Carbon Dioxide Activation with Sterically Pressured Mid- and High-Valent Uranium Complexes

Sterically pressured mid- to high-valent uranium complexes with an aryloxide substituted triazacyclononane ligand scaffold, [((RArO)3tacn)3–], were studied for carbon dioxide activation and transformation chemistry. The high valent uranium(V) imido species [((RArO)3tacn)U(NR)] (R = tBu, R′ = 2,4,6-trimethylphenyl (2-tBu); R = Ad, R′ = 2,4,6-trimethylphenyl (2-Ad); R = tBu, R′ = phenyl (3-tBu)) were synthesized and spectroscopically characterized. X-ray crystallography of the tert-butyl mesityl imido derivative, 2-tBu, reveals coordination of a bent imido fragment with a relatively long U−N bond distance of 2.05 Å. The mesityl imido complexes reacted with carbon dioxide, readily extruding free isocyanate to produce uranium(V) terminal oxo species, [((RArO)3tacn)U(O)] (R = tBu (4-tBu), Ad (4-Ad)), presumably through multiple bond metathesis via a uranium(V) carbimate intermediate. Using the smaller phenyl imido fragment in 3-tBu slowed isocyanate loss, allowing the uranium(V) carbimate intermediate to undergo a second metathesis reaction, ultimately producing the diphenyl ureate derivative, [((tBuArO)3tacn)U(NPh2)CO] (5-tBu). Single crystal X-ray diffraction studies were carried out on both uranium(V) terminal oxo complexes and revealed short U−O bonds (1.85 Å) indicative of a formal UO triple bond. The electronic structure of the oxo U(V) complexes was investigated by electronic absorption and EPR spectroscopies as well as SQUID magnetization and DFT studies, which indicated that their electronic properties are highly unusual. To obtain insight into the reactivity of CO2 with U−N bonds, the reaction of the uranium(IV) amide species, [((RArO)3tacn)U(NHMes)] (R = tBu (6-tBu), Ad (6-Ad) with carbon dioxide was investigated. These reactions produced the uranium(IV) carbamate complexes, [((RArO)3tacn)U(CO2NHMes)] (R = tBu (7-tBu), Ad (7-Ad)), resulting from insertion of carbon dioxide into U−N(amide) bonds. The molecular structures of the synthesized uranium carbamate complexes highlight the different reactivities due to the steric pressure introduced by the alkyl derivatized tris(aryloxide) triazacyclononane ligand. The sterically open tert-butyl derivative creates a monodentate η1-O bound carbamate species, while the sterically more bulky adamantyl-substituted compound forces a bidentate κ2-O,O coordination mode of the carbamate ligand.

本研究以空间受迫型芳氧基取代三氮杂环壬烷（aryloxide substituted triazacyclononane, tacn）配体骨架[((RArO)₃tacn)]³⁻构建的中高价铀配合物为研究对象，开展二氧化碳活化与转化化学相关研究。研究合成了系列高价铀(V)亚胺基配合物[((RArO)₃tacn)U(NR')]，其中R为叔丁基（tBu）、R'为2,4,6-三甲基苯基时对应物种记为2-tBu；R为金刚烷基（Ad）、R'为2,4,6-三甲基苯基时对应物种记为2-Ad；R为叔丁基（tBu）、R'为苯基时对应物种记为3-tBu，并通过光谱手段对上述配合物进行了表征。对叔丁基均三甲苯基亚胺基衍生物2-tBu的X射线晶体衍射分析显示，其亚胺基片段呈弯曲配位模式，U−N键长达2.05 Å。该类均三甲苯基亚胺基配合物可与二氧化碳反应，经铀(V)氨基甲酸酯中间体的多键复分解过程，快速释放游离异氰酸酯，生成铀(V)端基氧代配合物[((RArO)₃tacn)U(O)]，其中R为叔丁基（tBu）时对应物种记为4-tBu，R为金刚烷基（Ad）时对应物种记为4-Ad。当使用体积更小的苯基亚胺基片段（3-tBu）时，异氰酸酯的消除过程被延缓，使得铀(V)氨基甲酸酯中间体可发生第二次复分解反应，最终生成二苯基脲酸酯衍生物[((ᵗᴮᵘArO)₃tacn)U(NPh₂)CO]（5-tBu）。本研究对两种铀(V)端基氧代配合物开展了单晶X射线衍射研究，结果显示其U−O键长仅为1.85 Å，表明存在形式上的U≡O三键。通过电子吸收光谱、电子顺磁共振（electron paramagnetic resonance, EPR）、超导量子干涉器件（superconducting quantum interference device, SQUID）磁化率测量以及密度泛函理论（density functional theory, DFT）计算，对该类氧代铀(V)配合物的电子结构进行了探究，结果表明其电子性质极为特殊。为深入解析CO₂与U−N键的反应活性，本研究还考察了铀(IV)酰胺类物种[((RArO)₃tacn)U(NHMes)]（其中NHMes为(2,4,6-三甲基苯基)氨基配体，R为叔丁基（tBu）时对应物种记为6-tBu，R为金刚烷基（Ad）时对应物种记为6-Ad）与二氧化碳的反应。此类反应通过二氧化碳插入U−N(酰胺)键，生成铀(IV)氨基甲酸酯配合物[((RArO)₃tacn)U(CO₂NHMes)]，其中R为叔丁基（tBu）时对应物种记为7-tBu，R为金刚烷基（Ad）时对应物种记为7-Ad。所合成的铀氨基甲酸酯配合物的分子结构表明，烷基取代的三(芳氧基)三氮杂环壬烷配体所引入的空间位阻差异，会导致配合物呈现不同的反应活性：空间位阻较小的叔丁基衍生物会形成单齿η¹-O配位的氨基甲酸酯物种，而空间位阻更大的金刚烷基取代配合物则会迫使氨基甲酸酯配体采取双齿κ²-O,O配位模式。