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.

将1当量的二(叔丁基)酮亚胺锂（LiN=C(tBu)₂）或2-金刚烷基酮亚胺锂（LiN=CAd，Ad=2-adamantyl）加入至芳基碳二亚胺C(NDipp)₂（Dipp=2,6-二异丙基苯基，2,6-diisopropylphenyl）中，可高产率地得到锂代酮亚胺胍基配合物Li(THF)₂[(X)C(NDipp)₂]（其中X为N=C(tBu)₂ (1-tBu)、N=CAd (1-Ad)，THF为四氢呋喃，tetrahydrofuran）。这类新型配体可与铁发生金属化反应，得到N,N'-双齿螯合配合物[{(X)C(NDipp)₂}FeBr]₂（X为N=C(tBu)₂ (5-tBu)、N=CAd (5-Ad)），其中酮亚胺基团作为非配位骨架取代基存在。为探究酮亚胺基团对配体结构的潜在电子贡献，我们系统开展了结构与电子性质研究，对比了5-tBu、5-Ad与其胍基、脒基类似物[{(X)C(NDipp)₂}FeBr]₂（X为iPr₂N (6)、tBu (7)）的结构与性能。固态结构分析表明，N-酮亚胺氮原子几乎未提供电子效应；而5-tBu与5-Ad的溶液相电子吸收光谱与脒基配合物7在定性上高度相似。然而电化学测试显示，5-tBu中的酮亚胺胍基配体的给电子能力介于其胍基对应物6与脒基对应物7之间。初步反应性研究同样表明，5-tBu的还原反应行为与6、7存在显著差异。将5-tBu与过量镁或1当量石墨酸钾（KC8）反应，可得到铁(I)-铁(I)双核配合物[{μ-(tBu₂C=N)C(NDipp)₂}₂Fe₂] (11)，该配合物拥有极短的Fe-Fe键（2.1516(5) Å）；而6和7在还原过程中均未生成双核配合物。这一结果证明，酮亚胺胍基配体并非简单的脒基变体，而是可形成具有独特金属化学特性的配合物。