Redox Character and Small Molecule Reactivity of a Masked Titanium(II) Synthon

• Published in: Organometallics Vol. 39; no. 2; pp. 295 - 311
• Main Authors: Aguilar-Calderón, J. Rolando, Murillo, Jesse, Gomez-Torres, Alejandra, Saucedo, Carlos, Jordan, Aldo, Metta-Magaña, Alejandro J, Pink, Maren, Fortier, Skye
• Format: Journal Article
• Language: English
• Published: American Chemical Society 27.01.2020
• ISSN: 0276-7333; 1520-6041
• DOI: 10.1021/acs.organomet.9b00637

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.