Unprecedented Spectroscopic and Computational Evidence for Allenyl and Propargyl Titanocene(IV) Complexes: Electrophilic Quenching of Their Metallotropic Equilibrium

• Published in: Chemistry : a European journal 2016-02, Vol.22 (7), p.2427-2439
• Main Authors: Ruiz-Muelle, Ana Belén, Oña-Burgos, Pascual, Ortuño, Manuel A., Oltra, J. Enrique, Rodríguez-García, Ignacio, Fernández, Ignacio
• Format: Article
• Language: English
• Subjects: Aldehydes; Aliphatic compounds; Carbonyl compounds; Carbonyls; Chemistry; Computer applications; density functional calculations; Diffusion; DOSY; Einstein equations; Electronics; Equilibrium; Isomers; Ketones; Mathematical analysis; Mathematical models; metallotropic equilibrium; Monomers; NMR; NMR spectroscopy; Nuclear magnetic resonance; Organometallic compounds; Quenching; Structural analysis; Synthesis; titanocenes
• ISSN: 0947-6539; 1521-3765
• DOI: 10.1002/chem.201504281

The synthesis and structural characterization of allenyl titanocene(IV) [TiClCp2(CH=C=CH2)] 3 and propargyl titanocene(IV) [TiClCp2(CH2−C≡C−(CH2)4CH3)] 9 have been described for the first time. Advanced NMR methods including diffusion NMR methods (diffusion pulsed field gradient stimulated spin echo (PFG‐STE) and DOSY) have been applied and established that these organometallics are monomers in THF solution with hydrodynamic radii (from the Stokes–Einstein equation) of 3.5 and 4.1 Å for 3 and 9, respectively. Full 1H, 13C, Δ1H, and Δ13C NMR data are given, and through the analysis of the Ramsey equation, the first electronic insights into these derivatives are provided. In solution, they are involved in their respective metallotropic allenyl–propargyl equilibria that, after quenching experiments with aromatic and aliphatic aldehydes, ketones, and protonating agents, always give the propargyl products P (when carbonyls are employed), or allenyl products A (when a proton source is added) as the major isomers. In all the cases assayed, the ratio of products suggests that the metallotropic equilibrium should be faster than the reactions of 3 and 9 with electrophiles. Indeed, DFT calculations predict lower Gibbs energy barriers for the metallotropic equilibrium, thus confirming dynamic kinetic resolution. Light on titanocenes: The synthesis and structural characterization of allenyl titanocene(IV) [TiClCp2(CH= C=CH2)] and propargyl titanocene(IV) [TiClCp2(CH2−C≡C−(CH2)4CH3)] are described for the first time. Advanced NMR methods, including diffusion NMR methods, along with DFT calculations, and electrophilic quenching reactions, shed light on their metallotropic equilibrium (see scheme).