Daniel Stack

All Publications

• Imidazolate-Stabilized Cu(III): Dioxygen to Oxides at Type 3 Copper Sites. Angewandte Chemie (International ed. in English) Large, T. A., Keown, W., Gary, J. B., Chiang, L., Stack, T. D. 2024: e202416967

  Abstract

  Imidazole ligation of metals through histidine is extensive among metalloproteins and enzymes, yet the role of the imidazolate conjugate base is often neglected, despite its potential accessibility when bonded to a highly oxidized metal center. Using synthetic models of oxygenated tyrosinase enzymes with exclusive monodentate imidazole ligation, we find that deprotonation of the μ2-η2:η2-peroxidodicopper(II) species triggers redox isomerization to an imidazolate-ligated bis(μ2-oxido)dicopper(III) species. Formal two-electron oxidation to Cu(III) remains unprecedented in biological systems, yet is effected readily by addition of base in these model systems. Spectrophotometric titrations by UV/visible/near-IR and copper K-edge X-ray absorption spectroscopies are interpreted most simply as two cooperative, 2H+ transformations in which the peroxide O-O is cleaved in the first step. Elaboration from simple imidazoles to a protected histidine extends this isomerization into an amino acid environment. The role of phenolate as a base suggests this four-electron reduction of O2 is energetically viable in a biological context and requires only two copper centers, which act as two-electron shuttles when imidazole deprotonation assists. This existential precedent of viable imidazolate intermediates invites speculation into an alternative mechanism for phenol hydroxylation not previously considered at Type 3 copper sites such a tyrosinases.

  View details for DOI 10.1002/anie.202416967

  View details for PubMedID 39420537

• Fast and Versatile Functionalization of Glassy Carbon. Langmuir : the ACS journal of surfaces and colloids Ainsworth, J., Cook, T. C., Stack, T. D. 2022

  Abstract

  A rapid procedure for the functionalization of glassy carbon surfaces (GCSs) is disclosed. A three-step sequence of bromomethylation, azide displacement, and copper(I)-catalyzed azide-alkyne cycloaddition (CuAAC) allows ethynylated molecules to be attached covalently to the carbon surface through a methylene functional group. Redox-active ethynyl ferrocene and [RuII(Cl)(DMSO)(ethynyl-TPA)]1+ (DMSO = dimethylsulfoxide; TPA = tris(2-pyridylmethyl)amine) are attached with high coverages as assessed by cyclic voltammetry, and the elemental composition of the surface is confirmed by X-ray photoelectron spectroscopy. In less than 1 h, surface coverages of 1 × 1014 molecules/cm2 are possible that exhibit good durability in both acidic and basic media. Attached [RuII(Cl)(DMSO)(ethynyl-TPA)]1+ catalytically oxidizes alcohols, yet the currents and potentials are less impressive compared to an attachment without the intervening methylene group. The advantages of this covalent attachment procedure for GCSs are its short reaction times, mild reaction conditions, and the use of standard laboratory reagents and glassware, allowing for many types of ethynylated molecules to be attached rapidly to the surface.

  View details for DOI 10.1021/acs.langmuir.2c01964

  View details for PubMedID 36326209

• Phenolate-bonded bis(μ-oxido)-bis-copper(III) intermediates: hydroxylation and dehalogenation reactivities. Faraday discussions Kang, P., Lin, B. L., Large, T. A., Ainsworth, J., Wasinger, E. C., Stack, T. D. 2022

  Abstract

  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.

  View details for DOI 10.1039/d1fd00071c

  View details for PubMedID 35156114

• Electrocatalytic alcohol oxidation by covalently immobilized ruthenium complex on carbon. Journal of inorganic biochemistry Cook, T. C., Stenehjem, E. D., Ainsworth, J., Stack, T. D. 2022; 231: 111784

  Abstract

  A dearth of discrete immobilized metal complexes exist that electrocatalytically oxidize methanol. Reported here is the covalent immobilization of a tris(2-pyridylmethyl)amine ruthenium complex [RuII(Cl)(DMSO)(ethynyl-TPA)]+ (ethynyl-TPA = (5-ethynyl-2-pyridylmethyl)bis(2-pyridylmethyl)amine) to a glassy carbon (GC) electrode through a CuI catalyzed azide-alkyne cycloaddition (click) reaction between the ethynyl-TPA ligand and an azide derivatized carbon surface forming [RuII(Cl)(DMSO)(GC-click-TPA)]+. Following water substitution for DMSO and proton coupled electron transfer, [RuIV(O)(Cl)(GC-click-TPA)]+ electrooxidizes alcohols, including methanol, efficiently relative to other immobilized metal complexes. A primary kinetic isotope effect suggests rate-limiting Cα-H bond cleavage of benzyl alcohol. Approximately 40% of the [RuII(Cl)(DMSO)(GC-click-TPA)]+ undergoes the DMSO for water exchange to form an active oxidant, consistent with the 40% distribution of the more labile Cl-cis-amine isomer before immobilization. Using the benchmark of benzyl alcohol electrocatalytic oxidation, [RuIV(O)(Cl)(GC-click-TPA)]+ operates at ca. 250 mV lower overpotential, with a 15% increase in faradaic efficiency, and at least an order of magnitude increase in average turnover frequency (0.7 s-1 TOFavg) compared to the previously best immobilized discrete ruthenium complexes.

  View details for DOI 10.1016/j.jinorgbio.2022.111784

  View details for PubMedID 35298933

• Selective Oxidation of Exogenous Substrates by a Bis-Cu(III) Bis-Oxide Complex: Mechanism and Scope. Inorganica chimica acta Large, T. A., Mahadevan, V., Keown, W., Stack, T. D. 2019; 486: 782-792

  Abstract

  Cu(III)2(μ-O)2 bis-oxides (O) form spontaneously by direct oxygenation of nitrogen-chelated Cu(I) species and constitute a diverse class of versatile 2e-/2H+ oxidants, but while these species have attracted attention as biomimetic models for dinuclear Cu enzymes, reactivity is typically limited to intramolecular ligand oxidation, and systems exhibiting synthetically useful reactivity with exogenous substrates are limited. O tmpd (TMPD = N 1 , N 1 , N 3 , N 3 -tetramethylpropane-1,3-diamine) presents an exception, readily oxidizing a diverse array of exogenous substrates, including primary alcohols and amines selectively over their secondary counterparts in good yields. Mechanistic and DFT analyses suggest substrate oxidation proceeds through initial axial coordination, followed by rate limiting rotation to position the substrate in the Cu(III) equatorial plane, whereupon rapid deprotonation and oxidation by net hydride transfer occurs. Together, the results suggest the selectivity and broad substrate scope unique to O tmpd are best attributed to the combination of ligand flexibility, limited steric demands, and ligand oxidative stability. In keeping with the absence of rate limiting C-H scission, O tmpd exhibits a marked insensitivity to the strength of the substrate Cα-H bond, readily oxidizing benzyl alcohol and 1 octanol at near identical rates.

  View details for DOI 10.1016/j.ica.2018.11.027

  View details for PubMedID 31485082

  View details for PubMedCentralID PMC6724545

• Exclusive imidazole ligation to CuO2 and CuIIICuO2 cores. Chemical communications (Cambridge, England) Keown, W. n., Large, T. A., Chiang, L. n., Wasinger, E. C., Stack, T. D. 2019

  Abstract

  We disclose herein the synthesis and characterization of L2Cu(iii)2O2 and L3Cu(iii)Cu(ii)2O2 complexes with nitrogen ligation exclusively from imidazoles for the first time. Their accessibility by direct oxygenation of a L-Cu(i) precursor and the resulting Cu(iii) formation inform on the kinetic accessibility and thermodynamic superiority of imidazole in stabilizing Cu(iii).

  View details for DOI 10.1039/c9cc02982f

  View details for PubMedID 31173011

• Electronic Structure and Reactivity Studies of a Nonsymmetric One-Electron Oxidized CuII Bis-phenoxide Complex. Inorganica chimica acta Chiang, L., Wasinger, E. C., Shimazaki, Y., Young, V., Storr, T., Stack, T. D. 2018; 481: 151-158

  Abstract

  The tetradentate mixed imino/amino phenoxide ligand (N-(3,5-di-tert-butylsalicylidene)-N'-(2-hydroxyl-3,5-di-tert-butylbenzyl))-trans-1,2-cyclohexanediamine (salalen) was complexed with CuII, and the resulting Cu complex (2) was characterized by a number of experimental techniques and theoretical calculations. Two quasi-reversible redox processes for 2, as observed by cyclic voltammetry, demonstrated the potential stability of oxidized forms, and also the increased electron-donating ability of the salalen ligand in comparison to the salen analogue. The electronic structure of the one-electron oxidized [2]+ was then studied in detail, and Cu K-edge X-ray Absorption Spectroscopy (XAS) measurements confirmed a CuII-phenoxyl radical complex in solution. Subsequent resonance Raman (rR) and variable temperature 1H NMR studies, coupled with theoretical calculations, showed that [2• ]+ is a triplet (S = 1) CuII-phenoxyl radical species, with localization of the radical on the more electron-rich aminophenoxide. Attempted isolation of X-ray quality crystals of [2• ]+ afforded [2H]+, with a protonated phenol bonded to CuII, and an additional H-bonding interaction with the SbF6- counterion. Stoichiometric reaction of dilute solutions of [2• ]+ with benzyl alcohol showed that the complex reacted in a similar manner as the oxidized CuII-salen analogue, and does not exhibit a substrate-binding pre-equilibrium as observed for the oxidized bisaminophenoxide CuII-salan derivative.

  View details for DOI 10.1016/j.ica.2017.09.042

  View details for PubMedID 30581226

  View details for PubMedCentralID PMC6301013

• Simplest Monodentate Imidazole Stabilization of the oxy-Tyrosinase Cu2 O2 Core: Phenolate Hydroxylation through a Cu(III) Intermediate. Angewandte Chemie (International ed. in English) Chiang, L., Keown, W., Citek, C., Wasinger, E. C., Stack, T. D. 2016; 55 (35): 10453-10457

  Abstract

  Tyrosinases are ubiquitous binuclear copper enzymes that oxygenate to Cu(II) 2 O2 cores bonded by three histidine Nτ-imidazoles per Cu center. Synthetic monodentate imidazole-bonded Cu(II) 2 O2 species self-assemble in a near quantitative manner at -125 °C, but Nπ-ligation has been required. Herein, we disclose the syntheses and reactivity of three Nτ-imidazole bonded Cu(II) 2 O2 species at solution temperatures of -145 °C, which was achieved using a eutectic mixture of THF and 2-MeTHF. The addition of anionic phenolates affords a Cu(III) 2 O2 species, where the bonded phenolates hydroxylate to catecholates in high yields. Similar Cu(III) 2 O2 intermediates are not observed using Nπ-bonded Cu(II) 2 O2 species, hinting that Nτ-imidazole ligation, conserved in all characterized Ty, has functional advantage beyond active-site flexibility. Substrate accessibility to the oxygenated Cu2 O2 core and stabilization of a high oxidation state of the copper centers are suggested from these minimalistic models.

  View details for DOI 10.1002/anie.201605159

  View details for PubMedID 27440390

• Direct Copper(III) Formation from O2 and Copper(I) with Histamine Ligation. Journal of the American Chemical Society Gary, J. B., Citek, C., Brown, T. A., Zare, R. N., Wasinger, E. C., Stack, T. D. 2016; 138 (31): 9986-9995

  Abstract

  Histamine chelation of copper(I) by a terminal histidine residue in copper hydroxylating enzymes activates dioxygen to form unknown oxidants, generally assumed as copper(II) species. The direct formation of copper(III)-containing products from the oxygenation of histamine-ligated copper(I) complexes is demonstrated here, indicating that copper(III) is a viable oxidation state in such products from both kinetic and thermodynamic perspectives. At low temperatures, both trinuclear Cu(II)2Cu(III)O2 and dinuclear Cu(III)2O2 predominate, with the distribution dependent on the histamine ligand structure and oxygenation conditions. Kinetics studies suggest the bifurcation point to these two products is an unobserved peroxide-level dimer intermediate. The hydrogen atom reactivity difference between the trinuclear and binuclear complexes at parity of histamine ligand is striking. This behavior is best attributed to the accessibility of the bridging oxide ligands to exogenous substrates rather than a difference in oxidizing abilities of the clusters.

  View details for DOI 10.1021/jacs.6b05538

  View details for PubMedID 27467215

• Manganese(II)/Picolinic Acid Catalyst System for Epoxidation of Olefins ORGANIC LETTERS Moretti, R. A., Du Bois, J., Stack, T. D. 2016; 18 (11): 2528-2531

  Abstract

  An in situ generated catalyst system based on Mn(CF3SO3)2, picolinic acid, and peracetic acid converts an extensive scope of olefins to their epoxides at 0 °C in <5 min, with remarkable oxidant efficiency and no evidence of radical behavior. Competition experiments indicate an electrophilic active oxidant, proposed to be a high-valent Mn = O species. Ligand exploration suggests a general ligand sphere motif contributes to effective oxidation. The method is underscored by its simplicity and use of inexpensive reagents to quickly access high value-added products.

  View details for DOI 10.1021/acs.orglett.6b00518

  View details for Web of Science ID 000377319000003

  View details for PubMedID 27191036

• Metal complex assembly controlled by surface ligand distribution on mesoporous silica: Quantification using refractive index matching and impact on catalysis JOURNAL OF CATALYSIS Smith, B. J., Gallegos, P. A., Butsch, K., Stack, T. D. 2016; 335: 197-203

  View details for DOI 10.1016/j.jcat.2015.11.008

  View details for Web of Science ID 000371098200019

• Formation of hybrid guanidine-stabilized bis(μ-oxo)dicopper cores in solution: Electronic and steric perturbations. European journal of inorganic chemistry Herres-Pawlis, S., Haase, R., Verma, P., Hoffmann, A., Kang, P., Stack, T. D. 2015; 2015 (32): 5426-5436

  Abstract

  A series of new hybrid peralkylated-amine-guanidine ligands based on a 1,3-propanediamine backbone and their Cu-O2 chemistry is reported. The copper(I) complexes react readily with O2 at low temperatures in aprotic solvents with weakly coordinating anions to form exclusively bis(μ-oxo) dicopper species (O). Variation of the substituents on each side of the hybrid bidentate ligand highlights that less sterically demanding amine and guanidine substituents increase not only the thermal stability of the formed O cores but enhance inner-sphere phenolate hydroxylation pathways. TD-DFT analysis on selected guanidine-amine O species suggest that the additional visible feature observed is a guanidine π*→ Cu2O2 LMCT, which appears along with the classic oxo-ζu*→Cu(III) and πζ*→ Cu(III) LMCT transitions.

  View details for DOI 10.1002/ejic.201500884

  View details for PubMedID 27990098

  View details for PubMedCentralID PMC5158105

• Sequential "click'' functionalization of mesoporous titania for energy-relay dye enhanced dye-sensitized solar cells PHYSICAL CHEMISTRY CHEMICAL PHYSICS Unger, E. L., Fretz, S. J., Lim, B., Margulis, G. Y., McGehee, M. D., Stack, T. D. 2015; 17 (9): 6565-6571

  Abstract

  Energy relay dyes (ERDs) have been investigated previously as a mean to achieve panchromatic spectral response in dye-sensitized solar cells via energy transfer. To reduced the distance between the ERDs and energy-accepting injection dyes (IDs) on the surface of a mesoporous titanium dioxide electrode, the ERDs were immobilized adjacent to the IDs via a sequential functionalization approach. In the first step, azidobenzoic acid molecules were co-adsorbed on the mesoporous titanium dioxide surface with the ID. In the second step, the highly selective copper(I)-catalyzed 1,3-dipolar azide-alkyne cycloaddition "click" reaction was employed to couple an alkyne-functionalized ERD to the azidobenzoic acid monolayer. The cycloaddition step in the mesoporous electrode was slowed dramatically due to reactants and catalysts forming agglomerates. In solar cell devices, the close proximity between the surface-immobilized ERD and energy-accepting squaraine sensitizer dyes results in energy transfer efficiencies of up to 91%. The relative improvement in device performance due to the additional ERD spectral response was 124%, which is among the highest reported. The sequential functionalization approach described herein is transferrable to other applications requiring the functionalization of electrodes with complex molecules.

  View details for DOI 10.1039/c4cp04878d

  View details for Web of Science ID 000351435300045

  View details for PubMedID 25662612

• Primary amine stabilization of a dicopper(III) bis(μ-oxo) species: modeling the ligation in pMMO. Journal of the American Chemical Society Citek, C., Lin, B. L., Phelps, T. E., Wasinger, E. C., Stack, T. D. 2014; 136 (41): 14405-8

  Abstract

  Here we report the formation of the first examples of dicopper(III) bis(μ-oxo) complexes ligated by the primary amines, propylenediamine, and N,N,-dimethyl propylenediamine. Stabilization of these new compounds is effected at -125 °C by "core capture"- introduction of exogenous ligand to a preformed dicopper(III) bis(μ-oxo) complex supported by the peralkylated tetramethyl propylenediamine. Primary amine ligation in these compounds matches the single primary amine coordination of the putative active site of particulate methane monooxygenase (pMMO) and polysaccharide monooxygenase. Reactivity studies presented here show primary amine ligated cores are competent oxidants, capable of activating C-H bonds by an H-atom abstraction mechanism. Trends in spectroscopy, structure, and reactivity provide hints to the potential role of primary amine ligation in pMMO: increased substrate accessibility to the redox active orbitals of the Cu2O2 core and greater stabilization of the oxidant without attenuation of oxidizing power.

  View details for DOI 10.1021/ja508630d

  View details for PubMedID 25268334

• L-Edge X-ray Absorption Spectroscopy and DFT Calculations on Cu2O2 Species: Direct Electrophilic Aromatic Attack by Side-on Peroxo Bridged Dicopper(II) Complexes JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Qayyum, M. F., Sarangi, R., Fujisawa, K., Stack, T. D., Karlin, K. D., Hodgson, K. O., Hedman, B., Solomon, E. I. 2013; 135 (46): 17417-17431

  Abstract

  The hydroxylation of aromatic substrates catalyzed by coupled binuclear copper enzymes has been observed with side-on-peroxo-dicopper(II) (P) and bis-μ-oxo-dicopper(III) (O) model complexes. The substrate-bound-O intermediate in [Cu(II)2(DBED)2(O)2](2+) (DBED = N,N'-di-tert-butyl-ethylenediamine) was shown to perform aromatic hydroxylation. For the [Cu(II)2(NO2-XYL)(O2)](2+) complex, only a P species was spectroscopically observed. However, it was not clear whether this O-O bond cleaves to proceed through an O-type structure along the reaction coordinate for hydroxylation of the aromatic xylyl linker. Accurate evaluation of these reaction coordinates requires reasonable quantitative descriptions of the electronic structures of the P and O species. We have performed Cu L-edge XAS on two well-characterized P and O species to experimentally quantify the Cu 3d character in their ground state wave functions. The lower per-hole Cu character (40 ± 6%) corresponding to higher covalency in the O species compared to the P species (52 ± 4%) reflects a stronger bonding interaction of the bis-μ-oxo core with the Cu(III) centers. DFT calculations show that 10-20% Hartree-Fock (HF) mixing for P and ~38% for O species are required to reproduce the Cu-O bonding; for the P species this HF mixing is also required for an antiferromagnetically coupled description of the two Cu(II) centers. B3LYP (with 20% HF) was, therefore, used to calculate the hydroxylation reaction coordinate of P in [Cu(II)2(NO2-XYL)(O2)](2+). These experimentally calibrated calculations indicate that the electrophilic attack on the aromatic ring does not involve formation of a Cu(III)2(O(2-))2 species. Rather, there is direct electron donation from the aromatic ring into the peroxo σ* orbital of the Cu(II)2(O2(2-)) species, leading to concerted C-O bond formation with O-O bond cleavage. Thus, species P is capable of direct hydroxylation of aromatic substrates without the intermediacy of an O-type species.

  View details for DOI 10.1021/ja4078717

  View details for Web of Science ID 000327413300032

  View details for PubMedID 24102191

  View details for PubMedCentralID PMC3891796

• Squish and CuAAC: Additive-Free Covalent Mono layers of Discrete Molecules in Seconds LANGMUIR Pellow, M. A., Stack, T. D., Chidsey, C. E. 2013; 29 (18): 5383-5387

  Abstract

  A terminal alkyne is immobilized rapidly into a full monolayer by squishing a small volume of a solution of the alkyne between an azide-modified surface and a copper plate. The monolayer is covalently attached to the surface through a copper-catalyzed alkyne-azide cycloaddition (CuAAC) reaction, and the coverages of the immobilized electroactive alkyne species are quantified by cyclic voltammetry. A reaction time of less than 20 s is possible with no other reagents required. The procedure is effective under aerobic conditions using either an aqueous or aprotic organic solution of the alkyne (1-100 mM).

  View details for DOI 10.1021/la400172w

  View details for Web of Science ID 000318756200001

  View details for PubMedID 23551032

  View details for PubMedCentralID PMC3683963

• Gas-Phase Azide Functionalization of Carbon JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Stenehjem, E. D., Ziatdinov, V. R., Stack, T. D., Chidsey, C. E. 2013; 135 (3): 1110-1116

  Abstract

  Tailoring the surface and interfacial properties of inexpensive and abundant carbon materials plays an increasingly important role for innovative applications including those in electrocatalysis, energy storage, gas separations, and composite materials. Described here is the novel preparation and subsequent use of gaseous iodine azide for the azide modification of carbon surfaces. In-line generation of gaseous iodine azide from iodine monochloride vapor and solid sodium azide is safe and convenient. Immediate treatment of carbon surfaces with this gaseous stream of iodine azide provides a highly reproducible, selective, and scalable azide functionalization that minimizes waste and reduces deleterious side reactions. Among the possible uses of azide-modified surfaces, they serve as versatile substrates for the attachment of additional functionality by coupling with terminal alkynes under the mild copper-catalyzed azide-alkyne cycloaddition (CuAAC) "click" reaction. For instance, coupling ethynylferrocene to azide-modified glassy carbon surfaces achieves ferrocene coverage up to 8 × 10(13) molecules/cm(2) by voltammetric and XPS analyses. The 1,2,3-triazole linker formed during the CuAAC reaction is robust and hydrolytically stable in both aqueous 1 M HClO(4) and 1 M NaOH for at least 12 h at 100 °C.

  View details for DOI 10.1021/ja310410d

  View details for Web of Science ID 000314141200027

  View details for PubMedID 23301920

• Recent advances in phenoxyl radical complexes of salen-type ligands as mixed-valent galactose oxidase models. Coordination chemistry reviews Lyons, C. T., Stack, T. D. 2013; 257 (2): 528-540

  Abstract

  The interplay between redox-active transition metal ions and redox-active ligands in metalloenzyme sites is an area of considerable research interest. Galactose oxidase (GO) is the archetypical example, catalyzing the aerobic oxidation of primary alcohols to aldehydes via two one-electron cofactors: a copper atom and a cysteine-modified tyrosine residue. The electronic structure of the oxidized form of the enzyme (GO(ox)) has been investigated extensively through small molecule analogues including metal-salen phenoxyl radical complexes. Similar to GO(ox), one-electron oxidized metal-salen complexes are mixed-valent species, in which molecular orbitals (MOs) with predominantly phenolate and phenoxyl π-character act as redox-active centers bridged by mixing with metal d-orbitals. A detailed evaluation of the electronic distribution in these odd electron species using a variety of spectroscopic, electrochemical, and theoretical techniques has led to keen insights into the electronic structure of GO(ox).

  View details for DOI 10.1016/j.ccr.2012.06.003

  View details for PubMedID 23264696

  View details for PubMedCentralID PMC3524984

• Catalytic Phenol Hydroxylation with Dioxygen: Extension of the Tyrosinase Mechanism beyond the Protein Matrix ANGEWANDTE CHEMIE-INTERNATIONAL EDITION Hoffmann, A., Citek, C., Binder, S., Goos, A., Ruebhausen, M., Troeppner, O., Ivanovic-Burmazovic, I., Wasinger, E. C., Stack, T. D., Herres-Pawlis, S. 2013; 52 (20): 5398-5401

  View details for DOI 10.1002/anie.201301249

  View details for Web of Science ID 000318370200039

  View details for PubMedID 23609983

• Self-assembly of the oxy-tyrosinase core and the fundamental components of phenolic hydroxylation NATURE CHEMISTRY Citek, C., Lyons, C. T., Wasinger, E. C., Stack, T. D. 2012; 4 (4): 317-322

  Abstract

  The enzyme tyrosinase contains two Cu(I) centres, trigonally coordinated by imidazole nitrogens of six conserved histidine residues. The enzyme activates O(2) to form a µ-η(2):η(2)-peroxo-dicopper(II) core, which hydroxylates tyrosine to a catechol in the first committed step of melanin biosynthesis. Here, we report a family of synthetic peroxo complexes, with spectroscopic and chemical features consistent with those of oxygenated tyrosinase, formed through the self-assembly of monodentate imidazole ligands, Cu(I) and O(2) at -125 °C. An extensively studied complex reproduces the enzymatic electrophilic oxidation of exogenous phenolic substrates to catechols in good stoichiometric yields. The self-assembly and subsequent reactivity support the intrinsic stability of the Cu(2)O(2) core with imidazole ligation, in the absence of a polypeptide framework, and the innate capacity to effect hydroxylation of phenolic substrates. These observations suggest that a foundational role of the protein matrix is to facilitate expression of properties native to the core by bearing the entropic costs of assembly and precluding undesired oxidative degradation pathways.

  View details for DOI 10.1038/nchem.1284

  View details for Web of Science ID 000301983400019

  View details for PubMedID 22437718

• Deposition of Dense Siloxane Monolayers from Water and Trimethoxyorganosilane Vapor LANGMUIR Lowe, R. D., Pellow, M. A., Stack, T. D., Chidsey, C. E. 2011; 27 (16): 9928-9935

  Abstract

  A convenient, laboratory-scale method for the vapor deposition of dense siloxane monolayers onto oxide substrates was demonstrated. This method was studied and optimized at 110 °C under reduced pressure with the vapor of tetradecyltris(deuteromethoxy)silane, (CD(3)O)(3)Si(CH(2))(13)CH(3), and water from the dehydration of MgSO(4)·7H(2)O. Ellipsometric thicknesses, water contact angles, Fourier transform infrared (FTIR) spectroscopy, and electrochemical capacitance measurements were used to probe monolayer densification. The CD(3) stretching mode in the FTIR spectrum was monitored as a function of the deposition time and amounts of silane and water reactants. This method probed the unhydrolyzed methoxy groups on adsorbed silanes. Excess silane and water were necessary to achieve dense, completely hydrolyzed monolayers. In the presence of sufficient silane, an excess of water above the calculated stoichiometric amount was necessary to hydrolyze all methoxy groups and achieve dense monolayers. The excess water was partially attributed to the reversibility of the hydrolysis of the methoxy groups.

  View details for DOI 10.1021/la201333y

  View details for Web of Science ID 000293662800036

  View details for PubMedID 21721567

• Unexpected C(carbene)-X (X: I, Br, Cl) Reductive Elimination From N-Heterocyclic Carbene Copper Halide Complexes Under Oxidative Conditions. Organometallics Lin, B. L., Kang, P., Stack, T. D. 2010; 29 (17): 3683-3685

  Abstract

  The non-innocent behaviors of NHC ligands have attracted wide attention due to their important implications for catalyst designs and reaction mechanisms. Herein, we report facile C(carbene)-halogen reductive eliminations from NHC copper halide complexes at RT under oxidative conditions. Density functional calculations on a simplified model system suggest that the reactions occur through oxidation of Cu(I) to Cu(III) species followed by C(carbene)-halogen reductive eliminations from NHC Cu(III) halide complexes. Remarkably short C(carbene)-chloride contacts and rare interactions between the chloride lone pair electrons and the C(carbene)p(π) orbital were found for the calculated NHC Cu(III) chlorides. The facile C(carbene)-X reductive elimination reported here warrants consideration as a potential decomposition pathway in reactions involving NHC-supported high-valent metal complexes, especially with late transition metals.

  View details for DOI 10.1021/om1005726

  View details for PubMedID 21116469

  View details for PubMedCentralID PMC2992331

• Detailed Evaluation of the Geometric and Electronic Structures of One-Electron Oxidized Group 10 (Ni, Pd, and Pt) Metal(II)-(Disalicylidene)diamine Complexes INORGANIC CHEMISTRY Shimazaki, Y., Stack, T. P., Storr, T. 2009; 48 (17): 8383–92

  Abstract

  The geometric and electronic structures of a series of one-electron oxidized group 10 metal salens (Ni, Pd, Pt) have been investigated in solution and in the solid state. Ni (1) and Pd (2) complexes of the tetradentate salen ligand N,N'-bis(3,5-di-tert-butylsalicylidene)-1,2-cyclohexanediamine (H(2)Salcn) have been examined along with the Pt (3) complex of the salen ligand N,N'-bis(3,5-di-tert-butylsalicylidene)-1,2-ethylenediamine (H(2)Salen). All three oxidized compounds exist as ligand radical species in solution and in the solid state. The solid state structures of [1](+) and [3](+) exhibit a symmetric coordination sphere contraction relative to the neutral forms. By contrast, the coordination sphere of the Pd derivative [2](+) exhibits a pronounced asymmetry in the solid state. In solution, the oxidized derivatives display intense low-energy NIR transitions consistent with their classification as ligand radical compounds. Interestingly, the degree of communication between the phenolate moieties depends strongly on the central metal ion, within the Ni, Pd, and Pt series. Electrochemical measurements and UV-vis-NIR spectroscopy, in conjunction with density functional theory calculations provide insights into the degree of delocalization of the one-electron hole in these systems. The Pd complex [2](+) is the least delocalized and is best described as a borderline Class II/III intervalence complex based on the Robin-Day classification system. The Ni [1](+) and Pt [3](+) analogues are Class III (fully delocalized) intervalence compounds. Delocalization is dependent on the electronic coupling between the redox-active phenolate ligands, mediated by overlap between the formally filled metal d(xz) orbital and the appropriate ligand molecular orbital. The degree of coupling increases in the order Pd < Ni < Pt for the one-electron oxidized group 10 metal salens.

  View details for DOI 10.1021/ic901003q

  View details for Web of Science ID 000269313500046

  View details for PubMedID 19639970

  View details for PubMedCentralID PMC2778000

• Tyrosinase reactivity in a model complex: An alternative hydroxylation mechanism SCIENCE Mirica, L. M., Vance, M., Rudd, D. J., Hedman, B., Hodgson, K. O., Solomon, E. I., Stack, T. D. 2005; 308 (5730): 1890-1892

  Abstract

  The binuclear copper enzyme tyrosinase activates O2 to form a mu-eta2:eta2-peroxodicopper(II) complex, which oxidizes phenols to catechols. Here, a synthetic mu-eta2:eta2-peroxodicopper(II) complex, with an absorption spectrum similar to that of the enzymatic active oxidant, is reported to rapidly hydroxylate phenolates at -80 degrees C. Upon phenolate addition at extreme temperature in solution (-120 degrees C), a reactive intermediate consistent with a bis-mu-oxodicopper(III)-phenolate complex, with the O-O bond fully cleaved, is observed experimentally. The subsequent hydroxylation step has the hallmarks of an electrophilic aromatic substitution mechanism, similar to tyrosinase. Overall, the evidence for sequential O-O bond cleavage and C-O bond formation in this synthetic complex suggests an alternative intimate mechanism to the concerted or late stage O-O bond scission generally accepted for the phenol hydroxylation reaction performed by tyrosinase.

  View details for DOI 10.1126/science.1112081

  View details for Web of Science ID 000230120000034

  View details for PubMedID 15976297

• Structure and spectroscopy of copper-dioxygen complexes CHEMICAL REVIEWS Mirica, L. M., Ottenwaelder, X., Stack, T. D. 2004; 104 (2): 1013-1045

  View details for DOI 10.1021/cr020632z

  View details for Web of Science ID 000188934400022

  View details for PubMedID 14871148

• Efficient epoxidation of electron-deficient olefins with a cationic manganese complex JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Murphy, A., Dubois, G., Stack, T. D. 2003; 125 (18): 5250-5251

  Abstract

  The complex [MnII(R,R-mcp)(CF3SO3)2] is an efficient and practical catalyst for the epoxidation of electron-deficient olefins. This catalyst is capable of epoxidizing olefins with as little as 0.1 mol % catalyst in under 5 min using 1.2 equiv of peracetic acid as the terminal oxidant. A wide scope of substrates are epoxidized including terminal, tertiary, cis and trans internal, enones, and methacrylates with >85% isolated yields.

  View details for DOI 10.1021/ja0299962r

  View details for Web of Science ID 000182682700002

  View details for PubMedID 12720417

• C-H bond activation by a ferric methoxide complex: Modeling the rate-determining step in the mechanism of lipoxygenase JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Goldsmith, C. R., Jonas, R. T., Stack, T. D. 2002; 124 (1): 83-96

  Abstract

  Lipoxygenases are mononuclear non-heme iron enzymes that regio- and stereospecifcally convert 1,4-pentadiene subunit-containing fatty acids into alkyl peroxides. The rate-determining step is generally accepted to be hydrogen atom abstraction from the pentadiene subunit of the substrate by an active ferric hydroxide species to give a ferrous water species and an organic radical. Reported here are the synthesis and characterization of a ferric model complex, [Fe(III)(PY5)(OMe)](OTf)(2), that reacts with organic substrates in a manner similar to the proposed enzymatic mechanism. The ligand PY5 (2,6-bis(bis(2-pyridyl)methoxymethane)pyridine) was developed to simulate the histidine-dominated coordination sphere of mammalian lipoxygenases. The overall monoanionic coordination provided by the endogenous ligands of lipoxygenase confers a strong Lewis acidic character to the active ferric site with an accordingly positive reduction potential. Incorporation of ferrous iron into PY5 and subsequent oxidation yields a stable ferric methoxide species that structurally and chemically resembles the proposed enzymatic ferric hydroxide species. Reactivity with a number of hydrocarbons possessing weak C-H bonds, including a derivative of the enzymatic substrate linoleic acid, scales best with the substrates' bond dissociation energies, rather than pK(a)'s, suggesting a hydrogen atom abstraction mechanism. Thermodynamic analysis of [Fe(III)(PY5)(OMe)](OTf)(2) and the ferrous end-product [Fe(II)(PY5)(MeOH)](OTf)(2) estimates the strength of the O-H bond in the metal bound methanol in the latter to be 83.5 +/- 2.0 kcal mol(-1). The attenuation of this bond relative to free methanol is largely due to the high reduction potential of the ferric site, suggesting that the analogously high reduction potential of the ferric site in LO is what allows the enzyme to perform its unique oxidation chemistry. Comparison of [Fe(III)(PY5)(OMe)](OTf)(2) to other coordination complexes capable of hydrogen atom abstraction shows that, although a strong correlation exists between the thermodynamic driving force of reaction and the rate of reaction, other factors appear to further modulate the reactivity.

  View details for DOI 10.1021/ja016451g

  View details for Web of Science ID 000173217900022

  View details for PubMedID 11772065

• Aryl C-H activation by Cu-II to form an organometallic Aryl-Cu-III species: A novel twist on copper disproportionation ANGEWANDTE CHEMIE-INTERNATIONAL EDITION Ribas, X., JACKSON, D. A., Donnadieu, B., Mahia, J., Parella, T., Xifra, R., Hedman, B., Hodgson, K. O., Llobet, A., Stack, T. D. 2002; 41 (16): 2991-2994

  View details for Web of Science ID 000177597700023

  View details for PubMedID 12203435

• Stereospecificity and self-selectivity in the generation of a chiral molecular tetrahedron by metal-assisted self-assembly ANGEWANDTE CHEMIE-INTERNATIONAL EDITION Enemark, E. J., Stack, T. D. 1998; 37 (7): 932-935

  View details for Web of Science ID 000073339000013

• Ligand Self-Recognition in the Self-Assembly of a [{Cu(L)}2 ]2+ Complex: The Role of Chirality. Angewandte Chemie (International ed. in English) Masood, M. A., Enemark, E. J., Stack, T. D. 1998; 37 (7): 928-932

  Abstract

  The chirality alone of a conformationally restricted, bifunctional ligand (L) is the basis for the self-recognition process schematically represented below. A racemic mixture of these ligands reacts with Cu+ ions quantitatively to generate a racemic mixture of a [(CuL)2 ]2+ homochiral complex (represented by cubes), where each complex contains ligands with identical configurations.

  View details for DOI 10.1002/(SICI)1521-3773(19980420)37:7<928::AID-ANIE928>3.0.CO;2-T

  View details for PubMedID 29711475

• Stereospecificity and Self-Selectivity in the Generation of a Chiral Molecular Tetrahedron by Metal-Assisted Self-Assembly. Angewandte Chemie (International ed. in English) Enemark, E. J., Stack, T. D. 1998; 37 (7): 932-935

  Abstract

  The chiral bidentate ligand (S,S)-1 reacts stereospecifically with Ga3+ to generate a [Ga4 (L)6 ]12- molecular tetrahedron although similar ligands generate [Ga2 (L)3 ]6- complexes. The assembly of this complex is self-selective as a racemic mixture of the ligand sorts by chirality to generate an enantiomeric pair of homochiral complexes.

  View details for DOI 10.1002/(SICI)1521-3773(19980420)37:7<932::AID-ANIE932>3.0.CO;2-C

  View details for PubMedID 29711465

• Catalytic galactose oxidase models: Biomimetic Cu(II)-phenoxyl-radical reactivity SCIENCE Wang, Y. D., Dubois, J. L., Hedman, B., Hodgson, K. O., Stack, T. D. 1998; 279 (5350): 537-540

  Abstract

  Biomimetic functional models of the mononuclear copper enzyme galactose oxidase are presented that catalytically oxidize benzylic and allylic alcohols to aldehydes with O2 under mild conditions. The mechanistic fidelity between the models and the natural system is pronounced. Modest structural mimicry proves sufficient to transfer an unusual ligand-based radical mechanism, previously unprecedented outside the protein matrix, to a simple chemical system.

  View details for Web of Science ID 000071616000038

  View details for PubMedID 9438841

• Irreversible reduction of dioxygen by simple peralkylated diamine-copper(I) complexes: Characterization and thermal stability of a [Cu-2(mu-O)(2)](2+) core JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Mahadevan, V., Hou, Z. G., Cole, A. P., Root, D. E., Lal, T. K., SOLOMON, E. I., Stack, T. D. 1997; 119 (49): 11996-11997

  View details for Web of Science ID A1997YK70800036

• C-H bond activation by a ferric methoxide complex: A model for the rate-determining step in the mechanism of lipoxygenase JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Jonas, R. T., Stack, T. D. 1997; 119 (36): 8566-8567

  View details for Web of Science ID A1997XV70000022

• Trinuclear intermediate in the copper-mediated reduction of O-2: Four electrons from three coppers SCIENCE Cole, A. P., Root, D. E., Mukherjee, P., SOLOMON, E. I., Stack, T. D. 1996; 273 (5283): 1848-1850

  Abstract

  The reaction of metal complexes with dioxygen (O2) generally proceeds in 1:1, 21, or 41 (metal:O2) stoichiometry. A discrete, structurally characterized 31 product is presented. This mixed-valence trinuclear copper cluster, which contains copper in the highly oxidized trivalent oxidation state, exhibits O2 bond scission and intriguing structural, spectroscopic, and redox properties. The relevance of this synthetic complex to the reduction of O2 at the trinuclear active sites of multicopper oxidases is discussed.

  View details for Web of Science ID A1996VJ71300041

  View details for PubMedID 8791587