Selective oxo ligand functionalisation and substitution reactivity in an oxo/catecholate-bridged U(IV)/U(IV) Pacman complex

The oxo- and catecholate-bridged U(IV)/U(IV) Pacman complex [{(py)U(IV)OU(IV)(μ-O(2)C(6)H(4))(py)}(L(A))] A (L(A) = a macrocyclic “Pacman” ligand; anthracenylene hinge between N(4)-donor pockets, ethyl substituents on meso-carbon atom of each N(4)-donor pocket) featuring a bent U(IV)–O–U(IV) oxo bridge readily reacts with small molecule substrates to undergo either oxo-atom functionalisation or substitution. Complex A reacts with H(2)O or MeOH to afford [{(py)U(IV)(μ-OH)(2)U(IV)(μ-O(2)C(6)H(4))(py)}(L(A))] (1) and [{(py)U(IV)(μ-OH)(μ-OMe)U(IV)(μ-O(2)C(6)H(4))(py)}(L(A))] (2), respectively, in which the bridging oxo ligand in A is substituted for two bridging hydroxo ligands or one bridging hydroxo and one bridging methoxy ligand, respectively. Alternatively, A reacts with either 0.5 equiv. of S(8) or 4 equiv. of Se to provide [{(py)U(IV)(μ-η(2):η(2)-E(2))U(IV)(μ-O(2)C(6)H(4))(py)}(L(A))] (E = S (3), Se (4)) respectively, in which the [E(2)](2–) ion bridges the two U(IV) centres. To the best of our knowledge, complex A is the first example of either a d- or f-block bimetallic μ-oxo complex that activates elemental chalcogens. Complex A also reacts with XeF(2) or 2 equiv. of Me(3)SiCl to provide [{(py)U(IV)(μ-X)(2)U(IV)(μ-O(2)C(6)H(4))(py)}(L(A))] (X = F (5), Cl (6)), in which the oxo ligand has been substituted for two bridging halido ligands. Reacting A with either XeF(2) or Me(3)SiCl in the presence of O(Bcat)(2) at room temperature forms [{(py)U(IV)(μ-X)(μ-OBcat)U(IV)(μ-O(2)C(6)H(4))(py)}(L(A))] (X = F (5A), Cl (6A)), which upon heating to 80 °C is converted to 5 and 6, respectively. In order to probe the importance of the bent U(IV)–O–U(IV) motif in A on the observed reactivity, the bis(boroxido)-U(IV)/U(IV) complex, [{(py)(pinBO)U(IV)OU(IV)(OBpin)(py)}(L(A))] (B), featuring a linear U(IV)–O–U(IV) bond angle was treated with H(2)O and Me(3)SiCl. Complex B reacts with two equiv. of either H(2)O or Me(3)SiCl to provide [{(py)HOU(IV)OU(IV)OH(py)}(L(A))] (7) and [{(py)ClU(IV)OU(IV)Cl(py)}(L(A))] (8), respectively, in which reactions occur preferentially at the boroxido ligands, with the μ-oxo ligand unchanged. The formal U(IV) oxidation state is retained in all of the products 1–8, and selective reactions at the bridging oxo ligand in A is facilitated by: (1) its highly nucleophilic character which is a result of a non-linear U(IV)–O–U(IV) bond angle causing an increase in U–O bond covalency and localisation of the lone pairs of electrons on the μ-oxo group, and (2) the presence of the bridging catecholate ligand, which destabilises a linear oxo-bridging geometry and stabilises the resulting products.