Synthesis, Characterization, and Comparative Theoretical Investigation of Dinitrogen-Bridged Group 6-Gold Heterobimetallic Complexes

Abstract

High Resolution Image Download MS PowerPoint Slide We have prepared and characterized a series of unprecedented group 6–group 11, N 2 -bridged, heterobimetallic [ML 4 (η 1 -N 2 )(μ-η 1:η 1 -N 2 )Au(NHC)] + complexes (M = Mo, W, L 2 = diphosphine) by treatment of trans -[ML 4 (N 2 ) 2 ] with a cationic gold(I) complex [Au(NHC)] + . The adducts are very labile in solution and in the solid, especially in the case of molybdenum, and decomposition pathways are likely initiated by electron transfers from the zerovalent group 6 atom to gold. Spectroscopic and structural parameters point to the fact that the gold adducts are very similar to Lewis pairs formed out of strong main-group Lewis acids (LA) and low-valent, end-on dinitrogen complexes, with a bent M–N–N–Au motif. To verify how far the analogy goes, we computed the electronic structures of [W(depe) 2 (η 1 -N 2 )(μ-η 1:η 1 -N 2 )AuNHC] + ( 10 W + ) and [W(depe) 2 (η 1 -N 2 )(μ-η 1:η 1 -N 2 )B(C 6 F 5 ) 3 ] ( 11 W ). A careful analysis of the frontier orbitals of both compounds shows that a filled orbital resulting from the combination of the π* orbital of the bridging N 2 with a d orbital of the group 6 metal overlaps in 10 W + with an empty sd hybrid orbital at gold, whereas in 11 W with an sp 3 hybrid orbital at boron. The bent N–N–LA arrangement maximizes these interactions, providing a similar level of N 2 “push–pull” activation in the two compounds. In the gold case, the HOMO–2 orbital is further delocalized to the empty carbenic p orbital, and an NBO analysis suggests an important electrostatic component in the μ-N 2 –[Au(NHC)] + bond.