School of Chemistry, University of Nottingham, University Park, Nottingham NG7 2RD, UK.
Nat Commun. 2013;4:2323. doi: 10.1038/ncomms3323.
Despite the abundance of f-block-cyclopentadienyl, arene, cycloheptatrienyl and cyclo-octatetraenide complexes, cyclobutadienyl derivatives are unknown in spite of their prevalence in the d-block. Here we report that reductive [2+2]-cycloaddition reactions of diphenylacetylene and (2,2-dimethylpropylidyne)phosphine with uranium(V)-inverted sandwich 10π-toluene tetra-anion complexes results in the isolation of inverted sandwich cyclobutadienyl and diphosphacyclobutadienyl dianion uranium(IV) complexes. Computational analysis suggests that the bonding is predominantly electrostatic. Although the ψ4 molecular orbital in the cyclobutadienyl and diphosphacyclobutadienyl ligands exhibits the correct symmetry for δ-bonding to uranium, the dominant covalent contributions arise from π-bonding involving ψ2 and ψ3 orbital combinations. This contrasts with uranium complexes of larger arenes and cyclo-octatetraenide, where δ-bonding dominates. This suggests that the angular requirements for uranium to bond to a small four-membered ring favours π-bonding, utilizing 5f- instead of 6d-orbitals, over δ-bonding that is favoured with larger ligands, where 6d-orbitals can become involved in the bonding.
尽管 f 区环戊二烯基、芳基、环庚三烯基和环辛四烯化物配合物数量丰富，但 d 区中普遍存在的环丁二烯衍生物却未见报道。在这里，我们报告了二苯乙炔和（2,2-二甲基丙炔基）膦与铀（V）反转夹心 10π-甲苯四阴离子配合物的还原[2+2]-环加成反应，导致分离出反转夹心环丁二烯基和双膦环丁二烯基二阴离子铀（IV）配合物。计算分析表明，键合主要是静电的。尽管环丁二烯基和双膦环丁二烯基配体中的 ψ4 分子轨道对于与铀的 δ 键合具有正确的对称性，但主要的共价贡献来自涉及 ψ2 和 ψ3 轨道组合的 π 键合。这与较大的芳烃和环辛四烯化物的铀配合物形成对比，其中 δ 键合占主导地位。这表明，铀与小的四元环键合的角向要求有利于利用 5f-而不是 6d-轨道的 π 键合，而较大配体中有利于 δ 键合，其中 6d-轨道可以参与键合。
