Diniobium inverted sandwich complexes with μ-η⁶:η⁶-arene ligands: Synthesis, kinetics of formation, and electronic structure

Monometallic niobium arene complexes [Nb(BDI)(N^tBu)(R-C₆H₅)] (2a: R = H and 2b: R = Me, BDI = N,N′-diisopropylbenzene-β-diketiminate) were synthesized and found to undergo slow conversion into the diniobium inverted arene sandwich complexes [[(BDI)Nb(N^tBu)]₂(μ-RC₆H₅)] (7a: R = H and 7b: R = Me) in solution. The kinetics of this reaction were followed by ¹H NMR spectroscopy and are in agreement with a dissociative mechanism. Compounds 7a-b showed a lack of reactivity toward small molecules, even at elevated temperatures, which is unusual in the chemistry of inverted sandwich complexes. However, protonation of the BDI ligands occurred readily on treatment with [H(OEt₂)][B(C₆F₅)₄], resulting in the monoprotonated cationic inverted sandwich complex 8 [[(BDI^#)Nb(N^tBu)][(BDI)Nb(N^tBu)](μ-C₆H₅)][B(C₆F₅)₄] and the dicationic complex 9 [[(BDI^#)Nb(N^tBu)]₂(μ-RC₆H₅)][B(C₆F₅)₄]₂ (BDI^# = (ArNC(Me))₂CH₂). NMR, UV-vis, and X-ray absorption near-edge structure (XANES) spectroscopies were used to characterize this unique series of diamagnetic molecules as a means of determining how best to describe the Nb-arene interactions. The X-ray crystal structures, UV-vis spectra, arene ¹H NMR chemical shifts, and large J_CH coupling constants provide evidence for donation of electron density from the Nb d-orbitals into the antibonding π system of the arene ligands. However, Nb L_3,2-edge XANES spectra and the lack of sp³ hybridization of the arene carbons indicate that the Nb → arene donation is not accompanied by an increase in Nb formal oxidation state and suggests that 4d² electronic configurations are appropriate to describe the Nb atoms in all four complexes.