Dinitrogen Activation at Ambient Temperatures:  New Modes of H₂ and PhSiH₃ Additions for an “End-On-Bridged” [Ta(IV)]₂(μ-η:η¹-N₂) Complex and for the Bis(μ-nitrido) [Ta(V)(μ-N)]₂ Product Derived from Facile N⋮N Bond Cleavage

Chemical reduction of Cp*Ta[N( i Pr)C(Me)N( i Pr)](Cl) 3 (Cp* = η 5 -C 5 Me 5 ) ( 1 ) with 2.5 equiv of potassium graphite (KC 8 ) in tetrahydrofuran (THF) provides a 70% yield of {Cp*Ta[N( i Pr)C(Me)N( i Pr)]Cl} 2 (μη:η-ηN 2 ) ( 2 ). With 4 equiv of KC 8, reduction of 1 provides a 34% isolated yield of {Cp*Ta[N( i Pr)C(Me)N( i Pr)]} 2 (μη:η-N 2 ) ( 3 ). Single-crystal X-ray analyses of 2 and 3 provide N−N bond lengths of 1.288(10) and 1.313(4) Å, respectively, which is indicative of substantial hydrazido [μ-N 2 ] 4- character for these compounds. While 2 is thermally robust and chemically inert to hydrogenation and hydrosilylation in solution, above 0 °C, 3 spontaneously and quantitatively converts to {Cp*Ta[N( i Pr)C(Me)N( i Pr)N]} 2 ( 4 ) in a 7:1 ratio of cis and trans isomers. Crystallographic analyses of both cis - and trans - 4 reveal an absence of N−N bonding within the four-membered ring. Hydrosilylation of 3 at 25 °C using PhSiH 3 occurs with 1,4-addition across the Ta N−N Ta framework to provide a quantitative yield of the hydrosilylated product, 5 . Finally, hydrogenation of 3 in pentane at 0 °C, provides an 80% yield of the stereospecific 1,4-addition dihydride product 6 . Single-crystal X-ray analyses of 5 and 6 provide N−N bond lengths of 1.284(4) and 1.307(6) Å, respectively. Finally, cis - 4 was found to readily react with PhSiH 3 at 25 °C to provide 7 as the product of stereospecific ring-opening via σ-bond metathesis of a Ta−N single bond. Crystallographic analysis of 7 confirms the absence of bridging hydrides and the noncyclic nature of this compound.