| Abstract
| - Chemical reduction of Cp*Ta[N(iPr)C(Me)N(iPr)](Cl)3 (Cp* = η5-C5Me5) (1) with 2.5 equiv of potassium graphite (KC8) in tetrahydrofuran (THF) provides a 70% yield of {Cp*Ta[N(iPr)C(Me)N(iPr)]Cl}2(μη:η-ηN2) (2). With 4 equiv of KC8, reduction of 1 provides a 34% isolated yield of {Cp*Ta[N(iPr)C(Me)N(iPr)]}2(μη:η-N2) (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-N2]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(iPr)C(Me)N(iPr)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 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 °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 PhSiH3 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.
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