Amination of 2,2″-dibromo-p-terphenyl with 2,6-diisopropylaniline, through Pd mediated cross coupling, yields the p-terphenyl bis­(aniline) ligand H2LAr. Deprotonation of H2LAr with excess KH generates the dianion [K­(DME)2]2LAr as a dark red solid. Treatment of [K­(DME)2]2LAr with UI3(dioxane)1.5 produces the mononuclear U­(III) complex LArU­(I)­(DME) (1). Subsequent addition of the nucleophilic metal anion [CpFe­(CO)2]− (Fp–) gives the bimetallic U­(III) compound LArU­(Fp) (2) in modest yield which features a rare instance of an unsupported U–M bond. Inspection of the metrical parameters of the solid-state structures of 1·DME and 2·0.5DME from X-ray crystallographic analyses show a seemingly η6-interaction between the uranium and the terphenyl ligand (1: U1–Ccentroid = 2.56 Å; 2: U1–Ccentroid = 2.45 Å), spatially imposed as a consequence of the anilide N-donor atom coordination. Furthermore, the U–Fe bond length in 2 (U1–Fe1 = 2.9462(3) Å) is consistent with a metal–metal single bond. Notably, electronic structure analyses by CASPT2 calculations instead suggest that electrostatic, and not covalent, interactions dominate between the U–arene systems in 1 and 2 and between the U–Fe bond in the latter.

Two-electron reduction of the Ti compound ( guan)(Im N)Ti(OTf) (3) gives the arene-masked complex ( guan)(η -Im N)Ti (1) in excellent yield. Upon standing in solution, 1 converts to a Ti metallacycle (4) through dehydrogenation of a pendant isopropyl group. Spectroscopic evidence shows this transformation initially proceeds via the oxidative addition of a C(sp )-H bond and can be reversed upon exposure of 4 to H . Interestingly, treatment of 1 with cyclohexene gives cyclohexane and 4 via a titanium-mediated transfer hydrogenation reaction, a process that can be extended to catalytically hydrogenate other unsaturated hydrocarbons under mild conditions. These results, rare for the early-metals, suggest 1 possesses chemical characteristics reminiscent of noble, late-metals.

Transmetalation of the β-diketiminate salt [M]­[MenacnacPh] (M+ = Na or K; MenacnacPh– = {PhNC­(CH3)}2CH–) with UI3(THF)4 resulted in the formation of the homoleptic, octahedral complex [U­(MenacnacPh)3] (1). Green colored 1 was fully characterized by a solid-state X-ray diffraction analysis and a combination of UV/vis/NIR, NMR, and EPR spectroscopic studies as well as solid-state SQUID magnetization studies and density functional theory calculations. Electrochemical studies of 1 revealed this species to possess two anodic waves for the U­(III/IV) and U­(IV/V) redox couples, with the former being chemically accessible. Using mild oxidants, such as [CoCp2]­[PF6] or [FeCp2]­[Al­{OC­(CF3)3}4], yields the discrete salts [1]­[A] (A = PF6 –, Al­{OC­(CF3)3}4 –), whereas the anion exchange of [1]­[PF 6 ] with NaBPh4 yields [1]­[BPh 4 ].

The two-electron reduction of the Ti­(IV) guanidinate (Ketguan)­(ImDippN)­Ti­(OTf)2 (2 OTf ) (Ketguan = [( t Bu2CN)­C­(NDipp)2]−; ImDippN– = 1,3-bis­(Dipp)­imidazolin-2-iminato; Dipp = 2,6-diisopropylphenyl) with an excess of KC8 generates the masked complex (Ketguan)­(η6-ImDippN)Ti (1). Conversely, reduction of the chloride analogue (Ketguan)­(ImDippN)­TiCl2 (2 Cl ) with an excess of Na/Hg amalgam produces the Ti­(III) compound (Ketguan)­(ImDippN)­TiCl (3), while treatment of 2 Cl with 3.0 equiv of KC8 affords a complicated mixture from which (ImDippN)­(DippN)­[η2-( t Bu2C)NC(NDipp)]­(THF)Ti (4) is isolated as the product of reductive ligand cleavage. These results clearly indicate that the success of early metal reduction chemistry is highly sensitive to the halide coligands and reaction conditions. Complex 1, despite possessing a Ti­(IV) canonical form, behaves as a Ti­(II) synthon and appreciable reducing agent. For instance, 1 effects the one-electron reduction of benzophenone and pyridine to give the Ti­(III) products (Ketguan)­(ImDippN)­Ti­(η1-OC·Ph2) (6) and [(Ketguan)­(ImDippN)­Ti]2[μ2-(NC5H5–H5C5N)] (7), providing an approximate chemical redox potential range for 1 between ca. −2.3 to −3.1 V (vs [Cp2Fe]0/+). Additionally, treatment of 1 with π-acids such as CNCy (Cy = cyclohexyl) or NC t Bu leads to the formation of the Ti­(III) and Ti­(IV) products (Ketguan)­(ImDippN)­Ti­(CN)­(CNCy) (9) and (ImDippN)­[(DippN)­(2- i PrC6H3-6-(η1-CH3CHCH2)­N)­C­(NC t Bu2)]­Ti­[NC­(H) t Bu] (10), respectively, via reduction of the π-acid substrate. The two-electron reduction proclivity of 1 is demonstrated by its reactivity with chalcogen sources (e.g., N2O) and organoazides to give the Ti­(IV) products (Ketguan)­(ImDippN)­Ti­(E) (E = O (13), S (14), Se (15), S2 (16), NSiMe3 (17), NAd (18)). In addition to illustrating the versatile Ti­(II) synthon character of 1, the synthesis of these compounds shows that the 3N-coordinated [(Ketguan)­(ImDippN)­Ti] n+ manifold can readily accommodate metal–ligand multiple bonds, including relatively rare examples of terminally bound TiS and TiSe bonds. Taken altogether, the redox chemistry of 1, as a Ti­(II) synthon, clearly shows the chemical diversity of low-valent early metals (LVEMs) and their ability to reductively activate a wide range of substrates.