Dinitrogen activation at ambient temperatures:: New modes of H2 and PhSiH3 additions for an End-on-bridged [Ta(IV)]2(μ-η1: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(Pr-i)C(Me)N(Pr-i)](Cl)(3) (Cp* = eta(5)-C5Me5) (1) with 2.5 equiv of potassium graphite (KC8) in tetrahydrofuran (THF) provides a 70% yield of {Cp*Ta[N(Pr-i)C(Me)N(Pr-i)]Cl}(2)(mu eta:eta-eta N-2) (2). With 4 equiv of KC8, reduction of 1 provides a 34% isolated yield of {Cp*Ta[N(Pr-i)C(Me)N(Pr-i)]}(2)(mu eta:eta-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) A, respectively, which is indicative of substantial hydrazido [mu-N-2](4-) character for these compounds. While 2 is thermally robust and chemically inert to hydrogenation and hydrosilylation in solution, above 0 degrees C, 3 spontaneously and quantitatively converts to {Cp*Ta[N(Pr-i)C(Me)N(Pr-i)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 degrees C using PhSiH3 occurs with 1,4-addition across the TaN-NTa framework to provide a quantitative yield of the hydrosilylated product, 5. Finally, hydrogenation of 3 in pentane at 0 degrees 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) A, respectively. Finally, cis-4 was found to readily react with PhSiH3 at 25 degrees C to provide 7 as the product of stereospecific ring-opening via sigma-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.