Doubly Reduced Arene as a Platform for Th(II) and Zr(II) Synthons: M–Arene δ-Bonding Governs Divergent Two-Electron Reactivity

Low-valent Th complexes are extremely rare due to the high stability of Th(IV), yet are important to fully understand the intricacies of actinide–ligand bonding. Comparative results with Zr congeners are necessary to pinpoint differences between the reactivity of transition metal and actinide complexes. Here, we report the synthesis and characterization of two M(II) synthons, Th(NHAriPr6)2 (1) and Zr(NHAriPr6)2 (2) (NHAriPr6 = NH-2,6-(2,4,6-iPr3C6H2)2C6H3), where the central metal ion is coordinated to a two-electron-reduced, puckered arene (Arreduced) through an η6 interaction. The electrons stored in Arreduced offer stepwise, two-electron reduction of substrates which result in diverging products for 1 and 2. Enhanced reactivity was observed for 1 which reduces BnBr (Bn = PhCH2) to afford the dibromide Th(NHAriPr6)2Br2 (3), reduces azobenzene (N2Ph2) to yield the diazametallacyclopropane sandwich Th(NHAriPr6)2(η2-N2Ph2) (5), and activates H2 to form the bis(cyclohexadienyl) Th(η5-NHAriPr6-H)2 (6). For 2, reduction of BnBr produces the cyclohexadienyl Zr(η5-NHAriPr6-Bn)(NHAriPr6)(Br) (4), no reaction is observed with N2Ph2, and reduction of H2 yields the 1,2-hydrogenated Zr(η4-NHAriPr6-H2)(NHAriPr6) (7). Upon reduction of the substrates, the puckered Arreduced becomes planar, reflecting its redox non-innocence and supporting the assignment of 1 and 2 as Th(II) and Zr(II) synthons. DFT and multireference calculations reveal that the markedly destabilized HOMO, larger HOMO–ligand orbital energy gap, increased population of the Arreduced π* orbital via M–Arreduced δ-bonding, and reduced steric bulk of 1 collectively contribute to its divergent reactivity relative to 2.