The formation of the titanocene imido complexes 2b, 4, and 5 via double N−H bond activations at ambient temperature of sterically demanding amines (2,6-diisopropylaniline, tert-butylamine) by the use of the bis(η 5 :η 1 -adamantylidenepentafulvene)titanium complex (1) has been investigated. The titanium imido complex 2b exists in equilibrium with its corresponding titanium monoamide complex 2a. The equilibrium can be shifted to the side of the imide 2b by increasing the temperature. Starting from the equilibrium 2a/2b, or the titanium imido pyridine adduct 5, a broad range of [2 + 2] cycloaddition reactions are realized. In reactions with alkynes, the regioselective formation of the azacyclobutenes 6 and 7 are observed. By using carbodiimides, asymmetric (8 and 10a) and symmetric cycloaddition products (9 and 10b) are formed. In the reaction with isocyanates, dependent on the substitution pattern of the TiNR imido unit, the CO or the CN [2 + 2] cycloaddition occurs, resulting in the N,O-bound ureato(2-) complex 11 or the N,N-bound ureato(2-) complex 13. In the presence of isocyanates and nitriles, the equilibrium 2a/2b results in ring enlargement reactions to six-membered rings (12, 14) by insertion of a second equivalent of the substrate into the Ti−N bonds.

The π-η5:σ-η1 coordination mode of early transition metal pentafulvene ligands yields a strongly nucleophilic exocyclic carbon atom (Cexo). The substitution of the chlorido ligand of bis(η5:η1-(di-p-tolyl)pentafulvene)niobium chloride (1) by reaction...

The substitution of the chlorido ligand of bis(η 5 :η 1 -(di-p-tolyl)-pentafulvene)niobium chloride by reaction with (trimethylsilyl)methyllithium is followed by α-C−H bond activation. Because of the π-η 5 :σ-η 1 coordination mode, the exocyclic carbon atom of the pentafulvene ligand is strongly nucleophilic and therefore protonated, and the niobium alkylidene species (3) is isolated. The formed complex 3 shows cooperating reactions of the remaining pentafulvene and the alkylidene ligand in activating different E−H bonds. Under mild conditions, terminal imido complexes (4) and a hydrazido complex (5) are obtained by double N−H bond splitting. By the reaction of 3 with Nmethylaniline, a niobaaziridine ( 6) is formed via simultaneous N−H and C−H bond activation. By deuterium labeling, the N−H bond can be assigned to the pentafulvene moiety and the C−H bond to the alkylidene carbon center.

Transition-metal carbene complexes and their reactivities are a key topic of chemistry. They are an integral part of researches in catalysis, organic synthesis, coordination chemistry, and numerous other areas. In this context, we report the synthesis of a low-valent bis(η :η -(di-p-tolyl)-pentafulvene)niobium chloride. Owing to the π-η :σ-η coordination mode of the pentafulvenes and the resulting high nucleophilic character of the exocyclic carbon atom of the ligand, the bis(η :η -pentafulvene)niobium complex is able to achieve the umpolung of a coordinated vinyl unit and the resulting formation of the first η :η cyclic niobium Schrock carbene complex. This new synthetic route is, in comparison to classical α-hydrogen elimination reactions or thermolysis of diazo compounds, completely unprecedented. The reactivity of the cyclic carbene function and the remaining fulvene ligand is demonstrated by double N-H bond activation of primary amines to niobium imido complexes.

Transition-metal carbene complexes and their reactivities are akey topic of chemistry.They are an integral part of researches in catalysis,organic synthesis,coordination chemistry,a nd numerous other areas.I nt his context, we report the synthesis of al ow-valent bis(h 5 :h 1 -(di-p-tolyl)-pentafulvene)niobium chloride.O wing to the p-h 5 :s-h 1 coordination mode of the pentafulvenes and the resulting high nucleophilic character of the exocyclic carbon atom of the ligand, the bis(h 5 :h 1 -pentafulvene)niobium complex is able to achieve the umpolung of ac oordinated vinyl unit and the resulting formation of the first h 5 :h 1 cyclic niobium Schrock carbene complex. This new synthetic route is,incomparison to classical a-hydrogen elimination reactions or thermolysis of diazo compounds,c ompletely unprecedented. The reactivity of the cyclic carbene function and the remaining fulvene ligand is demonstrated by double NÀHb ond activation of primary amines to niobium imido complexes.