Dinitrogen Complexation and Extent of NN Activation within the Group 6 “End-On-Bridged” Dinuclear Complexes, {(η⁵-C₅Me₅)M[N(i-Pr)C(Me)N(i-Pr)]}₂(μ-η¹:η¹-N₂) (M = Mo and W)

Chemical reduction of Cp*M[N(i-Pr)C(Me)N(i-Pr)]Cl₃ (Cp* = η⁵-C₅Me₅) (1, M = Mo) and (2, M = W) using 0.5% NaHg in THF provided excellent yields of the diamagnetic dinuclear end-on-bridged dinitrogen complexes {Cp*M[N(i-Pr)C(Me)N(i-Pr)]}₂(μ-η¹:η¹-N₂) (6, M = Mo) and (8, M = W), respectively. Chemical reduction of Cp*Mo[N(i-Pr)C(NMe₂)N(i-Pr)]Cl₂ (4) with 3 equiv of KC₈ in THF similarly yielded diamagnetic {Cp*Mo[N(i-Pr)C(NMe₂)N(i-Pr)]}₂(μ-η¹:η¹-N₂) (7). Single-crystal X-ray analyses of 7 and 8 confirmed the dinuclear end-on-bridged μ-η¹:η¹-N₂ coordination mode and the solid-state molecular structures of these compounds provided d(NN) values of 1.267(2) and 1.277(8) Å for 7 and 8, respectively. Based on a comparison of ¹⁵N NMR spectra for ¹⁵N₂ (99%)-labeled 6 and ¹⁵N₂ (99%)-labeled 8, as well as similarities in chemical reactivity, a dinuclear μ-η¹:η¹-N₂ structure for 6 is further proposed. For comparison with a first-row metal derivative, chemical reduction of Cp*Ti[N(i-Pr)C(Me)N(i-Pr)]Cl₂ (9) with KC₈ in THF was conducted to provide {Cp*Ti[N(i-Pr)C(Me)N(i-Pr)]}₂(μ-η¹:η¹-N₂) (10) for which a d(NN) value of 1.270(2) Å was obtained through X-ray crystallography. Compounds 6−8 were all found to be thermally robust in toluene solution up to temperatures of at least 100 °C, and 6 and 8 were determined to be inert toward the addition of H₂ or H₃SiPh under a variety of conditions. Single-crystal X-ray analysis of meso-{Cp*Mo(H)[N(i-Pr)C(Me)N(i-Pr)]}₂(μ-η¹:η¹-N₂) (meso-11), which was serendipitously isolated as a product of attempted alkylation of Cp*Mo[N(i-Pr)C(Me)N(i-Pr)]Cl₂ (3) with 2 equiv of n-butyllithium, revealed a smaller d(NN) value of 1.189(4) Å that is consistent with two Mo(IV,d²) centers connected by a bridging diazenido, [μ-N₂]²⁻, moiety. Moreover, meso-11 was found to undergo clean dehydrogenation in solution at 50 °C to provide 6 via a first-order process. Chemical oxidation of 8 with an excess of PbCl₂ in toluene solution at 25 °C provided a 1:1 mixture of rac- and meso-{Cp*W(Cl)[N(i-Pr)C(Me)N(i-Pr)]}₂(μ-η¹:η¹-N₂) (12); both isomers of which provided solid-state structures through X-ray analyses that are consistent with an electronic configuration comprised of two W(IV,d²) centers linked through a bridging [N₂]²⁻ group [cf. for rac-12, d(NN) = 1.206(9) Å, and for meso-12, d(NN) = 1.192(3) Å]. Finally, treatment of 6 and 8 with either 4 equiv of CNAr (Ar = 3,5-Me₂C₆H₃) or an excess of CO in toluene provided excellent yields of Cp*M[N(i-Pr)C(Me)N(i-Pr)](CNAr)₂ (13, M = Mo and 14, M = W) and Cp*M[N(i-Pr)C(Me)N(i-Pr)](CO)₂ (15, M = Mo and 16, M = W), respectively. Single-crystal X-ray analyses of 13−16, along with observation of reduced IR vibrational ν_CN or ν_CO bond-stretching frequencies, provide strong support for the electron-rich character of the Cp*M[N(i-Pr)C(Me)N(i-Pr)] fragment that can engage in a high degree of back-donation with moderate to strong π-acceptors, such as N₂, CNR, and CO. The collective results of this work are analyzed in terms of the possible steric and electronic factors that contribute to preferred mode of μ-N₂ coordination and the extent of NN activation, including complete N−N bond scission, within the now completed experimentally-derived ligand-centered isostructural series of {Cp*M[N(i-Pr)C(Me)N(i-Pr)]}₂(μ-N₂) compounds where M = Ti, Zr, Hf, Ta, Mo, and W.