Titanium Hydrazinediido Half-Sandwich Complexes: Highly Active Catalysts for the Hydrohydrazination of Terminal Alkynes at Ambient Temperature

Abstract

Reaction of [Cp*Ti(N Xyl N)(N t Bu)(NH 2 t Bu)] with 1 molar equiv of diphenylhydrazine yielded the hydrazinediido complex [Cp*Ti(N Xyl N)(NNPh 2 )(NH 2 t Bu)] ( 1a ), whereas the orange pyridine adduct [Cp*Ti(N Xyl N)(NNPh 2 )(py)] ( 1b ) was obtained by reacting the imide [Cp*Ti(N Xyl N)(N t Bu)(NH 2 t Bu)] with diphenylhydrazine in the presence of pyridine. The tert -butylamine coordinated to the metal center in 1a could be removed by heating the solid at 60 °C and 10 −6 mbar for 72 h, yielding [Cp*Ti(N Xyl N)(NNPh 2 )] ( 1c ). In the presence of pyridine or 4-dimethylaminopyridine (dmap) as neutral co-ligands the hydrazinediido complexes [Cp*Ti(N Xyl N)(NNMePh)(py)] ( 2a ) and [Cp*Ti(N Xyl N)(NNMePh)(dmap)] ( 2b ) as well as [Cp*Ti(N Xyl N)(NNMe 2 )(dmap)] ( 3 ) were prepared. Upon replacement of dmap by the weaker donor ligand pyridine in the synthesis of the pyridine adduct analogous to 3, a mixture of [Cp*Ti(N Xyl N)(NNMe 2 )(py)] ( 4a ) and the dinuclear complex [Cp* 2 Ti 2 (N Xyl N) 2 (μ-η 1,η 1 -NNMe 2 )(μ-η 1,η 2 -NNMe 2 )] ( 4b ) was obtained. Reaction of the dimethylhydrazinediido complex 3 with phenylacetylene gave the Markovnikov cycloadduct, which had sufficient lifetime to allow its 1 H, 13 C, and 15 N NMR spectroscopic characterization in solution. All three hydrazinediido compounds 1a, 2a, and 3 were found to display remarkable activities in catalytic hydrohydrazinations at ambient temperatures. For catalyst loadings of 5 mol % complete conversions of the terminal alkynes and diynes with Markovnikov regioselectivities of over 99% selectivity were observed within 1 h.