Structure Activity Relationships for Reversible O2 Chemisorption by the Solid Phases of Co(salen) and Co(3F-salen)

The potential of solid-state materials comprising Co(salen) units for concentrating dioxygen from air was recognized over 80 years ago. While the chemisorptive mechanism at the molecular level is largely understood, the bulk crystalline phase plays important, yet unidentified roles. We have reverse crystal-engineered these materials and can for the first time describe the nanostructuring requisite for achieving reversible O₂ chemisorption by Co(3R-salen) R = H or F, the simplest and most effective of the many known derivatives of Co(salen). Of the six phases of Co(salen) identified, α-ζ: α = ESACIO, β = VEXLIU, γ, δ, ε, and ζ (this work), only γ, δ, ε, and ζ are capable of reversible O₂ binding. Class I materials (phases γ, δ, and ε) are obtained by desorption (40-80 °C, atmospheric pressure) of the co-crystallized solvent from Co(salen)·(solv), solv = CHCl₃, CH₂Cl₂, or 1.5 C₆H₆. The oxy forms comprise between 1:5 and 1:3 O₂:[Co] stoichiometries. Class II materials achieve an apparent maximum of 1:2 O₂:Co(salen) stoichiometries. The precursors for the Class II materials comprise [Co(3R-salen)(L)·(H₂O)_x], R = H, L = pyridine, and x = 0; R = F, L = H₂O, and x = 0; R = F, L = pyridine, and x = 0; R = F, L = piperidine, and x = 1. Activation of these depends on the desorption of the apical ligand (L) that templates channels through the crystalline compounds with the Co(3R-salen) molecules interlocked in a Flemish bond brick pattern. The 3F-salen system produces F-lined channels proposed to facilitate O₂ transport through the materials through repulsive interactions with the guest O₂. We postulate that a moisture dependence of the activity of the Co(3F-salen) series is due to a highly specific binding pocket for locking in water via bifurcated hydrogen bonding to the two coordinated phenolato O atoms and the two ortho F atoms.

Calcium-Ion Binding Mediates the Reversible Interconversion of Cis and Trans Peroxido Dicopper Cores

Coupled dinuclear copper oxygen cores (Cu₂ O₂ ) featured in type III copper proteins (hemocyanin, tyrosinase, catechol oxidase) are vital for O₂ transport and substrate oxidation in many organisms. μ-1,2-cis peroxido dicopper cores (^C P) have been proposed as key structures in the early stages of O₂ binding in these proteins; their reversible isomerization to other Cu₂ O₂ cores are directly relevant to enzyme function. Despite the relevance of such species to type III copper proteins and the broader interest in the properties and reactivity of bimetallic ^C P cores in biological and synthetic systems, the properties and reactivity of ^C P Cu₂ O₂ species remain largely unexplored. Herein, we report the reversible interconversion of μ-1,2-trans peroxido (^T P) and ^C P dicopper cores. Ca^II mediates this process by reversible binding at the Cu₂ O₂ core, highlighting the unique capability for metal-ion binding events to stabilize novel reactive fragments and control O₂ activation in biomimetic systems.

Tuning Oxygen Reduction Catalysis of Dinuclear Cobalt Polypyridyl Complexes by the Bridging Structure

The four-electron oxygen reduction reaction (4e^--ORR) is the mainstay in chemical energy conversion. Elucidation of factors influencing the catalyst's reaction rate and selectivity is important in the development of more active catalysts of 4e^--ORR. In this study, we investigated chemical and electrochemical 4e^--ORR catalyzed by Co₂(μ-O₂) complexes bridged by xanthene (1) and anthracene (3) and by a Co₂(OH)₂ complex bridged by anthraquinone (2). In the chemical ORR using Fe(CpMe)₂ as a reductant in acidic PhCN, we found that 1 showed the highest initial turnover frequency (TOF_init = 6.8 × 10² s^-1) and selectivity for 4e^--ORR (96%) in three complexes. The detailed kinetic analyses have revealed that the rate-determining steps (RDSs) in the catalytic cycles of 1-3 have the O₂ addition to [Co^II₂(OH₂)₂]⁴⁺ as an intermediate in common. In the only case that complex 1 was used as a catalyst, k_cat depended on proton concentration because the reaction rate of the O₂ addition to [Co^II₂(OH₂)₂]⁴⁺ was so fast as compared to that of the concerted PCET process of 1. Through X-ray, Raman, and electrochemical analyses and stoichiometric reactions, we found the face-to-face structure of 1 characterized by a slightly flexible xanthene was advantageous in capturing O₂ and stabilizing the Co₂(μ-O₂) structure, thus increasing both the reaction rate and selectivity for 4e^--ORR.

"Criss-crossed" dinucleating behavior of an N4 Schiff base ligand: formation of a μ-OH,μ-O2 dicobalt(III) core via O2 activation

We report the synthesis and structural characterization of a dicobalt(III) complex with a μ-OH,μ-O2 core, namely μ-OH,μ-O2-[Co(enN4)]2(X)3 [1(ClO4)3 and 1(BF4)3]. The dinuclear core is cross-linked by two N4 Schiff base ligands that span each cobalt center. The formally Co(III)-Co(III) dimer is formed spontaneously upon exposure of the mononuclear Co(II) complex to air and exhibits a ν(O-O) value at 882 cm(-1) that shifts to 833 cm(-1) upon substitution with (18)O2. The CV of 1(BF4)3 exhibits a reversible {Co(III)-Co(III)}↔{Co(III)-Co(IV)} redox process, and we have investigated the oxidized {Co(III)-Co(IV)} species by EPR spectroscopy (g = 2.02, 2.06; S = 1/2 signal) and DFT calculations.

A New Class of Complexes Possessing Cofacially-Oriented, Planar, Metal-Containing Subunits. Synthesis, Characterization, and Reactivity of [(MoO(2))(2)(&mgr;-O)](2+)-Linked, Catechol-Functionalized, Tetraazamacrocyclic and Salicylideneamine Complexes

A new synthetic route to molecules that contain cofacially oriented, [Mo(2)O(5)](2+)-bridged bis(catecholate) dianions is described. This synthesis has been useful in the preparation of supermolecular molecules containing catechol-functionalized, metalated macrocyclic [M(II)(TAD(OH)(2))] (M = Co, Ni) and SALPHEN [M(II)(R(2)R'(2)SALPHEN(OH)(2))] (M = H(2), Mn, Fe, Co, Ni, Cu) ligands. Of the former, the (Bu(4)N)(2)[Mo(2)O(5)[Ni(TAD(O)(2))](2)] (7) complex has been structurally characterized. The complex crystallizes in the triclinic space group P&onemacr; with unit cell dimensions a = 12.324(3) Å, b = 17.740(4) Å, c = 20.920(4) Å, alpha = 108.79(3) degrees, beta = 98.20(3) degrees, and gamma = 103.12(3) degrees. The nearly-planar macrocyclic ligands are essentially parallel, with a dihedral angle of 6.5(2) degrees. The Ni(1)--Ni(2) separation in the anion is 3.938 Å. The structure of (Bu(4)N)(2)[Mo(2)O(5)[Cu(EtO(2)H(2)SALPHEN(O)(2))](2)] (19) has also been determined. This complex crystallizes in the monoclinic space group P2(1)/c, with unit cell dimensions a = 20.821(4) Å, b = 23.133(5) Å, c = 20.056(4) Å, and beta = 117.71(3) degrees. The Cu(1)--Cu(2) separation is 4.110 Å, and the dihedral angle between SALPHEN "planes" is approximately 9.7(1) degrees. Analytical and spectroscopic properties are provided. Reactions of these molecules with oxidants and strongly coordinating ligands are presented. The ability of the Fe(II) and Co(II) analogues of 19 to bind ligands such as O(2)(-) or S(2)(-) and O(2), respectively, in the "pocket" of the complex is described, and the products have been characterized. The synthesis and characterization of the unique "mixed catecholate" complexes (Bu(4)N)(2)[Mo(2)O(5)(D(t)()BC)(M(II)((t)()Bu(4)SALPHEN(O)(2)))] (M = H(2), Fe, Co, Ni, Cu) is described, and comparisons between these latter systems and the bis(M(II)SALPHEN-catecholate) complexes are provided.