Spectroscopic Comparison of Dinuclear Ti + and Ti 2+ μ‐η 1 :η 1 Dinitrogen Complexes with Cp*/Pentafulvene and Amine/Amide Ligation: Moderate versus Strong Activation of N 2

The two dinuclear, end‐on dinitrogen‐bridged titanium complexes with cp*/pentafulvene ligation, [{(η 5 ‐C 5 Me 5 )Ti(η 6 ‐C 5 H 4 CR 2 )} 2 (μ‐η 1 :η 1 ‐N 2 )], R = p ‐tolyl ( 1 ) and >CR 2 = adamantylidene ( 2 ), are compared to an analogous titanium dinitrogen complex with a mixed amine/amide/chloride ligand set, [{(Me 3 Si) 2 NTiCl(TMEDA)} 2 (μ‐η 1 :η 1 ‐N 2 )] ( 3 ; TMEDA = tetramethylethylenediamine). The N–N stretching vibrations of complexes 1 and 2 are observed at 1749 and 1755 cm –1 , respectively, indicative of a moderate activation of the dinitrogen ligand. In contrast, the N–N stretch of 3 is located at 1284 cm –1 , reflecting a much more highly activated N 2 ligand. These findings are in qualitative agreement with N–N bond lengths that have been observed experimentally. DFT calculations reveal that the major difference in the electronic structures of 1 , 2 , and 3 is the fact that only one of the N 2 π* orbitals is (doubly) occupied in the case of 1 and 2 , whereas both π* orbitals are (doubly) occupied in the case of 3 . The reason for this finding is that in 1 and 2 two of the three electrons of each Ti + are involved in back‐bonding interactions with the terminal fulvene ligands, whereas no such interactions exist between the Ti 2+ centers of 3 and its terminal set of donor ligands. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006)