Fast O₂ Binding at Dicopper Complexes Containing Schiff-Base Dinucleating Ligands

A new family of dicopper(I) complexes [CuI2RL](X)2 (R = H, 1X, R = tBu, 2X and R = NO2, 3X, X = CF3SO3, ClO4, SbF6, or BArF, BArF = [B{3,5-(CF3)2C6H3}4]-), where RL is a Schiff-base ligand containing two tridentate binding sites linked by a xylyl spacer, has been prepared and characterized, and its reaction with O2 has been studied. The complexes were designed with the aim of reproducing structural aspects of the active site of type 3 dicopper proteins; they contain two three-coordinate copper sites and a rather flexible podand ligand backbone. The solid-state structures of 1ClO4, 2CF3SO3, 2ClO4, and 3BArF·CH3CN have been established by single-crystal X-ray diffraction analysis. 1ClO4 adopts a polymeric structure in the solid state while 2CF3SO3, 2ClO4, and 3BArF·CH3CN are monomeric. The complexes have been studied in solution by means of 1H and 19F NMR spectroscopy, which put forward the presence of dynamic processes. 1−3BArF and 1−3CF3SO3 in acetone react rapidly with O2 to generate metaestable [CuIII2(μ-O)2(RL)]2+ 1−3(O2) and [CuIII2(μ-O)2(CF3SO3)(RL)]+ 1−3(O2)(CF3SO3) species, respectively, that have been characterized by UV−vis spectroscopy and resonance Raman analysis. Instead, reaction of 1−3BArF with O2 in CH2Cl2 results in intermolecular O2 binding. DFT methods have been used to study the chemical identities and structural parameters of the O2 adducts, and the relative stability of the CuIII2(μ-O)2 form with respect to the CuII2(μ-η2:η2-O2) isomer. The reaction of 1X, X = CF3SO3 and BArF, with O2 in acetone has been studied by stopped-flow UV-vis exhibiting an unexpected very fast reaction rate (k = 3.82(4) × 103 M-1 s-1, ΔH⧧ = 4.9 ± 0.5 kJ·mol-1, ΔS⧧ = −148 ± 5 J·K-1·mol-1), nearly 3 orders of magnitude faster than in the parent [CuI2(m-XYLMeAN)]2+. Thermal decomposition of 1−3(O2) does not result in aromatic hydroxylation. The mechanism and kinetics of O2 binding to 1X (X = CF3SO3 and BArF) are discussed and compared with those associated with selected examples of reported models of O2-processing copper proteins. A synergistic role of the copper ions in O2 binding and activation is clearly established from this analysis.本研究合成并表征了一类新型二铜(I)配合物[CuI₂RL](X)₂（其中R=H时为1X，R=tBu时为2X，R=NO₂时为3X；X=CF₃SO₃、ClO₄、SbF₆或BArF，其中BArF=[B{3,5-(CF₃)₂C₆H₃}₄]⁻），RL为含两个三齿结合位点、并通过二甲苯间隔基连接的希夫碱配体（Schiff-base ligand），并对其与氧气(O₂)的反应进行了研究。该类配合物的设计目标是复刻3型双铜蛋白活性位点的结构特征，其包含两个三配位铜位点与柔性较强的podand配体骨架。研究人员通过单晶X射线衍射（single-crystal X-ray diffraction）分析确定了1ClO₄、2CF₃SO₃、2ClO₄及3BArF·CH₃CN的固态结构：1ClO₄在固态下呈聚合结构，而2CF₃SO₃、2ClO₄与3BArF·CH₃CN则为单分子结构。通过¹H与¹⁹F核磁共振（NMR）光谱对该类配合物的溶液状态进行表征，结果表明体系中存在动态过程。在丙酮溶剂中，1−3BArF与1−3CF₃SO₃可分别与O₂快速反应，生成亚稳态的[CuIII₂(μ-O)₂(RL)]²⁺（记为1−3(O₂)）与[CuIII₂(μ-O)₂(CF₃SO₃)(RL)]⁺（记为1−3(O₂)(CF₃SO₃)）物种，上述产物已通过紫外-可见（UV−vis）分光光度法与共振拉曼（resonance Raman）分析完成表征。与之相反，1−3BArF在二氯甲烷(CH₂Cl₂)中与O₂的反应则发生分子间氧结合。本研究采用密度泛函理论（DFT）方法，探究了氧加合物的化学本质与结构参数，以及CuIII₂(μ-O)₂构型相较于CuII₂(μ-η²:η²-O₂)异构体的相对稳定性。针对X为CF₃SO₃与BArF的1X配合物在丙酮中与O₂的反应，采用停流紫外-可见（stopped-flow UV-vis）光谱开展研究，结果显示其反应速率极快且出乎意料：k=3.82(4)×10³ M⁻¹·s⁻¹，ΔH⧧=4.9±0.5 kJ·mol⁻¹，ΔS⧧=−148±5 J·K⁻¹·mol⁻¹，反应速率比母体配合物[CuI₂(m-XYLMeAN)]²⁺快了近三个数量级。1−3(O₂)的热分解并不会产生芳香族羟基化产物。本文讨论了氧结合至1X（X=CF₃SO₃与BArF）的反应机理与动力学，并与已报道的若干氧气加工铜蛋白模型体系进行对比。通过本分析可明确得出，铜离子在氧结合与活化过程中存在协同作用。