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 (<b>1</b>) and NMe₂ (<b>2</b>), respectively, were developed. For <b>1</b>, synthesis proceeded via the amido, chloride intermediate Cp*Ta­[N­(ⁱPr)­C­(Me)­N­(ⁱPr)]­(Cl)­[NH­(^tBu)] (<b>4</b>), which was kinetically deprotonated with LiN­(ⁱPr)₂ to yield the enamido, amido species Cp*Ta­[N­(ⁱPr)­C­(CH₂)­N­(ⁱPr)­[NH­(^tBu)] (<b>5</b>). In toluene solution, <b>5</b> underwent quantitative tautomerization to the desired CpAm terminal imido <b>1</b>. For <b>2</b>, the amido, chloride intermediate Cp*Ta­[N­(ⁱPr)­C­(NMe₂)­N­(ⁱPr)]­(Cl)­[NH­(^tBu)] (<b>8</b>) 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)] (<b>9</b>) through oxidative hydrogen-atom abstraction of the amido group. Chemical reduction of <b>9</b> with potassium graphite (KC₈) then served to provide the desired CpGu terminal imido <b>2</b>. All new compounds were structurally characterized by single-crystal X-ray analysis. Although <b>1</b> and <b>2</b> proved to be unreactive toward hydrogenation or hydrosilyation involving the TaN bond, <b>2</b> 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 (<b>10</b>), I (<b>11</b>), and SPh (<b>12</b>). A similar radical-based reaction of {Cp*Ta­[N­(ⁱPr)­C­(Me)­N­(ⁱPr)]}₂(μ-η¹:η¹-N₂) (<b>I</b>) 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) (<b>13</b>)