Mechanistic Insights into Nitrile Activation by Cobalt(III)-Hydroperoxo Intermediates: The Influence of Ligand Basicity

The versatile applications of nitrile have led to the widespread use of nitrile activation in the synthesis of pharmacologically and industrially valuable compounds. We reported the activation of nitriles using mononuclear cobalt(III)-hydroperoxo complexes, [CoIII(Me3-TPADP)(O2H)(RCN)]2+ [R = Me (2) and Ph (2Ph)], to form cobalt(III)-peroxyimidato complexes, [CoIII(Me3-TPADP)(R-C(=NH)O2)]2+ [R = Me (3) and Ph (3Ph)]. The independence of the rate on the nitrile concentration and the positive Hammett value of 3.2(2) indicated that the reactions occur via an intramolecular nucleophilic attack of the hydroperoxide ligand to the coordinated nitrile carbon atom. In contrast, the previously reported cobalt(III)-hydroperoxo complex, [CoIII(TBDAP)(O2H)(CH3CN)]2+ (2TBDAP), exhibited the deficiency of reactivity toward nitrile. The comparison of pKa values and redox potentials of 2 and 2TBDAP showed that Me3-TPADP had a stronger ligand field strength than that of TBDAP. The density functional theory calculations for 2 and 2TBDAP support that the strengthened ligand field in 2 is mainly due to the replacement of two tert-butyl amine donors in TBDAP with methyl groups in Me3-TPADP, resulting in the compression of the Co-Nax bond lengths. These results provide mechanistic evidence of nitrile activation by the cobalt(III)-hydroperoxo complex and indicate that the basicity dependent on the ligand framework contributes to the ability of nitrile activation.

Reaction landscape of a mononuclear MnIII-hydroxo complex with hydrogen peroxide

Peroxomanganese species have been proposed as key intermediates in the catalytic cycles of both manganese enzymes and synthetic catalysts. However, many of these intermediates have yet to be observed. Here, we report the formation of a series of intermediates, each generated from the reaction of the mononuclear MnIII-hydroxo complex [MnIII(OH)(dpaq2Me)]+ with hydrogen peroxide under slightly different conditions. By changing the acidity of the reaction mixture and/or the quantity of hydrogen peroxide added, we are able to control which intermediate forms. Using a combination of electronic absorption, 1H NMR, EPR, and X-ray absorption spectroscopies, as well as density functional theory (DFT) and complete active space self-consistent field (CASSCF) calculations, we formulate these intermediates as the bis(μ-oxo)dimanganese(III,IV) complex [MnIIIMnIV(μ-O)2(dpaq2Me)2]+, the MnIII-hydroperoxo complex [MnIII(OOH)(dpaq2Me)]+, and the MnIII-peroxo complex [MnIII(O2)(dpaq2Me)]. The formation of the MnIII-hydroperoxo species from the reaction of a MnIII-hydroxo complex with hydrogen peroxide mimics an elementary reaction proposed for many synthetic manganese catalysts that activate hydrogen peroxide.

Systematic Electronic Tuning on the Property and Reactivity of Cobalt-(Hydro)peroxo Intermediates

A series of cobalt(III)-peroxo complexes, [Co^III(R₂-TBDAP)(O₂)]⁺ (1^R2; R₂ = Cl, H, and OMe), and cobalt(III)-hydroperoxo complexes, [Co^III(R₂-TBDAP)(O₂H)(CH₃CN)]²⁺ (2^R2), bearing electronically tuned tetraazamacrocyclic ligands (R₂-TBDAP = N,N'-di-tert-butyl-2,11-diaza[3.3](2,6)-p-R₂-pyridinophane) were prepared from their cobalt(II) precursors and characterized by various physicochemical methods. The X-ray diffraction and spectroscopic analyses unambiguously showed that all 1^R2 compounds have similar octahedral geometry with a side-on peroxocobalt(III) moiety, but the O-O bond lengths of 1^Cl [1.398(3) Å] and 1^OMe [1.401(4) Å] were shorter than that of 1^H [1.456(3) Å] due to the different spin states. For 2^R2, the O-O bond vibration energies of 2^Cl and 2^OMe were identical at 853 cm^-1 (856 cm^-1 for 2^H), but their Co-O bond vibration frequencies were observed at 572 cm^-1 for 2^Cl and 550 cm^-1 for 2^OMe, respectively, by resonance Raman spectroscopy (560 cm^-1 for 2^H). Interestingly, the redox potentials (E_1/2) of 2^R2 increased in the order of 2^OMe (0.19 V) < 2^H (0.24 V) < 2^Cl (0.34 V) according to the electron richness of the R₂-TBDAP ligands, but the oxygen-atom-transfer reactivities of 2^R2 showed a reverse trend (k₂: 2^Cl < 2^H < 2^OMe) with a 13-fold rate enhancement at 2^OMe over 2^Cl in a sulfoxidation reaction with thioanisole. Although the reactivity trend contradicts the general consideration that electron-rich metal-oxygen species with low E_1/2 values have sluggish electrophilic reactivity, this could be explained by a weak Co-O bond vibration of 2^OMe in the unusual reaction pathway. These results provide considerable insight into the electronic nature-reactivity relationship of metal-oxygen species.

DFT Mechanistic Insights into Aldehyde Deformylations with Biomimetic Metal-Dioxygen Complexes: Distinct Mechanisms and Reaction Rules

Aldehyde deformylations occurring in organisms are catalyzed by metalloenzymes through metal-dioxygen active cores, attracting great interest to study small-molecule metal-dioxygen complexes for understanding relevant biological processes and developing biomimetic catalysts for aerobic transformations. As the known deformylation mechanisms, including nucleophilic attack, aldehyde α-H-atom abstraction, and aldehyde hydrogen atom abstraction, undergo outer-sphere pathways, we herein report a distinct inner-sphere mechanism based on density functional theory (DFT) mechanistic studies of aldehyde deformylations with a copper (II)-superoxo complex. The inner-sphere mechanism proceeds via a sequence mainly including aldehyde end-on coordination, homolytic aldehyde C-C bond cleavage, and dioxygen O-O bond cleavage, among which the C-C bond cleavage is the rate-determining step with a barrier substantially lower than those of outer-sphere pathways. The aldehyde C-C bond cleavage, enabled through the activation of the dioxygen ligand radical in a second-order nucleophilic substitution (S_N2)-like fashion, leads to an alkyl radical and facilitates the subsequent dioxygen O-O bond cleavage. Furthermore, we deduced the rules for the reactions of metal-dioxygen complexes with aldehydes and nitriles via the inner-sphere mechanism. Expectedly, our proposed inner-sphere mechanisms and the reaction rules offer another perspective to understand relevant biological processes involving metal-dioxygen cores and to discover metal-dioxygen catalysts for aerobic transformations.

Influence of Thiolate versus Alkoxide Ligands on the Stability of Crystallographically Characterized Mn(III)-Alkylperoxo Complexes

The work described herein demonstrates the exquisite control that the inner coordination sphere of metalloenzymes and transition-metal complexes can have on reactivity. We report one of few crystallographically characterized Mn-peroxo complexes and show that the tight correlations between metrical and spectroscopic parameters, established previously by our group for thiolate-ligated RS-Mn(III)-OOR complexes, can be extended to include an alkoxide-ligated RO-Mn(III)-OOR complex. We show that the alkoxide-ligated RO-Mn(III)-OOR complex is an order of magnitude more stable (t_1/2^298 K = 6730 s, k_obs^298 K = 1.03 × 10^-4 s^-1) than its thiolate-ligated RS-Mn(III)-OOR derivative (t_1/2^293 K = 249 s, k₁^293 K = 2.78 × 10^-3 s^-1). Electronic structure calculations provide insight regarding these differences in stability. The highest occupied orbital of the thiolate-ligated derivative possesses significant sulfur character and π-backdonation from the thiolate competes with π-backdonation from the peroxo π*(O-O). DFT-calculated Mulliken charges show that the Mn ion Lewis acidity of alkoxide-ligated RO-Mn(III)-OOR (+0.451) is greater than that of thiolate-ligated RS-Mn(III)-OOR (+0.306), thereby facilitating π-backdonation from the antibonding peroxo π*(O-O) orbital and increasing its stability. This helps to explain why the photosynthetic oxygen-evolving Mn complex, which catalyzes O-O bond formation as opposed to cleavage, incorporates O- and/or N-ligands as opposed to ^cysS-ligands.

Structure and electrochemical properties of (μ-O)2Mn2(iii,iii) and (μ-O)2Mn2(iii,iv) complexes supported by pyridine-, quinoline-, isoquinoline- and quinoxaline-based tetranitrogen ligands

Seven new bis(μ-oxo)dimanganese complexes with Mn₂(iii,iii) or Mn₂(iii,iv) oxidation states were prepared using quinoline- and isoquinoline-based tetraamine ligands. The structures of the ligands include ethylenediamine, trans-1,2-cyclohexanediamine and tripodal amine, bearing two or three nitrogen-containing heteroaromatics. Regardless of the skeleton and number of aliphatic nitrogen atoms in the ligands, quinoline complexes stabilize the Mn₂(iii,iii) oxidation state, whereas, isoquinoline ligands afford Mn₂(iii,iv) complexes. A systematic comparison of the differences in structural parameters and redox potentials of a total of 14 complexes with a (μ-O)₂Mn₂ diamond core, which includes corresponding pyridine and quinoxaline derivatives as supporting ligands, highlights the distinct deviation of quinoline and tripodal amine motifs in this ligand series.

A side-on Mn(III)-peroxo supported by a non-heme pentadentate N3Py2 ligand: synthesis, characterization and reactivity studies

A mononuclear manganese(iii)-peroxo complex [MnIII(N3Py2)(O2)]+ (1a) bearing a non-heme N,N'-dimethyl-N-(2-(methyl(pyridin-2-ylmethyl)amino)ethyl)-N'-(pyridin-2-ylmethyl)ethane-1,2-diamine (N3Py2) ligand was synthesized by the reaction of [Mn(N3Py2)(H2O)](ClO4)2 (1) with hydrogen peroxide and triethylamine in CH3CN at 25 °C. The reactivity of 1a in aldehyde deformylation using 2-phenyl propionaldehyde (2-PPA) was studied and the reaction kinetics was monitored by UV-visible spectroscopy. A kinetic isotope effect (KIE) = 1.7 was obtained in the reaction of 1a with 2-PPA and α-[D1]-PPA, suggesting nucleophilic character of 1a. The activation parameters ΔH‡ and ΔS‡ were determined using the Eyring plot while Ea was obtained from the Arrhenius equation by performing the reaction between 288 and 303 K. Hammett constants (σp) of para-substituted benzaldehydes p-X-Ph-CHO (X = Cl, F, H, and Me) were linear with a slope (ρ) = 3.0. Computational study suggested that the side-on structure of 1a is more favored over the end-on structure and facilitates the reactivity of 1a.

How Metal Ion Lewis Acidity and Steric Properties Influence the Barrier to Dioxygen Binding, Peroxo O-O Bond Cleavage, and Reactivity

Herein we quantitatively investigate how metal ion Lewis acidity and steric properties influence the kinetics and thermodynamics of dioxygen binding versus release from structurally analogous Mn-O₂ complexes, as well as the barrier to Mn peroxo O-O bond cleavage, and the reactivity of Mn oxo intermediates. Previously we demonstrated that the steric and electronic properties of Mn^III-OOR complexes containing N-heterocyclic (N^Ar) ligand scaffolds can have a dramatic influence on alkylperoxo O-O bond lengths and the barrier to alkylperoxo O-O bond cleavage. Herein, we examine the dioxygen reactivity of a new Mn^II complex containing a more electron-rich, less sterically demanding N^Ar ligand scaffold, and compare it with previously reported Mn^II complexes. Dioxygen binding is shown to be reversible with complexes containing the more electron-rich metal ions. The kinetic barrier to O₂ binding and peroxo O-O bond cleavage is shown to correlate with redox potentials, as well as the steric properties of the supporting N^Ar ligands. The reaction landscape for the dioxygen chemistry of the more electron-rich complexes is shown to be relatively flat. A total of four intermediates, including a superoxo and peroxo species, are observed with the most electron-rich complex. Two new intermediates are shown to form following the peroxo, which are capable of cleaving strong X-H bonds. In the absence of a sacrificial H atom donor, solvent, or ligand, serves as a source of H atoms. With TEMPOH as sacrificial H atom donor, a deuterium isotope effect is observed (k_H/k_D = 3.5), implicating a hydrogen atom transfer (HAT) mechanism. With 1,4-cyclohexadiene, 0.5 equiv of benzene is produced prior to the formation of an EPR detected Mn^IIIMn^IV bimetallic species, and 0.5 equiv after its formation.

MnIII-Peroxo adduct supported by a new tetradentate ligand shows acid-sensitive aldehyde deformylation reactivity

The new tetradentate L7BQ ligand (L7BQ = 1,4-di(quinoline-8-yl)-1,4-diazepane) has been synthesized and shown to support MnII and MnIII-peroxo complexes. X-ray crystallography of the [MnII(L7BQ)(OTf)2] complex shows a monomeric MnII center with the L7BQ ligand providing four donor nitrogen atoms in the equatorial field, with two triflate ions bound in the axial positions. When this species is treated with H2O2 and Et3N at -40 °C, a MnIII-peroxo adduct, [MnIII(O2)(L7BQ)]+ is formed. The formation of this new intermediate is supported by a variety of spectroscopic techniques, including electronic absorption, Mn K-edge X-ray absorption and electron paramagnetic resonance methods. Evaluation of extended X-ray absorption fine structure data for [MnIII(O2)(L7BQ)]+ resolved Mn-O bond distances of 1.85 Å, which are on the short end of those previously reported for crystallographically characterized MnIII-peroxo adducts. An analysis of the X-ray pre-edge region of [MnIII(O2)(L7BQ)]+ revealed a large pre-edge area of 20.8 units. Time-dependent density functional theory computations indicate that the pre-edge intensity is due to Mn 4p-3d mixing caused by geometric distortions from centrosymmetry induced by both the peroxo and L7BQ ligands. The reactivity of [MnIII(O2)(L7BQ)]+ towards aldehydes was assessed through reaction with cyclohexanecarboxaldehyde and 2-phenylpropionaldehyde. From these experiments, it was determined that [MnIII(O2)(L7BQ)]+ only reacts with aldehydes in the presence of acid. Specifically, the addition of cyclohexanecarboxylic acid to [MnIII(O2)(L7BQ)]+ converts the MnIII-peroxo adduct to a new intermediate that could be responsible for the observed aldehyde deformylation activity. These observations underscore the challenges in identifying the reactive metal species in aldehyde deformylation reactions.

Spectroscopic and Computational Investigation of Low-Spin Mn(III) Bis(scorpionate) Complexes

Six-coordinate MnIII complexes are typically high-spin (S = 2), however, the scorpionate ligand, both in its traditional, hydridotris(pyrazolyl)borate form, Tp- and Tp*- (the latter with 3,5-dimethylpyrazole substituents) and in an aryltris(carbene)borate (i.e., N-heterocyclic carbene, NHC) form, [Ph(MeIm)3B]-, (MeIm = 3-methylimidazole) lead to formation of bis(scorpionate) complexes of MnIII with spin triplet ground states; three of which were investigated herein: [Tp2Mn]SbF6 (1SBF6), [Tp*2Mn]SbF6 (2SBF6), and [{Ph(MeIm)3B}2Mn]CF3SO3 (3CF3SO3). These trigonally symmetric complexes were studied experimentally by magnetic circular dichroism (MCD) spectroscopy (the propensity of 3 to oxidize to MnIV precluded collection of useful MCD data) including variable temperatures and fields (VTVH-MCD) and computationally by ab initio CASSCF/NEVPT2 methods. These combined experimental and theoretical techniques establish the 3A2g electronic ground state for the three complexes, and provide information on the energy of the "conventional" high-spin excited state (5Eg) and other, triplet excited states. These results show the electronic effect of pyrazole ring substituents in comparing 1 and 2. The tunability of the scorpionate ligand, even by perhaps the simplest change (from pyrazole in 1 to 3,5-dimethylpyrazole in 2) is quantitatively manifested through perturbations in ligand-field excited-state energies that impact ground-state zero-field splittings. The comparison with the NHC donor is much more dramatic. In 3, the stronger σ-donor properties of the NHC lead to a quantitatively different electronic structure, so that the lowest lying spin triplet excited state, 3Eg, is much closer in energy to the ground state than in 1 or 2. The zero-field splitting (zfs) parameters of the three complexes were calculated and in the case of 1 and 2 compare closely to experiment (lower by < 10%, < 2 cm-1 in absolute terms); for 3 the large magnitude zfs is reproduced, although there is ambiguity about its sign. The comprehensive picture obtained for these bis(scorpionate) MnIII complexes provides quantitative insight into the role played by the scorpionate ligand in stabilizing unusual electronic structures.

Synthesis, characterization, and reactivity of a side-on manganese(iii)–peroxo complex bearing a pentadentate aminopyridine ligand

A peroxomanganese(iii) species was prepared by the reaction of [Mn(ii)(Pro3Py)(OTf)](OTf) with H₂O₂/NEt₃ and characterized by UV-vis, EPR, ESI-MS and DFT, which exhibited nucleophilic reactivity in aldehyde deformylation.

Geometric and electronic structure of a peroxomanganese(III) complex supported by a scorpionate ligand

A monomeric Mn(II) complex has been prepared with the facially-coordinating Tp(Ph2) ligand, (Tp(Ph2) = hydrotris(3,5-diphenylpyrazol-1-yl)borate). The X-ray crystal structure shows three coordinating solvent molecules resulting in a six-coordinate complex with Mn-ligand bond lengths that are consistent with a high-spin Mn(II) ion. Treatment of this Mn(II) complex with excess KO2 at room temperature resulted in the formation of a Mn(III)-O2 complex that is stable for several days at ambient conditions, allowing for the determination of the X-ray crystal structure of this intermediate. The electronic structure of this peroxomanganese(III) adduct was examined by using electronic absorption, electron paramagnetic resonance (EPR), low-temperature magnetic circular dichroism (MCD), and variable-temperature variable-field (VTVH) MCD spectroscopies. Density functional theory (DFT), time-dependent (TD)-DFT, and multireference ab initio CASSCF/NEVPT2 calculations were used to assign the electronic transitions and further investigate the electronic structure of the peroxomanganese(III) species. The lowest ligand-field transition in the electronic absorption spectrum of the Mn(III)-O2 complex exhibits a blue shift in energy compared to other previously characterized peroxomanganese(III) complexes that results from a large axial bond elongation, reducing the metal-ligand covalency and stabilizing the σ-antibonding Mn dz(2) MO that is the donor MO for this transition.

Manganese(ii) complexes of tetradentate 4N ligands with diazepane backbones for catalytic olefin epoxidation: effect of nucleophilicity of peroxo complexes on reactivity

Mononuclear Mn(ii) complexes of linear 4N ligands have been studied as catalysts for deformylation of aldehydes and epoxidation (H₂O₂) of unfunctionalised olefins to understand the role of ligand stereoelectronic factors.

Reaction landscape of a pentadentate N5-ligated Mn(II) complex with O2˙- and H2O2 includes conversion of a peroxomanganese(III) adduct to a bis(μ-oxo)dimanganese(III,IV) species

Herein we describe the chemical reactivity of the mononuclear [Mn(II)(N4py)(OTf)](OTf) (1) complex with hydrogen peroxide and superoxide. Treatment of 1 with one equivalent superoxide at -40 °C in MeCN formed the peroxomanganese(III) adduct, [Mn(III)(O2)(N4py)](+) (2) in ~30% yield. Complex 2 decayed over time and the formation of the bis(μ-oxo)dimanganese(III,IV) complex, [Mn(III)Mn(IV)(μ-O)2(N4py)2](3+) (3) was observed. When 2 was formed in higher yields (~60%) using excess superoxide, the [Mn(III)(O2)(N4py)](+) species thermally decayed to Mn(II) species and 3 was formed in no greater than 10% yield. Treatment of [Mn(III)(O2)(N4py)](+) with 1 resulted in the formation of 3 in ~90% yield, relative to the concentration of [Mn(III)(O2)(N4py)](+). This reaction mimics the observed chemistry of Mn-ribonucleotide reductase, as it features the conversion of two Mn(II) species to an oxo-bridged Mn(III)Mn(IV) compound using O2(-) as oxidant. Complex 3 was independently prepared through treatment of 1 with H2O2 and base at -40 °C. The geometric and electronic structures of 3 were probed using electronic absorption, electron paramagnetic resonance (EPR), magnetic circular dichroism (MCD), variable-temperature, variable-field MCD (VTVH-MCD), and X-ray absorption (XAS) spectroscopies. Complex 3 was structurally characterized by X-ray diffraction (XRD), which revealed the N4py ligand bound in an unusual tetradentate fashion.

Characterization of metastable intermediates formed in the reaction between a Mn(II) complex and dioxygen, including a crystallographic structure of a binuclear Mn(III)-peroxo species

Transition-metal peroxos have been implicated as key intermediates in a variety of critical biological processes involving O2. Because of their highly reactive nature, very few metal-peroxos have been characterized. The dioxygen chemistry of manganese remains largely unexplored despite the proposed involvement of a Mn-peroxo, either as a precursor to, or derived from, O2, in both photosynthetic H2O oxidation and DNA biosynthesis. These are arguably two of the most fundamental processes of life. Neither of these biological intermediates has been observed. Herein we describe the dioxygen chemistry of coordinatively unsaturated [Mn(II)(S(Me2)N4(6-Me-DPEN))] (+) (1), and the characterization of intermediates formed en route to a binuclear mono-oxo-bridged Mn(III) product {[Mn(III)(S(Me2)N4(6-Me-DPEN)]2(μ-O)}(2+) (2), the oxo atom of which is derived from (18)O2. At low-temperatures, a dioxygen intermediate, [Mn(S(Me2)N4(6-Me-DPEN))(O2)](+) (4), is observed (by stopped-flow) to rapidly and irreversibly form in this reaction (k1(-10 °C) = 3780 ± 180 M(-1) s(-1), ΔH1(++) = 26.4 ± 1.7 kJ mol(-1), ΔS1(++) = -75.6 ± 6.8 J mol(-1) K(-1)) and then convert more slowly (k2(-10 °C) = 417 ± 3.2 M(-1) s(-1), ΔH2(++) = 47.1 ± 1.4 kJ mol(-1), ΔS2(++) = -15.0 ± 5.7 J mol(-1) K(-1)) to a species 3 with isotopically sensitive stretches at νO-O(Δ(18)O) = 819(47) cm(-1), kO-O = 3.02 mdyn/Å, and νMn-O(Δ(18)O) = 611(25) cm(-1) consistent with a peroxo. Intermediate 3 releases approximately 0.5 equiv of H2O2 per Mn ion upon protonation, and the rate of conversion of 4 to 3 is dependent on [Mn(II)] concentration, consistent with a binuclear Mn(O2(2-)) Mn peroxo. This was verified by X-ray crystallography, where the peroxo of {[Mn(III)(S(Me2)N4(6-Me-DPEN)]2(trans-μ-1,2-O2)}(2+) (3) is shown to be bridging between two Mn(III) ions in an end-on trans-μ-1,2-fashion. This represents the first characterized example of a binuclear Mn(III)-peroxo, and a rare case in which more than one intermediate is observed en route to a binuclear μ-oxo-bridged product derived from O2. Vibrational and metrical parameters for binuclear Mn-peroxo 3 are compared with those of related binuclear Fe- and Cu-peroxo compounds.

Correlation between structural, spectroscopic, and reactivity properties within a series of structurally analogous metastable manganese(III)-alkylperoxo complexes

Manganese-peroxos are proposed as key intermediates in a number of important biochemical and synthetic transformations. Our understanding of the structural, spectroscopic, and reactivity properties of these metastable species is limited, however, and correlations between these properties have yet to be established experimentally. Herein we report the crystallographic structures of a series of structurally related metastable Mn(III)-OOR compounds, and examine their spectroscopic and reactivity properties. The four reported Mn(III)-OOR compounds extend the number of known end-on Mn(III)-(η(1)-peroxos) to six. The ligand backbone is shown to alter the metal-ligand distances and modulate the electronic properties key to bonding and activation of the peroxo. The mechanism of thermal decay of these metastable species is examined via variable-temperature kinetics. Strong correlations between structural (O-O and Mn···N(py,quin) distances), spectroscopic (E(πv*(O-O) → Mn CT band), ν(O-O)), and kinetic (ΔH(‡) and ΔS(‡)) parameters for these complexes provide compelling evidence for rate-limiting O-O bond cleavage. Products identified in the final reaction mixtures of Mn(III)-OOR decay are consistent with homolytic O-O bond scission. The N-heterocyclic amines and ligand backbone (Et vs Pr) are found to modulate structural and reactivity properties, and O-O bond activation is shown, both experimentally and theoretically, to track with metal ion Lewis acidity. The peroxo O-O bond is shown to gradually become more activated as the N-heterocyclic amines move closer to the metal ion causing a decrease in π-donation from the peroxo πv*(O-O) orbital. The reported work represents one of very few examples of experimentally verified relationships between structure and function.