Small Molecule Activation by Uranium Tris(aryloxides): Experimental and Computational Studies of Binding of N2, Coupling of CO, and Deoxygenation Insertion of CO2 under Ambient Conditions
Small Molecule Activation by Uranium Tris(aryloxides): Experimental and Computational Studies of Binding of N2, Coupling of CO, and Deoxygenation Insertion of CO2 under Ambient Conditions
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DOI：
10.1021/ja2019492
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发表时间：
2011-06-15
影响因子：
15
通讯作者：
Arnold, Polly L.
中科院分区：
文献类型：
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作者：
Mansell, Stephen M.;Kaltsoyannis, Nikolas;Arnold, Polly L.
众所周知的铀三（aryloide） U(ODtbp)(3)， U(OC6H3-Bu-2(t)-2,6)(3)和三叔丁基类似物U(otbp)(3)， U(OC6H2-Bu-3(t)-2,4,6)(3)以桥接的形式呈现，侧对二氮配合物[U(OAr)(3)](2)(mu-eta(2):eta(2)-N-2)，其中三叔丁基N-2配合物是迄今为止分离出的最强大的U-2（N-2）配合物。尝试在N-2下还原三（芳基氧化物）配合物，由于配体重新分配，只得到铀（III）四（芳基氧化物）阴离子K[U(OAr)(4)]的钾盐。固态结构是由每个钾阳离子以eta(6)方式桥接相邻阴离子的两个芳烃形成的聚合链。同样的铀三（芳基氧化物）也被发现在环境条件下与一氧化碳偶联，在[U(OAr)(3)](2)(mu-eta(1):eta(1)- c2o2)(mu-eta（1):eta(1)- c2o2）中只产生炔二酸盐[OCCO]（2-）碘离子，直接类似于最近显示的还原偶联，产生[U{N(SiMe3)(2)}(3)](2)(mu-eta（1):eta(1)- c2o2）。然而，相关的U- iii配合物U{N(SiPhMe2)(2)}(3)和U{CH(siphme3)(2)}(3)在我们的研究中没有显示CO偶联化学。在芳基氧化物配合物中，只有U(OC6H2-Bu-3(t)-2,4,6)(3)与CO2反应生成含有桥接氧和碳酸芳基的插入产物U(otbp)(4)(mu-O)(mu-eta(1):eta(1)-O2COC6H2-Bu-3(t)-2,4,6)(2)，其结构已被表征。[U(OTtbp)(3)](2)（N-2）中配位N-2的存在阻止了与CO2的任何反应的发生，强调了N-2配合物的显著稳定性。二叔丁基羰基氧化物不插入CO2，仅分离到U(ODtbp)(4)。硅酰胺还与二氧化碳反应生成U(OSiMe3)(4)作为唯一的含铀物质。GGA和杂化DFT计算，结合电子密度的拓扑分析，表明U-N-2键是强极性的，并且唯一的共价U- > N-2相互作用是pi回键，导致电子结构的正式描述（U- iv）(2)(N-2)（2-）。N-N拉伸波数是首选的N-2还原到N-N键长度的度量，因为前者的理论和实验之间有很好的一致性，但由于x射线晶体学对r（N-N）的低估，后者的一致性较差。鉴定了CO与[U(OAr)(3)](2)（mu-C2O2）偶联途径上可能存在的中间体，势能表面扫描结果表明，炔二酸酯片段比辅助配体结合更弱，这可能对低温和高压催化CO化学的发展具有指导意义。
Previously unanticipated dinitrogen activation is exhibited by the well-known uranium tris(aryloxide) U(ODtbp)(3), U(OC6H3-Bu-2(t)-2,6)(3), and the tri-tert-butyl analogue U(OTtbp)(3), U(OC6H2-Bu-3(t)-2,4,6)(3), in the form of bridging, side-on dinitrogen complexes [U(OAr)(3)](2)(mu-eta(2):eta(2)-N-2), for which the tri-tert-butyl N-2 complex is the most robust U-2(N-2) complex isolated to date. Attempted reduction of the tris(aryloxide) complex under N-2 gave only the potassium salt of the uranium(III) tetra(aryloxide) anion, K[U(OAr)(4)], as a result of ligand redistribution. The solid-state structure is a polymeric chain formed by each potassium cation bridging two arenes of adjacent anions in an eta(6) fashion. The same uranium tris(aryloxides) were also found to couple carbon monoxide under ambient conditions to give exclusively the ynediolate [OCCO](2-) dianion in [U(OAr)(3)](2)(mu-eta(1):eta(1)-C2O2), in direct analogy with the reductive coupling recently shown to afford [U{N(SiMe3)(2)}(3)](2)(mu-eta(1):eta(1)-C2O2). The related U-III complexes U{N(SiPhMe2)(2)}(3) and U{CH(SiMe3)(2)}(3) however do not show CO coupling chemistry in our hands. Of the aryloxide complexes, only the U(OC6H2-Bu-3(t)-2,4,6)(3) reacts with CO2 to give an insertion product containing bridging oxo and aryl carbonate moieties, U-2(OTtbp)(4)(mu-O)(mu-eta(1):eta(1)-O2COC6H2-Bu-3(t)-2,4,6)(2), which has been structurally characterized. The presence of coordinated N-2 in [U(OTtbp)(3)](2)(N-2) prevents the occurrence of any reaction with CO2, underscoring the remarkable stability of the N-2 complex. The di-tert-butyl aryloxide does not insert CO2, and only U(ODtbp)(4) was isolated. The silylamide also reacts with carbon dioxide to afford U(OSiMe3)(4) as the only uranium-containing material. GGA and hybrid DFT calculations, in conjunction with topological analysis of the electron density, suggest that the U-N-2 bond is strongly polar, and that the only covalent U -> N-2 interaction is pi backbonding, leading to a formal (U-IV)(2)(N-2)(2-) description of the electronic structure. The N-N stretching wavenumber is preferred as a metric of N-2 reduction to the N-N bond length, as there is excellent agreement between theory and experiment for the former but poorer agreement for the latter due to X-ray crystallographic underestimation of r(N-N). Possible intermediates on the CO coupling pathway to [U(OAr)(3)](2)(mu-C2O2) are identified, and potential energy surface scans indicate that the ynediolate fragment is more weakly bound than the ancillary ligands, which may have implications in the development of low-temperature and pressure catalytic CO chemistry.