Experimental Evidence for Magnetic Exchange in Di- and Trinuclear Uranium(IV) Ethynylbenzene Complexes

We report the preparation and magnetic property investigations of a structurally related family of mono-, di-, and trinuclear U(IV) aryl acetylide complexes. The reaction between [(NN′3)UCl] and lithiated aryl acetylides leads to the formation of the hexacoordinate complexes [(NN′3)U(CCPh)2(Li·THF)] (1) and [(NN′3)2U2(p-DEB)(THF)] (2) as red-brown and yellow-green crystalline solids, respectively. In contrast, combining the uranacycle [(bit-NN′3)U] (bit-NN′3 = [N(CH2CH2NSitBuMe2)2(CH2CH2SitBuMeCH2]) with stoichiometric amounts of mono-, bis-, and tris(ethynyl) benzenes affords the yellow-green pentacoordinate arylacetylide complexes [(NN′3)U(CCPh)] (3), [(NN′3)2U2(m-DEB)] (4), [(NN′3)2U2(p-DEB)] (5), and [(NN′3)3U3(TEB)] (6), where NN′3 = [N(CH2CH2NSitBuMe2)3]. The measured magnetic susceptibilities for 1−6 trend toward non-magnetic ground states at low temperatures. Nevertheless, the di- and trinuclear pentacoordinate compounds 4−6 appear to display weak magnetic communication between the uranium centers. This communication is modeled by fitting of the direct current (DC) magnetic susceptibility data, using the spin Hamiltonian Ĥ = −2J(Ŝi·Ŝj). These results are consistent with weak ferromagnetic coupling for complexes 4−6 (J = 4.76, 2.75, and 1.11 cm−1, respectively), while the fit for 2 is consistent with a near-negligible exchange interaction (J = −0.05 cm−1). Geometry-optimized Stuttgart/6-31 g* B3LYP hybrid DFT calculations were carried out (spin−orbit coupling omitted) on model complexes of 3−5. The mononuclear complex shows a triplet ground state with singly occupied degenerate f orbitals. The meta- and para-bridged species are computed to show very weak ferro- and antiferromagnetic coupling, respectively. All three complexes show only small net spin density on the acetylide-containing ligands. The monomeric phenylacetylide complex 3 undergoes a reversible redox couple at −1.02 V versus [Cp2Fe]+/0, assignable to an oxidation of U(IV) to U(V).

本工作报道了一系列结构相关的单核、双核及三核芳基乙炔基四价铀(U(IV))配合物的合成与磁性质研究。[(NN′3)UCl]与锂化芳基乙炔基试剂反应，可分别得到红棕色和黄绿色结晶态的六配位配合物[(NN′3)U(CCPh)₂(Li·四氢呋喃(THF))]（1）以及[(NN′3)₂U₂(p-DEB)(THF)]（2）。与此不同，将铀杂环配合物[(bit-NN′3)U]（其中bit-NN′3 = [N(CH₂CH₂NSitBuMe₂)₂(CH₂CH₂SitBuMeCH₂)]）与化学计量比的单、双、三（乙炔基）苯混合，可得到黄绿色五配位芳基乙炔基配合物[(NN′3)U(CCPh)]（3）、[(NN′3)₂U₂(m-DEB)]（4）、[(NN′3)₂U₂(p-DEB)]（5）以及[(NN′3)₃U₃(TEB)]（6），其中NN′3 = [N(CH₂CH₂NSitBuMe₂)₃]。对1~6的磁化率测试结果表明，其在低温下趋近于非磁性基态。然而，双核及三核的五配位配合物4~6在铀中心之间表现出弱磁相互作用。通过采用自旋哈密顿量Ĥ = −2J(Ŝi·Ŝj)对直流(DC)磁化率数据进行拟合，可对该磁相互作用进行建模。拟合结果显示，配合物4~6存在弱铁磁耦合作用，其耦合常数J分别为4.76、2.75和1.11 cm⁻¹；而配合物2的拟合结果则表明其交换相互作用几乎可忽略（J = −0.05 cm⁻¹）。针对配合物3~5的模型体系，开展了斯图加特/6-31 g*基组下的杂化密度泛函理论(DFT) B3LYP几何优化计算（省略自旋轨道耦合）。单核配合物3表现为三重态基态，其简并f轨道上存在单电子占据。计算结果表明，间位桥联和对位桥联的配合物分别表现出极弱的铁磁和反铁磁耦合作用。上述三种配合物的含乙炔基配体上仅存在较小的净自旋密度。单体苯基乙炔基配合物3在相对于二茂铁(Cp₂Fe)+/0的-1.02 V电位下表现出可逆氧化还原电对，该电对可归因为U(IV)被氧化为U(V)。