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

Addition of 1 equiv of LiNCtBu2 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CtBu2 (1-tBu), 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CtBu2 (5-tBu), 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-tBu and 5-Ad to their guanidinate and amidinate
analogues [{(X)C(NDipp)2}­FeBr]2 (X = iPr2N (6), tBu (7)). Solid-state
structural analyses indicate little electronic contribution from the N-ketimine nitrogen atom, while solution-phase electronic
absorption spectra of 5-tBu 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-tBu to be intermediate between its guanidinate and amidinate counterparts
in 6 and 7. Preliminary reactivity studies
also show that the reduction chemistry of 5-tBu diverges significantly
from that of 6 and 7. Treatment of 5-tBu with excess magnesium or 1 equiv of KC8 leads to the
formation of the Fe­(I)–Fe­(I) complex [{μ-(tBu2C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.