Reductive assembly of cyclobutadienyl and diphosphacyclobutadienyl rings at uranium

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 10 pi-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 psi(4) molecular orbital in the cyclobutadienyl and diphosphacyclobutadienyl ligands exhibits the correct symmetry for delta-bonding to uranium, the dominant covalent contributions arise from pi-bonding involving psi(2) and psi(3) orbital combinations. This contrasts with uranium complexes of larger arenes and cyclo-octatetraenide, where delta-bonding dominates. This suggests that the angular requirements for uranium to bond to a small four-membered ring favours pi-bonding, utilizing 5f- instead of 6d-orbitals, over delta-bonding that is favoured with larger ligands, where 6d-orbitals can become involved in the bonding.