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

We have prepared and characterized a series of unprecedented group 6-group 11, N-2-bridged, heterobimetallic [ML4(eta(1)-N-2)(mu-eta(1):eta(1)-N-2)Au(NHC)](+) complexes (M = Mo, W, L-2 = diphosphine) by treatment of trans-[ML4(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)(eta(1)-N-2)(mu-eta(1):eta(1)-N-2)AuNHC](+) (10(W)(+)) and [W(depe)(2)(eta(1)-N-2)(mu-eta(1):eta(1)-N-2)B(C6F5)(3)] (11(W)). A careful analysis of the frontier orbitals of both compounds shows that a filled orbital resulting from the combination of the pi* 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 mu-N-2-[Au(NHC)](+) bond.

Automated summary

A series of unprecedented group 6-group 11, N-2-bridged, heterobimetallic complexes have been synthesized and characterized, which are very labile and exhibit structural similarities to Lewis pairs. Computational analysis reveals important electrostatic components in the mu-N-2-[Au(NHC)](+) bond in the gold complexes.