Donor Properties of a New Class of Guanidinate Ligands Possessing Ketimine Backbones: A Comparative Study Using Iron

Addition of 1 equiv of LiN═C(t)Bu2 or LiN═Ad (Ad = 2-adamantyl) to the aryl carbodiimide C(NDipp)2 (Dipp = 2,6-diisopropylphenyl) readily generates the lithium ketimine-guanidinates Li(THF)2[(X)C(NDipp)2] (X = N═C(t)Bu2 (1-(t)Bu), N═Ad (1-Ad)) in excellent yields. These new ligands can be readily metalated with iron to give the N,N'-bidentate chelates [{(X)C(NDipp)2}FeBr]2 (X = N═C(t)Bu2 (5-(t)Bu), N═Ad (5-Ad)), in which the ketimines behave as noncoordinating backbone substituents. In an effort to understand the potential electronic contributions of the ketimine group to the ligand architecture, a thorough structural and electronic study was conducted comparing the features and properties of 5-(t)Bu and 5-Ad to their guanidinate and amidinate analogues [{(X)C(NDipp)2}FeBr]2 (X = (i)Pr2N (6), (t)Bu (7)). Solid-state structural analyses indicate little electronic contribution from the N-ketimine nitrogen atom, while solution-phase electronic absorption spectra of 5-(t)Bu and 5-Ad are qualitatively similar to the amidinate complex 7. Yet, electrochemical measurements do show the donor properties of the ketimine-guanidinate in 5-(t)Bu to be intermediate between its guanidinate and amidinate counterparts in 6 and 7. Preliminary reactivity studies also show that the reduction chemistry of 5-(t)Bu diverges significantly from that of 6 and 7. Treatment of 5-(t)Bu with excess magnesium or 1 equiv of KC8 leads to the formation of the Fe(I)-Fe(I) complex [{μ-((t)Bu2C═N)C(NDipp)2}2Fe2] (11), which possesses an exceedingly short Fe═Fe bond (2.1516(5) Å), while neither 6 nor 7 forms dinuclear complexes upon reduction. This result demonstrates that ketimine-guanidinates do not simply behave as amidinate variants but can contribute to distinctive metal chemistry of their own.