Titanaaziridines or η(2)-imine titanium complexes are considered key intermediates of the titanium-catalyzed hydroaminoalkylation of alkenes. Herein, we present an efficient synthetic route to this class of compounds, starting from N-methylanilines and a bis(η(5):η(1)-pentafulvene)titanium complex. Consecutive reactions on the η(2)-methyleneaniline complexes, characterized for the first time, prove a high chemical versatility. In particular, hydroaminoalkylation products were found in reactions of the three-membered titanacycles with alkenes. For the first time, all the intermediates of the hydroaminoalkylation of alkenes were isolated and characterized.

We report the spontaneous coupling of N-heterocycles, initiated by C-H bond activation reactions. The reaction of quinoxalines and the titanocene acetylene complex Cp2Ti{eta2-C2(SiMe3)2}, as an excellent titanocene source, results in the formation of trinuclear 1,6,7,12,13,18-hexaazatrinaphthylene (HATN) titanium complexes. These HATN titanium complexes are thermally stable but sensitive to air and moisture. A three-fold dehydrogenative C-C coupling is proposed as the main step in the presented synthetic procedure. Particularly using commercial starting materials, an efficient route for the dehydrogenative coupling of N-heterocycles, leading to multidentate ligands, has been established.

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Two independent synthetic routes to η2-imine titanocene complexes were developed. On one hand side, ligand exchange reactions of bis(trimethylsilyl)acetylene by (p-Tolyl)HCNPh (3) employing the Rosenthal reagent Cp2Ti{η2-C2(SiMe3)2} (1) lead to Cp2Ti{η2-(p-Tolyl)CHNPh} (5), exhibiting a titanaaziridine structure. On the other hand, the direct reductive complexation of 3 by using Cp2TiCl2 (2) and Mg as reducing agent leads also to 5, one of the rare known titanoceneaziridines without additional ligands. By using the ketimine (p-Tolyl)2CNPh (4) instead of the aldimine 3, an unexpected coordination mode was found by X-ray diffraction, exhibiting an azatitanacyclopent-4-ene structure involving one tolyl fragment. In such a way, via the reductive complexation of 4, employing 2 or Cp*TiCl3 (12), the 1-aza-2-titanacyclopent-4-ene complexes 6 and 13 are formed. Density functional calculations at the M06-2X level identify these new complexes 6 and 13 as 1-aza-2-titanacyclopent-4-enes, in agreement with an analysis based on the experimental structural parameters. A theoretical study of the bonding between the titanocene fragment and the imine ligand reveals that steric factors are more pronounced for titanaaziridines and disfavor their formation compared to azatitanacyclopentenes. This provides a rationalization for the preferred formation of titanoceneaziridines in the case of aldimine ligands and azatitanacyclopentenes when ketimines are applied. Whereas titanoceneaziridine 5 undergoes insertion reactions into the Ti–C carbon σ-bond with aldehydes, ketones, or carbodiimides to the five-membered titanacycles 20 and 21, complex 6 appears to be inert in comparable reactions.

The series of bis(pentafulvene)titanium complexes (η6-C5H4CR2)2Ti (CR2 = C(p-tol)2 (3), adamantyl (4)) and their corresponding bis(benzofulvene) derivatives (η6-C9H6CR2)2Ti (CR2 = C(p-tol)2 (7), adamantyl (8)) have been synthesized by reaction of TiCl3·3THF with the pentafulvene ligands (2 equiv) and magnesium as reducing agent (1.5 equiv). The bis(fulvene) complexes 7 and 8 have been obtained as diastereomerically pure compounds. All complexes have been characterized by spectroscopy and by single-crystal X-ray diffraction. The bonding situation is best described as a π-η5:σ-η1 coordination mode. The bis(fulvene) complex 3 is approximately C 2 symmetric, whereas 7 crystallizes as a conglomerate in the monoclinic space group P21 and one of the enantiomers was identified as a (S,pS,pR) stereoisomer showing C 1 symmetry. The adamantyl benzofulvene complex 8 crystallizes in the monoclinic space group P21/c; both benzofulvene ligands are coordinated as optical antipodes, and a (S,pS,pR)/(R,pR,pS) configuration is found in the solid state. Reaction of 3 and 7 with HCl/Et2O leads to the corresponding bis(cyclopentadienyl)titanium chlorides (η5-C5H4−CHR2)2TiCl2 (R = p-tol (9)) and the bis(indenyltitanium) derivative (η5-C9H6−CHR2)2TiCl2 (R = p-tol2 (10)).