Mononuclear Tantalum(IV, d1) Imido Complexes Supported by the Monocyclopentadienyl, Amidinate and Guanidinate Ligand Sets As Models to Explore Dinitrogen Fixation by “End-On-Bridged” Dinuclear {[Ta(IV, d1)]}2(μ-η1:η1-N2) Complexes

Two independent synthetic routes to the monocyclopentadienyl, amidinate (CpAm) and guanidinate (CpGu) mononuclear Ta­(IV, d1) terminal imido complexes Cp*Ta­[N­(iPr)­C­(X)­N­(iPr)]­[N­(tBu)] (Cp* = η5-C5Me5) for X = Me (1) and NMe2 (2), respectively, were developed. For 1, synthesis proceeded via the amido, chloride intermediate Cp*Ta­[N­(iPr)­C­(Me)­N­(iPr)]­(Cl)­[NH­(tBu)] (4), which was kinetically deprotonated with LiN­(iPr)2 to yield the enamido, amido species Cp*Ta­[N­(iPr)­C­(CH2)­N­(iPr)­[NH­(tBu)] (5). In toluene solution, 5 underwent quantitative tautomerization to the desired CpAm terminal imido 1. For 2, the amido, chloride intermediate Cp*Ta­[N­(iPr)­C­(NMe2)­N­(iPr)]­(Cl)­[NH­(tBu)] (8) was first synthesized and then reacted with TEMPO to provide the Ta­(V) imido chloride Cp*Ta­[N­(iPr)­C­(NMe2)­N­(iPr)]­(Cl)­[N­(tBu)] (9) through oxidative hydrogen-atom abstraction of the amido group. Chemical reduction of 9 with potassium graphite (KC8) then served to provide the desired CpGu terminal imido 2. All new compounds were structurally characterized by single-crystal X-ray analysis. Although 1 and 2 proved to be unreactive toward hydrogenation or hydrosilyation involving the TaN bond, 2 was shown to engage in radical-based chemistry with MeI and PhS-SPh to yield the Ta­(V) imido complexes Cp*Ta­[N­(iPr)­C­(NMe2)­N­(iPr)]­(X)­[N­(tBu)], where X = Me (10), I (11), and SPh (12). A similar radical-based reaction of {Cp*Ta­[N­(iPr)­C­(Me)­N­(iPr)]}2(μ-η1:η1-N2) (I) with PhS-SPh yielded the product of formal 1,4-addition, {Cp*Ta­[N­(iPr)­C­(Me)­N­(iPr)]­(SPh)}2(μ-η1:η1-N2) (13).