Heterometallic Clusters with Uranium–Metal Bonds Supported by Double-Layer Nitrogen–Phosphorus Ligands

Abstract

ConspectusHeterometallic clusters with M–M bonds have significantly interested chemists because of their attractive structures and synergistic effects in small-molecule activation and catalysis. However, reports of the isolation of heterometallic clusters with uranium–transition metal (U–TM) bonds remain very limited. In this Account, we describe our research in the construction of heterometallic molecular clusters with multiple U–TM single or multiple bonds supported by novel double-layer N–P ligands. Multimetallic synergistic catalysis and small-molecule activation with these species are also summarized.First, according to the hard–soft acid–base theory, we employed a three-armed N–P ligand, which can be used to construct heterometallic clusters with four or six U–Ni bonds. This strategy was also effective in the construction of complexes with direct rare earth metal–TM bonding. The similar two-armed N–P ligands also are effective platforms for the synthesis of heterometallic complexes with U–Ni, U–Pd, and U–Pt bonds.Second, a set of heterometallic clusters featuring U≡Rh, U≡Co, and U≡Fe triple bonds were constructed under routine experimental conditions. X-ray diffraction analysis of these clusters exhibits the shortest U–TM bond distance (1.9693(4) Å for the U≡Fe triple bond) in these complexes. Theoretical studies reveal that the nature of the triple bond is one covalent σ bond and two TM → U dative π bonds. A large Wiberg bond index (WBI) of 2.93 and a significant degree of covalency for the U≡TM triple bonds were also found in these complexes.Third, these uranium complexes supported by the double-layer N–P ligands exhibit great potential in small-molecule activation. For instance, N2 cleavage without an external reducing agent was achieved by a U(III)–P(III) synergistic six-electron reduction. The synergism between U(III) and P(III) enables the activation of other small molecules, such as O2, P4, and As0(nano), and highlights the importance of the P atom in the double-layer N−P ligand for the activation of small molecules. A heterometallic cluster with U–Rh bonds can break the strong N≡N triple bond in N2 in the presence of potassium graphite, suggesting a synergistic effect between U and Rh. This multimetallic synergistic effect was also observed in catalytic processes. A heterometallic cluster with U≡Co triple bonds shows excellent selectivity and activity in the hydroboration of a series of alkynes under mild conditions. These results lead to effective methods for the construction of heterometallic molecular clusters with U–TM single or multiple bonds and could promote the application of heterometallic clusters with U–TM bonds in catalysis and the activation of small molecules.

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