A cobalt(ii) complex supported by the new tetradentate polypyridyl ligand PY4 is an electrocatalyst for the reduction of protons to hydrogen and can operate in 50% aqueous media.

The synthesis and reactivity of the end-on bound dinitrogen complex [(η 5 -C 5 H 3 -1,3-(SiMe 3 ) 2 ) 2 Ti] 2 (µ 2 ,η 1 ,η 1 -N 2 ) is described. The solid state structure of the dinitrogen compound reveals a weakly activated end-on bound N 2 ligand with an N-N bond length of 1.164(5) Å. Displacement of the N 2 ligand by organic azides has been used to prepare monomeric, basefree titanocene imido complexes, (η 5 -C 5 H 3 -1,3-(SiMe 3 ) 2 ) 2 TidNR (R ) SiMe 3 , 2,4,6-Me 3 -C 6 H 2 ). While unreactive toward C-H bonds, the Ti-N linkage is readily hydrogenated and participates in group transfer reactions with unsaturated organic molecules such as carbon monoxide and benzophenone. Reaction of the N 2 complex with Ph 2 CN 2 allowed isolation of (η 5 -C 5 H 3 -1,3-(SiMe 3 ) 2 ) 2 Ti(N 2 CPh 2 ), which exists as a mixture of interconverting η 2 and η 1 isomers in solution. The diazoalkane complex also participates in "imido-like" reactivity, producing (η 5 -C 5 H 3 -1,3-(SiMe 3 ) 2 ) 2 Ti(NHNdCPh 2 )H upon addition of H 2 . Changing the diazoalkane to Me 3 SiCHN 2 resulted in isolation of the double cyclometalated titanocene (η 5 -C 5 H 3 -η 1 -1-SiMe 2 CH 2 -3-SiMe 3 ) 2 Ti, arising from facile intramolecular C-H activation of the cyclopentadienyl substituent by a transient titanocene alkylidene.

The activation of dinitrogen by titanium sandwich complexes of the general form (eta5-C5Me4R)2Ti (R = CHMe2, CMe3, SiMe3) has been systematically investigated. Low-temperature, in situ, solution infrared spectroscopy has allowed detection of monomeric bis-dinitrogen complexes of titanium that are isostructural with more familiar dicarbonyl derivatives. One example, (eta5-C5Me4CHMe2)2Ti(N2)2, has also been characterized by X-ray diffraction and reveals weakly activated dinitrogen ligands. From the solution IR data, the relative azophilicity of the titanium sandwich complexes has been established and increases with smaller cyclopentadienyl substituents.

The titanocene dinitrogen complex [(η 5 -C 5 H 2 -1,2,4-Me 3 ) 2 Ti] 2 (µ 2 ,η 2 ,η 2 -N 2 ) (1-N 2 ) has been prepared both by dihydrogen reductive elimination from the in situ generated monohydride derivative and by alkali metal reduction of the corresponding monoiodide. In the solid state, 1-N 2 contains a side-on bound N 2 ligand with an N-N bond length of 1.216(3) Å, consistent with a significant contribution from a two-electron-reduced diazenido ([N 2 ] 2-) fragment. Attempts to maintain side-on coordination of the N 2 ligand but increase the wedge dihedral by introducing isopropyl or tert-butyl substituents induced a change in hapticity to end-on and a less activated bridging dinitrogen ligand. The ansa-titanocene dinitrogen complexes rac-[Me 2 Si(η 5 -C 5 H 2 -2-SiMe 3 -4-t Bu) 2 Ti] 2 (µ 2 ,η 1 ,η 1 -N 2 ) (4-N 2 ) and [Me 2 Si(η 5 -C 5 Me 4 )(η 5 -C 5 H 3 -3-t Bu)Ti] 2 (µ 2 ,η 1 ,η 1 -N 2 ) (5-N 2 ) have also been prepared and crystallographically characterized and shown to have weakly activated, end-on dinitrogen ligands. The latter example coordinates two additional equivalents of N 2 upon cooling under a dinitrogen atmosphere in pentane solution.