety of metal ions is given in relation to the catalytic reactivities. The mechanistic viability of metal ion catalysis in redox reactions of coenzyme analogs is described by showing a number of examples of both thermal and photochemical reactions that are made possible to proceed by controlling the redox reactivities of coenzymes with metal ions.