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, d(1)) terminal imido complexes Cp*Ta[N(Pr-i)C(X)N(Pr-i))[N(Bu-t)] (Cp* = eta(5)-C5Me5) for X = Me (1) and NMe2 (2), respectively, were developed. For 1, synthesis proceeded via the amido, chloride intermediate Cp*Ta[N(Pr-i)C(Me)N(Pr-i)](Cl)[NH(Bu-t)] (4), which was kinetically deprotonated with LiN(Pr-i)(2) to yield the enamido, amido species Cp*Ta[N(Pr-i)C(CH2)N(Pr-i)[NH(Bu-t)] (5). In toluene solution, 5 underwent quantitative tautomerization to the desired CpAm terminal imido 1. For 2, the amido, chloride intermediate Cp*Ta[N(Pr-i)C(NMe2)N(Pr-i)](Cl)[NH(Bu-t)] (8) was first synthesized and then reacted with TEMPO to provide the Ta(V) C(NMe2)N(Pr-i)](Cl)[N(Bu-t)] (9) through oxidative hydrogen-atom abstraction of the amido imido group. 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(Pr)](X)[N(Bu-t)], where X = Me (10), I (11), and SPh (12). A similar radical-based reaction of {Cp*Ta[N(Pr-i)C(Me)N(Pr-i)]}(2)(mu-eta(1):eta(1)-N-2) (I) with PhS-SPh yielded the product of formal 1,4-addition, {Cp*Ta[N(Pr-i)C(Me)N(Pr-i)](SPh)}(2)(mu-eta(1):eta(1)-N-2) (13).